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Passive Q-Switching and Mode-Locking of Erbium-Doped Fluoride Fiber Lasers at 2.7 μm
OPTICAL FIBER TECHNOLOGY ARTICLE NO.
2, 358]366 Ž1996.
0041
Passive Q-Switching and Mode-Locking of Erbium-Doped Fluoride Fiber Lasers at 2.7 m m CHRISTIAN FRERICHS
AND
UDO B. UNRAU*
Institut fur ¨ Hochfrequenztechnik, Technische Uni¨ ersitat, ¨ D-38106 Braunschweig, Germany Received May 15, 1996; revised July 18, 1996
Passive Q-switched and mode-locked operation of a 2.7-m m fluorozirconate fiber laser is reported. Both mode-locking and Q-switching were attained by means of InAs epilayers used as saturable absorbers. Another technique used was the so-called flying-mirror mode-locking, whereby an external resonator with a vibrating mirror is coupled to the active fiber resonator. Both techniques yielded Q-switched pulses containing trains of mode-locked pulses with durations shorter than 2.5 ns—the resolution limit of the detection unit. Q 1996 Academic Press, Inc.
Solid-state lasers at wavelengths around 3 m m have attracted much interest because of their potential applications in medicine and sensing. In the medical field, the strong absorption of radiation with wavelengths around 3 m m in water could lead to lasers excellently suited for the ablation of tissue w1x. An example in the wide field of sensing applications would be ice sensors for cars w2x. Spectroscopic applications are numerous and, especially pulse measurements, e.g., for the transient behavior of CO molecules on surfaces, are of interest w3x. Fiber lasers have certain advantages compared to other kinds of lasers in this wavelength region. They have potentially very low threshold powers and high efficiencies due to their large power densities in the fiber core and their large surface-to-volume ratio, which leads to good heat dissipation. They are simple devices compared to color center lasers, lead salt diodes, or optical parametric oscillators operating in the mid-infrared w4x. Unlike the latter devices, fiber lasers usually operate in the cw mode. Furthermore, they can be pumped with laser diodes, making the whole device potentially cheap and highly efficient. Since silicate glasses are opaque in the mid-infrared, fluoride glasses exhibiting low phonon energies are usually used as laser host in this spectral region. The most common material is the multicomponent glass called * To whom correspondence should be addressed. Fax: 49-531-3915841. E-mail: [email protected].
ZBLAN, one of the most stable and easily fabricated infrared glasses. The classical composition is 53% ZrF4 , 20% BaF2 , 4% LaF3 , 3% AlF3 , and 20% NaF. To raise the refractive index of the fiber core, an amount of PbF2 is added. Laser operation at 2.7 m m in an Er 3q-doped ZBLAN fiber was first demonstrated in 1988 by Brierley and France w5x. Fiber lasers at this wavelength have been realized with very low threshold powers and high efficiencies if pumped with a Ti:sapphire laser at about 795 nm or a DCM dye laser at about 650 nm. Tuning ranges of 160 nm from 2.67 to 2.83 m m were achieved using a diffraction grating w6x. Laser diode pumping at 800 nm was first published by Allen et al. w7x. A fiber laser at 2.7 m m pumped by an 800-nm laser diode with a threshold below 1 mW and a slope efficiency of more than 20% was achieved w8x. While there are numerous papers dealing with cw fiber lasers at 2.7 m m, very little research has been done on pulsed operation of these lasers. This operation mode could overcome some drawbacks of 2.7-m m cw fiber lasers and could open a wide field of applications to these lasers. Unfortunately, the output powers in cw fiber lasers at 2.7 m m are modest so far. Many applications, particularly in the medical field, need much higher powers. Pulses with peak powers some orders of magnitude higher than in the cw mode can be achieved by Q-switching the laser, thus overcoming the low-average output. Pulses of even shorter duration are needed in spectroscopy, where they can give new insights into ultrashort processes connected with these wavelengths w3x. Such ultrashort pulses can be generated by mode-locking the lasers. Q-switched and mode-locked fiber lasers were first demonstrated in 1986 w9, 10x. In the field of mode-locked NIR fiber lasers, sophisticated techniques have been developed that are capable of delivering pulses with durations below 100 fs w11x. Until recently, in the 3-m m region, Q-switching and mode-locking were achieved only with flashlamp-pumped crystal lasers such as Er:YAG or Er:YSGG using active and passive elements as Q-switches.
358 1068-5200r96 $18.00 Copyright Q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
Q-switched erbium crystal lasers around 3 m m have been realized with various techniques such as rotating mirrors w12x, acoustooptic modulators w13x, and electrooptic modulators w14x and passive techniques such as water]soap films w15x and InAs epilayers w16x used as saturable absorbers. Pulse durations down to 28 ns and pulse energies up to 180 mJ, corresponding to peak powers in the MW range, have been achieved w16x. The first operation of a Q-switched erbium-doped laser-diode pumped fluoride fiber laser at 2.7 m m was reported recently w17x. Utilizing an acoustooptic modulator, pulse durations down to 100 ns and peak powers exceeding 2 W could be measured. Using a rotating mirror as switching element yielded much poorer results. Mode-locked operation of flashlamp-pumped 3 m m erbium crystal lasers was performed utilizing techniques such as acoustooptic w18x and electrooptic mode-locking w19, 20x and passive mode-locking using InAs epilayers as saturable absorbers w21x. The best results reported were pulse durations down to 30 ps in a Q-switched envelope. First results of diode-pumped mode-locked and Qswitched fiber lasers at 2.7 m m were published recently by our group. The ‘‘flying-mirror’’ or kinematic mode-locking technique was utilized, whereby mode-locked pulses were generated by coupling a linear external resonator with a vibrating mirror to the active fiber resonator w22x or a simple passive modulation scheme with InAs epilayers as saturable absorbers w23x. Mode-locked pulse trains with detector-limited pulse durations down to 2 ns were presented w23x. This paper describes more mature experimental results of the two above-mentioned means of pulsed operation of 2.7 m m erbium-doped fluoride fiber lasers: The first is flying-mirror mode-locking, the second mode-locking and Q-switching by introducing a thin epilayer of InAs into the laser resonator acting as saturable absorber. Both techniques are passive modulation schemes: even the frequency of the vibrating mirror in the flying-mirror modelocking setup is not necessarily related to the resonator round-trip frequency of the laser. In active techniques, the switching element is driven by an external source, which in the case of mode-locking short lasers with high repetition rates is complicated. In contrast, the techniques applied here are simple and allow for very high repetition frequencies. Both techniques can yield not only modelocked but also Q-switched pulses simultaneously. The flying-mirror mode-locking technique has been realized already with many laser types, e.g., with nearinfrared fiber lasers w24x. The pulse generation mechanism may be explained in the following way: the mirror translation causes a frequency shift of the signal in the external cavity due to the Doppler effect. This frequency-shifted signal is fed back into the active resonator and amplified. In this way new frequency components, phase-locked to
359
each other, are added. After some thousands of resonator round-trips this leads to energy coupling between adjacent modes, hence mode-locking may start w24x. It has been shown theoretically that nonlinear effects are not required for pulse formation w25x. Although the technique is mostly used for the generation of mode-locked pulses, under certain circumstances also the formation of Qswitched pulses was reported and explained w26x. Passive mode-locking and Q-switching with saturable absorbers performed with InAs epilayers on GaAs substrate has the advantage of being an all-solid-state technology with no moving elements. The InAs epilayers used here have been described in w16x and w21x; they were grown 0.09]0.4 m m thick on GaAs substrates. The room temperature bandgap of InAs corresponds to a wavelength of 3.54 m m. Absorption saturation at lower wavelengths occurs due to the dynamic band-filling or Burstein]Moss effect. Fast recovery times in the order of 100 ps depending on the thickness of the samples have been reported w21x. These authors also reported problems due to the low damage intensities of the samples when used in flashlamp-pumped erbium lasers. Since the intensities in Er:ZBLAN fiber lasers are much lower, these problems should not pose too great a handicap. EXPERIMENTAL
The laser transition in Er 3q exploited here is 4 I 11r12 ª4 I 13r2 . Pump lasers were a 500-mW SDL AlGaAs laser emitting around 792 nm, a titanium]sapphire laser emitting around 800 nm, and a DCM dye laser emitting around 650 nm. The latter two lasers were used for higher pump power experiments with the InAs layers. Both pump wavelengths are well suited to pump the 2.7-m m laser since both have the advantage of depopulating the lower laser level by an excited state absorption process w6x. Two fibers, both manufactured by Le Verre Fluore, ´ were used: the first one was doped with 1000 ppm Er 3q, had a core diameter of 30 m m and a numerical aperture of 0.15, and had lengths decreasing from 140 to 115 cm during the experiments. The second fiber was doped with 3000 ppm Er 3q, had a core diameter of 6 m m and a numerical aperture of 0.4, and was used in lengths between 150 and 120 cm. The latter fiber was only used in the saturable absorber experiments. The coupling efficiencies were measured as about 37% Žlaser diode radiation into 30-m m fiber., 8.5% Žlaser diode radiation into 6-m m fiber., 45% ŽTi:sapphire laser radiation into 30-m m fiber. and 50% Ždye laser radiation into 6-m m fiber.. In the 30-m m fiber, about 70% of the launched power at 800 nm could be absorbed; in the 6-m m fiber, more than 95% could be absorbed at either wavelength. Figure 1 shows the flying-mirror mode-locking setup. The pump radiation was focused into the active fiber
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layer directly butted to the fiber ends. Pump source, input mirror setup, and fiber were the same as those described above. The output radiation was focused by an IRGN6 lens through a long-pass filter onto the detection units described above. The InAs epilayers were between 0.09 and 0.4 m m thick and were grown by molecular beam epitaxy on 0.25-mm-thick GaAs substrates w16x. Mounted in the way described, they served both as output mirror and as saturable absorber. Due to the refractive index n s 3.5 of InAs and n s 3.2 of GaAs, the Fresnel reflectivity at the InAs]glass interface amounts to about 16% and at the GaAs]air interface to about 29%. RESULTS AND DISCUSSION
Flying-Mirror Mode-Locking FIG. 1.
Flying-mirror mode-locking setup.
through the input mirror M 1 by a microscope objective MO. M 1 had a transmissivity T1 Ž792 nm. s 75% and a reflectivity R1 Ž2.7 m m. ) 99%. The input mirror M 1 and the output mirror M 2 with R 2 Ž2.7 m m. s 50]90% were butted to the fiber endfaces, forming the active resonator. The radiation diverging from the output endface of the fiber was collimated by an IRGN6 lens L Ž f s 20 mm.; the beam was folded by M 3 and led to an external mirror M 4 with R 4 Ž2.7 m m. ) 98%, reflecting it into the active cavity. This external resonator was about 200 cm long, which means that the optical path length was approximately equal to that of the active fiber resonator. M 4 was mounted on a loudspeaker LS driven by a sine generator. Frequencies between 20 Hz and several kHz could be applied at variable amplitudes. Power was coupled out of the external resonator by a 50% beamsplitter BS and was focused by an IRGN6 lens L through a long-pass filter F onto a detection unit. For monitoring the pulse shapes a fast InSb photodiode with a rise time of 2.3 ns was used. Its signal was preamplified and displayed on a 400-MHz storage oscilloscope. The response time of this detection system was about 2.5 ns. Power measurements were performed with a pyroelectric power meter. Figure 2 shows the setup for the InAs saturable absorber experiments. It was a standard Fabry]Perot fiber laser setup, the output mirror consisting of an InAs epi-
FIG. 2.
Setup with InAs saturable absorbers.
The flying-mirror mode-locking experiments were performed only with the multimode fiber Žcore diameter 30 m m.. Moving the external mirror M 4 manually caused trains of pulses. Switching on the loudspeaker also caused the laser to emit pulse trains. This behavior could be observed over a large range of the parameters pump power, loudspeaker frequency, loudspeaker amplitude, and loudspeaker maximum velocity. Figure 3 shows in millisecond timescale an oscilloscope shot of the fiber laser signal and the loudspeaker driving signal. The phase relation between both signals is offset due to a phase delay between the loudspeaker motion and its electric driving signal. Each spike of the fiber laser signal represents the envelope of a pulse train. Within each loudspeaker period several pulse trains occurred. No pulses were observed in the vicinity of the return points of the vibrating mirror. Each pulse train Žtypical length 2]10 m s. consisted of several hundreds of mode-locked pulses. The distances
FIG. 3. Oscilloscope trace of loudspeaker driving signal Žabove. and fiber laser signal Žbelow..
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
between the pulse trains were in the order of 10 to 50 m s, with longer distances occurring at lower pump powers. Figure 4 shows a photograph of a typical pulse train. Due to the response of the detector preamplifier the signal shows a ringing. The individual mode-locked pulses are not recognizable because of a large jitter in amplitude and phase between the single pulse trains superimposed on the depicted photograph. The FWHM duration of the pulse train was approximately 3 m s. Durations of this order indicate that the pulse train envelops were a kind of Q-switch pulses. Figure 5 shows a computer-recorded oscilloscope readout of two mode-locked pulses. The pulse width amounted to 3.6 ns, the decay time to 2.5 ns, the latter value representing exactly the response time of the detection unit. The noisy slope between the two pulses is due to the response characteristics of the preamplifier, and the noise is caused by the amplitude jitter of the pulses, which were triggered to their maximum value. The pulse repetition rate matched the resonator round-trip frequency amounting in this case to 75 MHz, which fits to a fiber length of 134 cm. Similar oscilloscope readouts with nearly equal pulse widths and fall times could be recorded over almost the entire range when mode-locking was possible. Determination of the pulse durations was not possible due to the lack of a faster detection system. The behavior described above was virtually independent of the operation parameters. Mode-locked pulses could be generated within a launched pump power range of 19 mW F Plaunch F 100 mW. The amplitude of the loudspeaker and therefore of M 4 could be varied from 1 to 100 m m; its frequency could be varied from 30 to 700 Hz without the pulsed operation ceasing. Calculation of the maximum mirror velocities revealed that mode-locking was possible between 0.5 and 25 mmrs, the latter being the highest velocity obtainable with the setup. Larger
FIG. 4. train.
Oscilloscope trace of a flying-mirror mode-locked pulse
FIG. 5.
361
Oscilloscope trace of two mode-locked pulses.
mirror velocities were favorable in order to achieve stronger pulse amplitudes and stable pulsed operation. Another parameter which seemed to have no influence on the laser operation was the length of the external resonator. A change of "4 cm of the optical length of the active resonator yielded apparently no change in the amplitude or the duration of the mode-locked pulses. Pulsed operation was also possible when the lengths of the resonators were matched exactly. A change of the active resonator output mirror M 2 w R 2 Ž2.7 m m. s 80%, 90%, and 50% were usedx yielded no substantial change in the mode-locked laser behavior either. These results say that by no means are there differences in the pulsing characteristics at different conditions. Possible differences, probably with regard to pulse duration and pulse peak power, could just not be measured with the detection setup used here. Figure 6 shows the average output power of the modelocked fiber laser versus the launched pump power. This characteristic was measured with a maximum mirror velocity of 11.2 mmrs at a loudspeaker frequency of 128 Hz and with R 2 Ž2.7 m m. s 80%. The output power is the sum of the radiation emitted to both sides of the beamsplitter. Launched threshold power was 3.4 mW, the slope efficiency was 6.1%, and the maximum power was 4.7 mW at a launched pump power of about 81 mW. This was the best characteristic measured. Operation of the flyingmirror mode-locking laser setup remained stable over a long time with only slight realignment necessary after one day. This favorable behavior may be due to the setup of the mode-locked fiber laser, which is in essence a Fabry]Perot-type fiber laser with an interfering external resonator. The fact that mode-locking occurs with this setup supports the position that nonlinear effects are not necessary
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FIG. 6. Average power characteristic of a flying-mirror mode-locked fiber laser.
for pulse formation w25x. Otherwise there would have been no mode-locking in a fiber laser with a rather thick core pumped by a laser diode at moderate powers. InAs Saturable Absorber Mode-Locking Experiments with InAs epilayers were carried out with both above-mentioned fibers. The high-NA fiber was assumed to guide predominantly the fundamental mode and is therefore called single-mode fiber in the following. The InAs epilayer samples used were 0.09, 0.1, 0.15, 0.2, and 0.4 m m thick. Their surfaces had been damaged by previous experiments w16x. Nevertheless there were enough areas suitable for experiments with fiber lasers. The fiber lasers, with the InAs layers used simultaneously as output couplers and saturable absorbers, operated generally in the Q-switched mode. Figure 7 shows a photograph of a series of Q-switched pulses. They were
FIG. 7.
Oscilloscope trace of a Q-switched pulse train.
recorded with a single mode fiber and an InAs sample of 0.4 m m thickness at Pabs s 32 mW Ž l pump s 793 nm.. The negative overshoot of the signal is due to the response characteristics of the detector preamplifier. The average pulse interval was in this case approximately 22 m s. Similar behavior could be observed with almost all fiber lasers with InAs saturable absorbers. Pulse trains recorded with the single-mode fiber lasers showed much less phase and amplitude jitter than those with the multimode fiber lasers. The reason for this is thought to be the stable single-mode operation without variation of the mode power distribution. Figure 8 shows a photograph of superimposed Qswitched pulses recorded with a single-mode fiber and a 0.09-m m InAs layer at Pabs s 250 mW Ž l pump s 647 nm.. The FWHM pulse width was 1.1 m s. In this case no mode-locked pulses could be detected under the envelope of the Q-switched pulse. Pulse widths around 1 m s could be generated at higher pump powers with both the single-mode fiber and the multimode fiber as active components. The dependence of the pulse repetition period ŽPRP. and the FWHM pulse width on InAs layer thickness, pump power, and type of fiber was investigated. As already mentioned, the lasers with the single-mode fiber yielded pulses with less jitter. The shortest pulses, however, could be recorded with the multimode fiber. Their FWHM duration amounted to 0.4 m s at Pabs s 175 mW Ž l pump s 793 nm.; in this case the InAs sample was 0.4 m m thick. The shortest pulse width obtained with laser diode pumping amounted to 1.2 m s, which was measured with the multimode fiber and a 0.4-m m InAs sample at Pabs s 57 mW. The pulse intervals were generally much longer with the multimode fiber, indicating that the lasers with these fibers have better energy storage properties.
FIG. 8.
Oscilloscope trace of superimposed Q-switched pulses.
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
Pulse widths and PRPs showed a decided dependence on the thickness of the InAs layers. With increasing thickness the PRP increased. At Pabs s 57 mW the PRP measured with the multimode fiber amounted to 60 m s with the 0.09-m m sample, 100 m s with the 0.1-m m sample, 120 m s with the 0.15-m m sample, and 900 m s with the 0.4-m m sample. The conditions were similar with the single-mode fiber, yet here the variation was not that strong. The PRP at Pabs s 32 mW Žthe maximum pump power with laser diode pumping with this fiber. amounted to 16 m s with the 0.1-m m sample and to 22 m s with the 0.4-m m sample. Increasing the layer thickness generally resulted in decreasing the pulse duration. At Pabs s 57 mW, the pulse widths measured with the 30-m m fiber amounted to ) 10 m s Ž0.09 m m., 8 m s Ž0.1 m m., 6 m s Ž0.15 m m., and 1.2 m s Ž0.4 m m.. The dependence was similar when using the single-mode fiber. The pulsing behavior was not always reproducible due to the poor quality of the InAs samples. With the 0.2-m m sample, pulse operation was very erratic indeed, and the pulse characteristics were worse than with the other samples. This sample was more damaged than the others. Increasing the pump power resulted in a decrease of pulse width and PRP. For example, with the single-mode fiber and a sample thickness of 0.09 m m, at Pabs s 250 mW the PRP amounted to 6.6 m s and the pulse width to 1.0 m s. At Pabs s 450 mW, the PRP was 5.2 m s and the pulse width was 0.9 m s Ž l pump s 647 nm.. The variation was even stronger with the multimode fiber. The reasons for these dependences lie in the saturation behavior of the InAs layers. Thicker layers have higher bleaching thresholds and thus also higher switching thresholds. Therefore, a higher level of inversion must be reached, which corresponds to larger pulse intervals. A higher inversion also produces more intensive Q-switched pulses: the stored energy is removed faster from the resonator, and thus the pulses are shorter. Increasing the pump power also yields higher inversion, since the amount of radiation necessary to reach the saturation level of the InAs layers has to be built up from the spontaneous emission, which develops on the timescale of the lifetime of the upper laser level Ž6.7 ms w6x.. As a result the pulses become shorter. At higher pump rates, the inversion can recover faster, and the PRPs are shorter. Mode-locked pulses could be generated only with lasers made of the multimode fiber. Figure 9 shows a photograph of a Q-switched pulse containing mode-locked pulses. The photograph was taken from the output of a laser with the multimode fiber and an InAs sample of 0.4 m m thickness at Pabs s 160 mW. In this case the detector preamplifier could be omitted; therefore the photograph shows no negative overshoot. The FWHM pulse width amounted to 0.5 m s. The repetition frequency of the
FIG. 9.
363
Q-switched pulse containing mode-locked pulse train.
mode-locked pulses was approximately 84 MHz, corresponding to the resonator length of 1.18 m. Two mode-locked pulses recorded under the same conditions as in Fig. 9 are shown in Fig. 10. The pulse width amounted to about 2.2 ns, which is the resolution limit of the InSb detector without preamplifier. At lower pump powers the resolution limit was 2.5 ns. Pulses of this duration could be recorded almost under all conditions with the multimode fiber. The real pulse widths could not be measured due to the lack of a faster detection setup. It can only be speculated that the real pulse durations could be in the range of the response time of the InAs layers of 100 ps. Even shorter durations could possibly be generated since the fluorescence bandwidth of the 2.7 m m transition is much broader than 100 nm w6x, which corresponds to bandwidth transformed pulse widths of less than 250 fs. In contrast to the results obtained with the multimode fiber lasers, the lasers with the single mode fiber showed no or only slightly mode-locked behavior. Sometimes there
FIG. 10.
Oscilloscope trace of two mode-locked pulses.
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FRERICHS AND UNRAU
were only Q-switched pulses as in Fig. 8, and sometimes the Q-switched pulses showed a modulation at the resonator round-trip frequency. Since this modulation showed a more-or-less sinosoidal shape, it can be assumed that it was only mode-beating. The reason for this behavior is clarified with the help of Fig. 11, which shows a schematic diagram of the fiber laser resonator with emphasis on the output at the InAs sample. The radiation diverging from the fiber is reflected to an amount of 16% at the glass]InAs interface. The remaining part of the radiation is emitted at a maximum angle of 2.68 Žmultimode fiber. and 7.28 Žsingle-mode fiber.. It is attenuated by the saturable absorber, undergoes 29% reflection at the GaAs]air interface, and then returns to the fiber end with a widened cross-section. Taking the cross-sectional ratio of fiber core and beam as coarse approximation of the input efficiency back into the fiber, 16% of the reflected radiation is coupled back into the multimode fiber and only 0.2% into the single-mode fiber. Calculating the amount of radiation coupled back into the multimode fiber Žassuming unsaturated InAs layers with attenuation coefficients of 2.4 dBrm m w27x. yields 18 to 19%, from which only 2 to 3% Ždepending on the thickness of the layer. is transmitted through the saturable absorber. Without considering the change in attenuation in the InAs layer due to the bleaching effect, one can see that the switching ratio is very small. Since the retroreflection into the single-mode fiber is nearly two orders of magnitude smaller than in the multimode case, the switching ratio should be almost negligible. Nevertheless, in both cases Q-switching is possible, with the multimode fiber also mode-locking. Average power characteristics of the modulated fiber lasers have been measured. Figure 12 shows the best curves recorded with both kinds of fiber. The InAs epilayer had a thickness of 0.15 m m. With the single-mode fiber, the threshold power amounted to 3.0 mW, the slope efficiency to 7.6%, and the maximum power to 2.1 mW at an absorbed power of 31 mW. With the multimode fiber,
FIG. 11. absorber.
Sketch of the fiber laser resonator with InAs saturable
FIG. 12. Average power characteristics of InAs saturable absorber modulated fiber lasers.
the threshold power was 19.7 mW, the slope efficiency 4.4%, and the maximum power 1.6 mW at 55 mW absorbed power. Fiber lasers with other InAs samples yielded slightly unfavorable characteristics. Much higher output powers could be measured when pumping the fiber lasers with the Ti:sapphire or the dye laser. Maximum average powers in these cases amounted to 16 mW at an absorbed power of 162 mW Ž l pump s 792 nm. with the multimode fiber and to 22 mW at an absorbed power of 450 mW Ž l pump s 647 nm. with the single-mode fiber. From the average powers, the pulse durations, and the repetition rate, the peak powers and the pulse energies of the Q-switched pulses could be calculated. Highest values calculated were obtained with the multimode fiber and the 0.4-m m InAs sample. At an absorbed power of 57 mW Žlaser diode pumped. a repetition rate of 1.1 kHz, a pulse duration of 1.2 m s, and an average output power of 1.4 mW could be measured, resulting in a peak power of 1.04 W and a pulse energy of 1.25 m J. These comparatively high values could be achieved mainly due to the low repetition rate and pulse width which were much longer with thinner samples. With all other samples, only peak powers of some tens of milliwatts and pulse energies around 100 nJ were calculated from the measured results either with the multimode fiber or with the single-mode fiber. Summarizing, the fiber lasers with the multimode fiber exhibited more favorable pulsing behavior than those with the single-mode fiber. Despite having higher thresholds and lower slope efficiencies compared to single-mode fiber lasers, they delivered shorter pulses at lower repetition rates. This results in higher peak powers and pulse energies. Furthermore, they are operating in the modelocked regime. The Q-switching results obtained here with the multimode fiber are comparable with results obtained
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
earlier with an acoustooptic modulator as switching element w17x. Using the InAs layers in the manner described here, the single-mode fiber lasers suffer from the very low reflection of radiation transmitted through the saturable absorber back into the fiber. This prevents efficient switching since no photon avalanche can develop. The inversion is quenched by a comparably slow process without the development of a giant pulse. Generally, the pulsed operation of the lasers was very stable and reproducible. The lasers operated over hours without degradation and alignment was easy. Problems arose when pumping with the dye laser at 647 nm at higher powers. Due to the high intensity at the layers}partly arising from unabsorbed pump power, partly from laser power}damage occurred at the InAs layers and at the fiber ends after short periods of regular operation. This either extinguished the laser operation or led to erratic operation. The damage was most likely to occur with thick layers. CONCLUSION
Passive Q-switching and mode-locking of erbium-doped fiber lasers at 2.7 m m have been demonstrated. The lasers were realized with laser diodes as pump sources. The first method presented, the flying-mirror modelocking scheme, seems to be especially suited for the generation of mode-locked pulses. It can be scaled by simple adaption of the fiber length to very high repetition rates. A drawback of this method is the required use of a mechanical device}in this case a loudspeaker. No continuous train of mode-locked pulses could be generated. Nevertheless it could be a simple method for generating subnanosecond pulses for applications in sensing and spectroscopy. Passive mode-locking of 2.7-m m fiber lasers with InAs epilayers as saturable absorbers proved to be a suitable method for the generation of Q-switched and mode-locked pulses. The technique is very favorable due to the simple resonator design with integrated saturable absorber. For single-mode fibers with high numerical aperture this design is not so suitable since the amount of radiation reflected back into the fiber, and hence the switching ratio, is too small for mode-locking and efficient Q-switching. This passive modulation scheme thus worked best with the multimode fiber, yielding maximum peak powers exceeding 1 W and pulse energies of 1.25 m J. Both techniques should have much potential for optimization. By careful design of the geometrical properties of the fiber and of the pump radiation input, an increase of the average powers should be possible. Especially the fiber lasers with the InAs saturable absorbers could be fitted to the desired applications by an advanced design of
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the InAs samples. A way to achieve this could be the utilization of antiresonant Fabry]Perot saturable absorber ŽA-FPSA. semiconductor devices, as already demonstrated in the near infrared w27x. This should also overcome the damage of the InAs samples at higher pump powers. Optimized diode pumped Q-switched and modelocked fiber lasers at this wavelength could serve as a simple, low-cost source for applications in medicine, sensing, and ultrashort spectroscopy. REFERENCES w1x L. Esterowitz and R. Allen, ‘‘Rare earth doped IR fiber laser for medical applications,’’ Proc. SPIE, vol. 1048, 129 Ž1989.. w2x K. Schmitt and W. Schaube, ‘‘Verfahren zur Ermittlung des Ž‘‘Method for Determination of Fahrbahnoberflachenzustandes’’ ¨ Roadway Surface Condition’’., German Patent Application DE 40 08 280 A1, Offenlegungsschrift, filed 19.9.1991. w3x A. G. Yodh, ‘‘Infrared methods for transient vibrational spectroscopy at surfaces and interfaces,’’ in OSA Annual Meeting Technical Digest, Albuquerque, NM, vol. 23, p. 184, 1992. w4x J. Hecht, ‘‘Tunable mid-infrared sources entice spectroscopists,’’ Laser Focus World, vol. 29, no. 9, 109 Ž1993.. w5x M. C. Brierley and P. W. France, ‘‘Continuous wave lasing at 2.7 m m in an erbium-doped fluorozirconate fibre,’’ Electron. Lett., vol. 24, 935 Ž1988.. w6x L. Wetenkamp, Ch. Frerichs, G. F. West, and H. Tobben, ‘‘Effi¨ cient CW operation of tunable fluorozirconate fibre lasers at wavelengths pumpable with semiconductor laser diodes,’’ J. NonCryst. Solids, vol. 140, 19 Ž1992.. w7x R. Allen, L. Esterowitz, and R. J. Ginther, ‘‘Diode-pumped singlemode fluorozirconate fiber laser from the 4 I 11r2 ª4 I 13r2 transition in erbium,’’ Appl. Phys. Lett., vol. 56, 1635 Ž1990.. w8x Ch. Frerichs, ‘‘All optical modulation of a 2.7 m m erbium-doped fluorozirconate fiber laser,’’ in OSA Proc. on Ad¨ anced Solid State Lasers, New Orleans, LA, vol. 15, pp. 399]402, 1993. w9x I. P. Alcock, A. C. Tropper, A. I. Ferguson, and D. C. Hanna, ‘‘Q-switched operation of a neodymium-doped monomode fibre laser,’’ Electron. Lett., vol. 22, no. 2, 84 Ž1986.. w10x I. P. Alcock, A. I. Ferguson, D. C. Hanna, and A. C. Tropper, ‘‘Mode-locking of a neodymium-doped monomode fibre laser,’’ Electron. Lett., vol. 22, no. 5, 268 Ž1986.. w11x D. Lytle, ‘‘Figure-8 laser makes ultrashort solitons,’’ Photon. Spectra, vol. 27, no. 8, 36 Ž1993.. w12x Kh. S. Bagdasarov, V. P. Danilov, V. I. Zhekov, T. M. Murina, A. A. Manenkov, M. I. Timoshechkin, and A. M. Prokhorov, ‘‘Pulse-periodic Y3 Al 5 O 12 :Er 3q laser with high activator concentration,’’ So¨ . J. Quantum Electron., vol. 81, no. 1, 83 Ž1978.. w13x S. Schnell, V. G. Ostroumov, J. Breguet, W. A. R. Luthy, H. P. ¨ Weber, and I. A. Shcherbakov, ‘‘Acoustooptic Q switching of erbium lasers,’’ IEEE J. Quantum Electron., vol. 26, no. 6, 1111 Ž1990.. w14x J. Breguet, A. F. Umyskov, W. A. R. Luthy, I. A. Shcherbakov, and ¨ H. P. Weber, ‘‘Electrooptically Q-switched 2.79 m m YSGG:Cr:Er laser with an intracavity polarizer,’’ IEEE J. Quantum Electron., vol. 27, no. 2, 274 Ž1991.. w15x J. Breguet, W. Luthy, and H. P. Weber, ‘‘Q-switching of YAG:Er ¨ laser with a soap film,’’ Opt. Commun., vol. 82, no. 5]6, 488 Ž1991..
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w16x F. Konz, ¨ M. Frenz, V. Romano, M. Forrer, H. P. Weber, A. V. Kharkovskiy, and S. I. Khomenko, ‘‘Active and passive Q-switching of a 2.79 m m Er:Cr:YSGG laser,’’ Opt. Commun., vol. 103, 398 Ž1993.. w17x Ch. Frerichs and T. Tauermann, ‘‘Q-switched operation of a laser diode pumped erbium-doped fluorozirconate fibre laser operating at 2.7 m m,’’ Electron. Lett., vol. 30, no. 9, 706 Ž1994.. w18x H. J. Eichler, J. Albertz, F. Below, A. Kummrow, T. Leitert, A. A. Kaminskii, and L. Jakab, ‘‘Acousto-optic mode locking of 3-m m Er lasers,’’ Appl. Opt., vol. 31, no. 24, 4909 Ž1992.. w19x K. L. Vodopyanov, L. A. Kulevskii, P. P. Pashinin, A. F. Umyskov, and I. A. Shcherbakov, ‘‘Bandwidth-limited picosecond pulses from a YSGG:Cr 3q:Er 3q laser Ž l s 2.79 m m. with active mode locking,’’ So¨ . J. Quantum Electron., vol. 17, no. 6, 776 Ž1987.. w20x B. Pelz, M. K. Schott, and M. H. Niemz, ‘‘Electro-optic mode locking of an erbium:YAG laser with a rf resonance transformer,’’ Appl. Opt., vol. 33, no. 3, 364 Ž1994.. w21x K. L. Vodopyanov, A. V. Lukashev, and C. C. Phillips, ‘‘Nano- and picosecond 3 m m Er:YSGG lasers using InAs as passive Qswitchers and mode-lockers,’’ Opt. Commun., vol. 95, 87 Ž1993..
w22x Ch. Frerichs, ‘‘Mode-locking of a 2.7 m m erbium-doped fluoride fiber laser,’’ in Proceedings of the NATO Ad¨ anced Study Institute on Trends in Optical Fiber Metrology and Standards ŽO. D. D. Soares, Ed.., pp. 840, 841, NATO ASI Series E, vol. 285, Dordrecht, 1995. w23x Ch. Frerichs and J. Urner, ‘‘Passive mode-locking and Q-switching of a 2.7 m m fluoride fiber laser,’’ in Congress ‘‘ Ad¨ anced Solid State Lasers’’ on the ‘‘LASER 95,’’ Munich, 19.6.1995. w24x G. Sargsjan, U. Stamm, C. Unger, and W. Zschocke, ‘‘Characteristics of a neodymium-doped fiber laser mode-locked with a linear external cavity,’’ Opt. Commun., vol. 86, 480 Ž1991.. w25x M. Muller and U. Stamm, ‘‘Numerical analysis of pulse formation ¨ in solid-state lasers mode-locked with a linear external cavity,’’ Appl. Phys. B., vol. 54, 136 Ž1992.. w26x P. M. W. French, D. U. Noske, N. H. Rizvi, J. A. R. Williams, and J. R. Taylor, ‘‘Characterisation of a cw titanium-doped sapphire laser mode-locked with a linear external cavity,’’ Opt. Commun., vol. 83, no. 1]2, 185 Ž1991.. w27x U. Keller, ‘‘Ultrafast all-solid-state laser technology,’’ Appl. Phys. B, vol. 58, 347 Ž1994..
2, 358]366 Ž1996.
0041
Passive Q-Switching and Mode-Locking of Erbium-Doped Fluoride Fiber Lasers at 2.7 m m CHRISTIAN FRERICHS
AND
UDO B. UNRAU*
Institut fur ¨ Hochfrequenztechnik, Technische Uni¨ ersitat, ¨ D-38106 Braunschweig, Germany Received May 15, 1996; revised July 18, 1996
Passive Q-switched and mode-locked operation of a 2.7-m m fluorozirconate fiber laser is reported. Both mode-locking and Q-switching were attained by means of InAs epilayers used as saturable absorbers. Another technique used was the so-called flying-mirror mode-locking, whereby an external resonator with a vibrating mirror is coupled to the active fiber resonator. Both techniques yielded Q-switched pulses containing trains of mode-locked pulses with durations shorter than 2.5 ns—the resolution limit of the detection unit. Q 1996 Academic Press, Inc.
Solid-state lasers at wavelengths around 3 m m have attracted much interest because of their potential applications in medicine and sensing. In the medical field, the strong absorption of radiation with wavelengths around 3 m m in water could lead to lasers excellently suited for the ablation of tissue w1x. An example in the wide field of sensing applications would be ice sensors for cars w2x. Spectroscopic applications are numerous and, especially pulse measurements, e.g., for the transient behavior of CO molecules on surfaces, are of interest w3x. Fiber lasers have certain advantages compared to other kinds of lasers in this wavelength region. They have potentially very low threshold powers and high efficiencies due to their large power densities in the fiber core and their large surface-to-volume ratio, which leads to good heat dissipation. They are simple devices compared to color center lasers, lead salt diodes, or optical parametric oscillators operating in the mid-infrared w4x. Unlike the latter devices, fiber lasers usually operate in the cw mode. Furthermore, they can be pumped with laser diodes, making the whole device potentially cheap and highly efficient. Since silicate glasses are opaque in the mid-infrared, fluoride glasses exhibiting low phonon energies are usually used as laser host in this spectral region. The most common material is the multicomponent glass called * To whom correspondence should be addressed. Fax: 49-531-3915841. E-mail: [email protected].
ZBLAN, one of the most stable and easily fabricated infrared glasses. The classical composition is 53% ZrF4 , 20% BaF2 , 4% LaF3 , 3% AlF3 , and 20% NaF. To raise the refractive index of the fiber core, an amount of PbF2 is added. Laser operation at 2.7 m m in an Er 3q-doped ZBLAN fiber was first demonstrated in 1988 by Brierley and France w5x. Fiber lasers at this wavelength have been realized with very low threshold powers and high efficiencies if pumped with a Ti:sapphire laser at about 795 nm or a DCM dye laser at about 650 nm. Tuning ranges of 160 nm from 2.67 to 2.83 m m were achieved using a diffraction grating w6x. Laser diode pumping at 800 nm was first published by Allen et al. w7x. A fiber laser at 2.7 m m pumped by an 800-nm laser diode with a threshold below 1 mW and a slope efficiency of more than 20% was achieved w8x. While there are numerous papers dealing with cw fiber lasers at 2.7 m m, very little research has been done on pulsed operation of these lasers. This operation mode could overcome some drawbacks of 2.7-m m cw fiber lasers and could open a wide field of applications to these lasers. Unfortunately, the output powers in cw fiber lasers at 2.7 m m are modest so far. Many applications, particularly in the medical field, need much higher powers. Pulses with peak powers some orders of magnitude higher than in the cw mode can be achieved by Q-switching the laser, thus overcoming the low-average output. Pulses of even shorter duration are needed in spectroscopy, where they can give new insights into ultrashort processes connected with these wavelengths w3x. Such ultrashort pulses can be generated by mode-locking the lasers. Q-switched and mode-locked fiber lasers were first demonstrated in 1986 w9, 10x. In the field of mode-locked NIR fiber lasers, sophisticated techniques have been developed that are capable of delivering pulses with durations below 100 fs w11x. Until recently, in the 3-m m region, Q-switching and mode-locking were achieved only with flashlamp-pumped crystal lasers such as Er:YAG or Er:YSGG using active and passive elements as Q-switches.
358 1068-5200r96 $18.00 Copyright Q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
Q-switched erbium crystal lasers around 3 m m have been realized with various techniques such as rotating mirrors w12x, acoustooptic modulators w13x, and electrooptic modulators w14x and passive techniques such as water]soap films w15x and InAs epilayers w16x used as saturable absorbers. Pulse durations down to 28 ns and pulse energies up to 180 mJ, corresponding to peak powers in the MW range, have been achieved w16x. The first operation of a Q-switched erbium-doped laser-diode pumped fluoride fiber laser at 2.7 m m was reported recently w17x. Utilizing an acoustooptic modulator, pulse durations down to 100 ns and peak powers exceeding 2 W could be measured. Using a rotating mirror as switching element yielded much poorer results. Mode-locked operation of flashlamp-pumped 3 m m erbium crystal lasers was performed utilizing techniques such as acoustooptic w18x and electrooptic mode-locking w19, 20x and passive mode-locking using InAs epilayers as saturable absorbers w21x. The best results reported were pulse durations down to 30 ps in a Q-switched envelope. First results of diode-pumped mode-locked and Qswitched fiber lasers at 2.7 m m were published recently by our group. The ‘‘flying-mirror’’ or kinematic mode-locking technique was utilized, whereby mode-locked pulses were generated by coupling a linear external resonator with a vibrating mirror to the active fiber resonator w22x or a simple passive modulation scheme with InAs epilayers as saturable absorbers w23x. Mode-locked pulse trains with detector-limited pulse durations down to 2 ns were presented w23x. This paper describes more mature experimental results of the two above-mentioned means of pulsed operation of 2.7 m m erbium-doped fluoride fiber lasers: The first is flying-mirror mode-locking, the second mode-locking and Q-switching by introducing a thin epilayer of InAs into the laser resonator acting as saturable absorber. Both techniques are passive modulation schemes: even the frequency of the vibrating mirror in the flying-mirror modelocking setup is not necessarily related to the resonator round-trip frequency of the laser. In active techniques, the switching element is driven by an external source, which in the case of mode-locking short lasers with high repetition rates is complicated. In contrast, the techniques applied here are simple and allow for very high repetition frequencies. Both techniques can yield not only modelocked but also Q-switched pulses simultaneously. The flying-mirror mode-locking technique has been realized already with many laser types, e.g., with nearinfrared fiber lasers w24x. The pulse generation mechanism may be explained in the following way: the mirror translation causes a frequency shift of the signal in the external cavity due to the Doppler effect. This frequency-shifted signal is fed back into the active resonator and amplified. In this way new frequency components, phase-locked to
359
each other, are added. After some thousands of resonator round-trips this leads to energy coupling between adjacent modes, hence mode-locking may start w24x. It has been shown theoretically that nonlinear effects are not required for pulse formation w25x. Although the technique is mostly used for the generation of mode-locked pulses, under certain circumstances also the formation of Qswitched pulses was reported and explained w26x. Passive mode-locking and Q-switching with saturable absorbers performed with InAs epilayers on GaAs substrate has the advantage of being an all-solid-state technology with no moving elements. The InAs epilayers used here have been described in w16x and w21x; they were grown 0.09]0.4 m m thick on GaAs substrates. The room temperature bandgap of InAs corresponds to a wavelength of 3.54 m m. Absorption saturation at lower wavelengths occurs due to the dynamic band-filling or Burstein]Moss effect. Fast recovery times in the order of 100 ps depending on the thickness of the samples have been reported w21x. These authors also reported problems due to the low damage intensities of the samples when used in flashlamp-pumped erbium lasers. Since the intensities in Er:ZBLAN fiber lasers are much lower, these problems should not pose too great a handicap. EXPERIMENTAL
The laser transition in Er 3q exploited here is 4 I 11r12 ª4 I 13r2 . Pump lasers were a 500-mW SDL AlGaAs laser emitting around 792 nm, a titanium]sapphire laser emitting around 800 nm, and a DCM dye laser emitting around 650 nm. The latter two lasers were used for higher pump power experiments with the InAs layers. Both pump wavelengths are well suited to pump the 2.7-m m laser since both have the advantage of depopulating the lower laser level by an excited state absorption process w6x. Two fibers, both manufactured by Le Verre Fluore, ´ were used: the first one was doped with 1000 ppm Er 3q, had a core diameter of 30 m m and a numerical aperture of 0.15, and had lengths decreasing from 140 to 115 cm during the experiments. The second fiber was doped with 3000 ppm Er 3q, had a core diameter of 6 m m and a numerical aperture of 0.4, and was used in lengths between 150 and 120 cm. The latter fiber was only used in the saturable absorber experiments. The coupling efficiencies were measured as about 37% Žlaser diode radiation into 30-m m fiber., 8.5% Žlaser diode radiation into 6-m m fiber., 45% ŽTi:sapphire laser radiation into 30-m m fiber. and 50% Ždye laser radiation into 6-m m fiber.. In the 30-m m fiber, about 70% of the launched power at 800 nm could be absorbed; in the 6-m m fiber, more than 95% could be absorbed at either wavelength. Figure 1 shows the flying-mirror mode-locking setup. The pump radiation was focused into the active fiber
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layer directly butted to the fiber ends. Pump source, input mirror setup, and fiber were the same as those described above. The output radiation was focused by an IRGN6 lens through a long-pass filter onto the detection units described above. The InAs epilayers were between 0.09 and 0.4 m m thick and were grown by molecular beam epitaxy on 0.25-mm-thick GaAs substrates w16x. Mounted in the way described, they served both as output mirror and as saturable absorber. Due to the refractive index n s 3.5 of InAs and n s 3.2 of GaAs, the Fresnel reflectivity at the InAs]glass interface amounts to about 16% and at the GaAs]air interface to about 29%. RESULTS AND DISCUSSION
Flying-Mirror Mode-Locking FIG. 1.
Flying-mirror mode-locking setup.
through the input mirror M 1 by a microscope objective MO. M 1 had a transmissivity T1 Ž792 nm. s 75% and a reflectivity R1 Ž2.7 m m. ) 99%. The input mirror M 1 and the output mirror M 2 with R 2 Ž2.7 m m. s 50]90% were butted to the fiber endfaces, forming the active resonator. The radiation diverging from the output endface of the fiber was collimated by an IRGN6 lens L Ž f s 20 mm.; the beam was folded by M 3 and led to an external mirror M 4 with R 4 Ž2.7 m m. ) 98%, reflecting it into the active cavity. This external resonator was about 200 cm long, which means that the optical path length was approximately equal to that of the active fiber resonator. M 4 was mounted on a loudspeaker LS driven by a sine generator. Frequencies between 20 Hz and several kHz could be applied at variable amplitudes. Power was coupled out of the external resonator by a 50% beamsplitter BS and was focused by an IRGN6 lens L through a long-pass filter F onto a detection unit. For monitoring the pulse shapes a fast InSb photodiode with a rise time of 2.3 ns was used. Its signal was preamplified and displayed on a 400-MHz storage oscilloscope. The response time of this detection system was about 2.5 ns. Power measurements were performed with a pyroelectric power meter. Figure 2 shows the setup for the InAs saturable absorber experiments. It was a standard Fabry]Perot fiber laser setup, the output mirror consisting of an InAs epi-
FIG. 2.
Setup with InAs saturable absorbers.
The flying-mirror mode-locking experiments were performed only with the multimode fiber Žcore diameter 30 m m.. Moving the external mirror M 4 manually caused trains of pulses. Switching on the loudspeaker also caused the laser to emit pulse trains. This behavior could be observed over a large range of the parameters pump power, loudspeaker frequency, loudspeaker amplitude, and loudspeaker maximum velocity. Figure 3 shows in millisecond timescale an oscilloscope shot of the fiber laser signal and the loudspeaker driving signal. The phase relation between both signals is offset due to a phase delay between the loudspeaker motion and its electric driving signal. Each spike of the fiber laser signal represents the envelope of a pulse train. Within each loudspeaker period several pulse trains occurred. No pulses were observed in the vicinity of the return points of the vibrating mirror. Each pulse train Žtypical length 2]10 m s. consisted of several hundreds of mode-locked pulses. The distances
FIG. 3. Oscilloscope trace of loudspeaker driving signal Žabove. and fiber laser signal Žbelow..
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
between the pulse trains were in the order of 10 to 50 m s, with longer distances occurring at lower pump powers. Figure 4 shows a photograph of a typical pulse train. Due to the response of the detector preamplifier the signal shows a ringing. The individual mode-locked pulses are not recognizable because of a large jitter in amplitude and phase between the single pulse trains superimposed on the depicted photograph. The FWHM duration of the pulse train was approximately 3 m s. Durations of this order indicate that the pulse train envelops were a kind of Q-switch pulses. Figure 5 shows a computer-recorded oscilloscope readout of two mode-locked pulses. The pulse width amounted to 3.6 ns, the decay time to 2.5 ns, the latter value representing exactly the response time of the detection unit. The noisy slope between the two pulses is due to the response characteristics of the preamplifier, and the noise is caused by the amplitude jitter of the pulses, which were triggered to their maximum value. The pulse repetition rate matched the resonator round-trip frequency amounting in this case to 75 MHz, which fits to a fiber length of 134 cm. Similar oscilloscope readouts with nearly equal pulse widths and fall times could be recorded over almost the entire range when mode-locking was possible. Determination of the pulse durations was not possible due to the lack of a faster detection system. The behavior described above was virtually independent of the operation parameters. Mode-locked pulses could be generated within a launched pump power range of 19 mW F Plaunch F 100 mW. The amplitude of the loudspeaker and therefore of M 4 could be varied from 1 to 100 m m; its frequency could be varied from 30 to 700 Hz without the pulsed operation ceasing. Calculation of the maximum mirror velocities revealed that mode-locking was possible between 0.5 and 25 mmrs, the latter being the highest velocity obtainable with the setup. Larger
FIG. 4. train.
Oscilloscope trace of a flying-mirror mode-locked pulse
FIG. 5.
361
Oscilloscope trace of two mode-locked pulses.
mirror velocities were favorable in order to achieve stronger pulse amplitudes and stable pulsed operation. Another parameter which seemed to have no influence on the laser operation was the length of the external resonator. A change of "4 cm of the optical length of the active resonator yielded apparently no change in the amplitude or the duration of the mode-locked pulses. Pulsed operation was also possible when the lengths of the resonators were matched exactly. A change of the active resonator output mirror M 2 w R 2 Ž2.7 m m. s 80%, 90%, and 50% were usedx yielded no substantial change in the mode-locked laser behavior either. These results say that by no means are there differences in the pulsing characteristics at different conditions. Possible differences, probably with regard to pulse duration and pulse peak power, could just not be measured with the detection setup used here. Figure 6 shows the average output power of the modelocked fiber laser versus the launched pump power. This characteristic was measured with a maximum mirror velocity of 11.2 mmrs at a loudspeaker frequency of 128 Hz and with R 2 Ž2.7 m m. s 80%. The output power is the sum of the radiation emitted to both sides of the beamsplitter. Launched threshold power was 3.4 mW, the slope efficiency was 6.1%, and the maximum power was 4.7 mW at a launched pump power of about 81 mW. This was the best characteristic measured. Operation of the flyingmirror mode-locking laser setup remained stable over a long time with only slight realignment necessary after one day. This favorable behavior may be due to the setup of the mode-locked fiber laser, which is in essence a Fabry]Perot-type fiber laser with an interfering external resonator. The fact that mode-locking occurs with this setup supports the position that nonlinear effects are not necessary
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FIG. 6. Average power characteristic of a flying-mirror mode-locked fiber laser.
for pulse formation w25x. Otherwise there would have been no mode-locking in a fiber laser with a rather thick core pumped by a laser diode at moderate powers. InAs Saturable Absorber Mode-Locking Experiments with InAs epilayers were carried out with both above-mentioned fibers. The high-NA fiber was assumed to guide predominantly the fundamental mode and is therefore called single-mode fiber in the following. The InAs epilayer samples used were 0.09, 0.1, 0.15, 0.2, and 0.4 m m thick. Their surfaces had been damaged by previous experiments w16x. Nevertheless there were enough areas suitable for experiments with fiber lasers. The fiber lasers, with the InAs layers used simultaneously as output couplers and saturable absorbers, operated generally in the Q-switched mode. Figure 7 shows a photograph of a series of Q-switched pulses. They were
FIG. 7.
Oscilloscope trace of a Q-switched pulse train.
recorded with a single mode fiber and an InAs sample of 0.4 m m thickness at Pabs s 32 mW Ž l pump s 793 nm.. The negative overshoot of the signal is due to the response characteristics of the detector preamplifier. The average pulse interval was in this case approximately 22 m s. Similar behavior could be observed with almost all fiber lasers with InAs saturable absorbers. Pulse trains recorded with the single-mode fiber lasers showed much less phase and amplitude jitter than those with the multimode fiber lasers. The reason for this is thought to be the stable single-mode operation without variation of the mode power distribution. Figure 8 shows a photograph of superimposed Qswitched pulses recorded with a single-mode fiber and a 0.09-m m InAs layer at Pabs s 250 mW Ž l pump s 647 nm.. The FWHM pulse width was 1.1 m s. In this case no mode-locked pulses could be detected under the envelope of the Q-switched pulse. Pulse widths around 1 m s could be generated at higher pump powers with both the single-mode fiber and the multimode fiber as active components. The dependence of the pulse repetition period ŽPRP. and the FWHM pulse width on InAs layer thickness, pump power, and type of fiber was investigated. As already mentioned, the lasers with the single-mode fiber yielded pulses with less jitter. The shortest pulses, however, could be recorded with the multimode fiber. Their FWHM duration amounted to 0.4 m s at Pabs s 175 mW Ž l pump s 793 nm.; in this case the InAs sample was 0.4 m m thick. The shortest pulse width obtained with laser diode pumping amounted to 1.2 m s, which was measured with the multimode fiber and a 0.4-m m InAs sample at Pabs s 57 mW. The pulse intervals were generally much longer with the multimode fiber, indicating that the lasers with these fibers have better energy storage properties.
FIG. 8.
Oscilloscope trace of superimposed Q-switched pulses.
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
Pulse widths and PRPs showed a decided dependence on the thickness of the InAs layers. With increasing thickness the PRP increased. At Pabs s 57 mW the PRP measured with the multimode fiber amounted to 60 m s with the 0.09-m m sample, 100 m s with the 0.1-m m sample, 120 m s with the 0.15-m m sample, and 900 m s with the 0.4-m m sample. The conditions were similar with the single-mode fiber, yet here the variation was not that strong. The PRP at Pabs s 32 mW Žthe maximum pump power with laser diode pumping with this fiber. amounted to 16 m s with the 0.1-m m sample and to 22 m s with the 0.4-m m sample. Increasing the layer thickness generally resulted in decreasing the pulse duration. At Pabs s 57 mW, the pulse widths measured with the 30-m m fiber amounted to ) 10 m s Ž0.09 m m., 8 m s Ž0.1 m m., 6 m s Ž0.15 m m., and 1.2 m s Ž0.4 m m.. The dependence was similar when using the single-mode fiber. The pulsing behavior was not always reproducible due to the poor quality of the InAs samples. With the 0.2-m m sample, pulse operation was very erratic indeed, and the pulse characteristics were worse than with the other samples. This sample was more damaged than the others. Increasing the pump power resulted in a decrease of pulse width and PRP. For example, with the single-mode fiber and a sample thickness of 0.09 m m, at Pabs s 250 mW the PRP amounted to 6.6 m s and the pulse width to 1.0 m s. At Pabs s 450 mW, the PRP was 5.2 m s and the pulse width was 0.9 m s Ž l pump s 647 nm.. The variation was even stronger with the multimode fiber. The reasons for these dependences lie in the saturation behavior of the InAs layers. Thicker layers have higher bleaching thresholds and thus also higher switching thresholds. Therefore, a higher level of inversion must be reached, which corresponds to larger pulse intervals. A higher inversion also produces more intensive Q-switched pulses: the stored energy is removed faster from the resonator, and thus the pulses are shorter. Increasing the pump power also yields higher inversion, since the amount of radiation necessary to reach the saturation level of the InAs layers has to be built up from the spontaneous emission, which develops on the timescale of the lifetime of the upper laser level Ž6.7 ms w6x.. As a result the pulses become shorter. At higher pump rates, the inversion can recover faster, and the PRPs are shorter. Mode-locked pulses could be generated only with lasers made of the multimode fiber. Figure 9 shows a photograph of a Q-switched pulse containing mode-locked pulses. The photograph was taken from the output of a laser with the multimode fiber and an InAs sample of 0.4 m m thickness at Pabs s 160 mW. In this case the detector preamplifier could be omitted; therefore the photograph shows no negative overshoot. The FWHM pulse width amounted to 0.5 m s. The repetition frequency of the
FIG. 9.
363
Q-switched pulse containing mode-locked pulse train.
mode-locked pulses was approximately 84 MHz, corresponding to the resonator length of 1.18 m. Two mode-locked pulses recorded under the same conditions as in Fig. 9 are shown in Fig. 10. The pulse width amounted to about 2.2 ns, which is the resolution limit of the InSb detector without preamplifier. At lower pump powers the resolution limit was 2.5 ns. Pulses of this duration could be recorded almost under all conditions with the multimode fiber. The real pulse widths could not be measured due to the lack of a faster detection setup. It can only be speculated that the real pulse durations could be in the range of the response time of the InAs layers of 100 ps. Even shorter durations could possibly be generated since the fluorescence bandwidth of the 2.7 m m transition is much broader than 100 nm w6x, which corresponds to bandwidth transformed pulse widths of less than 250 fs. In contrast to the results obtained with the multimode fiber lasers, the lasers with the single mode fiber showed no or only slightly mode-locked behavior. Sometimes there
FIG. 10.
Oscilloscope trace of two mode-locked pulses.
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were only Q-switched pulses as in Fig. 8, and sometimes the Q-switched pulses showed a modulation at the resonator round-trip frequency. Since this modulation showed a more-or-less sinosoidal shape, it can be assumed that it was only mode-beating. The reason for this behavior is clarified with the help of Fig. 11, which shows a schematic diagram of the fiber laser resonator with emphasis on the output at the InAs sample. The radiation diverging from the fiber is reflected to an amount of 16% at the glass]InAs interface. The remaining part of the radiation is emitted at a maximum angle of 2.68 Žmultimode fiber. and 7.28 Žsingle-mode fiber.. It is attenuated by the saturable absorber, undergoes 29% reflection at the GaAs]air interface, and then returns to the fiber end with a widened cross-section. Taking the cross-sectional ratio of fiber core and beam as coarse approximation of the input efficiency back into the fiber, 16% of the reflected radiation is coupled back into the multimode fiber and only 0.2% into the single-mode fiber. Calculating the amount of radiation coupled back into the multimode fiber Žassuming unsaturated InAs layers with attenuation coefficients of 2.4 dBrm m w27x. yields 18 to 19%, from which only 2 to 3% Ždepending on the thickness of the layer. is transmitted through the saturable absorber. Without considering the change in attenuation in the InAs layer due to the bleaching effect, one can see that the switching ratio is very small. Since the retroreflection into the single-mode fiber is nearly two orders of magnitude smaller than in the multimode case, the switching ratio should be almost negligible. Nevertheless, in both cases Q-switching is possible, with the multimode fiber also mode-locking. Average power characteristics of the modulated fiber lasers have been measured. Figure 12 shows the best curves recorded with both kinds of fiber. The InAs epilayer had a thickness of 0.15 m m. With the single-mode fiber, the threshold power amounted to 3.0 mW, the slope efficiency to 7.6%, and the maximum power to 2.1 mW at an absorbed power of 31 mW. With the multimode fiber,
FIG. 11. absorber.
Sketch of the fiber laser resonator with InAs saturable
FIG. 12. Average power characteristics of InAs saturable absorber modulated fiber lasers.
the threshold power was 19.7 mW, the slope efficiency 4.4%, and the maximum power 1.6 mW at 55 mW absorbed power. Fiber lasers with other InAs samples yielded slightly unfavorable characteristics. Much higher output powers could be measured when pumping the fiber lasers with the Ti:sapphire or the dye laser. Maximum average powers in these cases amounted to 16 mW at an absorbed power of 162 mW Ž l pump s 792 nm. with the multimode fiber and to 22 mW at an absorbed power of 450 mW Ž l pump s 647 nm. with the single-mode fiber. From the average powers, the pulse durations, and the repetition rate, the peak powers and the pulse energies of the Q-switched pulses could be calculated. Highest values calculated were obtained with the multimode fiber and the 0.4-m m InAs sample. At an absorbed power of 57 mW Žlaser diode pumped. a repetition rate of 1.1 kHz, a pulse duration of 1.2 m s, and an average output power of 1.4 mW could be measured, resulting in a peak power of 1.04 W and a pulse energy of 1.25 m J. These comparatively high values could be achieved mainly due to the low repetition rate and pulse width which were much longer with thinner samples. With all other samples, only peak powers of some tens of milliwatts and pulse energies around 100 nJ were calculated from the measured results either with the multimode fiber or with the single-mode fiber. Summarizing, the fiber lasers with the multimode fiber exhibited more favorable pulsing behavior than those with the single-mode fiber. Despite having higher thresholds and lower slope efficiencies compared to single-mode fiber lasers, they delivered shorter pulses at lower repetition rates. This results in higher peak powers and pulse energies. Furthermore, they are operating in the modelocked regime. The Q-switching results obtained here with the multimode fiber are comparable with results obtained
PASSIVE Q-SWITCHING AND MODE-LOCKING OF 2.7-m m LASERS
earlier with an acoustooptic modulator as switching element w17x. Using the InAs layers in the manner described here, the single-mode fiber lasers suffer from the very low reflection of radiation transmitted through the saturable absorber back into the fiber. This prevents efficient switching since no photon avalanche can develop. The inversion is quenched by a comparably slow process without the development of a giant pulse. Generally, the pulsed operation of the lasers was very stable and reproducible. The lasers operated over hours without degradation and alignment was easy. Problems arose when pumping with the dye laser at 647 nm at higher powers. Due to the high intensity at the layers}partly arising from unabsorbed pump power, partly from laser power}damage occurred at the InAs layers and at the fiber ends after short periods of regular operation. This either extinguished the laser operation or led to erratic operation. The damage was most likely to occur with thick layers. CONCLUSION
Passive Q-switching and mode-locking of erbium-doped fiber lasers at 2.7 m m have been demonstrated. The lasers were realized with laser diodes as pump sources. The first method presented, the flying-mirror modelocking scheme, seems to be especially suited for the generation of mode-locked pulses. It can be scaled by simple adaption of the fiber length to very high repetition rates. A drawback of this method is the required use of a mechanical device}in this case a loudspeaker. No continuous train of mode-locked pulses could be generated. Nevertheless it could be a simple method for generating subnanosecond pulses for applications in sensing and spectroscopy. Passive mode-locking of 2.7-m m fiber lasers with InAs epilayers as saturable absorbers proved to be a suitable method for the generation of Q-switched and mode-locked pulses. The technique is very favorable due to the simple resonator design with integrated saturable absorber. For single-mode fibers with high numerical aperture this design is not so suitable since the amount of radiation reflected back into the fiber, and hence the switching ratio, is too small for mode-locking and efficient Q-switching. This passive modulation scheme thus worked best with the multimode fiber, yielding maximum peak powers exceeding 1 W and pulse energies of 1.25 m J. Both techniques should have much potential for optimization. By careful design of the geometrical properties of the fiber and of the pump radiation input, an increase of the average powers should be possible. Especially the fiber lasers with the InAs saturable absorbers could be fitted to the desired applications by an advanced design of
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the InAs samples. A way to achieve this could be the utilization of antiresonant Fabry]Perot saturable absorber ŽA-FPSA. semiconductor devices, as already demonstrated in the near infrared w27x. This should also overcome the damage of the InAs samples at higher pump powers. Optimized diode pumped Q-switched and modelocked fiber lasers at this wavelength could serve as a simple, low-cost source for applications in medicine, sensing, and ultrashort spectroscopy. REFERENCES w1x L. Esterowitz and R. Allen, ‘‘Rare earth doped IR fiber laser for medical applications,’’ Proc. SPIE, vol. 1048, 129 Ž1989.. w2x K. Schmitt and W. Schaube, ‘‘Verfahren zur Ermittlung des Ž‘‘Method for Determination of Fahrbahnoberflachenzustandes’’ ¨ Roadway Surface Condition’’., German Patent Application DE 40 08 280 A1, Offenlegungsschrift, filed 19.9.1991. w3x A. G. Yodh, ‘‘Infrared methods for transient vibrational spectroscopy at surfaces and interfaces,’’ in OSA Annual Meeting Technical Digest, Albuquerque, NM, vol. 23, p. 184, 1992. w4x J. Hecht, ‘‘Tunable mid-infrared sources entice spectroscopists,’’ Laser Focus World, vol. 29, no. 9, 109 Ž1993.. w5x M. C. Brierley and P. W. France, ‘‘Continuous wave lasing at 2.7 m m in an erbium-doped fluorozirconate fibre,’’ Electron. Lett., vol. 24, 935 Ž1988.. w6x L. Wetenkamp, Ch. Frerichs, G. F. West, and H. Tobben, ‘‘Effi¨ cient CW operation of tunable fluorozirconate fibre lasers at wavelengths pumpable with semiconductor laser diodes,’’ J. NonCryst. Solids, vol. 140, 19 Ž1992.. w7x R. Allen, L. Esterowitz, and R. J. Ginther, ‘‘Diode-pumped singlemode fluorozirconate fiber laser from the 4 I 11r2 ª4 I 13r2 transition in erbium,’’ Appl. Phys. Lett., vol. 56, 1635 Ž1990.. w8x Ch. Frerichs, ‘‘All optical modulation of a 2.7 m m erbium-doped fluorozirconate fiber laser,’’ in OSA Proc. on Ad¨ anced Solid State Lasers, New Orleans, LA, vol. 15, pp. 399]402, 1993. w9x I. P. Alcock, A. C. Tropper, A. I. Ferguson, and D. C. Hanna, ‘‘Q-switched operation of a neodymium-doped monomode fibre laser,’’ Electron. Lett., vol. 22, no. 2, 84 Ž1986.. w10x I. P. Alcock, A. I. Ferguson, D. C. Hanna, and A. C. Tropper, ‘‘Mode-locking of a neodymium-doped monomode fibre laser,’’ Electron. Lett., vol. 22, no. 5, 268 Ž1986.. w11x D. Lytle, ‘‘Figure-8 laser makes ultrashort solitons,’’ Photon. Spectra, vol. 27, no. 8, 36 Ž1993.. w12x Kh. S. Bagdasarov, V. P. Danilov, V. I. Zhekov, T. M. Murina, A. A. Manenkov, M. I. Timoshechkin, and A. M. Prokhorov, ‘‘Pulse-periodic Y3 Al 5 O 12 :Er 3q laser with high activator concentration,’’ So¨ . J. Quantum Electron., vol. 81, no. 1, 83 Ž1978.. w13x S. Schnell, V. G. Ostroumov, J. Breguet, W. A. R. Luthy, H. P. ¨ Weber, and I. A. Shcherbakov, ‘‘Acoustooptic Q switching of erbium lasers,’’ IEEE J. Quantum Electron., vol. 26, no. 6, 1111 Ž1990.. w14x J. Breguet, A. F. Umyskov, W. A. R. Luthy, I. A. Shcherbakov, and ¨ H. P. Weber, ‘‘Electrooptically Q-switched 2.79 m m YSGG:Cr:Er laser with an intracavity polarizer,’’ IEEE J. Quantum Electron., vol. 27, no. 2, 274 Ž1991.. w15x J. Breguet, W. Luthy, and H. P. Weber, ‘‘Q-switching of YAG:Er ¨ laser with a soap film,’’ Opt. Commun., vol. 82, no. 5]6, 488 Ž1991..
366
FRERICHS AND UNRAU
w16x F. Konz, ¨ M. Frenz, V. Romano, M. Forrer, H. P. Weber, A. V. Kharkovskiy, and S. I. Khomenko, ‘‘Active and passive Q-switching of a 2.79 m m Er:Cr:YSGG laser,’’ Opt. Commun., vol. 103, 398 Ž1993.. w17x Ch. Frerichs and T. Tauermann, ‘‘Q-switched operation of a laser diode pumped erbium-doped fluorozirconate fibre laser operating at 2.7 m m,’’ Electron. Lett., vol. 30, no. 9, 706 Ž1994.. w18x H. J. Eichler, J. Albertz, F. Below, A. Kummrow, T. Leitert, A. A. Kaminskii, and L. Jakab, ‘‘Acousto-optic mode locking of 3-m m Er lasers,’’ Appl. Opt., vol. 31, no. 24, 4909 Ž1992.. w19x K. L. Vodopyanov, L. A. Kulevskii, P. P. Pashinin, A. F. Umyskov, and I. A. Shcherbakov, ‘‘Bandwidth-limited picosecond pulses from a YSGG:Cr 3q:Er 3q laser Ž l s 2.79 m m. with active mode locking,’’ So¨ . J. Quantum Electron., vol. 17, no. 6, 776 Ž1987.. w20x B. Pelz, M. K. Schott, and M. H. Niemz, ‘‘Electro-optic mode locking of an erbium:YAG laser with a rf resonance transformer,’’ Appl. Opt., vol. 33, no. 3, 364 Ž1994.. w21x K. L. Vodopyanov, A. V. Lukashev, and C. C. Phillips, ‘‘Nano- and picosecond 3 m m Er:YSGG lasers using InAs as passive Qswitchers and mode-lockers,’’ Opt. Commun., vol. 95, 87 Ž1993..
w22x Ch. Frerichs, ‘‘Mode-locking of a 2.7 m m erbium-doped fluoride fiber laser,’’ in Proceedings of the NATO Ad¨ anced Study Institute on Trends in Optical Fiber Metrology and Standards ŽO. D. D. Soares, Ed.., pp. 840, 841, NATO ASI Series E, vol. 285, Dordrecht, 1995. w23x Ch. Frerichs and J. Urner, ‘‘Passive mode-locking and Q-switching of a 2.7 m m fluoride fiber laser,’’ in Congress ‘‘ Ad¨ anced Solid State Lasers’’ on the ‘‘LASER 95,’’ Munich, 19.6.1995. w24x G. Sargsjan, U. Stamm, C. Unger, and W. Zschocke, ‘‘Characteristics of a neodymium-doped fiber laser mode-locked with a linear external cavity,’’ Opt. Commun., vol. 86, 480 Ž1991.. w25x M. Muller and U. Stamm, ‘‘Numerical analysis of pulse formation ¨ in solid-state lasers mode-locked with a linear external cavity,’’ Appl. Phys. B., vol. 54, 136 Ž1992.. w26x P. M. W. French, D. U. Noske, N. H. Rizvi, J. A. R. Williams, and J. R. Taylor, ‘‘Characterisation of a cw titanium-doped sapphire laser mode-locked with a linear external cavity,’’ Opt. Commun., vol. 83, no. 1]2, 185 Ž1991.. w27x U. Keller, ‘‘Ultrafast all-solid-state laser technology,’’ Appl. Phys. B, vol. 58, 347 Ž1994..