The realization of a precision frequency standard requires a laser system with a narrow linewidth and precise tunability of the output frequency, an atomic system which provides a narrow, stable, and symmetric reference line, and a spectroscopic method which gives an error signal for frequency stabilization with a sufficient signal-to-noise ratio. In this thesis we have designed and built single-mode Nd:YAG laser systems for optical frequency standards, and performed theoretical investigations of a two-level atomic system interacting with a polychromatic field as fundamental studies on the spectroscopic method.
We have proposed two designs of unidirectional single-mode lasers, i.e., a Nd:YAG laser with a planar semimonolithic ring cavity, and an intracavity second harmonic generator with a nonplanar ring cavity. The frequency of commercial monolithic Nd:YAG lasers is tuned mainly by thermal expansion, which is an inherently slow method of frequency tuning and a main obstacle to the realization of an optical frequency standard using the commercial monolithic Nd:YAG laser. Frequency tuning of the proposed laser systems can be directly achieved by changing the position of one mirror mounted on a piezoelectric transducer.
The unidirectional single-mode Nd:YAG laser with the planar semimonolithic ring cavity consists of a laser active medium with an applied magnetic field, a crystal quartz plate, and an output coupling mirror, which form an optical diode by acting as a Faraday rotator, a reciprocal polarization rotator, and a partial polarizer, respectively. An eigenpolarization theory for the cavity configuration is presented and losses for the eigenmodes are calculated in order to estimate appropriate design parameters for unidirectional operation. A pump-limited single-mode output power of 155 mW at 1064 nm and a slope efficiency of 17 % are obtained when the laser is pumped by a 1.2-W diode laser at 809 nm. A laser linewidth of less than 100 kHz is inferred from a beat note frequency spectrum between two identical laser systems and continuous tuning over greater than 2 GHz is observed.
The intracavity second harmonic generator with the nonplanar ring cavity consists of a Brewster-angled Nd:YAG block placed in a magnetic field, a KTP crystal, and two spherical mirrors. In this design the Nd:YAG block acts as both a nonreciprocal polarization rotator and a partial polarizer, and the nonplanar portion of the ring cavity, which is formed by a relative twist angle between the Brewster-angled end surfaces of the Nd:YAG block, serves as a reciprocal polarization rotator. An eigenpolarization theory for the cavity configuration is presented and suitable values of the relative twist angle for unidirectional operation are estimated. A single-mode output power of 22 mW at 532 nm and an optical to optical conversion efficiency of 1.8 % are obtained with a 1.2-W diode laser at 809 nm.
Theoretical investigations on the subjects of analytic solutions of the optical Bloch equations, physical interpretation of SNR (Super-Narrow Resonance), and dressed states of a two-level atom driven by a trichromatic field are performed. The analytic solutions of the optical Bloch equations are used to obtain the polarization spectrum of the atom. The spectrum is found to consist of a series of discrete peaks or dips (SNR's) superimposed on the continuous part of the spectrum. Physical interpretation of the SNR's is given using a perturbation approach for the simplest case where the SNR's are observed. Dressed states of a two-level atom interacting with a trichromatic field which corresponds to an amplitude modulated driving field resonant with the atomic transition are obtained by a fully quantum-mechanical treatment of the system. Some features of the dressed states are discussed in detail analytically and numerically. The dressed states are found to converge to the well-known two limiting cases of a resonant monochromatic and a resonant bichromatic driving fields.
