Spot-size conversion using uniform waveguide sections for efficientlaser-fiber coupling

Using numerically simulated results, it is shown that an efficient laser-to-fiber coupling is possible by incorporating a uniform spot-size converter (SSC) with two nonidentical but phase matched optical waveguides. Using a “diluted” waveguide with a low index contrast, the power coupling loss can be significantly reduced from 10.4 dB to only 1.15 dB and at the same time 1.0 dB alignment tolerances can also be improved to achieve ±1.8 μm. Besides the improved coupling, the beam divergence of the far-field is also reduced considerably to 9°, accompanied by a significant lowering of the reflected power from the fiber interface     

1.55-μm InGaAsP-InP spot-size-converted (SSC) laser with simpletechnological process

In this letter, we report the realization of a 1.55-μm spot-size-converted (SSC) laser using conventional SCH-MQW active layers and conventional photolithography. The laser consists of a 300-μm-long rectangular gain section, with compensated multiple-quantum-well (MQW) structure, and a 300-μm-long tapered passive waveguide, fabricated on lower SCH layer. The device exhibits a beam divergence of 13°×18° and 3.5-dB coupling loss with a cleaved single-mode fiber (SMF). The 1-dB alignment tolerance is ±2.3 μm in the vertical direction and ±1.9 μm in the lateral direction, respectively     

Pseudomorphic InP HEMTs with dry-etched source vias having 190 mWoutput power and 40% PAE at V-band

We report state-of-the-art V-band power performance of 0.15-μm gate length InGaAs/InAlAs/InP HEMT's which have 15 μm×23 μm dry-etched through-substrate source vias (substrate thickness 50 μm). The 500-μm wide InP HEMT's were measured in fixture at 60 GHz and demonstrated an output power of 190 mW with 40% power-added efficiency (PAE) and 6.8 dB power gain at an input power of 16 dBm. These results represent the best combination of power and PAE reported to date at this frequency for any solid state device. The results are achieved through optimization of the InP-based heterostructure which incorporates a graded pseudomorphic InGaAs channel and a graded pseudomorphic InAlAs Schottky barrier layer, and the use of 15 μm×23 μm dry-etched through-substrate source vias     

Intrinsic and extrinsic response of GaAs metal-semiconductor-metalphotodetectors

Simulation of GaAs metal-semiconductor-metal (MSM) photodetectors using a self-consistent Monte Carlo (MC) method is discussed. Intrinsic device properties are discussed in terms of MC electron and hole transport under low illumination intensity. Parasitic circuit elements are then introduced to more closely model realistic devices using the MC results in a circuit simulator. Intrinsic devices with 0.5- and 1.0-μm spacing between fingers are dominated by stationary high-field transport. Surprisingly, full-width-half-maximum (FWHM) of 0.5- and 1.0-μm detectors with parasitics is 4.3 and 3.8 ps, respectively. However, the 1-μm detector exhibits a long hole tail and transient oscillations. Thus, FWHM results (and intrinsic device response) can be inadequate predictors of ultimate frequency response and scaling behavior. However, an estimate of maximum repetition frequency gives f_max=92 GHz for the 0.5 μm device, consistent with experimental data     

A low-resistance self-aligned T-shaped gate for high-performancesub-0.1-μm CMOS

This paper describes the high performance of T-shaped-gate CMOS devices with effective channel lengths in the sub-0.1-μm region. These devices were fabricated by using selective W growth, which allows low-resistance gates smaller than 0.1 μm to be made without requiring fine lithography alignment. We used counter-doping to scale down the threshold voltage while still maintaining acceptable short-channel effects. This approach allowed us to make ring oscillators with a gate-delay time as short as 21 ps at 2 V with a gate length of 0.15 μm. Furthermore, we experimentally show that the high circuit speed of a sub-0.1-μm gate length CMOS device is mainly due to the PMOS device performance, especially in terms of its drivability     

Ion implantation enhanced metal-Si-metal photodetectors

The quantum efficiency and frequency response of simple Ni-Si-Ni metal-semiconductor-metal (MSM) photodetectors at long wavelengths are significantly enhanced with a simple, ion-implantation step to create a highly absorbing region ~1 μm below the Si surface. The internal quantum efficiency is improved by a factor of ~3 at 860 nm (to 64%) and a full factor of ten at 1.06 μm (to 23%) as compared with otherwise identical unimplanted devices. Dark currents are only slightly affected by the implantation process and are as low as 630 pA for a 4.5-μm gap device at 10-V bias. Dramatic improvement in the impulse response is observed, 100 ps vs, 600 ps, also at 10-V bias and 4.5-μm gap, due to the elimination of carrier diffusion tails in the implanted devices. Due to its planar structure, this device is fully VLSI compatible. Potential applications include optical interconnections for local area networks and multi-chip modules     

Investigation of broad-band quantum-well infrared photodetectorsfor 8-14-μm detection

Typical quantum-well infrared photodetectors (QWIPs) exhibit a rather narrow spectral bandwidth of 1-2 μm. For certain applications, such as spectroscopy, sensing a broader range of infrared radiation is highly desirable. In this paper, we report the design of four broad-band QWIPs (BB-QWIPs) sensitive over the 8-14-μm spectral range. Two n-type BB-QWIPs, consisting of three and four quantum wells of different thickness and/or composition in a unit cell which is then repeated 20 times to create the BB-QWIP structure, are demonstrated. The three-well n-type In_xGa_1-xAs-Al_yGa_1-yAs BB-QWIP was designed to have a response peak at 10 μm, with a full-width at half-maximum (FWHM) bandwidth that varies with the applied bias. A maximum bandwidth of Δλ/λ_p=21% was obtained for this device at V_b=-2 V. The four-well n-type In_xGa_1-xAs-GaAs BB-QWIP not only exhibits a large responsivity of 2.31 A/W at 10.3 μm and V_b=+4.5 V, but also achieves a bandwidth of Δλ/λ_p=29% that is broader than the three-well device. In addition, two p-type In _xGa_1-xAs-GaAs BB-QWIPs with variable well thickness and composition, sensitive in the 7-14-μm spectral range, are also demonstrated. The variable composition p-type BB-QWIP has a large FWHM bandwidth of Δλ/λ_p=48% at T=40 K and V_b=-1.5 V. The variable thickness p-type BB-QWIP was found to have an even broader FWHM bandwidth of Δλ/λ _p=63% at T=40 K and V_b=1.1 V, with a corresponding peak responsivity of 25 mA/W at 10.2 μm. The results show that a broader and flatter spectral bandwidth was obtained in both p-type BB-QWIP's than in the n-type BS-QWIP's under similar operating conditions     

Self-aligned InGaP/GaAs heterojunction bipolar transistors formicrowave power application

As an alternative to AlGaAs/GaAs heterojunction bipolar transistors (HBTs) for microwave applications, InGaP/GaAs HBTs with carbon-doped base layers grown by metal organic molecular beam epitaxy (MOMBE) with excellent DC, RF, and microwave performance are demonstrated. As previously reported, with a 700-Å-thick base layer (135-Ω/sq sheet resistance), a DC current gain of 25, and cutoff frequency and maximum frequency of oscillation above 70 GHz were measured for a 2-μm×5-μm emitter area device. A device with 12 cells, each consisting of a 2-μm×15-μm emitter area device for a total emitter area of 360 μm², was power tested at 4 GHz under continuous-wave (CW) bias condition. The device delivered 0.6-W output power with 13-dB linear gain and a power-added efficiency of 50%     

1.55-μm narrow-linewidth and high-power distributed feedbacklasers for coherent transmission systems

Stable operation of 1.55-μm distributed feedback lasers with a narrow linewidth and high power is demonstrated for coherent transmission systems. Narrow linewidth and high power were realized by improving the geometrical uniformity of the active region and thinning the active layer by using MOVPE and a dry-etching technique. A narrow linewidth of 1 MHz was achieved at a high power of 20 mW for a distributed feedback laser with a thin active layer of 0.07 μm and a long cavity of 1.2 mm     

Manufacturing study of yield and performance dependence on gatelength of submicron AlInAs-GaInAs HEMTs

The fabrication, characterization, and statistical analysis of the performance and yield of AlInAs-GaInAs on InP low-noise high electron mobility transistors (HEMTs) with subquarter-micron T-gates fabricated with electron beam lithography are reported. This was undertaken to establish the manufacturability of submicron AlInAs-GaInAs HEMT technology for various low-noise microwave receiver applications. Excellent DC device yield (up to 90%) was obtained from devices to gate widths 300 μm and 1000 μm. A range of minimum noise figures between 0.026 to 0.5 dB at 2 GHz and 0.39 to 0.8 dB at 12 GHz were obtained for 0.15-μm and 0.20-μm gate length devices. The results establish the correlation between the noise figure and yield for this new class of microwave devices     

Spot-size converted 1.3 μm laser with butt-jointed selectivelygrown vertically tapered waveguide

A 1.3 μm laser has been developed with a butt-jointed selectively grown spot-sire converter (SSC). The SSC vertically tapered waveguide and strained multiquantum well (MQW) active region are independently optimised. The laser was buried with semi-insulating InP to reduce optical loss in the SSC. A threshold current of 7 mA and an output power of >20 mW were obtained. Minimum coupling loss to a flat-end fibre of 1.06 dB was achieved. Long-term stability was also confirmed     

Length dependence of the saturation characteristics in 1.5-μmmultiple quantum well optical amplifiers

The dependence on amplifier length of gain and gain saturation characteristics in 1.5-μm multiple-quantum-well optical amplifiers is reported. Gain measurements are presented for amplifiers with lengths of 200 μm to 1 mm, and a simple model is introduced which relates gain and saturation characteristics to the amplifier length. The 1-mm-long device has superb properties, with a gain of 25.2 dB and a saturation output power of 40 mW     

A very short planar silica spot-size converter using a nonperiodicsegmented waveguide

To reduce the coupling loss of a fiber-to-ridge waveguide connection, a planar silica spot-size converter for a wavelength of 1.55 μm is implemented in the form of a nonperiodic segmented waveguide structure with irregular tapering. A simple single-step lithography process is sufficient for the fabrication of the planar structures. An evolutionary algorithm has been successfully applied for the optimization. The simulated results obtained with a three-dimensional (3-D) finite difference beam propagation method (FD-BPM) program are compared with measurements of implemented couplers, showing very good agreement. A waveguide-to-fiber coupling efficiency improvement exceeding 2 dB per converter is shown. Structures obtained with this approach are very short (~140 μm) and simple to integrate on the same wafer with other planar structures such as phased arrays or ring resonator structures     

InP-InGaAsP high-speed traveling-wave electroabsorption modulatorswith integrated termination resistors

Traveling-wave electroabsorption modulators for operation at 1.55 μm have been designed and fabricated. Devices of different lengths were characterized. Modulators with integrated termination resistors showed wide modulation bandwidths and excellent bandwidth-length products. A bandwidth of 43 GHz was measured for a 450-μm-long device, which corresponds to a 19.3-GHz·mm bandwidth length product. For a device length of 250 μm, a bandwidth of 67 GHz is extrapolated from measurements up to 45 GHz     

Device characteristics of the GaN/InGaN-doped channel HFETs

First dc, small signal, and RF power characteristics of GaN/InGaN doped-channel heterojunction field effect transistors (HFETs) are reported. HFETs with a 1-μm gate length have demonstrated a maximum drain current of 272 mA/mm, a flat G_m around 65 mS/mm in a V _GS between -0.65 V and +2.0 V, and an on-state breakdown voltage over 50 V. Complete pinchoff was observed for a -3.5 V gate bias. Devices with a 1-μm gate length have exhibited an f_T of 8 GHz and f_max of 20 GHz. A saturated output power of 26 dBm was obtained at 1.9 GHz for a 1 μm×1 mm device     

Self-aligned bottom-gate submicrometer-channel-length a-Si-:Hthin-film transistors

Fully self-aligned bottom-gate thin-film transistors (TFTs) fabricated by using a back substrate exposure technique combined with a metal lift-off process are discussed. Ohmic contact to the sources and drains is accomplished by a 40-nm-thick layer of phosphorous-doped microcrystalline silicon. Devices with channel lengths ranging from 0.4 to 12 μm are processed with overlap dimensions between the gate and the source and the gate and the drain ranging from 0.0 to 1.0 μm. Analysis of the conductance data in the linear voltage regime reveals a parasitic drain-to-channel and source-to-channel resistance that is 14% of the channel resistance for a 10-μm device and 140% for a 1-μm device. Thus, increase in the device speed caused by reducing the channel length does not follow expected behavior. A similar situation exists in the nonlinear regime. The on-current of the devices starts to saturate below channel lengths of 2 μm. Current on/off ratios taken at V_d=5 V and V_G=15 V and 0 V, respectively, are approximately 1×10⁶ for the 1- and 12-μm-long devices. The on/off ratio is reduced to 1×10⁵ for the 0.4-μm device     

Repeated wavelength conversion of 10 Gbit/s signal usingwavelength-tunable semiconductor lasers

Repeated wavelength conversion of 10 Gbit/s pseudorandom non-return-to-zero signals is demonstrated using superstructure grating distributed Bragg reflector lasers operating in the 1.55-μm wavelength region. Error-free and very low-power-penalty wavelength conversion can be achieved in both first and second wavelength conversion for a fixed converted wavelength over a broad wavelength range from 1.486 to 1.573 μm (about 90-nm wide). The power penalty of the transmitted signal light through the first wavelength conversion device increases when the converted wavelength is switched from 1.544 to 1.573 μm periodically at a repetition frequency of 40 MHz. The increase in power penalty, however, is less than 6 dB even when the bit error rate is 10^-12     

Effectiveness of the pseudowindow for edge-coupled InP-InGaAs-InPPIN photodiodes

We have shown that edge-coupled PIN photodiodes can benefit from the incorporation of a pseudowindow of appropriate thickness. In our experiments, a pseudowindow 2-3 μm thick can effectively protect the device and antireflection (AR) coating during the cleavage process without sacrificing the device efficiency and speed performance. Also, devices with a pseudowindow could have an increased coupling aperture, which results from the dielectric layers, and thus permits more light to enter the device. The light input facet of the device without a pseudowindow can be severely damaged during the cleavage process and AR coating, which may degrade the device dark current by several orders of magnitude. We also found that, even with partial recovery after rapid thermal annealing, devices without pseudowindows still suffer from damage and the maximum photocurrent is typically restricted to about 3 mA. The typical performance at -5 V of a device with a 100-μm junction length and a 3-μm pseudowindow is a responsivity of ~0.95 A/W responsivity and a ~5.5-GHz bandwidth at a wavelength of 1.3 μm under 100-μW illumination     

1.4-μm-band gain characteristics of a Tm-Ho-doped ZBLYAN fiberamplifier pumped in the 0.8-μm band

The gain characteristics of a 1.4-μm-band thulium-holmium-doped ZBLYAN fiber amplifier are described. Signal gain is obtained over the whole 1.4-μm band for a pump power level of 73.5 mW. A maximum signal gain of 18 dB is achieved at a signal wavelength of 6.46 μm for a pump power of 150 mW at 0.79 μm. The noise figure is 5.6 dB in the signal wavelength region from 1.45-1.50 μm. From a comparison of the gain characteristics of thulium-holmium-doped ZBLYAN fibers and thulium-doped ZBLYAN fibers, it is proved that holmium ions play an effective role in increasing the gain and widening the gain spectrum     

InP-based cylindrical microcavity light-emitting diodes

We have investigated the properties of InP-based microcavity light-emitting diodes (λ=1.6 μm). Our objective was mainly to study the effects of lateral confinement of optical modes, which was achieved by the wet oxidation of double In_0.52Al_0.48 As layers. The smallest devices had a cavity radius of 0.5 μm, which becomes comparable to λ/n, where n is the effective refractive index of the photon emitting heterostructure. Two types of devices were tested: the first without any mirrors in the vertical direction, and the second with a combination of MgF/ZnSe DBR (top) and silver (bottom) to produce a low Q~35-45. The latter type of devices exhibited higher output power and narrower spectral linewidth; otherwise, the characteristics were very similar The output slope efficiency monotonically decreases with reduction of lateral cavity size up to ~2-μm in diameter and then is enhanced again for smaller cavity sizes. The slope efficiency of the smallest device (aperture diameter 1 μm) is almost equal to that measured for the largest devices. The maximum output power measured from the devices is 30 μW. The far-field pattern of devices with aperture radii ranging from 1.5 to 20 μm shows an angular width (FWHM) of 50°. On the other hand, devices with smaller aperture (radius ~0.5 μm) exhibit an angular width of 20°. The measured small-signal modulation bandwidth increases from ~0.45 GHz for the larger devices to 0.8 GHz for the smallest devices. Our results indicate that microcavity effects can be observed with only lateral photon confinement, making device fabrication requirements less stringent compared to surface-emitting lasers