Stuart Gray

High rate, long-distance quantum key distribution over 250 km of ultra low loss fibres

We present a fully automated quantum key distribution prototype running at 625 MHz clock rate. Taking advantage of ultra low loss (ULL) fibres and low-noise superconducting detectors, we can distribute 6000 secret bits s−1 over 100 km and 15 bits s−1 over 250 km.

Al/Ge co-doped large mode area fiber with high SBS threshold

We propose a novel approach of making large effective area laser fiber with higher threshold for the stimulated Brillouin scattering (SBS) using Al/Ge co-doping in the fiber core. The increased SBS threshold is achieved by reducing the acoustic-optic overlap integral while keeping the optical refractive index profile with a step structure. The manipulation of the overlap integral is done by adjusting the relative doping level between Al(2)O(3) and GeO(2) in the core. The mechanism is validated by detailed examples of numerical modeling. An Yb-doped double clad fiber with the core co-doped with Al(2)O(3) and GeO(2) was fabricated by the OVD process. Measured acoustic velocity profile using a scanning acoustic microscope verified that the acoustic velocity in the fiber core changes with the design. An amplifier utilizing the fiber demonstrated that the proposed fiber yielded 6 dB higher SBS threshold than a fiber without using the co-doping scheme.

502 Watt, single transverse mode, narrow linewidth, bidirectionally pumped Yb-doped fiber amplifier

High power operation of narrow linewidth optical fiber amplifiers is usually limited by the onset of stimulated Brillouin scattering. In this paper, we present results demonstrating over 500 Watts of power in a single mode beam from a fiber designed to suppress stimulated Brillouin scattering through a reduction in the overlap of the optical and acoustic fields. Simulations demonstrate the potential for this fiber to achieve greater than 1000 Watts of output power.

Limit of Effective Area for Single-Mode Operation in Step-Index Large Mode Area Laser Fibers

Step-index (SI) fiber designs are commonly used in achieving large mode area (LMA) and single-mode (SM) operation in fiber lasers. These fibers can either be intrinsically single-moded fibers or few-moded fibers, which can be forced into SM operation through bending. In this paper we evaluate the limitation of the effective area for SM operation by taking into account the effects of practical constraints such as fiber bending loss, laser performance, and fiber mechanical reliability on these LMA fiber designs. It is shown that the effective area of these fibers cannot be arbitrarily scaled up with the size of the fiber core. We also use the modeling result to provide estimated upper limits to the core diameters and corresponding effective areas for conventional SI fiber designs taking into account fiber parameters that are achievable nowadays.

Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber.

Measurements of backscattered Raman amplified spontaneous emission in single-mode dual-hole-assisted fiber indicate suppression of Raman gain by more than two orders of magnitude compared to SMF. These results imply that fiber lasers based on the dual-hole-assisted fiber design are effectively immune to SRS, thus enabling significant power scaling beyond current limits from a single-mode core.

Effective Area Limit for Large Mode Area Laser Fibers

We analyze practical constraints of fibers designs for achieving large mode area and single mode operation for fiber lasers, and provide estimated upper limits for the core diameter and effective area.

Transient gain dynamics in wide bandwidth discrete Raman amplifiers

This paper has described measurements of transient gain effects caused by dropping channels in a wide bandwidth backward pumped discrete Raman amplifier. These measurements have shown that in a WDM amplifier the transients include two different features. There is a small but fast component due to signal-to-signal Raman crosstalk which can either increase or decrease the signal power depending on whether the surviving channels are at a longer or shorter wavelength than the dropped channels. There is also a larger, slower component due to pump readjustment which occurs on a time scale determined by the length of the amplifier. After changing the signal power at the input to the amplifier it requires up to three times the transit time of the amplifier for the new steady state to be reached.

Optical Fibers With Tailored Acoustic Speed Profiles for Suppressing Stimulated Brillouin Scattering in High-Power, Single-Frequency Sources

In this paper, we discuss the design of optical fibers with tailored acoustic speed profiles to suppress stimulated Brillouin scattering in high-power, single-frequency sources. We demonstrate the ability of these fibers to achieve single-mode output powers of greater than 500 W and discuss the potential to achieve powers beyond 1 kW.

Kilowatt-level, narrow-linewidth capable fibers and lasers

High power fiber lasers have been recently demonstrated at the kilowatt level. The spectral linewidths of these lasers oscillators can exceed 20 nm. Whereas, such broad spectra are fine for many applications, such as materials processing where raw power is the primary requirement, other applications, including coherent beam combination, harmonic generation, or gravitational wave detection, require high powers beams with much narrower linewidths. Amplification of narrow linewidth signals in optical fibers is limited by stimulated Brillouin scattering (SBS). We discuss novel fiber designs that limit SBS allowing the amplification of narrow linewidth signals to kilowatt power levels.

Detection of nitric oxide in air with a 5.2 μm distributed-feedback quantum cascade laser using quartz-enhanced photoacoustic spectroscopy

We demonstrate the detection of nitric oxide in both air and nitrogen atmospheres with a 5.2 μm distributed-feedback quantum cascade laser in a quartz-enhanced photoacoustic spectroscopy system. The photoacoustic signal generated by nitric oxide is shown to be several times larger in air than in nitrogen due to the faster vibrational-translational energy relaxation process induced by the presence of oxygen. A sensitivity of 2.5 parts-per-million by volume is achieved in air at atmospheric pressure.