Transmitting device, transmitting apparatus and optical transmission system for optically transmitting analog electrical signals

An optical transmission system (SYS), e.g. a cable television distribution network, contains at least one transmitting device (SEN), an optical transmission network (NET) and a number of receiving devices (EMP). In order to transmit analog electrical signals, e.g. television signals in the MHz range in such an optical transmission system, the analog electrical signals are first digitized in accordance with the pulse-duration modulation method (PDM) and are then routed to an electro-optic modulator (MOD). An optical pulse source (PULS) generates optical pulses whose repetition frequency rate is higher by the factor of 100 at least than the 3-dB cutoff frequency of the analog electrical signals, e.g. 10 GHz. The optical pulses are modulated with the digitized signals in the electro-optic modulator (MOD) and are then transmitted to the receiving devices (EMP). Only an optical-to-electrical converter OE and a passive electrical low-pass filter (TP) are required to recover the analog electrical signals. The modulated optical pulses from different transmitting devices (SEN) can be transmitted in the optical time-division multiplex method to increase the transmission capacity.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The invention concerns a transmitting device, a transmitting unit and an 
optical transmission system for optically transmitting analog electrical 
signals. 
2. Discussion of Related Art 
The analog signals are television signals for example. An optical 
transmission system for optically transmitting television signals is known 
from the article "Diamond-A digital optical television distribution 
network" in the publication `telecom praxis` 2/1993, pages 38 to 45. The 
television signals have a bandwidth of 7 MHz to 14 MHz. They are usually 
converted from analog to digital by means of pulse code modulation, then 
multiplexed electrically and converted electrical-to-optical; one optical 
pulse usually corresponds to one electrical pulse and thus to one bit. The 
now optical digital television signals are transmitted to several 
receiving devices via a passive optical transmission network of 
glass-fibers, i.e., optical fibers, and optical splitters. Each receiving 
device includes optical-to-digital converters and a digital-to-analog 
converter for recovering the analog electrical signals. The number of 
multiplexed television signals is limited by the resolution and speed of 
the available digital-to-analog converters. Each digital-to-analog 
converter is an active unit which must be controlled and synchronized. 
SUMMARY OF THE INVENTION 
It is therefore a task of the invention to optically transmit analog 
electrical signals. 
According to a first aspect of the invention, a transmitting device for 
optically transmitting analog electric signals in the form of modulated 
optical pulses, comprises a device for digitizing the analog electric 
signals by pulse-duration modulation, an optical pulse source for 
generating optical pulses whose repetition frequency is higher than the 
3-dB cutoff frequency of the analog electric signals by at least a factor 
of a hundred, and an electro-optic modulator for modulating the optical 
pulses with the digitized electric signals. 
According to a second aspect of the invention, a transmitting unit 
comprises at least two transmitting devices according to the first aspect 
of the present invention, and an optical multiplexer for optically 
time-division multiplexing the modulated optical pulses. 
According to a third aspect of the invention, an optical transmission 
system comprises at least one transmitting device, a passive optical 
transmission network, and a plurality of receiving devices, each of the 
receiving devices comprising an optical-to-electrical transducer and a 
passive electric filter for recovering the analog electric signals from 
the received modulated optical pulses. 
A special advantage of the invention is that it is applicable up to bit 
rates above 10 GHz. This is achieved, e.g., by using a mode-locked fiber 
ring laser as the optical pulse source. 
A further advantage of the invention is the reduction in the accuracy 
requirements regarding the linearity of the electro-optic modulator by 
using a fixed pulse repetition frequency. A fixed pulse repetition 
frequency allows the transmission of a number of analog electrical signals 
with different bandwidths. 
In addition the transmission capacity can be increased by optically 
time-division multiplexing modulated optical pulses from different 
transmission devices. It is furthermore advantageous if the receiving 
devices only contain passive components. The receiving devices are 
therefore cheaper, more insensitive to interference and have a simpler 
circuitry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The first configuration example will now be explained with the help of 
FIGS. 1 and 2. FIG. 1 illustrates an optical transmission system SYS 
according to the invention for the optical transmission of analog 
electrical signals. The optical transmission system SYS contains a 
transmitting device SEN which is connected to several receiving devices 
EMP1, EMP2, . . . , EMPn via a passive optical transmission system NET; n 
is a natural number. 
The analog electrical signals are television signals for example. For the 
optical transmission, the analog electrical signals are first converted 
from analog to digital in the transmitting device. This takes place by 
modulating the analog electrical signals in accordance with the 
pulse-duration modulation method. Subsequently the digitized electrical 
signals are converted from electrical to optical. The conversion takes 
place with an optical pulse source and an electrical-to-optical modulator. 
In this way the transmitting device is used for the analog-to-digital and 
the electrical-to-optical conversion of the analog electrical signals. 
From the transmitting device the analog electrical signals are fed in the 
form of modulated optical pulses to the passive optical transmission 
network NET. 
The passive optical transmission network NET is an optical cable television 
distribution network with optical fiber cables and optical splitters for 
example. The cable television distribution network provides a 
point-to-multiple point connection between the transmitting device SEN and 
the receiving devices EMP1, EMP2, . . . , EMPn. In this way the modulated 
optical pulses sent out by the transmitting device SEN reach all the 
receiving devices EMP1, EMP2, . . . , EMPn. The analog electrical signals 
can be television signals as well as video signals for transmission in a 
video-on-demand system. In the latter case the passive optical 
transmission network NET is preferably a distribution network with a 
feedback channel. 
The analog electrical signals are recovered from the respective received 
modulated optical pulse by the receiving devices EMP1, EMP2, . . . , EMPn. 
The recovery is accomplished with an optical-to-electrical converter and a 
passive electrical low-pass filter. 
The optical transmission system SYS provides a method of optically 
transmitting analog electrical signals which is independent of the optical 
transmission network NET structure, and where the recovery of the analog 
electrical signals only requires a passive lowpass filter. With a 
transmitting device SEN and a number of receiving devices EMP1, EMP2, . . 
. , EMPn, for example 1000, this is an advantage for the circuitry as well 
as for reasons of cost. 
The analog electrical signals have a bandwidth of 10 MHz for example. They 
are converted by the transmitting device into optical pulses with a 
repetition frequency rate of 10 Gbits for example. This oversampling 
results in the transmission being insensitive to interference. 
FIG. 2 illustrates the construction of a transmitting device SEN according 
to the invention and a receiving device EMP1 according to the invention 
for the optical transmission system in FIG. 1. 
The transmitting device SEN and the receiving device EMP1 are 
interconnected by an optical distribution network NET composed of optical 
splitters and optical fiber lines. 
The transmitting device SEN receives analog electrical signals via an 
antenna ANT. The received signals are television signals transmitted by a 
television station for example. They are routed to a PDM device which is 
used for digitization in accordance with the pulse-duration modulation 
method. The received signals for example have a 3-dB cutoff frequency of 
10 MHz and therefore a bandwidth of 10 MHz. The PDM device contains a 
signal generator GEN and a comparator OP. 
The signal generator GEN generates sampling signals which have a sawtooth 
or a triangular shape for example. The frequency of the sampling signals 
is at least twice as high as the 3-dB cutoff frequencies of the received 
analog signals, therefore 20 MHz for example. The amplitude of the 
sampling signals is at least equal to the amplitude of the received 
signals. 
The comparator OP is configured for example as an amplifier OP with a 
differential input which functions as a comparator. The received analog 
signals are routed to the positive input of amplifier OP and the generated 
sampling signals to the negative input of amplifier OP. When the analog 
electrical signals are compared with the sampling signals, each 
coincidence of the input voltages at the output of amplifier OP causes a 
change in the sign of the output voltage which produces pulse-duration 
modulated signals. The signals that are digitized in this manner are 
routed to the electrical input of an electro-optic modulator MOD. An adder 
ADD for adding d.c. voltage signals to the digitized signals is connected 
between the PDM device and the electro-optic modulator MOD. 
The d.c. voltage signals are generated by a d.c. voltage source DC. They 
are used to adapt the d.c. voltage level of the digitized signals to the 
modulation range of the electro-optic modulator MOD. 
The transmitting device SEN furthermore contains an optical pulse source 
PULS for generating optical pulses whose repetition frequency rate is much 
higher, preferably by a factor of at least 100 times higher than the 3-dB 
cutoff frequency of the analog electrical signals, for example 10 GHz. A 
mode-locked fiber ring laser is preferably used to generate such 10 GHz 
signals, whose amplifying element is doped with an element from the group 
of rare earths, preferably erbium. The generated optical pulses are routed 
to the optical input of the electro-optic modulator MOD. 
The electro-optic modulator MOD is a Mach-Zehnder modulator for example, 
whereby an intensity modulation can be performed. It is used to modulate 
the optical pulses with the digitized electrical signals. The 10 GHz 
optical pulses are therefore modulated with the digitized 20 MHz signals. 
A high resolution is achieved in the modulation of the digitized signals 
because of the large frequency difference of, e.g., preferably at least 
two orders of magnitude between optical pulses and digitized signals, 
which causes any distortion of the analog electrical signals that are 
recovered by the receiving device EMP1 to be very small. The modulated 
optical signals are transmitted by the optical distribution network NET to 
several receiving devices EMP1, EMP2, . . . , EMPn, only one of which is 
illustrated in FIG. 2. 
The receiving device EMP1 contains an optical-to-electrical converter OE 
which is followed by a passive electrical low-pass filter TP for the 
recovery of the analog electrical signals from the received modulated 
optical signals. 
The optical-to-electrical converter converts the optical signals into 
electrical signals by means of a photodiode. The sensitivity of the 
photodiode is at least twice the 3-dB cutoff frequency of the analog 
electrical signals, for example 20 MHz, enabling the envelope of the 
received optical signals to be detected and converted into electrical 
signals. 
The passive electrical low-pass filter TP is a 5th or a 7th order low-pass 
filter for example, constructed of a chain circuit of resistors and 
capacitors and with a cutoff frequency that corresponds to the 3-dB cutoff 
frequency of the analog electrical signals, for example 10 MHz. The analog 
electrical signals are recovered from the output signals of the 
optical-to-electrical converter OE by means of the passive electrical 
low-pass filter TP. No active units are therefore required to recover the 
analog electrical signals. 
The second configuration example will now be described with the help of 
FIG. 3. FIG. 3 illustrates an optical transmission system SYS according to 
the invention, with several transmitting devices SEN1, SEN2, . . . , SENn 
and several receiving devices EMP1 to EMP6, six of which are illustrated 
as an example. The optical transmission system SYS is used to transmit 
modulated optical pulses from individual transmitting devices SEN1 to SENn 
to individual receiving devices EMP1 to EMP6, or to groups of receiving 
devices EMP1, EMP2, EMP3, EMP4, EMP5, EMP6. The transmission takes place 
in the optical time-division multiplex method. Since not all transmitting 
devices SEN1 to SENn are active at the same time, an optical switching 
unit OS is provided and performs the switching functions for the optimum 
usage of the transmission capacity of the optical fiber lines. 
Each of the transmitting devices SEN1 to SENn corresponds to the 
transmitting device SEN in FIG. 2 with respect to construction and 
functionality. 
The number n of transmitting devices SEN1 to SENn is 100 for example. Each 
transmitting device SEN1 to SENn is connected to the optical switching 
unit OS via an optical fiber line. 
The task of the optical switching unit OS is to detect the active 
transmitting devices SEN1 to SENn and to forward their signals to an 
optical multiplexer OMUX. Assuming that only 6 of the 100 transmitting 
devices SEN1 to SENn are active simultaneously, 6 optical fiber lines are 
sufficient to transmit all modulated optical pulses from the switching 
unit OS to the optical multiplexer OMUX. For each transmitting device SEN1 
to SENn the optical switching unit OS for example contains an asymmetric 
coupler, an electro-optic modulator, a timing recovery unit and a 
photodiode. The optical switching unit OS furthermore contains a central 
electrical controller and 6 electro-optic modulators for the 6 optical 
fiber lines which are connected to the optical multiplexer OMUX. Part of 
the modulated optical pulses of an active transmitting device SEN1 to SENn 
are coupled out and routed to the central electrical controller via the 
optical timing recovery unit and the photodiode. The central controller 
then controls two electro-optic modulators in a way so that the modulated 
optical signals are routed to the optical multiplexer OMUX through a still 
unoccupied optical fiber line. The optical switching unit OS is a 
so-called optical cross-connect for example, whereby every input can be 
switched to every output; an optical fiber line may be required for the 
control signals. 
The optical multiplexer OMUX bundles the modulated optical signals from the 
6 optical fiber lines so that they can be transmitted in one optical fiber 
line. Each of the modulated optical signals has a bit rate of 10 Gbit/s. 
The 6 modulated optical signals are combined into a 60 Gbits signal in the 
time-division multiplex method and fed to an optical transmission network 
NET1 through an optical fiber line. 
The optical transmission network NET1 is a cable television distribution 
network of optical fiber lines and optical splitters for example, or a 
submarine cable. The structure of the optical transmission network NET1 is 
for example tree-shaped, bus-shaped or star-shaped. The optical 
transmission network NET1 is connected for example to several receiving 
devices EMP1, EMP2, two of which are illustrated, and to an optical 
demultiplexer ODMUX. 
Each receiving device contains an optical demultiplexer ODMUX and six 
optical-to-electrical converters and six passive electrical low-pass 
filters for the recovery of the analog electrical signals. 
The optical demultiplexer ODMUX is used to split the combined modulated 
optical signals into different transmission networks NET2, NET3. 
The optical transmission network NET2 is, for example, a cable television 
distribution network which routes the television signals from a television 
station, for example the transmitting device SEN1, to several receiving 
devices EMP3, EMP4, two of which are illustrated as an example. The 
receiving device EMP3 can also be a coaxial cable network interface for 
example. 
The optical transmission network NET3 is a submarine cable for example, 
whereby video signals, e.g. the latest movie films, are routed to 
individual movie installations, for example the receiving devices EMP5 and 
EMP6. 
In the first configuration example, the analog electrical signals received 
via the antenna ANT are routed directly to the PDM device for digitization 
of the analog electrical signals. Since the amplitude of the sampling 
signals must be adapted to the amplitude of the analog signals it can lead 
to errors in the modulation. This can be avoided in the following manner: 
An amplitude detector is added to the transmitting device SEN to detect the 
amplitude of the received analog electrical signals and to generate 
control signals for adjusting the amplitude of the output signals from the 
signal generator GEN, the comparator OP and/or the d.c. voltage source DC. 
In this way the amplitude of the received analog electrical signals is 
measured and the amplitude of the sampling signals is adapted to this 
amplitude. This could possibly require a readjustment of the amplitude of 
the output signals from the amplifier and from the d.c. voltage source. 
Both configuration examples are able to utilize a specified and not 
variable pulse frequency rate of optical pulses from the optical pulse 
source, e.g. 10 Gbits, for digitizing a number of analog electrical 
signals with different bandwidths, e.g. 1 MHz to 100 MHz. This can take 
place for example with a signal generator GEN whose sampling frequency is 
adjustable, and a frequency detector for detecting the 3-dB cutoff 
frequency of the received analog signals and for generating the adjustment 
signals for the signal generator in the transmitting device SEN of FIG. 2. 
The measured 3-dB cutoff frequency provides the value for adjusting the 
sampling frequency, which must be higher than the 3-dB cutoff frequency by 
a factor of 2 at least. 
Although the invention has been shown and described with respect to a best 
mode embodiment thereof, it should be understood by those skilled in the 
art that the foregoing and various other changes, omissions and additions 
in the form and detail thereof may be made therein without departing from 
the spirit and scope of the invention.