Noncritical Phase Matching Research Articles

Electro-optic Q-switched intracavity optical parametric oscillator at 1.53 μm based on KTiOAsO4

An eye-safe, high peak power optical parameter oscillator (OPO) intracavity pumped by electro-optic Q-switched Nd:YAG laser is presented. This OPO is based on a 20mm length KTiOAsO4 crystal with non-critical phase matching (θ=90°, ϕ=0°) cut. An aperture ∅3mm acted as limiting diaphragm to get good beam quality of pumping laser. The output energy of 25mJ at the signal wavelength 1.53μm was obtained with repetition rate of 1Hz. The highest peak power intensity was up to 88MW/cm2 with pulse width of 4ns. Without diaphragm, the maximum output energy of 90mJ was achieved with area of light spot (∅6mm) four times larger, but the peak power intensity was lower.

Tunable optical parametric oscillator based on a KTP crystal, pumped by a pulsed Ti3+:Al2O3 laser

We present the characteristics of an optical parametric oscillator based on a KTP crystal, pumped with noncritical phase matching by a pulsed Ti3+:Al2O3 laser, tunable in the range 677–970 nm. Tunable generation of signal and idler waves is obtained in the ranges 1030–1390 nm and 2690–3050 nm respectively. The efficiency of conversion of the pump to the signal wave is ≈23%, which for pulses of duration ≈8 nsec ensures an energy in the range 1.0–11.5 mJ. The width of the emission spectrum for the signal wave is within the range 0.8–1.8 nm and is predominantly determined by the linewidth of the Ti3+:Al2O3 pump laser.

All-solid-state continuous wave intracavity frequency-tripled Nd:YVO4–LiB3O4 blue laser using double-resonant approach

In the paper, a new way of intracavity frequency-tripled all-solid-state laser to generate continuous wave blue coherent radiation is firstly demonstrated. High-efficiency of continuous wave third harmonic generation using the approach of double-resonance at fundamental frequency and second harmonic is developed by insertion of one wedge prism for the phase control. The maximum output power at the wavelength of 447nm, which was generated with two long LiB3O4 crystals by noncritical phase matching, is about 1.15W with a beam quality factor of M2 of 1.05. From the experimental results, the generation of continuous wave blue light using this way with higher conversion efficiency can be achieved.

Investigations of NCPM second harmonic generation and self frequency doubling in Gd1−x−yRxNdyCa4O(BO3)3 (R = Sc or Lu) crystals

Pure and Nd3+ doped single crystals of Gd1−xRxCa4O(BO3)3 (R3+=Sc3+ or Lu3+) with large size and good quality have been grown by the Czochralski pulling technique. By changing the compositional parameter x of Gd1−xRxCa4O(BO3)3 crystals, their optical birefringence can be controlled in order to achieve non-critical phase matching (NCPM) in the ranges of 790–824nm along Y-axis and 920–963nm along Z-axis, during second harmonic generations. Nonlinear optical properties of these new biaxial borate crystals are reported. Preliminary investigations of self frequency doubling in NCPM condition in Gd1−x−yRxNdyCa4O(BO3)3 crystals have been carried out.

Second-harmonic generations of blue light in nonlinear optical crystals of Gd_1−xLu_xCa_4O(BO_3)_3 and Gd_1−xSc_xCa_4O(BO_3)_3 through noncritical phase matching

Nonlinear optical borate crystals of Gd1−xSCxCa4O(BO3)3 and Gd1−xLuxCa4O(BO3)3 have been grown and characterized. By changing the composition of these crystals, their optical birefringence can be controlled to obtain second-harmonic generations (SHG) in the ranges of 790-824 nm along the Y axis and 920-963 nm along the Z axis through noncritical phase matching (NCPM). The refractive indices of grown crystals were measured with a minimum-deviation technique. The experimental results of NCPM conditions for SHG are in good agreement with the theoretical predictions.

Monolithic wavelength converter for ultraviolet light by use of a GdxY_1-xCa_4O(BO_3)_3 crystal

Gd(x)Y(1-x)Ca4O(BO3)3 (GdYCOB) is a promising nonlinear optical crystal that shows high effective nonlinearity d(eff), noncritical phase matching, and high chemical stability. We report on the fabrication and characteristics of a monolithic wavelength converter, which generates ultraviolet light by the incidence of a 1.064 microm near-infrared laser. The converter consists of GdYCOB for third-harmonic generation, KTiOPO4 (KTP) for second-harmonic generation, and a wave plate. GdYCOB has the advantage of an extremely wide angular acceptance bandwidth, whereas KTP exhibits a high effective nonlinear coefficient and a broad temperature bandwidth. Consequently the combination of these crystals results in highly efficient and stable ultraviolet conversion for constructing a compact and robust ultraviolet laser.

Harmonic generation in bismuth triborate (BiB3O6)

We report a study of second- and third-harmonic generation in BiB3O6 (BiBO). The effective nonlinearity, phase-matching angle, acceptance bandwidth, and walk-off are calculated and analyzed within the principal planes of the optical indicatrix. In the experiment, the phase-matched harmonic generation is investigated within the xz and yz planes. Also, the temperature dependence of the noncritical phase matching for laser radiation propagating along the z axis is measured for second-harmonic generation (SHG) at crystal temperatures of 25–265^∘C. The corresponding wavelengths of the laser radiation are in the range of 1.16 to 1.34μm. In addition, SHG of 1342-nm radiation of a q-switched Nd:YVO4 laser system is investigated for noncritical and critical phase matching. The achieved conversion efficiencies are 59% and 20%, respectively. Besides SHG, third-harmonic generation (THG) of 1064-nm, ns laser pulses is investigated. The measured conversion efficiency is as high as 34%. For THG the properties of BiBO are compared with those of BBO and LBO.

Study of nonlinear-optical characteristics of AgGa1-xInxSe2 crystals

The nonlinear-optical characteristics of AgGa1-xInxSe2 crystals with x = 0.25–0.34 are studied. The experimental dependence of the wavelength of noncritical phase matching on the indium concentration is obtained upon second-harmonic generation by pumping the crystal by a CO2 laser. The quadratic nonlinear susceptibility is measured by the absolute method. The radiation resistance threshold for pulsed radiation from the CO2 laser is determined.

Optical Waveguide inGd0.275Y0.725Ca4O(BO3)3 Crystals Formed by MeV HeIon Implantation

A GdxY1−xCa4O(BO3)3 (GdxY1−xCOB)crystal, which belongs to rare-earth calcium oxyborate family, is usedfor non-critical phase-matching of visible and ultraviolet (UV) light.We employ 2.8-MeV He+ ions to form optical waveguides in theGd0.275Y0.725COB crystal at room temperature. The doses arevaried from 1.5×1016 ions/cm2 to 5×1016 ions/cm2. A model 2010 prism coupler is used to measurethe modes in Gd0.275Y0.725COB waveguides formed by MeVHe+ ion implantation. The refractive index profile is fitted basedon the reflectivity calculation method. The shape of refractive indexprofile is compared with one of nuclear energy loss obtained by TRIM'98simulation.

Source of polarization entanglement in a single periodically poled KTiOPO4 crystal with overlapping emission cones

We present a simple polarization entanglemenet source based on type-II phase-matched parametric down-conversion in periodically poled KTiOPO4. By use of noncritical phase matching in a noncollinear geometry the single-crystal source emits a cone of polarization-entangled photons. Two beams on opposite sides of the cone are selected for measurements to yield an observed flux of 820 pairs/s per mW of pump power with a two-photon fringe visibility of 96%. With this source we measure a violation of Bell's inequality of 140 standard deviations in 160 s.

Noncritical phase matching of Nb:KTP crystal for blue light generation

The relationships between the temperatures and the principal refractive indices of 7.5 mol% Nb:KTP crystal is achieved from the measured refractive indices. The cutoff wavelengths of noncritical phase matching at different temperatures are calculated. Compared with the KNbO3 crystal, the Nb:KTP crystal shows the better performance for generating the blue light.

Broadband generation of terahertz radiation in a waveguide.

We report a novel geometry that allows for the phase-matched generation of broadband terahertz radiation in a polymer-based parallel-plate metal waveguide by means of optical rectification. Both the optical pump beam and the generated terahertz radiation propagate in the fundamental mode of the waveguide. This allows for noncritical phase matching over a broad range of terahertz frequencies. We demonstrate guided-wave interaction lengths of up to 3 mm.

Ca4REO(BO3)3 crystals for green and blue microchip laser generation: from crystal growth to laser and nonlinear optical properties

We have taken advantage of congruent melting behavior of the nonlinear rare-earth oxoborate Ca4REO(BO3)3 family to perfect a process of collective fabrication of self-frequency doubling microchip laser based on Nd:GdCOB (Ca4Gd1−xNdxO(BO3)3) crystals. The process goes from Czochralski boule to 1 × 3 mm2 chips perfectly oriented (better than 0.1°) to the phase matching direction (θ=90°, φ=46°) in the XY principal plane, with dielectric mirrors directly deposited on both faces of the chips. 20 mW of self-frequency doubling output power at 530 nm was performed under 800 mW of diode laser as incident pump power at 812 nm. In addition, new compositions from the solid solution Ca4Gd1−xYxO(BO3)3 (Gd1−xYxCOB) (x=0.13, 0.16, 0.44) have been grown by the Czochralski pulling method, in order to achieve noncritical phase matching (NCPM) second harmonic generation of 4F3/2 → 4I9/2 Nd3+ doped laser hosts. Three types of laser wavelengths have been chosen: Nd:YAP (YAlO3) at 930 nm, Nd:YAG (Y3Al5O12) at 946 nm, and Nd:ASL (NdySr1−x Lax−yMgx Al12−xO19) at 900 nm. Angular acceptance measurements of these three types of compositions present very large values, compared to pure GdCOB or YCOB oriented in critical phase matching configurations.

Generation of 22.8 mW single-frequency green light by frequency doubling of a 50-mW diode laser

We report frequency doubling of an extended-cavity diode laser in an a-cut KTP crystal. Continuous-wave, single-mode green light at 540 nm with a power of 22.8 mW was generated from an input of 44.2 mW by type II non-critical phase matching. Stable operation with intensity fluctuations smaller than 1 % peak-to-peak for more than two hours was achieved by employing resonant optical feedback. The optical feedback also reduced the fundamental-wave linewidth to the upper limit of 41 kHz.

Monolithic Wavelength Converter for Ultraviolet Light Using GdYCOB Crystal

GdYCOB is a promising nonlinear optical crystal that possesses characteristics of an effective nonlinear coefficient, noncritical phase matching operation, and high chemical stability. In this paper, we report about fabrication and characteristics of a monolithic wavelength converter, which generates ultraviolet light by the incidence of near infrared laser. The converter consists of GdYCOB for third-harmonic generation, KTP for second-harmonic generation, and a wave plate for polarization-control. These constituents are integrated with an optical contact bonding. The extreme downsizing of ultraviolet generator is possible with this converter as compared to the conventional method. GdYCOB has the advantage of wide angular bandwidth whereas KTP exhibits wide temperature bandwidth. Consequently, the combination of these crystals results in highly efficient and stable ultraviolet conversion. The converter does not necessitate any high-temperature curing or desiccators since the constituent crystals are non-hygroscopic. Thus the monolithic wavelength converter enables a very compact and highly reliable ultraviolet laser.

Quadratic soliton collisions

The details of two soliton collision processes were investigated in detail in a 1 cm long periodically poled KTP crystal for the case when the solitons were excited by inputting only the fundamental beam. The effects on the collision outcomes of the distance of the collision into the sample, collision angle and phase mismatch were measured for different relative phases between the input beams. At small angles (around 0.4 degrees ) fusion, repulsion and energy transfer processes were observed, while at the collision angles approaching 3.2 degrees the two output soliton beams were essentially unaffected by the interaction. The phase mismatch was varied from 3.5 to -1.5pi for the 0.4 degrees collision angle case. The output solitons separation at pi input phase difference showed strongly asymmetric behavior with phase mismatch. In general, the measurements indicate a decrease in the interaction strength with increasing phase mismatch. All collision processes were performed in the vicinity of a non-critical phase matching.

Non-critical phase-matched second harmonic generation in GdxY1-xCOB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut GdxY1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd0.28Y0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser.

Noncritically phase-matched second-harmonic generation in cesium lithium borate.

Efficient generation of 236-nm light was demonstrated by use of noncritically phase-matched second-harmonic generation in cesium lithium borate. Noncritical phase matching provided approximately 20x the nonlinear drive for second-harmonic generation than beta-barium borate for 236-nm generation. The 236-nm wavelength is the fourth harmonic of a 946-nm Nd:YAG laser. Phase matching was accomplished at a crystal temperature of -15 degrees C.

Synchronously pumped femtosecond optical parametric oscillator based on an improved pumping concept

We present an improved pumping concept for a synchronously pumped optical parametric oscillator (OPO). The validity of this new concept has been verified by an experiment of a synchronously pumped femtosecond OPO based on periodically poled LiNbO 3 (PPLN). With this new concept, not only can we obtain high harmonic repetition rate from an OPO, but also we can design the OPO with one more freedom, that is to choose the cavity length of the OPO. We also propose a simple method to compensate for intracavity group velocity dispersion (GVD) in a synchronously pumped ring-cavity PPLN OPO at the telecom wavelengths (1.3–1.6 μm), that is to insert a piece of type-I noncritical phase-matching (NCPM) lithium tri-borate (LBO) ( θ=90°, φ=0°) in the ring cavity.

Two-dimensional type I quadratic spatial solitons in KNbO_3 near noncritical phase matching

Experiments on the properties of quadratic, two-dimensional spatial soliton properties in KNbO(3) near and at noncritical phase matching (NCPM) are reported. The NCPM geometry leads to unique features such as a large angular bandwidth for soliton generation, weak dependence of soliton composition on intensity and incidence angle, and unique multisoliton-generation properties.