CN101873172B - Millimeter wave generating device based on optic-fiber ring resonator and method thereof - Google Patents

Abstract

The invention discloses a millimeter wave generating device based on an optical fiber ring resonant cavity and a method thereof. Semiconductor optical amplifier, 1×2 polarization maintaining optical coupler, Mach-Zehnder intensity modulator, three-port polarization maintaining optical circulator, polarization maintaining optical fiber The output of the optical fiber ring resonator obtained from the other output end of the polarizing coupler is connected to the high-speed photodetector through the wavelength tunable optical comb filter, and the Mach-Zehnder intensity modulator is modulated by a microwave source with a frequency of f _m . By controlling the DC bias voltage of the Mach-Zehnder intensity modulator and fine-tuning the length of the polarization-maintaining optical delay line, a millimeter-wave signal with a frequency of 4f _m is generated at the RF output of the high-speed photodetector. The invention has the advantages of high millimeter wave efficiency, compact structure, stable operation, anti-electromagnetic interference and the like.

Description

A millimeter wave generating device and method based on optical fiber ring resonator

technical field

The invention relates to the fields of millimeter-wave optical fiber communication and millimeter-wave optical generation, in particular to a millimeter-wave generating device based on an optical fiber ring resonant cavity and a method thereof.

Background technique

Radio over fiber (ROF) is a communication technology that combines wireless communication and optical communication. The signal is loaded onto the optical carrier through subcarrier multiplexing technology, and the wireless signal is distributed from the central station to each base station (BS) through the optical fiber link. , and then transmit the base station signal back to the central station (CS). With people's higher and higher requirements for data transmission rates, the bandwidth of wireless signals has been extended to the millimeter wave band, especially the 60GHz band. Due to the limitations of electronic bottlenecks, there are a series of disadvantages in generating and processing millimeter-wave signals in the electrical domain, such as high loss, additional phase noise, and expensive high-gain amplifiers. Processing millimeter-wave signals in the optical domain has the advantages of high time-bandwidth product, small crosstalk between lines and devices, low phase noise, and anti-electromagnetic interference, and can naturally match with ROF link systems to reduce system costs. The advent of broadband, high-power photodetectors has enabled photonic methods to generate millimeter-wave signals and can replace traditional electronic RF signal generators.

So far, the millimeter-wave signals generated by photonics in the world can basically be divided into three categories. They are: (1) beat frequency for two phase-locked laser sources, and the realization methods include optical injection locking and optical phase locking. This method requires a laser source with narrow linewidth, and the system cost is too high. (2) Use the four-wave mixing effect of optical fiber or optical semiconductor amplifier to up-convert the reference microwave signal. The disadvantage of this method is that the threshold value of the four-wave mixing effect is very high, and changes in the external environment are likely to cause system instability. (3) Using an external optical modulator to achieve frequency doubling and quadrupling of the reference microwave signal. This technology is widely used in today's ROF systems, but the disadvantage of this technology is that the system requires a high-power reference microwave signal. Generating mmWave signals is less efficient, introducing additional microwave amplifiers and the resulting unwanted noise.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of existing photonics methods for generating millimeter-wave signals, and provide a stable and efficient millimeter-wave generating device and method based on optical fiber ring resonators.

The millimeter-wave generating device based on the fiber ring resonator includes semiconductor optical amplifier, 1×2 polarization-maintaining fiber optical coupler, Mach-Zehnder intensity modulator, microwave source, DC stabilized power supply, three-port polarization-maintaining optical circulator, polarization-maintaining optical fiber Grating, polarization-maintaining optical delay line, wavelength-tunable optical comb filter, high-speed photodetector; semiconductor optical amplifier is connected with 1×2 polarization-maintaining optical coupler, and one of the output terminals of 1×2 polarization-maintaining optical coupler is connected with Mach-Zehnder The first port of the intensity modulator and the three-port polarization-maintaining optical circulator are connected in sequence, the second port of the three-port polarization-maintaining optical circulator is connected with the polarization-maintaining fiber grating, and the third port is connected with the polarization-maintaining optical delay line and the semiconductor optical amplifier in sequence, forming A fiber optic ring resonator, the output of the fiber optic ring resonator obtained from the other output end of the 1×2 polarization-maintaining optical coupler is connected to the high-speed photodetector through the wavelength-tunable optical comb filter, and the Mach-Zehnder intensity modulator operates at the frequency In the case of f _m microwave source modulation, by controlling the DC bias voltage of the Mach-Zehnder intensity modulator and fine-tuning the length of the polarization-maintaining optical delay line, a millimeter-wave signal with a frequency of 4f _m can be obtained at the RF output of the high-speed photodetector .

The millimeter-wave generation method based on the fiber ring resonator is: composed of semiconductor optical amplifier, 1×2 polarization maintaining optical coupler, Mach-Zehnder intensity modulator, three-port polarization maintaining optical circulator, polarization maintaining fiber grating and polarization maintaining optical delay line In the optical fiber ring resonant cavity, the output light of the semiconductor optical amplifier that provides optical signal gain enters the Mach-Zehnder intensity modulator through an output port of the 1×2 polarization-maintaining optical coupler, and the light modulated by the microwave source with frequency f _m is from The output of the Mach-Zehnder intensity modulator passes through the optical bandpass filter composed of a three-port polarization-maintaining optical circulator and a polarization-maintaining fiber grating, and then reaches the polarization-maintaining optical delay line, and the light delayed by the polarization-maintaining optical delay line re-enters the semiconductor optical Amplifier, the fiber ring resonator output obtained from the other output end of the 1×2 polarization-maintaining optical coupler, enters the high-speed photodetector through the wavelength-tunable optical comb filter; when the Mach-Zehnder intensity modulator is subjected to f _m When the microwave source is modulated, adjust the length of the polarization-maintaining optical delay line so that the frequency f _m =(k+1/2)f _c , where k is an integer, the cavity frequency f _c =nc/L of the fiber ring resonator, and n is the fiber The effective refractive index of the ring resonator, L is the cavity length of the fiber ring resonator, the fiber ring resonator works in the second rational harmonic mode-locked state, and the output port of the 1×2 polarization-maintaining optical coupler obtains a repetition rate of 2f _m Optical pulse sequence, adjust the DC bias voltage to make the Mach-Zehnder intensity modulator work at the maximum transmission point, the optical pulse sequence with a repetition frequency of 2f _m has the largest energy and the most balanced amplitude, and the optical pulse sequence with a repetition frequency of 2f _m The spectrum with an interval of 4f _m reaches the high-speed photodetector through the wavelength-tunable optical comb filter, and the high-speed photodetector performs photoelectric conversion on the optical signal to generate a millimeter-wave signal with a frequency of 4f _m .

Compared with today's most common method of generating millimeter-wave signals based on an optical external modulator, thanks to the mode-locking effect of the optical fiber ring resonator, the present invention improves the quadrupling efficiency, and the power of the required microwave reference signal is lower , reducing the cost of the system. The invention has the advantages of compact structure, high working stability, good compatibility with the millimeter-wave optical fiber communication system, and broad application prospects.

Description of drawings

Figure 1 is a schematic structural diagram of a millimeter-wave generator based on a fiber optic ring resonator;

Fig. 2 is a schematic diagram of the frequency spectrum of the optical pulse at the output port of the optical fiber ring resonator;

Fig. 3 is a transmission curve diagram of a wavelength tunable comb filter;

In the figure: the solid arrow indicates the transmitted light sideband, and the dotted arrow indicates the optical frequency filtered by the filter;

Fig. 4 is a schematic diagram of the frequency spectrum of the light pulse after passing through the filter;

Fig. 5 is a schematic diagram of the spectrum of the generated millimeter wave signal;

In the figure: semiconductor optical amplifier 1, 1×2 polarization-maintaining fiber optical coupler 2, Mach-Zehnder intensity modulator 3, microwave source 4, DC regulated power supply 5, three-port polarization-maintaining optical circulator 6, polarization-maintaining fiber grating 7 , polarization maintaining optical delay line 8, wavelength tunable optical comb filter 9, high-speed photodetector 10.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing:

As shown in Figure 1, the millimeter-wave generating device based on the fiber ring resonator includes a semiconductor optical amplifier 1, a 1×2 polarization-maintaining fiber optic coupler 2, a Mach-Zehnder intensity modulator 3, a microwave source 4, and a DC stabilized power supply 5. Three-port polarization-maintaining optical circulator 6, polarization-maintaining fiber grating 7, polarization-maintaining optical delay line 8, wavelength-tunable optical comb filter 9, high-speed photodetector 10; semiconductor optical amplifier 1 and 1×2 polarization-maintaining optical coupler 2 connected, one of the output terminals of the 1×2 polarization-maintaining optical coupler 2 is connected to the first port of the Mach-Zehnder intensity modulator 3 and the three-port polarization-maintaining optical circulator 6 in turn, and the second port of the three-port polarization-maintaining optical circulator 6 It is connected to the polarization-maintaining fiber grating 7, and the third port is connected to the polarization-maintaining optical delay line 8 and the semiconductor optical amplifier 1 in turn to form a fiber ring resonator, and the fiber ring obtained by the other output end of the 1×2 polarization-maintaining optical coupler 2 The output of the resonant cavity is connected to the high-speed photodetector 10 through the wavelength-tunable optical comb filter 9, and the Mach-Zehnder intensity modulator 3 is controlled by controlling the Mach-Zehnder intensity modulator under the condition that the microwave source 4 with a frequency of f _m is modulated. 3 DC bias voltage and fine-tuning the length of the polarization-maintaining optical delay line 6, a millimeter-wave signal with a frequency of 4f _m is obtained at the radio frequency output end of the high-speed photodetector 10.

The millimeter-wave generation method based on the fiber ring resonator is as follows: a semiconductor optical amplifier 1, a 1×2 polarization maintaining optical coupler 2, a Mach-Zehnder intensity modulator 3, a three-port polarization maintaining optical circulator 6, a polarization maintaining fiber grating 7 and In the optical fiber ring resonator formed by the polarization-maintaining optical delay line 8, the output light of the semiconductor optical amplifier 1 that provides optical signal gain enters the Mach-Zehnder intensity modulator 3 through an output port of the 1×2 polarization-maintaining optical coupler 2, and passes through a frequency of The light modulated by the f _m microwave source 4 is output from the output port of the Mach-Zehnder intensity modulator 3, passes through the optical band-pass filter composed of the three-port polarization-maintaining optical circulator 6 and the polarization-maintaining fiber grating 7, and then reaches the polarization-maintaining optical delay line 8. The light delayed by the polarization-maintaining optical delay line 8 re-enters the semiconductor optical amplifier 1, and the output of the optical fiber ring resonator obtained from the other output end of the 1×2 polarization-maintaining optical coupler 2 enters through the wavelength-tunable optical comb filter 9 High-speed photodetector 10; when Mach-Zehnder intensity modulator 3 is modulated by microwave source 4 with frequency f _m , adjust the length of polarization-maintaining optical delay line 8 so that frequency f _m =(k+1/2)f _c , Wherein k is an integer, the cavity frequency _fc =nc/L of the fiber ring resonator, n is the effective refractive index of the fiber ring resonator, L is the cavity length of the fiber ring resonator, and the fiber ring resonator works at the second-order rational harmonic In wave mode-locked state, the output port of 1×2 polarization-maintaining optical coupler 2 obtains an optical pulse sequence with a repetition frequency of 2f _m , adjusts the DC bias voltage to make the Mach-Zehnder intensity modulator 3 work at the maximum transmission point, and the repetition frequency is The 2f _m optical pulse sequence has the largest energy and the most balanced amplitude, and the spectrum with an interval of 4f _m in the optical pulse sequence with a repetition frequency of 2f _m reaches the high-speed photodetector 10 through the wavelength-tunable optical comb filter 9, and the high-speed photoelectric The detector 10 performs photoelectric conversion on the optical signal to generate a millimeter wave signal with a frequency of 4f _m .

The working principle of the present invention is as follows:

1. When the RF port of the modulator does not add a microwave source signal, the fiber ring resonator works in the state of single-wavelength laser output, and the central wavelength is the reflection wavelength of the fiber grating. Set f ₀ to adjust the operating current of the semiconductor optical amplifier. The power of the single wavelength laser output can be controlled. The electric field of a single wavelength laser can be expressed as:

E(t)＝E ₀ exp(j2πf ₀ t+jθ)

where _E0 is the magnitude of the electric field, _f0 and θ are the frequency and initial phase of the electric field, respectively

2. Adjust the voltage of the DC stabilized voltage source to make the Mach-Zehnder intensity modulator work at the maximum point transmission state and suppress the generation of odd-numbered sidebands. When the microwave source signal f _m is applied, when the light with the center frequency f ₀ passes through the modulator for the first time, two light sidebands f ₀ +2f _m and f ₀ -2f _m are added, which have the same initial phase as the center wavelength . In order to ensure that the two optical sidebands of f ₀ +2f _m and f ₀ -2f _m maintain the same phase as the carrier f ₀ after one cycle in the cavity, the length of the polarization-maintaining optical delay line is adjusted so that f _m =(q+1/ 2) f _c , the optical fiber ring resonator works in the mode-locked state of the second rational harmonic, at this time, the two optical sidebands are modulated by the microwave source, and further side frequencies f ₀ +4f _m and f ₀ -4f _m will be generated , going round and round, there are more and more light sidebands f ₀ ±2nf _m in the cavity, where n is an integer. These optical sidebands and optical carriers are collectively called longitudinal modes, and the 3dB bandwidth of the fiber grating reflection spectrum and the magnification of the semiconductor optical amplifier determine the number of longitudinal modes in the cavity. As long as the gain of the longitudinal modes in the cavity is greater than the loss, these longitudinal modes run stably in the fiber resonator. In this example, the five longitudinal modes generated coherently superimpose to form an optical pulse with a repetition frequency of 2f _m . The schematic diagram of the frequency spectrum of the optical pulse at the output port of the optical fiber ring resonant cavity is shown in Figure 2. The electric field expression for these light pulses is:

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="HSA00000175709700011.png"\; state="\{\{state\}\}">j</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&theta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

Among them, E ₀ is the electric field magnitude of the optical carrier, and E _n besides that is the electric field magnitude of the nth optical sideband,.

3. The light pulse with a repetition frequency of 2f _m is output from the fiber ring resonant cavity and enters the comb filter with adjustable wavelength. The central wavelength of the wavelength tunable comb filter is adjusted so that the wavelength of a minimum transmitted power point is the same as the central wavelength of the laser in the fiber ring resonant cavity. Therefore, the wavelength tunable comb filter selects the optical sideband with frequency f ₀ ±2f _m , and the optical field after filtering can be expressed as:

E _L (t)＝E ₁ exp[j2π(f ₀ ±2f _m )t+jθ)]

See Figure 3 for the transmission curve of the wavelength-tunable comb filter, and Figure 4 for the schematic diagram of the spectrum of the light pulse passing through the filter.

4. The high-speed photodetector in the present invention is an envelope response, and the response bandwidth is greater than 4f _m , and the photoelectric current output by the photodetector can be expressed as:

I(t)＝ηE _L (t) · E _L ^* (t)＝2ηE ₁ ² +2ηE ₁ ² cos(8πf _m t)

Among them, η is the responsivity of the photodetector, which is generally 0.6A/W. The first part on the far right of the above formula represents the DC component of the photocurrent, and the second part is the detected millimeter-wave signal with a frequency of 4f _m . The schematic diagram of the frequency spectrum of the millimeter wave signal is shown in Figure 5.

Claims (1)

1. A millimeter-wave generating device based on an optical fiber ring resonator, characterized in that it comprises a semiconductor optical amplifier (1), a 1 × 2 polarization-maintaining optical fiber coupler (2), a Mach-Zehnder intensity modulator (3), a microwave Source (4), DC stabilized power supply (5), three-port polarization-maintaining optical circulator (6), polarization-maintaining optical fiber grating (7), polarization-maintaining optical delay line (8), wavelength-tunable optical comb filter (9) , a high-speed photodetector (10); a semiconductor optical amplifier (1) is connected to a 1×2 polarization-maintaining fiber optic coupler (2), and one of the output ends of the 1×2 polarization-maintaining fiber optic coupler (2) is connected to a Mach Zeng The first ports of the Del intensity modulator (3) and the three-port polarization-maintaining optical circulator (6) are connected in turn, the second port of the three-port polarization-maintaining optical circulator (6) is connected with the polarization-maintaining fiber grating (7), and the third port It is sequentially connected with the polarization-maintaining optical delay line (8) and the semiconductor optical amplifier (1) to form a fiber ring resonator, and the output of the fiber ring resonator obtained from the other output end of the 1×2 polarization-maintaining fiber optic coupler (2) , is connected to the high-speed photodetector (10) through the wavelength tunable optical comb filter (9), and the Mach-Zehnder intensity modulator (3) is modulated by the microwave source (4) at frequency f _m , by controlling The DC bias voltage of the Mach-Zehnder intensity modulator (3) and the length of the polarization-maintaining optical delay line (8) are finely adjusted to obtain a millimeter-wave signal with a frequency of 4f _m at the radio frequency output end of the high-speed photodetector (10).

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