The detection section:
After the input, the light goes through the filtering section which acts like a rainbow and separates colors. In order to get valuable wavelength readings, the goal is to only send one color to the detector. The reality is that no filter can be so precise. The high end laboratory grade units can offer resolution down to 15pm.
There are many aspects to account for in order to get the best possible reading at the detector. One of them is noise. All optical test systems are driven by electronic circuitry. The amount of light left after the filtering section is very low thus the conversion of light into current then into voltage delivers low level signals thus signals very sensitive to noise. To shelter the circuits from the electrical noise, shielding is used around most detection parts. On top of shielding, a chopper motor is used to “slice” the optical signal to a 50 percent duty cycle. The detection circuitry is synchronized with the chopping motor and expects complete darkness then light then darkness and so on. When no signal is expected but some is found, it is known to be noise and can be removed from the displayed results.
Part of the complexity leading to inaccuracy of the OSA is the signal amplification and data processing to cover such large wavelength range. The detector, most times an InGaAs is the last part of the detection section. In the case of an OSA which includes visible light measurement such as the Ando AQ6315E , an InGaAs detector can’t read the 350-700nm range and a detector switching mechanism is included after the monochromator. When reading the low wavelength range, a Silicon (Si) detector is used instead of the InGaAs. Such OSA is not used in telecom measurements as they are expensive, old and the visible range is of little value in such application.
Continue reading “Let’s demystify the OSA – Part 3”
The signal input section:
There are two very different inputs found in OSAs; free space inputs and fibered inputs. The most known free space (in air) input OSA is the Ando AQ6317B. That OSA is likely the most spread OSA worldwide and was sold in mass volume during the late 1990 and early 2000 which were the boom years of CDWM and Fiber optics based telecom systems. It is fit with an FC front adaptor and can accept either FCAPC or FCPC connector holding any kind of fiber so multimode or singlemode although the fiber used will create some uncertainty in the power readings. The use of free space for the input section offers the great attribute of fiber independence but forces the topology of the monochromator to have the filtering section directly behind the input adaptor. The fibered input OSAs do not have this limitation.
Other OSAs such as the Agilent/HP 86140A and the rest of the Agilent OSA family or the smaller Anritsu MS9710C and previous Anritsu OSA generation have a fibered input which means that the front mating sleeve is directly tied to a fiber right behind it or that the connector in the front of the unit has glass in it and, when so, there is normally a collimator or grin lens right behind it. For best results, the fiber used should be matched with the signal input section fiber. Most of those OSAs were built with singlemode fiber and 62.5um Multimode fibered OSAs such as the Agilent 86141B are rare find on the used market.
So if you are buying an OSA for measurement of “yet to be defined” input signals, you might want to favor a free space OSA while if your measurement is aimed at clearly known signals, a fibered OSA matching the fiber you intend to use will do you just as well as free air. Note that the price between the two input technologies is similar so the selection of one or the other will likely not be based on price.
Continue reading “Let’s demystify the OSA – Part 2”