Adjustable mounting unit for an optical element of a gas laser

Details Adjustable mounting unit for an optical element of a gas laser Adjustable mounting unit for an optical element of a gas laser

2000-02-22

2002-12-10

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

Mirror support or alignment structure

C372S108000, C372S055000, C372S065000

Reexamination Certificate

active

06493375

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to lasers, and more particularly to adjustable mounting units for the optical elements of gas lasers.
BACKGROUND OF THE INVENTION
Lasers have recently been applied to a large variety of technical areas, such as optical measurement techniques, material processing, medicine, etc.
Due to the special chemical, ablative, spectroscopic or diffractive properties of UV light, there is a big demand for lasers that generate laser beams having a short wavelength in the UV range.
Excimer lasers, such as the ones disclosed in U.S. Pat. Nos. 5,771,258 and 5,438,587, serve well as a laser for generating coherent, high intensity pulsed beams of light in the UV wavelength range.
The excimer lasers described in U.S. Pat. Nos. 5,771,258 and 5,438,587, are pulsed lasers. Pulsing is required in excimer lasers to allow sufficient time between pulses to replace the laser gas within the discharge region with fresh gas and allow the gas used for generating the previous pulse to recover before being used again for another gas discharge. In the discharge region (i.e., discharge gap), which in an excimer laser is typically defined between an elongated high voltage electrode and an elongated ground electrode which are spaced apart from each other, a pulsed high voltage occurs, thereby initializing emissions of photons which form the laser beam.
The laser beam is emitted along the extended ground electrode in a longitudinal direction of the laser tube. To achieve the desired amplification by stimulated emission of radiation, a resonator comprising a reflecting and a partially reflecting optical element disposed at opposite ends of the discharge gap is required. The laser beam leaves the tube through the latter.
If the reflective optical elements are provided outside the gas laser tube, a fully transparent window is provided in alignment with the discharge gap at each end of the tube to seal the tube, as can be seen in U.S. Pat. No. 5,438,587, for example. A mirror or other reflective optical element is then provided in axial alignment with one of the windows and its reflective side facing the window. A partially transparent, partially reflective mirror is positioned outside the tube so that it is aligned with and facing the other window. As a result, the faces of the two reflective optical elements are opposing one another and define a laser light resonator.
If the reflective optical elements are used to seal the tube, the mirror and the partially transparent, partially reflective mirror are integrated into the end walls of the tube at opposite ends of the discharge gap. As a result, no extra windows are required. For lasers emitting light in the ultraviolet range of the electromagnetic spectrum, extra windows have the disadvantage of significantly reducing the efficiency and increasing the operating costs, as the special window materials employed are expensive and deteriorate with use and time and need to be occasionally changed. In addition, the transparent windows closing the tube form extra optical elements resulting in extra losses and reflections on the surfaces. The latter can be removed by inclining the window at Brewster's angle as taught by U.S. Pat. No. 4,746,201, but invariably the laser output is reduced. Deterioration of the optical elements also cannot be entirely avoided, reducing output and giving rise to the need to replace the rather expensive optical elements after a certain time.
The reflective optical elements that form the resonator must be precisely positioned relative to one another to ensure optimal laser light output power, laser efficiency, and the quality of the laser beam. This is especially true with respect to the angular alignment of the reflective optical elements, not only with respect to each other, but also with respect to the laser tube. However, maintaining the appropriate angular alignment of the reflective optical elements is difficult in view of changes in the operating conditions, such as pressure or temperature of the gas and the temperature of the tube, the optical elements, and their supporting units. In addition, mechanical vibrations or shock to the laser may also affect the angular alignment of the reflective optical elements forming the laser resonator.
When the reflective optical elements are provided outside the laser tube, a very complex outer supporting structure for supporting the reflective optical elements must be provided. Such a supporting structure is very expensive and susceptible to damage. Furthermore, the length of the resonating path between the two opposing mirrors is longer than what is actually necessary. This reduces the output power of the laser, which in turn reduces the efficiency of the laser. In addition, the supporting structure is susceptible to deformation due to outer forces or thermal expansion. Such distortions may distort the angular alignment of the reflective optical elements, particularly the parallelism between the two opposing laser optical elements.
These disadvantages, which are attendant to external supporting structures, have lead to a demand to provide the reflective optical elements as an integral part of the laser tube. However, trying to provide the reflective optical elements as an integral part of the laser tube has caused a different set of problems.
Inside the laser tube, high gas pressures occur, thereby increasing the danger of deformation and damage of the rather sensitive laser optical element. The gas pressure is further increased as a result of the increasing temperature of the gas inside the laser tube caused by the emission of energy. This obviously makes the problem even worse. In addition, thermal expansion of the laser tube can generate a further distortion of the parallel disposition of the laser optical elements with respect to each other.
A mechanism for permitting the reflective laser optical elements to be adjusted with respect to each other is crucial, because light inside the resonator is reflected by the reflective optical elements forming the resonator numerous times. As a result, even a slight divergence from the ideal adjustment may cause a malfunction of the laser or at least a reduction of the laser light output power, and thus reduction in the efficiency of the laser and its beam quality.
A number of patents, including DE 3130399 A1, DBGM 297 15 466.4, U.S. Pat. No. 4,744,091, JP 61-047008, and DE 3710525 C2, teach the use of spacer bars or frames that surround the laser tube and support the reflective optical elements that form the laser's resonator. These spacer bars or frames also include adjustment mechanisms that permit the reflective optical elements to be adjusted relative to one another and the laser tube. Due to the spacing between the laser tube and these spacer bars or frames, these known mounting structures are not exposed to the operating conditions in the tube. Thus, the operating conditions of the laser do not tend to influence the position of the optical elements. However, such arrangements are difficult to manufacture and service. In addition, they are more prone to distortions resulting from external forces than a support structure for the resonator's optical elements that are directly coupled to the tube itself.
Thus, a need exists for an improved adjustable mounting unit for mounting the optical elements of a laser.
RELATED APPLICATIONS
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
Docket
Serial or
No.
Title
Inventors
Filing Date
Patent No.
249/300
Gas Laser
Claus Strowitzki
February 22,
09/510,539
Discharge Unit
and Hans Kodeda
2000
249/301
A Gas Laser
Hans Kodeda,
February 22,
09/511,649
and a
Helmut Frowein,
2000
Dedusting Unit
Claus Strowitzki,
Thereof
and Alexander
Hohla
249/302
Dedusting Unit
Claus Strowitzki
February 22,
09/510,667
for a Laser
2000
Optical
Element of a
Gas Laser and
Method For
Assembling
249/303
Shadow Device
Claus Strowitzki
February 22

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