Coupled laser mode locking by two-quantum interaction

Wencheng Wang

Murray Sargent

Mark P. Jelonek, Won B. Roh

Existing coupled-laser theories have been extended to analyze the case of two coupled lasers supporting two modes in each cavity. A set of eight coupled equations describing the amplitudes and phases of the two modes oscillating in each cavity has been solved numerically to evaluate the operating characteristics of the coupled-laser system. Locking ranges were computed for both single- and two-mode coupled-laser systems, and it was found that the presence of the additional mode reduces the locking range of the two-mode system to approximately one half that of the single-mode coupled lasers. A number of other oddities in behavior have been found, including a mutually reinforced hole-burning minimum in intensity and a phase-dependent shift of the laser frequency from the bare-cavity frequency.

Hermann Haken, H. Sauermann

Koji Masuda

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.

E. Krüger