A Comprehensive Guide to Understanding Ripple in SOAs
What is Spectral Ripple?
In the ASE (Amplified Spontaneous Emission) spectrum testing of SOAs, RSOAs and SLDs, we often observe periodic fluctuations in the spectrum, which are commonly referred to as ripple. In practical applications, the presence of such ripple is generally undesirable.
Figure 1: Example of ASE spectral ripple for a typical SOA
Differences Between ASE Spectral Ripple and Gain Ripple
During the operation of SOAs and RSOAs, the ASE spectrum is relatively easy to measure, whereas gain spectrum measurement is more complex.
Figure 2 shows a comparative test of the ASE spectrum and gain spectrum performed on the same SOA device (note: the rapid fluctuations in the gain spectrum are due to measurement errors).
It can be observed that the ripple waveform and period of the gain spectrum are consistent with those of the ASE spectrum, but the gain spectrum ripple is significantly smaller.
In Figure 2, the ASE ripple is approximately 2 dB, while the gain spectrum ripple is 0.2 dB.
Despite the large difference in magnitude, we can still use ASE spectral ripple to quickly evaluate gain spectrum ripple due to the simplicity of ASE spectrum measurement.
Figure 2: Comparison example of ASE spectral ripple and gain ripple for a typical SOA
Origin of Ripple
Ripple is directly related to facet reflectivity and gain.
The lasing condition is (GsR)2=1, where is the single-pass gain and denotes the facet reflectivity.
To avoid lasing, optical reflection back into the active region should be minimized.
If the reflectivity is not sufficiently low, ripple will appear in the gain spectrum.
As shown in the formula above, the ripple depth is a function of the front and rear facet reflectivities and the single-pass gain .
For example, when and are 10-4 and
