GB2228318A

GB2228318A - Optical photon counter

Info

Publication number: GB2228318A
Application number: GB9002145A
Authority: GB
Inventors: John Philip Dakin
Original assignee: York Ltd
Current assignee: York Ltd
Priority date: 1989-01-31
Filing date: 1990-01-31
Publication date: 1990-08-22
Also published as: GB8902080D0; JPH04503107A; EP0456710A1; WO1990008946A1; GB9002145D0

Abstract

An optical photon counter includes an optical amplifier (14) which feeds an optical detector (16) coupled to a pulse height discriminator (20). The amplifier may be a fibre optic laser, a parametric amplifier or a semiconductor device.

Description

OPTICAL DETECTION APPARATUS FOR COUNTING OPTICAL PHOTONS This invention relates to optical detection apparatus for counting optical photons.

It is well known to use either photomultipliers or semiconductor avalanche photodiodes (which exhibit gain processes where generated photoelectrons are accelerated to high enough velocity to produce additional electrons by collision with stationary molecules) in photon counting apparatus. Such apparatus enables a percentage of incident photons to trigger the count threshold of an electronic counting system due to the fact that the incident photons generate photo-electrons which in turn generate a sufficiently large packet or bunch of electrons by an electron amplification process. Such known photon counting apparatus is limited in performance.

The photomultiplier has a very high gain (typically 106 times) and hence it generates an output pulse amplitude sufficient to overcome the thermal noise of a following electronic amplifier. However, photomultipliers are large, fragile and easily destroyed by high levels of light.

Furthermore, the photomultipliers most importantly have a poor quantum efficiency so that they only detect a small fraction of incoming photons.

Avalanche photodiodes have a more acceptable size, are more robust and have a better quantum efficiency. However, the avalanche photodiodes suffer from the disadvantage of having lower photoelectron gain (typically about 500-1000 times) except when operated close to avalanche breakdown, when spontaneous bursts of amplified electrons cause "dark counts", i.e. one not corresponding to incident photons. The disadvantage of the low gain is that it is difficult to produce a photo-electron-induced packet of electrons which will be well above the thermal noise level of a following electronic amplifier, especially if a high frequency response is desired of the electronic amplifier in order to permit fast counting rates.

The use of optical amplifiers is also known. These optical amplifiers are based either on optical-fibre photon amplifiers (using either rare-earth doped fibres or Raman-gain or Brillouin gain processes) or on semiconductor travelling wave or laser type amplifiers. It is becoming common practice to experiment with such photon amplifiers as preamplifiers before a relatively noisy PIN-photodetector/electronic-ampllfier combination, in order to enhance the relatively poor receiver performance of the latter. However, no attempt has been made to use such photon amplifiers before a device with photoelectron gain, such as a photomultiplier, as for normal communication purposes (which do not use photon counting) the photon amplifier alone will normally suffice to cause the PIN detector output to dominate the thermal noise level of the following electronic amplifier.

Also, and more particularly, no attempt has been made to use the photon amplifiers as a preamplifier in photon counting systems, in spite of the advantages offered in doing so.

It is an aim of the present invention to provide optical detection apparatus for counting optical photons, which optical detection apparatus is able to operate with a high efficiency and a high bandwidth.

Accordingly, this invention provides optical detection apparatus for counting optical photons, which apparatus comprises receiver means and counter means for determining the number of optical photons arriving at the receiver means in a given period of time, the receiver means comprising amplifier means for amplifying the arriving optical photons and optical detector means for detecting the amplified photons and for producing a photocurrent, and the counter means being such that it operates by counting the number of output photocurrent pulses above a given threshold level in order to produce a measure of the number of photons arriving at the receiver means.

Preferably, the amplifier means operates by a stimulated optical photon amplification process.

The amplifier means may comprise at least one length of optical fibre waveguide containing energised electronic states with a population inversion of such excited states. compared to lower ground states, such that a net optical gain occurs during passage of the optical photons through the optical fibre waveguide.

The amplifier means may alternatively be a length of optical fibre with parametric gain when the length of optical fibre is pumped by an external optical source. The parametric gain may be due to either of the well-known Raman or Brlllouin processes.

The amplifier means may alternatively be a semiconductor amplifier with a population inversion of excited electronic sites.

The semiconductor with the population inversion of excited electronic sites may be in the form of a travelling-wave semiconductor amplifier, or a partially mirrored device of the Fabry-Perot type or a distributed Bragg feedback type of construction similar to that used in semiconductor lasers.

The optical detector means may be a PIN photodiode.

The optical detector means may alternatively be a photomultiplier.

The optical detection apparatus may include pulse height discriminator means which is provided after the optical detector means and which is capable of deciding which of the output photocurrent pulses to count according to the height of the output photocurrent pulses. The pulse height discriminator means will usually be an appropriate electronic circuit.

The output of the pulse height discriminator means may be used to produce a signal corresponding to the number of electron pulses above a given threshold which arise within a given time period.

When the output of the pulse height discriminator is used to produce a signal corresponding to the number of electron pulses above a given threshold which arise within a given time period, then the optical detection apparatus may be a communications receiver, or an optical time-domain reflector system, or an optical radar (i.e. Lidar) system or an apparatus used to examine the photo arrival statistics of scattered light from moving objects to determine the motion of said objects. (This latter application includes the methods of laser doppler velocimetry and photon correlation spectroscopy).

The optical detection apparatus of the present invention may provide a highly efficlent and high performance photon counting system which, as indicated above, utilises an optical photon amplifier before a conventional optical detector means. The optical detector means is preferably the above mentioned avalanche photodiode, but the above mentioned PIN photodiode may satisfactorily be used providing the photon gain of the amplifier means is sufficient.

The effective quantum efficiency of an optical amplifier is extremely high, and most incident photons will exhibit a gain in the amplifier. Also, by the time the optical signal from a single incident photon reaches the optical detector means, the optical signal will have been subjected to a substantial gain of from 30 - 1,000,000 times, depending on the gain available from the amplifier means which acts as a preamplifier. Thus substantially all amplified photon bursts will result in a burst of photoelectrons in the optical detector means, even if the quantum efficiency of the optical detector means were to be as low as 10%. Thus effective overall quantum efficiency of the combination will be close to 100%, virtually every incident photon resulting in a strong output burst of electrons from the optical detector means.

The response bandwidth of optical pre-amplifiers (with the exception of Brillouin fibre amplifiers) is generally exceptionally high and is physically at least several tens of GHz. Also, the bandwidths of the post-detector electronic amplifier can be extremely high, without the noise penalty that might otherwise arise where the total (i.e. photon times photoelectronic) galn-product not to be so high. The only limitation on speed is, therefore, that of the avalanche photodiode detector, with a typical upper limit with current technology being of the order of 1-2GHz. This limit compares very favourably with the upper photon-counting rates of conventional systems (using only photo-electron gain) which is typically of the order of only 1OMHz.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 illustrates conventional photon counting apparatus using only electronic multiplication: Figure 2 illustrates optical detection apparatus in accordance with the invention, the optical detection apparatus being for counting optical photons and being such as to use prior photon multiplication using an optical amplifier; and Figure 3 further illustrates the amplification of the photons in the optical detection apparatus shown in Figure 2.

Referring to Figure 1, there is shown conventional photon counting apparatus 2 which uses only electronic multiplication.

More specifically, incident light falls on a combined detector and electron-multiplication device 4 which may be an avalanche photodiode or a photo-multiplier. The device 4 is connected to an amplifier 6 which is in turn connected to a pulse-height discriminator 8. Pulses from the pulse-height discriminator 8 pass via line 10 to counter means (not shown).

Referring now to Figure 2, there is shown optical detection apparatus 12 for counting optical photons. The optical detection apparatus 12 comprises amplifier means in the form of an optical amplifier 14, optical detector means in the form of an optical detector 16, and amplifier 18 connected as shown to the optical detector 16, and a pulse-height discriminator 20.

Pulses output from the pulse-height discriminator 20 pass to counter means (not shown).

The optical detection apparatus 12 operates such that weak incident light 22 passes to the optical amplifier 14 where it is amplified and passes as amplified light 24 to the optical detector 16. Depending upon the gain of the optical amplifier 14, the optical detector 16 may or may not require to also contain an electron-multiplication stage indicated by the amplifier 18. With a high gain optical amplifier 14, a simple PIN photodiode may suffice as the optical detector 16.

Referring to Figure 3, an incident photon 26 is shown arriving at the optical amplifier 14 and being amplified as a packet or a burst 28 of amplified photons which are received by the optical detector 16. Figure 3 also illustrates how the packet or burst 28 of amplified photons arriving at the optical detector 16 may pass along line 30 as a burst 32 of photo-electrons. This burst 32 of photo-electrons may already have been subjected to internal electronic multiplication within the detector device if the option of using an avalanche photodiode detector or a photomultiplier detector has been chosen with the aim of overcoming amplifier noise.

It is to be appreciated that the embodiment of the invention described above with reference to Figures 2 and 3 of the accompanying drawings has been given by way of example only and that modifications may be effected.

Claims

1. Optical detection apparatus for counting optical photons, which apparatus comprises receiver means and counter means for determining the number of optical photons arriving -at the receiver means in a given period of time, the receiver means comprising amplifier means for amplifying the arriving optical photons and optical detector means for detecting the amplified photons and for producing a photocurrent, and the counter means being such that it operates by counting the number of output photocurrent pulses above a given threshold level in order to produce a measure of the number of photons arriving at the receiver means.

2. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said amplifier means comprises at least one length of optical fibre waveguide which includes material having energised electron states with a population inversion of excited states to effect optical gain during passage of radiation through said wave guide.

3. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said amplifier means comprises a length of optical fibre which exhibits parametric gain when pumped by an external optical radiation source.

4. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said amplifier means comprises a semiconductor amplifier.

5. Optical detection apparatus for counting optical photons as claimed in claim 4 wherein said semiconductor amplifier consists of a travelling-wave semiconductor amplifier.

6. Optical detection apparatus for counting optical photons as claimed in claim 4 wherein said semiconductor amplifier comprises a partially-mirrored device of the Fabry-Perot type.

7. Optical detection apparatus for counting optical photons as claimed in claim 4 wherein said semiconductor amplifier consists of a distributed Bragg feedback amplifier.

8. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said optical detector means consists of a PIN diode.

9. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said optical detector means consists of an avalanche photodiode.

10. Optical detection apparatus for counting optical photons as claimed in claim 1 wherein said optical detector means consists of a photomultiplier.

11. A communications receiver including optical detection apparatus as claimed in any one of the preceding claims.