1. Introduction

Femtosecond mode-locked lasers are applied in various fields of physics, engineering, chemistry, biology and medicine, with applications including ultrafast spectroscopy, metrology, superfine material processing and microscopy. Specific and interesting properties of the femtosecond laser pulses have been used in these applications. For example, femtosecond pulses have a very precise time resolution, and their strong electric field induces important and unusual nonlinear effects. For those applications, high-power, high-efficiency and compact femtosecond lasers are required. Ceramic materials are attractive for satisfying these requirements. YAG ceramics have 10% higher hardness than a YAG single crystal, and the fracture toughness of YAG ceramics is more than threefold that of the YAG single crystal. Therefore, the ceramics have a higher resistance to thermal shock than the single crystal. Ytterbium (Yb³⁺) also has interesting properties satisfying the above requirements. Its broad absorption and emission spectra allow the realization of a directly laser-diode (LD)-pumped femtosecond laser. Moreover, its small quantum defect, absence of excited-state absorption, upconversion and cross-relaxation reduce the thermal load and enable highly efficient operation. The emission and absorption spectra and thermal conductivity strongly depend on the host material.

Recently, various ceramic materials have been progressively investigated for use in ultrashort-pulse lasers [1-5]. A diode-pumped femtosecond Yb:Y₂O₃ ceramic laser was demonstrated, and 615 fs pulses at a center wavelength of 1076.5 nm were obtained with a 420 mW average power [1]. A diode-pumped passively mode-locked Yb:Lu₂O₃ ceramic laser was demonstrated, for which 357 fs pulses at a center wavelength of 1033.5 nm with a 352 mW average power were obtained [2]. A passively mode-locked femtosecond Yb³⁺-doped Y₃(Sc_0.5Al_0.5)₂O₁₂ (Yb:YSAG) ceramic laser pumped by a Ti:sapphire laser was also demonstrated, and 280 fs pulses at a center wavelength of 1035.8 nm with a 62 mW average power were obtained [3], but the laser was not diode-pumped. A diode-pumped Kerr lens mode-locked laser of Yb³⁺:Sc₂O₃ ceramics was demonstrated with 92 fs pulses at a center wavelength of 1042 nm and an 850 mW average power, and 90 fs pulses at a center wavelength of 1092 nm and a 160 mW average power [4]. A diode-pumped Kerr lens mode-locked laser with Yb³⁺:Lu₂O₃ and undoped Y₂O₃ combined ceramics was demonstrated, and 65 fs pulses at a center wavelength of 1032 nm with a 320 mW average power were obtained [5]. In previous reports on Yb:YAG ceramic lasers, there is no description of the mode locking of lasers. However, in some reports the femtosecond mode locking of Yb:YAG crystal lasers is discussed [6-10]. The average power and pulse energy have been scaled up to 76 W and 25.9 μJ using a mode-locked thin-disk Yb:YAG laser [9]. A Kerr lens mode-locked Yb:YAG crystal laser has been demonstrated, for which the pulse duration was as short as 100 fs at a center wavelength of 1051 nm [10]. The shortest pulse obtained using a diode-pumped ceramic laser without the Kerr lens effect was 357 fs [2].

In this paper we report a diode-pumped passively mode-locked Yb:YAG ceramic laser generating 286 fs pulses with an average power of 25 mW at a wavelength of 1033.5 nm using a 0.1% output coupler. 233 fs pulses with an average power of 20 mW were also obtained at a center wavelength of 1048.3 nm using a 0.1% output coupler. To the best of our knowledge, this is the first demonstration of a diode-pumped mode-locked Yb:YAG ceramic laser and the shortest pulse using diode-pumped ceramic lasers without Kerr lens mode locking.

2. Tunable properties of Yb:YAG ceramics

In our previous studies, we measured the tunability of Yb:YAG ceramics at room temperature (RT) [11-12]. The measurements of tunability at RT were carried out using an SF10 prism inserted in a v-fold cavity. Figure 1 shows the dependence of the output power versus the laser oscillation wavelength for output couplers of T = 0.1, 1, 5 and 10% when the absorbed pump power was 13.8 W. Maximum powers of 4.41 and 3.63 W were obtained at 1031.78 and 1034.14 nm, and tunable ranges of 52.64 and 77.83 nm were obtained for T = 10 and 5%, respectively. The maximum power of 1.40 W was obtained at 1033.42 nm, and a tunable range of 104.52 nm from 994.35 to 1098.87 nm was obtained for T = 1%. Finally, as shown in Fig. 1, T = 0.1% results in a maximum tunable range of 118.31 nm from 992.52 to 1110.83 nm. This broad tunability indicates the possibility of sub-100-fs pulse generation using Yb:YAG ceramics.

 

Fig. 1. Output power of ceramic Yb:YAG tunable laser as function of oscillation wavelength for various output couplers [12].

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3. Experimental setup

The experimental setup for the mode-locked Yb:YAG ceramic laser is shown in Fig. 2. An x-fold cavity configuration was used. A 940 nm fiber-coupled LD was used as a pumping source. The core diameter of the fiber was 200 μm. The numerical aperture (NA) of the fiber was 0.22. The maximum pump power was 26.6 W. The pumping beam was imaged by relay to the ceramics using lens L1 (f = 50 mm) and lens L2 (f = 70 mm). The 1-mm-thick Yb:YAG (C_Yb = 9.8 at.%) ceramic plate was arranged at the Brewster’s angle. The Yb:YAG plate was wrapped with indium foil and mounted in a water-cooled copper heat sink block. The copper block was cooled by flowing water at 20°C. The ceramic was placed between two high-reflectivity mirrors (M1, M2) that were anti-reflection (AR)-coated at 940 nm and had high reflectivity at 1030 nm with a 100 mm radius of curvature (ROC). The reference laser operated in cw mode with a 10% output coupler (OC1) and a high-reflectance end mirror (M3) along the dashed lines in Fig. 2. For passive mode locking, a 1 or 0.1% output coupler (OC2) and a semiconductor saturable absorber mirror (SESAM, BATOP) with 2% saturable absorption at 1030 nm, 70 μJ/cm² saturation fluence and 500 fs relaxation time constant were used in the respective arms. The total cavity length was 1620 mm. The laser beam was focused onto the SESAM by a concave mirror (M4, ROC = 250 mm). The distance among the mirrors and the folded angle of the mirrors are shown in Fig. 2. The astigmatism compensation was not considered. The spot sizes of laser mode in the laser crystal and on SESAM were estimated ~ 61 × 53 μm and ~ 450 × 330 μm, respectively. An SF10 Brewster prism pair (P1, P2) with 465 mm separation was inserted in the other arm to compensate for the dispersion. The total negative GDD of this cavity was about −2670 fs² per a round trip.

 

Fig. 2. Experimental setup of the mode-locked Yb:YAG ceramic laser. LD: fiber-coupled diode laser. L1, L2: focusing lenses. M1-M4: high-reflectivity mirrors. OC1, OC2: output couplers. P1-P2: SF10 Brewster prisms. SESAM: semiconductor saturable absorber mirror. Dashed lines indicate the beam lines in cw operation. Solid lines indicate the beam lines in mode-locked operation.

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4. Experimental Results

We demonstrated mode locking for three cases. The first case was mode locking at 1030 nm using a 1% output coupler. The second case was mode locking at 1030 nm using a 0.1% output coupler. The third case was mode locking at 1050 nm region with cw oscillation at 1030 nm region using a 0.1% output coupler.

4.1 Mode locking at 1030 nm region using a 1% output coupler

Figure 3 shows the intensity autocorrelation trace and the spectrum of mode-locked pulses using a 1% output coupler. The average output power was 250 mW at a pump power of 26.6 W. The sech²-fitted pulse width was 417 fs and the spectral width was 3.02 nm at the center wavelength of 1033.3 nm, which results in a time-bandwidth product of 0.353, slightly above the Fourier limit for a sech² pulse (0.315). The repetition rate was 91 MHz. The pulse energy and peak power were 2.75 nJ and 6.60 kW, respectively. When the 1% output coupler was used, mode locking at 1050 nm region was not observed but cw oscillation at 1050 nm region or simultaneous cw dual-wavelength oscillation at 1030 and 1050 nm was observed.

 

Fig. 3. (a). Measured autocorrelation trace and the sech² fitting, and (b) spectrum of the mode-locked pulses using a 1% output coupler.

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4.2 Mode locking at 1030 nm region using a 0.1% output coupler

Figure 4 shows the intensity autocorrelation trace and the spectrum of mode-locked pulses using a 0.1% output coupler. The average output power was 25 mW at a pump power of 26.6 W. The sech²-fitted pulse width was 286 fs and the spectral width was 4.51 nm, and centered at 1033.5 nm. This results in a time-bandwidth product of 0.363, which is 15% above the Fourier limit for a sech² pulse (0.315). This indicates that the pulse has potential to be further shortened by extracavity compression. The repetition rate was 91 MHz. The pulse energy and peak power were 0.275 nJ and 0.960 kW, respectively. The pulse width of 286 fs obtained using a 0.1% output coupler (Fig. 4) was shorter than that of 417 fs obtained using a 1% output coupler (Fig. 3) because the intracavity power is increased using a 0.1% output coupler, which makes it possible for the laser to oscillate with a broad spectral range. However, the average output power decreased to 25 mW owing to the low transmission of the 0.1% output coupler.

 

Fig. 4. (a) Measured autocorrelation trace and the sech² fitting, and (b) spectrum of the mode-locked pulses using a 0.1% output coupler.

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4.3 Mode locking at 1050 nm region with cw oscillation at 1030 nm region using a 0.1% output coupler

When the SESAM angle was changed slightly from the conditions discussed in Sec. 4.2, mode locking at 1033.5 nm suddenly stopped and simultaneous cw dual-wavelength oscillation at 1030 and 1050 nm occurred. When the intensity of the cw spectral component at 1050 nm was increased and the laser mode radius focused on the SESAM was optimized, mode-locked pulses were generated again. Figure 5 shows (a) the intensity autocorrelation trace, (b) the spectrum of mode-locked pulses, and (c) SH spectrum in the case of using a 0.1% output coupler. The output spectrum in Fig. 5(b) was measured by injection into the fiber-coupled spectrometer directly from OC2. The average output power was 20 mW at a pump power of 26.6 W. The sech²-fitted pulse width was 233 fs and the spectral width was 5.20 nm and centered at 1048.3 nm, which results in a time-bandwidth product of 0.330, slightly above the Fourier limit for a sech² pulse (0.315). The repetition rate was 91 MHz. The pulse energy and peak power were 0.220 nJ and 0.946 kW, respectively. Figure 5(c) shows the SH spectrum of mode-locked pulses by using a 0.1% output coupler. There is no peak at 516 nm in this spectrum. Therefore, the spectral component at 1030 nm is not related in mode locking. Figure 6 shows a pulse train of cw mode-locking in millisecond time scale. This indicates that the mode-locked pulses had a long-term stability. The pulse width of 233 fs at 1048.3 nm in Fig. 5 obtained using a 0.1% output coupler was the shortest among these results. This indicates that the laser has the potential to generate shorter pulses at 1050 nm region rather than 1030 nm region. However, the cw spectral component at 1032.4 nm was not quenched in our cavity during mode locking at 1048.3 nm. This occurred owing to the strong emission at 1030 nm.

 

Fig. 5. (a). Measured autocorrelation trace and the sech² fitting, (b) spectrum of the mode-locked pulses, and (c) its SH spectrum using a 0.1% output coupler.

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Fig. 6. Pulse train of cw mode-locking in millisecond time scale.

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5. Conclusion

A diode-pumped femtosecond ytterbium laser with a host material of YAG ceramic was demonstrated. We successfully achieved passive mode locking at wavelengths of 1033.5 and 1048.3 nm. At 1033.5 nm, passive mode locking by a semiconductor saturable absorber mirror generated 286 fs pulses with an average power of 25 mW using a 0.1% output coupler. This is the shortest pulse width in the 1030 nm region. At 1048.3 nm, shortest pulses of 233 fs with an average output power of 20 mW were generated using a 0.1% output coupler. To the best of our knowledge, this is the first mode-locked Yb:YAG ceramic laser, and the shortest pulse for diode-pumped ceramic lasers without Kerr lens mode locking was obtained.