We present and analyze a method of laser-frequency stabilization via steady-state patterns of spectral holes in Eu(3+)∶Y(2)SiO(5). Three regions of spectral holes are created, spaced in frequency by the ground-state hyperfine splittings of (151)Eu(3+). The absorption pattern is shown not to degrade after days of laser-frequency stabilization. An optical frequency comparison of a laser locked to such a steady-state spectral-hole pattern with an independent cavity-stabilized laser and a Yb optical lattice clock demonstrates a spectral-hole fractional frequency instability of 1.0×10(-15)τ(-1/2) that averages to 8.5(-1.8)(+4.8)×10(-17) at τ=73 s. Residual amplitude modulation at the frequency of the rf drive applied to the fiber-coupled electro-optic modulator is reduced to less than 1×10(-6) fractional amplitude modulation at τ>1 s by an active servo. The contribution of residual amplitude modulation to the laser-frequency instability is further reduced by digital division of the transmission and incident photodetector signals to less than 1×10(-16) at τ>1 s.
We present and analyze a method of laser-frequency stabilization via steady-state patterns of spectral holes in Eu(3+)∶Y(2)SiO(5). Three regions of spectral holes are created, spaced in frequency by the ground-state hyperfine splittings of (151)Eu(3+). The absorption pattern is shown not to degrade after days of laser-frequency stabilization. An optical frequency comparison of a laser locked to such a steady-state spectral-hole pattern with an independent cavity-stabilized laser and a Yb optical lattice clock demonstrates a spectral-hole fractional frequency instability of 1.0×10(-15)τ(-1/2) that averages to 8.5(-1.8)(+4.8)×10(-17) at τ=73 s. Residual amplitude modulation at the frequency of the rf drive applied to the fiber-coupled electro-optic modulator is reduced to less than 1×10(-6) fractional amplitude modulation at τ>1 s by an active servo. The contribution of residual amplitude modulation to the laser-frequency instability is further reduced by digital division of the transmission and incident photodetector signals to less than 1×10(-16) at τ>1 s.
We present and analyze four frequency measurements designed to characterize the performance of an optical frequency reference based on spectral hole burning in Eu3+:Y2SiO5. The first frequency comparison, between a single unperturbed spectral hole and a hydrogen maser, demonstrates a fractional frequency drift rate of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Optical frequency comparisons between a pattern of spectral holes, a Fabry-P\'erot cavity, and an Al+ optical atomic clock show a short-term fractional frequency stability of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ that averages down to ${2.5}_{\ensuremath{-}0.5}^{+1.1}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ at $\ensuremath{\tau}=540\text{ }\text{ }\mathrm{s}$ (with linear frequency drift removed). Finally, spectral-hole patterns in two different Eu3+:Y2SiO5 crystals located in the same cryogenic vessel are compared, yielding a short-term stability of $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ that averages down to ${5.5}_{\ensuremath{-}0.9}^{+1.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ at $\ensuremath{\tau}=204\text{ }\text{ }\mathrm{s}$ (with quadratic frequency drift removed).
There have recently been several studies of the performance of laser frequency stabilization using spectral holes in solids, instead of an external cavity, as a frequency reference. Here an analytical theory for Pound-Drever-Hall laser frequency stabilization using spectral hole-burning is developed. The interaction between the atomic medium and the phase modulated light is described using a linearized model of the Maxwell-Bloch equations. The interplay between the carrier and modulation sidebands reveals significant differences from the case of locking to a cavity. These include a different optimum modulation index, an optimum sample absorption, and the possibility to lock the laser in an inherent linear frequency drift mode. Spectral holes in solids can be permanent or transient. For the materials normally used, the dynamics and time scales of transient holes often depend on population relaxation processes between ground state hyperfine levels. These relaxation rates can be very different for different solid state materials. We demonstrate, using radio-frequency pumping, that the hyperfine population dynamics may be controlled and tailored to give optimum frequency stabilization performance. In this way also materials with initially non-optimum performance can be used for stabilization. The theoretical predictions regarding the inherent linear frequency drift is compared to experimental data from a dye laser stabilized to a spectral hole in a Pr(3+)3+:Y(2)SiO(5) crystal.
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.
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.
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.