CN112909715A

CN112909715A - Full polarization maintaining fiber ultrashort pulse laser

Info

Publication number: CN112909715A
Application number: CN202110073916.XA
Authority: CN
Inventors: 胡凯淋; 刘志弘; 颜及堃; 陈瑜; 刘少臻; 郭子悦; 曹涛; 彭家晖
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2021-01-20
Filing date: 2021-01-20
Publication date: 2021-06-04

Abstract

The invention discloses a full polarization maintaining fiber ultrashort pulse laser. The method comprises the following steps: a non-linear amplification loop mirror with an asymmetric splitting ratio such that pulses within the loop have different phase shifts to output high power ultrashort pulses; the linear arm part is composed of a saturable absorber and is used for forming a complete resonant cavity and improving the self-mode-locking characteristic of the laser; and the output end is used for directly outputting pulses or pre-amplifying the laser pulses, so that the laser outputs ultra-short pulses with higher power, or broadening the spectrum through pre-amplification to obtain narrower pulse width. The all-fiber ultra-short laser oscillator adopts a full polarization maintaining structure, does not introduce a space adjusting element, has good mode locking self-starting characteristic, is not easy to damage a saturable absorber, has the characteristics of small volume and high stability, and has the potential to be developed into a new generation of all-fiber ultra-short laser oscillators with excellent performance.

Description

Full polarization maintaining fiber ultrashort pulse laser

Technical Field

The invention belongs to the technical field of laser, and particularly relates to a full polarization maintaining optical fiber ultrashort pulse laser.

Background

Due to the advantages of extremely short time scale, extremely high peak power and the like, the ultrafast laser is widely applied to the fields of material processing, biomedical imaging, micro-spectroscopy and the like. Since the peak power of the laser pulse is limited due to the limitation of the damage threshold of the laser device, the single pulse energy of the picosecond laser pulse is often much higher than that of the femtosecond laser, so that the picosecond laser is used more efficiently for industrial applications such as ultrafast laser micromachining, material surface modification, medical treatment, and the like. On the other hand, the amplification of the picosecond laser does not need steps such as broadening, compressing and the like, so that the structure of the picosecond laser is simpler, the cost is lower than that of the femtosecond laser, and the market of the picosecond laser in the current ultrafast laser industry is far larger than that of the femtosecond laser.

The seed source technology (oscillator) of picosecond laser mainly comprises a Q-switched mode and a mode-locked mode, but the stability of the mode-locked laser is far higher than that of the Q-switched laser, so the mode-locked technology is more used in the seed source of picosecond laser for industrial application. In the picosecond laser seed source constructed by using the mode locking technology, part of lasers still obtain the seed source by filtering the femtosecond laser pulse of the mode locking and intercepting the picosecond pulse. The method has the advantages of low energy utilization rate and high cost of the seed source, and the situation is mainly caused by the fact that the market lacks a mode-locking picosecond seed source technology special for a high-power picosecond laser.

At present, the mode locking mode in the optical fiber laser is mainly a passive mode locking technology, and the realization modes of the passive mode locking mainly include saturable absorber mode locking, nonlinear polarization rotation mode locking and nonlinear optical loop mirror mode locking. Compared with the other two mode locking technologies, the nonlinear optical loop mirror mode locking technology has the advantages of high response speed, extremely high signal-to-noise ratio, low cost, high system stability, high bearable power, convenience in integration, small interference from the external environment and the like.

At present, most mode-locked lasers utilizing a nonlinear optical ring mirror mechanism adopt an 8-shaped structure, but because the lasers are of a full-closed ring structure, the cavity length cannot be adjusted, the physical quantity in a cavity and the influence relationship of the physical quantity on the mode-locking process are difficult to measure, and the improvement of the performance and the practical application of the mode-locked lasers are limited. In order to realize self-starting mode locking, a non-reciprocal space element can be inserted into the annular cavity, the additional phase of the pulse is related to the propagation direction, and the optimal self-starting mode locking effect is obtained by changing the phase shift of a wave plate in the element. However, the introduction of the space element reduces the stability of the system to some extent, and is not favorable for miniaturization and integration of the laser.

A saturable absorber (SESAM) is a material whose transmittance for incident light increases with increasing light intensity. When the laser pulse in the resonant cavity passes through the saturable absorber, the transmittance of the peak part with higher light intensity is higher, and the transmittance of the two wing parts with lower light intensity is lower. Therefore, the intensity modulation effect of the SESAM is utilized to start the mode locking process, and meanwhile, stable mode locking can be realized. However, the damage threshold of the saturable absorber of a general semiconductor material is low, and the SESAM is easily damaged by a high-intensity Q-switched pulse in the Q-switched mode locking stage at the initial stage of mode locking. This is also a problem with the picosecond laser seed source in the industry today.

Disclosure of Invention

Aiming at the defects of the prior art and the requirements of practical production, the invention provides a full polarization maintaining optical fiber ultra-short pulse laser, and aims to construct a full polarization maintaining optical fiber structure ultra-fast laser which is high in stability, capable of quickly and automatically starting mode locking, not prone to damage and easy to integrate.

In order to achieve the above object, the present invention provides a fully-polarization-maintaining fiber ultrashort pulse laser, including: the nonlinear amplification loop mirror, the linear arm and the output end. Wherein the nonlinear amplification ring mirror and the linear arm jointly form a complete laser resonant cavity, so that the laser oscillates and is amplified in the cavity in a reciprocating manner. All optical fiber elements contained in the nonlinear amplification ring mirror, the linear arm and the output end are polarization maintaining devices, and the polarization maintaining device has the advantages that the laser runs in a specific polarization state, and the output end outputs ultrashort laser pulses in the specific linear polarization state.

The nonlinear amplification loop mirror is formed by sequentially connecting a first output port of the coupler, a first gain optical fiber, a first wavelength division multiplexer, a first semiconductor laser and a second output port of the coupler into a loop. The first gain fiber is a gain medium of the laser, and the first semiconductor laser is connected to the first gain fiber through the first wavelength division multiplexer and serves as a pumping source of the laser. The laser has the beneficial effect that the laser stably works in a mode locking state.

Further, the gain fiber in the nonlinear amplification ring mirror is in a non-intermediate position in the ring cavity to increase asymmetry in the cavity, which has the advantage of making the laser easier to mode-lock.

The coupler of the nonlinear amplification loop mirror is a non-50: 50 coupler, and the nonlinear amplification loop mirror has the advantages of improving asymmetry in the nonlinear loop mirror and being beneficial to outputting pulses with higher power. The first input port (the output splitting ratio of the input light at the first output port and the second output port is larger than 1), so that the design has the beneficial effect that more energy firstly passes through the filter and then passes through the gain fiber, and the oscillation starting of frequency components except the designed wavelength is inhibited.

The filter bandwidth and center wavelength in the laser structure can be fine tuned within the gain bandwidth of the gain fiber. The bandwidth of the filter can be selected from a few tenths of nanometers to dozens of nanometers, the required bandwidth of the filter can be obtained through direct calculation by designing the pulse width according to the calculation of the Fourier transform limit pulse width. The laser has the advantages of enhancing the adjustability of the laser and expanding the application range of the laser.

Further, the linear arm of the laser is formed by connecting a polarization maintaining optical fiber, a first lens group, a second lens group and a saturable absorber to a first input port of the coupler. The linear arm is used for forming one end of the resonant cavity, and the distance between the lens groups can be adjusted, so that the repetition frequency can be finely adjusted, and the adjustability of the laser is improved. The saturable absorber in the linear arm can be reflective or transmissive, and its material can be semiconductor, graphene, carbon nanotube, molybdenum disulfide, or other material with function of introducing pulse phase modulation (or amplitude modulation). The saturable absorber has the function of reducing the condition of mode locking for providing an additional amplitude modulation mechanism, so that the laser can be started automatically and rapidly. In order to enable the laser to be started automatically quickly, the mode locking process can be started in a mode that instantaneous impulse current is introduced by programming the first semiconductor laser (4) or phase disturbance is introduced by adding piezoelectric ceramics and other mechanical methods, and when the laser is started automatically, a saturable absorber is not needed at the reflection end of the linear arm and a common total reflector is used.

Further, the focal length ratio of the lens group in the linear arm is preferably less than 1, so that light spots are focused on the reflective saturable absorber in a weak focusing mode, and the beneficial effects of avoiding damage to the saturable absorber and improving the reliability of the system are achieved.

Further, lens group and saturable absorber can be integrated into enclosed construction through the mode of machinery, and its beneficial effect is, reduces the interference of environment, promotes laser instrument stability. The focal length of the lens groups and the distance between the lens groups can be determined according to the cavity length of the laser, and a spherical lens, an aspherical lens or a GRIN lens can be used for considering the convenience of laser integration. The aspheric lens can better eliminate spherical aberration, has a smaller focal length while having a proper size, and is therefore a better choice.

Further, the output end of the laser can be directly output through the isolator, or be connected with a first amplification stage through the isolator and then output. The isolator functions to prevent forward propagating light from reflecting back into the laser oscillator, thereby causing damage to the oscillator. After the amplifier stage is connected, ultrafast laser with higher power can be output, or the spectrum is widened through the amplifier stage, self-similar amplification is introduced to the pulse, so that the pulse is evolved into an ultrashort pulse with shorter pulse width. As a picosecond laser oscillator, the power output through the isolator is already sufficient to serve as a seed source for a high power picosecond laser.

Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:

(1) the full polarization maintaining optical fiber structure is adopted, so that the stable output of the pulse of the polarization state of a specific line by the laser is ensured, and meanwhile, the integration and miniaturization of the laser are convenient to realize.

(2) The nonlinear amplification loop mirror is used as a mode locking element and matched with a saturable absorber or other modes to rapidly self-start. On one hand, the nonlinear amplification loop mirror mode locking technology improves the working stability of the laser and the signal-to-noise ratio of pulses, and reduces external interference; on the other hand, the saturable absorber is used for promoting the self-starting process, the introduction of a non-reciprocal element is avoided, the complexity of the system is improved, the laser can quickly enter the mode locking process, and extra interference is not brought to the system, so that the stability of the laser cannot be reduced.

(3) The light incident on the saturable absorber irradiates on the saturable absorber reflector in a weak focusing mode, so that the central light intensity of the light spot is lower than the damage threshold of the saturable absorber, and the damage of the saturable absorber is avoided.

(4) The laser has more adjustable parameters such as wavelength, bandwidth, pulse width, repetition frequency and the like, can be designed according to requirements, can be designed into a picosecond or femtosecond laser, and expands the application range of the laser.

(5) The laser has higher output average power, improves the energy conversion efficiency of the system and reduces the cost.

(6) The laser may operate with a variety of intra-cavity dispersion profiles including, but not limited to, intra-cavity all positive dispersion, intra-cavity net dispersion positive, intra-cavity net dispersion negative, and the like.

Drawings

Fig. 1 is a schematic structural diagram of a laser according to a first embodiment of the present invention;

fig. 2 is a schematic structural diagram of a laser according to a second embodiment of the present invention;

FIG. 3 is a measured spectrum of a laser provided by an oscillating stage according to a first embodiment of the present invention;

FIG. 4 is a measured spectrum of a laser provided by an amplifier stage according to a first embodiment of the present invention;

FIG. 5 is a measured spectrum of a laser provided by an oscillator stage according to a second embodiment of the present invention;

FIG. 6 is a measured spectrum of a laser provided by an amplifier stage according to a second embodiment of the present invention;

fig. 7 shows the measured spectrum of the laser provided by the oscillator stage according to the third embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be replaced with each other as long as they do not conflict with each other. In the present disclosure, the terms "first," "second," and the like (if any) are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order.

As shown in fig. 1, in a first embodiment of the present invention, a design structure of a fully polarization maintaining fiber ultrashort pulse laser provided by the present invention includes: the device comprises a nonlinear amplification loop mirror 1, a linear arm 2 and an output end 3. The nonlinear amplification loop mirror is formed by sequentially connecting a coupler 6, a first gain optical fiber 7 and a wavelength division multiplexer 8 into a loop; the linear arm 2 is composed of a polarization maintaining fiber 12, a first lens group 10, a second lens group 10', a saturable absorber 11 and a filter 5; the output end is formed by connecting a second input port 16 of the coupler 6, an isolator 9, a wavelength division multiplexer 8 ', a semiconductor laser pump 4', a gain optical fiber 14 and an output end 13. All optical elements are polarization maintaining devices.

In the nonlinear amplification loop mirror 1, a first gain fiber 7 is a gain medium of a laser, and a semiconductor laser 4 is connected to the gain fiber through a wavelength division multiplexer 8 and serves as a pumping source of the laser. The nonlinear amplification loop mirror 1 is also a mode locking element of the laser, different nonlinear phase shifts are accumulated on light transmitted clockwise and light transmitted anticlockwise in the nonlinear amplification loop mirror due to different intensities, then the light reaches the coupler simultaneously, interference occurs in the coupler so that reflection and transmissivity of the nonlinear amplification loop mirror are changed, the amplitude modulation effect similar to a saturable absorber is achieved, and the laser can stably work in a mode locking state.

In the non-linear amplification loop mirror, the gain fiber is not in the middle of the ring cavity, and the first gain fiber 7 is directly connected to the first output port 18 of the coupler 6 by welding so as to increase the asymmetry in the cavity. In a non-linear amplification ring mirror, the main reason for the non-linear phase shift is that the clockwise and counterclockwise transmitted light have different intensities, thereby accumulating different non-linear phase shifts within the ring cavity. The position of the gain fiber determines the position of gain of the light transmitted in opposite directions, so that the nonlinear phase difference of the two beams of light is determined, and the laser can more easily reach a mode locking region by larger nonlinear phase shift difference.

The coupling coefficient of the coupler 6 (i.e. the splitting ratio of the coupler) is 0.8, i.e. the splitting ratio of the second output port 17 of the coupler to the first output port 18 of the coupler is 80:20 when light is incident on the first input port 15 connected by the linear arm. In the nonlinear loop mirror, the coupling coefficient of the coupler determines the modulation depth of the nonlinear loop mirror, and when the coupling coefficient of the coupler is 0.5, the modulation depth of the nonlinear loop mirror reaches the maximum, in this case, when the mode locking condition is reached, the transmissivity of the laser is 1 (consistent with the reflectivity of the nonlinear loop mirror), so that most energy of the laser enters the linear arm to oscillate in the cavity, and only a very small part of the power is output. When the coupling coefficient of the coupler is not equal to 0.5, the transmissivity is less than 1 when the mode locking condition is achieved, and the output power of the laser is greatly improved.

The filter 5 is located in the linear arm 2, the filter 5 is connected to the first input port 15 of the coupler, and then is coupled to the saturable absorber 11 through a section of polarization maintaining fiber 12 and the first lens group 10, the second lens group 10'.

Wherein the output end 3 is constituted as: the isolator 9 is connected to the second input port 16 of the coupler 6 and the other end of the isolator is connected to the second wavelength division multiplexer 8 'and then to the second gain fiber 14 and the second semiconductor laser 4'. The output end forms an amplification stage through the second gain optical fiber, the second wavelength division multiplexer and the second semiconductor laser, so that the output power is higher, and the spectrum is widened to a certain extent.

The linear arm portion of the laser is connected to a length of polarization maintaining fiber 12 by a first input port 15 of coupler 6 and then coupled to the saturable absorber mirror through a first lens group 10 and a second lens group 10'. The saturable absorber mirror is a semiconductor material and the saturable absorber is a fast saturable absorber. The length of the polarization maintaining fiber 12 and the equivalent focal lengths of the first lens group 10 and the second lens group 10 'and the distance therebetween can be used to control the repetition frequency of the laser, but for easier integration of the laser, the focal length of the lens groups is small, the equivalent focal length ratio of the first lens group 10 to the second lens group 10' is 6.25mm:11mm, and the distance therebetween is small, about 20 mm.

The spectral width of picosecond laser is narrow, so that a band-pass filter with the bandwidth of less than 2nm can be used as the filter, the central wavelength can be selected within the gain bandwidth of the gain fiber, in the first embodiment, the gain fiber is an ytterbium-doped fiber, the gain spectrum is distributed between 1030nm and 1100nm, the central wavelength of the filter is 1064nm, the bandwidth is 2nm, and the output pulse width of the laser is about 800fs according to Fourier transform. The laser output was connected directly through isolator 9 to the coupler input 16 for output, in an example experiment of the present invention, a 2nm filter with a center wavelength of 1064nm was used, the spectrometer was used to monitor the spectrum at the output as shown in fig. 3, the full width at half maximum of the spectrum was about 0.3nm, the 10dB bandwidth was about 3.3nm, 40.1MHz pulses were output with a pump power of 150mW, and the average power was up to 14.15 mW. The amplified spectrum is shown in FIG. 4, and has a full width at half maximum of about 0.17nm, a 10dB bandwidth of about 3.2nm, and an average power of 150 mW.

A second embodiment of the present invention is shown in fig. 2, and its basic structure is the same as the first embodiment, and it is composed of a non-linear amplification loop mirror 1, a linear arm 2 and an output end 3.

The coupling coefficient (i.e. the splitting ratio of the coupler) of the coupler 6 is 0.7, i.e. the splitting ratio of the coupler second output port 17 to the coupler first output port 18 to which the filter 5 is close when light is incident on the linear arm connected first input port 15 is 70: 30.

Wherein the saturable absorber 11 of the linear arm 2 is a common total reflector, and the pumping source semiconductor laser 4 of the laser is controlled by programming to provide instantaneous high current when the laser is started and then to recover normal current supply. The instantaneous impulse current provides a large disturbance for the laser, so that the laser can smoothly complete self-starting.

In example two, using a filter with a bandwidth of 0.5nm and a center wavelength of 1064nm, the spectra obtained in the example experiments are shown in fig. 4 and 5, where fig. 5 is a spectrum with no amplification stage at the output, a full width at half maximum of the spectrum of about 0.3nm, and a 10dB bandwidth of about 1.1 nm. The 49.6MHz pulse is output under the pumping power of 150mW, and the average power reaches 10.54 mW. FIG. 6 shows the spectrum when the output terminal is connected to the amplifier stage, the full half height of the spectrum is about 3nm, the 10dB bandwidth is about 4nm, and the conversion limit pulse width is about 500fs corresponding to the 3dB bandwidth. If self-similar amplification is carried out, the spectral width can be further widened, and pulses with 100fs magnitude can be output.

The third embodiment of the present invention has the same structure as the second embodiment, and the third embodiment differs from the first embodiment in the filter. Example three a filter with a centre wavelength of 1030nm and a bandwidth of 2nm was used and the output spectrum is shown in figure 7 with a full width at half maximum of the spectrum of about 0.7nm and a 10dB bandwidth of about 3 nm. The 36.8MHz pulse is output under the pump power of 100mW, and the average power reaches 11.2 mW.

The technical solutions proposed to solve the same problem in the first embodiment, the second embodiment and the third embodiment of the present invention can be replaced with each other and combined to form lasers with different structures.

The fully polarization maintaining fiber ultrashort pulse laser provided by the invention has higher output power, can be directly used as an oscillator of a high-power picosecond laser so as to be applied to industrial production, and can also be directly applied to scientific research fields such as biophotonics, spectroscopy and the like as an ultrashort pulse light source.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A fully polarization maintaining fiber ultrashort pulse laser, comprising: the laser comprises a nonlinear amplification loop mirror (1), a linear arm (2), an output end (3) and a filter, wherein the filter is positioned in the nonlinear amplification loop mirror (1) or the linear arm (2), the nonlinear amplification loop mirror (1) and the linear arm (2) jointly form a complete laser resonant cavity, and all optical fiber elements contained in the nonlinear amplification loop mirror (1), the linear arm (2) and the output end (3) are polarization maintaining devices.

2. The ultrashort pulse laser with fully polarization-maintaining fiber according to claim 1, wherein the nonlinear amplification loop mirror (1) is formed by connecting a first output port (18) of a coupler (6) with a first gain fiber (7), a first wavelength division multiplexer (8), a first semiconductor laser (4) and a second output port (17) of the coupler (6) in sequence to form a loop; the first gain fiber (7) is a gain medium of the laser, and the first semiconductor laser (4) is connected to the first gain fiber (7) through the first wavelength division multiplexer (8) and serves as a pumping source of the laser.

3. The ultrashort pulse laser of claim 2, wherein the first gain fiber (7) is asymmetrically positioned in the nonlinear amplification loop mirror (1) such that the accumulated nonlinear phase shift of the pulse entering the nonlinear amplification loop mirror (1) from the first output port (18) is not equal to the accumulated nonlinear phase shift of the pulse entering the nonlinear amplification loop mirror (1) from the second output port (17).

4. The ultrashort pulse laser of claim 2, wherein the coupler (6) is a non-50: 50 coupler and is input from the first input port (15), and the output splitting ratio of the first output port (18) to the second output port (17) is greater than 1.

5. The ultrashort pulse laser of claim 2, wherein the central wavelength of the filter (5) is within the gain bandwidth of the first gain fiber (7).

6. The ultrashort pulse laser with fully polarization-maintaining fiber according to claim 1, wherein the linear arm (2) is composed of a first input port (15) of a coupler (6) connected to a polarization-maintaining fiber (12), a first lens group (10), a second lens group (10') and a saturable absorber (11).

7. The ultrashort pulse laser of claim 6, wherein the saturable absorber (11) element structure in the linear arm (2) is reflective, and the material is semiconductor, graphene, carbon nanotube or molybdenum disulfide; or a transmission type, and the material is semiconductor, graphene, carbon nano tube or molybdenum disulfide.

8. The ultrashort pulse laser of claim 7, wherein the saturable absorber (11) in the linear arm (2) is a reflective device without saturable absorption property, and the mode locking process is initiated by programming the first semiconductor laser (4) to introduce transient impact current or by introducing phase perturbation by adding piezoelectric ceramic.

9. The ultrashort pulse laser of claim 1, wherein the output (3) is formed by connecting the input of an isolator (9) directly to one end (16) of the coupler (6).

10. The ultrashort pulse laser of claim 1, wherein the output (3) further comprises a second wavelength division multiplexer (8'), a second gain fiber (14) and an output pigtail (13) connected in series to the output of the isolator (9) for pre-amplifying and spectrally broadening the laser pulses.