JP4844432B2 - Optical transmission apparatus and method - Google Patents

Description

  The present invention relates to an optical transmission apparatus and method for transmitting an electrical signal including a plurality of subcarriers by analog optical modulation.

  A system that transmits an optical signal after, for example, intensity modulation according to the amplitude of an electric signal in a radio frequency band has been proposed (for example, see Non-Patent Document 1).

  Non-Patent Document 1 uses Orthogonal Frequency Division Multiplexing (OFDM) technology, and an optical transmission apparatus first performs real and imaginary part processing by fast Fourier inverse transform processing and digital / analog conversion processing. The baseband signal is generated, and the baseband signal of the real part and the imaginary part are respectively frequency-converted by the same frequency sine wave signals orthogonal to each other and added to modulate the continuous light from the laser diode. Generating a modulated electrical signal.

  The optical signal whose intensity is modulated by the modulated electric signal has an optical spectrum in which the optical carrier signal, which is the emission frequency of the laser diode, is an axis of symmetry, and the sidebands carrying information are arranged on the high frequency side and the low frequency side. In the optical transmission device described in Non-Patent Document 1, one sideband is removed by a filter, that is, an optical signal is transmitted using a single sideband (SSB) system.

  In addition, the optical transmission device that has received the optical signal first converts the received optical signal into a modulated electrical signal by a photodiode, and multiplies the modulated electrical signal by a sine wave that is in phase and orthogonal to the modulated electrical signal. Thus, the baseband signals of the real part and the imaginary part are converted into baseband signals of the real part and the imaginary part, and the baseband signals of the real part and the imaginary part are converted into complex signals in the frequency domain by digital analog conversion processing and fast Fourier transform processing.

James Lowery et al, "Orthogonal-frequency-division multiplexing for dispersal compensation of long-haul optical systems," Optics Express. 14, no. 6, pp. 2079-2084, March 2006

FIG. 7 is a diagram illustrating a schematic optical spectrum of an optical signal transmitted by the optical transmission device described in Non-Patent Document 1. Note that the frequency of the sine wave signal for frequency conversion of the baseband signal is f _TX , and the frequency difference between the lowest subcarrier and the highest subcarrier included in the modulated electric signal, that is, the band of the modulated electric signal is f _SIG . According to FIG. 7, the center frequency of the upper sideband 30 including a plurality of subcarriers is separated from the optical carrier 20 by the frequency f _TX of the sine wave signal used for frequency conversion. Here, the frequency of the optical carrier 20 is equal to the emission frequency of the laser diode input to the external optical modulator.

As described in Non-Patent Document 1, when an optical signal having a sideband 30 including a plurality of subcarriers, such as OFDM, is converted into a modulated electric signal by photo diode, each is also referred to as intermodulation (intermodulation). Mutual mixing noise is generated by intermixing between subcarriers. FIG. 8 is a diagram showing a schematic frequency spectrum of an electric signal output when the optical signal shown in FIG. 7 is input to the photodiode. According to FIG. 8, the center frequency of the modulated electric signal 50 is equal to the frequency difference between the optical carrier 20 and the center of the sideband 30 and is f _TX , and the mutual mixing noise 40 is generated from the direct current to the frequency f _SIG. ing. Note that extrapolation noise of this intermixing noise 40 (Extrapolation) is, because leaking to a higher frequency range than the frequency _{f SIG,} signals present in higher areas than the frequency _{f SIG} also will be affected in practice .

According to Non-Patent Document 1, the intermixed noise 40 increases as the carrier-to-signal ratio (CSR) decreases. In other words, the intermixing noise 40 can be reduced by allocating much of the optical power that can be used for optical transmission to the optical carrier 20, but this is allocated to the sideband 30, that is, a signal having information to be transmitted. This means that the optical power that can be swung is reduced, and the signal-to-noise ratio (SNR) is deteriorated. For this reason, in Non-Patent Document 1, in order to distribute the same optical power to the optical carrier 20 and the sideband 30 and avoid the mutual mixing noise 40, f _{TX is set} to 1.5 times f _SIG , and the optical carrier 20 A guard band having a frequency width f _SIG is secured between the sideband 30 and the sideband 30.

  The guard band is a band that is not used for transmission of information, and therefore there is a problem that the band cannot be effectively used by the method according to the prior art. In particular, as shown in FIG. 9, when wavelength multiplexing the optical SSB signal, it is necessary to secure a guard band that does not transmit information for each channel, and this problem becomes significant. Furthermore, there is a problem that the quality of the low subcarrier deteriorates due to the influence of extrapolation noise.

  Therefore, the present invention provides an optical transmission device that can effectively use a band compared to the prior art and suppress the influence on the demodulation of mutual mixing noise when an electrical signal including a plurality of subcarriers is transmitted by analog optical modulation. And to provide a method.

According to the optical transmission device of the present invention,
Means for generating an optical carrier for each channel, means for frequency converting an electric signal including a plurality of subcarriers input to each channel, and the electric signal after frequency conversion of the optical carrier for each channel for the corresponding channel Means for generating an optical signal including sidebands corresponding to a plurality of subcarriers for each channel, and means for combining the optical carriers and sidebands of each channel. The wavelength-multiplexed optical signal output by the means for wavering has a frequency arrangement including an optical carrier of another channel or a sideband of another channel between the optical carrier and sideband of the same channel.

According to another embodiment of the optical transmission device of the present invention,
The frequency converting means is a signal in which the lowest frequency of the subcarrier included in the electrical signal is higher than n times the channel interval and the highest frequency is lower than n + 1 times the channel interval for an integer n of 1 or more. It is also preferable to perform frequency conversion. Furthermore, it is also preferable that n = 1.

According to the optical transmission method of the present invention,
A step of generating an optical carrier for each channel, and an optical signal including a sideband corresponding to the plurality of subcarriers are generated for each channel by modulating the optical carrier of each channel with an electric signal including a plurality of subcarriers. And a step of multiplexing the optical carrier of each channel and the sideband and transmitting a wavelength-multiplexed optical signal, the wavelength-multiplexed optical signal between the optical carrier of the same channel and the sideband. And having a frequency arrangement including an optical carrier of another channel or a sideband of another channel.

  The frequency arrangement of the wavelength multiplexed optical signal has an optical carrier of another channel or a sideband of another channel between the optical carrier and sideband of the same channel. That is, there is at least an empty band larger than the channel interval between the optical carrier and the sideband of the same channel, and therefore, when the one-sideband optical signal of each channel is converted into an electric signal, the electric signal obtained Is out of the band of mutual mixing noise, and the frequency utilization efficiency is improved as compared with the prior art in which each channel can be used less than half of the channel interval.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings.

  FIG. 1 is a block diagram of a transmission side of an optical transmission apparatus according to the present invention. Referring to FIG. 1, the optical transmission apparatus includes a plurality of optical signal generation units 1-1 to 1-n, a plurality of optical branching units 2-1 to 2-n, and a plurality of optical modulation units 3-1 to 3-n. n, optical multiplexing units 4 and 5, and an optical coupling unit 6. In addition, when it is not necessary to distinguish each optical signal generation part, it refers to as the optical signal generation part 1, and when it is not necessary to distinguish each optical modulation part, it refers to as the optical modulation part 3.

The optical signal generator 1 is, for example, a distributed feedback laser diode, is provided corresponding to each channel, and outputs continuous light. Here, the optical signal generated by each optical signal generation unit 1 differs from the optical signal generated by the optical signal generation unit 1 of the adjacent channel by the frequency f _CH corresponding to the channel interval. The optical signal generated by the optical signal generation unit 1 is branched in the corresponding optical branching units 2-1 to 2-n, and the branched optical signals are respectively transmitted to the optical multiplexing unit 5 and the corresponding optical modulation unit 3. Entered.

  The optical modulator 3 is an external optical modulator that modulates continuous light, such as a Mach-Zehnder modulator, for example, modulates continuous light with the modulated electrical signal 50 of each input channel, and includes an optical carrier, an upper sideband, Output optical signal including lower sideband. Note that examples of the modulated electric signal 50 including a plurality of subcarriers include an OFDM signal and a signal transmitted by an optical signal in a sub-carrier multiplexing (SCM) system, but are not limited thereto. It is not a thing.

FIG. 3 is a block diagram of a circuit for generating the modulated electrical signal 50 in the optical transmission apparatus according to the present invention. The modulation unit 10 inputs a baseband signal including a plurality of subcarriers to the frequency conversion unit 11, and the frequency conversion unit 11 performs frequency conversion of the baseband signal into an electric signal in a radio frequency (RF) band, A modulated electric signal 50 including a plurality of subcarriers is output to the corresponding optical modulator 3. In the following description, it is assumed that the band of the modulated electric signal 50 is f _SIG and the center frequency thereof is f _TX . The signal input to the frequency conversion unit 11 does not need to be a baseband signal, and the frequency conversion unit 11 can convert an arbitrary electric signal having a band of f _SIG so that its center frequency is f _TX. Anything can be used.

  The optical multiplexing unit 4 is, for example, an arrayed waveguide grating (AWG), and an optical carrier of one of the optical signals output from each optical modulation unit 3 and one sideband, in this embodiment, Suppresses side sidebands, combines only upper sidebands and outputs. The optical multiplexing unit 5 multiplexes and outputs the optical signals generated by the optical signal generation units 1 and branched by the optical branching unit 2. Further, the optical coupling unit 6 combines the optical signals output from the optical multiplexing unit 4 and the optical multiplexing unit 5 and outputs the combined optical signals to the optical transmission line. In the following description, an embodiment using the upper sideband will be described, but it is naturally possible to use the lower sideband.

  FIG. 4 is a diagram showing a schematic optical spectrum for explaining the present invention. 4A shows the optical carrier 21 and the upper sideband 31 of the optical signal output from the optical modulation unit 3-1 corresponding to the channel 1, and FIG. 4B shows the optical modulation unit 3 corresponding to the channel 2. 2 shows the optical carrier 22 and the upper sideband 32 of the optical signal output by -2. For simplicity, the lower sideband is omitted.

As shown in FIGS. 4A and 4B, the frequency difference between the center of the upper sideband 31 and the optical carrier 21 and the frequency difference between the center of the upper sideband 32 and the optical carrier 22 are It is equal to the center frequency _fTX of the modulated electrical signal 50. In the present embodiment, a frequency f _TX that satisfies the following equation is used.
_{f CH + f SIG / 2 +} f g <f TX <2f CH -f SIG / 2f g (1)
Note that f _g is a minimum required guard band between the sideband and the optical carrier.
f _CH > f _SIG + 2f _g .

Here, when the lowest frequency of the subcarrier of the modulated electric signal 50 is f _TXL and the highest frequency is f _TXH ,
f _TXL = f _TX -f _SIG / 2
f _TXH = f _TX + f _SIG / 2
Therefore, the equation (1) is
f _TXL > f _CH + f _g (3)
f _TXH <2f _CH- f _g (4)
It becomes.

  Therefore, the frequency conversion unit 11 of each channel converts the frequency of the baseband signal in a band less than the channel interval so as to satisfy the expressions (3) and (4), so that the optical signal output from the optical coupling unit 6 is 4 are arranged as shown in FIG. More specifically, for a system with n channels, the sideband of channel k is arranged between the optical carrier of channel k + 1 and the optical carrier of channel k + 2 for integer k from 1 to n. However, since there are no channels n + 1 and n + 2 for k = n−1, n, there are no optical carriers on both sides of the sideband of channel n.

As described above, in the optical transmission device according to the present invention, although the space between the optical carriers of channel 1 and channel 2 is vacant, it is not necessary to secure the same guard band as the sideband band between the optical carriers of other channels. in each channel, it is possible to enhance the frequency use efficiency as compared to less than half of the prior art band were only available for information transmission channel spacing f _CH.

  Next, the optical signal reception process will be described. FIG. 2 is a block diagram of the receiving side of the optical transmission apparatus according to the present invention. As shown in FIG. 2, the optical transmission device includes an optical branching unit 2, optical demultiplexing units 7 and 8, optical coupling units 6-1 to 6-n for each channel, and photoelectric conversion unit 9-1 for each channel. To 9-n. In the following description, the photoelectric conversion unit 9 is referred to when it is not necessary to distinguish the photoelectric conversion unit.

  The wavelength-multiplexed SSB optical signal from the optical transmission line is branched by the optical branching unit 2 and output to the optical demultiplexing units 7 and 8. The optical demultiplexing unit 7 is, for example, an AWG, suppresses the optical carrier of each channel included in the optical signal, and demultiplexes the sidebands of each channel, and the optical demultiplexing unit 8 is, for example, an AWG Then, each sideband included in the optical signal is suppressed and the optical carrier of each channel is demultiplexed.

  The optical couplers 6-1 to 6-n are provided corresponding to the respective channels, and the sidebands of the corresponding channels output from the optical demultiplexing unit 7 and the optical carriers of the corresponding channels output from the optical demultiplexing unit 8 Are output to the corresponding photoelectric converter 9. Therefore, the photoelectric conversion unit 9-1 corresponding to the channel 1 receives the optical signal shown in FIG. 4A, and the photoelectric conversion unit 9-2 corresponding to the channel 2 is shown in FIG. 4B. An optical signal is received.

The photoelectric conversion unit 9 is, for example, a photodiode, and converts an input optical signal into a modulated electric signal 50. FIG. 5 is a diagram illustrating a spectrum of a signal output from the photoelectric conversion unit 9. The center frequency of the modulated electrical signal 50 is f _TX , but as described above,
f _TX > f _CH + f _SIG / 2 + f _g and f _CH > f _SIG + 2f _g
Because
f _TX > 1.5 f _SIG +3 f _g
It becomes.
Therefore, the modulated electric signal 50 is out of the band of the mutual mixing noise 40 generated by photoelectric conversion. That is, the modulated electric signal 50 is not disturbed by the mutual mixing noise 40. Furthermore, 3f prior art _g only modulated electrical signal 50 is to shift to the high frequency side, the influence of the extrapolation noise is reduced compared to the prior art.

  As described above, the same guard band as the sideband is secured for each channel by setting the lowest frequency of the subcarrier of the modulated electric signal 50 higher than the channel interval and lowering the highest frequency less than twice the channel interval. There is no need to do this, and it is possible to avoid interference from the intermixed noise 40.

  In the above embodiment, the optical modulation unit 3 outputs an optical signal including an optical carrier and both sidebands. However, for example, the optical modulation unit 3 can generate an optical signal in which the optical carrier is suppressed. It is. Specifically, as shown in FIG. 6, the average value of the amplitude of the modulated electric signal 50 corresponds to the phase change point of the transmitted optical signal of the optical modulator 3, that is, the point where the optical signal is not transmitted. The bias voltage for the light modulator 3 is set. As a result, half of the optical signal output from the optical modulation unit 3 is phase 0, the other half is phase π, and the optical carrier is effectively suppressed even when the optical carrier is close to the sideband. An optical signal is output. In this case, the optical multiplexing unit 4 in FIG. 1 only has to suppress and multiplex only the sidebands different from the sidebands to be used, and the characteristics required for the optical multiplexing unit 4 are relaxed.

  Further, a known SSB optical modulator is used for the optical modulation unit 4, and the optical multiplexing unit 4 adjusts the bias voltage of the known SSB optical modulator even if it is configured to suppress only the optical carrier 20, You may output the SSB optical signal which suppressed the optical carrier directly from the optical modulation part 4. FIG.

Further, the sideband of channel k has been described as being arranged between the optical carrier of channel k + 1 and the optical carrier of channel k + 2, but the sideband of channel k is, for example, the optical carrier of channel k + 2. And a mode of being arranged between the optical carriers of the channel k + 3. That is, any wavelength-multiplexed optical signal in which at least one other-channel optical carrier and / or other-channel sideband is arranged between a certain-channel optical carrier and the side-band of that channel may be used. In this case, the lowest frequency and the highest frequency of the subcarrier of the modulated electric signal 50 are the integer n of 1 or more,
f _TXL > nf _CH + f _g
f _TXH <(n + 1) f _CH −f _g
It becomes.

It is a block diagram of the transmission side of the optical transmission apparatus according to the present invention. It is a block diagram of the receiving side of the optical transmission apparatus by this invention. It is another block diagram of the transmission side of the optical transmission apparatus by this invention. It is a figure explaining the process by the side of the transmission of the optical transmission apparatus of this invention. It is a figure which shows the spectrum of the signal which a photoelectric conversion part outputs. It is a figure explaining the carrier wave suppression optical signal production | generation in an optical modulation part. It is a schematic spectrum figure of an optical SSB-OFDM signal. FIG. 8 is a schematic spectrum diagram when the optical signal shown in FIG. 7 is converted into an electrical signal. It is a schematic spectrum diagram of a wavelength division multiplexing SSB optical signal according to the prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1-1 to 1-n Optical signal generation part 2, 2-1 to 2-n Optical branching part 3, 3-1 to 3-n Optical modulation part 4, 5 Optical multiplexing part 6, 6-1 to 6 -N Optical coupling unit 7, 8 Optical demultiplexing unit 9, 9-1 to 9-n Photoelectric conversion unit 10 Modulation unit 11 Frequency conversion unit 21, 22, 23 Optical carriers 31, 32, 33 Sidebands 40 Intermixing Noise 50 Modulated electrical signal

Claims (4)

Means for generating optical carriers for each channel;
Means for frequency converting an electrical signal including a plurality of subcarriers input to each channel;
Means for modulating the optical carrier of each channel with the electrical signal after frequency conversion of the corresponding channel, and generating an optical signal including sidebands corresponding to a plurality of subcarriers for each channel;
Means for combining optical carriers and sidebands of each channel;
With
Wavelength-multiplexed optical signal means for multiplexing outputs, during the optical carrier and the sidebands of the same channel, have a frequency arrangement comprising sidebands of the optical carrier, or other channels of other channels,
The means for generating generates an optical signal in which the optical carrier of each channel is suppressed . The frequency of the optical carrier in the adjacent channel differs by a predetermined channel interval,
The band of the electrical signal is less than the channel spacing;
The frequency converting means is configured to convert the lowest frequency of the subcarriers included in the electrical signal to a frequency higher than n times the channel interval and the highest frequency to n + 1 of the channel interval for an integer n of 1 or more. Convert to a frequency lower than double the frequency,
The optical transmission device according to claim 1.   The optical transmission apparatus according to claim 2, wherein n is 1. Generating an optical carrier for each channel;
Modulating the optical carrier of each channel with an electrical signal including a plurality of subcarriers, and generating an optical signal including a sideband corresponding to the plurality of subcarriers for each channel;
Combining the optical carrier and sidebands of each channel and transmitting a wavelength multiplexed optical signal;
With
Wavelength-multiplexed optical signals, between the optical carrier and the sidebands of the same channel, have a frequency arrangement comprising sidebands of the optical carrier, or other channels of other channels,
The generating step generates an optical signal in which an optical carrier of each channel is suppressed .

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