Multi Spectral Lidar (MSL)

We have developed and demonstrated both Ytterbium-doped and Erbium-doped, diode-pumped and seeded, fiber amplifiers at 1064 and 1570 nm, respectively. By pulse pumping a one-stage Erbium amplifier, we have shown greater than 20 W peak output power and high wall-plug efficiency. Our pulse-pumping approach improves energy efficiency up to 80% (at 1 kHz PRF) over the identical CW pumping scheme while suppressing amplified spontaneous emission (ASE). We report on the development of these rare-earth doped fiber amplifiers and the application of multi-stage fiber amplifiers to create a multi-spectral laser transmitter ideally suited for space and planetary lidar investigations. INTRODUCTION Laser remote sensing or lidar can make unique and important measurements related to NASA’s Vision for Space Exploration. Single wavelength lidar measurements enable three-dimensional maps of planetary surface topography, can profile aerosols, plumes, and clouds in planetary atmospheres and can provide altimetry and hazard avoidance information during lander decent and rover operations. In addition, rovers equipped with small imaging lidars can provide high resolution, 3-D images of rocks and interesting geological formations. Adding additional laser wavelength channels to a lidar, creating a multi-spectral lidar, enables active spectroscopy of permanently shaded regions of the moon, i.e. craters near the lunar south pole, trace gas measurement of planetary atmospheres, and spectroscopy of surface composition. Fiber amplifiers, initially developed for long-haul, fiber optic telecommunications have many potential advantages for use in space due to their compact, rugged and highly efficient nature. Operated at a low duty cycle, short pulse regime, fiber amplifiers at 1064 nm can produce peak powers in excess of 300 kW. Additionally, fiber amplifiers can access a broad wavelength region in the near infrared (1000-1600 nm) that can be extended into the visible region (500-800 nm) using nonlinear, second harmonic generation (i.e. frequency doubling). Research conducted at the Imperial College of London, UK has demonstrated efficient frequency doubling (>40%) of Yb-doped fiber amps to the 3-6 Watt level. Fiber amplifiers alleviate many issues that plague diode pumped solid-state (DPSS) lasers. Laser 2 of ICESAT/GLAS suffered a contamination related failure. Fiber amplifiers eliminate contamination related failures by fusion-splicing all components in a continuous, silica fiber optical path. There is only one glass/air interface, at the final output, that can be protected by fusing a large diameter (~1 mm) glass rod onto it, reducing the fluence. Conducting materials analysis to assure the long-term reliability of laser diodes can mitigate packaging related-failures in pump laser diodes. One important distinction for fiber amplifiers is the use of single element pump laser diodes over traditional diode bars. Currently, a photonics group at NASA Goddard is analyzing laser modules for their ability to withstand long term usage in harsh environments through materials analysis that includes a Destructive Physical Analysis (DPA). DPA was used to identify the packaging related failure modes in the GLAS laser diode arrays. DPA is currently being coordinated with a current internal research and development program to raise the technology readiness level (TRL) of diode laser pump technology. Rare-Earth (RE) doped gain fiber is being characterized for space flight radiation environments at NASA Goddard. The radiation study will not only raise the TRL of the proposed system by eliminating unanswered questions about radiation induced darkening but the study will include methods of adding pumps lasers for mitigating radiation induced effects using photo-bleaching. NASA Goddard is also providing expertise and knowledge of the physics of failure for mitigating the risk at the component level prior to actual space flight prototype development. Understanding the component level failure modes is necessary to avoid the possibility of these failures occurring during a flight mission. NASA Goddard is involved with fiber laser component level investigations for projects including the Laser Interferometer Space Antenna (LISA), the Mars Laser Communications Demonstration (MLCD), the Instrument Incubator Program (IIP), and the Air Force Research Labs (AFRL). Our goal is to demonstrate pulsed laser transmitters capable of 100 uJ pulses, 10 nS wide (peak power of 10 kW) and a PRF of 150-1000 Hz at both 1064 nm (Ytterbium) and 1570 nm (Erbium). APPROACH Our approach to developing a multi-wavelength laser transmitter for space and planetary lidar applications involves developing single and double stage amplifiers employing both Erbium and Ytterbium doped fiber operating at 1.5 μm and 1.06 μm respectively. The amplifiers are seeded and pumped with single mode laser diodes that are frequency stabilized with fiber Bragg gratings (FBG). Single mode gain fiber is used with co-propagating single-mode pump laser diodes coupled with WDM combiners for the first stage and large mode area (LMA) fibers pumped with single mode 980 pump diodes for the second stage. In order to maximize wall plug efficiency, we hypothesized that pulse pumping each stage of the amplifier would increase the overall efficiency by suppressing the amplified spontaneous emission (ASE). Frequency doubling the output of these seeded amplifiers with periodically-poled materials such a lithiumniobate or KTP will access the visible to near infrared portion of the spectrum (520 785 nm). Core pumping with single mode, 980 nm laser diodes was chosen over clad pumping due to the availability of high-power, single mode, fiber coupled pump lasers. 1. Erbium Fiber Amp Development A one stage, diode seeded, bi-directionally pumped (i.e. co-propagated pumps) fiber amp using Erbium doped fiber was assembled and tested. A block diagram depicting the one-stage amp is shown in Figure 1. A Wavelength Division Multiplexing (WDM) combiner was used to combine the 980 nm pump with the 1550 nm seed and an identical combiner was used to reversepropagate a second 980 nm pump laser while singling out the amplified 1550 nm signal. Modeling results using a vendorsupplied application designer were deemed too ambiguous to be useful so our approach was to experimentally establish the performance and behavior of the amplifier using decreasing lengths of gain fiber. Two meters of high concentration, Erbium-doped single mode gain fiber (Liekki Er110 4/125) were spliced into the amplifier. Data was acquired while varying the seed current effectively controlling the optical seed power coupled into the fiber amplifier while the output power was recorded. Figure 2 shows the CW output power as a function of seed laser current with no pumping, forward pumping only, reverse pumping only and with both pumps operating. Small signal gains in excess of 30 dB are evident when the seed source is coupling approximately –30dBm or 1 μW into the amplifier. With increasing seed power, the gain saturates and no additional output power is realized. Interestingly, reverse pumping alone improves performance over forward pumping alone by almost a factor of two (3 dB) while both forward and reverse pumping together increase performance -50 -40 -30 -20 -10 0 10 20 0.00 50.0 100 150 200 250 Single Mode Erbium Doped Fiber Amplifer No pumping F & R pumping Forward pumping Reverse pumping O ut pu t P ow er (d B m ) Seed Laser Current (mA) FORWARD WDM FORW ARD PUMP