Flat optical phased array receiver incorporating an on-chip metalens concentrator.

We propose and design a flat optical phased array (OPA) receiver that consists of a grating antenna, a free-propagation region (FPR) incorporating an on-chip metalens concentrator (OCMC), and an output port of a tapered waveguide. By concatenating the OCMC-integrated FPR with the antenna, the proposed OPA allows light coupled at a slanted ψ angle to be conveyed to the output, thereby resolving the challenges of phase-controlled light detection. To impose a space-dependent phase on the incident light from the antenna, the OCMC is constructed by laterally arranging subwavelength slot meta-atoms with varying slot lengths, which are created in the core layer of a slab and uniformly quantized at 16 phase levels. Hence, without the aid of phase modulators, the light beam emerging from the grating antenna can be focused on the output port through angle-tolerant coupling along the lateral direction. The miniaturized OCMC was confirmed to play a pivotal role in achieving enhanced in-plane coupling efficiency over the field of view.

Highly efficient broadband silicon nitride polarization beam splitter incorporating serially cascaded asymmetric directional couplers.

The implementation of a polarization beam splitter (PBS) on a silicon nitride platform remains challenging owing to its relatively low index. We therefore propose a silicon nitride PBS that exploits serially cascaded asymmetric directional couplers (ADCs), leading to a high polarization extinction ratio (PER) over a broad bandwidth. The ADC spatially routes incident light through polarization-selective mode coupling under a small footprint of 112 µm. The proposed PBS does not require an active phase control. It is thus effectively realized via a single-step lithography process. The measured transverse-electric and transverse-magnetic PERs were determined to be above 23 dB and 10 dB over an 80-nm bandwidth, respectively, spanning λ=1520-1600nm. The proposed device is thus anticipated to play a key role in providing polarization diversity in photonic-integrated circuits.

Silicon nitride optical phased array based on a grating antenna enabling wavelength-tuned beam steering.

An optical phased array (OPA) in silicon nitride (SiN) is conspicuously highlighted as a vital alternative to its counterpart in silicon. However, a limited number of studies have been conducted on this array in terms of wavelength-tuned beam steering. A SiN OPA has been proposed and implemented with a grating antenna that incorporated an array of shallow-etched waveguides, rendering wavelength-tuned beam steering along the longitudinal direction. To accomplish a superior directionality on a wavelength-tuned beam steering, the spectral beam emission characteristics of the antenna have been explored from the viewpoint of a planar structure that entails a buried oxide (BOX), a SiN waveguide core, and an upper cladding. Two OPA devices having substantially different thicknesses of the resonant cavities, established by combining the BOX and SiN core, were considered theoretically and experimentally to scrutinize the spectral emission characteristics of the antenna on beam steering. Both of the fabricated OPA devices steered light by an angle of 7.4° along the longitudinal direction for a wavelength ranging from 1530 to 1630 nm, while they maintained a divergence angle of 0.2°×0.6° in the longitudinal and lateral directions. Meanwhile, the OPA fabricated on a substantially thick BOX layer featured a limited steering performance to attain a stabilized response over a broad spectral region. We examined the influence of the cavity thickness on the spectral response of the antenna in terms of optical thickness. Based on the two antenna characteristics, it was confirmed that the grating antenna emitted the beam with a higher efficiency when the optical thickness of the cavity corresponded to odd integer multiples of the quarter wavelength. This work is a considerable strategy for designing a stabilized SiN OPA over a desired spectral region.

Temporal response of polymer waveguide beam scanner with thermo-optic phase-modulator array.

Solid-state light detection and ranging, capable of performing beam scanning without using any mechanical moving parts, requires a phase-modulator array. Polymers facilitate the fabrication of efficient phase modulators with low drive power, owing to their high thermo-optic (TO) effect and low thermal conductivity. We designed and fabricated a polymeric phase-modulator array and analyzed the temporal response of the TO phase modulator. The frequency response of the phase modulator was measured for a Mach-Zehnder interferometer (MZI), and the transfer function was modeled in terms of multiple poles and zeros. The frequency response of a fabricated beam-scanning device incorporating the TO phase modulator was also measured. The temporal response of the beam scanner was confirmed to coincide well with that of the MZI modulator. The device exhibited a fast rise time of 12 ms, accompanied by slight power variations appearing for a long period (over hundreds of seconds), which originated from the inherent viscoelastic effect of the polymer materials.

The Relationship between Superior Disc-Endplate Complex Injury and Correction Loss in Young Adult Patients with Thoracolumbar Stable Burst Fracture.

BACKGROUND: To determine the relationship between superior disc-endplate complex injury and correction loss after surgery in a group of young adult patients with a stable thoracolumbar burst fracture. METHODS: The study group was comprised of young adult patients who had undergone short-segment posterior fixation and bone grafting under the diagnosis of a stable thoracolumbar burst fracture from March 2008 to February 2014. Follow-up was available for more than 1 year. Before surgery, magnetic resonance imaging was performed to determine injury to the anterior longitudinal ligament, posterior longitudinal ligament, and superior and inferior intervertebral discs and endplates. Correction loss was evaluated by the Cobb angle, intervertebral disc height, upper intervertebral disc angle, vertebral wedge angle, and vertebral body height. RESULTS: No significant relation was noted between correction loss and an injury to the anterior longitudinal ligament, posterior longitudinal ligament, inferior intervertebral disc/endplate, and fracture site, whereas an injury to the superior endplate alone and superior disc-endplate complex showed a significant association. Specifically, a superior intervertebral disc-endplate complex injury showed statistically significant relation to postoperative changes in Cobb angle (p = 0.026) and vertebral wedge angle (p = 0.047). CONCLUSIONS: A superior intervertebral disc-endplate complex injury may have an influence on the prognosis after short-segment fixation in young adult patients with a stable thoracolumbar burst fracture.

Radiologic and Clinical Courses of Degenerative Lumbar Scoliosis (10°-25°) after a Short-Segment Fusion.

STUDY DESIGN: Retrospective study. PURPOSE: We report the surgical outcomes of small degenerative lumbar scoliosis (DLS) patients treated by a short-segment fusion and followed for a minimum of 5 years. OVERVIEW OF LITERATURE: Several surgical options are available for the treatment of DLS, such as decompression only, decompression plus a short-segment fusion, or decompression with a long segment fusion. Few studies have evaluated the results of a short-segment fusion in patients with DLS over time. METHODS: Seventy small DLS patients (Cobb's angle, 10°-25°) with a minimum follow-up of 5 years were treated with a short-segment fusion between March 2004 and February 2010. The mean patient age was 71 (male:female=16:54), with a follow-up of 6.5 years (range, 5.0-11.6). The Cobb's angle, 1 and 2 segment coronal upper intervertebral angle, 1 and 2 segment sagittal upper intervertebral angle, the lumbar lordosis angle, and the C7 plumb lines (coronal and sagittal) were evaluated using simple radiographs, and visual analog scale (VAS), back pain was assessed preoperatively, immediately after surgery, and at 3, 6, and 12 months and 3 and 5 years after surgery. To identify factors influencing the radiologic progression, age, number of fusion segments, vertebral levels of fusion, body mass index, lowest instrumented vertebra (L5 or S1), bone mineral density (>-2.5, ≤-2.5), and the presence of an interbody fusion were analyzed. RESULTS: The Cobb's angle and 1 segment coronal upper intervertebral angle showed more progression during follow up, particularly at 6 and 12 months after surgery. Clinical outcomes and radiological results were found to be significantly associated (p=0.041). No statistically significant association was found between other factors affecting radiologic progression from postoperative 6 months to 1 year. CONCLUSIONS: Radiologic variables (the Cobb's angle and coronal upper intervertebral angle-1) should be carefully considered and clinical caution exercised from 6 to 12 months after short-segment fusion in small DLS (10°-25°).

Highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator.

Mid-infrared of the 3 - 5 µm spectral region has attracted considerable attention in various fields. We report a highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator. Two hybrid etalon resonators are serially stacked to make a single peak resonance with a well-suppressed sideband. Each etalon resonator, which exploits a pair of cavities, includes not only a cavity featuring a resonance but also a functional cavity lowering the sideband of transmission spectrum. By taking advantage of double cavities, each etalon features a resonant characteristic with suppressed sideband. Consequently, the filter shows a peak transmission of 92% at resonance and well-suppressed sideband within the 3 - 5 µm spectral region. Transparent dielectric materials make the device highly efficient for both transmission and reflection. Furthermore, the filter utilizing a high-index cavity leads to an enhanced angular tolerance regardless of incident light polarization. The transmission of the filter exhibits center wavelength shifts of around 0.1 µm at an angle of 50° for both transverse electric and transverse magnetic modes, compared to normal incidence.