JP5421007B2 - Optical 90 degree hybrid circuit - Google Patents

Description

  The present invention relates to an optical 90-degree hybrid circuit constituting an optical receiver used for a coherent reception method in an optical transmission system. More specifically, the present invention relates to an optical 90-degree hybrid circuit that realizes wavelength independence of a phase difference between an In-phase output and a Quadrature output.

  An optical multi-level modulation method has attracted attention for realizing an ultrahigh-speed optical transmission system of 100 Gbit / s or more. In particular, coherent reception schemes such as DP-QPSK (Dual Polarization Quadrature Phase-Shift Keying) have attracted attention because of their superior optical noise resistance and superior chromatic dispersion distortion compensation capability through electrical signal processing after photoelectric conversion. Consideration for application is intensifying. The optical receiver used for the coherent reception method includes a local oscillation light generator that generates local oscillation light, a polarization splitter that separates signal light and local oscillation light into different output ports according to the polarization state, and signal light 90-degree hybrid circuit for combining optical oscillation and local oscillation light, a photoelectric conversion unit for converting an output signal from the optical 90-degree hybrid circuit into an electrical signal, and an electrical signal output from the photoelectric conversion unit for conversion into a digital signal And a digital signal processing (DSP) circuit that calculates a digital signal. By separating and detecting the In-phase component and the Quadrature component of the interference light between the input signal light and the local oscillation light, information included in the input signal light can be obtained.

  Among the components of the optical receiver used in the coherent reception system, as for the optical 90-degree hybrid circuit, a product composed of a spatial optical system combined with a bulk type optical component has already been developed and commercialized. On the other hand, a planar lightwave circuit (PLC) composed of an optical waveguide fabricated on a planar substrate is superior to the above-described spatial optical system in terms of mass productivity and reliability. In addition, the adoption of a PLC type optical 90-degree hybrid circuit increases the possibility of integration of, for example, a polarization beam splitter and a photoelectric conversion unit compared to a spatial optical system, and provides a smaller optical receiver. It becomes possible. From such a background, practical application of a PLC type optical 90-degree hybrid circuit is expected.

  FIG. 1 is a block diagram showing a conventional PLC type optical 90-degree hybrid circuit. This conventional PLC type optical 90-degree hybrid circuit is disclosed in Patent Document 1. Patent Document 1 relates to an optical delay interference circuit used for demodulation of a DQPSK (Differential Quadrature Phase-Shift Keying) signal. This itself does not correspond to the components constituting the optical receiver used in the coherent reception system, but it functions as an optical 90-degree hybrid circuit that multiplexes two light waves and separates them into an In-phase component and a Quadrature component. It is included in a part of the circuit. Hereinafter, the In-phase component is expressed as “I component” and the Quadrature component is expressed as “Q component”. FIG. 1 shows an extracted configuration of only the circuit portion necessary for realizing the optical 90-degree hybrid function in the optical circuit described in Patent Document 1.

  Here, the propagation process of the light input to the conventional PLC type optical 90-degree hybrid circuit of FIG. 1 will be described. The signal light input from the outside of the PLC is branched into two by the optical splitter 2a through the input waveguide 1a. The local oscillation light input from the outside of the PLC is branched into two by the optical splitter 2b through the input waveguide 1b. The light branched into two by the optical splitter 2a is input to the two optical couplers 3a and 3b via the arm waveguides 10a and 10b. The light branched into two by the optical splitter 2b is input to the two optical couplers 3a and 3b via the arm waveguides 11a and 11b. The signal light and the local oscillation light input to the optical coupler 3a and the optical coupler 3b are combined and interfere with each other, and are branched into two so that the phase difference of the interference light is 180 degrees. Is done. Interference light between the signal light and the local oscillation light output from the optical coupler 3a is formed as an external circuit via the output waveguides 4a and 5a, and is output to the differential light receiving unit 6a that functions as a photoelectric conversion unit. Is done. Interference light between the signal light and the local oscillation light output from the optical coupler 3b is formed as an external circuit via the output waveguides 4b and 5b and output to the differential light receiving unit 6b functioning as a photoelectric conversion unit. Is done.

  The 90-degree phase shift unit 7 is provided in any of the four arm waveguides 10a, 10b, 11a, and 11b. As a result, the interference light output from the optical coupler 3a and the optical coupler 3b via the output waveguides 4a, 4b, 5a, and 5b is differentially detected by the differential photodetectors 6a and 6b. It is possible to separate the I component and Q component of the modulated signal. Here, in order to simultaneously detect the I component and the Q component of the modulation signal, the waveguide lengths of the two arm waveguides 10a and 10b that transmit the signal light branched by the optical splitter 2a are equalized, and the optical splitter 2b It is necessary to make the waveguide lengths of the two arm waveguides 11 a and 11 b that transmit the branched local oscillation light equal except for the 90-degree phase shift unit 7. Further, by making the waveguide lengths of the four arm waveguides 10a, 10b, 11a, and 11b equal except for the 90-degree phase shift unit 7, light for receiving a differential phase modulation signal such as DQPSK. It can also be used as an optical 90-degree hybrid circuit constituting the delay interference circuit.

International Publication No. WO2003 / 063515 Pamphlet

S. H. Chang, H. S. Chung and K. Kim, "Impact of quadrature imbalance in optical coherent QPSK receiver", IEEE Photonics Technology Letters, vol. 21, no. 11, pp. 709-711, June 1, 2009 Katsumi Okamoto “Basics of Optical Waveguide” Corona, 1992

However, the structure described below is caused by the configuration of the 90-degree phase shift unit 7. The 90-degree phase shift unit 7 is installed for the purpose of changing the optical path length through which the propagation light passes by λ × (± 1/4 + m). Here, λ represents the wavelength of the signal light or the local oscillation light, and m represents an integer. As shown in Non-Patent Document 1, when the phase shift amount θ of propagating light in the 90-degree phase shift unit 7 deviates from 90 degrees, the reception characteristics deteriorate. For example, in the digital arithmetic processing, when the deviation from 90 degrees of the phase shift amount θ is not corrected, the OSNR (optical signal to noise ratio) penalty is suppressed to 0.5 dB or less at BER (Bit Error Rate) = 10 ⁻³ . For this purpose, it is necessary to suppress the deviation of the phase shift amount θ from 90 degrees within 5 degrees.

As a configuration of the 90-degree phase shift unit 7, there is a method of adjusting the effective refractive index of the core by loading a thin film heater on the upper surface of the PLC, applying electric power, and heating the periphery of the core constituting the waveguide. If the refractive index change amount due to heating of the core is ΔN and the heater length is L, the following equation is established.
L × ΔN = (± 1/4 + m) λ (1)

  In the case of this configuration, the phase shift amount θ can be accurately adjusted to 90 degrees by adjusting the applied power. On the other hand, in order to function as an optical 90-degree hybrid circuit, it is necessary to always apply constant power, which causes a problem of increasing the power consumption of the optical receiver. Further, in consideration of the influence of aging of the heater and the wiring portion, it is necessary to add a monitoring function of the phase shift amount θ, and there is a concern that the optical receiver and its control mechanism become complicated.

As a configuration of the 90-degree phase shift unit 7 different from the above, there is a method of adjusting the length of the waveguide. In this case, among the four arm waveguides 10a, 10b, 11a, and 11b, only the optical waveguide in which the 90-degree phase shift unit 7 is installed can be changed in length by ΔL so as to satisfy the following equation. That's fine.
ΔL × N (λ) = (± 1/4 + m) λ (2)

Here, N indicates the effective refractive index of the core constituting the waveguide, and is expressed as a function of the wavelength λ. In the case of this configuration, there is an advantage that no power is consumed and a monitoring function of the phase shift amount θ is unnecessary. However, since ΔL can only be set to a unique value, the phase shift amount θ depends on the wavelength λ of the propagation light. For example, when ΔL of the 90-degree phase shift unit 7 is designed so that a certain wavelength λ ₁ satisfies Expression (2) (θ = 90 degrees), the phase shift amount θ _{2 at} different wavelengths λ ₂ is expressed by the following expression. The

  As the wavelength band used as an optical 90-degree hybrid circuit expands, the shift of the phase shift amount θ from 90 degrees increases, and the influence on reception characteristics deterioration also increases.

  Here, the 90-degree phase shift unit 7 according to the related art will be described in detail. FIG. 2 is an enlarged view of FIG. 1 for explaining the configuration of the 90-degree phase shift unit 7 according to the prior art. Here, an example is shown in which the 90-degree phase shift unit 7 is installed in the arm waveguide 11b among the arm waveguides 10a, 10b, 11a, and 11b. In this example, the phase of light coupled to the optical coupler 3b out of the local oscillation light branched into two by the optical splitter 2b is shifted by 90 degrees compared to the phase of the light coupled to the optical coupler 3a. It is the structure for the purpose.

  When the absolute value of the phase difference between the local oscillation light coupled to the optical coupler 3a and the local oscillation light coupled to the optical coupler 3b is 90 degrees, the IQ phase difference is 90 degrees. The sign of the phase shift amount θ in FIG. Therefore, even if the 90-degree phase shift section having the same configuration as the 90-degree phase shift section 7 shown in this example is installed in the arm waveguide 11a, the same function as the configuration shown in FIG. 2 is realized. can do.

The wavelength dependency of the shift from 90 degrees of the phase shift amount θ in the 90-degree phase shift unit 7 of the prior art is represented by a linear function of wavelength and is represented by β × Δλ. Here, β is a constant of the unit [degree / nm]. Δλ represents the difference between the wavelength λ _{1 at} which the phase shift amount θ is 90 degrees and the wavelength λ of the light propagating through the 90-degree phase shift unit 7. Table 1 shows the relative phase relationship of the light output from the output waveguides 4b, 5a, and 5b when the phase of the light output from the output waveguide 4a is used as a reference.

  Here, it is assumed that the lengths of the arm waveguides 10a, 10b, 11a, and 11b are equal to each other. Here, the length of the arm waveguide 11b does not include the length of the waveguide constituting the 90-degree phase shift unit 7 among the waveguides connecting the optical splitter 2b and the optical coupler 3b. As the IQ phase difference of the optical 90-degree hybrid circuit, it is necessary to evaluate the phase difference of light output from the output waveguides 4a and 4b, 4b and 5a, 5a and 5b, 5b and 4a. Table 2 shows the phase difference of light output from the output waveguides 4a and 4b, 4b and 5a, 5a and 5b, 5b and 4a.

  Here, -270 degrees and 90 degrees are equivalent as the absolute value of the phase difference of light, and therefore converted so that each phase difference in the table is a positive number. It can be seen that the IQ phase difference is shifted from 90 degrees by ± β × Δλ in the configuration of the prior art.

  The present invention has been made in view of such problems, and an object thereof is to provide an optical 90-degree hybrid circuit that realizes wavelength independence. More specifically, an optical 90-degree hybrid circuit that realizes wavelength independence of a phase difference between an In-phase output and a Quadrature output (hereinafter referred to as “IQ phase difference”) is provided.

In order to achieve such an object, the present invention provides a first optical splitter for inputting a first light and branching it into two, and a second light for inputting the first light. A second optical splitter branched into two, a first arm waveguide that transmits one of the lights branched by the first optical splitter, and the other of the lights branched by the first optical splitter A second arm waveguide, a third arm waveguide that transmits one of the lights branched by the second optical splitter, and a fourth arm that transmits the other of the lights branched by the second optical splitter. A first optical coupler that generates interference light by combining the arm waveguide, the light transmitted by the first arm waveguide, and the light transmitted by the third arm waveguide; The light transmitted by the arm waveguide and the fourth arm waveguide A second optical coupler for generating interference light by combining the transmitted light, a phase difference between the light transmitted from the first arm waveguide and the light transmitted from the second arm waveguide; The absolute value of the sum θ of the phase difference between the light transmitted from the third arm waveguide and the light transmitted from the fourth arm waveguide is the center wavelength λ = λ _C in the wavelength band to be used. An optical 90-degree hybrid circuit comprising: a phase shift mechanism configured so that m is an integer of 90 + 360 m degrees and the absolute value of dθ / dλ is 0 at λ = λ _C.

  According to a second aspect of the present invention, in the optical 90-degree hybrid circuit according to the first aspect, the phase shift mechanism is provided in the first arm waveguide and the second arm waveguide. Degree hybrid circuit.

  The invention according to claim 3 is the optical 90-degree hybrid circuit according to claim 2, wherein the phase shift mechanism is further provided in the third arm waveguide and the fourth arm waveguide. It is a 90 degree hybrid circuit.

According to a fourth aspect of the present invention, in the optical 90-degree hybrid circuit according to any one of the first to third aspects, the phase shift mechanism is a waveguide having different lengths or widths. Degree hybrid circuit.

According to a fifth aspect of the present invention, in the optical 90-degree hybrid circuit according to any one of the first to third aspects, the phase shift mechanism is a waveguide having different lengths and widths. Degree hybrid circuit.

  According to the present invention, it is possible to realize wavelength independence of the IQ phase difference of the optical 90-degree hybrid circuit. A phase difference of light output from the first phase shift waveguide and the second phase shift waveguide, which employs a phase shift mechanism including the first phase shift waveguide and the second phase shift waveguide. By introducing a design that realizes wavelength independence, a circuit configuration suitable for realizing wavelength independence of the IQ phase difference is provided. Therefore, it is possible to prevent the reception characteristics from deteriorating.

It is a block diagram which shows the optical 90 degree | times hybrid circuit of a prior art. It is an enlarged view of FIG. 1 for demonstrating the structure of the 90 degree phase shift part based on a prior art. It is a schematic diagram showing an optical 90 degree hybrid circuit according to the present invention. It is an enlarged view of FIG. 3 for demonstrating the structure of the phase shift mechanism which concerns on this invention. 1 is a schematic diagram illustrating an optical 90-degree hybrid circuit according to Embodiment 1 of the present invention. It is a figure which shows the transmission spectrum measurement result of Example 1 of this invention and the optical 90 degree | times hybrid circuit based on a prior art. It is a figure which shows the evaluation result of IQ phase difference of Example 1 of this invention and the optical 90 degree | times hybrid circuit based on a prior art. It is a schematic diagram which shows the optical 90 degree | times hybrid circuit based on Example 2 of this invention. It is an enlarged view of FIG. 8 for demonstrating the structure of the phase shift mechanism which concerns on Example 2 of this invention. It is a figure which shows the evaluation result of IQ phase difference of Example 2 of this invention and the optical 90 degree | times hybrid circuit based on a prior art.

  The present invention provides an optical 90 degree hybrid circuit that realizes wavelength independence of an IQ phase difference caused by the wavelength dependence of the phase shift amount θ in the 90 degree phase shift unit 7.

  FIG. 3 is a configuration diagram of the optical 90-degree hybrid circuit according to the embodiment of the present invention. The optical 90-degree hybrid circuit according to the present invention includes an optical splitter 2a coupled to the input waveguide 1a, an optical splitter 2b coupled to the input waveguide 1b, and arm waveguides 10a and 10b coupled to the optical splitter 2a. Arm waveguides 11a and 11b coupled to the optical splitter 2b, a first phase shift waveguide 121 formed in the arm waveguide 11a, and a second phase shift formed in the arm waveguide 11b. A waveguide 122; an optical coupler 3a coupled to the arm waveguides 10a and 11a; an optical coupler 3b coupled to the arm waveguides 10b and 11b; and an output waveguide 4a coupled to the optical coupler 3a; 5a and output waveguides 4b and 5b coupled to the optical coupler 3b.

  Here, each component will be described. The signal light input from the outside of the PLC is branched into two by the optical splitter 2a through the input waveguide 1a. The local oscillation light input from the outside of the PLC is branched into two by the optical splitter 2b through the input waveguide 1b. One of the lights branched into two by the optical splitter 2a is input to the optical coupler 3a via the arm waveguide 10a, and the other is input to the optical coupler 3b via the arm waveguide 10b. One of the lights branched into two by the optical splitter 2b is shifted in phase by the first phase shift waveguide 121 formed in the arm waveguide 11a via the arm waveguide 11a, and the optical coupler. It is input to 3a. The other of the light branched into two by the optical splitter 2b is shifted in phase by the second phase shift waveguide 122 formed in the arm waveguide 11b via the arm waveguide 11b, and the optical coupler. It is input to 3b. The two lights input to the optical coupler 3a are combined to become interference light. The two lights input to the optical coupler 3b are combined to become interference light. The interference light output from the optical coupler 3a is output to the differential light receiving unit 6a via the output waveguides 4a and 5a. The interference light output from the optical coupler 3b is output to the differential light receiving unit 6b via the output waveguides 4b and 5b.

  The differential light receiving units 6a and 6b are formed as external circuits, function as photoelectric conversion units, detect differentially the interference light output from each of the optical coupler 3a and the optical coupler 3b, and input modulation signals The I component and the Q component are separated.

  The phase shift mechanism 12 is composed of the first phase shift waveguide 121 and the second phase shift waveguide 122. The light passing through the phase shift mechanism 12 is phase shifted. By appropriately designing the phase shift mechanism 12, it is possible to realize wavelength independence of the IQ phase difference. The design will be described below.

  Means for manifesting the effects of the present invention will be described. Comparing the optical hybrid circuit 8 of FIG. 3 with the conventional optical 90-degree hybrid circuit (FIG. 1), the difference is that the 90-degree phase shift unit 7 is omitted and the phase shift mechanism 12 is arranged.

  FIG. 4 is an enlarged view of FIG. 3 for explaining the configuration of the phase shift mechanism 12 according to the present invention. By installing the phase shift mechanism 12 instead of the 90 degree phase shift unit 7, it becomes possible to realize the wavelength independence of the IQ phase difference.

  As shown in FIG. 4, the phase shift mechanism 12 includes a first phase shift waveguide 121 and a second phase shift waveguide 122. In the phase shift mechanism 12, the relative phase difference of the light output from the first phase shift waveguide 121 with respect to the phase of the light output from the second phase shift waveguide 122, that is, generated in the phase shift mechanism 12. The phase shift amount θ is expressed by the following equation.

When i is a natural number, n _i (λ) is an effective refractive index in the i-th phase shift waveguide and is a function of the wavelength λ. L _i is the length of the i-th phase shift waveguide. Here, conditions necessary for making the phase shift amount θ independent of wavelength will be described. The wavelength dependence of the phase shift amount θ is expressed by the following equation.

  An important point for realizing the effect of the present invention is that θ (λc) in equation (4) is made to coincide with 90 ° at the center wavelength λc of the wavelength band using the optical 90 ° hybrid circuit, and the equation ( 5) to minimize the absolute value of dθ (λc) / dλ. In particular,

And

The effect of the present invention is maximized when the phase shift mechanism 12 is configured by the first phase shift waveguide and the second phase shift waveguide that minimize δθ defined in (1). Here, m is an integer, and β ₁ = dn ₁ (λ _c ) / dλ and β ₂ = dn ₂ (λ _c ) / dλ.

Next, a design method of the first phase shift waveguide and the second phase shift waveguide that satisfies Equation (6) and minimizes δθ represented by Equation (7) will be described. The lengths L ₁ and L ₂ of the waveguide can be easily controlled by adjusting the drawing pattern in a photomask used for the purpose of forming a waveguide pattern in the PLC manufacturing process. The effective refractive indexes n ₁ (λ _c ) and n ₂ (λ _c ) can be controlled by adjusting the concentration of the refractive index adjusting additive when forming the core layer in the PLC manufacturing process. However, considering that the first phase shift waveguide and the second phase shift waveguide are formed on the same substrate in the same manufacturing process, the effective refractive index is adjusted by adjusting the concentration of the refractive index adjusting additive. It is difficult to individually control n ₁ (λ _c ) and n ₂ (λ _c ).

On the other hand, as shown in Non-Patent Document 2, it is widely known that the effective refractive index depends on the waveguide structure. Therefore, the effective refractive indexes n ₁ (λ _c ) and n ₂ (λ _c ) can be controlled by adjusting the thickness or width of the waveguide core. However, considering that the first phase shift waveguide and the second phase shift waveguide are formed on the same substrate in the same manufacturing process, the effective refractive index can be adjusted by adjusting the thickness of the waveguide core layer. It is difficult to individually control n ₁ (λ _c ) and n ₂ (λ _c ).

On the other hand, the waveguide widths W ₁ and W ₂ can be easily controlled by adjusting the drawing pattern in a photomask used for the purpose of forming a waveguide pattern in the PLC manufacturing process. Here, W _i represents the core width of the i-th phase shift waveguide. In addition, as a factor of the wavelength dispersion of the refractive index, material dispersion and structural dispersion can be cited. Among these, as shown in Non-Patent Document 2, it is known that the structural dispersion depends on the waveguide structure. Therefore, by adjusting the thickness or width of the waveguide core, the wavelength dependence of the effective refractive indexes n ₁ (λ _c ) and n ₂ (λ _c ), that is, β ₁ and β ₂ can be controlled. Become. However, considering that the first phase shift waveguide and the second phase shift waveguide are formed on the same substrate in the same manufacturing process, β ₁ and It is difficult to control each β ₂ individually.

On the other hand, the waveguide widths W ₁ and W ₂ can be easily controlled by adjusting the drawing pattern in a photomask used for the purpose of forming a waveguide pattern in the PLC manufacturing process. The above explanation is summarized. In order to satisfy Equation (6) and minimize δθ expressed by Equation (7), L ₁ and L ₂ , n ₁ (λ _c ) and n ₂ (λ _c ), β ₁ and β Adjust ₂ As the adjustment method, adjusting the length and width of the first phase shift waveguide and the second phase shift waveguide is the simplest method. The actual design example will be described in detail in the embodiments according to the present invention described later.

  FIG. 5 is a schematic diagram of an optical 90-degree hybrid circuit according to Example 1 actually manufactured. PLC technology was used to fabricate the optical 90 degree hybrid circuit. Specifically, a quartz glass waveguide was produced on a silicon substrate by using a flame deposition method and reactive ion etching. The cross-sectional shape of the core is a 4.5 μm square, and the relative refractive index difference is 1.5%. The core was embedded with overclad glass having a thickness of 30 μm. In order to experimentally evaluate the IQ phase difference, the optical delay circuit unit 15 including the optical splitter 13 and the delay line 14 is coupled to the input waveguides 1a and 1b of the optical 90-degree hybrid of the present invention. This is because, instead of the signal light and the local oscillation light in FIG. 3, the light output from the same light source is branched, one of the branched lights is delayed through the delay line 14, and the optical 90-degree hybrid circuit is input. An object of the present invention is to construct an optical delay interference circuit by inputting to the waveguides 1a and 1b. By using the optical delay interference circuit, the relative phase difference of the light output from the output waveguides 4a, 4b, 5a, and 5b based on the transmission spectrum output from the output waveguides 4a, 4b, 5a, and 5b. Can be calculated. It is clear that the optical delay circuit unit 15 is removed after the IQ phase difference evaluation to function as an optical 90-degree hybrid circuit and the effect of the present invention is not impaired.

In the first embodiment, the length L ₁ of the first phase shift waveguide 121 is designed to be 574 μm and the width W _{1 is set} to 10.6 μm, and the length L ₂ of the second phase shift waveguide 122 is set to 574 μm and the width. By designing W ₂ to be 12.6 μm, it is expected that δθ satisfying Equation (6) and represented by Equation (7) will be zero. Here, it was designed as λc = 1550 nm. Further, Y-branch waveguides are used as the optical splitters 2a and 2b, and directional couplers are used as the optical couplers 3a and 3b.

  Here, it should be noted that the widths of the first phase shift waveguide 121 and the second phase shift waveguide 122 are different from the widths of the arm waveguides 11a and 11b, respectively. In such a case, by introducing a tapered waveguide into the connection portion between the first phase shift waveguide 121 and the arm waveguide 11a and the second phase shift waveguide 122 and the arm waveguide 11b, the propagation light can be obtained. It is possible to reduce the mode conversion loss.

  FIG. 6 shows a transmission spectrum measurement result of the produced optical 90-degree hybrid circuit. For comparison with the prior art, the optical 90-degree hybrid circuit of the prior art was fabricated by the same fabrication process as in Example 1 of the present invention, and the phase difference between the outputs was evaluated. The results are also shown in FIG.

  FIG. 7 is a diagram in which the phase difference between the outputs calculated from the transmission spectrum measurement results shown in FIG. 6 is plotted as a function of the input light wavelength. In the configuration of the prior art, the coefficient β of the wavelength dependence of the shift from the 90 degree phase shift amount θ of the 90 degree phase shift unit 7 is β = 0.04 [degrees / nm]. On the other hand, the IQ phase difference of the optical 90-degree hybrid circuit of this embodiment is 0 regardless of the wavelength, and it was experimentally confirmed that the wavelength dependence of the deviation from the 90-degree IQ phase difference was eliminated. .

  FIG. 8 is a schematic diagram of an optical 90-degree hybrid circuit according to Example 2 actually manufactured. The same PLC technology as in Example 1 was used to fabricate the optical 90-degree hybrid circuit. The cross-sectional shape of the core is a 4.5 μm square, and the relative refractive index difference is 1.5%. The core was embedded with overclad glass having a thickness of 30 μm. For the purpose of experimentally evaluating the IQ phase difference, as in the first embodiment, the optical delay circuit unit 15 including the optical splitter 13 and the delay line 14 is used as the input waveguide of the optical 90-degree hybrid of the present invention. Bound to 1a and 1b.

  It is clear that the optical delay circuit unit 15 is removed after the IQ phase difference evaluation to function as an optical 90-degree hybrid circuit and the effect of the present invention is not impaired. The difference from the first embodiment is that a phase shift mechanism 16 including a third phase shift waveguide 161 and a fourth phase shift waveguide 162 is installed together with the phase shift mechanism 12.

  FIG. 9 is an enlarged view of FIG. 8 for explaining the configuration of the phase shift mechanism 12 and the phase shift mechanism 16 according to the second embodiment. The relative phase difference of the light input from the arm waveguide 11b to the optical coupler 3b with respect to the light input from the arm waveguide 11a to the optical coupler 3a is θ <11a-11b>, and the arm waveguide 10b to the optical coupler 3b. The relative phase difference of the light input from the first arm waveguide 10a to the optical coupler 3a with respect to the input light is represented by θ <10b-10a>. The arrangement of the phase shift mechanism 12 and the phase shift mechanism 16 according to the second embodiment is designed so that the absolute value of the sum of θ <11a-11b> and θ <10b-10a> is 90 + 360 m degrees. Thus, by setting | θ <11a-11b> + θ <10b-10a> | = 90 + 360 m degrees, the IQ phase difference is 90 + 360 m degrees, and the function as an optical 90-degree hybrid circuit is realized. In the second embodiment, the phase shift mechanism 12 and the phase shift mechanism 16 are designed so that θ <11a-11b> = θ <10b-10a> = 45 degrees.

Specifically, the length L ₁ of the first phase shift waveguides 121 287Myuemu, the width W ₁ and 12.6Myuemu, the length L ₂ of the second phase shift waveguides 122 287Myuemu, the width W ₂ and 10.6 [mu] m, the third 287μm length L ₁ of the phase shift waveguide 161, the width W ₁ and 10.6 [mu] m, the fourth 287μm length L ₂ of the phase shift waveguide 162, the width W ₂ It was 12.6 μm. Thereby, expression of the effect of the present invention is expected. Here, it was designed as λc = 1550 nm. Further, Y branch waveguides are employed as the optical splitters 2a and 2b, and directional couplers are employed as the optical couplers 3a and 3b.

  FIG. 10 is a diagram in which the phase difference between the outputs calculated from the transmission spectrum measurement result is plotted as a function of the input light wavelength. In the configuration of the prior art, the coefficient β of the wavelength dependence of the shift from the 90 degree phase shift amount θ of the 90 degree phase shift unit 7 is β = 0.04 [degrees / nm]. On the other hand, the IQ phase difference of the optical 90-degree hybrid circuit of this embodiment is 0 regardless of the wavelength, and it was confirmed experimentally that the wavelength dependence of deviation from 90 degrees of the IQ phase difference was eliminated. did.

  As described above, according to the configuration of the present invention, the IQ phase difference is 90 degrees regardless of the wavelength, that is, the deviation of the IQ phase difference from 90 degrees is always 0 regardless of the wavelength. In the configuration of the present invention, the wavelength dependency of the phase shift amount θ in the 90-degree phase shift unit 7 is unavoidable in the prior art, whereas the phase shift mechanism 12 and the phase shift mechanism 16 are employed, and This is an effect of introducing a design that realizes wavelength independence of the phase difference of each light output from the phase shift mechanism 12 and the phase shift mechanism 16.

  The present invention can be used as an optical 90-degree hybrid circuit that is a component part of an optical receiver used in a coherent reception system in an optical transmission system.

1a, 1b Input waveguide 2a, 2b, 13 Optical splitter 3a, 3b Optical coupler 4a, 4b, 5a, 5b Output waveguide 6a, 6b Differential light receiving part 7 90 degree phase shift part 8 Optical 90 degree hybrid circuit 9 minutes Wave optical couplers 10a, 10b, 11a, 11b Arm waveguides 12, 16 Phase shift mechanism 121 First phase shift waveguide 122 Second phase shift waveguide 161 Third phase shift waveguide 162 Fourth phase Shift waveguide 14 Delay line 15 Optical delay circuit section

Claims (5)

A first optical splitter that receives the first light and branches it into two;
A second optical splitter that receives the second light and branches it into two;
A first arm waveguide for transmitting one of the lights branched by the first optical splitter;
A second arm waveguide for transmitting the other of the light branched by the first optical splitter;
A third arm waveguide for transmitting one of the lights branched by the second optical splitter;
A fourth arm waveguide for transmitting the other of the light branched by the second optical splitter;
A first optical coupler that combines the light transmitted by the first arm waveguide and the light transmitted by the third arm waveguide to generate interference light;
A second optical coupler for generating interference light by combining the light transmitted by the second arm waveguide and the light transmitted by the fourth arm waveguide;
The phase difference between the light transmitted from the first arm waveguide and the light transmitted from the second arm waveguide, the light transmitted from the third arm waveguide, and the fourth arm waveguide The absolute value of the sum θ with the phase difference of the light transmitted from is 90 + 360 m degrees where m is an integer at the center wavelength λ = λ _C in the wavelength band to be used, and the absolute value of dθ / dλ at λ = λ _C And a phase shift mechanism configured so that becomes 0. An optical 90-degree hybrid circuit.   2. The optical 90-degree hybrid circuit according to claim 1, wherein the phase shift mechanism is provided in the first arm waveguide and the second arm waveguide. 3.   The optical 90-degree hybrid circuit according to claim 2, wherein the phase shift mechanism is further provided in the third arm waveguide and the fourth arm waveguide. It said phase shifting mechanism, the optical 90-degree hybrid circuit according to any one of claims 1 to 3, characterized in that the length or width is different from the waveguide to each other. It said phase shifting mechanism, the optical 90-degree hybrid circuit according to any one of claims 1 to 3, wherein the length and width are different waveguides from one another.

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