Page 315 - Fiber Bragg Gratings

292 Chapter 6 Fiber Grating Band-pass Filters

A weak, 20-mm-long grating (A g = 547 nm) with a chirp in the period
of 1.92 nm (±0.96 nm) was used to tap ~5% of the light over a 35-nm
bandwidth. An index-matched prism was placed in contact with a fiber
of square cladding cross-section to promote good adhesion, to direct the
light to the photodiode array.
The effective focal length was 160 mm. The resolution of the spectrom-
eter was demonstrated to be 0.12 nm with a measured bandwidth of 14
nm, an insertion loss of <1 dB, a wavelength accuracy of ±0.03 nm, a
power level accuracy of <0.25 dB, and a dynamic range of 35 dB. The
spectrum of two resolved channels separated by 50, 100, and 200 GHz
using this spectrometer is shown in Fig. 6.48 [88].
While this device is an excellent example of a band-pass filter, which
is also tunable, the applications of the STG and the LPG in this area

Figure 6.48: The spectrum of two channels as the detuning is increased from
50 to 100 to 200 GHz (courtesy T. Strasser, from Wagener J. L., Strasser T. A.,
Pedrazzani J. R., and DeMarco J., "Fiber grating optical spectrum analyzer tap,"
In Tech. Digest ofECOC'97, Publn. No. 448, Vol. 5, pp. 65-68. © IEE 1997 Ref.
[88]).