JP3721132B2 - Optical communication device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength division multiplexing optical transmission system, and more particularly to an optical communication apparatus suitable for high density multiplexing.
[0002]
[Prior art]
In recent years, in optical transmission, information is digitized and optically modulated on the transmitting side and transmitted through an optical fiber as a transmission path, and an optical signal transmitted through the optical fiber is converted into an electrical signal on the receiving side. This is a communication system that restores information by returning it to a digital signal, can secure a wide band, enables high-speed transmission, and can transmit a large amount of information with high quality.
[0003]
With the advancement of optical fiber amplifiers, research and development of optical transmission systems have been carried out more and more, and have attracted attention as a large-capacity transmission system for realizing the future multimedia information age.
[0004]
By the way, NRZ (Non-Return-to-Zero) pulses are often used for optical transmission, but optical transmission based on intensity modulation (IM-DD: Intensity-Modulation Direct-Detection) using conventional NRZ pulses. In order to realize large-capacity transmission in a system, a wavelength division multiplexing optical transmission system that multiplexes optical signals of a plurality of channels in the optical region without increasing the transmission capacity per channel is promising. The bandwidth of the optical fiber, which is the optical region, extends to several thousand GHz, and it is possible to cope with future demands for large capacity by multiplexing a considerable number of channels.
[0005]
In a wavelength division multiplexing optical transmission system, in order to suppress interference due to crosstalk, it is necessary to open a band several times or more as large as the band of an optical signal to be transmitted between adjacent channels. As described above, the optical fiber band is very wide. However, when long-distance optical transmission is performed using an optical fiber amplifier as a repeater, the gain band of the optical fiber amplifier becomes narrower due to repeated repeating. The possible optical fiber bandwidth is narrowed.
[0006]
In such a case, the number of channels that can be multiplexed is greatly limited against the wide bandwidth of the optical fiber, so it is desirable to perform wavelength multiplexing at a higher density. Therefore, the optical signal using the frequency spectrum distribution of the optical signal in the normal double sideband shown in FIG. 14B and the residual sideband modulation that blocks a part of the sideband shown in FIG. As can be seen from the frequency spectrum distribution, a residual sideband modulation method that can suppress the spread of the optical signal band per channel is suitable for high-density wavelength multiplexing.
[0007]
Looking at the field of wireless communication, the filter for vestigial sideband modulation in wireless communication is according to the document “Communication Systems (second edition)” by Simon Haykin, published by John Wiley & Sons. H (f) is
H (f + f_sig ) + H (f−f_sig ) = 2H (f_sig ) = Constant ... (1)
And the carrier frequency f_sig Those having complementary symmetry with respect to are used. In other words, the condition of equation (1) is that the full width at half maximum with complementary symmetry is B_fil Is used, the transmission center frequency f of the filter is used._fil The
｜ f_sig -F_fil | = 0.5B_fil ... (2)
Will be set so that
[0008]
On the other hand, in the optical communication field, when considering an optical filter for performing residual sideband modulation, this optical filter has a large extinction ratio in order to suppress crosstalk from adjacent channels during wavelength multiplexing. It is necessary to use a type that can be taken. Examples of types that can increase the extinction ratio include dielectric multilayer films, diffraction gratings, and optical filters such as five-perot etalon.
[0009]
However, it is difficult to satisfy the conditions of complementary symmetry suitable for vestigial sideband modulation, and all of these have the problem that waveform distortion occurs because an optical filter with good complementary symmetry cannot be obtained. .
[0010]
[Problems to be solved by the invention]
Considering an optical filter for performing vestigial sideband modulation in optical communication, this optical filter is of a type that can have a large extinction ratio in order to suppress crosstalk from adjacent channels during wavelength multiplexing. It is necessary to use it.
[0011]
However, as described above, it is difficult to realize an optical filter having complementary symmetry required by residual sideband modulation. Further, when a narrower band optical filter is inserted in order to realize higher-density wavelength multiplexing, the waveform distortion becomes larger under the condition that satisfies the expression (2). And since waveform distortion means waveform deterioration, it becomes a big barrier in ensuring the reliability of communication, and is a big problem in realizing long-distance and large-capacity optical communication stably. Therefore, development of a technology that solves these problems is eagerly desired.
[0012]
SUMMARY OF THE INVENTION An object of the present invention is to provide an optical communication device that remarkably reduces waveform deterioration when performing residual sideband modulation using a narrow-band optical filter, and stably realizes long-distance and large-capacity optical communication. An object of the present invention is to provide an optical communication apparatus that can be realized.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as follows. That is, according to the present invention, in an optical communication apparatus using intensity modulation, an optical transmitter side includes an optical filter for modulating a residual sideband of an optical signal, and the frequency of the optical signal increases with time. When high frequency components gather at the rear end, the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig If the frequency of the optical signal decreases with time and low frequency components are collected at the rear end of the pulse, the transmission center of the optical filter is set to a higher frequency band. Frequency f_fil The carrier frequency f of the optical signal_sig Is set to a lower frequency band, and the lower sideband is mainly transmitted.
[0014]
The optical communication device having the above-described configuration has the transmission center frequency f of the optical filter._fil The carrier frequency f of the optical signal_sig On the other hand, in the optical signal, when the frequency increases with time and high frequency components are collected at the rear end of the pulse, the frequency decreases with time on the upper side band side, and low frequency components are collected at the rear end of the pulse. In this case, the lower sideband is set. Therefore, it is possible to perform residual sideband modulation even for an optical signal having a frequency chirp and having a biased frequency spectrum, and it is possible to generate an optical NRZ pulse in which the spread of the optical signal band is suppressed. In addition, it is possible to provide an optical communication apparatus that remarkably reduces waveform deterioration when performing residual sideband modulation using a narrow band optical filter, and to stably realize long-distance and large-capacity optical communication.
[0015]
Therefore, when performing residual sideband modulation using a narrowband optical filter, it is possible to provide an optical communication device that enables high-quality communication with little waveform deterioration and stable long-distance and large-capacity optical communication. In addition, it is possible to provide an optical communication apparatus that can be implemented in a high-density manner and that can multiplex wavelengths with high density and can secure a large number of channels.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific examples of the present invention will be described with reference to the drawings. The characteristics shown in the present invention are obtained by numerical simulation.
[0017]
(First specific example)
In the first specific example, in an optical communication apparatus using intensity modulation, the full width at half maximum B is placed on the optical transmitter side._fil And a transmission center frequency f of the optical filter._fil And the carrier frequency f of the optical signal_sig 0.5B_fil <| F_sig -F_fil It is a residual sideband modulation system that blocks one sideband of the optical signal and leaves a part of the sideband to be removed by setting within a range in which | The details will be described below.
[0018]
FIG. 1 is a block diagram showing a first specific example of the first invention. In the figure, 1 is an optical transmitter, 2 is a semiconductor laser light source, 3 is an external optical modulator, 5 is an optical fiber amplifier, 6a is an optical filter, and 7 is an optical fiber.
[0019]
Among these, the semiconductor laser light source 2 outputs CW (Continuous-Wave) light 101, and the external optical modulator 3 modulates the intensity of the CW light 101 with the transmission signal 4 to generate an optical signal having an NRZ pulse waveform. 102 is output. The optical fiber amplifier 5 amplifies and outputs the optical signal 102, and the optical filter 6a outputs the optical signal amplified by the optical fiber amplifier 5 while suppressing the spread of the optical signal band. The optical fiber 7 is a transmission line of the optical communication system, and transmits the output from the optical filter 6a.
[0020]
The optical transmitter 1 includes the semiconductor laser light source 2, the external optical modulator 3, the optical fiber amplifier 5, and the optical filter 6a. The operation of the optical transmitter 1 having such a configuration will be described. CW light 101 is output from the semiconductor laser light source 2, and the CW light 101 is intensity-modulated by the transmission signal 4 by the external optical modulator 3 to become an optical signal 102 having an NRZ pulse waveform. The transmission speed of the transmission signal 4 is B_sig Represented by
[0021]
The optical signal 102 having the NRZ pulse waveform is amplified by the optical fiber amplifier 5, and the optical filter 6a suppresses the spread of the optical signal band, and is input to the optical fiber 7 and transmitted.
[0022]
Here, the present invention is characterized by the optical filter 6a. In the description of the specific example of the present invention, the shape of the transmission characteristic of the optical filter 6a was considered for the Gaussian type, the third order Butterworth type, and the etalon type.
[0023]
Therefore, the details of the optical filter 6a will be described next. First, the optical signal 102 is an NRZ pulse waveform having no frequency chirp. When the transmission rate is 10 Gb / s (gigabit / second), the optical signal 102 has an eye pattern as shown in FIG. It has a frequency spectrum as shown in FIG.
[0024]
The eye pattern shown in the present invention is obtained after an ideal square wave detection by a photo detector as a photo detector._sig It is an eye pattern when it passes through a low-pass filter having a cutoff frequency of × 0.75.
[0025]
As can be seen from the figure, the frequency spectrum shown in FIG. 2B has signal components in both sidebands. Conventional optical signals are transmitted in such a double sideband, but in this case, the bandwidth required for transmission is 2B._sig A degree is required.
[0026]
In the present invention, in order to reduce the band required for transmission, one sideband is gently cut off by the narrow-band optical filter 6a to perform residual sideband modulation. For example, the transmission bandwidth of the optical filter 6a is 9.375 GHz (0.075 nm), and the shape of the transmission characteristic is a Gaussian type.
[0027]
Carrier frequency f of optical signal 102 incident on optical filter 6a_sig And the transmission center frequency f of the optical filter 6a_fil Detuning amount Δf = f_fil -F_sig FIG. 3 and FIG. 4 show the eye pattern and frequency spectrum of the emitted light signal 103 with respect to.
[0028]
FIG. 3A shows an example in which the detuning amount Δf is 0.0 GHz, and the full width at half maximum B of the optical filter 6a to be inserted is shown._fil Is the optical signal transmission speed B_sig In FIG. 3A, [a-I] indicates an eye pattern, and [a-II] in FIG. 3A indicates a frequency spectrum. As shown in FIG. 3A, the full width at half maximum B of the optical filter 6a to be inserted is shown._fil Is the optical signal transmission speed B_sig Especially if B is not wide enough_fil ≦ 3B_sig In the case of f_sig = F_filIs set, the high-frequency signal components in both sidebands are greatly attenuated, so that a pattern effect appears and waveform distortion occurs.
[0029]
FIG. 3B shows an example in which the detuning amount Δf is 3.125 GHz, [b-I] in FIG. 3B shows an eye pattern, and [b-II] in FIG. 3B shows a frequency spectrum. ing.
[0030]
4A shows an example in which the detuning amount Δf is 4.0 GHz, [a-I] in FIG. 4A is an eye pattern, and [a-II] in FIG. 4A is a frequency spectrum. Is shown.
[0031]
FIG. 4B shows an example in which the detuning amount Δf is 5.0 GHz, [b-I] in FIG. 4B shows an eye pattern, and [b-II] in FIG. 4B shows a frequency spectrum. ing.
[0032]
In the case of the detuning amount Δf = 3.125 GHz (0.025 nm) where the condition of the above-described equation (2) shown in FIG. 3B is satisfied, and the detuning amount Δf = 4.4 shown in FIG. The eye pattern and frequency chirp at 0 GHz (0.032 nm) are compared.
[0033]
In FIG. 3B, the full width at half maximum B of the inserted optical filter._fil Is optical signal band B_sig Therefore, the high-frequency signal component is attenuated, the pattern effect remains, and the waveform distortion is large. In FIG. 4C, although there is a pattern effect, the balance between the high-frequency signal component and the low-frequency signal component is maintained, so that the eye pattern is shown in FIGS. 3A, 3B, and 4B. You can see that it is open compared to).
[0034]
However, as shown in FIG._fil On the other hand, when the signal is detuned, the low-frequency signal component attenuates and causes a large waveform deterioration. Therefore, it is necessary to select an optimal Δf. The optimum Δf includes an optical signal transmission speed B_sigAnd full width at half maximum B of optical filter 6a_fil Is greatly involved.
[0035]
FIG. 5 shows the transmission characteristics normalized by the transmission speed of the optical signal when the shape of the transmission characteristic of the optical filter 6a is a Gaussian type, a 3rd butterworth type, and an etalon type (fabry-perot). Bandwidth B_fil / B_sig Detuning amount with respect to Δf / B_sig And eye penalty (loss). The eye penalty in the present invention is expressed in decibels by the ratio of the eye opening width obtained over the identification time zone of 20% width of the 1-bit time slot and the average “1” and “0” level difference in this identification time zone. It is a value expressed in [dB]. Since no noise at the receiver end or the like is taken into consideration, this eye penalty represents a received light power penalty.
[0036]
From FIG. 5, the transmission bandwidth B in each optical filter._fil / B_sig Detuning amount Δf / B_sig Are almost equal, but there are differences in the size and rate of change of the eye penalty. The Gaussian type and etalon type whose cutoff characteristics change slowly with respect to frequency include a relatively wide band before the extinction ratio becomes sufficiently large, so that the frequency spectrum of the optical signal may be largely cut off. No, the eye penalty changes slowly.
[0037]
On the other hand, a type having a steep cutoff characteristic such as a third-order Butterworth type has a sufficiently large extinction ratio in a not-so-wide band, and thus the eye penalty is rapidly deteriorated even if it is slightly deviated from the optimum Δf. Also, in the Gaussian and Butterworth types, the normalized full width at half maximum B_fil / B_sig Optimum detuning amount Δf / B when changing_sig Is shown in FIG.
[0038]
From FIG._fil / B_sig ≦ 2.0, optimal detuning amount Δf / B of Gaussian type and Butterworth type_sig Are approximately equal, but B_fil / B_sig When ≧ 2.0, the optimum Δf / B_sig It can be seen that there is a difference. This is considered to be due to the difference in the extinction ratio slope around the cutoff frequency in the transmission characteristics of the optical filter.
[0039]
From the above conditional expression (2), the optimum detuning amount Δf in the residual sideband modulation in wireless communication is Δf = 0.5B_sig Met. However, when a narrow-band optical filter is used, the detuning amount Δf is Δf> 0.5B from FIG. 5 and FIG._sig It is considered effective to set to.
[0040]
Further, in the Gaussian type and third order Butterworth type optical filters in this specific example, the full width at half maximum B of the optical filter normalized from FIG._fil / B_sig And optimal detuning amount Δf / B_sig The relationship of
Δf / B_sig = 0.575B_fil / B_sig -0.025 (3)
And it is known that optimal results are obtained when this relationship is maintained. When the optical signal contains frequency chirp, the optimum Δf varies depending on the size of the frequency chirp from the above approximate relationship.
[0041]
As described above, in this specific example, the optical communication device is an optical communication device using intensity modulation._fil And a transmission center frequency f of the optical filter._fil And the carrier frequency f of the optical signal_sig The
0.5B_fil <| F_sig -F_fil ｜
By using the residual sideband modulation method that blocks one sideband of the optical signal and leaves a part of the sideband to be removed The carrier frequency f of the optical signal at the optical transmitter side._sig And the transmission center frequency f of the optical filter_fil By shifting and setting within a specified range, one sideband of the optical signal is cut off, and residual sideband modulation that transmits some of the sidebands is performed, so that the optical signal waveform is not distorted. In addition, it is possible to generate an optical NRZ pulse in which the spread of the optical signal band is suppressed.
[0042]
However, the residual sideband modulation according to the present invention suppresses the spread of the optical signal band by a narrow-band optical filter, so that the full width at half maximum B_fil If it is too narrow, the signal band is largely cut off, resulting in a large eye penalty. If it is too wide, the sideband on one side transmits up to the secondary high-frequency signal component, so that the spread of the optical signal band cannot be suppressed. Therefore, the full width at half maximum B of the optical filter 6a_fil Is 0.3-3.0B_sig The range of is more effective.
[0043]
The above has suppressed the spread of the optical signal band without distorting the optical signal waveform by blocking one sideband of the optical signal and performing residual sideband modulation that transmits some of the sidebands. An example has been described in which an optical NRZ pulse can be generated.
[0044]
An example of enabling generation of an optical NRZ pulse that enables residual sideband modulation and suppresses the spread of the optical signal band even for an optical signal having a frequency chirp and having a biased frequency spectrum. This will be described as a second specific example.
[0045]
(Second specific example)
The configuration of the optical transmitter 1 in the second specific example is basically the same as that of FIG. 1, and includes the optical transmitter 1, the semiconductor laser light source 2, the external optical modulator 3, the optical fiber amplifier 5, and the optical filter 6a. However, the external optical modulator 3 is different from the first specific example only in that a frequency chirp is added to the modulated NRZ pulse.
[0046]
Here, the frequency chirp is represented by a parameter C. For the parameter C, a chirp parameter described in the literature, “Residual Chirp in Integrated-Optic Modulators” by Anders at IEEE Photo. Tech. Lett., Vol.4, no.
[0047]
When the frequency chirp C is positive, the frequency decreases with time and low frequency components gather at the rear end of the pulse. When the frequency chirp C is negative, the frequency increases with time and the pulse rear end. This refers to the case where high frequency components are collected.
[0048]
FIG. 7 is a diagram for explaining the difference in frequency spectrum when the frequency chirp exists. FIG. 7A shows an example of the spectrum when there is no frequency chirp C, and FIG. Shows an example of a spectrum when the frequency chirp C is positive, and FIG. 7B shows an example of a spectrum when the frequency chirp C is negative. At this time, the transmission center frequency f of the optical filter 6a for performing the residual sideband modulation_fil The carrier frequency f of the optical signal_sig On the other hand, there is a difference in eye penalty depending on whether the frequency band is set to a high band or a low band.
[0049]
FIG. 8 shows the transmission rate B_sig = 10 Gb / s, the full width at half maximum of the optical filter is 9.375 GHz (0.075 nm) as a third-order Butterworth type, and the detuning amount Δf / B when the frequency chirp is positive and negative_sig And the relationship between eye penalties.
[0050]
From FIG. 8, when the frequency chirp added to the optical signal is positive, the transmission center frequency f of the optical filter is obtained._fil The carrier frequency f of the optical signal_sig It can be seen that the eye penalty is smaller when set to a lower frequency band. Conversely, when the frequency chirp added to the optical signal is negative, the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig It can be seen that the eye penalty is smaller when set to a higher frequency band.
[0051]
Since the frequency chirp is added to the optical signal in the laser direct modulation system and the system using an external optical modulator, which are actual optical transmission systems, the transmission center frequency f of the optical filter 6a._fil The direction of the detuning is greatly affected by the transmission characteristics.
[0052]
As described above, the optical communication apparatus according to this specific example includes an optical filter on the optical transmitter side, and when the optical signal is subjected to residual sideband modulation with the optical filter, the optical signal has a frequency over time. Increases and high-frequency components gather at the rear end of the pulse, the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig If the frequency of the optical signal decreases with time and low frequency components are collected at the trailing edge of the pulse, the optical filter transmits the optical filter. Center frequency f_fil The carrier frequency f of the optical signal_sig Is set to a lower frequency band, and mainly transmits the lower sideband.
[0053]
That is, in this optical communication apparatus, the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig On the other hand, when the frequency of the optical signal increases with time and high frequency components are collected at the rear end of the pulse, the transmission center frequency f of the optical filter is obtained._fil The carrier frequency f of the optical signal_sig If the frequency band is set to a lower frequency band, the eye penalty is smaller, so that the frequency decreases on the upper side band side and with time, and when low frequency components are collected at the rear end of the pulse, the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig Since the eye penalty is smaller when set to a higher frequency band, the lower sideband is set. Therefore, even for optical signals with frequency chirp and biased frequency spectrum, it is possible to efficiently apply residual sideband modulation with reduced eye penalty, and to reduce optical signal NRZ pulses with reduced optical signal bandwidth. It is possible to generate.
[0054]
The example of the optical transmitter has been described above. Next, an example of an optical receiver that makes it possible to receive a vestigial sideband modulated optical signal without transmitting unnecessary bands will be described.
[0055]
(Third example)
The third specific example is the transmission center frequency f of the optical filter inserted at the input end of the optical receiver._fil And optical signal carrier frequency f_sig Is set to be shifted within a predetermined range, whereby an optical signal subjected to residual sideband modulation can be received without passing through an unnecessary band, which will be described in detail below.
[0056]
FIG. 9 is a block diagram showing a third specific example of the third invention. In the figure, 7 is an optical fiber as a transmission line, 8 is an optical receiver, 5b is an optical fiber amplifier, 6b is an optical filter, 9 is a photodetector, and 10 is a low-pass filter. The optical receiver 8 includes an optical fiber amplifier 5, an optical filter 6, a photodetector 9, and a low-pass filter 10.
[0057]
Among these, the optical fiber amplifier 5b is for amplifying and outputting the received optical signal 104 transmitted through the optical fiber 7, and the optical filter 6b is the amplified received optical signal. This is for limiting the bandwidth of the network. The photodetector 9 detects an optical signal transmitted through the optical filter 6b and converts it into an electrical signal. The low-pass filter 10 is a filter that passes a low-frequency component of the electrical signal and outputs it as an output signal. .
[0058]
In the optical receiver 8 having such a configuration, the received optical signal 104 by the residual sideband modulation transmitted from the transmitting side via the optical fiber 7 is amplified by the optical fiber amplifier 5b, and then amplified by the optical filter 6b. After the band is limited, it is detected by the photo detector 9 and converted into an electric signal. The low-frequency component obtained by passing the converted electric signal through the low-pass filter 10 becomes the reception signal 105.
[0059]
In general, an optical receiver is provided with an optical filter, but the transmission center frequency f of the optical filter 6b._fil Is the carrier frequency f of the received optical signal 104_sig It is normal to match. However, since the frequency spectrum of the optical signal 103 on the optical transmitter 8 side according to the present invention is vestigial sideband modulated, the carrier frequency f of the optical signal is_sig Is asymmetrical. Therefore, the frequency spectrum of the received optical signal 104 on the optical receiver side is also the carrier frequency f._sig Is asymmetrical.
[0060]
Therefore, when the optical filter 6b is not sufficiently wide-band, the carrier frequency f of the optical signal 104_sig And the transmission center frequency f of the optical filter 6b_fil If they are matched as in the prior art, it is possible not only to transmit unnecessary bands, but also to block necessary bands. Therefore, in the present invention, the transmission center frequency f of the optical filter 6b inserted on the optical receiver side._fil Is the carrier frequency f of the optical signal._sig Need to be detuned.
[0061]
At a transmission speed of 10 Gb / s, an optical signal obtained by detuning a Gaussian optical filter having a bandwidth of 12.5 GHz (0.1 nm) to Δf = + 7.25 GHz on the optical transmitter side and modulating the residual sideband is obtained. As shown in FIGS. 10 (a) and 10 (b) and FIGS. 11 (a) and 11 (b), assuming that the signal is received after passing through a Gaussian optical filter having a bandwidth of 12.5 GHz on the receiver side. The relationship between the eye pattern and the frequency spectrum with respect to the detuning amount Δf is shown. In both cases, [a-I] and [b-I] indicate an eye pattern, and [a-II] and [b-II] indicate a frequency spectrum.
[0062]
10A is an example in which the detuning amount Δf is “3.125 GHz”, and FIG. 10B is an example in which the detuning amount Δf is “0.0 GHz”. a) is an example in which the detuning amount Δf is “3.125 GHz”, and (b) in FIG. 11 is an example in which the detuning amount Δf is “6.125 GHz”.
[0063]
10 (a), (b), and FIG. 11 (b), the pattern effect appears greatly, and the eye pattern is distorted ([a-I] in FIG. 10 (a), FIG. 10). (B), see [b-I] in FIG. 11B), and the eye pattern (see [a-I] in FIG. 11A) is fine at Δf = + 3.125 GHz in FIG. You can see it is open.
[0064]
Thus, the optical filter 6 on the optical receiver side has a detuning direction and the bandwidth B_fil It is necessary to select an optimum detuning amount in consideration of the relationship between Δf and Δf.
[0065]
When the optical filter 6a used on the optical transmission side is a Gauss type, a Butterworth type, and a Fabry-Perot type, and on the reception side, the shape of the transmission characteristic of the optical filter 6b is a Gauss type, a third order Butterworth type, and an etalon type At half-width B of normalized optical filter_fil / B_sig Detuning amount for Δf / B_sig FIG. 12 shows the relationship between eye penalties.
[0066]
From FIG. 10, FIG. 11 and FIG. 12, the transmission center frequency f of the optical filter 6b on the optical receiver side._fil For the optical signal 104 to be received._sig Thus, it can be seen that the deterioration of the eye penalty is smaller when the frequency band on the sideband side that is mainly transmitted is matched.
[0067]
In addition to the optical filter 6b, when the optical receiver side further includes a narrow-band optical filter, the transmission characteristics of the entire optical transmission system including the narrow-band optical filter on the optical transmitter side are less narrow-band light. It becomes equal to the transmission characteristic of the filter. Therefore, the transmission center frequency f of the optical filter 6b on the optical receiving side._fil Is the carrier frequency f of the optical signal to be received._sig If the transmission characteristics including the optical filter 6a on the optical transmitter side are considered, the optical filter having a narrower transmission bandwidth is f._fil ~f_sig As a result, the pattern effect appears greatly. Further, when the optical filter 6b on the optical receiver side is detuned to the same size as the optical filter 6a on the optical transmitter side, it is equivalent to setting a narrower band optical filter with the same detuning amount. Therefore, as can be seen from the characteristics of FIG. 5, the deterioration of the eye penalty increases.
[0068]
Therefore, the detuning amount of the optical filter on the optical receiver side is set smaller than the detuning amount of the optical filter on the optical transmitter side. This is the same condition as when a narrower band optical filter is inserted with a smaller detuning amount, and there is a point where the eye penalty is minimized.
[0069]
In a specific example of the present invention, on the optical receiver side, the bandwidth of the optical filter is 0.3 to 3.0 B._sig In the case of Δf / B_sig The value does not vary so much, 0.15-0.40B_sig Detuning amount of f_sig On the other hand, it is known that a good eye penalty characteristic can be obtained by setting the frequency side of the sideband that is mainly transmitted.
[0070]
As described above, the optical communication apparatus according to the third specific example is an optical receiver that receives a residual sideband modulated optical signal by shifting the carrier frequency of the optical signal and the transmission center frequency of the optical filter by Δf on the optical transmitter side. The optical receiver is provided with an optical filter at the input end, and the transmission center frequency f of the optical filter_fil The carrier frequency f of the optical signal_sig On the other hand, when the optical signal is mainly an upper sideband signal,
0.0 <f_fil -F_sig <| Δf |
When the optical signal is mainly a lower sideband signal
− | Δf | <f_fil -F_sig <0.0
In this way, on the optical receiver side, the transmission center frequency f of the optical filter inserted at the input end of the optical receiver is set._fil And optical signal carrier frequency f_sig By shifting the value within a predetermined range, it becomes possible to receive a residual sideband-modulated optical signal without transmitting unnecessary bands, thereby significantly reducing waveform deterioration. In addition, long-distance and large-capacity optical communication can be realized stably.
[0071]
The above is an example for a single channel. However, when optical signals for a plurality of channels are multiplexed and transmitted, the following may be performed. This example will be described as a fourth specific example.
[0072]
(Fourth specific example)
The fourth specific example is an example in which optical signals for a plurality of channels are multiplexed and transmitted, and the optical transmitter side has an input port for each channel and is input to the input port. The optical signal to be transmitted has a different carrier frequency for each channel, and the optical combiner combines the plurality of optical signals having different wavelengths with the optical multiplexer. The carrier frequency f of the optical signal transmitted through the port at any one or more ports_sig And the transmission center frequency f of the port_fil By setting the offset, the optical signal transmitted through the port is configured to perform residual sideband modulation, and the optical signal transmitted through a port other than the port is combined to generate a wavelength multiplexed optical signal. The optical signal is provided for transmission, and the optical receiver side has an output port for each channel, and the transmitted residual-sideband modulated optical signal is demultiplexed for each channel to correspond. An optical demultiplexer that outputs to the output port of the channel, and any one or more ports of the optical demultiplexer includes a carrier frequency f of the optical signal transmitted through the port_sig And the transmission center frequency f of the port_fil This is an example in which a larger number of communication channels can be secured in the same system by demultiplexing the wavelength-multiplexed optical signal into a plurality of optical signals having different wavelengths, and the details will be described below. .
[0073]
FIG. 13 is a block diagram showing a fourth specific example. In the figure, reference numeral 11 denotes a wavelength division multiplexing optical transmission terminal station, which includes optical transmitters 12-1 to 12-N for N channels, an optical multiplexer 13, and an optical fiber amplifier 5a. Each of the N-channel optical transmitters 12-1 to 12-N includes a semiconductor laser light source, an external optical modulator, and an optical fiber amplifier. Each of the optical transmitters 12-1 to 12-N outputs CW light from the respective semiconductor laser light source, and intensity-modulates the CW light with a transmission signal by an external optical modulator to obtain an optical signal having an NRZ pulse waveform. These are optically amplified by an optical fiber amplifier and output.
[0074]
In this wavelength division multiplexing optical transmission terminal station 11, the optical signals from the optical transmitters 12-1 to 12-N for N channels are multiplexed by an optical multiplexer 13 for optical multiplexing, and this wavelength multiplexed optical signal is obtained. After the multiplexed optical signal is amplified via the optical fiber amplifier 5a, the multiplexed optical signal is sent to the optical fiber 7 serving as a transmission path to be transmitted to the receiving side.
[0075]
The wavelength division multiplexing optical receiving terminal station 14 includes optical fiber amplifiers 5b and optical demultiplexers 15 and optical receivers 16-1 to 16-N for each channel of 1 to N. Each of the channels 1 to N has a different carrier frequency for each channel. Here, the first channel has a carrier frequency f._sig1, The second channel has a carrier frequency f_sig2, To N-th channel are carrier frequency f_sigNIt is assigned and operated. Therefore, the optical multiplexer 13 has input ports for the first channel, the second channel, and the Nth channel, and the transmitters 12-1 to 12-N for each channel are input ports corresponding to the own channel. Connect to allow input. Further, the optical demultiplexer 15 has output ports for the first channel, the second channel, and the Nth channel, and outputs optical signals corresponding to each channel, which are demultiplexed for each channel, to the corresponding output ports. Therefore, the optical receivers 16-1 to 16-N for each channel are connected to the output port corresponding to the own channel.
[0076]
Accordingly, the wavelength division multiplexing optical receiving terminal station 14 receives the wavelength division multiplexed optical signal transmitted through the optical fiber 7 and amplifies it by the optical fiber amplifier 5b. In this configuration, each of the optical receivers 16-1 to 16-N corresponding to each channel can be demultiplexed. Each of the optical receivers 16-1 to 16-N amplifies the received optical signal, passes through an optical filter, detects and converts it into an electrical signal, extracts a necessary signal component through a low-pass filter, and outputs the channel. It has a function of receiving on the Internet.
[0077]
Next, the operation of the optical communication apparatus shown in FIG. 13 will be described. As the optical multiplexer / demultiplexer 13 of the wavelength division multiplexing terminal station 11 and the optical multiplexer / demultiplexer 15 of the wavelength division multiplexing reception terminal station 14, a Littrow type using an diffraction grating or an AWG (Arrayed Wavelength Grating) type is mainly used. These Littrow and AWG types have band-limiting characteristics with respect to wavelengths, unlike the characteristics of couplers and the like.
[0078]
Therefore, in this system, the optical signal transmitted from each of the optical transmitters 12-1 to 12-N for each channel is the carrier frequency f._sig1~ F_sigNIs the transmission center frequency f of the port for each channel of the optical multiplexer 13._sig1~ F_sigNAre set in advance so as to be detuned and input to the optical multiplexer 13, respectively. Then, the optical multiplexer 13 optically multiplexes these optical signals for each channel into wavelength multiplexed light and outputs it. The optical signal of each channel is transmitted by its corresponding carrier frequency f._sig1~ F_sigNOn the other hand, since each of them is detuned and input to the optical multiplexer 13, the output optical signal of the optical multiplexer 13 is a wavelength in which the optical signals of the respective channels are collectively subjected to residual sideband modulation. It becomes a multiplexed optical signal.
[0079]
Since the band of the vestigial sideband optical signal is narrower than the band of the normal double sideband optical signal, it is possible to multiplex at a higher density in wavelength multiplexing. Therefore, the communication system can secure more communication channels.
[0080]
FIG. 14 (a) shows the frequency spectrum distribution during wavelength multiplexing of a vestigial sideband modulated optical signal. For comparison, FIG. 14B shows a frequency spectrum distribution of a wavelength-division multiplexed optical signal by a conventional double sideband modulation which is a conventional example.
[0081]
The conventional optical signal has a carrier frequency f_sig The frequency spectrum with the signal component symmetrically distributed around the center is distributed, but the optical signal of the present invention gently cuts off the frequency spectrum on one side and leaves a part, so that the signal component is not lost Has a frequency spectrum concentrated almost on one side. Therefore, the spread of the entire frequency spectrum distribution of the optical signal according to the present invention is narrower than that of the conventional frequency spectrum distribution. In the wavelength division multiplexing optical transmission system to which the optical signal of the present invention is applied, the channels are multiplexed at a higher density. Therefore, a larger number of channels can be multiplexed within the same frequency band.
[0082]
Of course, also in the wavelength division multiplexing optical receiving terminal station 14, the carrier frequency f of each channel of the wavelength division multiplexed optical signal that has been transmitted._sig1~ F_sigNAnd the transmission center frequency f of the optical demultiplexer 16_sig1~ F_sigNBy detuning, it is possible to split a residual sideband optical signal wavelength-multiplexed in a lump by transmitting only a necessary band.
[0083]
In addition, the detuning amount of the optical multiplexer / demultiplexer 13 in the optical transmitting terminal station 11 and the optical multiplexer / demultiplexer 15 in the optical receiving terminal station 14 is the same as that of the transmission characteristics of the optical multiplexer / demultiplexers 13 and 15, Gauss type or third order Butterworth type. Therefore, in the wavelength division multiplexing optical transmission system in the fourth specific example, the same setting as in the first, second and third specific examples of the present invention may be used.
[0084]
In the wavelength division multiplexing optical transmission system according to the fourth embodiment of the present invention, it is not necessary to insert an optical filter for each channel, and only the optical multiplexer / demultiplexer necessary for wavelength multiplexing is used to perform residual operation. Sideband modulation, multiplexing, and demultiplexing can be performed, enabling higher-density wavelength multiplexing.
[0085]
As described above, the fourth specific example is an optical signal multiplexing of the optical communication device in the first to third specific examples, and the optical filter inserted on the optical transmitter side is an optical multiplexer, In addition, the optical filter inserted on the optical receiver side is an optical demultiplexer. Further, on the optical transmitter side, an optical multiplexer is provided, a plurality of optical signals having different wavelengths are inserted into the optical multiplexer, and light transmitted through the port at any one or more ports of the optical multiplexer Signal carrier frequency f_sig And the transmission center frequency f of the port_fil The optical signal transmitted through the port is modulated by vestigial sideband modulation and combined with the optical signal transmitted through the port other than the port to generate and transmit a wavelength multiplexed optical signal. It is characterized by.
[0086]
Further, an optical demultiplexer is provided on the optical receiver side, and a wavelength division multiplexed optical signal including an optical signal subjected to residual sideband modulation is inserted into the optical demultiplexer, and any one of the optical demultiplexers is inserted. In the above ports, the carrier frequency f of the optical signal transmitted through the port_sig And the transmission center frequency f of the port_fil The wavelength-multiplexed optical signal is demultiplexed into a plurality of optical signals having different wavelengths.
[0087]
In this optical communication device, on the optical transmitter side, by utilizing the band limiting characteristics of the optical multiplexer, the optical signals of a plurality of channels are batched, and the residual sideband modulation is performed to multiplex the optical signals. On the optical receiver side, a plurality of optical signals by wavelength side multiplexed residual sideband modulation are transmitted only in the necessary band on the optical receiver side. Branching at once.
[0088]
In this optical communication device, the optical signal of each channel is suppressed from spreading of the optical signal band by vestigial sideband modulation, so that the interval between adjacent channels can be narrowed. Therefore, according to the present system, it is possible to perform wavelength multiplexing at a higher density with the same device scale as the conventional wavelength division multiplexing optical transmission system, and it is possible to secure more channels.
[0089]
While various specific examples have been described above, the present invention is not limited to this specific example and can be implemented with various modifications.
[0090]
【The invention's effect】
As described above, according to the present invention, even when a narrow-band optical filter is used, a residual sideband modulated optical signal that suppresses the spread of the optical signal band on the optical transmitter side can be generated. It is possible to provide an optical communication device that can receive an optical signal modulated by vestigial sideband modulation on the optical receiver side while blocking unnecessary bands, and is expected to be used for long-distance and large-capacity transmission. Optical transmission can be realized.
[0091]
Furthermore, according to the present invention, even when a narrow-band optical filter is used, a residual sideband modulated optical signal that suppresses the spread of the optical signal band on the optical transmitter side can be generated. It is possible to provide an optical communication apparatus that can receive an optical signal subjected to residual sideband modulation on the receiver side through only a necessary band. Furthermore, when the present invention is applied to a wavelength division multiplexing transmission system, residual sideband modulation is performed on a plurality of channels at once in an optical multiplexer and an optical demultiplexer without adding a new component. The band-modulated optical signals can be separated and transmitted through only a necessary band at a time, and high-density wavelength multiplexing optical transmission expected as long-distance large-capacity transmission can be realized.
[Brief description of the drawings]
FIG. 1 is a block diagram for explaining the present invention and showing a schematic configuration of a first specific example of the present invention;
FIG. 2 is a diagram showing an eye pattern and a frequency spectrum of an optical signal.
FIG. 3 is a diagram for explaining the present invention and showing a relationship between an eye pattern of a transmission optical signal and a frequency spectrum with respect to a detuning amount according to a first specific example of the present invention;
FIG. 4 is a diagram for explaining the present invention, and showing a relationship between an eye pattern of a transmission optical signal and a frequency spectrum with respect to a detuning amount according to a first specific example of the present invention;
FIG. 5 is a diagram for explaining the present invention, and showing the relationship between the detuning amount and the eye penalty for each transmission characteristic of the optical filter in the first specific example of the present invention.
FIG. 6 is a diagram for explaining the present invention and showing a relationship among an optical filter, a bandwidth, and a detuning amount setting range in the first specific example of the present invention.
FIG. 7 is a diagram for explaining the present invention and showing a frequency spectrum when a frequency chirp exists.
FIG. 8 is a diagram for explaining the present invention, and shows a relationship between a detuning amount and an eye penalty in the presence of a frequency chirp with respect to each transmission characteristic of an optical filter.
FIG. 9 is a block diagram for explaining the present invention and showing a schematic configuration of a third specific example of the present invention;
FIG. 10 is a diagram for explaining the present invention and showing an eye pattern and a frequency spectrum of a received optical signal with respect to a detuning amount in a third specific example of the present invention.
FIG. 11 is a diagram for explaining the present invention and showing an eye pattern and a frequency spectrum of a received optical signal with respect to a detuning amount in a third specific example of the present invention.
FIG. 12 is a diagram for explaining the present invention, and showing the relationship between the detuning amount and the eye penalty for each transmission characteristic of the optical filter in the third specific example of the present invention.
FIG. 13 is a block diagram for explaining the present invention and showing a schematic configuration of a fourth specific example of the present invention;
FIG. 14 is a diagram for explaining the present invention, wherein the frequency spectrum distribution of the wavelength multiplexed optical signal in the fourth specific example of the present invention and the frequency spectrum distribution of the wavelength multiplexed optical signal in the conventional example for comparison; The figure which showed.
[Explanation of symbols]
1 ... Optical transmitter
2 ... Semiconductor laser light source
3 ... External light modulator
4 ... Transmission signal
5a, 5b ... Optical fiber amplifier
6a, 6b ... Optical filter
7. Optical fiber
8 ... Optical receiver
9 ... Photo detector
10 ... Low-pass filter
11 ... wavelength division multiplexing optical transmission terminal
12-1 to N: WDM optical transmitter
13 ... Optical multiplexer
14: Wavelength multiplexed optical receiving terminal
15 ... Optical demultiplexer
16-1 to N: Optical receiver for WDM
101 ... CW light
102 ... NRZ pulse optical signal
103: Transmitted optical signal
104: Received optical signal
105 ... Received signal

Claims (1)

In the optical communication device using intensity modulation, the optical transmitter side is a laser light source, an external optical modulator that outputs an optical signal by intensity-modulating light output from the laser light source with a transmission signal of transmission speed B _sig , and with FWHM B _fil for vestigial sideband modulation of the optical signal comprises an optical filter in the range of 0.3 to 3.0 B _sig, the optical signal is increased its frequency with time, pulse When high-frequency components are collected at the rear end , 0.5 B _fil between the full width at half maximum B _fil , the transmission center frequency f _fil of the optical filter, and the carrier frequency f _sig of the optical signal < ｜ f _sig − f _fil Within the range in which the relationship | is satisfied, the transmission center frequency f _fil of the optical filter is set to a frequency band higher than the carrier frequency f _sig of the optical signal to mainly transmit the upper side band, and the light When the frequency of the signal decreases with time and low frequency components gather at the rear end of the pulse , the transmission center frequency f _fil of the optical filter is changed to the carrier frequency f _{sig of the} optical signal within the range where the above relationship holds. An optical communication device characterized in that a lower frequency band is set mainly to transmit the lower sideband.

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