WO2011067566A1

WO2011067566A1 - Improvements in and relating to optical sensors and optical sensing

Info

Publication number: WO2011067566A1
Application number: PCT/GB2010/002216
Authority: WO
Inventors: Mark Pitter; Michael Somekh; Nicholas Johnston; Roger Light
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2009-12-05
Filing date: 2010-12-03
Publication date: 2011-06-09
Anticipated expiration: 2012-06-05
Also published as: GB2488082A; GB201209792D0; GB0921354D0

Abstract

A photo diode (12) is connected across a power supply (16,18) by a switch (19). A variable capacitance (14) is connected across the photo diode (12), through a switch (22). Accordingly, the capacitance (14) can be charged by closing the switches (19,22). Thereafter, the switch (19) is open, the photo diode (12) is exposed to illumination (24) and the switch (22) is closed for a measurement period. The degree to which the capacitance (14) discharges through the photo diode (12), during the measurement period, is a measure of the illumination intensity at the photo diode (12) and is output as a voltage at (26). The capacitance (14) is varied so that conversion gain of the apparatus can be increased as illumination intensity reduces, improving the contrast and signal-to-noise attributes of the output signal.

Description

Improvements in and Relating to Optical Sensors and Optical Sensing

The present invention relates to improvements in and relating to optical sensors and optical sensing.

Many situations arise in which optical sensing is required. Examples include cameras, which are typically required to provide high-quality output even at low incident light levels. Other examples include scientific measurement applications. In some measurement applications, optical sensors are required to provide high-quality measurement of relatively small variations in a relatively high intensity illumination. That is, the sensors are required to isolate a small signal within a large background. The examples to be described are considered to be particularly, but not exclusively, applicable to this latter type of measurement application.

Examples of the present invention provide apparatus comprising: a photosensitive element, and a capacitance associated with the photosensitive element, wherein the photosensitive element provides a discharge path for the capacitance while the photosensitive element is illuminated, and the photosensitive element has a conductivity which depends on the illumination intensity, and wherein the value of the capacitance is variable, the apparatus further comprising control means operable to select the value of the capacitance. The apparatus may further comprise at least one capacitor which is a component separate from the photosensitive element. There may be a plurality of capacitors which are each components separate from the photosensitive element. The apparatus may comprise switch means operable to connect and disconnect the photosensitive element with the or each separate capacitor. The switch means may be operable to connect at least one of the separate capacitors in parallel with the photosensitive element.

The switch means may comprise a plurality of switch elements, each separate capacitor having an associated switch element. Each switch element may be separately controllable. Each switch element may be controlled by a switch voltage. The switch means may further comprise a register operable to store information relating to the required state of each switch. The switch means may comprise a data input for receiving state information, and be operable to load state information into the register.

The photosensitive element may be a photodiode. At least the photosensitive element and the capacitance may be implemented in CMOS technology.

The apparatus may comprise a plurality of capacitances, as aforesaid, associated with a common photosensitive element. There may be processing means operable to combine signals representing the degree of discharge of the respective capacitances. The signals may be combined to form difference signals. The difference signals may be digitised. The signals may be generated and combined within an integrated circuit, wherein the output of the integrated circuit is provided as the difference signals.

In another aspect, examples of the invention provide a sensor comprising a plurality of. pixels, each pixel comprising apparatus as defined above.

Examples of the present invention also provide a method comprising: providing a photosensitive element; providing a capacitance associated with the photosensitive element, the photosensitive element providing a discharge path for the capacitance while the photosensitive element is illuminated, and the photosensitive element having a conductivity which depends on the illumination intensity, and controlling the value of the capacitance.

At least one capacitor which is a component may be provided separate from the photosensitive element. There may be a plurality of capacitors which are provided as components separate from the photosensitive element. Switch means may be provided, operable to connect and disconnect the photosensitive element with the or each separate capacitor. The switch means may be operated to connect at least one of the separate capacitors in parallel with the photosensitive element.

Each separate capacitor may be provided with an associated switch element. Each switch element may be separately controlled. Each switch element may be controlled by a switch voltage. Information relating to the required state of each switch may be stored in a register.

A plurality of capacitances, as aforesaid, and associated with a common photosensitive element may be provided. Signals representing the degree of discharge of the respective capacitances may be combined. The signals may be combined to form difference signals. The difference signals may be digitised. The signals may be generated and combined within an integrated circuit, wherein the output of the integrated circuit is provided as the difference signals.

In another aspect, the present invention provides apparatus comprising: a photosensitive element, and a plurality of capacitances associated with the photosensitive element, wherein the photosensitive element provides a discharge path for the capacitances while the photosensitive element is illuminated, and the photosensitive element has a conductivity which depends on the illumination intensity, and wherein the apparatus further comprises processing means operable to combine signals representing the degree of discharge of the respective capacitances.

The signals may be combined to form difference signals. The difference signals may be digitised. The signals may be generated and combined within an integrated circuit, wherein the output of the integrated circuit is provided as the difference signals.

Examples of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram of an example embodiment of the invention;

Fig. 2 illustrates the capacitance of Fig. 1 in more detail;

Fig. 3 illustrates the control arrangement for one capacitor of Fig. 2;

Fig. 4 illustrates an array of pixels formed from circuits as shown in Fig. 1 ; Fig. 5 illustrates an illumination profile which may be encountered in a measurement application;

Fig. 6 illustrates a temporal modulation in the profile of Fig. 5; and

Fig. 7 schematically illustrates a system constructed from circuits of Fig. 1

Fig. 1 illustrates apparatus 10 which is an example embodiment of the present invention. The apparatus 10 includes a photosensitive element 12 which, in this example, is a photodiode. A capacitance indicated at 14 is associated with the photodiode 12.

The photodiode 12 is connected across the power supply 16, 1 8, from which it can be isolated by a switch 19. The positive side of the capacitance 14 is connected to the positive terminal of the photodiode 12, through a switch 22, here called the shutter switch. Accordingly, the capacitance 14 can be charged to supply voltage when the switching components 19, 22 are closed, or isolated from the positive rail 16 of the power supply. The switch 22 also allows the capacitance 14 and the photodiode 12 to be connected or isolated from each other.

During use, the photodiode 12 is exposed to illumination 24. The conductivity of the photodiode 12 depends on the intensity of the illumination 24. While the photodiode 12 is illuminated, it therefore provides a discharge path for the capacitance 14, when the shutter switch 22 is closed and the switching component 19 is open.

Accordingly, a measurement of the intensity of the illumination 24 can be taken in the following manner. First, the apparatus 10 is prepared by closing the shutter switch 22 and closing the switching component 19 in order to charge up the capacitance 14 to the supply voltage at 16. The switching component 19 is then opened to isolate the capacitance 14 and photodiode 12 from the supply voltage at 16. The photodiode 12 is then exposed to the illumination 24. The capacitance 14 begins to discharge through the photodiode 12, at a rate which depends on the conductivity of the photodiode 12, and thus depends on the intensity of the illumination 24. At the end of the required measurement period, the shutter switch 22 is opened to prevent further discharge of the capacitance 14. The voltage at the positive terminal of the capacitance 14 is provided as an output at 26, through an amplifier 28. The output voltage at 26 represents the intensity of the illumination 24. The higher the intensity of the illumination 24, the more the capacitance 14 will have discharged in a given time, and the lower will be the output voltage at 26.

In accordance with the invention, the capacitance 14 has a value which is variable. Furthermore, a control arrangement 30 is provided, operable to select the value of the capacitance 14. The manner in which this is achieved, and the significance of it can now be described, turning first to the structure of the variable capacitance 14.

Fig. 2 illustrates the capacitance 14 in more detail. The capacitance 14 includes a line of individually identifiable capacitors 32. Each capacitor 32 is a component separate from the photosensitive element 12. However, it is to be noted that the internal capacitance of a photodiode can also be used to provide additional capacitance.

Each of the capacitors 32 is connected in parallel with each other and across the photodiode 12, from the positive terminal of the photodiode 12, to the negative supply rail 18. A switch 34 is associated with each capacitor 32 and in series with the capacitor 32 and the shutter switch 22, between the capacitor 32 and the positive terminal of the photodiode 12. When the shutter switch 22 is closed, each switch 34 serves to connect and disconnect the photodiode 12 with the corresponding capacitor 32, according to the state of the switch 34. Each switch 34 is separately controllable, as indicated at 36. In this example, the control at 36 is provided by a switch voltage from a register 38. The register 38 stores information relating to the required state of each switch 34. For example, the register 38 may contain a cell 42 corresponding with each switch 34, containing a binary vaiue for the corresponding switch 34, representing the required state (ON/OFF). The switch control voltages 36 are then generated in accordance with the binary values in the register 38, to open or close the switches 34, as required.

The register 38 has a data input 40 for receiving state information. In this example, the state information is a binary string containing a bit representing the required state of each switch 34. State information received at the data input 40 is stored in the register 38.

Fig. 3 shows one of the capacitors 32 in more detail, together with the associated control circuit, again in more detail. Fig. 3 shows one cell 42 of the register 38, in the form of a latching gate having a Value input 44, a Load input 46 and an output 48. The cell 42 is loaded by a change of voltage at the Load input 46 which results in the contemporaneous value at the Value input 44 being latched into the cell 42, thereafter appearing at the output 48 until a new value is loaded at a subsequent time. The value at the output 48 is applied at 50 to control the state of the switch 34. Thus, the device 34 is set ON or OFF according to the binary value held by the cell 42. Thus, the binary value held by the cell 42 causes the capacitor 32 to be connected or disconnected across the photodiode 12.

Accordingly, the choice and number of capacitors 32 which are switched into circuit with the photodiode 12 can be controlled by loading an appropriate binary string into the cells 42 of the register 38. Those capacitors 32 which are switched into circuit will all be in parallel with each other, so that the value of the capacitance 14, as seen by the photodiode 12, will be equal to the sum of the capacitances of those capacitors 32 which are switched into circuit.

The capacitors 32 may each have the same capacitance value, or other arrangements could be used.

The apparatus 10, as just described, can be used as one pixel in an array of pixels forming a sensor 54, as illustrated in Fig. 4. In the example of Fig. 4, a group of pixels, each formed as described above, forms a sensor 54 which is a linear array. Alternatively, a square or rectangular array could be formed. Each pixel 10 in the sensor 54 is individually addressable by a data input 58, used to load the register of the pixel, as described above.

The significance of the variable capacitance 14, and the ability to select the value of the capacitance, can now be described with particular reference to Fig. 5 and Fig. 6. Figure 5 illustrates a profile which may be encountered in an optical measurement application. The vertical axis represents the intensity of illumination 24 received at the photodiode 12. The horizontal axis represents another variable, which maybe position, angle or another parameter. As pan be seen, the illumination intensity 24 varies with the other parameter. The profile of illumination intensity may be Gaussian or another shape, and may have more than one peak. Many measurement applications exist in which the output is expected to follow the general shape of a known profile (such as a Gaussian), with useful information contained within small variations from the expected profile. Examples of such measurement applications include picosecond laser ultrasonics, ultrafast spectroscopy and other applications employing pump/probe methods.

Useful information can be recovered from the Gaussian (or other) expected output by applying a temporal modulation to the input of the measurement application and observing the effect at the output, which is the illuminating intensity 24 observed at the photodiode 12. The temporal variation is illustrated in Fig. 6. This represents the illumination intensity 24, over time, at a single value of the horizontal axis of Fig. 5, as the input to the experiment is modulated. It can be shown that by measuring the intensity at the four times indicated in Fig. 6 as , l₂, Ι3» and U, the amplitude of the temporal modulation signal 80 can be recovered as:

{Modulation Amplitude}² = {( l₃ - I1)² + ( - †) (Equation 1 )

When a main profile such as that illustrated in Fig. 5 is observed by the apparatus 10 in order to recover information from smaller amplitude modulations, issues arise in relation to the quality of information recovered, particularly as a result of signal-to-noise ratios. This arises because the voltage output from a pixel (at the output 26) is proportional to the number of photons received by the photodiode 12 and inversely proportional to the value of the capacitance 4. That is:

V = q/C where V is the output voltage;

C is the value of the capacitance 14; and

q is the value of charge drained from the capacitance 4, during exposure (and which is proportional to the number of photons received by the photodiode 12).

The value 1/C is known as the "conversion gain" for the system, and reduces as the value of the capacitance 14 increases. If the value C is chosen to be optimum for the peak of the profile shown in Fig. 5, and is thereafter kept constant, then the voltage output will reduce away from the peak. The voltage difference produced by the same change of illuminating intensity will also decrease, which results in a lower contrast in the output voltage, which will tend to increase problems of signal-to-noise ratio in subsequent processing stages. For example, with a lower contrast output signal, problems of read-out noise in later processing stages will become more significant. Accordingly, the modulation information contained in the profile may be more difficult to recover, away from the peak of the profile.

However, the apparatus 10 allows the value of the capacitance 14 to be changed. Thus, away from the peak of the profile, where the intensity is lower, some of the capacitors 32 can be switched out of circuit to reduce the value of the capacitance 14 and hence to increase the conversion gain of the apparatus 10. This boosts the voltage at the output 26. Furthermore, the contrast achieved in the output signal is also improved. That is, the voltage difference produced by the same change of illuminating intensity is increased over that which is achieved without the reduction in the capacitance value. This will enable a more beneficial response to be achieved, particularly in relation to signal-to-noise considerations. In particular, increasing the contrast will reduce read-out noise problems in later processing.

Accordingly, the apparatus of the invention allows the conversion gain of the pixel- to be controlled by controlling the value of the capacitance, so that the conversion gain is set in accordance with the expected illuminating intensity, in order to boost the gain when the intensity is expected to be low, and to reduce the gain, when the intensity is expected to be high.

An analogy can be helpful for understanding. The apparatus 10 provides a "well" in which photons are captured in the form of charge. Best measurement results are achieved when a measurement wholly or nearly fills the "well". Consequently, the size of the well (which is the value of the capacitance 14) is controlled so that for each measurement, the size of the well will result in the well being wholly or nearly filled. Adjusting the size of the well also allows the contrast in the output signal to be optimised. This is because the maximum output voltage which can occur is fixed by the voltage difference between the supply voltages 16, 18, so that the optimum contrast is achieved by setting the capacitance value so that the expected illumination intensity will create an output voltage at or near the maximum. Variations in illumination intensity will then create the largest possible change in output voltage, thus optimising the contrast.

Fig. 7 illustrates a system 70 constructed from multiple examples of apparatus 10 and which may be used for measurements of a signal of the type illustrated in Fig. 6. The system 70 represents one multi-channel pixel, which may form one pixel of an array. The system 70 includes four separate apparatus 0, each of the type described above, except that the same photodiode is connected into each of the apparatus 10. This allows four measurements to be taken at different times, corresponding with l₂, I3 and l₄, and ensures that the outputs of each channel have identical spatial alignment, arising from the same sensing device. Each apparatus 10 produces a respective output 26. The outputs 26 are used as inputs to differential amplifiers 72 whose outputs 74 are accordingly difference signals. The two difference signals 74 are digitised by analogue to digital converters 76 to form digital outputs 78. The whole of the system 70, including the apparatus 10, the differential amplifiers 72 and the converters 76 is built, in this example, as a single integrated circuit on a single semiconductor chip. Accordingly, the output of the integrated circuit is the digital difference signals 78, rather than the raw signals 26. The significance of this can now be described by returning to Fig. 6.

Fig. 6 shows only a single cycle of the temporal modulation signal 80 superimposed on the main profile 82 by modulation of the input to the experiment. The main profile 82 can be considered as a D.C. background. The amplitude of the temporal modulation signal 80 is much smaller than the amplitude of the D.C. background 82, symbolised by . breaking the vertical (Intensity) axis at 84. It can be shown that in a profile such as that of Fig. 6, arising from a measurement application of the type mentioned above, for example, the D.C. value of the main profile 82 does not contain useful information, but the amplitude and phase of the temporal modulation signal 80 are both useful. The nature of the profiles 80, 82 present signal processing problems, as noted above. For example, the D.C. amplitude of the main profile 82 might be 1000 times greater than the amplitude of the temporal modulation 80. Even ignoring the effect of noise, a conventional eight-bit analogue-to-digital converter, capable of resolving only 256 states, would not be able to measure a change in amplitude between points to l₄.

Consequently, much higher capacity converters would be required in order to send all of the intensity information of a profile such as that illustrated in Fig. 6, from the sensor to another location for further processing. However, higher capacity converters are very much more expensive, to the extent that they are available at all.

We have appreciated that data conversion requirements arising from the apparatus 10 can be obviated or mitigated by providing some pre-processing of data, prior to digitising the data for transmission from the integrated circuit.

Accordingly, the differential amplifiers 72 are used to form the two differences required for calculating the modulation amplitude (Equation 1 above), and for amplifying these two differences. The D.C. background of the main profile 82 is thus removed from these two difference signals. The amplitude of the difference signals will be much smaller than the absolute intensity of the incident light, allowing the difference signals to be amplified within the amplifiers 72. However, by removing the D.C. signal, problems of analogue-to-digital converter dynamic range are significantly reduced and consequently, the difference signals can be digitised more easily, without requiring excessive complexity, bandwidth or cost for the converters 76. It is to be understood that the apparatus 10 can be used without the differential amplifiers 72 and converters 76, and that the differential amplifiers 72 and converters 76 could be used in the manner described above, in conjunction with other apparatus.

The apparatus which has been described can be implemented in CMOS technology. Other technologies can be used. Many other circuit arrangements and component choices can be made within the scope of the invention.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. Apparatus comprising: a photosensitive element, and a capacitance associated with the photosensitive element, wherein the photosensitive element provides a discharge path for the capacitance while the photosensitive element is illuminated, and the photosensitive element has a conductivity which depends on the illumination intensity, and wherein the value of the capacitance is variable, the apparatus further comprising control means operable to select the value of the capacitance.

2. Apparatus according to claim 1 , further comprising at least one capacitor which is a component separate from the photosensitive element.

3. Apparatus according to claim 1 or 2, comprising a plurality of capacitors which are each components separate from the photosensitive element.

4. Apparatus according to claim 2 or 3, comprising switch means operable to connect and disconnect the photosensitive element with the or each separate capacitor.

5. Apparatus according to claim 4, wherein the switch means is operable to connect at least one of the separate capacitors in parallel with the photosensitive element.

6. Apparatus according to claim 4 or 5, the switch means comprising a plurality of switch elements, each separate capacitor having an associated switch element.

7. Apparatus according to claim 6, wherein each switch element is separately controllable.

8. Apparatus according to claim 6 or 7, wherein each switch element is controlled by a switch voltage.

9. Apparatus according to claim 6, 7 or 8, the switch means further comprising a register operable to store information relating to the required state of each switch element.

10. Apparatus according to claim 9, the switch means comprising a data input for receiving state information, and being operable to load state information into the register.

1 1 . Apparatus according to any preceding claim, wherein the photosensitive element is a photodiode.

12. Apparatus according to any preceding claim, wherein at least the photosensitive element and the capacitance are implemented in CMOS technology.

13. Apparatus according to any preceding claim, the apparatus comprising a plurality of capacitances, as aforesaid, associated with a common photosensitive element.

14. Apparatus according to claim 13, comprising processing means operable to combine signals representing the degree of discharge of the respective capacitances.

15. Apparatus according to claim 14, wherein the signals are combined to form difference signals.

16. Apparatus according to claim 15, wherein the difference signals are digitised.

17. Apparatus according to claim 14, 5 or 6, wherein the signals are generated and combined within an integrated circuit, wherein the output of the integrated circuit is provided as the difference signals.

18. A sensor comprising a plurality of pixels, each pixel comprising apparatus as defined in any preceding claim.

19. A method comprising: providing a photosensitive element; providing a capacitance associated with the photosensitive element, the photosensitive element providing a discharge path for the capacitance while the photosensitive element is illuminated, and the photosensitive element having a conductivity which depends on the illumination intensity, and controlling the value of the capacitance.

20. A method according to claim 19, wherein at least one capacitor is provided separate from the photosensitive element.

21 . A method according to claim 19 or 20, there being a plurality of capacitors provided as components separate from the photosensitive element.

22. A method according to claim 20 or 21 , wherein switch means are provided, operable to connect and disconnect the photosensitive element with the or each separate capacitor.

23. A method according to claim 22, wherein the switch means are operated to connect at least one of the separate capacitors in parallel with the photosensitive element.

24. A method according to claim 22 or 23, wherein each separate capacitor is provided with an associated switch element.

25. A method according to claim 24, wherein each switch element is separately controlled.

26. A method according to claim 24 or 25, wherein each switch element is separately controlled by a switch voltage.

27. A method according to claim 24, 25 or 26, wherein information relating to the required state of each switch is stored in a register.

28. A method according to any of claims 19 to 27, wherein the photosensitive element is a photodiode.

29. A method according to any of claims 19 to 28, wherein at least the photosensitive element and the capacitance are implemented in CMOS technology.

30. A method according to any of claims 19 to 29, wherein a plurality of capacitances, as aforesaid, are provided and are associated with a common photosensitive element.

31 . A method according to claim 30, wherein signals representing the degree of discharge of the respective capacitances are combined.

32. A method according to claim 31 , wherein the signals are combined to form difference signals.

33. A method according to claims 31 or 32, wherein the difference signals are digitised.

34. A method according to claims 31 , 32 or 33, wherein the signals are generated and combined within an integrated circuit, and wherein the output of the integrated circuit is provided as the difference signals.

35. Apparatus comprising: a photosensitive element, and a plurality of capacitances associated with the photosensitive element, wherein the photosensitive element provides a discharge path for the capacitances while the photosensitive element is illuminated, and the photosensitive element has a conductivity which depends on the illumination intensity, and wherein the apparatus further comprises processing means operable to combine signals representing the degree of discharge of the respective capacitances.

36. Apparatus according to claim 35, wherein the signals are combined to form difference signals.

37. Apparatus according to claim 36, wherein the difference signals are digitised.

38. Apparatus according to claims 36 or 37, wherein the signals are generated and combined within an integrated circuit, wherein the output of the integrated circuit is provided as the difference signals.

39. Apparatus substantially as described above, with reference to the accompanying drawings.

40. - A method substantially as described above, with reference to the accompanying drawings.

41 . Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.