The unique challenges of optical power measurement in telecom applications need unique solutions.
In general, optical telecom utilizes wavelengths in the near infrared region of the spectrum, typically in the range 1260nm to 1625nm, and the transport medium is usually optical fiber, though free space is also common in some applications.
Challenges in measuring and controlling these beams include:
- The very wide dynamic range in use
- The fact that the beams can be widely diverging
- The need to account for losses as the beam goes into and out of a fiber
Since the average powers in question are typically low (up to several watts), photodiode-based sensors are the usual solution. Ophir offers power sensors based on InGaAs as well as Germanium photodiode detectors, which have just the right spectral range and sensitivity range for telecom work. The PD300-IRG, for example, can measure powers as low as 10pW (or -80 dBm). These can be used to measure both free-space and fiber-coupled beams; Fiber Optic adapters are available for all the major connector types such as FC, SMA, etc.
For highly diverging beams, often one needs to use an integrating sphere to capture the entire beam. Ophir offers small Integrating Sphere sensors such as the IS-1-2W (Silicon), 3A-IS (Silicon), and 3A-IS-IRG (InGaAs), and a series of larger 6” Integrating Spheres such as the IS6-D-VIS. The 6” Integrating Spheres are available with and without built-in calibrated sensors, and they (as well as the 3A-IS series) also have auxiliary ports so you can sample light out from the sphere to additional measuring instruments (such as Pulse Characterization Sensors, spectrometers, etc.).
For low NA sources, one can use a photodiode sensor such as the PD300-IR, without needing an Integrating Sphere.
Important to note: These sensors generally include attenuating filters (both fixed and removable) to widen the useful dynamic range, and the attenuating filters are usually absorptive - which gives the sensors a roughly cosine sensitivity to incidence angle. This should be taken into account when using these sensors for measuring power of widely diverging beams. Graphs of the angle dependence are provided in the relevant sensor data sheets.
Since the beams are coupled from the fiber thru air into the receiver, there are no errors in measurement due to improper fiber mating. (Note that if in your application’s actual use, the fiber will be coupled with no loss to another element, then the reflective losses from the sensor’s fiber-to-air transmission should be added to the reading.) It also means that your measurements are unaffected by fiber facet angle and polish type (for example, the FC fiber adapter is also good for FC/APC).
In addition to the sensor, which is the input of the measurement instrument, you’ll also need an output. Ophir offers a wide range of meters, which can be used stand-alone or connected to a PC, as well as direct-to-PC interface devices. The choice will depend on the level of functionality your application requires; our “Meter Finder” comparison table can help you with that.
In some telecom applications, the beam’s spatial profile can play a critical role. For example, in DWDM systems, precise alignment of multi-fiber arrays in Add/Drop Multiplexers can make the difference between the signal reaching or not reaching its destination. Ophir offers Beam Analysis tools that can help, including for example Ophir-Photon’s NanoScan scanning slit profiler which was specifically designed to meet the requirements of the telecom industry.