JP4867306B2 - WDM signal monitor - Google Patents

Description

  The present invention relates to a WDM signal monitor that measures each of WDM (wavelength division multiplexing) optical signals, and more particularly to a WDM signal monitor that can measure an optical signal with a small number of photodiodes. It is.

  One type of optical communication system that transmits optical signals through optical fibers is WDM communication. The WDM communication is a communication method for transmitting a plurality of optical signals having different wavelengths through a single optical fiber. A plurality of optical signals having different wavelengths are also called WDM signals. The optical signals of the WDM signals are often counted as, for example, one channel and two channels from the short wave side.

  As the transmission capacity expands, high-density multiplexing of WDM signals advances, and not only the optical signal level and peak wavelength of each channel but also the optical SNR are important measurement parameters. Here, the optical SNR is individually defined for each channel of the optical signal, and is defined as a ratio between the optical signal level and the optical noise level at the same wavelength. However, in reality, since the optical noise level immediately 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.

  Further, the WDM signal is generally optically amplified using an optical fiber amplifier which is a kind of optical amplifier and transmitted through the optical fiber. The optical noise level in such an optical communication system is the optical amplifier. It often refers to the optical signal level of ASE (Amplified Spontaneous Emission: amplified spontaneous emission light), which is a noise source.

  The WDM signal monitor uses a wavelength dispersive element (for example, a diffraction grating) to divide the light to be measured including the WDM signal for each wavelength, obtain optical power existing in an arbitrary wavelength width, and from the obtained optical power, This is a device for measuring the parameters described above. In addition, some WDM signal monitors are configured using, for example, a small spectroscope so that they can be incorporated in a part of an optical communication system in an in-line format, and can be constantly monitored. Are detected by an array type detector (for example, a photodiode array) (see, for example, Patent Document 1).

  Here, FIG. 4 is a diagram showing the measured light converged on the light receiving surface of the photodiode array and the spectrum of the measured light. In FIG. 4, a plurality of photodiodes PDa to PDj are arranged along a predetermined direction. Here, the arrangement direction is taken as the x-axis. In FIG. 4, two optical signals s1 and s2 are shown as an example.

Each of the optical signals s1 and s2 in the light to be measured transmitted by the optical fiber is a line spectrum, but due to the response characteristics of the spectrometer, the light spots of the optical signals s1 and s2 formed on the photodiode array are The optical power is elliptical or circular with a Gaussian distribution. Here, the optical spot is a portion having an optical power that is 1 / e ² times the optical peak power of each optical signal. The distance from the center of the optical spot to the circumference (x-axis direction). Becomes the radius ω of the light spot. This light spot is set wider (for example, three or more elements) than the pitch r of the photodiodes PDa to Pdj.

  In addition, each of the photodiodes PDa to Pdj is assigned a wavelength in advance, and corresponds to a position where the light to be measured is dispersed by wavelength by the diffraction grating and converged on the surface of the photodiode array.

  Then, an arithmetic unit provided at the subsequent stage of the spectroscope reproduces the spectrum of the optical signal from the sampling data output from each of the photodiodes PDa to Pdj of the photodiode array, and obtains the wavelength, optical power, optical SNR, and the like.

  For example, the calculation unit detects sampling data having an output value that is the peak of the optical signal s1, and outputs data of the photodiode PDd corresponding to the detected peak, and the photodiodes PDc and PDe sandwiching the photodiode PDd. Find Gaussian distribution from data. Then, the center wavelength and optical power (total power) of the optical signal s1 are obtained from the obtained Gaussian distribution.

  On the other hand, the optical SNR needs to obtain an optical noise level, but the optical spots of the optical signals s1 and s2 extend over at least three elements in order to approximate the Gaussian. Therefore, the portion (photodiode PDg) located in the middle of the photodiodes PDd and PDj that has the peaks of the optical signals s1 and s2 has the lowest stray light level. However, under the influence of the optical signals s1 and s2 that increase the light spot, it is difficult to accurately determine the optical noise level even with the photodiode PDg at the intermediate position.

  Therefore, the response characteristic of the spectroscope is obtained in advance, and the calculation unit calculates the output value of the photodiode PDg, the response characteristic of the spectroscope, the center wavelengths of the optical signals s1 and s2, and the total power of the optical signals s1 and s2. The influence of the optical signals s1 and s2 on the photodiode PDg is obtained by simultaneous equations as parameters, and the optical noise level is calculated. Then, the optical SNR is obtained from the total power and the optical noise level (see, for example, Patent Documents 2 and 3).

Japanese Patent No. 3230565 Japanese Patent Laid-Open No. 2003-4531 Patent No. 3678201

  In this way, the wavelength, total power, and optical SNR are obtained by reproducing the spectrum shape of the optical signals s1 and s2 using the sampling data and further using the response characteristics of the spectrometer.

  The frequency (wavelength) of the optical signal of the WDM signal to be measured is determined by ITU (International Telecommunications Union), which is an international organization that establishes communication standards. In recent years, an ITU grid defined by the ITU is required in the WDM communication system, particularly when the transmission system side is appropriately adjusted due to technical progress of the laser light source, progress of technology for stably operating the laser light source, and the like. Thus, the wavelengths of the optical signals s1 and s2 are not greatly shifted, and the optical power and the optical SNR on the transmission system side are adjusted to have appropriate sizes. On the other hand, since multiple stages of optical amplification are performed by long-distance transmission, it is important to measure the optical power and optical SNR at the relay point and the receiving system side.

  On the other hand, the WDM signal is modulated laser light, and its spectrum is relatively narrow. Therefore, in order to accurately measure the optical power and optical SNR of each optical signal using the response characteristics of the spectrometer by reproducing the spectrum shape by approximating with a Gaussian distribution as in the past, the center of the optical signal is used. Wavelength calculation is required, sampling must be performed at an appropriate interval, and a photodiode array with a narrow pitch r is inevitably required.

  However, as WDM signals are multiplexed at higher density and the number of WDM signals (that is, the number of channels) is increased, the photodiodes PDa to PDj are further increased in density (minimization of the pitch r), and the photodiode PDa. There is a problem that a large-scale photodiode array is required due to an increase in the number of elements of .about.PDj. Furthermore, there is a problem that it takes time for the sweep time and the calculation time for sequentially reading the outputs from the photodiodes PDa to PDj, and high-speed measurement becomes difficult. In addition, the high-density and large-scale photodiode array has a problem that the yield in manufacturing is low and the cost is high.

  Accordingly, an object of the present invention is to realize a WDM signal monitor capable of measuring the optical power, optical SNR, and the like of an optical signal with a small number of photodiodes.

The invention described in claim 1
A WDM signal monitor for measuring a wavelength-division-multiplexed optical signal whose wavelength is predetermined by the International Telecommunication Union ,
A plurality of photodiodes are arranged in a predetermined direction, a collimating lens converts the optical signal into parallel light, a reflective diffraction grating wavelength-disperses the optical signal of the parallel light in the predetermined direction, and a focusing lens A spectroscope for collecting each wavelength-dispersed optical signal every other one of the photodiodes, and receiving the photodiode;
A noise level calculation means for obtaining an optical noise level from an output of a photodiode located next to the photodiode that receives the optical signal, and a total output of the optical signal by an output of the photodiode of the spectrometer that receives the condensed optical signal. Power calculation means for correcting power, and correcting the calculated total power with the optical noise level obtained by the noise level calculation means;
In a WDM signal monitor comprising SNR calculation means for obtaining an optical SNR from the total power obtained by correction by the power calculation means and the optical noise level obtained by the noise level calculation means,
The width of the photodiode in the predetermined direction is larger than the diameter of the light spot of the optical signal formed on the light receiving surface of the photodiode, and the width of the dead zone between the photodiodes is the light of the optical signal. The focal lengths of the collimating lens and the focusing lens are adjusted so that it is smaller than the diameter of the spot and the pitch of the photodiode is about twice the diameter of the light spot of the optical signal . It is characterized by.
The invention according to claim 2
A WDM signal monitor for measuring a wavelength-division-multiplexed optical signal whose wavelength is predetermined by the International Telecommunication Union ,
A plurality of photodiodes are arranged in a predetermined direction, a collimating lens converts the optical signal into parallel light, a reflective diffraction grating wavelength-disperses the optical signal of the parallel light in the predetermined direction, and a focusing lens A spectroscope for collecting each wavelength-dispersed optical signal every other one of the photodiodes, and receiving the photodiode;
Noise level calculation means for obtaining an optical noise level from an output of a photodiode located next to the photodiode that receives the optical signal;
Power calculating means for obtaining the total power of the optical signal by the output of the photodiode of the spectroscope that receives the optical signal;
In a WDM signal monitor comprising SNR calculation means for obtaining an optical SNR from the total power obtained by the power calculation means and the optical noise level obtained by the noise level calculation means,
The width of the photodiode in the predetermined direction is larger than the diameter of the light spot of the optical signal formed on the light receiving surface of the photodiode, and the width of the dead zone between the photodiodes is the light of the optical signal. The focal lengths of the collimating lens and the focusing lens are adjusted so that it is smaller than the diameter of the spot and the pitch of the photodiode is about twice the diameter of the light spot of the optical signal . It is characterized by.
The invention according to claim 3 is the invention according to claim 1 or 2,
The noise level calculation means is characterized in that the output of the photodiodes located on both sides of the photodiode receiving the optical signal is obtained by averaging.

The present invention has the following effects.
According to the first to third aspects , the spectroscope is adjusted to receive each dispersed optical signal every other photodiode element. And since the total power of the optical signal is obtained from the output of every other photodiode that receives the optical signal, even if the WDM signal is multiplexed at a high density, the number of photodiode elements is greatly increased as in the prior art. There is no need to increase. As a result, the optical signal can be measured with a small number of photodiodes, and the sweep time and calculation time of the photodiodes can be suppressed and high-speed measurement can be performed. Further, since it is not necessary to reduce the pitch and width of the photodiode as in the prior art, the yield in manufacturing is improved and the cost can be suppressed.
The noise level calculation means obtains the optical noise level from the output of the photodiode located next to the photodiode receiving the optical signal, and the power calculation means obtains the total power obtained by the noise level calculation means. Since the correction is made with the noise level, the net total power of the optical signal not including the optical noise level is obtained.
Further, since the SNR calculation means obtains the optical SNR from the total power from the power calculation means and the optical noise level from the noise level calculation means, even if the WDM signal is multiplexed with high density, the number of elements of the photodiode There is no need to increase the number of times as in the conventional case. As a result, the optical signal can be measured with a small number of photodiodes, and the sweep time and calculation time of the photodiodes can be suppressed and high-speed measurement can be performed. Further, since it is not necessary to reduce the pitch and width of the photodiode as in the prior art, the yield in manufacturing is improved and the cost can be suppressed.
According to the third aspect, since the noise level calculation means averages the outputs of the photodiodes located on both sides of the photodiode that receives the optical signal to obtain the optical noise level, the optical noise level can be obtained with high accuracy. .

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, a spectroscope 10 is of a polychromator type, receives a light to be measured 100 including a WDM signal, measures the light to be measured 100, and outputs measurement data as sampling data. The WDM signal monitor of the present invention is desirably used at the relay point and reception system side of the communication system, and the optical SNR on the transmission side is about 25 to 30 [dB]. Often deteriorates to about 15 [dB].

  The spectroscope 10 includes an optical fiber 11, a collimating lens 12, a diffraction grating 13 that is a wavelength dispersion element, a focusing lens 14, and a photodiode array module (hereinafter abbreviated as PDM) 15.

  The optical fiber 11 is a transmission path through which the measured light 100 enters the spectrometer 10. The collimating lens 12 is installed facing the exit of the optical fiber 11 and emits the measured light 100 emitted from the optical fiber 11 as parallel light.

  The diffraction grating 13 is tilted with respect to the collimating lens 12 in order to diffract the emitted light from the collimating lens 12 at a desired angle. In addition, the diffraction grating 13 divides and emits the light to be measured 100 at different angles for each wavelength. The focusing lens 14 is installed on the optical path of the outgoing light from the diffraction grating 13 and converges the outgoing light.

  The PDM 15 is installed at a position where the measured light 100 converges. The PDM 15 is provided with a photodiode array in which a plurality of photodiodes PD (1) to PD (N) which are strip-shaped or dot-shaped light receiving elements are arranged in a predetermined direction. Note that N is a natural number and (the number of channels of the WDM signal) × 2 ≦ N. The photodiodes PD (1) to PD (N) generate a current (photocurrent) corresponding to the optical power of the incident measurement light 100. The PDM 15 sequentially outputs measurement data obtained by converting photocurrents of the photodiodes PD (1) to PD (N) into voltages, for example, from the photodiode PD (1) on the short wavelength side.

  Further, each of the photodiodes PD (1) to PD (N) is assigned so as to correspond to the optical signal of the ITU grid. The allocation corresponds to a position where the measured light 100 is wavelength-dispersed for each wavelength in the arrangement direction of the photodiodes PD (1) to PD (N) by the diffraction grating 13 and converges in the photodiode array. The optical signal of the channel determined by the ITU is adjusted so as to be received by one photodiode PD (1) to PD (N) per channel.

  Here, FIG. 2 is a diagram illustrating an example of an ITU grid in a part of the photodiodes PD (1) to PD (N). In FIG. 2, photodiodes PD (1) to PD (N) are shown as five elements PD (1) to PD (5) with the direction in which the photodiodes PD (1) to PD (N) are arranged as the x-axis. Of course, the photodiode PD (6) and the subsequent ones are also arranged and irradiated with an optical signal, but the illustration is omitted here.

  For example, if the WDM signal is 100 [GHz] in the C-Band band, the shortest-wavelength side photodiode PD (1) corresponds to 190.05 [THZ], and the photodiode Pd (2) is , 190.1 [THz], and similarly, photodiodes PD (3), PD (4)... Correspond to 190.15 [THZ], 190.2 [THz].

  If the width of each photodiode PD (1) to PD (N) is Δp and the width of the dead zone between the photodiodes PD (1) to PD (N) is Δq, then (Δq) <(light spot (2ω)) <(Δp).

  For example, if the pitch of the photodiodes PD (1) to PD (N) is r and Δp: Δq = 7: 1 or so, (pitch r) / (2 · ω) = 2.0 may be used. Thereby, for example, the optical power received by the adjacent photodiode PD (j + 1) or PD (j−1) is about −30 [dB] with respect to the optical power received by the photodiode PD (j). If the optical SNR is about 15 [dB], the measurement is hardly affected.

  If the ITU grid has an interval of 100 [GHz] (about 0.8 [nm]), one pitch of the photodiodes PD (1) to PD (N) is 50 [GHz] (about 0.4 [ nm]) is adjusted and assigned to the focal lengths of the lenses 12 and 14.

  Further, the center in the x-axis direction of the light spot of the optical signal of the first channel (the light spot formed on the light receiving surface of the photodiode) is the central portion in the x-axis direction of the photodiode PD (2). , The optical fiber 11, so that the center of the light spot at the center of the photodiode PD (4) is the center of the photodiode PD (2 × j). The lenses 12, 14, the diffraction grating 13, the PDM 15 and the like are adjusted. That is, the photodiode PD (j) irradiated with the optical signal and the photodiode PD (j + 1) not irradiated with the optical signal are alternately arranged. In other words, each optical signal wavelength-dispersed by the diffraction grating 13 is received by the photodiodes PD (2), PD (4),..., PD (2 × j),.

  In the spectroscope 10, the portion to be measured is wavelength-dispersed in the direction in which the photodiodes PD (1) to PD (N) are arranged and converged on the PDA 15 (optical fiber 11, lenses 12, 14, The diffraction grating 13) is called a dispersion optical system, and the PDM 15 that receives wavelength-dispersed light to be measured is also called a light detection unit.

  The calculation unit 20 includes a noise level calculation unit 21, a power calculation unit 22, and an SNR calculation unit 23, and calculates the optical power (total power) and optical SNR of the optical signals s1 and s2 from the measurement data of the spectrometer 10. .

  The noise level calculation means 21 outputs the photodiodes PD (1), PD (3), PD (5),... Located next to the photodiodes PD (2), PD (4),. To obtain the optical noise level.

  The power calculation means 22 calculates the total power of the optical signal of each channel from the outputs of the photodiodes PD (1) to PD (N) corresponding to each channel and the optical noise level obtained by the noise level calculation means 21. .

  The SNR calculator 23 calculates the optical SNR from the total power determined by the power calculator 22 and the optical noise level determined by the noise level calculator 21.

The operation of such an apparatus will be described. First, an operation until the spectroscope 10 outputs measurement data to the calculation unit 20 will be described.
The light under measurement 100 is multiplexed with a plurality of optical signals whose frequency intervals (wavelength intervals) are determined by an ITU grid. The light to be measured 100 emitted from the optical fiber 11 becomes parallel light by the collimating lens 12 and enters the diffraction grating 13. Then, the light to be measured 100 is split by the diffraction grating 13 for each wavelength. The measured light 100 dispersed by the diffraction grating 13 is converged on the light receiving surface of the photodiode array of the PDM 15 by the focusing lens 14, but the converged position is shifted corresponding to the wavelength of the measured light 100.

  The photocurrent generated in each of the photodiodes PD (1) to PD (N) is sequentially output from the short wavelength side PD (1). A converter (not shown) of the PDM 15 converts the photocurrent output from the PD (1) to PD (N) into a voltage. Further, since the signal converted into this voltage is an analog signal, the conversion unit converts this analog signal into a digital signal and outputs it to the calculation unit 20 as measurement data. As described above, the measurement data is sampling data sampled by the photodiodes PD (1) to PD (N).

  FIG. 3 is a diagram showing the optical signals s1 and s2 of the light under measurement 100 converged on the light receiving surface of the photodiode array and the spectrum of the light under measurement. Here, the same components as those shown in FIG. In FIG. 3, the measurement data output from the photodiodes PD (1) to PD (N) are P (1) to P (N).

Next, the operation of the calculation unit 20 will be described. As an example, a description will be given from an example in which the center wavelength and optical power of the optical signal s1 of the first channel shown in FIG. 3 are obtained.
First, the calculation unit 20 determines the measurement data P from the spectroscope 10 based on the correspondence between the optical signals of the respective channels stored in the storage means (not shown) and the photodiodes PD (1) to PD (N). It is determined that (2) is for the optical signal s1 of the first channel.

  Then, the noise level calculation means 21 receives light from the outputs P (1) and P (3) of the photodiodes PD (1) and PD (3) adjacent to each other across the photodiode PD (2) that receives the optical signal s1. Find the noise level. For example, the noise level calculation means 21 calculates the average value of both outputs P (1) and P (3) as the optical noise level.

  Then, the power calculation means 22 obtains the total power from the output P (2) of the photodiode PD (2) that receives the optical signal s1, and the obtained total power is obtained with respect to the optical signal s1 obtained by the noise level calculation means 21. The noise level ((P (1) + P (3)) / 2) is subtracted and corrected. As a result, the net optical power (total power) of the optical signal s1 not including the optical noise level is obtained. That is, the output P (2) from the photodiode PD (2) includes not only the total power of the optical signal s1, but also an optical noise level (uniformly present in the vicinity of the optical signal s1).

  Further, the SNR calculation means 23 obtains the optical SNR of the optical signal s1 from the net total power of the optical signal s1 obtained by the power calculation means 22 and the noise level near the optical signal s1 obtained by the noise level calculation means 21.

  Similarly, for the optical signal s2 of the second channel, the noise level calculation means 21 outputs the output P (3) of the photodiodes PD (3) and PD (5) adjacent to each other across the photodiode PD (4) that receives the optical signal s2. ), P (5) to obtain the optical noise level. Then, the power calculation unit 22 subtracts the noise level obtained by the noise level calculation unit 21 from the output P (4) of the photodiode PD (4) that receives the optical signal s2, and does not include the optical noise level. The net optical power (total power) of s2 is obtained. Further, the SNR calculation means 23 obtains the optical SNR of the optical signal s2 from the net total power of the optical signal s2 obtained by the power calculation means 22 and the noise level near the optical signal s2 obtained by the noise level calculation means 21.

  Similarly for the third and subsequent channels, the noise level calculation means 21, power calculation means 22, and SNR calculation means 23 obtain the optical noise level, total power, and optical SNR, respectively.

  In this way, by adjusting the dispersion optical system of the spectrometer, the optical signal s1 to be measured is received by the photodiode PD (2), the optical signal s2 is received by the photodiode PD (4), and so on. Each optical signal dispersed by the dispersion optical system is received by every other photodiode. Since the total power of the optical signal is obtained from the outputs P (2) and P (4) of the photodiodes PD (2), PD (4),... That have received the optical signal, the WDM signal is multiplexed with high density. Even if this is the case, it is not necessary to increase the number of photodiode elements as in the prior art. As a result, the optical signal can be measured with a small number of photodiodes, and the sweep time and calculation time of the photodiodes can be suppressed and high-speed measurement can be performed. Further, since it is not necessary to reduce the pitch and width of the photodiode as in the prior art, the yield in manufacturing is improved and the cost can be suppressed.

  Further, the noise level calculation means 21 has photodiodes PD (1), PD (3), PD (5) located next to the photodiodes PD (2), PD (4),... That receive the optical signals s1, s2. ),...,..., And the power calculation means 22 corrects the obtained total power with the optical noise level obtained by the noise level calculation means 21, so that the optical signal s1, which does not include the optical noise level, The net optical power (total power) of s2 is obtained.

  Further, since the SNR calculation means 23 obtains the optical SNR from the total power from the power calculation means 22 and the optical noise level from the noise level calculation means 21, even if the WDM signal is multiplexed with high density, the photodiode There is no need to greatly increase the number of elements as in the prior art. As a result, the optical signal can be measured with a small number of photodiodes, and the sweep time and calculation time of the photodiodes can be suppressed and high-speed measurement can be performed. Further, since it is not necessary to reduce the pitch and width of the photodiode as in the prior art, the yield in manufacturing is improved and the cost can be suppressed.

  Further, the noise level calculation means 21 outputs, for example, outputs P (1) and P (3) of the photodiodes PD (1) and PD (3) located on both sides of the photodiode PD (2) that receives the optical signal s1. Is averaged ((P (1) + P (3)) / 2) to obtain the optical noise level, so that the optical noise level can be obtained with high accuracy.

The present invention is not limited to this, and may be as shown below.
The noise level calculation means 21 averages the output values P (1) and P (3) of the photodiodes PD (1) and PD (3) on both sides of the photodiode PD (2) irradiated with the optical signal s1. Although the configuration for obtaining the optical noise level is shown, the optical noise level may be obtained from the outputs P (1) and P (3) of either one of the photodiodes PD (1) and PD (3). Thereby, the calculation speed is improved.

  Moreover, although the configuration using the lenses 12 and 14 in the dispersion optical system is shown, a parabolic mirror may be used, and a fiber grating, a prism, or the like may be used for the wavelength dispersion element instead of the diffraction grating.

The beam radius ω of the light spot is the distance from the center of the light spot to the circumference (in the x-axis direction) at a portion where the optical power is 1 / e ² times the peak value of the optical signal. If the spectrum shape of the optical signal is Gaussian, the beam radius ω may be a full width half maximum (FWHM).

  Moreover, although the structure which makes an ITU grid an interval of 100 [GHz] was shown, what kind of interval may be sufficient.

  In addition, the power calculation means 22 has been shown to correct the calculated total power with the optical noise level determined by the noise level calculation means 21, but the correction is not made to subtract the calculated total power with the optical noise level. Also good.

  Moreover, although the structure which calculates | requires both the optical power (total power) and optical SNR of optical signal s1, s2 was shown, you may obtain | require only optical power. In this case, the SNR calculation means 23 may not be provided.

  In addition, the power calculation means 22 corrects the total power obtained by the optical noise level obtained by the noise level calculation means 21 and the SNR calculation means 23 shows the structure for obtaining the optical SNR. However, only the optical power is obtained. The total power need not be corrected with the optical noise level. In this case, the noise level calculation unit 21 and the SNR calculation unit 23 may not be provided.

It is the block diagram which showed one Example of this invention. It is the figure which showed an example of the ITU grid in a part of photodiode of the apparatus shown in FIG. It is the figure which showed the spectrum of the to-be-measured light and the to-be-measured light converged on the light-receiving surface of the photodiode array of the apparatus shown in FIG. It is the figure which showed the spectrum of the to-be-measured light and the to-be-measured light converged on the light-receiving surface of the photodiode array of the conventional WDM signal monitor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Spectrometer 21 Noise level calculating means 22 Power calculating means 23 SNR calculating means PD (1) -PD (5) Photodiode

Claims (3)

A WDM signal monitor for measuring a wavelength-division-multiplexed optical signal whose wavelength is predetermined by the International Telecommunication Union ,
A plurality of photodiodes are arranged in a predetermined direction, a collimating lens converts the optical signal into parallel light, a reflective diffraction grating wavelength-disperses the optical signal of the parallel light in the predetermined direction, and a focusing lens A spectroscope for collecting each wavelength-dispersed optical signal every other one of the photodiodes, and receiving the photodiode;
A noise level calculation means for obtaining an optical noise level from an output of a photodiode located next to the photodiode that receives the optical signal, and a total output of the optical signal by an output of the photodiode of the spectrometer that receives the condensed optical signal. Power calculation means for correcting power, and correcting the calculated total power with the optical noise level obtained by the noise level calculation means;
In a WDM signal monitor comprising SNR calculation means for obtaining an optical SNR from the total power obtained by correction by the power calculation means and the optical noise level obtained by the noise level calculation means,
The width of the photodiode in the predetermined direction is larger than the diameter of the light spot of the optical signal formed on the light receiving surface of the photodiode, and the width of the dead zone between the photodiodes is the light of the optical signal. The focal lengths of the collimating lens and the focusing lens are adjusted so that it is smaller than the diameter of the spot and the pitch of the photodiode is about twice the diameter of the light spot of the optical signal . WDM signal monitor. A WDM signal monitor for measuring a wavelength-division-multiplexed optical signal whose wavelength is predetermined by the International Telecommunication Union ,
A plurality of photodiodes are arranged in a predetermined direction, a collimating lens converts the optical signal into parallel light, a reflective diffraction grating wavelength-disperses the optical signal of the parallel light in the predetermined direction, and a focusing lens A spectroscope for collecting each wavelength-dispersed optical signal every other one of the photodiodes, and receiving the photodiode;
Noise level calculation means for obtaining an optical noise level from an output of a photodiode located next to the photodiode that receives the optical signal;
Power calculating means for obtaining the total power of the optical signal by the output of the photodiode of the spectroscope that receives the optical signal;
In a WDM signal monitor comprising SNR calculation means for obtaining an optical SNR from the total power obtained by the power calculation means and the optical noise level obtained by the noise level calculation means,
The width of the photodiode in the predetermined direction is larger than the diameter of the light spot of the optical signal formed on the light receiving surface of the photodiode, and the width of the dead zone between the photodiodes is the light of the optical signal. The focal lengths of the collimating lens and the focusing lens are adjusted so that it is smaller than the diameter of the spot and the pitch of the photodiode is about twice the diameter of the light spot of the optical signal . WDM signal monitor.   3. The WDM signal monitor according to claim 1, wherein the noise level calculation means obtains an average of outputs of photodiodes located on both sides of the photodiode receiving the optical signal.

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