On this page
Optical Space Communications — Physics and Receivers
1. Propagation and aperture coupling
For a circular, uniformly illuminated diffraction-limited aperture of diameter $D$, first-null angular radius is approximately
\[\theta_{\mathrm{null}}\approx1.22\lambda/D.\]In an ideal Fraunhofer setting, geometrical spreading, transmit/receive apertures and tracking determine received power. Atmospheric turbulence, clouds and aerosols dominate ground-to-space availability for optical links, so a vacuum-only link budget is insufficient.
Photon count: at received optical power $P_r$ and wavelength $\lambda$, incident photon rate is
\[\dot N=\frac{P_r\lambda}{hc}.\]At 1550 nm and $P_r=1$ pW, the rate is about $7.8\times10^6$ photons/s before efficiency and background losses.
2. Receiver architecture
Direct detection: telescope → optical bandpass filter → PIN/APD/SPAD or other detector → low-noise front end → clock recovery → decoding.
Coherent detection: telescope → optical local oscillator and hybrid → balanced photoreceivers → ADC/DSP → carrier/phase recovery → demodulation.
Coherent systems can measure field quadratures but require tight laser-frequency/phase control. Photon-counting systems may offer good low-flux performance yet suffer dark counts, afterpulsing and dead time.
3. Pointing, acquisition and tracking (PAT)
An optical beam may be microradian-scale. Design acquisition search, attitude knowledge, fine-steering mirrors, vibration suppression, point-ahead angle and tracking jitter together. Pointing errors reduce the overlap of the transmitted beam with the receiver aperture.
4. What to verify
Link availability; mean and fading optical power; detector NEP/dark counts; timing jitter; false acquisition; pointing-control bandwidth; BER or photon efficiency; background radiance; safety and atmospheric link effects. Optical link margin cannot be inferred from diffraction gain alone.
Advanced exercise: compare required transmitter diameter and rms pointing error for a laser link at two wavelengths and identical range, holding detector sensitivity and atmosphere assumptions explicit.