 Optical receiver

An optical receiver converts an incoming optical signal into an electrical signal that can be measured or processed. A typical direct-detection receiver combines a photodetector with readout electronics; optical filtering, electrical amplification and bandwidth limitation determine which input variations survive. An optical receiver can characterize a modulated beam as well as receive a communications signal.^[ref]

In a simple linear photodiode model, the signal current is $i(t)=\rho P(t)$, where $\rho$ is the spectral responsivity at the operating wavelength and $P$ the incident radiant power. A low-frequency transimpedance amplifier gives $v(t)=-R_{f}i(t)$ for its ideal inverting readout. For an original example, $\rho =0.8\mathrm{A}{\mathrm{W}}^{-1}$, mean power $20\mu \mathrm{W}$, modulation amplitude $4\mu \mathrm{W}$ and $R_f=10\mathrm{k\Omega }$ give mean photocurrent $16\mu \mathrm{A}$ and modulation amplitude $3.2\mu \mathrm{A}$. The corresponding output has mean $-0.16\mathrm{V}$ and modulation amplitude $0.032\mathrm{V}$, before offsets and frequency-response effects.

A motivating experiment measures the response to optical beating. Hale and colleagues combined two single-frequency lasers near $1319\mathrm{nm}$, with attenuators, polarization control and a beam splitter, into a single-mode fibre. Tuning their frequency difference supplied a tunable optical intensity modulation whose electrical response could be measured.^[ref]

For perfectly overlapping fields of equal polarization, the ideal incident power isA frequency difference of $100\mathrm{MHz}$ therefore produces a $10\mathrm{ns}$ intensity period; it is not the optical carrier period. Sweeping this difference probes modulation response. Measurement calibration must account for optical overlap, receiver gain, electrical losses and impedance mismatch. Spectral sensitivity at an optical wavelength and electrical modulation bandwidth are different specifications; a large direct-current output alone establishes neither low noise nor a fast receiver.