Bellows Improves Laser Resonator Seals

Brent D. Johnson

The stability and reliability of the argon-ion laser has made it a workhorse for a range of applications from writing masks for semiconductor circuits to measuring the flow rate of cell protoplasm.

As countless subtle improvements continue to refine the laser's performance, its only significant drawback, aside from its prodigal energy consumption, may be a tendency to collect dust while in operation. A new flexible pressure coupling has eliminated this once-vexing problem.

A nickel bellows helps keep ion laser resonator assemblies clean, producing an airtight seal that prevents dust from accumulating.

As an argon-ion laser heats up, dust particles can pass through air gaps between the Brewster window and the output coupler, condensing on the components and optics. An accumulation of this material can cause dispersion of the beam, which can have negative results in certain kinds of manufacturing.

"By keeping dust out of the resonator, you avoid dropout," said Tony Gutierrez, ion product manager for Coherent Inc. This is particularly important in applications such as DVD mastering. "One dropout can mean 18 bits are missing," he said. For one piece of dust, the whole master has to be restarted.

Servometer custom-designed a flexible electrodeposited nickel bellows that makes an airtight seal around the resonator. The spring-loaded device expands and contracts with temperature variations throughout a range of operating conditions, closing the gaps. This keeps the resonator clean and free from particulate matter.

Another benefit is that Coherent can keep the crystal housing and field board assemblies contained in an inert nitrogen environment. This is particularly important for frequency-doubled lasers that operate in the deep-UV. Moisture in the air can absorb these wavelengths.

Engineer Paul Hazlitt, who has designed similar systems for Isomet Corp. and Lucent Technologies, said the bellows technology is not a well-known concept. "We're not a product that's taught in school," he said.

Gutierrez said the bellows did not measurably improve the beam quality of the lasers that use them (including the Innova 90C and 300C series and the Innova Sabre line), but the benefit has improved the cost of ownership. By keeping the resonator clear of dust and debris, there is less downtime for servicing.