NORTHROP GRUMMAN Patara I User Manual

Contents

NORTHROP GRUMMAN Patara I User Manual Manual pdf 14 page image

Chapter One: Introduction© 2018 Cutting Edge Optronics, Inc. 2 Patara I and II Laser User ManualTheory of OperationPatara Laser is a Q-switched, intra-cavity, frequency doubled, laser oscillator. Theoutput consists of pulses of light at 532 nm wavelength and 60 ns to 100 ns pulseduration at repetition frequencies of 4,000 Hz to 10,000 Hz. It is a transverseelectromagnetic mode (TEM00) laser and therefore has very high brightness. Thecontinuous wave (CW) pumped neodymium-doped yttrium aluminum garnet(Nd:YAG) rod module in the oscillator is optically excited using arrays of laserdiode bars operating at wavelengths around 806 nm.A laser oscillator consists of an optical amplifier in an optical cavity which formsa laser optical mode that can oscillate when the gain of the amplifier is sufficientto overcome the losses in the optical cavity. One such loss is the amount thatescapes the oscillator cavity through one of its mirrors resulting in its usefuloutput. This basic approach suffices unless the output beam at a desiredwavelength differs from the fundamental wavelength (1064 nm) of the laseramplifier medium (Nd:YAG).Various nonlinear optic techniques can serve to convert this beam to anotherwavelength, but usually with considerable loss of power or with a residue ofunconverted fundamental power. This occurs when the fundamental radiation isallowed to leave the oscillator cavity and the beam is converted outside the cavity.However, if the nonlinear conversion is completed inside the oscillator cavity theleftover fundamental simply returns to the oscillator and is conserved, amplified,and returned to the nonlinear frequency converter. As a result nearly all the powerwhich could have been optimally coupled out of the cavity by a semi-transparentmirror, is now converted to another wavelength (provided that the semi-transparent mirror is replaced by one that is fully reflective at the fundamentalwavelength). Patara uses this intra-cavity technique to double the laser frequencyto a wavelength of 532 nm.The frequency doubling device is a crystal of lithium triborate (LBO) cut at aspecial angle providing phase matching between the fundamental waves in thebeam and the waves generated at 532 nm at around room temperature. Phasematching and efficient conversion is sensitive to the angle to the unidirectionalintra-cavity beam and to the temperature of the LBO crystal. As a result, it isnecessary to provide a stable mechanical and thermal environment for thedoubling crystal.A phase matching condition can be met by either the angular adjustment of thedoubling crystal mechanically or by the varying the temperature of the crystalelectrically. Stability and reliability considerations lead to a mechanically fixeddoubling crystal with fine tuning accomplished electrically.With the doubling crystal mounted in a cell on the top of the thermal electriccooler (TEC), the temperature of the doubling crystal can be controlled. Theperformance parameters are optimized with a crystal temperature between 20 ºC