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Mode-locked Nd:YAG laser with intracavity phase conjugate mirror
Volume 61, number 1
OPTICS COMMUNICATIONS
I Januart 1987
MODE-LOCKED Nd:YAG LASER WITH INTRACAVITY PHASE CONJUGATE MIRROR M. SZCZUREK Kaliski Institute o/'Plasma Physics and Laser Micr~?lusion. P O.
Bo.v49. 00-~08 I~ arsaw, Po/ami
Received 21 July 1986: revised manuscript received 9 October 1986
Phase conjugation of 30-ps Nd:YAG laser pulses by intracavity degenerate four-wave mixing in solutions of the transition metal complex bis-(4-dimethylaminodithiobenzil)-nickel (BDN) in 1,2-dichloroethane and 0.001 M Rh 6G in ethanol has been demonstrated.
The phenomenon of conjugate wave generation has been investigated very intensively for many years [ 1-3 ]. One o f the methods used for generation of a conjugate wave is degenerate four-wave mixing ( D F W M ) proposed by Hellwarth [4]. The geometry of this interaction is as follows: two counterpropagating pump waves and the probe wave with the same frequency interact in a nonlinear medium to produce a backward conjugate wave at the same frequency. In comparison to other methods used for generating the conjugate wave, D F W M has the following advantages: this process can apply regardless of the input angle o f the probe wave and turns out to be free of phase matching constraints by virtue of the interaction geometry. It is possible to generate a conjugate wave of the amplitude higher than the amplitude of the probe wave and the conjugate wave has the same frequency as the input waves [ 5 ]. The use o f pulse radiation in D F W M enables investigation of temporal characteristics of a nonlinear medium and shaping of the laser pulse parameters [ 1,3 ]. Particularly interesting are the experiments in which picosecond pulses are used for investigating D F W M [ 6-9 ]. Smirl and Boggess [ 6 ] measured the temporal evolution o f carrier density in germanium samples in the D F W M geometry using picosecond pulses from a Nd:YAG laser. Tocho el al. [7] investigated the process of conjugate wave generation in organic dye saturable absorbers using a picosecond pulse train from a passively mode-locked dye laser. 42
Venherzeele et al. [ 8 ] demonstrated a simultaneous Q-switching, mode-locking, and self-pumped phaseconjugation in a Nd:YAG laser cavity using the D F W M geometry. Wu et al. [9] investigated the temporal evolution of a conjugate wave generated by D F W M in selected liquids using picosecond pulses from a frequency doubled Nd:YAG laser. Natural conditions for generation of a conjugate wave in the D F W M process occur within a laser cavity because the intracavity laser fields form the counterpropating pump waves required for the realization of a phase conjugate mirror [ 10,12]. This paper presents preliminary results of an investigation of the process of conjugate wave generation in a passively mode-locked Nd:YAG laser with the intracavity phase conjugate mirror. The experimental arrangement used to generate and study the picosecond backward-travelling conjugate waves is shown schematically in fig. 1. The solid beam path in this figure represents a conventional Nd:YAG laser cavity, Q-switched and mode-locked by the Eastman Kodak A 9740 dye, producing a train of 30 ps ( fwhm ) pulses spaced by 13 ns. The time duration of these pulses was measured by the two-photon fluorescence method [13,14]. The Pockels cell (PC), dielectric polarizer ( D P ) and nonlinear medium ( N M ) were additionally placed within a conventional laser cavity. Controlling the polarization state of the laser radiation within a conventional laser cavity b,x applying voltage to the Pockels cell we can couple an 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume 61, number 1
OPTICS COMMUNICATIONS NM
M1
DP ~ J
~¢°~-
D~D , .J"
M~"
--'-- - ) ~ 3''; ~j. . . . z'"
M2
t
z a
Fig. 1. Schema of the experimental configuration of the modelocked Nd:YAGlaserwith intracavity phase conjugatemirror for the generation and study of a picosecond backward-travelling conjugate wave. adjustable amount of radiation out of this cavity producing a probe beam which is directed by the M3 mirror into the nonlinear medium at the angle 0 ~ 5 °. Temporal overlap of probe ( 1 ) and pump (2) pulses inside the nonlinear medium was done by adjusting the position of the M1 mirror. The conjugate pulse generated in the DFWM process in the nonlinear medium is then reinjected into the conventional laser cavity and it becomes a pump pulse, while the part of the pump pulse which was not transformed into the conjugate wave becomes a probe pulse. Thus there appears a phase conjugate cavity (dotted beam path), closed by the M2 mirror and phase conjugate mirror, connected with a conventional one. Round trips of these cavities are precisely equalized (i.e. distances N M - D P - M 3 - N M and N M - D P - M I - D P - N M are equal) in order to ensure good temporal overlap of the probe and pump pulses. Probe and conjugate pulses were observed by using beam splitter BS, photodiodes D 1 and D2 and a dual beam fast oscilloscope. Intensity distributions of the probe and conjugate waves were observed by using infrared to visible converters. In this way it was confirmed that phase conjugation takes place. By Pockels cell controlling it is possible to regulate the coupling degree of the conventional and phase conjugate cavities. If after generation of a probe pulse, half-wave voltage were applied to a Pockels cell, then the conventional cavity would be optically blocked and oscillations appear only in a conjugate resonator. In the described experimental setup the coupling between the two cavities was held constant by a static setting of the Pockels cell. A laser providing only phase conjugate oscillations by optical blocking of the conventional cavity is actually realized.
1 Januari 1987
Note that the constructed system of a Nd:YAG laser with intracavity phase conjugate mirror differs essentially from the system described in ref. [8] because it allows use of various media as a phase conjugate mirror and it enables investigation of temporal parameters of a conjugate wave by controlling the position of the M2 mirror. Moreover, the conjugate cavity system presented above can be used in an actively mode-locked Nd:YAG laser. A solution of transition metal complex bis(dimethyl-amino-dithiobenzil)-nickel (BDN) in 1,2dichloroethane was used as a nonlinear medium in this investigation. BDN is a multilevel saturable absorber used for Q-switching of Nd:YAG lasers [ 15,16 ]. Moses and Wu [ 17 ] demonstrated generation of an amplified conjugate wave by DFWM in BDN for Q-switched Nd:YAG laser pulses. In the system shown in the fig. 1 the BDN solution was placed in a 2 m m thick glass cell. The concentration of the solution was selected in such a way as to not disturb the process of picosecond pulse generation in the conventional cavity. The pump pulse (3) reached the nonlinear medium after time td ~ 1 ns since the moment of a probe (1) and pump (2) pulses overlap. This time results from a distance N M - M 2 (td = 2 ( N M - M 2 ) / c where c is the speed of light), and can be easily changed by adjusting the position of the M2 mirror. If the probe beam and the pump beam are polarized parallel to each other (P~,) then there appears a periodical optical structure which, as a result of resonant absorption in BDN, creates a population inversion grating. The life time of this grating is equal to the relaxation time of BDN in 1,2-dichloroethane tr=2.2 ns [ 18]. The backward conjugate wave (4) is produced by Bragg diffraction of pump wave (3) from this grating. Then, the population inversion grating, as a result of radiationless transitions, is transformed into thermal grating. If the probe beam and pump beam have perpendicular polarizations (P±) the interference is cancelled and three waves interact through the third-order nonlinear susceptibility of the medium to produce the backward conjugate wave. Controlling the state of a probe beam polarization and of the delay time td enables one to separate phenomena responsible for generation of a backward conjugate wave and to determine the response and decay times of these phenomena [ 9,19 ]. As a result of our investigation in this laser system, 43
Volume 61, number I
OPTICS COMMUNICATIONS
I ,lanuari 198 v
s o l u t i o n s o f B D N in 1 , 2 - d i c h l o r o e t h a n e a n d R h G G in e t h a n o l . T h e a u t h o r w i s h e s to t h a n k Prof. A. K u j a w s k i for his i n t e r e s t in t h i s w o r k a n d t h e referee for s o m e sugg e s t i o n s a n d Dr. J. Powell for a c r i t i c a l r e a d i n g o f t h e manuscript.
References
Fig. 2. The oscilloscope traces of the conjugate pulse train (a) and of the probe pulse train ( b ). The distance between pulses is 13 ns: P .
a b a c k w a r d c o n j u g a t e w a v e , in t h e f o r m o f a p i c o s e c o n d pulse t r a i n , w a s o b s e r v e d . Fig. 2 s h o w s t h e oscilloscope traces o f p r o b e pulses a n d o f c o n j u g a t e pulses. T h e s e t r a c e s c o r r e s p o n d to a s t a t i c s e t t i n g o f t h e P o c k e l s cell t h a t c o u p l e s ~ 4 0 % o f t h e r a d i a t i o n f r o m t h e c o n v e n t i o n a l l a s e r c a v i t y i n t o t h e p r o b e b e a m . In all m e a s u r e m e n t s l o w e r i n t e n s i t y o f t h e b a c k w a r d c o n j u g a t e w a v e was o b s e r v e d for P than for P. A d d i t i o n a l l y , s o l u t i o n o f 0.001 M R h 6 G in e t h a n o l was u s e d as a n o n l i n e a r m e d i u m . G e n e r a t i o n o f a low i n t e n s i t y b a c k w a r d c o n j u g a t e w a v e ( b e l o w t h e sensitivity threshold of the used oscilloscope) was observed. Quantitative measurements of temporal a n d spatial p a r a m e t e r s o f a b a c k w a r d c o n j u g a t e w a v e g e n e r a t e d in t h e d e s c r i b e d l a s e r s y s t e m will b e t h e subject of further investigations. In c o n c l u s i o n , a m o d e - l o c k e d N d : Y A G l a s e r w i t h an intracavity phase conjugate mirror has been cons t r u c t e d . W e h a v e d e m o n s t r a t e d t h a t it is p o s s i b l e to generate a backward conjugate wave by DFWM for p i c o s e c o n d p u l s e s at a w a v e l e n g t h o f 1.06 /,zm in
44
[1] R.A. Fisher, Optical phase conjugation (Academic Press. New York. 1983) [2] D.M. Pepper, Nonlinear optical phase conjugation, in: Laser handbook, Vol. 4, eds. M.L. Stitch and M. Bass (Elsevier Science Publishers, 1985 I. [3] B.Ya. Zel'dovich, N.F. Pilipetskii and V.V. Shkunov, Principles of phase conjugation ( Springer Verlag, 1985 ~. [4] R.W. Hellwarth, J. Opt. Soc. Am. 67 (1977) I. [5] D.M. Pepper, Opt. Eng. 21 (19821 155. [6] A i . Smirl, T.F. Boggess and F.A. Hopf, Optics Comm 34 (1980) 463. [ 7 ] J.O, Yocho, W. Sibbett and D.J. Bradley, Optics Comm, 34 (1980) 122. [ 8 ] H. Vcnherzeele, J.L. Van Eck and 4.E. Siegman, {)pries LeII. 6(1981)467. [9] C.K. Wu, P. Agostini, G. Petite and F. Fabre, Optics Letl. ,"; (1983) 67. [10] E.E. Bergmann, l.J. Bigio. G.J. Feldman and R.A. Fisher Optics Lett. 3 (1978) 82. [ 11 ] R.A. Fisher and B.J. Fetdman, Optics Lett. 4 ( 1979 ) 140. [ 12] A.E. Siegman. P.A. Belanger and A. Hardy, in: optical phase conjugation, ed. R.A. Fisher (Academic Press, New York, (1983) p. 465. [131J.A. Giordmaine, P.M. Rentzepis, S.L. Shapiro and K.W. Wecht, Appl. Phys. Len. 11 (1967) 216. [ 14] D.J. Bradley and G.H.C. New, Proc. I EEE 62 (1974l 313. [ 15] K.H. Drexhage and U.T. Muller-Westerhoff, IEEE J. Quantum Electron. QE-8 (1972) 759. [ 16 ] B. Fan and T.K. Guslafson, Optics Comm. 15 ( 1975 ) 32. [ 17 ] E,1. Moses and F.Y. Wu, Optics Lett, 5 (1980) 64. [ 18] H. AI-Obaidi, R.J. Dcwhursl, D. Jacoby, G.A. Oldershaw and S.A. Ramsden, Optics Comm. 14 ( 1975 ) 219. [19] K.A. Nelson, R.J.D. Miller, D.R. kutz and M.D. Fayer, J. Appl. Phys. 53 (1982) 1144.
OPTICS COMMUNICATIONS
I Januart 1987
MODE-LOCKED Nd:YAG LASER WITH INTRACAVITY PHASE CONJUGATE MIRROR M. SZCZUREK Kaliski Institute o/'Plasma Physics and Laser Micr~?lusion. P O.
Bo.v49. 00-~08 I~ arsaw, Po/ami
Received 21 July 1986: revised manuscript received 9 October 1986
Phase conjugation of 30-ps Nd:YAG laser pulses by intracavity degenerate four-wave mixing in solutions of the transition metal complex bis-(4-dimethylaminodithiobenzil)-nickel (BDN) in 1,2-dichloroethane and 0.001 M Rh 6G in ethanol has been demonstrated.
The phenomenon of conjugate wave generation has been investigated very intensively for many years [ 1-3 ]. One o f the methods used for generation of a conjugate wave is degenerate four-wave mixing ( D F W M ) proposed by Hellwarth [4]. The geometry of this interaction is as follows: two counterpropagating pump waves and the probe wave with the same frequency interact in a nonlinear medium to produce a backward conjugate wave at the same frequency. In comparison to other methods used for generating the conjugate wave, D F W M has the following advantages: this process can apply regardless of the input angle o f the probe wave and turns out to be free of phase matching constraints by virtue of the interaction geometry. It is possible to generate a conjugate wave of the amplitude higher than the amplitude of the probe wave and the conjugate wave has the same frequency as the input waves [ 5 ]. The use o f pulse radiation in D F W M enables investigation of temporal characteristics of a nonlinear medium and shaping of the laser pulse parameters [ 1,3 ]. Particularly interesting are the experiments in which picosecond pulses are used for investigating D F W M [ 6-9 ]. Smirl and Boggess [ 6 ] measured the temporal evolution o f carrier density in germanium samples in the D F W M geometry using picosecond pulses from a Nd:YAG laser. Tocho el al. [7] investigated the process of conjugate wave generation in organic dye saturable absorbers using a picosecond pulse train from a passively mode-locked dye laser. 42
Venherzeele et al. [ 8 ] demonstrated a simultaneous Q-switching, mode-locking, and self-pumped phaseconjugation in a Nd:YAG laser cavity using the D F W M geometry. Wu et al. [9] investigated the temporal evolution of a conjugate wave generated by D F W M in selected liquids using picosecond pulses from a frequency doubled Nd:YAG laser. Natural conditions for generation of a conjugate wave in the D F W M process occur within a laser cavity because the intracavity laser fields form the counterpropating pump waves required for the realization of a phase conjugate mirror [ 10,12]. This paper presents preliminary results of an investigation of the process of conjugate wave generation in a passively mode-locked Nd:YAG laser with the intracavity phase conjugate mirror. The experimental arrangement used to generate and study the picosecond backward-travelling conjugate waves is shown schematically in fig. 1. The solid beam path in this figure represents a conventional Nd:YAG laser cavity, Q-switched and mode-locked by the Eastman Kodak A 9740 dye, producing a train of 30 ps ( fwhm ) pulses spaced by 13 ns. The time duration of these pulses was measured by the two-photon fluorescence method [13,14]. The Pockels cell (PC), dielectric polarizer ( D P ) and nonlinear medium ( N M ) were additionally placed within a conventional laser cavity. Controlling the polarization state of the laser radiation within a conventional laser cavity b,x applying voltage to the Pockels cell we can couple an 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume 61, number 1
OPTICS COMMUNICATIONS NM
M1
DP ~ J
~¢°~-
D~D , .J"
M~"
--'-- - ) ~ 3''; ~j. . . . z'"
M2
t
z a
Fig. 1. Schema of the experimental configuration of the modelocked Nd:YAGlaserwith intracavity phase conjugatemirror for the generation and study of a picosecond backward-travelling conjugate wave. adjustable amount of radiation out of this cavity producing a probe beam which is directed by the M3 mirror into the nonlinear medium at the angle 0 ~ 5 °. Temporal overlap of probe ( 1 ) and pump (2) pulses inside the nonlinear medium was done by adjusting the position of the M1 mirror. The conjugate pulse generated in the DFWM process in the nonlinear medium is then reinjected into the conventional laser cavity and it becomes a pump pulse, while the part of the pump pulse which was not transformed into the conjugate wave becomes a probe pulse. Thus there appears a phase conjugate cavity (dotted beam path), closed by the M2 mirror and phase conjugate mirror, connected with a conventional one. Round trips of these cavities are precisely equalized (i.e. distances N M - D P - M 3 - N M and N M - D P - M I - D P - N M are equal) in order to ensure good temporal overlap of the probe and pump pulses. Probe and conjugate pulses were observed by using beam splitter BS, photodiodes D 1 and D2 and a dual beam fast oscilloscope. Intensity distributions of the probe and conjugate waves were observed by using infrared to visible converters. In this way it was confirmed that phase conjugation takes place. By Pockels cell controlling it is possible to regulate the coupling degree of the conventional and phase conjugate cavities. If after generation of a probe pulse, half-wave voltage were applied to a Pockels cell, then the conventional cavity would be optically blocked and oscillations appear only in a conjugate resonator. In the described experimental setup the coupling between the two cavities was held constant by a static setting of the Pockels cell. A laser providing only phase conjugate oscillations by optical blocking of the conventional cavity is actually realized.
1 Januari 1987
Note that the constructed system of a Nd:YAG laser with intracavity phase conjugate mirror differs essentially from the system described in ref. [8] because it allows use of various media as a phase conjugate mirror and it enables investigation of temporal parameters of a conjugate wave by controlling the position of the M2 mirror. Moreover, the conjugate cavity system presented above can be used in an actively mode-locked Nd:YAG laser. A solution of transition metal complex bis(dimethyl-amino-dithiobenzil)-nickel (BDN) in 1,2dichloroethane was used as a nonlinear medium in this investigation. BDN is a multilevel saturable absorber used for Q-switching of Nd:YAG lasers [ 15,16 ]. Moses and Wu [ 17 ] demonstrated generation of an amplified conjugate wave by DFWM in BDN for Q-switched Nd:YAG laser pulses. In the system shown in the fig. 1 the BDN solution was placed in a 2 m m thick glass cell. The concentration of the solution was selected in such a way as to not disturb the process of picosecond pulse generation in the conventional cavity. The pump pulse (3) reached the nonlinear medium after time td ~ 1 ns since the moment of a probe (1) and pump (2) pulses overlap. This time results from a distance N M - M 2 (td = 2 ( N M - M 2 ) / c where c is the speed of light), and can be easily changed by adjusting the position of the M2 mirror. If the probe beam and the pump beam are polarized parallel to each other (P~,) then there appears a periodical optical structure which, as a result of resonant absorption in BDN, creates a population inversion grating. The life time of this grating is equal to the relaxation time of BDN in 1,2-dichloroethane tr=2.2 ns [ 18]. The backward conjugate wave (4) is produced by Bragg diffraction of pump wave (3) from this grating. Then, the population inversion grating, as a result of radiationless transitions, is transformed into thermal grating. If the probe beam and pump beam have perpendicular polarizations (P±) the interference is cancelled and three waves interact through the third-order nonlinear susceptibility of the medium to produce the backward conjugate wave. Controlling the state of a probe beam polarization and of the delay time td enables one to separate phenomena responsible for generation of a backward conjugate wave and to determine the response and decay times of these phenomena [ 9,19 ]. As a result of our investigation in this laser system, 43
Volume 61, number I
OPTICS COMMUNICATIONS
I ,lanuari 198 v
s o l u t i o n s o f B D N in 1 , 2 - d i c h l o r o e t h a n e a n d R h G G in e t h a n o l . T h e a u t h o r w i s h e s to t h a n k Prof. A. K u j a w s k i for his i n t e r e s t in t h i s w o r k a n d t h e referee for s o m e sugg e s t i o n s a n d Dr. J. Powell for a c r i t i c a l r e a d i n g o f t h e manuscript.
References
Fig. 2. The oscilloscope traces of the conjugate pulse train (a) and of the probe pulse train ( b ). The distance between pulses is 13 ns: P .
a b a c k w a r d c o n j u g a t e w a v e , in t h e f o r m o f a p i c o s e c o n d pulse t r a i n , w a s o b s e r v e d . Fig. 2 s h o w s t h e oscilloscope traces o f p r o b e pulses a n d o f c o n j u g a t e pulses. T h e s e t r a c e s c o r r e s p o n d to a s t a t i c s e t t i n g o f t h e P o c k e l s cell t h a t c o u p l e s ~ 4 0 % o f t h e r a d i a t i o n f r o m t h e c o n v e n t i o n a l l a s e r c a v i t y i n t o t h e p r o b e b e a m . In all m e a s u r e m e n t s l o w e r i n t e n s i t y o f t h e b a c k w a r d c o n j u g a t e w a v e was o b s e r v e d for P than for P. A d d i t i o n a l l y , s o l u t i o n o f 0.001 M R h 6 G in e t h a n o l was u s e d as a n o n l i n e a r m e d i u m . G e n e r a t i o n o f a low i n t e n s i t y b a c k w a r d c o n j u g a t e w a v e ( b e l o w t h e sensitivity threshold of the used oscilloscope) was observed. Quantitative measurements of temporal a n d spatial p a r a m e t e r s o f a b a c k w a r d c o n j u g a t e w a v e g e n e r a t e d in t h e d e s c r i b e d l a s e r s y s t e m will b e t h e subject of further investigations. In c o n c l u s i o n , a m o d e - l o c k e d N d : Y A G l a s e r w i t h an intracavity phase conjugate mirror has been cons t r u c t e d . W e h a v e d e m o n s t r a t e d t h a t it is p o s s i b l e to generate a backward conjugate wave by DFWM for p i c o s e c o n d p u l s e s at a w a v e l e n g t h o f 1.06 /,zm in
44
[1] R.A. Fisher, Optical phase conjugation (Academic Press. New York. 1983) [2] D.M. Pepper, Nonlinear optical phase conjugation, in: Laser handbook, Vol. 4, eds. M.L. Stitch and M. Bass (Elsevier Science Publishers, 1985 I. [3] B.Ya. Zel'dovich, N.F. Pilipetskii and V.V. Shkunov, Principles of phase conjugation ( Springer Verlag, 1985 ~. [4] R.W. Hellwarth, J. Opt. Soc. Am. 67 (1977) I. [5] D.M. Pepper, Opt. Eng. 21 (19821 155. [6] A i . Smirl, T.F. Boggess and F.A. Hopf, Optics Comm 34 (1980) 463. [ 7 ] J.O, Yocho, W. Sibbett and D.J. Bradley, Optics Comm, 34 (1980) 122. [ 8 ] H. Vcnherzeele, J.L. Van Eck and 4.E. Siegman, {)pries LeII. 6(1981)467. [9] C.K. Wu, P. Agostini, G. Petite and F. Fabre, Optics Letl. ,"; (1983) 67. [10] E.E. Bergmann, l.J. Bigio. G.J. Feldman and R.A. Fisher Optics Lett. 3 (1978) 82. [ 11 ] R.A. Fisher and B.J. Fetdman, Optics Lett. 4 ( 1979 ) 140. [ 12] A.E. Siegman. P.A. Belanger and A. Hardy, in: optical phase conjugation, ed. R.A. Fisher (Academic Press, New York, (1983) p. 465. [131J.A. Giordmaine, P.M. Rentzepis, S.L. Shapiro and K.W. Wecht, Appl. Phys. Len. 11 (1967) 216. [ 14] D.J. Bradley and G.H.C. New, Proc. I EEE 62 (1974l 313. [ 15] K.H. Drexhage and U.T. Muller-Westerhoff, IEEE J. Quantum Electron. QE-8 (1972) 759. [ 16 ] B. Fan and T.K. Guslafson, Optics Comm. 15 ( 1975 ) 32. [ 17 ] E,1. Moses and F.Y. Wu, Optics Lett, 5 (1980) 64. [ 18] H. AI-Obaidi, R.J. Dcwhursl, D. Jacoby, G.A. Oldershaw and S.A. Ramsden, Optics Comm. 14 ( 1975 ) 219. [19] K.A. Nelson, R.J.D. Miller, D.R. kutz and M.D. Fayer, J. Appl. Phys. 53 (1982) 1144.