Abstract: A mode locked fiber laser gyroscope using a rare-earth doped fiber. The mode locked fiber laser gyroscope comprises a fiber amplifying unit, a dichroic mirror connected to one end of the fiber amplifying unit, and a fiber rotation sensing unit connected to the other end of the fiber amplifying unit in order to generate a signal in accordance with rotation of the laser gyroscope. The rare-earth doped fiber constitutes the fiber amplifying unit and is connected at one end thereof to the dichroic mirror and applied with the light of a pumping light source through the dichroic mirror. The other end of the rare-earth doped fiber is connected to the fiber rotation sensing unit. In the mode locked fiber laser gyroscope of this invention, the measurement of a rotation rate from a detect signal of a photo detector does not require a complex signal processing but is directly achieved by a time axis.

Abstract: Apparatus for measuring a physical quantity such as pressure, temperature, strain etc., by using a beat frequency of a longitudinal mode, a transverse mode or two polarization modes of the optical fiber laser. In the rare-earth optical fiber laser as a resonator, there are two different polarization modes normal to each other. Since the optical path lengths of the optical fiber laser for the two polarization modes are different, longitudinal mode frequencies of each polarization mode are different depending on the birefringence of resonator which is varied as an external physical quantity such as pressure, temperature, strain etc. Due to the variation in the birefringence of the resonator, the longitudinal mode frequency difference between two polarization modes becomes varied. This variation is linearly proportional to the physical quantity externally applied. The difference in longitudinal mode frequency between polarization modes can be measured from a beat signal of the polarization modes.

Abstract: Fiber-optic devices and sensors are implemented using fiber gratings. The fiber-optic device includes the fiber grating, an input mode stripper and an output mode stripper placed at both ends of the fiber grating, to admit only one mode therethrough, and a polarizer. The fiber grating of this invention is used in combination with a directional coupler instead of a mode stripper and a polarizer. The fiber-optic device outputs desired wavelengths, a switching signal or an interference signal. An external perturbation, such as a strain, a temperature, light pulse, and etc. may be applied to the fiber grating in its preparation to change a characteristic of the fiber grating. These perturbations are actively used to make fiber-optic devices and sensors.

Abstract: A Brillouin fiber optic gyroscope has a cavity loop that includes a first length of optical fiber wound as a first number of turns in a first direction to provide a selected system response that includes the dynamic range of gyroscope rotation rate, the resolution of the gyroscope rotation rate measurement, the relative lock-in range and the relative Kerr-effect-induced beat-frequency bias. The cavity loop further includes a second length of optical fiber wound as a second number of turns in the same direction as the first number of turns and wound as a third number of turns in an opposite direction to the winding direction of the first and second number of turns. The effect of the second and third number of turns is to increase the overall length of the optical fiber in the cavity loop to reduce the pump power required to generate Brillouin laser light within the cavity loop without increasing the Sagnac effect within the cavity loop.

Abstract: An interferometer used as a rotation sensor is constructed using a strand of optical fiber, a portion of which is formed into a sensing loop. A pair of light waves are caused to counterpropagate in the sensing loop and are combined to form an optical output signal that has an intensity that varies in accordance with the difference in the phases of the two counterpropagating light waves. A phase modulator is positioned on the optical fiber in the sensing loop at a location such that the two counterpropagating light waves are modulated approximately 180 degrees out of phase. The time-varying phase modulation causes a time-varying phase difference that is combined with a rotationally-induced Sagnac effect phase to provide a total phase difference that is detected by a photodetector. The photodetector provides an electrical output signal that is processed to determine the Sagnac phase difference. The rotation rate is then calculated from the Sagnac phase difference.

Abstract: A Brillouin fiber optic gyroscope having a feedback system which monitors the difference between counterpropagating Brillouin intensities and utilizes this difference in the form of a correction signal to control one of the circulating pump intensities so as to equalize the circulating pump intensities. The Brillouin fiber optic gyroscope further includes a second feedback system which detects electrical signals proportional to the phase-modulated, counterpropagating intensities in the gyroscope, and utilizes a combination of the electrical signals as an error signal to stabilize the resonant cavity at a length substantially equal to a length midway between the resonant lengths of the counterpropagating pump signals. The Brillouin fiber optic gyroscope of the present invention also provides a dynamic range of the gyroscope rotation rate that is twice the dynamic range of existing gyroscopes.

Abstract: An interferometer used as a rotation sensor is constructed using a strand of optical fiber, a portion of which is formed into a sensing loop. A pair of light waves are caused to counterpropagate in the sensing loop and are combined to form an optical output signal that has an intensity that varies in accordance with the difference in the phases of the two counterpropagating light waves. A phase modulator is positioned on the optical fiber in the sensing loop at a location such that the two counterpropagating light waves are modulated approximately 180 degrees out of phase. The time-varying phase modulation causes a time-varying phase difference that is combined with a rotationally-induced Sagnac effect phase to provide a total phase difference that is detected by a photodetector. The photodetector provides an electrical output signal this is processed to determine the Sagnac phase difference. The rotation rate is then calculated from the Sagnac phase difference.

Abstract: A laser source comprises an optical fiber doped with a homogeneously broadened lasing medium, preferably with Erbium, pumped by a laser pump source and an intracavity acousto-optic modulator. When the acousto-optic modulator is driven by a variable frequency source, the Erbium fluorescence line emitted by the Erbium-doped optical fiber can be electronically tuned. In another embodiment, an electronic sweep waveform is used to frequency modulate the acoustic signal produced by the acousto-optic modulator. Without the low-rate frequency modulation, Erbium in a silica optical fiber is a mostly homogeneously broadened gain medium with a narrow laser linewidth. When measured on a long time scale, low-rate frequency modulation provides a broader spectral width, on the order of 19 nm, which makes such a source an ideal source for certain optical applications such as fiber optic gyroscopes.

Abstract: An inexpensive fiber optic gyro using a fiber optic sensing coil that is fed from two single-mode light carriers such as an optical waveguide or optical fibre. The light carriers obtain their energy, through a mode splitter, from a double-mode light carrier. The double-mode carrier is, in turn, energized by an at least partially coherent light source through a single-mode light carrier. The light source is preferably a laser. Detectors are attached to light carriers which tap different predetermined positions along the double-mode waveguide to produce substantially sinusoidal signal functions of sensed rotation.

Abstract: A passive quadrature phase detection system for coherent fiber systems includes first and second optical detectors positioned to receive an output signal from the output end of an optical signal apparatus such as an interferometer, or the like. The optical signal from the apparatus includes light propagating in two propagation modes. As the light propagates away from the output end of the apparatus from the near field to the far field, the light in the two modes undergoes a relative phase shift of .pi./2 in accordance with the Guoy effect. The two detectors are positioned such that the first detector detects the intensity of light resulting from the interference between the two modes in the near field of the output signal, and such that the second detector detects the intensity of light resulting from the interference between the two modes in the far field of the output signal. The additional .pi.

Abstract: A laser source comprises an optical fiber doped with a homogeneously broadened lasing medium, preferably with Erbium, pumped by a laser pump source and an intracavity acousto-optic modulator. When the acousto-optic modulator is driven by a variable frequency source, the Erbium fluorescence line emitted by the Erbium-doped optical fiber can be electronically tuned. In another embodiment, an electronic sweep waveform is used to frequency modulate the acoustic signal produced by the acousto-optic modulator. Without the low-rate frequency modulation, Erbium in a silica optical fiber is a mostly homogeneously broadened gain medium with a narrow laser linewidth. When measured on a long time scale, low-rate frequency modulation provides a broader spectral width, on the order of 19 nm, which makes such a source on an ideal source for certain optical applications such as fiber optic gyroscopes.

Abstract: The present invention discloses a thermally stable rare-earth doped fiber source comprising an active medium such as Erbium or Neodymium. The thermal stability of the mean wavelength of such a source is determined by three contributions as expressed by the following differential equation: ##EQU1## The first term is the intrinsic temperature dependence of the active medium, the second term is the pump power dependence and the third term is a contribution that arises from the dependence of the emission wavelength on the pump wavelength. The method of the present invention minimizes the temperature dependence on the mean wavelength by using the above equation and optimizing the values of the pump power and the pump wavelength so that the three contributions in the governing equation cancel each other.

Abstract: Apparatus and methods for reducing and preventing polarization fading in unbalanced measuring interferometers. An extended interferometer having a plurality of sensors and a compensating interferometer are used. They are driven from a pulsed optical signal source wherein the optical signal comprises sequences of two pulses each. To prevent polarization fading the polarization of a predetermined one of each two-pulse sequence is switched, preferably orthogonally, from sequence to sequence. Interference pulse output groups are produced for each two-pulse driving sequence. Each output group has the same number of usable pulses as the number of sensors in the interferometer.

Abstract: An improved broadband light source for a Sagnac interferometer includes a waveguide, such as a fluorescent optical fiber, that is pumped by a pump source with a sufficient intensity to generate temporally incoherent light. The fluorescent optical fiber has first and second ends, one end being an input end of the fiber. The broadband light is provided at an output of the fluorescent optical fiber and is input to the interferometer. In order to prevent laser oscillations between the light source and the interferometer, one end of the fluorescent optical fiber is formed so as to prevent reflections. The light output from the fluorescent fiber to the interferometer comprises only that light that initially propagates toward the output of the optical fiber. In one embodiment of the light source, the pump light from the pump source is coupled into the fluorescent optical fiber in a direction so that it travels away from the output of the fluorescent optical fiber towards the first end.

Abstract: An improved broadband light source for a Sagnac interferometer includes a fluorescent optical medium that is pumped by light from a pump source with a sufficient intensity to cause the fluorescent optical medium to generate temporally incoherent light by superfluorescence. In the preferred embodiment, the superfluorescent optical medium comprises an optical fiber which is backward pumped. The signal output from the interferometer loop is amplified by the optical gain of the superfluorescent fiber which acts as a light source and as an amplifier. In order to avoid gain modulation in the superflorescent fiber, the modulation frequency is selected so that the modulation gain depth is substantially reduced.

Abstract: An optical fiber is subjected to a series of traveling flexural waves propagating along a length of the fiber. At least a portion of an optical signal propagating within the optical fiber in a first propagation mode is coupled to a second propagation mode. The optical signal in the second propagation mode has a frequency which is equal to either the sum of or the difference between the frequency of the optical signal in the first propagation mode and the frequency of the traveling flexural waves.

Abstract: An optical fiber laser includes a single-mode optical fiber doped with a lasing material such as Neodymium. The optical fiber is pumped with a pump optical signal having a pump wavelength selected to cause spontaneous emission of an optical signal at a second wavelength different from the pump wavelength. The optical fiber is formed into a laser cavity such as by including a suitable reflector at each of the two ends of a suitable length of the optical fiber so that the emitted optical signal oscillates therein. One of the reflectors has a reflectivity at the wavelength of the emitted light so that most (e.g., approximately 95%) of the emitted light is reflected back into the laser cavity and a smaller portion (e.g, approximately 5%) is transmitted through the mirror as a laser output signal. Alternatively, the optical fiber can be formed into a ring laser structure using an optical coupler that couples a substantial portion (e.g.

Abstract: A re-entrant fiber optic interferometer comprises an optical fiber, forming a loop for recirculating an optical signal in the loop. The loop of optical fiber comprises an active material which emits photons at a first wavelength and responds to pumping in a second wavelength. Signal light at the first wavelength is input to the loop for circulation therein, and pump light at the second wavelength is input to the loop to optically pump the active material to emit light at the first wavelength. The invention also includes a multiplexing coupler which has different coupling ratios for the pump light and the signal light, such that only a fraction of the signal light is coupled out of the loop on each circulation about the loop, but substantially all of the pump light is coupled out of the loop after a single circulation, thereby suppressing pump phase noise in the loop.

Abstract: A superfluorescent broadband fiber laser source comprises a fiber doped with laser material coupled to a multiplexing coupler. In the preferred embodiment, a source of pumping illumination provides pumping light to the doped fiber, and the coupler is adjusted to have a 0% coupling efficiency at the wavelength of the source. The pumping light is sufficiently intense to produce amplified spontaneous emission within the doped fiber, and gives rise to a forward signal and a backward signal. One of the superfluorescent signals is reflected back to the doped fiber by a reflector cemented to one end of the doped fiber or to one end of another fiber through the coupling function of the coupler. The coupler is adjusted to provide complete coupling at the frequency of the lasing light. The temperature dependence of the coupler can be selected to reduce or cancel the temperature dependence of the superfluorescent signal. Other arrangements utilizing the multiplexing properties of the coupler are also described.

Abstract: An apparatus utilizes a two-mode optical waveguide with a non-circular core to provide stable spatial intensity patterns in both propagation modes for light propagating therein. The light has a wavelength, and the non-circular core has cross-sectional dimensions selected such that (1) the waveguide propagates light at that wavelength in a fundamental mode and a higher order mode, and (2) substantially all of the light in the higher order mode propagates in only a single, stable intensity pattern. Embodiments of the invention include, for example, modal couplers, frequency shifters, mode selectors and interferometers. One of the interferometer embodiments may be used as a strain gauge.