Method and apparatus for light amplification exhibiting a flat gain spectrum

Abstract: A semiconductor light amplifier (21) comprises a substrate (1), a semiconductor activation layer (4) formed on the substrate for propagating a light, and amplifying a light propagated through the activation layer which has a wavelength within a predetermined range when a current is applied thereto, an electrode (13) for applying a current to at least a portion of the activation layer, and means (9) for imparting a loss to the light propagated through the activation layer which has a wavelength in a portion of the predetermined wavelength range.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor light amplifier used in a light communication system, and a method for light amplification by the light amplifier. Related Background Art In general, a semiconductor light amplifier refers to one which has a semiconductor laser structure and amplifies an external input light with a biasing current lower than a threshold. In a field of light communication, it has been developed to compensate for light loss which occurs in the connection in a fiber or between fibers. Further, since the semiconductor light amplifier has a gain over a wide wavelength range and can simultaneously amplify lights having different wavelengths, it is expected for use in simultaneous amplification of multi-wavelength signals. Where the semiconductor light amplifier is used for the simultaneous amplification of the multi-wavelength signals, it is desirable to impart the same gain to various input lights. However, it is known that in the prior art semiconductor light amplifier the gains are different from wavelength to wavelength. Such differences in the gains are harmful in constructing a multi-wavelength communication system. For example, after a signal light has passed through the semiconductor light amplifier, the output light levels of the signals of different wavelengths are different and hence an additional requirement such as broadening a dynamic range of a light receiver is imposed. On the other hand, based on the restriction imposed to the light receiver, the wavelengths of the signal lights, the wavelength multiplexity and the wavelength range are determined. This is disadvantageous in designing the system. The wavelength dependency of the gain, that is, the gain spectrum is mainly due to a laser structure. Accordingly, the gain spectrum is designed by designing an activation layer of the semiconductor laser structure. As a method for overcoming the above problems and improving the gain spectrum, Japanese Laid-Open Patent Application No. 1-179488 discloses a method of imparting a quantum well structure having different quantum wells to a light amplifier. On the other hand, US-A-4,680,769 discloses a method for constructing a laser structure by connecting two serially arranged activation layers. In those methods, the gains are superimposed on the spectrum to design a gain spectrum in a desired wavelength range, particularly in a wide wavelength range. However, in those methods, the design is relatively complex because the activation layer itself is a parameter to determine the gain spectrum. For example, in the method disclosed in Japanese Laid-Open Patent Application No. 1-179488, a desired result is not readily attained unless the design is made with the consideration to a current injection condition. In the method disclosed in US-A-4,680,769, it is difficult to connect the two activation layers and it is necessary to pay full attention to an optical alignment. Those methods may be effective when the semiconductor light amplifier is designed for a wide wavelength range of several tens to several hundreds nm, but since only one activation layer may have a broad gain spectrum distribution, it is not always wise to try to attain the equal effect in a relatively narrow wavelength range. In such a case, the superposition of the gain spectra described above may cause the degradation of other properties such as increase of noise. SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems encountered in the prior art and provide a light amplifier which has a simple construction and attains a flat gain spectrum without increasing noise, and a method for light amplification by using the light amplifier. According to the invention the above object is accomplished by a semiconductor light amplifier comprising: a substrate; a semiconductor activation layer formed on said substrate for propagating light; said activation layer amplifying the light propagated through said activation layer when a current is applied thereto, the light having a wavelength within a predetermined range; said activation layer having a non-flat gain spectrum in said predetermined wavelength range; an electrode for supplying said current to at least a portion of said activation layer; loss means for imparting a loss to the light propagating through said activation layer, the light having a wavelength in a portion of said predetermined wavelength range; said portion of wavelength range corresponding to a wavelength range having a relatively high gain in a gain spectrum of said activation layer; and said loss means flattening within said portion of said predetermined wavelength range a gain spectrum of the light output from said activation layer for the light input to said activation layer, relative to the gain spectrum of said activation layer. According to the invention the above object is also accomplished by a light communication system comprising: (a) a plurality of terminals each including at least one of a light transmitter and a light receiver; (b) a light transmission line for interconnecting said terminals; and (c) a semiconductor light amplifier according to any of the claims 1 to 17 arranged in a path of said light transmission line for amplifying a light signal transmitted through said light transmission line. Additonally, according to the invention this object is accomplished by a method for amplifying light by using a semiconductor light amplifier having a substrate, a semiconductor activation layer formed on said substrate, an electrode for supplying a current to at least a portion of said activation layer and loss means for imparting a loss to a portion of light propagated through said activation layer, comprising the steps of: applying light to said activation layer; propagating the input light through said activation layer; applying a current from said electrode to said activation layer to amplify the light propagated through said activation layer, the light having a wavelength within a predetermined range; said activation layer having a non-flat gain spectrum in said predetermined wavelength range; imparting a loss by said loss means for imparting the loss to the light propagated through said activation layer, the light having a wavelength in a portion of said predetermined wavelength range; and taking out the amplified light from said activation layer, said portion of wavelength range corresponding to a wavelength range having a relatively high gain in a gain spectrum of said activation layer, a gain spectrum of the light output from said activation layer for the light input to said activation layer being flattened within said portion of said predetermined wavelength range relative to the gain spectrum of said activation layer by imparting the loss to the light in said portion of said predetermined wavelength range. In the present invention, an optical filter region or wavelength selective loss means for losing light by separating or absorbing a light of a specific wavelength range or wavelength from a light amplification region having a wave guide structure for amplifying a guided light is provided so that a total gain spectrum between input and output may be designed independently from the light amplification region comprising an activation layer formed on a semiconductor substrate. In this manner, the gain spectrum of the light amplification region is improved for a desired wavelength range or wavelengths to substantially equalize the gain spectrum between input and output. Specifically, the wavelength-selective loss means may comprise a main wave guide directly connected to the light amplification region, an adjacent sub-wave guide and a grating or directive coupler which wavelength-selectively couples those wave guides, or it may comprise a grating or light absorption region which is directly connected to the light amplification region and formed obliquely. The light amplification region and the wavelength-selective loss means may be formed at separate positions or an overlapped one position as viewed from the direction of travel of the guided light. In order to isolate the light wave separated by the sub-wave guide from the input/output terminals, a reflection-free structure such as a large absorption region or an oblique end plane may be formed at the end of the sub-wave guide. Further, the sub-wave guide may be a channel wave guide or a slab wave guide, it may be laminated at a different position from the main wave guide as viewed from a lamination direction of the laminated layers on a substrate, or it may be aligned in the same lamination plane. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a side sectional view of a first embodiment of a light amplifier of the present invention; Fig. 2 shows a front sectional view of the semiconductor light amplifier of Fig. 1; Fig. 3 shows a propagation mode of an activation layer and a wave guide layer in the first embodiment; Fig. 4A shows a gain spectrum of the activation layer in the first embodiment; Fig. 4B shows a wavelength dependency of a power transfer of light from the activation layer to the wave guide layer by a diffraction grating in the first embodiment; Fig. 4C shows a gain spectrum of amplification of an output light to an input light in the first embodiment; Fig. 5 shows a side sectional view of a second embodiment of the light amplifier of the present invention; Fig. 6 shows a plan view of a third embodiment of the light amplifier of the present invention; Fig. 7 shows a plan view of a fourth embodiment of the light amplifier of the present invention; Fig. 8 shows a plan view of a fifth embodiment of the light amplifier of the present invention; Fig. 9A shows a gain spectrum of a light amplification region in the fifth embodiment; Fig. 9B shows a wavelength dependency of a light transmittance of a light absorption region in the fifth embodiment; Fig. 9C shows a gain spectrum of amplification of an output light to an input light in the fifth embodiment; and, Fig. 10 shows a configuration of a light communication system which uses the light amplifier of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention are now explained in detail with reference to the drawings in which the like elements are designated by the like numerals and detailed explanation thereof is omitted. Fig. 1 shows a side sectional view of a first embodiment of the light amplifier of the present invention. The light amplifier comprises a semiconductor light amplifier 21, and a power supply 20. A front sectional view of the light amplifier taken along a line A-A′ is shown in Fig. 2. In the semiconductor light amplifier 21 of Fig. 1, a buffer layer 2, a first clad layer 3, an activation layer 4 of a multi-quantum well (MQW) structure, a second clad layer 5, an optical wave guide layer 6, a third clad layer 7 and a cap layer 8 are sequentially laminated on a substrate 1. The activation layer 4 forms a cavity of a laser resonator by the opposite end planes which are parallel to each other. Anti-reflection (AR) coatings 15 are applied on those end planes. They are provided to prevent the occurrence of the laser resonation when a current is supplied under a condition of no input light to the activation layer 4. In the light amplifier of the present embodiment, a so-called travelling wave type laser amplifier is formed by providing the AR coatings 15. The light amplifier 21 is divided into an optical filter region 18 and a light amplification region 19 along a direction of light propagation. A diffraction grating 9 is formed on a boundary surface between the second clad layer 5 and the optical wave guide layer 6 in the optical filter region 18. The diffraction grating 9 couples a light which is propagated through the activation layer 4 and has a predetermined wavelength range to the optical wave guide layer 6. A first electrode 13 is formed on the cap layer 8 in the light amplification region 19. The first electrode 13 is connected to the power supply 20. A second electrode 14 is formed on the bottom surface of the substrate 1. The power supply 20 supplies a current to the activation layer 4 in the light amplification region through those electrodes 13, 14. The activation layer 4 amplifies the light other than a portion of the light coupled to the optical wave guide layer 6, by the supply of the current. A portion of the first clad layer 3, the activation layer 4, the second clad layer 5, the optical wave guide layer 6, the third clad layer 7 and the cap layer 8 are etched to leave a striped mesa extending along a direction of light propagation as shown in Fig. 2. The removed areas are buried by buried layers 10 and 11. An insulation layer 12 is formed on the buried layer 11 so that the current is injected from the electrode 13 only to the mesa. The semiconductor light amplifier may be manufactured in the following manner. The buffer layer 2 made of n-GaAs and the first clad layer 3 made of n-Al 0.5 Ga 0.5 As and having a thickness of 1.5 µm are formed on the substrate 1 made of n-GaAs, by an organic metal chemical vapor deposition (MO-CVD) method. Then, well layers made of non-doped GaAs and barrier layers made of non-doped Al 0.4 Ga 0.6 As are alternately and repeatedly laminated by the MO-CVD method to form the activation layer 4 of an MQW structure having a total thickness of 0.45 µm. The second clad layer 5 made of p-Al 0.5 Ga 0.5 As is grown on the activation layer 4 by the MO-CVD method. Then, the diffraction grating 9 is formed on the surface of the second clad layer 5 in the optical filter region 18 by photolithography. Then, well layers made of p-GaAs and barrier layers made of p-Al 0.5 Ga 0.5 As are alternately and repeatedly laminated on the second clad layer 5 by the MO-CVD method to form the optical wave guide layer 6 of the MQW structure having a total thickness of 0.2 µm. Then, the third clad layer 7 made of p-Al 0.5 Ga 0.5 As having a thickness of 1.5 µm and the cap layer 8 made of p-GaAs having a thickness of 0.2 µm are grown on the optical wave guide layer 6 by the MO-CVD method. In the above process, a molecule beam epitaxial (MBE) method may be used in place of the MO-CVD method. Then, the opposite sides are removed up to the first clad layer 3 by wet etching as shown in Fig. 2, and the buried layer 10 made of n-Al 0.5 Ga 0.5 As and the buried layer 11 made of p-Al 0.5 Ga 0.5 As are grown by a liquid phase epitaxial (LPE) method to form a buried structure. The insulation layer 12 is formed on the buried layer 11 by a plasma CVD method. Then, as shown in Fig. 1, the area which functions as a light input/output area is removed up to the vicinity of the optical wave guide layer 6 by wet etching. The first electrode 13 of p-type is formed in the area on the cap layer 8 which functions as the light amplification region 19, and the second electrode 14 of n-type is formed on the rear surface of the substrate 1. Finally, as shown in Fig. 1, the area which functions as the light input/output area is cleaved and ZrO₂ films are vapor-deposited to end planes thereof to a thickness of λ/4 ( λ : wavelength) by electron beam (EB) vapor deposition to form the AR coating films 15. In this manner, the light amplifier having the optical filter region 18 and the light amplifier region 19 is manufactured. An operation of the present embodiment is now explained. An incident light 16 is condensed by an end of an optical fiber or a lens and applied to the light amplifier. Since the area up to the optical wave guide layer 6 has been removed in the light input/output area, the incident light 16 is efficiently coupled to the activation layer 4. In the area where both the activation layer 4 and the wave guide layer 6 are present, propagation modes shown in Fig. 3 exist. Namely, they include an even mode 30 which is established around the activation layer 4 and an odd mode which is established around the wave guide layer 6. An ordinate represents an electric field strength and an abscissa represents a distance from the activation layer 4. The incident light 16 couples to the even mode 30 which has a peak power in the activation layer 4. In the area where there is no diffraction grating 9, the incident light 16 does not substantially couple to the odd mode 31 and it reaches to the other end plane while it is amplified by the activation layer 4, and produces the output light 17. However, in the area where there is the diffraction grating 9, the light power is transferred if the following condition is met between a propagation constant β o of the odd mode 31 and a propagation constant β e of the even mode 30. β e (λ) - β o (λ) = 2π/Λ where Λ is a pitch of the diffraction grating 9. An efficiency of the transferred light depends on the length L of the diffraction grating 9, and it is represented by η = sin² {(|K|²+Δ²) 1/2 L}/(1+Δ²/|K|²) where K = ∫E e A₁(x)E o d x Δ = β e (λ) - β o (λ)+2 π/Λ E e and E o are electric field distributions in the even mode 30 and the odd mode 31, A₁(x) is a component of Fourier progression development which corresponds to a primary diffracted light. Accordingly, the efficiency is maximum when a complete coupling length L c is π/2K, and when L < π/2K, a portion of the input light 16 can be transferred from the activation layer 4 to the wave guide layer 6. The light transferred to the wave guide layer 6 is guided to the output end without amplification and output from the output end. Since this non-amplified light has a different exit direction from that of the output light 17 from the activation layer 4, they can be separately handled. Thus, in order to attain the light amplifier which has no difference in the gain by the wavelength, that is, which has a flat gain spectrum, the light at the peak wavelength in the gain distribution of the activation layer region 4 is selectively transferred to the wave guide layer 6 to reduce a total gain of the region. Fig. 4A shows a gain spectrum of the activation layer 4 of the first embodiment, that is, a gain spectrum of amplification of the output light 17 to the input light 16 when no diffraction grating 9 is provided, Fig. 4B shows wavelength dependency of power transfer of the light from the activation layer 4 to the wave guide layer 6 by the diffraction grating 9 in the first embodiment, and Fig. 4C shows a gain spectrum of amplification of the output light 17 to the input light 16 in the first embodiment. In order to attain the gain spectrum as shown in Fig. 4C, it is necessary to properly design the diffraction grating 9 in accordance with the gain spectrum of the activation layer 4 and the propagation constants of the activation layer 4 and the optical wave guide layer 6. The pitch Λ of the diffraction grating 9 is determined by the formula (1) to cope with the peak wavelength of the gain spectrum. The length L of the diffraction grating 9 is an important parameter to determine the gain distribution. When L is small, the flattening effect shown in Fig. 4C is small, and when L approaches the complete coupling length L c , a dip of the gain takes place in the vicinity of the center wavelength of the diffraction grating 9. A desired flat gain is attained by properly selecting the length L. In this manner, by properly designing the activation layer 4 and the diffraction grating 9, a good multi-wavelength transmission system having a wavelength range of a flat gain can be constructed. Thus, the gains for the respective signals are substantially uniform, the design of the receiver is simplified, and the system can be constructed without paying much attention to the number of stages and the wavelength multiplexity when the light amplifiers are used in multiple stages. The construction of the present invention is particularly adapted to a traveling wave type light amplifier having a low end plane reflectivity, and a high gain characteristic which is a feature of the traveling wave type can be effectively utilized. In the present embodiment, no electrode is provided in the optical filter region 18 which is completely isolated from the light amplifier region 19. The above is equally applicable where the optical filter region 18 has an amplification function. It is rather preferable in that a difference between indices of refraction by the current injection in the optical filter region 18 and the light amplification region 19 is eliminated. In the first embodiment, the activation layer 4 and the optical wave guide layer 6 are of the MQW structure although they may be of other structure such as single quantum well (SQW) structure or bulk structure. Further, while one wave guide layer 6 and one diffraction grating 9 are used, a plurality of wave guide layers and diffraction gratings may be serially arranged along a direction of layers, in-plane or light travel. The design may be made to cope with the gain spectrum of the light amplification region 19. For example, where the gain spectrum is asymmetric, two diffraction gratings may be used and center frequencies thereof are set toward a shorter wavelength and a longer wavelength from the peak wavelength of the gain distribution, respectively, so that both affect at the peak wavelength of the gain. In this manner, a flat gain spectrum is attained as it is in the first embodiment. In essence, the structures and numbers of the diffraction gratings and the wave guide layers are properly designed in accordance with the gain spectrum of the light amplification region 19 so that the gain spectra of the input/output lights are equalized in the desired wavelength range or at a plurality of wavelengths. Fig. 5 shows a side sectional view of a second embodiment of the light amplifier of the present invention. In the second embodiment, steps are provided in the vicinity of the input/output and planes from the region in which a diffraction grating 51 is formed, along the direction of lamination, and the diffraction grating 51 is of chirp shape. Other features are identical to those of the first embodiment. By providing the steps between the input and the output, the coupling of the input light 16 to the optical wave guide layer 6 is prevented in the input area so that the light is effectively coupled to the activation layer 4. In the output area, the light transferred to the wave guide layer 6 is more lost than it is in the first embodiment by the curvature of the optical wave guide layer 6. Further, since the light propagated through the optical wave guide layer 6 is further positionally spaced from the output light 17 from the activation layer 4 at the exit end, it can be completely separated from the amplified output light 17. Further, since the diffraction grating 51 is of chirp shape, it is effective in designing any desired gain spectrum. In the first embodiment, the wavelength range for flattening is uniquely determined by the area length L of the diffraction grating 9. In the present embodiment, because the diffraction grating 51 is of chirp shape to enable the modulation of the pitch Λ, a parameter to the wavelength may be adjusted so that the gain can be flattened over any wavelength range. A basic operation is identical to that of the first embodiment. In the first and second embodiments, the optical filter extends along the direction of lamination and both the diffraction grating 9, 51 and the wave guide 6 are used. In the present invention, however, the optical filter for improving the gain spectrum need not extend along the direction of lamination and the diffraction grating 9, 51 and the optical wave guide 6 need not coexist. Embodiments in accordance with the above are shown below. Fig. 6 shows a third embodiment and Fig. 7 shows a fourth embodiment. In the third embodiment, the optical filter uses a directional coupler 61 between channel type optical wave guides arranged not along the direction of lamination but laterally (along the in-plane direction). In the present embodiment, a wavelength characteristic of the directional coupler 61 is utilized and the wave guide structure and the area length are designed as parameters such that a portion of the input light 16 is separated from the light amplification region 19 in order to attain the substantially same effect as that of the first embodiment. In the third embodiment, since the coupling is lateral, the activation layer is used as the wave guide layer as it is in the light amplification region 19 as the wave guide structure 62 for the non-amplified light. In this case, the separated non-amplified light is subjected to only the loss by the wave guide 62 including the non-amplifying activation layer. In the lateral coupling, the wave guide structure 62 for the non-amplified light may be relatively easily changed so that a new effect may be expected. For example, as shown in Fig. 6, the wave guide 62 for the non-amplified light may be deflected or applied obliquely to the end plane so that the return light to the input/output areas can be effectively eliminated. Further, it is effective to implant ions locally to the wave guide 62 so that the non-amplified light is subjected to larger loss while it is guided. In the fourth embodiment shown in Fig. 7, an oblique grating 71 is formed in a portion of a channel wave guide to impart a filter effect. Fig. 7 shows a lateral (in-plane) sectional view. At the grating 71, a light propagated through the channel wave guide (directly transmitted 0-order light) 72 and a slab wave guide light (diffracted light) 73 propagated through the slab wave guide externally of the channel are coupled at a Bragg condition. Where a period and an angle of inclination of the grating 71 are selected to meet the Bragg condition for the light wavelength to which a loss is to be imparted as the non-amplified light, the light of the wavelength to which the loss is to be imparted can be coupled from the channel wave guide 72 of the main wave guide to the slab wave guide as the diffracted light 73 to form a wavelength-selective loss area. In the fourth embodiment, it is effective to implant ions externally of the light amplification region 19 to make the light loss larger. The diffraction grating 71 is arranged in the wave guide, but in order to secure the desired wavelength selectivity and diffraction efficiency, the width of the wave guide in only the area in which the grating 71 is present may be increased. In the third and fourth embodiments, the method for flattening the gain spectrum is basically identical to that of the first embodiment shown in Figs. 4A to 4C. Fig. 8 shows a fifth embodiment. In the fifth embodiment, light absorption is utilized as means for wavelength-selective light loss. Accordingly, a light absorption region 81 is formed in the wave guide and the vicinity thereof. Fig. 8 shows a lateral (in-plane) sectional view. For example, where material having a light absorption spectrum as shown in Fig. 4B is available, the same effect as those in the above embodiments may be expected. Materials which exhibit such as absorption spectrum include exciter absorption and light chemical hole burning but they have a very narrow wavelength range. They are therefore not appropriate to the present invention which intends to flatten the gain over the range of several nm to several tens nm. However, it is possible to partially flatten on only one side of the gain distribution by utilizing a normal absorption spectrum (characteristic in the vicinity of the absorption end). Where the light absorption is used, a filter which absorbs light on the shorter wavelength side of a specific wavelength may be used. Thus, the fifth embodiment is somewhat different in its design from the previous embodiments. A design example is shown in Figs. 9A to 9C. Fig. 9A shows a gain spectrum of the light amplification region 19 in the fifth embodiment, Fig. 9B shows wavelength dependency of the light transmittance of the light absorption region 81, and Fig. 9C shows a gain spectrum of amplification of the output light 17 to the input light 16, that is, a total gain spectrum of the light amplifier of the fifth embodiment. By designing the filter transmittance or the absorption spectrum of the filter such that the optical filter compensates for the gradient on the longer wavelength side from the peak of the gain spectrum of the light amplification region 19, the total gain distribution can be flattened in the wavelength area including the gradient. In the present embodiment, the gain distribution of the light amplifier is flattened in this manner. On the boundary of the light absorption region 81 and the light amplification region 19, a refractive index apts to change and reflection apts to occur. In order to prevent it, the boundary of both regions is inclined as shown in Fig. 8. The light absorption region 81 may be formed by using different compositions in the light amplification region 19 and the wave guide. For example, where GaAs is used as the activation layer, Al 0.05 Ga 0.95 As may be used as the wave guide layer to provide a desired optical filter. The compositions may be of appropriate MQW structure. Fig. 10 shows a configuration of a light communication system which uses the light amplifier of the present invention. The light communication system comprises a plurality of transmitting/ receiving terminals 23₁, ..., 23 n-1 , 23 n , a bus line 24 for interconnecting the terminals, and a light amplifier 25 arranged in the path of the bus line 24. Each of the transmitting/receiving terminals 23₁ to 23 n includes at least one of a light transmitter and a light receiver to exchange light signals. The light signal emitted from the terminal is transmitted through the bus line 24, which comprises a light transmission medium such as an optical fiber. The light signal transmitted through the bus line 24 is gradually attenuated by the absorption by the optical fiber. Thus, the light amplifiers 25 are provided in the bus line 24 to amplify the light signal. The semiconductor light amplifier of the present invention is used as the light amplifier 25. Since the light amplifier of the present invention has a flat gain spectrum, it is particularly suitable to the multi-wavelength communication system which communicates by using a plurality of light signals having different wavelengths. The present invention may be applied in various ways other than those shown in the embodiments. For example, while GaAs and AlGaAs are used as the material to form the light amplifier, other III-V group semiconductor or II-VI group semiconductor may be used to form the light amplifier. The present invention includes all such modifications without departing from the scope of claims.

Claims: A semiconductor light amplifier comprising: a substrate (1); a semiconductor activation layer (4) formed on said substrate (1) for propagating light; said activation layer (4) amplifying the light propagated through said activation layer (4) when a current is applied thereto, the light having a wavelength within a predetermined range; said activation layer (4) having a non-flat gain spectrum in said predetermined wavelength range; an electrode (13) for supplying said current to at least a portion of said activation layer (4); loss means (9, 6; 51; 61, 62; 71; 81) for imparting a loss to the light propagating through said activation layer (4), the light having a wavelength in a portion of said predetermined wavelength range; said portion of wavelength range corresponding to a wavelength range having a relatively high gain in a gain spectrum of said activation layer (4);and said loss means (9, 6; 51; 61, 62; 71; 81) flattening within said portion of said predetermined wavelength range a gain spectrum of the light output from said activation layer (4) for the light input to said activation layer (4), relative to the gain spectrum of said activation layer (4). A semiconductor light amplifier according to claim 1, wherein a power supply (20) is connected to said electrode (13). A semiconductor light amplifier according to claim 1 or 2, wherein said semiconductor activation layer (4) propagates the light in a first propagation mode (30); a wave guide layer (6) is formed on said substrate (1) to be laminated on said activation layer (4) thicknesswise; said wave guide layer (6) propagates the light in a second propagation mode (31) different from said first propagation mode (30); and said loss means includes a diffraction grating (9; 51) which is formed in an overlapping area of said frist propagation mode (30) and said second propagation mode (31); said diffraction grating (9; 51) coupling the light propagated through said activation layer (4) to said wave guide layer (6), the light having a wavelength in a portion of said predetermined wavelength range; the light coupled to said wave guide layer (6) being separated from the light amplified by said activation layer (4). A semiconductor amplifier according to claim 1 or 2, wherein said substrate (1) is made of a n-type semiconductor; a first clad layer (3) is formed on said substrate (1) and is made of a n-type semiconductor; said activation layer (4) is formed on said first clad layer (3) and is made of a non-doped semiconductor for propagating light in a first propagation mode (30); a second clad layer (5) is formed on said activation layer (4) and is made of a p-type semiconductor; a wave guide layer (6) is formed on said second clad layer (5) and is made of a p-type seminconductor; said wave guide layer (6) propagates light in a second propagation mode (31) different from said first propagation mode (30); a third clad layer (7) is formed on said wave guide layer (6) and is made of a p-type semiconductor; said electrode (13) is formed on said third clad layer (7); a n-type electrode (14) is formed on a bottom surface of said substrate (1); and said loss means includes a diffraction grating (9; 51) being formed in an overlapping area of said first propagation mode (30) and said second propagation mode (31); said diffraction grating (9; 51) couples the light propagated through said activation layer (4) to said wave guide layer (6), the light having a wavelength in a portion of said predetermined wavelength range; the light coupled to said wave guide layer (6) is separated from the light amplified by said activation layer. A semiconductor light amplifier according to claim 1 or 2, wherein said loss means includes an optical wave guide layer (6) laminated on said activation layer (4) on said substrate (1) and a grating coupler (9; 51) for coupling the light propagated through said activation layer (4), the light having the wavelength in said portion of said predetermined wavelength range. A semiconductor light amplifier according to claim 1 or 2, wherein said activation layer (4) has a first channel type wave guide through which the light to be amplified is propagated, and said loss means includes a second channel type wave guide (62) formed in a portion of said activation layer (4) and having no amplification function and a directional optical coupler (61) for coupling the light propagated through said first channel type wave guide (4) to said second channel type wave guide (62), the light having the wavelength in said portion of said predetermined wavelength range. A semiconductor light amplifier according to claim 1 or 2, wherein said activation layer (4) has a channel type wave guide through which the light to be amplified is propagated, and said loss means includes a slab type wave guide formed externally of said channel type wave guide (4) and having no amplification function and a grating (71) for diffracting the light propagated through said channel type wave guide to couple the light to said slab type waveguide, the light having the wavelength in said portion of said predetermined wavelength range. A semiconductor light amplifier according to claim 1 or 2, wherein said loss means includes a light absorption region (81) formed on a portion of said activation layer (4) for absorbing the light having the wavelength in said portion of said predetermined wavelength range. A semiconductor light amplifier according to claim 3, further comprising: first, second and third clad layers (3, 5, 7); said first clad layer (3), said activation layer (4), said second clad layer (5), said wave guide layer (6) and said third clad layer (7) being laminated on said substrate (1) in this sequence. A semiconductor light amplifier according to claim 4 or 9, wherein said diffraction grating (9; 51) is formed on a boundary surface between said second clad layer (5) and said wave guide layer (6). A semiconductor light amplifier according to claim 4 or 9, wherein said third clad layer (7) and said wave guide layer (6) have opposite ends along the direction of light propagation etched away. A semiconductor light amplifier according to claim 4 or 9, wherein a portion of said first clad layer (3), said activation layer (4), said second clad layer (5), said waveguide layer (6) and said third clad layer (7) are mesa-etched except a stripe area extending along the direction of light propagation, and buried layers (10, 11) are formed on the opposite sides of the mesa. A semiconductor light amplifier according to any of the preceding claims 3, 4, 9 to 12, wherein said activation layer (4) and said waveguide layer (6) have multi-quantum well structure. A semiconductor light amplifier according to any of the preceding claims 3, 4, 9 to 13, wherein said wave guide layer (6) is curved so that a distance from said activation layer (4) is shorter at a center and longer at the opposite ends. A semiconductor light amplifier according to claim 3, 4, 9 to 14, wherein β₁ (λ) - β₂ (λ) = 2π/Λ is met, wherein λ is a wavelength of said portion of said predetermined wavelength range, β₁(λ) is a propagation constant in said first propagation mode (30), β₂(λ) is a propagation constant in said second propagation mode (31) and Λ is a pitch of said diffraction grating (9; 51). A semiconductor according to claim 15, wherein L < π/2K K = ∫E₁A₁(x)E₂dx is met, wherein E₁ is an electric field distribution in said first propagation mode (30), E₂ is an electric field distribution in said second propagation mode (31), A₁(x) is a component of Fourier progression development of said diffraction grating (9; 51) corresponding to a primary diffracted light and L is a length of said diffraction grating (9; 51). A semiconductor light -amplifier according to any of the preceding claims 1 to 16, wherein said substrate (1) and said layers are made of GaAs and AlGaAs, respectively. A light communication system comprising: (a) a plurality of terminals (23₁ to 23 n ) each including at least one of a light transmitter and a light receiver; (b) a light transmission line (24) for interconnecting said terminals (23₁ to 23 n ) ; and (c) a semiconductor light amplifier (25) according to any of the preceding claims 1 to 17 arranged in a path of said light transmission line (24) for amplifying a light signal transmitted through said light transmission line (24). A method for amplifying light by using a semiconductor light amplifier (25) having a substrate (1), a semiconductor activation layer (4) formed on said substrate (1), an electrode (13) for supplying a current to at least a portion of said activation layer (4) and loss means (9; 51; 61; 71; 81) for imparting a loss to a portion of light propagated through said activation layer (4), comprising the steps of: applying light to said activation layer (4); propagating the input light through said activation layer (4); applying a current from said electrode (13) to said activation layer (4) to amplify the light propagated through said activation layer (4), the light having a wavelength within a predetermined range; said activation layer (4) having a non-flat gain spectrum in said predetermined wavelength range; imparting a loss by said loss means (9, 6; 51; 61, 62; 71; 81) for imparting the loss to the light propagated through said activation layer (4), the light having a wavelength in a portion of said predetermined wavelength range; and taking out the amplified light from said activation layer (4), said portion of wavelength range corresponding to a wavelength range having a relatively high gain in a gain spectrum of said activation layer (4), a gain spectrum of the light output from said activation layer (4) for the light input to said activation layer being flattened within said portion of said predetermined wavelength range relative to the gain spectrum of said activation layer (4) by imparting the loss to the light in said portion of said predetermined wavelength range. A method for amplifying light according to claim 19, wherein said semiconductor light amplifier (25) further has a wave guide layer (6) formed on said substrate (1) to be laminated on said activation layer (4) thicknesswise, and said loss means includes a diffraction grating (9; 51) for optically coupling said activation layer (4) and said wave guide layer (6), further comprising the steps of: coupling the light propagated through said activation layer (4) to said wave guide layer (6) by said diffraction grating (9; 51), the light having a wavelength in a portion of said predetermined range; amplifying the remaining light propagated through said activation layer (4) other than the light coupled to wave guide layer (6).

Inventors: HANDA YUICHI C O CANON KABUSHI, NISHIMURA MICHIYO C O CANON KA, HANDA, YUICHI, C/O CANON KABUSHIKI KAISHA, NISHIMURA, MICHIYO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01S3
IPC codes: H01S5, G02F1, G02B6, H04B10