JP2708467B2 - Tunable semiconductor laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser having a narrow spectral line width, capable of tunable over a wide wavelength range, and suitable for use as a semiconductor laser for coherent communication.

[Conventional technology]

A review of the tunable semiconductor lasers published so far is provided by OCFC Technical Digest, TAHK, p. 1,176 (1988) (OFC '
88 Technical Digest, THKI, P. 176, 1988).

[Problems to be solved by the invention]

The wavelength tunable semiconductor laser introduced in the above-mentioned document is
It is roughly divided into two types.

First, the first type is DFB (distributed feedback) or D
In a BR (distributed reflection) semiconductor laser, a current is injected into a diffraction grating region to change the refractive index of the region, and as a result, the Bragg reflection wavelength is made variable. Generally, the Bragg reflection wavelength λ _D is λ _D = 2Λn
(Λ: period of diffraction grating, n: equivalent refractive index of a region where laser light propagates), the variable wavelength width of this type is at most about 4 nm. Another problem is that when a current is injected into the diffraction grating region to change the wavelength, the spectral line width is significantly increased due to the fluctuation of the carrier density or the contribution of spontaneous emission light.

The second type is an external resonator type configuration in which a diffraction grating is provided outside a semiconductor laser. That is, a laser beam is transmitted through space or inside an optical fiber and coupled to an external diffraction grating, and the angle or period of the diffraction grating is mechanically changed to change the Bragg reflection wavelength. In this type, the device itself is large (number
cm to several tens of cm) and strict accuracy is required for optical axis alignment.

As described above, in the conventional type of wavelength tunable semiconductor laser, as pointed out in the above-mentioned document, "it is problematic to maintain a narrow and constant spectral line width even when the wavelength changes." The problem remains.

An object of the present invention is to provide a wavelength tunable semiconductor laser which has eliminated the problems of the conventional technology as described above.

[Means for solving the problem]

The present inventors have realized a wavelength tunable semiconductor laser having a completely new configuration that has never existed, by making use of many years of experience in semiconductor laser research. That is, on a semiconductor substrate, a laser light generating portion capable of changing the emission angle of laser light from behind the active region and an optical waveguide portion having a diffraction grating are provided, and an optical waveguide layer is used between the two regions. Connected. Further, the period of the diffraction grating to which the laser light from the rear is combined at each emission angle was changed.

[Action]

The principle of operation will be described in detail with reference to FIG. The light emitted from the stripe-shaped active layer 2 has its output angle from the rear controlled by the output beam angle variable control region. The emitted light propagates through the optical waveguide layer 4, and when it reaches the area of the diffraction grating 5, only the light of the Bragg reflection wavelength is selectively subjected to distributed feedback, and the period of the reflected diffraction grating corresponds to each emission angle. In the region where Accordingly, the Bragg reflection wavelength can be changed by changing the emission angle of the laser (λ ₁ , λ in FIG. 1A).
₂ ).

Laser oscillation is caused by a feedback loop of reflection at the cleavage plane 10 at the front end face and Bragg reflection of the diffraction grating. That is,
By changing the emission angle from behind, the laser oscillation wavelength can be varied.

FIG. 1 shows an example in which the output beam varying electrode 7 is used as a means for changing the angle of the output beam. The principle of the change in the angle of the output beam in this example will be described with reference to FIGS. 1 (a) and 1 (c). Now, if the current injection amount into the electrode 7a is larger than 7b, the carrier density in the active layer 2 becomes
7a side (right side in FIG. 3C) is larger. Since the refractive index decreases as the carrier density increases, the refractive index on the left side of the stripe (in FIG. 3C) increases. Then, the laser beam splits to the left. In other words, the emission angle is also pulled in that direction due to the smaller current flowing from the electrode toward the smaller electrode. By using this phenomenon, the emission angle can be changed by adjusting the amount of current injected into the electrodes 7a and 7b.

Further, regarding the spectral line width, the resonator length of the cleavage plane and the diffraction grating can be set longer, so that the band can be narrowed as compared with the conventional laser (the spectral line width is inversely proportional to the resonator length). When a curve is provided on the diffraction grating as shown in FIG. 1 and its focal point is set at the boundary between the active region and the optical waveguide region, the light reflected by the diffraction grating by the lens effect is automatically fed back into the active layer stripe. Therefore, the coupling is improved.

Further, according to the present invention, since it can be formed on a semiconductor in a monolithic or hybrid manner, strict optical axis alignment unlike the prior art type 2 is unnecessary and the size can be reduced.

In order to continuously change the wavelength, it is only necessary to change the emission angle continuously. At that time, the phase of the light distributed and returned from the diffraction grating is slightly changed, and a mode hop of a longitudinal mode occurs. Atsuta. In order to suppress this, it has also been found that the optical waveguide 4 region or the active region may have a phase adjustment function.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

Embodiment 1 FIG. 1 shows a laser device according to an embodiment of the present invention.
InGaAsP active layer 2 and p-InP cladding layer 3 on n-InP substrate 1
Is formed, an active layer is formed in a stripe shape (width: 0.5 to 3 μm), and then buried with a p-InP layer 11 and an n-InP layer 12 (FIG. 4D). Then, after removing all the growth layers in the optical waveguide region, an InGaAsP optical waveguide layer (having a larger band gap than the active layer 2) 4 is grown in that region. Then about 50 from the active area
It has a curvature on the optical waveguide separated by 0 μm (curvature radius ~ 250
μm) A diffraction grating 5 was formed. The period of the diffraction grating is 238.5 nm in the region on the stripe axis and +10 from the stripe axis
At positions shifted by ° and -10 °, they were set to 237 nm and 240 nm, respectively, and the change of the period between them was gradually and linearly changed. The number of diffraction gratings was about 2,000.

Next, the first part is located at the center of the stripe in the output beam variable region.
As shown in FIG. 3C, after the insulating region 13 was provided by ion implantation of protons or boron, the electrodes 6, 7a, 7b, 8 were formed as shown in the figure. The length of the active region is 200-500 μm,
The length (axial direction) of the output beam variable electrodes 7a and 7b was set to about 1/10 to 2/3 of the active area length.

The prototype laser oscillated when 10-20 mA was injected into the current injection main electrode 6. Now, the injection current into the electrode 7a is Ia,
FIG. 2 shows a change in the emission angle and the oscillation wavelength when the injection current to the electrode 7b is Ib and the ratio is changed. Ia + Ib is about 3
~ 20mA. By changing the value of Ia / (Ia + Ib) from 0 to 1, the emission angle was continuously changed between -10 ° and + 10 °. As a result, the Bragg reflection wavelength also changed, and the oscillation wavelength continuously changed between 1.557 and 1.539 μm. The tunable wavelength range was about 18 nm, and over this period, a stable single longitudinal mode (side mode pressing ratio 〜40 dB) and a spectral line width of about 1 MHz were obtained at a light output of 5 mW.

Embodiment 2 Another embodiment of the present invention will be described with reference to FIG.

In this embodiment, all the active regions of the first embodiment have the structure shown in FIG. 1 (c). That is, the entire active region is an emission beam angle changing region. Electrode 7
Laser oscillation occurred when the current injection amount to a and 7b was about 20 mA. Further, as in the first embodiment, the emission angle was changed by changing the ratio of the amount of current injected to the electrodes 7a and 7b, and the oscillation wavelength became variable. Its characteristics were almost the same as in Example 1.

In the above embodiment, when the oscillation wavelength is changed by changing the ratio of the current injection amount, hopping in a longitudinal mode occurs in a certain element, and there is a case where the hopping does not change continuously. It has been found that this is caused by a shift in the phase of the laser light in the resonance between the Bragg reflection of the diffraction grating and the cleavage plane.

Embodiment 3 This embodiment is an example in which the problem of the hopping is solved.

FIG. 4 shows an embodiment having a laser light phase adjusting mechanism. The basic structure is the same as that of the first embodiment, except that a P-InP current injection layer 14 and a phase adjusting electrode 15 are provided on the optical waveguide. It is appropriate that the axial length of this region is about 20 to 50 μm. Since the refractive index can be adjusted by injecting a current into this region, the propagation constant in this region can be adjusted. Accordingly, the phase of the laser beam could be adjusted by adjusting the amount of current injection. The laser characteristics in this embodiment were almost the same as those in the first embodiment, and the wavelength variable range was almost the same as in FIG. Further, when the ratio of the current injection amount of the output beam angle varying electrode is changed to change the wavelength, the current injection amount to the phase adjustment electrode 15 is appropriately adjusted (about 0 to 50 mA), Vertical mode hopping could be suppressed.

Embodiment 4 An embodiment in which a semiconductor laser according to the present invention is used as a local light source for heterodyne detection in a coherent optical communication system will be described with reference to FIG.

The detection system is so-called heterodyne detection, in which signal light incident from an optical fiber 16 is combined with light from a wavelength tunable laser 25 serving as a local light source by an optical coupler 17, and converted into an electric signal by a photodiode 19 through a lens 18. The intermediate frequency is amplified by the amplifier 20 and decoded by the identification circuit 21.
Further, the oscillation wavelength of the tunable semiconductor laser 25 is stabilized by the frequency stabilizer 24 by the electric signal fed back through the intermediate frequency discriminator 22 and the amplifier 23. This detection system succeeded in detecting 1Gbit / SFSK modulated light with a wavelength interval of 1Å and 150 channels.

In the above embodiment, the example of the InGaAsP-based semiconductor laser has been described.
It is needless to say that the present invention can be applied to other material systems capable of continuous oscillation at room temperature, such as a system and an InGaP system.

〔The invention's effect〕

According to the present invention, it is possible to provide a wavelength tunable semiconductor laser having a narrow spectral line width and a large tunable wavelength width in a longitudinal single mode, so that a phenomenal effect as a local light source for heterodyne detection in coherent optical communication can be provided. is there.

[Brief description of the drawings]

1 (a) is a top view of a semiconductor laser according to an embodiment of the present invention, FIG. 1 (b) is a side sectional view thereof, and FIGS. 1 (c) and 1 (d) are AA 'and B- of FIG. FIG. 2 is a vertical sectional view of the line B ', FIG. 2 is a wavelength tunable characteristic of the laser of the embodiment of the present invention, FIG. 3 is a top view of a semiconductor laser of another embodiment of the present invention, and FIG. FIG. 5 is a side sectional view of a semiconductor laser according to an embodiment, and FIG. 5 is a block diagram of an embodiment of an optical communication system incorporating the laser of the present invention. Reference numeral 2 denotes an active layer, 4 denotes an optical waveguide layer, 5 denotes a diffraction grating, 7 denotes an output beam variable electrode, 9 denotes a backward output laser beam, 10 denotes a cleavage plane, 13 denotes an insulating region, 15 denotes a phase adjustment electrode, and 17 denotes a phase adjustment electrode. Optical couplers, 19 photodiodes, 24 frequency stabilizers, 25 tunable semiconductor lasers.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Naoki Kaine 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory of Hitachi, Ltd. (56) References JP-A-61-198792 (JP, A) JP-A-58 -48981 (JP, A) JP-A-58-92289 (JP, A)

Claims (1)

(57) [Claims] 1. A semiconductor substrate, a first region in which a stripe-shaped active layer and a cladding layer are laminated on the semiconductor substrate, and a first region having a cleavage surface on one side serving as an emission end face of a laser beam; And a second region formed by contacting an optical waveguide layer having a larger forbidden band width than the active layer on a first side surface of the active layer opposite to the cleavage plane. An insulating region formed to insulate the upper surface of the cladding layer in the stripe direction from the first side surface, and a pair of electrodes formed separately on the upper surface of each cladding layer separated by the insulating region. The second region includes a diffraction grating portion formed on the optical waveguide layer with a curvature with respect to a stripe axis of the active layer around the first side surface, the first side surface and the diffraction grating portion. The light between A phase adjusting portion formed by laminating a current injection layer and a phase adjusting electrode on the waveguide layer, wherein the period of the diffraction grating forming the diffraction grating portion is from one side to the other side with respect to the stripe axis; A wavelength tunable semiconductor laser characterized by linearly changing toward.