Technical Papers

» Commercialization of Interferometric Interrogation Techniques for Fiber Sensing Applications

JEFF BUSH and ALLEN CEKORICH
7652 Haskell Ave.
Van Nuys, CA 91406

Optiphase, Inc. develops and manufactures fiber optic interferometric sensing and measurement systems. Principal activities involve the commercialization of this technology in two distinct categories: 1) Interferometric Demodulation; and 2) White-Light Coherence Domain Systems. Instead of focusing on specific end-use sensing systems, we have targeted commercialization of the elements of these systems involved with the interrogation of the sensors and accurate conversion of (optical) sensor information into electronic formats. The motivating concept is that commercialization of interrogators will address multiple markets with common designs leading to cost reduction through higher volume manufacturing. This business approach pre-supposes that such standardization is acceptable to and meets the needs of the (fiber) sensing industry. This discussion reviews the commercialization efforts for the two interrogator categories.

Interferometric Demodulation

There are many applications that exist where the phase between two beams of light in an interferometer needs to be measured. The term "interferometric demodulation" is used to perform this measurement. For the case of this presentation, this specifically implies true-phase measurements made by inverse trigonometric means. Generally, in order to mechanize the demodulation process and maintain high measurement accuracy, the interrogation approach needs to provide for a controlled modulation method and synchronized signal sampling. The most general configuration depicting such an arrangement is shown in figure 1.

Here, the timing/control block serves to control the modulation levels and acquisition timing.

Figure 1. Generalized Demodulator Configuration

The Optiphase implementation for demodulation utilizes a unique sensor interrogation approach based on time domain sampling. It is suited for sinusoidal or discrete level modulation, and for single or multiple (time multiplexed) channel sensor systems. The demodulation circuitry is all digital except for the analog interfaces. It utilizes an embedded DSP for core processing and master control, programmable logic devices for timing and digital I/O, DAC’s for modulation drive and ADC’s for direct digitization of optically received signals. The merits of this design approach include:

• Open-Loop demodulation approach (applicability to most interferometer types);
• Self Correction (modulation depth, sample phasing errors, and intensity fluctuations);
• Time Domain process is both code and sample efficient;
• High dynamic range (micro-radians to thousands of radians via fringe counting);
• Measurements are precise (intensity independent, dc coupled, typical THD of 0.1%);
• It can be used by unskilled operators and importantly;
• It is Low Cost in Production.

Single-Channel Demodulators. Optiphase has commercialized two types of single channel demodulators and two types of multi-channel demodulators. Figure 2 shows, a low cost demodulator circuit with measurement rate capabilities out to 140 KHz. It provides an analog modulation output signal and a 32 bit digital output (measured optical phase signal) through the DSP’s high speed serial interface. Figure 3 shows the a more fully featured single channel demodulator. This device includes a three-channel polarization diversity (optical) receiver where the demodulator is configured to automatically select the (1 of 3) receivers having the highest interferometric visibility on a continuous basis with seamless signal output (no transients produced during receiver switch process). Additionally, this more enhanced demodulator provides both filtered analog and digital outputs, and is firmware ready for configuration into a CW multi-channel system.

Both demodulators in figures 2 and 3 have similar measurement performance capabilities. Key features are:

• self noise of 2-3 urad/rt-Hz;
• measurement range of +/- 12,000 radians;
• dc coupled measurement, 32 bit output word
• typical linearity is 0.1%.

Figure 3. Enhanced Single Channel Demodulator

Multi-Channel Demodulators. Optiphase has also commercialized multi-channel demodulators. Here the basic demodulation approach is the same as single channel, but the interrogation technique differs by implementation of a pulsed source so that Time Division Multiplexed (TDM) addressing of fiber sensors in a line array may be utilized. Figure 4 shows a generalized block diagram of an interrogator (including demodulator) for a multi-channel sensor system.

Figure 4. Generalized Multi-Channel Sensor System showing Interrogator with Demodulator

These multi-channel configurations can employ fiber based sensors configured (with couplers) to form Mach-Zehnder or Michelson type interferometers or with partial reflectors or placed between the sensors to form a “N” pulse, time separated return response to an input pulse from the interrogator. Current design interrogators manufactured by Optiphase employ CW lasers, Integrated optic (dual MZ) pulse forming elements and discrete level IOC based phase modulators configured within compensating interferometers. The demodulator remains as an embedded DSP circuit (TMS320C6201 based COTS board) with a more sophisticated (custom) timing circuit, which provides timing for the pulse and modulator drive, and the sampling electronics. The DSP controls the modulator drive and calculates optimum sample times for the returning pulses.

These type of interrogator designs have proven effective for multi-channel systems, however they are somewhat costly, as the specialized integrated optic devices and associated PM components used are not available as low cost items. Design efforts are underway to reduce the cost of multichannel interrogators by replacement of the IOC pulser with fiber coupled acousto-optic elements, and replacement of PM components with single mode components where applicable.

White Light Coherence Domain Systems

Traditional white-light scanning interferometers utilize bulk optic components mounted on a mechanical scanning mechanism. Many emerging applications for these interferometers require fiber optic probes. By design, such fiber compatible instruments are expensive and are limited to slow scan rates. Optiphase, Inc. has commercialized this new “all-fiber” design approach which reduces the cost of such designs and enables higher scan rates by use of highly efficient fiber wrapped piezoelectric cylinder elements. Applications for such instruments include:

• Medical - Intravascular monitoring, Tissue, Bone, Plaque and Lesion location or ID;
• Industrial - Thickness Gauge, Displacement Locator, Optical Inspection;
• Birefringent Material Measurements, Dental, Composite Structure Evaluation;
• Optical Coherence Tomography (OCT) – 3 dimensional measurements.

All-Fiber PM Michelson White-Light System The first system commercialized used a Michelson configuration with PM fiber. This is shown below in figure 5. The left side shows the optical schematic and the right shows the OEM “PM Scan Assembly.” The fiber leads shown are intended to be connected to a user developed probe and reference fibers which complete the interferometer. The unit also requires external power and will accept a wide variety of modulator control input signals. Scan rates for this device vary with the range desired and are roughly 200 Hz/mm for triangle waveforms and 500 Hz/mm for sine waveforms (in air).

The PM system in figure 5, although effective for its intended design has a number drawbacks relating to cost and performance. In order for the system to work with transform limited resolution (to the optical source) the PM fiber used in the two legs of the interferometer must be screened to match the dispersion. Additionally, since the design uses PM fiber, both the fiber and the coupler are relatively high cost. Lower cost Michelson designs made with single mode fiber cannot be used as interferometric visibility is not preserved and the PZT modulators create significant birefringence modulation which impair the measurement resolution.

All Fiber SMF Autocorrelator

A variation of the All-Fiber Michelson white-light interferometer is realized when the probe (fiber) is located external to the interferometer. In this case, the light returning from the sample is sent to a scanning interferometer and then processed. This is an autocorrelator configuration and has the advantage of using a probe of arbitrary length without having to match the length of a probe fiber to the length of a reference fiber. Additionally, this device can be made with single mode fiber if Faraday Reflector Mirrors (FRM) are used in the scanning section. This implementation defeats the birefringence modulation and interferometric visibility problems mentioned earlier for the Michelson configuration. A collaborative effort between Optiphase, Inc. and Eastman Kodak has been focused on investigating the effectiveness of an all-fiber autocorrelator to make precision displacement measurements, where a second (coherent) wavelength is co-propagated through the scanning section. This arrangement is shown in figure 6.

Signals created in the coherent channel are used to "strobe" the digitizer used for the measurement channel and post processing is implemented to resolve the location of specular reflections from the sample to accuracies significantly less than 1 um. This all fiber configuration can be manufactured at half the price of its bulk optic equivalent and provide measurement rates 5 to 10 times faster.