Routing to mid-infrared microcomb via near-infrared direct pump.

Mid-infrared (MIR) microcomb provides a new way into the "molecular fingerprint" region. However, it remains rather a challenge to realize the broadband mode-locked soliton microcomb, which is often limited by the performance of available MIR pump sources and coupling devices. Here, we propose an effective approach towards broadband MIR soliton microcombs generation via a direct pump in the near-infrared (NIR) region, through full utilization of the second- and third-order nonlinearities in a thin-film lithium niobate microresonator. The optical parametric oscillation process contributes to conversion from the pump at 1550 nm to the signal around 3100 nm, and the four-wave mixing effect promotes spectrum expansion and mode-locking process. While the second-harmonic and sum-frequency generation effects facilitate simultaneous emission of the NIR comb teeth. Both the continuous wave and pulse pump sources with relatively low power can support a MIR soliton with a bandwidth over 600 nm and a concomitant NIR microcomb with a bandwidth of 100 nm. This work can provide a promising solution for broadband MIR microcombs by breaking through the limitation of available MIR pump sources, and can deepen the understanding of the physical mechanism of the quadratic soliton assisted by the Kerr effect.

Simultaneous generation of a broadband MIR and NIR frequency comb in a GaP microring.

Midinfrared (MIR) optical frequency combs are of great significance as broadband coherent light sources used in extensive areas such as coherent communications and molecule detections. Conventional MIR combs are usually restricted in size and power, while most microcombs are focused in the near-infrared (NIR) region because of the limited accessible Q-factor of microrings and the poor performances of available pumps. In this paper, we numerically demonstrate the simultaneous generation of a broadband MIR and NIR comb in a GaP microring with an additive waveguide. The achieved octave-spanning (1890-4050 nm) MIR microcomb at a low pump power of 34 mW can be effectively converted to the second-harmonic NIR comb covering 1120-1520 nm with separate dispersion optimization of the ring cavity and straight waveguide. The proposed system has the advantage of simple structure and low power threshold, which could find potential in highly integrated MIR optical sources and related applications.

Mid-infrared optical parametric oscillation spanning 3.4-8.2µm in a MgF2microresonator.

Mid-infrared optical parametric oscillators (OPOs) offer a compelling route for accessing the 'molecular fingerprint' region and, thus, can find intensive applications such as precision spectroscopy and trace gas detection. Yet it still remains rather a challenge to realize broadband mid-infrared OPOs within a single cavity, usually limited by strict phase-matching conditions for wide spectral coverage and available pump power for adequate frequency generation. Here, we report the mid-infrared parametric oscillation spanning from 3.4 to 8.2µm, based on four-wave mixing in a high-QMgF2microresonator with optimized dispersion. The center wavelength at 4.78µm is determined by the continuous tunable quantum cascade laser source, which contributes to effective expansion towards longer wavelength, as well as systemic miniaturization with smaller pump module. Such results could not only shed light on new ultimates of crystal and other microresonators, but also inspire explorations on their growing potentials in near future.

Simple reformulation of the coordinate transformation method for gratings with a vertical facet or overhanging profile.

We reformulate the coordinate transformation method (C method) for gratings with a vertical facet or overhanging profile (overhanging gratings), in which no tensor concept is involved, only the knowledge of elementary mathematics and Maxwell's equations in the rectangular coordinate system is used, and we provide a detailed recipe for programming. This formulation is easy to understand and implement. It adopts the strategy of a rotating coordinate system from Plumey et al. [J. Opt. Soc. Am. A14, 610 (1997)JOAOD60740-323210.1364/JOSAA.14.000610] and expresses it with the method of changing variables from Li et al. [Appl. Opt.38, 304 (1999)APOPAI0003-693510.1364/AO.38.000304]. We investigate several typical overhanging gratings by the reformulated C method, and we validate and compare the results with the Fourier modal method, which shows that it is superior, especially for metal deep smooth gratings. This reformulation can facilitate the research in light couplers for optical engineers.

Acetylene sensing system based on wavelength modulation spectroscopy using a triple-row circular multi-pass cell.

A sensitive acetylene (C2H2) sensing system based on a novel triple-row circular multi-pass cell (CMPC) was demonstrated. This CMPC has an effective optical length of 21.9 m within an extremely small volume of 100.1 mL. We utilized wavelength modulation spectroscopy (WMS) for absorption spectroscopy detection of C2H2. The distance between the two minima of the second harmonic was used to normalize the maximum value of it, which makes the time to obtain stable output for continuous detection shorten dramatically. A fiber-coupled distributed feedback (DFB) diode laser emitting at 1.5316 µm was employed as a light source. An Allan deviation analysis yielded a detection sensitivity of 76.75 ppb with a normalized noise equivalent absorption coefficient of 8.8 × 10-10 cm-1 Hz-1/2 during an average time of 340 s. With a fast stable time, reduced size and high detection sensitivity, the proposed sensing system is suitable for trace gas sensing in a weight-limited unmanned aerial vehicle (UAV) and an exhalation diagnosis for smoking test.

Full-depth spectral domain optical coherence tomography technology insensitive to phase disturbance.

To achieve full-depth spectral domain optical coherence tomography in the case of strong environmental disturbance, the iterative phase-shifting (IPS) method and modified dispersion-coded (MDC) method are proposed in this work. In IPS, the precise amount of phase shift is retrieved by iteration, and the direction of the phase shift is determined by dispersion compensation. Conjugate mirror items and noise can be simultaneously eliminated by two captured interferograms, whereas only one of them can be removed in the traditional phase-shift method with two interferograms. In MDC, they are removed through dispersion compensation and signal extraction with a single interferogram. Full-depth images of a glass slide, an onion, and a live fish eye are obtained by the two methods. The advantages and disadvantages of each method are analyzed and compared. IPS is found to be more effective for removing conjugate artifacts, whereas MDC is more conducive to real-time imaging. For a 2 mm × 3.6 mm image of a fish eye (200 depth scans and 1200 spectral sampling points per depth scan), the mirror image artifact is reduced by 28.55 dB in MDC and 41.53 dB in IPS. Processing times are 5.1 seconds (20 iterations) for the IPS method and 0.91 seconds for MDC.

Simulation of penetration depth of Bessel beams for multifocal optical coherence tomography.

Multifocal Bessel beam optical coherence tomography (MBOCT) combines the advantages of Bessel beam OCT and multifocal OCT to increase imaging depth. For MBOCT, the penetration depth of the Bessel beam in highly scattering biological tissue limits the final imaging depth. In this paper, we theoretically analyze the penetration depth of the Bessel beams with different parameters to explore which kind of Bessel beam is more suitable for MBOCT in a scattering medium. The finite-difference time-domain method is used to simulate the field distribution of Bessel beams in the medium. We find that the MBOCT for more focus based on a Bessel beam with a smaller Fresnel number N has higher penetration depth and light intensity when its lateral resolution is fixed. Moreover, the Bessel beam with N reversely closer to unity is more advantageous for penetrating the highly scattering medium for a certain imaging depth, and the Bessel beam has larger penetration depth when its lateral size is close to the size of the object to be imaged.

Generalized Optical Design of the Double-Row Circular Multi-Pass Cell.

A new design of circular multi-pass cells (CMPC) with two rows of reflection spots on mirrors is presented. The effective optical path length (OPL) of this novel CMPC is double that of traditional CMPC with the same diameter and interval of spots. This OPL can be readily adjusted to have regular intervals by rough rotation adjustment. We achieved a spatial separation of pre- and post-transfer optical systems that was adequately large even in the cases with a large number of passes. Analytical chief ray tracing analysis and a generalized method for parameter determination for designing the cell are presented in detail. The stable condition of the double-row CMPC (DR-CMPC) is also derived by the ABCD matrix method. Designs with maximum effective OPL of 74.72 m, 48.67 m and 24.57 m are demonstrated and verified by ray tracing simulations within a 25 cm diameter DR-CMPC. An adjustment of the regular intervals to 1 m can be achieved in both designs. The overall astigmatism of the design with an effective OPL of 74.72 m is only 9.30 × 10-6 mm, which is four orders of magnitude smaller than that of the traditional CMPC with similar geometric parameters.

Degenerate critical coupling in all-dielectric metasurface absorbers.

We develop the theory of all-dielectric absorbers based on temporal coupled mode theory (TCMT), with parameters extracted from eigenfrequency simulations. An infinite square array of cylindrical resonators embedded in air is investigated, and we find that it supports two eigenmodes of opposite symmetry that are each responsible for half of the total absorption. The even and odd eigenmodes are found to be the hybrid electric (EH111) and hybrid magnetic (HE111) waveguide modes of a dielectric wire of circular cross section, respectively. The geometry of the cylindrical array is shown to be useful for individual tuning of the radiative loss rates of the eigenmodes, thus permitting frequency degeneracy. Further, by specifying the resonators' loss tangent, the material loss rate can be made to equal the radiative loss rate, thus achieving a state of degenerate critical coupling and perfect absorption. Our results are supported by S-parameter simulations, and agree well with waveguide theory.

Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection.

A methane sensor based on dispersion spectroscopy is presented in this paper. A standard Mach-Zehnder modulator working in carrier suppression mode is adopted to generate a spectrum of a carrier and two sidebands. We aim at detecting the phase shift of the beatnote generated by the two sidebands in a methane concentration evaluation process. We put forward an analytical model to describe the dual-sideband heterodyne scheme and carry out experiments to demonstrate the model. Long-term tests show that the sensor has a minimum detection limit of 0.4 ppm·mHz-0.5 at an average time of 1 s. And in the condition of 1 atm and room temperature, a linear measurement range from 0.4 to 44955 ppm·m is achieved.