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.
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.
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.
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.
Several properties of Eu 3+ :Y 2 SiO 5 spectral holes are measured, to assess the suitability of broad-band hole-patterns for use as laser-frequency references. We measure frequency shifts due to magnetic fields, side-features of neighboring spectral holes and changing optical probe power. A precise calibration of a temperature insensitive point is also performed, where the temperature-induced frequency shift is canceled to first order by the pressure-induced shift from the crystal's helium-gas environment.
Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu(3+):Y(2)SiO(5) at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5±3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.
Narrow spectral holes in the absorption lines of Er doped crystals have been explored as references for frequency stabilizing external cavity diode lasers at the important 1.5 μm optical communication wavelength. Allan deviations of the beat signal between two independent stabilized lasers as low as 200 Hz over 10 ms integration time have been achieved using regenerative spectral holes in Er:Y2SiO5 and Er:KTP, while drift was reduced to ~ 7 kHz/min by incorporating the inhomogeneous absorption line as a fixed reference. During active stabilization, the transient spectral hole was continuously regenerated as hole burning balanced relaxation. In contrast, persistent spectral holes in Er:D:CaF2, with lifetimes of several weeks, provided programmable and transportable secondary frequency references that maintained sub-kilohertz stability over several seconds and enabled 6 kHz stability over 3 1.6 10 s × . The error signal was derived from the spectral hole transmission using frequency modulation spectroscopy. A servo amplifier applied fast frequency corrections to the injection current of the laser diode and slower adjustments to the piezo-driven feedback prism plate. These stabilized lasers provide ideal sources for spectral hole burning applications based on optical coherent transients, where laser stability is required over the storage time of the material. Since the lifetime of the frequency reference is exactly the material storage time, this requirement is automatically met by using our technique. This was demonstrated in Er:Y2SiO5 and successfully transferred to high-bandwidth signal processing applications. The material Er:Y2SiO5 was optimized for these applications. The I15/2 and I13/2 crystal field levels were site-selectively determined by absorption and fluorescence spectroscopy. The excited state lifetime was measured to be 11.4 ms for site 1 and 9.2 ms for site 2. Zeeman experiments and two-pulse photon echo spectroscopy as a function of magnetic field orientation were used to determine the anisotropic electronic g-values for both Er sites and established a preferred magnetic field orientation for minimizing homogeneous line broadening by spectral diffusion. The spectral diffusion was characterized by stimulated photon echo spectroscopy and successfully described with established theories. In a 0.02 atomic percent Er:Y2SiO5 crystal at B = 0.8 T and T = 1.6 K, line broadening became significant after 10 μs, increasing the homogeneous linewidth from 7.5 kHz to 75 kHz after 120 μs. Spectral diffusion, primarily caused by direct phonon driven Er spin-flips in the ground state, can be controlled to negligible levels with proper magnetic field strength and orientation, temperature, and erbium concentration. In optimizing Er:Y2SiO5, the narrowest optical resonance in any solid-state material of 73 Hz was measured.
The agile generation and control of multiple optical frequency modes combined with the realtime processing of multi-mode data provides access to experimentation in domains such as optomechanical systems, optical information processing, and multi-mode spectroscopy. The latter, specifically spectroscopy of spectral-hole burning (SHB), has motivated our development of a multi-mode heterodyne laser interferometric scheme centered around a software-defined radio platform for signal generation and processing, with development in an entirely open-source environment. A challenge to SHB is the high level of shot noise due to the laser power constraint imposed by the spectroscopic sample. Here, we have demonstrated the production, detection, and separation of multiple optical frequency modes to the benefit of optical environment sensing for realtime phase noise subtraction as well as shot noise reduction through multi-mode averaging. This has allowed us to achieve improved noise performance in low-optical-power interferometry. Although our target application is laser stabilization via SHB in cryogenic temperature rare-earth doped crystals, these techniques may be employed in a variety of different contexts.