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Pulsed infrared difference frequency generation in CdGeAs2
Volume 15, number 1
OPTICS COMMUNICATIONS
September 1975
PULSED INFRARED DIFFERENCE FREQUENCY GENERATION IN CdGeAs 2 * Martin S. PILTCH, John RINK and Charles TALLMAN Los Alamos Scientific Laboratory, University o f California, Los Alamos, New Mexico 87544, USA
Received 26 May 1975 Pulsed, discretely tunable infrared difference frequency generation using CO and CO2 lasers was demonstrated in the chalcopyrite crystal CdGeAs2. The process of Type II phase-matched mixing was employed.
1. Introduction We report the first achievement of line-tunable pulsed infrared difference frequency generation in single crystal chalcopyrite CdGeAs 2 [1,2] t using CO and CO 2 laser radiation. As previously reported for the cw case [3] an infrared signal at the difference between the CO and CO 2 laser frequencies was obtained by the process of type II phase-matched mixing [4] in tile tetragonal (42m) crystal. The range of discrete line tunability was from 15.5 to 16.5/am in this experiment. The crystal was cut to maximize the interaction for difference frequencies in the neighborhood of 16/am. In principle, phase matched sum and difference frequency mixing are achievable in this crystal from wavelengths of 3.5 to 18/am [2]. The various combinations o f CO and CO 2 laser vibrational-rotational transitions makes possible dense coverage of the frequency band from 8 to 19/am with difference frequencies spaced less than 1 cm -1 apart.
2. Experimental apparatus and technique The experimental apparatus, shown in fig. 1, con-
* Work performed under the auspices of the U.S. Energy Research and Development Administration. "~Crystal grown by G. Iseler, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Mass. (process fixst suggested by H. Kildal, Lincoln Laboratory for use in performing saturation spectroscopy). 112
sisted of an electron beam preionized CO laser and a CO 2 TEA laser. In detail, the CO laser employed five plasma diode cathodest? to deliver preionization electrons to the laser gas. The plasma diodes were 10.0 cm in diameter. They were spaced as closely as possible in order to provide uniform preionization to the 2.5 cm high X 2.0 cm wide × 45 cm long interelectrode volume. The plasma diodes operated at a pressure of 0.055 torr of helium. They were energized by a 120 kV pulse forming network which supplied a 70 /as duration pulse. The laser gas mixture was composed of 85% argon and 15% high purity CO, at a total pressure of 280 torr and excited by an ignitron switched and crowbarred capacitor bank operating at 3 kV. The gas electrical loading was in the range of 2000 joules per liter-atmosphere of CO molecules. The laser was line tunable by means of an intracavity original ruled grating having 150 lines per mm and was capable of operation at wavelengths from 5.3 to 6.1/am with single line, single transverse mode energies of typically 50 mJ during its 50/as output pulse. The CO 2 laser utilized was a Lumonics type 103 double discharge laser employing an intracavity diffraction grating for line tunability. Tile device was operated at a pressureof 580 torr with the nitrogen rich mixture of 4: 2:1, He:N 2 :CO 2 and produced an output pulse of 2/as duration. The laser operated at a typical single line, single transverse mode output energy of 3 0 0 - 5 0 0 mJ with no evidence of spontanet'~ Plasma diode CO laser based on design of S. Byron, Mathematical Sciences NW, Seattle, Washington.
Volume 15, number 1
OPTICS COMMUNICATIONS
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Fig. 1. Pulsed optical mixer. ous mode locking. Both lasers were operated at a repetition rate of 1 pulse per second. In order to achieve maximum nonlinear conversion efficiency and to remain below the threshold for crystal damage (less than 10 MW/cm 2) the two laser beams were processed differently before impinging on the crystal. The CO 2 beam diameter was reduced to 0.5 cm diameter by means of an inverted telescope of ZnSe lenses. The output of the CO laser was brought to a focus 2 cm in front of the CdGeAs 2 crystal by means of a ZnSe lens of 30 cm focal length. The beams were spatially combined by a Ge dielectric coated dichroic beam splitter before impinging on the crystal. At the crystal surface the CO 2 laser intensity was kept below 0.5 MW/cm 2 while that of the CO laser was of the order of 0.2 MW/cm 2. At these intensity levels, no damage was observed to the crystal surfaces after a total of some 5 X 105 pulses. The proper laser polarization directions for type II phase matching in the positive uniaxial CdGeAs 2 were assured by the proper orientation of the Brewster angle windows of each laser. Within the crystal, the CO laser propagated as an ordinary wave while the CO 2 laser propagated an extraordinary wave. The crystal entrance face was cut and polished such that at normal incidence the lasers propagated in a direction 50 ° to the crystal axis. The crystal had dimen-
sions of 5 × 10 X 10 mm. There were no antireflection coatings on either the entrance or exit 5 X 10 mm faces. After the beams passed through the crystal and nonlinear mixing occurred, the output was filtered to remove the largely unconverted 5 and 10/am radiation. Two multilayer filters, blocked at wavelengths shorter than 10/am with a total attenuation of 106 were employed to assure isolation of the mixed signal. The mixed radiation was collected and focused on the detector by an f/1 ZnSe lens. A HgCdTe photovoltaic detector peaked for maximum sensitivity at 16/am cooled to 77 K was employed. For wavelength measurements a 0.5 meter Jarrell-Ash monochromator was used with the above detector. Energy measurements were made with a Laser Precision high sensitivity joulemeter, Type AK-2900.
3. Results and comparison with theory The results of the experiment can be summarized by the following typical properties. For peak CO and CO 2 laser powers of 1 kW and 200 kW, respectively, measured at the input crystal face, a usable output in the 16/am wavelength range of 2.5 W was measured. The measured mixed signal power can be most 113
Volume 15, number 1
OPTICS COMMUNICATIONS
easily compared to analytical predictions with the aid of the following relationship [4]. The formula is derived assuming no depletion of either of the pump waves and is couched in hybrid, but commonly used units.
52"2d2L 2Il I2 (s~___xx) 2 13 -- nln2n3X~ where I1,12,13 are the intensities of the mixed signal and the pumping signals, respectively (W/cm2), n 3, n2, n 1 are the indices of refraction of the crystal for these signals, X3 is the difference wavelength (cm), X2 and X1 are the pump wavelengths, d is the relevant effective nonlinear coefficient (cm/statvolt), L is the crystal length and
X-~"
X2
~
L.
Correction factors must be applied to account for 1) the large Fresnel reflection (31%) at each of the crystal surfaces; 2) the linear crystal absorption coefficient (0.5 cm -1 and 0.2 cm - 1 ) measured at the two pump wavelengths and (0.3 cm - 1 ) at the mixed wavelength. With these considerations, the measured output is within a factor of two of the predicted output. The remaining discrepancy is attributed to nonreproducible laser transverse mode structure and pulse-to-pulse output power.
114
September 1975
4. Summary In summary, we have demonstrated pulsed infrared difference frequency generation at power levels some 106 times greater than those previously reported using the chalcyporite crystals [3]. In spite of its complexity the system has been engineered to operate with very high reliability at rates of 1 pulse per sec. The power output is currently limited by the quality of available crystals insofar as their damage and linear attenuation properties are concerned.
Acknowledgement The authors gratefully acknowledge the technical assistance of J. Atencio, R. Clayton, R. Hinsley, G. Lindholm and R. Nickle. The high reliability electron beam laser electronics were engineered by G. Erickson and P. Mace. Thanks are due to D. Edwards for a critical reading of the manuscript. Helpful discussions with S. Byron, H. Kildal, and E. O'Hair are further acknowledged.
References [1] R.L. Byer, H. Kildal and R.S. Feigelson, Appl. Phys. Lett. 19 (1971) 237. [2] G. Boyd, E. Buehler, F. Strong and J. Wernick, IEEE J. Quantum Electronics QE-8 (1972) 419. [3] H. Kildal and J.C. Mikkelson, Opt. Commun. 10 (1974) 306. [4] F. Zernike and M. Midwinter, Applied Nonlinear Optics (John Wiley, New York, 1973) pp. 25-44.
OPTICS COMMUNICATIONS
September 1975
PULSED INFRARED DIFFERENCE FREQUENCY GENERATION IN CdGeAs 2 * Martin S. PILTCH, John RINK and Charles TALLMAN Los Alamos Scientific Laboratory, University o f California, Los Alamos, New Mexico 87544, USA
Received 26 May 1975 Pulsed, discretely tunable infrared difference frequency generation using CO and CO2 lasers was demonstrated in the chalcopyrite crystal CdGeAs2. The process of Type II phase-matched mixing was employed.
1. Introduction We report the first achievement of line-tunable pulsed infrared difference frequency generation in single crystal chalcopyrite CdGeAs 2 [1,2] t using CO and CO 2 laser radiation. As previously reported for the cw case [3] an infrared signal at the difference between the CO and CO 2 laser frequencies was obtained by the process of type II phase-matched mixing [4] in tile tetragonal (42m) crystal. The range of discrete line tunability was from 15.5 to 16.5/am in this experiment. The crystal was cut to maximize the interaction for difference frequencies in the neighborhood of 16/am. In principle, phase matched sum and difference frequency mixing are achievable in this crystal from wavelengths of 3.5 to 18/am [2]. The various combinations o f CO and CO 2 laser vibrational-rotational transitions makes possible dense coverage of the frequency band from 8 to 19/am with difference frequencies spaced less than 1 cm -1 apart.
2. Experimental apparatus and technique The experimental apparatus, shown in fig. 1, con-
* Work performed under the auspices of the U.S. Energy Research and Development Administration. "~Crystal grown by G. Iseler, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Mass. (process fixst suggested by H. Kildal, Lincoln Laboratory for use in performing saturation spectroscopy). 112
sisted of an electron beam preionized CO laser and a CO 2 TEA laser. In detail, the CO laser employed five plasma diode cathodest? to deliver preionization electrons to the laser gas. The plasma diodes were 10.0 cm in diameter. They were spaced as closely as possible in order to provide uniform preionization to the 2.5 cm high X 2.0 cm wide × 45 cm long interelectrode volume. The plasma diodes operated at a pressure of 0.055 torr of helium. They were energized by a 120 kV pulse forming network which supplied a 70 /as duration pulse. The laser gas mixture was composed of 85% argon and 15% high purity CO, at a total pressure of 280 torr and excited by an ignitron switched and crowbarred capacitor bank operating at 3 kV. The gas electrical loading was in the range of 2000 joules per liter-atmosphere of CO molecules. The laser was line tunable by means of an intracavity original ruled grating having 150 lines per mm and was capable of operation at wavelengths from 5.3 to 6.1/am with single line, single transverse mode energies of typically 50 mJ during its 50/as output pulse. The CO 2 laser utilized was a Lumonics type 103 double discharge laser employing an intracavity diffraction grating for line tunability. Tile device was operated at a pressureof 580 torr with the nitrogen rich mixture of 4: 2:1, He:N 2 :CO 2 and produced an output pulse of 2/as duration. The laser operated at a typical single line, single transverse mode output energy of 3 0 0 - 5 0 0 mJ with no evidence of spontanet'~ Plasma diode CO laser based on design of S. Byron, Mathematical Sciences NW, Seattle, Washington.
Volume 15, number 1
OPTICS COMMUNICATIONS
o..... ~,'~, • .':*, T.., ."
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ating•
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II ii Beam ReducinQ Telescope
I:
/
NaCI Beam Splitter - D~chrotc Beam Splitter X-Roy Shielding
Fig. 1. Pulsed optical mixer. ous mode locking. Both lasers were operated at a repetition rate of 1 pulse per second. In order to achieve maximum nonlinear conversion efficiency and to remain below the threshold for crystal damage (less than 10 MW/cm 2) the two laser beams were processed differently before impinging on the crystal. The CO 2 beam diameter was reduced to 0.5 cm diameter by means of an inverted telescope of ZnSe lenses. The output of the CO laser was brought to a focus 2 cm in front of the CdGeAs 2 crystal by means of a ZnSe lens of 30 cm focal length. The beams were spatially combined by a Ge dielectric coated dichroic beam splitter before impinging on the crystal. At the crystal surface the CO 2 laser intensity was kept below 0.5 MW/cm 2 while that of the CO laser was of the order of 0.2 MW/cm 2. At these intensity levels, no damage was observed to the crystal surfaces after a total of some 5 X 105 pulses. The proper laser polarization directions for type II phase matching in the positive uniaxial CdGeAs 2 were assured by the proper orientation of the Brewster angle windows of each laser. Within the crystal, the CO laser propagated as an ordinary wave while the CO 2 laser propagated an extraordinary wave. The crystal entrance face was cut and polished such that at normal incidence the lasers propagated in a direction 50 ° to the crystal axis. The crystal had dimen-
sions of 5 × 10 X 10 mm. There were no antireflection coatings on either the entrance or exit 5 X 10 mm faces. After the beams passed through the crystal and nonlinear mixing occurred, the output was filtered to remove the largely unconverted 5 and 10/am radiation. Two multilayer filters, blocked at wavelengths shorter than 10/am with a total attenuation of 106 were employed to assure isolation of the mixed signal. The mixed radiation was collected and focused on the detector by an f/1 ZnSe lens. A HgCdTe photovoltaic detector peaked for maximum sensitivity at 16/am cooled to 77 K was employed. For wavelength measurements a 0.5 meter Jarrell-Ash monochromator was used with the above detector. Energy measurements were made with a Laser Precision high sensitivity joulemeter, Type AK-2900.
3. Results and comparison with theory The results of the experiment can be summarized by the following typical properties. For peak CO and CO 2 laser powers of 1 kW and 200 kW, respectively, measured at the input crystal face, a usable output in the 16/am wavelength range of 2.5 W was measured. The measured mixed signal power can be most 113
Volume 15, number 1
OPTICS COMMUNICATIONS
easily compared to analytical predictions with the aid of the following relationship [4]. The formula is derived assuming no depletion of either of the pump waves and is couched in hybrid, but commonly used units.
52"2d2L 2Il I2 (s~___xx) 2 13 -- nln2n3X~ where I1,12,13 are the intensities of the mixed signal and the pumping signals, respectively (W/cm2), n 3, n2, n 1 are the indices of refraction of the crystal for these signals, X3 is the difference wavelength (cm), X2 and X1 are the pump wavelengths, d is the relevant effective nonlinear coefficient (cm/statvolt), L is the crystal length and
X-~"
X2
~
L.
Correction factors must be applied to account for 1) the large Fresnel reflection (31%) at each of the crystal surfaces; 2) the linear crystal absorption coefficient (0.5 cm -1 and 0.2 cm - 1 ) measured at the two pump wavelengths and (0.3 cm - 1 ) at the mixed wavelength. With these considerations, the measured output is within a factor of two of the predicted output. The remaining discrepancy is attributed to nonreproducible laser transverse mode structure and pulse-to-pulse output power.
114
September 1975
4. Summary In summary, we have demonstrated pulsed infrared difference frequency generation at power levels some 106 times greater than those previously reported using the chalcyporite crystals [3]. In spite of its complexity the system has been engineered to operate with very high reliability at rates of 1 pulse per sec. The power output is currently limited by the quality of available crystals insofar as their damage and linear attenuation properties are concerned.
Acknowledgement The authors gratefully acknowledge the technical assistance of J. Atencio, R. Clayton, R. Hinsley, G. Lindholm and R. Nickle. The high reliability electron beam laser electronics were engineered by G. Erickson and P. Mace. Thanks are due to D. Edwards for a critical reading of the manuscript. Helpful discussions with S. Byron, H. Kildal, and E. O'Hair are further acknowledged.
References [1] R.L. Byer, H. Kildal and R.S. Feigelson, Appl. Phys. Lett. 19 (1971) 237. [2] G. Boyd, E. Buehler, F. Strong and J. Wernick, IEEE J. Quantum Electronics QE-8 (1972) 419. [3] H. Kildal and J.C. Mikkelson, Opt. Commun. 10 (1974) 306. [4] F. Zernike and M. Midwinter, Applied Nonlinear Optics (John Wiley, New York, 1973) pp. 25-44.