CN114401047A - Chaos bandwidth expansion method based on optical fiber modulation instability - Google Patents
Chaos bandwidth expansion method based on optical fiber modulation instability Download PDFInfo
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- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2543—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to fibre non-linearities, e.g. Kerr effect
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Abstract
The invention discloses a chaotic bandwidth expansion method based on optical fiber modulation instability, which specifically comprises the following steps: firstly, the output light of a semiconductor laser is coupled to a grating array through a section of optical fiber to realize frequency spectrum cutting and reflection; then, the reflected light is fed back and injected into the laser through the polarization controller, and the chaotic seed signal is generated under the specific feedback condition; and then, the chaotic seed signal enters a chaotic spread spectrum module at the rear end through a coupler and an isolator, and the optical fiber modulation instability is excited under the condition of specific parameters, so that the chaotic frequency spectrum is flattened and expanded. The invention can realize the generation of chaos, is compatible with the cutting and the expansion of frequency spectrum, and can realize the increase of chaos bandwidth by several times to dozens of times only by adjusting control parameters on the premise of not increasing additional cost.
Description
Technical Field
The invention belongs to the technical field of chaotic frequency spectrum expansion, and particularly relates to a chaotic bandwidth expansion method based on optical fiber modulation instability.
Background
With the increasing demand for communication bandwidth, optical fiber communication becomes the underlying physical foundation of information networks due to its inherently large transmission bandwidth. As a host light source, semiconductor lasers play an important role in broadband fiber optic communication. For a semiconductor laser, traditional research is always dedicated to maintaining a free-running light-emitting state of the semiconductor laser and avoiding nonlinear jitter; with the research, the nonlinear dynamics of semiconductor lasers are gradually emphasized, and the new corner is exposed in some emerging applications. Unlike the free-emitting state, the nonlinear dynamics of a semiconductor laser can be excited by introducing external perturbations, and the simplest, classical way is external optical feedback. Therefore, the research and application of the nonlinear dynamics of the optical feedback semiconductor laser are widely concerned at home and abroad. By introducing external optical feedback, the semiconductor laser follows a quasi-periodic chaotic mode and gradually presents: stable locking, periodic oscillation, quasi-periodic oscillation and finally entering a chaotic state. The chaotic state is particularly concerned due to unique properties of random waveform, wide spectrum, high dimensionality, synchronicity and the like, and accordingly, the optical feedback chaotic laser is widely introduced into various hot research fields of laser radar, secret communication, high-speed physical random number generation, dynamic Brillouin grating sensing, secure key distribution, photon reserve pool calculation and the like.
Under external optical feedback, the nonlinear dynamics of semiconductor lasers are dominated by two fundamental time scales: external cavity feedback delay and laser relaxation oscillation period. The former determines the resonant mode of the external feedback cavity of the laser, while the latter limits the speed of photon and carrier energy exchange inside the laser. In the application based on the laser chaos, the chaos bandwidth directly corresponds to key indexes such as resolution in chaos ranging, data rate in chaos secret communication, generation rate of a safety key and the like. However, the chaotic bandwidth is limited by relaxation oscillation (usually about 10 GHz) of the laser, and has gradually become a key factor for limiting the performance of the chaotic laser, which is a difficult problem to be solved urgently in the development and application of the chaotic laser technology.
In the conventional chaotic spectrum spreading scheme, a multiple laser cascade scheme is usually adopted to realize aliasing and spreading of the chaotic spectrum. The apparatus of the method is shown in FIG. 1. The solution shown in fig. 1 consists essentially of 2 lasers and corresponding intermediate optical cascade devices. The laser 1 is responsible for chaotic seed signal generation, and the laser 2 is responsible for specific nonlinear processing of the chaotic seed, and finally the chaotic signal with expanded bandwidth is obtained at the output end. The implementation steps of the scheme are as follows: firstly, the output light of the laser 1 reaches the port A1 of the circulator 1, then the output light passes through the optical loop from the port B1 to reach the port C1, and then the output light is fed back to the laser 1 from the port A1 to form the chaotic seed signal generating device, and the polarization controller 1 is used for adjusting the polarization of the feedback light to be the same as the polarization of the output light of the laser 1; then, the feedback parameters (including feedback intensity and feedback time delay) of the laser 1 are controlled to enable the laser to output chaotic seed signals, a coupler divides partial seed signals, the chaotic seed signals are injected into the laser 2 through ports A2 and B2 of the circulator 2, and the polarization controller 2 is used for adjusting the polarization of the injected light to be the same as the polarization of the output light of the laser 2; finally, the dynamics control parameters (including injection intensity and injection frequency offset) of the laser 2 are debugged, so that the chaos seed signal injected from the outside and the self resonance signal of the laser 2 generate a four-wave mixing effect, the bandwidth is expanded through frequency spectrum aliasing, and the spread spectrum chaos signal is obtained at the output port.
The spectrum spreading mechanism of the prior art solution is to use the non-linear effect of the additional lasers to achieve the spectral overlap. However, the frequency response bandwidth of the laser participating in the nonlinear processing is also limited by the relaxation oscillation, so that the scheme does not essentially break through the limitation of the relaxation oscillation, and the spectrum spreading effect is approximately proportional to the number of the lasers, thereby inevitably bringing about the multiplication of the cost.
Interpretation of terms:
feedback strength: the ratio of the feedback optical power to the internal optical power of the laser.
Feedback delay: the light travels one round trip in the external feedback cavity of the laser for the time required.
Injection strength: the ratio of injected optical power to the optical power inside the laser.
Injecting frequency deviation: the frequency difference between the injected light and the light inside the laser.
Relaxation oscillation: relaxation oscillation is one of the basic characteristics of semiconductor lasers, and is a representation of photon interaction in a resonant cavity and carrier energy in a gain medium. Relaxation oscillation occurs when the photon dissipation time in the cavity is much less than the carrier dissipation time in the gain medium.
Self-phase modulation: the invention relates to a method for changing the refractive index of optical fiber by the instantaneous change of the amplitude of the transmitted optical signal, and further changing the phase of the signal.
Group velocity dispersion: the derivative of the group velocity of light in the medium to the optical frequency characterizes the correlation of the group velocity of light with its own frequency.
Modulation instability: the present invention refers to the fact that in an optical fiber, a change in the amplitude of the transmitted optical signal causes a frequency chirp by the fiber self-phase modulation effect, which is then converted into a modulation of the amplitude by the fiber dispersion effect.
Disclosure of Invention
In order to overcome the problems, the invention provides a chaotic bandwidth expansion method based on optical fiber modulation instability.
The invention discloses a chaotic bandwidth expansion method based on optical fiber modulation instability, which specifically comprises the following steps:
step 1: the chaotic seed signal is generated and spectrum is pre-shaped based on optical filtering feedback.
The output light of the laser is coupled to the grating array through a section of optical fiber to realize frequency spectrum cutting and reflection; then, the reflected light is fed back to the laser through a polarization controller to realize the generation of chaotic seed signals, and the polarization controller is used for adjusting the polarization of the reflected light to be matched with the polarization of the output light of the laser; adjusting feedback parameters to realize that the laser is in a chaotic dynamic state, and adjusting filter parameters to realize upper triangular spectrum output; and then the chaotic seed signal enters a chaotic spread spectrum module at the rear end through the coupler and the isolator.
Step 2: in the chaotic spread spectrum module, the chaos spectrum flatness and spread are realized by using the optical fiber modulation instability under the action of the optical fiber self-phase modulation and the anomalous dispersion effect.
In the chaotic spread spectrum module, the optical fiber self-phase modulation nonlinear effect is controlled by adjusting the optical power of the injected chaotic seed signal, and the dispersion effect is controlled by adjusting the wavelength of the chaotic laser and the length of the nonlinear optical fiber; the nonlinear and dispersion effects are regulated and controlled to excite the modulation instability of the optical fiber, and the M-type gain spectrum of the optical fiber is matched with the upper triangular spectrum of the chaotic seed signal, so that the energy near the central frequency domain of the input chaotic signal is converted into the edge spectrum region of the chaotic seed signal through the modulation instability of the optical fiber, the spectrum flattening is realized, and the spectrum spreading effect is further achieved.
The feedback parameters comprise feedback strength and feedback time delay, and the filter parameters comprise the center frequency and the reflection bandwidth of each grating.
The beneficial technical effects of the invention are as follows:
the invention provides a functional module combining chaos generation and spectrum pre-shaping, designs a grating array filtering feedback scheme, simultaneously realizes chaos spectrum cutting and optical feedback, and is compatible with chaos generation and spectrum regulation and control. In the chaotic spread spectrum module, M-type gain spectrum response of modulation instability is matched with the chaotic upper triangular spectrum, so that the flatness and the expansion of the spectrum are realized.
Drawings
Fig. 1 is a schematic diagram of a conventional dual-laser cascaded chaotic spread spectrum scheme system.
Fig. 2 is a schematic diagram of chaotic bandwidth expansion based on fiber modulation instability according to the present invention.
Fig. 3 is a graph of modulation instability gain intent based on fiber nonlinearity.
Detailed Description
The invention is described in further detail below with reference to the figures and the detailed description.
The invention discloses a chaotic bandwidth expansion method based on optical fiber modulation instability, which is shown in figure 2 and specifically comprises the following steps:
step 1: the chaotic seed signal is generated and spectrum is pre-shaped based on optical filtering feedback.
The invention provides an array filtering feedback to realize chaotic seed generation, which comprises the following implementation processes: first, the output light of the laser is coupled to the grating array through a section of fiber to achieve spectral clipping and reflection. And then, the reflected light is fed back to the laser through a polarization controller to realize the generation of the chaotic seed signal, and the polarization controller is used for adjusting the polarization of the reflected light to be matched with the polarization of the output light of the laser. The laser can be in a chaotic dynamic state under specific feedback parameters (including feedback intensity and feedback time delay), and the laser chaotic laser can realize upper triangular spectrum output under specific filter parameters (center frequency and reflection bandwidth of each grating). And then the chaotic seed signal enters a chaotic spread spectrum module at the rear end through the coupler and the isolator.
Step 2: in the chaotic spread spectrum module, the chaos spectrum flatness and spread are realized by using the optical fiber modulation instability under the action of the optical fiber self-phase modulation and the anomalous dispersion effect.
The oscillation time scale of the luminous intensity of the chaotic semiconductor laser is usually in the order of tens to hundreds of picoseconds, so the stimulated scattering effect (Brillouin scattering and Raman self-frequency shift) in the optical fiber transmission process can be generally ignored. According to the nonlinear schrodinger equation, the chaotic spreading subsystem can be simplified to be described by:
where d is the transmission distance in the fiber and A (d, t) is the slowly varying optical field in the fiber. The right side of the equation describes the power loss effect, group velocity dispersion effect, and kerr nonlinear effect, respectively, of the fiber. Wherein, alpha is the attenuation coefficient of the optical fiber, beta2Is a group velocity dispersion parameter, gamma, of the optical fiberIs a fiber kerr nonlinear parameter. When beta is2At negative (anomalous dispersion) the transmission of a continuous waveform will experience modulation instability effects. FIG. 3 shows an optical signal with 1W power at 1.55 μm wavelength in a common single mode fiber (. beta.)2＝-20ps2/km,γ＝2W-1/km), the theoretical gain curve of the modulation instability effect experienced when transmitting (without taking into account the power loss).
As can be seen from fig. 3, the modulation instability has an M-type gain spectrum, and can effectively convert the energy of the transmission light (pump light) at the center frequency into the gain of the two side frequency regions. The M-type gain spectrum and the range can be regulated and controlled by adjusting the nonlinear effect and the dispersion effect of the optical fiber. Furthermore, fig. 3 shows that the response spectrum of the fiber modulation instability far exceeds the relaxation oscillation of the semiconductor laser, typically by a factor of several to tens.
In order to realize the expansion of the bandwidth of the chaotic seed signal, the invention provides a chaotic spectrum expansion scheme based on the fiber modulation instability effect, and the specific implementation process of regulating and controlling the gain spectrum of a spectrum expansion module is as follows: on one hand, the power of the chaotic seed signal injected into the nonlinear optical fiber is adjusted through the optical amplifier, so that the nonlinear effect is controlled; on the other hand, the dispersion effect is controlled by adjusting the wavelength of the chaotic laser and the length of the nonlinear optical fiber; the M-type gain spectrum of the chaotic spread spectrum module is matched with the upper triangular spectrum of the chaotic seed signal, so that the energy near the central frequency domain of the input chaotic signal is converted into an edge spectrum region through the modulation instability of the optical fiber, the spectrum planarization is realized, and the spread spectrum effect is further achieved.
In the laser chaotic light source system, a laser is a main component of cost. The traditional chaotic spectrum spreading mechanism relies on the spectrum superposition of a plurality of lasers, and inevitably has the disadvantage that the cost is linearly increased along with the bandwidth. According to the scheme of the invention, the power of the chaotic signal is induced to migrate to the sideband frequency of the chaotic signal by virtue of the optical fiber modulation instability effect so as to realize the flatness of a frequency spectrum and further achieve the purpose of bandwidth expansion. The response bandwidth of the fiber modulation instability far exceeds the relaxation oscillation of the laser, and the chaos bandwidth can be increased by several to tens of times only by adjusting the control parameters on the premise of not increasing extra cost. The invention is innovative from the mechanism, avoids the increase of the number of the lasers along with the increase of the chaotic bandwidth, and has better cost advantage.
Claims (2)
1. A chaos bandwidth expansion method based on optical fiber modulation instability is characterized by comprising the following steps:
step 1: based on optical filtering feedback, chaotic seed signal generation and spectrum pre-shaping are realized;
the output light of the laser is coupled to the grating array through a section of optical fiber to realize frequency spectrum cutting and reflection; then, the reflected light is fed back to the laser through a polarization controller to realize the generation of chaotic seed signals, and the polarization controller is used for adjusting the polarization of the reflected light to be matched with the polarization of the output light of the laser; adjusting feedback parameters to realize that the laser is in a chaotic dynamic state, and adjusting filter parameters to realize upper triangular spectrum output; then the chaotic seed signal enters a chaotic spread spectrum module at the rear end through a coupler and an isolator;
step 2: in the chaotic spread spectrum module, the chaos spectrum flatness and spread are realized by utilizing the optical fiber modulation instability under the action of the optical fiber self-phase modulation and the anomalous dispersion effect;
in the chaotic spread spectrum module, the optical fiber self-phase modulation nonlinear effect is controlled by adjusting the optical power of the injected chaotic seed signal, and the dispersion effect is controlled by adjusting the wavelength of the chaotic laser and the length of the nonlinear optical fiber; the nonlinear and dispersion effects are regulated and controlled to excite the modulation instability of the optical fiber, and the M-type gain spectrum of the optical fiber is matched with the upper triangular spectrum of the chaotic seed signal, so that the energy near the central frequency domain of the input chaotic signal is converted into the edge spectrum region of the chaotic seed signal through the modulation instability of the optical fiber, the spectrum flattening is realized, and the spectrum spreading effect is further achieved.
2. The method of claim 1, wherein the feedback parameters include feedback strength and feedback delay, and the filter parameters include center frequency and reflection bandwidth of each grating.
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