JPH1013351A - Optical communication equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To save energy, to reduce cost and to miniaturize the equipment by using a modulation signal having a sinusoidal signal component at least at a mark part of a transmission signal through the signal processing at an electric stage for the transmission signal and the sinusoidal signal with a period equal to a clock period of the transmission signal. SOLUTION: A signal source 24 outputs a sinusoidal signal 5 and a transmission signal 6 being an NRZ signal. A transmission signal 6 is waveform-shaped by a waveform shaping device 7 of an optical transmitter 1. The relation of phase of the sinusoidal signal 5 and the transmission signal 6 is adjusted by a variable delay attenuator 8 and sinusoidal 5 is given to an adder 9 with the transmission signal, in which the both are added as to output of the adder and the power of the outputted modulation signal is amplified and added to a DC bias voltage at a bias T circuit 12, in which the modulation signal to be fed to an external light(EA) modulator 4 is obtained, the EA modulator 4 uses this modulation signal and applies intensity modulation to a CW optical signal 101 from a semiconductor laser light source 2, amplifies a transmission optical signal 102 and the transmission optical signal 103 is sent to an optical fiber 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication apparatus in which a signal from a signal source is an NRZ signal and an optical signal transmitted to a single mode optical fiber is an RZ pulse.

[0002]

2. Description of the Related Art In recent years, with the progress of optical fiber amplifiers, research and development of long-distance, large-capacity optical transmission systems have been actively carried out, and are expected as transmission means for realizing the future multimedia information age.

[0003] In optical transmission, the shapes of optical pulses that greatly affect the transmission characteristics are classified into NRZ pulses and RZ pulses. Among these, the RZ pulse is an optical MUX in the time domain that is impossible with the NRZ pulse.
/ DEMUX (multiplexing / demultiplexing) is possible, so that it is suitable for large-capacity transmission, more advantageous in light receiving sensitivity than NRZ pulses, and suitable for long-distance transmission such as a higher Burrian scattering threshold than NRZ pulses. is there.

There is an optical soliton transmission system which has attracted attention as a next-generation transmission system, and this optical soliton transmission system is a system using a kind of RZ pulse.
Also in the optical wavelength division multiplexing (WDM) transmission system, it has been reported that the influence of the cross phase modulation effect is smaller in the RZ pulse than in the NRZ pulse (for example, M. Suyama et al.,
“Improvement of WDM Transmission performance by N
on-Soliton RZ Coding ”, the Conference on Optical
Fiber Communication '96, PD26-2). As described above, the RZ pulse has become an effective choice as a transmission pulse waveform of the next-generation optical transmission system.

A transmission signal from a signal source which is an interface between an optical signal and an electric signal is mainly an NRZ signal.
The NRZ pulse was obtained by applying a modulation signal obtained by adjusting the amplitude of a transmission signal and adding a bias to a semiconductor laser light source or an external optical modulator in an optical transmitter having a configuration as shown in FIG.

That is, in FIG. 17, 1 is an optical transmitter, 2 is a semiconductor laser light source, 3 is a DC bias current source,
4 is an external optical modulator, 6 is a transmission signal, 7 is a waveform rectifier, 1
0 is an electric amplifier, 11 is a DC bias voltage, 12 is a bias tie, 13 is an optical fiber amplifier, 24 is a signal source, 1
01 is a CW optical signal (continuous optical signal), 102 is transmission light, 1
03 is a reception light.

An optical fiber 14 is an optical transmission line.
Reference numeral 15 denotes an optical receiver, which includes a photodetector 16 and an electric amplifier 1.
7, an equivalent filter 18 and the like.

In the optical transmitter 1, a CW (continuous-watt) signal is transmitted from a semiconductor laser light source 2, which is one of the components.
ave: continuous light) Outputs an optical signal 101. That is, the semiconductor laser light source 2 is a semiconductor laser light source that oscillates in a single longitudinal mode in a 1.5 [μm] band, and the semiconductor laser light source 2 uses a DC bias current supplied from a DC current source 3 to generate a CW optical signal. 101 is output. And this CW
The optical signal 101 is supplied to the external optical modulator 4, where it is modulated according to a modulation signal provided separately and transmitted optical signal 102
Becomes

Here, the external light modulator 4 has a predetermined modulation voltage −
This is an optical modulator having an extinction ratio characteristic.

The modulation signal applied to the external light modulator 4 is obtained as follows.

The transmission source 6 of the NRZ signal is transmitted from the signal source 24.
Is output and input to the optical transmitter 1. In the optical transmitter 1, first, the waveform of the transmission signal 6 is adjusted by the waveform shaper 7. Then, it is used as a modulation signal for the external optical modulator 4.
, And is added by the DC bias voltage from the DC voltage source 11 and the bias tee 12 to obtain a modulation signal to be applied to the external optical modulator 4.

The external optical modulator 4 uses this modulation signal to modulate the intensity of the CW optical signal 101 from the semiconductor laser light source, and amplifies the transmission optical signal 102 obtained by the optical fiber amplifier 13 so that: It is transmitted as an optical signal 103 for transmission to an optical fiber 14 having a zero-dispersion wavelength in the 3 [μm] band. The optical fiber 14 optically transmits the optical signal 103.

The optical signal 1 transmitted through the optical fiber 14
03 is the received light, which is detected by the optical receiver 15. The optical receiver 15 converts the received light into an electric signal using a photodiode 16, further amplifies the signal with an electric amplifier 17, and obtains a received signal through an equivalent filter 18.

That is, an optical transmitter using an NRZ pulse generates a modulation signal for generating a signal for modulating the CW light in correspondence with a laser light source of the CW light, an external optical modulator, and transmission information provided from a signal source. It is composed of a signal generation system and the like.

On the other hand, an optical transmitter using an RZ pulse has a configuration in which a modulation signal obtained by converting an NRZ signal, which is a transmission signal, into an RZ signal is applied to a semiconductor laser or an external optical modulator.

That is, when the RZ pulse is used,
An NRZ signal which is a transmission signal provided from a signal source is represented by R
It was necessary to convert the signal into a Z signal, and to apply the resulting modulated signal to an external optical modulator. However, the frequency spectrum of the RZ signal is larger than that of the NRZ signal, and the conversion of the transmission signal to the RZ signal in the electric stage requires a Gb / s (gigabit / second) class for high-speed transmission of this class. There is a problem that it is technically difficult to realize it when the response characteristics of the device are taken into consideration.

Further, even in an NRZ-RZ conversion device for converting an NRZ signal into an RZ signal, adjustment of the shape and width of the RZ signal cannot be performed.

Therefore, in the conventional RZ pulse transmission,
It is necessary to provide a portion for generating an RZ pulse train by a ring laser or a semiconductor electroabsorption optical modulator (EA modulator) driven by a sine wave, and an external modulation portion for data-modulating the RZ pulse train by a transmission signal which is an NRZ signal. Was.

FIG. 18 shows the configuration of an RZ pulse light transmitter using an EA modulator. In FIG. 18, the RZ pulse generator applies a sine wave signal 5 having a clock cycle to the external optical modulator 4 to generate an RZ pulse train (a pulse train that is not subjected to data modulation). In order to modulate the pulse train with data, an external optical modulator placed after
Apply NRZ signal as data and turn on pulse
/ Off (off).

At this time, at the position where the pulse exists, on / of
Therefore, in order to match the phases of the sine wave signal and the NRZ signal, phase control by the phase detector 22 and the phase shifter 23 is required.

That is, the transmitter for optical soliton transmission using the RZ pulse is different from the transmitter for NRZ pulse transmission in that the external optical modulator 4 generates the RZ pulse train by the clock signal and the data modulation by the transmission signal. For two systems,
In addition, in order to synchronize the RZ pulse train generation by the clock signal with the data modulation by the transmission signal, the apparatus configuration is complicated, such as using a clock signal phase detector and a transmission signal phase detector. And the cost of the device increases.

[0022]

The RZ pulse transmission has attracted attention as a carrier of next-generation optical transmission.
In the Z-pulse transmission, an RZ pulse train is generated by a ring laser or a sine-wave driven semiconductor electroabsorption optical modulator or the like, and an RZ signal is transmitted by an RZ signal.
An external modulation portion for performing data modulation of the pulse train is provided.

Therefore, as can be seen by comparing FIGS. 17 and 18, the transmitter for RZ pulse transmission has a larger number of components and a larger scale than the transmitter for NRZ pulse transmission. Since the phase adjustment between the generation unit and the data modulation unit is indispensable, there is a problem that the structure in the optical transmitter becomes complicated due to the addition of a phase control function or the like.

This means that the cost is increased and that the device is prevented from being downsized, and is not preferable from the viewpoint of energy saving. In order to prepare an environment in which RZ pulse transmission can be adopted, it is necessary to solve these problems.

Therefore, the object of the present invention is to
It is an object of the present invention to provide an optical communication device that employs RZ pulse transmission and has a simple optical transmitter, and thus can save energy and reduce the size of the device at low cost. .

[0026]

SUMMARY OF THE INVENTION According to the present invention, a transmission signal from a signal source in a single mode optical fiber is NRZ.
On the optical transmitter side of the optical communication device, which is a signal, the sine wave signal component is included in at least a mark portion of the transmission signal by signal processing in an electric stage of a transmission signal and a sine wave signal equal to the clock cycle of the transmission signal. It is an object of the present invention to provide an optical communication device capable of performing RZ pulse transmission by using a modulation signal without providing a conventional RZ pulse train generation unit and a data modulation unit.

The optical communication apparatus according to the present invention performs signal processing in an electrical stage of a transmission signal, which is an NRZ signal, and a sine wave signal equal to the clock cycle of the transmission signal. An optical transmitter for obtaining a modulated signal having a wave signal component, intensity-modulating the optical signal with the modulated signal, and transmitting the modulated signal to a transmission path.

In the optical communication apparatus according to the present invention, the signal processing of the transmission signal and the sine wave signal in the electric stage is performed by addition processing or multiplication processing or switching processing for selecting a sine wave signal corresponding to the transmission signal. Features.

Such an optical communication apparatus first obtains a modulation signal from signal processing in an electrical stage of a transmission signal, which is an NRZ signal from a signal source, and a sine wave signal having the same clock cycle as the transmission signal. This signal processing is mainly performed by adding, multiplying, or transmitting a signal of a sine wave signal by a mark or space.
An n / off switch is conceivable. Although addition is the easiest to realize as signal processing, since a sine wave signal is added even at the space level, it is necessary to use a semiconductor electroabsorption optical modulator capable of increasing the extinction ratio for the optical modulation means. . However, in the signal processing by the multiplication and the switch, since the signal level in the space of the transmission signal remains small, a LiNbO ₃ modulator which cannot obtain a large extinction ratio can also be used as the light modulation means.

Then, the modulated signal after the signal processing is applied to the optical modulating means, and the optical signal (continuous light) from the laser light source is intensity-modulated in accordance with the modulated signal, whereby the data-modulated R signal is modulated.
Generate a Z pulse. Therefore, unlike a conventional RZ pulse optical transmitter, there is no need to generate an RZ pulse train in an RZ pulse generation unit and data-modulate the generated RZ pulse train in a subsequent optical modulator. The number of components is small, and an optical transmitter capable of transmitting RZ pulses on almost the same scale as the optical transmitter for NRZ pulses can be realized. Further, thereby, downsizing, energy saving, and cost reduction can be achieved.

Further, in the optical communication apparatus according to the present invention, the modulated signal to be given to the optical modulating means may have a waveform adjusted through an electronic device having a non-linear characteristic whose amplification gain varies depending on an input level. It is characterized by.

In such a configuration, the optical communication device is
A modulated signal obtained by performing signal processing in an electric stage of a transmission signal as an NRZ signal and a sine wave signal having the same clock cycle as the transmission signal is passed through an electronic device having nonlinear characteristics. For example, when the above-described signal processing is addition, by giving the electronic device a characteristic of blocking a modulation signal having a threshold value or less, by setting the signal level when the transmission signal is a space to a threshold level or less, Eliminates signal level excitement during space. By doing so, it becomes possible to use a LiNbO ₃ optical modulator whose extinction ratio is not so large as an external optical modulator. In addition, when the electronic device has an amplification characteristic in which a gain changes according to the power of the input modulation signal, the modulation signal waveform can be deformed to change the pulse width of the RZ pulse.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, specific examples of the present invention will be described.
This will be described with reference to the drawings.

[First Embodiment] FIG. 1 is a block diagram of a first embodiment of the present invention. In FIG. 1, 1 is an optical transmitter, 2
Is a semiconductor laser light source, 3 is a DC bias current source, 4 is an external optical modulator, 5 is a sine wave signal, 6 is a transmission signal, 7 is a waveform rectifier, 8 is a phase / amplitude adjuster, 9 is an adder, 10 is An electric amplifier, 11 is a DC bias voltage, 12 is a bias tee, 13 is an optical fiber amplifier, 24 is a signal source, 101 is a CW optical signal (continuous optical signal), 102 is transmission light, and 103 is reception light.

An optical fiber 14 is an optical transmission line.
Reference numeral 15 denotes an optical receiver, which includes a photodetector 16 and an electric amplifier 1.
7, an equivalent filter 18 and the like.

The signal source 24 is a supply source of information to be transmitted such as transmission data and a clock signal, and the sine wave signal 5 is a sine wave signal generated by the signal source 24 as a clock signal. is there. The transmission signal 6 is output from the signal source 24, and is obtained by converting transmission data into an NRZ signal.

The waveform rectifier 7 rectifies the NRZ signal transmission signal 6 and outputs it. The phase / amplitude adjuster 8 adjusts the phase and amplitude of the sine wave signal 5 output from the signal source 24. For example, it is configured using a variable delay attenuator.

The adder 9 comprises a sine wave signal 5 whose phase and amplitude have been adjusted by the phase / amplitude adjuster 8 and a waveform rectifier 7.
The transmission signal 6 obtained by further waveform rectification is added and output, and the electric amplifier 10 amplifies and outputs the added output of the adder 9.
This is for adjusting the level by giving a required bias to the amplified output of the added output output from the amplifier 10.

The DC bias current source 3 is a supply source for supplying a DC bias current, and the semiconductor laser light source 2 receives a current from the DC bias current source 3 to generate a continuous light laser light (CW light) signal 101. What happens.

The external optical modulator 4 obtains the CW optical signal 101 from the semiconductor laser light source 2 and the signal from the bias tee 12, and modulates the CW optical signal 101 in accordance with the signal from the bias tee 12. And outputs the transmission light 102 modulated by the modulator.

The optical fiber amplifier 13 transmits the transmission light 10
2, for transmitting the optically amplified transmission light 102 to the optical fiber 14, which is an optical transmission line.

The optical transmitter 1 has the above-mentioned 2-4 and 7-1.
3 is comprised.

The optical receiver 15 comprises a photodetector 16, an electric amplifier 17, and an equivalent filter 18. Of these, the photodetector 16 detects an optical signal transmitted via the optical fiber 14. The electric amplifier 17 amplifies the signal, and the equivalent filter 18 obtains a received signal from the amplified signal.

The operation of the present system having such a configuration will be described. In FIG. 1, the operation of the optical transmitter 1 will be described first. The semiconductor laser light source 2 is a semiconductor laser light source that oscillates in a single longitudinal mode in a 1.5 [μm] band. The semiconductor laser light source 2 uses a DC bias current supplied from a DC current source 3 to generate CW light (continuous light). The signal 101 is output. Then, the CW optical signal 101 is provided to the external optical modulator 4, where it is modulated to correspond to a separately provided modulation signal and becomes a transmission optical signal 102.

Here, the external light modulator 4 is a semiconductor electro-absorption type optical modulator having a modulation voltage-extinction ratio characteristic as shown in FIG.
It is called a modulator.

The modulation signal applied to the EA modulator 4 is obtained as follows.

The signal source 24 outputs a sine wave signal 5 (see FIG. 3 (b)) as a clock signal and an NRZ transmission signal 6 (see FIG. 3 (a)) synchronized with the clock signal. .

The sine wave signal 5 is transmitted to the optical transmitter 1.
And the transmission signal 6 of the NRZ signal. First, the waveform of the transmission signal 6 is adjusted by the waveform shaper 7. The waveform shaper 7 has, for example, a configuration using a flip-flop. The transmission signal 6 of the NRZ signal is given to the flip-flop to be operated, whereby the transmission signal 6 having a distorted waveform is obtained.
Can be made into a regular waveform.

On the other hand, the sine wave signal 5 is supplied to the variable delay attenuator 8, where the phase relationship with the transmission signal 6 is adjusted to the operation state shown in FIG. 3 and input to the adder 9 together with the transmission signal 6. . Then, the two are added together to form an adder output as shown in FIG. 3C, and this adder output is used as a modulation signal for the external optical modulator 4.

The modulated signal output from the adder 9 as shown in FIG. 3C is amplified in power by an electric amplifier 10 and added to a DC bias voltage from a DC voltage source 11 by a bias tee 12. , EA modulator 4.

The EA modulator 4 uses this modulation signal to modulate the intensity of the CW optical signal 101 from the semiconductor laser light source 2 and amplifies the transmission optical signal 102 obtained by the optical fiber amplifier 13. It is transmitted as an optical signal 103 for transmission to an optical fiber 14 having a zero-dispersion wavelength in the 3 [μm] band. The optical fiber 14 optically transmits the optical signal 103.

The optical signal 1 transmitted through the optical fiber 14
03 is the received light, which is detected by the optical receiver 15. The optical receiver 15 converts the received light into an electric signal using a photodiode 16, further amplifies the signal with an electric amplifier 17, and obtains a received signal through an equivalent filter 18.

The above is the outline of the operation of the present system.

Next, the operation of the optical communication device will be examined in detail.

The signal source 24 has a clock cycle of 2.5 [GHz], and a sine wave signal 5 equal to the clock cycle and a transmission signal 6 which is a 2.5 [Gb / s] NRZ signal synchronized with the clock. Is output.

The optical fiber 14 serving as a transmission path has a 1.5 [μ]
[m] band and a normal dispersion fiber having a group velocity dispersion of 18 [ps / nm / km], and an optical transmission line having a transmission distance of 1500 [km] by such an optical fiber transmission line is assumed. In this optical transmission line, optical fiber amplifiers 13 are arranged at intervals of 60 [km] in order to compensate for optical loss.

Then, the reception states of the NRZ pulse and the RZ pulse at a transmission point of 1500 [km] over the optical fiber transmission line were simulated and compared.

As a result, in the case of optical transmission using an NRZ pulse at a transmission speed of 2.5 [Gb / s], the transmission waveform as shown in FIG. The time (indicating 10 ^-12 [sec]) is 1500 [km]
As shown in FIG. 5, which is the reception eye pattern at the point,
It is disturbed (in FIG. 5, the vertical axis indicates strength, and the horizontal axis indicates time (10 ⁻¹² [sec])).

That is, in the case of the optical transmission using the NRZ pulse, even if the transmission light power is optimally set, the influence of the dispersion of the optical fiber 14 is gradually affected, and the optical waveform is distorted.
At the point of 0 [km], the received eye pattern is greatly distorted as shown in FIG. Therefore, in the case of optical transmission using NRZ pulses, it is necessary to compensate for this dispersion of the optical fiber.

However, when the RZ pulse transmission by the optical communication apparatus (soliton transmission optical transmitter) of the present invention is performed under the same conditions, the waveform distortion is small even after 1500 [km] transmission as shown in FIG. A received waveform is obtained, and the eye pattern is clearly open (in FIG. 7, the vertical axis indicates strength, and the horizontal axis indicates time (10 ⁻¹² [sec])).

That is, the optical soliton transmission waveform as shown in FIG. 6 (in FIG. 6, the vertical axis represents intensity and the horizontal axis represents time (10 ⁻¹² [se
c]) is received at the point of 1500 [km] and its eye pattern is shown in FIG. 7, which shows a very good reception waveform in which the eye mask is cleanly opened and the eye mask is open. can get. This is due to the characteristics of soliton transmission.

[0062] Soliton transmission is defined as an abnormal dispersion region of an optical fiber (a region where a high frequency component in an optical signal propagates faster than a low frequency component) in a transmission line.
When transmitting an olic-secant RZ pulse, this transmission method utilizes the feature that the group velocity dispersion of an optical fiber and the self-phase modulation effect, which is a non-linear phenomenon, are compensated for each other to suppress waveform distortion.

The RZ pulse generated by using the conventional sine-wave driven EA modulator is a pulse having a shape close to a hyperbolic-secant type, and is suitable for soliton transmission. When the EA modulator is used as the modulator, the RZ pulse of the optical transmitter according to the present invention can generate an RZ pulse close to a hyperbolic-secant type, and transmission using the soliton feature is possible.

However, in the case of a transmission speed of 10 [Gb / s] or more, the effect of group velocity dispersion is usually large in a dispersion fiber, so that the self-phase modulation effect also needs to be increased.

Therefore, in order to quickly compensate for the loss of the optical signal power, an optical fiber amplifier must be inserted at an installation interval of about 20 to 30 [km]. This is undesirably too expensive for a typical optical transmission system.

However, as described above, according to the optical communication apparatus of the first embodiment according to the present invention, the NRZ described in FIG.
RZ with almost the same circuit scale configuration as the transmitter for pulse transmission
Pulse transmission is enabled, and it is possible to generate an RZ pulse that satisfies the same soliton condition as the RZ pulse generated by the conventional pulse train generation unit and data modulation unit, and the next generation optical soliton transmission system and WDM ( Application to optical wavelength division multiplexing (DM) transmission systems is expected.

That is, according to the present invention, only the phase / amplitude adjuster and the adder are provided, and the conventional system does not have the transmission signal modulation system and the clock signal modulation system, respectively. Therefore, the apparatus can be provided at a low cost because the configuration is simplified, and the configuration shown in FIG.
RZ pulse transmission can be performed with a circuit configuration substantially equal to that of the NRZ pulse transmission transmitter described in Section 6, and
A transmitter capable of generating an RZ pulse that satisfies the same soliton condition as the RZ pulse generated by the pulse train generator and the data modulator can be obtained.

Further, the simplification of the configuration means that energy saving and downsizing of the apparatus are enabled.

In the above, the main components of the external light modulation system (pulse train generation unit) using the clock signal and the external light modulation system (data modulation unit) for the transmission signal are shared. The composition with the clock signal (sine wave signal) whose phase and amplitude have been adjusted is performed by the adder.

However, the combination of the two can also be realized by a method other than the adder.
Will be described as a specific example.

[Second Specific Example] In a second specific example, a clock signal (sine wave signal) whose phase and amplitude are adjusted with the transmission signal 6 is used.
5 is composed by a multiplier.

FIG. 8 is a block diagram showing a second specific example according to the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The transmitter 1 receives the transmission signal 6 and the sine wave signal 5 from the signal source 24.
The sine wave signal 5 given from the phase 4
In step (1), the phase and amplitude are adjusted to adjust the phase relationship with the transmission signal 6 to the operating state shown in FIGS. 9 (a) and 9 (b). Then, the adjusted sine wave signal 5 is input to the multiplier 19 together with the transmission signal 6.

As the multiplier 19, for example, a double balanced mixer is used. By multiplying the sine wave signal 5 and the transmission signal 6 by the multiplier 19, a modulated signal as shown in FIG. 9C is obtained.

In the first specific example, since the adder is used, the sine wave signal 5 is added even when the content of the transmission signal 6 is space data. The level will be higher. Therefore, it was necessary to use an EA modulator capable of increasing the extinction ratio for the external light modulator 4.

[0075] However, in the second embodiment as adapted to use multipliers, also the signal level remains low, such in the case the content of the transmission signal 6 is a data space, LiNbO ₃ can not take a large extinction ratio Modulators can also be used. In other words, when the digital data is "0", the value is "0" regardless of what is multiplied by the digital data. Therefore, the signal level of the multiplication result remains low even in the analog multiplication system. Therefore, as the external optical modulator 4, L
An iNbO ₃ modulator can also be used.

The LiNbO ₃ modulator can easily select plus or minus of the frequency chirp. In general, in the case of the frequency chirp, the degree of the spread of the frequency spectrum of the optical signal is smaller in the minus direction, so that the influence of the group velocity dispersion from the optical fiber 14 is reduced. However, in the case of soliton transmission, since the balance between the group velocity dispersion and the self-phase modulation effect is important, a minus value cannot always be said to be good. for that reason,
It is preferable to select the code of the frequency chirp from the relationship with the transmission conditions and the optical signal power.

As described above, in the second specific example, the composition of the transmission signal 6 and the clock signal (sine wave signal) 5 whose phase and amplitude have been adjusted is performed by the multiplier. You can also. This will be described as a third specific example.

[Third Specific Example] FIG. 10 is a block diagram showing a third specific example according to the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

In this specific example, a switch 20 composed of, for example, a transistor is used, and the synthesis of the transmission signal 6 and the clock signal (sine wave signal) 5 whose phase and amplitude have been adjusted is replaced with the multiplier 19 in the second specific example. It is composed.

The phase relationship between the sine wave signal 5 and the transmission signal 6 is adjusted by the phase / amplitude adjuster 8 to the operation state shown in FIGS. 11A and 11B. It is input to the switch 20 used.

Here, the switch 20 sets the transmission signal 6
If the content of the signal is a mark, the sine wave signal 5 is passed, and if the content of the transmission signal 6 is a space, the sine wave signal 5 is controlled to be turned off. (C). That is, a pulse output corresponding to the signal content is obtained as a modulation signal. The CW optical signal 101 from the semiconductor laser light source 2 is intensity-modulated by applying this modulation signal to the external optical modulator 4 via the electric amplifier 10 and the bias tee 12 as a modulation signal, and the transmission light obtained by this is applied. The signal 102 is transmitted to the optical fiber amplifier 1
The signal is amplified at 3 and sent to the optical fiber 14 as an optical signal 103.

In the third specific example, as in the second specific example, even when the content of the transmission signal 6 is a space, the signal level remains low, so that the extinction ratio cannot be increased.
The iNbO ₃ modulator also has a configuration that can be used as the external optical modulator 4.

[Fourth Specific Example] FIG. 12 is a block diagram showing a fourth specific example according to the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In the fourth specific example, the signal level below a specific threshold is prevented from passing through the electronic device 21 having the non-linear characteristic, and the modulation signal is transmitted through the electronic device 21 to the external optical modulator 4.
Are provided as modulation signals.

The transmission signal 6 and the sine wave signal 5 are added by the adder 9.
Is input to the electronic device 21 having nonlinear characteristics. As the electronic device 21, for example, a device such as a diode having characteristics as shown in FIG. 13 (ideal characteristics in the drawing) is used.

The electronic device 21 is, for example, as shown in FIG.
If the electronic device 21 has such a characteristic as described above, the signal level below a specific threshold is not passed. Even if an input waveform as shown in c) is input, the output is as shown in FIG. 14B, and the modulated signal can be shaped as shown in FIG. 14B, so that the signal level difference between the mark and the space is increased. By changing the threshold value, the RZ pulse width can also take an arbitrary value.

This concept can be applied to the configuration shown in FIG. This will be described as a fifth specific example.

[Fifth Specific Example] FIG. 15 is a configuration diagram showing a fifth specific example according to the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In the fifth specific example, the modulated signal obtained by multiplying the transmission signal 6 and the sine wave signal 5 by the multiplier 19 (for example, FIG.
For example, a non-linear characteristic having a square characteristic (that is, a characteristic of y = X ² ) such that the output becomes “X × X” when the input is “X”, for example, Is input to the electronic device 21 having, for example, and is shaped by the electronic device 21 as shown in FIG.

That is, by using the electronic device 21, the pulse width can be narrowed by utilizing the fact that the passing gain varies depending on the signal level of the input signal, and a sharp waveform can be obtained.

Then, the modulated signal whose pulse width has been narrowed in this way and which has a sharp waveform is given to the external optical modulator 4, and
When the CW optical signal 101 from the semiconductor laser light source 2 is intensity-modulated in accordance with the modulation signal, a transmission optical signal 102 obtained is obtained.
Is as shown in FIG. 16C, and a sharper transmission optical signal 102 can be obtained.

As described above, the fourth and fifth specific examples block a signal of a level lower than a predetermined threshold value, and a signal of a level higher than the threshold value changes the signal value corresponding to the level. The pulse width is narrowed by passing a modulation signal to be supplied to the external optical modulator 4 through this electronic device, whereby a sharp transmission light signal 102 can be obtained.

The modulation signal applied to the external optical modulator 4 is obtained through a bias tee 12 for applying a required bias and adjusting the level. Therefore, the level applied to the modulation signal is adjusted according to the threshold value of the electronic device. Thus, the transmission optical signal 102 having a desired pulse width can be easily obtained. In particular, by providing the electronic device 21 with such a characteristic that the passing gain varies depending on the input signal level, the modulation signal has a width narrower than that of the sine wave signal 5 and has very little noise. Can be easily shaped into
The fact that the pulse width of the generated RZ pulse can be varied by such a modulation signal means that an RZ pulse with a good S / N can be obtained. This further improves the transmission characteristics of the optical soliton transmission.

While various examples have been described above, the present invention has various modifications in addition to these specific examples.

As the semiconductor laser light source 2, a light source in the 1.5 [μm] band is used in the specific example.
However, the present invention is not limited to this, and a 1.3 [μm] band or other wavelengths can be used. Also,
Although the transmission bit rate has been described as 2.5 [Gb / s], the transmission bit rate is not limited to this and may be another bit rate. Further, the zero dispersion of the optical fiber 14 as the transmission path is not limited to the 1.3 [μm] band. If the pulse width is changed according to the amount of dispersion in each case, the pulse with excellent transmission characteristics can be obtained by adjusting the amplitude of the sine wave signal, the gain of the amplifier of the modulation signal, and the bias voltage value. it can.

Further, in order to further simplify the structure of the transmitter 1, the semiconductor laser light source 2 can be directly modulated by the obtained modulation signal without using the external light modulator 4. Thus, an RZ pulse can be obtained.

[0095]

As described above, according to the present invention,
The RZ pulse, which is considered to be a promising transmission pulse waveform in next-generation optical soliton transmission and WDM transmission, is converted to a conventional R
Unlike the Z pulse optical transmitter, there is no need to provide an RZ pulse train generation unit and a data modulation unit, respectively, and the system configuration can be realized at a low cost, for example, it can be realized with a circuit configuration of the same scale as the NRZ pulse optical transmitter. It is possible to provide an optical transmitter that can be summarized and easily applied to various future RZ pulse transmissions.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the present invention, showing a configuration of an optical communication device for explaining a first specific example of the present invention.

FIG. 2 is a diagram for explaining the present invention, showing a modulation voltage-extinction ratio characteristic of an EA modulator used in the present invention.

FIG. 3 is a diagram for explaining the present invention, and is a timing chart showing a phase relationship between a transmission signal and a sine wave signal in the first specific example of the present invention.

FIG. 4 is a diagram showing an optical transmission waveform eye pattern of an NRZ pulse during transmission.

FIG. 5 is a diagram illustrating an eye pattern of a signal obtained by transmitting the NRZ pulse representing the optical transmission waveform eye pattern of FIG. 4 after 1500 [km] has been received;

FIG. 6 is a diagram showing an optical transmission waveform eye pattern of an RZ pulse during transmission.

7 is a diagram illustrating an eye pattern of a signal obtained by transmitting an RZ pulse indicating the optical transmission waveform eye pattern illustrated in FIG. 6 after 1500 [km] has been received.

FIG. 8 is a diagram for explaining the present invention, showing a configuration of an optical communication device as a second specific example of the present invention.

FIG. 9 is a diagram for explaining the present invention, and is a timing chart showing a phase relationship between a transmission signal and a sine wave signal in the second specific example of the present invention.

FIG. 10 is a diagram for explaining the present invention, showing a configuration of an optical communication device as a third specific example of the present invention.

FIG. 11 is a diagram for explaining the present invention, and is a timing chart showing a phase relationship between a transmission signal and a sine wave signal in a third specific example of the present invention.

FIG. 12 is a diagram for explaining the present invention, showing a configuration of an optical communication device as a fourth specific example of the present invention.

FIG. 13 is a diagram for explaining the present invention, and is an ideal characteristic diagram of an electronic device having a non-linear characteristic used in the fourth specific example of the present invention.

FIG. 14 is a diagram for explaining the present invention, showing a waveform shaping of a modulation voltage by an electronic device having a non-linear characteristic in a fourth specific example of the present invention.

FIG. 15 is a diagram for explaining the present invention, showing a configuration of an optical communication device as a fifth specific example of the present invention.

FIG. 16 is a diagram for explaining the present invention, showing an example of waveform shaping of a modulation voltage by an electronic device having a non-linear characteristic in the fifth specific example of the present invention.

FIG. 17 is a diagram showing a typical configuration example of a conventionally fixed NRZ pulse optical transmitter.

FIG. 18 is a diagram showing a typical configuration example of a conventionally fixed RZ pulse optical transmitter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical transmitter 2 semiconductor laser light source 3 DC bias current source 4 external optical modulator 5 sine wave signal (clock signal) 6 NRZ signal transmission signal 7 waveform rectifier 8 phase / amplitude adjuster 9 ... Adder 10 ... Electrical amplifier 11 ... DC bias voltage 12 ... Bias tee 13 ... Optical fiber amplifier 14 ... Optical fiber 15 ... Optical receiver 16 ... Photodetector 17 ... Electrical amplifier 18 ... Equivalent filter 19 ... Multiplier 20 ... Switch 21 ... Non-linear characteristic electronic device 22 ... Phase detector 23 ... Phase shifter 24 ... Signal source 101 ... CW optical signal 102 ... Transmission light 103 ... Reception light

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Claims (9)

[Claims] 1. A sine wave signal component in at least a mark portion of the transmission signal by performing signal processing of a laser light source that generates continuous light, a transmission signal based on an NRZ signal, and a sine wave signal equal to a clock cycle of the transmission signal. An optical transmitter comprising: a means for obtaining a modulation signal having the following: an optical modulation means for intensity-modulating the laser light from the light source with the obtained modulation signal; and a means for transmitting the intensity-modulated light to a transmission path. An optical communication device comprising: 2. A sine wave at least in a mark portion of the transmission signal by adding a laser light source that generates laser light as continuous light, a transmission signal based on an NRZ signal, and a sine wave signal equal to a clock cycle of the transmission signal. Means for obtaining a modulation signal having a wave signal component; light modulation means for intensity-modulating an optical signal from the laser light source with the obtained modulation signal; and transmission of an intensity-modulated optical signal of the light modulation means to a transmission path. An optical communication device comprising: 3. A laser light source that generates laser light as continuous light, and a multiplication process of a transmission signal based on an NRZ signal and a sine wave signal equal to a clock cycle of the transmission signal, the sine wave is added to at least a mark portion of the transmission signal. Means for obtaining a modulation signal having a wave signal component; light modulation means for intensity-modulating an optical signal from the laser light source with the obtained modulation signal; and transmission of an intensity-modulated optical signal of the light modulation means to a transmission path. An optical communication device comprising: 4. A laser light source for generating laser light as continuous light, and a switching process for selecting and outputting a sine wave signal equal to a clock cycle of the transmission signal in accordance with a transmission signal based on an NRZ signal, thereby performing transmission processing of the transmission signal. Means for obtaining a modulation signal having the sine wave signal component at least in a mark portion; light modulation means for intensity-modulating an optical signal from the laser light source with the obtained modulation signal; and intensity-modulated light of the light modulation means. An optical communication device, comprising: an optical transmitter having means for transmitting a signal to a transmission path. 5. An optical transmission line for amplifying and relaying an optical signal by an optical fiber amplifier.
An optical communication device according to any one of claims 1 to 4. 6. The optical communication device according to claim 1, wherein the laser light source generates light having a wavelength in an anomalous dispersion region of an optical fiber that is a transmission path. 7. The modulation signal applied to the optical modulation means, the waveform of which is adjusted via an electronic device having a nonlinear characteristic whose amplification gain varies according to an input level. An optical communication device according to claim 1. 8. The optical communication device according to claim 1, wherein the optical modulator is a semiconductor electro-absorption optical modulator. 9. The optical communication device according to claim 1, wherein said optical modulator is a LiNbO ₃ modulator.