We report on the development, testing, and initial space qualification of a 1.5-μm, high-power (6 W), high wall-plug efficiency (∼15%), pulse-position-modulated (PPM), polarization-maintaining, fiber laser transmitter subsystem for deep-space laser communication links. Programmable high-order PPM modulation up to PPM-128 formats, with discrete pulse slots ranging from 0.5 to 8 ns, satisfies variety of link requirements for deep-space laser communication to Mars, asteroids, and other deep-space relay links, as per the National Aeronautics and Space Administration’s space laser communication roadmap. We also present initial space qualification results from thermal-vacuum tests, vibration testing, radiation testing, and an overall reliability assessment.
High power, high energy pulsed fiber laser with precise control of individual pulse width and wavelength is an enabling source for coherent imaging and communication applications. Here a turn-key 1550 nm PM fiber amplifier generating 22 μJ pulse energy with near transform limited linewidth (600 MHz) is presented. Individual pulse wavelengths and pulse widths can be controlled with 30-120 pm wavelength separation and 2-10 nsec pulse width. The 22 W average power laser, based on COTS Er and ErYb doped LMA PM-fibers is optimized for high peak power (< 4 kW), low duty cycle (~0.1%) operation while maintaining diffraction limited beam quality (M2 < 1.1). High wall plug efficiency (<10%) for the FPGA controlled system is maintained by temporal and spectral ASE suppression. Pulse energies are limited by Stimulated Brillion Scattering and Four Wave Mixing. Dependence of the fiber nonlinearities on pulse width and wavelength separation is characterized.
We report on the development, testing and initial space qualification of a 1.5-μm, high-power (6W), high wall-plug efficiency (~15%), pulse-position-modulated (PPM), polarization-maintaining (PM), fiber laser transmitter subsystem for deep-space laser communication links. Programmable high-order PPM modulation up to PPM-128 formats, with discrete pulse slots ranging from 0.5- to 8-nsec, satisfies variety of link requirements for deep space laser communication to Mars, asteroids, and other deep-space relay links, per NASA's space laser communication roadmap. We also present initial space qualification results from thermal-vacuum tests, vibration testing, radiation testing and overall reliability assessment.
A cladding-pumped, LMA ErYb fiber-based, amplifier is presented for use in a LIDAR transmitter for remote sensing of atmospheric CO2 from space. The amplifier is optimized for high peak power, high efficiency, and narrow linewidth operation at 1572.3nm. Using highly reliable COTS components, the amplifier achieves 0.5kW peak power (440uJ pulse energy), 3.3W average power with transform limited (TL) linewidth and M2<1.3. The power amplifier supports a 30% increase in pulse energy when linewidth is increased to 100MHz. A preliminary conductively cooled laser optical module (LOM) concept has size 9x10x1.25 in (113 in3) and estimated weight of 7.2lb (3.2 kg). Energy scaling with pulse width up to 645uJ, 1.5usec is demonstrated. A novel doubleclad ErYb LMA fiber (30/250um) with high pump absorption (6 dB/m at 915nm) was designed, fabricated, and characterized for power scaling. The upgraded power amplifier achieves 0.8kW peak power (720uJ pulse energy) 5.4W average power with TL linewidth and M2<1.5.
A 1um fiber laser outputting high energy (<1mJ) pulse-bursts with high peak powers (<15kW) and narrow linewidth (<300MHz) is an attractive pump source for tunable periodically poled crystal (PPx) based OPA’s which are used in gas sensing, imaging and communication applications. Here a turn-key 1064nm PM Yb-doped fiber amplifier capable of generating high pulse burst energies with transform limited linewidth is presented. The ~20W average power capable laser is optimized for high energy (0.5-2mJ) and high peak power (<10kW) operation at low duty cycles (<0.1%). The laser is capable of operating at <10x the saturation energy level of the final stage gain fiber and achieves a high level of pulse-to-pulse peak power uniformity within pulse-burst. Stimulated Brillion Scattering (SBS) limited micro pulse energy up to 40uJ is achieved and SBS dependence on micro pulse width and separation are characterized. High wall plug efficiency (<20%) for the FPGA controlled system is maintained by temporal and spectral ASE suppression and by spreading the necessary pulse pre-shaping losses (~12dB) to three different amplitude modulation points in the amplifier chain.
Fibertek has developed a space qualifiable, highly efficient, high power (<5W), fiber based 1.5um laser optical module (LOM). The transmitter achieves 6W average and <1kW peak power out of a 2m long single mode delivery fiber with 8nsec pulses and <6Ghz linewidth. Stimulated Brillouin Scattering (SBS) is managed by precise linewidth control and by use of LMA gain fiber in the power stage while maintaining the required diffraction limited, and highly polarized (PER<20dB) output. Size and weight of the built LOM are 8”x10”x2.375” and 3 kg, respectively. With improvements in the modulation scheme and component specification, achieved LOM electrical to optical efficiency is over 17.0%. Highly efficient operation is sustained for a wide range of pulse-position modulation (16 to 128-ary PPM) formats with pulse widths varying from 8nsec to 0.5nsec and operation temperature 10-50C. Pressure stress analysis, random vibration analysis and thermal analysis of the designed LOM predicts compliance with NASA GEVS levels for vibration and thermal cycling in a vacuum environment. System will undergo both thermal vacuum and vibration testing to validate the design.
5W peak power at 911 nm is demonstrated with a pulsed Neodymium (Nd) doped fiber master oscillator power amplifier (MOPA). This result is the first reported high gain (16dB) fiber amplifier operation at 911nm. Pulse repetition frequency (PRF) and duty-cycle dependence of the all fiber system is characterized. Negligible performance degreadation is observed down to 1% duty cycle and 10 kHz PRF, where 2.5μJ of pulse energy is achieved. Continuous wave (CW) MOPA experiments achieved 55mW average power and 9dB gain with 15% optical to optical (o-o) efficiency. Excellent agreement is established between dynammic fiber MOPA simulation tool and experimental results in predicting output amplified spontaneous emission (ase) and signal pulse shapes. Using the simulation tool robust Stimulated Brillion Scattering (SBS) free operation is predicted out of a two stage all fiber system that generates over 10W's of peak power with 500 MHz line-width. An all fiber 911 nm pulsed laser source with >10W of peak power is expected to increase reliability and reduce complexity of high energy 455 nm laser system based on optical parametric amplification for udnerwater applications. The views expressed are thos of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.
Novel tapered Yb-doped polarization-maintaining (PM) large-mode-area (LMA) fibers were fabricated, with 25/250μm
and 40/400μm core/clad at each end, and used in the last stage of a multi-stage Yb-fiber MOPA. The fiber-MOPA is
controlled by high-speed FPGA for real-time control of the seed laser diode modulation, electro-optic modulator and
acousto-optic modulator. Stable 0.3nsec pulses at 10kHz rate with 0.3mJ pulse energy (1MW peak power) is
demonstrated. At 1nsec pulsewidth, scaling to >0.5mJ pulse energy is demonstrated. Polarization extinction ratio of
17dB is obtained with diffraction limited beam quality and excellent (<3%) power stability. Such compact and robust
pulsed fiber amplifiers enable next generation of airborne and space lidar transmitters.
We demonstrate a multi-channel architecture for nsec pulsed lidar transmitter, scalable to larger channel counts via
wavelength- and time-multiplexing in a multi-stage Yb-fiber amplifier. This technology enables lidar systems for
topographic mapping missions, requiring much greater spatial coverage and range resolution. We demonstrate prototype
hardware, where wavelength channels at 1060nm, 1061nm and 1064nm are multiplexed, and precise 1.3nsec pulse using
LiNbO3 electro-optic(EO) modulators, at a combined repetition rate of 1MHz, with equal time interleaving between the
wavelengths. The multiplexed pulses are amplified to >20W average power, in an optimized three-stage Yb-fiber
amplifier system. We show simple de-multiplexing and frequency-doubling of one of the wavelengths (1064nm). Highspeed
FPGA based control provides for independent and programmable control of the pulse rate, timing trigger,
pulsewidth, and the intra-pulse-pattern for improved detection schemes.
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.
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