U.S. patent application number 12/153755 was filed with the patent office on 2008-09-25 for method and apparatus for duplex communication in hybrid fiber-radio systems.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to A-Jung Kim.
Application Number | 20080232799 12/153755 |
Document ID | / |
Family ID | 28786975 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080232799 |
Kind Code |
A1 |
Kim; A-Jung |
September 25, 2008 |
Method and apparatus for duplex communication in hybrid fiber-radio
systems
Abstract
An apparatus and method for enabling cost-effective duplex
communication by diplexing one of down stream signals for frequency
up-conversion in a hybrid fiber-radio system includes diplexing an
unmodulated mode signal among beating signals between a master
laser and an injection-locked slave laser and using the diplexed
signal for down-conversion in upstream transmission, thereby
eliminating the need for expensive high-frequency local oscillators
for frequency conversion. Higher radio frequency signals can be
generated using beating between basic modes and satellite modes
such as FWM conjugates of the master laser and slave laser.
Cost-effective systems, stabilization of a light source and
improved transmission performance may be achieved by using a
diplexer instead of an expensive high-frequency local
oscillator.
Inventors: |
Kim; A-Jung; (Seoul,
KR) |
Correspondence
Address: |
LEE & MORSE, P.C.
3141 FAIRVIEW PARK DRIVE, SUITE 500
FALLS CHURCH
VA
22042
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
Suwon-city
KR
|
Family ID: |
28786975 |
Appl. No.: |
12/153755 |
Filed: |
May 23, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10423760 |
Apr 25, 2003 |
7379669 |
|
|
12153755 |
|
|
|
|
Current U.S.
Class: |
398/41 ; 398/118;
398/74 |
Current CPC
Class: |
H04B 10/25752
20130101 |
Class at
Publication: |
398/41 ; 398/74;
398/118 |
International
Class: |
H04B 10/24 20060101
H04B010/24; H04J 14/00 20060101 H04J014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2002 |
KR |
2002-22704 |
Claims
1. (canceled)
2. An apparatus for duplex communication in a hybrid fiber-radio
system, the apparatus at a remote station comprising: an
optical-to-electrical converter for converting received optical
signals into electrical signals; a diplexer for splitting data mode
signals containing user source data and unmodulated mode signals; a
radio transmitter for converting the data mode signals into
wireless radio signals and for transmitting the wireless radio
signals wirelessly; a radio receiver for converting received
wireless radio signals into electrical signals; a mixer for mixing
diplexed signals and the electrical signals and outputting mixed
electrical signals; and a laser for converting the mixed electrical
signals into optical signals.
3. (canceled)
4. A method of uplink transmission in a method for duplex
communication in a hybrid fiber-radio system comprising: (a)
converting signals wirelessly received by an antenna into
electrical signals; (b) mixing diplexed unmodulated mode signals
with the electrical signals to form mixed electrical signals; (c)
down-converting a signal band of data; (d) generating optical
signals with a laser driven by the mixed electrical signals; (e)
transmitting the optical signals over an optical fiber; and (f)
receiving the optical signals with a photodetector at a central
office.
5. (canceled)
6. A computer-readable recording medium on which a program for
executing the method claimed in claim 4 is recorded.
7. An apparatus for duplex communication in a hybrid fiber-radio
system as claimed in claim 2, wherein the data mode signals are
modulated data mode signals and the unmodulated mode signals are
unmodulated non-data mode signals.
8. An apparatus for duplex communication in a hybrid fiber-radio
system as claimed in claim 7, wherein the diplexed signals mixed by
the mixer are the unmodulated non-data mode signals.
9. An apparatus for duplex communication in a hybrid fiber-radio
system as claimed in claim 2, further comprising an optical routing
device for launching optical signals from the laser over an optical
transmission fiber.
10. A method of uplink transmission in a method for duplex
communication in a hybrid fiber-radio system as claimed in claim 4,
wherein unmodulated mode signals are unmodulated non-data mode
signals.
11. A communication system comprising a central station and one or
more remote stations, wherein each remote station includes: an
optical-to-electrical converter for converting received optical
signals into electrical signals; a diplexer for splitting data mode
signals containing user source data and unmodulated mode signals; a
radio transmitter for converting the data mode signals into
wireless radio signals and for transmitting the wireless radio
signals wirelessly; a radio receiver for converting received
wireless radio signals into electrical signals; a mixer for mixing
diplexed signals and the electrical signals and outputting mixed
electrical signals; and a laser for converting the mixed electrical
signals into optical signals.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a divisional application based on pending
application Ser. No. 10/423,760, filed Apr. 25, 2003, the entire
contents of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and apparatus for
enabling cost-effective duplex communication by diplexing one of
down stream signals for frequency upconversion in a hybrid
fiber-radio system.
[0004] 2. Description of the Related Art
[0005] Increasing demands for new mobile internet services
including video and interactive services have resulted in the
exhaustion of the available frequency band for wireless
communications, and so triggered the development of the 4th
generation wireless communication system. As the next generation
communication system of the IMT-2000 (International Mobile
Telecommunication), micro/millimeter-wave communication utilizing
signals in the band of 3 GHz to 300 GHz is drawing attention for
broadband wireless communication.
[0006] However, because of its limitation on the transmission
distance and need for high-frequency sources, the
micro/millimeter-wave communication system must be hybrided with
existing wire systems. Low-attenuation, EMI-free optical fiber
transmission is considered the most promising candidate for
long-haul transport of high frequency band wireless signals. Thanks
to the development of optical amplifiers and WDM wavelength
division multiplexing) technology, transmission capacity is
remarkably increased with optical fiber communication systems.
Employing optical fiber in transmitting and amplifying
micro/millimeter-wave signals is advantageous in utilizing existing
core optical fiber systems and mature technologies in more
economical ways. In such hybrid systems, the technology for
generating optical micro/millimeter-waves is the key technology for
broadband communication systems.
[0007] A main issue in hybrid fiber-radio transmission systems is
signal band conversion into a carrier frequency, because up/down
conversion between baseband and carrier frequency in
micro/millimeter-wave systems requires expensive equipment.
[0008] There are three system options: baseband signal
transmission, intermediate frequency (IF) signal feeder
transmission, and optical micro/millimeter-wave transmission. A
disadvantage of baseband transmission is the necessary use of
highly complex outdoor base stations including full SDH/SONET
compliant equipment. A disadvantage of the IF feeder system is the
use of moderately complex outdoor base stations including RF
up/down conversion. For high radio frequency systems, however, both
the baseband transmission and IF feeder transmission methods
require many local oscillators to up/down convert signal
frequencies into carrier frequencies. Therefore, neither baseband
signal transmission nor IF signal feeder transmission are
considered cost-effective solutions for pico-cell
communication.
[0009] Among the system design options for the hybrid fiber-radio
system, generating and transmitting optical micro/millimeter-wave
signals has many advantages. For example, the optical
micro/millimeter-wave transmission system requires relatively
simple outdoor base stations with compact remote antenna units. In
an optical micro/millimeter-wave transmission system, signals at
carrier frequencies are optically generated with light sources at a
central office (CO) and transmitted through optical fibers to a
remote station (RS), where the signals are simply optical to
electrical converted (O/E-converted). Therefore, using a
micro/millimeter-wave transmission system can reduce loads of many
RSs in pico-cell communication networks. Moreover, as the
high-frequency optical signals are generated at a central station,
the optical micro/millimeter-wave transmission system is not only
cost-effective and efficient, but also allows a centralized system
to be implemented. With control functions at the central station,
channel allocation, hand-over, and antenna positioning can be
easily controlled, and the number of subscribers that the system
can support is increased.
[0010] In addition to the advantage of easy system maintenance, the
optical micro/millimeter-wave transmission system has transparency
to modulation types. Since the generated optical signals are
modulated by electrical baseband signals, the optical
micro/millimeter-wave transmission system is flexible to any type
of modulation format. With these advantages, the optical
micro/millimeter-wave transmission system enables cost-effective
and efficient communication and provides flexibility to radio
access systems. Thus, the method of optical micro/millimeter-wave
generation and transmission has been applied to mobile
communications and wireless subscriber loops such as mobile LANs or
broadband wireless local loops (B-WLL) or LMDS systems.
[0011] Despite its many advantages, the optical
micro/millimeter-wave transmission system has problems in upstream
transmission from a subscriber to a head end, because in upstream
transmission, it is necessary to generate an optical
micro/millimeter-wave in a remote base station, which may increase
the cost of RSs. Therefore, it is difficult to apply the optical
micro/millimeter-wave transmission system to duplex communication
systems.
SUMMARY OF THE INVENTION
[0012] To solve the above-described problems, it is a feature of an
embodiment of the present invention to provide an apparatus and
method for enabling duplex communication by generating and
diplexing optical micro/millimeter signals in a hybrid fiber-radio
system. In particular, it is a feature of an embodiment of the
present invention to provide an efficient method for enabling
duplex communication by generating signals in a carrier frequency
mode and in side-band modes. One of the generated modes is diplexed
with a diplexer, and is used for down-conversion in upstream
transmission, thereby eliminating the need for expensive
high-frequency local oscillators for frequency conversion.
[0013] In an effort to achieve the feature described above, in one
embodiment of the present invention there is provided an apparatus
for duplex communication in a hybrid fiber-radio system, the
apparatus at a central office including a central station including
an electrical signal source unit for generating electrical RF
signals; a master laser (ML) driven by the electrical RF signals
for generating optical signals; a local oscillator for generating
intermediate frequency signals; a modulator for converting user
binary data into modulated data signals; a mixer for mixing the
intermediate frequency signals and the modulated data signals; a
slave laser (SL) for outputting the optical signals at a lasing
frequency and the modulated data signals at a down-converted
frequency; an optical routing device for feeding signals from the
ML to the slave laser 16 and for launching signals from the slave
laser 16 over an optical transmission fiber; and a photodetector
for receiving an optical signal from an upstream link.
[0014] In another embodiment, there is provided an apparatus for
duplex communication in a hybrid fiber-radio system, the apparatus
at a remote station including an optical-to-electrical (O/E)
converter for converting received optical signals into electrical
signals; a diplexer for splitting modulated data mode signals
containing user source data and unmodulated non-data mode signals;
a radio transmitter for converting the data mode signals into
wireless radio transmission signals and transmitting the radio
transmission signals wirelessly; a radio receiver for converting
received wireless radio transmission signals into electrical
signals; a mixer for mixing the unmodulated non-data mode signals
from the diplexer and the electrical signals from the radio
receiver; and an optical transmitter for generating optical signals
modulated by the mixed electrical signals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above features and advantages of the present invention
will become more apparent to those of ordinary skill in the art by
describing in detail preferred embodiments thereof with reference
to the attached drawings in which
[0016] FIG. 1 illustrates a configuration of a central station
according to an embodiment of the present invention;
[0017] FIG. 2 illustrates a remote base station according to an
embodiment of the present invention;
[0018] FIG. 3 is a flowchart of a transmitting method according to
an embodiment of the present invention;
[0019] FIG. 4 is a flowchart of a receiving method according to an
embodiment of the present invention;
[0020] FIG. 5 shows the spectrum of optical signals according to
the present invention;
[0021] FIG. 6 shows the spectrum of RF signals according to the
present invention;
[0022] FIGS. 7(a)-7(f) show the spectrum of optical signals
measured by a Fabry-Perot (F-P) interferometric analyzer for
various modulating RF source powers according to the present
invention; and
[0023] FIGS. 8(a)-8(f) show the spectrum of RF signals measured by
a RF analyzer for various modulating RF source powers according to
the present invention.
[0024] FIG. 9 illustrates a recording medium.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Korean Patent Application No. 2002-22704, filed on Apr. 25,
2002, and entitled: "Method And Apparatus For Duplex Communication
In Hybrid Fiber-Radio System" is incorporated by reference herein
in its entirety.
[0026] The present invention will now be described more fully with
reference to the accompanying drawings, in which preferred
embodiments of the invention are shown.
[0027] FIG. 1 illustrates a configuration of a central station in a
hybrid fiber-radio system for duplex communication. The central
station includes a master laser 12 and a slave laser 16. A radio
frequency source unit 11 generates electrical signals at a radio
frequency and is connected to the master laser 12. The radio
frequency source unit 11 drives the master laser 12 using a direct
modulation method. An optical signal output from the master laser
12 is injected into the slave laser 16 by the routing operation of
an optical circulator 17, and mode locks the slave laser 16. A
modulated data signal from a modulator 14 and a signal from a local
oscillator (LO) 13 are mixed by a mixer 15, which outputs a mixed
signal. For example, quadrature phase shift keying (QPSK) may be
employed as a typical modulation method for wireless systems, but
other modulation formats are acceptable for the implementation of
the present invention. QPSK is a digital frequency modulating
technique in which two bit data of 0 and 1 of digital signals are
modulated as one of four phases of a carrier frequency wave.
[0028] The mixed signal is further applied to the slave laser 16 to
modulate the slave laser and then modulated signals are generated
at a frequency translated from a lasing frequency of the slave
laser 16, f.sub.SL by the frequency of the local oscillator 13,
f.sub.LO. The output from the slave laser 16 is composed of a
non-data mode signal injection-locked by the master laser 12, and
data mode signals modulated and band-shifted by the modulator 14
and the local oscillator 13. The output signals are then
transmitted through an optical fiber by the routing operation of
the optical circulator 17. Instead of the optical circulator 17, a
coupler having an isolator may be used as a device for applying an
optical signal from the master laser 12 to the slave laser 16. The
slave laser 16 for the above operation may be either a Fabry Perot
Laser Diode (FP-LD) or a Distributed Feed Back Laser Diode (DFB-LD)
without an internal isolator so that an external optical signal can
be injected thereto. A cost-effective FP-LD is used when only
digital signals are forwarded, while a DFB-LD may be used for the
purpose of transmitting analog broadcasting signals such as in
video broadcasts.
[0029] For uplink transmission, the central station includes a
photodetector 18, which receives signals from a RS.
[0030] FIG. 2 illustrates a remote base station in a hybrid
fiber-radio system for duplex communication according to an
embodiment of the present invention. The optical signals output
from the slave laser 16 are transmitted over the optical fiber and
are received by the remote base station. Then, each optical signal
is converted into an electrical signal by an optical-to-electrical
(O/E) converter 21, and then demultiplexed by a diplexer 22 into an
unmodulated non-data mode signal and a modulated data mode signal
containing user source data. The modulated data mode signals are
transmitted to a radio transmitter 23. The radio transmitter 23
including an antenna converts the modulated data mode signals,
which are electrical, into wireless radio signals and broadcasts
them. The unmodulated non-data mode signal, which is band-shifted,
is diverted and used as a local oscillator for upstream
transmission.
[0031] In uplink communication, a radio receiver 24 converts
signals received through the antenna into electrical signals. A
mixer 25 mixes the electrical received signal and the unmodulated
non-data mode signal output from the diplexer 22. The unmodulated
non-data mode signal is used as a local oscillator that shifts a
carrier frequency during down conversion. The mixed intermediate
frequency (IF) signal is supplied to an optical transmitter 26, a
laser which generates an optical signal, which is then transmitted
through the optical fiber to the photodetector 18 corresponding to
a receiver in the central station. A filter for uplink and downlink
can be achieved using a wavelength division multiplexing (WDM)
coupler.
[0032] The electrical RF signal source unit 11 driving the master
laser 12 in the hybrid fiber-radio system for duplex communication
is employed to generate side-band modes of the master laser 12 and
to stabilize signals from the slave laser 16 by means of injection
locking. The photodetector 18, which is governed by a square law,
uses a direct detection method to detect a master laser signal, a
slave laser signal, and four-wave mixing (FWM) signals, and also
measures beat signals that are generated from beating between these
signals. That is, when a continuous wave is generated by an
adjustment in a bias current and operating temperature of the
master laser 12 and injected into the slave laser 16,
non-degenerated FWM conjugates having frequencies of a difference
between lasing frequencies of the master laser 12 and the slave
laser 16, i.e., f.sub.ML-f.sub.SL, and the sum of the lasing
frequencies of the master laser 12 and the slave laser 16, i.e.,
f.sub.ML+f.sub.SL, are generated due to the non-linear
characteristics of the semiconductor laser. The photon density in a
cavity of the slave laser 16 oscillates with a frequency of a
difference between the lasing frequencies of the master laser 12
and the slave laser 16. Beat signals between the NDFWM conjugate
signals as well as peak signals of the master laser 12 and the
slave laser 16 are detected. The beat signals have different
sources, and thus coherency is weak. Accordingly, they have low
stability or purity. In order to stabilize the beat signals and
reduce phase noise, the master laser 12 is connected to the RF
source unit 11 so that a signal can be directly and electrically
modulated. In this operation, side bands are formed around a lasing
frequency with frequency modulation (FM) in the master laser 12,
and these modes are injected into the cavity of a slave laser 16.
One of the side-band modes of the master laser 12 locks the lasing
mode of the slave laser 16 at a peak frequency and coherency
between the two sources is achieved. As a result, as well as the
basic mode of the slave laser 16 and the master laser 12, satellite
modes including FWM overlapping with the side bands of the master
laser 12 are mode-locked as well, and so the output beat signals
are stabilized, having low fluctuation and phase noise.
[0033] The present invention is based on the fact that FM side band
modes generated at a laser that is directly modulated by a RF
source signal are used to lock another laser to thus generate a
high frequency beat signal that is an unmodulated non-data signal.
The present invention is also based on the fact that a band-shifted
signal that is generated by mixing an IF signal generated from a
local oscillator with a modulated signal containing user source
data is transmitted as a downlink signal. Finally, the present
invention is based further on the fact that the unmodulated
non-data signal is diplexed so that it is used as a local
oscillator for down conversion of an uplink signal when an uplink
signal is transmitted.
[0034] FIG. 3 is a flowchart of a transmitting method performed
throughout a central station, an optical fiber, and a remote base
station during duplex fiber-radio communication according to an
embodiment of the present invention. A central station generates a
radio frequency source signal acting as a stable guide frequency in
step 301. A master laser converts the radio frequency source signal
into an optical signal in step 302. In the meantime, a local
oscillator generates an IF signal in step 303. User source data is
converted into a QPSK modulated signal in step 304. The IF signal
and the QPSK modulated signal are mixed in step 305. Next, a slave
laser converts the mixed signal into a non-data mode optical signal
and a data mode optical signal in step 306. The non-data mode
optical signal and the data mode optical signal are modulated using
the output of the master laser and transmitted to an optical fiber
by the routing operation of an optical circulator in step 307. The
optical signal received through the optical fiber is converted into
an electrical transmission signal in step 308. A diplexer divides
the electrical transmission signal into a data mode signal
containing the user source data and a non-data mode signal
containing the radio frequency source signal in step 309. The data
mode signal is converted into a radio transmission signal and is
then transmitted wirelessly in step 310.
[0035] FIG. 4 is a flowchart of a receiving method during duplex
fiber-radio communication according to an embodiment of the present
invention. A signal wirelessly received through an antenna is
converted into an electrical reception signal in operation 401. An
existing non-data mode signal containing a radio frequency source
signal, which has been obtained by the dividing operation of a
diplexer, is mixed with the electrical reception signal in
operation 402. A transmission laser converts the mixed signal into
an optical signal and transmits the optical signal to an optical
fiber in operation 403. An optical detector receives the optical
signal from the optical fiber in operation 404.
[0036] To summarize the operation of a down link signal
transmission, in a central station, a master laser driven by a RF
source generates an optical signal, which is injected into a slave
laser 16 and locks the lasing mode of the slave laser 16, and-in
turn, an unmodulated optical beat signal with low phase noise is
generated. A local oscillator generates an IF signal. User source
data is converted into a modulated signal by a modulator. The IF
signal and the modulated signal from the modulator are mixed. The
mixed signal is fed to a slave laser, and the output is a data
modulated signal at the lower side-band. The unmodulated mode
signal and the data mode signal are transmitted over an optical
fiber with the routing operation of an optical routing device. The
optical signal received through the optical fiber is converted into
an electrical transmission signal. A diplexer splits the electrical
transmission signal into the data mode signal containing the user
source data and an unmodulated mode signal. The data mode signal is
converted into a radio signal and is then transmitted
wirelessly.
[0037] The uplink transmission process in duplex fiber-radio
communication according to an embodiment of the present invention
is explained as follows. Signals that are wirelessly received by an
antenna are converted into electrical signals. Diplexed unmodulated
non-data mode signals are mixed with the received electrical
signals and the signal band of the data is down-converted. A laser
converts the electrical signals into optical signals and transmits
them over an optical fiber, and a photodetector at a central office
receives the optical signals.
[0038] FIG. 5 shows the spectrum of optical signals according to
the present invention and FIG. 6 shows the spectrum of RF signals
according to the present invention.
[0039] As described above, in a master laser to which a RF source
fm is connected, frequency modulated side bands are formed at
intervals of fm with a center frequency of the lasing frequency
f.sub.ML of the master laser 12 as the result of frequency
modulation. When outputs of the master laser are injected into a
slave laser and a lasing frequency of the slave laser is within
locking range of the modes of the inputs, a peak mode of the slave
laser 16 is locked.
[0040] On the other hand, if the slave laser is modulated by a
mixed signal of a data signal and an IF signal from a local
oscillator, the output signal of the slave laser is down-converted
at a frequency shifted by the frequency of the local oscillator
f.sub.LO from a frequency f.sub.SL of the slave laser, i.e.,
f.sub.SL-f.sub.LO. When these signals are transmitted to a
receiver, beat signals are detected in a photodetector. These beat
signals are induced from different sources but have coherency due
to mode locking of the slave laser 16 and the master laser 12 so
that stability and purity of the beat signal are enhanced. The
detected radio frequency signals include unmodulated mode signals
of f.sub.ML-f.sub.SL and modulated data mode signals of
f.sub.SL-f.sub.LO. The data mode signal is sent to a radio
(micro/mm) transmitter, converted into a wireless radio signal, and
then distributed. The unmodulated mode signal is diplexed by a
diplexer to substitute for a local oscillator.
[0041] In uplink communication, a signal received by an antenna is
converted into an electrical signal. The electrical signal is
down-converted by mixing with an unmodulated mode signal of
f.sub.ML-f.sub.SL and then modulates an uplink laser signal at a
remote station. An optical signal is transmitted over an optical
fiber and is received by a photodetector at a central office.
[0042] Another method to generate a high frequency signal such as
micro/mm-wave is by beating between satellite modes such as
FWM.
[0043] As described above, a high radio frequency can be generated
by beating among basic modes. However, higher radio frequency can
be generated using beating among the basic modes and a satellite
mode such as FWM.
[0044] A continuous wave is generated by adjusting operating
temperature or laser diode bias so that the lasing frequency of the
master laser is sufficiently higher than the lasing frequency of
the slave laser to achieve positive detuning
(f.sub.ML>f.sub.SL). When the continuous wave is applied to the
slave laser, almost pure FWM conjugate signals having frequencies
of various combinations of f.sub.ML and f.sub.SL are generated due
to non-linearity of a semiconductor laser. Here, since a photon
density within the cavity of the slave laser vibrates at a
frequency of about a difference in lasing frequency between the
master laser and the slave laser, beat signals among the FWM
conjugate signals are detected in a direct detector complying with
a square law.
[0045] FIG. 7(a) shows a spectrum of the peaks of continuous waves
each generated by the master laser 12 and the slave laser 16 and
FWM conjugate modes appearing when the continuous waves interact
with each other. The separation of the lasing frequencies of the
master laser 12 and the slave laser 16 does not allow injection
locking initially. f.sub.I and f.sub.J are FWM conjugate
frequencies.
[0046] An interval between f.sub.I and f.sub.ML is the same as an
interval between f.sub.J an f.sub.SL and the interval between the
lasing frequencies of the master laser 12 and the slave laser 16.
The frequency f.sub.SL of an optical signal generated by the slave
laser 16 shifts to a lower frequency due to a reduction in the
carrier density of the slave laser 16 when the laser light
generated by the master laser 12 is injected into the cavity of the
slave laser 16, the effect of which is called `red shift`. Since
the laser light injected into the slave laser 16 serves as a pump
signal for FWM, not as a master laser for locking, the beat signals
generated by the master laser 12 and the slave laser 16 have
serious fluctuation and phase noise because of lack of coherency.
In experiments, the fluctuation is in the order of tens of MHz for
the beat signals of tens of GHz.
[0047] FIGS. 7(b) and 7(c) are the spectrum of optical signals
generated by the master laser 12 and the slave laser 16,
respectively, when they are driven by the electrical signals at a
RF band. In direct modulation of RF signals, frequency modulation
as well as intensity modulation occurs only to cause sideband modes
appearing around the lasing frequencies of the master laser 12 and
the slave laser 16 at an interval of RF modulation frequency
fm.
[0048] FIG. 7(d) shows the red shift that appears in the frequency
f.sub.SL generated by the slave laser 16 due to injection locking.
When the signals of the master laser 12 are injected to the slave
laser 16, one of the sideband modes of the slave laser 16 is
coupled with and locked to one of those of the master laser 12.
Accordingly, f.sub.SL is shifted to f.sub.SL' which is located at a
multiple of RF-modulation frequency fm from f.sub.ML, and then the
output signals from the slave laser 16 are locked. The injection
locking is achieved when the sideband modes of the master laser 12,
which is injected to the slave laser 16, are superposed within the
locking range and coupled with some of the sidebands of the slave
laser 16. With the injection locking of the side modes of f.sub.ML,
f.sub.SL is shifted to f.sub.SL' the difference between which and
the master laser frequency f.sub.ML is a multiple of RF-modulation
frequency fm. The more increased the power Pm of the RF-modulation
signal is, the further red-shifted f.sub.SL and its side modes are,
and the stronger coupling and locking of the slave laser with the
master laser are.
[0049] As f.sub.SL is shifted to f.sub.SL', the FWM conjugate mode
frequencies f.sub.I and f.sub.J are shifted to f.sub.I' and
f.sub.J', respectively, and then locked with each other.
Accordingly, the difference between the master/slave laser and its
adjacent FWM conjugate mode is adjusted from f.sub.b to f.sub.b'
that is a multiple of the RF-modulation frequency fm. All the FWM
conjugate modes are locked with one another, and can generate
stable beat signals with reduced phase noise.
[0050] FIGS. 7(a)-7(f) and FIGS. 8(a)-8(f) are spectrum of the
outputs from the slave laser 16 which are measured with an optical
spectrum analyzer (not shown), and an RF spectrum analyzer (not
shown), respectively, when the laser light from the master laser 12
is injected into the slave laser 16 according to the present
invention. In detail, FIG. 7(a) and FIG. 8(a) are the output
spectrum from the slave laser 16 where no RF-modulated signal is
supplied to the slave laser 16. FIG. 7(b) through (f) and FIG. 8
(b) through (f) are the output spectrum from the slave laser 16
when the RF-modulated signals are supplied to the slave laser 16
with the power Pm of (b) 5 dBm, (c) 8 dBm, (d) 10 dBm, (e) 12 dBm
and (f) 16 dBm, respectively.
[0051] As shown in FIG. 7(a) and FIG. 8(a), when the RF-modulating
source is not connected to the slave laser 16, the outputs are the
master/slave laser signals of frequencies f.sub.SL, f.sub.ML, their
FWM conjugate mode signals, and the beat signals of a frequency of
13.8 GHz.
[0052] If the master and slave lasers are both directly modulated
with the RF source of a frequency fm of 3 GHz, the frequency
f.sub.SL is shifted to a lower frequency as shown in FIGS. 7(b) and
7(c). As the power Pm of the RF-modulating signal increases,
f.sub.SL is further shifted to the lower frequency. This red shift
of the lasing frequency of the slave laser 16 can also be shown in
FIGS. 8(b) and 8(c), in that the frequency of the beat signal
increases as the power Pm increases.
[0053] As seen in FIGS. 8(b), (c) and (f), because of the
electrical RF modulation, additional modes are generated and
measured at the intervals fm from the main beat signal (of a
frequency of a difference between the master and the slave laser
frequencies) and a multiple of the modulated frequency, which are
due to the higher order harmonics of the modulated signal. The
additional mode shown in FIGS. 8(b), (c) and (f) is not superposed
on the main beat signal mode, meaning that the main beat signal is
not locked to the modulated signal and the other modes are just the
higher order harmonics, that is, a multiple of fm.
[0054] Peripheral modes around the main beat signal are shown in
FIGS. 8(c) and 8(f), due to the back reflection by the surface of
the optical fiber pigtail of the slave laser 16 having no internal
isolator.
[0055] As the power Pm is increased, the sideband modes of the
slave laser 16 are red-shifted and locked to those of the master
laser 12. Once locking is achieved, f.sub.SL is fixed at the
spectral position 15 GHz (a multiple of fm) off from f.sub.ML as
shown in FIGS. 7(d) and 7(e).
[0056] In FIGS. 8(d) and 8(e), the beat signal generated within the
locking range has much less phase noise and no peripheral modes
around it. In the case of Pm=12 dBm (FIG. 8(e)), the phase noise of
the RF signal is -96 dBc/Hz at an offset frequency of 100 kHz from
15 GHz. In addition, while the line width of the unlocked signal in
FIG. 8(a) is about 4 MHz, that of the locked signal is limited
mainly by the resolution of the RF spectrum analyzer. Accordingly,
it is noted that the present invention can also contribute to
reducing line width of micro/millimeter-wave band signals. As shown
in FIGS. 8(d) and 8(e), the power of the beat signal increases
remarkably, which means that the output signal results from
locking. As long as the sideband modes of the slave laser 16 are
locked, the frequency f.sub.SL is not further shifted with the
increased Pm. In the experiment, the locking behavior in the slave
laser 16 was maintained over a range of 10 dBm<Pm<13.5 dBm.
But the locking condition was broken and the slave laser 16 got
further red-shifted when the power Pm was greater than 13.5 dBm, as
shown in FIG. 7(f) and FIG. 8(f).
[0057] In result, it is possible to generate stable
micro/millimeter-wave band signals having less phase noise over a
locking region of Pm of 10.about.13.5 dBm. Also, signals of 30, 45
and 60 GHz can be generated by beat signals between the FWM
conjugates. Thus, a wide frequency range of generated signals can
be achieved by the present invention.
[0058] According to the present invention, in optical
micro/millimeter-wave transmission in a hybrid fiber-radio system,
a satellite side mode generated by two lasers and an IF local
oscillator is diplexed and is then used to up-convert frequencies
of upstream signals, so efficient duplex communication can be
accomplished. By using a diplexer instead of an expensive
high-frequency local oscillator, a cost-effective system can be
built. The present invention also stabilizes a light source,
reduces signal deterioration by suppressing a non-linearity effect,
and allows radio frequency conversion.
[0059] In addition to the above-described effects, the present
invention provides a simple and efficient duplex transmission
system which does not need a plurality of high-frequency sub IF
oscillators in a remote base station, and increases flexibility of
a broadband radio communication system such as a broad-wireless
local loop (B-WLL) and so can be widely applied to broadband
wireless systems. When beat signals between satellite modes such as
FWM conjugate modes are used, the present invention can generate
signals at a higher frequency like, such as a millimeter wave
frequency. By the method according to the present invention, a
locking range is as wide as 30 GHz by feeding a signal from a
master laser 12 with a high power thereby allowing ease of control
and tuning. The present invention contributes to the stabilization
of a light source and so reduces phase noise and frequency
fluctuation of the light source. The present invention also reduces
third order inter modulation distortion (IMD) to increase a
spurious-free dynamic range (SFDR), thereby accomplishing
high-quality communication. Since a chirp of the light source is
reduced, transmission dispersion decreases so that signal
deterioration during transmission can be reduced. A load of an
antenna at a remote base station can be reduced allowing many base
stations or mobile devices in a pico-cell communication system to
be simple and compact, thereby lowering costs of the system.
Placement of controls and maintenance systems for a mobile
communication system in one central station-allows channel
allocation control, hand-over control and antenna position and
maintenance to be easily carried out.
[0060] Preferred embodiments of the present invention have been
disclosed herein and, although specific terms are employed, they
are used and are to be interpreted in a generic and descriptive
sense only and not for purpose of limitation. Accordingly, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made without departing from the
spirit and scope of the present invention as set forth in the
following claims.
* * * * *