IEEE Journal of Selected Topics in Quantum Electronics

Published by Institute of Electrical and Electronics Engineers
Online ISSN: 1558-4542
Print ISSN: 1077-260X
Publications
Nonlinear microscopy has become widely used in biophotonic imaging. Pulse shaping provides control over nonlinear optical processes of ultrafast pulses for selective imaging and contrast enhancement. In this study, nonlinear microscopy, including two-photon fluorescence, second harmonic generation, and third harmonic generation, was performed using pulses shaped from a fiber supercontinuum (SC) spanning from 900 to 1160 nm. The SC generated by coupling pulses from a Yb:KYW pulsed laser into a photonic crystal fiber was spectrally filtered and compressed using a spatial light modulator. The shaped pulses were used for nonlinear optical imaging of cellular and tissue samples. Amplitude and phase shaping the fiber SC offers selective and efficient nonlinear optical imaging over a broad bandwidth with a single-beam and an easily tunable setup.
 
Intravital microscopy has emerged in the recent decade as an indispensible imaging modality for the study of the micro-dynamics of biological processes in live animals. Technical advancements in imaging techniques and hardware components, combined with the development of novel targeted probes and new mice models, have enabled us to address long-standing questions in several biology areas such as oncology, cell biology, immunology and neuroscience. As the instrument resolution has increased, physiological motion activities have become a major obstacle that prevents imaging live animals at resolutions analogue to the ones obtained in vitro. Motion compensation techniques aim at reducing this gap and can effectively increase the in vivo resolution. This paper provides a technical review of some of the latest developments in motion compensation methods, providing organ specific solutions.
 
We present an innovative surface-enhanced Raman spectroscopy (SERS) sensor based on a biological-plasmonic hybrid nanostructure by self-assembling silver (Ag) nanoparticles into diatom frustules. The photonic-crystal-like diatom frustules provide a spatially confined electric field with enhanced intensity that can form hybrid photonic-plasmonic modes through the optical coupling with Ag nanoparticles. The experimental results demonstrate 4-6× and 9-12× improvement of sensitivities to detect the Raman dye for resonance and nonresonance SERS sensing, respectively. Such low-cost and high-sensitivity SERS sensors have significant potentials for label-free biosensing.
 
We demonstrate a miniature, near-infrared microscope (λ = 785 nm) that uses a novel dual axes confocal architecture. Scalability is achieved with post-objective scanning, and a MEMS mirror provides real time (>4 Hz) in vivo imaging. This instrument can achieve sub-cellular resolution with deep tissue penetration and large field of view. An endoscope-compatible version can image digestive tract epithelium to guide tissue biopsy and monitor therapy.
 
Since the early 1980's, the enhanced backscattering (EBS) phenomenon has been well-studied in a large variety of non-biological materials. Yet, until recently the use of conventional EBS for the characterization of biological tissue has been fairly limited. In this work we detail the unique ability of EBS to provide spectroscopic, polarimetric, and depth-resolved characterization of biological tissue using a simple backscattering instrument. We first explain the experimental and numerical procedures used to accurately measure and model the full azimuthal EBS peak shape in biological tissue. Next we explore the peak shape and height dependencies for different polarization channels and spatial coherence of illumination. We then illustrate the extraordinary sensitivity of EBS to the shape of the scattering phase function using suspensions of latex microspheres. Finally, we apply EBS to biological tissue samples in order to measure optical properties and observe the spatial length-scales at which backscattering is altered in early colon carcinogenesis.
 
Schematic of LEBS experimental instrument. A broadbaned 450 W Xenon source (S) is imaged onto an aperture of variable size which serves as a secondary source (SS). The size of the aperture is selected by appropriately positioning the aperture wheel. The beam is collimated with lens L 2 and passed through polarizer P 1 . A beam splitter (B) allows collection of backscattered light from the sample. The co-polarized light is selected with polarized P 2 . Lens L 3 then maps the angular distribution of the backscattered light onto the CCD camera detection chip. A liquid crystal tunable filter (LCTF) attached to the camera is used to select the wavelength of collection.  
Demonstration of speckle reduction in LEBS from a dried paint sample (ls* ~ 4μm). Panel a: EBS measurement obtained with Helium Neon (HeNe) laser. Panel b: average of 30 ensembles of EBS measurements collected with HeNe laser. The EBS signal begins to be visible over the speckle noise. Panel c: a single measurement with partial coherence illumination (L SC = 160μm). Panel d: a sinlge measurement with partial coherence measurement (L SC = 20μm).  
Low-coherence enhanced backscattering (LEBS) is a technique that has recently shown promise for tissue characterization and the detection of early pre-cancer. Although several Monte Carlo models of LEBS have been described, these models have not been accurate enough to predict all of the experimentally observed LEBS features. We present an appropriate Monte Carlo model to simulate LEBS peak properties from polystyrene microsphere suspensions in water. Results show that the choice of the phase function greatly impacts the accuracy of the simulation when the transport mean free path (ls*) is much greater than the spatial coherence length (L(SC)). When ls* < L(SC), a diffusion approximation based model of LEBS is sufficiently accurate. We also use the Monte Carlo model to validate that LEBS can be used to measure the radial scattering probability distribution (radial point spread function), p(r), at small length scales and demonstrate LEBS measurements of p(r) from biological tissue. In particular, we show that pre-cancerous and benign mucosal tissues have different small length scale light transport properties.
 
This article reports the evolution of scanning spectral imaging techniques using scattered light for minimally invasive detection of early cancerous changes in tissue and cell biology applications. Optical spectroscopic techniques have shown promising results in the diagnosis of disease on a cellular scale. They do not require tissue removal, can be performed in vivo, and allow for real time diagnoses. Fluorescence and Raman spectroscopy are most effective in revealing molecular properties of tissue. Light scattering spectroscopy (LSS) relates the spectroscopic properties of light elastically scattered by small particles, such as epithelial cell nuclei and organelles, to their size, shape and refractive index. It is capable of characterizing the structural properties of tissue on cellular and sub-cellular scales. However, in order to be useful in the detection of early cancerous changes which are otherwise not visible to the naked eye, it must rapidly survey a comparatively large area while simultaneously detecting these cellular changes. Both goals are achieved by combining LSS with spatial scanning imaging. Two examples are described in this article. The first reviews a clinical system for screening patients with Barrett's esophagus. The second presents a novel advancement in confocal light absorption and scattering spectroscopic (CLASS) microscopy.
 
Recent advances in the use of diffuse optical techniques for monitoring the hemodynamic, metabolic and physiological signatures of the neoadjuvant breast cancer therapy effectiveness is critically reviewed. An extensive discussion of the state-of-theart diffuse optical mammography is presented alongside a discussion of the current approaches to breast cancer therapies. Overall, the diffuse optics field is growing rapidly with a great deal of promise to fill an important niche in the current approaches to monitor, predict and personalize neoadjuvant breast cancer therapies.
 
Improved methods for imaging and assessment of vascular defects are needed for directing treatment of cardiovascular pathologies. In this paper, we employ magnetomotive optical coherence tomography (MMOCT) as a platform both to detect and to measure the elasticity of blood clots. Detection is enabled through the use of rehydrated, lyophilized platelets loaded with superparamagnetic iron oxides (SPIO-RL platelets) that are functional infusion agents that adhere to sites of vascular endothelial damage. Evidence suggests that the sensitivity for detection is improved over threefold by magnetic interactions between SPIOs inside RL platelets. Using the same MMOCT system, we show how elastometry of simulated clots, using resonant acoustic spectroscopy, is correlated with the fibrin content of the clot. Both methods are based upon magnetic actuation and phase-sensitive optical monitoring of nanoscale displacements using MMOCT, underscoring its utility as a broad-based platform to detect and measure the molecular structure and composition of blood clots.
 
Optical coherence microscopy (OCM) is a promising technology for high-resolution cellular-level imaging in human tissues. Line-scanning OCM is a new form of OCM that utilizes line-field illumination for parallel detection. In this study, we demonstrate improved detection sensitivity by using an achromatic design for line-field generation. This system operates at 830-nm wavelength with 82-nm bandwidth. The measured axial resolution is 3.9 μm in air (corresponding to ~2.9 μm in tissue), and the transverse resolutions are 2.1 μm along the line-field illumination direction and 1.7 μm perpendicular to line illumination direction. The measured sensitivity is 98 dB with 25 line averages, resulting in an imaging speed of ~2 frames/s (516 lines/s). Real-time, cellular-level imaging of scattering tissues is demonstrated using human-colon specimens.
 
Fiber lasers mode locked with large normal group-velocity dispersion have recently achieved femtosecond pulse durations with energies and peak powers at least an order of magnitude greater than those of prior approaches. Several new mode-locking regimes have been demonstrated, including self-similar pulse propagation in passive and active fibers, dissipative solitons, and a pulse evolution that avoids wave breaking at high peak power but has not been reproduced by theoretical treatment. Here, we illustrate the main features of these new pulse-shaping mechanisms through the results of numerical simulations that agree with experimental results. We describe the features that distinguish each new mode-locking state and explain how the interplay of basic processes in the fiber produces the balance of amplitude and phase evolutions needed for stable high-energy pulses. Dissipative processes such as spectral filtering play a major role in normal-dispersion mode locking. Understanding the different mechanisms allows us to compare and contrast them, as well as to categorize them to some extent.
 
Recent progress in the development of femtosecond-pulse fiber lasers with parameters appropriate for nonlinear microscopy is reviewed. Pulse-shaping in lasers with only normal-dispersion components is briefly described, and the performance of the resulting lasers is summarized. Fiber lasers based on the formation of dissipative solitons now offer performance competitive with that of solid-state lasers, but with the benefits of the fiber medium. Lasers based on self-similar pulse evolution in the gain section of a laser also offer a combination of short pulse duration and high pulse energy that will be attractive for applications in nonlinear bioimaging.
 
Fluorescence imaging in neurosurgery has a long historical development, with several different biomarkers and biochemical agents being used, and several technological approaches. This review focuses on the different contrast agents, summarizing endogenous fluorescence, exogenously stimulated fluorescence and exogenous contrast agents, and then on tools used for imaging. It ends with a summary of key clinical trials that lead to consensus studies. The practical utility of protoporphyrin IX (PpIX) as stimulated by administration of δ-aminolevulinic acid (ALA) has had substantial pilot clinical studies and basic science research completed. Recently multi-center clinical trials using PpIx fluorescence to guide resection have shown efficacy for improved short term survival. Exogenous agents are being developed and tested pre-clinically, and hopefully hold the potential for long term survival benefit if they provide additional capabilities for resection of micro-invasive disease or certain tumor sub-types that do not produce PpIX or help delineate low grade tumors. The range of technologies used for measurement and imaging ranges widely, with most clinical trials being carried out with either point probes or modified surgical microscopes. At this point in time, optimized probe approaches are showing efficacy in clinical trials, and fully commercialized imaging systems are emerging, which will clearly help lead to adoption into neurosurgical practice.
 
Polymer microring resonators fabricated by nanoimprinting are presented as a means of ultrasound detection. Acoustic waves impinging on a ring-shaped optical resonator cause strain in the ring dimensions, modulating optical output. Basic acoustic and optical characteristics of the microring sensor are presented. Measurements at several frequencies show a high sensitivity and low noise-equivalent pressure. The angular response is determined by sensing the optoacoustic excitation of a 49 μm polyester microsphere and shows wide-angle sensitivity. A 1-D array consisting of 4 microrings is demonstrated using wavelength multiplexing for addressing each element. The high sensitivity, bandwidth, and angular response make it a potentially useful sensor platform for many applications including high-frequency ultrasonic and photoacoustic imaging.
 
Energy diagram. (A) TPEF. (B) SFG. (C) CARS.  
Multimodal imaging of spinal cord white matter. (A) CARS image of myelin sheath (red). (B) SFG image of astroglial processes (blue) that surround parallel axons and microtubules (blue, indicated by arrows) inside the axon along a node of Ranvier. (C) Overlaid image of (A) and (B). (D) SFG image of astroglial processes (blue) that surround a blood vessel seen with CARS (red). ω p − ω s = 2840 cm −1 for each image.  
Simultaneous F-CARS imaging of axonal myelin (red) and TPEF imaging of Ca 2+ indicator (green) reveals influx of calcium ions into axons during myelin degradation induced by Lyso- PtdCho. (A) 0 min. (B) 61 min after treating the spinal cord white matter with 1% Lyso-PtdCho. Intensity traces along the line indicated are below each image. Solid trace: CARS intensity, dashed trace: TPEF intensity. ω p − ω s = 2840 cm −1 for each image. Ax: axon.  
Multimodal nonlinear optical (NLO) imaging is poised to become a powerful tool in bioimaging given its ability to capitalize on the unique advantages possessed by different NLO imaging modalities. The integration of different imaging modalities such as two-photon-excited fluorescence, sum frequency generation, and coherent anti-Stokes Raman scattering on the same platform can facilitate simultaneous imaging of different biological structures. Parameters to be considered in constructing a multimodal NLO microscope are discussed with emphasis on achieving a compromise in these parameters for efficient signal generation with each imaging modality. As an example of biomedical applications, multimodal NLO imaging is utilized to investigate the central nervous system in healthy and diseased states.
 
Three periods of Al<sub>0.1</sub>Ga<sub>0.9</sub>N/Al<sub>0.15</sub>Ga<sub>0.85</sub> N multiple quantum wells (MQWs) were used as the active region of a p-i-n diode fabricated on 6H-SiC substrate. Electroluminescence (EL) of these MQWs has been investigated in both injection and avalanche modes. Band-to-band luminescence of the Al<sub>0.1</sub>Ga<sub>0.9</sub>N wells was found to peak at 364 nm in the injection mode and in the range of 364-372 nm in the avalanche mode. The most striking phenomenon is that band-to-band EL of the Al<sub>0.15</sub>Ga<sub>0.85</sub>N barriers has also been observed in the injection mode, while it is not seen in the avalanche mode. This is explained by considering different sources of carriers and different carrier transportation mechanisms in the two modes. The luminescence intensity I <sub>EL</sub> has a power-law dependence on the current I by I <sub>EL</sub> prop I <sup>2</sup> in the injection mode and by I <sub>EL</sub> prop I <sup>4</sup> in the avalanche mode.
 
The main goal of this paper is to present experimental results on comparison of lasing efficiency for Li III 13.5 nm (2-1 transition) line in LiF microcapillary plasmas using 0.25 and 1 μm subpicosecond pumping lasers. A formula for soft X-ray laser efficiency calculation is presented and used for such comparisons. The results for discharge created preplasma in L=4 mm and L=14 mm microcapillaries are also presented and compared with the results for laser created preplasma
 
Record pulse energy (E~0.3 μJ) is reported for a gain-switched laser diode stack with subnanosecond electrical pump pulses. Pulsewidths near 100 ps are seen to be feasible. A detailed time-resolved analysis of emitted near-field patterns is presented, which provides guidelines for further improvements in time performances. A stack design for pulsed laser diodes of high energy is discussed
 
In this paper, we report on the design and fabrication of electrically-pumped circular-grating surface-emitting DBR lasers for operation in the 0.98-μm wavelength range. The layer structure with InGaAs-GaAs-AlGaAs strained multiquantum-wells was obtained by one-step epitaxial growth. Circular gratings are defined by electron-beam lithography around circular gain sections of different diameters. Low threshold CW operation as low as 26 mA for a 60-μm diameter gain section, and high-power pulsed operation of over 120 mW for a 100-μm diameter gain region are demonstrated. A quasi-circular far-field pattern with a divergence of about 1° is obtained
 
High spatial resolution imaging with terahertz pulses is implemented with a novel collection-mode near-field probe. The spatial resolution capabilities of the system are in the range of few micrometers. We demonstrate resolution of 7 μm using 0.5-THz pulses and discuss performance of the collection-mode near-field probes and image properties
 
We propose a novel continuous-time simultaneous-readout scheme for active imaging systems based on orthogonal modulation of photodetector signals. The superimposed-continuous-time approach presented here differs from the conventional scheduled-discrete-time scheme in that the photodetector signals are summed in a common bus and read concurrently. We show how that our proposed architecture may be advantageous, particularly in applications where bandwidth requirements for a time-multiplexed scheme are highly demanding. The active readout cell presented here is the kernel of the proposed orthogonal encoding architecture. We describe the cell operation principle, its properties and major design challenges. A 0.5-μm CMOS test chip has been fabricated to demonstrate functionality of the readout architecture. Test results show it to be a viable option for highly-integrated active imaging systems.
 
We describe the design, fabrication, and measured characteristics of the high-power optically pumped-semiconductor (OPS) vertical-external-cavity surface-emitting lasers (VCSELs). Using diode laser pumping, we have recently demonstrated operation of such lasers, which for the first time generate high (watt-level) power and a circular Gaussian beam directly from a semiconductor laser. These OPS-VECSELs have a strain-compensated multi-quantum-well InGaAs-GaAsP-GaAs structure and operate CW near λ~1004 nm with output power of 0.69 W in TEM <sub>11</sub> mode, 0.52 W in TEM<sub>00</sub> mode and 0.37 W coupled to a single-mode fiber. With multiple pump and gain elements, OPS-VCSEL technology is scalable to the multiwatt power levels. Such lasers will prove useful in a variety of applications requiring compact and efficient sources with high-power output in a single-mode fiber or with diffraction-limited beam quality
 
Results of a monolithically integrated optical receiver for optical data transmission and optical interconnects is presented. A 0.6-μm BiCMOS technology is used to realize the optoelectronic integrated circuit (OEIC). This OEIC can be used at data rates of 622 Mb/s, 1Gb/s, 1.25 Gb/s, and 1.5 Gb/s with a dual supply of 5 and 17 V with sensitivities of -24.5, -24.3, -24.1, and -22.1 dBm, respectively, at a bit-error rate of 10<sup>-9</sup>. With a single supply of 5 V, operation at 1.25 Gb/s with a sensitivity of -22.7 dBm was achieved.
 
Short-wavelength Al-free active-region compressively strained (Δa/a=1.6%) InGaAsP single-quantum-well diode lasers have been optimized for high continuous-wave (CW) output powers. The use of a highly misoriented substrate is shown to improve the low-temperature spectral characteristics of the active quantum well and result in higher laser performance. By employing strain compensated active regions and growth on highly misoriented substrates, record-high characteristic temperature coefficients T<sub>0</sub> (115-125 K) and T<sub>1</sub> (400-500 K) are achieved for this wavelength region (λ=0.73 μm). A broad waveguide laser design with In<sub>0.5</sub>(Ga<sub>0.5 </sub>Al<sub>0.5</sub>)<sub>0.5</sub>P cladding layers is utilized to achieve CW output powers of 3.2 W (100-μm wide, L=1 mm), with stable operation demonstrated at 0.5-W CW
 
The influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers is investigated for the first time over a large emission range of 0.78<λ<1.1 μm. GaAsP and InGaAs are used for tensile and compressive-strained quantum-well layers, respectively, while GaAs and GaInAsP lattice-matched to GaAs are applied for unstrained quantum wells. The laser structures were prepared by using gas-source molecular beam epitaxy, and broad-area and ridge waveguide Fabry-Perot laser diodes were fabricated. This study shows that applying both tensile and compressive strains in the quantum well reduces threshold current density for the Al-free strained-layer quantum-well lasers. However, it was found that the lattice relaxation set a limitation of maximum compressive strain (i.e., maximum lasing wavelength) for the compressive strained InGaAs lasers while the carrier confinement determined the acceptable maximum tensile strain (i.e., minimum lasing wavelength) and lasing performances for the tensile strained GaAsP lasers. Threshold current density as low as 164 A/cm<sup>2</sup> has been obtained for 1.4% compressive-strained InGaAs-GaInAsP-GaInP lasers having a 12-nm thick quantum well. However, excellent characteristics, such as low threshold current, high efficiency low internal loss, and high output power, have been achieved for the Al-free strained-layer quantum-well lasers
 
The 0.78- and 0.98-μm buried-ridge AlGaAs laser diodes (LD's) with a high Al-content AlGaAs confinement layer selectively grown by using a Cl-assisted MOCVD are demonstrated. By employing the AlGaAs confinement layer, the threshold current and the slope efficiency of the 0.78-μm LD are improved by ~40%, compared to those of the conventional loss-guided LD with the GaAs confinement layer. In addition, the stable fundamental mode up to 150 mW and the small astigmatic distance less than 1 μm are obtained. The 0.78-μm LD also shows the excellent high-power and high-temperature characteristic such as 100 mW CW operation at 100°C and the reliable 2,000-hour operation under the condition of 60°C and 55 mW. In the 0.98-μm LD, the narrow beam with the low aspect ratio of 1.86 and the stable fundamental transverse mode over 200 mW are exhibited. As a result, the 0.98-μm LD realizes the high fiber-coupled-power of 148 mW. Moreover, the high-power and high-temperature operation of 150 mW at 90°C is obtained. In the preliminary aging test, the LD's have been stably operating for over 900 hours under the condition of 50°C and 100 mW
 
We conduct a theoretical analysis of the design, fabrication, and performance measurement of high-power and high-brightness strained quantum-well lasers emitting at 0.98 μm. The material system of interest consists of an Al-free InGaAs-InGaAsP active region and AlGaAs cladding layers. Some key parameters of the laser structure are theoretically analyzed, and their effects on the laser performance are discussed. The laser material is grown by metal-organic chemical vapor deposition and demonstrates high quality with low-threshold current density, high internal quantum efficiency, and extremely low internal loss. High-performance broad-area multimode and ridge-waveguide single-mode laser devices are fabricated. For 100-μm-wide stripe lasers having a cavity length of 800 μm, a high slope efficiency of 1.08 W-A, a low vertical beam divergence of 34°, a high output power of over 4.45 W, and a very high characteristic temperature coefficient of 250 K were achieved. Lifetime tests performed at 1.2-1.3 W (12-13 mW/μm) demonstrates reliable performance. For 4-μm-wide ridge waveguide single-mode laser devices, a maximum output power of 394 mW and fundamental mode power up to 200 mW with slope efficiency of 0.91 mW/μm are obtained.
 
We have fabricated Al-free InGaAs-InGaAsP-GaAs strained quantum-well 0.98-μm lasers with a window structure. The window structure was obtained by Si ion-implantation-induced QW intermixing. The photoluminescence and photocurrent measurements show that an implantation energy of 100 keV and a dose of 1E13 cm<sup>-2</sup> are enough for the fabrication of the window structure in our laser structure. The threshold current of the fabricated 0.98-μm lasers with a window structure is 20 mA and a stable lateral mode is obtained up to 300 mW, and these results suggest that there is no scattering loss or absorption due to the introduction of a window structure. The reliability of the lasers is greatly improved by the introduction of the window structure: they exhibited stable operation for more than 1000 h at 240-mW output power at 50°C. And this results gives us an estimated lifetime of more than 200 000 h at 25°C
 
We compared 0.98-μm lasers with a strain-compensated active layer consisting of a compressive InGaAs well and tensile-strained InGaAsP barriers with identical lasers that have a conventional active layer with GaAs barriers. It was shown that the lasers with InGaAsP barriers have better temperature characteristics due to the larger energy gap difference between a well and barriers. Because of the high characteristic temperature, 200-mW operation was obtained with the InGaAsP-barrier laser even at 90°C without any significant deterioration. We also showed that the operation of the lasers with a strain-compensated active layer was highly reliable. The degradation rate of these lasers was four times smaller than that of the lasers with GaAs barriers due to the better crystal quality in their active laser. The estimated lifetime at 25°C for the lasers with a strain-compensated active layer was more than 170000 hours
 
We have demonstrated a 0.98-μm wavelength tapered broad-area amplifier with a monolithically integrated aspherical waveguide lens. CW output exceeding 1 W from the amplifier-lens chip was measured with 10 mW input from a 0.98-μm diode laser. The integrated semiconductor waveguide lens focused the amplifier output to a 8 μm×3 μm spot, which was measured at output power up to about 0.5 W, corresponding to 2.5 times the diffraction limit The beam propagation method was used to model the integrated amplifier-lens chip, and the calculated focal distances agree with the experiment to within 5%. The integrated lens may be used for output coupling to a single mode fiber with the requirement that the focal point should be positioned on the output facet. Based on BPM simulation, however, the focal point position becomes uncritical if a single mode output waveguide is integrated. Our results indicate that the waveguiding lens is a useful component for the design of high-power photonic integrated circuits
 
We have proposed a GaAs-InGaP superlattice optical confinement layer (SL-OCL), which replaces graded InGaAsP alloy layers in 0.98-μm InGaAs-InGaP graded-index separate-confinement-heterostructure (GRINSCH) strained quantum-well (QW) lasers. Theoretical study of the multiquantum barrier (MQB) effect of the GaAs-InGaP SL indicates that electrons in the GaAs OCL feel more than two times higher barrier height than the classical bulk barrier height. Actually, the increase of internal quantum efficiency and the decrease of threshold current density were confirmed. Furthermore, the extremely high characteristic temperature T <sub>0</sub> of 300 K around RT was obtained. These improvements of laser characteristics, especially high T<sub>0</sub>, is mainly owing to the enhancement of the carrier confinement due to the MQB effect of the SL-OCL
 
A 0.98-μm InGaAs-InGaAsP-GaAs strained quantum-well (QW) laser with an exponential-shaped flared stripe is proposed for high-power, highly reliable operation. The stripe width is wider at the front facet to reduce the optical density by widening the spot size. The stripe width is narrower at the rear facet for stable lateral-mode operation. The stripe width in the transient region is varied exponentially along the cavity for smooth mode transformation. We showed that this structure expands the spot size effectively without any deterioration in stable lateral-mode operation. The kink-occurrence output power is determined only by the stripe width at the rear facet, and the spot size at the front facet is a function only of the stripe width at the front facet. The maximum output power is 40-60% higher than that of ordinary straight-stripe lasers for the same kink-occurrence output power. Testing at 150 mW showed stable operation with an estimated lifetime of more than 200000 h at 25°C
 
(a) Conduction band structure and electron wave functions of MQW tunneling injection lasers and (b) an expanded view of the wave functions in the quantum-well regions.  
(a) Modulation frequency response of undoped MQW tunneling injection lasers under CW and (b) pulsed bias conditions.  
(a) Resonance frequency against square root of optical power and (b) damping against resonance frequency squared of undoped MQW tunneling injection lasers.  
Magnitude of measured electrical impedance of undoped tunneling injection lasers at various biases.  
We demonstrate GaAs-based 0.98-μm multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a “cold” carrier distribution in the quantum wells (QWs). The tunneling time (2 pS) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz
 
State-of-the-art femtosecond lasers have the potential to dramatically improve the effectiveness of interface nonlinear optics for diagnosing Si(001) interface characteristics that are relevant to Si microelectronics manufacturing. We present an analysis of signal acquisition rate and parasitic surface heating for SHG by ultrashort laser pulses at Si(001) interfaces, emphasizing their dependence on the pulse duration, energy, repetition rate, wavelength and focal geometry of the pulses. The results of the analysis are illustrated by several experimental examples of SHG by a Ti: sapphire femtosecond laser from a buried Si(001)-SiO<sub>2</sub> interface or a Si(001) surface during chemical vapor deposition
 
In the above titled paper (ibid., vol. 14, no. 1, pp. 118-125, Jan/Feb 08), Fig. 4 was not correct. The correct figure is presented here.
 
The author corrects errors made in the above titled paper (ibid., vol. 15, no. 6, pp. 1547-1569, Nov./Dec. 09), and withdraws the Note Added in Proof.
 
Optical properties of Zn(II) complex using 1,2-bis(8-hydoxyquinolin-2-yl)ethane (Zn(BQOEH)), such as optical absorption, photoluminescence, and electroluminescence, are studied. Zn complex film emits a yellow photoluminescence band centered at 564 nm and an electroluminescence band at 567 nm. Transient characteristic of the organic light-emitting devices (OLEDs) with Zn(BQOEH) as the emitting material are demonstrated. An optical pulse of more than 5 MHz is obtained from the OLED with Zn(BQOEH).
 
A new silole derivative, 1,2,3,4,5-pentaphenyl-1-(8-phenyl-1,7-octadiynyl)silole, is synthesized, characterized, and used as the electron-transport/emission layers in organic light-emitting diodes. Blue emission at 492 nm is observed, with a maximum luminance of 10 460 cd/m<sup>2</sup> at 18 V. The respective maximum current and power efficiencies are 8.47 cd/A and 3.8lm/W. A triple-layer composite cathode was used, consisting of tris(8-hydroxy-quinolne)aluminum (Alq<sub>3</sub>) lithium fluoride and aluminum. The dependence of emission efficiency on the thickness of TPD and Alq<sub>3</sub> is investigated and explained.
 
We propose a low-crosstalk multichannel wavelength conversion scheme based on a parametric process. Simultaneous wavelength conversion of 25 GHz spaced 103 channeltimes10 Gb/s (1.03 Tb/s) wavelength-division multiplexing signals with an 8- and 4-nm guard band is successfully demonstrated by using a quasi-phase-matched lithium niobate waveguide. The method is evaluated both theoretically and experimentally
 
The pumping and gain properties of Yb<sup>3+</sup>-doped Sr<sub>5 </sub>(PO<sub>4</sub>)<sub>3</sub>F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm<sup>2</sup> based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF<sub>4</sub> (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0×10<sup>-20</sup> and 1.5×10<sup>-20</sup> cm <sup>2</sup> for π and σ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3×10<sup>-20</sup> and 1.4×10<sup>-20</sup> cm<sup>2</sup> for π and σ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25×25×175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (<~1%/cm) and adequately large laser damage thresholds at 1064 nm (~20 J/cm<sup>2</sup> at 3 ns). Overall, diode-pumped Yb:S-FAP amplifiers are anticipated to offer a viable means of amplifying 1.047-μm light, and may be particularly well suited to applications sensitive to overall laser efficiencies, such as inertial confinement fusion energy applications
 
Previously, it has been demonstrated that resonant-cavity-enhanced, quantum-dot avalanche photodiodes can achieve a good gain and high quantum efficiency at 1.06 μm. In our new effort, these devices have shown RC-limited bandwidths of 35 GHz at low gain and gain-bandwidth products as high as 220 GHz. The achievable gain has been increased from ~18 to greater than 50 while keeping the quantum efficiency high. These photodiodes also exhibited low avalanche noise (k=0.24), low dark current (less than 100 nA at 90% of the breakdown voltage), and low-breakdown voltage (~17 V)
 
Ytterbium-doped silica fibers exhibit very broad absorption and emission bands, from ~800 nm to ~1064 nm for absorption and ~970 nm to ~1200 nm for emission. The simplicity of the level structure provides freedom from unwanted processes such as excited state absorption, multiphonon nonradiative decay, and concentration quenching. These fiber lasers therefore offer a very efficient and convenient means of wavelength conversion from a wide variety of pump lasers, including AlGaAs and InGaAs diodes and Nd:YAG lasers. Efficient operation with narrow linewidth at any wavelength in the emission range can be conveniently achieved using fiber gratings. A wide range of application for these sources can be anticipated. In this paper, the capabilities of this versatile source are reviewed. Analytical procedures and numerical data are presented to enable design choices to be made for the wide range of operating conditions
 
Room-temperature continuous-wave operation is demonstrated for the first time in a GaInNAs-GaAs laser grown by metal-organic chemical vapor deposition. A low-threshold current density of 660 A/cm<sup>2</sup> and a high characteristic temperature of 113 K emitting at 1.245 μm is achieved. Emitting at the longest wavelength of 1.225 μm is also demonstrated in a highly strained GaInAs-GaAs double-quantum-well laser on a GaAs substrate by increasing the In content up to 39%. A low-threshold current density of about 200 A/cm<sup>2</sup> in a wavelength range up to 1.2 μm is achieved
 
In free-space single-photon quantum key distribution (QKD), the error rate due to daytime background photons can be reduced with strong temporal filtering. In this case, the improvement in performance is determined by the receiver's ability to resolve signal-photon arrival times. We use fast clock recovery and commercially available single-photon detectors with timing resolution enhanced by additional electronic circuitry to implement temporal gating down to 50 ps in a free-space QKD system. The single-photon channel operates at 850 nm, and the improved timing resolution enables transmission rates of 1.25 GHz. We observe daytime quantum bit error rates of 0.04, which is less than one-third of the ungated error rate. We present the design and performance of the system and demonstrate its benefit to free-space QKD.
 
In this paper, we have newly developed an InGaAs metamorphic buffer on a GaAs substrate grown by metal-organic vapor-phase epitaxy, and realized a fully relaxed quasi-InGaAs substrate with low threading dislocation density. We have also successfully developed a 1.3-mu m-range ridge waveguide laser with InGaP upper cladding and InAlGaAs lower cladding layers. This laser has achieved the highest continuous-wave operating temperature (173degC) reported for a metamorphic laser. We measured the relaxation oscillation frequency from the relative intensity noise and undertook a 10-Gb/s direct modulation experiment.
 
Long wavelength GaInNAsSb-SQW lasers and GaInAsSb-SQW lasers that include a small amount of Sb were successfully grown by gas-source molecular beam epitaxy (GSMBE). We confirmed that Sb reacts in a highly strained GaInAs-GaAs system and GaInNAs-GaAs system like a surfactant, which increases the critical thickness at which the growth mode changes from two-dimensional (2-D) growth to three-dimensional (3-D) growth. The lasers were processed into ridge lasers. The GaInNAsSb lasers oscillated under continuous-wave (CW) operation at 1.258 μm at room temperature. The low CW threshold current of 12.4 mA and high characteristic temperature (T<sub>c</sub>) of 157 K were obtained for GaInNAsSb lasers, which is the best result for GaInNAs-based narrow stripe lasers. Further, the GaInNAsSb laser oscillated under CW conditions over 100°C. On the other hand, GaInAsSb lasers oscillated under CW operation at 1.20 μm at room temperature. The low CW threshold current of 6.3 mA and high characteristic temperature (T<sub>c</sub>) of 256 K were obtained for GaInAsSb lasers, which is also the best result for 1.2-μm-range highly strained GaInAs-based narrow stripe lasers. We can say that GaInNAsSb lasers are very promising material for realizing an access network. Further, the differential gain of GaInNAs-based SQW lasers was estimated for the first time. GaInNAsSb-SQW lasers have the extremely large differential gain of 1.06-10<sup>-15 </sup> cm<sup>2</sup> in spite of the single-QW lasers; therefore, GaInNAsSb lasers are also suitable for high-speed lasers in the long wavelength region
 
The 1.27-μm InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than ∼35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75°C. The VCSELs continuously operate up to 105°C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of ∼5.25GHz/(mA)<sup>1</sup>2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25°C to 70°C. We also accumulated life test data up to 1000 h at 70°C/10 mA.
 
We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al<sub>0.4</sub>Ga<sub>0.6</sub>As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570°C. We achieved 1.28-μm continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In<sub>0.13</sub>Ga<sub>0.87</sub>As strain-reducing layer whose p-cladding layer was grown at 560°C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer.
 
The effect of structural parameters on the lasing characteristics of 1.3-μm narrow beam lasers has been investigated. Monolithically integrated vertically tapered multiquantum-well (MQW) waveguide, fabricated by use of selective metal-organic chemical vapor deposition (MOCVD), is used for the expansion of the optical spot size. It is experimentally shown that the energy separation between the gain and waveguide regions that is formed simultaneously by selective MOCVD is shown to be an important parameter in order to achieve low-threshold current density and good temperature characteristics. The lengths of gain and waveguide regions have been investigated in terms of temperature characteristics of threshold current and far-field angle. A lower threshold current density and a higher characteristic temperature were obtained for longer gain region, We also have estimated the waveguide loss of the mode-field converter lasers diodes (MFC-LD's). High performance of 1.3-μm integrated vertically tapered waveguide lasers were achieved in an optimized device
 
We present stable polarization of a long-wavelength vertical-cavity surface-emitting laser (LW VCSEL). The polarization control was achieved through growing its active region on a (113)B InP substrate, which was integrated to [001] GaAs-based distributed Bragg reflectors by a wafer-bonding technique. Theoretical investigation showed that to achieve high polarization stability, a large dichroism such as an anisotropic gain is needed. It was also shown that the (113)B and other planes of the (11n) family have asymmetry, which results in asymmetric stress and anisotropic optical gain in a strained multiquantum well. An index-guiding mesa structure was fabricated in an asymmetric shape. The index guiding either enhanced or distracted the polarization stability originating from gain anisotropy, depending on its orientation of the asymmetry, as was confirmed by a statistical summary. Using a VCSEL with an appropriate index-guiding structure, we performed 1-Gb/s modulation and confirmed single polarization under large-signal modulation.
 
Top-cited authors
Evgueni Slobodtchikov
  • Q-Peak Inc., Bedford, MA USA
A. Carter
  • Nufern
Frank W Wise
  • Cornell University
William H Renninger
  • University of Rochester
F. Bugge
  • Ferdinand-Braun-Institut