JPH0566438A - Second higher harmonic generation device - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a second harmonic generation device, and an object thereof is to generate a second harmonic having a high light intensity and capable of being modulated up to a high frequency region. [Constitution] The YAG crystal 11 is constantly excited by the LD 14, and the oscillation mode of the YAG laser is set to several hundreds M by the modulated seed light 32 from the wavelength tunable LD 31.
Modulate in a frequency band as high as Hz. This makes YA
The G laser is a wavelength tunable laser whose wavelength tunable region is within the oscillation line width (0.7 nm). The allowable width of the KNbO ₃ crystal 13 and the oscillation line width described above make the former narrower than the latter.
The KNbO ₃ crystal 13 has high light intensity and several hundred MH.
It is configured to output the second harmonic wave 40 modulated in a frequency band as high as z.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a second harmonic generator.

The second harmonic of laser light is expected to enable high density recording in the field of optical recording, for example.

The second harmonic wave that can be used for optical recording or the like must have high light intensity and must be modulated at a high frequency of several hundred MHz, for example, in relation to the signal to be handled. ..

Therefore, the second harmonic generator can generate the second harmonic with high light intensity, and more than several hundred MH.
It is necessary to be able to modulate up to a frequency range as high as z.

[0005]

2. Description of the Related Art FIG. 10 shows an example of a conventional second harmonic wave generator 1, which is a laser diode (hereinafter referred to as LD) 2
And a crystal 3 of LiNbO ₃ are combined.

The laser light 4 from the LD 2 enters the crystal 3, and the crystal 3 outputs the second harmonic wave 5 of the laser light 4.

FIG. 11 shows another conventional second harmonic generation device 10.

11 is a YAG (Y ₃ Al ₅ O ₁₂ ) crystal, 1
Reference numeral 2 is an optical resonator, and 13 is a YAG laser.

Reference numeral 14 is a KNbO ₃ crystal.

Reference numeral 15 is an LD, which is for excitation and is driven by a frequency-modulated signal.

The laser beam 16 from the LD 15 is the optical resonator 1
2 inside and confined here, N of YAG crystal 11
Excite d atom.

When the Nd atom returns to the ground state, it emits light 17.

The emitted light 17 is intensified in the optical resonator 12.

The enhanced laser beam 18 is the YAG laser 1
3 (optical resonator 12), enters the crystal 14, and the crystal 14 outputs the second harmonic wave 19 of the laser light 18.

[0015]

In the device 1 shown in FIG. 10,
The output of the LD 2 is low, and therefore the light intensity of the second harmonic wave 5 is low.

In the apparatus 10 of FIG. 11, since the laser light 18 is obtained by exciting the YAG crystal 11, the second harmonic wave 18 having high light intensity can be obtained.

However, N when the LD 15 is turned off
The release of d is a spontaneous release, which takes a few 100 μs.

Therefore, the laser light 18 from the YAG laser 13 remains in FIG. 12A even after the LD 15 is turned off.
As indicated by the line 20, it will continue to come out despite weakening. Therefore, the LD 15 is excited by the line 2 in FIG.
When modulated as shown in FIG. 1, the light intensity of the laser light 18 from the YAG laser 13 is as shown by the line 22 in FIG. 7C, and the light intensity of the second harmonic wave 19 is shown in FIG. D) As indicated by the center line 23.

As described above, since the output of the laser light 18 and the output of the second harmonic wave 19 requires a time t ₁ of several 100 μs, the modulation frequency band is limited to several kHz.
Further modulation is impossible and it lacks usefulness.

Therefore, it is an object of the present invention to provide a second harmonic generation device which realizes high light intensity and high modulation frequency.

[0021]

According to a first aspect of the present invention, a solid-state laser medium having a solid-state laser medium having a predetermined oscillation line width is provided in an optical resonator, and the solid-state laser medium of the solid-state laser is excited. A first semiconductor laser that outputs laser light;
A second semiconductor laser that outputs seed light that has been frequency-modulated within the oscillation line width of the solid-state laser medium and makes it enter the solid-state laser medium, and has a wavelength dependence, and emits laser light from the solid-state laser. A non-linear optical crystal that is incident to generate its second harmonic, the solid-state laser medium maintains a state of being excited by the laser light from the first semiconductor laser, and the solid-state laser is The oscillation mode is modulated by the seed light from the second semiconductor laser, and the nonlinear optical crystal outputs a second harmonic whose light intensity is modulated by its wavelength dependence.

According to a second aspect of the present invention, the nonlinear optical crystal according to the first aspect is provided in the optical resonator.

According to a third aspect of the present invention, the solid-state laser of the first aspect is provided with a saturable absorber.

[0024]

According to the first semiconductor laser of the first aspect, the solid-state laser operates so as to always maintain the oscillation mode.

The second semiconductor laser acts so as to draw the oscillation mode of the solid-state laser by the seed light output from the second semiconductor laser.

Here, since the solid-state laser always maintains the oscillation mode, the stimulated emission lifetime is short, on the order of ns, and the oscillation mode of the solid-state laser is modulated in a high frequency band of several 100 MHz.

That is, the first and second semiconductor lasers act so that the solid-state laser is a wavelength tunable laser having a wavelength tunable range within its oscillation line width.

The structure utilizing the wavelength dependence of the nonlinear optical crystal acts so as to output the second harmonic wave whose light intensity is modulated at a frequency as high as several 100 MHz.

The configuration in which the non-linear optical crystal according to the second aspect is provided in the optical resonator acts to increase the light intensity of the laser light passing through the non-linear optical crystal.

The structure in which the supersaturated absorber is provided in the solid-state laser according to claim 3 functions so as to lock the solid-state laser in self mode.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a second harmonic generator 30 of the first embodiment of the present invention, which is used for outputting a laser beam for recording an information signal on an optical disk, for example.

In the figure, parts corresponding to those shown in FIG. 2 are designated by the same reference numerals.

Reference numeral 31 denotes a wavelength tunable semiconductor laser, which has a structure in which a diffraction grating is provided in a resonator and outputs a seed light 32.

The seed light 32 is applied to the YAG crystal 11, and the YAG crystal 11 performs the laser pull-in operation described later.

An information signal to be recorded on the magneto-optical disk is input to the wavelength tunable semiconductor laser 31 via the terminal 33. From the laser 31, as shown in FIG. 2B, the seed light 3 whose wavelength changes according to the above information signal.
2, that is, the frequency-modulated seed light 32 is output at several 100 MHz.

Further, the laser 31 is set so that the frequency modulation width a of the seed light 32 is within the oscillation line width b (see FIG. 4) of the laser light from the YAG crystal 11 which will be described later. As a result, the seed light 32 is frequency-modulated within the oscillation line width b as shown in FIG.

Next, the operation of the device 30 having the above configuration will be described.

On the YAG crystal 11, the laser light 35 from the pumping LD 34 shown in FIG.
And the seed light 32 shown in FIG.

The excitation LD 34 is not modulated as in the case of FIG. 11 and continuously oscillates in a constant state.

The YAG crystal 11 is excited by the laser light 35 and emits light 36 when the atoms of Nd ions return to the ground state.

Here, the oscillation wavelength of Nd ions of 1.06 μm is not a simple line. This line is said to transition _{^{4 F 3/2 → 4 I}} 11/2, the width of this line is about 0.7 nm in terms at room temperature 65cm ^-1, and the the wavelength.

Therefore, when only the laser beam 35 is incident on the YAG crystal 11, the oscillation line 38 of the laser beam 37 output from the YAG crystal optical resonator 12 has a width b of 0.7 nm as shown in FIG. Have.

That is, the YAG crystal optical converter 12 has a 0.7 nm
Though it is an extremely narrow range, it becomes a wavelength tunable laser.

Here, the pumping LD 14 is continuously operating, the laser light 35 is continuously incident on the YAG crystal 11, as shown in FIG.
3 continues stimulated release.

The seed light 32 is also incident on the YAG crystal 11, which causes the laser pull-in operation described below.

In general, the oscillation wavelength of the laser can be any value as long as it is within the line width for oscillation. The oscillation wavelength can be influenced by chance rather. However, if light of a specific wavelength is incident at the beginning of oscillation, it is drawn in and selectively stimulated and emitted by that wavelength, and the entire output oscillates at the specific wavelength. The light that is first injected from another laser for this purpose is called seed light. Therefore, the oscillation wavelength of the laser can be controlled by controlling the oscillation wavelength of this seed light.

Here, the YAG crystal 11 maintains the excited state, and the YAG laser 13 is connected to stimulated emission. The laser level lifetime is not the spontaneous emission lifetime but the stimulated emission lifetime. It will be much smaller, on the order of ns.

When the seed light 32 is incident on the YAG crystal 11, the high-power laser light 39 emitted from the YAG laser 13 is frequency-modulated by the seed light 32 as shown in FIG. 2C. It has been done.

As described above, since the emission life in the YAG crystal 11 is on the order of ns, the operation of returning the wavelength of the laser light 39 is instantaneously performed, and the YAG laser 13 emits the laser light 39 at a high frequency of several hundred MHz. Frequency modulation is possible up to the frequency band.

Therefore, the YAG laser 13 has several hundred
The modulation operation can be performed at a speed sufficient to follow the seed light 32 that is frequency-modulated by Mz. As shown in FIG. 2C, the light intensity is high and constant, and the wavelength is several 100 Mz. The laser beam 39 frequency-modulated by is emitted.

It may be argued here that the wavelength is controlled by the seed light immediately after the oscillation, but it cannot be controlled after the oscillation because stimulated emission occurs due to its own oscillation wavelength. However, continuous oscillation of a solid-state laser does not always produce a constant output over time. It is known that even if it is continuous oscillation, intermittent spike oscillation occurs on the order of ns. This is a phenomenon known as relaxation oscillation, which is characteristic of solid-state lasers. Each spike oscillates at an independent wavelength.

This laser light 39 is incident on the KNbO ₃ crystal 13.

In order to efficiently output the second harmonic from the crystal 13, it is necessary to achieve phase matching.

In this crystal 13, the allowable width c of the wavelength of the incident laser beam 39 for establishing phase matching is about
It is 0.06 nm · cm.

The relationship between the incident laser beam and the second harmonic on the crystal 13 is as shown in FIG.

When the wavelength of the incident laser light is changed without changing the light intensity as shown by the line 45, the light intensity of the second harmonic becomes as shown by the line 46.

That is, when the wavelength of the incident laser light falls within the allowable width c, the phases match and the crystal 13 outputs the second harmonic. When the wavelength of the incident laser light deviates from the allowable width c,
The phases are mismatched, and the output of the second harmonic becomes almost zero.

As described above, the crystal 13 has wavelength dependence.

The present invention utilizes this wavelength dependence.

In order to utilize this wavelength dependence, the allowable width c and the oscillation line width b have a relationship of c << b as shown in FIGS. 3 and 4.

Since the laser light 39 has a wavelength whose oscillation line width c is frequency-modulated, when it enters the crystal 13, the wavelength goes in and out of the allowable width c, and a phase matching and a mismatching state occur. Repeated.

The crystal 13 outputs the second harmonic only when the phases are matched, and does not output the second harmonic when the phases are mismatched.

As a result, from the crystal 13, FIG.
As shown in, the second harmonic wave 40 whose frequency is modulated at a high frequency with a light intensity of several 100 MHz is output.

Therefore, the above-mentioned device 30 is several hundred MH.
In response to the arrival of the information signal modulated at the high frequency of z, the light intensity is high and the wavelength is half of the laser light 39 of the YAG laser 13, and the light intensity is several hundred MHz corresponding to the information signal. The second harmonic 40 frequency-modulated by a high frequency is output.

The device 30 can be used not only for optical disks but also for other fields such as optical communication.

Further, instead of the YAG crystal 11, another solid-state laser crystal may be used.

Further, in place of the KNbO ₃ crystal 13, Li
NbO ₃ crystal or LiTiO ₃ crystal may be provided.

FIG. 6 shows a second harmonic generator 50 according to the second embodiment of the present invention.

This device 50 is different from the device 30 described above in that the KNbO ₃ crystal 13 is provided in the resonator 51.

As a result, the laser light passing through the inside of the crystal 13 becomes stronger and the second harmonic is generated more effectively. As shown in FIG. 6, the second harmonic having a high light intensity (P ₂ > P ₁ ) is generated. 52 is output.

Further, the length of the crystal 13 becomes equivalently long, and the allowable width of phase matching becomes narrow. Therefore, the wavelength variable width of the wavelength variable LD 31 may be narrowed by that amount, which is advantageous.

FIG. 7 shows a second harmonic generator 60 according to the third embodiment of the present invention.

This device 60 has a structure in which the saturable absorber 61 is provided inside the optical resonator 12 and on the emission side of the YAG crystal 11.

The supersaturated absorber 61 absorbs a portion of the laser light 39 output from the YAG crystal 11 that has a low light intensity.

As a result, the oscillation of the YAG laser 13 is self-mode-locked, and the YAG laser 13 operates as shown in FIG.
2, the peak of the light intensity is P _11, which is higher than P _{10 in} FIG. 2C, and the laser light 62 composed of regular pulses on the order of GHz is output.

Thus, since the peak is high, the crystal 13
Efficiently generate the second harmonic wave, and as shown in FIG. 8B, the second harmonic wave 6 having a larger light intensity than that of FIG.
3 is output.

Since the laser beam 62 has regularity,
The second harmonic 63 has less noise than the conventional one.

[0078]

As described above, according to the invention of claim 1, the oscillation mode of the solid-state laser is, for example, several hundred MHz.
It can be modulated in a very high frequency band. Also,
By utilizing the wavelength dependence of a nonlinear optical crystal,
It is possible to generate a second harmonic wave having a high light intensity and a light intensity modulated at a high frequency of, for example, several 100 MHz.

According to the second aspect of the invention, it is possible to generate a second harmonic wave having a higher light intensity than that of the first aspect of the invention.

According to the invention of claim 3, as compared with the invention of claim 1, it is possible to generate a second harmonic wave having a higher light intensity.

[Brief description of drawings]

FIG. 1 is a diagram showing a second harmonic generation device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a laser beam and a second harmonic of each part of the apparatus of FIG.

FIG. 3 is a diagram showing a frequency modulation width of seed light, an oscillation line width of laser light, and an allowable width of phase matching.

FIG. 4 is a diagram showing an oscillation line of laser light of a YAG laser in FIG.

5 is a diagram illustrating the wavelength dependence of KNbO ₃ crystal in FIG. 1. FIG.

FIG. 6 is a diagram showing a second harmonic generation device according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a second harmonic generation device according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a second harmonic generation device according to a third embodiment of the present invention.

9 is a diagram showing laser light and second harmonics in FIG.

FIG. 10 is a diagram showing a conventional example.

FIG. 11 is a diagram showing another conventional example.

FIG. 12 is a diagram illustrating the operation of the apparatus of FIG.

[Explanation of symbols]

11 YAG crystal 12, 51 Optical resonator 13 YAG laser 14 KNbO ₃ crystal 30, 50, 60 Second harmonic generation device 31 Wavelength tunable semiconductor laser 32 Seed light 33 Terminal 34 Excitation semiconductor laser 35 Laser light 36 Light 37 Laser light Laser light excited by 35 and output 38 Oscillation line 39,62 Laser light 35 and laser light emitted when seed light 32 is incident 40,52,63 Second harmonic wave 61 Saturated absorber a Seed Optical frequency modulation width b Laser light oscillation line width c Phase matching allowable width

Claims (3)

[Claims] 1. A solid-state laser (13) comprising a solid-state laser medium (11) having a predetermined oscillation line width (b) in an optical resonator (12), and exciting the solid-state laser medium of the solid-state laser. A first semiconductor laser (34) that outputs a laser light (35) that emits light, and a seed light (32) that is frequency-modulated within the oscillation line width (b) of the solid-state laser medium to output the solid-state laser medium. A second semiconductor laser (31) which is incident on the laser diode, and a nonlinear optical crystal (1) which has wavelength dependence and which is irradiated with laser light from the solid-state laser to generate its second harmonic.
3), wherein the solid-state laser medium maintains a state of being excited by the laser light from the first semiconductor laser, and the solid-state laser oscillates by the seed light from the second semiconductor laser. A second harmonic generation device characterized in that a mode is modulated and the nonlinear optical crystal outputs a second harmonic whose light intensity is modulated by its wavelength dependence. 2. A second harmonic generation device, characterized in that the nonlinear optical crystal according to claim 1 is provided in the optical resonator (51). 3. A second harmonic generation device, characterized in that the solid-state laser according to claim 1 is provided with a saturable absorber (61).