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Tunable, μs-pulsed ytterbium fiber laser system with a linewidth below 2.7 GHz
Optics Communications 279 (2007) 173–176 www.elsevier.com/locate/optcom
Tunable, ls-pulsed ytterbium fiber laser system with a linewidth below 2.7 GHz M. Engelbrecht *, D. Wandt, D. Kracht Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany Received 16 February 2007; received in revised form 15 June 2007; accepted 11 July 2007
Abstract We report on a widely tunable, pulsed laser system with narrow spectral linewidth based on a continuous wave ytterbium fiber oscillator, a pulse shaper and a power amplifier stage. The system is tunable from 1055 nm to 1085 nm and provides a maximum pulse energy of 155 lJ with a pulse duration of 1–5 ls. The linewidth is less than 2.7 GHz over the whole tuning range. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.55.Wd Keywords: Tunable laser; Fiber laser
1. Introduction The sensitive detection of gaseous components by using laser spectroscopy is generally performed in the ‘‘fingerprint’’ region of the appropriate molecules with their strong absorption lines beyond 3 lm. Unfortunately, no simple and reliable primary laser source is currently available in that wavelength range and with the small linewidth necessary. However, a nonlinear frequency conversion stage, e.g. an optical parametric oscillator, pumped by a high-power near-infrared laser can generate tunable radiation within the whole above mentioned wavelength range. Systems pumped with fixed wavelength Nd-doped solid state lasers have been demonstrated. They transfer the linewidth of the pump laser into the mid infrared and they are widely tunable by a change of the parameters of the nonlinear crystal or variation of the crystal temperature [1,2]. A disadvantage, especially of temperature tuning, is the slow tuning speed.
*
Corresponding author. Tel.: +49 511 2788 239; fax: +49 511 2788 100. E-mail address: [email protected] (M. Engelbrecht).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.07.018
Ytterbium-doped fiber lasers are a good alternative as pump sources for frequency conversion as they offer a high efficiency and allow the generation of high peak power in combination with excellent spatial and spectral beam quality [3,4]. Moreover, due to the amorphous structure of their glass host the gain profile of the active ions is significantly broadened [5]. This makes fiber lasers very attractive for the construction of tunable laser sources. Tunable lasers based on ytterbium-doped fibers are very common and the full gain spectrum of this material has been explored more than 15 years ago [6]. Since then, the continuous wave output power was increased strongly. As well, tunable and pulsed laser systems have been demonstrated [7,8]. Their fast tunability can be directly transformed into the mid infrared region by parametric frequency conversion [9]. Unfortunately, laser oscillators are either limited in the output power, or the high output power owing to the high gain allows a higher number of longitudinal modes to reach threshold, leading to a linewidth of at least several 10 GHz. A reduced linewidth increases the resolution of spectroscopic measurements, whereas a high output power ensures a good conversion efficiency in the frequency conversion and a higher detection sensitivity.
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M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
In this paper we report on a fast tunable, pulsed ytterbium-doped fiber laser system, delivering linewidth below 3 GHz. High peak output power is realized by a pulsed operation, suitable for efficient nonlinear frequency conversion. In combination, this laser system builds a powerful basis for trace gas detection systems. 2. Experimental set-up The experimental set-up is shown in Fig. 1. The laser consisted of a master-oscillator, power amplifier scheme, which separates the spectral properties from the power generation. The tunable master cw-oscillator consisted of a unidirectional ring resonator. The 25 m long active fiber was an ytterbium-doped double-clad large-mode-area fiber (Institut fu¨r Hochtechnologie, Jena, Germany). It was pumped in propagation direction through the end facets into the outer pump core (400 lm, NA: 0.38) by a fiber coupled diode module, delivering an output power of 15 W at 976 nm out of a fiber with a diameter of 400 lm and a NA of 0.22. The active fiber had an Yb2O3 doping concentration of 1000 mol parts in 106, a core diameter of 10 lm, a NA of 0.07, ensuring more than 95% pumplight absorption and single transverse mode operation. Both fiber ends were polished with an angle of 7° to eliminate signal feedback. The propagation direction was defined by an intracavity Faraday isolator. A double grating arrangement in Littman–Littrow configuration in combination with a telescope with a magnification of 5 defined the linewidth of the laser [10,11]. The advantage of this tuning mechanism is the combination of a small spectral filter with a wide tuning range, which can not be provided by other tuning mechanisms, like Lyot filters or etalons. Recently available fiber Fabry–Perot filters have the drawback of the very low power handling capability, but may become an attractive alternative in the future. In our setup wavelength tuning was provided by rotation of the Littrow grating, which was mounted on a motorized rotation stage, allowing a full spectral scan in 2 s. The holographic gratings had Pulse- Shaper
Master -Oscillator Pumpdiode
Power -Amplifier Yb-fiber
Yb-fiber
EOM
2 4 2
2
Tuning grating
2
Grating
Modematching
Pumpdiode
Telescope
Fig. 1. Setup of the laser system. The three modules of the laser system are separated by dashed lines.
1200 lines/mm and the incidence angle on the Littman grating was 80°. The polarization at the output of the fiber segment was adjusted by means of a quarter and a half waveplate towards the input polarizing beam splitter cube (PBS) of the isolator. Additionally the half waveplate was used to define the output coupling ratio through this PBS. With an external half waveplate the seeding power was reduced for the following modules. In the second stage, a Pockels cell was used for pulse shaping. It provided an extinction ratio of 35 dB at the center wavelength. In order to reduce the driving voltage of the Pockels cell, it was operated in a double pass arrangement. The beam was separated by a Faraday isolator, which also prevented the oscillator from back-reflections of the following amplification stage. To adjust the polarization between the isolator and the Pockels cell, an additional half waveplate was placed in between. The power amplifier stage was built up by the same ytterbium fiber used in the oscillator, with the same length. The fiber was pumped contra directional to the laser propagation with a diode module similar to the one used in the oscillator. To match the beam parameters between the pulse shaper and the amplifier, an additional telescope was placed in between. 3. Experimental results The laser system was continuously tunable from 1055 nm to 1085 nm. The tuning range was limited by the size and the efficiency of the gratings, which is reduced at the edges of the tuning range. A wider tuning range, possible for example by omitting the telescope, would lead to a broader linewidth. The oscillator was driven at a low pump power and delivered 580 mW of output power with an absorbed pump power of 5.1 W. This operation ensured saturated operation of the amplifier as well as a small linewidth of the oscillator, which increased with the output power. The pulses generated from the Pockels cell were rectangular shaped with a length of 1 ls and 5 ls, respectively. The repetition rate was set to 20 kHz and the pulses had a peak power of 500 mW at the entrance side of the amplifier due to some losses from the Pockels cell. The amplification stage was pumped with a pumping power of 14 W resulting in a maximum output power of 3.1 W at 1070 nm, independently from the pulse duration generated, as shown in Fig. 2. This corresponded to pulse energy of 156 lJ. The wavelength dependency of the average output power and the pulse energy of the laser system at 20 kHz repetition rate and 1 ls pulse duration is shown in Fig. 3. To measure the pulse energy Ep, the time signal was measured with a photodiode and the ratio A between the area under the pulse and under the time signal in between the pulses was calculated. To exclude the cw component, the pulse energy was then calculated to Ep = A Æ P/R, where P is the average power and R is the repetition rate of the amplifier. During the tuning process, the driving voltage for the Pockels cell was not adjusted, but optimized for about
M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
2.0
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100
1.5
75
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Pulse energy [µJ]
1070 nm 20 kHz Pulse duration: 5 µs 1 µs
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—50
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0
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175
14
1040
1060
1080
Pump power [W] Fig. 2. Output power and pulse energy of the laser system versus pump power at 1070 nm.
Fig. 4. Output spectra of the laser at different operating wavelength at 1 ls pulse duration and 20 kHz repetition rate.
200
4.0
1070 nm Average power 0.24 W 0.72 W 1.72 W 3.13 W
300 175 250
125 100
3.0
75 2.5
Average power Pulse energy
1060
1070
200 150 100
50 50 25 0
2.0
Power [W]
150
3.5
Pulse energy [µJ]
Average power [W]
1100
Wavelength (nm)
1080
Wavelength [nm] Fig. 3. Wavelength dependency of the average output power and the pulse energy at 14 W pump power. The pulse duration was 1 ls and the repetition rate 20 kHz. The Pockels cell was optimized for 1070 nm.
1070 nm wavelength. Therefore, the polarization rotation provided by the Pockels cell derived from the optimum of 45° for each pass to the edges of the tuning range, decreasing the signal suppression inbetween the pulses by the pulse shaping unit. This led to a continuous wave background, which is strongly amplified. However, a coupling between the driving voltage of the Pockels cell and the wavelength setting would avoid this problem. In Fig. 4 the optical spectra of the laser are shown for 1 ls pulse duration and 20 kHz repetition rate for the center wavelength and for 1050 nm and 1080 nm. A good ratio between the laser line and amplified spontaneous emission (ASE) for the center wavelength can be observed. Here, more than 98% of the power were inside the laser line. However, to the edges of the tuning range, the power content inside the ASE increased. For 1055 nm and 1085 nm the power inside the laserline was still above 84%. In Fig. 5 the pulse shape of the amplified pulses is shown at 1070 nm. It strongly deviated from the rectangular shape given by the input pulse. For the maximum pump power
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time [µs] Fig. 5. Pulse shape of the output pulses. For increasing average power, the pulse front gets more strongly amplified than the pulse tail.
and 1 ls pulse duration a peak power of 300 W was achieved in the leading edge of the pulse. The power decreased over the pulse duration to 90 W at the end of the pulse. This pulse shape is caused to the fact, that the stored energy is reduced during the pulse duration. It is in agreement with the theory of pulse propagation in saturated laser amplifiers as described by Frantz and Nodvik [12]. The strength of the decrease depends on the inversion generated in the active medium, which corresponds to the pump power. Therefore, the decrease is strongest for highest pump power. The power decrease during the pulse can not be avoided as long as the amplifier is seeded with rectangular pulses. However, recent works have shown that a shaping of the seed pulses can result in rectangular output pulses [13], albeit a loss of pulse energy can be expected due to a non saturated operation during such shaping processes. The pulse shaping was beyond the scope of this paper. The same qualitative behavior of the pulse shape was observed for 5 ls pulse duration with a maximum peak power of 63 W. The small ripples on top of the pulse shape
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M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
efficient frequency conversion. With a linewidth of 2.5 GHz, the system is a promising base for trace gas analysis systems. Overall, a peak power of 300 W was shown for pulses with a repetition rate of 20 kHz and a pulse duration of 1 ls. The fast tunability of the system over the range from 1055 nm to 1085 nm is an important feature for all kind of frequency scanning applications.
1.0
Intensity [a.u.]
0.8
0.6
0.4
Acknowledgement 0.2
We gratefully thank the European Commission for supporting this research under contract number 2504 (the optical nose).
0.0 -2.5
0.0
2.5
5.0
7.5
10.0
12.5
Frequency [GHz] Fig. 6. Linewidth measurement at 1070 nm using a scanning Fabry–Perot interferometer with 10 GHz free spectral range.
were caused by a small backcoupling into the oscillator due to the not sufficient suppression from the isolator inbetween the oscillator and the amplification stage. The linewidth of our laser system using the double grating arrangement was as narrow as 2.7 GHz FWHM at a center wavelength of 1055 nm and decreased to 2.2 GHz at 1085 nm. It was measured by a scanning Fabry–Perot cavity with a free spectral range of 10 GHz and a finesse of 100 and is shown in Fig. 6. This linewidth is the smallest linewidth observed to the best of our knowledge for pulsed tunable ytterbium fiber lasers with multi Watt level peak power. However, the linewidth is broad enough that no signs of Brillouin scattering were visible, neither in the laser output, nor in the backward direction. 4. Summary In summary, we have demonstrated a pulsed, tunable fiber laser system that delivers output power sufficient for
References [1] W.R. Bosenberg, A. Drobshoff, J.I. Alexander, L.E. Myers, R.L. Byer, Opt. Lett. 21 (1996) 1336. [2] P.E. Powers, T.J. Kulp, S.E. Bisson, Opt. Lett. 23 (1998) 159. [3] Y. Jeong, J. Nilsson, J.K. Sahu, D.B.S. Soh, C. Alegria, P. Dupriez, C.A. Codemard, D.N. Payne, R. Horley, L.M.B. Hickey, L. Wanzcyck, C.E. Chryssou, J.A. Alvarez-Chavez, P.W. Turner, Opt. Lett. 30 (2005) 453. [4] A. Liu, M.A. Norsen, R.D. Mead, Opt. Lett. 30 (2005) 67. [5] H.M. Pask, Robert J. Carman, David C. Hanna, Anne C. Tropper, Colin J. Mackechnie, Paul R. Barber, Judith M. Dawes, IEEE J. Sel. Top. Quant. Electron. 1 (1995) 2. [6] D.C. Hanna, R.M. Percival, I.R. Perry, R.G. Smart, P.J. Suni, A.C. Tropper, J. Mod. Opt. 37 (1987) 517. [7] Y.-X. Fan, F.-Y. Lu, S.-L. Hu, K.-C. Lu, H.-J. Wang, X.-Y. Dong, J.-L. He, H.-T. Wang, Opt. Lett. 29 (2004) 724. [8] C.C. Renaud, R.S. Selvas-Aguilar, J. Nilsson, P.W. Turner, A.B. Grudinin, IEEE Phot. Tech. Lett. 11 (1999) 976. [9] P. Gross, M.E. Klein, T. Walde, K.-J. Boller, M. Auerbach, P. Wessels, C. Fallnich, Opt. Lett. 27 (2002) 418. [10] I. Shoshan, U.P. Oppenheim, Opt. Commun. 25 (1978) 375. [11] D. Wandt, M. Laschek, A. Tu¨nnermann, H. Welling, Opt. Lett. 15 (1997) 390. [12] L.M. Frantz, J.S. Nodvik, J. Appl. Phys. 63 (1963) 2346. [13] K.T. Vu, A. Malinowski, D.J. Richardson, F. Ghiringhelli, L.M.B. Hickey, M.N. Zervas, Opt. Express 14 (2006) 10996.
Tunable, ls-pulsed ytterbium fiber laser system with a linewidth below 2.7 GHz M. Engelbrecht *, D. Wandt, D. Kracht Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany Received 16 February 2007; received in revised form 15 June 2007; accepted 11 July 2007
Abstract We report on a widely tunable, pulsed laser system with narrow spectral linewidth based on a continuous wave ytterbium fiber oscillator, a pulse shaper and a power amplifier stage. The system is tunable from 1055 nm to 1085 nm and provides a maximum pulse energy of 155 lJ with a pulse duration of 1–5 ls. The linewidth is less than 2.7 GHz over the whole tuning range. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.55.Wd Keywords: Tunable laser; Fiber laser
1. Introduction The sensitive detection of gaseous components by using laser spectroscopy is generally performed in the ‘‘fingerprint’’ region of the appropriate molecules with their strong absorption lines beyond 3 lm. Unfortunately, no simple and reliable primary laser source is currently available in that wavelength range and with the small linewidth necessary. However, a nonlinear frequency conversion stage, e.g. an optical parametric oscillator, pumped by a high-power near-infrared laser can generate tunable radiation within the whole above mentioned wavelength range. Systems pumped with fixed wavelength Nd-doped solid state lasers have been demonstrated. They transfer the linewidth of the pump laser into the mid infrared and they are widely tunable by a change of the parameters of the nonlinear crystal or variation of the crystal temperature [1,2]. A disadvantage, especially of temperature tuning, is the slow tuning speed.
*
Corresponding author. Tel.: +49 511 2788 239; fax: +49 511 2788 100. E-mail address: [email protected] (M. Engelbrecht).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.07.018
Ytterbium-doped fiber lasers are a good alternative as pump sources for frequency conversion as they offer a high efficiency and allow the generation of high peak power in combination with excellent spatial and spectral beam quality [3,4]. Moreover, due to the amorphous structure of their glass host the gain profile of the active ions is significantly broadened [5]. This makes fiber lasers very attractive for the construction of tunable laser sources. Tunable lasers based on ytterbium-doped fibers are very common and the full gain spectrum of this material has been explored more than 15 years ago [6]. Since then, the continuous wave output power was increased strongly. As well, tunable and pulsed laser systems have been demonstrated [7,8]. Their fast tunability can be directly transformed into the mid infrared region by parametric frequency conversion [9]. Unfortunately, laser oscillators are either limited in the output power, or the high output power owing to the high gain allows a higher number of longitudinal modes to reach threshold, leading to a linewidth of at least several 10 GHz. A reduced linewidth increases the resolution of spectroscopic measurements, whereas a high output power ensures a good conversion efficiency in the frequency conversion and a higher detection sensitivity.
174
M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
In this paper we report on a fast tunable, pulsed ytterbium-doped fiber laser system, delivering linewidth below 3 GHz. High peak output power is realized by a pulsed operation, suitable for efficient nonlinear frequency conversion. In combination, this laser system builds a powerful basis for trace gas detection systems. 2. Experimental set-up The experimental set-up is shown in Fig. 1. The laser consisted of a master-oscillator, power amplifier scheme, which separates the spectral properties from the power generation. The tunable master cw-oscillator consisted of a unidirectional ring resonator. The 25 m long active fiber was an ytterbium-doped double-clad large-mode-area fiber (Institut fu¨r Hochtechnologie, Jena, Germany). It was pumped in propagation direction through the end facets into the outer pump core (400 lm, NA: 0.38) by a fiber coupled diode module, delivering an output power of 15 W at 976 nm out of a fiber with a diameter of 400 lm and a NA of 0.22. The active fiber had an Yb2O3 doping concentration of 1000 mol parts in 106, a core diameter of 10 lm, a NA of 0.07, ensuring more than 95% pumplight absorption and single transverse mode operation. Both fiber ends were polished with an angle of 7° to eliminate signal feedback. The propagation direction was defined by an intracavity Faraday isolator. A double grating arrangement in Littman–Littrow configuration in combination with a telescope with a magnification of 5 defined the linewidth of the laser [10,11]. The advantage of this tuning mechanism is the combination of a small spectral filter with a wide tuning range, which can not be provided by other tuning mechanisms, like Lyot filters or etalons. Recently available fiber Fabry–Perot filters have the drawback of the very low power handling capability, but may become an attractive alternative in the future. In our setup wavelength tuning was provided by rotation of the Littrow grating, which was mounted on a motorized rotation stage, allowing a full spectral scan in 2 s. The holographic gratings had Pulse- Shaper
Master -Oscillator Pumpdiode
Power -Amplifier Yb-fiber
Yb-fiber
EOM
2 4 2
2
Tuning grating
2
Grating
Modematching
Pumpdiode
Telescope
Fig. 1. Setup of the laser system. The three modules of the laser system are separated by dashed lines.
1200 lines/mm and the incidence angle on the Littman grating was 80°. The polarization at the output of the fiber segment was adjusted by means of a quarter and a half waveplate towards the input polarizing beam splitter cube (PBS) of the isolator. Additionally the half waveplate was used to define the output coupling ratio through this PBS. With an external half waveplate the seeding power was reduced for the following modules. In the second stage, a Pockels cell was used for pulse shaping. It provided an extinction ratio of 35 dB at the center wavelength. In order to reduce the driving voltage of the Pockels cell, it was operated in a double pass arrangement. The beam was separated by a Faraday isolator, which also prevented the oscillator from back-reflections of the following amplification stage. To adjust the polarization between the isolator and the Pockels cell, an additional half waveplate was placed in between. The power amplifier stage was built up by the same ytterbium fiber used in the oscillator, with the same length. The fiber was pumped contra directional to the laser propagation with a diode module similar to the one used in the oscillator. To match the beam parameters between the pulse shaper and the amplifier, an additional telescope was placed in between. 3. Experimental results The laser system was continuously tunable from 1055 nm to 1085 nm. The tuning range was limited by the size and the efficiency of the gratings, which is reduced at the edges of the tuning range. A wider tuning range, possible for example by omitting the telescope, would lead to a broader linewidth. The oscillator was driven at a low pump power and delivered 580 mW of output power with an absorbed pump power of 5.1 W. This operation ensured saturated operation of the amplifier as well as a small linewidth of the oscillator, which increased with the output power. The pulses generated from the Pockels cell were rectangular shaped with a length of 1 ls and 5 ls, respectively. The repetition rate was set to 20 kHz and the pulses had a peak power of 500 mW at the entrance side of the amplifier due to some losses from the Pockels cell. The amplification stage was pumped with a pumping power of 14 W resulting in a maximum output power of 3.1 W at 1070 nm, independently from the pulse duration generated, as shown in Fig. 2. This corresponded to pulse energy of 156 lJ. The wavelength dependency of the average output power and the pulse energy of the laser system at 20 kHz repetition rate and 1 ls pulse duration is shown in Fig. 3. To measure the pulse energy Ep, the time signal was measured with a photodiode and the ratio A between the area under the pulse and under the time signal in between the pulses was calculated. To exclude the cw component, the pulse energy was then calculated to Ep = A Æ P/R, where P is the average power and R is the repetition rate of the amplifier. During the tuning process, the driving voltage for the Pockels cell was not adjusted, but optimized for about
M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
2.0
—20
125
—30
100
1.5
75
1.0
50
0.5
25
Intensity (dB)
Average power [W]
2.5
150
Pulse energy [µJ]
1070 nm 20 kHz Pulse duration: 5 µs 1 µs
3.0
4
6
8
10
12
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—50
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0
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175
14
1040
1060
1080
Pump power [W] Fig. 2. Output power and pulse energy of the laser system versus pump power at 1070 nm.
Fig. 4. Output spectra of the laser at different operating wavelength at 1 ls pulse duration and 20 kHz repetition rate.
200
4.0
1070 nm Average power 0.24 W 0.72 W 1.72 W 3.13 W
300 175 250
125 100
3.0
75 2.5
Average power Pulse energy
1060
1070
200 150 100
50 50 25 0
2.0
Power [W]
150
3.5
Pulse energy [µJ]
Average power [W]
1100
Wavelength (nm)
1080
Wavelength [nm] Fig. 3. Wavelength dependency of the average output power and the pulse energy at 14 W pump power. The pulse duration was 1 ls and the repetition rate 20 kHz. The Pockels cell was optimized for 1070 nm.
1070 nm wavelength. Therefore, the polarization rotation provided by the Pockels cell derived from the optimum of 45° for each pass to the edges of the tuning range, decreasing the signal suppression inbetween the pulses by the pulse shaping unit. This led to a continuous wave background, which is strongly amplified. However, a coupling between the driving voltage of the Pockels cell and the wavelength setting would avoid this problem. In Fig. 4 the optical spectra of the laser are shown for 1 ls pulse duration and 20 kHz repetition rate for the center wavelength and for 1050 nm and 1080 nm. A good ratio between the laser line and amplified spontaneous emission (ASE) for the center wavelength can be observed. Here, more than 98% of the power were inside the laser line. However, to the edges of the tuning range, the power content inside the ASE increased. For 1055 nm and 1085 nm the power inside the laserline was still above 84%. In Fig. 5 the pulse shape of the amplified pulses is shown at 1070 nm. It strongly deviated from the rectangular shape given by the input pulse. For the maximum pump power
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time [µs] Fig. 5. Pulse shape of the output pulses. For increasing average power, the pulse front gets more strongly amplified than the pulse tail.
and 1 ls pulse duration a peak power of 300 W was achieved in the leading edge of the pulse. The power decreased over the pulse duration to 90 W at the end of the pulse. This pulse shape is caused to the fact, that the stored energy is reduced during the pulse duration. It is in agreement with the theory of pulse propagation in saturated laser amplifiers as described by Frantz and Nodvik [12]. The strength of the decrease depends on the inversion generated in the active medium, which corresponds to the pump power. Therefore, the decrease is strongest for highest pump power. The power decrease during the pulse can not be avoided as long as the amplifier is seeded with rectangular pulses. However, recent works have shown that a shaping of the seed pulses can result in rectangular output pulses [13], albeit a loss of pulse energy can be expected due to a non saturated operation during such shaping processes. The pulse shaping was beyond the scope of this paper. The same qualitative behavior of the pulse shape was observed for 5 ls pulse duration with a maximum peak power of 63 W. The small ripples on top of the pulse shape
176
M. Engelbrecht et al. / Optics Communications 279 (2007) 173–176
efficient frequency conversion. With a linewidth of 2.5 GHz, the system is a promising base for trace gas analysis systems. Overall, a peak power of 300 W was shown for pulses with a repetition rate of 20 kHz and a pulse duration of 1 ls. The fast tunability of the system over the range from 1055 nm to 1085 nm is an important feature for all kind of frequency scanning applications.
1.0
Intensity [a.u.]
0.8
0.6
0.4
Acknowledgement 0.2
We gratefully thank the European Commission for supporting this research under contract number 2504 (the optical nose).
0.0 -2.5
0.0
2.5
5.0
7.5
10.0
12.5
Frequency [GHz] Fig. 6. Linewidth measurement at 1070 nm using a scanning Fabry–Perot interferometer with 10 GHz free spectral range.
were caused by a small backcoupling into the oscillator due to the not sufficient suppression from the isolator inbetween the oscillator and the amplification stage. The linewidth of our laser system using the double grating arrangement was as narrow as 2.7 GHz FWHM at a center wavelength of 1055 nm and decreased to 2.2 GHz at 1085 nm. It was measured by a scanning Fabry–Perot cavity with a free spectral range of 10 GHz and a finesse of 100 and is shown in Fig. 6. This linewidth is the smallest linewidth observed to the best of our knowledge for pulsed tunable ytterbium fiber lasers with multi Watt level peak power. However, the linewidth is broad enough that no signs of Brillouin scattering were visible, neither in the laser output, nor in the backward direction. 4. Summary In summary, we have demonstrated a pulsed, tunable fiber laser system that delivers output power sufficient for
References [1] W.R. Bosenberg, A. Drobshoff, J.I. Alexander, L.E. Myers, R.L. Byer, Opt. Lett. 21 (1996) 1336. [2] P.E. Powers, T.J. Kulp, S.E. Bisson, Opt. Lett. 23 (1998) 159. [3] Y. Jeong, J. Nilsson, J.K. Sahu, D.B.S. Soh, C. Alegria, P. Dupriez, C.A. Codemard, D.N. Payne, R. Horley, L.M.B. Hickey, L. Wanzcyck, C.E. Chryssou, J.A. Alvarez-Chavez, P.W. Turner, Opt. Lett. 30 (2005) 453. [4] A. Liu, M.A. Norsen, R.D. Mead, Opt. Lett. 30 (2005) 67. [5] H.M. Pask, Robert J. Carman, David C. Hanna, Anne C. Tropper, Colin J. Mackechnie, Paul R. Barber, Judith M. Dawes, IEEE J. Sel. Top. Quant. Electron. 1 (1995) 2. [6] D.C. Hanna, R.M. Percival, I.R. Perry, R.G. Smart, P.J. Suni, A.C. Tropper, J. Mod. Opt. 37 (1987) 517. [7] Y.-X. Fan, F.-Y. Lu, S.-L. Hu, K.-C. Lu, H.-J. Wang, X.-Y. Dong, J.-L. He, H.-T. Wang, Opt. Lett. 29 (2004) 724. [8] C.C. Renaud, R.S. Selvas-Aguilar, J. Nilsson, P.W. Turner, A.B. Grudinin, IEEE Phot. Tech. Lett. 11 (1999) 976. [9] P. Gross, M.E. Klein, T. Walde, K.-J. Boller, M. Auerbach, P. Wessels, C. Fallnich, Opt. Lett. 27 (2002) 418. [10] I. Shoshan, U.P. Oppenheim, Opt. Commun. 25 (1978) 375. [11] D. Wandt, M. Laschek, A. Tu¨nnermann, H. Welling, Opt. Lett. 15 (1997) 390. [12] L.M. Frantz, J.S. Nodvik, J. Appl. Phys. 63 (1963) 2346. [13] K.T. Vu, A. Malinowski, D.J. Richardson, F. Ghiringhelli, L.M.B. Hickey, M.N. Zervas, Opt. Express 14 (2006) 10996.