Atomic resonances had long been used as a frequency reference. We present resonant tuning of coherent population trapping in 87Rb with an intracavity Fabry-Perot etalon in a mode-locked laser.
Resonant interactions of a mode-locked pulse train with intracavity samples, namely rubidium-87 (Rb-87) vapor and Fabry-Perot etalon, placed inside a laser cavity are studied in the light of developing ultra-sensitive laser sensors to measure a small magnetic field and a minute change of index of refraction of a sample material, respectively. A Rb-87 vapor provides an opportunity for a compact high-sensitivity atomic magnetometer due to its accessibility by the standard laser sources and to the large ensemble magnetization. By employing the ultrasensitive interferometric technique utilizing the intracavity properties of a mode-locked laser, the performance of the Rb-87 magnetometer can be further improved. The fundamental properties of coherent interaction between a mode-locked pulse train and a Rb-87 vapor are studied in numerical calculations of 3 x 3 density matrix equations and the reduced wave equation, which are then examined in experiments. In particular, a coherent dark-state is created by the pulse train and is further enhanced by means of spectral shaping or polarization modulation of the excitation pulse train. Experiments performed inside a laser cavity show that the atomic coherence is still preserved due to the coherent nature of interaction between the Rb-87 vapor and the ultrashort pulses occurred within a short time scale compared to the atomic relaxation times, which results in nonlinear propagation of the pulses as well as an observation of the dark-line resonance inside the laser cavity. A Fabry-Perot etalon is a type of optical cavity and serves as a tuning element of the frequency of cw-lasers. By inserting a Fabry-Perot etalon inside a mode-locked laser, the cavity resonance modes are modified due to a coupling between the two cavities, which leads to unique temporal and spectral characteristics of the resultant pulse train and its frequency comb. Both the temporal and spectral properties of the pulse train are studied in detail in experiments as well as in numerical calculations. In particular, it will be shown that the coupling between low frequency modes from the laser repetition rate and high frequency modes from the Fabry-Perot pulse train can be exploited for a new interferometric technique to measure a minute change in the index of refraction of the etalon.
We investigate the use of phase modulation to generate an error signal for frequency stabilization to a reference atomic transition. The study focuses on the high-frequency modulation regime. Our theoretical analysis shows that, in the case of coherent population trapping, the maximal slope of the error signal remains constant as the modulation frequency increases, whereas it steadily decreases for other types of resonances. Experimental results with 87Rb atoms confirm this behavior under comparison with the double radio-optical resonance.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.