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Carbon nanotube mode-locking Cr: YAG laser;All solid state microstructure fiber
Source :China laser   Time :2017-6-20

       Efficient solution for 1.5 μm, high repetition rate femtosecond light source – carbon nanotube mode-locked Cr:YAG laser


       High-average power, ultra-high repetition frequency (> 500 MHz) femtosecond light source has broad application prospects in the fields of nonlinear spectroscopy and material processing. Such a femtosecond light source with high pulse peak power and high repetition rate can greatly increase the number of pulses per unit time, improve the signal-to-noise ratio when used in the field of nonlinear spectroscopy, and greatly improve the processing yield in the field of material processing.


       However, developing a compact femtosecond light source with high average power and high repetition rate is not easy. They require a more delicate balance between the nonlinearity of the gain medium, the dispersion in the cavity, and the characteristics of the saturable absorber compared to low repetition rate femtosecond sources. For a femtosecond laser with a Cr:YAG laser crystal as the gain medium, the laser crystal has a small stimulated emission cross section and a low thermal conductivity, so the laser is very sensitive to gain and loss in the cavity, which further requires More precise control of the parameters of the saturable absorber


       Recently, Prof. Fabian Rotermund from KAIST (Korean Institute of Science and Technology) and Prof. Guang-Hoon Kim from KERI (Korea Electric Research Institute) demonstrated a compact version of carbon nanotube (CNT) mode-locking with a repetition rate greater than 500 MHz. Femtosecond Cr: YAG laser. They prepared CNTs with a 0.51% modulation depth and a 28 μJ/cm2 saturation energy density around a 1.5 μm wavelength. Using this CNT as a saturable absorption mirror, a mode-locked femtosecond Cr:YAG laser that can stably operate at a wavelength of 1.5 μm is realized. According to the research team, compared to other CNT-mode-locked Cr:YAG lasers, the laser currently has the highest repetition rate, with an output power of 147 mW and a pulse width of 110 fs. The results of this study are published in Chinese Optics Letters, Vol. 16, No. 6 (J. W. Kim et al., "550-MHz carbon nanotube mode-locked femsecond Cr: YAG laser." Vol. 16, No. 6, 061404, 2018).



       Figure 1. Schematic and physical photograph of a high repetition rate femtosecond Cr:YAG laser with carbon nanotube mode-locking. The compact laser operates near a 1.5 μm wavelength with a repetition rate of 550 MHz, an output power of 147 mW and a pulse width of 110 fs.


       Fabian Rotermund pointed out: "The CNT-mode-locked 1.5 μm wavelength femtosecond coherent light source is compact and can provide ultra-short pulses with high average power and high repetition rate, providing an efficient solution for various material processing and nonlinear spectroscopy. The next step will be to develop ultra-compact solid-state lasers based on CNT-mode-locked, repetitive frequency GHz.


       Normally dispersive all-solid-state microstructured fiber that can be used to produce broadband, high-flat, high-coherence supercontinuum


       The supercontinuum generated in the fiber contains a variety of dispersion effects and nonlinear phenomena, and has characteristics such as wideband and high brightness, and can be applied to many fields, such as ultrashort pulse generation and optical frequency measurement. In some applications, high flatness and coherence are important.


       Generally, a supercontinuum spectrum is produced when the pumping pulse wavelength is in the anomalous dispersion region of the fiber. Due to the effects of soli splitting and modulation instability, the resulting supercontinuum has poor coherence and the spectrum has more fine structures. When the pumping pulse wavelength is in the normal dispersion region of the fiber, the supercontinuum generation mainly depends on the self-phase modulation and the light wave splitting effect, and they can maintain the integrity and coherence of the pumping pulse. However, producing a high flatness supercontinuum under wide pulse pumping conditions is a significant challenge.


       The research team of the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences has proposed an all-solid-state microstructured fiber. The structure of the fiber contains only regular hexagonal elements, and the optimized fiber has a small absolute and flat normal dispersion characteristic in the wavelength range around the wavelength of 1.55 μm. By numerical simulation, the research team used a pulse with a width of 200 fs, a peak power of 100 kW, and a center wavelength of 1.55 μm to pump the fiber, which can produce a super-continuous spectrum with high flatness and high coherence. In the -3 dB intensity range, the supercontinuum covers a wavelength of 1030 to 2030 nm, which is close to an octave. In theory, the supercontinuum pulse can be compressed to 13.9 fs (approximately 2.69 optical periods) by linear chirp compensation alone. The related results are published in Photonics Research, 2018, Vol. 6 (C. Huang, et al. Ultraflat, broadband, and highly coherent supercontinuum generation in all-solid microstructured optical fibers with all-normal dispersion).



       Left: Schematic diagram of the designed fiber structure; right: from top to bottom is the spectrum of the transmission lengths at 0, 5 and 100 cm when the pumping pulses are transmitted in the fiber. The parameters of the pump pulse are: pulse width 200 fs, peak power 100 kW, center wavelength 1550 nm.


       Compared with the reported fiber, the fiber can meet the needs of wider pulse pumping to produce broadband high flatness and high coherence supercontinuum. The team's researchers believe that the -3 dB intensity of the spectrum corresponds almost to the full width at half maximum of the spectrum, which is of great significance in applications; in addition, the use of this fiber as a nonlinear medium is expected to be achieved in the case of wide pulse pumping. Obtain an all-fiber broadband high-flat, high-coherence supercontinuum source. The next step is to achieve a supercontinuum source and ultrashort pulse compression for all fibers.