JP2007218628A - Polarization state measuring device and optical sampling device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization state measuring device of high sensitivity that can measure the polarization state of a light signal varying at high speed and can also measure a light signal of high speed and low power. <P>SOLUTION: Polarization splitting elements 11 and 12 split signal light and sampling light into P polarized waves and S polarized waves having polarization planes different by 90°, respectively. The split P polarized waves and S polarized waves output first and second currents corresponding to the P polarized waves and third and fourth currents corresponding to the S polarized waves with balanced type photodetectors 41-44 via optical 90° hybrids 31 and 32. Arithmetic processors 71 and 72 calculate the first to fourth currents and determine the P polarization component and S polarization component of the signal light, phase difference between P polarized waves, and phase difference between S polarized waves. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a polarization state measuring device that can measure the polarization state of an optical signal that fluctuates at high speed in an ultra-high-speed optical communication system, and a polarization-independent type that does not depend on the polarization state of an optical signal. The present invention relates to an optical sampling device.

  Measuring the polarization state of light is very important in the fields of optical communication, optical measurement, and optical properties. In particular, the signal speed is expected to increase in the future optical communication field, and the polarization dispersion of optical transmission lines and optical components (a phenomenon in which the group speed varies depending on the polarization state) degrades the signal quality of the optical signal. Can no longer be ignored. In evaluating and compensating for this polarization dispersion, measurement of the polarization state is indispensable.

  Conventional polarization state measuring devices using a polarizer and a wave plate are known and have already been commercialized. In this method, the light to be measured is branched into four and then received through a spatial optical system composed of a polarizer and a wave plate, and the electric field strength of the x-polarized component and the electric field strength of the y-polarized component of the measured light. The phase difference between x-polarized light and y-polarized light was obtained (for example, see Non-Patent Document 1).

  Further, as a conventional optical sampling optical waveform measuring device, for example, a technique for observing a repetitive optical signal at low power and at high speed by observing a linear correlation between the signal light and the sampling light is known (for example, Patent Documents 1 and 2).

JP-A-9-162808 JP 2004-132719 A Agilent Technologies, Inc., “Optical Meter Catalog 2000-2001”, p81-85

  With conventional polarization state measuring instruments, it is impossible to measure the polarization state of an optical signal that fluctuates at high speed in an optical line or optical device in an ultra-high-speed optical communication system. It was difficult to measure the polarization state of a weak signal because it received light.

  Moreover, in the conventional optical sampling optical waveform measuring apparatus, one of the polarization plane of the signal light and the polarization plane of the sampling light input to the optical 90-degree hybrid is linear polarization, and the other is circular polarization. When the directions of the polarization planes of the two lights are completely perpendicular, the output intensity from the balanced light receiver is maximized. Therefore, when it is desired to observe the waveform with high sensitivity and stability, it is necessary to adjust the polarization state of the signal light and the sampling light by the polarization direction controller, respectively, and keep the state stable.

  The present invention has been made in view of the above circumstances, and its object is to measure the polarization state of an optical signal that fluctuates at a high speed, and to measure an optical signal with high speed and low power. Another object of the present invention is to provide a polarization state measuring device having high sensitivity and a polarization independent optical sampling device that does not depend on the polarization state of an optical signal.

  To achieve the above object, the polarization state measuring device of the present invention includes a first polarization separation element that separates repeated signal light into P polarization and S polarization whose polarization planes are different by 90 degrees, and a sampling light pulse. From the second polarization separation element that separates the P-polarization and S-polarization whose polarization planes are different by 90 degrees, and from the P-polarization and the second polarization separation element emitted from the first polarization separation element The emitted P-polarized light is incident through the first 90-degree hybrid and outputs the first current and the second current, and the first balanced light receiver and the second balanced light receiver, The S-polarized light emitted from the first polarization separation element and the S-polarization emitted from the second polarization separation element are incident via the second optical 90-degree hybrid, and the third current and the second A third balanced photoreceiver that outputs a current of 4 and a fourth balanced photoreceiver; An arithmetic process is performed on the current value of the first current output from the first balanced light receiver and the current value of the second current output from the second balanced light receiver to obtain P of the signal light. A first arithmetic processing unit that obtains a polarization component and divides the current value of the second current by the current value of the first current to obtain a phase difference between the P polarizations of the signal light and the sampling light; The signal light is obtained by performing arithmetic processing on the current value of the third current output from the third balanced light receiver and the current value of the fourth current output from the fourth balanced light receiver. Second calculation processing for obtaining the S-polarized component of the signal light and dividing the current value of the fourth current by the current value of the third current to obtain the phase difference between the S-polarized light of the signal light and the sampling light And a device.

  In addition, the optical sampling device of the present invention includes a first polarization separation element that separates repetitive signal light into P-polarized light and S-polarized light having different polarization planes by 90 degrees, and P A second polarization separation element that separates into polarization and S polarization; a P polarization emitted from the first polarization separation element; and a P polarization emitted from the second polarization separation element. A first balanced light receiver and a second balanced light receiver that are incident via a first light 90-degree hybrid and output a first current and a second current; and the first polarization separation element. S-polarized light emitted from the second polarized light separating element and the S-polarized light emitted from the second polarization separation element are incident through the second optical 90-degree hybrid and output a third current and a fourth current. 3 balance type light receiver and fourth balance type light receiver, and the first balun. An arithmetic process is performed on the current value of the first current output from the type photoreceiver and the current value of the second current output from the second balanced photoreceiver to obtain the P polarization component of the signal light. And calculating the current value of the third current output from the third balanced light receiver and the current value of the fourth current output from the fourth balanced light receiver. And an arithmetic processing unit for obtaining an S polarization component of the signal light and obtaining an amplitude component of the signal light from the P polarization component and the S polarization component of the signal light.

  The polarization state measuring instrument of the present invention is capable of measuring the polarization state of an optical signal that fluctuates at high speed, and separates signal light and sampling light into S polarization and P polarization by a polarization separation element, In order to detect an interference effect that is a linear correlation between the separated signal light and the S-polarization component and the P-polarization component of the sampling light pulse, it is possible to obtain a time resolution depending on the pulse width of the sampling light at high speed. The polarization state of the fluctuating signal light can be measured. Further, the intensity required for the measurement of the signal light may be relatively small, and the high-speed and low-power signal light that can measure the polarization state of the signal light that fluctuates at high speed can be measured. A polarization state measuring instrument with high sensitivity can be realized. At the same time, a polarization-independent optical sampling device that does not depend on the polarization state of the signal light can be realized.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a configuration explanatory view showing a polarization state measuring device according to an embodiment of the present invention. In FIG. 1, reference numerals 11 and 12 denote polarization separation elements such as polarization beam splitters, and signal light that fluctuates at high speed in an ultrahigh-speed optical communication system is repeatedly incident on the first polarization separation element 11. . The polarization separation element 11 passes the P-polarized light, which is a polarization component in the 90-degree direction with respect to the reference direction (0-degree direction), in the incident signal light as it is, and the polarization component in the reference direction in the incident signal light. S-polarized light is reflected. The polarization planes of the P polarization and S polarization of the signal light separated by the polarization separation element 11 are 90 degrees different from each other.

  Further, the second polarization separating element 12 has a period slightly different from the repetition period of the signal light, and the optical pulse width is shorter than the reciprocal of the frequency fluctuation of the signal light, and has a 45 degree direction with respect to the reference direction. A sampling light pulse having a single polarization plane facing is incident. The polarization separation element 12 passes the P-polarized light component, which is a polarization component in the 90-degree direction with respect to the reference direction, in the incident sampling light pulse, and S, which is a polarization component in the reference direction, in the incident sampling light pulse. Reflect polarization. The polarization planes of the sampling light pulses separated by the polarization separation element 12 are 90 degrees different from each other.

  The P polarization of the signal light separated and emitted from the polarization separation element 11 is incident on the first optical 90-degree hybrid 31 and the P polarization of the sampling light pulse separated and emitted from the polarization separation element 12. Is incident on the first light 90-degree hybrid 31. The first optical 90-degree hybrid 31 includes optical couplers 21 to 24.

  The S polarization of the signal light separated and emitted from the polarization separation element 11 is incident on the second optical 90-degree hybrid 32 and the S light of the sampling light pulse separated and emitted from the polarization separation element 12. The polarized light is incident on the second light 90-degree hybrid 32. The second optical 90-degree hybrid 32 includes optical couplers 25 to 28.

  The light emitted from the first 90-degree hybrid 31 is incident on a first balanced light receiver 41 composed of photodiodes D1 and D2 and a second balanced light receiver 42 composed of photodiodes D3 and D4, respectively. A first current is output from the first balanced light receiver 41 and a second current is output from the second balanced light receiver 42.

  The light emitted from the second 90-degree hybrid 32 is incident on a third balanced light receiver 43 composed of photodiodes D5 and D6 and a fourth balanced light receiver 44 composed of photodiodes D7 and D8, respectively. Then, a third current is output from the third balanced light receiver 43 and a fourth current is output from the fourth balanced light receiver 44.

  The first current output from the first balanced light receiver 41 is waveform-equalized by the first low-pass filter 51 and then digitized by the first digitizing means 61 before the first current is outputted. To the arithmetic processing unit 71. The second current output from the second balanced light receiver 42 is waveform-equalized by the second low-pass filter 52 and then digitized by the second digitizing means 62. Input to the first arithmetic processing unit 71.

  The third current output from the third balanced light receiver 43 is waveform-equalized by the third low-pass filter 53 and then digitized by the third digitizing means 63. Input to the second arithmetic processing unit 72. The fourth current output from the fourth balanced light receiver 44 is waveform-equalized by the fourth low-pass filter 54 and then digitized by the fourth digitizing means 64. Input to the second arithmetic processing unit 72.

  In the first arithmetic processing unit 71, the current value of the first current digitized by the first digitizing unit 61 and the current value of the second current digitized by the second digitizing unit 62 are used. Then, the P-polarized component of the amplitude of the signal light is obtained by performing arithmetic processing, and the second arithmetic processing unit 72 calculates the current value of the third current and the third current numerically expressed by the third numerical means 63. The S-polarized component of the amplitude of the signal light is obtained by performing arithmetic processing on the current value of the fourth current that has been digitized by the four digitizing means 64.

  Further, the first arithmetic processing unit 71 converts the current value of the second current that has been digitized by the second digitizing means 62 into the current value of the first current that has been digitized by the first digitizing means 61. To obtain the relative phase difference between the P polarizations of the signal light and the sampling light, and the second arithmetic processing unit 72 uses the fourth numerical unit 64 to express the fourth phase. By dividing the current value of the current by the current value of the third current digitized by the third digitizing means 63, the relative phase difference between the S polarizations of the signal light and the sampling light is obtained.

  That is, the polarization state of the signal light at an arbitrary time is the P polarization component and the S polarization component of the amplitude of the signal light, the relative positions of the P polarization and the S polarization of the signal light and the sampling light, respectively. Determined by phase difference. Therefore, it is possible to realize a polarization state measuring device that can measure the polarization state of an optical signal that fluctuates at high speed in an ultra-high speed optical communication system.

  The polarization state measuring device in FIG. 1 depends on the pulse width of the sampling light in order to detect an interference effect that is a linear correlation between the separated S-polarization component and P-polarization component of the signal light and the sampling light pulse. The time resolution can be obtained, and the polarization state of the signal light changing at high speed can be measured. Further, the intensity required for the measurement of the signal light may be relatively small, and the polarization state can be measured with high sensitivity and excellent time resolution.

  FIG. 2 is a configuration explanatory view showing a polarization-independent optical sampling device according to an embodiment of the present invention. In FIG. 2, the same parts as those in FIG. In FIG. 2, reference numeral 73 denotes an arithmetic processing unit that obtains the P-polarization component and the S-polarization component of the amplitude of the signal light, and uses the P-polarization component and the S-polarization component of the amplitude of the signal light. By obtaining the above, the amplitude component of the signal light can be obtained. That is, since the intensity modulation information of the signal light can be obtained from the P-polarization component and the S-polarization component of the amplitude of the signal light, each polarization state of the signal light and the sampling light is adjusted by the polarization direction controller. Therefore, optical sampling can be performed in a polarization-independent type that does not depend on the polarization state of the signal light. Therefore, it is possible to realize an optical sampling apparatus that observes a low-power and high-speed repeated optical signal.

  Next, specific operations of the polarization state measuring device and the polarization-independent optical sampling device of the present embodiment configured as described above will be described. The sampling light pulse, which is a local light emission pulse, needs to satisfy the following conditions. These conditions and the action of the optical 90-degree hybrid are the same as those of the conventional technology (for example, see Patent Documents 1 and 2).

[Condition 1] The intensity of the signal light hardly changes within the time of the pulse width of the sampling light pulse.
[Condition 2] The frequency of the signal light hardly changes within the time of the pulse width of the sampling light pulse.
[Condition 3] The center frequency of the sampling light matches the center frequency of the signal light.

  From the above conditions 1 and 2, it is necessary to prepare a sampling light pulse shorter than the required time resolution in order to measure the polarization state of the signal light changing at high speed. When the above conditions are satisfied, it will be described below that the polarization state and amplitude of the signal are correctly detected by the polarization state measuring device and the optical sampling device of the present embodiment.

First, the signal light separated by the first polarization separation element 11 and the second polarization separation element 12 and the P polarization of a sampling light pulse having a single polarization plane facing a 45 degree direction with respect to the reference direction The waveform of the component and the S polarization component is expressed by the following equation.

Here, E ^(s) _p (t) and E ^(s) _s (t) are the P polarization component and the S polarization component of the amplitude of the signal light, and φ ^(s) _p and φ ^(s) _s are It is a relative phase difference between the P-polarized light and the S-polarized light of the signal light and the sampling light. ω is the center frequency of both. The polarization state of the signal light at an arbitrary time is uniquely determined by these four parameters {E ^(s) _p (t), E ^(s) _s (t), φ ^(s) _p , φ ^(s) _s }. It is determined.

At this time, the currents P ₁ (t) and P ₂ (t) flowing in the first balanced photoreceiver 41 and the second balanced photoreceiver 42 are expressed as follows.

The sampling light pulse has a value only in the vicinity of time τ, and is zero otherwise.

The current values P ₁ (t) and P ₂ (t) calculated as described above are the outputs from the first balanced light receiver 41 and the second balanced light receiver 42, and correspond to the first values. The low-pass filter 51 and the second low-pass filter 52 are low-pass filtered, the first numerical unit 61 and the second numerical unit 62 are digitized, and supplied to the first arithmetic processing unit 71. Is done.

となり、また、信号光のＰ偏波成分の位相φ^（ｓ） _ｐは

となる。上記演算処理は第１の演算処理装置７１で行われるものとする。 Thereby, the P polarization component E ^(s) _p (t) of the signal light is

And the phase φ ^(s) _p of the P polarization component of the signal light is

It becomes. It is assumed that the arithmetic processing is performed by the first arithmetic processing device 71.

Similarly, current values P ₃ (t) and P ₄ (t) flowing in the third balance type light receiver 43 and the fourth balance type light receiver 44 are expressed as follows.

The current values P ₃ (t) and P ₄ (t) calculated as described above are outputs from the third balanced light receiver 43 and the fourth balanced light receiver 44, and correspond to the third values. The low-pass filter 53 and the fourth low-pass filter 54 are low-pass filtered, and the third numerical unit 63 and the fourth numerical unit 64 are digitized and supplied to the second arithmetic processing unit 72. Is done.

となり、またＳ偏波の位相φ^（ｓ） _ｓは

となる。上記演算処理は第２の演算処理装置７２で行われるものとする。 As a result, the s polarization component E ^(s) _s (t) of the signal light is

And the S-polarization phase φ ^(s) _s is

It becomes. It is assumed that the arithmetic processing is performed by the second arithmetic processing device 72.

となり、これより信号光の振幅成分を求めることができる。上記演算処理は演算処理装置７３で行われるものとする。 The electric field E ^(s) (t) of the signal light is

Thus, the amplitude component of the signal light can be obtained. It is assumed that the arithmetic processing is performed by the arithmetic processing device 73.

The path length from the first polarization separation element 11 to the first optical 90-degree hybrid 31 is L ₁ , and the path length from the second polarization separation element 12 to the first optical 90-degree hybrid 31 is L _2. The path length from the first polarization separation element 11 to the second optical 90-degree hybrid 32 is L ₃ , and the path length from the second polarization separation element 12 to the second optical 90-degree hybrid 32 is L _If it is ₄ , the path differences | L ₁ −L ₂ | and | L ₃ −L ₄ | must satisfy the following conditions.

| L ₁ −L ₂ | = | L ₃ −L ₄ |
If the above condition is not satisfied, only the sampling light whose polarization state is known is incident on the experimental system, the initial phase difference depending on the path length difference is obtained, and the value is corrected as the reference value.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiment examples may be appropriately combined.

It is composition explanatory drawing which shows the polarization state measuring device which concerns on one Embodiment of this invention. 1 is a configuration explanatory diagram illustrating a polarization-independent optical sampling device according to an embodiment of the present invention. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... 1st polarization beam splitter, 12 ... 2nd polarization beam splitter, 21-28 ... Optical coupler, 31 ... 1st light 90 degree hybrid, 32 ... 2nd light 90 degree hybrid, 41 ... 1st 1. Balanced light receiver 42... 2nd balanced light receiver 43. 3rd balanced light receiver 44. 4th balanced light receiver 51. 1st low-pass filter 52. 2 low-pass filter, 53 ... third low-pass filter, 54 ... fourth low-pass filter, 61 ... first numerical means, 62 ... second numerical means, 63 ... third Numerical means 64, fourth numerical means 71, first arithmetic processing device 72, second arithmetic processing device 73, arithmetic processing device.

Claims (2)

A first polarization separation element that separates repetitive signal light into P polarization and S polarization whose polarization planes are different by 90 degrees;
A second polarization separation element that separates the sampling light pulse into P polarization and S polarization whose polarization planes are different by 90 degrees;
The P polarization emitted from the first polarization separation element and the P polarization emitted from the second polarization separation element are incident through the first optical 90-degree hybrid, and the first current and A first balanced light receiver and a second balanced light receiver that output a second current;
S-polarized light emitted from the first polarization separation element and S-polarized light emitted from the second polarization separation element are incident via the second optical 90-degree hybrid, and a third current and A third balanced light receiver and a fourth balanced light receiver for outputting a fourth current;
An arithmetic process is performed on the current value of the first current output from the first balanced photoreceiver and the current value of the second current output from the second balanced photoreceiver to obtain signal light. A first arithmetic processing unit that obtains a P-polarized component and obtains a phase difference between the P-polarized light of the signal light and the sampling light by dividing the current value of the second current by the current value of the first current When,
An arithmetic process is performed on the current value of the third current output from the third balanced photoreceiver and the current value of the fourth current output from the fourth balanced photoreceiver to obtain signal light. A second arithmetic processing unit that obtains an S-polarized component and obtains a phase difference between the S-polarized light of the signal light and the sampling light by dividing the current value of the fourth current by the current value of the third current A polarization state measuring device. A first polarization separation element that separates repetitive signal light into P polarization and S polarization whose polarization planes are different by 90 degrees;
A second polarization separation element that separates the sampling light pulse into P polarization and S polarization whose polarization planes are different by 90 degrees;
The P polarization emitted from the first polarization separation element and the P polarization emitted from the second polarization separation element are incident through the first optical 90-degree hybrid, and the first current and A first balanced light receiver and a second balanced light receiver that output a second current;
S-polarized light emitted from the first polarization separation element and S-polarized light emitted from the second polarization separation element are incident via the second optical 90-degree hybrid, and a third current and A third balanced light receiver and a fourth balanced light receiver for outputting a fourth current;
An arithmetic process is performed on the current value of the first current output from the first balanced photoreceiver and the current value of the second current output from the second balanced photoreceiver to obtain signal light. While obtaining the P polarization component, the current value of the third current output from the third balanced light receiver and the current value of the fourth current output from the fourth balanced light receiver are calculated. An optical sampling device comprising: an arithmetic processing unit that performs an arithmetic process to obtain an S-polarized component of signal light and obtains an amplitude component of the signal light from the P-polarized component and the S-polarized component of the signal light apparatus.

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