Light Controlled by Voltage
The electro-optic modulator is the workhorse of fiber-optic communications, converting electrical data signals into intensity-modulated light at rates exceeding 100 Gbps. The Mach-Zehnder interferometer geometry splits a laser beam into two paths, applies a voltage-dependent phase shift using the Pockels effect in lithium niobate or carrier effects in silicon, then recombines the beams. The interference between the two arms converts phase modulation into amplitude modulation.
Transfer Function
The modulator's output follows a cosine-squared transfer function: T = cos²(πV/2Vπ). This means the relationship between voltage and optical power is inherently nonlinear. For digital modulation (OOK), the nonlinearity is acceptable since we only use the ON and OFF states. For analog links — like cable TV distribution or radio-over-fiber — the modulator must be biased precisely at the quadrature point (Vπ/2) where the transfer function is most linear.
Extinction Ratio and Chirp
A perfect Mach-Zehnder modulator would completely extinguish light at the OFF state, but fabrication imperfections cause unequal splitting or different arm losses, limiting the extinction ratio to 20–35 dB in practice. Chirp — an unintended frequency modulation accompanying amplitude modulation — arises when the two arms experience unequal phase shifts. Dual-drive or push-pull configurations can achieve zero chirp, critical for long-distance transmission where chromatic dispersion converts chirp into signal distortion.
High-Speed Design
Modern modulators use travelling-wave electrodes where the microwave drive signal co-propagates with the optical wave. Velocity matching between the slow microwave (n_m ≈ 4 in LiNbO₃) and fast optical (n_o ≈ 2.2) modes is achieved through electrode geometry engineering. Thin-film lithium niobate on insulator has revolutionized the field, achieving Vπ below 2 V and bandwidth beyond 100 GHz in centimeter-scale devices.