Femtosecond pulse lasers at 780 nm play important roles in various fields including two-photon imaging^［1-2］， nanofabrication^［3-6］ and terahertz generation^［7-8］. In general， Ti∶sapphire lasers can emit red and near-infrared light from 650 nm to 1100 nm， such that they can directly generate femtosecond pulses at 780 nm through mode-locking^［9］. On the other hand， Ti∶sapphire lasers with free-space configurations， as the commonly used lasers， are known to be costly， large， and sensitive to environmental perturbations， preventing their clinical and industrial applications that require excellent reliability and low cost^［10-11］. Alternatively， fiber lasers are inexpensive， compact， and resistant to external perturbations， and they are promising for these applications with demanding environmental conditions. Recently， the direct generation of visible wavelengths via fiber lasers based on nonlinear amplifying loop mirror （NALM） has been reported^［12］. As there is no ready-to-use gain fiber for directly generating laser at 780 nm， frequency-doubling of femtosecond pulses from Er-doped fiber lasers at 1560 nm through second-harmonic generation （SHG） has been an effective method^［13］. In order to increase the average power at 780 nm， high peak power of the fundamental beam is required for the SHG process. Consequently， the chirped pulse amplification （CPA） technique is commonly adopted^［14］. In the meantime， it is possible to achieve high SHG efficiency by designing frequency-doubling crystals with appropriate thickness and poling period. So far， various laser systems based on Er-doped fiber lasers have been demonstrated for generating femtosecond pulses at 780 nm^［15-20］， with some of these systems exhibiting SHG efficiency even exceeding 40%^{［15，18-20］}. Yang et al. have achieved ~1 W average power at 780 nm by managing the pre-chirp using dispersion compensating fiber （DCF）^［20］， which is an inaugural achievement of a fiber-based 780-nm femtosecond pulse laser system with watt-level average power. Nevertheless， these 780-nm femtosecond pulse laser systems in prior works invariably incorporate non-polarization-maintaining （non-PM） fibers， which renders the laser system susceptible to environmental perturbations， leading to concerns about the reliability for clinical and industrial applications. To this end， the adoption of all-PM fiber structures could significantly enhance system reliability， and several research groups have implemented all-PM 780-nm femtosecond pulse laser systems^［21-23］. For example， by optimizing the temperature of the nonlinear crystal ［i.e.， periodically poled lithium niobite （PPLN）］， Jiang et al. obtained an average power of 1.1 W at 780 nm by SHG of an all-PM fiber femtosecond pulse laser at 1560 nm^［22］. For practical applications， a higher average power of the 780-nm femtosecond laser with full engineering integration is still interesting^［24-25］， but yet to be further explored.

Here， we introduce a high-power all-PM fiber femtosecond pulse laser source at 780 nm with full engineering integration for practical applications based on an all-PM fiber laser. The integrated laser system can deliver an output power of >2.22 W and a pulse width of <288 fs at 780 nm. The 780-nm femtosecond pulses are generated from the SHG of an all-PM fiber amplified mode-locked laser at 1560 nm， and the SHG efficiency is about 52%， which is the best performance to date. The power fluctuation of the integrated all-PM fiber laser system is measured to be only 0.23% ［root mean square （RMS）］ for 1 h. This all-PM fiber femtosecond pulse laser system provides optimal performance in both average power and SHG efficiency， and its practical application is further guaranteed by compact engineering integration.

The experimental configuration of the femtosecond pulse laser source is presented in Fig. 1. The fiber laser source mainly comprises a seed， stretcher， pre-amplifier， and main amplifier. The seed is a figure-9 mode-locked fiber laser utilizing a NALM as an artificial mode locker^［26］. A phase shifter （PS） is employed to modify the intensity-dependent transmission for passive mode-locking. Through a wavelength-division multiplexer （WDM）， a single-mode laser diode （SM-LD， 460 mW maximum output power at 976 nm） is utilized as the pump source of the oscillator. A 40-cm-long PM Er-doped fiber （EDF， Liekki Er80-4/125-HD-PM） is used as the gain fiber which has a normal group velocity dispersion （GVD） of 28 ps²/km at 1550 nm. To manage the intracavity dispersion， a 30-cm-long PM DCF （Nufern PM2000D） is employed， with a normal GVD of 76 ps²/km at 1550 nm. An optical coupler （OC） is utilized to loop the figure-9 cavity. The 70% port of the OC is connected with fiber-type dielectric film （DF） that has a reflectivity of 85% at 1550 nm， with 15% extracted as the output. The 30% port of OC is allocated for monitoring. The total length of the cavity is about 179 cm with a net group delay dispersion （GDD） of about +0.002 ps². A fiber-based isolator （ISO） is used to protect against back-reflection and ensure the stability of the seed. In the stretcher， the pulse is pre-chirped by a 40-m-long PM DCF （Nufern PM2000D）， i.e.， stretched from approximately 100 fs to 50 ps. The stretched pulse is subsequently amplified by the pre-amplifier and main amplifier. The pre-amplifier includes a 6-m-long PM EDF （Fibercore DHB1500）， WDM， and SM-LD （the same as before）， while the main amplifier involves a 4.5-m-long double-clad PM Er/Yb co-doped fiber （EYDF， Coherent PM-EYDF-12/130-HE）， combiner and multimode laser diode （MM-LD， 27 W maximum output power at 976 nm）. An ISO is placed between the pre-amplifier and main amplifier to avoid back-reflections. Following the two stages of amplifiers， laser pulses are launched into the free space by a collimator （focal length f=19 mm）. In the free space， the laser pulses are compressed by a pair of gratings （LightSmyth T-940CL-90）. The polarization state of the laser is adjusted by a half-wave plate （HWP） to align with the polarization direction of the crystal to obtain the highest frequency-doubling efficiency. A pair of double-glued achromatic lenses （f=19 mm） are used to focus and collimate the laser beam for frequency-doubling in a 0.5-mm-long PPLN crystal （CASTECH， the poling period is accessible in the range of 19.4‒19.7 μm in different channels）. Then the laser beam is passed through a dichroic mirror （DM） to remove the 1560-nm wavelength component， and subsequently， a bandpass filter （BPF） to filter out the green wavelength component.

The basic performance of the figure-9 mode-locked laser is depicted in Fig. 2. The self-started mode-locking is realized when the pump power is 300 mW. To ensure single-pulse operation， the pump power is automatically set to 200 mW by programming the pump current of the home-designed LD driver. When the seed operates in the single-pulse state， the output power is about 5.4 mW. Figure 2（a） presents the optical spectrum of the single-pulse seed measured by an optical spectrum analyzer （YOKOGAWA AQ6370D） with a resolution of 0.1 nm， indicating a center wavelength at 1561 nm and a 3-dB bandwidth of 24 nm. Figure 2（b） displays the radio-frequency （RF） spectrum of the seed measured by a 26.5-GHz frequency signal analyzer （Rohde & Schwarz FSWP26）， with a signal-to-noise ratio （SNR） of >100 dB， signifying a stable mode-locked state. The flat and broad optical spectrum results from the near-zero net intracavity dispersion. Figure 2（c） illustrates the oscilloscopic trace of the pulse sequence with a pulse spacing of 9 ns， equivalent to a repetition rate of 111.5 MHz， which is received by a 12.5-GHz photodetector （Newport 818-BB-51F） and recorded with a 20-GHz real-time oscilloscope （LeCroy WaveMaster 820Zi-B）. Figure 2（d） showcases the stability of the seed average power over 1 h， which exhibits a RMS of merely 0.076%. This notably good stability highlights the inherent advantage of the all-PM fiber structure. The seed is observed by a photodiode power sensor （Thorlabs S146C） in the experiment.

The pulses generated by the seed oscillator are then stretched and enter the pre-amplifier. With a pump power of 400 mW， the average power after the pre-amplifier reaches 125 mW with an optical efficiency of 31%. Figure 3（a） demonstrates the optical spectrum of the amplified pulses after the main amplifier. The optical spectrum is centered at 1561 nm with a 3-dB bandwidth of 13 nm. A narrower amplified optical spectrum results from the gain-narrowing effect of the amplification. The amplified optical spectrum also manifests modulation structures produced by the nonlinear effects of the optical fiber， mainly self-phase modulation （SPM）. As shown in Fig. 3（b）， the maximum power is about 5.37 W at a pump power of 17.9 W， corresponding to an optical efficiency of up to 30%. Figure 3（c） illustrates the power stability of the amplified laser for 1 h， wherein the RMS is calculated to be 0.16%. The slight decrease of the power stability can be attributed to the introduction of two stages of optical amplifiers. After the power amplification， a grating pair is used as the pulse compressor. The average power of the compressed pulses is 4.2 W， corresponding to a compression efficiency of 79%. Figure 3（d） displays the autocorrelation trace of the compressed pulses， measured by an autocorrelator （Femtochrome FR-103WS）， and the pulse duration is about 204 fs （assuming a hyperbolic-secant pulse shape）， corresponding to a peak power of 185 kW. The inset of Fig. 3（d） displays the autocorrelation trace before compression， which has a pulse width of about 30 ps. The compressed pulses show pedestals as the result of high-order dispersion and nonlinearity^［27］.

After pulse compression， the femtosecond pulses are focused onto the frequency-doubling crystal for SHG. The performance of the generated laser pulses at 780 nm is elucidated in Fig. 4. Figure 4（a） illustrates the optical spectrum of the generated laser at 780 nm. Since the spectral components that do not satisfy the phase-matching condition have limited contribution to the frequency-doubling process， the optical spectrum after frequency-doubling is smooth and narrowed. Figure 4（b） shows the frequency-doubling efficiency with respect to input power. Thanks to the appropriate thickness and poling period of frequency-doubling crystal， and the optimized focal length of the lens， the frequency-doubling efficiency reaches up to 52%， with a maximum average power of 2.22 W at 780 nm， corresponding to a single pulse energy of 20 nJ. As far as we know， this is the highest power and frequency-doubling efficiency obtained in 780-nm femtosecond pulse fiber laser systems. Figure 4（c） shows that the RMS of the 780-nm laser is as low as 0.23% over a 1-h power stability measurement， implying a high stability inherited from the all-PM fiber system design. The output power of the laser source at 780 nm is monitored using a thermal power sensor （Thorlabs S405C） in the experiment. Figure 4（d） is the autocorrelation trace of the SHG pulses， indicating a pulse width of 288 fs （assuming a hyperbolic-secant pulse shape）. The SHG pulses are clean without any visible pedestal compared with the autocorrelation trace of the 1560-nm femtosecond pulses. The beam qualities before and after the SHG are also measured， as shown in Figs. 4（e） and 4（f）. The M² factor of the laser beam before SHG is less than 1.05， which suggests a nice beam quality thanks to the single-mode all-fiber design. After the frequency-doubling， however， the beam quality degrades， yielding an M² factor of <1.5. There exists nonuniform heating in the nonlinear crystal due to linear and nonlinear absorption of fundamental laser and second-harmonic during the frequency-doubling process^［28-29］. This leads to a change in the refractive index of the crystal and thermal dephasing and thermal lensing， resulting in beam quality degradation^［30-31］.

We further conducted the measurement of the phase noise （PN） and relative intensity noise （RIN） for both the seed and power amplification in a frequency-offset range of 10 Hz‒10 MHz， as shown in Fig. 5. The timing jitter results before and after the amplification are 861 fs and 3.69 ps， respectively， and the integrated RIN values are 0.06% and 0.12%， respectively. The all-PM fiber architecture， as well as the intracavity dispersion management， contributes to the outstanding noise performance of the seed. The introduction of the stretcher and the two-stage amplifier， however， increases both the timing jitter and the integrated RIN after power amplification.

For practical applications， we finally integrate the fiber laser system in an optical enclosure with dimensions of 40 cm×25 cm×15 cm， as shown in Fig. 6. In this enclosure， the seed， pre-amplifier and main amplifier are packaged in three separate boxes， while the stretcher and pre-amplifier are placed in the same box. To improve heat dissipation， the fibers are covered with thermal adhesive in all three boxes. In addition， our home-design circuit boards that control the LD pump sources are placed outside of the three boxes along with all the LDs to minimize the impact of heat on the fiber system. A universal serial bus （USB） cable connects the circuit boards to the computer， and a home-design computer program is employed for laser control. For the free-space parts， the compressor and SHG module are integrated into an optical enclosure with dimensions of 30 cm×20 cm×10 cm， which is connected to the enclosure of the fiber laser system via a PM fiber cable. This integrated laser can stably operate in a temperature range of -10 ℃ to 50 ℃.

We have built a high-power， high-stability femtosecond pulse laser source at 780 nm with full engineering integration based on an all-PM fiber laser. We successfully generated a laser with a power up to >2.22 W and a pulse width of <288 fs at 780 nm. The power variation of the integrated laser system is only 0.23% （RMS） for 1 h. We anticipate that this stable femtosecond pulse laser source will find diverse applications in fields including two-photon imaging， nanofabrication， terahertz generation， etc.