High-SNR mid-infrared dual-comb spectroscopy using active phase control cooperating with CWs-dependent phase correction.

Mid-infrared (MIR) dual-comb spectroscopy (DCS) is a highly effective method for molecular metrology of rovibrational transition spectra in a quick accurate manner. However, due to limited comb frequency instability, manipulating coherence between two frequency combs to accomplish high-quality spectral analysis in the MIR region is a huge challenge. Here, we developed a comb-teeth resolved MIR DCS based on active phase control cooperating with a CWs-dependent (CWD) interferogram timing correction. Firstly, four meticulously engineered actuators were individually integrated into two near-infrared (NIR) seed combs to facilitate active coherence maintenance. Subsequently, two PPLN waveguides were adopted to achieve parallel difference frequency generations (DFG), directly achieving a coherent MIR dual-comb spectrometer. To improve coherence and signal-to-noise ratio (SNR), a CWD resampled interferogram timing correction was used to optimize the merit of DCS from 7.5 × 105 to 2.5 × 106. Meanwhile, we carried out the measurement of MIR DCS on the methane hot-band absorption spectra (v3 band), which exhibited a good agreement with HITRAN by a standard deviation on recording residual of 0.76%. These experimental results confirm that this MIR DCS with CWD interferogram timing correction has significant potential to characterize the rovibrational transitions of MIR molecules.

Temperature-dependent determination of NO2 dimerization reaction based on dual-comb spectroscopy.

Dimerization reactions play a critical role in various fields of research, including cell biology, biomedicine, and chemistry. In particular, the dimerization reaction of 2NO2⇌N2O4 has been extensively applied in pollution control and raw material preparation. Spectroscopy, as a powerful tool for investigating molecular structures and reaction kinetics, has been increasingly employed to study dimerization reactions in recent years. In this study, we successfully demonstrated the application of dual-comb spectroscopy (DCS) to analyze NO2 dimerization reactions, making the first report on the application of this technique in this context. Parallel measurements of NO2 and N2O4 fingerprints spectra with high resolution at 3000 cm-1 was performed, benefiting from the unprecedented broadband and high-precision capability of DCS. The absorption cross-sections of N2O4 from 296 to 343 K was obtained from the measured spectra, which contributes to further research on the molecular spectrum of N2O4. These results demonstrate the potential of DCS for studying the dimerization reaction mechanism.

Investigation of stable pulse mode-locking regimes in a NALM figure-9 Er-doped fiber laser.

We demonstrate three typical mode-locking processes of a nonlinear amplifying loop mirror (NALM) fiber laser via a general nonlinear Schrödinger equation-based (GNLSE) simulation model. First, the pulse evolutions in the NALM cavity were separately simulated under asymmetric and weakly asymmetric conditions. We found that the splitting ratio and positions of the gain fiber can result in a suitable phase bias between clockwise and counter-clockwise beams, enabling the realization of a self-starting low-threshold operating condition. To assess the roles of the splitting ratio and gain in the mode-locking process, we simulated three pulse formation processes: in the soliton, stretched-pulse, and dissipative soliton mode-locking regimes. The simulation results show that the splitting ratio, gain, and dispersion directly influence the mode-locking condition and pulse characteristics, thereby providing effective quantified guidance for high-quality pulse generation. Finally, an experimental NALM oscillation operating under stretched pulse conditions was established to investigate the impact of the splitting ratio and pump power on the pulse characteristics. The experimental results prove that the splitting ratio, gain, and dispersion can be used to manipulate the mode-locking threshold, self-starting threshold, nonlinear effects, and pulse characteristics.

Mid-infrared dual-comb spectroscopy for rapid temperature distribution characterization.

Due to the influence of chemical reactions, phase change, and other phenomena, the combustion system is a complicated high-temperature environment. Therefore, the spatio-temporally resolved monitoring of the temperature field is crucial for gaining a comprehensive understanding of the intricate combustion environment. In this study, we proposed a fast and high-precision temperature measurement technique based on mid-infrared (MIR) dual-comb spectroscopy with a high spectral resolution and fast refresh rate. Based on this technique, the spatio-temporally resolved measurement of a non-uniform temperature field was achieved along the laser path. To verify the capability of DCS for temperature measurement, the bandhead ro-vibrational lines of the CO2 molecule were acquired, and the 1-σ uncertainty of the retrieved temperature was 3.2°C at 800°C within 100 ms. The results demonstrate the potential of our fast and high-precision laser diagnostic technique which can be further applied to combustion kinetics.

208-µs single-shot multi-molecular sensing with spectrum-encoded dual-comb spectroscopy.

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 (C2H2, HCN, CO, CO2) 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.

Achieving Precise Spectral Analysis and Imaging Simultaneously with a Mode-Resolved Dual-Comb Interferometer.

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.

High-repetition-rate mid-IR femtosecond pulse synthesis from two mid-IR CW QCL-seeded OPAs.

Coherent pulse synthesis in the mid-infrared (mid-IR) domain is of great interest to achieve broadband sources from parent pulses, motivated by the advantages of optical frequency properties for molecular spectroscopy and quantum dynamics. We demonstrate a simple mid-IR coherent synthesizer based on two high-repetition-rate optical parametric amplifiers (OPAs) at nJ-level pump energy. The relative carrier envelope phase between the two OPAs was passively stable for a shared continuous wave (CW) quantum cascade laser (QCL) seed. Lastly, we synthesized mid-IR pulses with a duration of 105 fs ranging from 3.4 to 4.0 µm. The scheme demonstrated the potential to obtain broader mid-IR sources by coherent synthesis from multiple CW QCL-seeded OPAs.

130 W, 180 fs ultrafast Yb-doped fiber frequency comb based on chirped-pulse fiber amplification.

We report on a high-power fiber optical frequency comb consisting of a 250-MHz mode-locked fiber laser and a three-stage cascaded fiber chirped-pulse amplification system. After power scaling, the group velocity dispersion and third-order dispersion, generated in fiber stretcher and amplifiers, are compensated by a grism compressor, outputting a 132-W, 180-fs pulse train. The repetition rate and carrier-envelope offset frequency are locked to a Rb clock with the standard deviations of 1.07 and 0.87 mHz, corresponding to the fractional instability of 8.3×10-13 and 1.35×10-19, respectively. Moreover, we investigate the noise characteristics at high average powers, presenting a low-noise property of this high-power fiber OFC.

Octave mid-infrared optical frequency comb from Er:fiber-laser-pumped aperiodically poled Mg: LiNbO3.

In this Letter, we report an octave-spanning mid-infrared (MIR) comb generation with a difference frequency generation (DFG) approach optimized for aperiodically poled Mg:LiNbO3 and nonlinear spectral broadening. An Er:fiber comb is delivered to two branches and amplified in an Yb:fiber and an Er:fiber amplifier, respectively. We demonstrate that the two-branch DFG can yield the spectrum tuned over an octave in a fan-out periodically poled lithium niobate. Thus, we obtain an optimized poling period profile and design the aperiodically poled Mg:LiNbO3. The results demonstrate that broadband combs can be generated in the MIR atmospheric window.

High-repetition-rate femtosecond mid-infrared pulses generated by nonlinear optical modulation of continuous-wave QCLs and ICLs.

We demonstrate an effective method to obtain high-repetition-rate femtosecond mid-infrared (mid-IR) pulses by nonlinear optical modulation of mid-IR continuous-wave (CW) quantum and interband cascade lasers (ICLs and QCLs). In the experiment, a high-repetition-rate femtosecond ytterbium-doped fiber laser with nanojoule-level pulse energy was used as the pump source of optical parametric amplifiers to modulate and amplify the mid-IR CW laser. Near transform-limited 84 fs duration (7.3 cycles) mid-IR pulses were generated with above 200 mW average power and a repetition rate of 160 MHz. Moreover, the spectral tunability was demonstrated using CW QCL at different wavelengths. The scheme offered a simple method to produce high-repetition-rate ultrashort pulses and that can be flexibly adopted in other mid-IR regions.

Generation of 3.9-cycle pulses from the coherent synthesis of two continuous-wave injection seeded optical parametric amplifiers at 53 MHz.

A high repetition-rate, few-cycle light pulse is of great importance due to its potential for a variety of applications, including two-dimensional infrared spectroscopy and time-resolved imaging of molecular structures, which benefit from its ultrabroadband spectrum and ultrashort pulse duration. The generation of an ultrabroadband coherent spectrum is one of the frontiers of ultrafast optics, and accessing such few-cycle pulses is presently under active exploration. Here, we demonstrate a simple yet effective pulse synthesizer. It is based on two continuous-wave (cw) injection-seeded high-repetition-rate optical parametric amplification systems and the following self-phase-modulation dominated spectra-broadening processes. The combined spectrum spans from 1250 to 1670 nm, and a near Fourier-transform-limited 3.9-cycle (19.2 fs) synthesized pulse with a central wavelength of 1470 nm is obtained accordingly.

Dielectric-mirror-less femtosecond optical parametric oscillator with ultrabroad-band tunability.

We demonstrate a high average power, widely tunable, dielectric-mirror-less optical parametric oscillator (OPO) based on MgO:PPLN (MgO-doped periodically poled lithium niobate), which is synchronously pumped by a 1040 nm femtosecond fiber laser. The OPO does not require any dielectric coating mirrors. By exploiting the four-prism sequence system, combined with the gold mirrors, the oscillating laser pulses could span the spectral regions in both the signal and idler, and the output pulses of OPO can be tuned across 1367-1914 nm in the signal, and 2152-4480 nm in the idler as well. This device can deliver as much as 1.2 W of average power at 1482 nm in the signal and up to 411 mW at 3487 nm in the idler, respectively. The ultrabroad-band spectra tunability, along with the high average output property, makes the dielectric-mirror-less OPO an attractive alternative to conventional OPOs.

Line-scan spectrum-encoded imaging by dual-comb interferometry.

Herein, the method of spectrum-encoded dual-comb interferometry is introduced to measure a three-dimensional (3-D) profile with absolute distance information. By combining spectral encoding for wavelength-to-space mapping, dual-comb interferometry for decoding and optical reference for calibration, this system can obtain a 3-D profile of an object at a stand-off distance of 114 mm with a depth precision of 12 µm. With the help of the reference arm, the absolute distance, reflectivity distribution, and depth information are simultaneously measured at a 5 kHz line-scan rate with free-running carrier-envelope offset frequencies. To verify the concept, experiments are conducted with multiple objects, including a resolution test chart, a three-stair structure, and a designed "ECNU" letter chain. The results show a horizontal resolution of ∼22 µm and a measurement range of 1.93 mm.

Noise characteristics of high power fiber-laser pumped femtosecond optical parametric generation.

We study, both numerically and experimentally, the relative intensity noise (RIN) and timing jitter characteristics of optical parametric generation (OPG) process in MgO-doped periodically poled LiNbO3 (MgO:PPLN) pumped by fiber femtosecond laser. We directly characterize the RIN, and measure timing jitter spectral density of the OPG process based on the balanced optical cross-correlator (BOC) technique for the first time as well, which are both in a fairly good agreement with numerical simulation. Both the numerical and experimental study reveals that OPG can suffer from a smaller intensity fluctuation but a lager temporal jitter when it is driven into saturation. Furthermore, we demonstrate that with a 30 mW CW diode laser injection seeding the OPG output results in superior noise performance compared to the vacuum fluctuations seeded OPG.

Extended femtosecond laser wavelength range to 330 nm in a high power LBO based optical parametric oscillator.

We experimentally demonstrate a compact tunable, high average power femtosecond laser source in the ultraviolet (UV) regime. The laser source is based on intra-cavity frequency doubling of a temperature-tuned lithium tribotate (LBO) optical parametric oscillator (OPO), synchronously pumped at 520 nm by a frequency-doubled, Yb-fiber femtosecond laser amplifier system. By adjusting crystal temperature, the OPO can provide tunable visible to near-infrared (NIR) signal pulse, which have a wide spectral tuning range from 660 to 884 nm. Using a ß-barium borate (BBO) crystal for intra-cavity frequency doubling, tunable femtosecond UV pulse are generated across 330~442 nm with up to 364 mW at 402 nm.

Ultrafast axial scanning for two-photon microscopy via a digital micromirror device and binary holography.

In this Letter, we present an ultrafast nonmechanical axial scanning method for two-photon excitation (TPE) microscopy based on binary holography using a digital micromirror device (DMD), achieving a scanning rate of 4.2 kHz, scanning range of ∼180 µm, and scanning resolution (minimum step size) of ∼270 nm. Axial scanning is achieved by projecting the femtosecond laser to a DMD programmed with binary holograms of spherical wavefronts of increasing/decreasing radii. To guide the scanner design, we have derived the parametric relationships between the DMD parameters (i.e., aperture and pixel size), and the axial scanning characteristics, including (1) maximum optical power, (2) minimum step size, and (3) scan range. To verify the results, the DMD scanner is integrated with a custom-built TPE microscope that operates at 60 frames per second. In the experiment, we scanned a pollen sample via both the DMD scanner and a precision z-stage. The results show the DMD scanner generates images of equal quality throughout the scanning range. The overall efficiency of the TPE system was measured to be ∼3%. With the high scanning rate, the DMD scanner may find important applications in random-access imaging or high-speed volumetric imaging that enables visualization of highly dynamic biological processes in 3D with submillisecond temporal resolution.

Soft mold-based hot embossing process for precision imprinting of optical components on non-planar surfaces.

Patterning micro- and nano-scale optical elements on nonplanar substrates has been technically challenging and prohibitively expensive via conventional processes. A low-cost, high-precision fabrication process is thus highly desired and can have significant impact on manufacturing that leads to wider applications. In this paper, we present a new hot embossing process that enables high-resolution patterning of micro- and nano-structures on non-planar substrates. In this process, a flexible elastomer stamp, i.e., PDMS, was used as a mold to perform hot-embossing on substrates of arbitrary curvatures. The new process was optimized through the development of an automated vacuum thermal imprinting system that allows non-clean room operation as well as precise control of all process parameters, e.g., pressure, temperature and time. Surface profiles and optical properties of the fabricated components, including micro-lens array and optical gratings, were characterized quantitatively, e.g., RMS ~λ/30 for a micro-lens, and proved to be comparable with high cost conventional precision processes such as laser lithographic fabrication.

Fast 3-D temporal focusing microscopy using an electrically tunable lens.

In this paper, we present a 3-D temporal focusing microscope based on an electrically tunable lens (ETL) and a femtosecond regenerative laser amplifier. The focus-tunable lens provides a fast and compact way to perform non-mechanical z-scanning and resolves the blurry image issue compared with GVD-based z-scanning methods. The optical performance of the temporal focusing system, including z-scanning characteristics, the associated the magnification variation, and the lateral and axial resolution, has been studied and characterized using calibrated Rhodamine-6G thin film sample, fluorescent beads, and pollen samples. Lastly, we demonstrate the optical cross-sectioning and z-scanning capability with an in vivo experiment, where Ca(2+) imaging of neurons in GaCamp6 labeled zebrafish was performed.

High power tunable femtosecond ultraviolet laser source based on an Yb-fiber-laser pumped optical parametric oscillator.

We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO.

High-speed femtosecond laser beam shaping based on binary holography using a digital micromirror device.

In this Letter, we present a digital micromirror device (DMD)-based ultrafast beam shaper, i.e., DUBS. To our knowledge, the DUBS is the first binary laser beam shaper that can generate high-resolution (1140×912 pixels) arbitrary beam modes for femtosecond lasers at a rate of 4.2 kHz; the resolution and pattern rate are limited by the DMD. In the DUBS, the spectrum of the input pulsed laser is first angularly dispersed by a transmission grating and subsequently imaged to a DMD with beam modulation patterns; the transmission grating and a high-reflectivity mirror together compensate the angular dispersion introduced by the DMD. The mode of the output beam is monitored by a CCD camera. In the experiments, the DUBS is programmed to generate four different beam modes, including an Airy beam, Bessel beam, Laguerre-Gaussian (LG) beam, and a custom-designed "peace-dove" beam via the principle of binary holography. To verify the high shaping rate, the Airy beam and LG beam are generated alternately at 4.2 kHz, i.e., the maximum pattern rate of our DMD. The overall efficiency of the DUBS is measured to be 4.7%. With the high-speed and high-resolution beam-shaping capability, the DUBS may find important applications in nonlinear microscopy, optical manipulation, and microscale/nanoscale laser machining, etc.