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Observation of pulsating instabilities in an intracavity Raman laser
Volume 84, number
I,2
OPTICS
1 July 1991
COMMUNICATIONS
Observation of pulsating instabilities in an intracavity Raman laser V.N. Chizhevsky,
D.E. Gakhovich,
A.S. Grabchikov,
S.Ya. Kilin, V.A. Orlovich
and L.L. Tomilchik* Institute ofPhysics, BSSR Akudemy ofSciences, Leninsky Pr. 70, 220602 Minsk, USSR Received
8 November
1990; revised manuscript
received
18 February
An intracavity Raman laser as a dynamical system with stochastic locking and possible features of a chaotic behaviour of the Q-switched tering are observed.
Stimulated Raman scattering (SRS) in a resonator is an interesting example of a nonlinear system. A number of physical effects have been observed in the intracavity SRS: both in the system containing a Raman and laser medium in the mutual resonator (intracavity Raman laser) and in a system with only a Raman medium in the resonator (Raman laser). The laser pulse compression for nanosecond and subnanosecond pulses in the intracavity Raman laser were investigated in refs. [ I,2 1. Mode-locking and some aspects of the regular behaviour for this system were demonstrated in refs. [ 3,4]. The Raman laser as a dynamical system exhibiting instabilities and chaotic behaviour was explored in ref. [ 5 1. Additionally it is well known that the laser itself can demonstrate dynamical instabilities (see ref. [ 6 ] ). In this work we investigate the intracavity Raman laser as an example of a dynamical system with stochastic behaviour. We report the experimental observatidn of the instabilities, mode-locking and some features of chaotic behaviour induced by intracavity SRS in a Q-switched laser. The experimental setup is shown in fig. 1. A passive Q-switched ruby laser with a homogeneously broadened spectral line of emission was used as the main source of emission. The laser resonator con* Institute of Nuclear Problems, Byelorussian State University 220080 Minsk, Bobruyskaya 11,USSR. OO30-4018/91/$03.50
0 199
I - Elsevier Science Publishers
199 1
behaviour is investigated. Pulsating instabilities, moderuby laser induced by intracavity stimulated Raman scat-
Fig. 1. Scheme of the experimental setup. R,.* - mirrors, passive Q-switch, A - pinhole, L1,2 -lenses.
M -
tained two mirrors with reflections R, =70%, R2= 99% for the laser wavelength (694 nm) and R, =90%, R2=56% for the first Stokes wavelength (975 nm). A single transverse-mode was selected by a pinhole in the resonator. The Raman cell was placed into the resonator between two lenses L, and Lz with equal focal lengths. We used a compressed hydrogen gas (P=40 atm, T,= 180 ps) as the Raman medium. The Raman cell was 20 cm long. A passive absorber had the relaxation time TX,=40 ps. The experiments were performed using a laser cavity length of 120 cm. The spectral selection of Stokes and laser beams was realised by means of glass filters and selective mirrors. Temporal laser and Stokes output characteristics were measured with fast detectors and an oscilloscope with a resolution of about 1 ns. It is known that the single-mode laser with an absorber has a tendency to multimode operation characterized by oscillations in the output. We have investi-
B.V. ( North-Holland
)
47
Volume 84, number
1,2
OPTICS
COMMUNICATIONS
gated the shapes of the laser pulses when the Raman cell was empty and SRS did not develop. It was found that 90% of the laser pulses were single-mode with a gaussian temporal shape (fig. 2 a) and 10% of the pulses had modulated shapes. We believe both regimes (single-mode and multimode) have interesting aspects and can be the subject of investigations of an intracavity Raman laser. The frequency spectrum was obtained by computer processing of the registered oscillograms. In our experiments we varied the pumping level E (normalized pump energy, for the SRS-threshold E= 1) and transmittance T of the passive Q-switch. When the Raman cell was filled with hydrogen the output laser pulses changed drastically (fig. 2b) as a result of the SRS-generation. At the moment t, when the SRS-threshold is reached a sharp transition to different regimes occurs. The laser and Stokes pulses generated under these conditions have shown at least three characteristic intervals: (i) before SRS-generation; (ii) transitional interval when rather long SRS-pulses ap-
Fig. 2. Typical temporal shapes: (a) laser emission with an empty Raman cell; (b) laser and SRS-pulses with the Raman cell filled with hydrogen.
48
1 July 1991
peared and the laser intensity decreased abruptly; (iii) interval of the strong laser-SRS interaction with short pulsations. Typical laser intensity oscillograms and their spectra, calculated using a fast Fourier transformation, are presented in figs. 3, 4. Fig. 3 clearly shows the transition from modulated Q-switched oscillation to the SRS-induced pulse-train oscillation. The oscillograms demonstrate a situation where the laser radiation started as single-mode and the beginning of the laser pulse was smooth and nearly gaussian. The SRS build-up induced oscillation with different periods in parts A and B (fig. 3 a). The spectra of these parts (fig. 3 b, c) demonstrate a transition from the regime with a maximum frequency equal approximately 2fi (fig. 3 b) to one with a maximum frequency offi (fig. 3 c), wheref, = 119 MHz is the free spectral range of the laser resonator. The last part of the train on fig. 3 corresponds to the mode-locking regime with characteristic regular pulsation. It should
Fig. 3. Example of a laser emission oscillogram showing the transition to the mode-locked regime (a) and corresponding spectrums (b, cl) of the temporal intervals A and B. E= 1.028; T= 56%. Early laser emission is gaussian.
Volume 84, number
1,2
OPTICS
1July 1991
COMMUNICATIONS
be noted that the mode-locking regime excited for limited values of the system parameters. More complex temporal behaviour arose when the laser pulses were modulated from the onset of lasing (figs. 4, 5 ). Fig. 4 shows deep modulation. SRS-development changes the pulsation period and spectrum. The pulse in fig. 5 corresponds to slow modulation. The lasing demonstrates nonregular chaotic temporal behaviour. The spectrum of part A has a large maximum nearfi. The part B is characterized by irregular oscillations. Its spectrum has a low maximum at 3f, and a very low (almost noisy) intensity level. The possible reason for such a temporal behaviour can be the nonlinear interactions in a dynamical system (dynamical chaos), as well as the breaking of the periodicity in the amplitude spectrum due to the intracavity field mode interactions induced by SRS in the resonator (multiperiodical nonregular dynamics with irrational mode frequencies). As fol-
L
0 IOOCI Lau
50 ,
luu
15u 5oo1,a.u.
t,ns I
Fig. 5. Examples of a laser emission oscillogram showing nonregular chaotic behaviour (a) and corresponding spectra (b, c) of the temporal intervals A and B. E= 1.077; T= 56%. Early laser emission is modulated.
!
50
100 lb)
Fig. 4. Example of a laser emission oscillogram showing the transition induced by SRS (a) and corresponding spectra (b, c) of the temporal intervals A and B. E= 1.215; T=45%. Early laser emission is strongly modulated.
lows from the temporal behaviour of the signal, presented in fig. 5a, and its intensity spectrum on fig. 5c it is possible to interpret some features of the observed chaotical behaviour on the basis of the route to chaos characterized by the intermittency. The peculiarity which gives the ground for this point is the transition from the discrete to continuous intensity spectrum in its central part (compare fig. 5b and fig. 5c). But all these nonregular features can also be interpreted as multiperiodical nonregular movement. The basis of this interpretation is the rather often appearance of such nonregular behaviour in the case of initial laser pulse modulation. In this case the intracavity mode interaction increased and therefore the intracavity mode spectrum is strongly changed. Besides that the last reason gives a clear interpretation of the synchronized pulse modulation presented on fig. 4 as a result of the correlations between neighboring modes ( w, and w, +, ) and the modes spaced with the value of 2fi (w, and w,,+~). It should be 49
Volume 84, number 1,2
OPTICS COMMUNICATIONS
noted that the correlations between w, and wnkz are larger than the correction between w, and w,+, modes (fig. 4~). The intensity spectrum (fig. 5c) corresponding to the realization presented on fig. 5a shows the rather large diminishing of the w, - w,~, correlations as well as of the ( w, - w,? 2), ( w, - w, + 3) , ( w, - w, f 4) correlations (compare figs. 5c and 4~). The final assignment of the nonregular behaviour to one of the abovementioned types requires additional experimental studies. It should be noted that SRS has an effect on laser action when a nonlinear regime with rather high conversion efftciency is reached. So large-scale quantum fluctuations [ 7,8] observed mainly in the linear regime did not have a strong effect on the laser action. We believe that the observed instability has mainly a dynamical character as a result of the nonlinear laser-SRS interaction. Such nonregular pulsations and mode-locking we did observe also in an active Q-switched ruby laser in the independent experiment.
50
1July 1991
In conclusion, the observations reported here clearly identify the intracavity Raman laser as a candidate for the manifestation of pulsating instabilities, mode-locking and possible chaotic behaviour. We also note that the nonresonant SRS-process can be used as a mode-locking method in the UV-region.
References [ 1] R. Frey, F. Pradere and A. Martino, Optics Lett. 8 ( 1983) 431. AS. Grabchikov, R.G. Zaporozhchenko, A.V. Kachinskii and V.A. Orlovich, Sov. J. Quant. Electron. 12 (1985) 2360. N.V. Kravtzov and N.I. Naumkin, Sov. J. Quant. Electron. 6 (1979) 375. 1Y.B. Band, J.R. Ackerhalt, J.S. Krasinski and D.F. Heller, IEEE Quant. Electron. QE-25 (1989) 208. R.G. Harrison, I.A. Al-Saidi and D.J. Biswas, IEEE Quant. Electron. QE-21 (1985) 1491. [ 61 A.N. Oraevskii, Sov. J. Trudy Fiz. Akad. Nauk. 17 1 ( 1986) 3. [ 71 M.G. Raymer and I.A. Walmsley, Phhys. Rev. Lett. 50 ( 1983) 962. [S] AS. Grabchikov, S.Ya. Kilin, V.P. Kozich and N.M. Iodo, Sov. J. Pis’ma Zh. Eksp. Teor. Fiz. 43 ( 1986) 118.
I,2
OPTICS
1 July 1991
COMMUNICATIONS
Observation of pulsating instabilities in an intracavity Raman laser V.N. Chizhevsky,
D.E. Gakhovich,
A.S. Grabchikov,
S.Ya. Kilin, V.A. Orlovich
and L.L. Tomilchik* Institute ofPhysics, BSSR Akudemy ofSciences, Leninsky Pr. 70, 220602 Minsk, USSR Received
8 November
1990; revised manuscript
received
18 February
An intracavity Raman laser as a dynamical system with stochastic locking and possible features of a chaotic behaviour of the Q-switched tering are observed.
Stimulated Raman scattering (SRS) in a resonator is an interesting example of a nonlinear system. A number of physical effects have been observed in the intracavity SRS: both in the system containing a Raman and laser medium in the mutual resonator (intracavity Raman laser) and in a system with only a Raman medium in the resonator (Raman laser). The laser pulse compression for nanosecond and subnanosecond pulses in the intracavity Raman laser were investigated in refs. [ I,2 1. Mode-locking and some aspects of the regular behaviour for this system were demonstrated in refs. [ 3,4]. The Raman laser as a dynamical system exhibiting instabilities and chaotic behaviour was explored in ref. [ 5 1. Additionally it is well known that the laser itself can demonstrate dynamical instabilities (see ref. [ 6 ] ). In this work we investigate the intracavity Raman laser as an example of a dynamical system with stochastic behaviour. We report the experimental observatidn of the instabilities, mode-locking and some features of chaotic behaviour induced by intracavity SRS in a Q-switched laser. The experimental setup is shown in fig. 1. A passive Q-switched ruby laser with a homogeneously broadened spectral line of emission was used as the main source of emission. The laser resonator con* Institute of Nuclear Problems, Byelorussian State University 220080 Minsk, Bobruyskaya 11,USSR. OO30-4018/91/$03.50
0 199
I - Elsevier Science Publishers
199 1
behaviour is investigated. Pulsating instabilities, moderuby laser induced by intracavity stimulated Raman scat-
Fig. 1. Scheme of the experimental setup. R,.* - mirrors, passive Q-switch, A - pinhole, L1,2 -lenses.
M -
tained two mirrors with reflections R, =70%, R2= 99% for the laser wavelength (694 nm) and R, =90%, R2=56% for the first Stokes wavelength (975 nm). A single transverse-mode was selected by a pinhole in the resonator. The Raman cell was placed into the resonator between two lenses L, and Lz with equal focal lengths. We used a compressed hydrogen gas (P=40 atm, T,= 180 ps) as the Raman medium. The Raman cell was 20 cm long. A passive absorber had the relaxation time TX,=40 ps. The experiments were performed using a laser cavity length of 120 cm. The spectral selection of Stokes and laser beams was realised by means of glass filters and selective mirrors. Temporal laser and Stokes output characteristics were measured with fast detectors and an oscilloscope with a resolution of about 1 ns. It is known that the single-mode laser with an absorber has a tendency to multimode operation characterized by oscillations in the output. We have investi-
B.V. ( North-Holland
)
47
Volume 84, number
1,2
OPTICS
COMMUNICATIONS
gated the shapes of the laser pulses when the Raman cell was empty and SRS did not develop. It was found that 90% of the laser pulses were single-mode with a gaussian temporal shape (fig. 2 a) and 10% of the pulses had modulated shapes. We believe both regimes (single-mode and multimode) have interesting aspects and can be the subject of investigations of an intracavity Raman laser. The frequency spectrum was obtained by computer processing of the registered oscillograms. In our experiments we varied the pumping level E (normalized pump energy, for the SRS-threshold E= 1) and transmittance T of the passive Q-switch. When the Raman cell was filled with hydrogen the output laser pulses changed drastically (fig. 2b) as a result of the SRS-generation. At the moment t, when the SRS-threshold is reached a sharp transition to different regimes occurs. The laser and Stokes pulses generated under these conditions have shown at least three characteristic intervals: (i) before SRS-generation; (ii) transitional interval when rather long SRS-pulses ap-
Fig. 2. Typical temporal shapes: (a) laser emission with an empty Raman cell; (b) laser and SRS-pulses with the Raman cell filled with hydrogen.
48
1 July 1991
peared and the laser intensity decreased abruptly; (iii) interval of the strong laser-SRS interaction with short pulsations. Typical laser intensity oscillograms and their spectra, calculated using a fast Fourier transformation, are presented in figs. 3, 4. Fig. 3 clearly shows the transition from modulated Q-switched oscillation to the SRS-induced pulse-train oscillation. The oscillograms demonstrate a situation where the laser radiation started as single-mode and the beginning of the laser pulse was smooth and nearly gaussian. The SRS build-up induced oscillation with different periods in parts A and B (fig. 3 a). The spectra of these parts (fig. 3 b, c) demonstrate a transition from the regime with a maximum frequency equal approximately 2fi (fig. 3 b) to one with a maximum frequency offi (fig. 3 c), wheref, = 119 MHz is the free spectral range of the laser resonator. The last part of the train on fig. 3 corresponds to the mode-locking regime with characteristic regular pulsation. It should
Fig. 3. Example of a laser emission oscillogram showing the transition to the mode-locked regime (a) and corresponding spectrums (b, cl) of the temporal intervals A and B. E= 1.028; T= 56%. Early laser emission is gaussian.
Volume 84, number
1,2
OPTICS
1July 1991
COMMUNICATIONS
be noted that the mode-locking regime excited for limited values of the system parameters. More complex temporal behaviour arose when the laser pulses were modulated from the onset of lasing (figs. 4, 5 ). Fig. 4 shows deep modulation. SRS-development changes the pulsation period and spectrum. The pulse in fig. 5 corresponds to slow modulation. The lasing demonstrates nonregular chaotic temporal behaviour. The spectrum of part A has a large maximum nearfi. The part B is characterized by irregular oscillations. Its spectrum has a low maximum at 3f, and a very low (almost noisy) intensity level. The possible reason for such a temporal behaviour can be the nonlinear interactions in a dynamical system (dynamical chaos), as well as the breaking of the periodicity in the amplitude spectrum due to the intracavity field mode interactions induced by SRS in the resonator (multiperiodical nonregular dynamics with irrational mode frequencies). As fol-
L
0 IOOCI Lau
50 ,
luu
15u 5oo1,a.u.
t,ns I
Fig. 5. Examples of a laser emission oscillogram showing nonregular chaotic behaviour (a) and corresponding spectra (b, c) of the temporal intervals A and B. E= 1.077; T= 56%. Early laser emission is modulated.
!
50
100 lb)
Fig. 4. Example of a laser emission oscillogram showing the transition induced by SRS (a) and corresponding spectra (b, c) of the temporal intervals A and B. E= 1.215; T=45%. Early laser emission is strongly modulated.
lows from the temporal behaviour of the signal, presented in fig. 5a, and its intensity spectrum on fig. 5c it is possible to interpret some features of the observed chaotical behaviour on the basis of the route to chaos characterized by the intermittency. The peculiarity which gives the ground for this point is the transition from the discrete to continuous intensity spectrum in its central part (compare fig. 5b and fig. 5c). But all these nonregular features can also be interpreted as multiperiodical nonregular movement. The basis of this interpretation is the rather often appearance of such nonregular behaviour in the case of initial laser pulse modulation. In this case the intracavity mode interaction increased and therefore the intracavity mode spectrum is strongly changed. Besides that the last reason gives a clear interpretation of the synchronized pulse modulation presented on fig. 4 as a result of the correlations between neighboring modes ( w, and w, +, ) and the modes spaced with the value of 2fi (w, and w,,+~). It should be 49
Volume 84, number 1,2
OPTICS COMMUNICATIONS
noted that the correlations between w, and wnkz are larger than the correction between w, and w,+, modes (fig. 4~). The intensity spectrum (fig. 5c) corresponding to the realization presented on fig. 5a shows the rather large diminishing of the w, - w,~, correlations as well as of the ( w, - w,? 2), ( w, - w, + 3) , ( w, - w, f 4) correlations (compare figs. 5c and 4~). The final assignment of the nonregular behaviour to one of the abovementioned types requires additional experimental studies. It should be noted that SRS has an effect on laser action when a nonlinear regime with rather high conversion efftciency is reached. So large-scale quantum fluctuations [ 7,8] observed mainly in the linear regime did not have a strong effect on the laser action. We believe that the observed instability has mainly a dynamical character as a result of the nonlinear laser-SRS interaction. Such nonregular pulsations and mode-locking we did observe also in an active Q-switched ruby laser in the independent experiment.
50
1July 1991
In conclusion, the observations reported here clearly identify the intracavity Raman laser as a candidate for the manifestation of pulsating instabilities, mode-locking and possible chaotic behaviour. We also note that the nonresonant SRS-process can be used as a mode-locking method in the UV-region.
References [ 1] R. Frey, F. Pradere and A. Martino, Optics Lett. 8 ( 1983) 431. AS. Grabchikov, R.G. Zaporozhchenko, A.V. Kachinskii and V.A. Orlovich, Sov. J. Quant. Electron. 12 (1985) 2360. N.V. Kravtzov and N.I. Naumkin, Sov. J. Quant. Electron. 6 (1979) 375. 1Y.B. Band, J.R. Ackerhalt, J.S. Krasinski and D.F. Heller, IEEE Quant. Electron. QE-25 (1989) 208. R.G. Harrison, I.A. Al-Saidi and D.J. Biswas, IEEE Quant. Electron. QE-21 (1985) 1491. [ 61 A.N. Oraevskii, Sov. J. Trudy Fiz. Akad. Nauk. 17 1 ( 1986) 3. [ 71 M.G. Raymer and I.A. Walmsley, Phhys. Rev. Lett. 50 ( 1983) 962. [S] AS. Grabchikov, S.Ya. Kilin, V.P. Kozich and N.M. Iodo, Sov. J. Pis’ma Zh. Eksp. Teor. Fiz. 43 ( 1986) 118.