Electro-absorption modulator (EAM)
An optical intensity modulator that uses voltage-controlled absorption in a semiconductor (typically via the quantum-confined Stark effect) to attenuate transmitted light. Compact and used in low-power telecom transmitters.
An electro-absorption modulator uses the quantum-confined Stark effect (QCSE) in a semiconductor multiple quantum well (MQW) to shift the absorption edge under applied bias. At a properly chosen wavelength near the absorption edge, modest voltage swings (1–3 V) change the absorption coefficient enough to modulate transmitted light intensity by 10–20 dB.
EAM characteristics versus Mach–Zehnder modulators:
| Property | EAM | MZM (LNOI) |
|---|---|---|
| Drive voltage | 1 – 3 V | 2 – 5 V (single-arm), 1 – 3 V (push-pull) |
| Size | 200 μm | 5 – 20 mm |
| Insertion loss | 3 – 7 dB | 3 – 6 dB |
| Bandwidth | 30 – 80 GHz | 30 – 100 GHz |
| Extinction ratio | 10 – 20 dB | 20 – 30 dB |
| Wavelength operating range | 30 nm (band-edge constrained) | 100 nm |
| Chirp | Nonzero, voltage-dependent | 0 (LNOI push-pull) |
| Power handling | +5 dBm | +15 dBm |
EAMs are intrinsically chirped because changes in absorption (imaginary index) are accompanied by changes in real index (Kramers–Kronig). The chirp degrades performance in dispersion-limited links. However, the chirp parameter can be engineered to negative values, where chirp combined with positive chromatic dispersion produces partial pulse compression. This is used to extend transmission distance at 10 Gb/s and below.
Common deployments:
- Externally modulated lasers (EMLs): DFB laser monolithically integrated with EAM on the same InP chip. Standard short- and medium-reach transmitter, from 10G up to 100G and 200G per lane.
- InP integrated modulators: EAMs in larger PICs alongside lasers, splitters, and detectors
- Electro-absorption modulated photonic integrated circuits: for switching, gating, and pulse generation
EAMs operate at a specific wavelength range set by the bandgap of the MQW. Outside this range, either no modulation (longer wavelengths) or excessive insertion loss (shorter wavelengths) results. Each EML SKU is therefore designed for a specific WDM channel; the wavelength is not field-tunable.
References: S. L. Chuang, Physics of Photonic Devices, 2nd ed. (Wiley, 2009). L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012).