SCM type of optical signal transmission system without clipping distortion

A subcarrier multiplexed type of optical signal transmission system includes a transmitting unit, a receiving unit, and a single optical fiber. The transmitting unit includes a plurality of multiplexing circuits, each of which frequency-division-multiplex of analog image signals to produce an electric signal; a plurality of modulating units provided for the remaining multiplexing circuits, wherein each of the plurality of second modulating units performs analog-modulation of an optical oscillation signal in accordance with the electric signal without clipping to produce an optical signal; and a coupling unit for combining the optical signals to produce a transmission optical signal which is to be transmitted onto a single optical fiber. In this case, a distributed feedback type semiconductor laser is preferable as each of the plurality of modulating units. Further, each of the plurality of modulating units is preferably provided with an optical isolator at the output side for preventing a reflected optical signal from being inputted. In order to prevent any clipping distortion, it is preferable that an optical modulation is small for each of the plurality of modulating units, e.g., semiconductor lasers and an effective analog-modulation index of the optical oscillation signal in each of the remaining modulating units is equal to or less than 20%.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical signal transmission system, and 
more particularly, to an optical signal transmission system in analog data 
service such as a bidirectional cable television (CATV) in which high 
carrier to noise ratio (CNR) characteristic is requested. 
2. Description of the Related Art 
In the next generation CATV systems discussed recently from various 
viewpoints, a digital video signal needs to be transmitted for broad band 
bidirectional communication services by video on demand (VOD) signals from 
subscribers services, as well as conventional broadcasting image signals 
(vestigial sideband--analog modulation (VSB-AM) signals). Further, it is 
importance to implement such a transmission system with low cost. 
As a transmission system which satisfy the needs, there is a subcarrier 
multiplexed (SCM) system which is described by Robert Olshansky et al. in 
"Subcarrier Multiplexed Lightwave Systems for Broad-Band Distribution", 
(Journal of Lightwave Technology, Vol. 7, No. 9, September 1989, pp. 
1329-1341) as reference 1 and by Winston I. Way in "Subcarrier Multiplexed 
Lightwave System Design Considerrations For Subscriber Loop applications", 
(Journal of Lightwave Technology, Vol. 7, No. 11, November 1989, pp. 
1806-1818) as reference 2. 
This SCM system is an optical signal transmission system using a single 
mode optical fiber as a transmission medium. Base band analog or digital 
signals for a plurality of channels, e.g., 60 channels are first 
high-frequency converted or up-converted using local oscillation circuits 
(LOs) having different frequencies and then 
frequency-division-multiplexed. These up-converted signals are synthesized 
or combined and a laser unit is modulated in accordance with the 
synthesized high-frequency signal or a combined signal. The frequency of 
an oscillation signal from the local oscillation circuit is a subcarrier 
for an optical carrier frequency. In the receiving unit, one of the 
frequency-division-multiplexing (FDM) channels is selected by a subscriber 
through a tuning operation of a local oscillation circuit, like the usual 
tuning operation in a television receiver and radio. A high 
frequency/microwave signal of the selected FDM channel is 
low-frequency-converted or down-converted to restore an original base band 
analog or digital signal. 
In the above SCM system, in a case where analog image signal for 60 
channels and a VOD signal as a digital video signal are simultaneously 
transmitted, there is a problem in that noise is generated due to clipping 
distortion so that a bit error rate of the VOD signal is increased because 
an effective optical modulation exceeds 20% for the image signals for 60 
channels. 
In order to solve the problem, an optical SCM transmission system is 
proposed in the proceedings of the 1994 spring Conference of IEICE 
(B-1128) (reference 3). In the reference 3, there is discussed an optimal 
method in which the bit error rate due to the clipping distortion can be 
reduced in the transmission of the combined signal composed of a VSB-AM 
signal as analog image signals and multivalue digital carrier signal 
(16QAM) as the digital video signal (to be referred to as a VOD signal) 
and it is concluded that it is effective to separate an image signal 
transmission system and a VOD signal transmission system from each other 
such that the VSB-AM signal and the 16QAM signal are transmitted in 
parallel. 
FIG. 1 is a block diagram showing a conventional optical signal 
transmission system described in the reference 3. Referring to FIG. 1, the 
conventional optical signal transmission system includes an optical signal 
transmitting unit 101 for outputting an optical signal LF60 corresponding 
to image signals 60VA for 60 channels and an optical signal LV 
corresponding to a VOD signal VD independently, optical fibers 102 and 104 
for transferring the optical signals LF60 and LV, respectively, and an 
optical signal receiving unit 103 for receiving the optical signals LF60 
and LV and detecting a reception combined signal. 
The optical signal transmitting unit 101 includes frequency division 
multiplexing (FDM) circuits 111 for frequency-division-multiplexing the 
image signals 60VA for 60 channels to produce an FDM signal F60, a 
semiconductor laser (LD) 112 which is subjected to analog-modulation based 
on the FDM signal F60 and outputs an optical signal LF60 on the optical 
fiber 102 via an optical isolator 113, the optical isolator 113 for 
preventing reflected light of the optical signal LF60 from being inputted 
into the semiconductor laser 112, an FDM circuit 114 for 
frequency-division-multiplexing the VOD signal VD to produce an FDM signal 
FV, and a semiconductor laser (LD) 115 which is subjected to 
analog-modulation based on the FDM signal FV and outputs an optical signal 
LV on the optical fiber 104. The receiving unit 103 includes an optical 
signal detector (PD) 121 coupled to the optical fiber 102, for receiving 
and photoelectrically converting the optical signal LF60 into a reception 
signal R60 and an optical signal detector (PD) 122 coupled to the optical 
fiber 104, for receiving and photoelectrically converting the optical 
signal LV into a reception signal RV. 
Next, the operation of the conventional optical signal transmission system 
will be described with reference to FIGS. 2A and 2B which indicate the 
examples of analog-modulation of a semiconductor laser (LD). As shown in 
FIG. 2A, the analog-modulation of the semiconductor laser is a so-called 
small amplitude modulating operation in which the semiconductor laser is 
supplied with a current which satisfies the oscillation condition of the 
semiconductor laser, i.e., a constant bias current higher than a threshold 
current and a modulation current is superposed on the bias current. If the 
amplitude of the modulation current becomes greater so that the negative 
peak of the modulation current is decreased lower than the threshold 
current as shown in FIG. 2B, a part of the modulated light output signal, 
the negative peak in the example is cut so that the clipping distortion is 
caused. Also, when the output of the semiconductor laser is saturated at 
the positive peak, the clipping distortion is caused. Thus, the clipping 
distortion is caused when the optical modulation of the semiconductor 
laser is more than 100%. 
In this conventional optical signal transmission system, since an FDM 
signal F60 corresponding to the image signals for 60 channels needs to be 
transmitted, the optical signal outputted from the single semiconductor 
laser 112 is modulated in accordance with the FDM signal F60. In this 
case, an effective optical modulation .mu.e is represented by the 
following equation for the FDM signal F60. 
##EQU1## 
where N is the number of image signals which constitute the FDM signal F60 
and .mu.i is the optical modulation index for the image signal VAi, A 
relation between the effective modulation index .mu.e and the clipping 
distortion is experimentally obtained. For instance, when an optical 
modulation index is 5% for an image signal for one channel and image 
signals for 60 channels are supplied to the semiconductor laser 112 as the 
FDM signal F60, the effective optical modulation index .mu.e of the 
semiconductor laser 112 is about 27%. 
FIG. 3 is a graph showing a relation of a bit error rate (BER) to a carrier 
to noise ratio (CNR) when image signals of a plurality of channels and a 
VOD signal of one channel are simultaneously transmitted through 
modulation by a single semiconductor laser. It can be seen that a bit 
error is abruptly increased if the effective optical modulation index 
exceeds 20%. This is because the impulse noise due to the clipping 
distortion adversely influences to the modulated signal. 
SUMMARY OF THE INVENTION 
Therefore, the present invention has, as an object, to provide an optical 
signal transmission system in which image signals can be transmitted 
without any clipping distortion and a transmitting method for the 
transmission system. 
Another object of the present invention is to provide an optical signal 
transmission system in which image signals can be transmitted without 
adverse influence of beat noise and a transmitting method for the 
transmission system. 
In order to achieve an aspect of the present invention, a subcarrier 
multiplexed type of optical signal transmission system includes a 
transmitting unit, a receiving unit, and a single optical fiber. The 
transmitting unit includes a plurality of multiplexing circuits, each of 
which frequency-division-multiplex of analog image signals to produce an 
electric signal; a plurality of modulating units provided for the 
remaining multiplexing circuits, wherein each of the plurality of second 
modulating units performs analog-modulation of an optical oscillation 
signal in accordance with the electric signal without clipping to produce 
an optical signal; and a coupling unit for combining the optical signals 
to produce a transmission optical signal which is to be transmitted onto a 
single optical fiber. In this case, a distributed feedback type 
semiconductor laser is preferable as each of the plurality of modulating 
units. Further, each of the plurality of modulating units is preferably 
provided with an optical isolator at the output side for preventing a 
reflected optical signal from being inputted. 
A CATV system to which the subcarrier multiplexed type of optical signal 
transmission system is applied, may further includes an additional 
multiplexing circuits for frequency-division-multiplexing digital image 
signals for video on demand (VOD) to produce an additional electric 
signal; and an additional modulating unit provided for the additional 
multiplexing circuit, for performing analog-modulation of an optical 
oscillation signal in accordance with the additional electric signal to 
produce an additional optical signal. In the case, the coupling unit 
combines the optical signals and the additional optical signal to produce 
the transmission optical signal. 
In order to prevent any clipping distortion, it is preferable that an 
optical modulation index is small for each of the plurality of modulating 
units, e.g., semiconductor lasers and an effective analog-modulation of 
the optical oscillation signal in each of the remaining modulating units 
is equal to or less than 20%. 
It is preferable that the plurality of modulating units respectively 
include 1.3 .mu.m wavelength semiconductor lasers in which wavelengths of 
the optical oscillation signals are different from one after another by at 
least 0.1 nm, because any beat noise can be avoided when the transmission 
band of the optical channel signals is in a range of about 50 MHz to 1 
GHz. 
In order to achieve another aspect of the present invention, a method of 
transmitting image signals using a single optical fiber comprising the 
steps of: 
frequency-division-multiplexing sets of analog image signals into electric 
signals, respectively; 
analog-modulating of optical oscillation signals into optical signals in 
accordance with the electric signals, respectively; and 
synthesizing the optical signals into a transmission optical signal which 
is to be transmitted onto the single optical fiber. 
The method may further includes frequency-division-multiplexing digital 
image signals into an additional electric signal; and analog-modulating of 
an optical oscillation signal into an additional optical signal in 
accordance with the additional electric signal such that any clipping 
distortion does not occur. In this case, the optical signals and the 
additional optical signal are synthesized into the transmission optical 
signal. Further, an effective analog-modulation in the analog-modulating 
in accordance with the electric signals is preferably equal to or less 
than 20%. Further, the analog-modulating into the optical signals is 
preferably performed such that a difference in frequency between any two 
of the optical oscillation signals falls out of transmission bands of the 
analog image signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The optical signal transmission system according to the present invention 
will be described below in detail with reference to the accompanying 
drawings. 
First, the structure of the optical signal transmission system according to 
an embodiment of the present invention will be described with reference to 
FIG. 4. 
In FIG. 4, the optical signal transmission system includes an optical 
signal transmitting unit 1, an optical fiber 2, and an optical signal 
receiving unit 3. The optical signal transmitting unit 1 combines optical 
signals LF20A, LF20B and LF20C for three channels each of which 
corresponds to image signals 20VAA, 20VAB or 20VAC for 20 channels, and an 
optical signal LV corresponding to a VOD signal VD to produce a 
combination optical signal LF20ABCV and transmits the combination optical 
signal LF20ABCV onto the optical fiber 2. The optical signal receiving 
unit 3 receives the combination optical signal LF20ABCV to produce a 
reception combination signal R60. 
The optical signal transmitting unit 1 includes frequency divisional 
multiplexing (FDM) circuits 11, 12 and 13, semiconductor lasers (LDs) 14, 
15 and 16, an FDM circuit 17, a semiconductor laser (LD) 18, an optical 
isolator 19, 20 and 21, and optical couplers 22, 23 and 24. The FDM 
circuits 11, 12 and 13 frequency-divisional-multiplex image signals 20VAA, 
20VAB and 20VAC for 20 channels to produce FDM signals F20A, F20B and 
F20C, respectively. The semiconductor lasers (LDs) 14, 15 and 16 output 
optical signals LF20A, LF20B and LF20C from the FDM signals F20A, F20B and 
F20C, respectively. The FDM circuit 17 frequency-divisional-multiplexes a 
VOD signal VD to produce an FDM signal FV. The semiconductor laser (LD) 18 
outputs an optical signal LV from the FDM signal FV. The optical isolators 
19, 20 and 21 are provided at the output side of the semiconductor lasers 
(LDs) 14, 15 and 16 to prevent an optical signal reflected by a load side 
from being inputted, respectively. The optical coupler 22 of 3 dB combines 
the optical signals LF20A and LF20B outputted from the semiconductor 
lasers 15 and 16 to produce an optical signal LF20AB and the optical 
coupler 23 combines the optical signals LF20C and LV outputted from the 
semiconductor lasers 16 and 18 to produce an optical signal LF20V. The 
optical coupler 24 combines the optical signal LF20AB and the optical 
signal LF20V to produce the optical signal LF20ABCV for transmission of it 
to the optical signal receiving unit 3 via the optical fiber 2. The 
receiving unit 3 includes an optical signal detector (PD) 31 for receiving 
the optical signal LF20ABCV and performs photoelectric conversion to the 
received optical signal to produce a reception signal R60. 
Next, the operation of the optical signal transmission system according to 
the embodiment of the present invention will be described below. In this 
embodiment, assume that image signals for 60 channels and VOD signals for 
260 channels for requests from subscribers are transmitted via a single 
optical fiber in the optical signal transmission system for a CATV system. 
As described in the related art, in a case where these signals is 
transmitted using a single semiconductor laser, if an optical signal 
modulation index of the image signal is 5% per a channel, an effective 
optical signal modulation is about 27% for the image signals of 60 
channels. As a result, clipping distortion occurs. If the optical signal 
modulation is decreased in order to prevent the occurrence of clipping 
distortion, the effective optical signal modulation can be decreased for 
60 channels. In this method, however, there is a problem that a carrier to 
noise ratio (CNR) is reduced. 
In order to remove the influence by the clipping distortion and to keep a 
high CNR, a plurality of semiconductor lasers are provided to restrict the 
effective optical signal modulation index to 20% or below in the 
embodiment. If the optical signal modulation index of the image signal is 
5% per a channel, like the conventional technique, the effective optical 
signal modulation index .mu.e can be suppressed to 20% or below when up to 
32 channels are frequency-divisional-multiplexed by a single semiconductor 
laser. Therefore, it is preferable that at least two semiconductor lasers 
are used in a case of transmission of 60 channels of image signals and at 
least three semiconductor laser are used in a case of transmission of 80 
channels of image signals. 
In this embodiment, the three semiconductor lasers 14, 15 and 16 are 
provided for the image signals for 60 channels, that is, the 
analog-modulation is performed for the FDM signals F20A, F20B and F20C 
corresponding to the image signals for 20 channels 20VAA, 20VAB and 20VAC 
by the semiconductor lasers 14, 15 and 16, respectively. In this case, if 
the optical modulation index is 6.3% or below for one channel, the 
effective optical modulation index .mu.e is equal to or less than 20% from 
the above equation, resulting in no clipping distortion. On the other 
hand, the VOD signal is transmitted through the same optical fiber 2 using 
the semiconductor laser 18. The optical signals outputted from the 
semiconductor lasers 14 to 18 are synthesized or combined by the 3-dB 
optical coupler 22 to 24 to a single optical signal LF20ABCV which is 
transmitted through the single optical fiber 2. In this case, a single 
optical fiber having 4 inputs and 1 output may be used, if available. 
The semiconductor laser 14 to 16 need to have a specifically small 
intermodulation distortion for analog modulation of the FDM signals 
corresponding to analog image signals. In the embodiment, a semiconductor 
laser of a distributed feedback (DFB) type is used which is manufactured 
for the CATV system and has a distortion characteristic as low as a 
composite second order distortion (CSO) of -60 dB and a composite triple 
beat (CTB) less than -70 dB. On the other hand, a DFB type of 
semiconductor laser having a usual distortion characteristic is sufficient 
for the semiconductor laser 17 by which the FDM signal FV corresponding to 
the VOD signal is modulated. 
In a case that the optical signals outputted from the semiconductor lasers 
are synthesized by the optical couplers as in the embodiment, beat noise 
is generated corresponding to the difference in frequency between any two 
of the lasing wavelengths of the semiconductor lasers. Therefore, the 
frequencies of the beat noise need to be set outside of the transmission 
band for the signals, i.e., the FDM signals F20A, F20B, F20C and FV. The 
laser oscillation frequency or wavelength of each of the semiconductor 
lasers is set such that the beat noise frequency is sufficiently higher or 
lower than the transmission band. In the SCM type of CATV system, the 
transmission band is in a range of about 50 MHz to 1 GHz. In a case of 1.3 
.mu.m wavelength semiconductor laser, if the difference in wavelength 
between two laser oscillations is less than 0.00028 nm or more than 0.0057 
nm, the above condition can be satisfied. Therefore, the semiconductor 
laser is sufficient to have the wavelength difference of about 0.1 nm 
between the laser oscillations. In this embodiment, 1.3 .mu.m wavelength 
semiconductor lasers having the wavelength difference of laser oscillation 
of 0.1 nm or above are used as the semiconductor lasers 14-16 to 18. 
As described above, in the optical signal transmission system according to 
the present invention, since image signals for a plurality of channels are 
allocated to the semiconductor lasers each of which performs 
analog-modulation for the allocated image signals, the effective optical 
modulation of each semiconductor laser is sufficiently lower than a level 
at which clipping distortion is caused. Therefore, the 
frequency-division-multiplexed signals for the image signals and the 
digital video signal for the video on demand can be transmitted without 
occurrence of any clipping distortion.