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WO3-coated multiwall carbon nanotubes (MWCNTs) were fabricated by sputter-deposition of WO3 on MWCNT paste. The outer diameters of WO3-coated MWCNTs ranged from 20 to 40 nm and the lengths ranged up to a few tens of micrometers. The low-magnification TEM image of a typical WO3-coated CNT showed a CNT with an inner diameter of ~20 nm and a tube wall thickness of ~7 nm and WO3 shells with a thickness up to 10 nm at both edges of the tube. The WO3 shells were very nonuniform in thickness not only along the axis of the nanotube but also from one nanotube to the other. The sensing properties of multiple networked WO3-coated CNT sensors toward NO2 gas were examined. The WO3-coated MWCNT sensors showed responses of 120-221% over an NO2 concentration range of 1 to 5 ppm at room temperature. The responses were 1-2 fold higher than those of the pristine MWCNT sensor over the same NO2 concentration range. The origin of the enhancement of the MWCNTs in the response to NO2 by coating them with WO3 is discussed.

We report the use of a widely tunable detuned dual-mode laser diode (DML) as a compact and portable continuous-wave THz emitter. The wavelength difference between the two lasing modes of this DML can be tuned from 2.4 nm to 9.3 nm by using integrated microheaters. The power difference between these modes is less than 1 dB, and the side-mode suppression ratio is greater than 30 dB over the entire tuning range.

This study reports a portable and precision photonic biosensor reader that can measure the concentration of a particular antigen using an optical resonant reflection biosensor (ORRB). To create a compact biosensor reader, a compact tunable vertical-cavity surface-emitting laser (VCSEL) and a compact built-in wavelength meter were manufactured. The wavelength stability and accuracy of the compact built-in wavelength meter were measured to be less than 0.02 nm and 0.06 nm, respectively. The tunable VCSEL emission wavelength was measured with the compact built-in wavelength meter, it has a fast sweep time (~ 10 seconds) and a wide tuning range (> 4 nm) that are sufficient for biosensor applications based on ORRB. The reflection spectrum of a plastic based ORRB chip was measured by the fabricated portable photonic biosensor reader using the VCSEL and wavelength meter. Although the reader is the size of a palmtop device, it could make a precise measurement of the peak wavelength on equal terms with a conventional bulky optical spectrometer.

We report an experimental demonstration of 40 Gbps all-optical 3R regeneration with all-optical clock recovery based on InP semiconductor devices. We also obtain alloptical non-return-to-zero to return-to-zero (NRZ-to-RZ) format conversion using the recovered clock signal at 10 Gbps and 40 Gbps. It leads to a good performance using a Mach-Zehnder interferometric wavelength converter and a self-pulsating laser diode (LD). The self-pulsating LD serves a recovered clock, which has an rms timing jitter as low as sub-picosecond. In the case of 3R regeneration of RZ data, we achieve a 1.0 dB power penalty at 10 -9 BER after demultiplexing 40 Gbps to 10 Gbps with an eletroabsorption modulator. The regenerated 3R data shows stable error-free operation with no BER floor for all channels. The combination of these functional devices provides all-optical 3R regeneration with NRZ-to-RZ conversion.

All-optical clock recovery is performed using a multisection semiconductor laser that can generate two kinds of self-pulsation due to compound cavity mode beating and passive mode locking. To investigate factors to influence quality of optical clock, optical clocks recovered from an optical data are compared by using various self-pulsations with different characteristics. It is experimentally demonstrated that characteristics of optical clock depend on free-running characteristics of a RF linewidth and amplitude noise. In particular, optical clock with a subpicosecond jitter is recovered by use of a self-pulsation with a narrow linewidth and low amplitude noise of the monolithic semiconductor laser in the 10 GHz regime. The free-running characteristics can be utilized as a criterion of a potential of self-pulsating semiconductor lasers for all-optical clock recovery.

Our recent studies in regards of developing portable THz scanner for imaging and spectroscopy systems are presented. In the course, high power tunable continuous wave (CW) THz emitter and high sensitivity THz receiver platforms are presented. Those platforms can be realized with tunable optical beating source, broadband photomixer, arrayed photomixer and Schottky barrier diode, evanescently-coupled photodiodes with high saturation current, and semiconductor optical amplifier (SOA) integrated optical beating source. On the system level, our recent THz thickness measurement systems and the THz line scanner imaging system are presented.

An optical triplex transceiver (TRx) module, which consists of thin-film filter (TFF)-attached wavelength-division multiplexer (WDM) and photodiode (PD) carriers, has been fabricated using a silica planar lightwave circuit (PLC) hybrid integration technology. Two types of TFFs were attached to a diced sidewall of a silica-terraced PLC platform to realize the TFF-attached WDM. The PD carriers with a 45deg mirror, on which receiving surface-illuminated PDs were bonded, were assembled with the PLC platform to form receiver (Rx) parts. As the main performances of the packaged TRx module, a very clear transmitter (Tx) eye pattern and minimum Rx sensitivity of -25.7 dBm were obtained under a 1.25-Gb/s Tx-Rx operation for digital applications. For an analog Rx application, a module responsivity of about 0.8 A/W was achieved, and a second-order intermodulation distortion value of less than -70 dBc at an optical modulation index of 40% was obtained under a two-tone test of 400 and 450 MHz

We report characterization and experimental realization of recent InP-related semiconductor optical devices for all-optical 3R regeneration. A Mach-Zehnder interferometric wavelength converter (MZ IWC) with preamplifiers gives an extinction ratio (ER) improvement and a negative power penalty for all-optical reamplification and reshaping (2R) of pseudorandom bit sequence (PRBS) nonreturn-to-zero (NRZ) data at 10 Gb/s. In addition, we briefly describe the mechanism for injection locking of the multisection laser diode with a distributed feedback (DFB) reflector. Using this device, we demonstrate clock recovery from PRBS return-to-zero (RZ) and NRZ data. The achieved rms timing jitter is less than 1 ps for the input of 11.727 Gb/s PRBS RZ and NRZ. All-optical 3R regeneration leads to a good performance using the MZ IWC and multisection DFB LD. The combination of these functions provides all-optical 3R regeneration with NRZ to RZ conversion. For the realization of a small and compact 3R regeneration module, we propose the hybrid module packaging process, which is composed of 2R and 1R semiconductor chip in single butterfly-type module

We have demonstrated a high-power widely tunable sampled grating distributed Bragg reflector (SGDBR) laser integrated monolithically with a semiconductor optical amplifier (SOA) having a lateral tapered waveguide, which is the first to emit a fiber-coupled output power of more than 10 dBm using a planar buried heterostructure (PBH). The output facet reflectivity of the integrated SOA using a lateral tapered waveguide and two-layer AR coating of TiO 2 and SiO 2 was lower than 3 × 10 - 4 over a wide bandwidth of 85 nm. The spectra of 40 channels spaced by 50 GHz within the tuning range of 33 nm were obtained by a precise control of SG and phase control currents. A side-mode suppression ratio of more than 35 dB was obtained in the whole tuning range. Fiber-coupled output power of more than 11 dBm and an output power variation of less than 1 dB were obtained for the whole tuning range.

A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna- integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics technologies will envisage the bright future of THz systems.

We have fabricated thermal-induced wavelength- tunable vertical-cavity surface-emitting lasers with a 780-nm emission wavelength that is tunable by using an internal device heater. A 5.36-nm tuning range was achieved by means of a 70-mA tuning current and a 3.5-mA driving current while maintaining continuous single-mode operation and the optical power variation is less than 2.5 dB in the tuning range. In addition, a multimode tuning range of 9.1 nm was achieved with a 90-mA tuning current and a 7-mA driving current.

Harmonic signal generation and frequency up-conversion are demonstrated using a hybrid mode-locked multisection distributed-feedback (MS-DFB) laser. Hybrid mode-locking is realized by direct injection of electrical local oscillator (LO) signals into the laser gain control section. The harmonic signals are generated when multiple optical modes produced by the hybrid mode-locked MS-DFB laser are detected in a photodiode (PD). In addition, if data signals at fIF are applied to the device along with LO signals at fLO, optical sidebands separated from the optical modes by fIF are generated and harmonic up-converted signals are obtained by mode-beating in PD. Using this method, we demonstrate generation of the third-harmonic millimeter-waves at 30.79 GHz with fLO at 10.263 GHz, and up-conversion of 12.5-Mb/s 32 quadrature amplitude modulation data at 300-MHz fIF into 30-GHz band

A travelling-wave photomixer was realized. The responsivity was 0.42A/W for 15μm length device. It showed a 3dB bandwidth of more than 200GHz, limited by transit-time of photo-generated carriers. Also, it showed a successful THz wave generation up to frequency range of more than 500GHz.

We have investigated the direct modulation characteristics of a phase-shifted 1.3-μm dual-mode laser diode. Stable dual-mode operation and high spectral purity are maintained under 2.5-Gbps direct modulation.

We demonstrate the tunable continuous-wave (CW) terahertz generator based on the λ/4 phase-shifted 1.3 μm dual-mode laser diode (DML) and travelling-wave photodiode (TWPD). The DML and TWPD operate as an optical beat source and terahertz photomixer, respectively. The laser diodes (LDs) operating at the 1.3 μm have more suitable characteristics as optical beat sources than the LDs operating at 1.55 μm because of their high efficiency and better thermal stability. The micro-heaters are integrated on top of each DFB LD for mode beat frequency tuning. The fabricated DML was continuously tuned from 230 GHz to 1485 GHz by increasing the temperature of each DFB section independently via integrated micro-heaters. The high-speed TWPD with an InGaAs absorber was designed and fabricated to efficiently generate the photomixing terahertz CW. A complementary log-periodic antenna was integrated with the TWPD to radiate the generated terahertz wave with minimum reflection in the wide frequency range. The terahertz characteristics of the tunable CW terahertz generator based on the DML and TWPD were measured in a fiber-coupled, homodyne terahertz photomixing system. Our results of the tunable CW terahertz generator show the feasibility of a compact and highly efficient CW terahertz spectrometer and imager.

A sampled-grating distributed Bragg reflector laser module having an integrated multiwavelength locker has been developed and evaluated. The uniquely designed wavelength locker made of thermally controlled etalon has provided uniform wavelength monitoring and very stable wavelength locking in the 188-ITU grid channels (37 nm) with 25-GHz spacing. Over the case temperature from -5/spl deg/C to 65/spl deg/C, the laser wavelength was locked within /spl plusmn/0.5 GHz, and the total power consumption of the module was less than 4 W.

We have demonstrated a high-power widely tunable sampled grating (SG) DBR laser integrated monolithically with optical semiconductor amplifier (SOA), using planar buried heterostructure (PBH). The measured threshold current was 5 mA on average with 60 chips randomly selected which is lowest among the typical average values. Fiber-coupled output power was 12.4 dBm and the output power variation was ?1 dB for the whole tuning range.

Planar buried heterostructure (PBH) was adopted to fabricate a sampled grating distributed Bragg reflector laser diode (SGDBR-LD) having a low threshold current and a stable fundamental transverse mode. The etching process for butt-coupling was optimized to improve the reproducibility and the uniformity of the butt-coupled waveguide. The maximum output power of the fabricated SGDBR-LD was 20 mW at 200-mA continuous-wave operation at 25/spl deg/C. The output power was measured 10 and 9 mW higher than those of ridged waveguide (RWG) structure and buried ridge stripe (BRS), and the threshold current was slightly higher than those of RWG and BRS. The spectra of 25 channels spaced 50 GHz within the tuning range of 44.4 nm was obtained by a precise control of SG and phase control currents. The side-mode suppression ratio of more than 35 dB was obtained in the whole tuning range. The output power variation was less than 5 dB, which is 4 dB smaller than that of RWG structure.

We optimized the etching process for butt coupling to improve the reproducibility and the uniformity of the process for the integrated GaInAsP multiquantum-well laser with a butt-coupled waveguide. Three different ways of etching process were tested, which are reactive ion etching (RIE), RIE followed by a small amount (50 nm thick) of selective wet etching, and RIE followed by an adequate amount (125 nm thick) of selective wet etching. RIE followed by an adequate amount of selective wet etching showed the superior properties to the common expectation on RIE only, giving the measured coupling efficiency 96/spl plusmn/1.7% versus 34/spl plusmn/8%. The high coupling efficiency and the very small variation across a quarter of a 2-in wafer demonstrate that RIE coupled with an adequate amount of selective wet etching can also replace the conventional process for butt coupling, RIE followed by HBr-based nonselective wet etching, to fabricate high-quality integrated photonic devices.

It represents a viable solution for the realization of a portable biosensor platform that could screen/diagnose acute myocardial infarction by measuring cardiac marker concentrations such as cardiac troponin I (cTnI), creatine kinase MB (CK-MB), and myoglobin (MYO) for application to u-health monitoring system. The portable biosensor platform introduced in this presentation has a more compact structure and a much higher measuring resolution than a conventional spectrometer system. Portable guided-mode resonance (GMR) biosensor platform was composed of a biosensor chip stage, an optical pick-up module, and a data display panel. Disposable plastic GMR biosensor chips with nano-grating patterns were fabricated by injection–molding. Whole blood filtration and label-free immunoassay were performed on these single chips, automatically. Optical pick-up module was fabricated by using the miniaturized bulk optics and the interconnecting optical fibers and a tunable VCSEL (vertical cavity surface emitting laser). The reflectance spectrum from the GMR biosensor was measured by the optical pick-up module. Cardiac markers in human serum with concentrations less than 0.1ng/mL were analyzed using a GMR biosensor. Analysis time was 30min, which is short enough to meet clinical requirements. Our results show that the GMR biosensor will be very useful in developing lowcost portable biosensors that can screen for cardiac diseases.

A widely tunable dual mode laser diode with a single cavity structure is demonstrated. This novel device consists of a distributed feedback (DFB) laser diode and distributed Bragg reflector (DBR). Micro-heaters are integrated on the top of each section for continuous and independent wavelength tuning of each mode. By using a single gain medium in the DFB section, an effective common optical cavity and common modes are realized. The laser diode shows a wide tunability of the optical beat frequency, from 0.48 THz to over 2.36 THz. Continuous wave THz radiation is also successfully generated with low-temperature grown InGaAs photomixers from 0.48 GHz to 1.5 THz. (C) 2012 Optical Society of America

A high-speed travelling-wave photodetector (TWPD) with an InGaAs absorber was designed and realized. The bandwidth of the TWPD was measured as 210GHz using electro-optic sampling techniques. This kind of device can be used for THz photomixer

We have developed the λ/4 phase-shifted 1.3-µm dual mode laser as an optical beat source of continuous-wave (CW) terahertz (THz) generation. The mode beat frequency is continuously tuned from 230 to 1485 GHz via integrated micro-heaters. The CW THz system is also demonstrated with low-temperature-grown InGaAs photomixers.

We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated, low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based communication technologies because the well-developed InP-based optoelectronic technologies are already expected to enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers (SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize compact, cost-effective, and portable THz spectrometer.

We demonstrate continuous wave (CW) THz generation with a compact dual-mode laser diode (DML) and InGaAs-based photomixers (PMs). By tuning the output wavelengths of the DML, we achieved THz tuning more than 1 THz. We detected THz output from PM emitter using coherent detection scheme.

The phase shifted dual-mode laser diode (DML) is demonstrated for the optical beat source of continuous wave THz system. Optical beat frequency can be continuously tuned from 0.22 THz to 1.15 THz. High side mode suppression ratio of 40 dB is obtained through whole tuning range.

We propose a high-speed frequency-scanning optical beat source using a wavelength swept laser and a tunable laser for continuous THz wave generation. The beat frequency is scanned at 1 kHz over a range with a 2.16 THz width.

We have investigated the effects of electron injection efficiency on the electrical and optical performances of the nc‐Si LEDs. The current‐voltage curves of nc‐Si LED with 6‐periods of SiCN/SiC multiple electron injection layers was better than that of nc‐Si LED with single SiC electron injection layer. This can be attributed to the increase in an electron density of the 6‐periods of SiCN/SiC multiple electron layers compared to that of the single SiC electron injection layer because the electron density of SiCN layer is higher than SiC layer. Light output power of the LED with 6‐periods of SiCN/SiC multiple electron injection layers was improved by 50% compared to that of LED with single SiC electron injection layer. We show here that the use of the multiple electron injection structure could be used to realize a highly efficient nc‐Si LED.

The optimization of a waveguide photodiode (WGPD) having a thin absorption layer is discussed. Responsivity of 1.09 A/W was achieved. More than 100 A thick absorption layer is required for high responsivity and low polarization dependency. A formula for frequency response in a WGPD with a thin absorption layer is obtained.

The butt-coupled (BT) sampled grating distributed Bragg reflector laser diode (SGDBR-LD) was designed and fabricated using planar buried heterostructure (PBH), enabling a low threshold current and a stable fundamental transverse mode. The but-coupled SGDBR-LD's with target tuning ranges of 44.4nm was fabricated, and the tuning ranges were experimentally measured to be 44.4nm. The measured peak periods of the fabricated SGDBR-LD's were well matched with theoretical values and output power is close to calculated values. The side mode suppression ratio of more than 35dB was obtained in the whole tuning range. The output power variation was less than 5dB, which is 4dB smaller than that of RWG structure.

We report an experimental demonstration of all-optical 3R regeneration with a format conversion from NRZ-to-RZ. It shows a good performance using a Mach-Zehnder interferometer wavelength converter and a self-pulsating multi-section laser diode with DFB reflector

We have demonstrated a high-power widely tunable SG-DBR laser monolithically integrated with a SOA. The structure of SOA is a lateral tapered waveguide and a wideband AR coating. Fiber coupled output power is more than 10 dBm of CW operation, and the output power variation was /spl sim/1 dB.

We have developed a 1.3??m phase-shifted dual-mode laser diode (pDML) for a tunable continuous-wave terahertz (CW THz) emitter. The mode beat frequency of the pDML is continuously tuned from 250 to 1480?GHz by introducing monolithically integrated micro-heaters and multiple sections to an all-active structure. The pDML shows a narrow spectral linewidth and a strong four-wave mixing signal throughout the entire tuning range. By?combining InGaAs based photomixers, we have successfully generated continuous-wave THz radiation, and confirmed the precise beat frequency settings and fast sweeping of this. This pDML operating at 1.3??m shows excellent performance in terms of THz conversion efficiency, tuning range and thermal stability.

A simple thickness measurement method based on the coherent homodyne CW THz system was demonstrated; it does not require precise control of the frequencies of the beat source, and only accurate scanning of the optical delay line is needed. Three beat frequencies are sufficient for measuring the thickness of a sample without considering the modulo 2 pi ambiguity. A novel compact 1.55 mu m lambda/4 phase-shifted dual-mode laser (DML) was developed as an optical beat source for the CW THz system. The thickness of a sample was accurately estimated from the measurements using the proposed method. Our results clearly show the possibility of a compact, simple, and cost-effective CW THz system for practical applications. (C) 2012 Optical Society of America

We propose a rapidly frequency-swept optical beat source for continuous wave (CW) THz generation using a wavelength swept laser and a fixed distributed feedback (DFB) laser. The range of the sweeping bandwidth is about 17.3 nm (2.16 THz), 1541.42-1558.72 nm. The achieved side mode suppression ratio for both wavelengths within the full sweeping range is more than 45 dB. We observe CW THz signals for tunable optical beat sources using a fiber coupled CW THz measurement system to confirm the feasibility of using our frequency swept optical beat source as a CW THz radiation source. The THz output signal falls to the thermal noise level of the low-temperature grown (LTG) InGaAs photomixer beyond 1.0 THz. The rapidly frequency-swept optical beat source will be useful for generating high-speed tunable CW THz radiation.

The successful demonstration of an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is described in this study. The terahertz output power of the emitter with two optical band rejection filters was 132 nW, which is an improvement of 70% over the output power without any filters. This improvement is due to the suppression of an optical modulated signal that is reverse-generated when an alternating current bias exceeding a certain threshold is applied to the emitter. Under the optimal alignment conditions, the terahertz detector in a fiber-coupled THz-TDS system clearly measured water vapor dips in the free space.

We report the effects of introducing an undoped Si 1-x C x buffer layer between a silicon nanocrystal (nc-Si) active layer and an n-type SiC layer on the performance of the nc-Si light-emitting diodes (LEDs). The electrical property of an nc-Si LED with a buffer layer was greatly improved compared to that of an nc-Si LED without a buffer layer. Moreover, the light output power of the nc-Si LED with a buffer layer was enhanced by a factor of 2. By employing a buffer layer, the efficiency of electron injection into the nc-Si layer was enhanced, which resulted in an increase in the light output power. The data show that the introduction of an undoped Si 1-x C x buffer layer is a very effective way to improve the performance of nc-Si LEDs.

The optimization of a waveguide photodiode (WGPD) having a thin absorption layer is discussed. With an absorption layer thickness of less than 0.1μm, responsivity of 1.09A∕W was achieved for lensed-fiber coupling. For flat-ended fiber coupling, responsivity of 0.93A∕W was achieved. An absorption layer thickness of more than 100A is required for high responsivity and low polarization dependency. A formula for frequency response in a WGPD with a thin absorption layer is also obtained. This formula gives the optimized WGPD structure for high speed operation with high responsivity.

Coherent laser light was tuned by cumulative refraction of light waves in a semiconductor laser cavity, while the wavelength of electroluminescence was not tuned by any noticeable amount. This effect is explained by the fact that the laser light has perfect spatial coherence, while the electroluminescence has a spatial coherence of 0.7 µm, smaller than the width of the wavefront. The refraction angle of 2.4 deg means that more than 16 triangular electrodes are involved in the refraction. This observation of coherent tuning may lead to a new optoelectronic device, a coherent filter, which filters out only coherent light from a coherent-incoherent mixture.