JPS6353717B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a multi-wavelength semiconductor laser used as a light source for optical fiber communications in the field of optoelectronics.

Conventional configurations and their problems Semiconductor lasers have traditionally held great promise as light sources for optical fiber communications, optical information processing, optical application equipment, etc., and have already been put into practical use due to longer lifespans and dramatic improvements in performance. However, most of these devices emit light of a single wavelength from a single semiconductor laser.For example, in optical fiber communications, there are economical reasons such as the high price of optical fibers and the need for larger capacity. With the aim of achieving information transmission, the realization of multi-wavelength semiconductor lasers for wavelength division multiplexing communications is a future challenge.

In order to meet the requirements for a stable single transverse mode and low threshold current, the structure of a typical semiconductor laser is designed to improve the confinement of light within the active layer by providing a step in the refractive index in the direction parallel to the junction. In general, the active layer is made of a thin film, or it is made with a device that uses current blocking layers on both sides of the active layer to improve current confinement. A conceivable way to achieve a multi-wavelength semiconductor laser is to provide several active layers with different band gaps, but in a semiconductor laser with the structure described above, band gaps may be formed in parallel in the direction parallel to the junction. It is impossible to provide active layers with different band gaps due to crystal growth, and therefore active layers with different band gaps must be provided in the growth direction perpendicular to the junction. Figure 1 shows InP substrate 3
InGaAsP active layer 1 with different band gaps on top,
2 is a schematic cross-sectional view of a conventional example of a two-wavelength semiconductor laser having a double DH structure by providing two layers. 3 is a substrate, and 4, 5, and 6 are cladding layers. Two laser beams with different wavelengths generate a current I ₁ between terminals T ₃ - _{T 1} ,
By passing current I ₂ between terminals T ₃ − _{T 2} ,
InGaAsP first active layer 1, InGaAsP second active layer 2
taken from. In this structure, although it is possible to provide active layers with different band gaps, it is difficult to provide a step in the refractive index on both sides of each active layer in the direction parallel to the junction, or to provide a current blocking layer. However, it is difficult to stabilize and unify the transverse mode and to lower the threshold current. In other words, in conventional semiconductor lasers with a structure in which laser light is extracted from a cavity surface formed using cleavage, the cleavage process is extremely complicated, and in order to achieve multiple wavelengths, it is necessary to provide several active layers with different band gaps. However, there was a problem in that it was not possible to simultaneously stabilize and unify the transverse mode and reduce the threshold current.

OBJECTS OF THE INVENTION The object of the present invention is to obtain a multi-wavelength semiconductor laser that extracts laser beams of different wavelengths from the back surface of a substrate in a stable single transverse mode and a single longitudinal mode.

Structure of the Invention This invention does not extract laser light from a resonator surface formed by cleavage, but instead forms resonator surfaces at both ends of an optical waveguide that have an angle of approximately 45° with respect to the axial direction of the optical waveguide. , the laser beam is extracted from the mirror-polished back surface of the substrate, a reflective coating film is provided on the resonator surface having an angle of approximately 45 degrees and the back surface of the substrate, and a region with a different refractive index is provided in a part of the optical waveguide. By using the above structure in which a single longitudinal mode is achieved by providing a single longitudinal mode, the position of the gain peak changes depending on the thickness of the reflective coating film. By creating an arrayed structure in the axial direction, laser beams of different wavelengths can be stably unified by simply providing one layer of active layer that stabilizes and unifies the transverse mode in order to solve the problem mentioned above. This is a multi-wavelength semiconductor laser that can be extracted from the backside of a substrate in both a transverse mode and a single longitudinal mode.

DESCRIPTION OF EMBODIMENTS The present invention will be described in detail below with reference to the drawings.

First, this invention utilizes the following two points.

[1] When a reflective coating film is provided on the resonator surface, the position of the original gain peak, that is, the emission wavelength changes depending on the film thickness.

[2] Semiconductor laser (Internal-Reflection-
Interference-Laser: Hereafter abbreviated as IRI laser)
In this case, a stable single longitudinal mode is obtained.

First, to explain [1], a semiconductor laser normally has a pair of resonator surfaces formed using cleavage, and light within an optical waveguide is repeatedly reflected at the resonator surfaces against an air layer. At this time, the optical waveguide
In the case of InGaAsP, the refractive index for light with a wavelength of 1.3 μm is 3.529, and assuming that the refractive index of the air layer is equal to 1, the reflectance is about 31.2%. For example, if a SiO ₂ film is provided on this cavity surface, the light inside the optical waveguide will be reflected at the interface between the optical waveguide and the SiO ₂ film and the interface between the SiO ₂ film and the air layer, and the reflected light will change depending on the thickness of the film. A phase difference occurs. As a result, the position of the original gain peak changes, that is, the emission wavelength changes. Figure 2 shows, in the direction parallel to the junction, SiO ₂ /Au, SiO The relationship between the film thickness and the position of the gain peak when a reflective coating film made of ₂ /Si is provided is shown. Here the _SiO2 film is Au and
It is used as an insulating film to prevent leakage current through Si. Also, SiO ₂ for light with a wavelength of 1.3 μm
has a refractive index of 1.47, and when only the SiO ₂ film is provided, the reflectance is less than 20%, which does not increase the gain inside the cavity compared to the 31.2% mentioned above, so one cavity surface has a reflectance. Au with a high refractive index is placed on the SiO ₂ film to achieve almost total reflection, and the other cavity surface has a high refractive index.
Si, which is approximately the same as InGaAsP (3.5), is placed on the SiO ₂ film to increase the reflectance and serve as the laser beam extraction surface. FIG. 2 is a side view of the semiconductor laser 7. In this figure, the emission spectra under the following five conditions are shown in FIG.

No reflective coating.

On one end surface of the semiconductor laser 7, an Au (~3000 Å) layer 9 is formed on the SiO ₂ (2210 Å) layer 8.

On one end surface of the semiconductor laser 7, an Au (~3000 Å) layer 9 is placed on the SiO ₂ (2210 Å) layer 8, and the semiconductor laser 7
A SiO ₂ (2210 Å) layer 10 is formed on the other end surface.

In addition to layers 8 and ₉ , Si (2210
Å) Forming layer 11.

In addition to layers 8 to 11, there is also a Si (317 Å) layer 12.
form.

In Figure 3, the position of the original gain peak is
By changing the thickness of the reflective coating film, it is possible to shift the distance from 1.289 μm by 160 Å to 1.305 μm. Note that the SiO ₂ film has a film thickness of 1/4 wavelength (wavelength
1.3 μm light is divided by its refractive index of 1.47 in SiO ₂ , and the 1/4 wavelength is 2210 Å), and the phase of the reflected light at each interface is matched, so in the case of the condition, the gain peak The position has hardly changed. This change in the position of the gain peak depending on the thickness of the reflective coating film can be solved by providing several laser beam extraction surfaces, each with a reflective coating film of different thickness. It is possible to obtain multi-wavelength light emission from

Next, to explain [2], in general, the longitudinal mode of a normal semiconductor laser causes so-called mode hopping from one longitudinal mode to the next longitudinal mode depending on the temperature, and a stable single longitudinal mode is obtained. However, with the IRI laser, the gain spectrum obtained from the original resonator has two gain peaks because it has two resonators that are divided into two regions with different refractive indexes and effectively combined. It becomes a divided form, and each has a narrower spectral width than the original gain spectrum. Therefore, each of the two divided gain peaks has a larger intensity ratio between the peak longitudinal mode and the adjacent longitudinal mode than the original gain peak, and since the two gain peaks are further apart, As a result, one of the gain peaks becomes dominant, resulting in a stable single longitudinal mode.

Figure 4A shows a 1.3μm band InGaAsP with a structure designed to stabilize the transverse mode, similar to the one used in Figure 2.
system conductor laser 8, and a part of the optical waveguide is
Regions 9 having different refractive indexes are provided by Zn diffusion, and are divided into resonator lengths L~272 μm, _L1 ~153 μm, and _L2 ~112 μm (however, the Zn diffusion region width D~7 μm).
The frequency spectrum at that time is shown in B and C. When the operating current is small, two gain peaks are obtained by the two divided resonators, but when the operating current is increased, one becomes dominant and a single longitudinal mode is achieved. Here, the interval between the two gain peaks is 64 Å, but it can be made even narrower by changing L ₁ and L ₂ . Also, Figure 5 shows the temperature characteristics of the longitudinal mode, and the wavelength change due to temperature is ~3 Å/℃ compared to a normal semiconductor laser.
It is as small as 0.77 Å/°C and achieves a single longitudinal mode without mode hopping over a temperature range of 30°C. Achieving a stable single longitudinal mode in this way makes it possible to convert each light emission into a single longitudinal mode when multi-wavelength is achieved using the reflective coating film mentioned above, and is useful for multi-wavelength semiconductor lasers in wavelength division multiplexing communications. This is what makes this happen.

FIG. 6 shows a perspective view of a 1.3 μm band InGaAsP/InP based three-wavelength semiconductor laser based on the present invention, which utilizes the two points explained above. 10 is a resonator plane having an angle of approximately 45° to the <011> direction, which is the axial direction of the optical waveguide, and a V-shaped groove in the <011> direction was formed using normal photolithography technology and a Br-CH ₃ OH solution. It is formed by etching. 11a is a SiO ₂ vapor deposited film provided on the resonator surface 10, 11b,
11c was provided on the back surface of the mirror-polished board.
SiO ₂ vapor deposited films, 12a and 12b are Au vapor deposited films provided on the SiO ₂ vapor deposited films 11a and 11b, respectively.
a, 13b, 13c are Si vapor deposited films provided on the SiO ₂ vapor deposited film 11c, and the SiO ₂ vapor deposited film and the Au vapor deposited film 11
a/12a, 11b/12b and SiO ₂ vapor deposited film and Si vapor deposited film 11c/13a, 11c/13b, 1
1c/13c to form a reflective coating film.
14 is a diffusion region formed in a stripe shape in the <011> direction by Zn diffusion, which provides a region with a different refractive index to a part of the optical waveguide and also
Divide into lengths l ₁ and l ₂ . Reflective coating film 1
1a/12a, 11b/12b, 11c/13
a, 11c/13b, 11c/13c and the diffusion region 14 are all formed using normal photolithography techniques.

Figure 7 shows line A-A' in Figure 6, that is (011)
FIG. 15 is an n-type InP substrate,
21 is an n-type InP cladding layer, 22 is an InGaAsP active layer, 23 is a p-type InP cladding layer, and 24 is a p-type
InGaAsP cap layer, 25 is a P-type InP layer, and 26 is an n-type InP layer. P-type InP layer 25, n-type InP layer 2
6 is provided with a refractive index step on both sides of the InGaAsP active layer 22 in the direction parallel to the junction, and serves as a current blocking layer to stabilize and unify the transverse mode. FIG. 8 is a sectional view taken along the line B-B' of FIG. 6, that is, the (011) plane, and shows the pattern of optical waveguide within the resonator. As can be seen from FIG. 8, by forming the resonator surface 10, the resonator is C ₁ ,
The laser beams are individually divided into C ₂ and C ₃ and are extracted from the back side of the substrate. Here, as mentioned in the explanation of [1] above, the SiO ₂ vapor deposited film 11
The reflective coating films 11a/12a, 11b/12b are provided with film thicknesses of wavelengths a, 11b, and 11c to increase the gain inside the resonator, and the Si vapor deposited films 13a, 13
When the film thicknesses of b and 13c are changed, the reflective coating film 11 is formed from the divided resonators C ₁ , C ₂ , and C ₃ .
Laser beams λ ₁ , λ ₂ , and β ₃ of different wavelengths are obtained through c/13a, 11c/13b, and 11c/13c.

Figures 9 and 10 show the resonator lengths divided by diffusion regions l ₁ ~125 μm, l ₂ ~70 μm, diffusion region width d ~7 μm, SiO ₂ deposited film ~2210 Å, and Au deposited film ~3000 Å. , Si vapor deposited film of 152 Å and 315 Å, respectively.
The gain spectrum obtained as Å, 461 Å and the temperature characteristics of the longitudinal mode are shown. In the results shown in Figure 10, the distance between the
Three wavelengths of stable single longitudinal mode light emission are obtained without mode hopping in the practical temperature range up to 45℃. This has sufficient performance for wavelength division multiplexing communication in optical fiber communication,
It also has unprecedented characteristics in temperature characteristics and mode stability.

In addition, when forming the resonator surface 10 in the above-mentioned integrated semiconductor laser, by etching the V-shaped groove deeply enough, there is no need to use conventional cleavage when making chips, which also improves mass production efficiency. It is excellent. In addition, the material of the reflective coating film is SiO ₂ ,
In addition to Au and Si, it is also possible to control the thickness of the reflective coating film other than the back surface of the substrate.Furthermore, the method of providing regions with different refractive indexes in a part of the optical waveguide is not limited to diffusion. A material with a smaller refractive index (for example, InP in an InGaAsP optical waveguide) may be introduced.

Effects of the Invention As explained above, the present invention provides a resonator surface having a reflective coating film at both ends of an optical waveguide formed with a single transverse mode in the direction parallel to the junction, with respect to the axial direction of the optical waveguide. A reflective coating film is also provided on the back surface of the mirror-polished substrate, forming a resonator structure in which the laser beam is extracted from the back surface of the substrate, and a part of the optical waveguide has a different refractive index. By changing the thickness of the reflective coating film and arranging the above structure in the axial direction of the optical waveguide, laser light with different wavelengths can be transmitted from the back side of the substrate in a stable single transverse mode and single longitudinal mode. It has a removable effect.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the schematic configuration of a conventional two-wavelength semiconductor laser, Figure 2 is a diagram showing the conditions for providing a reflective coating film on the cavity surface, and Figure 3 is a diagram showing the conditions and the position of the gain peak. Characteristic diagrams showing the relationship; Figures 4A to 4C are gain spectrum diagrams when regions with different refractive indexes are provided in a part of the optical waveguide;
5 is a temperature characteristic diagram of the longitudinal mode of the semiconductor laser in FIG. 4, FIG. 6 is a perspective view of a three-wavelength semiconductor laser which is an embodiment of the present invention, and FIG. 7 is a line A-A' in FIG. 6. 8 is a sectional view taken along line B-B' in FIG. 6, FIG. 9 is a gain spectrum diagram of a three-wavelength conductor laser based on the present invention,
FIG. 10 is a temperature characteristic diagram in the longitudinal mode. 10... Resonator surfaces having an angle of approximately 45 degrees with respect to the axial direction of the optical waveguide, 11a, 11b, 11c...
...SiO ₂ vapor deposited film, 12a, 12b...Au vapor deposited film,
13a, 13b, 13c...Si vapor deposited film, 14...
Zn diffusion region, 15... n-type InP substrate, 21... n
InP cladding layer, 22... InGrAsP active layer, 2
3...P-type InP cladding layer, 24...P-type
InGaAsP cap layer, 25...P-type InP layer, 26
...n-type InP layer.

Claims (1)

[Claims] 1. A resonator surface provided with a reflective coating film at both ends of an internal striped optical waveguide formed with a step in the refractive index in a direction parallel to the junction is placed at an angle of approximately 45° with respect to the axial direction of the optical waveguide. A reflective coating film is also provided on the back surface of the substrate, which has been formed and mirror-polished, to create a resonator structure that extracts the laser beam from the back surface of the substrate.Furthermore, regions with different refractive indexes are provided in a part of the optical waveguide to form a single layer. By arraying the above structures in the axial direction of the optical waveguide and controlling the thickness of the reflective coating film of each laser, laser beams with different wavelengths can be converted into stable single transverse mode and single longitudinal mode. A multi-wavelength semiconductor laser that is extracted from the backside of the substrate using a mode.