250 Fiber Grating Band-Pass Filters are routed to A2, except for the channel at l j , which is dropped at C1 and rein- serted from C2, to be routed to A2. The inclusion of the "isolating" grating reduces the leakage between A1 and B1 from ­26 to ­71 dB. The improvement is evident in the add­drop function, by a reduction in the power penalty of 1 dB due to the elimination of coherent beat noise. The use of higher reflectivity grat- ings in the MZIs should eliminate the need for the "isolator" grating, although short-wavelength radiation loss will remain a problem. 6.5 THE OPTICAL CIRCULATOR-BASED OADM The optical circulator has become extremely important for applications with fiber gratings. While the use of a fiber grating with a circulator is an obvious method for converting the band stop to a band-pass filter, it is worthwhile to consider the benefits of such a configuration. The first reported use was as an ASE filter for an erbium amplifier [59]. The amplified signal is routed to the input of a circulator and reflected by a narrow-bandwidth grating in the second port. The grating filters the amplified spontaneous emission from the amplifier and routes the signal to the output port. The reduction in the out-of-band spon- taneous emission can be considerable, but is determined by the quality of the reflectivity spectrum of the grating. Optical circulators have an insertion loss of only $1 dB, turning this very simple device into a superb band-pass filter. Several gratings with different Bragg wavelengths may be cascaded to form a multiple band-pass filter. The addition of a second circulator leads to a simple method of performing an optical circulator based add-drop multiplexing (OC- ADM) function using gratings and is shown in Fig. 6.32. Channels injected at the "input" port are reflected by the gratings in between the two circulators OPTICAL CIRCULATORS INPUT l 1, l 2, l 3 Bragg gratings at l 1..................... l n OUTPUT l 1, l 2, l 3 DROP l 1.... INSERT l 1 Figure 6.32 An OC-ADM using an all optical circulator. This device allows several channels to be dropped or added according to the number of fiber gratings between the circulators. The signals at the Bragg wavelengths are reflected and appear at the drop port, while the same grat- ings may be used to insert the same channels for wavelength reuse from the insert port.