234 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.4: (a) Tilted-stripe and (b) buried-facet structures for nearly TW semiconductor optical
amplifiers.
an alternative scheme [10] a transparent region is inserted between the active-layer ends
and the facets [see Fig. 6.4(b)]. The optical beam spreads in this window region before
arriving at the semiconductor–air interface. The reflected beam spreads even further on
the return trip and does not couple much light into the thin active layer. Such a structure
is called buried-facet or window-facet structure and has provided reflectivities as small
as 10−^4 when used in combination with antireflection coatings.
6.2.2 Amplifier Characteristics
The amplification factor of SOAs is given by Eq. (6.2.1). Its frequency dependence
results mainly from the frequency dependence ofG(ν)when condition (6.2.4) is sat-
isfied. The measured amplifier gain exhibits ripples reflecting the effects of residual
facet reflectivities. Figure 6.5 shows the wavelength dependence of the amplifier gain
measured for a SOA with the facet reflectivities of about 4× 10 −^4. Condition (6.2.4) is
well satisfied asG
√
R 1 R 2 ≈ 0 .04 for this amplifier. Gain ripples were negligibly small
as the SOA operated in a nearly TW mode. The 3-dB amplifier bandwidth is about
70 nm because of a relatively broad gain spectrum of SOAs (see Section 3.3.1).
To discuss gain saturation, consider the peak gain and assume that it increases lin-
early with the carrier populationNas (see Section 3.3.1)
g(N)=(Γσg/V)(N−N 0 ), (6.2.5)