The yttrium lithium fluoride (YLF) host is an attractive alternative to the more common yttrium aluminum garnet (YAG) due to its weaker thermal lensing, naturally polarized output, and broader emission bandwidth. However, Yb:YLF still lags behind the impressive progress observed in mode-locking of Yb:YAG [1–3] in terms of demonstrated pulse widths and average/peak power levels. Recent progress achieved in Yb-systems, such as Yb:Lu₂O₃ [4], Yb:CALGO [5,6], also shows the promise of Yb-based oscillators in directly generating high-energy few-cycle pulses from low-cost oscillators. In earlier mode-locking studies with Yb:YLF using semiconductor saturable absorber mirrors (SESAMs) for mode-locking [7,8], Coluccelli et al. generated sub-200-fs pulses centered around 1028 nm with up to 120 mW of average power from a multimode diode (MMD) pumped Yb:YLF laser [9]. Later, by using a fused silica prism pair for dispersion tuning, Pirzio et al. achieved sub-90-fs pulses around 1052 nm with an average power of 35 mW [10]. Cryogenic operation has been used to scale-up average power levels to 28 W, where a SESAM mode-locked Yb:YLF laser produced 3-ps pulses around 1018 nm from a high-power diode module pumped Yb:YLF oscillator [11]. Additionally, tuning of the central wavelength of the pulses in the 1013.5–1019 nm range was demonstrated [11]. Recently, the cryogenic laser system was also mode-locked at 995 nm, where 105-ps pulses with 40-W average power were achieved [12].

In this Letter, we present detailed continuous wave (cw) and cw mode-locked lasing results from a compact low-cost MMD pumped Yb:YLF oscillator. Using passive mode-locking with a SESAM, we have generated 40-fs pulses with an average power of 265 mW around 1050 nm by employing a 1% output coupler (OC). At an increased dispersion setting, fs tuning is achieved with 200 fs and 500 fs level pulses in the 1025–1047 nm and 1019–1047 nm ranges, respectively. Using a 5% OC, the average power is scaled to 1.85 W for 380-fs long pulses centered around 1025 nm.

Figure 1 shows a schematic of the Yb:YLF laser that is used in cw and cw mode-locked laser experiments. The system is pumped by a single-emitter MMD providing up to 10 W of pump power at 960 nm. The diode has an emitter size of 1 × 100 µm (sagittal/fast × tangential/slow axes). The diode output beam is first collimated with a 4.5-mm focal length aspheric lens (f₁), then a cylindrical lens with a focal length of 50 mm (f_z, acting on the fast axis) is used to minimize the diode beams astigmatism. An achromatic doublet with a focal length of 80 mm (f₂) is employed to focus the pump beam to a waist of 25 µm × 50 µm inside the crystal. A 1.5-mm long 25% Yb-doped YLF crystal with 1.5-mm long undoped endcaps diffusion-bonded on both ends is used as the gain medium (total crystal length, ∼4.5 mm; crystal aperture, ∼5.5 × 7 mm²). The crystal is mounted with indium foil in a copper holder under water cooling at 15°C. Both surfaces of the crystal are antireflection coated for both pump and lasing wavelengths, and the crystal is placed under normal incidence. The a-cut crystal absorbs approximately 75% of the TM polarized pump light at 960 nm, and its c-axis is positioned vertical. Due to higher gain in the c-axis, the free running laser output is TE polarized.

 

Fig. 1. Experimental setup of the diode-pumped Yb:YLF laser used in cw and cw mode-locked operation.

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The cw Yb:YLF laser has a standard X-cavity, consisting of two curved high reflectors, each with a radius of curvature (ROC) of 75 mm (M1 and M2), a flat end high reflector (M3), and a flat OC. The laser is built on a 60 cm × 60 cm optical breadboard. The length of each resonator arm is approximately 25 cm, resulting in a beam waist of ∼30 µm inside the crystal. The reflectivity of mirrors M1–M3 covers the 1010–1200 nm spectral region (R > 99.9%) and are antireflection coated in the 800–970 nm range (R < 2%). An adjustable width mechanical slit is inserted near the OC to control the transverse output mode of the laser in the tangential axis. A 4-mm-thick off-surface optical axis crystal quartz birefringent filter (BRF) with a diving angle of 25° is inserted at Brewster’s angle for smooth tuning of the laser wavelength both in the cw and cw mode-locked regimes [13]. Once the BRF is inserted, the laser output becomes TM polarized and to employ the E//c axis for lasing, insertion of a half-wave plate (HWP) is also necessary. Finally, to increase the modulation depth of the BRF, an additional polarizer beam splitter cube is also inserted near the OC in cw tuning experiments (not shown in the figure).

For mode-locking experiments, to achieve soliton pulse shaping, mirrors M4 and M5 with negative group delay dispersion (GDD) are inserted. We adjust the number of bounces on the mirrors to acquire the desired amount of dispersion. For short pulse generation, double chirped mirrors (DCMs) with a GDD of −80 ± 20 fs² (900–1080 nm) are used. For high average power operation, we employed Gires–Tournois interferometer (GTI) mirrors with a dispersion of −1000 ± 200 fs² (1010–1050 nm) and −1300 ± 150 fs² (1015–1045 nm). Mode-locking is initiated and sustained by a SESAM, which is placed at a secondary focus generated by the curved mirror M6. Curved mirrors with a ROC of 100–150 mm and 600 mm are used in long (100–500 fs) and short (sub-100-fs) pulse configurations, respectively. The commercial SESAM (BATOP, SAM-1040-1.5-1ps) has a company specified modulation depth of 0.9%, a non-saturable loss of approximately 0.6%, a reflectivity range of 1000–1080 nm, a relaxation time constant of 1 ps, and a saturation fluence of 50 µJ/cm².

Figure 2 summarizes the cw laser performance of the Yb:YLF laser, where efficiency and tuning data taken with 1%, 5%, and 15% transmitting OCs are shown. The Yb:YLF laser produced 3.03 W of output power at an absorbed pump power of 7.78 W using the 5% OC (incident power, 9.87 W), and reached a slope efficiency close to 50% (similar power levels were obtained in earlier room-temperature studies [14–16]). Increasing the laser crystal holder temperature from 15°C to 40°C resulted in 10% reduction of output power, which showed that thermal effects are present, but they are not at a level to strongly diminish laser performance. This indicated that the obtainable power levels are limited by the available pump power. We believe that the power scaling improvement observed in this study is partly due to the undoped cap sections on both ends of the crystal, and partly due to the improved brightness of laser pump diodes over the last decades.

 

Fig. 2. (a) Measured cw laser performance of the room-temperature Yb:YLF laser using 1%, 5%, and 15% transmitting OCs. Near-field beam profiles of the cw Yb:YLF laser at different output power levels are shown for the 5% OC as an inset. The intracavity slit enables single-transverse mode (STM) operation at the expense of reduced output power. (b) Measured cw tuning curve of Yb:YLF laser with different OCs at an absorbed pump power of approximately 7.5 W.

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Due to the multimode pump beam, the Yb:YLF laser output was also multimode, and its variation with pump power is shown in the inset near the laser efficiency curve in Fig. 2(a). Adjusting the intracavity slit width carefully, single-transverse mode (STM) operation was feasible at the expense of reduced efficiency. The beam propagation factor (M²) was measured to be below 1.1 for the STM case. With the 5% OC, the laser was tunable in the 1000–1060 nm range. Broadest cw tuning was achieved with a 1% OC, where lasing in the 997–1075 nm range has been observed [17]. Note that, with this OC, the tuning curve is also smoother: a low output coupling usage is required to enable a smooth and broad gain profile in Yb:YLF, as the gain bandwidth depends strongly on inversion level in three-level systems [e.g., see Fig. 2(a) in [9] for inversion dependent gain profile of Yb:YLF in the E//c axis at room temperature].

We start by presenting the mode-locking results in Fig. 3, which shows the performance of the laser with the 5% OC. The aim for the trial with 5% output coupling was to reach high average powers, so the net negative cavity dispersion was relatively large, and set to approximately −2000 fs². A 100-mm ROC mirror (M6) was used to focus the beam on the SESAM; and the spot size was estimated to be 20 µm inside the crystal and 75 µm on the SESAM. The laser operated in the cw regime for pump powers up to approximately 4 W, and above that, stable cw mode-locked operation was achieved. The critical intracavity pulse energy for stable mode-locking was measured as 84 nJ, which is rather close to the theoretical estimate of 92 nJ [18]. At an absorbed pump power of 7.47 W, the Yb:YLF laser produced 380-fs pulses with 1.85 W of average power at a repetition rate of 190.3 MHz [assuming sech²-shaped pulse, Fig. 3(b)]. At this pump power level, the spectrum was centered around 1024.8 nm and had a FWHM of 3.15 nm, indicating a time bandwidth product of approximately 0.34. The corresponding pulse energy and peak power were 9.7 nJ and 22.5 kW, respectively. Note that the laser output was slightly multi-transverse mode at this output power level, and a TEM₀₀ beam profile was obtained at an output power of up to approximately 1 W. Using the intracavity BRF, the central wavelength of the pulses could be tuned in the 1020–1025 nm range. With the 5% OC, via adjusting the cavity dispersion carefully (using DCM mirrors), we could also generate pulses as short as 130 fs at an average power level of 450 mW at approximately 110 MHz (10 nm bandwidth centered around 1026 nm). Mode-locking was also feasible with 3% and 10% transmitting OCs, with almost similar performance to 5% OC (lower powers with 3% OC, and less stable ML operation with 10% OC).

 

Fig. 3. Summary of mode-locking performance of the Yb:YLF laser using the 5% OC. (a) Measured laser output power versus absorbed pump power in regular cw and cw mode-locked (CWML) cases. Inset shows the measured near-field beam profile at average power levels of 1 W and 1.85 W. (b) Measured microwave spectrum indicating clean mode-locked operation at 190.284 MHz. (c) Measured variation of laser optical spectra with average output power for power levels between 0.8 W and 1.85 W. At the highest power level, the spectrum is centered around 1024.8 nm and has a width of 3.15 nm. Estimated net cavity dispersion level is also shown. (d) Measured autocorrelation trace of the pulses for the broadest optical spectrum, indicating a pulse width of 380 fs at an average power level of 1.85 W.

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To investigate the limits of short pulse generation and fs tuning range, we focused our attention on mode-locking with the 1% transmitting OC, as Yb:YLF provides a smoother gain profile at low inversion levels [9,17]. Figure 4 summarizes the mode-locking results obtained with the 1% OC. For this data, a 600-mm ROC mirror (M4) was used to focus the beam on the SESAM; and the spot size was then approximately 150 µm on the SESAM (estimated critical intracavity pulse energy for stable cw mode-locking 190 nJ). As we see from the laser efficiency curve in Fig. 4(a), the Yb:YLF laser operated cw for absorbed pump powers up to 1.4 W, and above that, stable cw mode-locked operation was achieved up to 3 W of absorbed pump power. Beyond that, the laser stability started to degrade at first due to cw breakthrough (appearance of cw component in the optical spectra). Later, at higher pump power levels, pulse break up, double pulsing, and even SESAM damage were observed. At an absorbed pump power of 2.98 W, the Yb:YLF laser produced 263 mW of average power in stable cw mode-locked operation (4 W incident). The estimated net cavity dispersion was approximately −400 fs², and the optical spectrum was centered around 1050.8 nm and had a width of 34 nm. The laser pulse width was measured as 40 fs, indicating a time bandwidth product of approximately 0.36 (1.15 times the transform limit for a sech² pulse). Due to the higher number of bounces required on DCM mirrors, and the usage of a larger ROC focusing mirror (M6, 60 cm), the cavity length became rather long, and the repetition rate of the laser was reduced to 87.3 MHz. The corresponding pulse energy and peak power were 3.05 nJ and 68.8 kW, respectively. To our knowledge, these are the shortest pulses generated from Yb:YLF systems to date.

 

Fig. 4. Summary of mode-locked laser performance of the Yb:YLF laser using the 1% OC. (a) Measured laser efficiency showing different regimes of operation. Inset shows the measured near-field beam profile at an average power of 263 mW. (b) Measured microwave spectrum indicating clean mode-locked operation at 87.267 MHz. (c) Measured optical spectrum at an average power of 263 mW: the spectrum is centered around 1050.8 nm and has a width of 34 nm. Estimated net cavity dispersion level is also shown. (d) Measured autocorrelation trace indicating a pulse width of 40 fs assuming sech² pulse shape.

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Note that in the unstable mode-locking regime, we observed optical spectra with a width up to 40 nm that ideally supports sub-30-fs level pulses. However, these pulses were not long-term stable, due to cw spike appearance and or pulse breakup. We tried to use a larger spot size on the SESAM to minimize these nonlinearities, but it then became increasingly hard to initiate mode-locked operation. Mode-locking could be achieved at lower output coupling values of 0.6% and 0.3% as well, but this did not help with pulse width reduction, and the achievable average powers were lower due to larger intracavity pulse energies. We suspect that the pulse width obtained in the current study is limited by the dispersion bandwidth of the optics [Fig. 4(c), limited by GDD increase of pump mirrors M1 and M2 on the short wavelength side and by GDD decrease of DCM mirrors on the long wavelength side]. We believe that in future studies, by employing DCMs and dichroic mirrors optimized for Yb:YLF oscillators, sub-25-fs pulses might become feasible to generate in Kerr-lens mode-locked Yb:YLF systems [5].

We finalize our discussion with fs tuning studies performed with the 1% OC, which is summarized in Fig. 5. With the insertion of the BRF and HWP for fs tuning, the number of bounces on the DCM mirrors (10 in a round-trip) is not sufficient to provide the required negative dispersion level; hence, GTI mirrors were employed for dispersion compensation in these tuning trials. Tuning experiments were performed at two different dispersion settings. For a net cavity dispersion level of −3500 fs² [Fig. 5(a)], the laser produced 500-fs level pulses with an average power of approximately 250 mW at around 100 MHz, and the central wavelength of the laser could be smoothly tuned between 1019 nm and 1047 nm (28 nm tuning range). At a dispersion setting of approximately −1500 fs² [Fig. 5(b)], the laser generated sub-250-fs level pulses with an average power of approximately 200 mW, and the pulses were tunable in the 1025–1047 nm range (average pulse width, ∼200 fs; pulse width estimate is based on measured spectral width, assuming a sech² pulse shape). We believe that the decreased gain below 1020 nm limited the tuning in the short wavelength side [see the cw tuning curve with the 1% OC in Fig. 2(b)]. On the long-wavelength side, the dispersion bandwidth of the GTI mirrors limits further tuning above 1050 nm. In general, compared with the fs tuning results with 5% OC (1020–1025 nm tuning), due to the broader and smoother gain bandwidth at reduced inversion, the 1% output coupling provided a much broader tuning range.

 

Fig. 5. Typical spectra from the mode-locked Yb:YLF laser showing tunability of central wavelength from (a) 1019 to 1047 nm for sub-600 fs pulses, and (b) 1025 to 1047 nm for sub-300 fs pulses. Estimated total cavity dispersion is also shown for both cases. Data are taken with a 1% transmitting OC at an absorbed pump power of ∼5 W. Average pulse width along the tuning range is 500 fs in panel (a) and 205 fs in panel (b).

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In summary, we have achieved pulse widths down to 40 fs (265-mW average power, 68.8-kW peak power), and average powers up to 1.85 W (380-fs pulses) from a SESAM mode-locked room-temperature Yb:YLF system. To the best of our knowledge, these are the shortest pulses generated from Yb:YLF oscillators to date, and the achieved peak powers are approximately a 20-fold improvement over the earlier room-temperature mode-locking efforts. The Yb:YLF system is easy to work with due to its relatively high gain and availability of high-quality low-loss crystals and high-brightness diode pumps, and hence we believe that future work has the potential to further improve the performance achieved in this initial study. Compared with systems such as Ti:Sapphire [19] and Cr:LiSAF [20], the obtainable pulse widths are longer, but the system has advantages in terms of thermal effects and simplicity, which makes it an interesting candidate for applications such as ultralow timing jitter source development.