GB2383910A

GB2383910A - Optical free space signalling system

Info

Publication number: GB2383910A
Application number: GB0200243A
Authority: GB
Inventors: Alan Edward Green; Euan Morrison
Original assignee: Quantumbeam Ltd
Current assignee: Quantumbeam Ltd
Priority date: 2002-01-07
Filing date: 2002-01-07
Publication date: 2003-07-09
Also published as: GB0200243D0; TW200412738A; WO2003057221A1; AU2003201637A1; AU2003201637A8; WO2003057221A9

Abstract

An optical communications system is provided comprising first and second signalling devices 3, 7 in which a free space duplex communications link 11 between the devices is established. The first device 3 includes a modulating retro-reflector and the second device 7 includes at least one light source for directing light towards the retro-reflector. A full duplex link is described which uses small signal modulation techniques to differentiate the transmitted uplink modulation data from the reflected downlink modulation data. The strength of the reflected signal is monitored and used as part of a control loop to control the power level of the transmitted light. In one embodiment the modulation depth (the difference in modulated intensity levels p1, p2 in fig. 5) of the transmitted uplink signal is varied in accordance with the attenuation characteristics of the channel. A drive circuit for the light source is disclosed in detail (see figure 11). Use of lower signal frequencies, such as microwave, is also envisaged.

Description

OPTICAL FREE SPACE SIGNALLING SYSTEM This invention relates to a signalling system and components thereof. Embodiments of the invention relate to a signalling method and apparatus in which data is conveyed by modulating and retro-reflecting a free-space light beam.

International Patent Application WO 98/35328, the whole contents of which are incorporated herein by reference, describes a point-to-multipoint communication system using free-space light beams. In particular, WO 98/35328 describes a system in which a plurality of user stations (provided, for example, on respective houses in a street) emit unmodulated light beams which are directed to a local distribution node (provided, for example, on a post in the street). At the local distribution node, each of the incoming light beams is modulated in accordance with a data signal by a respective modulator element of an array of modulator elements which are individually drivable, and is reflected back to the user station from which it originated. At the user station, the modulated light beam is detected and the data signal is regenerated.

According to one aspect, the present invention provides a signalling system comprising first and second signalling devices, the first signalling device comprising means for receiving a signal output from the second signalling

device and carrying modulation data transmitted from the second signalling device; means for converting a first portion of the received signal into a corresponding electric signal; means for processing the corresponding electric signal to retrieve said uplink data; and means for reflecting a second portion of the received signal back to the second signalling device; and the second signalling device comprising means for generating a signal; means for varying the intensity of the generated signal between first and second non-zero intensity levels in dependence upon the uplink data to be transmitted to the first signalling device; and means for receiving the reflected signal from the first signalling device; wherein the second signalling device further comprises means for receiving a signal indicative of signal attenuation characteristics between the first and second signalling devices; and means for varying the difference in intensity between the first and second intensity levels in dependence upon the signal indicative of said attenuation characteristics.

According to another aspect, the present invention provides a drive circuit for applying a drive current to a signal generator, the drive circuit comprising a current source, a differential amplifier; means for differentially applying a data signal to said differential amplifier; and a capacitor connected between a base electrode of a first solid state switch forming part of said differential amplifier and the collector of

a second solid state switch forming part of said differential amplifier.

According to a further aspect, the present invention provides a signalling system comprising first and second signalling devices, the first signalling device comprising means for receiving a signal output from the second signalling device; means for reflecting a portion of the received signal back to said second signalling device; and the second signalling device comprising means for generating a signal; means for outputting the generated signal towards the first signalling device; and means for receiving the reflected signal from the first signalling device; wherein the second signalling device further comprises means for processing the reflected signal received at said second signalling device to determine a measure of the signal strength of the reflected signal; and means for dynamically varying the signal level of the generated signal in dependence upon the measured signal level of said reflected signal.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is schematic diagram of a point-to-multipoint communication system for distributing data between a central distribution system and a plurality of user

stations; Figure 2 is schematic diagram of a user station and associated user device which form part of the data distribution system shown in Figure 1; Figure 3 shows a perspective view of the user station illustrated in Figure 2; Figure 4 is a schematic diagram showing the detection surface of a detector forming part of the user station illustrated in Figure 2; Figure 5 is a plot illustrating the way that the power of a laser beam emitted by the user station is varied to achieve a small signal modulation for uplink data transmitted from the user device to the local distribution node; Figure 6 is an eye diagram schematically illustrating the effect of the small signal modulation on the downlink data transmitted from the local distribution node to the user device; Figure 7 is a schematic diagram of a local distribution node which forms part of the data distribution system illustrated in Figure 1; Figure 8A is a cross-sectional view of one modulator of a modulator array which forms part of the local

distribution node illustrated in Figure 7 in a first operational mode when no DC bias voltage is applied to the electrodes thereof; Figure 8B is a cross-sectional view of one modulator of the modulator array which forms part of the local distribution node illustrated in Figure 7 in a second operational mode when a bias voltage is applied to the electrodes thereof; Figure 9 is a signal diagram which schematically illustrates the way in which the light incident on the modulator shown in Figures 8A and 8B is modulated in dependence upon the bias voltage applied to the pixel electrodes; Figure 10 is a schematic diagram of a surface of the modulator array forming part of the local distribution node shown in Figure 7; Figure 11 is a circuit diagram illustrating the main components of a laser driver forming part of the user station shown in Figure 2; Figure 12 is a block diagram illustrating the main components of a central control unit forming part of the user station shown in Figure 2; Figure 13A is a plot showing the way in which the drive current applied to the laser diode shown in Figure 2

varies using a conventional laser driver; and Figure 13B is a plot of the drive current generated using the laser driver shown in Figure 11.

OVERVIEW Figure 1 schematically illustrates a data distribution system which employs a point-to-multipoint signalling system to transmit data to and receive data from a plurality of user stations. As shown, the data distribution system comprises a central distribution system 1 which transmits optical data signals to and receives optical data signals from a plurality of local distribution nodes 3a to 3c via respective optical fibres 5a to 5c.

At the local distribution node 3a, data streams received from the central distribution system 1 are transmitted to respective users stations 7a to 7d and data for transmission to the central distribution 1 is received from the user stations 7a to 7d using free-space optical links lla to lld, i. e. optical links in which light is not guided along an optical fibre path. Similarly, data is transmitted between the local distribution node 3b and user stations 7e to 7h using free-space optical links 11e to llh, and data is transmitted between the local distribution node 3c and user stations 7i to 71 using free-space optical links lli to 111. Each of the user stations 7 is connected to at least one user device (not shown). In this embodiment, the user devices include a

television set (not shown), which transmits channel information to the central distribution system 1 and in response receives corresponding television signals, and a computer system (not shown), which accesses the internet via the central distribution system.

In this embodiment, each user station 7 emits a low divergence, free-space light beam which has a small signal modulation applied to it in accordance with data to be conveyed to the local distribution node 3 and which is directed at the corresponding local distribution node 3. Each local distribution node 3 has a plurality of modulating elements (not shown in Figure 1) which modulate and retro-reflect the light beams from respective user stations 7 to convey data from the local distribution node 3 to the user station 7. One aspect of the invention relates to a drive circuit used to drive the light source used in the user stations 7. Another aspect of the invention relates to a feedback control loop used to regulate the power of the transmitted light beam in accordance with the signal level of the retroreflected light beam received back at the respective user stations 7. Another aspect of the invention relates to a feedback control loop used to regulate the modulation depth of the small signal modulation applied to the transmitted light beam.

User Station Figure 2 schematically illustrates in more detail the main components of one of the user stations 7 of the data

distribution system shown in Figure 1. As shown, the user station 7 comprises a laser diode 21 which outputs a beam 23 of coherent light. In this embodiment, the user stations 7 are designed so that they can communicate with a local distribution node 3 within a range of 200 metres with a link availability of 99.9. %. To achieve this, the laser diode 21 is a 50mw laser diode which outputs a laser beam having a wavelength of 780nm.

The output light beam 23 is passed through a lens 25, hereafter called the collimating lens 25, which reduces the angle of divergence of the light beam 23 to form a substantially low divergence light beam 27. The divergence of the low divergence light beam 27 can be varied by varying the distance between the collimating lens 25 and the light source 21. However, those skilled in the art will appreciate that the low divergence light beam 27 can not be perfectly collimated due to diffraction at the emitting aperture of the laser diode 21. In this embodiment, the collimating lens 25 is a low aberration lens, so that the low divergence beam 27 has a relatively uniform wavefront, with a 50mm diameter and a numerical aperture which is just large enough to collect all the light emitted by the laser diode 21.

Although the divergence of the light beam 27 is low, the size of the light beam 29 incident on the user station 7 after reflection by the local distribution node 3 is significantly larger than that of the low divergence light beam 27 leaving the user station 7. In this

embodiment, as shown in Figure 3, the beam size of the reflected light beam 29 is large enough to encompass a lens, hereafter called the downlink detection lens 31, which is provided adjacent to the collimating lens 25. In this embodiment, the entrance pupils of the collimating lens 25 and the downlink detection lens 31 are located in the same plane.

Returning to Figure 2, in which for clarity only the portion of the received light beam 29 incident on the downlink detection lens 31 is shown, the downlink detection lens 31 focusses light from the received light beam 29 onto a detector 33, which in this embodiment is an avalanche photodiode. The downlink detection lens 31 has a diameter of 100mm but is not required to be of as high quality as the collimating lens 25 because its primary purpose is simply to direct as much light as possible onto the detector 33. Figure 4 schematically shows the detection surface 61 of the detector 33 and the light spot 63 formed by the downlink detection lens 31 focussing light from the received light beam 29. In this embodiment, the diameter of the detection surface 61 is 500um whereas the diameter of the light spot 63 is approximately 50um.

The detector 33 converts the received light beam into a corresponding electrical signal which varies in accordance with the modulation provided at the local distribution node 3. The electrical signal is amplified

by an amplifier 35 and then filtered by a filter 37. The filtered signals are input to a central control unit 39 which performs a conventional clock recovery and data retrieval operation to regenerate the data from the central distribution system 1. The retrieved data is then passed to an interface unit 41 which is connected to the user device 43.

The interface unit 41 also receives data from the user device 43 and inputs the received data to the central control unit 39, which generates an appropriate message (DATA) for transmittal to the central distribution system 1 via the local distribution node 3. This message is output to a laser driver 43 which modulates the light beam 23 output by the laser diode 21 in accordance with the message. In this embodiment, the laser driver 43 applies a small signal modulation to the light beam 23 output by the laser diode 21. Figure 5 illustrates this modulation and shows the CW laser level 65 and the small signal modulation 67 (varying between laser power levels Pi and P2) applied to it. Due to the asymmetric path loss of a retro-reflecting system, the small signal modulation concept can be used to provide a"full" bandwidth uplink channel. As those skilled in the art will appreciate, this uplink modulation data will then become an additional noise source for the down link data.

This is illustrated in Figure 6 which shows an eye diagram for the downlink data 69, which includes the interfering uplink data 67, and the consequent reduction in the noise margin 70. However, if the uplink

modulation depth (defined by Pi-Pz) is kept sufficiently low, then both the uplink and the downlink can operate with equal bandwidth.

Returning to Figure 2, the central control unit 39 is also connected to a first motor driver 45a for supplying drive signals to a horizontal stepper motor 47, and to a second motor driver 45b for supplying drive signals to a vertical stepper motor 49. In this embodiment, the laser diode 21, the collimating lens 25, the detector 33 and the downlink detection lens 31 are mounted together to form a single optical assembly 51, and the horizontal stepper motor 47 is operable to rotate the optical assembly 51 about a vertical axis so that the collimated light beam 27 moves within a horizontal plane and the vertical stepper motor 49 is operable to rotate the optical assembly 51 about a horizontal axis so that the collimated light beam 27 moves in a vertical plane. In this way, the direction of the emitted light beam can be varied.

Local Distribution Node Figure 7 schematically illustrates the main components of one of the local distribution nodes 3. As shown, the local distribution node 3 comprises a communications control unit 71 which receives optical signals transmitted along the optical fibre 5 conveying data from the central distribution system 1 and regenerates the conveyed data from the received optical signals. The communications control unit 71 generates control signals

in accordance with the conveyed data which are output to a modulator drive circuit 73 which in turn supplies corresponding drive signals to a modulator array 75. In this embodiment, the modulator elements of the modulator array 75 are individually addressable by the modulator drive circuit 73, with the drive signals output by the modulator drive circuit 73 varying the reflectivity of the modulator elements.

In this embodiment, the modulator array 75 comprises a two-dimensional planar integrated array of Quantum Confined Stark Effect (QCSE) devices (which are sometimes also referred to as Self Electro-optic Devices or SEEDs).

Figure 8A schematically illustrates the cross-section of one of the QCSE devices 91. As shown, the QCSE device 91 comprises a transparent window 93 through which the light beam from the appropriate user station 7 passes, followed by three layers 95-1,95-2, 95-3 of Gallium Arsenide (GaAs) based material. Layer 95-1 is a pconductivity type layer, layer 95-2 is an intrinsic layer having a plurality of Quantum wells formed therein, and layer 95-3 is an n-conductivity type layer. Together, the three layers 95 form a p-i-n diode. As shown, the p-conductivity type layer 95-1 is connected to an electrode 101 and the n-conductivity type layer 95-3 is connected to a ground terminal 103. A reflective layer 97, in this embodiment a Bragg reflector, is provided beneath the n-conductivity type layer 95-3, and a substrate layer 99 is provided beneath the reflective layer 97.

In operation, the light beam from the user station 7 passes through the window 93 into the Gallium Arsenide based layers 95. The amount of light absorbed by the intrinsic layer 95-2 depends upon the DC bias voltage applied to the electrode 101. Ideally, when no DC bias is applied to the electrode 101, as illustrated in Figure 8A, the light beam passes through the window 93 and is totally absorbed within the intrinsic layer 95-2.

Consequently, when there is no DC bias voltage applied to the electrode 101, no light is reflected back to the corresponding user station 7. On the other hand, when a DC bias voltage of approximately-5 volts is applied to the electrode 101, as illustrated in Figure 8B, the light beam from the corresponding user station 7 passes through the window 93 and the Gallium Arsenide based layers 95 and is reflected by the reflecting layer 97.

Therefore, by changing the bias voltage applied to the electrode 101 in accordance with the drive signals from the modulator drive circuit 73, the QCSE modulator 91 amplitude modulates the received light beam and reflects the modulated light beam back to the user station 7.

In the ideal case, as illustrated in Figure 9, a zero voltage bias, resulting in no reflected light, is applied to the electrode 101 to transmit a binary 0 and a DC bias voltage of-5 volts is applied to the electrode 101, resulting in the light from the user station 7 being reflected back from the QCSE device 91, to transmit a binary 1. Typically, however, the QCSE modulator 91 will reflect 70% of the light beam when no DC bias is applied

to the electrode 101 and 95% of the light beam when-5 volts DC bias is applied to the electrode 101. Therefore, in practice, there will only be a difference of about 25% between the amount of light which is detected at the user station 7 when a binary 0 is transmitted and when a binary 1 is transmitted.

The amount of the received light beam absorbed by the intrinsic layer 95-2 can be increased by adding additional Quantum Wells to increase the depth of the intrinsic layer 95-2. However, if the depth of the intrinsic 95-2 is increased, then a higher voltage must be applied to the electrode 101 in order to produce the required electric field across the intrinsic layer 95-2 for allowing light to pass through the intrinsic layer 95-2. There is, therefore, a trade-off between the absorptivity of the intrinsic layer 95-2 and the voltage applied to the electrode 101.

By using the QCSE modulators 91, modulation rates of the individual modulator cells in excess of a Gigabit per second can be achieved.

Figure 10 shows the surface of the modulator array 75 used in this embodiment. As shown, the modulator array 75 is a two-dimensional array with sixteen modulator elements 91 provided in a Y-direction and two modulator elements 91 provided in a X-direction perpendicular to the Y-direction. Those skilled in the art will appreciate that, by having only two modulators in the X-

direction, the fabrication of the modulator array 75 is greatly simplified because the modulator elements 91 can be addressed from the longitudinal sides of the array.

In this embodiment, each modulator element 91 has a length of approximately lmm in the X-direction and a width of approximately lOOpm in the Y-direction. This layout has been selected to match the likely distribution of users within a building having many floors. In particular, the modulator array 75 is aligned so that the X-direction corresponds to the horizontal direction on the building and the Y-direction corresponds to the vertical direction on the building, and less modulator elements 91 are provided in the X-direction than in the Y-direction because the users are expected to be predominantly distributed in the Y-direction. The length of the modulator elements 91 in the X-direction is made longer than the width in the Y-direction to ensure adequate coverage of the sides of the building.

The local distribution node 3 also comprises a detector array 77 having a plurality of light detecting elements. Each detecting element converts incident light from a respective user station 7 into a corresponding electrical signal which is input to a detection circuit 79. In the detection circuit 79, the electrical signals from the detector array are amplified, and then the detection circuit 79 performs conventional clock retrieval and data regeneration processing to recover message data from the user stations 7. The recovered message data from all of

the user stations 7 is then output to the communications control unit 71 which transmits the message data to the central distribution system 1 as optical signals along the optical fibre 5.

As shown in Figure 7, in this embodiment, separate optical systems are provided for the modulator array 75 and the detector array 77. In particular, the modulator array 75 is located substantially at the back focal plane of a telecentric lens 79 (represented by the stop member 81 and the lens element 82). As those skilled in the art will appreciate, the telecentric lens 79 directs a low-divergence light beam received from a user station 7 towards a point within its back focal plane whose position depends upon the angle of incidence of the received light beam. In other words, the telecentric lens 79 maps different directions within its field of view to different positions on the modulator array 75.

In this way, the modulator array 75 is able to modulate and reflect light beams from a plurality of user stations 7 positioned in different locations within the field of view of the telecentric lens 79. The advantage of using a telecentric lens in front of the modulator array 75 is described in the applicant's earlier international application WO 98/35328, the contents of which are incorporated herein by reference.

The detector array 77 is positioned in the back focal plane of a lens, hereafter called the uplink detection lens 83. As those skilled in the art will appreciate,

it is not necessary for the principal rays passing through the uplink detection lens 83 to be incident perpendicular to the detector array 77. The uplink detection lens 83 is therefore designed simply to collect as much light from the user stations 7 as possible and to direct the collected light to respective detecting elements. In this embodiment, the uplink detection lens 83 is twice the size of the modulator lens 79 but has approximately the same focal length. In other words, the uplink detection lens 79 has approximately half the fnumber of the modulator lens 83.

LASER DRIVER As mentioned above, various aspects of the present invention relate to the control and operation of the laser driver 43. A more detailed description of the laser driver 43 will therefore be given.

Figure 11 is a circuit diagram illustrating the form of the laser driver 43 in combination with the laser diode 21 used in this embodiment. As shown, the laser driver 43 includes two current sources 221 and 223 which each provide current for driving the laser diode 21. Current source 223 is a voltage controlled current source which outputs a current in dependence upon the voltage control signal CTRL2 received from the central control unit 39 shown in Figure 2. The current generated by the current source 223 is a bias current and is applied to the laser diode 21 through a resistor Ri. The level of the bias current generated by the current source 223 determines

the lower output power level of the laser diode 21 (i. e.

P2 shown in Figure 5).

The second current source 221 is also a voltage controlled current source whose output current level is determined by the voltage control signal CTRL1 output by the central control unit 39 shown in Figure 2. As shown in Figure 11, the current output from current source 221 feeds into a differential amplifier formed by two p-n-p transistors Q1 and Q2. As shown, the uplink message (DATA) received from the central control unit 39 is buffered in a buffer 225 and differential outputs are taken from the buffer 225 and applied to respective bases of the transistors Q1 and Q2. In particular, the base of transistor Q1 is driven with the complement of the uplink message (NOT DATA) and the base of transistor Q2 is driven with the uplink message (DATA). In this way, when the message data has a low value transistor Q1 will be open and transistor Q2 will be closed so that the current generated by the current source 221 passes to ground via the resistor R2 and, when the message data has a high value, transistor Q2 will be open and transistor Q1 will be closed so that the current generated by the current source 221 passes through the transistor Q1 and adds with the bias current generated by the current source 223 which is then applied to the laser diode 21. As those skilled in the art will appreciate, the amount of current generated by the voltage controlled current source 221 determines the highest output power level of the laser diode 21 (i. e. P2 shown in Figure 5).

One feature of the retro-reflecting system of the present embodiment is that the signal strength of the reflected light beam received at the user station 7 is measured and used to control the laser power of the transmitted light beam so that the received signal level is just sufficient to achieve the desired signal-to-noise ratio or bit error rate. In this way, the transmitted light beam power level is set at the lowest level required for system operation, thereby maximising the efficiency of the system and imparting an eye safety benefit.

As those skilled in the art will appreciate, the received signal will include the modulated reflected beam together with background light from the environment (e. g. sunlight). The background light is fairly constant and will form part of the DC component of the received signal. In contrast, the reflected light beam will include both a DC and an AC component due to the amplitude modulation of the reflected beam with the downlink data. In this embodiment, the central control unit 39 measures the signal strength of the AC signal component of the received signal (in order to avoid false measurements caused by excessive ambient light), and uses this measurement to determine and to output the appropriate voltage control signal CTRL2 for controlling the bias current generated by the current source 223.

The way in which the central control unit 39 achieves this will now be described with reference to Figure 12 which shows the main components of the central control

unit 39. As shown, the signal received from the filter 37 is input to an average power determining unit 231. (In this embodiment, the filter 37 is a high pass filter which filters out the DC component but not the AC component of the received signal. ) In this embodiment, the average power determining unit 231 determines a running average power of the filtered signal. This measured power level is then subtracted from a desired power level (Pdes) to generate an error signal (e), which is input to controller A 233 which uses conventional control techniques to vary control signal CTRL2 in order to reduce the error signal (e) to zero. In this embodiment, the desired power level (Pdes) is set in advance and is determined by parameters of the optical receiver, such as its noise bandwidth, the input current noise and voltage noise spectral densities etc, which are well known parameters for the particular receiver design.

Figure 12 also shows the uplink data processor 235 which is used to process (encode etc. ) the uplink data (DATA) received from the user device 43 via the interface unit 41, so that it is in a suitable format for transmission to the local distribution node 3. Figure 12 also shows a second controller (controller B) 237 which generates control signal CTRL1 which is used to control the current source 221 and hence to control the modulation depth of the small signal modulation used to carry the uplink data to the local distribution node 3. As shown in Figure 12, controller B generates control signal CTRL1 using the control signal CTRL2 as an input. The reason for this

will now be described.

As mentioned above, the uplink data is transmitted to the local distribution node 3 by generating a small signal modulation current that is superimposed on the bias current required to provide light for the downlink. The uplink modulation also appears on the retro-reflected downlink signal and is seen as an additional noise source by the user station 7. To minimise the signal-to-noise penalty which the uplink modulation imposes on the downlink signal, it is advantageous to maintain the uplink modulation depth (i. e. Pi-P) at a level that is as small as possible whilst maintaining a sufficient uplink signal-to-noise ratio.

In a retro-reflecting system, the downlink signal traverses the atmosphere twice, whilst the uplink signal only traverses the atmosphere once. Therefore, the downlink signal suffers greater atmospheric loss (twice as many dBs) than the uplink signal. Therefore, in a simple case, if an increase in atmospheric loss requires that the laser power be increased by xdB to maintain the constant downlink received signal strength, then the uplink modulation depth need only be increased by x/2dB to maintain the performance of the uplink. In practice, however, the optical system geometrical losses (caused by beam divergence and the size of the system apertures) and the modulation efficiency of the QCSE modulators results in a more complicated relationship between the atmospheric losses and the required uplink modulation

depth. The inventors have calculated a power budget for both the uplink and downlink and determined the following relationship that relates the uplink modulation depth to the various losses and receiver sensitivities:

where muser represents the uplink modulation index which is a number ( < 1) representing the fraction of the optical power actually modulated, which (referring to Figure 5) is given by:

Snode is the receiver sensitivity of the local distribution node 3 (in dBm); Suser is the receiver sensitivity of the user station 7 (in dBm); mnode is the

modulation index of the QCSE modulators ; Aatmosphere is the atmospheric loss (in dB) ; Aup is the uplink (user station 7 to local distribution node 3) geometrical loss (in dB) which is given by:

where Dnode is the diameter of the receiver lens 83 at the local distribution node 3; R is the distance between the user station 7 and the local distribution node 3 and Ouser is the beam divergence of the light beam emitted from the user station 7.

Ateiecentricis the telecentric lens geometrical loss (in dB) which is given by:

where Dtelecentric is the diameter of the telecentric lens 79.

Adown is the downlink (local distribution node 3 to user station 7) geometrical loss (in dB) which is given by:

where Doser is the diameter of the receiving lens 29 of the user station 7 and 8node is the beam divergence of the reflected beam output from the telecentric lens 79.

In general, for a given installation Atelecentric, Aup and Adown are fixed losses due to the geometry of the system. The modulation index of the QCSE modulators is also a fixed

parameter and hence the atmospheric loss is the only parameter that is variable. Given a measure of the atmospheric loss, equation (1) above allows the calculation of the minimum uplink modulation index (muser). From this calculated modulation index, the appropriate control signal (CTRL1) can be determined for application to the voltage controlled current source 221 to achieve the calculated modulation index. In this embodiment, both of these calculations are performed by controller B 237 shown in Figure 12. As shown, in this embodiment controller B 237 calculates these solely from an input of the CTRL2 control signal and using equation (1) above.

In particular, in this embodiment the measure of the atmospheric loss (Atmosphere) is estimated from the current value of control signal CTRL2. This is possible because control signal CTRL2 varies with the atmospheric loss due to the action of controller A 233. In this embodiment, controller A 233 is designed so that the value of control signal CTRL2 is proportional to the atmospheric loss.

The constant of proportionality relating the value of CTRL2 to the atmospheric loss can then be determined empirically (in advance) and used by controller B 237 to determine a measure of the current atmospheric loss from the current value of CTRL2. Controller B 237 then determines the minimum uplink modulation index (mUSer) in the manner discussed above and then determines the appropriate value of control signal CTRLl (again using control signal CTRL2 which is proportional to the

transmitted laser power level P2) which will set the maximum transmission power level Pi at a level corresponding to the determined minimum uplink modulation index (muser) determined from equation (2).

Returning to Figure 11, another aspect of the drive circuit 43 used in this embodiment is the provision of the capacitor C connected between the base of transistor Q2 and the collector of transistor Ql. In particular, capacitor C is provided in order to reduce distortion of the modulation current applied to the laser diode 21.

Referring to Figure 13, Figure 13a is a plot showing the form of the drive current applied to the laser diode 21 generated by a laser driver identical to that shown in Figure 11 without the capacitor C. As shown, when the transistor Q1 opens and closes, spikes 241 are generated in the drive current applied to the laser diode 21.

The inventor has identified that these spikes 241 are caused by the inherent base to collector capacitance of transistor Ql, as it couples current from the base drive signal onto the laser diode 21. The inventor has realised that this distortion can be reduced by the addition of an appropriate capacitor C connected between the base of transistor Q2 and the collector of transistor Ql. This is because the signals used to drive the bases of transistors Q1 and Q2 are 180 out of phase and therefore when a data rising edge is presented to the base of transistor Q1 a corresponding falling edge is presented to the base of transistor Q2 and vice versa.

Therefore, by setting the value of capacitor C to be substantially equal to the base to collector capacitance of transistor Ql, the current that passes through capacitor C should substantially cancel the current that passes through the base to collector capacitance of transistor Ql. Figure 13B is a plot illustrating the drive current generated using the laser driver 43 shown in Figure 11 with the capacitor C connected in the manner shown. As can be seen from Figure 13B, the spikes 241 shown in Figure 13A have been substantially suppressed.

For a typical high-speed silicon transistor like those used in the present embodiment, the value of capacitor C required is approximately IpF.

MODIFICATIONS AND FURTHER EMBODIMENTS In the embodiment described above, the user station was in a fixed position relative to the local distribution node. As those skilled in the art will appreciate, the techniques described above can be applied to an embodiment in which the user station and/or the local distribution nodes move relative to each other. In this case, the user stations would preferably include an array of emitters such as vertical cavity surface emitting lasers (VCSEL) so that the direction of the emitted light beam can be steered electronically rather than mechanically as in the above embodiment. An array of detectors would also be preferably used in the user stations in such an embodiment, so that the direction of the local distribution node relative to the user station can be determined by determining which of the detector

elements in the detector array detects the modulated light from the local distribution node. Therefore, a tracking operation can be performed in which the VCSEL in the emitter array used to output the light beam to the local distribution node is selected in accordance with which of the detecting elements of the detector array detects the light returned from the local distribution node. The way in which this is carried out is described in International Patent Application WOOO/48338, the whole contents of which are hereby incorporated by reference.

In the above embodiment, separate optical systems are provided in the user station for the light emitter and the light detector to reduce back reflections falling on the light detector. Those skilled in the art will appreciate that the laser driver circuitry and the control algorithms described above could also be used in systems in which a beam splitter is used to optically align the optical axes of the collimating lens and the downlink detection lens, such as those described in WO 98/35328 and WO 00/48338.

As described above, providing separate optical systems for the light emitter and the light detector in the user station and for the modulator array and light detector in the local distribution node enables these lens systems to be separately optimised in accordance with their associated optical element. The particular details provided in the described embodiment are given for exemplary purposes only and are not essential to the

invention.

In the first embodiment, the light emitter and the light detector are mounted along with their associated optical systems as a single optical assembly which is moved by stepper motors in order to steer the emitted light beam.

Alternatively, the light emitter and associated lens system could be mounted separately from the light detector and associated lens system. The beam steering techniques described in WO 01/05072, the whole contents of which are incorporated herein by reference, could also be used. In another embodiment, the light beam could be steered by moving a lens element forming part of the lens of the emitter.

Those skilled in the art will appreciate that if the direction of the emitted light beam is varied then the return light beam will not generally be focussed at the centre of the detection surface of the detector.

However, if the detection surface is much larger than the focussed spot size, as in the first embodiment, this is not a problem. Those skilled in the art will also appreciate that for the arrangement in the first embodiment, it is not essential for the detection surface to be much larger than the focussed spot size because when the direction of the emitted light beam is varied, the optical axis of the detection lens is moved to match the direction of the emitted light beam.

In the described embodiment, light beams from a plurality

of user stations are incident on respective modulator elements of a modulator array in a local distribution node and are retro-reflected back to their originating user stations. Alternatively, a plurality of light emitters could be provided in the local distribution node and modulators provided in each of the user stations.

In the above embodiment, QCSE modulators are used. As those skilled in the art will appreciate, other types of reflectors and modulators could be used. For example, a plane mirror may be used as the reflector and a transmissive modulator (such as liquid crystal) could be provided between the lens and the mirror. Further, those skilled in the art will appreciate that the reflectors and/or modulators need not be integrated in a single device and it is also not essential for the reflectors and/or modulators to be located in a common plane, although these features are preferred for ease of device manufacture and alignment.

In the above embodiment, the modulator elements are arranged in a rectangular matrix. However, this is not essential and the modulator elements could be arranged in a different form of regular array or even in an irregular arrangement.

Those skilled in the art will appreciate that the term light includes electromagnetic waves in the ultra-violet and infra-red regions of the electromagnetic spectrum as well as the visible region. Although the embodiments

described above have used laser beams with a wavelength of about 780nm, other wavelengths could be used. In particular, a wavelength of 1. Sm is an attractive alternative because it is inherently more eye-safe and emitters and detectors have been developed for this wavelength for optical fibre communications.

Although the lenses in the user station and the local distribution, node have been schematically represented by a single lens, it will be appreciated that in practice each lens may have a plurality of lens elements.

In the above embodiment, a point-to-multipoint communication system has been described. As those skilled in the art will appreciate, the above laser driver and control circuit for controlling the laser driver may be used in other retro-reflecting systems, such as point-to-point communication systems or multipoint-to-point communication systems. Further, although the transmitted signal has been an optical signal, the above techniques can be used in systems which transmit at lower frequencies, such as at microwave frequencies. In this case, the retro-reflectors can be formed by appropriately lengthed microwave waveguides.

In the above embodiment, two control loops were used to control the drive current applied to the laser diode and one of the control signals was used to determine a measure of the atmospheric losses in the second control

loop. As those skilled in the art will appreciate, this is not essential. The measure of the atmospheric losses may be derived or obtained from an alternative source.

For example, a camera may be provided which generates image signals of the communication link between the user station and the local distribution node. In this case, appropriate image processing circuitry could be provided to process the image signals from the camera to derive an appropriate measure for the atmospheric loss.

However, as those skilled in the art will appreciate, such an embodiment is not preferred because of the additional complexity and circuitry required to determine the measure of the atmospheric loss.

In the embodiment described above, a telecentric lens was provided in front of the array of modulators in the local distribution node. As those skilled in the art will appreciate, the use of a telecentric lens is not essential and it may be replaced by a conventional lens or lens assembly.

In the above embodiment, a closed control loop was formed between the user station and the local distribution node using the retro-reflected optical beam generated at the user station. This control loop was used in order to try to maintain the dynamic range of variation of the reflected signal level at a predetermined level which allows the receiver electronics to be able to recover the downlink data. The measured signal used in the control loop was the average power level of the received signal.

As those skilled in the art will appreciate, other measures may be used, such as the signal-to-noise ratio. Further, in the above embodiments, the set point of the desired power level was fixed in advance. In an alternative embodiment, a mechanism may be provided to vary the set point either in response to a user input or automatically in response to historical performance data that can be analysed off-line.

Further, in the above embodiment, the central control unit monitored the average power level of the AC component of the received signal. Since the retroreflected beam also includes a DC component, the central control unit could instead or in addition monitor the DC component of the received signal to determine the appropriate CTRL2 control signal.

Further, in the above embodiment, p-n-p transistors were used in the laser drive circuit. As those skilled in the art will appreciate, n-p-n type transistors or any type of solid state switch may be used instead.

Claims

CLAIMS: 1. A signalling system comprising first and second signalling devices, the first signalling device comprising means for receiving a signal output from the second signalling device and carrying uplink data transmitted from the second signalling device; means for converting a first portion of the received signal into a corresponding electric signal; means for processing the corresponding electric signal to retrieve said uplink data; and means for modulating a second portion of the received signal with downlink data for the second signalling device and for reflecting the second portion of the received signal back to the second signalling device; and the second signalling device comprising means for generating a signal; means for varying the intensity of the generated signal between a first non-zero intensity level and a second non-zero intensity level in dependence upon said uplink data for the first signalling device; means for outputting the intensity modulated signal towards said first signalling device; means for receiving the reflected signal from said first signalling device carrying said downlink data; and means for retrieving the downlink data from said reflected signal; characterised in that said second signalling device further comprises means for receiving a signal indicative of current signal attenuation characteristics of a signalling channel between said first and second signalling devices through which said output signal and

<Desc/Clms Page number 34>

said reflected signal pass; and means for dynamically varying the difference in intensity between said first and second intensity levels in dependence upon the received signal indicative of the current signal attenuation characteristics of said signalling channel.
2. A system according to claim 1, wherein said means for modulating and reflecting comprises a Quantum Confined Stark Effect device.
3. A system according to claim 1 or 2, wherein said first and second signalling devices are operable to transmit said uplink data and said downlink data simultaneously.
4. A system according to any preceding claim, wherein said modulating means of said first signalling device is operable to intensity modulate the second portion of the received signal between third and fourth non-zero intensity levels, with the difference in intensity between the third and fourth intensity levels being greater than the difference in intensity level between the first and second intensity levels.
5. A system according to any preceding claim, wherein said means for generating a signal comprises a laser, a laser diode or a light emitting diode.
6. A system according to any preceding claim, wherein said second signalling device further comprises means for

<Desc/Clms Page number 35>

determining said signal indicative of the current signal attenuation characteristics of the signalling channel between the first and second signalling devices.
7. A system according to claim 6, wherein said determining means is operable to determine said measure using a measure of the signal level of the reflected signal received back from said first signalling device.
8. A system according to any preceding claim, wherein said second signalling device further comprises means for varying the lowest intensity level of said first and second non-zero intensity levels in dependence upon the strength of the reflected signal received back from said first signalling device.
9. A system according to claim 8, wherein said means for varying the difference in intensity between said first and second intensity levels is operable to vary said difference in dependence upon the lowest intensity level of said first and second intensity levels.
10. A system according to any preceding claim, wherein said varying means of said second signalling device comprises: a first current source for generating a drive current; a second current source for generating a bias current; a first solid state switch having first and second

<Desc/Clms Page number 36>

main electrodes and a control electrode, the first main electrode being connected, in use, to said first current source and the second main electrode being connected, in use, to a reference potential; a second solid state switch having first and second main electrodes and a control electrode, the first main electrode being connected, in use, to said first current source and the second main electrode being connected, in use, to said signal generator and to said second current source; means for receiving a first data signal corresponding to said uplink data and a second data signal corresponding to the inverse of said uplink data signal; means for applying said first data signal to the control electrode of said first solid state switch to open and close said first solid state switch in dependence upon the first data signal; means for applying said second data signal to the control electrode of said second solid state switch to open and close said second solid state switch in dependence upon the second data signal; whereby the current applied in use to said signal generator corresponds to (i) the bias current provided by said second current source summed with the current from the first current source when said second solid state switch is closed; and (ii) the bias current provided by said second current source when said second solid state switch is open.

<Desc/Clms Page number 37>
11. A system according to claim 10, wherein said varying means further comprises a capacitor connected between the control electrode of said first solid state switch and the second main electrode of said second solid state switch.
12. A drive circuit according to claim 11, wherein the capacitance of said capacitor is set in dependence upon an inherent capacitance between the control electrode and the second main electrode of said second solid state switch.
13. A drive circuit according to claim 12, wherein the capacitance of said capacitor is set to be approximately equal to the inherent capacitance between said control electrode and said second main electrode of said second solid state switch.
14. A system according to any preceding claim, wherein said first signalling device further comprises focussing means for focussing the second portion of the received signal onto said reflecting means.
15. A system according to claim 14, wherein said modulating means is transmissive and is located between said focussing means and said reflecting means.
16. A system according to any preceding claim, wherein said first signalling device further comprises focussing means for focussing the first portion of the received

<Desc/Clms Page number 38>

signal onto the converting means.
17. A system according to any of claims 14 to 16, wherein said focussing means comprises a telecentric lens and wherein said reflecting means and/or said converting means is located substantially at the focal plane of said telecentric lens.
18. A system according to any preceding claim, wherein said modulating means and said reflecting means are colocated.
19. A system according to any preceding claim, wherein said first signalling device comprises a plurality of modulating and reflecting means for modulating and reflecting optical signals received from a plurality of second signalling devices.
20. A system according to claim 19, wherein said plurality of modulating and reflecting means are arranged in an array.
21. A system according to any preceding claim, wherein said reflecting means comprises a retro-reflector.
22. A system according to any preceding claim, wherein the data rate of said uplink data and said downlink data is substantially the same.
23. A signalling system comprising first and second

<Desc/Clms Page number 39>

signalling devices, the first signalling device comprising means for receiving a signal output from the second signalling device and carrying uplink data transmitted from the second signalling device as a small signal modulation of the output signal; means for converting a first portion of the received signal into a corresponding electric signal; means for processing the corresponding electric signal to retrieve said uplink data; means for modulating a second portion of the received signal with downlink data for the second signalling device and for reflecting the second portion of the received signal back to the second signalling device; and the second signalling device comprising means for generating a signal; means for applying a small signal modulation to said generated signal with said uplink data for the first signalling device; means for outputting the modulated signal towards said first signalling device; means for receiving the reflected signal from said first signalling device carrying said downlink data; and means for retrieving the downlink data from said reflected signal; characterised in that said second signalling device further comprises: means for receiving a signal indicative of current signal attenuation characteristics of a communications channel between said first and second signalling devices through which said output signal and said reflected signal pass; and means for dynamically varying a modulation depth of the small signal modulation applied to said generated signal in dependence upon the

<Desc/Clms Page number 40>

received signal indicative of the current signal attenuation characteristics of said communications channel.
24. A drive circuit for applying a drive current to a signal generator, the drive circuit comprising: a current source for generating a drive current; a first solid state switch having first and second main electrodes and a control electrode, the first main electrode being connected, in use, to said current source and the second main electrode being connected, in use, to a reference potential; a second solid state switch having first and second main electrodes and a control electrode, the first main electrode being connected, in use, to said current source and the second main electrode being connected, in use, to said signal generator; means for receiving a first data signal and a second data signal, the second data signal being the inverse of the first data signal; means for applying said first data signal to the control electrode of said first solid state switch to open and close said first solid state switch in dependence upon the first data signal; and means for applying said second data signal to the control electrode of said second solid state switch to open and close said second solid state switch in dependence upon said second data signal; whereby current from said current source passes, in use, either through said first solid state switch to said

<Desc/Clms Page number 41>

reference potential or through said second solid state switch to said signal generator in dependence upon said first and second data signals; wherein the drive circuit further comprises a capacitor connected between the control electrode of said first solid state switch and the second main electrode of said second solid state switch.
25. A drive circuit according to claim 24, wherein the capacitance of said capacitor is set in dependence upon an inherent capacitance between the control electrode and the second main electrode of said second solid state switch.
26. A drive circuit according to claim 25, wherein the capacitance of said capacitor is set to be approximately equal to the inherent capacitance between said control electrode and said second main electrode of said second solid state switch.
27. A drive circuit according to any of claims 24 to 26, wherein said current source is a first current source, further comprising a second current source connected to said second main electrode of said second solid state switch and operable to apply, in use, a bias current to said signal generator, whereby the current applied in use to said signal generator corresponds to i) the bias current provided by said second current source summed with current from the first current source when said second solid state switch is closed; and ii) the bias

<Desc/Clms Page number 42>

current provided by said second current source when said second solid state switch is open.
28. A signalling system comprising first and second signalling devices, the first signalling device comprising means for receiving a signal output from the second signalling device; means for modulating a portion of the received signal with modulation data for the second signalling device and for reflecting the portion of the received signal back to said second signalling device; and the second signalling device comprising means for generating a signal; means for outputting the generated signal towards said first signalling device; means for receiving the reflected signal from said first signalling device carrying said modulation data; and means for retrieving the modulation data from said reflected signal; characterised in that said second signalling device further comprises means for processing the reflected signal received at said second signalling device to derive a measure of the dynamic range of variation of the reflected signal; means for comparing the derived measure with a predetermined threshold value to provide a comparison result; and means for dynamically varying the signal level of said generated signal in dependence upon the comparison result.
29. A signalling system according to claim 28, wherein said processing means of said second signalling device

<Desc/Clms Page number 43>

is operable to process said signal to determine a measure of the signal strength of the reflected signal received at the second signalling device.
30. A system according to claim 28 or 29, wherein the reflected signal received at the second signalling device comprises a DC component and an AC component and wherein said processing means comprises a filter for filtering out either the DC component or the AC component and is operable to process the filtered signal to derive said measure.
31. A system according to claim 30, wherein said filter is operable to filter out said DC component and wherein said processing means is operable to derive said measure by calculating an average power level of the AC component.
32. An apparatus according to any of claims 28 to 31, further comprising means for adjusting the predetermined threshold value.
33. An apparatus according to any of claims 28 to 32, wherein said varying means of said second signalling device is operable to control the transmitted signal level to be at a minimum necessary for said retrieving means to be able to retrieve the modulation data from said reflected signal.
34. A signalling device comprising:

<Desc/Clms Page number 44>

means for generating a signal; means for varying the intensity of the generated signal between a first nonzero intensity level and a second non-zero intensity level in dependence upon uplink data to be transmitted to a remote signalling device; means for outputting the intensity modulated signal towards said remote signalling device; means for receiving a reflected signal back from said remote signalling device carrying downlink data transmitted from the remote signalling device; and means for retrieving the downlink data from said reflected signal; characterised in that the signalling device further comprises means for receiving a signal indicative of current signal attenuation characteristics of a signalling channel between said signalling device and said remote signalling device through which said output signal and said reflected signal pass; and means for dynamically varying the difference in intensity between said first and second intensity levels in dependence upon the received signal indicative of the current signal attenuation characteristics of said signalling channel.
35. A signalling device comprising: means for generating a signal; means for outputting the generated signal towards a remote signalling device; means for receiving a reflected signal from said first signalling device carrying modulation data; and means for retrieving the modulation data from said

<Desc/Clms Page number 45>

reflected signal; characterised in that said signalling device further comprises means for processing the reflected signal to derive a measure of the dynamic range of variation of the reflected signal; means for comparing the derived measure with a predetermined threshold value to provide a comparison result; and means for dynamically varying the signal level of said generated signal in dependence upon the comparison result.
36. A signalling device comprising the technical second signalling device features of any of claims 1 to 33.
37. A signalling kit comprising one or more signalling devices according to any of claims 34 to 36.
38. An optical signalling method characterised by the use of a signalling system according to any of claims 1 to 33 or a signalling device according to any of claims 34 to 36.