J. A. Aust

Z‐propagating waveguide lasers in rare‐earth‐doped Ti:LiNbO3

A means of reproducibly fabricating stable cw lasers in rare‐earth‐doped Ti:LiNbO3 has been demonstrated through judicious choice of waveguide orientation. Z‐propagating waveguides have been fabricated in Nd‐ and Er‐diffused Ti:LiNbO3 and room‐temperature laser operation with greatly reduced photorefractive instability has been obtained. The reduced photorefractive damage susceptibility in this waveguide configuration has led to the realization of a 980 nm pumped laser in Er:Ti:LiNbO3, with a threshold of 10.5 mW of absorbed pump power and a slope efficiency of 8.5%.

Time-dependent modeling of erbium-doped waveguide lasers in lithium niobate pumped at 980 and 1480 nm

We have developed a rigorous phenomenological model for analyzing rare-earth doped waveguide lasers. The model is based on time-dependent laser rate equations for an arbitrary rare-earth-doped laser host with multiple energy levels. The rate equations are coupled with the laser signal and pump photon flux equations that have time-dependent boundary conditions. The formulation results in a large and stiff set of transcendental and coupled differential equations that are solved using finite difference discretization and the method of lines. Solutions for the laser signal power, pump power, and populations of ion energy levels as functions of space and time are obtained for waveguide lasers. We have used the model to predict the CW characteristics and Q-switched performance of waveguide lasers in lithium niobate pumped by a 980-nm source. Our analysis shows that hole burning can occur in erbium-doped lithium niobate lasers because of the intensity variation across guided transverse modes. We have predicted that Q-switch pulse peak powers can exceed 1 kW with pulsewidths less than 1 ns. Moreover, we have compared the CW and Q-switched performance of 980-nm pumped waveguide lasers and 1480 nm pumped waveguide lasers. An analysis of the effects of host- and fabrication-dependent parameters on CW 980-nm pumped lasers is included. These parameters include cooperative upconversion, excited state absorption, doping concentration, excess waveguide loss, cavity length, and mirror reflectance values. We demonstrate good quantitative agreement with waveguide laser experimental data obtained in our laboratory and with results from the literature.

Imaging of domain-inverted gratings in LiNbO3 by electrostatic force microscopy

Ferroelectric domains in LiNbO3 have been investigated by means of electrostatic force microscopy. Polarization-inverted gratings with 4 μm periodicity were fabricated by titanium diffusion into both +c and −c faces of single-domain LiNbO3 crystals. The distribution of the electric field in the vicinity of the sample surface was measured using scanning probe microscopy. The electrostatic force image was found to correlate with the shape of the domain-inverted profile observed by scanning electron and optical microscopies.

Nd:LiTaO 3 waveguide laser

Waveguide lasers operating near 1092 and 1076 nm were fabricated in Z-cut Nd-Ti codiffused LiTaO(3). The Nd diffusion was at 14000 degrees C for 120 h. Samples from two wafers were examined. The Nd film starting thickness was 7 nm in wafer 1 and 15 nm in wafer 2. Ti stripes, 8-15 microm wide, were diffused at 1500 degrees C for 4 h for wafer 1 (130-nm stripe thickness) and 2 h forwafer 2 (100-nm stripe thickness). Pumping was at 750 nm. Threshold occurred at 330 mW of absorbedpump power for the best waveguides from wafer 1 and100 mW for the best waveguides from wafer 2. The slope efficiency of the latter was 0.07%.

Passively Q-switched Nd-doped waveguide laser.

A passively Q-switched waveguide laser operating at 1.054 μm has been demonstrated in a Nd-doped phosphate glass. The channel waveguide was fabricated by K-ion exchange from a nitrate melt. Passively Q-switched pulses were achieved by placement of an acetate sheet containing an organic saturable-absorbing dye within the laser cavity. The resulting pulse train consisted of pulses with a FWHM of ~25 ns and peak powers of 3.04 W. With an 80% transmitting output coupler, cw operation of the laser provided 5.2 mW of output power at 1.054 μm for 229 mW of absorbed 794-nm pump power.

Crystal growth, characterization, and domain studies in lithium niobate and lithium tantalate ferroelectrics

Publisher Summary This chapter discusses the crystal structure and growth of noncongruent single crystals of lithium niobate and lithium tantalate using the double crucible Czochralski technique. The chapter focuses on the phenomena of ferroelectric domains and discusses ferroelectric hysteresis, internal fields, the role of nonstoichiometry in domain reversal, the structure of a domain wall and its interaction with lattice defects, and the phenomenological modeling of domain-wall structure in these materials. Detailed experiment and theory of using Maker fringe analysis as a characterization tool for wafer thickness, composition, strain, and internal field variations over an entire 2–3-in wafer of LiNb03 or LiTa03 are also discussed. From a technological viewpoint, the microengineering of ferroelectric domains for integrated optical applications also benefited from a real-time and nondestructive monitoring of the domain-engineering process during device fabrication.

Nonlinear optical characterization of LiNbO 3 . I. Theoretical analysis of Maker fringe patterns for x-cut wafers

Maker fringe analysis was adapted to x-cut LiNbO3 wafers to examine variations in birefringence, thickness, and photoelastic strain. The pump beam was polarized parallel to the crystalline y axis and produced e- and o-polarized Maker fringes, owing to d31 and d22, respectively, by rotation of the sample about the y axis. Fitting our model to the o-polarized data enabled computation of the sample thickness to an uncertainty of approximately ±0.01 μm. The accuracy was limited by an implicit ±2×10-4 uncertainty in no that exists in the commonly used Sellmeier equation of G. J. Edwards M. Lawrence , Opt. Quantum Electron. 16, 373 (1984). For a pump wavelength λp=1064 nm, fitting the model to the e-polarized fringes revealed that ne at 532 nm deviated from the Sellmeier result by typically -1.58×10-4. The uniformity of ne over a wafer 10 cm in diameter was approximately ±4×10-5. This result is consistent with that expected from compositional variations. Our model included multiple passes of the pump and second-harmonic waves. The effects of photoelastic strain in producing perturbations and mixing of the e- and o-polarized fringes was investigated. This was restricted to two experimentally motivated cases that suggested that strains produce rotations of the optic axis by typically ±0.05° about the x axis and y axis with the former assigned to an indeterminant combination of S1,S2, and S4 and the latter to an indeterminant combination of S5 and S6. In both cases the magnitude of the collective strains is of the order of 10-4. The birefringence variations that are due to strain are of the same magnitude as those expected from compositional variations. The formalism developed here is used in the subsequent mapping study of x-cut wafers.

Linewidth narrowing in an imbalanced Y-branch waveguide laser.

A Y-branch channel waveguide laser whose branch segments were mismatched in length by 2.4% was fabricated by electric-field-assisted ion exchange in Nd-doped, mixed alkali–silicate glass. The laser output wavelength was centered at 1057.3 nm, and the linewidth was 0.4 nm FWHM. Our similarly fabricated single-channel Fabry–Perot lasers and balanced Y-branch lasers display linewidths of 3–4 nm. Pumping was performed with a cw Ti:sapphire laser operating at 785 nm. The imbalanced Y-branch laser reached threshold with an absorbed pump power of 48 mW when a 2% transmitting output coupler was used. The slope efficiency was 2%. An extended cavity was used to imbalance the arms in a second laser by a ratio of 2.8:1. This device displayed a linewidth of approximately 3.7 GHz FWHM. The linewidth narrowing of these coupled-cavity lasers is analogous to that seen in a Michelson laser.

Maker fringe analysis of z-cut lithium niobate

Maker fringe (MF) analysis is the technique of examining the oscillatory SHG intensity as a function of pump angle of incidence. MF analysis was performed on a series of z-cut LiNbO/sub 3/ samples that were approximately 200 /spl mu/m thick.

Examination of domain-reversed layers in Z-cut LiNbO/sub 3/ using maker fringe analysis, atomic force microscopy, and high-resolution X-ray diffraction imaging

Domain-engineering of LiNbO3 is of importance for the fabrication of efficient cw and pulsed second-harmonic generators and optical parametric oscillators.