Gary A. Miller

Fundamental frequency noise properties of extended cavity erbium fiber lasers

A multimode linear cavity and a single-mode unidirectional ring cavity fiber laser with meter-long cavity lengths are shown to exhibit frequency noise limited by fundamental thermodynamic noise from 100 Hz to 100 kHz. Their measured spectra agree closely with theoretically derived thermodynamic noise and the characteristic dependence of the frequency noise power spectrum on the inverse of the cavity length is observed. The unidirectional ring laser exhibits a frequency noise of 2 Hz/Hz(1/2) at 1 kHz, one of the lowest published values to date from a free-running laser. The multimode linear cavity laser is shown to be a suitable candidate for thermal-noise-limited, meter-long fiber laser strain sensors with a strain resolution of 14 fϵ/Hz(1/2) at 1 kHz.

Shape sensing using distributed fiber optic strain measurements

The two-dimensional shape of a multicore optical fiber is estimated from distributed strain measurements. The fibers shape is calculated based on the local curvature of the fiber and assuming practical strain sensitivities.

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.

Inscription of Fiber Bragg Gratings in Multicore Fiber

We present solutions to unique problems associated with simultaneously writing FBGs in 4 cores of a multicore fiber used for bend and twist sensing, including exposure conditions, photosensitivity balance, and draw-induced refractive index changes.

Bend and Twist Sensing in a Multiple-Core Optical Fiber

A method for estimating an optical fibers shape from internal strain states is presented. Localized sensing of both bending and twisting requires a specialized multi- core fiber design, high-performance static strain sensing, and robust reconstruction algorithms.

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.

Fiber laser sensors: enabling the next generation of miniaturized, wideband marine sensors

Fiber laser strain sensors achieve fundamentally limited strain resolution, resulting in their ability to resolve axial fiber displacements at the sub-femtometer level. This ultra-high resolution enables the development of miniaturized sensors capable of achieving the performance necessary for high resolution marine sensing. The reduction in size also facilitates an increased operating bandwidth for broadband acoustic transducers. The last decade has seen considerable development of this technology, moving it from a laboratory curiosity to deployable demonstrations. Significant advances have been made in understanding the fundamental properties of erbium fiber laser strain sensors as well as laser multiplexing, signal demodulation and sensor design. This talk will describe the basic properties of fiber laser strain sensors and show how the technical challenges involved in developing deployable, multiplexed arrays of miniature transducers have been overcome. In particular the development of miniature wideband hydrophones, low power DC magnetometers with sub-nT resolution and miniature acoustic vector sensors will be described. Finally, a view towards potential future applications of this technology will be given.

Radiation-Induced Loss Predictions for Pure Silica Core, Polarization-Maintaining Fibers

We examined the room temperature radiation-induced optical attenuation of silica-core, polarization-maintaining fibers with boron stress rods. The fibers were exposed to 55 MeV protons to equivalent doses between 100 krad and 1 Mrad at dose rates of 2 to 100 rad/s. Spectra from 1250-1725 nm were recorded during both exposure and ~2,800 h recovery. The spectral response over time indicates two separate populations of defects that affect loss. Explicit kinetic modeling indicates that the growth of the induced loss is consistent with a power law in dose rate, and recovery is consistent with low (1st or 2nd) order annealing with a distribution of activation energies and a single attempt frequency for each population. For predicting the radiation-induced loss in exposure conditions of much lower dose rate and much longer time, our theory and numerical simulations indicate that the best experiments for reducing risk in our extrapolations include higher temperature anneals, and, unavoidably, longer exposures at lower dose rate.

High-Performance Sensing Using Fiber Lasers

Fiber laser sensors are capable of achieving fundamentally limited strain resolution. Their high sensitivity per unit length enables new sensing possibilities, miniaturization of existing sensors and simplification of sensing mechanisms.

Coherent light transmission properties of commercial photonic crystal hollow core optical fiber.

Photonic crystal hollow core fiber (PC-HCF) has enabled many exciting new applications in nonlinear optics and spectroscopy. However, to date there has been less impact in coherent applications where preservation of optical phase over long fiber lengths is crucial. This paper presents characteristics of three commercially available PC-HCFs relevant to coherent applications including higher-order mode analysis, birefringence and polarization-dependent loss, and their impact on coherent light transmission in PC-HCF. Multipath interference due to higher-order mode propagation and Fresnel reflection is shown to generate excess intensity noise in transmission, which can be suppressed by up to 20 dB through high frequency phase modulation of the source laser. To demonstrate the potential of PC-HCF in high performance sensing, a Mach-Zehnder interferometer (MZI) incorporating 10 m of PC-HCF in each arm is characterized and demonstrates a phase resolution (59×10(-9)  rad/Hz(1/2) at 30 kHz) close to the shot noise limit, which is better than can be achieved in a MZI made with the same length of single mode solid core fiber because of the limit set by fundamental thermodynamic noise (74×10(-9)  rad/Hz(1/2) at 30 kHz).