Unlike single mode laser, multimode light tends to spatially spread out in which each mode has its own distribution pattern and propagates light path. Therefore, without knowing the modal distribution, the measurement results can be drastically different using different light source. For example, a collimator that focuses a single mode laser to a small spot of 0.1mm may only achieve a spot size of 8mm for a LED source. Another common example is a multimode fiber optical device measured with 1dB loss by the manufacturer can have 5dB loss using a different laser at customer site.
The solution is to use the same light source to design, fabrication, and test the device. This will result in accurate and reproduce performance. A more convenient way is to use light sources with the same quantitively measured modal field number. There is a simple way to characterize the light source that everyone can perform using regular tools. Coupled Power Ratio (CPR) power ratio is an easier metric to understand and to do the test, although it has many deficits (a better way is available but more complex to do). It simply measures the propagating loss difference between a multimode fiber and a single mode fiber. The following lists the measurement steps.
Make two patch cords ( jumper cables) of SM28 single mode (SM) fiber, respectively.
(Note: use the same MM fiber as in your applications)
2. First measurement (MM)
Connect the MM cable to a power meter as in Figure 1, and zero the meter.
(Note: Keep the patch-lead as straight as possible as could affect the modal power distribution and hence the measurement accuracy).
3. Second measurement (SM)
Leave the MM lead connected to the light source, and connect the MM cable to the SM cable on one end and connect the other end of the SM to a power meter as in Figure 2. Make a 30 mm diameter loop, which acts as a mode filter. Measure & record the power output (L2) of the MM to SM pigtail combination.
1. CPR coefficient calculation
As meter was zeroed the CPR = – L2 (dB)
(Note: Otherwise the CPR coefficient is calculated as: CPR = L1 – L2 )