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.

Description

TECHNICAL FIELD

The present invention belongs to the technical field of superconducting nanowire photon detection, and particularly relates to an array imaging technology.

BACKGROUND

A superconducting nanowire single photon detector (SNSPD), a novel single photon detector, is applied to the fields of quantum information, space communication, laser radar, spectrum detection, time flight, depth imaging and the like, has the advantages of high sensitivity, low noise, low dark counting, low time jitter and the like.

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 to realize single photon detection. A bias current lb is applied to the nanowire in the superconducting state, and a “hot-spot” is locally formed after the nanowire absorbs photons; the density of the current around the “hot-spot” exceeds superconducting critical current density, the partial resistance is increased, so that the current of the nanowire is reduced, and meanwhile, the joule heat effect of a resistance area is weakened to dissipate heat to surrounding environment; the temperature of the resistance area is gradually reduced to the ambient temperature, the resistance area disappears, and the current of the nanowire is recovered to the initial state.

The formation of electrical pulses reflects the process that the current of the nanowire I_D changes along with time, and if an external bias current I_b is constant, the SNSPD is equivalent to a dynamic inductor L_k, a switch S and a time-varying resistor Rn. 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_L=0, and I_D=I_B; after the nanowire absorbs photons, S is opened to form the resistor Rn, and according to the current continuity theorem, I_L+I_D=I_B, I_D is rapidly reduced, and I_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 tens of nanoseconds, and S is closed.

The detection efficiency of a single SNSPD is higher than that of a semiconductor avalanche photodiode (APD), but the single SNSPD can only represent whether photons are absorbed or not and cannot accurately output a number and spatial position distribution of the photons. By encoding the spatial position of the photons, the information which can be represented by the single photon can be further increased, but a detection area of the SNSPD is increased, and the dynamic inductance of the detector is increased at the same time, which influences the speed of the detector.

The large-scale superconducting nanowire single-photon detector array is used to make the size of a single picture element to be as small as possible, and a large-area single photon detection is achieved through a plurality of pixels; however, at present, it is still difficult to form a large-scale array by a plurality of SNSPDs and read the same.

A conventional imaging system digitizes an analog signal, processes the digital signal, and restores the digital signal into an original image. Two methods for digitizing weak signals are common. In the first method, weak current signals are converted and amplified into voltage signals in real time, that is, I-V conversion is performed, then analog voltage signals are converted into digital signals by A/D conversion, signals outputted by a detector are completely restored, and carriers in a common semiconductor radio frequency amplifier do not migrate and lose amplification effect under a condition of superconducting low temperature. The second method is a time period processing method, a current-voltage real-time conversion or integral circuit is adopted at the front end to convert current into a voltage signal, then the voltage signal is converted into pulses by a circuit such as a V-F conversion or a comparator, a single pulse represents fixed charge quantity, and the total charge quantity is in direct proportion to the number of the pulses.

SUMMARY

The present invention adopts a second method to solve the problems in the prior art, provides a three-dimensional imaging method based on a superconducting nanowire photon detection array, converts outputted pulse signals into current signals, integrates the current signals within a period of time, then converts the current signals into pulse output, inverts a number of pulses according to the direct-proportion relation between the total charge quantity and the number of outputted pulses, has a working wave band of 750 nm to 1550 nm and has the highest photon detection efficiency of 98%. In order to achieve the above purpose, the present invention adopts the following technical solutions.

Superconducting nanowire single photon detector (SNSPDs) are adopted as array elements to form a superconducting nanowire photon detection array, and a number of the array elements is adjusted according to detection requirements; a lens array is adopted as an photon alignment system, transmitted lights are split into a plurality of beams with the same number as that of the array elements, and the plurality of beams are converged to a superconducting nanowire detection area; a surface of an object is detected by adopting a pulsed laser, different light pulses reflected by the surface of the object are transmitted through the lens array, and a round-trip time of each photon is recorded; the photons detected by each array element are collected, the array elements are taken as picture elements, and a gray value of the picture element is calculated according to a number of photons of the array elements; and a gray-scale image is plotted by taking the picture elements as pixel points, a distance between the object and the pixel points is calculated according to the round-trip time of each photon, and a three-dimensional image of the object is reconstructed according to the gray-scale image and the distance between the object and the pixel points.

The array element comprises a superconducting nanowire circuit, an amplifying circuit, a converting circuit, an integrating circuit and a buffer, wherein 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.

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 temporary resistance generated by internal superconducting disturbance, so that the nanowire is quickly restored to a superconducting state.

The amplifying circuit comprises a biasing circuit, a first stage amplifying circuit, a second stage amplifying circuit and a compensating circuit, wherein 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 transistor and a capacitor, 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.

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.

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.

The nanowire circuit is biased in a state slightly lower than a superconducting critical current of the nanowire at the superconducting low temperature; the nanowire absorbs photons at the superconducting low temperature, the superconducting state of an absorption area is damaged, a “hot-spot” occurs, a resistor is generated and is connected with the thin film resistor in parallel, and the resistance value is changed; the nanowire is cooled, the “hot-spot” disappears, the nanowire is restored to the initial state, and the resistance value is changed; the changes of the resistance value of the nanowire enables the circuit to generate electrical pulse signals, and the electrical pulse signals are amplified by the amplifying circuit through the superconducting delay lines; the voltage signals are converted into current signals by the converting circuit, and charge quantity of the current signals is obtained as the charge quantity of absorbed photons by the integrating circuit; and the charge quantity of absorbed photons is stored in the buffer, inputted into the computer by rows, and compared with charge quantity of single photon to obtain the number of the absorbed photons.

A picture element position of a nanowire for generating photon responses is set as x_a, a time point for absorbing photons as t_a, a transmission speed of electrical pulse signals generated by the picture element along the nanowire as v, time read by the computer are 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_a+(L−x_a)/v and τ′=t_a+x_a/v, and after photons are absorbed, x_a=((τ−τ′)v+L)/2 and t_a=((τ+τ′)−L/v)/2; and n pixels in each row simultaneously generating the photon responses are set, wherein the reading time is τ₁, τ₂, . . . , τ_n, and positions of the photon responses are calculated and obtained.

The pulsed laser is used to record total round-trip time of each photon τ_all, a distance between the actual position of the object and the pixel points is calculated according to I=c*τ_all/2, where c is light speed in free space.

The present invention not only has single photon detection function, but also can obtain the photon information reflected or directly emitted by a surface of the object, and restores the original photon resolution information of the object by an algorithm, thereby realizing the identification of a target distance and a three-dimensional image.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is voltage pulse signals;

FIG. 2 is an equivalent circuit of an pulse response;

FIG. 3 is a nanowire and resistor structure;

FIG. 4 is an integration imaging process;

FIG. 5 is a two-stage amplifying circuit;

FIG. 6 is an integrating circuit;

FIG. 7 is a lens condensing process;

FIG. 8 is a picture element processing process; and

FIG. 9 is an array circuit principle.

DETAILED DESCRIPTION OF EMBODIMENTS

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

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.

It can be seen from the formation of the electrical pulse that an external bias current I_b is constant, the SNSPD is equivalent to a dynamic inductor L_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.

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_L=0, and I_D=I_B; after the nanowire absorbs photons, S is opened to form the resistor Rn, and according to the current continuity theorem, I_L+I_D=I_B, I_D is rapidly reduced, and I_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.

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.

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.

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.

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.

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 Q₁₉ works in a linear region and is equivalent to a resistor, and Q₁₉ and C₁ 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_B is generated in a branch circuit.

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_D through gate voltage V_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.

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.

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.

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.

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.

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.

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.

After a photon is absorbed by a superconducting nanowire, assuming that the position of a picture element generating response on the nanowire is x_a, the time point of photon absorption is t_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 V₀ is outputted to a register R_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_a+(L−x_a)/v and τ′=t_a+x_a/v, and the relationship between the position x_a of the picture element and the time t_a is x_a=((τ−τ′)v+L)/2 and t_a=((τ+τ′)−L/v)/2 after photon absorption.

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 τ₁, τ₂, . . . , τ_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.

The pulsed laser is used to record total round-trip time of each photon τ_all, a distance between the actual position of the object and the pixel points is calculated according to I=c*τ_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.

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.

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 xa, a time point for absorbing photons as ta, 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=ta+(L−xa)/v and τ′=ta+xa/v, and after photons are absorbed, xa=((τ−τ′)v+L)/2 and ta=((τ+τ′)−L/v)/2; and setting n pixels in each row simultaneously generating the photon responses, wherein the reading time is τ1, τ2, . . . , τn, 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 τall and light speed as c, wherein the distance between the object position and the pixel points is calculated according to l=c*τall/2.