Polymer waveguide optical phased array (OPA) beam scanners can achieve stable beam steering with low driving power owing to the large TO coefficient and low thermal conductivity of the polymer material. However, polymeric OPAs with low refractive index contrast exhibit narrow field-of-view (FOV) because of the wide pitch of the output waveguide channels compared to their counterparts of Si and silicon nitride (SiN) photonic integrated circuit platforms. Meanwhile, SiN waveguides offer stronger mode confinement with smaller waveguide core sizes than polymer waveguides, and they can handle high optical power without nonlinear effects. Through monolithic integration of polymer and SiN waveguides using adiabatic transition tapers, a high-performance phase modulator (PM) is achieved for the purpose of demonstrating an OPA device with low power consumption, wide scanning angle, and fast response.
The real-time power information from the power plants and substations are required for smart grid infrastructure. However, the conventional current sensors are vulnerable to external electromagnetic interference (EMI) and surge current. Therefore, optical current sensors have received high attention for their advantages over the conventional system. Optical current sensors have various merits of insulating sensor materials, linear response over wide current range, and light weight for easy installment. Furthermore, they can be produced in small footprints and are immune to the external EMI, and they can be used in high power capacity. The only disadvantage of optical current sensor is the complicated interferometry structure, which could be resolved by adopting integrated optics. In this work, we designed and fabricated a polymeric integrated-optic device for producing the polarization rotated reflection interferometry (PRRI). By adopting integrated optics, the PRRI could be fabricated by a simple process and is advantageous for mass production. The polymeric waveguide device consists of phase modulators, TE-pass polarizers, directional couplers, and polarization converters integrated on a single chip. It was fabricated through a conventional semiconductor fabrication process, such as photolithography, spin-coating, UV-curing, etc. Finally, the fabricated polymeric integrated optic current sensor exhibited excellent linearity, fast response time, and high accuracy satisfying the 0.2 accuracy class (IEC 60044-8).
Optical phased array (OPA) beam scanners consisting of polymer waveguides have demonstrated precise beam steering with low driving power by taking advantage of the large TO coefficient of the polymer material. However, the slow response time of thermo-optic phase modulators limits the beam scanning time, and it has to be improved for the practical LiDAR application. In this study, a polyimide with a high refractive index is adopted to produce a waveguide with a small waveguide core size to reduce the response time of the thermo-optic phase modulator and improve the beam scanning speed of the OPA device.
Since the integrated optic current sensor (IOCS) is made of various optical components as a waveguide, it has advantages of small volume, reduced manufacturing cost, and is advantageous for mass production. In this work, we demonstrate the tolerance of device performance for providing stable sensing operation in long-term sensing experiment.
The phase error imposed on the optical phased arrays (OPAs) used for the beam scanner of LiDAR is unavoidable due to the minute dimensional fluctuations that occur during the waveguide manufacturing process. In this study, a fast-reacting beamforming algorithm is developed based on the rotating element vector method for compensating the phase error. The proposed algorithm is highly suitable for the OPA devices comprised of polymer waveguides, allowing each phase modulator to be controlled independently. Additionally using the least square approximation, the beamforming time is shortened to 16 seconds for a 32-channel polymer waveguide OPA device.
For constructing a quadrature phase interferometer for optical current sensors, we integrated various optical components on a single polymeric waveguide chip. To obtain stable output in quadrature interferometer, it is important to remove the scattered light inside the planar waveguide of integrated optics. In this work, we analyzed the mechanism of optical interference due to the radiated light, and improved the structure of optical waveguide device . After the improvement of waveguide structure, the sensor output signal was maintained within ±0.5% error range regardless of its operating point drift.
The evanescent field–based polymeric planar waveguide refractive index sensors having a high Q Fabry–Pérot (FP) cavity between identical dual Bragg gratings corrugated on the surface of the planar waveguide were developed. The FP Bragg gratings cavity was fabricated with a cavity size of 5 and 7 mm, respectively. The spectra of light reflected from fabricated Bragg gratings, which were butt joined, were measured and compared with different indices of surrounding media. It was obtained that the FP Bragg gratings cavity is more sensitive than the single Bragg grating. The sensor developed shows much promise in the application of biomedical diagnostics such as a biosensor and/or environmental monitoring systems.
This study demonstrates the high sensitivity of high Q polymeric planar waveguide refractive index sensors used on the
evanescent field. A Fabry-Perot Bragg gratings cavity was fabricated with a cavity size of 5 mm and 7 mm, respectively.
The spectra of light reflected from fabricated Bragg gratings, which were butt-joined, were measured and compared with
different indices of surrounding media. It was confirmed the FP Bragg gratings cavity is more sensitive than the single
Bragg grating. The sensor developed in this study shows much promise in the application of biomedical diagnostics such
as a bio-sensor and/or environmental monitoring systems.
Flexible polymer waveguide with an imbedded Bragg grating is incorporated to form an external cavity lasers operating at near infrared wavelength. The third-order Bragg reflection grating imbedded in a polymer waveguide is optimized to produce an appropriate reflection spectrum for NIR-ECL with a center wavelength of 840 nm. Compressive strain and tensile strain were imposed across the flexible polymer device so as to achieve the wavelength tuning of 32 nm. The wavelength peak position was almost linearly proportional to the imposed strain, and the efficiency of wavelength tuning by the imposed strain was 0.9 pm/ μɛ.
Polarization controllers are demonstrated by integrated polymer waveguide technology. The integrated-optic polarization controllers consist of three birefringence modulators and 45°-inclined quarter-wave plates inserted between them. The birefringence modulator by incorporating highly birefringent polymer material exhibits the difference in phase retardation for TE and TM guided modes in proportion to the heating power. Thin-film quarter-wave plates are fabricated by using a reactive mesogen, and inserted between the birefringence modulators to produce static phase retardation and polarization coupling. By applying a triangular AC signal to one birefringence modulator and a DC signal to another, general polarization conversion covering the entire surface of the Poincaré sphere is demonstrated.
Label-free biosensors based on polymeric waveguides with Bragg reflection grating are demonstrated for the purpose of
the highly sensitive protein detectors. Due to the unique processibility of the polymers in terms of nano-imprinting and
the injection molding, the polymer waveguide devices has large potential to provide cost-effective solution for the
disposable biosensors as long as robust immunoassay process is developed. The large contrast waveguide consisting of
low index fluorinated polymer exhibits enhanced sensitivity of detecting cover index variation by 1.9 times.
A tunable wavelength filters with Bragg grating are fabricated on a flexible substrate by using a post lift-off process
along with an absorbing layer in order to provide highly uniformity of Bragg grating patterns. When the Bragg grating is
fabricated on a flexible substrate, the highly elastic property of polymer is favorable to obtain much wider tuning range
than the silica fiber. The flexible Bragg reflector shows narrow bandwidth, which is convincing the uniform grating
structure fabricated on plastic film. By stretching the flexible polymer device, the Bragg reflection wavelength is tuned
continuously up to 45 nm for the maximum strain of 31,690 με which is determined by the elastic expansion limit of
waveguide polymer. From the linear wavelength shift proportional to the strain, the photoelastic coefficient of the ZPU
polymer is found.
Liquefied natural gas (LNG) transport carriers are exposed to a risk by the repeated bump in the LNG container during
the vessel traveling over the wave in ocean. The liquid inside the container, especially when it was not fully contained,
make a strong bump onto the insulation panel of the tank wall. The insulation panel consists of several layers of thick
polyurethane foam (PUF) to maintain the LNG below the cryogenic temperature, -162°C. Due to the repeated shock on
the PUF, a crack could be developed on the tank wall causing a tremendous disaster for LNG carriers. To prevent the
accidental crack on the tank, a continuous monitoring of the strain imposed on the PUF is recommended. In this work, a
fiber-optic Bragg grating was imbedded inside the PUF for monitoring the strain parallel to the impact direction. The
optical fiber sensor with a small diameter of 125 μm was suitable to be inserted in the PUF through a small hole drilled
after the PUF was cured. In-situ monitoring of the strain producing the change of Bragg reflection wavelength, a high
speed wavelength interrogation method was employed by using an arrayed waveguide grating. By dropping a heavy
mass on the PUF, we measured the strain imposed on the insulation panel.
Bragg reflecting waveguide devices are fabricated on a flexible substrate by using a post lift-off process in order to
provide highly uniform grating patterns on a wide range. In this process, the flexible substrate spin-coated on silicon
wafer is released after the final fabrication process of chip dicing. The fabricated flexible Bragg reflector shows very
sharp transmission spectrum with 3-dB bandwidth of 0.1 nm and 10-dB bandwidth of 0.4 nm, which proves the Bragg
reflector has excellent uniformity. To achieve athermal operation of the flexible Bragg reflector, thermal expansion
property of the plastic substrate is controlled by the thickness of two polymer materials constructing the plastic substrate.
The flexible substrate with 0.7-μm SU-8 layers sandwiching 100-μm NOA61 layer provides an optimized thermal
expansion property to compensate the thermo-optic effect of the waveguide made of ZPU polymer. The temperature
dependence of the Bragg reflector is decreased to -0.011 nm/°C through the incorporation of the plastic substrate.
To reduce the crosstalk of the polymer waveguide optical switches, waveguide attenuators are integrated with the switch on the same substrate. The switch and attenuator shares a single connected electrode which is controlled by a single current source. Due to the simple structure of the integrated attenuator, the device length is reduced to 10 mm so as to provide low insertion loss of 0.8 and 1.1 dB for 1300 and 1550 nm, respectively. Further radiation of the remained optical signal on the switch-off branch is induced by the integrated attenuator so that the switching crosstalk is reduced to -70 dB with an applied electrical power of 200 mW. The low crosstalk is maintained for the environmental temperature range of -10 to 55 °C.
The influence of charge injection on efficiency and operating voltage of organic light emitting device was investigated. Using UV-treated and un-treated ITO substrates, organic light emitting diodes were fabricated as we vary the thickness of LiF layer as the electron injection layer. The operating voltages and efficiencies were measured. When ITO surface is UV treated, operating voltage was decreased for all ranges of LiF thickness. By inserting a LiF thin layer with thickness of 5~10 Å lower operating voltages and higher power efficiencies were achieved. For a thicker LiF layer, power efficiency was decreased rapidly for UV treated ITO because of insulating properties of LiF. On the other hand, for untreated ITO - higher hole injection barrier, it was kept relatively high value up to the range of 20Å of due to the charge balance.
Biophotonic sensors based on polymer waveguide with Bragg reflection grating are demonstrated in this work.
Waveguide Bragg reflectors were designed by using the effective index method and the transmission matrix method.
The grating pattern was formed by exposing laser interference pattern on a photoresist. On top of the inverted rib
waveguide, the grating was inscribed into the core layer by the O2 plasma etching. In order to perform the bio-molecule
detection experiment, a calixarene molecule was self-assembled on top of thin Au film deposited on the waveguide
Bragg reflector. To measure the response of the sensor, several PBS solutions with different concentrations of potassium
ion from 1 pM to 100 μM were dropped on the sensor surface. The shift of Bragg reflection wavelength was observed
from the PBS solution with lowest potassium concentration, 1 pM.
To improve the reproducibility of passive alignment, large core single mode waveguides are demonstrated, which can be connected to thermally expanded core fibers so as to increase alignment tolerance. It is shown that polymer waveguide with a core dimension of 25 μm x 25 μm and an index contrast less than 0.001 can satisfy single mode condition. As a novel functional device incorporating the large core waveguide, variable optical attenuators (VOA) are designed and fabricated. For the fabrication of the thick core structure, a soft molding process is developed. Due to the small index contrast of the waveguide, efficient attenuation is expected for the smaller electrical power consumption, which is confirmed by 3-dimensional beam propagation method. From the fabricated VOA device, more than 20 dB of attenuation is obtained by applying 20 mW.
Material processing and device fabrication of many different electro-optic (EO) polymers developed at USC are reviewed. Detailed discussion is given to guest-host CLD/APCs, crosslinking perfluorocyclobutane (PFCB) polymer CX1, and thermally stable side-chain polymers CX2 and CX3. Excellent EO performance (1.4V at 1.31 μm, 2.1 V at 1.55 μm) was achieved in CLD/APC Mach-Zehnder modulators (2-cm, push-pull). CLD/APCs also possess low optical losses (1.2 dB/cm in slab waveguides and in thick core channel waveguides). However, the guest-host materials only have limited thermal stability (110-132 °C in short term, <60 °C in long term) and require special techniques in device fabrication. The crosslinking polymer CX1 was able to provide long-term stability at 85 oC when fully cured. It also has a low optical loss (comparable to CLD/APCs) before curing and decent EO coefficient when poled at 180 °C. However, after the films were poled at the crosslinking temperatures (200 °C or above), the transmissions of the waveguides and EO activity became very poor due to poling-induced chromophore degradation. By judicial molecular design of both chromophore and monomer structures to suppress thermal motion of polymer segments, we were able to realize the same or even better thermal stability in side-chain polymers CX2 and CX3. Since no curing is needed, devices can be poled at their optimal poling temperatures, and all good properties can be obtained simultaneously. Despite the excellent solubility in chlorinated solvents, these side-chain polymers are resistant to some other organic solvents or solutions such as acetone, photoresist and various UV-curable liquids.
Optoelectronic devices based on nonlinear optic (NLO) polymers, with electro-optic (EO) coefficients in excess of 100 pm/V at 1.06 μm and dielectric constants of < 3, have demonstrated 100+ GHz data rates with less than 4 volt operating voltages. This has gained interest from the space based applications community, since in addition to being tolerant to a space environment, electro-optic devices for space applications will also need to operate at high data rates and at low operational powers. We have investigated various NLO polymers for core materials as well as passive polymers with various conductivities, both ionic and electronic, suitable for use as optical cladding layers in NLO polymer based opto-electronic devices. Our goal was to find materials that would be tolerant to irradiation as well as maximizing the nonlinearity of the NLO core material, thus minimizing the total applied poling voltage, and minimize the optical absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, thus maximizing the EO coefficient with minimum applied voltage or power. We found, however, that it is necessary to balance the optical and electromagnetic properties of the materials with their processability and compatibility.
We have investigated various conductive and nonconductive polymer materials suitable for use as cladding layers in nonlinear optic (NLO) polymer based opto-electronic devices. Our goal was to maximize the nonlinearity of the NLO core materials, while minimizing the total poling voltage and minimizing the absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, realizing a maximum EO coefficient with minimum applied poling voltage. We found, however, that there are tradeoffs between absorption loss, conductivity, refractive index, materials processability and materials compatibility when using off-the-shelf materials. Results are presented for a 3-layer device structure using a conductive polymer material for both the top and bottom cladding layers.
We have investigated various conductive and non-conductive polymers suitable for use as cladding layers in nonlinear optic (NLO) polymer based opto-electronic devices. Our goal was to maximize the nonlinearity of the core material, minimize the total poling voltage, and minimize the absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, realizing a maximum EO coefficient with minimum applied voltage. We found, however, that there are tradeoffs between absorption loss, conductivity, refractive index, materials processability and materials compatibility when using off-the-shelf materials.
Photonic time-stretch has been proposed as a signal preprocessor to perform A/D conversion in otherwise inaccessible high frequency regimes. We have demonstrated time-stretching of MM-wave signals at frequencies up to 102 GHz down to 11 GHz , using an electrooptic modulator fabricated with the new polymer material PC-CLD. This application takes advantage of the inherent wideband capabilities of the PC-CLD material system, which has also demonstrated good optical insertion loss and high non- linearity at 1.55 micrometers . The dispersion penalty inherent to time-stretching imposes an additional bandwidth limit to that imposed by the modulator. A single-sideband modulator configuration is proposed to reduced the effect of this penalty.
Electro-optic waveguide arrays have been exploited to demonstrate a compact and low loss integrated optical M multiplied by N (M equals 4, N equals 8) space switch. The device is constructed with four units of 1 multiplied by N optical space switches aligned in parallel, which in turn comprises 1 multiplied by 2N beam splitter, 2N electro-optic phase shifters, and lens. The switching in each 1 multiplied by N space switch can be carried out in the lens focal plane by introducing an equal phase increment or decrement between the adjacent waveguides in the array, which results in the optical beam steering of the diffracted light. The number of input/output channels can be easily extended without sacrificing the device length by increasing the dimension of waveguide array in lateral direction.
Passive and active polarization converters have been designed and fabricated using electro- optic poled polymers. The devices utilize the controllability of the poling induced optic axis by using specially designed poling electrodes. In the passive TE-TM polarization mode converter, the optic axis of the poling induced waveguide is slowly rotated by using a slowly varying structure of poling electrodes. The polarization conversion is achieved as the guided mode propagates through the waveguide. Complete TE to TM mode conversion is observed and the polarization extinction ratio of the output light is higher than 30 dB. Because the device does not contain any periodic structure, it is wavelength insensitive and easy to fabricate. An active TE-TM mode converter utilizes a 45 degree off poling configuration, which can be obtained by arranging upper and lower stripe poling electrodes with proper lateral displacement. The poling electrodes themselvees can be used as driving electrode. The mode conversion properties and electro-optic effects are demonstrated by using a vector beam propagation method.
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