Page 256 - Fiber Bragg Gratings

6.1 Distributed feedback, Fabry-Perot, superstructure, and moire gratings 233

laser diode pumped fiber DFB devices producing ~1 mW single-frequency
radiation have already shown the promise of low noise of —160 dB/VHz
[9] (see Chapter 8).
Apodization of the grating leads not only to a smoothing of the side
mode structure but also to a reduction in effective length of the grating.
The reflectivity is reduced and the band-pass bandwidth increases. There
are several apodization envelopes to choose from (see Chapter 5), and
they all have a similar effect in unchirped DFB gratings: a reduced reflecti-
vity due to the reduction in the effective length of the grating. Figure 6.4
shows the effect of two such apodization envelopes: the cosine and raised
cosine (exponent = 2) for a 3-mm-long DFB with a refractive index modula-
3
tion amplitude of 1 X 10~ for both gratings.
Compared to Fig. 6.2, the side-mode structure has been smoothed
out. This effect is especially useful for concatenated fiber DFB lasers
operating at different wavelengths [10].
The shape of the band pass is not ideal for many applications; a flat
top is desirable. In order to increase the bandwidth of the band pass,
several concatenated phase steps may be used within a single grating.
The principle has been known in filter design [11], and its use was proposed

Figure 6.4: Transmission spectra for two 3-mm-long DFB gratings with co-
sine (A) and raised cosine (B) apodization. The stronger apodization (B) reduces
the effective length of the grating and therefore the reflectivity. The FWHM
bandwidths of the transmission peaks arc 0.1 nm (B) and 0.05 nm (A).