JP6028530B2 - Optical receiver - Google Patents

Description

  The present invention relates to an optical receiver.

  An optical transmission / reception module is used as a key component of a wavelength division multiplexing optical communication device or the like that realizes large-capacity and long-distance transmission using an optical fiber. In recent years, with the increase in communication traffic, digital coherent optical transmission technology effective for high-speed and large-capacity transmission has attracted attention. This technique is an optical transmission method in which a phase modulation signal is added to an intensity modulation signal of transmission light to transmit / receive in order to increase transmission information capacity. In such a communication method using the phase modulation method, it is necessary to detect the phase of the light, so that the signal light received after optical transmission interferes with the reference light of a separately prepared local oscillation light source having the same frequency, and this beat signal A homodyne detection method for detecting intensity modulation information and phase modulation information of the signal light is used. In a digital coherent optical receiver, highly stable optical reception is constructed by a combination of a phase diversity homodyne optical receiver and high-speed digital signal processing. The digital coherent optical transmission technology and the digital coherent optical receiver are described in, for example, Patent Document 1 and Non-Patent Document 1.

FIG. 1 shows the basic configuration of the optical circuit unit of the phase diversity homodyne optical receiver described in Non-Patent Document 1. It is assumed that the polarization of the reference light LO that is the outgoing light of the local oscillation light source having the same optical frequency as the signal light received after the optical fiber transmission is the same. The reference light LO is bifurcated into LO1 and LO2, and a phase difference of 90 ° (ie, π / 2) is given to LO1 and LO2 by a 90 ° optical hybrid. The two branched signal lights are multiplexed into LO1 and LO2, respectively, and become beat signals due to interference between the signal light and the reference light. The cosine component I _I of the signal light is extracted by optically detecting the orthogonally polarized components I1 and I2 of the interference light beat signal between the signal light and the LO1 based on the phase of the reference light LO, and the signal light and the LO2 sine component I _Q of the signal light is extracted by detecting an orthogonal polarization components of the interference light beat signal I3 and I4 light. The analog electric signals I _I and I _Q are converted into digital signals, and optical complex amplitude is obtained by digital signal processing.

  For example, Patent Document 2 describes a plurality of configuration examples of specific optical elements and optical systems of a 90 ° optical hybrid phase diversity homodyne optical receiver.

JP 2007-64860 A JP 2011-120030 A

Applied Physics, Vol. 78, No. 9 (2009), PP. 856-861

  In the optical electric field waveform observation apparatus using homodyne detection disclosed in Patent Document 1, it is necessary to use a plurality of polarization beam splitters at the time of phase detection, which increases the number of components. In addition, an optical coupler is used as the optical multiplexing / demultiplexing element. At this time, however, an optical loss and an increase in size due to the coupling between the optical fiber and the optical coupler are caused. Furthermore, the 90 ° optical hybrid disclosed in Patent Document 2 has a configuration in which a plurality of wave plates and a plurality of polarization separation elements are separately arranged, and the number of parts increases. As a result, it is difficult to reduce the size of the optical receiver, and there is a problem in that the assembly process at the time of manufacturing the optical receiver is complicated.

  Therefore, there has been a demand for an optical receiver suitable for 90 ° optical hybrid phase diversity homodyne optical reception that can be used for digital coherent optical transmission that can simplify the assembly process and is miniaturized.

The present invention provides an unpolarized light branching unit that branches incident light including s-polarized light, which is linearly polarized light, and p-polarized light orthogonal to the s-polarized light, into two traveling directions; Of the two lights branched by the non-polarization branching section, no phase difference is given to one light, and only the other light is between the s-polarized light and the p-polarized light, A wave plate having a birefringent material having optical axes aligned in the thickness direction so as to give a phase difference substantially equal to an odd multiple of 90 °, and light given a phase difference substantially equal to an odd multiple of 90 °. The first and second linearly polarized light beams that are incident and orthogonal to each other, a polarization branching unit that branches in two traveling directions, the non-polarization branching unit, and the polarization Four photodetectors for receiving each of the light branched into four by the sex branching unit , the non-polarization branching unit being An optical receiver , which is a polarization diffraction grating, wherein incident light is divided into + 1st order diffracted light and −1st order diffracted light in two traveling directions .

  In the invention, it is preferable that the incident light includes signal light and reference light having the same wavelength, and the polarization phase separation element is formed by the non-polarization branching unit, the wavelength plate, and the polarization branching unit. An optical receiver characterized in that the incident light is branched.

  Further, the present invention is configured such that the branch direction of the two lights branched by the non-polarization branch part and the branch direction of the light branched by the polarizing branch part form a predetermined angle. An optical receiver characterized.

  According to the present invention, the non-polarization branching unit is a non-polarization diffraction grating, and the + 1st order diffracted light and the −1st order diffracted light of the incident light are branched into two traveling directions. apparatus.

  Further, in the present invention, the polarizing branching portion is a polarizing diffraction grating and transmits the ordinary light polarization component of the incident light without diffracting it, and diffracts the extraordinary light polarization component, or the extraordinary light polarization component of the incident light. Is transmitted without diffracting the light and diffracts the ordinary polarized light component.

Further, the present invention, the polarizing diffraction grating, double that aligned in the thickness direction optical axis, consisting of birefringent material having a ordinary refractive index n _o and extraordinary refractive index _{n e (n o ≠ n e} ) refractive material layer and the periodic unevenness is formed by isotropic material layer made of an isotropic material having a refractive index becomes n _s, a refractive index n _s is the ordinary refractive index n _o or the extraordinary refractive substantially equal to the rate n _e, the direction of the optical axis of the birefringent material layer, and the direction of the optical axis of the wavelength plate, an optical receiving apparatus, characterized in that an angle of approximately 45 °.

  In the optical receiver according to the present invention, the longitudinal direction of the non-polarized diffraction grating and the longitudinal direction of the polarizing diffraction grating form a predetermined angle.

  In the present invention, the wave plate may include a first region that does not give a phase difference between the s-polarized light and the p-polarized light, the s-polarized light, and the p-polarized light. An optical receiver having a second region that gives a phase difference substantially equal to an odd multiple of 90 ° between them, and a polymer liquid crystal only in the second region.

  In the present invention, the wave plate may include a first region that does not give a phase difference between the s-polarized light and the p-polarized light, the s-polarized light, and the p-polarized light. Between the first region and the second region, the first region and the second region both have a polymer liquid crystal, and the second region gives a phase difference substantially equal to an odd multiple of 90 °. An optical receiver characterized in that the polymer liquid crystal in the region is aligned substantially parallel to the thickness direction.

  The present invention also includes a polarizing beam splitter that combines s-polarized light and p-polarized light that are orthogonal to each other, and combines the signal light and the reference light incident from different directions by the polarizing beam splitter. An optical receiver that generates incident light to a polarization phase separation element.

  According to the present invention, it is possible to obtain a compact and lightweight 90 ° optical hybrid phase diversity homodyne optical receiver with a small number of parts by using a spatially coupled small optical component in which a plurality of functions are integrated. it can. In addition, in the polarization phase separation element constituting the optical receiver, the assembly process at the time of manufacturing the optical receiver can be simplified by integrally forming the non-polarization branching part, the wave plate, and the polarizing branching part. Further, since the incident angle dependency of the wave plate in the polarization phase separation element is small, the incident light can be arranged in the optical path of the focused light that is collected on the light receiving surface of the photodetector. As a result, the optical receiving apparatus can be downsized. Also, in the polarization phase separation element that constitutes the optical receiver, unnecessary light out of the branched light generated by the non-polarized light splitting part made of the non-polarized diffraction grating and generated by the polarizing branch part made of the polarizing diffraction grating Since unnecessary light in the branched light does not enter the light receiving surface of the photodetector, an optical signal with a high S / N ratio can be detected.

Principle configuration diagram of optical circuit section of phase diversity homodyne optical receiver Configuration diagram of optical receiver in first embodiment Configuration diagram of a polarization phase separation element used in the optical receiver in the first embodiment Configuration diagram of wave plate used for polarization phase separation element of optical receiver in first embodiment 1 is a configuration diagram of a polarizing diffraction grating used in a polarization phase separation element of an optical receiver according to a first embodiment. Explanatory drawing (1) of the polarization separation functional element used for the optical receiver in the first embodiment Explanatory drawing (2) of the polarization separation functional element used for the optical receiver in 1st Embodiment Explanatory drawing (3) of the polarization separation functional element used for the optical receiver in 1st Embodiment Correlation diagram between incident angle and phase difference of polarization phase separation element used in optical receiver in first embodiment Another configuration diagram of the polarization phase separation element used in the optical receiver in the first embodiment The block diagram of the optical receiver in 2nd Embodiment

  The form for implementing this invention is demonstrated below. In addition, about the same member etc., the same code | symbol is attached | subjected and description is abbreviate | omitted.

[First Embodiment]
(Optical receiver)
The optical receiver in the first embodiment will be described. FIG. 2 is a configuration diagram of the optical receiving apparatus according to the present embodiment. The optical receiving apparatus 100 according to the present embodiment includes an optical transmission unit 70 such as an optical fiber or an optical waveguide that emits transmission light including linearly polarized signal light S in the X-axis direction and linearly polarized reference light R in the Y-axis direction, The condenser lens 80 condenses the light emitted from the light transmission unit 70 onto the photodetector 60, and the polarization phase separation element 10 disposed in the optical path of the condenser lens 80 and the photodetector 60. The polarization phase separation element 10 includes a non-polarization diffraction grating 20 serving as a non-polarization branch part, a wave plate 30, and a polarization diffraction grating 40 serving as a polarization branch part, and is incident on the polarization phase separation element 10 arranged in this order. The incident light 50 that is convergent light is branched into four light beams 53a, 53b, 53c, and 53d. These four light beams are condensed on the light receiving surfaces 60a, 60b, 60c, and 60d of the divided photodetectors of the photodetector 60, respectively. The function of the polarization phase separation element 10 will be described later.

  The signal light S and the reference light R that are emitted from the optical transmission unit 70 have the same wavelength so as to be suitable for homodyne detection, the signal light S includes optical information that is intensity-modulated and phase-modulated, and the reference light R is constant. Intensity and phase are aligned.

  Although there is no restriction on the wavelength suitable for the optical receiver in the present embodiment, the transmission light has a sufficiently narrow wavelength width and has a coherence length suitable for homodyne detection of the digital coherent optical receiver, for example, long-distance transmission It is a wavelength corresponding to each wavelength channel of S band (1460-1530 nm), C band (1530-1565 nm), and L band (1565-1625 nm) used for WDM (Wavelength Division Multiplexing) wavelength division multiplexing optical communication.

  The light intensity signal converted into an electrical signal by the photodetector 60 (60a, 60b, 60c, 60d) is subjected to digital signal processing after AD conversion using an electronic signal processing circuit (not shown), and the signal light S Are detected as an intensity modulation signal and a phase modulation signal.

  Since the polarization of the signal light transmitted over a long distance in the optical fiber in WDM optical communication is not uniform, the outgoing light of the optical fiber is separated into s-polarized light and p-polarized light using a polarization beam splitter PBS (Polarization Beam Splitter), and s-polarized light 2 may be applied to the signal light of p and the signal light of p polarization. In this case, the reference light is p-polarized for the s-polarized signal light and s-polarized for the p-polarized signal light so as to be orthogonal to the linearly polarized light of the signal light.

  Note that linearly polarized reference light orthogonal to the signal light can be efficiently combined on the same optical axis using PBS. The reference light is generated by separately preparing a laser that is a local oscillation light source having the same wavelength as that of the signal light and a long oscillation wavelength and a narrow oscillation wavelength width. Specifically, an oscillation wavelength width of 10 Hz or less is preferable.

  The photodetector 60 may be reduced in size in order to reduce noise from the outside, but in order to condense light to such a photodetector 60 sufficiently small, It is preferable to increase the image-side numerical aperture. For example, when condensing light having a wavelength of 1.5 μm to 30 μm or less, the numerical aperture needs to be 0.03 or more from the diffraction limit equation. In the case of a lens having a large numerical aperture, a light beam having a large incident angle enters the polarization phase separation element 10. When the numerical aperture is 0.03, the incident angle is 1.7 ° or less. When the numerical aperture is large, the incident angle to the polarization phase separation element 10 becomes large, so the incidence angle to the polarization phase separation element 10 can be 1.7 ° or more.

(Polarization phase separation element)
An embodiment of a polarization phase separation element used in the optical receiver according to the present embodiment will be described. FIG. 3 is a configuration diagram of the polarization phase separation element in the present embodiment. The polarization phase separation element 10 in the present embodiment has a non-polarization diffraction grating 20, which is a non-polarization branching part, a wave plate 30, and a polarization diffraction grating 40, which is a polarization branching part, as shown in FIG. The light is incident in this order.

  The non-polarized diffraction grating 20 is, for example, one in which periodic irregularities are formed on the surface of a transparent substrate made of an isotropic material exhibiting an optically isotropic refractive index, or on the surface of the transparent substrate, Other isotropic materials may be formed in an uneven shape. When the cross section of the diffraction grating is a rectangular unevenness, if the diffraction efficiency of the ± first-order diffracted light is set to be maximum, the light amounts of the + 1st-order diffracted light and the −1st-order diffracted light can be increased, and the light passes through straight. Since the 0th-order diffracted light (straight forward transmitted light) can be reduced, the light can be efficiently branched into two lights.

  In this case, since the light amounts of the + 1st order diffracted light and the −1st order diffracted light branched by the rectangular diffraction grating are equal, the light quantity ratio between the + 1st order diffracted light and the −1st order diffracted light is kept constant even when there is a variation in wavelength. Therefore, the phase difference detection process described later is facilitated. Further, when the cross section of the diffraction grating is a blazed concavo-convex shape or a quasi-blazed concavo-convex shape approximating a blazed shape like a staircase, the light amounts of the 0th-order diffracted light and, for example, the + 1st-order diffracted light can be increased. Since the folding light can be reduced, the light can be efficiently branched into two lights. The diffraction angle is determined by the wavelength of incident light and the period width (grating pitch) of the diffraction grating. For example, when the light is branched by increasing the diffraction angle, the grating pitch may be designed to be narrow.

Since the diffraction angle and the grating pitch follows the grating equation, when the light of wavelength λ is incident on the non-polarization grating 20 of the grating pitch P _g, the diffraction of the emitted m order diffracted light to the medium having a refractive index n _g The angle θ _g (m) satisfies the following formula.

_ng sin θ _g (m) = mP _g / λ
Also, if the distance to the wave plate 30 was light +1 order diffracted light and -1 order diffracted light branched with the diameter of the light and phi _w at the position of the wave plate 30 and L _gw, satisfies the following formula When,
_{L gw {tanθ g (+1)} -tanθ g (-1)}> φ w
It is preferable because the light branched on the wave plate 30 can be separated and the generation of stray light causing noise can be reduced. When the branched light is + 1st order diffracted light and 0th order diffracted light,
L _gw tan θ _g (+1)> φ _w
It is preferable because the light branched on the wave plate 30 can be separated and the crosstalk between the two branched lights causing noise can be reduced.

In addition, when the average refractive index _ng of the medium on which light is emitted is large, the diffraction angle is small, so that the thickness of the element is increased to reduce crosstalk between the two branched lights. End up. Accordingly, the average refractive index of the medium between the non-polarized diffraction grating 20 and the wave plate 30 is preferably small, and the average refractive index is preferably 1.6 or less, and is 1.45 or less. More preferably, it is further preferably 1.4 or less. A borosilicate glass such as BK7 can be used as a material having a refractive index of 1.6 or less, and a material such as quartz glass or fluorite can be used as a material having a refractive index of 1.45 or less. A material such as a fluororesin or air can be used as a material having a refractive index of 10%. The grating pitch of the non-polarized diffraction grating 20 is preferably 5 μm, and more preferably 2 μm or less.

  The diffracted light of the non-polarized diffraction grating 20 travels in a direction perpendicular to the longitudinal direction of the concavo-convex portion of the grating cross section and the plane formed by the incident light and in the plane including the traveling direction of the 0th-order diffracted light. That is, the branching direction of the 0th order diffracted light or ± 1st order diffracted light is orthogonal to the longitudinal direction of the grating irregularities. In FIG. 3, the incident light 50 is designed such that the longitudinal direction of the diffraction grating is a straight line and the X-axis direction so that ± first-order diffracted light is generated in the Y-axis direction by the non-polarization diffraction grating 20.

  The wave plate 30 has a first region 31 and a second region 32 corresponding to the two lights branched by the non-polarized diffraction grating 20, and the first region 31 or the second region 32. Any of these may be set so that a phase difference of 90 ° is given between the signal light and the reference light. For example, the first region 31 is made of an isotropic material, the second region 32 is made of a birefringent material, and a desired phase difference is given by adjusting the thickness of the second region 32. Can do. In addition, one of the first region 31 and the second region 32 is not limited to one made of a transparent isotropic material, and may be air. When one is air, it is only necessary to provide a birefringent material designed to give a desired phase difference only to the other region. A detailed configuration will be described later.

The polarizing diffraction grating 40 acts so that the light traveling in the polarization state orthogonal to each other among the incident light is transmitted or diffracted and the traveling directions are different from each other. For example, of the incident light, the first linearly polarized light has a function of allowing the first linearly polarized light to pass straight through and diffracting the second linearly polarized light orthogonal to the first linearly polarized light. The polarizing diffraction grating 40 is configured, for example, such that an optically birefringent birefringent material and an isotropic material form irregularities with a periodic cross section. Then, by combining a material in which the ordinary refractive index n _o or extraordinary refractive index n _e (n _o ≠ n _e ) of the birefringent material and the refractive index n _s of the isotropic material substantially match, The effect can be obtained.

Since the diffraction efficiency of the polarizing diffraction grating 40 depends on a factor obtained by dividing the product of (n _e −n _o ) and the thickness of the birefringent material by the wavelength λ of light, when (n _e −n _o ) is small, The influence of the thickness of the birefringent material is increased. When the thickness of the birefringent material is increased, the change in the optical path length is increased in accordance with the change in the incident angle of light, so that the variation in diffraction efficiency with respect to the incident angle of light is increased. _{Thus, (n} e -n o)> It is preferable to use a birefringent material of _0.01, more preferably the use of birefringent material comprising a _{(n e -n o)> 0.04} , (n e - n _o)> more preferably the use of birefringent material of _0.05, further preferable to use a birefringent material as the _{(n e -n o)> 0.065} .

  As in the case of the non-polarized diffraction grating 20, the diffraction angle may be designed to be narrow when the diffraction angle is increased and light is branched. The diffracted light of the polarizing diffraction grating 40 travels in a direction perpendicular to the longitudinal direction of the concavo-convex portion of the grating cross section and the plane formed by the incident light and in the plane including the traveling direction of the 0th-order diffracted light. That is, the branching direction of the 0th order diffracted light or ± 1st order diffracted light is orthogonal to the longitudinal direction of the grating irregularities. In the example shown in FIG. 3, with respect to incident light 50 traveling in the Z-axis direction, the longitudinal direction of the grating is linear and the X-axis direction so that diffracted light is generated in the Y-axis direction by the non-polarized diffraction grating 20. Designed so that the longitudinal direction of the grating is a straight line and the X-axis direction so that diffracted light is generated by the polarizing diffraction grating 40 in the Y-axis direction.

  In the case of the arrangement as shown in FIG. 3, each diffraction element prevents unwanted diffracted light generated by the non-polarized diffraction grating 30 from passing through the polarizing diffraction grating 40 so as to reach the same point as the desired light. It is preferable to design the lattice pitch and the element thickness. This is because when desired light and unnecessary diffracted light reach the same point, light interference occurs and the amount of light to be detected varies. Such a design is possible by using a grating equation and a ray tracing method. As an example of such a design, the grating pitch of the non-polarized diffraction grating 30 and the grating pitch of the polarizing diffraction grating 40 can be made not to be an integral multiple of each other.

  An element having a polarization separation function may be arranged in place of the polarizing diffraction grating 40 at the position of the polarizing diffraction grating 40 as long as the traveling directions of the polarized light beams orthogonal to each other are different from each other. Good. A walk-off of a material having birefringence can be used as the polarization separation function. Alternatively, a birefringent material having an inclined thickness and refracting in different directions depending on a difference in refractive index between two different optical axis directions may be used.

  Next, the operation of the polarization phase separation element 10 will be described. Incident light 50 incident on polarization phase separation element 10 in the present embodiment is split into two light beams 51 a and 51 b separated from each other by transmission and / or diffraction by non-polarization diffraction grating 20. The incident light 50 is a light obtained by combining the signal light and the reference light, and is incident on the polarization phase separation element 10 in the Z-axis direction in FIG. An arrow S indicates the polarization direction of the signal light, and an arrow R indicates the polarization direction of the reference light. In FIG. 3, the polarization direction of the signal light indicated by the arrow S is the X-axis direction, the polarization direction of the reference light indicated by the arrow R is the Y-axis direction, and the polarization direction of the signal light S and the polarization direction of the reference light R Is incident substantially orthogonally.

  The branched light 51 a is incident on the first region 31 of the wave plate 30, and the branched light 51 b is incident on the second region 32 of the wave plate 30. The light 51a incident on the first region 31 of the wave plate 30 is the light 52a without adding a phase difference between the signal light and the reference light, that is, the light component in the X-axis direction and the light component in the Y-axis direction. Emitted. On the other hand, the light 51b incident on the second region 32 of the wave plate 30 is emitted as light 52b with a 90 ° phase difference added between the signal light and the reference light. The phase difference is not limited to 90 °, but the same effect can be obtained by setting it to an odd multiple of 90 °. However, considering that the thickness of the wave plate can be reduced, the phase difference is 90 °. It is preferable.

Next, the light 52 a and the light 52 b enter the polarizing diffraction grating 40. The optical axis 40a of the birefringent material used for the polarizing diffraction grating 40 preferably forms an angle of approximately 45 ° between the orthogonal polarization direction of the signal light S and the polarization direction of the reference light R. Accordingly, the light 52a and the light 52b incident on the polarizing diffraction grating 40 are branched into linearly polarized light in the direction of the optical axis 40a and linearly polarized light in a direction orthogonal to the optical axis 40a, respectively. That is, the light 52a incident on the polarizing diffraction grating 40 is branched and emitted into light 53a having a polarization direction orthogonal to the optical axis 40a and light 53b having a polarization direction parallel to the optical axis 40a. The optical axis is either a slow axis or a fast axis. For example, a slow axis optical axis 40a, a direction in which the extraordinary refractive index n _e of the birefringent material, the refractive index n _s of the ordinary refractive index n _o and an isotropic material of the birefringent material Consider a case where there is a substantial match (n _o ≈n _s , n _e ≠ n _s ). At this time, since the light 53a is linearly polarized light orthogonal to the optical axis 40a, the light 53a travels straight without being diffracted by the polarizing diffraction grating 40, and the light 53b is a straight line parallel to the optical axis 40a. Since it is polarized light, it is diffracted and emitted by the polarizing diffraction grating 40.

  Therefore, the incident light 50 incident on the polarization phase separation element 10 is emitted as four lights 53a, 53b, 53c, and 53d having different traveling directions, and the four lights 53a, 53b, 53c, and 53d are emitted from the incident light 50. When the phase difference is used as a reference, the phase differences between the signal light S and the reference light R are 180 °, 0 °, 270 °, and 90 °, respectively.

Next, the function of the polarization phase separation element 10 in the present embodiment will be described using Jones vectors. Assuming that the reference light R having an electric field strength A times the electric field strength (= 1) of the signal light S is incident, the electric field E ₁ in the incident light 50 is expressed by the equation shown in Equation 1. The Here, Δφ is a phase difference of the signal light S based on the phase of the reference light R, and when the phase information modulated by the phase is included in the signal light S, it is only necessary to detect the phase difference Δφ.

この後、無偏光回折格子２０によって２つの光５１ａ及び光５１ｂに分岐され、更に、波長板３０を透過した、光５２ａにおける電場Ｅ_２、光５２ｂにおける電場Ｅ_３は、数２、数３に示す式で表される。

After that, the electric field E ₂ in the light 52 a and the electric field E ₃ in the light 52 b that are branched into the two light 51 a and the light 51 b by the non-polarized diffraction grating 20 and transmitted through the wave plate 30 are expressed by Equations 2 and 3. It is represented by the formula

尚、数２におけるα、数３におけるβは、光を分岐した際における強度の係数である。また、数３におけるＭは、ｓ偏光とｐ偏光との間において、９０°の位相差を与えるジョーンズ行列であり、例えば、数４に示されるものである。

Note that α in Equation 2 and β in Equation 3 are coefficients of intensity when light is branched. M in Equation 3 is a Jones matrix that gives a phase difference of 90 ° between s-polarized light and p-polarized light. For example, M is shown in Equation 4.

更に、偏光性回折格子４０によって、光５２ａは光５３ａと光５３ｂとに分岐され、光５３ａにおける電場Ｅ_４、光５３ｂにおける電場Ｅ_５は、数５、数６に示す式で表される。同様に、偏光性回折格子４０によって、光５２ｂは光５３ｃと光５３ｄとに分岐され、光５３ｃにおける電場Ｅ_６、光５３ｄにおける電場Ｅ_７は、数７、数８に示す式で表される。

Furthermore, the light 52a is branched into light 53a and light 53b by the polarizing diffraction grating 40, and the electric field E ₄ in the light 53a and the electric field E ₅ in the light 53b are expressed by the equations shown in Equations 5 and 6. Similarly, the light 52b is branched into light 53c and light 53d by the polarizing diffraction grating 40, and the electric field E ₆ in the light 53c and the electric field E ₇ in the light 53d are expressed by the equations shown in Equations 7 and 8. .

ここで、数５におけるＰ_１、数６におけるＰ_２は、偏光子を表すジョーンズ行列であり、例えば、数９、数１０に示す式で表される。尚、γ、δは強度の係数を示す。

Here, P ₁ in Equation 5 and P ₂ in Equation 6 are Jones matrices representing the polarizer, and are represented by, for example, the equations shown in Equations 9 and 10. Note that γ and δ indicate strength coefficients.

ここで、数９及び数１０に示す式を用い、α、β、γ、δを１とした場合、電場Ｅ_４、Ｅ_５、Ｅ_６及びＥ_７により得られる光信号強度Ｉ_４、Ｉ_５、Ｉ_６及びＩ_７は、以下のように、表すことができる。

Here, the optical signal intensities I ₄ and I ₅ obtained by the electric fields E ₄ , E ₅ , E _6, and E ₇ when α, β, γ, and δ are set to 1 using the equations shown in Equations 9 and 10. , I ₆ and I ₇ can be expressed as follows:

I ₄ = (1 + A ² −2A cos Δφ) / 4,
I ₅ = (1 + A ² + 2A cos Δφ) / 4,
I ₆ = (1 + A ² + 2AsinΔφ) / 4,
I ₇ = (1 + A ² −2 Asin Δφ) / 4,
Based on the optical signal intensities I ₄ , I ₅ , I ₆ and I ₇ , the optical intensity signal Sig enhanced by A times can be detected by performing the following calculation.

Sig = {(I ₄ −I ₅ ) ² + (I ₆ + I ₇ ) ² } ^1/2
= {(AcosΔφ) ² + (AsinΔφ) ² } ^1/2
= A
Accordingly, by detecting each of the four lights transmitted through the polarization phase separation element and further including a photodetector having a calculation function, the light intensity signal Sig can be detected with high sensitivity. Therefore, a high S / N ratio can be obtained in the detection of intensity-modulated optical information contained in the signal light S using this optical receiver.

  Furthermore, the phase difference Δφ between the reference light R and the signal light S can be detected by performing the following calculation.

(I ₆ -I ₇ ) / (I ₅ -I ₄ ) = {(Asin Δφ) / (Acos Δφ)}
= TanΔφ
Therefore,
Δφ = tan ⁻¹ {(I ₆ −I ₇ ) / (I ₅ −I ₄ )}
As a result, since the phase of the reference light R is constant, the phase-modulated information included in the signal light S can be detected with a high S / N ratio.

For example, when the value of Δφ is four values of 0 °, 90 °, 180 °, and 270 °, the photodetector 60 has a function of calculating I ₄ -I ₅ and calculating I ₇ -I _6. Assuming that Sig1 = I ₄ −I ₅ = AcosΔφ and Sig2 = I ₇ −I ₆ = AsinΔφ, the value of Δφ can be known based on the following.

When Δφ = 0 °, (Sig1, Sig2) = A (1, 0),
When Δφ = 90 °, (Sig1, Sig2) = A (0, 1),
When Δφ = 180 °, (Sig1, Sig2) = A (−1, 0),
When Δφ = 270 °, (Sig1, Sig2) = A (0, −1),
That is, it is possible to obtain a small-sized optical receiver 100 that can detect phase information and that can be easily assembled and manufactured.

(Wavelength plate)
Next, the wave plate 30 used for the polarization phase separation element 10 in the present embodiment will be described. The wave plate 30 may have various configurations. As shown in FIG. 3, the first region 31 and the second region 32 may be arranged so that the two lights 51a and 51b branched by the non-polarized diffraction grating 20 are incident on the first region 31 and the second region 32, respectively. The shape of the first region 31 and the second region 32 and the boundary line between these regions may be arbitrary.

  FIG. 4 illustrates a schematic cross-sectional view of the wave plate 30 a, the wave plate 30 b, and the wave plate 30 c as a specific configuration of the wave plate 30. These wave plate 30a, wave plate 30b and wave plate 30c can all be used as the wave plate 30 shown in FIGS. Specifically, as shown in FIG. 4A, the wave plate 30a includes an isotropic material 33a sandwiched between the transparent substrates 35a and 36a in the first region 31a, and in the second region 32a. A polymer liquid crystal 34a serving as a birefringent material and an isotropic material 33a are sandwiched between transparent substrates 35a and 36a. In FIG. 4A, the polymer liquid crystal 34a is aligned parallel to the Y-axis direction. The wave plate 30a is formed by uniformly forming a polymer liquid crystal film on the transparent substrate 36a, and then removing the polymer liquid crystal film in the first region 31a by photolithography and etching, so that only in the second region 32a. It can be formed by forming the polymer liquid crystal 34a and then filling an isotropic material 33a serving as a filler between the transparent substrate 35a and the transparent substrate 36a. The second region 32a may have an (air) structure in which the isotropic material 33a is not provided.

  As shown in FIG. 4B, the wave plate 30b includes a first region 31b having a vertically aligned liquid crystal 33b aligned in a direction perpendicular to the transparent substrate surface (Z-axis direction). The region 32b has horizontal alignment liquid crystal 34b aligned in the horizontal direction on the transparent substrate surface. In FIG. 4B, the horizontal alignment liquid crystal 34b is aligned parallel to the Y-axis direction. Such a wave plate 30b is formed by performing an alignment process on the transparent substrates 35b and 36b so that the liquid crystal is vertically aligned in the first region 31b and the liquid crystal is horizontally aligned in the second region 32b. The liquid crystal can be encapsulated with the surfaces subjected to the alignment treatment facing each other. In addition, as a method of alignment treatment, methods such as rubbing of an alignment film, photo-alignment treatment, ion beam irradiation, and groove formation for alignment can be used.

  Further, as shown in FIG. 4C, the wave plate 30c does not form anything in the first region 31c on the transparent substrate 33c, but birefringence only on the second region 32c on the substrate 33c. A phase difference generated by forming a material having a structural birefringence or a photonic crystal as the layer 34c is adjusted. In addition, a stretched polymer film may be formed as the birefringent layer 34c. In addition, the medium may be air assuming that no phase difference is given to the light transmitted through the first region or the second region. In this case, the wave plate is substantially the first region. Alternatively, a birefringent material that is disposed in any one of the second regions and that gives a phase difference of an odd multiple of 90 ° with respect to the incident light having the wavelength λ may be formed.

In addition, in the relationship between the ordinary light refractive index n _wo and the extraordinary light refractive index n _we (n _wo ≠ n _we ), it is preferable to use a birefringent material _satisfying (n _we −n _wo )> 0.01, n _we -n _wo)> more preferably the use of birefringent material serving as _{0.04, (n we -n wo)} > more preferably the use of birefringent material serving as _0.05, (n we _{-n wo)} It is more preferable to use a birefringent material with> 0.065. Further, from the viewpoint of a birefringent material that can be used, those _satisfying 0.3> (n _we −n _wo ) are preferable.

(Polarizing diffraction grating)
Next, the polarizing diffraction grating 40 will be described. As shown in FIG. 5, the polarizing diffraction grating 40 used in the polarization phase separation element 10 in the present embodiment has a blazed shape or a pseudo blazed shape in which the blazed shape approximates a step shape, and is a polymer. A birefringent material layer 44 made of liquid crystal and an isotropic material layer 45 in which an isotropic material is arranged so as to flatten the unevenness of the birefringent material layer 44 are formed on the transparent substrates 42 and 43. It has a sandwiched structure. The birefringent material layer 44 has a (pseudo) blazed inclination such that the convex portion becomes thick in the + Y direction. However, the birefringent material layer 44 may have a reverse inclination, and further, the direction of the optical axis is If the direction is 45 °, the longitudinal direction of the birefringent material layer 44 may be aligned in an arbitrary direction on the XY plane.

In addition, the orientation direction (for example, the slow axis) of the birefringent material layer 44 is a direction that forms an angle of 45 ° with respect to the X axis in the XY plane of FIG. Incidentally, the refractive index of the isotropic material when the n _s, isotropic material, in one of the polymer liquid crystal ordinary refractive index n _o or the extraordinary refractive index _{n e (n o ≠ n e} ) It is made of a substantially matching material. It is preferable that the polarizing diffraction grating has a (pseudo) blazed cross section because it can increase the utilization efficiency of light that is transmitted or diffracted and branched, but is not limited to this, and has a rectangular periodic unevenness. But you can.

Here, when the relationship between the refractive index n _s ≒ n _o, and n _s ≠ n _e, the light of the polarization component in the direction of the extraordinary refractive index of the slow axis direction of the polymer liquid crystal is diffracted, polymer The light of the polarization component in the direction of the ordinary refractive index that is the fast axis direction of the liquid crystal passes straight through without being diffracted. Such a polarizing diffraction grating 40 is formed by forming a polymer liquid crystal film on a transparent substrate 42 and then repeating photolithography and etching to form a birefringent material layer 44 made of polymer liquid crystal having a pseudo-blazed shape. After forming, an isotropic material layer 45 filled with an isotropic material as a filler can be formed between the transparent substrate 42 and the transparent substrate 43 to be manufactured. Further, as a method of forming the birefringent material layer 44, it is also possible to produce the birefringent material layer 44 by a method of forming a structural birefringence or a photonic crystal in a lattice shape.

  With respect to two lights branched by the non-polarization diffraction grating 20 that transmits the first region 31 and the second region 32 of the wave plate 30, the convex shape of the birefringent material layer 44 in the polarization diffraction grating 40 You may form so that a grating | lattice pattern (grating pitch and a grating | lattice longitudinal direction) may differ. Here, the light receiving surface of the photodetector 60 is arranged at a position where four branched lights generated according to the grating patterns of the non-polarization diffraction grating 20 and the polarization diffraction grating 40 are condensed.

  If the traveling directions of the polarized light beams that are orthogonal to each other are made different from each other in the incident light, an element having a polarization separation function is arranged in place of the polarizing diffraction grating 40 at the position of the polarizing diffraction grating 40. May be.

  As an example of such a polarization separation functional element, a polarization separation functional element 90a using a birefringent material walk-off is shown in FIG. In FIG. 6, the X ′ axis is a direction orthogonal to the 40a direction in FIG. 3, and the Y ′ axis is the same direction as 40a. The polarization separation functional element 90a uses a birefringent material 91, and its optical axis is oriented from the Y ′ axis in a direction that forms a predetermined angle in the Y′Z plane. When light is incident on such a birefringent material 91, a so-called walk-off occurs in which the pointing vector direction differs between light in the X ′ polarization direction and light in the Y ′ polarization direction. When the walk-off occurs, the light in the X ′ polarization direction and the light in the Y ′ polarization direction can be separated.

  Further, as another example of such a polarization separation functional element, FIG. 7 shows a polarization separation functional element 90b in which the thickness of the birefringent material is inclined. In FIG. 7, the X ′ axis is a direction orthogonal to the 40a direction in FIG. 3, and the Y ′ axis is the same direction as 40a. The polarization separation functional element 90b uses a birefringent material 92, and its optical axis is in the X′-axis direction. The birefringent material 92 is inclined in the Y′-axis direction. When light is incident on such a birefringent material 92, the refractive angle changes due to the difference in the refractive index in the X ′ polarization direction and the refractive index in the Y ′ polarization direction at the output-side interface of the birefringent material 92. The light in the X ′ polarization direction and the light in the Y ′ polarization direction are emitted in different directions. In this way, light in the X ′ polarization direction and light in the Y ′ polarization direction can be separated.

  As another example of such a polarization separation functional element, FIG. 8 shows a polarization separation functional element 90c in which the thickness of the birefringent material is inclined. In FIG. 8, the X ′ axis is a direction orthogonal to the 40a direction in FIG. 3, and the Y ′ axis is the same direction as 40a. The polarization separation functional element 90c uses a birefringent material 93, and its optical axis faces the X′-axis direction. The birefringent material 93 is inclined in the Y′-axis direction, and the polarization separation functional element 90 c is flattened by the isotropic material 94. When light is incident on such a polarization separation functional element 90c, the refractive angle in the X ′ polarization direction is different from the refractive index in the Y ′ polarization direction at the interface between the birefringent material 93 and the isotropic material 94. Changes, and light in the X ′ polarization direction and light in the Y ′ polarization direction are emitted in different directions. In this way, light in the X ′ polarization direction and light in the Y ′ polarization direction can be separated.

  FIG. 8 shows an example in which the refractive indexes in the optical axis direction of the isotropic material 94 and the birefringent material 93 coincide with each other. However, the optical axis of the isotropic material 94 and the birefringent material 93 is not limited to this case. Even when the refractive indexes of the directions do not match, it is possible to refract light in two polarization directions in different directions at the interface between the isotropic material 94 and the birefringent material 93. Further, in FIG. 8, the shape of a single saw blade-shaped birefringent material is used, but the shape is not limited to this, and the shape of a birefringent material having a plurality of saw blade-shaped irregularities may be used. By doing so, the thickness of the birefringent material can be reduced.

  As the birefringent materials 91, 92, 93, crystals having birefringence may be used. An organic material may be used, and a film material or a liquid crystal material can be used. When a liquid crystal material is used, it is preferable to use a polymer liquid crystal because temperature characteristics are stabilized. In the case where a liquid crystal material is used, the liquid crystal may be sealed in a cell made of a material such as glass. In such a case, the thickness of the liquid crystal material and the optical axis may be as shown in FIGS.

(Dependence on incident angle of polarization phase separation element)
Next, the incident angle dependency of the polarization phase separation element 10 in the present embodiment will be described. FIG. 9 shows the phase difference between p-polarized light and s-polarized light generated in the second region 32 of the wave plate 30 of the polarization phase separation element 10 according to the present embodiment, and the conventional polarization phase conversion separation element, that is, Patent Document 1. It is the result of having calculated the incident angle dependence about the phase difference of the p polarized light and s polarized light which arise in the angle selective polarization conversion element used as the polarization | polarized-light phase conversion separation element described. The calculation method uses a 4 × 4 matrix method, and the incident angle indicates an angle assuming incidence from the air.

  For a conventional angle selective polarization conversion element, the calculation was performed on a crystal whose optical axis coincided with the optical axis, and the ordinary light refractive index in the crystal was 1.557, the extraordinary light refractive index was 1.567, The thickness was 0.86 mm so that the phase difference generated at an incident angle of 10 ° was 90 °. On the other hand, with respect to the wave plate 30 in the present embodiment, the calculation of the incident angle dependence is performed for the case where a horizontally aligned polymer liquid crystal film is formed in the second region 32. The normal light refractive index in the liquid crystal was 1.510, the extraordinary light refractive index was 1.552, and the thickness was 2.4 μm.

  As shown in FIG. 9, in the wave plate 30 of the polarization phase separation element 10 in the present embodiment, even if the incident angle changes, the phase difference generated between the s-polarized light and the p-polarized light is In contrast to the angle selective polarization conversion element used in the conventional polarization phase conversion separation element, when the incident angle changes, the phase difference of the emitted light also changes.

  Such a difference in dependence on the incident angle depends on the direction of the optical axis of the birefringent material. When the optical axis of the wave plate is in the direction that coincides with the optical axis, the angle changes by changing the angle and changing the shape of the cross section of the refractive index ellipsoid. Produce. On the other hand, the optical axis of the wave plate in the present embodiment is oriented in the wave plate surface, and in particular, when the vector of the incident direction of incident light is projected into the wave plate surface, the projected vector Is set to be a direction orthogonal to the optical axis. By arranging in this way, the cross-sectional shape of the refractive index ellipsoid does not change even when the incident angle changes, so that it is possible to obtain good angular dependence of the phase difference caused by the wave plate.

  As described above, the wave plate 30 used in the present embodiment has extremely low dependency on the incident angle of light compared to the conventional angle-selective polarization conversion element, has a large degree of freedom in optical design, and is optical due to manufacturing variations. The characteristics do not change significantly.

  Next, the birefringent material layer of the polarizing diffraction grating in the polarization phase separation element is set so that the traveling direction of the diffracted light of the non-polarizing diffraction grating 20 and the traveling direction of the diffracted light of the polarizing diffraction grating 40 are not the same direction. For the polarization phase separation element formed so that the longitudinal direction of the convex portion made of 44 forms an angle with the X-axis direction that is the grating longitudinal direction of the non-polarization diffraction grating 20, the polarization phase separation element 11 shown in FIG. This will be described with reference to the configuration diagram.

  The difference between the polarization phase separation element 10 and the polarization phase separation element 11 is only in the longitudinal direction of the convex portion made of the birefringent material layer 44 of the polarizing diffraction grating 41, and the other configurations are the same. As a result, the incident light 50 is diffracted by the non-polarization diffraction grating 20, travels as two lights 51a and 51b, and is transmitted through a polarized light component 53a and 51a and 51b that is not diffracted by the polarizing diffraction grating 41 and passes straight. With respect to 53c, the optical axes of the light components 53b and 53d, which are the + 1st order diffracted light components of the polarization component diffracted by the polarizing diffraction grating 41, do not exist in the plane defined by the optical axes of the branched light beams 53a and 53c. .

  Except for the case where it is fabricated in an ideal blazed shape, the incident light of the diffractive element generates diffracted light at diffraction angles corresponding to a plurality of diffraction orders. The non-polarized diffraction grating 20 generates diffracted light other than ± first-order diffracted light for signal detection purposes. In the polarizing diffraction grating 41, the light for signal detection is + 1st order diffracted light generated by linearly polarized light parallel to the optical axis 40a of the birefringent material layer 44, and 0th order diffracted light of linearly polarized light orthogonal to the optical axis 40a. However, diffracted lights other than the + 1st order diffracted light and the 0th order diffracted light are generated due to manufacturing variations. When such unnecessary diffracted light is incident on the light receiving surface of the photodetector, the S / N ratio of the signal is lowered. In particular, a large number of multiple transmitted / diffracted light beams are generated, that is, transmitted / diffracted light of the non-polarized diffraction grating and transmitted / diffracted light of the polarizing diffraction grating, and therefore, stray light is easily generated.

  However, in the polarization phase separation element 11, since the direction of the transmitted / diffracted light of the non-polarized diffraction grating 20 and the direction of the transmitted / diffracted light of the polarizing diffraction grating 41 are different, the light 53a that is four branched signal lights, The generated stray light is not mixed in the light receiving surfaces of the photodetectors 60a, 60b, 60c, and 60d arranged at the condensing positions of 53b, 53c, and 53d, respectively. As a result, optical signal information is converted to electrical signal information while maintaining a high S / N ratio, and highly accurate intensity modulation information and phase modulation information can be detected. The polarization phase separation element 11 can be used in place of the polarization phase separation element 10 in FIGS.

[Second Embodiment]
Next, an optical receiver in the second embodiment will be described. FIG. 11 is a configuration diagram of the optical receiver in this embodiment. In the optical receiver 200 according to the present embodiment, the optical receiver 100a and the optical receiver 100b have substantially the same configuration as the optical receiver 100 according to the first embodiment shown in FIG.

  The signal light S including the optical information subjected to intensity modulation and phase modulation is emitted as divergent light 252 from the optical transmission unit 271 such as an optical fiber, and becomes parallel light by the condenser lens 283. The polarization state of the signal light S changes during optical fiber transmission. In FIG. 11, the p-polarized component that is linearly polarized in the Y-axis direction and the s-polarized component that is linearly polarized in the X-axis direction have a phase difference. It becomes the elliptically polarized light.

  The p-polarized component Sb of the signal light S passes through the PBS 293 and enters the light receiving unit 100b that is a homodyne detection unit. On the other hand, the s-polarized component Sa of the signal light S is reflected by the PBS 293 and the PBS 292 and then enters the optical receiving unit 100a which is a homodyne detection unit.

  The reference light R having the same wavelength as that of the signal light S and linearly polarized light is generated by using a laser light source 272 that is a local oscillation light source having a long coherence length and a narrow oscillation wavelength width, and the X axis in the XY plane of FIG. The light is emitted as linearly polarized divergent light 251 inclined by 45 ° with respect to the direction. The reference light R converted into parallel light by the condenser lens 281 has a polarization component in the X-axis and Y-axis directions having the same amplitude ratio, and the p-polarization component Ra that is linearly polarized light in the Y-axis direction passes through the PBS 291 and the PBS 292 Then, the light is incident on the optical receiver 100a which is a homodyne detector. On the other hand, the s-polarized component Rb, which is linearly polarized light in the X-axis direction, is totally reflected by the PBS 291 and the total reflection prism 294 that switches the optical path of 180 °, is reflected by the PBS 293, and then enters the light receiving unit 100b that is a homodyne detection unit. To do.

  The light incident on the light receiving unit 100a is detected by a photodetector 261a having a plurality of light receiving surfaces, and the light incident on the light receiving unit 100b is detected by a light detector 261b having a plurality of light receiving surfaces. .

  Compared to the first embodiment shown in FIG. 2, a laser light source 272 that generates reference light necessary for homodyne detection is provided in the optical receiver, and the number of components is small with only signal light input from the outside. A lightweight 90 ° optical hybrid phase diversity homodyne optical receiver can be realized. In addition, even if the polarization state of the signal light changes during transmission and the signal light intensity detected by the photodetectors 261a and 261b changes, there is no fluctuation in the combined signal intensity, so polarization diversity detection is performed. . The contents other than the above are the same as in the first embodiment.

Example 1
As an example, the optical receiver in the first embodiment will be described. The optical receiver in the present embodiment is an optical receiver 100 serving as a 90 ° optical hybrid phase diversity homodyne optical receiver in the C-band wavelength band in WDM optical communication as shown in FIG.

  The condensing lens 80, the polarization phase separation element 11, and the photodetector 60 having a wavelength of the signal light S and the reference light R of 1545 nm, an incident side and an output side NA of about 0.12, and an effective diameter of about 1 mm are arranged in this order. To do. The intervals between the condenser lens 80, the polarization phase separation element 11, and the photodetector 60 are about 0.5 mm and 4.5 mm, respectively. The polarization phase separation element 11 has the configuration and function shown in FIG.

  The non-polarized diffraction grating 20 is a diffraction grating obtained by performing periodic uneven processing on the plane of a quartz glass substrate having a thickness of 2 mm and having a longitudinal direction in the X-axis direction and a periodic direction in the Y-axis direction. Is processed to be 3 μm and the depth of the unevenness is about 1.76 μm. When the refractive index of quartz glass is 1.44, ± 1st order diffracted light is generated in the Y-axis direction with respect to light having a wavelength of 1545 nm, and its efficiency is about 40%, and the 0th order diffraction efficiency is about 0%. When the light beam of ± first-order diffracted light is traced, the + 1st-order diffracted light and the −1st-order diffracted light do not overlap spatially after passing through a quartz glass substrate having a thickness of 2 mm.

  The wave plate 30 has the same structure as the wave plate 30b of FIG. 4B, and has a structure in which a polymer liquid crystal is sandwiched between a pair of quartz glass substrates having a thickness of 0.3 mm. The pair of quartz glass substrates are subjected to a vertical alignment process for the first region 31b and the second region 32b, respectively, and a horizontal alignment process in which the X direction becomes the extraordinary light direction, and the planes subjected to the alignment process are opposed to each other. So that the gap is filled with liquid crystal. The extraordinary refractive index of the polymer liquid crystal is 1.65, the ordinary light refractive index is 1.53, and the thickness of the polymer liquid crystal is about 3.22 μm. At this time, in the first region 31b which is vertically aligned, no phase difference is generated with respect to the linearly polarized light in the X-axis direction and the Y-axis direction, and in the second region 32b, the linearly polarized light in the X-axis direction and the Y-axis direction On the other hand, a phase difference of about 180 ° is generated. Since the + 1st order diffracted light from the non-polarized diffraction grating is incident on the first region 31b and the -1st order diffracted light is incident on the second region 32b, the -1st order diffracted light is linearly polarized in the X-axis direction and the Y-axis direction. Is given a phase difference of about 180 °.

  The polarizing diffraction grating 40 is a 16-stage quasi-blazed polymer liquid crystal oriented in the 45-degree direction from the X-axis direction on the XY plane of a quartz glass substrate having a thickness of 0.3 mm. . Laminated with a 3 mm quartz glass substrate. The extraordinary light refractive index of the polymer liquid crystal is 1.65, the ordinary light refractive index is 1.53, and the refractive index of the filler is 1.53. The blaze shape has a longitudinal direction in the Y-axis direction and is periodically arranged with respect to the X-axis direction, the diffraction grating pitch is 10 μm, and the height of each stage of the blaze is about 0.8 μm. When the diffraction efficiency is calculated by the RCWA (Rigorous Coupled Wave Analysis) method, when linearly polarized light in the direction of ordinary light refractive index is incident, it is transmitted straight without being diffracted, and linearly polarized light in the direction of abnormal light refractive index is incident. In this case, + 1st order diffracted light is generated in the X-axis direction, and the efficiency is about 92%.

  The non-polarization diffraction grating 20, the wave plate 30, and the polarization diffraction grating 40 are integrated in this order from the light incident side, and the polarization phase separation element 11 having an element thickness of about 3.2 mm is manufactured.

  As described above, the incident light 50 incident on the polarization phase separation element 11 is branched into four light beams 53a, 53b, 53c, and 53d, and each light receiving surface of the photodetector 60 (60a, 60b, 60c, 60d). It is focused on. Since the other diffracted light of the non-polarized diffraction grating 20, the other diffracted light of the polarizing diffraction grating 40 and the multiple diffracted lights thereof are condensed at a position different from the light receiving surface of the photodetector 60, It is not stray light that leads to a decrease in the S / N ratio of light.

The optical signal intensities I ₄ , I ₅ , I ₆ and I ₇ converted into electric signals by the photodetector 60 are calculated using the method described in the embodiment, and I ₄ -I ₅ and I ₇ -I ₆ are calculated. Thus, Sig1 = I ₄ −I ₅ = AcosΔφ and Sig2 = I ₇ −I ₆ = AsinΔφ. As a result, the signal intensity of the signal light S is amplified A times by the reference light R, and the value of the phase difference Δφ of the signal light S with respect to the reference light R can be detected. That is, in a 90 ° optical hybrid phase diversity homodyne optical receiver that is effective in digital coherent optical transmission, an optical receiver that can detect light intensity information and phase information with a high S / N ratio and is easy to manufacture. Become.

  As the polarization phase separation element used in the optical receiver according to the present embodiment, the configuration example in which the non-polarization branch portion is made of a non-polarization diffraction grating and the polarization branch portion is made of a polarization diffraction grating has been described. An element may be used. For example, instead of a non-polarized diffraction grating, a triangular prism having a section of isosceles triangle divided into two spaces on the incident surface may be used, and incident light may be branched in two directions using refraction of prism slopes having different incident angles. Good. Further, the incident orthogonal polarization may be branched using a birefringent crystal such as a Wollaston prism or a Glan-Thompson prism instead of the polarizing diffraction grating.

(Example 2)
The optical receiver according to the second embodiment is obtained by changing only the non-polarized diffraction grating 20 according to the first embodiment, and the other configurations are the same as those of the first embodiment.

  The non-polarized diffraction grating 20 is a diffraction grating obtained by performing periodic uneven processing on the plane of a quartz glass substrate having a thickness of 0.5 mm and having a longitudinal direction in the X-axis direction and a periodic direction in the Y-axis direction. Is processed so that the pitch of 1.9 is 1.9 μm and the depth of the unevenness is about 1.76 μm. When the refractive index of quartz glass is 1.44, ± 1st order diffracted light is generated in the Y-axis direction with respect to light having a wavelength of 1545 nm, and its efficiency is about 40%, and the 0th order diffraction efficiency is about 0%. The non-polarized diffraction grating 20 and the wave plate 30 are installed so as to have an interval of 1.5 mm so as to be air. In this way, the average refractive index of the diffraction part of the non-polarization diffraction grating 20 and the wave plate part of the wave plate 30 is (0.5 × 1.44 + 1.5 × 1 + 0.3 × 1.44) / 2. .3 = 1.15. When the light beam of ± first-order diffracted light is traced, the + 1st-order diffracted light and the −1st-order diffracted light do not overlap spatially after passing through a quartz glass substrate having a thickness of 2 mm.

  As mentioned above, although the form which concerns on implementation of this invention was demonstrated, the said content does not limit the content of invention.

10, 10a, 10b, 11 Polarization phase separation element 20 Non-polarization diffraction grating 30, 30a, 30b, 30c Wave plate 31, 31a, 31b, 31c First region 32, 32a, 32b, 32c Second region 33a Material 33b Polymer liquid crystal (vertical alignment)
34a, 34b Polymer liquid crystals 40, 41 Polarizing diffraction grating 40a Optical axes 42, 43 Transparent substrate 44 Birefringent material layer 45 Isotropic material layer 50 Incident light 51a, 51b, 52a, 52b, 53a, 53b, 53c, 53d Light 60, 60a, 60b, 60c, 60d, 261a, 261b Photodetector 70, 271 Optical transmission unit 80, 281, 282, 283, 284 Condensing lenses 90a, 90b, 90c Polarization separation functional elements 91, 92, 93 Birefringent material 94 Isotropic material 100, 200 Optical receiver 251 252 Diverging light 272 Laser light source 291 292 293 Polarized beam splitter (PBS)
294 Total reflection prism S, Sa, Sb Signal light R, Ra, Rb Reference light

Claims (9)

A non-polarization branching unit that branches incident light including s-polarized light that is linearly polarized light and p-polarized light orthogonal to the s-polarized light in two traveling directions;
Of the two lights branched by the non-polarization branching section, no phase difference is given to one light, and only the other light is between the s-polarized light and the p-polarized light. A wave plate having a birefringent material with optical axes aligned in the thickness direction so as to give a phase difference substantially equal to an odd multiple of 90 °,
Two light beams including light having a phase difference substantially equal to an odd multiple of 90 ° are incident and are orthogonal to each other, in each of the first linearly polarized light and the second linearly polarized light, in two traveling directions. A polarizing branch that branches;
Four photodetectors for receiving each of the light branched into four by the non-polarization branching unit and the polarizing branching unit;
Equipped with a,
The non-polarization branching unit is a non-polarization diffraction grating, and is a light receiving device characterized in that the + 1st order diffracted light and the −1st order diffracted light of the incident light are branched into two traveling directions .   The incident light includes signal light and reference light having the same wavelength, and the incident light is branched by a polarization phase separation element formed by the non-polarization branching unit, the wave plate, and the polarizing branching unit. The optical receiver according to claim 1, wherein A polarization beam splitter that combines s-polarized light and p-polarized light that are orthogonal to each other, and combines the signal light and the reference light incident from different directions by the polarization beam splitter, to the polarization phase separation element. The optical receiver according to claim 2 , wherein incident light is generated. 2. The branching direction of the two lights branched by the non-polarization branching part and the branching direction of the light branched by the polarizing branching part are configured to form a predetermined angle. 4. The optical receiver according to any one of items 1 to 3 .   The polarizing branch is a polarizing diffraction grating, which transmits the ordinary light polarization component of incident light without diffracting and diffracts the extraordinary light polarization component, or transmits the extraordinary light polarization component of incident light without diffracting it. The optical receiver according to any one of claims 1 to 4, wherein the optically polarized component is diffracted together. The polarizing diffraction grating, aligned in the thickness direction is the optical axis, the ordinary refractive index n _o and the birefringent material layer made of a birefringent material having an extraordinary refractive index _{n e (n o ≠ n e} ) and, periodic unevenness is formed by isotropic material layer having a refractive index consisting of isotropic material as a n _s, a refractive index n _s is substantially equal to the ordinary refractive index n _o or the extraordinary refractive index n _e 6. The optical receiver according to claim 5, wherein an optical axis direction of the birefringent material layer and an optical axis direction of the wave plate form an angle of about 45 degrees.   7. The optical receiver according to claim 5, wherein the longitudinal direction of the non-polarized diffraction grating and the longitudinal direction of the polarizing diffraction grating form a predetermined angle.   The wave plate is 90 ° between the first region that does not give a phase difference between the s-polarized light and the p-polarized light, and between the s-polarized light and the p-polarized light. The optical receiver according to claim 1, further comprising: a second region that gives a phase difference that is substantially equal to an odd multiple of, and a polymer liquid crystal only in the second region. .   The wave plate is 90 ° between the first region that does not give a phase difference between the s-polarized light and the p-polarized light, and between the s-polarized light and the p-polarized light. A second region that gives a phase difference substantially equal to an odd multiple of the first region, the first region and the second region both have a polymer liquid crystal, and the polymer liquid crystal in the first region is The optical receiver according to any one of claims 1 to 7, wherein the optical receiver is oriented substantially parallel to the thickness direction.

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