A Wavelength Converter Integrated with a Discretely Tunable Laser ...

78 6. MWL with absolute wavelength control
a)
Standard
b)
Large FPR
c)
Second PHASAR
as filter
d)
Chirping
Gain
Loss
SOA Gain curve
Δλ
order m+1 order m
FSR
order m−1 order m−2
FSR
Figure 6.3: The figures illustrate how the lasing wavelengths are selected in different
PHASAR configurations. The dotted lines denote the net gain at the peak of the pass bands.
The gain curve applies to all figures, but it is shown in a) only.
the central order by making the focal length in the output FPR dependent on the wavelength.
Consequently, only the wavelengths in the central order are focussed exactly on the output
waveguides and transmitted with low loss. Wavelength dependency of the focal length is attained
by modifying the curvature of the phase front at the output aperture, i.e. by modifying
the differential arm lengths from a linearly increasing length to a parabolic increasing length
(Fig. 6.4). The parabolic shape is described by the function and the amount of curvature
of this function is determined by the chirp factor . The arm lengths of the chirped PHASAR
become [99]
¡
¢
¦
¨§
§ ¤
¤£
λ
λ
λ
λ
(6.1)
¦¥
¦©
were denotes the effective index of the waveguides in the PHASAR arms and is a
¦
(6.2)