- Email: [email protected]
Two-wavelength microsecond-time-delayed pulsed operation of a flash-lamp pumped titanium: sapphire laser
Infrared Phys. Fechnol.Vol. 36, No. 3. pp. 735-740, 1995
Pergamon
1350-4495(94)00109-X
Copyright ',~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights r~,-~rved 1350-4495/95 $9.50 + 0.00
TWO-WAVELENGTH MICROSECOND-TIME-DELAYED PULSED OPERATION OF A FLASH-LAMP PUMPED TITANIUM
:SAPPHIRE
LASER
M. M. ASIMOV, t A. D. DELEVA," Z. Y. PESHEV, 2 M. N. NENCHEV, z V. I. TZANEV 2 and T. B. PATRIKOV'qnstitute of Physics of Academy of Sciences of Belarus, 70 Boulevard Francisk Skorina, Minsk, Bclarus and "Institute of Electronics of Bulgarian Academy of Sciences, 72 Boulevard Tsarigradsko shosse, Sofia 1784, Bulgaria
(Received 27 April 1994) Al~tract--A coaxial flashlamp pumped titanium:sapphire laser, emitting two independently tunable wavelengths separately selected in a novel dual-channel cavity, is described. Two different modes of operation--with and without wavelength competition--are realized; they can easily be switched between. Generation of two simultaneous or successive pulses (produced by one pumping pulse), each corresponding to one of the emitted wavelengths, is accomplished. The time interval between the pulses is variable from 0 to about 3/~s. The generated wavelengths are different or identical. A coupled rate equation model is used to estimate the time characteristics of the two-wavelength operation.
I. I N T R O D U C T I O N
The titanium :sapphire (Ti:S) lasers emit in a wide tunable spectrum range (670-1200 nm), covering the absorption lines of a number of elements and compounds which are of interest for qualitative and quantitative chemical analyses, ecological monitoring, etc. for example Pb, 03, H~O, Cs, Rb, K, I, etc.). Hence Ti:S lasers are suitable light sources for various applications, such as intracavity laser spectroscopy (ILS)~t'2~and laser remote sensing of the atmosphere. °'4~ The best results in such investigations are obtained using differential methods [e.g. the differential absorption lidar systems (DIAL)], ~3'5~which ensure a high selectivity and reliability of the measurements. A laser for a DIAL lidar system should provide high energy radiation tunable at two or more wavelengths. Thus flashlamp pumped tunable Ti:S lasers are particularly suitable for such applications in the NIR spectral region. In this work the design and study of a two-wavelength tunable Ti: S laser pumped by a coaxial flashlamp is presented. Two independently tunable wavelengths were generated by a single rod in a novel dual-channel cavity where interference wedges were used as spectral selectors in the two cavity arms. The cavity design allows two modes of laser operation to be realized: (1) generation of two competitive wavelengths in a single output beam and (2) emission of two colinear juxtaposed output beams without wavelength competition. In both cases the individual wavelengths are generated in two simultaneous or successive pulses by one pumping pulse. The time interval between the pulses of the two wavelengths was varied from 0 to about 3/as by changing the optical losses in both cavity channels. A coupled rate equation model was used to determine the time intervals between the generation onset of both wavelengths and the pumping pulse maximum as a function of the optical losses under various conditions. II.
EXPERIMENTAL
The optical arrangement of the flashlamp pumped Ti:S laser is schematically shown in Fig. 1. The laser cavity has a dual-channel design. The length of each channel was about 45 cm. The first t~r ~;~-o
735
736
M. M. Asimov et al.
M2
Fig. 1. Optical arrangement of the two-wavelength Ti:S laser.
arm comprises an output mirror Mt (flat, reflectivity R = 90% in the range 700--800 nm), an interference wedge IWI (which serves as a selective element), and an end mirror M., (flat, R = 100% in the range 700--800 nm). The wedge IWI was adjusted at an incidence angle of 10-~with respect to the channel axis; the radiation wavelength (2,) generated in this part of the cavity (2,-arm) was tuned by translating the wedge 1W~. The second arm consists of the same output mirror Mj, the wedge 114,"I (serves as intermediate full reflecting mirror), an end mirror M~ (flat, R = 100% in the range 710--800nm) and an interference wedge IW.,, used as a selector. The wavelength (22) generated in the second arm (22-arm) was tuned by translating the wedge IW 2. Both wedges had a 5/Jm mean thickness of the air-gap (and angle of about 10-5 rad), enabling selection of spectral lines with 0.2 nm width. A laser head (active media Ti: S crystal, dia 6 ram, working length 85 mm, concentration of Ti-ions 0.13 weight % and a homemade coaxial flashlamp) is placed in the common part of the cavity arms. The mechanical design of the coaxial flashlamp is described earlier, tl6) Between the quartz cylinder containing the Ti:S rod and the internal flashlamp tube flowed a dye mixture (5 x 10 -4 moi/I ethanol solution of Coumarin 30 and PPO), which served as a spectrum converter of the pumping radiation from the UV range to that of the Ti: S absorption. The input electrical energy Ep (100+ 250J) was provided by a 6/~F capacitor bank. The typical flashlamp pulse duration (FWHM) was ~ 5/~s and the lamp operated either in the single pulse regime or at 0.2 Hz repetition rate. The lasing threshold (E~h) with the dye converter (for our experimental conditions) was ~ 140 J. Without the converter no lasing was obtained up to the maximum pumping energy. The output energy of each shot was measured by a Laser Precision Rj 7200 energy meter with RjP-735 probe. The spectral characteristics were obtained by a 9 ,~/mm dispersion spectrograph equipped with an optical multichannel analyzer (OMA). The time characteristics were determined using a pin-photodiode based detector (resolution 2 ns) and a 100 MHz storage oscilloscope. III.
RESULTS
AND
DISCUSSIONS
The interaction peculiarities of a large-aperture light beam with an interference wedge were used to realize both operation modes of the Ti:S laser (with and without wavelength competition). Within the laser spot on the wedge IW,-surface (when illuminating by the beam, see Fig. 1), 2-3 narrow parallel strips of resonance high transmission exist. These strips correspond to the equal wedge thicknesses. Dominance of one of these maxima (the strip "A" denoted by a dashed line within the laser spot, Fig. 1) was achieved by rotating the wedge IW~ (i.e. increasing the optical losses for the other strips). Thus for operation not too above the threshold pumping energy (Ep/E,h <~1.3) lasing through the lateral transmission maxima of the wedge was prevented; in case of greater pumping above the threshold, the opaque shields (slits with a variable width) $1 and S., (which block the lateral low transmission in both cavity channels) were used. The dominant part of the radiation which falls upon the area marked as "B" in Fig. 1 (within the laser spot) was
Pulsed operation of titanium:sapphire laser
[ J i
;kl
737
),2
Tt-0.30
TI=O
d L
,~
• T2-0.15
Tt-,O.07
|
i
750
i
• Tl~O
/\ ilj'l'~ll
J I
I
•
1 I
J I
755 760 765 Wavelength (nm)
I
I
I
I
770
750
o)
I
I
I
I
I
I
I
I'1
I
I
I
I
I
755 760 765 Wovelength (nm)
I
I
I
770
b)
Fig. 2. Spectrograms of the Ti:S laser two-wavelength radiation: (a) at two competitive wavelengths and (b) at two non-competitive wavelengths; 7t and 7: are the introduced optical losses into ~.t- and ,;.:-channels respectively.
reflected by the IWi-wedge surface. The area of resonance transmission (strip "2A") also reflected a part of the incident radiation namely, the one which corresponds to the non-resonance (for "A") wavelengths. The wavelength 2: is formed (within the )..,-channel) along the direction of one of the reflected (by "A" or by "B")beams. Ill. 1. Laser operation at two competitive wavelengths This operation mode was set by adjusting the interference wedge IW_, and the shield S: so that the 2.,-channel generates one of the non-resonance wavelengths reflected by the strip "A" of the IW~ surface. In this case 1 the ,~,- and )..,-beams spatially superimpose within the output beam and in the common part of the cavity, i.e. in the active media as well. The latter leads to occurrence of a strong competition between ).~ and ).,. wavelengths due to the Ti:S homogeneous gain broadening. The spectrum tuning (until two-wavelength operation still remained) covered range from 740 to 770 nm, limited by the wavelength competition• When a single wavelength was emitted (by blocking the channel of the other one) a tuning range (limited by the mirrors reflectivity only) from 715 to 790 nm was achieved for each spectral line. In Fig. 2(a) three spectrograms which illustrate the competitive wavelengths generation mode are shown• From the spectrograms one can see that with an increasing of the inserted optical losses in the ).rchannel from 0 to 15% (TK = 0 + 0.15) the amplitude of 2~ decreases while the one of 22 increases, until an inversion of their intensities ratio 1(21)/10.2) was reached. The change of this ratio was accompanied by a variation of the relative time delay between the two wavelengths pulses At12 = td()..,) -- td (21). This time interval (measured between the pulses maxima) varied from + 1.5. to - 1.0/~s in dependence on the above mentioned change of the losses. Consequently, a peculiarity of this mode of operation is a variation (caused by the mode competition) of the intensity and build-up time of the two wavelengths pulses when introducing losses into one of the channels• Thus, this mode of operation has two particular features: (1) a great part of the active media is not being utilized and (2) an instability (up to about 25%) of the intensity and the build-up time of the output ).1- and 22-pulses exist caused by competition. Therefore a laser, operating in this mode, is not suitable for the applications which require a higher power and/or time and intensity
738
M.M. Asimovet al.
stability of the output pulses, e.g. for DIAL. The active media volume utilized can be increased by replacing the interference wedges by Fabry-Perot etalons, but in this case the independent tunability of the output wavelengths becomes impossible. On the other hand, this mode of operation is distinguished by a high sensitivity to the intracavity losses in one of the channels. This gives a good opportunity for such a laser to be used for ILS and especially when the competitive beams method tT~is employed. Such a laser is also suitable for ILS based on a temporal approach, ts~ i.e. the detection of low concentrations is related to the build-up time of the absorbance wavelength pulse. Recently this approach was successfully applied using a laser-pumped Ti:S laser; tg~ it was found that the time delay between the output pulse and the pumping one varied from 0 to several hundreds of nanoseconds dependending on the absorbing sample concentration. In the case of a flashlamp pumped Ti: S laser the same time delay can vary within an interval up to several microseconds which, in addition to a wavelength competition, considerably increases the method's sensitivity.
111.2. Laser operation at two non-competitive wavelengths This mode of operation was set in a manner described below. The interference wedge IW2 was adjusted (by rotating) to maximize the transmission of that part of the beam which is reflected by the spot's area "B" (delineated by solid line in Fig. 1) located in the close proximity to the strip ("A") of resonance transmission. In this case the "~-t-and 2,-beams did not superimpose within the active media. Thus competition between the two wavelengths is avoided and they are emitted as two colinear juxtaposed beams. Wavelengths tuning was performed independently by translating the interference wedges IWt (in the ).~-channel) and IW2 (in the )..,-channel) which did not disturb the beams geometry. In this operation mode a range of tunability from 715 to 790 nm was achieved. In Fig. 2(b) three spectrograms which illustrate this mode of operation are shown. Optical losses up to 30% ()'2 = 0"0.3) were introduced in the ).z-channel by means of neutral density filters. The increased losses required relatively high initial Ep/Eth ratio. To prevent a CCD-line saturation by this wavelength, losses of about 15% (Yt = 0.15) per pass were introduced into the ).t-channel. From the spectrograms one can see that despite of a variation of ,~-zintensity within a wide range from saturation (~,2 = 0) to a suppression of the generation (72 = 0.3), the ).t intensity practically did not change. The spectra were recorded at a constant pump energy Ep = 230 J. The temporal behaviour of the ).,- and ).z-pulses was studied in two cases: (1) the pump energy was varied at constant conditions in both channels, (2) different calibrated losses were introduced into one of the channels at a constant pumping energy. In the case of 10% losses per pass introduced into the 2z-channel (Yz = 0.1) the pumping energy was varied from 150 to 250J which corresponds to the Ep/Eth ratio values from 1.1 to 1.8, respectively. The time delay of both pulses with respect to the maximum of the pumping pulse decreased from about 3/~s to 0, whereas the succession of the pulses remained. The duration of each output pulse varied from 100 ns (when it is emitted at the time close to the pumping maximum) to 250 ns (3 #s after it). The relative time delay between the output pulses Att2=ta(,~.2)-ta().l) was studied both experimentally and theoretically (Fig. 3) as a function of the losses inserted into the ).2-channel (up to 30%, i.e. Y2= 0 + 0.3) with a pump energy 1.8 times over threshold (for the ).t-channel). The losses were introduced by means of neutral density filters. The initial optical losses per pass (without introduced ones) in each channel was estimated to be about 40% (~0 = 0.4). A coupled rate equation model "°~ was used to evaluate the required dependence for an initial Ep/Eth ratio value of 1.8 under the conditions mentioned above. In Fig. 3 the experimental data (marked by points) and the theoretically estimated delay of 22-pulse with respect to ).t-pulse (denoted by solid line) are shown. Each experimental point is the averaged value of 50 pulses. From Fig. 3 one can see that the relative delay time between the two output pulses can be varied over a range of several
Pulsed operation of titanium:sapphire laser
739
3.5 00000
- -
-
,--,, 3.0
Experiment Theory
/
~L 2.5 2.0 >,
---~ 1.5 "13
E b-
1.0
0.5 d 0
0.0
i
0.3
f I
I
0.¢
o.5
o.6
!
Cavity
losses
i'
0.7
0.8
Fig. 3. Dependenceof the timedelay of..,-pulse with respectto 2t-pulse on the optical losses in 22-channel; pumping energy 250 J. microseconds. The buildup time of each (2~ and 22) output pulse can be controlled by combining properly the pumping level and the losses value in both channels. The operation mode without wavelengths competition differs from the previous one by the following main features: a greater part of the active media volume is utilized; the wavelengths are tunable in a greater range; a reliable independent variation of each wavelength intensity and of the corresponding pulses buildup time is achieved. A two-wavelength Ti: S laser operating in such a mode, is suitable for application in DIAL lidar systems. It might be also used for a treating of chemical, biological, etc. objects when selective irradiation by two independently tunable wavelengths is required. During the course of the work the laser was adjusted so that the same wavelength was selected in the both channels. This mode of operation with a controlled pulse buildup time is a useful tool for the study of fast transient processes on a microsecond time scale.
IV. C O N C L U S I O N We present a flashlamp pumped dual-channel Ti:S laser emitting two independently tunable wavelengths. Two operation modes were described, namely with and without wavelength competition. Two simultaneous or successive pulses, each of them corresponding to one of the generated wavelengths, were emitted during one pumping pulse. The time interval between the two pulses was varied from 0 to about 3 #s. Moreover, this control was accomplished in the case when the same wavelengths were selected in the both channels. The latter is of a great interest for investigation of fast transient phenomena in the microsecond time scale and is difficult to obtain by m e a n s of most of the well known laser cavity designs. The laser described can be used for a number of applications in the NIR region such as ILS, DIAL, chemical and biological studies and treatments, etc. Acknowledgements--We wish to express our gratitude to Professor B. K. Sevastianov, Institute of Crystallographyof the
Academy of Sciencesof Russia, for the helpfulcollaborationand valuablediscussion. This work was supported financially by the National Foundation for ScientificResearch (Bulgaria) under contract F-45. REFERENCES 1. D. A. Gilmore, P. V. Cvijin and G. H. Atkinson, Opt. Commun. 77, 385, (1990). 2. A. N. Kolerov, J. Appl. Spectrosc. (U.S.A.) 44, 211 (1986).
740
M . M . Asimov et al.
3. L. Stefanutti, F. Castagnoli. M. Del Guasta, M. Morandi, V. M. Sacco, V. Venturi. L. Zuccagnoli. J. Kolenda, H. Kneipp. P. Rairoux, B. Stein, D. Weidauer and J. Wolf, Appl. Phys,. B55, 13 (1992). 4. M. Nenehev, A. Deleva, E. Stoykova, Z. Peshev, T. Patrikov and A. Gizbrekht, Opt. Commun. 86, 405 (1991). 5. K. A. Fredriksson, Appl. Optics 24, 3297 (1985). 6. M: M. Asimov, A. G. Varpahovich and A. N. Rubinov, J. Appl. Spectrosc. (U.S.S.R,) 47, 389 (1987). 7. S. A. Batishchev, V. A. Mostovnikov and A. N. Rubinov, Sot. J. Quantum Electron. (U.S.A.)6, (1976). 8. J. M. Ramsey and W. B. Whitten, Analyt. Chem. 52, 2192 (1980). 9. A. D. Deleva and Z. Y. Peshev, Spectrosc. Lett. (in press). 10. Ch. Lin, IEEE dl Quantum Electron. QE-11, 602 (1985).
Pergamon
1350-4495(94)00109-X
Copyright ',~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights r~,-~rved 1350-4495/95 $9.50 + 0.00
TWO-WAVELENGTH MICROSECOND-TIME-DELAYED PULSED OPERATION OF A FLASH-LAMP PUMPED TITANIUM
:SAPPHIRE
LASER
M. M. ASIMOV, t A. D. DELEVA," Z. Y. PESHEV, 2 M. N. NENCHEV, z V. I. TZANEV 2 and T. B. PATRIKOV'qnstitute of Physics of Academy of Sciences of Belarus, 70 Boulevard Francisk Skorina, Minsk, Bclarus and "Institute of Electronics of Bulgarian Academy of Sciences, 72 Boulevard Tsarigradsko shosse, Sofia 1784, Bulgaria
(Received 27 April 1994) Al~tract--A coaxial flashlamp pumped titanium:sapphire laser, emitting two independently tunable wavelengths separately selected in a novel dual-channel cavity, is described. Two different modes of operation--with and without wavelength competition--are realized; they can easily be switched between. Generation of two simultaneous or successive pulses (produced by one pumping pulse), each corresponding to one of the emitted wavelengths, is accomplished. The time interval between the pulses is variable from 0 to about 3/~s. The generated wavelengths are different or identical. A coupled rate equation model is used to estimate the time characteristics of the two-wavelength operation.
I. I N T R O D U C T I O N
The titanium :sapphire (Ti:S) lasers emit in a wide tunable spectrum range (670-1200 nm), covering the absorption lines of a number of elements and compounds which are of interest for qualitative and quantitative chemical analyses, ecological monitoring, etc. for example Pb, 03, H~O, Cs, Rb, K, I, etc.). Hence Ti:S lasers are suitable light sources for various applications, such as intracavity laser spectroscopy (ILS)~t'2~and laser remote sensing of the atmosphere. °'4~ The best results in such investigations are obtained using differential methods [e.g. the differential absorption lidar systems (DIAL)], ~3'5~which ensure a high selectivity and reliability of the measurements. A laser for a DIAL lidar system should provide high energy radiation tunable at two or more wavelengths. Thus flashlamp pumped tunable Ti:S lasers are particularly suitable for such applications in the NIR spectral region. In this work the design and study of a two-wavelength tunable Ti: S laser pumped by a coaxial flashlamp is presented. Two independently tunable wavelengths were generated by a single rod in a novel dual-channel cavity where interference wedges were used as spectral selectors in the two cavity arms. The cavity design allows two modes of laser operation to be realized: (1) generation of two competitive wavelengths in a single output beam and (2) emission of two colinear juxtaposed output beams without wavelength competition. In both cases the individual wavelengths are generated in two simultaneous or successive pulses by one pumping pulse. The time interval between the pulses of the two wavelengths was varied from 0 to about 3/as by changing the optical losses in both cavity channels. A coupled rate equation model was used to determine the time intervals between the generation onset of both wavelengths and the pumping pulse maximum as a function of the optical losses under various conditions. II.
EXPERIMENTAL
The optical arrangement of the flashlamp pumped Ti:S laser is schematically shown in Fig. 1. The laser cavity has a dual-channel design. The length of each channel was about 45 cm. The first t~r ~;~-o
735
736
M. M. Asimov et al.
M2
Fig. 1. Optical arrangement of the two-wavelength Ti:S laser.
arm comprises an output mirror Mt (flat, reflectivity R = 90% in the range 700--800 nm), an interference wedge IWI (which serves as a selective element), and an end mirror M., (flat, R = 100% in the range 700--800 nm). The wedge IWI was adjusted at an incidence angle of 10-~with respect to the channel axis; the radiation wavelength (2,) generated in this part of the cavity (2,-arm) was tuned by translating the wedge 1W~. The second arm consists of the same output mirror Mj, the wedge 114,"I (serves as intermediate full reflecting mirror), an end mirror M~ (flat, R = 100% in the range 710--800nm) and an interference wedge IW.,, used as a selector. The wavelength (22) generated in the second arm (22-arm) was tuned by translating the wedge IW 2. Both wedges had a 5/Jm mean thickness of the air-gap (and angle of about 10-5 rad), enabling selection of spectral lines with 0.2 nm width. A laser head (active media Ti: S crystal, dia 6 ram, working length 85 mm, concentration of Ti-ions 0.13 weight % and a homemade coaxial flashlamp) is placed in the common part of the cavity arms. The mechanical design of the coaxial flashlamp is described earlier, tl6) Between the quartz cylinder containing the Ti:S rod and the internal flashlamp tube flowed a dye mixture (5 x 10 -4 moi/I ethanol solution of Coumarin 30 and PPO), which served as a spectrum converter of the pumping radiation from the UV range to that of the Ti: S absorption. The input electrical energy Ep (100+ 250J) was provided by a 6/~F capacitor bank. The typical flashlamp pulse duration (FWHM) was ~ 5/~s and the lamp operated either in the single pulse regime or at 0.2 Hz repetition rate. The lasing threshold (E~h) with the dye converter (for our experimental conditions) was ~ 140 J. Without the converter no lasing was obtained up to the maximum pumping energy. The output energy of each shot was measured by a Laser Precision Rj 7200 energy meter with RjP-735 probe. The spectral characteristics were obtained by a 9 ,~/mm dispersion spectrograph equipped with an optical multichannel analyzer (OMA). The time characteristics were determined using a pin-photodiode based detector (resolution 2 ns) and a 100 MHz storage oscilloscope. III.
RESULTS
AND
DISCUSSIONS
The interaction peculiarities of a large-aperture light beam with an interference wedge were used to realize both operation modes of the Ti:S laser (with and without wavelength competition). Within the laser spot on the wedge IW,-surface (when illuminating by the beam, see Fig. 1), 2-3 narrow parallel strips of resonance high transmission exist. These strips correspond to the equal wedge thicknesses. Dominance of one of these maxima (the strip "A" denoted by a dashed line within the laser spot, Fig. 1) was achieved by rotating the wedge IW~ (i.e. increasing the optical losses for the other strips). Thus for operation not too above the threshold pumping energy (Ep/E,h <~1.3) lasing through the lateral transmission maxima of the wedge was prevented; in case of greater pumping above the threshold, the opaque shields (slits with a variable width) $1 and S., (which block the lateral low transmission in both cavity channels) were used. The dominant part of the radiation which falls upon the area marked as "B" in Fig. 1 (within the laser spot) was
Pulsed operation of titanium:sapphire laser
[ J i
;kl
737
),2
Tt-0.30
TI=O
d L
,~
• T2-0.15
Tt-,O.07
|
i
750
i
• Tl~O
/\ ilj'l'~ll
J I
I
•
1 I
J I
755 760 765 Wavelength (nm)
I
I
I
I
770
750
o)
I
I
I
I
I
I
I
I'1
I
I
I
I
I
755 760 765 Wovelength (nm)
I
I
I
770
b)
Fig. 2. Spectrograms of the Ti:S laser two-wavelength radiation: (a) at two competitive wavelengths and (b) at two non-competitive wavelengths; 7t and 7: are the introduced optical losses into ~.t- and ,;.:-channels respectively.
reflected by the IWi-wedge surface. The area of resonance transmission (strip "2A") also reflected a part of the incident radiation namely, the one which corresponds to the non-resonance (for "A") wavelengths. The wavelength 2: is formed (within the )..,-channel) along the direction of one of the reflected (by "A" or by "B")beams. Ill. 1. Laser operation at two competitive wavelengths This operation mode was set by adjusting the interference wedge IW_, and the shield S: so that the 2.,-channel generates one of the non-resonance wavelengths reflected by the strip "A" of the IW~ surface. In this case 1 the ,~,- and )..,-beams spatially superimpose within the output beam and in the common part of the cavity, i.e. in the active media as well. The latter leads to occurrence of a strong competition between ).~ and ).,. wavelengths due to the Ti:S homogeneous gain broadening. The spectrum tuning (until two-wavelength operation still remained) covered range from 740 to 770 nm, limited by the wavelength competition• When a single wavelength was emitted (by blocking the channel of the other one) a tuning range (limited by the mirrors reflectivity only) from 715 to 790 nm was achieved for each spectral line. In Fig. 2(a) three spectrograms which illustrate the competitive wavelengths generation mode are shown• From the spectrograms one can see that with an increasing of the inserted optical losses in the ).rchannel from 0 to 15% (TK = 0 + 0.15) the amplitude of 2~ decreases while the one of 22 increases, until an inversion of their intensities ratio 1(21)/10.2) was reached. The change of this ratio was accompanied by a variation of the relative time delay between the two wavelengths pulses At12 = td()..,) -- td (21). This time interval (measured between the pulses maxima) varied from + 1.5. to - 1.0/~s in dependence on the above mentioned change of the losses. Consequently, a peculiarity of this mode of operation is a variation (caused by the mode competition) of the intensity and build-up time of the two wavelengths pulses when introducing losses into one of the channels• Thus, this mode of operation has two particular features: (1) a great part of the active media is not being utilized and (2) an instability (up to about 25%) of the intensity and the build-up time of the output ).1- and 22-pulses exist caused by competition. Therefore a laser, operating in this mode, is not suitable for the applications which require a higher power and/or time and intensity
738
M.M. Asimovet al.
stability of the output pulses, e.g. for DIAL. The active media volume utilized can be increased by replacing the interference wedges by Fabry-Perot etalons, but in this case the independent tunability of the output wavelengths becomes impossible. On the other hand, this mode of operation is distinguished by a high sensitivity to the intracavity losses in one of the channels. This gives a good opportunity for such a laser to be used for ILS and especially when the competitive beams method tT~is employed. Such a laser is also suitable for ILS based on a temporal approach, ts~ i.e. the detection of low concentrations is related to the build-up time of the absorbance wavelength pulse. Recently this approach was successfully applied using a laser-pumped Ti:S laser; tg~ it was found that the time delay between the output pulse and the pumping one varied from 0 to several hundreds of nanoseconds dependending on the absorbing sample concentration. In the case of a flashlamp pumped Ti: S laser the same time delay can vary within an interval up to several microseconds which, in addition to a wavelength competition, considerably increases the method's sensitivity.
111.2. Laser operation at two non-competitive wavelengths This mode of operation was set in a manner described below. The interference wedge IW2 was adjusted (by rotating) to maximize the transmission of that part of the beam which is reflected by the spot's area "B" (delineated by solid line in Fig. 1) located in the close proximity to the strip ("A") of resonance transmission. In this case the "~-t-and 2,-beams did not superimpose within the active media. Thus competition between the two wavelengths is avoided and they are emitted as two colinear juxtaposed beams. Wavelengths tuning was performed independently by translating the interference wedges IWt (in the ).~-channel) and IW2 (in the )..,-channel) which did not disturb the beams geometry. In this operation mode a range of tunability from 715 to 790 nm was achieved. In Fig. 2(b) three spectrograms which illustrate this mode of operation are shown. Optical losses up to 30% ()'2 = 0"0.3) were introduced in the ).z-channel by means of neutral density filters. The increased losses required relatively high initial Ep/Eth ratio. To prevent a CCD-line saturation by this wavelength, losses of about 15% (Yt = 0.15) per pass were introduced into the ).t-channel. From the spectrograms one can see that despite of a variation of ,~-zintensity within a wide range from saturation (~,2 = 0) to a suppression of the generation (72 = 0.3), the ).t intensity practically did not change. The spectra were recorded at a constant pump energy Ep = 230 J. The temporal behaviour of the ).,- and ).z-pulses was studied in two cases: (1) the pump energy was varied at constant conditions in both channels, (2) different calibrated losses were introduced into one of the channels at a constant pumping energy. In the case of 10% losses per pass introduced into the 2z-channel (Yz = 0.1) the pumping energy was varied from 150 to 250J which corresponds to the Ep/Eth ratio values from 1.1 to 1.8, respectively. The time delay of both pulses with respect to the maximum of the pumping pulse decreased from about 3/~s to 0, whereas the succession of the pulses remained. The duration of each output pulse varied from 100 ns (when it is emitted at the time close to the pumping maximum) to 250 ns (3 #s after it). The relative time delay between the output pulses Att2=ta(,~.2)-ta().l) was studied both experimentally and theoretically (Fig. 3) as a function of the losses inserted into the ).2-channel (up to 30%, i.e. Y2= 0 + 0.3) with a pump energy 1.8 times over threshold (for the ).t-channel). The losses were introduced by means of neutral density filters. The initial optical losses per pass (without introduced ones) in each channel was estimated to be about 40% (~0 = 0.4). A coupled rate equation model "°~ was used to evaluate the required dependence for an initial Ep/Eth ratio value of 1.8 under the conditions mentioned above. In Fig. 3 the experimental data (marked by points) and the theoretically estimated delay of 22-pulse with respect to ).t-pulse (denoted by solid line) are shown. Each experimental point is the averaged value of 50 pulses. From Fig. 3 one can see that the relative delay time between the two output pulses can be varied over a range of several
Pulsed operation of titanium:sapphire laser
739
3.5 00000
- -
-
,--,, 3.0
Experiment Theory
/
~L 2.5 2.0 >,
---~ 1.5 "13
E b-
1.0
0.5 d 0
0.0
i
0.3
f I
I
0.¢
o.5
o.6
!
Cavity
losses
i'
0.7
0.8
Fig. 3. Dependenceof the timedelay of..,-pulse with respectto 2t-pulse on the optical losses in 22-channel; pumping energy 250 J. microseconds. The buildup time of each (2~ and 22) output pulse can be controlled by combining properly the pumping level and the losses value in both channels. The operation mode without wavelengths competition differs from the previous one by the following main features: a greater part of the active media volume is utilized; the wavelengths are tunable in a greater range; a reliable independent variation of each wavelength intensity and of the corresponding pulses buildup time is achieved. A two-wavelength Ti: S laser operating in such a mode, is suitable for application in DIAL lidar systems. It might be also used for a treating of chemical, biological, etc. objects when selective irradiation by two independently tunable wavelengths is required. During the course of the work the laser was adjusted so that the same wavelength was selected in the both channels. This mode of operation with a controlled pulse buildup time is a useful tool for the study of fast transient processes on a microsecond time scale.
IV. C O N C L U S I O N We present a flashlamp pumped dual-channel Ti:S laser emitting two independently tunable wavelengths. Two operation modes were described, namely with and without wavelength competition. Two simultaneous or successive pulses, each of them corresponding to one of the generated wavelengths, were emitted during one pumping pulse. The time interval between the two pulses was varied from 0 to about 3 #s. Moreover, this control was accomplished in the case when the same wavelengths were selected in the both channels. The latter is of a great interest for investigation of fast transient phenomena in the microsecond time scale and is difficult to obtain by m e a n s of most of the well known laser cavity designs. The laser described can be used for a number of applications in the NIR region such as ILS, DIAL, chemical and biological studies and treatments, etc. Acknowledgements--We wish to express our gratitude to Professor B. K. Sevastianov, Institute of Crystallographyof the
Academy of Sciencesof Russia, for the helpfulcollaborationand valuablediscussion. This work was supported financially by the National Foundation for ScientificResearch (Bulgaria) under contract F-45. REFERENCES 1. D. A. Gilmore, P. V. Cvijin and G. H. Atkinson, Opt. Commun. 77, 385, (1990). 2. A. N. Kolerov, J. Appl. Spectrosc. (U.S.A.) 44, 211 (1986).
740
M . M . Asimov et al.
3. L. Stefanutti, F. Castagnoli. M. Del Guasta, M. Morandi, V. M. Sacco, V. Venturi. L. Zuccagnoli. J. Kolenda, H. Kneipp. P. Rairoux, B. Stein, D. Weidauer and J. Wolf, Appl. Phys,. B55, 13 (1992). 4. M. Nenehev, A. Deleva, E. Stoykova, Z. Peshev, T. Patrikov and A. Gizbrekht, Opt. Commun. 86, 405 (1991). 5. K. A. Fredriksson, Appl. Optics 24, 3297 (1985). 6. M: M. Asimov, A. G. Varpahovich and A. N. Rubinov, J. Appl. Spectrosc. (U.S.S.R,) 47, 389 (1987). 7. S. A. Batishchev, V. A. Mostovnikov and A. N. Rubinov, Sot. J. Quantum Electron. (U.S.A.)6, (1976). 8. J. M. Ramsey and W. B. Whitten, Analyt. Chem. 52, 2192 (1980). 9. A. D. Deleva and Z. Y. Peshev, Spectrosc. Lett. (in press). 10. Ch. Lin, IEEE dl Quantum Electron. QE-11, 602 (1985).