Optical device with an asymmetric dual quantum well structure

Abstract: An optical device includes an asymmetric dual quantum well (ADQW) structure which is comprised of a plur ality(at least two) of different semiconductor quantum well layers coupled to each other. In the ADQW structure, the width of a deeper quantum well layer (42) having a narrower band gap is made narrower than that of a less deeper quantum well layer (41) having a wider band gap such that the shift of an exciton wavelength is scarcely caused due to the quantum confined Stark effect by the application of an electric field in a predetermined range. As a result, only a refractive index of the ADQW structure is changed, but an absorption factor is scarcely changed for a given range of wavelength by the application of the electric field. Further, there is provided a member for applying an electric field to the asymmetric dual quantum well structure.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to optical devices in which a semiconductor quantum well structure is formed as an asymmetric dual quantum well (ADQW) structure including a plurality of different quantum wells (typically two wells) for achieving a specific property. According to the specific property, only a refractive index in the ADQW is changed, but not an absorption factor therein, by applying a reverse bias electric field thereto. Related Background Art In recent years there has been much interest in exploiting and developing optical communication systems which are capable of transmitting a great capacity of information at a high speed. Among them, wavelength or frequency division multiplexing systems and coherent optical communication systems are highlighted for their capability of communicating a greater capacity of information at a high speed which can be obtained by making a use of characterictics of light. Therefore, the intense effort has been made in developing devices for processing lights of different wavelengths or for modulating the phase of light and the like. In the wavelength division multiplexing systems, if semiconductor lasers are used as a light source, a wavelength range of tunable or changeable radiation lights is at most several nm (tens Å), so that when the wavelength division multiplexing communication is conducted within such wavelength range, devices for selecting or filtering light having a wavelength spectrum width less than 0,1 nm (1Å) at a resolution of 0,2∼0,3 nm (2∼3 Å) are required. Fig. 1 shows the structure of a waveguide type filter including a distributed reflector portion which is an example of the above-mentioned devices (see Numai et al.: Tunable filter using a phase shift type DFBLD, Informal Paper No. C-161 distributed at Autumnal Grand Meeting of Electronics Information Commun. Academy (1988)). In the device of Fig. 1 of the distributed feedback laser diode (DFBLD) type, there are provided, on a substrate 201, a waveguide 202 with a grating 203 and a partial active layer 204. The device is divided into two active portions 205a and 205b including the active layer 204 and a phase regulating portion 206 without the active layer 204 formed therebetween. Anti-reflection coats 207a and 207b made of SiN x are deposited on both end surfaces of the device. Reference letters N a , Np and  respectively denote refractive indices of the waveguide 202 in the active portions 205a and 205b and the phase regulating portion 206 and the length of the phase regulating portion 206. The refractive indices N a and N p in the waveguide 202 are changed by injecting an active current I a and a phase regulating current I p into the respective portions 205a, 205b and 206 to vary carrier densities in the waveguide 202 thereat. Thus, the wavelength of light reflected by the grating 203 or distributed reflector structure is varied to achieve a tunable filter. Such tunable filter, however, has drawbacks that a decrease of refractive index resulting from the carrier injection into the waveguide is reduced by an increase of refractive index resulting from a raise of temperature due to current flowing in the waveguide 202 and that thermal noise also occurs due to such raise of temperature. Moreover, there are several disadvantages that spontaneous emission appears due to the carrier injection, that a beat noise occurs from interaction between the spontaneous emission and an incident light, and others. Fig. 2 shows another prior art example which is a filter of Fabry-Perot type using a structure similar to surface emission type lasers (see Kubota et al.: Tunable filter of surface emission laser type, Informal Paper No. C-246 distributed at Vernal Grand Meeting of Electronics Information Commun. Academy (1990)). In the device of Fig. 2, there are provided, on a substrate 211, an etch-stop layer 212 and layers 213, 214, 215 and 216 of n-InP, p-GaInAsP, p-InP and p-GaInAsP. Anti-reflection coats 217 and 218 of TiO 2 /SiO 2 multi-layered film are respectively formed on the layer 216 of p-GaInAsP and the etch-stop layer 212 in a window etched in the substrate 211. In this device, selected or filtered wavelength of output light is changed by varying the refractive index by the injection of carriers, similar to the device shown in Fig. 1. Such filter, however, also has drawbacks similar to those of the device of Fig. 1 because carriers are injected into the device to shift the selected wavelength. That is, characteristics are degraded under adverse influences of the raise of temperature and spontaneous emission, similar to the device of Fig. 1. As a result, a tunable width of wavelength becomes narrow. Fig. 3 shows a prior art example which is a phase modulating device in which an electric field is applied to a superlattice layer 223 (intrinsic multiple quantum well (i-MQW) layer) to change the refractive index thereof by quantum confined Stark effect (QCSE) (see H. Nagai et al. : Field-induced modulations of refractive index and absorption coefficient in a GaAs/AlGaAs quantum well structure, Elect. Lett. 14th vol. 22, No. 17 (1986)). In the device of Fig. 3, there are formed, on an n-GaAs substrate 221 (dopant concentration: 4 × 10 18 cm -3 ) , an n-Al 0.4 Ga 0.6 As layer 222 (thickness: 35 µm and dopant concentration: 1×10 -15 cm -3 ), the i-MWQ layer 223 and an electrode 224 of Au thin film (thickness: 20 nm (200 Å)). In the substrate 221, a window is etched and another electrode 225 of Au thin film (thickness: 20 nm (200 Å)) is formed. The i-MQW layer 223 is comprised of ten periodes of GaAs layer (thickness: 10 nm (100 Å)) and Al 0.4 Ga 0.6 As layer (thickness: 15 nm (150 Å)). Such phase modulating device, however, also has the following drawbacks. Since an exciton wavelength of the i-MQW layer 223 is shifted to a longer one or greater value due to the QCSE by the applied electric field, absorption factor thereof as well as refractive index is changed and hence the intensity of an output varies if the wavelength of an input light is changed. Thus, a usable width of wavelength of the input light becomes narrow. Further, the absorption factor abruptly increases near the wavelength of light to be phase-modulated, so that the intensity of output light decreases greatly. GB-A-2 227 571 discloses an optical modulator with an asymmetric dual quantum well structure which are coupled via a barrier layer. The width of the deeper quantum well is smaller than the width of the less deep quantum well. With this device, it is intended to control the absorption coefficient by applying an electric field or a pump wave. The barrier layer separating the quantum wells has the same band gap as the barrier layers enclosing the quantum wells. Moreover, the Japanese Unexamined Laid-open Patent Application No. 1-204019 discloses an optical modulator having a multiple quantum well structure formed by sequentially laminating a first barrier layer consisting of a first semiconductor, a first well layer consisting of a second semiconductor having a smaller forbidden band width than said first semiconductor, a second barrier layer consisting of a third semiconductor having a larger forbidden band width than said second semiconductor, a second well layer consisting of a fourth semiconductor having a smaller forbidden band width than said second semiconductor, a third barrier layer consisting of said third semiconductor, a third well layer consisting of said second semiconductor, and a fourth barrier layer consisting of said first semiconductor. An object of the present invention is to provide an optical device with an ADQW which is capable of changing the refractive index of the ADQW by the application of an electric field thereto while suppressing variation of the absorption factor thereof in a given range of wavelength. According to the present invention the above object is achieved by the optical device, the light signal transmitter, the light signal receiver and optical communication systems of claims 1, 7, 8, 9, and 10, respectively. The advantages of the present invention as defined in the claims will be more readily understood in connection with the following detailed description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the structure of a prior art waveguide type tunable filter. Fig. 2 is a cross-sectional view showing the structure of a prior art surface emission type tunable filter. Fig. 3 is a cross-sectional view showing the structure of a prior art phase modulating device. Figs. 4A-4C are respectively views illustrating energy band structures and wave functions in respective electric field (F) -applied states (F=0, F=F 1 and F=F 2 ) for explaining the basic principle of the present invention. Fig. 5A is a representation illustrating the relationship between an absorption factor of a quantum well layer and an electric field applied thereto for explaining the basic principle of the present invention. Fig. 5B is a representation illustrating the relationship between a refractive index variation Δn of the quantum well layer and the electric field for explaining the basic principle of the present invention. Fig. 6A is a cross-sectional view of a first embodiment of the present invention. Fig. 6B is a representation illustrating the energy band structure of a quantum well layer of the first embodiment. Fig. 7 is a representation illustrating the electric field dependency of an absorption factor of the first embodiment. Figs. 8A-8D are respectively representations illustrating the electric field dependency of a refractive index variance Δn of the first embodiment. Fig. 9 is a cross-sectional view of a second embodiment of the present invention. Fig. 10 is a representation illustrating a transmission factor spectrum of the second embodiment. Fig. 11 is a cross-sectional view of a third embodiment of the present invention. Fig. 12 is a representation illustrating the wavelength dependency of an intensity of emitting light of the third embodiment. Fig. 13 is a block diagram showing an example of optical communication systems in which the optical device of the present invention is used as a filter and a light modulating device. DESCRIPTION OF THE PREFERRED EMBODIMENT First, the basic principle of the present invention will be explained by use of an example in which the ADQW consisting of two different quantum wells 1 and 2 (shallow well 1 of a wide width and deep well 2 of a narrow width) coupled to each other has an energy band structure as shown in Fig. 4A. In Figs. 4A-4C, wave functions ψ E1 and ψ E2 respectively having an electron ground level E 1 and an electron second level E 2 at the time of no electric field (F=0), electric field F=F 1 and electric field F=F 2 are respectively denoted by a solid line and dotted lines, and with respect of holes only a wave function ψ HH1 of heavy holes having a ground level HH 1 is shown. As the applied electric field increases, the wave functions ψ E1 and ψ E2 vary as shown in Figs. 4B and 4C. The wave function ψ E1 which is confined in the deep well 2 when no electric field is applied (Fig. 4A) begins to spread toward the wide, shallow well 1 as the applied electric field increases. In contrast, the wave function ψ E2 comes to have a dense part in the narrow, deep well 2 as the applied electric field increases. The wave function ψ HH1 of holes, however, scarcely varies even if the electric field increases. The absorption factor α ex of an exciton is proportional to an overlap integral between wave functions of electrons and holes as represented by: α ex ∝ ∫ ψ E × ψ H dz Therefore, as is seen from Figs. 4B and 4C, the absorption α ex abruptly decreases as the electric field increases because the overlap between the wave functions ψ E and ψ H abruptly decreases as the electric field is increased. In contrast, the energy or photon energy of exciton scarcely varies since the width of the deep well 2 is narrow and an energy gap between the levels E 1 and HH 1 scarcely varies. Namely, the intensity of an exciton peak clue to transition between the lowest energy level E 1 and the energy level of heavy hole HH 1 abruptly decreases as the electric field decreases. And the amount of a red shift of exciton peak in which the energy or photon energy shifts to a lower value is small, differently from normal QCSE. In contrast, the overlap between the wave functions ψ E2 and ψ HH1 increases, and the intensity of an exciton peak resulting from the transition between the levels E 2 and HH 1 increases. Fig 5A illustrates the above-discussed situation. The wavelength or photon energy dependency of the absorption factor α varies in the following manner as the electric field is increased from zero to F 1 and F 2 . When the electric field is F 1 , the exciton peak of transition between the levels E 2 and HH 1 begins to appear and the exciton peak of transition between the levels E 1 and HH 1 abruptly decreases as indicated by dotted lines. When the electric field is F 2 , the exciton peak between the levels E 1 and HH 1 almost disappears as indicated by dash and dotted lines. Meanwhile, the energy (wavelength or photon energy) of the exciton peak between the levels E 1 and HH 1 remains almost unchanged as shown in Fig. 5A. On the other hand, the spectrum of the refractive index variance Δn at this time is shown in Fig. 5B. The wavelength or photon energy dependency of the index variance Δn at the time of electric field F=F 1 is indicated by a solid line, and that at the time of electric field F=F 2 (>F 1 ) is indicated by dotted lines. For light of wavelength λ t whose photon energy is lower than that of the exciton peak between the levels E 1 and HH 1 at the time of electric field F=0, the absorption factor α remains almost unchanged since the red shift of the exciton peak between the levels E 1 and HH 1 scarcely occurs and only its intensity decreases as is explained referring to Fig. 5A. In contrast, the refractive index is considerably changed for the light of wavelength λ 1 as shown in Fig. 5B. Fig. 6A shows a first embodiment of a phase modulator which uses the above-discussed principle and in which the amplitude of modulation is large and the variation of output intensity is small. Fig. 6B illustrates the energy band structure of its quantum well layer structure. In Fig. 6A, on an n-GaAs substrate 31, an n-Al 0.5 Ga 0.5 As clad layer 32 having a thickness of 1.5 µm, an ADQW layer 33, a p-Al 0.5 Ga 0.5 As clad layer 32 having a thickness of 1.5 µm and an electrode 34 for applying an electric field to the ADQW layer 33. On the bottom surface of the substrate 31, another electrode 34' for applying the electric field to the ADQW layer 33 is formed. Anti-reflection coats 35 and 35' are respectively deposited on both end surfaces of the wafer for eliminating reflections at light input and output end surfaces. The ADQW layer 33 is comprised of, from the side of the substrate 31, an i-Al 0.16 Ga 0.84 As 41 well layer having a thickness of 8 nm (80 Å), an i-GaAs well layer 42 having a thickness of 4 nm (40 Å) and an i-Al 0.4 Ga 0.6 As barrier layer 43 having a thickness of 2 nm (20 Å) and formed therebetween as shown in Fig. 6B in the form of an energy band structure. Further, two i-Al 0.5 Ga 0.5 As barrier layers 44 and 44' having a thickness of 10 nm (100 Å) are respectively formed outside of the well layers 41 and 42 to build the ADQW structure. In Fig. 6B, the right side corresponds to an upper side of the wafer of Fig. 6A. In such structure, light from the outside couples to the ADQW layer 33 whose refractive index is larger than those of the n-type and p-type clad layers 32 and 32'. and propagates therethrough. For respective applied electric fields, the wavelength dependencies of the absorption factora of the quantum well layer 33 in the first embodiment are represented in Fig. 7. One scale on its ordinate indicates α =4x 1000 cm -1 , and respective indications on the ordinate also stand for origins for respective applied electric fields 0V/cm, 25kV/cm, 50 kV/cm, 75 kV/cm and 100 kV/cm. For all electric fields, the absorption factor α is in the vicinity of zero for photon energy of 1.50 eV. As has been explained in the basic principle, the exciton peak (indicated by black triangles) of the transition between the levels E 1 and HH 1 disappears at the electric field 75 kV/cm, and instead the exciton peak indicated by white triangles) of the transition between the levels E 2 and HH 1 appears. The photon energy of the exciton peak of the transition between the levels E 1 and HH 1 remains almost unchanged until the electric field of 50kV/cm. As a result, the absorption factor α for photon energies below 1.55 eV remains almost unchanged irrespective of the change in electric field. Figs. 8A-8D illustrate the wavelength dependencies of the refractive index variation Δn due to the electric field for respective applied electric fields. It is seen therefrom that the refractive index variation Δn of about 5 ×10 -3 is obtained even for photon energies below 1.53 eV at which the absorption factor ∝ is small. In the first embodiment, the length of the waveguide shown in Fig. 6A is set to 200µm. When light having a wavelength of 0.815 µm (photon energy: 1.52 eV) was propagated through the waveguide, the application of an electric field of 50 eV/cm sufficed for modulating the phase of the propagated light by π and the variance of intensity of an output light fell within 1 dB. Fig. 9 shows a second embodiment of a tunable filter which uses the above-discussed principle. In Fig. 9, on an n-GaAs substrate 61, an n-Al 0.5 Ga 0.5 As clad layer 62 having a thickness of 1.5 µm, an ADQW layer 63. a p-Al 0.5 Ga 0.5 As clad layer 62' having a thickness of 1.5 µm and an electrode 64 for applying an electric field to the ADQW layer 63. On the bottom surface of the substrate 61, another electrodes 64' for applying the electric field to the ADQW layer 63 is formed. Anti-reflection coats 65 and 65 ' are respectively deposited on both ends of the wafer for eliminating reflections at light input and output end surfaces. These layers and coats 61-65 and 65' are the Same as those 31-35 and 35' of the first embodiment shown in Fig. 6A in material and structure. In the second embodiment, there are further provided i-Al 0.4 Ga 0.6 As light guide layers 66 and 66' sandwiching the ADQW layer 63. A grating 67 is formed on the light guide layer 66' , and a portion 68 for shifting its phase by λ/2 is formed on the grating 67 for stabilizing a wavelength to be selected or filtered. When light is propagated through the device of the second embodiment, the light is diffracted by the grating 67 and a transmission spectrum as illustrated in Fig. 10 is obtained. Where a pitch of the grating 67 is Λ, a Bragg diffraction wavelength λ B is represented by λ B =2n eff Λ (n eff is an equivalent refractive index of the waveguide). Here, when a reverse bias voltage is applied between the electrodes 64 and 64' , only a refractive index of the waveguide is changed due to the above-discussed effect and the Bragg diffraction wavelength λ B is shifted by Δ λ B =2 Δn eff Λ. Also in the second embodiment, the application of an electric field of 50 V/cm sufficed for shifting the Bragg diffraction wavelength λ B by 1,2 nm (12 Å) and the variance of an intensity of an output light fell within 1 dB. Thus, a tunable filter which is capable of obtaining a filtering wavelength shift of 1,2 nm (12 Å) with a stable output is achieved. In the second embodiment, light having wavelengths remote from the Bragg wavelength λ B will be transmitted through the device as is seen from Fig. 10, so that the second embodiment could not be used as a filter when light having wavelength too remote from the Bragg wavelength λ B is incident on the device. Fig. 11 shows a third embodiment which can solve the drawback of the second embodiment. The third embodiment uses the filter of the second embodiment as a reflector part and further includes an amplifying part with an active layer 83'. Thus, a filter with an amplification function is obtained and the third embodiment is capable of amplifying light of the Bragg wavelength λ B and emitting the same. In Fig. 11, reference numerals 81-88 denote the same layers and portions 61-68 of the second embodiment of Fig. 9. There are further provided the i-Al 0.1 Ga 0.9 As active layer 83 having a thickness of 0.1 µm, an electrode 90 for injecting current therethrough and a groove 89 for dividing elctrodes 84 and 90 from each other. In the state in which current below a threshold for oscillation is caused to flow through the electrode 90, light is introduced into the active layer 83' . Then, only light of walength very near the Bragg wavelength λ B enters the filter part, so that the drawback of the second embodiment is solved. The wavelength dependency of an intensity of emitting light is as shown in Fig. 12. Only light of wavelength of λ B is amplified to be output, when no bias voltage is applied to the electrode 84 (F=0). Similar to the second embodiment, the application of an electric field of 50 V/cm sufficed for shifting the Bragg diffraction wavelength λ B by 1,2 nm (12 Å) (Δ λ B = 1,2 nm (12 Å)). Since the variance of an absorption factor due to the application of the electric field to a distributed reflector of the grating 87 is small as stated above, there is no need to control the current for amplification in the an amplifying part provided with the active layer 83' , for shifting the wavelength of emitting light as shown in Fig. 12. A tunable filter having a good performance is thus achieved. As discussed above, according to the present invention, a phase modulating device with a great modulation and a stable output is attained since there is provided a semionductor quantum well layer in which the shift of an exciton peak wavelength due to QCSE by the application of a reverse bias electric field almost disappears and only a refractive index is changed in a given range of wavelength by the application of the electric field while an absorption factor is very small and remains unchanged in such wavelength range. Further, a tunable filter with a wide width of tunable wavelength and a stable output is attained. Consequently, an electric field modulation type optical device with little output variation is achieved since the variation of absorption factor due to the application of electric field is small. Fig. 13 shows a block diagram illustrating an optical communication system in which the devices of the present invention can be used as a light modulator or a filter. In Fig. 13, reference numeral 186 is an optical fiber for transmitting light signals. A plurality of terminals 188 1 , 188 2 , ··· 188 n are connected to the optical fiber 186 through optical nodes 187 1 , 187 2 , ··· , 187 n , respectively. Terminal units 189 1 , 189 2 , ···, 189 n are connected to the respective terminals. Each terminal unit includes a keyboard, a display device, etc. In each terminal, there are provided a light signal transmitter consisting of a light modulator 182 and a modulator driver 183 and a light signal receiver consisting of a filter 179, a photodetector 180 and a demodulator 181. The transmitter and receiver are controlled by a controller 184 according to instructions from the terminal unit 189 1 . The light modulator 182 modulates a constant light from a laser diode (not shown) according to a signal from the driver 183. Namely, an external modulation is conducted instead a direct modulation. The filter 179 selects a signal of desired wavelength from plural signals of different wavelengths and outputs this desired signal to the photodetector 180 (in this case, the wavelength division multiplexing is conducted). While there has been shown and described what are considered preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the following claims.

Claims: An optical device comprising: at least two quantum wells (40, 41, 42, 80) having different depths, the width of the deeper quantum well layer being smaller than that of the less deep quantum well layer; a first barrier layer (43, 20) formed between said quantum well layers and having a band gap greater than the band gaps of said quantum well layers (40, 42, 42, 80), arranged such that the electron wave functions in said quantum well layers (40, 41, 42, 80) are capable of interfering with each other across said first barrier layer; second and third barrier layers (44, 44') including therebetween said first barrier layer (43, 20) and said quantum well layers (40, 41, 42, 80); and electric field applying means (34, 34'; 64, 64'; 84, 84') for applying an electric field to said quantum well layers (40, 41, 42, 80); characterized in that said first barrier layer (43, 20) has a band gap smaller than the band gaps of said second and third barrier layers (44, 44'), whereby upon application of said electric field, the device is capable of changing the refractive index while suppressing variation of the absorption factor thereof in a predetermined range of wavelengths. An optical device according to claim 1, further comprising p-type semiconductor layer (32', 62') and an n-type semiconductor layer (32, 62) for sandwiching said quantum well layers. An optical device according to claim 2, wherein the refractive indices of said p-type (32', 62') and n-type semiconductor layers (32, 62) are made smaller than that of said quantum well layers. An optical device according to any one of claims 1 to 3, further comprising light incident and light emitting end surfaces and anti-reflection coats (35, 35'; 65, 65') formed on said light incident and light emitting end surfaces. An optical device according to any one of claims 1 to 4, further comprising a grating (67) formed near said quantum well layers. An optical device according to any one of claims 1 to 5, further comprising a semiconductor active area (83') for amplifying an incident light, said active area including an active layer, and second applying means (90) for applying an electric field to said active layer separately from said first applying means (84, 84'). A light signal transmitter comprising: a light modulator; characterized in that said light modulator is the optical device according to any one of claims 1 to 6. A light signal receiver comprising: an optical filter; characterized in that said optical filter is the optical device according to any one of claims 1 to 6. An optical communication system comprising: a plurality of terminals (188 1 , 188 2 , 188 n ) each having a light signal transmitter and a light signal receiver; and an optical path connecting said plurality of terminals; characterized in that said light signal transmitter is the light signal transmitter according to claim 7. An optical communication system comprising: a plurality of terminals (188 1 , 188 2 , 188 n ) each having a light signal transmitter and a light signal receiver; and an optical path connecting said plurality of terminals; characterized in that said light signal receiver is the light signal receiver according to claim 8.

Inventors: FUJII KAZUHITO, ONO TAKEO, FUJII, KAZUHITO, ONO, TAKEO, FUJII, KAZUHITO, C/O CANON KABUSHIKI KAISHA, ONO, TAKEO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G02F1
IPC codes: G02F1, H01L31, H01S5