Efficient Nonlinear Frequency Conversion Research Articles

Laser-induced photochromic damage in potassium titanyl phosphate

We report studies of photochromic damage induced in KTiOPO4 single crystals by ultraviolet (UV) irradiation with peak intensities from 0.3 to 4.0 MW/cm2. The photochromic damage progressed to asymptotic values which were not linear with respect to UV intensity. Following UV exposure, transparency of damaged crystals recovered completely over tens of hours at room temperature, faster at elevated temperature. UV irradiation at significantly lower peak but higher average power did not produce damage. We also studied photochromic damage occurring during 1.064-μm second-harmonic generation. Fundamental infrared intensities ranged from 1.3 to 5.5 times larger than those producing highest nonlinear frequency conversion efficiency. Damage sites produced by the two methods exhibited similar absorption spectra.

Special Issue on Laser Parameter Control. Frequency Control of Semiconductor Lasers. Hihgly Efficient Phase Modulation and Nonlinear Frequency Conversion.

Performances of anoptical frequency comb generator and nonlinear optical frequency conversionof semiconductor lasers were reviewed. They are studied for realizing frequency referencegrids for an accurate and wideband optical frequency sweep. Semiconductor lasers wereused as light sources by improving their frequency stability to 2.1×10-12 and by reducing theirfield spectral linewidth to 7 Hz. A precise and continuous frequency sweep for 64GHz wasrealized by heterodyne-type optical phase locking. Frequency refrence grids with aspan of 0.1THz was realized by the optical frequency comb generator. Second harmonics, sum-frequencyand difference frequency signals were generated by using nonlinear optical crystals for the 180-577 THzrange, i. e., the wavelength range of 0.52-1.67μm.

Frequency control of semiconductor lasers

This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency. For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.

Diode-pumped microchip lasers electro-optically Q switched at high pulse repetition rates

We have obtained asymptotically equal to50 mW of time-averaged 1.064-microm output power from a diode-pumped, electro-optically Q-switched Nd:YAG microchip laser at pulse repetition rates between 5 and 500 kHz. The FWHM of the output pulses is proportional to the repetition rate, varying from <300 ps at low repetition rates to 13.3 ns at 500 kHz. The multikilowatt peak power obtained at low repetition rates allows for efficient nonlinear frequency conversion and is sufficient to ablate thin layers of metal.

Intracavity laser spectroscopy with nonlinear frequency conversion

A new absorption spectroscopy method is proposed. This method is based on intracavity generation of sum and difference frequencies and it is analogous, in respect of its physics and capabilities, to intracavity laser spectroscopy. An investigation is reported of the dependence of the sensitivity of the new method on the efficiency of nonlinear frequency conversion and on the phase relationships obeyed by waves interacting in a nonlinear crystal. It is reported that the sensitivity of the new method in determination of weak absorption is between two and three orders of magnitude higher than the extracavity variant.

Nonlinear optical frequency conversion in a resonator with random inhomogeneities

A study is made of the influence of phase inhomogeneities of resonator mirrors on the efficiency of nonlinear optical processes such as optical frequency doubling and parametric amplification of light waves. The results obtained make it possible to estimate the nonlinear frequency conversion efficiency of a resonator with real mirrors.