Bellows improve laser seal design

DN Staff

November 19, 2001

3 Min Read
Bellows improve laser seal design

To seal the intra-cavities of its laser resonator assemblies, Coherent Inc. uses flexible precision bellows instead of rigid metal parts. The electrodeposited (ED) nickel bellows, designed and manufactured by Servometer Corp. (Cedar Grove, NJ), expand and contract with variable temperatures during laser operation.

"The bellows make a reliable airtight seal, allowing cleaner laser resonator assemblies, which are easier to service and maintain," says Ion Product Manager Tony Gutierrez. "This enhanced cleanliness also translates into higher quality in many manufacturing applications of the lasers."

Coherent's continuous wave (CW) argon-ion lasers are used in a wide range of applications, such as measuring the flow rate of cell protoplasm for test and characterization of biomaterials. These lasers are also used to write diffraction gratings into the core of optical fibers for wave division multiplexing in communications, perform mask writing necessary for manufacturing ever-finer semiconductor circuits, and write the master CD and DVD disk media for entertainment and computer software.

Due to the sensitive nature of these applications, it is critical that the laser be protected from contamination in the external environment, Gutierrez explains. "Dust and air transients cause problems in lasers," he says. "The light from the excited argon bounces back and forth thousands of times before it passes an energy threshold and is emitted as a laser beam. If dirt interferes with the passage of light, the resulting dispersion of the beam prevents the proper functioning of the laser."

Not only must the path of the light be clear of dust, but the surface of the output coupler and other optics must also be clear. The elevated temperature may actually cause these components to attract several kinds of particulate materials. In the case of CD and DVD mastering, the atmosphere is prone to gaseous and solid airborne contamination generated by the action of the laser when it burns into the disk. A speck within the resonator on the output coupler can cause a "drop-out," or short interruption in the energy-transmitting laser beam. The result might be a software bug or playback skip.

The argon-ion lasers consist of a gain medium and a high-power energy source. The gain medium is an ion tube, a sealed component containing argon gas, which is excited by the introduction of electric energy. An output coupler and a highly reflective mirror are mounted at each end of the ion tube. This assembly, known as the resonator, concentrates the power that is introduced into the light beam that is emitted. In the argon-ion laser, this concentration is relatively inefficient compared with some other types of lasers. Some 45-50 kilowatts are introduced to obtain a continuous wave 5-watt beam of laser light. This inherent inefficiency causes heat to build up around the resonator.

The solid materials in the resonator assembly expand as heat rises. To allow for the expansion, the previous enclosure components had air gaps between metal-to-metal contact pieces made of low thermal expansion coefficient materials. The design also included temperature interlocks to turn off power if it got too hot inside, minimizing expansion. But excluding dust and other airborne contaminants from migrating into the resonator with was a persistent problem in the original design.

The new design uses the bellows to absorb the effects of thermal expansion while providing the required airtight seal. "Installed cost for the new bellows-type enclosure is the same as the solid tube component," says Gutierrez. "It works so well with lasers of various sizes that we've designed it into four different laser models so far."

For more information about bellows from Servometer Corp.: Enter 534

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