JPH04179933A - Second harmonics generator - Google Patents

Abstract

PURPOSE:To modulate a 2nd harmonic output by adjusting the phase matching by an electric field which is impressed on an electrode installed on a non-linear optical material. CONSTITUTION:Non-linear optical crystal KNbO34, etc., provided with the elec trode 9 for impressing the electric field is arranged on the optical axis of omegalight in an external resonator of a triangle ring. The KNbO34 is held at a temperature capable of performing the phase matching by a Peltier element in advance, and the light emitting intensity of 2omega light is modulated by the electric field which is impressed on the electrode 9. At this time, a resonating mirror for making omega light incident is inclined with reference to the optical axis of the omega light so that the reflected omega light may not return to a light source LD. Besides, the stabilization of the frequency of the LD is performed by returning scattered weak light generated in the external resonator to the LD. Thus, the light emitting intensity of the 2nd harmonics can be accurately con trolled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a second harmonic generation device using a nonlinear optical material having an external resonator.

[Prior Art] FIG. 3 shows a conventional second harmonic generation device using an external oscillator.

1 is a semiconductor laser (LD) for generating a fundamental wave, 8 is a collimating lens, and 3 is a fundamental wave (hereinafter referred to as ω light).
8' is a mode matching lens that matches the resonance mode in the external cavity with the incident beam, 4 is a nonlinear optical material such as KNb02 crystal,
5 is a dichroic mirror that transmits the second harmonic (hereinafter referred to as 2ω light) and reflects ω light, and 2 is a PZT element provided with a mirror for locking the oscillation frequency by feedback of ω light to the LDl. 6 is a photodiode (PD) that detects the ω light reflected by the dichroic mirror, and 7 is a controller that drives the PZT element 2 based on the detection signal of the PD 6 and adjusts the angle of the mirror provided therein.

[Problem to be solved by the invention] Conventionally, in the second harmonic generator as described above, ω
Matching the phases of the light and the 2ω light within the nonlinear optical material, that is, matching the refractive indices nω and n2ω of the ω light and the 2ω light within the nonlinear optical material, increases the emission intensity of the second harmonic. It is important to increase the

Conventionally, in order to achieve such phase matching, a nonlinear optical material is maintained at a constant temperature using a Peltier element or the like to achieve phase matching. The accuracy of temperature control must be ±0.1℃ or less, and since the generation of 2ω light will stop due to small temperature fluctuations, the accuracy is usually always maintained (adjusted to the temperature at which phase matching can be achieved. luminescence intensity.

There is also a method of modulating and adjusting the output of a semiconductor laser, etc., which is a light source of ω light, but with this method, the stabilization (locking) of the oscillation frequency of the semiconductor laser due to ω light feedback from an external resonator easily becomes unstable. , and was unable to generate 2ω light.

Therefore, it has conventionally been difficult to accurately control the emission intensity of 2ω light.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a light source for generating a fundamental wave and an external resonator to convert the fundamental wave into a second harmonic. a second nonlinear optical material having
In the harmonic generation device, the nonlinear optical material is provided with an electrode that adjusts the phase matching between the fundamental wave and the second harmonic and modulates the emission intensity of the second harmonic by applying an electric field. The present invention provides a second harmonic generation device having the following characteristics.

An embodiment of the present invention will be explained based on FIG. 1. Further, the overall configuration of the device is the same as that in FIG. 3. KNbo, a nonlinear optical crystal provided with an electrode 9 for applying an electric field, 4
etc. are arranged on the optical axis of the ω light inside the external resonator of the triangular ring! be done. KNbO, 4 is maintained in advance at a temperature that allows phase matching by a Peltier element, and the emitted light intensity of 2ω light is modulated by the electric field applied to the electrode 9. At this time, ω
The resonant mirror 3 for light incidence is tilted with respect to the optical axis of the ω light to prevent reflected light of the ω light from returning to the LDI, which is the light source. Further, the frequency of LDl is stabilized by returning weak scattered light generated inside the external resonator to LDI.

The external resonator can be triangular, quadrilateral or more polygonal, etc., but if there are too many mirrors, there will be a lot of loss, making it difficult to adjust the optical axis, and the number of parts will be large. Therefore, a triangular shape is preferable.As a nonlinear optical material, is KNbOs, β-BaB*04
．． KTiopo4. Nonlinear optical crystals such as KHz PO4 crystals and organic nonlinear optical materials can also be used. Although various solid state and gas lasers can be used as the light source, an LD is preferable in terms of compactness and weight reduction.

When using KNbOs crystal as a nonlinear optical material,
Due to the anisotropy, the polarization direction of ω light matches the b-axis direction of the crystal, and the polarization direction of 2ω light matches the c-axis direction. It is preferable to apply an electric field in the C-axis direction because the responsiveness of the refractive index change to the electric field is the best and the 2ω light can be efficiently modulated.

Alternatively, instead of holding the nonlinear optical material in advance at a temperature at which phase matching can be achieved, it is also possible to set the nonlinear optical material so that phase matching can be achieved using an electric field, and then modulate the nonlinear optical material by changing the electric field around that point.

[Function] As shown in FIG. 2, the polarization directions of ω light and 2ω light passing through a nonlinear optical crystal differ by 90°, and the responsivity of the refractive index to an electric field differs in the direction of the crystal axis. When the electric field strength is modulated along the C-axis direction, where the response to the electric field is most pronounced, the phase matching condition becomes thicker (changes, so
The emission intensity of 2ω light can be modulated with high precision. Furthermore, since the response of ω light to the electric field is gentle and coincides with the axis, it is hardly affected by the electric field strength.

Therefore, since the ω light is not affected by the electric field, the resonance state is not broken, and the locking of the LD's oscillation frequency by the ω light returning from the external resonator to the LD is not broken, making it possible to modulate the 2ω light. becomes.

[Example] An example of the present invention will be shown below based on FIGS. 1 and 3.

Semiconductor laser (LD) 1 wavelength approximately 860 nm output 10
0 mW ω light is made into parallel light by a collimator lens 8 and reflected by a PZT element 2 provided with a mirror, and a mode matching lens 8' is used to match the resonance mode in the external resonator and the incident beam. The light is then incident on a triangular ring-shaped external resonator made up of three mirrors 3. At this time, the light directly reflected by the incident side mirror 3 is shifted from the incident optical axis, and therefore does not return to the LDI. Inside the external resonator, resonance occurs in the clockwise direction in the figure, and the power of the ω light is multiplied. The ω light is converted into second harmonic 2ω light (λ = 430 nm) by a nonlinear optical crystal KNb0.4 (length 5 x width 1 x height 3 mm) placed at the beam waist position on the resonant optical axis.
) is converted to At this time, the KNbO3 crystal 4 is heated at 27°C by a Bertier element so that the ω light and the 2ω light are phase matched.
The ω light is made incident in a polarization direction parallel to the b-axis of the KNbO3 crystal 4, and the 2ω light is emitted in a polarization direction parallel to the c-axis direction perpendicular thereto.

At this time, when an AC voltage of 100 V peak to peak was applied at I KHz in the C-axis direction of the KNbO crystal 4 using a synthesizer and an amplifier, the output of 2ω light with a wavelength of about 430 nm and an output of 3 mW was modulated by 5%. Modulated by

As a method for frequency stabilization of LDI, an optical feedback method is used here. That is, in a triangular ring type external resonator, frequency stability is achieved by injecting a weak backward traveling wave (counterclockwise in the figure) of about 30 to 50 dB due to scattering inside the resonator into the LDI. The external resonator functions as a narrowband filter, and the oscillation frequency of the LDI is automatically pulled in and stabilized (locked) to the resonant frequency of the resonator.

At this time, in order to stabilize the system of optical feedback and external resonator, automatic phase adjustment of the reflected light is performed using the PZT element 2.

That is, the ω light reflected by the guichroic mirror 5 (ω light reflection, 2ω light transmission) is transmitted to the photodiode (PD
) 6 and controller 7 to maximize the
The PZT element 2 is controlled by. This locking was maintained almost unchanged even when an electric field was applied to the KNbO□ crystal 4.

Further, the feedback method of the present invention has the advantage that the returned light due to scattering and the incident light are coaxial, so there is no need to adjust the optical axis.

Further, if such a second harmonic generation device is used as a data detection light source for a pickup of an optical recording medium such as an optical disk or a magneto-optical disk, it becomes possible to read high-density data.

[Effects of the Invention] The present invention has an excellent effect in that the second harmonic specific power can be modulated by adjusting the phase matching by an electric field applied to an electrode provided on a nonlinear optical material.

[Brief explanation of drawings]

FIG. 1 is a side view of a KNbO crystal according to an embodiment of the present invention, and
FIG. 2 is an explanatory diagram showing the propagation state of the fundamental wave and the second harmonic in a KNbO crystal, and FIG. 3 is a block diagram of a conventional example. 1...LD 3...Mirror 4...KNb
O, crystal 9...electrode 1st figure C figure 2 -

Claims (3)

[Claims] (1) A second light source having a light source for generating a fundamental wave and a nonlinear optical material having an external resonator and converting the fundamental wave into a second harmonic.
In the harmonic generation device, the nonlinear optical material is provided with an electrode for adjusting the phase matching between the fundamental wave and the second harmonic and modulating the emission intensity of the second harmonic by applying an electric field. A second harmonic generator characterized by: (2) The second harmonic generator according to claim 1, wherein the nonlinear optical material is a KNbO_3 crystal, and the electric field is applied in the c-axis direction of the KNbO_3 crystal. (3) A pickup for an optical recording medium using the second harmonic generator according to claim 1 or 2 as a light source for detecting recorded information.