CN116540349A

CN116540349A - Mode filter, few-mode optical fiber amplifier and multistage optical fiber amplifier system

Info

Publication number: CN116540349A
Application number: CN202310440067.6A
Authority: CN
Inventors: 何飞; 邹锶; 万威; 陈抗抗
Original assignee: WUHAN ANYANG LASER TECHNOLOGY CO LTD
Current assignee: WUHAN ANYANG LASER TECHNOLOGY CO LTD
Priority date: 2023-04-21
Filing date: 2023-04-21
Publication date: 2023-08-04

Abstract

The utility model discloses a mode filter, a few-mode optical fiber amplifier and a multistage optical fiber amplifier system, wherein the mode filter and a polarization-preserving ytterbium-doped few-mode optical fiber are welded through an optical fiber welding machine, and the cross section of the mode filter comprises the following components: the fiber core area, the diameter of the fiber core is smaller than the diameter of the few-mode optical fiber; a cladding region surrounding the outside of the core region; the two stress rods are arranged on the cladding region and are symmetrically arranged relative to the upper and lower axes of the fiber core region; and the Bragg gratings are symmetrically distributed and inscribed on the edge of the fiber core region, and the positions of the Bragg gratings are overlapped with the high-order transverse mode part transmitted in the fiber core of the fiber core region. According to the utility model, the Bragg grating structure is inscribed around the fiber core of the few-mode optical fiber through the grating inscribing device, and the target position of the grating on the cross section of the optical fiber is matched with the high-order transverse mode field distribution of the optical fiber, so that the high-order mode in the optical fiber is subjected to the grating to obtain larger loss, and the mode filtering function is realized.

Description

Mode filter, few-mode optical fiber amplifier and multistage optical fiber amplifier system

Technical Field

The utility model relates to the technical field of lasers, in particular to a mode filter, a few-mode optical fiber amplifier and a multistage optical fiber amplifier system.

Background

The few-mode optical fiber refers to an optical fiber supporting transmission amplification of several higher-order modes in addition to the fundamental mode (LP 01) of the optical fiber. In the field of optical communications, few-mode optical fibers are used for mode division multiplexing to increase channel capacity, breaking through bandwidth limitations. In the field of optical fiber amplifiers, rare earth doped few-mode fibers are often used as gain media to amplify signals, breaking through the power limitations of small core single mode fibers.

In US patent publication No. 9595802B2, entitled multimode fiber amplifier, a method for realizing mode filtering by coiling optical fiber is proposed, which is based on the principle that loss is introduced into optical fiber mode by coiling optical fiber, so that loss introduced by higher order mode is larger than loss introduced by lower order mode, thereby realizing mode filtering function. However, this method has three disadvantages in practical use: firstly, axial torsion is inevitably generated when the optical fiber is coiled, stress is brought, torsion applied to the optical fiber after the optical fiber is fixed is changed along with time, and the mode filtering effect is changed; secondly, the system is sensitive to the influence of the environmental temperature, the mode loss can change along with the change of the temperature, and the system cannot guarantee the operation under the environment with temperature difference; third, coiling requires a certain fiber length to function, while for a femtosecond pulse chirped amplification system, an unnecessarily long fiber can cause non-linearity, resulting in reduced laser performance.

Chinese patent publication No. CN218240467U, entitled optical fiber mode multiplexing system, proposes a method for exciting different modes in a few-mode optical fiber by means of a reconfigurable super surface device unit, which can also be used for mode filtering when the reconfigurable super surface device unit is set to the filtering fundamental mode. However, the method uses multiple coupling from optical fiber to space to optical fiber, and has complex structure and poor coupling stability. The reconfigurable super-surface device has high unit price, limited bearable optical power and is not suitable for high-power laser amplification.

Chinese patent publication No. CN103928829a, entitled high-order mode acquisition device based on few-mode fiber bragg grating, proposes a method of reflecting different modes in few-mode fiber using fiber bragg grating. By selecting the center wavelength of the light source, the self-coupling resonance peak of the mode in the grating is matched, and reflection of different modes is realized. The device may also be used for mode filtering when all higher order modes are arranged to be reflected by the fiber bragg grating. However, this approach is limited to the use of narrow linewidth light sources, and for femtosecond pulse chirped amplification systems, the pulse spectral width is typically greater than 10nm, and this filtering approach is not applicable.

Mode filtering in a few-mode fiber amplifier is an effective solution to avoid inter-mode coupling crosstalk, however, a stable and reliable mode filter device suitable for a few-mode fiber amplifier in a femtosecond pulse chirped amplification system remains to be solved.

Disclosure of Invention

Aiming at the defects in the prior art, the utility model aims to provide a mode filter, a few-mode optical fiber amplifier and a multi-stage optical fiber amplifier system so as to solve the technical problems of pulse quality and pulse stability of a femtosecond pulse chirp amplifying system caused by inter-mode coupling crosstalk in the prior art.

In order to solve the above problems, a first object of the present utility model is to provide a mode filter, which is fusion-spliced with a polarization-maintaining ytterbium-doped few-mode fiber by an optical fiber fusion splicer, the cross section of the mode filter comprising:

the fiber core area is provided with a fiber core diameter smaller than the diameter of the ytterbium-doped few-mode fiber;

a cladding region surrounding the outside of the core region; and

the two stress rods are arranged on the cladding region and are symmetrically arranged relative to the upper and lower axes of the fiber core region;

and the Bragg gratings are symmetrically distributed and written at the edge of the fiber core region, and the positions of the Bragg gratings are overlapped with the high-order transverse mode part transmitted in the fiber core of the fiber core region.

Further, the bragg gratings are provided with four bragg gratings, wherein two bragg gratings are inscribed in the up-down direction of the fiber core area along the direction of the stress rod, and the other two bragg gratings are inscribed in the left-right direction of the fiber core area along the vertical direction of the stress rod.

Further, the Bragg grating is a reflection type grating, the reflection center wavelength of the Bragg grating is 1030-1080nm, the spectral bandwidth is 5-25nm, and the reflectivity is 50-100%.

Further, the writing diameter of the Bragg grating is 2 micrometers, the length is 2mm, and the writing position is 19 micrometers away from the center of the fiber core.

Further, the core region has a core diameter of 20 microns, the core numerical aperture NA of 0.04, the cladding region has a diameter of 125 microns, and the cladding region has a cladding numerical aperture NA of 0.46.

Furthermore, the fiber core of the fiber core region is made of pure quartz glass or mixed quartz glass doped with rare earth elements of erbium, ytterbium, neodymium and holmium.

Further, the mode filter is a step-index optical fiber or a photonic crystal optical fiber.

Further, the mode filter is a non-polarization-maintaining few-mode optical fiber or a polarization-maintaining few-mode optical fiber with the fiber core diameter of 10-40 microns and the cladding diameter of 125-400 microns.

A second object of the present utility model is to provide a few-mode optical fiber amplifier based on the above-mentioned mode filter, comprising: beam combiner, multimode pump, ytterbium doped few-mode fiber and mode filter, wherein:

the input end of the beam combiner is respectively connected with the few-mode transmission optical fiber and the multimode pump, the output mode of the multimode pump is multimode pump light of a fundamental mode, the output end of the beam combiner is connected with the ytterbium-doped few-mode optical fiber, and the beam combiner is used for optically coupling a signal to be amplified in the few-mode transmission optical fiber and the multimode pump with the fundamental mode and then injecting the signal into the ytterbium-doped few-mode optical fiber.

A third object of the present utility model is to provide a multistage optical fiber amplifier system comprising: the all-fiber mode locking seed source, the stretcher, the compressor and the few-mode fiber amplifier are described above, the output end of the all-fiber mode locking seed source is connected with the signal input end of the stretcher, the incident end of the few-mode fiber amplifier is connected with the signal output end of the stretcher, the reflection end of the few-mode fiber amplifier is connected with the input end of the compressor, and the output end of the few-mode fiber amplifier is collimated and then input to the compressor.

Compared with the prior art, the utility model has the following beneficial effects:

1. the mode filter is particularly suitable for a few-mode optical fiber amplifier, a Bragg grating structure is inscribed around a fiber core of a fiber core area through a grating inscribing device, so that the target position of the Bragg grating on the fiber cross section is matched with the high-order transverse mode field distribution of the fiber, and because the mode filter and the ytterbium-doped few-mode optical fiber are welded through an optical fiber welding machine, when pulses are transmitted in the few-mode optical fiber for a long distance, the few-mode optical fiber supports transmission of a plurality of modes, and coupling crosstalk can occur among the modes to cause pulse phase change. Particularly in a femtosecond pulse chirped amplification system, the uncontrolled pulse phase change can deform or destabilize the compressed pulse, when the compressed pulse is transmitted into a mode filter, the high-order mode in the few-mode optical fiber is effectively filtered under the action of a Bragg grating, so that the high-order mode in the optical fiber is subjected to larger loss after passing through the Bragg grating, and the mode filtering function is realized; therefore, in the optical fiber communication line, the few-mode optical fiber amplifier realizes signal amplification without complex processes such as photoelectric conversion, electro-optical conversion, signal regeneration and the like, can directly perform all-optical amplification on signals, and has good transparency.

2. The fiber filter based on the Bragg grating has the advantages of small volume, simple structure, high integration level and insensitivity to environmental change. On one hand, the device based on the fiber Bragg grating has high stability, and is more beneficial to the operation of the fiber amplifier in a scene with larger environmental temperature change; on the other hand, the all-fiber structure avoids using complex coupling devices, is very beneficial to the integration and maintenance of the amplifier, and reduces the manufacturing cost and the maintenance cost.

Drawings

FIG. 1 is a schematic view of a mode filter according to an embodiment of the present utility model;

FIG. 2 is a schematic diagram of a few-mode fiber amplifier in a femtosecond pulse chirped amplification system according to an embodiment of the present utility model;

FIG. 3 is a schematic cross-sectional view of a mode filter according to an embodiment of the present utility model;

fig. 4 is a schematic diagram of comparing output modes of the few-mode fiber without the mode filter with output modes of the few-mode fiber after the mode filter in the embodiment of the utility model.

Reference numerals illustrate:

1-few-mode optical fiber; 11-cores; 12-cladding; 2-bragg gratings.

Detailed Description

The following description of the embodiments of the present utility model will be made apparent and fully in view of the accompanying drawings, in which some, but not all embodiments of the utility model are shown. All other embodiments, which can be made by those skilled in the art based on the embodiments of the utility model without making any inventive effort, are intended to be within the scope of the utility model.

In the description of the present utility model, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; can be mechanically or electrically connected; the two components can be directly connected or indirectly connected through an intermediate medium, or can be communicated inside the two components, or can be connected wirelessly or in a wired way. The specific meaning of the above terms in the present utility model will be understood in specific cases by those of ordinary skill in the art.

In addition, the technical features of the different embodiments of the present utility model described below may be combined with each other as long as they do not collide with each other.

Because the fiber core of the optical fiber is small in size, energy is relatively concentrated, the transmission distance of pulses in the optical fiber is long, nonlinear effects become the most important factors for preventing the performance of the optical fiber femtosecond laser from being improved, and a large amount of nonlinear effects accumulate to cause the compressed pulses to split and distort. It is difficult to directly obtain high average power, high peak power femtosecond laser output in an optical fiber with conventional technology. The femtosecond fiber laser technology has been rapidly developed thanks to the utility model of chirped pulse amplification technology (CPA, chirped pulse amplification).

The chirped pulse amplification technique mainly consists of four parts: laser oscillators (seed sources), pulse stretcher, laser amplifiers, and pulse compressors. The basic principle is as follows: before the seed laser pulse is amplified, the seed laser pulse is stretched to hundred picoseconds or nanoseconds in the time domain by a dispersion device, then the power of the stretched pulse in a laser amplifier is improved, and finally the dispersion introduced by a front stage is compensated by a pulse compressor to compress the pulse to femtosecond magnitude. The purpose of widening the pulse is to reduce the intensity of the laser pulse in the amplifying process, so that the peak power of the pulse is below the damage threshold of the system element, thereby avoiding the damage of the ultra-short pulse to the optical element of the amplifier caused by the over-high power after the amplification, weakening or overcoming various nonlinear effects possibly caused by the high-intensity laser in the amplifying process, and improving the pulse energy in the optical fiber by several orders of magnitude. In particular, in the aspect of a high average power femtosecond fiber laser, the femtosecond pulse output with full optical fiber average power of hundred watts and pulse energy of micro-focus magnitude can be realized by CPA technology, and the peak power can reach ten megawatts magnitude.

Chirped amplification systems for infrared femtosecond pulses typically use ytterbium-doped few-mode fibers for pulse amplification. The power and pulse energy requirements of the application end on the femtosecond fiber laser are always improved, and the mode field area of the fiber is also required to be increased along with the improvement of the power and the energy due to the nonlinear effect of the fiber. When the mode field area of the optical fiber is increased to a certain degree and the single-mode condition is no longer satisfied, the optical fiber supports transmission amplification of a plurality of modes, and the optical fiber belongs to the few-mode optical fiber.

Based on the mode multiplexing of the few-mode optical fiber, the limited orthogonal mode in the few-mode optical fiber is used as an independent channel to load information, so that the transmission capacity of the system is multiplied. Meanwhile, the few-mode optical fiber has a relatively large mode field area, the nonlinear tolerance of the few-mode optical fiber is high, and adverse effects of nonlinear effects on a system can be well avoided.

In a few-mode fiber based mode multiplexing system, how to obtain higher order modes is important. Existing technologies for obtaining a high-order mode mainly include a Long Period Fiber Grating (LPFG) technology, a free space optical technology, a directional coupling technology (DC) based on a waveguide device, and the like. However, the structure of the free space optical technique is relatively complex, and the long period fiber grating technique requires a specific device to periodically change the topography of the optical fiber, which is easy to damage the optical fiber. Whereas directional coupling techniques (DC) based on waveguide devices can result in lower insertion loss, how to efficiently couple with fiber links is a challenging task. Therefore, designing a simple and efficient structural device to acquire a high-order mode is one of the important points of research on the mode multiplexing technology.

In general, the single mode condition of an optical fiber is generally determined by the V value, and the calculation formula is:

when V >2.405, the fiber starts to support high order mode transmission.

Such as Nufern's PLMA-YDF-10/125 ytterbium doped fiber, has a characteristic parameter of na=0.075, r=5.5 um, and a calculated V value of 2.516 when the central wavelength is 1030 nm. Under the above conditions, the optical fiber supports transmission and amplification of LP01 and LP11 modes, and is no longer a single mode optical fiber.

When pulses are transmitted over long distances in few-mode fibers, coupling crosstalk occurs between modes, resulting in a change in the phase of the pulses, as the fiber itself supports transmission in multiple modes. In particular, in a femtosecond pulse chirp amplification system, such uncontrolled pulse phase variation can deform or destabilize the compressed pulse, on the one hand, a few-mode fiber is needed in the femtosecond pulse chirp amplification system to increase the mode field area; on the other hand, pulses amplified by few-mode fiber amplifiers need to avoid inter-mode coupling crosstalk.

It can be seen that the few-mode fiber amplifier needs to incorporate an effective mode filtering method or mode filtering device to avoid inter-mode coupling crosstalk to ensure the pulse quality and pulse stability of the femtosecond pulse chirped amplification system.

In order to solve the above technical problems, referring to fig. 1, an embodiment of the present utility model provides a mode filter, which is welded with a polarization-maintaining ytterbium-doped few-mode fiber by an optical fiber welding machine, wherein a cross section of the mode filter includes a fiber core region, a cladding region, two stress rods and a bragg grating 2, and a fiber core diameter of the fiber core region is smaller than a fiber diameter of the ytterbium-doped few-mode fiber; the cladding region surrounds the outer side of the fiber core region; the two stress rods are arranged on the cladding region and are symmetrically arranged relative to the upper and lower axes of the fiber core region; a plurality of bragg gratings 2 are written in a symmetrical distribution at the edges of the core region, and the positions of the bragg gratings 2 coincide with the higher order transverse mode portions transmitted in the core of the core region.

Since the position of the bragg grating 2 coincides with the higher order transverse mode transmitted in the core of the few-mode fiber, the fundamental mode transmitted in the core 11 of the few-mode fiber 1 passes through the bragg grating region with low loss, whereas the higher order transverse mode transmitted in the core 11 of the few-mode fiber 1 passes through the bragg grating region with high loss.

The mode filter in the embodiment of the utility model is particularly suitable for a few-mode optical fiber amplifier, the Bragg grating 2 is inscribed around the fiber core of the few-mode optical fiber through the grating inscribing device, and the target position of the Bragg grating 2 on the cross section of the optical fiber is matched with the distribution of the high-order transverse mode field of the optical fiber, so that the high-order mode in the optical fiber is subjected to the Bragg grating 2 to obtain larger loss, thereby realizing the mode filtering function.

It should be further explained here that bragg gratings 2 now fall mainly into two main categories: fiber Bragg gratings (FBGs, collectively Fiber Bragg Grating) and Volume Bragg Gratings (VBGs), which are spatially and periodically phase distributed gratings formed in the fiber core, act essentially by forming a narrow band (transmissive or reflective) filter or mirror in the fiber core, which can be used to fabricate a number of unique optical fiber devices. The bragg grating 2 in the embodiment of the present utility model is an optical fiber bragg grating.

Referring to fig. 3, in the preferred embodiment of the present utility model, four bragg gratings are provided, two of which are inscribed in the up-down direction of the core region along the stress rod direction, and the other two of which are inscribed in the left-right direction of the core region along the stress rod perpendicular direction.

Specifically, the Bragg grating 2 in the embodiment of the utility model is a reflection type grating, the reflection center wavelength is 1030-1080nm, the spectral bandwidth is 5-25nm, and the reflectivity is 50-100%. Preferably, the center wavelength of the bragg grating 2 in this embodiment is 1030nm, the spectral bandwidth is 20nm, and the reflectivity is 99%.

Specifically, in the embodiment of the present utility model, the inscription diameter of the bragg grating 2 is 2 micrometers, the length is 2mm, and the inscription position is 19 micrometers from the center of the fiber core.

It will be appreciated by those skilled in the art that grating writing is a technique for forming minute structures using a pulsed light source and grating writing fiber surfaces. Grating writing can form three-dimensional structures on the surface of an optical fiber, including various folds, voids, pores, asperities, etc., to alter the optical properties of the optical fiber, such as refractive index, insulation thickness, surface refractive index, extinction coefficient, etc.

The preparation methods of the fiber gratings commonly used at present mainly comprise a double-beam interferometry method and a phase mask plate method. The method for preparing the fiber grating by using the phase mask plate is a common method for preparing the fiber grating at present, and the adopted phase mask plate is a one-dimensional periodic structure etched on the surface of a silicon wafer by electron beam lithography or holography; thus, after the photosensitive optical fiber is closely placed in the phase mask, the effective refractive index of the optical fiber is periodically modulated by utilizing interference fringes generated by near field diffraction of the phase mask, so that the optical fiber grating is formed.

As a preferred implementation of the embodiment of the present utility model, the bragg grating 2 is written inside the core of the optical fiber through a phase mask plate using an ultraviolet laser, or directly written inside the core of the optical fiber using a femtosecond laser.

Specifically, in embodiments of the present utility model, the core region has a core diameter of 20 microns, a core numerical aperture NA of 0.04, the cladding region has a diameter of 125 microns, and the cladding region has a cladding numerical aperture NA of 0.46.

Since optical signal transmission is different from electrical signal transmission, for electrical signals, the signal can be normally transmitted by connecting the output end of the amplifier with the transmission cable, but for optical communication, a part of light incident on the end face of the optical fiber cannot enter the optical fiber, and the light entering the end face of the optical fiber cannot be necessarily transmitted in the optical fiber, and only the light meeting a specific condition can be totally reflected in the optical fiber to be transmitted.

Therefore, when a beam of light, no matter how large an angle range is, only the light entering the optical fiber along a specific light cone angle can be normally transmitted, the sine value of the angle θ is called as the numerical aperture NA (Numerical Apeture) of the optical fiber, and the sine value is one of important optical parameters of the optical fiber, dimensionless and has no unit.

The numerical aperture NA of the fiber is calculated as follows:

NA＝n*Sinθ

where n is the refractive index of the medium and θ is the angle between the light and the fiber axis, commonly referred to as the half angle.

As can be seen from the formula, only light rays with a numerical aperture NA which is smaller than or equal to nsinθ can be coupled into the optical fiber for transmission, and the numerical aperture NA represents the light receiving capacity of the optical fiber.

The larger the NA, the more powerful the fiber is in receiving light, and the larger the NA is from the standpoint of increasing the optical power entering the fiber, since a larger numerical aperture of the fiber is advantageous for the butt-joint of the fibers. However, when NA is too large, the mode distortion of the optical fiber increases, which affects the bandwidth of the optical fiber. Therefore, in an optical fiber communication system, there is a certain requirement for the numerical aperture of an optical fiber.

Referring to fig. 1 and 2, the embodiment of the present utility model further provides a few-mode fiber amplifier 103, where the few-mode fiber amplifier 103 includes a combiner, a multimode pump, an ytterbium-doped few-mode fiber, and a mode filter, and the method further includes:

the input end of the beam combiner is respectively connected with the few-mode transmission optical fiber and the multimode pump, the multimode pump outputs multimode pump light with a fundamental mode, the output end of the beam combiner is connected with the ytterbium-doped few-mode optical fiber, and the beam combiner is used for optically coupling signals to be amplified in the few-mode transmission optical fiber and the multimode pump with the fundamental mode and then injecting the signals into the ytterbium-doped few-mode optical fiber.

The beam combiner in this embodiment is a (1+1) x1 polarization-maintaining single-mode optical signal/pump beam combiner, the signal fiber of the beam combiner uses PM980 polarization-maintaining single-mode optical fiber, the pump fiber of the beam combiner uses multimode optical fiber with the fiber core diameter of 105 micrometers and NA of 0.22.

The multimode pump is a 10W and 976nm lock wavelength multimode fiber output semiconductor laser, and the multimode pump output fiber uses a multimode fiber with a fiber core diameter of 105 microns. The pulse signal enters the fiber core with the diameter of 5.5 micrometers of the output fiber of the beam combiner through the beam combiner, and the pump light enters the cladding with the diameter of 125 micrometers of the output fiber of the beam combiner through the beam combiner.

The output optical fiber of the beam combiner (namely the output end of the beam combiner) and the polarization-maintaining ytterbium-doped few-mode optical fiber are welded by an optical fiber welding machine, the fiber core diameter of the polarization-maintaining ytterbium-doped few-mode optical fiber is 20 microns, the fiber core NA is 0.04, the cladding diameter is 125 microns, and the cladding NA is 0.46, so that the optical fiber does not meet the single-mode condition of the optical fiber, and can support the transmission and amplification of the LP01 mode and the LP11 mode.

The absorption coefficient of the multimode pump is 16dB/m, the use length is 0.5m, the coiling diameter is 8cm when the multimode pump is used, and the multimode pump is coiled for 2 circles. The pulse signal having an average power of 1mW can be amplified to 1W after passing through the few-mode fiber amplifier 103.

Specifically, the fiber core 11 of the fiber core region in the embodiment of the utility model is pure quartz glass or mixed quartz glass doped with rare earth elements of erbium, ytterbium, neodymium and holmium.

The optical fiber is actually a medium in which a fiber core made of transparent material and a cladding layer made of material with a slightly lower refractive index than the fiber core are adopted around the fiber core, and an optical signal injected into the fiber core is reflected by a cladding interface, so that the optical signal propagates in the fiber core.

Since the few-mode transmission fiber is a fiber supporting transmission amplification of several higher-order modes in addition to the fundamental mode (LP 01) of the fiber. In the field of optical communications, few-mode optical fibers are used for mode division multiplexing to increase channel capacity, breaking through bandwidth limitations.

In multimode fibers, the core diameter is 15um-50um, approximately corresponding to the thickness of human hair. The fiber core of the single-mode fiber is 8um-10um in diameter, a glass envelope with lower refractive index than the fiber core is surrounded outside the fiber core, so that the light is kept in the fiber core, and a thin plastic jacket is arranged outside the fiber core to protect the glass envelope. The optical fibers are usually bundled and protected by an outer jacket, and the core is usually a double concentric cylinder of small cross-sectional area made of quartz glass, which is brittle and subject to breakage, thus requiring an additional protective layer.

Preferably, the mode filter in the embodiment of the present utility model is a step-index optical fiber or a photonic crystal fiber.

Step-index fibers are a type of fiber having a step-index profile with a core index higher than the cladding index such that the input optical energy is continuously reflected and propagates at the core-cladding interface. The refractive index of the fiber core is uniform and the refractive index of the cladding is slightly lower. The refractive index from the central core of the optical fiber to the glass cladding is abrupt, and only has one step, so the optical fiber is called as a step-index multimode optical fiber, which is called as an abrupt optical fiber for short.

Photonic crystal fibers (photonic crystal fiber, PCF for short), also known as microstructured fibers (micro-structured fibers, MSF for short), have a relatively complex refractive index profile across their cross-section, and typically contain various arrangements of air holes having dimensions on the order of magnitude of the wavelength of the light wave that can be confined to the low refractive index core region of the fiber to propagate throughout the length of the device.

Photonic crystal fibers have many unique properties. For example, only one mode transmission may be supported over a wide bandwidth range; the arrangement mode of the air holes of the cladding region can greatly influence the mode property; the asymmetric arrangement of the air holes can also generate a great birefringence effect, thus providing possibility for designing a high-performance polarizing device.

Preferably, the mode filter in the embodiment of the utility model is a non-polarization-maintaining few-mode optical fiber or a polarization-maintaining few-mode optical fiber with a fiber core diameter of 10-40 micrometers and a cladding diameter of 125-400 micrometers.

Referring to fig. 2, further embodiments of the present utility model provide a multi-stage fiber amplifier system comprising an all-fiber mode-locked seed source 101, a stretcher 102, a compressor 104, and a few-mode fiber amplifier 103, wherein:

the output end of the all-fiber mode locking seed source 101 is connected with the signal input end of the stretcher 102, the incident end of the few-mode fiber amplifier 103 is connected with the signal output end of the stretcher 102, the reflecting end of the few-mode fiber amplifier 103 is connected with the input end of the compressor 104, and the output end of the few-mode fiber amplifier 103 is collimated and then input into the compressor 104.

The all-fiber mode-locked seed source 101 in this embodiment is a passive mode-locked laser, the central wavelength of the laser output is 1030nm, the spectrum width is 15nm, the average power is 1mW, the repetition frequency is 40MHz, and the pulse width is 5ps.

The mode-locked fiber laser is mainly divided into an active mode locking mode and a passive mode locking mode from the technical level. The active mode-locking fiber laser generally adopts a modulation device in a cavity, which can generate additional loss of the cavity, and the introduction of the modulation device is difficult to realize all-fiber integration because the modulation device is mostly a non-fiber element, so that the all-fiber development of the technology is restricted; meanwhile, active mode locking is easily affected by external environment such as temperature change, mechanical vibration, supermode noise, polarization state fluctuation in a resonant cavity and other factors, and many complex technologies are needed to improve the stability of the system, so that the complexity of the system is greatly increased and the cost of a laser is increased.

Therefore, the embodiment of the utility model adopts the passive mode-locking fiber laser, has the advantages of simple structure, stable performance, convenient integration and the like, is widely focused at home and abroad, and is more and more widely applied in a plurality of fields such as communication, medicine, processing, sensing, detection and the like.

The passive mode-locked fiber laser is mainly realized by adopting a nonlinear optical annular mirror, nonlinear polarization rotation, a semiconductor-based saturable absorption mirror and other mechanisms. The passive mode locking technology based on the semiconductor saturable absorber mirror (SESAM) has the advantages of flexible design, stable system, self-starting and the like, meanwhile, the semiconductor saturable absorber mirror can flexibly control key parameters such as modulation depth, recovery time, saturation flux and the like in the preparation process, and can be processed and integrated on an optical fiber end head according to requirements, so that the full-optical fiber is convenient, and therefore, the type of passive mode locking optical fiber laser is widely focused in the field of practical application.

In addition, the output end of the all-fiber mode-locking seed source 101 uses PM980 polarization-maintaining single-mode fiber, the fiber core diameter is 5.5 micrometers, the numerical aperture is 0.12, the cladding diameter is 125 micrometers, and the coating diameter is 245 micrometers.

In this embodiment, the stretcher 102 uses PM980 polarization-maintaining single-mode fiber, and the output pulse of the all-fiber mode-locked seed source 101 is subjected to pulse stretching by the stretcher 102, where the stretcher 102 is composed of a fiber circulator and a chirped grating. The pulse passing through the stretcher 102 maintains the shape of the input spectrum with a central wavelength of 1030nm and a spectral width of 15nm, the pulse is stretched to 500ps, and the stretched pulse of the stretcher 102 enters the few-mode fiber amplifier 103.

In this embodiment, the few-mode fiber amplifier 103 is composed of a combiner, a multimode pump, an ytterbium-doped few-mode fiber, and a mode filter.

The beam combiner is a (1+1) x1 polarization-maintaining single-mode optical signal/pump beam combiner, PM980 polarization-maintaining single-mode optical fiber is used as the signal optical fiber of the beam combiner, and a multimode optical fiber with the fiber core diameter of 105 microns and NA of 0.22 is used as the pump optical fiber of the beam combiner.

The multimode pump is a 10W 976nm lock wavelength multimode fiber output semiconductor laser, and the pump output fiber uses a multimode fiber with a fiber core diameter of 105 microns. The pulse signal enters the fiber core with the diameter of 5.5 micrometers of the output fiber of the beam combiner through the beam combiner, and the pump light enters the cladding with the diameter of 125 micrometers of the output fiber of the beam combiner through the beam combiner.

And the output optical fiber of the beam combiner and the polarization-maintaining ytterbium-doped few-mode optical fiber are welded by using an optical fiber welding machine. The fiber core diameter of the polarization-maintaining ytterbium-doped few-mode fiber is 20 microns, the fiber core NA is 0.04, the cladding diameter is 125 microns, and the cladding NA is 0.46. The optical fiber does not meet the single-mode condition of the optical fiber and can support the transmission and amplification of the LP01 mode and the LP11 mode.

The absorption coefficient of the multimode pump is 16dB/m, the use length is 0.5m, the coil diameter is 8cm when in use, the coil is 2 circles, and the pulse signal with the average power of 1mW is amplified to 1W after passing through the few-mode optical fiber amplifier 103.

The rear of the ytterbium-doped few-mode optical fiber is a mode filter, the mode filter and the ytterbium-doped few-mode optical fiber are welded by an optical fiber welding machine, the mode filter is made of a passive polarization-maintaining few-mode optical fiber, the fiber core diameter is 20 microns, the fiber core NA is 0.04, the cladding diameter is 125 microns, and the cladding NA is 0.46, and the size of the mode filter is completely matched with that of the polarization-maintaining ytterbium-doped few-mode optical fiber.

Referring to fig. 3, 4 fiber bragg gratings are written in the mode filter, the gratings are reflection gratings, the center wavelength is 1030nm, the spectral bandwidth is 20nm, and the reflectivity is 99%. The grating inscription diameter is 2 microns, and length is 2mm, and the position is 19 microns from the fiber core center, and along stress bar direction inscription 2, along stress bar vertical direction inscription 2.

Referring to fig. 3 and 4, it can be seen that the grating does not coincide with the power concentrating region of the LP01 fundamental mode, so that the fundamental mode can pass through the grating region without loss or with very low loss. The grating coincides well with the main power concentrating region of the LP11 high order modes, so the high order modes will be reflected by the grating and cannot pass through the grating region or pass through the grating region with very high losses. Thus, only the LP01 fundamental mode passes through the grating region, and the mode filtering function is realized.

The pulse signal after the mode filter enters the space light transmission through the optical fiber and enters the compressor 104, the compressor 104 consists of a volume grating, the pulse signal is transmitted in the space and realizes pulse compression, and the compressed pulse is about 300fs.

Referring to fig. 4, the mode filter can effectively filter out higher order modes in the few-mode fiber. The far field beam profile with and without a mode filter is shown in fig. 4. The beam quality is also significantly different, 1.22 without a mode filter and 1.01 with a mode filter.

Although the present disclosure is disclosed above, the scope of the present disclosure is not limited thereto. Various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the disclosure, and these changes and modifications will fall within the scope of the utility model.

Claims

1. A mode filter for fusion-splicing with ytterbium-doped few-mode optical fibers by an optical fiber fusion splicer, the cross section of the mode filter comprising:

a cladding region surrounding the outside of the core region;

the two stress rods are arranged on the cladding region and are symmetrically arranged relative to the upper and lower axes of the fiber core region; and

2. The pattern filter of claim 1, wherein four bragg gratings are provided, two of which are inscribed in the up-down direction of the core region along the stress rod direction, and the other two of which are inscribed in the left-right direction of the core region along the stress rod perpendicular direction.

3. The mode filter of claim 1, wherein the bragg grating is a reflective grating having a reflection center wavelength of 1030-1080nm, a spectral bandwidth of 5-25nm, and a reflectivity of 50-100%.

4. The pattern filter of claim 1, wherein the bragg grating has a inscription diameter of 2 microns, a length of 2mm, and an inscription location 19 microns from the center of the core.

5. The mode filter of claim 1, wherein the core region has a core diameter of 20 microns, the core numerical aperture NA is 0.04, the cladding region has a diameter of 125 microns, and the cladding region has a cladding numerical aperture NA of 0.46.

6. The pattern filter of claim 1, wherein the core of the core region is made of pure quartz glass or mixed quartz glass doped with rare earth elements erbium, ytterbium, neodymium and holmium.

7. The mode filter of any one of claims 1-6, wherein the mode filter is a step index fiber or a photonic crystal fiber.

8. The mode filter of any one of claims 1 to 6, wherein the mode filter is a non-polarization maintaining few-mode fiber or a polarization maintaining few-mode fiber having a core diameter of 10 to 40 microns and a cladding diameter of 125 to 400 microns.

9. A few-mode fiber amplifier based on the mode filter of claim 1, comprising: beam combiner, multimode pump, ytterbium doped few-mode fiber and mode filter, wherein:

10. A multi-stage fiber amplifier system, comprising: the all-fiber mode locking seed source, the stretcher, the compressor and the few-mode fiber amplifier according to claim 9, wherein the output end of the all-fiber mode locking seed source is connected with the signal input end of the stretcher, the incident end of the few-mode fiber amplifier is connected with the signal output end of the stretcher, the reflecting end of the few-mode fiber amplifier is connected with the input end of the compressor, and the output end of the few-mode fiber amplifier is collimated and then input into the compressor.