Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-02-04
2001-11-20
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S010000, C372S027000, C372S106000
Reexamination Certificate
active
06320894
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to lasers, and more particularly, to a neodymium laser tuned to emit pulsed light having a wavelength of about 0.9 &mgr;m using an optical filter that includes a polarizer and wave plate in the optical resonant cavity of the laser which selectively provides loss for wavelengths in the 1 &mgr;m band.
It is relatively easy to optically pump the
4
F
3/2
level of trivalent neodymium doped into various host materials using either flash lamps or more recently, laser diodes. A population inversion between the
4
F
3/2
level and the lower
4
I
9/2,
4
I
11/2,
and
4
I
13/2
levels can be readily obtained and laser emission between these levels has been demonstrated. The specific emission wavelengths depend on the host material and are typically near 0.9 &mgr;m, 1 &mgr;m, and 1.3 &mgr;m, respectively. The peak effective cross section is also host dependent and is typically about 10×times larger in the 1 &mgr;m band. than in the 0.9 &mgr;m band. Since these transitions all originate from the same upper level, the relative gain in a particular host at each of these wavelengths is fixed.
A neodymium laser is typically operated in the 0.9 &mgr;m band by using wavelength selective elements inside the laser cavity to suppress the stronger 1 &mgr;m band emissions. Reflective mirror coatings are designed to provide the required feedback at the desired wavelength and to be highly transmissive over the 1 &mgr;m band. This approach is generally adequate when the gain is not large such as the case with continuous wave or long pulse lasers. However, as the inversion increases, more loss is required over the 1 &mgr;m band in order to prevent parasitic lasing at that wavelength. This becomes more important with Q-switched operation. The degree of wavelength selective discrimination in the laser cavity effectively sets an upper limit on the energy which can be stored in the laser material.
At present, there do not exist adequate methods to provide the necessary wavelength discrimination for high power Q-switch operation of neodymium lasers in the 0.9 &mgr;m band. A number of standard approaches are available for general wavelength selection in lasers. Reflective coatings designed to give the best discrimination possible against unwanted wavelengths are generally used but it is difficult to design coatings to discriminate between closely spaced wavelengths. Other common techniques are also deficient for use in generating a high energy, Q-switched output light at 0.9 &mgr;m from a neodymium laser. For example, it is difficult to obtain sufficient wavelength dispersion using refractive elements such as prisms inside the laser cavity. Also, diffraction gratings generally cannot handle high power and tend to introduce significant loss for all wavelengths. Absorption filters which exhibit high transmission at 0.9 &mgr;m and sufficient absorption over the entire 1 &mgr;m band are not available. Generic birefringent tuning elements are routinely used for wavelength tuning of low-gain lasers. These devices consist of multiple crystal quartz plates having integral multiples of a common thickness. They allow convenient wavelength tuning by plate rotation but are not able to provide the necessary discrimination over the entire 1 &mgr;m band. Therefore, a need exists for an intracavity filter which efficiently suppresses 1 &mgr;m band laser action and allows efficient operation of Q-switched laser operation in the 0.9 &mgr;m band.
SUMMARY OF THE INVENTION
The present invention provides a Q-switched neodymium laser that emits optical energy in the 0.9 &mgr;m band at relatively high gain levels. The invention uses a birefringent filter which provides selective discrimination for wavelengths near 1 &mgr;m and prevents parasitic lasing in that wavelength region. The filter includes a specific wavelength dependent crystal quartz wave plate in combination with a linear polarizer. The linear polarizer is placed between two mirrors which define a laser cavity. The wave plate is designed to provide exactly 2 full waves of retardation at the desired wavelength &lgr;
1
in the 0.9 &mgr;m band, and exactly 1.75 waves of retardation at wavelength &lgr;
2
near the center of the 1 &mgr;m band for one pass. A Q-switching device, such as a Pockels cell, or acousto-optic cell is positioned in the laser cavity and selectively controls a pulsed output signal at &lgr;
1
.
In operation, &lgr;
1
light linearly polarized along an axis p
1
selected by the polarizer is unaffected by a double pass through the wave plate and reflection from the polarizer. When in an “OFF” state, the Q-switch inhibits laser action until a large inversion and high gain are built up in the laser gain medium. Laser action at &lgr;
1
begins when the Q-switch is switched to the “ON” state and a pulsed optical output signal is provided through one of the mirrors. Laser action at any of the relatively high gain spectral lines near 1 &mgr;m is prevented in part because of the loss added by the polarizer and wave plate filter. The specific additional loss due to the filter depends on the wavelength dependent loss of the polarizer for the polarization state orthogonal to axis p
1
and the wavelength dependent retardation characteristics of the wave plate. Since the wave plate is designed to be low-order and have only ¼ wave less retardation at &lgr;
2
than at &lgr;
1
, near the middle of the group of 1 &mgr;m high gain lines, the filter has a spectrally broad loss centered at &lgr;
2
which extends over the entire group of 1 &mgr;m spectral lines.
The invention includes a unique birefringent filter that includes a linear polarizer and crystal quartz compound wave plate. The filter enhances the operation of a Q-switched neodymium laser at 0.9 &mgr;m by suppressing all of the widely spaced high gain lines near 1 &mgr;m. The compound wave plate has the unique property of retarding &lgr;
1
light by two waves and retarding spectral components near &lgr;
2
by about 1.75 waves. The filter incorporated into the invention allows increased energy storage in the laser cavity before the onset of parasitic lasing in the 1 &mgr;m band and introduces minimal losses at the operating wavelength in the 0.9 &mgr;m band.
These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims.
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Hanson Frank E.
Poirier Peter M.
Arroyo Teresa M.
Fendelman Harvey
Kagan Michael A.
Lipovsky Peter A.
Monbleau Davienne
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