In this paper, we report a scheme providing precise spectral analysis and surface imaging, simultaneously, based on a high-coherence dual-comb interferometer. With two tightly phase-locking frequency combs, we demonstrate a high-coherence dual-comb interferometer (DCI) covering 188 to 195 THz (1538.5 to 1595.7 nm) with comb-tooth resolution and a max spectral signal-to-noise ratio (SNR) of 159.7. The combination of the high-coherence dual-comb spectrometer and a reference arm simultaneously enables gas absorption spectroscopy and for the absolute distance information to be obtained in one measurement. As a demonstration, we measure the spectrum of CO2 and CO. From the same interferograms, we demonstrate that distance measurement, by time-of-flight (TOF), can be resolved with an rms precision of 0.53 μm after averaging 140 images and a measurement time of 1 s. Finally, we demonstrate that non-contact surface imaging, using 2D mechanical scanning, reaches lateral resolution of 40 μm. The longitudinal precision is 0.68 μm with a measurement time of 0.5 s. It verifies that DCS has the potential to be applied in standoff detection, environmental pollution monitors, and remote sensing.
Dual-comb spectroscopy (DCS) is a powerful spectroscopic technique, which is developing for the detection of transient species in reaction kinetics on a short time scale. Conventionally, the simultaneous determination of multiple species is limited to the requirement of broadband spectral measurement at the cost of the measurement speed and spectral resolution owing to the inherent trade-off among these characteristics in DCS. In this study, a high-speed multi-molecular sensing is demonstrated and achieved through using a programmable spectrum-encoded DCS technique, where multiple narrow encoding spectral bands are reserved selectively and other comb lines are filtered out. As a dual-comb spectrometer with a repetition rate of 108 MHz is encoded spectrally over a spectral coverage range of 1520 to 1580 nm, the measurement speed is increased 6.15 times and single-shot absorption spectra of multiple molecules (C 2 H 2 , HCN, CO, CO 2 ) at a time scale of 208 µs are obtained. Compared to conventional single-shot dual-comb spectra, encoded dual-comb spectra have improved short-term signal-to-noise ratios (SNRs) by factors of 3.65 with four encoding bands and 5.68 with two encoding bands. Furthermore, a fiber-Bragg-grating-based encoded DCS is demonstrated, which reaches 17.1 times higher average SNR than that of the unencoded DCS. This spectrum-encoded technique can largely improve the DCS measurement speed, and thus is promising for use in studies on multi-species reaction kinetics.
We demonstrate a nonlinear polarization evolution (NPE) mode-locked polarization maintaining (PM) Yb-doped fiber laser with short NPE section segments by setting proper splicing angle. With a theoretical analysis, we propose that an appropriate deviation splicing angle exists to maximize the adjustable range of transmission modulation. The simulation results are highly consistent with theoretical conclusions. Experimentally, using the optimal splicing angle predicted by the theoretical calculation, we have achieved an environmentally stable mode-locking fiber laser at 111-MHz repetition rate with corresponding pulse energy of 0.47 nJ. Additionally, the noise performance of this PM fiber laser is characterized. The measured RMS timing jitter and amplitude noise are 6.41 fs and 0.0052% respectively (1 kHz-10 MHz), which are competitive to the low phase noise performance of the typical fiber laser.
In recent years, dual repetition-rate mode-locked lasers with slightly different pulse repetition rates, as newly developed ultrafast lasers, have attracted great interest and shown their applications in ultrafast dual-comb spectroscopy, asynchronous optical sampling without mechanical movement, etc. The traditional dual-comb system composed of a pair of independent optical frequency combs with slightly detuned comb spacing is still considered expensive, complex and fragile. It is imperative to develop practical and compact dual-comb devices. Dual repetition-rate ultrafast lasers generating asynchronous ultrafast pulses directly from a single cavity can be a promising alternative to the current dual-laser-based comb source. A dual-comb setup based on single laser has the advantages of compact structure, low cost and intrinsic mutual coherence. This technique paves the way for developing the compact, robust and environmental-immune dual-comb systems. In this paper we develop an alternative dual repetition-rate mode-locked Yb:YAG ceramic laser that emits a pair of pulses with spatially separated beams from a single cavity by using a semiconductor saturable absorber mirror and a dual-path pump configuration. In our experiment, a high quality transparent Yb:YAG ceramic prepared by non-aqueous taper-casting method is selected as the gain medium, which is pumped by a 940 nm laser diode. A dual-path pump configuration consisting of a pair of polarization beam splitters and a pair of half-wave plates is designed, in which total pump power from a laser diode is divided equally for pumping the two separate laser beams. When the total absorbed pump power is 5.6 W, dual repetition-rate continuous mode-locked laser operation is achieved under the gain-loss balanced cavity condition. The pulse repetition rates of Pulse1 and Pulse2 are 448.918 MHz and 448.923 MHz, respectively. The difference between repetition rates is 5 kHz mainly caused by the different optical path lengths in the cavity. Under an absorbed pump power of 7 W, the maximum total output power extracted from this laser reaches 170 mW, i.e., 89 mW for Pulse1 and 81 mW for Pulse2. The two mode-locked pulses have nearly identical spectral shapes centered at 1029.6 nm and 1029.8 nm, respectively. The spectral bandwidths for Pulse1 and Pulse2 are 1 nm and 1.16 nm, respectively. The corresponding pulse durations are 2.8 ps and 2.6 ps for the Pulse1 and Pulse2 respectively. Our scheme integrates the advantages of self-starting operation, high repetition-rate, suppression of gain competition. These results indicate that dual-path pump configuration is feasible for dual-repetition-rate mode-locked lasers. These co-generated, dual repetition-rate pulses from one laser cavity possess similar laser characteristics and can be operated independently by dual-path pump configuration. This laser has potential advantages of compact, cost-effective and high-stability for single-cavity-based dual-comb applications in dual-comb spectroscopy, distance ranging, etc.
Abstract Dual-comb spectroscopy is a powerful spectroscopic tool with ultrahigh-resolution, high-sensitivity properties, which opens up opportunities for the parallel detection of multi-species molecules. However, in its conventional form, highly stable laser combs with sophisticated control systems are required to perform dual-comb spectroscopy. Here, a passive mutually coherent dual-comb spectroscopy system via an optical-optical modulation method is addressed, where all fast phase-locking electronics are retired. Without post computer-based phase-correction, a high degree of mutual coherence between the two combs with a relative comb-tooth linewidth of 10 mHz is achieved, corresponding to a coherent time of 100 s. To demonstrate the performance and versatility of the system, the dual comb spectrometer is applied to record the mode-resolved single molecular spectra as well as parallel detected spectra of mixed gases including CO 2 , CO and C 2 H 2 that well agree with the established spectral parameters. Our technique exhibits flexible wavelength tuning capability in the near-infrared region and can be potentially extended to the mid-infrared region for more applications.
Abstract We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 × 10 −11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime.
We report on a low-noise, robust, all-polarization-maintaining (PM) mode-locked Er-doped fiber ring laser, indicating an effective method to further optimize the phase noise by adjusting the pump power. Integrated from 10 Hz to 10 MHz, the optimized timing jitter is as low as 269 fs with an amplitude noise of 0.028%. The experimental results agree well with the prediction in a series of simulations and numerical analyses, revealing that the phase noise performance is improved by rebalancing the interactions between the gain, dispersion, and nonlinearity, and the high pulse energy and short pulse duration contribute to a reduction in the phase noise until the nonlinearities begin to neutralize the benefit. Consisting of a packaged semiconductor saturable absorber mirror and an all-PM-fiber ring structure, the laser performs well with reliable output pulses in vibrational environments, and most of the technical noise from external disturbances is also suppressed.
A simple and robust dual-comb scheme with different repetition rates is demonstrated from a passively mode-locked polarization-maintain fiber cavity. With mutual coherent properties in a single cavity, mode-resolved spectroscopy is obtained with free-running state.
Efficient nonlinear Bragg diffraction was observed as an intense infrared femtosecond pulse was focused on a plasma grating induced by interference between two ultraviolet femtosecond laser pulses in air. The preformed electrons inside the plasma grating were accelerated by subsequent intense infrared laser pulses, inducing further collisional ionization and significantly enhancing the local electron density.
We report on laser actions of a new ytterbium-doped oxyorthosilicate Gd 2 SiO 5 (GSO). In the continuous-wave operation, Yb:GSO provided 2.7 W near 1094 nm. In mode-locked operation, a maximum average power of 638 mW was obtained.
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