Optical: systems and elements – Optical frequency converter
Reexamination Certificate
1999-08-04
2003-03-11
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C372S021000
Reexamination Certificate
active
06532100
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the frequency conversion of optical signals by nonlinear optical crystals and to laser systems that provide short-wavelength, frequency-multiplied outputs using such nonlinear optical crystals.
2. Description of the Related Art
Intense ultraviolet light sources can be used in a variety of different applications. For example, photolithography presently uses excimer lasers as light sources to take advantage of the intensity and narrow line width of excimer lasers to define features on semiconductor devices having widths of one quarter micron or smaller. Micro-machining applications use intense ultraviolet light sources, taking advantage of the flux of high-energy photons to efficiently machine fine structural details in objects. Photochemical applications can also use intense short-wavelength light sources to great advantage, because short wavelength or high-energy photons are particularly effective in driving certain types of photochemical reactions. All of these applications benefit from the use of high-intensity, short-wavelength light sources but to date the available light sources are not entirely satisfactory.
There are comparatively few light sources capable of outputting significant levels of optical output in the ultraviolet. Mercury lamps are a traditional short wavelength light source that has become increasingly inadequate for demanding or high intensity applications. Helium-cadmium and hollow cathode lasers can provide ultraviolet output, but with insufficient intensity for many applications. Excimer lasers are used in photolithography, but excimer lasers are large, expensive, and have poor beam quality. Perhaps the most significant limitation of excimer lasers for some applications, however, is the relatively low pulse rate of the laser. Typical excimer laser systems have a pulse rate on the order of one thousand Hertz, which can be disadvantageous.
Another possible ultraviolet light source is a frequency tripled or quadrupled solid state laser. The fundamental output of a solid state laser system such as Nd:YAG or Nd:YVO
4
is at a wavelength of approximately 1.064 &mgr;m. The frequency tripled or quadrupled output of such a solid state laser is thus in the ultraviolet, offering wavelengths of 355 nm (tripled) or 266 nm (quadrupled). Frequency multiplied laser systems are relatively complex. On the other hand, solid state laser systems have advantages compared to other intense ultraviolet light sources, making frequency multiplied solid state lasers desirable for at least some applications. The advantages of solid state lasers include excellent beam quality, relatively small size, good efficiency and the availability of high repetition rates. The reasons for certain of these advantages are discussed here to provide a better background for understanding certain aspects of the present invention.
Solid state lasers are a class of lasers that use as a gain medium a crystalline, glass or other solid material that serves as a host for an optically excitable material such as an ion of a rare earth element like neodymium. The crystalline, glass or other solid host material acts as a matrix fixing the optically excitable material in place. Examples of solid state laser systems include those that use neodymium as the excitable material within a matrix of yttrium aluminum garnet (Nd:YAG) or neodymium within a matrix of yttrium vanadate (Nd:YVO
4
) as the respective gain media of the solid state lasers. Pump sources for these solid state laser systems include broad band light sources such as xenon or other types of flash lamps. Often, however, the pump source for the solid state laser system is a diode laser or, more preferably, an array of diode lasers. Broad band flash lamp pumping can be inefficient because the pump light cannot be well focused into the gain region and because the spectrum of the light is so broad that light is absorbed that does not facilitate pumping of the gain medium and instead merely heats the gain medium. An alternative to broadband pump sources is the use of semiconductor laser diodes, including monolithic linear arrays of semiconductor lasers. Laser diode pump sources can be selected that output light at a wavelength closely matched to the gap between excitation levels within the gain medium. Such a matched pump source can efficiently pump the gain medium. Some solid state lasers utilize diode lasers to pump the solid state gain medium in an “end-pumped” configuration in which the gain medium has its longest dimension extending along the optical axis of the laser cavity and the diode laser pump light is introduced through an end face of the gain medium. This end pumped configuration can capture a relatively high portion of the input laser diode pump light to provide an efficient laser. Side pumped configurations of solid state lasers provide laser diode array pump sources along the sides of a solid state laser. In this configuration, it is typical to direct the output of the pump diode lasers generally perpendicular to the optical axis of the solid state laser and into the gain medium. Often, side pumped configurations capture less of the pump light but use simpler optics to couple the pump light into the gain medium. Nd:YVO
4
has a very high absorption coefficient at the most favorable wavelengths for efficiently pumping the Nd:YVO
4
gain medium. Solid state lasers using Nd:YVO
4
as a gain medium may be configured as end pumped lasers or as side pumped lasers.
FIG. 1
shows a frequency-tripled solid state laser and provides a useful illustration of several of the concepts involved in such a system. A laser diode array
10
provides pump light at a wavelength appropriate to efficiently pump the solid state gain medium and the pump light is collected and focused by a lens
12
into the gain medium
14
. The gain medium in this end pumped configuration might be, for example, Nd:YAG, Nd:YVO
4
or another gain medium. A coating, highly reflective at the fundamental output of the laser (1.06 &mgr;m) but transmissive of the pump radiation, is deposited on one end face
16
of the gain medium
14
to serve as the high reflecting mirror for the laser cavity. Alternately, the high reflector might be a freestanding mirror transmissive at the pump wavelength but highly reflective at the fundamental wavelength of the solid state laser. Output coupler
18
defines the other end of the resonant cavity. A Q-switch
20
may be provided within the resonant cavity to provide increased levels of peak output power in pulsed mode operation.
Light
22
at a wavelength of approximately 1.06 &mgr;m exits the resonant cavity through the output coupler
18
and is directed into a first nonlinear optical crystal, doubling crystal
24
. In this illustration, doubling crystal
24
might be KDP (potassium dihydrogen phosphate) configured in accordance with conventional type II phase matching conditions. It may be desirable to polarize the fundamental output of the gain medium if, for example, Nd:YAG is used as the gain medium
14
. On the other hand, if Nd:YVO
4
is used as the gain medium
14
, it may be unnecessary to provide a polarizer, since Nd:YVO
4
typically provides a polarized output. The output of the doubling crystal
24
is typically chosen to include light of the fundamental frequency and light of the doubled frequency. The two output light signals are passed through a rotation element
26
that adjusts the relative polarization between the two light beams to provide the desired relationship between the polarizations of the two light beams for the tripling crystal. From the rotation element
26
, the beams of fundamental and doubled light are provided into the second nonlinear optical crystal, tripling crystal
28
. The tripling crystal may also be KDP operated under type II phase matching conditions.
Photons from the fundamental beam and the frequency-doubled beam are combined in the tripling crystal to provide an optical output signal at a frequency tripled from the funda
Hug William F.
Partanen Jouni P.
Reynolds Gary
Wu Xingkun
3-D Systems, Inc.
D'Alessandro Ralph
Lee John D.
Wright Willaim H.
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