Timo worked on ultra-high sensitivity optical phase shift measurements using non-classical light at higher free spectral ranges. Optical resonators used in applications such as cavity spectroscopy or metrology require good frequency stability. This calls for accurate measurement of the phase shift of the resonator. The quality of the measurement depends on the signal-to-noise-ratio (SNR), with noise sources (e.g. stray light, seismic noise, thermal noise) as the most important cause of disturbance. Even in the absence of noise, however, SNR limitations still arise from the varying number of interacting photons on the photodiode – the shot-noise-limit. We want to demonstrate an experimental scheme that uses non-classical, “squeezed” sources of light to surpass this limit. Due to the Heisenberg uncertainty principle, squeezed light exhibits reduced noise in one quadrature component at the expense of increased noise in the other one.
We have built a squeezing-experiment with our sub-threshold optical parametric oscillator (OPO) that makes use of the existing weak pump depletion (WPD) for pump-phase locking purposes without corrupting the generated squeezed output. We have shown theoretically that there is an influence of the interaction between seed and pump field in the nonlinear medium on every possible outcoupling port of the cavity. However, by using WPD to generate an error signal for pump-phase locking, the detected squeezed states experience no degradation.
Deriving the cavity dynamics at frequencies far beyond cavity resonance show that the OPO generates squeezed light in every FSR, which is detectable with a high-bandwidth homodyne detector. We propose the idea of an enhanced spectroscopy setup consisting of up-shifted signals via a cascaded phase modulation and a Fabry-Pérot (FP) cavity with squeezed-light injection. The cascaded phase modulation leads to an amplitude modulation of the signal of interest being the sidebands of the sidebands of a phase modulation that is coincident with the FSR of the OPO and the FP cavity.
Timo simulated the cavity dynamics for squeezed states and signals at sideband frequencies at higher FSRs to show the possibility of detecting a small signal, which is masked by technical baseband noise. If the signal is up-shifted to sideband frequencies at the first FSR of the OPO and the FP cavity it will re-appear due to the absence of technical noise and by virtue of the reduced noise floor, which is lowered by the introduced squeezed light.
Timo worked on an experiment that combines squeezed light from the OPO with an artificial signal of interest, which then passes through a linear Fabry-Pérot-cavity, an optical resonator with high finesse. The transmitted signal is the desired signal with a reduced noise-floor provided by the squeezing light source and allows for a high precision phase measurement. All of the subsystems that are necessary for this enhanced spectroscopy setup were already assembled and tested. These soon-to-follow improved high-precision phase measurements in cavity spectroscopy have possible applications for cavity ring-down spectroscopy (CRDS) in the fields of optical frequency metrology or studies of light-matter interactions. The described work has been taken over by our new PhD student Jonas.
On 17.06.2016 he defended his doctoral studies on “High-precision metrology with high-frequency nonclassical light sources”.
In April 2017 Timo started a job in the automotive industry 🙂
- T. Denker, Dirk Schütte, Maximilian H. Wimmer, Trevor A. Wheatley, Elanor H. Huntington, and Michèle Heurs, “Utilizing weak pump depletion to stabilize squeezed vacuum states”, Opt. Express 23, 16517 – 16528 (2015)
- T. Denker: High-precision metrology with high-frequency nonclassical light sources, PhD thesis, Leibniz Universität Hannover (2016)