JP4129718B2 - WDM signal monitor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a WDM (Wavelength Division Multiplexing) signal monitor, and more particularly to an improvement in optical noise level measurement.
[0002]
[Prior art]
As the transmission capacity increases, WDM signals are multiplexed at a high density, and not only the optical signal level and wavelength of each channel, but also the optical SNR (signal to noise ratio) is an important measurement parameter. Here, the optical SNR is defined individually for each channel of the optical signal, and by definition is a ratio at the same wavelength, but in reality, the optical noise level directly below the optical signal level cannot be measured. It is represented by the ratio between the optical signal level and the optical noise level at a position away from the peak wavelength of this optical signal by a certain distance.
[0003]
Monitoring the level, wavelength, and optical SNR of these multiplexed WDM signals is indispensable for maintaining the quality of the transmission signal, and such a WDM signal monitor is in-line with a part of the optical communication system. For example, there is one configured by using a small spectroscope so that it can be integrated and monitored constantly.
[0004]
FIG. 6 is an example of a response characteristic with respect to the line spectrum of such a small spectroscope, and has a spread like a Gaussian distribution.
In FIG. 6, the 3 dB bandwidth is a parameter corresponding to the resolution of the spectrometer. The difference between the peak level of the optical signal and the optical signal level at the tail position having a characteristic distance from the peak wavelength by a certain distance Δλ _offset is the optical dynamic range of the spectrometer. The optical signal level at the bottom position becomes a stray light level that is an error factor with respect to the optical signal level of the peak wavelength to be measured, and determines the limit of the optical SNR measurement.
[0005]
FIG. 7 is a conceptual diagram of a WDM signal, in which a plurality of optical signals having different wavelengths are arranged at substantially equal intervals Δλ. For example, if the optical frequency interval is 50 GHz, Δλ = 0.4 nm.
Conventionally, in measuring and calculating the optical SNR for each of these optical signals, the point (offset position) for measuring the optical noise level for each of these optical signals is fixed at a predetermined position as shown in FIG. It was. Specifically, for example, in the case of 50 GHz, Δλ _offset = 0.2 nm is fixed as an intermediate position of the interval Δλ = 0.4 nm.
[0006]
[Problems to be solved by the invention]
However, the WDM signal monitor using a small spectroscope does not have a very large optical dynamic range because the resolution is limited as shown in FIG. Therefore, it is difficult to directly measure the optical noise level near the peak wavelength, and in order to accurately measure the optical noise level near the peak wavelength, some correction is necessary to cancel the stray light level.
[0007]
On the other hand, with the diversification of systems and the progress of networking, it is also required to cope with a mixture of optical signals having different channel intervals as shown in FIG. 8, for example, a mixture of optical frequency intervals of 50 GHz and 100 GHz. Yes. In FIG. 8, Δλ ₁ represents a series having a large optical frequency interval of, for example, 100 GHz, and Δλ ₂ represents a series having a small optical channel interval of, for example, 50 GHz.
[0008]
Here, when the noise measurement point NP21, NP22 to measure a sequence of [Delta] [lambda] ₂ is measuring and calculating a sequence of [Delta] [lambda] ₁ in the state of being fixed, the noise measuring point NP21 in the sequence of Δλ _1, NP22 is measured relative The stray light level that becomes an error factor with respect to the optical signal level of the peak wavelength to be measured becomes close to the target peak wavelength position. In this case, by setting the noise measurement points NP21 and NP22 further away, the amount of stray light level of the signal is reduced, and the correction error can be reduced.
[0009]
On the other hand, if the Δλ ₂ series is measured and calculated in a state where the noise measurement point is fixed in order to measure the Δλ ₁ series, the influence of the stray light level of the adjacent optical signal spectrum is increased in some cases, The calculation error may increase.
[0010]
The present invention solves such a problem, and one of its purposes is to provide a WDM signal monitor capable of correcting the influence of the stray light level.
Another object of the present invention is to provide a WDM signal monitor capable of measuring and calculating an optical SNR so as not to be affected by the stray light level as much as possible when optical signals having different channel intervals coexist.
[0011]
[Means for Solving the Problems]
The invention of claim 1 which achieves such an object,
In a WDM signal monitor using a spectroscope,
Means comprising a spectrometer for measuring the spectrum of a WDM signal;
Means for storing response characteristic data of the spectrometer with respect to a line spectrum of an optical signal;
Based on the spectrum measurement data of the WDM signal and the response characteristic data of the spectrometer, the optical signal level and the optical noise level of each channel are corrected while correcting the influence of the stray light level caused by the response characteristic of the spectrometer on the measurement data of the optical noise level. Means for calculating;
The optical noise level measurement calculation position in the means for calculating is from the midpoint of the channel interval of the WDM signal where the measurement error is reduced according to the channel interval of the optical signal of the WDM signal, or from the minimum point between the adjacent channels of the WDM signal or from the outside. Means to control to any of the specified positions ,
[0012]
Thereby, the influence of the stray light level caused by the response characteristics of the spectrometer with respect to the measurement data of the optical noise level can be corrected, and even when optical signals having different channel intervals are mixed, high-precision measurement can be performed.
[0013]
According to a second aspect of the present invention, in the WDM signal monitor according to the first aspect, the spectroscope including an array-type photodetecting element is used, and the output of the photodetecting element closest to the measuring point of the optical noise level is set to the optical noise level. It is characterized by a measured value .
[0015]
The invention of claim 3 is a WDM signal monitor characterized by dividing the measurement wavelength range of the WDM signal into arbitrary regions and applying claim 1 or claim 2 to each region.
[0020]
Accordingly, it is possible to realize a WDM signal monitor with a small measurement error due to the channel interval of the WDM signal.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing an example of an embodiment of the present invention.
The optical signal spectrum measurement unit 1 uses a small spectroscope, measures the spectrum of the WDM signal, and outputs measurement data to the calculation unit 2. Further, the response characteristic data of the spectroscope with respect to the line spectrum of the optical signal is input from the spectroscope response data storage unit 3 to the calculation unit 2.
[0022]
Based on the spectrum measurement data of the WDM signal and the response characteristic data of the spectrometer, the calculation unit 2 corrects the influence of the stray light level caused by the response characteristic of the spectrometer with respect to the measurement data of the optical noise level, and corrects the light of each channel. Calculate signal level and optical noise level.
[0023]
The optical noise level measurement position control unit 4 controls the measurement calculation position of the optical noise level in the calculation unit 2 to an appropriate position where the measurement error is reduced according to the channel interval of the optical signal of the WDM signal. The stray light that accompanies the optical signal of interest decreases as it moves away from a certain optical signal, but the stray light from the adjacent optical signal increases, so it is desirable that the total stray light is minimized as the measurement position of the optical noise level . These measurement calculation positions include an intermediate point of the channel interval of the WDM signal, a minimum point between adjacent channels of the WDM signal, a position designated from the outside, and the like.
[0024]
If the peak levels of adjacent optical signals are almost the same, the optical noise level is minimized at the midpoint between adjacent optical signals. When a plurality of WDM signals having different channel intervals coexist, errors can be suppressed as much as possible by independently applying appropriate methods and settings to the WDM signals of each sequence.
[0025]
FIG. 2 is a configuration diagram showing a specific example of a polychromator type spectrometer used as the optical signal spectrum measuring unit 1. In the figure, a WDM signal is incident through an optical fiber 11. The incident WDM signal is incident on the diffraction grating 13 through the lens 12. The diffraction grating 13 gives a certain phase difference to the optical signals of the respective wavelengths.
[0026]
The output light from the diffraction grating 13 is incident on the mirror 15 through the lens 14, and the reflected light from the mirror 15 is focused on the light receiving surface of the photodiode array 16.
[0027]
The photodiode array 16 converts an optical signal focused and incident on each photodetecting element into an electrical signal. Thus, the wavelength and level of each optical signal can be measured and monitored based on the output signal of the photodiode array 15.
[0028]
The resolution of the spectroscope configured in this way depends on the size of the dispersive element, and the resolution increases as the size increases, but it should be as small as possible in order to incorporate the spectrograph into the optical communication system. Is desirable.
As a result of reducing the size and limiting the resolution and dynamic range, the influence of the optical signal near the peak cannot be ignored in measuring the noise level near the peak of the measurement optical signal. Therefore, the following correction process is performed.
[0029]
FIG. 3 is a flowchart showing an example of the flow of measurement calculation processing for correcting the influence of the stray light level in the optical SNR measurement according to the present invention. First, in step 1, the spectrum of the dispersed light is sampled to measure the spectrum Pm (i) (i = 1 to M) of the optical signal of the WDM signal (ST1). Here, the sampling signal Pm (i) may be, for example, time-series data obtained by wavelength sweeping, or may be an element output data string of a photodiode array such as a polychromator, and within a predetermined resolution width. Is obtained as a superposition of the optical signal component Ps (k) and the optical noise component Pn (k) of the N channel wavelength.
[0030]
Next, the peak λk (k = 1 to N) is detected based on the spectrum Pm (i) (ST2).
[0031]
On the other hand, the profile f (Δλ) for measuring the response characteristics of the spectrometer with respect to the line spectrum for a plurality of wavelengths is measured offline (ST3), and profile data fk (Δλ) at an arbitrary wavelength k is obtained (ST4).
[0032]
Then, using these peak detection data λk (k = 1 to N) and profile data fk (Δλ), a simultaneous equation having the optical signal level Ps (k) and the optical noise level Pn (k) as unknowns is solved (ST5). Then, the optical signal level Ps (k) and the optical noise level Pn (k) are obtained (ST6).
[0033]
FIG. 4 is a flowchart showing an example of optical SNR measurement calculation processing performed while changing the measurement calculation position of the optical noise level according to the present invention. First, in step 1, the peak of the optical signal of the WDM signal is detected and the wavelength region is divided (ST1). Next, it is determined whether or not an optical noise level measurement calculation position is designated (ST2). If the measurement calculation position of the optical noise level is not designated, the intermediate point or the minimum point with the adjacent peak is detected, and these positions are set as the measurement calculation position of the optical noise level (ST3). If the optical noise level measurement calculation position is designated, the position is set as the optical noise level measurement calculation position (ST4). Then, the light detection element closest to the measurement calculation position of the optical noise level designated in step ST3 or ST4 is specified, and its output is detected (ST5). Then, the optical noise level is measured based on the outputs of these photodetecting elements, and the optical SNR is calculated (ST6).
[0034]
FIG. 5 is a configuration diagram showing another example of a spectroscope used as the optical signal spectrum measurement unit 1. In the figure, a WDM signal is incident on the input-side waveguide 17. The incident WDM signal is spatially expanded by the input-side slab waveguide 18 which is a two-dimensional waveguide, and is incident on the arrayed waveguide 19 with an equal phase. These arrayed waveguides 19 constitute an AWG (Arrayed Waveguide Grating), and give a certain phase difference to optical signals of respective wavelengths spread spatially.
[0035]
The light from the arrayed waveguide 19 is incident on the output side slab waveguide 20. The light incident on the output-side slab waveguide 20 is diffracted and separated for each wavelength, and is collected on the light receiving surface of a photodiode array 21 used as a photodetector.
[0036]
The photodiode array 21 converts an optical signal focused and incident on each photodetecting element into an electrical signal. Thus, the wavelength and level of each optical signal can be measured and monitored based on the output signal of the photodiode array 21.
[0037]
Here, the input side slab waveguide 18 and the output side slab waveguide 20 are arranged in parallel so as not to overlap each other, and the input port of the input side waveguide 17 is orthogonal to the input side slab waveguide 18 and the output side slab waveguide 20. It is provided in a positional relationship.
[0038]
With such a positional relationship, the multiplexed optical signal input to the input port of the input side waveguide 17 is prevented from leaking from the waveguide and being directly incident on the output side slab waveguide 20 or the photodiode array 21. is doing.
[0039]
Note that the photodiode array may be integrated on the common semiconductor substrate so as to face the output side of the output-side slab waveguide by using a semiconductor processing technology, and further for reading out the output signal of the photodiode array. Multiplexers may also be integrated to form a signal processing unit.
[0040]
Thus, by performing the optical SNR measurement calculation process while changing the measurement position of the optical noise level, it is possible to suppress the error as much as possible as compared with the case where the conventional measurement position is fixed.
【The invention's effect】
As described above, according to the present invention, the influence of the stray light level in the optical SNR measurement can be corrected, and when the optical signals having different channel intervals are mixed, the influence of the stray light level is minimized. A WDM signal monitor capable of measuring an optical SNR can be realized, and is suitable as a signal monitor for an optical communication system (particularly a WDM system).
[Brief description of the drawings]
FIG. 1 is a block diagram showing an example of an embodiment of the present invention.
2 is a configuration diagram showing a specific example of a spectroscope used as the optical signal spectrum measurement unit 1 of FIG.
FIG. 3 is a flowchart showing an example of the flow of measurement calculation processing for correcting the influence of the stray light level in the optical SNR measurement according to the present invention.
FIG. 4 is a flowchart showing an example of an optical SNR measurement calculation process performed while changing the measurement position of the optical noise level according to the present invention.
5 is a configuration diagram showing another specific example of a spectroscope used as the optical signal spectrum measuring unit 1 in FIG. 1. FIG.
FIG. 6 is an example of response characteristics with respect to a line spectrum of a small spectroscope.
FIG. 7 is a conceptual diagram of a WDM signal.
FIG. 8 is a conceptual diagram of a WDM signal in which optical signals having different channel intervals are mixed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical signal spectrum measurement part 2 Operation part 3 Spectrometer response data storage part 4 Optical noise level measurement position control part 11 Optical fiber 12, 14 Lens 13 Diffraction grating 15 Mirror 16 Photodiode array 17 Input side waveguide 18 Input side slab guide Waveguide 19 Array waveguide 20 Output side slab waveguide 21 Photodiode array

Claims (3)

In a WDM signal monitor using a spectroscope,
Means comprising a spectrometer for measuring the spectrum of a WDM signal;
Means for storing response characteristic data of the spectrometer with respect to a line spectrum of an optical signal;
Based on the spectrum measurement data of the WDM signal and the response characteristic data of the spectrometer, the optical signal level and the optical noise level of each channel are corrected while correcting the influence of the stray light level caused by the response characteristic of the spectrometer on the measurement data of the optical noise level. Means for calculating;
The optical noise level measurement calculation position in the means for calculating is from the midpoint of the channel interval of the WDM signal where the measurement error is reduced according to the channel interval of the optical signal of the WDM signal, or from the minimum point between the adjacent channels of the WDM signal or from the outside. Means to control to any of the specified positions ,
And a WDM signal monitor. 2. The WDM signal according to claim 1, wherein a spectroscope including an array-type photodetecting element is used, and an output of the photodetecting element closest to the measuring point of the optical noise level is used as a measured value of the optical noise level. monitor. A WDM signal monitor, wherein a measurement wavelength range of a WDM signal is divided into arbitrary regions, and claim 1 or claim 2 is applied to each region .

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