JP2013210380A - Optical signal monitoring apparatus and method for adjusting sampling frequency of the apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical signal monitoring apparatus capable of avoiding a lack of data of a time base direction of an eye waveform by adjusting a sampling frequency when using a self-oscillation type optical pulse generator, and to provide a method for adjusting the sampling frequency of the apparatus.SOLUTION: The optical signal monitoring apparatus includes: an optical pulse generator 2 for outputting a sampling optical pulse Ps; an electroabsorption type optical modulator 3 acting as an optical sampling gate for sampling an optical signal Px to be measured in accordance with the sampling optical pulse Ps; and an optical receiver 5 for converting an optical signal Py outputted from the electroabsorption type optical modulator 3 into an electric signal Ey to evaluate a waveform of the optical signal Px to be measured by an equivalent sampling method. The optical signal monitoring apparatus further includes optical path length changing means 22 for changing an effective optical path length of an optical resonator in the optical pulse generator 2 to change a cyclic frequency of the sampling optical pulse Ps by changing the effective optical path length of the optical resonator.

Description

  The present invention relates to an optical signal monitoring apparatus and a sampling frequency adjusting method for the apparatus, and more particularly to an optical signal monitoring apparatus for measuring an eye waveform of an optical signal to be measured using software synchronization and a sampling frequency adjusting method for the apparatus.

  In order to perform high resolution equivalent sampling, an optical sampling method has been proposed. As an example, an optical signal monitoring device using a mutual absorption saturation characteristic of an electroabsorption optical modulator has been proposed (see, for example, Patent Document 1).

  FIG. 4 is a schematic configuration diagram of an optical signal monitoring apparatus using the mutual absorption saturation characteristic of a conventional electroabsorption optical modulator. The conventional optical signal monitoring apparatus uses an optical pulse generator 2 that generates a sampling optical pulse Ps with a fixed period, and a measured light using a mutual absorption saturation characteristic between the optical signal Px to be measured and the optical pulse Ps for sampling. An electroabsorption optical modulator 3 that samples the signal Px and outputs an optical signal Py after sampling, a bias voltage generator 4 that applies a DC bias voltage to the electroabsorption optical modulator 3, and electroabsorption optical modulation A light receiving device 5 that receives and photoelectrically converts the sampled optical signal Py from the device 3 and outputs an electric signal Ey, and an AD converter 6 that converts the analog electric signal Ey into a digital signal Dy. By observing the digital signal Dy from the AD converter 6, the eye waveform of the measured optical signal Px is evaluated by an equivalent sampling method.

  Here, in order to obtain the eye waveform of the optical signal Px to be measured by the sequential sampling method, it is necessary to set the cycle of the sampling optical pulse Ps to a value slightly longer than an integer multiple of the cycle of the optical signal Px to be measured. . Usually, since the cycle of the sampling optical pulse Ps is much longer than the bit rate of the optical signal Px to be measured, it is necessary to set the cycle of the sampling optical pulse Ps with high resolution.

  A self-oscillation passive mode-locked fiber laser can generate short pulse light with a simple configuration. Therefore, it is conceivable to use a passive mode-locked fiber laser for the optical pulse generator 2.

International Publication 2008/088709 Japanese Patent Application No. 2008-311423

Mathias Westlund, Henrik Sunnerud, Magnus Karlsson, and Peter A.M. Andrewkson, “Software-Synchronized All-Optical Sampling for Fiber Communication Systems,” IEEE Journal of Lightwave Technology. 23, no. 3, pp. 1088-1099, March 2005

  However, since the passive mode-locked fiber laser is not a system that generates an optical pulse in synchronization with an external electric pulse, the repetition frequency of the sampling optical pulse Ps depends on the bit rate of the measured optical signal Px. Could not set.

  When the software synchronization method described in Non-Patent Document 1 or Patent Document 2 is used, it is not necessary to set the optical pulse repetition frequency exactly as in the sequential sampling method, but the bit rate and sampling of the optical signal Px to be measured. Depending on the frequency relationship, data loss may occur in the time axis direction of the eye waveform obtained from the electrical signal Ey. In this case, the omission can be avoided by slightly adjusting the sampling frequency.

  However, when the self-oscillation type optical pulse generator 2 such as a passive mode-locked fiber laser is used, the repetition frequency of the sampling optical pulse Ps cannot be changed as it is, and thus the omission cannot be avoided.

  In view of the above, an object of the present invention is to adjust the sampling frequency to avoid data loss in the time axis direction of the eye waveform when the self-oscillation type optical pulse generator 2 is used.

  In order to achieve the above object, the optical signal monitoring apparatus of the present invention is characterized by having optical path length varying means for changing the effective optical path length in the resonator of the self-excited oscillation type optical pulse generator.

  Specifically, the optical signal monitoring device of the present invention comprises an optical pulse generator that outputs a sampling optical pulse, an optical sampling gate that samples a measured optical signal according to the sampling optical pulse, and an optical sampling gate. An optical signal monitoring device for measuring an eye waveform of the optical signal under measurement using a software synchronization method with an equivalent sampling method, the optical signal monitoring device comprising: A missing detector for detecting a missing eye waveform in the time axis direction of the optical signal to be measured using an electrical signal from the optical signal, and generating the optical pulse when the missing detector detects a missing eye waveform in the time axis direction Optical path length variable means for changing the effective optical path length of the optical resonator in the optical device, and the optical path length variable means is an eye detected by the missing detection means. Until lack of time axis direction form is eliminated, and wherein the changing the repetition frequency of the sampling optical pulse by changing the effective optical path length of the optical resonator.

  In the optical signal monitoring device of the present invention, the optical sampling gate is preferably an electroabsorption optical modulator that samples the optical signal under measurement using a mutual absorption saturation characteristic.

  In the optical signal monitoring device according to the present invention, it is preferable that the optical path length varying means changes an effective optical path length of the optical resonator by changing a refractive index of an optical propagation medium constituting the optical resonator.

  In the optical signal monitoring device of the present invention, it is preferable that the optical path length varying means changes the effective optical path length of the optical resonator by changing the resonator length of the optical resonator.

  In the optical signal monitoring device of the present invention, the optical pulse generator is preferably a self-oscillation type passive lock fiber laser.

  In order to achieve the above object, a sampling frequency adjusting method of an optical signal monitoring apparatus according to the present invention includes an optical pulse generator that outputs a sampling optical pulse, and an optical sampling that samples a measured optical signal according to the sampling optical pulse. An optical signal monitor comprising: a gate; and a light receiver that converts an optical signal output from the optical sampling gate into an electrical signal, and measuring an eye waveform of the optical signal to be measured using a software synchronization method in an equivalent sampling method A sampling frequency adjustment method for an apparatus, comprising: a missing detection procedure for detecting missing in the time axis direction of an eye waveform of the optical signal under measurement using an electrical signal from the light receiver; and an eye waveform in the missing detection procedure. When missing in the time axis direction is detected, missing in the time axis direction of the eye waveform detected by the missing detection procedure Until resolved, characterized by having a a variable optical path length instructions for changing the repetition frequency of the sampling optical pulse by changing the effective optical path length of the optical resonator in the optical pulse generator in order.

  The present invention is not limited to the optical signal monitoring apparatus using the mutual absorption saturation characteristic of the electroabsorption optical modulator described in Patent Document 1, but is used for an optical signal monitoring apparatus using another type of optical sampling gate. It is also possible to do.

  The above inventions can be combined as much as possible.

  According to the present invention, the frequency of the sampling optical pulse output from the optical pulse generator is changed by changing the effective optical path length in the resonator of the optical pulse generator. The sampling frequency can be adjusted. Accordingly, it is possible to avoid the omission of data in the time axis direction of the eye waveform with a simple configuration using a self-oscillation mode-locked fiber laser.

It is a schematic block diagram of the optical signal monitor apparatus which concerns on this embodiment. It is a pick-up figure of a self-oscillation type mode-locked fiber laser. It is an example of an eye waveform, (a) shows the case where there is no missing data in the time axis direction, and (b) shows the case where there is a missing data in the time axis direction. It is a schematic block diagram of the conventional optical signal monitor apparatus.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  FIG. 1 is a schematic configuration diagram of an optical signal monitoring apparatus according to the present embodiment. The optical signal monitoring apparatus according to the present embodiment includes an optical pulse generator 2, an electroabsorption optical modulator 3 as an optical sampling gate, a bias voltage generator 4, a light receiver 5, an AD converter 6, An optical coupler 7, missing detection means 21, and optical path length variable means 22 are provided.

  The optical pulse generator 2 outputs a sampling optical pulse Ps. The measured optical signal Px and the sampling optical pulse Ps are input to the electroabsorption optical modulator 3. In this state, the bias voltage generator 4 applies a bias voltage to the electroabsorption optical modulator 3. Thereby, the electroabsorption optical modulator 3 samples the measured optical signal Px according to the sampling optical pulse Ps. The electroabsorption optical modulator 3 outputs the sampled optical signal Py.

  The optical pulse generator 2 is a self-excited oscillation type light source that outputs a sampling optical pulse using a resonator. As a result, the short-pulse sampling optical pulse Ps can be generated at low cost. The self-excited oscillation type light source is, for example, an optical fiber type mode-locked fiber laser or a semiconductor type integrated mode-locked semiconductor laser.

  The optical signal Py output from the electroabsorption optical modulator 3 is input to the light receiver 5. The light receiver 5 converts the optical signal Py into an electric signal Ey and outputs it. The AD converter 6 converts the analog electric signal Ey into a digital signal Dy. By collecting the digital signal Dy from the AD converter 6, the eye waveform of the measured optical signal Px can be observed. Thereby, the waveform evaluation of the optical signal Px to be measured can be performed by an equivalent sampling method.

  The missing detection means 21 accumulates the digital signal Dy from the AD converter 6. Then, the missing detection means 21 detects a missing data in the time axis direction of the eye waveform obtained from the digital signal Dy, and determines whether or not there is the missing data.

  The optical path length varying means 22 maintains the effective optical path length of the optical resonator in the optical pulse generator 2 when there is no missing data in the time axis direction of the eye waveform. On the other hand, when there is missing data in the time axis direction of the eye waveform, the effective optical path length of the optical resonator in the optical pulse generator 2 is changed. Thereby, the repetition frequency of the sampling optical pulse Ps is changed.

  Here, the lack of data in the time axis direction of the eye waveform occurs, for example, when the bit rate of the optical signal to be measured Px is an integral multiple of the repetition frequency of the sampling optical pulse Ps. In the present embodiment, the repetition frequency of the sampling optical pulse Ps is changed by changing the effective optical path length of the optical resonator in the optical pulse generator 2. As a result, the bit rate of the optical signal to be measured Px can be prevented from becoming an integral multiple of the repetition frequency of the sampling optical pulse Ps, and the omission of data in the time axis direction of the eye waveform can be avoided.

Next, details of the optical path length varying means 22 will be described.
FIG. 2 is a pickup diagram of the optical pulse generator. An example in which the optical pulse generator 2 is constituted by a self-oscillation passive mode-locked fiber laser is shown. The optical pulse generator 2 includes a pumping light source 31, a WDM coupler 32, an Er-doped optical fiber 33, isolators 34a and 34b, a saturable absorber 35, a single mode optical fiber 36, and a coupler 37. And outputs a sampling optical pulse Ps using a ring-type optical resonator.

  The excitation light output from the excitation light source 31 is input to the Er-doped optical fiber 33, and an optical gain is generated at a wavelength in the 1550 nm band. A WDM (Wavelength Division Multiplexing) coupler 32, an isolator 34a, a saturable absorber 35, a single mode optical fiber 36, a coupler 37, an isolator 34b, and an Er-doped optical fiber 33 constitute an optical resonator. Pulse oscillation occurs at a period corresponding to the effective optical path length. The light output from the coupler 37 to the outside of the optical resonator becomes the sampling optical pulse Ps.

  At this time, the optical path length varying means 22 changes the effective optical path length of the optical resonator in the optical pulse generator 2 by controlling the refractive index varying means 38 or the resonator length varying means 39. Thereby, the repetition frequency of the sampling optical pulse Ps can be made variable. Here, the effective optical path length means an optical path length obtained by adding the resonator length to the refractive index of the light propagation medium constituting the optical resonator.

  The refractive index varying means 38 is a means for varying the refractive index of the light propagation medium constituting the optical resonator. If the refractive index of the light propagation medium of the optical resonator changes, the effective optical path length of the optical resonator in the optical pulse generator 2 changes. Therefore, the optical path length variable means 22 can change the refractive index of the light propagation medium and change the effective optical path length of the optical resonator in the optical pulse generator 2 by controlling the refractive index variable means 38. .

  The refractive index varying means 38 is, for example, a Peltier element that changes the temperature of the single mode optical fiber 36 that is the light propagation medium of the optical resonator. In this case, the optical path length varying unit 22 can change the refractive index of the single mode optical fiber 36 by controlling the Peltier element and changing the temperature of the single mode optical fiber 36.

  The refractive index varying means 38 is, for example, a liquid crystal that changes the refractive index of the light propagation medium of the optical resonator. In this case, the optical path length variable means 22 can change the refractive index of the light propagation medium of the optical resonator by changing the voltage applied to the refractive index variable means 38.

  The resonator length varying means 39 is means for varying the resonator length of the optical resonator, that is, the length of the optical propagation medium of the optical resonator. If the resonator length changes, the effective optical path length of the optical resonator in the optical pulse generator 2 changes. Therefore, the optical path length varying unit 22 can change the resonator length by controlling the resonator length varying unit 39 to change the effective optical path length of the optical resonator in the optical pulse generator 2.

  The resonator length varying means 39 is, for example, a piezoelectric element that changes the tension applied to the single mode optical fiber 36 that is the light propagation medium of the optical resonator. In this case, the optical path length varying means 22 can change the length of the single mode optical fiber 36 by controlling the piezoelectric element and changing the tension applied to the single mode optical fiber 36.

  The resonator length varying means 39 is, for example, an optical circuit in which the length of the light propagation medium is variable. The optical circuit is, for example, a planar optical circuit in which an optical waveguide is formed on a substrate. Optical fiber transmission or optical space transmission may be used for the waveguide of the optical circuit. In this case, the optical path length varying means 22 can change the resonator length by changing the optical path length of the resonator length varying means 39. The change of the length of the light propagation medium is performed by, for example, an optical switch that provides a plurality of paths having different lengths of the light propagation medium and switches the paths. The length of the light propagation medium may be changed by changing the distance between the optical fiber and the mirror.

Next, details of the loss detection means 21 will be described.
FIG. 3 is an example of an eye waveform, where (a) shows a case where there is no missing data in the time axis direction, and (b) shows a case where there is a missing data in the time axis direction. Depending on the relationship between the bit rate of the optical signal to be measured Px and the repetition frequency of the sampling optical pulse Ps, as shown in FIG. 3B, omission of data in the time axis direction of the eye waveform may occur. Whether missing data can be detected from the time axis values x _i of the eye waveform. For example, if x _i is rearranged in order of increasing value and the maximum value of the interval | x _{i + 1} −x _i | is larger than a certain value, it can be determined that there is a missing part. Further, if the time axis is divided at regular intervals to create a histogram of x _i and there is an interval where the number of samples is 0, it can be determined that there is a missing portion.

  Next, a sampling frequency adjustment method of the optical signal monitoring apparatus according to the present embodiment will be described. The sampling frequency adjustment method of the optical signal monitoring apparatus according to this embodiment includes a missing detection procedure and an optical path length variable procedure in order.

  In the missing detection procedure, a missing data on the time axis of a part of the eye waveform is detected. For example, the missing detection unit 21 collects the digital signal Dy from the AD converter 6 and detects the missing data in the time axis direction of the eye waveform obtained from the digital signal Dy. Then, it is determined whether or not there is missing data in the time axis direction of the eye waveform. In addition, the detection and determination of the loss in the loss detection procedure may be performed visually by a person.

  When it is determined in the missing detection procedure that there is a missing data in the time axis direction of the eye waveform, the optical path length variable procedure is executed. In the optical path length variable procedure, the effective optical path length of the optical resonator in the optical pulse generator 2 is changed so that data in the time axis direction of the eye waveform of the optical signal to be measured Px obtained from the electrical signal Ey is not lost. . The change of the effective optical path length of the optical resonator in the optical pulse generator 2 is performed by, for example, the optical path length varying means 22. The optical path length variable procedure may be performed by a person controlling the refractive index variable means 38 or the resonator length variable means 39 described in FIG.

  Then, it is preferable to shift to a missing detection procedure. If there is missing data in the time axis direction of the eye waveform even after the effective optical path length of the optical resonator in the optical pulse generator 2 is changed, the effective optical path length of the optical resonator in the optical pulse generator 2 is further reduced. It is preferable to repeat the change. Since the probability of occurrence of missing data in the time axis direction of the eye waveform is small, if the effective optical path length of the optical resonator in the optical pulse generator 2 is changed several times, missing data in the time axis direction of the eye waveform is lost. Can be avoided practically.

  In the present embodiment, an example in which the electroabsorption optical modulator 3 that samples the optical signal to be measured Px using the mutual absorption saturation characteristic is described as an example of the optical sampling gate. However, the present invention is not limited to this. . For example, an optical modulator using nonlinear optical characteristics such as a nonlinear optical crystal or an optical modulator using supersaturated absorption characteristics such as a carbon nanotube can be used. Even if another type of optical sampling gate is used instead of the electroabsorption optical modulator 3, the same effect as the optical signal monitoring device according to the present embodiment and the sampling frequency adjusting method of the optical signal monitoring device can be obtained. it can.

  Since the present invention can perform high-resolution optical sampling, it can be applied to the information communication industry and various industries using light.

2: Optical pulse generator 3: Electroabsorption optical modulator 4: Bias voltage generator 5: Light receiver 6: AD converter 7: Optical coupler 21: Missing detection means 22: Optical path length variable means 31: Excitation light source 32: WDM coupler 33: Er-doped optical fibers 34a, 34b: isolator 35: saturable absorber 36: single mode optical fiber 37: coupler 38: refractive index varying means 39: resonator length varying means

Claims (6)

An optical pulse generator that outputs an optical pulse for sampling;
An optical sampling gate for sampling the optical signal under measurement according to the sampling optical pulse;
A light receiver that converts an optical signal output from the optical sampling gate into an electrical signal, and
An optical signal monitoring device that measures an eye waveform of the optical signal under measurement using a software synchronization method in an equivalent sampling method,
A missing detection means for detecting missing in the time axis direction of the eye waveform of the optical signal under measurement using an electrical signal from the light receiver;
An optical path length varying means for changing an effective optical path length of an optical resonator in the optical pulse generator when the missing detection means detects a missing time waveform direction of an eye waveform;
The optical path length varying means changes a repetition frequency of the sampling optical pulse by changing an effective optical path length of the optical resonator until the omission in the time axis direction of the eye waveform detected by the missing detection means is resolved. An optical signal monitoring device characterized by being changed. The optical signal monitoring apparatus according to claim 1, wherein the optical sampling gate is an electroabsorption optical modulator that samples the optical signal to be measured using a mutual absorption saturation characteristic. The optical path length varying means is
The optical signal monitoring apparatus according to claim 1, wherein an effective optical path length of the optical resonator is changed by changing a refractive index of an optical propagation medium constituting the optical resonator. The optical path length varying means is
4. The optical signal monitoring device according to claim 1, wherein an effective optical path length of the optical resonator is changed by changing a resonator length of the optical resonator. 5. The optical pulse generator is
5. The optical signal monitoring device according to claim 1, wherein the optical signal monitoring device is a self-oscillation type passive mode-locked fiber laser. An optical pulse generator that outputs an optical pulse for sampling;
An optical sampling gate for sampling the optical signal under measurement according to the sampling optical pulse;
A light receiver that converts an optical signal output from the optical sampling gate into an electrical signal, and
A sampling frequency adjustment method for an optical signal monitoring device that measures an eye waveform of the optical signal under measurement using a software synchronization method in an equivalent sampling method,
Missing detection procedure for detecting missing in the time axis direction of the eye waveform of the optical signal under measurement using an electrical signal from the light receiver;
If a missing eye waveform in the time axis direction is detected in the missing detection procedure, the optical resonator in the optical pulse generator until the missing eye waveform in the time axis direction detected in the missing detection procedure is resolved. An optical path length variable procedure for changing the repetition frequency of the sampling optical pulse by changing the effective optical path length of
A sampling frequency adjustment method for an optical signal monitoring device, comprising:

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