Switching to Light Speed

Moving production from semiconductors to fiber optic

by Alan Feinstein
President, Nanomotion Inc.

(editor's note: As of this writing, the Malaise of the New Millennium hasn't yet abated in the high-technology industry. The news reports each day the closings and layoffs, the NASDAQ and S&P 500 reports wobble and softly decline, and more dot coms turn into dot gones. The Semiconductor industry has been trying to ride out the downturns with continued advances in 300mm wafer manufacturing, as well as new packages for the burgeoning telecommunications industry. Still, as has been written on these pages previously, there approaches an upper limit to maximum use of silicon real estate--note that in May of this year, DARPA put out a solicitation for proposals on biochemical chip technology. OEM suppliers to the semiconductor industry are, of course, directly affected by the downturns in the market, but have found another outlet--the increased use of fiber optic technology for communication systems. Here's commentary from an inside player.--rm)

The need for speed and the bandwidth to carry huge volumes of data has resulted in projected average annual growth rate for the telecommunications industry of 50% per year, for the next decade. As the use of the internet, coupled with the ease of accessibility from hand held devices, continues to grow, it becomes a race as to who can produce more parts (fiber optic components) in a given amount of time.

Telecommunications, and more specifically, photonics (which refers to combined fiber optic and electronic components), has an unprecedented need to automate the manufacture of passive fiber optic components and assemblies, as well as develop micro adjusting devices for "active" components. Additionally, the semiconductor industry, while traditionally tied to "computer sales," is diverting significant resources to the production of laser diodes and other fiber optic components, which are based on chip production processes.

While there is fiber optic cable laid and operating in most industrial nations, there is still a demand to put down 11 million kilometers of cable in North America during 2001. The fiber networks are broken down into 3 basic categories; Long Haul (across the continent), Short Haul (around major cities), and Metro (within a neighborhood, bringing fiber to the home).

The impact of all this lies in the ability to produce the necessary components to support a fiber optic infrastructure. When fiber optic cable is stretched across a continent, the signal needs to be amplified every 50 to 100 kilometers. Moreover, routing, switching, and connecting to other signal devices has created a manufacturing nightmare, with huge production requirements in the rush to market.

Fiber Optic Components

Within the photonics industry there is a host of precision devices that are easiest to separate by component function--Passive or Active:

Passive components are devices that have no dynamic properties within them. They can pass signals through, amplify, direct, and so on, but there is no "tuning" or adjustment within the device. Some of these components are:

* Attenuators--used to match the optical power level to the dynamic range of receivers, adjust the input and output levels, equalize the power between different DWDM channels, and test the performance of the system.

* Couplers--split and combine light signals (without significant signal loss)

* Wavelength Division Multiplexer (WDM)--reads multi-channel analog and digital signals on the same fiber.

* Dense Wavelength Division Multiplexer (DWDM)--same as WDM, with higher channel count.

* Arrayed Waveguide Grating (AWG)--used with DWDM, waveguides are transparent dielectric structures that transport light. Like a fiber, they have a core and cladding (guiding structure) with different indices of refraction to guide light. These parts are constructed by patterning thin films deposited onto a substrate to form waveguides. Quite similar to semiconductor processes, using a photolithography process.

Active components are 'tunable' devices that require adjustment to condition an output. Devices such as digital switches, which may contain potentiometers or microelectromechanical systems (MEMS), which change with temperature, can be made into active devices. While some MEMS are passive, others can be active, with adjustable mirrors built inside optical switches to direct signals (optical relay). This requires miniature actuators or adjustment mechanisms buried inside that can respond to an input signal to make an adjustment. These mirror assemblies range down to 1mm square (or smaller) and can be configured with various reflecting angles.

Critical alignment

An optical fiber can range from 5 to 20 microns in diameter, and consists of a core (5 to 20 microns in diameter), a cladding (up to 125 microns in diameter) and a casing (protective outside). Fiber misalignment creates two situations affecting the performance of an optical network, causing slower speeds and reduced data flow. Dispersions refers to the haphazard and random scattering of light signals, resulting in less-than-optimal signal at the receiver. Backreflection (also called return loss, reflection coefficient, or reflectance) is a measurable attenuation of optical power that is reflected by the fiber connector interface back to a light source, rather than of power (light) that is transmitted or absorbed. Backreflections in optical systems can come from a number of sources. Primary sources include backscatter and reflections that occur at the junction of two materials having different refractive indices, such as connectors, fiber end faces, splices, improperly terminated couplers, fiber breaks/fractures, and detector surfaces. In simple terms, if the ends of the fibers are not cut properly or if they do not mate properly to the required components, backreflections will occur.

Backreflections affect laser sources by inducing power fluctuations, waveform distortions and phase noise. They also generate a phenomenon called mode hopping, which causes the laser's center wavelength to fluctuate. Overall these effects cause mix ups with competing signals and reduces the performance of the network.

Proper fiber termination techniques must be applied to separate the backreflections of the devices in the optical path from the backscatter of the fiber. Backreflections also stem from improperly terminated fiber optic couplings and dirty interfaces.

In simple terms, a small diameter fiber must be brought to a point on a component and powered. The signal strength is tested during a small alignment process to optimize the percentage of light being transmitted to the receiving device. This process is usually done in a small volumetric area, 25mm square, and typically involves movements down to 50nm.

After the alignment process, functions are performed from testing to attachment. In most processes, the fiber must be held perfectly stationary to perform the intended process. In laser diodes, the fiber must be held within 50nm once aligned, for testing the laser. During attachment processes (laser, epoxy, or soldering), the position stability is quite critical, down to 50nm or less.

For more information:

Circle 420 - Nanomotion, Inc., or connect directly to their website via the Online Reader Service Program at http://www.OneRS. net/106df-420

Motion considerations

There are several approaches to providing the motion control required for the manufacture of fiber optics. Typical motion criteria in fiber optic applications include:

• travels to 100mm. Most of the fiber optic work is done within 5mm to 10mm. As the quantity of parts being handled increase, so will the travel requirements.
• positioning resolution of 0.1µ to 0.05µ.
• position stability to 0.05µ or better. This requires that DC servo motors be turned off, or that drive technology be inherently stable.
• velocity range of 50 to 100mm/sec.
• general machine footprint less than 500mm square, as small as possible.

Today's processes of aligning a fiber to a laser diode or inserting a fiber into a narrow ferrule for attachment not only requires fine positioning, but ultimately perfect stablity during testing or attachment. Traditional servo dither in manufacturing systems will greatly affect the performance of the component. In fiber optics, the system needs to be able to move in very small increments of 50nm, and have less than 50nm of dither while positioned. While traditional technologies have tried to achieve this, most rotary motors and lead screws possess too much stiction to facilitate these miniscule movement requirements. Brushless DC linear motors, while capable of making small moves, will experience dither of ±1 count, requiring feedback resolutions to be ¾ 20nm.

The mirror assemblies in MEMS devices have similar positioning requirements during manufacturing as the fiber optic components. The mirrors are often adjusted and then set in place, requiring position stability and very small incremental moves. If attempted with a brushless DC linear motor, the servo would need to be turned off, to eliminate the dither, also eliminating all position holding capability.

Aligning optical fibers to components provide similar obstacles. The process requires exceptionally high resolution to position the cable in the optimum location, providing the highest signal strength. Once positioned, servo dither must be minimized, or eliminated while the cable is laser welded or epoxied into place.

Nanomotion piezoelectric linear motor

Alternatively, piezoelectric linear motors from Nanomotion Inc., Mountain View, CA, offers unlimited travel, exceptionally low resolutions below 5nm, and maintains absolute position stability. The piezoceramic devices generate an electrical field from mechanical strain. Under special electrical excitation drive and the ceramic geometry of Nanomotion motors, longitudinal extension and transverse bending oscillation modes are excited at close frequency proximity. The simultaneous excitation of the longitudinal extension mode and the transverse bending mode creates a small elliptical trajectory of the ceramic edge, thus achieving a dual-mode standing-wave motion. By coupling the ceramic edge to a precision stage, the resultant driving force is exerted on the stage, causing stage movement. The periodic nature of the driving force at frequencies much higher than the mechanical resonance of the stage allows continuous smooth motion for unlimited travel, while maintaining high resolution and positioning accuracy typical of piezoelectric devices. Travel can be linear or rotary, depending on the coupling mechanism. While the driving voltage is not applied, the ceramic plate is stationary and generates holding torque on the stage. Unlike any other braking device, the holding torque of the motor does not cause any position shift.

These motors operate as a closed loop servo with encoder feedback, or can operate in an open loop configuration, with feedback coming from external devices. For mirror assemblies and fiber alignment, a small stage driven with piezoelectric motors provide the resolution and stability necessary for the successful manufacture of the components.

Active components require the ability to adjust a device to compensate for signal loss. This typically involves adjusting a potentiometer or other device, as outside temperature may cause physical change that effects signal strength. In these instances, the piezo motor technology can be packaged into tiny actuators that can be used as a simple, cost effective device, which can turn a mechanism, without the use of a positioning system.

For more information:

Circle 420 - Nanomotion, Inc., or connect directly to their website at http://www.OneRS.net/106df-420