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.