The present invention provides self-collimating multiwavelength lasers (MWL) including a planar gain medium with a resonance cavity defined by superimposed gratings. These laser devices are characterized by lasing wavelengths with multiple peaks in their reflectance spectra defined by the superimposed gratings. The gratings also limit beam divergence, producing self-collimated multiwavelength output, with the potential for high power. These devices can be implemented in any planar gain medium in which multi-peak gratings can be produced, including but not limited to semiconductors and doped glasses. The self-collimated MWL's disclosed herein have applications in areas such as DWDM (dense wavelength division multiplexing) communications systems, free-space uses such as interconnects, range-finding, and inter-satellite infrared communication. In one aspect the device is a multiwavelength ring cavity laser including a planar gain medium with two orthogonal pairs of supergratings (SG) A and B, which emulate the superposition of M and N single-pitch gratings respectively. SG-A has a set of M diffraction wavelengths that depends on the incident angle .theta.; a similar dependence on .phi.=.theta.-90.degree. exists for the set of N diffraction wavelengths of SG-B. Wavelength-angle pairs common to both SG-A and SG-B will be resonant within the cavity as a whole. When combined with optical gain, the result is lasing at M by N wavelengths, collimated at a well-defined output angle.

A current switching circuit enabling current switching at a speed of several gigabits per second (Gbit/sec) or several giga Hertz (GHz) in analog integrated circuits and digital integrated circuits using GaAs FETs is so constructed that the current switching is effected by a pair of diodes, whose cathodes are connected in common and which are driven by a GaAs FET circuit (source follower circuit). This current switching circuit includes first and second FETs, each of which has a source, a drain and a gate. First and second input signals are connected to the gates of the first and the second FETs, respectively. First and second diodes are provided with anodes connected with the sources of the first and the second FETs, respectively. A constant current source is connected in common with the cathodes of the first and the second diodes, and a load circuit is connected with the drain of at least one of the first and the second FETs. By virtue of this arrangement, current flowing through the load circuit is switched, depending on the first and the second input signals, with an output signal being taken out from each load circuit.

A laser proximity sensor for a projectile includes a laser diode having front and rear facets. The diode generates a main laser signal and directs a first portion thereof out of the front facet as a source beam. Focusing means focuses the source beam on a target, and focuses the return beam reflected from the target into the laser diode through the front facet. The laser diode receives the return light beam, provides it with a positive gain, mixes it with the main laser signal, and guides it out the rear facet as a mixed beam. A detection focusing device focuses the mixed beam onto a PIN detector. The PIN detector coherently detects the mixed beam and provides an output signal having a perturbation where the target enters the focal field of the focusing optics. A processor detects the output signal from the PIN detector and may activate a fuse on the projectile. The processor is also capable of determining the relative velocity between the projectile and the target from measurement of the Doppler shifted signal or from the shape of the perturbation of the output signal from the PIN detector.

In a pulsed coherent Doppler radar, a Q-switched laser source develops a laser beam to have a high intensity pulse portion and a substantially lower intensity trailing tail portion. A switching device upon which the laser beam is incident directs substantially all of the energy of the pulse portion along a first propagation path towards a target, such as atmospheric aerosols, and substantially all of the energy of the tail portion along the second propagation path. The target reflects the pulse portion as a received signal. A heterodyne detector upon which each of the received signal and the tail portion is incident, determines the Doppler shift of the received signal with respect to the tail portion. The detector uses the tail portion as the local oscillator. The acousto-optic modulator also upshifts the frequency of the tail portion.

For accurately detecting a desired laser signal without being affected by ambient noise light pulse, a laser signal receiving apparatus, which receives a light pulse transmitted from a laser transmitter, includes a light receiving section having a photo-electric converter for converting a light pulse into an electric pulse signal, a pulse-width detecting section for detecting a pulse-width of the pulse signal derived from the light receiving section and a controlling section having a counter port supplied with the pulse signal and a pulse-width detector port supplied with an output of the pulse-width detecting section, so as to count only the pulse signal having a predetermined pulse-width, based on an output of the pulse-width detecting section.