GB2181832A - Improvements relating to optical detecting arrangements - Google Patents

Abstract

An optical detecting arrangement comprises one or more photo- detectors 1 constituted by avalanche photo-diodes or photo-multipliers, in which the or each photo-detector is biased from a capacitively de- coupled constant current source 2 thereby to maintain a constant short- term bias voltage for the photo- detector, in which the or each photo- detector is arranged to have applied to it a (continuous) optical signal IM to be measured as well as a stable (pulsed) optical bias control signal IBC, the time-averaged intensity of the bias control signal being relatively high compared with the time- averaged intensity of the optical signal to be measured so that the responsivity of the photo-detector is stabilised during the measurement periods when the bias pulses are not applied. An arrangement is also envisaged in which the optical bias control signal is of a constant level and the optical signal is pulsed. <IMAGE>

Description

SPECIFICATION Improvements relating to optical detecting arrangements This invention relates to optical detecting arrangements and relates more specifically to such detecting arrangements incorporating avalanche photo-diodes or photo-multipliers.

Avalanche photo-diodes and photo-multipliers exhibit good sensitivity to low intensity optical signals due to the charge-carrier multiplication produced by the avalanche effects. However, these photo-detectors have a multiplication gain and responsivity (i.e. electrical current output per unit of optical power input at a specified wavelength of incident radiation) which is strongly dependent upon a bias voltage applied to the photo-detector. Thus, variations in the bias voltage can cause instability in the response of such photo-detectors to the incident light. Moreover, the provision of matched pairs of such photo-detectors having the same responsivity versus bias voltage characteristics presents extreme difficulties.

According to the present invention therefore, there is provided an optical detecting arrangement comprising one or more photo-detectors constituted by avalanche photo-diodes or photomultipliers, in which the or each photo-detector is biased from a capacitatively de-coupled constant current source thereby to maintain a constant short-term bias voltage for the photodetector, in which the or each photo-detector is arranged to have applied to it an optical signal (continuous or pulsed) to be measured as well as a stable optical bias control signal (regular pulses of constant amplitude or a continuous constant power level) whereby the time-averaged intensity of the bias control signal is relatively high compared with the time-averaged intensity of the optical signal to be measured so that the responsivity of the photo-detector is stabilised during the measurement periods when the bias pulses are not applied.

In carrying out the present invention it is preferred to apply regular light pulses of constant amplitude (i.e. bias control pulses) to the or each of a number of photo-detectors in common.

The photo-detector(s) will also have applied to it (them) d.c. optical signal(s) to be measured which will be of low amplitude compared to the amplitude of the bias control pulses.

The biased photo-detector may be connected in series with a load resistor across which is connected a pre-amplifier which produces an electrical output representative of the input signal to be measured.

By way of example the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a circuit diagram of an optical detectirig arrangement; Figures 2 (a), and (b) show, respectively, the bias control optical pulses and the current/time waveform of the photo-detector in the arrangement of Fig. 1; and, Figure 3 shows photo-detector current/time waveform of an alternative biasing control arrangement.

Referring to Fig. 1, an avalanche photo-diode (or photo-multiplier) 1 which is to be used for the measurement of low intensity optical signals is biased from a constant current i source 2 capacitatively de-coupled by means of a capacitor 3 to maintain a constant short-term bias voltage for the photo-detector 1 and e load resistor 4 connected in series with it. In accordance with the invention this bias voltage of the photo-detector 1 is automatically controlled in order to stabilise the responsivity (R) of the photo-detector by applying a bias control optical signal to the photo-detector.

In the present embodiment the bias control signal takes the form of regular light pulses IBC (see Fig. 2(a)) derived from a stable pulsed light source 5. These light pulses of relatively high intensity 12 illuminate the photo-detector 1. The photo-detector also receives a continuous constant optical signal 1M which is the signal to be measured and has an intensity 11. However, the time-averaged intensity of this signal 1M is much lower than the time-averaged intensity of the bias control pulses.

Stabilisation of the responsivity of the photo-detector is achieved as follows.

The photo-detector current waveform is shown in Fig. 2(b). The current l,.R represents the signal current due to the input signal 1M of intensity Ii (assumed to be a signal of constant intensity) with a detector responsivity R. Over the period of measurement it is assumed that the detector avalanche gain is constant due to the de-coupling effect of the capacitor 3 which prevents the bias voltage changing significantly over the period between successive pulses 13C of the pulsed light source 5. Thus the average detector current is given by; (I,+12)R.T,+I, a R.T2 (id) Average = ------------- T1+T2 where T, is the pulse duration and T2 the pulse separation of the pulses 12.

If 12.T, is much greater than l1.T2, then l2.T1.R (id) Average = T1+T2 When switching on the detecting arrangement the initial voltage bias across the capacitor 3 will be zero. After a stabilisation period, dependent on the capacitive value of the capacitor 3, the mean photo-detector current will become equal to the constant current provided by the constant current source 2.That is to say, at the steady state condition the voltage across the capacitor 3 will stabilise to a value such that: l2.T1.R source = T,+T2 (T1 +T2) and hence R = . i source l2.T1 Thus, the responsivity R is stabilised to a level dependent only on the timing (T1 and T2) and intensity 12 of the pulses from the pulsed source 5 (or at least that part of the intensity of these pulses which reaches the photo-detector) and the magnitude of the constant current from the source 2.

Clearly, the responsivity R of two or more non-identical photo-detectors may be stabilised to the same level by allowing equal proportions of the light pulses from a single pulse source to impinge on each of these detectors.

A preamplifier 6 is connected across the load resistor 4 in series with the photo-detector 1 and the preamplifier provides an output which is representative of the signal 1M being measured.

It will be understood that the signal 1M to be measured must be weak compared to the pulse signals IBC as otherwise the responsivity R will also depend on the average level of the signal intensity 11. However, the pulse signals IBC must not be too intense or otherwise they may exceed the linear range of the photo-detector.

Although in the embodiment described it has been assumed that the signal 1M is of constant amplitude it could be a time variant signal but its magnitude can only be observed from the photo-detector current in the intervals between the pulses of amplitude 12 from the pulsed light source 5.

Ideally, the light signals of intensity 1, and 12 should be at the same wavelength. However, if the spectral response of the photo-detector is constant the responsivity may be controlled when the wavelengths are not the same. Thus, if two detectors are to be matched with the signals IBC and IM at different wavelengths then their spectral responses must each be well matched or their fixed spectral variations must be taken into account, as by adjusting their individual current bias sources.

If the pulse signals IBC are intense the effective bias voltage of the avalanche photo-diode may be reduced due to the voltage drop across the load resistor 4. The effect may be reduced further by using a transresistance amplifier in place of the load resistor 4 and the preamplifier 6 thereby creating a virtual earth point on the detector connection to the amplifier.

It may here be mentioned that if the avalanche photo-detector has a finite dark current 13 then it is neccessary that 12T, (1,+13)T2 in order to maintain the differential responsivity at a contolled level. Then, by subraction of each detectors dark current the signal currents may be obtained.

For use with detecting the alternating variations of chopped light signals 1, simple a.c. coupling will remove the d.c. dark current.

Although in the embodiment described the optical signal to be measured is a continuous d.c.

signal and the bias control signal comprises regular pulses the invention can also apply where the signal to be measured is a pulsed signal and the bias voltage signal is a constant d.c. signal provided the signals have the appropriate time-averaged intensities. The detector current waveform for such an alternative arrangement is depicted in Fig. 3 of the drawing. In this case it is necessary that 12T2 (1,+12)T,.

Claims (4)

1. An optical detecting arrangement comprising one or more photo-detectors constituted by avalanche photo-diodes or photo-muitipliers, in which the or each photo-detector is biased from a capacitively de-coupled constant current source thereby to maintain a constant short-term bias voltage for the photo-detector, in which the or each photo-detector is arranged to have applied to it an optical signal to be measured as well as a stable optical bias control signal whereby the time-averaged intensity of the bias control signal is relatively high compared with the time averaged intensity of the optical signal to be measured so that the responsivity of the photodetector is stabilised during the measurement periods when the bias pulses are not applied.

2. An optical detecting arrangement as claimed in claim 1, in which the bias control signal comprises light pulses of constant amplitude and in which the optical signal to be measured comprises a d.c. signal of low amplitude compared to the amplitude of the bias control pulses.

3. An optical detecting arrangement as claimed in claim 1 or claim 2, in which the biased photo-detector is connected in series with a load resistor across which is connected a preamplifier which produces an electrical output representative of the input signal to be measured.

4. An optical detecting arrangement substantially as hereinbefore described with reference to the accompanying drawings.