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Abstract
We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.
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1. Introduction
Laser sources emitting around 2.3 µm find applications in gas sensing and pollutant detection [1,2], combustion studies [3] and non-invasive glucose blood measurements [4] and they are also noteworthy for pumping of mid-infrared optical parametric oscillators [5]. This spectral range can be addressed by several types of laser sources. Chromium ions (Cr2+) in zinc chalcogenides (ZnS, ZnSe) offer extremely broad emission bands leading to broadband wavelength tuning and generation of femtosecond pulses [6,7]. However, the synthesis of these gain media of high optical quality is challenging and they suffer from moderate laser-induced damage thresholds and the need of special pump sources (Er-fiber lasers or semiconductor diode lasers [8]). Another possibility constitutes Vertical-Cavity Surface-Emitting Lasers (VCSELs) employing semiconductor heterostructures (GaInAs on InP or GaInAsSb on GaSb) however with very limited output powers [9,10]. As shown recently, optically pumped Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) feature better power scaling capabilities and enable femtosecond pulse generation at high repetition rates [11]. Another solution is the use of bulk and fiber thulium (Tm3+) lasers operating on the 3H4 → 3H5 electronic transition, Fig. 1 [12,13]. Although this laser scheme has been known for a long time, there has been growing interest in such sources over the past decade due to the availability of both laser gain media (Tm3+-doped fluoride crystals and glasses) and pump sources (AlGaAs laser diodes emitting at ∼0.8 µm).
The power scaling capabilities and the proof-of-principle of highly efficient laser operation of continuous-wave (CW) Tm-lasers operating on the 3H4 → 3H5 transition were recently demonstrated. Guillemot et al. reported on a Tm:KY3F10 laser delivering 0.84 W at 2.34 µm with a slope efficiency of 47.7% (under pumping by a Ti:Sapphire laser) [14]. Loiko et al. has shown that the slope efficiency for 2.3-µm Tm-lasers can exceed the Stokes limit owing to the positive effect of energy-transfer upconversion (ETU) from the terminal laser level, 3F4 + 3F4 → 3H4 + 3H6, refilling the upper laser manifold and boosting the pump quantum efficiency up to 2 (a two-for-one pump process) [15,16]. In their early study, Pinto et al. achieved continuous wavelength tuning of a Tm:LiYF4 laser across a broad range of 2.20–2.46 µm [17]. Recently, Yu et al. has presented a diode-pumped Tm:GdVO4 laser scaled to the multi-watt output power level (>6 W at 2.29 µm) representing a record-high output from this type of sources reported to date [18].
Fig. 1. (a) A partial energy-level scheme of Tm3+ ions in LiYF4 [23] illustrating the upconversion pumping scheme for the 3H4 → 3H5 laser transition: GSA / ESA – ground / excited state absorption, CR – cross-relaxation, ETU – energy-transfer upconversion, green arrow – laser transitions, NR – multiphonon non-radiative relaxation; (b) stimulated-emission cross-sections, σSE, for the 3H4 → 3H5 Tm3+ transition in LiYF4 for π and σ light polarizations, arrow indicates the laser wavelength.
The first mode-locked 2.3-µm Tm-laser was reported in 2017 by Soulard et al. [19] using a commercial SEmiconductor Saturable Absorber Mirror (SESAM) comprising the InxGa1−xAs quantum well (QW) technology and a Tm:LiYF4 laser crystal: 94 ps-long pulses were generated at 2306 nm with an average output power of 165 mW at a repetition rate of ∼100 MHz. In the study by Kowalczyk et al. [20], the negative etalon effect of the SESAM substrate in the inverted geometry was identified to modulate the laser spectrum and limit the pulse duration to ∼100 ps. Furthermore, in 2017, Canbaz et al. presented a femtosecond Kerr-lens mode-locked Tm:LiYF4 laser that generated 514-fs pulses at 2303 nm, albeit with a very low average output power of 14.4 mW at 41.5 MHz [21]. Subsequently, the same group of authors further developed Tm:LiYF4 and Tm:KY3F10 lasers passively mode-locked by graphene saturable absorbers, achieving slightly longer pulses but still at low average output powers [22]. Note that all these oscillators were pumped by Ti:Sapphire lasers at ∼0.78 µm (“direct” pumping to the upper laser manifold, 3H4). This naturally limited further development of such sources and their power scalability. Although spatially multimode fiber-coupled AlGaAs laser diodes are available, their use in pumping mode-locked laser cavities is limited. This limitation is due to low brightness, expected poor mode-matching efficiency, severe thermal problems, and low associated laser efficiency. Table 1 provides an overview of mode-locked 2.3-µm Tm-lasers reported to date. In the same table, for comparison, we also provide several milestone results on mode-locked lasers employing Cr2+-doped zinc chalcogenides (ZnS and ZnSe).
Table 1. Output Characteristicsa of Mode-Locked Thulium Lasers at ∼2.3 µm Reported So Far. Selected Results on Mode-Locked Cr2+-Ion Lasers are Given for Comparison
Recently, an original upconversion (UC) pumping scheme for Tm3+ ions was proposed with a goal of replacing Ti:Sapphire lasers as pump sources. It is based on a weak non-resonant ground-state absorption (GSA) 3H6 → 3H5 followed by a non-radiative step down to the 3F4 metastable state, as well as a resonant 3F4 → 3F2,3 excited-state absorption (ESA) [23], Fig. 1. This scheme is driven by a photon avalanche mechanism [24] involving a cross-relaxation process among adjacent Tm3+ ions, 3H4 + 3H6 → 3F4 + 3F4, recirculating the populations between the ground and metastable Tm3+ manifolds. The ESA transition of interest can be addressed by Yb-fiber lasers (YFLs) emitting slightly above 1 µm. The YFL technology combines the inherent advantages of fiber lasers, such as small footprint, flexible design, and high-power operation enabled by distributed heat management with good beam quality (high-brightness sources). Additionally, the characteristics of ytterbium (Yb3+) ions are beneficial, including a simple energy-level scheme leading to high laser efficiencies and weak heat loading. Furthermore, the large Stark splitting of the ground state in ytterbium ions enables broadband wavelength tunability around 1 µm. This tunability is crucial for addressing the very intense but narrow ESA peaks of Tm3+ ions within this spectral range.
Tyazhev et al. fully exploited this approach to demonstrate the first watt-level Tm-fiber laser at 2.3 µm with upconversion pumping enabled by a 1049-nm YFL. In the CW regime, the Tm:ZBLAN fiber laser delivered 1.24 W at 2.27 µm with a slope efficiency of 37% [25]. UC pumping has been employed in various bulk Tm-lasers using both fluoride and oxide crystals [26–28]. However, the output powers remained limited, raising questions about the viability of this approach for these types of laser sources. Dupont et al. proposed a modification of this pumping scheme via dual-wavelength pumping to initiate pump absorption from the metastable 3F4 Tm3+ manifold, but this resulted in only an incremental power improvement [29]. To date, no mode-locked Tm-lasers with UC pumping have been reported.
SESAM mode-locking offers several advantages. SESAMs are widely known for enabling reliable self-starting mode-locked lasers and for their use in robust laser cavities that operate effectively in the middle of their stability zones [30]. They can support high average output powers. Compared to physical saturable absorbers like nanomaterials (e.g., graphene), SESAMs have several benefits: they are commercially available, have a robust and solid design, offer better uniformity, and present a lower risk of degradation under environmental conditions and during laser operation. Furthermore, they allow for more precise design of their nonlinear optical properties, albeit within somewhat narrower operation ranges. In particular, SESAMs based on InxGa1−xAs quantum wells (QWs) have revolutionized the field of ultrafast oscillators in the near-infrared by solving the Q-switching instability problem [31]. However, their applications in the short-wave infrared spectral range remain limited.
In this spectral range, gallium antimonide (GaSb) offers a promising alternative [32]. The high refractive index difference of GaSb and AlAsSb enables fabrication of highly reflective distributed Bragg reflectors (DBRs) with wide stop bands. The bandgap in InGaAsSb QWs can be tailored to address laser wavelengths ranging from 1.9 µm to beyond 3 µm. The InGaAsSb Auger recombination rate increases significantly above 2 µm, making the SESAM recovery dynamics extremely fast with sub-ps recovery times [33]. In this case, no defect engineering is required to reduce the recovery time as in the case of InxGa1−xAs QWs. Recently, GaSb-based SESAM has been actively studied regarding their ultrafast nonlinear properties and femtosecond mode-locked laser performance in the spectral ranges around 2 µm [34,35] and 2.4 µm [6,8,36]. Barh et al. reported on a GaSb-based SESAM mode-locked Cr2+:ZnS laser generating 79 fs pulses at 2.37 µm with a watt-level average output power at a high repetition rate of ∼250 MHz [6]. Single-cavity dual-modelocked Cr2+:ZnS laser employing a GaSb-based SESAM has been reported also recently [36].
In the present work, we report on the first passively mode-locked thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping. A GaSb-based SESAM was used to initiate and stabilize the mode-locked operation.
2. GaSb-based SESAM
The saturable absorber was a GaSb-based SESAM (ETH Zürich) with a single ternary strained InGaSb quantum well (QW, 16 nm, 33% In) embedded in GaSb, Fig. 2. It thus had a type-I configuration with a potential well for holes and electrons existing in same material layer leading to high Auger recombination rate. The SESAM structure contained a highly reflective distributed Bragg reflector (DBR) consisting of 24 lattice-matched high- and low-index layer pairs (AlAs0.08Sb0.92/GaSb) on a GaSb substrate and an absorber region of single In0.33Ga0.67Sb QW embedded in GaSb on top of the DBR structure grown by molecular beam epitaxy (MBE) [32]. The high reflectivity with small non-saturable losses in this SESAM results in a standing-wave electric-field intensity pattern with well-defined nodes and antinodes as shown in Fig. 2.
Fig. 2. Design of the single-quantum-well GaSb-based SESAM: refractive index profile (light blue – GaSb, dark blue – AlAs0.08Sb0.92, pink – In0.33Ga0.67Sb (QW), red curve –electric field intensity normalized to 4 in air plotted over the thickness of the structure, where z is the distance from the GaSb substrate perpendicular to the QW layers.
The measured low intensity spectral reflectance of the SESAM exhibited a broad stopband exceeding 200 nm centered around 2.3 µm. To accurately characterize the SESAM’s nonlinear optical performance, which features low-loss and ultrafast response, the full optical characterization setup described in [37] was employed at the central wavelength of 2.35 µm. The nonlinear reflectivity curve measured using 100 fs-long pulses is shown in Fig. 3(a). This curve was fitted using a rate equation model for a two-level system, assuming a Gaussian beam profile.
Fig. 3. Characterization of the GaSb-based SESAM: (a) a nonlinear reflectance measurement, symbols – experimental data, curve – their fit (black solid) using Eq. (1), and (blue dashed) without rollover effects, i.e., for F2 = infinity, λ = 2350 nm; (b) a pump-probe trace, symbols – experimental data, curve – their biexponential fit using Eq. (2), λ = 2350 nm.
The pump-probe trace is shown in Fig. 3(b), obtained using a pulse duration of 100 fs and a pump fluence of 17.6 µJ/cm2. The ultrafast relaxation dynamics of the SESAM were characterized using a bi-exponential decay function:
where ΔR is the change of reflectivity, t is time, A is the weighting constant, and τ1 and τ2 are the characteristic recovery times. The best fit using Eq. (2) gave a fast recovery time τ1 of 0.2 ps with a high weighting constant A of >0.9 and a slow time constant τ2 of 1.6 ps, which are sufficiently short for stabilizing femtosecond soliton mode-locking [38] in the present work.3. SESAM mode-locked thulium laser
3.1 Laser set-up
As a gain medium, we employed a 6 at.% Tm:LiYF4 single-crystal grown by the Czochralski (Cz) method under Ar + CF4 atmosphere. This relatively high Tm3+ doping level was selected to ensure sufficient pump absorption under upconversion pumping. A Brewster-cut laser element (thickness: 6.0 mm, aperture: 4.0 × 4.0 mm2) was oriented for light propagation along the a crystallographic axis (a-cut) for π-polarization. This polarization state gives access to higher ESA cross-sections for the 3F4 → 3F2,3 pump transition, as well as higher gain for the 3H4 → 3H5 laser transition. The laser element was mounted in a Cu-holder cooled by circulating water (18 °C).
The home-made pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions (the UC pumping scheme) consisted of a wavelength-tunable Yb-fiber laser, a preamplifier stage and a polarization maintaining large-mode area fiber amplifier featuring a heavily Yb3+-doped fiber with a core diameter of 20 µm (Coherent PLMA-YDF-10/130-VII). The master-oscillator power-amplifier (MOPA) was broadly tunable from 1030 to 1064 nm and delivered up to 8.0 W at 1043 nm with a nearly diffraction limited beam quality (M2 ≈ 1.1) and a linear polarization (polarization extinction ratio, PER > 16 dB). High brightness pumping is essential for the development of mode-locked lasers as it enables good overlap between the pump and laser modes (being particularly relevant for long laser crystals), and tight focusing of the pump beam. For the direct pumping scheme at 0.8 µm, only Ti:Sapphire lasers can serve for this aim, as commercially available fiber-coupled spatially multimode AlGaAs laser diodes feature low brightness output. For UC pumping at 1 µm, Yb-fiber lasers benefit from (i) high brightness, (ii) power-scalable output and (iii) optional linear polarization of laser emission. UC pumping drains the population of the metastable Tm3+ state preventing unwanted colasing at ∼1.9 µm which is detrimental for stable mode-locking.
A Z-shaped astigmatically compensated standing-wave cavity was used, Fig. 4. The laser element was placed between two highly reflective (HR) curved folding mirrors (radius of curvature: RoC = -150 mm). The calculated size (diameter) of the laser mode inside the crystal was 63 × 202 µm2 (here and below, we specify the mode size for the cavity with an additional intracavity Tm:LiYF4 crystal) in the sagittal and tangential planes, respectively. The pump radiation (pumping into π-polarization) was focused into the crystal through a flat pump mirror (PM) coated for high transmission (HT) at 1.04 µm and HR at 2.15-2.6 µm using an antireflection (AR) coated plano-convex lens (focal length: f = 60 mm). The PM was placed between the crystal and one of the HR curved folding mirrors.
One cavity arm contained a dichroic flat folding mirror and a curved HR mirror (RoC = -300 mm) creating a secondary beam waist on the SESAM. The diameter of the laser mode on the SESAM was calculated to be 124 × 246 µm2 in the sagittal and tangential planes, respectively. The plane dichroic mirror was used to prevent heating the SESAM with the residual non-absorbed pump. Thus, pumping was in single-pass.
The other cavity arm was terminated by a plane-wedged output coupler (OC) with a transmission TOC = 0.5% or 1.5% at the laser wavelength. The HR and dichroic mirrors were coated for HR at 2.2-2.5 µm, HT at 1.04 µm and HT at 1.9 µm (to avoid oscillations on the competitive 3F4 → 3H6 Tm3+ transition).
Two approaches for the intracavity dispersion management were applied. First, we used a pair of uncoated CaF2 prisms placed at Brewster’s angle in the cavity arm terminated by the OC. The total cavity length was about 1.3 m (0.5% OC) and 1.6 m (1.5% OC). Then, the prisms were removed and an additional passive 6 mm-long Tm:LiYF4 crystal (Brewster plate, BP) was placed at Brewster’s angle. It was a-cut and oriented for light polarization π. The group velocity dispersion (GVD) of LiYF4 is -62.69 fs2/mm at 2.3 µm for e-wave [40]. The total geometrical length of the cavity with the BP was about 1.4 m. Tm3+ ions do not absorb the laser radiation at 2.3 µm and thus the doping of the available BP does not affect the laser performance.
The laser emission spectra were measured using an optical spectrum analyzer (Yokogawa, AQ6375). The pulse duration was measured using a second harmonic generation (SHG) based autocorrelator (FR-103XL, FEMTOCHROME Research). The radio frequency (RF) spectra were measured using a photodetector (UPD-5N-IR2-P, ALPHALAS, bandwidth: > 0.3 GHz) and an RF spectrum analyzer (R&S FSV7, Rohde & Schwarz).
3.2 Continuous-wave operation
In the continuous-wave (CW) regime (when replacing the SESAM with a plane HR mirror), the laser delivered an output power of 616 mW at 2312-2314 nm with a slope efficiency η of 10.0% (vs. the incident pump power) and a laser threshold of 1.10 W (for TOC = 1.5%), see Fig. 5(a). For smaller TOC of 0.5%, the laser generated 433 mW at 2316 nm with a lower slope efficiency of 7.0% and lower threshold of 0.96 W. The input-output dependences were linear; no signs of detrimental thermal effects were observed. No thermal fracture of the crystal was observed up to at least the maximum applied pump power. Further power scaling was limited by the available pump power.
Fig. 5. CW Tm:LiYF4 laser operating on the 3H4 → 3H5 transition with upconversion pumping: (a) input-output dependences, η – slope efficiency; (b,c) typical spectra of laser emission: (b) 0.5% OC, (c) 1.5% OC. The laser polarization is π.
The typical laser emission spectra are shown in Fig. 5(b,c). The spectra showed a slight dependence on the pump level and the emission occurred around 2.31 µm (the 3H4 → 3H5 Tm3+ transition). No unwanted colasing at 1.9 µm was observed.
The developed CW Tm:LiYF4 laser pumped by an Yb-fiber laser represents the record-high power for any upconversion-pumped 2.3 µm Tm laser, cf. Table 2, indicating the high potential of such high-brightness pump sources.
Table 2. Performancea of 2.3 µm Thulium Lasers with Upconversion Pumping Reported So Far
3.3 Passively mode-locked operation
First, the cavity design employing a prism pair for the intracavity dispersion management was studied as it provided more flexibility in varying the group delay dispersion (GDD). For 1.5% OC, upon increasing the incident pump power, the laser passed through CW, passively Q-switched ML (at Pinc > 2.2W) and CW ML (at Pinc > 2.8 W) operation regimes, Fig. 6(a). For this output coupling, the prism separation was optimized to be 345 mm. The round-trip negative GDD introduced by the CaF2 prisms was -2200 fs2. The SESAM ML Tm-laser delivered a maximum average output power of 450 mW at an incident pump power of 6.44 W, corresponding to an optical efficiency of 7.0%. Assuming a sech2-shaped spectral profile, an emission bandwidth (full width at half maximum, FWHM) of 3.1 nm was obtained at a central wavelength of 2306.5 nm, see Fig. 6(c). The recorded SHG based intensity autocorrelation trace was well fitted with a sech2-shaped temporal intensity profile, yielding an estimated pulse duration of 1.93 ps (FWHM), Fig. 6(e). The corresponding time-bandwidth product (TBP) amounted to 0.337 which was slightly above the Fourier-transform-limit (0.315). The laser operated at a pulse repetition rate of 95.1 MHz.
Fig. 6. Tm:LiYF4 laser operating on the 3H4 → 3H5 transition passively mode-locked by a GaSb-based SESAM (dispersion management by a prism pair): (a) power transfer characteristics; (b,c) spectra of the mode-locked pulses and (d,e) the corresponding SHG-based intensity autocorrelation traces for output coupler transmission TOC of (b,d) 0.5% and (c,e) 1.5%. Red and blue dashed curves – sech2 fits. The laser polarization is π.
For smaller TOC of 0.5%, the optimum prism separation was 60 mm. The round-trip negative GDD introduced by the CaF2 prisms was -650 fs2. On increasing the pump power, the transitions to passively Q-switched ML and further to CW ML were observed at 2.9 W and 4.4 W, respectively. At the maximum incident pump power of 7.0 W, the average output power from the SESAM ML Tm:LiYF4 laser dropped to 327 mW with an optical efficiency of 4.7%, Fig. 6(a). The CW ML laser operated at the central wavelength of 2308.4 nm with a spectral bandwidth of 4.8 nm (FWHM, for a sech2-shaped spectral intensity profile), see Fig. 6(b). The weak spectral modulation originated from the detection system. Using smaller output coupling allowed us to further shorten the pulse duration: the SHG based intensity autocorrelation measurement yielded a pulse duration of 1.19 ps assuming a sech2-shaped temporal intensity profile, Fig. 6(d). The corresponding TBP of 0.321 was slightly above the Fourier-transform-limit. The measurements of the radio frequency spectra revealed a fundamental beat node at 114.48 MHz with a high extinction ratio of >70 dB above the noise and uniform harmonics over a 1 GHz wide frequency span.
For the laser cavity configuration employing a prism pair for the dispersion management, for both studied OCs, mode-locking was self-starting above the CW ML threshold. Note that the distances between the cavity elements and the total cavity length were slightly different when using 0.5% and 1.5% OCs, and the cavity alignment targeted attaining high mode-locking stability at the expense of somewhat reduced average output power.
The excellent stability of the self-starting mode-locked operation together with the shortest pulse duration were observed when using the BP for the intracavity dispersion management. Its insertion resulted in a total round-trip negative intracavity GDD of -1438 fs2 at 2310 nm, Fig. 9. When using TOC = 0.5%, on increasing the pump power, the transitions to passively Q-switched ML and further to CW ML operation regimes were observed at the incident pump powers of 2.95 W and 3.96 W, respectively, i.e., the CW ML threshold was reduced as compared to the cavity employing the prism pair. At the incident pump powers above 7.0 W, the CW ML operation required a slight additional cavity adjustment to achieve self-starting behavior. Therefore, the CW ML laser for this cavity design was optimized at a pump power of 6.2 W yielding an average output power of 208 mW and an optical efficiency of 3.4%. The CW ML Tm-laser featured a substantially broader emission spectrum (FWHM: 6.51 nm) at a central wavelength of 2309.4 nm, which enabled us to obtain femtosecond pulses, Fig. 7(a). The background-free SHG based intensity autocorrelation trace was well fitted with the sech2-shaped profile giving a pulse duration of 870 fs, Fig. 7(b). The corresponding TBP was calculated to be 0.318 just above the Fourier-transform limit. The pulse energy directly generated out of the CW ML Tm:LiYF4 laser amounted to 1.98 nJ. No damage of the SESAM was observed under ML operation.
Fig. 7. Characterization of the shortest pulses from the Tm:LiYF4 laser operating on the 3H4 → 3H5 transition passively mode-locked by a GaSb-based SESAM (dispersion management by an intracavity BP): (a) spectrum of the shortest mode-locked pulses, grey solid curve – σSE spectrum for Tm3+ ions in LiYF4 (for π-polarized light); (b) the corresponding SHG-based intensity autocorrelation trace, TOC = 0.5%; (c) typical oscilloscope traces of laser emission with different time spans. Red and blue dashed curves – sech2 fits. The laser polarization is π.
Typical oscilloscope traces of laser emission from the CW ML laser are shown in Fig. 7(c) ruling out the beating effects. The RF spectra were measured to confirm the stability of the CW ML operation, Fig. 8. The recorded fundamental beat note at 105.08 MHz corresponding to fundamental ML exhibited a high extinction ratio of >80 dB above the noise. The uniform harmonics on a 1-GHz frequency span revealed high stability of the single-pulse mode-locked laser operation without any Q-switching instabilities, see Fig. 8(b). The CW ML operation of the Tm:LiYF4 laser was stable for hours without any degradation of the average output power or the pulse characteristics.
Fig. 8. RF spectra of the SESAM mode-locked Tm:LiYF4 laser operating on the 3H4 → 3H5 transition (dispersion management by an intracavity BP): (a) fundamental beat note recorded with a resolution bandwidth (RBW) of 10 Hz; (b) harmonics on a 1 GHz span, RBW = 100 Hz. TOC = 0.5%.
Fig. 9. Dispersion analysis for the SESAM mode-locked Tm:LiYF4 laser operating on the 3H4 → 3H5 transition (dispersion management by an intracavity BP): (a) the estimated round-trip GDD of the cavity elements and the measured reflectance curves of the SESAM (blue solid) and the OC (green solid); (b) a close look on the total round-trip GDD: (a,b) the spectral dependence of GDD: the SESAM (black dotted), the Tm:LiYF4 gain crystal, BP (black dashed), the total roundtrip cavity GDD (black solid). The red solid is the GDD needed to satisfy the soliton condition. The yellow rectangle marks the range of the mode-locked laser spectrum.
To confirm the proper choice of the dispersion management strategy used in this work, the GDD of the cavity elements and the ideal GDD for the soliton condition were calculated as shown in Fig. 9 for the shortest pulse duration (and lowest TBP) cavity configuration with the intracavity BP. The SESAM is designed to show a simulated GDD profile in the range of 0 − 200 fs2 over a 60-nm bandwidth near 2310 nm. The total cavity roundtrip GDD accounting for the contributions of the SESAM, the laser crystal and the BP is calculated to be -1438 fs2 at the center wavelength of the ML laser spectrum. This GDD with considering nonlinear phase shift from the self-phase modulation in the laser gain medium corresponds to an ideal soliton pulse duration limit of 834 fs (FWHM), as calculated using the following equations [21]:
The stimulated-emission cross-section, σSE, spectrum for Tm3+ ions in LiYF4 (for light polarization π) is shown in Fig. 7(a). The σSE value at the central laser wavelength is 0.47 × 10−20 cm2 and the gain bandwidth (FWHM) is as broad as 25.8 nm indicating a room for further pulse shortening from mode-locked Tm:LiYF4 lasers operating around 2.3 µm, e.g., via Kerr-lens mode-locking.
4. Conclusion
To conclude, we report a significant breakthrough in the development of ultrafast Thulium lasers operating on the 3H4 → 3H5 transition around 2.3 µm. This advancement is attributed to two key innovative approaches: (i) the upconversion pumping of bulk, heavily Tm3+-doped crystals using high-brightness, high-power, polarization-maintaining ∼1 µm Yb-fiber lasers and (ii) the use of GaSb-based SESAMs. These SESAMs feature wide stopbands, very low non-saturable losses, and fast recovery times, enabling the generation of femtosecond pulses at 2.3 µm. Specifically, by employing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb, we achieved soliton pulse shaping in an upconversion-pumped Tm:LiYF4 laser. This resulted in the generation of soliton pulses as short as 870 fs at the central wavelength of 2309.4 nm with an average output power of 208 mW and a pulse repetition rate of 105.08 MHz. This setup exhibited self-starting mode-locking behavior with excellent stability. Furthermore, we scaled the output power to 450 mW, though at the expense of a longer pulse duration of 1.93 ps. These results represent the first femtosecond SESAM mode-locked Tm-laser at 2.3 µm, and the highest average output power ever extracted from a mode-locked Tm oscillator operating on the 3H4 → 3H5 transition. This includes comparisons with lasers employing other saturable absorbers or Kerr-lens mode-locking.
Yb-fiber lasers emitting slightly above 1 µm are expected to boost the performance of Tm-lasers at 2.3 µm. This improvement is expected due to their inherent advantages. These include a commercially developed technology based on silica fibers, a small footprint, high output powers with excellent beam quality, and linearly polarized emission from polarization-maintaining fibers. Additionally, their sufficient wavelength tunability allows for precise addressing of the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. This leads to efficient upconversion pumping driven by the photon avalanche mechanism. In the CW regime, the output power achieved in the present work from the Tm:LiYF4 laser (616 mW at ∼2313 nm) sets a new record among all the previously reported upconversion-pumped bulk Tm-lasers at 2.3 µm. This suggests that this approach could be a viable solution for generating watt-level output from such laser sources. This is especially relevant considering the commercial availability of even kW-level single-mode Yb-fiber lasers.
Further pulse shortening is anticipated in mode-locked 2.3-µm Tm-lasers, thanks to the relatively broad gain bandwidth of Tm3+-doped fluoride crystals. These crystals support the generation of sub-250-fs for Tm:LiYF4 and even approximately 100-fs pulses for Tm:KY3F10. Achieving these pulse durations is expected to involve further optimization of the intracavity dispersion, reduction of the cavity losses through the use of specially designed mirrors, optimizing the crystal quality, or employing Kerr-lens mode-locking. Additionally, a reduction in thermal effects is expected by utilizing upconversion pumping at 1.45 µm with Raman fiber lasers, which address the 3F4 → 3H4 excited-state absorption transition of Tm3+ ions.
Funding
H2020 Marie Skłodowska-Curie Actions (101034329); Région Normandie (RIN NovaMAT); Agence Nationale de la Recherche (ANR-10-LABX-09-01, ANR-19-CE08-0028); European Regional Development Fund (Chaire RELANCE).
Acknowledgements
We thank FIRST clean room facility at ETH Zurich.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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