Yun-Li Li

Bio: Yun-Li Li is an academic researcher from National Taiwan University. The author has contributed to research in topics: Preamplifier & Light-emitting diode. The author has an hindex of 4, co-authored 6 publications receiving 122 citations.

Parasitic Stimulated Amplification in High-Peak-Power and Diode-Seeded Nanosecond Fiber Amplifiers

Abstract: The broadband parasitic amplification in a diode-seeded nanosecond ytterbium-doped fiber laser amplifier system is numerically and experimentally investigated. The amplification is originated from a weak and pulsed parasitic signal associated with the 1064-nm seed diode laser. Although the average power of the parasitic pulse is less than 5% of the total seed laser power, a significant transient spike is observed during the amplification. In agreement with the simulation, nonlinear effects caused by the transient spike limits the scaling of signal peak power in fiber preamplifiers. With the utilization of a narrow bandwidth filter to eliminate the parasitic pulse, the power and energy scalability of a multistage diode-seeded fiber amplifier laser system has been significantly improved. At 1064 nm, pulses with the peak power of 120 kW and energy of 1.2 mJ have been successfully generated in the multistage $\hbox{Yb}^{3+}$ -doped fiber amplifier with an energy gain of 63 dB and 56% conversion efficiency. In viewing of the parasitic pulse's 8.8-nm bandwidth, it has the potential to become a novel seed source for high-peak-power fiber amplifiers.

GaN-Based Miniaturized Cyan Light-Emitting Diodes on a Patterned Sapphire Substrate With Improved Fiber Coupling for Very High-Speed Plastic Optical Fiber Communication

Abstract: We demonstrate the performance of a novel cyan light-emitting diode (LED) on a patterned sapphire (PS) substrate as a light source for plastic optical fiber (POF) communications with the central wavelength at 500 nm. To further enhance the external quantum efficiency (EQE) and output power of this miniaturized high-speed LED, a LED with a PS substrate is adopted. Furthermore, by greatly reducing the number of active $\hbox{In}_{\rm x}\hbox{Ga}_{1 - {\rm x}}\hbox{N/GaN}$ multiple quantum wells (MQWs) to four and minimizing the device active area, we can achieve a record-high electrical-to-optical (E–O) bandwidth (as high as 400 MHz) among all the reported high-speed visible LEDs under a very small dc bias current (40 mA). The fiber coupling efficiency has been improved in 4 dB using lens with a 500- $\mu \hbox{m}$ diameter mounted on the LED chip. Thus, the maximum fiber-coupled power was $-$ 2.67 dBm at the bias current of 40 mA. The 1.07-Gb/s data transmissions over a 50-m SI-POF fiber have been successfully demonstrated using this device at the bias current of 40 mA.

High power broadband continuum source based on an all-PM-fiber master oscillator nonlinear power amplifier

Abstract: In an all-polarization-maintaining-fiber master oscillator power amplifier system at 1064 nm under all normal dispersion, intense nanosecond emission was generated with spectral broadening from 980 to 1600 nm. In such a fiber nonlinear power amplifier, efficient power scaling is able to be free from significant depletion because both laser amplification and nonlinear conversion are simultaneously employed. As a result, output peak power up to 117 kW with a pulse energy of 1.2 mJ is generated with a maximum core intensity of 30 GW cm 2 . In addition, the conversion efficiency is 66% for a pulse duration of 6.1 ns at the moderate repetition of 20 kHz. The output level is close to the damage threshold for long-term operation. The onset and interplay of constituted fiber nonlinearities can be addressed, especially from single mode to a few modes, stage by stage. Furthermore, the seeding influence on the spectral broadening reveals its versatility for enabling many potential applications. For seeding by a highly controlled diode laser at the nanojoule level, a double-pass preamplifier significantly improves the energy extraction, resulting in a high input level for an efficient nonlinear power amplifier. Such a linearly polarized light source composed of an intense 1064 nm pump and a broad sideband seed is beneficial for efficiently driving broadband tunable optical parametric amplification.

Very high-speed GaN-based cyan light emitting diode on patterned sapphire substrate for 1 Gbps plastic optical fiber communication

Abstract: We demonstrate novel cyan light-emitting-diodes (LEDs) on patterned-sapphire substrate with record-high electrical-to-optical bandwidth (0.38 GHz) among visible LEDs. 0.93Gbps error-free transmission in plastic-optical-fiber can be achieved by using such device under only 20mA bias current.

Sub-Bandgap Laser Light-Induced Excess Carrier Transport Between Surface States and Two-Dimensional Electron Gas Channel in AlGaN/GaN Structure

Abstract: Variations of the channel resistance of a AlGaN/GaN high electron mobilities transistor caused by subsequently incident photons with sub-bandgap energies after ultraviolet light-induced changes became stable were studied. Temperature-dependent measurements yielded a 0.342 eV thermal activation energy and wavelength-varying measurements yielded a 0.8 eV cut-off photon energy. The ratio between the two values is very close to the theoretical value of the ratio between the valence and conduction band discontinuities. A qualitative description about the transports of excess carriers through the band discontinuities, is also proposed and consistent with the experimental results.

A 3-Gb/s Single-LED OFDM-Based Wireless VLC Link Using a Gallium Nitride $\mu{\rm LED}$

Abstract: This letter presents a visible light communication (VLC) system based on a single 50- μm gallium nitride light emitting diode (LED). A device of this size exhibits a 3-dB modulation bandwidth of at least 60 MHz - significantly higher than commercially available white lighting LEDs. Orthogonal frequency division multiplexing is employed as a modulation scheme. This enables the limited modulation bandwidth of the device to be fully used. Pre- and postequalization techniques, as well as adaptive data loading, are successfully applied to achieve a demonstration of wireless communication at speeds exceeding 3 Gb/s. To date, this is the fastest wireless VLC system using a single LED.

A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

Abstract: The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined.

1.5 Gbit/s Multi-Channel Visible Light Communications Using CMOS-Controlled GaN-Based LEDs

Abstract: An on-chip multi-channel visible light communication (VLC) system is realized through a blue (450 nm) GaN-based micron-size light-emitting diode (μLED) array integrated with complementary metal-oxide-semiconductor (CMOS) electronics. When driven by a custom-made CMOS driving board with 16 independent parallel data input ports, this μLED array device is computer controllable via a standard USB interface and is capable of delivering high speed parallel data streams for VLC. A total maximum error-free data transmission rate of 1.5 Gbit/s is achieved over free space by modulating four μLED pixels simultaneously using an on-off key non-return to zero modulation scheme. Electrical and optical crosstalk of the system has also been investigated in detail and the further optimization of CMOS design to minimize the crosstalk is proposed.

2 Gbit/s data transmission from an unfiltered laser-based phosphor-converted white lighting communication system.

Abstract: We demonstrate data transmission of unfiltered white light generated by direct modulation of a blue gallium nitride (GaN) laser diode (LD) exciting YAG:Ce phosphors. 1.1 GHz of modulation bandwidth was measured without a limitation from the slow 3.8 MHz phosphor response. A high data transmission rate of 2 Gbit/s was achieved without an optical blue-filter using a non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. The measured bit error rate (BER) of 3.50 × 10(-3) was less than the forward error correction (FEC) limit of 3.8 × 10(-3). The generated white light exhibits CIE 1931 chromaticity coordinates of (0.3628, 0.4310) with a color rendering index (CRI) of 58 and a correlated color temperature (CCT) of 4740 K when the LD was operated at 300 mA. The demonstrated laser-based lighting system can be used simultaneously for indoor broadband access and illumination applications with good color stability.

Topological supramolecular network enabled high-conductivity, stretchable organic bioelectronics

Abstract: Intrinsically stretchable bioelectronic devices based on soft and conducting organic materials have been regarded as the ideal interface for seamless and biocompatible integration with the human body. A remaining challenge is to combine high mechanical robustness with good electrical conduction, especially when patterned at small feature sizes. We develop a molecular engineering strategy based on a topological supramolecular network, which allows for the decoupling of competing effects from multiple molecular building blocks to meet complex requirements. We obtained simultaneously high conductivity and crack-onset strain in a physiological environment, with direct photopatternability down to the cellular scale. We further collected stable electromyography signals on soft and malleable octopus and performed localized neuromodulation down to single-nucleus precision for controlling organ-specific activities through the delicate brainstem. Description Designing the right polymer blend Approaches to making soft electronics can involve putting rigid objects onto a soft substrate or finding ways to improve the conductivity and mechanical strength of inherently soft materials. Jiang et al. considered the systematic introduction of polyrotaxanes into soft conductive membranes made of the conductive polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The polyrotaxane consisted of a polyethylene glycol (PEG) backbone with cyclodextrins attached using PEG-methacrylate side chains. The cyclodextrins can slide back and forth along the chains, thus preventing crystallization of the PEG and providing better stretchability. The blended polymers could be photopatterned down to two-micrometer feature sizes and exhibited enhanced conductivity, making them suitable for surface-mounted and implanted bioelectronic devices. —MSL A stretchable conducting polymer allows stable monitoring of soft-bodied octopus and precise modulation of delicate brainstem.