Narrowband Spectral Filtering Research Articles

Cavity-enhanced narrowband spectral filters using rare-earth ions doped in thin-film lithium niobate

On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip.

Linearizing the vertical scale of an interferometric microscope and its effect on step-height measurement.

The vertical scale calibration of an interferometric microscope is important for establishing traceability of surface topography measurements to the International System of Units (SI) unit of length, the meter. Building on the calibration procedure for the amplification coefficient developed by de Groot and Beverage [Proc. SPIE 9526, 952610 (2015)], this paper describes a calibration procedure that yields the response curve for the entire vertical scan motion of a coherent scanning interferometric microscope. The method requires only a flat mirror as an artifact, a narrow band spectral filter, an aperture to reduce the effective numerical aperture, and the ability to raise and lower the microscope head so that the center of the interferogram can be varied within the scan range. The local frequency of the interferogram is determined by fitting sections of the interferogram to a sinusoidal function. The nonlinearity determined from the local frequency data can be used to estimate the uncertainty in uncorrected vertical height measurements. We describe how optical profile data can be corrected for nonlinearity due to dynamic effects in the scan motion and show that the correction improves the reproducibility of step height measurements by at least a factor of three and close to that of the repeatability.

Narrow-bandwidth picosecond dissipative soliton resonance in an all-anomalous-dispersion Er/Yb co-doped fiber laser

First, it is numerically demonstrated that narrow-bandwidth picosecond dissipative soliton resonance (DSR) pulses can be obtained in the all-anomalous-dispersion Er/Yb co-doped fiber (EYDF) laser, which is based on the nonlinear polarization rotation (NPR) mode-locking and narrow-band spectral filtering. The generated DSR pulses are characterized by a central peak on top of a rectangular pedestal temporally and two high-intensity spectral sidebands with less than 0.6-nm bandwidth. Second, the impacts of several key cavity parameters on characteristics of DSR pulses are investigated. Particularly, a new phenomenon of periodic evolution of the DSR pulse, featuring alternated high and low peak powers with the round-trips, is discovered by tuning filter bandwidth or net-cavity dispersion. Multi-pulse dissipative soliton (DS) and noise-like pulse (NLP) are also observed in the fiber laser. Finally, a 22.3-ps DSR pulse with 287.5-W peak power can be further compressed to 4.3 ps with 1126.5-W peak power by the dispersion compensation. This work further enriches the DSR pulse dynamics in the anomalous-dispersion region and paves a way to realize narrow-bandwidth picosecond DSR pulse fiber lasers.

Design of a Data Acquisition, Correction and Retrieval of Na Doppler Lidar for Diurnal Measurement of Temperature and Wind in the Mesosphere and Lower Thermosphere Region

Based on the resonance fluorescence scattering mechanism, a narrowband sodium (Na) lidar can measure temperature and wind in the mesosphere and lower thermosphere (MLT) region. By using a narrowband spectral filter, background light noise during the day can be suppressed, allowing for continuous observations. To obtain full-diurnal-cycle temperature and wind measurement results, a complex and precise retrieval process is required, along with necessary corrections to minimize measurement errors. This paper introduces the design of a data acquisition unit for three frequencies in three directions of the Na lidar system in the Chinese Meridian Project (Phase II) and investigates the calibration and retrieval methods for obtaining diurnal temperature and horizontal wind in the MLT region, using a Na Doppler lidar with Faraday anomalous dispersion optical filter (FADOF). Furthermore, these methods are applied to observations conducted by a Na lidar in Beijing, China. The wind and temperature results over full diurnal cycles obtained from the all-solid-state Na Doppler lidar are reported for the first time and compared with temperature measurements from satellite, as well as wind observations from a meteor radar. The comparison demonstrates a reasonable agreement between the results, indicating the rationality of the lidar-retrieved results and the feasibility and effectiveness of the data correction and retrieval method.

Portable diagnostic package for Thomson scattering and optical emission spectroscopy on Princeton field-reversed configuration 2 (PFRC 2).

An Advanced Research Projects Agency-Energy funded diagnostic system has been deployed to the Princeton field-reversed configuration 2 (PFRC-2) device, located at Princeton Plasma Physics Laboratory. The Portable Diagnostic Package (PDP), designed at Oak Ridge National Laboratory, allows for the measurement of Thomson Scattering (TS) for electron density and temperature and Optical Emission Spectroscopy (OES) for ion temperature, impurity density, and ion velocity. A tunable spectrometer on the PDP with three gratings provides the flexibility to measure low (1eV) and high (1000eV) electron temperature ranges from TS. Additionally, using a second spectrometer, the OES diagnostic can survey light emission from various ion excitation levels for wide wavelength ranges. The electron density (<2 × 1019m-3) of plasmas generated in PFRC-2 has been below the PDP TS discrimination threshold, which has made TS signal detection challenging against a high-background of laser stray light. The laser stray light was iteratively reduced by making modifications to the entrance and exit geometry on PFRC-2. Rayleigh scattering experiments on PFRC have yielded the TS discrimination sensitivity to be >1 × 1020m-3 for the PDP. A recently implemented narrow-band notch spectral filter that masks the second harmonic 532nm Nd:YAG laser wavelength has increased the system's TS light discrimination sensitivity 65 times compared to the instance when the notch filter was not implemented. The hardware implementation including design changes to the flight tubes and Brewster windows will be discussed, along with results from Rayleigh and rotational Raman scattering sensitivity analyses, which were used to establish a quantitative figure of merit on the system performance. The Raman scattering calibration with the notch filter has improved the PDP electron density threshold to 1 ± 0.5 × 1018m-3.

Polarization-independent long-wave infrared transmission bandpass filters based on guided mode resonance.

Subwavelength dielectric gratings based on guided mode resonances (GMRs) provide a compact, low-cost, and readily fabricable platform for narrowband spectral filters applied in numerous fields, such as sensors, spectrometers, and hyperspectral imaging. We numerically designed two long-wave infrared transmission bandpass filters based on Ge-ZnS-Ge thin film stacks and 2D zero-contrast gratings. The first filter is attributed to double GMR mode coupling, exhibiting a broadband high reflectance between 8 and 12 µm and a narrow transmission peak with a spectral linewidth of 53 nm and transmittance efficiency of 98% at the resonance. The other filter adopts a low-refractive-index material BaF2 as the substrate, which displays a lower-transmittance sideband from 8.6 to 9.4 µm, and a higher transmission efficiency up to 100% at the resonance. The physical mechanisms of two spectral responses are all relevant to guided mode coupling and Fabry-Perot resonance.

Harmonic Dissipative Soliton Resonance in an Er/Yb Co-Doped Fiber Laser Based On SESAM

Fundamental and harmonic mode-locking in the dissipative soliton resonance (DSR) regime is experimentally investigated in a net-normal-dispersion Er/Yb co-doped fiber laser based on the semiconductor saturable mirror (SESAM), where an equivalent narrow-band spectral filter is used for triggering multi-pulsing. By monotonically increasing the pump power with the fixed polarization controller (PC), the fundamental mode-locking operation of DSR pulses can be transformed into the harmonic mode-locking (HML) under the specific conditions. The main reason is the peak-power-clamping effect induced by the combined effects of the spectral filtering and saturable absorption of the SESAM at higher pump levels, which is applied to achieve the HML in the DSR regime for the first time. The DSR pulse broadens linearly with the pump power in a certain range, no matter whether it is in the fundamental mode-locking or HML state. By incorporating the dispersion compensation fiber (DCF) with different lengths, the pulse characteristics are discussed and compared in detail. The enhanced nonlinearity caused by increasing DCF length can lead to higher orders of HML pulses and even the generation of noise-like pulses (NLPs) with a long-enough DCF.

Integrated diffraction gratings on the Bloch surface wave platform supporting bound states in the continuum

Abstract We propose and theoretically and numerically investigate integrated diffraction gratings for the Bloch surface wave (BSW) platform, which have subwavelength or near-subwavelength period. We demonstrate that, in the oblique incidence geometry of a transverse-electric polarized BSW and with a properly chosen band gap configuration of the photonic crystal supporting the surface waves, the proposed structures operate in the scattering-free regime, when the energy of the incident BSW is divided between the reflected and transmitted BSWs with the same polarization corresponding to the propagating diffraction orders of the grating, and not scattered away from the propagation surface. In this regime, the studied integrated gratings support high-Q resonances and bound states in the continuum not only in the subwavelength case when only the specular (zeroth) diffraction orders propagate, but also in the case when non-evanescent zeroth and −1st diffraction orders satisfy the so-called Littrow mounting condition. The proposed integrated gratings on the BSW platform can be used as efficient narrowband spatial or spectral filters operating in reflection, or as BSW beam splitters or deflectors operating in transmission. The obtained results may find application in two-dimensional photonic circuits for steering the BSW propagation.

All-normal dispersion ytterbium-doped fiber laser mode locked by nonlinear multimode interference.

We report a mode-locking operation using a nonlinear multimode interference (MMI)-based saturable absorber in an ytterbium-doped fiber laser under an all-normal dispersion configuration. The MMI is observed in a step-index single mode-multimode-single mode (SMS) fiber structure. The intensity and wavelength-dependent transmission of the SMS structure has been studied numerically to understand the Kerr-induced saturable absorber properties of MMI in a step-index fiber. Numerical investigations show that presence of a narrow-band wavelength-tunable spectral filter is necessary for a nonlinear MMI-based saturable absorber to be effective for mode locking. A single-pulse mode-locking operation in the laser is realized at a very low pump power of ${\sim}{56.3}\;{\rm mW}$ at which the laser generates 180 ps duration pulses at ${\sim}{9}\;{\rm MHz}$ repetition rate with measured signal-to-noise ratio of ${\sim}{50}\;{\rm dB}$. This all-fiber integrated laser operates in multi-pulse and noise-like rectangular pulse regimes at increased pump power.

Diurnal variation of atmospheric metal Na layer and nighttime top extension detected by a Na lidar with narrowband spectral filters at Beijing, China

Diurnal observations of metal Na layer can be implemented by making use of narrowband Faraday filters. On one hand, the narrowband spectral filters help to effectively suppress the background noise; on the other hand, the narrow bandwidth leads to different transmissions of Na layer resonance fluorescence signal and stratosphere Rayleigh scattering signal. The influence of the narrow spectral transmission on Na density retrieval cannot be canceled out by Rayleigh normalization and would result in a systematic error. In this paper, the Na density retrieval method when narrowband Faraday filters are used in the lidar receiver is discussed based on the transmission spectrum of Faraday filters and echo spectra of Rayleigh scattering and Na fluorescence. The results of Na layer diurnal variations that have been obtained from Na lidar observations at Beijing, China are presented. Considerable diurnal variation in Na density on the layer top and bottom are clearly revealed on a logarithmic scale. Intriguingly, larger height-scale nighttime extensions on the Na layer topside have been frequently observed from early summer to autumn. These observational results can provide valuable evidence for studying solar effect on the metal layer variation, atmospheric dynamical and chemical processes in the mesosphere and lower thermosphere region.

Widely tunable ultra-narrow-linewidth dissipative soliton generation at the telecom band

Ultra-narrow-linewidth mode-locked lasers with wide wavelength tunability can be versatile light sources for a variety of newly emergent applications. However, it is very challenging to achieve the stable mode locking of substantially long, anomalously dispersive fiber laser cavities employing a narrowband spectral filter at the telecom band. Here, we show that a nearly dispersion-insensitive dissipative mode-locking regime can be accessed through a subtle counterbalance among significantly narrowband spectral filtering, sufficiently deep saturable absorption, and moderately strong in-fiber Kerr nonlinearity. This achieves ultra-narrow-linewidth (a few gigahertz) nearly transform-limited self-starting stable dissipative soliton generation at low repetition rates (a few megahertz) without cavity dispersion management over a broad tuning range of wavelengths covering the entire telecom C-band. This unique laser may have immediate application as an idealized pump source for high-efficiency nonlinear frequency conversion and nonclassical light generation in dispersion-engineered tightly light-confining microphotonic/nanophotonic systems.

High-resolution notch filters and diplexers based on SU-8 inverted rib waveguides.

We performed an end-to-end process, ranging from design, fabrication, and characterization of integrated polymeric optical devices under a mass production technology. Inverted rib waveguides formed by SU-8 photoresist deposited on top of full wafers, with trenches in silica, were used as a platform to implement such optical devices. Narrowband spectral filters based on microracetrack resonators and diplexers based on directional couplers, both with high extinction ratios, are demonstrated. Full wafers of those devices were processed in a complementary metal-oxide-semiconductor foundry's 150mm-facility. We believe the results are promising for applications ranging from telecommunication components to sensing devices.

Tunable filter and modulator with controlled bandwidth and wide dynamic range based on planar thin films structure.

Tunable narrowband spectral filters with high throughput and wide dynamic range are in high demand for many applications, however, a cost is usually associated with the filter narrowing either in the dynamic range, in the throughput or the manufacturability. Here it is shown for the first time that using the coupling between waveguide and lossy modes (LMs) in lossy ultrathin films through thin film coupling layer it is possible to obtain a reflection peak with controllable width (sub-Angstroms till tens of nm) and tunability over wide spectral range (>500nm in the visible and near infrared). The excitation of broadband LM is enabled using an ultrathin absorptive layer with high imaginary to real part ratio of the dielectric constant (i.e 6nm of Cr). The wider dynamic range and higher contrast are observed more with TE polarization than TM. The tuning is achieved by incidence angle scan of few degrees or by modulating the waveguide layer from the visible till the near infrared and in principle it can be designed to operate in any spectral range. Such a thin waveguide layer can allow tuning at ultrahigh speed using conventional electrooptic, magnetooptic, piezoelectric or thermooptic materials using relatively low external fields. The tuning sensitivity and range depend strongly on the waveguide layer thickness and the refractive index mismatch between the waveguide and the coupling layer. Under small index mismatch new peaks are seen via Rabi type splitting with gaps as high as 700nm or more, thus exhibiting ultrahigh tuning with negative sensitivity.

Toggling between active and passive imaging with an omni-resonant micro-cavity.

While passive illumination schemes often utilize a broadband spectral acceptance, the performance of active illumination with a laser is improved by narrowband spectral filtering at the sensor. We present an experimental demonstration of an optical cavity structure that is capable of toggling between two performance limits: narrowband resonant and broadband omni-resonant transmission. To achieve omni-resonance without modifying the cavity, the incident optical field is pre-conditioned by associating each wavelength with a particular incidence angle that enables a broad continuous spectrum to resonate with the cavity. This strategy can help seamlessly combine passive and active illumination in the same system.

Fabrication of buried nanostructures by atomic layer deposition

We present a method for fabricating buried nanostructures by growing a dielectric cover layer on a corrugated surface profile by atomic layer deposition of TiO2. Selecting appropriate process parameters, the conformal growth of TiO2 results in a smooth, nearly flat-top surface of the structure. Such a hard surface can be easily cleaned without damage, making the nanostructure reusable after contamination. The technique has wide applicability in resonance-domain diffractive optics and in realization of quasi-planar metamaterials. We discuss design issues of such optical elements and demonstrate the method by fabricating narrow-band spectral filters based on the guided-mode resonance effect. These elements have strong potential for, e.g., sensing applications in harsh conditions.

Single-shot multispectral imager using spatially multiplexed Fourier spectral filters

We demonstrate a single-shot multispectral imaging system using a fundamentally different approach compared with present-day methods. Conventional multispectral imagers utilize narrowband spectral filters, which makes the system application-specific. By using filters with sinusoidally varying transmittance, known as Fourier filters, we demonstrate an application-agnostic multispectral system. We designed and fabricated this Fourier filter array, integrated it with a commercial CCD image sensor, and acquired and reconstructed the spectral information from multiple targets. Compared with narrowband multispectral imagers, this system generalizes well to applications where we lack a priori knowledge of the spectral content of the scene, which makes it a versatile acquisition device for a wide range of applications.

High-Resolution and Low-Loss All-Fiber Spectral Filters Based on Biconic Tapers

A family of narrowband spectral filters based on biconic fiber tapers is investigated. These filters were made of highly-depressed-cladding single-mode fibers by a heat-and-pulling process, and using a cylindrical-tube graphite heater. The evolution of the taper profiles during the pulling of different elongations was modeled by a coupled system of partial differential equations governing the mass and axial momentum conservation. The optical responses were modeled by using the mode coupling theory. Theoretical results, ranging from prediction of the taper profiles to optical responses of these filters—transmission losses, free spectral range, and isolation—show strong accordance with experimental ones.

Effects of filtering on the purity of heralded single photons from parametric sources

Optical quantum protocols are reliant on the production of highly pure single photons. However, most parametric heralded single photon sources possess spectral correlations, limiting the purity of the produced photon. The most common method for restoring the purity is with narrowband spectral filtering, which comes at the cost of reducing the rate of heralding detection. Here, we characterize this trade-off analytically for a large class of parametric sources. It is possible to achieve up to 20% heralding success probabilities with 90% heralded photon purities.

Development and characterization of high suppression and high étendue narrowband spectral filters.

The present work addresses critical issues in constructing high suppression, narrowband spectral filters in rare-earth-ion-doped crystals, mainly targeting the application of ultrasound optical tomography but is also applicable for areas such as quantum memories, self-filtering of laser frequencies, and other applications relying on high absorption in rare-earth-ion-doped crystals. The polarization of light transmitted through a highly absorbing crystal is experimentally analyzed. Additionally, an existing wave propagation method is used to simulate beam propagation through a spectral hole to study the high étendue requirements of ultrasound optical tomography.

Etalon filters for Brillouin microscopy of highly scattering tissues.

Brillouin imaging of turbid biological tissues requires an effective rejection of the background noise due to elastic scattering of probe laser light. We have developed a narrowband spectral notch filter based on a pair of a free-space Fabry-Perot etalon and a mirror. The etalon filter in a 4-pass configuration is able to suppress elastically-scattered laser light with a high extinction ratio of > 40 dB and transmit inelastically-scattered light in a frequency shift range of 2-14 GHz with only 2 dB insertion loss. We also describe a simple etalon that enables us to use semiconductor diode laser sources for Brillouin microscopy by removing spontaneous emission noise. Using a clinically-viable Brillouin microscope employing these filters, we demonstrate the first Brillouin confocal imaging of the sclera and conjunctiva of the porcine eye.