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.

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.

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).

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).