Abstract:
An FM signal converter comprising: 
     an amplitude detecting unit for detecting amplitude variation of a plurality of signals that are multiplexed with subcarriers, as an amplitude variation signal; 
     a peak detection unit for determining from said amplitude variation signal whether a peak of the amplitude of said plurality of signals exceeds a threshold and for generating peak detection information that includes information about said peak of the amplitude; 
     a frequency signal source for providing signal with a predetermined frequency that differs from any of the frequencies of said subcarriers; 
     an amplitude phase control unit for adjusting amplitude and phase of the signal from the frequency signal source according to said peak detecting information and outputting the adjusted signal as a corrected signal; 
     signal combining means for combining said corrected signal and said plurality of signals multiplexed with subcarriers, with considering a time for generating the corrected signal; and 
     an FM modulator for modulating said signal from the signal combining means into an FM signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to FM signal converters and optical transmitting apparatus for use in telecommunications, CATVs, measurements, mobile communications and the like. 
     2. Releted art of the Invention 
     Recently, multi-channel optical transmission of video picture, voice or data that exploits low-loss broadband characteristics of optical fibers is made available in video monitoring systems, CATVs, subscriber lines and mobile communications. In such optical transmissions, multi-channel signal is electrically multiplexed by a plurality of sub-carriers with different frequencies into AM-SCM signal. The multiplexed signal is converted to optical signal by directly modulating e.g., semiconductor lasers to transmit via optical fiber. 
     Video optical transmission of AM signal is not costly because the modem configuration is simple. However, in order to achieve a C/N (carrier to noise ratio) associated with a desired video quality, high optical input power is required in the receiver side. Also, in mobile communication, since intensity level of transmitted voice and data signal vary substantially when a mobile terminal moves, high dynamic range against the signal variation is required. Further, distortion caused by reflected wave during optical conversion of semiconductor lasers and during optical fiber transmission may effect much, and an amplifier of AM signal requires good linear characteristics. 
     Following above needs, in order to improve anti-distortion and anti-noise of the optical transmission, an approach for optical transmission that converts subcarrier multiplexed AM signal together into FM signal is proposed. In order to get desired C/N value in optical transmission, modulation index is required to be large. 
     FIG. 9 shows a configuration of a prior art FM signal optical transmitting apparatus. FIG. 9 shows an optical transmitter  111 , an FM modulator  101 , a driver amplifier  201 , a semiconductor laser  210 , optical fiber  27 , an optical receiving unit  22 , a photodetector  23 , a preamplifier  24 , an FM demodulator  25  and a low-pass filter  26 . 
     Operation of such a prior art FM signal optical transmitting apparatus with such a configuration is as follows. Multi-channel frequency multiplexed AM video signal (AM-SCM signal) is provided to an FM modulator  101  and broadband FM modulated signal is outputted. 
     As shown in FIG.  7 ( a ), when the FM modulator directly modulates an FM laser (FM-LD)  81  via AM signal, not only amplitude of light is modulated but oscillating frequency of the light is also modulated. After this light is multiplexed by a multiplexer  83  with a local light source (LO-LD)  82  that has slightly different oscillating frequency, the light is provided to a photodiode  84  to be optical heterodyne detected, and broadband (for example, 1 to 6 GHz) FM modulated signal is obtained as the beat signal of two lasers. Using this FM modulated signal, a transmitting semiconductor laser  210  is directly modulated via a laser driving amplifier  201 , and the signal is converted to optical signal to be transmitted via optical fibers  72 . (For example, refer to Japanese Patent No. 2700622.) 
     Then, after the optical signal is amplified by e.g., optical amplifier, the signal is transmitted to a respective receiving unit via an optical branching unit. Illustratively, only one receiving unit  22  is shown. In the receiving unit  22 , after the optical signal is converted to electrical signal by the photodetector  23  and is amplified by the preamplifier  24 , the signal is demodulated into original AM-SCM signal in the FM demodulator  25  and a low pass filter. For example, the FM demodulator  25  is a delayed demodulating circuit and is composed of a high-speed logic IC (ex. AND gate) and a delaying unit, and a limiter amplifier. 
     As described, in prior art FM signal optical transmitting apparatus, in order to achieve the desired carrier-to-noise ratio (CNR) by FM modulation and demodulation, multi-channel AM-SCM signal with large amplitude variation is provided to an FM modulator since large frequency deviation is required. Multi-channel signal like this multi-channel AM-SCM signal generates large amplitude variation instantaneously if there are no correlation between each of the signals. 
     Therefore, in modulating operation of the FM modulator  101 , an instantaneous amplitude variation of a voltage or a current of the AM-SCM signal directly becomes same with the frequency deviation. 
     However, in FM modulators in general, oscillating frequency deviation does not have linear characteristic against input amplitudes. As shown in FIG. 8, regarding curve  71  that denotes oscillating frequency deviation against amplitude variation, suppose the instantaneous amplitude variation  61  had a large peek value at time “tp” instantaneously, it may exceed a range (“A 0 +Ath” in FIG. 8) that the relationship between the amplitude variation and the oscillating frequency signal deviation remains linear, and may be FM modulated in non-linear area. 
     If these FM modulated signal is demodulated, modulated components in the above non-linear area may appear as a distortion in the demodulated signal, and aggravate the co-modulation distortion of the AM-SCM signal. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a low cost FM signal optical transmitter that has a simple configuration, a low distortion and a good receiver sensitivity and that can transmit multi-channel AM-SCM signal, and an FM signal converter for enabling the above mentioned FM signal optical transmitter, and a low cost FM signal optical receiver. 
     An FM signal converter of the present invention comprises: 
     an amplitude detecting unit for detecting amplitude variation of a plurality of signals that are multiplexed with subcarriers, as an amplitude variation signal; 
     a peak detection unit for determining from said amplitude variation signal whether a peak of the amplitude of said plurality of signals exceeds a threshold and for generating peak detection information that includes information about said peak of the amplitude; 
     a frequency signal source for providing signal with a predetermined frequency that differs from any of the frequencies of said subcarriers; 
     an amplitude phase control unit for adjusting amplitude and phase of the signal from the frequency signal source according to said peak detecting information and outputting the adjusted signal as a corrected signal; 
     signal combining means for combining said corrected signal and said plurality of signals multiplexed with subcarriers, with considering a time for generating the corrected signal; and 
     an FM modulator form modulating said signal from the signal combining means into an FM signal. 
     The FM signal converter according to said present invention is also such that 
     the plurality of signals multiplexed with subcarriers can be obtained by suppressing a component of a carrier having said predetermined frequency out of a signal demodulated from said FM signal. 
     The FM signal converter according to said present invention is also such that 
     said amplitude phase control unit adjusts amplitude and phase of the signal from the frequency signal source in such manner that amplitude level of voltage or current of said corrected signal is smaller than said predetermined threshold. 
     The FM signal converter according to said present invention is also such that 
     said predetermined threshold is determined according to oscillating frequency deviation characteristics of said FM modulator. 
     The FM signal converter according to said present invention is also such that 
     the signal outputted by said frequency signal source has at least one carrier frequency. 
     The FM signal converter-according to present invention is also such that 
     the signals outputted by said frequency signal source are such signals that is converted from said plurality of signals multiplexed by subcarriers so that frequency band of converted signals do not overlap with those of said plurality of signals. 
     The FM signal converter according to said present invention is also such that 
     said signals having at least one carrier frequency are a plurality of signals-with different phases. 
     The FM signal converter according to said present invention is also such that 
     the signals outputted by the frequency signal source are signals converted from said plurality of signals multiplexed by subcarriers so that phase of converted signals do not overlap with those of said plurality of signals. 
     The FM signal converter according to said present invention can further comprise: 
     a signal switching unit for selectively outputting the signal outputted by the frequency signal source according to the magnitude of said peak of the amplitude. 
     The FM signal converter according to said present invention is also such that 
     said signal switching unit selectively outputs a signal with optimum phase out of signals provided from said frequency signal source. 
     The FM signal converter according to any one of said present inventions is also such that 
     a voltage-controlled oscillator is provided as said FM modulator. 
     The FM signal converter according to any one of said present inventions is also such that 
     two semiconductor laser devices are provided as said FM modulator each which oscillates with single mode frequency and frequencies which are oscillated from said tow laser devices are proximate to each other, and further comprises: 
     an optical detector for O(optical)/E(electric) converting such multiplexed signal that is obtained, 
     by directly modulating output signal from said signal combining means by one of said semiconductor laser device and multiplexing the modulated signal with signal from the other said semiconductor laser device, or 
     by dividing output said signal from said signal combining means into two signals, and multiplexing such signal which is generated by directly modulating one of said divided signals by one of said semiconductor laser device, with such signal which is generated by phase-inverting the other dicided signal and further directly modulating the phase-inverted signal by the other semiconductor laser device. 
     An FM signal optical transmitter of the present invention comprises: 
     the FM signal converter according to any one of said present inventions; and 
     an E/O converting unit for converting signal outputted from the FM signal converter into optical signal. 
     An FM signal optical receiver of the present invention comprises: 
     an O/E converting unit for converting optical signal transmitted from the FM signal optical transmitter according to claim 13 into electrical signal; 
     an FM demodulator for demodulating the FM signal converted into electrical signals; and 
     a filter for outputting a plurality of signals multiplesed with subcarriers by suppressing a component of a carrier having said predetermined frequency out of the signal from the FM demodulator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a configuration of an FM signal optical transmitting apparatus according to a first embodiment of the present invention; 
     FIG.  2 ( a ) shows an example of the time variation of AM-SCM signal amplitude transmitted according to the present invention; 
     FIG.  2 ( b ) shows a time variation of a carrier signal amplitude; 
     FIG.  2 ( c ) shows a time variation of a combined signal amplitude; 
     FIG.  3 ( a ) shows an example of the time variation of AM-SCM signal amplitude transmitted according to the present invention; 
     FIG.  3 ( b ) shows a signal which is one of such signals that generate the corrected signal; 
     FIG.  3 ( c ) shows a signal which is the other one of said such signals that generate the corrected signal; 
     FIG.  4 ( a ) shows a frequency location relationship between AM-SCM signal and carrier signal; 
     FIG.  4 ( b ) shows a frequency location relationship between AM-SCM signal and frequency-shifted AM-SCM signal; 
     FIG. 5 shows a configuration of an FM signal optical transmitting apparatus according to a second embodiment of the present invention; 
     FIG. 6 shows a configuration of an FM signal optical transmitting apparatus according to a third embodiment of the present invention; 
     FIG. 7 shows an exemplary configuration of an FM modulator according to the present invention; 
     FIG. 8 shows an FM modulation characteristics of an FM modulator according to the present invention; and. 
     FIG. 9 shows a configuration of a prior art FM signal optical transmitting apparatus. 
     Description of Symbols 
       1  Transmitting unit 
       2  FM converting unit 
       31 ,  32 ,  33  Signal inputting unit 
       41  Signal branching unit 
       42  Signal combiner 
       5  Amplitude detecting unit 
       6  Peak detection unit  6   
       7  Amplitude/phase controller 
       8  Carrier signal generating unit 
       9  Time delaying unit 
       10  FM modulator 
       20  Broadband amplifier 
       21  Semiconductor laser 
       22  Receiving unit 
       23  Photodetector 
       24  Preamplifier 
       25  FM demodulator 
       26  Signal band selective filter 
       27  Optical fiber 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
     FIG. 1 is a configuration of an FM signal optical transmitting apparatus according to a first embodiment of the present invention. The optical transmitting apparatus comprises a transmitting unit  1  and a receiving unit  22 . 
     The transmitting unit  1  comprises a signal inputting unit  31  that includes a signal branching unit  41 , a time delaying unit  9 , an amplitude detecting unit  5 , a peak detection unit  6 , a carrier signal generating unit  8 , an amplitude/phase controller  7  and a signal combiner  42 , and an FM converting unit  2  that includes the above mentioned signal inputting unit  31  and an FM modulator  10 , a broadband amplifier  20  and a semiconductor laser  21 . The receiving unit  22  comprises a photodetector (PD)  23 , a preamplifier  24 , an FM demodulator  25  and a signal band selective filter  26 . Optical signal from the transmitting unit  1  is transmitted via optical fiber  27  and provided to the receiving unit  22 . 
     The transmitting unit  1  corresponds to the FM signal optical transmitter according to the present invention, the FM converting unit  2  corresponds to the FM signal converter according to the present invention, the carrier signal generating unit  8  corresponds to the frequency signal source according to the present invention, the time delaying unit  9  and the signal combiner  42  corresponds to the signal combining means according to the present invention, and the receiving unit  22  corresponds to the FM signal optical receiver according to the present invention. This is also true with the following embodiments. 
     FIG. 2 shows a time variation of signal input to the FM modulator  10  for describing an operation of the optical transmission apparatus according to the present invention. 
     FIG. 4 shows spectrum charts of signal at the signal inputting unit  31 . Numeral  51  denotes multi-channel AM-SCM signal spectrum, and  52  denotes carrier signal spectrum that is generated in the carrier signal generating unit  8  at frequency apart from the frequency band of the multi-channel AM-SCM signal (Illustratively, higher frequency.). 
     FIG. 8 shows an oscillating frequency deviation of the signal provided to the FM modulator  10  and the instantaneous amplitude variation. The modulation characteristic denotes linear change in oscillating frequency of the FM carrier wave in a range with a width twice an amplitude variation range “Ath” and with a center of an amplitude level “A 0 ” (See curve  71 .). The amplitude level “A 0 ” corresponds to bias level that defines a carrier frequency f 0  during the FM modulation. 
     Operation of the FM signal optical transmitting apparatus according to the first embodiment will be described. 
     Referring to FIG. 1, multi-channel AM-SCM signal is provided to the signal branching unit  41 . Instantaneous amplitude variation of the signal component branched at the signal branching unit  41  is detected in the amplitude detecting unit  5 . For example, circuit with envelope detecting function can be used as the amplitude detecting unit  5 . The amplitude detecting unit  5  provides instantaneous amplitude variation signal like a curve  61  in FIG.  2 ( a ). 
     The instantaneous amplitude variation signal is provided to the peak detection unit  6 . In the peak detection unit  6 , the instantaneous amplitude level is detected together with the peak detecting signal, at a time point (“tp”) when the instantaneous variation of the amplitude level of the multi-channel AM-SCM signal exceeds the range from “A 0 ” to threshold “Ath” in FIG. 2, i.e., outside of an amplitude variation range that exhibits linear modulation characteristic. 
     In the peak detection unit  6 , a comparator is used for peak level detection in analog processing. In digital processing, the instantaneous amplitude variation value is quantized by an A/D converter, a significant bit in the binary representation is made as a threshold level, and the peek level detection is done if more significant bit than the significant bit is used for the quantized data. Thus, the instantaneous amplitude level and the time point are detected simultaneously. 
     Carrier signal from the carrier signal generating unit  8  is shaped in the amplitude/phase controller  7  so that the instantaneous phase of the amplitude level of the carrier signal forms instantaneous variation curve  62  that is of opposite phase and equal value to the amplitude phase during peak detection of the multi-channel AM-SCM signal based on the instantaneous amplitude level from the peak detection unit  6  and its time information. 
     The scheme to make the instantaneous variation curve  62  that is opposite phase and that has an equal value will be described referring to FIG.  3 . 
     That is for such part of the original signal that is over the threshold value (Ath) in the FIG.  3 ( a ), high frequency (fh) is full-wave rectificated and further the rectificated signal is amplitude-modulated in such manner that the envelope shape of the modulated signal becomes similar to the wave shape of the above mentioned part over the threshold value though the pahses are contrary to each other(FIG.  3 (b)). 
     Further for such part of the original signal that is over the threshold value (Ath) in the FIG.  3 ( a ), high frequency (fh) whose phase is shift by 90 degree to said above menteioned high frequency signal is full-wave rectificated also and further the rectificated signal is amplitude-modulated in such manner that the envelope shape of the modulated signal becomes similar to the wave shape of the above mentioned part over the threshold value though the pahses are contrary to each other (FIG.  3 ( b )) 
     Furthermore such obtained two amplitude-modulated signals are synthesized to each other and finally the synthesized singal is added to the original signal corresponding to said over part thereby to lower the level of the over part of the original sigal than the thereshold value. 
     Regarding the amplitude/phase controller  7 , in the amplitude control, values such as gain of the amplifier or attenuation of the attenuator may be made variable, and the phase control may use variable phase generator or may use e.g., the difference of transmitting line length like the time delaying unit  9 , and delay filters. 
     Multi-channel AM-SCM signal from the other side, i.e., from the signal branching unit  41 , is delayed by time τ in the time delaying unit  9  and combined with the carrier signal from the amplitude/phase controller  7  in the signal combiner  42 , wherein τ denotes the time needed in operation for defining the amplitude level and the phase relationship in the amplitude/phase controller  7 . 
     As shown in FIG.  4 ( a ), a spectrum of the combined signal will have a shape such that a multi-channel AM-SCM signal spectrum  51  and a carrier signal spectrum  52  is multiplexed in the frequency domain. The instantaneous amplitude variation after the combining will be like curve  63  and becomes lower than the peak level threshold, and be able to be in the range of the linear FM modulation characteristic in FIG.  8 . 
     Such combined signal is provided to the FM modulator  10 , and a broadband FM signal will be obtained. Then, this FM signal is converted to optical signal by a direct modulation that use the semiconductor laser  21  via the broadband amplifier  20 , and then transmitted via optical fiber  27 . 
     Operation of the receiving unit  22  side is described. The optical signal transmitted by the optical fiber  27  is provided to the receiving unit  22 . The optical signal provided to the receiving unit  22  is converted to electrical signal in the photo detector  23  that includes a light receiving element that is a photodiode or an avalanche photodiode, and the preamplifier  24 , and then provided to the FM demodulator  25 . FIG.  4 ( a ) shows a spectrum of the signal demodulated by the FM demodulator  25 . 
     There are several configurations for the FM demodulator  25  but a delay line type or a pulse count type demodulator with broadband and good linearity characteristics is preferred. The delay line type demodulator comprises a branch element for branching input signal, a delay circuit for providing time delay to a branched output signal, and a mixer for mixing an output from the delay circuit and the other branched output signal from the branch element (not shown). Broadband balanced type mixer may be used as the mixer. If necessary, a limiter amplifier for adjusting levels may be connected before the branch element in the FM demodulator  25 . 
     Signal from the FM demodulator  25  is obtained as the original multi-channel AM-SCM signal  51  by removing the carrier signal component  52  in the signal band selective filter  26 . 
     Thus, according to this embodiment, multi-carrier signal (AM-SCM signal) can be modulated in a range where an FM modulation characteristic is linear so that low distortion FM modulated signal is obtained. Also, since it suppresses large instantaneous amplitude variation, the oscillating frequency variation range, i.e., maximum frequency deviation of the FM modulator can be kept small. 
     Therefore, substantial band for the FM modulated signal of the multi-channel AM-SCM signal out of the transmitting FM signal band can be kept narrow so that the broadband response characteristic of the optical transmitting system can be relaxed. 
     Since the carrier signal can be automatically removed in the signal band selective filter in the receiver side, additional circuit components are not needed in particular. This is a great advantage of this embodiment. 
     The FM demodulator  25  is not limited to the above-described configurations. Circuits with a frequency discriminating function such as a multiple tuned frequency discriminator, a Foster-Seeley discriminator and a ratio envelope detector may be used. 
     In FM signal converted in the transmitting unit  1 , high frequency side (higher band) and low frequency side (lower band) appear symmetrical to the FM carrier wave. However, either the higher band or the lower band may be transmitted. If band of one side of the two sides is transmitted, a limiter amplifier is inserted before the FM demodulator  25  in the receiving unit  22  such that the original FM signal is reproduced. 
     Second Embodiment 
     FIG. 5 shows a configuration of the transmitting unit  1  and the receiving unit  22  in the FM signal optical transmitting apparatus according to a second embodiment of the present invention. Comparing with the first embodiment shown in FIG. 1, this embodiment includes a signal inputting unit  32  that includes signal switching unit  11  that passes or blocks signal based on the peak detection signal from the peak detection unit  6  between the amplitude/phase controller  7  and the signal combiner  42 . Same reference numerals denote same element with FIG.  1 . The receiving unit  22  is same with the first embodiment so it will not be described. 
     The signal switching unit  11  can be implemented as an on-off circuit that uses transistors or FETs. 
     Regarding the signal switching unit  11 , upon detecting peak that exceeds threshold by peak detection signal from the peak detection unit  6 , the amplitude/phase controller  7  adjusts amplitude phase and outputs opposite phase signal to the signal combiner  42 . If the peak is not detected, opposite phase signal will not be output to the signal combiner  42 . 
     Thus, in this embodiment, in addition to the advantages of the first embodiment, carrier signal is provided and combined into AM-SCM signal if necessary, and therefore, bandwidth expansion after the FM modulation may not be stationary. Also, while the carrier signal is not provided, power may be saved if power supply to the carrier signal generator is stopped. 
     Third Embodiment 
     FIG. 6 shows a configuration of an FM signal optical transmitting apparatus according to a third embodiment of the present invention. Comparing with the first embodiment shown in FIG. 1, instead of the carrier signal generating unit  8 , this embodiment includes a signal inputting unit  33  that includes a frequency converter  12  that provides the same signal as the multi-channel AM-SCM signal for providing to the signal branching unit  41 , and as in the second embodiment shown in FIG. 5, this embodiment includes the signal switching unit  11  that passes or blocks signal based on peak detection signal from the peak detection unit  6  between the amplitude/phase controller  7  and the signal combiner  42 . 
     The frequency converter  12  has a function to shift frequency band of the inputting multi-channel AM-SCM signal to higher frequency in this embodiment. For example, the frequency converter  12  comprises a successive multiplexer, or a local oscillator and a mixer. 
     Operation for this embodiment will be described except for the matter already described for the first embodiment. 
     In the multi-channel AM-SCM signal provided to the frequency converter  12 , the frequency band is shifted to higher frequency relatively to the original frequency band, and in addition, the amplitude component and the phase component is adjusted in the amplitude/phase controller  7  to correspond with the peak detection signal from the peak detection unit  6 , and combined with the original multi-channel AM-SCM signal in the signal combiner  42  synchronizing with the peak detected time by the signal switching section  11 . 
     FIG.  4 ( b ) shows a spectrum at this point of time. Spectrum  53  shows the multi-channel AM-SCM signal component that is shifted to higher frequency. 
     Thus, in this embodiment, in addition to the advantages of-the first embodiment, frequency shifted multi-channel AM-SCM signal is used as carrier signal for peak suppressing, and the instantaneous amplitude variation of the signal resembles the instantaneous amplitude variation of the original multi-channel AM-SCM signal so that the inner circuit configuration of the amplitude/phase controller  7  is made simple and synchronization during signal combining is made easier. 
     In this embodiment, a signal switching unit  11  is used but it is not required. Spectrum signal like the one shown in FIG.  4 ( b ) is provided to the FM modulator  10  steadily. 
     Instead of adjusting phase in the amplitude/phase controller  7 , a plurality of frequency-converted multi-channel AM-SCM signal or carrier signal with different phases may be provided. Then, frequency converted multi-channel AM-SCM signal or carrier signal with optimum phase is selected based on the peak detection signal from the peak detection unit  6  to combine with the original multi-channel AM-SCM signal in the signal combiner  42  (not shown). 
     In this embodiment, the band of the carrier signal is located higher than the band of the multi-channel AM-SCM signal but it may be located in lower frequency. The band may only need to remove carrier signal in the receiver side. 
     Signal to be transmitted is not limited to AM signal. It may be a subcarrier-multiplexed (SCM) multi-value modulation signal such as digital signal or QAM. 
     Particularly, the present invention is advantageous in the FM modulator using the voltage-controlled oscillator (VCO)  85  shown in FIG.  7 ( b ). Generally, a voltage-controlled oscillator uses variable capacity diodes and parasitic capacities of elements as means for changing oscillating frequency. Therefore, capacity changes corresponding to the input amplitude variation will not be linear. Thus, expansion of the input amplitude variation range that is deemed linear in FM modulation by voltage-controlled oscillator will not be easy. 
     The present invention relates to an FM signal optical transmitter that converts multi-channel AM-SCM signal into FM signal and an FM signal optical transmitting apparatus that includes such an FM signal optical transmitter, wherein instantaneous amplitude of the multi-channel AM-SCM-signal is detected to detect peak level, in order to suppress the peak level, amplitude/phase of the carrier signal is controllably combined, and after converting into FM signal in the FM converter, it is converted to optical signal for transmission. The present invention is not limited to processing AM-SCM signal. For example, FM signal and PCM signal may also be processed. 
     Generally, an FM modulator converts AM signal to FM signal, and voltage-controlled oscillator (VCO) or phase modulator may be used for the FM modulator. The FM modulator may not convert directly to FM signal. It may combine indirectly from AM signal or phase modulated signal. An oscillator that use varacters or reactance transistors, or a digital oscillator with multi-vibrator may be used for the voltage-controlled oscillator. Also, after integrating the AM signal to produce phase modulated signal, balanced modulator may be used to produce narrowband FM signal. 
     Exemplary FM modulator may be configured to optical heterodyne combine FM-LD  81  that were directly modulated by the multi-channel AM-SCM signal and LO-LD  82  by the multiplexer  83  and to optical-to-electrical (O/E) convert in PD  84  to produce FM signal (shown in FIG.  7 ( a )), or may be configured to FM modulate the multi-channel AM-SCM signal in VCO  85  and for example, to shift to lower frequency by frequency converter  86  (CONV.). 
     Semiconductor laser may be used for the E/O (electrical-to-optical) converting unit. For example, in the present invention, long wavelength laser of InP-series material with 1.2 to 1.6 μm band wavelength,semi conductor laser with 0.98 μm band wavelength, or laser of GaAsAl-series material with oscillating wavelength of 0.78 μm band may be used. 
     Further, in the present invention, ordinary optical fibers may be used. For example, ordinary optical fibers having core diameter of about 10 to 300 μm may be used. Also, multi-mode optical fibers or single-mode optical fibers can be used. 
     The frequency converter uses frequency mixing effect of non-linear circuit elements to convert to higher or lower frequency, and has local oscillating signal generating function. 
     The present invention has advantages of generating low distortion FM signal that is able to transmit via optical fiber, and having stability and reliability with low cost.