WO1999021303A1

WO1999021303A1 - Optical transmission system

Info

Publication number: WO1999021303A1
Application number: PCT/JP1998/004693
Authority: WO
Inventors: Hideyuki Omura; Nobuya Shinoda; Masafumi Tabara
Original assignee: The Furukawa Electric Co., Ltd.
Priority date: 1997-10-21
Filing date: 1998-10-16
Publication date: 1999-04-29
Also published as: JPH11191755A

Abstract

An optical transmission system comprises a communication center including a transmitter-receiver for transmitting and receiving optical signals, and a plurality of terminals which communicate with the communication center. The system also comprises a plurality of node devices provided between the communication center and the terminals. The plurality of node devices convert optical signals from the communication center into electrical signals for transmission to the terminals and convert electrical signals from the terminals into optical signals for transmission to the communication center. Each of the plurality of node devices is connected to a group of the terminals. The group of the terminals which are connected to one of the node devices sends information to said communication center by digital modulated electrical signals. Each of the node devices comprises a light-emitting element in which a bias point is at or near a threshold for light emission.

Description

DESCRIPTION

OPTICAL TRANSMISSION SYSTEM

Field of the Invention

The present invention relates to an optical communication technology wherein optical signals are modulated by electrical signals.

Background of the invention

Fig. 1 shows a model system in which a single receiver 2 is located in a center 1 and is connected to a plurality of terminals 4 via optical fibers 3. In such a system, when the plurality of terminals 4 transmit optical signals to the receiver 2 of the center 1 as shown in Fig. 2, electrical signals are supplied to a laser diode LD, which is a light-emitting element, at a predetermined bias level so as to intensity modulate the laser diode LD and transmit information in terms of variations of the intensity of optical signals.

In a system for providing long-distance transmission to a number of terminals such as CATV (Cable TV), a hybrid fiber coaxial (HFC) system has recently been used, which transmits optical signals to a certain point via coaxial cables, converts the optical signals to electrical signals, and distributes the electrical signals to the terminals via coaxial cables. Fig 3 illustrates a hybrid fiber coaxial system in which a plurality of node devices 5 are connected to a center 1 via optical fiber cables 3 configured in a star shape. Each node device 5 is connected to coaxial cables 8 and a plurality of coaxial amplifiers 7. Each coaxial amplifier 7 has a plurality of branched coaxial cables, to which subscriber's terminals 4 are connected. In such a system, optical signals are output from the center 1 for downward transmission from the center 1 to the terminals 4, are converted into electrical signals by the node devices 5 and are distributed to the subscribers' terminals 4. For upward transmission from the subscribers' terminals 4 to the center 1, electrical signals are output in time-division multiplexing, converted into optical signals by the node devices 5 and transmitted to the center 1. The electrical signals from the terminals 4 are analog signals which are modulated by the terminals 4 based on information to be sent. The waveforms of the signals are diagrammatically shown in a window 13. The relationship between the currents applied to the light- emitting element and the light-emitting levels is shown in a window 12 and is described below in detail. A window 11 diagrammatically shows the relationship between the intensity of the optical signals transmitted via optical fibers 3 and the currents which modulate the optical signals or the currents which will result by demodulating the optical signals.

In upward transmission of the signals, if a plurality of the node devices 5 operate in time-division multiplexing (TDM), the laser diodes of inactive mode devices do not emit light until such mode devices are made active and emit light at a predetermined bias point (an operating point), the intensity of the lights being modulated by the electrical signals. Delays may occur for each of the node devices 5 due to the time required to activate the laser from an inactive state to an active state at the bias point. Also, light-emission timing of the light-emitting element in the node devices 5 is difficult to control, resulting in deterioration of the system efficiency. To resolve this problem, wavelength division multiplexing scheme is preferred over the time-division multiplexing scheme. In such a scheme, the laser of each node device 5 continuously emits light of a unique wavelength, and the intensity of the light emission is modulated by the electrical signals from the terminals 4.

In this case, as shown in the window 12 of Fig. 3, the light-emitting element of each node devices 5 emits light of a unique wavelength at a predetermined bias point. When optical communication is implemented with such a method, optical beat with a wavelength equal to the difference between wavelengths of received optical signals occurs in a transmitter-receiver 6 of the communication center 1. Assuming that the wavelengths of the lights emitted from the laser diodes of a plurality of the node devices 5 are λ 1, λ 2,... λ n, the two lights of wavelengths λ l and λ 2 will generate an optical beat of a wavelength equal to the difference λ 1- λ 2 as shown in Fig. 12(a). If the optical beat occurs near the frequency of the transmission signals, it will result in a noise in the transmission signals, which reduces the quality of transmission. Preventing the optical beat from occurring near the transmission signal will require limitations in selecting wavelengths of lights emitted by the node devices 5 and limitations in the number of channels in the system.

The signals for upward transmission to be optically modulated are typically video signals, STM (synchronous transfer mode) signals used as a basic frame of information to be transmitted in a wide-band ISDN, and modem signals which are sent from the terminals of CATV-LAN, are time-division multiplexed in a repeater station and are output to a cable. Among them, the need for CATV-LAN signals is increasing.

The signals which are output from the cable modem are digital modulated signals, which are burst signals. Carriers are not normally generated but are generated when the signals need to be transmitted. The carriers generated are immediately modulated. In most case, the modem does not provide the information as to when the signals will be output.

The signals for upward transmission are converted into the optical signals in the node devices at the end of the cable, the optical signals being connected to an optical receiver with one-to-one relationship. If a single optical receiver receives mixed optical signals from a plurality of optical transmitters, optical beat will be generated by the lights which are simultaneously emitted when the wavelengths of the lights are close to each other, which causes disturbance of transmission.

Japanese patent publication 58- 142648 discloses an optical transmission device with a laser diode in which a bias current is set near a threshold current to reduce modal noise. However, configuration of the transmission system is not described in which limitations in the number of channels caused by the optical beat is reduced and the time delay in emitting light by the light-emitting element is prevented. Japanese patent publication 62- 136932 discloses an optical transmitter in which an averaged optical output from a laser diode is constant. Further, Japanese patent publication 59-91746 discloses a laser transmitter in which a bias circuit is provided with a feedback control circuit for adjusting levels of a bias current.

In general, an amplifier for driving a laser diode is a class A amplifier. With the class A amplifier, when the laser diode is biased at an operating point near the threshold for light emission, the positive-polarity component of input electrical signals is only used to drive the laser diode and the negative- polarity component is unused. Thus, the electrical power is wasted.

Therefore, it is an object of the present invention to provide an optical transmission system in which the effects of optical beat are reduced. It is another object of the invention to provide an optical transmission system in which the number of channels is not substantially limited.

Further, according to an aspect of the invention it is an object of the invention to provide a transmission method in which the time delay in emitting light by the light-emitting element of the node device does not appear for upward signal transmission with hybrid fiber coaxial (HFC).

Further, according to another aspect of the invention it is an object of the invention to reduce the power consumption of the laser transmitter.

Summary of the Invention

In order to resolve the above objects, the invention provides an optical transmission system having a plurality of transmitting stations, each including light-emitting elements for converting optical signals into electrical signals for transmission, said system comprising at least one optical transmission line for transmitting the optical signals from said plurality of transmitting stations, a receiving station for receiving a plurality of optical signals transmitted through said at least one optical transmission line; and each of said plurality of transmitting stations being set so that an operating bias point of a driving current of the light-emitting element is at or near a threshold for light emission.

According to such an aspect of the invention, since the operating bias point of the driving current of the light-emitting device in a plurality of transmitters are set at or near the threshold for light emission, generation of optical beat in the receiving station which collectively receives the optical signals is reduced and the quality of transmission is improved.

Further, in accordance with another aspect of the invention, the driving current which is substantially below the threshold for light emission are clipped prior to being applied to the light-emitting element. Since the driving currents below the threshold for light emission is substantially eliminated prior to being applied to the light-emitting element, an inverse voltage is not applied to the light-emitting element and thus deterioration of the light- emitting element which would otherwise be caused is reduced.

According to further another aspect of the invention, the invention provides an optical transmission system having a communication center including a transmitter-receiver for transmitting and receiving optical signals, and a plurality of terminals which communicate with the communication center, said system comprising a plurality of node devices provided between the communication center and the terminals, said plurality of node devices converting optical signals from the communication center into electrical signals for transmission to the terminals and converting electrical signals from the terminals into optical signals for transmission to the communication center, each of said plurality of node devices being connected to a group of the terminals, said group of the terminals which are connected to one of said node devices sending information to said communication center by electrical signals, the electrical signals being modulated such that information may be transported with one side of the signals.

Each of said node devices comprises a light-emitting element and means for biasing the light-emitting element at a bias point which is at or near a threshold for light emission, and each of said node devices has a light- transmitting device modulating the light-emitting element and converting the electrical signals from said terminals into the optical signals for transmission to the communication center.

According to such an aspect of the invention, the light-emitting element is biased at or near the threshold for light emission, and accordingly the amount of light emission is very small or nearly zero when there is no signal to be transmitted. When the light-emitting element is biased at or near the threshold for light emission, substantially only one side of the signals contributes optical modulation and the other side or the lower half component of the electrical signals does not cause light emission. The electric signals to be applied are modulated in such a manner that information can be transported with one side or one polarity of the signals, and thus information can be transported with the above described optical modulation.

According to such an aspect of the invention, the optical beat which will otherwise be generated in the communication center which receives optical signals from a plurality of node devices, is reduced and the quality of transmission is improved. Also, the possibility of optical interference is reduced since the electrical signals (RF signals) are transmitted in time- division multiplexing and a plurality of light-emitting elements do not simultaneously emit lights when a plurality of RF signals are not simultaneously output from the cable modems. Thus, the optical receiver may mix the optical signals from a plurality of optical transmitters, so that the number of the optical receiver can be reduced.

According to further another aspect of the invention, the invention provides a node device for optical transmission system, said optical transmission system having a communication center including a transmitter- receiver for transmitting and receiving optical signals, and a plurality of terminals which communicate with the communication center, said system comprising a plurality of node devices provided between the communication center and the terminals, said plurality of node devices converting optical signals from the communication center into electrical signals for transmission to the terminals and converting electrical signals from the terminals into optical signals for transmission to the communication center, each of said node devices being connected to a group of the terminals, said node device comprising a light-emitting element and means for biasing the light-emitting element at a bias point which is at or near a threshold for light emission, said node device modulating the light-emitting element by electrical signals arriving from said group of the terminals connected to the node device into optical signals for transmission to the communication center, the electrical signals being modulated such that information may be transported with a signal component of substantially single polarity or one side of the signals.

According to further another aspect of the invention, the invention provides a light-transmitting device comprising a clipping circuit for clipping electrical signals into a signal component of substantially single polarity, a light-emitting element means for biasing said light-emitting element at a bias point which is at or near a threshold for light emission, and transmitting means for driving said light-emitting element by signals output from said clipping circuit and transmitting the optical output signals.

According to such an another aspect of the invention, the electrical signals are clipped into components of essentially one polarity or one side of the signals, which effectively drive the light-emitting element, causing reduced power consumption.

According to further another aspect of the invention, the light- transmitting device comprises a clipping circuit for clipping the input electrical signal into a signal component of substantially single polarity and said communicating center comprises a light-receiving device for mixing and receiving the optical signals from a plurality of said light-transmitting devices.

Brief Description of Drawings

Fig. 1 is a block diagram showing a system configuration of 1 to n terminals of the prior art.

Fig. 2 is a block diagram showing the relationship between bias levels and applied signal levels.

Fig. 3 is a block diagram showing the relationship between the HFC system configuration and signals of the prior art. Fig. 4 is a block diagram showing a configuration of an embodiment of the invention.

Fig. 5 is a block diagram showing an embodiment of a light-transmitting device according to the invention.

Fig. 6 is a block diagram showing another embodiment of a light-transmitting device according to the invention.

Fig. 7 is a diagram showing operating characteristics of a class A amplifier and a class B amplifier.

Fig. 8 is a diagram showing an operation of a laser diode according to the embodiment of the invention. Fig. 9 is a diagram illustrating times-division multiplexing scheme.

Fig. 10 is a block diagram showing a configuration of the node device 5 of Fig.

4.

Fig. 11 is a block diagram showing a configuration of the transmitter-receiver

6 in the communication center 1 of Fig. 4. O 99/21303

10

Fig. 12 is a diagram showing the effect of optical beat.

Fig. 13 is a diagram showing an embodiment in accordance with invention in which optical fibers connect the center and the terminals. Fig. 14 is a block diagram of a repeater system according to the invention.

Detailed description of the invention

An embodiment of the invention is described below with reference to Fig. 4. The hybrid fiber coaxial system in Fig.4 is basically similar to that of Fig. 3 described above. The same reference numbers are used for the same components. The transmission system comprises a communication center 1, a plurality of node devices 5 connected to the center 1 via optical fibers 3, and a plurality of subscribers' terminals 4 connected to each node device 5 via coaxial cables 8 and coaxial amplifiers 7 to communicate using electrical signals. The coaxial cable 8 is branched at the coaxial amplifier 7 and each branched coaxial cable is connected to each subscriber's terminal 4. Note that the center 1 is also called a receiving station and the node 5 is also called a transmitting station.

On the upward signal line, a group of terminals 4 which are connected to a node device transmit information in the form of time-division multiplexed digital electrical signals. The electric signals are modulated by digital modulation such as a phase shift keying (PSK) scheme which converts digital signals into phase variation of analog carriers for transmission, a frequency shift keying (FSK) scheme which converts digital signals into frequency variation of analog carriers for transmission, and an amplitude shift keying (ASK) scheme which converts digital signals into amplitude variation of analog carriers. Any of these modulating schemes may be used to implement the invention, PSK with the least loss of information being preferred. As shown in a window 12' of Fig. 4, a bias point of the light-emitting element 52 is set at or near the threshold for light emission and therefore a positive component of the applied electrical signals is used to modulate the intensity of light. A model of this operation is shown in a window 11' of Fig. 4. The negative levels of the electrical signals do not contribute to optical modulation. The electrical signals are modulated in such a manner that information can be sent with the component of substantially one polarity or one side of the electrical signals, so that only one of positive and negative levels of the signals is used for optical modulation. The modulation factor of the intensity of the light is preferably below 200%. Thus, the modulating signals can efficiently be used.

Fig. 5 is a diagram showing the concept of an optical transmitter 40 according an aspect of the invention. The RF signals (sign wave in Fig. 5) modulated such that the information can be sent using the component of substantially one polarity, are clipped to discard the negative-polarity component and to leave the positive-polarity component. The electrical signals clipped into the component of one polarity are supplied to the laser diode 42 for optical modulation. ACC 43 is a constant-current circuit. The laser diode is biased at the operating point which is at or near the threshold of optical emission (the operating point may be below the threshold). Though clipping the RF signals reduces the power consumption, RF signals without such clipping may be supplied to the laser diode.

An auto temperature control (ATC) 44 controls temperature of the laser diode 32 in order to control wavelength of the optical signals. In the embodiment, ATC44 may be omitted since the optical beat is reduced and the wavelength of the optical signals need not be controlled. Fig. 6 (A) is a block diagram of an optical transmitter in the node device 5 according to another embodiment of the invention. The optical transmitter 5 comprises a class B amplifier 31, which is a clipping circuit for clipping input electrical signals (a) into components of substantially one polarity. The amplifier 31 only amplifies the substantially positive-polarity component of the signals and supply outputs (b) to the laser diode 32. The constant-current driving circuit 33 biases the laser diode 32 in such a manner that its operating point is at or near the threshold for light emission. The laser diode 32 is driven in response to the positive electrical signal (b) to supply optical output signals (c).

Fig. 6 (B) depicts a typical configuration of an amplifier circuit with a bipolar transistor. Fig. 7 illustrates operations of a class A amplifier and a class B amplifier, where the lateral axis indicates base-emitter voltage vBE and the vertical axis indicates collector current iC. The operating point of the transistor can be varied by varying resistance in a well-known amplifier having a bipolar transistor which is shown in Fig. 7(B). An amplifier which is biased such that an operating point P is at a position P on the iC-vBE characteristic curve as shown in Fig. 7(A) is called a class A amplifier. In such an amplifier, output signals (b) are obtained in a collector in response to input signals (a).

On the other hand, an amplifier which is biased such that an operating point is at a position P on the iC-vBE characteristic curve shown in Fig. 7(B) is called a class B amplifier. This operating point P is near the threshold at which the amplifier begins to operate. Thus, supplying the input signals to the amplifier causes output signals in which the component of one polarity is only amplified while the component of the opposite polarity is discarded. Thus, a class B amplifier only amplifies the component of one polarity of the input signals and discards the component of the opposite polarity. Therefore, in general, a push-pull circuit is constituted by combining a pair of class B amplifiers with opposite polarities, output of each amplifier being combined to obtain output signals with two polarities corresponding to input signals. In the present invention, only one class B amplifier is used to obtain output signal (b) of one polarity which is an amplified component of one polarity of the input signal.

Fig. 8 is a diagram showing a light emission characteristics of the laser diode, where a lateral axis indicates electrical inputs to the laser diode and a vertical axis indicates optical outputs. The laser diode 32 is biased such that an operating point is at a point P near the threshold for light emission. The output signals (b) from the class B amplifier are supplied to the laser diode and the laser diode correspondingly outputs output optical signals (c).

Fig. 9 shows a model of timing of time-division multiplexing from the group of terminals 4a, 4b, 4c and 4d, each of which is connected to a single node device 5. Fig. 10 is a block diagram showing a configuration of the node device 5. In Fig. 10, the PSK electrical signals from the terminal 4 input through a low side L (10 through 50 MHz) of a frequency-division filter 51 to a light-emitting element 52 for converting the electrical signals into optical signals for optical modulation. The light-emitting element is typically a laser diode. The optical signals modulated by the light-emitting element 52 are combined by an optical coupler 54 and transmitted through the optical fiber 3 to the center 1.

On the other hand, the optical signals from the center 1 are transmitted through the optical fiber 3, the optical coupler 54 of the node device 5, to the light-receiving element 53. The light-receiving element is typically a photodiode. The light-receiving element 53 converts the optical signals into electrical signals, which are then output to the coaxial cable 8 through a high side H (70 through 800MHz) of the frequency-dividing filter 51, to the terminal 4.

Fig. 11 is a block diagram showing a basic configuration of the transmitter-receiver 6 in the center 1. The optical signals from a plurality of node devices 5 are transmitted through optical coupler 61 to a light-receiving element 63, on a channel basis, i.e. on a basis of wavelength λ 1, λ 2... λ n of light described above. The optical signals are then converted into electrical signals, which are demodulated using demodulation technology for PSK signals of the prior art. Since the signals from the terminal 4 are transmitted in time-division multiplexing, the information is decoded in accordance with timing of time division.

Conversely, the signals from the center 1 to terminals 4 are converted into the optical signals by the light-emitting element 62 in the transmitter- receiver 6 and output to the optical fiber 3 through the optical coupler 61. Thus, the signals from the center 1 are transmitted through the node device 5, the coaxial cable 8 and the coaxial amplifier 7, to the terminals 4.

Fig. 12 is a diagram showing the effects of the optical beat generated by the lights of a plurality of wavelengths which are received by the transmitter-receiver 6 in the center 1. Fig. 12(a) shows optical beat of the prior art when the modulating bias point of the light-emitting element in the node device 5 is far from the threshold for light emission shown in the window 12 of Fig. 3. In this case, the optical signals of wavelengths λ 1 and λ 2 transmitted from different node devices interfere and generate optical beat of a wavelength λ 1- /1 2. The optical beat become larger as the levels of the optical signals of wavelengths λ 1 and λ 2 are higher. If the optical beat overlaps the frequency band, it will cause a noise in the transmission signals and the quality of transmission is reduced.

Fig. 12 (b) illustratively shows the effects of optical beat when the levels of the optical signals to cause interference are low. In the system according to the present invention, since the bias point of the light-emitting element of the node device 5 is at or near the threshold for light emission, light-emitting levels of the light-emitting element are extremely low or nearly zero when the signal to be transmitted is not arriving. Additionally, when the signal to be transmitted inputs to the light-emitting element, the level of light emission is reduced relative to that of the prior art in which the bias point is set at a higher level for light emission. However, the level for light emission is not necessarily set low when signals are to be transmitted, because such a level is set on the basis of various system requirements.

With the present invention, the light-emitting level of the node device when the signals are transmitted is extremely low or nearly zero, and additionally, the light-emitting level when the signals are transmitted to the center 1 can be set lower than that of the prior art depending on requirements. Thus, the optical beat caused by interference of the optical signals from the node devices 5 is reduced and the effects on the signals to be transmitted is reduced. Further, the electrical signals are transmitted in time division and a plurality of electrical signals are not simultaneously transmitted from the cable modem. Thus, a plurality of light-emitting elements do not simultaneously emit lights and the interference of lights are prevented. The optical signals from a plurality of optical transmitters can be mixed by the optical receiver, which reduces the number of optical receivers. In the above embodiment as shown in Fig. 4, the bias levels are set near the threshold for light emission and the light-emitting element emits high levels of light only when the signals are transmitted. Thus, the level of the optical beat is lowered relative to that of the prior system in which all the lights emitted are high level, and the power consumption is reduced.

Recently, a high frequency band (70 to 800 Hz) and a second frequency band (for example 800MHz to 900MHz) are used for upward transmission and such approach is called a double split scheme. The present invention is also applicable to the communication using the second frequency band. Further, the level of optical beat is lowered and the wavelength of lights need not be controlled.

The invention is not limited to the hybrid system as shown in Fig. 4 and is also applicable to a system in which a plurality of optical signals are transmitted in multiplexing and then received by a single photodiode PD. For example, the present invention may be applied to a system in which the optical fibers 3 are only connected between the center 1 and the terminals 4a-4d. Also, the invention may be applied to a repeater system using an optical repeater, which is combined with light-to-electricity converter (receiver), an electrical amplifier and an electricity-to-light converter (transmitter).

Fig. 14 illustrates a repeater system. In a repeater 70, optical signal transmitted from the left side of the drawing are converted into electrical signals by light-to-electricity converter (typically a photodiode) 71 and an electrical amplifier 72 amplifies the electrical signals. The amplified electrical signals are converted into optical signals by electricity-to-light converter (typically a laser diode) 73 and transmitted to an optical fiber commutation line. Thus, in the repeater 70, the light-to-electricity converter 71 corresponds to a receiver and the electricity-to-light converter 73 corresponds to a transmitter. The optical signals output from the optical repeater 70 are converted to the electrical signals by a light-to-electricity converter 71' in an optical repeater 17. The electrical signals are amplified by an electrical amplifier 72' and converted to optical signals by an electricity- to-optical converter 73'. The optical signals are transmitted to an optical transmission line.

The converter 73' comprises a light transmission device which includes a light-emitting element and which is biased to operate at or a near the threshold for light emission. The light emitting-element, typically a laser diode, is modulated by electrical signals from the amplifier 72'.

Thus, according to the invention, the effects of the optical beat are reduced in the optical transmission system.

While the invention was described with respect to specific embodiments, the scope of the invention is not limited to such embodiments.

Claims

1. An optical transmission system having a plurality of transmitting stations, each including light-emitting elements for converting optical signals into electrical signals for transmission, said system comprising: at least one optical transmission line for transmitting the optical signals from said plurality of transmitting stations; a receiving station for receiving a plurality of optical signals transmitted through said at least one optical transmission line; and each of said plurality of transmitting stations being set so that an operating bias point of a driving current of the light-emitting element is at or near a threshold for light emission.

2. The optical transmission system of claim 1 wherein a portion of the driving current below said threshold for light emission is clipped prior to being applied to said light-emitting element.

3. An optical transmission system having a communication center including a transmitter-receiver for transmitting and receiving optical signals, and a plurality of terminals which communicate with the communication center, said system comprising: a plurality of node devices provided between the communication center and the terminals, said plurality of node devices converting optical signals from the communication center into electrical signals for transmission to the terminals and converting electrical signals from the terminals into optical signals for transmission to the communication center, each of said plurality of node devices being connected to a group of the terminals, said group of the terminals which are connected to one of said node devices sending information to said communication center by electrical signals, the electrical signals being modulated such that information may be transported with substantially one side of the signal; and each of said node devices comprising a light transmission device which includes a light-emitting element and means for biasing the light- emitting element at a bias point which is at or near a threshold for light emission, said light-emitting element being modulated by the electrical signals arriving from said terminals for transmission to the communication center.

4. A node device for optical transmission system, said optical transmission system having a communication center including a transmitter-receiver for transmitting and receiving optical signals, and a plurality of terminals which communicate with the communication center, said system comprising: a plurality of node devices provided between the communication center and the terminals, said plurality of node devices converting optical signals from the communication center into electrical signals for transmission to the terminals and converting electrical signals from the terminals into optical signals for transmission to the communication center, each of said node devices being connected to a group of the terminals, said node device comprising a light-emitting element and means for biasing the light-emitting element at a bias point which is at or near a threshold for light emission, said light-emitting element being modulated, by electrical signals arriving from said group of the terminals, into optical signals for transmission to the communication center, said electrical signals being modulated such that information may be transported with substantially one side thereof.

5. A light-transmitting device comprising: a clipping circuit for clipping electrical signals into a signal component of substantially single polarity; a light-emitting element; means for biasing said light-emitting element at a bias point which is at or near a threshold for light emission; and transmitting means for driving said light-emitting element using output signals from said clipping circuit and transmitting the optical output signals.

6. The system of claim 3 wherein said light-transmitting device comprises a clipping circuit for clipping the input electrical signal into substantially one side thereof and said communicating center comprises a light-receiving device for receiving and mixing the optical signals from a plurality of said light- transmitting devices.

7. The system of claim 3 wherein said electrical signals are modulated by a digital modulation scheme selected from the group including phase shift keying, frequency shift keying and amplitude shift keying.