Optical mode converter integration with InP-InGaAsP active and passive waveguides using a single regrowth process

A new method for integration of optical mode converters with InP-based photonic integrated circuits is described and demonstrated. The mode converter is integrated to a Sampled Grating DBR (SG-DBR) laser to demonstrate integration and to facilitate accurate fiber-coupling loss measurements. The entire fabrication process requires a single MOCVD regrowth, making it compatible with low-cost integration with other photonic components. The mode converter utilizes a vertically tapered geometry and yields a record 86% coupling efficiency using a lensed, uncoated, single-mode fiber. Cleaved fiber to InP-based waveguide coupling loss sensitivity is measured to be better than 1 dB with /spl plusmn/ 1.85-/spl mu/m lateral fiber misalignment and /spl plusmn/1.5-/spl mu/m transversal fiber misalignment.     

Germania-based core optical fibers

Germania-glass-based core silica glass cladding single-mode fibers (/spl Delta/n up to 0.143) with a minimum loss of 20 dB/km at 1.9 /spl mu/m were fabricated by the modified chemical vapor deposition (MCVD) method. The fibers exhibit strong photorefractivity with the type-IIa-induced refractive-index modulation of 2/spl times/10/sup -3/. The Raman gain of 300 to 59 dB/(km/spl middot/W) was determined at 1.07 to 1.6 /spl mu/m, respectively, in a 75 mol.% GeO/sub 2/ core fiber. Only 3 m of such fibers are enough for the creation of a 10-W Raman laser at 1.12 /spl mu/m with a 13-W pump at 1.07 /spl mu/m. Raman generation in optical fiber at a wavelength of 2.2 /spl mu/m was obtained for the first time.     

Multiquantum-well lasers with tapered active stripe for direct coupling to single-mode fiber

We demonstrate narrow beam divergence in 1.3-/spl mu/m wavelength multiquantum-well (MQW) lasers with an active stripe horizontally tapered over the whole cavity, for direct coupling to single mode-fibers. The lasers have reduced output beam divergence in a simple structure which does not contain an additional spot-size transformer. The fabricated laser shows narrow beam divergence of /spl sim/12/spl deg/, while a low-threshold current of 6.9 mA and a high efficiency of 0.62 mW/mA are realized. Furthermore, a direct-coupling efficiency to a single-mode fiber is -4.0-dB and -3-dB alignment tolerance is /spl plusmn/2.5 /spl mu/m.     

Rearrangeable nonblocking SOI waveguide thermooptic 4/spl times/4 switch matrix with low insertion loss and fast response

A rearrangeable nonblocking 4/spl times/4 thermooptic silicon-on-insulator waveguide switch matrix at 1.55-/spl mu/m integrated spot size converters is designed and fabricated for the first time. The insertion losses and polarization-dependent losses of the four channels are less than 10 and 0.8 dB, respectively. The extinction ratios are larger than 20 dB. The response times are 4.6 /spl mu/s for rising edge and 1.9 /spl mu/s for falling edge.     

1.49-/spl mu/m-band gain-shifted thulium-doped fiber amplifier for WDM transmission systems

This paper describes in detail the amplification characteristics of gain-shifted thulium-doped fiber amplifiers (GS-TDFAs) operating in the 1480to 1510-nm wavelength region (1.49-/spl mu/m S-band) for use in wavelength-division-multiplexing (WDM) systems. Gain shifting of a TDFA, which normally has a gain band at 1.47 /spl mu/m (S/sup +/-band), is achieved by two types of dual-wavelength pumping: (1) 1.05 and 1.56 /spl mu/m or (2) 1.4 and 1.56 /spl mu/m. The main pump source at 1.05 or 1.4 /spl mu/m creates population inversion between /sup 3/F/sub 4/ (upper laser level) and /sup 3/H/sub 4/ (lower laser level), while the auxiliary pump source at 1.56 /spl mu/m reduces the average fractional inversion down to approximately 0.4, which is a desired level for gain shifting. We show experimentally that the former provides a low internal noise figure (<4 dB) due to high fractional inversion at the input end of a thulium fiber, while the latter provides a very high optical efficiency but a higher internal noise figure (/spl sim/5 dB) due to the lower fractional inversion at the input end. These characteristics were verified by numerical simulation based on a comprehensive rate equation modeling. We demonstrated a 1.4- and 1.56-/spl mu/m laser-diode-pumped GS-TDFA with an optical efficiency of 29.3% and high output power of +21.5 dBm. Gain flatness and tilt control were also investigated. These results strongly confirm the feasibility of using GS-TDFAs in practical ultralarge-capacity WDM networks.     

Fabrication and characteristics of GaP-AlGaP tapered waveguide semiconductor Raman amplifiers

We have fabricated GaP-AlGaP tapered waveguide semiconductor Raman amplifiers, and analyzed the effect of tapering in pulse-pumped high-gain operation. The finesse measurement and 80-ps pulse pumped Raman amplification experiment were performed. Although the tapering has caused additional optical loss, the highest gain of 23 dB has been obtained for a tapered waveguide with input facet of 6.0 /spl mu/m/sup 2/ and back facet of 2.9 /spl mu/m/sup 2/ at averaged input power of 170 mW (peak power 26 W). It is shown that the optical loss of the pump light is more severe than the linear optical loss of the signal light when the gain is higher than 20 dB.     

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.     

Parallel optical transmitter module using angled fibers and a V-grooved silicon optical bench for VCSEL array

We propose an advanced structure of optical subassembly (OSA) for packaging of the vertical-cavity surface-emitting laser (VCSEL) array, using (111) facet mirror of the V-groove ends formed in a silicon optical bench (SiOB) and angled fiber apertures. The feature of our OSA can provide a low optical crosstalk between neighboring channels, a low feedback reflection, and a large misalignment tolerance along the V-groove. We describe the optimized design of fiber angle, VCSEL position, and fiber position. The fabricated OSA structure consists of 12 channels of angled fiber array, 54.7/spl deg/ V-grooves, Au-coated mirrors on (111) end facet of the V-grooves, and flip-chip-bonded VCSEL array on a SiOB. In this structure, the beam emitted from the VCSEL is deflected at the 54.7/spl deg/ mirror of (111) end facet and propagated into the angled fiber. The angled fiber array was polished by 57/spl deg/. Fabricated OSAs showed a coupling efficiency of 30%-50% that is 25 times larger than that obtained from an OSA with a vertically flat fiber array. Our OSA showed large misalignment tolerance of about 90 /spl mu/m along the longitudinal direction in the V-groove. We fabricated a parallel optical transmitter module using the OSA and demonstrated 12 channels /spl times/2.5 Gb/s data transmission with a clear eye diagram.     

Fabrication of high-packing-density vertical cavity surface-emitting laser arrays using selective oxidation

We describe the nearly-planar processing of two-dimensional vertical cavity laser arrays based on the selective conversion of AlAs to Al/sub x/O/sub y/. The individual lasers of 8/spl times/8 and 2/spl times/2 arrays are defined by native Al/sub x/O/sub y/ to achieve 4-/spl mu/m square active regions on 12-/spl mu/m center-to-center spacings. Interelement thermal coupling is characterized along with the optical mode structure.     

Integrated folding 4/spl times/4 optical matrix switch with total internal reflection mirrors on SOI by anisotropic chemical etching

A folding rearrangeable nonblocking 4/spl times/4 optical matrix switch was designed and fabricated on silicon-on-insulator wafer. To compress chip size, switch elements (SEs) were interconnected by total internal reflection (TIR) mirrors instead of conventional S-bends. For obtaining smooth interfaces, potassium hydroxide anisotropic chemical etching of silicon was utilized to make the matrix switch for the first time. The device has a compact size of 20/spl times/1.6 mm/sup 2/ and a fast response of 7.5 /spl mu/s. The power consumption of each 2/spl times/2 SE and the average excess loss per mirror were 145 mW and -1.1 dB, respectively. Low path dependence of /spl plusmn/0.7 dB in total excess loss was obtained because of the symmetry of propagation paths in this novel matrix switch.     

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.     

Flexible pillars for displacement compensation in optical chip assembly

In chip-to-chip optical interconnect systems with surface mounted light-sources and detectors, thermal and mechanical effects can cause lateral displacements of the assembled devices. These displacements can result in optical signal losses that can critically deteriorate the bit-error-rate of the digital system. We demonstrate that, for a given loss budget of 1 dB, the use of flexible optical pillars with 150-/spl mu/m height and 50-/spl mu/m diameter can double the lateral displacement tolerance from about 15 to 30 /spl mu/m. The pillars fabricated from Avatrel polymer form an air-free path between the light source and the substrate and cause maximum optical power losses less than 0.2 dB.     

Optical coupling characteristics of laser diodes to thermally diffused expanded core fiber coupling using an aspheric lens

A discussion is presented of the optical coupling characteristics of a 1.3 mu m laser diode to a thermally diffused expanded core (TEC) single-mode fiber coupling using an aspheric lens. The misalignment tolerance width for a 0.5 dB loss increment of the expanded core fiber was enlarged to 8.4 mu m from 1.9 mu m of a nonexpanded core fiber without a remarkable coupling-loss increase. The TEC fibers will make the LD module assembly easier, and reduce the assembly cost.<>     

Fiber fuse phenomenon in step-index single-mode optical fibers

The unsteady-state thermal conduction process in step-index single-mode (SM) optical fiber was studied theoretically with the explicit finite-difference method. We considered a high-temperature loss-increase mechanism, which includes two factors that bring about an increase in the absorption coefficients: 1) electronic conductivity due to the thermal ionization of a Ge-doped silica core and 2) thermochemical SiO production in silica glass. The core-center temperature changed suddenly and reached over 4/spl times/10/sup 5/ K when a 1.064-/spl mu/m laser power of 2 W was input into the core layer heated at 2723 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The high-temperature core areas were enlarged and propagated toward the light source. The propagation rates of the fiber fuse, estimated at 1.064 and 1.48 /spl mu/m, were in fair agreement with the experimentally determined values. We found that the threshold power for initiating the fiber fuse increases from 0.98 to 1.26 W when the input laser wavelength is increased from 1.06 to 1.55 /spl mu/m.     

Self-heating and trapping effects on the RF performance of GaN MESFETs

RF power performances of GaN MESFETs incorporating self-heating and trapping effects are reported. A physics-based large-signal model is used, which includes temperature dependences of transport and trapping parameters. Current collapse and dc-to-RF dispersion of output resistance and transconductance due to traps have been accounted for in the formulation. Calculated dc and pulsed I-V characteristics are in excellent agreement with the measured data. At 2 GHz, calculated maximum output power of a 0.3 /spl mu/m/spl times/100 /spl mu/m GaN MESFET is 22.8 dBm at the power gain of 6.1 dB and power-added efficiency of 28.5% are in excellent agreement with the corresponding measured values of 23 dBm, 5.8 dB, and 27.5%, respectively. Better thermal stability is observed for longer gate-length devices due to lower dissipation power density. At 2 GHz, gain compressions due to self-heating are 2.2, 1.9, and 0.75 dB for 0.30 /spl mu/m/spl times/100 /spl mu/m, 0.50 /spl mu/m/spl times/100 /spl mu/m, and 0.75 /spl mu/m/spl times/100 /spl mu/m GaN MESFETs, respectively. Significant increase in gain compression due to thermal effects is reported at elevated frequencies. At 2-GHz and 10-dBm output power, calculated third-order intermodulations (IM3s) of 0.30 /spl mu/m/spl times/100 /spl mu/m, 0.50 /spl mu/m/spl times/100 /spl mu/m, and 0.75 /spl mu/m/spl times/100 /spl mu/m GaN MESFETs are -61, -54, and - 45 dBc, respectively. For the same devices, the IM3 increases by 9, 6, and 3 dBc due to self-heating effects, respectively. Due to self-heating effects, the output referred third-order intercept point decreases by 4 dBm in a 0.30 /spl mu/m/spl times/100 /spl mu/m device.     

Short-cavity distributed Bragg reflector laser with an integrated tapered output waveguide

A distributed Bragg reflector strained-layer multiple-quantum-well laser at 1.5- mu m wavelength with an integrated tapered waveguide beam expander is discussed. The far-field FWHMs in the lateral and vertical directions are 10 and 15 degrees , respectively. A quantum efficiency of 17% out the tapered facet was measured with a corresponding threshold current of 28 mA. The alignment tolerances for coupling into a cleaved single-mode fiber are 9.5 and 7.5 mu m FWHM in the lateral and vertical directions, respectively. A 3-dB improvement for single-mode fiber coupling using a SELFOC lens was obtained.<>     

Low-chirp and external optical feedback resistant characteristics in /spl lambda//8 phase-shifted distributed-feedback laser diodes under direct modulation

Clarifies that phase shift value in directly modulated distributed feedback laser diodes (DFB-LDs) affects their chirp characteristics and resistance to external optical feedback because of the feedback effect of mirror loss (FEML). Comparing /spl lambda//8, /spl lambda//4, and 3//spl lambda/8 phase-shifted DFB-LDs, theoretically and experimentally, showed that the negative FEML in /spl lambda//8 phase-shifted DFB-LDs improves their transmission performance and makes them more resistant to external optical feedback. It was demonstrated with 2.5-Gb/s directly modulated 1.55-/spl mu/m /spl lambda//8 phase-shifted DFB-LDs that a very low-power penalty for 100-km transmissions (less than 1 dB for BER=10/sup -10/) can be obtained within a wide extinction ratio from 8.5 to 14.5 dB. Excellent resistance to optical feedback was also demonstrated using /spl lambda//8 phase-shifted DFB-LDs: isolator-free 80-km transmission was successful under -14-dB external optical feedback. In addition, the relation of transmission characteristics to coupling coefficient /spl kappa/L and detuning between oscillation wavelength and gain maximum was also discussed here for designing low chirp /spl lambda//8 phase-shifted DFB-LDs.     

Wavelength stabilized single-mode lasers by coupled micro-square resonators

Wavelength stabilized single-mode lasers were realized by on-chip coupling of a ridge waveguide laser with two micro-ring resonators. The micro-ring resonators were fabricated by deeply etching to achieve high lateral optical confinement and to allow small ring diameters in the range between 20-60 /spl mu/m. A square-like ring geometry with 45/spl deg/ facets was used to improve design control and side-wall quality. By using micro rings with two different diameters of 50 and 60 /spl mu/m, a free spectral range of about 18 nm could be obtained which allow a stable single-mode operation with a side-mode suppression ratio of more than 40 dB and a continuous-wave output power of 30 mW per facet at room temperature.     

Evanescently coupled photodiodes integrating a double-stage taper for 40-Gb/s applications-compared performance with side-illuminated photodiodes

The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-/spl mu/m fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (/spl plusmn/1.6 /spl mu/m and /spl plusmn/2 /spl mu/m, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than +10 dBm and a high output voltage of 0.8 V. These capabilities allow the photodiode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes.