Thanks to their unprecedented frequency accuracy and stability as low as
Photonics Research, Volume. 9, Issue 7, 1358(2021)
Broadband mid-infrared molecular spectroscopy based on passive coherent optical–optical modulated frequency combs Editors' Pick
Mid-infrared dual-comb spectroscopy is of great interest owing to the strong spectroscopic features of trace gases, biological molecules, and solid matter with higher resolution, accuracy, and acquisition speed. However, the prerequisite of achieving high coherence of optical sources with the use of bulk sophisticated control systems prevents their widespread use in field applications. Here we generate a highly mutually coherent dual mid-infrared comb spectrometer based on the optical–optical modulation of a continuous-wave (CW) interband or quantum cascade laser. Mutual coherence was passively achieved without post-data processes or active carrier envelope phase-locking processes. The center wavelength of the generated mid-infrared frequency combs can be flexibly tuned by adjusting the wavelength of the CW seeds. The parallel detection of multiple molecular species, including
1. INTRODUCTION
Thanks to their unprecedented frequency accuracy and stability as low as
Nowadays, the development of DCS aims at the extension of the wavelength region and the practical applications of DCS in field environments [5,30,31]. Among all the spectral domains, most molecules built the strong fundamental vibrational transitions in the mid-infrared region. However, limited by the gain media, mid-infrared sources are generated mainly based on nonlinear processes, such as difference frequency generation (DFG) sources [32], optical parametric oscillators [33,34], chip-scale microresonators [35], interband or quantum cascade lasers (ICLs or QCLs) [36,37], and supercontinuum broadened sources [38,39]. Many promising proof-of-principle experiments have demonstrated their intriguing potentials for the generation of mid-infrared DCS. The spectral elements involved in these schemes should be precisely controlled to obtain stabilized mid-infrared comb teeth, but often at the cost of complexity and the demand of a well-maintained laboratory environment. The current trend of achieving fieldable DCS is to design systems with built-in passive mutual coherence, which get rid of the control of a significant freedom, carrier-envelope phase (CEP). The intrapulse DFG or the DFG between two pulse trains sharing common near-infrared oscillators can directly generate mid-infrared DCS with zero-offset CEP [40,41]. Generally, the supercontinuum technique is often implemented to reach the required frequency range for the DFG process. Ycas
In this paper, we develop and demonstrate a broadband mid-infrared DCS based on its built-in passive mutual coherence. By the optical-optical modulation of a CW ICL or QCL, instead of using EOMs [45], the generated optical-optical modulated frequency combs (OMFCs) achieved the output spectral coverage over 400 nm in the mid-infrared domain, and the output power was synchronously amplified to 400 mW in the process. The robust mid-infrared sources have the potential for broadband spectroscopy in the lossy and fieldable environments. Besides, the spectral range of these sources can be tuned flexibly by replacing the operating wavelengths of mid-infrared CW lasers, even to the far-infrared region. The mutual coherence can be well maintained in the full spectral range. The measurements of multiple gas species, including
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2. PRINCIPLE
Commercial ICLs or QCLs have achieved beam emissions from 2.5 to 24 μm. Mid-infrared OFCs based on multimode operating ICLs or QCLs are investigated to achieve a future integrated comb sensor [35,48]. The number of comb teeth is generally limited to several hundreds, which hinders its applications for broadband and high-resolution spectroscopy. Here we extend the concept of OMFC, proposed in our previous work on near-infrared OMFCs [47] to the mid-infrared region. As shown in Fig. 1, the mid-infrared CW laser is pulsed and amplified by a near-infrared femtosecond pump source in the optical parametric amplification (OPA) process. In the time domain, the CW-seeded OPA process can be explained as the optical–optical modulation process of a CW signal. The modulation frequency is determined by the repetition rate
Figure 1.Schematic of an individual mid-infrared OMFC. A mid-infrared CW ICL/QCL is combined with a near-infrared femtosecond laser, whose repetition rate
3. RESULTS AND DISCUSSION
The experimental setup of the mid-infrared DCS is shown in Fig. 2(a). Two near-infrared Yb-doped femtosecond pump sources served as the pump source for the OPA processes, whose repetition rates (
Figure 2.Mode-resolved DCS spectra. (a) Schematic of the DCS setup. Two OMFCs were combined and then passed through a multipass gas cell. After spectral filtering, the heterodyne signal was detected by a balanced HgCdTe detector retrieved to an optical domain. BS, beam splitter; G, mid-infrared grating. (b) Typical detector signal with multiple interferograms. (c) Retrieved DCS spectrum. One hundred spectra, each with a recording time of
To evaluate our mid-infrared DCS, the tunable CW QCL was initially used as the seed, and its operating wavelength was set as 3.85 μm. The repetition rate offset of the pump sources was set to 600 Hz to achieve a single spectral measurement of
Figure 3(a) shows a typical optical spectrum retrieved from a single interferogram coherently averaged at
Figure 3.DCS spectra of a mixture of gases. (a) Optical spectrum retrieved from a single interferogram coherently averaged 240,000 times. (b) Comparison results of the extracted gas absorption lines (blue line) and the theoretical profiles from the HITRAN database (light grey curve for
Our mid-infrared DCS has the well spectral tunability by adjusting the PPLN periods or the operating wavelength of the common CW laser. As shown in Fig. 4, a spectral coverage ranging nearly from 3.3 to 4.0 μm was achieved by stitching the measured DCS spectra, each of which was averaged in a time of 100 s. Here the DCS spectra seeded by QCL were measured mainly by adjusting the operating wavelength of the CW laser at a step of
Figure 4.Tunable DCS spectra. (a) Measured spectra by scanning PPLN periods and adjusting the operating wavelength of the common CW laser. (b) Comparison between extracted gas absorption lines of (a) and the theoretical gas absorption profiles from the HITRAN database. The gaps are due to the electronical filter with a bandwidth of 2–48 MHz in the data acquisition processes. The weak spectral intensity and the low-frequency noises of the mid-infrared detector result in the deviations of the absorption intensity near the two gaps. (c) Portions of gas absorption lines of five gases.
Mid-infrared DCS can be used for highly sensitive molecular detection owing to the strong rovibrational absorption lines. Here we used our DCS to detect
Figure 5.DCS spectra of
In the experiment, the commercial CW ICL/QCL was kept relatively stable by controlling the operating current and temperature. The frequency fluctuations of the CW seeds result in the entire frequency shift of the mid-infrared OMFCs, which also mainly influences the frequency accuracy of our DCS. In this study, we used a mid-infrared wavelength meter (Bristol 771B, 0.75 pm accuracy) to measure the optical frequency of the CW seeds. The standard deviation of the frequency fluctuations of the QCL was
Figure 6.Line parameter measurements. (a) Portions of the gas absorption lines of the gas mixture of
4. CONCLUSION
We proposed and demonstrated a broad mid-infrared DCS based on the OMFC technique, passively referenced to a common commercial CW QCL or ICL. Mutual coherence was established without any fast feedback circuits or post-phase corrections, which is favorable for simplifying the architecture and improving the robustness of the DCS. The direct output spectral range reached several hundred nanometers with an average output power exceeding several hundred milliwatts (mW), which can be applied in nonlinear spectroscopy or lossy measurement environments. By introducing a PPLN waveguide, an octave-spanning spectrum is expected to be achieved. Furthermore, the scheme can be extended to the far-infrared region, whereas the OMFC concept has been demonstrated in the near-infrared domain [48]. On the other hand, the measurements of multiple gas species such as
APPENDIX A: REAL-TIME COHERENT AVERAGING
Real-time coherent averaging was performed based on an FPGA module. The data acquisition process was triggered by a sharp falling edge of the interferogram to remove the possible time jitter, which can be treated as an elementary phase correction [
Figure 7.(a) Comparison of results with and without the adaptive sampling method at a measurement time of 10 s. A 30 cm optical path cell is filled with 10%
APPENDIX B: MID-INFRARED FREQUENCY-AGILE DCS
To verify the frequency agility of our OMFCs, the original streams at different injected CW wavelengths as shown in Fig.?
Figure 8.Mode-resolved DCS spectra at different CW operating wavelengths. The red curves show the profiles computed from the HITRAN database using experimental parameters.
APPENDIX C: OCTAVE SPECTRUM OF OMFCS
Broadband mid-infrared DCS is of great importance for the parallel measurement of multiple gas species. The output spectral range of our OMFCs was limited by the phase-matching bandwidth of the fan-out grating PPLN. In this study, we achieved the spectral broadening of our OMFCs by introducing a cascade OPA process and combining the supercontinuum technique. As shown in Fig.?
Figure 9.(a) Schematic of the spectral broadening of the OMFC. The generated near-infrared idler after the OPA process, which was first spectrally broadened, served as the signal in the next OPA process. Broadband mid-infrared pulses were obtained when a chirped PPLN crystal was used in the OPA process. HWP, half-wave plate; PBS, polarizing beam splitter; DM, dichroic mirror; PPLN, periodically poled lithium niobate crystal; LP, long-pass filter; HNLF, highly nonlinear fiber; D, delay line; APPLN, aperiodically poled lithium niobate crystal; Ge, AR-coated germanium window. (b) Measured mid-infrared spectrum after the cascade OPA processes.
APPENDIX D: FREQUENCY ACCURACY OF OMFCs
The frequency accuracy of the common mid-infrared CW seeds directly determined the frequency accuracy of our DCS. Here we used a mid-infrared wavelength meter (Bristol 771B, 0.75?pm accuracy) to measure the optical frequency of the two CW seeds. The frequency accuracy of the wavelength meter is guaranteed by continuous calibration with a built-in He–Ne laser. To further evaluate the inherent accuracy of the wavelength meter, the optical frequency of a near-infrared CW diode (1560?nm, OEwaves, linewidth
Figure 10.Measured frequency stability of (a) near-infrared CW laser referenced to a fiber frequency comb, (b) mid-infrared CW ICL, and (c) QCL.
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Zhong Zuo, Chenglin Gu, Daowang Peng, Xing Zou, Yuanfeng Di, Lian Zhou, Daping Luo, Yang Liu, Wenxue Li, "Broadband mid-infrared molecular spectroscopy based on passive coherent optical–optical modulated frequency combs," Photonics Res. 9, 1358 (2021)
Category: Spectroscopy
Received: Feb. 9, 2021
Accepted: May. 6, 2021
Published Online: Jul. 5, 2021
The Author Email: Chenglin Gu (clgu@lps.ecnu.edu.cn), Wenxue Li (wxli@phy.ecnu.edu.cn)