The present invention relates to an optical communication system, an optical transmitter, an optical receiver, and an optical transponder, and more particularly to an optical OFDM communication system and a multicarrier optical communication system. More specifically, the invention relates to an optical communication system, an optical transmitter, an optical receiver, and an optical transponder that reduce the PAPR (Peak-to-Average Power Ratio) in an optical OFDM (Orthogonal Frequency Division Multiplexing) communication system.
Optical communication systems put into practical use so far use binary modulation and demodulation technologies based on optical intensity. More specifically, the transmitting side converts digital information, i.e., “ONEs” and “ZEROs”, into ONs and OFFs in optical intensity and transmits them into an optical fiber, and the receiving side receives the light propagated through the optical fiber and recovers the original information by performing opto-electric conversion. In recent years, with the rapid expansion of the use of Internet, further increases in communication capacity of optical communication systems are increasingly required. In order to accommodate such a need for further increases in communication capacity, the rate at which light is turned on and off, i.e., the modulation speed, has been increased in the past. However, such an approach in which an increase in communication capacity is achieved by an increase in modulation speed typically has the following problems.
That is, increasing the modulation speed may cause a problem in that the achievable transmission distance, which is limited by the wavelength dispersion of the optical fiber, is reduced. In general, the achievable transmission distance limited by the wavelength dispersion is inversely proportional to the square of the bit rate. Therefore, doubling the bit rate may result in the achievable transmission distance limited by the wavelength dispersion being reduced by a factor of four. In addition, increasing the modulation speed may cause another problem in that the achievable transmission distance limited by the polarization mode dispersion of the optical fiber is also reduced. In general, doubling the bit rate may result in the achievable transmission distance limited by the polarization mode dispersion being reduced by a factor of two. To show the influence of the wavelength dispersion more specifically, when a standard single mode fiber is used, the achievable transmission distance limited by the wavelength dispersion is 60 km if the bit rate is 10 Gbps, and it may be reduced to about 4 km when the bit rate is increased to 40 Gbps. In the case of the next generation systems, i.e., 100 Gbps systems, the achievable transmission distance limited by the wavelength dispersion may be further reduced to 0.6 km, thus making it impossible to achieve a trunk-line optical communication system having a transmission distance of about 500 km. In order to construct a trunk-line optical communication system operating at a super high speed, a special optical fiber having a negative wavelength dispersion, i.e., a so-called dispersion compensation fiber, is now installed in repeaters, transmitters, and receivers so as to offset the wavelength dispersion of the transmission line.
However, such a special fiber not only is expensive but also needs a skilled design that can achieve the optimal usage of the dispersion compensation fiber (the optimal length of the dispersion compensation fiber to be used) in each site, and these problems increase the cost of the optical communication system.
In view of the foregoing situation, research on optical communication systems using the OFDM technology is recently attracting increasing attentions as an optical modulation-demodulation method for increasing communication capacity. According to the OFDM technology, multiple sinusoidal waves that are orthogonal to one another in one symbol time, i.e., that have a frequency corresponding to an integer multiple of the reciprocal of one symbol time, are used (these sinusoidal waves are referred to as subcarriers). Specifically, by setting the amplitude and phase of each subcarrier to predetermined values, information is first reflected on the subcarriers (i.e., the subcarriers are modulated by the information). The subcarriers are then multiplexed into a signal, and the signal is used to modulate a carrier for transmission. The OFDM technology has been practically used in VDSL (Very high bit rate Digital Subscriber Line) systems that provide communication between telephone switching stations and households, in power-line communication systems for home use, and in digital terrestrial television systems. It is also expected to be used in the next generation mobile telephone systems.
An optical OFDM communication system is a communication system in which the OFDM technology is applied with light used as the carrier. As described above, the OFDM technology uses multiple subcarriers. In addition, multi-level modulation methods, such as 4-QAM, 8-PSK, or 16-QAM, can be used to modulate the individual subcarriers. For this reason, one symbol time becomes much longer than the reciprocal of the bit rate. As a result, the achievable transmission distance limited by the above described wavelength dispersion and polarization mode dispersion may become sufficiently longer than the transmission distance that needs to be achieved in optical communication system (e.g., 500 km for domestic trunk-line systems), thus enabling the above described dispersion compensation fiber to be eliminated or the amount of usage thereof to be reduced. This provides a possibility of achieving a low cost optical communication system.
An optical transmitter 1-1 and an optical receiver 2-1 are connected through an optical fiber 3. Once data to be transmitted is input into the optical transmitter 1-1 via an input terminal 4, it is converted into a baseband OFDM signal by a transmitter-signal processing unit 100 included in the optical transmitter 1-1. This signal is amplified by a driver amplifier 13, and is then used to field-modulate or intensity-module light, i.e., a carrier, in the optical modulator 12, thus resulting in an optical OFDM signal being generated. The optical OFDM signal travels through the optical fiber 3, i.e., the transmission line, to the optical receiver 2-1. The optical OFDM signal is direct-detection received and converted by a photodiode 21 into an electric signal. The electric signal is ideally the above described baseband OFDM signal, and the electric signal is amplified by a pre-amplifier 22 and is then demodulated by a receiver-signal processing unit 200, resulting in the originally transmitted data being output through an output terminal 5.
Data to be transmitted is first converted into 2N parallel data components by a serial-parallel converting unit 110. Here, N denotes the number of subcarriers on which the data is reflected. Although when the subcarriers are modulated using 4-QAM, the data is converted into 2N parallel data components, the data is converted into 4N parallel data components when 16-QAM is used, for example. That is, the serial data is converted into “the number of bits in one symbol multiplied by the number of subcarriers” parallel data components. A subcarrier modulating unit 120 modulates the N subcarriers using these parallel data components. The modulated subcarriers are converted into time-series data by an inverse FFT unit 130, and the time-series data is converted into serial data by a parallel-serial converting unit 140. After receiving cyclic prefixes inserted by a cyclic prefix inserting unit 150, the serial data is converted into an analog signal by a D/A converting unit 160, and the analog signal is output to the driver amplifier.
In the receiver-signal processing unit 200, an A/D converting unit 210 converts the received electric signal amplified by the pre-amplifier into a digital signal. A cyclic prefix deleting unit 220 deletes the cyclic prefixes. A serial-parallel converting unit 230 converts the digital signal into N parallel data components. An FFT unit 240 separates these parallel data components into N subcarrier signals. A subcarrier demodulating unit 250 obtains data reflected on the subcarriers by demodulating the subcarrier signals, and the data is then converted into serial data by a parallel-serial converting unit 260.
Optical communication systems and RF radio communication systems share a problem in that the PAPR (Peak-to-Average Power Ratio) of the OFDM signal is high. For RF wireless communication systems, if the linearity of the power amplifier driving the transmission antenna is poor, the signal is distorted at power peaks, thereby reducing the receiving sensitivity or causing interference to the adjacent wireless channels due to spreading of the signal spectrum.
Optical communication systems have another problem due to the high PAPR, which cannot be found in RF wireless communication systems and is therefore unique only to the optical fiber communication. It is a phenomenon, called “nonlinear phase rotation”, in which the phase of light rotates more when the peak power is high than when the peak power is not high. This phenomenon is caused due to the fact that optical fibers serving as the transmission line have a weak nonlinearity. The nonlinear optical effect of optical fibers, i.e., so-called Kerr effect, can be represented by the following expression:
where, φ0 denotes the linear phase, φNL(t) denotes the nonlinear phase, γ denotes the nonlinear constant of the optical fiber, α denotes the loss factor of the optical fiber, P(t) denotes the optical power, Pave denotes the average optical power, and PAPR(t) denotes the peak-to-average power ratio (PAPR) at time t, respectively. It is to be noted that the symbols shown in italic type in the expression will be presented in non-italic type in the following description for convenience. As can be seen in the expression, the nonlinear phase of light rotates in proportion to the PAPR. For an optical communication system using light having a single wavelength, the peak power of the signal itself may cause the phase to rotate (self-phase modulation effect), causing waveform distortion due to the wavelength dispersion and increasing the error rate. On the other hand, for wavelength multiplexing optical communication systems, the signal peak powers of the adjacent wavelengths may induce phase rotation (cross-phase modulation effect), increasing the bit error rate as in the self-phase modulation effect. These phase rotations may cause the subcarrier phases of the OFDM signal to rotate. Speaking more precisely, a random phase rotation depending on the PAPR is induced in addition to the fixed phase rotation determined by the average power. When the random phase rotation exceeds a threshold value for symbol determination, the symbol is determined to be erroneous. For example, if the subcarriers are modulated using QPSK, a wrong symbol determination may be made when the phase rotates by ±π/4 from the ideal symbol point. Therefore, in order to reduce the error rate, it is important to perform optical transmission using a signal having a PAPR that is suppressed as much as possible.
A variety of technologies for PAPR reduction have been proposed for RF wireless transmission systems. Typical examples include, e.g., (1) a filter is used to suppress the spectral interference to the adjacent RF wireless channels while the PAPR is forcibly kept equal to or less than a predetermined value using a hard limiter, (2) data mapping to the subcarriers (i.e., modulation) is tried two or more times to select a modulation having a less PAPR, and (3) a pre-coding (such as the Trellis coding) is used to secure redundancy, thereby generating a signal having a low PAPR. Nonpatent Literature 1 comprehensively describes the principles, advantages, and disadvantages of these approaches. Furthermore, as described in Nonpatent Literature 2, a method in which the envelope of a wireless signal is kept constant (PAPR=0 dB) using phase modulation is also under study now.
Results of research works in which these PAPR reduction methods are applied to optical OFDM communication systems also have been already published (Nonpatent Literatures 3 and 4). Furthermore, in Japanese Unexamined Patent Application Publication No 2009-188510 (Patent literature 1), there has also been devised an optical OFDM communication system in which the above described phase modulation is used to keep the envelope constant.
Using the countermeasures described in Nonpatent Literatures 3 and 4 can merely provide a PAPR equal to or more than 6 dB, which is higher than the PAPR of the existing optical communication systems using OOK, and therefore their advantages are limited. Furthermore, for the technology disclosed in Japanese Unexamined Patent Application Publication No. 2009-188510, the receiving method is limited only to the coherent receiving method, which requires not only a receiver configuration four times larger than that of the direct-detection receiving method but also a complicated receiver-signal processing unit, thus resulting in an expensive communication system being obtained when compared with that using the direct-detection receiving method.
The present invention has been devised in view of the foregoing situations, and an object of the invention is to provide an optical communication system, an optical transmitter, an optical receiver, and an optical transponder that can provide a PAPR lower than the PAPR (6 dB) of the existing optical communication systems where the photoelectric power is high within the transmission line in an optical OFDM communication system and that can be also applied to the direct-detection receiving method. Another object of the present invention is to provide an optical communication system, an optical transmitter, an optical receiver, and an optical transponder that can provide a PAPR less than 6 dB.
According to the present invention, the phase of an RF sinusoidal wave is modulated using a baseband OFDM signal, the modulated sinusoidal wave is then used to modulate an optical wave, and the modulated optical wave is transmitted through an optical fiber. Then, the transmitted optical wave is converted into an electric signal, and the electric signal is synchronously detected using the RF sinusoidal wave, thereby recovering the baseband OFDM signal.
According to a first aspect of the invention, there is provided an optical communication system including an optical transmitter for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and transmitting an optical signal through an optical fiber and an optical receiver for converting the optical signal transmitted through the optical fiber into electric subcarrier signals and demodulating the subcarrier signals to recover the original digital data. The optical transmitter includes a transmitter-signal processing unit for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and generating a baseband OFDM signal by inverse fast Fourier transforming the modulated subcarrier signals, a first oscillator for outputting a sinusoidal wave having a predefined frequency, a phase modulating unit for phase-modulating the sinusoidal wave output from the first oscillator using the baseband OFDM signal, and an electro-optic converting unit for converting the sinusoidal wave output from the phase modulating unit into an optical signal. The optical receiver includes a opto-electric converting unit for converting the optical signal received from the optical transmitter through the optical fiber into an electric signal, a second oscillator for generating a sinusoidal wave having a frequency substantially corresponding to that of the first oscillator, a synchronous detecting unit for synchronously detecting the output of the opto-electric converting unit using the sinusoidal wave output from the second oscillator, and a receiver-signal processing unit for recovering the original digital data from the subcarrier signals obtained by fast Fourier transforming the output of the synchronous detecting unit.
According to a second aspect of the invention, there is provided an optical transmitter included in an optical communication system having an optical transmitter for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and transmitting an optical signal through an optical fiber and an optical receiver for converting the optical signal transmitted through the optical fiber into an electric subcarrier signals and demodulating the subcarrier signals to recover the original digital data. The optical transmitter includes a transmitter-signal processing unit for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and generating a baseband OFDM signal by inverse fast Fourier transforming the modulated subcarrier signals, an oscillator for outputting a sinusoidal wave having a predefined frequency, a phase modulating unit for phase-modulating the sinusoidal wave output from the oscillator using the baseband OFDM signal, and an electro-optic converting unit for converting the sinusoidal wave output from the phase modulating unit into an optical signal.
According to a third aspect of the invention, there is provided an optical receiver included in an optical communication system having an optical transmitter for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and transmitting an optical signal through an optical fiber and an optical receiver for converting the optical signal transmitted through the optical fiber into electric subcarrier signals and demodulating the subcarrier signals to recover the original digital data. The optical receiver includes a opto-electric converting unit for receiving through the optical fiber an optical signal obtained by phase-modulating a sinusoidal wave having a predefined frequency using a baseband OFDM signal and converting the optical signal into an electric signal, an oscillator, for which a frequency substantially corresponding to the above described frequency is set, for generating a sinusoidal wave having the frequency, a synchronous detecting unit for synchronously detecting the output of the opto-electric converting unit using the sinusoidal wave output from the oscillator, and a receiver-signal processing unit for recovering the original digital data from subcarrier signals obtained by fast Fourier transforming the output of the synchronous detecting unit.
According to a fourth aspect of the invention, there is provided an optical transponder including a transmitting section and a receiving section. The transmitting section includes a transmitter-signal processing unit for modulating a plurality of subcarriers orthogonal to one another over a symbol time by mapping digital data to the subcarriers and generating a baseband OFDM signal by inverse fast Fourier transforming the modulated subcarrier signals, a first oscillator for outputting a sinusoidal wave having a predefined frequency, a phase modulating unit for phase-modulating the sinusoidal wave output form the first oscillator using the baseband OFDM signal, and an electro-optic converting unit for converting the sinusoidal wave output from the phase modulating unit into an optical signal. The receiving section includes an opto-electric converting unit for converting the optical signal received through the optical fiber into an electric signal, a second oscillator for generating a sinusoidal wave having a frequency substantially corresponding to that of the first oscillating unit, and a synchronous detecting unit for synchronously detecting the output of the opto-electric converting unit using the sinusoidal wave output from the second oscillator, and a receiver-signal processing unit for recovering the original digital data from subcarrier signals obtained by fast Fourier transforming the output of the synchronous detecting unit.
The present invention can provide an optical communication system, an optical transmitter, an optical receiver, and an optical transponder that can reduce the PAPR where the photoelectric power is high within the transmission line in an optical OFDM communication system, thus enabling sensitivity degradation to be reduced. Furthermore, the invention can provide an optical communication system, an optical transmitter, an optical receiver, and an optical transponder that can reduce the PAPR, thus enabling long distance transmission.
For example, with an optical communication system having a PAPR of 3 dB according to the invention, the achievable transmission distance determined by the nonlinear phase noise induced by the PAPR is about three times the achievable transmission distance of the existing optical OFDM communication systems.
The principle of an embodiment of the present invention will now be described with reference to
The signals associated with the present embodiment will be described below using mathematical expressions. The output signal of the transmitter-signal processing unit 100 shown in
where Ck denotes the data (the signal space coordinates, e.g., four points consisting of ±1±i when the subcarriers are modulated using 4-QPSK). Furthermore, N denotes the number of subcarriers, Δf denotes the frequency spacing of the subcarriers, t denotes time, and Ts denotes one symbol time.
When the sinusoidal wave having a frequency fm output from the RF oscillator 6 is phase-modulated using this signal as the modulation signal, the output signal of the phase modulating unit 8 can be represented by Expression (2):
I(t)=cos(2π·fm·t+h·φ(t)) (2)
where h denotes the modulation depth of the phase modulation.
The electro-optic converting unit 10 converts this phase-modulated sinusoidal wave into an optical signal. Assume that a direct modulation semiconductor laser is used as the electro-optic converting device, for example. Then, if the current applied to the semiconductor laser is proportional to Expression (2) and an appropriate bias current is superimposed on the current, the output photoelectric power of the semiconductor laser can be represented by Expression (3):
P(t)=P0·(1+cos(2π·fm·t+h·φ(t))) (3)
where, P0 denotes the average photoelectric power.
As can be seen in Expression (3), the PAPR is calculated as 3 dB. Therefore, in this case, the PAPR can be reduced significantly when compared with the existing optical OFDM communication.
The optical signal denoted by Expression (3) travels through the optical fiber 3 serving as the transmission line and enters the optical receiver 2. In the optical receiver 2, the opto-electric converting unit 20 converts the optical signal into an electric current which is proportional to the photoelectric power of the optical signal (Expression (3)). The current is further converted into a voltage, and is then amplified. The output signal of the opto-electric converting unit 20 thus obtained is synchronously detected by the synchronous detecting unit 9 using the sinusoidal wave output from the RF oscillator 7. The frequency of the sinusoidal wave is equal to (or substantially equal to) the frequency fm of the RF oscillator 6 included in the transmitter 1. As shown in
The first term of the left portion of Expression (4) represents the AC component of the input into the synchronous detecting unit 9, the second term represents the output of the RF oscillator 7, and the left portion as a whole represents the operation of the mixer 90. This signal is output through the low-pass filter 91 of the synchronous detecting unit 9. The signal output from the low-pass filter can be represented by the middle portion of Expression (4). Here, when a small signal is phase-modulated (h<1), the signal can be represented by the right portion of Expression (4). This is proportional to the baseband OFDM signal of Expression (1). By demodulating this signal using the receiver-signal processing unit 200 included in the optical receiver 2, the transmitted data is output from the output terminal 5. This is the basic principle of the present embodiment.
When the small-signal approximation cannot be applied to the phase modulation, the output of the synchronous detecting unit 9 is represented by the middle portion of Expression (4). Then, the transmitted data can be obtained by replacing the receiver-signal processing unit 200 by a receiver-signal processing unit 200-1 shown in
Although the above solution is described as using direct modulation with a semiconductor laser such as that shown in
An MZ modulator 12-1 shown in
cos(2π·fm·t+φ(t))·cos(2π·fc·t)+K1·cos(2πfc·t) (5)
The first term of Expression (5) represents the field-modulated photoelectric field, while the second term represents the continuous photoelectric field which has not yet been modulated.
In an optical communication system in which field modulation is combined with direct-detection reception, the field-modulated light and the continuous light are transmitted at the same time through the optical fiber. When they are direct-detection received, the field-modulated light and the continuous light generate a beat, which is then converted into an electric signal. In this case, it is necessary to install a band-pass filter or a low-pass filter that blocks a harmonic having a center frequency of 2×fm, which is two times what is represented in Expression (2), at the output of the opto-electric converting unit included in the receiver.
In order to efficiently extract the electric signal from the beat between the field-modulated light and the continuous light generated during the direct-detection reception, it is preferable that K1 be set as 1+√2/2 about 1.7. Then, the PAPR of the light represented by Expression (5) is equal to or less than 6 dB where the photoelectric power is high within the optical fiber 3. This demonstrates that this approach can solve the problems.
The field intensity of the continuous light can be set in a manner shown in Expression (5) by adjusting the direct current bias for the MZ modulator 12-1.
As another example of available solutions, optical SSB (single Side Band) modulation can also be used in the electro-optic converting unit 10. The photoelectric field output from the transmitter using optical SSB modulation can be represented by the following expression.
cos(2π·(fm+fc)·t+φ(t))+K2·cos(2π·fc·t) (6)
The first term of Expression (6) represents the upper sideband wave, while the second term represents the field of the continuous light. Although the following description will be made using the upper sideband wave, the lower sideband wave can also be used in a similar manner.
When the light represented by Expression (6) is direct-detection received, the continuous wave and the upper sideband wave generate a beat, which is obtained as the signal. In order to extract this signal efficiently, it is preferable that the amplitude K2 in the second term of Expression (6) be set as about 1.0.
Then, the PAPR of Expression (6) is calculated as 3 dB. The fact that the PAPR where the photoelectric power is high within the optical fiber is calculated as 3 dB demonstrates that the above described approach using the optical SSB modulation can also solve the problems.
As shown in
As another approach to achieve the optical SSB modulation, as shown in
The above described solutions use the direct-detection reception. From among these solutions, the solutions using either the MZ modulator or the optical SSB modulation and the direct-detection reception are described as converting the beat generated between the continuous light and the modulated light during the direct detection into an electric signal. However, beats may also be generated among the subcarriers of the OFDM signal, thus resulting in the corresponding electric signals being generated. This phenomenon occurs in a range of 2×B from direct current on the frequency axis. Here, B denotes the bandwidth of the baseband OFDM signal, and can be represented as B=(N+1)×Δf using the symbols in Expression (1). These beat signals among the subcarriers may interfere with the original beat signal between the continuous light and the modulated light, impairing the reception error rate.
To solve this problem, a guard band may be provided between the frequency fc of the continuous light and the frequency of the modulated light. This is shown in
Although the above described solutions are mainly described as using the direct detection reception, the opto-electric converting unit of the receiver in the present embodiment is not limited thereto, and coherent reception can also be used as shown in
A first embodiment will now be described with reference to
The optical OFDM communication system includes, e.g., a transmitter (optical transmitter) 1, an optical fiber 3, and a receiver (optical receiver) 2. The transmitter 1 includes, e.g., a transmitter-signal processing unit 100, an RF oscillator 6, and an electro-optic converting unit 10. The transmitter 1 may also have an input terminal 4. The receiver 2 includes an opto-electric converting unit 20 and a receiver-signal processing unit 200. The receiver 2 may also have an output terminal 8. The transmitter 1 and the receiver 2 are connected through the optical fiber 3. The electro-optic converting unit 10 of the transmitter 1 may be achieved using a driver amplifier 13-1 and a direct modulation semiconductor laser 11-1 as shown in
The transmitter-signal processing unit 100 includes e.g., a serial-parallel converting unit (S/P) 110, a subcarrier modulating unit 120, an inverse FFT unit (inverse fast Fourier transforming unit) 130, a parallel-serial converting unit (P/S) 140, a cyclic prefix inserting unit (CPI) 150, and a digital-analog converting unit (D/A converting unit) 160.
Data to be transmitted is converted into 2N parallel data components by the serial-parallel converting unit 110. The subcarrier converting unit 120 modulates N subcarriers using the parallel data components. The modulated subcarriers (ck, K=1, 2, . . . , N) are input into the inverse FFT unit 130. The input signal is converted into time-series data by the inverse FFT unit 130, and the time-series data is converted into serial data by the parallel-serial converting unit 140. After receiving cyclic prefixes inserted by the cyclic prefix inserting unit 150, the serial data is converted by the D/A converting unit 160 into an analog signal to be output. This signal is referred to as a baseband OFDM signal.
A sinusoidal wave output from the RF oscillator 6 shown in
Here, consider the case in which small-signal approximation is possible for the phase modulation. In general, the phase-modulated signal can be represented by the following expression:
cos(ωc·t+φ(t))=cos((ωc·t)·cos(φ(t))−sin(ωc·t)·sin(φ(t)) (7)
where, ωc denotes the oscillation angular frequency of the RF oscillator, while φ(t) denotes the baseband OFDM signal.
If the small-signal approximation is performed for the phase modulation, Expression (A) can be represented by the following expression:
cos(ωc·t)+φ(t)·sin(ωc·t) (8)
As described above, the configuration of the electro-optic converting unit 10 can use the direct modulation (
This optical signal enters the receiver 2 through the optical fiber 3 serving as the transmission line. In the receiver, the optical signal is converted by the opto-electric converting unit 20 into an electric signal. This electric signal is synchronously detected by a synchronous detecting unit 9 using a sinusoidal wave output from an RF oscillator 7. The output signal of the synchronous detecting unit 9 is demodulated by the receiver-signal processing unit 200 so that serial data is output through the output terminal 10. The configuration of the receiver-signal processing unit 200 may be the same as, e.g., the configuration shown in
The configuration of the synchronous detecting unit 9 may be that shown in
It is also possible to use a receiver-signal processing unit 200-1 shown
A second embodiment will now be described with reference to
The electro-optic converting unit 10 in the second embodiment may be a unit 10-2 shown in
As another means for generating the optical SSB signal,
Furthermore, when small-signal approximation is possible for the phase modulation, the Hilbert transforming unit 15 shown in
cos(ωm·t+φ(t))=cos(ωm·t)·cos(φ(t))−sin(ωm·t)·sin(φ(t)) (9)
Here, when the small-signal approximation is possible for the phase modulation, Expression (9) can be represented by Expression (10):
cos(ωm·t)−φ(t)·sin(ωm·t) (10)
When Expression (10) is Hilbert transformed taking into account the fact that the baseband OFDM signal φ(t) is real (it is a real number when phase modulation is performed), it can be approximated by the following expression.
sin(ωm·t)+φ(t)·cos(ωm·t)≅sin(ωm·t+φ(t)) (11)
Furthermore, if the phase modulating unit corresponding to the Hilbert transforming unit shown in
A third embodiment will now be described with reference to
A receiver 2-3 according to the third embodiment includes e.g., an opto-electric converting unit 20-2, an RF oscillator 7-1, a synchronous detecting unit 9, a receiver-signal processing unit 200, a local oscillator semiconductor laser 50, and an optical combining unit 60. An optical signal transmitted from a transmitter 1 through an optical fiber 3 enters the receiver 2-3. This optical signal is combined with light output from the local oscillator semiconductor laser 50 installed in the receiver 2-3, and is received by the opto-electric converting unit 20-2 using the so-called coherent receiving method so as to be converted into an electric signal. This electric signal is synchronously detected by the synchronous detecting unit 9 using a sinusoidal wave output from the RF oscillator 7-1 included in the receiver 2-3, and the output of the synchronous detecting unit is demodulated by the receiver-signal processing unit 200 so that the transmitted data is output through a terminal 5.
The optical combining unit 60 in the present embodiment may be an optical coupler or an optical 90-degree hybrid, or may be a polarization beam splitter (PBS) capable of polarization diversity and two optical 90-degree hybrids. In addition, as well known, the photodiode 21 may be a balanced photodiode or a pair of photodiodes so as to suit the configuration of the optical combining unit 60.
As another embodiment,
In the present embodiment, a single RF oscillator can serve both as the oscillator included in the transmitter 1 and the oscillator included in the receiver 2. For example, as shown in
The embodiments described herein can be used for optical communication systems, for example.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2010/050474 | 1/18/2010 | WO | 00 | 7/17/2012 |