1. Introduction

Femtosecond lasers are used in many applications, e.g., laser processing, optical communication, metrology, and spectroscopy. They are especially important in ultrafast spectroscopy for investigating ultrafast phenomena in a variety of molecules [1–5]. Recently, Yb-doped optical fiber has become a popular gain medium because it can generate high-energy ultrashort light pulses with broad spectra at a high repetition rate in addition to its high pumping efficiency and its ability to deliver a high average output power [6]. Furthermore, ultrafast green light pulses can be generated from the output pulses using a simple second harmonic generation (SHG) technique. This light source is better suited for enabling ultrafast spectroscopy in the visible range than other femtosecond light sources, e.g., optical parametric oscillators (OPOs), optical parametric amplifiers (OPAs), and Ti:sapphire laser systems. However, the gain bandwidth of the Yb-doped material is narrower than that of Ti:sapphire, accentuating the gain-narrowing effect during the amplification process. In previous studies, the output spectrum generated by Yb-doped fibers was broadened by nonlinear effects such as self-phase modulation [7]. As an alternative approach, we compensated for gain-narrowing by inserting an optical filter consisting of multiple dielectric layers into the system [8]. This approach yields high quality pulses with relatively clean temporal shape and spectrum compared with supercontinuum generation during amplification. In our previous work, we demonstrated a 65-fs, 100-nJ light pulse generation utilizing a Yb-doped fiber amplifier with multiple dielectric layers [8].

This study demonstrates ultrafast green pulse generation from a Yb-doped fiber laser system with gain-narrowing compensation. The fiber laser system is similar to that in [8], but is optimized for a replaced fiber oscillator. The duration of the fundamental output pulse reconstructed from the measured spectrum and phase was 55 fs. The duration was shorter than that in [8]. The system also delivers pulses with a high repetition rate. Specifically, a 500 nm light source was obtained at a repetition rate of 3 MHz, spectral range of 515–538 nm, and pulse energy of 35 nJ. The duration of the second-harmonic output pulse reconstructed from the measured spectrum and phase was 41 fs. The post- and pre-pulse intensity was suppressed to less than 3% of the main pulse intensity. The output pulse duration was in agreement with the calculated duration of 40 fs for the ideal second harmonic generation from the fundamental output pulse. This shows that the almost ideal second harmonic generation was performed experimentally.

2. Method

Figure 1 shows a schematic of the laser system. In our previous work, a hundreds femtosecond solid-state oscillator followed by a white light generator was used [8]; however, here, a mode-locked Yb-doped fiber laser oscillator was used owing to its superior compactness, durability, and cleanness of the output spectrum compared to the previous seed. The oscillator delivered seed pulses at a repetition rate of 90 MHz. The cavity dispersion was compensated by a grating pair inside the cavity, and the polarization state of the mode-locked pulses was adjusted with three wave plates before the output coupler [9]. First, the generated pulses from the oscillator were stretched by passing them through the pulse stretcher to prevent damage to the optics in the amplifiers. In our previous work, we used a fiber-based stretcher [8]. However, here, we used a grating-based pulse stretcher to generate longer chirped pulses, which further prevented nonlinear effects in an amplifier chain.

 

Fig. 1 Schematic of laser system.

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Figure 2 shows a schematic of the pulse stretcher. The pulse stretcher consists of a transmission grating with 1000 grooves/mm (LightSmyth LSFSG-1000-3225-94), a roof mirror, and an improved Öffner-type telescope [10]. The telescope consists of a concave mirror with a radius curvature of 200 mm and a convex mirror with a radius curvature of 107 mm. The radius curvature and the position of the convex mirror were optimized to decrease the aberration of the telescope for the pulse stretcher [10]. The grating was used in a Littrow configuration to maximize the diffraction efficiency, with an incident angle of 31.3°. The diameter and thickness of the concave mirror were 2 in. and 0.375 in., respectively. The convex mirror has dimensions of 40 mm (W) × 3 mm (H) × 10 mm (D). The convex mirror should be thin to reduce the residual vertical and horizontal spectral distributions of the output beam. As the vertical positions of the beams over the convex mirror and those under the convex mirror become closer, the residual vertical and horizontal spectral distributions of the output beam become smaller [10]. For this purpose, the diameter of the input beam of the pulse stretcher was demagnified to less than 1 mm by the telescope, and the height difference between the highest beam and the lowest beam was 15 mm.

 

Fig. 2 Schematic of pulse stretcher.

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The pulse repetition of the seed chirped pulses was lowered to 3 MHz by an acousto-optic modulator to avoid accumulation of the excitation in the sample for ultrafast spectroscopy [11]. The stretched pulses were amplified in a preamplifier—a 1.6-m-long polarization-maintaining cladding-pumped Yb fiber with a core diameter of 10 μm. The preamplifier was pumped with a 976 nm diode. This step is responsible for spectrum narrowing. To compensate for this effect, we reduced the spectral components around the Yb gain bandwidth using a gain-narrowing compensation filter. The schematic and transmission characteristics of the filter were presented in a previous work [8]. The incident angle of the filter was designed to be 40° to be able to shift the peak wavelength of the attenuation to both longer and shorter wavelengths. The number of passes through the filter and the incident angle were optimized to minimize the duration of the fundamental output pulse from the laser system. The gain-narrowing compensation was followed by the use of a dispersion compensator to compensate for the dispersion of the fundamental output pulse from the laser system [8]. The pulses were amplified in a power amplifier—a 2-m-long polarization-maintaining cladding-pumped Yb fiber with a core diameter of 10 μm—that was pumped with a 976 nm diode.

The amplified chirped pulses were compressed to ultrafast pulses utilizing a pulse compressor consisting of two transmission gratings with 1000 grooves/mm (LightSmyth LSFSG-1000-3212-94). The incident angle of the grating was 31°. The optical length between the gratings was 120 mm. The spectral phase of the fundamental output pulse from this system was measured by two-dimensional spectral shearing interferometry (2DSI) [12]. The schematic of 2DSI for fundamental pulses is shown in our previous work [8]. The measured dispersion imposed by the fibers and other optical components is compensated using a dispersion compensator between the two amplifiers.

SHG was performed using a nonlinear crystal. Generally, beta barium borate (BBO) is used for SHG of ultrashort pulses due to the relatively low group delay dispersion. However, BBO has a relatively low acceptance angle, which when used for SHG with a strongly focused input beam to improve the SHG efficiency can result in an elliptical output beam shape. To avoid this phenomenon, we adopted lithium triborate (LBO) for SHG generation owing to its relatively wide acceptance angle compared with that of the BBO crystal.

We previously generated an ultrashort green pulse with a Fourier-transform-limit (FTL) pulse duration of 85 fs by utilizing a LBO crystal with a thickness of 2 mm. We also calculated the wavelength dependence of the SHG efficiency using Sellmeier equations of LBO [13]. Based on these results, we determined the optimal thickness of the LBO crystal. The thickness of the LBO crystal was 0.5 mm with type-I phase matching.

The spectral phase of the output pulse from this system was measured using 2DSI. Figure 3 shows the schematic of 2DSI for green pulses. The basic configuration of 2DSI for green pulses is the same as that for fundamental pulses. We adopted a pair of gratings with 2400 grooves/mm for chirped pulse generation. The number of grooves of the gratings is twice that used for fundamental pulses; hence, the dimensions of 2DSI for green pulses were the same as those for fundamental pulses for convenience of construction. A 0.02-mm-thick BBO crystal was used to mix the unchirped pulse with the two chirped pulses.

 

Fig. 3 Schematic of 2DSI for green pulses.

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The measured dispersion imposed by the LBO crystal and mirrors was compensated by a pair of fused silica prisms after the second harmonic generation.

3. Results

The spectrum output from the oscillator is shown in Fig. 4 (blue line). The FTL pulse duration calculated from this spectrum is 35 fs. The spectral bandwidth and pulse energy of the output pulse from the oscillator are 115 nm (edge to edge) and 0.7 nJ, respectively.

 

Fig. 4 Spectra of the oscillator (blue line), before (purple line) and after (green line) the gain-narrowing compensator.

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The pulse width is stretched to approximately 30 ps by the pulse stretcher. The preamplifier amplified the input pulses to 18 nJ. The spectral components of the Yb³⁺ gain bandwidth are well-amplified around 1030 nm, but longer-wavelength components are poorly amplified, and components shorter than 1030 nm are absorbed by Yb³⁺ [Fig. 4]. If the output pulse from the preamplifier is amplified without gain-narrowing compensation, the spectral components of the pulse are well-amplified around 1030 nm, but longer-wavelength components are poorly amplified. Therefore, the output pulse was passed through the gain-narrowing compensation filter four times, and the spectral peak around 1030 nm markedly decreased while the pulse energy was attenuated to 1.5 nJ [Fig. 4]. The chirped pulses passed through the dispersion compensator and were amplified to 203 nJ by the power amplifier. The compressed pulse energy decreased to 163 nJ with an 80% transmission efficiency of the grating compressor.

The output pulse duration was several hundred femtoseconds when the spatial light modulator did not work. Figure 5(a) shows the fundamental output spectrum from the entire system and the retrieved fundamental phases obtained using 2DSI after dispersion compensation using the spatial light modulator. The graph of the phase was almost flat between 1020 and 1085 nm. Figure 5(b) shows the reconstructed temporal shape and the FTL temporal shape of the fundamental output pulse. The pulse duration of the reconstructed (FTL) fundamental pulse is 55 fs (55 fs), and these temporal pulse shapes agree well with each other. This shows that the dispersion compensation of the fundamental pulses was effective. This also shows that the wings of the temporal shape were due to the spectrum profile, not to the residual dispersions, for example, the cubic phase.

 

Fig. 5 (a) Fundamental spectrum of the entire system (thick line) and phase with dispersion compensation (thin line), and (b) reconstructed temporal shape (solid line) and Fourier-transform-limit temporal shape (dotted line) of fundamental output pulse.

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Figure 6(a) shows the output spectrum for the entire system and the retrieved phases obtained using 2DSI after dispersion compensation by fused silica prisms. The graph of the phase was almost flat between 516 and 544 nm. The peak wavelength of the output spectrum agrees with the second-harmonic wavelength of the center of the fundamental spectra. This shows that the whole of the fundamental spectra contributed to the second harmonic generation. Figure 6(b) shows the reconstructed temporal shape and the FTL temporal shape of the output pulse. The pulse duration of the reconstructed (FTL) fundamental pulse is 41 fs (40 fs), and these temporal pulse shapes agree well with each other. We also performed the calculation of the ideal second harmonic generation from the fundamental output pulse using an ideal nonlinear crystal with no wavelength dependence of SHG efficiency and no group delay dispersion. The calculated pulse duration (40 fs) agreed with that of the experimental output pulse. Therefore, the almost ideal second harmonic generation was achieved experimentally. The post- and pre-pulse intensities were suppressed to less than 3% of the main pulse intensity. This shows that the dispersion compensation of the output pulses was also effective.

 

Fig. 6 (a) Output spectrum of the entire system (thick line) and phase with dispersion compensation (thin line), and (b) reconstructed temporal shape (solid line) and Fourier-transform-limit temporal shape (dotted line) of output pulse.

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4. Conclusion

In this paper, we demonstrated high pulse repetition, a broadband spectrum, and high energy of a green pulsed light source. This system consisted of a Yb-doped fiber oscillator and two Yb-doped fiber amplifiers. The gain-narrowing compensators were incorporated into a chirped pulse amplification system. We obtained 3-MHz pulse repetition, with a 515–538 nm spectrum, and a 35-nJ pulse energy, green light source. The almost ideal second harmonic generation was performed experimentally, and the reconstructed pulse duration was 41 fs.