Abstract: A polarization independent wavelength tunable filter in the form of a dual-waveguide directional coupler. The conditions necessary to achieve coupling of a selected wavelength including both TE and TM polarization modes from one waveguide to a second waveguide in close proximity are achieved through a combination of geometrical and compositional tailoring of the waveguides and birefringence compensation in the waveguide and coupling layer.

Abstract: A Type II heterojunction formed of semiconductor material which normally forms a Type I heterojunction. The Type II heterojunction is created by using a carefully chosen stack of epitaxial semiconductor materials with modifications to the band structure through quantum mechanical confinement effects. This virtual type II heterojunction is incorporated adjacent to the active region in a single-quantum-well, ridge-waveguide laser structure. An anomalous "Negative-T.sub.o " region is observed in which threshold current decreases with increasing temperature. A reduction in the temperature sensitivity of the threshold current of the laser structure results.

Abstract: A low noise linear amplifier and a microwave voltage controlled oscillator constructed from such amplifier. Each amplifier within the VCO utilizes a ratioed transistor configuration to generate a linear output over a wide range of inputs. Output current from the amplifier is split into a main output current and components of in-phase and 180.degree. out-of-phase current. A logarithmic tuning control combines the components of in-phase and 180.degree. out-of-phase currents in inverse ratio to provide a constant d.c. feedback current.

Abstract: A system for generating an ultra short optical pulse by pumping a semiconductor signal laser with an optical pulse from a semiconductor probe laser. The signal laser has dual segments each operated under different d.c. injection current levels. The probe laser is modulated with a pulsed input and the optical output thereof is coupled via optical means to the signal laser so as to Q-switch the latter. Optical pulses in the 65 ps range result.

Abstract: A gain-coupled distributed feedback (DFB) laser or amplifier structure having a gain-coupling grating in the form of grooves periodically etched through strained, multi-quantum wells. The etched grooves are regrown with buffer material having a bandgap selected to compensate for index-coupling aid to block carrier leakage. The low carrier leakage reduces non-radiative recombination which in turn increases quantum efficiency.

Abstract: A surface emitting, circular grating, semiconductor laser. Gain is generated by injecting current into an active, annular mesa region which surrounds a central, circular region having a circular grating for surface emission. The annular mesa region is surrounded by an outer annular region in which is etched a first order grating for creating feedback. Methods of fabricating the circular grating laser are also described.

Abstract: A linear bipolar junction transistor amplifier. The linear gain is achieved by an offset voltage created by ratioed transistors wherein the ratio is an integer. The ratios are achieved either by connecting equal transistors in parallel or by employing multiple emitters on a single transistor. This results in a linear amplifier which is substantially independent of manufacturing process tolerance variations as well as power supply and temperature variations.

Abstract: A current steering switch circuit responsive to a CMOS signal. In an specific embodiment the switch is incorporated in a hybrid BiCMOS multiplexer circuit using combined CMOS and CML/ECL signal types. The high speed CML/ECL logic signals are multiplexed under the control of a lower speed CMOS signal. A particular aspect of the circuit is that a CMOS to CML/ECL converter is not used. Additionally, a differential, logic commutation signal is not required.

Abstract: A 2-level logic `current injection` circuit topology suitable for heterojunction bipolar transistor (HBT) technology. The circuit requires lower supply voltage headroom and demonstrates smaller propagation delay than conventional 2-level emitter coupled logic/current mode logic (ECL/CML). Master-slave flip-flop circuits have been fabricated in AlGaAs/GaAs, HBT. Test results indicate that the circuits are fully functional at 10 Gbit/s and an operating clock frequency as high as 20 GHz is recorded.

Abstract: A circuit for generating a pair of quadrature output signals from a pair of quadrature input signals in which the frequency of the output signals is double that of the input. The circuit consists of two dual phase shifters, two symmetrical multipliers and a phase controller. The circuit is fabricated by conventional integrated circuit processing technology. A method of generating frequency doubled quadrature output signals is disclosed.

Abstract: A gyrator based resonator comprises first and second differential amplifiers cross coupled to form a quadrature phase voltage controlled oscillator. The differential amplifier circuits include matched transistor pairs and associated transconductance adjusting components to control the resonant frequency of the resonator. Transconductance delay and admittance loss elements are also provided and these elements serve to control the loop gain of the resonator. A loop gain greater than unity results in the resonator oscillating while a loop gain less than unity produces a bandpass filter. A resonator having gain less than unity may be slaved to a similar resonator having gain adjusted to be greater than unity thus creating a low noise oscillator and tracking filter.

Abstract: A method of reducing thermally-induced emission wavelength shift in laser diodes. In addition to the conventional ridge contact for supplying the modulation current to the laser diode a second ridge contact is formed on the device. A compensation current of substantially the same frequency as the modulation current but having an opposite phase is applied to the second contact. This compensation current is selected to substantially neutralize the transient temperature variations due to changes in the modulation signal and result in a generally steady state thermal situation.