Increased competition in the transceiver/transponder market along with customer demand for aggressive price reductions has continued to put pressure on optical components manufacturers to reduce costs while maintaining performance. This has particularly affected optical isolator manufacturers, who have seen prices for single-stage, free-space isolators erode from well over $100 to under $20 today.
To support these prices, pressure has been exerted over the material supply chain to squeeze margins, and manufacturing has predominantly moved to areas of low labor cost around the world. These efforts have resulted in best pricing of approximately $15 per isolator at this writing. Current manufacturing methods are unlikely to sustain continued price reductions, and thus a manufacturing paradigm shift will be necessary to enable isolators to drop below the $10 threshold.
A new isolator manufacturing method is now available that enables lower manufacturing and materials cost and thus lower isolator prices. This new method exploits the economies of scale enabled by semiconductor processing techniques, and benefits from an integrated manufacturing process that reduces assembly costs. Isolators manufactured by this method are shipping now with low insertion loss (0.25 dB), high isolation (25 dB), and a broad temperature range (-5 to +85 degrees C). These isolators will be deployed in the optical network during the first half on 2005 at prices well under $15 each, and are expected to be under $10 each by the end of the year.
Limitations of traditional technology
Single-stage, free-space isolators are typically manufactured from three optical components: two polarizers and a Faraday Rotator. The two polarizers "sandwich" the Faraday Rotator in an isolator core, and define the orientation of the input and output polarization states (Figure 1). Because of the inherent polarization sensitivity, these types of isolators are generally used at the output of source lasers in telecom systems where the polarization state is already defined by the source laser.
Faraday Rotators are non-reciprocal devices that rotate linearly polarized light in an amount that is proportional to the thickness. Faraday Rotators used at telecommunications wavelengths (e.g., 1310 nm, 1550 nm, etc.) are generally rare-earth iron garnet thick films. These garnet films are generally less than 1 mm thick and may require a saturating magnetic field. Garnets that require a saturating field typically perform better over the broad temperature ranges required by many telecom applications, so this type tends to be more common -- even though the addition of a magnetic field complicates the design.
Polarizers used on the entrance and exit faces of the garnet enable the device to function as a light valve (i.e., allow light to travel in only one direction). Traditional polarizers used in telecom transceiver/transponder applications rely on resonant absorption in metallic materials to define the transmitted polarization state. This technology has been perfected over several decades, and these polarizers are widely available at moderate cost with high transmission (≥98%) and high contrast (≥10,000:1). Unfortunately, limitations of current manufacturing technology result in a 0.2-mm-thick optic that is manufactured in a discrete process and must be assembled in the isolator core in a separate manufacturing process.
The discrete manufacturing processes (i.e., the fabrication of the polarizers and the assembly of the optics) of the isolator core drive manufacturing costs and limit the ability of isolator manufacturers to shrink the optics.
Nanofabrication offers a more cost-effective alternative to discrete manufacturing. The technology utilizes semiconductor-style, wafer-based manufacturing processes to tailor materials on the sub-micron scale. Nanofabrication processes enable sub-wavelength structures to be created in a variety of different shapes (e.g., rails, pillars, pyramids, cones) on a variety of different substrates (e.g., glass, fused silica, III-IV materials, garnet) and on a variety of different sizes (e.g., large circular wafers and large or small rectangular sheets). With the appropriate selection of materials, substrates, shapes, and sizes, a wide variety of optical functions can be created from nanostructures. Demonstrated functions include polarization, phase retardation, beamsplitting, optical filtering, and optical isolation.
A nanofabricated isolator core is constructed by integrating polarizing nanostructures directly on the garnet substrate (Figure 2), which leads to three benefits of interest to transceiver/transponder vendors: size reduction, improved reliability, and reduced cost.
Polarizing nanostructures are less than 1 micron thick (compared to the ~200-micron thickness of resonant absorption polarizers). The thin nanostructures enable the isolator core thickness to be cut in half (from ~0.9 mm to ~0.5 mm at 1550 nm and from ~0.7 mm to ~0.3 mm at 1310 nm). The thin size and monolithic construction enable the isolator core to be diced into smaller dimensions than laminated construction allows, supporting vendor requirements for more compact designs.
In addition, environmental reliability is greatly improved by the elimination of the epoxy layers frequently used to fabricate low-cost isolators. Costly manual alignment and assembly processes are eliminated, and packaging in a saturating magnetic field is simplified creating manufacturing efficiencies leading to an isolator cost of less than $10.
Reaching the market
Nanofabricated isolators manufactured with polarizing nanostructures on a garnet Faraday Rotator are now beginning commercial sampling. These isolators have successfully completed functional and preliminary environmental qualification testing and are shipping to transceiver/transponder vendors.
Environmental qualification testing will be complete in 2004, and nanofabricated isolators will begin deployment in the telecommunications network in the first half of 2005. The physics of nano-optics, the economies of scale enabled by semiconductor processing techniques, and the benefits of an integrated manufacturing process will result in savings of millions of dollars per year for transceiver/transponder manufacturers.
Alan Graham is the director of sales development at NanoOpto Corp. (Somerset, NJ).