Simultaneous interrogation of multiple cores in a shape sensor fiber with a graded index fiber micro-turnaround.

A critical limitation for optical fiber sensor technology is the complexity of the interrogators used in such measurements, which has driven continued interest in enhanced optical fibers and fiber assemblies that will simplify interrogator design. In this work, we report on a novel multicore fiber shape sensor utilizing a distal graded index (GRIN) fiber micro-turnaround. We show that four offset cores of this fiber can be interrogated simultaneously with a single high performance optical frequency domain reflectometry measurement. The GRIN turnaround is 498 µm in length and reflects signal from one offset core to an opposite core with a 2 dB roundtrip attenuation. We show that the bend sensing accuracy of our single measurement system is similar to the accuracy of sequential measurements of four individual cores. We also demonstrate fiber shape reconstruction with a single measurement over 0.55 m with 80 µm spatial resolution when the fiber is wrapped around two posts.

Polarization-maintained propagation of a 2200 µm2 effective-area higher-order-mode in a bent optical fiber.

We report on the excitation and polarization preserved propagation of a very large effective-area (Aeff ∼ 2240 µm2) higher-order-mode in an optical fiber. A laser signal operating in the 1 µm wavelength region is transported in a Bessel-like LP0,4 mode over a 10 m long section of the polarization-maintaining higher-order-mode fiber. We observe that the light propagates through the fiber with >10 dB polarization-extinction-ratio as the fiber is coiled into circular loops of 40 cm diameter.

Measuring the shape of microbends in optical fibers.

We report on the distributed shape measurement of small deformations produced along the length of an optical fiber. The fiber contains multiple waveguiding cores, each inscribed with weak continuous Bragg gratings. The distributed Bragg-reflectivity data for the fiber cores, obtained from the optical backscatter reflectometry, are used to estimate the local curvature and the position of the fiber. We successfully demonstrate the sensing of periodic microdeformations-approximately 1 µm or less in amplitude and a few hundred µm in length. Such microbends are known to cause attenuation in optical fibers, and the approach presented here can enable a detailed measurement of these microbends in applications ranging from telecommunications cable design to biotechnology, robotics, manufacturing, aerospace, and security.

Polarization-maintaining, large-effective-area, higher-order-mode fiber.

Higher-order-mode (HOM) fibers guiding light in large-effective-area (Aeff) Bessel-like modes have recently generated great interest for high-power laser applications. A polarization-maintaining (PM) version of HOM fibers can afford the added possibility of coherent beam combination, improved material processing, and polarization multiplexing of high-power fiber lasers. We report a PM-HOM fiber for guiding Bessel-like modes with Aeff ranging from 1200-2800 µm2. The fiber modes exhibit a birefringence value that compares well with that of a conventional single-mode PM fiber (2×10-4), and exhibit a polarization extinction ratio ranging from 13-23 dB over meter-long fiber lengths, practical for amplifier systems. This fiber presents a unique platform for next-generation high-power fiber systems, as well as for the fundamental studies on deterministically polarized Bessel-like modes in fibers.

Higher-order mode fiber enables high energy chirped-pulse amplification.

Energy scaling of femtosecond fiber lasers has been constrained by nonlinear impairments and optical fiber damage. Reducing the optical irradiance inside the fiber by increasing mode size lowers these effects. Using an erbium-doped higher-order mode fiber with 6000 µm(2) effective area and output fundamental mode re-conversion, we show a breakthrough in pulse energy from a monolithic fiber chirped pulse amplification system using higher-order mode propagation generating 300 µJ pulses with duration <500 fs (FWHM) and peak power >600 MW at 1.55 µm. The erbium-doped HOM fiber has both a record large effective mode area and excellent mode stability, even when coiled to reasonable diameter. This demonstration proves efficacy of a new path for high energy monolithic fiber-optic femtosecond laser systems.

Single-frequency Brillouin distributed feedback fiber laser.

We demonstrate a single-frequency Brillouin distributed feedback laser (DFB). The DFB laser cavity was a 12.4 cm long fiber Bragg grating with a π-phase shift offset from the grating center. It exhibited a threshold of 30 mW and conversion efficiency from pump to Stokes wave as high as 27%. Higher-order Stokes waves were suppressed by more than 20 dB. The Stokes output of the laser could be obtained in either the forward or backward direction, simply by changing the orientation of the offset of the discrete phase shift with respect to the pump propagation direction. The DFB laser operated over a pump frequency range of 1.2 GHz, more than 60 times larger than the SBS gain bandwidth.

Raman fiber distributed feedback lasers.

We demonstrate fiber distributed feedback (DFB) lasers using Raman gain in two germanosilicate fibers. Our DFB cavities were 124 mm uniform fiber Bragg gratings with a π phase shift offset from the grating center. Our pump was at 1480 nm and the DFB lasers operated on a single longitudinal mode near 1584 nm. In a commercial Raman gain fiber, the maximum output power, linewidth, and threshold were 150 mW, 7.5 MHz, and 39 W, respectively. In a commercial highly nonlinear fiber, these figures improved to 350 mW, 4 MHz, and 4.3 W, respectively. In both lasers, more than 75% of pump power was transmitted, allowing for the possibility of substantial amplification in subsequent Raman gain fiber.

Enhanced Optical Fiber for Distributed Acoustic Sensing beyond the Limits of Rayleigh Backscattering.

We report on engineered fibers with enhanced optical backscattering that exceeds Rayleigh scattering limits by more than one order of magnitude. We measure attenuation less than 0.5 dB/km from 1,300 to 1,650 nm. By controlling the enhanced backscatter over a 1.5-km length, we compensate for this attenuation, resulting in a higher backscatter signal at the end of the fiber. We demonstrate that the scattering strength may be stabilized for operation at temperatures above 200°C for at least 3 weeks. We show that the deleterious signal distortion due to the Kerr nonlinearity is within 10% of standard fiber. We then report on the use of these fibers in distributed acoustic sensing (DAS) measurements. A significant increase in acoustic signal-to-noise ratio leads to the possibility of improved spatial resolution in the enhanced fiber DAS system.

Polarisation dynamics of vector soliton molecules in mode locked fibre laser.

Two fundamental laser physics phenomena--dissipative soliton and polarisation of light are recently merged to the concept of vector dissipative soliton (VDS), viz. train of short pulses with specific state of polarisation (SOP) and shape defined by an interplay between anisotropy, gain/loss, dispersion, and nonlinearity. Emergence of VDSs is both of the fundamental scientific interest and is also a promising technique for control of dynamic SOPs important for numerous applications from nano-optics to high capacity fibre optic communications. Using specially designed and developed fast polarimeter, we present here the first experimental results on SOP evolution of vector soliton molecules with periodic polarisation switching between two and three SOPs and superposition of polarisation switching with SOP precessing. The underlying physics presents an interplay between linear and circular birefringence of a laser cavity along with light induced anisotropy caused by polarisation hole burning.

Two-photon fourth-order polarimetery.

We demonstrate a fourth-order polarimeter that employs two-photon detection to measure the fourth-order polarization coherency matrix (FOCM) of an optical signal. We measure the FOCM of picosecond pulses subjected to polarization mode dispersion (PMD). The fourth-order polarimeter distinguishes different PMD states even when the state of polarization, including the degree of polarization, is constant.

Scattering from nonuniform tilted fiber gratings.

Using volume current analysis, we describe the scattering from intracore fiber gratings with various nonuniformities and find that effects of cross-sectional and longitudinal variations can be roughly decoupled. We show that cross-sectional asymmetry affects the azimuthal pattern of scattering around the fiber and find good agreement with real gratings exhibiting UV-induced grating asymmetry across the core. We also show that longitudinal variations affect the spread of scatter angles with respect to the fiber axis, and this angular spread depends on the Fourier transform of the profiles of the grating period, the average index, and the index modulation of the grating.

Spectral polarimetry with a differential group delay bias.

We show that the addition of a polarization controller and bias differential group delay to a spectral polarimeter increases the amount of polarization information that can be extracted from a Stokes spectrum measurement. We use this biased spectral polarimetry on 40-Gbit/s signals to measure the polarization mode dispersion of the optical fiber through which they propagate. Our measurements do not require any control of the polarization at the fiber input. Averaging measurements from several settings of the polarization controller yields improved accuracy, and, after data rejection is applied, both the accuracy and the number of valid data points is increased when using several measurements.

Influence of the longitudinal mode field in grating scattering from weakly guided optical fiber waveguides.

We measured the polarization dependence of light scattered from a tilted fiber grating and found disagreement with previous volume-current perturbation analysis. However, by including the longitudinal E field of the guided wave we were able to obtain good agreement, demonstrating that, although it is small, this component cannot be neglected when scattering of weakly guided waves is considered. A first-order approximation formula for the polarization dependence was also obtained and is shown to be accurate within most of the resonance band of scattering.

Interferometric characterization of phase masks.

We demonstrate a novel interferometric technique for highly accurate characterization of phase masks used in optical fiber grating fabrication. The principle of the measurement scheme is based on the analysis of the interference pattern formed between the first- and zero-order beams transmitted through or reflected from the grating under test. For spatial resolution of a few millimeters, our methods allow the determination of local variations of the order of 1-microm grating period with an accuracy of a few picometers. These methods are applicable to a broad class of diffractive grating structures.