Operating method for a semiconductor optical device

Abstract: An optical device is provided having a large extinction ratio and being suitable for digital operation including first and second electrodes (7,8) formed on both sides of a waveguide structure, such that a carrier-injection region (3a) and a non-carrier injection regions (3b) are formed adjacent to each other in the waveguide structure. When mass carriers are stored in the carrier injection region, its refractive index is reduced to be lower than that of the non-carrier-injection regions. In this state, when a light wave with low optical power propagates through the carrier-injection region, since the amount of carriers consumed thereby is small, the refractive index of this region is still lower than the non-carrier-injection regions, and the input light wave is emitted sideways through the non-carrier-injection region. In contrast, when a light wave with high optical power propagates through the carrier-injection region, since the carrier consumption is large, the refractive index of this region (3a) becomes higher than the non-carrier-injection regions (3b), and the input light propagates through a waveguide which is optically induced to extend from the incident surface to the exit surface.

Description: The present invention relates to a method of operating an optical device which can be preferably used as a resource in the wavelength division multiplex (WDM) optical communication system. In accordance with recent abrupt progress in digital communication, development of a WDM optical communication system has been strongly required. This WDM optical communication system requires an optical wave converter in order to utilize a limited number of channels in an efficient manner by channel switching. In prior art wavelength converters, XGM type wavelength converters utilizing cross gain modulation and XPM type wavelength converters utilizing cross phase modulation are known. In the XGM type optical wavelength converter, an intensity-modulated input optical signal having a wavelength λ 1 and an optical signal having a wavelength λ 2 and a constant amplitude are supplied to a semi-conductor optical amplifier, and a polarity-inverted output optical signal having a wavelength λ 2 is produced by utilizing a difference in gain for an optical power impinging upon the semiconductor optical amplifier. The XPM type wavelength converter utilizes the principle of Mach-Zehnder interferometer. In this type of device, an input side of a waveguide having an input optical signal having a wavelength λ 2 is divided into two waveguides, a semiconductor optical amplifier is arranged in one of the waveguides, and these two waveguides are set to be in-phase for light having a wavelength λ 2 to be modulated. When an input optical signal having a wavelength λ 1 and an optical signal having a wavelength λ 2 and a constant amplitude propagate, there is produced a phase difference of π/2 between the two waveguides due to the function of the input signal. By utilizing this phase difference, an inverted optical output having a wavelength λ 2 is generated. Since the known XGM type wavelength converter utilizes the saturated gain of the optical amplifier, the extinction ratio of this optical waveguide converter is small. In addition, it has the inherent drawback that only the inverted optical output signal is produced and a non-inverted output signal could not be produced. In the XPM type optical wavelength converter, although it is possible to obtain a sufficiently large extinction ratio, since it reveals a periodical response, an extremely severe tolerance is required for device length. Therefore, through-put of the known XPM type optical wavelength converter in manufacture is substantially reduced. Furthermore, the above mentioned known optical wavelength converter is relatively large in size. That is to say, the typical size of the known optical converter is not smaller than several to ten millimeters, and thus it is practically difficult to integrate it as a single chip. JP 07 312460A, and the corresponding "Patent Abstract of Japan", describe an optical semiconductor amplifier element with a waveguide structure. The optical semiconductor amplifier element is provided with antireflection film on the ends. It is therefore desirable to realize a novel method of operating an optical device which can overcome or at least mitigate the aforementioned drawbacks and/or can have a large extinction ratio. It is also desirable to provide a method of operating an optical device which can produce an non-inverted output signal and can operate in a digital manner. It is still further desirable to provide a method of operating an optical device as a wavelength converter or a waveform shaper. The invention is defined in the attached independent claims to which reference should now be made. Further preferred features may be found in the sub-claims appended thereto. For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic perspective view showing the construction of an example of the optical device to be operated according to an embodiment of the present invention; Figs. 2A, 2B are sectional views of the optical device of Fig. 1 taken along a plane perpendicular to the direction of the light wave propagation and a plane parallel to the direction of the light wave propagation; Figs. 3A, 3B are graphs illustrating the relation between the location in a x refractive index; Figs. 4A, 4B are graphs illustrating the relations between the output power and the gain and the input power, respectively; Fig. 5 is a schematic view of a wavelength converter operated according to an embodiment of the invention; and Fig. 6 is a schematic view of a waveform shaper operated according to an embodiment of the invention. Fig. 1 and Figs. 2A, 2B show an optical device to be operated according to the present invention. Fig. 1 is a perspective general construction view of the optical device. Fig. 2A is a schematic sectional view along a plane perpendicular to the direction of the light wave propagation and Fig. 2B is a schematic sectional view taken along a plane parallel to the direction of the light wave propagation. As shown in Fig. 1, the direction of the light wave propagation is indicated as z direction, the direction of semiconductor layers stacking and carrier injection as y direction, and the direction perpendicular to the directions of the light wave propagation and the carrier injection as x direction. The optical device comprises a semi-conductor substrate 1 of a first conductivity type made of n-InP, and a wave-guide structure formed on the semiconductor substrate. The substrate 1 has a thickness of 100 µm and an impurity concentration of 2 x 10 18 atoms/cm 3 . The substrate 1 contains (for example) Sulfur as n-type impurity. A first cladding layer 2 is formed on the substrate 1, said first cladding layer having a thickness of 0.4 µm and being made of n-InP having an impurity concentration of 5 x 10 17 atoms/cm 3 . On the first cladding layer 2, is formed an active layer 3 of i-InGaAsP (1.55 µm) having a thickness of 0.2 µm. An etching stopper layer 4 of InGaAsP (1.25 µm.) is formed on the active layer with a thickness of 0.02 µm. On the etching stopper layer 4 is formed a second cladding layer 5 of a second conductivity type. This second cladding layer 5 has a thickness of 0.4 µm and an impurity concentration of 5 x 10 17 atoms/cm 3 . P-type impurity may be Zn. On the second cladding layer 5 is formed a cap layer 6 having a thickness of 0.1 µm and an impurity concentration of 2 x 10 19 atoms/cm 3 , on which a second electrode 7 of Ti-Au is formed. Finally, a first electrode 8 of Ti-Au is formed on an opposite surface of the substrate 1. Now a method of manufacturing the above mentioned optical device will be explained briefly. After formation of the second cladding layer 5, it is selectively removed by photolithography while the layer 4 is used as an etching stopper, leaving the portions corresponding to ridge parts of a semiconductor optical amplifier, thereby forming a ridge type waveguide structure. Then, a cap layer 6 and a second electrode layer 7 are formed. By a patterning process, the second electrode 7 is formed on the ridge portion of the semiconductor optical amplifier. Then, the first electrode 8 is formed on the rear surface of the semiconductor substrate 1 in the same manner. Finally, as shown in Fig. 2B, anti-reflection coatings are formed on a light incident end surface 9 and a light exit end surface 10 of the semiconductor optical amplifier respectively. An input side optical fiber (not shown) is arranged to face the light incident end surface 9 such that the signal light is incident upon the waveguide structure through the input optical fiber. An output side optical fiber (not shown) is arranged to face the light exit end surface 10 so that an optically amplified light wave is emitted through the output optical fiber. A DC bias source 11 is connected between the first electrode 8 and the second electrode 7. From the DC bias source 11, carriers are continuously injected into the central portion 3a of the active layer 3 at a constant rate. Electrons are injected into the central portion 3a of the active layer 3 through the first electrode 8, the n-type semiconductor substrate 1, and the n-type first cladding layer 2. Holes are injected into the active layer 3 through the second electrode 7, the cap layer 6, the second clad layer 5, and the etching stop layer 4. The thus injected carriers are stored within the central portion 3a of the active layer 3. Therefore, as shown in Fig. 2A, a carrier injection region 3a into which carriers are injected and stored is formed at the center of the active layer 3, and non-carrier injection regions 3b and 3c into which carriers are substantially not injected are formed on both sides of the carrier-injection region viewed in the propagating direction of the input optical signal. Now refractive index properties of the waveguide structure will be explained. The intrinsic InGaAsP constituting the active layer 3 has a refractive index of about 3.50 and the InP constituting the first and second cladding layers 2 and 4 has a refractive index of about 3.20. Therefore, the refractive index of the active layer 3 is larger than that of the first and second cladding layers adjacent to the active layer viewed in the y direction such that the light wave propagating along the waveguide structure is confined in the y direction and is guided along the waveguide. Next, the optical confinement effect as well as the radiation effect for the input optical signal in the x direction will be described. As shown in Fig. 2B, when the input optical signal is applied to the incident end surface 9, this light wave propagates along the z direction in the waveguide structure composed of the first cladding layer 2, the active layer 3, and the second clad layer 4. During the propagation through the waveguide structure, the light wave is optically amplified by carriers stored in the carrier injection region 3a of the active layer 3. When the power level of the input optical signal is low, the amount of carriers which are consumed by the amplifying function is small and the amount of carriers remaining in the carrier-injection region 3a of the active layer 3 is relatively "increased", and therefore the refractive index of the waveguide with respect to the propagating wave is relatively decreased. Contrary to this, when the power level of an input optical signal is high, a large amount of carriers are consumed by the optically amplifying function, thereby substantially reducing the amount of carriers remaining in the active layer. Therefore, the refractive index of the active layer is abruptly increased. As a result, the refractive index of the waveguide with respect to the propagating light wave is relatively increased. This state will be explained with reference to Fig. 3A and 3B. Figs. 3A and 3B are graphs schematically illustrating relations between the refractive index and location in the x direction in a plane perpendicular to the z direction which is the propagation direction of the signal light. Fig. 3A shows the relation between the refractive index and location in the x direction when signal light having a low power level propagates along the waveguide, and Fig. 3B shows the relation when signal light having a high power level propagates. The refractive index of intrinsic InGaAsP constituting the active layer 3 in a state that the carriers are not forced to be injected, that is, a refractive index of the non-carrier-injection regions 3b and 3c, is about 3.26. Meanwhile, when carriers are forced to be injected, a refractive index of the carrier injection region 3a is decreased to, for example about 3.258. Therefore, the refractive index of the carrier injection region as defined by the ridge type waveguide becomes lower than that of the surrounding non-carrier-injection regions 3b and 3c, whereby the refractive index profile shown in Fig. 3A is obtained. When an optical signal having a high power level is supplied, a quite large amount of carriers are consumed by the optically amplifying function, causing the amount of the carriers stored in the carrier-injection region 3a of the active layer 3 to be decreased, while the amount of carriers in the non-carrier-injection regions 3a and 3c is kept substantially constant. As a result, the refractive index of the carrier injection region 3a is increased to about 3.262 and becomes higher than that of the surrounding non-carrier-injection regions, as shown in Fig. 3B. As described above, when an input optical signal having a low power level is supplied, the refractive index of the propagation path of the light wave is lower than that of the adjacent side regions 3b and 3c. Owing to such a difference in refractive index, the light wave of the input signal is absorbed by the non-carrier-injection regions having a higher refractive index and is emitted to the exterior through the surrounding regions. This results in the optical device operating in the anti-guiding mode, and negligibly weak light is emitted from the output surface. In contrast, when an input optical signal having a high power level is supplied, the refractive index of the propagation path becomes higher than that of the non-carrier-injection regions 3b and 3c. This difference in refractive index gives rise to the optical confinement effect, forming a complete waveguide extending from the incident surface 9 to the exit surface 10. Thus, the optical confinement effect occurs both in the x and y directions, and the amplified light wave is confined both in the x and y directions and propagates through the waveguide and is emitted from the exit surface 10. Figs. 4A and 4B are graphs schematically illustrating the relation between the input power and the output power and also the relation between the input power and the gain (P out /P in ) when the optical signal propagates in the aforementioned optical device. In Fig. 4A, the horizontal axis represents input power of the optical signal and the vertical axis represents output power. The optical device to be operated according to the present invention has both the optical amplifying function and index guiding function. When input power of the input optical signal is low, the index guiding function does not act and almost all optical power of the input optical signal is radiated sideways and is not emitted from the exit surface. Thus, this optical device acts in the anti-guiding mode. Furthermore, in the case that the refractive index of the carrier injection region is lower than that of the surrounding regions, output power hardly changes in spite of the increase in the input power. Conversely, when input power is increased and the refractive index of the carrier injection region exceeds that of the non-carrier-injection regions, the index guiding effect abruptly appears and the input optical signal begins to propagate in the carrier injection region. By this propagation of the signal light, the optically amplifying function is abruptly enhanced and output power is sharply increased relative to the input power. Thus, as shown in Fig. 4A, a nonlinear input/output characteristic with a threshold value can be obtained owing to the multiplier effect of the index guiding effect and the optical amplification effect. As a reference, the relation between input power and the output power in a common (prior art) semiconductor optical amplifier is shown by a dashed line. In prior art semiconductor optical amplifiers, the refractive index of the carrier-injection region is set to be higher than that of the surrounding regions both in the x and y directions at the manufacturing stage, and thus the output power increases linearly in accordance with the input power. In Fig. 4B, the horizontal axis represents the input power and the vertical axis represents the gain (P out /P in ). The solid line indicates the characteristic of the optical device according to the present invention and the dashed.line denotes a general characteristic of a conventional semiconductor optical amplifier. In the optical device to be operated according to the present invention, optical amplification hardly occurs when the power level of the input optical signal is low level. However, when the input power further increases and the refractive index of the carrier injection region approaches that of the non-carrier-injection regions, both the guiding function and the optical amplifying function appear significantly and the optical amplification effect abruptly increases. After exceeding the carrier injection limit of the DC current source, the gain is gradually decreased. In contrast, since the gain of the prior art optical semiconductor amplifier is set constant irrespective of the input power, the gain is kept to be constant in spite of the increase in the input power and is decreased gradually after exceeding the carrier-injection limit. As is seen from the gain characteristics of Fig. 4B, the optical device to be operated according to the present invention has a specific gain characteristic which is different from that of the known semiconductor optical amplifiers. As described above, the optical device to be operated according to the present invention has a threshold characteristic which cannot be obtained by the known semiconductor optical amplifier; various optical devices can be realized by utilizing this special threshold characteristic. Now, an optical device operated according to the present invention as a wavelength converter will be explained. As shown in Fig. 5, an input optical signal having a wavelength λ 1 and light wave having a wavelength λ 2 and a constant amplitude are made incident upon an optical device according to the present invention. The power level of the light wave having wavelength λ 2 and constant amplitude is set at a value a little lower than the threshold value shown in Figs. 4A and 4B. When the power level of the optical signal with the wavelength λ 1 is high, a combined high level optical signal is made incident on the optical device and thus both of the light waves with wavelengths of λ 1 and λ 2 are optically amplified and are emitted as a high level signal. On the contrary, when a power level of the optical signal with the wavelength λ 1 is at a low level, the optical device operates in the anti-guiding mode and both light waves of λ 1 and λ 2 are not amplified, thereby both of light waves are emitted as a low level signal. As a result of this, an amplified optical signal with the wavelength λ 2 modulated according to the input optical signal can be emitted from the exit end surface. Fig. 6 shows an operated optical device according to the present invention used as a waveform shaper. An input optical signal having a wavelength λ 1 and a light wave having the same wavelength and a constant amplitude are supplied to the optical device according to the present invention. The power level of the light wave having constant amplitude is set at a value a little lower than the threshold value shown in Figs. 4A and 4B. When the input optical signal is at a high level, the device operates in the guiding mode and emits an optically amplified high level light signal. When the input optical signal is at a low level, a low level signal is output, because the power level of the input light wave is lower than the threshold value. As a result, an optical device operated, according to the present invention can function as a waveform shaper. This waveform shaper can be favourably used to optically amplify a weak input signal and generate an output signal having a high S/N ratio. That is to say, when only a weak optical signal is supplied to the optical device, it cannot be amplified. However, when the optical signal of weak power and a light wave with relatively high constant amplitude are both supplied to the optical device, the optical power of the input light wave can be increased up to the threshold value and thus an output optical signal having significantly increased amplitude and a high S/N ratio can be obtained by utilizing the threshold characteristic of the optical device. In addition, the optical device shown in Fig. 6 can function as a waveform shaper by supplying only the input optical signal light. Also in this case, an output optical signal having a high S/N ratio can be regenerated, because noise components with low power level can be removed as well as the wave distortion. The application of the present invention is not limited to the aforementioned devices and various alternations and modifications may be conceived by those skilled in the art. For instance, the material systems constituting the waveguide structure are not limited to InGaAs/InP system but semiconductor systems such as GaAlAs/GaAs system, InGaAlAs/10 system, and InGaAs/GaAs system may be employed. Also, a quantum well structure may be used as the active layer. In this case, for example, a first optical confinement layer of InGaAsP (1.55 µm) having a thickness of 200 nm is formed on the first cladding layer. On this optical confinement layer, a quantum well structure composed of 5 sets of a quantum well layer of alternate InGaAsP (1.75 µm) having a thickness of 10 nm and a barrier layer of InGaAsP (1.25 µm) having a thickness of 10 µm is formed. On this quantum well structure, a second optical confinement layer of InGaAsP (1.25 µm) having a thickness of 200 nm is formed. This quantum well structure can be used as an active layer. In summary, a method for operating an optical device is provided, the device having a large extinction ratio and being suitable for digital operation including first and second electrodes (7, 8) formed on both sides of a waveguide structure, such that a carrier-injection region (3a) and a non-carrier injection region (3b) are formed adjacent to each other in the waveguide structure. When mass carriers are stored in the carrier injection region, its refractive index is reduced to be lower than that of the non-carrier-injection regions. In this state, when a light wave with low optical power propagates through the carrier-injection region, since the amount of carriers consumed thereby is small, the refractive index of this region is still lower than the non-carrier-injection regions, and the input light wave is emitted sideways through the non-carrier-injection region. In contrast, when a light wave with high optical power propagates through the carrier-injection region, since the carrier consumption is large, the refractive index of this region (3a) becomes higher than the non-carrier-injection regions (3b), and the input light propagates through a waveguide which is optically induced to extend from the incident surface to the exit surface.

Claims: A method of operating an optical device, said device comprising a waveguide structure formed by a plurality of semiconductor layers (2-6) formed on a first major surface of a semiconductor substrate (1), and having an incident surface (9) and an exit surface (10) opposed to each other, said incident and exit surfaces (9, 10) being perpendicular to the semiconductor layers, said semiconductor layers being composed of semiconductor materials whose refractive indices vary according to the amount of carriers injected and stored therein, a first electrode (8) formed on the second major surface of the semiconductor substrate and a second electrode (7) formed on top of said waveguide structure such that the second electrode is opposed to said first electrode, said first and second electrodes being formed such that a carrier-injection region (3a) into which carriers are injected through the first and second electrodes and non-carrier-injection regions (3b, 3c) into which carriers are not substantially injected are formed in the waveguide structure such that the carrier injection region is laterally sandwiched between the non-carrier-injection regions, said method comprising the steps of applying a DC voltage across said first and second electrodes (8 and 7) to inject carriers into said waveguide structure and drive said waveguide structure into a carrier injection operation state in which the refractive index of the carrier-injection region (3a) is lower than that of the non-carrier-injection regions (3b, 3c); operating the optical device in a guiding mode by impinging an input light wave of a first power level higher than a threshold level upon the incident surface (9) to change the refractive index of the carrier-injection region (3a) to be higher than that of the non-carrier-injection regions (3b, 3c) so that the carrier-injection region (3a) constitutes an optical channel waveguide which guides the amplified input light wave from said incident surface (9) to said exit surface (10); and operating the optical device in an anti-guiding mode by impinging an input light wave of a second power level lower than the threshold level upon the incident surface (9) to keep the refractive index of the carrier-injection region (3a) lower than that of the non-carrier-injection regions (3b, 3c) so that the input light wave is emitted laterally through said non-carrier-injection regions. The method according to claim 1, wherein said optical device is operated as a waveform shaper by impinging an input optical signal having a higher level higher than the threshold level and a lower level lower than the threshold level upon the incident surface (9) and deriving an output optical signal having an amplified and modulated amplitude in accordance with the input optical signal from the exit surface. The method according to claim 1, wherein said optical device is operated as an wavelength converter by impinging an input optical signal having a wavelength λ 1 and a light wave having a wavelength λ 2 different from λ 1 and a constant amplitude slightly lower than the threshold level upon the incident surface (9) and deriving an amplified optical signal having a wavelength λ 2 and an amplitude modulated in accordance with the input optical signal of the wavelength λ 1 from the exit surface (10). The method according to claim 1, wherein said optical device is operated as waveform shaper by impinging an input optical signal having a wavelength λ 1 and light wave having the same wavelength λ 1 and a constant amplitude slightly lower than the threshold level upon the incident surface (9) and deriving an amplified optical signal having a wavelength λ 1 and an amplitude modulated in accordance with the input optical signal of the wavelength λ 1 from the exit surface (10).

Inventors: MA BYONG-JIN, NAKANO YOSHIAKI, MA, BYONG-JIN, NAKANO, YOSHIAKI
Applicants: UNIV TOKYO, THE UNIVERSITY OF TOKYO
Application year: 2000
Main classification: G02F1
IPC codes: G02F1, H01S5, G02F2, B65D85