JP2011243891A - Quantum dot semiconductor optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier which is capable of amplifying a narrow band for a 1.3 μm band desired for high-speed optical communication.SOLUTION: An optical amplifier is a semiconductor optical amplifier for a narrow band equal to or less than a half-value width of 30 nm for a 1.3 μm band, by providing a quantum dot structure consisting of over an N-GaAs substrate 1, an N-AlGaAs clad layer 2, a quantum dot active layer 3, a P-AlGaAs clad layer 4, and a P-GaAs contact layer 5 in the order and by constituting the active layer 3 with uniform quantum dots. By injecting current, a wavelength other than a gain wavelength can be attenuated and a deflection state can be switched.

Description

  The present invention relates to an optical amplifier made of a quantum dot semiconductor suitable for optical fiber communication.

  Conventionally, there are two types of optical amplifiers for communication: a semiconductor type and an optical fiber type. The conventional semiconductor optical amplifier is a 1.55 μm band, which is main in optical fiber communication, and has a wide band adapted for wavelength division multiplexing, Characteristics such as large high power have been emphasized.

  The active layer of a semiconductor optical amplifier includes a bulk structure, a quantum well structure, and a quantum dot structure. Prior art studies on optical amplifiers using quantum dot structures include Patent Documents 1 to 3.

  Conventionally, it is known that when a quantum dot structure is applied to a semiconductor optical amplifier, a broadband, high-saturation output device having an amplification band of 1.2 to 1.5 μm is obtained (Patent Document 1, paragraph 169). Etc.). In addition, semiconductor optical amplifiers have features such as being small and capable of high-speed response. Therefore, semiconductor optical amplifiers are expected as devices for realizing all signal processing. It is known to have excellent characteristics such as a wide band (broadband amplification) and high light output (high saturation light output) (see paragraphs 2 and 3 of Patent Document 2).

  The present inventor has conducted extensive research on the quantum dot structure, a semiconductor laser technology (see Patent Document 4) that uses the 1.3 μm wavelength band for optical communication, and a technology for controlling the energy level of the quantum dot (patent) Reference 5), and a technology for narrowing the spectral width of a quantum dot assembly (see Patent Document 6) has been proposed.

JP 2007-251089 A JP 2008-91420 A JP 2010-44413 A JP 2007-53322 A JP 2007-194378 A JP 2010-54695 A

  In the proposals related to the optical amplifier so far, the emission wavelength is mainly in the 1.55 μm band. From this year, the standard for 100 Gbps Ethernet will be determined by IEEE. This standard assumes high-speed communication in the communication wavelength band of 1.3 μm. Although it is not envisaged to use an optical amplifier in the current standard proposal, it is important to develop a 1.3 μm band optical amplifier in anticipation of the future, and the present invention realizes the next generation 1.3 μm band optical amplifier. It is something to try.

  A conventional semiconductor optical amplifier having a quantum dot structure has a wideband amplification, and therefore cannot be used as a next-generation optical amplifier. That is, the size variation of the quantum dots is increased, and a wide wavelength band is aimed.

  The present invention is intended to solve these problems, and an object of the present invention is to be able to amplify a narrow band in the vicinity of an amplification wavelength of 1.3 μm in a semiconductor optical amplifier. The 1.3 μm band refers to 1280 nm to 1340 nm.

The present inventor has intensively studied in order to solve the above-mentioned problems, paying attention to the fact that the quantum dot active layer has a delta function energy level, and by making the quantum dots highly dense and highly uniform, an amplification wavelength of 1 A semiconductor amplifier capable of amplifying a narrow band in the 3 μm band is provided.
In order to achieve the above object, the present invention has the following features.

  The apparatus of the present invention is a semiconductor optical amplifier, characterized in that it comprises a semiconductor structure in which the active layer is a uniform quantum dot. In particular, a narrow band is amplified for a wavelength band of 1.3 μm. The half bandwidth of the amplification band is preferably 30 nm or less. Since the optical amplifier of the present invention has a filter function, it is preferably used as a filter. The semiconductor structure includes an electrode, and can attenuate wavelengths other than the gain wavelength by injecting current. The semiconductor structure includes an electrode and can switch a deflection state by injecting a current.

Here, the quantum dots are nanometer-sized semiconductor fine particles and have a size of about several tens of nanometers. The quantum dot layer of the present invention has a uniform quantum dot size, and the emission half-value width of light generated from the quantum dot is within 30 meV in terms of energy level and in the range of 24 nm in terms of wavelength. The narrow half-value emission width indicates the uniformity of the quantum dots. The quantum dot layer of the present invention has a high density and a surface density in the range of 6 × 10 ¹⁰ cm ⁻² to 10 × 10 ¹⁰ cm ⁻² . The active layer of the present invention is composed of a layer in which a plurality of quantum dot layers are stacked, and is preferably as many as possible in order to obtain a large amplification, but it should be within 10 layers from the viewpoint of unifying light propagation modes. preferable. Specifically, the active layer of the present invention has 1 to 10 layers, preferably 5 to 10 layers.

  According to the present invention, it is possible to amplify a narrow band by making the quantum dots of the quantum dot structure of the active layer of the semiconductor amplifier highly dense and highly uniform. The quantum dot structure optical amplifier of the present invention has optical amplification characteristics in a communication wavelength band of 1.3 μm band corresponding to the standards of 40 Gbps and 100 Gbps Ethernet. Since the amplification band is as narrow as 30 nm or less, it is useful as an amplifier having a filter function.

  According to the present invention, the portion other than the gain wavelength can be attenuated by the carrier plasma effect by the injected current. Further, according to the present invention, since the quantum dot structure is a flat dot and has an incident polarization dependent characteristic, the deflection state can be selected by an injection current and can be used as an element for switching the deflection state.

It is a figure which shows the principal part of the structure of the semiconductor quantum dot optical amplifier of this invention. It is a figure which shows the structure of the quantum dot active layer of this invention. It is a figure which shows the device structure of the semiconductor optical amplifier of this invention. It is a figure which shows the transmission characteristic of the semiconductor optical amplifier in 1st Embodiment. It is a figure which shows the transmission characteristic when an electric current is sent in the semiconductor optical amplifier of 1st Embodiment. It is a figure which shows the transmission characteristic of wavelengths other than the amplification zone | band when an electric current is sent in the semiconductor optical amplifier of 1st Embodiment. It is a figure which shows the transmitted light characteristic in case the incident light is TE polarized light and TM polarized light in the semiconductor optical amplifier of 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail.

(First embodiment)
A first embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows a main part of the structure of the semiconductor quantum dot optical amplifier according to the present embodiment. As shown in FIG. 1, the main part of the structure of the semiconductor quantum dot optical amplifier is formed on a semiconductor substrate 1 in the order of a cladding layer 2, a quantum dot active layer 3, a cladding layer 4, and a contact layer 5. . Specifically, an N-AlGaAs cladding layer 2, a quantum dot active layer 3, a P-AlGaAs cladding layer 4, and a P-GaAs contact layer 5 are formed on an N-GaAs substrate 1 in this order. For example, on an N-GaAs substrate 1, an Si-doped N-AlGaAs cladding layer 2 having an aluminum (Al) composition of 40% and a gallium (Ga) composition of 60%, an active layer 3 composed of InAs quantum dots and a GaAs layer, aluminum ( A Be-doped P-AlGaAs cladding layer 4 and a Be-doped P-GaAs contact layer 5 having an Al) composition of 40% and a gallium (Ga) composition of 60% were formed. In the AlGaAs cladding layers 2 and 4, the mixing ratio of the group 13 element (Ga, Al) and the group 15 element (As) is half.

  The clad layers 2 and 4 are used to confine light in the active layer 3. The active layer 3 is a layer that converts electricity into light. The contact layer 5 is for reducing contact resistance with the electrode. Doping is performed for current injection.

  FIG. 2 shows a detailed structure of the quantum dot active layer 3. As shown in FIG. 2, the active layer 3 includes a GaAs layer 31, an InAs quantum dot layer 32, an InGaAs layer 33, a GaAs layer 34, an InAs quantum dot layer 35, an InGaAs layer 36, a GaAs layer 37, an InAs quantum dot layer 38, and an InGaAs. It consists of a layer 39 and the like. As shown in FIG. 2, the active layer includes, for example, a total of nine pairs of InAs quantum dot layers and InGaAs layers (collectively referred to as “one pair”). The InAs quantum dot layer is supplied with In, 2.4 ML (2.4 molecular layer), and an In22% InGaAs layer (2.5 nm) is grown on the InAs quantum dot layer. A GaAs layer (39 nm) is disposed between one pair of each quantum dot and the other pair. When the InAs quantum dot layer, the InGaAs layer, and the GaAs layer are combined and the number of stacked quantum dots is counted as one layer, a representative example is composed of nine layers.

  In FIG. 2, the layers represented as InAs quantum dot layers (32, 35, 38) may be composed of InAs thin film layers and InAs quantum dots. In FIG. 2, the InGaAs layers (33, 36, 39) provided on the InAs quantum dot layer are strain relaxation layers and are factors that determine the amplification wavelength. When the InGaAs layer is not provided, the amplification wavelength is 1.0 μm to 1.2 μm, not the 1.3 μm band.

  The number of quantum dot layers is preferably large in order to obtain large amplification, but from the viewpoint of unifying the light propagation mode, it is preferably 1 layer or more and 10 layers or less. A III-V compound semiconductor is used as the semiconductor material of the quantum dot layer. As a representative example, an InAs quantum dot layer is shown, but InGaAs can be used. Although an InGaAs layer is used as the strain relaxation layer, an InAlAs layer or an InGaAlAs layer can be used. As the barrier layer (31, 34, 37) located between the strain relaxation layer and the quantum dot layer on the quantum dot layer, a GaAs layer is used, but an AlAs layer or a GaAlAs layer can be used.

A method for manufacturing the structure of the semiconductor quantum dot optical amplifier according to the present embodiment will be described. A method of manufacturing the quantum dot active layer portion of FIG. 2 which is a feature of the present invention will be described. An InAs thin film layer and an InAs quantum dot 32 thereon are provided on a GaAs layer 31 provided on a semiconductor substrate, an InGaAs layer 33 is provided thereon for planarization, a GaAs layer 34 is provided, and this is repeated. Specifically, while supplying As ₂ onto the GaAs layer 31, the growth interruption was put in for 60 seconds, and In and As ₂ were then supplied to produce InAs quantum dots 32. Further, while supplying As ₂ , the growth temperature is lowered by about 50 ° C. while the growth is interrupted for 30 seconds, and then In, Ga, and As ₂ are supplied to grow the InGaAs layer 33. Further thereon, Ga and As ₂ were supplied to grow a GaAs layer 34. Thereafter, the same was repeated. The higher the In composition of the InGaAs layer, the more effective it is to reduce the strain between the InAs quantum dots and the GaAs layer, so that the characteristics of the quantum dots are improved. The composition of the InGaAs layer is best at In _0.5 Ga _0.5 As, which is the midpoint between the two strains. In this embodiment, the InGaAs layer is thinned to reduce the influence of misfit transition due to the critical film thickness.

If the layer structure is repeated, the GaAs layer after the InGaAs layer is less likely to be flat. For this reason, the growth rate of GaAs is set to a low growth rate of 1.0 μm / hour or less. The growth temperature of the upper layer (second layer and third layer) is preferably the same or higher by 5 ° C. than the lower layer (first layer and second layer). Nearly identical InAs quantum dots could be produced in each layer. A 9-layer stacked structure was manufactured using InAs quantum dots having an areal density of 8 × 10 ¹⁰ cm ⁻² or more.

  FIG. 3 shows a device structure of the semiconductor optical amplifier according to the present embodiment. The optical amplifier 7 amplifies the light incident from the optical fiber 6 and outputs the amplified light to the optical fiber 6. An enlarged structure of the optical amplifier 7 is shown in the figure. An example of a mesa waveguide structure is shown as an optical amplifier. The optical amplifier has electrodes on the upper and lower sides and has a structure that can efficiently inject current into the active layer. The waveguide width is 5 μm and the length of the resonator (waveguide length) is 2 mm. However, the width and length may be appropriately set according to the required amplification factor and power consumption.

  The amplification function of the amplifier of this embodiment will be described. The following measurement was performed for the amplification characteristics. An optical fiber is arranged at both ends of the waveguide of the semiconductor optical amplifier, and a measurement system is provided in which light is input from one side and receives light from the other side (FIG. 3). Light of an arbitrary wavelength and output was input from the input side using a wavelength tunable laser. The output light received by the optical fiber was observed with a power meter or spectrum analyzer. Further, the semiconductor optical amplifier has no heat sink.

  FIG. 4 shows transmission characteristics when incident light is 3 μW, TE polarized light (transverse wave), and injection current 0 mA. In FIG. 4, the horizontal axis represents the wavelength of the transmitted light, and the vertical axis represents the transmitted light power. From the measurement results, there is a valley of transmitted light in the vicinity of 1330 nm. This is light absorption by the quantum dot active layer. The result is theoretically paired with the photoluminescence method, and shows the characteristics of the active layer well. The coupling loss between the optical fiber and the semiconductor optical amplifier was 3.5 dB, and the waveguide loss of the semiconductor optical amplifier was 8 dB. FIG. 5 shows the result of transmission characteristics obtained by injecting a current of 50 mA. The horizontal axis of FIG. 5 is the wavelength of transmitted light, and the vertical axis is the transmitted light power. Light is amplified by injecting current. Optical amplification is performed at the center of the gain region near 1330 nm, and the half bandwidth of the amplification band is about 30 nm. At 1330 nm, the transmitted light was 1.65 nW before current injection, whereas it was 408.48 nW at 50 mA injection, which was amplified by 24 dB. The amplification factor for incident light of 3 μW is estimated to be 7 dB in consideration of coupling losses of 3.5 dB and 8 dB.

  Further, by injecting current outside the amplification band, the transmitted light is attenuated by the carrier plasma effect. FIG. 6 shows the result of incident light 30 μW and TE polarization. The horizontal axis in FIG. 6 is the input current (also called injection current), and the vertical axis is the transmitted light power. 1260 nm which is the short wavelength side of the amplification band and 1450 nm which is the long wavelength side were measured. From FIG. 6, it can be seen that at any wavelength, there is light attenuation due to an increase in current. Attenuation of about 20% was observed at 1260 nm and about 60% at 1450 nm. In particular, the conventional quantum well has no attenuation band on the short wavelength side, and is a phenomenon peculiar to quantum dots.

  Based on these measurement results, an optical amplifier having a filter function of amplifying only a specific wavelength and attenuating the other wavelengths can be realized. Although the specific wavelength is 1330 nm, the specific wavelength can be changed by changing the dot size and composition of the quantum dots and the composition and layer thickness of the strain relaxation layer. Further, it is known that the emission wavelength of a semiconductor element changes at a rate of 0.1 nm / K depending on the operating temperature. For example, if the operating temperature is set at a predetermined temperature within a range of 10 ° C. to 30 ° C., the specific wavelength can be changed by about 2 nm.

  The optical amplifier of the present embodiment has a filter function with a narrow amplification band in the 1.3 μm band which is particularly important in optical communication. By creating highly uniform quantum dots having a quantum dot structure, a narrow filter function with an amplification band of 30 nm or less can be realized. The high uniformity of quantum dots means that the emission half-value width of light generated from quantum dots is narrow. In the present invention, the emission half width is within 30 meV or less in terms of energy level and 24 nm in terms of wavelength.

  In addition, since the optical amplifier of this embodiment can increase the effect of attenuating other wavelengths by the carrier plasma effect by flowing an injection current, a filter that transmits a specific wavelength by adding the injection current. Improve functionality. The performance of the filter function can be changed according to the injection current value.

  In the 100 Gb Ethernet standard, it is expected that the 1.3 μm band is used at 4 × 25 Gb. Therefore, the optical amplifier according to the present embodiment can be used as a filter for extracting one specific wavelength from four wavelengths.

(Second Embodiment)
The second embodiment of the present invention is characterized in that it has a function of selecting a deflection state by an injection current to the optical amplifier. The semiconductor optical amplifier of this embodiment has the same structure as that of the first embodiment.

  The quantum dot of the present invention is not a sphere but a flat disk. For this reason, the direction of the light which reacts is a horizontal direction, and the polarization state of emitted light can be changed. Measurement was performed using the same measurement system as described in the first embodiment. FIG. 7 shows transmitted light characteristics when incident light is changed to TE polarized light (transverse wave) or TM polarized light (longitudinal wave). In FIG. 7, the horizontal axis represents the input current, and the vertical axis represents the transmitted light power. At this time, the incident light power is 3 μW and the wavelength is 1335 nm. While TE polarized light is optically amplified, transmitted light power hardly changes in TM polarized light. When the injection current is 0 mA, TM polarized light (95 nW) shows a larger transmitted light power value than TE polarized light (1.8 nW). On the other hand, at an injection current of 70 mA, TE polarized light (251 nW) shows a larger transmitted light power value than TM polarized light (84 nW). By selecting the injection current in this way, the transmitted light can be selected from either TE polarized light or TM polarized light. Further, when the magnitude of the injection current is selected to be, for example, 30 mA, it is possible to transmit TE polarized light and TM polarized light equally.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims, and technical means disclosed in different embodiments can be appropriately combined.

  The apparatus of the present invention is useful as an optical amplifier of 1.3 μm band adapted to the IEEE 40 Gb Ethernet and 100 Gb Ethernet standards.

1, N-GaAs substrate 2, N-AlGaAs cladding layer 3, quantum dot active layer 4, P-AlGaAs cladding layer 5, P-GaAs contact layer 6, optical fiber 7, optical amplifiers 31, 34, 37, GaAs layer 32, 35, 38, InAs quantum dot layer 33, 36, 39, InGaAs layer

Claims (4)

  A narrowband semiconductor optical amplifier comprising a semiconductor structure in which an active layer is a uniform quantum dot and having an optical amplification characteristic in a wavelength band of 1.3 μm.   2. The semiconductor optical amplifier for a narrow band according to claim 1, which has a filter function and has a half-value width of 30 nm or less in the amplification band.   2. The semiconductor optical amplifier for a narrow band according to claim 1, wherein the semiconductor structure includes an electrode and attenuates wavelengths other than the gain wavelength by injecting current.   2. The semiconductor optical amplifier for a narrow band according to claim 1, wherein the semiconductor structure includes an electrode and switches a deflection state by injecting a current.

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