Fiber Arrays
Fiber arrays are one- or two-dimensional optical arrays made of single or multiple optical fibers. These are typically used for coupling optical fibers to planar lightwave circuits, sensors or other photonic integrated circuit components such as power splitters and fiber optic connectors.
Fiber arrays are usually fabricated with extremely tight tolerance, often using V-grooves or micro-holes in quartz glass, ceramics or Al2O3. Connector options include SMA, FC, ST, SC, LC and MTP.
Linear
Linear fiber arrays are formed by a single or multiple bundles of optical fibers that have been carefully fabricated and packaged to be well-aligned in all dimensions. They are used for a variety of different applications, including light coupling, beam steering, and fiber optic switches in telecom networks.
One of the most common types of fiber array is a crossbar switch. The crossbar switch has an input and output port array and a number of actuators that move along these paths to switch the state of each port. When a mirror is introduced to fold the path length between the input and output arrays, a more compact switch form factor can be obtained.
For a number of reasons, it is important to align the ends of fiber arrays correctly and accurately, as misalignment can significantly affect the quality of the output beam. This is especially the case in optical imaging and laser applications where a linear array of fibers, with a closely matched numerical aperture, is combined with a lens or microlens array for collimation of the light.
Typically, the fibers are centered and surrounded by cladding material such as a glass or a polymer to reduce reflections. In addition, the cladding may have certain features that aid in achieving proper alignment of the fibers.
The end of the fiber bundle is often packaged with some kind of locking device, such as a cap that locks onto an adapter. The adapter is usually a tube and the cap is secured on it via external threads that attach to the internal threads of the directional adapter.
Once the cap is locked on the directional adapter, the tube is rotated to align the fiber bundle array with the spectrometer entrance slit. Once the slit is aligned, the spectrometer is turned on and the light from the sample is transferred to the fiber bundle.
Since the fiber core diameter is generally larger than the slit width, significant part of the light is blocked by the slit, which can seriously compromise system throughput and sensitivity. Moreover, the light is also scattered elastically by the slit, which can significantly lower the signal-to-noise ratio.
Two-dimensional
Two-dimensional fiber arrays are a type of arbitrary pattern of individual fiber array optical fibers arranged in a 2D dimensional space. These can be used in optical waveguide devices, integrated optics, optical imaging, and more. The resulting fiber arrays may be formed from a variety of materials, including a solid surface or a thin plastic film.
Many 2D fiber arrays are created by a process called V-grooves, in which a precise arrangement of holes is made on a piece of glass or polymer material. In other cases, 2D fiber arrays are fabricated using a technique known as laser drilling.
The most common two-dimensional fiber arrays are those that have a single X and Y axis and a symmetrical square lattice, but there are other patterns as well. They are often used in optical cross-connection, spectroscopy, and astronomy as well as biomedical imaging applications.
In some two-dimensional fiber arrays, a fine separation between the end of each fiber is achieved by shaping the cladding diameter of the fibers and by etching the cladding to decrease its size. This reduces the raster line spacing a and enables higher density data recording on the recording medium without being constrained by the field of view imposed by the imaging optics.
Another method of reducing the raster line spacing is to stagger multiple rows of equally spaced fibers such that data tracks are written at fine intervals. This approach is similar to the method described in US Patent 4,590,492.
However, the disadvantage of this scheme is that it requires the use of an opaque mask which can substantially decrease the system efficiency. It also does not take advantage of the wide range of variation in cladding and core diameters available on the market today.
To overcome these limitations, a new apparatus is provided which is capable of producing high density raster lines on the recording medium. It does so by staggering multiple rows of equally spaced fibers such as one can see in FIGS.
It further provides for the precise separation between the ends of the fibers by using a surface of the fiber cladding to maintain precise control over the position of each fiber within the rows and columns. In a preferred embodiment, each one of the rows and columns is slightly skewed in a direction that is reflected on a recording surface, such as a video recording device.
Core-less
Frequently, one needs to split one data signal into many outputs and to distribute this to multiple fibers. This is done with a planar waveguide circuit, the outputs of which are then connected to fibers using a fiber array (see Fig. 1).
The individual fibers in the array must be accurately placed in narrow-pitch V-groove substrates or micro-hole arrays formed by ultra-precision processing technology and polished on a submicron level, i.e., in a precise way such that light passes through them without any loss. The alignment must also be accomplished with high precision, usually utilizing active alignment techniques.
In addition, the end faces of the fibers in a 2D fiber array must be properly aligned to avoid light interference between the individual ends. This is often achieved with special end caps, such as blocks of optical glass material, which are shaped to aid the alignment.
However, this process is time consuming and can only be performed by a manual operator. In addition, the end faces need to be carefully packaged such that they can be conveniently handled and transported.
A common approach to the problem of phase distortion in MCFs is to employ digital optical phase conjugation (DOPC),40,41 which enables the generation of an arbitrary light field distribution through the MCF by compensating the OPDs. In order to do so, a CGH generated by a tailored GS algorithm26 is loaded to the SLM as an additional input along with the DOPC-generated phase.
The resulting holograms are then loaded to the SLM in real-time fiber array for generating the corresponding dynamic light fields at the MCF facet. The generated holograms can be used to display a complex holographic image through the lensless microendoscope with 10,000 single-mode fiber cores, as shown in Fig. 6.4.
This hologram generation is done in real-time via CoreNet, which is based on a neural network that is trained with a distribution map of the MCF phased array and the corresponding spatial amplitudes. The distribution map is extracted from experimental measurements on the MCF proximal and distal facet of the MCF, and a tailored CGH for a randomly distributed phased array is generated by CoreNet.
Custom
Fiber arrays are custom designed or manufactured to meet individual customer requirements. They can be manufactured using a variety of optical fiber types and can include industry-standard connectors as well as hermetic outer sheathing.
Typical applications of these optical fiber assemblies are in medical devices, bioanalytical instrumentation, spectroscopy, industrial sensing and low-power laser light delivery. Depending on the application, these assemblies may be equipped with metal (aluminum or gold) coated fibers to increase transmission performance and damage threshold in high temperature environments.
Another example of a fiber array application is in telecom, where data can be sent through many different fibers at extremely high bit rates. This requires interfaces (fiber connectors) that are based on fiber arrays, to avoid the possibility of unintentionally exchanging fibers during transmission.
One can also use mode-matching fibers for efficient coupling of light from photonic integrated circuits (PICs). These require fibers with a mode-field diameter (MFD) that is matched to the chip waveguides. This is usually achieved with lensed or tapered single-mode fibers (SMFs), where a glass lens is melted or etched to the end of each SMF. However, this is not always possible with standard SMFs, because they tend to have MFDs of around 10 um while most PICs require MFDs of several micrometers or less.
For some applications, such as the coupling to chip waveguides with very small cross-sections, one needs a special mode size converter. These can be implemented either with lensed or tapered fibers or with core-less end caps of a variety of shapes and sizes (e.g. with rectangular or circular cross-sections).
In the case of astronomical telescopes, it is also possible to use fiber arrays in conjunction with mirror arrays, fabricated with MEMS technology, for flexible routing of data signals. These are suitable for telecom providers as they allow flexible switching between data lines, but they can also be used in factory automation, infrastructure monitoring and other applications where fast switches are essential.
AMS Technologies manufactures a wide range of fiber optic assemblies that include single and multi-fiber bundles. Typically, each bundle or assembly features various inputs and outputs with unique cross-sections and geometries that require a different end-fitting and sheath to protect the optical component from damage.
