Designing with micro-optics

Wallace Latimer

Micro-optics are an integral part of fiberoptic systems: they are part of the devices that connect to the fiber and are the parts responsible for manipulating light. Whether you have been thrust into the world of light, or are an optical engineer experienced with regular lenses, you need to understand some basic issues related to micro-optics in fiber systems in order to specify the right components for your application.

Because network designers, manufacturers, and optical engineers think very differently, we need to define some terms before we can talk constructively across these disciplines. For example, what is throughput? Is it the bandwidth of data, the number of widgets that a production line makes in a day, or the intensity of light that passes through a lens? For the purposes of this article, we will use the terms of optical engineers-throughput is the intensity of light through a lens. Light intensity is an important factor because a lack of intensity creates a noisy or weak signal, which is not useful for carrying data.

Flavors of micro-optics

Micro-optics are lenses, mirrors, prisms, windows, and other elements, used to manipulate light, that have dimensions between 0.5 and 3 mm (see Fig. 1). Among the host of lens types are PCX (with one planar and one convex side), DCX (with two convex sides), ball, drum, and gradient index (GRIN) lenses. The latter are popular because they can be made to guide light toward their axis, which can be very useful for guiding light into a fiber core. There is more to the micro-optics for fiber than GRIN lenses, however (see sidebar).

Nearly all of these manipulative elements perform one of two basic functions: they either collimate light or couple light from one device to the next. Collimating optics catch and reshape the spreading beam that emerges from a laser diode. Coupling optics have more varied jobs: they are employed where the beam magnification needs to change, which is typical any time light moves between a fiber and another component (for example, multiplexers/demultiplexers, circulators, gratings, or switches).

Micro-optics are made of either glass or plastic. The most typical glass is BK7, and elements made from this material have standard characteristics. By using materials with higher refractive indexes, a lens with the same radius can have a shorter focal length and a higher numerical aperture. High-index materials include LASF9 and cubic zirconium.

Plastics, such as PMMA, SMMA, or polycarbonate, have considerably lower indices of refraction than BK7, but are used because they can be made easily by molding and are less expensive than glass. Furthermore, the molding process allows manufacturers to incorporate mechanical structures or aspheric surfaces into the elements.

Plastics, however, are difficult to coat, and coatings are essential for fiber applications. Standard vapor-deposition coating methods cannot be used on plastics because the materials cannot withstand the temperatures. Plastics can be coated by dipping techniques, but such coatings are not as complex as those possible with vapor deposition. Coatings maximize throughput, reduce reflection, and filter stray light.

More complex coatings can filter for polarization as well; this function cannot be done on plastic. For collimators, however, where narrow passbands are not needed, plastic microlenses work well.

Using micro-optics

Micro-optics increase throughput by fighting back-reflection and alignment errors. Reflections not only reduce efficiency but can cause feedback in the laser. For example, when coupling two fibers with plane faces or collimating light from a laser, one can reduce feedback by using discrete elements or coatings, or both. Antireflection coatings cut down on the amount of reflection at each surface. An 8∞ wedge on the back side of a coupling group can reduce feedback by redirecting reflected light out of the beam path.

Fiber coupling is subject to three types of misalignment (see Fig. 2): separation, offset, and tilt. In separation, the fibers may not be close enough together: if there is an unplanned-for distance along the z-axis between them, light from one fiber core will spread out and lose much of its intensity. When offset, the fiber cores may be displaced laterally along the x-axis, so that light from one core hits the cladding layer of the second fiber, also reducing the light throughput. Finally, one fiber may be tilted (rotated around the x- and z-axis) so that the light will hit the cladding of the second fiber when launched.

The mechanical effects and tolerancing of the way the fibers are held certainly prompt alignment errors. Optical tolerances apply to mounting devices as well as to the optics; they share the total error budget. If the mounts are made of molded plastic, it is hard to hold tight tolerances to the mounts, and the optics have to be much more precise in order to stay within the budget.

The gross error in molded plastic housings is sizeable. If you have ever pulled the cover off of patch panels, you can see that the efficiency of typical connectors needs improvement. To compensate for mechanical errors, optical tolerances are driven very hard. A better solution would be to improve the accuracy of molded plastic connectors, or at least to evaluate where more improvement can be made to meet the requirement for an application.

With optical fiber, axial positioning tolerances make a big difference. If you have a 50-µm fiber core, and the beam entering the fiber is decentered by 10 to 30 µm, the system may still work, but it will lose throughput.

Tolerancing

What about tolerancing in the micro-optics themselves? Glass tolerancing is well understood and built into popular optical design programs. Plastics are not as well characterized. After being molded, plastics shrink tremendously-for example, to get a 7-mm-diameter element, a 10-mm mold may be needed. This limits the feasible tolerances of plastic parts and certainly includes a different set of issues than for glass. Plastic`s material qualities also limit surface accuracy and centering (see table).

The issues apply to elements other than lenses. Windows have many of the same tolerancing and coating issues. Microprisms are manufactured and coated differently: special tools must be created to grind, polish, and coat the right-angle prisms used for switching. To some extent, the tolerances depend on the tooling.

As the size of the optic shrinks from macroscopic to micro-sizes, two conflicting tolerancing issues occur. First, the error budget is smaller because the system and its components are smaller. But small size also works for micro-optics. For example, consider a wedge: the total indicator runout for this part is the product of the angle times the diameter. Because the diameter is so small, the wedge is less sensitive to errors in the angle. Tolerancing issues do not translate directly from macro-optics.

Consider surface roughness. Imagine that a lens is specified to have a quarter-wave roughness. This specification means that at some points on the surface, the roughness may be as large as a quarter wave. As the aperture gets smaller, the smaller area is more likely to be within spec because it is less likely to contain one of the roughest spots. The probability of having a deviation within the field is smaller, for a smaller field. For a micro-optic, one might achieve the same quality by specifying only a half-wave surface roughness.

The scratch-dig surface requirements work the same way. Specifying a quality of 20-10 should not change the price. As the diameter decreases, so does the difficulty of holding quality over that smaller area, but it doesn`t completely solve the problem-obtaining a quality of 10-5 is more difficult.

Considerations

For your application, consider what kind of optics and coatings you need to get the performance you want. As the sidebar suggests, PCX lenses are inexpensive and readily available in many diameters and can solve many of the same problems as GRIN lenses.

When you specify the elements, pay some attention to the tolerancing: if you can inject some intelligence into the specifications to make them fit your application, you may bypass some expensive manufacturing problems that are not strictly necessary. Although the size of micro-optics suggests that they be more precise than macro-optics, the reality is that the tooling is more difficult and tighter tolerances may not be necessary.

FIGURE 2. Different types of misalignment in fiber couplers are caused by separation, offset, and tilt.

Typical tolerance for micro-optics

Tolerance Glass Plastic

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CT to Et ratio 2:1 4:1

Surface quality 20-10 40-20

Dimensional 0.03 0.015

Centering 20 min 20 min

EFL 2% 0.50%

Power 2F 2F

Irregularity 1/2F 1/2F

Battle of the lenses

Everyone who works with optical fiber seems to love gradient index (GRIN) lenses. They do have some excellent

characteristics: flat faces and no spherical aberration on axis. They are expensive, however, and at times PCX or ball lenses can work just as well at a lower cost.

GRIN lenses have other drawbacks aside from price. Although there is no spherical aberration on axis, as soon as the light path is off-axis, it contains the same aberrations as other types of lenses. GRINs are most beneficial for small single-mode core fiber, in which the beam is very close to paraxial.

For a 50-µm core fiber, one can use other types of lenses and get the same efficiency. Consider the following options:

- A drum lens, which is an edged-down ball lens (see figure). Drum lenses are readily available and comparable to the performance of GRIN lenses.

- A ball lens has the same effect as a drum lens, is compact, and the focal length is the diameter. Within the past year, ball lenses that are coated uniformly on all sides have been commercialized. The lens can be dropped into a system without worrying about an axis or uncoated region, and because of its shape, mounting is straightforward.

- PCX lenses, which are effectively half of a ball lens, work fine for efficiency in a lot of moderate bandwidth systems, such as OC-1 or less. They are available off the shelf, and their biggest drawback is spherical aberration. For a 50-µm core fiber, however, one can achieve 90% to 95% efficiency with a PCX lens at lower bandwidths. By keeping track of numerical aperture effects within the system, systems designers can maintain that efficiency.

In the 1980s, an analysis of three types of lenses by A. Nicia showed them comparable in fiber-coupling efficiency.1 In experiments, ball lenses were shown to reach the theoretical expectations. The efficiency of these devices really depends on the packaging.

REFERENCES

Drum lenses can be used for many fiber applications, with fiber-coupling efficiencies comparable to GRIN lenses. Drum lenses are easily available and inexpensive.