Jamming code development set-ups are generally employed for evaluating the jamming code effectiveness of Directed Infrared Countermeasure (DIRCM) Systems on seeker heads in laboratory conditions. In these set-ups, usually, the output beam of a mid-infrared (mid-IR) laser having similar properties with the DIRCM laser is expanded and collimated before directing it to the seeker head so that a beam with an almost-homogeneous intensity distribution is obtained at the seeker aperture. This simulates what would be expected in a real long distance engagement scenario where an infrared heat seeking missile is attacked by a countermeasure laser and the laser beam diverges to create an almost-homogenous intensity profile at the seeker aperture provided that the effect of atmospheric turbulence is neglected. Large aperture off-axis parabolic mirrors are often employed for the purpose of expanding and collimating the laser beam in these set-ups. However, instead, it is also possible to employ refractive beam shaping optical systems that are much smaller in size compared to these large collimating mirrors in an effort to reduce the laser power loss, required space, and cost for obtaining such uniform laser beam profiles. In this paper, we propose a simple design method for a Galilean beam shaping optical system that transforms a laser beam with a Gaussian intensity distribution into flattop, Super Gaussian, or Fermi-Dirac intensity distributions, hence facilitates obtaining an almost-homogenous intensity distribution for the purpose of future use in a jamming code development setup to be operating in mid-IR band. Similar to Galilean or Keplerian type conventional beam shaping optical systems, the method depends on using two aspheric lenses whose active and opposing surfaces shape the beam by refraction. The first surface redistribute the beam intensity distribution and the second surface collimates the beam. However, contrary to these conventional methods, we employ a simple numerical algorithm to generate the surface equations of the two aspheric lenses. Using our method, we present examples of optical system designs for chosen wavelengths in the mid-IR band. We use ZEMAX’s Physical Optics Propagation package to verify that our design method works.
Their exceptionally large jam-to-signal ratios (J/S) make directed infrared countermeasure (DIRCM) systems the most efficient means for defeating heat seeking missiles. Although several studies have performed analyses on DIRCM systems, influence of optical turbulence on the effectiveness of jamming waveforms is usually ignored. However, due to turbulence originating from the exhaust plume of the air platform and the atmosphere, a DIRCM laser’s beam may be exposed to time-varying intensity variations which may drastically reduce the effective J/S at the seeker aperture compared to the one at the platform. Furthermore, previous studies have usually focused on the signal processing stages of seeker heads while neglecting the diffractive and aberrative properties of the missile optics. An analysis of the impact of turbulent air on the DIRCM effectiveness from a wave-optics point of view is required. In this paper, we first investigate the time-varying impact of several degrees of turbulence on laser beams modulated with a jamming pattern along the optical path from the air platform to the missile’s dome. In the turbulence model, the laser beam emerging from the platform with an arbitrary quality factor of <i>M</i><sup>2</sup> transverses two different turbulent regimes when directed to the missile threat. First, it passes through the region of highly turbulent medium around a rotary-wing platform originating from the rotor downwash of the exhaust plume. Next, the beam travels much longer distances (on the order of few km’s) in the turbulent atmosphere until it reaches the missile. Beam propagation in both regions is simulated using the split-step method. Using the ZEMAX software and its wave-optics based Physical Optics Propagation (POP) package, we employ a generic model for the optical system of a first-generation spin-scan seeker and obtain the time-dependent intensity profiles of the laser beam at the focal point at various instants of the jamming pattern. Generic models for an uncooled lead-sulfide detector and the following signal processing stages have also been included in the model.
<p>We report a mid-infrared (mid-IR or 3.5 to 5.0 μm) laser source for testing jamming code effectiveness against heat-seeking missiles in open-field conditions. We developed an optical parametric oscillator (OPO) based on a ZnGeP<sub>2</sub> (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically <italic>Q</italic>-switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the <italic>Q</italic>-switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition frequency (PRF) of 50 kHz. We characterized the output beam in terms of its output power and wavelength spectrum. The output power of the pump laser can be modulated in a pattern of on/off pulses with a smaller frequency than the PRF. The frequency and the duty cycle of these on/off pulses can be adjusted. Identical on/off pulse modulation appears on the mid-IR output beam after the OPO. It is possible to create jamming patterns and codes in mid-IR band using this feature. We have presented examples of on/off modulation of the OPO output at arbitrary pattern frequencies and duty cycles.</p>
Mid-infrared (mid-IR or 3.5 – 5.0 μm) laser sources are widely employed in applications such as laser projectors, remote sensing of the atmosphere and countermeasures against heat seeking missiles. In this paper, we describe the development of a mid-IR laser source which can be used as a ground-based system for remote sensing applications and testing countermeasure effectiveness in the field. For this purpose, we developed an optical parametric oscillator (OPO) based on a ZnGeP<sub>2</sub> (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically Q-switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the Q-switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition rate (PRR) of 50 kHz. Between the pump laser and the OPO, we use a Faraday isolator for protecting the pump laser from back reflections of the output beam coupling into the pump laser cavity, we also use a polarization rotator and a lens which focuses the pump beam down to a spot of 275 μm (1/e2 , diameter) at the center of the 15-mm long and 55°-cut ZGP crystal located in the middle of a 19-mm long cavity formed by two flat mirrors. The OPO cavity is doubly resonant, the input mirror is a high transmitter at 2.1 μm and a high reflector in mid-IR whereas the output mirror is a high transmitter at 2.1 μm but a partial reflector with a reflectance of 50% in mid-IR. The mid-IR and residual pump beams are separated from each other using a dichroic mirror. We characterized the performance of the OPO in terms of the mid-IR power, power conversion efficiency and pump depletion as functions of the input pump power. We recorded the wavelength spectrum of the midIR beam.
In this paper, we compare the theoretical performance of two design methods that allow simultaneous phase matching of two arbitrary X<sup>(2)</sup> processes along with the capability of adjusting their relative strength. The crystal of these 1-D aperiodic gratings is chosen to be the orientation-patterned gallium arsenide (OP-GaAs), which has been recently used in several devices for high power infrared beam generation. These single gratings placed in an optical parametric oscillator (OPO) or an optical parametric generator (OPG) can simultaneously phase match two optical parametric amplification (OPA) processes, OPA-1 and OPA-2, for converting the 2.1-μm pump laser radiation into the long-wave infrared (8-12 μm) in an idler-efficiency enhanced scheme. The first aperiodic grating design method (Method-1) relies on generating an aperiodic grating structure that has domain walls located at the zeros of the summation of two cosine functions each of which has a spatial frequency that equals one of the phase-mismatches of the two processes. In this method some of the domain walls are discarded considering the minimum domain length (D<sub>min</sub>) that is achievable in the production process. The second design method (Method-2) relies on discretizing the crystal length with samples with a length that is much smaller than D<sub>min</sub> and testing each sample’s contribution in such a way that the sign of the nonlinearity maximizes the magnitude sum of the real and imaginary parts of the Fourier Transform of the grating function at the relevant phase-mismatch spatial frequencies. Also, during the procedure, the smallest domain length is imposed to be D<sub>min</sub>. In this paper, we present the results of Method-2 which we find that it produces a similar performance as Method-1 in terms of the maximization of the magnitudes of the Fourier peaks located at the phase-mismatches of the nonlinear processes while adjusting their relative strength. To our knowledge, we are the first to propose such aperiodic OP-GaAs gratings for efficient long-wave infrared beam generation based on simultaneous phase matching.
In this paper, we model the performance of a device with a simple architecture for high-power mid-wave infrared beam generation at a wavelength of 3.8 microns. We believe that this device can be used as an efficient frequency converter for an infrared countermeasure source. The device is a seeded idler eﬃciency-enhanced optical parametric generator (IEE-OPG) based on an aperiodically poled MgO-doped LiNbO3 (APMgLN) grating pumped by a high-repetition rate nanosecond-pulsed 1064-nm laser and seeded by a low-power 1478-nm distributed feedback diode laser. In the IEE-OPG, two optical parametric ampliﬁcation (OPA) processes, OPA-1 and OPA-2, are simultaneously phase matched in a single APMgLN grating. The signal at 1478 nm is ampliﬁed and the idler at 3800 nm is generated as a result of OPA-1, the signal acts as the pump for OPA-2 and the conversion efficiency of the idler is enhanced as a result of OPA-2. Also, a difference-frequency beam at 2418 nm is generated.
We characterized the device performance using a realistic model that takes the diffraction of the beams into account. We designed multiple aperiodic gratings with varying relative strengths of OPA-1 and OPA-2. For various crystal lengths, optimum relative strengths of the two processes and input pump power levels for achieving the maximum mid-wave infrared conversion efficiency and output power are determined.
Efficiency-enhanced mid-wave infrared beam generating optical parametric oscillators (OPOs) based on AP- MgLN gratings were reported before. However, no attempt was made for the optimization of the relative strengths of the simultaneously phase-matched processes in these devices. Our model calculations show that it is possible to reach and exceed the mid-wave infrared conversion efficiencies of these OPOs by correctly choosing the design parameters of the seeded OPGs based on relatively long APMgLN gratings.