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Detection Techniques and Fundamental Limits in Frequency Stabilization via Spectral Hole Burning

Michael Hartman, Xiuji Lin, Benjamin Pointard, Rodolphe Le Targat, Signe Louise Seidelin, Philippe Goldner, Bess Fang, Yann Le Coq

Published Nov 8, 2023• 0 citations
OpenAlex

Abstract

This poster presents the status of the development of an ultra-frequency stable laser via a spectroscopic technique known as Spectral Hole Burning (SHB). In short, we can use a pump laser to imprint, or ’burn’, transmissive spectral lines, or ’spectral holes’, in the otherwise inhomogeneously broadened absorptive spectral sites of a Eu3+ doped Y2SiO5 crystal. We can then lock a probe laser to these narrow spectral features achieving our frequency stabilized laser. Of great concern is the frequency stability of the spectral holes, such as their sensitivity to temperature changes which we characterize here. Additionally, a challenge this experiment faces is the restriction of probe laser power to avoid damaging the spectral features. This sets a relatively high level of shot noise, limiting the detection sensitivity. Here we present our solution to overcome this difficulty by using a multi-spectral- hole probing scheme to average the probing signals of several burned spectral holes.

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arXiv:2004.01752•Optics Letters•Feb 28, 2020•21 citations
This poster presents the status of the development of an ultra-frequency stable laser via a spectroscopic technique known as Spectral Hole Burning (SHB). In short, we can use a pump laser to imprint, or ’burn’, transmissive spectral lines, or ’spectral holes’, in the otherwise inhomogeneously broadened absorptive spectral sites of a Eu3+ doped Y2SiO5 crystal. We can then lock a probe laser to these narrow spectral features achieving our frequency stabilized laser. Of great concern is the frequency stability of the spectral holes, such as their sensitivity to temperature changes which we characterize here. Additionally, a challenge this experiment faces is the restriction of probe laser power to avoid damaging the spectral features. This sets a relatively high level of shot noise, limiting the detection sensitivity. Here we present our solution to overcome this difficulty by using a multi-spectral- hole probing scheme to average the probing signals of several burned spectral holes.
Techniques in frequency stabilization of lasers to fixed-spacer optical cavities have advanced to the point where the ultimate frequency stabilities are limited by thermal noise for standard cavity configurations at room temperature. In early experiments, laser stabilization via spectral-hole burning (SHB) has been shown to be a promising alternative. In this Letter, we explore the thermal-noise limits to frequency stability in spectral holes and compile known material parameters for a typical system used in SHB experiments (Eu3+ doped Y2SiO5) to make numerical estimates for the fundamental thermal-noise induced frequency instability in spectral holes for the liquid-helium temperature and dilution temperature cases. These efforts can guide the design of future SHB experiments and clarify which important material parameters remain to be measured.
Spectral hole burning in rare earth doped crystal can provide frequency references based on the optical transition of the dopant ions [1] . The narrow spectral patterns can be used to improve laser frequency stabilization [2] , [3] and provide a new generation of ultra-stable lasers with lower thermal noise benefiting from the cryogenic environment and better quality factor of the crystal than the amorphous glass typically used in the Fabry-Perot cavity. We utilize Europium doped yttrium silicon oxide (Eu:YSO) in our experiments, which allow realization of long lived (hours or more depending on temperature) narrow spectral holes ( kHz or less) well suited for high precision experiment applications. We will describe our novel double-heterodyne detection method [4] , used to decrease the detection noise, as well as experiments assessing quantitatively the sensitivity of the spectral pattern to crystal deformation [5] , [6] , application of electric field [7] and temperature fluctuations. We currently demonstrate a state-of-the-art laser frequency stability at 1.7 × 10 − 15 at 1s [4] , limited by the latter effect, for which improvement strategies have been demonstrated elsewhere [2] and which we are currently implementing. The other effects which we have quantitatively studied are compatible with a few 10 − 18 frequency stability at 1s time scale. This work is particularly promising in the context of optical frequency lattice clocks, for which the ultra-stable laser used for probing the clock transition is a strong limiting factor of the frequency stability.
Laser frequency stabilization giving a 500-Hz Allan deviation for a 2-ms integration time with drift reduced to 7 kHz/min over several minutes was achieved at 1536 nm in the optical communication band. A continuously regenerated spectral hole in the inhomogeneously broadened 4I15/2(1)4I13/2(1){}^{4}{I}_{15/2}{(1)}^{4}{I}_{13/2}(1)4I15/2​(1)4I13/2​(1) optical absorption of an Er3+:Y2SiO5{\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ crystal was used as the short-term frequency reference, while a variation on the locking technique allowed simultaneous use of the inhomogeneously broadened absorption line as a long-term reference. The reported frequency stability was achieved without vibration isolation. Spectral hole burning frequency stabilization provides ideal laser sources for high-resolution spectroscopy, real-time optical signal processing, and a range of applications requiring ultra-narrow-band light sources or coherent detection; the time scale for stability and the compatibility with spectral hole burning devices make this technique complementary to other frequency references for laser stabilization.
We describe a spectroscopic method applicable to inhomogeneously broadened transitions. As a test system we use suitable rare-earth-doped solids in which persistent spectral holes can be burned on weakly allowed electronic transitions. These systems are of interest as frequency references for the stabilization of laser frequencies. In all experiments so far, spectral holes are burned and read out individually. Here we present a demonstration of simultaneous burning and readout of sets of spectral holes, containing up to 1024 individual spectral holes, by a comb of laser frequencies. The simultaneous readout of such a set leads to an effective compound signal that has a higher signal-to-noise ratio. Here, an improvement by a factor of 20 was obtained for a set of 512 spectral holes. We use the method to stabilize the frequency of a laser and characterize its stability.
We report on the measurement of uniaxial stress sensitivities of the spectral holes in rare earth doped crystal. This provides the necessary information for ab initio calculation of the acceleration sensitivity for a given crystal geometry, and provides the necessary base for optimizing the mounting of the crystal so as to decrease the vibration induced frequency instability. Furthermore, we report on a novel double heterodyne detection scheme which we have shown can be used to decrease the detection noise when locking a laser to the central frequency of a spectral hole, and thereby allows us to obtain a laser with fractional frequency instability of 1.7×10-15@ 1 s.
We present an experimental technique for realizing a specific absorption spectral pattern in a rare-earth-doped crystal at cryogenic temperatures. This pattern is subsequently probed on two spectral channels simultaneously, thereby producing an error signal allowing frequency locking of a laser on the said spectral pattern. Appropriate combination of the two channels leads to a substantial reduction in detection noise, paving the way to realizing an ultra-stable laser for which the detection noise can be made arbitrarily low when using multiple channels. We use this technique to realize a laser with a frequency instability of 1.7 × 1 0 − 15 at 1 s, not limited by the detection noise but by environmental perturbation of the crystal. This is comparable with the lowest instability demonstrated at 1 s to date for rare-earth-doped crystal stabilized lasers.