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Light-induced grating formation in dye solutions by two independent laser beams
Volume 30, number 2
OPTICS COMMUNICATIONS
August 1979
LIGHT-INDUCED GRATING FORMATION IN DYE SOLUTIONS
BY TWO INDEPENDENT LASER BEAMS P.A. APANASEVICH and V.A. ZAPOROZHCHENKO Institute of Physics, BSSR Academy of Sciences, Minsk 220602, USSR
Received 15 February 1979 Revised manuscript received 23 April 1979
The distributed feedback laser effect was used for the detection of light-induced gratings created in organic dye solutions due to the interference of actively mode-locked ruby laser radiation and that of electrooptically Q-switched ruby laser. Radiation kinetics in different regions of the dye superfluorescence spectrum was studied. The distributed feedback effect was shown to be caused by the amplification coefficient grating in the conditions under discussion.
The direct observation of the interference of two independent laser pulses is usually very difficult because o f the short time of their mutual coherence. But the interference pattern indication is available in nonlinear media due to the light-induced grating formation. In dye solutions one can use for this aim the distributed feedback (DFB) laser effect provided by the gain coefficient grating or by that of the refractive index [1 ]. The picosecond pulse generation in DFB systems [2,3] is an evidence of the short lasing development time and thus the DFB laser effect can be used for the indication of short-lived interference patterns if their pe-
riods coincide with the luminescence wavelengths of the dye solution. In this communication we report the experimental results concerning the indication of light-induced gratings created in organic dye solutions due to the interference of the actively mode-locked ruby laser radiation and that o f an electroopticatly Q-switched ruby laser. Fig. 1 shows the experimental setup used for the grating creation and its detection by the DFB lasing. The ultrashort pulse train from the actively modelocked laser described in [4] and the giant pulse from an electrooptically Q-switched laser were directed into
~FABRY PEROT
-~0
@'
MODE-LOCKED PULSETRAIN Q-SWITCHEDPULSE
I F- 71PHOTOMU'T P' ER I SPECTROG APH I}I I Fig. 1. Experimental setup, 231
Volume 30, number 2
OPTICS COMMUNICATIONS
the prism dye cell, where they interfered in the dye solution which was in contact with the hypotenuse side of the prism. The cell windows were inclined relatively to the plane of the prism side to prevent light generation due to Fresnel reflection. Time coincidence of the mode-locked pulse train and the Q-switched pulse was provided by a special electronic device which permitted their mutual temporal shift. Their mutual position in time was controlled by means of two optical signals detection by the first coaxial vacuum photocell (see fig. 1). Spectra of ruby lasers were measured with a l-ram Fabry-Perot etalon. The dye radiation spectrum was recorded with the diffractiongrating spectrograph. The time behaviour of the total dye output was recorded by the second photocell. The investigation of time characteristics in different spectral ranges of dye radiation was also carried out. In this case the 1 mm slit was placed in the image plane of the spectrograph, used as a monochromator with spectral resolution 3 A. The optical signal after passing through the monochromator was detected by a high-speed photomultiplier with rise time 2 ns. The photocells and the multiplier output signals were recorded by the two-channel oscillograph. The mode-locked laser pulse width was measured by two-photon fluorescence technique. The peak intensity of ultrashort pulses reached 4 - 5 GW/cm 2 with pulse duration ~20 ps. The peak intensity of the Q-switched laser was ~ 2 0 MW/cm 2. The spectral width of mode-locked radiation was ~0.9 A, i.e. ultrashort pulses were near time-bandwidth limited. The bandwidth of the Q-switched laser radiation was ~0.5 A and the corresponding time interval of its coherence was longer than the ultrashort pulse duration. The middle wavelengths of two lasers differed by 0.23 .~. Two organic dye solutions were used for the observation of the DFB laser effect: cryptocyanine in ethanol (Solution 1) and 3,3-diethyl-6,7,6,7-di (2phenylthiasolo-4,5) thiacarbocyanine chloride in ethanol (Solution 2). Dye concentrations, population decay times and quantum yields were equal to 7 × 1017cm -3, 1.5 × 1 0 - 1 I s , 0.02 and 1.5 X 1017cm -3, 2.5 × 10 -9 s, 0.32 for solution 1 and solution 2, respectively. Fig. 2 shows the total time behaviour and spectrum of the dye radiation when different cases of pumping pulses synchronisation are realised. A substantial narrowing of the dye superfluorescence spectrum was ob232
August 1979
/I
11 t
b
£
Fig. 2. Oscillograms(a, b, c) and spectra (a', b', c' ) of superfluorescence of solution 1. served when the mode-locked pulse train was coincident with the Q-switched pulse (fig. 2b'). In other cases we could see a broad superfluorescence spectrum either without any structure (fig. 2a'), or with a weak line due to the interference between the residual pulses from the train and the leading edge of Q-switched pulse (fig. 2c' ). The comparison of figs. 2a and 2c shows that the superfluorescence intensity is lower when the ultrashort pulse train is incident on the dye solution later than the Q-switched pulse. This effect as well as the shorter length of the superfluorescence train in comparison with the pumping one can be attributed to an essential role of singlet-triplet conversion and to some photochemical processes in polymethine dye solutions [5]. The dye superfluorescence spectrum narrowing proves the light-induced grating presence which pro-
Volume 30, number 2
OPTICS COMMUNICATIONS
vides the DFB effect in dye solution. The superfluorescence wavelength corresponds to the interference pattern period and is easily tunable by variation of the angle between interfering beams (compare fig. 2b' and fig. 3e, where the DFB superfluorescence of solution 1 is obtained in different spectral ranges). As far as the wavelengths of pumping beams differ in 0.23 A the grating is moving. The distance travelled by it in 20 ps (the duration of the pulses) is small compared to the grating period (~0.29) and this movement does not lead to a substantial degradation of the DFB superfluorescence. The interaction of the interference field with the medium causes the gain coefficient grating as well as that of the refractive index. There are three mechanisms of the refractive index change by the radiation: optical Kerr-effect, electro-striction and thermal effect. It is known [6] that under the conditions of Q-switched pulse pumping the latter leads to the refractive index variation not only due to its direct dependence on the medium temperature but mainly because of the density change with temperature. It provides an accumulative growth of the grating amplitude due to a large time of heat diffusion along the grating (~10 -7 s). The contributions of electrostrictive and Kerr-effect index variations are small because of the insufficient radiation intensity of Q-switched pulses.
August 1979
When the grating is formed under the influence of the mode-locked pulse train [2] the thermal effect contribution decreases as a rule and that of the Kerreffect increases as a result of higher radiation intensity. The electrostrictive effect does not manifest in this case as the medium density is unchanged within the ultrashort pulse duration. Under our experimental conditions we could neglect both the electrostriction and the medium density variation under the heating as the short mutual coherence time of two lasers used made the process of the grating accumulation impossible. Every subsequent pulse could not increase the grating produced by the previous one. Therefore, the role of the thermal refractive index variation in our case could not be expected significant as its partial derivative with respect to temperature is very small. It could be proved that the refractive index grating caused by the optical Kerr-effect was also insignificant in our experiments. As is well-known [6], the presence of several gratings in the medium leads to the generation of several spectral components. In some of our experiments we observed also the two wavelength generation due to the presence of an additional grating in the medium which was caused by the fourfold Fresnel reflection of the mode-locked laser radiation inside the prism as shown in the right corner of fig. 1. The rereflected
Fig. 3. OsciUogramsof dye output in different spectral ranges (a,b, c) and spectra of solution 2 - (d) and solution 1 - (e). Pulses in train are separated by interval 11.1 ns. 233
Volume 30, number 2
OPTICS COMMUNICATIONS
radiation was incident on the dye solution at the angle 0' differing from the initial angle 0 by 0' - 0 = 261 + 46 2. The measurement of prism angles and the calculations of grating steps showed that the additional grating caused the left lines in fig. 3d, e. The ratio of the rereflected beam intensity to that of the initial one was 1.4 × 10 -4. The oscillograms of dye output in different spectral ranges (fig. 3a, b, c) show that DFB light generation provided by the initial beam-produced grating is observed when the first low intense ultrashort pulses from the train interfere with the leading edge of the Q-switched pulse (see fig. 3b). With an increase of ultrashort pulse intensity lasing conditions are violated for the main grating while they are attained for the additional one and rather a long pulse train is generated at the additional grating wavelength (see fig. 3a). This result proves the lack of the optical Kerr-effect contribution to the DFB formation process under our experimental conditions. An opposite statement would be in contradiction to the fact of the absence of lasing on the main grating while it does occur on the additional one, which is formed by a much lower field amplitude. The explanation of the dye output time behaviour is possible if the DFB is determined by the gain coefficient grating. The large difference of the interfering beam intensities in the experiments under discussion resulted in the low interference field modulation depth ~0.13 while dye solutions were under the saturation action of high intense picosecond pulses from the initial beam which could bring almost all particles to the excited state in a picosecond (Bup "" 1013 s - I , B is the Einstein coefficient, Up the pumping energy density). The gain grating is proportional to that of the working levN population AN 1 which one can calculate in the frame of a four-level system neglecting the superfluorescence and a transient period of level populating process as AN1 =~ dup ! Up=Ul+U2 2 ~ 1 u 2 2N
B ' 2 u~-~lU~
- T1 [ 2 / T I + B ( U l + U 2 ) 1 2 ,
234
(1)
August 1979
where N is the dye concentration, T 1 a decay time of working level, u 1 and u 2 are the energy densities of Q-switched and mode-locked pumping beams respectively. The above expression is written in the approximation of short time of vibrational relaxation compared to T 1 and low modulation depth of interference field 2~¢/UlU2/(Ul + u2) ~ 1. As far as the rereflected beam intensity is almost four orders lower then that of the initial one the numerator of (1) is two orders lower, while the denominator is at least four orders lower for the additional grating because the latter for the initial grating is determined by (Bu2)2 and for the additional one by either (2/T 1)2 or (BUl)2. So the additional gain grating amplitude must be at least two orders higher of that for the most intense pulses from the mode-locked pulse train. It should be noted that the expression (1) is quite rigorous for the additional grating because one has to take into account a transient behaviour of the populating level process as far as its duration is comparable or even larger than the ultrashort pulse duration (Bu 1 ~ 4 X 1010 s - l ) . Nevertheless the above consideration is severe enough to explain the results shown in fig. 3 by means of saturation effect which leads to the decrease of DFB superfluorescence caused by the initial grating with the increase of the mode-locked pulse intensity. Thus it seems probable that just the gain coefficient grating caused the DFB superfluorescence in the experiments under discussion.
References [1 ] H. Kogelnik and C.V. Shank, J. Appl. Phys. 43 (1972) 2327. [2] V.A. Zaporozhchenko, A.N. Rubinov and T.Sh. Efendiev, Pism. Zhurn. Techn. Fiz. 8 (1977) 114. [3] Y. Aoyagi and S. Namba, Optics Comm. 23 (1977) 330. [4] R.G. Zaporozhchenko and V.A. Zaporozhchenko, Zhurn. PriM. Spektros 26 (1977) 37. [5] S.A. Batishche and V.A. Mostovnikov, Izv. Akad. Nauk SSSR see fiz. 39 (1975) 2254. [6] A.N. Rubinov and T.Sh. Efendiev, Zhurn. Prikl. Spektrosk. 27 (1977) 634.
OPTICS COMMUNICATIONS
August 1979
LIGHT-INDUCED GRATING FORMATION IN DYE SOLUTIONS
BY TWO INDEPENDENT LASER BEAMS P.A. APANASEVICH and V.A. ZAPOROZHCHENKO Institute of Physics, BSSR Academy of Sciences, Minsk 220602, USSR
Received 15 February 1979 Revised manuscript received 23 April 1979
The distributed feedback laser effect was used for the detection of light-induced gratings created in organic dye solutions due to the interference of actively mode-locked ruby laser radiation and that of electrooptically Q-switched ruby laser. Radiation kinetics in different regions of the dye superfluorescence spectrum was studied. The distributed feedback effect was shown to be caused by the amplification coefficient grating in the conditions under discussion.
The direct observation of the interference of two independent laser pulses is usually very difficult because o f the short time of their mutual coherence. But the interference pattern indication is available in nonlinear media due to the light-induced grating formation. In dye solutions one can use for this aim the distributed feedback (DFB) laser effect provided by the gain coefficient grating or by that of the refractive index [1 ]. The picosecond pulse generation in DFB systems [2,3] is an evidence of the short lasing development time and thus the DFB laser effect can be used for the indication of short-lived interference patterns if their pe-
riods coincide with the luminescence wavelengths of the dye solution. In this communication we report the experimental results concerning the indication of light-induced gratings created in organic dye solutions due to the interference of the actively mode-locked ruby laser radiation and that o f an electroopticatly Q-switched ruby laser. Fig. 1 shows the experimental setup used for the grating creation and its detection by the DFB lasing. The ultrashort pulse train from the actively modelocked laser described in [4] and the giant pulse from an electrooptically Q-switched laser were directed into
~FABRY PEROT
-~0
@'
MODE-LOCKED PULSETRAIN Q-SWITCHEDPULSE
I F- 71PHOTOMU'T P' ER I SPECTROG APH I}I I Fig. 1. Experimental setup, 231
Volume 30, number 2
OPTICS COMMUNICATIONS
the prism dye cell, where they interfered in the dye solution which was in contact with the hypotenuse side of the prism. The cell windows were inclined relatively to the plane of the prism side to prevent light generation due to Fresnel reflection. Time coincidence of the mode-locked pulse train and the Q-switched pulse was provided by a special electronic device which permitted their mutual temporal shift. Their mutual position in time was controlled by means of two optical signals detection by the first coaxial vacuum photocell (see fig. 1). Spectra of ruby lasers were measured with a l-ram Fabry-Perot etalon. The dye radiation spectrum was recorded with the diffractiongrating spectrograph. The time behaviour of the total dye output was recorded by the second photocell. The investigation of time characteristics in different spectral ranges of dye radiation was also carried out. In this case the 1 mm slit was placed in the image plane of the spectrograph, used as a monochromator with spectral resolution 3 A. The optical signal after passing through the monochromator was detected by a high-speed photomultiplier with rise time 2 ns. The photocells and the multiplier output signals were recorded by the two-channel oscillograph. The mode-locked laser pulse width was measured by two-photon fluorescence technique. The peak intensity of ultrashort pulses reached 4 - 5 GW/cm 2 with pulse duration ~20 ps. The peak intensity of the Q-switched laser was ~ 2 0 MW/cm 2. The spectral width of mode-locked radiation was ~0.9 A, i.e. ultrashort pulses were near time-bandwidth limited. The bandwidth of the Q-switched laser radiation was ~0.5 A and the corresponding time interval of its coherence was longer than the ultrashort pulse duration. The middle wavelengths of two lasers differed by 0.23 .~. Two organic dye solutions were used for the observation of the DFB laser effect: cryptocyanine in ethanol (Solution 1) and 3,3-diethyl-6,7,6,7-di (2phenylthiasolo-4,5) thiacarbocyanine chloride in ethanol (Solution 2). Dye concentrations, population decay times and quantum yields were equal to 7 × 1017cm -3, 1.5 × 1 0 - 1 I s , 0.02 and 1.5 X 1017cm -3, 2.5 × 10 -9 s, 0.32 for solution 1 and solution 2, respectively. Fig. 2 shows the total time behaviour and spectrum of the dye radiation when different cases of pumping pulses synchronisation are realised. A substantial narrowing of the dye superfluorescence spectrum was ob232
August 1979
/I
11 t
b
£
Fig. 2. Oscillograms(a, b, c) and spectra (a', b', c' ) of superfluorescence of solution 1. served when the mode-locked pulse train was coincident with the Q-switched pulse (fig. 2b'). In other cases we could see a broad superfluorescence spectrum either without any structure (fig. 2a'), or with a weak line due to the interference between the residual pulses from the train and the leading edge of Q-switched pulse (fig. 2c' ). The comparison of figs. 2a and 2c shows that the superfluorescence intensity is lower when the ultrashort pulse train is incident on the dye solution later than the Q-switched pulse. This effect as well as the shorter length of the superfluorescence train in comparison with the pumping one can be attributed to an essential role of singlet-triplet conversion and to some photochemical processes in polymethine dye solutions [5]. The dye superfluorescence spectrum narrowing proves the light-induced grating presence which pro-
Volume 30, number 2
OPTICS COMMUNICATIONS
vides the DFB effect in dye solution. The superfluorescence wavelength corresponds to the interference pattern period and is easily tunable by variation of the angle between interfering beams (compare fig. 2b' and fig. 3e, where the DFB superfluorescence of solution 1 is obtained in different spectral ranges). As far as the wavelengths of pumping beams differ in 0.23 A the grating is moving. The distance travelled by it in 20 ps (the duration of the pulses) is small compared to the grating period (~0.29) and this movement does not lead to a substantial degradation of the DFB superfluorescence. The interaction of the interference field with the medium causes the gain coefficient grating as well as that of the refractive index. There are three mechanisms of the refractive index change by the radiation: optical Kerr-effect, electro-striction and thermal effect. It is known [6] that under the conditions of Q-switched pulse pumping the latter leads to the refractive index variation not only due to its direct dependence on the medium temperature but mainly because of the density change with temperature. It provides an accumulative growth of the grating amplitude due to a large time of heat diffusion along the grating (~10 -7 s). The contributions of electrostrictive and Kerr-effect index variations are small because of the insufficient radiation intensity of Q-switched pulses.
August 1979
When the grating is formed under the influence of the mode-locked pulse train [2] the thermal effect contribution decreases as a rule and that of the Kerreffect increases as a result of higher radiation intensity. The electrostrictive effect does not manifest in this case as the medium density is unchanged within the ultrashort pulse duration. Under our experimental conditions we could neglect both the electrostriction and the medium density variation under the heating as the short mutual coherence time of two lasers used made the process of the grating accumulation impossible. Every subsequent pulse could not increase the grating produced by the previous one. Therefore, the role of the thermal refractive index variation in our case could not be expected significant as its partial derivative with respect to temperature is very small. It could be proved that the refractive index grating caused by the optical Kerr-effect was also insignificant in our experiments. As is well-known [6], the presence of several gratings in the medium leads to the generation of several spectral components. In some of our experiments we observed also the two wavelength generation due to the presence of an additional grating in the medium which was caused by the fourfold Fresnel reflection of the mode-locked laser radiation inside the prism as shown in the right corner of fig. 1. The rereflected
Fig. 3. OsciUogramsof dye output in different spectral ranges (a,b, c) and spectra of solution 2 - (d) and solution 1 - (e). Pulses in train are separated by interval 11.1 ns. 233
Volume 30, number 2
OPTICS COMMUNICATIONS
radiation was incident on the dye solution at the angle 0' differing from the initial angle 0 by 0' - 0 = 261 + 46 2. The measurement of prism angles and the calculations of grating steps showed that the additional grating caused the left lines in fig. 3d, e. The ratio of the rereflected beam intensity to that of the initial one was 1.4 × 10 -4. The oscillograms of dye output in different spectral ranges (fig. 3a, b, c) show that DFB light generation provided by the initial beam-produced grating is observed when the first low intense ultrashort pulses from the train interfere with the leading edge of the Q-switched pulse (see fig. 3b). With an increase of ultrashort pulse intensity lasing conditions are violated for the main grating while they are attained for the additional one and rather a long pulse train is generated at the additional grating wavelength (see fig. 3a). This result proves the lack of the optical Kerr-effect contribution to the DFB formation process under our experimental conditions. An opposite statement would be in contradiction to the fact of the absence of lasing on the main grating while it does occur on the additional one, which is formed by a much lower field amplitude. The explanation of the dye output time behaviour is possible if the DFB is determined by the gain coefficient grating. The large difference of the interfering beam intensities in the experiments under discussion resulted in the low interference field modulation depth ~0.13 while dye solutions were under the saturation action of high intense picosecond pulses from the initial beam which could bring almost all particles to the excited state in a picosecond (Bup "" 1013 s - I , B is the Einstein coefficient, Up the pumping energy density). The gain grating is proportional to that of the working levN population AN 1 which one can calculate in the frame of a four-level system neglecting the superfluorescence and a transient period of level populating process as AN1 =~ dup ! Up=Ul+U2 2 ~ 1 u 2 2N
B ' 2 u~-~lU~
- T1 [ 2 / T I + B ( U l + U 2 ) 1 2 ,
234
(1)
August 1979
where N is the dye concentration, T 1 a decay time of working level, u 1 and u 2 are the energy densities of Q-switched and mode-locked pumping beams respectively. The above expression is written in the approximation of short time of vibrational relaxation compared to T 1 and low modulation depth of interference field 2~¢/UlU2/(Ul + u2) ~ 1. As far as the rereflected beam intensity is almost four orders lower then that of the initial one the numerator of (1) is two orders lower, while the denominator is at least four orders lower for the additional grating because the latter for the initial grating is determined by (Bu2)2 and for the additional one by either (2/T 1)2 or (BUl)2. So the additional gain grating amplitude must be at least two orders higher of that for the most intense pulses from the mode-locked pulse train. It should be noted that the expression (1) is quite rigorous for the additional grating because one has to take into account a transient behaviour of the populating level process as far as its duration is comparable or even larger than the ultrashort pulse duration (Bu 1 ~ 4 X 1010 s - l ) . Nevertheless the above consideration is severe enough to explain the results shown in fig. 3 by means of saturation effect which leads to the decrease of DFB superfluorescence caused by the initial grating with the increase of the mode-locked pulse intensity. Thus it seems probable that just the gain coefficient grating caused the DFB superfluorescence in the experiments under discussion.
References [1 ] H. Kogelnik and C.V. Shank, J. Appl. Phys. 43 (1972) 2327. [2] V.A. Zaporozhchenko, A.N. Rubinov and T.Sh. Efendiev, Pism. Zhurn. Techn. Fiz. 8 (1977) 114. [3] Y. Aoyagi and S. Namba, Optics Comm. 23 (1977) 330. [4] R.G. Zaporozhchenko and V.A. Zaporozhchenko, Zhurn. PriM. Spektros 26 (1977) 37. [5] S.A. Batishche and V.A. Mostovnikov, Izv. Akad. Nauk SSSR see fiz. 39 (1975) 2254. [6] A.N. Rubinov and T.Sh. Efendiev, Zhurn. Prikl. Spektrosk. 27 (1977) 634.