GaSb-based SESAM is successfully employed for passive mode locking of a Tm3+:CaGdAlO4 laser operating near 2 µm. The pulse duration is around 650 fs at a repetition rate ~100 MHz.
© 2015 Optical Society of America
Mode-locked lasers near 2 µm are drawing increasing attention recently , related to applications such as pump sources for mid-IR synchronously-pumped optical parametric oscillators , frequency comb generation and its down-conversion to the mid-IR [2,3], seeding of chirped pulse amplifier (CPA) laser  and near-degenerate optical parametric chirped-pulse amplifier (OPCPA) systems pumped near 1 µm for soft-X-ray generation , supercontinuum and THz generation , material processing of polymers , and medical treatment (highly localized surgery) . The characterization and assessment of different dopant (Tm3+ or Ho3+) and crystalline host combinations for mode-locked lasers proceeds under strong competition from all-fiber laser concepts. One of the most challenging issues in both cases is finding suitable saturable absorbers for this spectral range.
In terms of shortest pulse generation, the CaGdAlO4 (CALGO) crystal is one of the most successful hosts for Yb3+ mode-locked lasers operating slightly above 1 µm . This is due to the relatively broad and flat gain spectra related to the structural disorder of CALGO, nevertheless this material shows relatively good thermo-mechanical properties . Since Tm3+, similarly to Yb3+, presents a quasi-three level laser system, Tm:CALGO is also potentially interesting for generation of ultrashort pulses near 2 µm. While spectroscopic studies still do not provide emission cross sections for calculation of the gain spectral dependence, the fluorescence lifetime of Tm:CALGO amounts to ~6 ms  which, compared to Yb-doped systems, poses additional difficulties in relation to stabilization against Q-switching. In 2015, picosecond mode-locking (ML) of Tm:CALGO was reported using a commercial semiconductor saturable absorber (SESAM) . At 1949.5 nm, the mode-locked pulse duration amounted to 27 ps.
In the present work we employed for ML GaSb-based SESAMs designed as few quantum well (QW) or near-surface QW structures . They were AR-coated and optimized for fast carrier relaxation without introduction of additional losses. The characteristics of these SESAMs enabled the generation of pulses as short as ~650 fs at 2021 nm from the Tm:CALGO laser.
2. Laser setup and continuous-wave laser operation
The high quality 6 at.% (in the melt) Tm-doped CALGO crystal was grown by the Czochralski method . The active element was processed as a 3.7 mm long Brewster rhomb with an aperture of ~4 × 4 mm2 designed for σ polarization. An X-shaped cavity was used, see Fig. 1, with the Brewster crystal between two folding mirrors (M1 and M2) with radius of curvature (ROC) of −10 cm. The pump source was a tunable narrowband continuous-wave (CW) Ti:Sapphire (Ti:Sa) laser with maximum output power exceeding 3 W. The pump beam was focused with a lens (L) of 7.5 cm focal length and the crystal was pumped in a single pass.
The CW laser performance was characterized in a 4-mirror cavity with standard (broadband) plane output couplers (OCs) with transmission varying from 0.5 to 5%. In addition to them we used also a special output coupler (SOC) with characteristics similar to a cut-off filter highly transmitting below 2 µm. The transmission of the plane SOC at the actual laser wavelength, when the laser was constrained to oscillate above 2 µm, amounted to 1.84%. All output coupler substrates were wedged (~5′), to avoid etalon effects.
The CW laser characteristics recorded are shown in Fig. 2(a). The pump wavelength was 798 nm, corresponding to the absorption maximum of the crystal for σ polarization . The oscillation wavelength changed from 1974.5 to 1950 nm when the transmission of the standard OCs was increased from 0.5 to 5%. The threshold was in all cases below 100 mW of absorbed power. Best performance was achieved with the 5% OC (5.5% at the laser wavelength) which provided an output power of 337 mW at an absorbed power of 766 mW with a slope efficiency of 50%. This slope efficiency is much higher compared to previous reports based on diode pumping  indicating most probably better mode matching. The SOC shifted the oscillation wavelength to 2041.5 nm while the slope efficiency dropped to 16.8%, yielding a maximum output power of 110 mW at an absorbed pump power of 755 mW. We observed absorption bleaching in non-lasing conditions but the single pass crystal absorption in lasing conditions almost recovered to its small signal value. Thus the absorption at the maximum pump level applied (~1.2 W incident) varied only from 65.6% for the 0.5% OC down to 62.9% for the SOC which can be explained by the different intracavity intensity and the quasi-three level laser system of the Tm3+-ion.
The wavelength tunability was investigated by inserting a 3-mm thick Lyot filter close to the rear mirror M4. Figure 2(b) shows the results achieved with the 1.5% OC. A continuous tuning range was obtained that spans from 1811 to 2065 nm. Some structure is seen in the tuning curve which is most probably related to water absorption in air [see grey line in Fig. 2(b)]. The FWHM of the tuning curve amounts to 196 nm, exceeding the tuning capability of Tm:KLu(WO4)2 , the material that yielded the shortest pulses so far, 141 fs . The emission bandwidth of Tm:CALGO thus supports 20 fs pulse durations.
3. SESAM mode-locked Tm:CALGO laser
Recently, we studied GaSb-based SESAMs containing a low number of QWs [13,15,16]. The absorber region is anti-resonant at the operating wavelength of 2 µm and consists of 10-nm thick InGaSb QWs embedded in GaSb. The SESAMs are characterized by a near-surface placement of the QWs and additional AR coating. In this way fast surface recombination effects are exploited without introducing internal defects, resulting in relaxation times τ2<5 ps at 2 µm . Unfortunately, no measured values of modulation depth or non-saturable losses of such SESAM are available, yet. However, the estimated insertion loss in the laser cavity at the operation wavelength of ~2020 nm is between 3% and 6% depending on the number of QWs in the samples. From SESAM reflectivity measurements we deduced a maximum absorption of ~1.5% per QW. These SESAMs were initially tested without dispersion compensation in the Tm:CALGO laser cavity. They were employed as a cavity end mirror while the focusing mirror M3 used to create an additional waist had a ROC of −10 cm, Fig. 1. The SOC was employed with the SESAMs in order to avoid air absorption/dispersion effects in the absence of purging and operate them in their optimum spectral range.
Table 1 lists the ML results obtained with 4 different SESAMs. A tendency towards multi-pulsing was observed applying these SESAMs when the threshold of ML had been reached, however, at higher pump levels the ML stability was different. Best performance in terms of stability, damage resistivity and output power was achieved with SESAM no.2. The tendency towards multi-pulse operation was stronger for the 1-QW structures (no.3 and 4) exhibiting the lowest modulation depth. It shall be emphasized that the thinner cap layer seems to play an important role in terms of a shorter recovery time which affects the obtained pulse duration.
The results in Table 1 can be compared with the commercial SESAM (BATOP GmbH, Germany) used in . The relaxation time τ2 for this SESAM is specified as 34 ps while the obtained pulse duration τp for stable steady-state ML was 27 ps.
The SESAM no.2 selected for further study contained two QWs with a 10 nm barrier and a very thin GaSb cap layer of 5 nm. The mode-locked laser pulse duration was ~1 ps but the radio frequency spectra exhibited a limited contrast of the central peak (~40 dBc) and a broad pedestal indicative of timing jitter.
As a next step, the two CaF2 Brewster prisms (P1 and P2) for dispersion compensation were inserted into the cavity, see Fig. 1, at a separation of 15 cm. The total cavity length amounted then to ~1.5 m. As can be seen in Fig. 2(a), with SESAM no.2 and the prism pair inserted, stable and self-starting ML was observed in a broad power range starting from an absorbed pump power of ~500 mW (at an output level of ~20 mW) up to the maximum pump level of 961 mW applied (corresponding to an incident power of ~1.5 W) when the output power reached 58 mW. The slope efficiency and the output power dropped by factor of ~2 compared to the CW regime [Fig. 2(a)] which is attributed to the SESAM non-saturable losses.
In view of the relatively low slope efficiency in the ML regime (η = 8.8%) we did not further increase the pump power because changes in the pump mode profile were observed together with some roll over in the output power dependence of the Tm:CALGO laser which would not allow one to study the laser performance without additional realignment. The calculated beam waist radius at the position of the SESAM was 30 µm, resulting in an average fluence of ~350 µJ/cm2 at the ML threshold. The pulses got shorter with increasing pump level. An intensity autocorrelation trace of the shortest pulses obtained at maximum output power is shown in Fig. 3(a). It is almost perfectly fitted using a sech2-pulse shape profile which yields a pulse duration (FWHM) of 646 fs. The simultaneously recorded optical spectrum is shown in Fig. 3(b). The central wavelength is 2021 nm and the FWHM of 9.2 nm leads to a time-bandwidth product (TBP) of ~0.44.
Such a TBP (just about 1/3 above the Fourier limited value of 0.315) leaves little room for pulse compression by further optimizing the dispersion or using extracavity dispersive elements. However, the negative dispersion of the prism pair (both angular and material) was essential to reduce the noise level in the radio frequency spectra. Such spectra with a span of 1 GHz and 120 kHz are shown in Fig. 4. They were recorded with a resolution bandwidth (RBW) of 30 kHz and 300 Hz, respectively. Both records indicate clean CW ML without Q-switching instabilities notwithstanding the relatively long lifetime of Tm:CALGO compared to other hosts such as monoclinic double tungstates . The fundamental beat-note was 78 dB above noise level which is much better than the situation without the prism pair in the cavity. The optical cavity path length corresponds to a repetition rate of 100.6 MHz.
No dispersion relations are available for Tm:CALGO. The group velocity dispersion (GVD) for undoped CALGO near 2 µm is around −30 fs2/mm for σ-polarization (unpublished results). The calculated single pass contributions of the prism pair are also both negative, −15.2 fs2/cm for the angular part and −22 fs2/mm for the material part (about 4 mm). Thus, the round trip group delay dispersion (GDD), without the SESAM contribution, is estimated to be around −850 fs2. From the simulated GDD of the used SESAM structures, a rather low value of less than −300 fs2 can be estimated for its contribution . Thus it is most likely that the laser operates in the low GDD regime and the soliton shaping mechanism is weak. This is confirmed by the fact that with pulse duration of ~750 fs near the ML threshold the soliton theorem is not fulfilled when increasing the pump level. Thus the SESAM properties (temporal response) seem to play the major limiting role on the pulse duration for the present laser.
After completion of this work we became aware of a similar result in terms of pulse duration obtained with a SESAM mode-locked Tm:CaYAlO4 (CALYO) which was only briefly mentioned in . CALYO, also a disordered crystal, is an isomorph of CALGO which shows similar properties and initially was mode-locked only to produce ~35 ps pulses . The transition to the sub-ps regime was enabled by increasing the modulation depth of the commercial SESAM (BATOP GmbH, Germany). Also in this case the limit to the pulse duration, with dispersion compensation applied, seems to be set by the temporal characteristics of the SESAM. This Tm:CALYO laser operated at 1979 nm without spectral selective elements which indicates that for the present spectral bandwidths (of the order of 10 nm), the air absorption/dispersion is still not a crucial factor.
Femtosecond ML at 2 µm is demonstrated for the first time for the Tm:CALGO laser using GaSb-based SESAMs. The rather large tunability (254 nm) of the CW laser and the weak soliton-shaping mechanism of the pulse indicates a great potential for shorter pulses. At present the recovery time of the SESAM seems to be the limiting factor for the pulse duration. Apart from optimization of this parameter, further efforts will be focused on utilization of soliton shaping mechanisms. To this aim, the nonlinear refractive index of CALGO will be characterized and additional elements for intracavity self-phase modulation will be eventually introduced. We also plan to investigate in the future the performance of the π-polarization with a different active element.
At present stable ML of the Tm:CALGO laser using single-wall carbon nanotubes (SWCNTs) could not be achieved. However, due to their fast response time, combination of such a saturable absorber with a SESAM seems promising for achieving stable and shorter pulses with duration limited by the gain bandwidth.
We thank Ruijun Lan for help in some of the experiments, supported by the National Natural Science Foundation of China (NSFC), grant no. 61405171 and D. Rytz for providing unpublished information on the dispersion of the CALGO crystal.
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