Higher precision laser measurements with less power

Squeezing improves cavity spectroscopy with possible applications in trace gas analysis

February 11, 2021

It's not just gravitational-wave detectors that need high-precision laser measurements to detect miniscule signals with high signal-to-noise ratio. Detecting trace gases by laser absorption spectroscopy faces similar difficulties. These measurements are limited by technical and quantum mechanical noise. While technical noise can be suppressed with enough effort, quantum noise is a fundamental limit. An international team led by AEI researchers has now shown in a proof-of-concept experiment how using squeezed light in cavity spectroscopy can avoid technical noise and improve the signal-to-noise ratio through lowering the quantum noise floor without increasing the laser power. Their method also paves the way for more advanced techniques in molecular spectroscopy. It could have applications in trace gas detection where the specific gas might set an upper limit for the laser power used.

Paper abstract

In this article, we present a novel spectroscopy technique that improves the signal-to-shot-noise ratio without the need to increase the laser power. Detrimental effects by technical noise sources are avoided by frequency-modulation techniques (frequency up-shifting). Superimposing the signal on non-classical states of light leads to a reduced quantum noise floor. Our method reveals in a proof-of-concept experiment small signals at Hz to kHz frequencies even below the shot noise limit. Our theoretical calculations fully support our experimental findings. The proposed technique is interesting for applications such as high-precision cavity spectroscopy, e.g., for explosive trace gas detection where the specific gas might set an upper limit for the laser power employed.

Resolved cavity length modulation signals applied at a) 20 kHz and b) 100 Hz in the squeezed noise floor. The center frequency corresponds to Ω=199.733 MHz. The peaks located at this frequency are related to the PM sideband of the EOM and mainly originate from phase quadrature to amplitude quadrature coupling via phase noise. For the squeezed case, the peaks are enhanced compared to the classical case since the phase quadrature is amplified due to anti-squeezing. In a), the peaks at a distance of 2.6 kHz to the center peak originate from a FP cavity resonance. Parameters of the signal analyzer: a) two distinct measurements: span 400 kHz: RBW 1 kHz, VBW 10 Hz, avg 50; span 50 kHz: RBW 5.1 kHz, VBW 20 Hz, avg 10 (DN and SN avg 50), b) span 500 Hz: RBW 5.1 Hz, VBW 1 Hz, avg 300.

Other Interesting Articles

Go to Editor View