Multi-parameter ocular axial measurement system based on self-mixing vernier effect

Abstract

The application discloses a multi-parameter eye axis measurement system based on self-mixing vernier effect and relates to the field of optical fibers. A light beam emitted by a low-coherence light source is divided into a fiber reference light and a fiber measurement light through a coupler; the fiber measurement light is output as spatial measurement light through a first circulator, and the spatial measurement light scans an eyeball through a zoom lens; the fiber measurement light enters a fiber delay line through a second circulator and a wavelength division multiplexer and is output from a reflection end of the second circulator after reflection; the spatial measurement light is output as first low-coherence light from a reflection end of the first circulator after reflection by the eyeball; sample measurement light is output as second low-coherence light from the reflection end of the second circulator; the two beams of low-coherence light interfere through a second coupler; a light beam emitted by a high-coherence light source is self-mixed in a phase shift grating to serve as a scale for low-coherence measurement, and the refractive power of the eyeball is calculated by collecting a Hartmann array diagram. According to the scheme, the data signal-to-noise ratio of the measurement data is higher, and the data is more accurate.

Description

TECHNICAL FIELD

[0001] Embodiments of the present application relate to the field of optical fiber technology, and particularly relate to a multi-parameter eye axis measurement system based on self-mixing vernier effect. BACKGROUND

[0002] At present, human health problems gradually become the focus of the whole society, among which ophthalmic problems are particularly valued. Most of the ophthalmic problems can be reflected from the eye axial parameters and the refractive power. The monitoring of the eye axial parameters and the refractive power can help ophthalmic experts to understand the changes of the eyeball structure in time, and guide the treatment and management of related diseases. The eye axial parameters mainly include the axial length, the lens thickness, the corneal thickness and the anterior chamber depth, etc. The low coherence light interference technology, which combines the confocal microscope technology and the optical coherence tomography (OCT) technology with the advantages of white light coherence imaging, can realize the non-invasive measurement of the micron-level resolution in the biological tissue, effectively prevent the occurrence of secondary infection, etc., and has gradually become the mainstream technical solution for the non-contact and high-precision eye axial parameter measurement.

[0003] However, most of the related art eye axial biological measurement instruments adopt a spatial light path structure. The detection of the distance between the measured low coherence peaks needs to use a clock system to calibrate the position of the scanning delay line, which limits the utilization rate of the scanning light beam and the integration degree of the system, and affects the precision measurement of the signal-to-noise ratio and the sensitivity. Finally, the tested eyeball parameters cannot guarantee the high precision requirement. SUMMARY

[0004] Embodiments of the present application provide a multi-parameter eye axis measurement system based on self-mixing vernier effect, to solve the problem of inaccurate measurement of the traditional measurement system. The system comprises:

[0005] a low coherence light source, a light beam emitted by the low coherence light source is divided into a fiber reference light and a fiber measurement light through a first coupler; the fiber measurement light passes through a first circulator and a collimator to output a spatial measurement light, the spatial measurement light focuses a scanning light beam at different parts of the eyeball through a zoom lens; the fiber measurement light passes through a second circulator and a wavelength division multiplexer to enter a fiber delay line, and is output from the reflection end of the second circulator after reflection by the fiber delay line;

[0006] a high coherence light source, a light beam emitted by the high coherence light source passes through a doped fiber with a phase shift grating, and is reflected by a wavelength division multiplexer and a fiber delay line to generate self-mixing interference at the phase shift grating, so as to calibrate the low coherence peak distance; the spatial measurement light is reflected by the eyeball and then passes through the collimator and the first circulator to output the first low coherence light from the reflection end of the first circulator; the sample measurement light is output from the second circulator; two beams of low coherence light generate interference through a second coupler, and the interference data is received by a balanced photodetector, and the eyeball data is obtained by combining the low coherence peak distance measurement;

[0007] The spatial light reflected by the eyeball is incident on the Hartmann micro-lens group after focusing on the CCD light-sensitive surface, and the ocular refraction is calculated by the Hartmann array diagram collected by the Hartmann micro-lens group.

[0008] Specifically, a two-dimensional galvanometer is arranged in the spatial light path between the zoom lens and the eyeball, and the spatial light is focused on different regions of the eyeball through the two-dimensional galvanometer.

[0009] The two-dimensional galvanometer includes X galvanometer and Y galvanometer perpendicular to each other, and the light ray completes one cycle of X-Scan scanning through the X galvanometer, is reflected to the Y galvanometer, and then is deflected for multiple X-Scan scanning, so as to focus the light beam on the lens, the vitreous membrane, the fundus and the anterior segment interface of the eye tissue to be measured, and to perform two-dimensional scanning.

[0010] Specifically, a beam splitter and a mirror are arranged in the spatial light path between the zoom lens and the two-dimensional galvanometer, and part of the light beam reflected by the eyeball is introduced into the Hartmann micro-lens light path, and the ocular refraction is calculated by the Hartmann array diagram collected by the Hartmann light path.

[0011] Specifically, the signal intensity received by the balanced photodetector is I represented as follows:

[0012]

[0013]

[0014]

[0015] wherein and respectively represent the center wavelength and the full width at half maximum of the low-coherence light source, represents the energy of the received interference signal I ; represents the direct current components of the two low-coherence electrical signals I1 and I2, represents the change amount after interference; represents the change amount of the reference light phase;

[0016] In the process of measuring the signal intensity I , the change of the reference light phase change amount is changed by changing the internal posture of the optical fiber delay line, and the axial parameter is inversely deduced based on the adjustment posture size.

[0017] Specifically, the interference signal generated by the high-coherence light source presents periodic change, and the low-coherence peak distance D value is determined by comparison with the interference peak of the interference signal generated by the low-coherence light source, and is represented as follows:

[0018]

[0019] wherein is the wavelength of the high coherent laser, is the refractive index of air at the wavelength is the refractive index of air at the wavelength is the number of complete interference fringes, is the phase difference of incomplete interference fringes, is the overall phase difference.

[0020] Specifically, the high coherent light source, the phase-shifted grating, the doped fiber, the wavelength division multiplexer and the fiber delay line form a three-mirror cavity model, the phase-shifted grating is equivalent to the first mirror and the second mirror of the three-mirror cavity model, the fiber delay line is equivalent to the third mirror, and the adjustment posture of the fiber delay line is determined according to the self-mixing interference formula , as a signal scale of low coherent interference, is expressed as follows:

[0021]

[0022] represents the group refractive index, represents the line width broadening factor, represents the cavity length of the phase-shifted grating equivalent resonant cavity, represents the coupling efficiency, and respectively represent the optical frequency and the critical frequency, represents the speed of light, represents the equivalent external cavity round-trip delay between the phase-shifted grating and the fiber delay line.

[0023] Specifically, the high coherent light source is a 980nm pump, a wavelength division multiplexer is arranged on the fiber between the pump and the phase-shifted grating, and a photodetector is connected, and the signal scale is determined by detecting the self-mixing interference signal through the photodetector.

[0024] Specifically, the ocular refraction is calculated by the Hartmann array diagram collected, including:

[0025] When the wavefront to be measured is received by the microlens array, it is segmented into countless sub-wavefronts by the microlens array, and converged into a spot array diagram containing the wavefront information to be measured on the CCD;

[0026] The spot position offset is measured and calculated based on the spot array diagram;

[0027] The refraction and divergence data are calculated based on the spot position offset.

[0028] Specifically, the process of measuring and calculating the spot position offset is based on the construction of five groups of two-dimensional curved surfaces based on the Zernike polynomial, and the partial derivative of the curved surface is determined as the position offset of each sampling point. The curved surface polynomial and the partial derivative are expressed as follows:

[0029]

[0030] acquiring offset of at least one group of light points inverting coefficient value of five polynomials according to reference point coordinate offset is expressed as follows:

[0031]

[0032] derivative of sampling point is expressed as follows:

[0033]

[0034] wherein represents wave front function, and represents offset value, and is corresponding derivative, is focal length of lens, and n is 5.

[0035] Specifically, according to derivative of sampling point and corresponding series of matrix, polynomial coefficient identity is derived, and is expressed as follows:

[0036]

[0037] Zernike coefficient corresponding to wave front difference is obtained by using inverse matrix, and is expressed as follows:

[0038]

[0039] refractive power and divergence are calculated by using five coefficients , and are expressed as follows:

[0040]

[0041]

[0042] represents refractive power value, and represents divergence value.

[0043] The technical scheme provided by the embodiments of the present application has at least the following beneficial effects:

[0044] ​In the scheme, the parameters of the eyeball axis are measured by time-domain low-coherence interference. The time-domain low-coherence interference measurement system is composed of a laser, a reference arm, a measurement arm, a delay device and a detector. The information to be measured is carried by the measurement light, and the low-coherence light interference between the measurement light and the reference light obtains the interference peaks of the surfaces of the eyeball tissue. Finally, combined with the high-coherence interference technology, the high-coherence interference signal is used as a ruler to accurately calibrate the position information of the eyeball axis tissue. The interference measurement of the scheme reduces the complexity of the external optical path, and has strong resistance to environmental interference, and is suitable for high-precision and miniaturized sensor applications. The diopter measurement system composed of the Hartmann lens group introduced can measure the eyeball parameters while additionally measuring the diopter and divergence data of the eyeball, and perfect the system function. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 Fig. 1 is a structural schematic diagram of a multi-parameter eyeball axis measurement system based on self-mixing vernier effect provided by an application embodiment;

[0046] Figure 2 Fig. 2 is a theoretical schematic diagram of a three-mirror cavity model composed of a laser feedback interference technology;

[0047] Figure 3 Fig. 3 is a structural schematic diagram of an eyeball axis high-resolution measurement system with high-low coherence measurement combined with filter design provided by another application embodiment;

[0048] Figure 4 Fig. 4 exemplarily shows a schematic diagram of a Hartmann array diagram;

[0049] Figure 5 Fig. 5 lists a schematic diagram of an original data time-domain diagram and an original data frequency-domain diagram;

[0050] Figure 6 Fig. 6 shows a possible FIR band-pass filter diagram;

[0051] Figure 7 Fig. 7 is a possible Hilbert envelope diagram listed;

[0052] Figure 8 Fig. 8 is a frequency-domain diagram of the envelope after data processing;

[0053] Figure 9 Fig. 9 shows a possible filter effect diagram. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical scheme and advantages of the present application more clear, the embodiments of the present application will be further described in detail below with reference to the drawings.

[0055] The "multiple" mentioned in the present document refers to two or more than two. The "and / or" describes the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B can represent the three cases of A existing alone, A and B existing together, and B existing alone. The character " / " generally represents that the front and rear associated objects are in an "or" relationship.

[0056] Figure 1 is a structural schematic diagram of a multi-parameter eye axis measurement system based on self-mixing vernier effect provided by the embodiment of the present application. The whole contains a low-coherence interferometry system part composed of a low-coherence light source, a circulator, and a coupler, a high-coherence auxiliary measurement system part composed of a high-coherence light source and a fiber delay line, and a diopter measurement system part based on a Hartmann lens group. In addition, it also includes a balanced photodetector for measuring interference data, a filtering algorithm after photoelectric conversion, and the like. After filtering processing by the balanced photodetector, the relevant eyeball data can be obtained.

[0057] The low-coherence light source is responsible for emitting a light beam corresponding to a wavelength. The output thereof is connected to a first coupler through an optical fiber. The first coupler is divided into two paths, which are connected to a first and a second circulator through optical fibers, respectively. The direct output end of the first circulator is connected to a collimator through an optical fiber. After going out of the collimator, it is converted into spatial light transmission. The direct output end of the second circulator is connected to a wavelength division multiplexer (i.e., wavelength division multiplexer 2) through an optical fiber. The output end of the wavelength division multiplexer is connected to a fiber delay line. The spatial measurement light is reflected by the eyeball and then output from the first low-coherence light through the collimator and the reflection end of the first circulator. The sample measurement light is output from the second low-coherence light through the reflection end of the second circulator. The two beams of low-coherence light generate interference through the second coupler, and are detected by the balanced photodetector after the interference.

[0058] The optical path thereof is described as follows: the light beam emitted by the low-coherence light source is divided into fiber reference light and fiber measurement light through the first coupler. The fiber measurement light is output as spatial measurement light through the first circulator and the collimator. The spatial measurement light is focused on different parts of the eyeball through the zoom lens. The fiber measurement light enters the fiber delay line through the second circulator and the wavelength division multiplexer, and is output from the reflection end of the second circulator after being reflected by the fiber delay line.

[0059] In some embodiments, the low-coherence light source emits light of 850 nm, which is divided into two beams that can interfere with each other after passing through a 95:5 coupler 1. The polarization directions of the two beams are consistent and the frequencies are the same. Subsequently, the two beams are reflected by different reflecting surfaces and return to the coupler 3 to interfere. Within the interference distance, when the phase difference of the two light waves is 0 or The light intensity of the superimposed wave is the largest when the integer multiple. The phase of the reference light is adjusted by changing the position of the reference arm, so that the light reflected by different eye tissues in the measurement arm produces the largest interference light intensity in turn. Thus the ocular axis, cornea and other eye parameters such as preocular segment can be deduced from the moving distance of the reference arm. The reference arm and the measurement arm respectively refer to the output path part of the two circulators. Figure 1 In the present application, the reference arm is the optical fiber delay line after the output of the circulator 2; and the measurement arm is the spatial optical path device group after the collimating lens and the eyeball.

[0060] For the consideration of measurement accuracy, the system uses a low-coherence light source with a certain bandwidth. The signal received on the detector is the superposition of all wavelengths within the bandwidth. Therefore the signal intensity received by the balanced photodetector is the superposition of the interference intensity of each wavelength, which can be expressed as: I

[0061]

[0062]

[0063]

[0064] wherein and respectively represent the center wavelength and the full width at half maximum of the spectrum of the low-coherence light source, represents the energy of the received interference signal I ; represents the sum of the direct current components of the two low-coherence electrical signals, represents the change after interference; represents the change amount of the phase of the reference light.

[0065] Because the eye parameters are deduced from the moving distance of the reference arm, that is, the internal posture of the optical fiber delay line is changed during the measurement of the signal intensity I , the purpose is to change the change amount of the phase of the reference light , and finally the adjusted posture data and the measured related parameter size are used to deduce the eye parameters.

[0066] ​In order to meet the requirements of low coherence measurement accuracy and scanning speed, we use the fiber delay line to realize the high precision and high frequency scanning of the reference arm. In addition, in order to obtain the accurate phase change amount, the high coherence interference technology is introduced to assist the measurement. In this application, the fiber laser is selected as the high coherence light source, the phase shift grating is connected to the doped fiber, and the doped fiber is sent to the wavelength division multiplexer 2 and then input to the fiber delay line. That is, the fiber delay line is a common part of high coherence and low coherence light paths, and the fiber delay line reflection will interfere with the input beam, and the phase shift grating is located at the interference position. The interference generated at the phase shift grating can be calibrated based on the vernier effect.

[0067] Specifically, when the reference arm is scanned, the low coherence interference light path can detect the interference peaks of different optical surfaces, and the high coherence signal strength will present periodic changes. By comparing the two kinds of interference signals, the phase difference between the low coherence peak signals can be obtained. By comparing the interference peaks generated by the interference signals of the low coherence light source, the adjacent low coherence peak interval D value can be determined, which is represented as follows:

[0068]

[0069] Among them, is the wavelength of the high coherence laser, is the refractive index of air at the wavelength of the high coherence laser, is the number of complete interference fringes, is the phase difference of the incomplete interference fringes, is the overall phase difference.

[0070] Considering that high coherence technology is used in some experiments, the interference vernier effect and precision are limited, so in order to further improve the integration and sensitivity and reduce the complexity of the optical path, the self-mixing effect is used to generate interference. That is, the coherent light source, phase shift grating, doped fiber, wavelength division multiplexer and fiber delay line form a three-mirror cavity model, the phase shift grating is equivalent to the first and second mirrors of the three-mirror cavity model, and the fiber delay line is equivalent to the third mirror. According to the self-mixing interference formula, the adjustment posture of the fiber delay line is determined , which is used as a signal scale for low coherence interference.

[0071] Figure 2 is a theoretical diagram of a three-mirror cavity model using laser feedback interference technology, M1, M2 and M3 respectively represent three mirrors, laser emits a beam after passing through M2, and the beam is reflected at M3 Target and returns between M1 and M2, and self-mixing interference is generated between the incident beam and the reflected beam. represents the effective refractive index of the phase shift grating cavity, represents the effective refractive index of the external cavity.

[0072] Considering the intensity reflectivity of the output mirror of the laser, i.e. the second mirror M2 is where R2 represents the reflectivity of the mirror. The effect of the third mirror M3 on the second mirror is to change the reflectivity and the phase of the second mirror. The reflectivity of the second mirror after the effect of the third mirror of the external cavity can be expressed as:

[0073]

[0074] Here represents the coupling efficiency, represents the optical frequency and represents the round trip delay of the external cavity. At the same time, the stable condition of the laser cavity still needs to be satisfied, i.e. is a positive real number. Since the laser cavity still needs to be in a relatively stable state, the modulus of the imaginary part of is small. When the above relatively stable condition is satisfied, there is and it is generally defined that .

[0075] where when , it can be obtained that . Since the laser still maintains stable output, the phase change of the laser cavity for one round trip is 2π, so it can be obtained that:

[0076]

[0077] where is the eigen-wave number of the laser cavity, is the cavity length of the laser cavity, is the effective refractive index of the laser cavity, and is an integer. Since the effective refractive index of the internal cavity is , it is obtained that:

[0078]

[0079] The effect of the change of the optical field on the group refractive index is considered. The group refractive index is , and the line width broadening factor is replaced to obtain the adjustment attitude (displacement information) as the signal scale of low coherence interference, which is expressed as follows:

[0080]

[0081] represents the group refractive index, represents the line width broadening factor, represents the cavity length of the equivalent cavity of the phase-shifted grating, coupling efficiency, and respectively represent the optical frequency and the critical frequency, represents the speed of light, represents the equivalent round-trip delay of the external cavity between the phase-shifted grating and the fiber delay line. represents the equivalent round-trip delay of the external cavity between the phase-shifted grating and the fiber delay line.

[0082] In some embodiments, the sample arm of the spatial light part can measure the ocular parameters according to the actual situation. Since the distance from the cornea to the retina is more than 20 mm, the galvanometer scanning can only scan the image near the sample imaging point. If the detection light cannot be well focused on the layer of the ocular tissue to be measured, the light signal intensity reflected by the sample arm will be reduced, and then the obtained coherent light signal intensity will be reduced, thereby affecting the detection signal-to-noise ratio. Therefore, all imaging data cannot be obtained by one-time scanning. For example Figure 1 In order to realize high-contrast and high-resolution fundus and anterior segment images, the system in the embodiment additionally has a two-dimensional galvanometer in the spatial light path between the zoom lens and the eyeball. The spatial light is focused to different regions of the eyeball through the two-dimensional galvanometer, and the eyeball data of different regions are reflected. The two-dimensional galvanometer includes X galvanometer and Y galvanometer perpendicular to each other. The light ray completes one cycle of X-Scan scanning through the X galvanometer, is reflected to the Y galvanometer, and then completes multiple X-Scan scanning through one cycle of deflection. The light beam is focused on the lens, the vitreous membrane, the fundus, and the anterior segment interface of the eye tissue to be measured, two-dimensional scanning is performed, multiple measurement surfaces are scanned, and the signal-to-noise ratio is improved. In the focusing process, the system divides the measurement region into three parts: the first part contains the retina, the second part contains the posterior surface of the lens, and the third part contains the anterior surface of the lens and the cornea. The focusing position of the light ray is calculated in advance, and the zoom scanning can be realized by changing the position of the lens group.

[0083] It is particularly pointed out that the ocular parameters mentioned herein do not include the ocular refraction and dispersion, which need to be measured and calculated through the Hartmann lens group. Assuming that the ocular refraction and dispersion are not considered and are used only to measure the axial data, the system can also be simplified as Figure 3 The structure diagram of the axial high-resolution measurement system with high and low coherence measurement combined with filtering design is shown.

[0084] The axial high-resolution measurement system with high and low coherence measurement combined with filtering design also includes a low coherence interferometric measurement system part and a high coherence auxiliary measurement system part, and the difference lies in that the Hartmann lens group and the two-dimensional galvanometer are cancelled. That is, the axial data can be directly measured by adjusting the parameters, and the interference can be normally reflected and generated.

[0085] Because the power of the first low-coherence light is equal to the power of the second low-coherence light, the two interference signals enter the balanced photodetector for differential amplification and comparison processing after passing through the positive input and the negative input respectively, and the converted two current signals have a phase difference of 180 degrees; the differential amplification processing formula is as follows:

[0086]

[0087] denotes the differential amplification output, and denote two power signals respectively, denotes the responsivity of the photodiode at a given wavelength, G is a gain coefficient, and M is an optical gain factor.

[0088] Because the current signals and have a phase difference of π, the direct current signal is eliminated after differential processing, and the amplitude of the alternating current signal in the output signal is doubled. In addition, because differential amplification is used in balanced detection, the common-mode noise signal in the two detectors can be greatly suppressed, and the signal-to-noise ratio of the output signal of the multi-parameter ophthalmic biological measurement system can be improved.

[0089] It is particularly pointed out that, considering the eyeball tissue characteristics, the transmittance of light with a wavelength of 850 nm in water is very high, and the power loss of light with a wavelength of 850 nm in water caused by absorption is small, and the main component in the human eye tissue is water, and the water content in the vitreous body is more than 90%. Therefore, we selected 850 nm as the wavelength of the light source of the low-coherence interference system. The high-coherence light source of the present application is composed of a 980 nm pump and a wavelength division multiplexer (i.e., a wavelength division multiplexer 1), the wavelength division multiplexer 1 is on the optical fiber between the pump and the phase-shift grating, the wavelength division multiplexer 1 is connected to the photodetector, and the self-mixing interference signal is detected by the photodetector to determine the signal scale. It should be noted that the 980 nm pump will be excited to 1550 nm laser by the doped fiber, and because two different wavelength signals exist, the 980 nm pump will be matched with the wavelength division multiplexer as a whole light source device, i.e., a high-coherence light source. The wavelength division multiplexer 2 connected to the optical fiber delay line is used to cooperate with the wavelength division multiplexer 1 to allow the two different wavelength beams to be transmitted without interference.

[0090] In summary, the present system is divided into two parts, i.e., time-domain low-coherence measurement and high-coherence interference auxiliary measurement, and is connected by an all-fiber structure. This design not only saves the work of adjusting the spatial light path, but also is beneficial to reducing the system size and facilitating integration. At the same time, the polarization maintaining fiber is selected for connection in the interference light path part, so as to avoid the change of the polarization of the transmitted light caused by external environmental factors such as temperature, pressure or bending, ensure the consistency of the polarization states of the two interference lights, and further improve the signal-to-noise ratio of the system.

[0091] Further, considering that Figure 1 The system needs to calculate the diopter and divergence information, so it also needs to set a light splitting prism and a mirror in the spatial light path between the zoom lens and the two-dimensional galvanometer, and introduce the part of the light beam reflected by the eyeball into the Hartmann microlens light path. The Hartmann array graph collected by the Hartmann light path is used to calculate the diopter of the eyeball. The following are the steps of collecting the Hartmann array graph and calculating the diopter parameters:

[0092] S1, when the wavefront to be measured is received by the microlens array, it is divided into an infinite number of sub-wavefronts by the microlens array, and converged into a spot array graph containing the wavefront information to be measured on the CCD;

[0093] The Hartmann wavefront imaging light path is composed of a plurality of microlenses with the same parameters. When the wavefront to be measured is received by the microlens array, the microlens array divides it into an infinite number of sub-wavefronts, and finally converges into a spot array graph containing the wavefront information to be measured on the CCD.

[0094] S2, based on the spot array graph, the spot position offset is calculated;

[0095] The process of calculating the spot position offset is based on the construction of five groups of two-dimensional curved surfaces based on Zernike polynomials to back-propagate the curved surface function and calculate the diopter and divergence data. The partial derivative of the curved surface is determined as the position offset of each sampling point in the embodiment of the application. The polynomials and partial derivatives constructed based on the Zernike multi-curved surface are as follows:

[0096]

[0097] In the above formula, the polynomial is determined according to the selected number and Zernike polynomial, which will not be explained too much in this application. At least one group of light points The offset of the reference point coordinates is used to invert the coefficient values of the five polynomials; the offset is as follows:

[0098]

[0099] The sampling point partial derivative is as follows:

[0100]

[0101] Among them, represents the wavefront function, and represent the offset value, and are the corresponding derivatives, is the focal length of the lens, and n is 5 in this embodiment, indicating 5 curved surface polynomials.

[0102] S3, calculating the diopter and divergence data based on the spot position offset amount.

[0103] Further, after obtaining the above relationship, the polynomial coefficient identity is derived according to the sampling point partial derivative and the corresponding relationship matrix, and is expressed as follows:

[0104]

[0105] The Zernike coefficients corresponding to the wavefront phase difference are obtained by using the inverse matrix, and are expressed as follows:

[0106]

[0107] The diopter and divergence are calculated by using the five coefficients , and are expressed as follows:

[0108]

[0109]

[0110] The diopter value is expressed as , and the divergence value is expressed as .

[0111] Figure 4 An exemplary schematic diagram of a Hartmann array diagram is shown, in which a light beam is focused on a CCD photosensitive surface, and a Hartmann array diagram is obtained by a data acquisition unit of the CCD. By comparing a standard Hartmann array diagram (S=0C=0), accurate diopter (S) and divergence (C) data can be obtained. Figure 4 The left side in the middle is a standard Hartmann array diagram, and the middle and right sides are Hartmann array diagrams under different degrees of diopter and divergence.

[0112] Based on the above high and low coherence measurement and combined with the self-mixing vernier effect and the Hartmann microlens group, the measurement of the eye axis and the diopter can be realized, that is, a multi-parameter eye axis measurement system based on the self-mixing vernier effect is composed.

[0113] Considering that the light path of the high and low coherence light source is converted and reflected for many times, and finally the electrical signal converted by the balanced detector will have more interference. In order to ensure the accuracy of data processing and detection, the embodiment selects to filter the data. After comparing various filtering methods, the application finally designs a filtering frequency self-adjusting filtering method for processing interference signals, and composes a multi-parameter eye axis measurement and data processing system based on the self-mixing vernier effect. The system diagram is the same as Figure 1 The difference is that the content of post-data sorting filtering is added. The following is the data processing filtering step:

[0114] S1, the balanced photodetector converts the original current signal after photoelectric conversion, and converts the time domain into a frequency domain signal through FFT transformation;

[0115] The mathematical model of Fourier transform is as follows:

[0116]

[0117] Wherein is the Fourier transform result of the signal , indicating the frequency domain signal (spectrum); is the time domain signal, and w is the angular frequency; the inverse transform formula of Fourier transform is:

[0118]

[0119] This shows that the frequency domain signal can be restored to the original time domain signal through inverse Fourier transform. Figure 5 The schematic diagram of the original data time domain graph and the original data frequency domain graph is listed.

[0120] It should be noted that in the Fourier transform, there will be a symmetric peak (due to the symmetry of complex conjugate, the Fourier transform will appear symmetric peaks at positive and negative frequencies), and the essential reason is that the mathematical properties of real value signal lead to the conjugate symmetry of the spectrum. This phenomenon is very important in theory and actual signal processing (such as spectrum analysis, filter design, etc.), and provides important information about the signal components. In matlab, the negative frequency part will be symmetric to the positive frequency part, so there will be two peaks in the following figure, actually the right peak is the negative frequency symmetric peak.

[0121] S2, a bandpass FIR filter is designed according to the frequency range of the interference signal combined with the bandpass frequency, and a window is set;

[0122] According to the frequency range of the interference signal combined with the bandpass frequency, a bandpass FIR filter is designed, which mainly includes bandpass filtering, filter order and window function. The most important bandpass filtering includes some important parameters:

[0123] Lower cutoff frequency ( ): that is, the cutoff frequency of the low-pass filter, indicating the lowest frequency that can pass;

[0124] Upper cutoff frequency ( ): that is, the cutoff frequency of the high-pass filter, indicating the highest frequency that can pass;

[0125] Bandwidth (BW): refers to the frequency bandwidth that can pass through the bandpass filter, which is usually calculated as:

[0126]

[0127] Center frequency ( ): is the center point of the frequency through which the bandpass filter passes, and is usually calculated as:

[0128]

[0129] In this embodiment, the center frequency of the bandpass filter is periodically identified. The maximum frequency value identified in the frequency domain signal is then determined based on the bandwidth. and .

[0130] The frequency response of a bandpass FIR filter can be represented by a frequency response curve, which typically exhibits a bell-shaped curve (i.e., a Gaussian or Lorentz curve). This means that the signal is most effectively transmitted near the center frequency, while the signal is gradually attenuated outside the cutoff frequency. Its mathematical model is as follows:

[0131]

[0132] in This represents the frequency response of the bandpass filter.

[0133] In the algorithm implementation, frequency normalization is achieved by setting the target frequency and the ratio of the upper and lower cutoff frequencies to the sampling rate. Then, a window function is used to perform preliminary bandpass filtering. Simultaneously, a loop is introduced between the upper and lower cutoff frequencies and the filter order. By adjusting the loop range and its step size, the most suitable FIR filter combination for filtering is found through the loop system. Figure 6 A possible FIR bandpass filter diagram is shown.

[0134] S3. Extract the envelope and determine the location using Hilbert transform;

[0135] Hilbert transform offers significant advantages in extracting the envelope and determining the location, especially when processing non-stationary signals, noisy signals, and complex signals. It accurately extracts the instantaneous features of the signal without requiring additional preprocessing steps and is unaffected by manually set thresholds. Therefore, this embodiment performs Hilbert transform on the pre-filtered signal and extracts its envelope.

[0136] Set the real signal of the filtered interference signal to be Hilbert transform of imaginary signals It is expressed as follows:

[0137]

[0138] Essentially, it makes the signal This produces a 90-degree phase shift, in order to For the real part, constructing an analytic signal for the imaginary part The analytic signal constructed according to the real signal and the imaginary signal is expressed as:

[0139]

[0140] The modulus of the analytic signal represents the envelope of the original signal, and the calculation formula is:

[0141]

[0142] Figure 7 is a possible Hilbert envelope diagram listed, and the envelope signal usually represents the amplitude modulation of the signal, that is, the amplitude change of the signal over time.

[0143] S4, Fourier transform is performed on the extracted Hilbert envelope to obtain envelope spectrum data;

[0144] The Fourier transform is performed on the Hilbert envelope to obtain the frequency domain image of the envelope, as shown in Figure 8 , and the desired interference signal frequency range is found.

[0145] S5, a low-pass filter is set according to the interference signal frequency range to perform secondary filtering, and the filter combination is determined according to the sum of the signal-to-noise ratios.

[0146] The process is mainly to obtain the interference signals before and after the two times of filtering, calculate the signal-to-noise ratio according to the adjacent interference peaks. Then, the sum of the signal-to-noise ratios calculated under different setting parameters before and after the two times of filtering is sorted in descending order, and the filter combination with the largest sum of signal-to-noise ratios is selected as the target filtering strategy in the period. It specifically includes the upper and lower cutoff frequencies of the band-pass filter, the filter order, and the low-pass frequency of the Hilbert envelope filter.

[0147] For the determination method of the sum of the signal-to-noise ratios, the present application calculates the signal-to-noise ratio by using two interference peaks, and then obtains the maximum value of the sum of the two items as the determination output. Because if only one interference peak is used as the basis for judgment, the frequency selection of the other interference peak will be ignored, resulting in distortion of some signals. Such a determination method can simultaneously consider the frequency part of two peaks to meet the filtering requirements.

[0148] Based on the above design idea, the present application uses the filtering strategy for data processing, saves the calculation time on the basis of not affecting the filtering effect, improves the step size to reduce the number of cycles. Figure 9In the filter effect diagram shown, the calculation time is reduced from 35s to about 1.5s, while the sum of signal-to-noise ratios is still maintained at about 65dB, and the single peak signal-to-noise ratio is maintained at about 32dB. The red marked position is the front surface peak of the glass, and the green marked position is the back surface peak of the glass. Through the high signal-to-noise ratio measurement of the structure and the processing of the filter, the high and low coherence measurement combined with the filter design of the axial high-resolution measurement system is finally realized.

[0149] In summary, in the present scheme, the measurement of each parameter of the eyeball axis is realized by time-domain low coherence interference, and the time-domain low coherence interference measurement system is composed of a laser, a reference arm, a measurement arm, a delay device and a detector. The information to be measured is carried by the measurement light, and the low coherence interference between the measurement light and the reference light obtains the interference peaks of each surface of the eye tissue. Based on the time-domain low coherence interference fiber structure system, the interference signal is obtained, and the obtained interference signal is usually difficult to realize high-precision biological measurement without processing. Therefore, the signal usually needs to be filtered and processed. In the filtering technology, a series of operations such as Fourier transform of the original time-domain data, FIR band-pass filtering, Hilbert transform of the preliminary processed signal, low-pass filtering and the like are adopted, and the cycle process of frequency and filter order is introduced to select the best filter combination combined with the calculation of signal-to-noise ratio. Finally, the short-time and high signal-to-noise ratio processing of the obtained interference signal is realized, and the processing process and results are visualized to facilitate the observation of the change of the processed signal. Finally, combined with the high coherence interference technology, the high coherence interference signal is used as a ruler, and the accurate calibration of the position information of the axial tissue can be realized.

[0150] The specific embodiment is only an explanation of the present application, and is not a limitation of the present application. Those skilled in the art can make modifications to the present embodiment without creative contribution after reading the present specification, but as long as the present application is within the scope of the claims, it is protected by the patent law.

Claims

1. A multi-parameter ocular axial measurement system based on self-mixing vernier effect, characterized in that, The system comprises: a low-coherence light source, a light beam emitted by the low-coherence light source is split into a fiber reference light and a fiber measurement light by a first coupler; the fiber measurement light is output as a spatial measurement light by a first circulator and a collimator; the spatial measurement light is focused on different parts of an eyeball by a zoom lens; the fiber measurement light enters a fiber delay line through a second circulator and a wavelength division multiplexer, and is output from a reflection end of the second circulator after being reflected by the fiber delay line; a high-coherence light source, a light beam emitted by the high-coherence light source passes through a doped fiber provided with a phase-shift grating, is reflected by a wavelength division multiplexer and a fiber delay line, and generates self-mixing interference at the phase-shift grating to calibrate a low-coherence peak interval; the spatial measurement light is output as a first low-coherence light from a reflection end of the first circulator after being reflected by the eyeball; the sample measurement light is output as a second low-coherence light from a reflection end of the second circulator; the two beams of low-coherence light generate interference through a second coupler, interference data are received by a balanced photodetector, and eyeball data are obtained by combining the low-coherence peak interval measurement; a Hartmann microlens group, the spatial light reflected by the eyeball is focused on a CCD light-sensitive surface after being incident on the Hartmann microlens group, and the Hartmann array diagram obtained by the sampling is used to calculate the refractive power of the eyeball; The high-coherence light source, the phase-shift grating, the doped fiber, the wavelength division multiplexer and the fiber delay line constitute a three-mirror cavity model, the phase-shift grating is equivalent to the first mirror and the second mirror of the three-mirror cavity model, and the fiber delay line is equivalent to the third mirror; and the adjustment posture of the fiber delay line is determined according to a self-mixing interference formula As a signal scale of low-coherence interference, the following is indicated: denotes the group refractive index, denotes the line broadening factor, denotes the cavity length of the equivalent resonator of the phase-shifted grating, coupling efficiency, and denote the optical frequency and the critical frequency, respectively, denotes the speed of light, denotes the equivalent external cavity round-trip delay between the phase-shifted grating and the fiber delay line; the interference signal generated by the high-coherence light source presents periodic changes, and the low-coherence peak interval D value is determined by comparing the interference peak of the interference signal generated by the low-coherence light source, and is represented as follows: wherein is the wavelength of the high coherence laser, is the refractive index of air at the wavelength of the high coherence laser, is the number of complete interference fringes, is the phase difference of the incomplete interference fringes, is the overall phase difference.

2. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 1, wherein, a two-dimensional galvanometer is arranged in a spatial light path between the zoom lens and the eyeball, and the spatial light is focused on different regions of the eyeball by the two-dimensional galvanometer; the two-dimensional galvanometer comprises X and Y galvanometers perpendicular to each other, a light ray completes one cycle of X-Scan scanning after passing through the X galvanometer, is reflected to the Y galvanometer, and then completes multiple cycles of X-Scan scanning after being deflected for one cycle, so that the light beam is focused on the lens, the vitreous membrane, the fundus and the anterior segment interface of the eye tissue to be measured, and two-dimensional scanning is performed.

3. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 2, wherein, a beam splitter and a mirror are arranged in a spatial light path between the zoom lens and the two-dimensional galvanometer, part of the light beam reflected by the eyeball is introduced into a Hartmann microlens light path, and the Hartmann array diagram obtained by the Hartmann light path is used to calculate the refractive power of the eyeball.

4. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 1, wherein, The signal intensity received by the balanced photodetector I is represented as follows: wherein and respectively represent the center wavelength and the spectral full width at half maximum of the low coherence light source, represents the energy of the received interference signal I ; represents the direct current components of the two low coherence electrical signals and represents the change amount after interference; represents the reference light phase change amount; In measuring signal intensity I The reference light phase change amount is changed by changing the internal posture of the optical fiber delay line The eye axis parameters are inversely deduced based on the adjustment posture size.

5. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 1, wherein, the high-coherence light source is a 980 nm pump, a wavelength division multiplexer is arranged on a fiber between the pump and the phase-shift grating, and a photodetector is connected, and the signal scale is determined by detecting the self-mixing interference signal by the photodetector.

6. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 1, wherein, the Hartmann array diagram obtained by the sampling is used to calculate the refractive power of the eyeball, comprising: when the wavefront to be measured is received by the microlens array, it is segmented into an infinite number of sub-wavefronts by the microlens array, and is converged into a spot array diagram containing the wavefront information to be measured on the CCD; the spot position offset is calculated based on the spot array diagram; the refractive power and the divergence data are calculated based on the spot position offset.

7. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 6, wherein, the process of calculating the spot position offset is based on the Zernike polynomial to construct five groups of two-dimensional curved surfaces, and the curved surface partial derivative is determined as the position offset of each sampling point, and the curved surface polynomial and the partial derivative are represented as follows: acquiring at least one set of light points offsets from the reference point coordinates inverting the coefficient values of the five polynomials; offsets are represented as follows: the sampling point partial derivative is represented as follows: wherein represents a wavefront function, and represents an offset value, and is a corresponding derivative, is a focal length of the lens, and n has a value of 5.

8. The self-mixing vernier effect based multi-parameter ocular axial measurement system according to claim 7, wherein, the polynomial coefficient identity is derived according to the sampling point partial derivative and the corresponding series of matrices, and is represented as follows: the wavefront phase difference corresponding Zernike coefficient is obtained by using the inverse matrix, and is represented as follows: By five coefficients The diopter and the dispersion are calculated and expressed as follows: wherein is a coefficient matrix, is a gradient matrix, is a wavefront slope matrix; denotes a refractive value, denotes a divergence value.

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