U.S. patent application number 09/892605 was filed with the patent office on 2002-01-31 for optical transmission system for radio access and high frequency optical transmitter.
Invention is credited to Fuse, Masaru, Sasai, Hiroyuki.
Application Number | 20020012495 09/892605 |
Document ID | / |
Family ID | 27343903 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012495 |
Kind Code |
A1 |
Sasai, Hiroyuki ; et
al. |
January 31, 2002 |
Optical transmission system for radio access and high frequency
optical transmitter
Abstract
A plurality of modulators 110 respectively modulate inputted
baseband signals into IF signals having different frequencies. A
multiplexer 120 multiplexes the plurality of IF signals obtained by
the modulation. An electrical-optical converter 130 converts the
multiplexed IF signals into optical signals through intensity
modulation. A local oscillation signal source 140 outputs a
predetermined local oscillation signal. An external modulator 150
intensity-modulates the optical signal using the local oscillation
signal. An optical branching portion 160 branches the
intensity-modulated optical signal and respectively outputs optical
signals obtained by the branching to radio base stations. An
optical-electrical converter 21k converts the inputted optical
signal into an electric signal, to obtain an RF signal obtained by
frequency-converting the IF signal. Only a component having a
desired radio frequency extracted in a band filter 22k from the RF
signal is transmitted to a subscriber terminal from an antenna 23k.
Frequency conversion from the IF signal to the RF signal is thus
optically performed, thereby making it possible to share the
frequency converter or the electrical-optical converter among the
plurality of radio base stations.
Inventors: |
Sasai, Hiroyuki; (Katano,
JP) ; Fuse, Masaru; (Toyonaka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
27343903 |
Appl. No.: |
09/892605 |
Filed: |
June 28, 2001 |
Current U.S.
Class: |
385/24 ;
398/141 |
Current CPC
Class: |
H04B 10/25753 20130101;
H04B 10/25754 20130101 |
Class at
Publication: |
385/24 ;
359/173 |
International
Class: |
G02B 006/28 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2000 |
JP |
2000-197083 |
Sep 6, 2000 |
JP |
2000-269804 |
Nov 16, 2000 |
JP |
2000-349874 |
Claims
What is claimed is:
1. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting a plurality of radio base stations covering
different service areas and the center station through a plurality
of optical fibers, wherein said center station comprises at least
an electrical-optical conversion portion, receiving one or more
baseband signals as one or more modulated electric signals each
having a predetermined intermediate frequency, for converting the
electric signals into optical signals by intensity modulation, a
local oscillation signal source for outputting a predetermined
local oscillation signal, an external modulation portion for
intensity-modulating the optical signal obtained by the conversion
in said electrical-optical conversion portion using the local
oscillation signal outputted from said local oscillation signal
source, and an optical branching portion for branching the optical
signal intensity-modulated by said external modulation portion, and
respectively outputting optical signals obtained by the branching
to the plurality of optical fibers, and each of said plurality of
radio base stations comprises at least an optical-electrical
conversion portion for converting the optical signal transmitted
through said optical fiber into an electric signal in a radio
frequency band, and a band pass filter for extracting only an
electric signal component in a desired frequency band from the
electric signal obtained by the conversion in said
optical-electrical conversion portion, and feeding the extracted
electric signal component to said antenna portion.
2. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting a plurality of radio base stations covering
different service areas and the center station through a plurality
of optical fibers, wherein said center station comprises at least a
light source for outputting predetermined light, a local
oscillation signal source for outputting a predetermined local
oscillation signal, an external modulation portion for
intensity-modulating the light outputted from said light source
using the local oscillation signal outputted from said local
oscillation signal source, an optical branching portion for
branching an optical signal obtained by the intensity modulation in
said external modulation portion into optical signals whose number
corresponds to the number of said plurality of radio base stations,
and a plurality of IF modulation portions, receiving one or more
modulated electric signals each having a predetermined intermediate
frequency by one or more baseband signals for each of said radio
base stations to which the electric signal is to be transmitted,
for respectively intensity-modulating the optical signals obtained
by the branching in said optical branching portion using the
electric signals, and respectively outputting the modulated optical
signals to the plurality of optical fibers, and each of said
plurality of radio base stations comprises at least an
optical-electrical conversion portion for converting the optical
signal transmitted through said optical fiber into an electric
signal in a radio frequency band, and feeding the electric signal
to said antenna portion.
3. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting a plurality of radio base stations covering
different service areas and the center station through a plurality
of optical fibers, wherein said center station comprises at least
an electrical-optical conversion portion, one or more modulated
electric signals each having a predetermined intermediate frequency
by receiving one or more baseband signals for converting the
electric signals into optical signals by intensity modulation, a
local oscillation signal source for outputting a predetermined
local oscillation signal, a first external modulation portion for
intensity-modulating the optical signal obtained by the conversion
in said electrical-optical conversion portion using the local
oscillation signal outputted from said local oscillation signal
source, a first optical branching portion for branching the optical
signal intensity-modulated by said first external modulation
portion, and respectively outputting optical signals obtained by
the branching to a plurality of downstream optical fibers, a
plurality of first optical-electrical conversion portions for
respectively converting the optical signals transmitted from said
plurality of radio base stations through a plurality of upstream
optical fibers into electric signals in intermediate frequency
bands, and a plurality of demodulation portions for respectively
demodulating the electric signals obtained by the conversion in
said plurality of first optical-electrical conversion portions to
the baseband signals, and each of said plurality of radio base
stations comprises at least a second optical branching portion for
branching the optical signal transmitted through said downstream
optical fiber into two optical signals, a second optical-electrical
conversion portion for converting one of the optical signals
obtained by the branching in said second optical branching portion
into an electric signal in a radio frequency band, a band pass
filter for extracting only an electric signal component in a
desired frequency band from the electric signal obtained by the
conversion in said second optical-electrical conversion portion, a
circulator portion for outputting the electric signal component
extracted by said band pass filter and the radio signal received by
said antenna portion, respectively, to said antenna portion and a
second external modulation portion, and said second external
modulation portion for intensity-modulating the other optical
signal obtained by the branching in said second optical branching
portion using the radio signal outputted from said circulator
portion, and outputting the intensity-modulated optical signal to
said upstream optical fiber.
4. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting a plurality of radio base stations covering
different service areas and the center station through a plurality
of optical fibers, wherein said center station comprises at least a
light source for outputting predetermined light, a local
oscillation signal source for outputting a predetermined local
oscillation signal, a first external modulation portion for
intensity-modulating the light outputted from said light source
using the local oscillation signal outputted from said local
oscillation signal source, a first optical branching portion for
branching an optical signal obtained by the intensity modulation in
said first external modulation portion into optical signals whose
number corresponds to the number of said plurality of radio base
stations, and a plurality of IF modulation portions, receiving one
or more modulated electric signals each having a predetermined
intermediate frequency by one or more baseband signals for each of
said radio base stations to which the electric signal is to be
transmitted, for respectively intensity-modulating the optical
signals obtained by the branching in said first optical branching
portion using the electric signals, and respectively outputting the
modulated optical signals to the plurality of downstream optical
fibers, a plurality of first optical-electrical conversion portions
for respectively converting the optical signals transmitted from
said plurality of radio base stations through a plurality of
upstream optical fibers into electric signals in intermediate
frequency bands, and a plurality of demodulation portions for
respectively demodulating the electric signals obtained by the
conversion in said plurality of first optical-electrical conversion
portions to the baseband signals, and each of said plurality of
radio base stations comprises a second optical branching portion
for branching the optical signal transmitted through said
downstream optical fiber into two optical signals, a second
optical-electrical conversion portion for converting one of the
optical signals obtained by the branching in said second optical
branching portion into an electric signal in a radio frequency
band, a circulator portion for outputting the electric signal
obtained by the conversion in said second optical-electrical
conversion portion and the radio signal received by the said
antenna portion, respectively, to said antenna portion and a second
external modulation portion, and said second external modulation
portion for intensity-modulating the other optical signal obtained
by the branching in said second optical branching portion using the
radio signal outputted from said circulator portion, and outputting
the intensity-modulated optical signal to said upstream optical
fiber.
5. The optical transmission system according to claim 3, wherein a
downstream system through which the optical signal is transmitted
by radio from said radio base station to said subscriber terminal
and an upstream through which the optical signal is transmitting by
radio from said subscriber terminal to said radio base station are
made to differ in a radio frequency to be used.
6. The optical transmission system according to claim 1, wherein
the frequencies of the radio signals respectively used in said
radio base stations are set so as to differ.
7. The optical transmission system according to claim 2, wherein
the frequencies of the radio signals used in said radio base
stations which cover the adjacent service areas are set to differ
from each other.
8. The optical transmission system according to claim 1, wherein
the optical signal outputted from said external modulation portion
is an optical single-sideband signal with a carrier and a
single-sideband component.
9. The optical transmission system according to claim 1, wherein a
Mach-Zehnder type external modulator is used for said external
modulation portion, and a bias point in the external modulator is
set to a point at which light output power is the minimum or
maximum so that the optical signal is intensity-modulated by a
component which is twice the frequency of said local oscillation
signal.
10. The optical transmission system according to claim 1, wherein a
semiconductor laser for converting an electric signal into an
optical signal through direct modulation is used for said
electrical-optical conversion portion.
11. The optical transmission system according to claim 10, wherein
an optical fiber in which the wavelength of the optical signal
outputted from said electrical-optical conversion portion and the
zero dispersion wavelength almost coincide with each other is used
for said optical fiber.
12. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting first to n-th (n is an integer of not less
than two) radio base stations covering different service areas and
the center station through first to n-th upstream and downstream
optical fibers respectively provided so as to correspond to the
radio base stations, wherein said center station comprises first to
n-th electrical-optical conversion portions for respectively
converting one or more signals each having a predetermined
intermediate frequency into first to n-th optical signals having
different wavelengths .lambda.d1 to .lambda.dn uniquely
corresponding to said first to n-th radio base stations, a
wavelength multiplexing portion for multiplexing said first to n-th
optical signals obtained by the conversion, a local oscillation
signal source for outputting a local oscillation signal having a
predetermined frequency, an optical modulation portion for
intensity-modulating the multiplexed optical signals outputted from
said wavelength multiplexing portion using said local oscillation
signal, and a wavelength separation portion for
wavelength-separating the multiplexed optical signals
intensity-modulated into first to n-th modulated optical signals
having wavelengths .lambda.d1 to .lambda.dn, and sending out the
k-th (k=1 to n) modulated optical signal to said k-th downstream
optical fiber, and said k-th radio base station comprises an
optical-electrical conversion portion, receiving said k-th
modulated optical signal having the wavelength .lambda.dk
transmitted through said k-th downstream optical fiber, for
converting the modulated optical signal into an electric signal in
a radio frequency band, and outputting the electric signal.
13. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting first to n-th (n is an integer of not less
than two) radio base stations covering different service areas and
the center station through first to n-th upstream and downstream
optical fibers respectively provided so as to correspond to the
radio base stations, wherein said center station comprises first to
n-th electrical-optical conversion portions for respectively
converting one or more signals each having a predetermined
intermediate frequency into first to n-th downstream optical
signals having different wavelengths .lambda.d1 to .lambda.dn
uniquely corresponding to said first to n-th radio base stations,
first to n-th upstream light sources respectively outputting first
to n-th upstream optical signals having wavelengths .lambda.u1 to
.lambda.un which differ from any of the wavelengths .lambda.d1 to
.lambda.dn and differ from one another, a wavelength multiplexing
portion for multiplexing said first to n-th downstream optical
signals obtained by the conversion and said outputted first to n-th
upstream optical signals, a local oscillation signal source for
outputting a local oscillation signal having a predetermined
frequency, an optical modulation portion for intensity-modulating
the multiplexed optical signals outputted from said wavelength
multiplexing portion using said local oscillation signal, a
wavelength separation portion for wavelength-separating said
multiplexed optical signals intensity-modulated to the first to
n-th modulated downstream optical signals having the wavelengths
.lambda.d1 to .lambda.dn and the first to n-th modulated upstream
optical signals having the wavelengths .lambda.u1 to .lambda.un,
and sending out the k-th (k=1 to n) modulated downstream optical
signal, together with the k-th modulated upstream optical signal,
to said k-th downstream optical fiber, and first to n-th
optical-electrical conversion portions for respectively converting
the optical signals transmitted through said first to n-th upstream
optical fibers into electric signals, and said k-th radio base
station comprises a two-wavelength separation portion, receiving
the optical signal transmitted through said k-th downstream optical
fiber, for separating the optical signal into said k-th modulated
downstream optical signal having the wavelength .lambda.dk and said
k-th modulated upstream optical signal having the wavelength
.lambda.uk, an optical-electrical conversion portion for converting
the k-th modulated downstream optical signal obtained by the
separation in said two-wavelength separation portion into an
electric signal and outputting the electric signal, and an RF
modulation portion for intensity-modulating the k-th modulated
upstream optical signal obtained by the separation in said
two-wavelength separation portion using the inputted radio signal,
and sending out the k-th modulated upstream optical signal
intensity-modulated to said k-th upstream optical fiber.
14. The optical transmission system according to claim 13, wherein
the wavelengths .lambda.d1 to .lambda.dn of said first to n-th
downstream optical signals are set so as to belong to a
predetermined first wavelength band, the wavelengths .lambda.u1 to
.lambda.un of said first to n-th upstream optical signals are set
so as to belong to a predetermined second wavelength band, said
two-wavelength separation portion in said k-th radio base station
wavelength-separates the optical signal transmitted through said
k-th downstream optical fiber into an optical signal in said first
wavelength band and an optical signal in said second wavelength
band, to separate the optical signal into said k-th modulated
downstream optical signal having the wavelength .lambda.dk and said
k-th modulated upstream optical signal having the wavelength
.lambda.uk.
15. An optical transmission system, used for radio access for
transmitting information between a center station and a subscriber
terminal through a radio base station for transmitting and
receiving a radio signal to and from an antenna portion, for
optically transmitting radio signals bidirectionally by
respectively connecting first to n-th (n is an integer of not less
than two) radio base stations covering different service areas and
the center station through first to n-th upstream and downstream
optical fibers respectively provided so as to correspond to the
radio base stations, said center station comprises first to n-th
electrical-optical conversion portions for respectively converting
one or more signals each having a predetermined intermediate
frequency into first to n-th downstream optical signals having
different wavelengths .lambda.d1 to .lambda.dn belonging to a
predetermined first wavelength band uniquely corresponding to said
first to n-th radio base stations, first to n-th upstream light
sources respectively outputting first to n-th upstream optical
signals having wavelengths .lambda.u1 to .lambda.un which differ
from any of the wavelengths .lambda.d1 to .lambda.dn and belong to
a predetermined second wavelength band, a wavelength multiplexing
portion for multiplexing said first to n-th downstream optical
signals obtained by the conversion and said outputted first to n-th
upstream optical signals, a local oscillation signal source for
outputting a local oscillation signal having a predetermined
frequency, an optical modulation portion for intensity-modulating
the multiplexed optical signals outputted from said wavelength
multiplexing portion using said local oscillation signal, a
wavelength separation portion for wavelength-separating said
multiplexed optical signals intensity-modulated to first to n-th
modulated downstream optical signals having the wavelengths
.lambda.d1 to .lambda.dn and the first to n-th modulated upstream
optical signals having the wavelengths .lambda.u1 to .lambda.un,
and sending out the k-th (k=1 to n) modulated downstream optical
signal, together with the k-th modulated upstream optical signal,
to said k-th downstream optical fiber, and first to n-th
optical-electrical conversion portions for respectively converting
the optical signals transmitted through said first to n-th upstream
optical fibers into electric signals, and said k-th radio base
station comprises an electro-absorption type modulation portion,
receiving the optical signal transmitted through said k-th
downstream optical fiber to separate the optical signal into the
k-th modulated downstream optical signal having the wavelength
.lambda.dk and said k-th modulated upstream optical signal having
the wavelength .lambda.uk, converting said k-th modulated
downstream optical signal in said first wavelength band
representing an optical-electrical conversion function into an
electric signal and outputting the electric signal, and
intensity-modulating said k-th modulated upstream optical signal in
said second wavelength band representing an electrical-optical
conversion function using the inputted radio signal and sending out
said k-th modulated upstream optical signal intensity-modulated to
said k-th upstream optical fiber.
16. The optical transmission system according to claim 13, wherein
said first to n-th upstream light sources respectively output said
first to n-th upstream optical signals which uniquely correspond to
said first to n-th downstream optical signals and have wavelengths
.lambda.u1 to .lambda.un respectively different from the
wavelengths .lambda.d1 to .lambda.dn of the first to n-th downward
optical signals by predetermined amounts fs.
17. A high frequency optical transmitter used in a center station,
connected to a plurality of radio base stations respectively
covering different service areas using a plurality of optical
fibers, for optically transmitting radio signals, comprising a
three-branching portion for branching an inputted electric signal
into first and second electric signals which are the same in phase
and a third electric signal which has a phase difference of
90.degree. from the first and second electric signals; an
electrical-optical conversion portion for converting said third
electric signal into a light intensity modulated signal; a first
delay control portion for adjusting the propagation time of said
first electric signal; a second delay control portion for adjusting
the propagation time of said second electric signal; a
two-branching portion for branching an inputted local oscillation
signal into first and second local oscillation signals which are
opposite in phase; a third delay control portion for adjusting the
propagation time of said first local oscillation signal; a fourth
delay control portion for adjusting the propagation time of said
second local oscillation signal; a first multiplexing portion for
multiplexing said first electric signal outputted from said first
delay control portion and said first local oscillation signal
outputted from said third delay control portion; a second
multiplexing portion for multiplexing said second electric signal
outputted from said second delay control portion and said second
local oscillation signal outputted from said fourth delay control
portion; and a differential intensity modulator, having first and
second electrodes, for modulating said light-intensity modulated
signal outputted from said electrical-optical conversion portion by
respectively inputting signals obtained by the multiplexing in said
first and second multiplexing portions to said first and second
electrodes, said first to fourth delay control portions being
adjusted such that said first and second electric signals inputted
to said first and second electrodes of said differential intensity
modulator through said first and second multiplexing portions are
the same in phase, to subject the optical signal outputted from
said electrical-optical conversion portion to phase modulation and
subject the optical signal to optical modulation which is the same
in amount as and is opposite in phase to the frequency deviation
(an FM index) of a light frequency modulation component of the
optical signal.
18. A high frequency optical transmitter used in a center station,
connected to a plurality of radio base stations respectively
covering different service areas using a plurality of optical
fibers, for optically transmitting radio signals, comprising a
three-branching portion for branching an inputted electric signal
into first and second electric signals which are the same in phase
and a third electric signal which has a phase difference of
90.degree. from the first and second electric signals; an
electrical-optical conversion portion for converting said third
electric signal into a light intensity modulated signal; a first
delay control portion for adjusting the propagation time of said
first electric signal; a second delay control portion for adjusting
the propagation time of said second electric signal; a
two-branching portion for branching an inputted local oscillation
signal into first and second local oscillation signals which have a
difference of 90.degree. to each other; a third delay control
portion for adjusting the propagation time of said first local
oscillation signal; a fourth delay control portion for adjusting
the propagation time of said second local oscillation signal; a
first multiplexing portion for multiplexing said first electric
signal outputted from said first delay control portion and said
first local oscillation signal outputted from said third delay
control portion; a second multiplexing portion for multiplexing
said second electric signal outputted from said second delay
control portion and said second local oscillation signal outputted
from said fourth delay control portion; and a differential
intensity modulator, having first and second electrodes, for
modulating said light intensity modulated signal outputted from
said electrical-optical conversion portion by respectively
inputting signals obtained by the multiplexing in said first and
second multiplexing portions to said first and second electrodes,
said first and second delay control portions being adjusted such
that a phase difference between said first and second electric
signals inputted to said first and second electrodes of said
differential intensity modulator through said first and second
multiplexing portions is zero, to subject the optical signal
outputted from said electrical-optical conversion portion to phase
modulation and subject the optical signal to optical modulation
which is the same in amount as and is opposite in phase to the
frequency deviation of a light frequency modulation component of
the optical signal, said third and fourth delay control portions
being adjusted such that said first and second local oscillation
signals inputted to said first and second electrodes of said
differential intensity modulator through said first and second
multiplexing portions have a difference of 90.degree. to each
other, to subject said optical signal to optical side-band
modulation with a light carrier.
19. A high frequency optical transmitter used in a center station,
connected to a plurality of radio base stations respectively
covering different service areas using a plurality of optical
fibers, for optically transmitting radio signals, comprising: a
two-branching portion for branching an inputted electric signal
into first and second electric signals which have a difference of
90.degree. to each other; an electrical-optical conversion portion
for converting said first electric signal into a light intensity
modulated signal; a delay control portion for adjusting the
propagation time of said second electric signal; and an integrated
modulation portion, comprising a phase modulation portion and an
intensity modulation portion formed on the same substrate, for
modulating said light intensity modulated signal outputted from
said electrical-optical conversion portion by inputting said second
electric signal outputted from said delay control portion to the
phase modulation portion and inputting an inputted local
oscillation signal to the intensity modulation portion, in said
phase modulation portion, the optical signal outputted from said
electrical-optical conversion portion being subjected to phase
modulation and subjected to optical modulation which is opposite in
phase to the frequency deviation of a light frequency modulation
component of the optical signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to an optical
transmission system for radio access and a high frequency optical
transmitter, and more particularly, to an optical transmission
system for connecting, in radio access for coupling a center
station and a plurality of subscriber terminals with a radio signal
(a high frequency radio signal in a microwave band, a millimeter
wave band, or the like), the center station and a radio base
station by an optical fiber, and a high frequency optical
transmitter used for the system.
[0003] 2. Description of the Background Art
[0004] FIG. 17 illustrates an example of the configuration of a
conventional optical transmission system used for radio access for
connecting a center station and a subscriber terminal through a
radio base station for transmitting and receiving a radio
signal.
[0005] The conventional optical transmission system shown in FIG.
17 is constructed by respectively connecting a center station 600
to a plurality of radio base stations 701 to 70n (n is an integer
of not less than two; which is the same in the following present
specification) through a plurality of downstream (from the center
station to the radio base stations) optical fibers 801 to 80n. The
center station 600 includes a plurality of electrical-optical
converters 611 to 61n respectively corresponding to the plurality
of radio base stations 701 to 70n. The radio base stations 701 to
70n respectively include optical-electrical converters 711 to 71n,
modulators 721 to 72n, frequency converters 731 to 73n, local
oscillation signal sources 741 to 74n, and antennas 751 to 75n. The
operation of the conventional optical transmission system will be
described.
[0006] In the center station 600, information to be transmitted to
the subscriber terminal through the radio base station 70k is
inputted in the form of a baseband signal to an input terminal 6k
(k=1 to n; which is the same in the following present
specification). The electrical-optical converter 61k converts the
baseband signal inputted from the input terminal 6k into an optical
signal. The optical signal outputted from the electrical-optical
converter 61k is transmitted to the radio base station 70k from the
center station 600 through the downstream optical fiber 80k.
[0007] In the radio base station 70k, the optical signal
transmitted from the center station 600 is inputted to the
optical-electrical converter 71k. The optical-electrical converter
71k converts the inputted optical signal into an electric signal.
The modulator 72k converts the electric signal obtained by the
conversion into a signal having an intermediate frequency (an IF
signal). The local oscillation signal source 74k outputs a local
oscillation signal having a predetermined frequency. The frequency
converter 73k receives the IF signal and the local oscillation
signal, and converts the IF signal into a signal having a radio
frequency (an RF signal) using the local oscillation signal. The RF
signal is released to a space through the antenna 75k.
[0008] FIG. 18 shows, in a case where the center station 600 and
the plurality of radio base stations 701 to 707 are connected to
each other, the concept of a service area covered by each of the
radio base stations. Areas 901 to 907 shown in FIG. 18 represent
service areas respectively covered by the radio base stations 701
to 707.
[0009] Light signals respectively having different information are
respectively transmitted to the radio base stations 701 to 707 from
the center station 600 through the downstream optical fibers 801 to
807. In order to avoid interference between the adjacent service
areas, the plurality of radio base stations 701 to 707 respectively
change the frequencies of local oscillation signals outputted from
the local oscillation signal sources 741 to 747 provided therein
and convert IF signals into RF signals having different frequencies
(fd1 to fd7), to perform radio transmission to the subscriber
terminals. With respect to the radio base stations respectively
covering the service areas which are not adjacent to each other
(which correspond to the radio base stations 701, 705, and 706 in
the example of FIG. 18), the same radio frequency (fd1=fd5=fd6) may
be set.
[0010] However, when the different information are optically
transmitted from the center station 600 to the plurality of radio
base stations 701 to 70n and then to many subscriber terminals, as
shown in FIGS. 17 and 18, however, various problems arise as
follows.
[0011] The first problem is that the electrical-optical converters
611 to 61n, whose number (=n) corresponds to the number of the
radio base stations 701 to 70n, are required in the center station
600.
[0012] The second problem is that the expensive frequency
converters 731 to 73n for frequency-converting the IF signals into
the RF signals are respectively required in the plurality of radio
base stations 701 to 70n.
[0013] The third problem is that when information to be transmitted
to a lot of subscriber terminals are transmitted upon being
time-division multiplexed, a multiplexer is required in the center
station 600. In this case, separators are respectively required in
the radio base stations 701 to 70n, and high-speed modulation
processing is required for each of the modulators 721 to 72n.
[0014] The fourth problem is that when the capacity of the one
radio base station 70k (the amount of information which can be
transmitted from the antenna 75k to the subscriber terminal) is
increased for a new subscriber terminal, each of components other
than the antenna 75k must be additionally installed in the radio
base station 70k, and a multiplexer for multiplexing the RF signals
is also required. Particularly when the position of the subscriber
terminal to be added is the position where a substantially
unobstructed line-of-sight propagation path by cannot be ensured
from the existing radio base station 70k, the components shown in
FIG. 17 must be all newly installed such that the line-of-sight
propagation path can be ensured.
[0015] The fifth problem is that the frequencies of the local
oscillation signals outputted by the local oscillation signal
source 74k in the radio base station 70k must be made to differ in
order to avoid the interference between the adjacent service areas.
Therefore, different components or different adjustments (if with
the same components) are required for each radio base station
70k.
[0016] FIG. 19 illustrates the configuration of another
conventional optical transmission system in which the configuration
of each of radio base stations 701 to 70n is simplified.
[0017] The conventional optical transmission system shown in FIG.
19 is constructed by respectively connecting a center station 600
and the plurality of radio base stations 701 to 70n through a
plurality of downstream optical fibers 801 to 80n. The center
station 600 respectively include modulators 621 to 62n, frequency
converters 631 to 63n, local oscillation signal sources 641 to 64n,
external modulators 651 to 65n, and light sources 661 to 66n so as
to correspond to the radio base stations 701 to 70n. The radio base
stations 701 to 70n respectively include optical-electrical
converters 711 to 71n and antennas 751 to 75n. The operation of the
conventional optical transmission system will be described.
[0018] In the center station 600, information to be transmitted to
the radio base station 70k is inputted in the form of a baseband
signal to an input terminal 6k. The modulator 62k modulates the
baseband signal inputted from the input terminal 6k to an IF
signal. The local oscillation signal source 64k outputs a local
oscillation signal having a predetermined frequency. The frequency
converter 63k receives the IF signal obtained by the modulation in
the modulator 62k and the local oscillation signal outputted from
the local oscillation signal source 64k, and frequency-converts the
IF signal into an RF signal using the local oscillation signal. The
light source 66k generates an optical signal having a predetermined
wavelength. The external modulator 65k receives the RF signal
obtained by the conversion in the frequency converter 63k and the
optical signal outputted from the light source 66k, and
intensity-modulates the optical signal using the RF signal. The
intensity-modulated optical signal is transmitted to the radio base
station 70k through the downstream optical fiber 80k.
[0019] The optical signal transmitted from the center station 600
is inputted to the radio base station 70k upon propagating through
the downstream optical fiber 80k.
[0020] In the radio base station 70k, the optical-electrical
converter 71k converts the inputted optical signal into an electric
signal, to output an RF signal. The outputted RF signal is released
into a space from the antenna portion 75k to the subscriber
terminal as a radio signal.
[0021] In the conventional optical transmission system, therefore,
the IF signal is frequency-converted into the RF signal in the
center station 600. Accordingly, the radio base stations 701 to 70n
respectively require only the optical-electrical converters 711 to
71n in addition to the antenna portions 751 to 75n. Therefore, the
conventional light transmission system has the effect of
miniaturizing each of the radio base stations 701 to 70n.
[0022] In the configuration of the other conventional optical
transmission system shown in FIG. 19, however, the frequency
converters 631 to 63n, the external modulators 651 to 65n, and the
optical-electrical converters 711 to 71n must be high frequency
devices (active devices) respectively operating in radio frequency
bands. Such high-frequency devices are generally expensive. In such
a configuration that the center station 600 manages the plurality
of radio base stations 701 to 70n as in the other conventional
optical transmission system, therefore, n expensive devices are
required, so that the entire system becomes very expensive.
SUMMARY OF THE INVENTION
[0023] Therefore, an object of the present invention is to provide
an optical transmission system for radio access and a high
frequency optical transmitter in which the problems above are
solved by so constructing that transmission signals directed toward
a plurality of radio base stations are collectively
frequency-converted in a center station and optically performing
frequency-conversion from an IF signal to an RF signal (further
frequency conversion from an RF signal into an IF signal) through
an optical transmission path.
[0024] The present invention has the following features to attain
the object above.
[0025] A first aspect of the present invention is directed to an
optical transmission system, used for radio access for transmitting
information between a center station and a subscriber terminal
through a radio base station for transmitting and receiving a radio
signal to and from an antenna, for optically transmitting radio
signals bidirectionally by respectively connecting a plurality of
radio base stations covering different service areas and the center
station through a plurality of optical fibers, wherein
[0026] the center station includes at least
[0027] an electrical-optical converter, receiving one or more
baseband signals as one or more modulated electric signals each
having a predetermined intermediate frequency, for converting the
electric signals into optical signals by intensity modulation,
[0028] a local oscillation signal source for outputting a
predetermined local oscillation signal,
[0029] an external modulator for intensity-modulating the optical
signal obtained by the conversion in the electrical-optical
converter using the local oscillation signal outputted from the
local oscillation signal source, and
[0030] an optical branching portion for branching the optical
signal intensity-modulated by the external modulator, and
respectively outputting optical signals obtained by the branching
to the plurality of optical fibers, and
[0031] each of the plurality of radio base stations includes at
least
[0032] an optical-electrical converter for converting the optical
signal transmitted through the optical fiber into an electric
signal in a radio frequency band, and
[0033] a band pass filter for extracting only an electric signal
component in a desired frequency band from the electric signal
obtained by the conversion in the optical-electrical converter, and
feeding the extracted electric signal component to the antenna.
[0034] According to the first aspect, with respect to a downstream
system, the electrical-optical converter can be thus shared, and
the external modulator for performing optical frequency conversion
can be shared because no electrical frequency converter is
required. Further, the signals can be multiplexed more easily,
thereby making it possible to more easily increase the transmission
capacity with increasing the number of subscriber terminals, as
compared with the conventional optical transmission system.
[0035] The center station may include at least
[0036] a light source for outputting predetermined light,
[0037] a local oscillation signal source for outputting a
predetermined local oscillation signal,
[0038] an external modulator for intensity-modulating the light
outputted from the light source using the local oscillation signal
outputted from the local oscillation signal source,
[0039] an optical branching portion for branching an optical signal
obtained by the intensity modulation in the external modulator into
optical signals whose number corresponds to the number of the
plurality of radio base stations, and
[0040] a plurality of IF modulators, receiving one or more
modulated electric signals each having a predetermined intermediate
frequency by one or more baseband signals for each of the radio
base stations to which the electric signal is to be transmitted,
for respectively intensity-modulating the optical signals obtained
by the branching in the optical branching portion using the
electric signals, and respectively outputting the modulated optical
signals to the plurality of optical fibers, and
[0041] each of the plurality of radio base stations may include at
least an optical-electrical converter for converting the optical
signal transmitted through the optical fiber into an electric
signal in a radio frequency band, and feeding the electric signal
to the antenna.
[0042] In such a configuration, with respect to a downstream
system, the external converter for performing optical frequency
conversion can be shared because no electrical frequency converter
is required. Further, the signals can be multiplexed more easily,
thereby making it possible to more easily increase the transmission
capacity with increasing the number of subscriber terminals, as
compared with the conventional optical transmission system. In
addition, no band filter is required. Therefore, radio base
stations having the same configuration can be installed even when
service areas respectively covered by the radio base stations
differ.
[0043] The center station may include at least
[0044] an electrical-optical converter, receiving one or more
modulated electric signals each having a predetermined intermediate
frequency by one or more baseband signals for converting the
electric signals into optical signals by intensity modulation,
[0045] a local oscillation signal source for outputting a
predetermined local oscillation signal,
[0046] a first external modulator for intensity-modulating the
optical signal obtained by the conversion in the electrical-optical
converter using the local oscillation signal outputted from the
local oscillation signal source,
[0047] a first optical branching portion for branching the optical
signal intensity-modulated by the first external modulator, and
respectively outputting optical signals obtained by the branching
to a plurality of downstream optical fibers,
[0048] a plurality of first optical-electrical converters for
respectively converting the optical signals transmitted from the
plurality of radio base stations through a plurality of upstream
optical fibers into electric signals in intermediate frequency
bands, and
[0049] a plurality of demodulators for respectively demodulating
the electric signals obtained by the conversion in the plurality of
first optical-electrical converters to the baseband signals,
and
[0050] each of the plurality of radio base stations may include at
least
[0051] a second optical branching portion for branching the optical
signal transmitted through the downstream optical fiber into two
optical signals,
[0052] a second optical-electrical converter for converting one of
the optical signals obtained by the branching in the second optical
branching portion into an electric signal in a radio frequency
band,
[0053] a band pass filter for extracting only an electric signal
component in a desired frequency band from the electric signal
obtained by the conversion in the second optical-electrical
converter,
[0054] a circulator for outputting the electric signal component
extracted by the band pass filter and the radio signal received by
the antenna, respectively, to the antenna and a second external
modulator, and
[0055] the second external modulator for intensity-modulating the
other optical signal obtained by the branching in the second
optical branching portion using the radio signal outputted from the
circulator, and outputting the intensity-modulated optical signal
to the upstream optical fiber.
[0056] In such a configuration, with respect to both upstream and
downstream systems, the electrical-optical converter can be shared,
and the external modulator for performing optical frequency
conversion can be shared because no electrical frequency converter
is required. Further, the signals can be multiplexed more easily,
thereby making it possible to more easily increase the transmission
capacity with increasing the number of subscriber terminals, as
compared with the conventional optical transmission system.
[0057] The center station may include at least
[0058] a light source for outputting predetermined light,
[0059] a local oscillation signal source for outputting a
predetermined local oscillation signal,
[0060] a first external modulator for intensity-modulating the
light outputted from the light source using the local oscillation
signal outputted from the local oscillation signal source,
[0061] a first optical branching portion for branching an optical
signal obtained by the intensity modulation in the first external
modulator into optical signals whose number corresponds to the
number of the plurality of radio base stations,
[0062] a plurality of IF modulators, receiving one or more
modulated electric signals each having a predetermined intermediate
frequency by one or more baseband signals for each of the radio
base stations to which the electric signal is to be transmitted,
for respectively intensity-modulating the optical signals obtained
by the branching in the first optical branching portion using the
electric signals, and respectively outputting the modulated optical
signals to the plurality of downstream optical fibers, and
[0063] a plurality of first optical-electrical converters for
respectively converting the optical signals transmitted from the
plurality of radio base stations through a plurality of upstream
optical fibers into electric signals in intermediate frequency
bands, and
[0064] a plurality of demodulators for respectively demodulating
the electric signals obtained by the conversion in the plurality of
first optical-electrical converters to the baseband signals,
and
[0065] each of the plurality of radio base stations may include a
second optical branching portion for branching the optical signal
transmitted through the downstream optical fiber into two optical
signals,
[0066] a second optical-electrical converter for converting one of
the optical signals obtained by the branching in the second optical
branching portion into an electric signal in a radio frequency
band,
[0067] a circulator for outputting the electric signal obtained by
the conversion in the second optical-electrical converter and the
radio signal received by the antenna, respectively, to the antenna
and a second external modulator, and
[0068] the second external modulator for intensity-modulating the
other optical signal obtained by the branching in the second
optical branching portion using the radio signal outputted from the
circulator, and outputting the intensity-modulated optical signal
to the upstream optical fiber.
[0069] In such a configuration, with respect to both upstream and
downstream systems, the external modulator for performing optical
frequency conversion can be shared because no electric frequency
converter is required. Further, the signals can be multiplexed more
easily, thereby making it possible to more easily increase the
transmission capacity with increasing the number of subscriber
terminals, as compared with the conventional optical transmission
system. In addition, no band filter is required. Therefore, radio
base stations having the same configuration can be installed even
when the service areas respectively covered by the radio base
stations differ.
[0070] Preferably, a downstream system through which the optical
signal is transmitted by radio from the radio base station to the
subscriber terminal and an upstream system through which the
optical signal is transmitted by radio from the subscriber terminal
to the radio base station are made to differ in a radio frequency
to be used. Consequently, the radio signals may not interfere with
each other between the upstream system and the downstream
system.
[0071] Preferably, the frequencies of the radio signals
respectively used in the radio base stations are set so as to
differ, or the frequencies of the radio signals used in the radio
base stations which cover the adjacent service areas are set to
differ from each other. Consequently, the radio signals may not
interfere with each other between the adjacent service areas.
[0072] Preferably, the optical signal outputted from the external
modulator is an optical single-sideband signal with a carrier and a
single-sideband component. Alternatively, a Mach-Zehnder type
external modulator is used for the external modulator, and a bias
point in the external modulator is set to a point at which light
output power is the minimum or maximum so that the optical signal
is intensity-modulated by a component which is twice the frequency
of the local oscillation signal. Consequently, it is possible to
avoid the decrease in the level of the electric signal after
optical-electrical conversion which has conventionally occurred
when the optical fiber has dispersion characteristics. Further, in
the latter case, the oscillation frequency of the local oscillation
signal may be one-half that in the conventional example, and the
operation frequencies of the local oscillation signal source and
the external modulator may be one-half that in the conventional
example.
[0073] Preferably, a semiconductor laser for converting an electric
signal into an optical signal through direct modulation is used for
the electrical-optical converter. Consequently, the multiplexed
electric signals in intermediate frequency bands can be converted
into the optical signals through direct modulation, thereby making
it possible to perform the electrical-optical conversion simply and
at low cost.
[0074] More preferably, an optical fiber in which the wavelength of
the optical signal outputted from the electrical-optical converter
and the zero dispersion wavelength almost coincide with each other
is used for the optical fiber. Consequently, the wavelength of the
optical signal and the zero dispersion wavelength of the optical
fiber can almost coincide with each other. Accordingly, distortion
induced by the dispersion can be avoided, thereby making it
possible to realize high-quality transmission.
[0075] A second aspect of the present invention is directed to an
optical transmission system, used for radio access for transmitting
information between a center station and a subscriber terminal
through a radio base station for transmitting and receiving a radio
signal to and from an antenna, for optically transmitting radio
signals bidirectionally by respectively connecting first to n-th
radio base stations covering different service areas and the center
station through first to n-th upstream and downstream optical
fibers respectively provided so as to correspond to the radio base
stations, wherein
[0076] the center station includes
[0077] first to n-th electrical-optical converters for respectively
converting one or more signals each having a predetermined
intermediate frequency into first to n-th optical signals having
different wavelengths .lambda.d1 to .lambda.dn uniquely
corresponding to the first to n-th radio base stations,
[0078] a wavelength multiplexer for multiplexing the first to n-th
optical signals obtained by the conversion,
[0079] a local oscillation signal source for outputting a local
oscillation signal having a predetermined frequency,
[0080] an optical modulator for intensity-modulating the
multiplexed optical signals outputted from the wavelength
multiplexer using the local oscillation signal, and
[0081] a wavelength separator for wavelength-separating the
multiplexed optical signals intensity-modulated into first to n-th
modulated optical signals having wavelengths .lambda.d1 to
.lambda.dn, and sending out the k-th modulated optical signal to
the k-th downstream optical fiber, and
[0082] the k-th radio base station includes an optical-electrical
converter, receiving the k-th modulated optical signal having the
wavelength .lambda.dk transmitted through the k-th downstream
optical fiber, for converting the modulated optical signal into an
electric signal in a radio frequency band, and outputting the
electric signal.
[0083] According to the second aspect, the optical signals
wavelength-multiplexed are collectively externally modulated, to
frequency-convert the signals in the intermediate frequency bands
into the signals in the radio frequency bands. Therefore, the
electrical frequency converter, which has been conventionally
required, is not required, and the optical modulator for performing
the optical frequency conversion can be shared among the plurality
of radio base stations. Further, the signals in the radio frequency
bands to be transmitted to the plurality of radio base stations are
separated from each other in the light wavelength region, thereby
making it possible to easily separate the signals even if the radio
frequencies radiated from the plurality of radio base stations are
the same.
[0084] In this case, the center station may include
[0085] first to n-th electrical-optical converters for respectively
converting one or more signals each having a predetermined
intermediate frequency into first to n-th downstream optical
signals having different wavelengths .lambda.d1 to .lambda.dn
uniquely corresponding to the first to n-th radio base
stations,
[0086] first to n-th upstream light sources respectively outputting
first to n-th upstream optical signals having wavelengths
.lambda.u1 to .lambda.un which differ from any of the wavelengths
.lambda.d1 to .lambda.dn and differ from one another,
[0087] a wavelength multiplexer for multiplexing the first to n-th
downstream optical signals obtained by the conversion and the
outputted first to n-th upstream optical signals,
[0088] a local oscillation signal source for outputting a local
oscillation signal having a predetermined frequency,
[0089] an optical modulator for intensity-modulating the
multiplexed optical signals outputted from the wavelength
multiplexer using the local oscillation signal,
[0090] a wavelength separator for wavelength-separating the
multiplexed optical signals intensity-modulated to the first to
n-th modulated downstream optical signals having the wavelengths
.lambda.1 to .lambda.dn and the first to n-th modulated upstream
optical signals having the wavelengths .lambda.u1 .lambda.un, and
sending out the k-th modulated downstream optical signal, together
with the k-th modulated upstream optical signal, to the k-th
downstream optical fiber, and
[0091] first to n-th optical-electrical converters for respectively
converting the optical signals transmitted through the first to
n-th upstream optical fibers into electric signals, and
[0092] the k-th radio base station may include
[0093] a two-wavelength separator, receiving the optical signal
transmitted through the k-th downstream optical fiber, for
separating the optical signal into the k-th modulated downstream
optical signal having the wavelength .lambda.dk and the k-th
modulated upstream optical signal having the wavelength
.lambda.uk,
[0094] an optical-electrical converter for converting the k-th
modulated downstream optical signal obtained by the separation in
the two-wavelength separator into an electric signal and outputting
the electric signal, and
[0095] an RF modulator for intensity-modulating the k-th modulated
upstream optical signal obtained by the separation in the
two-wavelength separator using the inputted radio signal, and
sending out the k-th modulated upstream optical signal
intensity-modulated to the k-th upstream optical fiber.
[0096] In such a configuration, the first to n-th light sources for
outputting light having different wavelengths for transmitting an
upstream signal are provided, and the upstream optical signal and a
downstream optical signal are wavelength-multiplexed, so that the
optical modulator can be shared among the plurality of radio base
stations for optical modulation of not only the downstream optical
signal but also the upstream optical signal with the local
oscillation signal. Further, the upstream optical signal modulated
using the local oscillation signal is intensity-modulated using the
radio signal received by the antenna, so that mixing of the local
oscillation signal and the radio signal is performed in a light
region. Accordingly, the radio base station can also have a
frequency conversion function for converting the radio signal into
the signal having an intermediate frequency.
[0097] Preferably, the wavelengths .lambda.d1 to .lambda.dn of the
first to n-th downstream optical signals are set so as to belong to
a predetermined first wavelength band,
[0098] the wavelengths .lambda.u1 to .lambda.un of the first to
n-th upstream optical signals are set so as to belong to a
predetermined second wavelength band,
[0099] the second wavelength separator in the k-th radio base
station wavelength-separates the optical signal transmitted through
the k-th downstream optical fiber into a signal in the first
wavelength band and a signal in the second wavelength band, to
separate the optical signal into the k-th modulated downstream
optical signal having the wavelength .lambda.dk and the k-th
modulated upstream optical signal having the wavelength
.lambda.uk.
[0100] The wavelengths of the optical signals outputted from the
first to n-th electrical-optical converters and the wavelengths of
the optical signals outputted from the first to n-th light sources
are set so as to respectively belong to wavelength bands in
definite ranges, thereby making it possible to easily separate the
modulated downstream optical signal and the modulated upstream
optical signal in the two-wavelength separator in the radio base
station.
[0101] On the other hand, the center station may include first to
n-th electrical-optical converters for respectively converting one
or more signals each having a predetermined intermediate frequency
into first to n-th downstream optical signals having different
wavelengths .lambda.d1 to .lambda.dn, which belong to a
predetermined first wavelength band, uniquely corresponding to the
first to n-th radio base stations,
[0102] first to n-th upstream light sources respectively outputting
first to n-th upstream optical signals having wavelengths
.lambda.u1 to .lambda.un which differ from any of the wavelengths
.lambda.d1 to .lambda.dn and belong to a predetermined second
wavelength band,
[0103] a wavelength multiplexer for multiplexing the first to n-th
downstream optical signals obtained by the conversion and the
outputted first to n-th upstream optical signals,
[0104] a local oscillation signal source for outputting a local
oscillation signal having a predetermined frequency,
[0105] an optical modulator for intensity-modulating the
multiplexed optical signals outputted from the wavelength
multiplexer using the local oscillation signal,
[0106] a wavelength separator for wavelength-separating the
multiplexed optical signals intensity-modulated into first to n-th
modulated downstream optical signals having the wavelengths
.lambda.d1 to .lambda.dn and the first to n-th modulated upstream
optical signals having the wavelengths .lambda.u1 to .lambda.un,
and sending out the k-th modulated downstream optical signal,
together with the k-th modulated upstream optical signal, to the
k-th downstream optical fiber, and
[0107] first to n-th optical-electrical converters for respectively
converting the optical signals transmitted through the first to
n-th upstream optical fibers into electric signals, and
[0108] the k-th radio base station may include an
electro-absorption type modulator, receiving the optical signal
transmitted through the k-th downstream optical fiber to separate
the optical signal into the k-th modulated downstream optical
signal having the wavelength .lambda.dk and the k-th modulated
upstream optical signal having the wavelength .lambda.uk,
converting the k-th modulated downstream optical signal in the
first wavelength band representing an optical-electrical conversion
function into an electric signal and outputting the electric
signal, and intensity-modulating the k-th modulated upstream
optical signal in the second wavelength band representing an
electrical-optical conversion function using the inputted radio
signal and sending out the k-th modulated upstream optical signal
intensity-modulated to the k-th upstream optical fiber.
[0109] In such a configuration, the wavelengths of the optical
signals outputted from the first to n-th electrical-optical
converters and the wavelengths of the optical signals outputted
from the first to n-th light sources are suitably set, and the
electro-absorption type modulator for performing optical-electrical
conversion and electrical-optical conversion is installed in the
radio base station depending on the wavelength of the inputted
optical signal in place of the two-wavelength separator, the
optical-electrical converter, and the RF modulator, described
above. Consequently, it is possible to significantly simplify the
configuration of the radio base station in addition to the effect
obtained by the above-mentioned configuration.
[0110] Preferably, the first to n-th upstream light sources
respectively output the first to n-th upstream optical signals
which uniquely correspond to the first to n-th downstream optical
signals and have wavelengths .lambda.u1 to .lambda.un respectively
different from the wavelengths .lambda.d1 to .lambda.dn of the
first to n-th downstream optical signals by predetermined amounts
fs. Consequently, it is possible to simply together separate the
wavelengths .lambda.dk and .lambda.uk only by using an n output
wavelength separator for separating n optical signals multiplexed
at equal spacing in the configuration of the wavelength
separator.
[0111] A third aspect of the present invention is directed to a
high frequency optical transmitter, used in a center station
connected to a plurality of radio base stations respectively
covering different service areas using a plurality of optical
fibers, for optically transmitting radio signals, characterized by
comprising
[0112] a three-branching portion for branching an inputted electric
signal into first and second electric signals which are the same in
phase and a third electric signal which has a phase difference of
90.degree. to the first and second electric signals;
[0113] an electrical-optical converter for converting the third
electric signal into a light intensity modulated signal;
[0114] a first delay controller for adjusting the propagation time
of the first electric signal;
[0115] a second delay controller for adjusting the propagation time
of the second electric signal;
[0116] a two-branching portion for branching an inputted local
oscillation signal into first and second local oscillation signals
which are opposite in phase;
[0117] a third delay controller for adjusting the propagation time
of the first local oscillation signal;
[0118] a fourth delay controller for adjusting the propagation time
of the second local oscillation signal;
[0119] a first multiplexer for multiplexing the first electric
signal outputted from the first delay controller and the first
local oscillation signal outputted from the third delay
controller;
[0120] a second multiplexer for multiplexing the second electric
signal outputted from the second delay controller and the second
local oscillation signal outputted from the fourth delay
controller; and
[0121] a differential intensity modulator, having first and second
electrodes, for modulating the light-intensity modulated signal
outputted from the electrical-optical converter by respectively
inputting signals obtained by the multiplexing in the first and
second multiplexers to the first and second electrodes,
[0122] the first to fourth delay controllers being adjusted such
that the first and second electric signals inputted to the first
and second electrodes of the differential intensity modulator
through the first and second multiplexers are the same in phase, to
subject the optical signal outputted from the electrical-optical
converter to phase modulation and subject the optical signal to
optical modulation which is the same in amount as and is opposite
in phase to the frequency deviation (an FM index) of a light
frequency modulation component of the optical signal.
[0123] As described above, according to the third aspect, the
optical signal outputted from the electrical-optical converter is
phase-modulated using a part of the electric signal inputted to the
electrical-optical converter utilizing the external modulator for
performing frequency conversion. Consequently, the light frequency
modulation component (wavelength chirping) which occurs at the time
of the electrical-optical conversion can be canceled without using
an additional optical component, and distortion due to wavelength
dispersion which is induced by the function of the light frequency
modulation and the wavelength dispersion characteristics of the
optical fiber can be suppressed, thereby making it possible to
realize high-performance optical transmission.
[0124] A high frequency optical transmitter may include
[0125] a three-branching portion for branching an inputted electric
signal into first and second electric signals which are the same in
phase and a third electric signal which has a phase difference of
90.degree. from the first and second electric signals;
[0126] an electrical-optical converter for converting the third
electric signal into a light intensity modulated signal;
[0127] a first delay controller for adjusting the propagation time
of the first electric signal;
[0128] a second delay controller for adjusting the propagation time
of the second electric signal;
[0129] a two-branching portion for branching an inputted local
oscillation signal into first and second local oscillation signals
which have a difference of 90.degree. to each other;
[0130] a third delay controller for adjusting the propagation time
of the first local oscillation signal;
[0131] a fourth delay controller for adjusting the propagation time
of the second local oscillation signal;
[0132] a first multiplexer for multiplexing the first electric
signal outputted from the first delay controller and the first
local oscillation signal outputted from the third delay
controller;
[0133] a second multiplexer for multiplexing the second electric
signal outputted from the second delay controller and the second
local oscillation signal outputted from the fourth delay
controller; and
[0134] a differential intensity modulator, having first and second
electrodes, for modulating the light-intensity modulated signal
outputted from the electrical-optical converter by respectively
inputting signals obtained by the multiplexing in the first and
second multiplexers to the first and second electrodes,
[0135] the first and second delay controllers being adjusted such
that a phase difference between the first and second electric
signals inputted to the first and second electrodes of the
differential intensity modulator through the first and second
multiplexers is zero, to subject the optical signal outputted from
the electrical-optical converter to phase modulation and subject
the optical signal to optical modulation which is the same in
amount as and is opposite in phase to the frequency deviation of a
light frequency modulation component of the optical signal, and
[0136] the third and fourth delay controllers being adjusted such
that the first and second local oscillation signals inputted to the
first and second electrodes of the differential intensity modulator
through the first and second multiplexers have a difference of
90.degree. to each other, to subject the optical signal to optical
side-band modulation with a light carrier.
[0137] In such a configuration, the optical single-sideband
modulation is performed, thereby making it possible to avoid the
problem that an RF signal component obtained by
frequency-converting the electric signal is greatly decreased
because an upper sideband and a lower sideband of the
light-intensity modulation component are canceled by the effect of
wavelength dispersion which occurs in a case where the optical
signal which has been subjected to non-differential external
modulation (optical double-sideband modulation) is subjected to
optical-electrical conversion after being transmitted a long
distance.
[0138] Furthermore, a high frequency optical transmitter may
include
[0139] a two-branching portion for branching an inputted electric
signal into first and second electric signals which have a
difference of 90.degree. to each other;
[0140] an electrical-optical converter for converting the first
electric signal into a light intensity modulated signal;
[0141] a delay controller for adjusting the propagation time of the
second electric signal; and
[0142] an integrated modulator, comprising a phase modulator and an
intensity modulator formed on the same substrate, for modulating
the light intensity modulated signal outputted from the
electrical-optical converter by inputting the second electric
signal outputted from the delay controller to the phase modulator
and inputting an inputted local oscillation signal to the intensity
modulator,
[0143] in the phase modulator, the optical signal outputted from
the electrical-optical converter being subjected to phase
modulation and subjected to optical modulation which is opposite in
phase to the frequency deviation of a light frequency modulation
component of the optical signal.
[0144] In such a configuration, it is possible to easily make delay
adjustment for canceling the light frequency modulation component
by integrating the phase modulator for canceling the light
frequency modulation component and the intensity modulator for
performing frequency conversion. Further, the electric signal
inputted to the phase modulator and the local oscillation signal
inputted to the intensity modulator need not be multiplexed,
thereby making it possible to further reduce the power of the
electric signal inputted to the intensity modulator.
[0145] These and other objects, features, aspects and advantages of
the present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0146] FIG. 1 is a block diagram showing the configuration of an
optical transmission system according to a first embodiment of the
present invention;
[0147] FIG. 2 is a block diagram showing a configuration in a case
where a subscriber terminal is added in the optical transmission
system according to the first embodiment of the present
invention;
[0148] FIG. 3 is a block diagram showing the configuration of an
optical transmission system according to a second embodiment of the
present invention;
[0149] FIG. 4 is a block diagram showing the configuration of an
optical transmission system according to a third embodiment of the
present invention;
[0150] FIG. 5 shows diagrams each showing an example of a signal
spectrum processed by the optical transmission system according to
the third embodiment of the present invention;
[0151] FIG. 6 is a block diagram showing the configuration of an
optical transmission system according to a fourth embodiment of the
present invention;
[0152] FIG. 7 is a block diagram showing the configuration of an
optical transmission system according to a fifth embodiment of the
present invention;
[0153] FIG. 8 is a block diagram showing the configuration of an
optical transmission system according to a sixth embodiment of the
present invention;
[0154] FIG. 9 shows diagrams each for explaining an example of a
wavelength separating operation performed in a wavelength separator
460;
[0155] FIG. 10 is a block diagram showing another example of the
configurations of radio base stations 210 to 20n shown in FIG.
8;
[0156] FIG. 11 is a diagram showing an example of
optical-electrical conversion efficiency and electrical-optical
conversion efficiency obtained in a field effect absorption type
modulator 29k shown in FIG. 10;
[0157] FIG. 12 is a block diagram showing the configuration of a
high frequency optical transmitter according to a seventh
embodiment of the present invention;
[0158] FIG. 13 is a block diagram showing the specific
configuration of a differential light-intensity modulator 550 shown
in FIG. 12;
[0159] FIG. 14 shows diagrams each illustrating an example of a
light spectrum measured using the high frequency optical
transmitter according to the seventh embodiment of the present
invention;
[0160] FIG. 15 shows diagrams each showing the difference between
light spectra in a case where different external modulation schemes
are used in the high frequency optical transmitter according to the
seventh embodiment of the present invention;
[0161] FIG. 16 is a block diagram showing the configuration of a
high frequency optical transmitter according to an eighth
embodiment of the present invention;
[0162] FIG. 17 is a block diagram showing the configuration of a
conventional optical transmission system;
[0163] FIG. 18 is a diagram showing the concept of service areas
901 to 907 respectively covered by a plurality of radio base
stations 701 to 707; and
[0164] FIG. 19 is a block diagram showing the configuration of
another conventional optical transmission system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0165] (First Embodiment)
[0166] FIG. 1 is a block diagram showing the configuration of an
optical transmission system for radio access according to a first
embodiment of the present invention. In FIG. 1, the optical
transmission system according to the first embodiment is so
constructed that a center station 100 and a plurality of radio base
stations 201 to 20n are respectively connected to each other
through a plurality of downstream optical fibers 301 to 30n.
[0167] The center station 100 includes a plurality of modulators
110, a multiplexer 120, an electrical-optical converter 130, a
local oscillation signal source 140, an external modulator 150, and
an optical branching portion 160. The radio base stations 201 to
20n respectively include optical-electrical converters 211 to 21n,
band filters 221 to 22n, and antennas 231 to 23n. The operation of
the optical transmission system according to the first embodiment
will be described.
[0168] In the center station 100, different information to be
transmitted to subscriber terminals are respectively inputted to
input terminals 1 in the form of baseband signals. The plurality of
modulators 110 respectively modulate the baseband signals inputted
from the input terminals 1 to IF signals having different
predetermined frequencies. The frequencies are respectively
determined on the basis of the frequencies of radio signals to be
transmitted to the subscriber terminals from the radio base
stations 201 to 20n. The multiplexer 120 multiplexes the plurality
of IF signals outputted from the plurality of modulators 110. The
electrical-optical converter 130 converts the IF signals
multiplexed by the multiplexer 120 to an optical signal through
intensity modulation. The local oscillation signal source 140
outputs a local oscillation signal having a predetermined
frequency. The frequency of the local oscillation signal is
determined on the basis of the modulation frequencies of the
modulators 110 and the frequencies of the radio signals. The
external modulator 150 receives the optical signal obtained by the
conversion in the electrical-optical converter 130 and the local
oscillation signal outputted from the local oscillation signal
source 140, and intensity-modulates the optical signal using the
local oscillation signal. The optical branching portion 160
branches the optical signal intensity-modulated by the external
modulator 150 into optical signals, whose number corresponds to the
number (=n) of the radio base stations, and outputs the optical
signals, respectively, to the radio base stations 201 to 20n
through the downstream optical fibers 301 to 30n.
[0169] The optical signals outputted from the center station 100
are respectively inputted to the radio base stations 201 to 20n
after propagating through the downstream optical fibers 301 to 30n.
In the radio base station 20k, the optical-electrical converter 21k
converts the inputted optical signal into an electric signal. By
the conversion, an RF signal obtained by frequency-converting the
IF signal can be obtained. The reason for this is that in the
electrical-optical converter 130 and the external modulator 150 in
the center station 100, intensity modulation is doubly performed
using the IF signal and the local oscillation signal. The band
filter 22k receives the RF signal outputted from the
optical-electrical converter 21k, and extracts only an RF signal
component having a desired radio frequency therefrom. The extracted
RF signal component is released to a space from the antenna 23k to
the subscriber terminal.
[0170] In the present invention, the intensity modulation is thus
doubly performed in the center station 100. In each of the radio
base stations 201 to 20n, therefore, the RF signal obtained by
frequency-converting the IF signal can be obtained only by
converting the optical signal which has propagated into the
electric signal. Consequently, the electrical-optical converter 130
and the external modulator 150 for performing frequency conversion
can be shared among the plurality of radio base stations 201 to
20n. Consequently, the number of components in each of the radio
base stations 201 to 20n can be more significantly reduced, as
compared with that in the conventional example. Further, it is easy
to carry and install, for example, each of the radio base stations
201 to 20n.
[0171] Furthermore, the center station 100 frequency-division
multiplexes the information to be transmitted to the plurality of
subscriber terminals in the multiplexer 120 for analog optical
transmission. In the center station 100, therefore, the modulators
110, whose number corresponds to the number of information to be
multiplexed, are prepared. In the optical transmission system
according to the present invention, therefore, no
separator/multiplexer and high-speed modulator are required, unlike
the case where information to be transmitted is time-division
multiplexed for transmission as described in the Background Art
section.
[0172] Furthermore, in the present invention, the intensity
modulation is doubly performed. Accordingly, it is possible to use
a semiconductor laser which is applicable only to a relatively low
frequency signal but is superior in distortion characteristics to
the external modulator for intensity modulation of the
frequency-division multiplexed IF signals (performed by the
electrical-optical converter 130). Also it is possible to use an
external modulator which operates to a high frequency for intensity
modulation of the local oscillation signal (performed by the
external modulator 150). Further, the frequency of the local
oscillation signal is relatively high. Accordingly, an external
modulator which has been matched in a particular high frequency
band for modulation can also be used for the intensity modulation
of the local oscillation signal. Further, a Mach-Zehnder type
external modulator can also be used for the intensity modulation of
the local oscillation signal. When the Mach-Zehnder type external
modulator is used, a bias point is set to a point at which light
output power is the maximum or the minimum, thereby making it
possible to perform optical frequency conversion at a frequency
which is twice the frequency of the local oscillation signal. A
method of setting the bias point cannot be used for the intensity
modulation of the IF signals frequency-division multiplexed because
a second-order distortion component is increased. However, the
method can be used for the intensity modulation of the local
oscillation signal because the local oscillation signal is of only
one carrier. Further, in this method, the frequency of the local
oscillation signal used for the intensity modulation may be a
frequency which is one-half that in the normal frequency
conversion. Accordingly, it is possible to use as the local
oscillation signal source 140 a low-cost one.
[0173] On the other hand, a single mode fiber (SMF) having a zero
dispersion wavelength at 1.3 .mu.m is generally used for an optical
fiber used in optical transmission, for example. Generally when the
optical fiber is laid, not only an actually required number of
optical fibers but also preliminary optical fibers which are
currently not used are simultaneously laid in consideration of the
future use. Consequently, an unused SMF which has already been laid
may be thus used as the downstream optical fiber 30k in the present
invention. Further, when an optical amplifier must be used in order
to compensate for a branching loss and a transmission loss, a
wavelength of 1.5 .mu.m is used as the wavelength of a light source
used for the electrical-optical converter 130. At this time, when
double-sideband modulation (DSB modulation) is used as a modulation
scheme carried out in the external modulator 150, and a high
frequency signal in a microwave band or a millimeter wave band is
optically transmitted by the SMF, the power of the high frequency
signal after the optical transmission may be greatly decreased
depending on the transmission distance due to the effect of the
dispersion of the SMF. In order to avoid the effect of the
dispersion, an optical single-sided band modulation (optical SSB
modulation) with a carrier and a single-sideband component may be
used for the modulation scheme carried out in the external
modulator 150. Further, external modulation may be performed using
a Mach-Zehnder type external modulator as the external modulator
150 and setting a point at which light output power is the minimum
or the maximum to a bias point.
[0174] In a case where the downstream optical fiber 30(n+1) is
newly laid, for example, when the zero dispersion wavelength of the
downstream optical fiber 30(n+1) and the wavelength of light
outputted from the electrical-optical converter 130 can be
selected, it is desired that both the wavelengths be selected to
coincide with each other. In this case, even if the DSB modulation
is used as a modulation scheme, it is not affected by the
dispersion of the SMF. Further, in the electrical-optical converter
130, when the frequency-division multiplexed IF signals are
converted into the optical signal through direct modulation,
distortion due to dispersion corresponding to the frequency after
the frequency conversion is induced irrespective of the external
modulation scheme of the local oscillation signal. From the point
of view of preventing the distortion due to dispersion from being
induced, it is desired that the zero dispersion wavelength of the
downstream optical fiber 30(n+1) and the wavelength of the light
outputted from the electrical-optical converter 130 coincide with
each other.
[0175] Description is made of a case where a subscriber terminal is
added to increase the transmission capacity in the optical
transmission system. FIG. 2 is a block diagram showing a
configuration in a case where the subscriber terminal is added in
the optical transmission system according to the first embodiment
of the present invention.
[0176] As described above, the center station 100
frequency-division multiplexes information to be transmitted to a
plurality of subscriber terminals. As can be seen from comparison
between FIG. 1 and FIG. 2, when the subscriber terminal is added,
therefore, only required is a corresponding additional modulator
101 to be added to the center station 100, to input its output
signal to the multiplexer 120. Consequently, an IF signal outputted
from the additional modulator 101 is frequency-division multiplexed
and is optically transmitted to all the radio base stations 201 to
20n. Accordingly, the corresponding radio base station converts the
optically transmitted optical signal into an RF signal, and then
extracts the RF signal having a desired radio frequency through the
band filter 22k. Therefore, the present invention shows that it is
more simply feasible to increase the transmission capacity as the
subscriber terminal is added, as compared with that in the
Background Art section.
[0177] When the position of the subscriber terminal to be added is
the position where a line-of-sight propagation path cannot be
ensured from the existing radio base station, if there is room for
the number of branches by the optical branching portion 160, the
downstream optical fiber 30(n+1) and a new radio base station
20(n+1) maybe additionally installed, thereby making it possible to
more significantly reduce the number of components which must be
additionally installed, as compared with that in the
above-mentioned Background Art.
[0178] As described above, in the optical transmission system
according to the first embodiment of the present invention, the IF
signal obtained by frequency-division multiplexing the plurality of
IF signals is converted into an optical signal by intensity
modulation, and the optical signal is externally modulated using
the local oscillation signal, thereby collectively
frequency-converting the plurality of IF signals into the RF
signals in an optical signal state. Information to be transmitted
to the subscriber terminals are optically transmitted in the form
of the RF signals from the center station 100 to the plurality of
radio base stations 201 to 20n. Consequently, the following effect
is obtained in the present invention.
[0179] First, the plurality of IF signals are optically transmitted
upon being frequency-division multiplexed in the center station
100. Accordingly, the electrical-optical converter 130 can be
shared among the plurality of radio base stations 201 to 20n.
Second, the frequency conversion is optically performed by the
external modulator 150 in the center station 100. Accordingly, the
electrical frequency converter which is an indispensable component
in the Background Art is not required in each of the radio base
stations 210 to 20n, and the external modulator 150 can be shared
among the radio base stations 201 to 20n. Third, unlike the
conventional configuration, the modulator having a
multiplexer/separator and a high-speed modulation function is not
required in the present invention. Fourth, the capacity can be
easily increased as the subscriber terminal is added, thereby
making it possible to provide an optical transmission system
superior in expendability.
[0180] (Second Embodiment)
[0181] In the optical transmission system according to the first
embodiment, the plurality of IF signals are multiplexed by the
frequency division multiplexing, and the IF signals
frequency-division multiplexed are doubly intensity-modulated. In
the radio base stations 201 to 20n, therefore, the band filters 221
to 22n must be respectively used to extract RF signal components
having desired radio frequencies from the transmitted RF
signals.
[0182] In the second embodiment, description is made of an optical
transmission system which does not respectively require the band
filters 221 to 22n in the configurations of radio base stations 201
to 20n.
[0183] FIG. 3 is a block diagram showing the configuration of an
optical transmission system for radio access according to a second
embodiment of the present invention. In FIG. 3, the optical
transmission system according to the second embodiment is so
constructed that a center station 100 and a plurality of radio base
stations 201 to 20n are respectively connected to each other
through a plurality of downstream optical fibers 301 to 30n.
[0184] The center station 100 includes a plurality of modulators
110, a plurality of multiplexers 121 to 12n, a plurality of IF
modulators 181 to 18n, a light source 170, a local oscillation
signal source 140, an external modulator 150, and an optical
branching portion 160. The radio base stations 201 to 20n
respectively include optical-electrical converters 211 to 21n and
antennas 231 to 23n. The operation of the optical transmission
system according to the second embodiment will be described
below.
[0185] In the center station 100, different information to be
transmitted to subscriber terminals are respectively inputted to
input terminals 1 in the form of baseband signals. The plurality of
modulators 110 respectively modulate the baseband signals inputted
from the input terminals 1 to IF signals having different
predetermined frequencies. The frequencies are respectively
determined on the basis of the frequencies of radio signals
transmitted to the subscriber terminals from the radio base
stations 201 to 20n. Each of the multiplexers 121 to 12n
multiplexes the plurality of IF signals outputted from the
plurality of modulators 110 for each of the radio base stations 201
to 20n. The light source 170 generates an optical signal having a
predetermined wavelength. The local oscillation signal source 140
outputs a local oscillation signal having a predetermined
frequency. The frequency of the local oscillation signal is
determined on the basis of the modulation frequencies of the
modulators 110 and the frequencies of the radio signals. The
external modulator 150 receives the optical signal outputted from
the light source 170 and the local oscillation signal outputted
from the local oscillation signal source 140, and
intensity-modulates the optical signal using the local oscillation
signal. The optical branching portion 160 branches the optical
signal intensity-modulated by the external modulator 150 into
optical signals, whose number corresponds to the number (=n) of the
radio base stations, and respectively outputs the optical signals
to the plurality of IF modulators 181 to 18n. Each of the IF
modulators 181 to 18n receives the optical signal obtained by the
branching and an IF signal obtained by the multiplexing, and
intensity-modulates the optical signal depending on the IF signal.
The IF modulators 181 to 18n respectively correspond to the radio
base stations 201 to 20n. Each of the IF modulators 181 to 18n
intensity-modulates the optical signal depending on the IF signal
such that only an RF signal component used in a service area
covered by the corresponding radio base station is transmitted. The
optical signals intensity-modulated by the IF modulators 181 to 18n
are respectively transmitted to the radio base stations 201 to 20n
through the downstream optical fibers 301 to 30n.
[0186] The optical signals outputted from the center station 100
are respectively inputted to the radio base stations 201 to 20n
after propagating through the downstream optical fibers 301 to 30n.
In the radio base station 20k, the optical-electrical converter 21k
converts the inputted optical signal into an electric signal. By
the conversion, an RF signal obtained by frequency-converting the
IF signal and having a desired radio frequency can be obtained. The
reason for this is that in the external modulator 150 and the IF
modulator 18k in the center station 100, intensity modulation is
doubly performed using the local oscillation signal and the IF
signal. The RF signal obtained by the conversion is released to a
space from the antenna 23k to the subscriber terminal.
[0187] As described above, in the optical transmission system
according to the second embodiment of the present invention, the IF
modulators 181 to 18n are installed in the center station 100 for
the radio base stations 201 to 20n, and only the RF signal
component used in the service area covered by each of the radio
base stations 201 to 20n is transmitted to the radio base station.
In the radio base station 20k, therefore, radio transmission can be
performed at different frequencies for the adjacent service areas
without using the band filter 22k for extracting the RF signal
component having a desired radio frequency, as described in the
first embodiment. In the configuration, optical transmission is
possible even at the same frequency. Further, the band filter 22k
is not used for the radio base station 20k. Accordingly, the radio
base station from which the desired radio frequency can be
outputted need not be selected and used for each of the service
areas. Alternatively, the radio base station need not be adjusted
such that the desired radio frequency can be outputted for each of
the service areas (that is, in the service areas, the radio base
stations having the same configuration can be used). Consequently,
it is feasible to reduce the cost of the optical transmission
system.
[0188] (Third Embodiment)
[0189] The first and second embodiments were characterized by a
case where information is transmitted from the center station to
the subscriber terminal (in a downstream direction). Accordingly,
description was made of the optical transmission system comprising
the configuration for only downstream transmission.
[0190] In the third embodiment, description is then made of an
optical transmission system in which an RF signal in an optical
signal state obtained by frequency-converting an IF signal is
utilized for a case where information is transmitted from a
subscriber terminal to a center station 100 (in an upstream
direction), to simplify a configuration required for upstream
transmission.
[0191] FIG. 4 is a block diagram showing the configuration of an
optical transmission system for radio access according to a third
embodiment of the present invention. In FIG. 4, the optical
transmission system according to the third embodiment is so
constructed that a center station 100 and a plurality of radio base
stations 201 to 20n are respectively connected to each other
through a plurality of downstream optical fibers 301 to 30n and a
plurality of upstream optical fibers 351 to 35n.
[0192] The center station 100 includes a plurality of modulators
110, a plurality of demodulators 102, a multiplexer 120, an
electrical-optical converter 130, a local oscillation signal source
140, an external modulator 150, an optical branching portion 160,
and a plurality of optical-electrical converters 191 to 19n. The
radio base stations 201 to 20n respectively include optical
branching portions 261 to 26n, optical-electrical converters 211 to
21n, external modulators 251 to 25n, band filters 221 to 22n,
circulators 241 to 24n, and antennas 231 to 23n. The operation of
the optical transmission system according to the third embodiment
will be described below.
[0193] Description is now made of the downstream transmission from
the center station 100 to each of the radio base stations 201 to
20n.
[0194] Processing from the time when a plurality of baseband
signals are respectively inputted to input terminals 1 in the
center station 100 to the time when optical signals are
respectively outputted to the radio base stations 201 to 20n after
propagating through downstream optical fibers 301 to 30n in the
downstream transmission is the same as the processing described in
the first embodiment and hence, the description thereof is not
repeated.
[0195] In the radio base station 20k, the optical branching portion
26k branches the inputted optical signal into two optical signals,
and outputs one of the optical signals obtained by the branching
and the other optical signal, respectively, to the
optical-electrical converter 21k and the external modulator 25k.
The optical-electrical converter 21k converts the optical signal
obtained by the branching and outputted from the optical branching
portion 26k into an electric signal. By the conversion, an RF
signal obtained by frequency-converting an IF signal can be
obtained. The band filter 22k inputs the RF signal outputted from
the optical-electrical converter 21k, and extracts only an RF
signal component having a desired radio frequency. The extracted RF
signal component is released to a space from the antenna 23k to the
subscriber terminal through the circulator 24k.
[0196] Description is now made of upstream transmission from each
of the radio base stations 201 to 20n to the center station
100.
[0197] An RF signal transmitted from the subscriber terminal is
received by the antenna 23k. The received RF signal is outputted to
the external modulator 25k through the circulator 24k. In the
present invention, the circulator 24k is thus provided between the
optical-electrical converter 21k and the antenna 23k, so that the
antenna 23k is shared between the upstream transmission and the
downstream transmission. The external modulator 25kreceives the
optical signal obtained by the branching and outputted from the
optical branching portion 26k and the RF signal received by the
antenna 23k, and intensity-modulates the optical signal by the RF
signal. The optical signal intensity-modulated by the external
modulator 25k is outputted to the center station 100 through the
upstream optical fiber 35k.
[0198] The optical signals respectively outputted from the radio
base stations 201 to 20n are inputted to the center station 100
after respectively propagating through the upstream optical fibers
351 to 35n. In the center station 100, each of the
optical-electrical converters 191 to 19n converts the inputted
optical signal into an electric signal. By the conversion, an IF
signal obtained by frequency-converting an RF signal can be
obtained. The demodulators 102 respectively demodulate the IF
signals obtained by the conversion in the optical-electrical
converters 191 to 19n to baseband signals, and output the baseband
signals from output terminals 2.
[0199] In the radio base station 20k, the optical signal obtained
by the branching and outputted to the external modulator 25k is
intensity-modulated using the local oscillation signal generated by
the local oscillation signal source 140 in the external modulator
150 provided in the center station 100. In the external modulator
25k, therefore, the optical signal obtained by the branching and
outputted is intensity-modulated using the RF signal received by
the antenna 23k, so that the intensity modulation is doubly
performed using the local oscillation signal and the RF signal.
Accordingly, the optical signal propagating from the external
modulator 25k in the radio base station 20k is converted into an
electric signal by the optical-electrical converter 19k in the
center station 100, thereby making it possible to obtain the IF
signal obtained by frequency-converting the RF signal. In order to
avoid the interference between an upstream signal transmitted from
the subscriber terminal and a downstream signal transmitted from
the center station 100, the frequencies of the RF signals used in
the respective directions must be made to differ.
[0200] As described above, in the present invention, the downstream
optical signal which has already been intensity-modulated using the
local oscillation signal is branched into two optical signals using
the optical branching portion 26k installed in the radio base
station 20k, and one of the optical signals is utilized as the
upstream optical signal inputted to the external modulator 25k. In
the external modulator 25k, therefore, the RF signal transmitted
from the subscriber terminal is optically frequency-converted into
the IF signal, thereby making it possible to install the local
oscillation signal source 140 and the modulator 110 in the center
station 100. As a result, in the present invention, each of the
radio base stations 201 to 20n can be miniaturized.
[0201] Furthermore, processing performed in the optical
transmission system according to the third embodiment will be
specifically described with reference to FIG. 5.
[0202] (a) of FIG. 5 is a diagram showing an example of the
spectrum of the downstream signal outputted from the
optical-electrical converter 21k. In this example, it is indicated
that an IF signal 1 (frequency: f.sub.IF1) is converted into a
downstream RF signal 1 (frequency: f.sub.RF1) by an LO signal
(frequency:f.sub.L0) (b) of FIG. 5 is a diagram showing an example
of the spectrum of the upstream signal received by the antenna 23k.
In this example, it is indicated that an upstream RF signal 2
(frequency: f.sub.RF2) having a frequency difference of .DELTA.f
from the downstream RF signal 1 is received by the antenna 23k. In
the external modulator 25k, the optical signal transmitted from the
center station 100 is intensity-modulated using the RF signal 2, so
that mixing of the signals having the spectrums shown in FIGS. 5(a)
and 5(b) is performed in the state of light. A signal spectrum in a
case where an optical signal obtained by the mixing is converted
into an electric signal in the optical-electrical converter 19k is
as illustrated in (c) of FIG. 5. That is, an IF signal 2 having a
frequency f.sub.IF2 is obtained by mixing the LO signal and the RF
signal 2 in the external modulator 25k. Consequently, only a
component having the frequency f.sub.IF2 is separated by the band
filter, thereby making it possible to extract only the IF signal 2
obtained by down-converting the RF signal 2. At this time, the
frequency difference between the IF signal 1 and the IF signal 2 is
.DELTA.f.
[0203] The frequency of the upstream RF signal 2 is separated from
the frequency of the downstream RF signal 1 by .DELTA.f, so that
the upstream signal and the downstream signal can be down-converted
into signals having low frequencies without being adversely
affected by each other.
[0204] As described above, the optical transmission system
according to the third embodiment of the present invention also has
the effect of miniaturizing each of the radio base stations 201 to
20n with respect to the upstream system in addition to the effect
obtained with respect to the downstream described in the first
embodiment, so that it is easily installed outdoors.
[0205] In the first to third embodiments, the plurality of
modulators 110 are provided in order to respectively modulate the
inputted baseband signals into the plurality of IF signals having
different frequencies. When the baseband signal may be only
modulated to the IF signal having a single frequency, however, the
one modulator 110 may be provided. In this case, the IF signal
outputted from the modulator 110 may be directly inputted to the
electrical-optical converter 130 without providing the multiplexer
120.
[0206] (Fourth Embodiment)
[0207] FIG. 6 is a block diagram showing the configuration of an
optical transmission system for radio access according to a fourth
embodiment of the present invention. In FIG. 6, in the optical
transmission system according to the fourth embodiment, a center
station 100 and a plurality of radio base stations 201 to 20n are
respectively connected to each other through a plurality of
downstream optical fibers 301 to 30n.
[0208] The center station 100 includes a plurality of modulators
111 to 11n and a plurality of electrical-optical converters 131 to
13n respectively corresponding to the plurality of radio base
stations 201 to 20n, a wavelength multiplexer 430, a local
oscillation signal source 140, an optical modulator 450, and a
wavelength separator 460. The radio base stations 201 to
20nrespectively include optical-electrical converters 211 to 21n
and antennas 231 to 23n. The operation of the optical transmission
system according to the fourth embodiment will be described.
[0209] In the center station 100, information to be transmitted to
the radio base station 20k is inputted to an input terminal 1k in
the form of a baseband signal. The modulator 11k modulates the
baseband signal inputted from the input terminal 1k into an IF
signal having a predetermined frequency. The frequency is
determined on the basis of the frequency of a radio signal
transmitted to a subscriber terminal from the radio base station
20k. The electrical-optical converter 13k converts the IF signal
obtained by the modulation in the modulator 11k into an optical
signal through direct modulation. The wavelengths of the optical
signals obtained by the conversion in the electrical-optical
converters 131 to 13n are previously assigned so as to differ. The
electrical-optical converters 131 to 13n are set such that the
wavelengths of the optical signals are equally spaced, for example.
The wavelength multiplexer 430 wavelength-multiplexes the optical
signals having the different wavelengths respectively outputted
from the electrical-optical converters 131 to 13n. The local
oscillation signal source 140 outputs a local oscillation signal
having a predetermined frequency f.sub.L0. The frequency f.sub.L0
of the local oscillation signal is determined on the basis of the
modulation frequencies of the modulators 111 to 11n and the
frequencies of the radio signals to be respectively transmitted
from the radio base stations 201 to 20n to the subscriber
terminals. The optical modulator 450 receives the optical signals
wavelength-multiplexed in the wavelength multiplexer 430 and the
local oscillation signal outputted from the local oscillation
signal source 140, to collectively intensity-modulates the
wavelength-multiplexed optical signals using the local oscillation
signal. By the intensity modulation, it is possible to obtain the
optical signal having an RF component obtained by optically
converting the IF signal. The wavelength separator 460 separates
the optical signals intensity-modulated by the optical modulator
450 into a plurality of optical signals depending on the
wavelengths, and respectively transmits the corresponding optical
signals to the radio base stations 201 to 20n through the
downstream optical fibers 301 to 30n.
[0210] The optical signals transmitted from the center station 100
are respectively inputted to the radio base stations 201 to 20n
after propagating through the downstream optical fibers 301 to
30n.
[0211] In the radio base station 20k, the optical-electrical
converter 21k converts the inputted optical signal into an electric
signal, to output an RF signal. By the conversion, an RF signal
obtained by frequency-converting an IF signal can be obtained. The
outputted RF signal is released to a space from the antenna 23k to
the subscriber terminal as a radio signal after only its desired
frequency component is extracted therefrom.
[0212] As described above, in the optical transmission system
according to the fourth embodiment of the present invention, only
the IF signals having frequencies which are in close proximity to
or identical to each other are respectively converted into light
intensity modulated signals having different wavelengths. After the
light intensity modulated signals are wavelength-multiplexed, they
are collectively intensity-modulated using the local oscillation
signal through external modulation. Since the plurality of IF
signals are collectively optically frequency-converted into the RF
signals, therefore, no electrical frequency converter is required,
and the optical modulator for performing optical frequency
conversion can be shared among the plurality of radio base
stations, thereby making it possible to realize low-cost frequency
conversion.
[0213] Furthermore, the optical signals which have been
collectively intensity-modulated are respectively optically
transmitted to the radio base stations even after being separated
depending on the wavelengths. Even when the frequencies of the RF
signals are the same, therefore, the RF signals can be transmitted
without interfering with each other. In addition, the optical
signals can be easily separated in a light wavelength region,
thereby making it possible to provide a low-cost optical
transmission system for radio access.
[0214] Although in the fourth embodiment, description was made of
such a configuration that the wavelength separator 460 is installed
in the center station 100, the wavelength separator 460 need not be
necessarily installed in the center station 100. For example, the
wavelength separator 460 may be installed in separated fashion as
an independent relay station, or may be installed in each of the
radio base stations 201 to 220n. In the latter case, however, such
a configuration that the optical signal outputted from the optical
modulator 450 is branched into optical signals, and the optical
signals are respectively outputted to the radio base stations 201
to 20n must be newly provided in the center station 100.
[0215] Although description was made of a case where the IF signals
inputted to the electrical-optical converters 131 to 13n are of one
channel, multi-channel IF signals may be respectively inputted
thereto, in which case the same effect is obtained. In this case,
the IF signals on the channels may be inputted to the
electrical-optical converters 131 to 13n after being
frequency-division multiplexed.
[0216] Furthermore, in the fourth embodiment, the intensity
modulation is doubly performed. Accordingly, it is possible to use
a semiconductor laser which is only applicable to a relatively low
frequency signal but is superior in distortion characteristics to
an external modulator and an external modulator which operates to a
high frequency, respectively, for intensity modulation of the IF
signals wavelength-multiplexed (the electrical-optical converters
131 to 13n) and intensity modulation of the local oscillation
signal (the optical modulator 450). Further, the frequency of the
local oscillation signal is relatively high. Accordingly, an
external modulator which has been matched in a particular high
frequency band for modulation can be also used for the intensity
modulation of the local oscillation signal. Further, it is also
possible to use a Mach-Zehnder type external modulator for the
intensity modulation of the local oscillation signal. When the
Mach-Zehnder type external modulator is used, a bias point is set
to a point at which light output power is the maximum or the
minimum, thereby making it possible to perform optical frequency
conversion at a frequency which is twice the frequency of the local
oscillation signal. A method of setting the bias point cannot be
used for the intensity modulation of the IF signals
frequency-division multiplexed because a second-order distortion
component is increased. However, it can be used for the intensity
modulation of the local oscillation signal because the local
oscillation signal is of only one carrier. Further, in this method,
the frequency of the local oscillation signal used for the
intensity modulation may be the frequency which is one-half that in
the normal frequency conversion. Accordingly, it is possible to use
as the local oscillation signal source 140 and the optical
modulator 450 low-cost ones.
[0217] When an optical amplifier must be used in order to
compensate for a branching loss and a transmission loss, a
wavelength of 1.5 .mu.m is used as the wavelength of a light source
used for the electrical-optical converters 131 to 13n. At this
time, the optical fiber which has already been laid is an SMF. When
DSB modulation is used as a modulation scheme carried out in the
optical modulator 450, and a high frequency signal in a microwave
band or a millimeter wave band is optically transmitted by the SMF,
the power of the high frequency signal after the optical
transmission may be greatly reduced depending on the transmission
distance by the effect of the dispersion of the SMF. In order to
avoid the effect of the dispersion, optical SSB modulation with a
carrier and a sideband component may be used for a modulation
scheme carried out in the optical modulator 450. Accordingly, a
Mach-Zehnder type external modulator may be used as the optical
modulator 450, and a point at which light output power is the
minimum or the maximum may be set to a bias point, to perform
external modulation.
[0218] (Fifth Embodiment)
[0219] In the optical transmission system according to the fourth
embodiment of the present invention, in the electrical-optical
converters 131 to 13n, the IF signals are respectively converted
into the optical signals through direct modulation. When the
multi-channel IF signal is subjected to electrical-optical
conversion, therefore, wavelength chirping occurs. When there is
wavelength dispersion of the downstream optical fibers 301 to 30n,
distortion due to wavelength dispersion is induced, thereby
degrading the transmission characteristics.
[0220] In the fifth embodiment, description is made of an optical
transmission system in which no wavelength chirping occurs.
[0221] FIG. 7 is a block diagram showing the configuration of an
optical transmission system for radio access according to a fifth
embodiment of the present invention. In FIG. 7, in the optical
transmission system according to the fifth embodiment, a center
station 100 and a plurality of radio base stations 201 to 20n are
respectively connected to each other through a plurality of
downstream optical fibers 301 to 30n.
[0222] The center station 100 includes a plurality of modulators
111 to 11n, a plurality of light sources 171 to 17n, and a
plurality of IF modulators 481 to 48n which respectively correspond
to the plurality of radio base stations 201 to 20n, a wavelength
multiplexer 430, a local oscillation signal source 140, an optical
modulator 450, and a wavelength separator 460. The radio base
stations 201 to 20n respectively include optical-electrical
converters 211 to 21n and antennas 231 to 23n. In the optical
transmission system according to the fifth embodiment, the same
components as those in the optical transmission system according to
the fourth embodiment are assigned the same reference numerals and
hence, the description thereof is not repeated. In the optical
transmission system according to the fifth embodiment, description
is made, centered on the operations of the different
components.
[0223] The light sources 171 to 17n respectively output optical
signals having different wavelengths. For example, the light
sources 171 to 17n respectively output optical signals such that
the wavelengths are equally spaced. The IF modulator 48k receives
an IF signal modulated by the modulator ilk and the optical signal
outputted from the light source 17k, and intensity-modulates the
optical signal using the IF signal. The intensity-modulated optical
signals are wavelength-multiplexed in the wavelength multiplexer
430. Thereafter, the multiplexed optical signals are respectively
optically transmitted to the radio base stations 201 to 20n after
being subjected to the above-mentioned processing.
[0224] As described above, in the optical transmission system
according to the fifth embodiment shown in FIG. 7, the light
sources 171 to 17n and the IF modulators 48a to 48n are used, to
convert the IF signals into optical signals through external
modulation, so that no wavelength chirping occurs. Even when there
is wavelength dispersion of the downstream optical fibers 301 to
30n, therefore, the signals can be transmitted without degrading
transmission characteristics. The configuration as in the fifth
embodiment is particularly useful for a case where the
characteristics cannot be changed on the side of the optical fiber,
for example, a system is newly constructed using the optical fiber
which has already been laid.
[0225] (Sixth Embodiment)
[0226] The fourth and fifth embodiments were characterized by a
case where information is transmitted from the center station 100
to the subscriber terminals (in the downstream direction).
Accordingly, description was made of the optical transmission
system comprising a configuration for only downstream
transmission.
[0227] In the sixth embodiment, description is then made of an
optical transmission system in which an RF signal in an optical
signal state obtained by frequency-converting an IF signal is
utilized for a case where information is transmitted from a
subscriber terminal to a center station 100 (in an upstream
direction), to simplify a configuration required for upstream
transmission.
[0228] FIG. 8 is a block diagram showing the configuration of an
optical transmission system for radio access according to a sixth
embodiment of the present invention. In FIG. 8, in the optical
transmission system according to the sixth embodiment, a center
station 100 and a plurality of radio base stations 201 to 20n are
respectively connected to each other through a plurality of
downstream optical fibers 301 to 30n and a plurality of upstream
optical fibers 351 to 35n.
[0229] The center station 100 includes a plurality of modulators
111 to 11n, a plurality of electrical-optical converters 131 to
13n, a plurality of upstream light sources 471 to 47n, a plurality
of optical-electrical converters 421 to 42n, and a plurality of
demodulators 411 to 41n respectively corresponding to the plurality
of radio base stations 201 to 20n, a wavelength multiplexer 431, a
local oscillation signal source 140, an optical modulator 450, and
a wavelength separator 460. The radio base stations 201 to 20n
respectively include two-wavelength separators 271 to 27n, RF
modulators 281 to 28n, optical-electrical converters 211 to 21n,
circulators 241 to 24n, and antennas 231 to 23n. In the optical
transmission system according to the sixth embodiment, the same
components as those in the optical transmission system according to
the fourth embodiment are assigned to the same reference numerals
and hence, the description thereof is not repeated. The optical
transmission system according to the sixth embodiment will be
described, centered on the operations of the different
components.
[0230] Description is now made of the downstream transmission from
the center station 100 to each of the radio base stations 201 to
20n.
[0231] The upstream light sources 471 to 47n respectively output
optical signals used for transmitting upstream signals from the
radio base stations 201 to 20n to the center station 100. Here, the
upstream light sources 471 to 47n are set such that the wavelengths
of the outputted optical signals differ from each other and also
respectively differ from wavelengths previously assigned to the
electrical-optical converters 131 to 13n. The wavelength
multiplexer 431 wavelength-multiplexes the optical signals having
the different wavelengths respectively outputted from the
electrical-optical converters 131 to 13n and the optical signals
having the different wavelengths respectively outputted from the
upstream light sources 471 to 47n. The optical modulator 450
receives the optical signals wavelength-multiplexed by the
wavelength multiplexer 431 and a local oscillation signal outputted
from the local oscillation signal source 140, and collectively
intensity-modulates the wavelength-multiplexed optical signals
using the local oscillation signal. The wavelength separator 460
separates the optical signals intensity-modulated by the optical
modulator 450 into a plurality of optical signals depending on the
wavelengths, and respectively transmits the corresponding optical
signals to the radio base stations 201 to 20n through the
downstream optical fibers 301 to 30n. That is, the wavelength
separator 460 transmits the optical signal outputted from the
electrical-optical converter 13k and the optical signal outputted
from the upstream light source 47kto the radio base station 20k
through the downstream optical fiber 30k.
[0232] In the radio base station 20k, the two-wavelength separator
27k wavelength-separates the optical signal transmitted from the
center station 100, and outputs the optical signal outputted from
the electrical-optical converter 13k and the optical signal
outputted from the upstream light source 47k, respectively, to the
optical-electrical converter 21k and the RF modulator 28k. The
optical-electrical converter 21k converts the optical signal
wavelength-separated by the two-wavelength separator 27k into an
electric signal. By the conversion, an RF signal obtained by
frequency-converting an IF signal can be obtained. The RF signal is
released to a space from the antenna 23k to a subscriber terminal
through the circulator 24k.
[0233] Description is now made of the upstream transmission from
each of the radio base stations 201 to 20n to the center station
100.
[0234] An RF signal transmitted from the subscriber terminal is
received by the antenna 23k. The received RF signal is outputted to
the RF modulator 28k through the circulator 24k. In the present
invention, therefore, the circulator 24k is provided between the
optical-electrical converter 21k and the antenna 23k, so that the
antenna 23k is shared between the upstream transmission and the
downstream transmission. The RF modulator 28k receives the optical
signal wavelength-separated by the two-wavelength separator 27k and
the RF signal received by the antenna 23k, and intensity-modulates
the optical signal using the RF signal. The optical signal
intensity-modulated by the RF modulator 28k is outputted to the
center station 100 through the upstream optical fiber 35k.
[0235] The optical signal outputted from the radio base station 20k
is inputted to the center station 100 upon propagating through the
upstream optical fiber 35k.
[0236] In the center station 100, the optical-electrical converter
42k converts the inputted optical signal into an electric signal.
By the conversion, an IF signal obtained by frequency-converting
the RF signal can be obtained. The demodulator 41k demodulates the
IF signal obtained by the conversion in the optical-electrical
converter 42k to a baseband signal, and outputs the baseband signal
from an output terminal 4k.
[0237] In the radio base station 20k, the optical signal in the
upstream light source 47k which is outputted to the RF modulator
28k is intensity-modulated using the local oscillation signal
generated by the local oscillation signal source 140 in the optical
modulator 450 provided in the center station 100. In the RF
modulator 28k, therefore, the wavelength-separated optical signal
is intensity-modulated using the received RF signal in the RF
modulator 28k, so that the optical signal is doubly
intensity-modulated using the local oscillation signal and the RF
signal. Accordingly, the optical signal propagating from the RF
modulator 28k in the radio base station 20k is received by the
optical-electrical converter 42k in the center station 100 and is
detected, so that mixing of both the signals is performed, and the
RF signal is frequency-converted into the IF signal.
[0238] In the sixth embodiment, in order to transmit the radio
signal received by the antenna 23k, the optical signal outputted
from the upstream light source 47k, together with the optical
signal outputted from the electrical-optical converter 13k, is
intensity-modulated in the optical modulator 450, and is then used
for the intensity modulation in the RF modulator 28k. Consequently,
the RF signal is frequency-converted into the IF signal in the
optical-electrical converter 42k, so that the number of devices for
high-frequency signal processing can be reduced.
[0239] Referring now to FIG. 9, a wavelength separating operation
performed in the wavelength separator 460 in the center station 100
will be described by taking a specific example.
[0240] An n output wavelength separator for separating n optical
signals multiplexed at equal spacing in wavelength, for example,
can be used for the wavelength separator 460. It is possible to
use, as the n output wavelength separator, for example, an arrayed
waveguide grating (AWG) separator introduced in "Wavelength
Multiplexing Optical Semiconductor Component" (written by Yoshikuni
et al.) reported in a magazine "O plus E" published in November
1997. The n output wavelength separator having an AWG structure has
a periodic wavelength passband when it is viewed from one output
terminal.
[0241] As shown in (a) of FIG. 9, when the n output wavelength
separator is used, therefore, the wavelength .lambda.dk of the
optical signal outputted from the electrical-optical converter 13k
and the wavelength .lambda.uk of the optical signal outputted from
the upstream light source 47k are previously adjusted so as to
coincide with the periodic wavelength passband of the corresponding
output terminal. Consequently, the optical signals having the two
different wavelengths .lambda.dk and .lambda.uk respectively
outputted from the electrical-optical converter 13kand the upstream
light source 47k can be together taken out of the same output
terminal.
[0242] On the other hand, not the above-mentioned n output
wavelength separator having the periodic wavelength passband but a
separator having a wavelength passband having a predetermined width
can be also used as the wavelength separator 460 when it is viewed
from one output terminal. When such a separator is used, as shown
in (b) of FIG. 9, the wavelength .lambda.dk of the optical signal
outputted from the electrical-optical converter 13k and the
wavelength .lambda.uk of the optical signal outputted from the
upstream light source 47k are previously adjusted so as to be in
close proximity to each other within the wavelength passband of the
output terminal. Consequently, the optical signals having the two
different wavelengths .lambda.dk and .lambda.uk respectively
outputted from the electrical-optical converter 13k and the
upstream light source 47k can be together taken out of the same
output terminal.
[0243] Referring to FIGS. 10 and 11, another configuration used for
each of the radio base stations 201 to 20n will be described.
[0244] FIG. 10 is a block diagram showing the configuration of the
radio base station 20k using an electro-absorption type modulator
29k in place of the two-wavelength separator 27k, the
optical-electrical converter 21k, the circulator 24k and the RF
modulator 28k in the radio base station 20k shown in FIG. 8. The
electro-absorption type modulator 29k is equipment having both an
optical-electrical conversion function and an electrical-optical
conversion function. Accordingly, the configuration of the radio
base station 20k can be simplified, as shown in FIG. 10. The
electro-absorption type modulator 29k is described in "Full-Duplex
Fiber-Optic RF Subcarrier Transmission Using a Dual-Function
Modulator/Demodulator" (Andreas Stohr, et al.) reported in a
document "IEEE Trans. Microwave Theory Tech. Vol. 47, No. 7"
published in 1999, for example.
[0245] The optical-electrical conversion efficiency and the
electrical-optical conversion efficiency of the electro-absorption
type modulator 29k have wavelength dependency and are such
characteristics that high efficiency is obtained in different
wavelength areas, as indicated by a dotted line in FIG. 11.
Consequently, suitable wavelength setting is performed such that
optical signals to be subjected to optical-electrical conversion
which are outputted from the electrical-optical converters 131 to
13n and optical signals to be subjected to electrical-optical
conversion which are outputted from the upstream light sources 471
to 47n are respectively arranged on the side of a short wavelength
and a long wavelength (FIG. 11), thereby making it possible to make
effective use of the electro-absorption type modulator 29k.
[0246] As described above, in the optical transmission system
according to the sixth embodiment of the present invention, in
order to transmit the radio signal received by the radio base
station to the center station, the radio signal can be
frequency-converted into the IF signal as an optical signal state
by wavelength-multiplexing a plurality of unmodulated light having
different wavelengths on a downstream optical signal and previously
externally modulating the light signal using the local oscillation
signal. Consequently, no electric frequency converter is required
for the radio base station, and an optical modulator for optically
performing frequency conversion can be shared among the plurality
of radio base stations. Further, the light source need not be
installed in the radio base station, so that the optical
transmission system can be easily maintained. Further, the
electro-absorption type modulator is used for optical receiving and
optical modulation, thereby making it possible to simplify the
configuration of the radio base station.
[0247] (Seventh Embodiment)
[0248] As described above, in a center station 100 in an optical
transmission system according to the present invention, an inputted
IF signal is converted into an optical signal through direct
modulation in an electrical-optical converter (e.g., a
semiconductor laser), and the optical signal is further
intensity-modulated again using a local oscillation signal in an
external modulator.
[0249] Letting cos(.omega.t) be the IF signal, sin(.omega.0t) be
the optical signal, m be the degree of optical modulation at the
time of direct modulation, .beta.1 be a light frequency modulation
index based on the IF signal, and .beta.L0 be a light phase
modulation index at the time of external modulation, the doubly
modulated optical signal (electric field representation; E(t)) is
expressed by the following equation (1): 1 E ( t ) = [ { 1 + J1 (
L0 ) cos ( L0t ) } { 1 + m cos ( ( t - 1 ) ) } ] * sin [ 0 t + 1
sin ( t ) ] = [ { 1 + m cos ( ( t - 1 ) ) + J1 ( L0 ) cos ( L0t ) +
mJ1 ( L0 ) cos ( L0t ) cos ( ( t - 1 ) ) ] * sin [ 0 t + 1 sin ( t
) ] ( 1 )
[0250] As apparent from the foregoing equation (1), the doubly
modulated optical signal E(t) has a light-intensity modulation
component obtained by frequency-modulating a modulation frequency
(.omega.) by the electrical-optical converter by a modulation
frequency (.omega.L0) by the external modulator.
[0251] Generally, the electrical-optical modulator such as the
semiconductor laser has lower distortion characteristics but has a
relatively narrower frequency band, as compared with the external
modulator. Contrary to this, the external modulator has broad-band
characteristics but has inferior distortion characteristics.
Consequently, the electrical-optical converter and the external
modulator are connected in cascade, thereby making it possible to
make use of low distortion characteristics of the
electrical-optical converter and wide band characteristics of the
external modulator. Therefore, it is possible to realize low
distortion transmission of a high frequency signal.
[0252] In a case where the electrical-optical converter and the
external modulator are simply connected in cascade, however,
wavelength distortion is induced in an optical signal outputted
from a light source for direct modulation by the cause of the
exertion of a light frequency modulation component (wavelength
chirping) and wavelength dispersion characteristics on the optical
signal. Particularly, transmission characteristics of the optical
signal are greatly degraded at the time of long-distance
transmission.
[0253] In the seventh embodiment, therefore, description is made of
a high frequency optical transmitter, which is applicable to the
center station 100, in which a light frequency modulation component
of a directly modulated optical signal is suppressed, and
transmission characteristics are not degraded at the time of
long-distance transmission.
[0254] FIG. 12 is a block diagram showing the configuration of a
high frequency optical transmitter according to a seventh
embodiment of the present invention. In FIG. 12, the high frequency
optical transmitter according to the seventh embodiment includes a
three-branching portion 510, first to fourth delay controllers 521
to 524, first and second multiplexers 531 and 532, an
electrical-optical converter 540, a differential light-intensity
modulator 550, a local oscillation signal source 560, and a
two-branching portion 570.
[0255] The high-frequency optical transmitter is applicable in
place of the configurations of the electrical-optical converter
130, the local oscillation signal source 140, and the external
modulator 150 in each of the center stations 100 in the first and
third embodiments, and is applicable in place of the configurations
of the light source 170, the local oscillation signal source 140,
the external modulator 150, the optical branching portion 160, and
the corresponding IF modulator 18k in the center station 100 in the
second embodiment. The high frequency optical transmitter is
applicable by inputting the signal obtained by multiplexing the IF
signals outputted from the plurality of modulators 111 to 11n to an
IF input terminal 51, inputting the optical signal outputted by the
wavelength multiplexer 430 or 431 to the differential
light-intensity modulator 550 as an optical signal outputted from
the electrical-optical converter 540, and inputting an output
signal of an output terminal 54 to the wavelength separator 460 in
the center station 100 in each of the fourth to sixth embodiments.
The operation of the high frequency optical transmitter according
to the seventh embodiment of the present invention will be
described.
[0256] An IF signal having an intermediate frequency inputted from
the IF input terminal 51 is branched into first to third IF signals
in the three-branching portion 510. The first and second IF signals
and the third IF signal are respectively inputted to the first and
second delay controllers 521 and 522 and the electrical-optical
converter 540. The first and second IF signals are set so as to be
the same in phase and differ in phase by 90.degree. from the third
IF signal.
[0257] The third IF signal is converted into an optical signal by
direct modulation in the electrical-optical converter 540 and is
outputted. At this time, the directly modulated optical signal
outputted from the electrical-optical converter 540 is subjected to
light intensity modulation as well as light frequency modulation,
and its electric field representation: ELD(t) is given by the
following equation (2), letting cos(.omega.t) be the IF signal,
sin(.omega.0t) be the optical signal, m be the degree of optical
modulation, and .beta.1 be a frequency modulation index based on
the IF signal:
ELD(t)={square root}{square root over ( )}[1+m
cos(.omega.t)]sin[.omega.0t- +.beta.1 sin(.omega.t)] (2)
[0258] On the other hand, the first IF signal is outputted to one
terminal of the first multiplexer 531 after the amount of
propagation delay is adjusted to a predetermined value by the first
delay controller 521. Similarly, the second IF signal is outputted
to one terminal of the second multiplexer 532 after the amount of
propagation delay is adjusted to a predetermined value by the
second delay controller 522.
[0259] A local oscillation signal outputted from the local
oscillation signal source 560 is branched into first and second
local oscillation signals which differ in phase by 180.degree. in
the two-branching portion 570. The first local oscillation signal
is outputted to the other terminal of the first multiplexer 531
after the amount of propagation delay is adjusted in the third
delay controller 523 such that it is equal in propagation time to
the first IF signal. Further, the second local oscillation signal
is outputted to the other terminal of the second multiplexer 532
after the amount of propagation delay is adjusted in the fourth
delay controller 524 such that it is equal in propagation time to
the second IF signal. The first multiplexer 531 multiplexes the
first IF signal and the first local oscillation signal, and the
second multiplexer 532 multiplexes the second IF signal and the
second local oscillation signal. Respective multiplexed signals are
outputted to the differential light-intensity modulator 550.
[0260] The differential light-intensity modulator 550 is a
Mach-Zehnder type modulator having two optical waveguides, as
illustrated in FIG. 13, and is so constructed as to respectively
apply voltage signals to electrodes provided in correspondence with
the optical waveguides, change the refractive index of each of the
optical waveguides to provide the difference in propagation time of
light to optical signals, and then multiplex the optical signals.
At this time, a bias voltage is applied to each of the electrodes
such that the difference in propagation time of light passing
through the two optical waveguides is converted into {fraction
(.pi./2)} in terms of light phase, and the first and second local
oscillation signals are respectively applied to the electrodes in
opposite phases.
[0261] Letting cos(.omega.L0t) be the local oscillation signal, and
.beta.L0 be both light phase modulation indexes based on the first
and second local oscillation signals in the differential
light-intensity modulator 550, the optical signal (electric field
representation: EEMi(t)) outputted from the differential
light-intensity modulator 550 is expressed by the following
equation (3) when no IF signal is inputted: 2 EEMi ( t ) = [ { 1 +
J1 ( 2 L0 ) cos ( L0t ) } { 1 + m cos ( ( t - 1 ) ) } ] * sin [ 0 t
+ 1 sin ( t ) ] = [ { 1 + m cos ( ( t - 1 ) ) + J1 ( L0 ) cos ( L0t
) + mJ1 ( L0 ) cos ( L0t ) cos ( ( t - 1 ) ) ] * sin [ 0 t + 1 sin
( t ) ] ( 3 )
[0262] From the foregoing equation (3), it is found that the
optical signal outputted from the differential light-intensity
modulator 550 has a light-intensity modulation component obtained
by converting a modulation frequency (.omega.) by the
electrical-optical converter 540 by a modulation frequency
(.omega.L0) by the differential light-intensity modulator 550.
[0263] Then consider a case where only IF signals are inputted to
the differential light-intensity modulator 550.
[0264] In this case, the first and second IF signals are
respectively applied to the electrodes of the differential
light-intensity modulator 550 in the same phase. Letting .tau.1 be
a time period elapsed from the time when the third IF signal is
outputted from the three-branching portion 510 until it propagates
to the differential light-intensity modulator 550 after being
converted into an optical signal in the electrical-optical
converter 540, and .tau.2 be both time periods respectively elapsed
from the time when the first and second electric signals are
outputted from the three-branching portion 510 until they propagate
to modulate the optical signals in the differential light-intensity
modulator 550, and .beta.2 be light phase modulation indexes based
on the first and second IF signals in the differential light
intensity modulator 550, the optical signal (electric field
representation: EEMp(t)) outputted from the differential
light-intensity modulator 550 is expressed by the following
equation (4) when no local oscillation signal is inputted.
EEMp(t)={square root}{square root over ( )}[1+m
cos(.omega.(t-.tau.1))]sin- [.omega.0t+.beta.1
sin(.omega.(t-.tau.1)) +.beta.2 cos[.omega.(t-.tau.2)+{- fraction
(.pi./2)}]] (4)
[0265] In the first and second delay controllers 521 and 522, when
the amount of delay is adjusted such that .tau.2=.tau.1, the
foregoing equation (4) is changed into the following equation
(5):
EEMp(t)={square root}{square root over ( )}[1+m
cos(.omega.(t-1))]sin[.ome- ga.0t+(.beta.1-.beta.2)
*sin(.omega.(t-.beta.1))] (5)
[0266] From the foregoing equation (5), the light frequency
modulation index .beta.1 caused by direct modulation can be
decreased to .beta.1-.beta.2 using double modulation by the
differential light-intensity modulator 550. Further, the frequency
modulation component of the optical signal can be completely
removed by making the phase modulation index .beta.2 equal to
.beta.1.
[0267] As described above, when both the local oscillation signal
and the IF signal are inputted to the differential light-intensity
modulator 550, the optical signal (electric field representation:
EEM(t)) outputted from the output terminal 54 is expressed by the
following equation (6) when .beta.2=.beta.1: 3 EEM ( t ) = [ { 1 +
J1 ( 2 L0 ) cos ( L0t ) } { 1 + m cos ( ( t - 1 ) ) } ] * sin ( 0 t
) = [ { 1 + m cos ( ( t - 1 ) ) + J1 ( 2 L0 ) cos ( L0t ) + mJ1 ( 2
L0 ) cos ( L0t ) cos ( ( t - 1 ) ) ] sin [ 0 t ] ( 6 )
[0268] As apparent from the foregoing equation (6), a light
frequency modulation component caused in the electrical-optical
converter 540 is removed from the optical signal outputted from the
differential light-intensity modulator 550 and at the same time,
the optical signal has a light-intensity modulation component
obtained by frequency-converting the modulation frequency (.omega.)
by the electrical-optical converter 540 by the modulation frequency
(.omega.L0) by the differential light-intensity modulator 550.
[0269] FIG. 14 illustrates an example in which a light frequency
modulation component caused by direct modulation is actually
suppressed by external modulation. Illustrated in (a) of FIG. 14 is
a light spectrum with the light frequency modulation component
caused by the direct modulation, while illustrated in (b) of FIG.
14 is a light spectrum with the light frequency modulation
component canceled by the external modulation. In FIG. 14, no local
oscillation signal is inputted to the differential light intensity
modulator 550. From FIGS. 14(a) and 14(b), it can be confirmed that
the light frequency modulation component can be suppressed by using
the high frequency optical transmitter according to the seventh
embodiment.
[0270] Description was made of an example in which the same local
oscillation signals having a phase difference of 180.degree. are
respectively inputted to two local oscillation input terminals 52
and 53. At this time, the light spectrum is an optical
double-sideband (DSB) signal having upper and lower
double-sidebands, as shown in (a) of FIG. 15. Generally, the
optical fiber has such wavelength dispersion characteristics that
it varies in group speed depending on the light wavelength (the
light frequency). When the optical DSB signal is transmitted,
therefore, the group speeds of the upper sideband and the lower
sideband do not coincide with each other, and a phase difference
occurs in electric signal components respectively obtained as beat
components of the upper sideband and a light carrier and the lower
sideband and a light carrier at the time of square detection by the
optical receiver. Particularly at the time of long-distance
transmission, both the electric signals may be canceled upon being
opposite in phase.
[0271] Examples of a method of avoiding the phenomenon include an
optical double-sideband (SSB) modulation scheme with only a
single-sideband, as shown in (b) of FIG. 15, and an optical
double-sideband (DSB-SC) modulation scheme with a light carrier
suppressed, as shown in (c) of FIG. 15. In the above-mentioned
configuration of the seventh embodiment, the local oscillation
signals having a phase difference 90.degree. are respectively
inputted to first and second local oscillation input(c) terminals
52 and 53, thereby making it possible to easily realize optical SSB
modulation. Further, a bias voltage is applied such that the
difference in propagation time of light passing through two optical
waveguides in the differential light-intensity modulator 550 is
.pi. in terms of light phase, and local oscillation signals having
a phase difference 180.degree. are respectively inputted to the
first and second local oscillation input terminals 52 and 53,
thereby making it possible to easily realize optical DSB-SC
modulation. In the case of the optical DSB modulation and the
optical DSB-SC modulation, the local oscillation signal may be
inputted to only one of the two terminals in the differential
light-intensity modulator 550, in which case the same effect is
obtained.
[0272] As described above, according to the high-frequency optical
transmitter according to the seventh embodiment of the present
invention, the differential light-intensity modulator 550 is caused
to perform a light phase modulating operation, thereby making it
possible to cancel the light frequency modulation component caused
at the time of direct modulation using the IF signal and at the
same time, frequency-convert the electric signal into the high
frequency signal by the light-intensity modulating operation using
the local oscillation signal. Consequently, the differential
light-intensity modulator 550 can have two functions, that is, an
optical frequency conversion function and a function of canceling
frequency modulation, thereby making it possible to obtain good
transmission characteristics even when the high frequency signal is
transmitted a long distance by an optical fiber having dispersion
characteristics.
[0273] (Eighth Embodiment)
[0274] FIG. 16 is a block diagram showing the configuration of a
high frequency optical transmitter according to an eighth
embodiment of the present invention. In FIG. 16, the high frequency
optical transmitter according to the eighth embodiment includes a
two-branching portion 575, an electrical-optical converter 540, a
delay controller 525, a local oscillation signal source 560, and a
phase modulator 581 and an intensity modulator 582 constituting an
integrated modulator 580. The operation of the high frequency
optical transmitter according to the eighth embodiment of the
present invention will be described.
[0275] An IF signal inputted from an IF input terminal 51 is
branched into first and second IF signals having a phase difference
of 90.degree. therebetween in the two-branching portion 575. The
first IF signal and the second IF signal are respectively inputted
to the electrical-optical converter 540 and the delay controller
525. The first IF signal is converted into an optical signal by
direct modulation in the electrical-optical converter 540, and the
optical signal is outputted to the phase modulator 581. The second
IF signal is inputted to an optical waveguide of the phase
modulator 581 through the delay controller 525. A time period
.tau.3 elapsed from the time when the first IF signal is outputted
from the two-branching portion 575 until it propagates to the phase
modulator 581 after being converted into the optical signal in the
electrical-optical converter 540 and a time period .tau.4 elapsed
from the time when the second IF signal is outputted from the
two-branching portion 575 until it is modulated in the phase
modulator 581 are made equal to each other, thereby making it
possible to reduce a light frequency modulation component caused at
the time of direct modulation.
[0276] Generally, the amount of transmission delay is found by
measuring the level of the inputted signal and the level of the
received signal. In a case where the electric signal is
phase-modulated by the phase modulator 581, even if the optical
signal is converted into the electric signal on the side of light
receiving, no electric signal component is obtained. Accordingly,
the amount of transmission delay cannot be measured.
[0277] Therefore, the integrated modulator 580 constructed by
integrating the phase modulator 581 and the intensity modulator 582
is used to measure an amount of transmission delay .tau.4' in a
case where an IF signal is inputted to the intensity modulator 582
and is transmitted by intensity modulation. On the basis of the
results, the amount of delay of the delay controller 525 is first
coarsely adjusted such that .tau.3=.tau.4'. Thereafter, the amount
of delay of the delay controller 525 may be precisely adjusted such
that the light frequency modulation component is the minimum by
inputting the IF signal which has been inputted to the intensity
modulator 582 again to the phase modulator 581 to which the IF
signal is to be inherently inputted, and measuring a light spectrum
of the IF signal which has been subjected to direct modulation in
the electrical-optical converter 540 and phase modulation in the
phase modulator 581 using a light heterodyne method, for
example.
[0278] The eighth embodiment is superior to the seventh embodiment
in that a loss to the local oscillation signal may be small because
it can be directly applied to the intensity modulator 582 without
passing through a multiplexer or the like.
[0279] As described above, in the high frequency optical
transmitter according to the eighth embodiment of the present
invention, the phase modulator 581 and the intensity modulator 582
are integrated in the integrated modulator 580, thereby making it
possible to easily make delay adjustment for canceling the light
frequency modulation component. Further, a light phase modulating
operation for directly inputting the IF signal and the local
oscillation signal, respectively, to the phase modulator and the
intensity modulator without multiplexing the signals to suppress a
light frequency component caused at the time of direct modulation
and a light-intensity modulating operation for frequency conversion
using the local oscillation signal are performed, thereby making it
possible to reduce a loss to each of the signals and to perform
optical modulation more efficiently.
[0280] Although in the above-mentioned seventh and eighth
embodiments, description was made of a case where the high
frequency optical transmitter is applied to such a configuration
that information is transmitted in a downstream direction from the
center station to the subscriber terminals, the high frequency
optical transmitter is also similarly applicable to such a
configuration that information is transmitted in an upstream
direction from the subscriber terminals to the center station
100.
[0281] While the invention has been described in detail, the
foregoing description is in all aspects illustrative and not
restrictive. It is understood that numerous other modifications and
variations can be devised without departing from the scope of the
invention.
* * * * *