WO2005112307A1

WO2005112307A1 - Improved signal-to-noise ratio of free space optical communications by common mode rejection of atmospheric scintillation noise

Info

Publication number: WO2005112307A1
Application number: PCT/AU2005/000681
Authority: WO
Inventors: Bradley Adam Clare; Kerry Anne Mudge; Kenneth J. Grant
Original assignee: The Commonwealth Of Australia
Priority date: 2004-05-13
Filing date: 2005-05-13
Publication date: 2005-11-24

Abstract

A free space optical communications system that corrects for atmospheric scintillation noise using common mode rejection to remove co-channel noise, where each channel is transmitted on a separate but closely spaced wavelength. This significantly increases the SNR thereby increasing the range and/or bandwidth of the link and further compensates for errors of alignment. The system includes two laser beams having closely but spaced apart wavelength, the first laser beam being modulated to embed a signal, the two laser beams then emitted along a path to a receiver, said receiver including a means to distinguish between the two beam and further including a processor using an algorithm to remove co-channel noise by reference to the two signals such as the ratio of the signal.

Description

Improved signal-to-noise ratio of free space optical communications by common mode rejection of atmospheric scintillation noise.

FIELD OF THE INVENTION The present invention relates generally to a device and method to correct for atmospheric scintillation noise in free space communication systems and in particular to a free space optical communications using common mode rejection to remove co-channel noise, where each channel is transmitted on a separate but closely spaced wavelength.

BACKGROUND OF THE INVENTION Free space optical communication has been successfully used to transmit data through the atmosphere whether that data is of a passive type providing information such as laser sensing, or in fact data that is used to control or transmit data to or from equipment such as unmanned aerial vehicles (UAV's).

However, propagation through the atmosphere is subject to atmospheric scintillation noise affecting the signal-to-noise ratio (SNR), effectively reducing the range or bandwidth of the communication link. This scintillation is experienced even over relatively short propagation paths and is caused by small temperature variations in the atmosphere, resulting in index of refraction changes.

A number of solutions have been proposed to negate or at least minimise this degradation in signal. For example, aperture averaging is probably the most commonly used technique to minimise scintillation noise. Large aperture telescopes are used to collect the transmitted radiation and focus in on a detector. Although this averages out the scintillation noise to some extent, the telescopes required are expensive and bulky, which limits their application.

Other solutions have used multiple transmitters operating at the same wavelength to average out the scintillation noise. However, if there is no common mode noise on different channels the improvement in the SNR will increase as the square root of the number of transmitters. Thus with two transmitters the improvement will only be of the order of some 40%.

With free space optical communication being the preferred option in civilian applications such as broadcasting of sports games where multiple cameras can be optically connected to a central broadcasting box and defence applications such as remote control and data downloading from remote vehicles, the SNR has become critically important to enable the systems to be employable.

It is an object of the present invention to overcome at least some of the above-mentioned problems or provide the public with a useful alternative.

SUMMARY OF THE INVENTION

Accordingly it is an object of the present invention to correct for atmospheric scintillation noise in a free space optical communications using common mode rejection to remove co- channel noise, where each channel is transmitted on a separate but closely spaced wavelength. This significantly increases the SNR thereby increasing the range and/or bandwidth of the link. It further compensates for errors of alignment.

However the present invention can equally well be used for any application where the transmittal or retrieval of data occurs through free space such as remote laser sensing and it is not intended to limit the invention to communications systems.

These and other objects are achieved by providing a system including two laser beams having closely but spaced apart wavelength, the first laser beam being modulated to embed a signal, the two laser beams then emitted along a path to a receiver, said receiver including a means to distinguish between the two beam and further include a means to calculate the ratio of the signal and reference channels to remove co-channel noise. Equally well other well-known means may be used to remove unwanted co-channel noise. For example, one may use a voltage- controlled amplifier in analysing the two signals, or, in the case of digital signals various thresholds may be used to achieve the same or similar result. It is not the intention to limit the invention to a particular mathematical algorithm the choice of which would be well known to the relevant addressee.

Therefore in one form of the invention there is proposed an optical communication system for transmitting a signal including: a first laser beam and a second laser beam; a modulator adapted to embed the signal onto the first laser beam, the modulated first laser beam and the second laser beam transmitted to a receiver having a means to distinguish between the modulated first laser beam and the second laser beam and providing two outputs relating to the modulated first laser beam and to the second laser beam to a processing means that calculates the ratio of the two outputs to provide a final output.

In a further form of the invention there is proposed a free space optical communication system for communicating a signal including: first and second terminals, the first terminal includes a transmitter, the transmitter having a first laser source with a first wavelength, said transmitter having a second laser source with a second wavelength, the first and second wavelengths being different from each other; a modulator for modulating the signal onto the first laser source; the second terminal includes a receiver to receive the first and second laser sources, the receiver including a wavelength separator providing a first received signal at the first wavelength to a first sensor that provides a first output signal and a second received signal at the second wavelength to a second sensor that provides a second output signal; and a processing means that calculates the ratio of the first output signal and the second output signal to provide a final output signal._Preferably the first and second sensors are photodiodes.

Preferably the first and second wavelengths are closely spaced.

In preferences the modulator is a Mach-Zehnder modulator.

In preference the wavelength separator is an optical grating. the optical grating is an arrayed waveguide grating.

Preferably the wavelength separator is a sub-carrier multiplexer system.

In preference the difference between the first and the second wavelength is of the order of less than one nanometer.

In a further form of the invention there is proposed a method for the correction of atmospheric noise in a signal transmitted through free space said method including the steps of: embedding said signal onto a first laser signal having a first wavelength and transmitting the modulated first laser signal; transmitting a second laser signal having a second but closely spaced wavelength and; receiving and separating said modulated first laser and second laser signals and comparing the two signals to thereby remove atmospheric noise by applying well known techniques. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several implementations of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings,

Figure 1 is a schematic block diagram drawing illustrating the concept of the invention;

Figure 2 is a drawing of two time series, the top one the time series for the two channels, the bottom the corrected signal; and

Figure 3 is a schematic drawing illustrating one of the potential uses of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description of the invention refers to the accompanying drawings. Although the description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts.

Illustrated in Figure 1 is a schematic diagram of the present invention. Thus a first laser 10 operates at a wavelength of 1557.3 nm and a second laser 12 at a wavelength of 1558.1 nm. These wavelengths are preferred in an operational sense since being in the IR they are safest for the eye. Although wavelengths of around 850 nm may be preferred in optical communication systems they are somewhat unsafe. It is to be understood that the present invention is not to be limited to the wavelengths specified above. The wavelengths need simply be in the eye-safe region around 1550 nm and be closely spaced.

The two lasers 10 and 12 emit beams 14 and 16. A modulator 18 modulates the first laser beam 14. The modulator 18 may be for example a Mach-Zehnder modulator that modulates the laser beam with an analogue or digital signal. Such a modulator is preferred to directly modulating the laser since modulating the laser results in chirping and reduces the bandwidth.

The now modulated first laser beam and the second laser beam are typically fed to a single optical fibre 20 and the combined beam 22 emitted so that the two beams share the same propagation path thus experiencing the same atmospheric conditions. However, they could very well be emitted so that they follow closely adjacent paths.

For the purposes of testing the invention, as illustrated in Figure 1, the combined laser beam 22 passes through a circulator 24, collimator 26, and then to a retro-reflector 28, and back to the circulator 24 where it is re-directed to an arrayed waveguide grating 30 that separates the beam into its two discrete wavelengths 32 and 34, each of which is sampled by a photodiode 36 and 38 respectively. An appropriate electronic circuit 40 than calculates the ratio of the signal and the reference channels to remove co-channel noise and provide a final output signal 42 whose SNR is some 10 dB better then using a solitary laser beam.

An example of the improvement in the SNR can be observed in Figure 2 where there are shown two time series, the first including the combined modulated and reference signals, the second the corrected signal.

Of course, the above configuration of the circulator, collimator and retro-reflector can be used for testing purposes only. In real-life communication systems once the two laser beams have been combined, the received system only requires there to be a means to separate out the two signals. Thus, such as system may be employed in a situation as illustrated in Figure 3, where the SNR improvement enables real-life feed of video and audio signals from cameras at remote stadium locations, the signal then fed to a central broadcast facility. Thus there is shown a camera 44 feeding its signal to a last device 46 according to the present invention. Although two beams 48 and 50 are shown it is understood that typically they will pass through the same space. The signal is then received by tower 52 that can then broadcast using conventional means. The use of this means that expensive and complex cabling around the stadium 54 is eliminated.

The present invention could however be used to receive data and compensate for the atmospheric scintillation errors in that data. For example, in the case of UAV's, a light beam with two wavelengths is emitted from a base station and directed to a UAV including a retro- reflector, a multiple quantum well modulator, and drive circuitry that drives that multiple quantum well modulator. The multiple quantum well modulator, under the control of the drive circuitry, modulates only one wavelength of the light beam. The retro-reflector reflects this modulated light beam to the base station as well as the unmodulated other wavelength, where a receiver may detect them. The two measured signals may then be used to improve the SNR as detailed above. This configuration means that the UAV does not need to house heavy and complex laser emitting systems reducing the overall complexity and weight of the system.

Multiple quantum wells (MQW) are composed of alternating layers of different semiconductor materials. Currently, MQW are produced by growing these alternating semiconductor layers by molecular beam epitaxy or metal oxide chemical vapour deposition. Typical semiconductor materials used include GaAs, AlGaAs, and InGaAs, although others are possible. The material with the lowest conduction and valence band is called the well while the material with the higher conduction and valence band energy is called the barrier. Semiconductor materials exhibit a band-edge in absorption. At wavelengths longer than the band-edge, the material is transparent, while at shorter wavelengths, it is opaque. At the band edge, these materials exhibit a feature known as an exciton. The excitonic feature is generally broad and indistinct at room temperature in normal semiconductor materials. In a quantum well of suitable design, however, this feature becomes much narrower and its exact wavelength becomes a function of the thickness of the semiconductor layers and, a function of any applied electric fields. To ensure that only one of the wavelengths in a light beam of two is modulated requires careful selection of the thickness of the material and the applied field. To date MQW are limited to distinguishing wavelengths that are at least 10 nm apart.

Whist the above embodiment discussed physically separating the beams it is not the intention to limit the invention to that type of technique. For example, instead of physically separating the beams sub-carrier multiplexing (SCM) may be used to retrieve information on the multiple channels. The data are impressed on a subcarrier wave that is subsequently impressed on the optical carrier. SCM is conceptually similar to commercial radio, in which stations are placed at different RF such that a radio receiver can tune its filter to the appropriate subcarrier RF. The multiplexing and demultiplexing of the SCM channels is accomplished electronically, not optically. The obvious advantage is that several channels can share the same expensive optical components.

The reader should now appreciate the advantages of the present invention to significantly improve the SNR whether relating to digital or analogue signals. This technique can be used for many applications including but not limited to communication and remote sensing, basically anywhere where atmospheric noise has an effect on the propagating signal. Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

Claims

1. An optical communication system for transmitting a signal including: a first laser beam and a second laser beam; a modulator adapted to embed the signal onto the first laser beam, the modulated first laser beam and the second laser beam transmitted to a receiver having a means to distinguish between the modulated first laser beam and the second laser beam and providing two outputs relating to the modulated first laser beam and to the second laser beam to a processing means that calculates the ratio of the two outputs to provide a final output.

2. A free space optical communication system for communicating a signal including: first and second terminals, the first terminal includes a transmitter, the transmitter having a first laser source with a first wavelength, said transmitter having a second laser source with a second wavelength, the first and second wavelengths being different from each other; a modulator for modulating the signal onto the first laser source; the second terminal includes a receiver to receive the first and second laser sources, the receiver including a wavelength separator providing a first received signal at the first wavelength to a first sensor that provides a first output signal and a second received signal at the second wavelength to a second sensor that provides a second output signal; and a processing means that calculates the ratio of the first output signal and the second output signal to provide a final output signal.

3. A free space optical communication system as in claim 2 wherein the first and second sensors are photodiodes.

4. A free space optical communication system as in claim 2 where the first and second wavelengths are closely spaced.

5. A free space optical communication system as in claim 2 where the modulator is a Mach- Zehnder modulator.

6. A free space optical communication system as in claim 2 where the wavelength separator is an optical grating.

7. A free space optical communication system as in claim 2 where the optical grating is an arrayed waveguide grating.

8. A free space optical communication systems as in claim 2 where the wavelength separator is a sub-carrier multiplexer system.

9. A free space optical communication system as in claim 2 where the difference between the first and the second wavelength is of the order of less than one nanometer

10. A method for the correction of atmospheric noise in a signal transmitted through free space said method including the steps of: embedding said signal onto a first laser signal having a first wavelength and transmitting the modulated first laser signal; transmitting a second laser signal having a second but closely spaced wavelength and; receiving and separating said modulated first laser and second laser signals and comparing the two signals to thereby remove atmospheric noise by applying well known techniques.