Figure 1. The scheme of a ring OPO cavity. YAG: Nd3+— pumping laser; NC—nonlinear crystal LiNbO3; М1, М2, М3—mirrors; FPE—Fabry-Perot Etalon; AC—pumping laser absorber cell; ωs, ωi, ωp—signal and idler frequencies, and pumping laser frequency.
corresponds to the minimum OPO discontinuous tuning step of 0.133 nm. So, at the zero external field (Е = 0) there is no discontinuous tuning, while at the external field voltage of U = 4500 V the maximum discontinuous tuning value is equal to 12 nm.
OPO wavelength shift at NCL heating is controlled and compensated by software to a tolerance of 0.1˚C.
Figure 2 shows an optical scheme of OPO bandwidth and wavelength measurement.
The definition of absolute value of the measured OPO wavelength (λmeas) is performed by λmeas correction method at a known gas absorption line or this purpose part of OPO emission (~3%) reflects from plane-parallel СаF2-plate (3) and enters diffuse scattering sphere (8). Detector (5) receives stray radiation of the first channel after gas cell (7) with known gas (here methane 90%, at the pressure of 1 atm). Simultaneously the similar detector (5) receives stray radiation of the second channel. Then the electric signals run from the detectors to two inputs of ADC (4) which is connected to PC (10). The PC display shows a vibration rotation absorption spectrum υ3 of the methane band, which central branch Q is a benchmark. Specialized software processes the data received, correlates them with Q-branch and displays the true value of OPO wavelength (lmeas). For OPO wavelength calibration on the diffusing sphere there is a possibility to install IR-fibre (9) to transfer lasing to the entrance slit of monochromator МDR-12 (11).
OPO output energy was measured by calorimetric power meter S310 (USA).
3. Results and Discussion
The study of OPO power performance has been executed under extreme environment temperature conditions in the laboratory (Т = +30˚C) and in the open air (Т = –10˚C). Under the given conditions after 30 minutes of laser activity at a pulse frequency of 25 Hz the energy instability was within ±6%.
Figure 3 shows signal wavelength (l = 1.42 - 1.75 um) and idler wavelength (l = 2.9 - 4.2 um) dependence of OPO radiation energy.
An abrupt fall of the signal wave at 1.69 um and lack of idler wave oscillation at 2.85 are connected with heavy absorption of lithium niobate at 2.85 um. The implemented OPO scheme achieved the total conversion factor of 27%. The numerical value of beam angular divergence was calculated according to  as the relation of diaphragm diameter to the lens focus distance (d/l) with the diaphragm receiving 86% of total OPO pulse energy. Moreover the diaphragm was placed in the lens’ plane.
While OPO oscillation is given as superposition of signal and idler waves, knowing the far field beam parameters helps to evaluate separate wave energy distribution
Figure 2. The optical scheme of OPO bandwidth and wavelength measurement. 1—pumping laser YAG:Nd3+; 2—OPO unit; 3—CaF2 plane-parallel plate; 4—analog-digital converter (ADC); 5—FD-219 photodetector with PbSe preamplifier; 6—methane cell; 7—gas cell; 8—diffusing sphere; 9—IR-fiber of CaF2; 10—PC; 11—MDR-12 monochromator; 12—С1-91 os-cillograph.
character. Experimental OPO signal and idler wave divergence values were limited by 3.5 millirad within the whole oscillation range (Figure 3). At minor cavity length variation the value did not change significantly. The received result corresponded to the divergence calculation according to М2 method . Figure 4 shows OPO emission spectra obtained for signal and idler waves.
Figure 4(a) corresponds to OPO emission spectrum without FPE in the cavity. Figure 4(b) shows the degree of OPO emission spectrum narrowing at FPE introduction. Similar spectra were received within the whole range of OPO tuning wavelengths. The narrowing of OPO emission spectrum half width varied from 4 to 5 times. Here the change of OPO emission power was insignificant.
In conclusion some basic characteristics of IR OPO will be presented:
So, according to the study of non-linear optical characteristics of LiNbO3 and KTP crystals, as well as on the
Figure 3. Distribution of OPO emission energy in signal and idler waves. 1—signal wave; 2—idler wave.
Figure 4. OPO idler wave emission spectra. (а) without FPE (λ01 = 3.383 um) Dn = 3.6 cm–1; (b) with FPE (λ02 = 3.391 um) DnЭ = 0.69 cm–1.
basis of up-to-date engineering and software developments an IR OPO with continuous and/or discontinuous frequency tuning has been created and tested. Thanks to its characteristics the present IR OPO can be used both in lidar units and at solving of various spectroscopy tasks of fundamental research.