Optical transmission system

An optical transmission system comprises an optical transmitter (1) for launching optical signals into an optical fibre, and drive means (4 to 9) for driving the optical transmitter independently with first and second electrical signals. The drive means is such that the optical transmitter (1) transmits first and second optical signals corresponding to the first and second electrical signals.

This invention relates to an optical transmission system, and in particular 
to a system which provides for transmission of synchronous digital data 
over a continuously variable range of clock frequencies. 
As optical fibre transmission systems increasingly find applications in 
local area networks (LANs) and in-building environments, techniques to 
improve the network flexibility are increasingly being sought after by 
system planners. One such improvement would be to connect a number of 
different types of terminal equipment, such as telephones, fax machines, 
computers etc., to the same transmission line, which would result in a 
simpler and cheaper network. Additionally, such a system would be easier 
to maintain, and be physically smaller at the desk. Potentially, a single, 
optical terminal could provide ports for many different types of office 
equipment. 
A positive step towards this goal is a transmission system which transports 
clock and data over a range of data rates, without the need to change any 
of the system parameters. Known synchronous optical transmission systems, 
however, generally operate at one specific data rate, since clock recovery 
from the transmitted data stream is usually required to regenerate the 
data itself. Thus, a standard synchronous transmission system requires a 
narrow band-pass filter to extract the component at the clock frequency. 
However, a system operating at a different rate, would require a different 
band-pass filter to extract a clock component from the transmitted signal. 
Each conventional synchronous system is, therefore, restricted to operate 
at one data rate only. 
The problem preventing variable rate transmission systems originates 
through the requirement for a fixed narrow bandpass (high Q) filter to 
extract a component at the clock frequency from the data. Although phase 
locked loops can be used to track the frequency of the incoming signal, 
they are limited to a narrow spread of data rates by their tracking range. 
The present invention provides an optical transmission system comprising an 
optical transmitter for launching optical signals into an optical fibre, 
and drive means for driving the optical transmitter independently with 
first and second electrical signals, the drive means being such that the 
optical transmitter transmits first and second optical signals 
corresponding to the first and second electrical signals, wherein the 
first electrical signals are clock signals, and the second electrical 
signals are data signals. 
In a preferred embodiment, the first and second electrical signals are in 
separate regions of the radio frequency (RF) spectrum. Preferably, one of 
the optical signals in a baseband signal, the other optical signal being a 
sub-carrier multiplexed signal. 
Advantageously, each of the electrical signals is passed through a 
respective filter positioned upstream of the optical transmitter. Each of 
the filters may be a 3rd order Butterworth low-pass filter. Conveniently, 
said one optical signal corresponds to the first electrical signal, and a 
modulator is positioned between the filter for the second electrical 
signal and the optical transmitter. In this case, the system may further 
comprise an oscillator for supplying a carrier frequency to the modulator, 
the modulator using coherent FSK to modulate the second electrical signals 
onto the carrier. 
In a preferred embodiment, the clock signals and the data signals are 
provided by an externally-clocked data set providing clock signals and NRZ 
PRBS data signals over the range 100 kHz to 2.5 mHz. 
The optical transmission system may be combined with an optical receiver 
system, the optical receiver system including an optical receiver for 
converting the first and second optical signals received from the optical 
transmission system into third and fourth electrical signals corresponding 
thereto. 
This combination may further comprise means for retiming the fourth 
electrical signal with respect to the third electrical signal. 
Conveniently, a D-type flip-flop constitutes the means for retiming the 
fourth electrical signal with respect to the third electrical signal. 
Advantageously, the combination further comprises respective filters 
positioned in respective parallel baths from the optical receiver to the 
flip-flop, each filter being effective to filter out a respective one of 
the third and fourth electrical signals. 
Preferably, the filter for the third electrical signal is a 3rd order 
Butterworth low-pass filter, and the filter for the fourth electrical 
signal is a 4th order Butterworth band-pass filter. A demodulator may be 
positioned between the filter for the fourth electrical signal and the 
flip-flop. 
Advantageously, the third electrical signal is applied to the clock input 
of the D-type flip-flop, and the output signal of the demodulator is 
applied to the data input of the flip-flop. Thus, the output signal of the 
demodulator constitutes a demodulated fourth electrical signal, so that 
the output from the flip-flop corresponds to the retimed demodulated 
fourth electrical signal which has a well-defined phase relationship to 
the third electrical signal. 
The invention also provides an optical receiver system comprising an 
optical receiver for receiving independently transmitted first and second 
optical signals from an optical fibre, the optical receiver being 
effective to convert the first and second optical signals into first and 
second electrical signals corresponding to the first and second optical 
signals, and means for retiming the second electrical signal with respect 
to the first electrical signal. 
The invention further provides an optical transceiver system comprising an 
optical transmitter aparatus and an optical receiver apparatus, the 
optical transmitter apparatus compising an optical transmitter for 
launching optical signals into an optical fibre, and drive means for 
driving the optical transmitter independently with first and second 
electrical signals, the drive means being such that the optical 
transmitter transmits first and second optical signals corresponding to 
the first and second electrical signals, and the optical receiver 
apparatus comprising an optical receiver for receiving third and fourth 
optical signals from an optical fibre, the optical receiver being 
effective to convert the third and fourth optical signals into third and 
fourth electrical signals corresponding to the third and fourth optical 
signals, and means for retiming the fourth electrical signal with respect 
to the third electrical signal. 
The invention also provides a method of transmitting clock signals and data 
signals over an optical fibre by launching optical signals corresponding 
to the clock signals and the data signals into the optical fibre using an 
optical transmitter, the method comprising the step of launching the 
optical signals into the fibre by driving the optical transmitter 
independently with clock signals and data signals. 
Preferably, one of the optical signals is launched as a baseband signal, 
the other optical signal being launched in the form of a sub-carrier 
multiplexed signal.

Referring to the drawings, FIG. 1 shows the transmitter of the optical 
fibre transmission system, the transmitter including an LED 1 for 
launching optical signals into an optical fibre (not shown). The optical 
transmission system operates at 850 nm and has a power reduction of 3 dB 
at 18 MHz. The LED 1 is driven by clock signals (via an input line 2) and 
by modulated data signals (via an input line 3), the clock signals and the 
data signals being in separate regions of the RF spectrum. The clock 
signals and the data signals are provided by an externally-clocked data 
test set providing clock signals and non return to zero (NRZ) data signals 
as a pseudo random bit sequence (PRBS) over the range 100 kHz to 2.5 MHz. 
The clock signals are fed to the line 2 from an input line 4 via a 3rd 
order Butterworth low-pass filter 5 rated at 3 MHz. The data signals are 
fed to the line 3 from a data input line 6 via a 3rd order Butterworth 
low-pass filter 7 rated at 3 MHz and a Philips NE 564 modulator 8. The 
modulator 8 uses coherent frequency shift keying (coherent FSK) to 
modulate the data signals onto a carrier of frequency 7.35 MHz which is 
input to the modulator by an oscillator 9. Thus, by modulating the data 
signals onto the carrier, and transmitting the clock signals as a baseband 
signal, clock and data are transported independently. The filters 5 and 7 
limit the frequency content of the modulated transmitted optical signal, 
which leads to a reduction in the sideband signal and hence helps prevent 
crosstalk. 
The receiver (see FIG. 2) includes an optical receiver 10 which converts 
optical signals carried by the optical fibre into electrical signals on a 
line 11. These electrical signals are amplified by a National LM733 
amplifier 12. The output of the amplifier is fed, in parallel, to a 3rd 
order Butterworth low-pass filter 13 rated at 3 MHz, and to a 4th order 
Butterworth band-pass filter 14 having a pass range of from 5 MHz to 11 
MHz. The filter 13 extracts the clock signals, and the filter 14 extracts 
the carrier modulated by the data signals. The extracted clock signals are 
amplified by a National LM733 amplifier 15 whose output is fed to one 
input of a D-type flip-flop 16. The extracted carrier/data signals are 
passed to a Philips NE564 demodulator 17 which extracts the data signals 
from the carrier and feeds them to the other input of the flip-flop 16. 
The flip-flop 16 synchronises the clock and data signals at its outputs 
18. 
By modulating the carrier with the data signals, and transmitting the clock 
signals as a baseband signal, the two types of signal are transported 
independently. Consequently, there is no requirement for narrow band-pass 
filters for extracting the clock components at different operating rates, 
so that the system described above provides simultaneous clock and data 
transport at variable transmission rates. 
FIG. 3 is a graph showing the optical sensitivity (which is a measure of 
the received optical power) against data rate. For each data rate, the 
received optical power is adjusted to obtain a bit error rate (BER) of 
1.times.10.sup.-5). The graph shows a generally constant system 
sensitivity of -42 dBm up to a data rate of about 1 Mb/s. At higher data 
rates, a roll-off in sensitivity occurs as a result of a combination of 
performance--limiting factors, a rapid deterioration occuring after the 
data rate reaches about 2 Mb/s. 
The performance--limiting factors referred to above are: 
a) Crosstalk 
As the transmitted data rate is increased, the bandwidths of the baseband 
and modulated signals increase. At some rate, they will have bandwidths 
that begin to overlap. It will then be impossible to separate the two 
channels, without observing some degree of interference between them. This 
interference manifests itself as jitter, and causes system errors. 
Crosstalk is reduced by the use of the low pass filters 5 and 7. 
b) Modulated bandwidth 
Using FSK for modulation, the ratio f.sub.d /r (where f.sub.d is the 
frequency deviation and r is the data rate) is a useful parameter when 
discussing the bandwidth and power spectral density (psd) of the modulated 
signal. For low values of f.sub.d /r (e.g. 0.3), the FSK psd has a peak at 
the carrier frequency (f.sub.c) with smooth roll-off. The bandwidth is of 
the order of 2r. As f.sub.d /r increases, the bandwidth extends beyond 2r, 
and the psd displays two peaks at the deviated frequencies f.sub.c 
-f.sub.d and f.sub.c +f.sub.d. 
By pre-filtering the transmitted signals, these rates are attenuated on the 
band edge of the filter. This leads to a reduced sensitivity at higher 
rates, with a roll-off related to the filters 5 and 7 in the transmitter, 
as well as to the filters 13 and 14 used to separate the channels in the 
receiver. 
c) Phase relationship 
Because the clock is not recovered from the same signal as the data, but is 
transported independently, it will suffer delays through the various 
filters and recovery processes different from those experienced by the 
data, and this relative delay varies as the data rate is changed. Clock 
edges occuring too soon or too late relative to the centre of the data 
`eye` will result in a sensitivity penalty, if set up and hold times of 
any re-timing element are not met. 
A technique, such as automatically inverting the clock, could be used to 
overcome this problem. Thus, when one edge of the clock is too close to 
the data cross-over, inverting the clock shifts its phase, by n radians, 
towards the centre of the data `eye`. Either inverted or non-inverted 
clock should ensure no noticeable errors due to poor relative phase of the 
data and clock. 
The penalty paid in using higher order filters with steeper roll-offs to 
limit or recover the channels, is the effect on the group delay of the 
filters; a higher order filter having, in general, a larger peak in the 
group delay, where the filter begins to roll-off. This leads to large 
variations in the phase difference of the data and clock at the re-timing 
D-type flip flop 16. 
d) Linearity 
It is essential that, once the two signals are combined, the transmission 
system is linear, until after the signals are split in the receiver. This 
avoids interference between the two channels, due to changes in their 
frequency spectra caused by-non-linearities. The optical transmitter and 
receiver are designed to be linear, to have no noticeable effect on the 
transmitted spectrum. 
e) Modulation/demodulation 
The abilities of the modulator 8 and the demodulator 17 to transmit and 
recover the modulating signal eventually imposes a restriction on the 
maximum data rate r, due to r being too high a percentage of f.sub.c. This 
is an inherent limitation of the system. 
f) Carrier/frequency 
A major source of impairment as the transmission rate is increased is the 
interference between the baseband and carrier signals. 
The modulation index of the carrier signal is, therefore, chosen to yield 
optimum error performance at the approximate maximum rate. Thus, if T is 
the period of the modulating signal, W.sub.C is the carrier frequency, and 
W.sub.d is the single-sided frequency deviation, it can be shown that the 
minimum probability of an error occurs when: 
EQU 2W.sub.d T=3n/2, 
assuming that W.sub.C T&gt;&gt;1 and W.sub.C &gt;&gt;W.sub.d 
For a system operating at maximum rate of 2Mb/s, a frequency deviation, 
f.sub.d, of approximately 750 kHz is the theoretical optimum. To maintain 
the validity of the assumption that W.sub.c &gt;&gt;W.sub.d, the optimum carrier 
frequency for 2Mb/s operation is in the region of 7.5 MHz, this leading to 
the choice of 7.35 MHz for the carrier frequency in the system described 
above. 
The major contributor to the system limitation is the crosstalk between the 
channels. The crosstalk from the baseband channel (clock) to the carrier 
channel, has a slightly greater effect than that from the carrier channel 
to the baseband channel, probably because the clock harmonics in the 
region of the carrier frequency are of larger amplitude than that of the 
modulated signal at the clock frequency. 
The modulation/demodulation process operates successfully at data rates 
beyond 2 Mb/s. For example, a bit error-rate less than 1.times.10.sup.-8 
is achieved at a data rate of 2.2 Mb/s, when the received optical power is 
-34 dBm. 
The system shown in FIGS. 1 and 2 is a simplex system which could be used 
in an information--providing service. In this case, a single transmitter 
would service a plurality of receivers via a passive optical network 
(PON). Alternatively, where duplex operation is required, transceivers 
would be coupled to a PON, each transceiver being consituted by a 
combination of the transmitter of FIG. 1 and the receiver of FIG. 2. 
The system described above could be modified in a number of ways. For 
example, the data signals could be the baseband signal, and the clock 
signals could be modulated onto the carrier. The `clock on carrier` system 
exhibits similar characteristics to the `data on carrier` system, the 
overall sensitivity being -38 dBm. In addition to the crosstalk between 
channels, however, the modulation/demodulation process limits the system 
performance. This is because of the nature of the spectrum of a clock 
signal, and that of a signal when modulated by the clock. Thus, the clock 
information is mainly contained in a single frequency at the clock rate; 
and, when this is used to modulate the carrier, the bandwidth of the 
modulated signal is wider than in the case of data modulating the carrier. 
Clock information is then lost when the carrier channel band-pass filter 
14 attenuates important components of the modulated signal, resulting in 
reduced sensitivity. In order to achieve similar performance to the `data 
on carrier` technique, the `clock on carrier` system would require a 
higher carrier frequency, and a wider band-pass filter in the receiving 
circuitry, leading to overall wider system bandwidth. 
Although FSK is the prefered modulation technique, other methods of 
sub-carrier multiplexing (frequency division multiplexing) could be used 
instead. Moreover, the technique of sub-carrier multiplexing could be 
extended to provide LAN users with a series of flexible networks, using 
the same transmission system, by transmitting the signals from various 
items of office equipment on different carrier frequencies. The use of 
optical fibre in the LAN, and in the in-building environment, can thus be 
made more economical. 
It will be apparent that the system described above enables simultaneous 
transport of clock and data signals at variable rates of transmission. 
This is particularly advantageous in LANs which exploit the potentially 
enormous bandwidth of an optical fibre transmission system. Thus, using 
the system described above, many different signals from different sources 
could each be assigned a carrier frequency, and transported through the 
same optical fibre. At the receiver, various filters could pick off the 
desired carriers, which would undergo a demodulation process, to provide 
the user with the associated data signal. The system could also be used to 
provide an upgrading facility, allowing extra transmission capacity to be 
provided, without the need to change an existing transmission network. 
Faster electro-optic devices will, in general be required for the variable 
rate system described above, as compared to the electro-optic devices used 
in conventional systems operating at the maximum data rate of the variable 
system. As demand for suitable electro-optic devices for LANs increased, 
so the cost penalty for the extra speed should reduce, meaning greater 
network flexibility at little extra cost.