Abstract: There is provided by this invention an optical heterodyning device comprised of a semiconductor heterostructure having a forward biased electrode connected thereto for optically mixing an input signal frequency with a local oscillating frequency in the separate confined laser cavity and a reverse biase electrode connected thereto for detecting the intermediate frequency produced. The local oscillator provides preamplification in the laser cavity and can be effectively phase locked to the input signal frequency.

Abstract: A valve having dual seats and dual poppets operating in series to function as either a conventional check valve, or may be operated by an independent piston which may be styled to either combine or eliminate the check valve feature. One seat poppet combination employes a metal-to-metal seal. The other seat poppet combination utilizes a resilient o-ring seal vented so that when the seal is raised from the bottom of its groove, the upstream and downstream sides of the soft seat poppet can communicate. Alternative embodiments provide a family of zero leakage valves for different purposes, e.g. a check valve, pilot operated check valve, and an in line or balanced relief valve with or without the check valve feature by employing generally the same elements with modifications.