System for optically transmitting frequency-division-multiplexed signal and transmitter therefor
In an optical transmission system, a multiplexer frequency-division-multiplexes a plurality of signals, and outputs the resultant signal to an FM modulator. The FM modulator converts the frequency-division-multiplexed signal into an FM modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal. A frequency-divider converts the FM modulated signal into a frequency-divided FM modulated signal whose frequency is ½n (n is an integer of not less than 1) the frequency of the FM modulated signal. An optical modulator has a predetermined input-voltage vs. output-optical-power characteristic, and is biased at the minimum point (voltage) about the output optical power. The optical modulator modulates an unmodulated light fed from a light source with the applied frequency-divided FM modulated signal to produce an optical signal whose optical carrier component is suppressed, and sends the optical signal to an optical transmission line. An optical receiver receives the optical signal, and square-law detects the signal to convert into an FM modulated signal. A FM demodulator demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal. This configuration makes it possible to narrow the bandwidth of an FM modulated signal while increasing the frequency deviation thereof, and realize high-quality signal transmission as a result.
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This application is a reissue application of U.S. Pat. No. 6,486,986, issued Nov. 26, 2002.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly to a system that optically transmits a frequency-division-multiplexed signal obtained by frequency-division-multiplexing a plurality of signals.
2. Description of the Background Art
The operation of the conventional optical transmission system in the above configuration will be described below. The multiplexer 1100 frequency-division-multiplexes a plurality of signals, and outputs the resultant signal to the FM modulator 1101. The FM modulator 1101 converts the frequency-division-multiplexed signal into a frequency-modulated signal (hereinafter, referred to as “FM modulated signal”) having a predetermined frequency deviation through frequency modulation. After that, the FM modulator 1101 outputs the FM modulated signal to the electrical transmission line 1102. The optical transmitter 1104 receives the FM modulated signal through the electrical transmission line 1102, then converts the signal into an optical signal, and sends the optical signal to the optical transmission line 1105. The optical receiver 1106 receives the optical signal through the optical transmission line 1105, then converts the signal into an FM modulated signal which is an electrical signal, and outputs the FM modulated signal to the FM demodulator 1107. The FM demodulator 1107 demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal.
The optical transmission system in the above configuration is described in detail in “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems” by K. Kikushima et al. (IOOC'95 Technical Digest, Vol. 5 PD2-7, pp.33-34), and other documents. The optical transmission system converts a frequency-division-multiplexed signal into an FM modulated signal, and then optically transmits and demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal. The optical transmission system utilizes an FM gain in the FM transmission to improve the signal-to-noise power ratio (SNR) of the demodulated signal (i.e., the frequency-division-multiplexed signal), thereby enabling high-quality signal transmission.
Thus, the above-described optical transmission system can realize high-quality multi-channel signal transmission with an optical fiber.
However, the above-described system for optically transmitting an FM modulated signal has the following specific problems due to the properties of the FM modulated signal and the nonlinearity of an optical fiber.
SUMMARY OF THE INVENTIONAn FM modulation scheme increases a frequency deviation to acquire a greater FM gain, thereby enabling signal transmission of higher quality that other modulation schemes such as amplitude modulation. On the other hand, the increased frequency deviation requires a wider signal band. In addition, in the FM modulation scheme, linear distortion tends to occur under the influence of the group delay characteristic of a transmission line and the like (the characteristic that a propagation delay varies depending on a frequency). Therefore, the transmission line must be designed with particular attention. However, as a signal band becomes wider, the group delay variations in the band become more difficult to sufficiently suppress.
In a general optical modulation scheme, the optical frequency spectrum, of an optical signal is composed of a steep-shaped optical carrier component, which has narrow spectral line-width, and upper and lower sidebands, as shown in FIG. 12B. The upper and lower sidebands are geometrically similar to the frequency spectrum of a modulating signal. Therefore, if a wide-band signal like an FM modulated signal is used as a modulating signal in optical modulation, the optical freqeucy spectrum of the optical signal also becomes wider. The optical signal having such wide optical frequency spectrum becomes susceptible to the chromatic-dispersion of an optical fiber (the characteristic that a propagation delay varies depending on a wavelength). The affected optical signal component interacts with the optical carrier component to induce harmonic distortion in the FM modulated signal, resulting in waveform deterioration of the transmitted signal.
As is known from the above, the conventional optical transmission system has the specific problem that the quality of the transmitted signal is degraded due to the wide-band property of an FM modulated signal.
Therefore, an object of the present invention is to provide an optical transmission system capable of narrowing the bandwidth of an FM modulated signal while increasing the frequency deviation thereof to realize high-quality signal transmission. The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end. The transmission system comprises at the transmitting end, a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal, an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal, and an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in the optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to the receiving end. The transmission system also comprises at the receiving end, an optical receiver for receiving the optical-intensity-modulated signal from the optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal, and an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal.
As described above, in the first aspect, the FM modulated signal is obtained through frequency modulation using a frequency-division-multiplexed signal as an original signal. The FM modulated signal is converted into an optical-intensity-modulated signal at the transmitting end. The optical-intensity-modulated signal has an optical frequency spectrum in which upper and lower sidebands distribute geometrically similarly to the frequency spectrum of the original signal for the optical modulation and in which an optical carrier component is suppressed. Then, the optical-intensity-modulated signal is photodetected based on a square-law detection characteristic at the receiving end. At the receiving end, the optical transmission system thus obtains an FM modulated signal, having a frequency deviation twice as large as the one of the original FM modulated signal produced at the transmitting end, as a received FM modulated signal. In this manner, the optical transmission system can narrow (reduce in half) the bandwidth of the FM modulated signal at the transmitting end while securing the frequency deviation thereof large enough to acquire a sufficient FM gain in FM demodulation. As a result, it is possible to prevent the waveform of the transmitted signal from being deteriorated due to the group delay characteristic of the electrical transmission line and the chromatic-dispersion of the optical transmission line, and to realize signal transmission of good quality.
According to a second aspect, in the first aspect, the optical transmitter includes a light source for outputting an unmodulated light, and an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal. The optical modulator has the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and is biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the minimum.
As stated above, in the second aspect, the optical modulator used herein is an external optical modulator having the Mach-Zehnder interferometer structure. A modulating signal (an FM modulated signal) is applied to the optical modulator with respect to the “valley” where the output optical power is at the minimum in the input-voltage vs. output-optical-power characteristic (which is periodic like a sine wave) of the optical modulator. The optical modulator thus produces an optical-intensity-modulated signal whose optical carrier component is suppressed. The suppression of the optical carrier component prevents the waveform from being deteriorated by the chromatic-dispersion of the optical transmission line. In addition, the optical-intensity-modulated signal has an optical frequency spectrum in which upper and lower sidebands distribute geometrically similarly to the frequency spectrum of the original signal for the optical modulation. Therefore, after the optical-intensity-modulated signal is square-law detected at the receiving end, the freqeucy deviation of the FM modulated signal is doubled, thereby making it possible to realize high-quality signal transmission.
According to a third aspect, in the second aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
As described above, in the third aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain. The optical transmission system then converts the FM modulated signal into a frequency-divided FM modulated signal, and next converts the frequency-divided FM modulated signal into an optical-intensity-modulated signal for transmission. This reduces the phase noise in the FM modulated signal to be optically transmitted and FM demodulated. As a result, high-quality signal transmission can be realized.
According to a fourth aspect, in the first aspect, the optical transmitter includes a light source for outputting an unmodulated light, an optical branching circuit for branching the unmodulated light fed from the light source into first and second unmodulated lights, an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the maximum, and an optical combining circuit for combining the optical-intensity-modulated signal produced by the optical modulator and the second unmodulated light to cancel the optical carrier component of the optical-intensity-modulated signal with the second unmodulated light and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
As described above, in the fifth aspect, the optical-intensity-modulated signal produced by the optical modulator is combined with the second unmodulated light set in an opposite phase to the optical carrier component of the optical-intensity-modulated signal. The optical carrier component of the optical-intensity-modulated signal is thus canceled by the second unmodulated light. As a result, it is possible to produce an optical-intensity-modulated signal whose optical carrier component is suppressed.
According to a sixth aspect, in the fourth aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmission for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
As stated above, in the sixth aspect, as in the third aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain, then converts the FM modulated signal into a frequency-divided FM modulated signal, and converts the signal into an optical-intensity-modulated signal for transmission. It is therefore possible to reduce the phase noise in the FM modulated signal to be optically transmitted and FM demodulated.
According to a seventh aspect, in the first aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is 1½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
As described above, in the seventh aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain, then converts the FM modulated signal into a frequency-divided FM modulated signal, and next converts the signal into an optical-intensity-modulated signal for transmission. It is therefore possible to reduce the phase noise in the FM modulated signal to be optically transmitted and FM demodulated.
An eighth aspect of the present invention is directed to an transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, from a transmitting end to a receiving end. The transmitter comprises a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal, an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal, and an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in the optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to the receiving end.
According to a ninth aspect, in the eighth aspect, the optical transmitter includes a light source for outputting an unmodulated light, and an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the minimum.
According to a tenth aspect, in the ninth aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
According to an eleventh aspect, in the eighth aspect, the optical transmitter includes a light source for outputting an unmodulated light, an optical branching circuit for branching the unmodulated light fed from the light source into first and second unmodulated lights, an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the maximum, and an optical combining circuit for combining the optical-intensity-modulated signal produced by the optical modulator and the second unmodulated light to cancel the optical carrier component of the optical-intensity-modulated signal with the second unmodulated light, and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
According to a thirteenth aspect, in the eleventh aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
According to a fourteenth aspect, in the eighth aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through optical modulation using the frequency-divided FM modulated signal as an original signal.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
Next, referring to
An FM modulation scheme can arise an FM gain by increasing a frequency deviation, and improve the signal-to-noise ratio (SNR) of a demodulated signal. On the other hand, the increase in frequency deviation extends the spectrum bandwidth of an FM modulated signal, and also requires a wider band of a transmission line. The group delay characteristic of the transmission line with such wide band affects the FM modulated signal, to cause linear distortion thereof in some cases. That is, as the bandwidth of an FM modulated signal is wider, waveform distortion thereof increases. In short, a frequency deviation in FM modulated signal transmission has a trade-off relation between noise characteristic and waveform distortion of an FM modulated signal. Hence, the wide-band FM transmission system is difficult to optimally design.
The optical transmission system in
In the above first embodiment, an FM modulated signal is outputted from the FM modulator 101, and then directly applied to the optical modulator 104. On the other hand, in the present invention, the frequency-divider 408 divides each frequency component of the FM modulated signal outputted from the FM modulator 101 into 2n (n is an integer of not less than one) Namely, the frequency-divider 408 converts the FM modulated signal into a frequency-divided FM modulated signal whose frequency is ½n the frequency of the FM modulated signal. The frequency-divider 408 then outputs the frequency-divided FM modulated signal to the optical modulator 104.
In an FM modulation scheme, if an oscillator for generating an FM modulated signal has frequency drift, which is called “phase noise”,the frequency drift is converted into intensity noise at the FM demodulation to degrade the noise characteristic of the demodulated signal.
The optical transmission system shown in
As is clear from the above, the optical transmission system of the present embodiment can reduce the phase noise in the FM modulated signal after transmission, thereby realizing signal transmission of better quality.
Third EmbodimentIn the above first embodiment, an FM modulated signal is outputted from the FM modulator 101, and directly applied to the optical modulator 104. On the other hand, in the present embodiment, the amplitude controller 709 removes variations in amplitude of the FM modulated signal to adjust the amplitude to consistently remain constant, and then outputs the FM modulated signal to the optical modulator 104.
As described in the above, in the optical transmission system in
In the above-described first embodiment, the optical modulator 104 is biased at the “valley” in its input-voltage vs. output-optical-power characteristic to produce an optical signal whose optical carrier is suppressed. However, in the present embodiment, the optical modulator 804 is biased in its input-voltage vs. output-optical-power characteristic at the “peak” where the output optical power is at the maximum, as shown in FIG. 9. An FM modulated signal is applied to the optical modulator 804 with respect to the voltage of an operating point 1002 which is set by the above-mentioned bias. The optical modulator 804 thus produces an optical-intensity-modulated signal (hereinafter, referred to as “optical signal”) having an optical frequency spectrum as shown in FIG. 10A. The optical branching circuit 810 receives an unmodulated light (shown in
As is clear from the above, according to the optical transmission system in
The optical delay circuit 812 is interposed onto the path leading from the optical branching circuit 810 directly to the optical combining circuit 811 in the fourth embodiment, but may be interposed onto the path leading from the optical branching circuit 810 to the optical combining circuit 811 via the optical modulator 804 or interposed onto the both paths, or may be omitted.
In addition, it is possible to apply the change of the conditions for the optical modulation performed by the optical modulator 804, and the change and addition of the configuration accompanied therewith (the optical branching circuit 810, the optical combining circuit 811, and the optical delay circuit 812), all of which are described in the fourth embodiment, to the first embodiment and other embodiments. In such case, the same advantageous effects can be also provided.
While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.
Claims
1. A transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, said transmission system comprising:
- a transmitting end comprising a multiplexer operable to freqeuncy-division multiplex the plurality of signals to produce the frequency-division multiplexed signal, a FM modulator being operable to convert the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as a FM modulated signal, and an optical transmitter operable to convert the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to a receiving end; and
- said receiving end comprising an optical receiver operable to receive the optical-intensity-modulated signal from said optical transmitter, and convert the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to the output the electrical signal as a received FM modulated signal, and a FM demodulator operable to demodulate the received FM modulated signal to reproduce the frequency-division-multiplexed signal.
2. The transmission system according to claim 1, wherein said optical transmitter comprises:
- a light source being operable to output an unmodulated light; and
- an optical modulator being operable to modulate the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a minimum.
3. The transmission system according to claim 2, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, the n being an integer of not less than 1,
- wherein said optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
4. The transmission system according to claim 1, wherein said optical transmitter comprises:
- a light source being operable to output an unmodulated light;
- an optical branching circuit being operable to branch the unmodulated light fed from said light source into a first unmodulated light and a second unmodulated light;
- an optical modulator being operable to modulate the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a maximum; and
- an optical combining circuit being operable to combine the optical-intensity-modulated signal produced by said optical modulator and the second unmodulated light to cancel an optical carrier component to the optical-intensity-modulated signal with the second unmodulated light and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
5. The transmission system according to claim 4, wherein said optical transmitter further comprises an optical delay circuit provided between said optical branching circuit and said optical combining circuit, said optical delay circuit being operable to adjust a propagation delay of at least one of the first unmodulated light, the second unmodulated light, and the optical-intensity-modulated signal produced by said optical modulator such that the second unmodulated light and the optical carrier component of the optical-intensity-modulated signal produced by said optical modulator are set in opposite phases to each other.
6. The transmission system according to claim 4, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
7. The transmission system according to claim 1, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical transmitter comprises an optical modulator being operable to produce the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
8. A transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, said transmitter comprising:
- a multiplexer being operable to frequency-division-multiplex the plurality of signals to produce the frequency-division-multiplexed signal;
- a FM modulator being operable to convert the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as a FM modulated signal; and
- an optical transmitter being operable to convert the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to a receiving end.
9. The transmitter according to claim 8, wherein said optical transmitter comprises:
- a light source being operable to output an unmodulated light; and
- an optical modulator being operable to modulate the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a minimum.
10. The transmitter according to claim 9, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
11. The transmitter according to claim 8, wherein said optical transmitter comprises:
- a light source being operable to output an unmodulated light;
- an optical branching circuit being operable to branch the unmodulated light fed from said light source into a first unmodulated light and a second unmodulated light;
- an optical modulator being operable to modulate the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a maximum; and
- an optical combining circuit being operable to combine the optical-intensity-modulated signal produced by said optical modulator and the second unmodulated light to cancel an optical carrier component of the optical-intensity-modulated signal with the second unmodulated light, and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
12. The transmitter according to claim 11, wherein said optical transmitter further comprises an optical delay circuit provided between said optical branching circuit and said optical combining circuit, said optical delay circuit being operable to adjust a propagation delay of at least one of the first unmodulated light, the second unmodulated light, and the optical-intensity-modulated signal produced by said optical modulator such that the second unmodulated light and the optical carrier component of the optical-intensity-modulated signal produced by said optical modulator are set in opposite phases to each other.
13. The transmitter according to claim 11, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
14. The transmitter according to claim 8, further comprising a frequency-divider provided between said FM modulator and said optical transmitter, said frequency-divider being operable to convert the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is ½n of a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical transmitter includes an optical modulator being operable to produce the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
15. An optical transmission apparatus comprising:
- a multiplexer operable to multiplex a plurality of signals to produce a multiplexed signal;
- an FM modulator operable to convert the multiplexed signal into a frequency-modulated signal;
- an electrical-optical modulator operable to convert the frequency-modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and
- an optical transmitter operable to transmit the optical modulated signal
16. An optical transmitting method comprising:
- multiplexing a plurality of signals to produce a multiplexed signal;
- FM-modulating the multiplexed signal into a frequency-modulated signal;
- electrical-optical-modulating the frequency-modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and
- transmitting the optical modulated signal.
17. An optical transmission system comprising a transmission apparatus and a receiving apparatus,
- said transmission apparatus including: a multiplexer operable to multiplex a plurality of signals to produce a multiplexed signal; an FM modulator operable to convert the multiplexed signal into a frequency-modulated signal; an electrical-optical modulator operable to convert the frquency-modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and an optical transmitter operable to transmit the optical modulated signal, and said receiving apparatus including: a receiver operable to receive the optical modulated signal;
- an optical-electrical converter operable to convert the optical modulated signal into the frequency-modulated signal; and
- an FM demodulator operable to demodulate the frequency-modulated signal to produce the multiplexed signal.
18. An optical transmission/reception method comprising:
- multiplexing a plurality of signals to produce a multiplexed signal;
- FM-modulating the multiplexed signal into a frequency-modulated signal;
- electrical-optical-modulating the frequency-modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum;
- transmitting the optical modulated signal;
- receiving the optical modulated signal;
- optical-electrical converting the optical modulated signal into the frequency-modulated signal; and
- FM-demodulating the frequency-modulated signal to produce the multiplexed signal.
19. A transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- at said transmitting end, a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal; an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal; an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end; and a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n of a frequency of the FM modulated signal, n being an integer of not less than 1, and
- at said receiving end, an optical receiver for receiving the optical-intensity-modulated signal from said optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal; and an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal, wherein said optical transmitter includes: a light source for outputting an unmodulated light; and an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a minimum, and wherein said optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
20. A transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- at said transmitting end,
- a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal;
- an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal; and
- an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end, and
- at said receiving end,
- an optical receiver for receiving the optical-intensity-modulated signal from said optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal; and
- an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal,
- wherein said optical transmitter includes:
- a light source for outputting an unmodulated light;
- an optical branching circuit for branching the unmodulated light fed from said light source into first and second unmodulated lights;
- an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a maximum; and
- an optical combining circuit for combining the optical-intensity-modulated signal produced by said optical modulator and the second unmodulated light to cancel an optical carrier component of the optical-intensity-modulated signal with the second unmodulated light and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
21. The transmission system according to claim 20, wherein said optical transmitter further includes an optical delay circuit, provided between said optical branching circuit and said optical combining circuit, for adjusting a propagation delay of at least one of the first unmodulated light, the second unmodulated light, and the optical-intensity-modulated signal produced by said optical modulator such that the second unmodulated light and the optical carrier component of the optical-intensity-modulated signal produced by said optical modulator are set in opposite phases to each other.
22. The transmission system according to claim 20, further comprising,
- a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
23. A transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- at said transmitting end, a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal; an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal; an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end; and a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1, and
- at said receiving end, an optical receiver for receiving the optical-intensity-modulated signal from said optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal; and an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal, wherein said optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
24. A transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal;
- an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal;
- an opitical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end; and
- a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical transmitter includes:
- a light source for outputting an unmodulated light; and
- an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that the output optical power is at a minimum, and
- wherein said optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
25. A transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal;
- an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal; and
- an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end, and
- wherein said optical transmitter includes:
- a light source for outputting an unmodulated light;
- an optical branching circuit for branching the unmodulated light fed from said light source into first and second unmodulated lights;
- an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, said optical modulator having a Mach-Zehnder interferometer structure with a predetermined input-voltage versus output-optical-power characteristic, and being biased in the input-voltage versus output-optical-power characteristic such that an output optical power is at a maximum; and
- an optical combining circuit for combining the optical-intensity-modulated signal produced by said optical modulator and the second unmodulated light to cancel an optical carrier component of the optical-intensity-modulated signal with the second unmodulated light, and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
26. The transmitter according to claim 25, wherein said optical transmitter further includes an optical delay circuit, provided between said optical branching circuit and said optical combining circuit, for adjusting a propagation delay of at least one of the first unmodulated light, the second unmodulated light, and the optical-intensity-modulated signal produced by said optical modulator such that the second unmodulated light and the optical carrier component of the optical-intensity-modulated signal produced by said optical modulator are set in opposite phases to each other.
27. The transmitter according to claim 25, further comprising,
- a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1,
- wherein said optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
28. A transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, from a transmitting end to a receiving end, comprising:
- a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal;
- an FM modulator for converting the frequency-division-multiplexed signal to a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal; and
- an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in an optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to said receiving end; and
- a frequency-divider provided between said FM modulator and said optical transmitter for converting the FM modulated signal outputted from said FM modulator into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1, and
- wherein said optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
29. An optical transmission apparatus comprising:
- a multiplexer operable to multiplex a plurality of signals to produce a multiplexed signal;
- an FM modulator operable to convert the multiplexed signal into an FM modulated signal;
- a frequency-divider operable to convert the FM modulated signal into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1;
- an electrical-optical modulator operable to convert the frequency-divided FM modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and
- an optical transmitter operable to transmit the optical modulated signal.
30. An optical transmitting method comprising:
- multiplexing a plurality of signals to produce a multiplexed signal;
- FM-modulating the multiplexed signal to produce an FM modulated signal;
- frequency-dividing the FM modulated signal to produce a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1;
- electrical-optical-modulating the frequency-divided FM modulated signal to produce an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and
- transmitting the optical modulated signal.
31. An optical transmission system comprising a transmission apparatus and a receiving apparatus,
- said transmission apparatus comprising: a multiplexer operable to multiplex a plurality of signals to produce a multiplexed signal; an FM modulator operable to convert the multiplexed signal into an FM modulated signal; a frequency-divider operable to convert the FM modulated signal into a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1; an electrical-optical modulator operable to convert the frequency-divided FM modulated signal into an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and an optical transmitter operable to transmit the optical modulated signal, and
- said receiving apparatus comprising: a receiver operable to receive the optical modulated signal; an optical-electrical converter operable to convert the optical modulated signal into the FM modulated signal; and an FM demodulator operable to convert the FM modulated signal to produce the multiplexed signal.
32. An optical transmission and reception method including a transmitting method and a receiving method,
- said transmitting method comprising: multiplexing a plurality of signals to produce a multiplexed signal; FM-modulating the multiplexed signal to produce an FM modulated signal; frequency-dividing the FM modulated signal to produce a frequency-divided FM modulated signal whose frequency is 1/2n a frequency of the FM modulated signal, n being an integer of not less than 1; electrical-optical-modulating the frequency-divided FM modulated signal to produce an optical modulated signal whose optical carrier component is suppressed in an optical frequency spectrum; and transmitting the optical modulated signal; and
- said receiving method comprising: receiving the optical modulated signal; optical-electrical converting the optical modulated signal into the FM modulated signal; and
- FM-demodulating the FM modulated signal to produce the multiplexed signal.
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Type: Grant
Filed: Nov 26, 2004
Date of Patent: Aug 21, 2007
Assignee: Matsushita Electric Industrial Co., Ltd. (Osaka)
Inventor: Masaru Fuse (Osaka)
Primary Examiner: Hanh Phan
Attorney: Wenderoth, Lind & Ponack, L.L.P.
Application Number: 10/997,559
International Classification: H04B 10/18 (20060101); H04B 10/155 (20060101); H04B 10/152 (20060101); H04J 14/02 (20060101);