Japan's Oki Gets in the Groove

TOKYO -- Japan's Oki Electric Industry Co. Ltd. claims to have developed an optical assembly technology that could cut the prices of optical modules in half.

Oki's invention is a silicon microlens that sits between a laser diode and singlemode fiber to boost the coupling efficiency (see Oki Invents Mini Microlens). The microlens is unique because it is small enough to be positioned in an optical V-groove, enabling low cost, passive assembly processes to be used, says Takeshi Takamori, project leader of Oki’s Advanced Devices Laboratory.

Up until now, according to Takamori, coupling technologies for optical modules have fallen into two categories:

At the top end of the scale, there's technology used for high-end, gigabit-class optical units for long haul and longer metro systems. This usually relies on complicated laser sub-mounts that typically need two aspherical quartz lenses and an isolator, filter, or splitter to achieve the required 70 to 80 percent coupling efficiency between fiber and laser diode. Assembly is a highly labor-intensive process that requires customized parts and is expensive even for manufacturers that have moved production over to China, Takamori contends.

Modules for 2.5-Gbit/s systems typically ship for about ¥40,000 (about US$325), with about 60 percent of the cost coming from materials and 40 percent from fabrication, according to Oki. About 70 percent of the fabrication costs come from the tricky alignment process between lens and fiber, says Takamori.

At the bottom end of the scale are cheap-and-cheerful modules for, say, access systems. For these, many manufacturers rely on a so-called silicon V-groove SMT (surface mount) process. This uses automated machines to place optical devices such as laser diodes, microlenses, and optical fibers on silicon platforms. The fiber slots into a V-shaped groove etched into a silicon substrate adjacent to the laser diode chip. This cheap process enables Oki and competitors to turn out modules for less than ¥5,000 ($40). But the coupling efficiency is typically only about 10 percent, says Takamori.

Oki's new process attains some middle ground. By adding two 125-micron-diameter silicon lenses in the V-groove between the fiber and the laser diode chip, Oki is able to get about 50 percent coupling efficiency, or -3.0 dB output power. Takamori claims that the production cost is about 50 percent less than the high-end, optical lens assembly method.

One reason the method is so cost effective is that the lenses themselves are very cheap to produce. Diffraction (Fresnel) lenses can be made easily with a standard silicon fab process, such as micromachining and etching. A six-inch wafer yields about 10,000 of the tiny lenses with little waste. Oki already has pick-and-place technology that enables it to accurately line up and drop lenses in their slots.

“Yes, we have filed patents. Yes, we are the first to develop this technology,” Takamori assures Light Reading.

“These units can be easily produced; fabrication can be done on legacy lines; and we can produce high-end modules at reasonable prices. It’s a bit like Toyota making a new model car on one of their existing lines."

However, it does appear that at least one other vendor -- Infineon Technologies AG (NYSE/Frankfurt: IFX) -- has come up with something similar. It sells a range of uncooled, Fabry-Perot and Distributed Feedback (DFB) Lasers for applications up to 1.25 Gbit/s, which it claims are produced using a fully-automated micromechanical submount process. "The rather high coupling efficiencies of optical power into the singlemode fiber is realized by using a high-performance etched silicon lens," claims the company's Website.

Takamori is a bit shy about revealing how Oki’s optical module business is doing, compared with rivals such as Alcatel Optronics (Nasdaq: ALAO; Paris: CGO.PA), JDS Uniphase Corp. (Nasdaq: JDSU; Toronto: JDU), and NEC Corp. (Nasdaq: NIPNY). But he does concede that the market is so horrible that Oki is losing money on its low-end modules -- even with what he says is a competitive manufacturing process.

Oki plans to have samples of its microlens available by the end of this month. The company will license the technology to interested competitors and will go to production of its own modules with the new system by next summer, in time for an anticipated recovery in the metro/access markets in 2004, Takamori says.

— Paul Kallender, special to Light Reading
desikar 12/4/2012 | 9:12:46 PM
re: Japan's Oki Gets in the Groove Paul,

Si microlenses have been around for a while - have seen them being used in products targeted for telecom for over a year, including pretty small form-factors (though not quite in the sub-150 um dia scale). Nevertheless, if the coupling efficiency is 50% with truly PASSIVE alignment (i.e. pick-and-place with solder, weld or epoxy without optical power throughput feedback) this will be a big step forward for component manufacturing. If so, this industry needs more such breakthroughs.

There is then one primary question: Is there any active alignment at all being done? For example, is there an active alignment to set the spacings between the two Si lenses or to position them with respect to an active waveguide or w.r.t. fiber? This will partly depend on the tolerancing and agressiveness of the optical design.

A secondary question is : Is the 50% coupling efficiency the min, the median/mean or the maximum achieved?

And finally, did Oki share any notes on the optical characteristics (mode size, far-field angles, etc.) of the active waveguide used for the quoted measurements?

takamori455 12/4/2012 | 9:12:33 PM
re: Japan's Oki Gets in the Groove Dear dessikar,

My name is Takamori. I am in charge of developing this Si microlens in Oki.
Thank you for your comment.

As I am not supposed to provide the detailed information about our lens in public accoding to Oki's code of conduct.

You can reach us through the following email address:
[email protected]

Just briefly,
We are aiming to achieve the fully passive or visual assembly technology, so every parts such as lens are aligned by monitoring the alignment marks not optical power.

For the further information, please contact us directly.

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