Saturation spectroscopy of Iodine is used to lock the frequency of a Coherent 699-21 Ring Dye laser. Without an external reference the laser frequency drifts several tens of MHz per hour. With the system presented in this thesis the drift has been reduced to a maximum of ±lMHz. Theoretical description of saturation spectroscopy and hole burning is given and the experiments are presented. In preparation for future experiments in quantum computing hole burning in rare-earth-ion-doped crystals and broadening of the hole by excitation of ions at different intervals have also been studied in this master's thesis.
Saturation spectroscopy of Iodine is used to lock the frequency of a Coherent 699-21 Ring Dye laser. Without an external reference the laser frequency drifts several tens of MHz per hour. With the system presented in this thesis the drift has been reduced to a maximum of ±lMHz. Theoretical description of saturation spectroscopy and hole burning is given and the experiments are presented. In preparation for future experiments in quantum computing hole burning in rare-earth-ion-doped crystals and broadening of the hole by excitation of ions at different intervals have also been studied in this master's thesis.
In this thesis the application of slow light effects to the area of laser stabilisation is investigated. The frequency stability of lasers locked to external cavities is limited by the stability of the modes in the reference cavity. By using dispersive slow light effects the effective length of a cavity can be increased by several orders of magnitude, without a proportional increase in the sensitivity to fluctuations in cavity length, thereby yielding a net increase in stability. The effects are achieved through spectral hole burning in cryogenically cooled rare-earth ion doped crystals. Two sets of experiments are carried out, the first focusing on hole burning and the second focusing laser locking. Linewidths and frequency drift of the cavity spectrum are studied. Feedback control using an error signal generated from the slow light cavity is demonstrated, but successfully locking the laser is still yet to be achieved.
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.
Laser induced hole burning in optical absorption bands of low temperature solids has been investigated with emphasis on mechanisms for population hole burning, the interactions with the environment which limit the hole widths and the use of hole burning for ultra high resolution spectroscopy. Hole burning was observed in a wide range of solids including organic and organometallic molecules in crystals and rare earth ions in crystals and glasses. The mechanisms for hole burning also varied widely and a new mechanism involving optically induced nuclear spin flips of near neighbor nuclei was discovered. Examples of hole burning in new classes of materials are presented here for the first time, namely inorganic glasses (silicate glasses containing Nd(3+), Pr(3+) or Eu(3+)) and undoped stoichiometric compounds (e.g., EuP5O14). Hole recovery was used to determine nuclear spin lattice relaxation rates in praseodymium doped crystals. For allowed optical transitions (e.g., singlet states of zinc porphyrin, dibromoindigo and color centers in alkali halides) the hole widths at 2K are limited by population decay to approximately 50 MHz. For longer lived optical levels in crystalline materials, e.g., those containing rare earth ions, the hole widths are narrower (less than 10 MHz) and are limited by dynamical interactions with surrounding nuclei. In amorphous materials such as europium doped silicate glass, hole widths are much broader, being limited by low frequency tunneling motions. Hole burning measurements were complemented by time domain coherent transient (photon echo, and optical free decay) measurements of homogeneous linewidths which enabled us to measure optical resonances as narrow as 760 Hz in Eu(3+):Y2O3.
Solids containing rare earth (RE) ions have attracted spectroscopists' attention since the beginning of this century.In the last 30 years, their extremely narrow spectral features in combination with the use of non linear spectroscopy have served to sensitively measure ion-solid interactions and to progressively observe narrower homogeneous linewidths.The potential for using RE ion doped crystals as optical storage materials and processors has lately been a driving force in their investigations, Here, the ratio of inhomogeneous to homogeneous linewidth is regarded as an adequate measure of storage density and can become as big as 10 7 at cryogenic temperatures.Therefore, the analysis of line broadening is of fundamental as well as applied interest.This work is concerned with the investigation of interactions of RE ions with their environment.By means of a laser with a kilohertz resolution and non-linear spectroscopy, spectral dynamics as a function of intrinsic, external electric field, or light-pulse induced perturbations are analyzed on different time scales.In the first part, the theoretical background for the spectroscopy of RE ion doped crystals is outlined.The energy levels are presented and it is shown that due to their electronic properties RE ions have spectrally narrow electronic transitions, sometimes in the sub-kHz regime.The investigations of these narrow spectral features on the seconds to minutes time scale require very stable tunable laser sources.At present, such devices are not commercially available.In the second part, the performance of ahorne built laser system is characterized using persistent spectral holes, The results show that frequency selective materials can be used as powerful tools for the high resolution Vlll Abstract spectral analysis of laser sources.Complementary to that, they give a measure for the long term evolution of the spectral holes for the particular hole burning substance.In the third part, the effect of an external electric field on RE ion doped crystals is investigated.It is shown that for these systems the electric field induced frequency shifts are fully reversible.A general theory of Stark splittings of RE doped crystals is introduced and two methods are presented which allow for the determination of the magnitude and orientation of the electric dipole moment differences with sub-degree accuracy.Finally in the last part, spectral line broadening mechanisms of RE ion doped crystals are discussed and investigated by means of two-pulse photon echoes.Through variation of the experimental parameters, a discrimination between the different contributions is partly achieved and characteristic properties for different dephasing mechanisms are presented.In combination with the measurements from the third part, quantitative predictions of the electric dipole-dipole induced broadening are carried out, which are compared to the experimental results,
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.
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.