JP6251202B2 - Wavelength selective switch - Google Patents

Description

  The present invention relates to a wavelength selective switch having a spatial optical system using optical components such as a lens and a diffraction grating.

  With the development of wavelength division multiplexing (WDM) technology, it has come to be used not only for increasing the transmission capacity between two points, but also for setting a wavelength path between arbitrary nodes in a ring network. In particular, ROADM (Reconfigurable Optical Add / drop Multiplexer) ring network (hereinafter referred to as ROADM ring), which can reconfigure wavelength paths flexibly, can dramatically reduce the maintenance and management costs of the network and effectively use the wavelength paths. Therefore, in recent years, it has spread rapidly.

  Each node of the ROADM ring needs to have a function of connecting an arbitrary wavelength signal to an arbitrary port. As a device for this purpose, a wavelength selective switch (Wavelength Selective Switch) in which a diffractive element and a deflecting element are incorporated in a spatial optical system. : WSS). Deflection elements used in WSS are broadly classified into a specular reflection type based on MEMS (Micro Electro Mechanical Systems) technology and a phase modulation type represented by LCOS (Liquid crystal on silicon) technology. In particular, the LCOS type WSS has recently attracted attention because it has the feature that the transmission band of each wavelength channel can be flexibly controlled by controlling the liquid crystal phase in pixel units.

  FIG. 12 shows a configuration example of a general WSS optical system. For convenience, the chromatic dispersion operation surface is referred to as the DP surface, and the port switching (switching) operation surface is referred to as the SW surface. In addition, all the lenses in this example are cylindrical lenses, and only one of the DP surface and the SW surface exerts a light collecting action.

  FIG. 12A illustrates the operation of the DP surface. On the DP surface, members are arranged in the order of fiber 1211, DP surface collimating lens 1213, diffraction element 1214, DP surface main lens 1216, and deflection element 1217. The distance between the diffraction element 1214 and the DP surface main lens 1216, the DP surface main The distance between the lens 1216 and the deflection element 1217 matches the focal length of the DP surface main lens 1216. The SW plane collimating lens 1212 and the SW plane main lens 1215 are regarded as simple flat plates on the DP plane.

  First, the diffused beam emitted from the fiber 1211 corresponding to the input port is converted into a parallel beam by the DP plane collimating lens 1213. The parallel beam is incident on the diffraction element 1214 and then propagates at different diffraction angles corresponding to each wavelength signal. The parallel beams of the respective wavelength signals become convergent beams by the DP surface main lens 1216 and are incident on different areas of the deflecting element 1217.

  Here, by making the distance between the deflection element 1217 and the DP surface main lens 1216 coincide with the focal length of the DP surface main lens 1216, a beam waist (BW) can be formed on the deflection element 1217. The formation of BW on the deflection element 1217 is an indispensable condition for widening the WSS transmission band. Further, by making the distance between the diffraction element 1214 and the DP surface main lens 1216 coincide with the focal length of the DP surface main lens 1216, the beam optical axes of the respective wavelength signals after passing through the DP surface main lens 1216 are parallel to each other. Can be propagated as follows.

  Next, FIG. 12B illustrates the operation of the SW surface. On the SW surface, members are arranged in the order of the fiber 1221, the SW surface collimating lens 1222, the SW surface main lens 1224, and the deflection element 1226, and the distance between the SW surface main lens 1224 and the deflection element 1226 is the SW surface main lens 1224. Is consistent with the focal length. The DP surface collimating lens 1223 and the DP surface main lens 1225 are regarded as simple flat plates on the SW surface.

  First, the diffused beam emitted from the fiber 1221 corresponding to the input port is converted into a parallel beam by the SW plane collimating lens 1222. The beam that has reached the SW surface main lens 1224 has its propagation direction changed, and is incident on one point on the deflection element 1226 without depending on the input port.

  As described above, considering the operation of the DP surface and the SW surface in total, by controlling the beam deflection angle of the deflection element region corresponding to each wavelength signal according to each wavelength signal, an arbitrary wavelength signal beam can be changed to an arbitrary wavelength signal beam. Can be propagated and coupled to the output port.

  Next, FIG. 13 shows an example of member arrangement on the DP surface when one transmissive diffraction grating is used as the diffraction element. Since FIG. 12A is a schematic diagram for comparison with FIG. 12B, the incident light to the diffractive element is expressed by normal incidence, but in order to increase the diffraction efficiency of a specific diffraction order. In addition, it is necessary to make the light incident obliquely on the diffraction grating, and it is desirable to use the light at an angle where the incident angle and the diffraction angle substantially coincide.

  For example, as shown in FIG. 13, when the incident angle and the diffraction angle at the center wavelength are designed to be 45 °, in order to make the optical axis of the central wavelength ray coincide with the optical axis of the DP surface main lens 1304, the diffraction element It is necessary that 1303 is inclined by 45 ° with respect to the DP surface main lens 1304, and the input port 1301 and the DP surface collimating lens 1302 are inclined by 90 ° with respect to the DP surface main lens 1304.

  However, in this case, since the distance between the DP surface main lens 1304 and the diffraction element 1303 cannot be physically defined, the intersection of the optical axis of the wavelength multiplexed light and the diffraction element 1303 is defined as a “diffraction starting point”. By making the distance from the diffraction origin to the DP surface main lens 1304 coincide with the focal length of the DP surface main lens 1304, the beam optical axes of the respective wavelength signals after passing through the DP surface main lens 1304 are parallel to each other. It is possible to propagate. Accordingly, all the wavelength signal beams can be vertically incident and vertically reflected on the deflecting element 1305, so that an optical path from the input port 1301 to the deflecting element 1305 (hereinafter, “outward path”) and from the deflecting element 1305 to the output port 1301. These optical paths (hereinafter referred to as “return paths”) can be made completely the same on the DP plane.

  By the way, although the member arrangement on the DP surface shown in FIG. 13 is a general configuration in the case of using a deflection element having no polarization dependency such as a MEMS mirror, a deflection element having a polarization dependency such as LCOS is used. In this case, it is necessary to additionally arrange a polarization diversity function unit. 1C and 1D of Patent Document 1 disclose a wavelength selective optical switch having a polarization diversity function unit in which a cube-type polarization separation element and a 45 ° prism are bonded together.

  In a general WSS module, the arrangement of members on the SW surface is a restriction in the thickness direction, so it is disadvantageous from the standpoint of reducing the height to arrange the members on the SW surface. In addition, if the polarization diversity function unit is arranged in the region where the wavelength multiplexed light is demultiplexed, it is disadvantageous in terms of member size and cost. Therefore, if priority is given to the reduction in height, size, and cost, it is desirable that the polarization diversity function unit be disposed between the input (output) port and the diffraction element in the DP plane.

  FIG. 14 shows the configuration of the WSS in which the polarization diversity function unit is arranged on the DP plane from the above viewpoint. 13 is different from that of FIG. 13 in that the polarization diversity function unit is configured by a polarization separation element 1403, a bending mirror 1404, and a λ / 2 wavelength plate 1405 between the DP plane collimating lens 1402 and the diffraction element 1406. Is the point where is placed. The wavelength multiplexed light emitted from the input port 1401 is transmitted through the DP plane collimating lens 1402 and branched into two by the polarization separation element 1403, one passing through the λ / 2 wavelength plate 1405 and the other passing therethrough. Instead, both propagate toward the diffractive element 1406.

  Further, in the example shown in FIG. 13, there is one diffraction starting point, but in the example shown in FIG. 14, there are two diffraction starting points. Further, since the diffractive element 1406 is inclined with respect to the DP surface main lens 1407, the distance from each diffraction origin to the DP surface main lens 1407 cannot be made the same. Accordingly, since at least one of these distances is deviated from the focal length of the DP surface main lens 1407, the beam optical axes of the respective wavelength signals after passing through the DP surface main lens 1407 are not parallel to each other. Therefore, almost all wavelength signal beams need to be incident and reflected obliquely on the deflecting element, and the forward path and the return path are separated on the DP plane.

  Usually, the bending mirror is arranged so that the optical axes of the two light beams branched by the polarization separation element are parallel to each other. This is because there is a tolerance advantage that the distance between the two diffraction origins does not deviate from the design value even if the distance from the polarization separation element to the diffraction element is slightly deviated from the design value.

  Here, the operation principle of the polarization diversity function unit shown in FIG. 14 will be described with reference to FIGS. 15 and 16. 15 and 16, it is assumed that the deflecting elements 1508 and 1608 are LCOS that operates only with horizontal polarization, and the polarization separation elements 1503 and 1603 transmit vertical polarization and reflect horizontal polarization. In the following drawings, unless otherwise noted, the optical path diagram describes only the optical axis (principal ray) of the beam.

  First, in FIG. 15, attention is paid to the vertical polarization component transmitted through the polarization separation element 1503 among the wavelength multiplexed light emitted from the input (output) port 1501. The vertically polarized light beam is converted into a horizontally polarized wave component by a λ / 2 wavelength plate 1505 disposed immediately after the polarization beam splitting element 1503, and corresponds to each wavelength light beam via the diffraction element 1506 and the DP surface main lens 1507. A separate region of LCOS 1508 is reached. Each wavelength ray reaching LCOS 1508 is given a deflection angle for propagation to the target output port by phase control of LCOS 1508 in the SW plane, and is folded back at a reflection angle corresponding to the incident angle in the DP plane. . After that, the light passes through the DP surface main lens 1507, the diffraction element 1506, and the bending mirror 1504, and then enters the polarization separation element 1503 again. However, since it is converted into a horizontal polarization component in the return path, it is reflected by the polarization separation element. , Coupled to an input (output) port 1501 disposed at the same position in the DP plane.

  Next, in FIG. 16, attention is paid to the horizontal polarization component reflected by the polarization separation element 1603 among the wavelength multiplexed light beams emitted from the input (output) port 1601. The horizontally polarized light is corrected by the bending mirror 1604 in the same propagation direction as that before the polarization separation element 1603 is incident, and the LCOS 1608 corresponding to each wavelength light is separately transmitted via the diffraction element 1606 and the DP surface main lens 1607. Reach the area. Each wavelength ray reaching LCOS 1608 is given a deflection angle for propagation to the target output port by phase control of LCOS 1608 in the SW plane, and is folded back at a reflection angle corresponding to the incident angle in the DP plane. . Thereafter, the light enters the λ / 2 wavelength plate 1605 through the DP surface main lens 1607 and the diffraction element 1606. The wavelength multiplexed light converted into the vertical polarization component by the λ / 2 wavelength plate 1605 can pass through the polarization separation element 1603 and is coupled to the input (output) port 1601 arranged at the same position in the DP plane. To do.

  In this way, all polarization components emitted from the input port are once separated into a horizontal polarization component and a vertical polarization component, the forward path reaches LCOS through different optical paths, and the return path is the other polarization component. It reaches the output port via the optical path that has been the outbound path.

Japanese Patent No. 4365680 US Pat. No. 6,760,501

  By the way, the polarization diversity WSS optical system shown in FIGS. 14 to 16 has a problem that the wavelength dependency of the insertion loss becomes large, and the low loss property cannot be maintained in the entire use wavelength range. The principle will be described in order with reference to FIGS.

  FIG. 17 shows the relative relationship of the optical path of each wavelength signal demultiplexed by the diffraction element. As described above, the two light beams branched by the polarization separation element are incident on different positions of the diffraction element 1701, so that there are two diffraction origins. For convenience, of these two diffraction origins, the one far from the DP surface main lens 1702 is defined as “diffraction origin A”, and the one near the DP surface main lens 1702 is defined as “diffraction origin B”. Of the optical paths from the input port to the LCOS 1703, the direction including the diffraction origin A is defined as “optical path A”, and the direction including the diffraction origin B is defined as “optical path B”. That is, of the two orthogonal polarization components, one of the components has the optical path A as the forward path and the optical path B as the return path, and the other component has the optical path B as the forward path and the optical path A as the return path. Further, as described in the description of FIG. 14, the optical axes of the optical path A and the optical path B immediately before entering the diffraction element 1701 are parallel to each other.

  Next, attention is paid to an arbitrary wavelength ray after being demultiplexed by the diffraction element 1701. Since both the optical path A and the optical path B have the same wavelength and the same incident angle to the diffraction element 1701, the diffraction angles are also the same, and the optical axes of the optical path A and the optical path B are maintained in a parallel relationship even after demultiplexing. Therefore, if the reflective surface of the LCOS 1703 is arranged at the focal length of the DP surface main lens 1702, the optical axes of the same wavelength rays in the optical path A and the optical path B pass through the DP surface main lens 1702 and then on the reflective surface of the LCOS 1703. Intersect at one point.

  For convenience, the intersection of the optical path A and the optical path B is a “folding point”, the point where the optical path A is incident on the DP surface main lens 1702 is “incident point A”, and the point where the optical path B is incident on the DP surface main lens 1702 is “incident”. It is called “Point B”, and attention is paid to a triangle composed of a turning point, an incident point A, and an incident point B. Since the distance from the DP surface main lens 1702 to the turning point is constant regardless of the wavelength, the optical path length from the DP surface main lens 1702 to the reflecting surface of the LCOS 1703 (that is, the distance from the incident point A / B to the turning point) is incident. As the distance between the point A and the incident point B increases, the distance increases, and when the distance decreases, the distance decreases.

  Here, paying attention to the distance between the optical axes of the optical path A and the optical path B propagating between the diffractive element 1701 and the DP surface main lens 1702, the long-wave side light beam has a larger diffraction angle, so the distance between the optical axes becomes shorter. The distance between the incident point A and the incident point B is also shortened. On the other hand, since the diffraction angle of the light on the short wave side is small, the distance between the optical axes is long, and the distance between the incident point A and the incident point B is also long.

  Therefore, the optical path length from the DP surface main lens 1702 to the reflection surface of the LCOS 1703 tends to be shorter on the long wave side and longer on the short wave side with respect to the center wavelength, which increases the wavelength dependency of the insertion loss. It is a factor.

  The contents described above with reference to FIG. 17 will be described with reference to specific numerical examples shown in FIG. The transmission type diffraction grating 1801 has 1130 grooves / mm, a design center wavelength of 1550 nm, a long wavelength of 1580 nm, and a short wavelength of 1520 nm. In this case, in order to make the incident angle and the diffraction angle coincide at the design center wavelength, the incident angle may be set to 61.1 °. At this time, the distance between the optical axes of the optical path A and the optical path B after diffraction is 0.86 times shorter at the long wavelength and 1.12 times longer at the short wavelength. Next, assuming that the distance between the optical axes of the optical paths A and B at the design center wavelength is 10 mm and the focal length of the DP surface main lens 1702 (assumed to be an ideal thin lens) is 100 mm, the DP surface main lens 1702 to the LCOS 1703 The optical path length to the reflecting surface is 100.092 mm at the long wavelength, 100.125 mm at the design center wavelength, and 100.157 mm at the short wavelength. Therefore, when the design center wavelength is used as a reference, an optical path length difference of about ± 0.03 mm occurs from a long wavelength to a short wavelength.

  Next, in order to estimate the influence of the optical path length difference of about ± 0.03 mm exemplified in FIG. 18 on the wavelength dependence of loss, a model shown in FIG. 19 is considered. In FIG. 19, the optical path A and the optical path B are developed in one straight line for each wavelength, and the DP surface main lenses 1902 and 1904 (outward and return paths), the reflective surface of the LCOS 1903, etc. are arranged on the straight line. is there. The DP surface main lens passes through each wavelength ray twice in the optical path A and the optical path B, but in the long wavelength and the short wavelength optical path, the lens peripheral portion is set on either the optical path A or the optical path B. Will pass. Therefore, it is assumed that the focal length when passing through the peripheral edge of the lens is affected by spherical aberration and is shorter than ideal by δ mm.

  Therefore, when the focal length of the DP surface main lens is 100.1 mm and δ is 0.5 mm, the long wavelength (1580 nm) optical path is 200.184 mm for a lens with a focal length of 100.1 mm and a lens with a focal length of 99.6 mm. The optical path of the design center wavelength (1550 nm) is arranged with two lenses with a focal length of 100.1 mm arranged at a separation distance of 200.250 mm, and the optical path of a short wavelength (1520 nm) is This corresponds to a model in which a lens having a focal length of 99.6 mm and a lens having a focal length of 100.1 mm are arranged with a separation distance of 200.314 mm.

  Here, a beam waist having a spot size of 2.5 mm is formed in the optical path A, and after the beam has passed through the two DP surface main lenses 1902 and 1904, the beam waist is recombined with the beam waist having a spot size of 2.5 mm. Table 1 shows the calculation result of the coupling loss. It is assumed that the distance between each beam waist and the lens is 50 mm, and the results when there is no variation in the optical path length are also shown for reference.

  From this result, it can be seen that the maximum loss when there is no variation in the optical path length remains at 0.17 dB, but the maximum loss when there is a variation in the optical path length increases to 0.65 dB. That is, in the WSS optical system in which the polarization diversity function unit is arranged on the DP surface, the wavelength dependency of the insertion loss increases, and there is a problem that low loss cannot be maintained over the entire use wavelength range.

  As a method for reducing the wavelength dependency of loss due to the optical path length difference for each wavelength, there is a method of inserting a prism into the WSS optical system, that is, a method of inserting a prism between the DP surface main lens and the deflecting element. It is disclosed (see Patent Document 2).

  However, in this case, wavelength dependency occurs in the distance from the DP surface main lens to the turning point on the deflecting element. The entire deflection element must be tilted. Then, since the incident angle and the reflection angle on the deflecting element are not the same depending on each wavelength, it is necessary to control the beam deflection angle even in the DP plane, causing problems such as an increase in cost.

  The present invention has been made in view of such problems, and the object of the present invention is to make it possible to correct the wavelength dependency of loss caused by having a plurality of diffraction origins in the diffraction element, which is small, low profile, An object of the present invention is to provide an LCOS type wavelength selective switch capable of arranging low cost and low loss.

  In order to solve the above-described problems, the present invention provides a wavelength selective switch, an input port through which wavelength multiplexed light is emitted, a diffraction element that demultiplexes the wavelength multiplexed light into demultiplexed light, and the demultiplexing A prism that changes the propagation direction of a light beam according to an incident angle, and for the incident angle θ to the prism and the outgoing angle φ from the prism, θ> φ for the long wave side light beam of the demultiplexed light beam, and for the short wave side light beam A prism that satisfies the relationship θ <φ, a condensing element that converts the demultiplexed light beam that has passed through the prism into convergent light, a deflecting element that reflects the demultiplexed light beam that has passed through the condensing element, and The demultiplexed light beam reflected by the deflecting element includes an output port through which the condensing element, the prism, and the diffraction element are incident.

  The invention according to claim 2 is a wavelength selective switch, wherein an input port from which wavelength multiplexed light is emitted, and the wavelength multiplexed light are separated into two polarized light beams whose polarization directions are orthogonal to each other. A polarization diversity function unit that emits light so that the optical axes of the polarized light beams are parallel to each other, and diffraction in which the two polarized light beams are separated into separate demultiplexed light beams at the first and second diffraction origins, respectively. An element and a prism that converts the propagation direction of the demultiplexed light beam according to an incident angle, and the incident angle θ to the prism and the output angle φ from the prism are θ> The relationship of θ <φ holds for φ and short-wave rays, and a prism, a condensing element that converts the demultiplexed light beam that has passed through the prism into convergent light, and the demultiplexed light beam that has passed through the condensing element is reflected. Receiving deflection element and wavelength multiplexed light An output port, and the demultiplexed light beam reflected by the deflecting element is one of the condensing element, the prism, one of the first and second diffraction origins of the diffraction element, and the polarization element. The light beam is transmitted through a wave diversity function unit and is incident on the output port.

  According to a third aspect of the present invention, in the wavelength selective switch according to the first or second aspect, the separation distance between the condensing element and the deflecting element is the same as the focal length of the condensing element. Features.

  According to a fourth aspect of the present invention, in the wavelength selective switch according to any one of the first to third aspects, the prism is a center wavelength in a demultiplexed wavelength range among the demultiplexed light beams demultiplexed by the diffraction element. Is designed so that the incident angle to the prism and the exit angle from the prism are equal.

  According to a fifth aspect of the present invention, in the wavelength selective switch according to any one of the first to fourth aspects, the diffractive element is a transmission type and includes a plurality of diffraction gratings.

  The invention according to claim 6 is the wavelength selective switch according to claim 5, wherein the diffraction element includes a high refractive index material sandwiched between the plurality of diffraction gratings.

  The present invention relates to an LCOS type wavelength selective switch optical system having a polarization diversity function unit that splits a light beam into a horizontally polarized wave and a vertically polarized wave on a chromatic dispersion surface, and a loss caused by having a plurality of diffraction origins in the diffraction element. It has the effect of reducing the wavelength dependence and making it possible to combine small size, low profile, low cost, and low loss.

It is a figure explaining the outline of the principle of the present invention. It is a figure explaining operation | movement of a prism. 3 is a graph showing the θ dependence of the optical axis distance change rate shown in Formula 2. It is a graph showing the sum (θ + φ) of the incident angle θ and the outgoing angle φ with respect to the incident angle θ of the prism. FIG. 5 is an enlarged view of the graph of FIG. 4 when the apex angle α is 50 °. It is a figure which shows the specific example of the prism which correct | amends the wavelength dependence of the distance between optical axes supposing that it arrange | positions and uses immediately after the diffraction grating shown in FIG. (A) is a figure which shows the structure of WSS which has arrange | positioned the polarization diversity function part in DP surface, (b) is a figure which shows the prism inserted in the space from a diffraction element to DP surface main lens, (C) is a figure which shows the prism inserted in the space from a DP surface main lens to a deflection | deviation element. It is the optical path figure seen from DP surface about the WSS optical system at the time of using LCOS as a deflection | deviation element in WSS which concerns on the 1st Embodiment of this invention. It is the optical path figure seen from DP surface about the WSS optical system at the time of utilizing the MEMS mirror array as a deflection | deviation element in WSS which concerns on the 2nd Embodiment of this invention. It is the optical path figure seen from DP surface about the WSS optical system at the time of using LCOS as a deflection | deviation element in WSS which concerns on the 3rd Embodiment of this invention. It is the optical path figure seen from DP surface about the WSS optical system at the time of using LCOS as a deflection | deviation element in WSS which concerns on the 4th Embodiment of this invention. (A) is a figure which shows the structural example of the WSS optical system which has a general wavelength dispersion operation surface, (b) is a structural example of the WSS optical system which has a general port switching (switching) operation surface. FIG. It is a figure which shows the member arrangement | positioning example of DP surface in the case of using one transmission type diffraction grating as a diffraction element. It is a figure which shows the structure of WSS which has arrange | positioned the polarization diversity function part in DP surface. It is a figure explaining the principle of operation of WSS which has arranged a polarization diversity functional part on a DP surface. It is a figure explaining the principle of operation of WSS which has arranged a polarization diversity functional part on a DP surface. It is a figure which shows the relative relationship of the optical path of each wavelength signal demultiplexed with the diffraction element. It is a figure which shows the relative relationship of the optical path of each wavelength signal demultiplexed with the diffraction element. It is the figure which expand | deployed the optical path A and the optical path B to one straight line for every wavelength.

  FIG. 1 is a diagram for explaining the outline of the principle of the present invention. In the first place, the wavelength dependence of the insertion loss was caused by the occurrence of wavelength dependence in the “distance between optical axes” of the optical paths A and B after passing through the diffraction grating. Therefore, in order to correct the wavelength dependency of the distance between the optical axes, a prism 102 is disposed immediately after the diffraction element 101. With this prism, the distance between the optical axes is reduced in the short-wave optical path having a long distance between the optical axes, and conversely, the distance between the optical axes is increased in the long-wave optical path having a short distance between the optical axes. If this relationship is appropriately used, the wavelength dependence of the distance between the optical axes can be reduced.

  The details of the principle of the present invention will be described below with reference to FIGS.

  FIG. 2 is a diagram for explaining the operation of the prism. In a prism having a refractive index n and an apex angle α, φ is expressed as Equation 1 where θ is the incident angle of the light beam and φ is the output angle.

Further, when the distance between the optical axes when the prism is incident is L _IN , and the distance between the optical axes when the prism is emitted is L _OUT , the ratio (hereinafter referred to as “optical axis distance change rate”) is expressed as shown in Equation 2. Is done.

  Here, the refractive index n can be regarded as constant in the wavelength range of several tens of nanometers near 1550 nm used in wavelength division multiplexing communication. Further, since all wavelength rays are incident on a single prism, the apex angle α is also constant.

  FIG. 3 is a graph showing the θ dependency of the optical axis distance change rate expressed by Equation 2. However, the refractive index n was 1.5, and the apex angle α was plotted in 6 levels in the range of 20 ° to 70 °. What can be said from FIG. 3 is that, if the range of θ is appropriately selected, a design that can change the optical axis distance change rate from less than 1 to 1 or more, that is, the distance between the optical axes can be reduced and expanded. For example, when the apex angle α is 50 °, the incident angle θ is about 39 ° and the change rate of the optical axis distance is 1, and when used at a smaller incident angle, the distance between the optical axes is reduced and larger than that. If it is used at the incident angle, the distance between the optical axes is increased. The optical axis distance change rate is 1 when θ = φ.

Next, focus on θ + φ. If a prism is used at an incident angle in a range where θ + φ can be regarded as constant, the wavelength resolution obtained by the diffraction grating can be maintained. The reason is that the angle formed between the long wave ray and the short wave ray emitted from the diffraction grating coincides with the angle formed between the long wave ray and the short wave ray incident on the prism (| θ _{long wave−} θ _{short wave} |). This is because it becomes equal to the angle (| φ _{long wave−} φ _{short wave} |) formed by the long wave ray and the short wave ray emitted from the prism.

  FIG. 4 shows a graph representing the sum (θ + φ) of the incident angle θ and the outgoing angle φ with respect to the incident angle θ of the prism. As in FIG. 3, the refractive index n was 1.5, and the apex angle α was changed by 6 levels in the range of 20 ° to 70 °. The broken line is a straight line of θ = φ, and what can be said from this figure is that θ + φ can be regarded as constant in the vicinity of θ where θ = φ. Specifically, when the apex angle α is 50 °, the incident angle θ is around 40 °, when the apex angle α is 40 °, the incident angle θ is around 31 °, and when the apex angle α is 30 °, the incident θ If a prism is used at around 23 °, it can be seen that θ + φ can be regarded as almost constant.

  FIG. 5 shows an enlarged display of the graph of FIG. 4 when the apex angle α is 50 °. Even if the incident angle θ changes from 34 ° to 45 °, the change of θ + φ is within 0.5 °. It can also be seen that the change rate of θ + φ is minimized from 39 ° to 40 ° where the incident angle θ is θ = φ.

  2 to 5, the distance between the optical axes is reduced in the θ region where θ <φ, the distance between the optical axes is increased in the θ region where θ> φ, and near θ = φ. Θ + φ is constant in the θ region, that is, the wavelength dispersion is maintained. Therefore, (1) θ = φ at the center wavelength, (2) θ> φ to extend the optical axis distance with a long wavelength optical path with a short optical axis distance, and (3) a short wave optical path with a long optical axis distance. By designing the relative installation angle of the prism glass material, apex angle, and diffraction grating so that θ <φ in order to reduce the optical axis distance, the wavelength dependence of the optical axis distance is maintained while maintaining the wavelength resolution. Can be reduced.

  FIG. 6 shows a specific example of a prism that corrects the wavelength dependence of the distance between the optical axes, assuming that it is arranged immediately after the diffraction grating. The prism glass material was BK7, the apex angle was 61.8 °, the incident angle at a short wavelength of 1520 nm was 46 °, and the incident angle at a long wavelength of 1580 nm was 54.12 °. The optical axis relative angle of the short wavelength and the long wavelength is 8.12 ° when the prism is incident and is 8.32 ° when the diffraction grating is emitted, and 8.32 ° when the prism is emitted. Therefore, the wavelength resolution can be maintained.

  In FIG. 18, the distance between the optical axes is increased by 1.12 times by the diffraction grating in the short wavelength optical path of 1520 nm, but the distance between the optical axes is reduced by 0.86 times when passing through this prism. On the other hand, in the long wavelength optical path of 1580 nm, the distance between the optical axes is reduced by 0.86 times by the diffraction grating, but when the light passes through this prism, it is enlarged by 1.12 times. Therefore, when it finally passes through the prism, it becomes 0.96 times the distance between the optical axes before the diffraction grating is incident on both the short wavelength and the long wavelength, and the wavelength dependence of the distance between the optical axes can be greatly reduced.

  By the way, as a method of reducing the optical path length difference for each wavelength by inserting a prism into the WSS optical system, a method of inserting a prism between the DP surface main lens and the deflecting element is disclosed (see Patent Document 2). ).

  On the other hand, the present invention is characterized in that a prism is inserted between the diffractive element and the DP surface main lens, and the difference in action and effect resulting from the difference in configuration will be described with reference to FIG. For convenience, the space from the input (output) port 701 to the diffraction element 706 is “space 1”, the space from the diffraction element 706 to the DP surface main lens 707 is “space 2”, and the space from the DP surface main lens 707 to the deflection element 708 is The space is defined as “space 3”.

  FIG. 7A shows the configuration of the WSS in which the polarization diversity function unit is arranged on the DP plane. Thus, (1) if the optical axes of the optical path A and the optical path B are parallel to each other in the space 1, then (2) the optical axes of the optical path A and the optical path B are also parallel to each other in the space 2 if they have the same wavelength. It is. Accordingly, (3) in the space 3, the distance from the DP surface main lens 707 to the “turning point” (the intersection of the optical path A and the optical path B) is the same for all wavelength rays, so (4) the DP surface main lens There was no need to tilt the entire deflection element 708 relative to 707.

  However, when the prism shown in FIG. 7C is inserted into the space 3, the relations (3) and (4) are not maintained, and a change appears in the function required for the optical element. Specifically, in the space 3, since the wavelength dependence occurs in the distance from the DP surface main lens 707 to the turning point, the entire deflection element 708 must be inclined with respect to the DP surface main lens 707. Then, since the incident angle and the reflection angle on the deflection element 708 are not the same for each wavelength, the deflection element 708 needs to have a function of controlling the beam deflection angle even in the DP plane. When the deflecting element 708 is LCOS, it is necessary to change the specifications such as reducing the pixel size or increasing the DP surface beam size on the LCOS, which causes problems such as an increase in cost and a reduction in transmission band. End up.

  On the other hand, when the prism shown in FIG. 7B is inserted into the space 2 as in the present invention, the propagation direction of the optical path is changed, but the feature of the above (2) is maintained, so the above (3) ( The relationship of 4) is also maintained, and the function required for each optical element does not change before and after the prism insertion.

  Embodiments according to the present invention will be described below with reference to the drawings. However, the present invention is not limited to the specific configurations of the embodiments shown here.

(Embodiment 1)
FIG. 8 shows an optical path diagram seen from the DP plane in the WSS optical system when LCOS is used as a deflection element in the WSS according to the first embodiment of the present invention. The optical system of this embodiment includes a port array 801, a DP surface first cylindrical lens 802, a trapezoidal polarization beam splitter 803, a λ / 2 wavelength plate 804, a transmission diffraction grating 805, a prism 806, an SW surface cylindrical lens 807, It is composed of a DP surface second cylindrical lens 808 and an LCOS 809, the port array 801 is an input port and an output port, the transmission diffraction grating 805 is a diffraction element, the DP surface second cylindrical lens 808 is a condensing element, and the LCOS 809 is a deflection element. It corresponds to. Also, a prism 806 is disposed between the transmission type diffraction grating 805 and the DP surface second cylindrical lens 808, and is designed to reduce the wavelength dependency of the distance between the optical axes of the optical path A and the optical path B. Is a feature.

  The basic operation principle of the LCOS type WSS is the same as that described in the description of FIGS.

  Since the port array 801 needs to emit different NA beams on the SW surface and the DP surface, specific configurations include a combination of a fiber array and a cylindrical lens array, or a waveguide using PLC technology. An array chip or the like can be considered. In the latter case, advantages such as reduction in the number of parts can be obtained.

  The λ / 2 wave plate 804 can be arranged on either the optical path A or the optical path B of the trapezoidal polarization beam splitter, but the optical path A and the optical path B are selected and arranged so as to be aligned with the polarization direction in which the LCOS 809 operates. There is a need to.

  Since the DP surface second cylindrical lens 808 requires a high-performance lens having a large effective diameter, a doublet or a triplet lens may be used.

  In the present embodiment, the LCOS 809 is arranged such that the reflection surface is orthogonal to the paper surface. Generally, the vertical direction of the paper surface is the module height direction, and the LCOS area is a restriction for reducing the module height. End up. Therefore, if the reflective surface of the LCOS 809 is disposed parallel to the paper surface, a 45 ° tilt mirror is disposed immediately before the LCOS 809, and the optical path is converted by 90 °, the LCOS area can be prevented from becoming a restriction for reducing the module height.

(Embodiment 2)
FIG. 9 shows an optical path diagram of the WSS optical system when a MEMS mirror array is used as a deflection element in the WSS according to the second embodiment of the present invention as viewed from the DP plane. The difference from the first embodiment is that (1) the deflecting element is not the LCOS but the MEMS mirror array 909, so that the polarization diversity is unnecessary, and (2) the port array is arranged in two rows on the DP plane. And divided into a first port array 901 and a second port array 903. A first DP surface first cylindrical lens 902 and a second DP surface first cylindrical lens 904 are arranged for the first and second port arrays 901 and 903, respectively.

  In the present embodiment, since the MEMS mirror array 909 is used as a deflecting element, two-dimensional beam deflection of the SW surface and the DP surface is relatively easy. Therefore, it is possible to switch the ports such as only the optical path A or only the optical path B in both the forward path and the return path. Therefore, for example, assuming that 11 ports are arranged in the first port array 901 and 11 ports are arranged in the second port array 903, one port of the first port array 901 is an input port and all other ports are output. When used as a port, a large-scale WSS optical system of 1 input × 21 outputs can be configured. Here, the port array need not be limited to two rows, and may be three or more rows.

(Embodiment 3)
FIG. 10 shows an optical path diagram of the WSS optical system when LCOS is used as a deflection element in the WSS according to the third embodiment of the present invention as viewed from the DP plane. The difference from the first embodiment is that a cylindrical meniscus lens 1009 is disposed immediately before the LCOS 1010, whereby the field curvature aberration of the beam spot imaged on the reflective surface of the LCOS 1010 can be corrected.

(Embodiment 4)
FIG. 11 shows an optical path diagram of the WSS optical system when LCOS is used as a deflection element in the WSS according to the fourth embodiment of the present invention when viewed from the DP plane. The difference from the first embodiment is that the diffractive element is composed of two transmissive diffraction gratings 1105 and 1107, and the space 1106 sandwiched between the two diffraction gratings is made of a high refractive index material such as glass. It is a point that is filled. In other words, the WSS optical system uses a diffraction element block in which different transmission diffraction gratings 1105 and 1107 are attached to the front and rear surfaces of the prism.

  The wavelength dispersion power can be increased by increasing the number of passing diffraction gratings. Therefore, it is possible to obtain a degree of freedom in design that enlarges the transmission band or shortens the focal length of the DP surface second cylindrical lens 1110 to reduce the size of the optical system.

101, 601 Diffraction element 102, 602 Prism 701 Input (output) port 702 DP surface collimating lens 703 Polarization separation element 704 Bending mirror 705 λ / 2 wave plate 706 Diffraction element 707 DP surface main lens 708 Deflection element 801, 1001, 1101 Port array 901 First port array 903 Second port array 802, 1002, 1102 DP surface first cylindrical lens 902 First DP surface first cylindrical lens 904 Second DP surface first cylindrical lens 803, 1003, 1103 Trapezoidal polarization beam splitter 804, 1004, 1104 λ / 2 wavelength plate 805, 905, 1005, 1105, 1107 Transmission diffraction grating 806, 906, 1006, 1108 Prism 807, 907, 1007, 1109 SW surface cylindrical lens 808 , 908, 1008, 1110 DP surface second cylindrical lens 809, 1010, 1111 LCOS
909 MEMS mirror array 1009 Cylindrical meniscus lens 1106 Space 1211, 1221 Fiber 1212, 1222 SW surface collimating lens 1213, 1223 DP surface collimating lens 1214 Diffraction grating 1215, 1224 SW surface main lens 1222, 1302, 1402, 1502, 1602 DP surface collimating Lenses 1216, 1225 DP surface main lenses 1217, 1226 Deflection elements 1301, 1401, 1501, 1601 Input (output) ports 1302, 1402, 1502, 1602 DP surface collimating lenses 1303, 1406, 1506, 1606, 1701 Diffraction gratings 1304, 1407 , 1507, 1607, 1702, 1902, 1904 DP surface main lens 1305, 1408 Deflection element 1403, 150 , Folding mirror 1603 polarization splitter 1404,1504,1604 1405,1505,1605 λ / 2 wave plate 1508,1608,1703,1903 LCOS
1801 Transmission diffraction grating

Claims (6)

An input port from which wavelength multiplexed light is emitted;
A diffraction element for demultiplexing the wavelength-multiplexed light into demultiplexed light;
A prism that converts the propagation direction of the demultiplexed light beam according to an incident angle, and the incident angle θ to the prism and the output angle φ from the prism are θ> φ for the long wave side light of the demultiplexed light beam, and the short wave In the side rays, the relation θ <φ holds,
A condensing element that converts the demultiplexed light beam transmitted through the prism into convergent light;
A deflecting element that reflects the demultiplexed light beam that has passed through the condensing element;
An output port through which the demultiplexed light beam reflected by the deflecting element passes through the condensing element, the prism, and the diffractive element; and
A wavelength selective switch comprising: An input port from which wavelength multiplexed light is emitted;
A polarization diversity function unit that separates the wavelength-multiplexed light into two polarized light beams having orthogonal polarization directions and emits the two polarized light beams so that optical axes thereof are parallel to each other;
A diffractive element in which the two polarized rays are demultiplexed into separate demultiplexed rays at first and second diffraction origins, respectively;
A prism that converts the propagation direction of the demultiplexed light beam according to an incident angle, and the incident angle θ to the prism and the output angle φ from the prism are θ> φ for the long wave side light of the demultiplexed light beam, and the short wave In the side rays, the relation θ <φ holds,
A condensing element that converts the demultiplexed light beam transmitted through the prism into convergent light;
A deflecting element that reflects the demultiplexed light beam that has passed through the condensing element;
An output port for receiving wavelength multiplexed light;
The demultiplexed light beam reflected by the deflecting element includes: one of the light collecting element, the prism, the first and second diffraction origins of the diffraction element, and the polarization diversity function unit. A wavelength selective switch that transmits and enters the output port.   3. The wavelength selective switch according to claim 1, wherein a separation distance between the light collecting element and the deflecting element is equal to a focal distance of the light collecting element.   The prism is a demultiplexed light beam corresponding to the center wavelength of the demultiplexed wavelength range among the demultiplexed light beams demultiplexed by the diffraction element, so that the incident angle to the prism and the emission angle from the prism are equal. The wavelength selective switch according to claim 1, wherein the wavelength selective switch is designed as follows.   5. The wavelength selective switch according to claim 1, wherein the diffractive element is a transmissive type and includes a plurality of diffraction gratings.   The wavelength selective switch according to claim 5, wherein the diffraction element includes a high refractive index material sandwiched between the plurality of diffraction gratings.