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Self-stabilization of a hybridly mode-locked styryl-9 dye laser using coherent photon seeding
Optics Communications89 (1992) 240-244 North-Holland
OPTIC S COMMUNICATIONS
Self-stabilization of a hybridly mode-locked styryl-9 dye laser using coherent photon seeding J.Q. Bi, W. H o d e l a n d H.P. W e b e r Institute of Applied Physics, Universityof Bern, Sidlerstrasse5, CH-3012Bern, Switzerland
Received 16 December 1991; revised manuscript received22 January 1992
The self-stabilizationof a hybridly mode-lockedstyryl-9dye laser using the coherent photon seeding (CPS) technique is demonstrated experimentally. Using CPS the mode-lockingis improved and stable, transform limited pulses as short as 170 fs are generated. Further, the shape and width of these pulses are insensitive to unusually large cavity length variations of up to 40 I~m. As a consequence stable operation of the laser can be maintained over many hours without any readjustments.
1. Introduction Recently a new technique called coherent photon seeding (CPS) for the self-stabilization of synchronously pumped mode locked (SPML) lasers has been reported [ 1-6 ]. By using a linear external cavity [ 1 ] to feed back an extremely small fraction (typically 10- s ) of the pulse into the main cavity the laser stability is drastically improved and close to transform limited pulses are generated. Theoretical models [4,5] which have been developed to simulate the physical process suggest the following interpretation of how CPS works. The optimum performance of SPML lasers is achieved only when the main laser cavity is slightly longer than the pump laser cavity. The pulse envelope in the SPML laser cavity therefore moves forward with respect to the background radiation which develops due to spontaneous emission. Noise initially located just ahead of the pulse will then migrate into the pulse envelope and will progressively be amplified. The random amplitude and phase variations of the noise thus cause large variations of the pulse profiles. Theory [4 ] shows that the manifestations of these perturbations are the extended wings usually observed on the autocorrelation traces and large fluctuations of the pulse energy. By seeding a coherent signal on the leading edge of the pulse the deleterious influence of the spontaneous emission is suppressed. Consequently the 240
extended wings in the autocorrelation disappear and the pulse energy fluctuations are significantly reduced. Although the CPS technique has been applied to different lasers [ 1-7 ] and in different feedback schemes [ 1-3 ], to our knowledge the influence of CPS on a hybridly mode locked system has not yet been investigated experimentally. In this paper, we report on the successful application of coherent photon seeding to a hybridly mode-locked styryl-9 dye laser [ 8 ]. We demonstrate that the application of the CPS technique to the hybridly mode-locked laser leads to a further pulse shortening and mode-locking improvement. Furthermore, the laser pulses become insensitive to cavity length variations of up to 40 I~m. This insensitivity to cavity length variations results in a stable laser operation over many hours without any readjustments.
2. The hybridly mode-locked laser system The hybridly mode-locked laser investigated is a styryl-9 dye laser which is synchronously pumped by compressed and frequency doubled Nd:YAG laser pulses. The dye solution consists of 1.35X 10 -3 M styryl-9 (Lambda Physics) dissolved in a 4:1 mixture of ethylene glycol and propylene carbonate. The dye laser incorporates a nonlinear mirror inside the
0030-4018/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
laser cavity which acts as a saturable absorber. The nonlinear mirror (NLM) consists of a 200 nm layer of bulk GaAIAs (approximately 5% A1 content) which is grown on a linear Bragg mirror and acts as the saturable absorber. The laser is tuneable from 832 to 870 nm. Optimum operation of the laser is obtained for wavelengths near the room temperature band gap of the GaAIAs material which lies between 830 nm and 840 nm. In this wavelength range close to bandwidth limited pulses with a duration of typically 200 fs are produced (for further details see ref. [81). Not included in the previous report [ 8 ] is the laser performance for different cavity length detunings. The laser reaches threshold for a cavity length variation in the order of one millimetre but acceptable mode-locking operation is found only for two distinct cavity length settings. However, the optimum operation is obtained only for one of these cavity length settings which must be maintained within 1 ~tm. Fig. 1 illustrates this point. The pulses obtained for the shorter cavity length (chosen as position 0) are noisy, broad, and of low power. A typical autocorrelation trace of these pulses shown in fig. la is 323 fs wide. The corresponding spectrum (fig. lb) has a width of 4.67 nm. Assuming a sech2 pulse shape the pulse duration is 210 fs and time-bandwidth product is 0.42. Much better performance is obtained for a 30 ~tm longer cavity. The autocorrelation trace of the corresponding pulses shown in fig. lc is 283 fs wide and the width of the spectrum shown in fig. 1d is 5 nm. Assuming a sech 2 pulse shape the
(o)
6
C
' " " J
(c)
(b) ":. ~ , ,
~' :~,'".,,,. 323 fs
4.67 nm
~
~:
~
0
(d) j-~.
283 fs
_~' ~'
5 nm
¢-.
J ~"
"~ 820
Time Delay
+ ~ u.,,," 850
Wavelength j'nrn]
Fig. 1. Autocorrelation traces (a.u.) and the corresponding spectra (a.u.) for two acceptable mode-locked pulse trains without feedback.
1 May 1992
pulse duration is 184 fs and time-bandwidth product is 0.40. Although these pulses are close to bandwidth limited, they are very sensitive to thermal drifts and mechanical vibrations which requires a readjustment of the laser typically every few minutes.
3. CPS applied to the hybridly mode-locked laser The experimental set-up of the hybridly modelocked styryl-9 dye laser is shown in fig. 2. A Michelson interferometer with unequal arms (dashed box in fig. 2) is used to realize CPS. Arm 1 of the interferometer is part of the main laser cavity and contains the saturable absorber (NLM). Arm 2 is slightly longer than arm 1 and is used to feed back a replica of the pulses into the main laser cavity. A variable attenuator (A) is used to vary the feedback level. The laser performance for different cavity length detunings changes dramatically when CPS is applied. The right column in fig. 3 shows the autocorrelation traces obtained for cavity length detunings around the position 0 defined in the previous paragraph. For comparison the autocorrelation traces which result when CPS is not applied are shown in the left column of fig. 3. Note that clean mode-locking is achieved over a large range of resonator lengths for which no or unsatisfactory results were obtained without CPS. As shown in the right column of fig. 3 the laser performance is not symmetric with cavity length detuning. The autocorrelation trace drops very abruptly when the cavity length is increased from + 10 ~tm to + 15 ~m. A similar change in autocorrelation occurs slowly when the cavity length is changed from - 3 0 l~m to - 6 0 ~tm. The pulse disappears if the detuning is increased slightly above + 20 lxm and appears again at + 30 ~tm. At this position the pulse is very sensitive to a further cavity length detuning. The most striking feature, however, is that the autocorrelation profiles experience no deterioration and their shape and width are almost identical for cavity length variations from + 10 I~m to - 3 0 ~tm. This insensitivity to cavity length variations of up to 40 ~tm allows an extremely stable laser operation over many hours without any readjustment of the laser itself (although the pump laser has to be readjusted). Another important aspect is that the application 241
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS ~
Fiber-Grating Pulse Compressor
/
5atom
1.06~,m lOOps
Nd:YAG
I
I
532n 3.Sps
1 May 1992
50mm
'1
I
r--TA'CaAs
[ 50ram
80mm
Nonlinear Mirror
j
Fig. 2. The hybridly mode-locked laser system including CPS. The Michelson interferometer arrangement in the dashed frame is used to apply the CPS technique. no seed
with seed
A )~ \
-60
~,m
A
-50
#m
/4
//!"-
(o)
(b) ...., i
v
•
u =
-40 ~
:
i
263
4.83 nm
fs
E
//i\
-,30 p,m
/i:' k fA!
-20
,~! L ,$
!"d
.~
#m
820
~
k..SS..
Jl i~.
o
Time Delay
850 Wavelength [nm]
Fig. 4• Typical laser performance using CPS with a feedback level ~
/
~"
+10 #rn
A /
\ A
/,\
^ J:
~' . i ~
283
fs
--
--. i
!
, i
+15 ~,m +20 #m +25/m~
+35/a'n
Fig. 3. Autocorrelationtraces without (left) and with (fight) CPS technique for different cavity length settings. The feedback level is 1.2× 10-3 and the feedback pulse is reinjected l0 ps behind the main laser pulse. of CPS results in a further pulse shortening and modelocking improvement. To illustrate this point a typical autocorrelation trace o f the laser pulses at position 0 and the corresponding spectrum are shown in figs. 4a, b, respectively. The autocorrelation trace 242
of 1.2X l0 -3 and a feedback pulse delay of l0 ps: (a) is the autocorrelation trace and (b) is the spectrum. is 263 fs wide and the spectral width is 4.83 nm. Assuming a sech 2 pulse shape the pulse duration is 170 fs and the time-bandwidth product is 0.36 which is very close to the ideal value o f 0.32. Recall that at this cavity length setting the pulse without feedback is 210 fs wide and the time-bandwidth product is 0.42. A further pulse shortening and reduction o f the time-bandwidth product are therefore achieved by applying the CPS technique. The phenomena described above are observed over a large wavelength range. However, examination o f the laser tuning range shows that the nonlinearity o f the mirror is a requisite for the observed effects• Fig. 5 shows the pulse durations as a function o f wavelength with and without feedback, respectively. Also shown in fig. 5 is the reflectivity of the N L M for two intensity levels which correspond to the linear and
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
1000
•
e
0,8. v
O "~
o ~ IZ
600
0.6 ' =
0
0
•
0.4
+~
0.2
0
i
800
,
810
,
i
820
,
,
830
%.-
+ ,
._~
"6 L
0
i
840
Wovelength
200
,
,
850
,
,
8eO
,
,
870
,
D n
0 880
Into]
Fig. 5. Laser tuning properties. The reflectivityof the nonlinear mirror as a function of wavelengthis shown for an intensity of 30 kW/cm2 (O) and 1 GW/cm2 ( • ) , respectively.The pulse durations are shown without ( • ) and with a feedback (+) of 1.2>( 10-3 ( 10 ps delay), respectively. saturated response, respectively. The tuning of the laser is easily realized by simply turning the tuning element while the feedback pulse position and intensity level remain unchanged. It should be pointed out that not only the pulse shortening but also the insensitivity to cavity length variations are only observed in the wavelength range from 830 to 843 nm where the mirror exhibits nonlinearity. For the other part of the laser tuning range the influence of the feedback is the same as for purely synchronously pumped lasers, i.e. the laser pulses are greatly stabilized but they are still very sensitive to small cavity length variations.
4. Discussion
Finally we discuss the position and the level of the feedback applied in the experiment. The above mentioned effects are observed only for certain intensity levels of the feedback which is a typical feature of CPS. The feedback level must lie within the range from 2 × 10-3 to 5 X 10 -4. This is comparable to the feedback levels used in additive pulse mode locking (APM) [9,10]. However, in contrast to APM, the length of arm 2 is not critical and needs no active stabilization. In fact, the laser is insensitive to very large length variations of arm 2. The length difference between arm 1 and arm 2 can be determined by monitoring the autocorrelation of the laser output. A major part of the feedback pulse train is reflected at
1 May 1992
the laser output mirror (see fig. 1 ) and joins the laser output beam. This gives us the possibility to measure the autocorrelation traces of the main and feedback pulse simultaneously. Thus, by measuring the separation of the two corresponding autocorrelation peaks we can precisely determine the length difference between the two arms. We have found that the insensitivity to cavity length variations and the pulse shortening described above are observed when arm 2 is 50 ~tm up to 5 m m longer than arm 1. In the time domain this implies that the feedback pulse is reinjected into the main cavity 300 fs up to 30 ps behind the master pulse. At first sight these results seem to contradict our interpretation of the CPS technique given in the introduction. There we pointed out that the feedback pulse must be reinjected slightly in advance of the master pulse. The experimental results indicate that the saturable absorber considerably modifies the relative movement of the pulse profile with respect to the noise background: in contrast to the purely synchronously pumped laser, the pulse profile in the hybridly mode-locked laser moves backward with respect to the noise background. The feedback pulse therefore has to be reinjected with a delay into the main cavity where it is amplified and eventually evolutes into the master pulse from the rear. Due to the gain dynamics and the relatively long upper state lifetime of the dye (compared to the pump pulse duration) the level of the spontaneous emission behind the main laser pulse is larger than in front. As a consequence the seeding levels required are much higher for the hybridly mode-locked laser than for the purely synchronously pumped laser. It should be mentioned that a similar behaviour (insensitivity of the laser output to cavity length variations of up to 15 ~tm) was observed by other authors [ 11 ] for a synchronously pumped, hybridly mode-locked, antiresonant-ring dye laser. There the effect was attributed to an APM-like mechanism. As pointed out above, however, this is hardly possible in our case because when the two arms are exactly matched the pulse is destroyed. Further, the insensitivity to cavity length detunings can be achieved for extremely large length differences between the two arms. Our experiments suggest that the cavity length insensitivity is due to the combination of nonlinearity and CPS. However, a better understanding of 243
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
the underlying physical m e c h a n i s m s needs some further work which would go b e y o n d the scope o f this paper.
5. Conclusion We have d e m o n s t r a t e d experimentally the self stabilization o f a hybridly m o d e locked styryl-9 dye laser using CPS. T r a n s f o r m l i m i t e d pulses as short as 170 fs are produced. The shape a n d d u r a t i o n o f these pulses are insensitive to cavity length variations o f up to 40 ~tm. This insensitivity to cavity length variations leads to a stable operation o f the laser over m a n y hours without any readjustments. These significant i m p r o v e m e n t s o f the laser performance provide a further confirmation that the application range o f the CPS technique is much wider than anticipated.
Acknowledgements We would like to t h a n k M. Proctor, M.A. Dupertuis, D. Martin, F. M o r i e r - G e n o u d , and F.K. Rein-
244
1 May 1992
hart for the cooperation in the h y b r i d mode-locking o f the laser by designing a n d manufacturing the N L M . Thanks also go to M. H r s l e for the drawing o f the figures.
References [ 1] P. Beaud, J.Q. Bi, W. Hodel and H.P. Weber, Optics Comm. 80 (1990) 31. [ 2 ] D.S. Peter, P. Beaud, W. Hodel and H.P. Weber, Optics Lett. 16 (1991) 407. [ 3 ] C.J. Hooker, J.M.D. Lister and I.N. Ross, Optics Comm. 80 (1990) 375. [4 ] J.Q. Bi, W. Hodel and H.P. Weber, Optics Comm. 81 ( 1991 ) 408. [ fi] G.H.C. New, Optics Lett. 15 (1990) 1306. [6] D. Cotter, Optics Comm. 83 ( 1991 ) 76. [ 7 ] N. McCarthy and D. Gay, Optics Lett. 16 ( 1991 ) 1004. [8] J.Q. Bi, W. Hodel, P. Beaud, J. Schiitz, H.P. Weber, M. Proctor, M.A. Dupertuis, D. Martin, F. Morier-Genoud and F.K. Reinhart, Optics Comm. 89 (1992) 245. [9] P. N. Kean, X. Zhu, D. W. Crust, R. S. Grant, N. Langford and W. Sibbett, Optics Lett. 14 (1989) 39. [10]J. Mark, L.Y. Liu, K.L. Hall, H.A. Haus and E.P. Ippen, Optics Lett. 14 (1989) 48. [ l l ] W . T. Lotshaw, D. McMorrow, T. Dickson and G. A. Kenney-Wallace, Optics Lett. 14 (1989) 1195.
OPTIC S COMMUNICATIONS
Self-stabilization of a hybridly mode-locked styryl-9 dye laser using coherent photon seeding J.Q. Bi, W. H o d e l a n d H.P. W e b e r Institute of Applied Physics, Universityof Bern, Sidlerstrasse5, CH-3012Bern, Switzerland
Received 16 December 1991; revised manuscript received22 January 1992
The self-stabilizationof a hybridly mode-lockedstyryl-9dye laser using the coherent photon seeding (CPS) technique is demonstrated experimentally. Using CPS the mode-lockingis improved and stable, transform limited pulses as short as 170 fs are generated. Further, the shape and width of these pulses are insensitive to unusually large cavity length variations of up to 40 I~m. As a consequence stable operation of the laser can be maintained over many hours without any readjustments.
1. Introduction Recently a new technique called coherent photon seeding (CPS) for the self-stabilization of synchronously pumped mode locked (SPML) lasers has been reported [ 1-6 ]. By using a linear external cavity [ 1 ] to feed back an extremely small fraction (typically 10- s ) of the pulse into the main cavity the laser stability is drastically improved and close to transform limited pulses are generated. Theoretical models [4,5] which have been developed to simulate the physical process suggest the following interpretation of how CPS works. The optimum performance of SPML lasers is achieved only when the main laser cavity is slightly longer than the pump laser cavity. The pulse envelope in the SPML laser cavity therefore moves forward with respect to the background radiation which develops due to spontaneous emission. Noise initially located just ahead of the pulse will then migrate into the pulse envelope and will progressively be amplified. The random amplitude and phase variations of the noise thus cause large variations of the pulse profiles. Theory [4 ] shows that the manifestations of these perturbations are the extended wings usually observed on the autocorrelation traces and large fluctuations of the pulse energy. By seeding a coherent signal on the leading edge of the pulse the deleterious influence of the spontaneous emission is suppressed. Consequently the 240
extended wings in the autocorrelation disappear and the pulse energy fluctuations are significantly reduced. Although the CPS technique has been applied to different lasers [ 1-7 ] and in different feedback schemes [ 1-3 ], to our knowledge the influence of CPS on a hybridly mode locked system has not yet been investigated experimentally. In this paper, we report on the successful application of coherent photon seeding to a hybridly mode-locked styryl-9 dye laser [ 8 ]. We demonstrate that the application of the CPS technique to the hybridly mode-locked laser leads to a further pulse shortening and mode-locking improvement. Furthermore, the laser pulses become insensitive to cavity length variations of up to 40 I~m. This insensitivity to cavity length variations results in a stable laser operation over many hours without any readjustments.
2. The hybridly mode-locked laser system The hybridly mode-locked laser investigated is a styryl-9 dye laser which is synchronously pumped by compressed and frequency doubled Nd:YAG laser pulses. The dye solution consists of 1.35X 10 -3 M styryl-9 (Lambda Physics) dissolved in a 4:1 mixture of ethylene glycol and propylene carbonate. The dye laser incorporates a nonlinear mirror inside the
0030-4018/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
laser cavity which acts as a saturable absorber. The nonlinear mirror (NLM) consists of a 200 nm layer of bulk GaAIAs (approximately 5% A1 content) which is grown on a linear Bragg mirror and acts as the saturable absorber. The laser is tuneable from 832 to 870 nm. Optimum operation of the laser is obtained for wavelengths near the room temperature band gap of the GaAIAs material which lies between 830 nm and 840 nm. In this wavelength range close to bandwidth limited pulses with a duration of typically 200 fs are produced (for further details see ref. [81). Not included in the previous report [ 8 ] is the laser performance for different cavity length detunings. The laser reaches threshold for a cavity length variation in the order of one millimetre but acceptable mode-locking operation is found only for two distinct cavity length settings. However, the optimum operation is obtained only for one of these cavity length settings which must be maintained within 1 ~tm. Fig. 1 illustrates this point. The pulses obtained for the shorter cavity length (chosen as position 0) are noisy, broad, and of low power. A typical autocorrelation trace of these pulses shown in fig. la is 323 fs wide. The corresponding spectrum (fig. lb) has a width of 4.67 nm. Assuming a sech2 pulse shape the pulse duration is 210 fs and time-bandwidth product is 0.42. Much better performance is obtained for a 30 ~tm longer cavity. The autocorrelation trace of the corresponding pulses shown in fig. lc is 283 fs wide and the width of the spectrum shown in fig. 1d is 5 nm. Assuming a sech 2 pulse shape the
(o)
6
C
' " " J
(c)
(b) ":. ~ , ,
~' :~,'".,,,. 323 fs
4.67 nm
~
~:
~
0
(d) j-~.
283 fs
_~' ~'
5 nm
¢-.
J ~"
"~ 820
Time Delay
+ ~ u.,,," 850
Wavelength j'nrn]
Fig. 1. Autocorrelation traces (a.u.) and the corresponding spectra (a.u.) for two acceptable mode-locked pulse trains without feedback.
1 May 1992
pulse duration is 184 fs and time-bandwidth product is 0.40. Although these pulses are close to bandwidth limited, they are very sensitive to thermal drifts and mechanical vibrations which requires a readjustment of the laser typically every few minutes.
3. CPS applied to the hybridly mode-locked laser The experimental set-up of the hybridly modelocked styryl-9 dye laser is shown in fig. 2. A Michelson interferometer with unequal arms (dashed box in fig. 2) is used to realize CPS. Arm 1 of the interferometer is part of the main laser cavity and contains the saturable absorber (NLM). Arm 2 is slightly longer than arm 1 and is used to feed back a replica of the pulses into the main laser cavity. A variable attenuator (A) is used to vary the feedback level. The laser performance for different cavity length detunings changes dramatically when CPS is applied. The right column in fig. 3 shows the autocorrelation traces obtained for cavity length detunings around the position 0 defined in the previous paragraph. For comparison the autocorrelation traces which result when CPS is not applied are shown in the left column of fig. 3. Note that clean mode-locking is achieved over a large range of resonator lengths for which no or unsatisfactory results were obtained without CPS. As shown in the right column of fig. 3 the laser performance is not symmetric with cavity length detuning. The autocorrelation trace drops very abruptly when the cavity length is increased from + 10 ~tm to + 15 ~m. A similar change in autocorrelation occurs slowly when the cavity length is changed from - 3 0 l~m to - 6 0 ~tm. The pulse disappears if the detuning is increased slightly above + 20 lxm and appears again at + 30 ~tm. At this position the pulse is very sensitive to a further cavity length detuning. The most striking feature, however, is that the autocorrelation profiles experience no deterioration and their shape and width are almost identical for cavity length variations from + 10 I~m to - 3 0 ~tm. This insensitivity to cavity length variations of up to 40 ~tm allows an extremely stable laser operation over many hours without any readjustment of the laser itself (although the pump laser has to be readjusted). Another important aspect is that the application 241
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS ~
Fiber-Grating Pulse Compressor
/
5atom
1.06~,m lOOps
Nd:YAG
I
I
532n 3.Sps
1 May 1992
50mm
'1
I
r--TA'CaAs
[ 50ram
80mm
Nonlinear Mirror
j
Fig. 2. The hybridly mode-locked laser system including CPS. The Michelson interferometer arrangement in the dashed frame is used to apply the CPS technique. no seed
with seed
A )~ \
-60
~,m
A
-50
#m
/4
//!"-
(o)
(b) ...., i
v
•
u =
-40 ~
:
i
263
4.83 nm
fs
E
//i\
-,30 p,m
/i:' k fA!
-20
,~! L ,$
!"d
.~
#m
820
~
k..SS..
Jl i~.
o
Time Delay
850 Wavelength [nm]
Fig. 4• Typical laser performance using CPS with a feedback level ~
/
~"
+10 #rn
A /
\ A
/,\
^ J:
~' . i ~
283
fs
--
--. i
!
, i
+15 ~,m +20 #m +25/m~
+35/a'n
Fig. 3. Autocorrelationtraces without (left) and with (fight) CPS technique for different cavity length settings. The feedback level is 1.2× 10-3 and the feedback pulse is reinjected l0 ps behind the main laser pulse. of CPS results in a further pulse shortening and modelocking improvement. To illustrate this point a typical autocorrelation trace o f the laser pulses at position 0 and the corresponding spectrum are shown in figs. 4a, b, respectively. The autocorrelation trace 242
of 1.2X l0 -3 and a feedback pulse delay of l0 ps: (a) is the autocorrelation trace and (b) is the spectrum. is 263 fs wide and the spectral width is 4.83 nm. Assuming a sech 2 pulse shape the pulse duration is 170 fs and the time-bandwidth product is 0.36 which is very close to the ideal value o f 0.32. Recall that at this cavity length setting the pulse without feedback is 210 fs wide and the time-bandwidth product is 0.42. A further pulse shortening and reduction o f the time-bandwidth product are therefore achieved by applying the CPS technique. The phenomena described above are observed over a large wavelength range. However, examination o f the laser tuning range shows that the nonlinearity o f the mirror is a requisite for the observed effects• Fig. 5 shows the pulse durations as a function o f wavelength with and without feedback, respectively. Also shown in fig. 5 is the reflectivity of the N L M for two intensity levels which correspond to the linear and
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
1000
•
e
0,8. v
O "~
o ~ IZ
600
0.6 ' =
0
0
•
0.4
+~
0.2
0
i
800
,
810
,
i
820
,
,
830
%.-
+ ,
._~
"6 L
0
i
840
Wovelength
200
,
,
850
,
,
8eO
,
,
870
,
D n
0 880
Into]
Fig. 5. Laser tuning properties. The reflectivityof the nonlinear mirror as a function of wavelengthis shown for an intensity of 30 kW/cm2 (O) and 1 GW/cm2 ( • ) , respectively.The pulse durations are shown without ( • ) and with a feedback (+) of 1.2>( 10-3 ( 10 ps delay), respectively. saturated response, respectively. The tuning of the laser is easily realized by simply turning the tuning element while the feedback pulse position and intensity level remain unchanged. It should be pointed out that not only the pulse shortening but also the insensitivity to cavity length variations are only observed in the wavelength range from 830 to 843 nm where the mirror exhibits nonlinearity. For the other part of the laser tuning range the influence of the feedback is the same as for purely synchronously pumped lasers, i.e. the laser pulses are greatly stabilized but they are still very sensitive to small cavity length variations.
4. Discussion
Finally we discuss the position and the level of the feedback applied in the experiment. The above mentioned effects are observed only for certain intensity levels of the feedback which is a typical feature of CPS. The feedback level must lie within the range from 2 × 10-3 to 5 X 10 -4. This is comparable to the feedback levels used in additive pulse mode locking (APM) [9,10]. However, in contrast to APM, the length of arm 2 is not critical and needs no active stabilization. In fact, the laser is insensitive to very large length variations of arm 2. The length difference between arm 1 and arm 2 can be determined by monitoring the autocorrelation of the laser output. A major part of the feedback pulse train is reflected at
1 May 1992
the laser output mirror (see fig. 1 ) and joins the laser output beam. This gives us the possibility to measure the autocorrelation traces of the main and feedback pulse simultaneously. Thus, by measuring the separation of the two corresponding autocorrelation peaks we can precisely determine the length difference between the two arms. We have found that the insensitivity to cavity length variations and the pulse shortening described above are observed when arm 2 is 50 ~tm up to 5 m m longer than arm 1. In the time domain this implies that the feedback pulse is reinjected into the main cavity 300 fs up to 30 ps behind the master pulse. At first sight these results seem to contradict our interpretation of the CPS technique given in the introduction. There we pointed out that the feedback pulse must be reinjected slightly in advance of the master pulse. The experimental results indicate that the saturable absorber considerably modifies the relative movement of the pulse profile with respect to the noise background: in contrast to the purely synchronously pumped laser, the pulse profile in the hybridly mode-locked laser moves backward with respect to the noise background. The feedback pulse therefore has to be reinjected with a delay into the main cavity where it is amplified and eventually evolutes into the master pulse from the rear. Due to the gain dynamics and the relatively long upper state lifetime of the dye (compared to the pump pulse duration) the level of the spontaneous emission behind the main laser pulse is larger than in front. As a consequence the seeding levels required are much higher for the hybridly mode-locked laser than for the purely synchronously pumped laser. It should be mentioned that a similar behaviour (insensitivity of the laser output to cavity length variations of up to 15 ~tm) was observed by other authors [ 11 ] for a synchronously pumped, hybridly mode-locked, antiresonant-ring dye laser. There the effect was attributed to an APM-like mechanism. As pointed out above, however, this is hardly possible in our case because when the two arms are exactly matched the pulse is destroyed. Further, the insensitivity to cavity length detunings can be achieved for extremely large length differences between the two arms. Our experiments suggest that the cavity length insensitivity is due to the combination of nonlinearity and CPS. However, a better understanding of 243
Volume 89, number 2,3,4
OPTICS COMMUNICATIONS
the underlying physical m e c h a n i s m s needs some further work which would go b e y o n d the scope o f this paper.
5. Conclusion We have d e m o n s t r a t e d experimentally the self stabilization o f a hybridly m o d e locked styryl-9 dye laser using CPS. T r a n s f o r m l i m i t e d pulses as short as 170 fs are produced. The shape a n d d u r a t i o n o f these pulses are insensitive to cavity length variations o f up to 40 ~tm. This insensitivity to cavity length variations leads to a stable operation o f the laser over m a n y hours without any readjustments. These significant i m p r o v e m e n t s o f the laser performance provide a further confirmation that the application range o f the CPS technique is much wider than anticipated.
Acknowledgements We would like to t h a n k M. Proctor, M.A. Dupertuis, D. Martin, F. M o r i e r - G e n o u d , and F.K. Rein-
244
1 May 1992
hart for the cooperation in the h y b r i d mode-locking o f the laser by designing a n d manufacturing the N L M . Thanks also go to M. H r s l e for the drawing o f the figures.
References [ 1] P. Beaud, J.Q. Bi, W. Hodel and H.P. Weber, Optics Comm. 80 (1990) 31. [ 2 ] D.S. Peter, P. Beaud, W. Hodel and H.P. Weber, Optics Lett. 16 (1991) 407. [ 3 ] C.J. Hooker, J.M.D. Lister and I.N. Ross, Optics Comm. 80 (1990) 375. [4 ] J.Q. Bi, W. Hodel and H.P. Weber, Optics Comm. 81 ( 1991 ) 408. [ fi] G.H.C. New, Optics Lett. 15 (1990) 1306. [6] D. Cotter, Optics Comm. 83 ( 1991 ) 76. [ 7 ] N. McCarthy and D. Gay, Optics Lett. 16 ( 1991 ) 1004. [8] J.Q. Bi, W. Hodel, P. Beaud, J. Schiitz, H.P. Weber, M. Proctor, M.A. Dupertuis, D. Martin, F. Morier-Genoud and F.K. Reinhart, Optics Comm. 89 (1992) 245. [9] P. N. Kean, X. Zhu, D. W. Crust, R. S. Grant, N. Langford and W. Sibbett, Optics Lett. 14 (1989) 39. [10]J. Mark, L.Y. Liu, K.L. Hall, H.A. Haus and E.P. Ippen, Optics Lett. 14 (1989) 48. [ l l ] W . T. Lotshaw, D. McMorrow, T. Dickson and G. A. Kenney-Wallace, Optics Lett. 14 (1989) 1195.