Mode-locked solid-state lasers (SSLs) with femtosecond pulse durations are attracting great attention, due to powerful applications in various fields [1–3]. Although Kerr-lens mode-locked Ti:sapphire lasers are among the most frequently used systems in this family, they require a bulky and costly pump-laser system, generally a continuous-wave (CW) Nd:YVO⁴ laser. In contrast, ytterbium-doped SSLs based on Yb:YAG, Yb:KGW, or Yb:CALGO among others are economically and sizewise competitive, because they can be directly pumped by commercially available high-power diode lasers at around 980 nm [4–7]. Moreover, the small energy difference between the emission and absorption bands in Yb-doped crystals induces fewer thermal issues, which is a great advantage for high-power lasers.

Among the Yb-doped gain crystals, Yb:KGW is a popular choice for ultrashort lasers. The large emission cross section and comparatively good thermal conductivity of Yb:KGW crystal are beneficial to high-power lasers [8], and the spectral bandwidth (circa 25 nm) of the emission is favorable for generating ultrashort pulses of less than 100 fs. Recently the generation of 59-fs pulses in a semiconductor saturable-absorber mirror (SESAM)-assisted Kerr-lens mode-locked Yb:KGW was reported [9]. Also, a high-power Yb:KGW pulsed laser at the 10-W level, but with a pulse duration of over 200 fs, has been reported [10]. Among the reported mode-locked Yb:KGW laser systems, it is a general trend that very short pulses are delivered at relatively low powers, while high-power systems operate with comparatively long pulses. Because both high power and short pulse duration are important, lasers having moderate values in both properties may be better suited for some pulsed-laser applications than those that excel in one aspect but lag in the other.

Femtosecond Yb-doped SSLs commonly adopt a technique called passive mode locking, in which the intracavity group-velocity dispersion (GVD) is an important parameter. Mode-locking stability and pulse duration are determined by the interplay between self-phase modulation, due to the nonlinear optical processes in the gain crystal, and the GVD of the cavity elements [11, 12]. According to the solitonlike mode-locking model, the pulse duration as well as stable single-pulse-per-round-trip operation strongly depend on the total intracavity GVD [13].

In this paper, we report on a SESAM mode-locked Yb:KGW laser system producing ultrashort pulses around 1,030 nm that is pumped by a laser diode at 981 nm. As the negative intracavity GVD is varied in steps of −550 fs², the shortest pulses with a duration of 89 fs and average power of 3.6 W are obtained at a total GVD of −3,300 fs². The pulse broadens and the output rises as the total GVD is further lowered, reaching 154 fs and 5.6 W at a GVD of −6,600 fs². We find that the highest peak intensity is achieved at moderate GVD, because of the pulse broadening at high negative GVDs.

Figure 1 shows a schematic diagram of our SESAM mode-locked Yb:KGW oscillator that is constructed for six GVD mirror bounces per single pass. A Yb:KGW crystal with 3.6% Yb doping (EKSMA Optics, Vilnius, Lithuania) is cut along the Ng axis to a length of 3 mm. The end and front surfaces are antireflection-coated at both the pumping and lasing wavelengths. The crystal temperature is maintained at 20 ℃ by placing it inside a copper block cooled by chilled water. As a pump source, we use a laser diode (Han’s TianCheng Semiconductor Co., Ltd., Beijing, China) at a wavelength of 981 nm with a maximum power of 27 W, which is coupled to a 105-μm-core multimode fiber with NA = 0.22. A compact laser-diode driver (SF6090; Maiman Electronics LLC, Belgrade, Serbia) is used to operate the pump laser. The pump beam dispersing from the fiber end is collimated by an achromatic doublet with f = 50 mm, and then focused to the crystal through an off-axis parabolic mirror with f = 100 mm. As shown in Fig. 1, the parabolic mirror has a 3-mm-diameter hole to allow the lasing beam to pass through it. Spherical mirrors M1 and M2 placed on either side of the crystal have radii of curvature of 50 cm and 30 cm respectively, and are highly reflective (R > 99.9%) at 900–1,100 nm. The SESAM (model SAM-1040-4-800fs-25.4g; BATOP GmbH, Jena, Germany) located at the end of one cavity arm has a modulation depth of 2.6%, a saturation fluence of 120 μJ/cm², and a relaxation-time constant of 800 fs. The output coupler (OC) at the other end of the cavity arm has a partial reflectance of 80% at 1,000–1,100 nm. Two Gires–Tournois-interferometer (GTI) mirrors, each providing a negative GVD of −550 fs² per bounce at 1,030–1,040 nm, are inserted to manage the total intracavity GVD. We can vary the total intracavity GVD in steps of −550 fs² by increasing the number of bounces in the cavity from three to six per single pass. In the cases of three or five bounces, a low-GVD high-reflection mirror is used to direct the last beam to the OC. While varying the number of GTI mirror bounces we adjust the inter-GTI mirror distance accordingly, such that the total cavity length or the pulse repetition rate is maintained around a fixed value. The output lasing spectrum is measured using an optical-spectrum analyzer (AQ6319; Yokogawa Electric Co., Tokyo, Japan), and the duration of the mode-locked pulses is measured with a home-built interferometric autocorrelator based on two-photon absorption at a GaP photodiode.

Figure 1. Experimental setup of the diode-pumped SESAM mode-locked Yb:KGW laser, for the case of six GTI bounces per single pass (6GTI), and also showing the cavity modification for the case of five GTI bounces (5GTI). M1 and M2, spherical mirrors with R1 = 300 mm and R2 = 500 mm; PM, parabolic mirror with a hole; GTI, Gires–Tournois-interferometer mirror; HR, high-reflection mirror; SESAM, semiconductor saturable-absorber mirror; OC, output coupler; AL, achromatic lens; LD, laser diode.

To maximize the power-conversion efficiency of the lasing, we replace the SESAM in Fig. 1 with a high-reflection mirror and adjust the cavity parameters in CW mode. The output-power curve as a function of pump power under optimized conditions is shown in Fig. 2. CW lasing at a wavelength around 1030 nm is established above a threshold pump power of 5.0 W, and the optical-to-optical slope efficiency above the threshold is about 53%. The high power-conversion efficiency of pump photons to lasing indicates a satisfactory overlap of the pump spot with the cavity mode in the crystal, as well as good stability of the cavity design. The output polarization is horizontal to the floor, which is possibly due to a larger absorption cross section along the Nm axis of the Yb:KGW crystal.

Figure 2. Output power as a function of pump power under continuous-wave (CW)-mode lasing conditions, when the semiconductor saturable-absorber mirror (SESAM) is replaced by a high-reflection mirror.

After confirming the CW mode operation, we put the SESAM back in place for mode-locking operation. As the pump power is adjusted within a proper range, we can obtain pulsed-mode lasing. The transition from CW to pulsed mode, occurring during the pump-power sweep, can be easily noticed either from the spectral broadening in the pulsed mode or from the pulsed-intensity trace of the output as observed with a fast photodiode connected to an oscilloscope. The GVD is an important parameter that determines the pulse duration in mode-locked ultrashort-pulse lasers. In general, a pair of prisms or chirped-dispersion mirrors are used to manage the GVD in mode-locked lasers [14, 15]. Here we adjust the intracavity GVD by changing the number of bounces between the GTI mirrors, which provides a negative GVD of −550 fs² upon each bounce. We must mention that although higher output powers can be obtained with increasing pump power, the pulse breaks up into multiple pulses per round trip at high output powers, and there is a limit to the output power attainable under stable single-pulse-per-round-trip operation [16, 17]. Thus, we present here the lasing pulse characteristics obtained at the maximum output power before pulse breakup.

First, we measure the pulse characteristics for the condition of six GTI mirror bounces per single pass, with a total round-trip GVD of −6,600 fs². Under this condition an output power of 5.6 W can be achieved at a pump power of 17.3 W, with an optical-to-optical efficiency of 32%. Figures 3(a) and 3(b) show the laser spectrum and the autocorrelation trace of the mode-locked pulses at six GTI bounces respectively. The spectrum peaks at 1,027 nm with a bandwidth of 9.8 nm at full width at half maximum (FWHM). The pulse duration at FWHM, extracted from the autocorrelation trace assuming a sech-squared temporal profile, is measured to be 154 fs. The pulse train in Fig. 3(c), obtained with a fast photodiode, indicates a pulse repetition rate of 60 MHz. Then, the pulse energy and peak intensity at six GTI bounces are 93 nJ and 0.53 MW respectively.

Figure 3. Mode-locked pulse characteristics for 6 Gires–Tournois-interferometer (GTI). (a) Spectrum and (b) autocorrelation trace of the output pulses obtained when the lasing beam experiences six GTI mirror reflections per single pass inside the cavity. (c) Output pulse train obtained with a fast photodiode, indicating a pulse repetition rate of 60 MHz.

In Figs. 4(a) and 4(b) we show respectively the spectra and autocorrelation traces of the mode-locked pulses obtained at various GTI mirror-bounce numbers. As the number of GTI bounces decreases down to three, the spectrum bandwidth increases monotonically, while the pulse duration is accordingly shortened. From a further reduction to a two-bounce condition, we cannot succeed in mode locking. In the case of three GTI bounces, the pulse duration reaches 89 fs with a spectral bandwidth of 14 nm at FWHM. Figure 4(c) shows the pulse duration and output power as functions of the intracavity GVD. As the negative GVD increases, the attainable output power increases, but this is accompanied by pulse broadening. As a result, among the four different GTI bounce conditions, a maximum peak intensity of 0.68 MW is reached for the case of four GTI bounces. This tendency indicates that both high-power and short pulse operation cannot be achieved at the same time, and thus a proper choice of GVD is required for specific applications.

Figure 4. Intracavity group-velocity dispersion (GVD) dependence. (a) Spectra and (b) autocorrelation traces of the output pulses under the 3, 4, and 5 Gires–Tournois-interferometer (GTI) mirror-bounce conditions per single pass inside the cavity. (c) Pulse duration (black) and output power (red), and (d) pulse energy (pink) and peak intensity (blue), as a function of intracavity GVD for the laser cavity, with different numbers of GTI reflections.

To characterize the output beam quality of the mode-locked pulses, we measured the spatial beam profile and M ² factor using a commercial measurement system (Thorlabs), which measures the spatial beam shape at various distances after passing a lens with a focal length of 250 mm. The spatial beam shapes in the insets of Fig. 5, measured near the focal plane, indicate that the lasing mode is close to the TEM₀₀ Gaussian mode. As shown in Fig. 5, the M ² factor is measured to be about 1.2 in both horizontal and vertical axes. Although the beam profile is slightly elliptical, the Gaussian TEM₀₀ mode lasing with a relatively good M ² factor close to 1 indicates that the output beam quality is relatively good, to enable tight beam focusing. Accordingly, the laser system can be readily utilized for applications such as laser machining and nonlinear optical characterizations.

Figure 5. M ² factor of the output beam. Position-dependent variation of the 1/e² radius of the output beam, after passing a lens with 250-mm focal length. The solid lines are fittings with an M ² factor of 1.2 for both horizontal and vertical directions. The insets show the spatial beam shapes near the focal plane.

A SESAM mode-locked Yb:KGW laser, pumped by an inexpensive and compact diode laser at 981 nm, was demonstrated to generate ultrashort pulses around 1030 nm with an output power of over 5 W. The pulse duration and output power exhibited strong dependence on the total intracavity GVD. The shortest pulses of 89 fs, with an output power of 3.6 W, were achieved when the total GVD was −3300 fs², after which the pulse duration lengthened as the negative GVD increased. Although a larger pulse energy could be attained under high-negative-GVD conditions, the strongest peak intensity (which is an important parameter in nonlinear optical processes), was achieved at a moderate GVD of −4,400 fs². We note that the pulse duration may be further shortened by more precisely managing the GVD using low-GVD mirrors, while larger pulse energies may be achieved by changing the output coupling efficiency. We expect that high-power ultrashort pulses in the near-infrared wavelength region with sub-100 fs pulse durations, as demonstrated in this work, will be useful in various femtosecond-laser applications in materials processing and ultrafast diagnosis, and will contribute to the development of size- and cost-effective femtosecond SSL systems.

The authors declare no conflicts of interest.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.

Research Fund of 2020, Chungnam National University.