JP3297325B2 - Optical communication device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical transmission system, and more particularly to an optical communication device suitable for high-density multiplexing.

[0002]

2. Description of the Related Art In recent years, in optical transmission, information is digitized and light-modulated on a transmission side and transmitted through an optical fiber as a transmission path, and an optical signal transmitted through the optical fiber is transmitted on a reception side. This is a communication system capable of converting information into an electric signal and returning the signal to a digital signal to restore information. A wide band can be secured, high-speed transmission is possible, and large-capacity information can be transmitted with high quality.

[0003] With the progress of optical fiber amplifiers,
The research and development of optical transmission systems has been increasingly active,
It is attracting attention as a large-capacity transmission system that will realize the future multimedia information age.

[0004] By the way, NRZ (Non-
A Return-to-Zero pulse is often used, but the intensity modulation (IM-
DD: Intensity-Modulation D
In order to realize large-capacity transmission in an optical transmission system using the direct-detection method, a wavelength multiplexing optical transmission system that multiplexes optical signals of a plurality of channels in the optical domain without increasing the transmission capacity per channel is effective. It is. The band of the optical fiber which is the optical region is several thousand G
Hz, and a demand for a future large capacity can be dealt with by multiplexing a considerable number of channels.

[0005] In a wavelength division multiplexing optical transmission system, a band that is several times the band of an optical signal to be transmitted must be opened between adjacent channels in order to suppress interference due to crosstalk. As described above, the bandwidth of an optical fiber is very wide, but when performing long-distance optical transmission using an optical fiber amplifier as a repeater, the gain band of the optical fiber amplifier becomes narrower due to repeated relaying. Possible optical fiber bandwidth is reduced.

In such a case, the number of channels that can be multiplexed is greatly restricted against the wide band of the optical fiber. Therefore, the frequency spectrum distribution of the optical signal in the normal double sideband shown in FIG.
As can be seen from the frequency spectrum distribution of the optical signal using the residual sideband modulation that blocks a part of the sideband shown in (a), the residual sideband capable of suppressing the spread of the optical signal band per channel. The modulation method is suitable for dense wavelength multiplexing.

In the field of wireless communication, a filter for vestigial sideband modulation in wireless communication is disclosed in the document "Comm.
communication Systems (second
edition) ", by Simon Haykin, J
According to “Own Wiley & Sons Publishing”, the transfer function H (f) satisfies the following condition: 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. That is, the condition of the expression (1) is that, when a filter having complementary symmetry and a full width at half maximum of B _fil is used, the transmission center frequency f _fil of the filter is set to | f _sig −f _fil | = 0.5B _fil . )
Is set so that the relationship of holds.

On the other hand, in the field of optical communication, consider an optical filter for performing vestigial sideband modulation. In order to suppress crosstalk from an adjacent channel during wavelength multiplexing, this optical filter is quenched. It is necessary to use a type that can take a large ratio. As a type capable of obtaining a large extinction ratio, there are a dielectric multilayer film, a diffraction grating, and an optical filter such as a fiber-perot etalon.

However, it is difficult to satisfy the condition of complementary symmetry suitable for vestigial sideband modulation, and an optical filter with good complementary symmetry cannot be obtained, so that waveform distortion occurs. Have a problem.

[0010]

Considering an optical filter for performing vestigial sideband modulation in optical communication,
In order to suppress crosstalk from adjacent channels during wavelength multiplexing, it is necessary to use a type of optical filter that can increase the extinction ratio.

However, as described above, it is difficult to realize an optical filter having complementary symmetry required by vestigial sideband modulation. Also, in order to insert a narrower band optical filter in order to realize higher density wavelength multiplexing,
Under the condition satisfying the expression (2), the waveform distortion increases. And, since the waveform distortion means waveform deterioration, it becomes a big barrier in securing communication reliability, and is a big problem in stably realizing long-distance, large-capacity optical communication. Therefore,
There is a demand for the development of a technology that solves these problems.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical communication apparatus which significantly reduces waveform deterioration when performing vestigial sideband modulation using a narrow-band optical filter. Is to provide an optical communication device capable of stably realizing the above.

[0013]

In order to achieve the above object, the present invention is configured as follows. That is, the present invention firstly provides an optical communication device using intensity modulation,
The optical transmitter side includes an optical filter having a full width at half maximum B _fil , and sets the transmission center frequency f _fil of the optical filter and the carrier frequency f _sig of the optical signal to 0.5 B _fil <| f _sig −f _fil.
Is set within a range in which the relationship | is satisfied, so that one sideband of the optical signal is cut off, and a residual sideband modulation method is used in which a part of the sideband to be removed is left.

[0014]

[0015]

[0016] is further characterized in that the optical filter to be inserted into the optical transmitter side or the optical receiver side is the optical multiplexer.

[0017]

[0018]

[0019]

[0020]

[0021]

In the optical communication apparatus according to the fourth, fifth and sixth aspects of the present invention, the optical transmitter side collectively collects the optical signals of a plurality of channels by utilizing the band limiting characteristic of the optical multiplexer. It is possible to generate a wavelength multiplexed optical signal by modulating and multiplexing the residual sideband, and on the optical receiver side, utilizing the band limiting characteristics of the optical demultiplexer, the wavelength multiplexed residual sideband. A plurality of optical signals by band modulation are transmitted through only a necessary band and can be branched at once. In this optical communication device, since the spread of the optical signal band of the optical signal of each channel is suppressed by the vestigial sideband modulation, the interval between adjacent channels can be narrowed. It becomes possible to perform wavelength multiplexing at a higher density on a large scale.

Therefore, when performing vestigial sideband modulation using a narrow-band optical filter, it is possible to provide an optical communication apparatus which can perform high-quality communication with little waveform deterioration and can provide long-distance, large-capacity optical communication. It is possible to provide an optical communication device that can stably realize communication, can perform wavelength multiplexing at high density, and can secure many channels.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. The characteristics shown in the present invention were obtained by numerical simulation. (First Specific Example) In a first specific example, in an optical communication apparatus using intensity modulation, an optical filter having a full width at half maximum B _fil is provided on an optical transmitter side, and a transmission center frequency f of the optical filter is provided.
_fil and the carrier frequency f _sig of the optical signal are 0.5B _fil <
This is a residual sideband modulation system in which one sideband of the optical signal is cut off and a part of the sideband to be removed is left by setting within the range where the relationship | f _sig -f _fil | is satisfied. This will be described in detail below.

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.

Of these, the semiconductor laser light source 2 has a C
The external light modulator 3 outputs a W (Continuous-Wave) light 101, and the external optical modulator 3 intensity-modulates the CW light 101 with the transmission signal 4 to output an optical signal 102 having an NRZ pulse waveform. The optical fiber amplifier 5 amplifies and outputs the optical signal 102, and the optical filter 6a suppresses the expansion of the optical signal band and outputs the optical signal amplified by the optical fiber amplifier 5, The optical fiber 7 is a transmission line of the optical communication system and transmits an output from the optical filter 6a.

The optical transmitter 1 comprises a semiconductor laser light source 2, an external optical modulator 3, an optical fiber amplifier 5, and an optical filter 6a. The operation of the optical transmitter 1 having such a configuration will be described. CW light 101 from semiconductor laser light source 2
The CW light 101 is intensity-modulated by the external optical modulator 3 by the transmission signal 4 to become an optical signal 102 having an NRZ pulse waveform. The transmission speed of the transmission signal 4 is represented by B _sig .

The optical signal 102 having the NRZ pulse waveform is amplified by the optical fiber amplifier 5, the optical signal band is suppressed from being spread by the optical filter 6a, and is input to the optical fiber 7 and transmitted.

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 is considered to be a Gaussian type, a third-order Butterworth type, and an etalon type.

Therefore, the optical filter 6a will be described in detail below. First, the optical signal 102 has an NRZ pulse waveform without frequency chirp and has a transmission speed of 10 Gb.
/ S (gigabits / second), the optical signal 102
2A has an eye pattern as shown in FIG. 2A and a frequency spectrum as shown in FIG.

The eye pattern shown in the present invention is obtained by an ideal square detection by a photodetector as a photodetector,
_This is an eye pattern when passing through a low-pass filter having a cutoff frequency of _sig × 0.75.

As can be seen from the drawing, 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 that case, a band required for transmission requires about 2B _sig .

In the present invention, in order to reduce the band required for transmission, one of the sidebands has a narrow band.
The residual sideband modulation is performed by gently blocking by a. For example, the transmission bandwidth of the optical filter 6a is 9.375 GHz (0.075 nm), and the shape of the transmission characteristics is Gaussian.

The eye pattern and the frequency spectrum of the outgoing optical signal 103 with respect to the detuning amount Δf = f _fil −f _sig of the carrier frequency f _sig of the optical signal 102 incident on the optical filter 6a and the transmission center frequency f _fil of the optical filter 6a. Are shown in FIG. 3 and FIG.

FIG. 3A shows that the detuning amount Δf is equal to 0.
This is an example of 0 GHz, where the full width at half maximum B _fil of the optical filter 6a to be inserted is not sufficiently wide band with respect to the transmission speed B _{sig of the} optical signal, and [a−
I] indicates an eye pattern, and [a-II] in FIG. 3A indicates a frequency spectrum. As shown in FIG. 3A, when the full width at half maximum B _fil of the optical filter 6a to be inserted is not sufficiently wide with respect to the transmission speed B _{sig of the} optical signal,
In particular, in the case of B _fil ≦ 3B _sig , if f _sig = f _fil is set, the high-frequency signal components in both sidebands are greatly attenuated, so that a pattern effect appears and waveform distortion occurs.

FIG. 3B shows that the detuning amount Δf is 3.
It is an example of 125 GHz, and [b-
I] indicates an eye pattern, and [b-II] in FIG. 3B indicates a frequency spectrum.

FIG. 4A shows the detuning amount Δf.
Is an example of 4.0 GHz, and [a−
I] indicates an eye pattern, and [a-II] in FIG. 4A indicates a frequency spectrum.

FIG. 4B shows that the detuning amount Δf is 5.
This is an example of 0 GHz, and [b-I] in FIG. 4B shows an eye pattern, and [b-II] in FIG. 4B shows a frequency spectrum.

The detuning amount Δf = 3.125 GHz in which the condition of the above-mentioned equation (2) shown in FIG. 3B is satisfied.
(0.025 nm) and the detuning amount Δf = 4.0 GHz (0.032 nm) shown in FIG.
Then, the eye pattern and the frequency chirp are compared.

In FIG. 3B, since the full width at half maximum B _fil of the inserted optical filter is not sufficiently large as compared with the optical signal band B _sig , the high frequency signal component is attenuated, the pattern effect remains, and a large waveform distortion occurs. ing. FIG. 4 (c)
Has a pattern effect, but the balance between the high-frequency signal component and the low-frequency signal component is maintained, so that the eye pattern is wider than in FIGS. 3 (a), (b) and 4 (b). I understand.

However, as shown in FIG. 4B, when the f _fil is detuned too large, the low frequency signal component is attenuated conversely, causing a large waveform deterioration. Therefore, it is necessary to select the optimum Δf. The transmission rate B _sig of the optical signal and the full width at half maximum B _fil of the optical filter 6a are greatly involved in the optimum Δf.

FIG. 5 shows that the shape of the transmission characteristic of the optical filter 6a is Gauss-type (gauss), and the third-order Butterworth type (3rd butterw
orth) and the etalon type (fabry-perot), the relationship between the detuning amount Δf / B _sig and the eye penalty (loss) for the transmission bandwidth B _fil / B _sig normalized by the transmission speed of the optical signal. Is shown. The eye penalty in the present invention is the ratio between the eye opening width obtained over an identification time period of 20% width of a 1-bit time slot and the average "1", "0" level difference in the identification time period in decibels. [D
B]. Since no noise is considered at the receiver end or the like, this eye penalty represents the received light power penalty.

FIG. 5 shows that the optimum detuning amount Δf for the transmission bandwidth B _fil / B _sig in each optical filter.
Although the magnitude of / B _sig is almost equal, the magnitude and the rate of change of the eye penalty are different. Gaussian type and etalon type whose cutoff characteristics change slowly with frequency include a relatively wide band until the extinction ratio becomes sufficiently large, so that the frequency spectrum of the optical signal may be cut off greatly. Instead, the eye penalty changes slowly.

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 band that is not so wide, so that the eye penalty is sharply increased even if it is slightly deviated from the optimum Δf. to degrade. FIG. 6 shows the optimum detuning amount Δf / B _sig when the normalized full width at half maximum B _fil / B _sig is changed in the Gaussian type and the Butterworth type.

From FIG. 6, when B _fil / B _sig ≦ 2.0, the optimum detuning amount Δf / B _sig of the Gaussian type and the Butterworth type is almost equal, but B _fil / B _sig ≧
At 2.0, a difference appears in the optimum Δf / B _sig . This is thought to be due to the difference in the slope of the extinction ratio near the cutoff frequency in the transmission characteristics of the optical filter.

From the above-mentioned conditional expression (2), the optimum detuning amount Δf in the vestigial sideband modulation in wireless communication is:
Δf = 0.5 B _sig . However, when a narrow band optical filter is used, it is considered effective to set the detuning amount Δf to Δf> 0.5B _sig from FIGS. 5 and 6.

Further, in the Gaussian type and third-order Butterworth type optical filters in this embodiment, the relationship between the full width at half maximum B _fil / B _{sig of} the optical filter normalized from FIG. 6 and the optimum detuning amount Δf / B _sig is as follows. Approximately, Δf / B _sig = 0.575 B _fil / B _sig −0.025 (3) It is known that an optimum result is obtained when this relationship is maintained. Further, when the optical signal includes a frequency chirp, the optimum Δf varies according to the magnitude of the frequency chirp from the above approximate relationship.

As described above, in this specific example, the optical communication apparatus is an optical communication apparatus using intensity modulation, in which an optical filter having a full width at half maximum B _fil is provided on the optical transmitter side, and the transmission center frequency f By setting _fil and the carrier frequency f _sig of the optical signal within a range in which the relationship of 0.5B _fil <| f _sig −f _fil | is satisfied, one sideband of the optical signal is cut off and removed. It is one which comprises using a vestigial sideband modulation method to leave a portion of the sidebands, predetermined in the optical transmitter side, of the optical signal transmission center frequency f _fil carrier frequency f _sig and optical filter The sideband of one sideband of the optical signal is cut off, and the residual sideband modulation is transmitted through a part of the sideband, so that the optical signal waveform is not distorted. Optical NR with suppressed signal band expansion
This makes it possible to generate a Z pulse.

However, the vestigial sideband modulation according to the present invention is as follows.
To suppress the spread of the optical signal band by a narrow band optical filter, if the full width at half maximum B _fil is too narrow, the signal band is largely cut off, causing a large eye penalty. The transmission of the secondary high-frequency signal component does not suppress the expansion of the optical signal band. Therefore, the full width at half maximum B _fil of the optical filter 6a is 0.3 to 3.0.
The range of B _sig is more effective.

In the above, one sideband of the optical signal is cut off,
An example has been described in which by performing residual sideband modulation that transmits some sidebands, it is possible to generate an optical NRZ pulse in which the expansion of the optical signal band is suppressed without distorting the optical signal waveform. .

It is possible to perform vestigial sideband modulation even for an optical signal having a frequency chirp and a frequency spectrum bias, and to generate an optical NRZ pulse in which the expansion of the optical signal band is suppressed. An example will now be described as a second specific example.

(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 the optical transmitter 1, the semiconductor laser light source 2, and the external optical modulator 3 , An optical fiber amplifier 5 and an optical filter 6a. The only difference from the first specific example is that the external optical modulator 3 adds a frequency chirp to the modulated NRZ pulse. I have.

Here, the frequency chirp is represented by a parameter C. Parameter C is a reference, "Residual Ch
irp in Integrated-Optic M
odulators "by Anders at IE
EE Photo. Tech. Lett. , Vo
l. 4, no. 1, 1992, was quoted.

When the frequency chirp C is positive, it indicates that the frequency decreases with time and low-frequency components gather at the trailing end of the pulse. When the frequency chirp C is negative, the frequency increases with time and the pulse increases. Refers to the case where high-frequency components gather at the rear end.

FIG. 7 is a diagram for explaining a difference in frequency spectrum when a frequency chirp exists, and FIG. 7A shows an example of a spectrum when there is no frequency chirp C, and FIG. (c) is an example of a spectrum when the frequency chirp C is positive, FIG. 7 (b) frequency chirp C
Shows an example of a spectrum when is negative. At this time,
The transmission center frequency f _fil of the optical filter 6 a for performing the vestigial sideband modulation is determined with respect to the carrier frequency f _sig of the optical signal.
A difference in eye penalty occurs depending on whether the frequency is set to a high band or a low band.

FIG. 8 shows that the transmission rate B _sig = 10 Gb /
s, the full width at half maximum of the optical filter is 9.375 GHz (0.0
75 nm) as a third-order Butterworth type detuning amount Δf / B when the frequency chirp is positive and negative
_Shows the relationship between _sig and eye penalty.

FIG. 8 shows that when the frequency chirp added to the optical signal is positive, the transmission center frequency f _{fil of the} optical filter is obtained.
It can be seen that the eye penalty is smaller when is set to a frequency band lower than the carrier frequency f _sig of the optical signal. Conversely, when the frequency chirp added to the optical signal is negative, it can be seen that setting the transmission center frequency f _fil of the optical filter to a frequency band higher than the carrier frequency f _sig of the optical signal has a smaller eye penalty.

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 directivity of the detuning of the transmission center frequency f _fil of the optical filter 6a. Is considered to greatly affect the transmission characteristics.

As described above, the optical communication device according to this embodiment is
In the case where an optical filter is provided on the optical transmitter side, and the optical signal is subjected to vestigial sideband modulation by the optical filter,
When the frequency of the optical signal increases with time and high-frequency components gather at the rear end of the pulse, 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. Through the upper waveband,
Further, when the frequency of the optical signal decreases with time and low-frequency components gather at the trailing end of the pulse, the transmission center frequency f _fil of the optical filter is set to a lower frequency band than the carrier frequency f _sig of the optical signal. It is characterized by being set so as to transmit mainly the lower sideband.

That is, in this optical communication device, the transmission center frequency f _fil of the optical filter is changed to the carrier frequency f of the optical signal.
_In contrast, when the frequency of the optical signal increases with time in the optical signal and high-frequency components are collected at the rear end of the pulse, the transmission center frequency f _fil of the optical filter is changed to the carrier frequency f of the optical signal.
_{When the} frequency band is set lower than _{sig, the} eye penalty is smaller, so the frequency decreases with time in the upper waveband side, and also with time, and when low-frequency components gather at the pulse rear end, the transmission center frequency of the optical filter Since the eye penalty is smaller when f _fil is set to a frequency band higher than the carrier frequency f _sig of the optical signal, the f _fil is set to the lower side band side. Therefore, even for an optical signal having a frequency chirp and having a bias in the frequency spectrum, it is possible to efficiently apply the residual sideband modulation by suppressing the eye penalty and to generate an optical NRZ pulse in which the spread of the optical signal band is suppressed. Is what can happen.

The example of the optical transmitter has been described above. Next, an example of an optical receiver that can receive an optical signal subjected to vestigial sideband modulation without transmitting an unnecessary band will be described.

(Third Specific Example) A third specific example relates to the transmission center frequency f of the optical filter inserted at the input end of the optical receiver.
By setting the _fil and the carrier frequency f _sig of the optical signal so as to be shifted within a predetermined range, it is possible to receive the optical signal modulated in the vestigial sideband without transmitting an unnecessary band. Yes, the details will be described below.

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.

Of these, the optical fiber amplifier 5b
This is for amplifying and outputting the received optical signal 104 transmitted through the optical fiber 7, and the optical filter 6b
This is for limiting the band of the amplified and output received optical signal. The photodetector 9 detects an optical signal transmitted through the optical filter 6b and converts it into an electric signal. The low-pass filter 10 is a filter that passes a low-frequency component of the electric signal and outputs the signal. .

In the optical receiver 8 having such a configuration, the received optical signal 104 transmitted through the optical fiber 7 from the transmitting side and transmitted by the vestigial sideband modulation is transmitted to the optical fiber amplifier 5.
After being amplified by b, the band is limited by the optical filter 6b, then detected by the photodetector 9, and converted into an electric signal. The converted electric signal is supplied to a low-pass filter 10.
The low-frequency component obtained by passing the
It becomes 5.

In general, an optical receiver is provided with an optical filter, and the transmission center frequency f _fil of the optical filter 6b is generally adjusted to the carrier frequency f _sig of the received optical signal 104. However, the frequency spectrum of the optical signal 103 on the optical transmitter 8 side according to the present invention is left-right asymmetric with respect to the carrier frequency f _sig of the optical signal because of the residual sideband modulation. Therefore, the frequency spectrum of the received optical signal 104 on the optical receiver side is also asymmetrical with respect to the carrier frequency f _sig .

For this reason, when the optical filter 6b is not wide enough, if the carrier frequency f _sig of the optical signal 104 and the transmission center frequency f _fil of the optical filter 6b are matched as in the prior art, unnecessary bands are transmitted. not only,
It is conceivable to cut off the necessary band. Therefore, in the present invention, the optical filter 6b inserted on the optical receiver side
The transmission center frequency f _fil of the optical signal
_It is necessary to detune from _sig .

At a transmission rate of 10 Gb / s, a Gaussian optical filter having a bandwidth of 12.5 GHz (0.1 nm) is detuned to Δf = + 7.25 GHz on the optical transmitter side, and the residual sideband modulated optical signal is obtained. Are received after passing through a Gaussian optical filter having a bandwidth of 12.5 GHz on the optical receiver side, and FIG. 10A and FIG. 11A and FIG. 11A and FIG. The relationship between the eye pattern and the frequency spectrum with respect to the amount of detuning Δf under the condition (1) is shown. Both [a-I] and [b-I]
Represents the eye pattern, and [a-II] and [b-I
I] shows a frequency spectrum.

FIG. 10A shows the detuning amount Δf.
Is an example of “3.125 GHz”, and FIG.
FIG. 11A shows an example in which the detuning amount Δf is “0.0 GHz”, and FIG. 11A shows that the detuning amount Δf is “3.1 GHz”.
FIG. 11B shows an example in which the detuning amount Δf is “6.125 GHz”.

FIGS. 10A and 10B and FIG. 11B
In FIG. 10, a large pattern effect appears and the eye pattern is distorted ([a-I] in FIG.
0 (b), [b-I] of FIG. 11 (b)), FIG.
It can be seen that the eye pattern (see [a-I] in FIG. 11A) is clearly opened at Δf = + 3.125 GHz in 1 (a).

As described above, the optical filter 6 on the optical receiver side has the directionality of detuning, and the bandwidth B
_It is necessary to select an optimal amount of detuning in consideration of the relationship between _fil and Δf.

When the optical filter 6a used on the optical transmitting side is a Gaussian type, a Butterworth type, or a Fabry-Perot type, on the receiving side, the transmission characteristics of the optical filter 6b are Gaussian type, tertiary Butterworth type, and etalon type. , The full width at half maximum B _fil of the normalized optical filter
/ Dechu for B _sig - and training amount Delta] f / B _sig, shown in FIG. 12 the relationship eye penalty.

From FIGS. 10, 11 and 12, the transmission center frequency f _fil of the optical filter 6b on the optical receiver side is mainly transmitted from the carrier frequency f _sig to the optical signal 104 to be received. It can be seen that the eye penalty is less degraded when the frequency band is adjusted to the frequency band on the sideband side.

When a narrow band optical filter is further provided on the optical receiver side in addition to the optical filter 6b, the transmission characteristic of the entire optical transmission system including the narrow band optical filter on the optical transmitter side is narrower. It becomes equal to the transmission characteristic of the optical filter having a wide band.
Therefore, the transmission center frequency f of the optical filter 6b on the optical receiving side is obtained.
_{When fil is set} to a frequency band on the sideband side which is mainly cut off from the carrier frequency _{fsig of the} optical signal to be received, transmission characteristics including the optical filter 6a on the optical transmitter side are considered. In this case, an optical filter having a narrower transmission bandwidth is set by f _fil to f _sig , and a large pattern effect appears. 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, an optical filter having a narrower band can be obtained.
Since this is equivalent to setting with the same detuning amount, the deterioration of the eye penalty increases as can be seen from the characteristics in FIG.

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 equivalent to the case where a narrower band optical filter is inserted with a smaller detuning amount, and therefore, there is a point where the eye penalty is minimized.

[0076] In embodiments of the present invention, in an optical receiver side, when the band of the optical filter is 0.3～3.0B _sig, the value of Delta] f / B _sig is not fluctuated so, 0.15 Detune amount of 0.40B _sig to f
_It has been found that a good eye penalty characteristic can be obtained by setting the frequency side of the sideband mainly transmitting _sig .

As described above, the optical communication apparatus according to the third embodiment receives the optical signal modulated in the residual sideband 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. On the optical receiver side, an optical filter is provided at the optical receiver input end, and the transmission center frequency f _fil of the optical filter is set with respect to the carrier frequency f _sig of the optical signal.
When the optical signal is mainly an upper sideband signal, 0.0 <f
_When the optical signal is mainly a lower sideband signal in the range of _fil− f _sig <| Δf |, − | Δf | <f _fil −f _sig <
In this manner, the transmission center frequency f _fil of the optical filter inserted into the optical receiver input terminal and the carrier frequency f _sig of the optical signal are set on the optical receiver side. By shifting the optical signal within a predetermined range, it is possible to receive the optical signal modulated in the vestigial sideband without transmitting unnecessary bands, thereby significantly reducing waveform deterioration. At the same time, long distance and large capacity optical communication can be stably realized.

The above is an example for a single channel. However, when multiplexing and transmitting optical signals for a plurality of channels, the following method may be used. This example will be described as a fourth specific example.

(Fourth Specific Example) The fourth specific example is an example in which optical signals for a plurality of channels are multiplexed and transmitted. The optical transmitter has an input port for each channel. And an optical multiplexer for multiplexing the optical signal input to the input port. The optical signal to be transmitted has a carrier frequency set to a different wavelength for each channel, and a plurality of optical signals having different wavelengths are optically multiplexed. In synthesizing in the optical multiplexer, at any one or more ports of the optical multiplexer, the carrier frequency f _sig of the optical signal transmitted through the port and the transmission center frequency f _fil of the port are shifted and set. An optical signal transmitted through the port is configured to be modulated in a vestigial sideband, and an optical signal transmitted through a port other than the port is multiplexed to generate a wavelength-division multiplexed optical signal to be an optical signal to be transmitted. And also The receiver side has an output port for each channel, and has an optical demultiplexer that separates the transmitted vestigial sideband modulated optical signal for each channel and outputs it to the output port of the corresponding channel. ,and,
Any one or more ports of the optical demultiplexer are set by shifting the carrier frequency f _sig of the optical signal passing through the port and the transmission center frequency f _fil of the port, so that the wavelength division multiplexed optical signal has a different wavelength. This is an example in which a greater number of communication channels can be secured in the same system by splitting the signal into a plurality of optical signals. The details will be described below.

FIG. 13 is a block diagram showing a fourth specific example. In the figure, reference numeral 11 denotes a wavelength division multiplexing optical transmitting terminal, which comprises 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 its own semiconductor laser light source, and intensity-modulates the CW light with a transmission signal using an external optical modulator to obtain an NRZ pulse waveform optical signal. This is optically amplified by an optical fiber amplifier and output.

In this wavelength division multiplexing optical transmission terminal station 11,
The optical signals from the N-channel optical transmitters 12-1 to 12-N are multiplexed by an optical multiplexer 13 for optical multiplexing into a wavelength division multiplexed optical signal, and this wavelength division multiplexed optical signal is transmitted via an optical fiber amplifier 5a. After the amplification, the signal is sent to the optical fiber 7, which is a transmission path, and transmitted to the receiving side.

The wavelength multiplexing optical receiving terminal 14 comprises an optical fiber amplifier 5b, optical demultiplexers 15, and optical receivers 16-1 to 16-N for respective channels of 1 to N. 1 to above
Each of the N channels has a different carrier frequency for each channel, where the first channel is the carrier frequency f _sig1 and the second channel is the carrier frequency f _sig1 .
_{The sig2} , to the Nth channel are allocated and operated in such a _manner as the carrier frequency f _sigN . Therefore, the optical multiplexer 1
Reference numeral 3 denotes input ports for the first channel, the second channel, and the Nth channel, and the transmitters 12-1 to 12-N for the respective channels are connected to input to the input ports corresponding to the own channel. . The optical demultiplexer 15 has output ports for the first channel, the second channel, and the Nth channel, and outputs an optical signal corresponding to each channel demultiplexed for each channel to the corresponding output port. Therefore, the optical receivers 16-1 to 16-16 for each channel
-N is configured to connect to the output port corresponding to the own channel.

Therefore, the wavelength division multiplexing optical receiving terminal station 14 receives the wavelength division multiplexing optical signal transmitted through the optical fiber 7, amplifies the signal by the optical fiber amplifier 5b, and then demultiplexes the optical signal. At 15, the signals are demultiplexed into the respective channels, and the respective optical receivers 16-1 to 16-1 corresponding to the respective channels are obtained.
6-N. 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 electric signal, extracts a necessary signal component through a low-pass filter, and extracts It has the function of receiving data at

Next, the operation of the optical communication apparatus shown in FIG. 13 will be described. Optical multiplexer / demultiplexer 1 of wavelength multiplexing transmitting terminal 11
3 and the optical multiplexer / demultiplexer 15 of the wavelength division multiplex receiving terminal 14 are mainly a Littrow type using a diffraction grating or an AWG (AWG).
rayed Wavelength Gratin
g) type is used, but these Littrow type and AWG type have band limiting characteristics with respect to wavelength, unlike characteristics of couplers and the like.

Therefore, in the present system, the optical signals transmitted from the optical transmitters 12-1 to 12-N for each channel indicate their carrier frequencies f _{sig1 to} f _sigN and the port for each channel of the optical multiplexer 13. Transmission center frequencies f _{sig1 to} f _sigN
Are set in advance so as to be detuned and input to the optical multiplexer 13, respectively. And the optical multiplexer 13
The optical signal of each channel is optically multiplexed into wavelength-division multiplexed light and output, but the optical signal of each channel is
For its own carrier frequency f _{sig1 to} f _sigN ,
Since each is detuned and input to the optical multiplexer 13, the output optical signal of the optical multiplexer 13 is
The optical signals of each channel are collectively converted into a wavelength multiplexed optical signal that is subjected to vestigial sideband modulation.

Since the band of the vestigial sideband optical signal is narrower than the band of the ordinary double sideband optical signal, it is possible to perform multiplexing at higher density in wavelength multiplexing. therefore,
The communication system can secure more communication channels.

FIG. 14A shows the frequency spectrum distribution of the optical signal subjected to the vestigial sideband modulation at the time of wavelength multiplexing. 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.

In the optical signal of the conventional example, a frequency spectrum having a signal component is symmetrically distributed around the carrier frequency f _sig , but the optical signal of the present invention gradually blocks the frequency spectrum on one side. By leaving a part of the signal, the signal has a frequency spectrum that is concentrated on one side without reducing the signal component. Therefore, the optical signal according to the present invention has a narrower overall frequency spectrum distribution than the conventional frequency spectrum distribution. In a wavelength division multiplexing optical transmission system to which the optical signal according to the present invention is applied, channels are multiplexed at a higher density. And multiplexing at higher densities, it is possible to multiplex more channels within the same frequency band.

Of course, the wavelength multiplexing optical receiving terminal station 14 also determines the carrier frequencies f _{sig1 to} f _sigN of each channel of the transmitted wavelength multiplexing optical signal and the transmission center frequencies f _{sig1 to} f _sigN of the optical demultiplexer 16. By performing the detuning, it becomes possible to transmit and separate only the required band of the vestigial sideband optical signal that is collectively wavelength-multiplexed.

Further, the detuning amount of the optical multiplexer / demultiplexer 13 in the optical transmitting terminal 11 and the optical multiplexer / demultiplexer 15 in the optical receiving terminal 14 is such that the transmission characteristics of the optical multiplexers / demultiplexers 13 and 15 are Gaussian or Can be approximated by the following Butterworth type,
Also in the wavelength division multiplexing optical transmission system in the fourth specific example, the same settings as those in the first, second and third specific examples of the present invention are sufficient.

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 by using an optical multiplexer / demultiplexer necessary for wavelength multiplexing. In this way, it is possible to perform residual sideband modulation, multiplexing, and demultiplexing, thereby enabling higher-density wavelength multiplexing.

As described above, the fourth specific example is based on the first to fourth embodiments.
The optical communication device according to the third embodiment is an optical communication device that multiplexes an optical signal. The optical filter inserted into the optical transmitter is an optical multiplexer, and the optical filter inserted into the optical receiver is an optical demultiplexer. It was a container. An optical multiplexer is provided on the optical transmitter side, a plurality of optical signals having different wavelengths are inserted into the optical multiplexer, and light transmitted through the port is transmitted at any one or more ports of the optical multiplexer. The carrier frequency f _{sig of the} signal and the transmission center frequency f _fil of the port are set to be shifted from each other, and the optical signal transmitted through the port is subjected to the residual sideband modulation, and multiplexed with the optical signal transmitted through the ports other than the port. In this case, a wavelength multiplexed optical signal is generated and transmitted.

Further, an optical demultiplexer is provided on the optical receiver side, and a wavelength division multiplexed optical signal including an optical signal subjected to vestigial sideband modulation is inserted into the optical demultiplexer. In one or more ports, the carrier frequency f _sig of the optical signal passing through the port and the transmission center frequency f _fil of the port are set to be shifted, and the wavelength division multiplexed optical signal is demultiplexed into a plurality of optical signals having different wavelengths. It is characterized by doing.

In this optical communication apparatus, on the optical transmitter side, optical signals of a plurality of channels are collectively subjected to residual sideband modulation and multiplexed by utilizing the band limiting characteristics of an optical multiplexer to perform wavelength multiplexing. It is possible to generate optical signals, and on the optical receiver side, by using the band limiting characteristics of the optical demultiplexer, it is possible to convert a plurality of optical signals by wavelength-multiplexed vestigial sideband modulation to only the required band. , And it is possible to branch at once. In this optical communication apparatus, the spread of the optical signal band of the optical signal of each channel is suppressed by the 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 multiplexing optical transmission system, and it is possible to secure more channels. Although various specific examples have been described above, the present invention is not limited to these specific examples, and can be implemented with various modifications.

[0095]

As described above, according to the present invention, even when a narrow-band optical filter is used, an optical signal that has been subjected to vestigial sideband modulation to suppress the expansion of the optical signal band at the optical transmitter side. It is possible to provide an optical communication device capable of receiving an optical signal modulated in the vestigial sideband on the optical receiver side by cutting off an unnecessary band, and is expected as long-distance large-capacity transmission. High-density wavelength division multiplexing optical transmission can be realized.

Furthermore, according to the present invention, even when a narrow-band optical filter is used, an optical signal that has been subjected to vestigial sideband modulation that suppresses the expansion of the optical signal band can be generated on the optical transmitter side. Further, it is possible to provide an optical communication device that can receive an optical signal modulated in the vestigial sideband on the optical receiver side by transmitting only a necessary band. Furthermore, when the present invention is applied to a wavelength division multiplexing transmission system, without adding a new component, in an optical multiplexer and an optical demultiplexer,
Enables residual sideband modulation of multiple channels at once, and enables optical transmission of residual sideband modulation at once to transmit only the required band for demultiplexing, and is expected for long-distance large-capacity transmission High-density wavelength division multiplexing optical transmission can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the present invention, and is a block diagram 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, showing a relationship between an eye pattern of a transmission optical signal and a frequency spectrum with respect to a detuning amount according to the first specific example of the present invention.

FIG. 4 is a diagram for explaining the present invention, showing a relationship between an eye pattern of a transmission optical signal and a frequency spectrum with respect to a detuning amount according to the first specific example of the present invention.

FIG. 5 is a diagram for explaining the present invention, showing a relationship between a detuning amount and an 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, showing a relationship between an optical filter, a bandwidth, and a setting range of a detuning amount in the first specific example of the present invention.

FIG. 7 is a diagram for explaining the present invention, showing a frequency spectrum when a frequency chirp exists.

FIG. 8 is a diagram for explaining the present invention, and is a diagram showing a relationship between a detuning amount and an eye penalty for each transmission characteristic of an optical filter when a frequency chirp exists.

FIG. 9 is a diagram for explaining the present invention, and is a block diagram showing a schematic configuration of a third specific example of the present invention.

FIG. 10 is a diagram for explaining the present invention, 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, showing an eye pattern and a frequency spectrum of a received optical signal with respect to a detuning amount in the third specific example of the present invention.

FIG. 12 is a diagram for explaining the present invention, 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 diagram for explaining the present invention, and is a block diagram showing a schematic configuration of a fourth specific example of the present invention.

FIG. 14 is a diagram for explaining the present invention, showing a frequency spectrum distribution of a wavelength-division multiplexed optical signal in a fourth specific example of the present invention and a frequency spectrum distribution of a wavelength-division multiplexed optical signal in a conventional example for comparison; FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical transmitter 2 ... Semiconductor laser light source 3 ... External optical modulator 4 ... Transmission signal 5a, 5b ... Optical fiber amplifier 6a, 6b ... Optical filter 7 ... Optical fiber 8 ... Optical receiver 9 ... Photodetector 10 ... Low frequency Filter 11: WDM optical transmitting terminal 12-1 to N: WDM optical transmitter 13: Optical multiplexer 14: WDM optical receiving terminal 15: Optical demultiplexer 16-1 to N: WDM optical receiver 101 ... CW light 102 ... NRZ pulse light signal 103 ... Transmission light signal 104 ... Reception light signal 105 ... Reception signal

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-3456 (JP, A) JP-A-7-46187 (JP, A) JP-A-6-315010 (JP, A) Kazushige Yonaga, Seiji Norimatsu , High-density optical FDM-homodyne detection system using an optical filter, Transactions of the Institute of Electronics, Information and Communication Engineers, Japan, The Institute of Electronics, Information and Communication Engineers, May 25, 1994, Vol. J77-BI, No. 5,304-312 (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 JICST file (JOIS)

Claims (3)

(57) [Claims] In an optical communication apparatus using intensity modulation, an optical transmitter includes an optical filter having a full width at half maximum B _fil , a transmission center frequency f _fil of the optical filter, and a carrier frequency f _{sig of the} optical signal. Is 0.5B _fil <| f _sig
-F _fil | is a residual sideband modulation system that blocks one sideband of the optical signal and leaves a part of the sideband to be removed. Communication device. 2. An advance optical communication apparatus according to claim 1 Symbol mounting, the optical filter to be inserted to the optical transmitter side, light, wherein the carrier frequency is an optical multiplexer for multiplexing a plurality of optical signals different Communication device. 3. Keep the optical communication apparatus according to claim 1 Symbol mounting, wherein the optical filter is inserted into the optical receiver side, an optical demultiplexer for outputting separately-multiplexed optical signal by the carrier frequency Optical communication device.