JP4048635B2 - Wavelength division multiplexing communication system and light source - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength division multiplexing (WDM) system that simultaneously introduces and transmits light of a plurality of wavelengths into an optical transmission line, and a light source suitable for the WDM system.
[0002]
[Prior art]
In a conventional WDM system, in a 1.55 μm band, for example, 1.53 μm to 1.56 μm, a plurality of light sources that generate optical signals with a wavelength interval of 0.8 nm and N light sources that respectively receive light generated by each light source. The optical receiver is connected to the optical receiver using an optical fiber. In such a WDM system, N light beams generated by a light source are multiplexed, and then one optical fiber is transmitted. The transmitted light is received by an optical receiver for each optical wavelength and processed as a signal for each channel.
[0003]
The light source used in the WDM system must generate and transmit light in such a narrow interval. Conventionally, such a light source includes a device that has a wavelength selection unit whose refractive index is periodically changed inside the optical resonator, and thereby has a very narrow spectral width of light, such as a DFB laser or a DBR laser. Has been used.
[0004]
[Problems to be solved by the invention]
However, in a laser element having a wavelength selector inside the optical resonator, for example, a semiconductor laser, the laser oscillation wavelength is determined in the laser element manufacturing process. Wavelength selection is almost impossible. In the WDM system, since the wavelength used is strictly defined at intervals of 0.8 nm, the wavelength of the laser beam must be controlled with an accuracy of 5 digits. This entails considerable difficulty.
[0005]
A fiber grating laser (FGL) can be applied as a light source for a WDM system. In the FGL module, an internal diffraction grating in a DFB laser and a DBR laser is provided as an external diffraction grating (fiber grating, FG) in an optical fiber, and this diffraction grating and a light emitting element are combined to form an optical resonator. It is a module. In this optical module, the emission wavelength is substantially determined by the reflection wavelength of the FG, and the spectrum width can be narrowed to the same level as that of the DFB and DBR lasers. Further, since the diffraction grating wavelength of the FG is determined separately from the manufacturing process of the light emitting element, the emission light wavelength can be adjusted even after the light emitting element is completed.
[0006]
However, in such an FGL, the optical resonator length is longer than that of a DFB laser, a DBR laser, or the like. For this reason, when a high-speed signal is introduced into the optical module and the light emitting element is driven at a high frequency, the optical module exhibits a characteristic that the spectrum of the emitted light is broadened. In other words, as the modulation frequency increases, the phase of the injected carrier and the phase of the emitted light gradually shift, and as a result, a change in the refractive index with time is induced by the change in excess carrier, equivalently. The length of the optical resonator changes. This leads to an increase in the width of the oscillation spectrum of the optical module, that is, a chirp phenomenon.
[0007]
SUMMARY OF THE INVENTION An object of the present invention is to provide a WDM system using a light source that can easily set an oscillation wavelength, has a narrow spectrum, and has a reduced chirping phenomenon, and a light source therefor.
[0008]
[Means for Solving the Problems]
In the present invention The invention relates to a wavelength division multiplexing (WDM) communication system, The wavelength division multiplexing system For WDM communication Multiple light sources; For WDM communication A plurality of optical receivers, and an optical transmission path provided between the plurality of light sources and the plurality of optical receivers; The Prepare. The plurality of light sources generate light having different wavelengths. The plurality of optical receivers receive light corresponding to each of the wavelengths of light generated by the plurality of light sources. The optical transmission line includes a first end optically coupled to a plurality of light sources via a multiplexer, and a second end optically coupled to the plurality of optical receivers via a duplexer. Have
[0009]
The light source in such a system includes a semiconductor optical amplifier, wavelength selective light reflecting means, and modulating means. The semiconductor optical amplifier generates light of a predetermined wavelength when receiving an electric current. The wavelength selective light reflecting means selectively reflects light having a specific wavelength. The modulating means modulates light of a predetermined wavelength generated by the semiconductor optical amplifier according to the electric signal. The semiconductor optical amplifier and the wavelength selective light reflecting means are provided so as to form an optical resonator. The modulation means has an electroabsorption element that modulates light of a predetermined wavelength by absorbing light of a predetermined wavelength using electroabsorption caused in response to an electric signal, and the wavelength selective light reflection means is Have a fiber grating, The electroabsorption element modulates the light intensity of the incident light to the electroabsorption element, and the electroabsorption element is integrated with the semiconductor optical amplifier, The electroabsorption element is in the optical resonator.
[0010]
Thus, since the semiconductor optical amplifier and the wavelength selective light reflecting means form an optical resonator, the laser oscillation wavelength is determined by the wavelength selective light reflecting means. For this reason, it is set separately from the semiconductor optical amplifier, and this wavelength is not determined during the fabrication of the semiconductor optical amplifier. In addition, the laser oscillation wavelength can be changed by using wavelength selective light reflecting means for reflecting light of different wavelengths. Furthermore, an oscillation wavelength spectrum corresponding to the reflection spectrum of the wavelength selective light reflecting means is realized.
[0011]
In the wavelength division multiplexing system according to the present invention, it is preferable that the modulation means has an electroabsorption element that absorbs and modulates the light having the predetermined wavelength by using electroabsorption caused in response to an electric signal. Moreover, it is preferable that such an electric field absorption element is integrally formed on the same substrate as the light generation element. Such an electroabsorption element has an absorption layer capable of absorbing light, and the light generation element has an active layer capable of generating light.
[0012]
Such a light source has an optical fiber on which a fiber grating is formed as wavelength selective light reflecting means. Since one end of the optical fiber is optically coupled to the light emitting surface of the semiconductor optical amplifier, an optical resonator is formed from the light reflecting surface of the semiconductor optical amplifier and the fiber grating. Moreover, since the semiconductor optical amplifier has the light generating element and the electroabsorption element as the optical modulation element on the same substrate, the influence of the modulation signal applied to the electroabsorption element on the carrier density existing in the active layer is avoided. be able to. For this reason, since the refractive index fluctuation | variation based on the change of the carrier density by a modulation signal is reduced in an active layer, chirping is reduced.
[0013]
A light source in such a system is a light source for a wavelength division multiplexing (WDM) communication system, and a semiconductor optical amplifier that generates light of a predetermined wavelength when receiving an electric current. A modulation means for modulating, and a wavelength selective light reflecting means for reflecting light in a wavelength selective manner, wherein the semiconductor optical amplifier and the wavelength selective light reflecting means are provided so as to form an optical resonator. The modulation means includes a waveguide modulation element that modulates light passing through a waveguide provided on an electro-optic crystal substrate in accordance with the electric signal, and the wavelength selective light reflection means includes: A diffraction grating on a waveguide provided on the electro-optic crystal substrate; the waveguide modulation element and the diffraction grating are provided on the electro-optic crystal substrate; The waveguide modulation element modulates the light intensity of incident light to the waveguide modulation element, The waveguide modulation element is in the optical resonator.
[0014]
In such a light source, the light generating element has an active layer that generates light when carriers are injected, and the electroabsorption element is optically coupled to the active layer, and a voltage of a predetermined value is applied. It can have an absorbing layer that modulates light from the active layer by absorbing light. For example, a light generating element suitable for such a light source includes an active layer, a first conductivity type semiconductor layer provided on the substrate, and a second conductivity type provided on the substrate and having a conductivity type different from the first conductivity type. And a semiconductor layer. The active layer is provided between the first conductive type semiconductor layer and the second conductive type semiconductor layer, and generates light when carriers are injected from each of the first conductive type semiconductor layer and the second conductive type semiconductor layer. be able to. An electric field absorption element suitable for such a light source can have a pair of electrodes for applying a voltage to each of the first conductive semiconductor layer and the second conductive semiconductor layer. The absorption layer is provided between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer. Thus, if each of the active layer of the light generating element and the absorbing layer of the electroabsorption element is sandwiched between the common semiconductor layers, it is possible to enhance the optical coupling between the active layer and the absorbing layer. In addition, an electric field absorption element suitable for such a light source includes a third conductivity type semiconductor layer provided on the substrate, a fourth conductivity type semiconductor layer provided on the substrate and having a conductivity type different from the third conductivity type, A third conductive type semiconductor layer and a fourth conductive type semiconductor layer each have a pair of electrodes for applying a voltage. The absorption layer is provided between the third conductivity type semiconductor layer and the fourth conductivity type semiconductor layer. The fourth conductivity type semiconductor layer may be electrically insulated from the first conductivity type semiconductor layer.
[0015]
In the wavelength division multiplexing system according to the present invention, the modulation means preferably includes a waveguide modulator that modulates light passing through the waveguide provided on the electro-optic crystal substrate in accordance with an electric signal.
[0016]
A configuration suitable as such a modulation element is derived by applying a predetermined voltage between two electrodes sandwiching a planar waveguide formed on an electro-optic crystal substrate and changing the refractive index of the waveguide portion. A planar waveguide modulation element that modulates the amount of light passing through the waveguide can be used. Furthermore, if a diffraction grating is provided in the planar waveguide, it can be used as a wavelength selective light reflector. In this way, the wavelength selective reflector, the planar waveguide modulator, and the electro-optic crystal can be integrated together.
[0017]
When an electric field is applied to the electro-optic crystal, its refractive index changes and the phase of the guided light also changes. After splitting the incident light into two with a Y-branch coupler, applying an electric field to one of the waveguides, changing the phase of the light by 180 °, and then recombining them, when the electric field is applied, Since the light and the light from the other waveguide are 180 degrees out of phase with each other, they cancel each other. When no electric field is applied, the incident light is simply divided into two, and both lights are combined again. Therefore, assuming that there is no loss, the incident light intensity is maintained. Therefore, it becomes possible to modulate the emitted light according to the presence or absence of an electric field. In addition, the amount of phase change can be changed according to the applied electric field strength.
[0018]
In this case, the signal applied to the modulator can be a signal completely independent of the light generating element. Since the light generating element can completely operate in direct current, no change in the refractive index based on surplus carriers in the light generating element is induced. For this reason, in such a light source, chirp reduction of the emitted light based on surplus carriers does not occur. Furthermore, a diffraction grating formed in a planar waveguide using a wavelength selective light reflector can be used. This can be integrally formed on the same electro-optic crystal substrate as that of the planar waveguide modulator.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings. If possible, the same parts are denoted by the same reference numerals, and the description thereof is omitted.
[0020]
FIG. 1 is a block diagram of a wavelength division multiplexing (WDM) communication system according to the present invention. Referring to FIG. 1, the WDM communication system includes a plurality of light sources 1, a plurality of optical receivers 2, and an optical transmission path 4. In the description that follows, the number of light sources 1 is the same as the number of optical receivers 2.
[0021]
The plurality of light sources 1 in such a system generate light of different wavelengths. The light source 1 includes a light generating unit OA for generating light of a predetermined wavelength when receiving an electric current, a modulating unit OM for modulating light in accordance with an external electric signal (any one of CH1 to CHn), and a wavelength. It has selective light reflecting means FG. As the light generating means OA, for example, there is a semiconductor optical amplifier, and the semiconductor optical amplifier generates light of a predetermined wavelength when receiving an electric current. The wavelength selective light reflecting means selectively reflects light having a specific wavelength. The modulating means modulates light of a predetermined wavelength generated by the semiconductor optical amplifier according to the electric signal. Each light source 1 is optically coupled to a multiplexer 7 via an optical transmission line 5. The plurality of optical receivers 2 receive light having a wavelength corresponding to each of the light generated by the plurality of light sources 1. The optical receiver 2 includes a light receiving element PD that performs photoelectric conversion, such as a photodiode, and a preamplifier PreAMP that amplifies a current from the photodiode and outputs an electrical signal (OUT1 to OUTn). Each receiver 2 is optically coupled to a duplexer 9 via an optical transmission line 6. The optical transmission line 4 is optically coupled through a first end optically coupled with a plurality of light sources 1 via an optical multiplexer 7 and via a duplexer 9 with a plurality of optical receivers 2. Having a second end. As the optical transmission lines 2, 4, and 6, there are optical fibers.
[0022]
The light generating means OA and the wavelength selective light reflecting means FG are provided so as to form an optical resonator. Since the laser oscillation wavelength is substantially determined by the wavelength selective light reflecting means FG, it is set separately from the semiconductor optical amplifier OA. Therefore, this wavelength is not determined during the fabrication of the semiconductor optical amplifier OA. Further, if the wavelength selective light reflecting means FG that reflects light of different wavelengths is used, the laser oscillation wavelength can be changed. For this reason, it is suitable for a WDM communication system. The oscillation wavelength spectrum is determined according to the reflection spectrum of the wavelength selective light reflecting means FG.
[0023]
FIG. 2A is a configuration diagram of the duplexer. The duplexer 91 is an arrayed waveguide diffraction grating. The duplexer 91 includes n input ports (input 1 to input n) 92a to 92c, n output ports (output 1 to output n) 93a to 93c, and slab waveguide regions on the input side and the output side. 94 and 95, and n optical waveguides 96a to 96c connecting these waveguide regions 94 and 49. Only one input of the optical demultiplexer 91 is optically coupled to the optical transmission line 4. Each of the n outputs of the optical demultiplexer 91 is optically coupled to the optical transmission line 6. Such an optical demultiplexer 91 is realized as a quartz waveguide on a silicon substrate.
[0024]
The light introduced from the optical transmission line 4 to one input port of the optical demultiplexer 91 is introduced into the slab waveguide region 94. This light is equally divided from the slab waveguide region 94 into n optical waveguides 96a to 96c. The n optical waveguides 96a to 96c are different from each other in length by ΔL. The light evenly allocated to each of the optical waveguides 96a to 96c meets again in the slab waveguide 95. At this time, since the lengths from the respective optical waveguides 96 a to 96 c differ by ΔL, diffraction occurs in the slab waveguide region 95. As a result of diffraction, only light of a specific wavelength is output from an output port provided at a position corresponding to the diffraction angle of each light. Therefore, a plurality of lights having different wavelengths introduced from a single input port are dispersed.
[0025]
FIG. 2B is a configuration diagram of the multiplexer. The multiplexer 73 has a plurality of inputs 76 a to 76 d and a single output 77. The plurality of inputs 76 a to 76 d are optically coupled to the optical transmission paths 5 to the plurality of light sources 1, respectively. The output 77 is optically coupled to the optical transmission line 4. In order to optically couple the inputs 76 a to 76 d and the output 77, InGaAsP semiconductor optical waveguides 75 a to 75 d are provided on the InP semiconductor substrate 74. Since the optical waveguides 75a to 75d are merged on the semiconductor substrate 74, the light traveling along the optical waveguides 75a to 75d is merged to reach the output.
[0026]
A light source according to an embodiment of the present invention will be described with reference to FIGS. 3 and 4. This light source has a configuration of a fiber grating optical module. FIG. 3 is a perspective view of the fiber grating optical module, and is a partially broken view so that the inside of the fiber grating optical module becomes clear. 4 is a cross-sectional view taken along the line II of FIG. 3 showing the main part of the fiber grating laser.
[0027]
3 and 4, the fiber grating optical module 1 includes a housing 12 and a main part 10 of the fiber grating optical module.
[0028]
The housing 12 is a butterfly type package in the embodiment shown in FIG. The main portion 10 of the fiber grating optical module is disposed on the bottom surface in the housing 12. The main part 10 of the fiber grating optical module is sealed in the package 12 in a state in which an inert gas such as nitrogen gas is sealed. The housing 12 includes a main body portion 12a that houses the main portion 10 of the fiber grating laser module, a cylindrical portion 12b that guides the optical fiber 14 to the main portion 10, and a plurality of lead pins 12c. An optical fiber 14 is introduced from the tip of the cylindrical portion 12b. The optical fiber 14 passes through the inside of the cylindrical portion 12b and reaches the main portion 10 of the fiber grating laser module.
[0029]
The main part 10 of the fiber grating optical module includes mounting members 22, 24, and 26 for mounting the semiconductor optical elements 16 and 18, and an alignment mechanism 30 for optically coupling the optical fiber 14 to the semiconductor optical amplifier 16. Is provided. The alignment mechanism unit 30 includes a ferrule 32, a first support member 34, a second support member 36, and a third support member 38.
[0030]
The first support member 34 has a hole 34a penetrating from the contact plane 34a toward the facing surface 34b, and the ferrule 32 into which the optical fiber 14 is inserted is supported in the hole 34a. The second support member 36 is fixed to the ferrule 32 at a position away from the first support member 34 by a predetermined length. The ferrule 32 is supported in the hole 36 a of the second support member 36. The third support member has a cup shape having a cylindrical portion 38c having a through hole 38b in the central portion of the bottom surface 38a. In order to secure a predetermined length, the third support member 38 is sandwiched between the first support member 34 and the second support member 36.
[0031]
The main part 10 of the optical module includes an optical fiber 14 and a semiconductor optical amplifier 16. The semiconductor optical amplifier 16 is mounted on the chip carrier 22 so as to be optically coupled to the optical fiber 14. Heat generated during the operation of the semiconductor optical amplifier 22 is conducted through the chip carrier 22 and reaches the Peltier element 28. The Peltier element 28 operates as a cooling element when a current flows.
[0032]
The semiconductor optical amplifier 16 includes a light reflecting means 16b, a light generating element (40 in FIG. 5B), and a light modulating element (42 in FIG. 5B). The optical fiber 14 includes a fiber grating 14 a that selects and reflects the wavelength of light from the semiconductor optical amplifier 16. In the optical path of the light generated in the fiber grating optical module, a light generating element and a light modulation element are disposed between the light reflecting means 16b and the fiber grating (diffraction grating) 14a, and the light reflecting means 16b and the fiber grating 14a. Form an optical resonator. The semiconductor optical amplifier 16 will be described in detail later with reference to FIGS. 5 (a) and 5 (b).
[0033]
A light receiving element 18 such as a photodiode is mounted on a chip carrier 26. The chip carrier 26 is provided on the mounting member 24 so that the photodiode 18 is disposed at a position facing the end face 16 b of the semiconductor optical amplifier 16. The photodiode 18 operates as a monitoring photodiode for monitoring the light emission state of the semiconductor optical amplifier 16.
[0034]
The mounting member 24 is an L-shaped member having a base portion 24a and a wall portion 24b, and the wall portion 24b is provided on the mounting surface 24c of the base portion 24a. The chip carriers 22 and 26 are mounted on the mounting surface 24d of the base portion 24b. The mounting member 24 is installed on the Peltier element 28 with the installation surface 24 d facing the Peltier element 28.
[0035]
The optical fiber 14, the semiconductor optical amplifier 16, and the light receiving element 18 are arranged along a predetermined axis 20. The optical fiber 14 has a fiber grating 14a and two end portions 14b and 14c. One end 14 b of these end portions faces the first end face 16 a of the semiconductor optical amplifier 16. The optical fiber 14 is disposed at a position where the end 14b and the first end face 16a can be optically coupled. The end portion 14b optically coupled to the first end surface 16a has a lens shape that is processed into a tip so that light emitted from the first end surface 16a of the semiconductor optical amplifier 16 can be collected. ing. For this reason, it is not necessary to provide a separate lens between the semiconductor optical amplifier 16 and the optical fiber 14, so that optical coupling can be performed directly. As a result, the resonator length can be shortened, so that the high-frequency characteristics of the fiber grating laser are improved.
[0036]
The optical fiber 14 is provided with a diffraction grating (fiber grating) 14a at a position in the core portion that is separated from the one end portion 14b having a lens shape by a predetermined distance. The fiber grating is designed to have a reflection spectrum in the wavelength region used for alignment in the present application.
[0037]
The optical fiber 14 is inserted from one end into a through-hole extending from one end portion of the ferrule 32 to the other end portion, and the tip portion 14 b of the optical fiber protrudes from the other end portion of the ferrule 32 by a predetermined length. The ferrule 32 is fixed to the first support member 34 at a position away from the end portion from which the tip end portion 14b of the optical fiber protrudes. The ferrule 32 is also fixed to the second support member 36 at a position away from the one end where the optical fiber 14 is inserted by a predetermined distance. The first support member 34 and the second support member 36 are arranged at spatially separated positions by the third support member 38. The third support member 38 allows the second support member 36 to be disposed at a position spaced from the first support member 34. FIG. 5A is a perspective view of a semiconductor optical amplifier including a light generation element and a light modulation element. FIG.5 (b) is sectional drawing in the II-II cross section of Fig.5 (a).
[0038]
Referring to FIGS. 5A and 5B, in the semiconductor optical amplifier 16, the light generating element 40 and the light modulating element 42 are integrally formed.
[0039]
The light generating element 40 includes, on a substrate, an active layer 54 that generates and amplifies light when carriers are injected, and cladding layers 50 and 58 having a refractive index lower than that of the active layer 54 with the active layer 54 interposed therebetween. The light generating element 40 is a light emitting element that generates stimulated emission light by injecting carriers from the clad layers 50 and 58 on both sides into the active layer 54 to form an inversion distribution.
[0040]
The light modulation element 42 includes an absorption layer 56 that optically couples with the active layer 54 and absorbs light from the active layer 54 when a predetermined voltage is applied. The light modulation element 42 modulates light from the active layer 54 by applying a predetermined voltage to the absorption layer 56.
[0041]
The semiconductor optical amplifier 16 has a first end face 16a and a second end face 16b. The light reflectivity of the first end face 16a is smaller than the light reflectivity of the second end face. In order to realize this, a low reflection film is formed on the first end face 16a and a reflectance of about 1 to 0.1% is achieved, so that most of the light is transmitted. On the other hand, a highly reflective film is formed on the second end face 16b to achieve a reflectance of 85% or more. Therefore, the first end surface 16a is a light emitting surface, and the second end surface 16b is a light reflecting surface. Each surface can be formed as either a light reflecting surface or a light emitting surface as required, and both can be selected.
[0042]
The light generation element 40 and the light modulation element 42 included in the semiconductor optical amplifier 16 include configurations as will be described subsequently. Both the light generation element 40 and the light modulation element 42 are provided on the same semiconductor substrate 48. As the semiconductor substrate 48, for example, an n-type InP semiconductor substrate (hereinafter referred to as an n-InP substrate) can be used. Although the semiconductor optical amplifier 16 can include an n-type InP semiconductor buffer layer (not shown) on the n-InP substrate, hereinafter, a semiconductor optical amplifier is used when an n-InP substrate without a buffer layer is used. The structure of the light generation element 40 and the light modulation element 42 will be described while explaining the manufacturing method 16.
[0043]
The semiconductor optical amplifier 16 epitaxially grows an n-InP semiconductor layer over the n-InP substrate. The n-InP semiconductor layer becomes the first cladding layer 50.
[0044]
Subsequently, an insulating film mask for selectively forming an active layer and an absorption layer is formed on the first cladding layer 50 using a photolithographic technique.
[0045]
FIG. 6 is a plan view showing a planar pattern of the mask material 62 used when the active layer and the absorption layer of the semiconductor optical amplifier are formed by selective growth. Referring to FIG. 6, a portion (stripe portion) 62 a having a certain width in which the active layer and the absorption layer are to be formed is formed along the axis 61. This portion 62a is a portion from which the insulating film has been removed. The shaft 61 can be a straight line extending from the one end surface 44 of the semiconductor substrate 48 toward the other end surface 46. Although FIG. 6 illustrates the case where the shaft 61 is substantially orthogonal to the one end surface 44 and the other end surface 46, the shaft 61 can also intersect the one end surface 44 and the other end surface 46 at a predetermined angle.
[0046]
According to the pattern shown in FIG. 6, adjacent portions on both sides of the active layer and absorption layer forming portion 62 a, portions (mask regions) 62 b where the insulating layer is left extend along the axis 61. A portion 62c from which the insulating film is removed is provided adjacent to each portion 62b from which the insulating film remains. When the width of the mask region 62b is defined as the width in the direction orthogonal to the direction of the axis 61, the width d1 in the portion where the light generating element 40 is to be formed is larger than the width d2 in the portion where the light modulating element 42 is to be formed. It has become. In the region between the two elements 41 and 42, the width continuously changes from d1 to d2. Further, the composition in this region gradually changes from the composition of the active layer to the composition of the absorption layer, and the band gap also changes gradually.
[0047]
After such a mask is formed, the active layer 54 and the absorption layer 56 are formed in the same process. As the active layer 54 and the absorption layer 56, an undoped GaInAsP semiconductor is grown using the OMVPE method. When a GaInAsP semiconductor is grown using the above-described mask, the GaInAsP semiconductor is grown in the light generating element portion having the wider mask region and the light modulating element portion having the narrower mask region width. The composition is different. Therefore, an active layer having a band gap E1 and an absorption layer having a band gap E2 can be formed in a single growth process. As a result of adopting the mask shape of FIG. 6, the band gap E1 in the active layer 54 portion is smaller than the band gap E2 in the absorption layer 56 portion.
[0048]
Thus, if the active layer 54 and the absorption layer 56 are formed in a single growth step, there will be no positional deviation and no difference in thickness between the respective portions. Therefore, the active layer 54 and the absorption layer 56 can be realized with high efficiency and good reproducibility. If the cladding layers 50 and 58 sandwiching the active layer 54 of the light generating element 40 and the absorption layer 56 of the light modulation element 42 are formed of the same semiconductor layer, the active layer 54 and the absorption layer 56 It is possible to further strengthen the optical coupling.
[0049]
Next, a p-InP semiconductor layer is formed with the mask 62 left. The p-InP semiconductor layer is selectively formed only in the opening portion of the mask. The p-InP semiconductor layer becomes the second cladding layer 58. The second cladding layer 58, together with the first cladding layer 50, sandwiches the active layer 54 and the absorption layer 56. The first clad layer 50 and the second clad layer 58 have a smaller refractive index than the refractive index of the active layer 54 and the absorption layer 56, respectively, and thus act to confine light in the active layer 54 and the absorption layer 56. . Since the first cladding layer 50 is formed of an n-type semiconductor and the second cladding layer 58 is formed of a p-type semiconductor layer, a PN diode or a PN junction is formed with the active layer 54 interposed therebetween, and similarly absorbed. A PN diode or PN junction is formed with the layer 56 interposed therebetween. The conductivity types of the first and second cladding layers 50 and 58 are determined corresponding to the conductivity type of the semiconductor substrate 48.
[0050]
Subsequently, with the mask 62 left, p ⁺ -Growing InGaAs semiconductors. p ⁺ The -InGaAs semiconductor layer becomes a low resistance contact layer 60. The contact layer 60 is removed at the boundary between the light generating element 40 and the light modulating element 42 to form separate contact portions 60a and 60b. This prevents the light generating element 40 and the light modulating element 42 from being connected via a low-resistance semiconductor layer.
[0051]
After this, over the substrate 48, an insulating film such as SiO ₂ A film is formed. An insulating film 62 having an opening is formed in a portion where electrical connection to the contact portions 60a and 60b is to be made using a photolithographic technique.
[0052]
Subsequently, an electrode such as a Ti / Au electrode is formed on the insulating film 62. The electrodes include an electrode 64 for the light generation element 40 and an electrode 66 for the light modulation element 42. These electrodes 64 and 66 can be formed by using, for example, a lift-off method.
[0053]
Further, a back electrode 68 such as a Ti / Au electrode is formed over the back surface of the substrate 48. The electrode can be formed as a common electrode for the electrode 64 for the light generating element 40 and the light modulating element 42.
[0054]
Thus, a large number of semiconductor optical amplifiers were completed on the substrate 48. When this is separated into chips, a semiconductor optical amplifier having a shape as shown in FIGS. 5A and 5B can be obtained.
[0055]
The active layer 54 and the absorption layer 56 can also be formed in separate growth steps. For example, after the semiconductor layer having one composition of the active layer 54 and the absorption layer 56 is grown, this is partially removed, and then, after the semiconductor layer having the other composition of the active layer 54 and the absorption layer 56 is grown, a predetermined value is obtained. Remove leaving a portion. Such two growth steps can be used. Even in this case, the band gap E 1 of the semiconductor that is the material of the active layer 54 is smaller than the band gap E 2 of the semiconductor that is the material of the absorption layer 56.
[0056]
Thus, when the active layer 54 and the absorption layer 56 are not formed in a single growth step, there may be a difference in position and thickness between the portions 54 and 56. Since the refraction and reflection occur at the crystallographic boundary at the connecting portion of both layers, the light passing through this boundary tends to spread. For this reason, it is considered that the optical coupling between the active layer 54 and the absorption layer 56 is weakened. However, if each of the light generation element 40 and the light modulation element 42 is sandwiched between the clad layers 50 and 58 having a refractive index lower than that of the active layer 54 and the absorption layer 56 and integrally formed, the active layer 54 and The optical coupling of the absorption layer 56 can be strengthened. Further, high optical coupling between the active layer 54 and the absorption layer 56 can be realized with good reproducibility.
[0057]
When the thickness of the main part of such a semiconductor optical amplifier 16 is shown, for example,
First cladding layer: 1.0 μm
Active layer: 0.2 μm
Absorption layer: 0.2 μm
Second cladding layer: 2.0 μm
It becomes.
[0058]
The operation of the optical module including the semiconductor optical amplifier 16 and the optical fiber 14 having a fiber grating will be described with reference to FIG.
[0059]
The positive electrode of the power source 70 is connected to the electrode 64 of the light generating element 40, and the negative electrode of the power source 70 is connected to the electrode 68. The power source 70 is a power source for applying a current required for causing laser oscillation in the optical module to the light generating element 40. Since the power supply 70 biases the PN diode portion or the PN junction portion sandwiching the active layer 54 of the light generating element 40 in the forward direction, carriers are injected into the active layer 54 from the respective cladding layers 50 and 58. The light generating element 40 generates and amplifies light in the active layer 54 when carriers are injected from the first cladding layer 50 and the second cladding layer 58.
[0060]
The negative electrode of the power source 72 is connected to the electrode 66 of the light modulation element 42, and the positive electrode of the power source 72 is connected to the electrode 68. The power source 72 is a power source for applying a pulse voltage to the light modulation element 42. Since the power source 72 biases the PN diode portion or the PN junction portion sandwiching the absorption layer 56 of the light modulation element in the reverse direction, the absorption layer 56 to which a voltage is applied from the first cladding layer 50 and the second cladding layer 58 Depleted. An electric field corresponding to the applied voltage is generated in the depleted absorption layer 56. When a predetermined electric field is generated in the absorption layer 56, the fundamental absorption edge spectrum of the absorption layer 56 changes due to the Franz-Keldysh effect. This effect is effective for the absorption layer ⁺⁴ When an electric field of about V / cm is generated, it becomes noticeable. For this reason, for example, a voltage of 10 V is applied to an absorption layer having a thickness of 1 μm, and a voltage of about several volts is sufficient for an absorption layer of about 100 nm. Such a voltage value is a voltage value that can be handled in the optical module. If this voltage is used, a current flows between the power source 70 and the power source 72 via the second cladding layer 58, but the influence on the operation of the semiconductor amplifier 16 is considered to be small.
[0061]
When a semiconductor laser or a semiconductor optical amplifier is directly modulated at a frequency of several GHz, it is usually necessary to switch a large current of about several mA according to the modulation signal. However, the light modulation element 42 does not consume a current like the light generation element 40. For this reason, the current from the power source 72 is a very small value. Since the light modulation element 42 consumes little current, electromagnetic induction noise associated with current switching is also reduced. Furthermore, since there is no need to consider switching of a large current of about several mA as described above, the restrictions imposed on the pattern of the conductive layer on the wiring substrate and the semiconductor substrate can be reduced.
[0062]
FIG. 7 is a characteristic diagram showing the relationship between the light absorption rate and energy in the absorption layer 56 when an electric field is applied. Referring to FIG. 7, the solid line rising from the energy value Eg1 on the horizontal axis is the absorption characteristic when no electric field exists, and the broken line rising from the energy value Eg2 is the absorption characteristic when the electric field exists (electroabsorption characteristic). is there.
[0063]
Thus, since the fundamental absorption edge spectrum of the absorption layer 56 changes due to the Franz-Keldish effect, if the wavelength λ 0 of light received from the active layer 54 is included in the range of the spectrum change, the active layer 54 Is absorbed by the absorption layer 56 when a predetermined voltage is applied from the power source 72. That is, if the band gap value of the absorption layer 56 is made smaller than the band gap value of the active layer 54 by applying an electric field, the light from the active layer 54 changes to the fundamental absorption edge due to the electric field absorption of the absorption layer 56. Modulated accordingly. The absorption rate due to such electroabsorption is 10 ^-2 -10 ^-3 Although it is a value of the order, a desired absorption characteristic is realized if the length of the absorption layer 56 is sufficiently long.
[0064]
As described above, the fiber grating optical module having the configuration as described above has the light generation element 40 having the active layer 54 and the light modulation element 42 having the absorption layer 56 optically coupled to the active layer 54 on the same substrate 48. The semiconductor optical amplifier 16, and the optical fiber 14 having the first end portion 14 b and the fiber grating 14 a optically coupled to the light emitting surface 44 of the semiconductor optical amplifier 16, and the light reflecting surface 46 of the semiconductor optical amplifier 16. The fiber grating 14a forms an optical resonator.
[0065]
The operation of this optical module will be described with reference to FIG. When a voltage is applied to the light generating element 40 and carriers are injected into the active layer 54, light A and C are generated in the active layer 54. The light A becomes reflected light B toward the light reflecting surface 46 and propagates through the active layer 54 and reaches the absorption layer 56 in the same manner as the propagation light C.
[0066]
When no electric field is generated because no voltage is applied to the absorption layer 56, the propagation lights B and C propagate through the absorption layer 56 to reach the light emission surface 44 and are emitted from the light emission surface 44 and emitted light. D. When the emitted light D reaches the tip 14a of the optical fiber 14, it is introduced into the core as light E. When the light E reaches the fiber grating 14a, part of the light E becomes reflected light F and the rest becomes transmitted light G. The reflected light F becomes emitted light H from the tip 14 b of the optical fiber 14, and when it reaches the light emitting surface 44, becomes reflected light I introduced through the absorption layer 56. When the propagating light I reaches the active layer 54, light is stimulated and emitted in the active layer 54. In this way, laser light is generated in the optical module.
[0067]
On the other hand, the electric field (10 ⁺⁴ When V / cm) is applied, the light reaching the absorption layer 56 is attenuated by electroabsorption as it propagates through the absorption layer 56. This light substantially disappears while propagating through a sufficiently long absorption layer 56.
[0068]
In such a fiber grating optical module, since the semiconductor optical amplifier 16 has the light generating element 40 and the light modulating element 42 on the same substrate, the carrier density existing in the active layer 54 of the light generating element 40 is different from that of the light modulating element 42. It does not depend on the applied modulation signal. For this reason, since the refractive index fluctuation based on the change of the carrier density depending on the modulation signal does not occur in the active layer 54, the chirping based on the modulation is reduced.
[0069]
More specifically, the refractive index of the semiconductor layer depends on the number of carriers. Since the refractive index decreases as the number of carriers increases, the optical resonator becomes equivalently short, and the emission wavelength of the optical module shifts to a short wavelength. In a modulation operation with a low frequency, the number of carriers does not change so as to effectively change the optical resonator length. This is because even if a current (carrier) of a threshold value or more is applied to the semiconductor laser, it is converted into light in a time sufficiently shorter than the modulation frequency.
[0070]
On the other hand, in an optical module having a fiber grating, the optical resonator length is longer than, for example, the optical resonator length of a semiconductor laser. Therefore, the light generated in the optical resonator propagates between the resonators and has a phase. The time until the conditions are met becomes longer. Therefore, if the injection current changes before the phase condition is met, the current changes in a time that the phase condition is met. This means a change in the number of carriers. Thus, such changes cause chirping in the emission spectrum.
[0071]
However, in the present invention, there is no change in the number of carriers in the light generating element. In the light modulation element, since the reverse bias is applied to the absorption layer, the number of carriers does not change. Therefore, any element included in the semiconductor optical amplifier acts so as not to cause chirping in the emission spectrum. FIG. 8 is a characteristic diagram showing the relationship between the inputs V 1 and V 2 applied to the semiconductor optical amplifier 16 and the optical output I 0 generated from the semiconductor optical amplifier 16.
[0072]
Referring to FIG. 8, the voltage V1 volts is applied to the light generating element 40 at time t1 to maintain a constant value, so that light is generated in the active layer of the light generating element 40. In the period from time t1 to t2, the modulation signal low (L) is applied. Be Therefore, a voltage of 0 volts is applied to the light modulation element 42. For this reason, the absorption layer of the light modulation element 42 does not absorb the light input from the active layer, so that a light intensity I0 is obtained as the modulated light output.
[0073]
In the period from time t2 to t3, a modulation signal high (H) is given. This Therefore, the light modulation element 42 has a voltage. V2 Bolts are added. For this reason, since the light input from the active layer is absorbed in the absorption layer of the light modulation element 42, I1 that is weaker than the light intensity I0 is obtained in the modulated light output. If a sufficient amount of light absorption occurs in the absorption layer, I1 will be substantially zero.
[0074]
At time t4, when the voltage applied to the light generating element is cut to 0 volts, the intensity of the modulated light output becomes zero. Thereafter, during the period from time t5 to time t6, a modulation signal high (H) is applied, and the voltage V2 volts is applied to the light modulation element 42 and the absorption layer can absorb light. No light is input from 40. During the period from time t6 to t7, the modulation signal low (L) is applied. Being Although the voltage 0 V is applied to the light modulation element 42 and the absorption layer can transmit light, no light is input from the light generation element 40, so that the modulated light output becomes zero.
[0075]
As described above, the modulated light output is changed in accordance with the input to the light modulation element in the period from the time t1 to the time t4. However, since the voltage applied to the light generation element is constant, the carrier density of the active layer There is no change. For this reason, as already described, chirping does not occur in the modulated light output.
[0076]
FIG. 9 is a cross-sectional view of a semiconductor optical amplifier having another structure corresponding to the II-II cross section of FIG. Referring to FIG. 9, compared to FIG. 5, not only the contact layers 60 a and 60 b but also the second cladding layer is formed separately for each of the light generation element 40 and the light modulation element 42.
[0077]
The absorption layer 56 is sandwiched between the first cladding layer 50 and the second cladding layer 59a. The active layer 54 is sandwiched between the first cladding layer 50 and the third cladding layer 59b. Since the refractive indexes of the first cladding layer 50, the second cladding layer 59a, and the third cladding layer 59b are smaller than the refractive indexes of the active layer 54 and the absorption layer 56, respectively, these cladding layers 50, 59a, 59 b acts to confine light in the active layer 54 and the absorption layer 56. Since the first clad layer 50 is formed of an n-type semiconductor and the second clad layer 59a and the third clad 59b are formed of a p-type semiconductor layer, the P-type and N-type semiconductors sandwich the active layer 54 therebetween. The layers face each other, and similarly, the P-type and N-type semiconductor layers face each other with the absorption layer 56 interposed therebetween. In this embodiment, the anode of the PN junction diode is electrically insulated, but the cathode can also be insulated.
[0078]
As described above, since the second cladding layer 59a is electrically insulated from the third cladding layer 59b, the influence of the voltage applied to the light modulation element 42 on the light generating element 40 can be further reduced. .
[0079]
After this, over the substrate 48, an insulating film such as SiO ₂ A film is formed. A resist material is applied on the insulating film, and after a pattern having an opening at a portion where electrical connection to the contact portions 60a and 60b is to be made is transferred to the resist material using a photolithographic technique, the resist is The insulating film 62 is formed by removing the removed insulating film. The insulating film 62 is formed by separating the contact layer 60 and the p-InP semiconductor layer 59 between the two elements, the side surfaces of the contact portions 60a and 60b, the second cladding layer 59a, and the third cladding layer 59b. And the connection portion of the active layer 54 and the absorption layer 56 are covered.
[0080]
Note that the semiconductor optical amplifier may include the clad layers 59a and 59b separated as the semiconductor layers provided on the active layer 54 and the absorption layer 56 formed separately.
[0081]
As described above, the light source shown in the embodiment includes the optical fiber 14 and the semiconductor optical amplifier 16. The semiconductor optical amplifier 16 is a light generating element having an active layer that generates light when carriers are injected. The semiconductor optical amplifier 16 is optically coupled to the active layer and absorbs light when a predetermined voltage is applied. And a light reflecting surface for reflecting light from the active layer and a light emitting surface for emitting light from the absorbing layer. The optical fiber has a first end optically coupled to the light emitting surface, and a fiber grating provided at a predetermined distance from the first end.
[0082]
In such a fiber grating optical module, since one end of the optical fiber having the fiber grating is optically coupled to the light emitting surface of the semiconductor optical amplifier, optical resonance occurs from the light reflecting surface of the semiconductor optical amplifier and the fiber grating. A vessel is formed. Further, since the semiconductor optical amplifier has the separate light generating element and light modulating element on the same substrate, the carrier density existing in the active layer of the light generating element does not depend on the modulation signal applied to the light modulating element.
[0083]
For this reason, since the refractive index fluctuation based on the change of the carrier density depending on the modulation signal does not occur in the active layer, the chirping based on the modulation of the carrier density is reduced.
[0084]
Therefore, a fiber grating optical module with reduced chirping caused by modulation by a signal is provided.
[0085]
FIG. 10A to FIG. 13B are schematic views showing optical modules having various configurations. Since the optical module having such a configuration, the housing, the chip carrier, and the like that are the same as or similar to those shown in FIGS. 3 and 4 can be used, detailed description thereof will be omitted. The arrows shown in FIGS. 10 (a) to 13 (b) indicate the progress, reflection, and transmission of light as in FIG. 5 (b).
[0086]
FIGS. 10A and 10B are schematic views showing the configuration of the light source (fiber grating optical module 1) already described in detail. Referring to FIGS. 10A and 10B, the light source includes a semiconductor optical amplifier 16 and one end portion 14 b that is optically coupled to the light emitting surface 16 a of the semiconductor optical amplifier 16 along a predetermined axis 20. And an optical fiber 14 having The light reflecting surface 16b of the semiconductor optical amplifier 16 and the diffraction grating 14a included in the optical fiber 14 constitute an optical resonator. The semiconductor optical amplifier 16 includes a light generation element 40 and a light modulation element 42. The structural difference between FIG. 10A and FIG. 10B is the position of the light modulation element 42 in the optical resonator. The laser-oscillated light is extracted through the optical fiber 14.
FIG. 11A and FIG. 11B are schematic views showing the configuration of another light source suitable for the WDM communication system according to the present invention. Referring to FIG. 11A and FIG. 11B, the light source is light that is optically coupled to the optical fiber 15 having the diffraction grating 15a and one end portion 15b of the optical fiber 15 along a predetermined axis 20. A semiconductor optical amplifier 16 having an emission surface 16a. The laser-oscillated light can be extracted through the light semi-transmissive surface 16 c of the semiconductor optical amplifier 16. The light source for the WDM communication system preferably further includes an optical fiber 19 having an end portion 19a optically coupled to the light semi-transmissive surface 16c. The optical resonator includes a light semi-transmissive surface 16 c of the semiconductor optical amplifier 16 and a diffraction grating 15 a included in the optical fiber 15. The light semi-transmissive surface 16c has a characteristic of reflecting a part of received light and transmitting the remaining light. The structural difference between FIG. 11A and FIG. 11B is the position of the light modulation element 42 in the optical resonator. Between the semiconductor optical amplifier 16 and the optical fiber 19, an optical isolator 21 capable of passing light only in one direction (the direction of the arrow in the figure) can be provided. In this way, the return light can be blocked. The light semi-transmissive film 16c can transmit part of the light generated by the semiconductor optical amplifier 16. The light reflectance of the light semi-transmissive film 16c is larger than the light emitting surface 16a and smaller than the light reflecting surface 16b.
[0087]
FIGS. 12A to 12D are schematic views showing the configuration of a light source suitable for the WDM communication system according to the present invention. Referring to FIG. 12A, the light source is a semiconductor having an optical fiber 15 having a diffraction grating 15a along a predetermined axis 20 and a light emitting surface 17a optically coupled to one end 15b of the optical fiber 15. An optical amplifier 17 and a planar waveguide modulation element 80 optically coupled to the light semi-transmissive surface 17c of the semiconductor optical amplifier 17 are provided. The optical resonator includes a light semi-transmissive surface 17 c of the semiconductor optical amplifier 17 and a diffraction grating 15 a included in the optical fiber 15. The laser-oscillated light can be extracted via the output of the planar waveguide modulation element 80. It is preferable that the light source for such a WDM communication system further includes an optical fiber 19 having an end portion 19a optically coupled to the output of the planar waveguide modulation element 80. An optical isolator 21 can be provided between the semiconductor optical amplifier 17 and the planar waveguide modulation element 80 and between the planar waveguide modulation element 80 and the optical fiber 19 in order to block the return light.
[0088]
Referring to FIG. 12B, the light source includes an optical fiber 15 having a diffraction grating 15 a along a predetermined axis 20, and a planar waveguide modulation element 80 optically coupled to one end 15 b of the optical fiber 15. And a semiconductor optical amplifier 17 having a light emission surface 17a optically coupled to the planar waveguide modulation element 80. The optical resonator includes a light semi-transmissive surface 17 c of the semiconductor optical amplifier 17 and a diffraction grating 15 a included in the optical fiber 15. The laser-oscillated light can be extracted through the light semi-transmissive surface 17c of the semiconductor optical amplifier 17. It is preferable that the light source for such a WDM communication system further includes an optical fiber 19 having an end portion 19a optically coupled to the light semi-transmissive surface 17c of the semiconductor optical amplifier 17.
[0089]
Referring to FIG. 12C, the light source includes a semiconductor optical amplifier 16 having a light reflecting surface 17 b along a predetermined axis 20, and an end 23 b that is optically coupled to the light emitting surface 17 a of the semiconductor optical amplifier 17. And an optical fiber 23 having a diffraction grating 23a, and a planar waveguide modulation element 80 optically coupled to one end 23c of the optical fiber 23. The optical resonator includes a light reflecting surface 17 b of the semiconductor optical amplifier 17 and a diffraction grating 23 a included in the optical fiber 23. The laser-oscillated light can be extracted via the output of the planar waveguide modulation element 80. It is preferable that the light source for such a WDM communication system further includes an optical fiber 19 having an end portion 19a optically coupled to the output of the planar waveguide modulation element 80. An optical isolator 21 for blocking return light can be provided between the optical fiber 23 and the planar waveguide modulation element 80 and between the planar waveguide modulation element 80 and the optical fiber 19.
[0090]
Referring to FIG. 12D, the light source includes a semiconductor optical amplifier 17 having a light reflecting surface 17b along a predetermined axis 20, and a planar waveguide optically coupled to the light emitting surface 17a of the semiconductor optical amplifier 17. A modulation element 80 and an optical fiber 14 having a diffraction grating 14a and optically coupled to the planar waveguide modulation element 80 are included. The optical resonator includes a light reflecting surface 17 b of the semiconductor optical amplifier 17 and a diffraction grating 14 a included in the optical fiber 14. The laser-oscillated light can be extracted via the optical fiber 14.
[0091]
FIG. 13A and FIG. 13B are schematic diagrams showing a configuration of a light source suitable for a WDM communication system. Referring to FIGS. 13A and 13B, the light source includes a semiconductor optical amplifier 17 having a light reflecting surface 17b along a predetermined axis 20, and a light emitting surface 17a of the semiconductor optical amplifier 17 and an optical source. And a wavelength selective light reflector 90 having a diffraction grating 88 coupled thereto. The optical resonator includes a light reflecting surface 17 b of the semiconductor optical amplifier 17 and a diffraction grating 88 of the wavelength selective light reflector 90. Such a light source for a WDM communication system preferably further comprises an optical fiber 19 having an end 19a that is optically coupled to the output of the wavelength selective light reflector 90. The laser-oscillated light can be extracted through the optical fiber 19. In FIG. 13B, a modulation element 80 is provided in the optical resonator. On the other hand, in FIG. 13A, a modulation element 80 is provided outside the optical resonator. The modulation element 80 is provided in the wavelength selective light reflector 90 as shown in FIG.
[0092]
10 (a), 10 (b), 11 (a), 11 (b), 12 (a), 12 (c), 13 (a), and 13 (b). In the light source thus formed, the optical resonator is formed from two optical devices provided with reflecting means for constituting the optical resonator, so that the length of the optical resonator can be shortened. Further, such a light source can include a monitoring photodiode for monitoring the light emission state of the semiconductor optical amplifiers 16 and 17. The optical fiber ends 15b and 19a that are optically coupled to the semiconductor optical amplifiers 16 and 17, the planar waveguide modulator 80, and the wavelength selective light reflector 90 are similar to the end 14b of the optical fiber 14, If necessary, it can be tip-balled and have a lensed end.
[0093]
The semiconductor optical amplifiers 16 and 17, the planar waveguide modulator 80, the wavelength selective optical reflector 90, the optical fiber 19, the optical fibers 14, 15, and 23 provided with the fiber grating, and the isolator 21 are shown in FIGS. Can be disposed on or attached to the mounting member 24 shown in FIG. At this time, the optical fibers 14, 15, 19, and 23 can be fixed via a support member such as a ferrule as necessary.
[0094]
14A to 14C are perspective views showing the planar waveguide modulation element 80 and the wavelength selective light reflector 90. FIG. The planar waveguide modulation element 80 and the wavelength selective light reflector 90 are Mach-Zehnder type modulators.
[0095]
Referring to FIG. 14A, the planar waveguide modulation element 80 includes a planar waveguide 82, an input 82a and an output 82b provided at both ends of the planar waveguide 82, and two electrodes 83a sandwiching the planar waveguide 82c. , 83b. The planar waveguide 82 is LiNbO _Three These are formed by introducing Ti on the electro-optic crystal substrate 81. The planar waveguide modulation element 80 applies a predetermined voltage φ between the electrodes 83a and 83b, and modulates the amount of light passing through the waveguide by changing the refractive index of the waveguide portion.
[0096]
Incident light to the planar waveguide is bifurcated 82c and 82d by a Y-branch coupler. Thereafter, an electric field is applied to only one of the waveguides 82c to change the phase of the light by 180 °, and then the light is multiplexed again. When an electric field is applied, the light from one waveguide 82c and the light from the other waveguide 82d are out of phase with each other and thus cancel each other. When an electric field is not applied, the incident light is simply branched into two, and both lights are combined again. Therefore, assuming that there is no loss, the incident light intensity is maintained. Thus, when an electric field is applied to the electro-optic crystal, its refractive index changes. When this electric field is applied between the waveguides, the phase of the guided light can be changed. Therefore, it becomes possible to modulate the emitted light according to the presence or absence of an electric field. In addition, the amount of phase change can be changed according to the applied electric field strength.
[0097]
Referring to FIG. 14B, the incident light to the planar waveguide is branched into two branches 82c and 82d by a Y branch coupler. Thereafter, an electric field is applied to one of the waveguides 82c from the electrodes 85a and 85b by the power source φ1. An electric field is applied to the other waveguide 82d from the electrodes 85b and 85c by the power source φ2. The phases and amplitudes of the power sources φ1 and φ2 are adjusted so that the light phases of both waveguides are relatively changed by 180 °. Thereafter, when the signals are combined again, when an electric field is applied, the light from one waveguide 82c and the light from the other waveguide 82d are mutually different in phase by 180 °, and cancel each other. When no electric field is applied, the incident light intensity is maintained because the incident light is simply bifurcated and both lights are combined again.
[0098]
In this case, the signal applied to the modulator can be a signal completely independent of the light generating element. Since the light generating element can completely operate in direct current, no change in the refractive index based on surplus carriers in the light generating element is induced. For this reason, in such a light source, a chirp phenomenon of emitted light based on surplus carriers does not occur. FIG. 14C is a perspective view showing the wavelength selective light reflector 90. Referring to FIG. 14C, the wavelength selective light reflector 90 includes a diffraction grating 88 provided along the waveguide 82 on the same substrate 81 in addition to the planar waveguide modulation element 80. Thus, if the diffraction grating 88 is provided in the planar waveguide 82, the diffraction grating 88 and the planar waveguide modulation element 80 can be integrated together on the same electro-optic crystal. Therefore, a small wavelength selective light reflector 90 is obtained. As such a diffraction grating 88, a relief type grating formed by etching on the substrate surface, or a refractive index modulation type grating provided with a refractive index modulation part formed by spatially modulating the Ti concentration. , Each can be realized.
[0099]
FIG. 15 is a cross-sectional view of the semiconductor optical amplifier 17 including the light generating element 40 and corresponds to the II-II cross section of FIG. The semiconductor optical amplifier 17 in FIG. 15 is the same as the semiconductor optical amplifier 16 shown in FIG. 5B except that the semiconductor optical amplifier 17 has only the optical amplification element 40. For this reason, in FIG. 15, the same code | symbol is attached | subjected to the corresponding part of FIG.5 (b).
[0100]
【The invention's effect】
As described above in detail with reference to the drawings, in the WDM communication system according to the present invention, the semiconductor optical amplifier and the wavelength selective light reflecting means form an optical resonator. Determined by selective light reflecting means. For this reason, the optical resonator is set separately from the semiconductor optical amplifier, and this wavelength is not determined during the fabrication of the semiconductor optical amplifier. In addition, the laser oscillation wavelength can be changed by using wavelength selective light reflecting means for reflecting light of different wavelengths. Furthermore, an oscillation wavelength spectrum corresponding to the reflection spectrum of the wavelength selective light reflecting means is realized.
[0101]
Therefore, a WDM communication system using a light source that can easily set an oscillation wavelength, has a narrow spectrum, and has reduced chirping phenomena and a light source therefor are provided.
[Brief description of the drawings]
FIG. 1 is a block diagram of a wavelength division multiplexing (WDM) communication system.
2A is a configuration diagram of an optical demultiplexer, and FIG. 2B is a configuration diagram of an optical multiplexer.
FIG. 3 is a perspective view of the fiber grating optical module, and is a partially broken view so that the inside of the module becomes clear.
4 is a cross-sectional view taken along a line II in FIG. 3. FIG.
5A is a perspective view of a semiconductor optical amplifier including a light generation element and a light modulation element, and FIG. 5B is a cross-sectional view taken along a line II-II in FIG. 5A.
FIG. 6 is a plan view showing a planar pattern used when an active layer and an absorption layer of a semiconductor optical amplifier are formed by selective growth.
FIG. 7 is a characteristic diagram showing the relationship between the light absorption rate in the absorption layer and the energy of light when an electric field is applied.
FIG. 8 is a characteristic diagram showing a relationship between an input applied to the semiconductor optical amplifier and an optical output generated from the semiconductor optical amplifier.
FIG. 9 is a cross-sectional view of a semiconductor optical amplifier having another structure, corresponding to the II-II cross section of FIG. 5 (a).
FIGS. 10A and 10B are schematic views showing the configuration of the light source already described in detail.
FIGS. 11A and 11B are schematic diagrams showing the configuration of another light source suitable for the WDM communication system. FIGS.
FIGS. 12A to 12D are schematic views showing a configuration of a light source suitable for a WDM communication system.
FIGS. 13A and 13B are schematic diagrams showing a configuration of a light source suitable for a WDM communication system. FIGS.
14 (a) to 14 (c) are perspective views showing a planar waveguide modulation element and a wavelength selective light reflector.
FIG. 15 is a cross-sectional view of a semiconductor optical amplifier including a light generating element, and corresponds to the II-II cross section of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Optical receiver, 4 ... Optical transmission line, 7 ... Multiplexer, 9 ... Demultiplexer, 14, 15, 19, 23 ... Optical fiber, 16, 17 ... Semiconductor optical amplifier, 21 ... Light Isolator 40 ... light generating element 42 ... light modulation element 48 ... semiconductor substrate 50 ... first cladding layer 54 ... active layer 56 ... absorbing layer 58 ... second cladding layer 59 ... second Cladding layer, 60 ... contact layer, 62 ... mask material, 60a, 60b ... contact part, 62 ... insulating film, 64, 66, 68 ... electrode, 70, 72 ... power source, 80 ... planar waveguide modulation element, 90 ... wavelength Selective light reflector

Claims (4)

A wavelength division multiplexing (WDM) communication system comprising:
A plurality of light sources for generating light of different wavelengths for WDM communication; a plurality of optical receivers for WDM communication for receiving light of a wavelength corresponding to each of the light generated by the plurality of light sources; An optical transmission having a first end optically coupled to a plurality of light sources via a multiplexer and a second end optically coupled to the plurality of optical receivers via a duplexer. Road, and
Each of the plurality of light sources is generated by a semiconductor optical amplifier that generates light of a predetermined wavelength when receiving an electric current, wavelength selective light reflecting means for reflecting light in a wavelength selective manner, and the semiconductor optical amplifier. Modulation means for modulating light of a predetermined wavelength according to an electric signal, and the semiconductor optical amplifier and the wavelength selective light reflection means are provided so as to form an optical resonator,
The modulation means includes an electroabsorption element that modulates the light of the predetermined wavelength by absorbing the light of the predetermined wavelength using electroabsorption caused according to the electric signal,
The electroabsorption element modulates the light intensity of incident light to the electroabsorption element,
The electroabsorption element is integral with the semiconductor optical amplifier;
The wavelength selective light reflecting means has a fiber grating,
The wavelength division multiplex communication system, wherein the electroabsorption element is in the optical resonator. A wavelength division multiplexing (WDM) communication system comprising:
A plurality of light sources for generating light of different wavelengths for WDM communication; a plurality of optical receivers for WDM communication for receiving light of a wavelength corresponding to each of the light generated by the plurality of light sources; An optical transmission having a first end optically coupled to a plurality of light sources via a multiplexer and a second end optically coupled to the plurality of optical receivers via a duplexer. Road, and
Each of the plurality of light sources is generated by a semiconductor optical amplifier that generates light of a predetermined wavelength when receiving an electric current, wavelength selective light reflecting means for reflecting light in a wavelength selective manner, and the semiconductor optical amplifier. Modulation means for modulating light of a predetermined wavelength according to an electric signal, and the semiconductor optical amplifier and the wavelength selective light reflection means are provided so as to form an optical resonator,
The modulation means includes a waveguide modulation element that modulates light passing through a waveguide provided on an electro-optic crystal substrate according to the electric signal;
The wavelength-selective light reflecting means has a diffraction grating formed in a waveguide provided in the electro-optical crystal substrate,
The waveguide modulation element and the diffraction grating are provided on the electro-optic crystal substrate,
The waveguide modulation element modulates the light intensity of incident light to the waveguide modulation element,
The wavelength division multiplexing communication system, wherein the waveguide modulation element is in the optical resonator. A light source for a wavelength division multiplexing (WDM) communication system, comprising:
Semiconductor optical amplifier that generates light of a predetermined wavelength when receiving an electric current, modulation means for modulating the light of the predetermined wavelength according to an electric signal, and wavelength selective light reflection means for reflecting light in a wavelength selective manner The semiconductor optical amplifier and the wavelength selective light reflecting means are provided so as to form an optical resonator,
The modulation means includes an electroabsorption element that modulates the light of the predetermined wavelength by absorbing the light of the predetermined wavelength using electroabsorption caused in response to the electric signal,
The electroabsorption element modulates the light intensity of incident light to the electroabsorption element,
The electroabsorption element is integral with the semiconductor optical amplifier;
The wavelength selective light reflecting means has a fiber grating,
The light source, wherein the electroabsorption element is in the optical resonator. A light source for a wavelength division multiplexing (WDM) communication system, comprising:
Semiconductor optical amplifier that generates light of a predetermined wavelength when receiving an electric current, modulation means for modulating the light of the predetermined wavelength according to an electric signal, and wavelength selective light reflection means for reflecting light in a wavelength selective manner The semiconductor optical amplifier and the wavelength selective light reflecting means are provided so as to form an optical resonator,
The modulation means includes a waveguide modulation element that modulates light passing through a waveguide provided on an electro-optic crystal substrate according to the electric signal;
The wavelength selective light reflecting means has a diffraction grating on a waveguide provided on the electro-optic crystal substrate,
The waveguide modulation element and the diffraction grating are provided on the electro-optic crystal substrate,
The waveguide modulation element modulates the light intensity of incident light to the waveguide modulation element,
It said waveguide modulator element is in the optical resonator, light source, wherein the this.

Images