Carbon nanotube mode-locked Cr:YAG laser: High Efficient solution for high-repetition-rate femtosecond source near 1.5 μm
A schematic and photograph of carbon-nanotube mode-locked high-repetition-rate femtosecond Cr:YAG laser. The compact Cr:YAG laser delivers 147 mW output power and 110 fs pulses at 550 MHz repetition rate near 1.5 μm.
The femtosecond (fs) laser source with high average powers and high repetition rate (above 500 MHz) is very attractive for the applications like nonlinear spectroscopy and material processing. The femtosecond laser source with high pulse peak power and high repetition rate can increase pulse number in unit time. Both the signal to noise ratio in nonlinear spectroscopy and production of material processing can be substantially improved.
Compared with the low-repetition-rate femtosecond laser source, the road to high-average-power and high-repetition-rate version becomes tough, as it requires more precise balance between nonlinearity of the gain medium, intracavity dispersion and the performance of saturable absorber. In particular, as small gain and low thermal conductivity of laser crystal, the Cr:YAG laser is quite sensitive to intracavity gain and loss, and the precise control to the parameters of saturable absorber is unavoidable.
Recently, the researchers from the groups of Prof. Fabian Rotermund (KAIST, Korea) and Prof. Guang-Hoon Kim (KERI, Korea) demonstrated a carbon nanotube mode-locked compact femtosecond Cr:YAG laser with high repetition rate (>500 MHz). They fabricated a carbon nanotube (CNT) possessing 0.51% modulation depth and 28 μJ/cm2 saturation fluence near 1.5 μm. By employing the CNT as saturable absorber mirror (CNT-SAM), they realized the 1.5-μm stable mode-locked operation of Cr:YAG laser. To the best of the groups’ knowledge, the laser provides 147 mW output power and 110 fs pulses with the highest repetition rate among CNT mode-locked Cr:YAG lasers. These results are reported in Chinese Optics Letters (J. W. Kim et al., 550-MHz carbon nanotube mode-locked femtosecond Cr:YAG laser. Vol. 16, No. 6, 061404，2018).
"A compact 1.5 μm femtosecond coherent source which delivers high average output power, short pulse duration and high repetition rate simultaneously can provide an efficient solution for a variety of applications in material processing and nonlinear spectroscopy." says Fabian Rotermund.
Further work will be focused on the development of multi-GHz ultra-compact solid-state lasers, mode-locked by carbon nanostructures in various wavelength ranges.
COL封面故事：1.5 μm、高重复频率飞秒光源的高效解决方案——碳纳米管锁模Cr:YAG激光器
碳纳米管锁模的高重复频率飞秒Cr:YAG激光器原理图和实物照片。该紧凑型激光器工作在1.5 μm波长附近，重复频率为550 MHz，输出功率为147 mW，脉冲宽度为110 fs。
高平均功率、超高重复频率（> 500 MHz）的飞秒光源在非线性光谱和材料加工等领域有着广阔的应用前景。此类具有高脉冲峰值功率和高重复频率的飞秒光源可极大地增加单位时间内的脉冲数量，用于非线性光谱领域时可提高信噪比，在材料处理领域也可以极大地提升加工产量。
然而开发具有高平均功率、高重复频率的紧凑型飞秒光源并不容易。与低重复频率的飞秒光源相比，它们需要在增益介质的非线性、腔内色散和可饱和吸收体的特性之间取得更精妙的平衡。对于以Cr:YAG激光晶体为增益介质的飞秒激光器来说，该激光晶体受激发射截面较小、热导率较低，因此该激光器对腔内的增益和损耗非常敏感，这进一步要求对可饱和吸收体的参数进行更精确的控制。
最近，来自KAIST（韩国科学技术院）的Fabian Rotermund教授和KERI（韩国电气研究院）的Guang-Hoon Kim教授合作研究团队展示了重复频率大于500 MHz的碳纳米管（CNT）锁模的紧凑型飞秒Cr:YAG激光器。他们制备了在1.5 μm波长附近具有0.51％调制深度和28 μJ/cm2饱和能量密度的CNT。利用这种CNT作为饱和吸收镜，实现了可稳定工作在1.5 μm波长的锁模飞秒Cr：YAG激光器。据该研究团队所知，与其他的CNT锁模Cr:YAG激光器相比，该激光器目前具有最高的重复频率，输出功率为147 mW，脉冲宽度为110 fs。该研究结果发表在Chinese Optics Letters第16卷第6期 (J. W. Kim et al., 550-MHz carbon nanotube mode-locked femsecond Cr:YAG laser. Vol. 16, No. 6, 061404，2018)。
Fabian Rotermund指出：“基于CNT锁模的1.5 μm波长飞秒相干光源结构紧凑，能够同时提供高平均功率、高重复频率的超短脉冲，可为各种材料处理和非线性光谱提供高效的解决方案。”
下一步的工作将集中在开发基于CNT锁模、重复频率达数GHz的超紧凑型固体激光器。