There are two main ways for the optical signal emitted by the laser to enter the fiber: direct coupling and lens coupling. Lens coupling is further divided into single-lens coupling and multi-lens coupling. Using lens coupling can achieve higher coupling efficiency than direct coupling. The main advantage of using double-lens coupling is that the tolerance can be dispersed, so that the components on the optical path can have a larger displacement space.
tapered fiber
direct coupling
Direct coupling can be achieved using cleaved or tapered fibers. The split fiber is obtained by directly splitting the bare fiber. The end face of the fiber is flat and the price is relatively cheap. However, because the end face is flat, the reflection is large, and the insertion loss when coupled with the laser is also large (usually 9-12dB).
A tapered fiber is formed by combining a lens at the end of the fiber, which can be formed by the following two methods:
1. Melt and draw the fiber end into a tapered shape, which will tape both the core and the cladding. The fiber is usually heated using an electric arc or by extending the fiber into molten glass. The symmetry of the lens can be controlled by controlling the process. This method achieves an insertion loss of about 2-3dB.
2. Etching or grinding, this method keeps the diameter of the fiber core unchanged while forming the lens on the end face of the fiber. Also some other cross-sectional profiles (eg paraboloid) can be obtained instead of just spherical. This method can achieve better coupling efficiency, and the insertion loss can be as low as about 0.2-0.4dB when coupled with the laser.
For direct coupling, the fiber end is typically installed close to the laser. Therefore, the fiber must extend into the package, and if the device requires a hermetic package, the fiber must be metallized to seal with the package. Furthermore, the main factors affecting the coupling efficiency of the light source to the fiber in direct coupling are the divergence angle of the light source and the numerical aperture (NA) of the fiber. In addition, the size of the light-emitting surface of the light source, the size and shape of the fiber end face, and the distance between the two will also affect the coupling efficiency.
lens coupling
The lens coupling can be single lens or multiple lens. When using a single lens, the distance from the laser to the fiber end face is determined by the radii of the front and rear sides of the lens. In the case of using multiple lenses, the light beam is made parallel by the first lens and then focused by the second lens.
The isolator can be placed anywhere after the beam is collimated (i.e. anywhere between the two lenses) when tight control of reflection is required. In addition, lens coupling allows one of the lenses to be mounted on the envelope, so that the fiber does not have to protrude into the envelope and metallize the fiber.
Alignment technology
Alignment techniques are generally classified into "active alignment" and "passive alignment". In the active alignment technology, the laser or detector performs alignment of the optical axis and the like when the device is in a working state by applying an external bias voltage or current. For passive alignment, the active optical device does not need to work, but is aligned by some markings. In contrast, passive alignment is a newer alignment technology with advantages such as easy automation and reduced assembly equipment and procedures.
