This work presents a novel approach to the use of bidirectional swept sources. A MEMS-VCSEL light source with a bidirectional sweeping rate of 1.6MHz is used. Bidirectional sweeping poses a challenge as the tuning curve is different for each sweep. To exploit the full performance of the swept source, both sweeps within the bidirectional sweeping must be utilized and compensated. The proposed approach involves a software solution based on the Master-Slave protocol, where sets of theoretically inferred channelled spectra (masks) are prepared in advance for each sweep. The phase retrieval method called CMS (Complex Master Slave) is employed to handle bidirectional sweeping SS-OCT, involving the calculation of two sets of functions for each sweeping direction. The correct set of masks is used for each sweep direction to accurately represent A-Scan peaks. Differences in the sweep are analysed. Finally, the proposed method is successfully tested on posterior and anterior chambers.
In our paper, the design of a high contrast grating is optimized to obtain a wider reflectivity bandwidth. HCGs present an incidence angle dependence that can affect the reflectivity bandwidth. This dependence was mitigated by reducing the duty cycle of the HCG. A further optimized model was done by matching the reflectivity phase with the phase of the incident Gaussian wavefront. The broader bandwidth of the new designs was calculated using simulations. Subsequently the MEMS VCSELs with the new designs of HCG were fabricated. The highest tuning range obtained for the focusing design was 45nm and for the low duty cycle design 38nm. These ranges are lower than the standard design due to differences from the design. Further improvements in the fabrication process are required to demonstrate the new designs proposed.
Ocular aberrometry with a wide dynamic range for assessing vision performance and anterior segment imaging that provides anatomical details of the eye are both essential for vision research and clinical applications. Defocus error is a major limitation of digital wavefront aberrometry (DWA) as the blurring of the detected point spread function (PSF) results in a significant reduction of SNR beyond ±3 D range. With the aid of Badal-like precompensation of defocus, the dynamic defocus range of the captured aberrated PSFs can be effectively extended. We demonstrate a dual-modality MHz VCSEL-based swept-source OCT (SS-OCT) system, with an easy switch between DWA and OCT imaging mode. The system is capable of measuring aberrations with defocus dynamic range of D as well as providing fast anatomical imaging of the anterior segment at an A-scan rate of 1.6 MHz. The dual-mode system stands out for its modular design wherein simple hardware additions to an SS-OCT system enable the aberration measurement in DWA mode. In DWA mode, a diffraction-limited stationary spot is formed at the retina by a narrow illumination beam. The reflected light passes then through the full pupil of the eye, thereby the single path optical aberrations are captured. The OCT detection leads to volumetric PSFs, which are post-processed using the digital lateral shearing-based digital adaptive optics technique (DLS-DAO) to reconstruct the wavefront error. Capturing both optical and anatomical information of the eye can be potentially helpful for improved diagnosis and targeted treatment of ocular diseases.
In this report, a swept source optical coherence tomography (SS-OCT) instrument, equipped with a novel, multi-MHz tuning range swept source is presented. The source, based on an electrically pumped Micro Electro Mechanical System Vertical Cavity Surface Emitting Laser (MEMS-VCSEL) technology, is able to operate at 1.6 MHz with bidirectional sweeping, and emits light at a central wavelength of 1060 nm with a wavelength tuning range of 30 nm at -3 dB. The capabilities of the SS are investigated, and characterized, using an OCT instrument equipped with pupil tracking capabilities. The source provides an experimental axial resolution of 30 μm measured in air. From measuring the sensitivity drop-off, an axial imaging range longer than 90 mm was inferred. To estimate the wavenumber tuning non-linearities of the source and generate images, the Complex Master-Slave (CMS) method was employed. CMS also allowed for real-time visualization of the en-face images of the human retina, in-vivo, without computing the whole volume. By using the novel SS, in-vivo real-time images of the human retina are produced at 4 Hz volume rate when paired with a 2-D orthogonal galvanometer scanner. The increase in speed for A-scan and volume acquisition tends to reduce fragmented and blurry images. Apart from a montage of en-face images generated in real-time from various axial positions, we also present Bscans produced with a galvanometer scanner driven at 1 kHz from the optic nerve area.
MEMS VCSELs are one of the most promising swept source (SS) lasers for optical coherence tomography (OCT) and one of the best candidates for future integration with endoscopes, surgical probes and achieving an integrated OCT system. However, the current MEMS-based SS are processed on the III-V wafers, which are small, expensive and challenging to work with. Furthermore, the actuating part, i.e., the MEMS, is on the top of the structure which causes a strong dependence on packaging to decrease its sensitivity to the operating environment. This work addresses these design drawbacks and proposes a novel design framework. The proposed device uses a high contrast grating mirror on a Si MEMS stage as the bottom mirror, all of which is defined in an SOI wafer. The SOI wafer is then bonded to an InP III-V wafer with the desired active layers, thereby sealing the MEMS. Finally, the top mirror, a dielectric DBR (7 pairs of TiO2 - SiO2), is deposited on top. The new device is based on a silicon substrate with MEMS defined on a silicon membrane in an enclosed cavity. Thus the device is much more robust than the existing MEMS VCSELs. This design also enables either a two-way actuation on the MEMS or a smaller optical cavity (pull-away design), i.e., wider FSR (Free Spectral Range) to increase the wavelength sweep. Fabrication of the proposed device is outlined and the results of device characterization are reported.
The paper presents the design and fabrication of an optically pumped 1550nm tunable MEMS VCSEL with an enclosed MEMS. The MEMS is defined in SOI and the active material, an InP wafer with quantum wells are bonded to the SOI and the last mirror is made from the deposition of dielectric materials. The design brings in flexibility to fabricate MEMS VCSELs over a wider range of wavelengths. The paper discusses results from the simulations and bonding results from fabrication. The device will push the boundaries for wavelength sweep speed and bandwidth.
We report a new method for monitoring vapor concentration of volatile organic compounds using a vertical-cavity
surface-emitting laser (VCSEL). The VCSEL is coated with a polymer thin film on the top distributed Bragg
reflector (DBR). The analyte absorption is transduced to the electrical domain through modulation of the VCSEL
output power as the polymer swell. We have investigated the responsivity of this technique experimentally using
a plasma polymerized polystyrene coating and explain the results theoretically as a reflectance modulation of
the top DBR.
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