Orbital angular momentum spectrum coincidence measurement method and device based on time correlation two-photon source

Abstract

The invention belongs to the technical field of spectral measurement, and discloses an orbital angular momentum spectrum coincidence measurement method based on a time correlation two-photon source, which comprises the following steps of: obtaining a pair of down-conversion photons, namely signal photons and idler frequency photons, with a time correlation characteristic by utilizing a spontaneous parametric down-conversion process of a nonlinear crystal; loading orbital angular momentum information on the signal photons, and obtaining an orbital angular momentum spectrum of the signal photons by using an optical coordinate transformation method; and performing time marking and coincidence measurement on the orbital angular momentum spectrum signals of the signal photons by using the idler frequency photons. According to the invention, single-photon orbital angular momentum spiral spectrum detection is carried out by using the time correlation characteristic of two photons, and background stray light noise contained before signal light reaches a detector is suppressed by measuring correlated photon signals which reach the detector on two light paths of signal light and idler frequency light at the same time or in a short time; therefore, the detection sensitivity reaches the sensitivity threshold limit of a photoelectric detection system, namely a single photon level.

Description

Technical Field

[0001] This invention belongs to the field of spectral measurement technology and relates to a method and device for measuring orbital angular momentum spectrum coincidence based on time-correlated two-photon sources. Background Technology

[0002] A vortex beam is a novel type of laser beam with a spiral wavefront structure. Its wavefront complex amplitude expression contains a phase spiral term, which macroscopically reflects the orbital angular momentum of photons. The orbital angular momentum spiral spectrum is a characteristic parameter of a vortex beam. Any vortex light field distribution can be expanded according to a spiral spectrum function. For each orbital angular momentum quantum number contained in the vortex beam, there is a corresponding amplitude component, and the weights of all spiral spectrum amplitude components are added together to equal 1.

[0003] Due to their orbital angular momentum properties, vortex beams have been widely used in ultra-high-capacity optical communication, holographic storage, remote sensing, laser processing, and high-resolution imaging. In remote sensing applications using vortex beams, it is often necessary to detect the orbital angular momentum spiral spectrum contained in the echo field transmitted over long distances. Currently, methods for detecting orbital angular momentum spiral spectra can be broadly classified into three categories. The first is the diffraction and interferometry method, which involves passing the beam under test through a special diffraction grating or interfering with a specific reference beam, and then analyzing the relevant properties of the resulting diffraction or interference field to infer the orbital angular momentum spectrum of the beam under test. This method is only applicable to vortex beams with a single or two symmetrical orbital angular momentum spectra. The second is the projection detection method, which mainly utilizes a series of spiral phase plates or holograms with different topological charges to project and detect the amplitude intensity corresponding to different angular quantum numbers, thereby obtaining the orbital angular momentum spectrum. The third method is the mode beam splitting method, which is based on the principle of separating different OAM components in the beam to be measured in space through optical coordinate transformation and other technical means, and then measuring the intensity of each component, so as to obtain the orbital angular momentum spectrum at one time.

[0004] However, these methods for detecting orbital angular momentum spectra all suffer from low light utilization and are only suitable for detecting high-energy vortex beams. In remote sensing applications, long-distance transmission results in extremely weak echo light fields, and coupled with the influence of ambient background light noise, it is difficult to guarantee the effective detection of the orbital angular momentum spectrum in the echo signal using the three detection methods mentioned above. Summary of the Invention

[0005] (I) Purpose of the Invention

[0006] Current methods for detecting orbital angular momentum spectra suffer from low light utilization. Furthermore, in situations with very weak long-range echo signals, the system often experiences high background noise and low detection sensitivity. Therefore, there is an urgent need to explore a detection method based on quantum effects of the optical field, possessing high sensitivity and low background noise, to address the problems existing in current orbital angular momentum spiral spectrum detection at the single-photon level. This invention aims to overcome the shortcomings of existing technologies by proposing a method and apparatus for measuring orbital angular momentum spectrum coincidence based on a time-correlated two-photon source.

[0007] (II) Technical Solution

[0008] To address the aforementioned technical problems, this invention provides a method for measuring orbital angular momentum spectrum coincidence based on a time-correlated two-photon source, comprising the following steps:

[0009] S1: By utilizing the spontaneous parametric downconversion process of a nonlinear crystal, a pair of downconverted photons with time-correlated characteristics are obtained, namely, a signal photon and an idler photon.

[0010] S2: Load orbital angular momentum information onto the signal photon and obtain the orbital angular momentum spectrum of the signal photon using the optical coordinate transformation method;

[0011] S3: Time stamping and coincidence measurement of the orbital angular momentum spectrum signal of the signal photon using idler frequency photons.

[0012] Furthermore, in step S1, a pump light is incident on a non-centrosymmetric crystal with second-order nonlinear polarizability. A pump photon spontaneously annihilates in the crystal, simultaneously generating two photons with frequencies lower than the incident pump light frequency, namely a signal photon and an idler photon.

[0013] Furthermore, in step S1, the wavelength of the pump light is half of the wavelength expected to generate two photons.

[0014] Furthermore, in step S2, a spatial light modulator is used to load orbital angular momentum information containing multiple angular quantum numbers onto a signal photon, and then the signal photon is emitted to the surface of a remote target. The weak signal light reflected or scattered back from the surface of the remote target is collected by optical fiber coupling.

[0015] Further, in step S2, an optical coordinate transformation phase plate and a correction phase plate are used to spatially sieve the orbital angular momentum spectrum of the signal photons. A Fourier lens is placed after the correction phase plate to focus the rectangular beam obtained by the optical coordinate transformation onto a lateral position on the focal plane, forming a narrow lateral spot, i.e., the orbital angular momentum spectrum line. The lateral position of the spectrum line on the focal plane is positively correlated with the topological charge carried by the beam. A single-photon array detector is placed at the focal point of the Fourier lens to detect the single-photon signal of the orbital angular momentum spectrum line.

[0016] Furthermore, in step S3, the idler photon is coupled into the optical fiber and connected to a single-point detector for detection. After circuit delay, the electrical signal converted from the idler photon marks and coincides with the signal photon, thereby realizing the detection of the orbital angular momentum spectrum of the signal photon.

[0017] Further, in step S2, the orbital angular momentum information is loaded onto the signal photon using a spatial light modulator. The orbital angular momentum information contains one or more modes, and the orbital angular momentum spectrum is set symmetrically or asymmetrically. The orbital angular momentum spectrum of the signal light is spatially sieved using optical coordinate transformation sieving phase plates and correction phase plates. The working wavelengths of the two sieving phase plates are consistent with the wavelength of the signal photon, and the phase distribution satisfies the conformal mapping relationship, thus transforming the incident light field from the Cartesian coordinate system to the polar coordinate system.

[0018] This invention also provides a time-correlated two-photon source orbital angular momentum spectrum coincidence measurement device, which includes: a pump laser 1, a first lens 2, a nonlinear crystal 3, a second lens 4, a filter 5, and a polarization beam splitter 6 arranged coaxially from front to back along the optical path direction; a spatial light modulator 8, a reflector 9, an optical coordinate transformation sieving phase plate 10, a correction phase plate 11, a third lens 12, and a single-photon array detector 13 arranged sequentially along the optical path direction on the light-transmitting side of the polarization beam splitter 6; a single-photon point detector 7 arranged on the reflective side of the polarization beam splitter 6; the single-photon point detector 7 and the single-photon array detector 13 connected to a time-to-digital converter 14; and the time-to-digital converter 14 connected to a computer 15; the pump laser 1 emits laser light, which is focused onto the nonlinear crystal 3 by the first lens 2, and a spontaneous parametric down-conversion process occurs, generating time-correlated photon pairs; The generated down-conversion light is collimated and filtered using a second lens 4 and a filter 5 to remove pump light and interference light from the optical path. Then, a polarization beam splitter 6 splits the down-conversion light into two paths: a signal photon and an idler photon. The signal photon passes through a spatial light modulator 8 loaded with coherent superimposed orbital angular momentum information, is reflected by a mirror 9, and is then incident on an orbital angular momentum spectrum sieving module consisting of an optical coordinate transformation sieving phase plate 10, a correction phase plate 11, and a third lens 12 to extract orbital angular momentum information. Finally, it is incident on a single-photon array detector 13 operating in the near-infrared band for imaging detection. The other idler photon is detected by a single-photon detector 7. Finally, a time-to-digital converter 14 marks and measures the coincidence of the signal photon. After data processing by a computer 15, the amplitude of the orbital angular momentum spectrum of the weak vortex light is detected.

[0019] Furthermore, the pump laser 1 is one of a solid-state laser, a fiber laser, or a semiconductor laser, with a wavelength selection range of 350-1100 nm; the nonlinear crystal 3 is one of BBO, KTP, PPKTP, or PPLN; the filter 5 is a long-pass filter with a working cutoff wavelength greater than the pump light wavelength; the polarization beam splitter 6 has a splitting ratio adjustable in the range of 1:99 to 50:50; and the single-photon point detector 7 is a silicon-based single-photon detector or an InGaAs / InP single-photon detector.

[0020] Furthermore, different pump wavelengths are selected for different nonlinear crystals 3, including using a 405nm laser to pump a BBO crystal to prepare an 810nm correlated photon pair; using a 532nm laser to pump a KTP crystal to prepare a 1064nm correlated photon pair; and using a 775nm laser to pump a PPKTP or PPLN crystal to prepare a 1550nm correlated photon pair.

[0021] (III) Beneficial Effects

[0022] The above-mentioned technical solution provides a method and apparatus for measuring orbital angular momentum spectrum coincidence based on time-correlated two-photon sources, which has the following beneficial effects:

[0023] (1) In this invention, the time correlation characteristics of two photons are used to detect the orbital angular momentum spiral spectrum of a single photon. By measuring the correlated photon signals that arrive at the detector simultaneously or within a short time on the two optical paths of the signal light and the idler light, the background stray light noise contained in the signal light before it reaches the detector is suppressed, so that the detection sensitivity reaches the sensitivity threshold limit of the photoelectric detection system, i.e., the single photon level.

[0024] (2) In this invention, the orbital angular momentum spiral spectrum of a single photon is detected by coincidence measurement, which enhances the ability to measure weak vortex light fields and is beneficial to improving the detection efficiency of the vortex light measurement system.

[0025] (3) In this invention, during the coincidence counting process, the orbital angular momentum spectrum of the light field is mainly used to confirm the signal light. Compared with the traditional quantum detection method that uses photon energy to measure the signal light, the influence of ambient light on the system detection is reduced, thereby effectively reducing the false alarm rate of the detection.

[0026] (4) In this invention, the dual-sieving phase plate is selected to realize the sieving of the signal light orbital angular momentum spiral spectrum, which simplifies the optical path of the device compared with the traditional dual spatial light modulator sieving scheme. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the orbital angular momentum spectrum coincidence measurement device based on a time-correlated two-photon source, according to an embodiment of the present invention. Detailed Implementation

[0028] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0029] The core of the present invention, which is based on the orbital angular momentum spectrum coincidence measurement method and device of time-correlated two-photon source, lies in combining orbital angular momentum spectrum measurement with the time correlation characteristics of two-photon quantum light source, and using coincidence measurement to reduce background stray light noise and improve the detection efficiency of orbital angular momentum spectrum.

[0030] This embodiment of the orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources includes the following steps:

[0031] S1: By utilizing the spontaneous parametric downconversion process of a nonlinear crystal, a pair of downconverted photons with time-correlated characteristics are obtained, namely a signal photon and an idler photon.

[0032] Specifically, a strong pump beam is incident on a non-centrosymmetric crystal with a significant second-order nonlinear polarizability. One pump photon spontaneously annihilates within the crystal, simultaneously generating two photons with frequencies lower than the incident pump beam frequency—a signal photon and an idler photon—thus satisfying energy conservation. These two photons are correlated and synchronized in the time dimension. During implementation, a crystal medium with a large nonlinear coefficient and a matched pump beam should be selected. Energy conservation and phase matching must be satisfied between the pump beam and the two downconverted photons; therefore, the wavelength of the pump beam is half the expected wavelength for generating the two photons, and the incident angle between the pump beam and the crystal needs to be precisely adjusted to achieve phase matching. The power and pulse width of the pump beam need to be optimized in conjunction with the crystal characteristics. Increasing the pump power helps improve the brightness of the two-photon source but also increases background noise (such as fluorescence and Raman scattering) and the risk of optical damage to the crystal.

[0033] S2: Load orbital angular momentum information onto the signal photon and obtain the orbital angular momentum spectrum of the signal photon using the optical coordinate transformation method.

[0034] Specifically, a spatial light modulator (SLM) is used to load orbital angular momentum information containing multiple angular quantum numbers onto a signal photon. The signal photon is then emitted onto the surface of a remote target, and the weak signal light reflected or scattered back from the surface of the remote target is collected through optical fiber coupling.

[0035] Optical coordinate transformation phase plates and correction phase plates are used to spatially sieve the orbital angular momentum spectrum of signal photons. A Fourier lens is placed after the correction phase plate to focus the rectangular beam obtained from the optical coordinate transformation onto a lateral position on the focal plane, forming a narrow lateral spot, i.e., the orbital angular momentum spectral line. The lateral position of the spectral line on the focal plane is positively correlated with the topological charge carried by the beam. A single-photon array detector is placed at the focal point of the Fourier lens to detect the single-photon signal of the orbital angular momentum spectral line.

[0036] S3: Time stamping and coincidence measurement of the orbital angular momentum spectrum signal of the signal photon using idler frequency photons.

[0037] Specifically, idler photons are coupled into an optical fiber and directly connected to a single-point detector for detection. Due to the time-correlated synchronization between the signal photon and the idler photon, after precise circuit delay, the electrical signal converted from the idler photon can be used to mark and measure the coincidence of the signal photon, ultimately achieving high-sensitivity detection of the orbital angular momentum spectrum of the signal photon.

[0038] During the coincidence measurement process, since only related events that occur simultaneously or within a short period of time on the two optical paths of the signal light and the idler light are counted and measured, background stray light noise during the period when the signal photon has not yet reached the detector can be effectively suppressed.

[0039] In step S2, the orbital angular momentum information is loaded onto the signal photon using a spatial light modulator. The orbital angular momentum information contains one or more modes, and the orbital angular momentum spectrum can be symmetrical or asymmetrical.

[0040] The orbital angular momentum spectrum of the signal light is spatially sieved using optical coordinate transformation sieving phase plates and correction phase plates. The working wavelengths of the two sieving phase plates must be consistent with the wavelength of the signal photon, and the phase distribution should satisfy the conformal mapping relationship, transforming the incident light field from the Cartesian coordinate system to the polar coordinate system.

[0041] Idle and signal photons are collected by fiber optic coupling. To optimize the collection efficiency of idle and signal photons (>50%) and maximize the photon count reaching the receiver, good optical path alignment and fiber mode matching are required.

[0042] A single-photon array detector can also be replaced by an enhanced CCD camera. Its operating wavelength must match the wavelength of the signal photon, and it must also have high detection efficiency (>50% @ signal photon wavelength), low dark count (<100cps) and small time jitter (<1ns) to make the measured signal photon count as large as possible.

[0043] Among them, the use of a time-to-digital converter can achieve high-precision circuit delay (<100ps) for idler photons, so as to ensure accurate marking of signal photons in time and realize the coincidence measurement of signal photons and idler photons.

[0044] Combination Figure 1 The orbital angular momentum spectrum coincidence measurement device based on time-correlated two-photon sources in this embodiment includes: a pump laser 1, a first lens 2, a nonlinear crystal 3, a second lens 4, a filter 5, and a polarization beam splitter 6 arranged coaxially from front to back along the optical path. On the light-transmitting side of the polarization beam splitter 6, a spatial light modulator 8, a reflector 9, an optical coordinate transformation sieving phase plate 10, a correction phase plate 11, a third lens 12, and a single-photon array detector 13 are arranged sequentially along the optical path. On the reflective side of the polarization beam splitter 6, a single-photon point detector 7 is arranged. The single-photon point detector 7 and the single-photon array detector 13 are connected to a time-to-digital converter 14, and the time-to-digital converter 14 is connected to a computer 15.

[0045] The pump laser 1 can be of the type of solid-state laser, fiber laser, semiconductor laser, etc., and the wavelength selection range is 350-1100nm;

[0046] The nonlinear crystal 3 can include crystals with large nonlinear coefficients such as BBO, KTP, PPKTP, and PPLN;

[0047] For different nonlinear crystals, different pump wavelengths should be selected. Typical pumping schemes include: (1) using a 405nm laser to pump a BBO crystal to prepare an 810nm correlated photon pair; (2) using a 532nm laser to pump a KTP crystal to prepare a 1064nm correlated photon pair; and (3) using a 775nm laser to pump a PPKTP or PPLN crystal to prepare a 1550nm correlated photon pair.

[0048] The filter 5 is a long-pass filter, which can filter out the pump light; therefore, its operating cutoff wavelength is greater than the pump light wavelength. The splitting ratio of the polarization beam splitter 6 can be adjusted within the range of 1:99 to 50:50.

[0049] The single-photon point detector 7 can encompass silicon-based single-photon detectors and InGaAs / InP single-photon detectors, specifically selected based on the wavelength of the detected photon. The spatial light modulator 8 includes light field modulation elements such as a projection / reflection liquid crystal spatial light modulator, a holographic grating, and a vortex waveplate.

[0050] The optical coordinate transformation sieving phase plate 10 and the correction phase plate 11 can be replaced by a spatial light modulator loaded with specific phase information.

[0051] The single-photon array detector 13 can be replaced by an enhanced CCD camera.

[0052] In this embodiment, the pump laser 1 emits a laser beam with a center wavelength of 775 nm, which is focused onto the PPKTP crystal 3 by the first lens 2, undergoing a spontaneous parametric down-conversion process to generate time-correlated photon pairs with a center wavelength of 1550 nm. The generated down-converted light is collimated and filtered by the second lens 4 and the filter 5 to remove the pump light and other interfering light in the optical path. Then, the down-converted light is split into two paths by the polarization beam splitter 6, namely signal photons and idler photons. The signal photons pass through the spatial light modulator 8 loaded with coherent superimposed orbital angular momentum information, are reflected by the mirror 9, and are incident on the orbital angular momentum spectrum sieving module composed of the optical coordinate transformation sieving phase plate 10, the correction phase plate 11, and the third lens 12 to extract orbital angular momentum information. Finally, they are incident on the single-photon array detector / enhanced CCD camera 13 operating in the near-infrared band for imaging detection; the other idler photon is directly detected by the single-photon detector 7. Finally, the signal photons are marked and coincidence measured using a time-to-digital converter 14. After data processing by a computer 15, the detection of the orbital angular momentum spectrum amplitude of weak vortex light is finally achieved with high sensitivity and low noise.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for measuring orbital angular momentum spectrum coincidence based on time-correlated two-photon sources, characterized in that, Includes the following steps: S1: By utilizing the spontaneous parametric downconversion process of a nonlinear crystal, a pair of downconverted photons with time-correlated characteristics are obtained, namely, a signal photon and an idler photon. S2: Load orbital angular momentum information onto the signal photon and obtain the orbital angular momentum spectrum of the signal photon using the optical coordinate transformation method; S3: Time stamping and coincidence measurement of the orbital angular momentum spectrum signal of the signal photon using idler frequency photons.

2. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 1, characterized in that, In step S1, a pump light is incident on a non-centrosymmetric crystal with second-order nonlinear polarizability. A pump photon spontaneously annihilates in the crystal, generating two photons with frequencies lower than the incident pump light frequency, namely a signal photon and an idler photon.

3. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 2, characterized in that, In step S1, the wavelength of the pump light is half of the wavelength expected to produce two photons.

4. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 3, characterized in that, In step S2, a spatial light modulator is used to load orbital angular momentum information containing multiple angular quantum numbers onto a signal photon. Then, the signal photon is emitted to the surface of a remote target, and the weak signal light reflected or scattered back from the surface of the remote target is collected by optical fiber coupling.

5. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 4, characterized in that, In step S2, an optical coordinate transformation phase plate and a correction phase plate are used to spatially sieve the orbital angular momentum spectrum of the signal photons. A Fourier lens is placed after the correction phase plate to focus the rectangular beam obtained by optical coordinate transformation onto a lateral position on the focal plane, forming a narrow lateral spot, i.e., the orbital angular momentum spectrum line. The lateral position of the spectrum line on the focal plane is positively correlated with the topological charge carried by the beam. A single-photon array detector is placed at the focal point of the Fourier lens to detect the single-photon signal of the orbital angular momentum spectrum line.

6. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 5, characterized in that, In step S3, the idler photon is coupled into the optical fiber and connected to a single-point detector for detection. After circuit delay, the electrical signal converted from the idler photon marks and coincides with the signal photon, thereby realizing the detection of the orbital angular momentum spectrum of the signal photon.

7. The orbital angular momentum spectrum coincidence measurement method based on time-correlated two-photon sources as described in claim 6, characterized in that, In step S2, the orbital angular momentum information is loaded onto the signal photon using a spatial light modulator. The orbital angular momentum information contains one or more modes, and the orbital angular momentum spectrum is set symmetrically or asymmetrically. The orbital angular momentum spectrum of the signal light is spatially sieved using optical coordinate transformation sieving phase plates and correction phase plates. The working wavelengths of the two sieving phase plates are consistent with the wavelength of the signal photon, and the phase distribution satisfies the conformal mapping relationship, thus transforming the incident light field from the Cartesian coordinate system to the polar coordinate system.

8. A device for measuring orbital angular momentum spectrum coincidence based on a time-correlated two-photon source, characterized in that, include: A pump laser (1), a first lens (2), a nonlinear crystal (3), a second lens (4), a filter (5), and a polarization beam splitter (6) are arranged coaxially from front to back along the optical path. On the light-transmitting side of the polarization beam splitter (6), a spatial light modulator (8), a mirror (9), an optical coordinate transformation sieve phase plate (10), a correction phase plate (11), a third lens (12), and a single-photon array detector (13) are arranged sequentially along the optical path. On the reflective side of the polarization beam splitter (6), a single-photon point detector (7) is arranged. The single-photon point detector (7) and the single-photon array detector (13) are connected to a time-to-digital converter (14), which is connected to a computer (15). The pump laser (1) emits laser light, which is focused onto the nonlinear crystal (3) by the first lens (2), resulting in a spontaneous parametric down-conversion process and generating time-correlated photon pairs. The second lens (4) is used to... The downconversion light generated is collimated and filtered by a filter (5) to remove the pump light and interference light in the optical path. Then, the downconversion light is split into two paths by a polarization beam splitter (6), namely signal photons and idler photons. The signal photons pass through a spatial light modulator (8) loaded with coherent superimposed orbital angular momentum information, and after being reflected by a mirror (9), they are incident on an orbital angular momentum spectrum sieving module composed of an optical coordinate transformation sieving phase plate (10), a correction phase plate (11), and a third lens (12) to realize the extraction of orbital angular momentum information. Finally, they are incident on a single-photon array detector (13) working in the near-infrared band for imaging detection. The other idler photon is detected by a single-photon detector (7). Finally, a time-to-digital converter (14) is used to mark and measure the coincidence of the signal photons. After data processing by a computer (15), the amplitude of the orbital angular momentum spectrum of the weak vortex light is detected.

9. The orbital angular momentum spectrum coincidence measurement device based on a time-correlated two-photon source as described in claim 8, characterized in that, The pump laser (1) is one of a solid-state laser, a fiber laser, or a semiconductor laser, with a wavelength selection range of 350-1100nm; the nonlinear crystal (3) is one of BBO, KTP, PPKTP, or PPLN; the filter (5) is a long-pass filter with a working cutoff wavelength greater than the pump light wavelength; the polarization beam splitter (6) has a splitting ratio adjustable in the range of 1:99 to 50:50; and the single-photon dot detector (7) is a silicon-based single-photon detector or an InGaAs / InP single-photon detector.

10. The orbital angular momentum spectrum coincidence measurement device based on a time-correlated two-photon source as described in claim 9, characterized in that, For different nonlinear crystals (3), different pump wavelengths are selected, including using a 405nm laser to pump a BBO crystal to prepare an 810nm correlated photon pair; using a 532nm laser to pump a KTP crystal to prepare a 1064nm correlated photon pair; and using a 775nm laser to pump a PPKTP or PPLN crystal to prepare a 1550nm correlated photon pair.

Images