JP3433247B2 - Optical soliton generator and optical soliton transmission system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical soliton generator and an optical soliton transmission system.

[0002]

2. Description of the Related Art Light has a property of being concentrated in a place having a high refractive index, and it can be considered that a place having a higher refractive index for light has lower potential energy. For example, in an optical fiber, the potential energy of the core portion having a slightly higher refractive index is low, diffusion of light due to Fresnel diffraction in a homogeneous medium is suppressed, and light is confined in the core.

In the optical soliton, it is explained that a phenomenon similar to this occurs on the time axis. That is, the refractive index of quartz, which is the basic material of the optical fiber, is slightly non-linear, and the higher the light intensity, the higher the refractive index. FIG. 8 is a schematic diagram qualitatively showing the correspondence relationship between the optical soliton wave and the potential energy. FIG. 8A shows the optical soliton waveform, and FIG. 8B shows the potential energy (refractive index) on the time axis of the optical fiber in which the optical soliton propagates.
Shows the change of. The optical soliton maintains the isolated pulse state by competing the force to spread due to wavelength dispersion with the force to collect due to this nonlinearity. In other words, the optical soliton forms a portion with a low potential by itself, suppresses the spread of the pulse, and propagates stably.

In the conventional optical soliton transmission system, adjacent optical solitons are formed in phase or in opposite phase, and interference or interaction between adjacent optical solitons is a cause of deterioration of transmission characteristics of long distance transmission. Was becoming.

FIG. 9 shows two adjacent optical solitons A and B.
Shows a schematic diagram of the correspondence relationship between the waveform and the potential energy when they are formed with the same optical frequency and the same phase. FIG. 9A shows the intensity of the optical soliton on the time axis, and FIG. 9B shows the potential energy on the time axis. In the middle between the optical solitons A and B, as shown in FIG.
As shown in, the potential energy becomes lower than the outside of the optical solitons A and B. This is because the amplitude of the optical soliton A and the amplitude of the optical soliton B are added in phase. In this way, the potential energy in the middle between the optical solitons A and B becomes relatively low, so that the optical solitons A and B attract each other.

On the other hand, FIG. 10 is a schematic diagram of a correspondence relationship between a waveform and potential energy when two adjacent optical solitons C and D are formed in opposite phases (inverted phases) even at the same optical frequency. Show. FIG. 10A shows the intensity of the optical soliton on the time axis, and FIG. 10B shows the potential energy on the time axis. Contrary to the case of FIG. 9,
In the middle between the optical solitons C and D, since the amplitude phases of the optical solitons C and D are opposite to each other, the potential energy becomes completely zero as shown in FIG. It is higher than the outside of C and D. As a result, the optical solitons C and D repel each other and tend to separate from each other.

The attraction or repulsion of such adjacent optical solitons defines the upper limit of the transmission bit rate. As a means for reducing the interference or interaction between the adjacent optical solitons, an alternating amplitude method in which the amplitudes of the adjacent optical solitons are alternately increased and decreased, and a narrow band in which the center optical frequency is gradually slid to each repeater A sliding frequency guiding filter method for providing an optical filter has been studied and proposed.

[0008]

In the alternating amplitude method, it is difficult to provide a large difference in amplitude between adjacent optical solitons, and a large effect cannot be obtained. Further, in the sliding frequency guiding filter method, each repeater must be provided with a narrow band optical filter in which the center optical frequency is slid according to transmission. It is not easy to introduce such a narrow band optical filter into an actual transmission system.

It is an object of the present invention to provide an optical soliton generator that generates optical solitons that are unlikely to interfere with each other between adjacent optical solitons.

Another object of the present invention is to provide an optical soliton generator and an optical soliton transmission system that realize a higher transfer rate.

Another object of the present invention is to provide an optical soliton generator and an optical soliton transmission system which can realize a higher transmission rate even if the existing optical soliton transmission line is used as it is.

It is a further object of the present invention to present an optical soliton transmission system that realizes longer distances and / or faster transmission rates.

[0013]

By alternating-currently driving a laser element while continuously oscillating the laser element, the laser output light continuously changes its light wavelength in a cycle of alternating-current driving. The same thing can be realized by phase-modulating continuous output light by an external phase modulator. The pulse forming means forms an optical pulse suitable for optical soliton transmission from continuous laser light whose optical wavelength continuously changes. In the light pulse train thus formed, the light wavelengths are slightly different between the adjacent light pulses.

Optical pulses having slightly different optical wavelengths repeat a phase-matched state and a phase-reversed state during transmission.
Adjacent optical solitons attract each other when they are in the same phase and repel each other when they are out of phase, but because they repeatedly attract and repel, the interference or interaction between adjacent optical solitons as a whole. Therefore, the pulse interval can be made narrower than before, and the transmission rate can be improved and the distance can be increased.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a schematic block diagram of an embodiment of the present invention. Reference numeral 10 is a semiconductor laser, such as a DFB laser, which oscillates in a longitudinal single mode at a light wavelength of 1,500 nm band. The DC power supply 12 supplies a DC current for continuous laser oscillation to the semiconductor laser 10, and at the same time, the high frequency oscillation circuit 14 supplies a sine wave AC current having a frequency fa (for example, 10 MHz) to the semiconductor laser 10. In this embodiment, the DC power supply 12 supplies a DC current of about 60 mA to the semiconductor laser 10, and the high frequency oscillation circuit 14 supplies 10 MHz.
In this case, an alternating current of about 5 mA is supplied to the semiconductor laser 10.

FIG. 2 is a schematic view conceptually showing how the amplitude and the wavelength of the output light of the semiconductor laser 10 change. FIG. 2A shows the output light intensity of the semiconductor laser 10, FIG.
Indicates the light wavelength of the output light of the semiconductor laser 10. Both
The horizontal axis is time.

The semiconductor laser 10 continuously oscillates with the direct current from the direct current power source 12, but the semiconductor laser 1
Since a sinusoidal alternating current with a frequency fa (for example, 10 MHz) supplied from the high-frequency oscillation circuit 14 is superimposed on the drive current of 0, the output light of the semiconductor laser 10 has an amplitude and a light wavelength of frequency fa. It will be modulated by.
That is, the amplitude of the output light of the semiconductor laser 10 is as shown in FIG.
As shown in, the period fluctuates at 1 / fa. Further, the light wavelength of the output light of the semiconductor laser 10 fluctuates in a cycle of 10 MHz within a range defined by the frequency fa, as shown in FIG. When the original continuous laser oscillation light wavelength is λm, the wavelength of the output light of the semiconductor laser 10 is λm−Δ.
It cyclically changes with a cycle of 1 / fa in the wavelength range of λa to λm + Δλa. The shift amount Δλa of the light wavelength depends on the frequency and amplitude of AC driving.

The optical modulator 16 is an electro-absorption type optical modulator made of, for example, InGaAsP, and is a semiconductor laser 10.
It can be formed integrally with. The optical modulator 16 selectively absorbs the continuous wave laser light output from the semiconductor laser 10 with a clock having a clock frequency fb (for example, 10 GHz), and a predetermined pulse width (for example, 15) at a cycle 1 / fb.
ps) isolated light is formed. That is, the optical pulse train output from the optical modulator 16 consists of optical pulses obtained by sampling continuous laser light having the amplitude and the optical wavelength shown in FIGS. 2A and 2B at the clock frequency fb, and the adjacent optical pulses. The light wavelength of is slightly different.

When the oscillation frequency fa of the high-frequency oscillation circuit 14 is set to 5 GHz, the output of the optical modulator 16 is an optical pulse of wavelength λm + Δλa and a wavelength λm-Δλa.
The optical pulse train of the above becomes an alternating optical pulse train.

Although the semiconductor laser 10 is directly frequency-modulated by the output current of the high-frequency oscillation circuit 14 in the above embodiment, continuous wave laser light may be phase-modulated by an external phase modulation optical element. FIG. 3 shows a schematic block diagram thereof. A DC power supply 22 supplies a DC current equal to or more than a threshold value for continuous laser oscillation to a DC power source 20, and continuous laser oscillation is performed.
The phase modulator 24 phase-modulates the output light of the semiconductor laser 20 with the frequency fa. It goes without saying that the phase modulator 24 may amplitude-modulate the output light of the semiconductor laser 20. The optical wavelength of the output light of the phase modulator 24 is as shown in FIG.
Changes as shown in. Optical soliton modulator 26
Is composed of an optical element similar to the optical soliton modulator 16 and selectively absorbs the continuous laser light output from the phase modulator 24 with a clock having a clock frequency fb to obtain a period 1 / fb.
The isolated light having a predetermined pulse width (for example, 15 ps) is formed.

Since the semiconductor laser 20, the phase modulator 24, and the optical soliton modulator 26 can all be realized by InGaAsP-based compound semiconductors, all or part of them can be integrally formed. Obviously, it is preferable to form all of them integrally.

Two with slightly different optical frequencies (or optical wavelengths)
It is assumed that two optical solitons # 1 and # 2 are adjacent to each other as shown in FIG. In FIG. 4, the vertical axis represents light intensity and the horizontal axis represents time.
The optical frequencies of the optical solitons # 1 and # 2 are f1 and f, respectively.
Set to 2. As shown in FIG. 5, each of the optical solitons # 1 and # 2 has a substantially different refractive index due to the difference in the optical frequency.
Have different phases. The vertical axis of FIG. 5 represents the phase, and the horizontal axis represents the transmission distance. Therefore, even if the optical solitons # 1 and # 2 are incident in the same phase, the optical solitons # 1 and # 2 are in opposite phases when transmitted over a certain distance, and are in the same phase when further transmitted over the same distance. That is, the reverse phase and the same phase are repeated.

As described above, adjacent optical solitons attract each other in the same phase and repel in the opposite phase. Since the opposite phase and the same phase are repeated between the optical solitons # 1 and # 2 having slightly different optical frequencies, the inquiry and the repulsion are canceled as a whole. Therefore, when viewed as a whole, the influence of interference or interaction between the adjacent optical solitons is reduced, and the distance between the optical solitons # 1 and # 2 can be made narrower than the conventional one. The distance required for phase inversion depends on parameters such as optical frequency difference, but it is 1,000 to several thousand km.
It is a degree.

The group velocity 1 / (dβ / dω), which is the traveling velocity of the two optical solitons # 1 and # 2, is the optical frequency f1,
Although it varies depending on f2, as shown in FIG. 6, it is very small compared to the change of the phase velocity ω / β and can be neglected. Therefore, a substantial difference occurs in the propagation velocities of the optical solitons # 1 and # 2. Absent. FIG. 6 shows the relationship between the angular frequency ω and the propagation constant β of each of the optical solitons # 1 and # 2. The vertical axis represents the propagation constant β, and the horizontal axis represents the angular frequency ω.

FIG. 7 shows a schematic block diagram of an optical soliton transmission system using the optical soliton generator shown in FIG. 1 or 3. Reference numeral 30 is an optical soliton transmitter, 50 is an optical transmission line, and 60 is an optical soliton receiver.

First, the configuration of the optical soliton transmitter 30 will be described. Reference numeral 32 denotes the optical soliton generator shown in FIG. 1 or FIG. 3, which outputs a 10 GHz optical soliton pulse train. The data modulators 34 and 36 are the optical soliton generation circuit 32.
Optical pulse train output from the external 10Gbit
Modulate with s / s data. Data modulators 34, 3
6 passes an optical pulse for data "1" and blocks an optical pulse for data "0", for example. The data modulator 34 (or 36) has 20 Gbits at the output.
/ S, that is, an optical delay element corresponding to 50 ps is provided, and the data modulator 34 (or 36) is the data modulator 3
The optical pulse is output with a delay of 50 ps with respect to 6 (or 34).

The optical multiplexer 38 multiplexes the output pulses of the data modulators 34 and 36 on the time axis. As a result, the output of the optical multiplexer 38 becomes an optical soliton pulse train of 20 Gbits / s.

The optical pulse compressor 40 shortens the time width of each optical pulse output from the optical multiplexer 38. The optical pulse compressor 40 is composed of, for example, an optical fiber having a core diameter smaller than usual. Since an optical fiber having a small core diameter easily causes a non-linear phenomenon, the pulse width of each optical pulse can be reduced by utilizing this phenomenon.

The output optical pulse of the optical pulse compressor 40 is input to the polarization scrambler 42. Polarization scrambler 42
Is an optical element that rotates the plane of polarization of input light at a predetermined cycle. It is known that when the plane of polarization is fixed, optical noise increases on the plane orthogonal to the plane of polarization, and rotation of the plane of polarization by the polarization scrambler 42 can suppress the generation of such noise. The function and configuration itself of the polarization scrambler 42 are well known.

The output light of the polarization scrambler 42 is input to the optical transmission line 50 as the output light of the optical soliton transmitter 30. The optical transmission line 50 basically includes one or more dispersion shift fibers 52, one or more optical repeaters 54, and the dispersion shift fiber 52 mainly for the optical wavelength of the optical pulse to be transmitted.
One or more dispersion-compensating fibers (also called equalizing fibers) that compensate the accumulated chromatic dispersion due to ## EQU1 ##
56, and one or more narrow band (eg, 2.1 nm) optical bandpass filters 58. An optical amplifying fiber is usually incorporated in the optical repeater 54 to amplify an input optical signal as it is. The optical bandpass filter 58 is also
It is often incorporated in the optical repeater 54. In the optical soliton transmission experiment by the system shown in FIG. 4, the optical transmission distance of the optical transmission line 50 is about 8,100 km.

As is well known, the optical soliton transmission system is a system for transmitting an extremely short optical pulse over a long distance while maintaining its pulse waveform constant by balancing the nonlinearity and wavelength dispersion of an optical fiber. The dispersion shift fiber 52 is designed so that the chromatic dispersion is accumulated on the positive side, and the dispersion compensation fiber 56 is the dispersion shift fiber 5.
It is designed to compensate the accumulated chromatic dispersion at a predetermined distance so that the increase of the accumulated chromatic dispersion by 2 does not exceed a certain ratio. Therefore, the dispersion compensating fiber 56 is installed at every certain distance (equalization distance).

Needless to say, the transmission distance is
If it is short, the dispersion compensating fiber 56 may be unnecessary. The same applies to the optical repeater 54.

The optical soliton receiver 60 has the following structure. That is, the optical preamplifier 62 is connected to the transmission line 5
The optical signal from 0 is amplified. The output light of the optical preamplifier 62 is applied to the pin photodiode 64 and the optical separator 68. The output signal of the photodiode 64 is 20 GHz
Of the phase-locked loop (PLL) circuit 66. The PLL circuit 66 generates a 20 GHz clock in accordance with the output signal of the photodiode 64, and applies a half frequency clock of 10 GHz to the optical separator 68. In this way, the PLL circuit 66 generates the 20G
A clock recovery circuit for recovering a 10 GHz clock from a Hz clock is provided.

The optical separator 68 is composed of two systems of electro-absorption type optical modulators made of InGaAsP, for example.
An optical gate of about 50 ps width according to the 10 GHz clock from the circuit 66 separates the optical signal data train of 20 Gbits / s into two optical signal data trains of 10 Gbits / s. The input part of the optical splitter 68 has an optical branching element for splitting the input light into two systems, and the photodiode 6
4 and an optical delay device having a delay amount similar to the signal delay by the PLL circuit 66 are provided. The optical delay device is the PLL circuit 66.
It is a phase adjusting means for phase-synchronizing the clock from the optical preamplifier 62 with the optical signal from the optical preamplifier 62. The optical signal data strings separated by the optical separator 68 are applied to the data demodulation circuits 70 and 72, respectively. Data demodulation circuit 70,
Each 72 photoelectrically converts the input optical signal and outputs the electric signal of the data string in a predetermined format, for example, the NRZ format.

The operation of the optical transmission system shown in FIG. 7 will be described. In the optical soliton transmitter 30, the optical soliton generator 32 outputs an optical soliton pulse train of 10 GHz whose optical frequency is slightly different between the adjacent optical solitons, as described with reference to FIG. Data modulators 34, 36
Respectively modulate the output optical pulse train of the optical soliton generation circuit 32 with external data of 10 Gbits / s, and the optical multiplexer 38 multiplexes the outputs of the data modulators 34 and 36 on the time axis. Data modulator 34 (or 36)
Outputs the modulated optical signal after delaying it by 50 ps, the output of the optical multiplexer 38 is 2 channels of 10 Gbits /
It is an optical signal of 20 Gbits / s obtained by multiplexing the optical signal of s.

The output optical pulse of the optical multiplexer 38 has its pulse width compressed by the optical pulse compressor 40, and then is output to the optical transmission line 50 via the polarization scrambler 42.

In the optical transmission line 50, the optical soliton transmitter 3
The optical soliton output from (0) (of the polarization scrambler 42) propagates through the dispersion shift fiber 52, the optical repeater 54, the dispersion compensation fiber 56, and the optical bandpass filter 58. The dispersion compensating fiber 56 compensates the accumulated chromatic dispersion due to the dispersion shift fiber 52 (and other transmission medium) and suppresses the accumulated chromatic dispersion to a certain value or less. In this optical soliton transmission experiment, as described above, the optical transmission line 5
The optical transmission distance of 0 is about 8,100 km.

The optical soliton propagated through the optical transmission line 50 is input to the optical preamplifier 62 of the optical soliton receiver 60.
The optical preamplifier 62 amplifies the input light, and the output light thereof is input to the photodiode 64 and the optical separator 68. The photodiode 64 converts the output light of the optical preamplifier 62 into an electric signal and supplies it to the phase locked loop circuit 66.
The phase-locked loop circuit 66 forms a 10 GHz clock from the output signal of the photodiode 64 to generate an optical separator 6
8 and output to the outside.

The optical demultiplexer 68 is the 1 from the PLL circuit 66.
2 from the optical preamplifier 62 according to the 0 GHz clock
Two 10 Gbi optical signal data trains of 0 Gbits / s
The optical signal data string of ts / s is separated, and one is supplied to the data demodulation circuit 70 and the other is supplied to the data demodulation circuit 72. Each of the data demodulation circuits 70 and 72 photoelectrically converts the optical signal from the optical separator 68 and outputs the electric signal of the corresponding data string in a predetermined format, for example, the NRZ format to the outside.

In this way, it was confirmed that optical soliton transmission of 20 Gbits / s over an extremely long distance of 8,100 km was actually possible.

[0042]

As can be easily understood from the above description, according to the present invention, it becomes possible to further reduce the interval between optical solitons. This allows longer transmission distances and / or
Alternatively, a higher transmission rate can be realized.

Further, even if the existing optical transmission line designed and laid for optical soliton transmission is used as it is, a higher transmission speed can be realized, which is economical.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention.

FIG. 2 is a conceptual diagram showing how the output light of the semiconductor laser 10 changes in amplitude and light wavelength.

FIG. 3 is a schematic block diagram of another embodiment of the present invention.

FIG. 4 is a schematic diagram showing two optical solitons # 1 and # 2 having slightly different optical frequencies (or optical wavelengths).

5 is a schematic diagram showing how the phases of the optical solitons # 1 and # 2 shown in FIG. 4 advance.

FIG. 6 is a schematic diagram showing the relationship between the angular frequency ω of optical solitons # 1 and # 2 and the propagation constant β.

7 is a schematic block diagram of an optical soliton transmission system using the optical soliton generator shown in FIG. 1 or FIG.

FIG. 8 is a conceptual diagram of a correspondence relationship between an optical soliton wave and potential energy.

FIG. 9 is a schematic diagram showing a correspondence relationship between a waveform and potential energy when two adjacent optical solitons A and B are formed with the same optical frequency and the same phase.

FIG. 10 is a schematic diagram showing a correspondence relationship between a waveform and potential energy when two adjacent optical solitons C and D are formed with the same optical frequency and opposite phases.

[Explanation of symbols]

10: Semiconductor laser 12: DC power supply 14: High frequency oscillation circuit 16: Optical modulator 20: Semiconductor laser 22: DC power supply 24: Phase modulator 26: Modulator for optical soliton 30: Optical soliton transmitter 32: Optical soliton generator shown in FIG. 1 or FIG. 34, 36: Data modulator 38: Optical multiplexer 40: Optical pulse compressor 42: Polarization scrambler 50: Optical transmission line 52: Dispersion shifted fiber 54: Optical repeater 56: Dispersion compensating fiber (equalizing fiber) 58: Optical bandpass filter 60: Optical soliton receiver 62: Optical preamplifier 64: pin photodiode 66: Phase locked loop (PLL) circuit 68: Optical separator 70, 72: Data demodulation circuit

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H04B 10/18 (72) Inventor Itaro Morita 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (56) References JP-A-8-122834 (JP, A) JP-A-4-192386 (JP, A) JP-A-7-221706 (JP, A) JP-A-5-145487 (JP, A) JP-A-3 -214123 (JP, A) JP-A-6-326387 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H01S 3/10 H01S 5/30 G02F 1/35 JISST file (JOIS)

Claims (7)

(57) [Claims] And 1. A semiconductor laser device lasing a DC drive means for continuous lasing the semiconductor laser device DC drive to the AC driving means for AC driving the semiconductor laser device, the output of the semiconductor laser element It is composed of pulsing means for converting light into optical pulses according to a clock of a predetermined frequency.
The AC drive frequency of the current drive means is lower than the predetermined frequency.
Optical soliton generator, characterized in that again. 2. The optical soliton generator according to claim 1, wherein the pulsing means is an optical modulator that absorbs the input light in a predetermined period within one cycle of the clock according to the clock. 3. The semiconductor laser device is 1,500 nm
A semiconductor laser that oscillates in a single longitudinal mode in a band.
The optical soliton generator according to item 2. 4. A semiconductor laser device that oscillates a laser, a direct current driving means that drives the semiconductor laser element by direct current to continuously oscillate the laser, a phase modulating means that phase-modulates the output light of the semiconductor laser element, and the phase modulating means. An optical soliton generator, comprising: a pulse forming means for converting the output light of the means into an optical pulse according to a clock of a predetermined frequency. 5. The optical soliton generator according to claim 4, wherein the pulsing means is an optical modulator that absorbs the output light of the phase modulating means in a predetermined period within one cycle of the clock according to the clock. 6. The semiconductor laser device is 1,500 nm
5. A semiconductor laser that oscillates in a single longitudinal mode in a band.
Alternatively, the optical soliton generator according to item 5. 7. An optical transmission medium, an optical soliton transmitting unit that outputs an optical soliton modulated by data to be transmitted to the optical transmission medium, and an optical soliton transmitted through the optical transmission medium, and demodulates the data. An optical soliton transmission system comprising: an optical soliton receiving means for generating an optical soliton that generates optical solitons having different optical wavelengths between adjacent optical solitons at a predetermined rate, and the optical soliton transmitting means. the optical soliton generated by the generating means, for the data modulation means for modulating the data to be transmitted, characterized in that it consists an output means for outputting the optical soliton that is data-modulated by the data modulator to an optical transmission medium Optical soliton transmission system.