CN103070669A - Light-spectrum phase calibration system and method based on cascade Mach-Zehnder interferometer - Google Patents

Abstract

The invention discloses a light-spectrum phase calibration system and a light-spectrum phase calibration method based on a cascade Mach-Zehnder interferometer. A first reference interference signal with optical path difference d1 close to maximum imaging depth of the system and a second reference interference signal with optical path difference d2 close to zero optical path are separated from a cascade MZI (Mach Zehnder Interferometer) interference spectrum signal through Fourier transform, filtering and inverse Fourier transform; the first reference interference signal is used for demarcating and calibrating a fractional part of a light spectrum phase jump at the maximum imaging depth part in real time during a wave-number sampling process; the second reference interference signal is used for determining 2pi integer multiples of the light spectrum phase jump at the maximum imaging depth part, wherein the phase jump caused by an initial wave-number jumping is in linear relation with the depth; and the practical phase jump at any depth can be calibrated by combining the two reference interference signals. According to the light-spectrum phase calibration system and the light-spectrum phase calibration method disclosed by the invention, not only is real-time linear calibration realized, but also calibration precision of the light-spectrum phase jump is ensured, so that a 2pi confusion problem of the light-spectrum phase jump calibration is solved, and the real-time high-flexibility phase detection and the reconstruction of phase images can be realized.

Description

A kind of spectrum phase calibration system and method based on the cascading Mach Zehnder interferometer

Technical field

The present invention relates to the frequency sweep optical coherence tomography of optical coherent chromatographic imaging (OCT) technology and phase sensitive, relate in particular to a kind of spectrum phase calibration system and method based on the cascading Mach Zehnder interferometer.

Background technology

Optical coherence tomography (Optical Coherence Tomography, abbreviation OCT) imaging technique is a kind of novel optical image technology, can carry out non-intruding, noncontact, high-resolution imaging in vivo to organizational structure and the physiological function of tested live body sample interior, at the early diagnosis of disease with in body biopsy field extensive application prospect.

The OCT technology of phase sensitive can be measured amplitude and the phase place of rear orientation light simultaneously, the nm level displacement that outside stimulus is produced by measuring tissue, relatively traditional OCT technology can provide phase place relevant function comparative information, and the aspects such as, photo-thermal OCT, Doppler OCT micro-in quantitative phase, magnetic modulation OCT are widely used.The OCT technology of phase sensitive can be divided into the time domain OCT technology of phase sensitive and the frequency domain OCT technology of phase sensitive according to the difference of detection mode.The sample rate of the time domain OCT system of phase sensitive lower (50-250A-scans/s) and displacement detectivity limited (DS=17000pm); The frequency domain OCT of phase sensitive greatly improves on displacement detectivity (DS=5000pm) and sample rate (10k A-scans/s), and the frequency domain OCT technology of phase sensitive comprises the spectral coverage OCT surveyed based on spectrogrph and based on the frequency sweep OCT of swept light source.The spectral coverage OCT of phase sensitive adopts linear array to survey, be difficult to adopt the balance detection mode, comparing its dynamic range of detection and signal to noise ratio is restricted, the in addition CCD Technology Restriction of long-wave response, the InGaAs CCD sensitivity of surveying long wave is not high and expensive, and the spectral coverage OCT of phase sensitive is mainly used in the 800nm wavelength band at present; The frequency sweep OCT of phase sensitive is because instantaneous live width swept light source can be avoided these problems with the associating of some detection mode, and light scattering in biological tissue of comparing shortwave owing to long wave is less with absorption, better penetration depth is arranged, at 1060nm and 1310nm wave band very large advantage is arranged.But the frequency sweep interference spectrum is in non-linear resolution and the signal to noise ratio that has had a strong impact on imaging of k spatial sampling, and the stability of swept light source spectral scan is brought very big challenge to sensitivity and the stability of phase measurement in addition.

Make the interference spectrum signal sampling point of frequency sweep OCT be evenly distributed in the method for wave number space, it is the real-time calibration method of interference spectrum k space uniform sampling, research institution both domestic and external has proposed a lot of effectively solutions, such as the in real time even frequency clock method based on MZI, based on interference spectrum phase calibration method of MZI etc., repeat no more here.Patent of the present invention is paid close attention to the calibration problem of swept light source spectrum phase.

In frequency sweep OCT system, the unstable wave number saltus step that shows adjacent A-scan of the spectrum of swept light source, the wave number saltus step is from two aspects, one is that the triggering of each A-scan is not relevant with some fixing wave numbers, and another is the uncertainty of time delay between sampling clock and triggering signal; The phase hit that these two kinds of wave number saltus steps cause is all linear with the degree of depth, and the phase hit that the former causes may surpass 2 π at maximum imaging depth place, and the phase hit that the latter causes can not surpass π at maximum imaging depth place.

For the phase detection sensitivity of the frequency sweep OCT technology that improves phase sensitive, external a lot of research institutions propose a lot of solutions.

The J.G.Fujimoto research group of masschusetts, u.s.a Polytechnics adopts buffering Fourier domain mode-locking (buffered Fourier domain mode-locked, FDML) thus frequency-sweeping laser source improves the phase detection sensitivity that the spectrum stability of light source guarantees system, but the stability that the polarization mode dispersion of thousands of meters long optical fibers rings and light path float to phase measurement in the FDML laser cavity produces greatly challenge.

Based on the method for optics produce sampling clock can improve between swept light source and data collecting card synchronously, reduce the phase hit that time delay causes, the J.A.Izatt group of Duck university sends the fraction light of light source output into the one-period optical filter, produce optical clock signal and convert it into suitable TTL sampled clock signal, because sweeping laser is not linear scanning in the k space, thereby the easy occurrence frequency of sampling clock that produces is warbled, beginning a sampling clock frequency at the end of a frequency sweep cycle and next frequency sweep cycle changes arbitrarily, so between light generation clock and high-speed data acquisition card, have compatibling problem, need to insert phase-locked loop circuit in external sampling clock input.

The people such as the B.J.Vokoc of Harvard Medical School are by the additional calibration mirror generation calibration interference signal of an optical path difference near maximum imaging depth, and the phase place between the adjacent A-scan of Measurement and calibration interference signal changes, from the phase contrast of the sample signal that records, deduct the actual phase difference that the phase contrast of the calibrating signal that records obtains proofreading and correct by depth scale, but when initial wave number changes greatly, when causing the phase hit at maximum imaging depth place above 2 π, the spectrum phase calibration that the method produces may produce mistake, thereby generally only is used for calibrating the phase noise that the uncertainty of time delay between sampling clock and triggering signal causes.

The people such as R.K.Manapuram of Houston university utilize narrow band fiber Bragg grating (fiber Bragggrating, FBG, centre wavelength 1315nm, bandwidth 0.1nm) produces tunable TTL signal and come the Dynamic trigger Data Acquisition Card, thereby between light source and data acquisition, introduce perfect synchronization, reduced the phase hit noise that the sampling time delay causes.The accuracy of the method depends on the stability of FBG operation wavelength, and can't determine that initial wave number changes the phase hit that causes and whether surpasses 2 π.

The people such as R.V.Kuranov at Texas university health science center propose the phase sensitive frequency sweep OCT method of gas compartment reference, its system has adopted four interferometers, comprises that the main signal interferometer on common road, the common LUSHEN that triggers interferometer and the saltus step of calibration wave number based on the real-time clock interferometer of MZI, based on the spectrum of gas compartment examine interferometer.The method hardware configuration is complicated, the data capture card is required high, and the synchronous error of data acquisition may cause phase noise between the different data acquisition card, and 2 π that the calibration interference signal can not be got rid of phase hit obscure problem.

The people such as E.D.Moore of Colorado university propose the frequency sweep phase sensitive interferometer of self-reference and measure absolute distance, realize absolute distance measurement by the accurate calibration to the interferometer sampling interval, thereby it utilizes additional interferometer to monitor the instantaneous frequency real time calibration sampling interval of swept light source, but the frequency monitoring precision of additional interferometer need to be calibrated by the wavelength reference device of high spectral accuracy (0.1pm magnitude).

All there is intrinsic shortcoming in above-mentioned these methods, need to introduce complicated device, and 2 π that can not solve the spectrum phase saltus step obscure problem, therefore, be necessary that research is easy to realize, the high and good calibration steps of spectrum phase on a large scale of accuracy of phase hit calibration accuracy.

Summary of the invention

The object of the present invention is to provide a kind of spectrum phase calibration system and method based on the cascading Mach Zehnder interferometer.In the calibration arm of the frequency sweep optical coherence tomography system of phase sensitive, the cascading Mach Zehnder interferometer is set, by calculating two different fixed light path differences that the cascade Mach-Zehnder interferometer provides with reference to the phase contrast of the separately adjacent A-scan of interference signal, the OCT interference signal is carried out the spectrum phase calibration.

The objective of the invention is to be achieved through the following technical solutions:

A kind of spectrum phase calibration system based on the cascading Mach Zehnder interferometer comprises swept light source, broadband optical fiber coupler, spectrum phase calibration arm, OCT main interference instrument, OCT interference spectrum signal detecting device, data collecting card, computer.The low-coherent light that swept light source is sent enters respectively spectrum phase calibration arm and OCT main interference instrument after the first broadband optical fiber coupler light splitting, the sample arm of described OCT main interference instrument and reference arm be the road altogether; The sample arm on described altogether road and reference arm comprise band optical fiber circulator, collimating mirror, scanning galvanometer, condenser lens, beam-splitter or simultaneously plate thin Cover Glass and the sample of anti-reflection film, the second broadband optical fiber coupler the first outfan links to each other with the first end of band optical fiber circulator, the light of the second end output of band optical fiber circulator is behind collimating mirror, scanning galvanometer and condenser lens, be incident upon on the sample, light and the backward scattered light of sample of beam-splitter or the reflection of Cover Glass reference plane converge rear interference; Described OCT interference spectrum signal detecting device is the balance detection device, light and the sample rear orientation light of reference plane reflection converge rear formation OCT interference spectrum signal, enter an input of the 3rd broadband optical fiber coupler by the 3rd end of band optical fiber circulator, another input of the 3rd broadband optical fiber coupler links to each other with the second outfan of the second broadband optical fiber coupler, the two-way outfan of the 3rd wide-band coupler links to each other with the two-way input of balance detection device respectively, and the signal that the balance detection device is surveyed carries out data acquisition and date processing through data collecting card and computer.Described spectrum phase calibration arm is the cascading Mach Zehnder interferometer, is that 2 * 2 single-mode optical-fibre couplers and the cascading Mach Zehnder interferometer balance detection device of 1:1 forms linking arm l by three splitting ratios ₁, l ₂First bonder and second bonder are linked together linking arm l ₃, l ₄Second and the 3rd bonder are linked together, and the 3rd bonder is connected with cascading Mach Zehnder interferometer balance detection device.Low-coherent light after the first broadband optical fiber coupler light splitting can form one road MZI spectrum phase calibrating signal through the cascading Mach Zehnder interferometer, and it is d that optical path difference is provided simultaneously ₁First be d with reference to interference signal and optical path difference ₂Second with reference to interference signal, carry out data acquisition and date processing through data collecting card and computer.

A kind of spectrum phase calibration steps based on the cascading Mach Zehnder interferometer:

In the frequency sweep optical coherence tomography system, the light that swept light source is sent is told a part and is entered the Mach-Zehnder interferometer (MZI) that is comprised of three 2 * 2 single-mode optical-fibre coupler cascades, the interference signal that produces obtains MZI interference spectrum signal through balance detection, with frequency sweep optical coherence tomography (SSOCT) signal through the data collecting card synchronous acquisition.In computer, at first by Fourier transform, filtering and inverse Fourier transform MZI interference spectrum signal is isolated optical path difference d ₁Near the maximum imaging depth of system first with reference to interference signal and optical path difference d ₂Approach zero light path second with reference to interference signal.Then utilize first with reference to interference signal each road interference signal to be demarcated based on traditional spectrum calibration method, obtain respectively etc. wave number SSOCT signal (sample signal), first spaced apart with reference to interference signal and second with reference to interference signal.This three road signals wave number sample rate is identical, and phase hit and signal depth that the uncertainty of initial wave number causes are linear.Change with reference to the phase place between the adjacent A-scan of interference signal by calculating two-way, second changes 2 π integral multiple information of the phase hit that is used for determining that the maximum imaging depth place's light source wave number saltus step of system may cause with reference to the phase place of interference signal, and first change fractional part (the being no more than 2 π) information of the phase hit that is used for determining that the maximum imaging depth place's light source wave number saltus step of system may cause with reference to the phase place of interference signal, combination can be proofreaied and correct the large-scale phase hit in any imaging depth place according to the depth line sexual relationship, obtains the actual bit phase detectivity near ideal bit phase detectivity.Owing to adopted cascade MZI interferometer, the reference interference signal of two fixed light path differences can be provided simultaneously, both realized the real-time linear calibration of spectrum, guaranteed again the calibration accuracy of spectrum phase saltus step, 2 π that solved simultaneously spectrum phase saltus step calibration obscure problem, can realize the reconstruction of real-time, high-sensitive phase measurement and position phase images.The concrete steps of the method are as follows:

Step 1, in the frequency sweep optical coherence tomography system of phase sensitive, cascading Mach Zehnder interferometer (MZI) is set, linking arm l is set ₁, l ₂, l ₃And l ₄Length so that (l ₂-l ₁)+(l ₄-l ₃)=d ₁Near the maximum imaging depth of system, and (l ₂-l ₁)-(l ₄-l ₃)=d ₂Approach zero light path, then the spectrum interference signal of cascade MZI generation is that optical path difference is d ₁, d ₂The stack of two interference signals.

Step 2, spectrum interference signal and SSOCT spectrum interference signal by data collecting card synchronizing detection cascade MZI are transferred to and carry out date processing in the calculator memory;

Step 3, process by the spectrum interference signal of cascade MZI being carried out Fourier transform and Windowed filtering, obtaining optical path difference is d ₁And d ₂The spatial spectrum of interference signal, obtaining optical path difference through inverse Fourier transform again is d ₁First be d with reference to interference signal and optical path difference ₂Second with reference to interference signal;

Step 4, utilize first with reference to interference signal the SSOCT spectrum interference signal that collects and two-way to be carried out real time spectrum with reference to interference signal and demarcate, obtain sampled point at the equally distributed interference signal of wave number space;

Step 5, by phase extraction algorithms and the different phase hits that cause of initial wave number that relatively calculate with reference to the adjacent A-scan of interference spectrum signal, wherein

The first phase hit that provides with reference to interference signal is provided,

The second phase hit that provides with reference to interference signal is provided, then the phase hit of any depth of sample interference signal can be proofreaied and correct by following formula:

Wherein,

Be sample interference signal depth z _sThe phase contrast at place, n is the mean refractive index of sample.

Compare with background technology, the beneficial effect that the present invention has is:

1, adopted compact cascade MZI interferometer, the reference interference signal of two fixed light path differences can be provided simultaneously, improved the precision that initial wave number is proofreaied and correct, 2 π that solved again spectrum phase saltus step calibration obscure problem, have guaranteed calibration accuracy and the precision of spectrum phase saltus step.

2, do not need accurately Optical Clock triggering, only utilize minimum interferometer to realize simultaneously the calibration of the real-time linear calibration of spectrum and spectrum phase saltus step, alleviated the port pressure of data collecting card, saved the system hardware cost.

3, do not increase in the situation of extra interferometer, in real time each interference signal is carried out the equifrequent interval and demarcate, compensated frequency sweep spectrum not repeated that each frequency sweep cycle is counted the place, guaranteed higher axial resolution.

Description of drawings

Fig. 1 is phase sensitive frequency sweep optical coherence tomography system sketch map of the present invention;

Fig. 2 is cascading Mach Zehnder interferometer structural representation of the present invention;

Fig. 3 is phase sensitive frequency sweep optical coherence tomography system hardware controls structural representation of the present invention;

Fig. 4 (a) is original cascade MZI interference spectrum signal;

Fig. 4 (b) is process fft and filtered signal;

Fig. 4 (c) is that first after separating is with reference to interference signal;

Fig. 4 (d) is that second after separating is with reference to interference signal;

Fig. 4 (e) is for utilizing first to carry out wave number with reference to interference signal and demarcate, and waits until etc. that the wave number interval divides

Cloth first with reference to interference signal;

Fig. 4 (f) is for utilizing first to carry out wave number with reference to interference signal and demarcate, wait until etc. wave number spaced apart second with reference to interference signal;

Fig. 4 (g) is for utilizing first to carry out wave number with reference to interference signal and demarcate, and the wave number sample spaced apart/OCT signal such as waits until;

The schematic diagram of Fig. 5 for utilizing simultaneously two to carry out spectrum phase saltus step calibration with reference to interference signal;

Fig. 6 (a) changes for the unwrapped phase of the front OCT signal of spectrum phase saltus step calibration;

Fig. 6 (b) is d for optical path difference ₁The spectrum phase saltus step of reference interference signal;

Fig. 6 (c) is d for optical path difference ₂The spectrum phase saltus step of reference interference signal;

Light path is d to Fig. 6 (d) after the interference signal conjoint analysis for Radix Triplostegiae Grandiflorae is examined ₁The real spectrum phase hit of reference interference signal;

The unwrapped phase of OCT signal changes after Fig. 6 (e) spectrum phase saltus step calibration;

Among the figure: 1, swept light source, 2, the first broadband optical fiber coupler, 3, cascading Mach Zehnder interferometer, the 4, second broadband optical fiber coupler, 5, the band optical fiber circulator, 6, collimating mirror, 7, scanning galvanometer, 8, condenser lens, 9, beam-splitter or thin Cover Glass, 10, sample, the 11, the 3rd broadband optical fiber coupler; 12, balance detection device, 13, data collecting card, 14, computer, 15, cascading Mach Zehnder interferometer 2 * 2 broadband optical fiber couplers, 16,2 * 2 broadband optical fiber couplers, 17,2 * 2 broadband optical fiber couplers, 18, cascading Mach Zehnder interferometer balance detection device.

The specific embodiment

The present invention is further illustrated below in conjunction with drawings and Examples.

As shown in Figure 1 and Figure 2, a kind of spectrum phase calibration system based on the cascading Mach Zehnder interferometer comprises swept light source, broadband optical fiber coupler, spectrum phase calibration arm, OCT main interference instrument, OCT interference spectrum signal detecting device, data collecting card and computer.The low-coherent light that swept light source 1 is sent enters respectively spectrum phase calibration arm and OCT main interference instrument after 2 light splitting of the first broadband optical fiber coupler, the sample arm of described OCT main interference instrument and reference arm be the road altogether; The sample arm on described altogether road and reference arm comprise band optical fiber circulator 5, collimating mirror 6, scanning galvanometer 7, condenser lens 8, beam-splitter or simultaneously plate thin Cover Glass 9 and the sample 10 of anti-reflection film, the light of the second broadband optical fiber coupler 4 first outfans enters the first end of band optical fiber circulator 5, behind the second end, collimating mirror 6, scanning galvanometer 7 and the condenser lens 8 of band optical fiber circulator 5, be incident upon on the sample 10, light and the backward scattered light of sample of beam-splitter or the reflection of Cover Glass 9 reference plane converge rear interference; Described OCT interference spectrum signal detecting device is balance detection device 12, light and the sample rear orientation light of reference plane reflection converge rear formation SSOCT interference spectrum signal, enter an input of the 3rd broadband optical fiber coupler 11 by the 3rd end of band optical fiber circulator 5, another input of the 3rd broadband optical fiber coupler 11 links to each other with the second outfan of the second broadband optical fiber coupler 4, the two-way outfan of the 3rd wide-band coupler 11 links to each other with the two-way input of balance detection device 12 respectively, and the signal that the balance detection device is surveyed carries out data acquisition and date processing through data collecting card 13 and computer 14.Described spectrum phase calibration arm is cascading Mach Zehnder interferometer 3, is that 2 * 2 single-mode optical-fibre couplers and the cascading Mach Zehnder interferometer balance detection device 18 of 1:1 forms linking arm l by three splitting ratios ₁, l ₂ First bonder 15 and second bonder 16 are linked together linking arm l ₃, l ₄ Second bonder 16 and the 3rd bonder 17 are linked together, the 3rd bonder 17 is connected with cascading Mach Zehnder interferometer balance detection device 18, low-coherent light after the first broadband optical fiber coupler light splitting can form one road MZI spectrum phase calibrating signal through the cascading Mach Zehnder interferometer, and it is d that optical path difference is provided simultaneously ₁First be d with reference to interference signal and optical path difference ₂Second with reference to interference signal, carry out data acquisition and date processing through data collecting card and computer.

Figure 3 shows that frequency sweep optical coherence tomography system hardware controls structural representation of the present invention.Comprise: scanning galvanometer 7, balance detection device 12 and 18, data collecting card 13 and computer 14.Wherein data collecting card 13 links to each other with computer 14, and computer 14 links to each other with scanning galvanometer 7.Computer 14 control 13 pairs of balance detection devices 12 of data collecting card and the 18 interference spectrum signals that produce gather, and import computer 14 into by data/address bus and carry out date processing and image reconstruction.Computer 14 Quality control arm scanning galvanometers 7 realize that detecting light beam is to the transversal scanning of sample, with two dimension and the three-dimensional information data of obtaining sample.

Shown in Fig. 4 (a) and (b), (c), (d), (e), (f), (g), a kind of spectrum phase calibration steps based on the cascading Mach Zehnder interferometer, in the frequency sweep optical coherence tomography system of phase sensitive, cascading Mach Zehnder interferometer (MZI) is set, linking arm l is set ₁, l ₂, l ₃And l ₄Length so that (l ₂-l ₁)+(l ₄-l ₃)=d ₁Near the maximum imaging depth of system, and (l ₂-l ₁)-(l ₄-l ₃)=d ₂Approach zero light path, then the spectrum interference signal of cascading Mach Zehnder interferometer generation is that optical path difference is d ₁, d ₂The stack of two interference signals.

Synchronous acquisition optical coherence tomography interference spectrum signal and cascading Mach Zehnder interferometer interference spectrum signal, for ease of understanding initial wave number change to the impact of phase measurement, only consider that from sample depth z place reflectance be certain layer the interference fringe of R, then can be expressed as of the OCT interference spectrum signal of single axial scan (A-scanning) detection:

$i\left(t\right)\∝S\left(k\left(t\right)\right)\sqrt{R}\cos \left[2k\left(t\right)(z+\mathrm{\δz}\left(t\right))\right]---\left(1\right)$

Wherein, S (k) represents the spectral density function of swept light source, and k (t) represents time dependent wave number, and it is not the linear function of time t generally speaking.The optical path difference of corresponding reference arm sample arm is 0 when z representative sample depth coordinate and z=0.δ z (t) represents time resolution change in optical path length at depth z place, and we can't measure to note the variation of δ z (t) between an A-scan, and what General System will be surveyed is time resolution light path variable quantity of adjacent A-scan.

The analysis of cascading Mach Zehnder interferometer interference spectrum signal can be based on the collision matrix of melting cone type single-mode fiber directional coupler, S ₂Be the collision matrix of 2 * 2 broadband optical fiber couplers in the cascading Mach Zehnder interferometer, if the splitting ratio of 2 * 2 broadband optical fiber couplers is 1:1, then S ₂Can be expressed as respectively:

${\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="HDA00002749917300011.png,HDA00002749917300012.png"\; state="\{\{state\}\}">S</figure-callout>}}_2=\frac{1}{\sqrt{2}}\left[\begin{array}{cc}1& -\mathrm{expi}\frac{\mathrm{\&pi;}}{2}\\ -\mathrm{expi}\frac{\mathrm{\&pi;}}{2}& 1\end{array}\right]---\left(2\right)$

For a simple delay line system, its transmission matrix can be expressed as (has ignored common phase factor exp (ikl ₁)):

$H_{\mathrm{delay}}=\left[\begin{array}{cc}1& 0\\ 0& \exp (-\mathrm{ik}(l_2-l_1))\end{array}\right]---\left(3\right)$

Suppose that the input light field that enters into the cascading Mach Zehnder interferometer from broadband optical fiber coupler (11) is

The light symbol that this input light field is exported by 2 * 2 broadband optical fiber couplers at last through twice interference in interferometer

With

Expression is by S ₂And H _DeldyWrite out output light field

With

Expression formula be:

$\left[\begin{array}{c}{\mathrm{<figure-callout\; id="3"\; label="E"\; filenames="HDA00002749917300011.png"\; state="\{\{state\}\}">E</figure-callout>}}_3^{\mathrm{out}}\\ {\mathrm{<figure-callout\; id="4"\; label="E"\; filenames="HDA00002749917300011.png"\; state="\{\{state\}\}">E</figure-callout>}}_4^{\mathrm{out}}\end{array}\right]=S_2{H}_{\mathrm{delay}}^1{S}_2{H}_{\mathrm{delay}}^2{S}_2\left[\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">E</figure-callout>}}_1^{\mathrm{in}}\\ 0\end{array}\right]$

$=\frac{1}{2\sqrt{2}}\left[\begin{array}{cc}1& -i\\ -\mathrm{<figure-callout\; id="1"\; label="i"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">i</figure-callout>}& 1\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& \exp (-i{\mathrm{\&theta;}}_1)\end{array}\right]\left[\begin{array}{cc}1& -i\\ -\mathrm{<figure-callout\; id="1"\; label="i"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">i</figure-callout>}& 1\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& \exp (-i{\mathrm{\&theta;}}_2)\end{array}\right]\left[\begin{array}{cc}1& -i\\ -i& 1\end{array}\right]\left[\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">E</figure-callout>}}_1^{\mathrm{in}}\\ 0\end{array}\right]---\left(4\right)$

Wherein, θ ₁=k (l ₂-l ₁), θ ₂=k (l ₄-l ₃) represent two transmission phase contrasts of interfering arms.Ignored common exp (ikl in the formula (4) ₁) and exp (ikl ₃).The light intensity signal that balance detection device (18) is surveyed can be expressed as:

$P={\mathrm{<figure-callout\; id="4"\; label="E"\; filenames="HDA00002749917300011.png"\; state="\{\{state\}\}">E</figure-callout>}}_4^{\mathrm{out}}{\mathrm{<figure-callout\; id="4"\; label="E"\; filenames="HDA00002749917300011.png"\; state="\{\{state\}\}">E</figure-callout>}}_4^{\mathrm{out}*}-E_5^{\mathrm{out}}{E}_5^{\mathrm{out}*}$

$=\frac{1}{2}[\cos ({\mathrm{\&theta;}}_1+{\mathrm{\&theta;}}_2)-\cos ({\mathrm{\&theta;}}_1-{\mathrm{\&theta;}}_2)]{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">E</figure-callout>}}_1^{\mathrm{in}}{\mathrm{<figure-callout\; id="1"\; label="E"\; filenames="HDA00002749917300023.png"\; state="\{\{state\}\}">E</figure-callout>}}_1^{\mathrm{in}*}---\left(5\right)$

Again because (l ₂-l ₁)+(l ₄-l ₃)=d ₁, (l ₂-l ₁)-(l ₄-l ₃)=d ₂, d ₁Near the maximum imaging depth of system, d ₂Near zero optical path difference, then

$P\&Proportional;\frac{1}{2}(\cos {\mathrm{kd}}_1-\cos {\mathrm{kd}}_2)S\left(k\right)---\left(6\right)$

Can find out that from formula (6) the interference spectrum signal that spectrum phase calibration interferometer is surveyed comprises that optical path difference is respectively d ₁, d ₂Two cross-correlation items, this interference spectrum signal is carried out Fourier transform can obtain plural axial space signal, then carry out filtering, eliminate mirror shaft to spacing wave and separate two cross-correlation items, arrive spectral space through inverse Fourier transform again, namely can obtain respectively including only the complex signal of an interference term spectrum, namely optical path difference is d ₁First be d with reference to interference signal and optical path difference ₂Second with reference to interference signal; The first real part with reference to interference signal is equivalent to traditional MZI Real-Time Optical clock signal, utilizing this signal that OCT signal, first is carried out respectively real-time wave number space homogenization with reference to interference signal and second with reference to interference signal demarcates, obtain respectively etc. wave number OCT signal (sample signal), first spaced apart with reference to interference signal and second with reference to interference signal, the OCT signal can be expressed as through steps such as DC terms, self correlation item Transformatins again:

$I_{\mathrm{OCT}}\left(k\left(n\right)\right)\&Proportional;S\left(k\left(n\right)\right)\sqrt{R}\cos \left[2({\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002749917300043.png,HDA00002749917300044.png"\; state="\{\{state\}\}">k</figure-callout>}}_0+\mathrm{\&alpha;}(n+\mathrm{\&epsiv;}))(z+\mathrm{\&delta;z})\right]---\left(7\right)$

Etc. spaced apart first can being expressed as with reference to interference signal and the second complex signal with reference to interference signal of wave number:

$I_1\left(k\left(n\right)\right)=2S\left(k\left(n\right)\right)\sqrt{R_{R1}{R}_S}\&CenterDot;\exp \left(j\right[{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002749917300043.png,HDA00002749917300044.png"\; state="\{\{state\}\}">k</figure-callout>}}_0+\mathrm{\&alpha;}(n+\mathrm{\&epsiv;})]\&CenterDot;d_1)---\left(8\right)$

$I_2\left(k\left(n\right)\right)=2S\left(k\left(n\right)\right)\sqrt{R_{R2}{R}_S}\&CenterDot;\exp \left(j\right[{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002749917300043.png,HDA00002749917300044.png"\; state="\{\{state\}\}">k</figure-callout>}}_0+\mathrm{\&alpha;}(n+\mathrm{\&epsiv;})]\&CenterDot;d_2)---\left(9\right)$

K (n)=k wherein ₀+ α n, ε are the time delay between triggering signal and sampling clock, and it may be 0 and the sampling clock cycle T _DlBetween the change.The demarcation of real time spectrum space uniform can guarantee that the wave number linear sweep rate α of each A-scan is consistent, but the triggering of swept light source is not related with some fixed wave numbers, i.e. the initial wave number k of each A-scan ₀Difference is because k ₀Difference causes that adjacent A-scan is at the phase hit at depth z place

May surpass 2 π.From formula (7)～(9) as can be known, no matter initial wave number changes or the spectrum phase saltus step that the sampling clock delay variation causes is all linear with the degree of depth.Fig. 4 has provided the major part of the signal processing flow of formula (1)～(9).

Fig. 5 has explained and has utilized simultaneously optical path difference d ₁Near maximum imaging depth first with reference to interference signal and optical path difference d ₂Approach the second reason with reference to interference signal of zero light path.At same spectrum saltus step Δ k ₀Lower, the second phase hit with reference to the adjacent A-scan of interference signal

Can not surpass 2 π, and the first phase hit with reference to the adjacent A-scan of interference signal

May surpass 2 π; And under same phase noise level, directly utilize the second phase hit with reference to interference signal

Multiply by depth line sexual relationship z _s/ d ₂, phase calibration Noise will amplify z _s/ d ₂Doubly; And directly utilize the first phase hit with reference to interference signal

Multiply by depth line sexual relationship z _s/ d ₁, the noise of phase calibration is minimum, and is larger but the 2 π integral multiples that may exist are obscured the phase calibration error of bringing.Therefore need two phase hits with reference to interference signal of analysis-by-synthesis.

Concrete phase hit bearing calibration is as follows: formula (3) is carried out Hilbert transform and appropriate phase solution coiling algorithm can obtain the OCT signal and be changed in the phase place of the adjacent A-scan in depth z place

First has been plural form with reference to interference signal and second with reference to interference signal, can directly ask phase place to the reference interference signal in formula (8) and (9), utilizes phase unwrapping to obtain the phase mass that each A-scan changes with k around algorithm.Optical path difference with reference to interference signal is fixed, even the phase place of each wave number is different, the phase angle variations of the adjacent A-scan that each wave number is corresponding in principle should be identical, all be that the spectrum phase that the saltus step of time-out wave number causes changes, be to remove the window truncation effect of phase calculation and improve the computational accuracy of phase jumping, can compare the phase hit of the adjacent A-scan of each wave number and get in the middle of the phase contrast of one section wave number on average obtain first with reference to interference signal and the second phase hit with reference to interference signal

With

Then the phase hit of any depth of sample interference signal can be proofreaied and correct by following formula:

Fig. 6 (a) is that the phase place of the adjacent A-scan of OCT signal that records of typical case changes, and Fig. 6 (b) is the first phase contrast with reference to the adjacent A-scan of interference signal, and Fig. 6 (c) is the second phase contrast with reference to the adjacent A-scan of interference signal, Fig. 6 (d) for combination after depth d ₁The phase contrast that the place is actual, Fig. 6 (e) are the phase place variation based on the adjacent A-scan in depth z place after the spectrum phase calibration of cascading Mach Ceng Deyi.The contrast of Fig. 6 (a) and Fig. 6 (e) obviously show system bits mutually detectivity be greatly improved.

The invention discloses the more accurate method and system of a kind of spectrum phase based on the cascading Mach Zehnder interferometer, realize that under the prerequisite of guaranteeing acquisition of signal resolution spectrum phase is accurate, do not need accurately Optical Clock triggering and any complicated spectrum phase than standard apparatus, 2 π that solved spectrum phase saltus step calibration obscure problem, calibration accuracy and the precision of spectrum phase saltus step have been guaranteed, realized high-sensitive real-time phase measurement, in phase sensitive frequency sweep Optical coherence tomography, very strong practicality is arranged, be equally applicable to the phase-resolved Doppler blood flow measurement based on the frequency sweep Optical coherence tomography, the application such as photo-thermal phase information detection.

Claims (3)

1. spectrum phase calibration system based on the cascading Mach Zehnder interferometer, comprise light source, broadband optical fiber coupler, spectrum phase calibration arm, OCT main interference instrument, OCT interference spectrum signal detecting device, data collecting card, computer, it is characterized in that: the low-coherent light that swept light source (1) is sent enters the first broadband optical fiber coupler (2), enter respectively spectrum phase calibration arm and OCT main interference instrument after light splitting, the sample arm of described OCT main interference instrument and reference arm be the road altogether; Described altogether sample arm and the reference arm on road comprise band optical fiber circulator (5), collimating mirror (6), scanning galvanometer (7), condenser lens (8), beam-splitter or one side are plated thin Cover Glass (9) and the sample (10) of anti-reflection film, the second broadband optical fiber coupler (4) first outfans link to each other with the first end of band optical fiber circulator (5), the light of band optical fiber circulator (5) second ends is through collimating mirror (6), behind scanning galvanometer (7) and the condenser lens (8), be incident upon on the sample (10), light and the backward scattered light of sample of the reflection of beam-splitter or Cover Glass (9) reference plane converge rear interference; Described OCT interference spectrum signal detecting device is balance detection device (12), light and the sample rear orientation light of reference plane reflection converge rear formation OCT interference spectrum signal, enter an input of the 3rd broadband optical fiber coupler (11) by the 3rd end of band optical fiber circulator (5), another input of the 3rd broadband optical fiber coupler (11) links to each other with the second outfan of the second broadband optical fiber coupler (4), the two-way outfan of the 3rd wide-band coupler (11) links to each other with the two-way input of balance detection device (12) respectively, the signal that balance detection device (12) is surveyed carries out data acquisition and date processing through data collecting card (13) and computer (14), low-coherent light after the first broadband optical fiber coupler (2) light splitting is through the spectrum phase calibration arm, described spectrum phase calibration arm comprises cascading Mach Zehnder interferometer (3), described cascading Mach Zehnder interferometer (3) can form one road MZI spectrum phase calibrating signal, provides simultaneously optical path difference to be

First with reference to interference signal and optical path difference be

Second with reference to interference signal, carry out data acquisition and date processing through data collecting card and computer.

2. a kind of spectrum phase calibration system based on the cascading Mach Zehnder interferometer according to claim 1, it is characterized in that: described spectrum phase calibration arm is the cascading Mach Zehnder interferometer, be that 2 * 2 single-mode optical-fibre couplers and the cascading Mach Zehnder interferometer balance detection device of 1:1 forms linking arm by three splitting ratios

,

First bonder (15) and second bonder (16) are linked together linking arm

,

Second bonder (16) and the 3rd bonder (17) are linked together, and the 3rd bonder (17) is connected with cascading Mach Zehnder interferometer balance detection device (18).

3. spectrum phase calibration steps based on the cascading Mach Zehnder interferometer is characterized in that the method may further comprise the steps:

Step 1, in the frequency sweep optical coherence tomography system of phase sensitive, cascading Mach Zehnder interferometer (MZI) is set, linking arm is set

,

,

With Length so that

Near the maximum imaging depth of system, and

Approach zero light path, then the spectrum interference signal of cascading Mach Zehnder interferometer generation is that optical path difference is

,

The stack of two interference signals;

Step 2, spectrum interference signal and SSOCT spectrum interference signal by data collecting card synchronizing detection cascading Mach Zehnder interferometer are transferred to and carry out date processing in the calculator memory;

Step 3, process by the spectrum interference signal of cascading Mach Zehnder interferometer being carried out Fourier transform and Windowed filtering, obtain optical path difference and be With

The spatial spectrum of interference signal, obtain optical path difference through inverse Fourier transform again and be

First with reference to interference signal and optical path difference be

Second with reference to interference signal;

Step 4, utilize first with reference to interference signal the SSOCT spectrum interference signal that collects and two-way to be carried out real time spectrum with reference to interference signal and demarcate, obtain sampled point at the equally distributed interference signal of wave number space;

Step 5, by phase extraction algorithms and the different phase hits that cause of initial wave number that relatively calculate with reference to the adjacent A-scan of interference spectrum signal, wherein

The first phase hit that provides with reference to interference signal is provided,

The second phase hit that provides with reference to interference signal is provided, then the phase hit of any depth of sample interference signal can be proofreaied and correct by following formula:

，

Wherein,

Be the sample interference signal degree of depth The phase contrast at place,

Mean refractive index for sample.

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