Quantum-Noise-Limited Optical Frequency Comb Spectroscopy

Abstract

We achieve a quantum-noise-limited absorption sensitivity of 1.7×10-12 cm -1per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fastscanning Fourier transform spectrometer with an ultra-low-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380MHz are acquired within a few seconds. The measured absorption lineshapes are in excellent agreement with theoretical predictions.PACS numbers: 42.62.Fi, 42.50.Lc, 85.60.Gz, 42.79.Gn Continuous wave (cw) laser absorption spectroscopy is a well-established technique for quantitative measurements of various constituents in gas phase. The sensitivity of the technique can be improved either by employing modulation techniques, which shift the signal to audio [1, 2] or radio [3] frequencies, where the technical noise is reduced, or by implementing an external high-finesse enhancement cavity [4, 5], which increases the interaction length of the light with the sample, and thus the absorption signal. Successful combination of these two approaches has led to impressive shot-noise-limited absorption sensitivities [6]. The main constraint of cavity-2 enhanced cw techniques, however, is their inability to measure broadband spectra in short acquisition times. The optical frequency combs extend the benefits of cw-laser-based techniques to thousands of laser lines and remove the bandwidth limitation [7, 8]. The discrete spectrum of equidistant comb lines matches that of an external enhancement cavity, enabling efficient coupling of the comb light into the cavity [9,10]. Several detection schemes have been proposed for cavity-enhanced direct frequency comb spectroscopy (CE-DFCS), but none was able to suppress the technical noise down to the quantum limit. In most previous realizations of the technique, the light transmitted through the cavity has been detected using a dispersive element and a multidetector array [11][12][13]. With this approach the laser frequency-to-amplitude (FM-to-AM) noise conversion caused by the narrow cavity modes can be efficiently reduced, by measuring either the cavity ringdown time [11] or integrated cavity output [12]. However, the sensitivity is limited by the technical noise in the detector array because of the low optical power in each spectrally resolved element. An alternative is to use Fourier transform spectrometry (FTS), either with a mechanical Michelson interferometer [14,15] or with the dual comb approach [16]. The advantage of using FTS for CE-DFCS is that the spectral bandwidth of the detection system is limited only by the cavity dispersion, and not by the number of detection elements [17]. However, a photodetector and data acquisition board with a large dynamic range are necessary to obtain high signal-to-noise ratios [18]. Moreover, the use of FTS requires constant transmission through the cavity and thus the comb to b...

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared application to trace detection of hydrogen peroxide

“…Due to the similarity of the cavity mode structure to the OFC spectrum, the comb can be efficiently coupled into the optical cavity by proper control of the two radio frequencies f rep and f ceo [24]. Several detection schemes for cavity-enhanced (CE-) DFCS have been implemented [35][36][37], based on one- [38,39] or two-dimensional dispersion elements [36], on the Vernier method [40], or on Fourier transform spectroscopy (FTS) [41,42]. When dispersion elements are used to resolve the spectrum transmitted through the cavity, the frequency-to-amplitude noise conversion by the narrow cavity modes can be circumvented by measurement of cavity ringdown [39] or by fast modulation of either the comb mode frequencies or the cavity resonance frequencies while detecting the averaged cavity output [32,36,38].…”

“…This approach cannot be implemented in combination with FTS, which requires constant transmission through the cavity. The OFC-based FTS spectrometers are based either on a Michelson interferometer with a mechanical translation stage [37,[42][43][44] or on dual-comb spectroscopy [41,[45][46][47], which utilizes two femtosecond lasers with slightly different repetition rates, thus mimicking the effect of a fast-scanning delay stage. In this work we implement a recently developed detection scheme that efficiently suppresses the amplitude noise by combining an OFC tightly locked to a high-finesse cavity and a fastscanning FTS with autobalancing detector, allowing hours of uninterrupted operation [42].…”

Absolute absorption line-shape measurements at the shot-noise limit

“…This compromise is evident in the other spectroscopic techniques aimed at achieving shotnoise-limited detection such as frequency modulation (FM) spectroscopy or the extraordinarily sensitive noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) [21]. The exploitation of enhancement cavities to improve sensitivity will result in the measurement being sensitive to a mixture of absorption and dispersion [19,21]. The resulting loss in accuracy is not important if the ultimate goal is to achieve sensitivity.…”

“…[18]. This device can be used to reach the shot-noise limit in the presence of amplitude noise up to 70 dB larger, and was recently deployed in conjunction with an optical frequency comb to achieve detection sensitivity at the shot-noise limit [19]. However, it is important to realize that taking this approach often comes at the expense of linearity, and hence accuracy.…”

A digitally generated ultrafine optical frequency comb for spectral measurements with 0.01-pm resolution and 0.7-µs response time

“…For example, individual comb components can be tuned to scan over the frequency gap. [16][17][18][19][20][21] Other methods include spatially separating the comb lines using gratings, 21 Fourier transform spectroscopy of a single OFC 22 and dual-mode spectroscopy using two OFCs. 23,24 Nevertheless, few efforts have been made to decrease the comb spacing to improve the spectral resolution.…”