Abstract: A differential method of improving the carrier to noise ratio of optically transmitted amplitude modulated signals without affecting the fidelity of the transmitted signals is disclosed. The disclosed method improves over prior art methods by utilizing a single transmission fiber transmitting two wavelengths that are modulated out of phase by a single Mach Zehnder type modulator. The received light is separated into two components at two wavelengths and each wavelength is received by a photodetector. The two photodetector outputs are differentially combined to produce an output with improved signal to noise ratio resulting in improved bit error rates in digital transmissions. The invention is also useful in fiber optic analog transmission systems such as those used to distribute AM-VSB Cable Television programming, wherein the differential method improves the carrier to noise ratio without increasing the nonlinear components in the signal.

Abstract: An improved method of measurement of noise figure and gain of optical amplifiers is based on transforming the amplifier into an oscillator by applying optical feedback with known loss. The feedback consists of a tunable filter for wavelength control and of a variable attenuator for gain control. Measuring the output power at a given wavelength and the noise characteristics of the output signal, either with an optical spectrum analyzer or with a set of tunable filters and a power meter, provides data for full characterization of the optical amplifier. Another method is disclosed that utilizes the self-heterodyne setup and electrical spectrum analyzer to measure the spectral line width of the lasing amplifier, from which the noise figure is derived.

Abstract: Method and apparatus for reducing rotational bias error in a ring laser. The method includes the steps of varying a portion of an optical path for laser beams that emerge from a ring laser cavity along divergent paths and are subsequently combined along a common path to a detector. The apparatus of one embodiment includes a movable mirror positioned to intercept the laser beams emerging from the ring laser cavity before combination along a common path. The movable mirror may be piezo-electric transducive elements configured to bend on application of a voltage. The piezo-electric elements are provided with a reflective region that is positioned to redirect the laser beams from the ring laser back to an output coupling mirror so that re-reflection from the output mirror redirects the laser beams back into a beam combining optical element. In an alternative embodiment the detector is periodically displaced along an access parallel to the common path of the laser beam.