This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-282898, filed on Dec. 14, 2009, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein are related to a signal-light detection apparatus and method for detecting the presence or absence of a signal component of a light input through, for example, an optical amplifier.
For wavelength division multiplexing (WDM) transmission, with a transmission speed at or above 10 Gb/s, the effects of chromatic dispersion resulting from fiber transmission cause distortion to occur in the waveform of a signal and degrades reception characteristics of the signal light. To address this, for high-speed WDM optical fiber transmission, it is necessary to compensate for chromatic dispersion. Typically, chromatic dispersion is compensated for over the full range of wavelengths of a WDM light in a collective manner by the use of a dispersion compensating fiber (DCF). However, in the case of a WDM light that includes many channels (wavelengths), compensation for chromatic dispersion employing the DCF may be larger than necessary or insufficient, depending on the channel.
Remaining chromatic dispersion resulting from the above-described excess or deficiency of compensation for chromatic dispersion (hereinafter referred to as “residual dispersion”) can be at a level that does not virtually affect reception characteristics for, for example, a signal light of 10 Gb/s. However, in the case of a high-speed signal light at or above 40 Gb/s, the residual dispersion has unignorable effects on reception characteristics. Therefore, residual dispersion is compensated for by the provision of a tunable dispersion compensator to each channel of a WDM light.
In
Each of the optical receiver unit s 505 includes a single-wavelength optical amplifier 511, a tunable dispersion compensator (TDC) 512, an optical receiver (RX) 513, and a control circuit 514. The single-wavelength optical amplifier 511 amplifies a signal light obtained by the demultiplexer 504 to a necessary or desired level and outputs it to the TDC 512. The TDC 512 compensates for residual dispersion of the signal light output from the single-wavelength optical amplifier 511 and outputs the signal light to the optical receiver 513. The amount of dispersion compensation by the TDC 512 is variably controlled by the control circuit 514. The optical receiver 513 performs reception processing required for identifying and reproducing the signal light output from the TDC 512 and outputs information regarding a reception state of the signal light (hereinafter referred to as reception information) to the control circuit 514. The control circuit 514 performs feedback control for optimizing the amount of dispersion compensation by the TDC 512 on the basis of the reception information from the optical receiver 513. The single-wavelength optical amplifier may also be a WDM optical amplifier.
In the reception section of the above-described traditional WDM optical transmission system, an amplified spontaneous emission (ASE) light occurs in optical amplification at each of the WDM optical amplifier 502 and the single-wavelength optical amplifier 511 in the optical receiver unit 505. The amount of the ASE light occurring in the WDM optical amplifier 502 varies depending on the type of an optical fiber used in the optical transmission line 501, the transmission distance, and the number of operational wavelengths. If the WDM optical transmission system employs a multistage relay system in which an inline amplifier is arranged in the optical transmission line 501, an ASE light also occurs between relay sections, and the accumulated ASE light is provided to the WDM optical amplifier 502 in the reception section.
Typically, when the WDM optical amplifier 502 has no operational channel of an input WDM light, it is shut down. In this case, because no ASE light occurs in the WDM optical amplifier 502, no light is input into each of the optical receiver unit s 505. Accordingly, the optical receiver unit 505 can detect an interruption of a signal light by monitoring an input level. However, when there are one or more operational channels of a WDM light, the WDM optical amplifier 502 is activated and an ASE light occurs over the full amplified wavelength range. Therefore, an ASE light component within a transmission range that corresponds to each channel of the demultiplexer 504 is input to not only an optical receiver unit 505 corresponding to each of the operational channels but also another optical receiver unit 505 that does not correspond to the operational channel. The input of the ASE light to the optical receiver unit 505 that does not correspond to the operational channel causes a problem in which an interruption of a signal light cannot be detected if the level of the signal light in the optical transmission line 501 is small, for example.
Specifically, the single-wavelength optical amplifier 511 in the optical receiver unit 505 typically has the function of detecting the presence or absence of a signal component in response to an input optical level. The threshold used in that detection is set at a level smaller than the minimum reception level of a signal light by a specified value (e.g., 3 dB or 6 dB). That is, if the level of a light input to the single-wavelength optical amplifier 511 is lower than the threshold, an interruption of the signal light is detected; if it exceeds the threshold, the presence of the signal light is detected. When the interruption of the signal light is detected, in order to prevent an optical surge caused by the signal light returning thereafter, the single-wavelength optical amplifier 511 is deactivated and a loss of signal (LOS) alarm that indicates the interruption of the signal light is raised.
Under such circumstances where the function of detecting a signal light is activated, when the level of an ASE light input to the optical receiver unit 505 increases, even if it does not correspond to the operation channel, the level of the light input to the single-wavelength optical amplifier 511 exceeds the threshold, an interruption of the signal light cannot be detected. This disables the single-wavelength optical amplifier 511 from being shut down and a LOS alarm from being raised. If the single-wavelength optical amplifier 511 is not shut down, the optical receiver 513 may be broken by an optical surge, and needless power for operating the single-wavelength optical amplifier 511 is consumed.
One example of a traditional technique to avoid inaccurate detection of a signal light resulting from an ASE light is a technique of determining an input state of a signal light input to the optical receiver unit 505 on the basis of whether a clock signal in synchronization with the signal light has been detected in the optical receiver 513. Specifically, for an example configuration illustrated in
Another example traditional technique is extracting part of a light input to an optical receiver unit, separating it into four separated lights, extracting four polarization components having mutually different polarization parameters from the separated lights, and determining whether a signal light has been input on the basis of the polarization components (see, for example, Japanese Laid-open Patent Publication No. 2008-278082).
However, for the above-described traditional technique of detecting the presence or absence of a signal light on the basis of clock-signal detection information, an increase in residual dispersion in a signal light input to the optical receiver 513 will affect reproduction of a clock signal. To address this, it is necessary to check a detection state of the clock signal in the optical receiver 513 after feedback control for the TDC 512 converges to some extent. This results in a problem in which the time required for detecting the presence or absence of a signal light is long.
According to an aspect of the invention, a signal-light detection apparatus includes a polarization extractor that extracts a polarization component that is substantially in parallel with a specified axial direction from an input light, a polarization rotator that changes a relative angle between a direction of polarization of the input light and an axial direction of the polarization extractor, a photodetector that detects an optical power of the polarization component extracted by the polarization extractor, and a determination device that determines whether the input light includes a signal component, based on a variation in the optical power detected by the photodetector.
The object and advantages of the invention will be realized and attained by at least the features, elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
Embodiments are described in detail with reference to the accompanying drawings.
For above-described traditional technique of detecting the presence or absence of a signal light on the basis of four polarization components having mutually different polarization parameters, a complex optical system for extracting the polarization components is required, and processing for electric signals obtained by photoelectric conversion on the polarization components, specifically, arithmetic processing for calculating the degree of polarization (DOP) is also complicated.
Referring to
The splitter 11 branches part of a light that has been output from the single-wavelength optical amplifier 21 and then will be sent to the TDC 22 as a monitored light and outputs the monitored light to the polarization rotator 12.
The polarization rotator 12 rotates the direction of polarization (oscillation) of the monitored light obtained by the splitter 11 in an angle range at or above approximately 90°. Examples of the polarization rotator 12 may include a Faraday rotator, liquid crystal, a polarization scrambler, a polarization modulator, a wave plate, a birefringent plate, and a polarization-maintaining fiber. Examples of a method for rotating the direction of polarization (rotation pattern) may include approximately 360° continuous rotation in a single direction and continuous reciprocation of the angle of rotation between approximately 0° and 90° or between approximately 0° and 180°. The rotation speed may be set at any value in consideration of a response speed of the photodetector 14; specifically, it may preferably be set in the range from several hundred kilohertz to several megahertz.
The polarization extractor 13 extracts a polarization component in a certain direction from a monitored light output from the polarization rotator 12 and provides it to the photodetector 14. Examples of the polarization extractor 13 may include a polarizer and a polarization splitter. Here, the polarization extractor 13 is used in a fixed state where its transmission axis is in a specified direction. As illustrated in
The photodetector 14 converts a polarization component of a monitored light extracted by the polarization extractor 13 into an electric signal and outputs it to the determination device 15. The electric signal changes its level in accordance with the power of the polarization component input to the photodetector 14. That is, the photodetector 14 detects the power of the polarization component of the monitored light extracted by the polarization extractor 13.
The determination device 15 monitors the level of an electric signal output from the photodetector 14, determines the presence of a signal light when it detects a variation in the level within a specified measurement time, and determines an interruption of a signal light when it does not detect a variation in the level. The determination device 15 raises a LOS alarm when determining the interruption of the signal light. The determination device 15 outputs a signal that indicates determination of the presence or absence of the signal light to the control circuit 24 for the single-wavelength optical amplifier 21 and the control circuit 25 for the TDC 22.
The single-wavelength optical amplifier 21 amplifies a light input to the optical receiver unit 20 to a desired level and outputs the amplified light to the TDC 22 through the splitter 11. This optical amplification operation of the single-wavelength optical amplifier 21 is controlled in response to a signal output from the control circuit 24.
The TDC 22 compensates for residual dispersion of a signal light included in a light sent from the single-wavelength optical amplifier 21 through the splitter 11 and outputs the light to the optical receiver 23. The amount of dispersion compensation by the TDC 22 is variably controlled in response to a signal output from the control circuit 25.
The optical receiver 23 performs reception processing required for identifying and reproducing a light output from the TDC 22. The optical receiver 23 outputs information regarding a reception state of a signal light (e.g., a bit error rate or the number of corrected errors in forward error correction (FEC)) to the control circuit 25.
The control circuit 24 controls a driven state of the single-wavelength optical amplifier 21 on the basis of monitoring by an output monitor of the single-wavelength optical amplifier 21 such that an output optical power of the single-wavelength optical amplifier 21 is substantially constant at a desired level. The control circuit 24 shuts down the single-wavelength optical amplifier 21 when receiving a signal that indicates an interruption of a signal light from the determination device 15 of the signal-light detection apparatus 10. When receiving a signal that indicates the presence of a signal light from the determination device 15, the control circuit 24 cancels the shutdown of the single-wavelength optical amplifier 21.
The control circuit 25 generates a control signal for achieving the desired amount of dispersion compensation by the TDC 22 on the basis of reception information from the optical receiver 23 and outputs the control signal to the TDC 22. This feedback control for the amount of dispersion compensation by the TDC 22 performed by the control circuit 25 is suspended when the control circuit 25 receives a signal that indicates an interruption of a signal light from the determination device 15 of the signal-light detection apparatus 10 and is executed when the control circuit 25 receives a signal that indicates the presence of a signal light from the determination device 15.
Here, a case where, in response to determination by the signal-light detection apparatus 10, the control circuit 24 performs shutdown control for the single-wavelength optical amplifier 21 and the control circuit 25 switches (between suspension and execution of) feedback control for the TDC 22 is described. However, either one of the shutdown controls for the single-wavelength optical amplifier 21 and the switching of the feedback control for the TDC 22 may be performed in response to determination by the signal-light detection apparatus 10.
The above-described optical receiver unit 20 employing the signal-light detection apparatus 10 may be used in a reception section of each of a plurality of transponders (TRPNs) 111 included in a terminal apparatus 100 illustrated in
Each of the transponders 111 includes an optical transmission unit 30 including an optical transmitter (TX) 31 and a single-wavelength optical amplifier 32, in addition to the optical receiver unit 20.
The optical transmitter 31 generates a signal light having a wavelength corresponding to a reception channel for the optical receiver unit 20.
The single-wavelength optical amplifier 32 amplifies a signal light output from the optical transmitter 31 to a desired level. When an output optical power of the optical transmitter 31 is at a sufficient level, the single-wavelength optical amplifier 32 may be omitted.
The terminal apparatus 100 includes a transponder section 110 including a plurality of transponders 111 and a WDM section 120 for multiplexing and demultiplexing signal lights of different channels (wavelengths) transmitted and received by each of the transponders 111. The WDM section 120 includes a multiplexing unit 121 and a demultiplexing unit 122.
The multiplexing unit 121 multiplexes signal lights having mutually different wavelengths output from the transponders 111 using a multiplexer 41 to generate a WDM light, collectively amplifies the WDM light using a WDM optical amplifier 42, and outputs it. The WDM light output from the multiplexing unit 121 is transmitted to an optical transmission line connected to the terminal apparatus 100.
The demultiplexing unit 122 collectively amplifies a WDM light passing through the optical transmission line using a WDM optical amplifier 43, supplies the WDM light to a dispersion compensation fiber (DCF) 44, and collectively compensates for chromatic dispersion occurring in the signal light of each channel. The demultiplexing unit 122 demultiplexes the WDM light output from the DCF 44 in channels using a demultiplexer 45 and outputs the signal lights to the respective optical receiver unit s 20 of the corresponding transponders 111.
A WDM optical transmission system, for example, illustrated in
Next, an operation of the signal-light detection apparatus 10 according to the first embodiment is described.
A light input to the optical receiver unit 20 using the signal-light detection apparatus 10 is one of the lights demultiplexed in channels by the demultiplexer 45 in the demultiplexing unit 122 of the terminal apparatus 100 (
A light input to the optical receiver unit 20 further includes an additional ASE light by being amplified by the single-wavelength optical amplifier 21. This ASE light added in the amplification by the single-wavelength optical amplifier 21 is not filtered by the demultiplexer 45, unlike the ASE light occurring in the WDM optical amplifier 43, and therefore, it extends in a wide band corresponding to the amplification wavelength range of the single-wavelength optical amplifier 21. Each of the ASE light occurring in the WDM optical amplifier 43 and that in the single-wavelength optical amplifier 21 has a randomly polarized state. In contrast, a signal light passing through the optical transmission line has a substantially elliptically polarized state close to a linearly polarized state. The signal-light detection apparatus 10 utilizes a difference between the polarization state of the ASE light and that of the signal light and detects the presence or absence of a signal component in a light input to the optical receiver unit 20 through a procedure described below.
In the signal-light detection apparatus 10, the splitter 11 extracts part of a light output from the single-wavelength optical amplifier 21 as a monitored light and supplies the monitored light to the polarization rotator 12. The polarization rotator 12 rotates the direction of polarization of the monitored light in an angle range at or above approximately 90° and supplies it to the polarization extractor 13. The polarization extractor 13 extracts only a polarization component in a certain direction from the supplied monitored light. The polarization component extracted by the polarization extractor 13 is converted into an electric signal by the photodetector 14, and the signal is output to the determination device 15.
When a light input to the optical receiver unit 20 contains a signal light, if the direction of polarization of a signal-light component output from the polarization rotator 12 and the direction of the transmission axis of the polarization extractor 13 are substantially in parallel with each other, the optical power detected by the photodetector 14 is largest. The angle of rotation in the direction of polarization by the polarization rotator 12 in this case is indicated as 0° and 180° in
In contrast, when a light input to the optical receiver unit 20 does not include a signal light and includes only an ASE light, even if the randomly polarized ASE light is supplied to the polarization rotator 12, the ASE light output from the polarization rotator 12 is still a randomly polarized light. Therefore, of the ASE light supplied to the polarization extractor 13, only a slight component that is substantially in parallel with the direction of the transmission axis of the polarization extractor 13 is extracted by the polarization extractor 13. Accordingly, the optical power detected by the photodetector 14 does not depend on the angle of rotation by the polarization rotator 12 and is substantially constant at a low level.
The determination device 15 detects whether a substantial variation in the level has occurred in a signal output from the photodetector 14 within a specified measurement time on the basis of the above-described difference of characteristics between a signal light and an ASE light. The measurement time is set in consideration of a possibility that the polarization state of a signal light may be accidentally changed by, for example, application of a stress to the optical transmission line through which the signal light is conveyed. That is, a situation in which the accidental change of the polarization state cancels out a change in the direction of polarization caused by rotation by the polarization rotator 12 may occur, although such a situation is unlikely. Therefore, it may be preferable that a certain measurement time be prepared within a range that does not affect shutdown control for the optical amplifier to enable detection of a variation in the level resulting from rotation in the direction of polarization by the polarization rotator 12 even if the above-described situation occurs.
When detecting a variation in the level of a signal output from the photodetector 14, the determination device 15 determines that the light input to the optical receiver unit 20 includes a signal light. In contrast, when detecting no variation in the level of the signal output from the photodetector 14, the determination device 15 determines that the light input to the optical receiver unit 20 includes only an ASE light and a signal light is interrupted. When determining the interruption of the signal light, the determination device 15 raises a LOS alarm. Here, substantially in simultaneity with the raising of the LOS alarm, a signal indicating the interruption of the signal light is output from the determination device 15 to each of the control circuits 24 and 25. In response to this, the control circuit 24 performs shutdown control for the single-wavelength optical amplifier 21 and the control circuit 25 stops feedback control for the amount of dispersion compensation by the TDC 22. When a signal indicating the presence of a signal light is sent from the determination device 15 to each of the control circuits 24 and 25, the shutdown of the single-wavelength optical amplifier 21 is cancelled and the feedback control for the amount of dispersion compensation by the TDC 22 is executed.
Results of observation of how the level of a signal output from the photodetector 14 varies using a measurement system An illustrated in
The measurement system A in
With the above-described signal-light detection apparatus 10, whether a light input to the optical receiver unit 20 includes a signal light may be detected in a short time employing a simple optical system and simple detection of a variation in the level of an electric signal without being affected by an ASE light and, if an interruption of a signal light is determined, a LOS alarm may be reliably raised. The shutdown control for the single-wavelength optical amplifier 21 in the optical receiver unit 20 in response to the determination of the presence or absence of a signal light by the signal-light detection apparatus 10 may prevent or substantially attenuate an optical surge and reduce power consumption in the single-wavelength optical amplifier 21. Suspending feedback control for the TDC 22 when an interruption of a signal light is determined may avoid needless feedback control based on an ASE light, thus enabling a reduction in power consumption in the TDC 22. If the TDC 22 is a device that has a mechanical driving element (e.g., VIPA), its part life may also be extended.
Next, a signal-light detection apparatus according to a second embodiment is described.
Referring to
For the signal-light detection apparatus 10 having the above-described configuration, a monitored light obtained by the splitter 11 is directly supplied to the polarization extractor 13 rotated by the rotary driver 16. When the monitored light includes a signal light, as illustrated in
Accordingly, as in the case of the above-described first embodiment, the determination device 15 determines the presence of a signal light when detecting a variation in the level of a signal output from the photodetector 14 and determines an interruption of a signal light when detecting no variation in the level. When the interruption of the signal light is determined, the determination device 15 raises a LOS alarm. In response to the determination by the determination device 15, the control circuit 24 performs shutdown control for the single-wavelength optical amplifier 21 and the control circuit 25 switches (between suspension and execution of) feedback control for the TDC 22.
As described above, with the signal-light detection apparatus 10 according to the second embodiment, substantially the same advantages as in the above-described first embodiment are obtainable even by changing the direction of the transmission axis by rotating the polarization extractor 13 itself using the polarizer employing the rotary driver 16.
For the above-described first and second embodiments, an example in which the signal-light detection apparatus 10 is arranged between the single-wavelength optical amplifier 21 and the TDC 22 in the optical receiver unit 20 is described. However, the arrangement of the signal-light detection apparatus 10 in the optical receiver unit 20 is not limited to the described example. For example, the signal-light detection apparatus 10 may be arranged before the single-wavelength optical amplifier 21, as illustrated in
When the signal-light detection apparatus 10 is arranged after the TDC 22, a monitored light obtained by the splitter 11 has an optical spectrum corresponding to a transmission wavelength characteristic of the TDC 22. For example, when a VIPA is used as the TDC 22, because its transmission wavelength characteristic periodically varies, after a wide-band ASE light occurring in the single-wavelength optical amplifier 21 is filtered by the TDC 22, the ASE light is extracted as a monitored light by the splitter 11. In this case, because the power of the ASE light included in the monitored light is significantly reduced, the presence or absence of a signal light may be determined using a typical threshold for an input optical power. In contrast, when the TDC 22 has a nonperiodic transmission wavelength characteristic, most of a wide-band ASE light occurring in the single-wavelength optical amplifier 21 passes through the TDC 22 and is extracted as a monitored light by the splitter 11. In this case, with a typical threshold for an input optical power in determination, there is a strong possibility that the presence or absence of a signal light may be incorrectly determined on the basis of the power of an ASE light. In consideration of this respect, for determination of the presence or absence of a signal light after the TDC having a nonperiodic transmission wavelength characteristic, the use of the signal-light detection apparatus 10 according to the above-described embodiments is particularly effective.
For the above-described first and second embodiments, an example in which a single-wavelength signal light input to the optical receiver unit 20 has a substantially elliptically polarized state close to a linearly polarized state is described. For example, even in a case where a signal light subjected to polarization multiplexing is input to the optical receiver unit 20, the presence or absence of the signal light may also be detected using a signal-light detection apparatus having basically the same configuration as in the above-described embodiments.
Specifically, signal light components S1 and S2 in which two lights having substantially orthogonal directions of polarization are polarization-multiplexed are described.
In relation to the above-described first and second embodiments, when a WDM optical transmission system as illustrated in, for example,
With the above-described signal-light detection apparatus and method, the presence or absence of a signal component included in the input light may be detected utilizing a difference between the polarization state of an ASE light occurring in an optical amplifier and that of a signal light by the use of a simple configuration that changes a relative angle between the direction of polarization of an input light and the axial direction of a polarization extractor, extracts a specified polarization component from the input light, and detects the power may detect.
All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
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2009-282898 | Dec 2009 | JP | national |