CN107359497B - A method for dispersion management and chirp compensation based on micro-nano fiber - Google Patents

Abstract

The invention relates to a method for dispersion management and chirp compensation based on micro-nano optical fibers. The method includes the following steps: providing a micro-nano optical fiber with proper dispersion characteristics; properly encapsulating the micro-nano optical fiber in a box with certain sealing and mechanical strength; splicing the packaged micro-nano optical fiber on the optical path of the optical fiber desired location. The dispersion management and chirp compensation method proposed by the present invention utilizes the unique dispersion characteristics and extremely low transmission loss characteristics of micro-nano optical fibers, and is convenient for fusion splicing with ordinary optical fibers. The method is fully compatible with the currently used fiber technology, and can adjust the dispersion of the system in a large range to achieve the purpose of dispersion management and chirp compensation.

Description

A method for dispersion management and chirp compensation based on micro-nano fiber

technical field

The invention belongs to the technical field of ultrafast laser technology and optical dispersion compensation, in particular to a method for dispersion management and chirp compensation based on micro-nano fiber and a method for manufacturing a femtosecond laser.

Background technique

Fiber femtosecond lasers have many outstanding advantages such as low cost, compact structure, simple operation, good beam quality, high stability, and low environmental requirements. have a wide range of applications. By adjusting the total dispersion in the laser resonator, the working area of the laser can be changed to generate femtosecond pulses with different properties, such as sech-type soliton pulses in the negative dispersion region, Gaussian-type broadened pulses near zero dispersion, and full positive dispersion time spectra. An approximately rectangular pulse. In addition to the time-domain waveform and spectral shape, the total dispersion of the laser resonator has an important impact on the maximum single-pulse energy and noise performance of the laser, which is very important for large-scale equipment synchronization, precision measurement and other applications.

For the communication band, ordinary single-mode fiber has negative dispersion, while erbium-doped fiber can have positive dispersion, and dispersion management and chirp compensation are very convenient. However, for the 1-micron wavelength band, both the single-mode fiber and the Yb-doped gain fiber are positive dispersion fibers, while for the 2-micron wavelength band, the single-mode fiber and the Tm-doped or Tm:Ho co-doped fiber are both negative-dispersion fibers, which are difficult to pass through. Adjust the dispersion from positive to negative by changing the length of the fiber.

At present, in Yb-doped fiber mode-locked lasers and Tm-doped or Tm:Ho co-doped fiber mode-locked lasers, the commonly used dispersion adjustment methods are:

(1) Prism/prism pair: Generally, a prism/prism pair can provide negative second-order dispersion, which is very common in free-space Ti:sapphire lasers;

(2) Grating/grating pair: The grating has a strong dispersion, which can be used to provide a large positive or negative dispersion with a grating/grating pair in free space or in an optical waveguide;

(3) Chirped mirror: The surface of the flat mirror is coated with a specially designed multilayer film, which can also achieve dispersion compensation.

(4) Gires-Tournois interferometer: Using the reflection characteristics of the interferometer, changing the incident angle can adjust the dispersion of the interferometer.

Except for the chirped fiber grating, other methods are implemented in the free space optical path, which is incompatible with the optical fiber system, and it is not convenient to adjust, which reduces the original mechanical stability of the optical fiber system;

(5) Optical fibers with special structures: It can also be seen in the literature that there are optical fibers with special designed structures, such as photonic crystal fibers with hollow/solid structures, large numerical aperture fibers, etc., to compensate for dispersion. However, these fibers are generally incompatible with the pigtails of optical fiber devices, and the mode field matching is poor, which often has special requirements for the fusion technology, and the fusion loss is high;

(6) Utilize high-order modes of few-mode fibers: modes in ordinary fibers can be coupled to high-order modes in few-mode fibers through long-period fiber gratings in ordinary fibers, and all-fiber structures can be realized by utilizing the propagation characteristics of high-order modes dispersion compensation. However, this method requires additional long-period fiber gratings, which increases the complexity of the system.

Since the above methods have their own shortcomings, from a practical point of view, it is necessary to provide a simple, stable, convenient and reliable technology that is fully compatible with the existing fiber system to realize the dispersion management in the fiber femtosecond laser resonator cavity and the cavity. The method of external chirp compensation.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a method for dispersion management and chirp compensation based on micro-nano optical fibers. The micro-nano optical fibers drawn from ordinary optical fibers have unique dispersion characteristics. At the same time, it can generate strong positive dispersion or negative dispersion in the required waveband, so as to achieve the purpose of dispersion adjustment and chirp compensation; the manufacturing process of the invention is simple, the required optical fiber is completely compatible with the existing optical fiber system, and the insertion loss It is very low and the splice loss is negligible. Depending on the design parameters, the required dispersion can be provided in the required wavelength band, such as a large negative dispersion in the 1 micron band, or a large positive dispersion in the 2 micron band. It has great practical value for the dispersion adjustment in the ultrafast fiber laser cavity and the pulse compression outside the cavity. Specifically, the present invention utilizes a specific fabrication method, such as fusion taper method, to obtain micro-nano optical fibers with certain design parameters, which can achieve extremely low-loss fusion splicing with common optical fibers or gain optical fibers through pigtails. By drawing micro-nano fibers of different diameters and lengths, the dispersion provided by micro-nano fibers can be adjusted in a wide range. The micro-nano fiber is used in the resonator of the ultrafast laser, and can adjust the working region of the mode-locked state of the ultrafast laser (including the soliton region of negative dispersion, the broadened pulse region of near zero dispersion, and the region of positive dispersion). Used at the output end outside the laser cavity, it can compensate chirp and realize the compression or broadening of ultra-short pulses.

In order to achieve the above object, the technical scheme adopted in the present invention is:

A method of dispersion management and chirp compensation based on micro-nano fiber, design the diameter and length of the required micro-nano fiber, fabricate the micro-nano fiber, and package it, and combine the micro-nano fiber with the optical fiber in the part that needs dispersion management. And the pigtail fusion splicing of optical fiber devices to achieve dispersion management and chirp compensation.

The method specifically includes the following steps:

(1) Design: Design the geometric parameters of the micro-nano fiber by numerical calculation method, and obtain the required length and diameter of the micro-nano fiber.

Specifically, a typical design method is as follows: through the finite element method (such as Comsol Multiphysics software) or other optical waveguide mode calculation methods, the second-order dispersion curves of micro-nano fibers with different diameters are obtained, as shown in Figure 1. Select a diameter range with positive or negative dispersion in a specific wavelength band to obtain an approximate second-order dispersion value. According to the second-order dispersion value and the total dispersion amount to be compensated, the required length of the micro-nano fiber is calculated. For example, in Figure 1, in the 1.06-micron wavelength band, when the diameter of the micro-nano fiber is 1 to 2 microns, the second-order dispersion value is in the range of -150ps ² /km to -50ps ² /km. The diameter of the micro-nano fiber can be selected to be 1.5 microns, and the second-order dispersion value is -140ps ² /km. The total dispersion amount to be compensated is 0.02ps ² (corresponding to the total dispersion amount of a 1 meter long HI1060 fiber), then the required length of the micro-nano fiber with a diameter of 1.5 microns is about 15 cm.

(2) Tapering: remove the coating on the surface of the optical fiber, and fix both ends of the optical fiber on the fixture. Use a high temperature heat source to heat the exposed fiber area and stretch the fiber toward both ends. During the stretching process, the high temperature heat source moves back and forth, increasing the heating area. The stretched fiber consists of a taper transition region and a micro-nano fiber region; and then a certain dispersion measuring device is used to detect whether the second-order dispersion of the micro-nano fiber meets the design requirements.

The optical fiber is an ordinary single-mode or multi-mode optical fiber. The high temperature heat source is butane or isobutane-oxygen flame, hydrogen-oxygen flame, _CO2 laser, high-voltage arc, or high-temperature ceramic heater, and the temperature is 400℃-1000℃.

(3) Encapsulation: Properly encapsulate the prepared micro-nano fibers in a specially designed box. During the packaging process, it is necessary to ensure that the micro-nano fiber and the taper transition area are all within the box. The box should have certain air tightness and anti-ash function, and ensure the mechanical strength of the micro-nano fiber dispersion compensation device.

(4) Access: connect the encapsulated micro-nano optical fiber obtained through the above steps to a desired position. By using common optical fiber fusion splicing equipment and common optical fiber fusion splicing technology, low-loss access of micro-nano optical fibers in optical fiber systems can be realized.

In the present invention, the micro-nano optical fiber is composed of one or more sections of uniform diameter micro-nano optical fiber, or, it is composed of one or more sections of micro-nano optical fiber with tapered diameter.

The diameter of the micro-nano fiber is in the range of 500nm to 10μm, and the length is in the range of 1cm to 10m. The micro-nano fiber is continuously connected to the ordinary single-mode fiber through the tapered taper.

The invention can obtain micro-nano fibers with different dispersion characteristics by adjusting the process parameters for preparing micro-nano fibers, and change the dispersion value provided by the micro-nano fibers; To adjust the total dispersion value of micro-nano fiber and ordinary fiber.

Compared with the prior art, the beneficial effects of the present invention are:

1. The present invention adopts common optical fiber taper, and does not need to use special optical fiber. That is, through the taper technology of ordinary optical fibers, micro-nano optical fibers with dispersion compensation function can be obtained.

2. The micro-nano optical fiber used in the present invention can be continuously connected with the pigtail of the ordinary optical fiber after the taper drawing, which ensures extremely high optical transmission efficiency and extremely low subsequent fusion loss with the ordinary optical fiber. Insertion loss is extremely low.

3. The micro-nano optical fiber used in the present invention to be connected with the ordinary optical fiber with low loss can be realized by changing the process parameters for preparing the micro-nano optical fiber and adjusting the length of the ordinary optical fiber to be spliced when adjusting the dispersion.

Description of drawings

Figure 1 is the second-order dispersion diagram of micro-nano fibers with different diameters around 1 micron.

Figure 2 is the second-order dispersion diagram of micro-nano fibers with different diameters around 2 microns.

3 is a schematic diagram of the micro-nano fiber used to adjust the intra-cavity dispersion of the Yb-doped fiber femtosecond laser in Example 1 of the present invention.

FIG. 4 is a spectrogram obtained according to FIG. 3 .

FIG. 5 is a schematic diagram illustrating that the micro-nano fiber is used to adjust the extra-cavity dispersion of the Yb-doped fiber femtosecond laser in Embodiment 2 of the present invention.

FIG. 6 is an interference autocorrelation trace before and after pulse compression according to FIG. 5 .

Detailed ways

The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples.

Example 1

The present invention proposes a method for using micro-nano fiber as dispersion management method. Generally speaking, the present invention firstly prepares the micro-nano fiber with design parameters, and then splices the tail fiber of the micro-nano fiber with the ordinary fiber at the required position, In order to achieve the purpose of dispersion compensation.

Figure 1 shows the calculated second-order dispersion diagrams for micro-nano fibers with diameters of 1.0, 1.2, 1.5, and 2.0 microns, respectively. It can be seen from the figure that in the range of 1000nm to 1200nm, the dispersion of micro-nano fibers with the above diameters is always negative, and its absolute value is about 5-10 times that of ordinary fibers (such as Corning's HI1060 fiber, which is 23ps ² /km). Figure 2 shows the calculated second-order dispersion diagrams of micro-nano fibers with diameters of 1.0, 1.2, 1.5, and 2.0 microns, respectively. It can be seen from the figure that in the range of 1800nm to 2000nm, the dispersion of micro-nano fibers with the above diameters is always positive, and its absolute value is about 3-30 times that of ordinary fibers (such as Nufern's SM1950, which is -80ps ² /km).

Figure 3 shows a schematic diagram of inserting a micro-nano fiber into the resonator of a Yb-doped polarization-rotation mode-locked laser. FIG. 4 is a typical spectrum obtained according to FIG. 3 .

In the above-mentioned lasers, the self-starting of the mode-locked state is achieved by setting up an artificial saturable absorber composed of a quarter-wave plate, a half-wave plate and a polarization beam splitter PBS. The second-order dispersion value of the Yb-doped fiber used in the laser and the pigtail fiber of the fiber device used is about 23ps ² /km. Adding optical elements that can provide negative dispersion to the resonator can adjust the total dispersion of the laser cavity to about zero, so that the laser can work in the soliton region or the broadened pulse region to obtain better noise characteristics and output a stable pulse sequence. The above objects can be achieved by micro-nano fibers with a certain diameter and length.

The schematic diagram of the micro-nano fiber is shown in Figure 3, which is continuously connected to the ordinary single-mode fiber through a graded fiber taper. The drawing process of the micro-nano fiber ensures that the micro-nano fiber and the tapered tapered have extremely low optical transmission loss, and that the diameter and length of the micro-nano fiber are consistent with the design values. The diameter of the micro-nano fiber used in Figure 3 is about 1.6 microns and the length is about 10 cm. The pigtails connected at both ends of the micro-nano fiber ensure that it can use the fusion splicing process of ordinary optical fibers to achieve extremely low-loss fusion with ordinary single-mode fibers, so that it has extremely low insertion loss while adjusting the laser dispersion. The spectrogram in Figure 4 shows that the laser can work in the broadened pulse region after adjusting the dispersion using the micro-nano fiber with the above characteristics.

Example 2

The method of using the micro-nano fiber for dispersion management proposed by the present invention can be used not only in the cavity of the laser, but also in the chirped compensation optical path outside the cavity of the laser. This example presents a schematic demonstration as well as a practical effect for this purpose.

Figure 5 shows the schematic diagram of the optical path when the micro-nano fiber is used for the compensation of the extra-cavity chirp of the Yb-doped femtosecond fiber laser. In the output optical path after the PBS, micro-nano optical fibers with a certain diameter and length, as well as ordinary single-mode optical fibers for chirp compensation, are connected. The micro-nano fiber used for this purpose, the collimator pigtail and the chirped compensation single-mode fiber are spliced by ordinary fiber splicing methods to achieve extremely low-loss fusion. Since the micro-nano fiber provides positive dispersion in the 1-micron waveband, while the ordinary fiber provides negative dispersion, adjusting the length of the ordinary fiber used for chirp compensation can finely adjust the chirp compensation for the output chirped pulse. The interferometric autocorrelation in Figure 6 shows this result. After careful adjustment of the length of the ordinary fiber used for chirp compensation, the output pulse with a large chirp can be compensated to a pulse with almost no chirp, and the pulse width is about 100fs.

The above embodiments are used to demonstrate the principle and application of the present invention, but it should be understood that the above description does not limit the protection scope of the present invention, and other insubstantial modifications to the present invention are within the protection scope of the present invention. In fact, micro-nano fibers can be easily used in other systems, such as ultra-short pulse fiber lasers that can be integrated in all-fiber devices, for saturable absorbers such as carbon nanotubes, graphene, semiconductor saturable absorption mirrors, etc. In the dispersion management of the formed ultrashort pulse laser and the chirp compensation of the ultrashort pulse, it is convenient to realize the dispersion management and chirp compensation with small optical insertion loss, good stability, convenient fusion, and fully compatible with the existing optical fiber system. By designing the required diameter and length of the micro-nano fiber, the method for dispersion management and chirp compensation provided by the present invention is particularly suitable for the 1-micron waveband and the 2-micron waveband where it is difficult for ordinary optical fibers to achieve dispersion adjustment. When used in other frequency bands, they are all within the protection scope of the present invention.

Claims (6)

1. A method for dispersion management and chirp compensation based on micro-nano optical fibers is characterized in that the diameter and the length of the required micro-nano optical fibers are designed, the micro-nano optical fibers are manufactured and packaged, the micro-nano optical fibers are welded with optical fibers and tail fibers of optical fiber devices at the part needing dispersion management, dispersion management and chirp compensation are achieved, the micro-nano optical fibers are uninterruptedly connected with common single-mode optical fibers through gradual-change tapering, the micro-nano optical fibers are manufactured by adopting a fused tapering method, and the method comprises the following steps:

removing a coating layer on the surface of an optical fiber, fixing two ends of the optical fiber on a clamp, heating a bare optical fiber area by using a high-temperature heat source, stretching the optical fiber to the two ends, wherein the high-temperature heat source moves back and forth in the stretching process to enlarge a heating area, the stretched optical fiber consists of a tapered transition area and a micro-nano optical fiber area, and then detecting whether the second-order dispersion of the micro-nano optical fiber meets the design requirement by using a dispersion measuring device, the optical fiber is a common single-mode or multi-mode optical fiber, the high-temperature heat source is butane or isobutane-oxygen flame, oxyhydrogen flame, a CO2 laser, high-pressure electric arc or a high-temperature ceramic heater, and;

in the laser, the mode-locking state self-starting is achieved by setting an artificial saturable absorber consisting of an 1/4 wave plate, a half-wave plate and a polarization beam splitter PBS, an optical element for providing negative dispersion is added into a resonant cavity, and the total dispersion in the cavity of the laser is adjusted to be zero, so that the laser works in a soliton area or a pulse broadening area, excellent noise characteristics are obtained, and a stable pulse sequence is output;

the method comprises the steps that a micro-nano optical fiber with preset diameter and length and a common single mode fiber for compensating chirp are connected to an output light path behind the polarization beam splitter PBS, the micro-nano optical fiber and the collimator tail fiber, and the micro-nano optical fiber and the single mode fiber for compensating chirp are welded together through the common optical fiber to achieve extremely-low loss welding, the length of the common optical fiber for compensating chirp is adjusted, and therefore chirp compensation of output pulses is achieved.

2. The method for dispersion management and chirp compensation based on micro-nano fibers according to claim 1, wherein micro-nano fibers with appropriate diameters are selected according to second-order dispersion spectrums of different micro-nano fibers, and the length of the required micro-nano fibers is calculated according to the total dispersion amount to be compensated.

3. The method for dispersion management and chirp compensation based on micro-nano optical fibers according to claim 1, wherein the micro-nano optical fibers are composed of one or more sections of micro-nano optical fibers with uniform diameters, or are composed of one or more sections of micro-nano optical fibers with gradually changed diameters.

4. The method for dispersion management and chirp compensation based on micro-nano optical fibers according to claim 1 or 3, wherein the diameter of the micro-nano optical fibers is in a range of 500nm to 10 μm, and the length of the micro-nano optical fibers is in a range of 1cm to 10 m.

5. The method for dispersion management and chirp compensation based on micro-nano optical fibers according to claim 1, wherein micro-nano optical fibers with different dispersion characteristics are obtained by adjusting process parameters for preparing the micro-nano optical fibers, and the dispersion value provided by the micro-nano optical fibers is changed; the total dispersion value of the micro-nano optical fiber and the common optical fiber is adjusted by adjusting the length of the common optical fiber or the tail fiber of the optical fiber device connected with the micro-nano optical fiber.

6. The method for chromatic dispersion management and chirp compensation based on micro-nano optical fibers according to claim 1, wherein the micro-nano optical fibers and the fiber tapering transition region are packaged in a box with certain air tightness and mechanical strength.

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