Sean W. Moore

Diffraction-limited, 300-kW peak-power pulses from a coiled multimode fiber amplifier

We report a multimode, double-clad, Yb-doped fiber amplifier that produces diffraction-limited, 0.8-ns pulses with energies of 255 muJ and peak powers in excess of 300 kW at a repetition rate of ~8 kHz . Single-transverse-mode operation was obtained by bend-loss-induced mode filtering of the gain fiber.

A new method for side pumping of double-clad fiber sources

We report a new method for pumping of double-clad, rare-earth-doped fiber sources using diode lasers, diode bars, or fiber-coupled pump sources. In this technique, the pump beam is launched by reflection from a mirror that is embedded in a channel cut into the inner cladding. The mirror may be curved to reduce the divergence of the pump beam, thereby allowing the use of highly divergent pump sources and low-numerical-aperture inner claddings. Additional advantages include high coupling efficiency, little loss of brightness in coupling the pump beam, relatively low sensitivity to misalignment, no obstruction of the fiber ends, no loss for light propagating in the core, simplicity (low parts count), compact and rugged packaging, scalability to high power, and low cost. Because of the large alignment tolerances, the technique is uniquely well suited to the direct coupling of the output of a diode bar into one or more double-clad fibers. We present a detailed description and characterization of the technique. We also describe the performance of Yb-doped and Er-Yb-doped fiber amplifiers constructed with this method, including amplifiers with saturated output powers of 5.2 W (1064-nm wavelength) and 2.6 W (1550 nm) when pumped with two laser diodes.

Efficient second, third, fourth, and fifth harmonic generation of a Yb-doped fiber amplifier

We report generation of the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. We obtained high conversion efficiencies using a simple optical arrangement and critically phase-matched nonlinear crystals. Starting with 320 mW of average power at 1064 nm (86 μJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations.

Side-illuminated hollow-core optical fiber for atom guiding.

We demonstrate a technique for coupling guiding light into hollow-core optical fibers for atom guiding. Microprisms embedded into a multimode, double-clad hollow fiber, allow light to be coupled into the fiber at multiple locations along the length of the fiber. The technique offers significant advantages over end-pumped configurations.

Latest results from the 32 km maritime lasercom link at the Naval Research Laboratory, Chesapeake Bay Lasercom Test Facility

The Naval Center for Space Technology at the Naval Research Laboratory reports the latest results from the long-range, maritime, free-space lasercom test facility located between Chesapeake Beach, MD and Tilghman Island, MD. The two sections of the facility are separated by 16.2 km of the Chesapeake Bay. Using a new OC-48 receiver developed by NRL’s Optical Science Division with a sensitivity of -33dBm for 10-9 bit error rate at 2.5 Gbps, we have closed a 32.4 km maritime lasercom link (round trip across the Chesapeake Bay) and performed bit error rate testing while transmitting 1.13 Terabytes of data. Bit error rate testing was also performed at lower data rates when atmospheric conditions were not favorable for high speed (2.5 Gbps), including testing at 150 Mbps through light fog and rain. In addition, we have set up a system for digitizing and transmitting full-color, uncompressed, video along with six audio channels and three RS-232 data channels over the maritime link. The digital link operated at 311 Mbps and could be maintained indefinitely, depending on atmospheric conditions. Several complete videos were transmitted in entirety or in part as well as live video from a handheld camcorder to test the system operation and robustness. The transmitter and receiver were co-located on the western shore of the bay at the NRL Chesapeake Bay Detachment. The data for both the bit error rate testing and the video was transmitted across the bay and returned from an array of retroreflectors located on a tower at Tilghman Island on the eastern shore. The lasercom links were closed with static pointing and with no active atmospheric aberration mitigation such as adaptive optics or fast steering mirrors on the receiver optics.

Diffraction-limited, 300-kW-peak-power pulses from a Yb-doped fiber amplifier

Summary from only given. We demonstrate that bend loss-induced mode filtering is effective in scaling large-core-area fiber amplifiers to significantly higher pulse energies and peak powers while maintaining diffraction-limited beam quality, high repetition rates, and transform-limited temporal and spectral profiles. This system provides the highest single-transverse-mode pulse energy, peak power, and average power simultaneously extracted from a fiber amplifier to date.

Diode-bar side-pumping of double-clad fibers.

We demonstrate direct diode-bar side pumping of a Yb-doped fiber laser using embedded-mirror side pumping (EMSP). In this method, the pump beam is launched by reflection from a micro-mirror embedded in a channel polished into the inner cladding of a double-clad fiber (DCF). The amplifier employed an unformatted, non-lensed, ten-emitter diode bar (20 W) and glass-clad, polarization-maintaining, large-mode-area fiber. Measurements with passive fiber showed that the coupling efficiency of the raw diode-bar output into the DCF (ten launch sites) was ~84%; for comparison, the net coupling efficiency using a conventional, formatted, fiber-coupled diode bar is typically 50-70%, i.e., EMSP results in a factor of 2-3 less wasted pump power. The slope efficiency of the side-pumped fiber laser was ~80% with respect to launched pump power and 24% with respect to electrical power consumption of the diode bar; at a fiber-laser output power of 7.5 W, the EMSP diode bar consumed 41 W of electrical power (18% electrical-to-optical efficiency). When end pumped using a formatted diode bar, the fiber laser consumed 96 W at 7.5 W output power, a factor of 2.3 less efficient, and the electrical-to-optical slope efficiency was lower by a factor of 2.0. Passive-fiber measurements showed that the EMSP alignment sensitivity is nearly identical for a single emitter as for the ten-emitter bar. EMSP is the only method capable of directly launching the unformatted output of a diode bar directly into DCF (including glass-clad DCF), enabling fabrication of low-cost, simple, and compact, diode-bar-pumped fiber lasers and amplifiers.

Side-pumped Hollow-core Fiber for Atom Guiding

We demonstrate a side-pumping technique for high efficiency (>80%) light coupling into double-clad hollow-core optical fibers for evanescent wave atom-guiding. We examine the benefits of the scheme and techniques for optimizing atom throughput

Efficient visible and UV generation by frequency conversion of a mode-filtered fiber amplifier.

We have generated the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. The fiber amplifier employed multimode fiber (25 μm core diameter, V ~ 7.4) to provide high-peak-power pulses, but diffraction-limited beam quality was obtained by use of bend-loss-induced mode filtering. The amplifier output had a pulse duration of 0.97 ns and smooth, transform-limited temporal and spectral profiles (~500 MHz linewidth). We obtained high nonlinear conversion efficiencies using a simple optical arrangement and critically phase-matched crystals. Starting with 320 mW of average power at 1064 nm (86 µJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations. Significantly higher visible and UV powers will be possible by operating the fiber amplifier at higher repetition rates and pulse energies and by further optimizing the nonlinear conversion scheme.

Efficient UV and visible generation using a pulsed, Yb-doped fiber amplifier

We report efficient generation of the second, third, fourth, and fifth harmonics of a double-clad, Yb-doped fiber amplifier seeded by a pulsed Nd:YAG microchip laser. Up to 18 mW at 213 nm was obtained.