Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1996-01-17
2001-10-09
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S021000, C372S026000, C372S027000, C372S022000
Reexamination Certificate
active
06301276
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a laser light generating apparatus for radiating a continuous light of a short wavelength based upon the outgoing light of a laser light source.
An optical recording medium is employed for recording/reproduction of data such as video or audio signals. Although a large quantity of data may be recorded on an optical recording medium, it is desirable for an optical recording medium to have a recording capacity much larger than the current recording capacity.
For realizing an optical recording medium of a larger recording capacity, it is required to raise the information recording density of the information recording medium. One of the techniques of increasing the recording density is to shorten the wavelength of the laser light employed for high density recording.
Recently, a demand has been raised for a laser light of the ultraviolet wavelength as a short wavelength laser light. In particular, the laser light of the ultraviolet wavelength in the vicinity of the wavelength of 355 nm is desired most strongly. For generating the laser light of this wavelength range, there is known a method of generating third harmonics of the infrared wavelength used as the fundamental wavelength.
If, for generating the third harmonics (THG) by a laser light radiating device, Nd:YAG or Nd:YVO
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, a solid laser, is employed as a laser medium, the laser light of the fundamental wave generated by exciting the laser medium by a light pumping method and the second harmonics produced on wavelength conversion of the fundamental laser light are frequency-mixed for generating the third harmonics of the fundamental laser light, that is the laser light in the vicinity of the wavelength of 355 nm within the UV range.
The laser light in the vicinity of the wavelength of 355 nm is a laser light generated with the pulse mode. The reason the THG incurs the mode of pulse oscillation is that, when the outgoing laser light of the fundamental wavelength is frequency-mixed with the second harmonics, the laser light of the third harmonics is generated by a single pass in which the laser lights are passed only once through a non-linear crystal. The result is that the non-linear conversion of third harmonics is limited to only one pass resulting in pulse oscillation of the laser light of third harmonics.
Since the laser light radiated in the vicinity of the wavelength of 355 nm is a pulsed light, such laser light is difficult to apply to optical recording or optical display where continuous light is necessitated.
As a method for raising the non-linear conversion efficiency by the continuous light by the laser light generating apparatus, there has been proposed a method of arranging a non-linear crystal within the light path of the light resonator.
This method achieves short harmonics generation (SHG) by using a resonator having high finesse and high transmittance for the light of the wavelength equal to one-half the fundamental wavelength and having suppressed losses in level at the fundamental wavelength.
However, for generating the third harmonics as the continuous light, the fundamental laser light wave with a frequency of 1 &ohgr; and the second harmonics laser light by SHG with a frequency of 2 &ohgr; need to be resonated at the time of additive frequency mixing of the two laser light beams. In addition, a mirror constituting the resonator needs to be of low reflection with respect to the third harmonics. Furthermore, wavelength dispersion of the resonator needs to be synchronized for resonating the fundamental wavelength light generated by the laser light source with the second harmonics within the same resonator.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provided a laser light generating apparatus in which the laser light of the third harmonics of the fundamental wavelength may be radiated as a continuous laser light.
The laser light generating apparatus according to the present invention includes a first laser light source employing an Nd:YAG laser as a laser medium, as a laser light source radiating the continuous laser light of an infrared wavelength, a second laser light source for resonating a laser light from a laser medium of Nd:YVO
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within a resonator for generating second harmonics, as a laser light source radiating a continuous laser light of a green wavelength, and an external resonator consisting of a set of mirrors. The outgoing lights are combined by additive frequency mixing by a phase-matched non-linear crystal element BBO arranged within the external resonator as the outgoing lights are synchronized and resonated simultaneously within the external resonator.
The first laser light source has a wavelength of 1.05 to 1.07 &mgr;m.
As the laser medium producing the laser light with a wavelength of 1064 nm, LNP, YLF, GGG or YAP may be employed in place of Nd-YAG or Nd:YVO
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.
The second laser light source radiates second harmonics with a wavelength of &lgr;/2, where &lgr; is the wavelength of the first laser light source.
Specifically, the wavelengths of the outgoing lights of the first and second laser light sources are 1.064 &mgr;m and 0.532 &mgr;m, respectively.
The external resonator includes a mirror coated with a dielectric multi-phase layer and an input coupling mirror having reflectance appropriate for impedance matching. The dielectric multi-phase layer is formed by ion sputtering or ion plating. At least one of the mirrors of the external resonator is an output mirror having wavelength characteristics of transmitting the continuous light produced on additive frequency mixing. The input coupling mirror is used simultaneously as an output mirror.
The first laser light source and the second laser light source are provided with the wavelength controlling function of controlling the wavelength of the outgoing light.
Preferably, the external resonator is synchronized with the outgoing light from one of the first laser light source and the second laser light source by controlling its optical path length, and is also synchronized with the outgoing light from the other light source by controlling the oscillation wavelength of the outgoing light from the other light source.
As a light source having a phase modulator within at least one of the first laser light source and the second laser light source, it is possible to use the laser light from an injection synchronization laser light source or an external laser light source configured for producing second harmonics and to use the frequency sideband of one of the light sources for synchronization with the external resonator.
The non-linear optical crystal element arranged within the external resonator has a surface coating designed for lowering the reflectance at the wavelength of the outgoing lights from the first and second laser light sources.
As the non-linear optical crystal element, &bgr;-barium borate (BBO) crystal, for example, may be employed. The crystal may be cut at an angle &thgr;=31.3°, or the angle the main crystal axis makes with the optical axis may be set to 38.6° or 59.7° for meeting the type I or type II phase matching conditions for generating an outgoing light by additive frequency mixing.
As the non-linear optical crystal element, periodically poled lithium niobate (PPLN) may be employed. With PPLN, the poling period is set to an odd-number multiple of 1.9 &mgr;m or 3.6 &mgr;m and pseudo-phase matching conditions are met using the lights of the wavelength of 1.06 &mgr;m and 0.53 &mgr;m for generating an outgoing light by additive frequency mixing.
The external resonator is preferably configured so that the outgoing lights radiated from the two light sources, at least one of which is the S-polarized light, are resonated by at least three mirrors provided in the same plane.
Preferably, an even number of mirrors is provided for providing an arrangement invulnerable against disturbances.
The laser light generating apparatus includes a beam splitter for spatially splitting the reflected light and the lights of the two incident
Jr. Leon Scott
Kananen Ronald P.
Rader Fishman & Grauer
Sony Corporation
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