JP2917950B2 - Tunable semiconductor laser and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable semiconductor laser used for optical communication, optical information processing, optical measurement, and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art With the increase of digital information due to the spread of the Internet and multimedia services in recent years, the capacity required for transmission systems has been steadily increasing. Such a high-speed signal is approaching the limit of an electric circuit even if it is processed by a normal switching device, and a method of exchanging in a light state is being studied energetically.

[0003] There are two main types of systems for exchanging light. One is a method using an optical switch, and the other is a method for changing an output port by changing the wavelength of light using a wavelength filter. The latter does not use any electrical processing, so that high-speed processing can be expected.
For such wavelength exchange, a wavelength tunable light source capable of performing wavelength switching at high speed and capable of performing a wavelength tunable operation over a wide range is required. Since a wavelength tunable semiconductor laser can control the oscillation wavelength with a small control current, it is expected as a key device in such a wavelength switching system,
DBR-LD (Distributed Bragg Reflecto)
r-Laser Diode) and SSG-DBR-LD (Super Struc
ture Grating-Distributed Bragg Reflector-Laser Dio
Many structures have been proposed so far, such as de).

A three-electrode DBR-LD has an active region, a phase control region, and a DBR region divided in the resonator direction, and a diffraction grating is formed only in the DBR region (S.
Murata et al., Electronics letters, 23, 403 (1987)).
By injecting a current into the DBR region, the refractive index here is changed by the plasma effect, and the oscillation wavelength is roughly adjusted. Fine tuning of the oscillation wavelength is performed by injecting a current into a phase control region where no diffraction grating is formed. In the same structure, DBR
By independently controlling the injection of current into the region and the phase control region, a wavelength tunable operation without mode jump is possible, and a quasi-continuous wavelength tunable operation at 720 GHz (5.8 nm) has been reported.

[0005] To expand the wavelength variable width, an SSG-D using a super-periodic structure diffraction grating whose pitch changes periodically is introduced.
BR-LD is promising (Y. Tohmori et al., IEEE Phot
onics Technology Letters, 5 (2), 126 (1993)). In this method, a super-periodic structure diffraction grating is formed in a passive waveguide portion on both sides of an active layer region, and a wavelength tunable operation is realized by changing a current flowing through the passive waveguide portion.
Up to now, a wavelength tunable operation exceeding a maximum of 100 nm has been realized as a single wavelength tunable semiconductor laser.

[0006]

However, in the three-electrode DBR-LD, a quasi-continuous tunable operation can be realized by skillfully controlling the current flowing through the phase control region and the Bragg region. Remains at most about 10 nm. With such a wavelength variable width, the number of wavelength channels that can be used is small, and in a large-scale wavelength switching system or wavelength cross-connect system that requires many channels, it is necessary to prepare a plurality of light sources.

Further, in the SSG-DBR-LD, although the wavelength tunable operation can be performed in a wide range, the wavelength tunable operation is discontinuous. Therefore, not only the usable channels are limited, but also complicated control is required to adjust to a desired oscillation wavelength due to discontinuous wavelength tuning operation.

It is an object of the present invention to provide a semiconductor laser which overcomes these disadvantages of the prior art and realizes a quasi-continuous wavelength tunable operation over a wide range, and a method of manufacturing the same.

[0009]

Means for Solving the Problems The present inventor has made various studies in order to achieve the above object, and as a result, completed the present invention.

In the first invention, an active layer and a passive waveguide layer are formed on the same substrate, a diffraction grating is formed in a part of the passive waveguide layer, and the period of the diffraction grating is closer to the active layer. And shorter as the distance from the active layer increases, at least
In the area where the diffraction grating is formed, current is injected with a single electrode.
The present invention relates to a wavelength tunable semiconductor laser having a structure to be inserted .

A second invention relates to a method of manufacturing a wavelength tunable semiconductor laser according to the first invention, wherein a diffraction grating is formed by electron beam exposure and etching. .

A third aspect of the present invention is a passive waveguide layer in which an active layer and a passive waveguide layer are formed on the same substrate, a diffraction grating is formed in a part of the passive waveguide layer, and the diffraction grating is formed. Is thinner on the side closer to the active layer, and becomes thicker as the distance from the active layer increases, at least in the region where the diffraction grating is formed.
The invention relates to a wavelength tunable semiconductor laser having a structure in which current is injected by a single electrode .

A fourth invention is a method of manufacturing a wavelength tunable semiconductor laser according to the third invention, wherein a diffraction grating having a uniform period is formed, an active layer and a passive waveguide layer are formed, and an oxide film on a substrate is formed. By performing selective MOVPE growth on the opposing gap, the passive waveguide is formed collectively and further provided with a diffraction grating.
Narrowing the width of the opposing oxide film in a portion near the active layer,
The period can be effectively changed by reducing the total thickness of the passive waveguide, increasing the width of the opposing oxide film as the distance from the active layer increases, and increasing the total thickness of the passive waveguide. The present invention relates to a method for manufacturing a wavelength tunable semiconductor laser characterized by the following.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a three-electrode DBR-LD, a single-axis mode oscillation is realized by increasing the reflectance only at a specific wavelength by a diffraction grating formed in a passive waveguide region. By performing current injection,
The wavelength tunable operation is performed by changing the refractive index. In the present invention, the wavelength tunable range is expanded by changing the period of the diffraction grating introduced into the three-electrode DBR-LD in the traveling direction of light. Specifically, the diffraction grating is formed such that the period of the diffraction grating is longer on the side closer to the end face of the DBR region and shorter on the side farther from the end surface of the DBR region.

FIG. 8 shows the reflection characteristics of a waveguide having a diffraction grating used in the present invention. In a waveguide having this special diffraction grating, when current injection is not performed,
Light emitted from the active layer propagates to the vicinity of the end face of the DBR region deep in the waveguide, and the maximum reflectance is obtained near the end face of the DBR area in a long wavelength region mainly determined by a long cycle. On the other hand, as shown in FIG. 9, when current is injected into the waveguide, light cannot propagate to the vicinity of the end face of the DBR region due to an increase in absorption loss and is determined mainly by a short period of a portion near the phase control region. The highest reflectance is obtained in a short wavelength range.

The newly proposed diffraction grating is replaced with a conventional three-electrode D
When introduced into a BR-LD, the wavelength change obtained by the refractive index change due to current injection and the wavelength change due to the above-described equivalent diffraction grating period change are in the same direction. Significant expansion of the range can be realized.

The formation of a diffraction grating having a period changed in the light traveling direction can be performed by using an advanced fine processing technique using an electron beam (EB) exposure machine or the like.

In another embodiment which does not require the use of such advanced fine processing technology, by changing the thickness of the waveguide in the light traveling direction, the period of the diffraction grating is equivalently changed, A tunable light source having the same characteristics as the above light source can be manufactured even by using a diffraction grating having a uniform period formed by a normal interference exposure method.

To change the film thickness of the waveguide in the light traveling direction, it is necessary to use T.T. Sasaki et al., Journal of Crystal Growth, Volume 132, 435-44
Metal-organic vapor phase epitaxy (MOVPE), which is described on page 3 (1993) and selectively grows a waveguide in a gap portion of a stripe-shaped insulating film, is effective. The greatest feature of this selective growth technique is that the thickness of the waveguide can be changed by the width of the insulating film on both sides of the gap. Furthermore, if a multiple quantum well structure (MQW) is introduced into the waveguide, the bandgap wavelength of the waveguide can be changed only by changing the width of the insulating film, and the active layer and the passive waveguide layer can be formed simultaneously. It becomes possible. As described above, if the selective MOVPE growth technique is skillfully used, a wavelength tunable operation equivalent to that of the light source can be realized even when a diffraction grating having a uniform period manufactured by a conventional interference exposure method is employed.

[0020]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Embodiment 1 FIG. 1 shows a sectional structure view of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. In this structure, the active layer 1 and the passive waveguide layer 2 are provided on the same substrate. The active layer 1 and the passive waveguide layer 2 are divided into electrodes so that current can be independently injected. Further, the passive waveguide layer 2 has a DBR on which the diffraction grating 7 is formed.
The region and the electrode on the phase control region where the diffraction grating is not formed are divided.

As shown in FIG. 2, the diffraction grating 7 formed in the DBR region has a period of 2 in a portion near the phase control region.
40 nm and 243 near the end face of the DBR region.
nm.

Next, the steps of manufacturing the device will be described.
The process diagrams are shown in FIGS. First, as shown in FIG. 6A, the diffraction grating 7 was partially formed on the n-InP substrate 3. The diffraction grating has the change in period shown in FIG. In the formation of the diffraction grating 7, EB is first placed on the n-InP substrate.
After applying a resist and pre-baking, it was performed by direct drawing by EB exposure, development, and wet etching.

Subsequently, an oxide film was deposited to a thickness of 100 nm on the front surface by a thermal CVD method, and a pattern of the oxide film 8 shown in FIG. 6B was formed by a photolithography process using a photomask shown in FIG.

FIG. 7A shows the result of growing the active layer 1, the passive waveguide layer 2 and the cladding layer 4 by selective MOVPE growth using the pattern of the oxide film 8 shown in FIG. 6B. ing. The active layer 1 has a gap of 1.5 μm in width facing an oxide film of 50 μm, and the passive waveguide layer 2 composed of a phase control region and a DBR region has an oxide film of 4 μm in width. Both layers were collectively formed in a gap having a width of 1.5 μm.

The layer structure of the active layer 1 and the passive waveguide layer 2 is as follows.
An n-InGaAsP layer having a wavelength composition of 1.2 μm (n = 5 ×
10 ¹⁷ cm ^-3 ) of 0.1 μm, InP layer of 50 nm, InGaAsP of 1.2 μm wavelength composition of 100 nm, InGaAsP well layer of 1.6 μm wavelength composition and + 1.0% compression strain, and InGaAsP of 1.2 μm wavelength composition. Five periods of the quantum well composed of the barrier layer, the wavelength composition of 1.2 μm In
GaAsP was formed to a thickness of 100 nm. p-InP
The thickness of the cladding layer 4 (p = 5 × 10 ¹⁷ cm ⁻³ ) is 0.3
μm.

Again, by a photolithography step and etching with hydrofluoric acid, the oxide films on both sides of the waveguide on which the active layer and the passive waveguide layer were formed were each removed by 2 μm. Selective MOVPE growth is again performed using this wafer, and p-I
nP cladding layer 4 (p = 1 × 10 ¹⁸ cm ⁻³ ), InGa
As contact layers (p = 1 × 10 ¹⁹ cm ⁻³ ) were sequentially grown. After crystal growth, an oxide film is again deposited on the front surface of the wafer, and then a contact window is formed in the oxide film by a photolithography process and etching with hydrofluoric acid so that current flows only in the upper portion where the active layer and the passive waveguide layer are formed. Was opened. After that, 50 nm of Ti and 400 n of Au are formed on the entire surface.
m was deposited. As shown in FIG. 7B, the electrodes were separated into the respective regions by a photolithography process and wet etching so that current could flow independently in the active region, the phase control region, and the DBR region. After that, it is polished until the thickness of the substrate becomes about 150 μm, and further on the back surface,
Back surface n formed by forming 50 nm of Ti and 400 nm of Au
The electrode 6 was provided, and the device was completed through 400 ° C. electrode annealing.

This device was used for a total of 130 μm of an active region of 500 μm, a phase control region of 300 μm, and a DBR region of 500 μm.
It was cut to 0 μm, and a high reflection coating was applied to the end face on the DBR region side to evaluate the characteristics.

The manufactured wavelength tunable light source has a threshold current of 10 mA and a wavelength of 1.
Laser oscillation occurred at 55 μm. Maximum output is 50mW
And good values were obtained. When a wavelength tunable operation was performed by passing a current through the DBR region, a wavelength tunable range of 30 nm was obtained.

Fine adjustment of the wavelength was performed by injecting current into the phase control region, and by sequentially adjusting the current flowing through the DBR region, a quasi-continuous wavelength variable operation was realized over the entire variable wavelength range obtained.

Embodiment 2 In this embodiment, unlike the embodiment 1, even if a diffraction grating having a uniform period is used, the wavelength tunable operation over a wide range can be realized by skillfully using the selective MOVPE growth technique described above. The variable wavelength light source will be described.

FIG. 4 shows a sectional structural view of this embodiment. MOV is selectively applied to a 1.5 μm wide gap between oxide films.
This embodiment is characterized in that the thickness of the waveguide is changed in the light traveling direction by performing PE growth. As shown in FIG. 4, this embodiment has a structure in which the thickness of the passive waveguide layer 2 is small near the phase control region of the DBR region, and is large near the end face of the DBR region. ing. In such a waveguide structure, the effective diffraction grating period is short in a portion of the DBR region close to the phase control region, while the DBR region
Since the waveguide layer is thick near the end face of the R region, the equivalent refractive index is large, the effective diffraction grating period becomes long, and the same effect as in the first embodiment can be expected. That is, when current injection is not performed, light emitted from the active layer propagates deep into the DBR region and oscillates at a long wavelength determined by a diffraction grating having a long effective period. On the other hand, when current injection is performed, light cannot propagate deep into the DBR region due to large absorption in the waveguide, and laser oscillation occurs at a short-wavelength determined by a diffraction grating having a short effective period around the phase control region. .

Hereinafter, a method for fabricating the device in this embodiment will be described. First, the diffraction grating 7 was formed on the n-InP substrate 3 using the interference exposure method. This diffraction grating 7 has a D
This pattern was formed on the front surface of the 2-inch wafer, being formed over the BR region of 500 μm and not being formed on the active region and the phase control region of 800 μm.

Next, an oxide film was deposited on the entire surface of the n-InP substrate 3 on which the diffraction grating 7 was formed by thermal CVD. This was processed by a photolithography process and hydrofluoric acid etching using a photomask shown in FIG. 5 to form an oxide film pattern for selective MOVPE growth.

Subsequently, the active layer 1 and the passive waveguide layer 2 are formed in a gap of 1.5 μm in width facing the oxide film 8. In the waveguide layer 2, both layers are collectively formed at a portion where the width of the oxide film is 4 to 30 μm.

The layer structure of the active layer 1 and the passive waveguide layer 2 is as follows.
InGaAsP layer having a wavelength composition of 1.2 μm (n = 5 × 10
¹⁷ cm ⁻³ ) of 0.1 μm, InP layer of 50 nm, InGaAsP of 1.2 μm wavelength composition of 100 nm, InGaAsP well layer of 1.6 μm wavelength composition and + 1.0% compressive strain, and InGaAsP barrier of 1.2 μm wavelength composition Layer composed of 5 layers of quantum wells and InGa having a wavelength composition of 1.2 μm.
AsP was formed to a thickness of 100 nm. The p-InP cladding layer 4 (p = 5 × 10 ¹⁷ cm ⁻³ ) was formed to a thickness of 0.3 μm.

Again, the photolithography process and hydrofluoric acid
The active layer and the passive waveguide layer
The oxide films on both sides of the waveguide were removed by 2 μm. This
MOVPE growth is again performed using the wafer of p-I
nP cladding layer 4 (p = 1 × 10¹⁸cm^-3) Is 2μ thick
m, InGaAs contact layer (p = 1 × 10¹⁹c
m ^-3) Was grown by selective MOVPE of 0.3 μm each.

The subsequent manufacturing steps are the same as in Example 1. After the crystal growth, an oxide film is deposited again on the entire surface of the wafer, and then a current is applied only to the upper portion where the active layer and the passive waveguide layer are formed. A contact window was opened in the oxide film by a photolithography process and etching with hydrofluoric acid so as to flow. After that, Ti is 50 nm and Au is 400
Deposited to a thickness of nm. Active area, phase control area, DB
In order to allow a current to flow independently in the R region, each region was separated by a photolithography process and wet etching to form a P electrode 5 as shown in FIG. 7B. Thereafter, the substrate was polished to a thickness of about 150 μm, and a back surface n-electrode 6 formed of 50 nm of Ti and 400 nm of Au was provided on the back surface, and the device was completed through electrode annealing at 400 ° C.

This device was used for a total of 130 μm of an active region of 500 μm, a phase control region of 300 μm, and a DBR region of 500 μm.
It was cut to 0 μm, and a high reflection coating was applied to the end face on the DBR region side to evaluate the characteristics.

The tunable light source manufactured in this example oscillated at a wavelength of 1.55 μm at a threshold current of 10 mA when a current was applied only to the active region. The maximum output was as good as 50 mW. When a current was passed through the DBR region to perform the wavelength tunable operation, a wavelength tunable range of 30 nm was obtained, and characteristics comparable to those of the structure shown in Example 1 were obtained.

Fine adjustment of the wavelength was performed by injecting current into the phase control region, and by sequentially adjusting the current flowing through the DBR region, a quasi-continuous wavelength variable operation was realized over the entire variable wavelength range obtained.

As described above, in the embodiments of the present invention, an InGaAsP-based semiconductor laser device having a multiple quantum well structure manufactured on an InP substrate has been described. However, the semiconductor material is not limited to this.
The present invention is also applicable to s / AlGaAs and other material systems.

Although the element having a wavelength band of 1.55 μm has been described here, the present invention is effective in other wavelength bands.

[0044]

As is clear from the above description, according to the present invention, the wavelength variable range is quasi-continuous, and the wavelength variable range can be greatly expanded. As a result, in a wavelength switching system, a wavelength cross-connect system, or the like, the number of wavelength channels can be greatly increased, and a large-scale system having a large number of channels can be constructed.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a wavelength tunable semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a change in the period of a diffraction grating formed in a DBR region in Example 1.

FIG. 3 is a plan view of a selective MOVPE growth mask pattern used in Example 1.

FIG. 4 is a sectional structural view of a wavelength tunable semiconductor laser according to a second embodiment of the present invention.

FIG. 5 is a plan view of a selective MOVPE growth mask pattern used in Example 2.

FIG. 6 is a process chart of manufacturing the wavelength tunable semiconductor laser of the present invention.

FIG. 7 is a process chart of manufacturing the wavelength tunable semiconductor laser of the present invention.

FIG. 8 is a reflection characteristic diagram of a waveguide having a diffracted photon used in the present invention when no current is injected.

FIG. 9 is a reflection characteristic diagram of a waveguide having a diffracted photon used in the present invention in a state where a current is injected.

[Explanation of symbols]

 Reference Signs List 1 active layer 2 passive waveguide layer 3 n-InP substrate 4 p-InP cladding layer 5 p electrode 6 n electrode 7 diffraction grating 8 oxide film

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (4)

(57) [Claims] An active layer and a passive waveguide layer are formed on the same substrate, a diffraction grating is formed on a part of the passive waveguide layer, and a period of the diffraction grating is short on a side close to the active layer. The longer the distance from the active layer, the longer the diffraction grating
A structure in which current is injected with a single electrode in the formed region
1. A wavelength tunable semiconductor laser, comprising: 2. The method for manufacturing a wavelength tunable semiconductor laser according to claim 1, wherein the diffraction grating is formed by electron beam exposure and etching. 3. An active layer and a passive waveguide layer are formed on the same substrate, a diffraction grating is formed on a part of the passive waveguide layer, and the passive waveguide layer on which the diffraction grating is formed has a film thickness. thinner at close to the active layer side, also becomes thicker with distance from the active layer, a single conductive in a region at least diffraction grating is formed
A wavelength tunable semiconductor laser having a structure in which current is injected at a pole . 4. The method of manufacturing a wavelength tunable semiconductor laser according to claim 3, wherein a diffraction grating having a uniform period is formed, and the active layer and the passive waveguide layer are formed in a gap portion where an oxide film on the substrate faces. In a passive waveguide on which a diffraction grating is formed, the width of the opposing oxide film is reduced near the active layer, and the total thickness of the passive waveguide is reduced. A wavelength tunable semiconductor laser characterized in that the period is effectively changed by reducing the thickness, increasing the width of the opposing oxide film as the distance from the active layer increases, and increasing the total thickness of the passive waveguide. Manufacturing method.