Spectral Comb Research Articles

Ultrasensitive and Multiple Biomarker Discrimination for Alzheimer's Disease via Plasmonic & Microfluidic Sensing Technologies.

As the population ages, the worldwide prevalence of Alzheimer's disease (AD) as the most common dementia in the elderly is increasing dramatically. However, a long-term challenge is to achieve rapid and accurate early diagnosis of AD by detecting hallmarks such as amyloid beta (Aβ42). Here, a multi-channel microfluidic-based plasmonic fiber-optic biosensing platform is established for simultaneous detection and differentiation of multiple AD biomarkers. The platform is based on a gold-coated, highly-tilted fiber Bragg grating (TFBG) and a custom-developed microfluidics. TFBG excites a high-density, narrow-cladding-mode spectral comb that overlaps with the broad absorption of surface plasmons for high-precision interrogation, enabling ultrasensitive monitoring of analytes. In situ detection and in-parallel discrimination of different forms of Aβ42 in cerebrospinal fluid (CSF) are successfully demonstrated with a detection of limit in the range of ≈30-170 pg mL-1, which is one order of magnitude below the clinical cut-off level in AD onset, providing high detection sensitivity for early diagnosis of AD. The integration of the TFBG sensor with multi-channel microfluidics enables simultaneous detection of multiple biomarkers using sub-µL sample volumes, as well as combining initial binding rate and real-time response time to differentiate between multiple biomarkers in terms of binding kinetics. With the advantages of multi-parameter, low consumption, and highly sensitive detection, the sensor represents an urgently needed potentials for large-scale diagnosis of diseases at earlystage.

Rapid ultra-sensitive nucleic acid detection using plasmonic fiber-optic spectral combs and gold nanoparticle-tagged targets

Nucleic acid (NA) is a widely-used biomarker for viruses. Accurate quantification of NA can provide a reliable basis for point-of-care diagnosis and treatment. Here, we propose a tilted fiber Bragg grating (TFBG)-based plasmonic fiber-optic spectral comb for fast response and ultralow limit NA detection. The TFBG is coated with a gold film which enables excitation of surface plasmon resonance (SPR), and single-stranded probe NAs with known base sequences are assembled on the gold film. To enhance sensitivity of refractive index (RI) for sensing a chosen combination of probe and target NAs around the TFBG surface, gold nanoparticles (AuNPs) are bonded to the target NA molecules as “RI-labels”. The NA combination-induced aggregation of AuNPs induces significant spectral responses in the TFBG that would be below the detection threshold for the NAs in the absence of the AuNPs. The proposed TFBG-SPR NA sensor shows a fast response time of 30 s and an ultra-wide NA detection range from 1 × 10−18 mol/L to 1 × 10−7 mol/L. In the NA concentration range of 1 × 10−12 mol/L (1 pM) to 105 pM, an ultra-high sensitivity of 1.534 dB/lg(pM) is obtained. The sensor achieves an ultra-low limit of detection down to 1.0 × 10−18 mol/L (1 aM), which is more than an order of magnitude lower than the previous reports. The proposed sensor not only shows potentials in practical applications of NA detection, but also provides a new way for TFBG-SPR biochemical sensors to achieve higher RI sensitivity.

Broadband microwave photonic channelizer with high optical spectrum efficiency based on odd- and even-order local optical frequency combs

Microwave photonic channelizers assisted by coherent dual optical frequency combs (OFCs) have been widely considered for wideband radio frequency (RF) spectrum analysis. However, it requires dual OFCs with a large free spectral range (FSR) and multiple comb lines to support a wide RF coverage, leading to a complicated system structure and low optical spectrum efficiency. To this end, we propose a simplified, high optical spectrum-efficiency microwave photonic channelizer, wherein the odd-order and even-order OFCs with a double frequency spacing are generated and isolated to directly slice two frequency-shifted modulated optical spectra into a series of subbands. N comb lines can support 4 × N channels but take up a significantly reduced optical spectrum range. Partial experiments and simulations are presented to demonstrate the scheme. By exploiting a four-lines odd-order OFC and three-lines even OFC, a wideband RF spectrum range of 0–56 GHz is divided into 28 channels with a channel bandwidth of 2 GHz, and various wideband signals are spectrally analyzed with a high measurement accuracy and large image rejection suppression higher than 30 dB. Moreover, a 16-quadrature amplitude modulation (16QAM) vector signal with a center frequency of 7 GHz and a bit rate of 1 Gbps is channelized and successfully recovered with a good constellation diagram.

Fiber laser with intracavity parallel Mach-Zehnder interferometers towards Vernier refractive index sensing.

In this paper, dual-wavelength laser emission of an erbium-doped fiber laser (EDFL) with a tunable distinct wavelength selection of the simultaneously produced laser lines was achieved by applying a parallel comb filter configuration based on the optical Vernier effect. The intracavity inserted proposed comb filter consists of two parallel branches to generate the Vernier effect. Each branch is an in-line Mach-Zehnder interferometer (MZI) filter, which is composed of a polarization-maintaining fiber fusion spliced between single-mode fibers with sphere shapes at both ends. The tunability of the selected laser wavelength was realized by submerging the proposed filter in different sodium chloride/water mixtures. The proposed comb filter-based Vernier effect was used to independently achieve the selection of the dual-wavelength EDFL lines and for refractive index (RI) sensing applications. The in-line M Z I 1 and M Z I 2 structures show a wavelength shift sensitivity to RI variations of -88 and 79nm/RIU, respectively. Our proposed MZI structure presents a reliable, straightforward, and low-cost spectral comb filter for separate tunable dual-wavelength laser generation in the c-band region. Furthermore, the proposed filter structure-based Vernier effect presents a new perspective and method in the RI sensing application.

High-temperature and stress response behavior of femtosecond laser pulses inscribed eccentric fiber Bragg gratings

Eccentric fiber Bragg grating (EFBG) is inscribed in standard communication single-mode fiber using femtosecond laser pulses, and the temperature and strain sensing characteristics are experimentally demonstrated and analyzed. The EFBG exhibits strong thermal stability and good robustness in high-temperature measurement up to 1000 °C, and undergoes different thermal sensitivities during Bragg peak and the strong resonance coupled cladding spectral comb. The temperature sensitivity linearly increases with respect to the effective index of the resonant modes. Such a situation also occurs in axial strain measurement. These characteristics are of high interest for multiparametric sensing at high temperatures.

Slit Beam Shaping for Femtosecond Laser Point-by-Point Inscription of Highly Localized Fiber Bragg Grating

We propose and experimentally demonstrate a slit beam shaping method for femtosecond laser Point-by-Point (PbP) inscription of highly localized Fiber Bragg Gratings (FBGs). The influence of slit width on the shape and area of Refractive Index Modulation Region (RIMR) induced by a single femtosecond laser pulse was investigated. The shape of RIMR can be changed from a spot to a line with an enlarged RIMR, and hence enhances the coupling strength of core mode and cladding modes. The RIMRs were precisely assembled along the fiber core, producing highly localized FBGs with a spectral comb of pronounced Cladding Mode Resonances (CMRs) intensity of more than 30 dB, a wide wavelength span of 240 nm and a low insertion loss of 0.3 dB. Note that the total processing time for fabricating such a highly localized FBG only requires ∼3.7 s. Subsequently, by including tilted angle on the slit, highly localized tilted FBGs were fabricated and these FBGs show adjustable envelope on the CMRs. Moreover, we investigated the surrounding Refractive Index (RI) response and thermal characteristics of the fabricated highly localized FBGs, which exhibit a sensitivity of 510.87 nm/RIU in a wide RI measurement range and excellent high temperature resistance at 1000°C. Therefore, such highly localized FBGs could potentially be used for multi-parameter sensing in many extreme environments.

Sensing applications of fiber Bragg gratings in single mode fibers with as-drawn 25 μm diameter cladding

The sensing properties of fiber Bragg gratings (FBG) inscribed in single mode fiber with a 5 μm diameter core and 25 μm diameter cladding are studied experimentally for temperature, strain, bending, and surrounding refractive index. Compared to normal single mode fiber, the diameter of this fiber is 5 times smaller and it stretches 14.5 times more at the same applied load. Therefore, it is much more flexible and stretchable, while maintaining excellent optical quality at wavelengths near 1550 nm. In addition to a core mode back reflection resonance, strong FBGs inscribed in this fiber show a relatively small number of narrow bandwidth (0.7 nm) cladding mode resonances separated in wavelength by 2.5–6 nm. This relatively coarse spectral comb can then be used to sense many different kinds of perturbations involving core and cladding modes. In particular, unlike cladding-mode based sensors made from tilted FBGs, all resonances are of the same azimuthal order as the core mode (i.e. HE1m). This feature makes these gratings particularly sensitive to bending which causes the appearance of new resonances and reduced amplitudes of the original ones, each by up to 10 dB/mm−1 of curvature. On the other hand, the temperature sensitivities of all modes are similar to that of standard fiber (around 11 pm/oC) while strain sensitivities are somewhat higher (1.6–1.7 pm/μstrain). The surrounding refractive index sensitivity is also increased (by a factor of 3) over normal fiber, mostly due to the increased modal dispersion of the modes of the thinner cladding. Furthermore, it is possible to serially multiplex different gratings at different wavelengths by interleaving their resonance combs and preserving each grating identity in the combined spectrum.

Programmable High-Resolution Spectral Processor in C-band Enabled by Low-Cost Compact Light Paths

The flexible photonics spectral processor (PSP) is an indispensable element for elastic optical transmission networks that adopt wavelength division multiplexing (WDM) technology. The resolution and system cost are two vital metrics when developing a PSP. In this paper, a high-resolution 1 × 6 programmable PSP is investigated and experimentally demonstrated by using low-cost compact spatial light paths, which is enabled by a 2 K (1080p) liquid crystal on silicon (LCoS) and two cascaded transmission gratings with a 1000 line/mm resolution. For each wavelength channel, the filtering bandwidth and power attenuation can be manipulated independently. The total insertion loss (IL) for six ports is in the range of 5.9~9.4 dB over the full C-band. The achieved 3-dB bandwidths are able to adjust from 6.2 GHz to 5 THz. Furthermore, multiple system experiments utilizing the proposed PSP, such as flexible spectral shaping and optical frequency comb generation, are carried out to validate the feasibility for the WDM systems.

Direct-frequency-comb spectroscopy by a scanning Fabry-Pérot microcavity resonator

We present a direct-frequency-comb spectroscopy method able to resolve the spectral comb structure with an instrumental frequency resolution limited only by the coherence properties of the comb source itself. The proposed technique, based on a 50 000-finesse scanning Fabry-P\'erot microcavity resonator combined with a low-resolution diffraction grating, is characterized using a 250-MHz Er-fiber frequency comb demonstrating an ultimate frequency resolution as low as 50 kHz over an optical bandwidth up to 4 THz and with tens-of-milliseconds measurement times.

Coherent Stokes and anti-Stokes high-order components generation by biharmonic pumping via stimulated low-frequency Raman scattering.

In this Letter, we report on experimental effective coherent multiple Stokes and anti-Stokes high-order components generation by Stokes pulse injection at the process of stimulated low-frequency Raman scattering. This process can be used for effective generation the tunable spectral comb consisting of several equidistant spectral lines separated by a constant frequency spacing of several GHz.

Massively parallel coherent laser ranging using a soliton microcomb

Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser-based light detection and ranging (lidar)1 is used for long-range three-dimensional distance and velocimetry in autonomous driving2,3. FMCW lidar maps distance to frequency4,5 using frequency-chirped waveforms and simultaneously measures the Doppler shift of thereflected laserlight, similar to sonar or radar6,7 and coherent detection prevents interference from sunlight and other lidar systems. However, coherent ranging has a lower acquisition speed and requires precisely chirped8 and highly coherent5 laser sources, hindering widespread use of the lidar system and impeding parallelization, compared to modern time-of-flight ranging systems that use arrays of individual lasers. Here we demonstrate a massively parallel coherent lidar scheme using an ultra-low-lossphotonic chip-based solitonmicrocomb9. By fast chirpingof the pump laser in the solitonexistence range10 of a microcomb with amplitudes of up to several gigahertz and a sweep rate of up to ten megahertz, a rapid frequency change occurs in the underlying carrier waveform of the soliton pulse stream, but the pulse-to-pulse repetition rate of the soliton pulse stream is retained. As a result, the chirp from a single narrow-linewidth pump laser is transferred to all spectral comb teeth of the soliton at once, thus enabling parallelism in the FMCW lidar. Using this approach we generate 30 distinct channels, demonstrating both parallel distance and velocity measurements at an equivalent rate of three megapixels per second, with the potential to improve sampling rates beyond 150 megapixels per second and to increase the image refresh rate of the FMCW lidar by up to two orders of magnitude without deterioration of eye safety. This approach, when combined with photonic phase arrays11 based on nanophotonic gratings12, provides a technological basis for compact, massively parallel and ultrahigh-frame-rate coherent lidar systems.

Technique for generating broadband FM light.

We suggest a technique for using off-resonance spectral comb generation to produce broadband frequency-modulated (FM), and therefore amplitude-quieted, light. Results include closed-form formulas for the amplitudes and phases of all of the spectral components.

High-Speed and High-Resolution Microwave Photonic Interrogation of a Fiber-Optic Refractometer With Plasmonic Spectral Comb

Microwave photonic interrogation of a high-speed and high-resolution refractive index (RI) sensor based on a tilted fiber Bragg grating with surface plasmon resonance (TFBG-SPR) is proposed and experimentally demonstrated. Instead of demodulating the wavelength shift or intensity change of a TFBG-SPR spectrum in the optical domain, we convert the TFBG-SPR spectrum to the time domain based on spectral shaping (SS) and wavelength-to-time (WTT) mapping and use a digital signal processor (DSP) to extract the RI information at a high speed and high resolution. In the experiment, when an Au-coated TFBG is immersed in a solution, the TFBG-SPR spectrum will produce a dip. When the RI changes, the location of the dip in the TFBG-SPR spectrum will shift, which is a function of the RI. By passing a broadband frequency-chirped optical pulse generated by a frequency swept laser source to the TFBG-SPR and detecting the optical pulse at the output of the TFBG-SPR at a photodetector (PD), due to SS-WTT mapping, a temporal waveform with its shape identical to the optical spectrum is produced. A DSP is then used to extract the SPR envelope information from the temporal waveform. By monitoring the changes of the SPR envelope, the RI information is interrogated with a high resolution of 1.123 × 10−6 RIU at a high speed over 20 kHz.

High-contrast transparency comb of the electromagnetically-induced-transparency memory

Quantum interface of coherent optical field and atomic excitations plays an important role in quantum metrology and quantum information science. The electromagnetically-induced-transparency (EIT) technique has shown versatile and powerful capability in many applications during the last decades. By using efficient EIT-based memory, we directly observed single-photon-level Ramsey interference and also high-contrast spectral transparency comb with a visibility up to about 0.81 at around the resonant line in cold atoms. The interference fringes are mainly influenced by the pulse repetition period, the waveform overlap of the slowed and retrieved signal pulses, and the coherent lifetime of the EIT memory. All these factors have been investigated in detail. Such high-contrast interference fringe may offer us methods for precision measurement and potential applications in areas of atomic clock and magnetometry.

Wideband multi-stage CROW filters with relaxed fabrication tolerances.

We present wideband and large free spectral range optical filters with steep passband edges for the selection of adjacent WDM communication channels that can be reliably fabricated with mainstream silicon photonics technology. The devices are based on three cascaded stages of coupled resonator optical waveguides loaded on a common bus waveguide. These stages differ in the number of resonators but are implemented with exactly identical unit cells, comprised of a matched racetrack resonator layout and a uniform spacing between cells. The different number of resonators in each stage allows a high rejection in the through port response enabled by the interleaved distribution of zeros. Furthermore, the exact replication of a unique cell avoids the passband ripple and high lobes in the stopband that typically arise in apodized coupled resonator optical waveguide based filters due to fabrication and coupling induced variations in the effective path length of each resonator. Silicon photonics filters designed for the selection of 9 adjacent optical carriers generated by a 100 GHz free spectral range comb laser have been successfully fabricated with 248 nm DUV lithography, achieving an out-of-band rejection above 11 dB and an insertion loss of less than 0.5 dB for the worst channels.

Optical spectral sweep comb liquid flow rate sensor.

In microfluidic chip applications, the flow rate plays an important role. Here we propose a simple liquid flow rate sensor by using a tilted fiber Bragg grating (TFBG) as the sensing element. As the water flows in the vicinity of the TFBG along the fiber axis direction, the TFBG's spectrum changes due to its contact with water. By comparing the time-swept spectra of the TFBG in water to that of the TFBG with water flowing over it, a spectral sweep comb was formed, and the flow rate can be detected by selecting a suitable sweeping frequency. The proposed sensor has a high Q-value of over 17,000 for the lower rate and a large detectable range from 0.0058mm/s to 3.2mm/s. And the calculated corresponding lower detectable flow rate of 0.03nL/s is 3 orders magnitude better than that of the current fiber flowmeter. Meanwhile, the proposed sensor has the temperature self-compensation function for the variation of the external temperature. We believe that this simple configuration will open a research direction of the TFBG-deriving theory and configuration for lower flow rate measurements for microfluidic chip applications.

Analytical Model of Dual-Pumped Kerr Micro Resonators for Variable Free Spectral Range and Composite Frequency Comb Generation

We develop an analytical model to understand the generation of tunable free spectral range frequency comb (FC) and composite FC using two competing phenomena, namely nondegenerate four-wave mixing (FWM) and intensity-dependent modulation instability (MI) in dual-pumped micro-resonator configurations. Our analytical approach is based on the popular four-wave model. The MI rate at the onset, corresponding to the most susceptible frequency component that conditionally accompanies threshold-less comb generation in case of dual pump, is derived. Based on the analytical model, the mutual interplay of nondegenerate FWM and threshold intensity-dependent MI is investigated in detail for the comb generation process, and their regions of operations are clearly demarcated. The proposed analytical model provides a deterministic and quick estimation of micro-resonator parameters for generating a variety of FCs while avoiding the computationally intensive route of solving the Lugiato-Lefever equation. A close agreement between the analytical and numerical simulation is found.

Time and Fourier domain jointly mode locked frequency comb swept fiber laser

Fourier domain mode locked (FDML) fiber lasers enable megahertz wavelength sweeping rate but suffer from the short coherent length. Discretization of the swept spectrum by a comb filter was demonstrated effective to enhance the coherent length. In this paper, we propose a novel discretization method of the FDML signal with an intracavity intensity modulator. We propose and successfully demonstrate a time and Fourier domain jointly mode locked fiber laser with a Fabry-Perot comb filter and an intensity modulator in the cavity. A 50 GHz free spectral range comb filter in the Fourier domain mode locked fiber swept laser slices the spectrum into a series of comb lines and chops the swept signal into short pulses in time domain. The temporal signal is detected by a photodetector to generate a series of ultrashort pulses to drive the intensity modulator to further polish the intracavity pulses. We experimentally realized the proposed time and Fourier domain jointly mode locked fiber laser. Discrete wavelength swept laser output with a wavelength spacing of ~0.4 nm in a 41 nm sweeping range has been achieved.

Spectral comb of highly chirped pulses generated via cascaded FWM of two frequency-shifted dissipative solitons

Dissipative solitons generated in normal-dispersion mode-locked lasers are stable localized coherent structures with a mostly linear frequency modulation (chirp). The soliton energy in fiber lasers is limited by the Raman effect, but implementation of the intracavity feedback at the Stokes-shifted wavelength enables synchronous generation of a coherent Raman dissipative soliton. Here we demonstrate a new approach for generating chirped pulses at new wavelengths by mixing in a highly-nonlinear fiber of these two frequency-shifted dissipative solitons, as well as cascaded generation of their clones forming in the spectral domain a comb of highly chirped pulses. We observed up to eight equidistant components in the interval of more than 300 nm, which demonstrate compressibility from ~10 ps to ~300 fs. This approach, being different from traditional frequency combs, can inspire new developments in fundamental science and applications such as few-cycle/arbitrary-waveform pulse synthesis, comb spectroscopy, coherent communications and bio-imaging.

Wide Range Refractive Index Measurement Using a Multi-Angle Tilted Fiber Bragg Grating

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.