System Description - NORTHROP GRUMMAN IKA-012-QTG User Manual

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Table Of Contents

Table of Contents

switch to prevent the buildup of optical power in the oscillator cavity by introducing a

loss greater than the available gain allowing energy to accumulate in the gain medium.

The Q-switch then suddenly removes the loss, allowing the laser power to increase

exponentially with each round trip of the oscillator cavity until much of the stored energy

is exhausted and the gain once again falls below the loss. (It takes light only a few

nanoseconds to transit the laser cavity and results in releasing stored energy in a short

duration pulse.) The high peak power achieved in this manner is also essential to

achieving an adequate fraction of conversion to 532 nm. The conversion process is

dependent on the intensity of the circulating 1064 nm power.

The laser beam that circulates inside the oscillator cavity is determined by a process of

self-consistency. The beam characteristics at any given location in the cavity are repeated

after the beam completes a round trip back to that location. Oscillator beams are

composed of transverse modes of simple structure related to a fundamental Gaussian

intensity profile mode, which is of the smallest lateral extent and divergence in the set.

The aperture in the oscillator path limits the laser beam's transverse spatial extent. To

achieve maximum power and efficiency, this aperture should coincide with the aperture

of the amplifier medium so that the oscillating beam fills the gain medium to the

maximum extent. Laser beam quality will degrade if the diameter of the fundamental

mode is much smaller than the limiting aperture, causing higher order modes to oscillate

with a larger transverse extent and a higher divergence than the fundamental mode.

Beam quality is dependent upon the product of the diameter of the beam and its angular

divergence as it propagates into the far distance. This product is conserved with

propagation and transformation by optical elements such as lenses and mirrors. Beam

quality can also be rated by the reduction of spot size achieved by simple focusing with

higher quality beams providing spots with smaller diameters and higher intensities.

This factor also impacts the amount of achievable harmonic conversion and favors

restricting laser operation to its fundamental Gaussian mode, such as is used in the Iklwa

laser. The design of the laser cavity for fundamental mode operation involves the

selection of cavity length and the characteristics of the pumped amplifier medium's

diameter and focal power.

Once the design parameters are selected for optimum fundamental mode operation, they

cannot be altered without disrupting the fundamental mode size in the amplifier leading

to unacceptable changes in performance. This is particularly important to the intensity of

amplifier excitation, pump power, or drive current since these also determine focal

power. Consequently, the amplifier drive current must be kept within a close tolerance of

the design point.

In applications that require the laser output power be lowered to reduce gain without

sacrificing beam characteristics and focal power, the Iklwa laser achieves a

corresponding reduction of power by increasing the loss of the optical cavity while

maintaining the gain at a constant level.