High Power Laser Science and Engineering, Volume. 8, Issue 4, 04000e32(2020)

Mid-infrared optical frequency comb in the 2.7–4.0 μm range via difference frequency generation from a compact laser system

Lian Zhou1, Yang Liu1、*, Gehui Xie1, Chenglin Gu1, Zejiang Deng1, Zhiwei Zhu1, Cheng Ouyang1, Zhong Zuo1, Daping Luo1, Bin Wu3, Kunfeng Chen3, and Wenxue Li1,2、*
Author Affiliations
  • 1State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai200062, China
  • 2Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan030006, China
  • 3Science and Technology on Electronic Test & Measurement Laboratory, The 41st Research Institute of CETC, Qingdao266000, China
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    We report on the generation of a mid-infrared (mid-IR) frequency comb with a maximum average output power of 250 mW and tunability in the 2.7–4.0 μm region. The approach is based on a single-stage difference frequency generation (DFG) starting from a compact Yb-doped fiber laser system. The repetition rate of the near-infrared (NIR) comb is locked at 75 MHz. The phase noise of the repetition rate in the offset-free mid-IR comb system is measured and analyzed. Except for the intrinsic of NIR comb, environmental noise at low frequency and quantum noise at high frequency from the amplifier chain and nonlinear spectral broadening are the main noise sources of broadening the linewidth of comb teeth, which limits the precision of mid-IR dual-comb spectroscopy.

    Keywords

    1 Introduction

    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[1619]. 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[1620]. 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 (fr) 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.

    2 Experimental setup

    Schematic of the mid-IR comb. The mode-locked fiber oscillator serves as an NIR comb whose repetition rate is locked at 75 MHz. The CPA with two-stage fiber amplifiers scales the average power to 6.7 W. After the compressor, the system emits a pulse train with an average power of 4 W and a pulse duration of 194 fs, which corresponds to a pulse energy of 53 nJ. In the DFG module, the mid-IR pulse laser is generated in the PPLN by quasi-phase matching. SM LD, single-mode laser diode; YDF, Yb-doped fiber; WDM, wavelength division multiplexer; Col, collimator; FR, Faraday rotator; PBS, polarization beam splitter; ISO, isolator; MM LD, multimode laser diode; DC-YDF, double-cladding Yb-doped fiber; PCF, photonic crystal fiber; DM, dichroic mirror; PPLN, periodically poled lithium niobate.

    Figure 1.Schematic of the mid-IR comb. The mode-locked fiber oscillator serves as an NIR comb whose repetition rate is locked at 75 MHz. The CPA with two-stage fiber amplifiers scales the average power to 6.7 W. After the compressor, the system emits a pulse train with an average power of 4 W and a pulse duration of 194 fs, which corresponds to a pulse energy of 53 nJ. In the DFG module, the mid-IR pulse laser is generated in the PPLN by quasi-phase matching. SM LD, single-mode laser diode; YDF, Yb-doped fiber; WDM, wavelength division multiplexer; Col, collimator; FR, Faraday rotator; PBS, polarization beam splitter; ISO, isolator; MM LD, multimode laser diode; DC-YDF, double-cladding Yb-doped fiber; PCF, photonic crystal fiber; DM, dichroic mirror; PPLN, periodically poled lithium niobate.

    Characterization of the chirped pulse amplification. (a) Normalized optical spectrum of NIR oscillator (blue curve) and amplified pulse (green line), centered at 1030 and 1038 nm with a spectral width of 30 and 12 nm, respectively. (b) Measured autocorrelation trace (blue line) of the amplified pulse with corresponding sech fitting (dotted green line).

    Figure 2.Characterization of the chirped pulse amplification. (a) Normalized optical spectrum of NIR oscillator (blue curve) and amplified pulse (green line), centered at 1030 and 1038 nm with a spectral width of 30 and 12 nm, respectively. (b) Measured autocorrelation trace (blue line) of the amplified pulse with corresponding sech fitting (dotted green line).

    3 Results and discussion

    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.

    (a) The spectrum of the broadened signal laser after a long-pass filter at 1100 nm. (b) The spectrum and corresponding average power of the mid-IR comb. The mid-IR comb has a tunable coverage of 2.7–4.0 μm. The average powers are 30, 130, 190, 240, 250, and 187 mW centered at 2.7, 3.0, 3.3, 3.5, 3.7, and 4.0 μm, respectively.

    Figure 3.(a) The spectrum of the broadened signal laser after a long-pass filter at 1100 nm. (b) The spectrum and corresponding average power of the mid-IR comb. The mid-IR comb has a tunable coverage of 2.7–4.0 μm. The average powers are 30, 130, 190, 240, 250, and 187 mW centered at 2.7, 3.0, 3.3, 3.5, 3.7, and 4.0 μm, respectively.

    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).

    The autocorrelation of mid-IR pulse at 3.5 μm. The pulse duration is 174 fs with Gaussian fitting.

    Figure 4.The autocorrelation of mid-IR pulse at 3.5 μm. The pulse duration is 174 fs with Gaussian fitting.

    (a) Phase noise PSD and (b) relative intensity noise (RIN) of the repetition rate signal corresponding to NIR comb (origin), CPA (green), and mid-IR comb at 3.5 μm (blue).

    Figure 5.(a) Phase noise PSD and (b) relative intensity noise (RIN) of the repetition rate signal corresponding to NIR comb (origin), CPA (green), and mid-IR comb at 3.5 μm (blue).

    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) The measured repetition rate stability of NIR comb (blue) and mid-IR comb (orange) for 3 h. (b) The Allan variance of the NIR comb (blue) and mid-IR comb (orange).

    Figure 6.(a) The measured repetition rate stability of NIR comb (blue) and mid-IR comb (orange) for 3 h. (b) The Allan variance of the NIR comb (blue) and mid-IR comb (orange).

    4 Conclusions

    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.

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    Lian Zhou, Yang Liu, Gehui Xie, Chenglin Gu, Zejiang Deng, Zhiwei Zhu, Cheng Ouyang, Zhong Zuo, Daping Luo, Bin Wu, Kunfeng Chen, Wenxue Li. Mid-infrared optical frequency comb in the 2.7–4.0 μm range via difference frequency generation from a compact laser system[J]. High Power Laser Science and Engineering, 2020, 8(4): 04000e32

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    Paper Information

    Category: Letters

    Received: Jun. 4, 2020

    Accepted: Aug. 5, 2020

    Published Online: Oct. 9, 2020

    The Author Email: Yang Liu (yliu@lps.ecnu.edu.cn), Wenxue Li (wxli@phy.ecnu.edu.cn)

    DOI:10.1017/hpl.2020.32

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