JP2687464B2 - Semiconductor optical integrated device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor optical integrated device in which a wavelength tunable semiconductor laser and an optical modulator are integrated.

[Conventional technology]

It is effective to use a wavelength tunable laser as a transmission light source in a wavelength division multiplexing optical communication and a wavelength division optical switching system that are densely multiplexed. This is because the wavelength can be easily controlled even when the wavelength intervals of many light sources are close. However, in such a system, since wavelength chirping in the case of directly intensity-modulating a laser hinders high-density multiplexing, it is necessary to use an optical modulator having a small wavelength chirping also during modulation. Although various types of such optical modulators have been reported, they generally have the drawback of large coupling loss. Therefore, several optical integrated devices in which a laser and an optical modulator are integrated on one semiconductor substrate have been manufactured. Although an example of integrating a wavelength tunable laser and an optical modulator has not yet been seen, a distributed feedback laser (DFB laser) 500 and an optical modulator 200 as shown in FIG. Optical integrated devices have been reported.

The integrated optical device is easy to manufacture because the DFB laser 400 and the optical modulator 200 have the same layer structure. The light of the DFB laser 500 having the diffraction grating is intensity-modulated by changing the injection current to the optical modulator 200. In the optical modulator 200, absorption is large when there is no injection current, and gain occurs when there is injection current, so that a large extinction ratio is obtained. For more information on this integrated optical device, see Electronics Letters (M. Yamaguchi et al., Electron. Lett. 23) .
190 (1987)). On the other hand, there are some reports on the wavelength tunable laser itself.
Among them, the wavelength tunable distributed Bragg reflection laser (DBR laser) that utilizes the change in the refractive index of the optical guide layer due to the current injection is one of the best wavelength tunable lasers because it allows wavelength tuning in a wide range. The details of this tunable DBR laser have already been reported by the present inventors in the magazine Electronic Letters (Electron Lett. 23 , 403, (1987)).

[Problems to be solved by the invention]

The DFB laser 500 and the optical modulator 200 including the optical modulator 200 shown in the conventional example have the following problems. It is a light modulator 20
This is a point where the reflectance of the end face of 0 must be kept extremely small (0.1% or less). Therefore, the antireflection coating film 300
Was very difficult to manufacture. In general, the oscillation wavelength of a DFB laser changes greatly with a slight change in the end face phase. In the structure as shown in FIG. 3, if there is slight reflection on the end face of the optical modulator 200, the effective end face phase of the DFB laser 500 changes when the current of the optical modulator 200 is modulated. The oscillation wavelength changes. This causes the same effect as the wavelength chirping when the laser is directly modulated, and the advantage of integrating the optical modulator is lost. In fact, in the conventional example, the wavelength fluctuation is reduced only under the special modulation condition.

It is an object of the present invention to provide a semiconductor optical integrated device characterized in that, when an optical modulator is integrated in a wavelength tunable laser, the wavelength variation is small even during modulation, and the structure is simple and easy to manufacture.

[Means for solving the problem]

There are four types of semiconductor optical integrated devices of the present invention, one of which is a semiconductor optical device in which a wavelength tunable semiconductor laser and an optical modulator for intensity-modulating the output light of the semiconductor laser are formed on one semiconductor substrate. An integrated device, wherein the semiconductor laser includes at least an active region having a gain and a distributed Bragg reflector (DBR) region having a diffraction grating,
The effective refractive index of the BR region can be electrically controlled, and the optical modulator is provided adjacent to the DBR region. The second is the above-described semiconductor optical integrated device, wherein the optical modulator has the same layer structure as the active region,
In addition, the structure is characterized in that intensity modulation is possible by current injection. Thirdly, similar to the second one, the DBR region of the semiconductor optical integrated device and the optical modulator include the same optical guide layer, and a voltage is applied to the optical guide layer of the optical modulator. This is a configuration characterized in that intensity modulation is possible by. And the fourth is
The third optical guide layer is a semiconductor optical integrated device characterized by comprising a multiple quantum well structure.

〔Example〕

 Next, the details of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing a first embodiment of the present invention.
In the integrated optical device, a wavelength tunable DBR laser 100 and an optical modulator 200 are integrated on one substrate 111. The feature of this optical integrated device is that the optical modulator 200 has a DBR region 130 of the DBR laser 100.
The layer structure of the optical modulator 200 is the same as the layer structure of the active region 110 of the DBR laser 200. The structure of the DBR laser 200 is basically the same as that described in the conventional example, that is, the active region 110, the phase control region 12
0, DBR area 130 consists of three areas. The oscillation wavelength of the laser can be controlled by the current injected into the phase control region 120 and the DBR region 130. Laser light is active region 110 and DBR region
Although the light is emitted from either side of the 130, most of the output light is emitted from the DBR region 130 side by attaching a film 400 having high reflectance (high-reflection coating film) 400 to the end face of the active region 110 here. The light can be incident on the light modulator 200. DBR
The region 130 acts as a reflector for the laser light, but the ratio of the light incident on the optical modulator 200 can be adjusted by appropriately adjusting the length of this region and the depth of the diffraction grating 1116.

This optical integrated device is superior to the optical integrated devices of the DFB laser and the optical modulator described in the conventional example because of the following points. First of all, the wavelength tunable operation is possible. Next, the wavelength fluctuation during modulation is small. The reason why the wavelength fluctuation during modulation is small is that the oscillation wavelength of the DBR laser is mainly determined by the period of the diffraction grating 116 in the DBR region 130. Therefore, when the injection current to the optical modulator 200 is modulated and the intensity of the laser light is modulated, even if the reflectance of the antireflection coating film 300 is slightly large to about 1%, it hardly affects the laser. Therefore, it is not necessary to have a strict antireflection coating as is required for the optical integrated device of the DFB laser and the optical modulator. As described above, another feature of the present embodiment is that the active region 110 and the optical modulator 200 have the same layer structure, which has the advantage of simplifying the operation. The specific manufacturing procedure and device characteristics will be described below.

First, the diffraction grating 116 having a period of 239 nm is formed on a part of the n-type Inp substrate 111. Next, by liquid phase epitaxial growth, an InGaAsP optical guide layer (λ _g = 1.3 μm) 112, an InP etch stop layer 113, an InGaAsP active layer (λ _g = 1.54 μm)
114 and a P-type InP clad layer 115 are sequentially grown. Next, the clad layer 115 and the active layer 114 of the phase control region 120 and the DBR region 130.
After the selective removal of P,
The InP clad layer 115 is formed. Next, mesa etching is performed to form a DCPBH type embedded structure, and the embedded structure is formed by the third growth. DBR laser here 1
Both 00 and the optical modulator 200 have the same embedded structure.
Next, an electrode is attached to each region, and SiO ₂ /
A film (high-reflection coating film) 400 having a high reflectance made of an α-Si multilayer film was formed, and a non-reflection coating film 300 made of a SiN film was formed on the end face of the optical modulator 200. The reflectance is 80% and 1% respectively
It is. The length of each area is 300μ in the active area 110.
m, phase control region 120 is 100 μm, DBR region 130 is 300 μm
m, the optical modulator 200 is 300 μm. A groove having a width of 20 μm was formed between the respective regions except for the vicinity of the central mesa to electrically separate the regions. Of course, optically, the regions are coupled through a common light guide layer 112.

The characteristics of the tunable laser were that the threshold value was 20 mA, and continuous wavelength tuning of 4 nm was possible by the control current near the oscillation wavelength of 1.55 μm. The current of each region of the DBR laser 100 was kept constant, that is, the optical output and the wavelength were kept constant, and the current of the optical modulator 200 was modulated to examine the modulation characteristics. Modulation up to 600 Mb / S was possible with an extinction ratio of 10 dB or more. At this time, the wavelength variation due to the modulation was suppressed to 0.05 nm or less.

FIG. 2 is a simplified view of the cross section in the optical axis direction showing the second embodiment of the present invention. The point that the optical modulator 200 is adjacent to the DBRB region 130 is the same as the first embodiment. The difference from the first embodiment is mainly in the layer structure of the optical modulator 200 and how to apply a voltage. Here, the composition of the light guide layer 112 is slightly changed from that of the first embodiment, and the band gap is set to a value slightly larger than the energy of the laser light (λ _g = 1.45 μm). Light modulator
200 active layers have been removed. With this composition, since the light guide layer 112 is almost transparent to the laser light, the phase control region 120 and the DBR region 130 have the same function as in the first embodiment, that is, have a wavelength variable function. Where the light modulator
When reverse voltage is applied to 200, the light guide layer 1
Since the absorption edge of 12 shifts to the long wavelength side (Franz-Keldesh effect), it absorbs laser light. Therefore, the intensity modulation of light can be realized by modulating the voltage. Again, the oscillation wavelength of the DBR laser 100 is the optical modulator 200.
Therefore, the wavelength fluctuation due to the modulation is very small because it is hardly affected by the reflected light. Also, the optical modulator 200 and the phase control area 1
20. Since the DBR region 130 has the same layer structure, it is easy to manufacture.

The manufacturing procedure of the second embodiment is almost the same as that of the first embodiment. The difference is that the active layer of the optical modulator 200 is also removed before the second growth. Again, a DC-PBH type embedded structure was used. The characteristics of the DBR laser 100 were almost the same as those of the first embodiment. The optical modulator 200 is capable of performing modulation up to 2 Gd / S because the modulation band is not limited by the lifetime of the injected carrier as in the first embodiment. The wavelength fluctuation at this time was 0.03 nm or less.

Although not shown in the drawing, the third embodiment uses the optical guide layer 112 of the second embodiment having a multiple quantum well structure. The other structure is the same as that of the second embodiment. As is well known, when a reverse voltage is applied to a multiple quantum (well structure, a change in absorption edge similar to the Franz-Keldysh effect is obtained. Therefore, as in the second embodiment, an optical modulator is obtained. The intensity of light can be modulated by modulating the voltage of
Wavelength fluctuation during modulation is also small. Here, a 10-layer structure of InGaAs (layer thickness 6 nm) and InP (layer thickness 10 nm) was used as the multiple quantum well structure. Since the InGaAs layer that is the well layer is thin, the effective absorption edge is shifted to the short wavelength side (here, 1.47 μm), so that it is almost transparent to laser light when no voltage is applied. Further, in the multiple quantum well structure optical guide layer of the phase control region 120 and the DBR region 130, the same wavelength control as in the first and second effective examples can be performed by current injection.

The manufacturing procedure of the third embodiment is almost the same as that of the second embodiment, except that the MOVPE method is used for the first growth in order to grow the optical guide layer having the multiple quantum well structure. ing. As a characteristic, the wavelength fluctuation is 0.03n with 2Gb / S modulation.
m or less.

In the embodiments described above, the active region of the DBR laser and the D
Although the phase control region is provided between the BR regions, it is not always necessary if only the wavelength tuning is discontinuous. All DCBPH here for lateral mode control
Although a buried structure having a shape is used, other structures such as a ridge guide structure may be used. The DBR laser and the optical modulator may have different transverse mode control structures. Regarding the growth method, liquid phase epitaxial growth was mainly used here, but it is also possible to use other vapor phase growth or the like. Further, the material system is not limited to the InP system and can be applied to other material systems such as GaAs system.

〔The invention's effect〕

As described above, according to the present invention, when integrating a wavelength tunable laser and an optical modulator, it is possible to realize a semiconductor optical integrated device that has a small wavelength variation even during modulation, has a simple structure, and is easy to manufacture. In fact, when a tunable DBR laser and an optical modulator having the same layer structure as the active layer of the laser were integrated, the wavelength variation was 0.05 nm or less even when the current modulation was 600 Mb / S. In another example, the optical modulator was of a voltage-applied type, so that 2 Gd / S modulation was possible and the wavelength variation was 0.03 nm or less.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the first embodiment, FIG. 2 is a view showing the second embodiment, and FIG. 3 is a view showing a conventional example. 100 …… DBR laser, 200 …… Optical modulator, 300 …… Non-reflective coating film, 400 …… High-reflective coating film, 500 …… DFB laser, 1
10 ... Active region, 120 ... Phase control region, 130 ... DBR region, 111 ... Substrate, 112 ... Optical guide layer, 113 ... Etch stop layer, 114 ... Active layer, 115 ... Clad layer, 116
……Diffraction grating.

Claims (1)

(57) [Claims] 1. A semiconductor optical integrated device in which a wavelength tunable semiconductor laser and an optical modulator for intensity-modulating the output light of the semiconductor laser are formed on one semiconductor substrate, and the wavelength tunable semiconductor laser comprises: The active region having a gain, the phase control region, and at least a Bragg reflector (DBR) region having a diffraction grating, the phase control region and the
There is no active layer in the DBR region, the effective refractive index of the DBR region can be electrically controlled, and the optical modulator has the same layer structure and the current injection electrode as the active region, A semiconductor optical integrated device, which is provided adjacent to the DBR region.