CN102544985A - Optical fiber type terahertz wave generation device and method based on modulation instability - Google Patents

Abstract

The invention discloses an optical fiber type terahertz wave generation device and method based on modulation instability. The terahertz wave generation device based on modulation instability in optical fiber comprises a monomode laser, a phase modulation, a microwave source, an intensity modulator, an optical amplifier, a tunable filter, a photoelectric detector and an antenna. The optical fiber type terahertz wave generation device disclosed by the invention has simple and compact structure, can realize generation of tunable terahertz waves by adjusting the frequency of a microwave drive signal and has the advantages of greatly reducing the bandwidth requirements of the device and saving the cost of a system.

Description

Fiber-type terahertz wave generation device and method based on modulation instability

technical field

The invention relates to optically generated terahertz wave technology in the field of microwave photonics, in particular to a fiber-type terahertz wave generation device and method based on modulation instability.

Background technique

At present, the terahertz frequency band, which is in the microwave frequency band and the light wave frequency band, has attracted the attention of many scientific research institutions and companies. The terahertz wave in the 0.1-0.3THz frequency band has great development potential in high-speed wireless communication, military radar, imaging and other application fields due to its relatively small transmission loss in the atmosphere and relatively high power generation. It is a very popular emerging field. field of study. Since this frequency band is closer to the microwave frequency band, electronic technology is often used to generate terahertz waves, such as electronic lasers, electronic solid-state sources, etc. However, due to the limitation of the electronic bottleneck, the complexity and cost of the electronic system increase significantly with the increase of the output frequency band, which hinders the further development and application of terahertz technology.

Microwave photonics is a research field combining microwave and photonics. Among them, the generation of terahertz waves based on photonics is one of the important research directions. By combining the high bandwidth of the optical frequency band and the advantages of mature optical communication devices, it can significantly improve the situation that the terahertz system is limited by the bandwidth of electronic devices and system cost. At present, the more commonly used optically generated terahertz wave is the use of external modulation technology, using the intensity modulator to generate new coherent optical side bands to achieve optical frequency doubling, and the side band frequency difference can reach the order of terahertz, and then through photoelectric conversion. Terahertz wave signal, the terahertz wave generated in this way has a high signal-to-noise ratio and stable performance. However, the nonlinear response efficiency of a single commercial intensity modulator is limited, and it is generally more suitable for generating first-order optical sidebands to achieve optical frequency doubling. Due to the low conversion efficiency and low frequency multiplication of high-order optical sidebands, the device bandwidth can only be reduced by half. This method is relatively expensive for terahertz systems, which is not conducive to practicality.

Contents of the invention

The technical problem to be solved by the present invention is to provide a fiber-type terahertz wave generation device and method based on modulation instability, which can effectively improve the optical frequency multiplication factor in the optically generated terahertz wave technology, and greatly Reduce system component requirements and reduce system cost.

In order to solve the above technical problems, the technical solution adopted in the present invention is: a fiber-type terahertz wave generating device based on modulation instability, including a single-mode laser, a phase modulator, an intensity modulator, an optical amplifier, a tunable filter, Photodetectors and antennas, single-mode lasers, phase modulators, intensity modulators, and optical amplifiers are connected in sequence, optical amplifiers are connected to tunable filters through single-mode fibers, tunable filters, photodetectors, and antennas are connected in sequence, and the intensity The modulator is connected to a microwave source.

The single-mode laser is a distributed feedback laser, and the optical amplifier is an erbium-doped fiber amplifier.

Corresponding to the above-mentioned device, the present invention also proposes a fiber-type terahertz wave generation method based on modulation instability, which includes the following basic steps:

1) A single-mode laser is used to generate a continuous optical signal with a wavelength of 1550nm, and the spectrum of the optical signal is broadened by the phase modulator.

2) Adopting a microwave source capable of outputting 10-30 GHz frequency, and inputting the microwave frequency signal and the continuous optical signal broadened by the phase modulator into the intensity modulator to drive the intensity modulator to output two Optical signals modulated by double-sidebands of first-order sidebands and center carriers;

3) The optical signal output by the intensity modulator is input to the optical fiber amplifier, and the power is amplified to 0.45W.

2) The output amplified optical signal of the optical fiber amplifier is input to a 5-kilometer single-mode optical fiber for transmission to generate new second to fifth order sidebands, and the frequency interval between the new sidebands produced is equal to Driven microwave frequency.

3) The output signal of the single-mode fiber enters the tunable filter for filtering, and filters out two fifth-order sidebands;

4) beat the obtained optical fifth-order sideband through a photodetector to generate a terahertz wave electrical signal, and emit a terahertz wave from the antenna.

The invention aims at the low efficiency of high-order optical sidebands generated by a single commercial intensity modulator, and it is difficult to realize high-frequency multiplication to generate terahertz waves. After the double-sided band signal output by the intensity modulator is amplified by an optical power amplifier, it is connected to a 5km long Single-mode optical fiber, which uses the modulation instability effect in the optical fiber to generate fifth-order optical sidebands with a signal-to-noise ratio of up to 30dB. Then, the terahertz wave with high signal-to-noise ratio is obtained through the beat frequency of the photodetector and the antenna emission. The structure of the scheme is simple and compact, and tunable terahertz waves can be generated by adjusting the frequency of the microwave driving signal of the intensity modulator, which greatly reduces the bandwidth requirement of the device and saves the cost of the system.

Description of drawings

Fig. 1 is a schematic structural diagram of a fiber-optic terahertz wave generating device based on modulation instability according to an embodiment of the present invention;

in:

1: Distributed feedback laser (DFB-LD); 2: Phase modulator; 3: Intensity modulator; 4: Microwave source; 5: Erbium-doped fiber amplifier (EDFA); 6: Single-mode fiber (SMF); 7: Tunable filter; 8: photodetector (PD); 9: antenna; 10: terahertz wave.

Detailed ways

As shown in Figure 1, the device of an embodiment of the present invention includes a distributed feedback laser 1, a phase modulator 2, an intensity modulator 3, a microwave source 4, an erbium-doped fiber amplifier 5, a single-mode fiber 6, a tunable filter 7, A photodetector 8 and an antenna 9, the feedback laser 1 outputs a continuous optical signal and inputs it to the phase modulator 2, and after the phase modulator widens its continuous light width, it is consistent with the microwave drive signal output by the microwave source 4 And as the input signal of intensity modulator 3, described intensity modulator 3 outputs the optical double sideband signal that contains center carrier and two first-order sidebands, after being amplified by erbium-doped fiber amplifier 5 again, input to single-mode optical fiber (SMF ), the above-mentioned single-mode fiber transmission produces new second-order, third-order, fourth-order, and fifth-order optical signals, and then through the tunable optical filter 7, the output only contains fifth-order optical signals as the input signal of the photodetector 8, The output signal of the photodetector 8 is finally connected to the antenna 9 to output a terahertz wave 10 .

The specific description of each of the above modules is as follows:

A distributed feedback laser 1, used to generate an optical carrier signal with a specified narrow linewidth;

The phase modulator 2 is used to broaden the optical signal with a narrow linewidth, so as to suppress the Brillouin scattering effect in the optical fiber;

The intensity modulator 3 is used to perform double-sideband modulation on the specified optical carrier signal to generate two first-order sideband signals, which are used as modulated signals with unstable modulation in the optical fiber in the next stage;

Microwave source 4: used to generate a microwave source signal with a frequency of tunable from 10 to 30 GHz;

The erbium-doped fiber amplifier 5 is used to amplify the power of the optical double sideband signal;

A single-mode optical fiber 6 is used to generate modulation instability to a first-order sideband signal to generate a higher-order optical sideband;

The tunable filter 7 is used to retain the two optical fifth-order sideband signals generated by modulation instability, and filter out other sidebands and the center carrier optical signal;

The photodetector 8 is used to generate a terahertz electrical signal by beating two fifth-order optical signals;

The antenna 9 transmits the terahertz electrical signal in the form of electromagnetic waves.

Terahertz waves 10, generated terahertz waves.

A single-mode laser 1 is used to generate a continuous optical signal, and a phase modulator 2 is used to widen the line width of the optical signal. A microwave source 4 with a tunable output frequency of 10-30 GHz is adopted, and the continuous optical signal broadened by the phase modulator is input to an intensity modulator to drive the intensity modulator to generate two first-order sidebands and The optical signal modulated by the double sideband of the center carrier; the optical signal output by the intensity modulator is input to the erbium-doped fiber amplifier (EDFA) 5, and the power is amplified to 0.45W. After the output optical signal of the EDFA is amplified, it is input to the single-mode optical fiber 6, and after 5 kilometers of optical fiber transmission, a series of new high-order optical sidebands are generated, and the frequency interval between the new sidebands generated is equal to the driven microwave frequency. The output signal of the single-mode fiber enters the tunable filter 7 for filtering, and only two optical fifth-order sidebands are obtained; the obtained optical fifth-order sidebands are beaten by a photodetector 8 to generate high-frequency terahertz waves An electrical signal, a terahertz wave 10 is emitted from the antenna 9 .

The working principle and process of the present invention are: a continuous optical signal E ₀ =Ecos(ω _c t) is generated by a single-mode laser, and the wavelength is 1550nm. As an optical carrier, the phase modulator first broadens its line width, which can effectively Suppress the Brillouin scattering effect in the optical fiber, and then drive the intensity modulator with the microwave signal E _RF (t) = V _FR cos(ω _RF t), and modulate it on the optical carrier to generate an optical double-sideband modulation signal, that is, an optical Carrier and two first-order sideband optical signals, the expression can be:

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; DSB</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&chi;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&chi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; RF</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; RF</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>$

(1)

表示调制深度。Where ω _c is the frequency of the optical carrier, E represents the amplitude of the optical carrier, ω _RF represents the frequency of the microwave signal, _{10GHz≤ωRF≤30GHz} ,

Indicates the modulation depth.

After the double sideband signal is amplified by the optical amplifier to P ₀ =0.45W, it is input to the single-mode fiber for transmission. Ignoring the loss of the fiber, its propagation in the fiber satisfies the nonlinear Schrödinger equation:

$\mathrm{<span\; class="notranslate">\; i</span>}\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; A</span>}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{<span\; class="notranslate">\; \&PartialD;</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; A</span>}}{{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="T"\; filenames="HDA0000127728080000011.png"\; state="\{\{state\}\}">T</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&gamma;</span>}{<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}$

(2)

Among them, A(z, T) represents the amplitude of the light field envelope, _β2 represents the group velocity dispersion parameter, and γ represents the fiber nonlinear coefficient.

We use the optical carrier signal with frequency ω ₀ as the pump light, and the first-order sidebands of the two frequencies ω _c -ω _RF and ω _c +ω _RF as the probe light, which are in the anomalous dispersion region of the fiber at this time. When When the optical power is greater than the power threshold for modulation instability, new n-order optical sidebands, ω _c -nω _RF and ω _c +nω _RF , n≥2, will be generated according to the law of phase matching and energy conservation. The Akhmediev Breather (AB) solution is the exact analytical solution of the nonlinear Schrödinger equation. After performing Fourier transform on the AB solution, it can be used to represent the change of the optical n-order sideband with the optical fiber transmission distance:

${\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&xi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; ib</span>}\mathrm{<span\; class="notranslate">\; sinh</span>}\mathrm{<span\; class="notranslate">\; b\&xi;</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="p"\; filenames="HDA0000127728080000011.png"\; state="\{\{state\}\}">p</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; cosh</span>}\mathrm{<span\; class="notranslate">\; b\&xi;</span>}}{\sqrt{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="cosh"\; filenames="HDA0000127728080000011.png"\; state="\{\{state\}\}">cosh</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; b\&xi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}}}<span\; class="notranslate">\; \&times;</span>{<span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; cosh</span>}\mathrm{<span\; class="notranslate">\; b\&xi;</span>}<span\; class="notranslate">\; -</span>\sqrt{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="cosh"\; filenames="HDA0000127728080000011.png"\; state="\{\{state\}\}">cosh</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; b\&xi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}}}{\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}}}<span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; no</span>}}$

(3)

b＝[8a(1-2a)^1/2]，p＝2(ω_RF/ω_c)^1/2，L_NL＝(γP₀)^-1，ξ＝z/L_NL表示归一化的传输距离，sinh、cosh分别为双曲正弦和双曲余弦。in,

b=[8a(1-2a) ^1/2 ], p=2(ω _RF /ω _c ) ^1/2 , L _NL =(γP ₀ ) ^-1 , ξ=z/L _NL represents the normalized transmission Distance, sinh, cosh are hyperbolic sine and hyperbolic cosine respectively.

The parameters of the single-mode fiber we use are β ₂ =-21ps ² km ^-1 , γ = 1W ^-1 km ^-1 , z = 5km. It can be obtained by calculation that the fifth-order sideband signal-to-noise ratio caused by modulation instability is greater than 30dB, with a high signal-to-noise ratio. The two fifth-order sidebands are filtered out by the tunable optical filter, and the terahertz wave signal ten times that of the microwave driving signal is generated by the beat frequency of the high-speed photodetector:

E _out ＝μ·A ₅ (ξ)·cos(10ω _RF t)

(4)

where μ represents the response coefficient of the photodetector.

The modulation instability in the optical fiber is the result of the joint action of fiber dispersion and nonlinear effects, which is manifested in the splitting of continuous or quasi-continuous laser light into a series of ultrashort pulse trains. In the abnormal dispersion region, if the optical power is in a sufficiently large state, the nonlinear effect is sufficient. For continuous laser, the frequency will be split, and a series of frequency peaks will be found in the spectrum observation. However, due to the unstable and uncontrollable passive modulation, splitting will be random and difficult to control. If a stronger modulation is introduced before the condition of modulation instability is satisfied, the modulation instability will occur at the existing modulation frequency. In this way, the modulation instability is actively controlled, and a series of frequency peaks with stable frequency intervals are generated. The phases of these frequency peaks are locked. We drive the intensity modulator through a low-frequency microwave signal to generate two first-order sidebands. After the power is amplified by the optical amplifier, it is transmitted through a certain length of single-mode fiber. Due to the unstable modulation in the fiber Sexual effect, the first-order sideband will split multiple high-order sidebands, and the signal-to-noise ratio can reach 30dB when the fifth-order sideband is reached, and the frequency peak of the fifth-order sideband is filtered out through an adjustable optical filter With the beat frequency of the device and the antenna emission, the electromagnetic wave ten times that of the microwave signal can be obtained. Therefore, using this method to generate 0.1-0.3THz terahertz waves only requires 10-30G microwave signals, and the modulator bandwidth is only 30G, which is convenient and practical.

Claims (8)

1. A fiber-type terahertz wave generating device based on modulation instability, comprising a single-mode laser, a phase modulator, a microwave source, an intensity modulator, an optical amplifier, a tunable filter, a photodetector and an antenna, its characteristics The single-mode laser, the phase modulator, the intensity modulator, and the optical amplifier are connected in sequence, the optical amplifier is connected to the tunable filter through a single-mode fiber, the tunable filter, the photodetector, and the antenna are connected in sequence, and the intensity modulator is connected to the microwave source connection. 2. The fiber-type terahertz wave generating device based on modulation instability according to claim 1, wherein the single-mode laser is a distributed feedback laser, and the optical amplifier is an erbium-doped fiber amplifier. 3. A fiber type terahertz wave generation method based on modulation instability, characterized in that the steps of the method are: 1) A continuous optical signal is generated by a single-mode laser and input to a phase modulator to broaden the line width of the continuous optical signal; 2) Inputting the optical signal output by the phase modulator and the microwave signal generated by the microwave source to the intensity modulator to generate a double sideband modulator signal of two first-order sideband signals; 3) inputting the double sideband modulator signal into the optical amplifier and amplifying it for transmission through a single-mode optical fiber to generate two fifth-order sideband signals; 4) filter the fifth-order sideband signal with an adjustable optical harmonic filter, and select two fifth-order sidebands whose frequency difference is ten times that of the microwave signal; 5) The two fifth-order sidebands are beaten by the photodetector to generate a terahertz wave electrical signal, and then the terahertz wave is emitted by the antenna. 4. The fiber-type terahertz wave generation method based on modulation instability according to claim 3, characterized in that, in the step 1), the expression of the optical signal E ₀ is: E ₀ =Ecos(ω _c t) Its wavelength is 1550nm; where ω _c is the frequency of the optical carrier, and E represents the amplitude of the optical carrier. 5. the optical fiber type terahertz wave generation method based on modulation instability according to claim 3, is characterized in that, described step 2) in, the expression of double sideband modulator signal E _DSB is ${\mathrm{E.}}_{\mathrm{DSB}}\left(t\right)=\frac{\mathrm{E.}}{2}\{J_0\left(\mathrm{\&chi;}\right)\cos \left({\mathrm{\&omega;}}_{c}t\right)-J_1\left(\mathrm{\&chi;}\right)[\sin \left(({\mathrm{\&omega;}}_c-{\mathrm{\&omega;}}_{\mathrm{RF}})t\right)+\sin \left(({\mathrm{\&omega;}}_c+{\mathrm{\&omega;}}_{\mathrm{RF}})t\right)\left]\right\};$ Where: ω _RF represents the frequency of the microwave signal, 10GHz≤ω _RF ≤30GHz, Indicates the modulation depth, J ₀ indicates the zero-order Bessel function of the first kind, J ₁ indicates the first-order Bessel function of the first kind, V _RF indicates the voltage amplitude of the microwave driving signal, and V _π indicates the half-wave voltage of the modulator . 6. The fiber-type terahertz wave generation method based on modulation instability according to claim 3, characterized in that, in the step 3), the optical amplifier amplifies the output signal of the intensity modulator to 0.45W. 7 . The method for generating fiber-type terahertz waves based on modulation instability according to claim 3 , characterized in that, in the step 3), the signal-to-noise ratio of the fifth-order sideband signal is >30 dB. 8. The fiber-optic terahertz wave generation method based on modulation instability according to claim 3, characterized in that, in the step 5), the expression of the terahertz wave electrical signal E _out is: E _out =μ ·A ₅ (ξ)·cos(10ω _RF t); where: A _n (ξ) represents the change of optical n-order sideband with the transmission distance of the fiber, ξ represents the normalized transmission distance, μ represents the response of the photodetector coefficient.

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