Mid-infrared (mid-IR) laser sources are now becoming enabling tools for cutting-edge applications, including greenhouse gas sensing^[1,2], medical diagnosis^[3], and security and defense^[4]. Many molecules and molecular functional groups experience vibrational absorption in the mid-IR region. Indeed, spectroscopic applications would benefit from scaling the optical frequency comb to the mid-IR region, leading to numerous absorption lines being recorded with unprecedented accuracy and resolution^[5,6]. Moreover, ultrafast pulses with a stable carrier–envelope phase permit an exhaustive understanding of molecular structure and dynamics^[7,8], and they enable the soft X-ray generation to be scaled on a tabletop system^[9,10]. Over the last decade, steady progress has been made in the field of ultrafast mid-IR frequency comb generation. There is a wide array of innovative solutions to generate coherent mid-IR laser sources, with novel gain media^[11], quantum cascade lasers^[12] and micro-resonators^[13], and supercontinuum generation in waveguides and fibers^[14,15]. Compared with these approaches, detecting and controlling the offset frequency of the mid-IR sources is a challenge, which is the prerequisite for frequency comb spectroscopy. A more direct approach is to employ nonlinear frequency conversion of ultrashort pulses in the visible or near-infrared (NIR) regime to generate coherent mid-IR sources. Among the different nonlinear processes, difference frequency generation (DFG) with signal and pump from the same oscillator offers several advantages for a mid-IR frequency comb system, as it allows us to use compact and well-developed fiber laser technology, and also to achieve ultrashort pulses and intrinsic carrier–envelope phase stability, which reduce the complexity and improve the quality of the long-term performance^[16–19]. In DFG systems, the mid-IR spectrumd coverage depends on the NIR spectrum and the transmissivity of the crystal. Limited by the gain bandwidth of laser materials, the general NIR sources cannot directly emit pulses with such a broad spectrum. To achieve a broad mid-IR spectrum, highly nonlinear fiber (HNLF) is widely applied in DFG systems for NIR spectrum broadening. The oscillators split the output laser into two beams and nonlinearly broaden the NIR laser in HNLF^[16–20]. By phase matching, the signal and pump pulses are focused into a crystal to generate an idler pulse in the mid-IR region. The latest representative work demonstrated a high-power mid-IR femtosecond source with a broad spectrum from 1.6 to 10.2 μm by DFG^[21]. With such a broad spectrum, the high-power mid-IR femtosecond source will be a powerful tool for multispecies trace gas detection.

Benefitting from the development of dual-comb technology, high-speed comb-tooth-resolved broadband mid-IR dual-comb spectroscopy (DCS) has been presented^[6,22]. In a dual-comb system, a narrow linewidth of the comb teeth is expected for high-resolution DCS. The linewidth of the mid-IR comb generated by DFG is related to the phase noise of the NIR source. Compared with mid-IR combs, NIR combs are better at measuring and controlling phase noise. The noise of the mid-IR comb can be controlled by locking the repetition rate (f_r) of the NIR comb. Therefore, the low phase noise of two NIR sources is fundamental to achieve high-resolution DCS. Although various ultralow-noise NIR oscillators have been demonstrated^[23], excess phase noise from the NIR amplification and DFG process will lead to linewidth broadening of the mid-IR comb.

In this letter, we report on a mid-IR comb based on an NIR system. The NIR system includes an Yb:fiber comb and an Yb fiber chirped pulse amplifier (CPA) emits a pulse train with a 4 W average power and a 194 fs pulse duration. The NIR source is spilt into two beams as the signal and pump pulses of DFG. The spectrum of the signal pulse is nonlinearly broadened in a piece of HNLF with anomalous dispersion. The pump and signal pulses are focused into periodically poled lithium niobate (PPLN) for DFG. The mid-IR comb has a tunable spectrum from 2.7 to 4.0 μm and a maximum average power of 250 mW. To confirm the noise source, we measured the noise of each module and analyzed the noise source. This work has the potential to develop low-noise mid-IR combs, which may be applied in high-coherence DCS.

The NIR ultrafast pulse source is a home-made PM Yb-doped fiber laser based on a nonlinear amplifying loop mirror. By tuning the intracavity grating pair, the net dispersion of the cavity is controlled to near zero for low intrinsic phase noise^[24]. The laser emits 3 mW pulses with 30 nm full width at half maximum (FWHM) and 75 MHz repetition rate. As this pulse energy is insufficient for broad signal laser generation, CPA is employed to scale pulse energy. Before spectral broadening, the NIR pulse is coupled into a section of PM fiber for pulse duration stretching. After amplification, the seed spectral bandwidth is narrowed to less than 15 nm under the action of gain narrowing, as shown in Figure 2(a). The group delay and third-order dispersion accumulated in stretcher and fiber amplifiers are compensated by a grism pair. The detailed design and structure are described in Ref. [25]. At 4 W average power, the dechirped pulse has a temporal width of 152 fs as shown in Figure 2(b). The autocorrelation trace of the compressed pulses shows small distortions from the sech shape.

We optimize the conversion efficiency of quasi-phase matching by adjusting the optical delay line. The mid-IR comb is collimated by an uncoated CaF₂ lens with a focal length of 75 mm. A 3 mm thick Ge filter separates the mid-IR comb from the pump and signal lasers. The spectral, temporal, long-term frequency stabilization and phase noise performances were measured to characterize the mid-IR comb. The spectral coverage of the mid-IR comb was measured by a Fourier transform spectrometer as shown in Figure 3(b). The obtained tunable spectrum from 2.7 to 4.0 μm is obtained by tuning the poling period of the PPLN at fixed pump and signal laser power. The short-wavelength extension is blocked at 2.7 μm, which is limited by the longest signal wavelength generation from the PCF. A series of absorption lines in this spectrum are caused by water vapor in laboratory air (relative humidity ~45%, optical path length ~1 m). The maximum average output power is limited to 30 mW owing to a weak signal laser at 1.7 μm. Adjusting the PPLN channel to the poling period of 30.0 μm, the central wavelength can be tuned to 3.1 μm with an output power of 130 mW and an FWHM of 200 nm. With decreasing poling periods, a higher output power and a broader spectral bandwidth can be achieved. The maximum output power is 250 mW, which is obtained at a central wavelength of 3.7 μm with an FWHM of 300 nm. The longest wavelength we can achieve is limited by the poling period (28.0 μm) and increasing absorption of the crystal at long wavelength (>1 cm^–1 when the wavelength >4.0 μm).

Figure 5(b) shows the relative intensity noise (RIN) of the NIR comb, CPA, and mid-IR comb: the integrated RINs from 10 MHz to 1 Hz are 0.0076%, 0.1351%, and 3.9980%, respectively. It is demonstrated that saturation amplification will lead to an attenuation in the frequency range of 1 Hz–10 kHz^[28]. The RIN is not attenuated at low frequency in this CPA operating at saturation amplification. Therefore, the environmental perturbations (1–30 Hz) and pump noise (30 Hz–10 kHz) are the dominant noise sources, which cause a strong increase of RIN and offset the attenuation caused by saturation amplification. ASE and shot noise are the main noise sources at high frequency (10 kHz–10 MHz). After DFG, the obtained RIN of the mid-IR comb expresses a serious degradation with an integrated RIN of 3.9980%. The intensity noise of the input pulse will be amplified in the supercontinuum generation, the spectral broadening of which is sensitive to the peak power of the initial pulse^[29,30]. The intensity noise caused by unstable supercontinuum generation at low frequency is transmitted from the signal pulse to the mid-IR pulse. Moreover, environmental perturbations lead to a random time delay between the signal and pump pulses, which results in a random power instability to the mid-IR pulse. Silva de Oliveira et al.^[31] demonstrated that RIN is related to the temporal overlap in a DFG process, and the lowest RIN is obtained when the pump and signal pulses have no delay. Therefore, the mid-IR comb with free-running pump–signal delay has higher environmental noise in the low-frequency region. Therefore, the excess RIN at 1 Hz–1 kHz is related to the supercontinuum generation and environmental perturbations. The ‘flat’ RIN curve at 1 kHz–10 MHz is related to the quantum noise sources including broadband shot noise and the spontaneous Raman scattering^[29].

Therefore, the phase noise sources in this system are the environmental noise at low frequency and ASE-induced quantum noise at high frequency from the amplifier chain and nonlinear spectral broadening. The RIN sources are the environmental noise, pump noise of the amplifier, unstable supercontinuum generation, and quantum noise. For further applications in high-coherence DCS, the feedback control system, environmental isolation, power fluctuations of the pump laser, ASE suppression, and spectrum broadening need to be improved to reduce the phase noise of the mid-IR comb.

A mid-IR comb with tunable spectrum from 2.7 to 4.0 μm has been achieved by DFG. The maximum average power is 250 mW, which has been obtained at a central wavelength of 3.7 μm with an FWHM of 300 nm. By controlling the frequency and optimizing this system structure, this mid-IR comb system exhibits long-term frequency stability. The phase noise source has been analyzed by comparing the noise variation of Yb:fiber comb, CPA module, and mid-IR comb. Except for the intrinsic phase noise of the NIR comb, environmental noise at low frequency and quantum noise at high frequency from the amplifier chain and nonlinear spectral broadening are the additional noise source. Further measures, which consist of a precise feedback control system, good environmental isolation, suppression of the ASE, and so forth, will be taken to develop low-noise mid-IR combs in our future work for high-resolution spectroscopy in the mid-IR region.