Phosphate glass is an attractive material for rare-earth-doped fiber manufacturing because high doping levels are possible without introducing negative effects such as up-conversion or increased non-radiative recombination. In this paper we present a novel PM heavily Yb-doped polarization maintaining large mode area phosphate fiber and a < 100 W power level amplifier based on this fiber. The fiber was fabricated by a rod-in-tube technique. An 18 cm long piece of the fiber was used to build a high-power all-fiber amplifier. 106 W of output power at 1030 nm was achieved with 55 % slope efficiency with respect to the launched pump power. To the best of our knowledge, this is the highest average power ever demonstrated for short phosphate fiber lasers.
Heavy doping of common silica gain fibers is not practical; therefore long fibers are required for efficient amplification (usually 5-10m). This is undesirable due to nonlinearities that grow with fiber length. In contrast, NP Photonics phosphate-glass based fibers can be heavily doped without any side-effects, and hence can provide very high gain in short lengths (less than 0.5m). This enables an ideal pulsed fiber amplifier for a MOPA system that maximizes the energy extraction and minimizes the nonlinearities. We demonstrate 1W average power, 200μJ energy, and >10kW peak power from a SBS-limited all-fiber MOPA system at 1550nm, and 32W average power, 90μJ energy, and 45kW peak power from a SRS and SPM limited all-fiber MOPA system at 1064 nm. These results were limited by the seed and pump sources.
We report on the development of a fiber-optic pulsed coherent lidar transceiver for wind-velocity and aircraft
wake-vortex hazard detection. The all-fiber 1.5μm transmitter provides up to 560 μJ energy at 25 kHz with 800
ns pulse width (pump limited). Performance simulations indicate wake-vortex hazard signature detection up to
~2.5km range with a receiver sensitivity of ~2 fW (SNR=6), suited for an aircraft landing scenario. Furthermore,
the transceiver is implemented using high-speed FPGA based control and digital-signal-processing, enabling its
use as a flexible pulse-format multi-function in-flight lidar sensor. We present the latest laboratory results and
preliminary testing of this pulsed coherent lidar transceiver, along with the lidar performance simulation of
wake-vortex eddy models.
KW level fiber based MOPA laser system at 1064nm for uplink deep space communication is developed and
characterized. System achieves 11.5 kW peak power (600W average power) at 500 kHz (5% duty cycle) with >70%
optical conversion efficiency. Experiments using 16-ary PPM format are presented where without pre-pulse shaping
>±60% pulse energy variation is observed. Gain dynamics is identified as main source of pulse to pulse energy
fluctuation. Novel, FPGA implementable open loop pulse shaping algorithm is developed and demonstrated. Resulting
pulse energy statistics are reported, where <±7% pulse energy variation is achieved for 90% of pulses.
In this paper, we present results on a master-oscillator Yb-doped fiber amplifier with 1 kW cw output power (at
1064nm), and near-diffraction limited beam quality (M2<1.4), with internal quantum efficiency >83%. The final
amplifier stage uses a very high Yb-doped 35-um core LMA fiber, using a new process recipe that virtually eliminates
photo-darkening. As a result, high efficiency, SBS-free operation to 1 kW cw power level is obtained, with a phase
modulation bandwidth of only 450MHz, well below other reported results.
To enable single-frequency cw power scaling to kW levels, we investigate LMA fiber waveguide designs exploiting
gain-discrimination, using partially Yb-doped LMA fiber cores, with various diameters up to 80-um. SBS-free, singlefrequency
(few kHz) operation is demonstrated up to 0.9kW cw power. At the lower cw powers (<200W) neardiffraction
limited beam-quality is obtained, but is observed to deteriorate at higher cw powers. We discuss potential
causes, and present a detailed simulation model of kW large-core fiber-amplifiers, that includes all guided modes, fiber
bend, transverse spatial hole burning, gain-tailoring, mode-scattering, SBS nonlinearity, and various thermal effects. This
model shows good agreement with the observed single-frequency power scaling and beam-quality characteristics.
We present results on the design, development and initial testing of a fiber-optic based RF-modulated lidar transmitter
operating at 532nm, for underwater imaging application in littoral waters. The design implementation is based on using
state-of-the-art high-speed FPGAs, thereby producing optical waveforms with arbitrary digital-RF-modulated pulse
patterns with carrier frequencies ≥ 3GHz, with a repetition rate of 0.5-1MHz, and with average powers ≥5W (at 532nm).
Use of RF-modulated bursts above 500MHz, instead of single optical pulse lidar detection, reduces the effect of
volumetric backscatter for underwater imaging application, leading to an improved signal-to-noise-ratio (SNR) and
contrast, for a given range. Initial underwater target detection tests conducted at Patuxent River Naval Air Station, MD,
in a large water-tank facility, validates the advantages of this hybrid-lidar-radar (HLR) approach for improved
underwater imaging, over a wide range of turbidity levels and both white and black targets. The compact, robust and
power-efficient fiber laser architecture lends very well to lidar sensor integration on unmanned-underwater-vehicle
(UUV) platforms. HLR transmitters can also provide similar advantages in active-sensing situations dominated by
continuous backscatter, e.g. underwater communications, imaging through smoke and fire environment, rotor-craft
landing in degraded visual environment, and pointing-tracking of active-EO sensors through fog.
KEYWORDS: Frequency combs, Semiconductor lasers, Ultrafast phenomena, Signal processing, Radio optics, Fiber lasers, Mode locking, Time metrology, Phase modulation, Superposition
Fast and precise measurements of ultrafast optical waveforms are essential to the development of optical coherent
signal processing. In this paper, multi-heterodyne mixing of stabilized optical frequency combs is presented as a
simple technique for the measurement of ultrafast laser pulses and exotic arbitrary optical waveforms. This
technique takes advantage of both the broadband nature of the frequency comb and the narrow line-width of the
individual comb-lines to produce an array of radio-frequency beat-notes that share the characteristics of the optical
spectrum. Measurements of comb characteristics across THz of bandwidth are enabled by this method, while
maintaining the accuracy at the level of the individual comb-line width. Results show that both frequency
modulation and amplitude modulation combs can be measured using this scheme.
A novel scheme for recognition of electronic bit-sequences is demonstrated. Two electronic bit-sequences that are to be
compared are each mapped to a unique code from a set of Walsh-Hadamard codes. The codes are then encoded in
parallel on the spectral phase of the frequency comb lines from a frequency-stabilized mode-locked semiconductor laser.
Phase encoding is achieved by using two independent spatial light modulators based on liquid crystal arrays. Encoded
pulses are compared using interferometric pulse detection and differential balanced photodetection. Orthogonal codes
eight bits long are compared, and matched codes are successfully distinguished from mismatched codes with very low
error rates, of around 10-18. This technique has potential for high-speed, high accuracy recognition of bit-sequences, with
applications in keyword searches and internet protocol packet routing.
Mode-locked lasers have applications in signal processing and communications such as analog to digital conversion,
arbitrary waveform generation and wavelength division multiplexing. For such applications low noise and phase
coherent frequency stabilized optical combs are needed. In this work we report a low noise, Pound-Drever Hall
frequency stabilized, semiconductor mode-locked laser at 10.287GHz centered at 1550nm with 1000-Finesse sealed,
ultralow insertion loss intracavity etalon. The output optical power of the mode locked laser is ~5mW.
We demonstrate a chirped-pulse amplification system generating 25 μJ compressed pulses at a center wavelength of
1552.5 nm. The seed module and the amplifier chain are all in-fiber (with a few small fiber-pigtailed free-space
components), followed by a free-space diffraction grating pulse compressor. The amplifier chain contains a pre-amplifier
and a booster whose gain fibers are 45/125 μm core/cladding-diameter, core-pumped Er-doped fibers. The pump lasers
for both amplifiers are single-mode 1480 nm Raman lasers capable of up to 8 W output. The seed module generates up
to 2 ns chirped pulses that are amplified and subsequently compressed to <800 fs duration. At a repetition rate of 50 kHz,
the 2 ns pulses from the seed module were amplified to 72 μJ, resulting in 25 μJ after pulse compression. The
corresponding peak power levels after the amplifier chain and compressor were 36 kW and 31 MW, respectively.
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