CN113048968A - Polarization state control system and method of non-polarization-maintaining Sagnac interferometer - Google Patents

Polarization state control system and method of non-polarization-maintaining Sagnac interferometer Download PDF

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CN113048968A
CN113048968A CN202011249939.3A CN202011249939A CN113048968A CN 113048968 A CN113048968 A CN 113048968A CN 202011249939 A CN202011249939 A CN 202011249939A CN 113048968 A CN113048968 A CN 113048968A
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polarization
light
polarization controller
coupler
optical
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CN113048968B (en
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蔡志岗
王自鑫
赵伟鸿
王福娟
王嘉辉
李俊洋
黄柱源
张锡斌
张铭琛
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Sun Yat Sen University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
    • G01C19/58—Turn-sensitive devices without moving masses
    • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
    • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G02B27/283—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising used for beam splitting or combining
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/28—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising
    • G02B27/286—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for polarising for controlling or changing the state of polarisation, e.g. transforming one polarisation state into another

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Abstract

The invention provides a polarization state control system and method of a non-polarization-maintaining Sagnac interferometer, the system realizes the switching of single and double probe light paths by adding a light path gating device optical switch at the front end of a polarization beam splitter PBS; firstly, the optical switch is communicated with one path of the access optical probe, two polarization controllers are adjusted to enable the interference fringes to be locked at the position of pi/2 and disappear, then the path is switched to the path connecting the polarization beam splitter and the balanced detector, and the rotating shaft of the adapter is rotated or the other polarization controller at the same position is adjusted to enable the contrast of the interference fringes to be maximum, so that the optimization of the device is realized, and the highest sensitivity is achieved.

Description

Polarization state control system and method of non-polarization-maintaining Sagnac interferometer
Technical Field
The invention relates to the technical field of fiber optic gyroscopes, in particular to a polarization state control system and method of a non-polarization-maintaining type Sagnac interferometer.
Background
When the sound wave interacts with the substance, the sound velocity and the energy of the sound wave change, and some basic physical parameters of the substance can be determined through measuring the sound velocity and the attenuation of the sound wave. This is an ultrasonic detection technique, which is one of the basic methods for studying the structure and properties of a substance and has been successfully applied in many fields.
The conventional ultrasonic generation and reception methods are methods using ultrasonic transducers such as piezoelectric transducers, CMUT transducers, which are contact type measurement methods. Currently, the conventional contact transducer is increasingly replaced by a non-contact optical method, including an interference method, a beam deflection method, and the like. Due to the continuous improvement of optical technology, research on the aspect is also rapidly developing, and new detection technologies including optical fiber sensing technology are continuously generated.
Application number 201910885959.0 discloses a single-axis Sagnac interferometer phase offset control device and method, wherein the single-axis Sagnac interferometer phase offset control device comprises an optical pulse transmitter, an optical pulse receiver, a single-axis Sagnac optical fiber interferometer, a sensing optical cable, an optical reflector and a polarization controller; the optical pulse transmitter, the polarization controller, the single-axis Sagnac optical fiber interferometer, the sensing optical cable and the optical reflector are sequentially connected, and the single-axis Sagnac optical fiber interferometer is also connected with the optical pulse receiver; the optical pulse signal processing device comprises an optical pulse transmitter, a polarization controller, a sensing optical cable, a light reflector, a single-axis Sagnac optical fiber interferometer and an optical pulse receiver, wherein the optical pulse transmitter is used for generating a pulse optical signal, the polarization controller is used for changing the polarization state of the input optical signal, the sensing optical cable is used for sensing a vibration signal, the light reflector is used for reflecting the optical signal, the single-axis Sagnac optical fiber interferometer is used for enabling the signals scattered and reflected in the sensing optical cable to generate interference. The invention can bias the phase of the single-axis Sagnac interferometer to be close to the expected value, thereby obtaining better small-signal response sensitivity. However, the patent cannot realize that the optical switch of the optical path gating device is added at the front end of the PBS to realize the switching of the optical paths of the single probe and the double probes, so that the optimization of the device is realized, and the highest sensitivity is achieved.
Disclosure of Invention
The invention provides a polarization state control system of a high-sensitivity non-polarization-maintaining Sagnac interferometer.
It is still another object of the present invention to provide a polarization state control method for a non-polarization-maintaining Sagnac interferometer.
In order to achieve the technical effects, the technical scheme of the invention is as follows:
a polarization state control system of a non-polarization-maintaining Sagnac interferometer comprises a computer, an analog-to-digital converter, a balance detector, a polarization beam splitter PBS, a photoelectric detector, an optical switch, a polarization controller, a light source, a first polarization controller PC1, a circulator, a 1 × 2 coupler BS1, a long optical fiber ring, an acousto-optic modulator AOM, a second polarization controller PC2, a 1 × 2 coupler BS2, a collimator and a focusing lens; the computer, the analog-digital converter, the balance detector and the PBS are sequentially connected and then connected to one channel of the optical switch, one end of the photoelectric detector is connected with the analog-digital converter, and the other end of the photoelectric detector is connected to the other channel of the optical switch; the channel selection end of the optical switch is connected with the circulator through the polarization controller, the SLED broadband low coherence light source is connected with the circulator through the first polarization controller PC1, and the circulator is also connected with the 1 x 2 coupler BS 1; the 1 x 2 coupler BS1 is connected to the 1 x 2 coupler BS2 through a long fiber ring and then through an acousto-optic modulator AOM, the 1 x 2 coupler BS1 is further connected to the 1 x 2 coupler BS2 through a second polarization controller PC2, the 1 x 2 coupler BS2 is further connected to a collimator, and light emitted by the collimator is focused on an ultrasonic sample through a focusing lens.
Further, light emitted by the light source is projected onto an ultrasonic sample through the first polarization controller PC1, the circulator, the 1 × 2 coupler BS1, the long optical fiber ring, the acousto-optic modulator AOM, the 1 × 2 coupler BS2, the collimator and the focusing lens, and the optical path of the light is the optical path of CW light along the clockwise direction; after being reflected by the ultrasonic sample, the ultrasonic sample passes through a focusing lens, a collimator, a 1 x 2 coupler BS2, a second polarization controller PC2, a 1 x 2 coupler BS1, a circulator and a polarization controller, and then is selected by an optical switch to pass through a photoelectric detector and then an analog/digital converter to a computer, or is selected by the optical switch to pass through a polarization beam splitter PBS and a balance detector and then the analog/digital converter to the computer, and the optical path is the optical path of CCW light along the anticlockwise direction.
Preferably, the light source is a SLED broadband low coherence light source; the polarization beam splitter PBS adopts three optical fibers with polarization maintaining interfaces.
A control method of a polarization state control system of a non-polarization-maintaining Sagnac interferometer comprises the following steps:
s1: communicating an optical switch with a channel of a photoelectric detector, and adjusting a first polarization controller PC1 and a second polarization controller PC2 to enable interference fringes to be locked at pi/2 and disappear;
s2: and after the interference fringes disappear, the optical switch is communicated with the PBS channel of the polarization beam splitter, and the polarization controller is adjusted, so that the contrast of the interference fringes reaches the maximum.
Further, the CW light and the CCW light pass through the first polarization controller PC1 and the second polarization controller PC2 and then pass through the 1 × 2 coupler BS1, the 1 × 2 coupler BS2 and the circulator respectively to be converged, the sample surface is vibrating all the time, and the optical path difference between the CW light reaching the vibrating ultrasonic sample surface and the ultrasonic sample surface when the ultrasonic sample surface is stationary is set to be
Figure BDA0002771280650000031
The difference in optical path length when the CCW light reaches the vibrating ultrasonic sample surface compared to when the ultrasonic sample surface is stationary is
Figure BDA0002771280650000032
The fiber on the long fiber loop is lossless and the two couplers are arranged according to the following steps of 1: 1, the light intensity of CW light and CCW respectively account for 1/4, and the light field of CW light after passing through the second polarization controller PC2 is represented as
Figure BDA0002771280650000033
The optical field of CCW light after passing through the second polarization controller PC2 is shown as
Figure BDA0002771280650000034
Wherein the content of the first and second substances,
Figure BDA0002771280650000035
l is a unitary matrix
Figure BDA0002771280650000036
Theta is a polarization state incidence angle;
composite light field after the CW and CCW light merge:
Figure BDA0002771280650000037
the corresponding light intensities are:
Figure BDA0002771280650000038
is provided with
Figure BDA0002771280650000039
Then I0The expression of (c) is simplified as:
Figure BDA00027712806500000310
taking into account conditions
Figure BDA00027712806500000311
When the condition is satisfied, N is 0, the condition provides initial phase offset of interference fringe pi/2, and the expression of L is substituted into the condition
Figure BDA00027712806500000312
And simplifying to obtain the following relation:
Figure BDA00027712806500000313
and will beThe condition is substituted into the expression of N to obtain
Figure BDA00027712806500000314
Consider (M)2+|N2)max1, so when M is 0, | N tintmaxThe parameters corresponding to this condition being satisfied are:
Figure BDA0002771280650000041
these parameters are the parameters associated with the elements of the matrix L,
Figure RE-GDA0003015924420000034
when the input light field satisfies the condition θ of 0, the second polarization controller PC2 causes
Figure RE-GDA0003015924420000035
When θ and L satisfy the above relationship, M is 0, N is | N |, N is not calculationmaxThis condition provides an initial phase difference of pi/2 with the best sensitivity, and the maximum interference fringe contrast, so that the interferometer is in the best ideal operating condition.
Further, the light intensity expression relates to a linear combination of a cosine function and a sine function of Δ Φ, the two terms are interference terms, when the cosine component is 0 and only the sin component is present, the initial optical path difference of the interference fringes has the maximum sensitivity, whereas when there is no sin component and only the cos component is present, the sensitivity of the initial optical path difference of the interference fringes is just at a position where the slope is 0, the sensitivity is the worst, and in order to make the interference fringes have the maximum sensitivity and simultaneously have the maximum contrast of the interference fringes, it is necessary to maximize the coefficient in front of the sin component.
Further, when the optical switch is connected to the channel of the photodetector, the condition is satisfied
Figure BDA0002771280650000044
By replacement with
Figure BDA0002771280650000045
Then:
Figure BDA0002771280650000046
is verified at
Figure BDA0002771280650000047
At the time of the above-mentioned operation,
Figure BDA0002771280650000048
substituting the expression of M and N may result in M ═ N ═ 0, and satisfying the condition when M ═ N ═ 0 corresponds to
Figure BDA0002771280650000049
Further, after the optical switch is communicated with the PBS channel, the output light field needs to pass through the PBS and then respectively enter the balanced detector for differential detection, and the included angle between the fast axis and the slow axis of the PBS is set
Figure BDA00027712806500000410
If the included angle of the selected coordinate system is alpha, projecting the output light field to two directions of the fast and slow axes of the polarizing beam splitter PBS, taking the fast and slow axes of the polarizing beam splitter PBS as a new coordinate system, and expressing the CW light and the CCW light under the new coordinate system as follows:
Figure BDA0002771280650000051
will be in the condition of
Figure BDA0002771280650000052
Substituted into E'CWAnd E'CCWObtaining:
Figure BDA0002771280650000053
the CW light and the CCW light combine two polarization components of the light field in a new coordinate system:
Figure BDA0002771280650000054
output light intensity after differentiation:
Figure BDA0002771280650000055
thus when satisfying
Figure BDA0002771280650000056
When the contrast of the interference fringe is maximized, M is equal to N and equal to 0, corresponding to
Figure BDA0002771280650000057
The differentiated initial phase difference of the interference fringes depends on theta.
Further, when
Figure BDA0002771280650000058
The corresponding optimal conditions for the control of the time-modulated polarization controller 2 are
Figure BDA0002771280650000059
And the first polarization controller PC1 and the second polarization controller PC2 are adjusted at the same time, so that the matching of theta, L and alpha is realized, namely, the maximum sensitivity and the maximum differential fringe contrast are realized.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the invention realizes the switching of the single-probe and double-probe optical paths by adding an optical path gating device optical switch at the front end of the PBS; firstly, the optical switch is communicated with one path of the access optical probe, two polarization controllers are adjusted to enable the interference fringes to be locked at the position of pi/2 and disappear, then the path is switched to the path connecting the polarization beam splitter and the balanced detector, and the rotating shaft of the adapter is rotated or the other polarization controller at the same position is adjusted to enable the contrast of the interference fringes to be maximum, so that the optimization of the device is realized, and the highest sensitivity is achieved.
Drawings
FIG. 1 is a block diagram of the system of the present invention;
FIG. 2 is a schematic diagram of a portion of a loop of the system of the present invention, wherein CW light represents light propagating in a clockwise direction and CCW represents light propagating in a counterclockwise direction;
FIG. 3 is a graph showing an oscilloscope with an initial optical path difference slightly greater than π/2;
FIG. 4 is a graph showing an oscilloscope with an initial optical path difference slightly less than π/2;
FIG. 5 is a diagram of the oscilloscope following repeated adjustment of the first polarization controller PC1 of FIG. 3;
FIG. 6 is a diagram of the oscilloscope following repeated adjustments of the second polarization controller PC2 of FIG. 4;
FIG. 7 is a schematic diagram of birefringence effects after adjustment of a polarization controller;
FIG. 8 is a schematic diagram of the first polarization controller PC1 being trimmed to eliminate the analog DC component of the balanced detector output;
FIG. 9 is a schematic diagram of the second polarization controller PC2 being trimmed to eliminate the analog DC component of the balanced detector output;
FIG. 10 is a graph showing the behavior of the interferometer at different points in the parameter space consisting of M and N;
fig. 11 is a schematic diagram showing the initial phase difference of the differential interference fringes with respect to the path to the origin in addition to M and N.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the present embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
Example 1
As shown in fig. 1, a polarization state control system of a non-polarization-maintaining Sagnac interferometer includes a computer, an analog/digital converter, a balanced detector, a polarization beam splitter PBS, a photodetector, an optical switch, a polarization controller, a light source, a first polarization controller PC1, a circulator, a 1 × 2 coupler BS1, a long optical fiber ring, an acousto-optic modulator AOM, a second polarization controller PC2, a 1 × 2 coupler BS2, a collimator, and a focusing lens; the computer, the analog-to-digital converter, the balance detector and the PBS are sequentially connected and then connected to one channel of the optical switch, one end of the photoelectric detector is connected with the analog-to-digital converter, and the other end of the photoelectric detector is connected to the other channel of the optical switch; the channel selection end of the optical switch is connected with the circulator through the polarization controller, the SLED broadband low-coherence light source is connected with the circulator through a first polarization controller PC1, and the circulator is also connected with a 1 x 2 coupler BS 1; the 1 x 2 coupler BS1 is connected to the 1 x 2 coupler BS2 through a long optical fiber ring and then through an acousto-optic modulator AOM, the 1 x 2 coupler BS1 is further connected to the 1 x 2 coupler BS2 through a second polarization controller PC2, the 1 x 2 coupler BS2 is further connected to a collimator, and light emitted by the collimator is focused on an ultrasonic sample through a focusing lens.
Light emitted by a light source is projected onto an ultrasonic sample through a first polarization controller PC1, a circulator, a 1 x 2 coupler BS1, a long optical fiber ring, an acousto-optic modulator AOM, a 1 x 2 coupler BS2, a collimator and a focusing lens, and the optical path is the optical path of CW light along the clockwise direction; after being reflected by the ultrasonic sample, the ultrasonic sample passes through a focusing lens, a collimator, a 1 x 2 coupler BS2, a second polarization controller PC2, a 1 x 2 coupler BS1, a circulator and a polarization controller, and then is selectively transmitted to a computer through a photoelectric detector and an analog/digital converter by an optical switch, or is selectively transmitted to the computer through a polarization beam splitter PBS and a balance detector and then the analog/digital converter by the optical switch, and the optical path is the optical path of CCW light along the anticlockwise direction.
The light source is an SLED broadband low coherence light source; the polarizing beam splitter PBS uses three optical fibers with polarization maintaining interfaces.
Example 2
A control method of a polarization state control system of a non-polarization-maintaining Sagnac interferometer comprises the following steps:
s1: communicating an optical switch with a channel of a photoelectric detector, and adjusting a first polarization controller PC1 and a second polarization controller PC2 to enable interference fringes to be locked at pi/2 and disappear;
s2: and after the interference fringes disappear, the optical switch is communicated with the PBS channel of the polarization beam splitter, and the polarization controller is adjusted, so that the contrast of the interference fringes reaches the maximum.
The CW light and the CCW light respectively pass through the first polarization controller PC1 and the second polarization controller PC2 and then pass through the 1 x 2 coupler BS1, the 1 x 2 coupler BS2 and the circulator to be converged, the surface of the sample is always vibrated, and the optical path difference of the CW light when reaching the surface of the vibrated ultrasonic sample compared with the optical path difference of the CW light when the surface of the ultrasonic sample is static is set as
Figure BDA0002771280650000071
The difference in optical path length of the CCW light when it reaches the vibrating ultrasonic sample surface compared to when the ultrasonic sample surface is stationary is
Figure BDA0002771280650000081
The fiber on the long fiber loop is lossless and the two couplers are arranged according to the following steps of 1: 1, the light intensity of CW light and CCW respectively account for 1/4, and the light field of CW light after passing through the second polarization controller PC2 is represented as
Figure BDA0002771280650000082
The light field of the CCW light after passing through the second polarization controller PC2 is represented as
Figure BDA0002771280650000083
Wherein the content of the first and second substances,
Figure BDA0002771280650000084
l is a unitary matrix
Figure BDA0002771280650000085
Theta is a polarization state incidence angle;
composite light field after the CW and CCW light merge:
Figure BDA0002771280650000086
the corresponding light intensities are:
Figure BDA0002771280650000087
is provided with
Figure BDA0002771280650000088
Then I0The expression of (c) is simplified as:
Figure BDA0002771280650000089
taking into account conditions
Figure BDA00027712806500000810
When the condition is satisfied, N is 0, the condition provides initial phase offset of interference fringe pi/2, and the expression of L is substituted into the condition
Figure BDA00027712806500000811
And simplifying to obtain the following relation:
Figure BDA00027712806500000812
and substituting the condition into an expression of N to obtain
Figure BDA00027712806500000813
Consider (M)2+|N2)max1, so when M is 0, | N tintmaxThe parameters corresponding to this condition being satisfied are:
Figure BDA00027712806500000814
these parameters are the parameters associated with the elements of the matrix L,
Figure RE-GDA0003015924420000074
when the input light field satisfies the condition θ of 0, the second polarization controller PC2 causes
Figure RE-GDA0003015924420000075
When θ and L satisfy the above relationship, M is 0, N is | N |, N is not calculationmaxThis condition provides an initial phase difference of pi/2 with the best sensitivity, and the maximum interference fringe contrast, so that the interferometer is in the best ideal operating condition.
The light intensity expression is a linear combination of a cosine function and a sine function with respect to Δ Φ, which are interference terms, when the cosine component is 0 and only the sin component, the initial optical path difference of the interference fringes has the maximum sensitivity, whereas when there is no sin component and only the cos component, the sensitivity of the initial optical path difference of the interference fringes positively benefits at the position where the slope is 0, the sensitivity is the worst, and in order to make the interference fringes have the maximum sensitivity and simultaneously have the maximum contrast of the interference fringes, it is necessary to maximize the coefficient in front of the sin component.
When the optical switch is communicated with the channel of the photoelectric detector, the condition is satisfied
Figure BDA0002771280650000093
By replacement with
Figure BDA0002771280650000094
Then:
Figure BDA0002771280650000095
is verified at
Figure BDA0002771280650000096
At the time of the above-mentioned operation,
Figure BDA0002771280650000097
substituting the expression of M and N may result in M ═ N ═ 0, and satisfying the condition when M ═ N ═ 0 corresponds to
Figure BDA0002771280650000098
After the optical switch is communicated with the PBS channel of the polarization beam splitter, the output light field needs to pass through the PBS channel of the polarization beam splitter and then respectively enter the balance detectors for differential detectionIncluded angle between fast and slow axes of PBS
Figure BDA0002771280650000099
If the included angle of the selected coordinate system is alpha, projecting the output light field to two directions of the fast and slow axes of the polarizing beam splitter PBS, taking the fast and slow axes of the polarizing beam splitter PBS as a new coordinate system, and expressing the CW light and the CCW light under the new coordinate system as follows:
Figure BDA00027712806500000910
will be in the condition of
Figure BDA0002771280650000101
Substituted into E'CWAnd E'CCWObtaining:
Figure BDA0002771280650000102
the CW light and the CCW light combine two polarization components of the light field in a new coordinate system:
Figure BDA0002771280650000103
output light intensity after differentiation:
Figure BDA0002771280650000104
thus when satisfying
Figure BDA0002771280650000105
When the contrast of the interference fringe is maximized, M is equal to N and equal to 0, corresponding to
Figure BDA0002771280650000106
The differentiated initial phase difference of the interference fringes depends on theta.
When in use
Figure BDA0002771280650000107
The corresponding optimal conditions for the control of the time-modulated polarization controller 2 are
Figure BDA0002771280650000108
And the first polarization controller PC1 and the second polarization controller PC2 are adjusted simultaneously to realize the matching of theta, L and alpha, namely, the maximum sensitivity and the maximum differential fringe contrast are realized.
The CW light passes through PC1 first and then BS1, then the CW light passes through a long length of fiber, BS2, the sample probe, then returns to BS2, then goes the next way, enters the circulator through BS1, and the CCW light is similarly: the lower light path is firstly taken, and the upper light path is taken when the light path returns. As shown in fig. 2.
Because most of the optical paths taken by CW and CCW are identical, the polarization states of CW light and CCW light are different when CW light and CCW light pass through PC2 due to birefringence effect only when CW light and CCW light pass through PC2, and by using this effect, PC1 and PC2 and the rotation shaft of the adapter (or another polarization controller located at the same position) are adjusted to make CW light and CCW light in orthogonal polarization states, and then the CW light and CCW light are decomposed into two differential interference signals with a phase difference of pi/2 through PBS.
As shown in fig. 3 and 4, the SAGNAC interferometer with the optical switch connected to the channel of the photodetector, the sample is a mirror. The ultrasonic wave of 75kHz is excited by the piezoelectric plate, so that the surface of the reflector generates surface displacement with larger amplitude, the optical path difference between the long ring and the short ring of the interferometer is 2km, and the corresponding central frequency is 50 kHz. Observing the reading of the oscillograph to judge the initial optical path difference of the interference fringes, wherein the initial optical path difference is slightly larger than pi/2 when shown in figure 3, the initial optical path difference is slightly smaller than pi/2 when shown in figure 4, and the initial optical path difference can be considered to be pi/2 when the interference fringes are just in the middle of the two states.
As shown in fig. 5 and 6, describing the next flow following fig. 3 and 4, PC1 and PC2 are repeatedly adjusted so that (| M |, | N |) continuously approaches a point (| M |, | N |) (0,0) along Path1 of fig. 9. Adjusting PC1, the amplitude of the oscillations observed from the display becomes small, at which point adjusting PC2 repeats the operation to ensure that the initial phase difference is still pi/2, and then adjusting PC1 again continues to reduce the oscillations. The above steps are repeated until the interference fringes completely disappear, and the (| M |, | N |) approaches the origin along the | N | axis direction.
As shown in fig. 7, the birefringence effect caused by adjusting the polarization controller causes a to rotate. When the angle between the principal axes of the coordinates of the PBS (i.e., the fast and slow axes in the above figure) and the principal axes of the coordinates of the CW and CCW is
Figure BDA0002771280650000111
When the interference efficiency becomes maximum. It should be noted that the rotation axis of the polarization controller is also the polarization state change caused by the birefringence effect, and besides causing the rotation of the polarization principal axis, it also causes the change of the polarization plane (for example, the linear polarization may become elliptical polarization), but since the change of the polarization plane passed by the two lights is identical, the change of the polarization plane does not need to be considered too much, we still consider it as the same polarization plane as the original one, because when the polarization plane is changed, the two lights still pass through the same change of the polarization plane
Figure BDA0002771280650000112
Such a change of the plane of polarization only causes a difference in the phenomena observed during the conditioning process, but the end result of the experimental observation remains the same, i.e. the interference efficiency becomes maximum.
As shown in fig. 8 and 9, fig. 8 is a trim PC1 to remove the analog dc component of the balanced detector output. In the case of fig. 9, the formula of the interference fringe contrast V ═ I is usedmax-Imin/Imax+IminThe calculated V is 43.1%, considering that the loss of the fiber is 0.19dB/km, the experiment uses a 2km fiber long loop, so the theoretical maximum contrast is 49.96%. Because the experiment has various errors and the light source is a low-coherence light source, the experimental result shows that the control of various parameters is very close to an ideal state, and meanwhile, the device has the greatest advantages of high contrast and high sensitivity and has great value for practical application.
In fig. 10, in the parameter space composed of | M | and | N |, different points in the figure reflect the operating state of the interferometer. And adjusting the PC1 and the PC2 back and forth, and judging the working position of the system through the waveform of the oscilloscope. The ideal working position for a single probe is | N | ═ 1, | M | ═ 0, but is not easily reachable. The ideal operating position for the differential probe is | M | ═ 0 and | N | ═ 0, and experiments have shown that this is easily achievable. Fig. 11 shows that the initial phase difference of the differential interference fringes is related to the path to the origin, in addition to M and N: if the red Path (Path1) arrives along the | M | axis tangent, the initial phase difference is 0 or π, and the maximum sensitivity condition cannot be satisfied. Only a Path (Path2) along a tangent of the | N | axis, such as blue, reaches the origin with an initial phase difference of ± π/2.
(1) In the case of a single probe, the oscilloscope is used to observe the interference intensity signal, and the AOM is used to excite a larger waveform, and the PC1 is adjusted and the oscilloscope waveform is observed to determine whether the signal is in a state where M is equal to 0. When the waveform is in the middle state of fig. 3 and 4, it is illustrated that the two light paths participating in interference are in the state of the initial optical path difference of pi/2.
(2) In general, the result of one adjustment of PC2 is that a point on the M, N parameter space is in a state where M is 0 and N is not equal to 0. At this point, the PC1 is adjusted so that the oscillation amplitude is attenuated and the interference fringes detected by the single probe completely disappear when the two beams reach a perfectly orthogonal polarization state. It should be noted that the phase difference of pi/2 needs to be maintained in the process of adjusting the reduction of PC1, and this process needs to gradually approach the points of M-0 and N-0 by repeatedly adjusting PC1 and PC2 while ensuring the phase difference of pi/2. As shown in fig. 5 and 6.
(3) Under the condition that the interference fringes of the single probe completely disappear, the single probe is switched into the double probes, and the PBS incident port is rotated to connect the adapter rotating shafts of the polarization-maintaining optical fiber and the common optical fiber (or the polarization controllers positioned at the same position), so that the contrast ratio of the interference fringes is as large as possible. Fig. 8 shows an interference waveform in which the single-probe interference fringes are completely disappeared and the interference fringes are replaced with the double-probe interference fringes. Because of the influence of the non-interfering dc light, when the contrast of the interference fringes is maximum, the light intensities on both sides of the balanced detector may not be completely complementary. The dc component can be eliminated by trimming the PC1, but the overall efficiency is not affected much. Fig. 9 is an interference waveform of the dual probe after the dc component is removed.
The same or similar reference numerals correspond to the same or similar parts;
the positional relationships depicted in the drawings are for illustrative purposes only and are not to be construed as limiting the present patent;
it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. It will be apparent to those skilled in the art that other variations and modifications can be made on the basis of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims (10)

1. A polarization state control system of a non-polarization-maintaining Sagnac interferometer is characterized by comprising a computer, an analog-to-digital converter, a balance detector, a polarization beam splitter PBS, a photoelectric detector, an optical switch, a polarization controller, a light source, a first polarization controller PC1, a circulator, a 1 x 2 coupler BS1, a long optical fiber ring, an acousto-optic modulator AOM, a second polarization controller PC2, a 1 x 2 coupler BS2, a collimator and a focusing lens; the computer, the analog-to-digital converter, the balance detector and the PBS are sequentially connected and then connected to one channel of the optical switch, one end of the photoelectric detector is connected with the analog-to-digital converter, and the other end of the photoelectric detector is connected to the other channel of the optical switch; the channel selection end of the optical switch is connected with the circulator through the polarization controller, the SLED broadband low-coherence light source is connected with the circulator through the first polarization controller PC1, and the circulator is also connected with the 1 x 2 coupler BS 1; the 1 x 2 coupler BS1 is connected to the 1 x 2 coupler BS2 through a long fiber ring and then through an acousto-optic modulator AOM, the 1 x 2 coupler BS1 is further connected to the 1 x 2 coupler BS2 through a second polarization controller PC2, the 1 x 2 coupler BS2 is further connected to a collimator, and light emitted by the collimator is focused on an ultrasonic sample through a focusing lens.
2. The polarization state control system of the non-polarization-maintaining Sagnac interferometer of claim 1, wherein the light emitted from the light source is projected onto the ultrasound sample through the first polarization controller PC1, the circulator, the 1 x 2 coupler BS1, the long fiber ring, the AOM, the 1 x 2 coupler BS2, the collimator, and the focusing lens, and the optical path is the optical path of the CW light in the clockwise direction; after being reflected by the ultrasonic sample, the ultrasonic sample passes through a focusing lens, a collimator, a 1 x 2 coupler BS2, a second polarization controller PC2, a 1 x 2 coupler BS1, a circulator and a polarization controller, and then is selectively passed through a photoelectric detector and then is transmitted to a computer through an analog/digital converter by an optical switch, or is selectively passed through a polarization beam splitter PBS and a balance detector and then is transmitted to the computer through the analog/digital converter by the optical switch, and the optical path is the optical path of CCW light along the anticlockwise direction.
3. The polarization state control system of a non-polarization-maintaining Sagnac interferometer of claim 2, wherein said light source is a SLED broadband low coherence light source.
4. The polarization state control system of the non-polarization-maintaining Sagnac interferometer of claim 3, wherein said polarization beam splitter PBS employs three optical fibers with polarization maintaining interfaces.
5. A method of controlling the polarization state control system of a non-polarization-preserving Sagnac interferometer according to any of claims 1-4, comprising the steps of:
s1: communicating an optical switch with a channel of a photoelectric detector, and adjusting a first polarization controller PC1 and a second polarization controller PC2 to enable interference fringes to be locked at pi/2 and disappear;
s2: and after the interference fringes disappear, the optical switch is communicated with the PBS channel of the polarization beam splitter, and the polarization controller is adjusted, so that the contrast of the interference fringes reaches the maximum.
6. The polarization state control system of claim 5 for a non-polarization-preserving Sagnac interferometerThe control method is characterized in that CW light and CCW light respectively pass through a first polarization controller PC1 and a second polarization controller PC2 and then respectively pass through a 1 x 2 coupler BS1, a 1 x 2 coupler BS2 and a circulator to be converged, the surface of a sample is always vibrated, and the optical path difference of the CW light when the CW light reaches the surface of the vibrated ultrasonic sample compared with the optical path difference of the CW light when the surface of the ultrasonic sample is static is set as
Figure RE-FDA0003015924410000011
The difference in optical path length when the CCW light reaches the vibrating ultrasonic sample surface compared to when the ultrasonic sample surface is stationary is
Figure RE-FDA0003015924410000012
The fiber on the long fiber loop is lossless and the two couplers are arranged according to the following steps of 1: 1, the light intensity of CW light and CCW respectively account for 1/4, and the light field of CW light after passing through the second polarization controller PC2 is represented as
Figure RE-FDA0003015924410000013
The light field of the CCW light after passing through the second polarization controller PC2 is represented as
Figure RE-FDA0003015924410000014
Wherein the content of the first and second substances,
Figure RE-FDA0003015924410000015
l is a unitary matrix
Figure RE-FDA0003015924410000021
Theta is a polarization state incidence angle;
composite light field after the CW and CCW light merge:
Figure RE-FDA0003015924410000022
the corresponding light intensities are:
Figure RE-FDA0003015924410000023
is provided with
Figure RE-FDA0003015924410000024
Then I0The expression of (c) is simplified as:
Figure RE-FDA0003015924410000025
taking into account conditions
Figure RE-FDA00030159244100000211
N is 0 when the condition is satisfied, the condition provides initial phase offset of interference fringe pi/2, and the expression of L is substituted into the condition
Figure RE-FDA00030159244100000212
And simplifying to obtain the following relation:
Figure RE-FDA0003015924410000026
and substituting the condition into an expression of N to obtain
Figure RE-FDA0003015924410000027
Considering (| M2+|N|2)max1, so when M is 0, | N tintmaxThe parameters corresponding to this condition being satisfied are:
Figure RE-FDA0003015924410000028
these parameters are the parameters associated with the elements of the matrix L,
Figure RE-FDA0003015924410000029
when the input light field satisfies the condition θ of 0, the second polarization controller PC2 causes
Figure RE-FDA00030159244100000210
When θ and L satisfy the above relationship, M is 0, N is | N ∞maxThis condition provides an initial phase difference of pi/2 with the best sensitivity, and the maximum interference fringe contrast, so that the interferometer is in the best ideal operating condition.
7. The method of claim 6, wherein the expression of the intensity of light is a linear combination of a cosine function and a sine function with respect to Δ Φ, the two terms are interference terms, when the cosine component is 0 and only the sin component is present, the initial path difference of the interference fringes has the maximum sensitivity, whereas when there is no sin component and only the cos component is present, the sensitivity of the initial path difference of the interference fringes is just at a position where the slope is 0, the sensitivity is the worst, and in order to maximize the contrast of the interference fringes while maximizing the sensitivity of the interference fringes, it is necessary to maximize the coefficient in front of the sin component.
8. The method of claim 7, wherein the condition is satisfied when the optical switch is connected to the channel of the photodetector
Figure FDA0002771280640000036
By replacement with
Figure FDA0002771280640000037
Then:
Figure FDA0002771280640000038
is verified at
Figure FDA0002771280640000039
At the time of the above-mentioned operation,
Figure FDA00027712806400000310
and carrying in M and NThe expression may be such that M ═ N ═ 0, and the condition when M ═ N ═ 0 is satisfied corresponds to
Figure FDA00027712806400000311
9. The method of claim 8, wherein after the optical switch is connected to the PBS channel, the output optical field needs to pass through the PBS and then enter the balanced detector for differential detection, and the included angle between the fast axis and the slow axis of the PBS are set
Figure FDA0002771280640000041
If the included angle of the selected coordinate system is alpha, projecting the output light field to two directions of the fast and slow axes of the polarizing beam splitter PBS, taking the fast and slow axes of the polarizing beam splitter PBS as a new coordinate system, and expressing the CW light and the CCW light under the new coordinate system as follows:
Figure FDA0002771280640000042
will be in the condition of
Figure FDA0002771280640000043
Substituted into E'CWAnd E'CCWObtaining:
Figure FDA0002771280640000044
the CW light and the CCW light combine two polarization components of the light field in a new coordinate system:
Figure FDA0002771280640000045
output light intensity after differentiation:
Figure FDA0002771280640000046
thus when satisfying
Figure FDA0002771280640000047
When the contrast of the interference fringe is maximized, M is equal to N and equal to 0, corresponding to
Figure FDA0002771280640000051
The differentiated initial phase difference of the interference fringes depends on theta.
10. The method of claim 9, wherein the method further comprises controlling a polarization state control system of the non-polarization-maintaining Sagnac interferometer
Figure FDA0002771280640000052
The corresponding optimal conditions for the control of the time-modulated polarization controller 2 are
Figure FDA0002771280640000053
And the first polarization controller PC1 and the second polarization controller PC2 are adjusted at the same time, so that the matching of theta, L and alpha is realized, namely, the maximum sensitivity and the maximum differential fringe contrast are realized.
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