We present a detailed continuous-wave regime characterization and, for the first time to the best of our knowledge, SESAM mode-locked femtosecond operation with a monoclinic, Yb3+-doped, MgWO4 crystal. Pumping with a low-power, single-mode fiber-coupled laser diode emitting at 976 nm, we demonstrate threshold for continuous-wave (cw) operation as low as 50 mW (absorbed pump power) and slope efficiency up to ~60% (with respect to the absorbed pump power) for two of the principal emission polarizations. The output wavelength in the cw regime is continuously tunable over a ∼50 nm broad range. Mode-locking the laser with a SESAM, we achieve almost Fourier-transform limited pulses with a duration of 132 and 125 fs depending on the polarization.
© 2017 Optical Society of America
19 May 2017: A typographical correction was made to the author listing.
Magnesium tungstate (MgWO4) belongs to the crystal family of monoclinic (space group P2/c), divalent metal mono-tungstates, having the general chemical formula MWO4 where M = Mg, Mn, Ni, Cd or Zn [1, 2]. It shows a rather high thermal conductivity of ~8.7 W/mK . This value is almost three times higher compared to the monoclinic potassium double tungstates, KRE(WO4)2 [4, 5], and even higher compared to the tetragonal sodium or lithium double tungstates NaRE(WO4)2/LiRE(WO4)2 where RE denotes a “passive” rare earth element. The difference in ionic radius between the divalent Mg2+ and trivalent Yb3+ ions induces distortion of the crystal field. This leads to a broadening of the absorption and emission bands, which relaxes the pump wavelength stability requirements, and favors high power applications and broadly tunable operation at ~1 μm . For instance, if compared to Yb:KRE(WO4)2 [5,7], Yb:MgWO4 shows an almost two-times broader absorption peak with a full-width half-maximum (FWHM) of ~7 nm, centered at 975 nm .
The first laser operation of Yb3+- and Tm3+-doped MgWO4 crystals was reported only very recently [6,8]. By pumping with a multi-mode, multi-Watt fiber coupled laser diode at 975 nm, the cw Yb:MgWO4 laser generated an output power as high as 2.5 W, with a slope efficiency ≈53%, in a two-mirror, plano-convex resonator sustaining multiple transverse mode oscillation .
In this work we present a detailed cw and mode-locking (ML) characterization of a Yb:MgWO4 laser. To this purpose, we decided to exploit low-power, single-mode diode pumping , in order to investigate the laser material performance with no additional thermal issues that would complicate the interpretation of the results. For the first time to the best of our knowledge, we demonstrate stable and self-starting ML in the soliton regime, employing a semiconductor saturable absorber mirror (SESAM). Pulses of ~130-fs duration with ~12 nm FWHM optical spectrum centered at 1065 nm are obtained.
2. Experimental setup and results
The X-folded resonator layout for cw and soliton ML experiments is shown in Fig. 1.
The pump-diode module consisted of a single transverse-mode, fiber-coupled laser diode (JDSU S27-7602-400) emitting at 976 nm. The laser diode output beam was not polarized, but by properly coiling the fiber pig-tail we were able to induce a strongly elliptical polarization with a ratio of about 13:1 between the two axes. The laser diode beam was collimated by means of an aspherical lens (f = 15.3 mm, NA = 0.16) and focused into the laser crystal with a Gaussian waist radius of 12 μm by using a spherical lens with 50 mm focal length. The maximum incident pump power was 375 mW.
The active medium was a 3 × 3 × 3 mm3, Yb-doped MgWO4 crystal; the dopant ion concentration was ntot = 1.819 · 1020 cm−3, corresponding to 1.25-at.%. Details about the crystal growth and the spectroscopic characterization can be found in . In Table 1, we summarize the more relevant spectroscopic and thermo-optical properties of Yb:MgWO4 and, for comparison, Yb:KY(WO4)2 and Yb:YAG.
During laser experiments, the sample was placed on a metallic plate and oriented at Brewster-angle for minimization of insertion losses. Resonator curved mirror folding angles were optimized in order to compensate the cavity mode astigmatism in the active medium. In both cw and ML operation, we exploited the resonator stability region corresponding to a larger separation M1-M2. Therefore, we could control the cavity mode waist on mirror M4/SESAM by properly adjusting the distance M2-M4 and M1-M2 separation.
Through ABCD modeling of the resonator, we could estimate a fundamental cavity mode dimension (waist radius) in the active medium ranging from wg = 12 to 15 μm within the stability region, depending on the total resonator length.
2.1. Continuous-wave operation
We preliminarily tested the crystal in cw regime for both possible orientations, with laser emission polarization parallel to crystal X-axis (EL ║ X) or Y-axis (EL ║ Y). Note that XYZ denote the dielectric frame of the monoclinic MgWO4 but the principal values of the refractive index are still unknown – thus the chosen notation serves just to establish a correspondence between the laser results and the frame in which the spectroscopic properties were characterized . The gain cross section calculated for both orientations for different values of the inversion ratio  in the range of values typical for low loss/low output coupling femtosecond oscillators, are shown in Fig. 2. As it can be seen, the EL ║ Y crystal orientation presents a higher peak value and slightly broader gain cross section spectrum. Nevertheless, both crystal orientations ensure a broad gain spectrum suitable for femtosecond pulse generation.
Referring to Fig. 1, in the cw regime, the cavity arm lengths were set as follows: M2-M4 = 200 mm; M1-M2 = 94 mm; and M1-OC = 450 mm. In this configuration, from ABCD modeling of the resonator fundamental cavity mode, we expected a beam waist in the active medium wg ≃ 13 μm. The results obtained in the cw regime with a set of output couplers (OCs) with different transmission are shown in Fig. 3.
In good agreement with the gain cross-section spectra presented in Fig. 2, the emitted output wavelength was around 1060 nm for both crystal orientations, only slightly varying with different OC mirror transitivity. Pump absorption was higher for EL ║ X-axis crystal orientation, as expected because of the higher value of absorption cross section at the 976 nm pump wavelength for this polarization . Nevertheless, due to the higher emission cross section, the maximum output of 112 mW, was obtained for the EL ║ Y-axis crystal orientation employing the optimum Toc = 10% mirror. The corresponding slope efficiency, measured with respect to the absorbed pump power, was 59%. We also carried out a Caird analysis , in order to estimate the total round-trip resonator losses δ (reabsorption losses excluded) and the intrinsic slope efficiency η0, by fitting the measured slope efficiency η as a function of the reflectivity of OC, with the following equation:Fig. 4(a). The high values of η0 obtained for both crystal orientations (η0 ≲ 0.7) suggest a well optimized resonator design and a good crystal quality.
A single fused silica (FS) prism was used for wavelength tuning in cw regime. Employing the Toc = 0.8% output coupler, a broad tuning range of ~50 nm was obtained as shown in Fig. 4(b) for the more promising EL ║ Y-axis polarization.
2.2. Soliton mode-locking experiments
For soliton ML experiments, we replaced mirror M4 (see Fig. 1), with a SESAM specified with a modulation loss ΔR = 3% and a saturation fluence Fsat = 140 μJ/cm2. Intracavity group delay dispersion (GDD) compensation was provided either by GTI mirrors or by a single FS prism , as shown in Fig. 1.
Referring to Fig. 1, in the cavity configuration exploiting GTI mirrors, employing either a Toc = 0.8% or Toc = 0.4% OC, stable and self-starting soliton ML regime was obtained with a minimum number of 2 bounces per mirror on GTI2 and GTI3 and a single bounce on GTI1, corresponding to a total negative dispersion per round trip of ≈ −4800 fs2. For EL ║ Y-axis crystal orientation, shorter pulses were obtained by using a Toc = 0.4% OC, with a minimum pulse duration of 171 fs and an average output power of 15 mW. The corresponding optical spectrum was centered at 1058 nm with a FWHM of 9 nm, resulting in a time-bandwidth product of 0.41. Slightly longer pulses (~ 200-fs-long) with about 25 mW output power were obtained with Toc = 0.8% OC.
In order to ease the fine tuning of intracavity GDD and better exploit the available material bandwidth, we tested also a single dispersive prism resonator configuration. To this purpose, we removed the GTI mirrors and inserted a single FS prism at a distance of about 950 mm from the “virtual prism”, whose position calculated through ABCD modeling of the cavity and is close to mirror M1 (see  for a detailed explanation of the oscillator design principles). The maximum negative GDD corresponding to this “virtual”-to-real prism separation was ~−4500 fs2 per round trip.
Stable and self-starting ML was readily obtained when the FS prism was hit close to the tip. By finely adjusting the prism insertion we could optimize the amount of negative GDD and minimize soliton pulse duration. The shortest pulse autocorrelation trace and optical spectrum for both possible crystal orientations are shown in Fig. 5.
Employing the Toc = 0.4% OC, 125-fs-long (FWHM) pulses, assuming a sech2 shaped intensity profile, were obtained for EL ║ Y-axis crystal orientation, the corresponding optical spectrum was about 12 nm FWHM, centered at 1065 nm, resulting in a time-bandwidth product of 0.4. Only slightly longer pulses (132 fs) were obtained for the polarization EL ║ X-axis crystal orientation in the same cavity configuration. Pulse train average output power was about 20 mW for both crystal orientations.
The good stability of the ML regime was confirmed by the radio frequency (RF) spectrum of the pulse train. In Fig. 6 we show the fundamental beat note at ~117 MHz, with a high extinction ratio of ~50 dB, indicating the absence Q-switching instabilities. The RF spectrum in a 1-GHz span with a 10 kHz resolution bandwidth, shown in the inset of Fig. 6, is also indicative of clean single-pulse operation of the laser.
In conclusion, we presented a detailed performance characterization of a single-mode diode-pumped Yb:MgWO4 laser. For both EL ║ X-axis and Y-axis crystal orientations, we demonstrated low-threshold cw operation, with high intrinsic slope efficiency and, for the first time to the best of our knowledge, femtosecond pulse generation in a SESAM soliton ML oscillator. Pulses as short as 125 fs where obtained for the EL ║ Y-axis crystal orientation and only slightly longer (132 fs) for EL ║ X-axis. A better optimized combination of a shorter crystal with higher doping could easily lead to a further improvement of this already promising performance in the ML regime, fully exploiting the advantages of high-brightness, low-power single mode diodes. In addition, the good thermal properties of MgWO4 and the relatively broad absorption peak centered at 976 nm, are favorable properties for a significant power scaling with this promising novel laser material.
The National Natural Science Foundation of China (No.11404332, No.61575199); Key Project of Science and Technology of Fujian Province (2016H0045); the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDB20000000); the Instrument Project of Chinese Academy of Sciences (YZ201414); and the China Scholarship Council (CSC, No.201504910418 and No. 201504910629).
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