Waveguide optical element and its driving method
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a waveguide optical element in which a thin film such as an antireflection film or a high-reflectance film is formed at an end face of a waveguide and its driving method and, more particularly, to a waveguide optical element used as an optical amplifier in, e.g., an optical communication system, and its driving method. 
     2. Related Background Art 
     A conventional travelling wave type laser amplifier is formed in such a manner that a semiconductor active layer serving as a waveguide is formed on a substrate, and an electrode for injecting a current to the active layer is arranged. In this amplifier, the two end faces of the active layer are cleavaged to constitute a Fabry-Perot resonator. 
     When the amplifier is to be driven, a current equal to or lower than a threshold value, i.e., a current value for causing the amplifier itself to emit light is injected from the electrode to the active layer. Light is caused to be incident from one end face. The incident light is amplified by induced emission in the active layer, and emerges from the other end face. 
     In order to increase a gain of such a laser amplifier, it is known to form an antireflection film on the end face of the resonator. When an antireflection film is formed in this manner, the above-mentioned threshold value current is increased, and the density of carriers capable of being injected into the active layer can be increased. When the carrier density in the active layer is increased, a gain of light incident on the amplifier can be increased accordingly. The Fabry-Perot resonator has very severe wavelength selectivity in its light transmission characteristics. The antireflection film also has an effect of reducing the wavelength selectivity, and widening a wavelength range of light which can be used for the amplifier. 
     However, in the conventional amplifier, the refractive index, the film thickness, and the like of the antireflection film are determined to obtain a maximum antireflection effect against light having a predetermined wavelength at a predetermined temperature. For this reason, the conventional amplifier cannot obtain a sufficient gain when the wavelength of light to be actually used is different from the predetermined wavelength, or when a temperature during an operation is changed. The same problems are posed not only in the amplifier described above, but also in a laser diode in which a high-reflectance film is formed on an end face of a resonator. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a waveguide optical element which can solve the conventional problems, and can satisfactorily provide its function even when the wavelength of light to be used or the temperature in use is changed, and its driving method. 
     In order to achieve the above object, there is provided a waveguide optical element comprising: 
     a substrate; 
     a waveguide layer which is formed on the substrate, and through which light propagates, the waveguide layer having an end face from which the propagation light becomes incident on the waveguide layer, or emerges; 
     a thin film, formed on the end face of the waveguide layer, for increasing or decreasing a reflectance of light at the end face; and 
     control means for controlling a refractive index of a portion of the waveguide layer, which portion is in contact with the thin film. 
     There is also provided a semiconductor optical amplifier according to the present invention, comprising: 
     a substrate; 
     a semiconductor waveguide layer which is formed on the substrate, and through which light propagates, the waveguide layer having a pair of end faces constituting a laser resonator; 
     amplifier means for injecting a current into the waveguide layer to amplify the propagation light; 
     antireflection films formed on the two end faces of the waveguide layer; and 
     control means for controlling refractive indices of portions of the waveguide layer, which portions are in contact with the antireflection films. 
     There is also provided a method of driving the semiconductor optical amplifier, comprising the steps of: 
     inputting light from one end face of the waveguide layer; 
     causing the amplifier means to inject a current into the waveguide layer so as to amplify the input light; 
     detecting the amplified light emerging from the other end face of the waveguide layer; and 
     causing the control means to control refractive indices of the portions of the waveguide layer, which portions contact the antireflection films, in accordance with an intensity of the detected light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view showing a semiconductor optical amplifier according to the first embodiment of the present invention; 
     FIG. 2 is a schematic perspective view showing a semiconductor optical amplifier according to the second embodiment of the present invention; 
     FIG. 3 is a schematic sectional view of the second embodiment taken along a direction perpendicular to a resonance direction; 
     FIG. 4 is a schematic sectional view of the second embodiment taken along the resonance direction; and 
     FIG. 5 is a schematic diagram of a repeater of an optical communication system using the semiconductor optical amplifier of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic perspective view showing the first embodiment in which the present invention is applied to a semiconductor optical amplifier. In FIG. 1, the optical amplifier comprises a first electrode 1 formed of, e.g., an alloy of gold (Au) and germanium (Ge), a substrate 2 formed of, e.g., an n-type GaAs layer, a first (lower) cladding layer 3 formed of, e.g., an n-type Al 0 .3 Ga 0 .7 As layer, an active layer 4 formed of, e.g., a non-doped GaAs layer, a second (upper) cladding layer 5 formed of, e.g., a p-type Al 0 .3 Ga 0 .7 s layer, a capping layer 6 formed of, e.g., a p-type GaAs layer, window waveguide layers 7a and 7b which are located on two sides of the active layer 4 with respect to a light propagation direction, and are formed of, e.g., non-doped Al 0 .1 Ga 0 .9 As layers, a second electrode 8, formed of, e.g., an alloy of gold (Au) and chromium (Cr), for injecting a current to the active layer 4, third and fourth electrodes 9a and 9b, formed of, e.g., an alloy of gold (Au) and chromium (Cr), for injecting a current to the window waveguide layers 7a and 7b, respectively, antireflection films 10a and 10b formed of, e.g., an insulating material on the two end faces, and an insulating film 11 formed of, e.g., an SiO 2  layer. The active layer 4 and the window waveguide layers 7a and 7b constitute a light waveguide layer for transmitting light as a whole. The two end faces, where the antireflection films 10 a and 10b are formed, of the light waveguide layer serve as resonance surfaces, thus constituting a Fabry-Perot resonator. In a stripe-shaped portion extending along a light propagation direction, the second, third, and fourth electrodes 8, 9a, and 9b are in ohmic-contact with the capping layer. The stripe-shaped portion serves as an effective region of the amplifier. 
     The operation of the first embodiment with the above-mentioned structure will be described below. 
     The semiconductor optical amplifier is a so-called travelling wave type laser amplifier in which externally input light propagates through the effective region of the waveguide layer (the central portions of the window waveguide layer 7a, the active layer 4, and the window waveguide layer 7b), is amplified by induced emission when it passes through the active layer 4, and is output from the waveguide layer. Therefore, the antireflection films 10a and 10b are formed on the surface for receiving externally input light, and the surface for outputting light outside the amplifier, as described above. The refractive indices of waveguide portions contacting the antireflection films 10a and 10b, i.e., the window waveguide layers 7a and 7b are electrically controlled, so that reflectances of the antireflection films 10a and 10b are lower than the wavelength of input light or a peak wavelength in a gain spectrum of the active layer 4. For this purpose, according to this embodiment, a carrier is injected into the window waveguide layers 7a and 7b via the first, third, and fourth electrodes 1, 9a, and 9b, thereby changing equivalent refractive indices of the window waveguide layers 7a and 7b by a plasma effect. Thus, an injection current amount can be controlled so that the reflectances of the antireflection films 10a and 10b are lower than the input light wavelength. 
     In order to operate the amplifier of this embodiment as a travelling wave type laser amplifier, a current is injected from the first and second electrodes 1 and 8 into the active layer 4 to cause induced emission in the active layer 4. At this time, the injected current flows only through the effective region at the central portion of the active layer 4 due to the presence of the insulating film 11. Of course, a current injection amount at this time is set to be equal to or lower than a current threshold value of laser oscillation. Note that the antireflection films 10a and 10b do not perfectly inhibit reflection of light at the end faces, and the end faces cause slight reflection. Thus, the amplifier has a current threshold value of laser oscillation corresponding to the reflection at the end faces. 
     On the other hand, a current is injected into the window waveguide layers 7a and 7b via the first, third, and fourth electrodes 1, 9a, and 9b, and the refractive indices of the layers 7a and 7b are changed by the plasma effect. 
     In this manner, the reflectances of the end faces where the antireflection films 10a and 10b are formed can be decreased as much as possible with respect to a desired wavelength. Thus, a current amount capable of being injected into the effective region of the active layer 4 can be increased, and a larger gain than that of the conventional amplifier can be obtained. 
     FIG. 2 is a schematic perspective view showing the second embodiment in which the present invention is applied to a semiconductor optical amplifier. FIG. 3 is a schematic sectional view of the second embodiment taken along a direction perpendicular to a resonance direction, as indicated by a line segment A-A&#39; in FIG. 2. FIG. 4 is a schematic sectional view of the second embodiment taken along the resonance direction, as indicated by a line segment B-B&#39; in FIG. 2. 
     In FIGS. 2 to 4, the amplifier comprises a first electrode 21 formed of, e.g., an alloy of gold and germanium, a substrate 22 formed of, e.g., an n-type GaAs layer, a first cladding layer 23 formed of, e.g., a 2-μm thick n-type Al 0 .3 Ga 0 .7 As layer, quantum well structure (QWS) layers 24a and 24b obtained by alternately stacking, e.g., two sets of 100-Å thick GaAs well layers and 200-Å thick Al 0 .2 Ga 0 .8 As barrier layers, a second cladding layer 25 formed of, e.g., a 2-μm thick p-type Al 0 .3 Ga 0 .7 As layer, a capping layer 26 formed of a 0.1-μm thick p-type GaAs layer, a first buried layer 27 which is formed of, e.g., a p-type Al 0 .4 Ga 0 .6 As layer, and has a thickness from the substrate 22 to the QWS layers 24a and 24b, a second buried layer 28 formed of, e.g., an n-type Al 0 .4 Ga 0 .6 As layer on the first buried layer 27, antireflection films 29a and 29b  formed at the two end faces of the amplifier, a second electrode 30, formed of, e.g., an alloy of gold and chromium, for injecting a current into an active region, and third and fourth electrodes 31a and 31b, formed of, e.g., an alloy of gold and chromium, for applying reverse bias voltages to the QWS layers 24a and 24b near the two end faces, respectively. 
     As shown in FIGS. 3 and 4, an active layer 32 formed of, e.g., a 0.1-μm thick non-doped GaAs layer is formed between the QWS layers 24a and 24b on two sides to be sandwiched between the first and second cladding layers 23 and 25. 
     The refractive indices of the QWS layers 24a and 24b on two sides of the active layer 32 are changed by a quantum confinement Stark effect (QCSE) upon application of the reverse bias voltages from the first, third, and fourth electrodes 21, 31a, and 31b. This is because electron levels of subbands of the well layers in the QWS layers are shifted upon application of the voltage. When the refractive indices of the QWS layers are changed in this manner, the reflectances of the end faces where the antireflection films 29a and 29b are formed can also be changed. Therefore, according to the second embodiment, electric fields to be applied to the QWS layers 24a and 24b are adjusted, so that the reflectances of the end faces are lowered as much as possible with respect to a wavelength of light to be used, thus obtaining a large gain like in the first embodiment. 
     The basic operation of the second embodiment is substantially the same as that of the first embodiment, except that the lateral confinement of light is performed in the buried layers 27 and 28. 
     In the above embodiments, the refractive indices of the light waveguide portions near the antireflection films are changed by using the plasma effect or the QCSE, so that the reflectances of the antireflection films can be lowered with respect to a desired wavelength than those of such films which are simply formed on the end faces. In place of these effects, other effects, e.g., a Franz-Keldysh effect, may be used. 
     An application of the optical amplifier described above will be described below. 
     FIG. 5 shows an application example. In FIG. 5, reference numeral 51 designates an input signal, and reference numeral 52 designates an output signal. A semiconductor optical amplifier 53 according to the present invention comprises refractive index control sections 53-1 and 53-3, and an amplifier section 53-2. 
     The optical amplifier 53 has a structure described above with reference to FIG. 1 or FIGS. 2 to 4. 
     The amplifier 53 is connected to a control circuit 54 for controlling the optical amplifier 53, a beam sampler 55 for extracting some components of the output signal light 52, and a photodetector 56 for detecting the components of the output signal light 52 extracted by the beam sampler 55. 
     The control circuit 54 controls the refractive index control sections 53-1 and 53-3, and the amplifier section 53-2 of the semiconductor optical amplifier 53. 
     The arrangement shown in FIG. 5 can be used as a repeater in an optical communication system. The operation of the optical amplifier 53 in this case is as follows. 
     Prior to input of the input signal light 51, light which has the same wavelength as that of the signal light 51 and is modulated at a frequency ω 2  (adjustment signal light) is input to the semiconductor optical amplifier 53. Assume that a current modulated at a frequency ω 1  is injected into the amplifier section 53-2 of the amplifier 53. In this state, when the adjustment signal light is input to the amplifier 53, it is amplified by the amplifier section 53-2, and is output as the output signal light 52. 
     Some components of the output signal light 52 are separated by the beam sampler 55, and are input to the photodetector 56. The output signal from the photodetector 56 is input to the control circuit 54, and is separated into a signal component corresponding to a light intensity of light which has the frequency ω 1   and is emitted from the amplifier 53 itself (signal S 1 ), and a signal component corresponding to the amplified adjustment signal light having the frequency ω 2  (signal S 2 ). The control circuit 54 changes the refractive indices of the refractive index control sections 53-1 and 53-3 on the basis of these separated signals S 1  and S 2 , and controls to maximize the intensity of the amplified light without largely increasing light emitted from the amplifier 53 itself, i.e., without causing laser oscillation of the amplifier 53. In order to change the refractive indices of the refractive index control sections 53-1 and 53-3, when the first embodiment is adopted as the amplifier 53, a current to be injected into these control sections is changed. On the other hand, when the second embodiment is adopted as the amplifier 53, a voltage to be applied to the control sections 53-1 and 53-3 is changed. 
     Under the above-mentioned control, the semiconductor optical amplifier 53 can always provide a maximum gain corresponding to the current injected into the amplifier section 53-2. 
     In FIG. 5, light propagates in only one direction. However, the present invention can be applied to an arrangement in which light is input from two directions. If the input signal light 51 is separated by the beam sampler and the separated components are input to the control circuit 54, the gain can also be controlled. 
     Various other applications of the present invention may be made in addition to the embodiments described above. For example, the present invention may be applied to a laser diode in which high-reflectance films are formed at end faces of a resonator in addition to the optical amplifier. In this case, waveguide portions whose refractive indices are adjusted are arranged near laser resonance surfaces where the high-reflectance films are formed. When the refractive indices of the adjustment portions are controlled, the reflectances at the resonance surfaces can be maximized. The present invention includes such applications as long as they do not depart from the scope of claims.