Title: Method and apparatus for transmitting high-frequency signals in optical communication system
Abstract: A method for transmitting high-frequency signals in an optical communication system includes combining an optical signal, the first optical local component from a local light source and the second optical local component from the local light source having a predetermined frequency differential from the first optical local component, selecting the first high-frequency signal which consists of two predetermined electrical components from plural electrical components obtained by the optical frequency mixing process, and mixing the two selected electrical components included in the first high-frequency signal.
Patent Number: 7,006,772 Issued on 02/28/2006 to Kuri, et al.
| Inventors:
|
Kuri; Toshiaki (Tokyo, JP);
Chujo; Wataru (Tokyo, JP);
Kitayama; Ken-ichi (Tokyo, JP)
|
| Assignee:
|
National Institute of Information and Communications Technology (Koganei, JP)
|
| Appl. No.:
|
886257 |
| Filed:
|
June 22, 2001 |
Foreign Application Priority Data
| Jun 22, 2000[JP] | 2000-188363 |
| Current U.S. Class: |
398/204; 398/202 |
| Current Intern'l Class: |
H04B 10/00 (20060101) |
| Field of Search: |
398/202-214
|
References Cited [Referenced By]
U.S. Patent Documents
| 3530298 | Sep., 1970 | Hubbard et al.
| |
| 5140453 | Aug., 1992 | Tsushima et al.
| |
| 5546190 | Aug., 1996 | Hill et al.
| |
| 5896216 | Apr., 1999 | Kikushima et al.
| |
| Foreign Patent Documents |
| 6-152565 | May., 1994 | JP.
| |
| 2000/-244397 | Sep., 2000 | JP.
| |
Primary Examiner: Bello; Agustin
Claims
What is claimed is:
1. A method comprising the steps of:
modulating an optical signal with a high-frequency signal and sending the modulated
optical signal;
receiving the modulated optical signal;
combining the received modulated optical signal, a first optical local component
from a local light source and a second optical local component from the local light
source to produce an electrical signal which includes plural electrical components,
said second optical local component having a predetermined frequency differential
relative to a predetermined frequency of the first optical local component;
selecting a first electrical component from said plural electrical components,
said selected first electrical component being a frequency-shifted carrier component
obtained from the combining of the modulated optical signal with the first optical
local component;
selecting a second electrical component from said plural electrical components,
said selected second electrical component being a frequency-shifted sideband component
obtained from the combining of the modulated optical signal with the second optical
local component; and
demodulating to provide an output high-frequency signal using the first and second
electrical components.
2. A method comprising the steps of:
modulating an optical signal with a high-frequency signal of a predetermined
frequency to provide an optical carrier component and an optical sideband component;
sending the modulated optical signal;
receiving the modulated optical signal;
combining the received modulated optical signal, a first optical local component
from a local light source and a second optical local component from the local light
source to produce an electrical signal which includes plural high-frequency electrical
signal components, said second optical local component having a predetermined frequency
differential relative to a predetermined frequency of the first optical local component;
selecting a first high-frequency electrical signal component from said plural
high-frequency electrical signal components, said first high-frequency electrical
signal component including a frequency-shifted carrier component obtained from
the combining of the modulated optical signal with the first optical local component
and including a frequency-shifted sideband component obtained from the combining
of the modulated optical signal with the second optical local component;
combining the frequency-shifted carrier component and the frequency-shifted sideband component;
selecting a second high-frequency electrical signal component having a frequency
lower by an amount of the predetermined frequency differential relative to a carrier
frequency of the first high-frequency electrical signal component; and
demodulating to provide an output high-frequency electrical signal using the
second high-frequency electrical signal component.
3. A method for transmitting high-frequency signals in an optical communication
system, the method comprising the steps of:
transmitting an optical signal that includes an optical carrier component and
an optical sideband component obtained by modulation with a high-frequency signal
whose frequency is predetermined;
extracting an original high-frequency signal from the transmitted optical signal;
combining an optical signal, a first optical local component from a local light
source and a second optical local component from the local light source having
a predetermined frequency differential from the first optical local component;
making a carrier frequency of the extracted original high-frequency signal coincide
with the predetermined frequency differential; and
selecting two predetermined electrical components from plural electrical components
obtained by an optical frequency mixing process.
4. A method for transmitting high-frequency signals in an optical communication
system, the method comprising the steps of:
transmitting an optical signal that includes an optical carrier component and
an optical sideband component obtained by modulation with a high-frequency signal
whose frequency is predetermined;
combining an optical signal, a first optical local component from a local light
source and a second optical local component from the local light source having
a predetermined frequency differential from the first optical local component;
selecting, as a first optical signal, lights containing the optical sideband
component included in the optical signal and the first optical local component
from the local light source;
selecting, as a second optical signal, lights containing the optical carrier
component included in the optical signal and the second optical local component
from the local light source; and
selecting a signal with a relatively low frequency after mixing the first and
second optical signals.
5. An apparatus comprising:
means for modulating an optical signal with a high-frequency signal and sending
the modulated optical signal;
means for receiving the modulated optical signal;
means for combining the received modulated optical signal, a first optical local
component from a local light source and a second optical local component from the
local light source to produce an electrical signal which includes plural electrical
components, said second optical local component having a predetermined frequency
differential relative to a predetermined frequency of the first optical local component;
means for selecting a first electrical component from said plural electrical
components, said first electrical component being a frequency-shifted carrier component
obtained from the combining of the modulated optical signal with the first optical
local component;
means for selecting a second electrical component from said plural electrical
components, said second electrical component being a frequency-shifted sideband
component obtained from the combining of the modulated optical signal with the
second optical local component; and
means for demodulating to provide an output high-frequency signal using the first
and second electrical components.
6. An apparatus comprising:
means for modulating an optical signal with a high-frequency signal of a predetermined
frequency to provide an optical carrier component and an optical sideband component;
means for sending the modulated optical signal;
means for receiving the modulated optical signal;
means for combining the received modulated optical signal, a first optical local
component from a local light source and a second optical local component from the
local light source to produce an electrical signal which includes plural high-frequency
electrical signal components, said second optical local component having a predetermined
frequency differential relative to a predetermined frequency of the first optical
local component;
means for selecting a first high-frequency electrical signal component from said
plural high-frequency electrical signal components, said first high-frequency electrical
signal component including a frequency-shifted carrier component obtained from
the combining of the modulated optical signal with the first optical local component
and including a frequency-shifted sideband component obtained from the combining
of the modulated optical signal with the second optical local component;
means for combining the frequency-shifted carrier component and the frequency-shifted
sideband component;
means for selecting a second high-frequency electrical signal component having
a frequency lower by an amount of the predetermined frequency differential relative
to a carrier frequency of the first high-frequency electrical signal component; and
means for demodulating to provide an output high-frequency signal using the second
high-frequency electrical signal component.
7. An apparatus for transmitting high-frequency signals in an optical communication
system, the apparatus comprising:
means for transmitting an optical signal that includes an optical carrier component
and an optical sideband component obtained by modulation with a high-frequency
signal whose frequency is predetermined;
means for extracting an original high-frequency signal from the transmitted optical signal;
means for combining an optical signal, a first optical local component from a
local light source and a second optical local component from the local light source
having a predetermined frequency differential from the first optical local component;
means for making a carrier frequency of the extracted original high-frequency
signal coincide with the predetermined frequency differential; and
means for selecting two predetermined electrical components from plural electrical
components obtained by an optical frequency mixing process.
8. An apparatus for transmitting high-frequency signals in an optical communication
system, the apparatus comprising:
means for transmitting an optical signal that includes an optical carrier component
and an optical sideband component obtained by modulation with a high-frequency
signal whose frequency is predetermined;
means for combining an optical signal, a first optical local component from a
local light source and a second optical local component from the local light source
having a predetermined frequency differential from the first optical local component;
means for selecting, as a second optical signal, lights containing the optical
carrier component included in the optical signal and the second optical local component
from the local light source; and
means for selecting a signal with a relatively low frequency after mixing the
first and second optical signals.
9. An apparatus according to claim 6, further comprising:
means for generating the optical signal with a single-mode light source that
produces a single-mode optical carrier; and
means for generating the modulated optical signal, by modulating the optical
signal by the high-frequency signal by optical modulation.
10. An apparatus according to claim 6, wherein the means for combining projects
plural light beams onto a photo-detector to combine the plural light beams and
convert the combined beams to the electrical signal.
11. An apparatus according to claim 10, wherein the photo-detector has a configuration
of a balanced receiver.
12. An apparatus according to claim 6, wherein the local light source comprises:
means for generating the first optical local component and the second optical
local component; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source.
13. An apparatus according to claim 6, wherein the local light source comprises:
means for generating the first optical local component and the second optical
local component; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source with an optical injection locking.
14. An apparatus according to claim 6, wherein the local light source comprises:
means for generating the first optical local component and the second optical
local component; and
a light source that emits two adjacent light waves.
15. An apparatus according to claim 6, wherein the local light source comprises:
means for generating the first optical local component and the second optical
local component; and
means for selectively producing two light waves by modulating light from a single-mode
light source.
16. A method comprising the steps of:
modulating an optical signal with a high-frequency signal and sending the modulated
optical signal;
receiving the modulated optical signal;
combining the received modulated optical signal, a first optical local component
from a local light source and a second optical local component from the local light
source to produce an electrical signal which includes plural high-frequency electrical
signal components, said second optical local component having a predetermined frequency
differential relative to a predetermined frequency of the first optical local component;
selecting from said plural high-frequency electrical signal components a frequency-shifted
carrier component obtained from the combining of the modulated optical signal with
the first optical local component and a frequency-shifted sideband component obtained
from the combining of the modulated optical signal with the second optical local
component; and
combining the frequency-shifted carrier component and the frequency-shifted sideband component.
17. The method according to claim 16, wherein the combined frequency-shifted
carrier component and the frequency-shifted sideband component form a first high-frequency
electrical signal component.
18. The method according to claim 17, further comprising the step of:
selecting a second high-frequency electrical signal component having a frequency
lower by an amount of the predetermined frequency differential relative to a carrier
frequency of the first high-frequency electrical signal component.
19. The method according to claim 16, wherein the first and second optical local
components are generated using an original high frequency signal extracted from
the high frequency signals transmitted.
20. The method according to claim 16, wherein the step of selecting and the step
of combining the frequency-shifted carrier component and the frequency-shifted
sideband component are substituted with the steps of selecting a first optical
signal and a second optical signal, mixing the first and second optical signals
and selecting a signal with a lower frequency, in which the first optical signal
is light including an optical sideband component included in the the first optical
local component from the local light source, and the second optical signal is light
including an optical carrier component included in the second optical signal and
the second optical local component from the local light source.
21. An apparatus comprising:
means for modulating an optical signal with a high-frequency signal and sending
the modulated optical signal;
means for receiving the modulated optical signal;
means for combining the received modulated optical signal, a first optical local
component from a local light source and a second optical local component from the
local light source to produce an electrical signal that includes plural high-frequency
electrical signal components, said second optical local component having a predetermined
frequency differential relative to a predetermined frequency of the first optical
local component;
means for selecting from said plural high-frequency electrical signal components
a frequency-shifted carrier component obtained from the combining of the modulated
optical signal with the first optical local component and a frequency-shifted sideband
component obtained from the combining of the modulated optical signal with the
second optical local component; and
means for combining the frequency-shifted carrier component and the frequency-shifted
sideband component.
22. The apparatus according to claim 21, wherein the combined frequency-shifted
carrier component and the frequency-shifted sideband component form a first high-frequency
electrical signal component.
23. The apparatus according to claim 22, further comprising:
means for selecting a second high-frequency electrical signal component having
a frequency lower by an amount of the predetermined frequency differential relative
to a carrier frequency of the first high-frequency electrical signal component.
24. The apparatus according to claim 21, wherein the first and second optical
local components are generated using an original high frequency signal extracted
from high frequency signals transmitted.
25. The apparatus according to claim 21, wherein the means for selecting and
the means for combining are substituted with means for selecting a first optical
signal and a second optical signal, means for mixing the first and second optical
signals and means for selecting a signal with a lower frequency, in which the first
optical signal is light including an optical sideband component included in the
first optical local component from the local light source, and the second optical
signal is light including an optical carrier component included in the second optical
local component from the local light source.
26. The apparatus according to claim 22, further comprising:
means for generating the optical signal with a single-mode light source that
produces a single-mode optical carrier; and
means for generating the modulated optical signal, by modulating the optical
signal by the high-frequency electrical signal component by optical modulation.
27. The apparatus according to claim 23, further comprising:
means for generating the optical signal with a single-mode light source that
produces a single-mode optical carrier; and
means for generating the modulated optical signal, by modulating the optical
signal by the high frequency signal by optical modulation.
28. The apparatus according to claim 24, further comprising:
means for generating the optical signal with a single-mode light source that
produces a single-mode optical carrier; and
means for generating the modulated optical signal, by modulating the optical
signal by the high frequency signal by optical modulation.
29. The apparatus according to claim 25, further comprising:
means for generating the optical signal with a single-mode light source that
produces a single-mode optical carrier; and
means for generating an optical sideband component, which is the modulated optical
signal, by modulating the optical carrier component by the high frequency signal
by optical modulation.
30. The apparatus according to claim 22, wherein the means for combining projects
plural light beams onto a photo-detector, thereby combining the plural light beams
and converting the combined beams to the electrical signal.
31. The apparatus according to claim 23, wherein the means for combining projects
plural light beams onto a photo-detector, thereby combining the plural light beams
and converting the combined beams to the electrical signal.
32. The apparatus according to claim 24, wherein the means for combining projects
plural light beams onto a photo-detector, thereby combining the plural light beams
and converting the combined beams to the electrical signal.
33. The apparatus according to claim 25, wherein the means for combining projects
plural light beams onto a photo-detector, thereby combining the plural light beams
and converting the combined beams to the electrical signal.
34. The apparatus according to claim 30, wherein the photo-detector has a configuration
of a balanced receiver.
35. The apparatus according to claim 31, wherein the photo-detector has a configuration
of a balanced receiver.
36. The apparatus according to claim 32, wherein the photo-detector has a configuration
of a balanced receiver.
37. The apparatus according to claim 33, wherein the photo-detector has a configuration
of a balanced receiver.
38. The apparatus according to claim 22, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source.
39. The apparatus according to claim 23, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source.
40. The apparatus according to claim 24, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source.
41. The apparatus according to claim 25, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source.
42. The apparatus according to claim 22, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source with an optical injection locking.
43. The apparatus according to claim 23, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source with an optical injection locking.
44. The apparatus according to claim 24, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source with an optical injection locking.
45. The apparatus according to claim 25, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for extracting two predetermined continuous waves from an optical spectrum
emitted from a pulsed light source with an optical injection locking.
46. The apparatus according to claim 22, wherein the local light source comprises:
means for generating the first and second optical local components that utilizes
a light source that emits two adjacent light waves.
47. The apparatus according to claim 23, wherein the local light source comprises:
means for generating the first and second optical local components that utilizes
a light source that emits two adjacent light waves.
48. The apparatus according to claim 24, wherein the local light source comprises:
means for generating the first and second optical local components that utilizes
a light source that emits two adjacent light waves.
49. The apparatus according to claim 25, wherein the local light source comprises:
means for generating the first and second optical local components that utilizes
a light source that emits two adjacent light waves.
50. The apparatus according to claim 22, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for selectively producing two light waves by modulating light from a single-mode
light source.
51. The apparatus according to claim 23, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for selectively producing two light waves by modulating light from a single-mode
light source.
52. The apparatus according to claim 24, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for selectively producing two light waves by modulating light from a single-mode
light source.
53. The apparatus according to claim 25, wherein the local light source comprises:
means for generating the first and second optical local components; and
means for selectively producing two light waves by modulating light from a single-mode
light source.
54. A method comprising the steps of:
modulating an optical signal with a high-frequency signal having a frequency f
RF;
sending the modulated optical signal;
receiving the modulated optical signal;
mixing the received modulated optical signal including a frequency f
1,
a first optical local component having a first frequency f
2+f
LO/2
from a local light source, and a second optical local component having a second
frequency f
2-f
LO/2 from the local light source to produce
an electrical signal which includes plural electrical components, the first frequency
and the second frequency being different from each other by a predetermined frequency
differential f
LO;
selecting a first electrical component from said plural electrical components,
said selected first electrical component being a frequency-shifted carrier component
having a frequency f
1-f
2+f
LO/2 obtained from the
mixing of the modulated optical signal with the first optical local component;
selecting a second electrical component from said plural electrical components,
said selected second electrical component being a frequency-shifted sideband component
having a frequency f
1-f2+f
RF-f
LO/2
obtained the mixing of the modulated optical signal with the second optical local component;
mixing the first and second electrical components to produce a high-frequency
electrical signal having a frequency f
RF-f
LO; and
demodulating to provide an output signal having the frequency f
RF
with the high-frequency electrical signal having the frequency f
RF-f
LO.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for transmitting high-frequency
signals in an optical communication system. Especially, the invention relates to
a method and apparatus for transmitting high-frequency subcarrier signals in an
optical communication system that utilizes the optical heterodyne detection technique
with a dual-mode local light source and is hard to be affected by phase noise of
light sources.
2. Description of the Prior Art
It has been theoretically proven that receiving sensitivity of coherent optical
communication systems with strong local light is superior to that of conventional
intensity-modulation/direct-detection systems. Therefore, the coherent optical
communication systems had been looked to as a next-generation optical communication
system. However, optical amplifiers have been developed in recent years in order
to improve the receiving sensitivity of existing systems, and this, together with
the difficulty of controlling the local light source to reduce phase-noise effect
from light sources in coherent optical communication systems, means that coherent
optical communication systems are not yet practical.
In conventional optical fiber systems transmitting high-frequency signals, an
optical carrier from a light source is modulated, amplified if necessary, and transmitted
to a remote site. The optical signal is again amplified if the transmission attenuates
the signal power, and then demodulated. FIG. 2 shows an example of such a configuration,
which comprises a single-mode light source
101, a high-frequency signal
102, an optical modulator
103, an optical amplifier
104, an
optical transmission line
105, another optical amplifier
106, a compensator
107 that compensates for the optical-fiber dispersion effect, a photo-detector
108, and a demodulator
109 that demodulates the high-frequency signal
detected by the photo-detector
108.
An optical carrier emitted by the light source
101 is modulated with the
optical modulator
103 by the high-frequency signal
102 with payload
data. The optical amplifier
104 amplifies the modulated optical signal up
to the required power for transmission, and then the amplified optical signal is
transmitted along the optical transmission line
105. To compensate for transmission
loss and insertion loss due to the optical-fiber dispersion compensator
107,
the optical amplifier
106 in front of the optical-fiber dispersion compensator
107 amplifies again the optical signal. To eliminate any effect from optical-fiber
dispersion arising in the following photo-detection stage, the optical-fiber dispersion
compensator
107 performs compensation on a wavelength-by-wavelength basis.
The received optical signal is directly detected by the photo-detector
108,
and then the photo-detected signal is demodulated with the demodulator
109
to recover the payload data.
In the conventional system, in order to extend the transmission distance, it
has
been necessary to use multiple optical amplifiers to increase the received signal
power to a level large enough to ensure the desired communication quality. However,
light that is spontaneously emitted from an optical amplifier is amplified with
the following optical amplifiers, causing accumulation of spontaneously emitted
light. This spontaneously emitted light cannot be removed any longer in the case
of analog optical communication systems. Once it is detected by the photo-detector,
the spontaneously emitted light forms noise that corrupts the desired signal quality.
The noise is well-known as an amplified spontaneous emission (ASE) noise. Moreover,
in the conventional system additional optical-fiber dispersion compensators must
be added to nullify the effect of the optical-fiber dispersion. However, since
the dispersion effect strongly depends on both the wavelength and the transmission
distance involved, the optical-fiber dispersion compensators must have been individually
adjusted, making the configuration of the optical communication systems more complex.
In view of the foregoing, an object of the present invention is to provide an
optical communication system not only that does not need to use any optical amplifiers
that give rise to ASE noise, but also in which phase noise of light sources is
removed in principle.
Another object of the invention is to provide a system that is not affected
by optical fiber dispersion effect, thereby eliminating the use of additional optical-fiber
dispersion compensators that are required in the conventional system.
SUMMARY OF THE INVENTION
The optical communication system according to this invention transmits an optical
carrier that is modulated by a high-frequency signal with payload data. On the
receiving side, the optical signal is detected by the optical heterodyne detection
technique with a dual-mode local light. The optical carrier component and a desired
optical sideband component are extracted from photo-detected signals in the first
intermediate-frequency-band, and the two extracted signals are cross-multiplied,
converting to a desired signal in the second intermediate-frequency-band. The same
amount of the inherent phase noise of both the light source generating the optical
carrier and the dual-mode local light source generating the local light, which
is included each in the extracted signal from the first intermediate-frequency-band
signals, is differentially removed when the cross-multiplying is performed.
In accordance with the first aspect of the invention, the above objects are attained
by a method for transmitting high-frequency signals in an optical communication
system, comprising the steps of combining an optical signal, a first optical local
component from a local light source, and a second optical local component from
the local light source having a predetermined frequency differential from the first
optical local component, selecting a first high-frequency signal which consists
of two predetermined electrical components from plural electrical components obtained
by an optical frequency mixing process, and mixing the two selected electrical
components included in the first high-frequency signal.
The second aspect of the method of the invention relates to the use of intermediate-frequency-band
signals, and comprises the steps of transmitting an optical signal that includes
an optical carrier component and an optical sideband component obtained by modulation
with a high-frequency signal whose frequency is predetermined, combining the optical
signal, a first optical local component from a local light source, and a second
optical local component from the local light source having a predetermined frequency
differential from the first optical local component, selecting a first high-frequency
signal which consists of two predetermined electrical components from plural photo-detected
electrical components obtained by an optical frequency mixing process, and selecting
a second high-frequency signal whose frequency is lower by an amount of predetermined
frequency differential than a carrier frequency or the first high-frequency signal
obtained by the optical frequency mixing process.
The third aspect relates to demodulation by optical frequency mixing, and comprises
the steps of transmitting an optical signal that includes an optical carrier component
and an optical sideband component obtained by modulation with a high-frequency
signal whose frequency is predetermined, extracting an original high-frequency
signal from the transmitted optical signal, combining the optical signal, a first
optical local component from a local light source and a second optical local component
from the local light source having a predetermined frequency differential from
the first optical local component, making a carrier frequency of the extracted
original high-frequency signal coincide with the predetermined frequency differential,
and selecting two predetermined electrical components from plural electrical components
obtained by an optical frequency mixing process,
In accordance with a fourth aspect, which relates specifically to the use of
optical
filtering, the method of the invention comprises the steps of transmitting an optical
signal that includes an optical carrier component and an optical sideband component
obtained by modulation with a high-frequency signal whose frequency is predetermined,
combining the optical signal, a first optical local component from a local light
source and a second optical local component from the local light source having
a predetermined frequency differential from the first optical local component,
selecting, as a first optical signal, lights containing the optical sideband component
included in the optical signal and the first optical local component from the local
light source, selecting, as a second optical signal, lights containing the optical
carrier component included in the optical signal and the second optical local component
from the local light source, and selecting a signal with a relatively low frequency
after mixing the first and second optical signals.
In accordance with a fifth aspect of the invention, the above objects are attained
by an apparatus for transmitting high frequency signals in an optical communication
system, the apparatus comprising means for combining an optical signal, a first
optical local component from a local light source and a second optical local component
from the local light source having a predetermined frequency differential from
the first optical local component, means for selecting a first high-frequency signal
which consists of two predetermined electrical components from plural electrical
components obtained by an optical frequency Axing process, and means for mixing
the two selected electrical components included in the first high-frequency signal.
In a sixth aspect relating specifically to the use of intermediate-frequency-band
signals, the apparatus comprises means for transmitting an optical signal that
includes an optical carrier component and an optical sideband component obtained
by modulation with a high-frequency signal whose frequency is predetermined, means
for combining the optical signal, a first optical local component from a local
light source and a second optical local component from the local light source having
a predetermined frequency differential from the first optical local component,
means for selecting a first high-frequency signal which consists of two predetermined
electrical components from plural electrical components obtained by an optical
frequency mixing process, and means for selecting a second high-frequency signal
whose frequency is lower by an amount of predetermined frequency differential than
a carrier frequency of the first high-frequency signal obtained by the optical
frequency mixing process.
In a seventh aspect, the apparatus comprises means for transmitting an optical
signal that includes an optical carrier component and an optical sideband component
obtained by modulation with a high-frequency signal whose frequency is predetermined,
means for extracting the original high-frequency signal from the transmitted optical
sisal, means for combining the optical signal, a first optical local component
from a local light source and a second optical local component from the local light
source having a predetermined frequency differential from the first optical local
component, means for making a carrier frequency of the extracted original high-frequency
signal coincide with the predetermined frequency differential, and means for selecting
two predetermined electrical components from plural electrical components obtained
by an optical frequency mixing process.
In an eighth aspect, relating to the use of an optical filter, the apparatus
comprises
means for transmitting an optical signal that includes an optical carrier component
and an optical sideband component obtained by modulation with a high-frequency
signal whose frequency is predetermined, means for combining the optical signal,
a first optical local component from a local light source and a second optical
local component from the local light source having a predetermined frequency differential
from the first optical local component, means for selecting, as a first optical
signal, lights containing the optical sideband component included in the optical
signal and the first optical local component from the local light source, means
for selecting, as a second optical signal, lights containing the optical carrier
component included in the optical signal and the second optical local component
from the local light source, and means for selecting a signal with a relatively
low frequency after mixing the first and second optical signals.
In a ninth aspects the arrangement used for generating optical signals that are
to be transmitted comprises means for generating an optical carrier component with
a single-mode light source that produces a single-mode optical carrier, and means
for generating an optical sideband component obtained by modulating the optical
carrier component by a high-frequency signal by means of optical modulation.
In a tenth aspect, which relates to the means of optical frequency mixing, the
apparatus uses the combining means which has a configuration that projects plural
light beams onto a photo-detector to be combined and extracts at least part of
combined optical signal as electrical signals.
In an eleventh aspect, in order to eliminate the intensity fluctuations of light
emitted from light sources, the photo-detector is configured as a balanced receiver.
In accordance with a twelfth aspect relating to the use of a dual-mode local
light
source, in the means for generating the first optical local component and the second
optical local component having a predetermined frequency differential from the
first optical local component, two desired continuous waves are obtained from an
optical spectrum emitted from a pulsed light source; from an optical spectrum emitted
from a pulsed light source using optical injection locking; by utilizing a light
source that emits two adjacent lightwaves; and by selectively producing two lightwaves
by modulating light from a single-mode light source.
The above and other objects and features of the invention will become apparent
from the following description made with reference to the drawings.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an optical communication system of a high-frequency
Signal, according to the present invention.
FIG. 2 is a schematic diagram of an example of a conventional radio-on-fiber system.
FIG. 3 is a diagram showing an example of optical spectra before photo-detection
in the invented system.
FIG. 4 is a diagram showing an example of spectra of electrical signals in the
first intermediate-frequency-band.
FIG. 5 shows the measured spectra of a received optical signal and a dual-mode
local light.
FIG. 6 shows the measured spectra of electrical signals in the first intermediate-frequency-band.
FIG. 7 shows the measured spectra of electrical signals in the second intermediate-frequency-band.
FIG. 8 shows the measured bit error rate.
FIG. 9 is a schematic diagram of the second embodiment of the present invention.
FIG. 10 is a schematic diagram of the third embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following, embodiments of the present invention are explained with reference
to examples shown in the drawings. Throughout the embodiments illustrated in the
drawings, identical reference symbols indicate parts having identical or similar
functions or configurations. It is to be understood that the invention is not limited
to the specific examples described hereinafter.
An optical communication system according to this invention transmits an optical
carrier modulated by a high-frequency signal with payload data. This transmission
is done without the signal being amplified. On the receiving side, a dual-mode
local light source is utilized to implement signal detection by optical heterodyne
detection. The optical carrier component and a desired optical sideband component
are extracted from photo-detected signals in the first intermediate-frequency-band.
The two extracted signals are cross-multiplied, converting the high-frequency signal
in the first intermediate-frequency-band to a desired high-frequency signal in
the second intermediate-frequency-band. The same amount of the inherent phase noise
of both the light source generating the optical carrier and the dual-mode local
light source generating the local light, which is included each in the extracted
signal in the first intermediate-frequency-band, is differentially removed when
the cross-multiplying is performed. With this invention, since only two optical
components which are the optical carrier and one of the first-order optical sidebands
are demodulated, there is little degradation of high-frequency signals caused by
optical-fiber dispersion, compared to conventional methods.
The first embodiment of the present invention will now be described with reverence
to the drawings. FIG. 1 shows an embodiment of the apparatus for transmitting high-frequency
signals in an optical communication system, according to the present invention.
In the drawing, reference numeral
201 denotes a single-mode light source
driving at an operating wavelength of 1550.27 nm and an output power of 5 mW. Denoted
by
202 is a high-frequency signal with the subcarrier frequency of 59.6
GHz and the principal bandwidth of 156 MHz×2 due to the payload data, by
203
is an optical intensity modulator, by
204 is an optical transmission line
teat is five meters long, and by
205 is a dual-mode light source with operating
wavelengths of 1549.92 nm and 1550.42 nm, each having an output power of 0.03 mW.
References numeral
206 denotes an optical coupler, and numeral
207
denotes a photo-detector with the 3-dB bandwidth of 50 GHz. Denotes by
208,
211 and
212 are electrical filters with transfer characteristics
ranging from 0.05 to 50 GHz. Reference numeral
209 denotes an electrical
mixer with radio-frequency (RF) and local inputs having the bandwidth of 8 to 12.4
GHz and an intermediate-frequency output having the bandwidth of 0 to 3 GHz. Denoted
by
210 is an electrical filter with a transfer characteristic in the 2 to
4 GHz region, by
213 is an electrical mixer having the same characteristics
as the electrical mixer
209, and by
214 is an electrical demodulator
in the second intermediate-frequency-band, Reference numeral
221 denotes
the first phase-noise-canceling circuit and numeral
222 denotes the second
phase-noise-canceling circuit.
In FIG. 1, an optical carrier f
1 emitted from the single-mode light
source
201 is modulated in the optical intensity modulator
203 by
the high-frequency signal
202 with the carrier frequency f
RF and
payload data. The optical intensity modulator
203 can be replaced with an
amplitude modulator, a frequency modulator or a phase modulator. The high-frequency
signal
202 intends to be a subcarrier signal. The modulated optical signal
is transmitted to the receiving side via the optical transmission line
204.
On the receiving side, the dual-mode light source
205 emits local lights
in a two-frequency mode, with an frequency gap f
LO that provides a slightly
different frequency from the subcarrier frequency of the high-frequency signal
202; the frequencies being (f
2-f
LO/2) and (f
2+f
LO/2).
The optical coupler
206 combines the local lights with the modulated optical
signal. FIG. 3 illustrates the optical signal and the local lights in front of
the photo-detector
207. The photo-detector
207 carries out optical
heterodyne detection, resulting in that the local lights are mixed with the modulated
optical signal. Here, the mixing means a field conversion Grieved by means of a
substance having non-linear response characteristics. As a result of the detection,
the photo-detector
207 generates plural photo-detected signals in the first
intermediate-frequency-band, as shown in FIG. 4. The photo-detector
207
can be a photodiode or phototransistor. Using a photo-detector that is known to
be a balanced receiver makes it possible to reduce the effect of intensity fluctuations
in the optical carrier.
The photo-detected signals in the first intermediate-frequency-band are put into
the first phase-noise-canceling circuit
221, where the hatched portion is
removed by the electrical filter
208 to thereby extract only the desired
signals of frequency (f
1-f
2+F
LO/2) and (f
1-f
2+f
RF-f
LO/2),
and the two extracted signals are put into the electrical mixer
209 with
a square-law response. The electrically mixed and down-converted signal is then
passed through the next electrical filter
210 to obtain a desired signal
in the second intermediate-frequency-band that has the desired frequency (f
RF-f
LO).
The second phase-noise-canceling circuit
222 can be replaced with the
first phase-noise-canceling circuit
221, which might be preferable because
of the better noise characteristics. In the second phase-noise-canceling circuit
222, the photo-detected signals in the first intermediate-frequency-band
shown in FIG. 3 are split, with the electrical filter
211 being used to
extract the first electrical component with the frequency (f
1-f
2+f
LO/2)
and the electrical filter
212 being used to extract the second electrical
component with the frequency (f
1-f
2+f
RF-f
LO/2).
These electrical components are multiplied together with the electrical mixer
213
and extracted via the electrical filter
210 as the second intermediate-frequency-band
signal with the frequency (f
RF-f
LO).
The phase-noise-canceling circuits shown here are examples of the usable configurations.
Provided they can extract the second intermediate-frequency-band signal whose phase
noise originating from light sources has been completely removed, there is no limitation
on methods of the bandwidth limitation of the photo-detected signals in the first
intermediate-frequency-band, the methods of the multiplication or the methods of
the extraction of the second intermediate-frequency-band signal.
The second intermediate-frequency-band signal with the frequency of (f
1-f
2+f
RF-f
LO/2)
which is extracted by the electrical filters
221 and
222 is demodulated
with the electrical demodulator
214 working in the second intermediate-frequency-band,
by means of the well-known demodulating techniques.
There are a number of methods that can be used to produce the first optical
local component from a local light source and the second optical local component
from the local light source having a predetermined frequency differential from
the first optical local component. These can be categorized as; (a) a method of
extracting two continuous waves from the spectrum from a pulsed light source, (b)
a method of extracting two continuous waves from the spectrum from a pulsed light
source that uses an optical injection-locking technique, (c) a method of utilizing
a light source that emits two adjacent lightwaves, and (d) a method of modulating
light from a single-mode light source and selectively generating two lightwaves.
Especially, methods that can be used include (1) using an optical filter to extract
two desired lightwaves from a mode-locked laser diode; (2) using a dual-mode laser
diode; (3) using an optical filter to extract two desired lightwaves from a pulsed
optical-fiber laser; (4) generating two lightwaves by optical injection locking
of a laser; (5) generating two desired lightwaves by four-wave-mixing in an optical
non-linear device; (6) selecting two optical sideband components by sinusoidal
modulation of an optical carrier from a single-mode laser; and (7) generating two
desired lightwaves by resolving degeneration caused by Zeeman division.
From the above explanation, the modulation scheme of high-frequency signals
can be of analog or digital. There is no particular limitation on modulation methods,
multiplexing methods or access methods. However, the electrical demodulator
214
in the second intermediate-frequency-band should be a demodulator to cope with
the modulation format of the original high-frequency signal. Considering such analog
modulation systems that an optical carrier is modulated by a high-frequency signal,
either linear or non-linear modulation may be employed. When frequency modulation
is used, the phase-noise-canceling circuit can be provided with a frequency discriminator,
as in the case of phase modulation.
FIG. 5 shows the measured optical spectrum in the case of the above configuration.
Here, the local light is obtained by suppressed-carrier double-sideband modulation
using a Mach-Zehnder type intensity modulator. In this example, because the carrier
suppression was insufficient, the undesired optical carrier component at frequency
f
2 reined.
FIG. 6 shows the measured spectrum of photo-detected signals in the first intermediate-frequency-band
in the above configuration. The spectrum broadening included in each photo-detected
signals is caused by phase noise of light sources. The undesired optical carrier
component in the local light produces an undesired electrical component with the
frequency of (f
2-f
1) after photo-detection. However, since
the electrical filters
208,
211, and
212 can easily filter
out the Undesired component, the undesired component will not affect the following
electrical processing. FIG. 7 shows the measured spectrum of the second intermediate-frequency-band
signal, which is the output of the electrical mixer
209 in the above configuration.
The multiplication operation differentially removes the phase noise originating
from light sources, leaving just the second intermediate-frequency-band signal
purely with the desired frequency (f
RF-f
LO). Measurements
show that the line-width of the second intermediate-frequency-band signal was less
than 30 Hz, and single-sideband phase noise was -73 dB/Hz at 10 kHz apart from
the carrier. These good results show that the optical heterodyne detection in this
invention is not substantially affected by any phase noise of light sources.
FIG. 8 shows the measured bit error rate for transmission of a millimeter-wave-band,
high-frequency signal with the differential-phase-shift-keying-format data of 155.52
Mb/s and the carrier frequency of 59.6 GHz. The bit error rate is shown as a function
of the optical power put into the photo-detector. From the figures it can be seen
that when there is an attenuation of 2 dB, that is, when the transmission is along
a 10-kilometer-long optical fiber with the transmission loss of 0.2 dB/km, a bit
error rate of less than 10
-9 can be achieved.
FIG. 9 illustrates the second embodiment of the present invention. In this configuration,
an optical signal received at the optical coupler
206 and dual-mode local
light from the dual-mode local light source
205 are combined. Then, an optical
filter
215 is used to select the first group of optical components comprising
an optical sideband component included in the optical signal and the first local
light component, and to select the second group of optical components comprising
an optical carrier component included in the optical signal arc the second local
light component. By individually Mixing optical components each in the group with
the photo-detector
207, photo-detected signals in the first intermediate-frequency-band
are generated. By passing the photo-detected signals through electrical filters
211 and
212, the desired electrical component at (f
1-f
2+f
RF-f
LO/2)
and (f
1-f
2+f
LO/2) are respectively filtered out.
After processing the electrical component at (f
1-f
2+f
RF-f
LO/2)
and (f
1-f
2+f
LO/2) with the multiplier
213
and passing the result through the electrical filter
210, just a desired
signal in the second intermediate-frequency-band at the frequency of (f
RF-f
LO)
is obtained. The filter
215 can be a Fabry-Perot type filter, an arrayed
waveguide grating, and so on.
FIG. 10 illustrates the third embodiment of the present invention. In this arrangement,
the received optical signal is demodulated by being mixed directly with the local
light, without a step of the electrical processing in the second intermediate-frequency-band.
In the configuration shown in FIG. 10, the optical signal received at the optical
coupler
206 and the dual-mode local light from a local light source
220
that is functionally equivalent to a dual-mode light source are combined and passed
through an optical splitter
216. A fraction of the optical signal is used
to extract a high-frequency signal with the frequency of f
RF from the
photo-detector
207, and a subcarrier regenerator
219 is used to extract
the subcarrier frequency f
RF thereof, and the output is made to coincide
with the frequency gap between the local lights from the local light source
220.
This coincidence is accomplished by the well-known phase-locking method. Providing
the optical local source
220 with the ability to make the signals coincide
with each other enables a phase-locked loop to be formed by the optical local source
220, optical coupler
206, optical splitter
216, optical detector
207 and subcarrier regenerator
219. Therefore, when the signals coincide,
the information on the high-frequency signal
202 and f
RF can
be directly demodulated by using another electrical filter
217 to extract
the signals in the first intermediate-frequency-band at around (f
1-f
2+f
RF/2)
from the other photo-detector
207 and multiplying the signals themselves
in the electrical mixer
209 with the square-law response. The frequency-
and phase-locking methods in the above phase-locking loop at f
RF can
be performed by using the other methods already known, and are not limited to the
above method.
In the invention according to the first and fifth aspects, in coherent optical
communication, if optical heterodyne detection using a dual-mode local light is
adopted, phase noise from light sources does not affect the demodulation. Cost
Deduction is also possible because of the cost-effective electrical circuit in
the receiver working at lower frequency. In accordance with the second and sixth
aspects, a high-frequency signal can be transmitted and demodulated even without
optical amplifiers, by getting the gain of the optical heterodyne receiver.
