JPS62245692A - Distributed feedback semiconductor laser with external resonator - Google Patents

Abstract

PURPOSE:To obtain a light source having a narrow spectral line width and a controllable frequency by a method wherein an external resonator, wherein the refractive index of a partial region of the passive light waveguide is electrically designed variably and a distributed feedback (DFB) laser are integrated on the same semiconductor substrate. CONSTITUTION:Diffraction gratings of the prescribed period are formed in a DFB laser 100 on an N-type InP substrate 300, and an N-type InGaAsP light guide layer 310, an InGaAsP active layer 320 and a P-type InP clad layer 330 are epitaxially formed in order. Then, the layers 330 and 320 are selectively removed except the DFB laser part 100 and the whole is covered with a P-type InP clad layer 340. Then, a mesa etching is performed for conducting the embedding and growth and electrodes are each attached on the DFB laser part 100 and a phase control part 400. Moreover, the electrical insulation of each part is performed by providing an etching groove in parts of the growing layer. As a P-N junction is ready-formed in the guide layer 310, the refractive index of the phase control part 400 can be varied by injecting carriers into this part. Both end faces of an element are formed by performing a cleavage. If the injection current into the phase control part 400 is slightly changed, the frequency of the light source can be modulated leaving as the spectral line width is roughly constant.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a distributed feedback semiconductor laser with an external cavity.

(Prior Art) Distributed feedback semiconductor lasers (hereinafter referred to as DFB lasers) have excellent axial mode stability and are therefore expected to be used as light sources for coherent optical transmission in the future.

Light sources for coherent optical transmission particularly require narrow spectral linewidth and frequency controllability (or stability). When a single DFB laser was used, the spectral line width was at most about 10 MHz, which was insufficient in terms of characteristics. Also, in terms of frequency controllability, there was a problem in that flat frequency modulation characteristics could not be obtained. Several methods have been considered in the future to overcome these problems.

First, the problem of spectral linewidth can be solved by integrating a passive optical waveguide into a DFB laser.
A B laser has been realized, and the spectral linewidth has been reduced by about one order of magnitude. This has been reported at the Japan Society of Applied Physics (Murata et al., Proceedings of the 46th Society of Applied Physics Academic Conference: TP-M-6). In such an FB laser, the effective resonator length is increased by the external resonator, and the spectral linewidth can be reduced.

Regarding the second problem of frequency controllability, a flat frequency modulation characteristic was obtained by integrating a modulator into the DFB laser and effectively changing the phase of the laser end face. Regarding this, Electronics Letters magazine (S, YAMA
ZAKI et, al, Electron, Le
tt, 21 (1985) 283).

(Problems to be Solved by the Invention) However, the conventional examples described above still have many problems when viewed from the viewpoint of narrow spectral linewidth and frequency controllability required as a light source for coherent optical transmission. In other words, in the case of the first DFB laser with an external cavity, although a narrow spectral linewidth could be obtained, there was a problem in terms of frequency stabilization because there was no function to control the frequency, and strict temperature and current control was required. , and could only be used in a steady state. On the other hand, although the end facet phase controlled DFB laser allows frequency control within certain conditions, the spectral linewidth is comparable to that of a normal DFB laser, and an insufficient value has been obtained.

An object of the present invention is to improve the above-mentioned problems and provide a light source for coherent optical transmission that has a narrow spectral linewidth and is frequency controllable.

(Means for solving the problem) A DFB laser and an external resonator using a passive optical waveguide,
The above-mentioned problems are solved by a DFB laser with an external cavity, which is characterized in that the refractive index of a part of the passive optical waveguide is electrically variable in an integrated semiconductor laser integrated on the same semiconductor substrate. can.

(Function) FIG. 1 is a cross-sectional view in the axial direction of a laser resonator showing the basic structure of the present invention. The device consists of a DFB laser section 100 and an external resonator section 200, both of which are connected to a semiconductor substrate 3.
00 and is integrated.

Laser oscillation is performed by injecting current into the DFB laser section 100. While the laser output is emitted from the laser side end face, it is also introduced into the passive optical waveguide 310 of the external resonator. The introduced light is reflected by the external cavity side end face and then fed back to the laser active layer 320. By increasing the resonator length of the laser in this way, the spectral linewidth is significantly reduced compared to the case of a single DFB laser. The longer the external resonator length is, the greater the effect of reducing the spectral line width is, so the external resonator length usually needs to be several times or more the DFB laser length. One problem arises.

This is because the external resonator length is long, so the axial mode spacing becomes narrow, and the oscillation mode shifts to another axial mode due to slight fluctuations in the current or temperature of the DFB laser section 100. At this time, the spectral linewidth becomes extremely wide. Furthermore, even if the mode does not shift to another axis mode, the spectral linewidth widens simply by shifting the phase condition of the feedback light. Therefore, in the present invention, an electrode is attached to a part of the external resonator part 200, and this part is used as the phase control part 400. By changing the refractive index of the optical waveguide 310 of the phase control section 400 by carrier injection or electro-optic effect, the phase of the feedback light can be controlled. Due to this,
Even if the current or temperature of the DFB laser section 100 fluctuates and the laser frequency is excellent, it is possible to control the phase of the feedback light to suppress the jump in the axial mode and the increase in the spectral line width.

Conversely, the frequency can be controlled by changing the phase of the feedback light within a range where the spectral linewidth of the laser is stable. The phase control section 400 of the present invention includes the external resonator section 2
The principle is the same no matter where it is formed in 00. However, the structure in which the entire external resonator section 200 is a phase control section usually requires a long external resonator of 1 to 2 mm or more, and therefore is inferior in terms of optical loss and response speed.

(Example) FIG. 2 is a perspective view showing an embodiment of the present invention.

Basically, the structure is the same as that shown in Fig. 1, and the DFB laser section 10
0 and an external resonator section 200, and a phase control section 400 is provided in a part of the external resonator section 200.

The lengths of the DFB laser section 100, external resonator section 200, and phase control section 400 are 30011 m and 20011 m, respectively.
m, 1.5 mm.

The manufacturing procedure will be briefly described below. First, n-type InP substrate 3
A diffraction grating with a period of 240 nm is formed in the DFB laser section 100 on the DFB laser section 100. Next, an n-type InGaAsP optical guide layer 310, an InGaAsP active layer 320, and a p-type InP cladding layer 330 are sequentially grown by first LPE growth. Next, excluding the DFB laser section 100 with the diffraction grating,
Other parts of the cladding layer 330 and active layer 320 are selectively removed. The light guide layer 310 is left. Second L
A p-type InP cladding layer 340 is formed throughout the PE growth. Next, in order to form a buried structure, mesa etching is performed, and then buried growth is performed by a third LPE growth. In this embodiment, a double channel planar embedded type was used, but other striped structures can be used with essentially the same effect. Next, electrodes are formed on the substrate side and the DFB laser section 100 and phase control section 400 on the growth side.

Electrical isolation of each portion was achieved by etching a portion of the grown layer and providing grooves. Since the optical guide layer 310 is transparent to the oscillation wavelength of the laser, it becomes a passive optical waveguide. Further, since a pn junction is formed in the optical guide layer 310, by injecting carriers into the phase control section 400, the refractive index of this portion can be changed. It was formed by cleaving the end faces on both sides of the element.

An example of the characteristics of the device manufactured in this way is shown below. Threshold value 20mA, external differential quantum efficiency 18%, oscillation wavelength 1.
It was 5511m. The spectral line width is 0.7 MHz at 5 mW output, which is about one order of magnitude narrower than that of a single laser. The spectral linewidth changes periodically as the injection current to the laser increases, but by controlling the injection current of the phase control section at the same time as the injection current to the laser, it can be controlled over a relatively wide range of injection current to the laser. Spectral linewidth I
I was able to keep it below MHz. For example, when phase control is not performed, the laser current range is 5 m when the line width is below IMHz.
A became approximately 20 mA through phase control, improving stability. Furthermore, by slightly changing the injection current of the phase control section 400 within this current range, frequency modulation could be performed while keeping the spectral linewidth at a substantially constant value (approximately IMHz). Frequency deviation amount is 0.5GHz/yn
A flat frequency modulation characteristic was obtained up to around 800 MHz.

In the above-described embodiment, the phase control section 400 is placed adjacent to the DFB laser section 100, but the phase control section 400 is basically the same no matter where it is formed in the external resonator section 200. Furthermore, by forming a highly reflective film on the end face on the external resonance side, the amount of feedback light can be increased and the spectral linewidth can be further reduced. The method for changing the refractive index of the phase control section 400 is not limited to carrier injection, and may also use an electro-optic effect. In this case, higher performance can be expected by forming the optical guide layer 310 into a quantum well structure.

(Effects of the Invention) As described above, according to the present invention, the spectral line width is narrow;
In addition, a light source for coherent optical transmission with excellent frequency controllability can be realized. The PSK heterodyne system requires a narrow spectral linewidth and a stable frequency. Furthermore, the FSK heterodyne system requires a narrow spectral linewidth and good frequency modulation characteristics. The present invention is applicable to either of them. In the example, a stable spectral linewidth can be obtained over a wide current range, and 800MHz
A flat frequency modulation characteristic was obtained.

Note that these characteristics are important not only for optical transmission but also for various types of measurements, and it goes without saying that the present invention can be applied to applications other than optical transmission in terms of materials and applications.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the basic structure of the present invention, and FIG. 2 is a perspective view showing an embodiment. In the figure, 100 is a DFB laser section, 200 is an external resonator section, 400 is a phase control section, 30
0 is a substrate, 310 is a light guide layer, 320 is an active layer, 33
0.340 is the cladding layer.

Claims (1)

[Claims] In an integrated semiconductor laser in which a distributed feedback semiconductor laser and an external resonator consisting of a passive optical waveguide connected to a light emitting end of the distributed feedback semiconductor laser are formed on one semiconductor substrate, the passive optical waveguide 1. A distributed feedback semiconductor laser with an external cavity, comprising means for electrically varying the refractive index of at least a part of the wavepath.