Broadly tunable femtosecond pulses around 2.06 μm from a diode-pumped Tm-doped solid-state laser source

We report on a broadly tunable diode-pumped femtosecond Tm:LuScO3 laser source around 2.06 μm. Tuning was obtained through the use of a steeply diving birefringent filter, maintaining sub-600 fs pulses over a tuning range of 2019–2110 nm. The minimum pulse duration of 240 fs was recorded at a central wavelength of 2080 nm with an average output power of 93 mW. Higher output coupling of 2% resulted in a narrower tuning range of 2070–2102 nm with generated pulses as short as 435 fs and an average output power of 119 mW at 2090 nm. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.


Introduction
Laser sources capable of generating high peak power ultrashort pulses in the ~2-2.1 µm spectral region are important for the development of new techniques and technologies in many fields from the mid-infrared photonics sector.Such sources can be used to efficiently access the deeper mid-infrared region through optical parametric frequency conversion techniques [1,2] or supercontinuum generation [3], enabling benefits to developments and applications in the areas of coherent X-ray sources [4], minimally invasive surgery [5], materials processing [6,7], and sensing [8].The current laser amplifier systems required to achieve these high pulse energies tend to utilize Ho 3+ -doped gain media such as Ho:YAG and Ho:YLF.The performance of these gain media have been widely reported in various amplifier configurations.However, the existing ultrashort pulse laser sources used to seed these amplifiers are rather complex.For example, the seed lasers used in [9][10][11][12][13] involve multiple nonlinear energy conversion schemes before a pulse covering a suitable spectral range is generated (Ho:YLF, 2050/2060 nm; Ho:YAG, 2090 nm).An alternative approach has been taken with the development of ultrashort pulse Ho:fibre lasers [14,15] which have been used to seed Ho:YLF amplifiers in single- [16] and multi-pass [17] configurations.Whilst such sources allow the seed optical spectrum to directly match the gain peak of Ho:YLF near 2050 nm, they require multiple pump stages with a Tm:fibre laser being involved.Commercially available Tm:fiber ultrashort pulse seed systems have also been coming to the market in recent years (Menlo Systems, InnoLas Photonics, and AdValue Photonics).While such fiber sources can offer a stable and turn-key operation, they struggle to operate efficiently at >2 μm wavelengths without some additional spectral shifting process and amplification stages to reach >100mW power level.These result in their high complexity and price.
A more efficient way to reduce the complexity and cost of these seed sources would be to develop an ultrashort pulse system that could benefit from direct diode-pumping whilst also being able to target the gain spectra of existing Ho:YAG and Ho:YLF amplifiers in the range of 2050-2100 nm.Recently it has been shown that Tm 3+ -doped sesquioxide RE 2 O 3 (RE = Lu, Sc, Y, or any Lu a Sc b Y c composition, where a + b + c = 1) gain media could support efficient laser operation at >2 μm wavelength under direct diode pumping [18].In contrast to other Tm 3+ -doped gain media, they possess ultrabroad and smooth gain features reaching the 2.1 μm spectral region avoiding strong water vapor absorption bands in the ~1800-2000 nm spectral range, which would prevent stable mode-locked operation.These features make Tm 3+ -doped sesquioxides highly attractive for the development of compact and efficient femtosecond sources in the 2-2.1 µm spectral window, utilizing various mode-locking techniques and laser diode pump sources.
In particular, Tm:Lu 2 O 3 has been reported in crystalline and ceramic forms to have produced sub-200 fs pulses near 2070 nm, employing a single-walled carbon nanotube saturable absorber [19] and an InGaAsSb quantum-well-based semiconductor saturable absorber mirror (SESAM) [20].Employing the Tm:LuScO 3 crystalline gain media, we have previously demonstrated minimum pulse durations of 105 fs at 2010 nm [21].More recently, the ceramic form of the mixed sesquioxide was reported to produce pulses as short as 63 fs at 2057 nm [22].All of these systems were pumped using Ti:sapphire lasers operating around 800 nm.With the aim of making these systems more practical and less costly, laser diode pumping should be employed.However, the development of diode-pumped ultrashort pulse Tm 3+ -doped lasers is not a straight forward process.Poor pump beam quality can lead to lower efficiencies, high thermal loads, Q-switching instabilities, and weaker self-phase modulation thus requiring precise cavity and saturable absorber engineering for stable modelocking.In previous work [23], we reported on a diode-pumped Tm:Lu 2 O 3 ceramic laser that generated pulses as short as 242 fs with an average output power of 500 mW at a central wavelength of 2068 nm and, more recently, a diode-pumped crystalline Tm:LuScO 3 laser capable of producing near-transform-limited 170 fs pulses at 2093 nm with an output power of 113 mW has been demonstrated [24].
Here we report on further development of diode-pumped ultrafast Tm 3+ -doped sesquioxide lasers.In particular, broadly tunable femtosecond pulses spanning a >90 nm range around 2.06 µm from a diode-pumped, mode-locked Tm:LuScO 3 laser are demonstrated.With a 1% output coupler in use, a tuning range of 2019-2110 nm was achieved with an average output power as high as 96 mW and a corresponding pulse duration of 245 fs at 2090 nm.Under higher output coupling conditions of 2%, a maximum average output power of 119 mW with a corresponding pulse duration of 435 fs was achieved.Tuning of the femtosecond pulses was realized in a compact configuration through the use of a steeply diving birefringent filter (SD-BRF).To the best of our knowledge, this is the first use of such technique for the tuning of ultrashort pulses in the 2-2.1 µm region and paves the way for the development of compact, efficient, and versatile seed sources for the existing Ho 3+doped laser amplifiers.

Steeply diving birefringent filter design
In the design of conventional BRFs the birefringent material is cut so that the optic axis runs parallel to the surface of the plate (θ = 90° in Fig. 1).Tuning of the laser emission wavelength can be achieved by rotating the plate around the surface normal, changing the angle of rotation (α).Assuming the plate is inserted into the cavity at Brewster's angle (β e ≈57° for quartz at 2.1 µm), then the wavelength-dependent polarization changes due to the material's birefringence induce Fresnel reflection losses at the plates surfaces for the s-polarized component of the beam.This causes a wavelength dependent transmission loss as the plate is rotated.The thickness of the plate (t) defines the free spectral range (FSR) and the transmission p bandwidth de In the cas optic axis div larger FSR a conventional way as a conv with different When such S ability is gain bandwidth an Cr:LiSAF las delay dispersi lasers with im schemes.

Diode-pu
Initial charac continuous w [24] was con later point, th second waist Additionally, HR-coated Gi a total roundcurvature fold plane-plane, 4 crystal (LC).the gain medi −741 fs 2 , resp factor of 18 a mm focal len µm × 22 µm.possible overl >99.9% in th having simila With the q incident pump from 1986 nm SD-BRF orde supporting the that the tunin 2.06 µm at the    nm tuning range with average output powers of between 36 mW and 63 mW.This increase in pulse duration is presumed to be the result of lower output power.However, the narrower transmission bandwidth provided by the 2nd order of the SD-BRF could be an additional factor which limited optical bandwidth and duration of the generated pulses.As with the 1st order, the mode-locking stability was confirmed by examining widespan autocorrelation traces and RF spectra.
Increasing the output coupling to 2% resulted in higher average output powers but with a narrower tuning range and longer pulse durations.Operating in the 1st order, a tuning range of 2070-2102 nm was recorded, with output power and pulse durations varying from 78 fs to 119 mW and 435-670 fs, respectively [Fig.5(d)].It can be seen that the pulse durations and output power followed a similar profile to that seen with the 1% output coupler, with the maximum output power and minimum pulse duration found around 2090 nm.Figures 5(e) and 5(f) show the optical spectrum and intensity autocorrelation traces for a pulse recorded at 2090 nm, respectively.A pulse duration of 435 fs with an associated optical bandwidth of 11 nm were measured, giving a time-bandwidth product of 0.33.Moving to the 2nd order with the 2% output coupler gave a similar tuning range of 2072-2108 nm, with output powers varying between 74 mW and 103 mW and near-transform-limited pulses ranging from a maximum pulse duration of 811 fs to a minimum pulse duration of 563 fs.As with the 1% output coupler, clean RF spectra and widespan autocorrelation traces confirmed stable, single pulse mode-locked operation throughout the tuning range at a pulse repetition frequency of 114.3 MHz.
Only picosecond pulse durations were realized when operating in the 3rd order of the SD-BRF and using the 1% output coupler.A narrower tuning range than that recorded from the 1st or 2nd orders was observed with relatively unstable operation.No mode-locked operation was achieved when operating in the 4th order of the SD-BRF.These results can be associated with the relatively narrow optical bandwidths of the SD-BRF at higher orders that could be preventing pulse spectral broadening and stable soliton mode-locking.
It can be seen that the SD-BRF can be used as a means to not only to set the emission wavelength but also coarsely control the pulse duration of mode-locked pulses.Indeed, pulse durations of 245 fs, 363 fs, and 1 ps have been experimentally demonstrated around 2090-2095 nm for the 1st, 2nd, and 3rd orders, respectively.It was quite clear that the pulse durations increase and the corresponding optical bandwidths decrease with the higher orders.However, it is difficult to separate the SD-BRF narrowing effect from the effects of lower intracavity pulse energy when explaining the reduced bandwidth recorded with higher orders of the filter.It is believed that stable operation can be realized with higher orders but that the laser cavity parameters such as beam waists and dispersion would have to be additionally tailored for such mode-locking regimes.

Summary and outlook
We have demonstrated femtosecond pulses tunable over >90 nm around 2.06 µm from a diode-pumped, mode-locked Tm:LuScO 3 laser.Using a 1% output coupler and operating in the 1st order of the SD-BRF, tuning between 2019 nm and 2110 nm was achieved with the pulse durations varying from 240 fs to 538 fs and average output powers from 41 mW to 96 mW.Moving to the 2nd order gave a similar tuning range but with longer pulse durations of 349-685 fs.A 2% output coupler realized higher output powers over a narrower tuning range with minimum pulse durations of 435 fs and 563 fs recorded in the 1st and 2nd orders of the SD-BRF, respectively.To the best of our knowledge this is the first demonstration of a SD-BRF being used in the 2-2.1 µm region for broadband tuning of femtosecond pulses.In addition, the utilization of a SD-BRF to achieve broad femtosecond pulse tuning allows for a less complex and more robust cavity design compared with the traditionally used slit and prism pair combination.
Based on the demonstrated performance, it can be concluded that the mode-locked Tm:LuScO 3 laser could be used as a seed source for existing Ho 3+ -doped amplifiers in the 2050-2090 nm range.In particular, the >100 mW average output power level and ~570 fs pulses (Δλ ≈8.5 nm) achieved with the 2% output coupler and the 2nd order of the BRF would be particularly suitable for the narrow gain bands of Ho:YAG at 2090 nm.Additionally, when using the 1% output coupler, the gain peaks of Ho:YLF around 2050/2060 nm can be reached as well with an average mode-locked power of >50 mW.There is also potential for the Tm:LuScO 3 ultrafast laser source to be used as a seed for further amplification in a Tm 3+ -doped sesquioxide gain medium, in particular Tm:Lu 2 O 3 which benefits from excellent thermo-mechanical properties and broadband gain in the 2060-2080 nm range.
Fig. 1 angle indica to sur The theor providing opt Demirbas pro Naganuma et near 24° angl axis angle of performance chosen to giv contrast for a Figure 2(a be seen that th α of 28.2°, ha 4th shown in percentage of intended wav transmission b 28 nm, 14 nm shows tuning Fig. 2 angles transm Fig. 3 2.06 mirror in [24

Fig. 4 .
Fig. 4. CW tuning Fig. 5 of the spectr (f) sho order Rotating t of tuning ran However, com