Postgrowth tuning of semiconductor vertical cavities formultiple-wavelength laser arrays

Combined lateral-vertical oxidation of AlGaAs is investigated as a means of tuning the resonant wavelength of a semiconductor microcavity after the epitaxial growth. It is shown that this technique can provide arrays with a wavelength spread equal to the cavity's free spectral range with a single postgrowth processing step. Design issues for multiple-wavelength vertical-cavity laser arrays using this postgrowth tuning technique are discussed, comparing the performance of devices with all-semiconductor and partially or totally oxidized Bragg mirrors. Experimental results are presented on arrays with a 48-nm lasing span around 970 nm, using partially and totally oxidized mirrors     

Wavelength-graded horizontal cavity laser array with postgrowthadjustment of wavelength

Wavelength-graded emission from a visible red GaInP-AlGaInP laser diode array has been achieved with the contradirectional surface-mode coupling technique. The wavelength control is attained by postgrowth adjustment of the thickness of the surface waveguide. The horizontal cavity lasers show both edge and surface emission (beam divergence 0.12°). The thermal red-shift of the wavelength is 0.028±0.002 nm/K. They show single-mode emission with a typical spectral linewidth of 0.09 nm and a sidemode suppression ratio up to 29 dB. The wavelength spacing between the individual lasers is 0.76±0.08 nm yielding a total range across the array of 5.4 nm (from 681.5 to 686.9 nm)     

Resonant cavity-enhanced InGaAs-AlGaAs heterojunctionphototransistors with an optical design for high uniformity and yield

We present uniformity data on resonant cavity-enhanced InGaAs-AlGaAs heterojunction phototransistors (HPT's) with an optical design that promotes high uniformity and yield. The HPT's operate in the wavelength region where the GaAs substrate is transparent and the data show the HPT's to be suitable for vertical integration with optical emitters or modulators to form two-dimensional arrays of smart pixels operating in transmission mode. The absorbing region of the HPT consists of an InGaAs multiple-quantum-well structure where the quantum wells (QW's) have been distributed to make the total absorption in the cavity insensitive to growth variations as well as the spatial matching of the standing wave and absorbing QW's. Theoretically, we estimate the absorption to be 39%±1% of the incident optical power, even at wafer nonuniformities of 12.5%. With these nonuniformities, the resonant wavelength moves ±25 nm, making postgrowth tuning of the wavelength necessary. Experimentally, we show postgrowth tuning of the resonance wavelength without loss in uniformity. The arrays have good uniformity as well as very high responsivities. The average responsivity is 160 A/W ±15% from 927-955 nm. The standard deviation of a typical array is 0.5 nm in resonant wavelength and about 5% of the average responsivity. The difference between maximum and minimum values for an array is typically 3 nm in resonant wavelength and ±10% of the average responsivity     

Wavelength tuning of high-speed InGaAs-GaAs-AlGaAs pseudomorphicMQW lasers via impurity-free interdiffusion

The impurity-free interdiffusion technique has been utilized to modify the operating wavelength of high-speed In_0.3Ga_0.65As-GaAs multiple quantum well lasers. Modulation bandwidths of up to 26 GHz (I_bias=50 mA) and modulation current efficiency factors of 5 GHz/mA^1/2 are demonstrated for 3×100 μm² ridge waveguide lasers following wavelength shifts of 32 nm (34 meV). These results demonstrate the feasibility of fabricating monolithic multiple wavelength laser arrays in which each element is capable of low-bias-current direct modulation at bandwidths exceeding 20 GHz     

Multiple-wavelength vertical-cavity surface-emitting laser arrays with a record wavelength span

Multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) arrays are fabricated using a modified patterned-substrate growth technique in a molecular beam epitaxy (MBE) system. We achieved a large lasing wavelength span of 62.7 nm and highly uniform threshold currents with an average of 2.16/spl plusmn/0.81 mA. High repeatibility of wavelength spacing between 15 arrays with sharp wavelength shift rate 117.14 nm/mm is achieved.     

Wavelength shift in vertical cavity laser arrays on a patternedsubstrate

The authors demonstrate a spatially chirped emission wavelength in vertical cavity surface emitting laser (VCSEL) arrays grown by molecular beam epitaxy. The wavelength shift is due to a lateral thickness variation in the Al_0.2Ga_0.8As cavity, which is induced by a substrate temperature profile during growth. A 20 nm shift in lasing wavelength is obtained in a VCSEL array     

Monolithic oxide-confined multiple-wavelength vertical-cavitysurface-emitting laser arrays with a 57-nm wavelength grading rangeusing an oxidized upper Bragg mirror

Monolithic, oxide-confined, multiple-wavelength vertical-cavity surface-emitting laser arrays with a very large periodic, wavelength grading span of 57 nm (from 968 to 1025 nm) have been achieved under room temperature, continuous-wave operation, with threshold currents of 4.5 mA±1.5 mA. Almost linear wavelength grading is achieved by organometallic vapor phase epitaxial growth on a patterned substrate. An extended wavelength range is achieved by minimizing the optical loss dispersion by scaling the growth rate of all the epilayers and using a selectively-oxidized upper DBR mirror with a flattened optical reflectance spectrum, plus the higher differential optical gain provided by compressively-strained In_0.2Ga_0.8As-GaAs quantum wells     

Integration of waveguide-type wavelength demultiplexingphotodetectors by the selective intermixing of an InGaAs-InGaAsPquantum-well structure

Using the selective intermixing of an InGaAs-InGaAsP multiquantum-well (MQW) structure, a wavelength demultiplexing photodetector which can demultiplex two widely separated wavelengths was fabricated. An InGaAs-InGaAsP MQW with a u-InP cladding layer and a n-InGaAs cap layer, grown by metal organic chemical vapor deposition was used. Selective area intermixing of the InGaAs-InGaAsP MQW structure was done by a rapid thermal annealing after the deposition and patterning of the SiO₂ dielectric layer on the InGaAs cap layer. The integrated structure consists of shorter and longer wavelength sections, separated by an absorber section. Shorter wavelength and absorber sections were intermixed with the SiO₂ dielectric layer. At a wavelength of 1477 nm, the output photocurrent ratio was enhanced as the length of the absorber region increased and a ratio of over 30 dB was observed, while at a wavelength of 1561 nm, an output photocurrent ratio of 18.9 dB was observed     

Reduction of reflections in surface-wave delay lines with quarter-wave taps

The reflections and back voltages present in a multiply tapped LiNbO3delay line have been measured. Reflections from taps spaced by one-quarter wavelength were 5 to 15 dB less than those spaced by the customary half wavelength.     

Monolithic photovoltaic PbS-on-Si infrared-sensor array

The growth of epitaxial narrow-gap PbS-on-Si substrates using a stacked CaF₂-BaF₂ intermediate buffer layer and the fabrication of linear arrays of photovoltaic infrared (IR) sensors in the PbS layer are discussed. The sensors of the array exhibit resistance-area products at zero bias of 3 Ω-cm² at 200 K (3.4-μm cutoff wavelength) and 2×10⁵ Ω-cm ² at 84 K (4-μm cutoff), with corresponding detectivities of 2×10¹⁰ and 1×10¹³ cm-√Hz/W, respectively     

Dynamic programming applied to unequally spaced arrays

The application of the optimization technique known as dynamic programming to the design of "thinned" arrays with unequally-spaced elements is described. Dynamic programming is a systematic procedure for efficiently utilizing the capabilities of modern high-speed digital computing machines to find solutions to problems not computationally feasible by conventional means. In this paper it is applied to the design of linear arrays of 25 elements spaced within a 50 wavelength aperture. The results obtained compare favorably with similar results found by other design techniques. The effect on the sidelobes of varying the angular region of optimization and the spacing-quantization is also explored.     

A generalized suboptimum unequally spaced channel allocationtechnique. I. In IM/DD WDM systems

Four-wave mixing (FWM) is the most serious fiber nonlinearity associated with low-input optical power levels in long-haul multichannel optical systems employing dispersion-shifted fiber. To reduce the crosstalk due to FWM, a generalized suboptimum unequally spaced channel allocation (S-USCA) technique is proposed and investigated. Even though the developed technique is useful in combating FWM crosstalk in wavelength division multiplexing (WDM) lightwave systems with up to 12 channels, its main virtue is in designing multichannel WDM lightwave systems with more than 12 channels. Comparisons of power penalty due to FWM between equal channel spacing (ECS) systems and the S-USCA systems are presented. It is shown that for an intensity modulation/direct detection (IM/DD) transmission system operating in an optical bandwidth of 16 nm with 0 dBm (1 mW) peak optical input power per channel, while a conventional ECS WDM system with 0.84-nm channel spacing cannot even achieve a bit-error rate (BER)=10^-9, the suboptimum technique developed in this paper, for the same minimum channel spacing, can achieve a BER=10^-9 with an FWM crosstalk power of less than 1 dB at the worst channel in a 20-channel WDM system     

Fabrication of the InGaN-Based Light-Emitting Diodes Through a Photoelectrochemical Process

InGaN-based light-emitting diodes (LEDs) with a lateral-etched (LE) undercut structure were fabricated through a photoelectrochemical-etching process. The LE-LED was fabricated with an undercut structure, where the InGaN layer acted as a sacrificial layer without reducing the effective emission area. The electroluminescence (EL) spectrum of the LE-LED had a wavelength blueshift phenomenon of 4.6 nm when compared with a standard-LED (ST-LED) at a 30-$hbox{A/cm}^{2}$ current density. The wavelength blueshift phenomenon of the EL emission peaks were measured as 5.5 and 4.3 nm by varying the injection current density from 1.5 to 30 $hbox{A/cm}^{2}$ for the ST-LED and the LE-LED. In a bias-dependent micro-photoluminescence measurement, the blueshift phenomenon of a peak wavelength for the LE-LED was smaller than for the ST-LED. These results were attributed to a partially reduced piezoelectric field in the InGaN active layer and larger light extraction efficiency in the LE-LED structure.      

On the design of nonuniformly spaced slot arrays

A technique for the design of nonuniformly spaced waveguide slot arrays is proposed. The slots are in active resonance, and their position is adjusted so that the sources of the array will have the desired amplitudes and phases. The conductances of the slots depend on the desired pattern and the efficiency. Applications on Taylor and cosec ²&thetas; patterns have shown the validity of the method     

Simple multiwavelength device fabrication technique using a single-grating holographic exposure

A simple technique, using a single-grating holographic exposure associated with localized selective etching steps, has been developed for multiwavelength device fabrication. Four-wavelength DBR laser arrays with a 5 nm Bragg wavelength spacing have been fabricated for wavelength division multiplexing (WDM) applications with this method. These devices exhibit uniformly low-threshold currents (10-15 mA), high-output powers (15 mW) and wide tunabilities (12 nm), leading to an overall accessible wavelength domain of 28 nm for the array.     

Self-electro-optic effect device using Wannier-Stark localisationin an unstrained InGaAs/InAlAs superlattice grown on GaAs substrate

The authors report the first clear observation of Wannier-Stark localisation in an unstrained In_0.15Ga_0.85As/In _0.15Al_0.85As superlattice on a strain-relaxed InGaAs buffer layer grown on GaAs substrate. Optical bistability is demonstrated for a resistor-SEED (self-electro-optic effect device) and a symmetric SEED system at an operating wavelength of 877 nm     

发射二维方向信息的时空调制OFDM通信测向系统

通信与测向是现代电子系统中的两项重要技术,为了设计通信与测向综合发射源,本文提出一种发射信号载有二维方向信息的空间调制OFDM通信测向方案.其思想是利用间距大于半个波长的平面8元天线阵列,二元一组分组激励发射,使合成电磁场是一个被方位角和仰角信息调制的OFDM信号,单天线接收机只需解调信号就可以实现数字通信和测向任务.文中阐述了二维空间调制原理,选择了调制信号星座,设计了信号状态的差分编码结构,给出了单天线接收测向和数字信息解调算法.最后,通过对高斯信道下通信误码率和测向精度的仿真,证明了系统设计的有效性.     

Wavelength-selective coupling among three vertically integratedoptical waveguides by grating couplers

Selective coupling between two specified optical waveguides in a multistory device via radiation modes was proposed and discussed. Three single-TE-mode waveguides consisting of a Corning 7059 glass guiding core layer and a SiO₂ waveguide-separation layer were stacked on a Si substrate with grating couplers. The coupling length was 0.5 mm. Contradirectional couplings from the first story to the second story at a wavelength of 814 nm and from the first story to the third story at 819 nm with sharp wavelength dependence of 0.5-nm full-width at half-maximum (FWHM) were experimentally demonstrated     

610-nm band AlGaInP single quantum well laser diode

The short-wavelength limits of AlGaInP visible laser diodes with Al_0.5In_0.5P cladding layers, and Ga_xIn _1-xP single quantum well (QW) active regions are investigated. Good performance is maintained throughout the 620 nm band, but the characteristics rapidly degrade in the 610 nm band. Biaxial-compression and -tension were compared, with the case of tension yielding slightly better performance. Using a 25 Å Ga_0.45 In_0.55P QW, a wavelength of 614 nm was obtained, while a 50 Å Ga_0.6In_0.4P QW emitted at 620 nm with a threshold current density of 0.8 kA/cm². These results with thin single QWs indicate the effectiveness of using an Al_0.5In_0.5P cladding layer to reduce electron leakage     

Selective quantum-well intermixing in GaAs-AlGaAs structures usingimpurity-free vacancy diffusion

Impurity-free vacancy disordering (IFVD) using SiO₂ and SrF₂ dielectric caps to induce selective quantum-well (QW) intermixing in the GaAs-AlGaAs system is studied. The intermixing rate of IFVD was found to be higher in n-i-p and intrinsic than in p-i-n structures, which suggests that the diffusion of the Group III vacancy is not supported in p-type material. Single-mode waveguides have been fabricated from both as-grown and bandgap-tuned double-quantum-well (DQW) laser samples. Propagation losses as low as 8.5 dB cm^-1 were measured from the bandgap-tuned waveguides at the lasing wavelength of the undisordered material, i.e., 860 nm. Simulation was also carried out to study the contribution of free-carrier absorption from the cladding layers, and the leakage loss induced by the heavily p-doped GaAs contact layer. It was found that the leakage loss contributed by the GaAs cap layer is significant and increases with wavelength. Based on IFVD, we also demonstrate the fabrication of multiple-wavelength lasers and multichannel wavelength division multiplexers using the one-step “selective intermixing in selected area” technique. This technique enables one to control the degree of intermixing across a wafer. Lasers with bandgaps tuned to five different positions have been fabricated on a single chip. These lasers showed only small changes in transparency current, internal quantum efficiency, or internal propagation loss, which indicates that the quality of the material remains high after being intermixed. Four-channel wavelength demultiplexers based on a waveguide photodetector design have also been fabricated. Photocurrent and spontaneous emission spectra from individual diodes showed that the absorption edge was shifted by different degrees due to the selective degree of QW intermixing. The results obtained also imply that the technique can be used in the fabrication of broad-wavelength emission superluminescent diodes