Periodic poling spreads output wavelengths

Peacock tails are usually associated with form as opposed to function, but a broad array of mid-infrared (IR) laser frequencies shows promise for helping military aircraft avoid heat-seeking missiles. Confusing the enemy with a ruffle of laser tail feathers is just one among many potential applications for IR-emitting optical parametric oscillators (OPOs) based on periodically poled lithium niobate (PPLN).

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Periodic poling spreads output wavelengths

Hassaun Jones-Bey

Peacock tails are usually associated with form as opposed to function, but a broad array of mid-infrared (IR) laser frequencies shows promise for helping military aircraft avoid heat-seeking missiles. Confusing the enemy with a ruffle of laser tail feathers is just one among many potential applications for IR-emitting optical parametric oscillators (OPOs) based on periodically poled lithium niobate (PPLN).

This material is enabling engineers in industrial, academic, and military laboratories to replace the design limitations of inherent material characteristics, such as birefringence, with the design flexibility of quasi-phase-matching (see Laser Focus World, May 1997, p. 105). Engineers are also designing the new-found frequency-generation capabilities into components that will perform in demanding functional environments, such as military aircraft.

Development of an 808-nm diode-pumped, high-gain 1064-nm laser oscillator, for example, for pumping of an intracavity OPO has allowed engineers at Light Solutions (Mountain View, CA) to achieve 6 W of output power. The PPLN OPO spreads the output across the 1500- to 5000-nm spectrum concurrently at three signal and idler wavelengths, according to company president Larry Marshall. Rapid tuning by temperature change or physical movement of the PPLN OPO can also--through nonlinear mixing--provide a near-continuum of wavelengths across the visible spectrum (see photo). When deployed in the mid-IR frequency range, the technique displays an intriguing potential for countering IR missile-guidance technology.

Heat-seeking missiles chase the thermal signature generated by a ship or aircraft, and sophisticated heat-seekers use narrow IR filters to seek specific thermal peaks at wavelengths characteristic of an aircraft engine, for example. Sophisticated heat-seekers can also seek in several bands simultaneously to avoid being fooled by flares or other decoys and can reject background and other spurious sources by comparing signal strengths in the various frequency bands.

Just as a heat-seeker sweeps ahead on its flight path for a predetermined signal periodicity, however, a target ship or aircraft displaying a properly engineered peacock tail of IR frequencies could actually redirect the heat-seeker to a different target--possibly even back to the heat-seeker`s home base, Marshall said.

Engineers at Wright Patterson Air Force Base (Dayton, OH) have used the same high-gain laser oscillator to demonstrate 0.6 W of 3500-nm idler output from a truly continuous-wave PPLN OPO, Marshall said. And researchers at the University of Dayton have developed a novel intracavity arrangement allowing separate alignment of the OPO and laser cavities.

Because intracavity OPO operation greatly increases the available pump power, it offers the possibility of generating total OPO output approaching the total pump output of an optimally coupled pump laser. The combination of a high-gain laser and nonlinear material makes intracavity OPO operation highly promising for powerful continuous-wave mid-IR sources, Marshall said. "Ultimately such compact devices are attractive for use onboard advanced aircraft," he added.

Considering the healthy egos that most people associate with military fighter pilots, laser peacock tail feathers may also serve as a psychologically appropriate defense mechanism. The range of potential applications is quite broad, however. Applications mentioned in just one of several presentations on quasi-phase-matching and OPO technology at the 1997 Conference on Lasers and Electro-Optics in May (Baltimore, MD) include eye-safe operations, trace-gas monitoring, as well as wind-speed and wind-shear measurements.

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Diode-pumped high-gain oscillator for pumping an intracavity periodically poled lithium niobate OPO can stimulate a nearly continuous spread of wavelengths across the 1500- to 5000-nm spectrum, which is made visible above by frequency-doubling.

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