Under the condition of Gaussian approximation, the near-field distribution of normalized ld fundamental mode output field can be expressed as
Where ωLX and ωly are the light field radii (also called beam waist radius) in X (perpendicular to the junction plane) and Y (parallel to the junction plane) on the light-emitting end face of ld, with a typical value of 0.6 μ m× 4 μ m..
In the direction parallel to the pn junction, the divergence angle is small (θ ∨? About 5 ~ 6), so all optical power can enter the fiber.
In the direction perpendicular to the pn junction, the divergence angle θ ⊥? Very large (generally greater than θc), only the optical power in the middle part of θc angle can be coupled into the fiber.
The light field of edge-emitting ld is elliptical, which is seriously mismatched with the circular mode field of fiber, and? The beam divergence angle of LD is larger than the receiving angle of optical fiber. According to the calculation, will? When LD and SMF are directly coupled, the theoretical limit of coupling efficiency is only about 20%. The direct end-face coupling efficiency between LD and optical fiber is very low. In order to reduce the optical coupling loss between LD and SMF, a lens can be placed between the end face of optical fiber and the end face of LD die in practical application, which can transform the elliptical mode field of laser into the circular mode field of optical fiber, shape the laser beam and improve the coupling efficiency. ?