E. J. Friebele

Optical fiber sensors for spacecraft: applications and challenges

Optical fiber sensors offer a number of advantages for spacecraft applications, including freedom from electromagnetic interference, light weight, and low power consumption. One application is strain sensing, where high sensitivity and bandwidth and the ability to individually interrogate tens of multiplexed sensors via a single fiber lead has been demonstrated. This paper will describe 2 recent NRL uses of distributed strain sensing using arrays of fiber Bragg gratings (FBGs) on spacecraft parts, structures, and ground test hardware: distributed dynamic strain monitoring of a lightweight reflector during acoustic qualification tests and high-frequency, high-sensitivity strain measurements of a latch fixture. A second fiber sensor being seriously considered for spacecraft is the interferometric fiber optic gyroscope (IFOG). Although its performance in a benign environment is quite attractive, deployment of this and other optical fiber sensors requires addressing issues such as the deleterious effects of the space radiation environment. These challenges, unique to this application, will be discussed.

Radiation-induced coloring of erbium-doped optical fibers

The radiation-induced coloring of erbium-doped optical fibers (EDFs) is reported. The radiation hardness of the EDFs is observed to be strongly dependent on composition. The implications for erbium-doped fiber amplifier (EDFA) performance is modelled.

Modified F-matrix calculation of Bragg grating spectra and its use with a novel nonlinear index growth law

Reflection spectra and index structures created by the growth of fiber Bragg gratings (FBGs) are modeled using a modified piecewise-uniform approach that can accommodate realistic index growth behavior. Because grating formation generally involves nonlinear index growth, models that assume sinusoidal modulation shapes do not accurately predict the evolution of the grating spectra during the writing exposure. The authors first present a generally applicable treatment of arbitrarily shaped index modulations such that their reflection spectra can be accurately treated with an established modeling technique. This approach examines the actual photoinduced index modulation shape at each subregion of the grating and identifies the ac and dc coupling coefficients (from coupled-mode theory) of an equivalent sinusoidal modulation at the fundamental Bragg resonance. These derived coupling coefficients are then used to compute the grating spectrum via the fundamental matrix (F-matrix) method. Given an accurate description of index at each point along the grating, the modified F-matrix method can efficiently model grating spectra that result from complex exposure schemes including scanned exposures, various apodization profiles, chirp, and postexposures with fringeless light. Additionally, this paper presents a method for determining the detailed index profiles formed by arbitrary exposures. To obtain realistic index modulation profiles, a new index growth model consisting of a three-dimensional (3-D) surface of induced index (versus exposure time and intensity) and a rule for linking complex sequences of index growth under differing intensities is introduced. Using the index growth surface, the compound growth rule, and the modified F-matrix technique, the spectra of weak FBGs similar to those found in distributed fiber sensor systems are numerically synthesized.

Sensor grating array demodulation using a passively mode-locked fiber laser

Proven compatibility with civil engineering applications and composite structures has spurred increasing interest in fiber Bragg gratings (FBGs) for distributed optical fibre strain sensing. We demonstrate a fundamentally different demodulation scheme based on the high-energy, ultrafast, broadband pulses generated by a passively mode-locked erbium fiber laser as an alternative approach for high-speed interrogation of large grating arrays.

Noncontact measurement of optical fiber draw tension

A simple technique for measuring the tension on an optical fiber during the draw process without contacting the fiber surface is reported. It is based on detecting the resonant vibrational frequency of the fiber during draw using the position output of the fiber diameter monitor and requires no specialized circuitry. The tension, determined from the fundamental resonant frequency of the length of fiber between the neckdown region in the furnace and the coating cup, is accurate to within 1% and repeatable to >

Bend and twist sensing in a multi-core optical fiber

We present a method for sensing both bending and twisting in an optical fiber which is compatible with extensive multiplexing. The technique employs a multi-core fiber design, high-performance static strain sensing, and robust reconstruction algorithms.

Cooperative implementation of a high-temperature acoustic sensor

The current status and results of a cooperative program aimed at the implementation of a high-temperature acoustic/strain sensor onto metallic structures are reported. The sensor systems that are to be implemented under this program will measure thermal expansion, maneuver loads, aircraft buffet, sonic fatigue, and acoustic emissions in environments that approach 1800 F. The discussion covers fiber development, fabrication of an extrinsic Fabry-Perot interferometer acoustic sensor, sensor mounting/integration, and results of an evaluation of the sensor capabilities.

Sensor grating demodulation using a passively mode locked fiber laser

A fiber Bragg grating sensor array is interrogated using a broad bandwidth passively mode locked fiber laser source. A novel demodulation scheme is demonstrated using highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses. The mode locked fiber laser was then modified and operated in the square pulse regime, where 4 W, 10 ns pulses with bandwidths greater than 60 nm were used successfully to illuminate 2% fiber Bragg gratings.

Radiation effects in optical communications systems: fibers exposed at low-dose rates and Selfoc lenses

Fiber waveguides will be exposed to low-dose- rate nuclear environments in many commercial systems: typical values are ~0.1 rad/year for sea level cosmic ray background and ≳1 rad/day in some spacecraft and nuclear power applications. Most previous studies of radiation effects on optical fibers have employed either pulsed sources or steady-state sources with moderate (10-104-rad/ min) dose rates.1

Radiation effects in polarization-maintaining fibers

One likely application for polarization-maintaining (PM) fiber is in fiber-optic gyroscopes onboard spacecraft. Severe performance degradation could occur if the PM fiber is sensitive to the space radiation environment, which depends on such factors as the spacecraft orbit, shielding by the skin and structure, and sunspot activity. An approximate dose rate for a fiber gyro in the spacecraft interior is ~1 krad/yr.