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Abstract
So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
A wide range of applications in biology, medicine and physics strongly benefit from ultrashort pulses. To date, Ti:sapphire based laser systems have usually been the working horse for those applications. Passive mode-locking techniques have been applied to this gain material since the early nineties and have allowed for the generation of few-cycles pulses routinely [1–3]. However, Ti:sapphire suffers from two limitations. First, while cost-effective diode pumping starts to be developed [4,5], Ti:sapphire laser oscillators are still typically optically pumped at 532 nm using frequency-doubled diode-pumped solid-state lasers operating at 1064 nm, which increases the cost and the complexity of the systems. Second, their relatively high quantum defect limits the optical-to-optical (opt.-to-opt.) efficiency and the average output power due to thermal issues. An alternative are Yb-doped gain materials. Thanks to their excellent thermal and spectral properties [6], combined with the availability of multi-watt pump diodes for those gain materials allowing for cost efficient and more compact systems, they have experienced tremendous successes for more than a decade, allowing to develop laser systems with unprecedented performances (see e.g. [7–9]). Among them, Yb:CaGdAlO4 (Yb:CALGO) is one of the most promising candidates for delivering ultrashort sub-30-fs pulses at high average output power directly from an ultrafast bulk laser oscillator.
Yb:CALGO is a disordered uniaxial crystal providing a broad emission bandwidth [10–12] combined with comparably good thermal properties [10,13]. Since the first demonstration of a mode-locked laser oscillator based on this gain material in 2006 [11], there have been numerous research developments dedicated to improve its performance, both in terms of average output power and pulse duration. In the standard collinearly-pumped configuration with pulse durations below 100 fs, the highest average output power so far was 12.5 W in 94-fs pulses [14]. Very recently, pulses as short as 17.8 fs have been obtained [15]. This is an outstanding result which further confirms the suitability of Yb:CALGO to deliver few-cycle pulses directly from the output of a bulk laser oscillators based on Yb-doped gain materials (Yb-based laser oscillators). However, the average output power was only 26 mW with an opt.-to-opt. efficiency of 3.3% and the center wavelength of 1118 nm was outside of the favorable gain region centered at about 1050 nm. Additionally, in a standard collinearly-pumped configuration, similar results have been recently obtained using Yb:CaYAlO4, allowing for 10.4 W in 98-fs pulses [16] and 17-fs pulse duration [17]. However, also here the 17-fs pulses were limited to 18 mW of average output power with an opt.-to-opt. efficiency of only 0.5%.
Typical dichroic mirrors used in collinear pumping [Fig. 1(a), top] are required to have a high transmission for the pump wavelength and a high reflectivity for the laser wavelength. Since the pump and the laser wavelengths are close to each other for gain materials with a low quantum defect, this sets a boundary on the possible spectral expansion towards shorter wavelengths. Additionally, the sharp change in reflectivity between the pump and the laser wavelength also leads to a strongly varying group delay dispersion (GDD) as shown in Fig. 1(b) which further limits the expansion of the optical spectrum towards short wavelengths. This effect was experimentally reported in [18], where the authors demonstrated that such a dichroic mirror forces a red shift of the optical spectrum towards the spectral region where the intra-oscillator negative GDD is more flat. Furthermore, to reach higher power levels, higher output coupling is required, resulting in a stronger population inversion. For common broadband Yb-doped gain materials, this usually shifts the gain maximum towards shorter wavelengths until reaching the spectral edge of the dichroic mirror. This typical gain shift behavior is linked to the quasi-three-level system and the inversion-dependent shape of the gain cross section (see e.g. [12,18,19]). Both reasons set a severe limitation to efficiently exploit the available gain bandwidth for the generation of even shorter pulses and hinder further power scaling of Yb-based bulk laser oscillators in the sub-30-fs regime. Figure 1(c) and Fig. 1(d) show an overview of the opt.-to-opt. efficiency and the average output power versus the pulse duration of bulk laser oscillators based on Yb-doped and Ti:sapphire gain materials. Those plots clearly show that for Yb-based bulk laser oscillators, both parameters significantly drop for pulse durations below 40 fs, while this decrease is not as significant for laser oscillators based on Ti:sapphire for which the opt.-to-opt. efficiency still remains above 10%. For Yb-based bulk laser oscillators, this effect ultimately leads to a trade-off between the pulse duration and the average output power.
Fig. 1. (a) Schematic of collinear pumping using a standard dichroic mirror (top) and cross-polarization pumping (bottom). (b) Reflectivity (solid lines) and group delay dispersion (dashed lines) of a standard dichroic mirror (red), of the novel pump mirror presented here for a s-polarized laser light (blue) and for a p-polarized pump light (green). The typical wavelength for optical pumping is shown with the purple line. (c) Optical-to-optical efficiency and (d) average output power in function of pulse duration for several bulk laser oscillators based on various Yb-doped gain materials and Ti:sapphire [1,3,11,16–19,24–38]. s-pol.: s-polarization; p-pol.: p-polarization; AoI: angle of incidence; HR: highly reflective; HT: highly transmissive; OC: output coupler.
So far, there have been a few alternatives attempting to mitigate the spectral limitation of the dichroic mirror. The first one is to combine the effect of self-phase modulation (SPM) with stimulated Raman scattering in the gain medium. This effect allows to partially Stokes-shift the optical spectrum and to generate additional spectral components further in the infrared. In a Raman-assisted Kerr-lens mode-locked (KLM) laser oscillator, pulses as short as 22 fs have been generated but at the expense of a low average output power of 3 mW and an opt.-to-opt. efficiency of only 0.35% [20]. Furthermore, the widely modulated optical spectrum leads to pulses with low temporal contrast. A second alternative is to use a drilled mirror to direct the pump beam towards the gain crystal [21]. The pump mirror is placed such that the laser beam passes through the hole and therefore the laser cavity and the pump optics do not share any optical component besides the gain medium. A third alternative also decoupling the optical path of the pump beam from the laser beam is the thin-disk geometry. As the pump delivery and the laser cavity share no optics except of the disk, this concept allows for the optimization of the intra-cavity components for shorter pulses independently of the pump wavelength [22]. Very recently, this allowed for the generation of 27-fs pulses at 3.3 W average power from a Kerr-lens mode-locked Yb:YAG thin-disk laser oscillators [23].
In this paper, we report on a novel pump mirror concept relying on cross-polarization pumping [Fig. 1(a), bottom], allowing to bypass the spectral limitation of collinear pumping using standard dichroic mirrors. The developed pump mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond [Fig. 1(b)]. In contrast to a standard thin-film polarizer, it provides a low and flat GDD over a broad spectral range for the laser s-polarized light [Fig. 1(b)]. Implementing this mirror in a soft-aperture KLM bulk laser oscillator based on Yb:CALGO, we demonstrate an order of magnitude higher opt.-to-opt. efficiency and the highest average output power of any Yb-based laser oscillators operating in the sub-30-fs pulse duration regime.
2. Experimental setup
The laser oscillator cavity is shown in Fig. 2. A 3-mm-long a-cut anti-reflection coated Yb(3 at.%):CALGO crystal is placed between two concave mirrors (CM1 and CM2) with a 100-mm radius of curvature. The novel pump mirror is placed between the crystal and CM2. The crystal is pumped at a wavelength of 976 nm by a commercial diffraction-limited fiber laser from Azur Light Systems. The p-polarized pump is focused into the gain material through the novel pump mirror under an angle of incidence (AoI) of 60° with a transmission higher than 99%. The Yb:CALGO crystal is optically pumped along its c-axis in p-polarization for maximum pump absorption and the laser operates along its a-axis in s-polarization for highest gain cross section [12]. The pump beam diameter at the crystal position is ∼64 µm. One cavity arm (400-mm long) ends with a dispersive mirror (DM) in a first configuration optimized for the shortest pulse duration (config. 1). In a second configuration optimized for highest average power (config. 2) a highly reflective mirror (HR) replaces the DM. Finally, the latter is exchanged for a standard dichroic mirror in a third configuration to study its effect on the laser performance (config. 3). The other cavity arm (600-mm long) ends with a broadband output coupler (OC). To enforce laser operation in s-polarization, a 1-mm thick fused silica window is placed at Brewster angle with respect to its incidence plane which is perpendicular to the plane of incidence of all the other laser components [Fig. 2]. The SPM in the crystal and the positive GDD from both the gain material and the fused silica window are balanced by four −100 fs2 DMs (3× double-pass and 1× single-pass per cavity roundtrip) in config. 1 and by two −100 fs2 DM and two −150 fs2 DM (one of each in both cavity arms) in config. 2 and config. 3. Except for the anti-reflection coatings on the crystal facets, the optical coatings of every cavity component have been designed and manufactured in our ion-beam-sputtering facility. Soft-aperture Kerr-lens mode-locking is initiated by slightly pushing one cavity end mirror and the pump power is adjusted in each configuration for the maximum average output power, slightly below the appearance of parasitic continuous-wave (cw) lasing oscillations. Two −100 fs2 DMs are placed in the beam line before characterization to compensate for a positive chirp attributed to the transmission through the OC substrate and the collimating lens in the detection beam line.
Fig. 2. Experimental setup of the soft-aperture Kerr-lens mode-locked Yb:CALGO laser oscillator implementing our novel cross-polarization pumping approach. Config. 1 enabled the generation of 22-fs pulses at 729 mW of average output power. Config. 2 enabled the highest optical-to-optical efficiency and the highest average output power. Config. 3 was used to demonstrate the limiting effect of a standard dichroic mirror. OC: broadband output coupler; CM: curved mirror; DM: dispersive mirror; HR: highly reflective mirror.
For all configurations, the opt.-to-opt. efficiency was calculated using the pump power measured just before the lens focusing the pump beam into the gain material. To determine the unabsorbed pump power in mode-locked operation we placed an additional pump mirror under an AoI of 60° in the output beam line after the OC. The measured transmitted pump power is then corrected by the OC transmission (TOC) and the transmission through the fused silica plate for the pump wavelength and polarization (TBP ≈ (1–0.127)2) was also considered. For the opt.-to-opt. efficiency with respect to the absorbed pump power, we estimated two extreme scenarios in which either all of the pump which was not absorbed during the first pass and reflected by the OC and transmitted by the fused silica plate is absorbed in the gain material or none of it.
3. Experimental results for the shortest pulse duration
In config. 1 optimized for the shortest pulse duration and using a TOC = 11.5%, the laser oscillator delivers an average output power of 729 mW in 22-fs pulses and a peak power of 218 kW at 134 MHz repetition rate with an opt.-to-opt. efficiency of 25%. At the center wavelength of 1041.9 nm, this pulse duration corresponds to 6.4 optical cycles. The optical spectrum has a full width at half maximum (FWHM) bandwidth of 82 nm and extends even beyond the pump wavelength [Fig. 3(a)]. A least-square sech2 fit shown in comparison agrees well with the expected spectral shape for soliton pulses. A SHG-FROG measurement confirms the generation of almost ideal transform-limited soliton pulses [Fig. 4] and agrees well with an additional intensity autocorrelation measurement [Fig. 3(b)]. The radio-frequency spectra [Fig. 3(c)] shows stable mode-locking operation. Single-pulse operation was confirmed by a 60-ps autocorrelator scan and by observing the pulse train on a 40-GHz sampling oscilloscope with an 18.5-ps-rise-time photodetector [Fig. 3(d)]. Finally, the output beam is of high spatial quality with a measured beam quality factor of M2 ≤ 1.1.
Fig. 3. Characterization of the soft-aperture Kerr-lens mode-locked Yb:CALGO laser oscillator in config. 1 delivering 22-fs pulses at 729 mW of average output power. (a) Measured optical spectrum of the laser (blue) and the pump (purple). The normalized Yb:CALGO gain cross section for an inversion level of β = 0.15 is shown for reference (left y-axis). Estimated total group delay dispersion per cavity round trip (right y-axis). (b) Intensity autocorrelation trace. (c) Radio-frequency (RF) spectrum of the laser fundamental repetition rate measured with a 10-Hz resolution bandwidth (RBW). Inset: RF spectrum of the higher repetition rate harmonics measured with 100-Hz RBW. (d) 1.2-ns and 9-ns (inset) sampling oscilloscope trace.
Fig. 4. SHG-FROG characterization of the soft-aperture Kerr-lens mode-locked Yb:CALGO laser oscillator in config. 1 delivering 22-fs pulses at 729 mW of average output power. (a) Experimental and (b) Retrieved SHG-FROG trace. (c) Retrieved intensity profile (blue curve) and temporal phase (light blue curve) of the output pulses. (d) Retrieved (blue curve) and experimental (grey curve) spectra and spectral phase (light blue curve). We attribute the slight discrepancy in wavelength between the measured and retrieved spectra to the reconstruction error in the FROG algorithm combined with the limited accuracy of the FROG spectrometer.
4. Experimental results for the highest average output power
In order to optimize both the average output power and the pulse duration, we studied the laser performance in mode-locked operation for various TOC ranging from 2.4% to 23.0%. In comparison to the shortest pulse configuration, this study was performed with a slightly increased intra-oscillator negative GDD estimated to be ≈ −320 fs2 and using a HR mirror as one end mirror [Fig. 2, config. 2]. For each setup, the laser was operated with a pump power just below the threshold for the appearance of parasitic cw lasing oscillations. The obtained results are summarized in Table 1 and shown in Fig. 1(c), Fig. 1(d), and Fig. 5. For each setup, a beam quality factor of M2 ≤ 1.1 was measured.
Table 1. Laser performance in mode-locked operation in the three configurations obtained for various output coupler transmissions.a
By optimizing TOC, we achieved watt-level average output power with opt.-to-opt. efficiencies above 25% and pulse durations of around 30 fs. The highest opt.-to-opt. efficiency of 36.6% with an average output power of 1.63 W in 31-fs pulses was obtained for TOC = 16.4%. The highest average output power of 1.93 W in 33-fs pulses with an opt.-to-opt. efficiency of 29.9%, as well as the highest peak power of 385 kW were obtained for TOC = 23.0%. In comparison to previous Yb-based bulk laser oscillators operating at a similar pulse duration [15,17,18,20], the opt.-to-opt. efficiency and the average output power of the here presented results are more than one order of magnitude higher as depicted in Fig. 1(c) and Fig. 1(d).
From Table 1, Fig. 5(a) and Fig. 5(b), one can observe a blue shift of the optical spectrum when TOC is increased while at the same time the spectral bandwidth decreases. Here, when TOC is increased by almost an order of magnitude from 2.4% to 23.0%, the FWHM spectral bandwidth decreases by 13% from 49.3 nm to 42.9 nm. At the same time, the center wavelength shifts by 12.6 nm from 1049.9 nm to 1036.7 nm. While similar trends have been observed in a previous study using a standard dichroic mirror [18], here it appears less pronounced thanks to our novel pumping scheme. However, to unambiguously demonstrate the benefit of our novel pumping scheme, it is important to study the effect of a standard dichroic mirror in the same laser oscillator.
5. Limiting impact of a standard dichroic mirror
To demonstrate more clearly the limiting effect of a standard dichroic mirror, we modified our laser cavity by replacing the (HR) cavity end mirror with a standard dichroic mirror keeping the same cavity geometry and mirror positions [Fig. 2, config. 3]. Then we repeated the previous study with the same TOC. For each setup, the laser was again operated with a pump power just below the threshold for the appearance of parasitic cw lasing oscillations and the beam quality factor remained very good with a measured M2 ≤ 1.1. Overall, we can observe in this second study the same trends as in the previous one but more clearly pronounced [Table 1 and Fig. 5]. With a dichroic mirror in the cavity, the achievable FWHM spectral bandwidth is narrower and decreases when TOC is increased. With TOC = 2.4% the FWHM spectral bandwidth is 6% narrower when a dichroic mirror is inserted. Then, when TOC is increased from 2.4% to 23.0%, the FWHM spectral bandwidth decreases by 27% from 46.5 nm to 34.0 nm, twice as much as the previous configuration without dichroic mirror [Fig. 5(a)]. This inherently leads to an increase in the pulse duration [Fig. 5(c)]. Additionally, on average, the center wavelength is shifted by about 8 nm towards longer wavelengths [Fig. 5(b)].
Fig. 5. Evolution of the spectral bandwidth (a), center wavelength (b), pulse duration (c) and average output power of the pulses delivered by the laser oscillator with (config. 3, red) and without (config. 2, blue) dichroic mirror. (d) Optical spectra with TOC = 2.4% (e) and TOC = 23.0% (f) in the absence (config. 2, blue) or presence (config. 3, red) of a standard dichroic mirror. δλ shows the difference between the central wavelengths of both configurations.
A similar discrepancy is observed with the average output power. While with TOC = 2.4% the average output power is only 1% lower when the dichroic mirror is inserted, this difference increases with TOC and the average output power significantly drops by 21% for TOC = 23.0% [Fig. 5(d)]. In the configuration enabling the highest average power, again reached for TOC = 23.0%, the dichroic mirror limits the pulse duration to 40 fs while, 33 fs was achieved without dichroic mirror in config. 2 [Fig. 5(c)].
These observations can be explained by the shift of the gain cross section towards shorter wavelengths with higher inversion level [12], as it is the case with higher TOC. This leads to a shift of the optical spectrum towards the shorter wavelengths [Fig. 5(b)]. However, the lower reflectivity and the rising GDD of the dichroic mirror around 1 µm limit this spectral shift and can additionally give rise to a pedestal on the short wavelength side of the optical spectrum [Fig. 5(e) and Fig. 5(f)]. Eventually, these effects lead to a decrease in the maximum achievable average output power with ultrashort pulses for higher TOC because the optical spectrum cannot follow the maximum of the gain cross section which shifts towards shorter wavelengths with increased inversion level [Fig. 5(d)]. Ultimately the dichroic mirror can be clearly identified as the limiting cavity component for achieving higher average output powers and opt.-to-opt. efficiencies with shorter pulse durations. Our study demonstrates unambiguously the strong benefit of our cross-polarization pumping approach compared to using a standard dichroic mirror to generate ultrashort pulses with high opt.-to-opt. efficiencies and high average output power from Yb-based bulk laser oscillators at a center wavelength between 1030 nm and 1050 nm, corresponding to the bandwidth where the gain cross section is the highest.
6. Conclusion
In conclusion, we have presented a simple and effective way to by-pass previous limitations in the achievable spectral bandwidth of Yb-based bulk laser oscillators. By using a novel pump mirror design based on cross-polarization pumping which allows laser operation where the lasing wavelength overlaps the pump wavelength, we were able to mitigate the previous limitations on the possible spectral expansion towards shorter wavelengths. We have implemented this mirror in a soft-aperture KLM Yb:CALGO laser oscillator and obtained record high opt.-to-opt. efficiencies for sub-50-fs laser oscillators. We believe that this technique can be implemented for any gain material where the quantum defect is small. Furthermore, our pump source operated only at a wavelength of 976 nm. Yb:CALGO presents a ≈1.7 times higher maximal absorption cross section at a wavelength of 980 nm. Therefore, optical pumping at this wavelength should allow for a further increased opt.-to-opt. efficiency of the system, as indicated by the significant amount of unabsorbed pump power in some configurations. Finally, while the results presented here have been obtained using a high brightness fiber laser as pump source, both soft (see e.g. [16,32,37]) and hard (see e.g. [36,39,40]) aperture KLM have been reported using multimode pump diodes with lower brightness and we believe that using those will allow for cost efficient multi-watt few-cycle laser oscillators.
Funding
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200020_179146/1, 200021_200774, R'Equip 206021_198176, SPARK CRSK-2_190593); Institut Universitaire de France.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are available in Ref. [41].
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