JPH1051072A - Semiconductor laser device having reflection preventive film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To materialize low reflectance independent of the dispersion in manu facture of silicon nitrides. SOLUTION: A reflection preventive film is composed of the silicon nitride 101 of the first layer where the refractive index is 1.82 to 2.00 and the thickness of an optical film is seventeen-hundredths to twenty three-hundredths as large as the oscillated wavelength and the silicon oxide 102 of the second layer where the thickness of the optical film is three-hundredths to fifteen-hundredths as large as the oscillated wavelength. Here, selecting the silicon nitride and the silicon oxide of such film thickness and refractive indexes that the reflectance becomes minimum to the laser oscillating wavelength will enable the reflectance to be under 0.6% even if the dispersion in manufacture of silicon nitrides and silicon oxides occurs. This way, a reflection preventive film which has stable low reflectance independent of the refractive index and the film thickness of the silicon nitride 101 is provided, and the yield in single vertical mode of a DFB (distributed feedback type) can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device for transmitting information used for optical communication and the like.

[0002]

2. Description of the Related Art Today, as optical communication systems are expanding on a worldwide scale, semiconductor lasers with a high production yield are required for lasers for optical communication in order to reduce costs. In medium- and long-distance optical communications, distributed feedback (DF) with a diffraction grating adjacent to the active layer to obtain single longitudinal mode oscillation
B) A laser is used. However, in a DFB laser having a uniform diffraction grating having the same period, whether the laser oscillates in a single longitudinal mode or has two modes is determined by the phase of the diffraction grating at the end face. Generally, the grating period is 200 to 2
Since it is about 40 nm, the phase of the end face of the diffraction grating cannot be controlled with the accuracy of the current cleavage technology. Therefore, the ratio of oscillation in the single longitudinal mode is determined stochastically. This probability also varies greatly depending on the reflectivity of the laser end face. FIG. 2 shows the dependence of the single longitudinal mode yield obtained by calculation on the reflectance of the front end face. As shown in the drawing, the yield increases as the reflectance of the front end face decreases. Regarding the anti-reflection film, he described “Dependence of the error rate characteristic of DFB laser on the facet reflectivity” (The IEICE Fall National Convention C, 1988)
-153), silicon nitride is generally used. FIG. 3 shows a perspective view of a conventional example. FIG.
Reference numeral 302 denotes a crystal part of the semiconductor laser, and a silicon nitride thin film 301 is formed on the emission end face thereof. Reference numeral 305 denotes a stripe electrode, and reference numerals 303 and 305 denote the other electrode. However, the reflectance of silicon nitride is at least about 1%, and furthermore, the ratio of nitrogen in silicon nitride is not constant and changes with each production, so that the refractive index varies by about 5%, and the film thickness also varies. Each time, the reflectivity changes by about 5%, so that the reflectivity often becomes higher than 1%, which hinders the improvement of the yield of the single longitudinal mode oscillation of the DFB laser.

A modulator-integrated laser light source in which a semiconductor laser and an electro-absorption modulator are integrated is used for long-distance communication because of its excellent characteristics. . In other words, there is a problem that if there is a reflection of 0.05% or more, the reflected light intensity-modulated by the electroabsorption modulator returns to the laser unit, and the oscillation wavelength of the laser unit fluctuates. In order to solve this, a configuration is generally used in which a window region is formed in a part of the front end face of the light emission, and the reflectance is reduced by a combination of the antireflection film and the window region. In this configuration, the amount of the antireflection film on the end face of the window region needs to be 0.2% or less, and it is difficult to form the antireflection film using a silicon nitride single layer film.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an antireflection film having a structure realizing a low reflectance independent of manufacturing variations of silicon nitride, and to realize a single longitudinal mode with a high yield in a DFB laser. It is in. In addition, an anti-reflection film that realizes a stable low reflectivity is provided on the front end face of the window region of the modulator integrated laser light source, and a modulator integrated laser light source having good high-frequency characteristics without returning light to the laser unit is provided. Is to make it happen.

[0005]

The above object is achieved by the following means.

As shown in FIG. 1, at least a silicon nitride thin film 101 and a silicon oxide thin film 102 are sequentially formed on a light emitting end face of a semiconductor laser crystal part 103. In FIG.
4 is an upper electrode, 105 is a stripe, and 106 is a lower electrode. As described above, the refractive index of the silicon nitride film varies every time it is manufactured, but the refractive index of the silicon oxide film is almost constant at 1.45. The refractive index of silicon nitride, which may vary, is used as the average value of those formed by the film forming apparatus, and the reflectance of laser light emitted from the end face of the semiconductor laser to the outside through the dielectric thin film is minimized. When the thicknesses of silicon nitride and silicon oxide are determined, and the antireflection film is formed with these film thicknesses, the reflectance is reduced to 0.
Does not exceed 6%. Table 1 shows examples of the dependency of the reflectance on the film thickness and refraction variation. This is an example in which the reflectance of the present invention is applied to a 1.3 μm band DFB laser. The calculation of the reflectivity used the effective Fresnel method which requires an exact solution. The effective refractive index of the semiconductor laser was 3.241, and the refractive index of silicon oxide was calculated as a constant. From the table, even if the film thickness and refractive index of silicon nitride change by 5% and the film thickness of silicon oxide changes by 5%, the reflectivity remains unchanged.
It turns out that it is less than 0.55%. The above-mentioned problem is solved by the above means.

[0007]

[Table 1]

In order to further reduce the reflectivity irrespective of manufacturing variations, the following means are used. First, the silicon nitride of the first layer and the second
The thickness of the silicon oxide layer is determined. A first layer of silicon nitride is formed on the end face of the semiconductor laser with a thickness obtained by calculation. After the film formation, the thickness and the refractive index of the silicon nitride are measured using an ellipsometer or the like. Next, the film thickness of the silicon oxide in the case of the reflectance which minimizes the measured value as a constant is calculated again by the effective Fresnel method of the exact solution or the vector method which is the approximate solution. The object of the present invention is achieved by forming the calculated film thickness of the second layer silicon oxide on the first layer silicon nitride. Also, simply after silicon nitride deposition,
It is also possible to measure the optical film thickness by measuring the reflectance and to correct the film thickness of the second layer silicon oxide by using the average refractive index of silicon nitride.

[0009] In order to quantify the manufacturing variation of the refractive index of the silicon nitride thin film, the refractive index of the silicon nitride thin film was statistically investigated. The result is shown in FIG. From FIG. 4, the average refractive index of the silicon nitride thin film is 1.91 and the standard deviation is 2σ = 0.091.
Met. Thus, it can be seen that the silicon nitride thin film varies from 1.82 to 2.00. The effective refractive index in the stripe of the semiconductor laser is 3.1 to 3.27 depending on the oscillation wavelength.
To change. When the refractive index changes in such a range, the value obtained by dividing each optical film thickness of the two-layer film of the silicon nitride thin film and the silicon oxide thin film at the time of obtaining the minimum reflectance by the laser oscillation wavelength is obtained. FIG. 5 shows the result calculated for the range that can be taken. Here, the optical film thickness is obtained by multiplying the refractive index of the thin film by the film thickness. From Fig. 5, the silicon nitride thin film has an optical thickness of 0.17 times the oscillation wavelength of the laser.
It was found that the thickness of the silicon oxide thin film varied from 0.03 times to 0.15 times the laser oscillation wavelength at the optical thickness. Therefore, calculation may be performed by the effective Fresnel method or the vector method within this range, and a combination of the optical thickness of the silicon nitride thin film and the silicon oxide thin film having the minimum reflectance may be selected. The minimum reflectance within this range is 1 × 10 ⁻³ % or less, which is sufficient for an antireflection film.

[0010]

FIG. 6 shows a first embodiment of the present invention.
Shown in In FIG. 6, reference numerals 605 and 608 denote an upper electrode and a lower electrode of the semiconductor laser, respectively. The laser structure may be well known in the art, for example using an embedded DFB laser 603 on a p-type InP substrate. The oscillation wavelength is 1.3 μm. The active layer has a multiple quantum well structure of InGaAsP having a compressive strain in the well layer. The effective refractive index of the stripe including the active layer 607 was 3.241. A diffraction grating 604 having a period of 202.0 nm is provided adjacent to the active layer. Rear end face 606
Has a reflective coating made of amorphous silicon and silicon oxide with a reflectance of 70%. Front end face 60
Reference numerals 1 and 602 are antireflection films according to the present invention. According to the calculation of the effective Fresnel method, the film thicknesses of silicon nitride and silicon oxide at the minimum reflectance are 0.134 μm and 0.087 μm, respectively.
m. At this time, the refractive index of silicon nitride was 1.901. Based on this calculation, the first
A layer silicon nitride thin film and a second layer silicon oxide thin film were formed. When the film thickness and the refractive index of the first layer and the second layer were measured after the film formation, the silicon nitride film had a refractive index of 1.940 and a film thickness of 0.129 μm.
As a result, the silicon oxide film had a refractive index of 1.45 and a thickness of 0.085 μm. The reflectivity was 0.09% by recalculation and 0.1% by measurement. The yield of single longitudinal mode oscillation of the semiconductor DFB laser using the antireflection film was as high as 44%.

FIG. 6 shows a second embodiment according to the present invention.
The laser structure may be well known in the art, for example, n-type In.
A ridge type DFB laser 603 on a P substrate is used. The oscillation wavelength is 1.55 μm. Active layer has compressive strain in well layer
It is a multiple quantum well structure of InGaAsP. The effective refractive index of the multilayer structure including the active layer was 3.195. A diffraction grating 604 having a period of 241.0 nm is provided adjacent to the active layer. The rear end face 608 is provided with a reflective coating made of amorphous silicon and silicon oxide with a reflectivity of 90%. The front end faces 601 and 602 are antireflection films according to the present invention. According to the vector calculation, the film thicknesses of silicon nitride and silicon oxide at the minimum reflectance are 0.151 μm and 0.118 μm, respectively.
m. Based on this calculation, a first-layer silicon nitride thin film was formed by an RF sputtering method. After the formation of silicon nitride, the refractive index and the film thickness were measured by an ellipsometer and found to be 1.853 and 0.162 μm, respectively. Based on this measured value, the reflectance which becomes the minimum when silicon oxide was applied to silicon nitride by the vector method was determined again, and a film thickness of 0.101 μm was obtained. Next, silicon oxide was formed on the silicon nitride with this thickness. As a result, the reflectance of the antireflection film was 0.18%. The yield of single longitudinal mode oscillation of the semiconductor laser provided with the antireflection film of the present invention was as high as 62%.

FIG. 7 shows a third embodiment according to the present invention.
FIG. 7 is a schematic view of a modulator integrated laser light source on an n-type InP substrate. In FIG. 7, reference numerals 705 and 708 denote an upper electrode and a lower electrode of the semiconductor laser portion of the integrated laser light source, respectively, and 706 denotes a rear end face reflection film of the semiconductor laser portion. The oscillation wavelength of the laser unit 703 is 1.55 μm. The active layer 707 has a multiple quantum well structure of InGaAsP. 704
A single oscillation mode is obtained by the diffraction grating. In the modulator integrated laser light source, the laser unit always emits laser light, and the modulator 709 in front of the laser unit modulates the laser light at high speed. The multiple quantum well layer 710 in the modulator is manufactured so that the energy band gap is larger than that of the multiple quantum well layer in the laser unit. When a reverse voltage is applied to the electrode 711 of the modulator, the laser light is absorbed by the modulator due to the quantum confined Stark effect, and the laser light does not go out. When no voltage is applied to the modulator upper electrode 711, the laser light is output to the outside without being absorbed by the modulator. 7
Reference numeral 12 denotes an InP window region, and reference numerals 701 and 702 denote antireflection films according to the present invention. The refractive index of the InP window region is 3.17.
According to the calculation of the effective Fresnel method, the film thickness of silicon nitride and silicon oxide at the time of the minimum reflectance is 0.158 μm, respectively.
0.108 μm. At this time, the refractive index of silicon nitride is 1.8
99 was used. Based on this calculation, a first-layer silicon nitride thin film and a second-layer silicon oxide thin film were formed by RF sputtering. When the film thickness and the refractive index of the first layer and the second layer were measured after the film formation, the silicon nitride film had a refractive index of 1.870 and a film thickness of 0.2.
At 150 μm, the silicon oxide film had a refractive index of 1.45 and a thickness of 0.112 μm. Reflectivity is 0.09% by recalculation and 0.11 by measurement
%. The reflectivity including the window area is less than 0.03%, and the high frequency response characteristic of this modulator integrated laser light source is 1
Flat and good characteristics were obtained up to 3 GHz. In addition, the transmission speed 2.
The maximum transmission distance to a normal dispersion fiber at 5 Gb is from 200 km using a single-layer film of silicon nitride to 600 km.
It increased above.

[0013]

According to the present invention, a stable anti-reflection film (reflectivity <0.6%) regardless of manufacturing variations of silicon nitride.
Can be easily manufactured, and the yield of single longitudinal mode oscillation of the DFB laser can be improved. In the modulator integrated laser light source, return light can be reduced, and high frequency characteristics can be easily improved.

[Brief description of the drawings]

FIG. 1 shows an antireflection film on a semiconductor laser end face according to the present invention.

FIG. 2 is an example of a calculated value of a front end face reflectance dependency of a single longitudinal mode yield of a DFB laser.

FIG. 3 is a schematic view of a conventional example.

FIG. 4 is a diagram showing manufacturing variations in the refractive index of a silicon nitride thin film.

FIG. 5 shows the dependency of the optical film thickness / laser oscillation wavelength on the silicon nitride thin film when the silicon nitride thin film and the silicon oxide thin film have the minimum reflectance.

FIG. 6 is a schematic diagram according to first and second embodiments according to the present invention.

FIG. 7 is a schematic diagram according to a third embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 101 silicon nitride thin film 102 silicon oxide thin film 103 semiconductor laser 104 upper electrode 105 stripe 106 lower electrode 301 silicon nitride thin film 302 semiconductor laser 303 upper electrode 305 stripe 306 lower electrode 601 silicon nitride thin film 602 silicon oxide thin film 603 semiconductor laser (p-type InP substrate) Top embedded type DFB laser or ridge type DFB laser on n-type InP substrate 604 Diffraction grating (202.0 nm or 241.0 nm) 605 Upper electrode 606 Rear end reflection film 607 Active layer (effective refractive index 3.241 or 3.195) 608 Lower electrode 701 Silicon nitride thin film 702 Silicon oxide thin film 703 Semiconductor laser unit (DFB laser on n-type InP substrate) 704 Diffraction grating (202.0 nm) 705 Upper electrode 706 Rear end reflection film 707 Active layer 708 Lower electrode 709 Modulator unit 710 Modulator unit multiplex Child well layer 711 modulator section upper electrode 712 window region part.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideaki Takano 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd.

Claims (4)

[Claims] The present invention has a stripe structure for confining light and a resonator structure for obtaining a laser beam from generated light, and an effective refractive index which is an effective refractive index of the stripe portion is 3.1 to 3.27.
Semiconductor laser comprising: a first and a second dielectric thin film sequentially formed on at least one or more light emitting end faces, wherein the first dielectric thin film has an optical thickness of an oscillation wavelength of the semiconductor laser. A semiconductor laser device comprising silicon nitride of 0.17 times to 0.23 times, and a second dielectric thin film having an antireflection film made of silicon oxide. 2. The antireflection film according to claim 1, wherein the first dielectric thin film has a refractive index of 1.82 to 2.00.
2. A semiconductor laser device having an anti-reflection film according to claim 1, wherein: 3. The anti-reflection coating for a semiconductor laser according to claim 2, wherein the optical thickness of the silicon oxide of the second dielectric thin film is 0.03 to 0.15 times. A semiconductor laser device having: 4. After forming a first dielectric thin film on the antireflection film, the first and second dielectric thin films are measured from the end face of the semiconductor laser in accordance with the observed refractive index and film thickness of the first dielectric thin film. An anti-reflection thin film for a semiconductor laser, wherein the thickness of the second dielectric thin film has a corrected value when the reflectance of the laser light emitted outside through the dielectric thin film is minimized. The semiconductor laser device according to claim 1.