Theory Of Operation - NORTHROP GRUMMAN IKA-012-QTG User Manual

Theory of Operation

The Iklwa 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 12,000 Hz. It is a transverse
electromagnetic mode (TEM
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). The Iklwa 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 to 50 ºC. Once this
temperature is identified and fixed, the temperature of the crystal can be stabilized to ±0.1
ºC with the TEC controller.
The Nd:YAG laser amplifier is capable of storing a considerable amount of energy due to
the long upper state lifetime of the Nd ion. The Iklwa laser uses an acousto-optic (AO) Q-
© 2019 Cutting Edge Optronics, Inc.
Approved for Public Release; NG19-2099
) laser and therefore has very high brightness. The
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Chapter One: Introduction
Iklwa Laser User Manual
CEO-UMAN-0055E