Cavity Mode Width Research Articles

Spectroscopic Gas Sensor Based on a Fiber Fabry-Perot Cavity

Optical spectroscopic sensors are powerful tools for analysing gas mixtures in industrial and scientific applications. Whilst highly sensitive spectrometers tend to have a large footprint, miniaturized optical devices usually lack sensitivity or wideband spectroscopic coverage. By employing a widely tunable, passively stable fiber Fabry-Perot cavity (FFPC), we demonstrate an absorption spectroscopic device that continuously samples over several tens of terahertz. Both broadband scans using cavity mode width spectroscopy to identify the spectral fingerprints of analytes and a fast, low-noise scan method for single absorption features to determine concentrations are exemplary demonstrated for the oxygen A-band. The novel scan method uses an injected modulation signal in a Pound-Drever-Hall feedback loop together with a lock-in measurement to reject noise at other frequencies. The FFPC-based approach provides a directly fiber coupled, extremely miniaturized, light-weight and robust platform for analyzing small analyte volumes that can straightforwardly be extended to sensing at different wavelength ranges, liquid analytes and other spectroscopic techniques with only little adjustments of the device platform.

High stability in near-infrared spectroscopy: part 1, adapting clock techniques to optical feedback

Optical feedback is an efficient way to narrow and stabilise semi-conductor lasers. As a step forward for ultra-stable, yet highly tunable sources, we developed a new prototype of a three-mirror V-shaped optical cavity (VCOF). It is made of a precisely machined Zerodur spacer, that holds 3 optical contacted mirrors forming a high finesse V-shaped cavity. This arrangement allows the resonant light to be sent back to the seeding laser, triggering a drastic narrowing of the emission linewidth well below the cavity mode width without the need for high-bandwidth active electronic lock of the laser on the cavity. Low expansion material and precise temperature control of the reference cavity leads to a source with Hz level frequency drift and 70 Hz-level emission linewidth at 215 THz. We discuss the benefits of this new prototype compared to its predecessor for high-sensitivity cavity ring down spectroscopy (CRDS). To serve this purpose, the frequency stabilised laser was characterised on short, mid and long time scales, using a high finesse etalon, a self referenced optical frequency comb and a long term saturated CRDS absorption Lamb dip measurement on water.

Dual-comb cavity-mode width and shift spectroscopy

Optical frequency comb spectrometry has become one of the leading fields devising new ways to investigate molecular structure and characterize optical elements due to its advantage in speed and accuracy. In this work we demonstrate the first dual-comb realization of cavity mode-width and mode-dispersion spectroscopies, achieving single-spectrum acquisition time in millisecond range, maintaining high spectral resolution limited by the comb tooth width. Interleaving of a few tens of spectra allows us to measure shapes of 13 kHz-wide enhancement cavity modes and determine their widths and positions with Hz-level precision. Thanks to this we can simultaneously measure the loss and dispersion of both the cavity and the intracavity medium. As a demonstration, we measure spectra of a gas mixture of methane and nitrogen and compare them with a simulation based on the HITRAN database. This novel method has many possible applications, including gas metrology, high-reflectivity mirror characterization or trace gas sensing.

Cavity mode-width spectroscopy with widely tunable ultra narrow laser

We explore a cavity-enhanced spectroscopic technique based on determination of the absorbtion coefficient from direct measurement of spectral width of the mode of the optical cavity filled with absorbing medium. This technique called here the cavity mode-width spectroscopy (CMWS) is complementary to the cavity ring-down spectroscopy (CRDS). While both these techniques use information on interaction time of the light with the cavity to determine absorption coefficient, the CMWS does not require to measure very fast signals at high absorption conditions. Instead the CMWS method require a very narrow line width laser with precise frequency control. As an example a spectral line shape of P7 Q6 O₂ line from the B-band was measured with use of an ultra narrow laser system based on two phase-locked external cavity diode lasers (ECDL) having tunability of ± 20 GHz at wavelength range of 687 to 693 nm.

Asymmetric temporal profile of optical Kerr signal from GaAs/AlAs multilayer with λ/2 phase shift layer

AbstractOptical Kerr response for a short‐input‐pulse of a GaAs/AlAs multilayer cavity which has a narrow cavity mode width was measured by an optical pump and probe technique. We observed a strong asymmetric temporal profile of the optical Kerr signal in the vicinity of Δt = 0. The initial rising time of the signal was 1.0 ps for Δt < 0, while the decay time was 0.25 ps for Δt > 0. The asymmetric profile of the Kerr signal was analyzed with the pulse shapes of the pump and probe pulses in the cavity. We found that the decay of the Kerr signal for Δt > 0 was faster than that of the profile explained by the pump and probe pulses. This suggests that the strong asymmetric profile of the Kerr signal was occurred by the strong phase shift due to the higher order term of the susceptibility. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cavity QED with Semiconductor Nanocrystals

We report on a strongly coupled cavity quantum electrodynamic (CQED) system consisting of a CdSe nanocrystal coupled to a single photon mode of a polymer microsphere. The strong exciton-photon coupling is manifested by the observation of a cavity mode splitting variant Planck's over 2piOmega(exp) between 30 und 45 microeV and photon lifetime measurements of the coupled exciton-photon state. The single photon mode is isolated by lifting the mode degeneracy in a slightly deformed microsphere cavity and addressing it by high-resolution imaging spectroscopy. This cavity mode is coupled to a localized exciton of an anisotropically shaped CdSe nanocrystal that emits highly polarized light in resonance to the cavity mode and that was placed in the maximum electromagnetic field close to the microsphere surface. The exciton confined in the CdSe nanorod exhibits an optical transition dipole moment much larger than that of atoms, the standard system for CQED experiments, and a low-temperature homogeneous line width much narrower than the high-Q cavity mode width. The observation of strong coupling in a colloidal semiconductor nanocrystal-cavity system opens the way to study fundamental quantum-optics phenomena and to implement quantum information processing concepts that work in the visible spectral range and are based on solid-state nanomaterials.

Experimental evidence for nonequilibrium Bose condensation of exciton polaritons

We observe dramatic changes in the near-field and far-field emission from a semiconductor microcavity excited by a pulsed and nonresonant optical pump with varying power. Above a threshold pumping power, light is emitted by a single quantum state lying at the bottom of the lower exciton-polariton band. Its intensity increases exponentially with the pump power and its linewidth becomes narrower than the cavity mode width. Near-field spectroscopy shows that the stimulated emission comes from several bright spots in the cavity plane, but no diffraction-induced angular broadening of the emission is observed. This is direct evidence for spontaneous formation of a nonequilibrium Bose condensate of coherent exciton polaritons with their wave function sharply peaked at structural imperfections.