JP2014199995A - Optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical receiver with a simplified structure and small dependence on wavelength.SOLUTION: An optical receiver demodulates a multi-value phase modulation signal corresponding to each polarization using signal light S in which each independent multi-value phase modulation signal is added to two types of orthogonal polarization, and local oscillation light R with the same wavelength as the signal light (linear polarization). The optical receiver comprises: composite wave signal generation means that separates the signal light into two separate signal lights S1, S2 corresponding to each polarization, converts the two separate signal lights into circular polarization, separates the local oscillation light into two separate local oscillation lights R1, R2, combines the circular-polarization converted separation signal lights and the separation local oscillation lights such that their polarization planes will form a prescribed angle, and forms and outputs two composite wave signals L1-L4 per separate signal light; and IQ output means that is made up of polarization and separation elements for separating and outputting an I signal and a Q signal corresponding to each polarization from the composite wave signals L1-L4. The polarization direction of the local oscillation light has a prescribed angle with the polarization axis of the polarization and separation elements.

Description

  The present invention relates to an optical receiver, and more particularly to an optical receiver that demodulates a multilevel phase modulation signal that is independent of two orthogonally polarized waves.

DP-QPSK (Dual Polarization) is one of the optical communication technologies.
Quadrature Phase Shift Keying), BPSK (Binary Phase Shift)
There has been proposed a communication method in which independent multilevel phase modulation signals are applied to two orthogonally polarized waves represented by Keying) and QAM (Quadrature Amplitude Modulation). In order to demodulate such signal light, light having a phase difference of 90 degrees between polarized waves is polarized and separated by a polarizing beam splitter (PBS), and an I signal (In-phase signal) and a Q signal are separated. An optical output of a signal (Quadrature-phase signal) is obtained.

  In the patent document 1, the present applicant proposed an optical receiver using an optical 90-degree hybrid that can be reduced in size. The structure of the light 90-degree hybrid is shown in FIG. The operation of the optical receiver shown in FIG. 1 will be described. Vertically polarized local oscillation light is input to a λ / 4 wavelength plate, converted into circularly polarized light, and branched by the next half mirror. On the other hand, the signal light is input to a polarization beam splitter (PBS) in an arbitrary polarization state, and is separated into vertical polarization and horizontal polarization. The vertically polarized wave and the horizontally polarized wave obtained by the PBS are respectively input to the λ / 2 wavelength plate and converted into an oblique 45-degree linearly polarized wave. The signal light converted into 45-degree obliquely linearly polarized light is combined with the circularly polarized local oscillation light and then input to the PBS to obtain an I signal and a Q signal having a phase difference of 90 degrees. Yes.

  The optical receiver of FIG. 1 requires different wavelength plates on the local oscillation light input side and the signal light input side, respectively, which complicates the configuration of the optical receiver. Further, since the wave plate has wavelength dependency, the use of many types of wave plates strengthens the wavelength dependency of the optical modulator and leads to deterioration of optical characteristics.

JP 2012-182790 A

  The problem to be solved by the present invention is to provide an optical receiver that solves the above-described problems, can simplify the structure, and has less wavelength dependency.

In order to solve the above problems, the present invention has the following technical features.
(1) Using signal light (S) in which independent multi-level phase modulation signals are assigned to two orthogonal polarizations, and local oscillation light (R) having the same wavelength (linear polarization) as the signal light In the optical receiver that demodulates the multi-level phase modulation signal corresponding to each polarization, the signal light (S) is separated into two separated signal lights (S1, S2) corresponding to each polarization, Two separated signal lights (S1, S2) are converted into circularly polarized light, the local oscillation light (R) is separated into two separated local oscillation lights (R1, R2), and the separated signal light (S1) and the separation are separated. The combined oscillation light is combined with the local oscillation light (R1) to generate combined light, and the separated signal light (S2) and the separated local oscillation light (R2) are combined to generate combined light. A synthesized wave signal generating unit configured by a spatial optical system that forms and outputs two synthesized wave signals (L1 to L4) every time, and the synthesized wave signal (L 1 to L4) and an IQ output means composed of a spatial optical system composed of polarization separation elements for separating and outputting the I signal and Q signal corresponding to each polarization, and the local oscillation light (R) The polarization direction has a predetermined angle with the polarization axis of the polarization separation element.

(2) In the optical receiver described in (1) above, the predetermined angle formed by the polarization direction of the local oscillation light (R) and the polarization axis of the polarization separation element is 45 degrees. .

(3) In the optical receiver described in the above (1) or (2), the IQ output means is configured to separate an I signal and a Q signal using a polarization beam splitter.

(4) In the optical receiver according to any one of the above (1) to (3), the optical component constituting the combined wave signal generating means and the optical component constituting the IQ output means have a linear expansion coefficient. It is characterized by being substantially equal.

  As in the present invention, signal light (S) in which independent multi-level phase modulation signals are given to two orthogonal polarizations, and local oscillation light (R) having the same wavelength (linear polarization) as the signal light, In the optical receiver that demodulates the multi-level phase modulation signal corresponding to each polarization, the signal light (S) is separated into two separated signal lights (S1, S2) corresponding to each polarization Then, the two separated signal lights (S1, S2) are converted into circularly polarized light, the local oscillation light (R) is separated into two separated local oscillation lights (R1, R2), and the separated signal light (S1) And the separated local oscillation light (R1) are combined to generate combined light, and the separated signal light (S2) and the separated local oscillation light (R2) are combined to generate combined light, Synthetic wave signal generation means composed of a spatial optical system that forms and outputs two synthetic wave signals (L1 to L4) for each combined light, and the synthesis IQ output means configured by a spatial optical system composed of polarization separation elements for separating and outputting the I signal and Q signal corresponding to each polarization from the signals (L1 to L4), and the local oscillation light ( The polarization direction of R) has a predetermined angle with the polarization axis of the polarization separation element, so that the number of wave plates can be reduced, the structure is simplified, and an optical receiver with less polarization dependency is provided. It becomes possible.

10 is a diagram illustrating an example of an optical receiver disclosed in Patent Document 1. FIG. It is a figure explaining the outline of the optical receiver of this invention. It is a figure which shows the example which uses an optical path length adjustment board with the optical receiver of this invention. It is a figure which shows the example which uses an optical path length adjustment board as a positioning means with the optical receiver of this invention. It is a figure which shows the example which uses a combination block with the optical receiver of this invention. It is a figure which shows the example which has arrange | positioned the optical path length adjustment board in the last stage with the optical receiver of this invention. It is a figure which shows the example which comprised IQ output means with the birefringent material with the optical receiver of this invention. It is a figure which shows the example which uses a birefringent material and an optical path length adjusting plate with the optical receiver of this invention.

Hereinafter, the optical receiver of the present invention will be described in detail using preferred examples.
In the present invention, as shown in FIG. 2, signal light (S) in which independent multi-level phase modulation signals are applied to two orthogonal polarizations, and local oscillation having the same wavelength (linear polarization) as the signal light In an optical receiver that demodulates the multi-level phase modulation signal corresponding to each polarization using light (R), the signal light (S) is divided into two separated signal lights (S1) corresponding to each polarization. , S2), the two separated signal lights (S1, S2) are converted into circularly polarized light, the local oscillation light (R) is separated into two separated local oscillation lights (R1, R2), and the separation The signal light (S1) and the separated local oscillation light (R1) are combined to generate combined light, and the separated signal light (S2) and the separated local oscillation light (R2) are combined to combine light. Generating a composite wave signal composed of a spatial optical system that generates and outputs two composite wave signals (L1 to L4) for each of the combined lights. And IQ output means composed of a spatial optical system composed of polarization separation elements for separating and outputting the I signal and the Q signal corresponding to each polarization from the combined wave signals (L1 to L4), The polarization direction of the local oscillation light (R) has a predetermined angle with the polarization axis of the polarization separation element.

  When the local oscillation light is 45-degree linearly polarized light and the wavelength plate on the signal light side is changed from the λ / 2 plate to the λ / 4 plate as in the present invention, it is compared with the conventional optical 90-degree hybrid optical system. It becomes possible to delete the λ / 4 wavelength plate on the oscillation light side. As a result, the configuration of the optical system can be simplified, and the wavelength plate having wavelength dependency can be reduced, so that the wavelength dependency of the optical 90-degree hybrid can be suppressed.

  In FIG. 2, the signal light (S) and the local oscillation light (R) are incident from the same side (upper side of the drawing) to the composite wave signal generation means (A), and the composite wave signals (L1 to L4) are opposite. The light is emitted to the side (the lower side of the drawing). Further, the IQ signal is output to the IQ output means (B) from the side (upper side of the drawing) on which the composite wave signal is incident (lower side of the drawing). With these configurations, the light emitted from the combined wave signal generation unit and the IQ output unit does not spread more than necessary, and the size of the optical receiver can be reduced. In FIG. 2, the incident direction of signal light and the like and the emission direction of the synthesized wave signal, the I signal, and the Q signal are configured to be parallel to each other. However, in the present invention, they are not necessarily parallel. It is not necessary, and can be appropriately designed according to the optical system after the output light is output.

  In the combined wave signal generating means, first, the signal light (S) is separated into two polarized waves (S1, S2) by a polarization beam splitter (PBS), and one polarized wave S2 is separated by a total reflection mirror and the other polarized wave S1. Are aligned in the same direction. Next, in order to make the polarized waves (S1, S2) circularly polarized light, it is passed through a λ / 4 wavelength plate. The polarized waves (S1, S2) that have passed through the λ / 4 wavelength plate are reflected in the horizontal direction of the drawing by a total reflection mirror disposed on the lower side of the wavelength plate, and the polarized wave S1 is polarized in the left direction. S2 is guided to the right.

  On the other hand, the local oscillation light (R) has the same wavelength as that of the signal light (linearly polarized wave, particularly 45 degree linearly polarized light), and is a half mirror that passes first. Separated into two of oscillation light. The downwardly oscillating local oscillation light is combined with the polarized wave S1 whose polarization plane is adjusted by the half mirror, and further forms two combined wave signals (L1, L2). Further, the local oscillation light directed to the right direction is reflected by the total reflection mirror and directed downward, and is combined by the half mirror with the polarized wave S2 whose polarization plane is adjusted, and further, two combined wave signals (L3) , L4).

  In the embodiment of FIG. 2, the IQ output means (B) is configured to separate into an I signal and a Q signal using a polarization beam splitter (PBS).

  In order to manufacture the spatial optical system as shown in FIG. 2, the distance between the components, which are arranged as necessary, includes a polarizing beam splitter (PBS), a half mirror (BS), a total reflection mirror, a wave plate, and the like. It is necessary to configure the spacer parts using BK7 as a base material and to attach the optical parts with high accuracy.

  In the optical receiver of the present invention, an optical film (reflection film, half mirror, polarization separation mirror, etc.) having little wavelength dependency in C-BAND (1530 nm-1565 nm), L-BAND (1565 nm-1625 nm), or both bands. A spatial optical system can be formed using In addition, the spatial optical system used in the present invention, which is composed of a glass member such as BK7, is superior in wavelength dependency and polarization dependency compared to a case where it is composed of a polarization dependent waveguide element. Demonstrate.

  FIG. 3 shows an optical path length adjusting means (optical path length adjusting plate) for adjusting the optical path length difference of each synthesized wave signal (L1 to L4) between the synthesized wave signal generating means (A) and the IQ output means (B). It is the Example which has arranged.

  The combined wave signal generation means (A) combines the signal light and the local oscillation light with a half mirror and generates two combined wave signals (L1 and L2, L3 and L4). At the time of output from the means (A), an optical path length difference is generated between the two synthesized wave signals. In order to correct this optical path length difference, optical path length adjusting means (C) is provided.

  As shown in FIG. 3, the optical path length adjusting means (C) can be arranged not only in close contact with the combined wave signal generating means (A) but also in a spaced arrangement. However, as shown in FIG. 3, the optical components can be positioned with high accuracy by being placed in close contact, and assembly adjustment of the optical receiver is facilitated.

  In the embodiment of FIG. 3, in the IQ output means (B), in order to make the optical path lengths after being separated into the I signal and the Q signal by the polarization beam splitter (PBS), A trapezoidal transparent optical component is arranged in the latter part of the means. The IQ output means shown in FIG. 2 is not adjusted in the optical path length as shown in FIG. 3, so the arrangement position of the light receiving element is adjusted, or the optical path length adjustment means is separately arranged on the downstream side of the IQ output means. It is necessary to do.

  In the embodiment of FIG. 4, the synthesized wave signal generating means (A), the optical path length adjusting means (C), and the IQ output means (C) are fixedly arranged in close contact with each other, and the entire spatial optical system constituting the optical receiver. Are integrated. Thereby, not only can the optical receiver be reduced in size, but also the positioning of each optical component can be performed with high accuracy and ease. By adopting the configuration of FIG. 4, an optical receiver including a polarization separation function can be formed in a space of about 15 × 15 mm.

  In the embodiment of FIG. 5, the IQ output means (B) is provided with two optical systems for IQ output on the left and right sides of the drawing, but it is complicated to position these components separately. The connecting block is arranged between the two so that the whole becomes one block body.

  In the embodiment of FIG. 6, an optical path length adjusting plate that complements the optical path length difference of each synthesized wave signal generated by the synthesized wave signal generating means is arranged on the downstream side of the IQ output means (B). As described above, the optical path length adjusting plate can be provided anywhere on the downstream side of the combined wave signal generating means (A). However, as shown in FIG. 6, the optical path length adjusting plate is provided on the downstream side of the IQ output means. In the case of arrangement, since the number of signals to be optically output is increased, the number of parts of the optical path length adjusting plate is increased, and the adjustment becomes complicated as compared with that in FIG.

  As shown in FIG. 7, the IQ output means can be a single optical component made of a birefringent material. The four combined wave signals (L1 to L4) incident on the birefringent material are polarized and separated, and output light of an I signal and a Q signal having a phase relationship of 90 degrees as shown in FIG. The crystal axis direction of the birefringent material is adjusted with a phase relationship of 90 degrees in accordance with the traveling direction of the polarized light beam.

  Furthermore, by adjusting the length of the birefringent material in the light traveling direction, the pitch between the optical outputs of the I signal and the Q signal can also be adjusted. As shown in FIG. 7, parallel light of the IQ signal is obtained, and the output pitch can be freely changed, so that the optical output of the I signal and the Q signal can be combined and optically coupled to a photodetector such as a light receiving element. It becomes.

  Each light wave output from the IQ output means is converted into an electrical signal corresponding to the I signal and the Q signal by a photodetector such as a balanced light receiving element.

  As shown in FIG. 7, when the IQ output means is composed of a single optical component made of a birefringent material, a plurality of optical components combined with PBS and total reflection mirrors are arranged as shown in FIGS. Compared to products, the number of parts can be greatly reduced. Thereby, the manufacturing cost in the optical receiver can be reduced, and complicated adjustment work between components can be omitted, so that deterioration of optical characteristics can be prevented. However, the use of the birefringent material increases the length in the vertical direction of the drawing, which increases the size as compared with that of FIG.

  When the IQ output means is composed of a single optical component, the IQ output means can be used as a positioning reference. As shown in FIG. 7, the synthesized wave signal generating means is provided on the birefringent material. By fixing and arranging the plurality of optical components to be configured, it becomes possible to arrange each optical component with high accuracy.

  It is preferable that the optical component constituting the synthetic wave signal generating means and the optical component constituting the IQ output means have substantially the same linear expansion coefficient. This makes it possible to suppress the occurrence of problems such as the occurrence of misalignment between the synthesized wave signal generating means and the IQ output means due to the temperature change of the use environment and the deterioration of optical characteristics. . Of course, the same applies to the optical path length adjusting means.

  Specifically, the optical component constituting the synthetic wave signal generating means can be mainly constituted by the optical glass BK7. Further, when the IQ output means is made of a birefringent material, YVO4 can be used. Since YVO4 and BK7 have substantially the same linear expansion coefficient, it is possible to suppress the deterioration of the optical characteristics accompanying the temperature change in the use environment.

  As shown in FIG. 8, an optical path length adjusting means such as an optical path length adjusting plate is arranged between the synthesized wave signal generating means and the IQ output means, and the optical path length (between L1 and L2) of each synthesized wave signal. The optical path length difference between the synthesized wave signals can be matched by adjusting the optical path length difference or the optical path length difference between L3 and L4. As described in the embodiment of FIG. 3, each polarization wave and local oscillation light are combined by a half mirror and then incident on a birefringent material as parallel light. This is because the optical path lengths in () are different. In this way, by adjusting the optical path length difference of the composite wave signal incident on the birefringent material, the light wave emitted from the birefringent material can be directly incident on the photodetector, and an appropriate I signal or The Q signal can be easily obtained.

  Further, the second optical path length adjusting means is arranged after the IQ output means, and the optical path lengths of the (I-1, I-2) signal and the (Q-1, Q-2) signal output from the IQ output means. It is also possible to adjust the difference. Thereby, more appropriate I signal and Q signal can be obtained.

  As described above, according to the present invention, it is possible to provide an optical receiver with a simpler structure and less wavelength dependency.

Claims (4)

Using each of the signal light (S) to which each of the two orthogonal polarizations is provided with an independent multilevel phase modulation signal and the local oscillation light (R) having the same wavelength (linear polarization) as the signal light, each polarization is used. In an optical receiver that demodulates the multi-level phase modulation signal according to a wave,
The signal light (S) is separated into two separated signal lights (S1, S2) corresponding to each polarization, the two separated signal lights (S1, S2) are converted into circularly polarized light, and the local oscillation light (R) is separated into two separated local oscillation lights (R1, R2);
The separated signal light (S1) and the separated local oscillation light (R1) are combined to generate a combined light, and the separated signal light (S2) and the separated local oscillation light (R2) are combined. A combined wave signal generating unit configured by a spatial optical system that generates combined light, and forms and outputs two combined wave signals (L1 to L4) for each combined light;
IQ output means configured by a spatial optical system composed of polarization separation elements that separate and output the I signal and Q signal corresponding to each polarization from the combined wave signals (L1 to L4),
An optical receiver characterized in that the polarization direction of the local oscillation light (R) has a predetermined angle with the polarization axis of the polarization separation element.   2. The optical receiver according to claim 1, wherein a predetermined angle formed by the polarization direction of the local oscillation light (R) and the polarization axis of the polarization separation element is 45 degrees.   3. The optical receiver according to claim 1, wherein the IQ output means is configured to separate into an I signal and a Q signal using a polarization beam splitter.   4. The optical receiver according to claim 1, wherein the optical component constituting the combined wave signal generating means and the optical component constituting the IQ output means have substantially the same linear expansion coefficient. Optical receiver.

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