22-W,240-fs High-Power Mid-Infrared Femtosecond Optical Frequency Comb
Objective The femtosecond optical frequency comb(FOFC)comprises a series of ultra-short laser pulses with the same temporal separation in the time domain and discrete,equidistant,and stable phase-related frequency components in the frequency domain.The FOFC can accurately measure the absolute frequency of an atomic clock and serve as a natural time-frequency reference.Currently,the most stable and compact light source is the mode-locked erbium-doped fiber laser with a central wavelength of 1.55 μm,typically employing highly nonlinear fibers to broaden the spectrum across the entire transparent range of silica fiber(350-2400 nm).However,the output power of the erbium-doped fiber FOFC is generally in the range of a few hundred milliwatts.Therefore,increasing the output power of the FOFC remains a crucial challenge.The mid-infrared FOFC holds significant application value in next-generation spectroscopy,as it can be used to detect gases such as carbon dioxide and ammonia and extend the FOFC wavelength to the molecular fingerprint spectrum range(3-20 μm)through nonlinear crystals.This spectrum range is vital for chemical composition analysis,making the development of high-power mid-infrared FOFCs a pressing need.Methods This system comprises an erbium-doped fiber FOFC,a super-continuum converter,a double-cladding thulium-doped fiber amplifier system,and a transmission diffraction grating pulse compressor.Initially,the erbium-doped fiber FOFC utilizes a highly nonlinear fiber with normal dispersion for frequency broadening.Additionally,a self-pump amplifier composed of thulium-doped fiber generates a femtosecond seed with a central wavelength of 1925 nm.This seed is injected into a chirped pulse amplification system comprising a 55 m long highly nonlinear fiber with normal dispersion,a three-stage thulium-doped fiber amplifier,and a transmission diffraction grating pulse compressor.To characterize the noise of the high-power mid-infrared FOFC,we analyze the relative intensity noise and the phase noise of the pulse train using a signal source analyzer.Moreover,we co-couple the super-continuum laser generated by the high-power mid-infrared FOFC in the fluorotellurite fiber with a 1064 nm iodine-stabilized Nd∶YAG laser to detect the beat signal and verify the performance of the high-power mid-infrared FOFC.Results and Discussions The 1.55 μm femtosecond laser output from the erbium-doped fiber femtosecond optical frequency comb is symmetrically broadened to the spectral range of 1100-2200 nm by the highly nonlinear fiber with normal dispersion(Fig.2).The resultant super-continuum laser is injected into the self-pump pre-amplifier to obtain a femtosecond seed with a central wavelength of 1925 nm and an average power of 50 mW[as indicated by the dashed line in Fig.3(a)].This seed is then broadened to hundreds of picoseconds through the normal dispersion fiber and amplified by the three-stage double-cladding thulium-doped fiber amplifier to yield a picosecond pulse with a central wavelength of 2000 nm and an average power of 36.07 W.After compression,a femtosecond pulse with an average power of 22.72 W and a pulse width of 240 fs is obtained[Fig.3(b)].The integral values of relative intensity noise and timing jitter are 1.16％and 472 fs,respectively(integral range of 10 Hz-1 MHz)(Figs.4 and 5).The super-continuum laser(Fig.6)generated by the high-power mid-infrared FOFC and the 1064 nm laser produce a beat signal with a signal-to-noise ratio of 40 dB,meeting the counting requirements of the counter(Fig.8).Conclusions We demonstrate a high-power FOFC based on an erbium-doped FOFC,generating a 2 μm femtosecond seed through a highly nonlinear fiber with normal dispersion and self-pump pre-amplifier.The highly nonlinear optical fiber with normal dispersion effectively overcomes noise sensitivity issues associated with nonlinear dynamics of abnormal dispersion,such as soliton self-frequency shift and Raman soliton,during super-continuum generation.The femtosecond pulse,obtained with an average power of 22.72 W and a pulse width of 240 fs,marks a significant advancement in developing high-power mid-infrared FOFCs.This development contributes to the spectroscopic analysis of molecular structures and dynamics and facilitates the expansion of optical frequency combs into the molecular fingerprint spectrum range(3-20 μm).
nonlinear opticsoptical frequency combfemtosecond laserchirped pulse amplificationmid-infrared band
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