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Improved performance of blue synchronously pumped dye lasers by operating at an increased repetition rate
Volume 38, number 5,6
OPTICS COMMUNICATIONS
1 September 1981
IMPROVED PERFORMANCE OF BLUE SYNCHRONOUSLY PUMPED DYE LASERS BY OPERATING AT AN INCREASED REPETITION RATE A.I. FERGUSON Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
Received 11 May 1981 Krypton and argon ion lasers have been actively mode locked at the third harmonic of their normal mode spacing. This has increased the average power by almost a factor of three. The performance of coumarin 102 and stilbene 3 dye lasers when synchronously pumped by these ion lasers is described. They exhibit several important improvements over the standard system including higher stability, easier alignment, larger mode spacing and an increase in average power of over an order of magnitude. A simple modification to a commercially available synchronously pumped dye la~er system which shows these advantages is described.
Synchronous pumping o f dye lasers b y actively mode-locked nobel gas ion lasers has become a very convenient and reliable way o f producing ultrashort fight pulses throughout the visible region o f the spectrum [ 1 - 4 ] . To cover this wide spectral region, high power ion pump lasers are required. These lasers have cavity lengths which are typically in the region o f 2 m and operate at repetition rates of about 75 MHz which is the longitudinal mode spacing. This means that a dye laser which is to be synchronously pumped with such a laser has to have its cavity extended to match that o f the ion laser. This has lead to some difficulty in maintaining a mechanically rigid structure over this length. The two most popular solutions to this problem have been either to support the extended dye laser output mirror on a rigid optical bench or to incorporate a muttipass delay line in the laser cavity. A difficulty with the extended cavity approach is that although the long term stability is quite good, vibrations from the surface of the table are easily picked up and this can upset the short term mode stability of the laser. A major disadvantage of the delay line is that it introduces loss into the cavity. This limits the tuning range o f certain dyes and can severely reduce the output power from some o f the less efficient dyes. Some of these problems have been overcome by operating the dye laser at half the ion cavity length but this has lead to no dramatic improvement in performance [7].
The above problems can be overcome and several further advantages result if the repetition rate o f the ion pump laser is increased. This has recently been recognised b y one laser manufacturer [5]. I have made a very simple modification to a commercially available synchronously pumped dye laser system which enables the ion laser to operate at three times the normal repetition rate. This means that at any given time there are three light pulses in the ion laser cavity and that the ion laser mode spacing is a factor o f three higher at about 230 MHz. This leads to almost a factor o f three improvement in the ion laser average output power. This increase in power is thought to be due to the fact that at the normal 75 MHz repetition rate there is 12 ns between pulses so that the inversion between the pulses is not fully utilised and is diminished due to spontaneous emission. An alternative, frequency domain picture o f what is going on is that at the higher repetition rate the mode spacing is much larger than the homogeneous linewidth but less than the inhomogeneous linewidth and so there is less mode competition. The modification to the ion laser has had a profound effect on the pumping o f weak dyes such as the coumarin 102 and stilbene 3 where an increase from average power o f greater than an order o f magnitude has been observed. Other advantages o f the modified system is that the dye laser cavity length can now be
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
387
Volume 38, number 5,6
OPTICS COMMUNICATIONS
%
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Fig. 1. Sampling oscilloscope traces of laser pulses at 228 MHz. Top and bottom: Dye laser pulses from the coumarin 102 laser at 480 nm. Centre: krypton ion laser pulses at 413 nm. made so short that the whole cavity can be mounted within the standard dye laser package and that no delay line or refocusing of optics is required. This has 388
1 September 1981
lead to much greater stability and ease of alignment. Finally, a most important advantage is that the cavity mode spacing is now of the order of 230 MHz. This means that in multiple pulse spectroscopy using picosecond pulses, the free spectral range of the method is increased by a factor of three without any loss in resolution [6,7]. Furthermore, operation at this higher frequency will make active stabilization of the mode structure much easier, since, for a given pulse duration, the number of modes is reduced by a factor of three. This fact, allied with the overall increase in average power means that the average power in each mode must be much larger. Whether active stabilization is accomplished either by heterodyning one of the laser modes against a fixed frequency laser or by prefiltering the laser so that only a single mode passes and then stabilizing on a reference cavity, active stabilization should be much easier using the modified system [6,8]. I shall now describe the modified system. I have modified both an argon ion (CR18 UV) laser operating at 364 nm and a krypton ion (CR3000 K) laser operating at 413 nm. The system consisted of a frequency synthesizer (CR4675SE) which drives an acousto-optic modulator. There was a synchronising output available on the commercial synthesizer which was a highly distorted sine wave at the synthesizer frequency. This synchronizing output was passed through a narrowband amplifier tuned to the third harmonic of the synthesizer frequency. This amplified the third harmonic distortion to a level of about 1.5 W. The output from the amplifier drove a standard acousto-optic modulator (CR467 SEM) at 114.34 MHz which is approximately three times its normal frequency of 38.18 MHz. By carefully changing the cavity length of the ion laser, to match the mode-locker frequency, pulses of much less than 200 ps at 413 nm have been obtained at a repetition rate of 228.68 MHz. Sampling oscilloscope traces of these pulses can be seen in fig. 1. The response time of the photodiode (Telefunkent BPW28) to dye laser pulses whose autocorrelated widths are about 8 ps, was 180 ps full width at half maximum (fwhm). These pulses are also shown for comparison. The continuous wave power from the krypton laser at 413 nm was 1.2 W. When mode locked at 38.18 MHz the average power fell to 250 mW. However, when mode-locked at 114.34 MHz the average power was 720 mW. Thus the average power was almost a factor of three greater
Volume 38, number 5,6
z
OPTICS COMMUNICATIONS
~8psec
20
10DELAY{psec) 0 10 20
Fig. 2. Autocorrelation trace of the coumarin 102 dye laser operating at 480 nm. The autocorrelation full width at half maximum is 8 ps and corresponds to a pulse duration of between 4 ps and 6 ps.
when mode-locked at three times the frequency. Similar behaviour was exhibited on the 364 nm line in Ar but the maximum mode locked power to be obtained on this line was 400 mW. The k r y p t o n and argon lasers were used to pump coumarin 102 and stilbene 3 respectively. This was done rather conveniently b y simply reversing the mirror holder in the commercial dye laser (CR599-01) and using the adjusting micrometer provided. There was no need to use a delay line or to mount a mirror externally. The average power from the coumarin 102 dye laser was 150 mW at the centre o f its tuning range. This compares with 7 mW when the entire system was set up and optimised at the standard 38.18 MHz using a delay line. The pulse duration in b o t h cases was very similar. An autocorrelation of the modified system is shown in fig. 2 using lithium formate as the frequency doubler. This has an auto-correlation full width at half m a x i m u m (afwhm) of 8 ps which corresponds to a pulse duration of between 4 ps and 6 ps depending on the assumed pulse shape. The tuning element was a
1 September 1981
two plate birefringent filter. No attempt has been made to obtain shorter pulses. This would be accomplished using a broader bandwidth tuning element. The tuning range o f the laser was from 525 nm to 460 nm. With the stilbene 3 laser an average power o f about 50 mW was obtained as compared with about 7 mW from the unmodified system. The tuning range was 410 nm to 475 nm. No autocorrelation o f these pulses has yet been made but there is no reason to expect them to be any poorer than has been previously obtained [3]. I believe that operation of synchronously pumped dye lasers at three times the normal frequency gives a major improvement in the performance o f the laser particularly when weaker dyes are in use. The system enjoys several advantages including much higher average power, better stability, ease o f alignment and larger mode spacing. These advantages are particularly beneficial to coherent multiple pulse spectroscopy. I have shown that a commercially available system can be easily modified to operate at three times its normal operating frequency. I wish to gratefully acknowledge the financial support o f the Science Research Council.
References [1] J.P. Heritage and R.K. Jain, Appl. Phys. Letter 32 (1978) 727. [2] A.I. Ferguson, J.N. Eckstein and T.W. H~insch, J. Appl. Phys. 49 (1978) 5389. [3] J.N. Eckstein, A.I. Ferguson, T.W. H//nsch, G.A. Minord and C.K. Chan, Optics Comm. 27 (1978) 466. [4] J. Kuhl, H. Klingenberg and D. yon der Linde, Appl. Phys. 18 (1979) 279. [5] Spectra Physics: private communication. [6] A.I. Ferguson, J.N. Eckstein and T.W. H~insch, Appl. Phys. 18 (1979) 257. [7] J.N. Eckstein, A.I. Ferguson and T.W. H/insch, Phys. Lett. 40 (1978) 847.
389
OPTICS COMMUNICATIONS
1 September 1981
IMPROVED PERFORMANCE OF BLUE SYNCHRONOUSLY PUMPED DYE LASERS BY OPERATING AT AN INCREASED REPETITION RATE A.I. FERGUSON Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
Received 11 May 1981 Krypton and argon ion lasers have been actively mode locked at the third harmonic of their normal mode spacing. This has increased the average power by almost a factor of three. The performance of coumarin 102 and stilbene 3 dye lasers when synchronously pumped by these ion lasers is described. They exhibit several important improvements over the standard system including higher stability, easier alignment, larger mode spacing and an increase in average power of over an order of magnitude. A simple modification to a commercially available synchronously pumped dye la~er system which shows these advantages is described.
Synchronous pumping o f dye lasers b y actively mode-locked nobel gas ion lasers has become a very convenient and reliable way o f producing ultrashort fight pulses throughout the visible region o f the spectrum [ 1 - 4 ] . To cover this wide spectral region, high power ion pump lasers are required. These lasers have cavity lengths which are typically in the region o f 2 m and operate at repetition rates of about 75 MHz which is the longitudinal mode spacing. This means that a dye laser which is to be synchronously pumped with such a laser has to have its cavity extended to match that o f the ion laser. This has lead to some difficulty in maintaining a mechanically rigid structure over this length. The two most popular solutions to this problem have been either to support the extended dye laser output mirror on a rigid optical bench or to incorporate a muttipass delay line in the laser cavity. A difficulty with the extended cavity approach is that although the long term stability is quite good, vibrations from the surface of the table are easily picked up and this can upset the short term mode stability of the laser. A major disadvantage of the delay line is that it introduces loss into the cavity. This limits the tuning range o f certain dyes and can severely reduce the output power from some o f the less efficient dyes. Some of these problems have been overcome by operating the dye laser at half the ion cavity length but this has lead to no dramatic improvement in performance [7].
The above problems can be overcome and several further advantages result if the repetition rate o f the ion pump laser is increased. This has recently been recognised b y one laser manufacturer [5]. I have made a very simple modification to a commercially available synchronously pumped dye laser system which enables the ion laser to operate at three times the normal repetition rate. This means that at any given time there are three light pulses in the ion laser cavity and that the ion laser mode spacing is a factor o f three higher at about 230 MHz. This leads to almost a factor o f three improvement in the ion laser average output power. This increase in power is thought to be due to the fact that at the normal 75 MHz repetition rate there is 12 ns between pulses so that the inversion between the pulses is not fully utilised and is diminished due to spontaneous emission. An alternative, frequency domain picture o f what is going on is that at the higher repetition rate the mode spacing is much larger than the homogeneous linewidth but less than the inhomogeneous linewidth and so there is less mode competition. The modification to the ion laser has had a profound effect on the pumping o f weak dyes such as the coumarin 102 and stilbene 3 where an increase from average power o f greater than an order o f magnitude has been observed. Other advantages o f the modified system is that the dye laser cavity length can now be
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
387
Volume 38, number 5,6
OPTICS COMMUNICATIONS
%
,i~
@
Fig. 1. Sampling oscilloscope traces of laser pulses at 228 MHz. Top and bottom: Dye laser pulses from the coumarin 102 laser at 480 nm. Centre: krypton ion laser pulses at 413 nm. made so short that the whole cavity can be mounted within the standard dye laser package and that no delay line or refocusing of optics is required. This has 388
1 September 1981
lead to much greater stability and ease of alignment. Finally, a most important advantage is that the cavity mode spacing is now of the order of 230 MHz. This means that in multiple pulse spectroscopy using picosecond pulses, the free spectral range of the method is increased by a factor of three without any loss in resolution [6,7]. Furthermore, operation at this higher frequency will make active stabilization of the mode structure much easier, since, for a given pulse duration, the number of modes is reduced by a factor of three. This fact, allied with the overall increase in average power means that the average power in each mode must be much larger. Whether active stabilization is accomplished either by heterodyning one of the laser modes against a fixed frequency laser or by prefiltering the laser so that only a single mode passes and then stabilizing on a reference cavity, active stabilization should be much easier using the modified system [6,8]. I shall now describe the modified system. I have modified both an argon ion (CR18 UV) laser operating at 364 nm and a krypton ion (CR3000 K) laser operating at 413 nm. The system consisted of a frequency synthesizer (CR4675SE) which drives an acousto-optic modulator. There was a synchronising output available on the commercial synthesizer which was a highly distorted sine wave at the synthesizer frequency. This synchronizing output was passed through a narrowband amplifier tuned to the third harmonic of the synthesizer frequency. This amplified the third harmonic distortion to a level of about 1.5 W. The output from the amplifier drove a standard acousto-optic modulator (CR467 SEM) at 114.34 MHz which is approximately three times its normal frequency of 38.18 MHz. By carefully changing the cavity length of the ion laser, to match the mode-locker frequency, pulses of much less than 200 ps at 413 nm have been obtained at a repetition rate of 228.68 MHz. Sampling oscilloscope traces of these pulses can be seen in fig. 1. The response time of the photodiode (Telefunkent BPW28) to dye laser pulses whose autocorrelated widths are about 8 ps, was 180 ps full width at half maximum (fwhm). These pulses are also shown for comparison. The continuous wave power from the krypton laser at 413 nm was 1.2 W. When mode locked at 38.18 MHz the average power fell to 250 mW. However, when mode-locked at 114.34 MHz the average power was 720 mW. Thus the average power was almost a factor of three greater
Volume 38, number 5,6
z
OPTICS COMMUNICATIONS
~8psec
20
10DELAY{psec) 0 10 20
Fig. 2. Autocorrelation trace of the coumarin 102 dye laser operating at 480 nm. The autocorrelation full width at half maximum is 8 ps and corresponds to a pulse duration of between 4 ps and 6 ps.
when mode-locked at three times the frequency. Similar behaviour was exhibited on the 364 nm line in Ar but the maximum mode locked power to be obtained on this line was 400 mW. The k r y p t o n and argon lasers were used to pump coumarin 102 and stilbene 3 respectively. This was done rather conveniently b y simply reversing the mirror holder in the commercial dye laser (CR599-01) and using the adjusting micrometer provided. There was no need to use a delay line or to mount a mirror externally. The average power from the coumarin 102 dye laser was 150 mW at the centre o f its tuning range. This compares with 7 mW when the entire system was set up and optimised at the standard 38.18 MHz using a delay line. The pulse duration in b o t h cases was very similar. An autocorrelation of the modified system is shown in fig. 2 using lithium formate as the frequency doubler. This has an auto-correlation full width at half m a x i m u m (afwhm) of 8 ps which corresponds to a pulse duration of between 4 ps and 6 ps depending on the assumed pulse shape. The tuning element was a
1 September 1981
two plate birefringent filter. No attempt has been made to obtain shorter pulses. This would be accomplished using a broader bandwidth tuning element. The tuning range o f the laser was from 525 nm to 460 nm. With the stilbene 3 laser an average power o f about 50 mW was obtained as compared with about 7 mW from the unmodified system. The tuning range was 410 nm to 475 nm. No autocorrelation o f these pulses has yet been made but there is no reason to expect them to be any poorer than has been previously obtained [3]. I believe that operation of synchronously pumped dye lasers at three times the normal frequency gives a major improvement in the performance o f the laser particularly when weaker dyes are in use. The system enjoys several advantages including much higher average power, better stability, ease o f alignment and larger mode spacing. These advantages are particularly beneficial to coherent multiple pulse spectroscopy. I have shown that a commercially available system can be easily modified to operate at three times its normal operating frequency. I wish to gratefully acknowledge the financial support o f the Science Research Council.
References [1] J.P. Heritage and R.K. Jain, Appl. Phys. Letter 32 (1978) 727. [2] A.I. Ferguson, J.N. Eckstein and T.W. H~insch, J. Appl. Phys. 49 (1978) 5389. [3] J.N. Eckstein, A.I. Ferguson, T.W. H//nsch, G.A. Minord and C.K. Chan, Optics Comm. 27 (1978) 466. [4] J. Kuhl, H. Klingenberg and D. yon der Linde, Appl. Phys. 18 (1979) 279. [5] Spectra Physics: private communication. [6] A.I. Ferguson, J.N. Eckstein and T.W. H~insch, Appl. Phys. 18 (1979) 257. [7] J.N. Eckstein, A.I. Ferguson and T.W. H/insch, Phys. Lett. 40 (1978) 847.
389