Won‐Tien Tsang

dc analysis of parallel arrays of two and three Josephson junctions

Arrays of Josephson junctions are becoming of increasing interest, and the dependence of the maximum zero‐voltage current on various factors is important in applications. When the array carries the maximum possible zero‐voltage current it is said to be in the critical state. Sets of equations describing the critical‐state behaviors of arrays of two and three junctions are derived. Account is taken of possible differences of the individual critical currents, self‐induced flux, and asymmetric current feed. In a space having the phase differences across the junctions as coordinates, one can construct a locus relating the phase differences that are obtained when the array is in the critical state. It is shown that the periodic, symmetric, and other properties of the various relationships between variables can be inferred from the properties of the phase‐difference locus.

GaAs‐Ga1‐xAlxAs double‐heterostructure injection lasers with distributed Bragg reflectors

Laser oscillation in injection‐pumped GaAs‐Ga1−xAlxAs double‐heterostructure distributed Bragg reflector (DBR) lasers is reported. Current threshold density as low as 890 A/cm2 in pulsed operation has been obtained at 183 °K. In this laser diode, the corrugations are placed outside the active part of the laser in the form of an end pair of parallel Bragg reflectors. The observed characteristics of the DBR lasers are found to agree well with the theory.

Preferentially etched diffraction gratings in silicon

Here, we describe the fabrication of diffraction gratings in the {100} surfaces of silicon by preferential etching. The gratings thus made have grooves with well‐defined geometric shape and bounded by nearly perfect reflecting walls. Meanwhile, the importance of utilizing this technique in integrated optics is also discussed.

Optical Waveguides Fabricated by Preferential Etching

We introduced a new technique of fabricating optical waveguides by first preferentially etching the waveguide grooves, which can be either cusp-shaped or cup-shaped in cross section, and then filling the grooves with organic films by solution-deposition technique. With the chemical etching technique, perfectly smooth reflecting waveguide walls and well-defined waveguide profiles can be easily produced and reproduced with an accuracy that no other existing fabrication techniques can achieve. In our experiments, silicon was chosen as the base material for possible future integration of optical and electronic components.

Profile and groove‐depth control in GaAs diffraction gratings fabricated by preferential chemical etching in H2SO4‐H2O2‐H2O system

We studied in detail the fabrication of diffraction gratings in GaAs by preferential chemical etching and demonstrated that different grating profiles can be obtained by proper choice of substrate orientation and direction of grating‐mask groove openings or by controlling the width of these groove openings and/or etching time. We have also obtained experimental curves relating the etched groove depth to etching time for gratings with different periodicities at different etchant temperatures. In our experiments, the H2SO4‐H2O2‐H2O system was used as the preferential etchant together with Shipley AZ‐1350J as the resist. This combination enables us to use the resist grating directly as a protective mask during chemical etching.

Simultaneous exposure and development technique for making gratings on positive photoresist

A simultaneous exposure and development technique for forming diffraction gratings and recording holograms in positive photoresist is described. This technique not only reduces the exposure time significantly, but is also able to produce gratings with deep grooves and sharp ridges. With the photoresist placed in a liquid‐filled prismlike container, the smallest period obtainable by this method is reduced by a factor equal to the index of refraction of the liquid.

Mode properties of GaAs‐Ga1−xAlxAs heterostructure inverted‐ridge optical waveguides

Clean mode excitation and transmission have been observed in inverted‐ridge waveguides fabricated by liquid‐phase epitaxy of Ga0.7Al0.3As‐GaAs structures over preferentially etched channels.

A thin-film prism as a beam separator for multimode guided waves in integrated optics

Abstract We discuss a new application of a thin-film prism as a beam separator for multimode guided waves in integrated optics. For each mode of propagation, the angle of deflection by the thin-film prism is different and each deflected beam can be processed subsequently. Deflection angles for different modes can be predicted by using the concept of effective index of refraction. The thickness and index of refraction of the guiding film are determined simultaneously by measuring the coupling angles of the laser beam and a simple graphical method. Calculated and observed angles of deflection are in good agreement. The thin-film prism is made by using preferential etching technique in order to obtain linear, well defined, tapered edges. Furthermore, silicon is used as the base material for possible future integration of optical and electronic components.

Analysis of ring distributed-feedback lasers

Here a structure for ring distributed feedback (DFB) lasers is proposed and analyzed. The threshold condition for such lasers is derived and compared with that for plain DFB lasers. It is shown that the etalon and feedback effects provided by the uniform section of a ring DFB laser should narrow the laser bandwidth and lower the threshold gain.

Thin‐film beam splitter and reflector for optical guided waves

We report the fabrication and operation of a new and simple beam splitter and reflector for thin‐film optical guided waves.