Abstract: A fiber optic switch module that can be incorporated into switch designs characterized by negligible crosstalk. The switch module is capable of operation over two wavelength bands. A 2.times.2 switch module has an open or bar state in which polished portions of the fiber segments are separated from each other and a closed or cross state where the polished portions are in optical contact with each other. The module has arbitrarily low crosstalk in the bar state, but typically some crosstalk in the cross state. In one embodiment, the third port of a first 2.times.2 module is coupled to the first port a second 2.times.2 module while no connections are-made to the second ports of the modules or to the third port of the second module.

Abstract: A Brillouin fiber optic gyroscope includes an intensity modulator in the optical loop which periodically attenuates the Brillouin light waves counterpropagating in the optical loop so that the counterpropagating Brillouin waves each propagate as square waves. The use of square wave modulation for the counterpropagating light wave reduces the cross-effect of the Brillouin waves to substantially the same magnitude as the self-effect so that the non-reciprocal Kerr effect is substantially reduced or eliminated. In order to support the counterpropagating square waves, the optical loop is pumped with pump light having frequency components selected to pump the optical fiber to provide Brillouin light at frequencies necessary to generate square waves in the counterpropagating Brillouin light waves. In addition, the Brillouin light must be generated at the correct intensity and phase relationship to form the square wave.

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 optical mode coupling apparatus includes an Erbium-doped optical waveguide in which an optical signal at a signal wavelength propagates in a first spatial propagation mode and a second spatial propagation mode of the waveguide. The optical signal propagating in the waveguide has a beat length. The coupling apparatus includes a pump source of perturbational light signal at a perturbational wavelength that propagates in the waveguide in the first spatial propagation mode. The perturbational signal has a sufficient intensity distribution in the waveguide that it causes a perturbation of the effective refractive index of the first spatial propagation mode of the waveguide in accordance with the optical Kerr effect. The perturbation of the effective refractive index of the first spatial propagation mode of the optical waveguide causes a change in the differential phase delay in the optical signal propagating in the first and second spatial propagation modes.

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: 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: A mode converter comprises an a-axis LiNbO.sub.3 optical fiber exhibiting a ferroelectric bi-domain structure. The fiber is subject to an electrical field that induces a +.pi./2 phase retardation in one domain of the fiber and a -.pi./2 phase retardation in the other domain. A light signal launched in the fundamental mode of the fiber is converted into a light signal propagating in the second order mode. When the electrical field is selected so that the phase retardations are not multiples of .pi./2, the mode conversion is partial and the LiNbO.sub.3 fiber can operate as an optical switch or as an amplitude modulator. The mode converter can also be operated as a second harmonic generator. The fiber is heated to a phase matching temperature so that a signal launched in the fundamental mode of the fiber and at a frequency .omega. is converted to the second order mode at a frequency 2.omega.. The LiNbO.sub.3 fiber can also simultaneously operate as an optical switch and as a second harmonic generator.

Abstract: Methods and apparatus are shown for cladding grown single crystal optical fibers. Neodymium YAG fibers are clad with a high index glass, either melted around the fiber in a trough or extruded over the fiber surface. Lithium niobate fibers are clad through an impregnation process. The lithium niobate fiber is first coated with magnesium oxide and then heated to a temperature and for a time sufficient for the magnesium oxide dopant material to diffuse into the fiber. The dopant lowers the intrinsic refractive indices of the fiber material around its circumference, creating a cladding region around the fiber core. Single crystal fibers clad by these methods and combined with suitable pumping means or with deposited electrodes provide low-loss single mode optical components useful for amplification, electro-optical effects and acousto-optical effects.

Abstract: A side pumped, fiber optic amplifier comprises an optical fiber, having a first refractive index, formed of a laser material, such as Nd:YAG. A jacket, which surrounds the optical fiber, has a second refractive index, lower than the first refractive index. This jacket is cone shaped and tapers from a large end to a small end. High power laser diodes are mounted on the large end to introduce pump light to pump the optical fiber material. The cone-shaped jacket focuses this pump light to an interaction region at the small end, where the jacket material is quite thin, e.g. on the same order of magnitude as the diameter of the optical fiber. The focused light is absorbed by the optical fiber in this interaction region, and causes an electronic population inversion in the laser fiber material. A signal propagating through the optical fiber stimulates spontaneous emission from the optically excited laser material, thereby resulting in amplification of the signal.

Abstract: Methods and apparatus are shown for cladding grown single crystal optical fibers. Neodymium YAG fibers are clad with a high index glass, either melted around the fiber in a trough or extruded over the fiber surface. Lithium niobate fibers are clad through an impregnation process. The lithium niobate fiber is first coated with magnesium oxide and then heated to a temperature and for a time sufficient for the magnesium oxide dopant material to diffuse into the fiber. The dopant lowers the intrinsic refractive indices of the fiber material around its circumference, creating a cladding region around the fiber core. Single crystal fibers clad by these methods and combined with suitable pumping means or with deposited electrodes provide low-loss single mode optical components useful for amplification, electro-optical effects and acousto-optical effects.

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: A periodic grating structure is placed on a facing surface formed on an optical fiber so that the grating structure is within a portion of the evanescent field of an optical signal propagating through the optical fiber. The spatial periodicity of the grating structure is selected to be equal to one-half the propagation wavelength of the optical signal. The grating structure causes the optical signal to be reflected at an angle of 180 degress and thus to propagate in a reverse direction from its original direction of propagation.

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 optical mode coupling apparatus includes an optical waveguide in which an optical signal at a signal wavelength propagates in a first spatial propagation mode and a second spatial propagation mode of the waveguide. The optical signal propagating in the waveguide has a beat length. The coupling apparatus includes a source of perturbational light signal at a perturbational wavelength that propagates in the waveguide in the first spatial propagation mode. The perturbational signal has a sufficient intensity distribution in the waveguide that it causes a perturbation of the effective refractive index of the first spatial propagation mode of the waveguide in accordance with the optical Kerr effect. The perturbation of the effective refractive index of the first spatial propagation mode of the optical waveguide causes a change in the differential phase delay in the optical signal propagating in the first and second spatial propagation modes.

Abstract: A fiber optic rotation sensor using birefringent optical fiber includes an uncorrelating element, an equalizing element and a polarizer in the common input and output fiber portions of the sensor to reduce or eliminate the intensity type phase errors caused by interference between lightwaves originally in the same polarization mode on entry to the sensor loop that cross couple into another polarization mode. In the preferred embodiment, the uncorrelating element comprises a birefringence modulator and a length of birefringent fiber. The equalizing element comprises a birefringent fiber having a splice at which the axes of birefringence of the spliced portions of the fiber are positioned at 45.degree. relative to each other.

Abstract: An amplifier for use with fiber optic systems comprises a neodymium YAG crystal placed in series with a signal-carrying optical fiber. The ND:YAG crystal is supplied by the optical fiber with both the signal to be amplified, and pumping illumination. The pumping illumination is coupled onto the optical fiber by a multiplexing coupler which is used to combine the signal to be amplified and illumination from a pumping illumination source onto a single optical fiber. The pumping illumination inverts the neodymium ions within the ND:YAG crystal. The signal to be amplified propagates through this crystal to stimulate emission of coherent light from the neodymium ions, resulting in amplification of the signal.

Abstract: A closed loop optical fiber interferometer is used in sensing a quantity, Q, by applying a time varying or modulated measure of, Q, asymmetrically to the closed loop (24) and detecting phase shift between two counterpropagating optical signals in the closed loop. The closed loop (24) can be used in the sensing element or a separate sensor (68, 70) can develop a time varying signal which is then applied to the closed loop interferometer.