THREE-DIMENSIONAL IMAGING METHOD BASED ON SUPERCONDUCTING NANOWIRE PHOTON DETECTION ARRAY

Abstract

A superconducting nanowire photon detection array adjusts a number of the array elements, a lens array as an photon alignment system, splits transmitted lights into a plurality of beams, and converges the plurality of beams to a superconducting nanowire detection area; detects a surface of an object by adopting a pulsed laser, transmits different light pulses reflected by the surface of the object through the lens array, and records a round-trip time of each photon; collects the photons detected by each array element, takes the array elements as picture elements, and calculates a gray value of the picture element; and plots a gray-scale image by taking the picture elements as pixel points, calculates a distance between the object and the pixel points, and reconstructs a three-dimensional image of the object and the distance between the object and the pixel points.

Claims

1. A three-dimensional imaging method based on a superconducting nanowire photon detection array, the method comprising: adopting superconducting nanowire single-photon detectors (SNSPDs) as array elements to form a superconducting nanowire photon detection array; adopting a lens array as an optical alignment system, splitting transmitted lights into a plurality of beams with the same number as that of the array elements, and converging the plurality of beams to a superconducting nanowire detection area; detecting a surface of an object by adopting a pulsed laser, transmitting different light pulses reflected by the surface of the object through the lens array, and recording a round-trip time of each photon; integrating pulse signals outputted by the detector array elements by adopting integral charges, obtaining a number of photons absorbed by the array elements by a corresponding relation between integral amplitude and a number of photons, and calculating gray values of picture elements by the number of photons of the array elements; and plotting a gray-scale image by taking the picture elements as pixel points, calculating a distance between the object and the pixel points according to the round-trip time of each photon, and reconstructing a three-dimensional image of the object according to the gray-scale image and the distance between the object and the pixel points.

2. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 1, wherein the array element adopts a superconducting nanowire circuit, an amplifying circuit, a converting circuit, an integrating circuit and a buffer; and the superconducting nanowire circuit is connected with an input end of the amplifying circuit, an output end of the amplifying circuit is connected with an input end of the integrating circuit through the converting circuit, and an output end of the integrating circuit is connected with a computer through the buffer.

3. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the step of collecting a number of photons detected by each array element comprises: placing the array elements at a superconducting low temperature, and biasing the superconducting nanowire circuit to be in a state slightly lower than a superconducting critical current of nanowire; allowing the nanowire to absorb photons and inflicting a damage on a superconducting state of an absorption area, then resulting in a “hot-spot”, and leading to changes in a resistance value; cooling the nanowire, resulting in the disappearance of the “hot-spot”, allowing the nanowire to restore to an initial state, and leading to changes in the resistance value; enabling the circuit to generate electrical pulse signals by the changes of the resistance value of the nanowire, and amplifying the electrical pulse signals by the amplifying circuit through superconducting delay lines; converting voltage signals into current signals by the converting circuit, and obtaining the amount of charge in current signals as charge on the absorbed photons by the integrating circuit; and storing the charge on the absorbed photons in the buffer and inputting the charge on the absorbed photons into the computer by rows, and comparing the charge on the absorbed photons with charge on single photon to obtain the number of the absorbed photons.

4. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the superconducting nanowire circuit is located at a center of the array element, and is connected with coplanar superconducting delay lines by connecting the superconducting nanowires and a thin film resistor in parallel; each row of the superconducting delay lines are connected with each other, the thin film resistor has a resistance value of 10Ω to 10000Ω, and the resistance generated by superconducting nanowire photon responses is in the order of kΩ to MΩ; the thin film resistor short-circuits the resistance generated by the nanowire, and releases a temporary resistance generated by internal superconducting disturbance, so that the nanowire is quickly restored to a superconducting state.

5. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the amplifying circuit adopts a first stage amplifying circuit, a second stage amplifying circuit, a compensating circuit and a biasing circuit; the first-stage amplification circuit adopts differential input, and the second-stage amplification circuit adopts a common-source amplifier; the compensating circuit consists of an MOS (Metal-Oxide-Semiconductor) transistor and a capacitor, wherein the MOS transistor works in a linear region and provides a constant bias current; and resistance is added into the biasing circuit by a source of the MOS transistor, and each array element shares a constant current source to generate a stable current.

6. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the converting circuit adopts a comparator and an MOS transistor, an input voltage is connected to a non-inverting input end of the comparator, a reference voltage is connected to an inverting input end of the comparator, an output end of the comparator is connected to a gate of the MOS transistor through a pull-up resistor, a drain of the MOS transistor is used as an output current, and if the input voltage is higher than the reference voltage, the MOS transistor conducts the output current.

7. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the integrating circuit adopts an MOS transistor and a capacitor, and an input current charges the capacitor through the MOS transistor to realize integration; and the circuit is reset to a low potential before integrating, and forced reset by a switch or reset by an MOS transistor is adopted.

8. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the step of plotting a gray-scale image by taking the picture elements as pixel points comprises: setting a picture element position of a nanowire for generating photon responses as x.sub.a, a time point for absorbing photons as t.sub.a, a transmission speed of electrical pulse signals generated by the picture element along the nanowire as v, adopting a superconducting nanowire as a delay line to connect a detector of each picture element, and reading time by the computer as τ and τ′ after the delay through a peripheral circuit and a register, wherein each row of detector delay lines have an equivalent length of L, thus τ=t.sub.a+(L−x.sub.a)/v and τ′=t.sub.a+x.sub.a/v, and after photons are absorbed, x.sub.a=((τ−τ′)v+L)/2 and t.sub.a=((τ+τ′)−L/v)/2; and setting n pixels in each row simultaneously generating the photon responses, wherein the reading time is τ.sub.1, τ.sub.2, . . . , τ.sub.n, and calculating to obtain positions of the photon responses.

9. The three-dimensional imaging method based on the superconducting nanowire photon detection array according to claim 2, wherein the step of calculating a distance between the object and the pixel points according to the round-trip time of each photon comprises: setting the round-trip time of the photon as τ.sub.all and light speed as c, wherein the distance between the object position and the pixel points is calculated according to l=c*τ.sub.all/2.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is voltage pulse signals;

[0020] FIG. 2 is an equivalent circuit of an pulse response;

[0021] FIG. 3 is a nanowire and resistor structure;

[0022] FIG. 4 is an integration imaging process;

[0023] FIG. 5 is a two-stage amplifying circuit;

[0024] FIG. 6 is an integrating circuit;

[0025] FIG. 7 is a lens condensing process;

[0026] FIG. 8 is a picture element processing process; and

[0027] FIG. 9 is an array circuit principle.

DETAILED DESCRIPTION OF EMBODIMENTS

[0028] The technical solutions of the present invention are described in detail below with reference to the drawings.

[0029] SNSPD adopts a nanowire prepared from an ultrathin superconducting material, forms a local hot-spot by absorbing photons, and generates voltage pulse signals at two ends of the nanowire (as shown in FIG. 1) to realize single photon detection.

[0030] It can be seen from the formation of the electrical pulse that an external bias current I.sub.b is constant, the SNSPD is equivalent to a dynamic inductor L.sub.k, a switch S and a time-varying resistor Rn, and the whole process is shown in FIG. 2 under the action of an external circuit.

[0031] When the SNSPD does not generate photon responses, the nanowire is in a superconducting state, S is closed, current passes through the nanowire to ground, I.sub.L=0, and I.sub.D=I.sub.B; after the nanowire absorbs photons, S is opened to form the resistor Rn, and according to the current continuity theorem, I.sub.L+I.sub.D=I.sub.B, I.sub.D is rapidly reduced, and I.sub.L is rapidly increased; due to the existence of an electrothermal feedback mechanism in the nanowire, the nanowire is recovered to a superconducting state after hundreds of picoseconds, and S is closed.

[0032] Superconducting nanowire single photon detectors (SNSPDs) are adopted as array elements to form an array, and incident photons are detected; array elements are taken as picture elements, voltage pulse signals outputted by each picture element are amplified, and the voltage signals are converted into current signals by adopting an MOS transistor; an integrating capacitor is adopted, the charge quantity is obtained by current signal integration, and a number of photons are calculated according to the charge quantity of the picture elements; the gray scale of each picture element is defined according to the number of photon number of each pixel in the array, the gray scale image of the array element is generated, and the gray scale image is converted into an original image.

[0033] In the center of a single picture element, a parallel structure formed by a superconducting nanowire and a thin film resistor is adopted, as shown in FIG. 3, two ends of the superconducting nanowire and the thin film resistor are connected with a coplanar superconducting delay transmission line and used as an input end of a two-stage amplifying circuit, an output end of the amplifying circuit is connected with a base of a triode, and an emitter of the triode is connected with an integrating circuit.

[0034] The working principle of the integration imaging circuit is shown in FIG. 4. The circuit is biased in a state slightly lower than the superconducting critical current of the nanowire when the picture elements are at a superconducting low temperature; the superconducting state of an absorption area is damaged after the nanowire absorbs photons, a “hot-spot” appears, and resistance is generated, and at this time, the superconducting nanowire and the resistor are considered to be connected in parallel, so that the resistance value of the whole circuit is changed; with the cooling of the nanowire and the substrate, the “hot-spot” disappears and the nanowire is recovered to an initial state; the process is represented as electrical pulse signals on an external circuit, the electrical pulse signals are amplified by a two-stage amplifying circuit with a superconducting delay line, and voltage signals are converted into current signals by an MOS transistor; and the current signals are integrated to obtain the charge on the picture element, and the number of photons of the picture element is calculated and obtained according to the charge quantity of a single pulse.

[0035] The resistor in the picture element circuit is made of metal materials or other resistance materials and the resistance value is 10Ω to 10000Ω. After superconducting nanowire photon responses, the nanowire resistor is changed from a superconducting state to a resistance state, the resistance is in the order of kΩ to MΩ; at this time, the nanowire is in short circuit connection with the resistance, the resistance plays a good shunting role, a temporary resistance state formed by superconducting disturbance inside the nanowire is released, the nanowire is prevented from being in a latch state, the superconducting current of the nanowire is improved, the current reduce time of the nanowire is shortened, and the nanowire is enabled to be rapidly recovered to the superconducting state.

[0036] As shown in FIG. 5, the two-stage amplifying circuit mainly comprises four parts: a biasing circuit, a first stage amplifying circuit, a second stage amplifying circuit and a compensating circuit; wherein the first-stage amplifying circuit adopts differential input, so that common-mode signal interference is effectively suppressed; the second-stage amplifying circuit adopts a common-source amplifier, a constant bias current is provided by an MOS transistor, the MOS transistor Q.sub.19 works in a linear region and is equivalent to a resistor, and Q.sub.19 and C.sub.1 form a Miller compensation circuit; a resistor R is added to the source of an MOS transistor in the bias circuit, and a stable current source I.sub.B is generated in a branch circuit.

[0037] A voltage/current conversion circuit is realized by a field effect transistor, an SNSPD array in the circuit works at extremely low temperature, a common semiconductor amplifier cannot work normally, and the field effect transistor is a voltage control device and controls a drain to output current I.sub.D through gate voltage V.sub.GS; voltage outputted by a front end circuit is connected to a non-inverting input end of a comparator, reference voltage is connected to an inverting input end of a comparator, an output end of the comparator is connected to the G pin through a pull-up resistor, and if the voltage outputted by the front end circuit is controlled to be higher than the reference voltage, the MOS transistor conducts the output current.

[0038] As shown in FIG. 6, the integrating circuit is composed of a PMOS transistor and an integrating capacitor, and the current outputted by the front-end circuit is charged to the integrating capacitor through an injection transistor to realize integration; the gain of the circuit is mainly related to the size of the capacitor and is also limited by the voltage of the power supply, the circuit is reset to a low potential before integration, and the circuit adopts a switch to reset forcibly and can also reset by adopting an MOS transistor.

[0039] The signals outputted by each picture element are collected to a buffer, a number of photons of picture element (i.e., pixel point) is calculated by a coefficient of which charge quantity is in direct proportion to the number of photons, and for a large array detector, the larger the array is, the more the picture elements are, the higher the pixels are, and the higher the image restoration is.

[0040] By adopting a special photon alignment system, as shown in FIG. 7, a lens array splits incident lights into a plurality of beams with the same number as that of picture elements, and the plurality of beams are converged to a superconducting nanowire detection area, so that the filling rate of the array device is improved.

[0041] The picture element structure is shown in FIG. 8, the picture element array is shown in FIG. 9 and is formed by arranging a plurality of picture elements, and each picture element in the array is provided with a single nanowire, a signal processing circuit and an integrating circuit; each column is connected with a constant current source to provide a bias current for the superconducting nanowires; the output end of each row of picture elements is connected with a buffer for a computer to read data, and the picture elements in each row are connected with each other by superconducting delay lines.

[0042] A photon detection is performed on a surface of an object, and different photon pulse signals reflected by the surface of the object are emitted into a picture element detection area of an integral imaging device through the lens array.

[0043] Assuming that all picture elements in the array simultaneously respond to photons and generate electrical pulse signals, electrical pulse signals of each picture element are integrated in a period using a time sequence circuit, output voltages are transmitted into a buffer, and the signals are read into a computer by rows; the longer the integration period is, the more photons the detector array detects each period, but the much longer the integration period is, the much more photons are, which results in the integrated signal being too large to be processed, so an integration period should be chosen appropriately.

[0044] After a photon is absorbed by a superconducting nanowire, assuming that the position of a picture element generating response on the nanowire is x.sub.a, the time point of photon absorption is t.sub.a, an electrical pulse signal is generated at the position of the picture element, the electrical pulse signal is processed by a peripheral circuit of the picture element, a voltage V.sub.0 is outputted to a register R.sub.a, the electrical pulse signal is transmitted to the other end of the nanowire at a fixed speed v, the time read by a computer is τ and τ′ after subtracting fixed delays such as access transmission time and reading delay time of electrical pulse in the peripheral circuit and the register; and assuming that each row of detector delay lines have an equivalent length of L, based on a relationship between distance, time and speed, it can be concluded that τ=t.sub.a+(L−x.sub.a)/v and τ′=t.sub.a+x.sub.a/v, and the relationship between the position x.sub.a of the picture element and the time t.sub.a is x.sub.a=((τ−τ′)v+L)/2 and t.sub.a=((τ+τ′)−L/v)/2 after photon absorption.

[0045] When n pixels in the same row generate photon response at the same time, the position of the photon response is determined according to the time τ.sub.1, τ.sub.2, . . . , τ.sub.n of reading picture elements; after a fixed time T, according to direct proportion of charge quantity to a number of photons, the voltage signal received by each picture element (pixel point) is restored to the number of photons to generate a statistical graph with gray scale, and when the superposition times are enough, the gray scale graph can reflect specific photon information of the identified object.

[0046] The pulsed laser is used to record total round-trip time of each photon τ.sub.all, a distance between the actual position of the object and the pixel points is calculated according to I=c*τ.sub.all/2, where c is light speed in free space, and a three-dimensional image of an object can be reconstructed after obtaining spatial position information of the object.

[0047] The above embodiments are not limiting of the present invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are included in the protection scope of the present invention.