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.

Reconversion of higher-order-mode (HOM) output from cladding-pumped hybrid Yb:HOM fiber amplifier.

We demonstrate operation of a cladding-pumped hybrid ytterbium-doped HOM fiber amplifier and reconversion of the HOM output to Gaussian-like beam by using an axicon based reconversion system. The amplifier was constructed by concatenating single-mode and HOM ytterbium-doped double clad fibers, and was excited by a common multimode pump source. A continuous wave (cw) input signal of 97mW was amplified to 100W at the amplifier output, which yielded a gain of more than 30dB. The LP0,10 output of the HOM amplifier could be converted to a Gaussian-like beam with 67% conversion efficiency. We present, both analytically and numerically, the effects of scaling the beam size on axicon's apex angle, and how shape imperfections affect the mode converter's performance.

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.

Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber.

We demonstrate a seven-core erbium-doped fiber amplifier in which all the cores were pumped simultaneously by a side-coupled tapered multimode fiber. The amplifier has multicore (MC) MC inputs and MC outputs, which can be readily spliced to MC transmission fiber for amplifying space division multiplexed signals. Gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band.

Crosstalk in multicore fibers with randomness: gradual drift vs. short-length variations.

Random perturbations play an important role in the crosstalk of multicore fibers, and can be captured by statistical coupled-mode calculations. In this approach, phase matching contributes a multiplicative factor to the average crosstalk, depending on the perturbation statistics and any intentional heterogeneity of neighboring cores. The impact of perturbations is shown to be qualitatively different depending on whether they are gradually varying, or have short-length (centimeter-scale) variations. This insight implies a novel crosstalk suppression strategy: fast modulation of a bend perturbation by spinning the fiber can disrupt the bend-induced phase matching.

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.

Real time monitoring of a fiber fuse using an optical time-domain reflectometer.

We propose and experimentally demonstrate monitoring of a fiber fuse in real time using an optical time domain reflectometer (OTDR). When a fuse starts, a weak reflection of light occurs from the leading edge of the fuse where plasma and voids are being formed in the core. In this work, we examined the possibility of monitoring a fiber fuse from a remote location using an OTDR. We demonstrate a method that allows us detect a fuse progressing at remote locations (over kilometers away). It was found to be effective even in the presence of strong spurious backscattering, such as spontaneous Raman scattering due to a strong continuous wave pump. Moreover, from the progress of the reflection edge monitored by the OTDR, the fuse velocity could be readily determined.

Backreflected radiation due to a propagating fiber fuse.

The properties of backreflected light due to voids in a fiber fuse were studied using optical coherence-domain reflectometry of a damaged fiber and real-time monitoring of the electrical (RF) and optical frequency spectrum. Light reflected backward at the interface of a propagating fiber fuse acquired low-frequency broadband amplitude modulation, which can be detected remotely at the source end, using an RF spectrum analyzer. For the light backreflected during propagation of a fuse, we derived an analytical expression that well explained the spectral features observed experimentally in the RF spectrum. Finally, a novel method that allowed us to rapidly terminate the fuse propagation (in a few milliseconds) is also shown.

Measurements of spectral broadening and Doppler shift of backreflections from a fiber fuse using heterodyne detection.

We report broadening in spectrum as light is backreflected from a propagating fiber fuse. The formation of micrometer-size voids results in periodic changes in Fresnel reflection occurring in the leading part of the fuse. High-resolution measurement of the optical spectrum via heterodyne detection revealed an upshift (Doppler shift) in the optical frequency and discrete frequency components that are characteristic of amplitude modulation due to periodic void formation.

Gain-assisted superluminal propagation in tellurite glass fiber based on stimulated Brillouin scattering.

We demonstrate superluminal propagation of optical pulses with amplification in optical fibers based on stimulated Brillouin scattering. A triple gain peak configuration is used for the generation of narrowband anomalous dispersion in 2 m tellurite glass fiber, where the group index change as much as -1.19 is achieved with 6.9 dB amplification in 34 ns Gaussian pulses, leading to the group index of 0.84.

High-speed all-optical modulation using an InGaAs/AlAsSb quantum well waveguide.

All-optical cross-phase modulation through the intersubband transition in an InGaAs/AlAsSb coupled quantum well was examined by using a sinusoidal intensity-modulated pump. For probe waves in the wavelength range of 1330-1620 nm and a pump modulation at a repetition rate of 76 GHz, cross-phase modulation (XPM) occurred with an efficiency eta, of 0.653-0.142 rad/W. Based on the efficiency we derived a transfer function for the intersubband transition (ISBT) modulator that can be conveniently used in simulating the characteristics of the modulated output (probe) for long excitation pulses with arbitrary waveforms.

Single-frequency Brillouin lasing using single-mode As(2)Se(3) chalcogenide fiber.

Brillouin lasing in a single-longitudinal mode at 1.55 mum is demonstrated using As2Se3 single-mode fiber for the first time. The As(2)Se(3) fiber provides sufficient Brillouin gain for the Stokes wave to initiate single frequency oscillation in a 2-m long fiber Fabry-Perot cavity with a nonresonant pump power of 56 mW. For a pump power of 78 mw, 12 mW of Stokes power was obtained, which corresponded a conversion efficiency of 15%.

Stimulated Brillouin scattering in single-mode tellurite glass fiber.

Stimulated Brillouin scattering properties in a single-mode tellurite glass fiber were studied using a cw laser with an operating wavelength of 1.54 mum. The Brillouin frequency shift v(B) and the gain linewidth Deltav(B) were 7.882 GHz and 23.6 MHz, respectively. A Brillouin gain coefficient g(B) in the range of 1.47 x 10(-10)-2.16 x 10(-10) m/W was measured. The higher gain coefficient of the tellurite fiber, together with its relatively low loss compared with other non-silica fibers, makes it a suitable candidate for realizing efficient, all-optical, slow-light devices.

Highly efficient Brillouin slow and fast light using As(2)Se(3) chalcogenide fiber.

We demonstrate the generation of slow and fast light based on stimulated Brillouin scattering in As(2)Se(3) chalcogenide fiber with the best efficiency ever reported. The Brillouin gain of 43 dB is achieved with only 60-mW pump power in a 5-m single-mode chalcogenide fiber, which leads to the optical time delay of 37 ns with a 50-ns Gaussian pulse.

Observation of strong stimulated Brillouin scattering in single-mode As2Se3 chalcogenide fiber.

Strong stimulated Brillouin scattering in single-mode As2Se3 chalcogenide fiber is observed using a cw laser at 1.55 microm wavelength region. Brillouin threshold for a 5-m-long fiber is as small as 85 mW. The Brillouin frequency shift vB and the gain linewidth DeltavB are 7.95 GHz and 13.2 MHz, respectively, measured with heterodyne detection and an RF spectrum analyzer. A Brillouin gain coefficient gB of 6.0 x 10(-9) m/W, about 134 times larger than that of fused silica fiber, is obtained for As2Se3 single mode fiber from measurements of Brillouin threshold power and the gain linewidth.

Brillouin amplification and lasing in a single-mode As2Se3 chalcogenide fiber.

Brillouin amplification and lasing are demonstrated in a single-mode As2Se3 chalcogenide fiber. A Brillouin gain of 42 dB was measured in a 4.9 m long fiber for a pump power of 68 mW at a 1.56 mum wavelength. In addition, a compact As2Se3 fiber-based Brillouin laser with a threshold power of 35 mW and a slope efficiency of 38% for nonresonant pumping is demonstrated.

Ultrafast pulse retiming by cross-phase modulation in an anomalous-dispersion polarization-maintaining fiber.

Retiming of signal pulses by orthogonally polarized control pulses copropagating in an anomalous-dispersion low-birefringence polarization-maintaining fiber is demonstrated. Through walk-off-free cross-phase modulation in a 500 m long fiber, optical pulses at a 10 GHz rate are retimed by as much as +/- 2 ps.

Widely tunable femtosecond soliton pulse generation at a 10-GHz repetition rate by use of the soliton self-frequency shift in photonic crystal fiber.

We demonstrate a soliton self-frequency shift of approximately 120 nm in a fiber with 1.56-microm pulses generated at a 10-GHz repetition rate by an actively mode-locked laser. A highly nonlinear photonic crystal fiber with a length of only 12.6 m and a nonlinear coefficient of 62 W(-1) km(-1) is used to achieve such broadband operation. The wavelengths of the resulting sub-300-fs solitons can be tuned effectively by adjusting the input power. The maximum output power of the solitons exceeds 200 mW.