AU648682B2 - Multiple-beam quantum noise-correlated lightwave system - Google Patents

Description

P/00/01 1 2W6101 AUSTRLLA.Regulation 3.2 Pate nts Act 1990 ORIGINAL 4 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Multiple-Beam.uant.m Noise-Correlated L4gt~W.ye.

*:The following statement is a full description of this invention, including the best mehod of performing it known to me:- -2- MULTIPLE-BEAM QUANTUM NOISE-CORRELATED LIGHTWAVE SYSTEM This invention relates to a method of generating and utilizing a plurality of lightwave beams which exhibit strongly correlated quantum noise fluctuations.

The present invention provides a multiple lightwave-beamgenerating device comprising two or more electrically coupled, high quantum efficiency light-emitting semiconductor junction and/or laser diodes which are optically coupled to one or more lightwave detectors with high overall current transfer efficiencies.

S" In one form of the invention, two high quantum efficiency laser diodes are electrically connected in series and driven from a common power supply and modulator. Each laser diode is separately optically coupled to a high quantum efficiency p-i-n diode detector with high overall current transfer efficiency.

In another form of the invention the two laser diodes are connected in parallel (rather than in series), optically coupled as before, and driven from a common power supply and modulator.

The invention may be implemented using either visible or infrared light-emitting diodes or laser diodes, providing these are of high quantum efficiency.

For example, high quantum efficiency infrared emitting diodes and photon counting detectors may be substi-,'i for the laser diodes and p-i-n diode detectors previously mentioned.

The Prior Art Current state-of-the art devices for generating noise-correlated light wave beams are technically complex. They usually employ nonlinear crystals operating as two-mode optical parametric oscillators or optical parametric down converters. These devices are usually bulky and inefficient, limited to two beams, and do not permit independent tuning of the wavelengths of the two lightwave beams.

The present invention provides for the more efficient and versatile generation of a plurality of intense noise-correlated light beams by a novel method which may be incorporated into lightwave instrumentation and communications systems.

is Background to the Invention The inventor was the first research worker in Australia (and the second in the world) to demonstrate, in 1990, the generation of quantum noise below the usual shot-noise level using light-emitting diodes driven from a high impedance current source. In the course of the analysis and generalisation of this work, the inventor was led to the realisation that laser diodes and/or light emitting diodes, when suitably electrically coupled generate quantum noisecorrelated lightwave beams. The utilisation of noise-correlated 25 beams of this type for precise measurement and secure communication has been foreshadowed by several workers.

However, such utilisation has been dependent on the use of optical crystal parametric down converters or similar devices. The present invention provides a new way of generating multiple beams in which the quantum noise in each beam is at, or below, the shotnoise level when detected with high quantum efficiency. This fact taken together with the high quantum correlation between the plurality of beams permits the employment of noise reduction techniques and algorithms to reduce system noise. For example, a dual (or multiple) beam spectrophotometric instrument can utilise the invention to eliminate, or greatly reduce, quantum noise and other noise originating in the lightwave sources. Convwentional -4instruments can reduce noise down to, but not below, the shot noise level. The inventor has developed and demonstrated a prototype of the invention.

Brief Description of the Drawings To assist with understanding the invention, reference will now be made to the accompanying diagrams which show two examples of the invention. The drawings are not meant to limit the scope of the invention in any way.

In the diagrams: Figure shows one example of a dual-beam lightwave system in which the detected lightwave intensities are negatively correlated.

Figure shows an example of a dual-beam lightwave system in which the detected lightwave intensities are positively correlated.

Referring to Figure the current supplied by the modulated high-impedance power supply is split between the two semiconductor junction light sources 3) which are optically S. coupled to p-i-n diode detectors 5) with high current transfer efficiencies. These diode detectors are connected to a current summing device Providing the impedance of the modulated current supply in Figure is very much higher than the diode resistance and the quantum transfer efficiencies are both unity, the intensity fluctuations are reduced below the normal shot-noise level negatively correlated with correlation coefficient of-1.

This means that, in the ideal case, the sum of the photodetector currents is then free of quantum noise and contains only the r.aodulator signal.

Thus in this case the system could be used to measure differential optical attenuation between the two beams which traverse light paths containing optical cells 7) with an effectively noise-free source leading to a consequent increase in precision and sensitivity.

Referring to Figure the diodes 3) are connected to a common current supply Only one diode need be modulated If both diodes are modulated, they must be modulated out of phase. In this case, referring to the previous discussion, the detected noise fluctuations will again be below the shot-noise level.

However, in this case, the correlation coefficient for tie ideal case. The difference current derived from the detectors will then contain the modulation and will be quantum noise-free.

The difference current, as shown in Figure is then a measurement of differential absorption which may be made as a consequence of placing an optical sample in one or both of the optical cells In double or multiple beam sources of the type shown in Figure the quantum noise correlation may be ideally maintained at +1, independently of the number of series diode elements.

The invention covers double and multiple beam lightwave systems 25 element which incorporate as their essential feature a double or multiple element array of lightwave emitting semiconductor junction diodes with correlated quantum noise fluctuations, used (for example) in measurement and communications applications.

Current state-of-the-art generators of this type are technically complex. Current experimental dual-beam sources of this type include two-mode optical parametric oscillators (TMOPO) and optical parametric down converters.

The present invention in the form shown in Figure is analogous to the TMOPO.

Claims (9)

1. A quantum noise-correlated, multiple beam, lightwave generating and detecting system comprising: An array of electrically coupled semiconductor junction light emitters of high quantum efficiency. An array of high quantum efficiency light detectors optically coupled to the array of light-emitters referred to in above. S(c) A means of optically coupling members of the array of light emitters to members of the array of light 15 detectors referred to above with high efficiency. A low noise, modulated current supply to drive the array of light emitters. Provision for the placement of one or more optical cells in the optical coupling paths between members of the light emitting array and members of the light detecting array. 25 A means of combining, or otherwise operating upon, the plurality of detected beam signals in order to enhance the performance of the system by virtue of the quantum noise correlation existing between the beam signals.

2. A system as claimed in Claim 1 where the light emitting array comprises laser diodes.

3. A system as claimed in Claim 1 where the light-emitting array comprises light-emitting diodes.

4. A system as claimed in Claims 1-3 where the light-emitting array comprises series-coi_---'ed light emitters.

A system as claimed in Claims 1-3 where the light-emitting array comprises shunt-connected light emitters.

6. A system as claimed in Claims 1-5 where the means of optical c.,'ling referred to in Claim 1(c) includes optical wave guides.

7. A system as claimed in Claims 1-6 where the optical cells referred to in Claim 1(e) include light-modulating devices.

S8. A system as claimed in Claims 1-7 where the optical cells referred to in Claim l(e) contain material samples whose properties are to be measured. p

9. A system as claimed in Claims 1-8 where the array of lightwave detectors includes photon counting detectors. A multiple beam correlated lightwave system substantially as herein described with reference to the accompanying drawings. *o Q e,• PAUL J EDWARDS 13 AUGUST 1992 Abstract A multiple beam lightwave system is disclosed in which a strong correlation is introduced between the detected quantum noise fluctuations in the beams. An array of coupled laser diodes or light-emitting diodes is optically coupled with high quantum transfer efficiency to one or more lightwave detectors Because of the quantum noise correlation, signal processing operations such as subtraction of photo detected beam intensities allow the system noise due to quantum noise in the sources to be significantly reduced below the shot noise level. There is a consequent improvement in the precision of lightwave attenuation and other measurements. S S 0 S oo o S o *0 5 0 se o S S g* oo