JP2014149323A - Semiconductor optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor optical modulator which suppresses polarization dependency, and includes a function of an optical amplification function enabling optical modulation at high speed.SOLUTION: A semiconductor optical modulator comprises: a light input/output section which inputs incident light and outputs emitted light; a polarization separation section which separates the incident light input from the light input/output section into first polarization mode light of first polarized light and second polarization mode light of second polarized light that have mutually different polarization modes; a polarization conversion section which converts the polarization mode of the first polarization mode light into the second polarized light, and outputs the converted light as first input light, and outputs the second polarization mode light as second input light; a semiconductor optical modulation amplification section which applies a modulation signal, and modulates and amplifies the first input light and the second input light; and a modulation signal application section which applies the modulation signal to the semiconductor optical modulation amplification section. In the semiconductor optical modulation amplification section, an amplification gain of the second polarized light is larger than an amplification gain of the first polarized light.

Description

  The present invention relates to a semiconductor optical modulator having an optical amplification function.

  In recent years, in a short-distance access network such as FTTH (Fiber To The Home) that does not use a repeater such as an optical amplifier, a wavelength division multiplexing passive optical network (WDM-PON) is used. The method is drawing attention. In this system, seed light transmitted from a base station, that is, an optical line terminal (OLT), is optically modulated on the terminal side, that is, an optical network unit (ONU) side, and then transmitted to the base station side. An optical modulator that can be sent back is required. Further, this optical modulator has an optical amplifier function for amplifying seed light attenuated due to loss in the optical fiber and each optical component, that is, a function of a semiconductor optical amplifier (SOA). desirable.

  Here, as for the SOA, an SOA with a high saturation output and a low noise figure has been reported (for example, Non-Patent Document 1). The amplification gain of the SOA generally varies depending on the polarization direction of incident light, and the amplification gain for the TE polarization mode parallel to the semiconductor lamination surface of the SOA is larger than the amplification gain for the TM polarization mode perpendicular to the SOA lamination direction of the SOA. large. This polarization dependent gain (PDG) causes a communication error if the intensity of the signal light output from the SOA is not constant. Therefore, a polarization independence technique that suppresses variations in output signal light intensity due to PDG has been proposed (for example, Patent Documents 1 and 2 and Non-Patent Document 2).

  In addition, as a method of making SOA independent of polarization in the prior art, there are a method of applying an extension strain to a bulk semiconductor active layer, a method of applying an extension strain to a quantum well having a multiple quantum well (MQW) structure, and the like. . This is because the TE polarization mode and the TM polarization mode orthogonal to the TE polarization mode are optimized by optimizing the active layer structure and the waveguide structure (in the case of MQW structure, well / barrier thickness and strain amount, waveguide width). Are designed to have the same amplification gain (for example, Non-Patent Document 2).

JP-A-8-64904 JP 2008-250021 A

K. Morito and S.M. Tanaka, “Record High Saturation Power (+22 dBm) and Low Noise Figure (5.7 dB) Polarization-Insensible SOA Module, IEEE HOTONICS TECHNOLOGY 98, ETH M.M. Itoh, Y. et al. Shibata, T .; Kakitsuka, Y. et al. Kadota, and Y.K. Tohmori, “Polarization-Insensitive SOA With a Strained Bulk”, IEEE PHOTOTONICS TECHNOLOGY LETTERS, 14, 6, 765-767, (2002)

  However, the method disclosed in Non-Patent Document 1 has an upper limit on the sum of the amplification gains of the TE and TM polarization modes. Therefore, when the amplification gain of the TE and TM polarization modes is designed to be equal, the gain of the TM polarization mode is As TE increases, the gain of the TE polarization mode decreases. Then, when the differential gain coefficient with respect to the TE polarization mode is lowered and further the differential gain coefficient is lowered, the relaxation oscillation frequency is lowered. Therefore, when a semiconductor optical modulator having an optical amplification function is configured using such an SOA having a low relaxation oscillation frequency, a new problem arises that high-speed optical modulation cannot be performed.

  The present invention has been made in view of the above, and an object of the present invention is to provide a semiconductor optical modulator having an optical amplification function in which polarization dependency is suppressed and optical modulation can be performed at high speed.

  In order to solve the above-described problems and achieve the object, a semiconductor optical modulator according to the present invention includes a light input / output unit that inputs incident light and outputs emitted light, and is input from the light input / output unit. A polarization separation unit that separates the incident light into a first polarization mode light of a first polarization and a second polarization mode light of a second polarization having different polarization modes; and a second polarization mode of the first polarization mode light. A polarized light conversion unit that converts the light into polarized light and outputs the light as first input light, and outputs the second polarization mode light as second input light. A modulation signal is applied to the first input light and the second input. A semiconductor light modulation amplification unit that modulates and amplifies light; and a modulation signal application unit that applies the modulation signal to the semiconductor light modulation amplification unit, wherein the semiconductor light modulation amplification unit The amplification gain is larger than the amplification gain of the first polarization. To.

  The semiconductor optical modulator according to the present invention is the waveguide according to the above invention, wherein the semiconductor optical modulation amplification unit has a first input / output unit at one end and a second input / output unit at the other end. The first input light is input from the first input / output unit, modulated and amplified while passing through the waveguide, and output from the second input / output unit, and the second input light is It is inputted from the second input / output unit, modulated and amplified while passing through the waveguide, and outputted from the first input / output unit.

  Further, in the semiconductor optical modulator according to the present invention, in the above invention, the semiconductor optical modulation amplification unit has an input / output direction between the first input / output unit and the second input / output unit being substantially parallel. It is characterized by.

  In the semiconductor optical modulator according to the present invention, the first input / output unit and the second input / output unit output from the polarization conversion unit are combined in the semiconductor optical modulation amplification unit in the above invention. The semiconductor light modulation amplification unit includes a waveguide structure having a light input / output unit at one end and a light reflection unit at the other end, and the semiconductor light modulation amplification unit includes: The signal light input from the optical multiplexing unit to the optical input / output unit is modulated and amplified while passing through the waveguide structure, reflected by the light reflecting unit, and output from the optical input / output unit. Features.

  In the semiconductor optical modulator according to the present invention as set forth in the invention described above, the modulation signal applying unit applies a modulation signal of 5 Gb / s or more.

  In the semiconductor optical modulator according to the present invention as set forth in the invention described above, the modulation signal applying unit applies a modulation signal of 10 Gb / s or more.

  In the semiconductor optical modulator according to the present invention as set forth in the invention described above, the modulation signal applying unit applies an RZ modulation signal.

  In the semiconductor optical modulator according to the present invention as set forth in the invention described above, the modulation signal applying unit applies an NRZ modulation signal.

  In the semiconductor optical modulator according to the present invention, in the above invention, the semiconductor optical modulation amplifying unit has an absolute value of a change in gain per unit intensity of input light of 0.16 dB / dBm or less. Features.

  An optical network unit according to the present invention comprises the semiconductor optical modulator according to the present invention.

  A wavelength division multiplexing passive optical network according to the present invention includes the optical network unit of the present invention.

  According to the present invention, it is possible to realize a semiconductor optical modulator having an optical amplification function capable of suppressing polarization dependency and optically modulating light at high speed.

FIG. 1 is a diagram showing a specific configuration of the semiconductor optical modulator according to the first embodiment of the present invention. FIG. 2 is a diagram illustrating an example of a current profile of the modulation current applied by the modulation signal applying unit. FIG. 3 is a plan view of the semiconductor optical modulation amplification unit in the semiconductor optical modulator according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view taken along line AA of BH-SOA of the semiconductor optical modulation amplification unit in the semiconductor optical modulator according to the first embodiment of the present invention. FIG. 5 is a schematic diagram showing an optical communication system using the semiconductor optical modulator according to the present invention. FIG. 6 is a diagram showing the relationship between the signal light reception intensity and the error rate when the semiconductor optical modulator according to the first embodiment is applied to the optical communication system shown in FIG. FIG. 7 is a plan view of a semiconductor light modulation amplification unit in a semiconductor light modulator according to a modification of the present invention. FIG. 8 is a diagram showing a specific configuration of the semiconductor optical modulator according to the second embodiment of the present invention.

  Embodiments of a semiconductor optical modulator according to the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the embodiments. In the description of the drawings, the same or corresponding elements are appropriately denoted by the same reference numerals. Further, the drawings are schematic, and it should be noted that the relationship between the thickness and width of each layer, the ratio of each layer, and the like may differ from the actual situation. Even between the drawings, there are cases in which portions having different dimensional relationships and ratios are included.

(Embodiment 1)
First, the semiconductor optical modulator according to the first embodiment of the present invention will be described. FIG. 1 is a diagram showing a specific configuration of the semiconductor optical modulator according to the first embodiment of the present invention. As shown in FIG. 1, a semiconductor optical modulator 100 includes, for example, an optical input / output unit 10 that inputs and outputs incident light that is incident on an optical fiber by adjusting an optical axis, and a planar lightwave circuit (PLC: Planner Lightwave Circuit). A polarization separation unit 30 and a polarization conversion unit 40 integrally formed as the element 20, a semiconductor optical modulation amplification unit 50 having a buried heterostructure (BH) SOA 55 (hereinafter referred to as BH-SOA 55), and a modulation signal And an application unit 60.

  The optical input / output unit 10 is connected to an input optical waveguide 31 of a polarization separation unit 30 configured as a PLC element 20. Thus, the light input / output unit 10 is configured to adjust the optical axis of incident light incident on the optical fiber and minimize the coupling loss between the optical fiber and the input optical waveguide 31.

  The PLC element 20 includes a polarization separation unit 30 and a polarization conversion unit 40. The PLC element 20 includes various optical waveguides configured by a core portion made of silica glass having a refractive index higher than that of the clad layer in a clad layer made of silica glass laminated on a silicon substrate. It is formed. Specifically, the PLC element 20 includes an input optical waveguide 31 constituting the polarization separation unit 30, a 3 dB coupler 32 including an optical waveguide type directional coupler, two arm optical waveguides 33 and 34, and an optical waveguide type directionality. As a Mach-Zehnder Interferometer (MZI) type polarization separation / combination element, a 3 dB coupler 35 composed of a coupler and output optical waveguides 41 and 42 constituting the polarization conversion unit 40 are sequentially connected. It is configured. A half-wave plate 43 is inserted into the output optical waveguide 41 of the polarization converter 40. Heaters 36 and 37 are formed on the surface of the cladding layer above the arm optical waveguides 33 and 34, respectively. The heaters 36 and 37 are for heating the arm optical waveguides 33 and 34 locally to change the effective refractive index, thereby adjusting the characteristics of the PLC element 20 as the polarization separation / combination element after the waveguide structure is formed. Is. The heaters 36 and 37 are formed of a metal thin film, for example.

  The semiconductor optical modulation amplifying unit 50 is connected to the InP semiconductor multilayer structure stacked on the InP substrate from the first input / output unit 51 and the second input / output unit 52 via the R units 53 and 54 having a bending radius. The optical waveguide is connected to BH-SOA 55 which is a buried type optical amplification waveguide. The R portions 53 and 54 are preferably set to have a bending radius of 125 μm or more in order to avoid the influence of bending loss.

  The BH-SOA 55 includes an active layer made of, for example, a GaInNAs semiconductor material, a GaInAsP semiconductor material, or an AlGaInAs semiconductor material, and the composition thereof is set so that light having a wavelength of 1.55 μm band can be amplified. ing. An n-side electrode and a p-side electrode are formed on the back surface of the InP substrate and the surface of the InP semiconductor multilayer structure, respectively. In FIG. 1, only the p-side electrode 56b of the electrodes is shown. A specific configuration of the BH-SOA 55 will be described later. Moreover, said semiconductor material is an illustration and the semiconductor material used can be suitably selected according to the wavelength of the light used.

  Bonding surfaces of the first input / output unit 51 and the second input / output unit 52 of the semiconductor optical modulation amplification unit 50 and the output optical waveguides 41 and 42 of the PLC element 20 are bent waveguides. This prevents the light reflected by the joint surface from being coupled again to each waveguide. Further, an antireflection film is formed on the bonding surface between the first input / output unit 51 and the second input / output unit 52 of the semiconductor optical modulation amplification unit 50 and the output optical waveguides 41 and 42 of the PLC element 20. Is desirable.

  The modulation signal applying unit 60 includes, for example, a high-frequency circuit 62 that generates a modulation current modulated with a high-frequency signal of 5 Gb / s or more, preferably 10 Gb / s or more, and a semiconductor optical modulation amplification unit that generates the modulation current generated by the high-frequency circuit 62. The modulation signal input unit 61 is applied to 50 electrodes.

  Next, an example of the operation of the semiconductor optical modulator 100 will be described. First, incident light L <b> 11 input from an optical fiber is incident on the input optical waveguide 31 by the light input / output unit 10. Incident light L11 incident on the light input / output unit 10 is 1.55 μm band signal light having an arbitrary polarization state. When the incident light L11 is input from the input optical waveguide 31, the polarization separation unit 30 receives the incident light L11 as the first polarization in the TM polarization mode (polarization perpendicular to the substrate surface of the PLC element 20). The polarized light is split into the mode light L31 and the second polarization mode light L32 in the TE polarization mode (polarization parallel to the substrate surface of the PLC element 20), which is the second polarization, and output to the output optical waveguides 41 and 42, respectively.

  When the first polarization mode light L31 is input, the half-wave plate 43 of the polarization conversion unit 40 rotates the polarization direction by 90 degrees and outputs the first polarization light as a TE polarization mode first light L41. Input to the first input / output unit 51 of the amplification unit 50. On the other hand, the second polarization mode light L32 is input to the second input / output unit 52 as the second input light L42 in the TE polarization mode.

Here, in the semiconductor optical modulation amplification unit 50, the modulation current generated by the modulation signal application unit 60 is applied between the n-side electrode and the p-side electrode. The modulation current corresponds to an NRZ (Non Return to Zero) method or an RZ (Return to Zero) method in OOK (On-Off Keying) which is a kind of intensity modulation. As an example of the modulation current profile, FIG. 2A shows an NRZ modulation current profile, and FIG. 2B shows an RZ modulation current profile. _{Tbit in} FIG. 2 represents a time width per _bit in the applied current.

  When a current is supplied by the modulation current and the BH-SOA 55 including the active layer is in a state of having an optical amplification function, the light is amplified when incident. On the other hand, when no current is supplied due to the modulation current, when light is incident, the light is absorbed by the active layer of the BH-SOA 55.

  Therefore, the BH-SOA 55 guides the first input light L41 and the second input light L42 input by the applied modulation current while performing optical modulation and amplification, and is opposite to one end of the input side. Output from one end. At this time, since both the first input light L41 and the second input light L42 are guided and modulated and amplified through the BH-SOA 55 in the TE polarization mode, the first input light L41 and the second input light L42 are affected by the PDG of the semiconductor optical modulation amplification unit 50. Without light modulation and amplification. Note that in order to modulate the first input light L41 and the second input light L42 at the same timing, the optical path lengths of the first input light L41 and the second input light L42 are equal, and the BH-SOA 55 is located at the center of the U-turn portion. It is desirable that the first input light L41 and the second input light L42 are disposed and guided to the BH-SOA 55 after being guided through the same optical path length.

  Thereafter, the optically amplified first input light L41 is input to the output optical waveguide 42 in the TE polarization mode. On the other hand, the optically amplified second input light L42 is input to the output optical waveguide 41 in the TE polarization mode, and is set to the TM polarization mode by the half-wave plate 43. Thereafter, the polarization separation unit 30 outputs a combination of the polarizations of the first input light L41 and the second input light L42 from the input optical waveguide 31 to the light input / output unit 10 as amplified outgoing light L12.

  Here, the modulation characteristics of the SOA will be described. In general, the modulation band of the semiconductor laser and the SOA is limited by the relaxation oscillation frequency or the parasitic capacitance and differential resistance of the element. In particular, in the case of SOA, in the modulation band up to 10 GHz, the modulation band is limited by the former relaxation oscillation frequency.

また、（１）式におけるＲ^’ _ｓｐは、Ｒ^’ _ｓｐ＝Γν_ｇｇｎ_ｓｐ／Ｖで与えられる。このとき、ｇは材料利得、ｎ_ｓｐは反転分布係数、Ｖは活性層の体積を示している。 First, the relaxation oscillation angular frequency is ω _r , the group velocity is ν _g , the differential gain coefficient for carriers is a, the differential gain coefficient for photons is a _p , the confinement coefficient is Γ, the photon density is N _p , and the differential carrier lifetime is τ _ΔN. When the photon lifetime is τ _p , the relaxation oscillation angular frequency ω _r satisfies the relationship of the following equation (1).

In addition, R in equation (1) _^'sp is, ^R' is given by _{sp = Γν g gn sp / V} . At this time, g represents a material gain, _nsp represents an inversion distribution coefficient, and V represents a volume of the active layer.

Then, when the relaxation oscillation frequency put the _{f r,} the relationship between ω _r is a ω _r = 2πf _r. Further, in the above formula (1), in the modulation band up to 10 GHz, the first term becomes dominant, and therefore a is an important parameter for determining _fr . Here, a which is a differential gain coefficient is expressed as a = dg / dN by the material gain g and the carrier density N.

In the prior art, when the amplification gain of the TE and TM polarization modes is designed to be equal in order to suppress the influence of PDG, the g of the TE polarization mode is reduced by about ½. That is, the _{computation f r} from equation becomes 1 / √2.

Thus, for example, with respect to SOA _{f r} it was 10 GHz, using conventional techniques, if the amplification gain of the TE and TM polarization modes is the equal design, _{f r} is 7.1GHz, and the modulation bandwidth limitations Will be. In this way, if the polarization gain independence is designed so that the amplification gain of the TE and TM polarization modes becomes equal due to stretching distortion or the like in the prior art, PDG can be suppressed, but high-speed modulation cannot be performed.

  On the other hand, the semiconductor optical modulator 100 according to the first embodiment is not designed to equalize the amplification gains of the TE and TM polarization modes by applying an extensional distortion as in the prior art. As a result, a method of making the polarization independence without degrading the differential gain coefficient of the semiconductor active layer due to the independence of the polarization, that is, reducing the relaxation oscillation frequency has been proposed.

In the semiconductor optical modulator 100 according to the first embodiment, the polarization separation unit 30 and the polarization conversion unit 40 constitute a polarization diversity circuit, and the polarization mode of the incident light L11 is changed by the amplification gain of the semiconductor light modulation amplification unit. PDG is suppressed by aligning with the maximum TE polarization mode. At this time, since the tensile strain is not applied, f _r remains of 10GHz, which enables high-speed modulation. The PDG in this polarization diversity circuit is considered to be caused by the linearity of the SOA gain, and it is desirable to use an SOA chip with good linearity.

In the first embodiment, when the time width T _bit per bit in the applied current shown in FIG. 2 is 200 ps or less, a high speed of 5 Gb / s or more is achieved by the polarization independent SOA using the polarization diversity circuit. Modulation can be realized. Further, when T _{bit is set} to 100 ps or less, high-speed modulation of 10 Gb / s or more can be realized.

  As described above, the semiconductor optical modulator 100 according to the first embodiment is a semiconductor optical modulator having an optical amplification function in which polarization dependency is suppressed and light modulation can be performed at high speed.

  Next, a specific configuration of the semiconductor light modulation amplification unit 50 will be described. FIG. 3 is a plan view of the semiconductor optical modulation amplification unit in the semiconductor optical modulator according to the first embodiment of the present invention. The first input / output unit 51 and the second input / output unit 52 include buried heterostructure spot size converter (BH-SSC) units 51a and 52a, high-mesa passive optical waveguide units 51b and 52b, and Consists of. The semiconductor optical modulator 100 according to the first embodiment desirably modulates the first input light L41 and the second input light L42 at the same timing. For this reason, the first input / output unit 51 and the second input / output unit 52 are formed to have equal optical path lengths. As a result, the first input light L41 and the second input light L42 are guided through the same optical path length and then enter the BH-SOA 55.

  FIG. 4 is a cross-sectional view taken along line AA of BH-SOA of the semiconductor optical modulation amplification unit in the semiconductor optical modulator according to the first embodiment of the present invention. As shown in FIG. 4, the BH-SOA 55 has a BH structure with a trench. First, a substrate 55b having an n-side electrode 56a formed on the back surface is formed on the silicon base 55a. A lower clad layer 55c made of, for example, n-InP is formed thereon, and further, a lower three-stage SCH layer 55d, an active layer 55e, and an upper three-stage SCH layer 55f, for example, an upper clad layer 55g made of p-InP are formed. Has been. A p-side electrode 56b is formed on the top. Further, in the BH-SOA 55, in order to protect the semiconductor layer, a passivation film 55j made of, for example, SiN is formed in a region where current is not injected.

  Further, the BH-SOA 55 has a trench structure in order to reduce parasitic capacitance. The semiconductor layer is removed by etching or the like as shown in the trench portion T of FIG. Further, one side of the trench portion T is filled with polyimide 55k in order to form the p-side electrode 56b and further improve the mechanical strength.

Further, the BH-SOA 55 has a buried mesa structure in order to efficiently interact the input signal light and the injected current. First, if not buried mesa structure, the current injected by the n-side electrode 56a and the p-side electrode 56b flows in width W _t of FIG. Here, a buried mesa structure can be formed by scraping the semiconductor layer deeper than the active layer 55e by etching or the like to form the buried layer 55h and the current blocking layer 55i. At this time, the buried layer 55h is p-InP opposite to the lower clad layer 55c, and the current blocking layer 55i is n-InP opposite to the upper clad layer 55g, so that the n-side electrode 56a and the p-side electrode 56b are formed. When a positive current is applied, carriers are accumulated in the active layer 55e, while carriers are depleted at the interface between the buried layer 55h and the current blocking layer 55i, and the buried layer 55h and the current blocking layer 55i serve as insulating layers. Function. Therefore, in the case of the buried mesa structure, the current injected by the n-side electrode 56a and the p-side electrode 56b flows while being _confined to Wact in FIG. 4, so that the incident light and the injected current interact efficiently. Can do.

  Next, the active layer 55e is made of, for example, GaInAsP having a different composition, and eight layers of well layers having a width of 6 nm and barrier layers having a width of 10 nm are alternately stacked. The active layer 55e is made of, for example, GaInAsP having a different composition, and is composed of a lower three-stage SCH layer 55d and an upper three-stage het-structure heterostructure (SCH: Separated Confinement Heterostructure) composed of three layers of 6/12/60 μm. A step SCH layer 55f is provided. At this time, the light confinement factor is 9.1%.

  Next, the operation of the semiconductor optical modulation amplification unit 50 will be described. First, the first input light L41 and the second input light L42 are incident on the first input / output unit 51 and the second input / output unit 52 from the output optical waveguides 41 and 42 of the polarization conversion unit 40, respectively. Then, the spot sizes of the first input light L41 and the second input light L42 are converted by increasing the core width of the waveguides of the BH-SSC units 51a and 52a from 0.5 μm to 3 μm, for example. Are input to the high mesa passive optical waveguide portions 51b and 52b and are guided through the high mesa passive optical waveguide portions 51b and 52b. In the high mesa passive optical waveguide portions 51b and 52b, for example, the core width of the waveguide is set to about 2 μm. The R portions 53 and 54 are desirably set to a bending radius of 125 μm or more in order to suppress bending loss. The end portions of the high mesa passive optical waveguide portions 51b and 52b have a core width of 3 μm, for example, in order to suppress coupling loss, and allow the first input light L41 and the second input light L42 to enter the BH-SOA 55. Then, the first input light L41 and the second input light L42 incident on the BH-SOA 55 are guided while being optically modulated and amplified, and are output from one end opposite to the input end.

Here, signal light having a wavelength of 1.55 μm is incident on the semiconductor light modulation amplification unit 50. The bias current of the modulation current is I _dc = 100 mA, and the maximum value of the modulation current is I _max = 200 mA. At this time, assuming that the intensity of the output signal light is P _out = 10 dBm (in I _max ) and the intensity of the input signal light is P _in = −15 dBm, the average carrier density is 2.2 × 10 ¹⁸ cm ⁻³ . Become. Therefore, when the relaxation vibration frequency is calculated by the equation (1), the relaxation vibration frequency is 9.85 GHz.

  On the other hand, in the SOA according to the related art, when the polarization gain is made independent as a design in which the amplification gains of the TE and TM polarization modes are equal due to stretching distortion or the like, the relaxation oscillation frequency becomes 1 / √2. Therefore, the relaxation oscillation frequency of the SOA according to the prior art is 6.97 GHz, and high-speed modulation cannot be performed.

  As described above, the semiconductor optical modulator according to the first embodiment can realize a semiconductor optical modulator having an optical amplification function in which polarization dependency is suppressed and light modulation can be performed at high speed.

  Here, FIG. 5 is a schematic diagram showing an optical communication system using the semiconductor optical modulator according to the present invention. As shown in FIG. 5, the semiconductor optical modulator according to the present invention can be used as an ONU transmitter (Tx) in WDM-PON.

  The WDM-PON 1000 includes an OLT 1001 that is an operator system, a transmission fiber 1010, an arrayed waveguide grating (AWG) wavelength multiplexer 1020, and ONUs 1030, 1040, 1050, and 1060 that are subscriber systems. Consists of. First, signal light having different wavelengths is oscillated at Tx / Rx 1002, 1003, 1004, and 1005 of the OLT 1001. The oscillated signal light is multiplexed by the AWG 1007. Further, the combined signal light is combined in the coupler 1008 with the ASE light output from the broadband light source 1006 using ASE (Amplified Spantanous Emission) light.

  Next, the combined signal light and ASE light are transmitted from the operator system to the subscriber system by the transmission fiber 1010. Then, in AWG 1020, the signals are demultiplexed into ONUs 1030, 1040, 1050, and 1060 that are subscriber systems. Here, the ASE light is filtered by the AWG 1020 to be ASE light having a different wavelength band for each subscriber system. In each ONU, signal light is transmitted and received by Tx / Rx 1031, 1041, and 1051. Note that Tx / Rx specifically includes WDM filters 1061a, Rx1061b, and Tx1061c, as in Tx / Rx1061. The WDM filter 1061a demultiplexes the signal light and the filtered ASE light. The Rx 1061b receives the signal light and extracts information contained in the signal light. The Tx 1061c includes the semiconductor optical modulator 100 according to the first embodiment, modulates and amplifies the ASE light demultiplexed by the WDM filter 1061a, and transmits the modulated ASE light as upstream signal light to the business operator system.

  Next, the result of actually manufacturing the semiconductor optical modulator according to the first embodiment and transmitting the signal light will be described. FIG. 6 is a diagram showing the relationship between the signal light reception intensity and the error rate when the semiconductor optical modulator according to the first embodiment is applied to the optical communication system shown in FIG. FIG. 6 shows an error rate (BER: Bit Error Rate) with respect to received signal light intensity of wavelengths 1530, 1540, 1550, and 1565 nm at a back-to-back (abbreviated as BB in the figure) and 20 km transmission. Yes (however, the error rate on the vertical axis is expressed in logarithm). As can be seen from FIG. 6, the difference in reception strength between back-to-back and 20 km transmission for the same error rate is small. As a result, it is understood that communication with few errors can be performed. As described above, the semiconductor optical modulator according to the present invention can be used as an ONU of a WDM-PON.

The optical amplification gain in the polarization diversity circuit according to the first embodiment is such that the incident light L11 has an inclination of 45 ° with respect to the TE polarized light and the TM polarized light, and the first input / output unit 51 and the second input / output unit To the BH-SOA 55, the incident light L11 is TE polarized light and TM polarized light, and either the first input / output unit 51 or the second input / output unit BB- It becomes the minimum when entering the SOA 55. For this reason, the intensity | strength of the emitted light L12 will change with the polarization modes of the incident light L11. Therefore, it is preferable that the error rate does not exceed 10 ⁻⁹ due to the difference in output due to the polarization mode of the incident light L11.

Here, the relationship between input light intensity and gain will be considered. First, the absolute value of the change in gain per unit intensity of input light is defined as the amount of gain increase when the input light intensity increases by 1 dB. That is, the absolute value of the change in gain per unit intensity of input light is expressed as a slope when the horizontal axis is input light intensity (dBm) and the vertical axis is gain (dB). When the absolute value of the change in gain per unit intensity of the input light is 0.16 dB / dBm or less, an approximate calculation is that the difference in output due to the polarization mode of the incident light L11 is 0.5 dB or less. Sought by. Furthermore, it has been confirmed by experiments by the inventors of the present invention that the error rate is 10 ⁻⁹ or less when the difference in output is 0.5 dB or less.

  Therefore, in the BH-SOA 55 used for the semiconductor optical modulator 100, the absolute value of the change in gain per unit intensity of the input light is 0.16 dB / dBm or less within the range of the input light intensity. Is preferred. The intensity of the input light is, for example, not less than −30 dBm and not more than 0 dBm.

(Modification)
Next, as a modified example according to the first embodiment, a configuration in which the U-shaped portion of the semiconductor optical modulation amplification unit 50 is composed of a high mesa structure SOA will be described. FIG. 7 is a plan view of a semiconductor light modulation amplification unit in a semiconductor light modulator according to a modification of the present invention. The first input / output unit 51 and the second input / output unit 52 of the present modification include BH-SSC units 51a and 52a. In the first embodiment, the U-shaped portion constituted by the R portion and the BH-SOA is formed as a high-mesa SOA 55A having a high-mesa structure having the U-shaped portion in the present modification. Further, the high mesa SOA 55A is embedded in polyimide in order to improve the mechanical strength of the semiconductor element. A p-side electrode 56b is formed thereon. When the R parts 53 and 54 are configured in the SOA, since the bending loss is not increased, it is necessary to confine the light more strongly in the waveguide. Therefore, the waveguide is transferred from the BH-SOA having the embedded structure to the high mesa SOA having the high mesa structure. The structure of was changed. The R portions 53 and 54 are desirably set to a bending radius of 125 μm or more in order to suppress bending loss.

  Next, the operation of this modification will be described. The first input light L41 and the second input light L42 incident from the output optical waveguides 41 and 42 of the polarization conversion unit 40 are a first input / output unit 51 and a second input / output, respectively, including BH-SSC units 51a and 52a. The light enters the portion 52, and the core width of the waveguide is converted from 0.5 μm to 3 μm, for example. Furthermore, the end portion is widened to 3 μm and the core width other than the end portion is input to the high mesa SOA 55A having a width of 2 μm. The high mesa SOA 55A is applied with modulation signals from p-type electrodes and n-type electrodes (not shown) formed on the front and back surfaces, and the first input light L41 and the second input light L42 incident on the high mesa SOA 55A are optically modulated and The light is guided while being amplified, and is output from one end on the side opposite to the one on the input side.

(Embodiment 2)
Next, a semiconductor optical modulator 200 according to Embodiment 2 of the present invention will be described. FIG. 8 is a diagram showing a specific configuration of the semiconductor optical modulator according to the second embodiment of the present invention. As shown in FIG. 8, the semiconductor optical modulator 200 according to the second embodiment differs from the configuration in which light is incident from both ends into the semiconductor optical modulation amplification unit 50 having the U-shaped waveguide according to the first embodiment. In this configuration, the light emitted from the separation unit 30 is multiplexed by the optical multiplexing unit 270 and is incident on the semiconductor light modulation amplification unit 250 having a linear reflection type waveguide.

  First, the structure of the semiconductor optical modulator 200 according to the second embodiment of the present invention will be described. The optical input / output unit 10 has the same structure as that of the first embodiment. The PLC element 220 includes a polarization separation unit 30, a polarization conversion unit 240, and an optical multiplexing unit 270. The polarization separation unit 30 has the same structure as that of the first embodiment. The output optical waveguide 241 and the output optical waveguide 242 of the polarization conversion unit 240 are connected to the optical multiplexing unit 270. The optical multiplexing unit 270 includes a 3 dB coupler 271 and multiplexes the first input light L241 and the second input light L242 aligned in the TE polarization mode. The output end of the optical multiplexing unit 270 is connected to the optical input / output unit 251 of the semiconductor optical modulation amplification unit 250. The semiconductor light modulation amplification unit 250 has a linear light input / output unit 251 at one end, a light reflection unit 57 at the other end, and a BH-SOA 55 having the same structure as that of the first embodiment. Have. The modulation signal applying unit 60 has the same configuration as that of the first embodiment.

  Next, the operation of the semiconductor optical modulator 200 according to the second embodiment of the present invention will be described. First, as in the first embodiment, the incident light L11 is polarized and separated by the polarization separation unit 30, and further, the first input light L241 and the second input that are aligned in the TE polarization mode by the polarization conversion unit 240. Let it be light L242. Then, the first input light L241 and the second input light L242 output from the polarization converter 240 are combined by the 3 dB coupler 271 of the optical combiner 270. Thereafter, the semiconductor optical modulation amplification unit 250 modulates and amplifies the signal light input from the optical multiplexing unit 270 to the optical input / output unit 251 by the BH-SOA 55 while passing through the linear waveguide structure. At this time, a modulation current is applied to the BH-SOA 55 by the modulation signal applying unit 60, and the signal light is modulated by the modulation current. Further, the signal light is reflected by the light reflecting portion 57 and emitted from the light input / output portion 251. Then, the PLC element 220 outputs, from the input optical waveguide 31, the light input / output unit 10 as a modulated and amplified outgoing light L <b> 12 that is obtained by polarization combining the first input light L <b> 241 and the second input light L <b> 242.

  As described above, according to the present embodiment, it is possible to provide a semiconductor optical modulator having an optical amplification function in which polarization dependency is suppressed and optical modulation can be performed at high speed.

  In the present embodiment, the SOA is arranged in the center of the U-shaped portion in order to amplify the first input light and the second input light evenly. Here, at 10 GHz, for example, when the light is guided through a medium having a refractive index of 1.5, the wavelength is 2 cm. On the other hand, the distance between the first input / output unit, the second input / output unit, and the BH-SOA waveguide constituting the semiconductor optical modulation amplification unit is on the order of several hundred μm to several mm. Therefore, SOA should just be formed in the approximate center part of U character part. Further, two or more SOAs may be formed, and only one of them may be modulated and amplified. Further, it may be configured to modulate and amplify by a plurality of formed SOAs by adjusting the timing of modulation and amplification.

  In the present embodiment, the intensity modulation current generated in the modulation signal applying unit is applied to the semiconductor optical modulation amplification unit. However, the present invention is not limited to such intensity modulation, but can be applied to various optical modulators such as phase modulation and frequency modulation. Even in such optical modulation, the polarization diversity circuit aligns the polarization of the signal light with the TE polarization mode, so that the polarization dependency is suppressed, and an optical amplification function capable of optical modulation at high speed is provided. A semiconductor optical modulator can be realized.

  In the present embodiment, the polarization separation unit separates the incident light into the TM polarization mode first polarization mode light and the TE polarization mode second polarization mode light, and the polarization conversion unit separates the first polarization mode light and the first polarization mode light. Although the two-polarization mode light is aligned with the TE polarization mode, the first input light incident on the BH-SOA and the first input light are used in order to achieve the effect of realizing the light speed light modulation while suppressing the polarization dependency of the present invention. The two-input light may be a polarization mode with a larger amplification gain. Therefore, the polarization separation unit separates the incident light into the first polarization mode light and the second polarization mode light having arbitrary two different polarization directions, and the polarization conversion unit separates the first polarization mode light and the second polarization. A configuration in which the polarization direction with the mode light is aligned with a polarization mode with a larger amplification gain may be employed.

  Further, the present invention is not limited by the above embodiment. What was comprised combining each component mentioned above suitably is also contained in this invention. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

DESCRIPTION OF SYMBOLS 10 Optical input / output part 20, 220 PLC element 30 Polarization separation part 31 Input optical waveguide 32, 35, 271 3 dB coupler 33, 34 Arm optical waveguide 36, 37 Heater 40, 240 Polarization conversion part 41, 42, 241, 242 Output light Waveguide 43 Half-wave plate 50, 250 Semiconductor optical modulation amplification unit 51 First input / output unit 51a, 52a BH-SSC unit 51b, 52b High mesa passive optical waveguide unit 52 Second input / output unit 53, 54 R unit 55 BH -SOA
55a Silicon base 55b Substrate 55c Lower clad layer 55d Lower three-stage SCH layer 55e Active layer 55f Upper three-stage SCH layer 55g Upper clad layer 55h Buried layer 55i Current blocking layer 55j Passivation film 55k Polyimide 55A High mesa SOA
56a n-side electrode 56b p-side electrode 57 light reflection unit 60 modulation signal application unit 61 modulation signal input unit 62 high frequency circuit 270 optical multiplexing unit 100, 200 semiconductor optical modulator 251 optical input / output unit 1000 WDM-PON
1001 OLT
1002, 1003, 1004, 1005, 1031, 1041, 1051, 1061 Tx / Rx
1006 Broadband light source 1007, 1020 AWG
1008 Coupler 1010 Transmission fiber 1030, 1040, 1050, 1060 ONU
1061a WDM filter 1061b Rx
1061c Tx
L11 incident light L12 outgoing light L31 first polarization mode light L32 second polarization mode light L41, L241 first input light L42, L242 second input light T trench part

Claims (11)

A light input / output unit for inputting incident light and outputting emitted light;
A polarization separation unit that separates the incident light input from the light input / output unit into a first polarization mode light of a first polarization and a second polarization mode light of a second polarization having different polarization modes;
A polarization conversion unit that converts a polarization mode of the first polarization mode light into second polarization, outputs the first polarization light as first input light, and outputs the second polarization mode light as second input light;
A semiconductor light modulation amplifier for modulating and amplifying the first input light and the second input light by applying a modulation signal;
A modulation signal applying unit that applies the modulation signal to the semiconductor optical modulation amplification unit,
The semiconductor optical modulator amplifier is characterized in that the amplification gain of the second polarization is larger than the amplification gain of the first polarization. The semiconductor optical modulation amplification unit includes a waveguide structure having a first input / output unit at one end and a second input / output unit at the other end,
The first input light is input from the first input / output unit, modulated and amplified while passing through the waveguide, and output from the second input / output unit,
The second input light is input from the second input / output unit, modulated and amplified while passing through the waveguide, and output from the first input / output unit. Semiconductor optical modulator.   3. The semiconductor optical modulator according to claim 2, wherein an input / output direction of the first input / output unit and the second input / output unit of the semiconductor optical modulation amplification unit is a substantially parallel direction. An optical combining unit that combines the first input / output unit and the second input / output unit output from the polarization conversion unit and outputs the combined signal to the semiconductor light modulation amplification unit;
The semiconductor light modulation amplification unit includes a waveguide structure having a light input / output unit at one end and a light reflection unit at the other end,
The semiconductor optical modulation amplification unit modulates and amplifies the signal light input from the optical multiplexing unit to the optical input / output unit while passing through the waveguide structure, reflects the signal light by the light reflection unit, and 2. The semiconductor optical modulator according to claim 1, wherein the output is output from an output unit.   5. The semiconductor optical modulator according to claim 1, wherein the modulation signal applying unit applies a modulation signal of 5 Gb / s or more.   The semiconductor optical modulator according to claim 5, wherein the modulation signal applying unit applies a modulation signal of 10 Gb / s or more.   The semiconductor optical modulator according to claim 1, wherein the modulation signal applying unit applies an RZ modulation signal.   The semiconductor optical modulator according to claim 1, wherein the modulation signal applying unit applies an NRZ modulation signal.   9. The semiconductor optical modulator according to claim 7, wherein the semiconductor optical modulation amplification unit has an absolute value of gain change per unit intensity of input light of 0.16 dB / dBm or less.   An optical network unit comprising the semiconductor optical modulator according to claim 1.   A passive optical network comprising the optical network unit according to claim 10.

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