Tandem Mach Zehnder Directional Coupler Design and Simulation on Silicon Platform for Optical Coherence Tomography Applications.

We design and compare the splitting ratio wavelength flatness of directional coupler (DC), Mach-Zehnder directional coupler (MZDC), and tandem MZDC. All coupler responses are analyzed, and tandem MZDC performance is the best in the wavelength insensitivity compared with the other two. An MZDC with any coupling ratio could be utilized to match the maximum flatness in a 40-nm wavelength range. To extend a broad flatness range, the tandem MZDC is proposed and still follows the Mach Zehnder structure taking two MZDCs as couplers connected through a decoupled region. Unlike DC, MZDC with the flat wavelength response has a non-linear output phase. Hence, using two wavelength-insensitive MZDCs as the coupling function in a tandem MZDC could demonstrate a more extensive decoupled phase term to maximize the flat wavelength response. The tandem MZDC theoretically demonstrates the splitting ratio with 100-nm flatness in the wavelength range from 1250 nm to 1350 nm. Finally, a point spread function through the tandem MZDC shows a 24-dB signal-to-noise ratio improvement in optical coherence tomography applications.

Two Interrogated FBG Spectral Linewidth for Strain Sensing through Correlation.

The spectral linewidth from two cross-correlated fiber Bragg gratings (FBGs) are interrogated and characterized using a delayed self-homodyne method for fiber strain sensing. This approach employs a common higher frequency resolution instead of wavelength. A sensitivity and resolution of 166 MHz/µÎµ and 50 nε were demonstrated from 4 GHz spectral linewidth characterization on the electric spectrum analyzer. A 10 nε higher resolution can be expected through random noise analyses when the spectral linewidth from two FBG correlations is reduced to 1 GHz. Moreover, the FBG spectrum is broadened during strain and experimentally shows a 0.44 pm/µÎµ sensitivity, which is mainly caused by the photo elastic effect from the fiber grating period stretch.

Surface plasmon resonator using high sensitive resonance telecommunication wavelengths for DNA sensors of Mycobacterium tuberculosis with thiol-modified probes.

Various analytes can be verified by surface plasmon resonance, thus continuous improvement of this sensing technology is crucial for better sensing selection and higher sensitivity. The SPR sensitivity on the wavelength modulation is enhanced with increasing wavelengths. The telecommunication wavelength range was then utilized to detect Mycobacterium tuberculosis (MTB) deoxyribonucleic acid (DNA) under two situations, without immobilization and with 5'-thiol end labeled IS6100 DNA probes, for SPR sensitivity comparison. The experimental data demonstrated that the SPR sensitivity increased more than 13 times with the wavelength modulation after immobilization. Since the operating wavelength accuracy of a tunable laser source can be controlled within 0.001 nm, the sensitivity and resolution on immobilized MTB DNA were determined as 1.04 nm/(µg/mL) and 0.9 ng/mL, respectively.

Biosensing using microring resonator interferograms.

Optical low-coherence interferometry (OLCI) takes advantage of the variation in refractive index in silicon-wire microring resonator (MRR) effective lengths to perform glucose biosensing using MRR interferograms. The MRR quality factor (Q), proportional to the effective length, could be improved using the silicon-wire propagation loss and coupling ratio from the MRR coupler. Our study showed that multimode interference (MMI) performed well in broad band response, but the splitting ratio drifted to 75/25 due to the stress issue. The glucose sensing sensitivity demonstrated 0.00279 meter per refractive-index-unit (RIU) with a Q factor of ~30,000 under transverse electric polarization. The 1,310 nm DFB laser was built in the OLCI system as the optical ruler achieving 655 nm characterization accuracy. The lowest sensing limitation was therefore 2 × 10-4 RIU. Moreover, the MRR effective length from the glucose sensitivity could be utilized to experimentally demonstrate the silicon wire effective refractive index with a width of 0.45 mm and height of 0.26 mm.

Mycobacterium tuberculosis DNA detection using surface plasmon resonance modulated by telecommunication wavelength.

A surface plasmon resonance sensor for Mycobacterium tuberculosis (MTB) deoxyribonucleic acid (DNA) is developed using repeatable telecommunication wavelength modulation based on optical fiber communications laser wavelength and stability. MTB DNA concentrations of 1 µg/mL and 10 µg/mL were successfully demonstrated to have the same spectral half-width in the dip for optimum coupling. The sensitivity was shown to be -0.087 dB/(µg/mL) at all applied telecommunication wavelengths and the highest sensitivity achieved was 115 ng/mL without thiolated DNA immobilization onto a gold plate, which is better than the sensor limit of 400 ng/mL possible with commercial biosensor equipment.

Sensitivity Enhancement of Group Refractive Index Biosensor through Ring-Down Interferograms of Microring Resonator.

In recent years, silicon-on-insulator substrates have been utilized for high-speed and low-power electronic components. Because of the high refractive index contrast of the silicon wire, its photonic device footprint can be significantly reduced. Moreover, the silicon photonic process is compatible with a complementary metal-oxide-semiconductor fabrication, which will benefit the high-density optoelectronic integrated circuits development. Researchers have recently proposed using the microring resonator (MRR) for label-free biosensing applications. The high-quality factor caused by the substantial electric field enhancement within the ring makes the MRR a good candidate for biomolecule detection under low analyte concentration conditions. This paper proposes an MRR chip to be a biosensor on the silicon platform through the relative displacement between the spatial ring-down interferograms at various cladding layers. The higher-order ring-down of the spatial interference wave packet will enhance the biosensing sensitivity after optimizing the coupling, MRR length, and the optical source bandwidth at the fixed optical waveguide loss. Finally, a typical sensitivity of 642,000 nm per refractive index unit is demonstrated under 0.1 µW minimum optical power detection for an MRR with a 100 µm radius. Higher sensitivity can be executed by a narrow bandwidth and lower silicon wire propagation loss.

Correction: Lai et al. Sensitivity Enhancement of Group Refractive Index Biosensor through Ring-Down Interferograms of Microring Resonator. Micromachines 2022, 13, 922.

In the original publication [...].

Bidirectional Coupler Study for Chip-Based Spectral-Domain Optical Coherence Tomography.

A chip-based spectral-domain optical coherence tomography (SD-OCT) system consists of a broadband source, interferometer, and spectrometer. The optical power divider flatness in the interferometer's wavelength is crucial to higher signal-to-noise ratios. A Mach-Zehnder directional coupler (MZDC) structure could be utilized to smoothly maximize the splitting ratio of 50:50 on a silicon platform, with a sub-micrometer of decoupler optical path difference insensitive to the process variation up to 20 nanometers. However, the optical signal reflected from the reference and sample will go back to the same interferometer MZDC. The so-called bidirectional coupler MZDC will not illustrate a flat optical power response in the operating wavelength range but could still demonstrate at least 20 dB signal-to-noise ratio improvement in OCT after the echelle grating spectrum compensation is applied. For maintaining the axial resolution and sensitivity, the echelle grating is also insensitive to process shifts such as MZDC and could be further utilized to compensate a 3 dB bidirectional MZDC structure for a broad and flat 100 nm wavelength response in the interferometer-based on-chip SD-OCT.

Waveguide coupled photodiode using reflector and metal coplanar waveguide for optical triplexing applications.

The monitoring photodiode is the key building block for an optical triplexer at wavelengths of 1310, 1490, and 1550 nanometers. The InGaAs/InP photodetectors were proposed and fabricated to be monolithi-cally integrated with AlGaAs/GaAs optical waveguides using total internal reflection coupling. The metal coplanar waveguides on top of the polyimide planarization and passivation layer were then connected to illustrate the high speed monitoring functions. The full width half maximum of the temporal response and 3-dB bandwidth for the optical waveguide coupled photodiodes demonstrated 29.5 ps and 11 GHz, respectively. The bit error rate performance of this integrated photodiode at 10 Gbit/s with 2(7)-1 long pseudo-random bit sequence NRZ input data also showed error-free operation.

Reflectively coupled waveguide photodetector for high speed optical interconnection.

To fully utilize GaAs high drift mobility, techniques to monolithically integrate In0.53Ga0.47As p-i-n photodetectors with GaAs based optical waveguides using total internal reflection coupling are reviewed. Metal coplanar waveguides, deposited on top of the polyimide layer for the photodetector's planarization and passivation, were then uniquely connected as a bridge between the photonics and electronics to illustrate the high-speed monitoring function. The photodetectors were efficiently implemented and imposed on the echelle grating circle for wavelength division multiplexing monitoring. In optical filtering performance, the monolithically integrated photodetector channel spacing was 2 nm over the 1,520-1,550 nm wavelength range and the pass band was 1 nm at the -1 dB level. For high-speed applications the full-width half-maximum of the temporal response and 3-dB bandwidth for the reflectively coupled waveguide photodetectors were demonstrated to be 30 ps and 11 GHz, respectively. The bit error rate performance of this integrated photodetector at 10 Gbit/s with 2(7)-1 long pseudo-random bit sequence non-return to zero input data also showed error-free operation.

Polarization-dependent loss compensation on silicon-wire waveguide tap by complex refractive index of metals.

A photodetector can be applied onto a silicon-wire waveguide tap to monitor light signals on waveguides. To meet the complexity of optical integrated circuits, the proposed photodetector would be positioned onto a wafer base instead of being employed and terminated at the edge end of an optical component. Because the silicon-wire-based optical directional coupler shows an undesirably high level of polarization-dependent loss on the tap port compared with the primary port, the complex refractive index of the reflective metal layer was proposed integrated into the direction-changing tap region, made using a 54.7 degrees angle from anisotropic silicon wet etching. This structure compensates for the polarization dependent loss of the tapping signal power for the primary port monitoring.

Repeat percutaneous vertebroplasty at cemented vertebra with fluid sign and recurrent pain.

SUMMARY: Six patients (three females and three males) were referred from their clinicians for evaluation with complaints of recurrent pain. A followup MRI showed fluid at the cemented vertebral bodies. Repeat percutaneous vertebroplasty (PV) was performed in these six patients at the cemented vertebrae. Pain scores, mobility scores, and spine MRIs before the 1st PV, prior to the repeat PV, and 1 and 3 months after the repeat PV were obtained. One month after the repeat PV, the six patients had a mean pain score reduction of 6.2 points and a mean postoperative pain level reduction of 2.8 points. Four of the six patients demonstrated an improvement in mobility with a 1.7 point mean decrease one month after the repeat PV. There was decreased fluid and bone marrow edema in four of the six patients on the follow-up MRIs one and three months after the repeat PV. Repeat PV at cemented vertebrae with fluid signs may offer therapeutic benefits for recurrent pain.