JPH03214123A - Photosoliton generating method and soliton transmitting method - Google Patents

Abstract

PURPOSE:To allow the broad application to communication using the intensity modulation of light by passing the pulse train of high repetitions obtd. by the direct modulation of a semiconductor laser, converting the pulse train to specific transformable pulses and amplifying the pulses by an erbium fiber amplifier, thereby obtaining photosolitons. CONSTITUTION:The light pulses generated by sinousoidally or impulsively changing the value of the current supplied to the semiconductor laser are passed through the band-bass optical filter limited in the band and are converted to the light pulses having 0.32 to 0.44 product of the pulse width and the spectral width thereof. The light pulses are then photoamplified by using the erbium fiber amplifier and the photosoliton pulse train, the peak output P of which is given by equation I, is obtd. In the formula I, N2 denotes a nonlinear refractive index; lambda denotes the wavelength of the light pulses; tau is the half width of the light pulses; ¦D¦ is the group speed dispersion of the optical fiber to propagate the photosolitons; Aeff denotes the effective sectional area of the fiber to propagate the photosolitons; C denotes the velocity of light. The optical communication is possible in this way.

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention aims to generate optical soliton pulses from a semiconductor laser by filtering them using a simple interferometer, which was previously impossible to generate. and (b) relates to a soliton generation method and soliton transmission method for propagating within an optical fiber.

``Prior Art'' ② FIG. 9 is a diagram for explaining a method of generating optical solitons using a color center laser. In this diagram,
l is the wavelength of 15 μm excited by the mode-locked YAG laser
Color center laser that generates pulses at the tip of the band 2.
2 is a coupling lens, 3 is a soliton transmission fiber (
(single mode fiber), 4 is a photodetector.

Color center laser! The optical pulse generated from the
In order to reach 0 W, a pulse power of several tens of W can be easily excited within the soliton transmission fiber 3. N
The waveform of the fundamental optical soliton represented by =I is S ech(
However, since the output waveform from the color center laser j is expressed as S ech(τ),
Optical soliton pulses are easily generated. Regarding optical solitons, see Masataka Nakazawa, “Nonlinear optics in optical fibers,” Applied Physics Vol. 56, No. 10, P, 1265-P.
+288 (+987) or refer to “Compression of Optical Pulses and Soliton Lasers”, Laser Research, Commentary, Vol. 15, No. 11, P, pp. 861-886 (+987).

The first change in the waveform of the optical soliton obtained by this method is
Shown in Figure 0. In this case, the length of the soliton transmission fiber 3 is 70,011. In addition, the input optical pulse is the 10th
It is as shown in Figure (a). First, as shown in FIG. 3B, when the peak power P is 0.3 W, it is clearly observed that the pulse width of the output pulse is wider than the human-powered optical pulse. Furthermore, as shown in FIG. 13C, it can be seen that when the peak power P is 1.2 W, the waveform of the output pulse is the same as that of the input optical pulse.

That is, it can be seen that in this soliton transmission fiber 3, N=1 solitons are excited at peak power P=1.2W. On the other hand, as shown in Figures (D) and (E), when the peak power P is increased to 5 to 11.4 W, it is clearly seen that higher-order solitons are excited.

(2) Next, FIG. 11 is a diagram for explaining a method of generating short pulses by sinusoidal modulation. In this figure, 5 is a sine wave generator, 6 is an electric amplifier, and 7 is a semiconductor laser. Note that the coupling lens 2 and the soliton transmission fiber 3 are the same as those in the above configuration.

In this method, by sinusoidally modulating the semiconductor laser 7, a pulse train is generated with high repetition rate and guided through the soliton transmission fiber 31.

■ Next, FIG. 12 is a diagram explaining the method of generating short pulses using a comb generator. As shown in this figure, a comb generator 8 is inserted between an electric amplifier 6 and a semiconductor laser 7. ing.

The comb generator 8 drives the semiconductor laser 7 with electric pulses to generate optical pulses at a high repetition rate of several GHz.

(2) Next, FIG. 13 is a diagram for explaining a method of generating pulses by an optical modulator. As shown in this figure, a semiconductor laser 7 is driven with a direct current 9 to extract CW light,
This extracted CW light is I.

The light is modulated by an ultrafast optical modulator 10 using the Stark effect of a 1Nbos or MQW (multilayer quantum well structure) semiconductor. As a result, a repeating pulse train of 5~lOG t(z) can be obtained.

"Problems to be Solved by the Invention" By the way, among the conventional methods mentioned above, the method in item 0 uses the color center laser 1 to generate optical solitons, or the repetition rate is as low as about 100 MHz. However, since it is large and expensive, it is difficult from a practical standpoint.

Furthermore, in the methods (2) and (0), it is not possible to obtain pulses with ideal transform limits. That is, although a pulse of about 10 to 30 ps can be generated, the spectral width is greatly expanded, so that the difference between the pulse width Δτ and the spectral width Δν is about Δτ−1 to 3. This means that ΔνΔ, which is the condition for transform-limited pulses, deviates considerably from -0.32 to 0.44, so the soliton transmission fiber 3
When propagating inside the fiber 3, the pulse spreads due to the group velocity dispersion of the fiber 3, causing a major problem in that information cannot be transmitted.

In addition, in the method (2), the pulse width is wide, 100 ps, and there is no advantage as a soliton.

The present invention has been made in view of the circumstances related to -, and an object of the present invention is to provide an optical soliton generation method and a soliton transmission method that can solve the above-mentioned problems.

[Means for Solving the Problems] The present invention converts a highly repetitive pulse train obtained by direct modulation of a semiconductor laser into a transform-limited pulse by passing it through a Fabry-Perot or Matsuhatsu Interferometric filter. This pulse train is amplified by an erbium fiber amplifier to obtain an optical soliton.

Then, soliton transmission is performed by directly modulating the obtained soliton pulse train.

"Effect -j A high-repetition pulse train is obtained by direct modulation of the semiconductor laser. Then, the spectrum of the obtained pulse train is controlled, and the product of the pulse width Δτ and its spectrum Δν becomes Δ
Pulsing with a transform limit such that τΔC is 0.32 to 0.44 should be performed. By further optically amplifying this, an ultra-high repetition soliton pulse train can be obtained. Soliton transmission is then performed by directly modulating the resulting soliton pulse train.

"Example" Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram for explaining a first embodiment of the present invention. Note that in this figure, parts common to each of FIG. 9, FIG. 1I, and FIG. 12 described above are given the same reference numerals, and the explanation thereof will be omitted. In this figure, 11 is a pulse signal generator, 12 is a 71-air low-pass filter that allows only the fundamental wave component of pulse signal generator II to pass through, and 13 is a narrow band optical filter (for example, a Fabry-Perot interferometer or a Matsuhatsu Ender interferometer, etc.). In the example Fabry-Perot
14 is a semiconductor laser beam,
The optical coupler 16 for combining the optical signal output from the erbium excitation light source 15 is the above-mentioned erbium fiber. 17 is an optical filter for removing spontaneous emission light that removes spontaneous emission noise other than optical pulses of the signal. be.

In the above configuration, first, when a pulse signal is output from the pulse signal generator 11, the electric low-pass filter 12
Only the fundamental wave component is extracted. and,
The fundamental wave component is amplified by the electric amplifier 6 and supplied to the semiconductor laser 7 as a pulse for driving the semiconductor laser. As a result, the semiconductor laser 7 is driven. Here, FIG. 2 shows how the semiconductor laser driving pulses supplied to the semiconductor laser 7 are generated. 3A shows the output waveform of the pulse signal generator 11, and FIG. 1B shows the output waveform of the electric low-pass filter 12.

Further, FIG. 3C shows the output waveform of the electric amplifier 6.

On the other hand, FIG. 3 shows the output of the semiconductor laser 7. The figure (a) shows the spectrum, and the figure (b) shows the time waveform of the pulse. As shown in these figures (a) and (b), in the experiment, the pulse width was Δλ-1,5nn, and the time width was Δτ-2.
4 ps is obtained.

By the way, when electrons are injected into the semiconductor laser 7, the refractive index decreases, so the oscillation frequency of the laser chirps to the longer wavelength side by -2, as shown in FIG. ). Note that a transform-limited pulse has only the width obtained by Fourier transform of the pulse, and the extra width.

In general, a pulse with a spectrum ΔνΔτ is a pulse that does not contain torque. In a semiconductor laser, the waveform is Gaussian, and the distance between the torque width Δν and the pulse width Δτ is ΔλcXΔτ = 0.44... ...There is a relationship of ■λ2. When the Δν·Δτ product of the pulse obtained in FIG. 3 is calculated, it becomes =4.6...■, which is about 10 times as large as the result of the formula (■). Therefore, the pulses obtained by direct modulation are far from the transform limit. This is an essential drawback of direct modulation of semiconductor lasers.

In the present invention, attention is paid to the chirb characteristic of this semiconductor laser, and a Fabry-Perot interferometer 13 is used to transform the spectrum into a transform limit. In this case, in the experiment, the transmission band of the Fabry-Bello interferometer 13 was set to 0.22 nm, and when inserted, it was possible to convert the Fabry-Bello output into a pulse of about 17 ps. Estimating the ΔνΔτ product of this output, we get 0.47...
...■, and this value is very close to the result of the formula ■,
This shows that a transform-limited pulse is obtained. Because it's Chitora and Charb Pulse 0
．． In principle, even with a width of 22r+m, there is a slight chirp, but in order to completely compensate for this chirp, Fabry-Perot
An optical fiber having normal dispersion for negative Chirb compensation may be inserted between the interferometer 13 and the optical coupler I4.

For example, if there is a chirp of 2 ps in the 0.2 nm band, I Ops/nm, but this means that the amount of dispersion of the fiber whose zero dispersion wavelength has been softened to the 1.6 to 1.8 μm band is 50 ps/km/ Using the fact that the length of the optical fiber is 200 m, the length of the optical fiber for this first compensation is preferably set to 200 m.

By inserting the Fabry-Bello interferometer 13 in this manner, a transform-limited pulse can be obtained. However, if left as is, the transmission output will be reduced by about 5 to 10 dB. This is because, firstly, there is a loss due to limiting the spectral width, and secondly, there is a transmission loss of the Fabry-Bello interferometer 13. In the experiment, the Fries vector range of the Fabry-Bello interferometer 13 was set to approximately 61,111. That is, if the fleece vector range is Δλ, then when L=20011rn, flztm...■. Therefore, Distributed Feedba
In the case of ck La5er (DPr3), it is a single spectrum oscillation (including chirb), and its width is at most 2
Since it is approximately r+m, a filter spacing of 6 nm is sufficient.

In other words, other filter components do not enter.

Regarding the band of the Fabry-Perot interferometer 13, the finesse may be adjusted to a band of about 0.1 to 10 m by appropriately setting the reflectance of the mirrors constituting the Fabry-Perot interferometer 13.

By doing the above, a pulse train with a weak pulse output but with a complete transform limit can be produced.

Next, these are optically amplified to the power level of a soliton pulse train. This is performed using an erbium fiber amplifier (a combination of an optical coupler 14, an erbium excitation light source 15, and an erbium fiber 16). here,
The pulse peak power of the standard soliton with N=1 is PN81
Then, p Nfl is...■ In this case, n is the nonlinear refractive index, λ is the wavelength of the optical pulse,
τ is the full width at half maximum of the optical pulse, ID+ is the group velocity dispersion of the soliton transmission fiber 3, Aeff is the effective cross-sectional area of the soliton transmission fiber 3, and C is the speed of light.

The output of the Fabry-Bello interferometer 13 has a peak power of about 1 mW. For example, the standard for the 2I redon transmission fiber 3 that transmits the solenoid is ID j
= 3 ps/km-nm dispersion-shifted fiber and can be set to A erf = 4 Considering the given 4゛, I'') N, , f: J ×4 × 10−+1・9.2 [+1W
...■. However, if there is a power equal to -FJ, a Sech type soliton with N=1 can propagate through the long soliton transmission fiber 3.

Next, the required erbium fiber amplifier filiI
The voice is 2〇, which means that about 10d13 is sufficient based on the input condition of 1mW. In this case, as shown in FIG.
Since a gain of 13 to 1- can be easily obtained in an Erium fiber of about 3 to 100 m, it can be seen that this method is very effective. For details on this amplification, see Masataka Nakazawa, Optics, Vol. 18, No. 6, P291-P, 296 “
Refer to "Light Amplification Using Optical Fiber".

The wavelength of the erbium excitation light source 15 is 05741
+1.0,6I1m, 0.877m, 0.98μm
and 1.48 μm band. Figure 5 shows 1.48μ
m InGaAsP semiconductor laser was used as the excitation light source (7).

Here, noise caused by spontaneous emission light other than the signal light is removed by an optical filter 17 for removing spontaneous emission light.

In addition, as other noise, a small amount of non-soliton components is generated due to the loss in the soliton transmission fiber 3. However, by adding a supersaturated absorber such as an fnGaAs MQW semiconductor to the filter 17 for removing spontaneous emission light, this problem can be solved. - The non-soliton part is completely transparent, and the non-soliton part is completely absorbed, allowing stable soliton transmission. The situation is shown in FIG. It can be seen that noise is completely removed in the figure (a) before passing through the supersaturated absorber, and in figure (b) after passing through the supersaturated absorber.

The pulses thus obtained propagate as solitons through the soliton transmission fiber 3, and then are regenerated and relayed using the pre-emphasis method (Patent Application No. 1-68619, Optical Sonotono Transmission System, Hirokazu Kubota, Masataka Nakazawa, Kazunobu Suzuki). After that, the information is finally extracted by the photodetector 1, and the optical communication using the soliton is completed.

Next, a second embodiment of the present invention will be described.

This second embodiment is based on an ultra-high repetition soliton pulse 5QJ.
This method uses a plurality of optical couplers of 11 couplings to multiplex 2N times the pulse repetition of the semiconductor laser 7 on the time axis. FIG. 7 shows its configuration. The instep and conductor laser 7 are pulse-driven by a sinusoidally modulated electric signal by a pulse signal generator 11 and an electric amplifier 6,
Narrow the spectral components to create a 3dB coupler n.
Lead to 18. When a coupler of N fls is used,
Multiplexing can be performed 2N times on the time axis -L.

Here, in order to generate a time delay and multiplex the pulses, the length of one of the two arms of the connected 3 dB coupler is changed. The difference in the length of the arms of the 3 dB coupler is ゛('-If the repetition period of the conductor laser 7 is shifted from 'r',
Set the time delay to be 'r/2' (i = 1.2·N-1). For example, to provide a time delay of 50 ps, it is sufficient to provide Icm(7)4''. In this way, pulses can be easily multiplexed. However, in this case, passing through N 3di3 couplers reduces the peak intensity of the pulse to l/2, which can be compensated by the erbium fiber amplifier mentioned above.

When multiplexed using this method, signals are turned on and off using ultra-high speed optical modulation BIO. This enables ultrahigh-speed optical soliton communication.

Next, FIG. 8 is a diagram for explaining a third embodiment of the present invention, in which instead of directly modulating the semiconductor laser 7 as in FIG. 7, a LiNbO3 optical modulator or a semiconductor of MQ to V is used. The used absorption type optical modulator 3m1Oa is inserted between the Fabry-Perot interferometer 13 and the optical coupler 14, thereby turning the optical soliton on and off. Although this method requires the use of the absorption type optical modulator 10a, the semiconductor laser 7
This has the advantage that it is not necessary to modulate directly.

"Effects of the Invention" As explained above, generation of high-power soliton pulses with a transfer limit from a semiconductor laser, which was conventionally considered impossible, can be achieved by using a narrowband filter such as a Fabry-Perot interferometer and an erbium fiber amplifier. This has the advantage that it can be widely applied to communications using optical intensity modulation. In other words, L r N , which has been indispensable for high-speed optical communication,
A high speed optical modulator with a bo 3 or MQW semiconductor is no longer required and the semiconductor laser can simply be directly modulated at high speed. Therefore, the optical soliton transmission system has the advantage of being very simple and simple.

If the pulse waveform is disturbed when directly modulating the semiconductor laser with a pulse code, a high-speed optical modulator can be inserted between the narrowband filter and the erbium fiber amplifier. In this case, since the semiconductor laser is not directly modulated, more stable soliton transmission may be possible.

In addition, this method is very effective because it can realize a soliton pulse with a transform limit as long as direct modulation of the semiconductor laser is possible in that frequency band, even if it is repeated at an ultra-high speed of 30 to 100 G I-1z. It is valid.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining the first embodiment of the present invention, and FIG.
The figure is a waveform diagram showing the circuit for driving the semiconductor laser (7) and the output of each part thereof, Figure 3 is the output waveform diagram of the semiconductor laser (7), and Figure 4 is the chirb characteristic of the semiconductor laser (7). 5 is a diagram showing the optical amplification characteristics of an erbium fiber, FIG. 6 is a diagram for explaining a method for removing non-soliton components using a supersaturated absorber, and FIG. 7 is a diagram showing a second embodiment of the present invention. 8 is a diagram for explaining the third embodiment of the present invention, and FIG. 9 is a diagram for explaining the color center laser (
A diagram for explaining the method of generating optical solitons according to 1),
Figure 1θ is a diagram for explaining the waveform change of a soliton generated by the configuration shown in Figure 9, Figure 11 is a diagram for explaining the short pulse generation method using sine wave modulation, and Figure 12 is a diagram for explaining the short pulse generation method using a comb generator. FIG. 13 is a diagram for explaining the short pulse generation method, and FIG. 13 is a diagram for explaining the pulse generation method by the optical modulator. ■・・・Color center laser 2・・・・・・
Coupling lens, 3... Soliton transmission fiber, 4...
・Photodetector, 5...Sine wave generator, 6...
...Electric amplifier, 7... Semiconductor laser, 8...
... Comb generator, 9 ... DC power supply, 10 ... Ultra high-speed optical modulator, 10a ... Absorption type optical modulator, 11 ...... Pulse signal generator,! 2...Electric low-pass filter 13...
...Narrowband optical filter (Fabry-Perot interferometer, etc.), 14...Optical coupler! 5...Light source for erbium excitation, 16...
... Erbium fiber 17 ... Optical filter for removing spontaneous emission light 18 ... 3 dB power puller group.

Claims (4)

[Claims] (1) An optical pulse generated by changing the value of the current supplied to the semiconductor laser sinusoidally or in a pulsed manner is passed through a band-limited narrow band optical filter, so that the product of the pulse width and its spectral width is 0. 32 to 0.44, then the optical pulse is converted to an optical width using an erbium fiber amplifier, and its peak output P is P=0.776×(λ^3/π^2n_2C)× (｜D
|/τ^2)×Aeff(n_2; nonlinear refractive index, λ;
Wavelength of the optical pulse, τ; Full width at half maximum of the optical pulse, |D|; Group velocity dispersion of the optical fiber that propagates the optical soliton, Aef
f; effective cross-sectional area of the fiber that propagates the optical soliton, C
; an optical soliton generation method characterized by obtaining an optical soliton pulse train given at the speed of light). (2) a 3d branching ratio of 1:1 for time division multiplexing between the narrowband optical filter and the erbium fiber amplifier;
2. The optical soliton generation method according to claim 1, wherein a 2^N^-^1 times as many optical soliton pulse train is obtained by consecutively coupling a plurality of B optical couplers and giving a time delay. . (3) Instead of changing the current supplied to the semiconductor laser sinusoidally or in a pulsed manner, by directly supplying a coded pulse signal, the coded optical soliton pulse train is converted to the optical soliton according to claim 1. 1. A soliton transmission method characterized in that an optical soliton pulse train is generated by a generation method, the generated optical soliton pulse train is passed through a long single mode fiber, and further detected by a high-speed photodetector to perform optical communication. (4) The optical soliton pulse train obtained by the optical soliton generation method according to claim 1 is applied to LiNbO_3 or MQW.
(Multi-quantum well structure) An optical soliton pulse train encoded by an absorption type optical intensity modulator using a semiconductor, passed through a long single mode fiber, and further detected by a high-speed photodetector. A soliton transmission method characterized by optical communication using