JPWO2006085650A1 - Wavelength monitoring device - Google Patents

Abstract

Provided is a wavelength monitor device capable of achieving an increase in size and a reduction in cost by simplifying an optical system. A wavelength monitor device is configured by a collimating lens, a birefringent element, a Faraday rotator, a diffraction grating having a grating surface in which a plurality of grooves or protrusions are formed in parallel, and a linear image sensor. The crystal axis of the birefringent element and the diffraction grating are positioned so that the formation direction of the part is parallel to the extraordinary ray separation direction in the birefringent element, and the optical signal from the WDM device is collimated with a collimating lens, The optical signal is separated into an ordinary ray and an extraordinary ray by a birefringent element, only the ordinary ray is transmitted to the Faraday rotator, the polarization direction is rotated by 90 degrees, and the polarization directions of the ordinary ray and the extraordinary ray are made the same direction, While diffracting at the same groove or projection, the spectral image of the two diffracted light components after diffraction is received by one light receiving portion of the linear image, and the sum of the intensity of the optical signal is output.

Description

  The present invention relates to a wavelength monitor device for measuring the wavelength of an individual channel in a wavelength division multiplexing optical communication system.

  Currently, in order to cope with rapidly increasing communication traffic in an optical communication system, multiplex communication methods such as time division multiplexing (TDM) and wavelength division multiplexing (WDM) are employed. Among them, the WDM method allows a large amount of information to be transmitted by substantially increasing the optical communication bandwidth by simultaneously transmitting optical signals of many different wavelengths through a single medium such as an optical fiber. It is a multiplex communication system.

  Currently, the wavelength interval and frequency interval of individual channels in the WDM system (hereinafter collectively referred to as “interval” as necessary) are reduced to about 0.8 nm or 100 GHz. By narrowing the interval, the number of wavelengths that can be multiplexed is increased to cope with an increase in communication traffic. However, the wavelength of the light source of each channel may gradually deviate from the nominal center frequency in the long term, and on the other hand, the interval tends to be narrowed as communication traffic increases as described above. .

  Therefore, there is an increased possibility that crosstalk will occur due to interference between channels adjacent to each other in terms of wavelength scale, or between channels physically adjacent to each other. In order to prevent such interference, a normal wavelength drift of ± 0.1 nm must be guaranteed. Therefore, the stability of the wavelength and intensity (power) of the light source of each channel is particularly important, and for this reason, a wavelength monitoring device that monitors the wavelength and intensity of the light source with high accuracy is required.

  As a wavelength monitoring device for monitoring the wavelength of a WDM channel, various structures have been proposed. As an example, an array optical waveguide (hereinafter referred to as an “AWG waveguide”) using a quartz optical waveguide as shown in FIG. ”), The optical signal of each wavelength is demultiplexed for each optical waveguide (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2003-140212 (page 3, FIG. 5)

  When an optical signal having a wavelength of λ1 to λ4 is input to one of the optical fibers 101 on the input side, the wavelength monitor device 100 in FIG. 10 passes through the first lens 102, the AWG optical waveguide 103, and the second lens 104, and then the output side. The optical signals of λ1, λ2, λ3, and λ4 are separately output from the plurality of optical fibers 105, and the output optical signals are received by a light receiver (not shown) to monitor the wavelength of the optical signals. Yes.

  On the other hand, as shown in FIG. 11, the optical signal is separated into two polarization components by a birefringent element, and the two polarization components are diffracted and condensed by a Littrow lens and a diffraction grating, respectively, and completely separated. There has also been proposed a wavelength monitor device that forms two spectral images of a state and detects the sum of the intensities of the two spectral images with a linear image sensor (see, for example, Patent Document 2).

JP 2003-194627 A (page 1, FIG. 1)

  11 includes an incident optical fiber 107, two birefringent elements (indicated as “Savart plate” in Patent Document 2) 108, a Littrow lens 109, a diffraction grating 110, a linear image sensor ( In Patent Document 2, the optical signal L1 from the incident optical fiber is separated into two polarization components L21 and L22 by the birefringent element 108, and the Littrow lens 109 After collimating and entering the diffraction grating 110 and diffracting into the two diffracted light components L41 and L42, the light is again incident on the Littrow lens 109 and condensed to form two completely separated spectral images. By receiving light on the light receiving surface of the image sensor 111, the intensity of each wavelength of the optical signal L1 is monitored.

  However, the wavelength monitor apparatus of FIG. 10 has the drawback that the fabrication accuracy of the array structure is severe, and that the entire array becomes larger as the spacing becomes smaller, such as 50 GHz, in the future as the density of the WDM system increases. is there. In addition, the cost of the AWG optical waveguide is high, and the entire wavelength monitor device becomes expensive.

  In addition, since the wavelength monitor apparatus of FIG. 11 uses two birefringent elements, the polarization axes of each other must be precisely adjusted, and the assembly process becomes complicated. Furthermore, since the optical signal is diffracted by the diffraction grating and then incident on the Littrow lens again, if the wavelength of the optical signal incident from the incident optical fiber changes and the diffraction angle at the grating surface of the diffraction grating changes for each wavelength, In addition, since it is necessary to make all of the diffracted light having different diffraction angles incident on the Littrow lens over the wavelength range to be monitored, a large-diameter Littrow lens must be used, resulting in an increase in the size of the optical system. is there.

  Also, when trying to reduce the diameter of the Littrow lens, it is necessary to optimize the angle of the grating surface for each wavelength by moving the diffraction grating for each wavelength of the optical signal, so that the diffracted light always enters the Littrow lens. A separate drive mechanism for changing the angle is required, which increases the cost and increases the size of the entire wavelength monitor. Furthermore, the drive control of the diffraction grating must be performed for each wavelength, and the drive of the wavelength monitor device becomes complicated and complicated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a wavelength monitor device that can achieve prevention of enlargement and cost reduction by simplifying the optical system. .

  The invention according to claim 1 of the present invention includes a collimating lens, a birefringent element that separates an optical signal into two polarization components of an ordinary ray and an extraordinary ray whose polarization directions are orthogonal to each other, and a Faraday having a rotation angle of 90 degrees. A rotor, a diffraction grating having a grating surface in which a plurality of grooves or protrusions are formed in parallel, and a linear image sensor, and the forming direction of the grooves or protrusions is within the birefringent element. The crystal axis of the birefringent element and the diffraction grating are positioned so as to be parallel to the extraordinary ray separation direction. Further, an optical signal from the WDM apparatus is incident on the collimator lens and collimated. A signal is separated into the ordinary ray and the extraordinary ray by the birefringent element, and only the polarization component emitted as the ordinary ray from the birefringent element is transmitted to the Faraday rotator. By rotating the polarization direction by 90 degrees, the polarization direction of the two polarization components is made the same direction, and the polarization component in this state is diffracted by the same groove or projection, A wavelength monitor device characterized in that a spectral image of a diffracted light component is received by one light-receiving unit of the linear image in a state where the spectral images are completely separated from each other, and an intensity sum of the optical signal is output. It is.

  According to the wavelength monitoring device of the present invention, the polarization direction of the separated polarization component is adjusted to the same direction, and the optical signal is incident on the grating surface of the diffraction grating, so that the diffraction angle changes for each polarization component. It is possible to receive the spectral images of the two diffracted light components at the light receiving portion of the linear image sensor at the same diffraction angle. Therefore, it is possible to receive two diffracted light component spectral images of the same order on one light receiving portion of the linear image sensor, and two diffracted light components having the same frequency and same order on different light receiving portions of the linear image sensor. Therefore, the wavelength resolution of the wavelength monitor device can be improved.

  Furthermore, even if there are variations in characteristics for each of the plurality of light receiving portions of the linear image sensor, two spectral images having the same frequency can be received by the same light receiving portion, so that the influence can be prevented.

  Further, since the optical system of the wavelength monitor device is composed of a collimating lens, a birefringent element, a Faraday rotator, a diffraction grating, and a linear image sensor, like an AWG waveguide, as in the conventional wavelength monitor device. An optical system can be formed without using expensive components. Therefore, the cost of the wavelength monitor device can be reduced.

  In addition, by forming an optical system that does not include lens components such as a rerout lens as in the conventional wavelength monitor device just before the diffraction grating, it is possible to prevent an increase in the size of the optical elements that constitute the optical system. Further, it is possible to prevent an increase in the size of the wavelength monitor device.

  In addition, since no lens parts are arranged, even if the wavelength of the optical signal incident from the WDM device changes, the light receiving unit of the linear image sensor can receive light without driving the diffraction grating each time. It is possible to achieve downsizing and cost reduction by reducing the number of parts of the apparatus and facilitating drive control of the wavelength monitor apparatus.

  Also, by making the polarization directions of the two polarization components of the optical signal incident on the diffraction grating the same direction, the two polarization components can always be diffracted at a constant angle when diffracting in the diffraction grating. Further, the two polarization components are diffracted by the same groove or protrusion. As described above, the optical path length difference between the two diffracted light components after diffraction can be made zero.

  Furthermore, when the two polarization components are incident on the grooves or projections on the grating surface, the diffracted light after diffraction is obtained by making the polarization directions of the two polarization components parallel to the formation direction of the grooves or projections. Since the polarization component in the other direction can be prevented from entering the component as noise, the wavelength resolution of the wavelength monitor device can be improved.

1 is a schematic perspective view schematically showing a wavelength monitor device according to a first embodiment of the present invention. The schematic plan view which looked at the wavelength monitor apparatus of FIG. 1 from the arrow a direction in FIG. The block diagram of the WDM apparatus in which the wavelength monitor apparatus of FIGS. 1-2 is integrated. FIG. 3 is a state diagram showing a polarization plane of an optical signal viewed from the z-axis direction in each cross section indicated by broken lines (a) to (c) in FIG. 2. FIG. 2 is a schematic partial perspective view showing only a diffraction grating and a linear image sensor constituting the wavelength monitor device of the first embodiment. FIG. 5 is a state diagram schematically showing a light receiving state of two spectrum images received by a linear image sensor. The schematic perspective view which shows typically the wavelength monitor apparatus of 2nd Embodiment. FIG. 5 is a schematic partial perspective view showing only a diffraction grating and a linear image sensor constituting the wavelength monitor device of the second embodiment. FIG. 9 is a schematic partial side view of a diffraction grating and a linear image sensor when FIG. 8 is viewed from the direction of arrow b. The schematic diagram which shows an example of the conventional wavelength monitor apparatus by AWG waveguide. The perspective view which shows an example of the other conventional wavelength monitor apparatus.

Explanation of symbols

1, 16 Wavelength monitoring device 2 Birefringent element
3a, 3b Faraday rotator 4, 17 Diffraction grating 5 Collimator lens 6 Linear image sensor 7 WDM device 8 Optical transmitter 9 Multiplexer
10 Optical amplifier
11 duplexer
12 Optical receiver
13, 15 Optical fiber
14 coupler

<First Embodiment>
A first embodiment according to the wavelength monitoring device of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic perspective view schematically showing the wavelength monitor device of the first embodiment, and FIG. 2 is a schematic view when the wavelength monitor device of FIG. 1 is viewed from the plane direction indicated by arrow a in FIG. FIG. 3 is a configuration diagram schematically showing a WDM apparatus in which the wavelength monitoring apparatus of FIGS. 1 to 2 is incorporated. FIGS. 4 (a) to 4 (c) are respectively broken lines (a ) To (c) are diagrams showing the polarization plane of the optical signal viewed in the z-axis direction in each of the cross sections, and FIG. 5 is a diffraction grating and a linear image sensor constituting the wavelength monitoring device of the first embodiment. FIG. 6 is a state diagram schematically showing a light receiving state of two spectrum images received by the linear image sensor. 1, 2, and 5 indicate an optical path, and an optical path portion drawn by a relatively thin line indicates that the optical path portion is propagating through the components of the optical system.

  As shown in FIGS. 1, 2 and 5, the wavelength monitor device 1 of the first embodiment includes one birefringent element 2, two Faraday rotators 3 a and 3 b, a diffraction grating 4, and a collimator. A lens 5 and a linear image sensor 6 are included.

  Next, a WDM apparatus 7 to which the wavelength monitor apparatus 1 is optically connected will be described with reference to FIG. The WDM apparatus 7 includes a plurality of optical transmitters 8 each having a light source (not shown) that transmits an optical signal, a multiplexer 9 that wavelength-multiplexes optical signals of a plurality of channels transmitted from the optical transmitter 8, and a combination thereof. An optical amplifier 10 connected in a plurality of stages to amplify and repeat the optical signal wavelength-multiplexed by the wave multiplexer 9, and a demultiplexer 11 for wavelength-separating the optical signal amplified by the optical amplifier 10 for each channel; And a plurality of optical receivers 12 for receiving the optical signals wavelength-separated by the demultiplexer 11.

  On the transmission side, the optical signals emitted from the light sources of the plurality of optical transmitters 8 having different wavelengths are multiplexed on the wavelength axis by the multiplexer 9, and the multiplexed optical signals are received by the single optical fiber 13. Send to. On the receiving side, the optical signal multiplexed on the wavelength axis is separated for each wavelength by the demultiplexer 11 and received by the optical receiver 12. When an optical signal is transmitted to the receiving side by the optical fiber 13, a part of the optical signal is branched to the optical fiber 15 by the coupler 14. Examples of the branching ratio include 5:95. 95% of the light is guided to the optical fiber 13 and sent to the receiving side, and 5% of the optical signal is guided to the wavelength monitor device 1 via the optical fiber 14.

  The optical fiber 15 is, for example, a single mode optical fiber, and the core has a diameter of, for example, 10 μm. When the optical signal s1 is incident from the optical fiber 15, the optical signal s1 is separated into two polarization components of an ordinary ray s2 and an extraordinary ray s3 whose polarization directions are orthogonal to each other by the birefringent element 2 (hereinafter referred to as necessary). S2 is described as “optical signal” or “ordinary ray”, and s3 is described as “optical signal” or “abnormal ray”. The plane of polarization of the optical signal s1 is shown in FIG.

  The birefringent element 2 is composed of a birefringent crystal such as rutile, and the crystal axis X1 direction on the optical surface 2a is set to 0 degrees so as to be parallel to the x-axis direction which is the horizontal direction, and the crystal in the thickness direction The direction of the axis X2 is set so as to exhibit an angle of 45 degrees from the optical surface 2a. The thickness of the birefringent element 2 is set so that the incident optical signal s1 is completely separated into the ordinary ray s2 and the extraordinary ray s3. The birefringent element 2 configured as described above is disposed immediately after the collimating lens 5 and separates the collimated optical signal s1 incident on the collimating lens 5 into two polarization components s2 and s3.

  Since the crystal axes X1 and X2 are set as described above, the ordinary ray s2 goes straight in the z-axis direction as it is inside the birefringent element 2, and the extraordinary ray s3 has an angle φ with respect to the z-axis direction. The light is refracted and propagated in a direction parallel to the x-axis direction.

  Then, as shown in FIG. 4B, the Faraday rotator in which only the polarization component s2 emitted from the birefringent element 2 and separated as an ordinary ray is separated and separated into two polarization components s2 and s3. Permeate through 3a and 3b. The two Faraday rotators 3a and 3b each have a rotation angle of 45 degrees, and the rotation directions are set to be the same when viewed from the propagation direction of the polarization component s2. Therefore, the Faraday rotators 3a and 3b have a rotation angle of 90 degrees in total. Further, as shown in FIG. 2, the Faraday rotators 3a and 3b are magnetically saturated by magnets M arranged adjacent to each other (not shown in FIG. 1).

  Therefore, when the polarization component s2 is transmitted through the two Faraday rotators 3a and 3b, the polarization direction is rotated by 90 degrees in total, and the same direction as the polarization direction of the polarization component s3 as shown in FIG. And the polarization directions of the two polarization components s2 and s3 are the same. In this way, in the state where the polarization directions of the two polarization components s2 and s3 are aligned, the optical signal is further incident on the grating surface of the diffraction grating 4 disposed in the subsequent stage.

  The diffraction grating 4 is a reflection type diffraction grating, and a plurality of grooves 4a elongated in the x-axis direction are formed one-dimensionally in parallel with each other on the grating surface as shown in FIG.

  When the diffraction grating 4 configured in this way is viewed from the birefringent element 2 side, as shown in FIG. 1, the separation direction of the extraordinary ray s3 within the birefringent element 2 and each of the diffraction gratings 4 are It can be seen that the directions of the crystal axes X1 and X2 and the position of the diffraction grating 4 are determined such that the formation direction (x-axis direction) of the groove 4a is parallel to the same direction. The two polarized components s2 and s3 incident on the grating surface are reflected and diffracted by the grating surface and converted into two diffracted light components s21 and s31.

  As is well known, when the polarization direction of an optical signal (here, two polarization components s2 and s3) incident on the diffraction grating 4 changes, the reflection angle on the grating surface changes even for polarization components of the same wavelength. Resulting in. However, in the wavelength monitor device 1 of the present embodiment, the polarization directions of the two polarization components s2 and s3 incident on the diffraction grating 4 are the same regardless of the polarization state of the optical signal s1 emitted from the optical fiber 15 as described above. Can be made. That is, even if the polarization state of the optical signal s1 changes, it is possible to keep the polarization directions of the two polarization components s2 and s3 in the same direction.

  For this reason, the two polarization components s2 and s3 incident on the diffraction grating 4 are always diffracted at a constant reflection angle when diffracted by the diffraction grating 4, and therefore, between the two diffracted light components s21 and s31 after diffraction. The optical path length difference can be made zero. Further, at the time of incidence on the groove 4a, the polarization directions of the polarization components s2 and s3 are parallel to the formation direction of the groove 4a. Accordingly, it is possible to prevent the polarization component in the other direction from entering the diffracted light components s21 and s31 after diffraction as noise, so that the wavelength resolution of the wavelength monitor device 1 can be improved.

  Further, the polarization components s2 and s3 are diffracted by the same groove portion (for example, the hatched groove portion in the example of FIGS. 1 and 5) 4a in the state where the polarization directions of the two polarization components s2 and s3 are the same direction. Thus, the optical path length difference between the two diffracted light components s21 and s31 after diffraction can be made zero. The intensity ratio of the two diffracted light components s21 and s31 after diffracting the diffraction grating 4 is equal to the intensity ratio of the two polarization components s2 and s3 before entering the diffraction grating 4.

  The two diffracted light components s21 and s31 diffracted from the diffraction grating 4 are propagated toward the linear image sensor 6 as shown in FIG. 5, and the two diffracted light components s21, s21, The spectral images sp1 and sp2 generated by s31 are received in a completely separated state. The completely separated state is a state where there is no overlapping portion between the two spectral images sp1 and sp2. 1 and 2, the linear image 6 is omitted.

  In FIG. 1 to FIG. 6, assuming that the optical signal s1 propagated through the optical fiber 15 is monochromatic light (light having an arbitrary single wavelength), spot-like spectral images sp1 and sp2 are illustrated. For example, when the optical signal s1 is not monochromatic light but includes two types of light having different wavelengths, the two spectral images sp1 and sp2 are spread along the wavelength dispersion direction (z-axis direction) on the light receiving surface. . Further, when an optical signal s1 including many kinds of wavelengths is incident, a large number of spot-like spectral images are discretely arranged along the wavelength dispersion direction. The chromatic dispersion direction is orthogonal to the separation direction (x-axis direction) of the two spectral images sp1 and sp2.

  The separation distance D1 (see FIG. 6) between the centers of the two spectral images sp1 and sp2 is proportional to the thickness of the birefringent element 2. In the wavelength monitor device 1 of the present embodiment, the two spectral images sp1 and sp2 are completely separated, and only one polarization component s2 can be transmitted to the Faraday rotators 3a and 3b. It is necessary to set the thickness of the birefringent element 2 so as to be a distance.

  The linear image sensor 6 has a large number of light receiving portions 6a elongated in the x-axis direction, which is the separation direction of the spectral images sp1 and sp2, on the light receiving surface, and these light receiving portions 6a are one-dimensionally arranged along the wavelength separation direction. Light receiving element. Each light receiving portion 6a has an elongated strip shape. The size of each light receiving unit 6a in the separation direction (x-axis direction) is set to a size capable of simultaneously receiving the two spectral images sp1 and sp2. For example, the separation distance between the two spectral images sp1 and sp2 It is set to be larger than the sum of D1 and the size D2 in the x-axis direction of each of the spectral images sp1 and sp2.

  Further, the size of each light receiving portion 6a in the separation direction can be set smaller than the sum of the separation distance D1 of the spectral images sp1 and sp2 and the size D2. In this case, the spectral images sp1 and sp2 are partially vignetted, but if the linear image sensor 6 is arranged so that the center of the separation distance D1 is aligned with the center of each light receiving portion 6a, the spectral images sp1 and sp2 The amount of each vignetting can be made equal.

  Further, the size of each light receiving unit 6a in the z-axis direction is set according to the wavelength resolution necessary for measuring the spectral images sp1 and sp2. The number of the light receiving parts 6a is set so that the wavelength range can be received without omission according to the wavelength range necessary for the measurement of the spectral images sp1 and sp2. In addition, it is preferable to use the linear image sensor 6 having uniform light receiving sensitivity of each light receiving unit 6a (or having the same light receiving sensitivity at the light receiving positions of at least two spectrum images sp1 and sp2).

  When the linear image sensor 6 receives the two spectral images sp1 and sp2 at the same time, if there is an overlapping portion between the two spectral images sp1 and sp2, interference occurs in the overlapping portion, and an optical signal is generated in the overlapping portion. A polarization state similar to the original polarization state of s1 appears. That is, the polarization state at the overlapping portion changes according to the polarization state of the incident optical signal s1. For this reason, when the light receiving sensitivity of the linear image sensor 6 has some polarization dependence, the output of the linear image sensor 6 changes according to the polarization state in the overlapping part of the spectrum images sp1 and sp2. Further, the output of the linear image sensor 6 also changes when the image intensity at the overlapping portion changes as a result of interference.

  However, in the wavelength monitor device 1 of the present embodiment, the two spectral images sp1 and sp2 are received by the linear image sensor 6 in a completely separated state. As a result, the linear image sensor 6 can always output the sum of the intensities of the two spectral images sp1 and sp2.

  Therefore, in the WDM apparatus 7, it is possible to monitor the intensity characteristics (intensity sum) for each wavelength of the optical signal emitted from the optical transmitter 8. By feeding back the measurement result by the wavelength monitor device 1 of the present embodiment to the optical transmitter 8, the intensity of the optical signal emitted from the optical transmitter 8 can be kept constant for each wavelength, and stable optical communication can be performed. It becomes possible.

  As described above, since the wavelength monitor device 1 of the present embodiment adjusts the polarization direction of the separated polarization component to the same direction and makes the optical signal incident on the grating surface of the diffraction grating 4, the diffraction angle is different for each polarization component. Without changing, it is possible to receive the spectral images of the two diffracted light components at the light receiving portion of the linear image sensor at the same diffraction angle. Therefore, it is possible to receive two diffracted light component spectral images of the same order on one light receiving portion of the linear image sensor, and two diffracted light component spectral images of the same order at the same frequency on different light receiving portions. Since it is possible to prevent light from being received, the wavelength resolution of the wavelength monitor device 1 can be improved.

  Furthermore, even if there is a slight characteristic variation for each of the plurality of light receiving portions of the linear image sensor 6, two spectral images having the same frequency can be received by the same light receiving portion, and thus are not affected.

  Furthermore, since the optical system of the wavelength monitor device 1 is composed of the collimating lens 5, the birefringent element 2, the Faraday rotators 3a and 3b, the diffraction grating 4, and the linear image sensor 6, it is like a conventional wavelength monitor device. In addition, an optical system can be formed without using expensive components such as an AWG waveguide. Therefore, the cost of the wavelength monitor device 1 can be reduced.

  In addition, by forming an optical system that does not include lens components such as a rerout lens, just like the conventional wavelength monitor device, just before the diffraction grating 4, an increase in the size of the optical elements that constitute the optical system is prevented. Therefore, an increase in the size of the wavelength monitor device 1 can be prevented.

  Further, since no lens parts are arranged, the light receiving unit of the linear image sensor 6 can receive light without driving the diffraction grating 4 each time even if the wavelength of the optical signal s1 changes. It is possible to achieve downsizing and cost reduction by reducing the number of points and facilitating drive control of the wavelength monitor device 1.

<Second Embodiment>
Next, a second embodiment according to the wavelength monitoring device of the present invention will be described with reference to FIGS. FIG. 7 is a schematic perspective view schematically showing the wavelength monitor device of the second embodiment, and FIG. 8 is a schematic partial perspective view showing only the diffraction grating and the linear image sensor constituting the wavelength monitor device of the second embodiment. FIG. 9 is a schematic partial side view of the diffraction grating and the linear image sensor when FIG. 8 is viewed from the direction of arrow b. The same parts as those in the first embodiment are denoted by the same reference numerals, and overlapping descriptions are omitted or simplified. 7, 8, and 9 indicate an optical path, and an optical path portion drawn by a relatively thin line indicates that the optical path portion is propagating through the components of the optical system.

  The wavelength monitor device 16 of the second embodiment is different from the wavelength monitor device 1 of the first embodiment in that a transmission type is used for the diffraction grating 17. On the lattice plane, as shown in FIG. 9, a large number of convex portions 17a are formed in a rectangular wave shape in a one-dimensional shape that is elongated in the x-axis direction and parallel to each other.

  When the diffraction grating 17 configured in this manner is viewed from the birefringence element 2 side, as shown in FIG. 7, the separation direction of the extraordinary ray s3 in the birefringence element 2 and the diffraction grating 17 The directions of the crystal axes X1 and X2 and the position of the diffraction grating 17 are determined so that the formation direction (x-axis direction) of the projections 17a is parallel to the same direction. The two polarization components s2 and s3 incident on the grating surface are transmitted through the grating surface and diffracted by the convex portion 17a, and converted into two diffracted light components s21 and s31 as shown in FIG. 7 to 9, only the 0th-order diffracted light component is shown for simplification.

  Also in the wavelength monitor device 16 of the present embodiment, the polarization directions of the two polarization components s2 and s3 incident on the diffraction grating 17 can be made the same regardless of the polarization state of the optical signal s1 emitted from the optical fiber 15. That is, even if the polarization state of the optical signal s1 changes, it is possible to keep the polarization directions of the two polarization components s2 and s3 in the same direction. For this reason, when the two polarization components s2 and s3 incident on the diffraction grating 17 are diffracted by the diffraction grating 17, the diffracted light components of the same order are always diffracted in the same direction.

  Further, at the time of incidence on the convex portion 17a, the polarization directions of the polarization components s2 and s3 are parallel to the forming direction of the convex portion 17a. Accordingly, it is possible to prevent the polarization component in the other direction from entering the diffracted light components s21 and s31 after diffraction as noise, so that the wavelength resolution of the wavelength monitor device 16 can be improved.

  Further, by diffracting the polarization components s2 and s3 by the same convex portion 17a in the state where the polarization directions of the two polarization components s2 and s3 are the same direction, the two diffracted light components s21 and s31 after the diffraction are diffracted. The optical path length difference at can be made zero. The intensity ratio between the two diffracted light components s21 and s31 after diffracting the diffraction grating 17 is equal to the intensity ratio between the two polarization components s2 and s3 before entering the diffraction grating 17.

  As shown in FIG. 8, the two diffracted light components s21 and s31 diffracted from the diffraction grating 17 are propagated toward the linear image sensor 6, and the two diffracted light components s21, The spectral images sp1 and sp2 generated by s31 are received in a completely separated state.

  7 to 9, assuming that the optical signal s1 propagated through the optical fiber 15 is monochromatic light (light having an arbitrary single wavelength), the spot-like spectral images sp1 and sp2 are illustrated. In the wavelength monitor device 16 of the present embodiment, the two spectral images sp1 and sp2 are received by the linear image sensor 6 in a completely separated state. As a result, the linear image sensor 6 can always output the sum of the intensities of the two spectral images sp1 and sp2.

  Therefore, in the WDM apparatus 7 shown in FIG. 3, it is possible to monitor the intensity characteristics (intensity sum) for each wavelength of the optical signal emitted from the optical transmitter 8. By feeding back the measurement result by the wavelength monitor device 13 of this embodiment to the optical transmitter 8, the intensity of the optical signal emitted from the optical transmitter 8 can be kept constant for each wavelength, and stable optical communication can be performed. It becomes possible.

  As described above, the wavelength monitor device 16 of the present embodiment adjusts the polarization direction of the separated polarization components to the same direction, and then causes the optical signal to enter the grating surface of the diffraction grating 17, so that the diffraction angle is different for each polarization component. Without changing, it is possible to receive the spectral images of the two diffracted light components at the light receiving portion of the linear image sensor at the same diffraction angle. Therefore, it is possible to receive two diffracted light component spectral images of the same order on one light receiving portion of the linear image sensor, and two diffracted light component spectral images of the same order at the same frequency on different light receiving portions. Since it is possible to prevent light from being received, the wavelength resolution of the wavelength monitor device 16 can be improved.

  Furthermore, even if there is a slight characteristic variation for each of the plurality of light receiving portions of the linear image sensor 6, two spectral images having the same frequency can be received by the same light receiving portion, and thus are not affected.

  Also, the optical system of the wavelength monitor device 16 is composed of the collimating lens 5, the birefringent element 2, the Faraday rotators 3a and 3b, the diffraction grating 4, and the linear image sensor 6, so that it looks like a conventional wavelength monitor device. In addition, an optical system can be formed without using expensive components such as an AWG waveguide. Accordingly, the cost of the wavelength monitor device 16 can be reduced.

  In addition, by forming an optical system that does not include lens components such as a rerout lens just like the conventional wavelength monitor device just before the diffraction grating 17, an increase in the size of the optical elements constituting the optical system is prevented. Therefore, an increase in the size of the wavelength monitor device 16 can be prevented.

  Further, since no lens parts are arranged, the light receiving part of the linear image sensor 6 can receive light without driving the diffraction grating 17 each time even if the wavelength of the optical signal s1 changes. It is possible to achieve downsizing and cost reduction by reducing the number of points and facilitating drive control of the wavelength monitor device 16.

  The present embodiment can be variously changed based on the technical idea, and the shape of the convex portion formed on the grating surface of the transmission type diffraction grating 17 is changed to the above-described rectangular wave shape, for example, a triangular wave convex shape. You may form in a part.

  The wavelength monitor device of the present invention can be used not only as a monitor device for intensity characteristics (intensity sum) for each wavelength of individual channels of a WDM device but also as an optical spectrum analyzer or a spectroscope.

Claims (1)

A collimating lens, a birefringent element that separates an optical signal into two polarization components of an ordinary ray and an extraordinary ray whose polarization directions are orthogonal to each other, a Faraday rotator having a rotation angle of 90 degrees, and a plurality of grooves or projections A diffraction grating having a grating surface formed in parallel, and a linear image sensor;
The crystal axis of the birefringent element and the diffraction grating are positioned so that the forming direction of the groove or the convex part is parallel to the separating direction of extraordinary rays inside the birefringent element,
Further, the optical signal from the WDM device is incident on the collimating lens to be collimated, and the collimated optical signal is separated into the ordinary ray and the extraordinary ray by the birefringent element, and the ordinary ray from the birefringent element. The polarization direction of the two polarization components is made to be the same direction by rotating only the polarization component emitted as follows through the Faraday rotator to rotate the polarization direction by 90 degrees,
The polarization component in this state is diffracted by the same groove or convex portion, and the spectral images of the two diffracted light components after diffraction are converted into one of the linear images with the spectral images completely separated from each other. A wavelength monitor device that receives light at a light receiving unit and outputs a sum of intensity of the optical signals.

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