The present invention relates to signal converter, optical transmitter and optical fiber transmission system for transmitting a multichannel analog signal, a digital video signal or the like through an optical fiber.
In a suggested method for transmitting and distributing a multichannel video signal to users' homes, the output light of a semiconductor laser is directly modulated with the multichannel video signal, and an optical signal resulting therefrom is transmitted through an optical fiber and directly detected by an optical receiver. An amplitude-modulated (AM) video signal transmission technique is currently in high demand, because this technique has excellent compatibility with existing CATV's. However, in accordance with this technique, excellent carrier-to-noise ratio (CNR) and distortion characteristics are required. Accordingly, in employing this technique, the transmission distance and the number of branches of an optical fiber are adversely limited and the resistance of an optical fiber connector to reflection is disadvantageously low.
In order to solve these problems, a method for distributing a multichannel video signal using an AM/FM simultaneous converter of an optical heterodyne detection type is proposed. Such a method is described by K. Kikushima et al., “Super-wide-band Optical FM Modulation Scheme and its Application to Multichannel AM Video Transmission Systems, IEEE Photonics Technology Letters, pp. 839-841, 1996, for example.
Hereinafter, a conventional optical fiber transmission system of an AM/FM simultaneous conversion type will be described with reference to FIGS. 1A through 1C.
As shown in FIG. 1A, an optical transmitter 1 of this optical fiber transmission system includes an AM/FM converter 2 and a semiconductor laser 3 for transmission. The optical transmitter 1 generates an optical signal. The intensity of the optical signal has been modulated with a microwave signal, the frequency of which has been modulated with an AM multichannel video signal. The optical signal output from the optical transmitter 1 is amplified by an optical fiber amplifier 4. The amplified signal is branched by an optical fiber coupler 5 into respective paths of an optical fiber 6, through which the optical signal is transmitted. The optical signal, which has been transmitted through the optical fiber 6, is received by an optical receiver 7. Specifically, an avalanche photodiode (APD) 8 of the optical receiver 7 receives the optical signal. The APD 8 converts the optical signal into a microwave signal, the frequency of which has been modulated with the AM multichannel video signal. And the microwave signal is demodulated by an FM demodulator 9 into the AM multichannel video signal.
FIG. 1B illustrates the internal configuration of the AM/FM converter 2. In the AM/FM converter 2, first, a semiconductor laser 10 is subjected to frequency modulation. Next, the output light of a local oscillator laser 11 is coupled with the output light of the semiconductor laser 10 at an optical coupler 12. Part of the coupled light is irradiated to a photodiode 14, which performs a heterodyne detection on the light so as to output a microwave signal having had the frequency modulated with the AM multichannel video signal. The carrier frequency of the microwave signal is equal to a beat frequency (υ2−υ1), which is the difference between the frequency υ1 of the semiconductor laser 10 and the frequency υ2 of the local oscillator laser 11.
The other part of the coupled light is irradiated to the other photodiode 13, which also performs a heterodyne detection on the light so as to output a microwave signal. The microwave signal is fed back to the semiconductor laser 10 through an auto frequency control (AFC) loop 15. This feedback loop can control the driving current of the semiconductor laser 10 and stabilize the carrier frequency. The AFC loop 15 includes an FM modulator 16 and an FM laser current controller 17.
Next, the semiconductor laser 3 for transmission (i.e., a distributed feedback (DFB) laser) is subjected to intensity modulation with the output signal of the AM/FM converter 2. As a result, the optical signal is output from the optical transmitter 1.
FIG. 1C illustrates the configuration of the FM demodulator 9 in the optical receiver 7. The FM demodulator 9 includes AND gates 18, 19, a delay line 20 and an amplifier 21 and demodulates the microwave signal output by the APD 8 into a multichannel video signal.
In accordance with such a transmission system, the minimum light-receiving level can be increased by about 10 dB and the reflective resistance of the optical fiber connector can be considerably improved as compared with a conventional AM transmission technique.
This conventional converter uses the AFC loop 15. However, if the temperature of the environment surrounding the semiconductor laser 10 changes, then the respective frequencies υ1 and υ2 of the semiconductor laser 10 and the local oscillator laser 11 also change. Accordingly, the intermediate frequency fIF (=υ2−υ1) is greatly variable with the changing environmental temperature. FIG. 2A illustrates the spectra of the laser light emitted from the semiconductor laser 10 and the laser light emitted from the local oscillator laser 11. As for the laser light emitted from the semiconductor laser 10, both the spectrum of the laser light at the frequency υ1 (having a relatively narrow distribution) and the spectrum of the AM multichannel video signal carried by the laser light (having a relatively broad distribution) are illustrated. FIG. 2B illustrates the spectrum of the carrier of the microwave signal at the intermediate frequency fIF (=υ2−υ1) and the spectrum of the AM multichannel video signal carried by the carrier. As described above, as the intermediate frequency fIF (=υ2−υ1) greatly changes with the changing environmental temperature, the spectrum of the intermediate frequency fIF shown in FIG. 2B has a broader width. FIG. 2C illustrates the intensity ratio of carrier to noise. As the width of the spectrum of the intermediate frequency fIF becomes broader, the intensity of the noise component increases relative to the intensity of the carrier. As a result, the CNR (carrier-to-noise ratio) decreases and the signal quality deteriorates.
In addition, in the conventional optical receiver 7, the optical signal needs to be received by the APD 8 operating in a broad band (e.g., 6 GHz) and the FM demodulator 9 needs to convert the optical signal into an electrical signal by using high-speed AND gates 18 and 19 that can operate at 6 GHz. In connecting high-speed devices such as these on multiple stages, since the frequency characteristics of the respective devices deviate from each other in terms of amplitude and group delay, such deviations are added to each other, thereby deteriorating noise and distortion characteristics. The broader band operation is advantageous in improving the noise characteristics. However, so long as electric devices such as AND gates are used, there is a limit on the high-speed operation. Since a band limitation is imposed, the deterioration of noise and distortion characteristics is inevitable.