Frequency Doubling Process Research Articles

Frequency doubling in gallium-lanthanum-sulphide optical glass with microcrystals

Second harmonic generation in gallium-lanthanum-sulphide (Ga:La:S) and germanium sulphide + Ga:La:S glasses is investigated. It is shown that microcrystals of Ga:La:S and α phase of gallium-sulphide (α-Ga2S3), whose presence in the glass matrix is revealed by energy dispersive spectroscopy and x-ray diffraction analysis, are responsible for the frequency doubling process.

Harmonic generation and frequency upconversion in Pr 3+-doped germanosilicate optical fibers pumped at 1.319 μm

We report on the generation of visible light signals through harmonic generation and frequency upconversion in Pr 3+-doped germanosilicate single-mode optical fibers pumped by a Q-switched and modelocked Nd:YAG laser operated at 1.319 μm. Growth rate of the frequency-doubling process seeded by upconversion fluorescence signal and instantaneous efficient frequency-tripling is also demonstrated.

Intracavity second harmonic generation of 0.532 μm in bulk periodically poled lithium niobate

We report on CW quasi-phase-matched intracavity frequency doubling of a 1.064 μm Nd:YAG laser. The 5 mm long and 0.2 mm thick crystal of periodically poled LiNbO3 was Brewster angle cut. When located intracavity, it presented an effective nonlinear coefficient d33 of 4.5 pm/V and an estimated loss per pass less than 10%. The observed temperature bandwidth of ∼ 5 K compares favourably with the theoretical value and indicates that the whole length of the crystal was contributing to the frequency doubling process.

Novel glass-forming ferroelectric liquid-crystal material of high second-order nonlinearity

We report on a novel ferroelectric liquid-crystal material with an exceptionally large and stable nonlinear optical response. The material shows a phase transition from theSmC* mesophase to the glassy state near room temperature. This allowed us to state-freeze a stable nonlinear optically active configuration close to room temperature at 10° C for one day. The stored orientations can easily be forced to relax back by heating slightly above the glass-transition temperature. The material and the freezing process have been characterized in detail by analyzing the frequency-doubling process of the fundamental radiation of a Nd: YAG laser in the liquid crystal. Under favourable conditions, we obtained second-order susceptibilities as large as χzzz = 0.5 pm/V.

Phase-matched second-harmonic generation in planar waveguides.

Second-harmonic generation exploiting evanescent field interaction has been investigated for a planar dielectric waveguide covered with a thin organic layer. The coherent nature of the frequency-doubling process in such a structure was studied experimentally by systematic ablation of the nonlinear film in an in situ setup. An increase in the conversion efficiency by two orders of magnitude has been observed for a nonoptimized second-order susceptibility of the coverage layer

Theoretical study of a modulator for a waveguide second-harmonic generator

A novel intensity modulator for second-harmonic waveguide lasers is proposed and theoretically analyzed. Noise caused by spectrum broadening in a diode-laser direct-modulation scheme is avoided in the suggested structure by using the electro-optic effect to modulate the phase mismatch in the frequency-doubling process. In the analysis the influence of waveguide parameters and electrode dimensions on the switching voltage, the output power, and the modulation bandwidth is evaluated. The results suggest that a switching voltage below the 5-V transistor-transistor logic level in digital systems is feasible and that, in a traveling-wave strip-electrode configuration, a modulation bandwidth in the gigahertz range can be obtained.

Frequency doubling and phase matching with II–VI microcrystals

A theoretical study on the use of composite materials with semiconductor quantum dots for phase-matching and/or generation of second harmonic light is presented. Approximate near-resonance susceptibilities of quantum dots are given up to the third order under a DC electric field. Generation and propagation of the near-resonant second harmonic and fundamental fileds through such a material are investigated by ignoring local field effects. We find that the efficiency and stability of frequency doubling process are sensitively dependent on absorption and self-phase-modulation at the harmonic frequency.

Highly efficient YSGG:Cr:Nd laser with frequency doubling in a KTP crystal

The output characteristics were determined for a pulsed YSGG:Cr:Nd laser with polarization coupling of radiation out of the resonator and conversion of its frequency to the second harmonic in a KTP (potassium titanyl phosphate) crystal. The efficiency was ~ 1% at the second-harmonic frequency. The average output power was ~ 6 W and the energy of single pulses of the output radiation was ~ 0.12 J. The efficiency of the frequency doubling process in the KTP crystal pumped by a beam with a homogeneous transverse intensity distribution was ~ 80%.

Optimization of a cw mode-locked frequency-doubled Nd:LiYF_4 laser

Several drawbacks of a cw-pumped mode-locked Nd:LiYF(4) laser are outlined, and it is shown how to overcome them. For the first time, attention is paid to thermally induced astigmatism and its compensation in a Nd:LiYF(4) rod. An optimized system generates an average output power similar to that of a typical Nd:YAG laser, however, with an increased stability and with pulse durations that are significantly shorter. The higher peak power in Nd:LiYF(4) can result in a higher conversion efficiency in a frequency-doubling process, provided the doubling crystal can handle the high average power. It will be demonstrated that a well-engineered Nd:LiYF(4) system may become the preferred choice for many applications, including seeding regenerative amplifiers and pumping synchronously mode-locked dye laser oscillators and amplifiers.

Second-harmonic generation in a multimode laser cavity

We present a theoretical study of a multilongitudinal-mode laser with an intracavity-doubling crystal. The longitudinal modes are coupled to one another through frequency-doubling processes (because of the crystal) and through sum-frequency processes (because of the laser cavity). This leads to a mode–mode competition that significantly alters the nature of the output solutions.

Analysis of frequency-doubling processes in optical fibers using Raman spectroscopy

We have experimentally found a correlation between the nonlinear characteristics of frequency-doubling optical fibers and a specific modification of their Raman spectra. We propose a tentative interpretation of the physical origin of these phenomena, based on the assumption of the existence of small Si-O rings inside the fiber material. These structures would explain the nonzero value of the second-order nonlinear susceptibility.

Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers

The output of a laser-diode-pumped Nd:YAG laser with an intravcavity-doubling crystal exhibits large-amplitude fluctuations and longitudinal-mode instabilities. The results of an experimental and theoretical investigation of these instabilities are described. The instabilities arise from coupling of the longitudinal modes of the laser oscillator by sum-frequency generation in the nonlinear crystal. A rate-equation theory of a multilongitudinal-mode laser oscillator with an intracavity-doubling crystal is presented that includes terms modeling the sum-frequency and frequency-doubling processes.

The statistical behaviour of nonresonant cars intensities

The statistical properties of CARS light generated by the mixing of incoherent pump and Stokes sources have been investigated analytically. The intensity probability densities for CARS generation from the nonresonant susceptibility are presented for several cases. It is shown that when second harmonic generation is used to provide pump sources of the right frequency, the resulting non-gaussian intensity fluctuations give rise to large enhancements of the nonresonant signal. The magnitude of the effect is dependent on the conversion efficiency of the frequency-doubling process, and is in good agreement with experiment.

Efficient frequency doubling of radiation emitted by a multistage neodymium laser

A detailed analysis is made of the process of frequency doubling in large neodymium glass systems with the aim of determining the maximum efficiency. A system of equations describing frequency doubling is solved numerically allowing for the limiting factors. Various forms of the radiation intensity profile are considered in the process of solution. The calculated results are used to identify the conditions under which the maximum efficiency can be attained. These results agree well with those obtained in an experiment carried out on a multistage neodymium laser.