PHOTOCONDUCTOR READOUT CIRCUIT

Abstract

Disclosed herein is a device including at least one photoconductor configured for exhibiting an electrical resistance R.sub.photo dependent on an illumination of a light-sensitive region of the photoconductor; at least one photoconductor readout circuit, where the photoconductor readout circuit is configured for determining a differential voltage related to changes of the electrical resistance R.sub.photo of the photoconductor, where the photoconductor readout circuit includes at least one bias voltage source configured for applying at least one periodically modulated bias voltage to the photoconductor such that the electric output changes its polarity at least once; and at least one electrical circuit configured to balance the differential voltage at a given illumination level.

Claims

1. A device comprising at least one photoconductor configured for exhibiting an electrical resistance R.sub.photo dependent on an illumination of a light-sensitive region of the photoconductor; at least one photoconductor readout circuit, wherein the photoconductor readout circuit is configured for determining a differential voltage related to changes of the electrical resistance R.sub.photo of the photoconductor, wherein the photoconductor readout circuit comprises at least one bias voltage source configured for applying at least one periodically modulated bias voltage to the photoconductor such that the electric output changes its polarity at least once; and at least one electrical element configured to balance the differential voltage at a given illumination level.

2. The device according to claim 1, wherein the photoconductor readout circuit comprises a Wheatstone bridge or a sample-and-hold circuit.

3. The device according to claim 1, wherein the bias voltage modulation is unipolar or bipolar.

4. The device according to claim 1, wherein the bias voltage modulation has a frequency of the power line frequency especially 50 Hz or 60 HZ.

5. The device according to claim 1, wherein the device comprises at least one coupling to at least one evaluation device.

6. The device according to claim 1, wherein the illumination is modulated.

7. The device according to claim 1, wherein the light-sensitive region comprises at least one photoconductive material selected from the group consisting of lead sulfide (PbS); lead selenide (PbSe); mercury cadmium telluride (HgCdTe); cadmium sulfide (CdS); cadmium selenide (CdSe); indium antimonide (InSb); indium arsenide (InAs); indium gallium arsenide (InGaAs); extrinsic semiconductors, and organic semiconductors.

8. A detector comprising at least one device according to claim 1, wherein the detector comprises at least one evaluation device configured for determining an output signal of at least one output of the photoconductor readout circuit of the device, wherein the evaluation device is configured for determining an illumination intensity by evaluating the output signal.

9. The detector according to claim 8, wherein the evaluation device is configured for performing one or more operations selected from the group consisting of: at least one Fourier transformation; a counting of frequency, an edge detection, and a measurement of the period length.

10. A method of using a device according to claim 1, for a purpose of readout of one or more of at least one PbS sensor, at least one PbSe sensor, or at least one pixelated sensor array comprising a plurality of pixels, wherein each of the pixels comprises at least one PbS or PbSe sensor.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0053] Further optional details and features of the invention are evident from the description of preferred exemplary embodiments which follows in conjunction with the dependent claims. In this context, the particular features may be implemented alone or with features in combination. The invention is not restricted to the exemplary embodiments. The exemplary embodiments are shown schematically in the figures. Identical reference numerals in the individual figures refer to identical elements or elements with identical function, or elements which correspond to one another with regard to their functions.

[0054] Specifically, in the figures:

[0055] FIG. 1a illustrates an electrical circuit to measure the differential voltage VDiff with DC;

[0056] FIG. 1b illustrates an electrical circuit to measure the differential voltage VDiff with AC;

[0057] FIGS. 2a to 2d illustrates an electrical circuit to measure the differential voltage VDiff with AC and modulated light intensity, FIGS. 2a-d show time dependent source voltage (2a), biased voltage (2b), measured voltage (2c), and light modulation (2d); and

[0058] FIG. 3 illustrates the new electrical circuit to measure the differential voltage VDiff in dependence of the photo resistance R.sub.photo by a sample and hold circuit.

EXEMPLARY EMBODIMENTS

[0059] FIG. 1 shows the most commonly used way measure to the differential voltage 107 in dependence of the resistor 105 by a Wheatstone Bridge in DC 101 operation. A Wheatstone bridge is a standard for a circuit, where differential voltage could be balanced to 0V by adjustment of the resistors 102, 103 and 104.

[0060] One of the resistors of the Wheatstone Bridge can be replaced by a photoconductor 105 with a suitable resistance but darkened. Photoconductors are sensors, which require an external excitation signal to generate an electrical output depending on the measured physical quantity. In the case of photoconductors is this physical quantity the illumination. Most commonly, a voltage V.sub.Bias 106 is applied to the photoconductor 105 as excitation signal.

[0061] The photoconductor 105 change their resistance depending on the illumination. The change itself is relatively small compared to the total resistance value of the photoconductor. As an example, a PbS-detector with dimension of 2 mm×2 mm featuring a resistance of about 1 MΩ changes its resistance due to infrared radiation at 1550 nm with an irradiance of 16 μW/cm2 about 10 kΩ, which corresponds 1% change. Thus, the excitation signal will be orders of magnitude greater than the electrical output change due to the illumination. Without any filtering, the read-out electronics should be able to measure the whole signal but still solve the change of 1% with a relatively good resolution. Such read-out electronics are commercially available, yet very expensive.

[0062] Photoconductors, PbS, PbSe etc., as other types of resistors like carbon composite, thick film resistors etc., exhibit a strong 1/f noise, also known as flicker noise, which is dominant at smaller frequencies. 1/f noise depends strongly on the DC part of the current IDC, flowing through the photoconductor.

[0063] The 1/f noise dominates at lower frequencies. The change of R.sub.photo should be measured at higher frequencies to eliminate the 1/f-noise. The measurement noise can be reduced by modulating the current flowing through the photoconductor, by modulating the resistance of the conductor R.sub.Photo or modulating the excitation signal.

[0064] Most commonly, either the light source is modulated or the light path from the light source to the detector is chopped, for example with a mechanical setup, such as a chopper or shutter etc. The modulated signal is then demodulated in analog or digital domain but keeps the 1/f noise at the modulation frequency, thus reducing the 1/f noise.

[0065] When illuminated the resistor R.sub.photo is changing its properties, and so the Wheatstone Bridge results in a differential voltage different from 0V. Any drift of the bias voltage 106 due to a temperature instability of the system can be balanced with the darkened photoconductor 105 and the differential voltage 107 will remain 0V during the on-phase or the off-phase of the light modulation.

[0066] Changing the polarity of the source voltage 108 changes polarity of the bias voltage 106. This setup is shown in FIG. 1b. Since the change of the electrical output due to the illumination is measured by changing the polarity of the bias voltage more than once, the asymmetry of resistance of the photoconductor which was mentioned above may not affect further measurement. Measurement is done between two changes of polarity of voltage.

[0067] FIGS. 2 and 3 show embodiments of a device according to the present invention.

[0068] FIG. 2 shows the photoconductor readout circuit to measure the differential voltage 207 in dependence of the resistance R.sub.photo 205 by a Wheatstone Bridge. The resistance R.sub.photo 205 changes its value ΔR.sub.photo as a function of the incident illumination with a given modulation frequency, while the bias voltage V.sub.Bias changes its polarity multiple times. Therefore a photoconductor readout circuit as described before is used with time dependent outer parameters as shown in FIGS. 2a-d. A permanent supply voltage 201 of 8V is applied to the circuit. FIG. 2a shows the supply voltage as a function of time. The supply voltage is switched via triggered switches to get a biased voltage 206 of +/−4V in the Wheatstone Bridge. This biased voltage is shown in FIG. 2b. The measured differential voltage as a function of time is shown in FIG. 2c. The illumination of the photo resistor is modified by modulation of the illumination source, such as a lamp, with a period shown I FIG. 2d. The frequency of the trigger 208 is much higher than the frequency of the lamp modulation.

[0069] The resulting output voltage 207 is shown in FIG. 2d. It is an overlay of both modulated inputs whereas the high trigger frequency 208 reduces the 1/f noise as proposed.

[0070] In FIG. 2, the photoconductor readout circuit comprise the Wheatstone Bridge as described with respect to FIGS. 1. The Wheatstone Bridge may comprise four resistors R.sub.1 202, R.sub.2 203, R.sub.3 204 and 205, wherein the resistance 205 is the resistance of the photoconduction R.sub.Photo. The Wheatstone Bridge may be connected to the voltage source Vs, denoted with reference number 201. The photoconductor may be connected to the bias voltage source configured for applying the bias voltage V.sub.Bias 206 across the photoconductor. The bias voltage 206 is a periodically modulated bias voltage. The bias voltage source may be configured for generating a permanent supply voltage. The photoconductor readout circuit may comprise at least one switching element for generating the periodically modulated bias voltage. For example, the switching element may comprise triggered switches. The triggered switches may act depending and/or in response to the trigger 208, in particular a trigger signal. The trigger signal may be an external signal and/or may be generated by at least one element of the photoconductor readout circuit. For example, in the embodiment of FIG. 2, the photoconductor readout circuit may comprise two sub-circuits for applying the bias signal to the Wheatstone Bridge between resistors 202 and 203 and on the right side between resistors 204 and 205. Each sub-circuit may comprise a triggered switch comprising a connection to the trigger 208 and at least one logic gate such as a not gate. For example, the trigger signal may switch on the triggered switch one of the sub-circuits, e.g. the sub-circuit on the left side of FIG. 2, in case the trigger signal is 1. At the same the triggered switch of the other sub-circuit is switched off. If the trigger signal is 0, the switches may be switched off and on the other way round.

[0071] Even if the differential voltage cannot be balanced by means of a Wheatstone Bridge, the resulting offset can be compensated with the help of further electronic components such as “Offset DAC”. Another possible implementation is a sample and hold circuit, which measures (samples) the DC part while the modulated light is off and use it as the reference potential of the differential voltage measurement while the modulated light is switched on, or vice versa. Independent of any temperature or electrochemical drift of R.sub.Photo, the reference voltage V.sub.Ref will always be recalibrated when a sample and hold circuit is used.

[0072] An example for the inventive photoconductor readout circuit is shown in FIG. 3. A supply voltage 301 is applied via switches 312-316 to the resistors 302 and the photoresistor 305. The applied voltage is triggered by outer signal 308 and the negated signal 309, whereas the lamp is modulated by the trigger signal 310. The resistance 305 R.sub.photo changes its value ΔR.sub.Photo as a function of the incident illumination with a given modulation frequency 301, while the bias voltage V.sub.Bias 306 changes its polarity multiple times during one period of lamp modulation, whereas the sample and hold circuit samples a reference voltage V.sub.Ref 311 while the lamp is off 312 and holds it for the measurement of V.sub.Diff 307 while the lamp is on. The reference voltage measurement can be performed while the lamp is on, and the difference voltage measurement can be performed while the lamp is off. The lamp modulation can be sinusoidal, square, triangular or in any other waveforms. The lamp can be turned off for the reference measurement in the beginning of a measurement set, while multiple measurements of V.sub.Diff may be performed at different illumination intensities and/or at different wavelengths, as long as the change in the intensity or wavelength of the illumination is slower than the modulation frequency of the bias voltage.

LIST OF REFERENCE NUMBERS

[0073] 101 source voltage V.sub.S [0074] 102 resistor R.sub.1 [0075] 103 resistor R.sub.2 [0076] 104 resistor R.sub.3 [0077] 105 photoresistor R.sub.Photo [0078] 106 bias voltage V.sub.Bias [0079] 107 output voltage V.sub.Diff [0080] 108 source AC [0081] 201 source V.sub.S [0082] 202 resistor R.sub.1 [0083] 203 resistor R.sub.2 [0084] 204 resistor R.sub.3 [0085] 205 photoresistor R.sub.Photo [0086] 206 bias voltage V.sub.Bias [0087] 207 output voltage V.sub.out [0088] 208 trigger [0089] 301 source V.sub.S [0090] 302 resistor R.sub.1 [0091] 305 photoresistor R.sub.Photo [0092] 306 bias voltage V.sub.Bias [0093] 307 output voltage V.sub.out [0094] 308 trigger AC [0095] 309 trigger AC [0096] 310 lamp trigger [0097] 311 component [0098] 312 switch [0099] 313 switch [0100] 314 switch [0101] 315 switch [0102] 316 switch [0103] 317 ground [0104] 318 ground