JP2006148977A - Fm signal optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an FM signal optical receiver capable of suppressing distortion caused by the distortion of phases in transmission. <P>SOLUTION: An FM optical signal inputted into an optical receiver 2013 from an optical fiber cable 92 is inputted into an optical fiber grating 2011 via an optical circulator 2010 placed in an optical/electrical converter 2015. After the optical signal has been converted into an electrical signal by an optical/electrical converter 2016 comprising a photodiode or an avalanche photodiode and a preamplifier 2017, the converted signal is inputted into a limiter amplifier 2018, a branching element 2019, a delay circuit 2020, a mixer 2021, and a low-pass filter 2022. At this stage, the signal is amplified to predetermined signal intensity and demodulated to an original AM signal 21. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an FM signal light receiving apparatus used for optical communication, CATV, optical measurement, mobile communication, and the like.

  In recent years, in video surveillance systems, CATV, subscriber systems, mobile communications, etc., it has been put into practical use that optical transmission of multi-channel video, audio, or data is made utilizing the low loss and wideband characteristics of optical fibers. Yes. In such an optical transmission device, a multi-channel signal is electrically multiplexed by a plurality of subcarriers (subcarriers) each having a different frequency to form an AM signal, and a semiconductor laser or the like is directly modulated by the AM signal. Then, it is converted into an optical signal and transmitted through an optical fiber. The optical transmission of AM signals is characterized in that the modulator / demodulator is simple and low-cost especially in the transmission of video signals.

  However, such an optical transmission device has the following disadvantages. That is, in video optical transmission, it is necessary to ensure a sufficient C / N (carrier-to-noise ratio) to ensure desired signal characteristics (video quality, etc.). In order to obtain a high C / N, high optical input power is absolutely necessary on the receiver side.

  Also, in mobile communication, the intensity level of voice and data signals to be transmitted varies greatly with the movement of the terminal, so a high dynamic range with respect to signal variations is required. Furthermore, it is susceptible to distortion caused by reflected waves during optical conversion with a semiconductor laser or during optical fiber transmission. Furthermore, an amplifier with good linearity is required for an AM signal amplifier.

  In order to eliminate such inconveniences and to improve distortion resistance and noise resistance, conventionally, there has been proposed an optical transmission apparatus that converts subcarrier-multiplexed AM signals into FM signals in a lump for optical transmission. . Furthermore, in order to obtain a desired C / N by further increasing the modulation index in the proposed apparatus, it has also been proposed to directly modulate the frequency of the semiconductor laser to obtain an FM signal having a high modulation index. FIG. 17 shows the structure of the optical transmission apparatus improved in this way.

  This optical transmission apparatus outputs an optical frequency modulation signal by directly modulating a semiconductor laser 41 with a multi-channel AM signal (for example, an AM video signal) 30 in an AM / FM converter 82 included in the optical transmitter 81. ing. At this time, the semiconductor laser 41 is directly modulated with the AM signal 30 to modulate the oscillation frequency simultaneously with the amplitude modulation of the light. Light having a slightly different transmission frequency is generated by the local oscillation light source 42 with respect to the optical frequency modulation signal thus created, and this light and the optical frequency modulation signal are combined by the multiplexer 43, and then combined. By inputting the waved light to the photodiode 44 and performing optical heterodyne detection, a broadband (for example, 1 to 6 GHz) FM modulation signal is created as a beat signal of the two lasers and output to the electrical / optical conversion unit 83.

  In the electrical / optical converter 83, the FM modulated signal is input to the semiconductor laser drive amplifier 88, and the semiconductor laser 89 for transmission is directly modulated by the output of the semiconductor laser drive amplifier 88 to create an FM optical signal to produce an optical fiber cable 92. (For example, see Japanese Patent No. 2700622 for the above configuration).

  The FM optical signal transmitted to the optical fiber cable 92 is amplified by an amplifier (not shown) provided in the middle portion of the optical fiber cable 92, and then the optical branching device (not shown) also provided in the middle portion of the optical fiber cable 92. ) Is transmitted to each optical receiver 93 via an optical fiber.

  In the optical receiver 93, first, the FM / AM converter 96 and the preamplifier 97 constituting the optical / electric converter 95 convert and amplify the FM optical signal into an electric signal, and then the FM / AM demodulator 94. Is demodulated into an AM signal 31. The FM / AM demodulator 94 is a delay type demodulator circuit, and is composed of high-speed logic ICs 51 and 53 (for example, AND gates), a delay unit 52, and a low-pass filter 54 via a limiter amplifier 50, enabling wideband demodulation. It is.

  However, in such a conventional FM transmission system, when the AM video multiplexed signal is converted into an FM optical signal by the semiconductor laser 41, the phase noise of the semiconductor laser 41 is added to the FM optical signal, so that the CNR (carrier to noise) Ratio) is significantly degraded. Therefore, even if the optical reception intensity to the optical receiver 93 is increased, the improvement in sensitivity beyond a certain CNR value cannot be obtained. In order to obtain a desired CNR in the optical receiver 93, a semiconductor laser having a line width of about 1/10 as compared with the conventional system is required, and a semiconductor laser having an external resonator structure is used. There is a need. Therefore, these semiconductor lasers themselves are expensive, and there is a problem that a plurality of semiconductor lasers must be used.

  Although a method of directly converting an AM signal into an electrical FM signal in a low frequency band is conceivable, if the modulation index in the FM modulator is increased (modulation degree ≧ 10%), distortion occurs in the FM modulator. However, the signal quality deteriorates due to this distortion, and there is a problem that good optical transmission cannot be performed.

  On the other hand, in the conventional optical transmission device, in order to transmit a broadband FM signal of 1 to 6 GHz, the uniformity of the phase of each frequency band of the signal is such as an amplifier in the optical transmitter 81, an optical fiber cable 92, and the like. There is a problem that the AM signal 31 demodulated by the optical receiving unit 93 is distorted due to the phase delay.

  An object of the present invention is to provide an FM signal light receiving apparatus that can suppress distortion caused by a phase shift during transmission as compared with the conventional technique.

According to a first aspect of the present invention (corresponding to the invention described in claim 1), a modulation means for converting a multiplexed signal obtained by subcarrier multiplexing a plurality of signals into an FM signal having a predetermined carrier frequency, and the modulation means A frequency conversion means for shifting the converted FM signal to a lower frequency side than the carrier frequency;
An FM signal comprising: an optical modulation unit that converts an optical signal into an FM optical signal by modulating the optical signal based on the FM signal output from the frequency conversion unit, and transmits the FM optical signal via an optical fiber cable. It is an optical transmission device.

In the second invention (corresponding to the invention described in claim 2), the carrier frequency is sufficiently higher than the frequencies of the plurality of signals,
The conversion to the FM signal is to convert the carrier signal having the carrier frequency into a narrowband FM signal by performing FM modulation with a modulation index that generates only the first sideband,
The shift of the narrowband FM signal to the low frequency side is to convert the narrowband FM signal into a lowband converted narrowband FM signal having a frequency sufficiently lower than the carrier frequency. It is FM signal optical transmission apparatus of invention.

  According to the above configuration, for example, since an optical signal for transmission is obtained by performing narrowband FM modulation, lowband conversion, and then intensity modulation, an additional circuit and an optical heterodyne detection circuit are unnecessary, and therefore As compared with the conventional example, an FM signal transmission system having a simple circuit configuration, excellent stability and reliability, and inexpensive can be provided. Further, since the narrow band FM signal is used, the modulation means hardly generates modulation distortion, so that the signal quality is not deteriorated.

According to a third aspect of the present invention (corresponding to the invention described in claim 3), the frequency multiplication means for multiplying the FM signal output from the modulation means by a plurality of times and outputting a multiplied signal;
The FM according to the first aspect of the present invention, further comprising: first band filtering means for band-filtering a desired narrowband FM signal out of the multiplied signal output from the multiplication means and outputting it to the frequency conversion means. This is a signal light transmission device.

  According to the above configuration, for example, since an optical signal for transmission is obtained by performing narrowband FM modulation, low band conversion, and then intensity modulation, an additional circuit and an optical heterodyne detection circuit are unnecessary, and therefore As compared with the conventional example, an FM signal transmission system having a simple circuit configuration, excellent stability and reliability, and inexpensive can be provided. Further, since the narrow band FM signal is used, the modulation means hardly generates modulation distortion, so that the signal quality is not deteriorated. Further, since the narrowband FM signal is multiplied, an FM signal having a larger modulation index than that of the conventional example can be obtained, and a desired CNR can be obtained by the optical receiver.

  According to a fourth aspect of the present invention (corresponding to the fourth aspect of the present invention), the first upper sideband and the first lower sideband included in the first sideband of the FM signal output from the modulating means. The FM signal light transmission apparatus according to the first or third aspect of the present invention, further comprising second band filtering means for band filtering only one of them.

  According to the above configuration, for example, since transmission can be performed in a narrower band, transmission efficiency can be increased, and power consumption for transmission driving can be reduced.

  According to a fifth aspect of the present invention (corresponding to the fifth aspect of the present invention), the modulation means is a voltage controlled oscillator or a relaxation oscillator, and the FM signal light according to any one of the first to fourth aspects of the present invention. It is a transmission device.

  According to the above configuration, for example, the circuit configuration can be simplified as compared with the conventional example, and an inexpensive device can be provided.

  According to a sixth aspect of the present invention (corresponding to the sixth aspect of the present invention), the modulation means narrows down the multiplexed signal by phase-modulating the multiplexed signal and converting it to a phase-modulated signal, and then combining the phase-modulated signal. The FM signal optical transmission apparatus according to any one of the first to fourth aspects of the present invention, wherein the FM signal is converted into a band FM signal and output.

  According to the above configuration, for example, the circuit configuration can be simplified as compared with the conventional example, and an inexpensive device can be provided.

  The seventh aspect of the present invention (corresponding to the invention of claim 7) includes suppression means for suppressing a center frequency component in the FM signal output from the modulation means, and the output from the suppression means is The FM signal light transmission apparatus according to any one of the above-described present invention, which is input to the frequency conversion means and is to be shifted.

  According to the above configuration, it is possible to further improve the quality of the transmission signal compared to the prior art while having a simple configuration.

  According to a ninth aspect of the present invention (corresponding to the ninth aspect of the present invention), any one of the above-described aspects of the present invention further comprises compensation means for performing dispersion compensation or group delay compensation on the transmitted FM optical signal. This is an FM signal light transmission apparatus.

  According to the above configuration, an optical transmission device with excellent phase characteristics during transmission can be realized at low cost.

According to a fifteenth aspect of the present invention (corresponding to the invention described in claim 15), compensation means for performing dispersion compensation or group delay compensation on an FM optical signal transmitted through an optical fiber;
An optical / electrical conversion means for converting an FM optical signal output from the compensation means into an FM electrical signal;
An FM signal light receiving device comprising: demodulation means for demodulating the FM signal converted by the optical / electrical conversion means into an AM signal.

  According to the above configuration, an optical receiver / transmitter that can suppress distortion due to a phase shift during transmission can be realized at low cost.

Embodiments according to the present invention will be described below with reference to the drawings.
First Embodiment FIG. 1 is a block diagram showing a configuration of an FM optical transmission system according to a first embodiment of the present invention. The FM optical transmission system according to the present embodiment includes an optical transmitter 1 and an optical receiver 2 connected by an optical fiber cable 92. Here, the optical transmitter 1 includes a narrowband FM modulator 3, a frequency converter 4, and an electrical / optical converter 8, and the optical receiver 2 includes an optical / electrical converter 10, an FM demodulator. 11 and a low-pass filter 12.

  In FIG. 1, the signal input to the narrowband FM modulator 3 is, for example, an AM multiplexed video signal 20 in which a multi-channel AM video signal is sub-carrier multiplexed as shown in FIG. The band FM modulator 3 internally generates a high-frequency carrier signal having a sufficiently high frequency compared to the frequency band of the AM multiplexed video signal 20, for example, a millimeter wave band, and the carrier wave according to the input AM multiplexed video signal 20. A relatively small modulation such that the signal is generated substantially only by the carrier of the FM signal and the first sideband (the first sideband includes the first upper sideband and the first lower sideband). By performing frequency modulation with an index (modulation degree corresponding to the modulation index is 10% or less), it is converted into a narrowband FM signal 27 shown in FIG. 2B and output to the mixer 4a of the frequency converter 4. Here, since the frequency of the carrier signal of the narrowband FM signal 27 is sufficiently higher than that of the original input signal, there is an advantage that a large modulation index necessary for optical transmission can be obtained.

  As the narrowband FM modulator 3, for example, a voltage controlled oscillator using a varactor or a reactance transistor may be used, or a digital relaxation oscillator using a multivibrator may be used. Alternatively, the AM multiplexed video signal 20 may be integrated and then phase modulated, and a carrier wave signal may be combined with the phase modulated signal by a balanced modulator to generate a narrowband FM signal.

  A narrowband FM signal 27 output from the narrowband FM modulator 3 is a mixer 4a including a nonlinear element such as a pin diode having nonlinear voltage-current characteristics, for example, and a local oscillation signal from the local signal oscillator 4b. After being mixed with the low-pass filter 5, for example, as shown in FIG. 3, the narrowband FM signal 27 having a center frequency (carrier frequency) of 25 GHz is, for example, the center frequency (carrier frequency). Are down-converted (frequency conversion to a lower frequency) into a low-frequency conversion narrow-band FM signal 27a having a low frequency of 5 GHz.

  The reason why the FM signal 27 is converted into the FM signal 27a in this way is as follows. That is, in general, the upper limit of the frequency at which signal conversion between an electric signal and an optical signal can be performed is limited by the limit of the frequency response speed of the conversion element. Normally, if the carrier frequency is not 10 GHz or less, frequency response is impossible and signal conversion cannot be performed, so that the above-described Dunn conversion is necessary.

  Next, the low-frequency conversion narrowband FM signal 27a down-converted as described above is amplified by the drive amplifier 6 in the electrical / optical converter 8 and then input to the semiconductor laser 7 having a laser diode. .

  The semiconductor laser 7 converts the internally generated optical signal into an intensity-modulated optical signal according to the input low-frequency converted narrowband FM signal, and converts the converted optical signal to the optical fiber cable 92. To the optical receiver 2 of the other party.

  Here, in the preferred embodiment, the semiconductor laser 7 is, for example, a long wavelength semiconductor laser of an InP-based material having a wavelength of 1.2 to 1.6 μm, a semiconductor laser of 0.98 μm, and an oscillation wavelength of 0.78 μm. A semiconductor laser of a GaAlAs material is used. In the preferred embodiment, for example, an optical fiber cable having a core diameter of about 10 to 300 μm, or a multimode optical fiber cable or a single mode optical fiber cable is used as the optical fiber cable 92.

  The optical signal received by the optical receiver 2 via the optical fiber cable 92 is input to the optical / electrical converter 10 and is photoelectrically converted into an electrical signal by the photoelectric converter 13 having a photodiode or an avalanche photodiode. Thereafter, the low-noise amplifier 14 amplifies the electric signal having a desired signal strength. The amplified electric signal is frequency demodulated by the FM demodulator 11 and returned to the original AM multiplexed video signal 20. Here, the FM demodulator 11 is desirably a delay line type or pulse count type having a wide bandwidth and good linearity, and FIG. 1 shows the delay line type.

  In the present embodiment, the narrowband FM modulator 3 frequency-modulates the carrier signal with a relatively small modulation index such that substantially only the carrier of the FM signal and the first sideband are generated. Since the FM signal 27 is obtained, the narrowband FM signal 27 is a signal that is substantially similar to the AM signal, and therefore, the first upper sideband and the first lower sideband of the narrowband FM signal 27. Only one of them may be filtered by a band pass filter for transmission. In this case, the optical receiver 2 can reproduce the original narrowband FM signal 27 by passing it through an amplitude limiter (limiter) after photoelectric conversion. Therefore, since transmission can be performed in a narrower band, transmission efficiency can be increased, and power consumption for transmission driving can be reduced.

  In this embodiment, as shown in FIG. 1, the delay line type is shown, and the two-output element 16, which is a high-speed digital element, the AND element 17, and the delay circuit 160 are used. Further, the FM demodulator 11 is not limited to the above configuration, and a circuit having a frequency discriminating function such as a double-tuned frequency discriminator, a Foster-Sealey discriminator, or a ratio detector may be used.

As described above, according to the present embodiment, an optical signal for transmission is obtained by narrow band FM modulation and low band conversion, and then intensity modulation by the semiconductor laser 7, so that an additional circuit and an optical heterodyne are obtained. A detection circuit is unnecessary, and therefore, an FM signal transmission system can be provided that has a simpler circuit configuration than that of the conventional example, is excellent in stability and reliability, and is inexpensive. Further, since the narrowband FM signal 27 is used, almost no modulation distortion is generated in the narrowband FM modulator 3, so that the signal quality is not deteriorated.
(Second Embodiment)
FIG. 4 is a block diagram showing the configuration of the optical transmitter 1a of the FM optical transmission system according to the second embodiment of the present invention. In FIG. 4, the same components as those in FIG. is doing. Note that the optical receiver 2 shown in FIG. 1 of the first embodiment is used. In the second embodiment, a multiplier 21 and a low-pass filter 22 are inserted between the narrowband FM modulator 3 and the frequency converter 4 as compared with the first embodiment of FIG. The narrowband FM signal 27 is multiplied a plurality by the multiplier 21, and a desired band signal is band-filtered by the band pass filter 22, and then the frequency converter 4 converts the signal to a lower frequency. It is characterized by frequency conversion. Hereinafter, differences from the first embodiment will be particularly described in detail.

In FIG. 4, an AM multiplexed video signal 20 formed by sub-carrier multiplexing a multi-channel AM video signal is input to the narrowband FM modulator 3, and the narrowband FM modulator 3 receives the original AM video signal frequency (500 MHz). The carrier wave signal having a higher carrier frequency (f ₀ = 5 GHz) than that of the other) is subjected to narrowband FM modulation according to the input AM multiplexed video signal 20 and converted to a narrowband FM signal 27. The narrowband FM converter 3 does not cause signal degradation due to distortion, and the FM conversion spectrum for each channel of the multi-channel subcarrier multiplexed signal is such that sidebands other than the first sideband do not appear remarkably. Narrow band FM modulation with a low modulation index is performed.

  The narrowband FM signal 27 is input to the multiplier 21 and multiplied by a plurality of N, and the modulation index of the narrowband FM signal 27 is increased. As a result, compared to the original AM multiplexed video signal 20, as shown in FIG. 5, it is converted into an FM signal having a sufficiently high carrier frequency (25 GHz when the multiplication number N = 5) and a wide signal band. Will be. Therefore, an FM signal having a modulation index for obtaining a CNR required in the optical receiver 2 of the FM optical transmission system can be obtained by this multiplication action.

  Next, this FM signal is input to the band pass filter 22, and after one narrowband FM signal 27 is extracted so as to remove the spectrum of unnecessary bands other than the FM signal band, the frequency converter 4 performs the first operation. Similar to the embodiment, as shown in FIG. 5, it is down-converted to a low-frequency conversion narrowband FM signal 27a. The following processing is the same as that of the first embodiment.

  As described above, according to the present embodiment, since the narrowband FM signal 27 is multiplied, an FM signal having a larger modulation index than that of the conventional example can be obtained. A desired CNR can be obtained.

As described above, even when the narrowband FM modulator 3 described in the first embodiment cannot generate a carrier signal having a sufficiently high frequency compared to the original AM multiplexed video signal 20, FIG. By adding the multiplier 21 as shown in the figure, the same effects as in the first embodiment are exhibited.
(Third embodiment)
FIG. 6 is a configuration diagram of an FM optical transmission system according to an embodiment of the present invention.

  The configuration of this embodiment will be described below with reference to FIG.

  As shown in the figure, the FM optical transmission system includes an optical transmitter 1001 and an optical receiver 1019. The optical transmitter 1001 includes an FM modulator 1004a, a band elimination filter 1005a, a multiplier 1006a, a band pass filter 1007a, an FM converter 1002 including a frequency converter 1008a, and an electric / optical converter including a broadband amplifier 1009 and a semiconductor laser 1010. 1003. An optical signal from the optical transmitter 1001 is transmitted through the optical fiber 92 and input to the optical receiver 1019. The optical receiver 1019 includes an optical / electrical converter 1014, an optical / electrical converter 1012 including a preamplifier 1015, an FM demodulator 1013, and a filter 1018. The suppression means of the present invention corresponds to the band elimination filter 1005a.

  The operation of the present embodiment will be described below with the above configuration.

  That is, the AM signal 20 which is a multi-channel subcarrier multiplexed video signal as shown in FIG. 7A is converted into an FM signal 300 shown in FIG. 7B by the FM modulator 1004a.

  The FM modulator 1004a performs narrowband FM modulation with no distortion and a low modulation degree. Here, the degree of modulation is such a low value that sidebands other than the first sideband 1032 do not appear remarkably as the spectrum of the FM converted signal 1300 for each channel of the multichannel subcarrier multiplexed signal 20.

Accordingly, as shown in FIG. 7 (b), the first sideband 1032 appears in regions separated from the FM carrier 1031 (frequency f ₀ ) by f ₁ . FIG. 7C shows the signal 1301 after the FM signal 1300 has passed through the band elimination filter 1005a. The center frequency of the band elimination filter 1005a is adjusted to the frequency f ₀ of the FM carrier 1031 in FIG. 7B, and the degree of suppression at the center frequency is not large. Therefore, after passing through the band elimination filter 1005a, the FM carrier 1034 is lowered by a level difference 1035 from the FM carrier 1031. Further, since the frequency of the sideband wave 1032 is f ₁ away from the FM carrier wave 1034, the influence of the phase change or level fluctuation is not caused by the band removal filter 1005a. Therefore, only the FM carrier 1034 portion is selectively suppressed by the level difference 1035 by the band elimination filter 1005a. As a result, since the level difference 1036 between the carrier wave and the sideband wave is smaller than the level difference 1033 before passing through the band elimination filter 1005a, the signal subjected to the narrow band FM modulation by the FM modulator 1004a becomes smaller by the band elimination filter 1005a. Apparently, it increased.

This signal 1301 is input to a multiplier 1006a and is multiplied by N, thereby further increasing the degree of modulation. As a result, the FM signal 1302 is converted into an FM signal 1302 having a sufficiently high FM carrier wave (for example, N × f ₀ = 25 GHz) and a wide signal band as shown in FIG. Therefore, an FM signal 1302 having a modulation degree for obtaining a C / N necessary for an optical receiver in optical transmission can be obtained by this multiplication action. Then, after removing the spectrum of the unnecessary band other than the FM signal band by the band transmission filter 1007a, the signal component is down-converted to the low frequency side by the frequency converter 1008a, for example, a mixer. In this way, an FM converted signal 1027 as shown in FIG. This FM converted signal 1027 is converted into an optical signal by the semiconductor laser 1010 via the broadband amplifier 1009 and propagated through the optical fiber cable 92.

  The operation of the optical receiver 1019 is the same as that of the optical receiver 2 described in the first embodiment (see FIG. 1). In this embodiment, as shown in FIG. 6, a delay line type is shown, and a two-output element 1016, an AND element 1017, and a delay circuit 1160, which are high-speed digital elements, are used. Further, the FM demodulator 1013 is not limited to the above-described configuration, and a circuit having a frequency discrimination function such as a double-tuned frequency discriminator, a Foster-Sealey discriminator, or a ratio detector may be used.

With the configuration as described above, FM conversion is performed using an FM modulator having a small FM modulation degree, the primary FM modulation degree is increased by a band elimination filter, and the secondary FM modulation degree is increased by subsequent multiplication. Thus, by obtaining a desired FM modulation degree, AM multichannel signals multiplexed by subcarriers are converted into FM signals collectively by signal processing by an electric circuit having a simple configuration without increasing the multiplication number of the multiplier. it can.
(Fourth embodiment)
FIG. 9 is a configuration diagram of an FM optical transmission system according to an embodiment of the present invention. Hereinafter, the configuration of the present embodiment will be described with reference to FIGS.

  In FIG. 9, the optical transmitter 1001 includes an FM converter 1004b, a frequency converter 1008b, a band elimination filter 1005b, a multiplier 1006b, a band pass filter 1007b, an FM converter 1002 including a frequency switch 1008a, and the above embodiment. 3 is constituted by the same electrical / optical switching unit 1003 as in FIG. Since the optical receiver 1019 is the same as that of the third embodiment, the description thereof is omitted.

  The operation of the present embodiment will be described below with the above configuration.

That is, the multi-channel subcarrier multiplexed AM signal 20 is processed by the narrowband FM modulator 1004b having an FM carrier sufficiently higher than the original AM signal frequency (for example, f _h = 25 GHz) as shown in FIG. It is converted into a signal 1300a.

Narrowband FM modulator 1004b does not cause signal degradation due to distortion, and the degree of modulation is such that sidebands other than first sideband 1032a do not appear as FM conversion spectra for each channel of the multichannel subcarrier multiplexed signal. Narrow band FM modulation is performed. This signal is down-converted to the low frequency side by the frequency converter 1008b to obtain a spectrum 1300b. For convenience of explanation, keep the f ₀ the center frequency when down-converted by the frequency converter 1008b.

  Thereafter, only the FM carrier 1031b is selectively suppressed by the level difference 1035 by the band elimination filter 1005b, and the signal of the spectrum 1301 in FIG. 8B is obtained. The subsequent operation is the same as that of the third embodiment.

  As described above, in this embodiment, in addition to the effects of the third embodiment, a frequency variable range is expanded by using an FM modulator having a sufficiently high FM carrier, and the signal input to the FM modulator. Can be made wider.

By the way, it is desirable that the modulation degree in the FM modulator in each of the above-described embodiments is 10% or less. If the degree of modulation is 10% or less, the signal energy in the frequency range from frequency f ₀ −f ₂ to frequency f ₀ + f ₂ of the FM modulation spectrum 1300 of the multi-channel subcarrier multiplexed signal of FIG. 98% or more can be secured, and a narrow band FM is possible.

  As described above, according to the above-described embodiment, for example, a plurality of subcarrier multiplexed signals are converted into FM signals by the FM modulator, and the center frequency component in the FM signal is suppressed by the band elimination filter. The FM signal in which the center frequency component is suppressed is multiplied by a frequency multiplier, and a signal having a desired modulation degree of the frequency multiplied signal is shifted to an optical transmission band on the low frequency side by a frequency converter and converted into an optical signal. And transmit.

  Further, according to the above-described embodiment, in the FM modulator that generates a plurality of subcarrier-multiplexed AM signals to generate a high-frequency carrier signal in the millimeter wave band having a sufficiently higher frequency than the AM signal that is the modulated wave. An FM signal is converted, the FM signal is shifted to a low frequency side by a frequency converter, and a center frequency component in the shifted FM signal is suppressed by a band elimination filter, and a desired modulation degree is apparently increased. Thereafter, the signal is multiplied by a frequency multiplier, and the frequency-multiplied signal having a desired modulation degree is shifted to a low-frequency side optical transmission band by a frequency converter, converted into an optical signal and transmitted.

  In the above-described embodiment, the configuration has been described in which the center frequency component in the FM signal is suppressed, and then the FM signal in which the center frequency component is suppressed is multiplied and shifted to the low frequency side. For example, a configuration that only suppresses the center frequency component in the FM signal may be used. In this case, a signal using a single subcarrier or a plurality of signals multiplexed by a plurality of subcarriers is converted to an FM signal using FM modulation means, and the center frequency in the FM signal is suppressed by suppression means. A component is suppressed, an optical signal is generated based on the FM signal in which the center frequency component is suppressed, and the optical signal is transmitted. As a result, when the AM signal is directly converted into an electrical FM signal, the modulation index in the FM modulator can be apparently increased, so that the C / N value can be further increased without increasing the distortion in the FM modulator. Demonstrate the effect of improvement.

  In the above embodiment, the band suppression filter is, for example, a filter that selectively suppresses the spectrum near the center frequency when FM conversion is performed, and the suppression degree may be about 10 dB.

  The suppression means of the present invention is a band elimination filter in the above embodiment, and is shifted to the low frequency side by the first frequency conversion means (corresponding to the frequency converter 1008b in FIG. 9 in the above embodiment). In the above description, the center frequency component of the FM signal is suppressed. However, the present invention is not limited to this. For example, the suppression unit is a stage before the shift of the center frequency component of the FM signal converted by the FM modulation unit. It is also possible to adopt a configuration in which the signal is suppressed by and then shifted to the low frequency side as described above. For example, the configuration in this case is as follows. That is, a plurality of subcarrier multiplexed AM signals are converted to FM signals in an FM modulator having FM carriers in a millimeter wave band having a sufficiently high frequency compared to an AM signal whose carrier frequency is a modulated wave. After suppressing the center frequency component in the FM signal and apparently increasing the desired modulation degree, the signal is shifted to the low frequency side by the first frequency converter, and the signal is multiplied by the frequency multiplier, The frequency-multiplied signal having a desired modulation degree is shifted to an optical transmission band on the low frequency side by a second frequency converter, converted into an optical signal and transmitted. The configuration diagram in this case is the same as that in FIG. 9 in which the arrangement order of the frequency elimination filter 1005b and the frequency converter 1008b is changed.

In the above embodiment, a plurality of sub-carrier multiplexed signals, that is, a plurality of channel signals multiplexed by a plurality of sub-carriers are converted into FM signals using FM modulation means. As described above, the present invention is not limited to this. For example, the signal converted into the FM signal may be a single channel signal using a single subcarrier. Even in this case, the same effect as described above is exhibited.
(Fifth embodiment)
FIG. 10 is a block diagram showing a configuration of the FM optical transmission system according to the fifth embodiment of the present invention. The configuration of the present embodiment will be described below with reference to FIG.

  As shown in the figure, the FM optical transmission system includes an optical transmitter 2001 and an optical receiver 2013, and the optical transmitter 2001 and the optical receiver 2013 are connected by an optical fiber cable 92. The optical transmitter 2001 includes an AM / FM converter 2002 and an electrical / optical converter 2003. The AM / FM converter 2002 includes an AM / FM converter 2004, a multiplier 2005, a band pass filter 2006, and a frequency converter 2007. The electrical / optical converter 2003 includes a broadband amplifier 2008, a semiconductor laser 2009, an optical fiber grating 2011, and an optical circulator 2010.

  The FM optical transmission system is substantially the same as the configuration of the above-described embodiment, but is characterized in that an optical fiber grating 2011 is provided.

  FIGS. 11 and 12 are conceptual diagrams of signal spectra for explaining the operation of the FM optical transmission system, and FIG. 13 is a configuration diagram of a group delay compensation unit including an optical fiber grating 2011. FIG. 14A to 14C are conceptual diagrams for illustrating the operation of the optical fiber grating 2011. FIG.

  Next, the operation of the optical fiber grating 2011 of the present embodiment will be mainly described with reference to these drawings.

The contents shown in FIGS. 11A to 12B are basically the same as those described in the above embodiment. That is, in FIGS. 11A and 11B, 20 is the original AM signal (bands f _{1 to} f ₂ ) described above, and 2061 is an FM signal converted by the AM / FM converter 2004. . Reference numeral 2062 denotes a first sideband in the spectrum of the FM signal 2061, and appears in regions separated by f ₁ around the FM carrier 2063 (frequency f ₀ ). 12A and 12B, reference numeral 2064 denotes an FM multiplied signal having a wide signal band having a carrier 2063 ′ obtained by multiplying the FM signal 2061 N times by the multiplier 2005. . The frequency of the carrier 2063 ′ is, for example, NF ₀ (NXf ₀ ) = 25 GHz, which is sufficiently higher than the original AM signal 20.

  That is, the FM frequency conversion signal 2065 obtained in the same manner as in the above embodiment is input to the semiconductor laser 2009 via the broadband amplifier 2008, where it is converted into an FM optical signal 2066. Then, the created FM optical signal 2066 is input to the optical fiber grating 2011 via the optical circulator 2010.

  As shown in FIG. 13, the optical fiber grating 2011 is configured so that the group delay characteristic in the optical signal reflected by the optical fiber grating 2011 can be arbitrarily set by changing the interval of the refractive index distribution in the longitudinal direction. Yes.

  Here, an example of the group delay characteristic of the FM optical signal 2066 when not passing through the optical fiber grating 2011 is shown in FIG. When the FM optical signal 2066 passes through the optical transmitter 2001 and the optical fiber 92, the group delay characteristic becomes uneven depending on the frequency. In the example shown in FIG. 14A, the group delay amount decreases as the frequency increases.

  On the other hand, the delay adjustment amount of the above-described optical fiber grating 2011 is set so as to have an opposite phase relationship to the group delay characteristic of FIG. 14A as shown in FIG. As a result, the group delay characteristic shown in FIG. 14A and the group delay adjustment amount shown in FIG. 14B are canceled out, and the group delay characteristic becomes uniform regardless of the frequency as shown in FIG. 14C. Is done.

  The FM optical signal 2066 whose group delay characteristic is adjusted by the action of the optical fiber grating 2011 as described above is transmitted to the optical fiber cable 92 via the optical circulator 2010.

  On the other hand, an FM optical signal 2066 inputted from the optical fiber cable 92 to the optical receiver 2013 is composed of an optical / electric conversion unit 2015 (an optical / electric converter 2016 composed of a photodiode or an avalanche photodiode and a preamplifier 2017). Is converted into an electrical signal, and then input to an FM / AM demodulator 2014 (which includes a limiter amplifier 2018, a branching element 2019, a delay circuit 2020, a mixer 2021, and a low-pass filter 2022). And is demodulated into the original AM signal 21. The demodulated AM signal 21 has a good phase characteristic because the optical fiber grating 2011 cancels the group delay in the transmission system.

  With the configuration as described above, AM / FM conversion is performed using the AM / FM converter 2004 having a small FM modulation degree, and a desired FM modulation degree is obtained by a secondary FM modulation degree increase by subsequent multiplication operation. be able to. As a result, AM multi-channel signals multiplexed by subcarriers can be collectively converted into FM signals by signal processing by an electric circuit having a simple configuration without increasing the multiplication number of the multiplier 2005, and the modulation / demodulation system and transmission system can also be converted. Can be demodulated into an AM signal 21 with excellent phase characteristics by a group delay compensation means comprising an optical fiber grating 2011 and an optical circulator 2010.

  In general, the group delay characteristic shown in FIG. 14A is about tens to hundreds of psec in the required band. Therefore, the group delay amount compensated by the optical fiber grating 2011 takes into account the refractive index distribution in the order of several hundreds psec / nm in the reverse phase in the optical fiber grating 2011, considering the wavelength broadening (several to several tens of GHz) of the semiconductor laser 2009 It may be formed so as to have the compensation characteristics shown in FIG.

By implementing such a group delay compensation function and performing group delay compensation in the entire transmission system, FM light having excellent group delay characteristics as a whole can be obtained even if inexpensive circuit elements having poor group delay characteristics are used. The transmission system could be configured. Thereby, cost reduction can be promoted, and a configuration that can be easily introduced into an access network including a subscriber system can be realized.
(Sixth embodiment)
FIG. 15 is a block diagram showing the configuration of the FM optical transmission system according to the sixth embodiment of the present invention. This FM optical transmission system basically has the same configuration as the configuration of the FM optical transmission system according to the fifth embodiment, and the same reference numerals are given to the same or similar configurations, and description thereof will be omitted. Omitted.

  This FM optical transmission system is characterized in that the optical fiber grating 2011 includes a temperature control unit 2025. The temperature control unit 2025 has a configuration such as a heater and a thermostat, and arbitrarily changes the atmospheric temperature of the optical fiber grating 2011. The temperature control unit 2025 adjusts the linear expansion of the optical fiber grating 2011 by changing the atmospheric temperature. The size in the longitudinal direction can be expanded and contracted. In this FM optical transmission system, it is possible to change the refractive index distribution interval of the optical fiber grating 2011 by expanding and contracting the longitudinal dimension of the optical fiber grating 2011.

  In each of the above-described embodiments, the optical fiber grating 2011 is used as the group delay compensation element. However, when the group delay characteristic of the transmission system changes linearly, a dispersion compensation fiber may be used. Not too long. Further, in each of the above-described embodiments, in the FM optical transmission system that transmits a plurality of subcarrier multiplexed signals after performing processing such as FM conversion → multiplication processing → low frequency frequency conversion processing → optical signal conversion Although the present invention has been implemented, the present invention is not only applicable to such an FM optical transmission system, and is an optical device that converts a plurality of subcarrier multiplexed signals into FM optical signals and transmits them collectively. It goes without saying that any configuration with an FM optical transmission system including a transmitter can be implemented in the same manner.

  By providing such a configuration (temperature control unit 2025), in addition to the effects of the fifth embodiment, the group delay compensation characteristic generated by the optical fiber grating 2011 can be varied, and according to the variation in transmission delay on the system. In addition, it is possible to finely adjust the degree of group delay compensation, and it is also possible to avoid fluctuations in the degree of dispersion compensation due to fluctuations in the installation environment temperature of the optical fiber grating 2011.

  In the present embodiment, the temperature control unit 2025 is provided in the optical fiber grating 2011. However, the present invention is not limited to this, and the same effect can be obtained by changing the tension in the longitudinal direction of the fiber.

Further, the group delay compensation characteristic only in a predetermined frequency range can be changed if the optical fiber grating 2011 has a function of locally heating / cooling within a certain range of the refractive index distribution or a function of changing the tension. .
(Seventh embodiment)
FIG. 16 is a configuration diagram of an FM optical transmission system according to the seventh embodiment of the present invention.

  The FM optical transmission system basically has the same configuration as that of the fifth embodiment, and the same or similar parts are denoted by the same reference numerals, and description thereof is omitted.

  This FM optical transmission system is characterized in that an optical fiber grating 2011 and an optical circulator 2010 are provided in an optical / electrical converter 2015 in the optical receiver 2013. By providing such a configuration, in addition to the effects of the fifth embodiment, it is possible to further compensate for group delay variation (phase shift) that occurs in the optical fiber cable 92 and the optical receiver 2013. .

  The frequency multiplier of the above embodiment is, for example, one that increases the frequency to n times the input frequency, and generates harmonics by using non-linearity such as a transistor, FET, or variable capacitance diode. It is.

  The frequency converter of the above embodiment converts the frequency to the high frequency side or the low frequency side by utilizing the frequency mixing action of the nonlinear circuit element as in the case of the frequency multiplier.

  In the above-described embodiment, the case where the compensation means is provided in the optical transmission device having one semiconductor laser as described above is described. However, the present invention is not limited to this. For example, the conventional FM signal light described in FIG. Of course, a configuration provided on the transmission side or the reception side of the transmission system is also possible.

  As is apparent from the above description, the present invention has advantages in that the circuit configuration is simpler than that of the prior art, and the stability and reliability are excellent, and the signal quality is good.

  Further, the present invention has an advantage that distortion caused by a phase shift during transmission can be suppressed.

  The FM signal light receiving apparatus according to the present invention has an effect of suppressing distortion caused by a phase shift during transmission, and is useful as an FM signal light receiving apparatus or the like.

The block diagram which shows the structure of the FM optical transmission system of the 1st Embodiment of this invention (A): Spectrum diagram showing frequency characteristics of the AM multiplexed video signal 20 which is an input signal of the narrowband FM modulator 3 of FIG. 1 (b): A narrowband FM signal 27 which is an output signal of the narrowband FM modulator 3 Spectrum showing the frequency characteristics of Spectrum diagram showing frequency conversion processing by frequency converter 4 of FIG. The block diagram which shows the structure of the optical transmitter 1a of the FM optical transmission system of the 2nd Embodiment of this invention. Spectrum diagram showing processing of optical transmitter 1a in FIG. Configuration diagram of FM signal light transmission apparatus of third embodiment of the present invention (A): An explanatory diagram of an AM signal transmitted according to the prior art and the present invention (b): An explanatory diagram of an FM signal after passing through an FM modulator of this embodiment (c): A band elimination filter of this embodiment Spectral diagram of later signal (A)-(c): Frequency conversion diagram for explaining the function of each part of the FM signal in the third and fourth embodiments of the present invention The block diagram of the FM signal optical transmission apparatus of the 4th Embodiment of this invention The block diagram which describes the structure of the FM optical transmission system of the 5th Embodiment of this invention (A): Diagram showing AM signal transmitted according to the present invention (b): Spectrum diagram of FM signal after passing through AM / FM converter (A), (b): Frequency conversion diagram for explaining the function of each part of the FM signal in the present invention The figure which shows the structure of the group delay compensation means using the optical fiber grating of the 5th Embodiment of this invention (A)-(c): The conceptual diagram of the group delay characteristic of the transmission system in this Embodiment 5. The block diagram which shows the structure of the FM optical transmission system of the 6th Embodiment of this invention The block diagram which shows the structure of the FM optical transmission system of the 7th Embodiment of this invention The block diagram which shows the whole structure of the FM optical transmission system of a prior art example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a ... Optical transmitter 2 ... Optical receiver 3 ... Narrow band FM modulator 4 ... Frequency converter 4a ... Mixer 4b ... Local signal oscillator 5 ... Low-pass filter 6 ... Drive amplifier 7 ... Semiconductor laser 8 ... Electricity Optical fiber cable 10 optical / electrical converter 11 FM demodulator 12 low pass filter 13 photoelectric converter 14 low noise amplifier 20, 20a AM multiplexed video signal 21 multiplier 22 ... Band pass filter 27 ... Narrow band FM signal 27a ... Low band converted narrow band FM signal

Claims (6)

Compensation means for performing dispersion compensation or group delay compensation on the FM optical signal transmitted via the optical fiber;
An optical / electrical conversion means for converting an FM optical signal output from the compensation means into an FM electrical signal;
Demodulation means for demodulating the FM signal converted by the optical / electrical conversion means into an AM signal;
An FM signal light receiving apparatus comprising:   The compensation means compensates for an anti-phase characteristic of the group delay of the FM optical signal, which occurs in the electric circuit unit and / or the optical fiber in the FM optical signal transmitter that transmits the FM optical signal. The FM signal light receiving device according to claim 1, wherein:   3. The FM signal light receiving apparatus according to claim 1, wherein the compensation means is an optical fiber grating.   4. The FM signal light receiving apparatus according to claim 3, further comprising a control unit that controls compensation characteristics of the optical fiber grating.   The FM signal light receiving apparatus according to claim 3, further comprising a tension control unit that controls a tension of an optical fiber constituting the optical fiber grating.   3. The FM signal light receiving apparatus according to claim 1, wherein the compensating means is a dispersion compensating fiber.

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