Digital Optical Frequency Comb Research Articles

Parallel interrogation of the chalcogenide-based micro-ring sensor array for photoacoustic tomography

Photoacoustic tomography (PAT), also known as optoacoustic tomography, is an attractive imaging modality that provides optical contrast with acoustic resolutions. Recent progress in the applications of PAT largely relies on the development and employment of ultrasound sensor arrays with many elements. Although on-chip optical ultrasound sensors have been demonstrated with high sensitivity, large bandwidth, and small size, PAT with on-chip optical ultrasound sensor arrays is rarely reported. In this work, we demonstrate PAT with a chalcogenide-based micro-ring sensor array containing 15 elements, while each element supports a bandwidth of 175 MHz (−6 dB) and a noise-equivalent pressure of 2.2 mPaHz−1/2. Moreover, by synthesizing a digital optical frequency comb (DOFC), we further develop an effective means of parallel interrogation to this sensor array. As a proof of concept, parallel interrogation with only one light source and one photoreceiver is demonstrated for PAT with this sensor array, providing images of fast-moving objects, leaf veins, and live zebrafish. The superior performance of the chalcogenide-based micro-ring sensor array and the effectiveness of the DOFC-enabled parallel interrogation offer great prospects for advancing applications in PAT.

On-Chip Self-Referenced Micro-Resonators Enhanced by Digital Optical Frequency Comb for Ultra-Sensitive C-Reactive Protein Detection

An on-chip self-referenced transducer assisted by digital optical frequency comb (DOFC) technology is presented for the biomolecules detection in the lower detecting limit. This self-referenced stricture eliminates temperature cross sensitivity between the detecting and reference micro-resonators fabricated on the chalcogenide glass platform. The high-resolution DOFC technology has the ultra-fine-comb spacers offering for visualized spectral analysis of micro-resonators and capturing the subtle changes of ultra-low concentrations. Combining the self-referenced and DOFC technology, the detection of C-reactive protein in the concentration from 0.1 ng/mL to 500 ng/mL with the theoretically limit of detection of 0.0146 ng/mL is achieved. Given its small footprint, compactness and high analytical performances, the proposed transducing platform has the potential for applications on integration and minimization sensing on a single integrated chip.

High spatial resolution fast Brillouin optical time-domain analysis enabled by frequency-agility digital optical frequency comb.

A significant spatial resolution enhancement scheme for digital optical frequency comb (DOFC)-based fast Brillouin optical time-domain analysis (BOTDA) is proposed and experimentally demonstrated by using frequency-agility probes, without sacrificing the frequency resolution. The proposed system ensures high spatial resolution by using short frame duration, meanwhile enabling high frequency resolution retrieval of the Brillouin gain spectrum using frequency interleaving of multiple frequency-agility DOFC probes. Additionally, quadratic phase coding is introduced to release the influence of the high peak to average power ratio of the probes. Eventually, the proposed BOTDA sensor achieves a record 5-m spatial resolution over 10-km fiber with less than 2-MHz frequency uncertainty, and a 1-GHz dynamic measurement range. For proof of concept, 10-Hz vibration sensing is also successfully demonstrated at a 40-Hz sampling rate, showing great potential for fast measurement. It is worth mentioning that a higher spatial resolution can be achieved by using more frequency-agility DOFC probes, albeit at the expense of increasing the measurement time.

Ultrasound Measurement Using On-Chip Optical Micro-Resonators and Digital Optical Frequency Comb

Optical resonator-based ultrasound detection plays a significant role in the modern ultrasonography. It consists of two steps including the optical readout of the acoustic wave signal and the electrical demodulation of the acoustic response. In this article, we conduct innovative researches on both of them for improved performance. High quality-factor (Q-factor) chalcogenide glass (ChG) microdisk and microring resonators as optical readout configurations are designed and fabricated. High-precision digital optical frequency comb (DOFC) technique is proposed to monitor the minor resonance frequency shift. The sensitivity up to 2.86 dB/Pa and noise equivalent pressure (NEP) down to 4 Pa are obtained under the 40 KHz ultrasound vibration using a 100-μm radius Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11.5</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">24</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">64.5</sub> microdisk resonator with the Q-factor of 1.15 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . As a proof-of-concept, three cascaded SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> microring resonators-based ultrasound detector is fabricated and characterized to further validate the versatility of DOFC in the arrayed microcavities. Successful realization of the 3-D ultrasound detection paves the way to the development of high-performance photoacoustic imaging.

Microbubble resonators combined with a digital optical frequency comb for high-precision air-coupled ultrasound detectors

Fast and sensitive air-coupled ultrasound detection is essential for many applications such as radar, ultrasound imaging, and defect detection. Here we present a novel approach based on a digital optical frequency comb (DOFC) technique combined with high- Q optical microbubble resonators (MBRs). DOFC enables precise spectroscopy on resonators that can trace the ultrasound pressure with its resonant frequency shift with femtometer resolution and sub-microsecond response time. The noise equivalent pressure of air-coupled ultrasound as low as 4.4 mPa / √ Hz is achieved by combining a high- Q ( ∼ 3 × 10 7 ) MBR with the DOFC method. Moreover, it can observe multi-resonance peaks from multiple MBRs to directly monitor the precise spatial location of the ultrasonic source. This approach has a potential to be applied in 3D air-coupled photoacoustic and ultrasonic imaging.

Multi-Dimensional Optical Fiber Sensing Enabled by Digital Coherent Optical Technologies

Optical fiber sensing (OFS) systems can be used to accurately measure temperature, strain, pressure, and other physical parameters by detecting the change in the field of lightwave. As they are used in more application areas, a new type of OFS with the ability of providing multi-dimensional, highly sensitive, ultra-fine, and real-time measurement is necessary. This is possible with the application of digital coherent optical technologies in the OFS systems. In this paper, we show that many aspects of OFS systems can be replaced or enhanced by digital coherent optical technologies. For example, digital optical frequency comb (DOFC) can replace the sweeping laser source, and the digital coherent optical receiver can replace the direct detection receiver to enable simultaneous detection of all the dimensions of lightwave. Based on these techniques, we present our research progresses on OFS with multi-dimensional, real-time, ultra-fine spectral analysis, and single-shot Brillouin optical time-domain analysis based DOFC for distributed OFS. We show that the OFS systems can be bought to a whole new level with the capabilities of real-time time–frequency analysis, ultra-fine spectral resolution, and ultra-low sensitivity. Opportunities and challenges of using digital coherent optical technologies in OFS systems are also discussed.

Spatial resolution improvement of single-shot digital optical frequency comb-based Brillouin optical time domain analysis utilizing multiple pump pulses.

A spatial resolution improvement scheme for the digital optical frequency comb (DOFC)-based single-shot Brillouin optical time domain analysis (BOTDA) by utilizing multiple pump pulses is demonstrated. Assisted by four pump pulses, the spatial resolution can be improved by four times without decreasing the detection resolution. The response time, and the spatial resolution of our scheme over 10km fiber, are 0.1ms and 12.5m. Distributed temperature and strain measurements are carried out with detection resolutions of 1.6°C and 44 μϵ, respectively, and the capability of dynamic measurement of this proposed BOTDA is also demonstrated by probing a vibration with frequency up to 1.2kHz.

Single-measurement digital optical frequency comb based phase-detection Brillouin optical time domain analyzer.

A single-measurement sweep-free distributed Brillouin optical time domain analyzer (BOTDA) sensor based on phase detection is proposed and experimentally demonstrated employing digital optical frequency comb (DOFC) probe signal. Brillouin Phase Spectrum (BPS) of DOFC probe induced by Brillouin interaction is measured using coherent detection in a single acquisition, without any frequency scanning and data averaging. Single-measurement BOTDA sensor based on BPS in 10km long fiber is demonstrated with a response time of 100 μs, which is limited only by the fiber length. The spatial resolution is 51.2m, determined by the duration of DOFC. And the Brillouin frequency shift (BFS) uncertainty is estimated to be~1.5 MHz at the end of fiber under test (FUT). Benefiting from the fast response time, dynamic measurement up to 1 kHz vibration frequency has been demonstrated.

Scanning-free BOTDA based on ultra-fine digital optical frequency comb.

We realize a scanning-free Brillouin optical time domain analyzer (BOTDA) based on an ultra-fine digital optical frequency comb (DOFC) with 1.95MHz frequency spacing and 2GHz bandwidth. The DOFC can be used to reconstruct the Brillouin gain spectrum (BGS) and locate the Brillouin frequency shift (BFS) without frequency scanning and thus can improve the measurement speed about 100 times compared with the conventional BOTDA. This scanning-free BOTDA scheme has also been demonstrated experimentally with 51.2m spatial resolution over 10km standard single mode fiber (SSMF) and with resolution of 1.5°C for temperature and 43.3με for strain measurement respectively.