Invar Alloy Resonator Support Structure Maintains Pinpoint Accuracy

High-power, water-cooled ion lasers, used widely in research, manufacturing, science, medicine and entertainment, are extremely sensitive to temperature variation. If not controlled, small temperature changes can cause serious problems in the end-user application.

That's why Coherent Laser Group, Santa Clara, CA, takes such great care in designing and building the resonator structures that are key to the fundamental stability of its ion laser systems.

The resonator maintains critical alignment of the mirrors that define the laser's optical cavity. In the typical ion laser, this cavity is constructed with a flat mirror and a long-radius, concave, curved mirror.

Coherent has used three rods of Carpenter Super Invar 32-5 alloy as integral supports in its resonators because of the material's extremely low coefficient of thermal expansion (CTE) and its high thermal mass.

No other material offered this combination of essential properties. Graphite and aluminum provided satisfactory CTE's, Coherent reported, but they both exhibited low thermal mass.

Low Expansion Alloy

Super Invar 32-5 alloy, developed by Carpenter Technology Corp., Reading, PA, is a magnetic, austenitic, solid solution alloy containing iron, nickel and cobalt. While the alloy offers minimal expansion at room temperature, it also exhibits austenite stability to service temperatures at least -67 ^oF (-55 ^oC) and thermal expansion properties less than Carpenter Invar "36"® alloy (36% nickel-iron) when used in the -67/203 ^oF (-55/95 ^oC) temperature range.

In an ion laser, the optical alignment of the laser cavity must be held to extremely tight angular tolerances. A very small - tens of microradians - misalignment of the mirrors can cause the output power to drop to half its full strength.

To better appreciate the level of accuracy required here, consider that an angular change of one microradian can cause a positional error of only 0.1 millimeter at a distance of 100 meters.

The resonator of a large-frame ion laser is about two meters long. Two key components, the plasma tube and magnet, are also supported inside this structure by the Super Invar 32-5 alloy rods. The plasma tube creates light, generating heat in the process.

The plasma tube typically converts as much as 40 kilowatts of electrical power into heat that must be dissipated into a water jacket. These unique requirements, combined, make it a challenge to design a mechanically stable resonator.

Ion lasers are notoriously inefficient. For 25 watts of output light, an energy expenditure of 40 kilowatts is required. The difference is waste heat which must be removed from the system with water.

Coherent, the leading supplier of ion lasers for scientific, OEM and commercial applications, explains what could happen if a mirror in the laser resonator were to become misaligned. In a nutshell, the laser efficiency would be seriously impaired.

When a resonator goes out of alignment, several things happen: 1. the ouput power drops, 2. output noise increases, 3. transverse mode (the laser's intensity profile across the beam axis) deteriorates, and 4. the beam moves.

A power drop could occur, with concurrent degradation of the transverse beam profile, loss of focusability and inability to produce desired results. Loss of power could reduce illumination in optical experiments and cause malfunction in various processing operations. In a photo-resist operation, lines may not be drawn straight. Inadequate power could generate interference patterns and consequent irregularities in holography.

Design specifications allow for 0.5% to 1.0% laser beam noise. With the slightest deviation in mirror alignment, that noise or intensity fluctuation would double or triple.

Making the Supports

Carpenter, processing its Super Invar 32-5 alloy to achieve the lowest possible CTE characteristics and optimum stability, supplied heat treated and ground bars of 3/8", 1/2" and 5/8" diameter in various lengths to the laser equipment manufacturer. They were then cut to length, straightened, annealed and plated with nickel for corrosion resistance.

Coherent then heat treated and verified the alloy's CTE by interferrometric evaluation. . . a method considered more exact than readings taken by a dilatometer. The laser company measured the thermal expansion of every Carpenter Super Invar 32-5 alloy rod, then matched three rods with precisely the same expansion characteristics to create an ultra-stable resonator structure.

Rods 3/8-in. in dia. by 2½ ft. long have been used generally on resonators less than one meter long. Half-inch diameter rods about a meter long have been selected for medium length resonators, and 5/8-in. dia. rods have been installed on resonators about 1¾ meters long. All rods were matched to the resonators by diameter and length for maximum longitudinal stiffness. The set of three rods is mounted horizontally on each resonator.

In relation to each other, some are mounted in an L-shaped configuration. Others resemble an equilateral triangle. The rods support mirrors that are gimbal mounted to the ends of each resonator.

Coherent was pleased that all the resonator rods made from the Carpenter alloy had high thermal mass because this characteristic made them more resistant to temperature transients. This means that a quick change in temperature - which might occur with the startup of a nearby air conditioner or a sudden diversion of water for the cooling system - would not cause an appreciable change in rod temperature. The high mass buffers temperature change, thus protecting the system from what Paul Ginouves, senior product manager at Coherent, called a temperature "hiccup".

Heat Generated

Ion lasers, Ginouves pointed out, use a powerful discharge of electrical energy to stimulate photon emission from inert gas. This discharge generates a great amount of heat, particularly in the plasma tube, which must be cooled by water. As a laser is started and warmed up, the resonator rods come to equilibrium with their environment.

The objective, says Ginouves, is to keep everything in the environment as constant as possible - water flow and temperature, discharge current and ambient air temperature - so there is minimal effect on laser performance. It is only when these elements are reliably and continuously stabilizied that a high quality laser beam, with minimal noise, is achievable.

Coherent has several stabilization mechanisms which negate the effect of some environmental perturbations, but those mechanisms are effective only because the fundamental stability of the laser is high. That, adds Ginouves, is due to the inherent mechanical stability of the resonator.

"We particularly like the Carpenter Super Invar 32-5 alloy rods for our resonators," commented Ginouves, "because they give us the highest immunity to thermal transients. They give us a sound, reliable resonator structure and good performance over a wide range of conditions.

"With these rods in our design," he continued, "we can often save the expense of elaborate and costly stabilizing systems, and even expand our product offering."