Fiber Arrays in Telecom Applications
Fiber arrays (also fiber-optic arrays) are one- or two-dimensional arrays of optical fibers. These fiber-based interfaces (fiber connectors) are used in a variety of telecom applications, including data communication and wavelength division multiplexing.
A fiber array can be formed from a single bundle of fibers, or from several bundles with different lengths. It is most commonly used for encapsulation of opto-electrical integrated circuits, optical planar structures or sensors.
Linear fiber arrays may be formed by placing a set of fibers into V-grooves made on some solid surface (see Figure 1). The resulting structure is typically very regular and can be very simple, or can even be quite complex. The arrays can be used to combine beams from a laser diode or VCSEL.
A linear fiber array can also be used to deflect beams from a laser or photodiode at a particular angle, which can be especially important for high-speed transceiver applications. In order to achieve this, the fibers are placed into V-grooves which have a small bending radius that is less than 8mm.
Typical applications include optical fiber switching, and fiber to chip edge coupling. For both, small, 90deg-bent fiber array connectors are necessary, which can be used to deflect the beams from the surface-emitting laser or photodiode in a silicon photonics transceiver module and connect to a high speed photonic circuit board.
These small optical fiber switches are arranged in crossbar structures, where one input fiber is mated to a collimating lens array at one facet of the switch and an output fiber is mated to a collimating mirror array at the other facet of the switch. The beams from the input and output fibers then form a grid with a very high degree of multiaxiality.
For these types of applications, it is important to ensure that the fiber ends of the array are well aligned. This can be done by forming the end of the array into a block of suitable shape and possibly adding features that aid the alignment, such as a metal flange around the fibers, or by applying anti-reflection coating.
In many cases, the light loss from misalignment of the fiber-slit interface is so high that it greatly reduces system throughput and sensitivity. The resulting reduction in system performance is particularly noticeable for Raman scattering.
To overcome this problem, a new type of mount for linear fiber arrays has been invented which provides an efficient means of light coupling and improves system sensitivity. The mount, which is attached to the fiber bundle and to the spectrometer, ensures that the fiber slit aligns with the entrance slit of the spectrometer.
Fiber arrays are used in a variety of applications. They are particularly useful in coupling light to a number of different components, such as an array of planar waveguides on a fiber array photonic integrated circuit or the outputs of an optical signal splitter.
Two-dimensional arrays are also useful for high-resolution spectral analysis. For example, they can be used to transport light from an astronomical telescope to a high-resolution detector for spectral analysis. They are especially useful for telescopic applications, where a high-resolution spectral analysis can be done in multiple viewing directions at the same time.
Many of these arrays are made from single-mode or multimode fibers, depending on the application. For some of these applications, polarization-maintaining fibers are also used.
Some of the most common applications of fiber arrays are as components in photonic integrated circuits, where their precise positioning and mode adaptation are key issues. In addition, they are often used in laser material processing.
Another important use of two-dimensional arrays is in the production of a wide range of optical sensors, such as for monitoring and tracking of various aspects of industrial processes. These sensors can be fabricated with microlens arrays or movable mirror arrays.
Moreover, they are also used for distributing optical signals from one data source to a number of different outputs in a telecom system. This is particularly relevant in the cable-TV industry, where the same signal is bundled and distributed to multiple customers.
A second technique for reducing the effective track spacing on the recording surface is provided by using a two dimensional array of optical output sources which are aligned at an angle with respect to a scan direction. The geometry reduces the effective raster spacing and allows a higher scan rate to be achieved.
To achieve this, a spacer element is used to maintain the precise relative distance between the individual fiber ends. This can be achieved with a spacing fixture, or by using the diameter and shape of the fiber cladding itself.
Another way to improve the efficiency of a two-dimensional array is to bend the fibers so that for each pair of fibers, the center-to-center separation of the output ends is less than the center-to-center separation of the input ends of the adjacent fibers. This reduces the loss associated with the bending of the fibers and enables the array to emit modulated signals with high optical efficiency.
There are a number of different types of fiber arrays. These can be single- or multi-dimensional, and they may be linear or asymmetric. The most common type is the V-grooved array (VGA). This is an array of fibers that are positioned by using a V-shaped hole in a substrate. This is used for optical communication systems such as optical fiber cable assemblies.
For biosensing, these fibers have been fabricated into a variety of structures. For example, they can be patterned to contain beads, cells, and single molecules. These are useful in many different applications including cell-based screening, DNA analysis, and toxicity testing.
These arrays can also be used in a variety of different imaging systems, including fluorescence microscopes and confocal microscopy. They are particularly useful for monitoring single cells over time.
This ability to monitor single cells allows researchers to identify variations in gene and protein expression as well as obtain information about cell-to-cell variability. In addition, these arrays can be used to perform cell-based screening for toxicity assays and in yeast two-hybrid screens.
The individual cells in these arrays are labeled with a fluorescent dye. They are then allowed to adhere to the fiber surface. This allows researchers to follow the cells and their movements over time, thereby improving their accuracy and reducing assay times.
Another unique application for these fibers is in migration assays. These allow scientists to determine the effectiveness of anti-migratory compounds. This can be a valuable tool in tumor research.
A third application for these arrays is in spectral imaging. These can be a highly sensitive and accurate way to observe fiber array light at very narrow wavelengths. These arrays are made of silica or specialty fibers that can be used in spectral regions from near-infrared to the ultraviolet.
These arrays are manufactured using a precision assembly process with a high-precision V groove substrate that ensures the core position of each fiber is extremely precise and has an ultra fine surface finish. This allows the array to be produced to a variety of custom or standard demands.
Various applications in telecom require the use of multi-fiber assemblies, for example for wavelength division multiplexing (WDM) or network routing. The multi-fiber assemblies, which have a very large number of fibers and a high core spacing, are often arranged in a pattern that may be quite complex, limited only by the imagination of the customer.
The resulting optical beam is collimated by a lens array and needs to be precisely aligned to the lens spacing, a critical step in the optical assembly. Often, the spacing between the fibers is also very small. This results in a problem of ensuring that the fibers are positioned accurately on the surface of the lens, particularly when the fibers are stacked in a V-groove.
In addition, if the fibers are to be bonded to each other in the vertical direction, it is very difficult to control the positions of the substrates in consideration of the influence of the adhesive shrinking during bonding. Therefore, it is important to provide a two-dimensional fiber array that can be molded with very little error and with a low cost.
This issue is addressed by an embodiment of the present invention, which provides a two-dimensional optical fiber array having an alignment substrate having a plurality of guide holes which are two-dimensionally arrayed and extend through the substrate, a plurality of cylindrical ferrules which are respectively inserted into the guide holes in the same direction and have through holes in central portions, and a plurality of optical fibers fitted and held in the respective through holes. The guide holes are formed into a cylindrical shape having a diameter substantially equal to an outer diameter of the ferrules, and a light incident/exit end face of each optical fiber is exposed on one end face of the ferrules.
Further, the present invention provides an optical fiber array having a plurality of fibers, each having a distal end portion attached to the respective distal end portions of the respective fibers and having an outer diameter that is slightly larger than an inner diameter of the corresponding guide hole. The fibers are inserted into the respective guide holes and fixed to each other with an adhesive injected through the corresponding adhesive filling hole.