CN1811503A - Method for producing non-linear chirp optical fibre grating for 40 Gb/S optical communication system - Google Patents

Abstract

The invention belongs to the technical field of fiber grating production, and is characterized in that it comprises the following steps in order to design a nonlinear chirped fiber grating with tunable characteristics, that is, the reflectivity and wavelength change in a Gaussian curve, and the group delay and wavelength change in a quadratic curve ; Use the grating reconstruction principle to obtain the refractive index modulation function of the grating, and then determine the sampling point position and exposure time; make a nonlinear chirped fiber grating for 40Gb/s optical communication system according to the design result, and then use the current The center wavelength of the grating is shifted by the thermal effect generated by the metal coating on the surface of the fiber grating, and the amount of dispersion compensation is controlled. Experiments prove that the power cost of the optical signal after passing through the invention is less than 0.7dB, which is lower than the upper limit of 1dB power cost stipulated in the optical communication technology.

Description

Fabrication method of nonlinear chirped fiber grating for 40Gb/s optical communication system

technical field

The technology belongs to the technical field of optical fiber communication, and in particular relates to the field of fiber grating production.

Background technique

Generally, fiber dispersion compensation can be realized by using dispersion compensating fiber or dispersion compensating devices, such as dispersion compensating fiber grating, dispersion compensating etalon, etc. The implementation method of this dispersion compensation is fixed, and once the system is designed, the amount of dispersion compensation cannot be changed. The accumulation of dispersion is a process that changes with time. The system will be affected by many external factors, such as temperature changes, pressure, etc., resulting in dynamically changing dispersion. This requires a dynamically tunable dispersion compensation device to adjust the changing dispersion. Compensation is done by way of monitoring and tracking. Among them, the implementation of tunable dispersion is the core content of the dynamically tunable dispersion compensation device. There are several ways to realize tunable dispersion, such as changing the dispersion of fiber grating by using thermal effect to generate dynamically changing chirp in fiber grating, and using variable delay curves generated by multiple etalons to adjust dispersion by linear superposition, etc. Among them, the tunable dispersion based on nonlinear chirp has become one of the attractive methods because of its simplicity and effectiveness.

The time delay curve of nonlinear chirped fiber grating is nonlinear, and the time delay of the fiber grating can be changed by heating the fiber grating. When the time delay of the fiber grating changes in a quadratic curve with the wavelength, the dispersion is proportional to the wavelength. Linear relationship, its time delay (group delay) and dispersion (dispersion) and light wavelength (wavelength) relationship curves are shown in Figures 1 and 2. At this time, the tuning control of the fiber grating can be simpler. Therefore, designing nonlinear chirped fiber gratings with different structures can achieve better compensation for system dispersion. Because high-quality nonlinear chirped fiber gratings need high-quality nonlinear chirped templates to make high-quality nonlinear chirped fiber gratings, and nonlinear chirped templates need to be produced by electron beam exposure technology, it is very difficult to obtain high-quality nonlinear chirped templates, and The price is also quite expensive. Moreover, different nonlinear chirped fiber gratings require different nonlinear chirped phase templates, which limits the design and application of the above-mentioned tunable devices.

Due to the difficulty in obtaining high-quality nonlinear chirp templates, a method using uniform phase templates and sampling fiber grating technology to obtain nonlinear chirp has emerged. In 2004, Jiang Dianjie, Chen Xiangfei and others proposed to use the chirp sampling period (CSP) of the sampling grating to obtain the equivalent chirp Grating period (CGP) and methods for achieving tuning using precision mechanics.

Although this method realizes the use of chirped phase templates to design and fabricate equivalent nonlinear chirped fiber gratings, it can only control the group delay curve of the grating and cannot control the reflection of the grating at the same time. rate curve, which degrades the performance of the grating. In addition, it uses a precision mechanical device for tuning, but the precision mechanical device is expensive and bulky, which limits the application of this tunable dispersion compensator to a certain extent.

Contents of the invention

The purpose of the present invention is to break through the existing technology and overcome the deficiencies of the existing technology, and propose a brand-new method to design and manufacture a special sampling fiber grating with equivalent nonlinear chirp effect, and The grating is tuned in a new way, enabling tunable dispersion compensation. The fiber grating adopts a uniform template design, and realizes simultaneous control of the group delay curve and the reflectivity curve through the reconstruction and equivalent of the grating. Then, uniform metal coating is carried out on the surface of the finished fiber grating, and the thermal effect generated when the current passes through the metal coating is used to realize the movement of the central wavelength of the grating, thereby controlling the compensation amount of dispersion.

We use the -1 order group delay spectrum of the reflection peak of the sampling grating for dispersion compensation.

The reflection characteristics of the designed grating are as follows:

Reflectance Curve:

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="e"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">e</figure-callout></span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Group delay curve: τ(λ)=a(λ-λ ₀ ) ² +b(λ-λ ₀ )+c (2)

In the formula, R(λ) is the reflectivity of the grating; λ is the wavelength of light; _λ0 is the central wavelength, which is taken as 1553.5mm; B is the 3dB bandwidth of the grating reflection spectrum, which is taken as 2nm; τ(λ) is the group of the grating Time delay; a, b, c are the design tuning parameters of the compensator, take a=-100ps/nm ² , b=-300ps/nm, c=900ps; m is the super-Gaussian apodization coefficient, take m=4. The group delay of the grating changes in a quadratic curve with the wavelength, while its dispersion has a linear relationship with the wavelength, so its tuning control can be simpler and more stable. In addition, in the design process of the grating, its group delay characteristics and reflectivity characteristics are considered at the same time, and the joint control of the two is realized, and a relatively flat reflection band is obtained, so a higher performance grating can be obtained.

Using the Fourier transform relationship between the AC refractive index modulation function of the grating and the reflection characteristics, the AC refractive index modulation function of the grating is obtained:

$\mathrm{<span\; class="notranslate">\; Ac</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">j</figure-callout></span>}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; \&Lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; l</span>}}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j\&theta;</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">j</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&sigma;z</span>}<span\; class="notranslate">\; ]</span>\mathrm{<span\; class="notranslate">\; d\&sigma;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

in, $\mathrm{\&theta;}={\&Integral;}_0^{\mathrm{\&lambda;}}\frac{2\mathrm{\&pi;}}{{{\mathrm{\&lambda;}}_0}^2}\mathrm{\&tau;}\left({\mathrm{\&lambda;}}^{\&prime;}\right)d{\mathrm{\&lambda;}}^{\&prime;},$ $\mathrm{\&sigma;}=\frac{2\mathrm{n\&pi;}}{\mathrm{\&lambda;}}-\frac{\mathrm{\&pi;}}{{\mathrm{\&Lambda;}}_0},$ Λ ₀ =λ ₀ /2n, n is the average refractive index of the grating.

Write Ac(z) as a complex exponential function:

Ac(z)＝A(z)exp[j(z)]                 (4)

The above-mentioned refractive index modulation is realized by using a sampling grating, and the position z _k of each sampling point is calculated by equation (5):

In the formula, P is the sampling parameter, usually P=0.12mm; k represents the kth sampling point of the sampling grating, k=1, 2, 3,....

The exposure time T _k of each sampling point is determined by formula (6):

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; max</span>}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \}</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Where T _max is the maximum exposure time of the sampling point, usually T _max =100s; max{A(z _k )} represents the maximum value of the refractive index modulation A(z _k ) of all the sampling points z _k .

The design method of the tunable dispersion compensator described in the present invention is characterized in that it contains the following steps successively:

(1) Design a nonlinear chirped grating with tunable characteristics, that is, the reflectivity and wavelength change in a super-Gaussian curve, and the group delay and wavelength change in a quadratic curve. The reflection characteristics of the grating are:

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="e"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">e</figure-callout></span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; ]</span>$

τ(λ)＝a(λ-λ ₀ ) ² +b(λ-λ ₀ )+c

Among them, R (λ) is the reflectivity of the grating; λ is the wavelength of the light wave; λ ₀ is the center wavelength, the set value; B is the 3dB bandwidth of the grating reflection spectrum, the unit is nm; τ (λ) is the group delay of the grating , the unit is ps; a, b, c are design parameters and set values, the unit of a is ps/nm ² , the unit of b is ps/nm, and the unit of c is ps.

(2) According to the reflection characteristics of the grating, the refractive index modulation function of the grating is obtained by using the reconstruction principle of the grating, and the refractive index modulation function of the grating is calculated by the following formula:

$\mathrm{<span\; class="notranslate">\; \&Delta;\; n</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; Ac</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">j</figure-callout></span>}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;z</span>}}{{\mathrm{<span\; class="notranslate">\; \&Lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>$

Among them, z is the axial length of the optical fiber; Δn(z) is the refractive index modulation function of the grating; Λ ₀ is the refractive index modulation period of the grating, which is determined by the central wavelength of the grating: Λ ₀ =λ ₀ /2n, n is the grating average refractive index,

Ac(z) is called the AC refractive index modulation function of the grating, which can be calculated by the following formula:

$\mathrm{<span\; class="notranslate">\; Ac</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">j</figure-callout></span>}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; \&Lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; l</span>}}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j\&theta;</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="A20061001141100162.png"\; state="\{\{state\}\}">j</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&sigma;z</span>}<span\; class="notranslate">\; ]</span>\mathrm{<span\; class="notranslate">\; d\&sigma;</span>}$

in, $\mathrm{\&theta;}={\&Integral;}_0^{\mathrm{\&lambda;}}\frac{2\mathrm{\&pi;}}{{{\mathrm{\&lambda;}}_0}^2}\mathrm{\&tau;}\left({\mathrm{\&lambda;}}^{\&prime;}\right)d{\mathrm{\&lambda;}}^{\&prime;},$ $\mathrm{\&sigma;}=\frac{2\mathrm{n\&pi;}}{\mathrm{\&lambda;}}-\frac{\mathrm{\&pi;}}{{\mathrm{\&Lambda;}}_0}$

Write Ac(z) as a complex exponential form, as: Ac(z)=A(z)exp[j(z)]

Among them, Ac(z), A(z), and (z) can all be obtained by using computer numerical calculation methods according to the designed reflection characteristics of the grating in step (1).

(3) The sampling point position z _k of the sampling grating is determined by the following equation:

In the formula, P is the sampling parameter, usually P=0.15mm; k represents the kth sampling point of the sampling grating, k=1, 2, 3,.... , the exposure time T _k of each sampling point is determined by the following formula:

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; max</span>}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \}</span>}$

Where T _max is the maximum exposure time of the sampling point, usually T _max = 100 seconds; max{A(z _k )} represents all sampling points

The refractive index of z _k modulates the maximum in A(z _k ).

The difference between the design and manufacturing method of the fiber grating of the present invention and the patent of "Design and Manufacturing Method of Tunable Dispersion Compensator" lies in:

The invention realizes joint control of the grating group delay curve and reflectivity curve through the equivalent and reconstruction of the grating, so as to obtain the optical fiber grating with ideal reflection characteristics. In addition, in the process of realizing fiber grating tuning, instead of using precision mechanical devices for tuning, we use the method of uniform coating on the surface of the grating, and use the thermal effect to adjust the dispersion of the grating. Such a design and manufacturing method can not only simplify the grating manufacturing process, but also greatly reduce the cost and volume of the tunable dispersion compensator.

The manufacture of the tunable dispersion compensator is characterized in that it contains the following steps in sequence:

(1) Carry out hydrogen-carrying treatment on ordinary optical fiber and strip off a section of coating layer;

(2) Fix the above-mentioned optical fiber behind the uniform template and make it close to;

(3) adjust the optical power of the laser output to 50mW;

(4) Adjust the optical path so that the light spot reflected by the scanning mirror is irradiated on the fiber core;

(5) Open the scanning mobile platform and laser control program of the microcomputer, input and set the following parameters:

Exposure point position (given by formula (5)):

Exposure time (given by equation (6)): $T_k=T_{\max }\&Center\; Dot;\frac{A\left(z_k\right)}{\max \left\{A\left(z_k\right)\right\}}$

(6) Start the scanning platform to make it run according to the parameters set in (5), so that the exposed optical fiber becomes a fiber grating with nonlinear chirp characteristics.

(7) Place the finished fiber grating in the degreasing lye and heat it in a water bath at 75°C for 30 minutes. The degreasing lye is prepared from the following four solutions in a volume ratio of 1:1:1:1:

Sodium hydroxide: 40g/L, sodium silicate: 40g/L, sodium carbonate: 30g/L, sodium phosphate: 30g/L

(8) Soak the cleaned optical fiber grating in the sensitizing solution for 10 seconds, then transfer to the activating solution and soak for 10 seconds, and then transfer to the sensitizing solution. Repeat this 4-5 times until the surface of the fiber grating turns dark brown. The preparation of sensitization solution and activation solution is as follows:

Sensitizing solution: stannous chloride, 30g/L, 20ml

Activation solution: palladium chloride, 0.1g/L, 20ml

(9) Put the optical fiber grating in the nickel plating solution, and heat it in a water bath at 50° C. for 2 hours, so that the surface of the optical fiber grating is covered with a layer of uniform metallic nickel coating. The preparation of the nickel plating solution is as follows:

Nickel sulfate: 30g/L, 35ml

Sodium pyrophosphate: 90g/L, 30ml

Sodium hypophosphite: 30g/L, 25ml

Ammonia water: 40ml/L, 5ml

(10) Welding the fiber grating covered with a uniform metal coating to the electrode and encapsulating it in a metal sleeve.

(11) Connect the RS-232 port of the computer to the communication pin of the single-chip microcomputer AT89C52, and control the digital signal output of the single-chip microcomputer through the computer.

(12) Connect the digital signal output pin of the single-chip microcomputer to the digital input pin of the DA converter MAX508, and use the DA converter to convert the digital signal of the single-chip microcomputer into an analog signal. The range of the analog signal output voltage of the DA converter is 0 to 10V .

(13) Connect the voltage analog signal output pin of the DA converter to the input pin of the power transistor (Darlington tube), and use the power transistor to amplify the voltage signal.

(14) Connect the output port of the power triode to the electrodes in the above step (10), so as to change the voltage at both ends of the electrodes through the computer, and then change the dispersion provided by the compensator through the computer.

Using this tunable dispersion compensator to test in a 40Gb/s optical communication system, it is measured that the power penalty of the optical signal after dispersion compensation is less than 0.7dB, which is lower than the upper limit of 1dB power penalty specified in the communication standard, indicating that the tunable Dispersion compensators have fairly good practical performance.

In summary, the present invention can flexibly design a high-performance fiber grating according to the needs of the system and users, and the grating manufacturing process is relatively simple and reliable, and the tuning device has a simple structure and stable performance. More importantly, the tunable dispersion The compensator is low in cost and has great application potential.

Description of drawings:

Figure 1: The relationship between the group delay (Group Delay) and the light wavelength (Wavelength) of the designed grating;

Figure 2: The relationship between the dispersion (Dispersion) and the light wavelength (Wavelength) of the designed grating;

Figure 3: The reflectivity curve of the grating;

Figure 4: Group delay curve for grating fabrication;

Figure 5: Schematic diagram of the fiber grating manufacturing device;

Figure 6: Schematic diagram of the tuning device of the tunable dispersion compensator;

Figure 7: Schematic diagram of the control device of the tunable dispersion compensator;

Figure 8: The relation curve between the electrode voltage of the tunable dispersion compensator and the amount of dispersion provided;

Figure 9: Fabrication flow chart of 40Gb/s tunable dispersion compensator.

Specific implementation examples:

The manufacturing method of the tunable dispersion compensator of the present invention is divided into two parts: one is the design of a sampling grating with a quadratic time delay curve and a super-Gaussian reflectivity curve, and the other is the manufacture of the tunable dispersion compensator. The specific implementation examples are described in detail below in conjunction with the accompanying drawings.

1. Design of fiber grating used in tunable dispersion compensator

For formula (1), take B=2nm, λ ₀ =1553.5nm, m=4; for formula (2), take a=-100ps/nm ² , b=-300ps/nm, c=900.

Using Fourier transform, A(z) and (z) in the AC refractive index modulation function of the grating are obtained from formula (3).

The position z _k of the kth sampling point of the sampling grating is obtained by using the formula (5), and the exposure time T _k at the kth sampling point is obtained by using the formula (6).

The reflectivity curve and time delay curve of the fabricated grating are shown in Fig. 3 and Fig. 4 respectively: within the bandwidth range of 2nm, its dispersion changes from -260ps/nm to -60ps/nm.

2. Fabrication of Tunable Dispersion Compensator

The device for making gratings in this implementation example is shown in FIG. 5 . Wherein, the light source is a continuous 244nm frequency-doubled argon ion laser 51 (produced by Coherent, USA). The scanning mirror 52 is fixed on the ESP6000 scanning mobile platform (manufactured by Newport Company) 53, and the motion precision of the scanning mobile platform is 0.1mm. The mirror 52 has the function of scanning and reflecting the light beam, and reflects the ultraviolet light output by the laser 51 onto the uniform phase template 54. The length of the phase template is 60 mm. mode fiber 55. The ESP6000 scanning mobile platform is connected with the PIO port of the microcomputer (not marked in the figure). By running the pre-designed driver software on the microcomputer, changing the motion state (movement distance, motion time, etc.) of the mobile platform to make it run according to a certain motion law, the required fiber grating can be obtained.

2. The manufacture of the tunable dispersion compensator is characterized in that it contains the following steps in sequence:

(1) Carry out hydrogen-carrying treatment on ordinary optical fiber and strip off a section of coating layer;

(2) Fix the above-mentioned optical fiber behind the uniform template and make it close to;

(3) adjust the optical power of the laser output to 50mW;

(4) Adjust the optical path so that the light spot reflected by the scanning mirror is irradiated on the fiber core;

(5) Turn on the scanning mobile platform of the microcomputer and the laser control program, input and set the following parameters: exposure point position and exposure time.

(6) Start the scanning platform to make it run according to the parameters set in (5), so that the exposed optical fiber becomes a fiber grating with nonlinear chirp characteristics.

(7) Place the finished fiber grating in the degreasing lye and heat it in a water bath at 75°C for 30 minutes. The degreasing lye is prepared from the following four solutions in a volume ratio of 1:1:1:1:

Sodium hydroxide: 40g/L, sodium silicate: 40g/L, sodium carbonate: 30g/L, sodium phosphate: 30g/L

(8) Soak the cleaned optical fiber grating in the sensitizing solution for 10 seconds, then transfer to the activating solution and soak for 10 seconds, and then transfer to the sensitizing solution. Repeat this 4-5 times until the surface of the fiber grating turns dark brown. The preparation of sensitization solution and activation solution is as follows:

Sensitizing solution: stannous chloride, 30g/L, 20ml

Activation solution: palladium chloride, 0.1g/L, 20ml

(9) Put the optical fiber grating in the nickel plating solution, and heat it in a water bath at 50° C. for 2 hours, so that the surface of the optical fiber grating is covered with a layer of uniform metallic nickel coating. The preparation of the nickel plating solution is as follows:

Nickel sulfate: 30g/L, 35ml

Sodium pyrophosphate: 90g/L, 30ml

Sodium hypophosphite: 30g/L, 25ml

Ammonia water: 40ml/L, 5ml

(10) Welding the fiber grating covered with a uniform metal coating to the electrode and encapsulating it in a metal sleeve.

(11) Connect the RS-232 port of the computer to the communication pin of the single-chip microcomputer AT89C52, and control the digital signal output of the single-chip microcomputer through the computer.

(12) Connect the digital signal output pin of the single-chip microcomputer to the digital input pin of the DA converter MAX508, and use the DA converter to convert the digital signal of the single-chip microcomputer into an analog signal. The range of the analog signal output voltage of the DA converter is 0 to 10V .

(13) Connect the voltage analog signal output pin of the DA converter to the input pin of the power transistor (Darlington tube), and use the power transistor to amplify the voltage signal.

(14) Connect the output port of the power triode to the electrodes in the above step (10), so as to change the voltage at both ends of the electrodes through the computer, and then change the dispersion provided by the compensator through the computer. The relationship curve between the electrode voltage (Voltage) and the dispersion provided by the compensator is shown in FIG. 8 .

A schematic diagram of a tuning device used to manufacture a tunable dispersion compensator is shown in FIG. 6 . Metal sleeve 61 is made of steel material, and other main parts are all directly or indirectly fixed on it. The two electrodes are fixed on both ends of the metal casing by insulating glue. The two ends of the grating 62 covered with the metal nickel layer 63 are welded to the electrodes 64, and two sections of wires 65 are led out from the two electrodes 64. The voltage between the two electrodes 64 is provided by a digitally controlled voltage source circuit 66 , and the digitally controlled voltage source circuit 66 is connected to the RS232 communication port of the computer through a serial port line 67 . 68 is an optical fiber connector, and 68 is connected with the optical communication system during use.

A schematic diagram of a control device for a tunable dispersion compensator is shown in FIG. 7 . The RS232 port 71 of the computer is connected with the digital signal input pin of the single-chip microcomputer 72 (AT89C52) through the serial port line 67; Digital signal input pin of DA converter 73 (MAX508). The DA converter 73 converts the digital signal into an analog voltage signal, which is sent to the power transistor 74 through its analog voltage output port for power amplification. The output pin of the power transistor 74 is connected with the electrode 64 through the wire 65 .

Using this tunable dispersion compensator to test in a 40Gb/s optical communication system, it is measured that the power penalty of the optical signal after dispersion compensation is less than 0.7dB, which is lower than the upper limit of 1dB power penalty specified in the communication standard, indicating that the tunable Dispersion compensators have fairly good practical performance.

In summary, the present invention can flexibly design a high-performance fiber grating according to the needs of the system and users, and the grating manufacturing process is relatively simple and reliable, and the tuning device has a simple structure and stable performance. More importantly, the tunable dispersion The compensator is low in cost and has great application potential.

Claims (1)

1. The method for manufacturing the nonlinear chirped fiber grating for the 40Gb/s optical communication system is characterized by sequentially comprising the following steps of:

step 1: a sampling grating with the following reflection characteristics and group delay characteristics is designed according to the following setting parameters:

reflectance curve: <math> <mrow> <mi>R</mi> <mrow> <mo>(</mo> <mi>&lambda;</mi> <mo>)</mo> </mrow> <mo>=</mo> <mfrac> <mi>e</mi> <msqrt> <mn>2</mn> </msqrt> </mfrac> <mi>exp</mi> <mo>[</mo> <mo>-</mo> <msup> <mrow> <mo>(</mo> <mfrac> <mrow> <mi>&lambda;</mi> <mo>-</mo> <msub> <mi>&lambda;</mi> <mn>0</mn> </msub> </mrow> <mrow> <mi>B</mi> <mo>/</mo> <mn>2</mn> </mrow> </mfrac> <mo>)</mo> </mrow> <mrow> <mn>2</mn> <mi>m</mi> </mrow> </msup> <mo>]</mo> <mo>,</mo> </mrow> </math>

group delay curve: τ (λ) ═ a (λ - λ)₀)²+b(λ-λ₀)+c，

Wherein: r (λ) is the reflectivity of the grating,

lambda is the wavelength of the light wave,

λ₀λ is set as the center wavelength₀＝1553.5nm，

B is the 3dB bandwidth of the grating reflection spectrum, and B is set to be 2nm,

τ (λ) is the group delay of the grating,

a, b and c are design tuning parameters, and let a be-100 ps/nm²，b＝-300ps/nm，c＝900ps，

m＝4；

Step 2: the refractive index modulation function Δ n (z) of the sampled fiber grating was calculated by a computer based on the design parameters of step 1, with the following set parameters:

<math> <mrow> <mi>&Delta;n</mi> <mrow> <mo>(</mo> <mi>z</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>Ac</mi> <mrow> <mo>(</mo> <mi>z</mi> <mo>)</mo> </mrow> <mi>exp</mi> <mrow> <mo>(</mo> <mo>-</mo> <mi>j</mi> <mfrac> <mrow> <mn>2</mn> <mi>&pi;z</mi> </mrow> <msub> <mi>&Lambda;</mi> <mn>0</mn> </msub> </mfrac> <mo>)</mo> </mrow> <mo>,</mo> </mrow> </math>

wherein z is the axial coordinate of the sampling grating,

Λ₀the refractive index modulation period of the sampled grating: lambda₀＝λ₀/2n，

n is the average refractive index of the sampled fiber grating,

the function Ac (z) is an AC refractive index modulation function of the sampled grating, and is calculated by a computer according to the following formula:

<math> <mrow> <mi>Ac</mi> <mrow> <mo>(</mo> <mi>z</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>j</mi> <mfrac> <mrow> <mn>2</mn> <mi>n</mi> <msub> <mi>&Lambda;</mi> <mn>0</mn> </msub> </mrow> <mi>&pi;</mi> </mfrac> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>l</mi> </msubsup> <mi>R</mi> <mrow> <mo>(</mo> <mi>&lambda;</mi> <mo>)</mo> </mrow> <mi>exp</mi> <mo>[</mo> <mo>-</mo> <mi>j&theta;</mi> <mo>]</mo> <mo>&CenterDot;</mo> <mi>exp</mi> <mo>[</mo> <mi>j</mi> <mn>2</mn> <mi>&sigma;z</mi> <mo>]</mo> <mi>d&sigma;</mi> <mo>,</mo> </mrow> </math>

wherein, <math> <mrow> <mi>&theta;</mi> <mo>=</mo> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>&lambda;</mi> </msubsup> <mfrac> <mrow> <mn>2</mn> <mi>&pi;</mi> </mrow> <msup> <msub> <mi>&lambda;</mi> <mn>0</mn> </msub> <mn>2</mn> </msup> </mfrac> <mi>&tau;</mi> <mrow> <mo>(</mo> <msup> <mi>&lambda;</mi> <mo>&prime;</mo> </msup> <mo>)</mo> </mrow> <mi>d</mi> <msup> <mi>&lambda;</mi> <mo>&prime;</mo> </msup> <mo>,</mo> <mn>0</mn> <mo>&le;</mo> <msup> <mi>&lambda;</mi> <mo>&prime;</mo> </msup> <mo>&le;</mo> <mi>&lambda;</mi> <mo>,</mo> </mrow> </math>

<math> <mrow> <mi>&sigma;</mi> <mo>=</mo> <mfrac> <mrow> <mn>2</mn> <mi>n&pi;</mi> </mrow> <mi>&lambda;</mi> </mfrac> <mo>-</mo> <mfrac> <mi>&pi;</mi> <msub> <mi>&Lambda;</mi> <mn>0</mn> </msub> </mfrac> <mo>,</mo> </mrow> </math>

l is the length of the sampled grating,

the complex exponential form of the function ac (z) is:

Ac(z)＝A(z)exp[j*(z)]；

and step 3: using the refractive index modulation function obtained in step 2 according toThe position z of each sample point is determined by_k：

Wherein, P is a sampling parameter, and P is 0.12 mm;

k denotes the kth sampling point of the sampled grating, k being 1, 2, 3.;

calculating the exposure time T of each sampling point according to the following formula_k：

<math> <mrow> <msub> <mi>T</mi> <mi>k</mi> </msub> <mo>=</mo> <msub> <mi>T</mi> <mi>max</mi> </msub> <mo>&CenterDot;</mo> <mfrac> <mrow> <mi>A</mi> <mrow> <mo>(</mo> <msub> <mi>z</mi> <mi>k</mi> </msub> <mo>)</mo> </mrow> </mrow> <mrow> <mi>max</mi> <mo>{</mo> <mi>A</mi> <mrow> <mo>(</mo> <msub> <mi>z</mi> <mi>k</mi> </msub> <mo>)</mo> </mrow> <mo>}</mo> </mrow> </mfrac> </mrow> </math>

Wherein, T_maxFor maximum exposure time of the sample point, take T_max100 seconds;

max{A(z_k) Denotes all the sampling points z_kRefractive index modulation amplitude A (z)_k) Maximum of (1);

and 4, step 4: the nonlinear chirped fiber grating for the 40Gb/s optical communication system is manufactured according to the following steps:

step 4.1: carrying out hydrogen loading treatment on a common optical fiber and stripping a section of coating layer;

step 4.2: fixing the optical fiber obtained in the step 4.1 on the uniform template to enable the optical fiber to be close to the uniform template;

step 4.3: adjusting the output of the laser to 50mW of optical power;

step 4.4: adjusting the light path to make the light spot reflected by the scanning reflector irradiate on the fiber core of the optical fiber;

step 4.5: opening a scanning mobile platform and a laser control program of a microcomputer, and setting and inputting the following parameters according to the calculation result in the step 3:

exposure point position:

exposure time: <math> <mrow> <msub> <mi>T</mi> <mi>k</mi> </msub> <mo>=</mo> <msub> <mi>T</mi> <mi>max</mi> </msub> <mo>&CenterDot;</mo> <mfrac> <mrow> <mi>A</mi> <mrow> <mo>(</mo> <msub> <mi>z</mi> <mi>k</mi> </msub> <mo>)</mo> </mrow> </mrow> <mrow> <mi>max</mi> <mo>{</mo> <mi>A</mi> <mrow> <mo>(</mo> <msub> <mi>z</mi> <mi>k</mi> </msub> <mo>)</mo> </mrow> <mo>}</mo> </mrow> </mfrac> <mo>;</mo> </mrow> </math>

step 4.6: starting a scanning platform, enabling the platform to operate according to the parameters set in the step 4.5, and enabling the exposed optical fiber to become an optical fiber grating with nonlinear chirp characteristics;

step 4.7: placing the manufactured fiber grating in oil-removing alkali liquor, heating the fiber grating in water bath at 75 ℃ for 30 minutes, wherein the oil-removing alkali liquor is prepared by the following four solutions according to the volume ratio of 1:

sodium hydroxide: 40g/L, sodium silicate: 40g/L, sodium carbonate: 30g/L, sodium phosphate: 30 g/L;

step 4.8: soaking the cleaned fiber grating in a sensitizing solution for 10 seconds, then soaking the fiber grating in an activating solution for 10 seconds, then transferring the fiber grating into the sensitizing solution, and repeating the steps for 4-5 times until the surface of the fiber grating is dark brown; the sensitizing solution and the activating solution are prepared as follows:

sensitizing solution: 30g/L of stannous chloride, 20 ml;

activating solution: palladium chloride, 0.1g/L, 20 ml;

step 4.9: putting the fiber bragg grating in a nickel plating solution, and heating the fiber bragg grating in water bath for 2 hours at 50 ℃ to cover a uniform metal nickel coating on the surface of the fiber bragg grating; the nickel plating solution is prepared as follows:

nickel sulfate: 30g/L, 35 ml;

sodium pyrophosphate: 90g/L, 30 ml;

sodium hypophosphite: 30g/L, 25 ml;

ammonia water: 40ml/L, 5 ml;

step 4.10: welding the fiber bragg grating covered with the uniform metal coating on the electrode and packaging the fiber bragg grating in the metal sleeve;

step 4.11: connecting an RS-232 port of a computer with a communication pin of a single chip microcomputer AT89C52, and controlling the digital signal output of the single chip microcomputer through the computer, wherein the range of binary digital signals output by the single chip microcomputer is 000000000000-111111111111;

step 4.12: connecting a digital signal output pin of the singlechip with a digital input pin of a DA converter MAX508, converting the digital signal of the singlechip into an analog signal by using the DA converter, wherein the range of the voltage analog signal output by the DA converter is 0-10V;

step 4.13: connecting a voltage analog signal output pin of the DA converter with an input pin of a power triode (Darlington tube), and performing power amplification on a voltage signal by using the power triode;

step 4.14: and (4) connecting an output port of the power triode with the electrode in the step 4.10 to change the dispersion of the sampling grating by a computer.

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