With the ever-increasing demand for bandwidth in the optical network, systems de signers at original equipment manufacturers (OEMs) are looking for ways to increase the number of optical ports available on their cards (in other words, the port density). This, in turn, is forcing component manufacturers to find innovative ways to reduce the physical size of optical components while maintaining competitiveness in this high-volume, price-sensitive market.
The demand for low-cost transceiver products has been growing at a remarkable rate, and the keen competition in this market has benefited customers by driving down cost and improving quality and reliability. Also, the early adoption of component-level standardization has freed OEMs to procure transceivers from multiple vendors with the confidence of knowing they will work in their system boards without modification. Probably the largest share of the current transceiver market is generated by the requirement for multisourced 1 x 9 or 2 x 9 products using the duplex SC optical connector.
However, the current design of transceiver products is likely to be superseded by products that offer OEM companies higher port density in their systems. Yet, increased port density is not the only trend in transceivers. The use of 3.3V of current is a rapidly emerging standard in the industry, as OEMs need to reduce overall system power consumption. In the past, OEM system designers were required to supply 5V rails purely for the optical transceiver in addition to the 3.3V system supply. The requirement for two power supplies added cost and complexity. New transceiver products are required to operate from a single 3.3V supply to remove this unnecessary complexity from the system board.
Hewlett-Packard adopted the multisourced 2 x 5 pin configuration for its new small-form-factor transceiver products, which contain passively aligned optics and use the reduced physical dimensions of the Mini MT-RJ connector. The increasingly demanding requirements for cost, quality, consistency, and statutory compliance in transceivers has justified a radical re-design at all levels if the products are to find a home in such applications as Fast Ethernet, Gigabit Ethernet, and Synchronous Optical Network/Synchronous Digital Hierarchy.
The majority of Hewlett-Packard`s singlemode components employ the company`s strained multiple quantum well (S-MQW) laser diodes. Functionally, the S-MQW lasers met the electro-optic requirements of the new transceiver products--but design changes such as the inclusion of special alignment features were implemented to enhance manufacturing automation. The lasers are picked from the wafer, visually inspected, electro-optically tested, and segregated by performance. All of these functions are performed automatically by a laser die inspection system.
Meanwhile, to fully realize the size reduction afforded by the MT-RJ connector, the optical platform had to be similarly reduced in size. Therefore, the company developed a new optical subassembly for use with the connector. The subassembly consists of a micro-machined silicon sub-bench onto which a laser diode, monitor photodiode, and singlemode fiber stub are mounted. For the receiver, a multimode fiber stub and high-speed substrate-entry photodiode are used. Silicon was chosen for the sub-bench because of the stability of its planar structure and its low material cost, as well as to leverage off a well-established capability to grow silicon parts.
An etching process is used to produce precise alignment features and a v-groove on the silicon. The V-groove is used to support a singlemode (9/125-micron) fiber stub; the alignment features are used during the laser die-attach process to align the laser to the V-groove, into which the fiber is then epoxied in place.
The size of the silicon sub-bench is kept to a minimum (1.1 x 2.5 mm) to reduce cost and allow for more flexibility in final packaging. The silicon sub-bench has a gold-bond pad, coated with a thin layer of high-temperature solder for the laser die, a bond pad for the rear facet monitor photodiode, and gold tracking for all of the inboard wire bonds. The transmitter sub-bench uses an edge-entry photodiode for the rear facet monitor.
The receiver sub-bench also employs a V-groove and gold tracking for wire bonding. But a high-speed substrate-entry photodiode is used and the fiber stub is multimode. The laser, monitor photodiode, and high-speed photodiode are all bonded directly to the silicon without any submount.
The transmitter and receiver sub-benches are identical in size, which assisted the automation of the manufacturing line. The sub-benches are delivered in ESD-protective 2-inch waffle trays holding 128 parts. The line has been developed to use the parts directly from these waffle trays, removing the need for manual handling. The decision to design the laser diode and sub-bench for automation was important, as it allows the optical subassembly to be produced more quickly, at lower cost, and with greater quality and consistency.
One of the most important features of this subassembly design is the absence of any lenses or other focusing elements. Removing the spherical lens is key to the cost reduction. It not only eliminates the high cost of the lens itself but, more importantly, removes the unnecessary process of performing active alignment.
Key to achieving good coupled powers without the use of any focusing components is the ability to align the laser die to the fiber with sufficient precision. To meet this need, a new, completely automated laser die-attach process, with a target accuracy of 2 microns in all three axes independent of the manufacturing tolerances of the piece parts, was developed.
The silicon substrate is vacuum-picked from the 2-inch waffle tray--visually located via a high-precision vision system--and the substrate`s thickness is accurately measured. Protected from oxidization by a local atmosphere of inert cover gas, it is then heated to reflow the solder pad ready for the laser die-attach. Similarly, the 350 x 200-micron laser die is picked using a gripper assembly--visually inspected--and measured for thickness. The laser is then presented to the substrate, followed by initial alignment to a precalculated optimum position relative to the empty V-groove .
The laser is lowered into the molten solder and positioned at the final alignment height. After final X, Y, and height correction, the substrate is cooled rapidly until the solder freezes. The alignment in all three axes--as well as the rotation--is monitored and adjusted throughout the cooling process until the moment just before the solder freezes to avoid introducing stresses in the solder joint. This process routinely achieves accuracies of better than 1 micron in X, Y, and height and angular errors of better than 1. This alignment accuracy allows coupled powers of more than 400 microwatts at 15 mA above laser threshold into a singlemode fiber. Work is being carried out to increase the coupling efficiency further to enable this optical-subassembly design to be used in long-reach transceiver products.
The subassembly is replaced in the waffle tray to be passed on to the rest of the assembly line. This manufacturing equipment is capable of processing hundreds of subassemblies without human intervention.
After the laser is attached, the monitor photodiode is expoxied into position behind the laser chip and both the laser and monitor photodiode are wire-bonded to gold tracks on the silicon substrate. A cleaved length of fiber is placed in the V-groove and anchored with epoxy. An automated functional test is performed to confirm alignment and guarantee quality. This testing completes the subassembly process. Then the subassemblies can be used in a variety of transmitter, receiver, or transceiver products.
Another important break from tradition is that this assembly does not need to be performed in a clean room with all its associated costs. During processes where any optical surfaces are exposed, local clean laminar flow is used. At all other times, the waffle trays remain covered and stored in a dry, inert environment.
The transceiver assembly combines the optical subassembly with the necessary control and drive electronics to provide the user with a simple electrical interface and eliminate the need for complex drive electronics on the OEM`s system board.
The completed optical subassemblies are attached to a transceiver printed circuit board (PCB). The fiber stubs are inserted into the Mini MT-RJ ferrule, which is also mounted on the PCB, and are "precision polished" to produce a high-quality optical interface.
The PCB carries the high-speed pre-amplifier for the receiver photodiode and a post amplifier to provide the user with a positive-referenced emitter-coupled logic (PECL) electrical interface. A laser-driver integrated circuit also is present, which provides the DC bias and modulation control for the transmitter, automatically maintains optimum performance over temperature, and gives the user a PECL interface to the transmitter.
The optical subassembly is wire-bonded to the PCB and the active optoelectronic devices are protected with semiconductor-grade silicone encapsulant. The attachment of metal shielding to protect the transceiver and guarantee its emi performance, followed by automated functional testing, completes the manufacturing process for this high-port-density transceiver product.
A process plug is applied to the Mini MT-RJ connector to make the transceiver compatible with flow solder and wash processes. Once the transceiver is soldered to the system board, the process plug can be discarded.
Andy McGhee is a senior R&D engineer at Hewlett-Packard Ipswich Components Operation (Ipswich, Suffolk, UK). He can be reached at [email protected].
Acknowledgements: Gary Trott, Richard Tella, and Bill Gong, Hewlett-Packard Laboratories (Palo Alto, CA).