Photodiode Coupling Technology

Our business is optical fiber alignment. We can fiber-couple and package virtually any photodiode in TO-can.

We can make coupling to various types of optical fibers: multimode (MM), single-mode (SM), polarization maintaining (PM).

Photodiode modules are thoroughly controlled at every production stage. We take care of cleanliness and ESD safety. Design of modules is being tested according to Telcordia GR-468-CORE.

Photodiode coupling

The Challenge: Fiber-coupling in photodiodes introduces the inevitable issue of back-reflection, which can disrupt or damage active components within the optical fiber line. A straight cleave or polish (Figure 1) results in a 4% (RL = 16 dB) reflectance at the fiber-air interface (yellow arrow in Figure), plus additional reflectance from the photodiode surface (green arrow in Figure).

Our RM Technology: Using our RM (= Refractive-index Matching) technology (see Figure 2) we fill the gap between a fiber and a photodiode with a special medium with the refractive index exactly equal to that of optical fiber core and cancel Fresnel reflection at fiber-air interface. The photodiode modules assembled with RM technology demonstrate larger responsivity (for example, 1.1 A/W for PD4B-80-P10-2G-K-RM-…) and very small polarization-dependent loss (PDL). However, there is still a significant reflectance (return loss RL = 20 to 25 dB) from the photodiode surface.

Our R50 Technology: To mitigate back-reflection, we've developed a technique where the fiber facet is cleaved or polished at an angle
(Figure 3), ensuring reflected beam does not return to the fiber. This approach significantly improves RL to >50 dB. However, it introduces a new complication: polarization-dependent variations due to the angle polishing. Angle polishing makes unequal transmission conditions for TE and TM polarizations (electric field or magnetic field in plane of angle-polished fiber interface, respectively). The transmittance difference reaches 0.5% at 8 degrees angle and increases with an angle. For a single-mode (SM) fiber in which polarization changes randomly reacting to mechanical stress, temperature changes, vibrations etc. this results in a random change of the photodiode signal.

A New Solution - LPR Technology: Our latest advancement, Low Polarization dependent loss & low back-Reflection (LPR) technology (Figure 4), strikes a perfect balance, achieving a photodiode signal variation as low as 0.1% (PDL = 0.004 dB), return loss between 30 to 35 dB, and maintaining high responsivity (e.g., 1.0 A/W at 1550 nm).