JP2692577B2 - Tunable semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical communication, optical information processing,
The present invention relates to a wavelength tunable semiconductor laser used for optical measurement control and the like.

[0002]

2. Description of the Related Art A wavelength tunable semiconductor laser is a key device of a wavelength division multiplexing transmission system (WDM) in optical fiber communication and a coherent optical transmission system. This is because the wavelength tunable semiconductor laser is small and has many advantages as compared with other lasers such that the oscillation wavelength can be easily controlled by current injection. However, regarding this, DBR-LD and SSG-DB with a three-electrode structure are used.
Although many structures such as R-LD have been proposed, there are many problems to be solved such as wavelength tunable width, wavelength controllability, and continuous wavelength tuning.

The former three-electrode structure DBR-LD has an active region, a phase control region and a DBR region which are divided in the resonator direction, and the diffraction grating is formed only in the DBR region. D
The Bragg wavelength can be changed by passing a current through the BR region, and the wavelength tunable operation without mode jump can be performed by passing a current through the phase control region independently.
It realizes quasi-continuous operation at 0 GHz (5.8 nm). Electronics Letters Volume 23 p. 403 (198
7) See (S. Murata et al., Electronics Letters, 2
3, 1987, p.403).

The latter SSG-DBR-LD is promising for further expansion of the wavelength variable width. This is because distributed reflection areas in which the pitch of the diffraction grating is periodically changed are formed on both sides of the active layer area. By changing the current flowing in each distributed reflection area, a maximum wavelength of 63 nm can be tuned. Has achieved width. Eye Triple E Photonics Technology Letters Volume 5 p. 126 (199
3) Reference (Y. Tohmori et al., IEEE Photonics Technolo
gy Letters, VOL.5, NO.2, 1993, p.126).

[0005]

The DBR-LD can realize a wavelength tunable operation in a quasi-continuous manner by controlling the current injected into the Bragg region in the phase control region.
The wavelength tunable width remains at most about 10 nm. Also, S
The SG-DBR-LD allows a wide range of variable wavelength operation, but has a discontinuous wavelength variable operation.

The present invention realizes quasi-continuous wavelength tunable operation over 200 nm, that is, in the entire gain range of a semiconductor laser, by skillfully utilizing the selective MOVPE growth technique.

[0007]

A wavelength tunable semiconductor according to the present invention
The body laser is a conductive laser in which a semiconductor laser and a directional coupler are integrated on the same conductive type substrate to form the directional coupler.
Laser light emitted from the semiconductor laser is coupled to the waveguide
It is characterized in that one of the waveguides forming the directional coupler has a diffraction grating whose period changes with respect to the traveling direction of light.

[0008] Wavelength-tunable semiconductor lasers of the present invention, on the same substrate on which the diffraction grating partially uniform periods are formed, the semiconductor laser and the directional coupler is integrated, the directional coupler
Emitted from the semiconductor laser into a waveguide forming a container
The laser beam is configured to be coupled, the diffraction grating having the uniform period is formed on one of the waveguides forming the directional coupler, and the direction in which the directional coupler is formed is guided. It is characterized in that it changes with respect to the traveling direction of light in the waveguide.

The problem is solved by the means (1) and (2).

[0010]

In the DBR-LD, the diffraction grating formed in the passive region increases the reflectance only at a specific wavelength,
A single mode oscillation is realized, and a current is injected into this region to change the refractive index, thereby performing a wavelength tunable operation.

In the present invention, a directional coupler is arranged in the passive region, and a diffraction grating whose period is changed with respect to the traveling direction of light is introduced into one waveguide forming the directional coupler. Thus, the wavelength tunable operation over an extremely wide range is realized. The wavelength tunable operation mechanism according to the present invention will be described below.

As is well known, when light of electric field intensity A (0) enters from one waveguide of the directional coupler, the waveguide II at the point where the other waveguide I has traveled a distance z. The electric field strength B (z) at can be calculated by the following formula 1.

[0013]

(Equation 1)

Here, k and Δ represent the coupling coefficient and the propagation constant difference between the two waveguides, respectively. As is clear from Equation 1, the waveguide I to the waveguide I are completely coupled with a full coupling length of π / 2β _c.
A maximum transition of lightwave power occurs at I. When the diffraction grating is formed in the waveguide II, strong reflection occurs at a specific wavelength determined by these periods. Furthermore, the waveguide I
If a diffraction grating whose period changes in the traveling direction of light is formed in I, the position at which light is most strongly coupled from the waveguide I to the waveguide II can be changed by injecting current into the waveguide, It is possible to control the wavelength at which maximum reflection occurs. The present invention takes advantage of this phenomenon and integrates the directional coupler and the semiconductor laser on the same substrate to perform a quasi-continuous wavelength tunable operation over an extremely wide range.

Further, in order to manufacture a diffraction grating whose period changes in the traveling direction of light, an advanced fine processing technique such as electron beam exposure (EB) is required. However, even if a diffraction grating with a uniform period is used, it is possible to change the period of the diffraction grating in a pseudo manner by changing the direction in which the waveguide of the directional coupler is formed. The characteristics equivalent to those of the wavelength tunable semiconductor laser can be obtained.

[0016]

Embodiments of the present invention will be described below. FIG.
FIG. 1 is a structural diagram of a wavelength tunable semiconductor laser which is a first embodiment of the present invention. In this figure, 1 is an MQW active layer, 2 is an InP cladding layer, 3 is a p-electrode, 4 is n-I.
An nP substrate, 5 is an n electrode, 6 is an n-InGaAsP optical guide layer, and 7 is a p-InGaAs cap layer. 2 and 3 are structural views of the cross section AB and CD of the device, respectively, and 8 shows a diffraction grating whose period changes in the traveling direction of light.

In the embodiment of the invention of claim 1 shown in FIG. 1, the period in the range of 219 nm to 250 nm in the <110> direction on the n-InP substrate 4 and on the (001) substrate as shown in FIG. After partially forming the changed diffraction grating by electron beam exposure and chemical etching, as shown in FIG.
The QW active layer 1 is selectively grown. This selective growth of the MQW active layer 1 can be easily carried out by using the selective MOVPE growth technique disclosed by Sasaki et al. In Japanese Patent Laid-Open No. 303982/1992.

The layer structure of the MQW active layer 6 is shown in FIG. I
An n-InGaAsP optical guide layer 6 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) of 0.05 μm, an undoped InGaAsP layer 9 of 0.05 μm, and a thickness of 7 nm of InG are formed on an nP substrate.
A quantum well consisting of an aAs well layer 10 and a 1.15 μm wavelength composition InGaAsP barrier layer 9 with a thickness of 10 nm has 5 periods, and an undoped InGaAsP layer 9 has a thickness of 0.05 μm.
The p-InP layer 2 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) was set to 0.
1μm grows sequentially. Again by MOVPE selective growth,
The MQW active layer 1 was embedded with a p-InP clad layer 2 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) having a width of 5.5 μm, and InG
An aAs cap layer 7 (carrier concentration 1 × 10 ¹⁹ cm ⁻³ ) is grown to 0.2 μm. At the same time, the p-InP clad layer 2 forms two ridge type optical waveguides each having a width of 5.5 μm and a thickness of 1 μm with an interval of 10 μm to form a directional coupler.

Next, Ti / Au is vapor-deposited on the p-side of the substrate by sputtering, and the electrode is divided by wet etching into one place on the semiconductor laser side and two places on the directional coupler. The n side of the substrate is polished until the total thickness of the substrate is 150 μm, and Ti / Au is again sputtered on the n side of the substrate.
The electrode is vapor-deposited and alloyed at 460 ° C. to complete the device.

FIG. 5 is a top view of a wafer forming pattern of an optical waveguide of a wavelength tunable laser according to a second embodiment of the present invention. With this structure, an LD structure can be realized by adopting the same buried structure by selective MOVPE growth as in the first embodiment. The difference from the first embodiment is that a diffraction grating having a uniform period of 212 nm is formed in one of the waveguides of the directional coupler, and the direction of the ridge waveguide formed on the (001) substrate is < The change is from the 110> direction to the <-110> direction at a rate of 0.03 deg / μm. By this, the period of the diffraction grating is equivalently continuously changed.

The semiconductor laser according to the first and second embodiments described above has a cavity length of 600 μm, and the directional coupler has a length of 100.
The device was mounted on a diamond heat sink by "p-side up" with a threshold current of 20 mA and an external differential quantum efficiency of 0.
A device having good characteristics of 18 W / A and maximum optical output of 30 mW was stably obtained. When current was injected into the waveguide in which the diffraction grating of the directional coupler was formed, a quasi-continuous wavelength tunable operation of 200 nm with a wavelength of 1.5 μm as the center could be realized. The wavelength variable width obtained here is limited by the gain bandwidth of the semiconductor laser fabricated on the same substrate.

As described above, by using the wavelength tunable semiconductor laser structure of the present invention, a wider wavelength tunable characteristic can be obtained as compared with the conventional one. Therefore, the coherent optical transmission light source, the WDM transmission light source, the optical measurement It can be said that it is extremely promising as a control light source.

[0023]

According to the present invention, a semiconductor laser capable of quasi-continuous wavelength tunable operation over the entire gain range can be obtained.

[Brief description of the drawings]

FIG. 1 is a structural diagram of a wavelength tunable semiconductor laser that is a first embodiment of the present invention.

FIG. 2 is a cross-sectional structural view taken along the line AB in FIG. 1 of the wavelength tunable semiconductor laser according to the first embodiment of the present invention.

FIG. 3 is a sectional structural view of the wavelength tunable semiconductor laser according to the first embodiment of the present invention, taken along the CD section in FIG. 1.

FIG. 4 is a layer structure diagram around the MQW active layer.

FIG. 5 is a structural diagram of a wavelength tunable semiconductor laser which is a second embodiment of the present invention.

[Explanation of symbols]

1 MQW active layer 2 p-InP clad layer 3 p electrode 4 n-InP substrate 5 n electrode 6 n-InGaAsP light guide layer 7 p-InGaAs cap layer 8 diffraction grating 9 InGaAsP layer 10 InGaAs layer 11 waveguide 12 SiO ₂

Claims (2)

(57) [Claims] 1. A semiconductor laser and a directional coupler are integrated on the same conductive type substrate to form the directional coupler.
The laser light emitted from the semiconductor laser is coupled to the waveguide.
Together it is configured as a wavelength tunable semiconductor laser, characterized by having a diffraction grating that changes in the period with respect to the traveling direction of the light in one of the waveguides forming the directional coupler. 2. A partially uniformly periodic same substrate on which the diffraction grating is formed of the semiconductor laser and the directional coupler is integrated is
The semiconductor waveguide in the waveguide forming the directional coupler.
The laser light emitted from the laser is configured to be combined.
The uniform-period diffraction grating is formed on one of the waveguides forming the directional coupler, and the direction in which the directional coupler is formed is relative to the traveling direction of the light in the waveguide. A wavelength tunable semiconductor laser characterized by being changed.