Optical Frequency Fluctuations Research Articles

Doppler narrowing, Zeeman and laser beam-shape effects in Λ-type electromagnetically induced transparency on the 85Rb D2 line in a vapor cell

We study Λ-type Electromagnetically Induced Transparency (EIT) on the Rb D2 transition in a buffer-gas-free thermal vapor cell without anti-relaxation coating. Experimental data show well-resolved features due to velocity-selective optical pumping and one EIT resonance. The Zeeman splitting of the EIT line in magnetic fields up to 12 Gauss is investigated. One Zeeman component is free of the first-order shift and its second-order shift agrees well with theory. The full width at half maximum (FWHM) of this magnetic-field-insensitive EIT resonance is reduced due to Doppler narrowing, scales linearly in Rabi frequency over the range studied, and reaches about 100 kHz at the lowest powers. These observations agree with an analytic model for a Doppler-broadened medium developed in (Javan et al 2002 Phys. Rev. A 66 013805; Lee et al 2003 Appl. Phys. B, Lasers Opt. (Germany) B 76, 33–9; Taichenachev et al 2000 JETP Lett. 72, 119). Numerical simulation using the Lindblad equation reveals that the transverse laser intensity distribution and two Λ-EIT systems must be included to fully account for the measured line width and line shape of the signals. Ground-state decoherence, caused by effects that include residual optical frequency fluctuations, atom-wall and trace-gas collisions, is discussed.

Nonlinear cavity optomechanics with nanomechanical thermal fluctuations

Although the interaction between light and motion in cavity optomechanical systems is inherently nonlinear, experimental demonstrations to date have allowed a linearized description in all except highly driven cases. Here, we demonstrate a nanoscale optomechanical system in which the interaction between light and motion is so large (single-photon cooperativity C0≈103) that thermal motion induces optical frequency fluctuations larger than the intrinsic optical linewidth. The system thereby operates in a fully nonlinear regime, which pronouncedly impacts the optical response, displacement measurement and radiation pressure backaction. Specifically, we measure an apparent optical linewidth that is dominated by thermo-mechanically induced frequency fluctuations over a wide temperature range, and show that in this regime thermal displacement measurements cannot be described by conventional analytical models. We perform a proof-of-concept demonstration of exploiting the nonlinearity to conduct sensitive quadratic readout of nanomechanical displacement. Finally, we explore how backaction in this regime affects the mechanical fluctuation spectra.

Impact of local oscillator frequency noise on coherent optical systems with electronic dispersion compensation.

A theoretical investigation of the equalization-enhanced phase noise (EEPN) and its mitigation is presented. We show with a frequency domain analysis that the EEPN results from the non-linear inter-mixing between the sidebands of the dispersed signal and the noise sidebands of the local oscillator. It is further shown and validated with system simulations that the transmission penalty is mainly due to the slow optical frequency fluctuations of the local oscillator. Hence, elimination of the frequency noise below a certain cut-off frequency significantly reduces the transmission penalty, even when frequency noise would otherwise cause an error floor. The required cut-off frequency increases linearly with the white frequency noise level and hence the linewidth of the local oscillator laser, but is virtually independent of the symbol rate and the accumulated dispersion.

Repetition rate spectroscopy of the dark line resonance in rubidium

Spectroscopy of a Λ structure in 87Rb is performed with a mode locked laser. A dark line, resulting from population trapping between the hyperfine levels, is observed when the repetition rate is 1/57th of the hyperfine splitting. Simulation of coherent interaction of Rb atoms with laser pulses shows darklines corresponding to different submultiple of the hyperfine splitting. It is demonstrated experimentally and theoretically that the resonance width is insensitive to fluctuations in optical frequency.

Long-term optical phase locking between femtosecond Ti:sapphire and Cr:forsterite lasers

Long-term optical phase-coherent two-color femtosecond pulses were generated by use of passively timing-synchronized Ti:sapphire and Cr:forsterite lasers. The relative carrier-envelope phase relation was fixed by an active feedback loop. The accumulated phase noise from 10 mHz to 1 MHz of the locked beat note was 0.43 rad, showing tight phase locking. The optical frequency fluctuation between two femtosecond combs was submillihertz, with a 1 s averaged counter measurement over 3400 s, leading to a long-term femtosecond frequency-comb connection.

Phase locking of a continuous-wave optical parametric oscillator to an optical frequency comb for optical frequency synthesis

A continuous-wave optical parametric oscillator (OPO) was phase locked to an optical frequency comb in the 830-nm region. The optical frequency of the OPO was controlled by changing the cavity length of the pump laser. The residual phase noise under phase locking was 220 mradrms and the energy concentration to the carrier was 95%. Furthermore, the optical frequency fluctuations of a free-running OPO were measured by using an optical frequency comb that was phase locked to an atomic clock. The measured fluctuations were around 10 MHz in an hour.

Frequency stabilization of an external cavity laser diode using electrooptic effect

An external cavity laser diode, which consists of a LiNbO/sub 3/ phase modulator, was frequency-stabilized to a resonance frequency of a Fabry-Perot interferometer using the electrooptic effect. The optical frequency fluctuation was detected through a Fabry-Perot interferometer and the detected signal was negatively fed back to the phase modulator to reduce the frequency fluctuation. The rear facet of the electrooptic phase modulator was coated with gold and used as an external mirror. At feedback-off state, the peak-to-peak frequency fluctuation of the laser diode was 52 MHz in 2 min. At feedback-on state, the peak-to-peak frequency fluctuation was reduced to 1.5 MHz. This condition was maintained for the duration of the observation, 1 h and 40 min.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A very wide-band frequency response measurement system using optical heterodyne detection

A very-wide band frequency response measurement system for optical devices has been established using the optical heterodyne detection method utilizing two distributed feedback-laser diodes (DFB-LDs). By controlling the laser diode (LD) temperature, the beat frequency is swept from DC to beyond several tens of gigahertz and the measurable bandwidth can be extended to that frequency without any additional calibration. In this system, the spectral linewidth of each LD is stabilized by the two-stage optical isolator and the lightwaves are mixed with the same polarization by a polarization-maintaining optical coupler. The optical frequency fluctuation of the LD is detected by the change of the transmitted optical power through Fabry-Perot etalon and the frequency is controlled by the negative feedback to the LD temperature. Included are a comparison of measurement methods, features of the developed measurement system, and the measurement results. >