Semiconductor Laser Devices

This invention relates to semiconductor laser devices and in particular to the stabilization in narrowing the spectrum linewidth, suitable for the light emitting source in coherent optical transmission systems.

Heretofore the narrowing of the emission spectrum linewidth of semiconductor laser devices with a external cavity is effected by utilizing coupling between the modes of the cavity within the laser and the external cavity. However, the condition to get this effect is very critical and it is known that widening of spectrum linewidth, wavelength jumping or output power fluctuations are caused by slight variations in injection current and temperature. An example therefor may be the one reported in an article in the preliminary reports for the national meeting of the Institute of Electronics and Communication Engineers of Japan 4, 1985, p. 22. Further, it is conceivable to control the length of the external cavity in submicron order by jointing a piezo element with the reflecting surface of the external cavity and by utilizing electro-striction as a method for varying the length of the external cavity in order to control the phase of feedback light to the internal cavity. This method is described e.g. in IEEE J. Quantum Electron. Vol. QE-16, p. 347, (1980). However, since, according to this method, optical axis alignment, mounting process, etc. need very precise techniques, this method is not practical.

An object of this invention is to provide a semiconductor laser device, which has a narrow spectrum linewidth at an arbitrary oscillation wavelength and few fluctuations in wavelength and in linewidth.

The inventors of this invention have paid attention to the fact that the phase matching at the junction between the two cavities must be controlled with a high precision for narrowing the spectrum linewidth of the semiconductor laser using an external cavity, because the phase matching between the lights the internal and external cavities of the laser is utilized therefor. In fact, when injection current is varied in a semiconductor laser having a fixed external cavity, phase matching is achieved at an appropriate injection current and light is emitted with a very narrow spectrum linewidth. It is thought that this is due to the fact that the refractive index varies with variations of the carrier density within the cavity and the phase of the light varies at the junction interface so that the phase matching condition is satisfied. However, when the optical path length in the external cavity is constant, the phase matching condition for narrowing spectrum linewidth varies with variations in refractive index due to variations in laser injection current and temperature. Thus, the spectrum linewidth and the oscillation wavelength can change easily. Therefore, the inventors of this invention have changed their point of view and intended to control electrically the phase and the intensity in the external cavity. Thus it has been studied to obtain light emission having a stable and narrow spectrum linewidth by controlling electrically the optical path length in the external cavity so that the phase in the external cavity is always matched with that in the internal cavity. In order to control the optical path length in the external cavity, a material the refractive index of which can be changed electrically is used for forming the cavity. Further, it has been also studied to effect the light intensity adjustment within the external cavity of the same material. In addition, attention has been paid to the fact that, if this external cavity can be constructed to be monolithic, its temperature controllability and the coupling efficiency between the internal and external cavities are ameliorated and a method for monolithic integration has been studied. In this way this invention has been done for realizing an integrated semiconductor laser, by which control of feedback light intensity (light intensity within the external cavity) and phase is possible by utilizing the principle of the carrier injection type optical switch.

According to this invention, it is possible to control easily the oscillation spectrum linewidth of the semiconductor laser so as to obtain the narrowest state by controlling the optical path length in the external cavity. Furthermore, it is possible to obtain easily a narrow spectrum linewidth having a stable oscillation wavelength by controlling feedback light intensity (light intensity within the external cavity). As stated above, an effect can be obtained to provide a semiconductor laser suitable for the light emitting source in coherent optical transmission systems.

FIG. 1 is a cross-sectional view illustrating a semiconductor laser and an external cavity made of LiNbO.sub.3 and jointed therewith;

FIG. 2a is a cross-sectional view illustrating a semiconductor laser and a phase controllable external cavity constructed in a monolithic form;

FIG. 2b is a perspective view of the device indicated in FIG. 2a;

FIG. 3 is a top view illustrating an element, in which a semiconductor laser and a feedback light intensity and phase controllable external cavity are constructed in a monolithic form;

FIG. 4a is a top view of an element used in Embodiment 4; and

FIG. 4b is a cross-sectional view along the line A--A' of the element indicated in FIG. 4a.

FIG. 1 illustrates a well-known semiconductor laser 1 and an optical waveguide path 3 for the external cavity made of a crystal of LiNbO.sub.3, which is an electro-optic material and in contact with a facet at one side thereof. The semiconductor laser 1 and LiNbO.sub.3 2 for the external cavity are so aligned and fixed that light reflected by a reflective end 9 of the LiNbO.sub.3 returns to the semiconductor laser 1. This external cavity is 7 mm long and 6 .mu.m wide. The incidence end 6 of the external cavity is in contact with the semiconductor laser. A reflective facet 9 is disposed at the end surface opposite to the incidence end of the external cavity. The LiNbO.sub.3 is Z-cut and the optical waveguide path 3 is formed by Ti diffusion. Electrodes 4 and 5 are so formed that electric field is applied along the y-axis of the crystal. The semiconductor laser 1 is made of InGaAsP/InP and its oscillation wavelength is 1.54 .mu.m. It is possible to control narrowing of the spectrum linewidth with respect to an arbitrary laser output and wavelength by varying the voltage applied to the external cavity 2 within a range of 0 to 3 V so as to control the optical path length of the external cavity. Further the similar result can be obtained, even if LiNbO.sub.3 is replaced by LiTaO.sub.3.

Explanation will be made while referring to FIGS. 2a and 2b. A diffraction grating having a period of 234 nm is cut on the surface of an n-conductivity type InP substrate 14. A guiding layer 10 (InGaAsP having a band gap wavelength .lambda.g=1.3 .mu.m) for the distributed feedback type laser, an active layer 11 (.lambda.g=1.53 .mu.m), an antimeltback layer 12 (.lambda.g=1.3 .mu.m) and a p-conductivity type InP layer 16-1 are superposed on each other by liquid phase growth. Then, after having removed the grown layers located on the external cavity 2 by selective etching, an optical waveguide layer 15 (.lambda.g=1.3 .mu.m) and a p-conductivity type InP layer 16-2 are superposed on each other by liquid phase growth. A part of the grown layers is removed so that a mesa stripe about 1 .mu.m wide, which stretches over both the two regions (1 and 2), is formed. After that, p-, n-, and p-conductivity type InP layers are grown one after another as a clad layer for blocking a current and for forming a waveguide by distributed refractive index. In order to isolate electrically the laser and the external cavity, unnecessary portions of the grown layers are removed by etching, remaining the optical waveguide path. An SiO.sub.2 layer is formed thereon, in which a window is formed at the region for current injection, where an anode electrode 4 is formed by evaporation. After having formed a cathode electrode 5 by evaporation on the back (rear) side surface, an SiO.sub.2 layer for isolation is superposed on the end surface of the external cavity and an Au film for reflection mirror is formed thereon by evaporation. In the case where the external cavity is 2 mm long, when an electric current of 20 mA flows through the external cavity (the refractive index being controlled by such injection level), it is confirmed that the mode in the external cavity is changed by 1. Furthermore, even in the case where oscillation wavelength of the laser varies in a range of 1.5455-1.5465 .mu.m due to variations in injection current, the condition for continuously narrowing the spectrum linewidth can be realized, through the observation in varying the current level flowing through the external cavity in the range of 0 to about 100 mA.

FIG. 2b is a perspective view of the element of this embodiment.

As explained above, by forming the monolithic form it is easy to control the spectrum linewidth narrowing.

Explanation will be made while referring to FIG. 3. FIG. 3 is a top view of an element, in which an optical waveguide path 22, a reflective and transparent area 23 for adjusting the feedback light intensity (i.e. the ratio of reflected light intensity to refracted or transmitted light intensity being adjustable), a phase controlling area (i.e. an external cavity) 25, an extra power reduction area 24 for reducing the feedback light intensity upon adjustment thereof and a reflective coating 26, all of which, except the coating 26, are made of III-V compound crystals. The intensity and the phase of the light within the external cavity can be controlled by varying the refractive index by allowing an electric current to flow through the reflective area 23 for adjusting the feedback light intensity and either by applying a voltage to the phase controlling area 25 or by making an electric current to flow therethrough. Owing to this construction, it is possible to suppress the mode fluctuation within the internal cavity at an arbitrary laser output and wavelength and to obtain a stable and narrow spectrum linewidth at a constant wavelength. Absorption of light by semiconductor and absorption of light by free carriers can be also utilized for this control of feedback power within the external cavity.

Another embodiment of this invention will be explained, referring to FIGS. 4a and 4b.

An external resonator is constructed by forming electrodes 14, 15 for applying a voltage at both the sides of an optical waveguide path 13, which is formed by diffusing Ti in an LiNbO.sub.3 substrate 27, Z-cut as indicated in FIG. 4a, beside a DFB (distributed feedback type) laser 1 having an oscillation wavelength 1.55 .mu.m, comprising positive and negative electrodes 30 and 33, respectively, and buried multiple layers 31 including an active layer formed on an InP substrate 32. This laser 1 and the external resonator are so regulated that the optical waveguide path receives the laser output and is secured on a fixed base 34. This external resonator is 7 mm long and 6 .mu.m wide and reflective coating of Au 35 is disposed on its end surface. Electric current for driving the DFB laser is set at an intensity two times as high as its threshold intensity. Variations in output intensity and spectrum linewidth accompanied by variations in phase of the returning light are confirmed by varying the voltage between the two electrodes of the external resonator in a range of 0-3 V.

The similar results can be obtained also, when a semiconductor of GaAs family and Si family, is used instead of those of InP family in the above described embodiments.