Semiconductor optical amplifier (SOA)
An optical amplifier based on a semiconductor gain medium driven by injection current. Provides broad gain bandwidth in a compact form factor; used for amplification, switching, and wavelength conversion.
A semiconductor optical amplifier has the same active region as a laser diode but with anti-reflection coatings (or angled facets) on both ends to suppress lasing. Input optical signal is amplified by stimulated emission as it propagates through the gain medium.
Compared to EDFAs, SOAs are:
- More compact — chip-scale rather than meters of fiber
- More broadband — gain bandwidth typically 50–100 nm (vs 35 nm for C-band EDFA)
- Faster — sub-nanosecond gain dynamics (vs millisecond for EDFA)
- Higher noise — typical NF 7–12 dB (vs 4–6 dB for EDFA)
- Polarization-dependent — gain difference of 0.5–3 dB between TE and TM (vs 0.5 dB for EDFA)
- Lower saturation power — typically +5 to +15 dBm (vs +20 dBm for EDFA)
- Higher coupling loss — fiber-to-chip coupling adds 2–6 dB on each side
The fast gain dynamics make SOAs unsuitable as in-line amplifiers in WDM systems (gain saturation by one channel reduces gain for others within nanoseconds, causing crosstalk). However, the same dynamics enable applications EDFAs cannot serve:
- Optical switches — bias on/off in nanoseconds
- Wavelength converters — four-wave mixing or cross-gain modulation
- Burst-mode amplifiers — fast power equalization for PON networks
- Signal regenerators — exploiting nonlinear gain compression
Typical SOA specifications:
| Parameter | C-band telecom SOA |
|---|---|
| Gain | 15 – 30 dB |
| 3 dB gain bandwidth | 50 – 80 nm |
| Saturation output power | +8 to +13 dBm |
| Noise figure | 7 – 10 dB |
| Polarization-dependent gain | 0.5 – 2 dB |
| Drive current | 100 – 500 mA |
| Operating range | C-band (1530–1565 nm) or O-band (1260–1360 nm) |
SOAs operating in the O-band fill a niche EDFAs cannot serve (no equivalent doped fiber amplifier exists at 1310 nm with usable performance), making them the dominant amplification choice for short-reach datacenter interconnects.