Experimental demonstration of high coupling efficiency between wide ridge waveguides and single-mode photonic Crystal waveguides

The experimental demonstration of a high efficiency coupling technique based on setting a single defect within a photonic crystal (PhC) taper is reported. The samples were fabricated on a Silicon-on-insulator substrate and a 3-/spl mu/m-wide ridge waveguide was used to couple the light into and out of a single-mode PhC waveguide. Transmission efficiencies higher than 70% for wavelengths at 1.55 /spl mu/m are demonstrated which sharply improves the transmission efficiency achieved with butt-coupling and conventional PhC tapers.     

Experimental verification of numerically optimized photonic crystal injector, Y-splitter, and bend

We present the experimental measurement of a photonic crystal (PhC) device comprising an injector, Y-splitter, and 60/spl deg/ bend. The complete device consists of a 9-/spl mu/m-long injector tapering down from 5 /spl mu/m into a triangular-lattice-of-holes single-line defect waveguide with period a=430 nm and 36.2% air filling factor (corresponding to a radius over period (r/a) ratio of 0.30), an optimized Y-junction, 60/spl deg/ bend and output injectors, with a total device footprint of 30 /spl mu/m. This is etched into a GaAs/AlGaAs heterostructure using chlorine/argon chemically assisted ion beam etching (CAIBE). An erbium-doped fiber amplifier (EDFA)-based source and Fabry-Perot technique are used to characterize the device. The device displays a bandwidth of approximately 110 nm in the 1.55 /spl mu/m window, and a transmission of 70% relative to the same length of 5-/spl mu/m-wide waveguide. This is compared with three-dimensional finite-difference time-domain (3-D FDTD) results, which have a bandwidth and transmission of 120 nm and 75%, respectively. The highlight of this paper is the close agreement of the numerically optimized complete microcircuit with its experimental equivalent, and the significant improvement in bandwidth over previous work on Y-junctions.     

High-efficiency broad-band transmission through a double-60/spl deg/ bend in a planar photonic Crystal single-line defect waveguide

We present two designs to improve the transmission of a conventional double-60/spl deg/ bend in a single-line defect planar photonic crystal waveguide by locally optimizing the shape and the size of air holes of a photonic crystal lattice at the corners. We fabricate these devices on a silicon-on-insulator substrate and characterized them using tunable laser sources over a wavelength range from /spl lambda/=1.259 /spl mu/m to /spl lambda/=1.641 /spl mu/m. As we show, over a 9% bandwidth, less than 1-dB loss/bend was observed. In order to theoretically validate these experimental results, the three-dimensional finite-difference time-domain simulations are performed and found to agree with the experimental results.     

An optimized InGaAsP/InP polarization converter employing asymmetric rib waveguides

In this paper, we report on the design of a compact (/spl ap/ 226 /spl mu/m) on-chip InGaAsP/InP polarization converter based on an asymmetric rib waveguide. Our theoretical analysis demonstrates that the device displays a conversion efficiency of < -25 dB (> 99.68% power conversion between orthogonal polarization) at 1550-nm wavelength with a nearly flat response over the optical C band. Regarding fabrication tolerances, we predict that the most sensitive design parameter is the waveguide width as the conversion efficiency drops to 10 dB for a deviation of /spl plusmn/ 0.1 /spl mu/m from the optimized value.     

1.55-/spl mu/m spot-size converter integrated laser diode with conventional buried-heterostructure laser process

A 1.55-/spl mu/m spot-size converter integrated laser diode is demonstrated with conventional buried-heterostructure laser process. For the spot-size converter, we employed a double-waveguide structure in which a ridge-based passive waveguide was incorporated. The passive waveguide was optically combined with a laterally tapered active waveguide to control mode size. The threshold current was measured to be 5 mA together with high slope efficiency of 0.45 W/A. The beam divergence angles in the horizontal and vertical directions were as small as 9.0/spl deg/ and 7.8/spl deg/, respectively.     

PCB-compatible optical interconnection using 45/spl deg/-ended connection rods and via-holed waveguides

In this paper, a new architecture for a chip-to-chip optical interconnection system is demonstrated that can be applied in a waveguide-embedded optical printed circuit board (PCB). The experiment used 45/spl deg/-ended optical connection rods as a medium to guide light paths perpendicularly between vertical-cavity surface-emitting lasers (VCSELs), or photodiodes (PDs) and a waveguide. A polymer film of multimode waveguides with cores of 100/spl times/65 /spl mu/m was sandwiched between conventional PCBs. Via holes were made with a diameter of about 140 /spl mu/m by CO/sub 2/-laser drilling through the PCB and the waveguide. Optical connection rods were made of a multimode silica fiber ribbon segment with a core diameter of 62.5 and 100 /spl mu/m. One end of the fiber segment was cut 45/spl deg/ and the other end 90/spl deg/ by a mechanical polishing method. These fiber rods were inserted into the via holes formed in the PCB, adjusting the insertion depth to locate the 45/spl deg/ end of rods near the waveguide cores. From this interconnection system, a total coupling efficiency of about -8 dB was achieved between VCSELs and PDs through connection rods and a 2.5 Gb/s /spl times/ 12-ch data link demonstrated through waveguides with a channel pitch of 250 /spl mu/m in the optical PCB.     

Two-step laterally tapered spot-size converter 1.55-/spl mu/m DFB laser diode having a high slope efficiency

We demonstrate a two-step lateral tapered 1.55-/spl mu/m spot-size converter distributed feedback laser diode (SSC DFB LD) having slope efficiencies as high as 0.457 and 0.319 mW/mA measured at 25 /spl deg/C and 85 /spl deg/C, respectively. The SSC DFB LD fabricated by using a nonselective grating process has a double core waveguide structure including a planar buried heterostructure type active waveguide and a ridge type passive waveguide. The fabricated SSC DFB LD operates at 1.553-/spl mu/m wavelength and shows a far-field pattern in horizontal and vertical directions of 7.3/spl deg/ and 11.6/spl deg/, respectively.     

Electroabsorption waveguide modulators at 1.3 /spl mu/m fabricated on GaAs substrates

An InGaAs-InAlAs multiple-quantum-well (MQW) electroabsorption (EA) waveguide modulator fabricated on a GaAs substrate has been designed and characterized at 1.3-/spl mu/m wavelength for microwave signal transmission on an analog fibre-optic link. The modulator structure with a lattice constant 2.5% larger than that of GaAs is grown upon a 0.7-/spl mu/m-thick three-stage compositionally step-graded In/sub z/Al/sub 1-z/As relaxed buffer. The waveguide modulator exhibits a high-electrooptic slope efficiency of 0.56 V/sup -1/, a 3-dB electrical bandwidth of 20 GHz, and a large optical saturation intensity in excess of 17 mW. These high-speed optoelectronic modulators could potentially be integrated with on-chip GaAs electronic driver circuits.     

1.5-/spl mu/m InGaAsP-InP slab-coupled optical waveguide lasers

We report the demonstration of high-power semiconductor slab-coupled optical waveguide lasers (SCOWLs) operating at a wavelength of 1.5 /spl mu/m. The lasers operate with large (4/spl times/8 /spl mu/m diameter) fundamental mode and produce output power in excess of 800 mW. These structures have very low loss (/spl sim/0.5 cm/sup -1/) enabling centimeter-long devices for efficient heat removal. The large fundamental mode allows 55% butt-coupling efficiency to standard optical fiber (SMF-28). Comparisons are made between SCOWL structures having nominal 4- and 5-/spl mu/m-thick waveguides.     

Continuous-wave operation of GaInAsSb-GaSb type-II ridge waveguide lasers emitting at 2.8 /spl mu/m

We have realized compressively strained GaInAsSb-GaSb type-II double quantum-well lasers with an emission wavelength of 2.8 /spl mu/m. Using broad area devices, an internal absorption of 9.8 cm/sup -1/ and an internal quantum efficiency of 0.57 is determined. For the increase of the threshold current with temperature, a T/sub 0/ of 44 K is obtained. Narrow ridge waveguide lasers show continuous-wave laser operation at temperatures up to 45 /spl deg/C, with room-temperature (RT) threshold current of 37 mA. At RT, the maximum optical output power per facet of an uncoated 800/spl times/7 /spl mu/m/sup 2/ ridge waveguide laser exceeds 8 mW.     

Estimation of polarization crosstalk at a micro-bend in Si-photonic wire waveguide

The singlemode Si-photonic wire waveguide allows sharp bends, which significantly expands the design flexibility of optical devices and circuits. Here, the suppression of the polarization crosstalk at a sharp bend will be an important issue, since a large crosstalk affects the performance of devices and circuits. In this study, the three-dimensional (3-D) finite-difference time-domain (FDTD) simulation showed that the crosstalk at a 90/spl deg/-bend with a radius of 0.35-1.75 /spl mu/m is less than -25 dB at a wavelength of 1.55 /spl mu/m. In the experiment, the crosstalk from TE-like to TM-like polarization was evaluated to be -13 dB to -10 dB. This large value was explained by a small tilt of waveguide sidewalls, which seriously increased the crosstalk. In addition, it was found in the calculation that some combinations of bends increase or decrease the crosstalk, and that a U-shape bend is the most effective for the suppression of the crosstalk.     

Resonant-phonon terahertz quantum-cascade laser operating at 2.1 THz (/spl lambda//spl sime/141 /spl mu/m)

The development of quantum-cascade lasers (QCLs) at 2.1 THz (/spl lambda//spl sime/141 /spl mu/m), which is the longest wavelength QCL to date without the assistance of magnetic fields, is reported. This laser uses a structure based on resonant-phonon depopulation, and a metal-metal waveguide to obtain high modal confinement with low waveguide losses. Lasing was observed up to a heatsink temperature of 72 K in pulsed mode and 40 K in continuous-wave (CW) mode, and 1.2 mW of power was obtained in CW mode at 17 K.     

Band-edge lasing in gold-clad photonic-crystal membranes

We investigate the possibility to achieve band-edge lasing in optically thick gold-clad photonic-crystal (PhC) membranes, with a dielectric thickness of around 1 /spl mu/m. We have performed a two-dimensional eigenmode-expansion analysis of band-edge resonators in one-dimensional PhCs. Material thresholds, quality factors, and emission efficiencies have been calculated for TE band-edge laser resonances on the second and third /spl Gamma/-point. The second /spl Gamma/-point sustains band-edge laser modes with quality factors above 2500 for a membrane thickness of 1 /spl mu/m and a cavity length of 20 periods, however, with a very poor surface-emission efficiency. Band-edge laser modes located on the third /spl Gamma/-point have lower quality factors but higher surface-emission efficiencies. In both cases, the PhC should be designed specifically to avoid coupling with lossy, higher order modes.     

Monolithic integration of an InGaAsP-InP MQW laser/waveguide using a twin-guide structure with a mode selection layer

We demonstrate a monolithically integrated 1.55-/spl mu/m wavelength InGaAsP-InP multiple-quantum-well (MQW) laser with a passive Y-branch waveguide in a vertical twin-waveguide structure. To reduce the sensitivity of the device performance characteristics to laser cavity length and variations in the layer structure, we introduce an In/sub 0.53/Ga/sub 0.47/As absorption, or "loss" layer. This layer eliminates the propagation of the even mode, while having minimal effect on the odd mode. The threshold current densities and differential efficiencies of the devices are unaffected by the loss layer. A record high coupling efficiency of 45% from the laser to the external passive waveguide is obtained.     

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.     

Large-signal performance of deep submicrometer AlGaN/AlN/GaNHEMTs with a field-modulating plate

This is the first time that the microwave performance of a 0.1-/spl mu/m gate in a silicon nitride window opening, with a field-modulating plate on an AlGaN/AlN/GaN heterojunction structure, is reported. The material structure was grown by organometallic vapor phase epitaxy on SiC substrates with an averaged channel sheet resistance of 313.5 ohms/square. Approximately 80-nm-thick plasma-enhanced chemical vapor deposition silicon nitride is used as the dielectric between gate metal extension and semiconductor surface. Transistors of a total gate width of 250 /spl mu/m and a 0.1 /spl mu/m gate footprint, with a 0.36 /spl mu/m long overhang on top of the silicon nitride, can be operated at a drain bias of 40-V high. Output power density of 9.5 W/mm, with 36% power-added efficiency in class AB regime, was demonstrated at 10 GHz in a continuous wave power measurement.     

Thermo-optical modulation at /spl lambda/=1.5 /spl mu/m in an /spl alpha/-SiC-/spl alpha/-Si-/spl alpha/-SiC planar guided-wave structure

A planar waveguide based on an amorphous silicon-amorphous silicon carbide heterostructure is proposed for the realization of passive and active optical components at the wavelengths /spl lambda/=1.3-1.5 /spl mu/m. The waveguide has been realized by low temperature plasma enhanced chemical vapor deposition and is compatible with the standard microelectronic technologies. Thermo-optical induced modulation at /spl lambda/=1.5 /spl mu/m is demonstrated in this waveguide. Numerical simulations predict that operation frequencies of about 3 MHz are possible. The measurements have also allowed the determination of the previously unknown thermo-optical coefficient of undoped amorphous silicon at this wavelength.     

Low-cost 1.55-/spl mu/m InGaAsP-InP spot size converted (SSC) laser with conventional active layers

We report the realization of a low cost 1.55-/spl mu/m spot size converted (SSC) laser using conventional SCH-MQW active layers. The laser consists of a rectangular gain section, a linear taper and a passive waveguide. The lateral taper and the passive waveguide are fabricated on the same lower SCH layer, using conventional photolithography and RIE (reactive ion etching). The device exhibits low beam divergence of 6.6/spl deg//spl times/10.9/spl deg/ and -2.2-dB coupling loss with a cleaved single-mode fiber. The 1-dB alignment tolerance is /spl plusmn/2.15 /spl mu/m in vertical direction and /spl plusmn/2.3 /spl mu/m in lateral direction, respectively.     

Analysis of partially depleted absorber waveguide photodiodes

This paper presents the analysis and characterization of partially depleted absorber (PDA) photodiodes. Coupling to these photodiodes is achieved through a planar short multimode waveguide (PSMW) structure. Electric transport in the PDA structure has been investigated and an equivalent electric circuit was developed. Measurements on 5/spl times/20 /spl mu/m/sup 2/ PSMW PDA photodiodes have shown 0.80 A/W responsivity with a fiber mode diameter as high as 6 /spl mu/m. The transverse electric/transverse magnetic polarization dependence was <0.5/spl plusmn/0.3 dB with -1-dB input coupling tolerances as high as /spl plusmn/2.0 and /spl plusmn/1.3 /spl mu/m for horizontal and vertical directions. The -3-dB bandwidth was 50 GHz, and the -1-dB compression current at 40 GHz was 17 mA corresponding to +4.5 dBm radio frequency (RF) power. Compared to similar evanescently coupled p-i-n photodiodes, the saturation current has been significantly improved while maintaining comparable bandwidth and high responsivity.     

A broad-band waveguide grating coupler with a subwavelength grating mirror

A new diffractive device for light coupling between a planar optical waveguide and free space is proposed. The device utilizes a second-order waveguide grating to diffract the fundamental waveguide mode into two free propagating beams and a subwavelength grating (SWG) mirror to combine the two free propagating beams into a single beam. The finite-difference time-domain (FDTD) simulations show that the SWG mirror improves the coupling efficiency of the waveguide fundamental mode into the single out-coupled beam from about 30% to 92%. A high efficiency (>90%) is predicted for a broad wavelength range of 1520-1580nm. The proposed device is compact (/spl sim/80 /spl mu/m in length) and it eliminates the need for blazing the waveguide grating.