|Microlens arrays home in on parallel optics|
FibreSystems - April 2006
Microlens arrays can ease optical coupling in parallel-optics-based transceivers and the like. But are module vendors willing to bet on MLAs? Roy Rubenstein finds out.
"It was a covert launch." That is how Steve Drumm, market development manager at Omron Electronic Components Europe, describes the introduction of its microlens array (MLA) product for singlemode fibre. You would think that a photopolymer-based lens array formed using a nanostructured powder to minimize imperfections would be a product worth hollering about. But Drumm's explanation is straightforward: "This MLA is used in our own products, and we sell it as part of a black box to key OEMs [original equipment manufacturers]," he says.
In contrast, Omron's other MLA offering, the P1L for multimode fibre, is sold openly. It received a fanfare introduction at ECOC 2004, since when, Omron has expanded the family to include half a dozen products. Drumm says that Omron's is the first single-piece lens array that enables passive alignment when used with the MT ferrules employed to align ribbon cable within an MTP/MPO connector or a parallel-optics transceiver. "You take the lens array, push and fit it over two pins and it works. No one has done that before," he explains.
All in one
MLAs offer manufacturers an alternative to using separate ball or radial-graded-index lenses. They're typically used to couple two or more light sources or detectors to fibres, for backplane interconnects, for example, or within an optical switch or multiplexer.
Omron forms its lens arrays using a micromachined or laser-indented mould. Tiny beads of polycarbonate are injected into the mould and heated until they congeal and take on the shape of the mould, after which the material is compacted and cooled carefully. The resulting single-piece MLA comprises a base with the lens profile - for instance, a single or dual row of 12 aspherical lenses - sat on top.
"The free-shape capability enables us to create MLAs with lenses providing different functions," says Drumm. "With this [injection moulding] process you are making all of the lenses at once, with high repeatability." Omron's P1L family currently offers a lens position tolerance of 5 ?m and an insertion loss of less than 2.5 dB.
There are two P1L MLA types available: flat and right-angled, both with focusing or collimating functions for LEDs and vertical-cavity surface-emitting lasers, as well as for detectors. "Transceiver makers are keen on getting the size, price and power consumption down," says Drumm. "Design of real-estate is becoming an issue and they want the smallest solutions possible."
Over the past 18 months, demand has increased for lens arrays for use in interconnect and backplane applications. "The demand is for more fibre-to-board, board-to-board and passive backplanes," explains Drumm, adding that he is even seeing interest in optical chip-to-chip connections. However, the leading market for Omron's multimode lens array is parallel optics for the POP4 and SNAP12 transceiver form-factors. Such transceivers also include the InfiniBand standard used for high-speed computing interconnect applications.
Omron's new singlemode MLA, meanwhile, is targeted at parallel-optics transceivers for storage-area and campus networks. It's also suitable for WDM transceivers such as CWDM, 10 Gbit/s 10GBASE-LX4 Ethernet and Fibre Channel, and small-form-factor 1 × 2 and 2 × 2 optical protection switches.
Omron's goal is to win over the transceiver manufacturers that currently use their own lens-based subassemblies and actively align the lenses with the light sources. One such company is US optical components and modules player Opnext. Ed Cornejo, director of product marketing, says that Opnext does not use microlens arrays.
"We do not make parallel-optics transceivers," he explains, although the company does manufacture LX4 transceivers.
Cornejo says that the advent of the single optical subcomponent array to replace discrete lenses still doesn't solve the biggest problem - placing the lenses accurately and making sure that they stay aligned to the discretes over the required temperature range. This process must meet tight positional tolerances of ±1-2 ?m, which is why Opnext chose to make its own lens assembly and use active alignment when it designed its LX4 transceiver more than a year ago. However, if moulded optics can now meet the tight tolerances needed for higher-speed transceivers such as LX4, "it could be very useful", Cornejo admits. "It would be very inexpensive and a breakthrough for us."
Drumm says that Omron has the capability for these micron-level tolerances. "Right now Omron is working towards perfecting its techniques in ultraprecision micromachining and moulding, using nanoparticle composite resins to realize surface and pitch accuracies of 1 ?m for multimode MLAs and 0.5 ?m for singlemode MLAs," he says.
Drumm reckons that the European market is lagging behind North America and the Far East in the adoption of the latest networking equipment. "There is less kick in Europe," he says, "we are at the starting block and ready to run." However, there are signs that service providers are struggling to meet growing storage network demands and Drumm expects that this will consequently drive the demand for transceivers, parallel optics and CWDM, with a resulting growth in microlens sales in Europe evident from the middle of 2006.
Opnext also expects to see a growing demand for MLAs, but is uncertain whether the product will be adopted in its own second-generation LX4 transceiver design. "It's also hard to anticipate how much longer there will be increasing demand for LX4, as it faces growing competition from the [serial] 10GBASE-LRM standard," adds Cornejo.
There are other opportunities emerging, however. One is the newly evolving quad SFP (QSFP) transceiver standard, which will use SNAP12 fibre-optic ribbon cable. "QSFP is going after a real problem," explains Cornejo - increasing the port density of OC-3 SDH/SONET and 1 Gbit/s Ethernet interfaces on switches and routers.
Another prospect could lie in the forthcoming hike in Ethernet speed beyond 10 Gbit/s. The next Ethernet standard could be 40 or 100 Gbit/s, although Cornejo stresses that system vendors tend to favour 100 Gbit/s. "The first 100 Gbit/s interface implementations will inevitably call for WDM-based parallel optics," he says.