JP2013134308A - Optical 90° hybrid circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical 90° hybrid circuit capable of suppressing deterioration of reception characteristics depending on wavelength of light to be input.SOLUTION: An optical 90° hybrid circuit includes: a 1/2 wavelength plate pair 10 on which signal light 6 is made incident; a 1/4 wavelength plate 3 on which the signal light 6 emitted from the 1/2 wavelength plate pair 10 is made incident; a half mirror 4 on which both of the signal light 6 emitted from the 1/4 wavelength plate 3 and local light 7 are made incident; and a polarization separator 5 on which the signal light 6 and the local light 7 transmitted or reflected on the half mirror 4 are made incident, and separately emits the light as a plurality of pieces of output light 8, the 1/2 wavelength pair 10 has a first 1/2 wavelength plate 1 and a second 1/2 wavelength plate 2, a first optical axis of the first 1/2 wavelength plate is arranged at an angle of (45+p1×90)/2° (p is an integer) to a second optical axis of the second 1/2 wavelength plate 2.

Description

  The present invention relates to an optical 90 ° hybrid circuit that gives a predetermined phase difference to a plurality of incident lights and emits a plurality of interference emission lights.

  Phase-modulated optical communication is suitable for increasing the transmission distance and increasing the transmission capacity. For this reason, various communication systems such as 10G-DPSK (Differential Phase Shift Keying) system and 40G-DQPSK (Differential Quadrature Phase Shift Keying) system have been promoted. ing.

  In the 10G-DPSK system or the 40G-DQPSK system, the optical receiver receives the modulated optical signal and demodulates the optical signal. For the demodulation, a phase difference between signal bits is detected. In order to detect this phase difference, an optical interferometer called a 1-bit delay device is used.

  In recent years, digital coherent methods such as DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) method, which is more suitable for long-distance transmission and large-capacity transmission, have been developed.

  In this digital coherent system, an optical 90 ° hybrid circuit is used in place of the 1-bit delay device in order to demodulate the modulated optical signal. The optical 90 ° hybrid circuit is a circuit that detects an I signal (in-phase component) and a Q signal (quadrature component) by mixing local light and signal light in coherent optical communication. So far, several proposals have been made regarding the configuration of an optical 90 ° hybrid circuit.

  For example, in Patent Document 1, a spatial optical system light 90 ° including a quarter-wave plate for obtaining circularly polarized light, a half-wave plate for obtaining 45 ° linearly polarized light, and an interference half mirror. Hybrid circuits have been proposed.

  Patent Document 2 proposes an optical 90 ° hybrid circuit for a digital coherent system that can control the voltage of the phase difference of a quarter-wave plate and feedback-control the output optical phase difference from a detection signal. Yes.

  In Patent Document 3, a half-wave plate is used to change the polarization method of light. However, a spatial optical system type optical 90 ° hybrid circuit that widens the wavelength band of the half-wave plate used in the optical head. Has been proposed.

JP 2011-120030 A JP 2008-278249 A Japanese Patent No. 4407495

  The phase difference imparted to the output light by the optical 90 ° hybrid circuit is ideally 90 °, but if a phase shift occurs from the phase difference 90 ° that should have been imparted, the light using this optical 90 ° hybrid circuit The reception characteristics of the receiver will deteriorate.

  Therefore, it is desirable to realize a phase difference of 90 ° in the output light so that a phase shift from a predetermined phase difference (90 °) is as small as possible, for example, so that the phase shift is within ± 1 ° within a wide wavelength range.

  In a spatial optical system type optical 90 ° hybrid circuit, a phase difference of 180 ° is generated by a phase difference between transmitted light and reflected light of a half mirror, and the phase difference of 90 ° is retardation of a quarter wavelength plate. (A phase difference generated when two linearly polarized lights parallel to the optical axis are transmitted).

  In the optical 90 ° hybrid circuit as described in the above-mentioned Patent Document 1, the retardation of the quarter wavelength plate varies depending on the wavelength of the input light, so that the 90 ° phase difference has wavelength dependency. . Therefore, there is a problem that the reception characteristic of the optical receiver using the optical 90 ° hybrid circuit deteriorates depending on the wavelength of the input light.

  Patent Document 2 proposes an optical 90 ° hybrid circuit that can variably control the retardation of a quarter-wave plate and that is calculated by demodulating a detection signal. The phase difference of output light is 90 °. A method of feedback control is described so as to be the interval.

  However, in order to be able to control the retardation of the quarter-wave plate, an increase in size and complexity of the device is inevitable, and there is a problem that the cost of the entire receiver including the addition of a control circuit is increased.

  In addition, the configuration of the half-wave plate described in Patent Document 3 is not effective for widening the wavelength range of the quarter-wave plate, although the half-wave plate itself has a wide wavelength range. It was not possible to suppress the deterioration of the reception characteristics depending on the wavelength of the input light.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical 90 ° hybrid circuit that can suppress deterioration of reception characteristics depending on the wavelength of input light. .

  The present invention is a light 90 ° hybrid circuit that gives a predetermined phase difference to a plurality of incident lights that are linearly polarized beams and emits a plurality of interference emission lights, and is a first incident light of the plurality of incident lights. Are incident on the half-wave plate, the quarter-wave plate on which the first incident light emitted from the half-wave plate pair is incident, and the quarter-wave plate is emitted from the quarter-wave plate. The first incident light and a half mirror on which the second incident light of the plurality of incident lights is incident, and the plurality of incident lights transmitted or reflected by the half mirror are incident, and the plurality of the lights are incident on the half mirror. A polarization separator that separates and emits as the interference emission light, and the half-wave plate pair includes a first half-wave plate and a second half-wave plate, The first optical axis that is the optical axis of one half-wave plate is the second half-wave plate optical For the second optical axis is characterized in that it is arranged at an angle of (45 + p × 90) / 2 ° (p is an integer).

  According to the present invention, there is provided a light 90 ° hybrid circuit that gives a predetermined phase difference to a plurality of incident lights that are linearly polarized beams and emits a plurality of interference emission lights, and is a first of the plurality of incident lights. A half-wave plate pair on which incident light is incident, a quarter-wave plate on which the first incident light emitted from the half-wave plate pair is incident, and a quarter-wave plate The first incident light and the half mirror on which the second incident light of the plurality of incident lights is incident, and the plurality of incident lights transmitted or reflected by the half mirror are incident on the light. A polarization separator that separates and emits a plurality of interference outgoing lights as the interference outgoing light, and the half-wave plate pair includes a first half-wave plate and a second half-wave plate, The first optical axis that is the optical axis of the first half-wave plate is a second half-wave plate. Arranging at an angle of (45 + p × 90) / 2 ° (p is an integer) with respect to the second optical axis, which is the optical axis, suppresses the input light wavelength dependency of the phase difference of the output light. it can. Therefore, it is possible to suppress the deterioration of the reception characteristics due to the wavelength dependency of the input light.

1 is a diagram showing a schematic configuration of an optical 90 ° hybrid circuit according to Embodiment 1. FIG. 2 is an explanatory diagram of a configuration of an optical 90 ° hybrid circuit according to Embodiment 1. FIG. FIG. 6 is a diagram showing another schematic configuration of the optical 90 ° hybrid circuit according to the first exemplary embodiment; It is a figure which shows the wavelength dependence of the ellipticity of the transmitted signal light in the optical 90 degree hybrid circuit concerning Embodiment 1. FIG. It is a figure which shows the wavelength dependence of the phase error of the output light in the optical 90 degree hybrid circuit concerning Embodiment 1. FIG. FIG. 6 is a diagram showing the wavelength plate installation angle dependency of the phase error of output light in the optical 90 ° hybrid circuit according to the first embodiment. It is the figure which showed the structure of the optical 90 degree hybrid circuit as a premise technique of this invention. It is a figure which shows the 1/2 wavelength plate installation angle dependence of the ellipticity of the transmitted signal light in the optical 90 degree hybrid circuit concerning Embodiment 1. FIG.

  FIG. 7 is a diagram for explaining the structure of a spatial optical system type optical 90 ° hybrid circuit as a prerequisite technique of the present invention.

  As shown in FIG. 7, in the digital coherent method, QPSK signal light 100 and local light 101 (local oscillation light) are input to an optical 90 ° hybrid circuit. The optical 90 ° hybrid circuit distributes and outputs each of the signal light 100 and the local light 101 to four output ports I1, output ports I2, output ports Q1, and output ports Q2 using a polarization separator.

  That is, the optical 90 ° hybrid circuit shown in FIG. 7 gives a predetermined phase difference between two input lights (signal light 100 and local light 101), and outputs four interference output lights.

  The difference between the phase difference of the light i1 output from the output port I1 and the phase difference of the light i2 output from the output port I2 is 180 °. Similarly, the difference between the phase difference of the light q1 output from the output port Q1 and the phase difference of the light q2 output from the output port Q2 is also 180 °.

  On the other hand, the difference between the phase difference of the light i1 and the phase difference of the light q1 and the difference between the phase difference of the light i2 and the phase difference of the light q2 are each 90 °.

  Because the frequency difference between the signal light 100 and the local light 101 causes a beat in the intensity of the optical signal output from each output port, the intensity of the output light vibrates.

  Between the output port I1 and the output port I2, the intensities of the two output lights (light i1 and light i2) vibrate in an opposite phase relationship. Similarly, the intensity of the two output light (light q1 and light q2) signals vibrate between the output port Q1 and the output port Q2 in an opposite phase relationship.

  Light i1 and light i2 output from each of the output port I1 and the output port I2 are detected by a pair of photodetectors. The light q1 and the light q2 output from each of the output port Q1 and the output port Q2 are detected by another pair of photodetectors.

  By differentially detecting the detection signals in these two pairs of photodetectors, a signal having an intensity proportional to the product of the electric field amplitudes of the signal light 100 and the local light 101 is heterodyne detected. The heterodyne detection is realized by a known means.

  In the digital coherent method, the phase of the signal light 100 is digitally detected in real time from the detection signal.

  Thus, the optical 90 ° hybrid circuit is indispensable for the digital coherent system and is an important optical signal receiving circuit.

  However, since the retardation of the quarter-wave plate has the wavelength dependency of the input light, there has been a problem that the reception characteristics of the optical receiver using the optical 90 ° hybrid circuit deteriorate. In the following embodiment of the present invention, an optical 90 ° hybrid circuit capable of suppressing deterioration of reception characteristics depending on the wavelength of input light will be described.

<Embodiment 1>
<Configuration>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, the same reference number is attached | subjected to the same or an equivalent part, and the detailed description is abbreviate | omitted.

  The optical 90 ° hybrid circuit according to the present embodiment is used for demodulating an optical signal modulated by, for example, a differential phase modulation (QPSK) system, a differential quaternary phase modulation (DQPSK) system, or the like. It is an optical 90 ° hybrid circuit included in the container.

  FIG. 1 is a diagram illustrating a schematic configuration of an optical 90 ° hybrid circuit according to the first embodiment. As shown in FIG. 1, the optical 90 ° hybrid circuit according to the first embodiment includes a ¼ wavelength plate 3 that functions as a ¼ wavelength plate at a specific first wavelength, and a specific second wavelength. A first half-wave plate 1 and a second half-wave plate 2 that function as half-wave plates, a half mirror 4, and a plurality of polarization separators 5 are provided.

  The first half-wave plate 1 and the second half-wave plate 2 are arranged such that the angle formed by the optical axes of each other is (45 + p × 90) / 2 ° (p is an integer). A two-wave plate pair 10 is formed. Here, the optical axis of the first half-wave plate 1 is the first optical axis, and the optical axis of the second half-wave plate 2 is the second optical axis.

  The wavelength range of input light is, for example, the C band and the L band used in optical communication, and is approximately 1520 to 1620 nm.

  The specific first wavelength is, for example, 1570 nm, and the quarter-wave plate 3 has a retardation capable of converting linearly polarized light into circularly polarized light at this wavelength.

  The specific second wavelength is also 1570.0 nm, for example, and the first half-wave plate 1 and the second half-wave plate 2 used for the half-wave plate pair 10 are linearly polarized at this wavelength. It has a retardation which can be converted into linearly polarized light having different polarization plane angles. The specific second wavelength may be different from the specific first wavelength, for example, 1570.1 nm.

  However, the specific wavelength is preferably near the center of the used wavelength range of the optical 90 ° hybrid circuit, but may be any wavelength within the used wavelength range or outside the used wavelength range.

  The input light is signal light and local light modulated by the DP-QPSK method, and is within the above-described wavelength range.

  These are emitted from the end faces of the two optical fibers (not shown), and are each made collimated light having a beam diameter of about 0.18 to 1.0 mm by a collimating lens. The two collimated lights are incident on the polarization separator, and are separated and emitted in two directions as linearly polarized light (not shown).

  The polarization separator is, for example, a cube-type polarization beam splitter with a non-reflective coating on the surface, or a flat-plate polarization beam splitter with a polarization separation film laminated on a glass substrate and a non-reflection coating on the back surface.

  The local light is emitted from the end face of the optical fiber as linearly polarized light through the polarization maintaining fiber, and the angle of the polarization plane is adjusted so that the intensity is roughly divided by the polarization separator.

  One of the signal light emitted from the polarization separator and one of the local light becomes two incident lights to the optical 90 ° hybrid circuit.

  One incident light (first incident light, signal light 6) that is a linearly polarized beam is a half-wave plate pair 10 (first 1/2) whose optical axis is set at a predetermined angle with respect to the plane of polarization. The light passes through the wave plate 1 and the second half wave plate 2), and further passes through the quarter wave plate 3 whose optical axis is set at a predetermined angle, and enters the half mirror 4.

  As the half mirror 4, for example, a cube-type non-polarization half mirror or a flat plate type half mirror in which a half mirror film is laminated on a glass substrate is used. The half mirror 4 is desirably configured such that reflection on the surface is reduced by a non-reflective coating or the like.

  The half-wave plate is, for example, a zero-order wave plate, and a known half-wave plate in which optical axes of quartz substrates having different thicknesses by a predetermined size are bonded to each other is used. A half-wave plate made of a material other than quartz or a known half-wave plate made of an oriented polymer film may be used. It is desirable that the half-wave plate be reduced in reflection on the surface by a non-reflective coating or the like.

  In the half-wave plate pair 10, the angle formed by the optical axes (the first optical axis and the second optical axis) of the half-wave plates is (45 + p × 90) / 2 ° (p is an integer). For example, 22.5 ° (p = 0).

  The quarter-wave plate 3 is, for example, a zero-order wave plate, and a known quarter-wave plate in which optical axes of quartz substrates having different thicknesses by a predetermined size are orthogonally bonded to each other is used. A quarter wavelength plate of a material substrate other than quartz or a known quarter wavelength plate made of an oriented polymer film may be used. The quarter-wave plate 3 is desirably made to reduce reflection on the surface by a non-reflective coating or the like.

  FIG. 2 schematically shows the direction of the polarization plane when the signal light 6 passes through the half-wave plate pair 10 and enters the quarter-wave plate 3. In the figure, the plane of polarization of the signal light 6 is formed in the direction of the arrow indicated by the bold line.

  The half-wave plate pair 10 includes, for example, a second half-wave plate in which the first optical axis of the first half-wave plate 1 is at an angle of 48.4 ° with respect to the polarization plane of the signal light 6. The second optical axis 3 is installed at an angle of 70.9 ° with respect to the plane of polarization of the signal light 6 (see the thin lines in FIG. 2). The half-wave plate pair 10 has optical axes that intersect each other at an angle of 22.5 °, and rotates the plane of polarization of transmitted light by approximately 45 ° (see FIG. 2).

  At the specific first wavelength, the plane of polarization of the signal light 6 transmitted through the half-wave plate pair 10 is rotated by 45 ° while maintaining linear polarization. The quarter-wave plate 3 is installed such that the optical axis is at an angle of 45 ° with respect to the plane of polarization of the light transmitted through the half-wave plate pair 10 (see the thin line in FIG. 2). At the specific first wavelength, the signal light 6 transmitted through the half-wave plate pair 10 and the quarter-wave plate 3 is circularly polarized.

  Also, as shown in FIG. 3, it is desirable that two half-wave plates are bonded and fixed in this arrangement as in the half-wave plate pair 9 because the assembly of the optical 90 ° hybrid circuit can be simplified.

  Further, as shown in FIG. 3, it is further desirable to bond and fix the half-wave plate pair 9 and the quarter-wave plate 3 in this arrangement because the assembly of the optical 90 ° hybrid circuit can be simplified.

  Next, the other incident light (second incident light, local light 7) which is a linearly polarized beam is in the same position as the signal light 6 is incident on the half mirror 4 with the polarization plane inclined by 45 °. Incident (see FIGS. 1 and 3). In the half mirror 4, the transmitted signal light 6 and the reflected local light 7 overlap, and the reflected signal light 6 and the transmitted local light 7 overlap each other.

  With the above configuration, output light emitted from the half mirror 4 in two directions is obtained. A plurality of polarization separators 5 are arranged at positions where the respective output lights are incident.

  Four output lights 8 emitted from the polarization separator 5 are obtained by such a spatial optical system. These output lights 8 are all interference lights of two incident lights (signal light 6 and local light 7). The relative phase differences between the output lights are 0 °, 90 °, 180 °, and 270 °, respectively.

  The angle of the half-wave plate pair is as described above, that is, the first optical axis of the first half-wave plate 1 is relative to the second optical axis of the second half-wave plate 2 ( By adjusting the angle so that the angle is 45 + p × 90) / 2 ° (p is an integer), even if the wavelength of the signal light 6 is changed, the fluctuation of the relative phase difference between the output lights is suppressed, and in a wide wavelength range. The deviation of the output light 8 from the predetermined phase difference can be suppressed.

  In such an optical 90 ° hybrid circuit, output light pairs with phase differences of 0 ° and 180 ° correspond to I signals, and output light pairs with 90 ° and 270 ° correspond to Q signals. Demodulated light with excellent phase error characteristics of DP-QPSK signal receiver by coupling to optical detector of balanced receiver that detects signals differentially using coupling lens such as lens array and optical element such as mirror It can be suitably used as a circuit.

<Operation>
Next, the operation mechanism of the optical 90 ° hybrid circuit according to the present embodiment will be described. In the spatial optical system type light 90 ° hybrid circuit described above, the phase difference between the output light (transmitted light and reflected light), which is a half mirror characteristic, is 180 °, and it is generated using a quarter-wave plate. By using the fact that the circularly polarized light that is orthogonalized has a phase difference of 90 ° between the orthogonally polarized waves, 0 °, 90 °, 180 °, and 270 ° output optical phase differences are obtained.

  Among these, the phase difference due to the half mirror 4 has a property of deviating from 180 ° as the loss due to the half mirror 4 (loss other than transmitted light and reflected light) increases. Therefore, by using the half mirror 4 having a low loss characteristic such as a dielectric multilayer film, the phase difference between the transmitted light and the reflected light can be set to 180 ° with high accuracy in a wide wavelength range.

  In contrast, the phase difference between the orthogonal polarizations in the signal light 6 transmitted through the quarter-wave plate 3 has a wavelength dependency in principle. The retardation of the quarter-wave plate 3 is determined by the ratio between the optical path length difference and the wavelength for polarized light in the direction of the two optical axes.

  In the case of the quarter-wave plate 3 bonded with a quartz substrate, if the thickness difference of the quartz plate is ΔL, and the refractive index difference in the direction of the two optical axes is Δn, the optical path length difference = Δn · ΔL, and the wavelength is When λ is set and retardation is expressed by phase Φ, Φ = 2πΔn · ΔL / λ is obtained.

  Since the wavelength dependence of Δn is small, the retardation changes in inverse proportion to the wavelength. For this reason, even if the retardation becomes 90 ° (= π / 2) at a specific wavelength, the retardation shifts from 90 ° at other wavelengths.

  Since the four output optical phase differences of the optical 90 ° hybrid circuit are 0 °, Φ, 180 °, 180 ° + Φ, the output optical phase difference of the optical 90 ° hybrid circuit shifts from the 90 ° interval when the wavelength changes. In the optical receiver using this optical 90 ° hybrid circuit, there is a problem that the reception characteristic is deteriorated.

For example, the Q penalty of the reception characteristic when the error rate is 10 ⁻³ is 0.02 dB when the phase error is 1 °, 0.07 dB when the phase error is 3 °, and 0.18 dB when the phase error is 5 °. When the phase error is 7 °, it is estimated to be 0.33 dB. As the phase error is larger, the Q penalty is increased and the reception characteristic is deteriorated.

  In order to obtain a good reception characteristic, it is desirable that the Q penalty due to the phase error is 0.02 dB or less. For this purpose, the phase error needs to be within 1 °.

  In the optical 90 ° hybrid circuit according to the present embodiment, a half-wave plate pair is inserted before the quarter-wave plate 3.

  When a single half-wave plate is inserted in front of the quarter-wave plate 3, the angle at which the polarization plane is rotated so that the signal light 6 is incident on the quarter-wave plate 3 with a polarization plane inclined by 45 °. Therefore, it is necessary to incline the angle of the optical axis of the half-wave plate with respect to the plane of polarization by ½ (that is, 22.5 °), and the angle cannot be changed to another angle.

  On the other hand, when the half-wave plate pair is inserted in front of the quarter-wave plate 3, the elliptical azimuth angle of the transmitted signal light 6 is 2 which is the angle between the half-wave plates and the optical axes of each other. At a wavelength that is rotated from the angle of the polarization plane of the signal light 6 and the retardation of the half-wave plate is 180 °, the polarization plane of the signal light 6 and the optical axis of the pair of half-wave plates are formed. It does not depend on the angle.

  However, at the wavelength where the retardation of the half-wave plate deviates from 180 °, the signal light 6 transmitted through the pair of half-wave plates is close to linearly polarized light but is strictly elliptically polarized. The rate is a bisector between the polarization plane of the signal light 6 and the optical axes of the two half-wave plates of the half-wave plate pair, that is, a bisection between the first optical axis and the second optical axis. It changes depending on the angle formed by the line.

  FIG. 4 shows the signal light 6 transmitted through the quarter-wave plate 3 in the optical 90 ° hybrid circuit having the half-wave plate pair and the optical 90 ° hybrid circuit not having the half-wave plate pair. The difference of the ellipticity depending on the wavelength is shown. The horizontal axis represents wavelength (nm) and the vertical axis represents ellipticity.

  In an optical 90 ° hybrid circuit that does not have a half-wave plate pair, the ellipticity decreases to near 0.90 in the range of 1520-1620 nm, which is the wavelength range of the C band and the L band. In the configuration having a pair of plates, the ellipticity is kept approximately 1 (1 ± 0.03 or less) in the same wavelength range. Therefore, it can be seen that the transmitted signal light 6 is generally circularly polarized light regardless of its wavelength.

  FIG. 5 shows the difference in wavelength dependence of the phase error of output light between an optical 90 ° hybrid circuit having a half-wave plate pair and an optical 90 ° hybrid circuit having no half-wave plate pair. . The horizontal axis represents wavelength (nm) and the vertical axis represents phase error (deg).

  In an optical 90 ° hybrid circuit that does not have a half-wave plate pair, the phase error is about ± 3 ° at the maximum, and reception characteristics are deteriorated due to the phase error. In the optical 90 ° hybrid circuit having a half-wave plate pair, the phase error is 0.1 ° or less, as shown in FIG. 4, that the transmitted light is substantially circularly polarized. Reception characteristic deterioration is suppressed.

  In an optical 90 ° hybrid circuit including a quarter-wave plate and a single half-wave plate, there is a problem in the phase difference of output light due to a change in wavelength of incident light.

  In the present embodiment, the provision of the ¼ wavelength plate and the ½ wavelength plate pair can cancel the contribution due to the wavelength change that causes the circularly polarized light to become elliptically polarized light. The quarter-wave plate can be operated as a broadband quarter-wave plate. Therefore, it is possible to configure an optical 90 ° hybrid circuit that can suppress the wavelength dependency of the input light and can maintain the phase difference of the output light in the vicinity of 90 °.

  On the other hand, FIG. 6 shows the phase error of the output light in the optical 90 ° hybrid circuit according to the present embodiment as the dependence of the installation angle on the incident light polarization plane of the half-wave plate pair. The horizontal axis represents wavelength (nm) and the vertical axis represents phase error (deg).

  The two half-wave plates of the pair of half-wave plates are bonded and fixed, and the angles of the optical axis with respect to the incident light polarization plane are 48.4 ° and 70.9 °, respectively (that is, the optical axis is 2nd etc. The segment has an angle of 59.65 ° with respect to the incident light polarization plane), and the angle shift of the half-wave plate pair with respect to the incident light polarization plane from the arrangement was used as a parameter.

  Referring to FIG. 6, it can be seen that in order to make the phase error within 1 ° in the wavelength range, the half-wave plate pair should be installed with an angular accuracy within ± 2 °.

  FIG. 8 shows a half-wave plate with respect to the angle of the optical axis of the half-wave plate on the light incident side of the two half-wave plates of the half-wave plate pair with respect to the plane of polarization of the incident light. The result of having calculated the ellipticity of the light which permeate | transmitted the pair and the quarter wavelength plate is shown.

  However, among the half-wave plates, the optical axis of the half-wave plate on the light exit side is inclined by 22.5 ° with respect to the half-wave plate on the incident side. .

  In FIG. 8, when the retardation of the wave plate is shifted by 50 nm from the wavelength (1570 nm) at 90 ° for the quarter wave plate and 180 ° for the half wave plate, that is, 90% for the quarter wave plate. × (1-50 / 1570) = 87.13 °, when the retardation is 180 × (1-50 / 1570) = 174.26 ° with a half-wave plate, and when the deviation is 25 nm, that is, The result when the retardation is 90 × (1-25 / 1570) = 88.41 ° with the quarter-wave plate and 180 × (1-25 / 1570) = 176.82 ° with the half-wave plate. Shown as an example.

  As shown in FIG. 8, when the angle of the optical axis of the half-wave plate on the light incident side is 48.4 °, the ellipticity of the transmitted light is very close to 1 regardless of the wavelength. It can be seen that ideal circularly polarized light can be obtained in a wide wavelength range.

  At this time, the angle of the optical axis of the half-wave plate on the light emission side is set to 48.4 + 22.5 = 70.9 °.

  As described above, the change in the ellipticity of the transmitted light due to the wavelength is canceled by the signal light 6 transmitted through the quarter-wave plate 3 and the signal light 6 transmitted through the half-wave plate pair 1 / The angle formed by the bisector of the optical axis of the two half-wave plates of the two-wave plate pair and the polarization plane of the signal light 6 is adjusted (for example, as described above, with respect to the polarization plane of the signal light 6, The angle of the first optical axis of the first half-wave plate 1 is 48.4 °, the angle of the second optical axis of the second half-wave plate 2 is 70.9 °, and the optical axis is 2 etc. By making the branch line have an angle of 59.65 °, the interval between the four output optical phase differences of the optical 90 ° hybrid circuit can be 90 ° in a wide wavelength range.

  By using such an optical 90 ° hybrid circuit, it is possible to configure a DP-QPSK receiver in which reception characteristic deterioration due to a phase error is reduced.

<Effect>
According to the embodiment of the present invention, in the optical 90 ° hybrid circuit, the half-wave plate pair on which the signal light 6 as the first incident light out of the signal light 6 and the local light 7 is incident, The ¼ wavelength plate 3 to which the signal light 6 emitted from the / 2 wavelength plate pair is incident, the signal light 6 emitted from the ¼ wavelength plate 3, and the local light 7 as the second incident light. The half mirror 4 that is incident together, and the polarization separator that receives the signal light 6 and the local light 7 transmitted or reflected by the half mirror 4 and separates and emits the light as output light 8 that is a plurality of interference emission lights. The half-wave plate pair includes a first half-wave plate 1 and a second half-wave plate 2, and the optical axis of the first half-wave plate 1 is A certain first optical axis is (45 + p × 90) with respect to the second optical axis which is the optical axis of the second half-wave plate 2. ) / 2 ° (p is an integer) is arranged at a predetermined angle between a plurality of output lights 8 having a predetermined phase difference (90 °) that are output by interference of the input signal light. Deviation from the phase difference can be suppressed in a wide wavelength range (a used wavelength range). Therefore, it is possible to suppress the deterioration of the reception characteristics due to the wavelength dependency of the input light.

  Further, according to the embodiment of the present invention, in the optical 90 ° hybrid circuit, the angle formed between the polarization plane of the signal light 6 and the first optical axis is 48.4 °, and the polarization plane of the signal light 6 And the second optical axis is 70.9 °, and the angle between the plane of polarization of the signal light 6 and the third optical axis that is the optical axis of the quarter-wave plate 3 is 0 °. Thus, the shift with respect to the predetermined phase difference between the plurality of output lights 8 having the predetermined phase difference (90 °) is not dependent on the wavelength of the input light in a wide wavelength range (the used wavelength range). Can be suppressed.

  It should be noted that the present invention can be modified or omitted in any constituent elements in the present embodiment within the scope of the invention.

  DESCRIPTION OF SYMBOLS 1 1st half wavelength plate, 2nd 1/2 wavelength plate, 3 1/4 wavelength plate, 4 Half mirror, 5 Polarization separator, 6,100 Signal light, 7,101 Local light emission, 8 Output light, 9,10 1/2 wavelength plate pair, I1, I2, Q1, Q2 output port.

Claims (2)

A light 90 ° hybrid circuit that gives a predetermined phase difference to a plurality of incident lights that are linearly polarized beams and emits a plurality of interference emission lights,
A half-wave plate pair into which the first incident light of the plurality of incident lights is incident;
A quarter-wave plate on which the first incident light emitted from the half-wave plate pair is incident;
A half mirror on which the first incident light emitted from the quarter-wave plate and the second incident light of the plurality of incident lights are incident together;
A plurality of the incident light transmitted or reflected by the half mirror is incident, and a polarization separator that separates and emits the light as the plurality of interference emission lights,
The half-wave plate pair includes a first half-wave plate and a second half-wave plate,
The first optical axis that is the optical axis of the first half-wave plate is (45 + p × 90) / 2 ° (p) with respect to the second optical axis that is the optical axis of the second half-wave plate. Is an integer) angle,
Light 90 ° hybrid circuit. The angle formed between the plane of polarization of the first incident light and the first optical axis is 48.4 °,
The angle formed between the plane of polarization of the first incident light and the second optical axis is 70.9 °,
An angle formed between a polarization plane of the first incident light and a third optical axis that is an optical axis of the quarter-wave plate is 0 °,
The optical 90 ° hybrid circuit according to claim 1.

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