CN104506237A - Soliton optical transmission system - Google Patents

Abstract

The invention relates to an optical arcon optical transmission system. With the continuous expansion of the power grid business, the transmission rate of the power communication transmission system is getting higher and higher, and the distance is getting longer and longer. With the improvement of the optical fiber manufacturing process, the loss of the optical fiber It is close to the theoretical limit, so fiber dispersion has become a problem to be solved in realizing ultra-large-capacity fiber-optic communication. Since photon communication can adopt various multiplexing technologies such as optical time division, optical polarization division, and optical wavelength division, its transmission code rate has great potential for improvement. Photon is a specific transmission mode of light wave energy. The first-order soliton transmission in an ideal fiber can keep the fiber pulse shape unchanged, which is an ideal way for high-speed and long-distance optical communication. The present invention utilizes the optical soliton for communication and can well solve the problem of dispersion in optical transmission.

Description

A photon light transmission system

technical field

The invention relates to the field of communication, in particular to a photon light transmission system.

Background technique

With the continuous expansion of power grid business, the transmission rate of power communication transmission system is getting higher and higher, and the distance is getting longer and longer.

How to ensure the quality of transmission signals in the case of high-speed and long-distance has become a top priority. In the optical transmission system, the main factors that limit the high-speed and long-distance transmission of optical fiber communication are the attenuation and dispersion of the optical fiber. For conventional linear optical fiber communication systems, the main factors that limit the transmission capacity and distance of the optical fiber are the loss and dispersion of the optical fiber. Dispersion.

With the improvement of optical fiber manufacturing technology, the loss of optical fiber is close to the theoretical limit, so the dispersion of optical fiber has become a problem to be solved in the realization of ultra-large-capacity optical fiber communication. The dispersion of the optical fiber makes the light propagation speeds of different wavelengths in the optical pulse inconsistent, resulting in the broadening of the optical pulse, which limits the transmission capacity and transmission distance. The optical solitons generated by the nonlinearity of the optical fiber can offset the effect of the optical fiber dispersion.

For chromatic dispersion, it is mainly realized by dispersion compensating fiber (DCF) or fiber Bragg grating (FBG) at present. Dispersion compensation using DCF is the most widely used compensation scheme at present. Since the dispersion compensation fiber has a negative dispersion coefficient, it can offset the dispersion caused by the conventional fiber. The structure of DCF is simple and the technology is mature, but the disadvantage is that the effective cross-sectional area of the fiber is reduced, the transmission loss is increased, and the nonlinear effect is affected, which needs to be compensated by the redundant gain of the optical amplifier. Using FBG for dispersion compensation is to make light of different wavelengths go through different transmission paths. The wavelength with fast transmission speed passes through a longer distance in the grating, while the wavelength with slow speed travels through a shorter distance. FBG has the characteristics of low insertion loss and no nonlinear effect, and can be used as an alternative to DCF.

When the signal rate is low, fixed dispersion compensation can be used, but with the increase of the signal rate and the number of system channels, the system will be more sensitive to small changes in some time-varying parameters, such as device aging, routing changes, etc. , so it is necessary to adjust the dispersion compensation of the system. In addition, since the dispersion of the optical fiber varies with the wavelength, attention should also be paid to the residual dispersion problem during dispersion compensation, and dispersion slope compensation is required, which increases the difficulty of compensation.

Contents of the invention

The fundamental difficulty encountered in the development of high-speed and long-distance optical fiber communication is that the optical pulse signal broadening caused by optical fiber dispersion leads to adjacent code crosstalk. Theory and experiments have proved that the intensity modulation for ultra-long-distance transmission directly detects the optical fiber communication system, and the transmission speed is difficult to exceed 10Gbit/s. In fact, since photon communication can adopt multiple multiplexing technologies such as optical time division, optical polarization division, and optical wavelength division, its transmission code rate has great potential for improvement. Photon is a specific transmission method of light wave energy. The first-order soliton transmission in an ideal fiber can keep the fiber pulse shape unchanged, which is an ideal way for high-speed and long-distance optical communication. Since the waveform of high-order solitons changes periodically during transmission, choosing an appropriate fiber length can realize optical pulse compression, which is an effective means to obtain picosecond and femtosecond optical pulses. Therefore, using optical solitons for communication can solve this problem well.

When the light pulse propagates in the optical fiber, when the light intensity is high enough, the light pulse will be narrowed, and the pulse width is less than 1 Ps. This is a phenomenon in nonlinear optics, called the optical soliton phenomenon. If optical solitons are used for communication, the bandwidth of optical fibers can be increased by 10 to 100 times, and the communication distance and speed can be greatly improved.

For conventional linear optical fiber communication systems, the main factor limiting its transmission capacity and distance is the loss and dispersion of the optical fiber. With the improvement of optical fiber manufacturing technology, the loss of optical fiber is close to the theoretical limit, so the dispersion of optical fiber has become a problem to be solved in the realization of ultra-large-capacity optical fiber communication. The dispersion of the optical fiber makes the light propagation speed of different wavelengths in the optical pulse inconsistent, resulting in the broadening of the optical pulse, which limits the transmission capacity and transmission distance. Optical solitons generated by the nonlinearity of optical fibers can counteract the effect of optical fiber dispersion.

The principle of soliton transmission, the formation of solitons is attributed to the nonlinear self-phase modulation of optical fiber and the balance effect of dispersion effect on optical pulses. When the optical pulse with a certain spectral width is transmitted in the dispersion fiber, its different frequency components have different transmission speeds, so the optical pulse width is continuously expanded. If the optical pulse power is strong, there will be a significant nonlinear effect (Kerr effect) when transmitting in the optical fiber, and the transmission characteristics will change significantly. Due to the Kerr effect, the refractive index of the fiber can be expressed as:

n=n ₀ +n ₂ I (1)

In the formula, n ₀ is a constant, and the second item indicates that the refractive index of the optical fiber is related to the transmitted light intensity I, n: is the nonlinear refractive index coefficient, and for the silica optical fiber, n ₂ =3×10 ^-20 m ² /W. When the light pulse travels a distance z in the fiber, a phase shift related to the light intensity occurs

Light pulses have different light intensities at different frequency instants, and thus have different phase shifts. The phase shift caused by the change of the light field itself is called self-phase modulation. The frequency shift caused by self-phase modulation is

$\mathrm{<span\; class="notranslate">\; \&Delta;\&omega;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; z</span>}\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; I</span>}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

light pulse leading edge Δω<o, trailing edge Δω>o. In the main area with strong light intensity, the frequency of the front edge is low and the frequency of the trailing edge is high. In the anomalous dispersion region of the fiber, the dispersion parameter (V _g is the group velocity), the group velocity of the components with low frequency of optical pulse is small, and the group velocity of components with high frequency is large, so the front edge of the pulse becomes slower, the trailing edge becomes faster, and the optical pulse is compressed. When the nonlinear compression effect and dispersion effect are balanced, the optical pulse keeps its waveform unchanged during transmission, which is called the first-order soliton. Since the nonlinear refractive index is related to the light intensity, only when the peak power of the light pulse exceeds a certain value, solitons can be formed.

The theoretical basis of soliton transmission is the nonlinear Schrödinger equation, which can be solved to obtain the transmission characteristics of light pulses in the form of various electric field envelopes in the optical fiber. The stable solution of this equation is the normalized amplitude A=1 double Curve positive secant light pulse, which corresponds to first-order light solitons, while the waveforms of other high-order solitons change periodically during transmission. The determined constant P ₁ satisfies:

$\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; P</span>}}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}}$

P ₁ =0.776λ ³ A _e D/πCn ₂ τ ² (4)

In the formula, λ and τ are the wavelength and pulse width of the optical pulse respectively, and A _e and D are the effective core area and anomalous dispersion of the optical fiber. In a lossless optical fiber, if the peak power of the incident light pulse is P=P ₁ , the first-order solitons can be transmitted without distortion. If there is a loss in the optical fiber, the amplitude of the optical pulse decays exponentially during transmission, the pulse width increases exponentially, and the pulse area remains unchanged. As long as energy is added to the optical pulse, the shape can be restored, thereby realizing long-distance transmission. A lumped erbium-doped fiber amplifier (EDFA) can be used to periodically replenish energy, that is, in the fiber link, a fiber amplifier is set at every distance L _a , and its gain just complements the fiber loss.

The photoarc source is a gain-switched semiconductor laser, modulated with a 2.5GHz sinusoidal current, and dechirped through an FP spectral window to obtain near-conversion-limited optical pulses with a wavelength of 1.553 μm, a pulse width of 18 ps, and a time-bandwidth product of 0.324. The light pulse is injected into 21km dispersion-shifted fiber (DSF) after being amplified by two fiber amplifiers. EDFA ₁ and EDFA ₂ are pumped by 1480nm and 980nm LD respectively, and the pump light is injected into the erbium-doped fiber through a wavelength division multiplexing coupler. Under the action of the pump light, ^{the 4} I _15/2 and ⁴ I of Er ⁺³ ions The particle number inversion is formed between the _13/2 energy levels, which can amplify the 1550nm signal light. DSF consists of three sections of anomalous dispersion (D>0) optical fiber fusion splicing, the average dispersion D=2.4ps/km·nm, and the total loss (including solder joints and two optical fiber joints) is 7dB.

EDFA is mainly composed of a section of erbium-doped optical fiber (about 10-30m long) and a pumping light source. A conversion medium loads light energy on the input optical signal. The optical signal enters the unit with a certain power, and the power increases when it goes out. The optical isolator filters out the unnecessary reflected signal.

Both the width of the incident light pulse and the output light pulse transmitted through the optical fiber are measured by a second harmonic intensity autocorrelator (SHG). The basic method for judging whether the optical soliton transmission is realized is to compare the width of the optical fiber input and output optical pulses. If the output optical pulse width τ _out ≤ the width τ _in of the cluster input optical pulses, the optical soliton transmission is realized.

Description of drawings

Fig. 1 is the structural representation of photon light transmission system of the present invention;

Fig. 2 is a schematic diagram of optical pulse waveforms of different distances transmitted by the optical arcon optical transmission system of the present invention.

Detailed ways

The photon light transmission system of the present invention mainly consists of power supply (AC, DC), photon source, spectral window (FP), controller, coupler, optical amplifier (EDFA ₁ , EDFA ₂ ), dispersion-shifted optical fiber, power Composed of a meter and an autocorrelator, the photon source is a gain-switched semiconductor laser (DFB), modulated with a 2.5GHz sinusoidal current, and chirped through the FP spectral window, to obtain a wavelength of 1.553μm, a pulse width of 18ps, and a time-bandwidth product of 0.324 for near-transform-limited light pulses. The light pulse is amplified by two fiber amplifiers EDFA ₁ and EDFA ₂ and injected into 21km dispersion-shifted fiber (DSF). EDFA ₁ and EDFA ₂ are pumped by 1480nm and 980nm LD respectively, and the pump light is injected into the erbium-doped fiber through a wavelength division multiplexing coupler. Under the action of the pump light, ^{the 4} I _15/2 and ⁴ I of Er ⁺³ ions The particle number inversion is formed between the _13/2 energy levels, which can amplify the 1550nm signal light. DSF consists of three sections of anomalous dispersion (D>0) optical fiber fusion splicing, the average dispersion D=2.4ps/km·nm, and the total loss (including solder joints and two optical fiber joints) is 7dB.

Both the width of the incident light pulse and the output light pulse transmitted through the optical fiber are measured by a second harmonic intensity autocorrelator (SHG). The basic method for judging whether the optical soliton transmission is realized is to compare the width of the optical fiber input and output optical pulses. If the output optical pulse width τ _out ≤ the width τ _in of the cluster input optical pulses, the optical soliton transmission is realized.

It is calculated that when the average power of the incident light is small, the nonlinear self-phase modulation effect of the fiber is extremely weak and the dispersion effect of the fiber is dominant, and the light pulse is broadened from 18Ps to 27.ZPs when it is incident. Since the self-phase modulation effect is enhanced with the increase of the incident light power, when the average power is 1.smw, the self-phase modulation effect and the dispersion modulation effect are just compensated, and the fiber output is equivalent to the input pulse width, forming a first-order light soliton transmission. When the input power is greater than 1.smw, the high-order soliton effect will narrow the pulse width. When the average power is 6.Zmw, the pulse width is compressed to 4.sps, and the compression ratio is 3.75.

In order to analyze the characteristics of the transmission system of the gain-switched DFB laser as the source of solitons and whether the near-conversion-limited optical pulses obtained after dechirping can be used for long-distance soliton transmission, we use the nonlinear Schrödinger equation The transmission of optical pulses in 5000km lossy fiber after passing through FP spectral window is simulated and calculated. An amplifier (EDFA) is set every 25km in the transmission fiber to compensate for the loss. The optical pulse waveforms for different distances are shown in Figure 2. It can be seen that the above optical pulses can be transmitted stably for 5000km. It can be shown that after the gain-switched semiconductor laser is detuned, it can not only realize short-distance photon transmission, but also realize long-distance stable transmission of thousands of kilometers when used as a photon source.

Claims (3)

1. A photon light transmission system, characterized in that: by power supply (AC, DC), photon source, spectral window (FP), controller, coupler, optical amplifier (EDFA ₁ and EDFA ₂ ), Composed of dispersion-shifted optical fiber, power meter and autocorrelator, the soliton source is a gain-switched semiconductor laser (DFB), modulated by a 2.5GHz sinusoidal current, and chirped through the FP spectral window to obtain a wavelength of 1.553μm and a pulse width of 18ps , a near-transform-limited light pulse with a time-bandwidth product of 0.324; _The light pulse is amplified by two fiber amplifiers EDFA ₁ and EDFA ₂ and injected into 21km dispersion-shifted fiber (DSF) _. Under the action of pump light, the erbium optical fiber forms a population inversion between ^{the 4} I _15/2 and ⁴ I _13/2 energy levels of Er ⁺³ ions, which can amplify the 1550nm signal light, and the DSF consists of three sections of anomalous dispersion (D>0) composed of optical fiber fusion, the average dispersion D=2.4ps/km·nm, the total loss is 7dB; Both the width of the incident light pulse and the output light pulse after transmission through the optical fiber are measured by the second harmonic intensity autocorrelator (SHG). If the width τ _{out of} the output light pulse ≤ the width τ _in of the input light pulse of the cluster, then the transmission of light solitons is realized. 2. A kind of photon light transmission system according to claim 1, characterized in that: there is a loss in the dispersion-shifted optical fiber, the amplitude of the optical pulse is exponentially attenuated during transmission, the pulse width is exponentially increased, and the pulse area remains unchanged. The shape can be restored by adding energy to the optical pulse, so as to realize long-distance transmission. The lumped erbium-doped fiber optical amplifier (EDFA) can be used to periodically replenish energy, and an optical fiber amplifier is set every distance La in the optical fiber link. The gain just complements the fiber loss. 3. A photon light transmission system according to claim 1, characterized in that: the optical amplifier is composed of a section of erbium-doped optical fiber (about 10-30m in length) and a pumping light source.