Small ball (full sphere) lenses in the 0.5-mm to 3.0-mm diameter range offer a number of practical advantages for fiber-to-fiber coupling and fiber collimation. Ball lenses are more physically compact and less expensive than commonly used gradient index (GRIN) lenses. Furthermore, the complete rotational symmetry of ball lenses makes them easier to mount, position, and align to a fiber than the cylindrically shaped GRIN lenses. Despite these advantages, the wide-scale use of ball lenses in the fiber telecommunications business has been limited, because of difficulties in depositing high-performance antireflection (AR) coatings on these lenses. Specifically, coatings produced using traditional evaporative technology are highly nonuniform; also, the tooling used leaves an uncoated stripe on the part. Together, these factors then require that the micro-ball lenses be precisely oriented during coupler or collimator assembly. In work performed at Deposition Sciences Incorporated (DSI), low-pressure chemical vapor deposition (LPCVD) technology has been adapted to uniformly coat the entire surface of ball lenses. The economics of this process make it viable for the volume production of components for the telecommunications market.
In a traditional evaporative coating chamber, one or more racks hold the substrates to be coated. The first step in the process is to pump all air out of the chamber to achieve a very high vacuum (10-6 Torr or better). A succession of coating materials is then evaporated, typically using either resistive heating or electron-beam bombardment. Because the chamber is in near vacuum, the mean free path of evaporated atoms or molecules is several meters. They stream out and recondense onto any surface in the chamber that has a direct line of sight to the source. This process makes the deposition rate highly dependent upon the distance from the source to the substrate surfaces, as well as their relative angular orientation. In order to maximize layer uniformity, the racks are rotated during coating, so that every part experiences the same "average" distance from the source. Complex internal masking is also often used to enhance deposition uniformity throughout the chamber.
There are two significant drawbacks to applying this evaporative technique to coat small ball lenses. First, highly curved ball lenses cannot be coated uniformly because of the significantly different orientation that the center and edge of each lens has with respect to the coating source. Second, processing an entire ball requires coating one side first, and then turning the balls 180° and repeating the procedure to coat the second side. Furthermore, in order for the chamber tooling to securely hold a ball lens, it must cover slightly more than half the sphere, so the end result is an uncoated stripe that runs around the entire circumference of the finished part. Together, these factors require that the tiny ball lenses be precisely oriented during the assembly process to ensure that the right part of the coated surface is facing correctly.
To successfully address this coating problem, DSI has developed a form of LPCVD, called IsoDyn™, that avoids the limitations of evaporative technology. LPCVD is widely used in the semiconductor industry to produce thin films during integrated circuit production, but had not been widely used for optical thin films.
In the IsoDyn process, parts are placed in an evacuated chamber and heated to about 500 °C. The chamber is then filled with a chemical vapor at relatively high (usually 0.1 to 5.0 Torr) pressure. The heat causes a chemical reaction in the precursor gas, resulting in deposition of the desired substance on all exposed surfaces of the substrate. This procedure can be repeated using various precursor gases to build up a multilayer coating.
The high gas pressure results in a short mean free path for the gas molecules, causing deposition to occur at the same rate on every surface within the chamber, regardless of its position or orientation. Consequently, this technique can produce extremely uniform coatings on highly curved or unusually shaped parts. Furthermore, there is no need to hold parts in traditional tooling and move them during the coating process. The lack of tooling that masks off some of the part during coating enables the production of uniform thin films covering virtually the entire surface of ball lenses.
Another benefit of the IsoDyn process is that the entire volume of the reactor can be filled with parts, and all surfaces of a part can be coated in a single run. This translates directly into higher throughput, and hence lower production cost.
DSI is now utilizing IsoDyn coating technology to produce a range of ball lenses for telecommunications, called IsoSpheres™, including BK 7, spinel, sapphire, LaSF N9, LaSF N18, LaSF N35, and stabilized cubic zirconia. The standard multilayer coating for these products has a specified insertion loss of less than 0.01 dB per surface at either 1310 nm or 1550 nm. A dual-wavelength antireflection coating for both these wavelengths (see graph), as well as other single-wavelength and dual-wavelength combinations, is also available.