Kouta Tateno

Optical interconnection using VCSELs and polymeric waveguide circuits

We investigated the waveguide loss and transmission characteristics for optical interconnection using vertical-cavity surface-emitting lasers (VCSELs) and multimode polymeric waveguide circuits with crossings. The excess loss with 100 crossings is 2.2 dB when the image magnification from a VCSEL to a waveguide is 2.3. We obtained error-free (i.e., bit error rate <10/sup -11/) optical interconnection at 1.0625 Gbps regardless of the number of crossings or the magnification. These results suggest the practicality of large-scale optical interconnection between VCSEL-based smart-pixel chips using multimode waveguides with more than 100 crossings.

Flip-chip bonded 0.85-μm bottom-emitting vertical-cavity laser array on an AlGaAs substrate

We report high-performance 0.85-/spl mu/m bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) on an AlGaAs substrate with 2.1 mA threshold current density 4.2 mW maximum output power, 11.7% power conversion efficiency and a maximum operating temperature of 130/spl deg/C. We also demonstrate a flip-chip bonded 0.85-/spl mu/m bottom-emitting VCSEL array, and confirm all pixels across the 8/spl times/8 VCSEL array operate at a f/sub 3/ dB bandwidth of 2.6 GHz at only 4.2 mA.

Carbon doping and etching effects of CBr4 during metalorganic chemical vapor deposition of GaAs and AlAs

Abstract We have investigated carbon (C) doping and the reduction in the growth rate for GaAs and AlAs using CBr 4 by metalorganic chemical vapor deposition (MOCVD). The C concentration is proportional to [CBr 4 ], [AsH 3 ] −1 and exp ( −E a RT ) ( E a = − 1.4 eV in GaAs and −1.3 eV in AlAs). These activation energies ( E a ) are equal to the Gibbs free energy of CBr 4 (1.3 eV) around the growth temperature. This indicates that the C incorporation is determined by the equilibrium of the vapor CBr 3 , which is easily decomposed from CBr 4 in the gas phase. The C concentration of AlAs is about one order of magnitude higher than that of GaAs. The activity of C as an acceptor is close to 100% in GaAs and 80% in AlAs, and slightly depends on the growth temperature ( T g ) and the V III ratio. The reduction in the growth rate (γ) for GaAs is proportional to [CBr 4 ], [AsH 3 ] −0.5 and exp ( −E a RT ) ( E a = 1.2 eV), while for AlAs it is proportional to [CBr 4 ], [AsH 3 ] −1.0 and exp ( −E a RT ) ( E a = −1.5 eV). These findings suggest that there are different types of etching mechanisms: for GaAs, the etching may be limited by the As removal, which is followed by fast Ga removal by HBr from CBr 4 decomposition. On the other hand, for AlAs, the etching may be performed by adsorbed CBr x ( x = 1−3) during the doping process. The hole concentration ( p ) of AlGaAs is equal to the average of those of GaAs and AlAs. However, γ for AlGaAs is not a simple average of those for GaAs and AlAs. This suggests that there is some alloy effect in addition to the above two etching processes.

Optical beam direction compensating system for board-to-board free space optical interconnection in high-capacity ATM switch

In interconnections between bookshelf-assembled asynchronous transfer mode (ATM) switch boards, a large number of micro-optical signal beams can be optically interconnected by a beam direction compensating system using a vertical cavity surface emitting laser, an x-y beam positioning sensor and a beam deflector made of an adjustable liquid prism. This beam direction compensating system can suppress the decrease in coupling efficiency between transmitters and receivers to no more than 15%, even if boards are inserted and extracted repeatedly, and are shocked repeatedly at a high intensity of 100 G. The compensation is very fast (20 ms). Furthermore, various optical interconnections necessary for ATM switching networks can be achieved by beam deflectors of a liquid crystal microprism array. In this preliminary study, we fabricate no more than eight-channel optical interconnections. However, since one surface emitting laser diode can have many channels in a small area (64 channels per 4 mm/sup 2/) and the aperture of this adjustable liquid prism is wide (28 mm in diameter) and uniform, a huge number of optical interconnection channels is possible using this system.

Novel technology for hybrid integration of photonic and electronic circuits

We have developed a new three-dimensional integration technology which involves hybrid integration of photonic and electronic circuits by means of polyimide bonding. To demonstrate this technology, we fabricated a GaAs metal-semiconductor-metal photodetector on a silicon substrate. Each photodetector on a polyimide layer is electrically connected to the electrode on the silicon substrate. The electrical interconnection between the photodetector and electrode on the silicon substrate consists of electroplated gold through a through-hole. The photoresponsivity of the photodetector is 0.3 A/W.

Packaging for a 40-channel parallel optical interconnection module with an over 25-Gb/s throughput

NTT is currently working on a project aimed at developing an interconnection module which has high throughput and is both compact and cost-effective. This project is called parallel interboard optical interconnection technology, or ParaBIT. The ParaBIT module being developed as the first step in this project is a front-end module with 40 channels, throughput of over 25 Gb/s, and transmission over 100 m along multimode fibers. One major feature of this module is the use of vertical-cavity surface-emitting laser (VCSEL) arrays as very cost-effective light sources. These arrays also enable a packaging structure that includes transmitter and receiver in one package. To achieve super-multichannel performance, new high-density multiport Bare Fiber (BF) connectors have been developed for the optical interface of the modules. Unlike conventional optical connectors, the BF connectors do not need a ferrule or spring. This ensures physical contact with excellent insertion loss of less than 0.1 dB for every channel. A polymeric optical waveguide film with a 45/spl deg/ mirror for coupling to the VCSEL/PD arrays by passive optical alignment has also been developed. Also to ensure easy coupling between the VCSEL/PD array chips and the waveguide, a packaging technique has been developed to align and diebond the optical array chips on a substrate. This technique is called Transferred Multichip Bonding (TMB), and can be used to mount optical array chips on a substrate with a positioning error of only several micrometers. These packaging techniques offer the performance of an ultra-parallel interconnection in prototype ParaBIT modules.

Hybrid integration of smart pixels by using polyimide bonding: demonstration of a GaAs p-i-n photodiode/CMOS receiver

The fabrication procedure of smart pixels based on a hybrid integration of compound semiconductor photonic devices with silicon CMOS circuits is described. According to the 0.8-/spl mu/m design rule, CMOS receiver/transmitter circuits are designed for use in vertical-cavity surface-emitting laser (VCSEL)-based smart pixels, and 16/spl times/16 and 2/spl times/2 Banyan-switch smart-pixel chips are also designed. By using our polyimide bonding technique, we integrated GaAs pin-photodiodes hybridly on the CMOS circuits. The photodetector (PD)/CMOS hybrid receiver operated error free at up to 800 Mb/s. Successful optical/optical (O/O) operation (a bit rate up to 311 Mbit/s) of the 2/spl times/2 Banyan-switch smart-pixel chip implemented with another VCSEL chip is also demonstrated.

An 850-nm InAlGaAs strained quantum-well vertical-cavity surface-emitting laser grown on GaAs (311)B substrate with high-polarization stability

The polarization stability of 850-nm InAlGaAs strained quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) grown on GaAs (311)B substrate was investigated by comparing it with that of GaAs unstrained QW VCSEL grown on GaAs (311)B substrate. Photoluminescence measurement showed that strained QWs grown on GaAs (311)B substrate had larger anisotropy in optical gain than unstrained QWs. The VCSEL with strained QWs showed an orthogonal polarization suppression ratio (OPSR) as high as 21 dB under CW operation. Time-dependent OPSR measurement indicated that the strained QW VCSEL had higher polarization stability than the unstrained QW VCSEL under zero-bias modulation.

Polarization-controlled 850-nm-wavelength vertical-cavity surface-emitting lasers grown on [311]B substrates by metal-organic chemical vapor deposition

We demonstrate the polarization stability of 850-nm-wavelength vertical cavity surface-emitting lasers (VCSELs) grown on [311]B substrates under continuous-wave (CW) and dynamic operation. To clearly verify the polarization stability of VCSELs on [311]B substrates due to the anisotropic optical gain, the characteristics of both VCSELs on [311]B and [100] substrates were compared experimentally. Under CW operation, very small difference in both orthogonal polarization suppression ratio and the distribution of polarization direction was observed between VCSELs on [311]B and [100] polyimide-buried structures. On the other hand, significantly larger orthogonal polarization suppression ratio was obtained for VCSELs on [311]B substrates than those on [100] substrates under zero-bias modulation. Time-dependent orthogonal polarization suppression ratio measurements also showed that the orthogonal polarization suppression ratios of the VCSEL on [311]B substrates were more stable than those on [100] substrates. The data transmission characteristics also indicate large differences in the dependence of the bit error rate on bias current and the power penalty between polarization resolved and unresolved systems between VCSELs on [311]B and [100] substrates. The beneficial effect of the polarization stability of VCSELs on [311]B substrates due to their anisotropic optical gain is clearly demonstrated.

Investigation of dynamic polarization stability of 850-nm GaAs-based vertical-cavity surface-emitting lasers grown on [311]B and [100] substrates

The polarization stability of 850-nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) under dynamic operation was investigated by comparing the characteristics of VCSELs grown on [311]B and [100] GaAs substrates. Significantly larger suppression ratios of the two orthogonal polarization modes was obtained for VCSELs on [311]B substrates than those on [100] substrates under zero-bias modulation. Time-dependent orthogonal polarization suppression ratio measurements also showed that the polarization direction was more stable in the VCSEL on [311]B substrates than that on [100] substrates. Error-free transmission was realized from VCSELs on [311]B substrates with and without a polarizer in both back-to-back and 100-m multimode fiber transmission.