JPH0728112A - Waveguide type second harmonic generation light source - Google Patents

Abstract

PURPOSE:To provide a second harmonic generating light source which involves less deterioration in the efficiency for varying laser wavelength, is easy to conduct light collection, provides less wave-front abberation, and is easy manufacturable, by furnishing a plurality of polarized inversion/refractive index modulation parts having different periods. CONSTITUTION:A second harmonic generating light source of waveguide path type includes a light waveguide part 4 composed of at least two regions having different periods, with which a semiconductor laser 7 makes oscillation with a wavelength meeting Eq. I by Bragg's reflection from the region with a period LAMBDAl where the light waveguide part 4 exists, and setting is made so that at least one of modulation periods LAMBDA2 other than this region meets Eq. II and does not meet Eq. I. In Eqs., N(lambda) is the effective index of refraction of the waveguide light at wavelength A and N(lambda/2) is the refractive index of the waveguide light at the wavelength lambda/2. Because one of them is used for generation of second harmonic while the other is used as a diffraction lattice, the second harmonic generating part can be provided separately from the reflection part of the fundamental wave, so that an optimum design can be made for respective conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type second harmonic generation light source for converting a semiconductor laser light having a wavelength of about 800 nm into a light having a wavelength of about 400 nm as a light source for an optical application device such as an optical disk device. It is about.

[0002]

2. Description of the Related Art In order to improve the recording density of an optical recording / reproducing apparatus and to increase the definition of a laser beam printer, there is a strong demand for shortening the wavelength of a semiconductor laser from the conventional 800 nm to 500 nm or less. However, in order to do this, semiconductors for lasers have been replaced by conventional semiconductors of the III-5 group
It is necessary to change to a Group 6 semiconductor, and the prospect is not yet obtained.

Therefore, at present, for example, a wavelength of 800n
Attention has been paid to a method of converting a semiconductor laser beam of m (infrared light) into a second harmonic having a wavelength of 400 nm by using a second harmonic generating element (SHG. Second Harmonic Generator). The nonlinearity of the dielectric constant is used in the wavelength conversion. However, since the refractive index of the optical material generally changes depending on the wavelength, there is a problem that the law of conservation of momentum cannot be satisfied if the law of conservation of energy is satisfied.
I was unable to extract the harmonics. Therefore, it has been considered to perform phase matching between the fundamental wave and the second harmonic, that is, to synthesize innumerable second harmonic components generated in the second harmonic generating element in the same phase in the optical waveguide. There is.

[0004] For example, in IEICE Autumn National Convention Proceedings C-249 of 1989, as shown in FIG. 9, lithium tantalate (LiTaO ₃₎ on the substrate 31, the liquid phase of lithium niobate doped with magnesium The optical waveguide 32 is grown to form a fundamental wave 33 (TE polarized light) that is polarized 35 horizontally to the substrate surface from one end thereof,
From the other end surface, vertically polarized light 36 (TM polarized light) second harmonic wave 34
Is disclosed.

Further, Japanese Patent Laid-Open No. 61-18964 discloses a LiNbO ₃ single crystal substrate 41 as shown in FIG.
An optical waveguide 42 is formed by a proton exchange method (a method in which Li ions of LiNbO ₃ are partially replaced with protons), and a fundamental wave 43 polarized perpendicularly to the substrate surface is made incident from one end of the optical waveguide 42. A method of extracting the vertically polarized second harmonic 44 generated by Cherenkov radiation in 42 is disclosed.

In addition, the Electronics Letters (Elec
tronics Letters) Vol. 25, pp. 731-732, as shown in FIG. 11, for example, a polarization reversal in which the spontaneous polarization direction is reversed at an equal pitch on a ferroelectric material having spontaneous polarization such as a LiNbO ₃ substrate 51. A layer 53 and an optical waveguide 52 formed by a proton exchange method are provided, a fundamental wave 54 polarized in the z direction is incident from one end of the optical waveguide 52, and z is applied from the other end.
A method of extracting the second harmonic wave 55 polarized in the direction has been proposed.

[0007]

In the conventional method shown in FIG. 9, the wavelength dispersion of the refractive index of LiNbO ₃ is too large, so that the phase is sufficiently matched when the wavelength of the second harmonic wave 34 is 500 nm or less. However, there was a problem that pure blue light could not be obtained. Further, since the temperature coefficients of the refractive indexes of the fundamental wave 33 and the second harmonic 34 in the respective polarization directions are largely different, the propagation speed changes depending on the temperature and the phase matching condition collapses. Therefore, the allowable temperature range is about 0.2 ° C. And the film thickness accuracy of the optical waveguide 32 is 0.01μ.
There is also a problem that an unrealistic value such as m or less is required.

In the conventional method shown in FIG. 10, since the second harmonic wave output 44 has a crescent-shaped cross section and a large wavefront aberration, it cannot be narrowed down to a minute light spot for an optical disk device or the like. was there.

On the other hand, in the second harmonic generating element of the conventional method shown in FIG. 11, the second harmonic is confined in the optical waveguide 52, so that the emitted light can be easily narrowed down,
Moreover, since the fundamental wave 54 and the second harmonic wave 55 have the same polarization direction, the difference in the refractive index and the temperature coefficient due to the difference in the polarization direction does not occur, and the allowable temperature range is relaxed. But,
In addition to being practically inadequate, there is a problem that the wavelength selectivity is severe and the efficiency becomes almost zero even if the wavelength of the semiconductor laser changes by only 1 nm.

As a measure for reducing the above-mentioned wavelength fluctuation, it is reported that the temperature of the laser is controlled to ± 0.5 ° C. or less in Proceedings of the Autumn Meeting of Applied Physics of Japan 1991 Proceedings 11p-ZN-9. Also, as another measure, IEEE Journal of Quantum Electronics, Volume 26,
On pages 1265 to 1276 of the seventh issue, it is proposed to gradually change the period Λ of the polarization inversion grating to broaden the wavelength selectivity. However, since the above method shifts the period in the long polarization inversion grating by 1/100 or less of the order of several microns, it is practically difficult to manufacture, and it is the polarization inversion grating that contributes to the generation of the second harmonic. Since it is an extremely short portion, there is a problem that the phase shift occurs in other portions and the efficiency is reduced.

Further, in Japanese Patent Laid-Open No. 2-63026,
Instead of changing the polarization inversion period as shown in FIG. 12, there has been proposed a method of changing the width or depth of the optical waveguide to relax the wavelength selectivity. But this way too
After all, only the short part of the optical waveguide contributes to the generation of the second harmonic wave, so that there is a problem in that the efficiency is significantly reduced.

Recently, the Institute of Electronics, Information and Communication Engineers Technical Report, OQ
On pages 31 to 36 of E91-23, as shown in FIG. 13, the second harmonic generating element shown in FIG. 11 and the semiconductor laser 71 are coupled by a lens system 73 and an optical fiber 72 to form the optical fiber 72. A method has been proposed in which the second harmonic generation element is used as a laser resonator that also functions as an output-side reflection mirror of the semiconductor laser 71, and the laser oscillation wavelength is pulled to a wavelength at which the second harmonic can be generated. Although the laser wavelength is stabilized by this method, increasing the reflectance of the domain-inverted grating with respect to the laser wavelength means increasing the loss of the waveguide, resulting in a large decrease in the second harmonic generation efficiency. There is a problem that it ends up. Also,
Due to various errors generated during the production of the second harmonic generation element, for example, the design wavelength and the actual operating wavelength deviate from each other, so that the semiconductor laser may be selected specially or the crystal temperature may be adjusted to adjust the design wavelength to There was also a secondary problem that the actual operating wavelengths had to match.

The object of the present invention is to solve the problems of the conventional device shown in FIG. 13 above, the deterioration of the efficiency η with respect to the change of the laser wavelength λ is slight, the light can be easily collected, and the wavefront aberration is small.
It is to obtain a waveguide type second harmonic generation light source that is easy to manufacture.

[0014]

The above-mentioned object is to provide a semiconductor laser beam, which is a fundamental wave, in an optical substrate having spontaneous polarization, and the direction of the spontaneous polarization and the refractive index of the substrate are periodically modulated. In a waveguide type second harmonic generation light source that is incident on an optical waveguide section and converts into a second harmonic wave, the optical waveguide section is composed of at least two regions having mutually different periods, and the semiconductor laser is Period with optical waveguide Λ1
By the Bragg reflection from the area having

[0015]

[Equation 1]

The laser beam is oscillated at a wavelength λ that satisfies

[0017]

[Equation 2]

(Where N (λ) is the effective refractive index of the guided light at the wavelength λ and N (λ / 2) is the refractive index of the guided light at the wavelength λ / 2), and equation (1) is satisfied. It is achieved by setting not to satisfy. Further, the modulation region having a period satisfying the above formula (2) is provided between the semiconductor laser and the modulation region satisfying the formula (1), or has the period Λ1 of the optical waveguide section. The area other than the area is at least 1 of the modulation period Λ2.
Is set so as not to satisfy the above formula (1), and a voltage is applied between a pair of electrodes provided through an optically transparent thin film on at least a modulation region satisfying the above formula (1). By controlling the refractive index of the optical waveguide section under the electrode by the electric field and changing the oscillation wavelength of the semiconductor laser, at least one of the modulation periods Λ2 satisfies the equation (2), and the period Λ2 is This is achieved by providing the modulation region having the semiconductor laser and the modulation region satisfying the expression (1).

Further, means for transmitting only the semiconductor laser light and reflecting the second harmonic of the semiconductor laser is provided between the semiconductor laser light incident end face of the optical substrate or the semiconductor laser, and the optical substrate is also provided. Means for transmitting the second harmonic of the semiconductor laser is provided on the end surface opposite to the semiconductor laser light incident end surface, and further, means for reflecting the semiconductor laser light on the end surface of the semiconductor laser opposite to the optical substrate. It is also achieved by providing the pair of electrodes in parallel with the light propagation direction of the optical waveguide section.

[0020]

In the optical substrate having the optical waveguide section including two or more regions having different periods according to the present invention, the incident semiconductor laser light is converted into the second harmonic in the modulation region having the period Λ2. In the modulation region having the period Λ1, the semiconductor laser light that has not been converted into the second harmonic wave is reflected to the semiconductor laser side. Further, the reflecting means provided in the semiconductor laser incident portion of the optical substrate reflects the second harmonic component of the semiconductor laser and transmits the fundamental wave component, but the reflecting means of the end face of the semiconductor laser is the semiconductor laser light. Is reflected by 90% or more, for example.

[0021]

Embodiments of the present invention will now be described with reference to the drawings. 1 is a sectional view showing a first embodiment of a waveguide type second harmonic generation light source according to the present invention, FIG. 2 is a plan view of the first embodiment, and FIG. 3 is a polarization inversion period and a diffraction grating of the above embodiment. FIG. 4 is a diagram showing the relationship between the period and the wavelength of the semiconductor laser, FIG. 4 is a diagram showing the deviation from the diffraction grating reflectance and the Bragg condition of the above-mentioned embodiment, and FIG. FIG. 6 is a sectional view showing a third embodiment of the present invention,
FIG. 7 is a diagram showing an example in which the light source of the present invention is applied to an optical disk, and FIG. 8 is a diagram showing an example in which the light source of the present invention is applied to a laser beam printer.

First, the characteristics of the present invention will be clarified by comparing the waveguide type second harmonic generation light source according to the present invention with the conventional light source element shown in FIG. In FIG. 1, the light of the semiconductor laser 7 is incident on the end face of the optical waveguide portion 4 provided in the substrate 1 via the collimating lens system 9, the polarizing plate 11, and the condenser lens system 10. Therefore, the collimating lens system 9,
The polarizing plate 11, the condenser lens system 10, and the like correspond to the optical fiber 62 of the conventional element shown in FIG. Even if the collimating lens system 9, the polarizing plate 11, the condenser lens system 10 and the like are replaced with optical fibers, the action and effect of the present invention do not change.

The substrate 1 is provided with a polarization inversion / refractive index modulator 3 having a period Λ2 and a polarization inversion / refractive index modulator 5 having a period Λ1 along the optical waveguide 4, and the polarization inversion / refractive index modulator is provided. The fundamental wave laser light is converted into the second harmonic wave by 3 and is reflected to the fundamental wave side by the polarization inversion / refractive index modulator 5. Therefore, the semiconductor laser 7 and the polarization inversion / refractive index modulator 5
The oscillation wavelength (fundamental wavelength) λ of the semiconductor laser 7 is determined by the propagation characteristics between the two. That is, this portion operates as a resonator of the semiconductor laser 7.

On the other hand, in the conventional light source element shown in FIG. 13, the polarization inversion grating 53 has the polarization inversion / refractive index modulating section 5.
Also has the role of. As a result, although the structure shown in FIG. 13 is simplified as described above, it is difficult to simultaneously increase the second harmonic conversion efficiency η and the reflectance for the fundamental wave. .

In the present invention, the polarization inversion / refractive index modulator 3 having the period Λ2 and the polarization inversion / refractive index modulator 5 having the period Λ1 are separately provided, so that the second harmonic wave generating portion and the fundamental wave reflecting portion are provided. Can be separated, so that each can be optimally designed. Further, in the present invention, the polarization inversion / refractive index modulator 5
A pair of electrodes 17 are arranged in parallel with the optical waveguide portion 4 via an optically transparent thin film. By applying an electric field to the electrode 17, the effective refractive index of the guided light propagating in the polarization inversion / refractive index modulator 5 can be changed,
The Bragg reflection wavelength of the polarization inversion / refractive index modulator 5 can be changed. Accordingly, even if the second harmonic generation wavelength and the laser oscillation wavelength deviate due to an element manufacturing error, they can be matched by applying a voltage. On the contrary,
By applying a high-frequency AC electric field to the electrode 17, the laser oscillation wavelength can be changed at a high frequency, so that the laser oscillation wavelength can be deviated from the second harmonic generation wavelength, and the second harmonic output. The intensity can be modulated.

First Embodiment A first embodiment of the waveguide type second harmonic generation light source according to the present invention will be described in detail with reference to FIG. Substrate 1 is ZcutLiT
It is an aO ₃ single crystal plate, and its surface is the −c plane. The spontaneous polarization of the substrate 1 is usually downward,
The polarization inversion / refractive index modulators 3 and 5 are simultaneously formed therein by the proton exchange method. The above LiTaO ₃ and LiN
The bO ₃ single crystal is a ferroelectric crystal belonging to the space group R3c, and the sign of the nonlinear optical coefficient thereof is periodically inverted in accordance with the direction of the spontaneous polarization (Journal of Applied Physics). of Applied Physics)
Vol. 40, No. 2, pp. 720-734). The semiconductor laser 7 has a reflection film 8 at one end, and the polarization direction is set to 9 by a polarizing plate 11.
It is rotated by 0 ° and enters the optical waveguide unit 4. In the polarization inversion / refractive index modulator 3, the fundamental laser light is converted into the second harmonic. The coating film 12 allows the fundamental wave to pass through and reflects the second harmonic light. The fundamental wave component emitted from the polarization inversion / refractive index modulator 3 is reflected by the polarization inversion / refractive index modulator 5 and fed back to the semiconductor laser 7 side to oscillate the semiconductor laser 7 at a wavelength of 830 nm, for example. The second harmonic component 15 emitted from the polarization inversion / refractive index modulator 3 passes through the coating film 13 and the collimator lens 14 for preventing its reflection and is emitted. The above optical elements are positioned by the holder 16 and mounted compactly.

Next, the characteristics of the above-mentioned waveguide type second harmonic generation light source according to the present invention will be theoretically derived, and the characteristics will be specifically described. The change ΔN in the effective refractive index of the optical waveguide section 4 is caused by the change in wavelength. Refractive index n of a substance in the visible light region
The wavelength dependence of follows the Sellmeiner formula. For example LiTa
For O ₃ , it becomes as shown in equation (3).

[0028]

[Equation 3]

However, A ₁ = 4.5789, A ₂ = 9.9
304 × 10 to ¹⁴ , A ₃ = 4.2275 × 10 to ¹⁴ , A ₄ =
2.2400 × 10 ¹⁰ , and the unit of λ is m.

The values of A _{1 to} A ₄ etc. are values with respect to the refractive index ns of the substrate 1 in the embodiment shown in FIG. 1 obtained by measurement using a Ti-sapphire laser, a dye laser and an argon laser. . Using the value of the equation (3), ΔN with respect to the deviation Δλ from the design wavelength λ ₀ = 850 (nm) is obtained, and the equation (4) is obtained.

[0031]

[Equation 4]

In order to maintain the efficiency at 80% or more of the maximum value in the second harmonic generating element having a length of 10 mm, ΔN is 2.5 × 10.
It is necessary to set it to ~ ⁵ or less.
It turns out that it is necessary to suppress to 0.043 nm or less when converted to. However, the wavelength of the semiconductor laser is usually 1 nm
It is almost impossible to satisfy the above conditions because it changes in the order of.

Therefore, according to the present invention, the oscillation wavelength of the semiconductor laser 7 is locked by using the Bragg reflection from the polarization inversion / refractive index modulator 5 as described above, and at the same time, the change of the injection current value of the semiconductor laser 7 and the longitudinal mode. I try to prevent hopping. FIG. 3 is a diagram showing the relationship between the oscillation wavelength λ of the semiconductor laser 7, the polarization inversion period Λ2, and the Bragg diffraction period Λ1. The Bragg diffraction period Λ1 is given by the equation (1). From FIG. 3, if q = 1, then λ = 850 nm, whereas Λ1 has a value of 0.19 μm, which is difficult to fabricate. Therefore, in this embodiment, q = 17 is set and the polarization inversion / refractive index modulator 5 is set. Was set to Λ1 = 3.4 μm. The polarization inversion period Λ2 is given by the equation (2). λ = 850 nm
It can be seen that in order to generate the second harmonic, it is necessary to set Λ2 = 3.9 μm. In this embodiment, the period of the polarization inversion / refractive index modulator 3 is set to Λ1 = 3.9 μm.

In this embodiment, the polarization inversion / refractive index modulator 3 is used as a polarization inversion grating and the polarization inversion / refractive index modulator 5 is used as a Bragg diffraction grating as described above. The rate modulation unit 3 may act as a Bragg diffraction grating. However, from FIG. 3, Λ2 = 3.9μ
The wavelength that may oscillate at a period of m is 805
nm or 885 nm, which is a wavelength range in which the laser gain is extremely small, it is considered that oscillation does not occur at these wavelengths. Therefore, laser oscillation occurs only in the vicinity of the wavelength of 850 nm, and Bragg reflection does not occur in the polarization inversion / refractive index modulator 3 having the period Λ2 = 3.9 μm, and only the second harmonic is generated.

By the way, the complex reflectance r of the Bragg reflection in the polarization inversion / refractive index modulator is given by the equation (5).

[0036]

[Equation 5]

Where K is the coupling coefficient, L _b is the length of the Bragg diffraction grating, and Δβ is the amount of phase mismatch.

[0038]

[Equation 9]

Α is an optical loss coefficient.

The reflection coefficient of the diffraction grating 5 is represented by R = | r | ² and takes the maximum value R ₀ shown in the equation (6) when Δλ = 0 which satisfies the Bragg condition.

[0041]

[Equation 6]

The coupling coefficient κ of the polarization inversion / refractive index modulator 5 is expressed by the equation (7) (see Nishihara, Haruna, and Suhara “Optical Integrated Circuits”, Ohmsha, page 77).

[0043]

[Equation 7]

Where n _f is the refractive index of the optical waveguide, and H is
_y (x) is the field distribution of the magnetic field, n (x) is the refractive index at the coordinate x, and q is the Bragg diffraction order.

In this embodiment, the Bragg diffraction order is q = 17.
And Further, since the polarization inversion / refractive index modulation portion is manufactured by the proton exchange method, Δn = 0.01 or so.
= A ^{5.0 × 10 ~ 4 μm ~ 1} . When the light propagation loss of the optical waveguide 4 is set to about 0.5 dB / cm and L _b = 4000 μm, the reflectance R thereof is obtained, and FIG. 4 is obtained. In FIG. 4, when the Bragg condition is satisfied (Δλ = 0), the reflectance R is 0.913, and the range of the wavelength Δλ where the value of R is kept at 80% or more is ± 0.05 nm. Has good wavelength selectivity. Therefore, the oscillation wavelength of the semiconductor laser is pulled into this range, and stable second harmonic oscillation can be realized.

Further, the generation efficiency η of the second harmonic can be made significantly higher than that of the conventional element shown in FIG. The first reason is that the polarization inversion / refractive index modulator 5 required as an external resonator of the semiconductor laser 7 is separated from the polarization inversion / refractive index modulator 3 which generates the second harmonic, so that the light propagation This is because the polarization inversion / refractive index modulator 3 can be manufactured so that the loss is reduced. The second reason is that the polarization inversion / refractive index modulator 3 can be manufactured so that the reflectance with respect to the fundamental wave is disregarded and the second harmonic generation efficiency is increased. The third reason is that the polarization inversion / refractive index modulator 3 that generates the second harmonic is placed between the semiconductor laser 7 and the polarization inversion / refractive index modulator 5 that acts as a diffraction grating. This is because the second harmonic wave is generated on the outward path and the return path. Further, the second harmonic wave is reflected by the coating film 12 so that the coating film 13 is almost completely removed.
Is also contributed to the generation efficiency η of the second harmonic.

The oscillation characteristics of the waveguide resonance type SHG light source according to the present invention produced based on the above-mentioned examination results are as follows. When the reflectivity of the output mirror is 0.1% and the reflectivity of the rear reflective mirror is 90%, the maximum output is 100 mW with an input current of 200 mA and a striped GaAlAs semiconductor laser is used. After all 9
When 0% and the output side is coated with an antireflection film, the threshold value is about 50 mA and the injection current is 200 mA.
At 40 mW, an extremely strong second harmonic output of wavelength 425 nm (blue) was obtained. Since the output of the conventional second harmonic generation element is at the level of 2.0 mW with respect to the same level of input, the present invention can increase the second harmonic output level by almost 20 times. The output level may be further improved by optimizing the structures of the reflecting mirror 8, the coating film 12, the external resonator, and the like.

Next, the correction effect of the drawing wavelength by the electrode 17 will be described. As mentioned above, the fundamental wave wavelength band of the second harmonic generation is ± 0.043 nm, which is extremely narrow.
For example, if various fabrication errors occur in the device fabrication process, the wavelength of the second harmonic generation may deviate from the wavelength of the semiconductor laser pulled by the polarization inversion / refractive index modulator 5. Therefore, by applying a DC voltage to the electrode 17, the effective refractive index of the guided light propagating on the diffraction grating 5 can be corrected and the pull-in wavelength can be adjusted.

The change ΔN in effective refractive index due to voltage application is
Equation (8) can be roughly expressed.

[0050]

[Equation 8]

Here, r is an electro-optic coefficient, for example, Li
In the case of TaO ₃ , it has a value of about 30 pm / V. n is the refractive index of the optical waveguide 4, V is the applied voltage, g is the distance between the electrodes,
Γ is a coefficient representing the overlap between the electric field of the guided light and the electric field of the applied electric field, and its value is 0.3 to 0.7, although it depends on the arrangement of the electrode and the optical waveguide. Therefore, by applying the TTL level voltage ± 5V, for example ΔN electrode distance when the 5μm = 8 × 10¯ ⁵ next, than this pull wavelength can be corrected as ± 0.5 nm. If a voltage of about 50 V is applied, a large correction of ± 5 nm is possible.

On the contrary, when the pull-in wavelength of the semiconductor laser and the wavelength of the second harmonic generation coincide with each other, the intensity of the second harmonic of the output can be modulated by applying the voltage. As described above, the output of the second harmonic becomes almost zero when the wavelength of the semiconductor laser deviates from the predetermined wavelength by only 0.15 nm. Therefore, if the semiconductor laser wavelength is shifted by 0.5 nm by applying a voltage of ± 5 V at the TTL level, the output of the second harmonic can be made zero. Further, by adjusting the voltage value and changing the refractive index of the fundamental wave, it is possible to obtain an arbitrary second harmonic intensity according to the equation (2).

It should be noted that the polarization inversion / refractive index modulator 3 having the period Λ2 and the polarization inversion / refractive index modulator 5 having the period Λ1 have the same effect even if the setting positions with respect to the semiconductor laser 7 are reversed. can get.

Next, a method of manufacturing the light source in this embodiment will be briefly described. -LiTaO with mirror-polished c-plane
₃ A Ta film having a thickness of 30 nm is formed on the substrate 1 by the sputtering method, and the hatched portions of the polarization inversion / refractive index modulation portions 3 and 5 shown in FIG. 2 are patterned on a photoresist using a photomask having an opening. The Ta film is patterned by using the photoresist as a mask and a reactive ion etching method, and then the photoresist is removed. Then, the substrate 1 is placed on a platinum plate, and the substrate is placed on a plate type heater together with the platinum plate to drop an appropriate amount of pyrophosphoric acid (H ₂ P ₄ O ₇ ) on the substrate.
Heat treatment is performed at 0 ° C. to 260 ° C. for about 30 minutes to 60 minutes, and the platinum is lowered from the above heater and rapidly cooled. After that, the substrate is thoroughly washed with water, and the Ta film is removed using an aqueous sodium hydroxide solution heated to 70 ° C. Thus, the spike-shaped polarization inversion grating and the refractive index of 0.01
The polarization inversion / refractive index modulators 3 and 5 modulated by about 0.02 to 0.02 are simultaneously formed. The polarization inversion / refractive index modulators 3 and 5 have the same propagation loss of about 0.5 dB / c.
The optical waveguide 4 has a good m. Therefore, according to the above manufacturing method, the device can be manufactured by an extremely simple process as compared with the conventional manufacturing in which the polarization inversion grating, the diffraction grating, and the optical waveguide are manufactured separately. After the polarization inversion / refractive index modulators 3 and 5 are formed, a buffer layer 6 of, for example, silicon dioxide is formed to a thickness of about 100 nm by a sputtering method, and then the electrode pattern 17 of the polarization inversion / refractive index modulator 5 having a period Λ1 is formed. Form.

Finally, the element was cut into a chip of about 20 mm × 10 mm, and the two end faces of the side of 10 mm were optically polished. After that, the above-mentioned device, the semiconductor laser 7, the lens system and the like are positioned on the holder 16 and mounted to complete the production of the light source.

Second Embodiment A second embodiment of the present invention shown in FIG. 5 is an example in which balanced phase matching is used as the phase matching method. In FIG. 5, 10
1 is, for example, a KTP (PO ₄ ) substrate, 102 and 103
Is a polarization inversion / refractive index modulator formed by a rubidium (Rb) ion diffusion method, 102 is a portion for generating a second harmonic (corresponding to 3 in the first embodiment), and 103 is a laser wavelength fixed. This is a portion that acts as a diffraction grating (corresponding to 5 in the first embodiment). All other parts are above 1st
This is the same as the example, and the phase matching of the fundamental wave and the second harmonic is performed by the balance of the plurality of refractive index regions.

Third Embodiment A third embodiment of the present invention shown in FIG. 6 is an example of achieving phase matching by periodically reducing the nonlinear optical coefficient. In FIG. 6, 111 is, for example, a LiNbO ₃ substrate, and 112 and 113 are portions that generate a second harmonic wave (corresponding to 3 in the first embodiment), each of which is composed of a portion that has undergone proton exchange and a portion that does not. Also, the portion (corresponding to 5 in the first embodiment) that acts as a diffraction grating for fixing the laser wavelength is shown, and all other portions are the same as in the first embodiment. The nonlinear optical coefficient of the proton-exchanged portion is about half of the nonlinear optical coefficient of the non-proton-exchanged portion, which makes it possible to achieve quasi phase matching.

Fourth Embodiment FIG. 7 shows a fourth embodiment in which the second harmonic generation light source shown in the first to third embodiments is used for recording or reproducing on an optical disk. The light source of the present invention is modulated by the signal to be recorded on the optical disc. The modulation method is the first.
As shown in the embodiment, a predetermined voltage is applied to the electrode 17 in accordance with the above signal to change the oscillation frequency of the laser light source and forcibly break the phase matching condition. The light beam emitted from the end face of the waveguide of the light source 122 according to the present invention and then modulated by the DC voltage applying means 121 reaches the beam splitter 124 and the mirror 125 through the second high frequency collimator lens 123, and the objective lens 126.
Then, a pit row is formed on the optical disk 127 according to the modulation signal. When reproducing this, the optical power is lower than that at the time of recording, and the optical disc 1
The reflected light from 27 is split by the beam splitter 124,
When the photodetector 128 receives the light, the original modulated signal is demodulated.

Fifth Embodiment FIG. 8 shows a fifth embodiment in which the second harmonic generation light source shown in the first to third embodiments is used as a light source for a laser beam printer. As in the case of the fourth embodiment, the modulated light emitted from the second harmonic generation light source 122 is condensed by the lens, the light beam is swung left and right by the rotating polyhedron 131, and the scanning objective lens 126 causes the photosensitive drum to rotate. 132
A spot is formed on the top. The toner is sprinkled on the latent image formed on the photosensitive drum 132 by the modulation signal and is transferred onto the paper.

[0060]

As described above, the second waveguide type according to the present invention is used.
The harmonic generation light source emits the light of the semiconductor laser, which is the fundamental wave,
Waveguide-type second harmonic generation that is incident on an optical waveguide section provided in an optical substrate having spontaneous polarization and in which the direction of spontaneous polarization and the refractive index of the substrate are periodically modulated and converted into a second harmonic In the light source, the optical waveguide section is composed of at least two regions having different periods from each other, and the semiconductor laser is
By Bragg reflection from the region having the period Λ1 of the optical waveguide

[0061]

[Equation 1]

It oscillates at a wavelength λ that satisfies

[0063]

[Equation 2]

Is satisfied (where N (λ) is the effective refractive index of the guided light at the wavelength λ and N (λ / 2) is the refractive index of the guided light at the wavelength λ / 2), and the formula (1) is satisfied. By setting so as not to, or by providing a modulation region having a period satisfying the above formula (2) between the semiconductor laser and the modulation region satisfying the formula (1),
Since two or more polarization inversion / refractive index modulators having mutually different periods are produced and one of them is used for generating the second harmonic and the other is used as a diffraction grating, the second harmonic generating part and the reflecting part of the fundamental wave are formed. Can be separated so that an optimum design can be made for each.

Since the oscillation frequency of the semiconductor laser is controlled by the voltage applied to the electrodes, the oscillation frequency of the semiconductor laser is tuned to the second harmonic generation frequency, or conversely, the intensity of the second harmonic light is adjusted. It can also be directly modulated. Also, with the output improvement of the semiconductor laser,
The generation efficiency of the second harmonic can be remarkably increased as compared with the conventional device.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of a waveguide type second harmonic generation light source according to the present invention.

FIG. 2 is a plan view of the first embodiment.

FIG. 3 is a diagram showing the relationship between the polarization inversion period and the diffraction grating period and the semiconductor laser wavelength in the first embodiment.

FIG. 4 is a diagram showing a deviation from a diffraction grating reflectance and a Bragg condition in the above embodiment.

FIG. 5 is a sectional view showing a second embodiment of a waveguide type second harmonic generation light source according to the present invention.

FIG. 6 is a sectional view showing a third embodiment of a waveguide type second harmonic generation light source according to the present invention.

FIG. 7 is a diagram showing a fourth embodiment in which the light source of the present invention is applied to an optical disc.

FIG. 8 is a diagram showing a fifth embodiment in which the light source of the present invention is applied to a laser beam printer.

FIG. 9 is a perspective view showing a second harmonic generation element using a conventional birefringence phase matching method.

FIG. 10 is a second example using the conventional Cherenkov phase matching method.
It is a perspective view of a harmonic generation element.

FIG. 11 is a perspective view of a second harmonic generation element using a conventional polarization inversion grating.

FIG. 12 is a perspective view of a second harmonic generation element in which the width of a conventional waveguide is modulated.

FIG. 13 is a configuration diagram of a conventional polarization inversion grating resonance type second harmonic generation element.

[Explanation of symbols]

 1 Optical Substrate 3, 5 Polarization Inversion / Refractive Index Modulator 4 Optical Waveguide 6 Transparent Thin Film 7 Semiconductor Laser 8 Reflective Film 12, 13 Coating Film 15 Second Harmonic Wave 17 Electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroshi Kaede 1-280, Higashi Koikeku, Kokubunji, Tokyo 1-280, Central Research Laboratory, Hitachi, Ltd. (72) Inventor Takumi, 1-280, Higashi Koikeku, Kokubunji, Tokyo Hitachi, Ltd. Central Research Institute (72) Inventor Satoshi Makio 5200 Sankejiri, Kumagaya, Saitama Prefecture, Hitachi Metals Co., Ltd.Magnetic Materials Research Laboratories (72) Inventor Fumio Futanda, 5200, Sankeijiri, Kumagaya, Saitama Hitachi Metals Co., Ltd.

Claims (9)

[Claims] 1. A semiconductor laser light, which is a fundamental wave, is incident on an optical waveguide portion provided in an optical substrate having spontaneous polarization, in which the direction of spontaneous polarization and the refractive index of the substrate are periodically modulated, In the waveguide type second harmonic generation light source for converting into the second harmonic, the optical waveguide section is composed of at least two regions having different periods from each other, and the semiconductor laser has a period Λ1 in which the optical waveguide section is present. By the Bragg reflection from the area having, (However, N (λ) is oscillated at a wavelength λ that satisfies the effective refractive index of the guided light at the wavelength λ, and at least one of the modulation periods Λ2 other than the above region is (However, N (λ / 2) is the refractive index of the guided light at the wavelength λ / 2) and is set so as not to satisfy the formula (1). light source. 2. A region having a modulation period satisfying the above formula (2) is set between the semiconductor laser and a region having a modulation period satisfying the above formula (1). 2. The waveguide type second harmonic generation light source described in 1. 3. A modulation region satisfying the above formula (1) is provided with a pair of electrodes on the modulation region via an optically transparent thin film, and a direct current voltage is applied between the electrodes so that the light guide The waveguide type second harmonic generation light source according to claim 1, wherein the refractive index of the wave portion is adjusted. 4. A DC voltage applied between the pair of electrodes is
3. The intensity of the second harmonic output is modulated by controlling the refractive index in the optical waveguide section below the electrode by changing the obtained electric field and changing the oscillation wavelength of the semiconductor laser. Waveguide type second harmonic generation light source. 5. In a region other than the region having the period Λ1 of the optical waveguide section, at least one of the modulation periods Λ2 is the above (1).
It is set not to satisfy the formula, and at least the above (1)
By providing a pair of electrodes through an optically transparent thin film on the modulation region satisfying the formula, and applying an electric voltage between the pair of electrodes, the refractive index of the optical waveguide section below the electrodes is changed by an electric field obtained. Control is performed to change the oscillation wavelength of the semiconductor laser so that at least one of the modulation periods Λ2 satisfies the equation (2), and the modulation region having the period Λ2 and the semiconductor laser are modulated to satisfy the equation (1). The waveguide type second harmonic generation light source according to claim 1, wherein the waveguide type second harmonic generation light source is installed between the area and the region. 6. The optical substrate is provided with a means for transmitting only the semiconductor laser light and reflecting the second harmonic of the semiconductor laser between the incident end surface of the semiconductor laser light or the semiconductor laser. The waveguide type second according to claim 1.
Harmonic generation light source. 7. The conductor according to claim 1, wherein the optical substrate is provided with a means for transmitting a second harmonic of the semiconductor laser on an end face opposite to an incident end face of the semiconductor laser light. Waveguide type second harmonic generation light source. 8. The semiconductor laser according to claim 1, further comprising a means for reflecting the semiconductor laser light on an end face opposite to the optical substrate.
2. A waveguide type second harmonic generation light source according to any one of 1. 9. The waveguide type second harmonic generation according to claim 2, wherein the pair of electrodes are arranged in parallel with a light propagation direction of the optical waveguide section. light source.