Optical: systems and elements – Optical amplifier
Reexamination Certificate
2001-12-10
2003-12-23
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
C359S330000
Reexamination Certificate
active
06667828
ABSTRACT:
TECHNICAL FIELD
This invention relates to nonlinear optical crystals, and more particularly to the conversion of optical radiation having a first frequency to optical radiation having a second frequency using nonlinear optical crystals.
BACKGROUND
There are many known sources of optical radiation, which can be characterized by a frequency, or frequency spectrum. A method of generating optical radiation of a desired frequency is to generate optical radiation of a first frequency, different from the desired frequency, and then to convert this to optical radiation having the desired frequency. For example, a pump laser can generate optical radiation having a frequency &ohgr; (i.e., fundamental frequency). This optical radiation can then be converted to optical radiation having a frequency 2&ohgr; (i.e., harmonic frequency) by appropriate illumination of a nonlinear frequency doubling crystal with the optical radiation having frequency &ohgr;.
The conversion efficiency of a pump laser beam into its harmonics is generally low. The power of a harmonic beam is related to the power of the fundamental pump beam in a nonlinear way. Hence it is not uncommon for high power pump lasers to be tightly focused onto a nonlinear crystal in order to generate sufficient power in the harmonic. For example, a 25 watt (W) pump laser may be focused onto a nonlinear crystal to power densities of about 250,000 W/cm
2
, generating about 10 to 20 milliwatts of power in the frequency doubled output beam.
The high power densities of pump laser beams in these systems can locally damage the nonlinear crystal. This, in turn, can lead to degradation of the power levels of the output beam. For example, in some cases the damage to the nonlinear crystal results in increased absorption of the pump beam by the nonlinear crystal.
A technique commonly used to overcome undesirable degradation of the power levels of the output beam is to vary the area of the nonlinear crystal on which the pump beam is focused. This can be achieved, for example, by translating the nonlinear crystal. In addition, increasing the power of the fundamental pump wavelength can compensate for absorption losses. However, the amount of additional power available may be limited and will depend on the laser source.
In applications where high pump beam power is required to maintain sufficient harmonic output power, the pump lasers used are typically large, complex, expensive systems, demanding expensive utilities (e.g., 3-phase power, flowing cooling water and high purity nitrogen). Such pump lasers are limiting in applications having space, utility, and/or budget constraints.
SUMMARY
The invention features a nonlinear optical crystal assembly and a gas mixture that surrounds the nonlinear crystal. The gas mixture reduces photochemical degradation of the nonlinear crystal caused by exposure of the nonlinear crystal to a high power light source. The assembly may be incorporated into a light source, and applications requiring a light source, such as, e.g., applications requiring ultraviolet light. In some embodiments, the nonlinear crystal assembly may be placed inside an optical cavity. Generally, the nonlinear crystal converts optical radiation from a pump source having a first frequency, to optical radiation having a second frequency, different from the first frequency.
In general, in one aspect, the invention features an optical system including: a light source providing a pump beam having a first frequency; a nonlinear optical crystal positioned to transform at least a portion of the pump beam into an output beam having a second frequency different from the first frequency; and an enclosure filled with gas and surrounding the nonlinear optical crystal, the gas including hydrogen and oxygen in amounts sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam. In some embodiments, the gas is sealed within the enclosure. In other embodiments, for example, the system further includes a gas source coupled to the enclosure for flowing the gas into the enclosure.
In general, in another aspect, the invention an optical system including: a light source providing a pump beam having a first frequency; a nonlinear optical crystal positioned to transform at least a portion of the pump beam into an output beam having a second frequency different from the first frequency; an enclosure surrounding the nonlinear optical crystal; and a gas source of hydrogen and oxygen coupled to the enclosure, wherein during operation the gas source provides the enclosure with amounts of hydrogen and oxygen sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam.
Embodiments of either optical system may include any of the following features.
The enclosure may surround the nonlinear optical crystal and the light source.
The system may further include a plurality of mirrors defining an optical cavity surrounding the nonlinear optical crystal. For example, the optical cavity may be resonant at the first frequency. Furthermore, the light source may located within the optical cavity. Moreover, the light source may include a gain medium and the optical cavity may resonantly enhance emission from the gain medium to generate the pump beam. For example, the light source may include a gas tube (e.g., an Argon ion gas tube) and electrical source coupled to the gas tube, and wherein during operation the electrical source produces an ion discharge in the gas tube. The gas tube may be air-cooled. Alternatively, the light source (e.g., a single frequency laser) may be located outside of the optical cavity, and wherein during operation the light source couples the pump beam at the first frequency into the optical cavity. In either case, the enclosure may also surround the optical cavity.
The nonlinear optical crystal may include Boron and Oxygen, for example, it may be one of Barium Beta Borate, Lithium Triborate, and Cesium Lithium Triborate.
The second frequency may be a harmonic of the first frequency. For example, the second frequency may be in the UV portion of the electromagnetic spectrum.
The gas including hydrogen and oxygen may further include a buffer gas, such as, for example, Argon or Nitrogen. The ratio of hydrogen to oxygen in the enclosure gas may about one to one. Furthermore, the gas including hydrogen and oxygen may have a hydrogen concentration of less than or equal to about 10%. Also, the gas including hydrogen and oxygen may have an oxygen concentration of less than or equal to about 10%. Furthermore, both the hydrogen and oxygen may have a concentration of less than or equal to about 10%. Similarly, the respective concentrations may be less than or equal to about 3%, and may be as low as about 0.1%. The hydrogen may include, e.g., hydrogen molecules or hydrogen ions. The oxygen may include, e.g., oxygen molecules, oxygen ions, or ozone. Furthermore, for example, the gas may include about 95% Argon, about 2.5% oxygen, and about 2.5% hydrogen. The concentration refers to the partial pressure concentration of the respective gases.
Furthermore, the gas in the enclosure may have a pressure greater than ambient pressure (i.e., greater than about 1 atmosphere), for example, the gas pressure may be greater than the ambient pressure by an amount up to 10 Psi.
The optical system may further include a heating element thermally contacted to the nonlinear optical crystal and a temperature controller coupled to the heating element.
For example, during operation the temperature controller may cause the temperature of the nonlinear optical crystal to be at least 50° C., or to be at least 70° C.
The light source may be an Argon ion laser, a Krypton ion laser, a YAG laser, or an Alexandrite laser, or it may include the corresponding gain medium when the system includes an optical cavity and the light source is positioned within the cavity. The light source may be a continuous wave laser. The light source may be an air-cooled laser.
In another aspect, the invention features an optical microscopy sy
Fish & Richardson P.C.
Hellner Mark
Zygo Corporation
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