L. Brandon Shaw

Large Raman gain and nonlinear phase shifts in high-purity As 2 Se 3 chalcogenide fibers

Third-order Kerr nonlinearities and Raman gain are studied experimentally in high-purity As2Se3 optical fibers for wavelengths near 1.55 μm. Kerr nonlinear coefficients are measured to be nearly 1000 times higher than those for silica fibers. In pulsed mode, nonlinear phase shifts near 1.2-π rad are measured in fibers only 85 cm long with peak pulse powers near 3 W. However, there are nonlinear losses near 20% for nonlinear phase shifts near π. By use of a cw optical pump, large Raman gains nearly 800 times that of silica were measured. In the cw case there were losses in the form of index gratings formed from standing waves at the exit face of the fiber. Discrete Raman amplifiers and optical regenerators are discussed as possible applications.

Chalcogenide Glass-Fiber-Based Mid-IR Sources and Applications

The Naval Research Laboratory (NRL) is developing chalcogenide glass fibers for applications in the mid-and long-wave IR wavelength regions from 2 to 12 mum. The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the IR, possess low phonon energies, are chemically durable, and can be drawn into fiber. Both conventional solid core/clad and microstructured fibers have been developed. Chalcogenide glass compositions have been developed that allow rare earth doping to enable rare-earth-doped fiber lasers in the IR. Also, highly nonlinear compositions have been developed with nonlinearities ~1000times silica that enables nonlinear wavelength conversion from the near IR to the mid-and long-wave IR. In this paper, we review rare-earth-doped chalcogenide fiber for mid-and long-wave IR lasers, and highly nonlinear chalcogenide fiber and photonic crystal fiber for wavelength conversion in the mid-and long-wave IR.

Maximizing the bandwidth of supercontinuum generation in As 2 Se 3 chalcogenide fibers

We describe in detail a procedure for maximizing the bandwidth of supercontinuum generation in As(2)Se(3) chalcogenide fibers and the physics behind this procedure. First, we determine the key parameters that govern the design. Second, we find the conditions for the fiber to be endlessly single-mode; the fiber should be endlessly single-mode to maintain high nonlinearity and low coupling loss. We find that supercontinuum generation in As(2)Se(3) fibers proceeds in two stages--an initial stage that is dominated by four-wave mixing and a later stage that is dominated by the Raman-induced soliton self-frequency shift. Third, we determine the conditions to maximize the Stokes wavelength that is generated by four-wave mixing in the initial stage. Finally, we put all these pieces together to maximize the bandwidth. We show that it is possible to generate an optical bandwidth of more than 4 microm with an input pump wavelength of 2.5 microm using an As(2)Se(3) fiber with an air-hole-diameter-to-pitch ratio of 0.4 and a pitch of 3 microm. Obtaining this bandwidth requires a careful choice of the fibers waveguide parameters and the pulses peak power and duration, which determine respectively the fibers dispersion and nonlinearity.

Applications of chalcogenide glass optical fibers

Abstract Chalcogenide-glass fibers based on sulfide, selenide, telluride and their rare-earth-doped compositions are being actively pursued worldwide. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite this, chalcogenide-glass fibers are enabling numerous applications that include laser power delivery, chemical sensing, and imaging, scanning near field microscopy/spectroscopy, IR sources/lasers, amplifiers and optical switches.

Computational study of 3–5 μm source created by using supercontinuum generation in As 2 S 3 chalcogenide fibers with a pump at 2 μm

We present simulation results for supercontinuum generation using As(2)S(3) chalcogenide photonic crystal fibers. We found that more than 25% of input power can be shifted into the region between 3 microm and 5 microm using a pump wavelength of 2 microm with a peak power of 1 kW and an FWHM of 500 fs. The broad dispersion profile and high nonlinearity in As(2)S(3) chalcogenide glass are essential for this application.

All-fiber mid-IR supercontinuum source from 1.5 to 5 µm

An all-fiber supercontinuum source extending from 1.5 to 5 μm has been demonstrated in single-mode step-index As2S3 fiber using a Raman shifted erbium doped mode-locked silica fiber laser pump source. 140 mW broadband power was demonstrated with a spectral intensity variation of 10 dB from 1.9 to 4.4 μm and 20 dB from 1.65 to 4.78 μm.

Anti-reflective surface structures for spinel ceramics and fused silica windows, lenses and optical fibers

Anti-reflective surfaces structures (ARSS) have been successfully fabricated on fused silica windows, lenses and fibers, and spinel ceramics. The reflection loss for spinel was reduced from 7% per surface to 0.9%. For fused silica with ARSS, the reflection loss was reduced to 0.02% near 1 µm. Pulsed laser damage thresholds at 1.06 µm were measured and thresholds as high as 100 J/cm2 were obtained for fused silica windows of up to 10 cm in diameter with ARSS and 850 J/cm2 for silica fibers with ARSS on the end faces. Spinel samples with ARSS showed damage thresholds more than two times higher than that of spinel with traditional AR coatings.

As-S and As-Se based photonic band gap fiber for IR laser transmission

IR transmissive As-S glass and As-Se glass triangular photonic band gap fiber structures are theoretically modeled. The potential for propagation of air-guided modes in the defect regions of these fibers is demonstrated by the large out of plane two-dimensional photonic band gaps found in these structures. Fiber design for IR light propagation is discussed.

Highly efficient cascaded amplification using Pr 3+ -doped mid-infrared chalcogenide fiber amplifiers

We computationally investigate cascaded amplification in a three-level mid-infrared (IR) Pr(3+)-doped chalcogenide fiber amplifier. The overlap of the cross-sections in the transitions (3)H(6)→(3)H(5) and (3)H(5)→(3)H(4) enable both transitions to simultaneously amplify a single wavelength in the range between 4.25 μm and 4.55 μm. High gain and low noise are achieved simultaneously if the signal is at 4.5 μm. We show that 45% of pump power that is injected at 2 μm can be shifted to 4.5 μm. The efficiency of using a mid-IR fiber amplifier is higher than what can be achieved by using mid-IR supercontinuum generation, which has been estimated at 25%. This mid-IR fiber amplifier can be used in conjunction with quantum cascade lasers to obtain a tunable, high-power mid-IR source.

Infrared transmitting fiber optics for biomedical applications

The availability of low loss and high strength chalcogenide fibers is enabling many applications, including biomedical. We report the fabrication and use of chalcogenide fibers for biomedical spectroscopy, scanning near field IR microscopy (SNIM) and laser power delivery. For example, lateral resolution of 20 nm and optical resolution of about 100 nm have been demonstrated for SNIM. The preliminary results are very encouraging and more work is being performed in lowering the losses and improving the performance of the fibers in the appropriate applications.