Continuously tunable 1.55-/spl mu/m VCSEL implemented by precisely curved dielectric top DBR involving tailored stress

We present an optically pumped and continuously tunable 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL). The device shows 26-nm spectral tuning range, 400-/spl mu/W maximum output power, and 57-dBm side-mode suppression ratio. The VCSEL is implemented using a two-chip concept. The movable top mirror membrane is precisely designed to obtain a tailored air-gap length (L'=16 /spl mu/m) and a radius of curvature (ROC=4.5mm) in order to efficiently support the fundamental optical mode of the plane-concave resonator. It consists of a distributed Bragg reflector (DBR) with periodic, differently stressed silicon nitride and silicon dioxide multilayers implemented by plasma-enhanced chemical vapor deposition. The lower InP-based part, comprising the InP-InGaAsP bottom DBR and the active region, is grown monolithically using metal-organic vapor phase epitaxy.     

An algorithm using visible and 1.38-/spl mu/m channels to retrieve cirrus cloud reflectances from aircraft and satellite data

The Moderate Resolution Imaging Spectro-Radiometer (MODIS) on the Terra spacecraft has a channel near 1.38 /spl mu/m for remote sensing of high clouds from space. The implementation of this channel on MODIS was primarily based on previous analysis of hyperspectral imaging data collected with the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We describe an algorithm to retrieve cirrus bidirectional reflectance using channels near 0.66 and 1.38 /spl mu/m. It is shown that the apparent reflectance of the 1.38-/spl mu/m channel is essentially the bidirectional reflectance of cirrus clouds attenuated by the absorption of water vapor above cirrus clouds. A practical algorithm based on the scatterplot of 1.38-/spl mu/m channel apparent reflectance versus 0.66-/spl mu/m channel apparent reflectance has been developed to scale the effect of water vapor absorption so that the true cirrus reflectance in the visible spectral region can be obtained. To illustrate the applicability of the present algorithm, results for cirrus reflectance retrievals from AVIRIS and MODIS data are shown. The derived cirrus reflectance in the spectral region of 0.4-1 /spl mu/m can be used to remove cirrus contamination in a satellite image obtained at a visible channel. An example of such an application is shown. The spatially averaged cirrus reflectances derived from MODIS data can be used to establish global cirrus climatology, as is demonstrated by a sample global cirrus reflectance image.     

Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 /spl mu/W of SHG light at 3.75 /spl mu/m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 /spl mu/m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 /spl mu/W/W/sup 2/. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 /spl mu/m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 /spl mu/W/W/sup 2/. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7/spl times/10/sup -5/ esu (2/spl times/10/sup 4/pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.     

Submilliwatt optical bistability in wafer fused vertical cavity at 1.55-/spl mu/m wavelength

We report on low-power optical bistability in a vertical cavity structure at 1.55-/spl mu/m wavelength due to the bandgap resonant dispersive optical nonlinearity of InGaAsP. In this structure, a GaAs-AlAs Bragg reflector grown on GaAs and the nonlinear medium grown on InP are bonded through wafer-fusion, leading to a high-quality vertical cavity after deposition of a top dielectric mirror. This device shows interesting characteristics for optical switching applications at fiber communication wavelengths, such as a switching contrast higher than 8:1 in the reflective mode and a bistability threshold power as low as 0.6 mW. True steady-state memory effect is observed with continuous-wave input, and the device switching time is in the ns range.     

A high-efficiency CMOS image sensor with air gap in situ MicroLens (AGML) fabricated by 0.18-/spl mu/m CMOS technology

The air gap in situ microlens (AGML) above-pixel sensor with 0.18-/spl mu/m CMOS image sensor technology has been successfully developed to dramatically improve the optical crosstalk and pixel sensitivity. We demonstrated excellent crosstalk diminution with the structure on small pixels. Compared with conventional 2.8 /spl mu/m square pixel, adopting the AGML can reduce the optical crosstalk up to 64%, and provide 21% in enhancement of photosensitivity at 0/spl deg/ incident angle. Furthermore, under 20/spl deg/ incident angle the optical crosstalk reduction and sensitivity enhancement are increased to 89% and 122%, respectively. Therefore, the AGML structure makes pixel size be further scaled down to less than 2.8 /spl mu/m square and maintain good performance.     

Laterally coupled DBR laser emitting at 1.55 /spl mu/m fabricated by focused ion beam lithography

By using focused ion beam lithography high performance 1.55-/spl mu/m emitting distributed Bragg reflector lasers were realized suitable for high-speed optical telecommunication. Threshold currents of 8 mA and continuous-wave efficiencies of 0.37 W/A for 600-/spl mu/m-long devices were achieved. Stable single-mode emission with sidemode suppression ratios of > 40 dB were observed for the entire operation range. By relative intensity noise measurements an intrinsic 3-dB modulation frequency of > 10 GHz was estimated for a single-mode output power of 23 mW.     

AlGaAsSb-InGaAsSb HPTs with high optical gain and wide dynamic range

Novel heterojunction phototransistors based on AlGaAsSb-InGaAsSb material systems are fabricated and their characteristics are demonstrated. Responsivity of a phototransistor is measured with applied bias voltage at four different wavelengths. The maximum responsivity around 1400 A/W and minimum noise equivalent power of 1.83/spl times/10/sup -14/ W/Hz/sup 1/2/ from this phototransistor with bias of 4.0 V at a wavelength of 2.05 /spl mu/m were measured at 20/spl deg/C and -20/spl deg/C, respectively. Noise equivalent power of the phototransistor is considerably lower compared with commercially available InGaAs p-i-n photodiodes. Collector current measurements with applied incident power are performed for two phototransistors. Currents of 400 nA at low intensity of 0.425 /spl mu/W/cm/sup 2/ and of 30 mA at high intensity of 100 mW/cm/sup 2/ are determined. Collector current increases nearly by five orders of magnitude between these two input intensities. High and constant optical gain of 500 in the 0.46-nW to 40-/spl mu/W input power range is achieved, which demonstrates high dynamic range for such devices at these power levels.     

Polarization independent bulk active region semiconductor optical amplifiers for 1.3 /spl mu/m wavelengths

Semiconductor optical amplifiers for 1.3 /spl mu/m are realized combining single-step grown bulk InGaAsP active region with ridge-waveguides. Achieved fiber-to-fiber gains are in excess of 27 dB with spectral ripples below 0.2 dB. Gain is polarization insensitive to within 1 dB over the entire range of driving current, 1.28 /spl mu/m to 1.34 /spl mu/m wavelength and 10/spl deg/C to 50/spl deg/C heat sink temperature. Intrinsic noise figure is 6.3 dB. Gain saturates at +10 dBm.     

Improved coupling efficiency of a strained InGaAs-AlGaAs quantum-well laser into a fiber Bragg grating

We describe a highly efficient coupling of up to 78% into a single mode fiber (SMF) by utilizing a low-divergence-angle InGaAs-AlGaAs laser and a new high-NA aspheric lens, resulting in the coupled optical output of over 300 mW with a kink-free performance. The kink-free 300 mW of fiber-coupled power is the largest power reported from a diode laser wavelength stabilized using a fiber Bragg grating. A 3% fiber Bragg grating makes the optical output stabilize at a wavelength of 1.016 /spl mu/m for an optical output power up to 310 mW and without any distinct decrease of the optical output. The coupling tolerance for lateral offsets is improved by utilizing a thermal-diffused expanded-core technique even in the case of using a high-NA coupling aspheric lens. We have also achieved a 300-mW fiber-coupled wavelength-stabilized InGaAs-AlGaAs semiconductor laser butterfly module using a fiber Bragg grating.     

Two- and three-photon absorption based optical limiting and stabilization using a liquid dye

It is experimentally shown that both two- and three-photon absorption in a high-molar-concentration chromophore system can be more efficiently utilized to accomplish optical power limiting and stabilization at laser wavelengths of 1.064 and /spl sim/1.3 /spl mu/m, respectively. The nonlinear absorbing medium is a novel liquid dye system, trans-4-[4-(dihexylamino)styryl]-N-(2-{2-[2-(2-hydroxy-ethoxy) -ethoxy]-ethoxy}ethyl)-pyridini um p-tosylate (abbreviated as ASEPT), consisting of chromophore molecules capable of multiphoton absorption in the near IR range. The nonlinear transmission property and output/input characteristics have been studied based on this liquid dye system, using nanoseconds 1.064-/spl mu/m laser pulses for two-photon excitation and /spl sim/1.3-/spl mu/m subpicoseconds laser pulses for three-photon excitation. A fairly good optical stabilization capability of this new material has been demonstrated at both laser wavelengths. The relative intensity fluctuation of the laser pulses can be remarkably reduced by simply passing through this multiphoton absorbing medium.     

Dramatic reduction of optical crosstalk in deep-submicrometer CMOS imager with air gap guard ring

A dielectric structure, air gap guard ring, has been successfully developed to reduce optical crosstalk thus improving pixel sensitivity of CMOS image sensor with 0.18-/spl mu/m technology. Based on refraction index (RI) differences between dielectric films (RI = 1.4 /spl sim/ 1.6) and air gap (RI = 1), total internal reflection occurred at dielectric-film/air-gap interface, thus the incident light is concentrated in selected pixel. Excellent optical performances have been demonstrated in 3.0 /spl times/ 3.0 /spl mu/m pixel. Optical spatial crosstalk achieves 80% reduction at 20/spl deg/ incidence angle and significantly alleviates the pixel sensitivity degradation with larger angle of incident light.     

Widely tunable 850-nm metal-filled asymmetric cladding distributed Bragg reflector lasers

Broadly tunable distributed Bragg reflector (DBR) lasers utilizing metal-filled surface-etched diffraction gratings were fabricated on a GaAs-unstrained quantum well with AlGaAs core and cladding layers. Devices with 200- and 400-/spl mu/m gratings sections were fabricated. Single-mode devices are fabricated over a broad spectral range, exhibiting over 55 mW of single-facet output power. The optical properties of this structure are analyzed in terms of effective index step between the peak and valley of the grating and scattering loss in the DBR. Discrete devices were fabricated with lasing wavelengths between 846.6 and 862.9 nm at intervals of 1.44nm, representing a range of 16.3 nm. Wavelength tuning by current injection into the DBR section is explored, and a broad tuning range of over 18 nm is measured while single-mode performance is maintained. The spectral linewidth of these devices is measured. The temperature dependence of light versus current, threshold current values, and spectral characteristics are also examined.     

Continuous room-temperature operation of optically pumped two-dimensional photonic crystal lasers at 1.6 /spl mu/m

In this letter continuous operation is realized from two-dimensional slab photonic crystal lasers at room temperature. The laser structure is prepared by wafer fusion of an InGaAsP MQW active layer with an AlAs layer that is wet oxidized into an Al/sub 2/O/sub 3/ layer subsequently. The incident threshold pump power at 0.98 /spl mu/m is 9.2 mW for a /spl sim/10-/spl mu/m-diameter hexagonal cavity lasing at 1.6 /spl mu/m.     

Narrow-stripe distributed reflector lasers with first-order vertical grating and distributed Bragg reflectors

Narrow-stripe 1.55-/spl mu/m wavelength distributed reflector lasers consisting of both distributed Bragg reflectors and vertical grating of the first Bragg order were fabricated. A low threshold current I/sub th/ of 2.8 mA, a differential quantum efficiency /spl eta//sub d/ of 28% from the front facet, and a submode suppression ratio of 44 dB were obtained for structures with a stripe width of 1.3 /spl mu/m and a cavity length of 150 /spl mu/m.     

Beam steering in high-power CW quantum-cascade lasers

We report the light-current (L-I), spectral, and far-field characteristics of quantum cascade lasers (QCLs) with seven different wavelengths in the /spl lambda/=4.3 to 6.3 /spl mu/m range. In continuous-wave (CW) mode, the narrow-stripe (/spl ap/13 /spl mu/m) epitaxial- side-up devices operated at temperatures up to 340 K, while at 295 K the CW output power was as high as 640 mW with a wallplug efficiency of 4.5%. All devices with /spl lambda//spl ges/4.7 /spl mu/m achieved room-temperature CW operation, and at T=200 K several produced powers exceeding 1 W with /spl ap/10% wallplug efficiency. The data indicated both spectral and spatial instabilities of the optical modes. For example, minor variations of the current often produced nonmonotonic hopping between spectra with envelopes as narrow as 5-10 nm or as broad as 200-250 nm. Bistable beam steering, by far-field angles of up to /spl plusmn/12/spl deg/ from the facet normal, also occurred, although even in extreme cases the beam quality never became worse than twice the diffraction limit. The observed steering is consistent with a theory for interference and beating between the two lowest order lateral modes. We also describe simulations of a wide-stripe photonic-crystal distributed-feedback QCL, which based on the current material quality is projected to emit multiple watts of CW power into a single-mode beam at T=200 K.     

InGaAs-AlAs-AlAsSb coupled quantum well intersubband transition all-optical switch with low switching energy for OTDM systems

We report a novel intersubband transition all-optical switch with low switching energy using InGaAs-AlAs-AlAsSb coupled double quantum wells (C-DQWs) with AlAs spacer layers. Very low saturation energy density of 34 fJ//spl mu/m/sup 2/ was observed for bulk transmittance with wavelength of 1.62 /spl mu/m. Using a waveguide, whose core is 93 periods of the C-DQWs, an all optical switch with a low switching energy of 10 pJ for 10-dB extinction ratio was realized.     

Light guide for pixel crosstalk improvement in deep submicron CMOS image sensor

Light guide, a novel dielectric structure consisting of PE-Oxide and FSG-Oxide, has been developed to reduce crosstalk in 0.18-/spl mu/m CMOS image sensor technology. Due to the difference in refraction index (1.46 for PE-Oxide and 1.435 for FSG-Oxide), major part of the incident light can be totally reflected at the interface of PE-Oxide/FSG-Oxide, as the incidence angle is larger than total reflection angle. With this light guide, the pixel sensing capability can be enhanced and to reduce pixel crosstalk. Small pixels with pitch 3.0-/spl mu/m and 4.0-/spl mu/m have been characterized and examined. In 3.0-/spl mu/m pixel, optical crosstalk achieves 30% reduction for incidence angle of light at 10/spl deg/.     

A surface micromachined optical scanner array using photoresist lenses fabricated by a thermal reflow process

This paper presents the design, fabrication, and operation of a newly developed micromechanical optical scanner array using a translating microlens. We have used photoresist reflow technique to form a microlens on a surface micromachined XY-stage of the scratch-drive actuation mechanism. The lens scanner is placed at the focal length from an incident optical fiber to collimate the transmitting light. The collimated beam is steered two-dimensionally by the XY-motion of the microlens with respect to the incident fiber. We also have developed a theoretical model to predict appropriate initial resist thickness and diameter for the scanning lens. An optical scanning angle of /spl plusmn/7/spl deg/ has been demonstrated by sliding a microlens of 670-/spl mu/m focal length at a physical stroke of /spl plusmn/67 /spl mu/m. Typical angular positioning resolution has been estimated to be 0.018/spl deg/.     

High-efficiency 1.3 /spl mu/m InAsP-GaInP MQW electroabsorption waveguide modulators for microwave fiber-optic links

High-efficiency electroabsorption waveguide modulators have been designed and fabricated using strain-compensated InAsP-GaInP multiple quantum wells at 1.32-/spl mu/m wavelength. A typical 200-/spl mu/m-long modulator exhibits a fiber-to-fiber optical insertion loss of 9 dB and an optical saturation intensity larger than 10 mW. The 3-dB electrical bandwidth is in excess of 20 GHz with a 50-/spl Omega/ load termination. When used in an analog microwave fiber-optic link without amplification, a RF link efficiency as high as -38 dB is achieved at 10 mW input optical carrier power. These analog link characteristics are the first reported using MQW electroabsorption waveguide modulators at 1.32 /spl mu/m.