Light-initiated quasi-instant solidification of a liquid polymer is attractive for its ultra-precise spatial and temporal control of the photochemical reaction. In this paper we present microlenses structured by femtosecond laser-induced photopolymerization. Due to nonlinear phenomena the fabrication resolution is not restricted to the diffraction limit for the applied laser excitation wavelength but is determined by the intensity of a focused beam. Furthermore, pin-point structuring enables one to produce three-dimensional structures of any form from the photopolymer. The smallest structural elements of 200 nm lateral dimensions can be achieved reproducibly by using high numerical aperture oil immersion focusing optics (NA = 1.4). Axial resolution (which is fundamentally a few times worse than lateral resolution due to the distribution of light intensity in the focal region) can be controlled to a precision of a few hundred nanometers by decreasing the scanning step. In our work we applied the commercially available and widely used zirconium–silicon based hybrid sol–gel photopolymer (Ormosil, SZ2080). Arrays of custom-parameter spherical microlenses for microscopy applications have been fabricated. Their surface roughness, focal distance and imaging quality were tested. The obtained results show potential for fast and flexible fabrication of custom-parameter microlenses by the proposed technique.

https://iopscience.iop.org/article/10.1088/2040-8978/12/3/035204/pdf

A femtosecond laser-induced two-photon photopolymerization technique for structuring microlenses

Analytical formulas for the power spectra of return-to-zero (RZ) optical signals generated by Mach-Zehnder (MZ) modulators are derived. Pulse duty cycles of 33%, 50%, and 67%, in conjunction with several modulation techniques, including binary ON-OFF keying (OOK), duobinary OOK, and M-ary differential phase-shift keying (DPSK), phase-shift keying (PSK), and quadrature-amplitude modulation (QAM), are considered. Spectral characteristics and bandwidth requirements of these different schemes are compared

/pdf/power-spectra-of-return-to-zero-optical-signals-4327fmn771.pdf

Power spectra of return-to-zero optical signals

This work demonstrates the fabrication of a simple, low-cost microlens array (MLA) diffuser film with controllable haze distribution (diffusion effect) by a combination of "breath figures" (BFs) and micro-replica molding methods. Polystyrene (PS) molds obtained by BFs method contain concave, hexagonal packed air holes formed by the condensation of water vapor on cooling surfaces in a chamber in which relevant influence factors can be controlled. The sizes of the air holes in the BFs PS molds can be controlled by varying such factors as chamber temperature, chamber relative humidity, substrate temperature and others. The temperature distribution on the substrate affects the distribution of diameters of the air holes formed in a BFs PS mold. Convex PDMS (poly-dimethylsiloxane) MLAs were obtained by molding from the BFs PS molds. The focal lengths of MLAs were measured and compared with theoretical values. The diffusion effect of the diffuser films with MLAs of diameters 6 microm and 3 microm were compared. The results indicate that an MLA with a smaller diameter has a larger diffusion effect.

Fabrication of microlens array diffuser films with controllable haze distribution by combination of breath figures and replica molding methods

This paper proposes an efficient bandwidth utilization (EBU) algorithm that utilizes the unused bandwidth in dynamic bandwidth allocation (DBA) of a 10-gigabit-capable passive optical network (XGPON). In EBU, an available byte counter of a queue can be negative and the unused remainder of an available byte counter can be utilized by the other queues. In addition, EBU uses a novel polling scheme to collect the requests of queues as soon as possible. We show through analysis and simulations that EBU improves performance compared to that achieved with existing methods. In addition, we describe the hardware implementation of EBU. Finally we show the test results of the hardware implementation of EBU.

https://onlinelibrary.wiley.com/doi/pdf/10.4218/etrij.13.0112.0061

Development of Efficient Dynamic Bandwidth Allocation Algorithm for XGPON

All-optical format conversion of 10 Gb/s non-return-to-zero on-off keying (NRZ-OOK) to RZ-OOK has been successfully achieved, for the first time to our knowledge, utilizing either cross-phase modulation (XPM) or four-wave mixing (FWM), in a Silicon (Si) nanowire. A 10-9-bit-error-rate (BER) receiver sensitivity gain of ~2.8 dB was obtained for converted RZ-OOK relative to NRZ-OOK using XPM, whereas a receiver sensitivity gain of ~2.5 dB was found for the FWM anti-Stokes RZ-OOK. Simultaneous wavelength and pulse format conversions were possible with FWM. No evidence of an error floor for BER <10-10 was observed in either technique. The converted RZ-OOK signal was also correctly encoded and of the correct polarity.

All-Optical Format Conversion of NRZ-OOK to RZ-OOK in a Silicon Nanowire Utilizing Either XPM or FWM and Resulting in a Receiver Sensitivity Gain of $\sim$ 2.5 dB

This letter presents a novel configuration for return-to-zero (RZ) differential phase-shift keyed (DPSK), carrier-suppressed (CS) RZ DPSK, and chirped nonreturn-to-zero (CNRZ) signal generation, which only requires a single-stage dual-electrode Mach-Zehnder modulator (DE-MZM). RZ-DPSK, CSRZ-DPSK, or CNRZ signals can be generated via the same configuration by changing the DE-MZM operating conditions. Analytical derivation and simulation on signal generation and wavelength-division-multiplexed transmission was also carried out to justify the equivalence of the proposed scheme and conventional methods. The proposed scheme is expected to be more cost-effective due to the reduction in required modulator number.

RZ/CSRZ-DPSK and chirped NRZ signal generation using a single-stage dual-electrode Mach-Zehnder modulator

We present a novel configuration for carrier-suppressed return-to-zero (CSRZ) signal generation, which only requires a single-stage Mach-Zehnder modulator in conjunction with an electrical mixer. An electrical band-limiting filter is also introduced to further reduce the spectral width of the generated CSRZ signal. The proposed scheme is potentially cost-effective, and simulation results show that the electrically band-limited CSRZ signal exhibits larger dispersion tolerance than a conventional CSRZ signal without any degradation in its nonlinearity tolerance.

Electrically band-limited CSRZ signal with simple generation and large dispersion tolerance for 40-Gb/s WDM transmission systems

Design and analysis of spatially variant microlens-array diffuser with uniform illumination for short-range infrared wireless communications using photometric approach

This letter presents a compact 2.5 Gb/s burst- mode receiver using the first reported monolithic amplifier IC developed with 0.25 μm SiGe BiCMOS technology. With optimum avalanche photodiode gain, the receiver module can obtain a fast response, high sensitivity and wide dynamic range, satisfying the overhead timing and various power specifications for a 2.5 Gb/s next-generation passive optical network (PON), as well as a legacy 1.25 Gb/s PON in the upstream.

https://onlinelibrary.wiley.com/doi/pdf/10.4218/etrij.09.0209.0227

Dong-Soo Lee

Papers

Development of Efficient Dynamic Bandwidth Allocation Algorithm for XGPON

RZ/CSRZ-DPSK and chirped NRZ signal generation using a single-stage dual-electrode Mach-Zehnder modulator

Electrically band-limited CSRZ signal with simple generation and large dispersion tolerance for 40-Gb/s WDM transmission systems

Design and analysis of spatially variant microlens-array diffuser with uniform illumination for short-range infrared wireless communications using photometric approach

Compact 2.5 Gb/s Burst-Mode Receiver with Optimum APD Gain for XG-PON1 and GPON Applications