JP4814147B2 - Optical signal processor - Google Patents

Description

  The present invention relates to an optical signal processing device. More specifically, the present invention relates to an optical signal processing circuit including a lens.

  As the speed and capacity of optical communication networks have increased, there has been an increasing need for optical signal processing apparatuses such as those represented by processing of wavelength division multiplexing (WDM) transmission signals. For example, there is a demand for a function of switching the path of multiplexed optical signals between nodes. An optical signal processing apparatus has been increased in speed by performing path conversion without changing the optical-electrical conversion.

  On the other hand, from the viewpoint of miniaturization and integration of an optical signal processing device, research and development of a waveguide type optical circuit (PLC: Planar Lightwave Circuit) is in progress. In a PLC, for example, a core made of quartz glass is formed on a silicon substrate, and various functions are integrated on one chip, thereby realizing an optical functional device with low loss and high reliability. Furthermore, a complex optical signal processing component (apparatus) that combines a plurality of PLC chips and other optical functional components has also appeared.

  For example, Patent Document 1 discloses an optical signal processing device in which a waveguide type optical circuit (PLC) including AWG or the like and a spatial modulation element such as a liquid crystal element are combined. More specifically, a wavelength blocker including a PLC and a collimating lens arranged symmetrically with respect to the liquid crystal element, a wavelength equalizer, a dispersion compensator, and the like are being studied. In these optical signal processing devices, optical signal processing is performed independently for each wavelength for a plurality of optical signals having different wavelengths.

  FIG. 11 is a conceptual diagram illustrating an example of an optical signal processing device. In this optical signal processing apparatus, an optical signal is input / output via the spectroscopic element 51. The spectroscopic element 51 demultiplexes a plurality of optical signals having different wavelengths at an emission angle θ corresponding to the wavelengths. The demultiplexed optical signal is emitted toward the condenser lens 52. The optical signal condensed by the condensing lens 52 is condensed on each condensing point at a predetermined position of the signal processing element 53 having a function of intensity modulation, phase modulation or deflection corresponding to the emission angle θ. . That is, it should be noted that the optical signal is collected at different positions of the signal processing element depending on the wavelength of the input optical signal. The signal processing element 53 is, for example, a liquid crystal element composed of a plurality of element elements (pixels). By controlling the transmittance of each element, the optical signal of each wavelength is subjected to intensity modulation and the like, and a predetermined signal processing function is realized. The optical signal subjected to the signal processing is reflected by the mirror 54 to reverse the traveling direction. The optical signal further passes through the condenser lens 52 and is multiplexed again in the spectroscopic element 51. As is generally well known, the spectroscopic element 51 can also multiplex optical signals according to the traveling direction. The combined optical signal of each wavelength is output again as output light to the outside of the optical signal processing device.

  In FIG. 11, the spectroscopic element 51 is conceptually shown as long as it can perform demultiplexing and multiplexing according to the wavelength of the optical signal. For example, the spectroscopic elements include a grating, a prism, and an arrayed waveguide grating (AWG). The signal processing element only needs to be capable of modulating the intensity or phase of the optical signal, or the intensity and phase, or capable of deflecting the traveling direction of the optical signal. For example, the signal processing element includes a liquid crystal element, a MEMS (Micro Electro Mechanical Systems) mirror, and a nonlinear crystal.

  The optical signal processing device shown in FIG. 11 has a configuration in which both optical signal demultiplexing and multiplexing are performed by a single spectroscopic element by folding the optical signal using a mirror. This configuration is generally called a reflection type. An apparatus that performs optical signal processing such as a wavelength block is not limited to this configuration. For example, without using the mirror of FIG. 11, the signal processing element is positioned at the symmetry axis, and an output system composed of another lens and a spectroscopic element is provided on the opposite side of the incident system on the extension line of the incident optical path axis. Arranged configurations are also possible. This configuration is a configuration that performs demultiplexing and multiplexing of optical signals via independent incident and outgoing systems, respectively, and is called a transmission type. Furthermore, in the apparatus configuration shown in FIG. 11, it is possible to combine the optical signals by changing the direction of the mirror and using an emission system including another lens and a spectroscopic element arranged at an arbitrary position. For example, the reflecting surface of the mirror can be inclined by 45 degrees with respect to the incident optical path of the optical signal, and the exit system can be configured by a lens and a spectroscopic element arranged in a direction perpendicular to the incident optical path. When the signal processing element has a deflection function, a plurality of emission systems can be provided.

  In FIG. 11, the spectroscopic element 51 and the condensing lens 52 are arranged apart from each other by a front focal length FFL, and the signal processing element 53 and the condensing lens 52 are arranged apart from each other by a rear focal length BFL. The focal point of the light collected by the condenser lens 52 must be on the surface of the mirror 54 at all wavelengths used. When deviating from the mirror surface, there arises a problem of causing a coupling loss between input and output light. At the same time, since the beam spot diameter of the collected optical signal is increased, there arises a problem that the wavelength resolution is lowered.

  In addition, the signal processing element 53 needs to have a spatially periodic structure in order to selectively modulate each wavelength of the optical signal. For example, when the signal processing element 53 is a liquid crystal element, the structure of the element element of the liquid crystal element must be designed according to the optical characteristics of the spectroscopic element and the condenser lens.

  More specifically, it is known that the wavelength dependence of the condensing position on the signal processing element follows that obtained by multiplying the angular dispersion value of the spectroscopic element by the focal length of the condensing lens. The wavelength dependence of the light collection position is also called a linear dispersion value of the spectroscopic optical system. The linear dispersion value of the optical system determined by the spectroscopic element and the condensing lens needs to be sufficiently coincident with the linear dispersion value used for designing the structure of the signal processing element. If there is a deviation in these linear dispersion values, the position of the condensing point of the actual optical signal does not coincide with the position of each element element (for example, a pixel of the liquid crystal shutter element) of the signal processing element. This mismatch results in a wavelength error in the processed optical signal.

Japanese Patent Laid-Open No. 2002-250828 (pages 16, 19, 20, 20, 27, 29D, etc.) Japanese Patent Application Laid-Open No. 2004-239991 JP-A-8-75948 JP-A-9-90216

  However, the conventional optical signal processing apparatus has a problem that the optical coupling loss generated in the optical system including the lens is not yet sufficiently reduced. As shown in FIG. 11, the conventional reflective optical signal processing apparatus has a configuration in which one condenser lens is disposed between the spectroscopic element and the signal processing element. In the transmission type optical signal processing apparatus, one condensing lens is arranged in each of the optical path between the demultiplexing spectral element and the signal processing element and the optical path between the signal processing element and the multiplexing spectral element. It was the composition to do. The lens that has been used in such a conventional optical signal processing apparatus is an inexpensive single lens. A general imaging lens has not been optimized in consideration of the optical characteristics required for the optical system of the optical signal processing apparatus described above.

  In the optical signal processing device including the spatial modulation element described above, the following optical characteristics are required from the viewpoint of the lens arrangement position. In a normal imaging lens, the stop position is generally at the center of the lens or inside the group lens. On the other hand, in the present optical signal processing device, the outgoing beam width is determined by the structure of the spectroscopic element. Therefore, since it is considered that there is a stop position at the emission end of the spectroscopic element, it is necessary to optimize the lens specifications different from those of a normal imaging lens.

  Since the present optical signal processing apparatus has a configuration of an optical coupling system, the light beam needs to be incident on the signal processing element and the mirror perpendicularly. In the imaging lens, the image telecentric lens satisfies this requirement. From this condition, it is necessary to dispose the condensing lens so that the emission end of the spectroscopic element is not only at the stop position, but also at the FFL (front focal point) position of the condensing lens.

  Further, from the viewpoint of the characteristics unique to the lens, it is necessary to study chromatic aberration, distortion, spherical aberration, astigmatism, and the like. In particular, astigmatism affects the light collection position of the signal processing element on the optical axis, and thus greatly affects the optical coupling loss.

FIG. 12 is a diagram for explaining astigmatism of a lens. The left end Z _{1 in} FIG. 12a corresponds to the position of the emission end of the spectroscopic element, and the condenser lens 52 is disposed at the position Z ₂ and the mirror 54 is disposed at the position of the right end Z ₃ . For simplicity, signal processing elements are omitted. WOx is the exit beam radius of the spectroscopic element, θo is the maximum exit angle, and Wix is the radius of the beam spot focused on the signal processing element. In FIG. 12a, three optical paths 55a, 55b, and 55c having different emission angles are shown, and the light is condensed at each point on the x-axis. These optical paths correspond to optical signals having different wavelengths.

  FIG. 12 b is an enlarged view of the beam spot where the optical signal is collected in the vicinity of the mirror. The optical paths 55a, 55b, and 55c correspond to, for example, optical signals having wavelengths of 1604 nm, 1587 nm, and 1570 nm, respectively. The optical path 55b passing through the center of the condenser lens corresponds to the optical signal having the center wavelength λc of the system (communication wavelength band) handled by the optical signal processing device. The optical paths 55a and 55c are emitted from the spectroscopic element in the state of the maximum emission angle θo, and respectively correspond to the wavelengths λe at both ends of the system handled by the optical signal processing device. H is the maximum displacement of the beam spot position from the optical axis center position (the condensing point of the optical path 55a) on the signal processing element. Astigmatism (TAS: Tangential Astigmatism) is an aberration caused by focusing on the front of the mirror surface, not on the mirror surface, when the optical signal is emitted in the state of the maximum displacement H. Due to TAS, the condensing point on the mirror surface will spread. Thereby, the optical coupling loss varies depending on the position of the condensing point on the x-axis, that is, depending on the wavelength. Therefore, wavelength dependence occurs in the optical coupling loss.

  FIG. 13 is a diagram illustrating an example of the TAS characteristic of a conventional double spherical single lens. The abscissa indicates the bending radius (f / R2) when the curvature radius R2 of the second surface of the lens and the focal length are f, and the ordinate indicates TAS / f. The lens material is BK7 (n = 1.5), and the TAS is calculated with an angle of view of ± 2.4 degrees.

  As is apparent from FIG. 13, even if the bending is optimized, the minimum value of | TAS | / f is only about 0.005. Even if the bending is optimized for one lens, TAS cannot be kept sufficiently small. For example, when f = 50 mm and | TAS | /f=−0.005, the maximum coupling loss due to TAS of the lens is estimated to be 2.5 dB. Considering that the overall excess loss of the optical signal processing apparatus of FIG. 11 is about 6 dB, TAS accounts for a large proportion of the overall excess loss, and it is necessary to further reduce TAS.

  As another conventional technique, there is a lens using a high refractive index material, and it is well known that there is little aberration (Patent Document 3). However, a silicon lens (n≈3.5) is difficult to process and the manufacturing cost of the lens is high. An aspherical lens having an aspherical surface optimized for each optical system also reduces aberrations, but the manufacturing method is complicated and expensive. An achromatic doublet is known as a lens having a small spherical aberration. However, it is necessary to bond two kinds of materials, which is difficult to process, and the manufacturing cost of the lens is high. Furthermore, a configuration using three or more lenses has been proposed (Patent Document 4). Although the lenses according to any of the above-described techniques have improved TAS performance as compared with a spherical single lens, they are expensive.

  The conventional optical signal processing apparatus has another problem. In the optical signal processing apparatus as shown in FIG. 11, the optical signal is condensed on the signal processing element at a different position for each wavelength of the optical signal. As described above, the linear dispersion value of the optical system determined by the spectroscopic element and the condensing lens needs to be sufficiently coincident with the linear dispersion value used for designing the structure of the signal processing element. In the case of an optical signal processing device configured with an optical system composed of one condenser lens as shown in FIG. 11, for example, when performing signal processing of one communication wavelength band (5000 GHz) with a resolution of 5 GHz, An error accuracy of at least 0.1% or less is required for the linear dispersion value of the optical system. However, the focal length error of the condenser lens by a general manufacturing method reaches about 1%. For this reason, in order to obtain a high accuracy with an error of 0.1% or less with respect to the linear dispersion value of the optical system, special adjustment processing of the optical system is required in the manufacturing process of the optical signal processing degree device. . For this reason, there has been a problem that the manufacturing cost of the optical signal processing device becomes high.

  In addition, the optical signal processing device for communication is required to have a large amount of return loss and a small crosstalk, unlike other optical devices intended for imaging. For example, when light reflected by each surface of the lens is combined with an incident beam, the return loss is reduced. Further, in the arrangement configuration of the reflection type optical signal processing device, when the light reflected by each surface of the lens is combined with the outgoing beam, the amount of crosstalk is increased. Therefore, there has been a demand for optical signal processing with a sufficiently large return loss and a sufficiently small crosstalk amount.

  As described above, in the conventional optical signal processing apparatus, there is a problem that the optical coupling loss generated in the spatial optical system including the single lens is not sufficiently reduced, and the adjustment processing at the time of manufacturing the optical signal processing apparatus is complicated. There was a problem. In addition, there is a problem that the cost of the apparatus and the manufacturing method is high. Furthermore, there has been a demand for securing a reflection attenuation amount in the optical system and reducing the crosstalk amount.

  In order to achieve such an object, the present invention provides a spectroscopic unit that splits a plurality of optical signals having different wavelengths at an emission angle corresponding to the wavelength of the optical signal, and Two spherical condensing lenses for condensing the optical signal separated by the spectroscopic means, the two spherical condensing lenses being arranged on the spectroscopic means side on the propagation optical path of the optical signal. 1 condenser lens and a second condenser lens arranged on the signal processing means side, and the radius of curvature of the first lens surface of the first condenser lens on the spectroscopic means side is R1, The radius of curvature of the second lens surface on the signal processing means side is R2, the radius of curvature of the third lens surface on the spectroscopic means side of the second condenser lens is R3, and the fourth lens on the signal processing means side The curvature radius of the surface is R4, and the refraction of the first condenser lens Is n1, the refractive index of the second condenser lens is n2, the first lens surface refractive power is D1 = (n1-1) / R1, and the second lens surface refractive power is D2 = (1- n1) / R2, the third lens surface refractive power is D3 = (n2-1) / R3, and the fourth lens surface refractive power is D4 = (1-n2) / R4. When the combined focal length of the lens is f, (1) 0.15 <(D1f + D2f) <0.70, (2) 0.22 <D3f <0.76, (3) -0.25 <(D2f-D1f ) <1.0, and an optical signal comprising signal processing means for intensity-modulating, phase-modulating or deflecting the optical signal collected by the two spherical condenser lenses It is a processing device. It is possible to realize an optical signal processing apparatus with extremely little wavelength dependency by suppressing TAS and greatly reducing optical coupling loss.

  According to a second aspect of the present invention, in the signal processing device according to the first aspect, the D1, D2, D3, and D4 are: (4) 0.32 <(D1f + D2f) <0.62, (5) 0.32 < Each condition of D3f <0.62 and (6) −0.05 <(D2f−D1f) <0.80 is further satisfied.

  According to a third aspect of the present invention, in the signal processing apparatus according to the first or second aspect, the D1 further satisfies a condition of D1> 0. In the optical signal processing apparatus, it is possible to sufficiently increase the return loss and sufficiently reduce the crosstalk amount.

  According to a fourth aspect of the present invention, in the signal processing device according to any one of the first to third aspects, the first condenser lens is n1 <1.6, and the second condenser lens is n2 <1. Each condition of 6 is further satisfied.

  According to a fifth aspect of the present invention, in the signal processing device according to any one of the first to fourth aspects, the composite focal length of the two spherical condenser lenses is variable by adjusting the interval between the two spherical condenser lenses. Thus, the position of the condensing point of the optical signal on the signal processing means is adjusted. Manufacturing variations in the wavelength dispersion characteristics of the spectroscopic elements and long-term fluctuations in the center wavelength can be compensated by simple adjustment.

  According to a sixth aspect of the present invention, in the signal processing device according to any one of the first to fifth aspects, the spectroscopic means is a grating, a prism, or an arrayed waveguide diffraction grating.

  According to a seventh aspect of the present invention, in the signal processing device according to any one of the first to sixth aspects, the spectroscopic means is a grating, a prism, or an arrayed waveguide diffraction grating.

  The invention according to claim 8 is a spectroscopic unit that splits a plurality of optical signals having different wavelengths at an emission angle corresponding to the wavelength of the optical signal, and two spherical surfaces that collect the optical signal split by the spectroscopic unit. A condenser lens, and signal processing means for intensity-modulating, phase-modulating, or deflecting the optical signal collected by the two spherical condenser lenses, and adjusting an interval between the two spherical condenser lenses; The optical signal processing apparatus is characterized in that the position of the condensing point of the optical signal on the signal processing means is adjusted by changing the combined focal length of the two spherical condenser lenses. Manufacturing variations in the wavelength dispersion characteristics of the spectroscopic elements and long-term fluctuations in the center wavelength can be compensated by simple adjustment.

  As described above, according to the present invention, the optical coupling loss in the spatial optical system of the optical signal processing apparatus can be greatly reduced. In addition, the adjustment of the optical signal processing device can be simplified. Furthermore, it is possible to secure the amount of reflection loss of the optical signal processing device and reduce the amount of crosstalk.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The optical signal processing apparatus of the present invention includes a condensing lens having a characteristic configuration found by the inventor by sufficiently considering the specific optical characteristics required for the optical system. Hereinafter, the configuration of the condenser lens according to the present invention will be described with particular attention to the reduction of TAS.

  FIG. 1 is a diagram conceptually showing the configuration of the optical system of the optical signal processing apparatus of the present invention. The configuration of the related art shown in FIG. 11 and FIG. 12 is different in that it includes two condensing lenses having a specific radius of curvature. FIG. 1 shows the configuration of a reflection type optical signal processing apparatus. That is, the optical signals of the respective wavelengths demultiplexed by the spectroscopic element 1 are condensed by the first condenser lens 2 and the second condenser lens 3. Each collected optical signal is selectively deflected by the signal processing element 4 for each wavelength in its intensity or phase or intensity and phase, or in its traveling direction. The optical signal processed by the signal processing element 4 is reflected by the mirror 5 to reverse the traveling direction. Since the operation after inversion is the same as in the prior art, a description thereof will be omitted.

  Examples of the spectroscopic element 1 include a grating and an AWG. Examples of the signal processing element 4 include a liquid crystal element, a MEMS mirror, and a nonlinear crystal. A feature of the present invention is the configuration of the condenser lens. Although the optical signal processing apparatus shown in FIG. 1 has a reflective configuration, it is needless to say that the optical signal processing device can also be applied to a transmissive configuration, as will be described later as an example. In the transmissive configuration, the optical system of the incident system and the output system can be arranged in a straight line by using two spectroscopic elements. In addition, the incident and exit optical systems can be arranged in different directions. It should also be noted here that the signal processing elements also include deflection elements. It should be noted that the present invention includes a case where there are three or more spectroscopic elements in the entire apparatus because a configuration including a plurality of emission systems is possible by using a deflection element.

  The above-described optical signal processing apparatus to which the present invention can be applied generally has an inevitable transmission loss of several dB or more due to the inherent light transmission loss of the spectroscopic element and the signal processing element. In the design of such an optical signal processing apparatus, an excess loss that is an order of magnitude smaller than the transmission loss can be allowed as a design loss value. Consider a case where the focal length of the condensing lens and the angular dispersion value of the spectroscopic element are set so that signal processing of one communication wavelength band (5000 GHz) can be performed with a resolution of several GHz. Here, in order to suppress the excess loss to 0.5 dB or less, it is necessary that | TAS | / f <0.002. f is the focal length of the condenser lens.

  In the signal processing by the optical signal processing device according to the present invention, the bandwidth of the communication wavelength band is narrow with respect to the center wavelength, so that chromatic aberration is not a problem. In addition, the optical signal used in the signal processing according to the present invention is a Gaussian beam of a basic mode in which the intensity of ambient light is small. Therefore, spherical aberration (LAS) has less influence than astigmatism (TAS), and | LSA | is sufficient if it is less than or equal to | TAS |.

  The inventor has focused on the reduction of TAS and found the lens configuration of the present invention using the refractive power (D1, D2, D3, D4) of each lens surface as a parameter. In the two-group two-lens rather than the single lens of the prior art, the focal lengths of the two lenses are close (D1f + D2f≈D3f + D4f≈0.5), and the second condenser lens has the convex side as the first lens. It has been found that the wavelength-dependent optical coupling loss is remarkably reduced in the case of a plano-convex lens facing the condenser lens side (D3f≈0.5). Therefore, in order to obtain a range in which the effect of reducing the optical coupling loss is large, the (D1f + D2f) dependency and the D3f dependency of the TAS value are obtained by ray tracing calculation using (D2f−D1f) as a parameter. D4f is determined automatically by determining D1f, D2f and D3f.

In the simulation calculation by ray tracing, the lens material used was a general-purpose optical glass (≡n <1.6), which is particularly inexpensive, and only a spherical lens was examined. As shown in FIG. 1, the curvature radius of the first lens surface (spectral element side) of the first condenser lens arranged on the spectroscopic element side is R1, and the curvature radius of the second lens surface on the opposite side is R1. R2, the curvature radius of the third lens surface (spectral element side) of the second condenser lens disposed on the signal processing element side is R3, and the curvature radius of the fourth lens surface on the opposite side is R4. The refractive indexes of the first condenser lens and the second condenser lens are n1 and n2, respectively. Here, the sign of the radius of curvature is R> 0 when convex toward the spectroscopic element, and R <0 when convex toward the signal processing element. Further, the refractive power Dn of each lens surface is defined as in Formula 1 to Formula 4.
D1 = (n1-1) / R1 (Formula 1)
D2 = (1-n1) / R2 (Formula 2)
D3 = (n2-1) / R3 (Formula 3)
D4 = (1-n2) / R4 (Formula 4)

  2 and 3 are diagrams showing simulation results of lenses used in the optical signal processing device according to the present invention. Both FIG. 2 and FIG. 3 are expressed by mapping the TAS value distribution when the horizontal axis is (D1f + D2f) and the vertical axis is D3f. The value of TAS / f is shown in the range of 0 to 0.010 in increments of 0.001.

  2a, 2b, and 2c show TAS distributions when D2f−D1f = −0.05, 0.50, and 0.80, respectively. A region represented by a rectangle is a region that satisfies the condition of | TAS | / f <0.002.

  3a, 3b, and 3c show TAS distributions when D2f−D1f = −0.25, 0.50, and 1.0, respectively. A region represented by a rectangle is a region that satisfies the condition of | TAS | / f <0.005.

  As can be seen from FIG. 2 and FIG. 3, in the space having (D1f + D2f), D3f, and (D2f−D1f) as three axes, the region satisfying the condition | TAS | / f <0.002 is (D2f− D1f) It can be seen that the shape is close to a spheroid, such as a rugby ball shape that is long in the axial direction. A rectangular parallelepiped range including this region, that is, a range of 0.32 <(D1f + D2f) <0.62 and 0.32 <D3f <0.62 and −0.05 <(D2f−D1f) <0.80. For example, the condition of | TAS | / f <0.002 is satisfied. Similarly, in the range of 0.15 <(D1f + D2f) <0.70 and 0.22 <D3f <0.76 and −0.25 <(D2f−D1f) <1.0, the conventional single lens is used. TAS is smaller than that, and the condition of | TAS | / f <0.005 is satisfied.

  Further, the beam propagating from the spectroscopic element to the first condenser lens is parallel or slightly spread. When the first surface of the first condenser lens is a flat surface (D1 = 0) or a concave surface (D1 <0), the parallel light or the light having a spread is easily combined with the beam on the spectroscopic element side. In the optical signal processing apparatus having the reflective arrangement, if the first surface of the first condenser lens is a convex surface (D1> 0), the spread angle of the reflected light is increased to prevent the above-described coupling, and the cross There is an effect that the amount of talk can be reduced.

  Various embodiments of the optical signal processing apparatus according to the present invention using a condensing lens that satisfies the above-described conditions will be described in detail below.

  Embodiment 1 FIG. 4 is a diagram showing a reflection-type optical signal processing degree apparatus and an optical coupling loss measurement system according to Embodiment 1 of the present invention. Since the optical signal processing apparatus of the present embodiment is basically the same as the configuration shown in FIG. 1, only differences from the configuration of FIG. 1 will be described. The spectroscopic element is AWG1, and a collimating lens 6 is disposed on the emission side of AWG1. The emitted light of each wavelength demultiplexed by the AWG 1 and converted into parallel rays by the collimator lens 6 is collected by the condenser lens including the first condenser lens 2 and the second condenser lens 3. Other configurations are the same as those in FIG. FIG. 4A is a top view of the AWG 1 as viewed from the vertical direction toward the demultiplexing surface (xz plane). The outgoing light travels in the Z-axis direction at an outgoing angle corresponding to the wavelength from the outgoing end of the AWG 1. FIG. 4b is a side view.

  In order to measure the optical coupling loss of the optical signal processing apparatus of this embodiment, a broadband light source 8 and an optical spectrum analyzer 7 are further connected to the AWG 1 via a circulator 9. The detailed parameters of AWG1 are as follows: the center wavelength is C-band center wavelength of 1547.3 nm, the angular dispersion value is 0.1435 deg / nm, the X-axis direction output beam radius is 3.0 mm, and the Y-axis direction output beam radius is 4.5 μm. The Y-axis focal length of the collimating lens 6 is 0.9 mm.

  FIG. 5 is a diagram showing a table comparing the design values of the conventional single lens (A and B) used for the comparative experiment and the condenser lens (C) of the present invention. As the lens material, the cheapest BK7 is used for all lenses. T1 is the thickness of the lens. The condenser lens A is a conventional biconvex lens. The condenser lens B is a conventional plano-convex lens.

  Since the purpose is to evaluate the optical coupling loss, a transparent glass plate is arranged as a dummy in place of the signal processing element 4 in the actual measurement with the configuration of FIG. The relative positions of the AWG 1, the collimating lens 6, the condensing lens, and the mirror 5 were adjusted so that the optical coupling loss was minimized at the design center wavelength (1547.3 nm). Further, the distance between the collimating lens 6 and the condenser lens was adjusted so that the optical coupling loss was minimized at the center wavelength ± 17 nm.

  FIG. 6 is a diagram showing the transmission characteristics of the optical signal processing device according to the present invention as compared with a conventional single lens. Each transmission characteristic in FIG. 6 corresponds to each lens shown in FIG. The horizontal axis indicates the wavelength (nm) of the C band (1530-1565), and the vertical axis indicates the light transmittance (dB) of the optical signal processing device. The unavoidable loss due to the AWG, collimating lens, and mirror is about 4 dB, and the transmission loss value is substantially the same among the three lenses at the center wavelength. However, in the case of the condensing lens A and the condensing lens B of the prior art, the transmission loss is greatly increased in the bands at both ends of the C band. That is, it can be seen that the optical coupling loss has a large wavelength dependency. For example, at 1565 nm, which is the band edge on the long wavelength side, the optical coupling loss is increased by 3 dB or more for the condenser lens A and 6 dB or more for the condenser lens B, compared to the center wavelength. On the other hand, in the case of the condensing lens C according to the present invention, it can be seen that there is only a deviation of 0.14 dB over the entire wavelength band of the C band.

  Example 2 Next, a reflective wavelength blocker based on the configuration of the optical signal processing apparatus according to the present invention will be described. The basic apparatus and the configuration of the evaluation system are the same as those shown in FIG. In the case of a wavelength blocker, a liquid crystal shutter element is used as the signal processing element 4.

  FIG. 7 is a structural diagram in the vicinity of the liquid crystal shutter and the mirror. FIG. 7A is a structural diagram of the liquid crystal shutter element viewed from the condenser lens side. In the liquid crystal shutter element 10, the liquid crystal pixels 11 are arranged in a line at a constant pitch Wp. Each of the liquid crystal pixels corresponds to each wavelength of a channel in the communication wavelength band. The pitch Wp is designed according to the wavelength dispersion value of the spectroscopic optical system (the angle dispersion value is 0.1435 deg / nm as in the first embodiment). In this embodiment, the pitch Wp of the liquid crystal pixels is 0.1 mm, and the number of channels is 40. FIG. 7 b is a side view of the liquid crystal shutter element and the mirror.

  FIG. 8 is a diagram showing the transmission characteristics of the wavelength blocker in the first channel. The center wavelength of the first channel is 1531.73 nm. Each transmission characteristic in FIG. 8 corresponds to each lens (A, B, C) shown in FIG. In the case of the condensing lens A and the condensing lens B, it can be seen that the optical coupling loss is larger than that of the condensing lens C according to the present invention, and the transmission bandwidth is narrowed due to defocus caused by TAS. . In addition, it is understood that the central wavelengths of the condenser lenses A, B, and C are randomly shifted from 1531.73 nm.

  According to the condenser lens C according to the present invention, since the positions of the two lenses can be easily adjusted, the transmission spectrum can be adjusted to a desired center wavelength position as shown in FIG. For example, as shown in FIG. 4, the center wavelength can be easily adjusted by adjusting the distance z1 between the first condenser lens 2 and the second condenser lens 3.

In general, it is known that the combined focal length f of _two lenses (focal lengths f ₁ and f ₂ ) arranged at a distance d is approximately expressed by the following equation (5).
f ⁻¹ = f ₁ ⁻¹ + f ₂ ⁻¹ −df ₁ ⁻¹ f ₂ ⁻¹ (Formula 5)
By making the distance d between the two lenses adjustable, the combined focal length f can be adjusted with high accuracy. For example, in the case of f ₁ = f ₂ = 100 mm, the adjustment accuracy of the inter-lens distance d necessary for adjusting the combined focal length f with an error accuracy of 0.1% is about 0.2 mm. Therefore, the adjustment can be easily performed with the adjustment accuracy that can be realized in the adjustment assembly process of the normal optical signal processing apparatus.

  As described above, according to the condensing lens configuration of the optical signal processing device according to the present invention, it is possible to easily adjust the center wavelength of the communication channel to a desired value by combining the two condensing lenses. Exhibits excellent effects. The focus of the conventional achromatic doublet and aspherical single lens (single lens) is fixed. Therefore, in order to match the wavelength dispersion characteristic of the spectroscopic element with the wavelength dispersion characteristic of the signal processing element, it is necessary to accurately match the wavelength direction pitch of the signal processing element. In the two-lens condensing lens according to the present invention, the focal length can be adjusted by changing the distance between the first condensing lens and the second condensing lens. Therefore, manufacturing variations in wavelength dispersion characteristics of the spectroscopic elements and long-term fluctuations in the center wavelength can be compensated by simple adjustment.

  Embodiment 3 Next, a wavelength selective switch based on the configuration of the optical signal processing device according to the present invention will be described. The optical signal processing apparatus according to the present invention may be any type as long as it condenses optical signals demultiplexed at different emission angles for each wavelength and performs an operation for modulating or deflecting the optical signal in a wavelength selective manner. It can be applied to other devices.

  FIG. 9 is a configuration diagram of a wavelength selective switch based on the optical signal processing device of the present invention. FIG. 9a is a top view of the surface on which the MEMES mirrors are arranged as viewed vertically. FIG. 9b is a side view. This wavelength selective switch has a reflective configuration using a transmissive diffraction grating 14 as a spectroscopic element and a MEMS deflection mirror 13 as a mirror. The optical signal is input from the plurality of optical fibers 16 to the transmissive diffraction grating 14 via the fiber collimator 15. The optical signal selectively switched by the MEMS deflection mirror 13 is output to the optical fiber 16 in the reverse direction again.

  In the wavelength selective switch having the above-described configuration, the radii of curvature of the condenser lenses 2 and 3 are sequentially set to R1, R2, R3, and R4 from the transmission diffraction grating 14 toward the MEMS deflection mirror 13.

The lens material is F2, the refractive index is 1.59, the lens surface radius of curvature is R1 = ∞ mm, R2 = −35.4 mm, R3 = 35.4 mm, R4 = ∞ mm, and the focal length f is 30 mm. At this time, each lens surface refractive power is as follows: D1 = 0.0000 (mm ⁻¹ ), D2 = 0.167 (mm ⁻¹ ), D3 = 0.167 (mm ⁻¹ ), D4 = 0.0000 (mm) ^-1 ). In the present invention, each evaluation function value that defines the TAS value is D1f + D2f = 0.50, D3f = 0.50, and D2f−D1f = 0.50. At this time, the astigmatism TAS / f is 0.00067. The angular dispersion value of the diffraction grating was 0.1435 deg / nm, and the center wavelength of the device was 1547.3 nm (C-band communication wavelength band).

  In the wavelength selective switch shown in FIG. 9, the deviation of the optical coupling loss in the channel at a maximum distance of 20 nm from the center wavelength was 0.10 dB. In the case of the wavelength selective switch using the lens of the prior art, the deviation is suppressed to a very small value as compared with the case where the deviation is 3 dB or more, and a wavelength selective switch having very little wavelength dependency can be realized.

  Example 4 A transmission type configuration of the optical signal processing device according to the present invention will be described. The description has so far centered on the reflection type configuration, but it goes without saying that the present invention can be applied to a transmission type optical signal processing apparatus. In the transmissive configuration, the condenser lens according to the present invention is disposed in both the incident system and the output system, with the position of the signal processing element as the center of symmetry.

  FIG. 10 is a configuration diagram of a transmissive optical signal processing device according to the fourth embodiment of the present invention. A plurality of optical signals having different wavelengths are input to the first AWG 21 from an optical fiber or the like and are demultiplexed for each wavelength. The demultiplexed optical signal passes through a collimator lens 22 and is collected by an incident side condenser lens 30 including a first condenser lens 23 and a second condenser lens 24 configured according to the present invention. The condensed optical signal is subjected to intensity modulation in different pixels of the liquid crystal shutter element 25 for each wavelength. The optical signal that has been subjected to the intensity modulation and passed through the liquid crystal shutter element 25 is collimated again by the output side condensing lens 31 including the third condensing lens 26 and the fourth condensing lens 27 configured according to the present invention. The The collimated optical signal passes through the second collimating lens 28, is input to the second AWG 29, is multiplexed, and is output as an optical output to the optical fiber.

  In the transmission type optical signal processing apparatus described above, the incident side condenser lens 30 and the emission side condenser lens 31 are configured as follows. Here, the incident side condensing lens 30 and the emission side condensing lens 31 are symmetrically configured with the liquid crystal shutter element 25 as the center. Accordingly, the curvature radii are R1, R2, R3, R4 (incident side), R4, R3, R2, R1 (exit side) in the order in which the optical signal passes.

The lens material is SiO ₂ , the refractive index is 1.44, the lens surface curvature radii are R1 = 264 mm, R2 = −264 mm, R3 = 206.25 mm, R4 = −366.56 mm, and the focal length f is 150 mm. At this time, the refractive powers of the lens surfaces are: D1 = 0.0017 (mm ⁻¹ ), D2 = 0.0017 (mm ⁻¹ ), D3 = 0.0021 (mm ⁻¹ ), D4 = 0.0012 (mm ^{− 1} ). The evaluation function values that define the TAS value in the present invention are D1f + D2f = 0.50, D3f = 0.32, and D2f−D1f = 0.00. At this time, the astigmatism TAS / f is 0.0011. The angular dispersion value of AWG was 0.1435 deg / nm, and the center wavelength of the apparatus was 1587.0 nm (L band communication wavelength band).

  In the optical signal processing device of FIG. 10, the deviation of the optical coupling loss in the channel separated from the center wavelength by a maximum of 20 nm was 0.26 dB. In the case of a wavelength blocker using a conventional condensing lens, the deviation is suppressed to a very small value compared to 3 dB or more, and a wavelength blocker with very little wavelength dependency can be realized.

  Furthermore, in the present example of the transmissive configuration, R1> 0 and the return loss was 40 dB or more.

  As described above in detail, according to the optical signal processing device of the present invention, the optical coupling loss in the spatial optical system can be greatly reduced. The wavelength dependence of the optical coupling loss can be greatly reduced. In addition, the adjustment of the optical signal processing device can be simplified. Furthermore, it is possible to secure the amount of return loss in the optical system and reduce the amount of crosstalk.

  The present invention can be applied to an optical signal processing device used for optical communication. It can be applied to wavelength blockers, wavelength equalizers, dispersion compensators, etc.

It is a block diagram of the optical signal processing apparatus of this invention. It is a figure which shows TAS distribution of the lens which concerns on the optical signal processing apparatus of this invention. It is a figure which shows TAS distribution of the lens which concerns on the optical signal processing apparatus of this invention. 1 is a configuration diagram of a reflective optical signal processing device according to a first embodiment of the present invention. FIG. It is a figure which shows the comparison table of the design value of the conventional single lens and the condensing lens which concerns on this invention. It is a figure which shows the light transmission spectrum of the optical signal processing apparatus of Example 1. FIG. It is a structural diagram of a liquid crystal shutter and a mirror. It is a figure which shows the transmission characteristic of the wavelength blocker which concerns on Example 2. FIG. 6 is a structural diagram of a wavelength selective switch according to Embodiment 3. FIG. 6 is a structural diagram of a transmission wavelength blocker according to Embodiment 4. FIG. It is a notional block diagram of the conventional optical signal processing apparatus. It is a figure explaining the astigmatism of a lens. It is a figure which shows the TAS characteristic of the conventional double spherical single lens.

Explanation of symbols

1, 51 Spectroscopic element 2, 3, 23, 24, 26, 27, 52 Condensing lens 4, 10, 13, 25, 53 Signal processing element 5, 54 Mirror 6, 15, 22, 28 Collimating lens 7 Optical spectrum analyzer 8 Broadband light source 9 Circulator 14 Transmission type diffraction grating 21, 29 AWG

Claims (8)

A spectroscopic means for splitting a plurality of optical signals having different wavelengths at an emission angle according to the wavelength of the optical signal;
Two spherical condensing lenses for condensing the optical signal dispersed by the spectroscopic means, wherein the two spherical condensing lenses are arranged on the spectroscopic means side on a propagation optical path of the optical signal. The first condensing lens and the second condensing lens arranged on the signal processing means side, and the radius of curvature of the first lens surface of the first condensing lens on the spectroscopic means side is R1, The radius of curvature of the second lens surface on the signal processing means side is R2, the radius of curvature of the third lens surface on the spectroscopic means side of the second condenser lens is R3, and the fourth radius on the signal processing means side. The radius of curvature of the lens surface is R4, the refractive index of the first condenser lens is n1, the refractive index of the second condenser lens is n2, and the first lens surface refractive power is D1 = (n1-1). ) / R1, and the second lens surface refractive power is D2 = (1-n1) / R 2. The third lens surface refractive power is D3 = (n2-1) / R3, the fourth lens surface refractive power is D4 = (1-n2) / R4, and the two spherical condenser lenses are combined. If the focal length is f,
(1) 0.15 <(D1f + D2f) <0.70
(2) 0.22 <D3f <0.76
(3) -0.25 <(D2f-D1f) <1.0
Satisfying each condition of
An optical signal processing apparatus comprising: signal processing means for intensity-modulating, phase-modulating, or deflecting the optical signal collected by the two spherical condenser lenses. The D1, D2, D3 and D4 are:
(4) 0.32 <(D1f + D2f) <0.62
(5) 0.32 <D3f <0.62
(6) -0.05 <(D2f-D1f) <0.80
The optical signal processing apparatus according to claim 1, further satisfying the following conditions:   The optical signal processing apparatus according to claim 1, wherein the D1 further satisfies a condition of D1> 0.   The said 1st condensing lens further satisfy | fills each conditions of n1 <1.6, and a said 2nd condensing lens further satisfies n2 <1.6. Optical signal processing device.   Adjusting the position of the condensing point of the optical signal on the signal processing means by adjusting the interval between the two spherical condensing lenses and varying the combined focal length of the two spherical condensing lenses. 5. The optical signal processing apparatus according to claim 1, wherein   6. The optical signal processing apparatus according to claim 1, further comprising a mirror that reflects the optical signal modulated by the signal processing means and bends the optical path of the optical signal.   7. The optical signal processing apparatus according to claim 1, wherein the spectroscopic means is a grating, a prism, or an arrayed waveguide diffraction grating. A spectroscopic means for splitting a plurality of optical signals having different wavelengths at an emission angle according to the wavelength of the optical signal;
Two spherical condenser lenses for condensing the optical signal dispersed by the spectroscopic means;
Signal processing means for intensity-modulating, phase-modulating or deflecting the optical signal collected by the two spherical condenser lenses;
Adjusting the position of the condensing point of the optical signal on the signal processing means by adjusting the interval between the two spherical condensing lenses and varying the combined focal length of the two spherical condensing lenses. An optical signal processing device.

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