Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser by sweeping the pulse repetition rate

www.nature.com/scientificreports OPEN received: 04 December 2014 accepted: 01 October 2015 Published: 27 October 2015 Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser by sweeping the pulse repetition rate Keunwoo Lee, Joohyung Lee†, Yoon-Soo Jang, Seongheum Han, Heesuk Jang, Young-Jin Kim‡ & Seung-Woo Kim Femtosecond lasers allow for simultaneous detection of multiple absorption lines of a specimen over a broad spectral range of infrared or visible light with a single spectroscopic measurement. Here, we present an 8-THz bandwidth, 0.5-GHz resolution scheme of Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser. A resolving power of 1.6 × 104 about a 1560-nm center wavelength is achieved by sweeping the pulse repetition rate of the light source on a fiber MachZehnder interferometer configured to capture interferograms with a 0.02-fs temporal sampling accuracy through a well-stabilized 60-m unbalance arm length. A dual-servo mechanism is realized by combining a mechanical linear stage with an electro-optic modulator (EOM) within the fiber laser cavity, enabling stable sweeping control of the pulse repetition rate over a 1.0-MHz scan range with 0.4-Hz steps with reference to the Rb clock. Experimental results demonstrate that the P-branch lines of the H13CN reference cell can be observed with a signal-to-noise ratio reaching 350 for the most intense line. Diverse femtosecond pulse lasers are available nowadays as broad spectral light sources suited for simultaneous spectroscopic detection of multiple absorption lines of a specimen1–5. Dispersive-type spectroscopy techniques can be adopted for such broad spectral sources, but special care is needed to prepare precision prisms or gratings that has to be tailored to cover the enlarged spectral bandwidth with a high resolving power6–9. On the other hand, Fourier-transform (FT) spectroscopy techniques enable flexible adjustment of the spectral bandwidth and the resolving power during the sampling process of the cross-correlation interferogram using a Michelson-type or equivalent two-arm interferometer10–14. Traditional FT spectroscopy techniques produce the interferogram mechanically by elongating the interferometer reference arm in scanning mode. Nonetheless, for the sake of high precision spectroscopy using femtosecond lasers, the reference arm scanning has to be performed with a sub-wavelength positioning accuracy over a long travel distance of up to several meters14. The burden of precise mechanical scanning can be relieved by combining a pair of femtosecond lasers of slightly different pulse repetition rates to form a multi-heterodyne light source15–22. This dual-comb FT spectroscopy technique permits acquisition of the interferogram by sampling the heterodyne beat-frequency signal down-converted to the radio-frequency regime. However, in order to achieve high precision spectroscopy without the frequency ambiguity or aliasing caused by the Nyquist sampling limit, both the combs have to be stabilized Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Science Town, Daejeon, 305-701, South Korea. †Present Address: Center for Space Optics, Korea Research Institute of Standards and Science (KRISS), Science Town, Daejeon, 305-340, South Korea. ‡Present Address: School of Mechanical and Aerospace Engineering, Nanyang Technological University (NTU), 639798, Singapore. Correspondence and requests for materials should be addressed to S.-W.K (email: ) Scientific Reports | 5:15726 | DOI: 10.1038/srep15726 1 www.nature.com/scientificreports/ in synchronization to each other20, or the heterodyne beat signal has to be monitored by combining a set of continuous-wave lasers of known wavelengths22. Repetition rate sweeping FT spectroscopy In this investigation, as an alternative method, FT spectroscopy is performed by sweeping the repetition rate (fr) of the femtosecond laser being used as the light source. This fr-sweeping method eliminates the interferometer arm scanning of conventional FT by employing a PZT micro-actuator so as to stretch the cavity length of the femtosecond laser23. In fact, the fr-sweeping method has already been successfully demonstrated not only for spectroscopy24 and lidar applications25 but also for long distance measurements26 and pulse duration estimation27,28. In principle, the interference overlap between pulses scales proportionally with the frequency range of fr-sweeping, but it is usually restrained to a few hundred Hz by the maximum elongation of the PZT micro-actuator. It is known that the interference overlap can be magnified significantly by providing a long unbalance length between the interferometer arms. However, such a long unbalance length is susceptible to the ambient temperature fluctuation which deteriorates the sampling accuracy and also the low signal-to-noise ratio in the resulting Fourier-transformed spectrum. Sampling errors may be corrected by incorporating a separate reference interferometer for elaborate tracing of the actual temporal variation of the unbalance length throughout the sampling process23. Such a posteriori error compensation requires excessive data recording and subsequently a large amount of computation to reconstruct the undistorted interferogram; besides these problems, quantifying the uncertainty of the utilized reference interferometer with traceability to a certified frequency standard remains an essential task. In our study, to cope with the problems of the previous attempts of fr-sweeping FT spectroscopy, special care was given to the real-time stabilization of the unbalance length by devising a dedicated length-monitoring interferometer. At the same time, the fr-sweeping was conducted by phase-locked control so that the FT sampling accuracy can be traceable to the Rb clock. In addition, emphasis was given to constructing a compact, robust FTS system demonstrating the effectiveness of femtosecond lasers as the light source for diverse spectroscopic applications over broad spectral bandwidths. Results Overall FT spectrometer system design. For this study, we intended to demonstrate an enhanced fr-sweeping scheme of FT spectroscopy using an Er-doped fiber femtosecond laser. First, our FT spectrometer system was designed to detect interferograms by configuring a two-arm fiber interferometer of Mach-Zehnder type with a 60-m unbalance arm length (Fig. 1). Second, sampling errors caused by the ambient disturbance of temperature and vibration were minimized by stabilizing the unbalance arm length within a fluctuation level of ~8 nm by embedding a homodyne length-locking control scheme inside the fiber interferometer (Fig. 2). Third, the required fr-sweeping was accomplished over a 1.0-MHz repetition rate range in 0.4-Hz steps with reference to the Rb clock. To accomplish this, a dual-servo control method was devised by combining a linear mechanical micro-stage and an el (...truncated)