Fiber Arrays in the Optical Field
In the optical field, fiber arrays are a good way to connect different optical components in a single device. They can be used for a wide variety of applications, including signal splitting, photonic integrated circuits, and beam collimation.
Fiber arrays are also used for cell-based biosensors, which allow the monitoring of individual cell responses over a long time period. The cells are usually engineered to express a fluorescent protein, but it is also possible to use enzymes that catalyze the production of the fluorescent product.
Optical waveguides
Optical waveguides, particularly segmented or photonic crystal waveguides, are an important part of most fiber arrays. They can be made from various optical materials such as silica and can span the whole range of spectral regions from the near-infrared to the ultraviolet.
Usually, the cross-sectional geometry of the waveguide is constant along its direction of propagation. However, it is also possible to form periodic changes in the cross-section, e.g. through the use of so-called Bloch modes. This allows the use of fibers with different mode profiles or symmetry, and can lead to more effective signal transmission and diffraction compensation, in comparison to waveguides which are only 1D or 2D in cross-section.
For this reason, some optical waveguides have been fabricated using lithographic techniques combined with, e.g., epitaxy or ion exchange. These processes can lead to a somewhat increased refractive index of the material, which can be useful for guiding light.
Another approach is to bury the waveguide in the surrounding medium, which is commonly done with glass, but may also be applied to other transparent media. This reduces propagation losses and leads to a more symmetric mode profile, but it can also have side effects such as heating or indiffusion of the material.
Finally, it is possible to align the fiber ends of a fiber array, such that they are all well oriented and bonded together. This is often crucial for the coupling of a fiber array to a photonic integrated circuit, where precise positioning is essential.
This requires careful design and fabrication, including careful packaging to allow for safe handling. It is especially important to ensure that the input or output end of the fiber fiber array array is properly aligned, which has an impact on both the resulting beam and the collimation of the output.
Photonic integrated circuits
Photonic integrated circuits (PICs) are an emerging technology that integrates active and passive components on a single substrate. These are often silicon-based and include lasers, modulators, detectors, and filters. PICs have several advantages over traditional photonic circuits, including faster data transfer, smaller size, and less power consumption.
The material that is used to fabricate the final PIC determines many of its properties, such as its transparency, refractive index, and ability to generate light. Some materials, such as indium phosphide (InP), have lower propagation losses than other materials, making them ideal for PICs.
Currently, InP-based monolithic integration is the most common PIC technology but it faces some technical problems, such as laser emission efficiency and electro-optic modulation. Companies such as Infinera, Alcatel-Lucent Bell Labs, and Huawei are working on improving these issues and making breakthroughs in PICs.
As the number of applications grows, so does the need to improve PICs. These applications span data communications, sensing (for agriculture and autonomous driving, for example), and biomedical applications such as lab-on-a-chip devices.
PICs are also being developed for other fields, such as optical metrology and terahertz imaging. These areas can profit from highly compact and robust PICs that are capable of generating and detecting terahertz waves, as well as processing the signals.
Photonic integrated circuits are a growing technology that is quickly maturing, driven by high-bandwidth communication applications and mature fabrication facilities. The smallest sized PICs can be fabricated using the same tools used for existing electronic circuits, such as silicon-on-insulator (SOI). These chips will move information faster and more efficiently, reducing power consumption.
Signal splitting
The ability to split an optical signal into multiple signals is essential in the transmission of data. For example, in cable TV, different programs are broadcast to different subscribers, and it is essential that the same data can be delivered in a way that ensures a high quality viewing experience for each user.
In order to split the one signal into a multitude of outputs, a planar waveguide circuit is often used. However, the outputs of this circuit need to be coupled to fibers in order to deliver the desired signal.
To achieve this, a fiber array is often used. A fiber array is a densely packed arrangement of optical fibers that have been arranged with precision in V-grooves on some solid surface.
The fibers are typically bonded together with a special bonding process that provides precise positioning and high reliability. The V-grooves are thermal expansion coefficient-matched to ensure stress-free operation and no fiber shift at high temperatures.
This method is particularly useful for 2D arrays, as it allows for much more flexible alignment of the fiber ends to the solid surface. It is also possible to use bare fiber ends and coatings on the fibers, which greatly reduces coupling losses and parasitic reflections.
Adaptive wavelength division multiplexers are an innovative concept that enables optical signals launched into one input fiber port to be multiplexed and dynamically routed into many output fiber ports with arbitrary splitting ratios. The concept is demonstrated on a 1×2 adaptive structure that is driven by computer-generated multicasting phase holograms uploaded onto an Opto-VLSI processor.
A variable fiber optical splitter/coupler fiber array is also proposed, which splits an incoming optical signal into two output optical fibers with a continuously variable ratio controlled by an electrical input voltage. The device preserves the total optical signals without loss and can be adapted to various pre-determined ratios with special orders.
Active alignment
In optical systems, active alignment is a process that uses lasers to energize individual fibers and monitor the light they produce. This method can be used in a variety of applications, including laser coupling to fibers and other passive optical components and pigtailing of fibers to other devices.
In order to optimize the position of the energized lasers, it is necessary to make careful measurements. For example, the distance between each energized fiber and the photodetector must be adjusted in order to maximize the amount of light that emerges from the back end of the fiber.
This is done by using a microscope objective and a camera to take measurements of the position of each energized fiber. This information is then used to determine the best position for the energized lasers to be in.
Compared to passive alignment methods, which are usually performed in a series of steps, active alignment is faster and requires less labor. It also cuts down on real estate and operating costs, which is important for companies that manufacture complex optics and photonics components.
A major advantage of this technology is that it can be used to align a wide range of devices. It can be used to assemble lasers, lens assemblies, and sensors.
For aligning fiber arrays, it is important to use a high-precision laser that can accurately energize the individual fibers. This can be done by using a laser with 1310-nm or 1550-nm wavelengths, depending on the application.
In addition, it is important to choose a good adhesive that can bond the fiber array to the device. A DUV epoxy is a great choice since it can withstand any possible stress and provides strong fixation against peel, cleavage, or shear.
Collimation
Collimation is a process that converts a divergent beam into a parallel one. It is important for a number of optical devices, including laser diodes, micro-optical crystals and fibers.
In the case of fiber arrays, collimation is a necessary step in transmitting light through multiple strands of fiber. This process can be difficult for a person to perform on their own, so many manufacturers use automatic collimation systems.
A fiber collimator is a device that enables the transfer of light in a right direction, which decreases the loss of an optical link. Typically, this device is made of a single-mode pigtail fiber and a collimating lens.
The simplest way to collimate a fiber is through the use of a single optical lens, which can be either spherical or aspheric in nature. This type of lens is usually used for standard telecom fibers, as it is relatively cheap and small.
However, larger beam diameters require more advanced collimators. In addition, the size of these lenses has to be carefully chosen, as a large lens can cause significant optical losses.
To minimize the insertion loss and improve coupling efficiency, collimators must be assembled properly. This includes both lateral and angular misalignment, which can affect the performance of the assembly.
An ideal arrangement would have both collimators have the same collimated beam size, and be aligned with a tolerance distribution that allows for good mode matching.
For example, if a fiber array is based on a fiber collimating lens and specialty fibers, the output of the specialty fibers should be confined to the multimode core to avoid refraction and large optical losses. This is a crucial step in transmitting large amounts of power through the system.
