JP5059221B2 - Optical limiter circuit and optical receiver circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical limiter circuit and an optical reception circuit capable of being easily integrated with other optical functional circuit and has a wide operating wavelength band. <P>SOLUTION: An optical limiter circuit comprises: an input waveguide 107, a semiconductor waveguide 108 with an enhanced absorption coefficient; and an output waveguide 109, which has an optical fuse function for protecting following optical circuits when input power exceeds several hundred mW. In the semiconductor, since the band gap wavelength shifts to longer wavelength side due to a temperature raise, in a specific wavelength, the absorption coefficient is further increased as the temperature rises. That is, the semiconductor waveguide having an absorption coefficient of certain extent has an optical limiter characteristic by itself. In this circuit, since the temperature rises up to 100&deg;C or more, the waveguide is fused by an excessive input and this circuit serves also as an optical fuse. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to an optical limiter circuit and an optical receiver circuit, and more particularly to an optical limiter circuit and an optical receiver circuit that reduce deterioration of a light receiving element due to an optical surge.

  In an optical network using an optical amplifier, the optical signal power suddenly increases after a state in which the average optical signal power to the optical amplifier is very small for several tens of milliseconds or longer, such as switch switching or instantaneous interruption of the device. An optical surge may occur and damage the receiving circuit. Conventionally, system design is made so that the input power to the optical amplifier does not become zero, or the optical signal level is monitored and the pumping laser light output in the optical amplifier is adjusted, so that a large optical surge does not occur. Has been devised. Also proposed is a method of suppressing an optical surge generated during amplification of an upstream signal by gain clamping with the downstream signal (Non-Patent Document 1).

  However, these methods require a large control system and monitor system in the communication apparatus, and are expensive. Further, the feedback by adjusting the excitation laser beam has a slow time constant, and there are cases where the optical surge cannot be sufficiently suppressed.

  Instead of these, by providing an optical limiter circuit in front of the light receiving element, damage to the receiving circuit due to optical surge can be reduced. As a conventional optical limiter circuit, a nonlinear etalon in which an optical nonlinear material is inserted in an optical resonator, or a nonlinear element in which a saturable absorber is inserted in an asymmetric Fabry-Perot resonator is used. Examples include an optical limiter circuit.

Yasuhiko Nakanishi, Takashi Nakanishi, Yoichi Fukada, Kenichi Suzuki, Katsumi Iwatsuki, "Optical Surge Suppression in Single EDF Bidirectional Access Optical Amplifier", IEICE General Conference Proceedings, 2007, B-8-8

  However, these elements have a surface type configuration, and there is a problem that it is difficult to integrate them into various optical circuits including waveguides. In addition, since it has a resonant structure, there is a problem that it is necessary to individually adjust the wavelength division multiplexed signal having a wide wavelength range for each wavelength used.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical limiter circuit and an optical receiver circuit that can be easily integrated with other optical functional circuits and have a wide wavelength band. It is to provide.

In order to achieve such an object, an invention according to claim 1 is an optical limiter circuit, wherein a first clad layer, a first core layer, and a second clad layer are sequentially laminated. in a single waveguide mode, said part of a single waveguide core, the core and the absorption coefficient of the surrounding Ri is Do different, at least said single waveguide is formed by using a semiconductor material, impurities of the semiconductor material The absorption coefficient is adjusted by changing the concentration .

The invention according to claim 2 is an optical limiter circuit, in a single mode single waveguide comprising a first cladding layer, a core layer, and a second cladding layer covering the periphery of the core layer. part of the core of a single waveguide, the core and the absorption coefficient of the surrounding Ri is Do different, at least said single waveguide is formed by using a semiconductor material, adjust the absorption coefficient by varying the impurity concentration of the semiconductor material It is characterized by that.

According to a third aspect of the present invention, there is provided an optical limiter circuit comprising: a single mode single waveguide in which a first cladding layer, a first core layer, and a second cladding layer are sequentially stacked; Part of the core of one waveguide has an absorption coefficient different from that of the surrounding core. At least the single waveguide is formed using a semiconductor material, and the absorption coefficient is adjusted by changing the amount of crystal defects in the semiconductor material. It is characterized by that .

The invention according to claim 4 is an optical limiter circuit, in a single mode single waveguide comprising a first cladding layer, a core layer, and a second cladding layer covering the periphery of the core layer. Part of the core of the single waveguide has an absorption coefficient different from that of the surrounding core. At least the single waveguide is formed using a semiconductor material, and the absorption coefficient is adjusted by changing the amount of crystal defects in the semiconductor material. It is characterized by that.

  According to the present invention, it is easy to integrate with other optical functional circuits, and an optical limiter function that operates over a wide wavelength range is possible. In addition, an optical fuse function can be provided in addition to the optical limiter function.

It is a figure which shows the structure of the optical limiter circuit which concerns on Embodiment 1 of this invention. It is a figure which shows the optical input / output characteristic of the optical limiter circuit which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the optical limiter circuit which concerns on Embodiment 2 of this invention. It is a figure which shows the optical input / output characteristic of the optical limiter circuit which concerns on Embodiment 2 of this invention. It is a figure which shows the structure of the optical receiver circuit provided with the optical limiter circuit of this invention. It is a figure which shows the optical input / output characteristic of the optical limiter circuit based on one Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted.

[Embodiment 1]
FIG. 1 shows the configuration of an optical limiter circuit according to Embodiment 1 of the present invention. A branching waveguide 102 is connected to the input waveguide 101, a combining waveguide 105 is connected to the output waveguide 106, and the first arm guide is connected between the branching waveguide 102 and the combining waveguide 105. A waveguide 103 and a second arm waveguide 104 are formed. Thus, the present optical circuit constitutes a Mach-Zehnder type optical interference waveguide.

  When the first and second arm waveguides have the same length and the same absorption coefficient, the branching ratio of the branching waveguide 102 is equal to that of the Y-branching waveguide, etc., and the branched lights have the same phase, In the waveguide 105, the branched lights are coupled in phase, and almost 100% of the input light is output from the output waveguide 106.

  Here, these optical circuits are constituted by semiconductor waveguides (for example, silicon Si, indium phosphide InP, gallium arsenide GaAs, etc.), and the first arm waveguide 103 is increased in defects by impurity doping, low temperature growth, ion implantation, or the like. To increase the absorption coefficient in the first arm waveguide 103. That is, the respective absorption coefficients are set so that there is a difference between the absorption coefficients of the first arm waveguide 103 and the second arm waveguide 104. Further, by adjusting the branching ratio of the branching waveguide 102 and the lengths of the first arm waveguide 103 and the second arm waveguide 104, the first arm waveguide 103 and the second arm waveguide when the optical input is weak. The light intensity immediately before coupling 104 to the multiplexing waveguide 105 is made equal and in phase.

When the optical circuit is configured in this way and the optical input power is increased, only the temperature of the first arm waveguide 103 locally increases due to light absorption. A semiconductor generally has a large thermo-optic coefficient and is about 10 ⁻⁴ K ⁻¹ . That is, if the first arm waveguide 103 has a length of about 1 mm, the phase of light shifts by about π with a temperature increase of about 10 ° C. Since this circuit constitutes an interference system, the coupling efficiency to the output waveguide 106 decreases unless the phases of the light from the first arm waveguide 103 and the second arm waveguide 104 are in phase immediately before the multiplexing waveguide 105. To do. Therefore, as shown in FIG. 2, in this circuit, the output power of the transmitted output light increases in proportion to the incident power in the region where the incident power is low, but when the incident power exceeds a predetermined intensity, Limiter type light input / output characteristics.

  Here, the manufacturing method of Embodiment 1 will be described with reference to examples. Each waveguide has a laminated structure of a lower clad layer, a core layer, and an upper clad layer on an InP substrate, which are laminated by the MOVPE method. The lower cladding layer is InP (layer thickness: 500 nm), the core layer is InGaAsP (composition wavelength is 1.4 μm, layer thickness: 300 nm), and the upper cladding layer is InP (layer thickness: 500 nm).

Next, SiO ₂ (layer thickness: 300 nm) is formed on the upper cladding layer by plasma CVD. By a photoresist process and CF-based RIE, SiO ₂ is processed into a mask having a region including a region corresponding to the first arm waveguide 103 in the Mach-Zehnder optical interference waveguide as an opening. That is, SiO ₂ is processed so as to mask regions corresponding to the input waveguide 101, the branch waveguide 102, the second arm waveguide 104, the multiplexing waveguide 105, and the output waveguide 106.

  Next, p-type impurities such as Zn, Be, Cd, and Mg are ion-implanted from the opening of the mask. As a result, the p-type impurity is ion-implanted only in the mask opening region including the region corresponding to the first arm waveguide 103. Thereafter, the p-type impurity is activated by normal lamp annealing.

Here, the SiO ₂ mask is once removed by hydrofluoric acid treatment, and then SiO ₂ (layer thickness: 300 nm) is again formed on the surface of the upper cladding layer by plasma CVD. This newly formed SiO ₂ film is processed into a Mach-Zehnder optical interference waveguide shape by CF-based RIE (Reactive Ion Etching) using a resist mask formed by a photoresist process.

Using the SiO ₂ processed into the Mach-Zehnder type optical interference waveguide shape as a mask, the InP / InGaAsP / InP layer is processed into a Mach-Zehnder type optical interference waveguide by chlorine-based plasma etching (RIE, RIBE: Reactive Ion Beam Etching). The waveguide widths are all 3 μm, and the arm lengths of the first arm waveguide 103 and the second arm waveguide 104 are 1 mm.

  In the region corresponding to the first arm waveguide 103, in addition to ion implantation of p-type impurities, defects may be introduced by ion implantation of impurities other than p-type such as Si and Fe, proton irradiation, electron beam irradiation, and the like. Good. Alternatively, p-type impurity ion implantation may be substituted by forming a p-type InP / InGaAsP / InP layer in a region corresponding to the first arm waveguide 103 by selective growth.

InP was used for the substrate and InP / InGaAsP / InP was used for the waveguide, but GaAs, Si, etc. could be used for the substrate, and compounds such as InP / InGaAs / InP and AlGaAs / GaAs / AlGaAs could be used for the waveguide. The same effect can be obtained by using a semiconductor, Si / SiGe / Si, SiO ₂ / Si / SiO ₂ or the like.

  When the calculation was performed with the wavelength of the incident light being 1.55 μm, the input / output characteristics as shown in FIG. 6 were obtained. Even if the wavelength is not 1.55 μm, it can be applied to other long wavelength bands such as 1.3 μm by adjusting the composition wavelength of InGaAsP. Moreover, it can apply also to wavelengths other than a long wavelength band by using the material corresponding to another wavelength for the optical nonlinear layer of the 1st arm waveguide 103. FIG.

[Embodiment 2]
FIG. 3 shows a configuration of a limiter circuit according to the second embodiment of the present invention. FIG. 4 shows the light input / output characteristics. Here, 107 is an input waveguide, 108 is a semiconductor waveguide having a high absorption coefficient, and 109 is an output waveguide.

  This circuit is an optical limiter having an optical fuse function for protecting the optical circuit in the subsequent stage when the input power is larger than the optical power (several tens of mW) assumed in the first embodiment and becomes several hundred mW or more. is there. Since the band gap wavelength of a semiconductor shifts to a long wavelength with a rise in temperature, the absorption coefficient further increases with a rise in temperature at a specific wavelength. That is, a semiconductor waveguide having a certain absorption coefficient has optical limiter characteristics by itself. In this circuit, since the temperature rise is 100 ° C. or more, the waveguide is melted by an excessive input and also functions as an optical fuse. That is, as shown in FIG. 4, hysteresis characteristics are exhibited.

  As described above, in this circuit, in the region where the incident power is low, the output power of the transmitted output light increases in proportion to the incident power, but when the incident power exceeds a predetermined intensity, the output power is relatively It has a limiter-type optical input / output characteristic that is suppressed, and also has an optical fuse function that blocks the light by blocking the waveguide when the incident power increases.

  Here, a manufacturing method of the second embodiment will be described with reference to examples. Each waveguide has a laminated structure of a clad layer, a core layer, and a clad layer on an InP substrate laminated by the MOVPE method. The lower cladding layer is InP (layer thickness: 500 nm), the core layer is InGaAsP (composition wavelength is 1.4 μm, layer thickness: 300 nm), and the upper cladding layer is InP (layer thickness: 500 nm). Here, the core layer is simply sandwiched between the lower and upper clad layers by the lower and upper clad layers, and there is no clad layer on the side surfaces, but all the side surfaces other than the input / output surface of the core are covered by the clad. A simple waveguide may be used.

Next, SiO ₂ (layer thickness: 300 nm) is formed on the upper cladding layer by plasma CVD. By a photoresist process and CF-based RIE, SiO ₂ is processed into a mask having a region including a region corresponding to the semiconductor waveguide 108 as an opening. That is, the SiO ₂ is processed so as to mask regions corresponding to the input waveguide 107 and the output waveguide 109.

  Next, p-type impurities such as Zn, Be, Cd, and Mg are ion-implanted from the opening of the mask. As a result, the p-type impurity is ion-implanted only into the mask opening region including the region corresponding to the semiconductor waveguide 108. Thereafter, the p-type impurity is activated by normal lamp annealing.

Here, the SiO ₂ mask is once removed by hydrofluoric acid treatment, and then SiO ₂ (layer thickness: 300 nm) is again formed on the surface of the upper cladding layer by plasma CVD. This newly formed SiO ₂ film is processed into a single waveguide shape by CF RIE using a resist mask formed by a photoresist process.

The InP / InGaAsP / InP layer is processed into a single waveguide by chlorine-based plasma etching using the SiO ₂ stripe processed into a single waveguide shape as a mask. The waveguide widths are all 3 μm, and the length of the semiconductor waveguide 108 having a high absorption coefficient is 1 mm.

  In order to enhance the effect as an optical fuse, it is effective to increase the p-type doping concentration or to narrow the waveguide width (less than 1 μm).

  As described above, the optical limiter circuit according to the present invention can operate over a wide wavelength range and can be configured with an optical waveguide such as a PLC, so that it can be easily integrated with other optical functional circuits.

[Embodiment 3]
FIG. 5 shows a configuration of an optical receiver circuit according to Embodiment 3 of the present invention. An input spot conversion circuit 110, an optical limiter circuit 112, and an output spot conversion circuit 114 are connected in cascade through single mode waveguides 111 and 113. The optical limiter circuit 112 is the optical limiter circuit according to any one of the first and second embodiments of the present invention. A photodiode 115 is coupled to the subsequent stage of the output spot conversion circuit 114. The third embodiment is a front end of an optical receiver, which enhances the resistance of the optical receiver to an optical surge.

  Here, a manufacturing method of the third embodiment will be described with reference to examples.

  The optical limiter according to any one of Embodiments 1 and 2 is mounted on a planar lightwave circuit (PLC) formed of quartz waveguides 111 and 113 and spot conversion circuits 110 and 114 formed on a Si substrate by a normal mounting method. A circuit 112 and a 1.55 μm photodiode (avalanche photodiode: APD) 115 are mounted. The photodiode 115 may be a pin photodiode.

  As described above, since the optical limiter circuit of the present invention can be formed in the PLC chip together with other optical functional circuits, a highly integrated light receiving circuit having an optical limiter function can be easily manufactured.

DESCRIPTION OF SYMBOLS 101 Input waveguide 102 Branching waveguide 103 1st arm waveguide 104 2nd arm waveguide 105 Combined waveguide 106 Output waveguide 107 Input semiconductor waveguide 108 Semiconductor waveguide 109 with a high absorption coefficient Output waveguide 110 Input spot conversion Circuits 111 and 113 Single mode waveguide 112 Optical limiter circuit 114 Output spot conversion circuit 115 Photodiode

Claims (4)

In a single-mode single waveguide in which a first cladding layer, a first core layer, and a second cladding layer are sequentially stacked, a part of the core of the single waveguide has an absorption coefficient that is equal to the surrounding core. but depends, at least the single waveguide is formed by using a semiconductor material, optical limiter circuit, characterized in that the adjusted absorption coefficient by varying the impurity concentration of the semiconductor material. In a single-mode single waveguide comprising a first cladding layer, a core layer, and a second cladding layer covering the periphery of the core layer, a part of the core of the single waveguide is absorbed by the surrounding core coefficient Ri is Do different, at least said single waveguide is formed by using a semiconductor material, optical limiter circuit, characterized in that the adjusted absorption coefficient by varying the impurity concentration of the semiconductor material. In a single-mode single waveguide in which a first cladding layer, a first core layer, and a second cladding layer are sequentially stacked, a part of the core of the single waveguide has an absorption coefficient that is equal to the surrounding core. The optical limiter circuit is characterized in that at least the single waveguide is formed using a semiconductor material, and an absorption coefficient is adjusted by changing a crystal defect amount of the semiconductor material . In a single-mode single waveguide comprising a first cladding layer, a core layer, and a second cladding layer covering the periphery of the core layer, a part of the core of the single waveguide is absorbed by the surrounding core An optical limiter circuit having different coefficients, wherein at least the single waveguide is formed using a semiconductor material, and an absorption coefficient is adjusted by changing a crystal defect amount of the semiconductor material.

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