Differences Between SOA Gain Spectrum and ASE Spectrum
Introduction
In the promotion of SOA products, customers frequently raise the following questions:
- "Why does the measured center wavelength of the C-band SOA fall around 1500 nm?"
- "Why is the center wavelength of the 1310 nm SOA at 1270 nm?"
- "Why is the ripple of our SOA higher than the nominal value listed in the datasheets?"
These confusions largely stem from a misunderstanding between the gain spectrum and the ASE spectrum of an SOA.
1. Definitions of SOA Gain Spectrum and ASE Spectrum
The gain spectrum of an SOA refers to a plot illustrating the gain characteristics versus wavelength under the condition of input optical signal injection.
For conventional SOAs (without integrated optical isolators), the optical path is reversible. The gain spectrum can be measured by interchanging the input and output ports.
ASE (Amplified Spontaneous Emission) describes the spectral density versus wavelength curve generated by the spontaneous emission amplified within the SOA in the absence of an input signal.
ASE emission in an SOA propagates bidirectionally; thus, ASE exists at both the input and output sides of the chip. If the SOA waveguide is symmetric, the material is homogeneous, and the current density is uniform, the ASE spectra measured at the input and output sides should be essentially consistent.
Figure 1 presents a comparison of the ASE spectrum and gain spectrum for a specific SOA chip.
Figure 1. Comparison of ASE spectrum and gain spectrum of a specific SOA
2. Comparison of ASE Spectrum and Gain Spectrum of SOAs
The ASE spectrum and gain spectrum of an SOA are fundamentally different. To characterize spectral performance, we typically evaluate three key parameters: center wavelength, spectral width, and ripple. Below, we compare the similarities and differences between the ASE spectrum and gain spectrum across these three dimensions.
2.1 Center Wavelength Comparison
As shown in Figure 1, there is a clear difference between the center wavelengths of the SOA’s ASE spectrum and gain spectrum. The center wavelength of the ASE spectrum is approximately 1515 nm, while that of the gain spectrum is 1535 nm—a difference of about 20 nm.
The center wavelength of the ASE spectrum is primarily affected by bias current and temperature, whereas the gain spectrum is additionally influenced by input optical power. To ensure comparability, all comparisons in this paper are performed under identical current and temperature conditions.
2.2 Spectral Width Differences
Compared with the ASE spectrum, the gain spectrum of an SOA typically exhibits a wider spectral width. This is because gain saturation readily occurs in high-gain regions, inducing a gain compression effect that flattens the gain profile across the spectral range. As the input optical power increases, this gain compression becomes more pronounced, further broadening and smoothing the gain spectrum. In practice, the spectral width of the gain spectrum is usually evaluated at an input optical power of -10 dBm.
2.3 Ripple Differences
Figure 2 presents a comparison of the ripple characteristics of the same SOA device under ASE spectrum and gain spectrum conditions (Note: High-frequency fluctuations in the gain spectrum are primarily due to measurement errors).
It can be observed that the ripples of the two spectra are nearly identical in waveform shape and period, but the ripple amplitude of the gain spectrum is significantly lower than that of the ASE spectrum. For example, the ripple of the ASE spectrum in the figure is approximately 2 dB, while that of the gain spectrum is only 0.2 dB.
Despite this notable difference in ripple amplitude, the ripple of the ASE spectrum remains a valid reference indicator for evaluating the ripple of the gain spectrum, given the simplicity and convenience of ASE spectrum testing.
Figure 2 Ripple Comparison Between the ASE Spectrum and Gain Spectrum of a Specific SOA
3. Case Study: Direct Comparison
We use the C+L-band SOA from our company (Janhoo) as an example to directly compare its ASE spectrum and gain spectrum.
Figure 3 C+L-band SOA from our company (Janhoo) as an example to directly compare its ASE spectrum and gain spectrum
| Parameter | ASE Spectrum | Gain Spectrum (@Pin = -10 dBm) |
|---|---|---|
| Center Wavelength | 1545 nm | 1579 nm |
| Spectral Width | 58 nm | 121 nm |
| Ripple | 0.5 dB | 0.1 dB |
4. Conclusion
In summary, significant differences exist between the gain spectrum and ASE spectrum of an SOA in terms of key parameters such as center wavelength, spectral width, and ripple, while a certain degree of correlation between them is still observed. In comparison, the gain spectrum typically exhibits a longer center wavelength, a broader spectral width, and a notably smaller ripple amplitude compared to the ASE spectrum.
Given the simplicity of the measurement method for the ASE spectrum, although it cannot fully replace the gain spectrum, it is commonly used for the preliminary evaluation of SOA gain performance in practical applications.
Tianjin Janhoo Optoelectronics Co., Ltd.
We are a high-tech enterprise specializing in the R&D and manufacturing of chinese Semiconductor Optical Amplifiers (SOAs). Currently, the company has launched a wide range of SOA products (including models at 850nm, 1060nm, 1270nm, 1310nm, 1550nm, and 1625nm) as well as Gain Chip and RSOA products (850nm, 1310nm, 1550nm).
The company has established a Class 10,000 cleanroom laboratory equipped with comprehensive optoelectronic chip production, processing, testing, and packaging equipment, and possesses the capability for hybrid integrated micro-packaging of optoelectronic chips.
At present, the company is engaged in the R&D of NLL/ECL + SOA hybrid integrated devices and high-power SOA devices, and also provides external services including testing, packaging, and processing for various optoelectronic devices.
