1. Introduction

Ultrashort pulse Yb-doped fiber (YDF) lasers with excellent performance of both high peak power (MW) and good beam quality (close to diffraction limited) have greatly promoted the development of industrial processing, scientific research, and defense [1–3]. However, the main challenges for peak power scaling of standard fiber amplifier are the onset of detrimental nonlinear effects, such as stimulated Brillouin scattering (SBS) [2], stimulated Raman scattering (SRS) [1] and transverse mode instability (TMI) [4–6].

Crucially, the impact of all these nonlinear effects scales inversely with the effective mode-field diameter (MFD). In general, scaling the MFD of optical fibers can efficiently help mitigate peak power scaling limitations imposed by nonlinear effects and increase facet damage threshold due to the reduction of the local power intensity [7]. Additionally, the impact of nonlinear effects can be further mitigated by using shorter fiber lengths. However, employing an active fiber with a large mode area (LMA) may lead to a degradation of the beam quality and excite higher-order modes (HOMs). In order to maintain single-mode operation in LMA fibers and obtain a high-power diffraction-limited output, the tremendous progress made in fabrication technology of specialty LMA fibers has been proposed, such as a photonic crystal fiber (PCF) [8–10], chirally coupled core (CCC) fiber [11,12], large-pitch fiber (LPF) [13,14], all-solid photonic bandgap fiber (AS-PBF) [15,16] and leakage channel fiber (LCF) [17,18]. At present, the peak power of the picosecond or femtosecond fiber laser can be megawatts (MW) [19] or several GW level. This specialty LMA fibers present considerable difficulties with fiber manufacture, fiber handling, and integration into multi-kilowatt all-fiber laser systems due to the fabrication processes and the refractive index profile control of the LMA fibers are relatively complex [20], even though they are efficient to offer an LMA with robust single-mode (SM) behavior.

Peak power scaling in Yb -doped fiber lasers is limited mainly by characteristics of Yb -doped fiber utilized in the final amplifier stage. While most kW lasers rely on small-core 20/400 LMA fibers, ultrafast lasers instead feature active fibers with much larger cores (≥30 µm), in which case keeping diffraction limited output is often a challenge. Consequently, tapered Yb-doped fibers (T-YDFs) with novel promising design of a nonuniform longitudinal geometry become an economical and practical option to adopt and realize fundamental mode coupling and the creation of compact and robust all-fiber systems with low bend sensitivity and simple power scaling [21–23]. Additionally, the core and the cladding increase in diameter along the fiber length, with a robust single mode core at the thin end and a core that is several times larger in diameter at the thick end, which, therefore, resulting in a high pump absorption capability and the inherent advantage of nonlinear effect suppression [24]. In this way, the HOMs excitation can be inhibited and achieve excellent beam quality in ultrafast fiber lasers as the output power increases [25]. Up to now, by using LMA T-YDFs, some achievements have been made in high-power ultrafast fiber laser and amplifiers. Bobkov et al. have done a series of research work with tapered fiber of different lengths in recent years. Using a hybrid Er/Yb fiber system, they have demonstrated an active tapered fiber-based CPA system that generated 100 kW of peak power in 7-ps pulses centered at 1040 nm, with possibility of compression down to 130 fs [23]. In 2017, they have investigated pulse energy of 6.8 µJ operating at 1056 nm that corresponding to the peak power was 10.5 MW by using a 1.05 m tapered fiber. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated with a 2.2 m tapered fiber [26]. In 2021, they have reported a ∼2.45 m tapered fiber with standard master-oscillator power- amplifier (MOPA) scheme allows the achievement of an average power level of 150 W (limited by the available pump power) and a peak power of 0.74 MW for 22 ps pulses [27]. Recently, they have demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The amplified pulses were compressed to 670 fs duration with 3.5 W average power, corresponding to the peak power of 92 MW [28]. Petrov et al. have demonstrated a compact picosecond MOPA system based on a 50 µm core and 2.5 m Yb-doped polarization-maintaining multi-clad tapered fiber delivering pulses with over 1.26 MW peak power and average output power up to 200 W preserving near diffraction limited beam quality [29]. By employing a Yb-doped PM-T-YDF with length of approximately 7.5 m, the maximum achieved amplified average power at 20 MHz repetition rate was measured to be 513 W corresponding to 481.9 kW of peak power and 25.7 µJ pulse energy have been shown in MOPA laser system by Ustimchik. The fraction of signal radiation decreased to about 50% at maximum achieved power of 338.9 W at a repetition rate of 1 MHz and the peak power and the pulse energy was estimated as 3.2 MW and 169.5 µJ with 50 ps duration [30]. In 2022, the result in terms of obtaining high peak and average power simultaneously by utilizing a tapered fiber was obtained in [31] where 141 W average power and ∼1.3 MW peak power delivering 3.4 ps pulse was demonstrated.

However, most of the reported tapered fiber lasers use several meters long tapers. It is difficult to precisely control the taper drawing condition and tapering ratio is limited, although this kind of long taper contribute to decrease optical power limits of several unwanted parasitic non-linear effects. Sufficiently long T-YDF with large mode diameter (LMD) for reducing of thermal load on the fiber facilitates high average power (several hundred watts). On the contrary, high peak power requires as short fibers as possible. Therefore, the main goal of the present paper is to investigate the prospects of tapered fiber design for simultaneous achievement of high peak and average powers in femtosecond pulses, while maintaining diffraction-limited beam quality. In our work, we have introduced the generation of femtosecond (fs) pulses in a CPA system by employing short local adiabatic tapers (70 cm taper length with an input end of∼35/250 µm) with LMA fibers (70 cm fiber length with an output end of∼56/400 µm) for peak power scaling while maintaining good beam quality. A homemade mode locked fiber laser with a spectral width of 14.8 nm (full width at half maximum) is selected as the seeder to offer a stretched pulse width of more than 1 ns. The efficient compressed of 266 fs pulse amplification with peak power of up to 132 MW at a repetition rate of 2 MHz thereafter using the chirped-fiber Bragg gratings (CFBG) with the fine-tuned capacity of second-order dispersion and higher-order dispersion. Good beam quality with a mean M² value of ∼1.17 was observed at the maximum achieved average power of 79.4 W. The generated ultrafast lasers combining high average power and high pulse energy for industry and science of laser micro processing, X-ray and THz generation, attosecond pulse generation, etc.

2. Experiment setup

Figure 1 shows the schematic diagram of the tapered LMA Yb-doped fiber with multi-clad structure, which was fabricated with the effective mode area ranging from 500 µm² for the 35/250 end up to over 1000µm² for the 56/400 end and 0.07 core numerical aperture (NA), which is commercially available from the INO (Canada). The tapered optical fiber (see Fig. 1(b)) is an Yb-doped PM LMA fiber comprising straight sections of 250 µm and 400 µm linked by a tapered section having a length of ∼0.7 m. The fiber core/cladding has a diameter of 35/250 µm at the smaller end and 56/400 µm at the larger end, thus resulting in a taper ratio of 1.6 (see Fig. 1(b)). Since the tapered optical fiber is drawn from a single preform uniform along its length, the core/cladding diameter ratio (CCDR = 0.14), core numerical aperture (NA = 0.07) and absorption (8 dB/m @ 975 nm) remains the same along the length of the taper. The PM tapered fiber is designed for M² lower than 1.2 making it the perfect choice for applications requiring superior beam quality. The fiber design features a confined core for selective gain amplification and multi-layer cladding for enhanced suppression of higher order modes. The used tapered optical fiber consists of a region around the core having a lowered refractive index compared to that of the fiber’s raised-index 1st cladding (see Fig. 1(c)). The purpose of this depressed-cladding is to enhance the differential bending losses between LP₀₁ and the HOMs by taking advantage of the fact that the evanescent field of HOMs extends further than that of the fundamental mode. By carefully engineering the depth and thickness of the depressed cladding, it is possible to ensure that the fundamental mode remains well confined in the core in a coiled fiber, while the evanescent field of the HOMs reaches the outer cladding region, thus reducing the effective numerical aperture of the HOMs. Another feature of the used tapered optical fiber helpful to improve the mode quality is the use of a confined core, which consists of a GeO₂-doped silica outer ring in the fiber’s core having the same NA as the active region. The confined core is represented by the dotted lines inside the core on Fig. 1(c). The purpose of this confined core design is to enhance the overlap of the fundamental mode with the gain compared to that of the HOM’s, thus providing better amplification for the fundamental mode and helping maintaining a good mode quality along the length of the fiber. The tapered fiber core and the cladding diameters monotonically increase along the fiber length to several times their original size, resulting in a high pump absorption capability and the inherent advantage of nonlinear effect suppression [24]. Initially this type of fiber was used to control dispersion along the single -mode fiber length (so called DDF - dispersion decreasing fibers) [32]. Later, the fiber design was applied to propagates the fundamental mode excited at the thin (single-mode) end towards the thick end of the tapered fiber without excitation of high-order modes (HOMs), meaning that excellent beam quality would be maintained as the output power increases, which may contribute to improving the TMI threshold [21,33,34]. A larger first cladding diameter at the output tapered fiber end makes possible counter-pumping using a low-brightness cheap pump diode [2].

 

Fig. 1. Schematic diagram of the tapered LMA YDF. (a) A cross section of the T-YDF; (b) The measured core/cladding diameters along the tapered optical fiber; (c) Schematic representation of the refractive index profile of an optical fiber featuring a depressed cladding and confined core.

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The experimental setup for the high peak power PM-T-YDF-based CPA laser system is depicted in Fig. 2. The seed laser is a homemade mode locked broadband fiber oscillator based on semiconductor saturable absorption mirror (SESAM), the schematic diagram of the experimental setup is shown in Fig. 3(a). The SESAM mode locked oscillator operates at a center wavelength of 1031.1 nm with a 14.8 nm spectral bandwidth (see Fig. 3 (b)) and no obvious spectrum modulation, the corresponding repetition rate of 37.7 MHz, an output power of 5 mW and pulse duration of 3.8 ps (see Fig. 3(c)). This broadband fiber mode locked source offers the possibility of obtaining a femtosecond laser pulse with a short pulse duration. This broadband oscillator is a very stable fiber laser oscillator for long term industrial application, and its spectrum band is substantially wider than that of the commercially available SESAM mode locked fiber oscillator.

 

Fig. 2. Schematic of the high peak power femtosecond fiber CPA amplification laser system.

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Fig. 3. (a) Schematic diagram of the homemade SESAM mode locked broadband fiber oscillator; (b) The spectrum of the mode locked fiber oscillator; (c) The pulse duration of the mode locked fiber oscillator.

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To lower the nonlinearity accumulated in the amplification, the stretcher is selected under the consideration of the large dispersion and the spectrum preservation, so the CFBG with a reflection band of 20 nm is selected to assure no filtration of the seeder spectrum; however, its maximum stretch ratio is approximately 43.5 ps/nm. To offer a large dispersion, two temperature tuning chirped-fiber Bragg gratings (TCFBGs) are connected by a four-port circulator with a large second order dispersion of ∼87 ps/nm are used to broaden the pulse duration. In additionally, the compressor requires accurate dispersion match in order to ensure the beam quality of the output pulse. Therefore, a temperature gradient along the fiber grating was added to offer second-order dispersion tuning and higher-order dispersion tuning of two CFBGs. The CFBGs can offer a second order dispersion of ∼48.73 ps² and a tuning range of more than 1.2 ps² with adding temperature gradient along the CFBGs. It also has a third order dispersion of − 0.422 ps³ and a tuning range of more than 0.07 ps³ with adding temperature gradient along the CFBGs. After the stretcher, the pulse duration is stretched to a favorable pulse duration of more than 1 ns. After two-stage YDF amplifier of a core pumped 6 µm/125 µm Yb-doped single mode gain fiber preamplifier and a 10 µm/125 µm cladding pumped power amplifier, the laser power reaches to approximately 1 W. Then, we set an acoustic-optic modulator (AOM) to reduce the pulse repetition rate from 37.7 MHz to 200 kHz for ease of obtaining a higher pulse energy in subsequent amplification stages. A Yb-doped polarization-maintaining multi-clad fiber with a core diameter of 10 µm and cladding diameter of 125 µm (Nufern, PLMA-YDF-10/125-M), which is same fiber type with the first pre-amplification stage was used to enhance the power to approximately 121.3 mW. The main amplifier is a one stage T-YDF amplifier and spliced with preamplifier stage and effectively guarantee compact and robust all-fiber systems. The tapered optical fiber at the heart of the gain module is based on the low photodarkening core chemistry and features a distinctive and proprietary refractive index profile. The result is a TMI free operation at up to 100 W of average output power, with excellent beam quality and good polarization maintenance. The module can be easily integrated to a pigtailed oscillator with its standard 10/125 PM input fiber. The T-YDF amplifier is end pumped by a 976 nm fiber-coupled laser diode that provided up to 200 W within a NA of 0.22. The highly divergent pump laser is focused into the T-YDF with a beam diameter of 365 µm by dichroic mirror (HT@976 nm, HR@1030 nm) and two lenses whose focal length is 25 mm (L1) and 50 mm (L2) respectively. The counter-pumped tapered fiber amplifier was conducted (where the pump power was space coupled into the thick end and the signal power was fiber coupled into the thin end) at the final amplification stage. The tapered fiber amplifier showed a decrease in the seed signal power required to saturate the amplifier by orders of magnitude compared to conventional LMA fibers. Counter-pumped configurations on the other hand deliver higher quality pulses with less energy in the pedestals. The T-YDF was in a water-cooled heat sink at 23 ^°C for efficient thermal management. Then, after amplification, the amplified power is compressed by a grating pair with a groove density of 1600 line/mm to offer dispersion compensation (two diffraction gratings are 30 mm × 20 mm × 6.35 mm and 135 mm × 20 mm × 6.35 mm).

A 500 MHz digital oscilloscope (Tektronix, MDO3052, USA) was employed to record pulse repetition. An Optical Spectrum Analyzer (Yokogawa AQ6370D 600∼1700nm) with a high resolution of 0.02 nm was used to monitor the output optical spectrum. The pulse width was recorded by a commercial autocorrelator (Pulse check, APE). A Spiricon beam squared beam quality analyzer (M2-200s-FW), together with a CCD camera (Dataray WinCamD-LCM) were used to record the beam quality and the profile of the generated laser pulses.

3. Experimental results

Chirped pulse amplification (CPA) is nowadays the most common approach to realize peak power scaling and achieve highly energetic ultra-short pulses [35]. Before the last stage of amplification to reduce peak power level inside the fiber amplifier substantially below stimulated raman scattering (SRS) threshold [36]. Two CFBGs are connected by a four-port circulator and offer a total dispersion ratio of ∼87 ps/nm and a reflection bandwidth of 20 nm are used to stretch the pulse duration to more than 1 ns. In additionally, the compressor requires accurate dispersion match in order to ensure the beam quality of the output pulse. Therefore, a temperature gradient along the fiber grating was added to offer second-order dispersion tuning and higher-order dispersion tuning of two CFBGs. Then, the stretched seed pulses are step-by-step amplified from 1 mW to 11.8 mW in the core-pumped 6 µm/125 µm Yb-doped single mode gain fiber and amplified from 11.8 mW to approximately 700 mW in the 10 µm/125 µm cladding-pumped power amplifier. For ease of obtaining a higher pulse energy in subsequent amplification stages, the pulse repetition rate was reduced to 200 kHz with the power of only approximately 1.43 mW after AOM. A Yb-doped polarization-maintaining double cladding (DC) fiber with a core diameter of 10 µm and cladding diameter of 125 µm was used to enhance the power to approximately 121.3 mW. The gain medium in the amplifier stage is a multi-clad PM-T-YDF, which has a core/cladding diameter of 35/250 µm and 56/400 µm at the thin and thick ends, respectively. The core/clad NA is 0.07/0.47. The tapered fiber was spliced with preamplifier stage and effectively guarantee compact and robust all-fiber systems. In addition, the thick end has an invariant core-size region, which is designed to mitigate nonlinear effects. The cladding absorption coefficient is measured to be ∼8 dB/m at 975 nm.

Figure 4(a) shows the dependence of the amplification performance on the incident pump power in the T-YDF, with the power changes from 6.1 to 144 W. Increasing the pump power to 144 W, we obtained an ultrashort pulse with a maximum average output power of 79.4 W. The amplifier output power increased linearly as the pump power, with a slope efficiency of 57%. We do not, however, investigate the laser system’s maximal capabilities due to the thorough parameter and stability requirements of the system. In our experiment, we conducted a straightforward of single pass, which has an excellent performance when the injection signal is quite low and consider the signal absorption of the T-YDF [37]. The spectrum evolution is measured. As shown in Fig. 4(b), the spectrum after AOM and T-YDF amplification at an output power of 79.4 W are measured, respectively. A spectral gain narrowing effect is observed in this CPA system.

 

Fig. 4. The output power of the T-YDF amplifier versus the pump power (a) and the spectrum evolution in the amplification chains (b).

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The beam profiles and beam quality of different output laser powers in the T-YDF amplifier are shown in Fig. 5. The waist width becomes larger and the beam quality deteriorates as the output power increases from 59.5 W to 79.4 W, which results from the thermal-optical effect. The M² factors of the T-YDF amplifier in the horizontal and vertical planes were measured to be 1.161 and 1.224 (see Fig. 5(a)), 1.176 and 1.229 (see Fig. 5(b)) with output power of 59.5 W and 79.4 W, respectively, which is close to the diffraction-limited. Obviously, the beam profile is excellent at the low pump power and the deterioration in beam profile was caused by the thermal-optical effect with increasing the pump power. During the high-power pump, the T-YDF serves as a thermal-lens, which makes the mode mismatch between the pump power and the seed laser. Consequently, optimizing the diameter of the seeder laser and the pump laser to maintain a better beam overlap in the T-YDF amplifier for further improving the beam profile.

 

Fig. 5. Beam quality of different output power in the T-YDF amplifier.

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After amplification, the laser is isolated by the high-power polarization dependent isolator and compressed by the diffraction grating pair, as shown in Fig. 6(a). We conducted experiments at a relatively low repetition rate of 1 MHz. By accurately adjusting the distance between the grating pair of 1.67 m, which can offer the second order dispersion of − 48.77 ps², the third dispersion of 0.209 ps³, and the fiber itself in the CPA system can also introduce the positive second order dispersion and third order dispersion. We obtained a pulse energy of 52.4 µJ with the peak power of 70.7 MW, corresponding to the compression efficiency was ∼85.6% with consideration of four-time diffraction of gratings. During the compression, we have observed the spectral distribution after the grating diffraction and did not find the spectrum of SRS at wavelength of 1080 nm, and the high compression efficiency also proves that the proportion of SRS is very small. Assuming the Lorentz-shaped hypothesis of the measured autocorrelation trace, the pulse duration was measured to be 741 fs (Fig. 6(b)).

 

Fig. 6. (a): The output power of the T-YDF amplifier and compressed output power versus the pump power; (b): The compressed pulse curve under output power of 52.4 W.

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To offer an ideal laser for the micromachining system, the beam quality and pulse duration are important parameters. By increasing the repetition rate to 2 MHz, an output average power of 70.6 W with peak power of 132 MW was achieved, the corresponding beam quality (M²) of the compressed laser was shown in Fig. 7(a). The M² factors of in the horizontal direction and vertical direction were measured to be 1.182 and 1.290. In our experiment, we tuned the dispersion by adding a temperature gradient on the CFBGs to make a precise dispersion compensation, and the pulse duration was shortened to 266 fs with Lorentz fitting as showed in Fig. 7(b), indicating the obtained compressed pulses have a very high temporal conference and the potential of this broadband fiber CPA system that can output a short pulse duration at a high energy level.

 

Fig. 7. Measurement of the beam quality (a) and compressed pulse curve (b) with peak power of 132 MW.

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4. Discussion

Nowadays, the reported tapered fiber lasers often use several meters long tapers. Peak power scaling for standard fiber amplifier is still limited because of parasitic nonlinear effects due to small mode field diameters and long effective interaction lengths [1,38], and low facet damage threshold due to high intensity. Crucially, the impact of all these nonlinear effects scales inversely with the effective mode-field area. Consequently, high average power requires the exploiting of sufficiently long T-YDF with a large surface area for reducing of thermal load on the fiber. On the contrary, in order to obtain high peak power, it is necessary to use as short fibers as possible; large mode area to avoid stimulated Raman scattering (SRS) appearance is also required. It is this contradiction that explains the fact that currently reported amplifier systems with T-YDF have been demonstrated either with MW peak power but at the same time with low average power and low repetition rate, or with high average power but with sub-MW peak power. However, our work reported the femtosecond pulse output in such T-YDF amplifier with excellent performance of both hundreds of MW-levels peak power and good beam quality of close to diffraction limited.

However, nonlinear effects induced by high laser power limit further scaling of the output power. The SRS makes it impossible for further pulse amplification due to transferring the amplified laser pulse energy to the new generated Stockes wavelength. Therefore, suppressing nonlinear effects is particularly critical for further scaling of the output power from high-power fiber lasers and amplifiers. For further explorations about the pulsed tapered fiber amplifier, the SRS gain coefficient of the first Stokes can be calculated by formula [26]:

Here, ${g_R}$ is the material Raman gain (${g_R}$. ≈ 10⁻¹³ m/W); L is the length of the tapered fiber amplifier; ${A_{eff}}(z )$ is the effective area of the fundamental mode at the position z; ${P_{peak}}(z )= P(z )/({f\cdot \tau } )$ is the peak power at the position z of the tapered fiber, where f is the pulse repetition rate and $\tau $ is the pulse width); $P(z )$ is the average power. The condition that defines the threshold of the SRS for the case of a passive fiber is ${G_{ls}} = 16$, the corresponding equation is as follows:

The SRS threshold is growing with the tapered fiber length. The considerably long taper fiber is able to absorb all the coupled pump power in the pump input part, while the other side of the fiber remains unpumped and no growth of signal takes place in this part, thus signal amplification occurs in the last ∼1 m of the fiber near the pump input end. Therefore, high amplification at this part results in a quick onset for nonlinear effects. The increase in the length of the tapered fiber suppresses the evolution of the nonlinear effects in the thin part of the fiber, which is numerically forced to operate in the single mode regime. The tapered active fiber with convex tapered structure presents large effective mode area exceeding 1000 µm² in its thick end, which provide some margin for achieving higher pulse energy and peak power while pushing back the threshold for adverse nonlinear effects such as SRS [39].

Additionally, we conduct numerical calculation by considering the B integral of the taper fiber amplification section. The B integral, which is used to describe the amount of accumulated nonlinear phase, can be expressed below:

Considering the high pump absorption of 8 dB/m at 975 nm, and the pump is end pumped from the larger end, so the pump absorption occurs almost in the last sections of the fiber comprising straight sections of 400 µm with a length of 0.7 m and tapered section with a length of ∼0.7 m near the pump input end. In the smaller-end, there is nearly no pump exists. We assume that in the 35/250 µm fiber section with length of 1 m, the signal remains constant of 0.5W, in the tapered fiber section and 56/400 fiber section, the amplification process is a small signal amplification process to facilitate numerical simulation, then can be expressed as:

Here, $g$ is the gain coefficient, $l$ is the position along the fiber amplifier, ${P_{in}}$ is the injection power. We set the injected power as 0.5 W at 2 MHz, considering that the signal amplification occurs almost in the last sections of the fiber comprising straight sections of 400 µm with a length of 0.7 m and tapered section with a length of ∼0.7 m near the pump input end, the calculated is 3.6197 /m.

During the amplification, based on the core and the cladding diameters monotonically increase along the tapered fiber, we separate the taper fiber into 3 parts, the 35/250 fiber amplification section, the tapered fiber amplification section and 56/400 fiber amplification section. Then the g parameter is calculated based on the above considerations, which $g$=0 in the 35/250 fiber amplification section and $g$=3.6197 /m in the tapered fiber amplification section and 56/400 fiber amplification section. The calculated amplified output power is 0.5 W, 6.3 W and 79.4 W from the three sections, respectively. With considering of spectral narrowing effect, the pulse width is dependent on the average spectrum width due to the chirped pulse amplification system. Through the measured input spectrum width and output spectrum width, we reasonably set the average spectrum width are 10 nm, 8 nm and 6 nm in these three fiber amplification sections, the corresponding pulse duration are about 870 ps, 696 ps and 522 ps (the dispersion parameter is ∼87 ps/nm). The nonlinear refractive index coefficient is set as 2.7 × 10⁻²⁰, and the average mode area diameter are set as 25 µm, 30.34 µm and 35.68 µm at these three amplification sections. We calculated the integral operation of the B parameter with the values are 0.0974 rad, 0.2679 rad and 2.4407 rad. The total B integral is 2.806 rad. It can be seen that the total B integral is relatively large, shortening the length of the thin part of the taper fiber and suppress unwanted non-linear effects for extremely high peak power will be improved in the following study. The tapered structure and amplification scheme have an effect on the mode characteristics and laser performance, which provides theoretical foundations for developing compact high-power fiber lasers and amplifiers.

5. Conclusions

In conclusion, ultrashort pulse Yb-doped fiber (YDF) lasers with excellent performance of both high peak power (hundreds of MW) and good beam quality (close to diffraction limited) have greatly promoted the development of industry, medicine and fundamental science. Here, we report generation of a high-energy sub 300 fs polarization maintaining fiber chirped pulse amplification (CPA) system by using a Yb-doped large mode area tapered PM optical fiber of the short local adiabatic taper (70 cm taper length with an input end of∼35/250 µm) and LMA fibers (70 cm fiber length with an output end of∼56/400 µm) for peak power scaling while maintaining good beam quality. The efficient compressed of 266 fs pulse amplification with peak power of up to 132 MW at a repetition rate of 2 MHz thereafter using the CFBG’s fine-tuned capacity of second-order dispersion and higher-order dispersion. Good beam quality with a mean M² value of ∼1.17 was observed at the maximum achieved average power of 79.4 W. To the best of our knowledge, it is the highest peak power reported in such T-YDF amplifier in the femtosecond regime. Further, short local adiabatic taper is an urgent need to suppress unwanted non-linear effects for extremely high peak power. The generated high-peak-intensity femtosecond fiber lasers paves the way for further development of high energy physics, such as laser particle acceleration.