JP5669140B2 - Optical performance monitor - Google Patents

Description

  The present invention relates to a waveguide type optical performance monitor for monitoring the transmission state of an optical signal in an optical network of a wavelength division multiplexing communication system.

  The Wavelength Division Multiplexing (WDM) system is a system that multiplexes optical signals of wavelengths close to one optical fiber, and as a means to support the explosive spread of the Internet and increase the capacity of information. Widely used.

  On the other hand, in an optical network based on the WDM system, a connection is established between nodes using a certain wavelength signal (wavelength channel) by branching and inserting light (optical add / drop), and the state of the connection is determined. The introduction of ROADM (Reconfigurable Optical Add / Drop Multiplexing) that changes over time according to the use status of the network is being promoted. In the future, there is an opportunity to introduce an optical cross-connect (OXC) system for cross-connecting with light.

  In order to realize these, each node of the optical network monitors the wavelength signal (wavelength channel) transmission status in addition to the wavelength selective switch (WSS) and the above-described OXC device, and always operates normally. An optical performance monitor that manages whether the optical network is operating is essential.

The optical performance monitor is required to measure and monitor the following items (1) to (3) and report it to the network management layer device.
(1) Optical signal wavelength being transmitted (2) Optical power of each optical signal (3) Optical signal to noise ratio (hereinafter referred to as OSNR)

  In addition, the optical performance monitor needs to measure the items (1) to (3) described above at high speed, share the information in the optical network, and cope with transmission troubles.

  FIG. 10 shows an example of a conventional optical performance monitor.

  In the optical performance monitor 101 shown in FIG. 10, the light emitted from the optical fiber 102 is dispersed by a diffraction grating 103 having a spectral function, and then collected by a lens 104, and a MEMS (Micro-Electro-Machining System) mirror 105. Is incident on. The light incident on the MEMS mirror 105 is reflected at each angle for each wavelength, light having an arbitrary wavelength enters the light receiver 106, and the optical power of that wavelength is measured by the light receiver 106.

  The MEMS mirror 105 can be rotated in the illustrated θ and φ directions by an applied voltage. By changing the reflection angle of light having an arbitrary wavelength and entering the light receiver 106, the optical power of each wavelength can be changed. Can be measured.

FIG. 11A shows an example of a wavelength spectrum measured by the conventional optical performance monitor 101. By obtaining the wavelength spectrum of FIG. 11A, the transmitted signal wavelength and the untransmitted signal wavelength can be confirmed, and the optical power of the transmitted signal wavelength can be monitored. The OSNR can be approximately obtained from the following expressions (1) and (2).
OSNR = 10 × log (Pi / Ni) (1)
Pi: signal power (W)
Ni: Noise power (W)
Ni = {N (λi−Δλ) + N (λi + Δλ)} / 2 (2)

  Note that N (λi−Δλ) and N (λi + Δλ) in Equation (2) represent the minimum values of the optical power before and after the wavelength λi of the optical signal for measuring the OSNR, as shown in FIG. Yes. In Expression (2), the noise power Ni at the wavelength λi is estimated by taking the average of these N (λi−Δλ) and N (λi + Δλ). That is, this method is a method of interpolating optical noise between signal wavelengths and estimating optical noise Ni of wavelength λi.

  In addition, there exists patent document 1 as prior art document information relevant to invention of this application.

JP 2010-107377 A

  In the conventional optical performance monitor 101 of FIG. 10, when the power levels of adjacent signal wavelengths are greatly different or when the wavelength interval is narrow, the noise power Ni is affected by the tail of the signal wavelength having a large adjacent power. However, there is a problem that cannot be measured accurately.

  Accordingly, an object of the present invention is to provide an optical performance monitor that can accurately measure the wavelength of the transmitted optical signal, the optical power of each optical signal, the noise power, and the OSNR by solving the above-described problems. It is in.

  The present invention has been developed to achieve the above object, and includes an input waveguide into which light to be monitored for transmission status is incident, a slab waveguide connected to the input waveguide, and a slab waveguide. A waveguide-type spectroscope that has a plurality of waveguides connected to a waveguide, and that splits and emits light incident on the input waveguide from an output end of the arrayed waveguide; and A first lens that is disposed at a distance F1 from the emission end of the waveguide spectrometer and collects light emitted from the emission end is F1, and at a distance F1 from the first lens. A polarization separation element that is disposed and separates the light emitted from the emission end of the waveguide spectrometer and transmitted through the first lens into X-polarized light and Y-polarized light that are orthogonal to each other; and the waveguide-type spectroscopy Provided between the polarizer and the polarization separation element A variable retarder that adjusts the optical power ratio of the X-polarized light and the Y-polarized light by adjusting a phase difference between the polarized light component in one birefringence axis direction and the other polarized light component orthogonal thereto, and the polarization separating element A second lens having a focal length F2 that is arranged to face the polarization separation element at a distance F2 from the lens and condenses the X-polarized light and the Y-polarized light, and the X-polarized light and the Y-polarized light emitted from the second lens. A half-wave plate that rotates only one of the polarizations spatially by 90 degrees to make the polarization direction the same, and is disposed at a distance F2 from the second lens, and from the second lens. A reflective optical phase modulator that reflects the one polarized light emitted through the half-wave plate and the other polarized light emitted from the second lens to the second lens; From the second lens The one polarized light and the other polarized light, which are arranged on the same side as the light separation element and facing the second lens at the position of the distance F2, reflected by the optical phase modulator and condensed by the second lens. An optical performance monitor comprising a light receiving element array for detecting interference fringes.

  A reflection mirror that is provided in parallel with the light receiving element array, reflects the one polarized light and the other polarized light reflected by the optical phase modulator and collected by the second lens, and is incident on the second lens again. And reflected by the reflecting mirror and reflected by the second lens, the optical phase modulator, the half-wave plate, the one polarized light passing through the second lens, and the reflecting mirror, and the second lens. The optical phase modulator and the other polarized light that has passed through the second lens are combined by the polarization separation element, and the variable retarder and the plurality of waveguides into which light that has passed through the first lens is incident. A waveform-type spectrometer for waveform monitoring, comprising: an arrayed waveguide, a slab waveguide connected to the arrayed waveguide, and a plurality of output waveguides connected to the slab waveguide; Waveguide spectroscopy Wherein a plurality of output waveguides waveform monitor light-receiving element array in which the light-receiving elements are arranged so as to correspond to the may further comprise a.

  The variable retarder adjusts the phase difference between the polarization component in one birefringence axis direction of the variable retarder and the other polarization component orthogonal thereto so that the optical power ratio of the X polarization and Y polarization is 1: 1. It may be made to do.

  ADVANTAGE OF THE INVENTION According to this invention, the optical performance monitor which can measure the optical signal wavelength currently transmitted, the optical power of each optical signal, and OSNR accurately can be provided.

(A) is a schematic block diagram of the optical performance monitor which concerns on one embodiment of this invention, (b) is the side view. It is a top view of the low resolution waveguide type | mold spectrometer used for the optical performance monitor of FIG. (A) is sectional drawing of LCOS used for the optical performance monitor of FIG. 1, (b) is the front view, (c) is a figure which shows an example of refractive index distribution in the cell arranged in the X-axis direction. It is a figure which shows an example of the interference fringe detected with PD array of the optical performance monitor of FIG. FIG. 9 is a schematic configuration diagram of an optical performance monitor according to another embodiment of the present invention, where (a) shows an optical path from a low-resolution waveguide spectrometer to a PD array and a reflecting mirror, and (b) shows an optical path from the reflecting mirror. It is a figure which shows the optical path which leads to 2PD array. FIG. 6 is a side view of the optical performance monitor of FIG. 5. FIG. 6 is a plan view of a high resolution waveguide spectrometer used in the optical performance monitor of FIG. 5. It is a figure which shows the fine structure of the signal spectrum in each modulation system. It is a block diagram of the communication system using the optical performance monitor of this invention. It is a schematic block diagram of the conventional optical performance monitor. (A), (b) is a figure explaining the measuring method of the transmitted optical signal wavelength, the optical power of each optical signal, and OSNR in the conventional optical performance monitor of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1A is a schematic configuration diagram of an optical performance monitor according to the present embodiment, and FIG. 1B is a side view thereof.

  As shown in FIGS. 1A and 1B, an optical performance monitor 1 includes a low-resolution waveguide spectrometer 2 as a waveguide spectrometer, a first lens 3, a variable retarder 4, and polarization separation. An element 6, a second lens 7, a half-wave plate 8, an LCOS (Liquid Crystal On Silicon) 9 as an optical phase modulator, and a PD (Photo Diode) array 10 as a light receiving element array are provided. ing. The variable retarder 4 may be disposed between the first lens 3 and the waveguide spectrometer 2 (position 5 in FIG. 1). Further, for example, a low-resolution waveguide spectrometer 2 having a free spectral range (FSR) of about 10 THz is used.

  The low-resolution waveguide spectrometer 2 has one or more input ports (not shown), and an input optical fiber 11 for entering light to be monitored for transmission status is input to one of the input ports. Connected.

  As shown in FIG. 2, the low-resolution waveguide spectrometer 2 has a structure in which a core 21 having a high refractive index is embedded in a clad 22 having a lower refractive index on a flat substrate (not shown). A waveguide type spectroscopic circuit 23 is laminated. The waveguide-type spectroscopic circuit 23 has a confinement structure only in one or more input waveguides 24 into which light enters from the input optical fiber 11 and the thickness direction (paper surface direction) connected to the input waveguides 24. It has a certain slab waveguide 25 and an arrayed waveguide 27 composed of a plurality of waveguides 26 having different lengths sequentially connected to the slab waveguide 25 by a certain length ΔL. Since the direction of the light emitted from the emission end 28 of the arrayed waveguide 27 varies depending on the wavelength, the light input to the input waveguide 24 is separated from the emission end of the arrayed waveguide 27 (X in FIG. 1A). The light is emitted in the axial direction. The direction (angle) of the emitted light depends on the difference ΔL in the length of the waveguides 26 of the arrayed waveguide 27. When ΔL is increased, even a slightly different wavelength can be dispersed at a large angle.

  Returning to FIG. 1, the first lens 3 is disposed so that the center thereof is located at a distance F <b> 1 from the emission end 28 of the low-resolution waveguide spectrometer 2. The focal length of the first lens 3 is F1 on both sides. The first lens 3 condenses the light emitted from the emission end 28 of the low resolution waveguide spectrometer 2 and has a function of collimating in the X-axis direction and the Y-axis direction shown in the figure. As the first lens 3, a spherical lens, a cylindrical lens, or the like can be used.

  The polarization separation element 6 is disposed so that one end face thereof is located at a distance F1 from the center of the first lens 3. The polarization separation element 6 comprises a Wollaston prism, and separates the light emitted from the emission end 28 of the waveguide spectrometer 2 and transmitted through the first lens 3 into X-polarized light and Y-polarized light (in the Y-axis direction shown in the figure). And the light is emitted.

  Between the waveguide spectrometer 2 and the polarization separating element 6, the birefringence axis is at an angle of π / 4 + m × π / 2 (m is an integer) with respect to the birefringence axis of the waveguide spectrometer 2. The optical power ratio of the X-polarized light and the Y-polarized light separated by the polarization separating element 6 is adjusted by adjusting the phase difference between the polarized light component in one birefringence axis direction and the other polarized light component orthogonal thereto. A variable retarder 4 to be adjusted is arranged. As the variable retarder 4, for example, a liquid crystal panel is used, and a liquid crystal variable retarder (LVR) that generates an arbitrary retardance (phase difference) by controlling a voltage applied to the liquid crystal panel can be used. In the variable retarder 4, for example, a retardance (phase difference) of about 2π can be obtained by applying a voltage of about 5V. Although details will be described later, in order to maximize the coherence (visibility) of the interference fringes detected by the PD array 10, the variable retarder 4 is set so that the optical power ratio of X-polarized light and Y-polarized light becomes 1: 1. To adjust the litter dance.

  In general, the polarization state of an optical fiber changes with time. For this reason, the polarization state of the light emitted from the emission end 28 of the low resolution waveguide spectrometer 2 to which the optical fiber is connected also changes. This temporal change changes relatively slowly (with a period of several seconds) in a normal state. For this reason, here, by using the variable retarder 4 to change the retardance of the passing light at a high speed in the order of milliseconds, the polarized light separating element can be applied to any polarization state incident from the optical fiber 11. 6, the amplitude ratio of X-polarized light and Y-polarized light (power is the square of the amplitude) can be 1: 1. Actually, as described above, when the coherence (visibility) of the interference fringes detected by the PD array 10 is maximized, the amplitude ratio of X-polarized light and Y-polarized light (power is the square of the amplitude) is 1: 1. It is analytically shown that

  By the above method, the orthogonally polarized lights whose optical power ratios are adjusted by the variable retarder 4 are separated as X-polarized light and Y-polarized light by the polarization separating element 6.

  The second lens 7 is arranged so that the center thereof is at a distance F2 from the center of the polarization separation element 6 and the upper half of the second lens 7 faces the polarization separation element 6. The focal length of the second lens 7 is F2 on both sides. The second lens 7 collects X-polarized light and Y-polarized light, respectively, and has a function of collimating in the X-axis direction and the Y-axis direction shown in the drawing. As the second lens 7, a spherical lens, a cylindrical lens, or the like can be used.

  The LCOS 9 used as an optical phase modulator can change the refractive index only with respect to a polarization component that vibrates in a uniaxial direction, that is, can change only the phase of a single polarization. However, in general, light has components of X-polarized light and Y-polarized light, and the ratio thereof changes with time. Therefore, it is necessary to similarly control the phase of X-polarized light and Y-polarized light. Therefore, in the present embodiment, the half-wave plate 8 is disposed only in either the X-polarized light path or the Y-polarized light path so as to cover only one polarized light between the second lens 7 and the LCOS 9. Then, only one of the X-polarized light and the Y-polarized light emitted from the second lens 7 is spatially rotated 90 degrees so that the polarization directions are the same. Here, the X-polarized light is converted to Y-polarized light by the half-wave plate 8 so that the polarization direction is aligned with the Y-axis direction. However, the Y-polarized light is converted to X-polarized light and the polarization direction is changed to the X-axis direction. You may make it arrange to. The half-wave plate 8 is arranged so that its birefringence axis is π / 4 + n × π / 2 (n is an integer) with respect to the polarization direction (here, the X-axis direction) incident.

  The LCOS 9 is arranged so that the reflective film 33 is located at a distance F2 from the center of the second lens 7. However, since the distance F2 is on the order of centimeters and the films 33 to 37 (see FIG. 3A) constituting the LCOS 9 are on the order of several microns, the end face of the LCOS 9 located on the second lens 7 side is There is substantially no problem even if it is disposed at a distance F2 from the center of the two lenses 7. The LCOS 9 converts the Y-polarized light condensed by the second lens 7 and the Y-polarized light obtained by converting the X-polarized light condensed by the second lens 7 by the half-wave plate 8 at an arbitrary angle for each cell. It consists of a plurality of cells, and the refractive index is variable for each cell. By controlling the refractive index distribution of the LCOS 9 by a control circuit (not shown), the phase change of the reflected light can be given to each cell.

  As shown in FIG. 3A, the LCOS 9 has an electrode (for example, ITO) 32, a reflective film 33, a SiO2 film 34, an alignment film 35, a liquid crystal layer 36, and a SiO2 film on a Si substrate 31 on which an electronic circuit is formed. 34, an electrode 32, and a thin glass substrate 37 are sequentially laminated.

  As shown in FIG. 3B, the LCOS 9 includes cells 38 arranged in a plurality of vertical and horizontal directions, and the refractive index of each cell 38 can be controlled independently. Specifically, the orientation direction (birefringence) of the liquid crystal layer 36 is controlled by applying a voltage to each cell 38, and thereby the phase of the light beam incident and reflected on the upper surface of the LCOS 9 is controlled for each cell 38. Can be modulated.

  The maximum phase change required to reflect the light beam incident on each cell 38 of the LCOS 9 is about 2π. Therefore, in the cells 38 aligned in the X-axis direction, the phase applied to the light beam does not exceed 2π, and a sawtooth voltage is applied to give a sawtooth refractive index distribution as shown in FIG. . In addition, the LCOS 9 can be provided with a refractive index distribution that corrects wavefront distortion due to the aberration of the first lens 3 and the second lens 7 by changing the voltage applied to each cell 38. That is, LCOS 9 provides a refractive index distribution in which a sawtooth refractive index distribution in the X-axis direction and a refractive index distribution for correcting aberrations are superimposed.

  The light reflected from the LCOS 9 passes through the second lens 7 and forms an image on the same plane as the PD array 10. At this time, the imaging position differs for each wavelength. By applying this refractive index distribution to the LCOS 9, the image formation position of these images can be shifted in the X-axis direction, and the shift amount can be controlled by changing the period of the sawtooth refractive index distribution. Accordingly, a light beam having a predetermined wavelength among the incident light beams can be incident on the PD array 10. The LCOS 9 is desirably temperature-controlled so as to have a constant temperature by a heater or a Peltier element.

  The PD array 10 is disposed on the same side as the polarization separation element 6 of the second lens 7 and faces the lower half of the second lens 7 so that the end surface thereof is at a distance F2 from the center of the second lens 7. Arranged. Light having a wavelength selected by the LCOS 9, that is, both X-polarized light converted to Y-polarized light and Y-polarized light are incident on the PD array 10. These two lights interfere with each other to form interference fringes, and the PD array 10 detects the interference fringes.

  Next, the operation of the optical performance monitor 1 will be described.

  When light of various wavelengths (wavelength multiplexed optical signal, light to be monitored for transmission status) input from the input optical fiber 11 enters the low-resolution waveguide spectrometer 2, each wavelength is output from the output end 28. The light is emitted in different directions (split and emitted). When these spectrally separated light passes through the first lens 3, the light of each wavelength enters the variable retarder 4 as parallel beams offset from each other.

  The light of each wavelength incident on the variable retarder 4 is emitted after the retardance is adjusted by the variable retarder 4 and the optical power ratio of the X-polarized light and the Y-polarized light is adjusted to 1: 1 (here, X Polarized light and Y-polarized light are polarized light separated by the polarization separating element 6).

At this time, assuming that the optical fields of the input light are Ex and Ey, the X-polarized light power Px and the Y-polarized light power Py after passing through the polarization separating element 6 are expressed by the following equations (3) and (4), respectively.
Px = (Ex ² + Ey ² −2ExEycosφ) / 2 (3)
Py = (Ex ² + Ey ² + 2ExEycosφ) / 2 (4)
Given in. In Expressions (3) and (4), φ is a phase difference that combines the outputs from the variable retarder 4 and the input optical fiber 11. Note that Px + Py = Ex ² + Ey ² , and the equations (3) and (4) satisfy the energy conservation law.

  From equations (3) and (4), even if light having any phase difference enters from the input optical fiber 11, the phase difference is changed from 0 to 2π by the variable retarder 4 so that φ = ± π / 2. Thus, Px = Py can be set.

  The light of each wavelength incident on the polarization separation element 6 is separated into an X-polarized light group and a Y-polarized light group, and is incident on the upper half of the second lens 7. The two polarized light groups (X polarized light group and Y polarized light group) that have passed through the second lens 7 become parallel beams and enter the LCOS 9. At this time, the X-polarized light group, which is one of the two polarized light groups, passes through the half-wave plate 8 before entering the LCOS 9 and the polarization direction is spatially rotated by 90 °. Y-polarized light then enters the LCOS 9.

  The reason why the polarization group of X polarization passes through the half-wave plate 8 and the polarization group of Y polarization does not pass through the half-wave plate 8 and is incident on the LCOS 9 is as follows. This is because it acts only on polarized light (here, Y-polarized light) (controls the direction of reflected light). When the LCOS 9 acts only on the X-polarized light, it is preferable that only the Y-polarized light pass through the half-wave plate 8.

  The light of each wavelength incident on the LCOS 9 is condensed on the LCOS 9 as X-polarized light (Y-polarized light after conversion) and one image for each Y-polarized light (shown as group A and group B in FIG. 1A). ) The image on the LCOS 9 is an image obtained by converting the image on the end face of the low resolution waveguide spectrometer 2 at a certain magnification. This is because the beam image is the result of the relationship of two Fourier transforms by the two lenses, that is, the first lens 3 and the second lens 7. The magnification B at this time is determined by the ratio of the focal lengths F1 and F2 of the first lens 3 and the second lens 7, and is given by B = F2 / F1.

  The light reflected by the LCOS 9 enters the lower half of the second lens 7 and is received by the PD array 10 disposed on the opposite focal plane (position of F2). In this focal plane, the light distribution on the LCOS 9 is converted into its Fourier image. For this reason, the light of each wavelength reflected by the LCOS 9 at different angles for each wavelength is dispersed in the X-axis direction, and the light distribution that is an ellipse by the LCOS 9 is an image in which the direction of flatness is changed by 90 °. Further, in this focal plane, the light of the two groups A and B on the LCOS 9 is incident on each wavelength at a certain angle by the second lens 7 and forms an image, so that interference fringes are formed. .

  In the present embodiment, the variable retarder 4 sets the power ratio of X-polarized light and Y-polarized light to 1: 1, and the optical powers of the two groups A and B are the same. The coherence (visibility) of the fringes is maximized.

  The PD array 10 disposed on the focal plane of the second lens 7 detects the spectrally divided interference fringes. FIG. 4 shows an example of interference fringes detected by the PD array 10 at this time.

  The vertical axis in FIG. 4 is the output current value at each PD in the PD array 10, and the horizontal axis is the number of each PD in the PD array 10. In this way, interference fringes (solid lines in the figure) can be detected from the output of each PD of the PD array 10. The broken lines in FIG. 4 represent interference fringes detected when the voltage applied to the variable retarder 4 is not appropriate and the maximum visibility cannot be obtained.

  When the background level in the interference fringes (interference fringes with the maximum visibility) shown in FIG. 4 is converted into a voltage, a noise component VB is obtained, and when the current amplitude of the PD array 10 is converted into a voltage, a signal component VA is obtained. The OSNR can be obtained from the ratio between the signal component VA and the noise component VB. Further, when the signal component VA and the noise component VB are added together, the optical power can be obtained.

  The wavelength at which light is transmitted and the wavelength ratio of light incident on the PD array 10 is changed (sweep) by changing the voltage distribution applied to the LCOS 9 to obtain the optical power and OSNR for each wavelength in the PD array 10. A signal can be detected.

  As described above, in the optical performance monitor 1 according to the present embodiment, light of each wavelength separated by the low-resolution waveguide spectrometer 2 is incident on the variable retarder 4, and the X-polarized light and Y-polarized light are input by the variable retarder 4. After adjusting the phase difference of the polarized light, it is separated into X polarized light and Y polarized light by the polarization separating element 6, and the separated one polarized light is converted to Y polarized light by the half-wave plate 8, and the polarization direction Are incident on the LCOS 9 in the Y-axis direction, both polarized lights reflected by the LCOS 9 are caused to interfere, and the interference fringes are measured by the PD array 10.

  Although the polarization state of the light transmitted through the input optical fiber 11 (light whose transmission status is to be monitored) changes from moment to moment, according to the optical performance monitor 1, the polarization direction using the half-wave plate 8. , The reflection direction (phase) can be controlled by the LCOS 9 regardless of the polarization state of the input light, and the phase difference is adjusted by the variable retarder 4 to adjust the X-polarized light and the Y-polarized light. By adjusting the optical power ratio to 1: 1, the visibility of interference fringes detected by the PD array 10 can always be maximized regardless of the polarization state of the input light. Therefore, by sweeping light of each wavelength using LCOS 9, it is possible to simultaneously detect the optical power and OSNR for each wavelength from the interference fringes detected by the PD array 10, and to detect the transmitted optical signal wavelength. It becomes possible. Thus, according to the optical performance monitor 1, it is possible to always measure the transmitted optical signal wavelength, the optical power of each optical signal, and the OSNR.

  Further, the present optical performance monitor 1 can directly measure the OSNR of the desired wavelength itself, and is not an interpolation estimation method performed in the conventional optical performance monitor. It is not affected by the tail of the optical wavelength signal, and measurement with extremely high accuracy is possible.

  Further, according to the optical performance monitor 1, the OSNR is always detected with a simple configuration in which the separated light is separated into X-polarized light and Y-polarized light, and the X-polarized light is converted into Y-polarized light, and then both polarized lights interfere with each other. It becomes possible. Therefore, a low-priced, compact, and high-performance optical performance monitor 1 can be realized, and future optical systems and optical networks can be greatly upgraded.

  Next, another embodiment of the present invention will be described.

  The optical performance monitor 51 shown in FIGS. 5 and 6 includes a channel identifier (Channel identification) indicating what modulation signal is transmitted in the channel in addition to the transmitted optical wavelength, the optical power of each optical signal, and the OSNR. ) Can be detected.

  In addition to the optical performance monitor 1 of FIG. 1, the optical performance monitor 51 further includes a reflection mirror 52, a high-resolution waveguide spectrometer 53 as a waveform monitor waveguide spectrometer, and a waveform monitor light receiving element array. The second PD array 54 is provided.

  The reflection mirror 52 is arranged in parallel with the PD array 10 in the X-axis direction so as not to interfere with the PD array 10, and the surface thereof is arranged at a distance F <b> 2 from the second lens 7. The reflection mirror 52 reflects the light reflected by the LCOS 9 and collected by the second lens 7 (X-polarized light converted to Y-polarized light and Y-polarized light) and reenters the second lens 7. is there.

  The high-resolution waveguide spectrometer 53 is arranged in parallel above the low-resolution waveguide spectrometer 2 (so that the end faces of both waveguide spectrometers 2 and 53 are aligned), and the incident end 55 is the first. One lens 3 is disposed at a distance F1 from the center of the lens 3. 5 and 6, the low resolution waveguide spectrometer 2 and the high resolution waveguide spectrometer 53 are drawn apart from each other for simplification of the drawings. They are stacked in close proximity. Although the case where the high resolution waveguide spectrometer 53 is disposed above the low resolution waveguide spectrometer 2 will be described here, it may be disposed below the low resolution waveguide spectrometer 2.

  As shown in FIG. 7, the high-resolution waveguide spectrometer 53 includes an arrayed waveguide composed of a plurality of waveguides 71 each having a certain length ΔL ′, into which the light reflected by the reflecting mirror 52 is incident. 72, a slab waveguide 73 having a confinement structure only in the thickness direction (in the drawing paper direction) connected to the arrayed waveguide 72, and a plurality of output waveguides 74 connected to the slab waveguide 73.

  The difference ΔL ′ in the length of the waveguide 71 in the arrayed waveguide 72 of the high-resolution waveguide spectrometer 53 is several in comparison with the difference ΔL in the length of the waveguide 26 in the low-resolution waveguide spectrometer 2. The size is double to several tens of times. Further, the output waveguide 74 of the high resolution waveguide spectrometer 53 is several tens to several hundreds (in FIG. 7, it is shown in a simplified manner). For example, a high-resolution waveguide spectrometer 53 having a free spectral range (FSR) of about 100 GHz is used.

  The second PD array 54 is formed by arranging light receiving elements (PD) 54 a so as to correspond to the plurality of output waveguides 74 of the high resolution waveguide spectrometer 53. As a result, the light emitted from each output waveguide 74 is received by the corresponding PD 54 a of the second PD array 54.

  The operation of the optical performance monitor 51 will be described.

  As shown in FIG. 5A, the light input from the input optical fiber 11 is split by the low-resolution waveguide spectrometer 2, sequentially passes through the first lens 3 and the variable retarder 4, and the polarization separation element 6 Is separated into X-polarized light and Y-polarized light, and enters the LCOS 9 through the second lens 7. At this time, only X-polarized light passes through the half-wave plate 8, is converted to Y-polarized light, and enters the LCOS 9. Both polarized lights incident on the LCOS 9 are incident on the PD array 10 and the mirror 52 through the second lens 7 with the reflection direction controlled for each wavelength by the LCOS 9. In the PD array 10, an interference fringe of incident light having a certain wavelength is detected, and optical power and OSNR at that wavelength are detected. The variable retarder 4 may be disposed between the first lens 3 and the waveguide spectrometer 2 (position 5 in FIG. 5).

  On the other hand, as shown in FIG. 5B, the light having a certain wavelength incident on the reflection mirror 52 is reflected by the reflection mirror 52, passes through the second lens 7 again, and enters the LCOS 9. One polarized light (X-polarized light converted to Y-polarized light) incident on the LCOS 9 is reflected by the LCOS 9, converted again to X-polarized light by the half-wave plate 8, and passes through the second lens 7 to the polarization separation element 6. Incident. The other polarized light (Y-polarized light) incident on the LCOS 9 is reflected by the LCOS 9 and enters the polarization separation element 6 via the second lens 7 without passing through the half-wave plate 8. Both polarized lights are combined by the polarization separation element 6, pass through the variable retarder 4 and the first lens 3, and enter the incident end 55 of the high resolution waveguide spectrometer 53. In other words, the light reflected by the reflecting mirror 52 travels back through the incident optical path and enters the high-resolution waveguide spectrometer 53. By appropriately adjusting the angle of the reflection mirror 52, the light reflected by the reflection mirror 52 can be incident on the high-resolution waveguide spectrometer 53.

  The high resolution waveguide spectrometer 53 splits the input light for each wavelength with higher resolution, and emits the split wavelength light from each output waveguide 74. The light emitted from each output waveguide 74 of the high-resolution waveguide spectrometer 53 is received by the corresponding PD 54a of the second PD array 54, and the fine structure (modulation) of the signal spectrum based on the output of each PD 54a. To detect the fine structure of the spectrum spread.

  Here, FIG. 8 shows a fine structure of a signal spectrum in each modulation method. In FIG. 8, NRZ (Non Return Zero), Duobinary, CS-RZ (Carrier Suppressed Return to Zero), RZ-DPSK (Return to Zero Differential Phase Shift Keying), RZ-DQPSK (Return to Zero Differential Quadrature) The fine structure of the signal spectrum in each modulation method of (Phase Shift Keying) is shown.

  As shown in FIG. 8, since the spectrum spread, that is, the fine structure of the signal spectrum differs depending on the modulation method, the modulation method, that is, the channel identification (Channel identification) is determined from the fine structure of the signal spectrum detected by the second PD array 54. It becomes possible to detect.

  By changing (sweeping) the wavelength of light incident on the reflection mirror 52 by changing the voltage distribution applied to the LCOS 9, the fine structure of the signal spectrum at each wavelength (each channel) is detected, and the channel identifier is detected. It becomes possible.

  As described above, according to the optical performance monitor 51, in addition to the transmitted optical wavelength, the optical power of each optical signal, the noise power, and the OSNR, it is possible to detect a channel identifier (Channel identification). .

  Conventionally, in order to detect the fine structure of the signal spectrum, it is necessary to use an extremely expensive device such as converting an optical signal into an electrical signal using a high-speed receiver and then performing a Fourier transform on the electrical signal. However, according to the present invention, the optical performance monitor 52 having the function of detecting the channel identifier can be realized at a low cost.

  Next, a method for using the optical performance monitor 1, 51 of the present invention will be described. As shown in FIG. 9, the optical performance monitors 1 and 51 are used in each node 502 of the metro core 501 and can be applied to an optical cross-connect system in addition to a normal optical signal branching / adding (optical add / drop) system. Is possible. Conventional optical performance monitors are used in relatively large-scale systems such as trunk lines or metro cores. However, if significant cost reduction is enabled by the present invention, a wide range of systems for metro edges and access systems can be used. Can be introduced, leading to innovative development of optical networks.

  The present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the spirit of the present invention.

  For example, although not mentioned in the above embodiment, since the demultiplexing characteristic of the low resolution waveguide spectrometer 2 changes depending on the temperature, the change of the demultiplexing characteristic due to this temperature characteristic is corrected by the LCOS 9. You may make it do. In this case, a temperature sensor for monitoring is arranged in the optical performance monitors 1 and 51, and the temperature detected by the temperature sensor and a low-resolution waveguide spectrometer for each temperature measured in advance and written in a memory of an electronic circuit or the like. Based on the demultiplexing characteristic of 2, the LCOS 9 may have an additional phase distribution to correct the demultiplexing wavelength shift of the low resolution waveguide spectrometer 2 due to temperature change.

1 Optical performance monitor 2 Low resolution waveguide spectrometer (waveguide spectrometer)
3 First lens 4 Variable retarder 6 Polarization separating element 7 Second lens 8 Half-wave plate 9 LCOS (optical phase modulator)
10 PD array (light receiving element array)
28 Emission end 52 Reflecting mirror 53 High resolution waveguide spectrometer (waveguide monitor waveguide spectrometer)
54 2nd PD array (light receiving element array for waveform monitoring)

Claims (3)

An input waveguide into which light to be monitored for transmission status is incident, a slab waveguide connected to the input waveguide, and an array waveguide composed of a plurality of waveguides connected to the slab waveguide. A waveguide-type spectrometer that splits the light incident on the input waveguide from the output end of the arrayed waveguide and emits the light;
A first lens that is disposed at a distance F1 from the exit end of the waveguide spectroscope and that has a focal length F1 that condenses light emitted from the exit end;
The light that is disposed at a distance F1 from the first lens and is emitted from the exit end of the waveguide spectrometer and transmitted through the first lens is separated into X-polarized light and Y-polarized light, and is emitted. A polarization separation element;
The X-polarized light and the Y-polarized light are provided between the waveguide-type spectroscope and the polarization separation element, and the phase difference between the polarization component in one birefringence axis direction and the other polarization component orthogonal thereto is adjusted. A variable retarder that adjusts the optical power ratio of polarized light;
A second lens arranged to face the polarization separation element at a position of a distance F2 from the polarization separation element and having a focal length F2 for condensing the X-polarized light and the Y-polarized light,
A half-wave plate that makes the polarization direction the same by spatially rotating only one of the X-polarized light and the Y-polarized light emitted from the second lens 90 degrees;
The one polarized light emitted from the second lens and emitted through the half-wave plate and the other polarized light emitted from the second lens are disposed at a distance F2 from the second lens. A reflective optical phase modulator that reflects polarized light to the second lens;
The one polarized light that is disposed facing the second lens at a distance F2 on the same side as the polarization separation element from the second lens, reflected by the optical phase modulator, and condensed by the second lens And a light receiving element array for detecting interference fringes of the other polarization,
An optical performance monitor characterized by comprising A reflection mirror that is provided in parallel with the light receiving element array, reflects the one polarized light and the other polarized light reflected by the optical phase modulator and collected by the second lens, and is incident on the second lens again. When,
Reflected by the reflecting mirror, reflected by the second lens, the optical phase modulator, the half-wave plate, the one polarized light passing through the second lens, and reflected by the reflecting mirror, the second lens, An array composed of a plurality of waveguides in which the optical phase modulator and the other polarized light that has passed through the second lens are combined by the polarization separation element, and the light that has passed through the variable retarder and the first lens is incident thereon A waveguide-type spectrometer for waveform monitoring having a waveguide, a slab waveguide connected to the arrayed waveguide, and a plurality of output waveguides connected to the slab waveguide;
A light-receiving element array for waveform monitoring in which light-receiving elements are arranged so as to correspond to the plurality of output waveguides of the waveguide-type spectrometer for waveform monitoring;
The optical performance monitor according to claim 1, further comprising: The variable retarder has a phase difference between a polarization component in one birefringence axis direction of the variable retarder and the other polarization component orthogonal thereto so that the optical power ratio of the X-polarized light and the Y-polarized light is 1: 1. The optical performance monitor according to claim 1, wherein the optical performance monitor is adjusted.