阅读说明：本技术 一种可饱和吸收体锁模方法以及可饱和吸收体锁模器件 (Mode locking method for saturable absorber and mode locking device for saturable absorber ) 是由 赵卫 王屹山 王虎山 张伟 胡晓鸿 张挺 赵凤艳 周凯明 于 2020-05-28 设计创作，主要内容包括：本发明提供了一种可饱和吸收体锁模方法以及可饱和吸收体锁模器件,利用一对大角度倾斜光纤光栅实现光和可饱和吸收材料的相互作用,弥补了现有可饱和吸收体锁模器件存在的缺陷,实现高性能可饱和吸收体锁模器件,对于超快光纤激光技术的发展具有推动作用。本发明基于大角度倾斜光纤光栅的可饱和吸收体锁模器件,包括沿光传输方向依次连接的第一个大角度倾斜光纤光栅和第二个大角度倾斜光纤光栅,所述第一个大角度倾斜光纤光栅和第二个大角度倾斜光纤光栅连接区域的光纤包层上沉积或涂覆可饱和吸收材料,其中,第一个大角度倾斜光纤光栅和第二个大角度倾斜光纤光栅的倾斜角度为66～89度。(The invention provides a mode locking method of a saturable absorber and a mode locking device of the saturable absorber, which utilize a pair of large-angle inclined fiber gratings to realize the interaction of light and a saturable absorption material, make up the defects of the existing mode locking device of the saturable absorber, realize the mode locking device of the high-performance saturable absorber and have a promoting effect on the development of an ultrafast fiber laser technology. The saturable absorber mode locking device based on the large-angle inclined fiber grating comprises a first large-angle inclined fiber grating and a second large-angle inclined fiber grating which are sequentially connected along the light transmission direction, saturable absorption materials are deposited or coated on fiber cladding layers of a connection region of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating, and the inclination angles of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are 66-89 degrees.)

1. A mode locking method for a saturable absorber is characterized in that: light transmitted in the fiber core of the optical fiber is coupled into a cladding of the optical fiber for transmission when passing through the first large-angle inclined fiber grating; then the interaction between the cladding evanescent field and the saturable absorption material deposited or coated on the cladding surface is carried out to generate saturable absorption effect; then the optical fiber is coupled back to the optical fiber core for continuous transmission when passing through a second large-angle inclined optical fiber grating; the inclination angles of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are 66-89 degrees.

2. The saturable absorber mode locking method according to claim 1, wherein: the inclination directions of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are the same or opposite along the axial direction of the optical fiber.

3. The saturable absorber mode locking method according to claim 1 or 2, wherein: the inclination angle and/or the grating period of the first large-angle inclined fiber grating is the same as that of the second large-angle inclined fiber grating.

4. The utility model provides a saturable absorber mode locking device based on wide-angle slope fiber grating which characterized in that: the fiber grating structure comprises a first large-angle inclined fiber grating and a second large-angle inclined fiber grating which are sequentially connected along the light transmission direction, wherein a saturable absorption material is deposited or coated on a fiber cladding of a connection region of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating, and the inclination angles of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are 66-89 degrees.

5. The mode-locked device of saturable absorber based on large-angle tilted fiber grating of claim 4, wherein: the inclination directions of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are the same or opposite along the axial direction of the optical fiber.

6. The saturable absorber mode-locked device based on the large-angle tilted fiber grating as claimed in claim 4 or 5, wherein: the inclination angle and/or the grating period of the first large-angle inclined fiber grating is the same as that of the second large-angle inclined fiber grating.

7. The saturable absorber mode-locked device based on large-angle tilted fiber grating according to claim 6, wherein: the first large-angle inclined fiber grating, the second large-angle inclined fiber grating and the fiber cladding between the two gratings, which is not deposited or coated with saturable absorption materials, are coated with low-folding glue, so that light of the cladding is prevented from leaking to the outside of the fiber, and extra loss is caused.

8. The mode-locked device of saturable absorber based on large-angle tilted fiber grating of claim 7, wherein: the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are manufactured on the same optical fiber, or are manufactured respectively and then are subjected to optical fiber fusion.

9. The saturable absorber mode-locked device based on large-angle tilted fiber grating of claim 8, wherein: manufacturing optical fibers of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating to be common single-mode optical fibers, few-mode optical fibers or multi-mode optical fibers; the saturable absorption material is carbon nano tube, graphene oxide, transition metal sulfide, transition metal oxide or black phosphorus.

10. An ultrashort pulse fiber laser is characterized in that: the mode locking device of the saturable absorber based on the large-angle tilted fiber grating, which comprises any one of claims 4 to 9, is fused in the fiber laser cavity to realize the mode locking of the fiber laser.

Technical Field

The invention relates to the field of optical fiber devices and optical fiber lasers, in particular to a mode locking method of a saturable absorber and a mode locking device of the saturable absorber.

Background

The fiber laser has unique technical advantages of high beam quality, high efficiency, high integration, high reliability and the like, and becomes an important development direction in the technical field of laser. The ultrashort pulse fiber laser has the remarkable characteristics of narrow pulse width, high repetition frequency, high peak power, wide spectral range and the like, and has wide application and huge market value in the fields of precision machining, medicine, high-speed imaging, strong-field physics, precision measurement, optical fiber sensing and the like.

The mode locking technology is the most efficient and concise mode for realizing ultrashort pulse output, wherein the passive mode locking technology has the advantages of simple structure, small volume, low cost, good compatibility, high conversion efficiency and self-starting, and becomes the most common ultrashort pulse generation technology. The passive mode locking technology mainly comprises nonlinear polarization rotation mode locking, nonlinear fiber ring mirror mode locking and saturable absorber mode locking. The nonlinear polarization rotation mode locking and the nonlinear fiber ring mirror mode locking are easily affected by ambient temperature, stress and the like, and have poor environmental stability. Therefore, the mode locking technology of the saturable absorber is a main technology for realizing the ultrashort pulse fiber laser at present.

The saturable absorber is a device with the transmittance related to the light intensity, and the mode locking principle of the saturable absorber is as follows: when a saturable absorber is placed in the laser cavity, the low intensity laser light is attenuated, however, due to the random variation in intensity of the unlocked mode laser light, the randomly generated intensity is large enough to transmit the saturable absorber. This process is repeated endlessly as the light oscillates in the laser cavity, so that high intensity laser light is amplified and low intensity light is absorbed. After oscillating many times, a series of optical pulses is generated and the laser is mode-locked.

In recent years, various nanomaterials such as carbon nanotubes, graphene oxide, transition metal sulfides, transition metal oxides, black phosphorus and the like have the characteristics of ultra-fast relaxation time, wide absorption spectrum lines, low cost and the like, and are used as saturable absorption materials to realize saturable absorber mode-locked devices. At present, for realizing the saturable absorber device, the adopted structural design scheme mainly has two types: first, the saturable absorber is made into a film shape and placed between two optical fiber patch cord connectors or between the end face of an optical fiber and a reflector, and the following problems exist in the structural design: the all-fiber structure is destroyed; easily cause mechanical damage to the material; the laser directly acts on the saturable absorption material, the acting area is small, the thermal damage threshold is reduced, high-quality mode locking pulse is difficult to realize, and devices are easy to damage. Secondly, the mode locking is realized by utilizing the side surface interaction between the saturable absorption material and the evanescent field to generate loss modulation in the laser, and the scheme can bear higher optical power and is beneficial to generating high-energy and high-power mode-locked fiber laser.

At present, the saturable absorption material carriers adopted in the second scheme are mainly micro-nano optical fibers and tapered optical fibers, and the two types of optical fibers have corresponding problems. The micro-nano optical fiber adopted in the saturable absorber device mainly comprises a D-type optical fiber and a side-polished optical fiber, the manufacturing of the optical fiber needs complex technology and expensive equipment, the fusion welding of the optical fiber and the common single-mode optical fiber in the optical fiber laser is difficult, the loss is generally large, and the proportion of light acted by an evanescent field and a material is difficult to accurately control and regulate. For the tapered optical fiber, the core diameter of the optical fiber at the tapered position is very thin and is easy to damage, which is not beneficial to practical application.

Disclosure of Invention

The invention provides a mode locking method of a saturable absorber and a mode locking device of the saturable absorber, which utilize a pair of large-angle inclined fiber gratings to realize the interaction of light and a saturable absorption material, make up the defects of the existing mode locking device of the saturable absorber, realize the mode locking device of the high-performance saturable absorber and have a promoting effect on the development of an ultrafast fiber laser technology.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a mode locking method for a saturable absorber is characterized in that light transmitted in a fiber core of an optical fiber is coupled into a cladding of the optical fiber for transmission when passing through a first large-angle inclined fiber grating; then the interaction between the cladding evanescent field and the saturable absorption material deposited or coated on the cladding surface is carried out to generate saturable absorption effect; then the optical fiber is coupled back to the optical fiber core for continuous transmission when passing through a second large-angle inclined optical fiber grating; the inclination angles of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are 66-89 degrees.

Meanwhile, the invention provides a saturable absorber mode locking device based on a large-angle inclined fiber grating, which comprises a first large-angle inclined fiber grating and a second large-angle inclined fiber grating which are sequentially connected along the light transmission direction, wherein saturable absorption materials are deposited or coated on fiber cladding layers of connecting areas of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating, and the inclination angles of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are 66-89 degrees.

Further, the inclination directions of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are the same or opposite along the axial direction of the optical fiber.

Further, the tilt angle and/or the grating period of the first large-angle tilted fiber grating and the second large-angle tilted fiber grating are the same.

Further, the first large-angle inclined fiber grating, the second large-angle inclined fiber grating and the fiber cladding between the two gratings, which is not deposited or coated with saturable absorption material, are coated with low-folding glue to prevent light of the cladding from leaking to the outside of the fiber and causing extra loss.

Furthermore, the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are manufactured on the same optical fiber, or are manufactured respectively and then are subjected to optical fiber fusion.

Furthermore, the optical fibers for manufacturing the first large-angle inclined fiber grating and the second large-angle inclined fiber grating are common single-mode optical fibers, few-mode optical fibers or multi-mode optical fibers; the saturable absorption material is carbon nano tube, graphene oxide, transition metal sulfide, transition metal oxide or black phosphorus.

In addition, the invention also provides an ultrashort pulse fiber laser, which comprises the saturable absorber mode locking device based on the large-angle inclined fiber grating, wherein the saturable absorber mode locking device based on the large-angle inclined fiber grating is welded in a fiber laser cavity to realize mode locking of the fiber laser.

Compared with the prior art, the invention has the following technical effects:

1. the invention adopts the large-angle inclined fiber grating to realize the interaction of light and a saturable absorption material, replaces the traditional schemes of D-shaped optical fiber, tapered optical fiber and the like, and realizes the high-performance saturable absorber mode locking device: the mode locking device of the saturable absorber has the following characteristics: (a) the saturable absorption material can be deposited or coated on the optical fiber cladding, compared with the traditional scheme, the surface area of the optical fiber cladding is larger, and the transmitted light can fully act with the saturable absorption material; (b) the coupling efficiency of the large-angle inclined fiber grating can be tuned in a certain range through the polarization controller, so that the action degree of light and the saturable absorption material can be controlled, the balance of interaction and loss of the light and the saturable absorption material in a laser cavity is achieved, and the mode locking of the laser is easier to realize; (c) the large-angle inclined fiber grating does not damage the physical structure of the optical fiber, and the mode locking device has higher stability; (d) the mode locking device can be directly manufactured based on single-mode optical fibers, the problem of optical fiber fusion is solved, and the insertion loss is low; (e) the large-angle inclined fiber grating has a filtering effect, can inhibit mode competition in a laser cavity, is beneficial to realizing mode locking of laser, and can control the wavelength range of output pulses; (f) the large-angle inclined fiber grating is relatively simple to manufacture and low in cost, and batch manufacturing of the saturable absorber mode locking device can be realized.

2. The invention adopts two large-angle inclined fiber gratings to realize the saturable absorber mode locking device, so that the saturable absorber mode locking device has the following characteristics: (a) two large-angle inclined fiber gratings can be respectively designed and optimized, different coupling efficiencies are realized, and parameters of a mode locking device are further easier to optimize; (b) the single large-angle inclined fiber grating is generally short in length, the length of the deposited saturable absorption material is limited in a mode locking device scheme of depositing the saturable absorption material on the single fiber grating, optical coupling from a fiber core to a cladding and from the cladding to the fiber core is achieved through the pair of large-angle inclined fiber gratings, the saturable absorption material is deposited or coated on a common fiber cladding between the two large-angle inclined fiber gratings, the larger deposition or coating length can be achieved, and the action degree of the light and the saturable absorption material is further improved; (c) by designing two large-angle inclined fiber gratings with different coupling wavelengths and bandwidths, narrow-spectrum ultrashort pulse laser output can be realized, and the application requirements of more lasers are met.

Drawings

FIG. 1 is a schematic diagram of a mode-locked device of a saturable absorber based on a large-angle inclined fiber grating according to the present invention (the directions of the grating inclination angles are the same);

FIG. 2 is a schematic diagram of a mode-locked device based on a saturable absorber with a large-angle tilted fiber grating according to the present invention (the tilt directions of the gratings are different).

Detailed Description

The invention is described in further detail below with reference to the figures and specific embodiments.

The invention provides a scheme for realizing a mode locking device of a saturable absorber by utilizing a pair of large-angle inclined fiber gratings, which can effectively improve multiple performances of the saturable absorber device, further realize a stable ultrashort pulse mode locking fiber laser and promote the development of an ultrafast fiber laser technology.

As shown in fig. 1, the mode-locked device of the saturable absorber based on the large-angle tilted fiber grating includes a first large-angle tilted fiber grating and a second large-angle tilted fiber grating connected in sequence along the optical transmission direction, and a saturable absorbing material is deposited or coated on the fiber cladding of the connection region of the first large-angle tilted fiber grating and the second large-angle tilted fiber grating. The working principle is as follows: due to the mode coupling effect of the large-angle inclined fiber grating, light transmitted in the fiber core of the optical fiber is coupled into the cladding of the optical fiber for transmission when passing through the first large-angle inclined fiber grating, the light transmitted in the cladding plays a role of saturable absorption through the interaction of the cladding evanescent field and a saturable absorption material deposited or coated on the surface of the cladding, and the light after the action is coupled back into the fiber core of the optical fiber from the fiber cladding through the second large-angle inclined fiber grating for continuous transmission.

The large-angle inclined fiber grating adopted in the saturable absorber mode locking device is different from a common fiber grating, an inclined periodic structure is arranged on a fiber core, in order to realize the coupling of transmission light from a forward fiber core to a forward cladding and from the forward cladding to the forward fiber core, the inclination angle is generally larger than 66 degrees and can be specifically 66-89 degrees, and the specific inclination angle and the grating period are determined according to the wavelength and the bandwidth of a required ultrashort pulse fiber laser.

The two large-angle inclined fiber gratings in the saturable absorber mode locking device can be manufactured on the same optical fiber or respectively manufactured and then subjected to optical fiber fusion. The inclination directions of the two large-angle inclined fiber gratings can be the same along the axial direction of the optical fiber (as shown in figure 1) or opposite along the axial direction of the optical fiber (as shown in figure 2), and both saturable absorber mode-locked devices can be realized. The two large-angle inclined fiber gratings can have the same parameters of inclination angle, grating period and the like, so that the two large-angle inclined fiber gratings have the same or similar coupling wavelength and bandwidth, and the maximum response bandwidth and transmission efficiency of the device are realized. The two large-angle inclined fiber gratings also can have different parameters, namely different coupling wavelengths and bandwidths, the filtering effect is realized through the dislocation of the response wavelengths and the bandwidths of the two large-angle inclined fiber gratings, the narrow-spectrum ultrashort pulse laser output can be realized, and the application requirements of more lasers are met.

In the mode locking device of the saturable absorber, the optical fiber adopted for manufacturing the large-angle inclined fiber grating can be selected from common single mode optical fiber, few mode optical fiber or multimode optical fiber, and is determined according to the requirements of the optical fiber laser.

In the mode locking device of the saturable absorber, the saturable absorbing material is deposited or coated on the surface of a cladding of the optical fiber at the joint of the first large-angle inclined fiber grating and the second large-angle inclined fiber grating after a coating layer of the optical fiber is removed. Compared with the traditional saturable absorber structure based on the tapered or D-shaped optical fiber, the surface area of the cladding is larger, and the saturable absorber material which transmits light and is deposited or coated on the surface of the cladding has more sufficient effect. In addition, the length of the coated or deposited optical fiber can be longer, flexible and easy to adjust, and the performance of the saturable absorber device can be further improved.

In the mode locking device of the saturable absorber, if the adopted large-angle inclined fiber grating and the optical fiber between the two gratings are provided with the coating layers, and the coating layers are high-folding glue, the coating layers of the optical fibers which are not deposited or coated with saturable absorption materials on the two large-angle inclined fiber gratings and the optical fiber between the two gratings still need to be removed or low-folding glue is coated after the coating layers are removed, so that light of the coating layers is prevented from leaking to the outside of the optical fiber, and extra loss is caused. If the large-angle inclined fiber grating and the fiber between the two gratings are not coated, the low-folding glue is directly coated on the cladding of the fiber region which is not deposited or coated with the saturable absorption material.

In the invention, the saturable absorber mode locking device is of an all-fiber structure, and fibers at two ends of the device can be welded in the fiber laser cavity to realize mode locking of the fiber laser. The ultrashort pulse fiber laser can adopt a linear cavity or a ring cavity. In addition, the laser can be used together with a polarization controller, and mode locking of the laser is easier to realize.

The invention adopts the large-angle inclined fiber grating to realize the interaction of light and a saturable absorption material, replaces the traditional schemes of D-shaped optical fiber, tapered optical fiber and the like, and realizes a high-performance saturable absorber mode locking device. The mode locking device of the saturable absorber has the following characteristics: (a) the saturable absorption material can be deposited or coated on the optical fiber cladding, compared with the traditional scheme, the surface area of the optical fiber cladding is larger, and the transmitted light can fully act with the saturable absorption material; (b) the coupling efficiency of the large-angle inclined fiber grating can be tuned in a certain range through the polarization controller, so that the action degree of light and the saturable absorption material can be controlled, the balance of interaction and loss of the light and the saturable absorption material in a laser cavity is achieved, and the mode locking of the laser is easier to realize; (c) the large-angle inclined fiber grating does not damage the physical structure of the optical fiber, and the mode locking device has higher stability; (d) the mode locking device can be directly manufactured based on single-mode optical fibers, the problem of optical fiber fusion is solved, and the insertion loss is low; (e) the large-angle inclined fiber grating has a filtering effect, can inhibit mode competition in a laser cavity, is beneficial to realizing mode locking of laser, and can control the wavelength range of output pulses; (f) the large-angle inclined fiber grating is relatively simple to manufacture and low in cost, and batch manufacturing of the saturable absorber mode locking device can be realized.

The invention adopts two large-angle inclined fiber gratings to realize the saturable absorber mode locking device, so that the saturable absorber mode locking device has the following characteristics: (a) two large-angle inclined fiber gratings can be respectively designed and optimized, different coupling efficiencies are realized, and parameters of a mode locking device are further easier to optimize; (b) the single large-angle inclined fiber grating is generally short in length, the length of the deposited saturable absorption material is limited in a mode locking device scheme of depositing the saturable absorption material on the single fiber grating, optical coupling from a fiber core to a cladding and from the cladding to the fiber core is achieved through the pair of large-angle inclined fiber gratings, the saturable absorption material is deposited or coated on a common fiber cladding between the two large-angle inclined fiber gratings, the larger deposition or coating length can be achieved, and the action degree of the light and the saturable absorption material is further improved; (c) by designing two large-angle inclined fiber gratings with different coupling wavelengths and bandwidths, narrow-spectrum ultrashort pulse laser output can be realized, and the application requirements of more lasers are met.