Objective Optical frequency comb (OFC) is a coherent light source with excellent time-frequency stability. In frequency domain, an OFC is a set of frequency comb teeth with narrow linewidth and equidistant distribution. Because of this characteristic, the OFC has revolutionized the approach to molecular spectroscopy, making it possible to simultaneously and rapidly measure hundreds of molecular spectral lines with high resolution. The OFC spectroscopy technology has become a hot spot in the field of spectral technology, and has promoted the technological changes in the fields of optical coherence tomography, trace analysis and optical remote sensing. However, due to the limitation of the light source gain bandwidth, the comb spectrum is still unable to compare with the traditional Fourier transform spectroscopy technology in ultra-wideband spectrum measurement, which limits its applications in, such as chemical analysis and atmospheric environment monitoring. At present, supercontinuum (SC) generation technology is the main way to expand the spectral width of light sources. However, the strong nonlinear effect and phase modulation instability inside nonlinear fibers will not only cause strong amplitude oscillation and jitter of SC spectrum, but also introduce additional phase (frequency) noise, resulting in obvious decoherence. These problems seriously limit the application of SC generation technology in comb and spectrum measurement. Here, the generation of near infrared SC comb with high coherence and flat spectrum is investigated. The coherence is confirmed by beat note measurements and nonlinear Schr?dinger equation (NLSE) based numerical simulation. The source could be useful for broadband gas sensing in monitoring of electrical installations.
Methods First, an erbium-doped fiber comb is harnessed for producing a flat SC with spectral coverage of 1250--1900 nm in a high nonlinear dispersion shifted fiber(HNL-DSF). A home-made nonlinear-polarization-rotation (NPR) mode-locked fiber laser comb delivered a train of 650 fs pulses at a repetition rate of 54.5 MHz with 10 mW of average output power (Fig. 1). The output pulses are temporally chirped with a 2 m-long single mode fiber (SMF) and then amplified by a bidirectional pumped erbium-doped fiber amplifier (EDFA) with an output power up to ~100 mW. The EDFA also behaves as a fiber compressor, producing pulses of sub-100 fs which is directly injected into a 200 m-long HNL-DSF (Yofc) for SC generation with a flat spectral profile. The HNL-DSF had a zero dispersion wavelength around 1550 nm (dispersion slope of 0.03 ps/(nm 2·km); loss coefficient <1.5 dB/km; nonlinear coefficient >10 W -1·km -1). Second, beat note measurements between the original spectrum (and SC) and two stable continuous-wave (CW) lasers at 1550.5 nm or 1535.4 nm are performed for confirming the coherence of the SC. For investigating the coherence beyond the CW laser wavelengths, numerical simulation is performed, revealing pulse propagation inside the fiber, by solving NLSE using split step Fourier method. Wide-band first-order spectral coherence is confirmed. Finally, the SC is launched for spectral measurements of a mixed gas sample, containing HF (6666.1 Pa), 12CO (19998.3 Pa), 12C2H2 (6666.1 Pa), and the natural water vapor in the laboratory. The measurements are carried out with a home-made Fourier-transform spectrometer at a spectral resolution of 0.1 cm -1. The temperature is 297 K.
Results and Discussions In the experiment, a broadband SC comb, spectrally spanning from 1250--1900 nm (Fig. 2) with good flatness around 1400 nm and 1800 nm, is achieved in a piece of DSF. The high coherence of SC is verified by theoretical simulation and experimental measurements. Simulation results show that the SC comb had a high coherence, g(ω)~1, and low phase noise (<10 mrad) over a wide spectral range (Fig. 3). Experimentally, the beat notes between the 1550.5 nm/1535.4 nm laser and the comb and SC show linewidths at -3 dB of (2.7±1.0) kHz/(2.0±1.0) kHz and (2.6±1.0) kHz/(1.8±1.0) kHz, respectively (Fig. 4). No obvious linewidth broadening is found, indicating the coherence remain unchanged in the nonlinear spectral broadening process. The absorption peak of water vapor in the near infrared region is measured around 1365 nm (Fig. 5). Meanwhile, the transition lines for HF (around 1300 nm), 12CO (1580 nm), 12C2H2 (1560 nm) are measured simultaneously. For spectroscopic validation, the results of HF are compared with the simulation based on the parameters given by the HITRAN2012 database. The experimental results are consistent with the simulation results (deviation less than 1.6%, as shown in Fig. 6), indicating that the SC comb could be an excellent light source for multi-gas sensing as required by monitoring of electrical power equipment.
Conclusions In conclusion, ultra-broadband near-infrared SC generation is performed using a dispersion shifted high nonlinear fiber, pumped by 1.55 μm comb pulses. The excellent flatness and coherence of SC are investigated by numerical simulation and experimental beat-note measurements. As a result, high spectral coherence and low phase noise (<10 mrad) are demonstrated for a wide spectral coverage. Particularly, the beat note linewidths between the initial comb and SC with two CW lasers confirm that the phase noise induced by the DSF, to a certain extent, is negligible, comparing to the comb line spacing (54.5 MHz). Furthermore, based on the SC comb source, Fourier transformed frequency comb spectroscopy is carried out for measuring ultra-broadband and high-resolution molecular spectra of mixed gases (HF、 12C2H2、 12CO、H2O). The experiment data are consistent with that simulated in HITRAN2012 database with standard deviation of less than 1.6%. We believe that flat SC based frequency comb spectroscopy will provide a promising approach for multi-gas sensing with high precision and high resolution.
