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FIR generation in LiNbO3 by optical difference-frequency mixing
InfraredPhys.Vol. 31, No. 4, pp. 319-322,1991 Printedin GreatBritain.All rightsreserved
0020-0891/91$3.00+ 0.00 CopyrightQ 1991PergamonF+ress pk
FIR GENERATION IN LiNbO, BY OPTICAL DIFFERENCE-FREQUENCY MIXING R. K. TYAGIand V. V. RAMPAL Defence Electronics Application Laboratory, Dehra Dun-248001, India
and G. C. BHAR Burdwan University, Laser Laboratory, Physics Department, Burdwan-713104, India (Received 26 September
1990)
Abstract-Generation of far-infrared radiation by nonlinear optical laser frequency mixing is considered in LiNbG, crystals. In one method, self-beating of a frequency-doubled Nd : YAG laser is used to generate radiation at 80&900 pm, while in the other, a dual frequency Nd:YAG laser-pumped dye laser is used for generation of radiation near 500-775 pm.
INTRODUCTION Various schemes have been reported for the generation of electromagnetic radiation in the submillimeter range. (I-QThese include optically pumped 1asers,(i4) mixing between a pump laser and a tunable Stokes light, ova)radiation from free electrons in a magnetic field”) and difference frequency mixing in a nonlinear medium.(‘*8)Due to different fabrication techniques for the source and/or to low power output, conventional electronic sources are not convenient beyond 300 GHz. Other techniques suffer from one or more of the following undesirable features: (1) requirement of a large magnetic field, (2) discrete rather than continuous tuning, (3) difficult to set up and operate. Difference frequency mixing of two laser frequencies inside a nonlinear medium provides a useful technique for generating continuously tunable submillimeter wave at room temperature. It has been studied in isotropic crystals GaAs, InAs, ZnSe and ZnTe@) and also in anisotropic crystals like ZnGeP.@)
EXPERIMENTAL
DETAILS
AND
RESULTS
We have generated submillimeter waves using a LiNbO, crystal as a nonlinear mixing medium at room temperature by (i) self-beating of laser frequencies and (ii) mixing of two closely spaced, collinear, orthogonally polarized and distinctly known frequencies inside the nonlinear medium. Yang et u/.(‘O)have used a dual dye laser system for the generation of 52.6500 pm radiation using DTTC iodide dye in DMSO. We have used, in our studies a dual frequency dye laser arrangement for the generation of submillimeter waves in the 500-775 pm region using Rhodamine 640 Perchlorate dye, which, to our knowledge has not been reported earlier. In our work on optical self-beating for submillimeter wave generation, we utilized a frequency doubled Nd:YAG laser delivering 2 MW, 15 ns pulses at 10 Hz at the crystal face (Fig. 1). The FIR radiation generated was reflected by a quartz plate into a cone condenser. The quartz plate acted as a transmission window for 532 nm radiation. The output from the 10: 1 cone condenser is passed through a polyethylene blacksheet to allow FIR to pass while blocking the green. The output then is made to fall on a pyroelectric detector array followed by an amplifier having a responsivity 125 V/W. No signal is observed when the nonlinear crystal is at angles other than the phase matching angle. Since the mixing frequencies are not sharply defined they are assumed to be around the half-power points in the spectral width of 0.532 pm radiation. This gives 1,=531.85nm and &= 532.2 nm which correspond to FIR at 808.7 pm. From this it 319
R. K. TYAGI et al.
320
Attn
NL
Crystal
Fig. 1. Experimental
0.532 pm \
Nd: YAG LASER
set up for submillimeter wave generation by optical beating. FD-Frequency Doubler; NL-Nonlinear.
is estimated that the generated FIR is lying around 800-9OOpm. Sticient power at FIR would only be available if modes lying between half power points took part in mixing, Experimental -results indicate an FIR signal of 2.7mW in 800-900 pm range at the phase matching angle of 6.3”. The generated FIR power agrees within reasonable limits, with theoretically expected value of 3.11 mW. Ex~~mentally observed angular width of phase matching angle, A0 = 0.8” for full band of self-beating modes, is also in general agreement with the theoretically expected value of 0.51” for beating of half power frequencies in the laser spectral width. This difference in angular width is possible on account of spectral spread of mixing wavelengths. The second set up required development of a dual frequency dye laser system. This was pumped by a frequency doubled Q-switched Nd:YAG laser delivering 10.7 MW, 15 ns, 10 Hz pulses at 0.532 pm (Fig. 2). The twin dye laser oscillator is followed by an amplifier. Two gratings at grazing incidence with tunable mirrors, are used in the laser cavities of the twin dye laser system. Polarization corresponding to the two wavelengths, are made orthogonal with the help of an intracavity Glan-Thomson polarizer. The twin dye laser pulsed system generated 1.9 MW (peak) and 1.7 MW (peak) in orthogonal polarization at the wavelengths of 0.6157 and 0.6162 pm respectively. Output wavelengths of the twin dye laser are accurately measured with the help of a wave meter (type SOPRA FIOOO). Rhodamine 640 perchlorate in methanol has been used with a molar concentration of I .2 x 10T3 M. Spot size of the laser radiation is 4mm dia on the input face of the nonlinear crystal. The
DET AND AMP
Wave
El
Mater
I TRIO.
SIQNAL
FROM
LASER
Fig. 2. Experimental set up for submillimeter wave generation with dual frequency dye laser. Ml, M2, M-Mirrors; DCl, DC2-Dye Cells, GT~Ian-~omson prism; Gl, G2-Gratings; FD-Frequency Doubler; NLX-Nonlinear crystal.
321
FIR generation in LiNbO,
93
A, = 6w.2 ml
P
P
- Lxpwlm*ntol
--Theoretical 12
B 0 -I\, a
11
P
&
10
-
\
I
9
I
I
615.5
A,
I
615.7
615.6
615.6
(nm)
Fig. 3. Variation of phase matching angle with input wavelength.
LiNb03 crystal (7 mm cube) was cut so that the C-axis was at an angle of 10” to face normal. The theoretical phase matching curve for angle tuning is shown in Fig. 3 and expected angular width of phase matching at 0.6157 pm input is shown in Fig. 4. Output of the crystal was passed through a 10: 1 cone condenser. A visible rejection filter (black polyethylene sheets) is used to cut off the visible dye laser radiation. The pyrodetector with amplifier is used for detecting the submillimeter wave radiation at room temperature. With the input wavelengths at 0.6162 and 0.6157 pm, a submillimeter wave output signal of 21 ,uW was obtained at 758.7 pm. Measured phase matching angle and angular width of phase matching were 19.9 and 1.8” respectively. When one of the two input wavelengths was changed from 0.6157 to 0.6155 pm, keeping the other fixed at 0.6162 pm, a 24 PW signal at 541.8 pm was obtained at phase matching angle of 12.8” (Fig. 3). However, no appreciable change in the angular width of phase matching angle was observed (Fig. 4). Further work for improving the output power and spectral range of the generated submillimeter waves is in progress. -
Thaor.tlcal
I
I
I
I
-1.0
-0.6
-0.6
-0.4
8 CM -42
0.19 0
0.2
I
I
I
I
0.4
0.6
0.6
1.0
Fig. 4. Normalized submillimeter output vs angular detuning in both theoretical and experimental situations. 0: theoretical-l 1.02 deg, experimental-10.9” PM.
322
R. K. TYAGI et al.
REFERENCES 1. R. K. Tyagi, V. V. Rampal and G. C. Bhar, Infrared Phys. 26, 29 (1986). 2. M. R. Schubert, M. S. Dursehlag and T. A. DeTemple, IEEE J. quant. Electron. QE13, 455 (1977). 3. D. T. Hodges, Infrared Phys. 18, 375 (1978). 4. T. Y. Chang, IEEE Trans. Microw. Theory Tech. m-22, 983 (1974). 5. C. K. N. Pate1 and E. D. Shaw, Pkys. Reo. B3, 1279 (1971). 6. G. D. Boyd, T. J. Bridges, E. Buehler and C. K. N. Patel, Appi. Phys. L&t. 21, 553 (1972). 7. S. C. Zhang and S. Yau, IEEE J. quant. Electron. QE23, 1646 (1987). 8. Y. R. Shen (editor) Nonlinear infrared generation. Topics in Applied Physics, Vol. 16. Springer, Berlin (1977). 9. M. A. Pies&up, R. N. Fleming and R. H. Pantell, Appl. Phys. L&r. 26, 418 (1975). 10. K. H. Yang, J. R. Morris, P. L. Richards and Y. R. Shen, Appl. Phys. Len. 23, 669 (1974).
0020-0891/91$3.00+ 0.00 CopyrightQ 1991PergamonF+ress pk
FIR GENERATION IN LiNbO, BY OPTICAL DIFFERENCE-FREQUENCY MIXING R. K. TYAGIand V. V. RAMPAL Defence Electronics Application Laboratory, Dehra Dun-248001, India
and G. C. BHAR Burdwan University, Laser Laboratory, Physics Department, Burdwan-713104, India (Received 26 September
1990)
Abstract-Generation of far-infrared radiation by nonlinear optical laser frequency mixing is considered in LiNbG, crystals. In one method, self-beating of a frequency-doubled Nd : YAG laser is used to generate radiation at 80&900 pm, while in the other, a dual frequency Nd:YAG laser-pumped dye laser is used for generation of radiation near 500-775 pm.
INTRODUCTION Various schemes have been reported for the generation of electromagnetic radiation in the submillimeter range. (I-QThese include optically pumped 1asers,(i4) mixing between a pump laser and a tunable Stokes light, ova)radiation from free electrons in a magnetic field”) and difference frequency mixing in a nonlinear medium.(‘*8)Due to different fabrication techniques for the source and/or to low power output, conventional electronic sources are not convenient beyond 300 GHz. Other techniques suffer from one or more of the following undesirable features: (1) requirement of a large magnetic field, (2) discrete rather than continuous tuning, (3) difficult to set up and operate. Difference frequency mixing of two laser frequencies inside a nonlinear medium provides a useful technique for generating continuously tunable submillimeter wave at room temperature. It has been studied in isotropic crystals GaAs, InAs, ZnSe and ZnTe@) and also in anisotropic crystals like ZnGeP.@)
EXPERIMENTAL
DETAILS
AND
RESULTS
We have generated submillimeter waves using a LiNbO, crystal as a nonlinear mixing medium at room temperature by (i) self-beating of laser frequencies and (ii) mixing of two closely spaced, collinear, orthogonally polarized and distinctly known frequencies inside the nonlinear medium. Yang et u/.(‘O)have used a dual dye laser system for the generation of 52.6500 pm radiation using DTTC iodide dye in DMSO. We have used, in our studies a dual frequency dye laser arrangement for the generation of submillimeter waves in the 500-775 pm region using Rhodamine 640 Perchlorate dye, which, to our knowledge has not been reported earlier. In our work on optical self-beating for submillimeter wave generation, we utilized a frequency doubled Nd:YAG laser delivering 2 MW, 15 ns pulses at 10 Hz at the crystal face (Fig. 1). The FIR radiation generated was reflected by a quartz plate into a cone condenser. The quartz plate acted as a transmission window for 532 nm radiation. The output from the 10: 1 cone condenser is passed through a polyethylene blacksheet to allow FIR to pass while blocking the green. The output then is made to fall on a pyroelectric detector array followed by an amplifier having a responsivity 125 V/W. No signal is observed when the nonlinear crystal is at angles other than the phase matching angle. Since the mixing frequencies are not sharply defined they are assumed to be around the half-power points in the spectral width of 0.532 pm radiation. This gives 1,=531.85nm and &= 532.2 nm which correspond to FIR at 808.7 pm. From this it 319
R. K. TYAGI et al.
320
Attn
NL
Crystal
Fig. 1. Experimental
0.532 pm \
Nd: YAG LASER
set up for submillimeter wave generation by optical beating. FD-Frequency Doubler; NL-Nonlinear.
is estimated that the generated FIR is lying around 800-9OOpm. Sticient power at FIR would only be available if modes lying between half power points took part in mixing, Experimental -results indicate an FIR signal of 2.7mW in 800-900 pm range at the phase matching angle of 6.3”. The generated FIR power agrees within reasonable limits, with theoretically expected value of 3.11 mW. Ex~~mentally observed angular width of phase matching angle, A0 = 0.8” for full band of self-beating modes, is also in general agreement with the theoretically expected value of 0.51” for beating of half power frequencies in the laser spectral width. This difference in angular width is possible on account of spectral spread of mixing wavelengths. The second set up required development of a dual frequency dye laser system. This was pumped by a frequency doubled Q-switched Nd:YAG laser delivering 10.7 MW, 15 ns, 10 Hz pulses at 0.532 pm (Fig. 2). The twin dye laser oscillator is followed by an amplifier. Two gratings at grazing incidence with tunable mirrors, are used in the laser cavities of the twin dye laser system. Polarization corresponding to the two wavelengths, are made orthogonal with the help of an intracavity Glan-Thomson polarizer. The twin dye laser pulsed system generated 1.9 MW (peak) and 1.7 MW (peak) in orthogonal polarization at the wavelengths of 0.6157 and 0.6162 pm respectively. Output wavelengths of the twin dye laser are accurately measured with the help of a wave meter (type SOPRA FIOOO). Rhodamine 640 perchlorate in methanol has been used with a molar concentration of I .2 x 10T3 M. Spot size of the laser radiation is 4mm dia on the input face of the nonlinear crystal. The
DET AND AMP
Wave
El
Mater
I TRIO.
SIQNAL
FROM
LASER
Fig. 2. Experimental set up for submillimeter wave generation with dual frequency dye laser. Ml, M2, M-Mirrors; DCl, DC2-Dye Cells, GT~Ian-~omson prism; Gl, G2-Gratings; FD-Frequency Doubler; NLX-Nonlinear crystal.
321
FIR generation in LiNbO,
93
A, = 6w.2 ml
P
P
- Lxpwlm*ntol
--Theoretical 12
B 0 -I\, a
11
P
&
10
-
\
I
9
I
I
615.5
A,
I
615.7
615.6
615.6
(nm)
Fig. 3. Variation of phase matching angle with input wavelength.
LiNb03 crystal (7 mm cube) was cut so that the C-axis was at an angle of 10” to face normal. The theoretical phase matching curve for angle tuning is shown in Fig. 3 and expected angular width of phase matching at 0.6157 pm input is shown in Fig. 4. Output of the crystal was passed through a 10: 1 cone condenser. A visible rejection filter (black polyethylene sheets) is used to cut off the visible dye laser radiation. The pyrodetector with amplifier is used for detecting the submillimeter wave radiation at room temperature. With the input wavelengths at 0.6162 and 0.6157 pm, a submillimeter wave output signal of 21 ,uW was obtained at 758.7 pm. Measured phase matching angle and angular width of phase matching were 19.9 and 1.8” respectively. When one of the two input wavelengths was changed from 0.6157 to 0.6155 pm, keeping the other fixed at 0.6162 pm, a 24 PW signal at 541.8 pm was obtained at phase matching angle of 12.8” (Fig. 3). However, no appreciable change in the angular width of phase matching angle was observed (Fig. 4). Further work for improving the output power and spectral range of the generated submillimeter waves is in progress. -
Thaor.tlcal
I
I
I
I
-1.0
-0.6
-0.6
-0.4
8 CM -42
0.19 0
0.2
I
I
I
I
0.4
0.6
0.6
1.0
Fig. 4. Normalized submillimeter output vs angular detuning in both theoretical and experimental situations. 0: theoretical-l 1.02 deg, experimental-10.9” PM.
322
R. K. TYAGI et al.
REFERENCES 1. R. K. Tyagi, V. V. Rampal and G. C. Bhar, Infrared Phys. 26, 29 (1986). 2. M. R. Schubert, M. S. Dursehlag and T. A. DeTemple, IEEE J. quant. Electron. QE13, 455 (1977). 3. D. T. Hodges, Infrared Phys. 18, 375 (1978). 4. T. Y. Chang, IEEE Trans. Microw. Theory Tech. m-22, 983 (1974). 5. C. K. N. Pate1 and E. D. Shaw, Pkys. Reo. B3, 1279 (1971). 6. G. D. Boyd, T. J. Bridges, E. Buehler and C. K. N. Patel, Appi. Phys. L&t. 21, 553 (1972). 7. S. C. Zhang and S. Yau, IEEE J. quant. Electron. QE23, 1646 (1987). 8. Y. R. Shen (editor) Nonlinear infrared generation. Topics in Applied Physics, Vol. 16. Springer, Berlin (1977). 9. M. A. Pies&up, R. N. Fleming and R. H. Pantell, Appl. Phys. L&r. 26, 418 (1975). 10. K. H. Yang, J. R. Morris, P. L. Richards and Y. R. Shen, Appl. Phys. Len. 23, 669 (1974).