Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-03-17
2001-05-15
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S020000, C372S098000, C372S092000, C372S032000
Reexamination Certificate
active
06233260
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a solid laser source in the field of optoelectronics and a laser application device using the solid laser source, particularly to reduction of noise in an output of a second-harmonic generator.
(ii) Description of the Related Art
With the progress of the highly information-oriented age, in the field of optical recording such as optical disk devices and laser printers, for improving the recording density or for meeting the requirements of high-speed printing, needs of laser source of shorter wavelength rise. But, in a blue range (wavelength of 400 to 480 nm) wherein there are many needs at practical product level, gas laser sources such as He—Cd (helium—cadmium) lasers and Ar (argon) lasers are only put to practical use. When a gas laser source is put in an optical disk device or the like, though the recording density can be considerably improved because of a short wavelength, since the size of the laser source is larger than that of the device for being equipped with it and the power consumption becomes great, putting it to practical use is hindered. But, though there is an example in which a gas laser source is put in a certain kind of laser printer, it is a limited kind for a special application. In view of such conditions, it is of urgent necessity to downsize the short-wavelength laser source and lower the power consumption of it.
Well, since the laser oscillation in the blue range is very difficult in a solid laser as described above, an optical second-harmonic generation (hereinafter called SHG for short) method using a nonlinear optical crystal as a method for obtaining a short wavelength not directly but indirectly is proposed and developed for practical use. In an SHG system, at least a solid laser crystal and a nonlinear optical crystal are disposed in an oscillator composed of a pair of mirrors, a base wave of a long wavelength is first generated by exciting the solid laser crystal from the exterior of the oscillator, and a second harmonic of the base wave is next generated by the nonlinear optical crystal. Besides, because the performance of SHG is closely connected with the characteristics of a semiconductor laser as an exciting light source, it is in a relation that the study for improvement of SHG is advanced always after increasing the power of the semiconductor laser or improving the high stability or the like of it. But, with improvement of the performance of the semiconductor laser, the SHG system has focussed the spotlight of attention. Putting the advantages of the above-described SHG in order, (1) it can be constructed in a small size; (2) a low power consumption can be realized; (3) a high stability of the SHG output by solidity can be intended; and (4) a long duration becomes possible.
As a solid laser source by which a light in the blue range is obtained, there is an internal oscillator type SHG system as shown in
FIG. 13
for example.
In
FIG. 13
, an exciting light
31
from a semiconductor laser (not shown) is introduced into a solid laser crystal
4
to excite the solid laser crystal
4
. The excited solid laser crystal emits a specific light according to its composition. This is a base wave light. Accordingly, the wavelength or range of the base wave is determined by the material of the solid laser crystal. But, although there is arbitrariness in material, since the kinds of solid laser crystals are not so much, the range of wavelength to select is necessarily limited. For example, when YAG (Nd:Y
3
Al
5
O
12
) is used as the solid laser crystal, a base wave of approximately circular polarization is obtained in a fairly narrow range of wavelength with the center of 1064 nm, while, in an LiSAF crystal described later, it is a base wave of nearly linear polarization with a considerably wide range of wavelength of 750 to 1000 nm. Further, the excited base wave light is introduced into a nonlinear optical crystal (SHG crystal)
6
. In the nonlinear optical crystal, a wavelength that meets the phase-matching conditions determined by the relation between the refractive index and the length of optical path in each crystal axis, namely, a second harmonic (SHG output)
33
is emitted. The first and second laser mirrors
3
and
7
constituting an oscillator are given the following characteristics of wavelength selection. That is, the first laser mirror
3
allows the exciting light
31
to pass but reflects a base wave beam
32
and the SHG output
33
. On the other hand, the second laser mirror
7
reflects the base wave beam
32
but is made to have a good transmission characteristic for the SHG output
33
. In short, the oscillator has the construction by which, while the base wave is shut up, the generated second harmonic is taken out to the exterior of the oscillator. By this construction, mixing of an outer disturbance such as a reflected return light from the exterior of the oscillator can be restrained without providing an optical isolator on the emission side, as a result, the influence of the outer disturbance on the oscillation wavelength of the base wave can be made small and there is another merit related to a stable oscillation.
Well, as disclosed in U.S. Pat. No. 4,811,349, a laser device in which an LiSAF (Cr:LiSrAlF
6
; fluorolithium-strontium-aluminum with addition of chromium) crystal that oscillates in a wide range of wavelength of 750 to 1000 nm is used as a solid laser crystal is proposed. By using this LiSAF, by which the band of oscillation wavelength becomes remarkably broad in comparison with a conventional crystal, as the above-described internal oscillator type solid laser crystal, it becomes possible selectively to obtain a short wavelength of 375 to 500 nm and the possibility of a variable-wavelength laser source becomes open.
The present inventors have been at grips with development of an SHG light source of blue range as the second oscillation wave obtained by a nonlinear optical crystal wherein an LiSAF crystal is applied to such a semiconductor laser-exciting system as shown in FIG.
13
and an excited laser light is used as the first oscillation wave (base wave), for many years. After this, although the application scope of the SHG light does nothing but extend, in particular, needs to improve the performance of the SHG light source applied to a precisely measuring instrument become intensive more and more, and the present invention is to open the way for a solution to the stability or reduced noise of the output light. The noise in the SHG system is a low-frequency side component of 3 MHz or less in the output light. It was found that increase of noise of low frequency in the SHG laser light has a serious influence from sides of stability or accuracy of the device. Hereinafter, the mechanism of generating noise in a prior art will be described in detail.
FIG. 14
shows a construction of an SHG system in which an LiSAF crystal is applied as a solid laser crystal. Although it is basically the same construction as that in
FIG. 13
, a wavelength selection element
5
is provided in front of the SHG crystal
6
in the oscillator, and an SHG output
33
of a proper wavelength can be selected by this wavelength selection element
5
. An exciting light
31
emitted from the semiconductor laser
11
passes through a convergent optical system
12
and the first laser mirror
3
and then is gathered in the solid laser crystal
4
of an LiSAF crystal to excite the solid laser crystal
4
. Further, base wave beams
32
A of required wavelengths among base wave beams
32
emitted from the solid laser crystal pass through the wavelength selection element
5
to be incident on the SHG crystal
6
. A part of the base wave beams
32
A is converted into an SHG light by the SHG crystal and the major part reaches the second laser mirror
7
and is reflected. Because mirror films of dielectric multilayer films for reflecting 99% or more of the base wave beams
32
A are formed on the first and second laser mirrors
3
and
7
, the base wave beams
32
A rep
Ishida Hidenobu
Makio Satoshi
Matsumoto Hironari
Sato Masayoshi
Hitachi Metals Ltd.
Jr. Leon Scott
Staas & Halsey , LLP
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