Yuk Tak Chow

Characterization of the laser crystal Nd:GdVO 4

Nd:GdVO4 crystal was grown by the Czochralski method. The absorption and fluorescence spectra of the crystal were measured at room temperature. The thermal expansion and the specific heat of the crystal were also measured. Laser outputs at 1.06 and 1.34 µm were achieved when a Nd:GdVO4 crystal sample with a high Nd doping concentration was pumped by a low-power laser diode (LD) at 808.5 nm, and visible green and red laser outputs of intracavity frequency doubling at 0.53 and 0.67 µm were also achieved when nonlinear KTiOPO4 and LiB3O5 crystal, respectively, were used. The highly Nd-doped Nd:GdVO4 crystal was pumped by a high-power LD, and a greater than 5-W laser output power at 1.06 µm was obtained. A low-Nd-doping concentration Nd:GdVO4 crystal sample was pumped by a high-power LD, yielding laser output powers at 1.06 and 0.53 µm; 0.53-µm green laser output was obtained when a KTiOPO4 crystal was used, and the output beam’s values were M2=1.76 at an output power of 14.3 W at 1.06 µm and M2=1.55 at an output power of 3.3 W at 0.53 µm. Acousto-optical Q-switched laser outputs at 1.06 and 0.53 µm were also achieved. A thermal lens made from a Nd:GdVO4 crystal was measured; it was weaker than that of a Nd:YVO4 crystal. Some important material parameters, such as temperature-induced changes in refractive index, material constant, thermal-stress resistance figure of merit, and power per unit length at the stress fracture limit, have been estimated.

Thermal and laser properties of Nd:YVO4 crystal

Nd:YVO 4 crystal has been grown by Czochralski method. The data of thermal expansion and specific heat have been measured. The thermal expansion coefficients along a- and c-axis are α 1 = 2.2 × 10 -6 /K, and α 3 = 8.4 × 10 -6 /K respectively. The specific heat is 24.6 cal/mol K at 330 K. The large anisotropy along c- and a-axis of thermal expansion coefficients is explained by the structure of YVO 4 crystal. 921 mW output laser at 1.06 μm has been obtained with a 3 mm × 3 mm × 1 mm crystal sample when pumped by 1840 mW cw laser diode, and the slope efficiency is 55.5%.

Growth, spectra and influence of annealing effect on laser properties of Nd:YVO4 crystal

Abstract Nd:YVO 4 single crystal has been grown by the Czochralski method. The effective segregation coefficients k eff of Nd, Y and V elements have been measured for crystals with different neodymium-doped concentrations. The absorption and fluorescence spectra of Nd:YVO 4 crystal have been measured. The lasers at 1.06 μm have been demonstrated when the crystal samples of annealed and non-annealed Nd:YVO 4 are pumped by laser diode (LD). The influence of annealing effect on laser properties is discussed.

Efficient self-frequency doubling of Nd:GdCOB crystal by type-I phase matching out of its principal planes

Abstract We report on the efficient self-frequency doubling of Nd:GdCOB crystal by type-I phase-matching at θ=66.3° and ϕ=134.4°, which is out of its principal planes. A CW output power of 225 mW at 530.5 nm is obtained at an absorbed pump power of 1.56 W with an optical conversion efficiency of 14.4% and an absorbed threshold pump power of 1.8 mW. To our knowledge, this is the highest output power, the highest efficiency and the lowest threshold obtained from a self-frequency-doubled Nd:GdCOB laser to date.

Experimental demonstration of intermodal dispersion in a two-core optical fibre

Abstract The recent prediction that intermodal dispersion could affect the propagation of short pulses in a two-core optical fibre was confirmed experimentally. A picosecond pulse at 1.548 μm launched into one core of a meters-long two-core fibre was found to come out of either core of the fibre as two temporally separate pulses. By measuring the time delay between these two pulses, the intermodal dispersion in the fibre was estimated to be 1.13 ps/m, in good agreement with theory.

Characterization and modeling of Bragg gratings written in polymer fiber for use as filters in the THz region

We demonstrate fiber Bragg gratings written in polymer fiber for use in the THz window for the first time. A KrF excimer laser operating at 248 nm was used to inscribe notch-type gratings in single component Topas subwavelength fiber. A transmission loss at the centre wavelength of the grating of 60 dB is observed in short gratings containing only 192 notches. Experimental results and modeling are presented. The gratings are expected to find use in THz signal filtering and chemical or biosensing applications.

A comparative study of crystal growth and laser properties of Nd:YVO4, Nd: GdVO4 and Nd :GdxLa1-xVO4(x = 0.80, 0.60, 0.45) crystals

Abstract The crystal growth of vanadate crystals is realised. Nd:YVO 4 , Nd:GdVO 4 and Nd:Gd x La 1− x VO 4 ( x =0.80, 0.60, 0.45) are determined effective segregation coefficients and laser properties of these crystals. All the crystals have the zircon structure with the space group of I 4 1 /amd. We find the Nd:YVO 4 crystal is the easiest one to grow among these crystals, the laser and thermal properties of Nd:GdVO 4 are the best, and Nd:Gd x La 1− x VO 4 ( x =0.80, 0.60, 0.45) crystals are the most difficult to be grown among the vanadate crystals. Under the pumping power of a 3 W laser diode, output powers of 1.27 W at 1.06 μm and 800 mW at 1.34 μm for the Nd:GdVO 4 crystal, 1.14 W at 1.06 μm and 780 mW at 1.34 μm for the Nd:YVO 4 crystal, and 1.18 W at 1.06 μm and 671 mW at 1.34 μm for the Nd:Gd 0.8 La 0.2 VO 4 crystal were obtained, and intracavity frequency-doubling to 532 and 670 nm was also demonstrated using KTiOPO 4 and LiB 3 O 5 crystals.

Refractive-index profiling of graded-index planar waveguides from effective indexes measured for both mode types and at different wavelengths

It is well known that, under the WKB approximation, the refractive-index profile of a graded-index planar waveguide can be calculated from the corresponding effective-index function, which can be found approximately by least-squares fitting of a set of measured effective indexes. In the previous work, effective indexes measured for a particular type of modes (the TE or TM modes) at a single wavelength are used. This requires that the waveguide supports at least three modes of the same type. In this paper, techniques of combining the effective indexes measured for both mode types and at different wavelengths are proposed and demonstrated with examples. With these techniques, single-mode and two-mode waveguides can be profiled.

Generation of phase-only Fresnel hologram based on down-sampling

We present a novel non-iterative method for generating phase-only Fresnel holograms. The intensity image of the source object scene is first down-sampled with uniform grid-cross lattices. A Fresnel hologram is then generated from the intensity and the depth information of the sampled object points. Subsequently, only the phase component of the hologram is preserved, resulting in a pure phase hologram that we call the sampled-phase-only hologram (SPOH). Experimental evaluation reveals that the numerical, as well as the optical reconstructed images of the proposed phase-only hologram derived with our method are of high visual quality. Moreover, the reconstructed optical image is brighter, and less affected by phase noise contamination on the hologram as compared with those generated with existing error-diffusion approaches.

Growth and laser properties of Nd:Ca4YO(BO3)3 crystal

Abstract Nd:Ca 4 YO(BO 3 ) 3 (Nd:YCOB) crystal was grown by the Czochralski method, and its structure was measured by using a four circle X-ray diffractometer. The transparent spectrum from 200 to 2600 nm was measured at room temperature. The fluorescence spectrum near 1.06 μm showed that the main emission wavelength of Nd:YCOB crystal was centered at 1060.8 nm. Laser output at 1.06 μm has been demonstrated when it was pumped by a Ti:sapphire laser at the wavelength of 794 nm, the highest output power was 68 mW under pumping power of 311 mW, the pumping threshold was 163 mW and slope efficiency was 46.9%. The self-frequency doubled green light has been observed when it was pumped by a Ti:sapphire or a laser diode (LD). A 14.5 mm Nd:YCOB crystal sample cut at ( θ , φ )=(90°, 33°) was used for type I second-frequency generation (SHG) of the 1.06 μm laser pulse. The SHG conversion efficiency was 22%.