ARGOS MODEL 2400 SF SERIES TUNABLE SINGLE-FREQUENCY MID-INFRARED SPECTROSCOPIC SOURCE
UNMATCHED OUTPUT POWER AND TUNING RANGE One of Lockheed Martin s innovative laser solutions, Argos TM Model 2400 is a single-frequency, continuous-wave (CW) optical parametric oscillator (OPO) that provides multi-watt, widely tunable, infrared (IR) output in a wavelength range inaccessible to other laser technologies (Figure 1). Argos has output power and tuning range unmatched by any other system making it an enabling technology for many high-resolution spectroscopy applications. ARGOS FEATURES 3 Ultra-narrow linewidth designed for IR spectroscopy Wide wavelength tuning from near-ir to mid-ir Multi-Watt output power Wide mode-hop-free tuning Excellent Gaussian beam quality Stable monolithic cavity Interchangeable alignment-free OPO wavelength modules Portable without need for re-alignment Room temperature operation; air cooled Operates from 100 240V AC power Figure 1: SF-15 Power output vs. wavelength
Argos SF-15 Argos SF-10 Mode of operation Idler wavelength range Signal wavelength range* Linewidth** (both signal and idler) Beam quality Power stability Mode-hop-free tuning range Maximum tuning rate*** Cooling method Power SPECIFICATIONS Signal power: >1 W Idler power: >1 W Signal power: >600 mw Idler power: >600 mw CW Module A: 2.3-2.5 μm Module B: 2.5-3.2 μm Module C: 3.2-3.9 μm Module D: 3.9-4.6 μm Module A: 1.85-1.98 μm Module B: 1.60-1.85 μm Module C: 1.46-1.60 μm Module D: 1.39-1.46 μm < 1 MHz TEMoo < 5% rms 100 GHz Up to 30 Hz Forced air 100-240 V AC 50-60 Hz DIMENSIONS (FIGURE 2) Controller 3U 19 equipment rack Seed laser Fiber laser OPO head 2U 19 equipment rack 3U 19 equipment rack 4.9 H x 7.5 W x 13.9 L Specifications subject to change without notice * Mode-hop-free tuning characteristics apply to idler only ** Linewidth of < 60 khz measured on 500 μs timescale *** Higher modulation frequencies are available with smaller range Power specs do not apply to D module VISIBLE AND INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SC ATTERED RADIATION CLASS 4 LASER PRODUC T 6.295 HIGHEST POWER OPO The Argos system generates the highest power output of any OPO to date, enabling a new generation of spectroscopic applications for the scientific research community. While Argos power output is 1W across the entire available wavelength range, actual generated power is substantially higher. Overall power output declines according to photon energy 1/λ. Temperature variation of the nonlinear crystal provides limited overlap in the wavelength coverage between Argos three modules. ULTRA-NARROW LINEWIDTH SUPERIOR FREQUENCY STABILITY The Argus OPO idler linewidth was measured by performing a fast- Fourier-transform of the beat frequency generated when combining beams from two OPOs on an IR detector. The linewidth on a 500 microsecond timescale is less than 60 khz (Figure 3). On a longer timescale of 80 milliseconds, Figure 3: Linewidth of OPO the linewidth is ~ 1 idler measured at 3 μm MHz. The frequency stability on longer timescales was measured using a wavemeter. Mode-hop-free operation is proven up to 30 hours. Under lab conditions (temperature variation of ~3 C), a peak-to-peak idler frequency drift of less than 400 MHz over 30 hours was recorded. The system operates without any active frequency locking this passive stability is achieved only with temperature control of the seed laser for the pump source and OPO cavity (Figure 4). 3.475 1.700 7.500 1.683 2.270.405 4.900 3.304 3.989 4.680 2.100 2.538 13.870 Figure 2: OPO module dimensions (inches) Figure 4: Wavelength stability of Argos CW OPO operating at 3 microns wavelength - OPO demonstrates this level of stability without any active frequency lock of pump source or OPO 4
ENABLING HIGH-RESOLUTION, NEAR- TO MID-IR SPECTROSCOPY Argos wide, mode-hop-free, mid-ir wavelength tuning is unmatched by any other laser source in the 2- to 4-micron range. Rapid tuning of the OPO pump source allows up to 100 GHz mode-hop-free tuning of the OPO idler frequency. Using the standard seed source, rapid tuning is possible at rates up to 30 Hz, providing the ability to record continuous spectra of molecular species with absorption features in the mid-ir (Figure 5). Employing alternate seed lasers with higher modulation frequencies for the pump source allows acquisition of highfrequency 2f spectra (Figure 6). Survey scans of complete molecular absorption bands can be accomplished by recording multiple adjacent 100 GHz sweeps. The OPO controller provides a straightforward method of setting the center wavelength, allowing collection of overlapping spectra. Figure 5: Direct absorbance spectrumof CO2 recorded using Argos References A. Henderson and R. Stafford, Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source, Opt. Express 14, 767-772 (2006). Figure 6: 2f spectrum of H2O recorded with a modulation frequency of 16 khz using Argos and photoacoustic technique A. Henderson and R. Stafford, Spectral broadening and stimulated Raman conversion in a continuous-wave optical parametric oscillator, Optics Letters 32, 1281-1283 (2007). 5
THE ARGOS SYSTEM Argos is a single-frequency, CW OPO pumped by a fiber laser. The system consists of a pump laser, OPO tuning controller and OPO wavelength converter module. The pump laser and tuning controller are each 3U, 19-inch equipment racks connected via optical fiber and electrical cables to a compact OPO module. The system runs on 100 240V AC power and does not require water or cryogenic cooling. PUMP SOURCE A 10W or 15W fiber-based source operating at 1064 nm, the pump source consists of a low-power distributed feedback (DFB) fiber laser that seeds a 10W/15W fiber amplifier. These devices are separate 19-inch rack mount units connected via an FC/APC fiber connector. Alternate seed sources can connect to this fiber connector. Fine wavelength tuning the pump source (and thus the OPO) is performed by applying a strain to the DFB fiber laser via a piezo element. The pump source output is a collimator lens barrel keyed to the OPO module input, allowing repeatable alignment-free pump source swapping from module to module (Figure 7). Figure 8 : OPO modules is frequency converted to two longer wavelengths known as signal and idler. The shorter signal wavelength is resonant within the cavity, while both pump and idler exit the cavity on a single pass. A Fabry-Perot etalon within the cavity fixes the resonant wavelength for intermediate tuning via angle adjustment using a galvanometer. An output coupling cavity mirror extracts signal power from a separate output port, while a pair of dichroic mirrors separate a pump and idler. OPO CONTROL UNIT To maximize OPO stability, this unit controls temperature of the MgO:PPLN crystal and the OPO cavity block. The unit also electronically controls intermediate (galvanometer/etalon). Control parameter set points are varied via a front panel interface or PC via RS232 port (Figure 9). Figure 9: OPO controller front panel interface Figure 7: Pump source and OPO configuration OPO Three interchangeable OPO modules, usable with the same pump laser, cover the available wavelength range (Figure 8). Other than their specific coated optics and nonlinear crystals, these modules are identical. The OPO is a four-mirror ring resonant cavity using a temperature-controlled Magnesiumdoped Periodically Poled Lithium Niobate (MgO:PPLN) crystal as the nonlinear element. Up to 70 percent of the pump power Does not require water or cryogenic cooling 6
WAVELENGTH TUNING ARGOS Argos offers the widest tuning range of any laser source in the 2- to 4-micron range with wavelength tuning performed by different means depending on the required range. As described below, coarse tuning is performed manually on the OPO module, while intermediate and fine tuning are performed electronically via the OPO controller. COARSE TUNING By varying the poling period of the MgO:PPLN crystal, which transmits the pump beam, coarse tuning is performed manually on each OPO module. In the Argos, each wavelength module uses a specific PPLN crystal poled with a linearly varying poling period across the crystal width. By translating the crystal linearly with a finepitch screw (accessed via a port on the top of the module enclosure), the wavelength of maximum gain (phasematching wavelength) is coarsely tuned (Figure 10). The specific wavelength pair produced by the OPO is defined by energy conservation according to 1/λp = 1/λs + 1/λi, where λp is the pump wavelength λs and λi are the signal and idler wavelengths. The wavelength range for each module is primarily defined by the bandwidth of the optical coatings applied to optical components. Four modules are required for near-complete coverage of the 1.38 to 4.6 micron range. Switching between wavelength modules to access adjacent tuning ranges of the A, B, C or D modules is accomplished without alignment by swapping the keyed fiber collimator. FINE MODE-HOP-FREE TUNING The pump laser for the Argos Model 2400 is a singlefrequency source in master-oscillator power-amplifier (MOPA) configuration. The master oscillator is a DFB fiber laser tunable via piezoelectric strain of the fiber. Tuning of the pump frequency is directly transferred to the idler frequency, while the signal frequency is fixed by the cavity longitudinal mode and by the etalon angle. 100 GHz mode-hop-free tuning of the idler frequency is obtained. Piezoelectric transducer (PZT) tuning requires supplying an external high voltage to a BNC input on the seed laser. FREQUENCY MODULATION AND LOCKING Frequency modulation of the OPO idler is provided by modulating the frequency of the pump source, while the signal frequency remains fixed by the OPO etalon (Figure 11). Figure 11: Idler tuning follows pump tuning, while signal remains fixed by etalon The standard DFB fiber seed laser attains the full 100 GHz idler tuning by applying a 200V ramp signal to the fiber s PZT element. Alternate seed lasers for the pump source generate differing modulation characteristics for rate and range. Rapid modulation with bandwidth >10 khz is achieved using singlefrequency semiconductor lasers as the seed source. Using the frequency modulation input of the seed laser, and appropriate servo electronic circuitry, the user can lock the OPO idler frequency to a molecular absorption feature or reference cavity. 7 Figure 10: OPO tuning vs. PPLN poling period INTERMEDIATE-LEVEL TUNING The OPO controller performs this function electronically, which allows tilting of a thin etalon attached to a galvanometer. This etalon is mounted in the path of the resonant signal wave of the OPO, and its angle defines the wavelength of lowest loss and thus the wavelength of oscillation. Etalon tuning results in discrete wavelength jumps of ~30 GHz and allows stepwise monotonic tuning over a range equivalent to its free spectral range, 400 GHz or more depending on module. TUNING THE SIGNAL WAVELENGTH The signal wavelength is resonant within the OPO cavity and, therefore, defined by the cavity s longitudinal mode. In Argos, it may be coarsely tuned via varied poling period or etalon adjustment. By these methods, both signal and idler wavelength are tuned simultaneously while the pump wavelength remains fixed. Achieving mode-hop-free tuning requires continuous variation of the cavity length. Lockheed Martin offers an optional PZT module for one of the OPO cavity mirrors, allowing fine signal tuning of one free spectral range to be performed by applying a ramp voltage to this PZT. This tuning mechanism provides an alternate means of frequency modulation and locking.
BRINGING PIONEERING RESEARCH AND PROVEN SOLUTIONS TO THE INDUSTRY Since 1993, Lockheed Martin Laser and Sensor Systems has used its extensive laser expertise to focus on our customers most difficult challenges. Whether it s increasing the visibility for helicopter pilots in brownout conditions or developing next generation high power lasers for Directed Energy, Lockheed Martin delivers innovative laser solutions for the defense, aerospace, industrial, and scientific markets. As one of the industry s leading laser centers of excellence including design and production, Lockheed Martin Laser and Sensor Systems is recognized around the world for design, development and production of robust, reliable and innovative lasers. Through our expertise in high power fiber lasers and long history with pulsed fiber lasers, Lockheed Martin has built a set of core capabilities for applications including: directed energy; electro-optic countermeasure systems (EOCM); short-wave infrared (SWIR); active and passive imaging and targeting; 3D mapping; proximity operations; scientific application; and online fluid monitoring sensors for condition-based maintenance. 8
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