JP2014206557A - Electric field absorption modulator, semiconductor optical element, and optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an EA modulator, a semiconductor optical element, and an optical module which allow for both a large extinction ratio and low chirp characteristics.SOLUTION: An electric field absorption modulator comprises a multiple quantum well having a band diagram in which a second energy is substantially equal to a first energy and a third energy is smaller than the second energy, when a barrier layer height relative to a ground level of electrons in a well layer is defined as the first energy, the barrier layer height relative to a light-hole ground level of the well layer is defined as the second energy, and the barrier layer height relative to a heavy-hole ground level of the well layer is defined as the third energy, respectively.

Description

  The present invention relates to an electroabsorption modulator.

In recent years, for example, in the information communication field, an electroabsorption modulator (hereinafter referred to as an EA (Electro-Absorption) modulator) has been used. The EA modulator uses the quantum confinement Stark effect (hereinafter referred to as QCSE) to selectively apply an electric field to the active region of the EA modulator, thereby reducing the amount of light absorption. It is a modulator that changes the ON / OFF control of light. When an electric field is applied to the active region, the absorption spectrum is shifted to the long wavelength side by QCSE, and the light absorption rate at the laser wavelength λ ₀ is changed. At this time, since the refractive index of the active region changes at the same time, chirping occurs in the output light of the EA modulator. Here, the active region includes a so-called single-quantum well (hereinafter referred to as SQW) layer or a multiple-quantum well (hereinafter referred to as MQW) layer. Hereinafter, in this specification, MQW includes SQW in addition to normal MQW.

  In recent years, there has been a demand for an EA modulator that has a lower chirp characteristic while maintaining a large extinction ratio. In order to realize a lower chirp characteristic, the chirp parameter of the EA modulator must be negative and the chirp parameter must be a smaller value (the absolute value of the chirp parameter is a larger value).

  The refractive index change Δn in the EA modulator is proportional to the chirp parameter in the EA modulator, and the refractive index change Δn is expressed by the following (Formula 1).

Here, λ ₀ is the laser wavelength and α is the absorption coefficient. Δα is a change amount of the absorption coefficient α from when no electric field is applied (signal ON state) to when an electric field is applied (signal OFF state). P represents principal value integration.

  Both the first term and the second term of the principal value integration of (Formula 1) have a positive denominator. Therefore, the refractive index change Δn can be made negative by increasing the negative part of Δα in the first term of the main value integration and increasing the positive part of Δα in the second term of the main value integration.

  An EA modulator in which the absorption coefficient α effectively changes between when no electric field is applied and when an electric field is applied is disclosed in Patent Document 1. Patent Document 1 shows a case where an MQW layer stacked on an InP wafer is formed of an InGaAsP-based material, and discloses an InGaAsP composition of the MQW layer.

JP-A-10-260381

  In order to realize a larger extinction ratio characteristic than that of the InGaAsP-based MQW layer, for example, it is desirable to form the MQW layer with an InGaAlAs-based material. InGaAlAs MQW has a different band offset from InGaAsP MQW. The band offset of the conduction band of the InGaAlAs-based MQW is characterized by being larger than the band offset of the conduction band of the InGaAsP-based MQW. Since the electron confinement becomes strong, the amount of change Δα of the absorption coefficient α expressed by (Equation 1) is There is a tendency to become smaller. Therefore, in the InGaAlAs MQW, the value of the chirp parameter increases compared to the InGaAsP MQW, and it is difficult to obtain low chirp characteristics.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an EA modulator, a semiconductor optical device, and an optical module that achieve both a large extinction ratio and low chirp characteristics.

  (1) In order to solve the above-described problem, the electroabsorption modulator according to the present invention is configured such that the height of the barrier layer with respect to the ground level of electrons in the well layer is the first energy, and the base of the light hole in the well layer. When the height of the barrier layer with respect to the level is a second energy, and the height of the barrier layer with respect to the ground level of the heavy hole of the well layer is a third energy, the second energy is the second energy. The third energy comprises a multiple quantum well substantially equal to one energy, the third energy being less than the second energy.

  (2) In the electroabsorption modulator according to (1), both the first energy and the second energy may be 25 meV or more and 50 meV or less.

  (3) In the electroabsorption modulator according to (1) or (2), the well layer of the multiple quantum well may be formed of an InGaAlAs-based material.

  (4) The electroabsorption modulator according to any one of (1) to (3), wherein light having a wavelength band of 1550 nm may be modulated.

  (5) A semiconductor optical device according to the present invention may include the electroabsorption modulator according to any one of (1) to (4).

  (6) The optical module which concerns on this invention may be provided with the semiconductor optical element as described in said (5).

  The present invention provides an EA modulator, a semiconductor optical device, and an optical module that achieve both a high extinction ratio and low chirp characteristics.

1 is a top view of an EA modulator integrated DFB laser according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view of an EA modulator integrated DFB laser according to a first embodiment of the present invention. 1 is a cross-sectional view of an EA modulator integrated DFB laser according to a first embodiment of the present invention. It is a figure which shows the band diagram of a quantum well. It is a figure which shows the band diagram of a quantum well. It is a figure which shows the band diagram of a quantum well. It is a figure which shows the absorption spectrum of the EA modulator part of the EA modulator integrated DFB laser which concerns on the 1st Embodiment of this invention. It is a figure which shows the measurement result about the alpha characteristic of the 1st example of the EA modulator integrated DFB laser 1 which concerns on the 1st Embodiment of this invention. It is a figure which shows the measurement result about the alpha characteristic of the 2nd example of the EA modulator integrated DFB laser 1 which concerns on the 1st Embodiment of this invention. It is a schematic diagram which shows the structure of the optical module which concerns on the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described specifically and in detail based on the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted. In addition, the drawings shown below are merely examples of the embodiment, and the size of the drawings and the scales described in this example do not necessarily match.

[First Embodiment]
The semiconductor optical device according to the first embodiment of the present invention includes an EA in which an EA modulator and a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser (Distributed FeedBack Laser)) are monolithically integrated on the same substrate. This is a modulator integrated DFB laser 1. The EA modulator integrated DFB laser 1 according to this embodiment is a laser element to be mounted on an optical transmission / reception module for 80 km transmission having a wavelength band of 1550 nm and a transmission speed of 10.7 Gbit / s.

  FIG. 1A is a top view of the EA modulator integrated DFB laser 1 according to the embodiment, and FIGS. 1B and 1C are cross-sectional views of the semiconductor optical device according to the embodiment. 1B shows a cross section indicated by the IB-IB line in FIG. 1A, and FIG. 1C shows a cross section indicated by the IC-IC line in FIG. 1A. The EA modulator integrated DFB laser 1 according to this embodiment includes an EA modulator unit 106 and a DFB laser unit 111. The DFB laser unit 111 oscillates continuous light, and the EA modulator unit 106 modulates the continuous light and emits it as an optical signal to the outside. The structure of the EA modulator integrated DFB laser 1 according to this embodiment will be described later.

  2A to 2C are diagrams showing band diagrams of quantum wells. FIG. 2A shows a band diagram C of the conduction band of MQW. FIG. 2B shows a band diagram Vl of a light hole (hereinafter referred to as LH) in the valence band of MQW. FIG. 2C shows a band diagram Vh of a heavy hole (hereinafter referred to as HH) in the MQW valence band. 2A to 2C, the vertical axis is the energy level E, the horizontal axis is the position Z in the stacking direction, and the band diagrams of the well layer W and the barrier layer B located on both sides thereof are shown in the figure. Yes.

The band offset ΔE _c of the conduction band shown in FIG. 2A is the difference between the lower end of the conduction band of the barrier layer B and the lower end of the conduction band of the well layer W, and the conduction band of the well layer W Depth of conduction band. Here, the lower end of the conduction band of the well layer W and the barrier layer B represents the lower end of the conduction band of the material (bulk) forming each layer. Further, if the difference between the lower end of the conduction band of the well layer W and the ground level of electrons in the well layer W is c0, c0 is a height of the ground level of electrons relative to the bottom of the conduction band of the well layer W. It represents. If the depth of the ground level of the electrons of the well layer W with respect to the lower end of the conduction band of the barrier layer is defined as the first energy, the first energy can be expressed by ΔE _c −c0, It may be said that the height of the barrier layer with respect to the ground level of electrons in the well layer W. The first energy is an absolute value of the difference between the two energy levels and takes a positive value.

The band offset ΔE _vl of the valence band shown in FIG. 2B is the difference between the upper end of the valence band of the barrier layer B and the upper end of the valence band (light hole) of the well layer W. It represents the depth of the electron band (light hole) relative to the valence band of the barrier layer B. Here, the upper end of the valence band of the well layer W and the barrier layer B represents the upper end of the valence band of the material (bulk) forming each layer. If the difference between the upper end of the valence band of the well layer W and the ground hole level of the well layer W is lh0, lh0 is the valence band of the well layer W at the ground level of the light hole. This represents the height relative to the upper end of the (light hole). If the ground level of the light hole of the well layer W with respect to the upper end of the valence band of the barrier layer is defined as the second energy, the second energy can be expressed by ΔE _vl -lh0, and the second energy is It may be said that it is the height of the barrier layer with respect to the ground level of the light hole of the layer W.

The band offset ΔE _vh of the valence band V shown in FIG. 2C is the difference between the upper end of the valence band of the barrier layer B and the upper end of the valence band (heavy hole) of the well layer W. It represents the depth of the valence band (heavy hole) relative to the valence band of the barrier layer B. Here, the upper end of the valence band of the well layer W and the barrier layer B represents the upper end of the valence band of the material (bulk) forming each layer. Further, when the difference between the upper end of the valence band of the well layer W and the ground hole ground level of the well layer W is hh0, hh0 is the valence band of the well layer W at the ground hole level. This represents the height relative to the upper end of the (heavy hole). When the ground energy level of the heavy hole of the well layer W with respect to the upper end of the valence band of the barrier layer is defined as the third energy, the third energy can be expressed by ΔE _vh −hh0. It may be said that it is the height of the barrier layer with respect to the ground level of the heavy hole of the layer W. In addition, 2nd energy and 3rd energy take a positive value similarly to 1st energy.

The main features of the EA modulator integrated DFB laser 1 according to this embodiment are that the MQW layer 104 of the EA modulator unit 106 is formed of an InGaAlAs-based material, and the band diagram of the MQW layer 104 has a second energy The third energy is substantially equal to the first energy (ΔE _c −c0 = ΔE _vl −lh0), and the third energy is smaller than the second energy (ΔE _vh −hh0 <ΔE _vl −lh0). Here, the second energy being substantially equal to the first energy means that the second energy coincides with the first energy or is within a range of ± 10% of the first energy. Furthermore, it is preferably within a range of ± 3%. Furthermore, it is more desirable that the first energy and the second energy are equal.

FIG. 3 is a diagram showing an absorption spectrum of the EA modulator unit 106 of the EA modulator integrated DFB laser 1 according to this embodiment. The vertical axis is the absorption coefficient α of the figure, the abscissa represents the wavelength lambda, represents respectively, a wavelength indicated by lambda ₀ in the figure, DFB laser 111 is a wavelength of the laser beam oscillated. FIG. 3 shows an absorption spectrum when no electric field is applied as a spectrum S1, and an absorption spectrum when an electric field is applied as a spectrum S2. Since the band diagram of the MQW layer 104 of the EA modulator unit 106 has the above-described structure, the portion where the change amount Δα of the absorption coefficient α on the shorter wavelength side than the laser wavelength λ ₀ is negative is increased. The portion where the amount of change Δα of the absorption coefficient α on the longer wavelength side than λ ₀ is positive can be increased. That is, when an electric field is applied to the MQW layer 104 of the EA modulator unit 106, electrons and light holes in the quantum well layer move in opposite directions, and the wave function of the electrons and the wave function of the light holes are It is possible to reduce the spatial overlap integral along the stacking direction. In order to easily apply spatial bias to the wave function by applying an electric field, the quantum well depth and effective mass of the quantum well depend. In this embodiment, paying attention to the fact that the effective masses of electrons and light holes are approximately the same, tensile strain is introduced into the well layer. In the band diagram of the MQW layer 104, the height of the barrier layer (first energy) with respect to the ground level of electrons having the same effective mass and the height of the barrier layer (second energy) with respect to the ground level of the light hole. Energy) and the height of the barrier layer (third energy) relative to the ground level of heavy holes with a heavy effective mass, the height of the barrier layer relative to the ground level of electrons, and the ground level of light holes. It is shallower (smaller) than the height of the barrier layer relative to the position. By forming the MQW layer 104 from an InGaAlAs-based material, the first energy (the height of the barrier layer with respect to the electron ground level) is larger than the first energy of, for example, an InGaAsP-based material, and a large extinction ratio characteristic is obtained. Furthermore, by using the MQW layer 104 of the EA modulator unit 106 as such a band diagram, even in an InGaAlAs-based MQW having a large extinction ratio, lower chirp characteristics can be realized. Further, by using an EA modulator integrated DFB laser, downsizing and low power consumption are further realized.

In recent years, the amount of information transmission has increased dramatically due to the spread of information terminals such as smartphones, and the EA modulator integrated DFB laser 1 according to this embodiment is optimal as a communication device supporting the communication infrastructure. is there. In order to realize 80 km transmission with a transmission speed of 10 Gbit / s in the 1550 nm wavelength band, the first energy and the second energy are preferably 25 meV or more and 50 meV or less (25 meV ≦ ΔE _c −c0 = ΔE _vl −lh0 ≦ 50 meV). ), Element design may be performed so that the first energy and the second energy are within a range.

Here, if the layer thickness of the well layer of the MQW layer 104 is L _w , the MQW layer 104 has the composition of the well layer and the barrier layer In _(1-xy) Ga _(x) Al _(y) As x and y is MQW given by (Formula 2) and (Formula 3) below.

^{_{x = (a · L w 2}} + b · L w + c) · (ΔEc-c0) + (d · L w 2 + e · L w + f) ··· ( Equation 2)

^{_{y = (a · L w 2}} + b · L w + c) · (ΔEc-c0) + (d · L w 2 + e · L w + f) ··· ( Equation 3)

Here, the coefficients a to f of the well layer and the barrier layer of the MQW layer 104 are given by the following Table 1, respectively. That is, it is preferable that x and y of the well layer and the barrier layer satisfy the expressions for substituting the values of the coefficients shown in Table 1 into (Equation 2) and (Equation 3), respectively. Even if each of x and y of the layer has an error of ± 3% of the value obtained by such an expression, it is only necessary that ΔEc-c0 of the manufactured MQW layer satisfies 25 meV or more and 50 meV or less. For example, if it is desired to produce an EA modulator such that ΔEc−c0 = 25 meV with L _w = 10 nm, x and y are obtained from (Equation 2), (Equation 3) and Table 1, and the x and y The well layer and the barrier layer of the MQW layer may be formed with the composition represented by At this time, an error may occur due to process variations even if the composition is determined by the composition determined by x and y, but ΔEc-c0 of the fabricated MQW layer is 25 meV even if the error is shifted by several percent. The effect of the present application can be obtained as long as it satisfies 50 meV or less.

The structure of the first example and the second example of the EA modulator integrated DFB laser 1 according to the embodiment will be described. In the first example, the first energy and the second energy are 50 meV (ΔE _c −c0 = ΔE _vl −lh0 = 50 meV), and the third energy is 26 meV (ΔE _vh −hh0 = 26 meV). As shown in FIG. 1B, the EA modulator unit 106 includes an n-type InP buffer layer 102, an n-type InGaAlAs lower guide layer 103, an MQW layer 104, a p-type InGaAlAs upper guide layer 105, p on an n-type InP substrate 101. A semiconductor multilayer composed of a type InP cladding layer 112 and a p-type contact layer 113 is sequentially laminated. Here, the well layer and the barrier layer of the MQW layer 104 are both undoped layers. Then, the well layer has a thickness _{L w} is 10 nm, a composition _In 0.473 _Ga 0.514 _{Al 0.013} As, the barrier layer, in a layer thickness _{L b} is 4 nm, the composition is _{an In 0.606} Ga _0.188 Al _0.206 As, and the MQW layer 104 includes seven well layers. Of the semiconductor multilayer, a region outside the active region forming the optical waveguide is removed to a height at which it becomes a part of the n-type InP buffer layer 102, thereby forming a mesa stripe structure. Further, the EA modulator section 106 has a buried hetero (hereinafter referred to as BH) structure in which both side surfaces of the mesa stripe structure are buried with a semi-insulating InP layer 114. A passivation film 115 is formed in a predetermined region on the upper surface of the semi-insulating InP layer 114, and an upper electrode 116 (p-type electrode) is formed so as to be in contact with the uppermost surface of the mesa stripe structure. Further, a lower electrode 117 (n-type electrode) common to the EA modulator unit 106 and the DFB laser unit 111 is formed on the back surface of the n-type InP substrate 101.

  In contrast, as shown in FIG. 1C, the DFB laser unit 111 includes an n-type InP buffer layer 102, an n-type InGaAsP lower guide layer 107, an MQW layer 108, and a p-type InGaAsP upper guide on an n-type InP substrate 101. A semiconductor multilayer including a p-type InGaAsP diffraction grating layer 110, a p-type InP clad layer 112, and a p-type contact layer 113 having a thickness of about 10 nm to 20 nm is sequentially stacked. In the MQW layer 108, both the well layer and the barrier layer are made of undoped InGaAsP. The semiconductor multilayer of the DFB laser unit 111 has a mesa stripe structure as in the case of the EA modulator unit 106. Further, the DFB laser unit 111 has a BH structure, and further includes a passivation film 115 and an upper side. An electrode 116 is formed.

  As shown in FIG. 1A, in the EA modulator integrated DFB laser 1 according to the embodiment, an EA modulator unit 106 and a DFB laser unit 111 are monolithically integrated on the same substrate. A reflection film 118 is formed on the opposite end surface (the right end surface in FIG. 1A), and a reflection suppression film 119 is formed on the output end surface (the left end surface in FIG. 1A).

Next, the structure of the second example of the EA modulator integrated DFB laser 1 according to this embodiment will be described. In the second example, the first energy and the second energy are 30 meV (ΔE _c −c0 = ΔE _vl −lh0 = 30 meV) and the third energy is 17 meV (ΔE _vh −hh0 = 17 meV). The layer has a layer thickness L _w of 10 nm and a composition of In _0.485 Ga _0.494 Al _0.022 As, and the barrier layer has a layer thickness L _b of 4 nm and a composition of In _0.595 Ga _{0. 227} Al _0.178 As. The MQW layer 104 includes seven well layers. Otherwise, the second example has the same structure as the first example.

  FIG. 4 is a diagram showing measurement results for the α characteristics of the first example of the EA modulator integrated DFB laser 1 according to the embodiment, and FIG. 5 is a diagram showing measurement results for the α characteristics of the second example as well. It is. The α characteristic is a characteristic that serves as an index of the chirp parameter of the EA modulator unit 106. Note that the α characteristic is completely different from the absorption coefficient α. As shown in FIGS. 4 and 5, both the first and second EA modulator integrated DFB lasers achieved α <0 at Vea <−0.4V. As a result, a transmission test was performed at a transmission rate of 10 Gbit / s and chromatic dispersion of 1600 ps / nm (that is, transmission equivalent to 80 km). As a result, with respect to 2 dB, which is a general transmission penalty specification, A transmission penalty of 1.5 dB could be obtained. At the same time, an extinction ratio of 11.2 dB was achieved at an electrical amplitude voltage Vmod = 1.2 V, and a large extinction ratio and a low chirp characteristic (low transmission penalty characteristic) were achieved.

  A method for manufacturing the EA modulator integrated DFB laser 1 according to this embodiment will be described below. An n-type InP buffer layer 102 and an n-type InGaAlAs lower side are formed on a wafer-shaped n-type InP substrate 101 by metal organic chemical vapor deposition (hereinafter referred to as MO-CVD (Metal-Organic Chemical Vapor Deposition) method). The guide layer 103, the MQW layer 104, the p-type InGaAlAs upper guide layer 105, and the p-type InP cap layer are grown in order (first multilayer growth of the EA modulator section 106).

The thermal vapor deposition method (hereinafter referred to as T-CVD (Thermal-Chemical Vapor Deposition) method) is applied only on the region to be the EA modulator portion 106 of the EA modulator integrated DFB laser 1 in the entire wafer thus formed. Thus, an oxide film (SiO ₂ ) is formed. Then, dry etching and wet etching are performed to remove the multilayer portion above the n-type InP buffer layer 102 located in a region other than the region to be the EA modulator portion 106.

  Thereafter, the n-type InGaAsP lower guide layer 107, the MQW layer 108, and the p-type InGaAsP upper guide are formed on the entire wafer by using a butt joint technique on the region to be the DFB laser unit 111 of the EA modulator integrated DFB laser 1. The p-type InGaAsP diffraction grating layer 110 and the p-type InP cap layer having a layer thickness of about 10 nm to 20 nm and a p-type InP cap layer are grown in order (first multilayer growth of the DFB laser unit 111).

  Here, it is assumed that the semiconductor multilayer is formed in the order of the EA modulator unit 106 and the DFB laser unit 111, but either order may be formed. After the EA modulator unit 106 and the DFB laser unit 111 are formed, only the p-type InP cap layer of the DFB laser unit 111 is etched. A diffraction grating is formed on the p-type InGaAsP diffraction grating layer 110 by the interference exposure method only on the region to be the DFB laser part 111 of each EA modulator integrated DFB laser. Note that the diffraction grating may be formed by an EB drawing method.

  Further, the p-type InP cladding layer 112, the p-type contact layer 113, and the p-type InP protective layer are sequentially grown by MO-CVD (second multilayer growth). In both the first multilayer and the second multilayer, zinc (Zn) is used as a dopant for the p-type semiconductor.

Next, dry etching is performed using a SiO ₂ film formed by patterning in a stripe shape as a mask to form a mesa stripe structure having a predetermined width in the EA modulator unit 106 and the DFB laser unit 111. At this time, the lowest point of the mesa stripe structure reaches a height at which it becomes a part of the n-type InP buffer layer 102. In the EA modulator unit 106 and the DFB laser unit 111, the MQW layers 104 and 108 serving as active layers. Are respectively located above the lowest point of the mesa structure. Thereafter, a semi-insulating InP layer 114 using ruthenium (Ru) as a dopant is formed on both side surfaces of the mesa stripe structure. The temperature at the time of formation is set between 550 and 600 ° C. so that the resistivity and the embedding shape of the semi-insulating InP layer 114 are optimized.

Thereafter, the p-type contact layer 113 in the region between the EA modulator unit 106 and the region to be the DFB laser unit 111 is removed, and the p-type contact layer 113 of the EA modulator unit 106 and the p-type contact layer of the DFB laser unit 111 are removed. The contact layer 113 is electrically disconnected. Then, the entire wafer is temporarily protected with a passivation film 115 (SiO ₂ film), and the passivation film 115 formed on the uppermost surface of the mesa stripe structure is partially removed in the EA modulator section 106 and the DFB laser section 111. Then, after forming a through hole, the upper electrode 116 is deposited and electrode patterning is performed by ion milling. Finally, the lower surface of the n-type InP substrate 101 is polished until the thickness of the entire wafer becomes 100 to 150 μm, the lower electrode 117 is deposited, and the wafer process is completed.

  Subsequently, the wafer is cleaved so as to have a bar shape, and a reflective film 118 having a reflectance of 90% or more is formed on the cleaved surface on the rear side (laser part side) of the laser element. A cleaving surface on the EA modulator side) is coated with a reflection suppressing film 119 having a reflectance of 1% or less. Thereafter, the chip is further cleaved to produce each of the EA modulator integrated DFB lasers 1.

  The EA modulator integrated DFB laser 1 according to the embodiment has been described above. In the EA modulator unit 106 of the EA modulator integrated DFB laser 1 according to the present embodiment, a larger extinction ratio is realized by forming the MQW layer 104 with an InGaAlAs-based material, and the band diagram of the MQW layer 104 is as described above. By doing so, low chirp characteristics are also realized. Lowering the chirp characteristics and increasing the extinction ratio generally have a trade-off relationship, so the first energy and the second energy are between 25 meV and 50 meV depending on the purpose and application of the device product. By adjusting in step (b), a semiconductor optical device that realizes both a large extinction ratio and a low chirp characteristic is realized.

  The present invention is used in the 1550 nm wavelength band, and when the MQW layer 104 is formed of an InGaAlAs material, a large extinction ratio and low chirp characteristics can be realized. The material to be used is not limited to the InGaAlAs system. Even when MQW is formed using other materials, if the same band diagram can be realized, the same effects as those of the embodiment can be obtained by the behavior of electrons and holes. In this embodiment, the EA modulator and the DFB laser are monolithically integrated on the same substrate. However, the present invention is not limited to this. Another laser may be used, and the EA modulator may be an external modulator to modulate the continuous light output from the semiconductor laser element. Even in such a case, it goes without saying that the same effects as those of the embodiment can be obtained.

[Second Embodiment]
The optical module according to the second embodiment of the present invention is an optical module including the EA modulator integrated DFB laser 1 according to the first embodiment. FIG. 6 is a schematic diagram illustrating a configuration of the optical module according to the embodiment. The optical module according to this embodiment includes an EA modulator integrated DFB laser 1, two lenses 10, an isolator 11, an optical fiber 12, a thermistor 13, and a monitor PD 14. The EA modulator integrated DFB laser 1 outputs a modulated optical signal. The optical signal output from the EA modulator integrated DFB laser 1 is made closer to parallel light by the first lens 10, passes through the isolator 11, and near the input end of the optical fiber 12 by the second lens 10. And is coupled to the optical fiber 12. The temperature of the EA modulator integrated DFB laser 1 is maintained at a predetermined constant temperature by the thermistor 13. Further, the optical output of the EA modulator integrated DFB laser 1 is monitored by a monitor PD 14 (pin-diode: light receiving element), and the drive current is controlled by feeding back the optical output amount. In the EA modulator integrated DFB laser 1 according to the first embodiment, a large extinction ratio and a low chirp characteristic are realized. In the optical module according to the embodiment, the optical fiber 12 is normally dispersed. However, good transmission characteristics can be obtained to a degree sufficient for long-distance transmission of 80 km. That is, ON / OFF of an optical signal can be identified even for light after long-distance transmission with the optical fiber 12. The optical module according to the embodiment is the optical transmission module (TOSA) shown in FIG. 6, but is not limited thereto, and other optical modules such as an optical transmission / reception module may be exceptional. Needless to say, the effects of

[Third Embodiment]
The optical transmission apparatus according to the third embodiment of the present invention is an optical transmission apparatus (optical transmission / reception system) including the optical module according to the second embodiment. The router according to this embodiment may be a router including the optical module according to the second embodiment. The optical transmission device or the router according to the embodiment can realize a large-capacity communication at a small size and at a low cost by including the optical module according to the second embodiment.

  The EA modulator, semiconductor optical device, and optical module according to the embodiment of the present invention have been described above. It goes without saying that the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of the invention.

  1 EA modulator integrated DFB laser, 10 lens, 11 isolator, 12 optical fiber, 13 thermistor, 14 monitor PD, 101 n-type InP substrate, 102 n-type InP buffer layer, 103 n-type InGaAlAs lower guide layer, 104 MQW layer 105 p-type InGaAlAs upper guide layer, 106 EA modulator section, 107 n-type InGaAsP lower guide layer, 108 MWQ layer, 109 p-type InGaAsP upper guide layer, 110 p-type InGaAsP diffraction grating layer, 111 DFB laser section, 112 p-type InP cladding layer, 113 p-type contact layer, 114 semi-insulating InP layer, 115 passivation film, 116 upper electrode, 117 lower electrode, 118 reflection film, 119 antireflection film.

Claims (6)

The height of the barrier layer with respect to the ground level of electrons in the well layer is a first energy, the height of the barrier layer with respect to the ground level of the light hole in the well layer is a second energy, and the height of the heavy hole in the well layer. When the height of the barrier layer with respect to the ground level is set as the third energy, respectively,
The electroabsorption modulator comprising a multiple quantum well, wherein the second energy is substantially equal to the first energy, and the third energy is smaller than the second energy. The electroabsorption modulator according to claim 1,
The first energy and the second energy are both 25 meV or more and 50 meV or less.
An electroabsorption modulator characterized by that. The electroabsorption modulator according to claim 1 or 2,
The well layer of the multiple quantum well is formed of an InGaAlAs-based material.
An electroabsorption modulator characterized by that. The electroabsorption modulator according to any one of claims 1 to 3,
Modulates light in the 1550 nm wavelength band,
An electroabsorption modulator characterized by that.   A semiconductor optical device comprising the electroabsorption modulator according to claim 1.   An optical module comprising the semiconductor optical device according to claim 5.

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