CN113916269A - Coherent polarization analyzer and measuring method - Google Patents
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Abstract
The invention relates to the technical field of optical fiber measurement, in particular to a coherent polarization analyzer and a measurement method, and the coherent polarization analyzer comprises a first laser, a first optical fiber coupler, a reference arm optical unit, a second optical fiber coupler, a first polarization analyzer, a second polarization analyzer and a data acquisition and processing unit; the laser emitted by the first laser firstly passes through the first optical fiber coupler to be branched into one path of light to be detected. The other path of light of the first optical fiber coupler is coherent with the light returned by the measured unit in the second optical fiber coupler through a standard optical unit, then the polarization states of two output ends of the second optical fiber coupler are respectively measured by the first polarization analyzer and the second polarization analyzer, finally two groups of Stokes parameters are connected to the data acquisition and processing unit in parallel, and the relative phase difference and the polarization state of the measured light, and the transmission matrix and the integral phase shift of the measured optical unit are calculated at one time.
Description
Technical Field
The invention relates to the technical field of measuring equipment, in particular to a coherent polarization analyzer and a measuring method.
Background
In recent years, with the development of coherent optical fiber communication, polarization multiplexing has been emphasized, and therefore a receiving apparatus capable of detecting both a phase and a polarization state, that is, a coherent stokes receiver, has become a new demand. On the other hand, among the optical fiber sensing technologies, the distributed Acoustic wave sensor das (distributed Acoustic sensor) and the phase-based optical time domain reflectometry technology have been widely used in the fields of earthquake, intrusion prevention fence, pipeline leakage monitoring, etc., and they are a phase-based optical fiber sensing technology. A sensor capable of detecting both phase change and polarization state change has become a new need.
Especially when the optical fiber is used as a sensing element, when the optical fiber is subjected to the external environment (stress, vibration, temperature), the optical fiber changes from an isotropic medium to an anisotropic medium, and the dielectric constant (or refractive index) of the optical fiber changes from a single constant to a second-order tensor. This second order tensor can fully reflect the direction of the external action (stress or vibration) and the relative magnitude of each component. Previous DAS systems only detect the longitudinal component of stress or external effects along the fibre, and cannot detect the lateral effect of the external forces on the fibre, and therefore the direction of the external forces. The external lateral action on the fiber causes a change in polarization state. Therefore, the magnitude and direction of the action can be detected comprehensively only by detecting the change of the polarization state and the overall phase simultaneously.
The existing measurement method is implemented by using two independent interferometers and a polarization analyzer respectively, so that a unit to be measured needs to be switched between the two instruments, the polarization and the phase are sensitive to optical fiber connection, and the optical fiber switching causes the polarization state and the phase to change, thereby causing inaccurate measurement.
In summary, it can be seen that a comprehensive coherent polarization analyzer capable of detecting both phase and polarization state without switching optical fibers is a current urgent need.
Disclosure of Invention
The invention aims to provide a coherent polarization analyzer and a measurement method, and aims to solve the problem that the existing analyzer cannot detect phase change and polarization state change simultaneously.
In order to achieve the above object, in a first aspect, the present invention provides a coherent polarization analyzer, including a first laser, a first fiber coupler, a reference arm optical unit, a second fiber coupler, a first polarization analyzer, a second polarization analyzer, and a data collecting and processing unit, where the first fiber coupler has a first port, a second port, a third port, and a fourth port, the first laser is connected to the first port and is connected to an input port of a unit under test through the third port, the reference arm optical unit is connected to the fourth port, the second fiber coupler has a fifth port, a sixth port, a seventh port, and an eighth port, an output end of the optical unit under test is connected to the fifth port of the second fiber coupler, and the reference arm optical unit is connected to the sixth port, the first polarization analyzer is connected with the seventh port, the second polarization analyzer is connected with the eighth port, and the data acquisition and processing unit is connected with the first polarization analyzer and the second polarization analyzer.
The coherent polarization analyzer further comprises a circulator, wherein the circulator is provided with a ninth first port, a ninth second port and a ninth third port, the ninth first port of the circulator is connected with the third port of the first optical fiber coupler, the ninth second port of the circulator is connected with the optical unit to be tested, and the ninth third port of the circulator is connected with the eighth port of the second optical fiber coupler.
The reference arm optical unit comprises a first polarization controller and a first optical fiber delay line, wherein the input end of the first polarization controller is connected with the fourth port of the first optical fiber coupler, the output end of the first polarization controller is connected with the input end of the first optical fiber delay line, and the output end of the first optical fiber delay line is connected with the sixth port of the second optical fiber coupler.
The reference arm optical unit is an active unit and comprises a second laser with the same wavelength as the first laser, a first polarization controller and a first optical fiber delay line, wherein laser emitted by the second laser is injected into the first polarization controller and then reaches a fifth port of the second optical fiber coupler through the first optical fiber delay line.
The first polarization analyzer comprises a second polarization controller, a 2x2 optical fiber coupler, a 1x3 optical fiber coupler, a third polarization controller, a fourth polarization controller, a fifth polarization controller, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a photoelectric conversion and amplification unit and a digital acquisition card;
the second polarization controller is connected with a seventh port of the second optical fiber coupler, an output end of the second polarization controller is connected with an input end of the 2x2 optical fiber coupler, the 2x2 optical fiber coupler is provided with a fifth-second output port and a fifth-second output port, the fifth-second output port is connected with one path of the photoelectric conversion and amplification unit and sends data to the data acquisition card, the fifth-second output port is connected with an input end of the 1x3 optical fiber coupler, the 1x3 optical fiber coupler is provided with a fifth-third output port, a fifth-third output port and a fifth-third output port, the fifth-third output port is connected with an input end of the third polarization controller, the fifth-third output port is connected with an input end of the fourth polarization controller, and the fifth-third output port is connected with an input end of the fifth polarization controller, the output ends of the third polarization controller, the fourth polarization controller and the fifth polarization controller are respectively connected with the input ends of the first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter, the output ends of the first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter are connected to the corresponding ends connected with the photoelectric conversion and amplification unit, and the output electric signals of the photoelectric conversion and amplification unit are collected and processed by the data acquisition card.
The first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter adopt Polarization Beam Splitters (PBS) of the same type, and the data acquisition card comprises an interface USB and a coaxial cable interface SMA with a computer.
In a second aspect, the present invention provides a method for measuring a coherent polarization analyzer, comprising:
disconnecting a connecting light path of the optical unit to be measured, measuring a transmission matrix of the reference arm optical unit, and adjusting the first polarization analyzer and the second polarization analyzer to enable the measurement results of the first polarization analyzer and the second polarization analyzer to be the same to obtain a first measurement result;
the first polarization analyzer and the second polarization analyzer respectively measure the polarization states of the seventh port and the eighth port of the second optical fiber coupler to obtain a second measurement result;
respectively calculating the polarization states of a fifth port and a sixth port of the second optical fiber coupler by using the second measurement result to obtain a first calculation result;
calculating power amplification factors of the fifth port reference arm optical unit and the sixth port reference arm optical unit of the second optical fiber coupler relative to the optical unit to be measured by using the first measurement result and the first calculation result to obtain a second calculation result;
and calculating the transmission matrix of the optical unit to be measured and the integral phase difference relative to the optical unit of the reference arm by using the first calculation result and the second calculation result.
According to the coherent polarization analyzer, coherence is carried out through the second optical fiber coupler, then the polarization state is measured through the first polarization analyzer and the second polarization analyzer, and finally output quantities (total 8 Stokes parameters) are connected to the data acquisition and processing unit in parallel, so that the relative phase difference and the polarization state of the measured light are measured at one time, and the problem that the existing analyzer cannot detect phase change and polarization state change at the same time is solved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic structural diagram of a dual port of a coherent polarization analyzer according to the present invention.
Fig. 2 is a schematic structural diagram of a single port of a coherent polarization analyzer according to the present invention.
Fig. 3 is a schematic configuration diagram of the first polarization analyzer.
Fig. 4 is a schematic structural view of the reference arm optical unit.
Fig. 5 is a schematic diagram of the structure of an active reference arm optical unit.
Fig. 6 is a flow chart of a measurement method of a coherent polarization analyzer according to the present invention.
In the figure: 1-a first laser, 2-a first optical fiber coupler, 3-a reference arm optical unit, 4-a second optical fiber coupler, 5-a first polarization analyzer, 6-a second polarization analyzer, 7-a data acquisition and processing unit, 8-a tested optical unit, 9-a circulator, 21-a first port, 22-a second port, 23-a third port, 24-a fourth port, 31-a first polarization controller, 32-a first optical fiber delay line, 33-a second laser, 41-a fifth port, 42-a sixth port, 43-a seventh port, 44-an eighth port, 51-a second polarization controller, 52-2x2 optical fiber coupler, 53-1x3 optical fiber coupler, 54-a third polarization controller, 55-fourth polarization controller, 56-fifth polarization controller, 57-first polarization beam splitter, 58-second polarization beam splitter, 59-third polarization beam splitter, 60-photoelectric conversion and amplification unit, 61-digital sampling card, 91-ninth first port, 92-ninth second port, 93-ninth third port, 521-fifth two-output port, 522-fifth two-output port, 531-fifth three-output port, 532-fifth three-output port and 533-fifth three-output port.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
In the description of the present invention, it is to be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on the orientations or positional relationships illustrated in the drawings, and are used merely for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and thus, are not to be construed as limiting the present invention. Further, in the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
[ example 1 ]
Referring to fig. 1 to 6, in a first aspect, the present invention provides a structure of a coherent polarization analyzer, including a first laser 1, a first fiber coupler 2, a reference arm optical unit 3, a second fiber coupler 4, a first polarization analyzer 5, a second polarization analyzer 6, and a data acquisition and processing unit 7, where the first fiber coupler 2 has a first port 21, a second port 22, a third port 23, and a fourth port 24, the first laser 1 is connected to the first port 21, the third port 23 outputs light to a measured optical unit 8, the reference arm optical unit 3 is connected to the fourth port 24, the second fiber coupler 4 has a fifth port 41, a sixth port 42, a seventh port 43, and an eighth port 44, light returned by the measured optical unit 8 is connected to the analyzer through the fifth port 41, the reference arm optical unit 3 is connected to the sixth port 42, the first polarization analyzer 5 is connected to the seventh port 43, the second polarization analyzer 6 is connected to the eighth port 44, and the data acquisition and processing unit 7 is connected to the first polarization analyzer 5 and the second polarization analyzer 6.
In this embodiment, the first port 21 and the second port 22 serve as input ports of the first optical fiber coupler 2, the third port 23 and the fourth port 24 serve as output ports of the first optical fiber coupler 2, the fifth port 41 and the sixth port 42 serve as input ports of the second optical fiber coupler 4, and the seventh port 43 and the eighth port 44 serve as output ports of the first optical fiber coupler 2. The first laser 1 emits coherent light with a coherence length larger than the transmission length of the optical unit 8 to be tested, and the coherent light is connected to the first port 21 of the first fiber coupler 2 through an optical fiber; the optical connection output by the third port 23 of the first optical fiber coupler 2 is connected to the input end of the tested optical unit 8, and the fourth port 24 is connected to the input end of the reference arm optical unit 3; the light returned from the tested optical unit 8 returns to the analyzer through the fifth port 41 of the second fiber coupler 4, and the output end of the reference arm optical unit 3 is connected with the sixth port 42 of the second fiber coupler 4; the first polarization analyzer 5 and the second polarization analyzer 6 are connected to two output ends (the seventh port 43 and the eighth port 44) of the second fiber coupler 4 respectively; the outputs (total of 8 stokes parameters) of the first polarization analyzer 5 and the second polarization analyzer 6 are connected in parallel to the data acquisition and processing unit 7. The first optical fiber coupler 2 and the second optical fiber coupler 4 are both optical fiber couplers with a splitting ratio of 50:50, no birefringence and no coupling in different polarization directions, light emitted by the first laser 1 reaches the second optical fiber coupler 4 through different paths for coherence, and then the polarization state is measured by the first polarization analyzer 5 and the second polarization analyzer 6, so that the problem that the existing analyzers cannot detect phase change and polarization state change at the same time is solved.
In a second aspect, the present invention provides a method for measuring a coherent polarization analyzer, comprising:
s101, disconnecting a connecting light path of the optical unit 8 to be measured, measuring a transmission matrix of the reference arm optical unit 3, adjusting the first polarization analyzer 5 and the second polarization analyzer 6 to enable the measurement results of the first polarization analyzer 5 and the second polarization analyzer 6 to be the same, and obtaining a first measurement result; the transmission matrix of the reference arm optical unit 3 is defined as the output light field vector
And the input light field vector
Correlation matrix between
The method comprises the following steps:
theoretical analysis of the invention proves that any one-dimensional optical element (comprising the reference arm optical unit 3 and the optical unit 8 to be measured) without polarization-dependent gain (or loss) has a transmission matrix
Having the form:
in the formula (I), the compound is shown in the specification,
the ratio of the output power to the input power can be obtained by simply measuring the optical power at the two ends; phase portion of the optical element transmission matrix
Where i is in imaginary units,
and e is the base of the natural logarithm,
(the same applies below); and matrix
Has the following form:
it has only 3 unknowns
. Obtaining 3 different output polarization states by inputting 3 different independent polarization states, such as horizontal linear polarization state, vertical linear polarization state and circular polarization state, and calculating the polarization state of the optical element by 3-point method
. When the optical unit under test 8 is switched off, it can be verified that the measurement results of the first polarization analyzer 5 and the second polarization analyzer 6 are the same. And obtaining a first measurement result after adjustment.
S102, connecting the optical unit 8 to be measured, and measuring the polarization states of the seventh port 43 and the eighth port 44 of the second optical fiber coupler 4 by the first polarization analyzer 5 and the second polarization analyzer 6 respectively to obtain a second measurement result;
in this case, the measurement results of the first polarization analyzer 5 and the second polarization analyzer 6 are generally different. Each polarization analyzer can obtain one set of results. Let the Stokes parameter measured by the first polarization analyzer 5 be
The Stokes parameter measured by the second polarization analyzer 6 is
The light field vector of the seventh port 43 can be calculated by the following formula
And the light field vector of the eighth port 44
The calculation formula is as follows:
in the above formula, the first and second carbon atoms are,
、
、
optical field vectors of the seventh ports 43 of the second fiber coupler 4, respectively
The magnitude of the x component, the magnitude of the y component, and the phase difference therebetween;
is 4 components of the stokes parameter measured by the first polarization analyzer 5;
、
、
optical field vectors of the eighth ports 44 of the second fiber coupler 4, respectively
The magnitude of the x component, the magnitude of the y component, and the phase difference therebetween;
are the 4 components of the stokes parameter measured by the second polarization analyzer 6.
S103, respectively calculating the polarization states of the fifth port 41 and the sixth port 42 of the second optical fiber coupler 4 by using the second measurement result to obtain a first calculation result;
using the measurement and calculation results in step S102, the polarization states of the two input ports (the fifth port 41 and the sixth port 42) of the second fiber coupler 4 are calculated respectively as follows:
in the formula (I), the compound is shown in the specification,
is the optical field vector of said fifth port 41 of said second fiber coupler 4,
is the optical field vector of said sixth port 42 of said second fiber coupler 4. Then, the polarization state (Stokes parameter) of the output light from the optical unit under test 8 of the probe arm 10 is calculated according to the definition of the Stokes parameter
In the formula, T represents transposition.
In the formula (I), the compound is shown in the specification,
respectively representing output light fields
The magnitude of the x-component and the y-component of (c),
representing the phase difference of the two components.
The stokes parameters of the polarization states of the outputs of the reference arm optical unit 3 (the seventh port 43 and the eighth port 44) are calculated with the same algorithm as the steps,
s104, calculating the power amplification factor of the reference arm optical unit 3 relative to the measured optical unit 8 by using the first measurement result and the first calculation result
Obtaining a second calculation result;
is calculated by the formula
。
And S105, calculating the transmission matrix of the tested optical unit 8 and the integral phase difference relative to the reference arm optical unit 3 by using the first calculation result and the second calculation result.
Is calculated by the formula
Thus, the measurement of the polarization state of the measured optical unit 8 and the overall phase difference with respect to the reference arm optical unit 3 is completed.
[ example 2 ]
Whereas embodiment 1 can only measure the optical unit 8 under test with one input port and one output port, it is sometimes necessary to measure the optical unit 8 under test with only one port, which serves as both the input port and the output port. Example 2 was designed by the present invention.
In this embodiment, the coherent polarization analyzer further includes a circulator 9, the ninth port 91 of the circulator 9 is connected to the third port 23 of the first optical fiber coupler 2, laser light is transmitted to the optical unit 8 to be measured via the ninth second port 92 of the circulator 9, light reflected by the optical unit 8 to be measured still returns to the circulator 9 via the ninth second port 92, and is connected to the eighth port 44 from the ninth third port 93 of the circulator 9; the rest is the same as in example 1.
In this embodiment, the modified one-dimensional optical system can be used for a reflective one-dimensional optical system, that is, the input end and the output end of the optical system use the same port, but respectively correspond to the incident light and the reflected light,
the invention has the advantages that:
1. the invention can measure the relative phase difference and the polarization state of the measured optical unit 8 at one time, which are the performances that the common interferometer and the polarization analyzer do not have, and the performances of the two are single and can only be respectively and independently measured. When the existing interferometer and polarization analyzer are used for measurement, because the two instruments need to be switched, the polarization and the phase are very sensitive to the connection of the optical fiber, the polarization state and the phase of the optical fiber are randomly changed due to the switching of the optical fiber, and the measurement is inaccurate. For example, after the phase is measured, the polarization analyzer is switched to, and the original phase is changed in the switching process.
2. The present invention also can measure the relative gain or loss of the measured optical unit 8, which is also a performance that common interferometers and polarization analyzers do not have.
3. The invention also enables the transmission matrix of the measured optical unit 8 to be measured, thereby measuring all polarization characteristics of the measured optical unit 8. The invention proves a one-dimensional optical transmission system theoretically, which can be expressed by the product of a whole phase shift, a transmission matrix (U matrix) and an amplification factor (or attenuation coefficient), and lays a theoretical foundation for measuring all polarization characteristics of the measured optical unit 8.
4. The invention can be used in a polarization-coherent multiplexing communication system as a coherent Stokes receiver, can ensure the polarization stability of the system and eliminate polarization fading.
5. The invention can be used in seismic wave detection systems, seabed seismic detection and alternative systems to DAS systems to improve their performance.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
Claims (6)
1. A coherent polarization analyzer is characterized by comprising a first laser, a first optical fiber coupler, a reference arm optical unit, a second optical fiber coupler, a first polarization analyzer, a second polarization analyzer and a data acquisition and processing unit, wherein the first optical fiber coupler is provided with a first port, a second port, a third port and a fourth port, the first laser is connected with the first port and is connected to an input port of an optical unit to be tested through the third port, the reference arm optical unit is connected with the fourth port, the second optical fiber coupler is provided with a fifth port, a sixth port, a seventh port and an eighth port, an output end of the optical unit to be tested is connected with the fifth port of the second optical fiber coupler, the reference arm optical unit is connected with the sixth port, and the first polarization analyzer is connected with the seventh port, the second polarization analyzer is connected with the eighth port, and the data acquisition and processing unit is connected with the first polarization analyzer and the second polarization analyzer.
2. The coherent polarization analyzer of claim 1,
the coherent polarization analyzer further comprises a circulator, wherein the circulator is provided with a ninth first port, a ninth second port and a ninth third port, the ninth first port of the circulator is connected with the third port of the first optical fiber coupler, the ninth second port of the circulator is connected with the optical unit to be tested, and the ninth third port of the circulator is connected with the fifth port of the second optical fiber coupler.
3. The coherent polarization analyzer of claim 1,
the reference arm optical unit comprises a first polarization controller and a first optical fiber delay line, wherein the input end of the first polarization controller is connected with the fourth port of the first optical fiber coupler, the output end of the first polarization controller is connected with the input end of the first optical fiber delay line, and the output end of the first optical fiber delay line is connected with the sixth port of the second optical fiber coupler.
4. The coherent polarization analyzer of claim 3,
the reference arm optical unit is an active unit and comprises a second laser with the same wavelength as the first laser, a first polarization controller and a first optical fiber delay line, wherein laser emitted by the second laser is injected into the first polarization controller and then reaches an eighth port of the second optical fiber coupler through the first optical fiber delay line.
5. The coherent polarization analyzer of claim 1,
the first polarization analyzer comprises a second polarization controller, a 2x2 optical fiber coupler, a 1x3 optical fiber coupler, a third polarization controller, a fourth polarization controller, a fifth polarization controller, a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a photoelectric conversion and amplification unit and a digital acquisition card;
the second polarization controller is connected with a seventh port of the second optical fiber coupler, an output end of the second polarization controller is connected with an input end of the 2x2 optical fiber coupler, the 2x2 optical fiber coupler is provided with a fifth-second output port and a fifth-second output port, the fifth-second output port is connected with one path of the photoelectric conversion and amplification unit and sends data to the data acquisition card, the fifth-second output port is connected with an input end of the 1x3 optical fiber coupler, the 1x3 optical fiber coupler is provided with a fifth-third output port, a fifth-third output port and a fifth-third output port, the fifth-third output port is connected with an input end of the third polarization controller, the fifth-third output port is connected with an input end of the fourth polarization controller, and the fifth-third output port is connected with an input end of the fifth polarization controller, the output ends of the third polarization controller, the fourth polarization controller and the fifth polarization controller are respectively connected with the input ends of the first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter, the output ends of the first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter are connected to the corresponding ends connected with the photoelectric conversion and amplification unit, and the output electric signals of the photoelectric conversion and amplification unit are collected and processed by the data acquisition card.
6. A measurement method of a coherent polarization analyzer, applied to the coherent polarization analyzer of any one of claims 1 to 5, comprising:
disconnecting a connecting light path of the optical unit to be measured, measuring a transmission matrix of the reference arm optical unit, and adjusting the first polarization analyzer and the second polarization analyzer to enable the measurement results of the first polarization analyzer and the second polarization analyzer to be the same to obtain a first measurement result;
the first polarization analyzer and the second polarization analyzer respectively measure the polarization states of the seventh port and the eighth port of the second optical fiber coupler to obtain a second measurement result;
respectively calculating the polarization states of a fifth port and a sixth port of the second optical fiber coupler by using the second measurement result to obtain a first calculation result;
calculating power amplification factors of the fifth port reference arm optical unit and the sixth port reference arm optical unit of the second optical fiber coupler relative to the optical unit to be measured by using the first measurement result and the first calculation result to obtain a second calculation result;
and calculating the transmission matrix of the optical unit to be measured and the integral phase difference relative to the optical unit of the reference arm by using the first calculation result and the second calculation result.
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