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Abstract
We report on the feasibility of MXene and platinum diselenide (PtSe2) as novel saturable absorbers for the development of wavelength stabilized passively mode-locked mid-infrared fibre laser systems. After evaluating the performance of individual absorbers in a test cavity, we demonstrate a linear all-fibre laser cavity that utilizes a high reflective chirped fibre Bragg grating for wavelength selective feedback. The observed mode-locked pulse train from this Er3+:ZBLAN fibre laser has a 37 MHz repetition rate with an average power of 603 mW and a spectral width of 721 pm. Our results show that MXene and PtSe2 are promising nonlinear materials for all-integrated ultrafast fibre laser cavities for the important mid-infrared spectral region.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lasers emitting at mid-infrared (mid-IR) wavelengths in the 2.5-5 $\mathrm{\mu}$m range have emerged as ideal tools for various important applications such as laser surgery in human tissue, spectroscopy, defence and micro-machining [1,2]. Rare-earth doped optical fibres are known to be an excellent platform for the generation of laser emission with high output power and excellent beam quality, i.e. high brightness. However, when moving beyond a wavelength of about 2.5 $\mathrm{\mu}$m, commonly used silica fibres become virtually opaque due to the high phonon energy (1100 cm$^{-1}$) of the host glass which makes those fibres incompatible with the development of mid-IR fibre lasers and alternative host glasses are required [2]. The most promising host materials are zirconium fluoride based glasses with the most stable composition being ZBLAN (ZrF$_{4}$-BaF$_{2}$-LaF$_{3}$-AlF$_{3}$-NaF) with a phonon energy of 565 cm$^{-1}$ [2,3]. Mid-IR ZBLAN fibre laser technology for light emission in the 3 $\mathrm{\mu}$m range has seen enormous progress during the last decade [2,4]. Amongst other areas, those lasers have demonstrated great potential for medical applications such as retinal surgery, dental tissue ablation, and cardiovascular surgery, due to the high absorption coefficient of water in body tissues at this particular wavelength [5–7].
A variety of continuous wave (CW) lasers at various wavelengths in the 2.7-4 $\mathrm{\mu}$m range have been demonstrated using ZBLAN fibres doped with different laser-active ions such as Er$^{3+}$, Ho$^{3+}$ and Dy$^{3+}$ [8–12]. More recently, great efforts are underway to increase the output power of CW fibre lasers in this wavelength regime [13]. However, for specific applications such as mid-IR nonlinear wavelength conversion and medical surgery, pulsed fibre lasers that are capable of producing high-peak power/energy pulses are needed. In addition, it has been shown that shorter pulses can significantly reduce collateral damage during ablation [4]. In order to generate ultrashort laser pulses from a laser resonator, a fast saturable absorber needs to be inserted into the laser cavity, a technique called passive mode-locking. One of the most crucial aspects for passive mode-locking is the choice of the material (or technique in the case of an artificial saturable absorber like Kerr-lens mode-locking) that is used to periodically modulate the laser intracavity losses. Compared to active mode-locking, passive mode-locking allows the generation of much shorter pulses due to the ultrafast recovery times of a passive saturable absorber (SA) which introduces a strong intensity-dependent absorption in the laser cavity. While promising SA materials are extensively accessible in the near-IR region, the optimum choice of nonlinear SA materials for the mid-IR region remains an open research question. To date, various mode-locking mechanisms have been demonstrated in the mid-IR regime that are based on artificial SAs such as nonlinear polarization rotation (NPR) [14,15] and nonlinear amplifying loop mirror (NALM) [16]. However, these demonstrations require free-space bulk optical elements in the laser cavity which demand precise alignment and thus make the systems inherently more complex and lossy. The most commonly used saturable absorber materials for mode-locked lasers are based on semiconductor quantum wells (semiconductor saturable absorber mirrors, SESAMs). However, there are only a few SESAMs that can be used in the mid-IR and these not only feature relatively high non-saturable losses, but also relatively long recovery times which favours Q-switched mode-locked over pure continuous wave (CW) mode-locked operation. In contrast to the near-IR, where a large number of studies has been published in the past and where a large number of different saturable absorber materials for fibre laser systems have been suggested, only a few narrow-bandgap saturable absorber materials such as e.g. black phosphorous [17] and crystalline Cd$_{3}$As$_{2}$ [18], or more recently also gold nanowires [19] have been used to demonstrate mode-locked operation in mid-IR fibre lasers and more research is therefore urgently needed. Since no ideal SA material for the mid-IR region has been identified to date, in this paper, we have investigated the suitability of MXene and platinum diselenide (PtSe$_{2}$), two novel two dimensional (2D) as mid-IR SA materials. A few layer PtSe$_{2}$ sample used in this work was successfully developed by chemical vapor deposition (CVD) method [20] onto a CaF$_{2}$ substrate. Similarly, MXene was ink-jet printed onto a CaF$_{2}$ glass as illustrated in Fig. 1 [21]. Ink-jet printing is emerging as a promising technique which has good substrate compatibility, scalability and low cost [21].
The newly developed 2D materials PtSe$_{2}$ and MXene have displayed remarkable optical properties and are relatively stable under ambient conditions [20–25]. The narrow and tuneable band gap of PtSe$_{2}$, allows this material to be used in nonlinear optics and ultrafast photonics [26]. As the number of layers increases, the bulk-structured PtSe$_{2}$ becomes a semi-metal with zero band gap [26–28]. The carrier mobility of PtSe$_{2}$ is comparable to black phosphorus and superior to most transition metal dichalcogenides (TMD) materials [29] and this property plays a critical role in generating narrow pulses. MXene on the other hand is composed of early transition metal carbides and/or carbonitrides [20,30]. Their general formula is M$_{n + 1}$X$_n$T$_x$ where n=1,2 and 3, M represents an early transition metal (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, or Mo), X is carbon and/or nitrogen, and T$_x$ stands for the surface terminations (hydroxyl, oxygen, or fluorine) [20]. Compared with the typical 2D materials such as graphene and molybdenum sulfide, MXenes have various advantages like good hydrophilicity, metallic conductivity, high damage thresholds and adjustable chemical composition [20,24,31]. In our studies, we have used the particular MXene Ti$_3$C$_2$T$_x$ which exhibits excellent thermal conductivity, high elastic moduli, high electric capacity, tunable bandgap and tailorable optical properties [25]. In particular, this material has been found to exhibit broadband saturable absorption, with increased transmittance at higher light fluences [32], makes it a promising candidate to operate in the longer wavelength regime.
In this paper, we experimentally demonstrate that the novel 2D SA nanomaterials PtSe$_{2}$ and MXene can offer improved performance over a commercially available SESAM for the development of a stable, passively mode-locked laser in the mid-IR regime using an Er$^{3+}$ doped fluoride fibre laser cavity at an emission wavelength around 3 µm.
2. Passively mode-locked mid-IR test laser cavity
As an initial step towards the goal to realise an all-fibre passively mode-locked stable Fabry-Perot laser cavity, we have developed a test laser cavity that enabled us to evaluate the performance of various SA materials and to directly compare different samples. Detailed measurements of the nonlinear optical properties of our PtSe$_{2}$ and MXene absorbers can be found in [20] and [25], respectively. The test laser linear cavity was formed by a 4 m long Er$^{3+}$ doped double-clad ZBLAN fluoride fibre as the gain medium as shown in Fig. 2. This cavity was pumped by a commercial silica fibre-coupled 980 nm laser diode with a maximum output power of 10 W. Due to the low tensile strength of silica to ZBLAN fibre splice joints, we have used a robust method to connectorize these different fibers, similar to what was very recently demonstrated by Zou et al. [33]. In contrast to butt-coupling that requires careful alignment with precision translation stages, the use of fixed connectors provides a robust and stable solution to the problem of pump/laser fiber coupling. During the connectorization process, the fibres were first secured in a commercial ferrule connector by using epoxy. Later, the end facet of the fibres were finely polished by using various polishing pads and a motor-driven fibre polishing machine. During this process different forces were applied to both ZBLAN and silica fibres due to the difference in material composition. This connectorization (refer Fig. 2) resulted in an improved pump coupling efficiency as compared to free-space coupling and we could obtain a repeatable connector loss of less than 0.3 dB.
To obtain wavelength-stabilized passive mode-locked operation, a mechanically robust Type-I chirped fibre Bragg grating (CFBG) with a reflectivity of 93% and a 3 dB bandwidth of 3 nm (see Fig. 3) was inscribed into the input end of the active fibre through the polymer coating using direct femtosecond laser inscription [34,35]. At the output end, the fibre was angle cleaved at an angle of 8$^{\circ }$ to avoid parasitic lasing, followed by a pump filter to separate the signal from the unabsorbed pump. The resonator was completed by a free-space section containing a confocal setup formed by two CaF$_{2}$ lenses of 12.5 mm focal lengths followed by a plane output coupler (OC) of 50% reflectivity. We have studied the test cavity with and without pump filter and confirm that the residual pump power does not influence the mode-locking stability and does not accelerate the saturable absorber damage at the high pump power.
Firstly, a commercial SESAM (BATOP GmbH) which operates in transmission (i.e. without the back reflecting mirror) was placed at the focal point of the confocal setup. As a result, the laser operated in Q-switched mode-locked regime for an incident pump power in the range between 2 W and 9 W as shown in Fig. 4. No CW mode-locking was observed at the available pump power. The SESAM was used in the test laser cavity as a bench-mark for our mode-locked laser measurements since it is a relatively well established technology.
As a next step, we have replaced the SESAM with PtSe$_{2}$ samples various physical CVD layers to study the mode-locking performance of the laser. PtSe$_{2}$ SA samples with 2, 4, 6, 8, and 10 physical CVD layers (not atomic layers) were tested inside the confocal setup between two CaF$_{2}$ lenses (L$_{1}$ and L$_{2}$, refer Fig. 2) of the laser cavity. Later, the output of the laser cavity was directed to a fast detector (VIGO FIP-1k-1G-F amplifier with a PVI-4TE-6 HgCdTe (MCT) photovoltaic detector) in combination with a fast oscilloscope to measure the temporal pulse train. We have observed that as the number of layers of PtSe$_{2}$ deposition increases, the laser is now entering a continuous wave (CW) mode-locked regime (except for 2 layers) above 2.5 W threshold pump power. There was also a reduction in pulse-to-pulse fluctuations from 21.8% to 10.9% with an increase in CVD layers as shown in Fig. 5. The increase in the number of physical layers increases the effective loss modulation to a point where the laser can enter a CW mode-locked regime with more or less strong suppression of Q-switching instabilities. As mentioned in [36], the Q-switched mode-locking threshold is proportional to the square root of the product of saturation fluence and modulation depth. From our experiment results it can be concluded that while an increase in the number of layers leads to an increased modulation depth, the corresponding reduction in saturation fluence overcompensates this change, and thicker layers thus provide a more robust protection against Q-switching instabilities. Similarly, Fig. 6 shows that the output power decreases as the number of physical layers increases at a constant pump power of 9W. This is due to the fact that as the number of layers increases, the effective cavity modulation becomes stronger, but also the non-saturable losses are increased as shown by the reduction in power [37].
Fig. 5. Pulse-to-pulse fluctuations in the temporal pulse trains with (a) 8 layers and (b) 10 layers of PtSe$_2$.
Fig. 6. Pule-to-pulse fluctuation and variation in output power with various physical layers of PtSe$_2$.
Similarly, for MXene, various physical ink-jet printing layers (4, 6, 8, 10 and 12) were deposited onto a CaF$_{2}$ substrate and we have observed stable CW mode-locking behaviour with all samples with 4-12 physical layers when the pump power exceeds 2.5 W under otherwise identical conditions. This stable mode-locked train of pulses at 30 MHz repetition rate was sustained up to 9 W of pump power as shown in Fig. 7. Figure 8 illustrates the measured pulse-to-pulse fluctuation with respect to different layers of MXene at constant pump power of 9 W. The pulse-to-pulse fluctuation decreases from 31% to 4.5% with the number of physical layers increases from 4 to 12. When compared to PtSe$_{2}$, MXene samples gave less pulse-to-pulse fluctuations as illustrated in Fig. 8.
Fig. 7. Mode-locked pulse train with 30 MHz repetition rate using MXene SA material (a) 6 layers (b) 12 layers.
Fig. 8. Pule-to-pulse fluctuation and variation in output power with various physical layers of MXene.
3. True all-fibre mid-IR laser cavity
From the test laser cavity, it was concluded that the MXene absorber with 12 physical layers gave stable operation with below 5% pulse-to-pulse fluctuations. While MXene is cost-effective to fabricate, the tests also showed an improved laser performance in terms of passive CW mode-locking when compared to a mature technology like a SESAM. Therefore, we have deposited 12 physical layers of MXene directly onto a 50% OC via ink-jet printing to realize a true all-fibre mid-IR laser cavity. While the input arrangement of the laser remained the same as for the test laser cavity, the output end of the fluoride fibre was now perpendicularly cleaved and directly butt coupled to the output coupler (therefore minimising Fresnel reflection) with SA deposited as shown in Fig. 9. Based on this approach, we have obtained stable CW mode-locked operation at a pump power threshold of 2.5 W and the pulse train remained stable (well below 5% pulse-to-pulse fluctuations) over several hours without showing any signs of Q-switching or Q-switched mode-locking regimes. The pulse repetition rate was observed as 37 MHz due to the decrease in the cavity length. The output of the laser was directed to various detection systems such as an oscilloscope to measure the temporal pulse train, an optical spectrum analyser, and an RF spectrum analyser.
For an incident pump power of 9 W, the average output power of the CW mode-locked laser was 603 mW with a slope efficiency of 7% and a CW mode-locking threshold of 2.5 W. The RF spectral trace of the pulsed laser output shows the fundamental repetition rate of 37 MHz and its harmonics up to 2.75 GHz (refer Fig. 10). The signal to noise ratio obtained is greater than 65 dB which confirms the laser’s stable operation in a fundamental mode-locked regime. The laser (peak wavelength was 2796 nm) output spectrum was first characterized without the SA and the measured full width half maximum (FWHM) of the optical spectrum was 464 pm. However, after incorporating MXene SA into the linear laser cavity we observed a spectral broadening to 721 pm as shown in Fig. 11. This corresponds to a Fourier transform-limited pulse duration of 11.4 ps. However, due to the unavailability of a suitable autocorrelator we were unable to measure the actual pulse width.
4. Conclusions
In conclusion, to the best of our knowledge, this is the first demonstration of a stable, true all-fibre passively mode-locked mid-IR laser using the novel 2D saturable absorber materials PtSe$_{2}$ and MXene. The observed mode-locked pulse train from this Er$^{3+}$ doped fluoride fibre laser cavity has a 37 MHz repetition rate with an average power of 603 mW and a spectral broadening to 721 pm. These results show the feasibility for developing advanced all-fibre mid-IR laser sources using the novel two-dimensional nanomaterials such as MXene and PtSe$_{2}$ as promising candidates.
Funding
Air Force Office of Scientific Research (FA2386-19-1-4049).
Acknowledgements
This work was performed in-part at the OptoFab node of the Australian National Fabrication Facility, utilizing NCRIS and NSW state government funding.
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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