JPH02219029A - Soliton generating method - Google Patents

Abstract

PURPOSE:To generate light soliton by the use of a small-sized, lightweight device of simple constitution by modulating the output of a single-longitudinal- mode semiconductor laser directly and generating a very-high-speed pulse train, and amplifying this pulse train by an optical fiber where erbium is added to the core part. CONSTITUTION:Current modulation is imposed directly on the semiconductor laser 4 with pulses of a laser driving pulse generating device 2 to generate the signal pulse train. This light pulse train is coupled with the Er-doped optical fiber 9 through a coupling lens 6, a dichroic mirror 7, and a coupling lens 2 and the Er-doped optical fiber 9 is excited with the exciting light of a light source 8 for excitation. When the pulse train which is amplified sufficiently by the Er-doped optical fiber 9 is made incident on a single-mode optical fiber 3 which has abnormal dispersion with the wavelength of signal light, a soliton pulse train can be propagated. Consequently, the light soliton can be generated by the simple method and small-sized device.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a soliton generation method for generating an optical soliton pulse train serving as a signal in ultrahigh-speed optical communication.

[Prior art] Conventional methods for generating optical solitons (nonlinear solitary waves) include: ■ coupling the output pulse of a high-power laser to an optical fiber (hereinafter referred to as the first method); ■ coupling the output pulse of a high-power laser to an optical fiber; A method using the output (hereinafter referred to as the second method), (2) a method using the output pulse of a gain-switched semiconductor laser (hereinafter referred to as the third method), etc. have been proposed.

[Problems to be Solved by the Invention] The configuration of the first method described above is shown in FIG. In this method, a light pulse from a high-power light source 1 is incident on an optical fiber 3 using a coupling lens 2. Here, the wavelength of the optical pulse is set to a wavelength that gives negative group velocity dispersion in the optical fiber 3. An optical soliton can be created by balancing the self- and self-phase modulation effects and negative group velocity dispersion within the optical fiber 3. Usually, a mode-locked laser capable of generating short coherent optical pulses is used as the high-output light source 1. However, since the repetition period of a mode-locked laser pulse is determined by the resonator length of the laser, it has been difficult to generate a high-speed pulse train of several GHz or more.

A second method using the soliton laser described above is the Raman soliton laser method (tl, A, Haus
andM, Nakazawa, “Theory of
the fiber Raman 5oli-ton
1aser” J, Opt, Soc, Amer,,B
4. [152 (1987))] and a parametric soliton laser system (Japanese Patent Application No. 1987-1945).

Both methods involve inserting an optical fiber that provides negative group velocity dispersion into a laser resonator, creating an optical soliton effect within the optical fiber, and obtaining steady laser oscillation. However, all of these methods use a mode-locked laser, so although pulses of lps or less can be easily obtained, the pulse repetition is limited to 1
It was difficult to make it higher than 0 GHz.

Further, the configuration of the third method described above is shown in FIG. In this figure, 4 is a semiconductor laser, 5 is a laser driving pulse generator, 2 is a coupling lens, and 3 is a single mode optical fiber. The laser driving pulse generator 2 generates a current pulse with a pulse width of 100 ps and a peak current of about 2 A, and this current pulse drives the semiconductor laser 4 to WIA! J
JI. As shown in FIG. 7(A), the semiconductor laser 4 generates a light pulse with a half width of 27 ps and a peak intensity of 40 mW. When this optical pulse is coupled through the coupling lens 2 to a single mode optical fiber 3 having a negative group velocity dispersion at the oscillation wavelength of the semiconductor laser 4, the optical pulse becomes A soliton is generated.

The pulse intensity PNI11 necessary for the soliton to propagate within the optical fiber 3 is expressed by the following equation (1).

Here, E is the wavelength of the soliton, C is the speed of light + n2 is the nonlinear refractive index, D is the group velocity dispersion, τ is the pulse width of the soliton, and 80 is the effective cross-sectional area.

For example, wavelength λ = 1.55 μm, pulse width = 27p
s light pulse is D-5 ps/km/r+I11. 8a
When propagating through an optical fiber of rt-4X 10-"m2, solitons are generated in the optical fiber 3 by the optical pulse of the semiconductor laser 4 using the above equation (1) at a peak intensity of 8 mW. However, after the optical pulse propagates through the optical fiber 3 with a length of 27?o, the waveform becomes as shown in Fig. 7(B).Optical fiber 3
At the output of
It can be seen that the pulse generated by the semiconductor laser 4 does not propagate to the end of the optical fiber 3 as a soliton with a narrow pulse width.

This phenomenon of widening the pulse width is caused by the semiconductor laser 4
This phenomenon is observed not only when using a single longitudinal mode semiconductor laser such as a Fabry-Bello type but also a DFB (distributed return type) or a DBR (distributed black reflection) type. This phenomenon is caused by the following causes. That is, when a current is injected into the semiconductor laser 4, the refractive index of the active layer changes over time according to the change in carrier concentration, and as a result, the oscillation wavelength of the semiconductor laser 4 changes over time ( Therefore, for example, during CW (continuous wave) operation, the half-width is 100 MHz, as shown in Figure 8 (^).
When the semiconductor laser 4, which had the following wavelength, is driven by the pulse generator 5, it has a wavelength width of 1.2 nm due to the chirping described above, as shown in FIG. 8(B). At this time, the product of the pulse width and frequency width of the optical pulse generated from the semiconductor laser 4 is 4.5, and both do not satisfy the conditions for Fourier transform. As described above, since the wavelength width of the pulse is wide, it is impossible to cancel out the pulse broadening caused by the group velocity dispersion of the optical fiber 3 due to nonlinearity. Furthermore, the chirping of the semiconductor laser 4 tends to increase as the pulse width is narrowed and the peak output is increased, which is a major problem when performing large-capacity, long-distance soliton communication. .

Furthermore, as shown in equation (1) above, when the pulse width τ is narrowed to increase the transmission speed, the pulse power P required to form a soliton is inversely proportional to the square of the pulse width τ.
. 1 increases, but with the capabilities of current semiconductor lasers, there is a limit to the power that can be extracted.

On the other hand, there is a method that uses optical amplification to obtain the power necessary to form solitons. Conventional optical amplification methods include amplification using a semiconductor laser, and methods using nonlinear optical effects in optical fibers such as stimulated Raman scattering and single-pass conduction four-photon mixing. However, since gain saturation occurs in amplifiers using semiconductor lasers at a power of 1 mW or less, there is a limit to the power that can be extracted, and there is a drawback that solitons cannot be formed depending on the optical fiber used for transmission. In addition, in optical amplification using nonlinear optical effects in optical fibers, a pumping power greater than that of IW is required to obtain a gain of 10dll, and in order to achieve this, large lasers such as YAG lasers are required. This made it difficult to downsize.

The main problems to be solved with the conventional techniques described above are summarized as follows.

■ In the conventional direct modulation method using a gain switch, the laser is driven with a large-amplitude current pulse in order to satisfy the pulse width and intensity required to generate an optical soliton, resulting in deviation from the transform limit condition. However, it was difficult to apply this method to optical soliton communication because only small pulses could be obtained.

■ Furthermore, optical soliton generation methods using mode-locked lasers or soliton lasers have a limit on the pulse repetition frequency, which limits the transmission capacity of soliton communication.

■ Furthermore, as a method to amplify small amplitude pulse trains,
Conventionally, there are optical amplification methods using amplifiers using semiconductor lasers and nonlinear optical effects such as stimulated Raman scattering and stimulated four-photon mixing, but it is difficult to obtain a gain of 1 odB or more, and the maximum that can be extracted is difficult. Beak power was low.

In view of the above-mentioned points, an object of the present invention is to solve the problems of chirping and insufficient optical output when applying a semiconductor laser to a light source for soliton communication, and to generate a soliton generation method using a small and lightweight device. Our mission is to provide
The goal is to provide a light source for future ultrafast soliton communications.

[Means for Solving the Problem] In order to achieve such an object, the present invention has the following features: 1.52
It has a single longitudinal mode semiconductor laser having an oscillation wavelength in the range from μm to 1.57 μm, and a modulation means that amplitude modulates the output of the semiconductor laser to form a pulse train optical signal, and the optical signal is pulsed by the modulation means. and erbium (
The light No. 42 amplified by the single-mode optical fiber is transferred to a single-mode optical fiber having a negative group velocity dispersion at the oscillation wavelength of the semiconductor laser. It is characterized by generating optical solitons by combining.

Further, in one aspect of the present invention, the modulation means directly modulates the current of a single longitudinal mode semiconductor laser,
It is characterized by forming a transform-limited pulse train optical signal by inputting the output of a semiconductor laser into an optical fiber having negative group velocity dispersion.

Further, in another aspect of the invention, the single longitudinal mode semiconductor laser continuously oscillates laser light, and the modulation means is an external modulator that vibrationally modulates the continuous light output from the semiconductor laser to form a pulse train optical signal. It is characterized by

[Function] As in the above configuration, the wooden invention generates an ultrafast pulse train with a transform limit by directly modulating the optical output of a single longitudinal mode semiconductor laser with a current or intensity modulating it using an external modulator. The most important feature is that this pulse train is amplified to the intensity at which optical solitons are generated using an optical fiber whose core is doped with erbium (Er) to generate optical solitons. At this time, it is an important condition for realizing the present invention that the pulse width and frequency width of the pulse train before amplification satisfy the conditions given by Fourier transform.

As described above, the present invention solves the problem of chirping and insufficient light output when applying a semiconductor laser to a soliton light source by adding Er (erbium) to the core part of the light source. This problem was solved by amplifying with an optical fiber to obtain high-output ultrashort pulses with high spectral purity. Therefore, according to the present invention, optical solitons can be generated using a device that is simple in structure, small in size, and lightweight.

Note that amplifiers have traditionally used semiconductor lasers.

There are optical amplification methods that amplify small-amplitude pulse trains using nonlinear optical effects such as 8-guide Raman scattering and 4-photon mixing, but compared to these conventional methods, the The method of the invention is significantly different in that a gain of 10 dB or more can be easily obtained by using a continuous wave semiconductor laser for pumping, and the maximum peak power that can be extracted is large.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

A. First embodiment FIG. 5 is a diagram illustrating the first embodiment of the present invention.

In this figure, 4 is a semiconductor laser, and 5 is a semiconductor laser 4.
6. A pulse generator for laser driving that directly modulates the current.
6' is a coupling lens, 7 is a dichroic mirror, 8
is a light source for exciting an Er-doped optical fiber. 2 is a coupling lens; 9 is a single-mode E-doped optical fiber doped with erbium (Er) in the core; 3 is a single-mode soliton having negative group velocity dispersion at the oscillation wavelength of the semiconductor laser 4; It is an optical fiber for transmission.The semiconductor laser 4 is a DFB or DBTt type single longitudinal mode semiconductor laser, and its oscillation wavelength is 1.52 to 1.57μ.
Use one between m.

To operate the apparatus shown in FIG. 1, first, the semiconductor laser 4 is directly current-modulated with pulses from the laser driving pulse generator 2 to generate a signal pulse train (optical signal). In this way, when the semiconductor laser 4 is directly modulated and a pulse of ]Ops is generated, the spectral width is 1.0 mW at the peak output of 40 mW.
20 m and a peak output of 5 mW, it becomes 0.15 nm, and as the drive current of the semiconductor laser 4 is lowered, the spectrum broadening due to chirping decreases. Therefore, if the drive current of the semiconductor laser 4 is adjusted so that the peak output is 0.1 mW, a pulse train with a spectrum width of 0.80 m and an approximately transform limit can be obtained from the semiconductor laser 4.

The optical pulse train obtained by the semiconductor laser 4 (for example, 0.8
(with a spectral width of ηm) is coupled to an E-doped optical fiber 9 through a coupling lens 6, a dichroic mirror 7, and a coupling lens 2.The E-doped optical fiber 9 is excited by excitation light from an excitation light source 8. As the excitation light #8, as already proposed by Kijun inventor Moku et al. (Japanese Patent Application No. 53-311910),
When the Er-doped optical fiber 9 is pumped using a Fabry-Bello type high-power 1 nGa 8sP semiconductor laser having an oscillation wavelength of 1.49 μm, the pulse can be efficiently amplified. Note that the dichroic mirror 7 can also be replaced with a fiber type multiplexer. Further, in order to reduce the polarization dependence of the amplification factor, the excitation light beam 8 was arranged and used so that the polarization directions of the two semiconductor lasers were orthogonal to each other.

With the Er-topped optical fiber 9, a gain of about 20 dB can be easily obtained with a pumping power of about 100 mW.
For human power of an optical pulse train with a peak intensity of 0.1 mW,
A pulse train with a peak output of 10 mW is obtained. In this way, when a pulse train sufficiently amplified using the Er-doped optical fiber 9 is input into the single mode optical fiber 3 having anomalous dispersion at the wavelength of the signal light, a soliton pulse train can be propagated. The optical fiber 3 has group velocity dispersion D.
-5ps/km/nm effective cross section Aart-4X 10
-11 m2, the power PHI11 required to form a soliton is 8 mW (see equation (1)).

FIGS. 2A to 2C show how optical solitons are formed in this example by changes in pulse waveforms.

The broken line in this figure is the power PN required to form a soliton.
! +1 level (light intensity) is shown. FIG. 2(A) shows the output pulse train of the semiconductor laser 4, and its peak output is P88. much smaller than. Figure 2 (B) shows ErF
- This is the waveform of the optical pulse train after being amplified by the optical fiber 9, and its peak output is much higher than that of PNII+.
It can be seen that the intensity has been amplified to be sufficient to excite the fundamental soliton. FIG. 2(C) shows the waveform after propagation through the soliton transmission optical fiber 3 having a length of 27 krn, and it can be seen that the pulse train propagates within the optical fiber as an optical soliton.

In order to increase the transmission capacity of optical communications using optical solitons, it is necessary to narrow the soliton pulse width. The power PN = 1 for forming an optical soliton increases in inverse proportion to the square of the pulse width, as shown in the above-mentioned equation (11). It is necessary to increase the amplification degree of the amplifier.In order to achieve this using the method of this embodiment, the length of the Er-tope optical fiber 9 is increased and the pumping light power of the pumping light source 8 is increased. good.

In addition, a coupling lens 6.6', a dichroic mirror 7
, coupling lens 2, E'doped optical fiber 9, excitation light source 8
The necessary power can also be obtained by inserting one or two more stages of optical amplification sections in series.

The optical amplifier using the Er-doped optical fiber 9 has a gain bandwidth large enough to amplify soliton pulses with a pulse width of 1ps or less without causing pulse broadening, so it has a transmission capacity of 1oOGbit/s or more. The method of this embodiment can also be applied to communication of soliton pulse trains.

As is clear from the above description, in this embodiment, it is possible to generate optical solitons in a simpler way than in the conventional technology, and since the only light source used is a semiconductor laser 4.8, it is possible to It becomes possible to generate optical solitons with the device.

B. Second embodiment FIG. 3 is a diagram illustrating a second embodiment of the present invention.

In this figure, 4 is a signal semiconductor laser that continuously oscillates laser light, IO is a light intensity modulator that amplitude modulates the continuous light output from this semiconductor laser 4 to form a pulse train optical signal, and II is a light intensity modulator. This is a modulator drive circuit that drives the modulator 10. The semiconductor laser 4 is a DFB or DBR type single longitudinal mode laser, and its oscillation wavelength is 1.
A material between 52 and 1.57 μm is used. Other constituent elements are the same as those in the embodiment shown in FIG. 1, so their explanation will be omitted.

To operate the device shown in FIG. 3, first the semiconductor laser 4 is
The cw oscillation output is modulated by the optical intensity modulator 10. The optical intensity modulator 10 includes a drive circuit 1
It is driven by a drive circuit 11 in accordance with a human-powered electric signal pulse train supplied to the drive circuit 11. As the optical intensity modulator lO,
A waveguide type modulator using an electro-optic effect, a bulk type modulator using an electro-optic effect, an optical modulator using an acousto-optic effect, etc. can be used. In particular, waveguide modulators are
Since there are some whose modulation band exceeds 10GIIz,
By using this, an ultrahigh-speed pulse train exceeding 10 Gb/s can be easily obtained.

FIG. 4(8) shows the output waveform of the semiconductor laser 4, and FIG. 4(It) shows the waveform of the optical pulse train after being modulated by the optical intensity modulator IO. Since the spread of the spectrum of the pulse after being modulated by the optical intensity modulator 10 is determined by the bandwidth of the modulated signal, it is possible to obtain a pulse with a higher transform limit than in the first embodiment of the present invention. can.

Next, in order to form a soliton within the soliton transmission optical fiber 3, a coupling lens 6,6', an excitation light source 8, a dichroic mirror 7, a coupling lens 2, and an Er-tope optical fiber 9 are combined. The output pulse of the optical intensity modulator 10 is amplified by the optical amplification section. Figure 4 (C) shows the waveform of the optical pulse train after it has been amplified by this optical amplification section, and it can be seen from this figure that the pulse intensity has been amplified enough to propagate an N=1 soliton. . FIG. 4(D) shows the pulse waveform after propagating through the soliton transmission optical fiber 3, and FIG.
) is confirmed to propagate to the end of the optical fiber 3 as an optical soliton.

[Effects of the Invention] As explained above, according to the present invention, an ultrahigh-speed pulse train with a transform limit is generated by direct modulation or external modulation of a semiconductor laser, and this pulse train is applied to a semiconductor laser whose core portion is doped with Er. Excitation optical fiber (E
Since optical solitons are generated by amplifying the intensity to a sufficient level to form optical solitons using an r-doped optical fiber, the configuration is simple, and ultra-high speed can be achieved using small and lightweight equipment. The effect of generating optical solitons suitable for optical transmission A3 can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram explaining the arrangement of the first embodiment of the present invention, FIGS. 2(A) to (C) are waveform diagrams showing pulse waveforms of each part in FIG. 1, and FIG. A schematic diagram illustrating the arrangement of the second embodiment of the invention, Figures 4 (8) to (D) are waveform diagrams showing pulse waveforms of each part in Figure 3, and Figure 5 is a conventional high-power laser Figure 6 is a schematic diagram illustrating a short optical pulse generation method using a gain switch of a conventional semiconductor laser. 7(A) is a waveform diagram showing the output pulse I waveform of a semiconductor body laser, FIG. 7(B) is a waveform diagram of the pulse after fiber propagation, and FIG. ^), (B) is a waveform diagram showing the change due to direct modulation of the spectrum of the semiconductor laser in Fig. 6.
- High output light source, 2... Coupling lens, 3... Single mode optical fiber, 4... Semiconductor laser, 5... Pulse generator for driving semiconductor laser, 6.6'
... Coupling lens, 7... Dichroic mirror 8... Light source for excitation of Er tope optical fiber, 9... E
"Taupe optical fiber, 1O...light intensity modulator, 11...modulator drive circuit. Patent applicant: Nippon Telegraph and Telephone Corporation agent, patent attorney Yoshiru Tani. Straight ice fl = 3, juice, ru ° 8' coma 0 no

Claims (1)

[Claims] 1) A single longitudinal mode semiconductor laser having an oscillation wavelength in the range from 1.52 μm to 1.57 μm, and modulation means for amplitude modulating the output of the semiconductor laser to form a pulse train optical signal. and amplifying the optical signal pulsed by the modulation means using a single mode optical fiber doped with erbium (Er) in its core, and amplifying the optical signal amplified by the single mode optical fiber. 1. A soliton generation method comprising generating an optical soliton by coupling a semiconductor laser to a single mode optical fiber having a negative group velocity dispersion at an oscillation wavelength. 2) In claim 1, the modulating means directly modulates the current of the single longitudinal mode semiconductor laser, and inputs the output of the semiconductor laser into the optical fiber having negative group velocity dispersion. A soliton generation method characterized by forming a transform-limited pulse train optical signal. 3) In claim 1, the single longitudinal mode semiconductor laser continuously oscillates a laser beam, and the modulating means vibrationally modulates the continuous light output from the semiconductor laser to form the optical signal of the pulse train. A soliton generation method characterized by being a container.