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Optimum cavity length and absolute cavity detuning in acousto-optically mode-locked argon-ion lasers
Volume 63, number 5
OPTICS COMMUNICATIONS
I September 1987
OPTIMUM CAVITY LENGTH AND ABSOLUTE CAVITY DETUNING IN ACOUSTO-OPTICALLY MODE-LOCKED ARGON-ION LASERS
I.S. RUDDOCK and R. ILLINGWORTH Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 ONG, UK
Received 6 April 1987
Acousto-optic mode-locking in an argon-ion laser was investigated in detail. Measurement of the discharge current is shown to be an accurate means of locating the optimum cavity length which depends strongly on level of excitation. The absolute cavity mismatch between the optimum length and that corresponding to c/4vRFwas determined by direct measurement and by using a cw dye laser as an active interferometer.
1. Introduction
The acousto-optically mode-locked argon-ion laser at 5 14.5 nm is routinely used in experimental quantum electronics. It is employed mainly as a pump source for synchronously mode-locked cw dye [ l] and colour centre lasers [ 21 and also as a source of pulses for time resolved fluorescence studies employing photon counting [ 31. The argon-ion laser can generate pulses of = 70 ps in duration but only when the cavity has been adjusted in length to within a tolerance of N 100 pm. This adjustment can be difficult unless there is access to a synchronous scan streak camera [ 41 or to a synchronously mode-locked dye laser for producing the sum frequency cross-correlation of the argon-ion and dye laser pulses [ 51. Much has been reported in recent years conceming the operation of synchronously excited lasers, in particular the synchronously pumped dye laser and its cavity mismatch or detuning characteristics [ 6-81. The topics of interest have been the effect on pulse profile and duration caused by small (pm) changes in the dye laser cavity length and the roles played by spontaneous emission and gain saturation in maintaining synchronism between the circulating intracavity dye laser pulse and the argon-ion laser pump pulses. There has been debate as to whether the dye laser cavity length for optimum pulse generation is greater than or equal to half the pump pulse separation. Gain saturation can advance the pulse enve-
lope in local time each transit, so consequently the dye laser cavity must be increased in length to compensate. However, since the pulse content travels at a rate determined by the cavity length, any substructure present moves in increments through the envelope from the leading edge to the trailing edge after each round trip. McDonald et al. [ lo] studied this phenomenon by means of recording the second harmonic cross-correlation of a pulse with another further along the pulse train. They argued that the optimum condition is when the dye laser cavity length is the same as half the pump pulse spacing because then the pulse substructure is stationary within the envelope. In the results reported by these authors the measurement resolution ( f 10 pm) of the cavity length and of the position of the pulse structure within the envelope was not really sufficient to establish if this was so because in the synchpumped dye laser adjustments are typically a few microns in magnitude. In this paper we discuss the analogous situation for the acousto-optically mode-locked argon-ion laser. Simple theory, ignoring the effects of gain saturation, states that the optimum length is equal to c/4v,, where vRF is the acoustic resonance frequency. By accurately measuring the discharge current as a function of cavity length the optimum length is identified and is shown to be greater than c/4vRF and to be dependent on the level of excitation. The absolute magnitude of the cavity detuning, measured from
0 030-4018/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
315
Volume 63. number 5
1 September 1987
OPTICS COMMUNICATIONS
C/&J,,, is determined by both using a dye laser as a monitor and by locating the minimum threshold length of the mode-locked argon-ion laser.
2. Experimental A 5W Spectra-Physics Model 165 argon-ion laser was acousto-optically mode-locked at 68.8840 MHz by a Model 45 l/342 mode-locker driver and head. It pumped an extended cavity cw dye laser either with the mode-locker RF on or off. With the RF on, the dye laser was synchronously mode-locked resulting in a train of ultrashort pulses. These were mixed with a portion of the 5 14.5 nm pump beam in an angle tuned ADP crystal to obtain the sum frequency. By introducing a variable time delay between the two beams, the sum frequency cross-correlation was obtained giving a true measure of the pulse duration of the mode-locked argon-ion laser. When the pump power was reduced such that the dye laser was brought to threshold, then the length of the dye laser for minimum threshold was deduced to equal half the separation of the argon-ion pulses [ Ill. In a synchronously mode-locked dye laser, the repetition frequency of the intra-cavity pulse matches that of the pump pulses irrespective of cavity length. Spontaneous emission and nonlinear amplification adjusts the position of the pulse envelope in local time to maintain this synchronisation each transit [ 121. At threshold the intra-cavity intensity tends to zero but at minimum threshold the contribution of nonlinear amplification to maintaining synchronism between the dye and argon-ion pulses becomes vanishingly small. A clear “resonance” between the dye and argon-ion pulses may be observed at this length. It is the same for all wavelengths but is different from that for maximum dye laser output power. With the RF off, the free-running cw dye laser was frequency doubled, also in ADP. This second harmonic signal peaked at a particular dye laser cavity length. Assuming self-locking was taking place in the free-running argon-ion laser, then this maximum was assumed to be the result of partial synchronous modelocking. Under these conditions, the length of the dye laser was used to indicate the actual length of the argon-ion laser cavity. 316
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1 -400
0
400
600
1200
1600 AL,+
Fig. 1. The dependence of the mode-locked argon-ion laser pulse duration + and discharge current 0 on the change in cavity length AL, at a fixed average power of 400 mW. Arbitrary zero for AL,.
3. Results and discussion 3.1 Determination of optimum cavity length by measurement of discharge current
Fig. 1 shows that the variation of the mode-locked argon-ion laser pulse duration and the required discharge current as the argon-ion cavity length was adjusted at a constant output power. A small maximum occurred in the current plot at the cavity length which produced the shortest pulses. We have discussed this in detail elsewhere [ 131 and explained it on the basis of spectral holeburning in the Doppler broadened 5 14.5 nm argon-ion emission line. When the comb of modes is symmetrically positioned on the emission line, corresponding modes on either side of line centre interact with the same groups of ions in the velocity distribution. The output power is reduced and the discharge current required for constant output power is increased. This increase in current for an ideal mode-locked laser occurs at cavity lengths separated by 1214close to the optimum length.
OPTICS COMMUNICATIONS
Volume 63, number 5
01
I
’
0
300
90;’
600 AL/\/
Pm
Fig. 2. The argon-ion laser optimum discharge current + and output power 0 for different cavity lengths. Arbitrary zero for AL,.
In the absence of active cavity length stabilisation, as the optical length of the cavity and the mode-locker frequency fluctuate, the longitudinal modes sweep continuously through the hole-burning resonances. Hence only a small averaged increase in discharge current for constant output power is observed as the number of coherent modes increases and decreases with change in cavity length through the optimum. This small increase in the required current has been found to be a reproducible and reliable method of locating the cavity length for shortest pulses in the acousto-optically mode-locked argon-ion laser at a particular output power. Alternatively the output power can be monitored as the current is kept constant giving a small decrease at the correct cavity length. 3.2. Dependence of optimum cavity length on excitation Fig. 2 shows the optimum length for acousto-optic mode-locking in the argon-ion laser as identified by this spectral hole-burning method for a range of currents and powers. On average the optimum length increased with excitation at a rate of N 6 1 pm/A. The length of the laser was then adjusted such that the current required for it to just oscillate was a minimum (15 A). This adjustment was the same as that predicted from the extrapolation of the curves in fig. 2 to zero output power. Assuming that the change in optimum cavity length was due to gain saturation experienced by the pulse envelope, as in the syn-
1 September 1987
chronously mode-locked dye laser, then at minimum threshold the argon-ion cavity length equals half the pulse spacing corresponding to the chosen vRF, i.e. LA= c/k+ This interpretation was confirmed by changing vRF and measuring the argon-ion length adjustment necessary to locate the new minimum threshold. Hence for mode-locking at, for example 0.4 W, fig. 2 shows that the optimum absolute cavity detuning is N 420 urn from the length given by c/4vRF. That the spectral hole-burning results of fig. 2 actually represented the optimum cavity lengths was confirmed by recording the sum frequency cross-correlation of the argon-ion and dye laser pulses over a time delay of N 2ns; sufficiently long to detect multiple pulsing. Correlation profiles were obtained for currents of 21 A, 23 A, 25 A, 27 A and 29 A and cavity lengths adjustments over a range of 1800 urn in 100 urn increments. They clearly showed that the lengths for which the shortest single pulses were produced were in agreement with those predicted in fig. 2. In addition they showed that as the cavity was then shortened a secondary pulse developed, separated from the main pulse by N 770 ps. This spacing was more or less independent of excitation and cavity length. It appears to be related to the physical separation between the ends of the plasma tube and the cavity mirrors and probably reflects the balance between gain recovery in the discharge and pulse timing in the mode-locker. As the cavity length was further decreased there came a point where single pulses were only generated close to threshold and then subsequent shortening caused collapse of the pulse envelope. This length was approximately the same as for minimum mode-locked threshold, i.e. c/4r+r as deduced above. Cavity lengths greater than optimum caused longer single pulses to be produced with slight modulation on the leading and trailing edges depending on the exact parameters. 3.3. Determination of absolute argon-ion cavity detuning by means of the cw dye laser Fig. 3 shows how the minimum threshold length of the mode-locked dye laser depended on lIvRF was varied over the range 68.7498-68.9394 MHz for a fixed argon-ion laser cavity length. This was limited by the extent ot the acoustic resonance in the modelocker. A least squares fit to the data points yields a 317
Volume 63, number 5
OPTICS COMMUNICATIONS
1 September 1987
400 AL$m 350
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Fig. 3. The change in the mode-locked dye laser cavity length dLo for minimum threshold plotted against I/vRF. The zero for dLo corresponds to Lo= c/4yRF,vRF= 68.8840 MHz. Some points have been omitted for clarity.
straight line of slope t(2.99f0.01 x 10’ ms-I). Fig. 4 (line a) shows that the length of the mode-locked dye laser at minimum threshold was constant to within 5 ym as the argon-ion laser cavity length was changed by 700 pm for a fixed v,,. Thus, since the length of the dye laser at minimum threshold is assumed to equal half the pump pulse separation, the results of fig. 3 and fig. 4 (line a) confirm that the repetition frequency of the pulses within the acoustooptically mode-locked argon-ion laser depends, as expected, on vRGand not on its cavity length. With the mode-locker RF now switched off, the
1
-500
-250
0
250
%
500 +
Fig. 4. The change in the dye laser cavity length dLD for (a) minimum threshold when mode-locked +, and (b) maximum second harmonic power when cw 0 at 1.OW (29 A) corresponding to changes in the argon-ion laser cavity length AL.,. The zero for ALD indicates the matched condition for the mode-locked dye laser i.e. LD=c/4uRF where vR,=68.8840 MHz. The zero for AL, corresponds to the optimum for mode-locking at 0.4 W.
318
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I 0.5
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1.5
2.5
20 PA/w
Fig. 5. The dependence on the argon-ion power of the dye laser cavity length for maximum second harmonic power with the mode-locker switched off. The zero for AL,, corresponds to the matched length, L,=c/~v,, vRF= 68.8840 MHz. The dashed line indicates the threshold for cw operation of the dye laser.
length of the cw dye laser for maximum second harmonic was found to be directly proportional to that of the free-running argon-ion laser at a constant power (1.0 W). As discussed in section 2 (experimental), this maximum was a result of self-locking in the argon-ion laser and indicated the matching of the cavity lengths. Fig. 4 (line b) shows the dye laser length for maximum second harmonic plotted against the argon-ion laser length as the latter was varied over the same 700 pm as in line a but with the mode-locker off. The straight line has a slope of 0.972 kO.004 confirming that the dye laser was faithfully monitoring the length of the argon-ion laser. The argonion laser cavity length at which lines a and b intersect equals c/4v,, because from above, this is the minimum threshold length of the mode-locked dye laser for a fixed VRF. However, as can be seen from fig. 4 this is N 325 pm shorter than the optimum for mode-locking at 0.4 WI23 A. This value of the absolute cavity detuning (LA-c/4VRF) is to be compared with the 420 pm obtained in subsection 3.1 by locating the minimum mode-locked threshold length. That these two methods do not exactly agree can be explained by including the effect on line b in fig. 4 of gain saturation in the unmode-locked argon-ion laser. Fig. 5 shows the change in the dye laser length for maximum second harmonic as the argon-ion power varied with the mode-locker off. The dye laser length had to be decreased with increasing argon-ion power at a rate of 43 pm/W. By misaligning the argon-ion
Volume 63, number 5
OPTICS COMMUNICATIONS
laser cavity it was confirmed that change in power, not current was the important parameter. Gain saturation within the dye laser can be ruled out because its length would have had to have been increased to compensate. The length reduction is consistent with gain saturation in the argon-ion laser causing the selflocked pulses to be advanced in local time. Consequently line b in fig. 4 must be repositioned such that all the dye laser measurements are increased by N 43 pm. This now gives an absolute detuning of - 370 pm for mode-locking at 0.4 WI23 A. Since the optimum argon-ion cavity lengths were measured to k 50 l.trnthen reasonable agreement is found between these two methods of determining the matched length and the absolute detuning.
I September 1987
pulse envelope collapses and grossly structured envelopes are obtained. It is clear from these results that the exact adjustment of an acousto-optically mode-locked ion laser depends on the use to which it is to be put. For example, time resolved fluoresence measurements [ 31 or the study of stimulated Raman scattering in optical fibres [ 141 demand the availibility of pulses of simple shape whereas synchronous pumping of dye and colour centre lasers possessing long fluorescence lifetimes can be achieved with more complex pulses 1151. Acknowledgement
Overall financial support was provided by the UK Science and Engineering Research Council. 4. Conclusion Results have been presented which clarify the operation of the acousto-optically mode-locked argon-ion laser. As with synchronously pumped dye laser gain saturation dominates its behaviour. Direct recording of the pulse profile and indirect current measurements both showed that the optimum length is dependent on the level of excitation. In the case of the Spectra-Physics Model 165 described, this variation is -60 pm/A. Extrapolation of this information gave a value for the absolute magnitude of the cavity detuning between the length corresponding to the RF drive frequency, c/4vRF, and the optimum length. Contirmation was obtained by locating the minimum modelocked threshold length for the argon-ion laser and by using the dye laser as an active interferometer to monitor the difference between its minimum modelocked threshold length (c/4vRF) and its maximum cw second harmonic length (LA). Sum frequency cross-correlation measurements showed that cavity detunings greater than optimum generate single but longer pulses whereas positive detunings less than optimum result in double pulsing. When the detuning becomes negative then the
References [ 1] C.K. Chan and S.O. Sari, Appl. Phys. Lett. 10 (1974) 25. [ 21 L. Isganitis, M.G. Sceats and K.R. German, Optics Lett. 5 (1980) 7.
[ 31 K.G. Spears, L.E. Cramer and L.D. Hoffland, Rev. Sci. Inst. 49 (1978) 255. [4] M.C. Adams, W. Sibbett and D.J. Bradley, Advances in electronics and electron physics, Vol. 52 (Academic Press, London 1979) p. 265. [ 51H. Mahr and M. Hirsch, Optics Comm. 13 (1975) 96. [6] N.J. Frigo, T. Daly and H. Mahr, IEEE J. Quant. Electron. QE-13 (1977) 101 [ 71 C.P. Ausschnitt, R.K. Jain and J.P. Heritage, IEEE J. Quant. Electron. QE-15 (1979) 912. [ 81 Z.A. Yasa, Appl. Phys. B 30 (1983) 134. [9] K. Smith, J.M. Catherall and G.H.C. New, Optics Comm. 58 (1986) 118. [lo] D.B. McDonald, W. Walbeck and G.R. Fleming, Optics Comm. 34 (1980) 127. [ II] T.F. Lillico, I.S. Ruddock and R. Illingworth, Optics Comm. 56 (1986) 354. [ 121 R.S. Putnam, J. Opt. Sot. Am. Bl (1984) 771. [ 131 I.S. Ruddock and R. Illingworth, J. Phys. E: Sci. Instrum. 18 (1985) 121. [ 141 R.H. Stolen, C. Lee and R.K. Jain, J. Opt. Sot. Am. B 1 (1984) 652. [ 15 ] R. Illingworth and IS. Ruddock, Optics Comm. 6 1 (1987) 120.
319
OPTICS COMMUNICATIONS
I September 1987
OPTIMUM CAVITY LENGTH AND ABSOLUTE CAVITY DETUNING IN ACOUSTO-OPTICALLY MODE-LOCKED ARGON-ION LASERS
I.S. RUDDOCK and R. ILLINGWORTH Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 ONG, UK
Received 6 April 1987
Acousto-optic mode-locking in an argon-ion laser was investigated in detail. Measurement of the discharge current is shown to be an accurate means of locating the optimum cavity length which depends strongly on level of excitation. The absolute cavity mismatch between the optimum length and that corresponding to c/4vRFwas determined by direct measurement and by using a cw dye laser as an active interferometer.
1. Introduction
The acousto-optically mode-locked argon-ion laser at 5 14.5 nm is routinely used in experimental quantum electronics. It is employed mainly as a pump source for synchronously mode-locked cw dye [ l] and colour centre lasers [ 21 and also as a source of pulses for time resolved fluorescence studies employing photon counting [ 31. The argon-ion laser can generate pulses of = 70 ps in duration but only when the cavity has been adjusted in length to within a tolerance of N 100 pm. This adjustment can be difficult unless there is access to a synchronous scan streak camera [ 41 or to a synchronously mode-locked dye laser for producing the sum frequency cross-correlation of the argon-ion and dye laser pulses [ 51. Much has been reported in recent years conceming the operation of synchronously excited lasers, in particular the synchronously pumped dye laser and its cavity mismatch or detuning characteristics [ 6-81. The topics of interest have been the effect on pulse profile and duration caused by small (pm) changes in the dye laser cavity length and the roles played by spontaneous emission and gain saturation in maintaining synchronism between the circulating intracavity dye laser pulse and the argon-ion laser pump pulses. There has been debate as to whether the dye laser cavity length for optimum pulse generation is greater than or equal to half the pump pulse separation. Gain saturation can advance the pulse enve-
lope in local time each transit, so consequently the dye laser cavity must be increased in length to compensate. However, since the pulse content travels at a rate determined by the cavity length, any substructure present moves in increments through the envelope from the leading edge to the trailing edge after each round trip. McDonald et al. [ lo] studied this phenomenon by means of recording the second harmonic cross-correlation of a pulse with another further along the pulse train. They argued that the optimum condition is when the dye laser cavity length is the same as half the pump pulse spacing because then the pulse substructure is stationary within the envelope. In the results reported by these authors the measurement resolution ( f 10 pm) of the cavity length and of the position of the pulse structure within the envelope was not really sufficient to establish if this was so because in the synchpumped dye laser adjustments are typically a few microns in magnitude. In this paper we discuss the analogous situation for the acousto-optically mode-locked argon-ion laser. Simple theory, ignoring the effects of gain saturation, states that the optimum length is equal to c/4v,, where vRF is the acoustic resonance frequency. By accurately measuring the discharge current as a function of cavity length the optimum length is identified and is shown to be greater than c/4vRF and to be dependent on the level of excitation. The absolute magnitude of the cavity detuning, measured from
0 030-4018/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
315
Volume 63. number 5
1 September 1987
OPTICS COMMUNICATIONS
C/&J,,, is determined by both using a dye laser as a monitor and by locating the minimum threshold length of the mode-locked argon-ion laser.
2. Experimental A 5W Spectra-Physics Model 165 argon-ion laser was acousto-optically mode-locked at 68.8840 MHz by a Model 45 l/342 mode-locker driver and head. It pumped an extended cavity cw dye laser either with the mode-locker RF on or off. With the RF on, the dye laser was synchronously mode-locked resulting in a train of ultrashort pulses. These were mixed with a portion of the 5 14.5 nm pump beam in an angle tuned ADP crystal to obtain the sum frequency. By introducing a variable time delay between the two beams, the sum frequency cross-correlation was obtained giving a true measure of the pulse duration of the mode-locked argon-ion laser. When the pump power was reduced such that the dye laser was brought to threshold, then the length of the dye laser for minimum threshold was deduced to equal half the separation of the argon-ion pulses [ Ill. In a synchronously mode-locked dye laser, the repetition frequency of the intra-cavity pulse matches that of the pump pulses irrespective of cavity length. Spontaneous emission and nonlinear amplification adjusts the position of the pulse envelope in local time to maintain this synchronisation each transit [ 121. At threshold the intra-cavity intensity tends to zero but at minimum threshold the contribution of nonlinear amplification to maintaining synchronism between the dye and argon-ion pulses becomes vanishingly small. A clear “resonance” between the dye and argon-ion pulses may be observed at this length. It is the same for all wavelengths but is different from that for maximum dye laser output power. With the RF off, the free-running cw dye laser was frequency doubled, also in ADP. This second harmonic signal peaked at a particular dye laser cavity length. Assuming self-locking was taking place in the free-running argon-ion laser, then this maximum was assumed to be the result of partial synchronous modelocking. Under these conditions, the length of the dye laser was used to indicate the actual length of the argon-ion laser cavity. 316
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26
1 -400
0
400
600
1200
1600 AL,+
Fig. 1. The dependence of the mode-locked argon-ion laser pulse duration + and discharge current 0 on the change in cavity length AL, at a fixed average power of 400 mW. Arbitrary zero for AL,.
3. Results and discussion 3.1 Determination of optimum cavity length by measurement of discharge current
Fig. 1 shows that the variation of the mode-locked argon-ion laser pulse duration and the required discharge current as the argon-ion cavity length was adjusted at a constant output power. A small maximum occurred in the current plot at the cavity length which produced the shortest pulses. We have discussed this in detail elsewhere [ 131 and explained it on the basis of spectral holeburning in the Doppler broadened 5 14.5 nm argon-ion emission line. When the comb of modes is symmetrically positioned on the emission line, corresponding modes on either side of line centre interact with the same groups of ions in the velocity distribution. The output power is reduced and the discharge current required for constant output power is increased. This increase in current for an ideal mode-locked laser occurs at cavity lengths separated by 1214close to the optimum length.
OPTICS COMMUNICATIONS
Volume 63, number 5
01
I
’
0
300
90;’
600 AL/\/
Pm
Fig. 2. The argon-ion laser optimum discharge current + and output power 0 for different cavity lengths. Arbitrary zero for AL,.
In the absence of active cavity length stabilisation, as the optical length of the cavity and the mode-locker frequency fluctuate, the longitudinal modes sweep continuously through the hole-burning resonances. Hence only a small averaged increase in discharge current for constant output power is observed as the number of coherent modes increases and decreases with change in cavity length through the optimum. This small increase in the required current has been found to be a reproducible and reliable method of locating the cavity length for shortest pulses in the acousto-optically mode-locked argon-ion laser at a particular output power. Alternatively the output power can be monitored as the current is kept constant giving a small decrease at the correct cavity length. 3.2. Dependence of optimum cavity length on excitation Fig. 2 shows the optimum length for acousto-optic mode-locking in the argon-ion laser as identified by this spectral hole-burning method for a range of currents and powers. On average the optimum length increased with excitation at a rate of N 6 1 pm/A. The length of the laser was then adjusted such that the current required for it to just oscillate was a minimum (15 A). This adjustment was the same as that predicted from the extrapolation of the curves in fig. 2 to zero output power. Assuming that the change in optimum cavity length was due to gain saturation experienced by the pulse envelope, as in the syn-
1 September 1987
chronously mode-locked dye laser, then at minimum threshold the argon-ion cavity length equals half the pulse spacing corresponding to the chosen vRF, i.e. LA= c/k+ This interpretation was confirmed by changing vRF and measuring the argon-ion length adjustment necessary to locate the new minimum threshold. Hence for mode-locking at, for example 0.4 W, fig. 2 shows that the optimum absolute cavity detuning is N 420 urn from the length given by c/4vRF. That the spectral hole-burning results of fig. 2 actually represented the optimum cavity lengths was confirmed by recording the sum frequency cross-correlation of the argon-ion and dye laser pulses over a time delay of N 2ns; sufficiently long to detect multiple pulsing. Correlation profiles were obtained for currents of 21 A, 23 A, 25 A, 27 A and 29 A and cavity lengths adjustments over a range of 1800 urn in 100 urn increments. They clearly showed that the lengths for which the shortest single pulses were produced were in agreement with those predicted in fig. 2. In addition they showed that as the cavity was then shortened a secondary pulse developed, separated from the main pulse by N 770 ps. This spacing was more or less independent of excitation and cavity length. It appears to be related to the physical separation between the ends of the plasma tube and the cavity mirrors and probably reflects the balance between gain recovery in the discharge and pulse timing in the mode-locker. As the cavity length was further decreased there came a point where single pulses were only generated close to threshold and then subsequent shortening caused collapse of the pulse envelope. This length was approximately the same as for minimum mode-locked threshold, i.e. c/4r+r as deduced above. Cavity lengths greater than optimum caused longer single pulses to be produced with slight modulation on the leading and trailing edges depending on the exact parameters. 3.3. Determination of absolute argon-ion cavity detuning by means of the cw dye laser Fig. 3 shows how the minimum threshold length of the mode-locked dye laser depended on lIvRF was varied over the range 68.7498-68.9394 MHz for a fixed argon-ion laser cavity length. This was limited by the extent ot the acoustic resonance in the modelocker. A least squares fit to the data points yields a 317
Volume 63, number 5
OPTICS COMMUNICATIONS
1 September 1987
400 AL$m 350
300
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14.510
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14.530
14.550
l4.540
Fig. 3. The change in the mode-locked dye laser cavity length dLo for minimum threshold plotted against I/vRF. The zero for dLo corresponds to Lo= c/4yRF,vRF= 68.8840 MHz. Some points have been omitted for clarity.
straight line of slope t(2.99f0.01 x 10’ ms-I). Fig. 4 (line a) shows that the length of the mode-locked dye laser at minimum threshold was constant to within 5 ym as the argon-ion laser cavity length was changed by 700 pm for a fixed v,,. Thus, since the length of the dye laser at minimum threshold is assumed to equal half the pump pulse separation, the results of fig. 3 and fig. 4 (line a) confirm that the repetition frequency of the pulses within the acoustooptically mode-locked argon-ion laser depends, as expected, on vRGand not on its cavity length. With the mode-locker RF now switched off, the
1
-500
-250
0
250
%
500 +
Fig. 4. The change in the dye laser cavity length dLD for (a) minimum threshold when mode-locked +, and (b) maximum second harmonic power when cw 0 at 1.OW (29 A) corresponding to changes in the argon-ion laser cavity length AL.,. The zero for ALD indicates the matched condition for the mode-locked dye laser i.e. LD=c/4uRF where vR,=68.8840 MHz. The zero for AL, corresponds to the optimum for mode-locking at 0.4 W.
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1.5
2.5
20 PA/w
Fig. 5. The dependence on the argon-ion power of the dye laser cavity length for maximum second harmonic power with the mode-locker switched off. The zero for AL,, corresponds to the matched length, L,=c/~v,, vRF= 68.8840 MHz. The dashed line indicates the threshold for cw operation of the dye laser.
length of the cw dye laser for maximum second harmonic was found to be directly proportional to that of the free-running argon-ion laser at a constant power (1.0 W). As discussed in section 2 (experimental), this maximum was a result of self-locking in the argon-ion laser and indicated the matching of the cavity lengths. Fig. 4 (line b) shows the dye laser length for maximum second harmonic plotted against the argon-ion laser length as the latter was varied over the same 700 pm as in line a but with the mode-locker off. The straight line has a slope of 0.972 kO.004 confirming that the dye laser was faithfully monitoring the length of the argon-ion laser. The argonion laser cavity length at which lines a and b intersect equals c/4v,, because from above, this is the minimum threshold length of the mode-locked dye laser for a fixed VRF. However, as can be seen from fig. 4 this is N 325 pm shorter than the optimum for mode-locking at 0.4 WI23 A. This value of the absolute cavity detuning (LA-c/4VRF) is to be compared with the 420 pm obtained in subsection 3.1 by locating the minimum mode-locked threshold length. That these two methods do not exactly agree can be explained by including the effect on line b in fig. 4 of gain saturation in the unmode-locked argon-ion laser. Fig. 5 shows the change in the dye laser length for maximum second harmonic as the argon-ion power varied with the mode-locker off. The dye laser length had to be decreased with increasing argon-ion power at a rate of 43 pm/W. By misaligning the argon-ion
Volume 63, number 5
OPTICS COMMUNICATIONS
laser cavity it was confirmed that change in power, not current was the important parameter. Gain saturation within the dye laser can be ruled out because its length would have had to have been increased to compensate. The length reduction is consistent with gain saturation in the argon-ion laser causing the selflocked pulses to be advanced in local time. Consequently line b in fig. 4 must be repositioned such that all the dye laser measurements are increased by N 43 pm. This now gives an absolute detuning of - 370 pm for mode-locking at 0.4 WI23 A. Since the optimum argon-ion cavity lengths were measured to k 50 l.trnthen reasonable agreement is found between these two methods of determining the matched length and the absolute detuning.
I September 1987
pulse envelope collapses and grossly structured envelopes are obtained. It is clear from these results that the exact adjustment of an acousto-optically mode-locked ion laser depends on the use to which it is to be put. For example, time resolved fluoresence measurements [ 31 or the study of stimulated Raman scattering in optical fibres [ 141 demand the availibility of pulses of simple shape whereas synchronous pumping of dye and colour centre lasers possessing long fluorescence lifetimes can be achieved with more complex pulses 1151. Acknowledgement
Overall financial support was provided by the UK Science and Engineering Research Council. 4. Conclusion Results have been presented which clarify the operation of the acousto-optically mode-locked argon-ion laser. As with synchronously pumped dye laser gain saturation dominates its behaviour. Direct recording of the pulse profile and indirect current measurements both showed that the optimum length is dependent on the level of excitation. In the case of the Spectra-Physics Model 165 described, this variation is -60 pm/A. Extrapolation of this information gave a value for the absolute magnitude of the cavity detuning between the length corresponding to the RF drive frequency, c/4vRF, and the optimum length. Contirmation was obtained by locating the minimum modelocked threshold length for the argon-ion laser and by using the dye laser as an active interferometer to monitor the difference between its minimum modelocked threshold length (c/4vRF) and its maximum cw second harmonic length (LA). Sum frequency cross-correlation measurements showed that cavity detunings greater than optimum generate single but longer pulses whereas positive detunings less than optimum result in double pulsing. When the detuning becomes negative then the
References [ 1] C.K. Chan and S.O. Sari, Appl. Phys. Lett. 10 (1974) 25. [ 21 L. Isganitis, M.G. Sceats and K.R. German, Optics Lett. 5 (1980) 7.
[ 31 K.G. Spears, L.E. Cramer and L.D. Hoffland, Rev. Sci. Inst. 49 (1978) 255. [4] M.C. Adams, W. Sibbett and D.J. Bradley, Advances in electronics and electron physics, Vol. 52 (Academic Press, London 1979) p. 265. [ 51H. Mahr and M. Hirsch, Optics Comm. 13 (1975) 96. [6] N.J. Frigo, T. Daly and H. Mahr, IEEE J. Quant. Electron. QE-13 (1977) 101 [ 71 C.P. Ausschnitt, R.K. Jain and J.P. Heritage, IEEE J. Quant. Electron. QE-15 (1979) 912. [ 81 Z.A. Yasa, Appl. Phys. B 30 (1983) 134. [9] K. Smith, J.M. Catherall and G.H.C. New, Optics Comm. 58 (1986) 118. [lo] D.B. McDonald, W. Walbeck and G.R. Fleming, Optics Comm. 34 (1980) 127. [ II] T.F. Lillico, I.S. Ruddock and R. Illingworth, Optics Comm. 56 (1986) 354. [ 121 R.S. Putnam, J. Opt. Sot. Am. Bl (1984) 771. [ 131 I.S. Ruddock and R. Illingworth, J. Phys. E: Sci. Instrum. 18 (1985) 121. [ 141 R.H. Stolen, C. Lee and R.K. Jain, J. Opt. Sot. Am. B 1 (1984) 652. [ 15 ] R. Illingworth and IS. Ruddock, Optics Comm. 6 1 (1987) 120.
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