Recently, the development of high-energy thulium (Tm)-doped femtosecond laser systems has attracted abundant research interest, driven by their applications in medicine, material processing, and biomedical imaging [1–3]. Especially, combined with second-harmonic generation (SHG), ultrafast Tm-doped lasers operating around a wavelength of 1.9 µm can be highly attractive for two-photon microscopy (2PM). Compared to traditional solid-state 2PM sources such as Ti:sapphire lasers, fiber-based sources can be more compact and user-friendly by minimizing free-space laser alignments [4]. Furthermore, fiber-based lasers for 2PM can be designed with versatile repetition rates, especially around 1–10 MHz [5]. Due to the lack of any available mainstream gain fibers in the wavelength window around 950 nm, most fiber-based systems rely on nonlinear conversion processes such as frequency-doubling [6] or self-phase modulation [7]. One approach for generating such ultrashort pulses consists of soliton self-frequency shifting (SSFS) of erbium-doped fiber lasers [5,8] before second-harmonic generation. However, SSFS-based 2PM sources require two stages of frequency conversion, which can increase the overall system complexity. Furthermore, the pulse energy and center wavelength become coupled in the SSFS process, which can limit the power scaling at certain (especially shorter) wavelengths [9]. The highest demonstrated pulse energy around 950 nm from such a SSFS-based fiber system is currently 72 nJ [5]. On the other hand, frequency-doubled ultrafast Tm lasers can allow further power scaling at 950 nm, which can be beneficial for high-speed, large-field-of-view 2PM through spatiotemporal multiplexing [10].

So far, ultrashort pulses from 1.7 µm to 2.0 µm have been generated directly from Tm-doped chirped-pulse amplification (CPA) systems [11,12] due to the broad gain bandwidth of thulium [13]. However, due to signal re-absorption [14] and detrimental atmospheric absorption [15], it has been challenging to generate high-energy femtosecond sources operating close to the short-wavelength edge of the Tm gain emission spectrum. As such, most Tm-doped CPA systems operate between wavelengths of 1920 nm to 1980 nm and there is a significant difference in available pulse energy from Tm-doped CPA systems at different wavelengths even when the fiber mode-field diameter is comparable. Currently, although ${\gt}{1}\;{\rm mJ}$ pulses have been demonstrated from Tm-doped femtosecond lasers with large-pitch fiber rods and coherent beam combination at longer wavelengths [16], the highest pulse energy from short-wavelength (${\lt}{1920}\;{\rm nm}$) Tm-doped femtosecond fiber CPA systems has amounted to ${\sim}{1.1}\;{\unicode{x00B5}}{\rm J}$ using ZBLAN fibers and ${\sim}{130}\;{\rm nJ}$ for silica fibers [17,18]. For a frequency-doubled Tm-doped CPA system around 950 nm, to the best of our knowledge, the highest demonstrated pulse energy of femtosecond pulses is 12.9 nJ (at a repetition rate of 22 MHz) [19].

Here, we present a compact Tm-doped CPA fiber laser system at the wavelength of 1901 nm seeded by dissipative solitons with a repetition rate of 9.26 MHz. With a large-mode-area (LMA) Tm-doped amplifier optimized for pulse amplification at our signal wavelength, a maximum output power of 6.5 W is obtained. After pulse compression, pulses with an energy of 394 nJ and a 490 fs duration are demonstrated. After frequency-doubling in a 1 mm periodically poled lithium niobate (PPLN) crystal, a maximum pulse energy of 138 nJ is obtained at the wavelength of 954 nm with a pulse duration of 390 fs. To the best of our knowledge, our results represent a three-fold improvement of the highest pulse energy for a silica-based Tm-doped fiber CPA system operating below 1920 nm [18] and a 10-fold increase for a frequency-doubled fully Tm-doped CPA system around 950 nm [19] (or a two-fold increase compared to any frequency-doubled SSFS setup at this wavelength [5]).

The schematic of our laser system is illustrated in Fig. 1. The system has two main sections: an amplified high-energy Tm-doped ultrafast fiber laser and a second-harmonic-generation setup. The Tm-doped CPA system consists of a mode-locked Tm-doped dissipative soliton oscillator, a fiber stretcher, three fiber amplifiers, and a free-space pulse compressor. An all-fiber configuration is maintained until the pulse compression stage, ensuring a compact system design.

 

Fig. 1. Experimental setup of the ultrafast 1.9 µm thulium-doped CPA system, consisting of a dissipative soliton oscillator, a single-mode stretcher based on ultrahigh numerical aperture fibers (UHNA), pre-amplifiers, and a large-mode-area (LMA) power amplifier, followed by a frequency-doubling stage to convert the wavelength to 950 nm. WDM, wavelength-division multiplexer; PD-ISO, fast-axis blocking polarization-dependent isolator; PC, polarization controller; SBR, saturable Bragg reflector; CIR, circulator; OC, output coupler; PBS, polarization beam splitter; MFA, mode field adapter; MPC, multi-mode pump combiner; HWP, half-wave plate; G, reflection grating; M, mirror; L, lens; F, filter.

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The unidirectional all-fiber ring oscillator incorporates a fast-axis blocking isolator and two in-line polarization controllers to facilitate mode-locking by nonlinear polarization evolution (NPE). Combined with a saturable Bragg reflector connected to a fiber circulator, reliable self-starting is supported in a hybrid mode-locking scheme. For a coupled pump power of 400 mW (at a wavelength of 1565 nm), the all-fiber laser cavity can generate stable up-chirped dissipative soliton pulses with an average power of 1.5 mW (pulse energy of 0.16 nJ) via a 25 % output coupling port. Using a relative short segment (1.6 m length) of Tm/Ho-doped gain fiber (Coractive, TH512, 5/125 µm core/cladding diameter, ${\beta _2}=-70\,\,\rm ps^{2}/\rm km$), the laser operates with a center wavelength of 1901 nm, close to the short-wavelength edge of the Tm-doped gain fiber emission spectrum. This emission wavelength is further supported by the spectral filtering of the SBR, fiber components, and a tunable Lyot filter whose transmission is optimized by two intracavity polarization controllers. The combination of an 8.3 m long segment of normal dispersion fiber (Nufern, UHNA4, ${\beta _{2{\rm UHNA}4}}=120\,\,\rm ps^{2}/\rm km$) with SMF-28e (${\beta _2}= - {67}\;{{\rm ps}^2}/{\rm km}$) results in an overall slightly normal net-cavity dispersion (${0.02}\;{{\rm ps}^2}$) which enables dissipative soliton mode-locking operation.

The optical spectrum of the seed pulses is shown in Fig. 2(a), featuring the characteristic steep spectral edges of dissipative solitons with a 10 dB spectral bandwidth of 35 nm. The oscilloscope trace in Fig. 2(b) presents a uniform pulse train with a roundtrip time of 108 ns, consistent with the cavity length of 22.2 m. The RF spectrum of the fundamental repetition rate at 9.26 MHz with a high SNR of 70 dB, see Fig. 2(c), indicates stable operation of the seed pulses in a single-pulsing regime, as shown in Fig. 2(d). The pulse train and RF spectra are measured by a 20-GHz digital oscilloscope and a radio-frequency analyzer together with a 12.5-GHz InGaAs photodetector, respectively. The output pulses are highly linearly polarized with a polarization extinction ratio of ${\gt}{30}\;{\rm dB}$, suitable for seeding CPA systems.

 

Fig. 2. Mode-locking performance of the all-fiber Tm-doped oscillator. (a) Optical spectrum with a 35 nm bandwidth (resolution of 0.05 nm). (b) Oscilloscope trace, indicating a roundtrip time of 108 ns. (c) RF spectrum showing the fundamental repetition rate of 9.26 MHz (resolution bandwidth of 100 Hz). (d) Long-range RF spectrum indicating stable single-pulsing mode-locking (resolution bandwidth of 100 kHz).

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For the stretcher, two types of ultrahigh numerical aperture fiber (51.5 m of UHNA4; 20 m of UHNA7, ${\beta _{2{\rm UHNA}7}}={41}\;{{\rm ps}^2}/{\rm km}$) with positive second-order dispersion values are used to temporally broaden the seed pulses to 94 ps. The opposite sign of the third-order dispersions of these two fibers [20] can lead to a better pulse shape at the system output. Due to the intrinsic birefringence in the fiber stretcher, some polarization degradation occurs during the stretching process. To compensate for the polarization degradation in the non-polarization-maintaining stretching fibers, the stretched pulses are converted by two in-line PCs and a fiber polarization beam splitter back to linear polarization. Due to the intrinsic propagation loss in UHNA fibers, the splicing losses, and the loss from depolarization, the average power drops to 0.9 mW at the output of the stretcher.

Thus, two cascaded stages of single-mode pre-amplifiers are used to increase the average pulse power first to 12 mW and up to 210 mW in the second pre-amplifier (with coupled pump powers of 400 mW and 1.6 W at 1565 nm, respectively) such that the power amplifier can be saturated. The first pre-amplifier comprises a 1.6 m long gain fiber section of TH512 in a backward pumping scheme while the second pre-amplifier consists of a highly Tm-doped fiber (13.5 cm, Coherent TSF-5/125) in a forward pumping regime. With such a design, any nonlinear phase shift accumulation during pre-amplification is minimized by reducing both the gain fiber and its subsequent passive fiber length in the second amplifier stage. The spectrum of the pre-amplified pulses, as shown by the green curve in Fig. 3(b), is free from any nonlinear spectral distortion with the same 10 dB spectral bandwidth of 35 nm as the seed. A slight spectral amplitude shaping effect favoring the shorter wavelength is achieved to balance the gain filtering effect in the power amplifier.

 

Fig. 3. Characterization of the amplifier and compressor performance. (a) Output power of the LMA amplifier for different pump power values with a maximum output of 6.5 W. (b) The optical spectra of the output pulses of the oscillator (blue dashed), after the pre-amplifiers (green) and the LMA amplifier (orange) show spectral shaping across all stages, leading to the flattest spectral profile at the fiber system output. (c) The autocorrelation (AC) trace and the corresponding Gaussian fit of the 490 fs compressed pulses at a wavelength of 1.9 µm.

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A PC is used to align the light polarization to the transmission axis of a PD-ISO to seed linearly polarized light into the power amplifier. The power amplifier consists of a backward-pumped segment (120 cm) of polarization-maintaining (PM) double-clad large-mode-area Tm-doped fiber with a larger core diameter of 25 µm (Coherent, PLMA-TDF-25P/400-HE). Two fiberized 793 nm CW pump diodes are spliced to a PM $({2} + {1}) \times {1}$ pump combiner to provide a maximum coupled pump power of 52 W. Before the LMA Tm-doped fiber, a PM cladding pump stripper is used to remove any residual pump power and to protect the preceding fiber components. The combination of a relative short length of the LMA gain fiber with a backward pumping scheme is optimal for signal amplification around a wavelength of 1.9 µm with reduced signal re-absorption at shorter wavelengths (that can be more prominent in a forward pumping scheme). Passive forced air cooling of the gain fiber and the splicing points between the gain and passive fibers prevents excessive heat accumulation and improves the amplifier efficiency. The fiber system output is angle polished (8°) to avoid back-reflection at the air–glass interface.

To improve the optical beam quality of the amplified pulses, a PM mode field adapter ensures the excitation of only the fundamental mode before the LMA gain fiber. To suppress the excitation of higher-order modes (HOMs, LMA fibers are known to support especially LP11 and LP02 modes) in the gain fiber, the splice between the PM LMA passive and active fibers has been optimized to reduce any distortion of the fiber refractive index profile by using a short arc time. Furthermore, the gain fiber is wrapped into circles with a diameter of 10 cm to create a stronger bending-induced loss for the HOMs.

The PM-LMA amplifier boosts the average power to 6.5 W, corresponding to a pulse energy of 703 nJ. In Fig. 3(a), the output power of the LMA amplifier with respect to the coupled 793 nm pump power is shown. The slope efficiency of the power amplifier amounts to 17.6%, limited by the short gain fiber. Considering the 793 nm cladding pump absorption of 4.8 dB/m, the effective slope efficiency is 24.1% with respect to the absorbed pump power in the LMA gain fiber (73% of total pump power). Comparing the spectra at different stages of the system, as shown in Fig. 3(b), the LMA system output features the flattest spectral profile across the wavelength from 1891 nm to 1930 nm due to the balance between the seed spectral shape and the gain filtering effects at different amplification stages. The spectral modulation of the LMA system output is likely due to atmospheric absorptions and interferences between the fundamental mode and weak residual HOMs, but it has negligible effects on the temporal pulse quality. The B-integral is estimated as $0.21\pi ,\;0.04\pi$, and $0.59\pi$ for the fiber stretcher, the pre-amplifiers, and the power amplifier, respectively, and an overall system value of $0.84\pi$.

The amplified pulses are compressed in free space, after passing through a half-wave plate, with a pair of reflection gratings (Zeiss, 750 l/mm) in a double-pass Treacy configuration with a theoretical single-pass efficiency of 90%. An overall compression efficiency of 63% is reached, leading to an average compressed power of 3.7 W, corresponding to a pulse energy of 394 nJ. Measured with a commercial intensity autocorrelator (APE), the compressed pulse, as shown in Fig. 3(c), has a pulse duration of 490 fs. Based on the ratio between the area under the Gaussian fit and the AC trace, it is estimated that ${\sim}{89}\%$ of the pulse energy resides in the main peak.

During the SHG stage, the compressed 1.9 µm pulses are focused into a fanout periodically poled lithium niobate crystal (HCPhotonics). The crystal thickness is chosen to be 1 mm such that the spectral acceptance bandwidth of the PPLN matches well with the signal spectral width and a high conversion efficiency can be achieved. To maximize SHG efficiency, the focusing into the crystal is optimized (focal length of L1 ${ f} = {50}\;{\rm mm}$, beam waist at focus ${\omega _{0\:}} = {30}\;\unicode{x00B5}{\rm m}$). For a poling period of 28.4 µm with the temperature tuned to 38°C, an optimal quasi-phase-matching is achieved. A second lens L2 (${f} = {15}\;{\rm mm}$) is used to collimate the frequency-doubled output after the crystal. The light then passes through a long-pass filter F1 (700 nm cutoff) and a short-pass filter F2 (1600 nm cutoff) to eliminate spurious higher-order harmonics and the residual fundamental wavelength, respectively.

A maximum pulse energy of 138 nJ is achieved for the frequency-doubled pulses with a conversion efficiency of 35%, as shown in Fig. 4(a). In the inset, a well-defined Gaussian beam is presented after second-harmonic generation, as measured by a beam profiler (Newport). The high beam quality of the frequency-doubled pulses is further confirmed by an ISO-compliant ${M^2}$ measurement. The spatial output mode is close-to-diffraction-limited with $M_X^2 = 1.25$ and $M_Y^2 = 1.17$ at its maximum power. The spectrum of the frequency-doubled pulses is centered at 954 nm with a 10 dB spectral width of 5 nm [Fig. 4(b)]. In Fig. 4(c), by measuring the AC trace of the frequency-doubled pulses, it is confirmed that the pulse duration is reduced to 390 fs after the SHG.

 

Fig. 4. Performance of the SHG system. (a) Energy of the frequency-doubled pulses (with a maximum of 138 nJ) w.r.t. the input 1.9 µm light pulse energy. Inset: spatial profile of the frequency-doubled beam. (b) Optical spectrum of the frequency-doubled pulses centered at 954 nm. (c) The AC trace of the frequency-doubled pulses with a pulse duration of 390 fs along with a Gaussian fit.

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To conclude, we reported a 1.9 µm thulium-doped chirped-pulse amplification fiber laser system capable of generating up to 394 nJ compressed pulses with a repetition rate of 9.26 MHz and a pulse duration of 490 fs based on a linearly polarized dissipative soliton seed source. For an efficient amplification at the desired wavelength, unique designs for the amplifiers were combined with spectral filtering. After frequency-doubling in a PPLN crystal, the system can deliver 138 nJ pulses with a pulse duration of 390 fs centered at a wavelength of 954 nm with a close-to-diffraction-limited spatial beam quality. For a different oscillator input seed at longer wavelengths, higher gain from the same Tm-doped fibers can lead to an even higher pulse energy. Currently, the system performance is determined by the maximum available pump power for the LMA amplification stage (and not any excessive nonlinearity accumulation), so that further power scaling is possible.

Overall, the presented source marks a compact fiber laser system for high-energy pulses at 1.9 µm that can be of interest for a wide range of applications, including material processing and biomedical surgery, e.g., in urology. The high pulse energy of the frequency-doubled output combined with the megahertz repetition rate makes our laser a strong candidate for two-photon microscopy and any multiplexed imaging systems.