Variable optical attenuator (VOA)
A device that introduces a controllable, wavelength-flat optical loss into a signal path. Used to set absolute power levels, equalize channels in WDM systems, and protect downstream optics.
A variable optical attenuator imposes a controlled optical loss with electrically or mechanically settable magnitude. The ideal VOA has flat wavelength response, low polarization-dependent loss, and fast response over the rated attenuation range.
Implementation technologies:
| Technology | Mechanism | Typical range | Response time |
|---|---|---|---|
| MEMS (mirror/shutter) | Mechanical beam interception | 0 – 30 dB | 1 – 10 ms |
| Magneto-optic (Faraday rotation + polarizer) | Variable polarization rotation | 0 – 30 dB | s – ms |
| Liquid crystal | Polarization-dependent transmission | 0 – 20 dB | 10 – 100 ms |
| PIC thermo-optic Mach–Zehnder | Voltage-controlled phase interference | 0 – 30 dB | s |
| PIC electro-optic Mach–Zehnder | Carrier injection or depletion | 0 – 30 dB | ns – s |
| Bulk-optical absorbing wedge | Continuous neutral-density gradient | 0 – 40 dB | Mechanical, slow |
Performance specifications for telecom-grade VOAs:
| Parameter | Typical value |
|---|---|
| Attenuation range | 0 – 30 dB (above intrinsic 0.5 – 1 dB minimum loss) |
| Resolution | 0.01 – 0.1 dB |
| Wavelength flatness over C-band | 0.2 dB |
| Polarization-dependent loss (PDL) | 0.2 dB |
| Polarization mode dispersion | 0.1 ps |
| Return loss | 50 dB |
Applications:
- WDM channel equalization: per-channel VOA arrays flatten the power profile across all channels after EDFA amplification, since EDFA gain is not flat with wavelength
- Pre-emphasis at the transmitter: launch power adjustment per channel to optimize OSNR after long-haul transmission
- Detector protection: limit power into a fast photodetector or coherent receiver to avoid damage or saturation
- Lab measurements: set absolute power for receiver sensitivity testing, BER curves, etc.
- System debugging: introduce known loss to test margin and locate degradation
Distinct from a fixed optical attenuator (FOA), which provides a single specified attenuation value as a pigtailed inline device. FOAs are non-adjustable; they are simpler, cheaper, and used wherever the required attenuation is known in advance and will not change.