Power, Pulse Width, and Repetition Rate Agile Low-cost Multi-spectral Semi-active Laser Simulator

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1 Power, Pulse Width, and Repetition Rate Agile Low-cost Multi-spectral Semi-active Laser Simulator Jason K. O Daniel, Preston Young* Photodigm, Inc., 55 E. Collins Blvd., Ste. 2, Richardson, TX 7582 Capt. Eric Golden, Robert Barton, Donald Snyder USAF AFRL\RWGG, West Eglin Blvd., Eglin, FL Public Release Affairs Case #96ABW-2-6-SAL ABSTRACT The emergence of spectrally multimode smart missiles requires hardware-in-the-loop (HWIL) facilities to simulate multiple spectral signatures simultaneously. While traditional diode-pumped solid-state (DPSS) sources provide a great basic testing source for smart missiles, they typically are bulky and provide substantially more power peak power than what is required for laboratory simulation, have fied pulse widths, and require some eternal means to attenuate the output power. HWIL facilities require systems capable of high speed variability of the angular divergence and optical intensity over several orders of magnitude, which is not typically provided by basic DPSS systems. In order to meet the needs of HWIL facilities, we present a low-cost semi-active laser (SAL) simulator source using laser diode sources that emits laser light at the critical wavelengths of 64 nm and 55 nm, along with light in the visible for alignment, from a single fiber aperture. Fiber delivery of the multi-spectral output can provide several advantages depending on the testing setup. The SAL simulator source presented is capable of providing attenuation of greater than 7 db with a response time of a few milliseconds and provides a means to change the angular divergence over an entire dynamic range of.2-6º in less than 4 ms. Further, the SAL simulator is pulse width and pulse repetition rate agile making it capable of producing both current and any future coding format necessary. Keywords: Semi-active Laser Simulator, Hardware-in-the-Loop, SAL, HWIL. INTRODUCTION Spectrally multimode seeker systems used in laser guided weapons, those currently in production and those to be used in future weapons, must endure rigorous testing to ensure proper functionality. This testing requires a versatile semi-active laser (SAL) simulator to be integrated into the test loop to make sure these seeker systems perform according to specifications. Hardware-in-the-Loop (HWIL) facilities require a SAL simulator capable of emitting at 64 nm for current seeker systems, and at 55 nm for future seeker systems; it is also advantageous for the system to emit in the visible for alignment purposes. Furthermore as coding format changes, it is necessary for systems to have the ability to evolve and emit pulse widths and repetition frequencies to keep up with technological changes. It is possible to use custom designed SAL simulator system based on commercially available diode-pumped solid-state (DPSS) systems for HWIL testing. Systems designed and constructed in such a manner have several shortcomings. First, due to the fied pulse width associated with Nd:YAG and Er:Glass DPSS systems, these systems cannot be pulse width agile. Second, these systems have high output peak powers that are typically far greater than what is needing in a laboratory or production floor environment. Therefore, facilities using these systems employ a high number of safety precautions in order to ensure the well being of testing personnel. Third, eternal systems of attenuation and beam control must be used in order to achieve the large dynamic range needed for appropriate seeker testing; due to the fact that the attenuation systems are typically large and mechanical, the overall system has a slow response time. Fourth, due to the initial cost of the DPSS system and the cost of designing and implementing eternal systems for beam control and safety precautions, the final system can be etremely epensive. Fifth, theses systems will be rather bulky and cumbersome to move and may be difficult to keep aligned to the test system. *pyoung@photodigm.com; phone ; fa ; photodigm.com

2 Due to the high costs associated with such custom built DPSS systems, Photodigm with input from the Air Force Research Laboratory (AFRL) Kill Vehicle HWIL Simulator (KHILS) facility developed a low-cost low-power SAL simulator source. The details of this SAL simulator source will be presented in this work. Section 2 will present the specifications of the SAL simulator source. Section 3 will present specifics of the SAL simulator subsystems. Section 4 will present measured characteristics of one of these SAL simulator systems, followed by the conclusion in Section SPECIFICATIONS The SAL simulator source developed integrates three separate fiber-coupled laser diodes at wavelengths of 66 nm, 64 nm, and 55 nm. The 64 nm is a high-power DBR laser diode as described in []. These sources are spectrally combined using wavelength division multipleers to provide the laser light emission from one single-mode fiber; the ability to emit all these wavelengths from a single fiber aperture reduces alignment difficulty. The electronic laser diode drivers for each of these lasers are directly integrated into the system. These drivers were designed such that each laser could be enabled individually, both locally and remotely; further, these drivers were designed such that the laser diodes could be pulsed at any necessary pulse repetition frequency with any desired pulse width. The SAL simulator source also contains remotely controlled variable optical attenuators for the 64 nm and 55 nm sources, such that the optical intensity can be varied by more than seven orders of magnitude or 7 db. Lastly, the output fiber is coupled to an analog voltage controlled variable divergence module (VDM) that allows the beam divergence angle to be controlled from.2º to 6º, with the module, shown in Figure, able to cover the full dynamic range within 4 ms. The ability to electronically control laser output intensity and beam divergence in real time makes it possible to simulate seeker fly-in accurately. The entire system, with eception of the variable divergence module, can fit in a 2U height of a 9 rack mount cabinet; the system is shown in Figure. The features and specifications for the SAL simulator source are shown in Table. Figure. Photodigm SAL simulator source system control unit (left) and divergence variability module (right) The output power specifications listed for the system in Table are reduced by as much as three times from the output power from the individual laser devices due to using non-optimized off-the-shelf components as wavelength division multipleers (WDMs). Using custom WDMs would result in a higher output power, but would also result in a higher cost. In addition, the output peak powers of both the 64 nm and 55 nm sources could also be boosted to -2 W peak power by including a simple two stage fiber amplifier in the system for each source for those applications requiring higher power levels. One of these SAL simulator systems can cost $35, to $4, for single units; this cost is mainly driven by designing to custom specifications, system assembly, and qualifications testing. However, systems in quantities of s could easily reduce the system cost to approimately $25,. Considering that Nd:YAG systems can easily have costs in the range of $5, without the eternal attenuation and beam control, even a $4, unit is a substantial savings.

3 Table. Features and specification for the SAL simulator source FEATURE SPECIFICATION Source Wavelengths 66 nm +/- nm 64 nm +/- nm 55 nm +/- nm Output Peak Power 64 nm >25 mw- no attenuation Output Peak Power 55 nm >5 mw - no attenuation Output Power Visible >2 mw Laser Biasing Constant current or peak current setting, not user selectable Laser Modulation TTL input for digital modulation up to MHz Attenuation Control 6 db or greater Response time <2 ms Divergence Variability.3- mrad (.2-6º) Response time <4 ms for full range Unit Power On-Off Switch, Safety Interlock Enable Required Individual Laser Power On-Off Toggle or TTL Level Control Optical Head Mount ¼-2 Tapped Hole Power Input 2 VAC, < 5 A Dimensions 9 Rack Mount Width, 8 Deep, 3.5 (2U) High 3. Electronics 3. SUBSYSTEMS SPECIFICS The SAL simulator system is powered by 2 VAC. This AC voltage is converted by two AC/DC converters into 5 VDC and 2 VDC which power the systems integrated electronics boards. These boards consist of the main control board and two thermo-electric cooler (TEC) control boards. The internal layout of the electronics can be seen in Figure 2. The main control board electronics consists of three laser diode drivers, one servo controller for the VDM, the safety interlock control, and attenuation control. AC/DC Converter Main Control Board TEC Controller AC/DC Converter TEC Controller Figure 2. Internal layout of electronics boards in SAL simulator system

4 3.. Laser Diode Controls Each of the three laser diodes used in the system has its own separate laser diode driver. The laser diode driver for the 66 nm source is a constant current source based on a voltage regulator integrated circuit (IC) with transient and reverse bias protection. The drivers used for the 64 nm and 55 nm laser diodes are identical laser drivers using commercially available laser diode driver ICs. The drive current limits are set individually for the different laser diodes to protect them from electrical overstress. The enable function of each laser diode driver is an OR function between a switch on the front panel and a control pin on a D-sub connector on the back panel. These digitally triggered drivers have modulation capabilities up to MHz. The modulation inputs of the drivers are tied to the SMA connectors in the front panel of the SAL simulator system; a TTL level high at this SMA input turns the laser diode output on, and a TTL level low turns the laser diode output off Servo Controller The servo controller controls the position of the moving stage on the VDM. The user inputs an analog setpoint voltage between -5 V and 5 V to set the position of the stage. This setpoint voltage is fed into an analog proportional integral differential (PID) feedback circuit consisting of a network of operational amplifiers to generate an error signal. The error signal feeds into a high-current operation amplifier that directly drives the motor of the VDM. The feedback signal is provided by a linear potentiometer connected to the stage of the VDM. The outputs and input of the VDM servo controller are provided on the pins of the D-sub connector on the front panel of the SAL simulator system Safety Interlock Control The safety interlock control on the unit consists of a single pin on the D-sub connector on the back panel that must be a TTL level high for the unit to operate. Driving this control pin high closes a relay allowing 5 VDC and 2 VDC power to be supplied to the power planes of the main control board Attenuation Control Attenuation control of the 64 nm and 55 nm output is provided on separate pins on the D-sub connector on the back panel. The level of attenuation is set by an analog input voltage ranging from V to 6 V. No voltage input results in no attenuation and 6 V results in maimum attenuation. 3.2 Optical Path The entire optical subsystem internal to the SAL simulator system is in optical fiber. All components in the system are fiber coupled components, including the laser diodes. Furthermore, all fiber coupled components are connected by splicing the fiber pigtails together. The light emitted from the 64 nm and 55 nm laser diodes is coupled into analog voltage controlled fiber optical attenuators based on micro-electro-mechanical systems (MEMS) technology. The paths for both sources contain two attenuators in series with each attenuator capable of providing greater than 4 db of attenuation; thus, the two in series can provide greater than 8 db of dynamic range. The optical paths, after attenuation control, are combined using off-the-shelf components as WDMs. The 66 nm / 64 nm WDM is a 6 nm fiber optic splitter that combines some of the 66 nm output with some of the 64 nm output. The 66 nm \ 64 nm light is combined using an off-the-shelf 64 nm \ 55 nm WDM, which passes approimately 5 % of the 66 nm light. A schematic of the optical path is shown in Figure 3. The final optical output of the control unit is provided through a FC/PC port on the front panel of the control unit. A stainless steel armored fiber optic patchcord is used for routing the control unit output to either the VDM or user determined test setup.

5 66nm 66/64 WDM 64nm ATT ATT2 64/55 WDM 55nm ATT3 ATT4 Output 66nm >2mW 64nm >25mW 55nm >5mW X Splice, yields.3db ATT Variable Optical Attenuator Figure 3. Optical subsystem fiber optic pathway for source combining 3.3 Variable Divergence Module The VDM is a custom low-cost positioning stage consisting of a moving lens, with a focal length of, moving with relation to the fied fiber output of the SAL simulator control unit. The VDM is shown in Figure 4 with important components labeled. The stage moves on a pair of precision crossed roller bearing slides that ensure the lens stays precisely in the center of the optical path during translation; the stage has a translation range of. The stage was designed such that the output light is collimated slightly before the maimum range of the translation stage is reached and maimum divergence occurs when the lens is closest to the fiber output; the divergence angles at minimum are approimately.2º and 6º at maimum for the 64 nm output. Since the lens is a refractive element, the three different wavelengths will not be collimated simultaneously; the end user must be aware of this fact for proper use of the system. The translation stage is connected to a geared servo motor that turns an actuation arm to move the stage back and forth. The control voltage to the motor is provided by the servo controller in the control unit, and the feedback position of the stage is provided by the linear potentiometer connected to the moving stage. The potentiometer rod is encompassed by a compression spring that serves as a backlash reduction element for the stage. The stage feedback voltage versus setpoint voltage was measured to determine repeatability. Based on the specifications of the potentiometers repeatability, the stages repeatability is approimately +/- 2 µm. This type of accuracy is more than sufficient for control of the output beam divergence. In order to determine the response time of the VDM, the control setpoint of was driven with a triangular signal and the feedback signal was monitored. The feedback signal started to distort from a triangular response at a repetition rate of.4 Hz or a ramp time in either direction of 36 ms. The stage operates more smoothly at high speeds when running only in one direction, as would be the case for seeker fly-in simulation.

6 Lens Fiber Output Potentiometer Bearing Slides Motor Actuation Arm Figure 4. DVM parts 4. MEASURED CHARACTERISTICS Several basic measurements were carried out in order to characterize the SAL simulator system. The first measurement taken was a measure of the optical pulses emitted from the system at various pulse widths with a GHz photodiode connected to an oscilloscope; the recorded optical pulses are shown in Figure 5 (left). These pulses have substantial ringing due to the initial design of the driver circuit; this ringing was substantially reduced in later SAL simulator systems as can be seen in the improved optical pulse shown in Figure 5 (right). The second measurement taken was the optical spectrums of the 64 nm and 55 nm sources under temperature control and 2ns pulse conditions; the spectrum for each of the sources is shown in Figure 6. The third measurement was a measure of the performance of the variable optical attenuators (VOAs) versus voltage; this measurement is shown in Figure 7. The attenuation measurement was limited by the minimum detection level of the power meter used in the measurement. The specifications of the VOAs used include a maimum response time of 2 ms and a maimum attenuation of at least 4 db. The last measurement taken was the relative variation in energy per pulse. This measurement was taken to ensure the system pulse-to-pulse energy was stable from pulse to pulse. The optical pulse trace was recorded several times at time intervals on the order to 3 s to min, the trace was integrated, and Figure 8 shows the variation in percent from the pulse energy average. 25ns 2ns ns.8.8 Amplitude [A.U.].6.4 Amplitude [A.U.] Time [nsec] Time [nsec] Figure 5. The optical pulses measured from the first SAL simulator system at various pulse widths (left), and the optical pulse measured from a second generation SAL simulator system.

7 Normalized Spectral Density Normalized Spectral Density Wavelength [nm] Wavelength [nm] Figure 6. Optical spectrum of the 64 nm source (left) and the 55 nm source (right). 64 nm 55 nm - -2 Attenuation [db] Voltage [V] Figure 7. Attenuation versus control voltage of the variable optical attenuators for both 64 nm and 55 nm Percent Change Sample Number Figure 8. Percentage change in relative pulse energies for pulse samples taken at several s of seconds apart.

8 5. CONCLUSION A low-cost SAL simulator source using laser diode sources that emits laser light at the critical wavelengths of 64 nm and 55 nm, along with light in the visible for alignment, has been presented. This SAL simulator source, emitting these three wavelengths from a single fiber aperture, was designed to meet the developing need of HWIL facilities to simulate multiple spectral signatures simultaneously. This SAL simulator source has several advantages over traditional DPSS sources for use in production or laboratory environments including pulse width agility, low power, integrated intensity and beam divergence control, cost, and system size. The overall system has been designed for simplicity with basic controls making it easy to use. The optical subsystem also has a straight forward design, integrating VOAs to allow for high speed intensity control of the optical output and off-the-shelf components for combining the wavelengths. The VDM has a maimum response time near 4 ms and has a divergence range of.2-6º. The recorded measurements present show that the system is pulse width agile, has spectral signatures as designed, has a dynamic range in terms of intensity of at least 7 db, and has great pulse energy stability from pulse-to-pulse. ACKNOWLEDGEMENTS This work was paid for by the Air Force under SBIR Phase II Contract # FA865-5-C-99. Public Release Affairs Case #96ABW-2-6-SAL REFERENCES [] Achtenhagen, M., Amarasinghe, N.V., and Evans, G.A.: "High-power distributed Bragg reflector lasers operating at 65 nm", Electron. Lett., 27, 43, (4), pp