Showing papers in "IEEE Photonics Technology Letters in 2009"

100-Mb/s NRZ Visible Light Communications Using a Postequalized White LED

Abstract: This letter describes a high-speed visible light communications link that uses a white-light light-emitting diode (LED). Such devices have bandwidths of few megahertz, severely limiting the data rates of any communication system. Here, we demonstrate that by detecting only the blue component of the LED, and using a simple first-order analogue equalizer, a data rate of 100 Mb/s can be achieved using on-off keying nonreturn-to-zero modulation.

PAM-DMT for Intensity-Modulated and Direct-Detection Optical Communication Systems

Abstract: A novel concept of pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) is presented. It is shown that by pulse amplitude modulation of the subcarriers used, the resulting PAM-DMT time signal can be clipped asymmetrically to achieve a unipolar signal for transmission. The distortion resulting from this asymmetric clipping is orthogonal to the information signal itself, causing no additional penalty. Especially in intensity-modulated direct-detection optical channels, where a unipolar signal is to be transmitted, it is shown that this idea can improve system performance.

Low Dark Count Single-Photon Avalanche Diode Structure Compatible With Standard Nanometer Scale CMOS Technology

Abstract: A single-photon avalanche diode structure implemented in a 130-nm imaging process is reported. The device employs a p-well anode, rather than the commonly adopted p+, and a novel guard ring compatible with recent scaling trends in standard nanometer scale complementary metal-oxide-semiconductor technologies. The 50-mum 2 active area device exhibits a dark count rate of 25 Hz at 20 degC and a photon detection efficiency peak of 28% at 500 nm.

Soliton Thulium-Doped Fiber Laser With Carbon Nanotube Saturable Absorber

Abstract: We report stabilization of a thulium-holmium codoped fiber soliton laser with a saturable absorber based on carbon nanotubes. The laser generates transform-limited 750-fs pulses with 0.5-nJ energy.

In-Line Abrupt Taper Optical Fiber Mach–Zehnder Interferometric Strain Sensor

Abstract: A Mach-Zehnder interferometer with two abrupt single-mode fiber tapers is simulated, constructed, and demonstrated. The interferometer has an insertion loss of 5 dB and an extinction ratio over 15 dB. The interferometer is tested as a strain sensor based on the maximum attenuation wavelength shift with a comparable sensitivity (slope: 2000 nm/ epsiv, R 2 = 0.996) with long-period-grating-type sensor and promises low fabrication cost.

Demonstration of Broadband Wavelength Conversion at 40 Gb/s in Silicon Waveguides

Abstract: We present ultra-broadband wavelength conversion in silicon photonic waveguides at a data rate of 40 Gb/s. The dispersion-engineered device demonstrates a conversion bandwidth spanning the entire S-, C-, and L-bands of the ITU grid. Using a continuous-wave C-band pump, an input signal of wavelength 1513.7 nm is up-converted across nearly 50 nm at a data rate of 40 Gb/s, and bit-error-rate measurements are performed on the converted signal.

Relative Humidity Sensor Based on Tilted Fiber Bragg Grating With Polyvinyl Alcohol Coating

Abstract: A relative humidity (RH) sensor based on tilted fiber Bragg grating (TFBG) is proposed by utilizing polyvinyl alcohol (PVA) as the sensitive cladding film. RH increasing in the PVA coating will result in reduction of refractive index. Due to the TFBG's sensitivity to ambient refractive index, the spectral properties of PVA-coated TFBG are modified under exposure to different ambient humidity levels ranging from 20% to 98% RH. The transmission power of TFBG has different linear behaviors for two different humidity ranges (20%-74% RH and 74%-98% RH), and the sensitivity for each humidity range reaches as high as 2.52 and 14.947 dBm/%RH, respectively. Combining the advantages of optical fiber grating and PVA as a smart material, this design involves simple configuration, low cost, compactness, a small degree of hysteresis, stability, and wide dynamic sensing range as well. Therefore, the sensor could be applied in real-time RH monitoring for normal as well as extremely humid environments.

White Light Wireless Transmission at 200 ${+}$ Mb/s Net Data Rate by Use of Discrete-Multitone Modulation

Abstract: Spectrally efficient data transmission with white light-emitting diodes (LEDs) is a topic of increasing interest. In this letter, we report a visible light communication link operating at 200 ${+}$ Mb/s net transmission rate (230 ${+}$ Mb/s gross) with a bit-error ratio for uncoded data below ${10^{-3}}$ . The link is based on a thin-film high-power phosphorescent white LED and offline signal processing of discrete multitone signals. Transmission at the brightness levels of about 1100 and 550 lx was investigated. Our results indicate that the achievable data rates are limited by detector noise.

Microwave Photonic Technique for Frequency Measurement of Simultaneous Signals

Abstract: A novel microwave photonic frequency measurement (MPFM) receiver is reported. The technique directly maps the frequency information of the incoming simultaneous signals to a distinct set of relative time delays utilizing a dispersive medium. The measurement of the observed time delay directly equates to the value of the input signal frequency. The operation of the MPFM receiver concept is successfully verified for two simultaneous signals at 20 and 40 GHz, demonstrating a potential capability for use in a spectrally cluttered environment.

A Frequency-Doubling Optoelectronic Oscillator Using a Polarization Modulator

Abstract: A novel realization of a frequency-doubling optoelectronic oscillator (OEO) using a polarization modulator (PolM) is proposed and experimentally demonstrated. In the proposed system, the PolM in combination with two optical polarizers connected via two polarization controllers (PCs) is operating as a two-output intensity modulator. One output of the intensity modulator is connected to the radio-frequency port of the PolM, to form an optoelectronic loop for the generation of a microwave signal with the fundamental frequency determined by the center frequency of a narrowband electronic filter. The other output of the intensity modulator provides a fundamental or frequency-doubled optically modulated microwave signal depending on the static phase term introduced by the PC before the polarizer. The proposed OEO is experimentally demonstrated. A fundamental microwave signal at 10 GHz or a frequency-doubled microwave signal at 20 GHz is generated. The phase noise performance of the generated microwave signal is also investigated.

Fiber Inline Core–Cladding-Mode Mach–Zehnder Interferometer Fabricated by Two-Point CO $_{2}$ Laser Irradiations

Abstract: We report a fiber inline Mach-Zehnder-type core-cladding-mode interferometer fabricated by two-point CO2 laser irradiations on a single-mode fiber. The laser irradiations caused efficient light coupling from the core mode to the lower order cladding modes and vise versa. High-quality interference spectra with a fringe visibility of about 20 dB were observed for four different interferometer lengths (5, 10, 20, and 40 mm). The temperature sensitivity of the device with a length of 5 mm was measured to be 0.0817 nm/degC. The sensitivity for refractive index measurement of the device was comparable with a long-period fiber grating of LP04 cladding mode.

Dispersion Relation and Loss of Subwavelength Confined Mode of Metal-Dielectric-Gap Optical Waveguides

Abstract: We have investigated the dispersion relation of a novel metal-dielectric-gap optical waveguide. This structure confines the optical field strongly in the gap region between metals and dielectric materials, and its size can be reduced to less than the wavelength of the transmitted light. In addition, the propagation length of light extends much greater than that of the surface plasmon modes on metal surfaces. We show that this mode of propagation has a cut-off at zero wavenumber, and that it is hollow-waveguide-like for small wavenumbers, while it approaches a surface-plasmon-like mode for large wavenumbers. A typical propagation length at around the communication wavelength is 10-20 mum, and optical fields are confined into an approximately 100 times 200 nm2 cross section.

Optical Static RAM Cell

Abstract: We demonstrate an optical static random access memory cell that provides read and write functionality at 5 Gb/s. The circuit comprises a hybridly integrated semiconductor optical amplifier-Mach-Zehnder interferometer (SOA-MZI) flip-flop serving as the memory unit and two additional SOA-based cross-gain modulation switches for controlling access to the memory cell.

A Tunable Broadband Photonic RF Phase Shifter Based on a Silicon Microring Resonator

Abstract: We propose and demonstrate a tunable broadband photonic radio frequency (RF) phase shifter based on a silicon microring resonator. This scheme utilizes the thermal nonlinear effect of the silicon microring to change the electrical phase of the RF signal with a wide tuning range. A prototype of the phase shifter is experimentally demonstrated for a 40-GHz signal with a 0-4.6-rad tuning range.

Ge-on-Si p-i-n Photodiodes With a 3-dB Bandwidth of 49 GHz

Abstract: Fast Ge-on-Si p-i-n photodiodes are fabricated and their frequency response is measured up to 67 GHz at a wavelength of 1550 nm. At a bias voltage of -2 V, a 3-dB bandwidth (BW) of 49 GHz is achieved. This is to the best of the authors' knowledge the highest BW ever published for Ge photodiodes.

Athermalizing and Trimming of Slotted Silicon Microring Resonators With UV-Sensitive PMMA Upper-Cladding

Abstract: This letter experimentally demonstrates significant reduction in the resonance thermal sensitivity of the slotted silicon microring resonators by exploiting polymethyl methacrylate (PMMA) as upper-cladding. We investigate the resonance temperature dependence on the slot width with and without PMMA upper-cladding. The experimental results show that the temperature dependence is reduced from 91 pm/degC for a regular microring resonator to 27 pm/degC for the PMMA-clad slotted microring resonator. The ultraviolet (UV) sensitive PMMA upper-cladding also allows the resonance wavelengths of slotted microring resonators shift by 0.5 nm under UV light trimming. With the demonstrated schemes, each individual optical component can be athermalized and precisely registered to standard wavelengths for a future multiwavelength optically interconnected computing system-on-a-chip.

A General ADE-FDTD Algorithm for the Simulation of Dispersive Structures

Abstract: A finite-difference time-domain general algorithm, based on the auxiliary differential equation (ADE) technique, for the analysis of dispersive structures is presented. The algorithm is suited for cases where materials having different types of dispersion are modeled together. While having the same level of accuracy, the proposed algorithm finds its strength in unifying the formulation of different dispersion models into one form. Consequently, savings in both memory and computational requirements, compared to other ADE-based methods that model each dispersion type separately, are possible. The algorithm is applied in the simulation of surface plasmon polaritons using the multipole Lorentz-Drude dispersion model of silver.

A Novel OFDMA-PON Architecture With Source-Free ONUs for Next-Generation Optical Access Networks

Abstract: This letter proposes a novel architecture for next-generation passive optical networks based on the employment of orthogonal frequency-division multiple access The key new feature of the proposed approach is source-free single-wavelength upstream transmission enabled by carrier suppression, which has been experimentally verified to achieve 10-Gb/s upstream throughput

Transversal Load Sensing With Fiber Bragg Gratings in Microstructured Optical Fibers

Abstract: We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

Selection Transmit Diversity for FSO Links Over Strong Atmospheric Turbulence Channels

Abstract: In this letter, transmit laser selection for multiple-input-single-output free-space optical communication systems with intensity modulation and direct detection over strong atmospheric turbulence channels is analyzed. Assuming channel side information at the transmitter and receiver, a transmit diversity technique based on the selection of the optical path with a greater value of scintillation has shown to be able to extract full diversity as well as providing better performance compared to other alternatives recently proposed such as orthogonal space-time block codes and repetition codes. Simulation results are further demonstrated to confirm the analytical results obtained for a diversity order of two.

Field Experiment on a “Star Type” Metropolitan Quantum Key Distribution Network

Abstract: Quantum key distribution (QKD) networks have recently attracted growing attention. The topology of the local QKD network is the basis of the next-generation global secure communication network. In this letter, we report a realization of a wavelength-routing star type QKD network which can span a metropolis using a commercial backbone optical fiber network without trusted relays. The longest and the shortest fiber lengths between two geographically separated nodes are 42.6 and 32 km, respectively, and the maximum average quantum bit-error rate is below 8%. A novel analysis model with experimental validation is also proposed to evaluate the user's performance in this network under the condition of maximum multiuser crosstalk.

Thin Photo-Patterned Micropolarizer Array for CMOS Image Sensors

Abstract: We fabricated and characterized a thin photo-patterned micropolarizer array for complementary metal-oxide-semiconductor (CMOS) image sensors. The proposed micropolarizer fabrication technology completely removes the need for complex selective etching. Instead, it uses the well-controlled process of ultraviolet photolithography to define micropolarizer orientation patterns on a spin-coated azo-dye-1 film. The patterned polymer film micropolarizer (10 mum x 10 mum) exhibits submicron thickness (0.3 mum) and has an extinction ratio of ~ 100. Reported experimental results validate the concept of a thin, high spatial resolution, low-cost photo-patterned micropolarizer array for CMOS image sensors.

Proposal for a Grating Waveguide Serving as Both a Polarization Splitter and an Efficient Coupler for Silicon-on-Insulator Nanophotonic Circuits

Abstract: A grating waveguide is introduced and designed to serve as both a polarization splitter and an efficient vertical coupler between a fiber and a silicon-on-insulator (SOI) nanophotonic waveguide. Through this grating waveguide, the light from the fiber can be efficiently coupled into an SOI chip where the two orthogonally polarized waves are separated to travel towards the opposite directions along the waveguide. According to our simulations, the optimized structure can give a high coupling efficiency of about 50% for both polarizations, as well as a large bandwidth of over 70 nm and a very low polarization crosstalk below -22 dB at the output ports.

Microwave Photonic Delay Line With Separate Tuning of the Optical Carrier

Abstract: Significant performance improvement for photonic delay lines in microwave-photonics based on a new concept of separately tuning the optical carrier is proposed and analyzed. Optical microresonator delay lines using a balanced SCISSOR design with additional resonators to separately tune the carrier phase delay to provide a true-time-delay for the broadband microwave signal are presented. Significant improvements in broadband tunable delay and devices losses are predicted.

Ultrashort Polarization Splitter Using Two-Mode Interference in Silicon Photonic Wires

Abstract: An ultrashort polarization splitter based on the zero-gap directional coupler is proposed and realized. Its interference section is 8.8 mum long. It is also shown that the length of the interference section can be reduced to about 2.1 mum. The crosstalk for both transverse-electric (TE) and transverse-magnetic (TM) polarizations is 16 dB at a wavelength of 1.55 mum. The extinction ratios of TE and TM polarizations are 18.2 and 13.7 dB, respectively. The device has 3-dB bandwidth of 43 nm in TE polarization and 50 nm in TM polarization with the center wavelength at 1.55 mu m.

Silicon-on-Insulator Echelle Grating WDM Demultiplexers With Two Stigmatic Points

Abstract: We present ultracompact integrated optical echelle grating wavelength-division-multiplexing (de)multiplexers for on-chip optical networks, fabricated using high-index-contrast silicon-on-insulator photonic waveguide technology. These devices are based on a design with two stigmatic points, which enables compact geometries with reduced aberrations. In the example presented here, this design allows us to achieve an eight-channel (de)multiplexer with 3.2-nm channel spacing, within an ultracompact footprint of 250 times 200 mum. The channel-to-channel isolation of the devices is 19 dB. The minimum insertion loss, relative to a straight waveguide, is 3 dB with a channel-to-channel variation of 0.5 dB.

Individually Addressable AlInGaN Micro-LED Arrays With CMOS Control and Subnanosecond Output Pulses

Abstract: We report the fabrication and characterization of an ultraviolet (370 nm) emitting AlInGaN-based micro-light- emitting diode (micro-LED) array integrated with complementary metal-oxide-semiconductor control electronics. This configuration allows an 8 t 8 array of micro-LED pixels, each of 72-mum diameter, to be individually addressed. The micro-LED pixels can be driven in direct current (dc), square wave, or pulsed operation, with linear feedback shift registers (LFSRs) allowing the output of the micro-LED pixels to mimic that of an optical data transmitter. We present the optical output power versus drive current characteristics of an individual pixel, which show a micro-LED output power of up to 570 muW in dc operation. Representative optical pulse trains demonstrating the micro-LEDs driven in square wave and LFSR modes, and controlled optical pulsewidths from 300 ps to 40 ns are also presented.

Ultracompact SOI Microring Add–Drop Filter With Wide Bandwidth and Wide FSR

Abstract: We report the design and fabrication of an ultracompact silicon-on-insulator add-drop microring filter with a 1.5-mum radius for wavelength-division-multiplexing on-chip interconnect applications. The fabricated device was measured to have a wide free-spectral range of 52 nm and a wide channel bandwidth of 210 GHz, with a 2.9-dB insertion loss and a deep through-port extinction of nearly 25 dB.

Widely Tunable Microwave Photonic Notch Filter Based on Slow and Fast Light Effects

Abstract: A continuously tunable microwave photonic notch filter at around 30 GHz is experimentally demonstrated and 100% fractional tuning over 360deg range is achieved without changing the shape of the spectral response. The tuning mechanism is based on the use of slow and fast light effects in semiconductor optical amplifiers assisted by optical filtering.

Optofluidic Waveguides in Teflon AF-Coated PDMS Microfluidic Channels

Abstract: We report a new method for fabricating an optofluidic waveguide that is compatible with polydimethylsiloxane (PDMS). The light path follows the microfluidic channels, an architecture that can maximize detection efficiency and make the most economic use of chip area in many lab-on-chip applications. The PDMS-based microfluidic channels are coated with Teflon amorphous fluoropolymers (Teflon AF) which has a lower refractive index (n = 1.31) than water (n = 1.33) to form a water/Teflon AF optical waveguide. Driven by a vacuum pump, the Teflon AF solution was flowed through the channels, leaving a thin (5-15 mum) layer of coating on the channel wall as the cladding layer of optical waveguides. This coating process resolves the limitations of spin-coating processes by reducing the elasticity mismatch between the Teflon AF cladding layer and the PDMS device body. We demonstrate that the resulting optofluidic waveguide confines and guides the laser light through the liquid core channel. Furthermore, the light in such a waveguide can be split when the fluid flow is split. This new method enables highly integrated biosensors such as lab-on-chip flow cytometers and micro-fabricated fluorescence-activated cell sorter with on-chip excitation.