1. Field of the Invention
The present invention relates generally to laser apparatus and, more particularly to a laser light generating apparatus suitable for generating a second harmonic laser light by a nonlinear optical crystal element.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows an arrangement of an example of a laser light generating apparatus according to the prior art.
As shown in FIG. 1, a laser diode 1 is driven by a laser diode driver 8 to generate a pumping laser light and the pumping laser light becomes incident on a convex lens 2. The pumping laser light is converged by the convex lens 2 and travels through a concave surface mirror 3 and a quarter wave plate 4 to a laser medium 5 such as an Nd:YAG laser or the like. The concave surface mirror 3 passes the pumping laser light from the laser diode 1 and reflects a fundamental wave laser light generated by the laser medium 5 which will be described later. When irradiated with the pumping laser light, the laser medium 5 generates a fundamental wave laser light within a resonator composed of mirrors 3 and 7. At that time, a heat lens 5a is formed in the laser medium 5. The fundamental wave laser light is introduced through a nonlinear optical crystal element 6 such as KTP (KTiOPO.sub.4) or the like into the plane mirror 7. The reflection mirror 7 is designed to reflect the fundamental wave laser light generated by the laser medium 5 and to pass a second harmonic laser light which will be described later. The fundamental wave laser light reflected on the reflection mirror 7 becomes incident on the laser medium 5 one more time through the nonlinear optical crystal element 6.
The fundamental wave laser light emitted from the laser medium 5 in the lefthand direction of FIG. 1 is introduced through the quarter wave plate 4 to the concave surface mirror 3 and thereby reflected. The fundamental wave laser light thus reflected is introduced again into the laser medium 5 through the quarter wave plate 4. In this way, the fundamental wave laser light travels between the concave surface mirror 3 and the plane mirror 7. That is, the concave surface mirror 3, the quarter wave plate 4, the laser medium 5, the nonlinear optical crystal element 6 and the plane mirror 7 constitute a laser light resonator 9.
The positions in which the fundamental wave laser light travels are concentrated due to the action of the concave surface mirror 3 and the heat lens 5a, thereby the energy being increased. Thus, the KTP such as the nonlinear optical crystal element 6 generates a second harmonic laser light of frequency as high as twice the frequency of the fundamental wave laser light on the basis of type II phase matching. The plane mirror 7 reflects almost all of the fundamental wave laser light but allows almost all of the second harmonic laser light with the result that the second harmonic laser light is output from the resonator 9. An optical axis of the quarter wave plate 4 in the extraordinary ray direction is adjusted so as to become 45 degrees relative to the optical axis of the nonlinear optical crystal element 6 in the extraordinary ray direction. If an azimuth angle .theta. of the quarter wave plate 4 is set to 45 degrees, then it is possible to avoid a coupling phenomenon from taking place between two polarizing modes of the fundamental wave laser light. If a phase delay amount by the KTP provided as the nonlinear optical crystal element 6 falls within .+-..pi./2, then a space hole burning within the laser medium 5 can be alleviated and a longitudinal mode having a single polarizing light can be presented. As a consequence, the second harmonic laser light can be stabilized (see U.S. Pat. No. 4910740, for example).
It is proposed that this laser light generating apparatus is miniaturized more as, for example, shown in FIG. 2. According to the example shown in FIG. 2, the quarter wave plate 4, the laser medium 5 and the nonlinear optical crystal element 6 are unitarily formed. The concave surface mirror 3 is convexed at its lefhand surface in the quarter wave plate 4 (i.e., concaved to the laser medium 5 side) and deposited on the surface of the quarter wave plate 4. Similarly, the plane mirror 7 is deposited on the righthand end face of the nonlinear optical crystal element 6.
The laser light resonator 9 thus arranged, the lens 2 and the laser diode 1 are mounted on a base 12 and housed within a housing 11. A second haromic laser light is emitted from an opening portion 11a formed through the housing 11.
When the laser medium 5 is excited by the pumping laser light emitted from the laser diode 1, a reflected-back light such as a laser light, which results from reflecting the pumping laser light on the surfaces of the lens 2, the surface of the concave surface mirror 3 or the like, the fundamental wave laser light leaked from the concave surface mirror 3 or the like occurs in the laser diode 1. When supplied with the reflected-back light, the laser diode 1 is increased in noise level. If a noise occurs in the laser diode 1, when the noise produced in the laser diode 1 appears in the second harmonic laser light.
FIG. 4 of the accompanying drawings shows frequency characteristics of noise when the laser medium 5 of the laser diode 1 is not excited (no reflected-back light is produced). In FIG. 4, an upper line represents a noise of the laser diode 1 and a lower line represents a noise generated from a measuring equipment. This is also true in FIGS. 5 and 6. Whereas, FIG. 5 shows frequency characteristics of noise when the laser light resonator 9 is excited by the laser diode 1 to emit the second harmonic laser light (i.e., when the reflected-back light is produced). Having compared FIGS. 4 and 5, it is to be understood that a noise is increased by 10 dB to 15dB in the case of FIG. 5 It is considered that this increase of noise is caused by the reflected-back light.
FIG. 6 of the accompanying drawings shows a frequency characteristic of noise in the second harmonic laser light. Study of FIG. 6 teaches that a noise of a relatively large level appears in the frequency band less than about 1 MHz. This noise may be considered such that the noise produced in the laser diode as shown in FIG. 5 occurs in response to the frequency characteristic (see FIG. 3) of the laser light resonator 9.
As shown in FIG. 3, a ratio (in ordinate) of an output (P2) of the second harmonic laser light relative to an output (P1) of the pumping laser light output from the laser diode 1 is substantially constant in a range of a relatively low frequency in which the frequency (in abscissa) of the second harmonic laser light reaches a transition oscillation frequency fr. However, the above-mentioned ratio is attenuated in proportion to the square off the frequency in a range of frequency higher than the transition oscillation frequency fr. In other words, since the second harmonic laser light never responds to the fluctuation of the pumping laser light in the range of frequency higher than the transition oscillation frequency fr, there is then substantially no risk that the noise of the pumping laser light appears in the second harmonic laser light.
However, the noise of the laser diode 1 appears in the second harmonic laser light in the vicinity of the transition oscillation frequency fr or in the frequency lower than the transition oscillation frequency fr. As a consequence, if this conventional laser light generating apparatus is used to play back an optical disc, a signal-to-noise (S/N) ratio is deteriorated in the vicinity of the transition oscillation frequency fr or in the frequency lower than the transition oscillation frequency fr.