Elimination of drift in a single-mode optical fiber interferometer using a piezoelectrically stretched coiled fiber

Birefringence modulation in fiber-optic phase modulators

We report an unusually large birefringence modulation in fiber-optic phase modulators formed with cylindrical piezoelectric transducers when driven at a few hundred kilohertz. The magnitude of the birefringence modulation is a strong function of modulation frequency. A simple model is used to explain the phenomenon, and the theoretical predictions agree well with the experimental results.

Experimental demonstration of an optical phased array antenna for laser space communications

The feasibility of an optical phased array antenna applicable for spaceborne laser communications was experimentally demonstrated. Heterodyne optical phase-locked loops provide for a defined phase relationship between the collimated output beams of three single-mode fibers. In the far field the beams interfere with a measured efficiency of 99%. The main lobe of the interference pattern can be moved by phase shifting the subaperture output beams. The setup permitted agile beam steering within an angular range of 1 mr and a response time of 0.7 ms. We propose an operational optical phased array antenna fed by seven lasers, featuring high transmit power and redundance.

Densely spaced two-channel wavelength division demultiplexer with an S-shaped two-coupler optical fiber ring resonator

It is proposed that an S-shaped two-coupler single-mode optical fiber ring resonator be used as a densely spaced two-channel wavelength division demultiplexer. The dependence of the channel separation and the cross talk on the parameters of the couplers and the fiber, such as the intensity-coupling coefficients and the transmission coefficients of the couplers and the attenuation of the fiber, are studied theoretically.

Partially multiplexing sensor network exploiting low coherence interferometry

A partially multiplexed fiber-optic-based sensor network based on low coherence interferometry is reported for which the scanning interferometer simultaneously sweeps the coherence-matched region of each sensor. The advantage of this approach is that the signal-to-noise ratio is maximized while the cross talk between sensors is minimized. The normal requirement that each sensor must be fabricated with a different length also does not apply. The concept is demonstrated by multiplexing two bulk optic Michelson interferometers. A resolution of ~ 50 nm with an operational range of 200 microm was achieved in the prototype system.

Temporal averaging of phase measurements in the presence of spurious phase drift: application to phase-stepped real-time holographic interferometry

A technique that compensates for low spatial frequency spurious phase changes during an interference experiment is developed; it permits temporal averaging of multiple phase measurements, made before and after object displacement. The method is tested with phase-stepped real-time holographic interferometry applied to cantilever bending of a piezoelectric bimorph ceramic. Results indicate that temporal averaging of the corrected data significantly reduces the white noise in a phase measurement without incurring systematic errors or sacrificing spatial resolution. White noise is reduced from 3 degrees to less than 1 degrees (lambda/360) using these methods.

Temporal averaging of phase measurements in the presence of spurious phase drift: application to phase-stepped real-time holographic interferometry

A technique that compensates for low spatial frequency spurious phase changes during an interference experiment is developed; it permits temporal averaging of multiple phase measurements, made before and after object displacement. The method is tested with phase-stepped real-time holographic interferometry applied to cantilever bending of a piezoelectric bimorph ceramic. Results indicate that temporal averaging of the corrected data significantly reduces the white noise in a phase measurement without incurring systematic errors or sacrificing spatial resolution. White noise is reduced from 3 degrees to less than 1 degrees (lambda/360) using these methods.

Interrogation of a remote elliptical-core dual-mode fiber strain sensor by using a tandem interferometer configuration

We report on the interrogation of a lead-insensitive dual-mode fiber sensor using a white-light interferometric technique with a multimode laser-diode source. An elliptical-core dual-mode fiber acts as a remote-sensing interferometer, and a second dual-mode fiber acts as a receiver interferometer. We establish the sensor design for optimized performance and demonstrate the capability of accurate detection of periodic strain signals in the presence of, e.g., temperature-induced low-frequency phase drifts.

Miniaturized fiber-optic Michelson-type interferometric sensors

We present a novel, miniaturized Michelson-type fiber-optic interferometric sensor that is relatively insensitive to temperature drifts. A fused-biconical tapered coupler is cleaved immediately after the coupled length and polished down to the region of the fused cladding, but short of the interaction region. The end of one core is selectively coated with a reflective surface and is used as the reference arm; the other core serves as the sensing arm. We report the detection of surface acoustic waves, microdisplacements, and magnetic fields. The sensor is shown to be highly stable in comparison to a classic homodyne, uncompensated Michelson interferometer, and signal-to-noise ratios of 65 dB have been obtained.

High-speed fiber-optic temperature sensor

The temporal response of a fiber-optic Fabry-Perot-type interferometer, formed by cleaving the ends of a length of highly birefringent (Hi-Bi) fiber, to high-frequency heat pulses is investigated both theoretically and experimentally. The heat pulses are produced by amplitude modulating the output of an argon laser with a variable frequency Bragg cell. This beam is focused at one of the cleaved ends of the interferometer; the fastest temporal response occurs when the beam is focused onto the core of the fiber. A time response of ~2.5 micros is measured.

Modified J(1) ... J(4) method for linear readout of dynamic phase changes in a fiber-optic homodyne interferometer

Fiber-optic interferometers have been studied extensively for sensing applications. Recently a technique described as the J(1) ... J(4) technique was reported for the linear measurement of dynamic phase changes in a fiber interferometer that requires no phase bias and for which the measurement is independent of random phase fluctuations. However, the implementation of the J(1) ... J(4) technique is limited because only the magnitude of the J(1) ... J(4) Bessel components can be measured on a spectrum analyzer without information available on the sign of the Bessel function. Here a modified signal-processing technique that overcomes the limitations mentioned above is described.

Interferometric measurement of fiber strain hysteresis and nonlinearity for sensitivity optimization

A simple and reliable fringe-counting method is used to determine the fiber static elastic response in fiber-optic sensors and modulators. Large strain hysteresis and nonlinearity have been observed in the commonly employed configuration of bonding a bare fiber onto a strain element. Jacketed fibers show reduced hysteresis and nonlinearity in their elastic response. Reductions in hysteresis from 23% to 1% of the maximum strain and in the nonlinearity by a factor of 3 have been measured.

Fiber-optic interferometric sensor for the detection of acoustic emission within composite materials

An optical-fiber Michelson interferometric acoustic emission sensor is described. The sensor uses ordinary singlemode fiber and is embedded in the composite material under test. Signals are demodulated through the active homodyne. This system provides a novel approach for material nondestructive evaluation.

Electronic speckle-pattern interferometry using single-mode fibers and active fringe stabilization

Optical fibers are used to provide object and reference illumination in the context of the electronic speckle-pattern interferometry technique. An all-fiber Michelson interferometer is used, with an electronic servo and a piezoelectric phase modulator, to obtain compensation for environmentally induced phase coupling into the fiber arms.

Lensless focusing with an array of phase-adjusted optical fibers

A bundle of monomode fibers can be used for transmission of coherent radiation at power levels beyond the damage threshold of an individual fiber. The coherence at the bundle output can be reestablished by proper correction of the phases at the individual fiber ends. For a phase adjustment that leads to a convergent wave the focusing efficiencies are calculated for various geometric parameters of the fiber bundle. For practical situations concentration of almost 40% of the power into the focal spot seems realistic.

Linear readout of dynamic phase change in a fiber-optic homodyne interferometer

A new technique that provides linear measurement of dynamic phase change in a no-feedback, no-phase-bias fiber-optic interferometer is described. The phase measurement is unaffected by random changes in phase, source intensity, and fringe visibility. A minimum detectable phase shift of 0.1 rad has been measured for the configuration reported.

Extended-range fiber polarimetric strain sensor

We describe a frequency-modulation technique that is applicable to two-beam interferometric systems illuminated by semiconductor diode lasers. The technique permits a determination of the optical path difference between the two arms of the interferometer and is used here to extend the range of a fiber polarimetric strain sensor by determining the order of the particular polarimetric fringe under consideration.

Optical fiber calorimeter for measuring CO(2) laser power

A new method of measuring CO(2) laser power by an optical fiber interferometer (calorimeter) is described. The linearity of the phase change vs the irradiated power of the CO(2) laser was observed. The sensitivity was found experimentally to be 0.33 pirad/mW. The response time was almost independent of the CO(2) laser power.

Quadrature outputs from fiber interferometer with 4 x 4 coupler

A broad stripe waveguide of width W and length L acts as a self-imaging 4 x 4 directional coupler if L = NW(2)/lambda, where N is the effective index. When the arms of a fiber-optic Mach-Zehnder interferometer are connected to two input ports of this coupler, the powers at the four output ports vary in phase quadrature.

Synchronous sampling demodulation scheme for nonlinear fiber-optic sensors

A new demodulation scheme for nonlinear fiber-optic sensors that uses a synchronous sampling technique is proposed and demonstrated. By using this scheme in a fiber-optic magnetometer, a sensitivity of <4 x 10(-5) Oe of minimum detectable magnetic field has been obtained.

Remote displacement measurement using a passive interferometer with a fiber-optic link

Remote displacement measurement is demonstrated using a Fabry-Perot cavity with a multimode optical fiber link. The sensing cavity modulates, as a function of its length, the spectrum of a light-emitting diode (LED). The light returns via the fiber and is analyzed by a tunable reference cavity. A closed-loop control causes the reference cavity to track the sensing cavity length within 2 x 10(-12) m. Displacement range is 2 x 10(-6) m. The reference cavity length is measured interferometrically, using a laser, to obtain the sensing cavity length. Advantages of this sensing technique include compatibility with multimode fiber-optic components, high immunity to optical losses, and large dynamic range.

Unlimited phase compensator for fiber-optic interferometric detection of slow temperature change

Measurement of slow temperature change by means of a Michelson-type fiber-optic interferometric sensor exploiting unlimited phase compensation is presented. A new simple phase demodulation system, based on a rotating half-wave retarder controlled by a feedback loop, permits real-time detection of a slowly induced phase change of any magnitude. A theoretical analysis of the demodulator is given, together with experimental data showing that a precision better than l/10 degrees C was achieved on a 1-h thermic cycle.

Fiber-end interferometric sensor using cooperative retroreflectors

The well-known fiber Doppler velocimeter is examined as a high-resolution sensor of small displacements. The system is greatly improved through the use of thin-film retroreflective elements on the moving object. Range and sensitivity are increased, and problems in obtaining optical alignment become trivial. The interferometer is used here to study the flight dynamics of an electromagnetic constitutes a more general sensor coupling arrangement. actuator, although the fiber-retroreflector combination constitutes a more general sensor coupling arrangement.

Sensitive, high-speed thermometry using optical fibers

The use of aluminum-jacketed optical fibers for sensitive, high-speed thermometry is demonstrated. Sensitivity of the fiber to thermal variations is observed at frequencies up to 30 kHz. Although the presence of configurationdependent mechanical resonances between 10 and 22 kHz precludes a simple analysis of the thermal response at these frequencies, the response between 5 Hz and 4 kHz is in reasonable agreement with analytic results. Assuming a minimum-detectable phase change of 10(-6) rad, the observed sensitivity at 4 kHz corresponds to a minimum detectable temperature variation of 1 microK for a 1-cm length of fiber. This performance represents orders-of-magnitude improvement in both sensitivity and frequency response relative to those of commercially available temperature-smeasuring systems.

Phase compensation in interferometric fiber-optic sensors

A new method of keeping interferometric fiber-optic sensor systems in quadrature by tuning the emission frequency of the laser-diode source is described. The compensation scheme has a better than 10(-5)-rad sensitivity and a usable tracking range.

Optical techniques to solve the signal fading problem in fiber interferometers

In single-mode optical fiber interferometer sensors environmental effects such as ambient temperature fluctuations and static pressure changes result in signal fading. Three different optical techniques which eliminate the signal fading problem are presented and experimentally verified. Comparison of the three techniques in terms of their technical merits is presented.

Fiber gyroscope with nonreciprocally operated, fiber-coupled LiNbO(3) phase shifter

We describe a fiber gyroscope operating at maximum sensitivity that is due to a nonreciprocal pi/2 phase bias effected with a pulsed voltage applied to an electro-optic LiNbO(3) channel waveguide phase shifter. With a He-Ne laser source, the noise was equivalent to a 0.1 degrees /sec rotation, for a 10-Hz bandwidth and 2% reflectivity at each fiber-LiNbO(3) interface. The phase shifters showed polarization-independent behavior that was characterized by measurements in a fiber interferometer. They were flip-chip coupled with single-mode fiber that were cemented to the channel ends to give a stable module.