PHOTONICS INSIGHTS
OPTICAL heterodyNE detection
ω AOM. Larger shifts can be achieved with two modulators. Alternatively, electro-optic phase or intensity modulators can generate symmetrical optical sidebands, from which a component can be selected. Further improvements can be obtained through balanced detection. The two complementary outputs of a beam splitter produce interference terms of opposite signs. Subtracting the corresponding photocurrents suppresses the continuous optical background and rejects common-mode fluctuations, particularly local-oscillator intensity noise. Meanwhile, the useful interference terms add constructively. For an ideal 50:50 beam splitter and matched photodiodes, the balanced output is therefore proportional only to the heterodyne term, thereby improving both the dynamic range and the signalto-noise ratio. This can enable operation near the shot-noise limit provided detector imbalance, electronic noise, residual intensity noise, and phase fluctuations remain sufficiently small.
Modern developments of optical heterodyne detection
The versatility of optical heterodyne detection is reflected in its wide range of applications, which enable measurements that cannot be achieved by direct intensity detection alone by providing access to the complex optical field. While the underlying physical principle has remained essentially unchanged since its introduction to radio engineering, the way it is implemented has evolved considerably alongside advances in narrow-linewidth lasers, high-speed optoelectronics, integrated photonics, digital signal processing, and quantum technologies. One of its most well-established applications is in coherent optical fibre communication [ 3 ]. In a coherent receiver, a narrow-linewidth local oscillator enables the simultaneous detection of the in-phase and quadrature components of the modulated optical signal. Combined with polarization-diverse architectures and digital signal processing, this
POUND-DREVER-HALL LASER STABILIZATION
Pound-Drever-Hall( PDH) laser frequency stabilization has been used for four decades to improve the frequency stability and phase noise of laser sources [ 6 ]. An optical resonator, typically a Fabry-Pérot cavity with highly reflective mirrors, and hence a very high Finesse, provides a narrow and highly sensitive frequency reference. An electro-optic-modulator( EOM) is used to imprint sidebands on a laser signal before coupling the light into the cavity. The phase-sensitive beat signal between the reflected carrier and sidebands is used to determine which side of the resonance the laser is tuned to. This is where the heterodyne beat between beam at resonance and its sidebands, which are not on resonance, becomes important. The beat signal demodulated with the RF signal of the EOM, generates an antisymmetric error signal with a steep slope, allowing a direct and fast feedback to the laser in order to stabilize it on the center of the cavity resonance- in practice with a factor of several thousand better than the linewidth of the resonance. PDH locking is a critical component for precision experiments, such as gravitational wave detectors based on large interferometers, and for stabilization of lasers to the mHz level for the most stable and accurate optical atomic clocks. State-of-theart turn-key systems reach fractional frequency stabilities of < 10-16 at 1 second and linewidth of below 40 mHz.
Figure 2.( a) Concept of PDH stabilization showing EOM for adding sidebands and heterodyne error signal detection after reflection from the reference cavity,( b) monocrystalline silicon cavity operated at cryogenic temperatures for achieving < 40 mHz linewidth and < 10-16 stability in a Menlo Systems product, based on the joint effort development between PTB( Germany) and JILA, Colorado( USA) [ 7 ].
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