JP2007248901A - Optical transceiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable wavelength optical transceiver having functions of variable wavelength laser and a variable wavelength filter in combination. <P>SOLUTION: This variable wavelength optical transceiver 100, with a Mach-Zehnder interferometer as an optical coupler, is composed of two dual-input and dual-output type optical couplers 101, 102, two arm waveguides 103, 104, and four waveguides 105, 106, 107, 108. In the two arm waveguides 103, 104, there are formed equally structured gratings 109, 110 respectively. An AR coat 111 is applied to one end face of the optical transceiver while an HR coat 112 is applied to the other end face as an optical reflector. Further, in the two waveguides 107, 108 on the HR coat side, there are arranged semiconductor optical amplifiers 113, 114 as gain mediums respectively. By selecting ON (gained state) and OFF (absorbed state) of the current in the gain mediums, the laser operation and the filter operation can be selected as a choice of two alternatives. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention generally relates to optical transceivers, and more particularly to a wavelength tunable optical transceiver that implements the functions of a wavelength tunable laser and a wavelength tunable filter in a single device.

  In the transmission between today's nodes, the transmission capacity is increased by using a wavelength division multiplexing transmission system in order to cope with an increase in traffic on the Internet. A wavelength tunable laser and a wavelength tunable filter are important optical components indispensable for wavelength multiplexing transmission. Furthermore, demand for optical transceivers capable of both transmitting and receiving optical signals is expected to increase rapidly, especially in optical subscriber systems.

  FIG. 16 shows a configuration diagram of an exemplary optical transceiver 1600 realized by combining a wavelength tunable laser and a wavelength tunable filter. In the configuration illustrated in FIG. 16, the wavelength tunable laser 1601 and the wavelength tunable filter 1602 are arranged in parallel, and an optical signal is transmitted and received via the splitter 1603.

  Currently, various types of wavelength tunable lasers have been researched and developed. If roughly classified by the structure for selecting the oscillation wavelength, a wavelength tunable laser using a sampled grating (see Non-Patent Document 1), Tunable laser using super-structure grating (see Non-Patent Document 2), Tunable laser using grating assisted co-directional coupler (Non-patent) Reference 3).

  An example of the wavelength tunable laser is a wavelength tunable laser using a grating assisted codirectional coupler. This is because a number of layers having different semiconductor compositions and structures are formed in each region in the plane, and therefore, a regrowth process such as metal organic chemical vapor deposition (MOCVD) is required to form this. Must be repeated 5 times or more. That is, the manufacturing process of the element has to be complicated. Moreover, since it is designed and manufactured as a wavelength tunable laser, it cannot operate as a wavelength tunable filter.

  An example of the semiconductor wavelength tunable filter is the wavelength tunable filter described in Non-Patent Document 4. However, since the wavelength tunable filter is designed and manufactured, it cannot operate as a wavelength tunable laser.

  Therefore, when an optical transceiver is to be configured, as shown in FIG. 16, a configuration in which a wavelength tunable laser and a wavelength tunable filter are arranged in parallel is conceivable. However, each manufacturing process is complicated and independent. It is very difficult to optimize the performance of both on the device and make it one chip while maintaining it.

  An example of an optical transceiver having both the functions of an optical transmitter and an optical receiver is the optical transceiver described in Non-Patent Document 5. In this optical transceiver, a laser active layer and a photodetection layer are stacked and a single chip is realized on a semiconductor element. However, the laser oscillation wavelength is fixed, and the optical receiver is a filter function only with a photodetector. Does not have. In other words, it does not have wavelength tunability structurally, and cannot cope with wavelength multiplexing transmission systems that will be developed in the future.

IEEE, J. Quantum Electron., 29 (16), pp. 1824-1834, 1993. IEEE, J. Quantum Electron., 32 (3), pp. 433-441, 1996. IEEE, Photonics Technology Letters, 7 (7), pp.697-698, 1995. IEEE, Photonics Technology Letters, 17 (1), pp.139-141, 2005. IEEE, J. of Selected Topics in Quantum Electronics, 5 (3), pp. 627-630, 1999.

  In view of such a background, the present invention realizes the functions of an optical transmitter (laser) and an optical receiver (filter or filter and detector) having wavelength variability with a single component, and is compact and high-performance. An object of the present invention is to provide an optical transceiver capable of reducing the number of parts of a system.

  In order to achieve such an object, an invention according to claim 1 is an optical transceiver having a laser operation and a filter operation, and includes a first 2-input 2-output optical coupler, and a second 2 An input two-output optical coupler; two arm waveguides connected at both ends to each of the first and second two-input two-output optical couplers; and the first two-input two-output optical light One end is connected to the coupler, the other end is connected to the end face of the optical transceiver that has been coated with the AR coat, and one end is connected to the second two-input two-output optical coupler. A Mach-Zehnder interferometer configured using two waveguides whose other ends are connected to the HR-coated end face of the optical transceiver, a grating formed on the two arm waveguides, HR coat of Mach-Zehnder interferometer It said electrically consists of a waveguide arranged gain medium, characterized by having a means for selecting the absorption and gain at the gain medium two.

  The invention according to claim 2 is an optical transceiver having a laser operation and a filter operation, and one end of the two-input / two-output optical coupler and one end of the two-input / two-output optical coupler are provided. Four waveguides connected to each other and connected to the end face of the optical transceiver that has been coated with AR coating, gratings formed in two of the four waveguides, and the grating formed The gain medium disposed in the two waveguides and the photodetector disposed in one of the four waveguides where the grating is not formed. It has the means to select a gain and absorption, It is characterized by the above-mentioned. That is, as compared with the invention described in claim 1, one optical coupler positioned between the grating and the end face is reduced, and a short resonator is realized. As a result, the longitudinal mode interval of the element is expanded, and the oscillation wavelength is stabilized during laser operation.

  The invention according to claim 3 is an optical transceiver having a laser operation and a filter operation, and one end of the one-input / two-output type optical coupler and one end of the one-input / two-output type optical coupler are connected to each other. Three waveguides connected to each other and connected to the end face of the optical transceiver that has been coated with AR coating, a grating formed in two of the three waveguides, and the grating formed The gain medium is disposed in the two waveguides, and has means for selecting gain and absorption in the gain medium. By using one 1-input 2-output optical coupler as the optical coupler, the laser oscillation wavelength can be stabilized by shortening the resonator as in the second aspect.

  The invention according to claim 4 is an optical transceiver having a laser operation and a filter operation, and one end of the two-input one-output optical coupler and one end of the two-input one-output optical coupler are provided. Two waveguides connected to each other and connected to the end face of the optical transceiver on which the AR coating is applied, and one end of the two-input one-output optical coupler are connected to each other, and the other end is connected to the optical waveguide. One waveguide connected to the end surface of the transceiver that has been coated with HR coating, a grating formed on the waveguide on the HR coating side of the one two-input one-output optical coupler, and the grating were formed A gain medium disposed in the one waveguide, and a photodetector disposed in one waveguide on the AR coat side of the one two-input one-output optical coupler; Select absorption and It characterized by having a stage. With the configuration using one two-input one-output optical coupler as the optical coupler, the laser oscillation wavelength can be stabilized by shortening the resonator as in the second aspect. Further, since the refractive index control electrode is reduced to one for variable wavelength, the control circuit of the present optical transceiver can be greatly simplified and reduced in cost.

  According to a fifth aspect of the present invention, in the optical transceiver according to the first to fourth aspects, the optical transceiver includes a first refractive index changing means for changing a refractive index of the waveguide, and the first refractive index changing means The phase of the light passing through the output waveguide is changed to switch the output waveguide during the laser operation or the filter operation.

  According to a sixth aspect of the present invention, in the optical transceiver according to the first to fifth aspects, two or more Mach-Zehnder interferometers or the optical couplers are connected in series by a waveguide. In this way, the refractive index of the waveguide located between the optical coupler and the grating is changed, instead of switching between the laser operation and the filter operation by selecting the gain state and the absorption state in the gain medium. Thus, the laser operation and the filter operation can be switched. That is, the gain medium is always in the gain state, and the optical signal can be amplified even during the filter operation.

  According to a seventh aspect of the present invention, in the optical transceiver according to the first to sixth aspects, the grating is a super-periodic structure grating or a sampled grating. Thereby, the wavelength variable range of the optical transceiver is increased by the vernier effect.

  According to an eighth aspect of the present invention, in the optical transceiver according to any one of the first to seventh aspects, the optical transceiver includes a second refractive index changing unit that changes a refractive index of the waveguide in which the grating is formed. The reflection characteristic of the grating is changed by the refractive index changing means to change the selection wavelength at the time of laser operation or filter operation.

  According to a ninth aspect of the present invention, in the optical transceiver according to the first to eighth aspects, the grating is formed on a side wall of a waveguide. Using this structure, it is possible to modulate the amplitude of the grating, which was impossible with an embedded grating, and to combine the transmission characteristics of the filter with desired characteristics.

  According to a tenth aspect of the present invention, in the optical transceiver according to the first to ninth aspects, the amplitude of the grating is modulated so as to become maximum at a central portion of the grating and to decrease as the distance from the central portion increases. It is characterized by that. Thereby, the side rope extinction ratio of the grating can be improved.

  According to an eleventh aspect of the present invention, in the optical transceiver according to any one of the first to tenth aspects, the function for performing the modulation is one of a Gaussian function, a trigonometric function, a Hamming window function, a Hanning window function, and a Sinc function. It is characterized by using the product of one or more or their reciprocals. By using these functions, the oscillation wavelength during laser operation is stabilized.

  According to a twelfth aspect of the present invention, in the optical transceiver according to the first to eleventh aspects, the refractive index control means uses one or more of a thermo-optic effect, current injection, electric field application, and pressure application. It is characterized by changing the refractive index.

  According to the present invention, it is possible to provide a small and high-performance wavelength tunable optical transceiver capable of realizing the functions of a wavelength tunable laser and a wavelength tunable filter used in wavelength division multiplexing transmission with a single component.

  In the present invention, an optical transceiver is realized using an optical coupler, a grating, an optical reflector, and a gain medium. Due to the light coherence of the optical coupler, it is possible to switch light combining / demultiplexing and light input / output during filter operation or laser operation. In addition, the grating functions as a filter element because it has a wavelength-dependent reflection characteristic. Further, a laser resonator is configured using a grating, a gain medium, and an optical reflector, and the laser oscillation wavelength is determined by the reflection characteristics of the grating. By using the optical coupler and the grating together, the grating can be used as a filter element at the time of filter operation or as a wavelength selection element at the time of laser operation. With this configuration, a laser and a filter having wavelength tunability can be realized with one component.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a diagram illustrating a structure of a first tunable optical transceiver 100 according to an embodiment of the present invention. This wavelength tunable optical transceiver 100 has two 2-input two-output MMI (multi-mode interference) couplers 101 and 102, and both ends thereof connected to the two MMI couplers 101 and 102, using a Mach-Zehnder interferometer as an optical coupler. And two waveguides 105, 106, 107, 108 having one end connected to the two MMI couplers 103, 104 and the two MMI couplers 101, 102 and the other end connected to the end face. Has been. Gratings (diffraction gratings) 109 and 110 having the same structure are formed in the two arm waveguides 103 and 104 of the Mach-Zehnder interferometer, respectively. Also, an AR (anti-reflection) coat (antireflection film) 111 is provided on one end face of the optical transceiver, and an HR (high-reflection) coat (high reflectivity film) 112 is provided as an optical reflector on the other end face. Has been given. Further, semiconductor optical amplifiers (SOA) 113 and 114 are arranged as gain media in the two waveguides 107 and 108 on the HR coat 112 side of the Mach-Zehnder interferometer, respectively. By selecting ON (gain state) and OFF (absorption state) of the current in the SOAs 113 and 114, laser operation and filter operation can be selected alternatively. Further, a refractive index control electrode 115 for switching the output waveguide is formed immediately above the waveguides 103 and 104 positioned between the first MMI coupler 101 and the gratings 109 and 110. A refractive index control electrode 116 for changing the wavelength is formed immediately above.

  During the filter operation, the wavelength-multiplexed input light enters the waveguide 105 on the AR coat 111 side. The incident input light is first equally split into both arms by the first MMI coupler 101. From the equally branched input light, only the selection light is reflected by the grating disposed on the arm, and is incident on the first MMI coupler 101 again. Light other than the selection light (non-selection light) passes through the grating and enters the second MMI coupler 102. Since the two gratings formed on both arms have the same structure, the reflected light (selected light) and the transmitted light (non-selected light) enter the MMI coupler while maintaining the phase difference. Therefore, the selection light is output to the waveguide 106, and the non-selection light is output to the waveguide 107 on the first SOA 113 side. Since the current in the first SOA 113 is OFF during the filter operation, the first SOA 113 functions as an absorbing medium, and all non-selective light is absorbed by the first SOA 113 and reflected by the end face to which the HR coat 112 is applied. However, only the selected light is output.

  The non-selected light transmitted through the grating is mainly output to the waveguide 107 on the first SOA 113 side, but leaks to the waveguide 108 on the second SOA 114 side by about 1% due to a manufacturing error, thereby degrading the filter performance. Cause it. In order to prevent this, in the present embodiment, the second SOA 114 having the same structure as the first SOA 113 is arranged, and similarly, the current is turned off to prevent crosstalk.

  During the laser operation, the current in the first SOA 113 or the second SOA 114 is turned on, and the SOA in which the current is turned on functions as a gain medium. The two gratings 109 and 110 formed on the two arm waveguides 103 and 104 and the end face provided with the HR coat 112 function as a reflecting mirror, and both constitute a resonator indispensable for laser oscillation. Therefore, when the SOA gain exceeds the light intensity loss of the element, laser oscillation occurs. The wavelength of the laser oscillation light is determined by the wavelength at which the reflectance in the grating is highest. The laser oscillation light is output from the waveguide 105 on the AR coat 111 side when the current in the first SOA 113 is ON, and from the waveguide 106 when the current in the second SOA 114 is ON. When both of the two SOAs are ON, laser oscillation light is output from both waveguides. As described above, it is also a feature of the present optical transceiver that the output waveguide of the laser oscillation light can be switched by ON / OFF selection of current in the two SOAs.

FIG. 2 is a diagram showing a waveguide structure of the optical transceiver in the present embodiment. An optical transceiver having this waveguide structure is produced as follows.
First, a 1.55Q composition i-InGaAsP active layer 203 is grown in advance on a compound semiconductor n-InP substrate 201 by metal organic chemical vapor deposition (MOCVD). Next, the remaining region except the active layer in the SOA region is removed to the bottom of the active layer by dry etching and wet etching, and a 1.4Q composition i-InGaAsP core layer in the passive region is formed by using selective growth of the MOCVD method. Grow 203. Subsequently, a grating pattern having a period of about 240 nm is formed on the 1.4Q composition i-InGaAsP core layer 203 by electron beam lithography and wet etching. Further, the p-InP layer 205 is grown on the entire surface of the substrate including the SOA region and the waveguide region of the Mach-Zehnder interferometer including the grating by MOCVD. Further, a waveguide stripe pattern of Mach-Zehnder interferometer including SOA and grating is formed by photolithography and dry etching, and finally, it is buried and grown by Fe-doped semi-insulating InP (SI-InP) 204 by MOCVD method. By doing so, the waveguide structure shown in FIG. 2 is produced. After the waveguide formation process, current injection Au electrodes are formed on the SOA and the grating portion of the Mach-Zehnder interferometer, respectively. In addition, a refractive index control electrode 115 having a length of 100 μm is formed immediately above the waveguides 103 and 104 positioned between the first MMI coupler 101 and the gratings 109 and 110. Finally, the device is cut out by cleavage and an HR / AR coat is formed by vapor deposition.

  FIG. 3 shows the transmission characteristics of the optical transceiver in this embodiment during the filter operation. When the currents in the two SOAs are turned off and light is input from the waveguide 105, filter selection light is output from the waveguide 106. When current is injected into the arm waveguides 103 and 104 in which the two gratings 109 and 110 are formed and the refractive index of the core layer is reduced by 0.3% by the plasma effect, the center wavelength changes by about 4.5 nm. This represents that the optical transceiver is operating as a wavelength tunable filter.

  Further, when a current of about 100 mA is passed through the first SOA 113, laser oscillation light is emitted from the waveguide 105. When a current is injected into the grating portion, the laser oscillation wavelength changes as the reflection characteristic of the grating shifts to the short wavelength side as in the case of the filter operation. This indicates that the present optical transceiver operates as a tunable laser. Further, when current is injected into one of the refractive index control electrodes 115, the emitted laser oscillation light is switched from the waveguide 105 to the waveguide 106. This is because the phase of light propagating through the arm waveguides 103 and 104 has changed by π. This effect is the same in the case of the filter operation, and the selection light output from the waveguide 106 can be switched so as to be output from the waveguide 105. In other words, this means that the output waveguide during filter operation and laser operation can be arbitrarily selected electrically. For example, due to this effect, at least one optical fiber is required for input / output of this optical transceiver. This can contribute to a reduction in mounting cost and a reduction in the number of system components.

In the present embodiment, 1.0 .mu.m waveguide stripe pattern width 206, to produce 600μm length of SOA, 1500 .mu.m length of the grating, 50 nm depth _{D g} of the grating, the optical transceiver period as 240 nm. Each numerical value is an important parameter that determines the characteristics of the present optical transceiver.

In realizing the wavelength tunable optical transceiver, it is preferable that the following conditions are satisfied.
1) The height and width of the core layer of the waveguide are preferably such that light propagates through the waveguide in a single mode, and in this embodiment, the height and width are 0.1 μm to 3.0 μm.
2) The length of the SOA determines the magnitude of gain, gain bandwidth, and saturation gain characteristics, and is 20 μm to 2000 μm in this embodiment.
3) The length of the grating determines the light intensity loss during the filter operation, the transmission bandwidth, and the wavelength selection performance during laser oscillation, and is 50 μm to 4000 μm in the present embodiment.
4) the amplitude D _g of the grating determines the coupling coefficient of the grating, influence the wavelength selection performance in transmission bandwidth and the laser oscillation during the filter operation. In the present embodiment, it is 0.01 μm to 2.0 μm.
5) The period determines the wavelength band used for the optical transceiver, and in this embodiment, the center wavelength is 1.55 μm and is 240 nm.

  Further, in this embodiment, the resonator is configured by functioning the end surface coated with HR as a reflecting mirror, but it is also possible to configure the laser resonator by replacing the end surface coated with HR with a grating. is there. In that case, AR coating may be applied to both end faces. Further, although a 1.55Q composition i-InGaAsP active layer is used as a gain medium, a gain band can be expanded by using a multiple quantum well structure or a strained multiple quantum well structure for the active layer, TE polarization or TM It is also possible to increase the gain by offsetting the gain to the polarization. In addition, Fe doped semi-insulating InP is used to form the waveguide, but other systems that block current, for example, pn buried that forms InP pn junctions in the opposite direction to the active layer. A structure (thyristor structure) may be used. The waveguide structure for realizing the optical transceiver is a buried structure, but a structure other than the buried structure, for example, a high mesa structure or a ridge structure may be used.

(Second Embodiment)
FIG. 4 shows a structural diagram of a second tunable optical transceiver 400 according to an embodiment of the present invention. In the present embodiment, not a configuration using a Mach-Zehnder interferometer, but a configuration in which one MMI coupler 401 is used as a 2-input 2-output optical coupler. That is, as compared with the first embodiment, one optical coupler located between the grating and the end face is reduced, and a short resonator is realized. Thereby, the longitudinal mode interval of the element is widened, and the oscillation wavelength during laser operation is stabilized. In the first embodiment, the end face provided with the HR coat is used as the reflecting mirror. However, in this embodiment, the AR coat 412 is provided on both end faces and the structure is the same, but the length is only 200 μm. A long grating 402 is arranged to constitute a laser resonator. The reason why the gratings having different lengths are used is to make the reflectance different between the gratings constituting the resonator and to output laser oscillation light to the waveguides 403 and 404 side. Further, as in the first embodiment, a refractive index control electrode 407 for switching an output waveguide having a length of 100 μm is formed immediately above the waveguides 405 and 406 positioned between the MMI coupler 401 and the grating 402. A refractive index control electrode 411 for changing the wavelength is formed immediately above the grating 402. In addition, a photodetector (photodiode: PD) 408 is disposed in the waveguide 404. The PD 408 can be realized by the same layer structure and waveguide structure as the SOA used as the gain medium, and an optimal reverse bias may be applied.

  During the filter operation, the wavelength-multiplexed input light is incident on the waveguide 403 as in the first embodiment. The incident input light is equally branched into the waveguides 405 and 406 by the MMI coupler 401. Only the selection light is reflected from the grating 402 arranged in the output waveguide of the MMI coupler 401 and the input light that is equally branched is incident on the MMI coupler 401 again. Light other than the selection light (non-selection light) is transmitted through the grating 402 and is incident on and absorbed by the first SOA 409 and the second SOA 410 in which the current is OFF. The reflected selection light is incident on the waveguide 404 as in the first embodiment, and optical / electrical conversion is performed by the PD 408. When a forward bias is applied to the PD 408 while the selection light is incident on the waveguide 404, the PD 408 can operate as a gain medium (SOA) and amplify the selection light. Further, when a current is injected into the refractive index control electrode 407 and the phase of the light is changed by π, it is possible to output selective light to the waveguide 403.

  During laser operation, laser current is emitted from the waveguide 403 and the waveguide 404 when a current of about 100 mA is passed through the first SOA 409. At this time, since the light intensities output from the waveguide 403 and the waveguide 404 are equal, the light intensity output from the waveguide 403 can be monitored by the photocurrent flowing through the PD 408 disposed in the waveguide 404. is there. Further, when current is injected into the two grating portions constituting the resonator, the laser oscillation wavelength changes as the reflection characteristic of the grating shifts to the short wavelength side as in the filter operation. This indicates that the present optical transceiver operates as a tunable laser.

(Third embodiment)
FIG. 5 shows a structural diagram of a third tunable optical transceiver 500 according to an embodiment of the present invention.

  The optical transceiver 400 in the second embodiment has a configuration using a two-input two-output optical coupler as an optical coupler, but the optical transceiver 500 in this embodiment has one input as one MMI coupler 501. A two-output optical coupler is used. Therefore, in this embodiment, there is only one input / output waveguide.

  Reference numeral 502 denotes a grating, reference numeral 503 denotes a waveguide, reference numeral 504 denotes a first SOA, reference numeral 505 denotes a second SOA, reference numeral 506 denotes a refractive index control electrode for changing the wavelength, and reference numeral 507 denotes a refractive index control electrode. Represents an AR coat, and each is configured in the same manner as in the second embodiment.

  Similarly to the second embodiment, one optical coupler positioned between the grating and the end face is reduced as compared with the first embodiment, and a short resonator is realized. Thereby, the longitudinal mode interval of the element is widened, and the oscillation wavelength during laser operation is stabilized.

  The manufacturing method of the optical transceiver and the waveguide structure in the present embodiment are the same as those in the first embodiment.

(Fourth embodiment)
FIG. 6 shows a structural diagram of a fourth tunable optical transceiver 600 according to an embodiment of the present invention. The optical transceiver 600 according to the present embodiment has a configuration using one MMI coupler 601 as a 2-input 1-output optical coupler. With this configuration, as in the case of the second embodiment, it is possible to stabilize the laser oscillation wavelength by shortening the resonator. Furthermore, since the refractive index control electrode 602 is only one for variable wavelength, it is possible to greatly reduce the cost of manufacturing an electric circuit for controlling the optical transceiver.

  During the filter operation, the input light wavelength-multiplexed as in the second embodiment is incident on the waveguide 604 on the AR coat 603 side. The incident input light passes through the MMI coupler 601, and only the selection light is reflected by the grating 607 disposed in the output waveguide 606 of the MMI coupler 601 and enters the MMI coupler 601 again. Light other than the selection light (non-selection light) passes through the grating 606, enters the SOA 608 in which the current is in an OFF state, and is absorbed without being reflected by the end face to which the HR coat 610 is applied. The reflected selection light is equally output from the waveguide 604 and the waveguide 605. Since the output light intensity is equal, the light intensity output from the waveguide 604 can be monitored by the photocurrent flowing through the PD 609 disposed in the waveguide 605. Further, when current is injected into the grating portion, the reflection characteristic of the grating 607 is shifted to the short wavelength side as in the filter operation.

  During laser operation, laser current is emitted from the waveguide 604 and the waveguide 605 when a current of about 100 mA is passed through the SOA 608. At this time, since the light intensities output from the waveguide 604 and the waveguide 605 are equal, the light intensity output from the waveguide 604 can be monitored by the photocurrent flowing through the PD 609 as in the filter operation. .

  The manufacturing method of the optical transceiver and the waveguide structure in this embodiment are the same as those in the first embodiment and the second embodiment.

(Fifth embodiment)
FIG. 7 shows a grating structure diagram of a wavelength tunable optical transceiver according to an embodiment of the present invention. In the present embodiment, a 1.55Q composition i-InGaAsP active layer and a p-InP upper cladding layer are grown in advance by MOCVD on an n-InP substrate that is a compound semiconductor. Next, the other regions except the SOA region are removed to the bottom of the active layer by dry etching and wet etching, and the 1.4Q composition i-InGaAsP core layer and the p- The InP upper clad layer is grown at once. Next, SiO ₂ is deposited on the substrate by plasma CVD, and an SiO ₂ grating pattern shown in FIG. 7A is formed by electron beam lithography and dry etching. Further, using this as a mask, a high mesa optical waveguide having a primary grating on the side wall as shown in FIG. At the end of the waveguide fabrication process, a ridge structure, which is the SOA waveguide structure shown in FIG. 8, is fabricated by photolithography and dry etching.

  Using a structure in which a grating is formed on the side wall of a high-mesa waveguide makes it possible to modulate the amplitude of the grating, which is impossible with an embedded grating, and to combine the transmission characteristics of the filter with the desired characteristics. Is the purpose. In general, when forming an embedded grating, etching is performed in the depth direction of the substrate to form the grating. However, when the above synthesis is performed, it is necessary to modulate the amplitude of the grating, that is, to give an in-plane distribution to the etching depth, which is very difficult in production. Therefore, by using a structure in which a grating is formed on the side wall of the high mesa waveguide, the waveguide formation pattern of electron beam lithography is adjusted, and the high mesa waveguide is formed using the mask pattern, thereby modulating the amplitude of the grating. become able to.

In the present embodiment, the depth D _g of the grating shown in FIG. 7 (a), a sine function (variable 0～Pai) modulated by the square of. As a result, the coupling coefficient of the grating is modulated so that it becomes maximum at the center of the filter and becomes smaller as it goes away from the center at the periphery. A grating distribution of amplitudes D _g of of the total length L of the grating in Figure 9 shows the transmission characteristic of the optical transceiver manufactured in FIG. The overall length L of the grating is 1500 μm. FIG. 10 simultaneously shows the transmission characteristics of an optical transceiver manufactured with a uniform distribution without modulating the amplitude of the grating. As is clear from FIG. 10, the side rope extinction ratio is about 8 dB in the uniform distribution, but by making the coupling coefficient of the grating a sinusoidal distribution, 30 dB required for the wavelength filter used for wavelength division multiplexing optical transmission. The above side rope extinction ratio can be obtained. Further, by adopting the structure in which the grating is formed on the side wall of the waveguide, the step of embedding the grating by the MOCVD method, which has caused the yield reduction and the cost increase, becomes unnecessary.

In the present embodiment, the high-mesa waveguide width of 1.0 .mu.m, 2.0 .mu.m of SOA ridge waveguide width, 600 .mu.m and the length of the SOA, 1500 .mu.m length of the grating, 50 nm the maximum grating depth _{D g} The optical transceiver was fabricated with a period of 240 nm. Each numerical value is an important parameter that determines the characteristics of the present optical transceiver. In realizing the wavelength tunable optical transceiver, it is preferable that the following conditions are satisfied.
1) The height and width of the core layer of the waveguide are preferably such that light propagates through the waveguide in a single mode, and in this embodiment, the height and width are 0.1 μm to 3.0 μm.
2) The length of the SOA affects the magnitude of gain, gain bandwidth, and saturation gain characteristics, and is 20 μm to 2000 μm in this embodiment.
3) The length of the grating determines the light intensity loss during the filter operation, the transmission bandwidth, and the wavelength selection performance during laser oscillation, and is 50 μm to 4000 μm in the present embodiment.
4) the amplitude D _g of the grating determines the coupling coefficient of the grating determines the transmission bandwidth and wavelength selection performance in the laser oscillation during the filter operation. In this embodiment, it is 0 μm to 3.0 μm.
5) The period Λ determines the wavelength band used for the optical transceiver, and in this embodiment, the center wavelength is 1.55 μm and is 240 nm.

  In the present embodiment, the waveguide structure of the grating is manufactured as a high mesa structure. However, in the structure other than the high mesa structure, for example, in the ridge type structure shown in FIG. Also good.

(Sixth embodiment)
FIG. 11 shows the structure of a fifth tunable optical transceiver 1100 according to an embodiment of the present invention. The optical transceiver 1100 in the present embodiment is connected to each other so that the Mach-Zehnder interferometer in which the grating and the refractive index control electrode in the first embodiment are formed face each other. The SOA 1101 is arranged between them. Further, AR coating 1102 is formed on both end faces.

  In the present embodiment, a first sampled grating 1103 and a second sampled grating 1104 having a plurality of reflection peaks are respectively formed on the arms of the Mach-Zehnder interferometer. Further, the reflection peak interval is slightly different between the first sampled grating 1103 and the second sampled grating 1104. Therefore, the output is obtained at the wavelength where the reflection peaks coincide, and the reflection peak interval of the entire element is the least common multiple of the first sampled grating 1103 and the second sampled grating 1104. Furthermore, if the refractive index of the waveguide in which the first sampled grating 1103 and the second sampled grating 1104 are formed is slightly changed by current injection, the matching peak like a vernier scale is greatly shifted, and the wavelength is variable. The area can be increased.

  During the filter operation, the wavelength-multiplexed input light enters the waveguide 1105. The incident input light is first branched equally into both arms by the first MMI coupler 1107. From the equally branched input light, only the selection light is reflected by the first sampled grating 1103 disposed on the arm, and is incident on the first MMI coupler 1107 again. Light other than the selection light (non-selection light) passes through the first sampled grating 1103. Since the sampled gratings 1103 formed on both arms have the same structure, the reflected light (selected light) and the transmitted light (non-selected light) are incident on the second MMI coupler 1108 while maintaining the phase difference. Therefore, the selection light passes through the waveguide 1111 in which the SOA 1101 is disposed, and further enters the Mach-Zehnder interferometer at the subsequent stage. The non-selective light passes through the end surface opposite to the incident end surface on which the AR coat is formed, and is emitted out of the element.

  In the present embodiment, the current in the SOA 1101 is always ON during both the filter operation and the laser operation, and the signal light selected by the preceding Mach-Zehnder interferometer is amplified during the filter operation. Therefore, the signal light further incident on the subsequent Mach-Zehnder interferometer and further selected by the Mach-Zehnder interferometer is output to the waveguide 1106. Switching between the laser operation and the filter operation is performed by injecting current into the refractive index control electrodes 1112, 1113 formed immediately above the arm waveguide located between the MMI coupler and the sampled grating. It is the greatest feature of the optical transceiver in the form. With this configuration, it is possible to amplify the optical signal using the gain medium even during the filter operation.

  Even during the laser operation, the current in the SOA 1101 is turned ON and the SOA 1101 functions as a gain medium, as in the filter operation. In this embodiment, it is possible to inject current into one of the refractive index control electrodes 1114 formed on the arms of the front and rear Mach-Zehnder interferometers to change the light phase by π. By doing so, a feedback action works between the first sampled grating 1103 and the second sampled grating 1104 formed in the former stage and the latter stage Mach-Zehnder interferometer, and both provide a resonator indispensable for laser oscillation. Composed. Therefore, when the SOA gain exceeds the light intensity loss of the element in this state, laser oscillation occurs. The wavelength of the laser oscillation light is determined by the wavelength with the highest reflectance product in both sampled gratings, and the laser oscillation light is output from the waveguide 1105 and the waveguide 1104, respectively.

The manufacturing method of the optical transceiver and the waveguide structure in the present embodiment are the same as those in the fifth embodiment. However, the sampled grating has a structure in which the presence and absence of the grating are periodically repeated in the light propagation direction. An example of the distribution of the grating amplitude D _g of the total length L of the grating in this case is shown in FIG. 12. In the present embodiment, for the purpose of improving the side rope extinction ratio, the amplitude _Dg of the grating is maximized at the central portion of the grating, and the sinusoidal function (variable is 0 to π) so that the amplitude decreases as the distance from the center decreases. Modulate with square.

  FIG. 13 shows the transmission characteristics of the fabricated optical transceiver during filter operation. At this time, when a current of about 100 mA is injected into the SOA and light is input from the input / output waveguide 1, filter selection light is output from the input / output waveguide 2. FIG. 13 also shows transmission characteristics of a single Mach-Zehnder interferometer in which sampled gratings having peak intervals of 3.2 nm and 4.0 nm are formed, respectively. From FIG. 13, it can be seen that the peak interval of the entire element is 16 nm, which is the least common multiple of the peak intervals of the sampled grating 1 and the sampled grating 2. Furthermore, when current is appropriately injected into the waveguides in which the sampled gratings 1 and 2 are formed, all wavelengths in the variable region 16 nm centering on the wavelength 1550 nm can be selected. That is, a wavelength variable amount that greatly exceeds the maximum value of the refractive index change amount by current injection can be obtained.

  During laser operation, if a current of about 100 mA is passed through the SOA, and current is injected into one of the refractive index control electrodes to change the phase of the light by π, the laser oscillation light from the input / output waveguide 1 and the input / output waveguide 2 Is emitted. In this case, the light intensity output from the input / output waveguide 1 and the input / output waveguide 2 is equal. The light intensity output from the waveguide 1 can be monitored. Further, when current is injected into the grating portion of the Mach-Zehnder interferometer at the front stage and the rear stage constituting the resonator, the laser oscillation wavelength changes as the reflection characteristic of the grating is largely shifted as in the filter operation. This indicates that the present optical transceiver also operates as a broadband wavelength tunable laser.

(Seventh embodiment)
14 shows according to an embodiment of the present invention, of the total length L of the grating of the tunable optical transceiver, the distribution of the amplitude D _g of the grating. As in the case of the sixth embodiment, the optical transceiver in this embodiment is connected to the input / output waveguides so that the Mach-Zehnder interferometers on which the grating and the refractive index control electrode are formed face each other. The SOA is arranged between the input / output waveguides. Furthermore, an AR coat is formed on both end faces. However, for the purpose of stabilization of the oscillation wavelength during laser operation, the amplitude D _g of the grating was modulated by the square of the product of Sinc function, the inverse of Sinc function, and the sine function shown by the following equation.

  N represents the number of reflection peaks, and is 5 in this embodiment. L represents the total length of the grating, which is 1500 μm in this embodiment. P is a parameter that determines the reflection peak interval, and is obtained from the following equation.

Here, n _eff represents an effective refractive index and is 3.8 in the present embodiment. λ _B represents the center wavelength, which is 1.55 μm in this embodiment. Δλ represents the reflection peak interval, which is 3.2 nm in FIG.

  FIG. 15 shows the transmission characteristics of the optical transceiver in this embodiment during the filter operation. At this time, when a current of about 100 mA is injected into the SOA and light is input from the input / output waveguide 1, filter selection light is output from the input / output waveguide 2. FIG. 15 also shows the transmission characteristics of a single Mach-Zehnder interferometer having peak intervals of 3.2 nm and 4.0 nm, respectively. In the optical transceiver having the transmission characteristics shown in FIG. 13, a periodic optical output is obtained every 16 nm, which is the least common multiple of the peak interval between the sampled grating 1 and the sampled grating 2. Is limited to only five reflection peaks by the above modulation function, and has a single transmission characteristic as shown in FIG. That is, in the optical transceiver according to the sixth embodiment, a transmission band appears at each periodic wavelength interval. Therefore, if this transmission peak interval becomes narrower than the SOA bandwidth, the oscillation operation becomes unstable due to mode hopping. Become. However, this problem is solved by the optical transceiver obtaining a single transmission characteristic by the modulation function.

Finally, although an InP-based compound semiconductor is used for the optical transceiver in each embodiment, a fiber bragg grating in which a grating is formed in an optical fiber, and an erbium doped fiber amplifier (erbium doped) as a gain medium It can also be realized by combining a fiber amplifier). Further, a GaAs-based waveguide, a Si / SiO ₂ -based silicon fine wire waveguide, or a waveguide made of polyimide or the like can be similarly realized.

  In each embodiment, the refractive index change due to current injection into the semiconductor waveguide is used. However, the wavelength variable operation can be obtained even when the refractive index change is applied by applying voltage, heat, or pressure to the waveguide. Obviously you can.

It is a figure which shows the structure of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the optical waveguide structure of the optical transceiver which concerns on one Embodiment of this invention. It is a characteristic view which shows the transmission spectrum of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the structure of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the structure of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the structure of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the grating structure of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the waveguide structure of SOA of the optical transceiver which concerns on one Embodiment of this invention. A grating distribution of amplitudes D _g of the optical transceiver of the total length L of the grating according to an embodiment of the present invention. FIG. It is a characteristic view which shows the transmission spectrum of the optical transceiver which concerns on one Embodiment of this invention. It is a figure which shows the structure of the optical transceiver which concerns on one Embodiment of this invention. A grating distribution of amplitudes D _g of the optical transceiver of the total length L of the grating according to an embodiment of the present invention. FIG. It is a characteristic view which shows the transmission spectrum of the optical transceiver which concerns on one Embodiment of this invention. A grating distribution of amplitudes D _g of the optical transceiver of the total length L of the grating according to an embodiment of the present invention. FIG. It is a characteristic view which shows the transmission spectrum of the optical transceiver which concerns on one Embodiment of this invention. It is a block diagram of the optical transceiver which has arrange | positioned the wavelength variable laser and the wavelength variable filter in parallel.

Explanation of symbols

100 Wavelength Tunable Optical Transceiver 101, 102 2-input 2-output MMI Coupler 103, 104 Arm Waveguide 105, 106, 107, 108 Waveguide 109, 110 Grating 111 AR Coat 112 HR Coat 113, 114 SOA
115, 116 Refractive index control electrode

Claims (12)

An optical transceiver having a laser operation and a filter operation,
A first two-input two-output optical coupler;
A second 2-input 2-output optical coupler;
Two arm waveguides connected at both ends to each of the first and second two-input two-output optical couplers;
Two waveguides having one end connected to the first two-input two-output optical coupler and the other end connected to an end surface of the optical transceiver on which an AR coating is applied;
Mach-Zehnder interference configured using two waveguides having one end connected to the second two-input two-output optical coupler and the other end connected to the end face of the optical transceiver on which the HR coating is applied. Total
A grating formed on the two arm waveguides;
The gain medium disposed in the two waveguides on the HR coat side of the Mach-Zehnder interferometer,
An optical transceiver comprising means for selecting gain and absorption in the gain medium. An optical transceiver having a laser operation and a filter operation,
One 2-input 2-output optical coupler;
Four waveguides having one end connected to the one two-input two-output optical coupler and the other end connected to an end surface of the optical transceiver on which an AR coating is applied;
A grating formed in two of the four waveguides;
A gain medium disposed in the two waveguides in which the grating is formed;
A photodetector arranged in one of the four waveguides where the grating is not formed;
An optical transceiver comprising means for selecting gain and absorption in the gain medium. An optical transceiver having a laser operation and a filter operation,
One 1-input 2-output optical coupler;
Three waveguides having one end connected to the one-input two-output optical coupler and the other end connected to an end surface of the optical transceiver on which an AR coating is applied;
A grating formed in two of the three waveguides;
A gain medium disposed in the two waveguides in which the grating is formed,
An optical transceiver comprising means for selecting gain and absorption in the gain medium. An optical transceiver having a laser operation and a filter operation,
One 2-input 1-output optical coupler;
Two waveguides having one end connected to the one two-input one-output optical coupler and the other end connected to an end face of the optical transceiver that has been coated with an AR coat;
One waveguide having one end connected to the one two-input one-output optical coupler and the other end connected to an end face of the optical transceiver on which HR coating is applied;
A grating formed in a waveguide on the HR coat side of the one two-input one-output optical coupler;
A gain medium disposed in the one waveguide in which the grating is formed;
A photodetector arranged in one waveguide on the AR coat side of the one two-input one-output optical coupler;
An optical transceiver comprising means for selecting gain and absorption in the gain medium.   5. The optical transceiver according to claim 1, further comprising first refractive index changing means for changing a refractive index of the waveguide, wherein the phase of light passing through the output waveguide is changed by the first refractive index changing means. And changing the output waveguide during laser operation or filter operation.   6. The optical transceiver according to claim 1, wherein two or more Mach-Zehnder interferometers or optical couplers are connected in series through a waveguide.   7. The optical transceiver according to claim 1, wherein the grating is a super-periodic structure grating or a sampled grating.   8. The optical transceiver according to claim 1, further comprising second refractive index changing means for changing a refractive index of the waveguide in which the grating is formed, wherein the second refractive index changing means is used to change the refractive index of the grating. An optical transceiver characterized by changing a reflection characteristic and changing a selection wavelength during laser operation or filter operation.   9. The optical transceiver according to claim 1, wherein the optical transceiver has a structure in which the grating is formed on a side wall of a waveguide.   10. The optical transceiver according to claim 1, wherein the amplitude of the grating is modulated so as to become maximum at a central portion of the grating and to become smaller as the distance from the center decreases.   11. The optical transceiver according to claim 1, wherein one or a plurality of Gaussian functions, trigonometric functions, Hamming window functions, Hanning window functions, Sinc functions, or their reciprocals are used as the function for performing the modulation. An optical transceiver characterized by using a product. 12. The optical transceiver according to claim 1, wherein the refractive index control means changes the refractive index using one or more of a thermo-optic effect, current injection, electric field application, and pressure application. Optical transceiver.

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