CN102636337A - Method for measuring optical fiber dispersion - Google Patents

Abstract

The invention discloses a method for measuring optical fiber dispersion and relates to the field of optical fiber loss measurement. The invention is based on the femtosecond laser pumping and detection technology, and is realized by a femtosecond laser system, a beam splitter, a reflector, a time delay line, a chopper, a photodetector, a lock-in amplifier, an optical fiber, a sample, and a computer system. A photodetector is used to detect the change of the transient reflected light on the sample surface before and after loading the optical fiber, and the moment when the transient reflectance changes abruptly in the two cases is obtained, and the dispersion of the optical fiber is calculated. The invention is applicable to the measurement of optical fiber dispersion of different materials, and the device has simple structure and good repeatability.

Description

A Method for Measuring Optical Fiber Dispersion

technical field

The invention relates to the field of optical fiber loss measurement, in particular to a method for measuring optical fiber dispersion, which is applicable to the measurement of any type of optical fiber dispersion.

Background technique

As we all know, the development of optical communication is inseparable from the development of optoelectronic devices. Many transmission devices in optical communication require good dispersion characteristics. The chirp caused by the high-order dispersion of optical fiber directly affects the transmission of optical pulses, and also affects the Soliton formation and compression of optical pulses. In addition, the rapid growth of global business volume requires a substantial increase in the capacity and rate of optical fiber communication, which requires the control of optical fiber dispersion. Because of the existence of dispersion, not only will the output laser light pulse be broadened, especially in ROF optical communication, it will cause the output light amplitude to change periodically with the change of the fiber length, resulting in unstable output energy and lower output bit error rate. Increase. Dispersion compensation devices such as fiber gratings and waveguide microring resonators are usually used to compensate the dispersion of optical fibers. Therefore, with the deepening of research on optical fiber and its wide application, the testing of related parameters of optical fiber has become a technical problem that needs to be solved urgently. The emergence of lasers, especially femtosecond lasers, provides a powerful tool for the development of various testing technologies. Femtosecond laser has the characteristics of ultra-fast time response and ultra-high peak value. On the one hand, it can fundamentally avoid the existence and influence of thermal diffusion when light is transmitted in the optical fiber. On the other hand, it makes the nonlinear interaction between laser and matter dominate. Therefore, the femtosecond laser has the characteristics of high precision and three-dimensional controllability. After the femtosecond laser interacts with matter, the excitation and relaxation of carriers are both ultrafast processes. The starting point of femtosecond spectroscopy technology is the transient coherent effect of ultrashort pulse laser and matter. It can be used to detect the signal light field Amplitude, phase and wave vector can also measure other information of matter and laser transient coherence, such as the change of excited state population number, electron motion and nuclear vibration. Invention patent "A Device and Method for Measuring Dispersion of Fiber Optic Devices" (Chinese patent, patent application number: 201110268543.8) provides a fiber optic dispersion test system and method based on microwave optoelectronics. The test method used is based on light wave interference , mainly to make the light wave signal pass through the fiber optic device with dispersion, and obtain a certain optical path difference during the transmission process. After passing through the detector, the two microwave signals interfere, and the size of the dispersion is judged by the relationship between the interference signal and the dispersion. The invention uses a femtosecond laser as a light source for measurement, provides a brand-new dispersion measurement method without interference, has convenient operation, simple data processing, and can measure the dispersion of various types of optical fibers.

Contents of the invention

The technical problem to be solved by the present invention is to propose a method for measuring fiber dispersion based on femtosecond laser pumping detection technology. Utilizing the characteristics of ultrafast energy density and ultrafast time response of femtosecond laser, combined with pump detection technology, the dispersion of optical fiber can be accurately measured. This method is simple in principle, easy to operate, good in repeatability, can measure the dispersion of various types of optical fibers, and has a wide range of applications.

The technical solution adopted in the present invention is to provide a method for measuring optical fiber dispersion. The sample is excited by pump light. After the electrons in the sample absorb the laser energy, non-equilibrium relaxation of electrons occurs, which is manifested as a transient state of the probe light. A sudden change in emissivity. The probe light is used to characterize changes in the emitted light from the sample surface. Through the detector, lock-in amplifier and computer system, the transient reflectance signal of the sample surface is collected, denoised and processed, and the transient reflectance change (△R/R) curve is drawn, and a sudden change occurs according to the value of △R/R The corresponding step size calculates the dispersion of the fiber.

The method for measuring optical fiber dispersion based on femtosecond laser pumping detection technology proposed by the present invention mainly includes the following contents:

(1) The femtosecond laser pulse emitted by the titanium sapphire femtosecond laser system is divided into two beams, the pump light and the probe light, after passing through the beam splitter. The energy density ratio of the two is controlled at about 10:1, and the pump light enters the pump The light transmission channel, the detection light enters the detection light transmission channel.

(2) In the pump light transmission channel, the continuous pump light is modulated by a chopper into a discontinuous square wave sequence pulse. If the measurement is the transient reflection signal before loading the fiber, the intermittent square-wave pump light pulse is directly transmitted to the focusing lens. If the measurement is the transient reflection signal after loading the optical fiber, the pump light pulse output from the chopper is first focused into the optical fiber through the focusing lens for transmission, and then transmitted to the focusing lens after exiting the optical fiber.

(3) After the probe light passes through a right-angle mirror, it enters the time-delay optical path. The delay time of the probe light is mainly controlled by a stepping motor, so that an optical path difference is formed between the pump light and the probe light.

(4) The pump light and the probe light are respectively emitted from their respective transmission channels and then converged to the same point on the sample surface through the focusing lens. The surface reflected light is received by the detector and enters the lock-in amplifier and the computer system.

(5) Draw the change curve of the transient reflectance of the film through computer software, and calculate the dispersion of the optical fiber according to the step size corresponding to the sudden change of reflectivity in the curve.

In this method, the femtosecond laser pumping detection technology with high sensitivity and strong anti-interference ability is used to test the dispersion of the optical fiber. Such as chromium, nickel, etc.) to detect and study the ultrafast reflectivity change process, so as to calculate the dispersion of the optical fiber to be tested. The experimental data obtained by this technique has high accuracy and strong reliability, and the test method is simple and repeatable, and is suitable for measuring the dispersion of various types of optical fibers.

Description of drawings

The structural block diagram and accompanying drawing of the present invention are as follows:

Figure 1 Schematic diagram of optical fiber dispersion measurement system;

Figure 2 Schematic diagram of the pump light transmission path;

Figure 3 is a schematic diagram of the detection light transmission path;

Figure 4 The change curve of the transient reflectance of the sample surface under the action of femtosecond laser;

In the figure, 1. Ti:Sapphire femtosecond laser system 2. Beam splitter 3. Pump light transmission path 4. Focusing lens 5. Sample 6. Mirror 7. Detection light transmission path 8. Computer 9. Lock-in amplifier 10. Detector 11. Chopper 12. Focusing mirror 13. Optical fiber 14. Time delay line 15. Mirror 16. Mirror.

Detailed ways

1. The Vedi CW laser with a maximum output power of 5W is used as the pump source of the Ti:Sapphire laser, and its working wavelength is 532nm. The center wavelength of the Ti:Sapphire femtosecond laser obtained after mode locking is 800nm, and the repetition frequency is 82 MHz. The single pulse width is 30fs, and the output laser energy is controlled at about 330mW.

2. The femtosecond laser beam emitted by the femtosecond laser system 1 passes through the beam splitter 2 and is divided into two beams, the pump light and the probe light. The optical power of the pump light is about 300mW, and the energy is relatively high. The optical power is about 30mW, which is mainly used to detect the change signal of the transient reflectivity of the excited sample surface.

3. The pump light first passes through the chopper 11, and the pump light is modulated at a frequency of 2000 Hz, and the continuous pump light is modulated into an intermittent square wave sequence pulse with a duty ratio of 1:1, and then the pump light is After passing through the focusing lens 12, the pump pulse is focused into the optical fiber 13 for transmission, emerges from the optical fiber 13, converges through the lens 4, and acts on the surface of the sample 5.

4. The probe light first passes through the reflector 6 and reflector 15, and then enters the time delay line 14. The time delay line is mainly composed of a stepping motor, and the motion accuracy is 1 step, each step is 10 μm, and the converted time resolution is 66.7fs , the probe light passes through the stepping motor and then exits through a reflector 16 , and is also converged by the lens 4 before acting on the surface of the sample 5 .

5. The converging spot of pump light and the converging spot of probe light on the surface of sample 5 must overlap together, and the smaller the spot, the better.

6. Use the detector 10 to detect the transient reflection signal on the sample 5, and convert it into an electrical signal, then input it into the lock-in amplifier 9 for denoising processing, and finally input it into the computer 8, and draw the transient reflection signal on the surface of the sample with special computer software. State reflectivity (ΔR/R) change curve.

表示。在样品表面瞬态反射率变化曲线中，曲线的横坐标表示探测光发生瞬态反射时对应的具体时刻，用步长（step）表示，1步长等于66.7飞秒。测量泵浦光传输通道中不加光纤前样品表面瞬态反射率变化发生突变时对应的步长

，以及泵浦光传输通道中加载光纤后样品表面瞬态反射率发生突变时对应的步长

，光纤色散引起的时间延迟用步长表示即为：。根据色散定义：，即可求出光纤的色散

。 7. Calculate the dispersion value of the optical fiber to be tested. Optical fiber dispersion, that is, the time delay generated after the light pulse passes through the optical fiber, is used

express. In the transient reflectance change curve of the sample surface, the abscissa of the curve indicates the specific moment when the probe light is transiently reflected, and is represented by a step, and one step is equal to 66.7 femtoseconds. Measure the step size corresponding to the sudden change in the transient reflectance of the sample surface before the optical fiber is added to the pump light transmission channel

, and the corresponding step size when the transient reflectance of the sample surface changes abruptly after the optical fiber is loaded in the pump light transmission channel

, the time delay caused by fiber dispersion is represented by the step size: . According to the definition of dispersion: , the dispersion of the fiber can be obtained

.

Claims (3)

1. the method for a measuring optical fiber chromatic dispersion is characterized in that, concrete steps are following:

A, employing peak power output are the pumping source of the Vedi CW laser instrument of 5W as ti sapphire laser; Its operation wavelength is 532nm; The centre wavelength of the titanium jewel femtosecond laser that behind locked mode, obtains is 800nm; Repetition frequency is 82 MHz, single pulse width 30fs, and the output laser energy is controlled at about 330mW;

Be divided into pump light behind the femtosecond laser beam process beam splitter (2) that B, fs-laser system (1) send and survey light two bundles, the luminous power 300mW of pump light is used for excited sample; Survey light luminous power 30mW, be used to survey the variable signal that is swashed sample surfaces transient state reflectivity;

C, pump light at first pass through chopper (11); Is that 2000 Hz modulate to pump light with frequency; Continuous pump light is modulated into the interrupted square wave train pulse that dutycycle is 1:1; Then pumping pulse line focus lens (12) backs is focused on and gets into optical fiber (13) transmission, from optical fiber (13) outgoing after lens (4) act on sample (5) surface after converging;

D, detection light at first pass through catoptron (6) and catoptron (15); Entry time lag line (14) then, the time delay line mainly is made up of a stepper motor, 1 step of kinematic accuracy; Per step is 10 μ m; Being converted into temporal resolution is 66.7fs, surveys light through a warp catoptron (16) outgoing again behind the stepper motor, after lens (4) are assembled, acts on sample (5) surface equally then;

The pump light focused light spot on E, sample (5) surface overlaps with detection optical convergence hot spot;

F, survey the transient state reflected signal on the sample (5) with detector (10); And convert it into electric signal; Import lock-in amplifier (9) then and carry out denoising Processing; Be input to computing machine (8) at last, draw transient state reflectivity (Δ R/R) change curve of sample surfaces with the computing machine special software;

The dispersion values of G, calculating testing fiber; Fibre-optical dispersion; Be that the time delay that produces behind the optical fiber is passed through in light pulse; The expression with

; Measure and not add step-length corresponding when sample surfaces transient state reflectance varies is undergone mutation before the optical fiber in the pump light transmission channel; And loading step-length corresponding when sample surfaces transient state reflectivity is undergone mutation behind the optical fiber in the pump light transmission channel, the time delay that fibre-optical dispersion causes is represented to be with step-length: .

2. the method for a kind of measuring optical fiber chromatic dispersion according to claim 1; It is characterized in that; Calculate the dispersion values of testing fiber, in sample surfaces transient state reflectance varies curve, the horizontal ordinate of curve representes to survey the corresponding concrete moment of light generation transient state reflex time; With step-length (step) expression, 1 step-length equals 66.7 femtoseconds.

3. the method for a kind of measuring optical fiber chromatic dispersion according to claim 2; It is characterized in that CHROMATIC DISPERSION IN FIBER OPTICS

.

Images