Figure 1: Measuring the Incident Optical Power

Figure 2: Measuring the Reflected Optical Power

Thorlabs Lab Fact: Fiber to Free Space Return Loss Comparison Test Data

In fiber-to-free-space and free-space-to-fiber applications, Fresnel reflections typically occur at the glass to air interface because of the index of refraction discontinuity. These reflections are specified as the return loss, or signals reflected back to the light source instead of exiting the fiber. For a standard, uncoated fiber, the reflectivity (or return loss expressed as a decimal) can be calculated using the following formula, assuming normal incidence:

where R is the reflectivity, n₀ is the index of refraction of air (~1), and n_s is the index of refraction of the fiber's silica core (~1.5). Using these typical values, an uncoated fiber will experience a typical reflectivity of approximately 4%. AR-coated cables use a dielectric stack antireflective coating on the fiber tip to minimize back reflections, thus lowering return loss and increasing transmission.

Experimental Setup
AR-coated and standard SM and PM patch cables were tested for return loss in a fiber-to-free space application. First, the light source was connected to a 2x2 50:50 SM coupler, while the opposite legs of the coupler were connected to a power meter and terminator as shown in Figure 1. In this configuration the power meter measures the optical power in that leg of the coupler. Next, the cable's uncoated connector was connected to the coupler, and the coated connector was left unconnected and open to the air. The power meter was moved to the open leg on the light source side and used to measure the reflected power, as shown in Figure 2. Using this data and taking into account the insertion loss of the coupler, return loss in dB can be calculated:

Where RL is the return loss in dB or percent, P_i is the incident optical power measured in Figure 1, P_r is the reflected optical power measured in Figure 1, and IL_c is the insertion loss of the coupler. Return loss in percent (reflectivity) can also be calculated:

Results
Both the AR-coated cables and the equivalent uncoated cables were tested in the same fashion, and the results are shown in the table to the right. The improvement in return loss when using AR-coated cables is apparent.