||NFP is an abbreviation for Near Field Pattern and is also called a near-field image. The NFP can be used by the surface distribution of the spot light intensity at the emitting end face of the light emitting element such as an optical fiber or LD, the MFD measurement of the single mode fiber, the evaluation of the excitation state of the GI multimode fiber and the emission point size measurement of the light emitting element, etc.
||FFP is an abbreviation for Far Field Pattern and is also called a far-field image. The FFP can be used by intensity (angle) distribution of the light emitted from the light emitting element such as an optical fiber or LD at a location sufficiently distant from the emission end face, measurement of the divergence angle of the emitted light of the single mode fiber or the light emitting element, evaluation of the excitation state of the SI multimode fiber, etc. In addition, before, it was difficult to obtain a highly accurate two-dimensional image sensor in the communication band wavelength (1.31/1.55µm), so the method of calculating MFD in the communication band SMF from FFP measurement data has been standardized.
||A lens designed so that the image height at the imaging position of incident light at a certain angle θ is f×θ. It has barrel-shaped distortion that makes it an equidistant projection. It is used as a lens for FFP measurement and as a lens for laser lithography equipment because it has a linear relationship between the amount of change in the image position and the amount of change in the angle.
|FFP measurement with objective lens
||Our optical system (M-Scope Type D) can measure NFP and FFP at the same time. In this optical system, the FFP is measured using an objective lens instead of the f-θ lens. However, since the objective lens is used, there is a deviation from the f-θ characteristic in the peripheral part of the field of view, which causes a slight decrease in measurement accuracy.
||A device that reflects a small amount of light by using the surface reflection of optical glass and crystal substrates, and performs optical sampling for monitoring. There is a parallel plate or wedge plate type, and the wedge plate type can prevent the influence of reflection on the back side. In our company, there is a beam sampler that uses a dielectric multilayer mirror to transmit a small amount of light instead of using the reflection on the substrate surface.
||It is an optical device that safely blocks high-power laser beams and is also called a beam block or beam trap. A light beam is terminated by absorbing a laser beam and converting it into heat by using a material having an absorption characteristic for the internal structure. We use it to terminate unnecessary light other than sampling light when analyzing the beam profile of high-power light.
||The N.A. is abbreviation for numerical aperture. It called the numerical aperture.
- ●In case of lens
- The N.A. of the lens is expressed by the following formula, where θmax is the maximum angle of the light beam incident on the objective lens from the object with respect to the optical axis, and n is the refractive index of the medium between the object and the objective lens. (Note that this is not the refractive index of the lens)
- ●In case of optical fiber
- The N.A. of an optical fiber is given by the following formula when the maximum incident angle θmax is the incident angle at which light is totally reflected in the core. (When the refractive index on the output side is 1.) Light incident from an angle outside this range cannot propagate in the optical fiber.
||The W.D. is abbreviation for working distance. It called working distance. It is the distance from the lens tip to the subject.