GB2479002A

GB2479002A - A Photon Detector

Info

Publication number: GB2479002A
Application number: GB201005171A
Authority: GB
Inventors: Oliver Edward Thomas; Andrew James Shields; Zhiliang Yuan
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-03-26
Filing date: 2010-03-26
Publication date: 2011-09-28
Anticipated expiration: 2030-03-26
Also published as: GB201005171D0; GB2479002B

Abstract

A photon detector comprising an avalanche photodiode and a voltage source for the avalanche photodiode. The voltage source provides a gating bias to the avalanche photodiode with a voltage component which is static with respect to time (dc) and a voltage component which varies with time (ac). The voltage source applies a static dc component and an ac component to the photodiode via a inductance and a capacitance respectively on a bias tee. The time varying component is cyclical; each cycle has a high voltage portion where said avalanche photodiode is reverse biased above its breakdown voltage and a low voltage portion where said avalanche photodiode is biased below said breakdown voltage. The duration of the high voltage portion is longer than the duration of the low voltage portion. An output circuit removes a time varying voltage component from the output signal. Multiple photons may be detected.

Description

A Photon Detector The present invention relates to the field of photon detectors and methods for detecting weak light signals.

There is a pressing need in a number of applications for optical light detectors which can register a response at the level of individual photons. Single photon detectors are threshold devices which detect the presence of I or more photons on the device, but cannot determine the number of photons. They are used for general low light level detection, as well as for various applications based around determining the arrival time of the photon at the detector.

The applications of single photon detectors include industrial inspection, environmental monitoring, testing of fibre optic cables and components, medical imaging, chemical analysis and scientific research. Many of these applications use the ability of a single photon detector to measure the arrival time of a single photon. In industrial inspection systems a bright laser pulse is directed at the object under inspection and the time for single photons from the pulse to be reflected are recorded. From the time of flight data it is possible to build a 3D image of the object. Similar techniques involving single photon detectors are used to determine the location of faults in optical fibres and components, and to measure particles in the atmosphere.

Single photon detection is also used in various forms of x-ray and radioisotope imaging in medical imaging, as well as in laser optical imaging at infra-red wavelengths.

Lifetime fluorescence measurements using single photon detection can be used in the diagnosis of some medical conditions. It is employed in analytical chemistry for determining the chemical recipe of a sample. Single photon detection is also used in scientific research in the field of particle physics, astrophysics and materials science.

The photon detector of the present invention allows low noise light detection over a range of different wavelengths, for example, in the visible, near infra-red or mid-infrared regions of the spectrum. In particular, using a silicon APD it may be used to detect in the range 300-1 lOOnm or using an lnGaAs APD it may be used to detect in the range 800-1600 nm. Wavelengths up to 2400 rim and 5000 nm can also be detected using GaAsSb/GalnAs/lnP and HgCdTe -based APDs respectively.

Photon number resolving detectors, not only detect the presence of photons, but are able also to count the number of photons in an incident light pulse. Like single photon detectors they are also able to determine the arrival time of the photons at the detector.

Photon number resolving detectors are required for low noise light detection based on photon counting. Here they have the advantage over single photon detectors that they can operate effectively with light intensities higher than the single photon level.

The ability to resolve the number of photons in the incident pulse is also very important for many applications in quantum information technology. In a quantum relay, for example, it is necessary to distinguish between 0-, 1-and 2-photon detection events in each detector. A similar detector capability is needed for many of the gates used in linear optics quantum computing.

The present invention provides a photon number resolving detector which can operate in the commercially useful visible to near infra-red part of the spectrum, as well as at longer wavelengths. The ability to detect weak optical signals and resolve the number of photons in a signal in this wavelength range has applications for linear optics quantum computing, quantum relays and repeaters, quantum cryptography, photon number state generation and conditioning, and characterisation of photon emission statistics of light sources.

Current photon number resolving detectors at visible/near infra-red wavelengths include Transition Edge Sensors (TES); Parallel Nanowire Detectors (PND); Visible Light Photon Counter (VLPC); Time-multiplexed Detectors (TMD); and Multi-pixel Silicon Avalanche Photodiodes (MPPC). However, these are unsuitable for practical applications. Some of their disadvantages are: 1. TES detectors have very low maximum count rates (0.05 MHz), large timing jitter, long integration times, can only operate at milli-Kelvin temperatures 2. PND detectors have very low efficiency, low maximum count rates (80 MHz), can only operate at Liquid Helium temperature 3. VLPC detectors have very low maximum count rates (0.015 MHz), large timing jitter, high dark count rates, can only operate at liquid Helium temperatures 4. For TMD detectors the probability of successful measurement is strongly limited by the detection efficiency and decreases exponentially with measured photon number 5. MPPC detectors allow only low-speed operation and low efficiency due to the geometrical fill-factor of the pixel array.

Geiger Mode Avalanche Photodiodes (APDs) are also used for low noise light detection at visible/near infra-red wavelengths.

In the Geiger mode an APD is biased with a DC voltage which is above the breakdown voltage condition of the diode, requiring an additional electronic circuit to quench the current avalanches induced by detection events. Geiger Mode APDs can be used as threshold single photon detectors, i.e. they can detect when 1 or more photons in a light pulse is incident on a device. However, they do not work as photon number resolving detectors, i.e. they do not distinguish between 1, 2, 3 or more photons incident on the device. As a result Geiger Mode APDs can be used to detect very weak optical signals (ie much less than I photon per incident pulse), but they cannot be used for stronger signals.

In addition, Geiger Mode APDs have the disadvantages of low maximum counting rates, the requirement for complicated quenching circuits, high dark count levels and poor timing jitter.

In W0200811 04799 the content of which are herein incorporated by reference an APD is operated in a mode which is different to Geiger mode, where a time varying bias as well as a steady state bias is applied. In unpublished patent application GB 0919588.4 the contents of which are herein incorporated by reference, a Silicon APD is operated in a mode where both a time varying bias and a steady state bias are applied. This APD was found to have good photon number resolving properties.

The present invention provides an improvement upon the photon number resolving performance of W02008/104799 and unpublished patent application number GB 0919588.4 and in a first aspect provides a photon detector comprising an avalanche photodiode and a voltage source for said avalanche photodiode, wherein said voltage source is configured to bias said avalanche photodiode with a voltage component which is static with respect to time and a voltage component which varies with time, and wherein said time varying component is cyclical and each cycle has a high voltage portion where said avalanche photodiode is reverse biased above its breakdown voltage and a low voltage portion where said avalanche photodiode is biased below said breakdown voltage, and the duration of the high voltage portion is longer than the duration of the low voltage portion.

The detector of the present invention provides an enhanced photon number resolution.

This is achieved due to the antisymmetric time varying bias signal which has a high voltage portion which is longer than its low voltage portion.

Although the time varying component is cyclical, the period of the cycles does not have to be constant. The time varying component may be rectangular in profile or may have a more rounded profile.

Preferably, the high voltage portion is at least 2 times longer than the low voltage portion, more preferably at least 4 times longer, even more preferably at least 8 times longer.

In an embodiment, an output circuit compares the output voltage of the APD with that in a preceding period. In a further embodiment, the circuit combines the output voltage of the APD in the first and second half of the gating bias period.

In a further embodiment, a self differencing circuit is provided comprising a signal divider to split the signal into two parts, an electrical line to delay one of parts relative to the other and a signal differencer to output the difference between the two parts.

The delay is preferably an integer number of gate periods.

In yet a further embodiment, a self differencing circuit is provided comprising a signal divider to split the signal into two parts, a phase shifter to shift the phase of one of the two parts by 180 degrees, or a signal inverter, and a signal combiner to output the sum of the two parts.

If a self differencing circuit is provided, it may further comprise a control unit configured to balance the strength of the two parts and/or vary the length of the delay.

An amplifier or combination of amplifiers may be provided to amplify the output of the self-differencing circuit.

The output circuit may also comprise a filter or series of filters to remove a periodic background or use lock-in' techniques to retrieve the avalanche signal.

As previously mentioned, the present invention may be used for photon counting.

Therefore, in a further embodiment an output circuit is provided to discriminate the output voltage between multiple predetermined levels. For example, such a circuit may be configured to measure the height of the avalanche peak, as the height of the peak is dependent on the number of photons detected. In a further example, the output circuit is provided with a discriminator, said discriminator being configured to set multiple discrimination levels, each discrimination level equivalent to a value of the output signal corresponding to 1, 2, 3, 4 photons etc. In a further embodiment the photon detection system further comprises a cooler to lower the temperature of the avalanche photodiode.

In a further embodiment the photon detection system comprises beamsplitters to subdivide the incident pulse into several component pulses, which are detected using multiple avalanche photodiodes. In a further embodiment the detection system comprises an array of APE) elements.

Light may be coupled to the avalanche photodiode through an optical fibre.

In a second aspect, the present invention provides a method of detecting photons, the method comprising: providing an avalanche photodiode; and applying a bias to said avalanche photodiode with a voltage component which is static with respect to time and a voltage component which varies with time, and wherein said time varying component is cyclical and each cycle has a high voltage portion where said avalanche photodiode is reverse biased above its breakdown voltage and a low voltage portion where said avalanche photodiode is biased below said breakdown voltage, and the duration of the high voltage portion is longer than the duration of the low voltage portion.

The present invention will now be described with reference to the following preferred embodiments in which: Figure 1 is a schematic of an avalanche photodiode configured for self differencing; Figure 2 is a plot of a known bias signal for an avalanche photodiode of the type shown in figure 1; Figure 3 is a plot of a bias signal for biasing an avalanche photodiode in accordance with an embodiment of the present invention; Figure 4 is a plot of the probability distribution reflecting the statistics of the avalanche peak height of the self.-differencer output, Vd, for a fixed average photon flux; Figure 5 is a plot of the probability distributions measured for bias signals having differing characteristics; Figure 6 is a schematic of a photon detector in accordance with a further embodiment of the present invention; and Figure 7a is a schematic of a photon detector in accordance with an embodiment of the present invention in which the detector is mounted on a thermo-electric cooler viewed from a side and figure 7b is a schematic of the same photon detector of figure 7a viewed from the front.

Figure 1 depicts a photon detector in accordance with an embodiment of the present invention. The photon detector is an Avalanche Photodiode (APD) which is arranged in a configuration which allows self-differencing of the output signal. Self differencing will be described in more detail later in the description. The detector is referred to as a SeIf-Differencing Avalanche Photodiode (SD-APD).

APDs are well known in the art and the internal layer structure of the APD will not be described here.

An avalanche photodiode (APD) 117 is connected such that it is reverse biased. The bias voltage comprises both a DC component VDC 111 from DC bias source 113 and an AC component VAC 107 from AC bias source 109.

The AC 107 and DC 111 components are combined using bias-tee 105. Bias tee 105 comprises, on a first arm of the tee, a capacitor 101 connected to the AC source 109 and, on the second arm of the tee, an inductor 103 connected to the DC source 113.

The output of the APD 117 is divided between a resistor 119 which leads to ground and self differencing circuit 123.

When a photon is incident on the APD 117, a macroscopic avalanche photocurrent is induced by avalanche charge multiplication arising from photon detection and leads to a voltage across a series resistor 119, which corresponds to the output voltage, V0t, 121.

To isolate the periodic capacitive response of the APD 117 to a gating modulation, which masks small avalanches resulting from high-speed operation, a seif-differencing circuit is used 123, comprising a signal divider 125, two electrical lines 127 and 129 and a signal differencer 131.

The APD output voltage, 121 is input into signal divider 125, which divides the signal into two close to equal components. A potentiometer 135 is used to balance the dividing ratio and further equate the two components. Since one of the electrical lines 127 is longer than the other 129, one of these components is delayed.

The delay is chosen to be an integer number of gating periods T supplied by the AC voltage source 111, and the delay line 127 is chosen to be adjustable in order to tune the delay independently of T. When these two signals are input into the signal differencer 131, they are subtracted one from the other and the strong periodic capacitive background is largely cancelled in the self-differencer output voltage, V5d, 133. It is common to use a low-pass filter 137, for example with a 780 MHz frequency cut-off, and amplifier 139 to further improve the retardation of the capacitive background.

This allows small avalanches to be revealed in the self-differencer output, VSd, 133.

The amplitude of these small avalanches is dependent upon the incident photon number.

As an alternative to the set-up in Figure 1, the electrical delay between the electrical lines 127 and 129 may be chosen to be an odd integer number of half the gating period T. In this case the signal differencer 131 is replaced by a signal combiner, which adds the two signals. This also has the effect of cancelling the capacitive response of the APD 117, leaving only the weak photon induced avalanche.

The APD 117 may comprise Silicon and may be of the deep junction or shallow junction variety. It may also be a Silicon -Germanium heterostructure. Alternatively the APO may be designed to work at longer wavelengths and be grown on an lnP substrate. It may consist of a lnGaAsP heterostructure. It may also consist of a GalnAs/GaAsSb or HgCdTe heterostructure for use at yet longer wavelengths.

Preferably the APD has a diameter of its active area of between 10 and 200 microns.

Preferably it has a junction capacitance less than 10 pF.

Figure 2 is a plot of the bias signal in voltage (arbitrary scale) over time. This bias signal is already known from W02008/1 04799.

The APD has a reverse breakdown voltage Vb,. 201 above which a macroscopic avalanche gain of photoexcited carriers can occur. Depending on the operation temperature and the particular device structures, the breakdown voltage for APDs can vary from 20 to 300 V. Preferably, the breakdown voltage is between 20 V and 50 V. (Note that we write this as a positive number, although it is actually a reverse bias applied to the p-n junction of the APD).

The APD bias voltage Va 115, comprises an AC voltage with peak-to-peak amplitude Vac 107 and a period T 203 superimposed on a DC voltage VdC 111.

The period of the AC bias is sometimes referred to as the gating period or the clock period and is the inverse of the gating frequency or clock frequency. For example, the SD-APD may be operated with an AC gating period of 2 ns, corresponding to a gating frequency of 0.5 GHz.

VdC 111 can also be set above, at or below the breakdown voltage and a value of Vac 107 is chosen such that the APD bias voltage 115, lies above the breakdown voltage 201 at its highest values Vhj9h 209 and below the breakdown voltage 201 at its lowest values V10, 205.

It should be noted that the bias signal is symmetric in time, with the duration for which the bias is above the breakdown voltage Thigh 213 being approximately equal to the time duration for which it is less than the breakdown voltage 211.

Figure 3 is a plot of bias signal against time, which can be used to drive an APD in accordance with an embodiment of the present invention. Comparing the bias signal of figure 3 with that of figure 2, the duration Thigh 303 for which the APD bias is above the breakdown voltage Vb,. 201 in figure 3 is extended with respect to that of figure 2 and the duration T10 301 for which the signal of figure 3 is below the breakdown voltage Vbr 201 is reduced compared to that of figure 2. This means that Thigh> T10, unlike the case in figure 2 where they are essentially equal.

For example, if the gating period T is 2 ns, possible values for Thigh 303 and T,0 301 are ThIgh 1.8ns and T,0 = 0.2 ns.

The gating period or clock period may be synchronised with that of a photon source.

In one embodiment, the gating frequency of the detector is varied by a small amount e.g. 50 kHz, which is used to desynchronise the detector from the photon source, essentially broadening the time window over which the detector is capable of detecting photons.

Figure 4 shows the avalanche statistics, arising from the avalanche probability plotted as a function of the self-differencer output, VSd 115.

The probability distribution is obtained from around 6 million samples, and accumulated in real-time using a fast digital oscilloscope.

Peak 401 at 0 mV corresponds to the 0-photon contribution from gates in which no photon was detected.

The width of this feature (-18 mV) is attributed to a residual component of the capacitive response of the diode, due to the imperfect cancellation of the self-differencer circuit.

The feature around 30.5 mV, peak 403, is due to avalanches arising from the absorption of one photon and peaks 405, 407 and 409 at 53.1 mV, 69.9 mV and 84.7 mV respectively correspond to the detection of two, three and four photons.

Figure 5 shows the probability distribution of the self-differencer output, VSd 115 for five different detector bias signals. A constant period of 2 ns is used for each signal In the uppermost trace, components of Th9h = l000ps and T0 =l000ps correspond to a symmetric bias signal. In the second trace from the top Thigh = l200ps and Tk,W = 800ps. It can already be seen that this small change in the symmetry of the pulse with time greatly increases the separation of the peaks arising from the absorption of different numbers of photons. The lowest 3 traces in descending order are for Thigh = l400ps, l600ps and 1830ps, for which 7 = 600ps, 400ps and l7Ops respectively.

It can be seen that the photon number peaks move to higher voltage with increasing Thigh, and that the peaks can be more clearly discerned as a result.

Figure 6 shows a variant on the SD-APD of figure 1. Here the electrical lines 127 and 129 of Figure 1 are replaced with a phase shifter 1001, so as to induce a phase shift of 180 degrees in one (1005) of the two outputs (1005, 1007) of the power splitter 125.

The 180° phase shifter acts as a signal inverter.

The signal differencer 131 of Figure 1 is replaced with a signal combiner 1003, whose function is to add the two signals.

Since they have a relative phase shift of 1800 this has the effect of cancelling the capacitive response of the APD.

This allows the detection of weak avalanches in a similar fashion to that described in

the preceding description.

Figures 7a and b show a side view and a front view (the front being the side where a fibre is coupled to said detector) respectively of a SeIf-Differencing Avalanche Photodiode 117 mounted on a thermo-electric cooler 1107.

Thermal contact is provided to the case of the packaged device 117 through a copper heat-sink 1103 and conductive screws 1105. A temperature of T -30 °C is commonly used.

Optical access to the sample is provided by an optical fibre pigtail 1101. Electrical access to the Si-APD, Vapd 115 and 121 is provided by metallic pins.

Claims

CLAIMS: 1. A photon detector comprising an avalanche photodiode and a voltage source for said avalanche photodiode, wherein said voltage source is configured to bias said avalanche photodiode with a voltage component which is static with respect to time and a voltage component which varies with time, and wherein said time varying component is cyclical and each cycle has a high voltage portion where said avalanche photodiode is reverse biased above its breakdown voltage and a low voltage portion where said avalanche photodiode is biased below said breakdown voltage, and the duration of the high voltage portion is longer than the duration of the low voltage portion.
2. A photon detector according to claim 1, wherein the time varying bias has a period less than 100 ns.
3. A photon detector according to any preceding claim, where the time varying bias has an amplitude larger than I Volt.
4. A photon detector according to any preceding claim wherein the difference between the static bias and the breakdown voltage of the avalanche photodiode is less than half the maximum amplitude of the time varying bias.
5. A photon detector according to any preceding claim, further comprising an output circuit configured to receive an output signal from said avalanche photodiode and process said output signal to remove a time varying component from said output signal.
6. A photon detector according to claim 5, wherein said output circuit is configured to compare the output voltage of the avalanche photodiode with that in a preceding period.
7. A photon detector according to either of claims 5 or 6, wherein said output circuit is configured to combine the output voltage of the APD in the first and second half of the gating bias period.
8. A photon detector according to any of claims 5 to 7, wherein the output circuit comprises a signal divider to split the signal into two parts, an electrical line to delay one of parts relative to the other and a signal differencer to output the difference between the two parts.
9. A photon detector according to 8, wherein the delay is an integer number of gate periods.
10. A photon detector according to any of claims 5 to 7, where the output circuit comprises a signal divider to split the signal into two parts, a phase shifter to shift the phase of one of the two parts by 180 degrees, or a signal inverter, and a signal combiner to output the sum of the two parts.
11. A photon detector according to any of claims 5 to 10, wherein the period of the time varying modulation varies as a function of time.
12. A photon detector according to either of claims 8 or 10 further comprising a controller configured to balance the strength of the two parts.
13. A photon detector according to either of claims 8 or 10, further comprising a controller configured to vary the length of the delay.
14. A photon detector according to any of claims 5 to 13, further comprising an amplifier to amplify the output of the self-differencing circuit.
15. A photon detector according to any of claims 5 to 14, further comprising a circuit to discriminate the output voltage of the output circuit between multiple predetermined levels.
16. A photon detector according to any preceding claim, further comprising a cooler to lower the temperature of the avalanche photodiode.
17. A photon detector according to any preceding claim, comprising multiple avalanche photodiodes or photodiode elements.
18. A photon detector according to any preceding claim, further comprising means to couple the light to the avalanche photodiode through an optical fibre.
19. A photon detector according to any preceding claim, where the avalanche photodiode comprises Silicon or its heterostructures, Indium Gallium Arsenide or Indium Phosphide or their heterostructures
20. A method of detecting photons, the method comprising: providing an avalanche photodiode; and applying a bias to said avalanche photodiode with a voltage component which is static with respect to time and a voltage component which varies with time, and wherein said time varying component is cyclical and each cycle has a high voltage portion where said avalanche photodiode is reverse biased above its breakdown voltage and a low voltage portion where said avalanche photodiode is biased below said breakdown voltage, and the duration of the high voltage portion is longer than the duration of the low voltage portion.