WO2007077451A1 - Communications device - Google Patents
Communications device Download PDFInfo
- Publication number
- WO2007077451A1 WO2007077451A1 PCT/GB2007/000020 GB2007000020W WO2007077451A1 WO 2007077451 A1 WO2007077451 A1 WO 2007077451A1 GB 2007000020 W GB2007000020 W GB 2007000020W WO 2007077451 A1 WO2007077451 A1 WO 2007077451A1
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- WO
- WIPO (PCT)
- Prior art keywords
- optical
- fibre
- electrical
- host
- analogue
- Prior art date
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
- H04B10/25752—Optical arrangements for wireless networks
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
Definitions
- the present invention relates to a communication device for use in or with an optical/electrical transmission system, to a transmission system, to a locking device and to a recognition system.
- Such digital pluggable optical transceivers are typically used at both ends of an optical fibre and cooperate with host devices, for example at each end of the optical fibre.
- the host device may be a hub device, for example for routing purposes, or it may be a remote device, for example at an antenna unit of a distributed antenna system.
- the optical transceiver module is provided with an external housing and has at one end one part of a two-part electrical connector and at the opposite end, a receptacle for a fibre counter.
- the electrical connector has a counterpart in the host device.
- the two-part electrical connector is arranged so that all of the electrical connections necessary for power, management, communication and any other control is provided by the electrical connector so that no other electrical connections are necessary.
- the receptacle at the other end of the module typically allows an optical fibre or optical fibres to be connected in pluggable fashion to the module.
- the host device may be powered up, and, if appropriate, any software booted up.
- a transceiver is then plugged in to one of the designated locations on the host device.
- the host device typically includes logic circuitry responsive to plugging in a correctly-configured transceiver module that enables communication between the host device and the transceiver module to take place.
- An optical fibre or optical fibres may then be connected to the receptacle of the module.
- a communications device having an electrical input node configured to receive an analogue electrical input from a host device, a lasing device having an optical output configured for coupling laser light from the lasing device to an optical fibre and circuitry for applying current to the lasing device so that the laser light corresponds to the electrical input, the device being configured to pluggably connect to the host device.
- the input node in an embodiment is configured to receive an analogue radio- frequency electrical input. There is a releasable optical connection for plugged coupling to the optical fibre.
- the host device is a hub for a distributed antenna system (DAS); in another it is an antenna unit of a DAS.
- DAS distributed antenna system
- a communications system comprising a device of the first aspect, a host device, an optical fibre and an output device configured to receive light from the fibre and convert it to electromagnetic radiation corresponding to the analogue electrical input, and coupling means for coupling laser light between the lasing device and the optical fibre.
- the optical fibre may be a multimode fibre
- the coupling means may comprise a lens and be arranged to provide a restricted set of modes in the fibre.
- the lasing device may be arranged to provide a restricted set of modes in the fibre.
- the restricted mode set may comprise less power in low mode orders and high mode orders and relatively more power in intermediate orders.
- the lasing device may be arranged to provide a desired polarisation state of radiation in the fibre.
- the fibre may be a single mode fibre.
- a communications device having an electrical output node configured provide an analogue electrical input to a distribution hub, a photosensor having an optical input configured to receive laser light having analogue variations, circuitry for providing signals corresponding to the analogue variations in the laser light to the electrical output node, the device being configured to pluggably connect to the distribution hub.
- an optical communication device having means for connecting to a host device and means for connecting to at least one optical fibre, the optical communication device including at least one semiconductor device for producing optical radiation, electrical circuitry for powering the semiconductor device and means for modulating the optical radiation in accordance with electrical signals received from the host device, the modulating means being configured to provide one of a linear relationship between the optical radiation and the electrical signals and a defined non-linear relationship therebetweeen.
- the host device may be a hub for a distributed antenna system (DAS); in another it may be an antenna unit of a DAS.
- DAS distributed antenna system
- the modulating means may include a circuit affecting a bias current to the semiconductor device, whereby direct modulation is effected.
- indirect modulation may be effected.
- the semiconductor device may be operated in a lasing mode.
- the optical communication device may include an optical system coupling an output of the semiconductor device to a fibre so as to provide a desired launch condition.
- the semiconductor device may have one or more outputs configured to provide desired modes in the fibre.
- the population of modes in the fibre may be selected to overcome nulls in transmission in the fibre.
- the optical system may be selected to provide desired polarisation state of optical radiation in the fibre.
- the desired polarisation state may be selected to overcome nulls in transmission in the fibre.
- a method of operating a communications system in which analogue signals are received at an electrical host, and in response thereto, analogue signals are transferred over an optical fibre to a remote location, the method comprising plugging an electro-optical transducer module into a corresponding socket of the electrical host, providing a signal from the transducer module to the host whereby the host recognises that the transducer module, connecting an optical fibre to the transducer module and transferring said analogue signals over the optical fibre.
- One embodiment comprises an analogue electrical-optical transducer module for plug-in use with an electrical hub device and with an optical output for a multimode optical fibre.
- Another embodiment is an analogue optical-electrical transducer module for plug-in use with an electrical hub device and with an optical input for a multimode optical fibre.
- Other embodiments are for use with single mode fibre.
- analogue as used herein implies correspondence between electrical signals and optical signals in the fibre, with the electrical signals modulated on an rf carrier using typically a modulation format such as BPSK, QPSK or QAM. These formats are however only examples.
- Correspondence means that either amplitude or phase or both amplitude and phase of the applied electrical signal can be retrieved from the content of the received signal. It will be understood by those skilled in the art that the information content of digital signals can be obtained without such correspondence as it is largely the transition information that is of importance in digital systems, whereas for analogue systems the correspondence is required.
- optical fibres are specified by a modal bandwidth, e.g. 500 MHz.km.
- a modal bandwidth e.g. 500 MHz.km.
- the performance is such that for 1 km of fibre, a bandwidth of 500 MHz is guaranteed.
- the limitation is largely due to modal dispersion, which results in perceived smearing of a pulse due to the different group delays of different transmission modes in the fibre.
- Other launch conditions have been found to give rise to improved performance for digital signals, where transition information is sufficient for signal reconstitution.
- optical fibres especially pre-installed MMFs, can be used to carry analogue signals - for example 3 G mobile phone signals without frequency conversion, with acceptable signal performance even where fibre defects exist.
- At least some of the fibres are known to respond to restricted mode launch, as has been shown by EVM (error vector magnitude) measurements on different fibres with different restricted modes.
- the optical output device itself typically a laser diode
- the optical output device will be intensity modulated about a mean intensity.
- Intensity deviations will be related to the amplitude deviations of the incoming electrical signals.
- the relationship is linear but this is not essential to all systems. Indeed, it may be desirable to have a non-linear relationship, of known non-linearity.
- Some embodiments are y-axis pluggable and some are z-axis pluggable.
- Fig. 1 shows a cut away perspective view of an SFP module
- Fig. 2a shows a front cut-away perspective view of a distribution hub
- Fig. 2b shows a rear cut-away perspective view of a distribution hub
- Fig. 3 a shows a top perspective view of an SFP module showing a known latch mechanism
- Fig. 3b shows a bottom perspective view of an SFP module showing details of an embodiment of a latch mechanism
- Fig. 3 c shows a top cut-away perspective view of an SFP module showing the latch mechanism of Fig. 3b;
- Fig. 3c shows a detailed view of the latch mechanism of Figs 3b and 3c;
- Fig. 4 shows isolation between transmitter and receiver as a function of frequency
- Fig. 5 shows a block schematic diagram of a transceiver module
- Fig. 6 shows a general schematic diagram of a diagnostic module
- Fig. 7 shows a cut away perspective view showing an example of a laser coupling system.
- the hub/router may be supplied with less than all the slots populated. This means allows low entry point costs.
- Pluggability may offer an upgrade path without the need to replace the system, simply replacing the pluggable component with suitable alternatives in the optical or electrical domain.
- the supplier of the hub can make software capable of recognising and responding to only allowable optical module manufactures to prevent inferior product use.
- DAS distributed antenna system
- the pluggable component comprises the optical sub assemblies, circuit board and housing.
- the housing provides a latching mechanism for an optical connector for connecting an optical fibre or cable;
- the circuit board provides an interface to an electrical connector for connecting to a host device.
- a number of standards exist for pluggable components, and the invention may be applied to any of these as required. Amongst those currently envisaged are: a) SC duplex 1x9 or 2x9 configurations. b) Small form factor pluggable + (SFP) c) Gigabit interface converter d) XFP e) X2 f) Xenpak g) Small-form-factor pin-through-hole (SFF).
- an example of a small form factor pluggable device 1 uses an LC-type optical connector 2 and a 20 pin electrical connector 3 extending on an electrical circuit board 4.
- the circuit board 4 provides the mounting for electrical components (on the underside of the board and thus not shown) and contacts 5.
- the contacts link to two optical sub-assemblies 6a, 6b.
- the device 1 has an elongate housing 7 having a generally rectangular cross-section.
- the electrical connector 3 is supported by the housing 7 within it recessed from a proximal end wall 7a, so that the circuit board 4 extends away from the proximal end wall 7a towards a distal end 7b.
- the optical connector 2 extends inwardly from the distal portion 7a, and the optical sub- assemblies 6a, 6b are disposed in parallel side-by-side between the circuit board 4 and the optical connector 2.
- the circuit board 4 carries components (described later herein) to give the required transfer function.
- the optical sub-assemblies 6a, 6b may be conventional.
- One optical sub-assembly 6a is a transmitter device.
- it houses a DFB but various other optical sources such LED, laser (FP and VCSEL), EML may be used where appropriate in other embodiments.
- the wavelength may be arbitrarily selected but in some embodiments it is centred at one of 850nm, 1310nm and 1550nm.
- the optical operation in the present embodiment is single wavelength, but in other embodiments coarse WDM or dense WDM may be used.
- the transmitter sub assembly 6a contains other components for example, to impedance match the RF impedance of the assembly, or to provide gain.
- the other optical sub-assembly 6b is a receiver optical sub-assembly.
- the optical sub-assembly 6b has a PIN diode and preamplifier combination.
- the pre-amplifier is highly linear for an analogue application and therefore differs to that used in the digital domain.
- the transmitter optical sub-assembly 6a and receiver optical sub-assembly 6b may be supplied as components in their own right to service the analogue market. These can be fibre pigtailed or have receptacles to accept a suitable optical connector type. Different embodiments have different connector receptacles.
- a hub 10 is a generally flat rectangular box 11 having a front sidewall 21 and a rear sidewall 12.
- the front sidewall 21 has eight sockets 22 into which may be plugged respective devices of the type described with respect to Fig. 1 so as to output signals to an optical fibre cable.
- into one or more sockets may be plugged an electrical connector module (not shown) so that the respective socket 22 provides an output to a conductor for distribution.
- the rear sidewall 12 has four RF inputs 13, four RF outputs 14 and a power input 15 with associated power switch 15a.
- the RF inputs 13 from the rear of the hub module are switched to one or more of the front sockets 22.
- the RF input is conditioned by passing through gain/ attenuation stages.
- Electronic broadband switches are used to divert the RF input to the chosen output or outputs.
- the RF inputs may be mixed to provide multiple RF signals to one or more front sockets.
- the switches, in the hub module are controlled by an on-board microprocessor.
- the nature of the radiation producing element dictates the medium to be used and ultimately the distance over which it is possible to transmit data.
- the standardised nature of the connector also allows provision of a non-optical diagnostic module allowing measuring of system response and simulation of system signals, useful for diagnosing faults or in system installation. Such a module allows investigation of system operation and evaluation of parameters not readily available by the user.
- the module has an electrical connector in place of the optical connector.
- the electrical connector is a USB port.
- a "module present" indication is provided by grounding pin 6 in the transceiver module.
- the host device has a pull-up connected to its pin 6, so that provided there is no module connected, the host pin 6 is at a logic high level.
- the host pin 6 is grounded and a logical low level is present. Responding to this enables the host to recognise that a transceiver is connected.
- Other module definition pins are provided.
- pin 6 is connected to Vcc within the optical module.
- the host can interrogate this pin and determine that an analogue module is present.
- pin 6 is connected to earth (NOTTTL) in a digital optical module, and the corresponding host pin has a pull-up to Vcc, so that upon insertion of a digital module, a logic change is noted to indicate the presence of a digital device. If such a digital device were connected to the host of the present system, no change of state would occur and hence the host would not power up the (incorrectly used) digital module.
- Fig. 3 a shows a known de-latching mechanism for securing an optical connector of a small form factor pluggable device in place in its socket.
- the latching mechanism is used in conjunction with the reference cage for the pluggable component. When inserted into such a cage, the latch engages in an aperture of a tang the cage, thus making removal impossible.
- the de-latch mechanism provides the means for disengagement. When the wire handle is pulled the spring element is compressed. The legacy component has a different spring mechanism. The latching tang in the reference cage is lifted by the presence of the raised components. The module can therefore be removed. Upon release of the wire handle the spring mechanism returns the de-latch to the neutral position.
- the latch device 40 has a longitudinal axis IH-III'. Transverse to the axis III-III ' is disposed a generally rectangular plastics block portion 41 with opposed endwalls 41a, 41b defining pivot recesses 42a, 42b for opposing ends of a steel wire portion 45 formed into generally square loop.
- the endwalls 41a, 41b are connected by first- fourth relatively larger sidewalls 44a,44b,44c, 44d.
- From the third sidewall 44c two legs 46a, 46b extend parallel to the axis III-III', so as to define a space 47 therebetween.
- the length of the legs 46a, 46b is about twice the width of the block portion 41.
- the legs extend into distal feet portions 48a, 48b. About one quarter of the way from the feet portions 48a, 48b to the block portion 41, a transverse connecting portion 49 connects the two legs 46a, 46b together.
- the connecting portion 49 carries a sprung tongue 50 that extends into the space 47, and that is curved upwardly out of a plane defined by the legs 46a, 46b.
- the tongue 50 acts as a spring mechanism to ensure the latch is always engaged.
- transceiver device 100 consists of two portions, an optical transmitter portion 100a and an optical receiver portion 100b. Each portion is provided with its own power supply VccT, VeeT for the transmitter portion 100a and VccR, VeeR for the receiver portion 100b. In the present embodiment, these are not connected together within the transceiver device 100, so as to reduce crosstalk between transmitter portion 100a and receiver portion 100b. In other embodiments, connection of the power supplies to one another within the transmitter device is made.
- the transceiver 100 also has a digital processing section 190, described later herein.
- the optical transmitter portion 100a has a laser 120.
- the laser 120 is a DFB (Distributed Feedback) laser.
- the laser is a directly modulated FP (Fabre Perot) laser, VCSEL (Vertical Cavity Surface Emitting Laser), LED (light emitting diode or EML (externally modulated Laser).
- the laser 120 is connected to an optical fibre (not shown) for transferring the resultant optical radiation to a remote location.
- Interface between the laser 120 and the fibre may be effected in a number of ways.
- the aim is to achieve a restricted mode set in the fibre.
- one subset of embodiments uses a standard laser diode, or the like, and uses optical components configured and arranged to provide the desired modal launch.
- the launch is offset from the fibre centre, so as to provide reduced low order modes and reduced high order modes.
- the launch is a centre launch to give rise to mainly low order modes.
- Fig. 7 shows an example of a launch.
- a set of modules may be provided, each having a respective specified launch characteristic destined for a specific fibre type. Alternatively a range of modules may be tried in a particular application until the best, or an acceptable, response is achieved.
- Modules may be provided for single mode fibre transmission.
- the laser 120 is operated by a laser bias circuit 121, including a two-input first amplifier 110 having an inverting input 101, a non-inverting input 102 and an output 103, an npn bipolar transistor 140 having its emitter coupled to a the transmitter lower power supply voltage Veet via a current source resistor 150, and a monitor photodiode 130.
- the collector of the transistor 140 is connected directly to the cathode of the laser 120.
- the monitor photodiode 130 has its anode connected to VccT and its cathode connected to the inverting input 101.
- a signal source (here connected via a host in the form of a distribution hub - see Figs 2a, 2b) inputs differential mode signals TD+ TD- on an rf carrier to input nodes 161, 162 of a conditioning circuit 160.
- the conditioning circuit 160 processes the rf signals to cause them to be of suitable amplitude and frequency to modulate the laser, and has an output 163 connected directly to the cathode of the laser diode 120 via an inductance 113.
- the inductance 113 serves to reduce noise.
- the conditioning circuit 160 also converts the data from differential mode to single-ended at the output 163.
- the Rf signal is superimposed onto the dc optical signal in such a way as to maintain linearity of the signal to be transmitted. This is achieved by maintaining the correct depth of modulation current as controlled by the conditioning circuit 160.
- the conditioning circuit may receive a single-ended rf signal and itself be single-ended.
- the monitor photodiode 130 captures light from the rear facet 129 of the laser, and is connected to the inverting input 101 of the bias amplifier 110. In conjunction with the first amplifier 110 and current source 150 the monitoring photodiode 130 forms a feedback loop to maintain a constant current through the monitor photodiode 130. If the power output from the laser 120 drops (for example due to a rise in temperature) the current of the monitor photodiode 130 falls in like fashion. The inverting input 101 to the first amplifier 110 falls causing the output 103 to rise. This in turn provides additional base current to the transistor 140 increasing current to the laser 120.
- a transmit disable input 155 is hard-wired input to the conditioning circuit 160 and the first amplifier 110.
- the output 103 of the amplifier 110 is pulled down to VeeT to disable the laser 120, and the conditioning circuit is tristated to isolate it.
- the receiver portion 100b has a receiver photodiode 170 having sufficient bandwidth to detect the RF signal in question from the return fibre (not shown).
- the receiver photodiode 170 has its cathode connected to VccR and its anode connected to VeeR via a bias resistor 172, and to the input 173 of a second amplifier 175, in this embodiment.
- the second amplifier 175 includes an automatic gain control loop.
- the output from the receiver photodiode 170 may be sufficiently large to not require amplification, and sufficiently constant not to require AGC.
- any necessary amplification may be formed downstream, for example in the host (distribution hub).
- the second amplifier 175 has an output 177 that is connected as input 181 to a single-ended to differential mode converter 180. This in turn has two output 183,184 providing the analogue rf outputs.
- This device is a linear amplifier with gain, highly linear to prevent signal distortion and low signal to noise ratio. It also includes logic circuitry for providing an output indicating loss of signal (LOS) when no light is received or the level is too low for detection. This LOS signal is used by the host as a warning for instance that the fibre is broken.
- LOS loss of signal
- the power supply, Vcct-Veet; Vccr-VeeR can be as low as 3 V. However to achieve a good bandwidth performance it is advisable to use a minimum of 5 V for the receiver photodiode 170 to ensure full depletion of the active area and therefore optimal bandwidth required for a 6GHz RoF system.
- Power supply filtering external to the module is provided to ensure safe operation of the module.
- the digital processing section 190 forms a system management block for handling a digital interface and itself contains memory.
- Digital optical monitoring is a means of the device holding information about itself and performance. It holds both static and dynamic data.
- the static data can be date code, part number, manufacturer, serial number.
- the dynamic data may be for example laser conditions, temperature etc.
- Data relating to the optical devices is read and loaded into specific memory location and then accessed externally via a 2-wire interface (I 2 C). This memory can also be use to hold information relating to manufacturer, parts numbers etc as documented by the module multi-sourced agreement.
- the device 100 that has been described includes both a transmit portion and a receive portion. It is to be housed in a single physical housing. However for improved immunity from cross-talk it may be advantageous to provide a transmit device and a separate receive device, each to be housed and powered separately.
- the isolation characteristic is shown in Fig. 4.
- Module 200 is a service module that allows connection to the same electrical socket but allows interrogation and reporting of parameters from the system.
- module 200 has an SFP electrical connector 201 connected via a bus 202 to a microprocessor 210.
- the bus 202 connects to the microprocessor 210 via A-to- D and D-to A converters 205.
- the microprocessor 210 forms an interface adaptor and is configured to allow connection to a remote monitoring system or computer (not shown) via USB, Ethernet or RS232 interface.
- the converters 205 allow parameters and signals present at the connector 201 to be measured and reported to the monitoring system.
- the digital to analogue converter allows voltages to be applied to the electrical connector 201.
- the system I C bus is present at the electrical connector 201, communication is possible to a Hub or antenna unit microprocessor. This may be of particular interest in the case of an antenna unit which is malfunctioning.
- the laser 120 is fitted to a pedestal 210 and launches light into a ball lens 220. Light is thereby focussed at the end of a fibre stub 230.
- the stub has a single mode core contained in a ceramic ferrule. It serves to minimise optical reflections when mating with the user patch chord due to the precision physical connection.
- the split sleeve serves as a precision alignment aid.
- the position of the laser 120 or ball lens 220 may be manipulated such as to focus the light on the end of the fibre stub to stimulate centre or offset launch within the transmitter assembly.
- the invention is not limited to any particular launch device or assembly.
- the fibre measurements show that with a standard launch several nulls appear that are potentially at the frequency of interest.
- the use of a pluggable module enables the use of different laser launch conditions to suite a particular application. This may encompass centre launch, offset launch, vortex launch or other such solutions.
- a range of modules be available off the shelf, and that one of these may be selected for use based upon known fibre characteristics.
- the installer may select by trial and error between a subset of available modules.
- the host device is powered up and, if appropriate, the software is allowed to boot up to an operating condition. Then a module embodying the invention is hot-plugged into the host device.
- the host device recognises the fact that the module is a module configured according to the invention and not a digital module and communicates with the internal digital controller. After connection of the necessary optical fibre or optical fibres, the system is ready for testing.
- the module is known to be good for the particular fibre; however, in other embodiments the effect of connecting the module to the fibre is then checked with a view to determining whether the transmission is being properly effected. If it is not being properly effected then it is possible to substitute a module having different launch conditions without powering down the host device.
- the fibre bandwidth profile is critically dependent upon the laser bias current when modulated with an rf carrier. Using the I 2 C interface on the transceiver it is possible remotely to send commands to increase or decrease the laser drive conditions and therefore modify the bandwidth profile.
- the optical signals are at the prevailing rf data rate and are broad band. All frequencies within the pass band may be transmitted and received. This approach puts significant design constraints on the optical components due to the requirement for wide bandwidth and gain flatness across a wide band.
- alternative embodiments -
- Frequency conversion is used.
- the system modifies the carrier frequency to a lower frequency for transmission.
- At the receiver end it is then returned to the original carrier frequency for radiation via the antenna.
- This benefits the optical component in that the bandwidth requirement can be made to be relatively low.
- a penalty is increased complexity in both up and down conversion of the carrier frequency.
- the RoF system is channelised then the optical components only have to deal with specific defined frequencies rather than broadband.
- the RoF system is designed to transport only the service provider frequencies of interest. The system is no longer agnostic of the data it transports but the system and optical components can be optimised at specific frequencies. This makes it simpler to provide signal balance and gain flatness.
- specific optical modules are designed for specific channels of frequencies. This reduces complexity and ultimately cost of the optical components. To make the most of this performance advantage, the RoF system is channelised to specific frequencies.
- the embodiments described have Z pluggability — i.e. the module is introduced perpendicular the plane of a relevant panel of the host device.
- Other connections are also possible, and specifically include Y pluggability, where the module is connected by movement parallel to the plane.
- Non pluggable modules may be appropriate: features of embodiments described here in the context of pluggable modules may be applied to non- pluggable modules. In some systems a non-modular approach may be used, in which case some or all of the components may be incorporated along with features of the host device described above into a single device. Although direct modulation is described, other modulation techniques, for example including external modulators, may be used.
- mode selection may also occur at the receive end.
- polarisation state at the launch end may be selected by providing a polarisation-selective coupling whose angular orientation can be varied - e.g. among a fixed number of possible orientations - to set the input polarisation.
- Embodiments of the invention may carry signals of many different frequency types, for example 2G; 3G; DECT; HSDPA; WiFi; WiMAX; TETRA; PMR; DVB-H; and DMB.
Abstract
A communications device has an electrical input for an analogue radio-frequency electrical input from a host device, a lasing device with an optical output for coupling to an optical fibre and circuitry for applying current to the lasing device so that the laser light corresponds to the analogue electrical input. A releasable electrical connection allows plugged coupling to the host device and a releasable optical connection allows plugged coupling to the optical fibre. A similar optical-to-electrical device is also disclosed, so as to enable a distributed antenna system with pluggable elements to be formed.
Description
COMMUNICATIONS DEVICE
The present invention relates to a communication device for use in or with an optical/electrical transmission system, to a transmission system, to a locking device and to a recognition system.
The art is replete with digital pluggable optical transceivers for cooperation with optical fibres. These operate at baseband, typically using a laser source that is switched between on and off states.
Such digital pluggable optical transceivers are typically used at both ends of an optical fibre and cooperate with host devices, for example at each end of the optical fibre. The host device may be a hub device, for example for routing purposes, or it may be a remote device, for example at an antenna unit of a distributed antenna system.
Typically the optical transceiver module is provided with an external housing and has at one end one part of a two-part electrical connector and at the opposite end, a receptacle for a fibre counter. The electrical connector has a counterpart in the host device. The two-part electrical connector is arranged so that all of the electrical connections necessary for power, management, communication and any other control is provided by the electrical connector so that no other electrical connections are necessary. There may of course be mechanical means provided to secure the transceiver in place on the host. The receptacle at the other end of the module typically allows an optical fibre or optical fibres to be connected in pluggable fashion to the module.
In use, the host device may be powered up, and, if appropriate, any software booted up. A transceiver is then plugged in to one of the designated locations
on the host device. The host device typically includes logic circuitry responsive to plugging in a correctly-configured transceiver module that enables communication between the host device and the transceiver module to take place. An optical fibre or optical fibres may then be connected to the receptacle of the module.
In one aspect there is provided a communications device having an electrical input node configured to receive an analogue electrical input from a host device, a lasing device having an optical output configured for coupling laser light from the lasing device to an optical fibre and circuitry for applying current to the lasing device so that the laser light corresponds to the electrical input, the device being configured to pluggably connect to the host device. The input node in an embodiment is configured to receive an analogue radio- frequency electrical input. There is a releasable optical connection for plugged coupling to the optical fibre.
In one embodiment the host device is a hub for a distributed antenna system (DAS); in another it is an antenna unit of a DAS.
In another aspect there is provided a communications system comprising a device of the first aspect, a host device, an optical fibre and an output device configured to receive light from the fibre and convert it to electromagnetic radiation corresponding to the analogue electrical input, and coupling means for coupling laser light between the lasing device and the optical fibre.
The optical fibre may be a multimode fibre, the coupling means may comprise a lens and be arranged to provide a restricted set of modes in the fibre.
Launching means other than a lens may be used.
The lasing device may be arranged to provide a restricted set of modes in the fibre.
The restricted mode set may comprise less power in low mode orders and high mode orders and relatively more power in intermediate orders.
The lasing device may be arranged to provide a desired polarisation state of radiation in the fibre.
The fibre may be a single mode fibre.
In a further aspect there is provided a communications device having an electrical output node configured provide an analogue electrical input to a distribution hub, a photosensor having an optical input configured to receive laser light having analogue variations, circuitry for providing signals corresponding to the analogue variations in the laser light to the electrical output node, the device being configured to pluggably connect to the distribution hub.
In another aspect there is provided an optical communication device having means for connecting to a host device and means for connecting to at least one optical fibre, the optical communication device including at least one semiconductor device for producing optical radiation, electrical circuitry for powering the semiconductor device and means for modulating the optical radiation in accordance with electrical signals received from the host device, the modulating means being configured to provide one of a linear relationship between the optical radiation and the electrical signals and a defined non-linear relationship therebetweeen.
In one embodiment, the host device may be a hub for a distributed antenna system (DAS); in another it may be an antenna unit of a DAS.
The modulating means may include a circuit affecting a bias current to the semiconductor device, whereby direct modulation is effected.
Alternatively, indirect modulation may be effected.
The semiconductor device may be operated in a lasing mode.
The optical communication device may include an optical system coupling an output of the semiconductor device to a fibre so as to provide a desired launch condition.
The semiconductor device may have one or more outputs configured to provide desired modes in the fibre.
The population of modes in the fibre may be selected to overcome nulls in transmission in the fibre.
The optical system may be selected to provide desired polarisation state of optical radiation in the fibre.
The desired polarisation state may be selected to overcome nulls in transmission in the fibre.
In a yet further aspect there is provided a method of operating a communications system in which analogue signals are received at an electrical
host, and in response thereto, analogue signals are transferred over an optical fibre to a remote location, the method comprising plugging an electro-optical transducer module into a corresponding socket of the electrical host, providing a signal from the transducer module to the host whereby the host recognises that the transducer module, connecting an optical fibre to the transducer module and transferring said analogue signals over the optical fibre.
One embodiment comprises an analogue electrical-optical transducer module for plug-in use with an electrical hub device and with an optical output for a multimode optical fibre. Another embodiment is an analogue optical-electrical transducer module for plug-in use with an electrical hub device and with an optical input for a multimode optical fibre. Other embodiments are for use with single mode fibre.
The term "analogue" as used herein implies correspondence between electrical signals and optical signals in the fibre, with the electrical signals modulated on an rf carrier using typically a modulation format such as BPSK, QPSK or QAM. These formats are however only examples. "Correspondence" means that either amplitude or phase or both amplitude and phase of the applied electrical signal can be retrieved from the content of the received signal. It will be understood by those skilled in the art that the information content of digital signals can be obtained without such correspondence as it is largely the transition information that is of importance in digital systems, whereas for analogue systems the correspondence is required.
As is known to those skilled in the art, optical fibres are specified by a modal bandwidth, e.g. 500 MHz.km. Typically for MMF this is the guaranteed performance for an over- filled launch condition, and is measured under pulse conditions. In this example the performance is such that for 1 km of fibre, a
bandwidth of 500 MHz is guaranteed. However, for 2 km, only 250 Mhz is guaranteed. The limitation is largely due to modal dispersion, which results in perceived smearing of a pulse due to the different group delays of different transmission modes in the fibre. Other launch conditions have been found to give rise to improved performance for digital signals, where transition information is sufficient for signal reconstitution. The present applicants have identified that optical fibres, especially pre-installed MMFs, can be used to carry analogue signals - for example 3 G mobile phone signals without frequency conversion, with acceptable signal performance even where fibre defects exist. At least some of the fibres are known to respond to restricted mode launch, as has been shown by EVM (error vector magnitude) measurements on different fibres with different restricted modes.
Typically in an analogue optical communications system, the optical output device, itself typically a laser diode, will be intensity modulated about a mean intensity. Intensity deviations will be related to the amplitude deviations of the incoming electrical signals. In some systems, the relationship is linear but this is not essential to all systems. Indeed, it may be desirable to have a non-linear relationship, of known non-linearity.
Some embodiments are y-axis pluggable and some are z-axis pluggable.
Others have fibre connectors and yet others are fibre pigtailed. It is also possible to adopt a similar analogue optical module but soldered into electrical sub-assemblies rather than pluggable.
It is also envisaged to provide pluggable diagnostic modules having a connector interface such as a USB interface.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 shows a cut away perspective view of an SFP module;
Fig. 2a shows a front cut-away perspective view of a distribution hub; Fig. 2b shows a rear cut-away perspective view of a distribution hub;
Fig. 3 a shows a top perspective view of an SFP module showing a known latch mechanism;
Fig. 3b shows a bottom perspective view of an SFP module showing details of an embodiment of a latch mechanism; Fig. 3 c shows a top cut-away perspective view of an SFP module showing the latch mechanism of Fig. 3b;
Fig. 3c shows a detailed view of the latch mechanism of Figs 3b and 3c;
Fig. 4 shows isolation between transmitter and receiver as a function of frequency; Fig. 5 shows a block schematic diagram of a transceiver module;
Fig. 6 shows a general schematic diagram of a diagnostic module; and
Fig. 7 shows a cut away perspective view showing an example of a laser coupling system.
Today the digital world uses pluggable electro-optical components to populate host systems having switching hubs or routers. If any pluggable component should fail the user may replace the component without the need for extensive servicing, and in some cases without opening the box.
1. The hub/router may be supplied with less than all the slots populated. This means allows low entry point costs.
2. Pluggability may offer an upgrade path without the need to replace the system, simply replacing the pluggable component with suitable alternatives in the optical or electrical domain.
3. The supplier of the hub can make software capable of recognising and responding to only allowable optical module manufactures to prevent inferior product use.
The advantages apply to an analogue system. In addition the deployment of a fibre or copper distributed antenna system (DAS) can be created or expanded as required using the concept of a pluggable analogue electro-optical, or electrical, modules.
The pluggable component comprises the optical sub assemblies, circuit board and housing. The housing provides a latching mechanism for an optical connector for connecting an optical fibre or cable; the circuit board provides an interface to an electrical connector for connecting to a host device. A number of standards exist for pluggable components, and the invention may be applied to any of these as required. Amongst those currently envisaged are: a) SC duplex 1x9 or 2x9 configurations. b) Small form factor pluggable + (SFP) c) Gigabit interface converter d) XFP e) X2 f) Xenpak g) Small-form-factor pin-through-hole (SFF).
These standards employ various shapes and sizes of enclosure to cope with power dissipation. They also envisage different fibre connector types such as MT, MT-RJ, LC, SC in both duplex and simplex configurations. They may be adapted to suite pigtailed optical assemblies as required.
Referring Io Fig. 1, an example of a small form factor pluggable device 1 uses an LC-type optical connector 2 and a 20 pin electrical connector 3 extending on an electrical circuit board 4. The circuit board 4 provides the mounting for electrical components (on the underside of the board and thus not shown) and contacts 5. The contacts link to two optical sub-assemblies 6a, 6b.
As seen the device 1 has an elongate housing 7 having a generally rectangular cross-section. The electrical connector 3 is supported by the housing 7 within it recessed from a proximal end wall 7a, so that the circuit board 4 extends away from the proximal end wall 7a towards a distal end 7b. The optical connector 2 extends inwardly from the distal portion 7a, and the optical sub- assemblies 6a, 6b are disposed in parallel side-by-side between the circuit board 4 and the optical connector 2.
The circuit board 4 carries components (described later herein) to give the required transfer function. The optical sub-assemblies 6a, 6b may be conventional.
One optical sub-assembly 6a is a transmitter device. In the described embodiment it houses a DFB but various other optical sources such LED, laser (FP and VCSEL), EML may be used where appropriate in other embodiments. The wavelength may be arbitrarily selected but in some embodiments it is centred at one of 850nm, 1310nm and 1550nm.
The optical operation in the present embodiment is single wavelength, but in other embodiments coarse WDM or dense WDM may be used.
In the present embodiment this completes the transmitter sub-assembly but in other embodiments the transmitter sub assembly 6a contains other components
for example, to impedance match the RF impedance of the assembly, or to provide gain.
The other optical sub-assembly 6b is a receiver optical sub-assembly. In the present embodiment, the optical sub-assembly 6b has a PIN diode and preamplifier combination. The pre-amplifier is highly linear for an analogue application and therefore differs to that used in the digital domain.
The transmitter optical sub-assembly 6a and receiver optical sub-assembly 6b may be supplied as components in their own right to service the analogue market. These can be fibre pigtailed or have receptacles to accept a suitable optical connector type. Different embodiments have different connector receptacles.
Referring to Fig. 2a, a hub 10 is a generally flat rectangular box 11 having a front sidewall 21 and a rear sidewall 12. The front sidewall 21 has eight sockets 22 into which may be plugged respective devices of the type described with respect to Fig. 1 so as to output signals to an optical fibre cable. Alternatively, into one or more sockets may be plugged an electrical connector module (not shown) so that the respective socket 22 provides an output to a conductor for distribution.
Referring to Fig. 2b, the rear sidewall 12 has four RF inputs 13, four RF outputs 14 and a power input 15 with associated power switch 15a.
The RF inputs 13 from the rear of the hub module are switched to one or more of the front sockets 22. The RF input is conditioned by passing through gain/ attenuation stages. Electronic broadband switches are used to divert the RF input to the chosen output or outputs. Also by similar means the RF inputs
may be mixed to provide multiple RF signals to one or more front sockets. The switches, in the hub module are controlled by an on-board microprocessor.
The nature of the radiation producing element dictates the medium to be used and ultimately the distance over which it is possible to transmit data.
The standardised nature of the connector also allows provision of a non-optical diagnostic module allowing measuring of system response and simulation of system signals, useful for diagnosing faults or in system installation. Such a module allows investigation of system operation and evaluation of parameters not readily available by the user. In an embodiment - see later herein - the module has an electrical connector in place of the optical connector. In an embodiment the electrical connector is a USB port.
In the art of signal distribution, some types of devices may be hot-plugged into host systems (i.e. plugged in for maintenance or configuration changes without powering down). To accomplish this it may be desirable to use certain connector pins to characterise the device. For example under the terms of the SFP Transceiver Multisource Agreement, a "module present" indication is provided by grounding pin 6 in the transceiver module. The host device has a pull-up connected to its pin 6, so that provided there is no module connected, the host pin 6 is at a logic high level. When a transceiver is connected, the host pin 6 is grounded and a logical low level is present. Responding to this enables the host to recognise that a transceiver is connected. Other module definition pins are provided.
It is proposed to connect pin 6 to Vcc within the optical module. When the module is connected to the host card, the host can interrogate this pin and determine that an analogue module is present. In one prior art device, pin 6 is
connected to earth (NOTTTL) in a digital optical module, and the corresponding host pin has a pull-up to Vcc, so that upon insertion of a digital module, a logic change is noted to indicate the presence of a digital device. If such a digital device were connected to the host of the present system, no change of state would occur and hence the host would not power up the (incorrectly used) digital module.
Fig. 3 a shows a known de-latching mechanism for securing an optical connector of a small form factor pluggable device in place in its socket. The latching mechanism is used in conjunction with the reference cage for the pluggable component. When inserted into such a cage, the latch engages in an aperture of a tang the cage, thus making removal impossible. The de-latch mechanism provides the means for disengagement. When the wire handle is pulled the spring element is compressed. The legacy component has a different spring mechanism. The latching tang in the reference cage is lifted by the presence of the raised components. The module can therefore be removed. Upon release of the wire handle the spring mechanism returns the de-latch to the neutral position.
Referring to Fig. 3 c an improved latch is shown. The latch device 40 has a longitudinal axis IH-III'. Transverse to the axis III-III ' is disposed a generally rectangular plastics block portion 41 with opposed endwalls 41a, 41b defining pivot recesses 42a, 42b for opposing ends of a steel wire portion 45 formed into generally square loop. The endwalls 41a, 41b are connected by first- fourth relatively larger sidewalls 44a,44b,44c, 44d. From the third sidewall 44c two legs 46a, 46b extend parallel to the axis III-III', so as to define a space 47 therebetween. The length of the legs 46a, 46b is about twice the width of the block portion 41. The legs extend into distal feet portions 48a, 48b. About one quarter of the way from the feet portions 48a, 48b to the block portion 41, a
transverse connecting portion 49 connects the two legs 46a, 46b together. The connecting portion 49 carries a sprung tongue 50 that extends into the space 47, and that is curved upwardly out of a plane defined by the legs 46a, 46b. The tongue 50 acts as a spring mechanism to ensure the latch is always engaged.
Referring now to Fig. 5., transceiver device 100 will now be described. It consists of two portions, an optical transmitter portion 100a and an optical receiver portion 100b. Each portion is provided with its own power supply VccT, VeeT for the transmitter portion 100a and VccR, VeeR for the receiver portion 100b. In the present embodiment, these are not connected together within the transceiver device 100, so as to reduce crosstalk between transmitter portion 100a and receiver portion 100b. In other embodiments, connection of the power supplies to one another within the transmitter device is made. The transceiver 100 also has a digital processing section 190, described later herein.
The optical transmitter portion 100a has a laser 120. In this embodiment, the laser 120 is a DFB (Distributed Feedback) laser. In other embodiments, the laser is a directly modulated FP (Fabre Perot) laser, VCSEL (Vertical Cavity Surface Emitting Laser), LED (light emitting diode or EML (externally modulated Laser). The laser 120 is connected to an optical fibre (not shown) for transferring the resultant optical radiation to a remote location.
Interface between the laser 120 and the fibre may be effected in a number of ways. In one family of embodiments, where the optical fibre is a multimode fibre, the aim is to achieve a restricted mode set in the fibre. To this end, one subset of embodiments uses a standard laser diode, or the like, and uses optical components configured and arranged to provide the desired modal launch. In one embodiment of this subset, the launch is offset from the fibre centre, so as to provide reduced low order modes and reduced high order modes. In another
embodiment the launch is a centre launch to give rise to mainly low order modes. In another subset, there is used a laser diode that has an output or set of outputs selected to provide a desired mode pattern. For example a reduced set of high and low order modes may be achieved by providing a laser diode having a radiation null at the centre of its emission facet and radiation outputs circularly disposed around that centre. Fig. 7 shows an example of a launch.
A set of modules may be provided, each having a respective specified launch characteristic destined for a specific fibre type. Alternatively a range of modules may be tried in a particular application until the best, or an acceptable, response is achieved.
Modules may be provided for single mode fibre transmission.
The laser 120 is operated by a laser bias circuit 121, including a two-input first amplifier 110 having an inverting input 101, a non-inverting input 102 and an output 103, an npn bipolar transistor 140 having its emitter coupled to a the transmitter lower power supply voltage Veet via a current source resistor 150, and a monitor photodiode 130. The collector of the transistor 140 is connected directly to the cathode of the laser 120. The monitor photodiode 130 has its anode connected to VccT and its cathode connected to the inverting input 101.
A signal source (here connected via a host in the form of a distribution hub - see Figs 2a, 2b) inputs differential mode signals TD+ TD- on an rf carrier to input nodes 161, 162 of a conditioning circuit 160. The conditioning circuit 160 processes the rf signals to cause them to be of suitable amplitude and frequency to modulate the laser, and has an output 163 connected directly to the cathode of the laser diode 120 via an inductance 113. The inductance 113 serves to reduce noise. The conditioning circuit 160 also converts the data
from differential mode to single-ended at the output 163. The Rf signal is superimposed onto the dc optical signal in such a way as to maintain linearity of the signal to be transmitted. This is achieved by maintaining the correct depth of modulation current as controlled by the conditioning circuit 160.
Although this embodiment uses single-ended RF, it is also possible for embodiments to use differential mode signals. Equally, the conditioning circuit may receive a single-ended rf signal and itself be single-ended.
The monitor photodiode 130 captures light from the rear facet 129 of the laser, and is connected to the inverting input 101 of the bias amplifier 110. In conjunction with the first amplifier 110 and current source 150 the monitoring photodiode 130 forms a feedback loop to maintain a constant current through the monitor photodiode 130. If the power output from the laser 120 drops (for example due to a rise in temperature) the current of the monitor photodiode 130 falls in like fashion. The inverting input 101 to the first amplifier 110 falls causing the output 103 to rise. This in turn provides additional base current to the transistor 140 increasing current to the laser 120.
A transmit disable input 155 is hard-wired input to the conditioning circuit 160 and the first amplifier 110. When a logical low level is applied at the disable input 155, the output 103 of the amplifier 110 is pulled down to VeeT to disable the laser 120, and the conditioning circuit is tristated to isolate it.
The receiver portion 100b has a receiver photodiode 170 having sufficient bandwidth to detect the RF signal in question from the return fibre (not shown). The receiver photodiode 170 has its cathode connected to VccR and its anode connected to VeeR via a bias resistor 172, and to the input 173 of a second amplifier 175, in this embodiment. In this embodiment the second amplifier
175 includes an automatic gain control loop. In other embodiments, the output from the receiver photodiode 170 may be sufficiently large to not require amplification, and sufficiently constant not to require AGC. In yet other embodiments any necessary amplification may be formed downstream, for example in the host (distribution hub).
The second amplifier 175 has an output 177 that is connected as input 181 to a single-ended to differential mode converter 180. This in turn has two output 183,184 providing the analogue rf outputs. This device is a linear amplifier with gain, highly linear to prevent signal distortion and low signal to noise ratio. It also includes logic circuitry for providing an output indicating loss of signal (LOS) when no light is received or the level is too low for detection. This LOS signal is used by the host as a warning for instance that the fibre is broken.
In some embodiments the power supply, Vcct-Veet; Vccr-VeeR, can be as low as 3 V. However to achieve a good bandwidth performance it is advisable to use a minimum of 5 V for the receiver photodiode 170 to ensure full depletion of the active area and therefore optimal bandwidth required for a 6GHz RoF system.
Power supply filtering external to the module is provided to ensure safe operation of the module.
The digital processing section 190 forms a system management block for handling a digital interface and itself contains memory. Digital optical monitoring (DOM) is a means of the device holding information about itself and performance. It holds both static and dynamic data. The static data can be date code, part number, manufacturer, serial number. The dynamic data may
be for example laser conditions, temperature etc. Data relating to the optical devices is read and loaded into specific memory location and then accessed externally via a 2-wire interface (I2C). This memory can also be use to hold information relating to manufacturer, parts numbers etc as documented by the module multi-sourced agreement.
The device 100 that has been described includes both a transmit portion and a receive portion. It is to be housed in a single physical housing. However for improved immunity from cross-talk it may be advantageous to provide a transmit device and a separate receive device, each to be housed and powered separately. The isolation characteristic is shown in Fig. 4.
Referring to Fig. 6, in another module 200 no optical components are provided. Module 200 is a service module that allows connection to the same electrical socket but allows interrogation and reporting of parameters from the system. In the presently described system where SFP modules are used, module 200 has an SFP electrical connector 201 connected via a bus 202 to a microprocessor 210. The bus 202 connects to the microprocessor 210 via A-to- D and D-to A converters 205.
The microprocessor 210 forms an interface adaptor and is configured to allow connection to a remote monitoring system or computer (not shown) via USB, Ethernet or RS232 interface. The converters 205 allow parameters and signals present at the connector 201 to be measured and reported to the monitoring system. Likewise the digital to analogue converter allows voltages to be applied to the electrical connector 201.
As the system I C bus is present at the electrical connector 201, communication is possible to a Hub or antenna unit microprocessor. This may be of particular interest in the case of an antenna unit which is malfunctioning.
The use of a pluggable optical module will enable specific optical components to overcome deficiencies in the installed base MM fibre.
Referring now to Fig. 7, the laser 120 is fitted to a pedestal 210 and launches light into a ball lens 220. Light is thereby focussed at the end of a fibre stub 230. The stub has a single mode core contained in a ceramic ferrule. It serves to minimise optical reflections when mating with the user patch chord due to the precision physical connection. The split sleeve serves as a precision alignment aid.
The position of the laser 120 or ball lens 220 may be manipulated such as to focus the light on the end of the fibre stub to stimulate centre or offset launch within the transmitter assembly.
As noted above, the invention is not limited to any particular launch device or assembly.
The fibre measurements show that with a standard launch several nulls appear that are potentially at the frequency of interest. The use of a pluggable module enables the use of different laser launch conditions to suite a particular application. This may encompass centre launch, offset launch, vortex launch or other such solutions.
It is envisaged in one set up that a range of modules be available off the shelf, and that one of these may be selected for use based upon known fibre
characteristics. In another set-up, the installer may select by trial and error between a subset of available modules.
In use, the host device is powered up and, if appropriate, the software is allowed to boot up to an operating condition. Then a module embodying the invention is hot-plugged into the host device. The host device recognises the fact that the module is a module configured according to the invention and not a digital module and communicates with the internal digital controller. After connection of the necessary optical fibre or optical fibres, the system is ready for testing. In one embodiment, as mentioned above, the module is known to be good for the particular fibre; however, in other embodiments the effect of connecting the module to the fibre is then checked with a view to determining whether the transmission is being properly effected. If it is not being properly effected then it is possible to substitute a module having different launch conditions without powering down the host device.
The fibre bandwidth profile is critically dependent upon the laser bias current when modulated with an rf carrier. Using the I2C interface on the transceiver it is possible remotely to send commands to increase or decrease the laser drive conditions and therefore modify the bandwidth profile.
In the preceding embodiments, the optical signals are at the prevailing rf data rate and are broad band. All frequencies within the pass band may be transmitted and received. This approach puts significant design constraints on the optical components due to the requirement for wide bandwidth and gain flatness across a wide band.
In alternative embodiments :-
1. Frequency conversion is used. In this case the system modifies the carrier frequency to a lower frequency for transmission. At the receiver end it is then returned to the original carrier frequency for radiation via the antenna. This benefits the optical component in that the bandwidth requirement can be made to be relatively low. A penalty is increased complexity in both up and down conversion of the carrier frequency.
2. If the RoF system is channelised then the optical components only have to deal with specific defined frequencies rather than broadband. In this case the RoF system is designed to transport only the service provider frequencies of interest. The system is no longer agnostic of the data it transports but the system and optical components can be optimised at specific frequencies. This makes it simpler to provide signal balance and gain flatness. In some systems specific optical modules are designed for specific channels of frequencies. This reduces complexity and ultimately cost of the optical components. To make the most of this performance advantage, the RoF system is channelised to specific frequencies.
The embodiments described have Z pluggability — i.e. the module is introduced perpendicular the plane of a relevant panel of the host device. Other connections are also possible, and specifically include Y pluggability, where the module is connected by movement parallel to the plane.
Non pluggable modules may be appropriate: features of embodiments described here in the context of pluggable modules may be applied to non- pluggable modules.
In some systems a non-modular approach may be used, in which case some or all of the components may be incorporated along with features of the host device described above into a single device. Although direct modulation is described, other modulation techniques, for example including external modulators, may be used.
In some embodiments it is envisaged that mode selection may also occur at the receive end. In yet other, polarisation state at the launch end may be selected by providing a polarisation-selective coupling whose angular orientation can be varied - e.g. among a fixed number of possible orientations - to set the input polarisation.
Embodiments of the invention may carry signals of many different frequency types, for example 2G; 3G; DECT; HSDPA; WiFi; WiMAX; TETRA; PMR; DVB-H; and DMB.
The embodiments described are not intended to limit the invention.
Claims
1. A communications device having an electrical input node configured to receive an analogue radio-frequency electrical input from a host device, a lasing device having an optical output configured for coupling laser light from the lasing device to an optical fibre and circuitry for applying current to the lasing device so that the laser light corresponds to the analogue electrical input, the device having a releasable electrical connection for plugged coupling to the host device and a releasable optical connection for plugged coupling to the optical fibre.
2. A communications system comprising a device according to claim 1, a host device, an optical fibre and an output device configured to receive light from the fibre and convert it to electromagnetic radiation corresponding to the analogue electrical input, and coupling means for coupling laser light between the lasing device and the optical fibre.
3. A communications system according to claim 2, wherein the optical fibre is a multimode fibre, the coupling means comprises a lens and is arranged to provide a restricted set of modes in the fibre.
4. A communications system according to claim 2 or 3, wherein the lasing device is arranged to provide a restricted set of modes in the fibre.
5. A communications system according to claim 4, wherein the restricted mode set comprises less power at low mode orders and high mode orders and relatively more power in intermediate orders.
6. A communications device having an electrical output node configured provide an analogue electrical input to a host device, a photosensor having an optical input configured to receive laser light having analogue variations, circuitry for providing signals corresponding to the analogue variations in the laser light to the electrical output node, the device having a releasable electrical connection for plugged coupling to the host device.
7. A method of operating a communications system in which analogue signals are received at an electrical host, and in response thereto, analogue signals are transferred over an optical fibre to a remote location, the method comprising plugging an electro-optical transducer module into a corresponding socket of the electrical host, providing a signal from the transducer module to the host whereby the host recognises that the transducer module, connecting an optical fibre to the transducer module and transferring said analogue signals over the optical fibre.
8. An optical communication device having means for connecting to a host device and means for connecting to at least one optical fibre, the optical communication device including at least one semiconductor device for producing optical radiation, electrical circuitry for powering the semiconductor device and means for modulating the optical radiation in accordance with electrical signals received from the host device, the modulating means being configured to provide one of a linear relationship between the optical radiation and the electrical signals and a defined non-linear relationship therebetweeen.
9. An optical communication device according to claim 8, wherein the modulating means includes a circuit affecting a bias current to the semiconductor device, whereby direct modulation is effected.
10. An optical communication device according to claim 8, wherein external modulation is effected.
11. An optical communication device according to claim 8, 9 or 10 wherein the semiconductor device is operated in a lasing mode.
12. An optical communication device according to any of claims 8-11, wherein the optical communication device includes an optical system coupling an output of the semiconductor device to a fibre so as to effect a desired launch condition providing desired modes in the fibre.
13. An optical communication device according to any of claims 8-11, wherein the semiconductor device has one or more outputs configured to provide desired modes in the fibre.
14. An optical communication device according to claim 12 or 13, wherein a population of modes in the fibre is selected to overcome nulls in transmission in the fibre.
15. An optical communication device according to any of claims 8-14, wherein the optical system is selected to provide desired polarisation of light in the fibre.
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GB0600162A GB0600162D0 (en) | 2006-01-05 | 2006-01-05 | Communications device |
GB0600162.2 | 2006-01-05 |
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