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. 2017 Dec 20;8:2216. doi: 10.1038/s41467-017-02392-5

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — Resonance modulation spectroscopy. a Sketch of the circuit used for resonance modulation spectroscopy. On the tuning probe input (right side), a modulated signal (V _AC) at frequency f coming from the lock-in is superimposed on a tuneable DC voltage (V _DC). On the detector output (left side), the photocurrent (I _ph) produces a voltage drop on a load resistor (R _L) and its in-phase component (X-channel) and its phase (φ) at the frequency (f) are measured by the lock-in amplifier. b Comparison of a laser line recorded using two operation modes of the sensor: spectrometer mode described earlier (red dots) and resonance modulation mode (blue dots). Both measurements were performed simultaneously, by sweeping the tuning voltage and reading the photocurrent DC value (red) and the in-phase component measured by the lock-in amplifier (blue), using a load resistor R _L = 30 kΩ. Since the blue curve is proportional to the derivative of the red curve, the constant background is eliminated. The cavity is modulated with V _AC = 5 mV_pp at f = 608 Hz, and excited at a fixed laser frequency of 1322 nm (the power incident on the sample is estimated to be 125 μW); Inset: zoom-in of the zero crossing in the resonance modulation mode