CN1614455A - Optical fibre phase modulation and feedback white light interference system - Google Patents

Abstract

A method for modulating optical fibre phase sets a light feedback device at optical fibre end face of phase modulator to let light wave pass through the phase modulator twice. A white light interference system in feedback type is structured based on the method.

Description

A kind of optical fibre phase modulation and feedback white light interference system

Technical field

The invention belongs to the optical fiber technology field, be specifically related to a kind of new fiber phase modulator approach and the novel white-light interference system of structure thereof.

Background technology

The external world is detected the principles such as stress-strain effect, elasto-optical effect, electrooptical effect, thermal strain effect and thermo-optic effect of signal by optical fiber changes parameters such as the size of optical fiber and refractive index, thereby the phase change that causes light conducting in the optical fiber, realization is to the phase modulation (PM) of light conducting, by specific interference system, detect the phase information of light, thereby realize measurand such as temperature, acoustic pressure, strain, the detection of speed, in view of the above, can make up the full Fibre Optical Sensor of so-called functional form.This method is simple and practical, has a wide range of applications in fields such as Fibre Optical Sensor, optical fiber communications at present, particularly has a wide range of applications in fields such as fibre optic hydrophone, the detection of fiber ultrasonic ripple, fibre strain monitorings.But have a lot of limitation in the intrinsic fibre optic interferometer that adopts phase modulating method to construct, such as when increasing induction fiber lengths raising phse sensitivity, optical fiber is tediously long; And the problems such as random variation of polarization state often appear, and especially in the process of outer signals modulation optical fiber phase place, polarization state of light often all can change, and existing polarization state controller just seems powerless, causes interference fringe to disappear when serious; In distributed optical fiber sensing system, sensor array technology, remote sensing network system, ordinary optic fibre Intrinsical phase modulation (PM) generally all needs two optical fiber to realize signal extraction, and optical fiber is tediously long, and complexity is installed, and the cost height.

Summary of the invention

The objective of the invention is to propose new fiber phase modulator approach, to overcome in the existing fiber phase modulation (PM), optical fiber is tediously long, and complexity is installed, and the deficiency that cost is high proposes the white light interference system of structure thus simultaneously.

The fiber phase modulator approach that the present invention proposes, be to add at the fiber end face of fibre optic phase modulator the light feedback assembly is set, make light wave successively twice by in the phase-modulator, middle switching time is extremely short, can ignore, and construct novel feedback white light interference system.

The light wave that conducts in the optical fiber is realized both forward and reverse directions by fibre optic phase modulator by the feedback assembly that is located at fiber end face, can improve one times of phse sensitivity (under the environment of identical outer signals excitation); And can effectively avoid of the influence of phase-modulator polarization state random variation to interference system; Simultaneously, the simple optical fiber that can realize phase-modulated information extracts.Its principle is as follows:

When light wave was the optical fiber one time of l by length, the phase delay of generation was

Φ＝βl (1)

β is the propagation constant of light wave in optical fiber in the formula, β=nk ₀Wherein n is the effective refractive index that light wave is propagated in optical fiber, k ₀Be light wave wave number in a vacuum.

When length is that the optical fiber of l is subjected to outer signals when modulation as sensing unit, the variation φ that produces phase of light wave is

φ＝ΔΦ＝Δ(βl)＝βΔl+lΔβ (2)

Further arrangement turns to

$\mathrm{\φ}=\mathrm{\Φ}(\frac{\mathrm{\Δl}}{l}+\frac{\mathrm{\Δ\β}}{\mathrm{\β}})---\left(3\right)$

Outer signals causes that the phase differential of light wave can describe with following formula, mainly comprises two, and first phase differential that the length for optical fiber takes place to cause when changing relatively generally caused by strain effect; Second phase differential that causes for the relative variation of light wave propagation constant mainly caused by electrooptical effect, elasto-optical effect, poisson effect and thermo-optic effect etc.The phase delay Φ that while phase difference and optical fiber cause is proportional.Under the certain situation of extraneous pumping signal, promptly $\frac{\mathrm{\Δl}}{l}+\frac{\mathrm{\Δ\β}}{\mathrm{\β}}$ Necessarily, in order to wait until higher phse sensitivity, obviously can realize by the method that increases fiber lengths l, general realization approach is to increase actual fiber lengths, and the present invention adopts the light wave method that twice is passed through in optical fiber, do not increasing under the optical fiber physical length situation, it is 2 Φ effects that actual phase postpones, and the phase differential that obviously adopts the method to obtain is

${\mathrm{\φ}}^{\′}=2\mathrm{\Φ}(\frac{\mathrm{\Δl}}{l}+\frac{\mathrm{\Δ\β}}{\mathrm{\β}})=2\mathrm{\φ}----\left(4\right)$

As seen under identical outer signals excitation, adopt light wave in sensor fibre, to pass through twice technical scheme, obtain the multiplication effect of phase differential, realized that promptly phse sensitivity doubles.So this type of phase modulating method can be applied in Fibre Optical Sensor, fiber optic communication field widely.Application in the Fibre Optical Sensor mainly shows as and can be used for surveying the physical quantity that sound wave, strain, temperature, vibration, voice or the like can cause phase differential in the formula (3); Optical fiber communication is beneficial to mainly to show as and utilizes lithium niobate to realize aspects such as photoswitch, optical routing and full optical exchange network as external modulator.

The light feedback assembly of realizing said method is by two kinds of selections, one makes that for adopting the mode at sensor fibre end face plating reflectance coating the light in the optical fiber arrives end face, utilizes the retroactive effect of reflectance coating, being coupled to again in the optical fiber, the reflectance coating of this moment can be used as level crossing and handles; It two adopts faraday's rotating mirror at the sensor fibre end face, makes when light is reflected, and the polarization surface half-twist, and then be coupled to and realized double modulation in the sensor fibre.When fibre optic phase modulator is inserted the demodulated interferential light path, all can run into the random perturbation problem of polarization state, sometimes especially severe causes interference fringe to disappear.Yet adopt above-mentioned feedback assembly, can effectively overcome the polarization state variation that fibre optic phase modulator causes, the polarization state at random that particularly takes place in the process of outer signals effect phase-modulator develops.At top two kinds of light feedback assemblys, carry out the analysis of system below.

With the single-mode fiber is example, and we know and only propagate HE in the single-mode fiber ₁₁A pattern is actually double degenerate, can be decomposed into two orthogonal polarization components HE independently of one another, that be independent of each other ₁₁ ^xAnd HE ₁₁ ^y, the polarization state of describing in the optical fiber with Jones (Jones) vector J is

β wherein _x, β _yThe propagation constant of light wave on two orthogonal polarization orientation.Because the imperfection of optical fiber production and use makes the propagation characteristic of optical fiber show as anisotropic.Mainly show as linear birefringence and circular birefringence (oval birefringence is the situation of both stacks).In most cases, because the light wave plane of polarization that circular birefringence causes rotates at random, and linear birefringence just causes the phase differential of pairwise orthogonal polarization state, thus at fiber lengths more in short-term, circular birefringence will be much larger than the influence of linear birefringence to the optical interference behavior to the influence of optical interference.Above-mentioned optical fiber can be described with Jones matrix the influence of the polarization state of light wave.

The transmission matrix that linear birefringence is

$M_l=\left[\begin{array}{cc}{e}^{-j{\mathrm{\δ}}_1}& 0\\ 0& e^{j{\mathrm{\δ}}_1}\end{array}\right]----\left(6\right)$

δ wherein ₁=(δ _y-δ _x) z/2, suppose that herein selected coordinate direction overlaps with the birefringence fast and slow axis.

The transmission matrix of circular birefringence is

$M_c=\left[\begin{array}{cc}\cos \mathrm{\θ}& \sin \mathrm{\θ}\\ -\sin \mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]---\left(7\right)$

θ=(β wherein _R-β _L) z/2, β _R, β _LBe right-handed rotation, laevorotatory propagation constant.

Yet, describe birefringent Jones matrix and can write following form for the optical system that the linear optical element of lossless (mainly referring to Polarization Dependent Loss) is formed

$J=\left[\begin{array}{cc}{J}_{\mathrm{XX}}& -J_{\mathrm{YX}}^{*}\\ J_{\mathrm{YX}}& J_{\mathrm{XX}}^{*}\end{array}\right]---\left(8\right)$

Wherein conjugation is got in subscript * representative, and | J _XX| ²+ | J _YX| ²=1

Because the birefringence that causes of external interference such as temperature variation, vibration etc. generally all has reciprocity, establish in the optical fiber conduction light wave from along single direction position 1 to the position 2, describing the transmission matrix that polarization state changes is J ₁₂, equally in opposite direction from the position 2 to the position 1 to describe the transmission matrix that polarization state changes be J ₂₁If, the birefringence performance reciprocity character that external interference causes, the pass of two Jones matrixs is so $J_{21}=J_{12}^T,$ Subscript T represents transpose of a matrix, below calculates and all is assumed to the reciprocity birefringence.

So when using the fiber end face plated film to form reflecting surface or level crossing, our supposition reflection fully when lossy (be that change procedure is taken advantage of a coefficient), corresponding Jones matrix is

$M_M=\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]---\left(9\right)$

When having linear birefringence, the transmission matrix J of this feedback light path _FLFor

$J_{\mathrm{FL}}=M_l^T{M}_M{M}_l=\left[\begin{array}{cc}{e}^{-j{\mathrm{\&delta;}}_1}& 0\\ 0& e^{j{\mathrm{\&delta;}}_1}\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]\left[\begin{array}{cc}{e}^{-j{\mathrm{\&delta;}}_1}& 0\\ 0& e^{j{\mathrm{\&delta;}}_1}\end{array}\right]=\left[\begin{array}{cc}{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20041008903300104.png,A20041008903300112.png"\; state="\{\{state\}\}">j</figure-callout>}2{\mathrm{\&delta;}}_1}& 0\\ 0& e^{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20041008903300104.png,A20041008903300112.png"\; state="\{\{state\}\}">j</figure-callout>}2{\mathrm{\&delta;}}_1}\end{array}\right]---\left(10\right)$

As seen, this moment, linear birefringence was not cancelled, but multiplication occurred.

When having circular birefringence

$J_{\mathrm{FC}}=M_c^T{M}_M{M}_c=\left[\begin{array}{cc}\cos \mathrm{\&theta;}& -\sin \mathrm{\&theta;}\\ \sin \mathrm{\&theta;}& \cos \mathrm{\&theta;}\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]\left[\begin{array}{cc}\cos \mathrm{\&theta;}& \sin \mathrm{\&theta;}\\ -\sin \mathrm{\&theta;}& \cos \mathrm{\&theta;}\end{array}\right]=\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]---\left(11\right)$

Have reciprocity circular birefringence and offset fully this moment.

When the light wave feedback assembly adopts faraday rotator mirror, with the plane of polarization half-twist, faraday rotated the transmission matrix of transmitting mirror and was this moment

$M_F=\left[\begin{array}{cc}0& 1\\ -1& 0\end{array}\right]---\left(12\right)$

Adopt matrix J to represent Jones matrix this moment, and then the transmission matrix of feedback light path is

$J_F=J^T{M}_{F}J=\left[\begin{array}{cc}{J}_{\mathrm{XX}}& J_{\mathrm{YX}}\\ -J_{\mathrm{YX}}^{*}& J_{\mathrm{XX}}^{*}\end{array}\right]\left[\begin{array}{cc}0& 1\\ -1& 0\end{array}\right]\left[\begin{array}{cc}{J}_{\mathrm{XX}}& -J_{\mathrm{YX}}^{*}\\ J_{\mathrm{YX}}& J_{\mathrm{XX}}^{*}\end{array}\right]=\left[\begin{array}{cc}0& 1\\ -1& 0\end{array}\right]----\left(13\right)$

As seen birefringence this moment is offset fully.

Sum up conclusion: when the light feedback method that adopts plated film etc. not change the reflected version of plane of polarization is connected fibre optic phase modulator when terminal, this moment, not only the multiplication effect appearred in the fiber phase modulation, phse sensitivity can double, and can effectively offset the influence of circular birefringence to light path.Though multiplication appears in linear birefringence, and is very little to interference effect generally speaking, and simple in structure, the convenient realization; When adopting faraday rotator mirror as the light feedback assembly, can eliminate the birefringence of any reciprocity in the phase-modulator fully, system's antijamming capability is strong.Structure is compared with the former and is shown complicated slightly.

Novel phase modulating method of the present invention can be applied in any Intrinsical fibre optic interferometer.Using the present invention of this modulator approach has also constructed feedback white light interference system and it has been tested.

Adopt three fiber couplers, adopt light reaction type phase modulating method, constructed feedback white light interference system.As depicted in figs. 1 and 2.

Among Fig. 1, the light path feature of this feedback white light interference system is: after the light source 1 is fiber coupler 4, through beam split, the beam split of coupling mechanism 4 ports 8 is through chronotron 11, be coupling mechanism 5 afterwards, arrive and close fine phase-modulator 6, and by light feedback assembly 7 reflections of phase-modulator end, feedback light is got back to the port 9 of coupling mechanism 4 by coupling mechanism 5: the beam split of coupling mechanism port 9 arrives phase-modulator 6 through coupling mechanism 5, and through reflecting surface 7 reflections, feedback light is got back to coupling mechanism 4 ports 8 through chronotron 11.Two light beams form the light signal that carries the measurand feature in fiber coupler 4, this signal is detected device 2,3 and receives.By the inverting interference signal, obtain the physical characteristics of measurand.

Among Fig. 2, this feedback white light fiber optic interferometric system features light path is: after the light source 1 is coupling mechanism 4, after light is coupled the device beam split, the beam split of coupling mechanism port 8 is through chronotron 11, be coupling mechanism 5 afterwards, arrive fibre optic phase modulator 6, through the fiber reflector of fiber coupler 10 structures, reflection comes back to phase modulation (PM) 6 and has realized twice of light beam through 6, and feedback light is got back to the port 9 of coupling mechanism 4 by coupling mechanism 5; The beam split of coupling mechanism port 9 arrives phase-modulator 6 through coupling mechanism 5, and the fiber reflection mirror reflection of process coupling mechanism 10 structures, and feedback light is passed through coupling mechanism 5 successively, chronotron, and 11 get back to coupling mechanism 4 ports 8.Two light beams form the light signal that carries the measurand feature in 3 * 3 fiber couplers 4, this signal is detected device 2,3 and receives.By the inverting interference signal, obtain the physical characteristics of measurand.

Among Fig. 1 and Fig. 2, coupling mechanism 4 adopts 3 * 3 fiber couplers, if coupling mechanism 4 adopts 2 * 2 coupling mechanisms, then feedback white light interference system is for as shown in Figure 3 and Figure 4.

Structure and the principle of Fig. 3 and Fig. 1 are similar, and structure and the principle of Fig. 4 and Fig. 2 are similar.

The light feedback assembly that adopts in the above-mentioned white light interference system can be the catoptron in formation such as fiber end face plated films, also can be faraday rotator mirror.

A kind of method of the realization of fibre optic phase modulator of the present invention is to twine a circle or multiturn optical fiber on the elastic body of column type, when elastic wave effect optical fiber, can utilize the elasto-optical effect of optical fiber to realize phase modulation (PM).Herein, not only can be, and can be based on the Intrinsical fibre optic phase modulator of other physical influence, the crystal that optical index can change with the impressed voltage signal is as lithium columbate crystal etc.

The novel optical fiber phase modulating method that the present invention proposes not only can be used for constructing white light interference system, and can be used for any needs and utilize optical fiber to realize the fibre optic interferometer of phase modulation (PM), such as, the M-Z interferometer.

Feedback white light interference system of the present invention, connected mode between being connected of fiber coupler and optical fiber, the optical fiber is that the mode of fusing connects, light source is connected with the connected mode FC/PC wire jumper of interference system, and interference system also is that the FC/PC wire jumper is connected with the connected mode of detector.

It can be tapered fiber coupler that the present invention relates to the fiber coupler that is.

Among the present invention, the luminous power of coupling mechanism is divided equally, and promptly 3 * 3 fiber coupler luminous power splitting ratios are 1: 1: 1, and 2 * 2 fiber coupler luminous power splitting ratios are 1: 1.

Single-mode fiber, multimode optical fiber all are applicable to system of the present invention.

Among the present invention, stabilized light source (laser instrument) can be any in following: operation wavelength is the semiconductor laser diode (LD) of 1.31 μ m or 1.55 μ m; Semiconductor light-emitting-diode (LED) laser instrument; Super-radiance light emitting diode (SLD) laser instrument etc.

Outstanding advantage of the present invention is to improve one times of phse sensitivity (under the environment of identical outer signals excitation); And can effectively avoid of the influence of phase-modulator polarization state random variation to interference system; Simultaneously, the simple optical fiber that can realize phase-modulated information extracts.When the light feedback method that adopts plated film etc. not change the reflected version of plane of polarization is connected fibre optic phase modulator when terminal, and can effectively offset the influence of circular birefringence to light path; When adopting faraday rotator mirror as the light feedback assembly, can eliminate the birefringence of any reciprocity in the phase-modulator fully, system's antijamming capability is strong.Gou Zao feedback white light interference system thus, under the situation that does not change fiber lengths, phse sensitivity can double, and can be used for the extraction of weak physical signalling.And can realize the process of the uni-core bidirectional transmission of signal extraction, in distributed optical fiber sensing system, Fibre Optical Sensor group battle array, remote fiber sensor-based system, what show is particularly important.And adopt the light feedback assembly, and the birefringence that causes in the light path can be eliminated by system, and especially circular birefringence phenomenon is so system can be used for complexity, rugged environment.Adopt the feedback white light interference system of novel phase modulation technique can be applied in Fibre Optical Sensor, fiber optic communication field widely.

Description of drawings

Fig. 1 is a structural representation of the present invention.Wherein, fiber coupler 4 is 3 * 3 coupling mechanisms.

Fig. 2 is a structural representation of the present invention.Wherein, fiber coupler 4 is 3 * 3 fiber couplers.

Fig. 3 is a structural representation of the present invention.Wherein, fiber coupler 4 is 2 * 2 fiber couplers.

Fig. 4 is a structural representation of the present invention.Wherein, fiber coupler 4 is 2 * 2 fiber couplers.

Fig. 5 realizes phase modulation (PM) for the employing optical fiber of feedback white light interference system is wrapped on the elastic body, when adopting interference structure Fig. 1, and output interference signal and pumping signal when adopting the dextrorotation acoustic wave action.

Fig. 6 when not taking feedback assembly, the output interference signal and the pumping signal that record.

Number in the figure: the 1st, light source, the 2, the 3rd, photodetector, the 4th, fiber coupler, the 5th, fiber coupler, the 6th, the Intrinsical phase-modulator, the 7th, the light feedback assembly, the 8, the 9th, two ports of fiber coupler, 10 is 2 * 2 fiber couplers, the 11st, fibre delay line.

Embodiment

Further specifically describe the present invention below by embodiment.

In the present embodiment, used laser instrument is SO3-B type super radiation light emitting tube (SLD) the type stabilized light source (1) that 44 research institutes of main office of electronics group produce.Fiber coupler is the single-mode optical-fibre coupler that Wuhan post and telecommunications research institute produces.Photodetector (2) (3) is that 44 models of being produced are the InGaAs photodetector of GT322C500.Used optical fiber is " healthy and free from worry " G652 type single-mode fiber produced in USA.Adopt the audio signal generator of Agilent.Output signal is through amplifying.Light source is that the FC/PC wire jumper is connected with interference system, interference system with the connected mode of detector.Phase-modulator adopts optical fiber is wrapped on the elastic body, along with the realization phase modulation (PM) is shunk in the excitation of the outer sound pressure signal of elastic body down.The light feedback assembly adopts the fiber end face plated film to form the mode of catoptron.Adopt the mode of Fig. 1 to fuse, link together, the interference curve of realization as shown in Figure 5.

Simultaneously, if phase-modulator does not add feedback assembly, but the feedback end of Fig. 1 is directly received the port one 2 of coupling mechanism (5), adopt identical experiment condition among Fig. 5, the interference curve that obtains is Fig. 6.

As seen, feedback white light interference system gained interference curve Fig. 5 with do not adopt white light interference system gained interference curve Fig. 6 of light reaction type, the former has obtained number of interference fringes and has Duoed one times than the latter, shows as more crypto set of striped, and has improved signal to noise ratio (S/N ratio).

Claims (8)

1. a fiber phase modulator approach is characterized in that at the fiber end face of phase-modulator one smooth feedback assembly being set, and makes light wave successively pass through phase-modulator twice.

2,, it is characterized in that said smooth feedback assembly adopts the catoptron that forms at the fiber end face plated film, perhaps adopts faraday rotator mirror according to the described method of claim 1.

3, a kind of feedback white light interference system that constitutes based on method according to claim 1, it is characterized in that light path is as follows: light source (1) is fiber coupler (4) afterwards, after light is coupled device (4) beam split, the beam split of coupling mechanism port (8) is through chronotron (11), be coupling mechanism (5) afterwards, arrive fibre optic phase modulator (6), and by the reflection of the light feedback assembly (7) of phase-modulator end, feedback light is got back to the port (9) of coupling mechanism (4) by coupling mechanism (5); The beam split of coupling mechanism port (9) arrives phase-modulator (6) through coupling mechanism (5), and through reflecting surface (7) reflection, feedback light is got back to coupling mechanism (4) port (8) through chronotron (11); Two light beams form the light signal that carries the disturbing source physical features in fiber coupler (4), this signal is detected device and receives.

4, feedback white light interference system according to claim 3 is characterized in that described smooth feedback assembly is a level crossing, or faraday rotator mirror.

5, feedback white light interference system according to claim 3 is characterized in that coupling mechanism adopts taper coupler.

6, feedback white light interference system according to claim 3, it is characterized in that used laser instrument is that operation wavelength is semiconductor laser diode or the semiconductor light-emitting-diode laser instrument of 1.31 μ m or 1.55 μ m, or the super-radiance light emitting diode laser instrument, or wavelength is the gas laser of 850 μ m.

7, feedback white light interference system according to claim 3 is characterized in that used optical fiber or fibre delay line are single-mode fibers, or multimode optical fiber.

8, according to the described feedback white light interference system of claim 3, it is characterized in that the connected mode between being connected of coupling mechanism and optical fiber, the optical fiber is that the mode of fusing connects, being connected of light source and optical fiber, detector adopt wire jumper to connect with being connected of optical fiber.

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