G. A. Magel

Quasi-phase-matched second harmonic generation: tuning and tolerances

The theory of quasi-phase-matched second-harmonic generation is presented in both the space domain and the wave vector mismatch domain. Departures from ideal quasi-phase matching in periodicity, wavelength, angle of propagation, and temperature are examined to determine the tuning properties and acceptance bandwidths for second-harmonic generation in periodic structures. Numerical examples are tabulated for periodically poled lithium niobate. Various types of errors in the periodicity of these structures are then analyzed to find their effects on the conversion efficiency and on the shape of the tuning curve. This analysis is useful for establishing fabrication tolerances for practical quasi-phase-matched devices. A method of designing structures having desired phase-matching tuning curve shapes is also described. The method makes use of varying domain lengths to establish a varying effective nonlinear coefficient along the interaction length. >

Laser‐heated miniature pedestal growth apparatus for single‐crystal optical fibers

We have designed and built a single‐crystal fiber growth apparatus. The apparatus employs novel optical, mechanical, and electronic control systems that enable the growth of high optical quality single‐crystal fibers. We have grown oriented single‐crystal fibers of four refractory oxide materials, Al2O3, Cr:Al2O3, Nd:YAG, and LiNbO3. These materials exhibit similar growth characteristics and yield fibers of comparable quality. Fibers as small as 20 μm in diameter and as long as 20 cm have been grown. Measured optical losses at 1.06 μm for a 10‐cm‐long, 170‐μm‐diam Cr:Al2O3 fiber were 0.074 dB/cm.

Quasi‐phase‐matched second‐harmonic generation of blue light in periodically poled LiNbO3

LiNbO3 crystals with periodically alternating ferroelectric domains have been produced using laser‐heated pedestal growth. Domain thicknesses as small as 1 μm have been achieved. This material was applied to room‐temperature, quasi‐phase‐matched frequency doubling to generate light at wavelengths as short as 407 nm, using the d33 and d22 nonlinear coefficients. The measured conversion efficiencies and wavelength and temperature tuning bandwidths are consistent with an effective interaction length of ≊320 μm (>230 domains). An initial test with high‐intensity focused blue beams showed that the periodically poled material exhibits no discernible photorefractive damage effect.

Periodically poled LiNbO3 for high‐efficiency second‐harmonic generation

Quasi‐phase‐matched room‐temperature frequency doubling to generate blue, green, and red light was demonstrated using periodically poled LiNbO3 crystals. A 1.24‐mm‐long sample in an external resonant cavity generated 1.7 W of green power from an input of 4.2 W at the 1.064 μm Nd:YAG laser line when heated to 140 °C to compensate for a slight error in periodicity.

Quasi-Phase-Matched Interactions In Lithium Niobate

Phase-matching nonlinear interactions by periodic variations in the nonlinear susceptibility, quasi-phase-matching, offers advantages in accessible tuning range and in choice of nonlinear coefficients over the conventional birefringent technique. Periodically reversed ferroelectric domains can be used to create monolithic structures with the necessary high-spatial-frequency variations in the nonlinear susceptibility. We present two techniques for the fabrication of periodically-poled lithium niobate crystals, and results for bulk and guided-wave second harmonic generation of blue and green light.

High-speed high-resolution fiber diameter variation measurement system

A fiber diameter variation measurement system is described which is capable of measuring transparent fibers with 0.02% diameter resolution and 6-microm axial resolution at a measurement rate of 1 kHz and with a working distance of >100 mm. The principles of its operation are discussed in detail, and experimental confirmation of its performance is reported. A theoretical calculation of the optimum obtainable diameter resolution for a given set of experimental parameters is also presented.

Low-Loss Single-Crystal Sapphire Optical Fibers

Single-crystal sapphire (α-Aℓ203) fibers are potentially useful in a wide variety of optical applications, particularly those involving high-power or high temperature light-guiding, over the wavelength range from 0.24 μm in the ultraviolet to 4.0 μm in the mid-infrared. These fibers are routinely grown at rates of up to 8 mm/min, and with diameter stability of better than 0.5% rms under feedback control. Recent measurements on unclad 150 μm diameter fibers show fundamental mode scattering losses of about 0.3 dB/m in the visible and less than 0.07 dB/m at 3.39 μm.

Laser Assisted Growth Of Optical Quality Single Crystal Fibers

Single crystal fibers of four refractory oxide materials (Aℓ203 , Cr:AZ203 , Nd:YAG and LiNb03) have been grown by a miniature pedestal growth technique. The growth apparatus employs novel electronic control, mechanical and optical systems enabling growth of high optical quality fibers. All four materials exhibit similar growth characteristics and yield fibers of comparable quality. Measured optical waveguide losses at 632.8 nm for a 5 cm long 170 μm diameter CrℓAZ203 fiber were 0.04 dB/cm.

Nonlinear Optics in Single Crystal Fibers

Since the advent of low loss optical fibers fifteen years ago, considerable research effort has been directed towards the study of nonlinear interactions in fibers. A variety of devices have taken advantage of the combination of transverse confinement and long interaction lengths available in glass fibers to operate efficiently at relatively low pump powers. Because glasses are inherently centro-symmetric, only third-order nonlinear processes, e.g. Raman1 and Brillouin2 scattering, optical Kerr effect,3 self-phase modulation,4 or extremely weak quadrupole second order processes5 are allowed. Thus, the combination of fiber geometry and the second order susceptibility of non-centro-symmetric single crystals would open the door to a broad range of nonlinear applications not possible in glass fibers.