1. Introduction

Femtosecond amplifiers with high repetition rates and high average output power levels have been widely investigated for various industrial and scientific applications, such as nonlinear or ultrafast spectroscopy with a high signal-to-noise ratio, micromachining for transparent materials, and non-thermal high-precision material processing, among others [1–6]. In these studies, 1 µm femtosecond lasers based on Yb-doped gain media have been broadly investigated as the major type in the femtosecond laser market, as Yb-doped gain media possess a broad gain bandwidth near the 1 µm wavelength range, high thermal conductivity, a long fluorescence lifetime, and low quantum defects. In addition, fiber-coupled high-power laser diodes are commercially available as a pumping source [7–9]. Yb:YAG, one of the most intensively researched and widely used laser materials, possesses high thermal conductivity and a large emission cross-section but also a relatively narrow gain bandwidth [10–12]. To complement the gain bandwidth of Yb:YAG, the Yb:Y₂O₃ ceramic material has been studied as the one of the alternatives to Yb:YAG due to broad gain bandwidth [13]. After the first demonstration of the Yb:Y₂O₃ ceramic femtosecond oscillator [14], femtosecond oscillators with bulk and thin-disk schemes have been reported [14–18]. More recently, studies of femtosecond amplifiers with Yb:Y₂O₃ ceramic materials have appeared [13,19,20].

In the research on high-repetition-rate femtosecond amplifiers, thermal management of the gain medium with a simple system configuration is important for efficient laser operation. Additionally it is essential to increase the surface-to-volume ratio of the gain medium for better thermal dissipation. To increase this ratio, numerous amplification schemes involving thin-disk, InnoSlab, fiber, and thin-rod components have been proposed. Thin-disk and InnoSlab amplifiers have been successfully demonstrated as femtosecond amplifiers with high average output power capabilities [9,10]. However, they inherently possess complex pump and amplifier configurations because their short lengths of the gain medium forces a non-collinear multi-pass amplification scheme of the pump and seed beam to extract sufficient levels of gain. The optical fiber approach exhibits a simple amplification scheme with a large surface-to-volume ratio, but the propagation length through nonlinear media is long, on the meter scale, and it is difficult to avoid the accumulated nonlinear phase shift, which induces unwanted self-focusing and temporal distortion in the femtosecond regime [21]. A thin-rod amplifier presents a compromise as a solution when choosing between a solid-state bulk multi-pass amplifier and a fiber amplifier. A thin-rod, which possesses sub-mm diameter and a length of a few cm, has advantages in terms of thermal management and the short length of the nonlinear media. In addition, it ultimately provides simple pumping and an amplification scheme with collinear overlap between the pump and the seed beam. It is a great advantage for a simple and compact amplification system configuration with price competitiveness [22–25]. For femtosecond pulse amplification, there are noticeable demonstrations adopting a thin-rod structure, but most focus on Yb:YAG amplifiers, whereas there are few results pertaining to other thin-rod amplifiers based on Yb-doped materials [20]. In addition, the demonstrations of Yb:Y₂O₃ ceramic femtosecond amplifiers have required additional complexity like thin-disk structure [13], cryogenic cooling [19], and a Pockel cell for a regenerative amplification scheme [13,20].

In this research, we present the study of the direct amplification of femtosecond pulses with diode-pumped Yb:Y₂O₃ ceramic thin-rod amplifier which does not adopt chirp pulse amplification (CPA) scheme. A simple single-stage amplifier with a four-pass amplification scheme delivers maximum output power of 8.1 W at a center wavelength of 1030.5 nm and spectral bandwidth of 4.8 nm. Assuming a sech²-shaped pulse, a pulse duration of 239 fs is achieved with a time-bandwidth product of 0.324. To the best of our knowledge, the proposed Yb:Y₂O₃ ceramic thin-rod femtosecond amplifier exhibits the shortest pulse duration among all Yb:Y₂O₃-based femtosecond amplifiers with Watt-level output power.

2. Experimental setup

A schematic of the Yb:Y₂O₃ ceramic thin-rod amplifier is shown in Fig. 1. As a master oscillator (MO), a custom-made SESAM mode-locked Yb:KGW femtosecond master oscillator, similar to the model in our previous research, is used [26,27]. To increase the gain, a center wavelength of MO close to 1030 nm is favored, which is the peak of the emission cross-section of the Yb:Y₂O₃ ceramic [13]. In this experiment, the center wavelength MO is blue-shifted such that it is close to 1030 nm by increasing the transmission of the output coupler from 8% to 10%. From the modified MO, a seed beam exhibits output power up to 2 W, a pulse duration of 170 fs, a repetition rate of 80 MHz, a center wavelength of 1030.5 nm and a spectral bandwidth of 7.5 nm. The seed beam from the MO is focused onto a Yb:Y₂O₃ ceramic thin-rod by a plano-convex lens with a focal length of 400 mm. The beam diameter of the focused seed beam varies from 160 µm to 300 µm in terms of the full width half maximum (FWHM), which is controlled by changing the beam size of incident seed beam before focusing lens. The seed beam is incident onto a half-wave plate and a Faraday isolator, where the Faraday isolator serves as the output coupler of the amplifier. The seed beam then passes the half-wave plate and a thin film polarizer to change the beam path for collinear multipass amplification. As a pump source, a fiber-coupled laser diode (Han’s TCS) (LD) with output power of 150 W, a center wavelength of 976 nm, a core size of 105 µm, and a N.A. value of 0.22 is used. To keep the center wavelength near 976 nm when the LD reaches the maximum pump power, the temperature of the LD is set to 31.5 °C, which is stabilized by a thermoelectric cooling device. The pump beam is incident on a long-wave pass filter (LWPF) which shows high reflection near 976 nm and high transmission near 1030 nm. The pump beam is collimated by an achromatic doublet lens with focal length of 50 mm, and the diameter of the pump beam near the area of focus is controlled by an achromatic doublet convex lens with focal length of 100 mm, 125 mm, and 150 mm. Because the spatial beam profile of the pump beam does not have a Gaussian shape, the diameter corresponding to the FWHM of the pump and seed beam are matched in this research. As a gain medium, a 1.0-mm-diameter, 27-mm-long Yb:Y₂O₃ ceramic thin-rod is prepared by cutting and polishing a Yb:Y₂O₃ ceramic slab which possesses 0.5 at. % Yb³⁺ doping ratio (Konoshima Chemical Co., LTD). Two sides of the thin-rod are antireflection coated near 980 nm and 1030 nm wavelength ranges. The Yb:Y₂O₃ ceramic thin-rod is mounted onto a water-cooled copper holder with a cooling temperature of 20 °C. The pump and seed beam from the thin-rod after amplification becomes separated by a LWPF. A separated seed beam goes back to the thin-rod after passing a quarter-wave plate and a concave mirror (CM) with radii of curvature of -250 mm. The seed beam which underwent two-pass amplification is reflected by a TFP and returned to the thin-rod by a CM. Finally, the seed beam which undergoes collinear four-pass amplification emits by a Faraday isolator.

 

Fig. 1. Schematic of the Yb:Y₂O₃ thin-rod amplifier: L1, plano-convex lens with f = 400 mm; λ/2, half-wave plate; F-iso, Faraday isolator; TFP, thin-film polarizer; CM, concave mirror with ROC=-250 mm; LWPF, long-wave-pass filter; Pump LD, 976 nm, 150 W fiber-coupled laser diode; L2, L3, achromatic doublet convex lens with f = 50 mm, 100 mm, 125 mm, 150 mm; Yb:Y₂O₃ thin-rod, 0.5 at. % doping ratio, 1-mm-diameter, 27-mm-long Yb:Y₂O₃ thin-rod module; λ/4 quarter-wave plate.

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3. Experimental results

For efficient amplification of the pulse, the beam overlap between the pump and seed with a proper volume of the active region is most important. Consequently, beam size optimization is initially accomplished in this research. Figure 2(a) shows the output power measurement depending on the beam diameter of pump and seed in the four-pass amplification scheme. Both the pump and seed beam is varied in size while matching FWHM of the pump and the seed be the same. At a beam diameter of 200 µm and with a maximum pump power of 123.7 W, maximum output power level of 8.1 W is achieved. For beam diameters of 160 µm and 300 µm with a maximum pump power, maximum output power levels of 5.7 W and 5.3 W are confirmed, respectively. During the output power measurements, for both cases, no saturation effects are observed. From the beam size optimization process, it is confirmed that a beam diameter of 200 µm is proper for our system due to the limited pump power of 123.7 W. Assuming a Gaussian-shaped beam profile, the peak intensity corresponding to the maximum pump power is 263.1 kW/cm². Figure 2(b) shows the transmitted pump power after the Yb:Y₂O₃ ceramic thin-rod corresponding to a beam diameter of 200 µm. When the 123.7 W pump is incident on the thin-rod, the transmitted pump power is 13.4 W and the transmission is approximately 10.8%. This indicates that our Yb:Y₂O₃ thin-rod exhibits proper absorption of the pump beam, a condition which can effectively overcome the reabsorption of the seed beam.

 

Fig. 2. Yb:Y₂O₃ thin-rod amplifier: (a) output power as the beam diameter with the four-pass amplification scheme, and (b) transmitted pump power corresponding to a beam diameter of 200 µm (without seed beam).

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Figure 3(a) shows the output power corresponding to the one-, two-, and four-pass amplification schemes with a pump and seed beam diameter of about 200 µm. The maximum output power levels after one-, two-, and four-pass amplification are 2.9 W, 4.2 W, and 8.1 W, respectively. The corresponding gains are 1.5, 2.2 and 4.2. From the output power measurements, saturation of the output power as the incident pump power was not observed in any case. The gain value of two-pass amplification scheme are similar to the second power of the gain value of one-pass amplification scheme. In case of four-pass amplification scheme, the gain value is lower than the fourth power of the gain value of the one-pass amplification scheme because the linear loss of the TFP, HWP and the Faraday isolator affects to the gain value after final amplification pass. The M² values and the far-field beam profile corresponding to the maximum output power are measured, as shown in Fig. 3(b). From the M² measurements, M² values of 1.18 for the x-axis and 1.10 for the y-axis are confirmed. Considering the M² values and far-field beam profile, our amplifier shows a high beam quality level.

 

Fig. 3. (a) Output powers with one- two-, four-pass amplification corresponding to a beam size of 200 µm, (b) M² and far-field beam profile measurements corresponding to the maximum output power of four-pass amplification.

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The spectrum and corresponding pulse duration of the four-pass amplification scheme are shown in Figs. 4(a) and 4(b). From the spectrum measurement outcomes shown in Fig. 4(a), a center wavelength of 1030.5 nm and a spectral bandwidth of 4.8 nm are confirmed. The corresponding pulse duration as the dispersion value is depicted in Fig. 4(b). Assuming a sech²-shaped pulse, the pulse duration without dispersion compensation is determined to be 391 fs with a time-bandwidth product value of 0.531. For dispersion compensation, a negative-chirp mirror with a -10000 fs²/bounce is applied. With three bounces of the chirp mirror, a minimum pulse duration of 239 fs is achieved. At this time, a time-bandwidth product value of 0.324, very close to the Fourier transform limited value of 0.315, is achieved. This indicates that our seed beam undergoes gain narrowing which efficiently reduce nonlinear chirp during the amplification process.

 

Fig. 4. (a) Spectrum and (b) corresponding autocorrelation trace at an applied dispersion of + 0 fs² (blue) and -30000 fs² (red).

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Figure 5 shows the output power and gain as the seed power in the four-pass amplification scheme with a maximum pump power of 123.6 W. When the incident seed power ranges from 0.05 W to 1.95 W, the output power increases from 0.39 W to 8.1 W and the gain value decreases from 7.8 to 4.2.

 

Fig. 5. Output power and gain as a seed power.

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4. Conclusions

In this research, we studied the direct amplification of femtosecond pulses from a diode-pumped Yb:Y₂O₃ ceramic thin-rod amplifier. From beam diameter optimization of the seed and pump beam, maximum output power of 8.1 W and gain of 4.2 are achieved in a single-stage amplifier using a four-pass amplification scheme. At the maximum output power, our Yb:Y₂O₃ ceramic thin-rod amplifier delivers a 239 fs pulse at a center wavelength of 1030.5 nm and a spectral bandwidth of 4.8 nm. To the best of our knowledge, our Yb:Y₂O₃ ceramic thin-rod femtosecond amplifier exhibits the shortest pulse duration among Yb:Y₂O₃-based femtosecond amplifiers with Watt-level output power. Considering the time-bandwidth product value of 0.324 and the tendency of the gain as the seed power, our system is able to use not only a high-power seed laser for another amplifier which possesses high gain near 1030 nm but also a second-stage amplifier. Considering the limited pump power, much higher output power can be achieved from this Yb:Y₂O₃ ceramic thin-rod amplifier. We hope that Yb:Y₂O₃ thin-rod femtosecond amplifier will efficiently complement the Yb:YAG femtosecond amplifier in terms of gain bandwidth and pulse duration.