David L. Veasey

Arrays of distributed-Bragg-reflector waveguide lasers at 1536 nm in Yb/Er codoped phosphate glass

We have demonstrated an array of monolithic, single-frequency-distributed-Bragg-reflector (DBR), waveguide lasers operating near 1536 nm wavelengths. The lasers were fabricated by forming waveguides in Yb/Er-codoped phosphate glass by ion exchange. The slope efficiency for each laser as a function of launched pump power is 26% and the thresholds occur at 50 mW of launched pump power. An output power of 80 mW was achieved with 350 mW of coupled pump power. Each laser exhibits stable operation on a single longitudinal mode and all have linewidths less than 500 kHz. A comb of waveguides with varying effective indices allows the selection of wavelength using a single-period grating.

Ion-exchanged waveguide lasers in Er 3+ /Yb 3+ codoped silicate glass

We investigated an Er(3+)/Yb(3+) codoped silicate glass as a host material for waveguide lasers operating near 1.5 microm. Spectroscopic properties of the glass are reported. Waveguide lasers were fabricated by K(+)-ion exchange from a nitrate melt. The waveguides support a single transverse mode at 1.5 microm. An investigation of the laser performance as a function of the Yb:Er ratio was performed, indicating an optimal ratio of approximately 5:1. Slope efficiencies of as great as 6.5% and output powers as high as 19.6 mW at 1.54 microm were realized. The experimental results are compared with a waveguide laser model that is used to extract the Er(3+) upconversion coefficients and the Yb(3+)-Er(3+) cross-relaxation coefficients. The results indicate the possibility of obtaining high-performance waveguide lasers from a durable silicate host glass.

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.

Hybrid glass substrates for waveguide device manufacture

Hybrid glass substrates were prepared by a novel, low-temperature process joining active (Er-Yb codoped) and passive phosphate glass. The resulting hybrid substrates are chemically and physically robust; they can be cut, ground, and polished by conventional, water-based techniques. The entire substrate can be immersed in a molten-salt bath to produce waveguides simultaneously in the active and passive regions. A low reflectance of -34+/-2 dB was measured at the joint interface with 1531.2-nm light by optical low-coherence reflectometry. Further, a hybrid laser waveguide device exhibited a slope efficiency of 33% at 1540 nm when pumped at 975 nm.

High-power waveguide lasers

During the past few years high power lasers which incorporate intracavity optical waveguiding have been demonstratedin a number of different geometric formats. These include rectangular planar waveguide structures, two-dimensional multielement waveguide array lasers and annular waveguide devices, all of which dependcrucially on the operational flexibility of the transverse radiofrequency excitation technique. Here, we review the fundamental issues which underlie the attractions of the use of waveguiding structures in the design and construction of ultracompact, diffusion-cooledlasers which are efficient and operate at high average power levels. In particular, we review the properties oflarge area discharge planar waveguide C02/CO lasers, where multi-kilowatt cw power levels have been demonstrated with excellent beam quality and efficiency. It is shown that similar concepts may also be applied to solid state lasers. In addition, theuse of the multi-element array concept for high power scaling will be examined, and the operating characteristics ofan ultra-compact 64 (4 x 16) element array laser operating at 2kW cw output power will be described.

Waveguide polarizers with hydrogenated amorphous silicon claddings.

We have fabricated TE- and TM-pass waveguide polarizers with polarization isolations of 42 and 35 dB, respectively. The devices were fabricated by the growth of hydrogenated amorphous silicon claddings on K(+)-Na(+) ionexchanged channel waveguides in glass. Cladding thicknesses were accurately tuned to permit optimum coupling of either a TE or a TM mode to the cladding. We have also demonstrated that a waveguide polarizer attenuation as high as 760 dB/cm can be measured by using a photothermal deflection technique.

Waveguide lasers by ion-exchange in Er-doped glass

Erbium and erbium/ytterbium co-doped silicate glass waveguide lasers have been fabricated by silver ion-exchange and their characteristics analyzed. We report on measurements and comparisons made in the lasing properties of these devices, including threshold, slope-efficiencies and pump tuning ranges. The results presented show that through proper choice of host glass, it is possible to make low-threshold lasers both in singly and co-doped devices.

Rigorous scalar modeling of Er- and Er/Yb-doped waveguide lasers

A rigorous scalar model for predicting the characteristics of rare-earth-doped waveguide lasers has been developed. The model consists of two nonhomogeneous wave equations: one for the forward-propagating laser signal power, the other for the backward-propagating laser signal. These equations are coupled with one forward-propagating, nonhomogeneous wave equation representing the pump signal. The three wave equations are coupled with the space dependent laser rate equations to form a system of time dependent differential equations. This large system of equations is solved, using appropriate initial and boundary conditions, by the method of lines using collocation for the spatial approximation. The solutions to this system yield data which predict the time and position-dependent laser signal power, pump power, and population densities in a waveguide laser cavity supporting an arbitrary guided mode. The assumptions made in this new model are that the transverse field maintains the same shape as a function of longitudinal position in the laser cavity and that the effects of spatial hole burning and standing waves are neglected. We have used this model to predict continuous wave and Q-switched laser performance for Er an Er/Yb-doped lasers. We have achieved favorable comparisons with actual laboratory operation of cw Yb/Er-co- doped waveguide lasers. Results from simulations of Er-doped and Yb/Er-doped Q-switched lasers are presented which show that high peak powers on the order of 500 W and 1 ns pulse widths can be achieved.

Integrated optical polarization-discriminating receiver in glass

We have successfully demonstrated an integrated optical TE-TM mode discriminator using waveguide polarizers and guided-wave photodetectors for use in polarimetric optical sensor and positioning systems. The photonic integrated circuit consists of a Y-branch waveguide splitter formed by potassium-sodium ion exchange in silicate glass. Hydrogenated amorphous silicon claddings were deposited on each branch of the splitter to act as polarizers. One output cladding was trimmed to a thickness which attenuated the TE polarization, while the other cladding was trimmed to attenuate light having TM polarization. The thickness trimming was accomplished using a process of localized plasma etching which allows in situ extinction optimization by monitoring transmitted light. Optical extinction ratios of up to 27 dB were demonstrated on Y-branch waveguides for polarizers with claddings 1.2 mm in length. The integrated receiver was completed with the deposition of metal-semiconductor-metal photodetectors on each of the output waveguide branches following the polarizers. Amorphous silicon claddings were contacted with chrome-gold interdigitated Schottky contacts to form the waveguide detectors.

Glasses for waveguide lasers

Waveguide lasers formed by ion exchange in rare-earth-doped glasses have emerged as an attractive new technology on the threshold of commercial insertion. These devices can be used as both laser oscillators and optical amplifiers. In this article, we review ion exchange and glass composition. We then discuss the performance of ion-exchanged waveguide lasers made in silicate and phosphate glasses.