A brief discussion on the working principle of fiber optic collimators
Definition:
A device that collimates light coming out of an optical fiber, or emits collimated light into an optical fiber.
Often times, it is necessary to convert the output light from optical fibers into a collimated beam in free space. In theory, using a collimating lens (as shown in Figure 1) can meet this requirement. However, the lens requires that the distance between the fiber optic port and it be fixed, approximately equal to its focal length. In practice, using a fiber collimator is more convenient. There are two different types of fiber collimators:
One type is a collimator that can be directly in contact with bare optical fibers. This is the cheapest and simplest method, but this collimator is usually permanently connected to the optical fiber.
There is a mechanical cross-section between another type of fiber collimator and optical connector, such as FC or SMA type; Usually not used for bare fiber optics. This collimator can be easily installed or removed from fiber optic cables with connectors.
The same device can also emit collimated beams into optical fibers or be used for fiber to fiber coupling: first, a first collimator is used to collimate the light emitted from the first fiber, and then another collimator is used to focus the collimated beam into the second fiber. Generally speaking, fiber optic connectors can be seen as the natural cross-section between fiber optics and free space optics.
Figure 1
Figure 1: A lens can collimate the output light of a fiber or emit a collimated beam into the fiber.
Another application is to combine a back facing mirror with additional optical components. For example, a Faraday rotator can be inserted to obtain a fiber Faraday reflector, or a quarter wave plate can be inserted to obtain a half wave plate reflector. In some other cases, fiber filters or saturation absorbers can be used.
catalogue
The size of the collimated beam
The type of lens used
Insertion loss
The purpose of tilting fiber optic ports
Size of collimated beam
The beam radius of the collimated beam obtained in different situations is different. Sometimes, the beam diameter is equivalent to the fiber diameter, for example, 125 μ m; At this point, the Rayleigh length is less than 1 cm. In some cases, a beam diameter of several millimeters or even larger is required.
Single mode fiber is a relatively simple case to calculate. At this point, the following formula can be used to accurately calculate the beam radius:
Here, it is assumed that the beam shape of the fiber mode is approximately Gaussian, so the formula corresponding to the half angle θ fiber of the beam divergence angle can be used.
Assuming that the distance between the fiber optic port and the lens is approximately equal to the focal length f. If the distance is too small, the beam will diverge, while if the distance is too large, the beam will converge to the focal point at a certain distance. It is usually better to have a slightly larger distance, where the beam focus can be obtained within the working distance. The longer the focal length, the less important the front to back position.
The smaller the size of the fiber mode, the larger the divergence angle of the obtained beam, so the collimated beam obtained with a given focal length is larger. The shorter the wavelength, the smaller the obtained pattern size and the larger the output beam. When the wavelength is short enough, this effect becomes more pronounced when the fiber enters the multimode region. Due to the above reasons, the visible guiding beam of the infrared beam cannot provide the accurate size of the infrared beam. In addition, the accurate fiber position required for collimation is wavelength dependent, especially when achromatic prisms are not used.
The divergence angle (i.e. collimated beam size) of the output beam of multimode fiber is related to the light emission conditions, especially to the fiber (e.g. bending). Generally speaking, its beam divergence angle is larger than that of single-mode fiber.
Fiber optic collimators can be used for different collimated beam sizes, corresponding to different focal lengths. The larger the size of the collimated beam, the longer and larger the diameter of the device is required.
Some fiber collimators have adjustment screws to control the direction of the beam (combined with angle adjustment), or to fine tune the front and rear positions (adjusting the focus or working distance). For collimators without adjustment screws, additional optomechanical devices are required to place the collimators.
The type of lens used
Common lenses include:
GRIN lens: small in size and low in cost, widely used in communication optical fibers, suitable for scenes with spot diameters within a few millimeters;
Spherical/non spherical lenses: suitable for applications with large light spots and long working distances (such as free space optical communication), providing longer Rayleigh lengths and good wavefront quality;
Achromatic lens: used in broadband systems to suppress collimation deviation caused by chromatic aberration;
Non spherical lens: suitable for large numerical aperture optical fibers, can effectively suppress aberrations and improve beam quality.
Insertion loss
The insertion loss of a single fiber collimator is usually very small, at the level of 0.2 dB or even lower. It is related to many factors, such as anti reflective coatings and dirt on lenses. However, when using bare fiber or fiber with connectors, the above factors will not have an impact.
The insertion loss of a pair of couplers used for coupling between optical fibers will be greater than the sum of the insertion losses of two devices. Especially in the case of single-mode fibers, achieving good mode matching is very important. Of course, the collimator should have collimated beams of the same size. According to the specific front and rear positions of the optical fibers in the collimator, the ideal situation is that the distance between the collimators is not zero. In this case, other optical components such as optical filters or polarizers can also be inserted.
The purpose of tilting fiber optic ports
Tilted fiber optic ports are commonly used to suppress the light reflected back into the fiber core from the fiber end face, that is, to maximize the return loss. However, tilted ports can also reflect the output beam.
Some fiber optic connectors have tilted fiber optic interfaces, which can be compensated using tilted fiber optic fixtures. Otherwise, the light beam emitted from the optical fiber will be directed towards the lens at a certain angle. After passing through the lens, the beam direction is in the same direction as the fiber (assuming accurate front back and lateral positions), but there will be a certain offset relative to the center of the lens. This will also increase insertion loss and reduce beam quality if reflections, scattering, etc. occur at the edges.