Xian Jin

Wireless Indoor Optical Positioning With a Differential Photosensor

An indoor optical positioning technique using a differential photosensor device is presented. The method is based on angle of arrival information estimated by the differential photosensor in an indoor environment with fixed optical beacons. A photocurrent is generated by each of the three photodiodes in the photosensor by incident light from the optical beacons. The amplitudes of these photocurrents are a function of the incident angle of the light. Previously derived equations that express photocurrent amplitudes as a function of the azimuthal arrival angle, φ, and the polar arrival angle, θ, are modeled with second- and third-order polynomials, respectively, to determine the φ and θ angles from measured photocurrents. Testing with optical beacons in various positions with respect to a fixed photosensor resulted in a root mean squared error for all estimated angles φ and θ of 2.8°. A positioning accuracy of better than 4 cm is achieved.

Microlenses with tuned focal characteristics for optical wireless imaging

Microlenses are fabricated and investigated for integrated imaging applications. The microlenses are fabricated by an in situ polymer electro-dispensing technique that enables user-controlled microlens sizes and shapes, by direct-dispensing and voltage-tuning with a metal micro-needle tip in a filler solution. Theoretical and experimental analyses are carried out for three limiting-cases of electro-dispensed microlenses: an acute-angle microlens with a 30° contact angle, a right-angle microlens with a 90° contact angle, and an obtuse-angle microlens with a 120° contact angle. It is found that the right-angle microlens, with a 500 μm diameter, yields an especially short focal length (700 μm) and exceedingly large numerical aperture (0.533). These characteristics can meet the needs of emerging applications, such as optical wireless devices, which demand compact device integration and broad field-of-view imaging. The microlenses are tested in optical wireless imaging receivers, for signal-to-noise ratio perf...

Differential Retro-Detection for Remote Sensing Applications

A retro-detection technique is introduced for simultaneous optical retroreflection, detection, and control in bidirectional sensor links. An architecture with three orthogonal photodiodes (PDs) is used to retroreflect incident light and sample the incident optical signals. Such a system establishes a two-way (bidirectional) optical link. Moreover, the three photocurrent signals can be used in differential combinations for active control and optimization of the alignment. The theory and methodology for the optical retroreflection, detection, and control characteristics are presented, and the technique is verified experimentally with a silicon PD retro-detection prototype. The active remote sensing capabilities of the retrodetector are found to be successful.

An ultrafast hydrogel photocrosslinking method for direct laser bioprinting

Photocrosslinking is a widely-used method to generate cell-laden hydrogels for tissue engineering. At present, it usually takes more than 30 seconds to crosslink hydrogels using UV illumination, and this delay makes it more likely that damage will occur in the DNA. With this in mind, we introduce an ultrafast photocrosslinking method using a low-cost blue laser diode. Experimental results show that a hydrogel with a diameter of 8 mm can be crosslinked using this process within 10 seconds with over 90% cell viability. Moreover, it is shown that the laser can be focused for the implementation of bioprinting. A microscale cell-laden microtube was successfully fabricated with this laser-based system, demonstrating its feasibility for bioprinting.

Ultrafast Photoexcitation and Transient Mobility of GaP for Photoconductive Terahertz Emission

The prospects for photoconductive (PC) terahertz (THz) generation are studied for wide-bandgap semiconductors exhibiting transient mobility. Such semiconductors offer practical benefits (by resisting dielectric breakdown and minimizing Joule heating) as well as improved frequency responses (by accentuating high-frequency PC THz emission). It is shown that GaP can offer these wide-bandgap and transient mobility characteristics. The ultrafast photoexcitation and subsequent transient mobility are investigated for a GaP PC THz emitter with photoexcitation fluences of 18, 36, and 72 μJ/cm2. The 100 fs rise and 700 fs fall in the transient photocurrent, due to the respective photoexcitation and transient mobility responses, yields far-field THz emission that improves upon that of the well-established GaAs PC THz emitter. It is ultimately found that semiconductors with both wide-bandgap and transient mobility characteristics can offer strategic improvements for emerging high-power PC THz technologies.

Ultrafast Refractometry for Characterization of Nanocomposite Material Systems

An ultrafast refractometry technique is introduced for direct characterization of refractive indices, absorption coefficients, and refractive index variances in nanocomposite (NC) assemblies. The system samples bulk NC optical characteristics by probing phase delay, absorption, and spatial coherence effects on an ultrashort laser probe pulse. The integrated system is demonstrated for representative samples of 20-nm SiC nanoparticles in a polymer host and is found to successfully sample the desired optical characteristics.

Ultrafast transient responses of optical wireless communication detectors

In this work, fundamental ultrafast transient responses are studied for optical wireless communication (OWC) detectors. It is shown that material impulse responses, associated with transient photoconductivity, and geometrical input responses, associated with transient optical power, must be considered in tandem when OWC photodetection is pursued with broad spectral and directional characteristics. An OWC detector, composed of GaAs photoconductive gaps in a corner-cube geometry, is fabricated and analyzed. The GaAs material response times are investigated experimentally and found to range from approximately 3 ps to 200 fs for 390 nm (violet) to 780 nm (red) photoexcitation. The geometrical response times are investigated theoretically and found to range from approximately 2 to 20 ps for device dimensions from 1 to 10 mm. The overall response times manifest themselves in two distinct dimensional regimes, with differing levels of wavelength and dimension dependence. The relevance of these findings is discussed for future ultrafast OWC detectors.

Characterization of Image Receivers for Optical Wireless Location Technology

In this letter, image receivers are characterized to address the challenges of indoor positioning with optical wireless location systems. Image receivers are studied according to their field-of-view (FOV) characteristics: 1) the first image receiver uses a wide-FOV (95°) microlens and 2) the second image receiver uses an ultrawide-FOV (130°) microlens. An angle of arrival characterization of the image receiver is used to quantify azimuthal, φ, and polar, θ, angles of incident light from optical beacons. A dilution of position (DOP) characterization is used to quantify geometrical effects of the optical beacon distribution. It is found that the ultrawide-FOV microlens (with a mean positioning error of 1.5 cm) can better image widely separated optical beacons, and thus operate at a lower DOP, compared with the wide-FOV microlens (with a mean positioning error of 3.2 cm).

Ultrafast charge-carrier and phonon dynamics in GaP

The ultrafast energy relaxation of GaP is analyzed through charge-carrier and phonon dynamics. Early timescales show hot electron intervalley scattering from the Γ valley into the X sidevalley, with 700 and 4000 fs time constants for scattering to and from the X7 valley. Later timescales show carrier-phonon interactions in the X6 valley with hot phonon and screening effects. Fluence-dependent relaxation is observed over 30 to 52 ps for 2.3 to 72 μJ/cm2 fluences. The prolonged relaxation of GaP is due to impeded (hot) phonon decay and screening at low and high fluences, respectively.

Multitone Photoconductive Sensors for Free-Space Optics

A multitone frequency-biasing technique is applied to an integrated photoconductive (PC) sensor for application to free-space optical (FSO) communication systems. The introduced technique and integrated device provide the required optical transmission, detection, and retroreflection capabilities for both passive uplink and active downlink operations. The physical structure incorporates three triangular PC switches, with 450- μm GaAs gaps, assembled in a corner-cube retroreflector architecture. The retroreflective nature of the element establishes a bidirectional passive uplink between the PC sensor and transmitter/source, whereas the three PC switches together provide unidirectional reception of incident optical signals during active downlink operation. At the same time, the threefold symmetry of the structure, together with the multitone biasing, provides a frequency-domain balancing mechanism to optimize the sensor alignment. The prototype is presented with both theoretical and experimental analyses, and proof-of-concept FSO communication is demonstrated over an indoor 5-m link length.