Draw Tower Gratings have a reflection which is as standard specified larger than 15% (the typical reflectivity value is between 20% and 25%). This is about 6 dB lower compared to table top written FBGs, which have a reflectivity of >90%. The question that is frequently asked is “does the lower reflectivity of DTG® s cause problems in the measurement set-up?” In practice, for almost all commercial FBG measurement devices no problems are experienced for static nor for high speed measurement devices:
STATIC MEASUREMENT DEVICE
Static measurement devices have in general a large dynamic range (>30dB). The figure below shows a FBG chain spectrum measured with a high reflective (>90%), low reflective (25%) and very low reflective (4%) FBG. As can be observed, due to the dynamic range of the device, even the 4% reflective grating can still be measured with a Signal to Noise ratio of almost 13 dB, which is well above the safe detection limit of 10 dB.
DYNAMIC MEASUREMENT DEVICE
Dynamic measurement devices in general have a lower dynamic range (< 25 dB). However, these systems can typically be adjusted to modify either the laser light power (for tunable laser based systems) or the integration time (for photo-diode based spectrometers). Both parameters are for most measurement devices tunable, in order to optimize the Signal to Noise ratio of the measured configuration set-up.
The lower reflectivity of DTG® s can therefore easily be compensated by increasing one of these parameters. The next figure. shows a spectrum of a standard DTG® (Reflectivity about 15%), High reflective FBG (>90% reflectivity) and a low reflective FBG (reflectivity about 3%), all of them measured with a Micron Optics SM130 device.
The SM130 is a dynamic system based on a tunable laser. The light power of the laser was optimised for each grating type using the gain adjustment in the software. The higher the gain setting, the higher the optical power supplied to the fiber for detecting the FBG wavelength.
As can be observed, for high reflective FBGs, the gain had to be set to 1 dB in order to achieve optimal Signal to Noise ratio. A higher gain value would saturate the detection system of the measurement device.
For the DTG® , the optimal gain was 11.5 dB. Also here, the use of a higher gain value would saturate the detectors and make the readings unusable. Knowing that the maximum gain of this device is 20 dB, it is clear that there is still room for dealing with additional losses introduced by the network (e.g. bad connectors, longer fiber lengths, optical splitters…).
It can also be seen that even the low reflective FBG could easily become measured when using a gain of 18.5 dB.
Finally it should be noted that Time Domain Multiplexed (TDM) measurement devices are using even lower reflectivity than those obtained with standard DTG®s. TDM systems have the advantage that they can interrogate several FBGs with the same wavelength (> 100 FBGs on one single fiber).
However, in order to avoid ‘shadowing’ the FBGs at the end of the fiber line with those at the beginning , the reflectivity of the FBG typically needs to be between 1 and 5%. Excellent results can also be obtained here with respect to measurement stability.
In conclusion, it can be stated that in order to have good detection of an FBG, it is not the absolute reflectivity of the FBG that is important but it is rather the Signal to Noise ratio with which the FBG is measured that matters. For the vast majority of commercially available measurement devices, the S/N ratio is more than sufficient to detect even low reflective gratings below 4% reflectivity.