Lateral-mode discrimination by misaligned total internal reflection (TIR) mirrors in multimode ridge waveguide structures is proposed. The effects of angular misalignment in TIR mirrors and index steps in ridge waveguides on the modal reflectivity is analyzed by a plane wave expansion method. The modal reflectivity of a high order mode decreases more rapidly than that of a fundamental mode as the angular misalignment of TIR mirrors increases. It is found that a fundamental mode can have high modal reflectivity (/spl sim/0.9) with significant lateral-mode discrimination over high-order modes if the ridge waveguides have large index steps. >

Lateral-mode discrimination in ridge waveguides by misaligned total internal reflection mirrors

A NEW APPROACH to the electrooptical gyroscope, based on a semiconductor ring laser, has recently been proposed [1]. This solution allows for a very compact realization, much smaller than the well-known RLG (Ring Laser Gyro) and FOG (Fiber Optic Gyro) [2, 3], and should offer a better sensitivity with respect to other recently proposed small-sized gyroscopes, i.e. the passive integrated optical gyro [4] and the micromachined gyro [5]. Recent research work [6] has gained a deep insight into the design and fabrication of semiconductor ring lasers for optical communications, which now appear as mature areas. The application of the semiconductor ring laser as a gyroscope requires particular features, such as large enclosed area, narrow linewidth, single-mode operation [1], not yet investigated in a systematic way. In the present work we report progress on the design, fabrication and characterization of large-radius ring lasers and of a prototype version of the integrated ring laser gyroscope (IRLG), for which the functions of counter-propagating mode recombination and of signal detection are integrated on the same substrate. 2. Technology

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Progress on the GaAlAs Ring Laser Gyroscope

The interest in semiconductor ring lasers (SRLs) has been steadily growing in the last few years because of several unique properties such as ultrafast directional bistability, stable single mode operation and potential for integration. However, most of the mode dynamical behavior as well as the optimum device design are still far from a complete understanding. This thesis reports on the design, technological development and characterization of SRLs emitting at 1.55 um, which are monolithically integrated with a number of other optical elements such as tunable couplers, optical amplifiers, Bragg reflectors and distributed feedback lasers (DFBs). A detailed analysis on the device design is presented with particular emphasis on its robustness with respect to fabrication tolerances and to the optical feedback from the output waveguides. The complete processing technology is developed with a focus on selective dry etching to achieve very accurate control of the waveguide bending losses. Three completely novel and monolithically integrated SRL devices are fabricated and characterized. The first is a master-slave device based on the monolithic integration of an SRL with a DFB that shows highly efficient cavity enhanced four-wave mixing up to detuning frequencies of 1.5 THz. In a second geometry, a Bragg reflector defined on one of the output waveguides selects the lasing mode of the SRL. The device shows world-record wavelength switching speeds as low as 450 ps and strong immunity to thermal fluctuations of the grating. The
third device is an SRL with tunable couplers for active Q-switching applications. Pulses as short as 120 ps at a repetition rate of 1.8 GHz are obtained by injecting
only a few mA of current into the tuning section.

Monolithic integration of semiconductor ring lasers

The complete analytic solution to the problem of coupling between a slab waveguide and a ring has been derived. The two-configuration problem is reduced to two simultaneous single-configuration problems by the method of equivalence. These are independently solved to yield the exact solution in terms of modal content. The results are illustrated with a numerical example and corroborated independently by a variational approach.

Coupling of a planar waveguide and a dielectric disk [laser resonator]

We present the first analysis of the mode structure and thresholds of index-guided ring lasers, and on longitudinal mode discrimination of X-junction-coupled ring lasers. Submilliampere thresholds can be achieved with single-lateral-mode ring lasers having small refractive index steps. In addition, strong longitudinal mode discrimination can be obtained with ring lasers of different cavity lengths coupled via an X junction. >

Lateral and longitudinal mode discrimination in index-guided circular ring semiconductor lasers

Circular ring semiconductor lasers are promising sources of coherent light for integrated optoelectronics. There have been a number advances in these lasers [1-3]. We believe that the structures with relatively smal index step (Δn=0.1-0.2), for instance ridge- waveguide of buried heterostructure ring lasers, will have better performance characteristics than the devices with deep etching through active region. We present in this paper a WKB analysis of mode structure and thresholds of ridge-waveguide semiconductor lasers, and coupled-mode theory of radially coupled dual ring lasers. We show strong suppression of higher-order lateral modes due to leakage. We show also that radial coupling of concentric two ring lasers results in discrimination of longitudinal modes.

R. F. Nabiev

Papers

Lateral and longitudinal mode discrimination in index-guided circular ring semiconductor lasers

Lateral and Longitudinal Mode Discrimination in Index-Guided Circular Ring Semiconductor Lasers