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| United States Patent |
5,838,473
|
|
Wake
|
November 17, 1998
|
Optical communications system
Abstract
An optical communication system used for the distribution of a signal
includes a central station having an optical source. One or more remote
stations are connected to the central station by an optical network. The
central station outputs onto the network a carrier signal and an
intermediate frequency signal. The remote station includes an optical
mixer/detector which mixes the carrier and intermediate frequency signal
both in the optical domain and outputs an RF electrical signal. The
optical mixer/detector may be a heterojunction bipolar phototransistor
(HBT). The optical source in the central station may be a dual-mode
semiconductor laser.
| Inventors:
|
Wake; David (Levington, GB)
|
| Assignee:
|
British Telecommunications public limited company (London, GB2)
|
| Appl. No.:
|
622345 |
| Filed:
|
March 25, 1996 |
Foreign Application Priority Data
| Oct 13, 1995[GB] | 9520946 |
| Nov 29, 1995[EP] | 95308583 |
| Current U.S. Class: |
398/141; 398/204 |
| Intern'l Class: |
H04B 010/00; H04B 010/06 |
| Field of Search: |
359/125,137,133,145,162,167-168,191,325-326
455/92,118
379/56
|
References Cited [Referenced By]
U.S. Patent Documents
| 4868894 | Sep., 1989 | Gilden et al. | 359/162.
|
| 5535044 | Jul., 1996 | Takeshima et al. | 359/191.
|
Other References
Pavlidis, D "Millimeter-Wave and Optoelectronic Applications of
Heterostructure Integrated Circuits" Proceedings of the SPIE, Oct. 28.
1990.
Pavlidis "Millimeter-Wave and Optoelectronic Applications of
Heterostructure Integrated Circuits", SPIE, vol. 1362, Physical Concepts
of Materials for Novel Optoelectronic Device Aplications II: Device
Physics and Applications (1990).
|
Primary Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Claims
I claim:
1. An optical communications system for the distribution of a signal, said
system comprising:
a central station including an optical source and arranged to output
carrier frequency and intermediate frequency signals modulated on
respective optical carriers:
an optical signal distribution network;
at least one remote station connected to the central station by the optical
network, the at least one remote station including an integral
mixer/detector arranged to receive at an input the carrier frequency and
intermediate frequency signals in the optical domain and to output an
electrical RF signal produced by the mixing of the carrier and
intermediate frequency signals.
2. A system as in claim 1 in which the integral mixer/detector is a
phototransistor.
3. A system as in claim 2 in which the integral mixer/detector is a
heterojunction bipolar phototransistor (HBT).
4. A system as in claim 1 in which the optical network comprises a passive
optical network (PON) which links a plurality of remote stations to the
central station.
5. A system as in claim 1 in which an optical source for the carrier signal
comprises a dual-mode semiconductor laser.
6. A system as in claim 1 comprising part of a cellular mobile radio
system.
7. A remote station for use in an optical communications system, said
station comprising:
a) an optical input for connection to an optical network;
b) an electrical output for outputting an RF signal; and
c) an integral mixer/detector arranged to receive from the optical input
carrier frequency and intermediate frequency signals in the optical domain
and to supply to the electrical output an electrical RF signal produced by
the mixing of the carrier and intermediate frequency signals.
8. A remote station as in claim 7 in which the integral mixer/detector is a
phototransistor.
9. A method of operating an optical communications system comprising a
central station including an optical source, at least one remote station,
and an optical network connecting the central station with each remote
station, the method comprising:
(a) outputting from the central station onto the network a carrier
frequency optical signal and intermediate frequency signal both modulated
onto respective optical carriers; and
(b) at the remote station feeding both the said carrier and intermediate
frequency signals in the optical domain to an integral mixer/detector and
outputting from the integral mixer/detector an RF electrical signal
corresponding to the mixing product of the intermediate and carrier
frequency signals.
10. A method as in claim 9 in which in step (b) the signals in the optical
domain are fed to the optical input of a phototransistor.
11. A method as in claim 10 in which the phototransistor is a
heterojunction bipolar transistor (HBT).
12. An optical communications system for the distribution of a signal, said
system comprising:
a central station including an optical source and arranged to output
carrier frequency and intermediate frequency signals modulated on
respective optical carriers;
an optical signal distribution network; and
a multiplicity of remote stations connected to the central station by the
optical network,
each remote station including an integral mixer/detector comprising a
phototransistor arranged to receive at an input the carrier frequency and
intermediate frequency signals in the optical domain and to output an
electrical RF signal produced by the mixing of the carrier and
intermediate frequency signals.
13. An optical communications system as in claim 12 comprising part of a
cellular mobile radio system.
14. A hybrid optical-optical/electrical domain method of mixing LO and IF
optical signals to produce an RF electrical output signal in an optical
communication system, said method comprising the steps of:
using one integral optical-to-electrical signal transducer with an included
non-linear response characteristic to process two input optical LO and IF
signals and to therein produce corresponding LO and IF signals; and
further mixing said LO and IF signals within said one integral
optical-to-electrical signal transducer to recover and output a modulated
Rf electrical signal therefrom.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communications system, and in
particular to a system suitable for distributing signals to be transmitted
at radio frequencies to mobile or cordless users.
2. Related Art
It has been proposed to use millimetre-wave radio as a short-range
"final-drop" access medium for future telecom services requiring both
medium to high data capacity and the use of mobile or cordless terminals
by the end-user. Such a system might be used to extend services such as
video telephony, or other multimedia services beyond the fixed network to
mobile users. Current proposals suggest the use of radio frequencies above
30 GHz, and particularly in the region of 60 GHz in a cellular network
comprising small cells having a diameter of, e.g., 100 to 200 meters.
The small size of the cell implies a need for a large number of cell
transmitter sites and accordingly it is important that the cost of the
cell transmitter sites, and of the associated signal distribution systems,
are kept as low as possible.
Fibre-fed systems in which signals at the carrier frequency are distributed
optically from a central location potentially offer significant capital
cost savings in terms of infrastructure, as well as simplified management
and control. There are many different ways of configuring the optical link
in these systems, ranging from conventional approaches using external
optical modulators and PIN photodiodes, to more exotic approaches, for
example using harmonic techniques for optical carrier generation, ›1!.
Conventionally, data modulation of the carrier has been carried out at the
source located in the central station, prior to the transmission of the
signal over the optical link to a remote site in a given cell. There has
also been a proposal for the use of a system in which the carrier and the
modulated IF signal are transmitted separately across the optical link and
mixed at the remote site, after the conversion of one or other of the
signals to the electrical domain. This approach however, has resulted in a
significant increase in the cost and complexity of the remote site, and so
has not found commercial acceptance.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an
optical communications system for the distribution of a signal comprising
a central station including an optical source, at least one remote
station, and an optical network connecting the central station to the or
each remote station, the central station outputting onto the optical
network a carrier signal and an intermediate frequency signal, and the
remote station including means for mixing signals and thereby generating
an RF signal,
characterised in that the means for mixing comprise an optical
mixer/detector arranged to mix the carrier and intermediate frequency
signals both input in the optical domain and to output an RF electrical
signal.
The present inventor has realised that by carrying out the mixing of the
signals at the remote site entirely input in the optical domain it is
possible to combine the functions of mixer and photodetector in a single
device, enabling a great saving in the cost and complexity of the remote
site. At the same time, the approach adopted in the present invention is
found to minimise problems with dispersion which, in prior art systems,
have led to signal fading and other forms of signal degradation. The term
"optical network" as used herein encompasses a simple point-to-point link,
as well as more complex topologies.
Preferably the optical mixer/detector is a phototransistor, and more
preferably a heterojunction bipolar phototransistor (HBT).
The use of a phototransistor is preferred as being able to provide a
significant level of gain, together with the functions of mixing the
signals and converting to the electrical domain. The use of an HBT is
found to give a particularly good performance and can be constructed to
facilitate optical access to the device. The present invention is not
however limited to the use of phototransistors, and other devices may be
used. For example, an avalanche photodiode (APD) may be used as the
mixer/detector.
Preferably the network comprises a passive optical network (PON) linking a
plurality of remote stations to the central station.
Where a PON and optical splitting are used to take signals from a single
central station to a number of remote stations then the PON will in
general use erbium-doped fibre amplifiers (EDFA). However, since these
have a fixed wavelength of operation, at around 1550 nm, the transmission
wavelength cannot then be freely chosen to minimise dispersion in the
fibre. Accordingly, it is particularly advantageous in this context to be
able to transmit the carrier and IF signal separately and thereby
substantially eliminate problems due to dispersion.
Preferably the optical source for the carrier signal comprises a dual-mode
semiconductor laser.
The use of a signal from a dual-mode semiconductor laser, as described in
the paper by C. R. Lima, D. Wake and P. A. Davies, Electronics Letters,
2nd Mar. 1995, Vol 31 No. 5 pp 364-365, further optimises the dispersion
performance of the system.
According to a second aspect of the present invention there is provided a
remote station for use in an optical communications system, the remote
station including an input for connection to an optical network, in use
the remote station receiving a carrier signal and an IF signal from the
optical network, and means for mixing the carrier signal and IF signal
received at the input and thereby generating an RF signal, characterised
in that the means for mixing comprise an optical mixer/detector arranged
to mix the carrier and intermediate frequency signals both input in the
optical domain and to output an RF electrical signal.
According to a third aspect of the present invention, there is provided a
method of operating an optical communications system comprising a central
station including an optical source, at least one remote station and an
optical network connecting the central station to the or each remote
station, the method including outputting from the central station onto the
network a carrier signal and an intermediate frequency signal, and the
remote station mixing signals at the carrier frequency and intermediate
frequency and thereby generating an RF signal, characterised in that the
step of mixing the signals is carried out in the optical domain in an
optical mixer/detector and the resulting signal is converted by the mixer
to the electrical domain and output as an RF electrical signal.
BRIEF DESCRIPTION OF THE DRAWINGS
Systems embodying the present invention will now be described in further
detail, by way of example only, and contrasted with the prior art, with
reference to the accompanying drawings in which:
FIGS. 1A and 1B are schematics of a prior art system and a system embodying
the present invention respectively;
FIG. 2 is a plot of frequency of response curves for a photoHBT and a
photodiode;
FIGS. 3A and 3B are charts showing signal and noise levels for a prior art
system and for a system embodying the present invention;
FIG. 4 is a diagram showing a system embodying the present invention and
incorporating a multiple access PON;
FIG. 5 is a schematic cross section of an HBT for use in systems embodying
the present invention;
FIGS. 6A and 6B are graphs illustrating the relative performance at a
photoHBT and a photodiode; and
FIG. 7 shows a dual mode semiconductor laser.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
As shown in FIG. 4, an optical communication system comprises a central
station 1 linked by a fibre network 2 to a number of remote sites 3. Each
remote site 3 includes an antenna 31 and transmits RF signals to mobile
terminals 4 within a respective cell. In the present example, the RF
signals are transmitted in the 62-63 GHz or 65-66 GHz frequency bands. The
mobile terminals may include, for example, audio/video transceivers and
input/output devices for use with video telephony or interactive
multimedia services (IMS).
The central station includes a carrier source 11 and a modulated IF source
12, the IF source being modulated with the data which is to be transmitted
by the remote station to the users. For the carrier frequency source, a
dual-mode semiconductor laser is used. As described in the above-cited
paper by Lima, Wake and Davies, this is a specially modified DFB device in
which oscillation occurs simultaneously on both sides of the Bragg
frequency. The laser is shown in FIG. 7. The 2 mm-long laser has a uniform
grating, operates in the region of 1560 nm and is divided into four
sections of lengths 85,610,610 and 730 .mu.m respectively. It has four
electrical contacts. The sections are independently biased at 20-110 mA
and modulation is applied via the shortest section in order to provide the
highest current density for a given level of drive power. The optical
output is taken from the laser via a lens ended fibre. The facets of the
laser have anti-reflection coatings.
As shown in FIG. 1B, the remote station in this example uses a photo HBT as
a combined mixer/detector. The HBT is shown in FIG. 5 and is a
two-terminal, edge-coupled InGaAs/InP device developed at BT Labs and
designed with low parasitics and efficient optical access ›2!. The HBT has
a dc responsivity of around 200 A/W.
The device and its fabrication are described in further detail in ›2!.
Alternative mixer/detector devices include APDs, and FET phototransistors.
For example, the mixer/detector may be a Germanium or InGaAs APD's.
The technique of the present invention was verified experimentally, and
compared to a more conventional approach using two photodiodes and an
electrical mixer. FIG. 1 shows these configurations; A is the prior art
two photodiode plus mixer approach, and B is the single phototransistor
alternative. The remote end of configuration B embodying the present
invention is much simpler than for A, requiring no mixer or WDM coupler,
and only requiring one photodetector and one amplifier. In each
configuration, the carrier (LO) and data (IF) modulate separate lasers,
whose outputs are combined optically using a WDM coupler. In configuration
A, the signals are separated at the remote site using another WDM coupler,
and detected using photodiodes. Low noise amplifiers (LNA) are used to
bring the signals to a power level required by the electrical mixer. In
configuration B, the photoHBT 32 is used to detect both signals, and the
internal nonlinearities are used to provide the mixing function. A low
noise amplifier 33 is used to bring the signal to the same level as in
configuration A. To compare these approaches, a link similar to
configuration B was constructed, and measurements of signal and noise
levels were performed using a spectrum analyser with a low noise
preamplifier. An optical attenuator was used in place of a long span of
optical fibre to reproduce the effects of losses due to fibre attenuation
and potential splitting losses. A photodiode was also used in place of the
photoHBT for measurements relevant to configuration A. The photodiode used
was developed at BT Labs, and has a dc responsivity of 0.75 A/W and a 3 dB
bandwidth of around 20 GHz ›3!. The frequency response curves of these
devices are shown in FIG. 2. The vertical scale in this figure shows the
response in dB (electrical) referred to a responsivity of 1 A/W. The
photoHBT response has a gain (compared to the photodiode) of around 45 dB
at 140 MHz, and 25 dB at 4 GHz. These frequencies were used in this
experiment (IF of 140 MHz and LO of 4 GHz) in order to demonstrate the
concept. This technique, however, is equally applicable and particularly
relevant for use at mm-wave frequencies.
FIG. 3 shows the signal and noise levels (in 1 Hz bandwidth) obtained for
each configuration as the signal passes through from IF at the source to
RF at the remote site. These levels were obtained from measurement of the
experimental optical link and adjusted downstream for typical amplifier
and mixer performance. The noise level at the photodiode for configuration
A was too low to measure, and was assumed to be negligible compared to the
noise generated by the LNA (since the photocurrent was only 7.5 .mu.A).
The noise at the source was dominated by phase noise, and the level at 1
MHz offset from the carrier was used (-130 dBm). An RF signal level of
OdBm was chosen as the final output power. At each position in the link,
the signal to noise ratio is also shown in the figure. For the photodiode
configuration (A), the signal level drops by 73 dB across the optical
link, and 83 dB of amplification must be used to provide sufficient output
power if a mixer gain of -10 dB is assumed. A S/N ratio of 92 dB is
obtained; most of the degradation being due to the optical link. For the
photoHBT configuration (B), the signal level drops by only 28 dB due to
the high internal gain of the device. However, this gain is also
accompanied by higher noise (-135 dBm) which limits the S/N ratio to only
13 dB more than in the case of the photodiode. After the internal mixing
process the S/N ratio becomes almost identical that of the photodiode
configuration, but only 65 dB of amplification is required (compared to 83
dB for configuration A).
The table below lists the output powers obtained for the IF signal, the
carrier signal (LO) and the RF signal at different bias currents and
voltages. In the table, Iph is the photocurrent. These results are further
illustrated in the graphs of FIGS. 6A and 6B. Although for experimental
convenience, a relatively low radio frequency of 4 GHz is used, in
commercial implementations the higher frequencies of around 60 GHz
referred to above would be used.
REFERENCES
1. D. Wake, I. C. Smith, N. G. Walker, I. D. Henning, and R. D. Carver,
`Video transmission over a 40 GHz radio-fibre link`, Electron. Lett., vol.
28, pp. 2024-2025, 1992.
2. D. Wake, D. J. Newson, M. J. Harlow, and I. D. Henning,
`Optically-biased edge-coupled InP/InGaAs heterojunction
phototransistors`, Electron. Lett., vol. 29, pp. 2217-2219, 1993.
3. D. Wake, R. H. Walling, I. D. Henning, and D. G. Parker, `Planar
junction, top-illuminated GaAs/InP pin photodiode with bandwidth of 25
GHz`, Electron. Lett., vol. 25, pp. 967-968, 1989.
TABLE
__________________________________________________________________________
PERFORMANCE DATA
HPT mixing expts 5 18-5/95
Set-up:
LO LASER
ATTN OUT S Analyser
IF LASER
LO Laser S/N 00811
l1 = 90 mA
l2 = 22 mA
f = 4GHz
P = 14 dBm
IF Laser S/N 00637
l1 = 80 mA
l2 = 22 mA
f = 140MHz
P = 15 dBm
LO Osc. 83620A
IF Osc. 8341B
Sp. Analyser
HP71000
DUT HPT #33723 (AR1099 2)
HSP 1628/2
Device
Attn. dB
iph, mA
Bias, V
P, dBm
IF LO RF
__________________________________________________________________________
0.14 4 4.14
HSP 0 0.52 -5 -36.3
-26.4 -81.3
15 0.016
-5 -67.2
-55.8 -111.3
18 0.008
-5 -73.2
-61.8 -117.3
HPT 18 0.67 -0.25 -34.5
-52.5 -75
1 -0.5 -30.2
-46.3 -88.8
1.3 -0.75 -28.5
-43.4 -66.5
1.6 -1 -27.9
-42.3 -64.9
__________________________________________________________________________
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