Characteristics of Self-Focusing Lens

Introduction to Self-Focusing Lenses Self-focusing lenses, also known as gradient refractive index lenses, are cylindrical optical lenses whose internal refractive index distribution decreases gradually along the radial direction, with focusing and imaging functions.

Because the gradient refractive index lens has end-plane collimation, coupling, and imaging characteristics, coupled with its cylindrical and compact shape, it can be used in a variety of different micro-optical systems more convenient. And in the field of integrated optics such as miniature optical systems, medical optical instruments, optical copiers, fax machines, scanners and other equipment has a wide range of applications.

Gradient refractive index lenses are essential basic components in passive devices for optical communication. Applied to various occasions requiring focusing and collimating functions, they are used in optocouplers, collimators, optical isolators, optical switches, lasers, etc. respectively.

1.1 Characteristics of self-focusing lenses

When the light propagates in the air and encounters different media, the different refractive indices of the media will change the direction of propagation. The traditional lens is to control the curvature of the lens surface, using the resulting optical range difference to make the light converge to a point.

The difference between a self-focusing lens and an ordinary lens is that the distribution of refractive index of the self-focusing lens material decreases gradually along the radial direction, which enables continuous refraction of light transmitted along the axial direction, thus realizing the smooth and continuous convergence of the outgoing light to a point.

The self-focusing lens utilizes the gradient variable refractive index distribution along the radial direction of the gradient gradually decreases the characteristics of the change in its refractive index change is expressed by Equation 1. Its refractive index distribution curve is shown in FIG. 3.

No ......... Refractive index of the center of the self-focusing lens

r ............ Radius of the self-focusing lens

Root A ......... Refractive index distribution constant of the self-focusing lens

1.2 Propagation trajectories of light in self-focusing lenses with different pitches:

1.3 Main parameters of self-focusing lens

Pitch: In a self-focusing lens, the length of a sinusoidal cycle of a light beam propagating along a sinusoidal trajectory is called a pitch.

Lens length: the distance between the center axes of the two ends of the self-focusing lens lens.

Refractive index distribution constant (root A): the refractive index of a self-focusing lens is distributed along the radial direction.

Numerical Aperture (N.A.):

n............ refractive index of the medium in which the incident light is located

am...... ... the maximum aperture angle of the incident light

1.4 Principle of application of self-focusing lenses?

Focusing: According to the light transmission principle of self-focusing lens, for a 1/4 pitch self-focusing lens, when a parallel beam of light is input from one end face, the light will converge on the other end face after passing through the self-focusing lens. This end face focusing function is not possible with conventional curved lenses. As shown in Figure 5 below:

Collimation: Collimation is a reversible application of the focusing function. According to the light transmission principle of self-focusing lens, for 1/4 pitch self-focusing lens, when the converging light is input from one end face of self-focusing lens, it will be transformed into parallel light after passing through the self-focusing lens. As shown in Figure 6 below:

Self-focusing lens is essential for optical fiber communication passive components in the basic devices, applied to the requirements of the focusing and collimation function of the various occasions, such as: collimator, coupler, optical isolator, optical switches, wavelength-division multiplexer and so on. For example, in Figure 7, two self-focusing lenses are used for collimation and focusing, so that we can add a variety of optical devices between the two self-focusing lenses, such as: filters, polarizers, Faraday rotator, etc., to constitute a variety of optical passive devices.

Coupling focus: because the self-focusing lens can be completed through the end face focusing function, coupled with its simple cylindrical shape, so that it has a very wide range of uses in the optical energy connection and conversion. For example: optical fiber and light source, optical fiber and photodetector, as well as optical fiber and optical fiber coupling and so on.

Figure 8 indicates that L1 for the light source or fiber to the distance from the end of the self-focusing lens, Z for the length of the self-focusing lens, L2 for the distance from the end of the self-focusing lens to the fiber. In order to make the light emitted from the light source or optical fiber can be effectively coupled into the optical fiber after focusing the self-focusing lens, the distance between L1 and L2 needs to be adjusted to achieve the best coupling efficiency. However, in the actual coupling process, the coupling efficiency is less than its theoretical value, the reason is that the coupling efficiency and the structure of the device and the use of direct relationship.

Single-lens imaging: In addition to the general imaging function of curved lenses, self-focusing lenses also have the characteristics of end-face imaging. For the P/2 pitch self-focusing lens, its end-face imaging mechanism is shown in Figure 9:

According to this imaging principle, the use of P/2 integer length lens can be realized in the microscope camera system end face to end face image relay transmission. Therefore, low chromatic aberration self-focusing lenses have been more and more widely used as objective and relay lenses in various medical endoscopes and industrial endoscopes.

Self-focusing lens array imaging: when using a combination of spherical lenses to transmit a large image, the purpose is to get the 1:1 image, but its **** yoke distance is generally 4 times the focal length. Through the use of self-focusing lens array, can greatly shorten the *** yoke distance, so that the entire device size miniaturization; on the other hand into a linear arrangement of self-focusing lens array in the entire straight line imaging resolution is the same, and the entire field of view of the transfer function of the value of a more uniform, greatly improving the quality of imaging, and at the same time to overcome the ordinary spherical lens in the lens periphery of the imaging resolution and clarity of the lower than the center, as well as the combination of assembly work Complexity of the shortcomings of the assembly work. Therefore, the self-focusing lens array has become an important device for photocopiers, fax machines, scanners and other instrumentation.