Hak-Soon Lee

Silicon photonic temperature sensor employing a ring resonator manufactured using a standard CMOS process

An ultra-small integrated photonic temperature sensor has been proposed and demonstrated which incorporates a silicon ring resonator linked to a vertical grating coupler. It was manufactured using a 0.18 μm standard CMOS process, rendering a homogeneous integration into other electrical/optical devices. The temperature variation was measured by monitoring the shift in the resonant wavelength of the silicon resonator, which was induced by the thermo-optic effect and the thermal expansion effect. The dependence of its sensing capability upon the waveguide width of the resonator was intensively probed both theoretically and experimentally. The best achieved sensitivity was about 83 pm/°C for a waveguide width of 500 nm, while the sensitivity was boosted by ~10 pm/°C by adjusting the waveguide width from 300 nm to 500 nm. Finally, the response speed of the sensor was as fast as ~6 μs.

Passively Aligned Transmit Optical Subassembly Module Based on a WDM Incorporating VCSELs

A short-reach transmit optical subassembly module enabled by a passive alignment has been proposed and manufactured which takes advantage of a filter free coarse wavelength-division multiplexer (WDM). A four-channel data transmission was accomplished by employing only two plastic optical fibers and four vertical-cavity surface-emitting lasers (VCSELs). A multiaxis beam combination was introduced to overcome the use of WDM filters. The proposed module was constructed via a passive alignment; the overall alignment tolerance was examined by assessing the optical coupling efficiency in terms of the VCSEL displacement. The achieved tolerance was ~20 and ~150 μ m in the horizontal and vertical direction respectively, while the demonstrated peak coupling was ~41%. Finally, a high-speed 2.5-Gb/s signal was successfully delivered via the demonstrated module.

Ribbon plastic optical fiber linked optical transmitter and receiver modules featuring a high alignment tolerance

Ribbon plastic optical fiber (POF) linked four-channel optical transmitter (Tx) and receiver (Rx) modules have been proposed and realized featuring an excellent alignment tolerance. The two modules share a common configuration involving an optical sub-assembly (OSA) with vertical cavity surface emitting lasers (VCSELs)/photodetectors (PDs), and their driver ICs, which are integrated onto a single printed circuit board (PCB) substrate. The OSA includes an alignment structure, a beam router and a fiber guide, which were produced by using plastic injection molding. We have accomplished a fully passive alignment between the VCSELs/PDs and the ribbon POF by taking advantage of the alignment structure that serves as a reference during the alignment of the constituent parts of the OSA. The electrical link, which largely determines the operation speed, has been remarkably shortened, due to a direct wire-bonding between the VCSELs/PDs and the driver circuits. The light sources and the detectors can be individually positioned, thereby overcoming the pitch limitations of the ribbon POF, which is made up of perfluorinated graded-index (GI) POF with a 62.5 μm core diameter. The overall alignment tolerance was first assessed by observing the optical coupling efficiency in terms of VCSEL/PD misalignment. The horizontal and vertical 3-dB alignment tolerances were about 20 μm and 150 μm for the Tx and 50 μm and over 200 μm for the Rx, respectively. The VCSEL-to-POF coupling loss for the Tx and the POF-to-PD loss for the Rx were 3.25 dB and 1.35 dB at a wavelength of 850 nm, respectively. Subsequently, a high-speed signal at 3.2 Gb/s was satisfactorily delivered via the Tx and Rx modules over a temperature range of -30 to 70°C with no significant errors; the channel crosstalk was below -30 dB. Finally, the performance of the prepared modules was verified by transmitting a 1080p HDMI video supplied by a Bluelay player to an LCD TV.

Refractometric sensor utilizing a vertically coupled polymeric microdisk resonator incorporating a high refractive index overlay.

A refractometric sensor resorting to a vertically coupled polymeric microdisk resonator was demonstrated, estimating the refractive index (RI) of an analyte by monitoring the resonant wavelength shift in its transfer characteristics. The disk resonator was especially overlaid with a high RI TiO2 film, thereby reinforcing the interaction of the evanescent field of its guided mode with the analyte. The sensitivity of the sensor was theoretically and experimentally confirmed to be enhanced by adjusting the overlay thickness. The fabricated sensor provided the maximum sensitivity of approximately 294 nm/RIU (refractive index unit) with the 40-nm-thick overlay, which is equivalent to an improvement of 150% compared with the case without the overlay.

Temperature Compensated Refractometric Biosensor Exploiting Ring Resonators

A temperature compensated refractometric biosensor in polymeric waveguides was demonstrated by integrating a vertically coupled ring resonator with a laterally coupled ring resonator playing the role of monitoring the temperature. The proposed main sensing part was evaluated by observing the concentration of an aqueous glucose solution, and it was found to work decently around room temperature, yielding the sensitivity of ~120 pm/(g/dL), and the temperature monitoring part offered a temperature sensitivity of -172 pm/degC. With the help of the temperature compensation, the measurement error of the main sensor resulting from the temperature variation was substantially reduced from 1.33 (g/dL)/degC down to 0.03 (g/dL)/degC.

CWDM based HDMI interconnect incorporating passively aligned POF linked optical subassembly modules

A four-channel transmitter OSA (TOSA) and a receiver optical sub-assembly (ROSA) module were presented. They take advantage of a coarse WDM (CWDM) scheme, employing two types of VCSELs at 780 and 850 nm, where no wavelength filters are involved in the TOSA. The ROSA and TOSA were constructed through a fully passive alignment process using components produced by virtue of a cost effective plastic injection molding technique. In order to build a high quality optical HDMI interconnect, four channel optical links between these modules ware established via two graded-index plastic optical fibers (GI-POFs). The HDMI interconnect was thoroughly evaluated in terms of the alignment tolerance, the light beam propagation, and the data transmission capability. For the ROSA, the measured tolerance, as affected by the photodiode alignment, was ~45 μm and over 200 μm for the transverse and longitudinal directions, respectively. For the TOSA, the tolerance, which is mostly dependent upon the VCSEL alignment, was ~20 μm and more than 200 μm for the transverse and longitudinal directions, respectively. The beam profiles for the TOSA and ROSA were monitored to confirm their feasibility from the optical coupling perspective. A digital signal at 2.5 Gb/s was efficiently transmitted through the HDMI interconnect with a bit error ratio of below 10-16. A 1080p HDMI signal from a Blu-ray player was delivered through the interconnect to an LCD monitor and successfully displayed a high quality video.

Reflective Optical Encoder Capitalizing on an Index Grating Imbedded in a Compact Smart Frame

A reflective optical encoder has been proposed and realized capitalizing on a miniaturized sensing head, which integrates both a pair of index gratings and a beam router in a plastic smart frame. The index gratings play a role in selectively transmitting the incident beam carrying the periodic grating pattern of a code scale. The beam router, consisting of a collimating lens in conjunction with a prism, is used to collimate and steer the beam originating from a vertical-cavity surface-emitting laser (VCSEL) toward the code scale. The proposed encoder is rigorously designed by ray optic simulations and manufactured by passively aligning a smart frame, which is produced by plastic injection molding, with a VCSEL at λ = 850 nm and two semicircular photodetectors. The index grating is created by forming an Al grating pattern of a 10- μm pitch on a silica substrate. Two output signals provide sinusoidal signals, which are 90 ° out of phase from each other. The period of the signals is 4 μs at a frequency of 250 kHz, which is equivalent to a period of 10 μm. As expected, the relative phase relationship is reversed by altering the direction of rotation of the code scale. By examining either of the two output signals, we can simply demonstrate positional and angular resolutions of about 10 μm and 0.04 °, respectively. The attained resolutions can be readily enhanced to 2.5 μm and 0.01 °, by simultaneously taking into account the two signals with a distinct 90 ° phase difference.

Tunable-Resonator-Based Temperature Sensor Interrogated through Optical Power Detection

An interrogation scheme based on optical power monitoring was proposed and implemented to demonstrate a compact photonic temperature sensor, which incorporates a tunable silicon resonator with a heating electrode, a single-wavelength light source, and a photodetector. The temperature is basically determined by tracking the notch of the resonator, which is displaced by the thermal effects. By utilizing the proposed interrogation scheme, which did not require any complex spectral scanning, the ambient temperature was successfully estimated with a resolution of better than 0.1 °C over the range of 20 to 50 °C, and the electrical power consumption was ~600 mW.

Reflective-type photonic displacement sensor incorporating a micro-optic beam shaper

A reflective-type photonic displacement sensor has been proposed and realized by taking advantage of a compact optical sensing head that incorporates a micro-optic beam shaper in conjunction with a rotary scale. The miniature beam shaper, which includes a pair of aspheric lenses, plays the role of optimally focusing a light beam emitted by a VCSEL source onto a rotary scale by utilizing efficient collimating optics. The focused beam is selectively reflected by a periodic grating pattern relevant to the scale; the beam then arrives at the photodetector (PD) receiver. Hence, an arbitrary displacement, encoded by the scale, could readily translate into an output signal available from the receiver. The proposed sensor was thoroughly designed through ray tracing based simulations and then analyzed in terms of the alignment tolerance for the VCSEL and code scale. The slim beam shaper was cost effectively constructed using plastic injection molding, and it was precisely integrated with the VCSEL and PD in a passive alignment manner, in order to complete the optical sensing head. In order to construct the displacement sensor, a code-wheel type scale containing alternate patterns of high- and low-reflection, was integrated with the optical head. The sensor was primarily characterized with respect to the evolution of generated beams for single-mode (SM) and multi-mode (MM) VCSELs, taking into consideration that the modulation depth of the output signal is elevated with decreasing focused beam size. For an embodied displacement sensor based on an SM VCSEL, leading to a focused beam spot of ~30 μm, a well-defined output with a modulation depth of 7% was obtained in response to the displacement of the rotary scale engraved with a grating of 10-μm pitch. The linear and angular resolutions were accordingly estimated to be better than 5 μm and 0.02°, respectively.

High Tolerance Receptacle Type Active Optical Interconnect Incorporating Collimated Beam Based Optics

A miniaturized short-reach active optical interconnect, taking advantage of collimated beam based optics, is presented, that features a superior structural tolerance. A receptacle type transmitter (Tx) and receiver (Rx) are introduced, to precisely interface with a standard multimode fiber in a flexible manner while they are efficiently operating based on equivalent collimated beams. In particular, the dependence of the tolerance and optical coupling upon the alignment of constituent elements is rigorously analyzed through ray optic simulations. Plastic injection molding was used to produce crucial parts associated with the optical modules; the arrangement tolerance for the components, such as VCSEL sources, photodetectors (PDs), fibers, and collimating/focusing lenses, was intensively investigated, by monitoring the beam patterns in conjunction with the optical coupling. A compact collimated beam was observed to be generated, providing a divergence of ~ 1.5°. The measured positional shift for the focused beam was found to be only below 10 μm, in response to a misalignment of over 300 μm between the collimating and focusing lens. Thanks to the proposed collimated beam optics, the overall 3-dB alignment tolerance was substantially enhanced to ~ 25 μm. The optical interconnect was finally completed by passively aligning the Tx and Rx modules, with an optical loss of ~ 2.9 and 0.85 dB for the VCSEL-to-fiber and fiber-to-PD coupling, respectively. Its feasibility was practically ensured by delivering a high speed digital signal at 2.5 Gb/s and moreover HD-SDI video data, over a 100-m long fiber.