JP2006113464A - Demultiplexer and multi-wavelength optical transmission module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical wavelength multiplexer/demultiplexer and an optical transmission module that have a high diffraction efficiency in a desired wavelength range and that can reduce change in characteristic caused by polarization. <P>SOLUTION: The demultiplexer 10 is equipped with a light incident part 12 on which a signal light L1 is made incident, a diffraction grating 13 which diffracts the signal light L1 made incident from the light incident part 12, and a light receiving part 14 which has a plurality of photodetectors for detecting diffracted signal light L2. In this demultiplexer 10, in a prescribed wavelength range, the difference of diffraction efficiency between the TE polarization and TM polarization of the diffraction grating 13 is made smaller on the short wavelength side and larger on the long wavelength side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical device that demultiplexes a wavelength-multiplexed optical signal for each wavelength or combines optical signals of different wavelengths, and particularly relates to a demultiplexer and a wavelength-multiplexed optical transmission module that separate wavelengths using a diffraction grating.

  In the field of optical communication, in order to transmit high-speed and large-capacity information, wavelength division multiplexing optical communication is performed in which optical signals of a plurality of wavelengths are multiplexed and transmitted on a single optical fiber. One of the important devices used for wavelength division multiplexing communication is an optical demultiplexer. The optical demultiplexer demultiplexes light in which a plurality of wavelengths are multiplexed into light of a narrow band wavelength that can be regarded as a substantially single wavelength.

  For example, in an optical demultiplexer including an optical fiber and a diffraction grating, the diffraction efficiency of the polarization component (TM polarization) perpendicular to the diffraction grating groove in the incident light and the polarization component (TE polarization) in the parallel direction are Since they are different, the light intensity of the demultiplexed light changes under the influence of the polarization state of the incident light that changes due to disturbance or the like. In order to stabilize the light intensity of the demultiplexed light, the diffraction grating from the optical fiber is used by using a diffraction grating having a blaze wavelength such that the diffraction efficiencies of the TE polarized light and the TM polarized light are equal to the wavelength of the incident light. There is an optical demultiplexer that prevents the influence of the polarization state of the light incident on (see, for example, Patent Document 1).

JP 2001-201654 A

  However, the above-described optical demultiplexer also has a problem that polarization dependency occurs when the wavelength of the incident light is other than the wavelength at which the diffraction efficiency between the TE and TM polarized light is equal. In particular, on the shorter wavelength side than the wavelength at which the diffraction efficiency of TE-polarized light and TM-polarized light are equal, so-called anomaly in which the diffraction efficiency of TM-polarized light is greatly reduced is likely to occur, and large polarization dependency is likely to occur. Therefore, the diffraction efficiency in the optical demultiplexer is lowered due to the polarization dependent loss in the diffraction grating, and there is a problem that the wavelength demultiplexing performance of the optical demultiplexer is limited.

  In addition, the conventional optical spectrometer uses a uniform diffraction grating having a constant lattice constant, and the incident light from the optical fiber is converted into parallel light by a collimator lens. Therefore, the incident angle of incident light to the diffraction grating is constant.

  As the diffraction grating, there is a chirped diffraction grating having a different lattice constant in order to give the diffraction grating itself a condensing function. In the chirped diffraction grating, since the incident angle, the emission angle, and the grating constant of the light beam change depending on the position of the optical demultiplexer, the wavelength dependency and polarization dependency of the diffraction efficiency differ depending on the location. For this reason, the chirped diffraction grating has a problem that it is difficult to control the variation in polarization dependence accompanying the change in wavelength.

  Accordingly, an object of the present invention is to provide a demultiplexer and a wavelength division multiplexing optical transmission module which solves the above-described problems and has high diffraction efficiency and reduced characteristic variation due to polarization in a desired wavelength range.

  Another object of the present invention is to provide a demultiplexer and a wavelength division multiplexing optical transmission module in which fluctuations in diffraction efficiency due to polarization are reduced even in a chirped diffraction grating.

  In order to achieve the above object, the invention of claim 1 is directed to a light incident part for receiving signal light, a diffraction grating for diffracting signal light incident from the light incident part, and a photodetector for detecting diffracted signal light. In a demultiplexer including a plurality of light receiving units, a difference in diffraction efficiency between TE polarized light and TM polarized light of the diffraction grating is small on the short wavelength side and large on the long wavelength side in a predetermined wavelength range.

  According to a second aspect of the present invention, a light incident portion that receives signal light, a diffraction grating that diffracts the signal light incident from the light incident portion, and a light receiving portion that includes a plurality of photodetectors that detect the diffracted signal light are provided. In the demultiplexer, the maximum value of the loss in the TM polarization is smaller than the maximum value of the loss in the TE polarization of the diffraction grating.

  The invention of claim 3 is the demultiplexer according to claim 2, wherein the loss of the TM polarization at the shortest wavelength is larger than the loss of the TE polarization of the longest wavelength of the diffraction grating.

  The invention of claim 4 is the demultiplexer according to claim 2, wherein the maximum value of the TM polarization loss of the diffraction grating is smaller than the maximum value of the TE polarization loss and larger than the minimum value of the TE polarization loss.

  According to a fifth aspect of the present invention, a light incident portion that receives signal light, a diffraction grating that diffracts the signal light incident from the light incident portion, and a light receiving portion that includes a plurality of photodetectors that detect the diffracted signal light are provided. The demultiplexer is a demultiplexer in which a light filter is provided with a polarizing filter for reducing TM polarization of signal light incident on the diffraction grating.

  The invention of claim 6 is the demultiplexer according to any one of claims 1 to 5, wherein the diffraction grating is a chirped diffraction grating having a different grating period.

  According to a seventh aspect of the present invention, the diffraction grating is an Eschlet type diffraction grating, the diffraction grating is divided into a plurality of regions along the groove, and the blaze angle is different for each region. Demultiplexer.

  The invention of claim 8 is the demultiplexer according to claim 7, wherein the number of regions of the diffraction grating is 4-9.

  The invention according to claim 9 is the demultiplexer according to claim 5 or 6, wherein the diffraction grating is an Eschlet type diffraction grating, and the blaze angle continuously changes.

  In the invention of claim 10, on the short wavelength side of the predetermined wavelength range, the blaze angle of the diffraction grating is such that the direction in which the signal light incident on the diffraction grating is regularly reflected on the grating surface and the diffraction direction of the signal light are substantially the same. The demultiplexer according to any one of claims 7 to 9, wherein the demultiplexer is larger than a specular reflection blaze angle that is an angle to be transmitted and smaller than an angle at which the diffraction efficiency of TE polarized light is maximized.

  In the eleventh aspect of the invention, the diffraction grating has a wavelength of signal light in a predetermined wavelength range λ, a grating constant d, an incident angle of the signal light to the diffraction grating i, and a diffraction angle of the diffraction grating of the signal light. When θ is

かつ、

And,

It is a demultiplexer in any one of Claims 5-10 which satisfy | fills.

  The invention of claim 12 is provided with the demultiplexer according to any one of claims 1 to 11, and forms a joint portion connected to the light receiving portion and an external optical transmission line, and the light receiving portion of the demultiplexer is connected to the light receiving portion from This is a wavelength division multiplexing optical transmission module to which a receiving circuit for amplifying and shaping a signal is connected.

  According to a thirteenth aspect of the present invention, there are provided a plurality of light sources, a transmission circuit connected to these light sources for driving and controlling the light sources, and wavelength multiplexing connected to the plurality of light sources to multiplex signal light from the plurality of light sources. 13. The wavelength division multiplexing optical transmission module according to claim 12, further comprising a device.

  The invention according to claim 14 is the wavelength multiplexed light according to claim 13, wherein the junction and the demultiplexer are connected by a multimode optical transmission line, and the junction and the wavelength multiplexing device are connected by a single mode optical transmission line. It is a transmission module.

  According to the present invention, excellent effects such as high diffraction efficiency and reduced polarization-dependent loss can be achieved in a desired wavelength range.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a top view showing a preferred embodiment of a demultiplexer according to the present invention, and FIG. 2 is a side view of FIG.

  As shown in FIGS. 1 and 2, the demultiplexer 10 includes a light incident portion 12, a diffraction grating 13, and a light receiving portion 14 that are fixedly provided on a base material (base) 11.

  The light incident part 12 is a member that introduces the signal light L1 into the demultiplexer 10 and enters a signal toward the diffraction grating 13, and includes a light transmission path connected to a light source and an external optical element. Here, the optical transmission line is a means for propagating signal light, and indicates an optical fiber, an optical waveguide, or the like. In this embodiment, the incident optical fiber 15 is a fiber on the substrate 11 as the optical transmission line. It is bonded and fixed via a holding table 16.

  The incident optical fiber 15 was a graded index (GI) type multimode optical fiber (MMF) having a core diameter of 62.5 μm and a numerical aperture of 0.275. The multimode optical fiber has a large numerical aperture and a large divergence angle of the signal light L1. In the optical fiber 15 of the present embodiment, the divergence angle obtained from the numerical aperture is ± 16 °.

  A cylindrical lens 18 and a polarizing filter 19 are fixedly provided on the optical system fixing base 21 on the emission end 17 side of the optical fiber 15. The cylindrical lens 18 is a lens that converts the signal light L1 emitted from the optical fiber 15 into a parallel beam, and a cylindrical lens is used.

  The polarizing filter 19 limits the amount of light transmitted through one of the orthogonally polarized light (p-polarized light and s-polarized light) components of the signal light L1. The polarizing filter 19 is formed by forming a dielectric multilayer film 22 as a filter surface in a prism 23 and applying an antireflection coating to the incident / exit surfaces 24 a and 24 b of the prism 23.

  Here, FIG. 3 shows the wavelength characteristics of the filter 19 of the present embodiment. The filter 19 transmits almost 100% of p-polarized light (TE-polarized light) and transmits almost 100% of s-polarized light (TM-polarized light) on the short wavelength side of the used wavelength within the usable wavelength range of the signal light. The transmittance decreases toward the wavelength side.

  The diffraction grating 13 is an echelette in which straight grating grooves are formed in the direction of the arrow shown in FIG. 2, the grating interval is different depending on the position, and the grating groove has a sawtooth shape in which a plurality of right-angled triangles are continuous. This is a plane diffraction grating of a mold.

  The light receiving unit 14 includes a photodetector array 26, a light receiving optical waveguide 27, and a cylindrical lens 28, and the height of each member is adjusted via a holding base and is fixed to the base material 11.

  The photo detector array 26 is an array type photo detector provided with a plurality of photo detector elements and arranged in a row in order to detect signal light at different positions.

  The light receiving optical waveguide 27 is disposed in front of the photodetector array 26 (on the diffraction grating 13 side), and is connected to a slab waveguide portion 29 provided on the diffraction grating 13 side and a subsequent stage of the slab waveguide portion 29. , And a plurality of tapered waveguide portions 31 that are bonded and fixed to the photodetector array 26.

  The cross section of the slab waveguide portion 29 is a right triangle, and the plurality of tapered waveguide portions 31 gradually increase the waveguide width from the emission end (the hypotenuse of the right triangle) 32 of the slab waveguide portion 29. It is narrowed and formed to extend toward the photodetector array 26.

  The thickness (core thickness) of the light receiving optical waveguide 27 is determined in consideration of the light receiving diameter of the photodetector array 26 so that the signal light is efficiently coupled to the photodetector array 26.

  The tapered waveguide portion 31 has a tapered shape in which the width of the core decreases from the incident side (connection portion with the slab waveguide portion 29) 33 to the photodetector array 26 side, and has a structure for narrowing light to the photodetector. In order to efficiently propagate an optical signal to the photodetector array 26, it is desirable that the numerical aperture NA of the slab waveguide portion 29, particularly the numerical aperture of the side surface is large, and it is desirable to set NA to 0.5 or more. For this reason, the numerical aperture is increased so that the core is in direct contact with air without providing a clad on the side surface of the tapered waveguide portion 31.

  The width of the tapered waveguide portion 31 on the incident side is the same as the pitch of the light detection elements of the photodetector array 26 on the side joined to the slab waveguide portion 29, and the efficiency of the light detection elements on the photodetector array 26 side. The width is such that it can be combined well. In this way, by narrowing down the signal light L2 of each wavelength that has been demultiplexed to the light detection element, it is possible to use a light detection element with a fast response speed and a small light receiving area, and it can be applied to high-speed optical communication. .

  The cylindrical lens 28 is the same as the cylindrical lens 18 of the light incident portion 12, and is disposed in parallel to the incident end 32 of the slab waveguide portion 29.

  Now, the diffraction grating 13 of the demultiplexer 10 of the present embodiment will be described in detail.

  The diffraction grating 13 chirps the grating interval so as to reflect and diffract the signal light L1 that is spread from the optical fiber 15 and collects the diffracted signal light L2 on the light receiving unit 14. As shown in FIGS. 4 and 5, the diffraction grating 13 is chirped with a grating period (lattice constant) as a function of position so that the signal light L2 can be condensed at a desired position (graph 51). Here, the lateral direction x of the diffraction grating in FIG. 4 corresponds to the grating position x in FIG.

  The position of each grating groove is determined from the emission position of the signal light L1 and the light receiving position of the signal light L2, that is, the positional relationship between the light incident part 12 and the light receiving part 14. In addition to the chirping of the grating period of the diffraction grating 13, the blaze angle is also determined from the positional relationship between the light incident part 12 and the light receiving part 14 at each grating position in order to increase the diffraction efficiency.

  As shown in FIGS. 4 and 5, the Eschlet type diffraction grating 13 is divided into a plurality of regions 41a to 41f, the blaze angle is constant in the same region, and the blaze angle is different for each region (graph). 52).

  Specifically, the number of divisions (the number of regions) was set to 6, and the blaze angle was made the smallest in the region 41a on the left side in FIG. 4 and increased as the region on the right side in FIG.

  As in the graph 51, the lattice constant is continuously narrowed from the left side in the drawing in the regions 41a to 41d, and the spacing is continuously narrowed from the left side in the drawing also in the regions 41e to f. Further, the diffraction order to be used for each region is selected so as to increase the diffraction efficiency and reduce the polarization dependence. In the regions 41a to 41d on the left side of the diffraction grating, the diffraction order is the first order, and the right side in the diagram. In the regions 41e and 41f, the lattice constant is doubled discontinuously at the boundary between the regions 41d and 41e so that the second-order diffracted light is used.

  In general, when the grating constant is small, it is desirable to use high-order diffracted light, and it is desirable to use high-order diffracted light with a grating constant that is comparable to or less than the incident wavelength. Corresponding to the present embodiment, the lattice constant is increased in order to use higher-order diffracted light in the regions 41e and 41f where the lattice constant decreases when the chirp is made monotonously.

  In this embodiment, even in a plurality of regions having the same diffraction order of diffracted light, processing is performed by continuously chirping the grating period across the regions so that the diffracted beam is focused at a desired position. is doing. Furthermore, by switching the diffraction order at the blaze angle switching position, the grating constant can be switched without increasing the number of divisions of the diffraction grating.

  The blaze angle is the diffraction efficiency of TE-polarized light (polarized light whose electric field direction is parallel to the marking direction) and TM-polarized light (polarized light whose electric field direction is perpendicular to the marking direction) on the short wavelength side in the wavelength range of the signal light L1. It is determined for each region so that the difference from the efficiency is small. With TM polarized light, the diffraction efficiency is maximized when the blaze angle is a specular reflection blaze angle that satisfies the specular reflection condition. The regular reflection condition indicates when the direction in which the signal light L1 incident on the diffraction grating 13 is regularly reflected by the grating surface (blazed surface) 34 coincides with the diffraction direction of the signal light L2. In TE polarization, the diffraction efficiency is maximized at a blaze angle larger than the specular reflection blaze angle. The specular reflection blaze angle α is defined as an incident angle i (see FIG. 1), a grating constant d, a diffraction order m, and a refractive index n of the optical waveguide.

により求まる（グラフ５３）。

(Graph 53).

  Therefore, as shown in the graph 52 in FIG. 5, near the center of each region, the blaze angle is determined to be larger than the specular reflection blaze angle α and smaller than the blaze angle at which TE polarization becomes maximum. Thereby, the maximum value of the diffraction efficiency of TE-polarized light approaches the maximum value of the diffraction efficiency of TM-polarized light, and the difference in diffraction efficiency between the polarized lights becomes small, so that the polarization-dependent loss due to the diffraction grating 13 can be reduced.

  As the number of divisions of the regions 41a to 41f increases, the blaze angle distribution can be determined more finely with respect to the wavelength of the signal light, and the polarization dependent loss can be further reduced.

  The number of divisions is preferably 4-9, more preferably 6-9. This is because if the number of divisions of the region is 4 or more, the loss is reduced, and if it is 6 or more, the loss is substantially constant. Even if the number of divisions is increased, the loss is not significantly reduced when the number of divisions exceeds a certain number. Moreover, if the number of divisions is increased too much, the number of times that the blaze angle is changed during the production of the diffraction grating is increased, and the processing accuracy of the connecting portion between the regions may be reduced, and the loss may be reduced. In the present embodiment, since the grooves of the diffraction grating are processed on a flat surface, positional displacement in the plane direction and the vertical direction at the connecting portions of the regions hardly occurs in the processing of the diffraction grating grooves, and the processing can be performed with high accuracy. it can. Even so, if the number of divisions of the area is greater than 9, errors at the joints accumulate and the condensing characteristics deteriorate significantly.

  In the present embodiment, the diffraction grating 13 is formed by using a die that is mechanically formed with a groove using a ruling engine. The lattice grooves were processed while changing the lattice constant at a constant blaze angle for each region. That is, the diffraction grating 13 is a replica produced using this mold as a master. Depending on the processing method, there may be a shift in the connecting part of the region, and the grating constant of the chirped diffraction grating may be slightly discontinuous. However, if the desired light collection performance can be obtained, it is considered to be substantially continuous. Can do.

  In the Eschlett type diffraction grating, when the wavelength is λ and the lattice constant is d, λ / d is

And the blaze angle is selected in the range of 25 ° to 35 °, the polarization dependency can be particularly reduced and the diffraction efficiency can be increased. At this time, the incident angle i and the outgoing angle θ (see FIG. 1) of the signal light L1 are

It is necessary to. Between the incident angle i and the outgoing angle θ,

There is a relationship. Where n _s is the refractive index of the diffraction grating.

  That is, the diffraction grating is formed so that the lattice constant satisfies the formulas (2) and (3) and the blaze angle is in the range of 25 to 35 ° in the vicinity of the principal ray having the strong light intensity of the signal light L1. When arranged, the loss due to diffraction loss and polarization dependence can be further reduced.

  Next, the operation of the present embodiment will be described.

  When the signal light L1 transmitted to the incident optical fiber 15 as wavelength multiplexed light is spread and emitted from the emitting end 17, the spreading of the cylindrical lens 18 in the direction of the engraving direction of the diffraction grating 13 is reduced, and the light becomes substantially parallel light. , Passes through the filter 19 and enters the diffraction grating 13.

  In the filter 19, the filter surface 22 transmits almost 100% of the p-polarized light and reflects a part of the s-polarized light, so that the amount of s-polarized light is limited. Therefore, the amount of light L1 incident on the diffraction grating 13 is reduced by the filter 19 in the amount of polarized light (TM polarized light) perpendicular to the grooves of the diffraction grating 13.

  The signal light L1 is reflected and diffracted by the diffraction grating 13, and the diffracted signal light (emitted light beam) L2 is reflected with different diffraction angles for each wavelength. The signal light L <b> 2 is narrowed down by the cylindrical lens 28 and condensed on the incident end 32 of the slab waveguide portion 29.

  The light incident on the slab waveguide portion 29 propagates freely in the waveguide plane (in the cross section), and is split at different angles for each wavelength by the diffraction grating 13, and thus is provided at each position. The light is focused on the tapered waveguide portion 31. The signal light L2 enters the tapered waveguide portion 31 corresponding to each wavelength, and is detected by the light detection element of the photodetector array 26, respectively.

  In the demultiplexer 10 of the present embodiment, the diffraction grating 13 and the filter 19 are used to increase the diffraction efficiency and reduce the polarization dependent loss. Here, conditions for TM and TE polarization for reducing the polarization-dependent loss will be described.

  When polarized signal light is incident, the polarization incident on the diffraction grating changes, so the maximum loss condition (worst condition) is a condition that results in the greater loss of TE-polarized light and TM-polarized light. It is desirable that the loss under the maximum loss condition is low.

  TM polarized light has higher peak diffraction efficiency than TE polarized light, but has a large wavelength dependency. On the other hand, TE-polarized light has a small wavelength dependency in peak diffraction efficiency. Further, the wavelength at which the diffraction efficiency of TM polarized light reaches a peak is on the longer wavelength side than the wavelength at which the diffraction efficiency of TE polarized light reaches a peak. Therefore, in order to reduce the loss under the maximum loss condition, it is preferable to make the maximum value of the TM polarized light smaller than the maximum value of the TE polarized light loss in the wavelength range of the signal light used in the demultiplexer 10. In order to obtain such signal light characteristics, the grating constant and blaze shape of the diffraction grating are optimized and determined by numerical calculation or the like.

  Furthermore, it is desirable that the maximum value of the TM polarized light loss is larger than the minimum value of the TE polarized light loss. Here, the loss of the diffraction grating may be the loss of the entire diffraction grating in the assumed incident pattern from the optical fiber when the loss varies depending on the chirped-type equal grating position.

  In the wavelength band used for optical communication, as the wavelength becomes longer, the loss of TE polarization tends to increase and the loss of TM polarization tends to decrease. Therefore, the maximum value of TM polarization is made smaller than the maximum value of loss of TE polarization by making the loss of TM polarization at the shortest wavelength smaller than the loss of TE polarization at the longest wavelength in a predetermined wavelength range. That's fine.

  Furthermore, it is more desirable to increase the loss of the TM polarized light at the shortest wavelength than the loss of the TE polarized light at the shortest wavelength, because the polarization dependency is reduced.

  In addition, the polarization dependent loss can be further reduced by using the filter 19 that reduces the amount of TM polarized light as in the present embodiment.

  FIG. 6 is a diagram showing the wavelength loss characteristics of both TE and TM polarized light of the demultiplexer 10 of this embodiment, and is compared with the wavelength loss characteristics of a demultiplexer not provided with a filter.

  As shown in FIG. 6, the characteristic of the TE polarization component (graph 61) is that the loss does not depend on the wavelength regardless of the presence or absence of the filter 19, and a substantially constant light amount is detected in any wavelength band. On the other hand, the TM polarized light is as shown in the graph 62 when the incident light L1 is not transmitted through the filter 19, and the loss on the long wavelength side is small. Therefore, on the long wavelength side, a loss difference occurs between the TE polarized light and the TM polarized light, and the polarization dependent loss is increased.

  On the other hand, the loss of TM polarized light through which the signal light L1 has passed through the filter 19 is substantially the same as the loss of TE polarized light regardless of the wavelength (graph 63). In the diffraction grating 13, the loss is reduced on the long wavelength side of the TM polarized light, but the signal light L 1 is transmitted through the filter 19 and attenuated on the long wavelength side of the TM polarized component, so that the signal light can be obtained even at different wavelengths. The loss of L2 is made substantially equal. Therefore, by passing through the filter 19, it is possible to further reduce the polarization dependent loss of the signal light L2 demultiplexed by the demultiplexer 10.

  As described above, the diffraction grating 13 of the present embodiment can reduce the diffraction loss and also reduce the difference in loss between polarized light. Specifically, the diffraction loss is 1.5 dB or less, a diffraction efficiency of 70% or more can be obtained, and the polarization-dependent loss in the wavelength range of 85 nm can be reduced to 0.5 dB or less.

  In this embodiment, the grating constant can be increased by using high-order diffracted light, and the number of grating grooves can be reduced, so that the diffraction grating can be easily processed by the ruling engine. is there. However, if the diffraction order is increased in a region where the blaze angle is large, the wavelength dependence of the diffraction efficiency increases and the peak diffraction efficiency also decreases. Therefore, the diffraction order is preferably 4 or less.

  The present invention is not limited to the embodiment described above.

  In the present embodiment, the diffraction grating 13 is formed in the eschlet type and the chirp type, but is not limited to the eschlet type, and in the case of a diffraction grating having a sinusoidal cross section, a rectangular binary diffraction grating, or any other arbitrary The present invention can also be applied to a groove-shaped diffraction grating. The diffraction grating is not limited to a planar diffraction grating, but can be similarly applied to a concave diffraction grating. Further, not only the chirped diffraction grating but also the equally spaced diffraction grating can similarly realize a reduction in polarization dependence and a reduction in loss.

  The grating groove pattern of the diffraction grating 13 may be processed using a three-dimensional NC processing machine. Further, it may be formed on a semiconductor substrate such as Si by using a photolithography technique.

  Alternatively, an external transmission optical fiber that is inserted into and removed from the demultiplexer 10 without fixing the incident optical fiber 15 to the base 11 may be used. In that case, an insertion guide or the like may be used so that the optical fiber 15 fixed to the optical connector is abutted to a predetermined position even if the optical fiber 15 is inserted or removed. However, fixing the incident optical fiber 15 to the base 11 has an effect of excellent long-term stability of characteristics. When the incident optical fiber 15 is fixed, the transmission optical fiber may be abutted against the other end side of the incident optical fiber 15 and light may be incident on the demultiplexer 10. In this case, the numerical aperture and the diameter of the transmission optical fiber may be equal to or smaller than those of the incident optical fiber 15. Therefore, the transmission optical fiber may be connected to the same GI type multimode optical fiber as that of the incident optical fiber 15, such as a single mode optical fiber (SMF) having a small fiber diameter and numerical aperture, and a multimode having a core diameter of 50 μm. An optical fiber (50 MMF) can also be connected.

  When the light incident part 12, the diffraction grating 13, and the light receiving part 14 are arranged close to a Littrow arrangement in which the diffracted light is reflected in the substantially original direction with respect to the incident light, the number of cylindrical lenses is one, The signal lights L1 and L2 may be condensed.

  In the present embodiment, the signal light L1 propagates through the air and reaches the diffraction grating 13. However, a diffraction grating is formed on an optical member such as glass or plastic, and a cylindrical lens is integrated with the optical member. The signal light L1, L2 may be propagated using the optical member as an optical transmission path. In that case, the lattice constant and the blaze angle are preferably determined in accordance with the refractive index of the optical member.

  In this embodiment, the filter 19 is provided in the light incident part 12, but the same effect as the demultiplexer 10 can be obtained even if the filter 19 is provided in the light receiving part 14. The form of the filter 19 is not limited to the prism type, but may be a form formed on a flat substrate.

  Although the demultiplexer 10 has been described so far, a light source such as a laser diode array is provided instead of the photodetector array 26 of the light receiving unit 14 and is used as a multiplexer (optical multiplexer) for combining signal lights having different wavelengths. You can also. At this time, it is desirable to provide a tapered waveguide formed so as to narrow down the signal light at the incident end (for example, the emission end 17 in FIG. 1) of the optical fiber that emits the wavelength-multiplexed light.

  Next, a demultiplexer according to another embodiment will be described.

  The demultiplexer of the present embodiment has almost the same configuration as the demultiplexer 10 of FIG. 1 described above, and only the diffraction grating 13 is different.

  The diffraction grating 13 provided in the demultiplexer 10 of the previous embodiment has a constant blaze angle in the divided region, but as shown in FIG. 7, the diffraction grating provided in the demultiplexer of the present embodiment. In FIG. 5, in addition to changing the lattice constant substantially continuously (graph 71), the blaze angle is also changed continuously (graph 72). In the present embodiment, the position of the groove in the diffraction grating is a position to the left of the 1 mm point (regions 41a to c in FIG. 4), the diffraction order is the first order diffracted light, and the right side from the point of the position 1 mm (region 41d in FIG. 4). In f), the lattice constant was doubled discontinuously at the boundary so as to use the second-order diffracted light. The blaze angle becomes discontinuous so as to increase continuously from the right side in the drawing in each of the regions 41a to 41c and 41d to 41f and at a point of 1 mm (boundary between the region 41c and the region 41d). It is formed as follows.

  As in the previous embodiment, it is desirable that λ / d is in the range of 0.6 to 1.0 and the blaze angle is in the range of 25 ° to 35 °. In this case, the incident angle i and the outgoing angle θ of the signal light are ( 3) It is necessary to satisfy the equation.

  The diffraction grating of this embodiment was processed by changing the grating period and blaze angle for each grating groove using a three-dimensional NC processing machine.

  Also in the demultiplexer using the diffraction grating of the present embodiment, the diffraction loss and polarization dependent loss of the demultiplexed signal light can be reduced. Furthermore, the polarization dependent loss can be further reduced by providing a filter as in the previous embodiment.

  As shown in FIG. 8, the wavelength characteristic of the loss of the demultiplexer using the above-mentioned diffraction grating and not using the filter has almost no change in the loss due to the wavelength in the TE polarized light (graph 81). In TM polarized light, there is almost no difference in loss from TE polarized light on the short wavelength side of the used wavelength, but the loss is small on the long wavelength side and smaller than the loss of TE polarized light (graph 82).

  The loss wavelength characteristic of the demultiplexer of the present embodiment hardly changes the loss of TE polarization (graph 82). On the other hand, the loss of TM polarized light is small on the long wavelength side in the diffraction grating, but the incident light on the long wavelength side is reduced because the incident light is transmitted through the filter. This reduces the loss dependency due to (graph 83). Therefore, the loss due to the filter and the diffraction grating (demultiplexer loss) reduces the difference between the TE-polarized light and the TM-polarized light within the usable wavelength range, and can further reduce the polarization-dependent loss.

  Next, a wavelength division multiplexing optical transmission module including the demultiplexer 10 of FIG. 1 will be described.

  As shown in FIG. 9, the wavelength division multiplexing optical transmission module 90 includes a receiving unit 92 that demultiplexes and receives wavelength division multiplexed light from the outside, and a wavelength division multiplexing unit that multiplexes the signal light of each wavelength. A transmission unit 93 that transmits to the outside as light, and a joint unit 96 that connects the reception unit 92 and the transmission unit 93 to external transmission optical fibers 94 and 95, respectively.

  The receiving unit 92 includes the demultiplexer 10 illustrated in FIG. The incident optical fiber 15 of the demultiplexer 10 is connected to the transmission optical fiber 94 at the joint 96. The receiving circuit 97 is a circuit that performs amplification and waveform shaping of the detected electric signal, and is electrically connected to the photodetector array 26 of the demultiplexer 10. The receiving circuit 97 is connected to an external electronic device via a parallel-serial conversion circuit 98.

  The transmission unit 93 includes a wavelength multiplexing device 99, a light source, and a transmission circuit 101. The wavelength multiplexing device 99 has a plurality of incident ends 102 and one output end 103, and forms an optical coupler in which optical waveguides extending from the respective incident ends 102 are coupled to each other, and coupled to one optical waveguide. It is an optical waveguide device. An outgoing optical fiber 104 connected to the transmission optical fiber 95 is connected to the outgoing end 103 at the joint 96, and a light source is connected to the multiple incoming ends 102. The light source is optically connected to the incident end 102 of the optical multiplexing device 99 and electrically connected to the transmission circuit 101 for driving the light source. A laser diode array 105 in which a plurality of laser diodes are arranged in an array is used as a light source. The transmission circuit 101 is a circuit for driving and controlling the laser diode array 105, and is connected to an external electronic device via a serial-parallel conversion circuit 106.

  The joining part 96 joins the transmission optical fiber 94 for reception and the incident optical fiber 15, and joins the transmission optical fiber 95 for transmission and the outgoing optical fiber 104. In this embodiment, a detachable optical connector is provided as the joint portion 96, and in the optical connector, transmission optical fibers 94 and 95 for transmission and reception optical fibers 15 and 104, respectively. It is matched. The transmission optical fibers 94 and 95 were used as single mode optical fibers.

  The wavelength multiplexed optical signal propagated through the receiving transmission optical fiber 94 enters the incident optical fiber 15, is demultiplexed for each wavelength by the demultiplexer 10, and is detected by the photodetector array 26. The detected signal light is amplified and shaped by the receiving circuit 97. Next, the parallel-serial conversion circuit 98 converts parallel signals received in parallel into serial signals and sequentially outputs them.

  On the other hand, when transmitting signal light, the signal input to the serial-parallel conversion circuit 106 is separated into parallel signals, and each signal drives each laser diode of the diode array 105 for each electrical signal by the transmission circuit 101. , Signal lights having different wavelengths are emitted. Each signal light is wavelength-multiplexed by the wavelength multiplexing device 99, and the signal propagates through the outgoing optical fiber 104 and is emitted to the transmission optical fiber 95 for transmission.

  In the present embodiment, since the demultiplexer 10 having small polarization dependence and small loss wavelength dependence is used, even if linearly polarized light is incident from the transmission optical fiber 95 that is a single mode optical fiber, Variations in optical characteristics due to variations in direction can be reduced. As a result, the signal can be amplified and shaped in the receiving circuit 97 more precisely.

  Examples of the joining portion 96 include optical connectors that can be attached to and detached from each other, but the transmission optical fibers 94 and 95 and the incoming and outgoing optical fibers 15 and 104 are bonded or fused together so that they cannot be attached or detached. May be.

  Since the single-mode optical fiber and the multi-mode optical fiber are used as the input / output optical fibers 15 and 104 and are connected to the transmission optical fibers 94 and 95, respectively, the transmission optical fibers 94 and 95 include a multi-mode fiber. Either a single mode fiber or a single mode fiber may be used. In the present embodiment, a GI type multimode fiber having a core diameter of 62.5 μm is used as the incident optical fiber 15, and a single mode optical fiber having a core diameter smaller than that of the optical fiber 15 is used as the transmission optical fiber 94. Multimode optical fibers of 50 μm and 62.5 μm may be used.

  In the present embodiment, the four wavelengths of the central wavelengths 1275.7, 1300.2, 1324.7, and 1349.2 nm that have been multiplexed and transmitted are separated. Each of these center wavelengths is assumed to vary by a maximum of ± 6.7 nm due to wavelength variations due to light source manufacturing errors, temperature, and spectral distribution. The wavelength variation is determined by the production and characteristics of the light source, and is necessary as a range that does not require strict selection of the light source.

  In the present embodiment, the wavelength division multiplexing optical transmission module 90 having both transmission and reception functions has been described. However, the present embodiment is characterized in the demultiplexer 10 included in the reception unit 92 and includes only the reception unit 92. The effect of the present invention can also be obtained as a wavelength division multiplexing optical transmission module. Further, as described above, a demultiplexer may be used as an optical multiplexer, and may be used instead of the optical multiplexing device of the transmission unit 93.

It is a top view of the demultiplexer of this Embodiment. It is a side view of the demultiplexer of FIG. It is a figure which shows the transmittance-wavelength characteristic of the filter with which the demultiplexer of FIG. 1 is provided. It is a front view of the diffraction grating with which the demultiplexer of FIG. 1 is provided. It is a figure which shows the relationship between the grating position of the diffraction grating of FIG. 4, a grating period, and a blaze angle. It is a figure which shows the wavelength characteristic of the loss of the demultiplexer of FIG. It is a figure which shows the relationship between the grating position of the diffraction grating in another embodiment, a grating period, and a blaze angle. It is a figure which shows the wavelength characteristic of the loss of the demultiplexer of other embodiment. It is a block diagram of the wavelength division multiplexing optical transmission module of this Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Demultiplexer 12 Light incident part 13 Diffraction grating 14 Light receiving part 15 Optical fiber 18 Cylindrical lens 19 Filter 26 Photodetector array 27 Optical waveguide for light reception 90 Wavelength multiplexing optical transmission module 96 Junction part 97 Receiving circuit 99 Wavelength multiplexing device 101 Transmission circuit

Claims (14)

  In a demultiplexer comprising: a light incident portion that receives signal light; a diffraction grating that diffracts the signal light incident from the light incident portion; and a light receiving portion that includes a plurality of photodetectors that detect the diffracted signal light. A demultiplexer characterized in that, in the wavelength range, a difference in diffraction efficiency between TE polarized light and TM polarized light of the diffraction grating is small on the short wavelength side and large on the long wavelength side.   In the demultiplexer, comprising: a light incident portion that receives signal light; a diffraction grating that diffracts the signal light incident from the light incident portion; and a light receiving portion that includes a plurality of photodetectors that detect the diffracted signal light. A demultiplexer characterized in that the maximum loss in TM polarization is smaller than the maximum loss in TE polarization of the grating.   The demultiplexer according to claim 2, wherein the loss of the TM polarized light at the shortest wavelength is larger than the loss of the TE polarized light of the longest wavelength of the diffraction grating.   3. The demultiplexer according to claim 2, wherein a maximum value of TM polarization loss of the diffraction grating is smaller than a maximum value of TE polarization loss and larger than a minimum value of TE polarization loss.   In the demultiplexer, comprising: a light incident portion that receives signal light; a diffraction grating that diffracts the signal light incident from the light incident portion; and a light receiving portion that includes a plurality of photodetectors that detect the diffracted signal light. A demultiplexer characterized in that a polarizing filter for reducing TM polarization of signal light incident on the diffraction grating is provided at an incident portion.   The demultiplexer according to claim 1, wherein the diffraction grating is a chirped diffraction grating having a different grating period.   7. The demultiplexer according to claim 5, wherein the diffraction grating is an Eschlet type diffraction grating, and the diffraction grating is divided into a plurality of regions along the groove, and the blaze angle is different for each region.   8. The demultiplexer according to claim 7, wherein the number of the diffraction grating regions is 4 to 9.   7. The demultiplexer according to claim 5, wherein the diffraction grating is an Eschlet type diffraction grating, and the blaze angle is continuously changed.   A specular reflection blaze in which the blaze angle of the diffraction grating is an angle at which the direction in which the signal light incident on the diffraction grating is specularly reflected by the grating surface and the diffraction direction of the signal light substantially coincide with each other on the short wavelength side of the predetermined wavelength range. The demultiplexer according to any one of claims 7 to 9, wherein the demultiplexer is larger than an angle and smaller than an angle at which the diffraction efficiency of TE polarized light is maximized. When the wavelength of the signal light in the predetermined wavelength range is λ, the grating constant is d, the incident angle of the signal light to the diffraction grating is i, and the diffraction angle of the signal light diffraction grating is θ,

And,

The demultiplexer according to claim 5, wherein   A demultiplexer according to any one of claims 1 to 11, comprising: a junction for connecting the light receiving unit and an external optical transmission line; a signal from the light receiving unit is amplified and waveformd at the light receiving unit of the demultiplexer; A wavelength division multiplexing optical transmission module, wherein a receiving circuit to be molded is connected.   A plurality of light sources, a transmission circuit that is connected to the light sources, drives and controls the light sources, and a wavelength multiplexing device that is connected to the plurality of light sources and combines the signal lights from the plurality of light sources. The wavelength division multiplexing optical transmission module according to claim 12. The wavelength division multiplexing optical transmission module according to claim 13, wherein the junction and the demultiplexer are connected by a multimode optical transmission line, and the junction and the wavelength multiplexing device are connected by a single mode optical transmission line.

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