Nam Hwang

Laser weldability analysis of high-speed optical transmission device packaging

A 2.5 Gb/s distributed feedback laser diode (DFB-LD) module was used as a test package to develop laser welding techniques. Various package weld joint geometries, such as lap, fillet, butt, and fillet-lap, were designed and welded to investigate the optimal package configuration that leads to the maximum coupling efficiency and minimum weld shift. Furthermore, the welded joints at a given set of laser parameters were cross-sectioned for metallurgical analysis, such as weld penetration and microcracks, for verification of the weld joint integrity. Through such analysis, some important laser welding parameters, such as depth-of-penetration (DOP), heat-affected zone (HAZ), shear-strength, and solidification formation of the weld pool were analyzed. Also, as the result of investigating 56 laser welded DFB-LD submodules, optimal laser parameters, and the suitable joint geometry for this package could be determined with the average weld shift less than 0.19 dB, which translates to less than 1.0 /spl mu/m in radial displacement. These laser welded packages exhibited an excellent tracking stability during environmental testing, which consequently resulted in obtaining a desirable bit-error rate (BER) during the data transmission performance analysis.

Deep trench guard technology to suppress coupling between inductors in silicon RF ICs

Trench guard technology was proposed to suppress the coupling between inductors, and to reduce crosstalk in silicon RF ICs. The test structure adopting a deep trench guard ring showed a coupling less than -40 dB in the frequency range of 0.5-20 GHz, and negligible change in designed inductance value up to 10 GHz. Furthermore, it can be also used to isolate a noisy circuit block from other quiet blocks in mixed signal silicon RF ICs.

Fabrication of semiconductor optical switch module using laser welding technique

The 4/spl times/4, 1/spl times/2, and 1/spl times/4 semiconductor optic-switch modules for 1550 nm optical communication systems were fabricated by using the laser welding technique based on the 30-pin butterfly package. For better coupling efficiency between a switch chip and an optical fiber, tapered fibers of 10-15 /spl mu/m lens radius were used to provide the coupling efficiency up to 60%. The lens to lens distance of the assembled tapered fiber array was controlled within /spl plusmn/1.0 /spl mu/m. A laser hammering technique was introduced to adjust the radial shift, which was critical to obtain comparable optical coupling efficiencies from all the channels at the same time. The fabricated optical switch modules showed good thermal stability, with less than 5% degradation after a 200 thermal cycling. The transmission characteristics of the 4/spl times/4 switch module showed good sensitivities, providing error free transmissions below -30 dBm for all the switching paths. The dynamic ranges for the 4/spl times/4 and 1/spl times/2 switch modules were about 8 dB for a 3 dB penalty and about 17 dB for a 2 dB penalty, respectively.

High frequency modeling for 10 Gbps DFB laser diode module packaging

In packaging the laser diode several electrical requirements must be satisfied. The conventional packaging method which consists of transmission line, matching resistor and wire bonding interconnect showed good result for the modulation bandwidth requirement of greater than 10 GHz. However the return loss requirement was not content with less than -10 dB. To analyze the effects of package parasitics on return loss, we proposed a small signal circuit model of a laser diode module. From the small signal measurement data, we obtained the small signal circuit model of the laser diode for each bias current, which contains package parasitics and intrinsic laser diode characteristics. Considering both the intrinsic laser diode characteristics and total parasitic effects of the packaged module, the overall frequency response of the laser diode module was simulated and compared to the experimental results. The simulation results on the small signal modulation bandwidth and return loss showed a good agreement with measurements. However, the return loss does not meet the requirement for 10 Gbps laser diode module. Therefore, the wire bonding effect on the return loss was analyzed. The characteristics of wire bonding inductance which affect the return loss were simulated using an interconnect analysis program. As a result, shorter wire bonding length would be the best step to reduce the package induced inductance.

Reliability considerations of laser diodes for optical communication system application

The purpose of this paper is to propose a reliability parameter of laser diodes for optical communication system applications. Based on the alarm function in optical communication systems, we have developed a reliability parameter; /spl rho//sub a/(=/spl eta//sub a///spl eta//sub 0/.Ith/sub a//Ith/sub 0/)/spl ges/1.5, which is more applicable to estimate reliability projections and to design LD modules for optical communication systems.

A novel bidirectional optical coupling module for subscribers

To overcome drawbacks and limitations of planar lightwave circuit based modules for bidirectional communications, such as the demand for several chips and in consequence more packaging efforts, we have recently developed a novel optical coupling technique using our unique 155 Mbps bidirectional laser chip. Since the chip is structured with a pin-photodiode monolithically integrated on a laser diodes waveguide, the optical coupling requires only the alignment of the chip with a fiber. To optically couple the laser diode and photodiode simultaneously with a single fiber, we have designed an unusual coupling structure using a fiber having a cleaved surface whose normal is 35/spl deg/ angled to the fiber core axis, and using an index-controlling medium with a refractive index of /spl sim/1.3. The bidirectional chip is flip-chip bonded and the fiber is passively aligned using a V-groove on the same substrate of 2.5/spl times/1.3 mm/sup 2/ in size. Even with this extremely small and simple scheme for bidirectional optical coupling, we could obtain an optical output power of -7/spl sim/-10 dBm and a responsivity of <-30 dBm, which are satisfactory to the STM-1 level telecommunications specifications.

Fabrication of optical Tx/Rx subscriber modules incorporating passive alignment technique

Compact optical transmitter/receiver modules for STM-1 transmission systems have been developed incorporating custom ASICs (application specific integrated circuit) and novel packaging technologies. Coupling of optical chips and fibers for both modules is accomplished utilizing passive alignment method to lessen package cost and size. A fully integrated module is as compact as 3 cc in volume.

Fabrication and transmission experiments of distributed feedback laser modules for 2.5 Gb s -1 optical transmission systems

A distributed feedback (DFB) laser module has been developed for 2.5 Gbs-1 optical transmission systems. The DFB laser has a multiple-quantum-well (MQW) active layer and a planar buried heterostructure (PBH) for a low threshold current and stable singlemode operation with low chirping. A PBH DFB laser module with a single-mode fibre pigtail and an optical isolator was designed and fabricated by employing a single lens and a laser welding method. The fabricated MQW PBH DFB laser module is shown to be a suitable light source for 2.5 Gb s-1 optical transmission systems with a minimum received power of-33 dBm after 47 km conventional optical fibre transmission.

Quantitative evaluation of passive-aligned laser diode-to-fiber coupling

Quantitative evaluation of passive-aligned laser diode-to-fiber coupling is presented. From the assembly of 250 transmitter optical subassemblies, flip-chip bonding of laser chips and fiber fixing yields are reported. The coupling characteristics of passive-aligned laser diode modules are evaluated in comparison with the active alignment data and the misalignment tolerances. The assembly and coupling results have showed about 80% overall yield and 1.79 dB/spl plusmn/1.55 dB excess coupling loss, respectively.

An empirical lifetime projection method for laser diode degradation

An empirical method for lifetime projection of 1.55 /spl mu/m InGaAs-InP MQW-DFB laser diodes (LD) is presented. On the basis of experimental results of an accelerated aging test for 1500 hours, relationship between LD degradation, operating voltage, and ambient temperature has been determined. The presented method makes it possible to predict the lifetime of LDs by determining the thermal voltage ratio.