1. Introduction

The eye-safe laser emission region around 2 µm covered by Tm³⁺, Ho³⁺_, and codoped (Tm³⁺-Ho³⁺) active media is important for medical applications, mainly due to the strong optical absorption by water, and remote sensing (LIDAR) of CO₂ and water in the atmosphere [1]. Ultrashort pulse laser sources in the 2 µm spectral region are of particular interest as pump sources for synchronously-pumped optical parametric oscillators operating in the mid-IR above 5 µm, as seeders of chirped-pulse optical parametric amplifiers pumped near 1 µm for high-order harmonic and soft-X-ray generation, for IR supercontinuum or THz generation, and for time-resolved molecular spectroscopy in the molecular fingerprint region [2].

In recent years the direct generation of ultrashort pulses near 2 μm was demonstrated using Tm- and Tm,Ho-doped solid-state bulk or fiber lasers and semiconductor disk lasers (SDL). The latter are limited to low pulse energies due to their intrinsically high repetition rates in the GHz range. The only demonstrated fs-SDL, containing GaSb semiconductor structures, delivered pulses with a duration of 384 fs at 1960 nm [3]. Tm³⁺ and Ho³⁺ both operate as quasi-three-level systems near 2 µm but the advantage of the Tm-laser is the possibility of direct diode pumping with AlGaAs diodes. The first sub-ps 2-µm lasers were based on mode-locked Tm- and Tm,Ho-doped fiber lasers. In 1995, Nelson et al. demonstrated generation of 500-fs pulses [4] using a spectral filter and the nonlinear polarization rotation approach. Shortly afterwards, Sharp et al. achieved 190-fs pulses with a semiconductor saturable absorber mirror (SESAM) mode-locked Tm-doped fiber laser, the shortest pulses to date out of a 2-µm oscillator [5]. Higher pulse energy of 4 nJ at 41 MHz was demonstrated using a Tm-fiber laser operating in the stretched-pulse regime. The pulses with a duration of 1.2 ps could be compressed outside the cavity to 173 fs [6].

Tm- and Tm,Ho-doped bulk gain materials are an alternative choice for the development of high-power ultrashort-pulse lasers in the 2-μm spectral region. Among them, the Tm-doped sesquioxide single crystals stand out because of their high thermal conductivity, even when doped with high Tm³⁺-concentrations. However, in most laser crystals high doping concentrations lead to a strong decrease of the thermal conductivity like, e.g., observed in YAG. This in turn leads to a strong heating of the crystal which is detrimental, in particular for quasi-three level lasers. The strong decrease of the thermal conductivity is not observed in thulium-doped Lu₂O₃, since the masses of the thulium and lutetium ions differ by only 3% and thus the phonon propagation gets only slightly constrained. Furthermore, Tm:Lu₂O₃ exhibits the advantages of broad absorption and emission spectra, high emission cross sections, and low phonon energies [7]. In continuous-wave (CW) regime a diode-pumped Tm:Lu₂O₃ laser at 2065 nm delivering 75-W output power with a slope efficiency of 39% and spectral tuning from 1922 nm to 2134 nm was already demonstrated [8].

The first passively mode-locked Tm-doped bulk laser using single-walled carbon nanotubes as saturable absorber (SWCNT-SA) was demonstrated in 2009. The Tm:KLu(WO₄)₂ laser produced 10-ps pulses at 1944 nm [9]. SWCNT-SAs were also successfully employed to mode-lock Tm-doped fiber lasers [10, 11] whereby the shortest pulse duration amounted to 750 fs [11]. Recently femtosecond lasers around 2 µm containing bulk gain media were realized. A Tm-doped fluorogermanate glass [12] and KY(WO₄)₂ laser [13] yielded pulse durations of 410 fs and 386 fs, respectively. Lasers based on Tm,Ho codoped KY(WO₄)₂ [14] and NaY(WO₄)₂ [15], both operating around 2060 nm, delivered pulses as short as 570 fs and 191 fs, respectively. All reported femtosecond bulk lasers operating at 2 µm were passively mode-locked by SESAMs.

SWCNT-SAs are unique nanostructures which, due to their fast third order optical nonlinearity and saturable absorption effects, can act as a potential replacement for SESAMs. While SESAMs have to be fabricated by very complex epitaxial processes, SWCNT-SAs profit from simple production technology and low manufacture cost. In addition, natural mixture of SWCNTs of different diameters and chiralities provides much broader absorption band than that of SESAMs. SWCNT-SAs have proven their suitability for use as SAs for mode-locking during the last ten years. A number of different active materials, fiber as well as bulk lasers, were mode-locked by SWCNT-SAs in the wavelength range between 800 nm and 2000 nm [16–18].

Here we report picosecond and femtosecond mode-locking of Tm:Lu₂O₃ employing SWCNTs as saturable absorbers. We demonstrate the generation of pulses as short as 175 fs, the shortest to date for any laser oscillator around 2 µm. Furthermore we present results achieved with different Tm³⁺ doping levels.

2. Experimental setup

High-quality Tm-doped Lu₂O₃ single crystals with dopant concentrations of 1 at.%, 1.8 at.%, and 5 at.% were grown by the Heat-Exchanger Method [19]. The uncoated samples had a thickness of 2 mm with an aperture of 3 mm × 3 mm. The SWCNT-SA used in the present work was based on arc-made SWCNTs and can basically also be applied in the 1 µm spectral range. While the absorption band around 1 µm corresponds to the E₂₂-transition of SWCNTs, the E₁₁-transition is utilized near 2 µm. The SWCNT-SAs were fabricated by the spin coating method on a quartz substrate as described in Ref. 18.

The linear transmission of the SWCNT-SA used for mode-locking is shown in Fig. 1 . We have characterized the nonlinear response of similar SWCNT-SAs at 2 µm, resulting in a very fast relaxation time of about 1 ps. The preparation for the measurement of the nonlinear reflectivity near 2 µm is still in progress. We expect a saturation fluence of <10 μJ/cm² and a modulation depth of <1%, similar to the values for the E₂₂-transition of the same SWCNTs [9]. The experimental laser set-up was similar to that described in [17]. A Ti:sapphire laser emitting up to 2 W of output power at 798 nm served as a pump source. The actively cooled Tm:Lu₂O₃ crystal was placed at Brewster angle in between two folding mirrors with radius-of-curvature (ROC) = 10 cm. One resonator arm was additionally folded using two highly reflecting focusing mirrors (ROC = 10 cm and 5 cm). The transmission-type SWCNT-SA was placed at Brewster angle in the vicinity of this second resonator waist to enhance the intensity on the absorber. The other resonator arm contained a plane output coupler and prisms could be introduced to control the dispersion.

 

Fig. 1 Linear transmission spectrum of the arc-discharge SWCNT saturable absorber indicating the E₁₁- and E₂₂-transitions of semiconducting nanotubes.

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3. SWCNT-SA mode-locked Tm:Lu₂O₃ laser

Initially we studied the SWCNT-SA mode-locked operation in the picosecond regime using the 1 at.% Tm³⁺-doped Lu₂O₃ crystal. In this configuration, without the prism pair in the cavity, the laser delivered 31 ps at a repetition rate of 128 MHz. The average output power applying an output coupling of 1.5% amounted to 88 mW at an absorbed pump power of approximately 1 W. The intensity autocorrelation and the corresponding fit, assuming a sech² pulse shape, are shown in Fig. 2 . The optical spectrum had a FWHM below 0.5 nm at a center wavelength of 1965 nm. The radio frequency spectra recorded at a resolution bandwidth (RBW) of 100 kHz and 1 kHz, respectively, do not show any spurious modulation and indicate clean CW mode-locking with the fundamental beat-note being 49 dB above noise level.

 

Fig. 2 Picosecond Tm:Lu₂O₃ laser mode-locked by SWCNT-SA: (a) autocorrelation trace and sech²-fit, (b) first beat note of the RF spectrum; inset: 1 GHz scan.

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In contrast to the study in the picosecond regime at 1965 nm, for the femtosecond regime the emission wavelength had to be shifted to >2000 nm because the water vapor absorption in the 1800-2000 nm region prevented broadband mode-locking. For this purpose we decided to operate the laser at low inversion levels where the gain cross section of Tm:Lu₂O₃ is highest around 2065 nm, cf. Figure 3 in Ref. 8. This approach was not successful when using the 1 at. % doped Tm:Lu₂O₃ crystal, despite applied output coupler transmissions as low as 0.2%. Increasing the Tm-doping level and hence the reabsorption loss we were able to operate the laser in the desired spectral range above 2000 nm.

 

Fig. 3 Femtosecond Tm:Lu₂O₃ laser mode-locked by SWCNT-SA: (a) autocorrelation trace (dots) and fit (line) assuming a sech²-pulse shape, (b) optical spectrum.

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First the 1.8 at.% Tm-doped Lu₂O₃ sample was investigated in the femtosecond regime. The pump absorption increased to 57% in the single pass resulting in higher output powers compared to the previously used 1 at.% Tm-doped crystal. For the femtosecond regime we optimized the cavity design and introduced two CaF₂ prisms resulting in a repetition rate of 90 MHz. Using a 0.2% output coupler stable CW mode-locking was achieved yielding 30 mW of average output power. The optical spectrum was centered at 2061 nm and had a bandwidth of 15.5 nm (FWHM). From the measured autocorrelation trace a pulse duration of 279 fs was derived assuming a sech²-pulse shape.

The best femtosecond laser performance was found for the Lu₂O₃ crystal with the highest available doping concentration of 5 at.% Tm³⁺ and 0.2% of output coupler transmission. The overall cavity dispersion was negative with the main contribution of the prism sequence (−950 fs²) compared to the Tm:Lu₂O₃ crystal (−64 fs²). At an absorbed pump power of 600 mW, which is 250 mW above the lasing threshold, stable and self-starting mode-locking was observed. This low mode-locking threshold is a feature of SWCNT-SA mode-locked lasers and related to the low saturation fluence of the SWCNT-SAs. The intensity autocorrelation trace and the corresponding fit of the shortest pulses are shown in Fig. 3(a). The deconvolved FWHM of the pulse was 175 fs and the corresponding optical spectrum, shown in Fig. 3(b), was centered at 2070 nm and had a bandwidth of 27.4 nm. This results in a time-bandwidth product (TBP) of 0.335, corresponding to nearly transform-limited sech²-pulses. The pulses are shorter than those reported for 2 µm oscillators so far, compared e.g. with 191 fs for a Tm:NaY(WO₄)₂ laser [15] and 190 fs for a Tm:fiber laser [5]. The output power of 36 mW in the mode-locked regime was obtained for a pump level of 1.35 W at 798 nm, of which roughly 82% were absorbed in the crystal. The output power could be slightly increased to 50 mW when using a 0.5% transmission output, but at the expense of longer pulses with a duration of about 300 fs.

The radio-frequency (RF) spectra of the SWCNT-SA mode-locked Tm:Lu₂O₃ laser are shown in Fig. 4 . Measured at a resolution bandwidth of 1 kHz in a ~200 kHz span, the fundamental beat note at 87.83 MHz displays an extinction ratio of 65 dB above carrier (Fig. 4(a)). As further evidence for stable CW single-pulse operation without Q-switching, Fig. 4(b) depicts a 1 GHz wide-span RF measurement.

 

Fig. 4 RF spectrum of the femtosecond Tm:Lu₂O₃ laser mode-locked by SWCNT-SA: (a) first beat note, (b) 1 GHz scan.

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Table 1 summarizes the best output parameters achieved with the three Tm:Lu₂O₃ crystals in the mode-locked regime at repetition rates of 128, 90, and 88 MHz, respectively.

Table 1. Parameters of Different Mode-Locked Tm:Lu₂O₃ Lasers

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4. Conclusions

Mode-locking of Tm-doped sesquioxide crystals of different doping levels using SWCNT-SAs in the picosecond and femtosecond regime was reported. We demonstrated 175 fs pulses from a Tm:Lu₂O₃ laser what we believe to be the shortest pulse duration ever reported for a 2-µm oscillator. This was achieved using a saturable absorber based on single-walled carbon nanotubes. Previous CW laser experiments on Tm:Lu₂O₃ indicated a wide tuning range sufficient to host pulses of sub-50 fs duration [7]. Consequently, further experiments will be directed towards exploitation of this bandwidth potential for which the combination of Tm:Lu₂O₃, SWCNT-SAs and direct diode-pumping appears promising.