JPH0667234A - Waveguide internal resonance type shg light source - Google Patents

Abstract

PURPOSE:To increase the 2nd higher harmonic conversion efficiency of a waveguide internal resonance type 2nd higher harmonic generating element which converts infrared laser light into visible light and to stabilize oscillation wavelength. CONSTITUTION:An optical waveguide 4 is formed in an epitaxial thin film on a substrate and a polarization inversion part 3 and a diffraction grating 5 are installed adjacently in the optical waveguide 4; and light from a semiconductor laser 7 is made incident on the polarization inversion part 3 and converted into a 2nd higher harmonic, the following diffraction grating 5 reflects the fundamental wave to the side of the semiconductor laser 7, and only the 2nd higher harmonic is outputted. Further, a voltage is impressed to electrodes provided on the diffraction grating 5 to adjust the semiconductor laser wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source having a short wavelength so that a semiconductor laser light having a wavelength of about 800 nm can be converted into a blue light having a wavelength of about 400 nm, and in particular, an optical disk device and a laser. Waveguide type second harmonic generation element (SHG: Second Ha) in printers and other optical applications
rmonic Generator), a bulk type optical head using the same, an integrated optical head, a disk device, a laser beam printer or the like and a generating element.

[0002]

2. Description of the Related Art As a semiconductor light source in the long wavelength band, GaA
III-V group semiconductors such as s, InP, AlSb, AlSb and GaP are used. These semiconductor materials are 500 nm to 1
It can emit light at a wavelength of 000 nm. But,
In order to improve the recording density of the optical recording / reproducing apparatus and to enhance the definition of the laser beam printer, it is desired to shorten the semiconductor laser wavelength from the conventional 500 to 1000 nm to 500 nm or less. . However, for that purpose, it is necessary to change the laser semiconductor from the conventional III-V group semiconductor to the II-VI group semiconductor, but there is no prospect yet.
Therefore, under the present circumstances, for example, a semiconductor laser with a wavelength of 800 nm is used.
A method of converting the light (infrared light) into a second harmonic having a wavelength of 400 nm is drawing attention. In the above wavelength conversion, the non-linearity of the dielectric is used. That is, the dielectric polarization P
Polarizes + and-, but consists of a term proportional to the electric field and a term proportional to the square of the electric field as in the following equation.

[Equation 1] In the present invention, in order to convert to the second harmonic, the above formula (1)
Use a term proportional to the square of. 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 energy-conservation law is satisfied, and the second harmonic cannot be simply extracted. That is, the electric field E in the above equation (1) is expressed by the following equation.

[Equation 2] Ω in the above equation (2) is expressed by the following equation. ω = 2πν = 2πc / λ ... (3) where ω is the angular frequency, ν is the frequency, c is the speed of light, and λ is the wavelength. is there. Then, λ in the above (3) is expressed by the following equation. λ = λ ₀ / n (λ) (4) where λ ₀ is the wavelength in air, λ is the wavelength in the substance, and n (Λ) is a refractive index. Therefore, studies have been conducted on phase matching between the fundamental wave and the second harmonic, that is, combining the innumerable second harmonic components generated in the second harmonic generating element with the same phase in the optical waveguide.

[0003] For example, in the 1989 IEICE Autumn National Convention Proceedings C-249, the second as shown in FIG.
A birefringent phase matching element that emits a harmonic component is disclosed. In FIG. 3, lithium tantalate (LiTa
O ₃ ) Lithium niobate doped with magnesium is liquid-phase-grown on a substrate 31 to form an optical waveguide 32, and a fundamental wave 33 polarized horizontally from one end thereof to the substrate surface.
(TE polarized light) is incident, and vertically polarized (TM polarized) second harmonic wave 34 is emitted from the other end surface. In addition, JP-A-6
Japanese Unexamined Patent Publication No. 1-18964 discloses a Cherenkov phase matching element as shown in FIG. In FIG. 4, Li
On the NbO ₃ single crystal substrate 41, a proton exchange method (LiNb
An optical waveguide 42 is formed by a method of partially replacing the protons with Li ions of O ₃ ), and a fundamental wave 43 polarized perpendicularly to the substrate surface is incident from one end of the optical waveguide 42 and generated by Cherenkov radiation in the optical waveguide. Vertically polarized second harmonic 4
Taking out 4. Also, "Electronics, Letters", Vol. 25, pp. 731-932 discloses an SHG element using a polarization inversion grating as shown in FIG. In FIG. 5, for example, a polarization inversion layer 53 in which the spontaneous polarization direction is inverted at an equal pitch and an optical waveguide 52 formed by a proton exchange method are provided on a ferroelectric material having spontaneous polarization such as a LiNbO ₃ substrate 51. , Optical waveguide 5
The substrate wave 54 polarized in the z direction is incident from one end of the second wave 2, and the second harmonic wave 55 polarized in the z direction is extracted from the other end. FIG. 2 is a diagram showing the principle of the phase matching operation. Figure 2
When light propagates through the optical waveguide in the direction of the arrow as shown in (a), at the point A, the second line as shown by the solid line in FIG.
A higher harmonic wave is generated, and at point B, a second higher harmonic wave is generated as indicated by the broken line in FIG. FIG. 2C shows the waveform of the fundamental wave. Thus, in terms of different optical waveguides,
A shift occurs in the waveform of the generated second harmonic.

[0004]

As described above, there have heretofore been proposed some methods for extracting the second harmonic component in the optical waveguide. However, in the method shown in FIG.
Since the wavelength dispersion of the refractive index of iNbO ₃ is too large, there is a problem that the phase matching cannot be sufficiently performed and the pure blue light cannot be obtained when the wavelength of the second harmonic 34 is 500 nm or less. In addition, since the temperature coefficient of the refractive index in each polarization direction of the fundamental wave 33 and the second harmonic 34 is significantly different,
Since the propagation velocity changes depending on the temperature and the phase matching condition is broken, the allowable temperature range is narrowed to about 0.2 ° C., and the film thickness accuracy of the optical waveguide 32 is 0.01 μm or less, which is unrealistic. There was also a problem that a large value was required. Next, the method shown in FIG. 4 has a problem that the second harmonic wave output 44 is of a crescent type and has a large wavefront aberration, so that it cannot be narrowed down to a minute light spot for an optical disk device or the like. Next, in the method shown in FIG. 5, since the second harmonic is confined in the optical waveguide 52, the emitted light can be easily narrowed down, and the fundamental wave 54 and the second harmonic 55 have the same polarization direction. Therefore, 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. However, there is a problem that the efficiency is almost zero even if the wavelength of the semiconductor laser is changed by only 1 nm, in addition to being still insufficient for practical use. That is, as shown in FIG. 8, when the change ΔN in the semiconductor laser wavelength is ± 4 × (1/10 ⁵ ), SH
The efficiency of G is about 0.6, and ΔN is ± 6 × (1/1
0 ⁵ ), the efficiency is about 0.1.

Therefore, as a measure for reducing the above wavelength fluctuation,
"Proceedings of the 1991 Periodical Lecture Meeting of the Society of Applied Physics" 11
It has been reported in p-ZN-9 that the temperature of the semiconductor laser is controlled to ± 0.5 ° C. or less to use it in a narrow portion. As another measure, "IEEE Journal of Quantum Electronics" Vol. 26, No. 7,
pp.1265-1276, it is proposed to gradually change the period Λ of the polarization inversion grating to broaden the wavelength selectivity. However, in the above method, the period in the long polarization inversion grating is shifted by 1/100 or less of the order of several microns, which is difficult to manufacture in practice, and it is the polarization that contributes to the generation of the second harmonic. There is a problem in that the efficiency is lowered due to the occurrence of phase shift in the other part, which is a very short part of the inversion grating. Furthermore, JP-A-2-63026
The publication discloses a domain-inverted SHG element in which the width of the waveguide is modulated as shown in FIG. That is, FIG.
In the above, the wavelength selectivity is relaxed by changing the width and depth of the optical waveguide instead of changing the polarization inversion period. In FIG. 6, 53 is a polarization inversion lattice,
51 is a LiNbO ₃ substrate, and 61 is an optical waveguide whose width and depth are changed. However, even with this method, only the short portion of the optical waveguide contributes to the generation of the second harmonic, and there is a problem in that the efficiency is significantly reduced.

Recently, "Technical Report of IEICE"
OQE91-23, pp.31-36 discloses an SHG light source using a polarization inversion grating as an external resonator as shown in FIG. In FIG. 7, 71 is a semiconductor laser, 7
2 is an optical fiber, 73 is a lens system, and the external resonator on the right side is an SHG element using the polarization inversion grating shown in FIG. Among the SHG elements, 51 is a LiNbO ₃ substrate, 52
Is an optical waveguide, and 53 is a domain inversion layer. A lens system 73 and an optical fiber 72 are arranged between the second harmonic generating element and the semiconductor laser 71, and they are coupled together.
Then, by using the optical fiber 72 and the second harmonic generation element as a laser resonator which also has the function of the output side reflection mirror of the semiconductor laser 71, the laser oscillation wavelength is changed to the second harmonic. It pulls in a wavelength that can generate waves. But,
In this method, the laser wavelength is stabilized, but increasing the reflectance of the polarization inversion grating with respect to the laser wavelength means increasing the loss of the waveguide, so that the second harmonic generation efficiency is improved. There is a problem that it will drop significantly.
Also, due to various errors that occur during device fabrication, for example, there is a gap between the design wavelength and the actual operating wavelength, and a semiconductor laser is selected specially or the temperature of the crystal is adjusted to obtain the design wavelength. There is a secondary problem of having to match the actual operating wavelength. The object of the present invention is to solve such a conventional problem, to reduce the deterioration of the efficiency η with respect to the change of the laser wavelength λ, to easily collect light, to reduce the wavefront aberration, and to easily manufacture the inside of the waveguide. It is to provide a resonant SHG light source.

[0007]

In order to achieve the above object, in the waveguide internal resonance type SHG light source of the present invention, (a) the direction of the spontaneous polarization is periodically inverted in the optical substrate having the spontaneous polarization. In the SHG light source that converts the fundamental wave component of the semiconductor laser light into the second harmonic wave in the optical waveguide having the polarization inversion section, in the optical waveguide on the second harmonic wave output side of the polarization inversion section, Semiconductor laser
A diffraction grating for reflecting the light is provided on the diffraction grating, and a pair of electrodes for adjusting the refractive index of the optical waveguide by applying a DC voltage through the optically transparent thin film is provided on the diffraction grating. Is characterized by. Further, (b) Furthermore, only the fundamental wave of the semiconductor laser light is transmitted between the end face of the optical substrate on which the semiconductor laser light is incident or between the optical substrate and the semiconductor laser, and the semiconductor laser light It is also characterized in that means for reflecting the second harmonic wave is provided, and means for reflecting the fundamental wave of the semiconductor laser light is provided on the end surface of the optical substrate opposite to the end surface on which the semiconductor laser light is incident. I am trying. In addition, (c) the diffraction grating is provided in the refractive index modulation section in which a plurality of regions having different refractive indexes are periodically repeated in the optical substrate instead of the optical waveguide on the second harmonic output side of the polarization inversion section. It is also characterized in that it is provided in the optical waveguide on the second harmonic output side. Further, (d) the diffraction grating is provided on the second harmonic wave output side of the spontaneous polarization modulator in which the magnitude of the spontaneous polarization is periodically modulated, instead of the optical waveguide on the second harmonic wave output side of the polarization inversion section. It is also characterized in that it is provided in the optical waveguide. Also (e)
The optical substrate includes lithium niobate (LiNbO ₃ ), magnesium doped lithium niobate (MgO: L).
iNbO ₃ ), lithium tantalum niobate (LiTaN
bO ₃ ) or lithium tantalate (LiTaO ₃ )
It is also characterized in that it is formed using any of the above.
In addition, (f) the optical waveguide is provided in a ferroelectric thin film having the same spontaneous polarization direction as that of the optical substrate, and the diffraction grating is characterized by having a periodically etched structure. Another feature is that a means for reflecting the semiconductor laser light is provided on the end surface of the (g) semiconductor laser opposite to the optical substrate. Further, (h) the polarization inversion part and the diffraction grating have the same grating period, and the pair of electrodes arranged on the diffraction grating may be arranged in parallel with the light propagation direction of the optical waveguide. It has a feature. Also,
(I) The electric field obtained by applying a voltage between the pair of electrodes is
It is also characterized in that the Bragg wavelength of the diffraction grating is made to coincide with the wavelength of the SH light oscillated by the polarization inversion grating by controlling the refractive index of the diffraction grating below the electrodes. Further, an electric field obtained by applying a voltage between a pair of (nu) electrodes controls the refractive index of the diffraction grating below the electrodes to change the Bragg wavelength of the diffraction grating to that of SH light oscillated by the polarization inversion grating. It is also characterized in that the intensity of the output second harmonic is modulated by shifting the wavelength.

[0008]

In the present invention, for example, wavelengths of 750 to 110
A near-infrared semiconductor laser light of 0 nm is incident on an optical waveguide of an optical substrate, and a polarization inversion part in which the direction of spontaneous polarization is periodically inverted along the optical waveguide and a diffraction grating are arranged next to each other. Furthermore, through an optically transparent thin film on the diffraction grating 1
Place a pair of electrodes. Semiconductor ray incident on the optical substrate
The light may be a polarization inversion part of the optical waveguide part, an optical waveguide part in which a plurality of regions having different refractive indexes provided in the optical substrate are periodically repeated, or a spontaneous polarization part provided in the optical substrate having spontaneous polarization. It is converted into the second harmonic by the optical waveguide whose size is periodically modulated, and the remaining semiconductor laser light is reflected to the semiconductor laser light side by the adjacent diffraction grating. The optical substrate is made of lithium niobate (LiNbO ₃ ) or magnesium doped lithium niobate (M
gO: LiNbO ₃ ), lithium tantalum niobate (LiTaNbO ₃ ), lithium tantalate (LiTaO ₃ ), or the like, and the nucleus of the spontaneous polarization part is arranged on one side of the ferroelectric material substrate, and periodically on the other side. The etched structure is placed, and the ferroelectric thin film whose refractive index is higher than that of the optical substrate is placed on the etched structure, and the direction of the spontaneous polarization is the same as the nucleus of the spontaneous polarization part of the ferroelectric material substrate. A domain-inverted portion having a high aspect ratio is formed, and the periodic etching structure portion is formed as a diffraction grating. The reflecting means arranged at the semiconductor laser light incident part of the optical substrate reflects the second harmonic component of the semiconductor laser and passes the fundamental wave component, and the reflecting means at the end face of the semiconductor laser is The semiconductor laser light is reflected by 90% or more, for example. With such a configuration,
Since the second harmonic wave generating portion and the fundamental wave reflecting portion can be separated, each can be optimally designed. In addition, the output efficiency of the semiconductor laser can be improved and the generation efficiency of the second harmonic can be significantly increased.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings. 1 (a) and 1 (b) are a cross-sectional view of a waveguide internal resonance type SHG light source showing an embodiment of the present invention and a top view of an element portion viewed from above. In FIG. 1A, 7
Is a semiconductor laser, 9 is a collimating lens system, 11 is a polarizing plate, 10 is a condenser lens system, 12 is a coating film, 3 is a polarization inversion part, 2 is a LiNbO ₃ single crystal thin film, and 5 is a diffraction grating. , 6 is a protective film, 1 is a substrate, 17 is an electrode, 13 is a coating film, 14 is a collimating lens, 15 is a second harmonic component, and 16 is a holder. Light from the semiconductor laser 7 is incident on the end face of the optical waveguide portion 4 provided in the substrate 1 via the collimation lens system 9, the polarizing plate 11 and the condenser lens system 10. Comparing the SHG light source of FIG. 1 with the conventional SHG light source of FIG. 7, the collimating lens system 9, the polarizing plate 11, and the condenser lens system 10 of FIG. 1A correspond to the optical fiber 72 of FIG. , The optical waveguide portion 4 in the substrate 1 of FIG.
Corresponding to the external resonator of. 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. On the other hand, on the substrate 1 of FIG. 1A, a polarization inversion part 3 and a diffraction grating 5 are installed along the optical waveguide part 4, and the polarization inversion part 3 transmits laser light (fundamental wave) to the second harmonic wave. Converted to the diffraction grating 5
Is reflected to the fundamental wave side. Therefore, the oscillation wavelength (fundamental wavelength) λ of the semiconductor laser 7 is determined by the propagation characteristics between the semiconductor laser 7 and the diffraction grating 5. That is, this portion operates as a resonator of the semiconductor laser 7. On the other hand, in the conventional structure shown in FIG. 7, the polarization inversion grating 53 also serves as the diffraction grating 5. Therefore, although the structure of the element of FIG. 7 is simplified, the second harmonic conversion efficiency η and
There is a problem that it is difficult to increase the reflectance for the fundamental wave at the same time.

On the other hand, in the structure of the present invention shown in FIG. 1, by adding the diffraction grating 5 to the polarization inversion section 3, the second harmonic generation section and the reflection section of the fundamental wave can be separated from each other, so that each is optimal. It is possible to design Further, since the oscillation frequency of the semiconductor laser 7 changes in the domain-inverted portion 3 according to the temperature change of the diffraction grating 5, it is possible to widen the allowable deviation range of the frequency. Although it is the same in the element of FIG. 7 that the allowable deviation width of the frequency can be widened,
In the element shown in FIG. 7, since the reflectance for the fundamental wave is low, the degree of coupling between the semiconductor laser 71 and the polarization inversion grating 53 becomes weak, and as a result, the temperature fluctuation followability of the laser oscillation frequency is poor. Further, in the element of FIG. 7, the temperature of the optical fiber 72 does not follow the temperature of the polarization inversion grating 53.
There is a problem that the temperature fluctuation followability of the laser oscillation frequency is deteriorated by the temperature fluctuation of No. 2. In the SHG light source of FIG. 1,
As shown in (b), a pair of electrodes 17 are arranged in parallel with the optical waveguide 4 via an optically transparent thin film on the diffraction grating 5. By applying an electric field to the electrode 17, the effective refractive index of the guided light propagating on the diffraction grating 5 can be changed, and the Bragg wavelength of the diffraction grating 5 (the wavelength most reflected by the diffraction grating) can be changed. it can. As a result, even if the second harmonic generation wavelength and the laser oscillation wavelength deviate due to an element manufacturing error, it is possible to make them coincide with each other by applying a voltage.

The structure of FIG. 1 will be described in more detail. Substrate 1 is a ZcutLiNbO ₃ single crystal plate of 1 mol% MgO doping, and its surface is + c plane. On the substrate 1, a LiNbO ₃ single crystal thin film 2 whose spontaneous polarization is generally upward is provided, and a polarization inversion part 3 whose polarization direction is downward is provided therein.
And a diffraction grating 5 are provided. Then, the upper portion is protected by the protective film 6. Further, the optical waveguide section 4 is manufactured by proton exchange. The LiNbO ₃ or LiTaO ₃ single crystal is a ferroelectric crystal belonging to the space group R ₃ C, and the sign of the nonlinear optical coefficient thereof is periodically inverted in accordance with the direction of the spontaneous polarization (for this, for example, 『Lournal of Appli
ed Physics ”Vol. 40. No. 2, pp. 720-734). In addition, the period of the polarization inversion unit 3 is Λ, and the diffraction grating 5 is
Let be the period of Λr. The semiconductor laser 7 has a reflection film 8 at one end.
The light is propagated in one direction by being reflected by the reflection film 8, the polarization direction is rotated by 90 degrees by the polarizing plate 11, and the light is incident on the polarization inversion unit 3. In the polarization inversion unit 3, the laser light (fundamental wave) is converted into the second harmonic wave.
The coating film 12 transmits the fundamental wave and reflects the second harmonic light. The fundamental wave component emitted from the domain-inverted portion 3 is reflected by the diffraction grating 5 and fed back to the semiconductor laser 7 side, so that the semiconductor laser 7 has a wavelength of, for example, 830.
Oscillate at nm. The second harmonic component 15 emitted from the polarization inversion unit 3 passes through the coating film 13 and the collimating lens 14 for preventing its reflection and is emitted. The above optical elements are positioned by the holder 16 and mounted compactly. The polarization inversion unit 3 and the diffraction grating 5 may have the same grating period.

The characteristics of the device of the present invention will be logically described below. The change ΔN in the effective refractive index of the domain-inverted portion 3 is caused by the change in wavelength. In this case, the wavelength dependence of the refractive index n of the substance in the visible light region follows the Sellmeiner equation. As an example, for 5% MgO-doped LiNbO ₃ , the following formula (5) is obtained. n ² = A ₁ + A ₂ / (λ ² −A ₃ ) −A ₄ λ ² (λ: nm) (5) where A ₁ = 4.57 A ₂ = 9.10 × 10 ⁴ A ₃ = 2.14 × 10 ₂ A ₄ = 3.0 × (1/10 ⁸ ) In addition, values such as A _{1 to} A ₄ are Ti-sapphire laser, dye laser, argon laser. It is a value with respect to the refractive index ns of the substrate 1 (FIG. 1) obtained by measurement using a square. When the value ΔN with respect to the deviation Δλ from the design wavelength λ ₀ = 830 (nm) is obtained using the value of the above expression (5), the following expression (6) is obtained. ΔN = −4.24 × (1/10 ⁴ ) Δλ ······· (6) (Δλ: nm) An SHG element with a length of 10 mm and an efficiency of 80% or more of the maximum value. In order to keep the above value, it is necessary to make ΔN 2.5 × (1/10 ⁵ ) or less. From the above equation (6), it can be seen that when converted into Δλ, it is necessary to suppress it to 0.043 nm or less. However, since the wavelength of the semiconductor laser usually changes in the order of 1 nm, it is almost impossible to satisfy such a condition.

Therefore, in the present invention, as described above, the oscillation wavelength of the semiconductor laser 7 is locked by using the diffraction grating 5, and at the same time, the change of the injection current value of the semiconductor laser 7 and the longitudinal mode are controlled. I try to prevent hopping. Figure 9
It is a figure which shows the relationship of the oscillation wavelength (lambda) of a semiconductor laser, the period (lambda) of a polarization inversion part, and the period (lambda) r of a diffraction grating. The upper solid line in FIG. 9 is the period Λr of the diffraction grating, and the lower solid line is the period Λ of the polarization inversion part.
And both solid lines intersect at one point. Period Λ of diffraction grating 5
r is represented by the following formula (7). Here, q is an integer indicating the diffraction order. Λr = λ / (2N) × q (7) where N is the effective refractive index of the guided light. Now, if q = 1, then Λr is 0.19 for λ = 0.83 (μm).
The value is μm, which is difficult to manufacture. Therefore, in this embodiment, q = 17 and Λ = Λr = 3.2 (μm) are set.
Next, the complex reflectance r of the diffraction grating 5 is given by the following equation (8). However, δ is a deviation from the Bragg diffraction wave number. r = [-iktanh (γLb)] / [γ + (α / 2 + iδ) tanh (γLb)] (8) where k: coupling coefficient α: optical Optical loss coefficient of waveguide Lb: Length of diffraction grating δ: Deviation from Bragg wave number γ = [(α / 2) + iδ] ² + k ²

Next, the reflection coefficient of the diffraction grating 5 is R = | r |
^When δ = 0, which is represented by ² , and satisfies the Bragg condition, the maximum value R ₀ shown in the following equation (9) is taken.

[Equation 3] Further, the coupling coefficient k is expressed by the following equation (10), and q = 1
7, when h = 0.1 (μm), k = 7.05 ×
(1/10 ⁴ ) (1 / μm) (for example, "Optical Integrated Circuit", Nishihara, Haruna, and Suhara, published by Ohmsha, pp.77.
See). k = 0.1199 (h / q) (1 / μm) (10) (h: μm) FIG. 10 shows the relationship between the reflectance and the deviation of the wave number from Bragg diffraction. FIG. When the light propagation loss of the optical waveguide 4 in FIG. 1B is set to about 0.8 (dB / cm) and the length Lb of the diffraction grating is set to Lb = 2600 μm, the reflectance R thereof is obtained as shown in FIG. In FIG. 10, when the Bragg condition is satisfied, the reflectance R at δ = 0 is 0.875, and the range of δ at which the value of the reflectance R is kept at 80% or more is ± 0.001. is there.

When the permissible fluctuation range Δλ of the laser wavelength is calculated from the half-value width of R, it is about ± 0.1 nm, which shows good wavelength selectivity. Therefore, the oscillation wavelength of the semiconductor laser is pulled into this range, and stable second harmonic oscillation can be realized. Also, the generation efficiency η of the second harmonic is
It can be made significantly larger than the conventional device of FIG. The first reason is that the diffraction grating 5 required as an external resonator of the semiconductor laser 7 is separated from the polarization inversion section 3, so that the polarization inversion section 3 can be manufactured so as to reduce the optical propagation loss. It has become. The second reason is that the reflectance for the fundamental wave is disregarded, and the second
This is because the polarization inversion part 3 can be manufactured so as to increase the harmonic generation rate. As a result, a rectangular domain-inverted lattice with a high aspect ratio can be used by the liquid layer epitaxial growth method. Furthermore, the third reason is that the polarization inversion portion 3 is arranged between the semiconductor laser 7 and the diffraction grating 5, so that the second harmonic wave is generated in the forward path and the return path of the fundamental wave. Furthermore, the fourth reason is that the second harmonic is
This is because the fact that the light is reflected by the coating film 12 and is almost completely emitted from the coating film 13 contributes to the generation efficiency η of the second harmonic.

Based on the above-mentioned examination results, the experimental results of the oscillation characteristics of the waveguide internal resonance type SHG light source of the present invention are as follows. When the reflectance of the output mirror is 10% and the reflectance of the back reflection mirror is 90%, the maximum output is 100 mW of stripe type GaAl at an input current of 200 mA.
Rear mirror-8 using an As semiconductor laser (see Fig. 1)
When the output side is coated with an antireflection film, the threshold value is about 50 mA and 40 mA at 200 mA.
An extremely strong second harmonic wave output of wavelength 410 nm (blue) of mW was obtained. The output of the conventional SHG element was 2.0 mW level for the same level of input, but in the present invention, the second harmonic output level can be increased about 20 times. This output level is
This can be further improved by optimizing the structures of the coating film 12, the external resonator, and the like. In the case of FIG. 1, the diffraction grating 5 has a structure in which the substrate 1 is directly processed, that is, a diffraction grating having a so-called relief type structure. However, this is a refractive index for periodically modulating the refractive index of the waveguide 4. Even if it is a modulation type diffraction grating, the effect is exactly the same. next,
The effect of correcting the pull-in wavelength by the electrode 17 shown in FIG. 1B will be described. As described in the above formula (6), the fundamental wavelength band of the second harmonic generation by the polarization inversion grating 3 is ±
Since it is as extremely narrow as 0.043 nm, for example, when various manufacturing errors occur in the device manufacturing process, the drawing wavelength of the semiconductor laser drawn by the diffraction grating and the second
It is possible that the wavelength of harmonic generation may shift. Therefore, by applying a DC voltage to the electrode 17, it is possible to correct the effective refractive index of the guided light propagating on the diffraction grating 5 and adjust the pull-in wavelength.

The change ΔN in the effective refractive index due to the voltage application to the electrode 17 can be expressed by the following equation (11). ΔN = − (1/2) n ³ ΓrV / g ··········································· (11) where r is an electro-optic coefficient, for example, in the case of LiNbO ₃ / V). n is the refractive index of the optical waveguide 4, V is the applied voltage, g is the distance between the electrodes, and Γ is a coefficient representing the overlap between the electric field of the guided light and the electric field of the applied electric field. Depending on the value, the value is 0.3 to 0.7. Therefore, by applying a TTL level voltage of ± 5 V, for example, the oscillation wavelength is 0.5 nm.
Can be corrected. If a voltage of about 50 V is applied, a large correction of ± 5 nm is possible. vice versa,
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 output second harmonic can be modulated by applying a voltage. That is, 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 above equation (6).

Hereinafter, a waveguide internal resonance type SH according to the present invention will be described.
A method of manufacturing the G light source will be described. FIG. 11 is a manufacturing process diagram of an SHG light source showing an embodiment of the present invention. First, in FIG. 11A, MgOlmo 1% doped LiN
_On the substrate 1 of bO ₃ , 5 nm Ti film 111 and 100 n
A Cr film 112 of m is sequentially sputtered, a photoresist pattern 113 is formed on it by a normal photolithography technique, and the Cr film 112 and the Ti film 111 are sequentially etched to remove the photoresist. This Cr
The left side of the / Ti pattern corresponds to the domain-inverted part 3, its period is 3.2 μm, and the line / space ratio is set to 2: 8 in consideration of blurring due to diffusion. The right side corresponds to the diffraction grating 5, which has a period of 3.1 μm and a line / space ratio of 1: 1. Next, as shown in FIG.
After the domain inversion part 3 is covered again with the photoresist 113, the LiNbO ₃ substrate 1 is etched by 0.1 μm by ion milling to form the diffraction grating 5. Next, FIG.
As shown in (c), the Cr film 112 and T
The i film 111 is removed by etching, and then the polarization inversion part 3 is removed.
Is covered with a photoresist to form a Ti film 111 of the domain inversion portion 3, and the photoresist is removed by etching.

Next, as shown in FIG. 11D, heat treatment is carried out in the diffusion furnace 114 to form the polarization inversion portion 3 having a depth of about 0.5 μm. In order to prevent outdiffusion of Li ₂ O during the heat treatment, argon gas Ar and oxygen gas O _{2 which} have been passed through warm water heated to 80 ° C. in the atmosphere of the diffusion furnace 114
And heat treatment conditions were 1070 ° C. and 45 minutes. Further, the Ti film 111 is formed by ion implantation at room temperature, and then heat treatment is performed, whereby the precision of the domain inversion portion 3 can be further improved. The same effect can be obtained by using a transition metal such as Ta or Cr in addition to Ti. The polarization inversion part 3 can also be manufactured by an ion exchange method in which lithium ions in LiNbO ₃ are replaced with alkali ions such as H, Na, K, Rb and Cs.

Further, the polarization inversion part 3 can be manufactured by the proton exchange method. In the proton exchange method,
A 100 nm Cr film is formed on the substrate 1 by sputtering, and the Cr film is patterned by the usual photolithography technique and etching technique. Next, a 100 nm Cr film is formed in the region of the diffraction grating 5 to prevent its proton exchange. Then, the substrate 1 is immersed in a quartz container obtained by heating and melting the benzoic acid powder, and a proton exchange treatment is performed to form a proton exchange layer having a depth of about 0.1 μm. The treatment temperature at this time was 200 ° C. and the treatment time was 15 minutes.
There is no problem if it is between 0 ℃ and 220 ℃, and 200
At 220 ° C, the proton exchange rate can be increased. Further, if the temperature is 220 ° C. or higher, the substrate surface is roughened in the subsequent heat treatment step, which is not preferable. Moreover, the proton exchange rate can be freely changed by adding 0.1 to 5 mol% of lithium benzoate to benzoic acid. next,
After cooling the substrate, ultrasonic cleaning is performed in ethanol to remove the Cr film. Next, as in FIG. 11 (b),
The domain-inverted portion 3 is covered with a photomask, and the LiNbO ₃ substrate is etched by 0.1 μm ion milling. Finally, the photoresist is removed, the Cr film is removed by etching, and heat treatment is performed at 1030 ° C. for 15 minutes in a heat treatment furnace to form a domain-inverted portion 3 having a depth of about 1 μm.

Each of the above methods, that is, FIGS. 11A to 11D.
11 or after the polarization inversion grating 3 is formed on the substrate 1 by the above method, the ion exchange method, or the proton exchange method.
Go to step (e). In FIG. 11E, the single crystal thin film liquid layer 2 is epitaxially grown on the substrate 1 on which the domain inversion lattice 3 is formed. The melt for the liquid layer epitaxial growth is 20 mol% L which is the material of the optical waveguide 4.
A flux material of iNbO ₃ and 80 mol% lithium borate Li ₂ B ₂ O ₄ was used as a raw material, and lithium carbonate Li
₂ CO ₃ , boric acid H ₃ BO ₃ , niobium pentoxide Nb ₂ O ₅ , magnesium oxide MgO, and other powders were mixed, mixed thoroughly, and then placed in a platinum crucible, and heated in an electric furnace in an atmosphere of oxygen and water vapor for 1,200. It is manufactured by heating at ℃ for 3 hours. Next, this melt is cooled to 800 ° C. at a cooling rate of 30 ° C./h, and a 5 mol% MgO-doped ZcutLiNbO ₃ single crystal substrate 1 having a + c plane optically polished is immersed therein. Next, the substrate 1 is taken out and gradually cooled to room temperature in an electric furnace at a cooling rate of 30 ° C./h, and 1 mo is left on the substrate 1.
A 1% MgO doped LiNbO ₃ thin film 2 is epitaxially grown to a thickness of 2.5 μm. As shown in FIG.
The polarization inversion direction formed in (d) is transferred,
A rectangular domain inversion part 3 having a high aspect ratio is formed.
The amount of the flux material added is 70 to 90 mol.
%, The film thickness is 0.5 to 3 μm for the immersion time.
Then, it is 10 to 30 minutes. Further, as the flux material, in addition to lithium borate, lithium fluoride Li
It is also possible to use F, potassium fluoride KF, vanadium pentoxide V ₂ O ₅ or the like. Next, the thin film 2 is annealed in an oxygen atmosphere containing water vapor to compensate for the oxygen deficiency.

Next, the optical waveguide 4 is manufactured. First, Fig. 1
As shown in FIG. 1 (f), Ta of 100 nm is formed on the thin film 2.
A film 115 is formed by sputtering, a photoresist 116 is applied on the film 115, and the photoresist 116 is patterned by a photomask 117 having an opening in the optical waveguide 4. Next, as shown in FIG. 11G, the Ta film 115 is etched by reactive ion etching using the photoresist 116 as a mask to form a groove having a width of 3 μm. Next, the photoresist 116 is removed, and the optical waveguide 4 is formed by the same proton exchange method as described above. However,
1 mol% of lithium benzoate is added to benzoic acid, and heat treatment is performed at a heat treatment temperature of 220 ° C. for 15 minutes.
Next, after cooling the substrate 1, Ta115 is removed by ultrasonic cleaning in sodium hydroxide, and 45 minutes in the atmosphere.
The optical waveguide 4 is manufactured by annealing at 400 ° C. It should be noted that when the annealing temperature is 425 ° C. or higher, the proton exchange waveguide 4 disappears due to thermal decomposition, and when the annealing temperature is 370 ° C. or lower, the heat treatment time becomes long and the light propagation loss increases. Is 370-425
The temperature is preferably in the range of 30 to 60 minutes.

Finally, as shown in FIG. 11 (h), a SiO ₂ thin film of about 0.1 μm is formed as a protective layer 6 by plasma CV.
Deposit by the D method. For the optical waveguide 4 manufactured in this manner, a wavelength 83 polarized in a direction perpendicular to the substrate surface is obtained.
When a 0 nm Ti: sapphire laser beam was incident and the optical propagation loss was measured, a good value of 0.5 dB / cm was obtained. The reason for this is that the thin film 2 having a quality very close to the stoichiometric composition could be grown by liquid phase epitaxial growth, and the optical waveguide 4 was formed by the proton exchange method.
It is based on the reduction of the propagation loss of each. Next, an Al film having a predetermined thickness is formed by the vapor deposition method or the sputtering method, the electrode 17 is formed by the photolithography technique and the etching technique, the element is cut out, and the surface perpendicular to the optical waveguide 4 is optically polished. Finally, a coating film 12 that allows only the fundamental wave to pass therethrough and reflects the second harmonic wave is provided on the end face of the polarization inverting portion 3, and a coating film that transmits the second harmonic wave to the end face of the diffraction grating 5. 13 is provided and mounted on the holder-16, the semiconductor laser 7 is similarly mounted and wired, other parts are also mounted to adjust the optical axis, and the collimator lens 14 is attached to the inside of the waveguide. Complete the resonant SHG light source.

FIG. 12 is a side sectional view of a waveguide internal resonance type SHG light source showing a second embodiment of the present invention. In this embodiment, balanced phase matching is used as the phase matching method.
In FIG. 12, 121 is a KTP substrate, 122,
Reference numeral 123 is a refractive index segmented waveguide formed by the Rb (rubidium) ion diffusion method and having different refractive indexes. All other parts are the same as in the first embodiment shown in FIG. In this embodiment, in order to suppress reflection from the refractive index segment, the waveguides 122 and 1 having different refractive indexes are used.
It is necessary to reduce the difference in the refractive index of the portion 23. FIG.
Is a waveguide internal resonance type SH showing a third embodiment of the present invention.
It is a sectional side view of a G light source. In this embodiment, phase matching is achieved by periodically reducing the nonlinear optical coefficient. In FIG. 13, 131 is, for example, LiNb
This is an O ₃ substrate, and 132 and 133 respectively indicate a proton-exchanged portion and a non-proton-exchanged portion. All other parts are the same as in the first embodiment shown in FIG. The nonlinear optical coefficient of the proton-exchanged portion 132 is about half that of the non-proton-exchanged portion, which allows quasi phase matching to be achieved.

FIG. 14 shows an application example of the present invention, which is an application when the light sources of the first to third embodiments are used for recording or reproduction of an optical disk. In FIG. 14, 1
41 is a DC voltage applying means, 142 is a light source according to the present invention,
143 is a second harmonic collimating lens, 144 is a beam splitter, 145 is a mirror, 146 is an objective lens, 1
Reference numeral 47 is an optical disk, and 148 is a photodetector. With the signal to be recorded on the optical disk 147, that is, the signal applied by the DC voltage applying means 141, the light source 1 of the present invention
42 is modulated. The modulation method is performed by applying a predetermined voltage to the electrode 17 according to the signal, changing the oscillation frequency of the laser light source, and forcibly breaking the phase matching condition, as shown in the first embodiment. . The modulated optical beam emitted from the end face of the waveguide 4 is a second harmonic collimating lens 143, a beam splitter 144, and a mirror 14.
5 and the objective lens 146 to form a spot, and a row of pits is formed on the optical disc 147 according to the modulation signal. When reproducing this, the reflected light from the disk 147 is branched by the beam splitter 144 as an optical power lower than that at the time of recording, and the photodetector 148 is used.
The light is received by and the original modulated signal is demodulated. FIG. 15 shows another application example of the present invention, which is an application when the light sources of the first to third embodiments are used as a light source for a laser beam printer. In FIG. 15, reference numeral 151 denotes a rotary polygon mirror, and 1
52 is a photosensitive drum, 141 is a DC voltage applying means, 142
Is a light source according to the present invention, 143 is a second harmonic collimating lens, and 146 is an objective lens. The modulated light is condensed by the collimating lens 143, the light beam is swung left and right by the rotary polygon mirror 151, and a spot is formed on the photosensitive drum 152 by the scanning lens 146. A toner is applied to the latent image formed on the photosensitive drum 152 by the modulation signal, and the toner is transferred to a sheet and recorded.

[0026]

As described above, according to the present invention,
Since a diffraction grating can be added to the polarization inversion section to separate the second harmonic wave generating section and the fundamental wave reflecting section, it is possible to design each of them optimally. Moreover, since the oscillation frequency of the semiconductor laser is controlled by the voltage applied to the electrodes,
It is possible to tune the oscillation frequency of the semiconductor laser to the second harmonic generation frequency or, conversely, directly modulate the intensity of the second harmonic light. Further, as the output of the semiconductor laser is improved, the generation efficiency of the second harmonic can be significantly increased as compared with the conventional method.

[0027]

[Brief description of drawings]

FIG. 1 is a waveguide internal resonance type SH showing an embodiment of the present invention.
It is a sectional side view and a top view of a G light source.

FIG. 2 is an explanatory diagram of the principle of phase matching.

FIG. 3 is a perspective view of a second harmonic generation element using a conventional birefringence phase matching method.

FIG. 4 is a perspective view of a second harmonic generation element using a conventional Cherenkov phase matching method.

FIG. 5 is a perspective view of a second harmonic generation element using a conventional polarization inverter.

FIG. 6 is a perspective view of a conventional second harmonic generation element in which the width of a waveguide is modulated.

FIG. 7 is a configuration diagram of a conventional polarization inversion grating resonance type second harmonic generation element.

FIG. 8 is a diagram showing a relationship between a change in effective refractive index and efficiency in FIG.

9 is a diagram showing the relationship between the polarization inversion period, the diffraction grating period and the semiconductor laser wavelength in FIG.

10 is a diagram showing the relationship between the reflectance of the diffraction grating in FIG. 1 and the deviation from the Bragg condition.

FIG. 11 is a waveguide internal resonance type S showing an embodiment of the present invention.
It is a manufacturing-process figure of a HG light source.

FIG. 12 is a side sectional view of a waveguide internal resonance type SHG light source showing a second embodiment of the present invention.

FIG. 13 is a side sectional view of a waveguide internal resonance type SHG light source showing a third embodiment of the present invention.

FIG. 14 is a perspective view showing an application example of the present invention when a light source is used for an optical disc.

FIG. 15 shows another application example of the present invention, and is a configuration diagram when a light source is applied to a laser beam printer.

[Explanation of reference numerals] 1 substrate 2 LiNbO ₃ single crystal thin film 3 polarization inversion part 4 optical waveguide 5 diffraction grating 6 protective film 7 semiconductor laser 8 reflector 9 collimating lens system 10 condensing lens system 11 polarizing plates 12, 13 Coating film 14 Collimating lens 15 Second harmonic component 17 Electrode 111 Ti film 112 Cr film 113 Photoresist 115 Ta film 117 Photomask 121 KTP substrate 122, 123 Refractive index segmented waveguides 131 LiNbO ₃ substrate 132 , 133 Proton-exchanged part and non-proton-exchanged part 141 DC voltage applying means 142 Light source of the present invention 143 Second harmonic collimating lens 144 Beam splitter 145 Miller 146 Objective lens 147 Optical disc 148 Photodetector 151 Rotation polyhedral Mirror 152 photosensitive drum

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takumi Tada 1-280, Higashi Koigokubo, Kokubunji, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd.

Claims (10)

[Claims] 1. A fundamental wave component of semiconductor laser light is incident on an optical waveguide having a polarization inversion section in which the direction of spontaneous polarization is periodically inverted in an optical substrate having spontaneous polarization.
In the SHG light source for converting into a harmonic, a diffraction grating for reflecting the fundamental wave component of the semiconductor laser light to the semiconductor laser is provided in the optical waveguide on the second harmonic output side of the polarization inversion section. At the same time, a pair of electrodes for adjusting the refractive index of the optical waveguide by providing a direct current voltage through an optically transparent thin film is provided on the diffraction grating. Type SHG light source. 2. A fundamental wave component of semiconductor laser light is incident on an optical waveguide having a polarization inversion part in which the direction of spontaneous polarization is periodically inverted in an optical substrate having spontaneous polarization.
In the SHG light source for converting into a harmonic, a diffraction grating for reflecting the fundamental wave component of the semiconductor laser light to the semiconductor laser is provided in the optical waveguide on the second harmonic output side of the polarization inversion section. At the same time, a pair of electrodes for adjusting the refractive index of the optical waveguide by applying a DC voltage through an optically transparent thin film is provided on the diffraction grating, and the semiconductor substrate of the optical substrate is provided. Means for transmitting only the fundamental wave of the semiconductor laser light and reflecting the second harmonic of the semiconductor laser light between the end face on which the laser light is incident or between the optical substrate and the semiconductor laser. And an end surface of the optical substrate opposite to the end surface on which the semiconductor laser light is incident,
A waveguide internal resonance type SHG light source, characterized in that means for reflecting the fundamental wave of the semiconductor laser light is provided. 3. The waveguide internal resonance type SHG light source according to claim 1 or 2, wherein the diffraction grating has a refractive index in an optical substrate instead of the optical waveguide on the second harmonic output side of the polarization inversion section. Is provided in the optical waveguide on the second harmonic output side of the refractive index modulation section in which a plurality of different regions are periodically repeated. 4. The waveguide internal resonance type SHG light source according to claim 1, wherein the diffraction grating has a spontaneous polarization in place of the optical waveguide on the second harmonic output side of the polarization inversion section. A waveguide internal resonance type SHG light source, which is provided in an optical waveguide on a second harmonic output side of a periodically-polarized spontaneous polarization modulator. 5. The waveguide internal resonance type SHG light source according to claim 1, wherein the optical substrate comprises lithium niobate (LiNbO ₃ ), and magnesium doped lithium niobate (MgO). : LiNbO ₃ ), lithium tantalum niobate (LiTaNbO ₃ ), or lithium tantalate (LiTaO ₃ ), which is characterized by being formed in the waveguide internal resonance type SH.
G light source. 6. The waveguide internal resonance type SHG light source according to claim 1, wherein the optical waveguide is provided in a ferroelectric thin film having the same spontaneous polarization direction as the optical substrate, and The diffraction grating has a periodically etched structure, and the waveguide internal resonance type SH is characterized.
G light source. 7. The waveguide internal resonance type SHG light source according to claim 1, wherein the semiconductor laser light is applied to an end surface of the semiconductor laser opposite to the optical substrate. A waveguide internal resonance type SHG light source, characterized in that a reflection means is provided. 8. The waveguide internal resonance type SHG light source according to claim 1, wherein the polarization inversion section and the diffraction grating have the same grating period and are arranged on the diffraction grating. The waveguide internal resonance type SHG light source, wherein the pair of electrodes are arranged in parallel with the light propagation direction of the optical waveguide. 9. The waveguide internal resonance type SHG light source according to claim 1, wherein an electric field obtained by applying a voltage between the pair of electrodes is a refraction of a diffraction grating below the electrodes. A waveguide internal resonance type SHG light source, characterized in that the Bragg wavelength of the diffraction grating is matched with the wavelength of the SH light oscillated by the polarization inversion grating by controlling the index. 10. The waveguide internal resonance type SHG light source according to claim 1, wherein an electric field obtained by applying a voltage between the pair of electrodes is a refraction of a diffraction grating below the electrodes. The waveguide internal resonance type SHG characterized in that the intensity of the output second harmonic is modulated by shifting the Bragg wavelength of the diffraction grating with respect to the wavelength of the SH light oscillated by the polarization inversion grating by controlling the ratio. light source.