Fiber lens, also known as fiber microlens or lens fiber. The so-called fiber lens is that a certain lens shape is processed at the end face of the fiber. The function of the fiber lens is to change the optical path or convert the mode in the optical fiber or optical system.
In different application fields, the fiber lens also has different requirements. Generally, the shape of the fiber lens is divided into bevel (double bevels, or four bevels), wedge, spherical, conical and so on. From the composition of the fiber lens, it can be divided into the single fiber lens and the fiber lens combination.
It is mainly divided into two categories: one with a bevel angle of 6° to 10°, which is used to prevent interference or damage caused by the reflected light on the surface of the optical fiber when it returns to the optical path; the other with a bevel angle of 40° to 50° or even greater. The bevel fiber lens can change the optical path, and even make the optical path produce a large-angle total reflection, which can also increase the light-receiving area of the fiber, so that more light can enter the fiber. It is mainly used in fiber laser, fiber communication, traditional optics, fiber sensing and other fields.
In most cases, wedge fiber lens combination is used for optical path coupling. Among them, the commonly used fiber lens combination: the front end of wedge fiber lens is made into micro-cylindrical fiber lens, or made into four-bevel fiber lens, and tapered wedge fiber lens, etc. We collectively refer to this type of fiber lens as wedge fiber lens.
Because the spot of many LD output beams is elliptical, and the ratio of the long and short axes of the ellipse is proportional to the output power, generally the ratio is 3 to 5, high-power LD can be greater than 10, and the highest can reach more than 50. Obviously, such a beam is difficult to couple into the fiber. In order to adapt to the shape of the output beam of the LD, a wedge fiber lens can be used. The two large wedges should face the direction of the larger divergence angle of the LD, thus increasing the efficiency of the LD coupling into the fiber.
The ordinary wedge lens needs to be coated with an anti-reflection film on the end face, so as to minimize the reflected light, while the tapered wedge fiber lens has a special geometric shape, so that the fiber lens and the LD form a refraction angle without affecting the transmission of the optical path. Thus, the influence of the reflected light on the LD is avoided, and the noise interference caused by the reflected light is also eliminated. Compared with the technology of coating ordinary wedge fiber lens with anti-reflection film, the cost problem caused by the coating technology and the instability caused by the poor controllability of the coating technology are reduced.
There are many fields of application of spherical fiber lens, such as optical coupling field, biological field, medical field, sensor field and many other fields. For example, making a bevel lens on a spherical fiber lens can be used for an imaging diagnostic technology OCT, optical coherence tomography, which has developed rapidly in recent years.
Because the conical fiber lens achieves the purpose of expanding the numerical aperture of the fiber and increasing the light-receiving capacity, it is very suitable for coupling with LD, DFB, SLD lasers or VCSELs with circular or nearly circular output beam cross-sections. In addition, high-precision conical fiber lenses are also used to make medical laser micro-surgery systems and micro-illumination systems.