Laser remote sensing has become a versatile and widely applied tool for the detection and characterization of a variety of hard and soft targets. For applications where eye safety is an issue these sources are usually in the infrared. Mission needs often dictate transmitter parameters that require unusual laser wavelengths and waveforms that are not commercially available. Waveform requirements can include ns-class pulses for precision ranging; single-frequency 100s-ns pulses for velocity measurements; frequency-agile sources for chemical sensing; high-energy, J-class, pulses for long-range environmental sensing; and adaptive waveforms for in-situ transmitter optimization or multi-function sensors. Additionally, in the case of airborne and space-based sensors, platforms often dictate stringent size, weight and power constraints. This paper discusses laser transmitter requirements for a variety of laser remote sensing missions and describes novel pulsed solid-state sources under development at CTI to meet these needs. Current work includes the development of high-efficiency Nd:YAG, Er:YAG and Ho:YAG lasers, high power Tm:YALO lasers, high-energy Tm:YAG amplifiers, broadly tunable Cr:ZnSe lasers, short-pulse Raman lasers, and wavelength-agile optical parametric oscillators.
Broadly tunable near- and mid-infrared lasers are of interest for a variety of applications including high-resolution spectroscopy, metrology, pumping of nonlinear optical frequency converters such as optical parametric oscillators (OPOs) and standoff chemical sensing. Tunable laser sources in the 2-3 um region include Cr2+ doped chalcogenide lasers; cryogenic systems, such as color center lasers; limited tunability devices, such as Tm and Ho lasers, gas or chemical lasers, and diode lasers; and nonlinear optical devices such as OPOs. Transition-metal-doped chalcogenide lasers are of high interest because of their high versatility, broad room-temperature wavelength tunability, high optical efficiencies, and their potential to be scaled to high powers via direct diode or fiber laser pumping. To date, continuous-wave, gain-switched, Q-switched and mode-locked laser operation has been demonstrated. Material advantages include broad absorption and emission bands, high fluorescence quantum efficiencies at room temperature, high gain cross-sections, and minimal loss mechanisms such as excited-state absorption or upconversion. Additionally, the materials can be produced by a variety of methods, including several direct growth techniques and diffusion doping. The principal material disadvantages include a relatively large change in refractive index with temperature (large dn/dT), which can induce thermal lensing, and a short, microseconds, energy storage time. In this paper we review fundamental material properties, the current state-of-the-art of continuous-wave and pulsed Cr2+ doped chalcogenide lasers, and recent research results.
Broadly tunable infrared laser sources are of interest for a variety of applications including differential absorption lidar, differential scattering lidar, multi-spectral detection and imaging, hard target identification and discrimination, optical communications in poor visibility conditions, and spectroscopy. For chemical sensing applications, sources are particularly sought in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral regions. A variety of laser and nonlinear optical devices have been demonstrated that access these wavelengths. In particular, CTI is developing novel, tunable, narrow linewidth transmitters for coherent and direct detection lidar measurement applications. An example is a multi-watt Cr:ZnSe laser that is tunable over the 2.1 to 2.8 micrometers wavelength region. This laser has been used to pump-tune optical parametric oscillators (OPOs) that are broadly tunable across the MWIR and LWIR. We are also developing tunable Yb lasers that can be used to pump OPOs that emit signal beams in the eyesafe 1.55 micrometers region while generating idler beams that access the 3 to 4 micrometers MWIR band. This paper describes these sources.
HF mirror metrology is currently costly and time consuming, requiring laser component delivery to an HF laser site, and operation of another HF laser to reach relevant wavelengths. Coherent Technologies, Inc. has developed a solid state Cr:ZnSe laser pumped by a Tm:YALO laser that provides up to 1.1W of output power with 1.1nm linewidth at 2.64micrometers , an HF laser line. The laser can also tune to other HF laser liens in the wavelength range of 2.64micrometers to 2.8micrometers . The Cr:ZnSe laser was used to measure the reflectivity of HF mirror samples provided by TRW. Examples of other possible applications of this source include beam train alignment and preliminary testing of diagnostic subsystems that measure HF laser output power, wavefronts, and beam profiles. Such a direct laser source is simple and can potentially achieve high intensity stability, allowing for a robust and compact HF laser surrogate. Moreover, power scaling is straightforward.
Tunable single-frequency sources in the 2-4 micron wavelength region are useful for remote DIAL measurements of chemicals and pollutants. We are developing tunable single-frequency transmitters and receivers for both direct and coherent detection lidar measurement applications. We have demonstrated a direct-diode-pumped PPLN-based OPO that operates single frequency, produces greater than 10 mW cw and is tunable over the 2.5 —3.9 micron wavelength region. This laser has been used to injection seed a pulsed PPLN OPO, pumped by a 1.064 micron Nd:YAG laser, producing 50-100 microJoule single-frequency pulses at 100 Hz PRF near 3.6 micron wavelength. In addition, we have demonstrated a cw Cr:ZnSe laser that is tunable over the 2.1 —2.8 micron wavelength region. This laser is pumped by a cw diode-pumped Tm:YALO laser and has produced over 1.8 W cw. Tm- and Tm,Ho-doped single-frequency solid-state lasers that produce over 50 mW cw and are tunable over approximately 10 nm in the 2 —2.1 micron band with fast PZT tuning have also been demonstrated. A fast PZT-tunable Tm,Ho:YLF laser was used for a direct-detection column content DIAL measurement of atmospheric CO2. Modeling shows that that all these cw and pulsed sources are useful for column-content coherent DIAL measurements at several km range using topographic targets.
A systematic spectroscopic study of 22 rate-earth-ion doped ZBLAN glass samples was conducted to investigate the feasibility of sensitizing Tm:ZBLAN with Yb to facilitate the development of an efficient and conveniently pumped blue upconversion fiber laser. It was determined that, under conditions of single-color pumping, 480 nm emission from Tm3+ is strongest when Yb, Tm:ZBLAN is excited at a wavelength of approximately 975 nm. In this case, the strongest blue emission was obtained from a ZBLAN glass sample with a nominal dopant concentration of approximately 2.0 wt percent Yb + 0.3 wt percent Tm. Additionally, it was demonstrated that for weak 975 nm pump intensities, the strength of the blue upconversion emission can be greatly enhanced by simultaneously pumping at approximately 785 nm. This increase in upconversion efficiency is due to a reduction in the number of energy transfer steps needed to populate the Tm3+1G4 energy level. Measurements of fluorescence lifetimes as a function of dopant concentration wee also made for Yb3+, and Pr3+ transitions in ZBLAN in order to better characterize concentration quenching effects. Energy transfer between Tm3+ and Pr3+ in ZBLAN is also described.