Fiber Optic Interferometric System Research Articles

Research on the Linear Demodulation Range and Background Noise of Fiber-Optic Interferometer System

The linear demodulation range and background noise of the Michelson interferometer system are investigated with a laser phase noise measurement system. We have theoretically and experimentally analyzed the performance of the interferometer system by changing the frequency modulation amplitude of the laser and the optical path difference (OPD) of the interferometer, respectively. It is shown that the linear demodulation range of the Michelson interferometer system is finite, which depends on the parameters of the system, such as the sample frequency, the delay time between two interferometer arms, and the system bandwidth. Furthermore, the experimental results indicate that the background noise of the interferometer system can be reduced by using a sufficiently long OPD so that the smaller true phase information can be detected with the demodulation system. The parameters of the measurement system could be optimized to satisfy the demand of the phase demodulation with different levels, which is of great significance for the phase monitoring interrogator, such as fiber-optical interferometer sensors and distributed acoustic sensors.

A bio-inspired optical directional microphone with cavity-coupled diaphragms.

A bio-inspired acoustic sensor for sound source localization is presented, mimicking the internally coupled ears found in many terrestrial vertebrates and insects. It consists of two aluminum diaphragms coupled by a U-shaped cavity and detected by a low-coherence fiber optic interferometer system. A large-scale prototype with a center-to-center separation of 1″ is fabricated and experimentally demonstrated to amplify the interaural phase difference by a factor of 2 to 4 for a wide frequency range (0.5-2 kHz), which agrees well with simulation. This work presents a mechanism of using cavity-coupled diaphragms to develop acoustic sensors for sound source localization.

Optical excitation of atomic force microscopy cantilever for accurate spectroscopic measurements

Reliable operation of frequency modulation mode atomic force microscopy (FM-AFM) depends on a clean resonance of an AFM cantilever. It is recognized that the spurious mechanical resonances which originate from various mechanical components in the microscope body are excited by a piezoelectric elemen that is intended for exciting the AFM cantilever oscillation and these spurious resonance modes cause the serious undesirable signal artifacts in both frequency shift and dissipation signals. We present an experimental setup to excite only the oscillation of the AFM cantilever in a fiber-optic interferometer system using optical excitation force. While the optical excitation force is provided by a separate laser light source with a different wavelength (excitation laser : λ=1310 nm), the excitation laser light is still guided through the same single-mode optical fiber that guides the laser light (detection laser : λ=1550 nm) used for the interferometric detection of the cantilever deflection. We present the details of the instrumentation and its performance. This setup allows us to eliminate the problems associated with the spurious mechanical resonances such as the apparent dissipation signal and the inaccuracy in the resonance frequency measurement.

Complete-noncontact photoacoustic microscopy by detection of initial pressures using a 3×3 coupler-based fiber-optic interferometer.

We demonstrate a 3×3 coupler-based fiber-optic interferometric system to detect the local initial photoacoustic pressure. In contrast with the existing interferometric photoacoustic microscopy (PAM) relying on the measurement of the phase change of the probe light caused by the sample surface vibration, the present method measures the intensity change of the probe light caused by the initial photoacoustic pressure. Compared with the conventional interferometric PAMs, this method has the advantages: (1) it is free from the influence of the rough tissue surface, achieving complete noncontact in vivo imaging; (2) the probe light and the excitation light are focused at a same point below the sample surface, and the confocal configuration makes it more convenient for in vivo imaging; and (3) there is no need for phase stabilization, allowing a high imaging speed. These advantages show that the method will be a promising technique for in vivo imaging. This method is verified by imaging of a resolution test target and in vivo imaging of the blood vessels in a mouse ear.

A bio-inspired sound source localization sensor with internal coupling

The mechanism of using internal coupling to enhance directional hearing has been found in various animals across multiple length scales, including crickets, lizards, frogs, birds, and alligators. For each eardrum, the acoustic stimuli impinge not only on the front side but also on the opposing side via the connecting cavity. The combination of these two stimuli renders a much higher directional sensitivity than the case with two uncoupled independent receivers. Inspired by this mechanism found in Nature, here we present a bio-inspired sound source localization sensor which consists of two pre-tensioned membranes on a three-dimensional printed housing. The vibration of the two membranes is detected by a low coherence fiber optic interferometric system. The experimental results from this prototype will be demonstrated to validate the feasibility of developing miniature bio-inspired devices for sound source localization.

High-spatial-resolution localization algorithm based on cascade deconvolution in a distributed Sagnac interferometer invasion monitoring system.

In the Sagnac fiber optic interferometer system, the phase difference signal can be illustrated as a convolution of the waveform of the invasion with its occurring-position-associated transfer function h(t); deconvolution is introduced to improve the spatial resolution of the localization. In general, to get a 26 m spatial resolution at a sampling rate of 4×106 s-1, the algorithm should mainly go through three steps after the preprocessing operations. First, the decimated phase difference signal is transformed from the time domain into the real cepstrum domain, where a probable region of invasion distance can be ascertained. Second, a narrower region of invasion distance is acquired by coarsely assuming and sweeping a transfer function h(t) within the probable region and examining where the restored invasion waveform x(t) gets its minimum standard deviation. Third, fine sweeping the narrow region point by point with the same criteria is used to get the final localization. Also, the original waveform of invasion can be restored for the first time as a by-product, which provides more accurate and pure characteristics for further processing, such as subsequent pattern recognition.

High resolution temperature independent refractive index measurement using differential white light interferometry

In this work a fiber optic interferometric system for differential refractive index measurement is described. The system is based on a white light Mach–Zehnder configuration, with serrodyne phase modulation, used to interrogate two similar non-adiabatic tapered optical fiber sensors in a differential scheme. In this situation the system is able to measure the refractive index independent of temperature. Signal processing with low cost digital instrumentation developed in Labview environment allows a detectable change in refractive index of Δn≈1.46×10−6, which is, from the best of our knowledge the highest resolution achieved using a bare fiber taper device for a range of refractive index close to the water index. The results demonstrate the potential of the proposed scheme to operate as a self-referenced chemical and biological sensing platform.

Fiber-optic interferometric two-dimensional scattering-measurement system

We present a fiber-optic interferometric system for measuring depth-resolved scattering in two angular dimensions using Fourier-domain low-coherence interferometry. The system is a unique hybrid of the Michelson and Sagnac interferometer topologies. The collection arm of the interferometer is scanned in two dimensions to detect angular scattering from the sample, which can then be analyzed to determine the structure of the scatterers. A key feature of the system is the full control of polarization of both the illumination and the collection fields, allowing for polarization-sensitive detection, which is essential for two-dimensional angular measurements. System performance is demonstrated using a double-layer microsphere phantom. Experimental data from samples with different sizes and acquired with different polarizations show excellent agreement with Mie theory, producing structural measurements with subwavelength accuracy.

Development of a closed‐loop micro‐/nano‐positioning stage with small tilting angles

This study is to develop a PC‐based closed‐loop micro‐/nano‐positioning stage with small tilting angles. The developed system mainly consists of a piezoelectric actuator, a displacement magnification mechanism, a ball joint, a micro‐/nano‐positioning stage body with four folded leaf springs, a fiber optic interferometer system, and a PC. After executing the optimization analyses of the stress, the displacement, and the frequency, using ANSYS software, the dimensions of the stage body have been designed and determined. A set of software, programmed using the LabView programming interface, was developed to conduct the PID control of the positioning system. According to the test results, the maximum tilting angles using the ball joint were about 1 arc‐second within a travel of 35 μm. A positioning capability of 10 nm under a tracing speed of 5 nm/s of the developed closed‐loop positioning stage, was possible based on the test results.

Performance analysis of a tandem of white-light fiber-optic interferometers

A white-light fiber-optic interferometric system composed of sensing and receiving interferometers separated with a dichroic-type linking interferometer was analyzed in order to determine the minimum group imbalance necessary to separate signal and noise patterns. For this purpose the analytical formulas for contrasts of the cross-interference patterns as well as the patterns produced by the sensing and receiving interferometers were derived. The contrast behavior was analyzed as a function of alignment at all coupling points and the dichroism of the linking interferometer. It is shown that the minimum group imbalance, which is the parameter responsible for temperature stability of the sensor, may be decreased to half for certain alignments.

Fiber-optic techniques for generating and detecting ultrasonic waves for quantitative NDE

This paper describes a method that uses pulsed laser light and coherent light interferometry in a fiber-optic-based system for non-contact generation and detection of ultrasound. Fiber-optic bundles are used to construct a rugged and versatile system to deliver geometrically conditioned, high-energy laser pulses capable of generating ultrasonic signals. In addition, an actively stabilized, multi-channel fiber-optic interferometric system for detecting these signals is described.

Large range coherence tuned fiber optic interferometric system for application in accelerometers

A low coherence interferometric system with large tracking range and self-initialization to be used for remote signal processing of fiber optic accelerometers is investigated. The tracking range of the low coherence system is 1.1 mm at low frequency. Phase resolution better than 1 mrad/√Hz at frequencies below 600 Hz is obtained, with a dynamic range of ≊90 dB. Data are presented which show that if the system were used to process the output from a compliant cylinder-type accelerometer, resolutions of ∼10−7 g/√Hz could be achieved.