Theory Of Operation - NORTHROP GRUMMAN Patara I User Manual

Theory of Operation

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