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Cavity ring-down spectroscopy with an optical feedback frequency stabilized laser

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Absorption line shape metrology is one of the hottest topics of high resolution spectroscopy at present.The accurate measurement of molecular absorption features both on the frequency (X) and on the absorption (Y) axis has crucial implications in various fields, including atmospheric remote sensing[1], isotopic ratio measurements[2] and a redetermination of the Boltzmann constant[3]. This has fostered considerable theoretical progress (e.g. [4]) in quest of a line shape model, which accounts correctly for collision-induced velocity changes and dephasings.

Our group is committed to contributing high quality experimental absorption lines for testing models at ever-increasing signal-to-noise ratios and to tackle the aforementioned urgent and fascinating applications. To this end, we have
proposed and developed a novel spectrometer combining a narrow, stable and tunable laser with an ultra-sensitive [5] cavity ring-down absorption measurement: Optical Feedback Frequency Stabilized Cavity Ring-Down Spectroscopy (OFFS-CRDS).

Experimental setup for OFFS-CRDS: The CRDS cavity is Pound-Drever-Hall locked to a distributed-feedback diode laser tuned by single-sideband modulation using an integrated Mach-Zehnder modulator. The DFB laser, in turn, is optical feedback locked to a highly stable V-shaped reference cavity.

As the heart of our setup, we have developed a subkilohertz linewidth laser[6], which is based on feedback photons coming from inside a home-made highly stable V-shaped reference cavity.

Highly stable V-shaped reference cavity developed at LIPhy: Ultra-low expansion materials and tightly contacted, intrinsically aligned, precision-machined mirror holder flanges guarantee automatic V-cavity alignment and excellent dimensional stability.

Being reinjected into the laser medium, they counteract the random walk of laser phase due to spontaneous emission. This leads to a linewidth narrowing by almost four orders of magnitude and all-optical locking to a reference cavity resonance, thereby reducing frequency drifts to below 20 Hz/s.

Optical feedback locked laser frequency noise and linewidth: Measured by means of a stable high finesse etalon, the frequency noise power spectral density of the optical feedback stabilized laser implies a linewidth below 530 Hz (down from a free-running linewidth of about 2 MHz).

This laser source is continuously tunable over 1 THz around 1590 nm by selecting a reference cavity mode and using an innovative single-sideband suppressed-carrier modulation scheme[6], which allows for frequency shifting over up to 40 GHz with millihertz accuracy and excellent spectral purity. Its physical basis are multiple voltage-controlled interferences in an integrated electro-optic Mach-Zehnder modulator

Single-sideband spectral purity: The transmission from a scanning Fabry-Pérot cavity reveals that for optimal control voltages (black curve) the single-sideband intensity is more than 28 dB above the suppressed optical carrier and other sidebands.

In a final step, the ring-down spectroscopy cavity is locked onto our single-sideband tuned optical feedback stabilized laser, thereby transferring its frequency stability and optimizing CRDS transmission. The versatility of the MZM makes the use of an acousto-optic modulator for triggering ring-down events obsolete. The same holds for an additional electro-optic modulator normally used for generating the PDH error signal.Because of its stable and narrow probe laser, high repetition rate and shot-noise-limited ring-down acquisition, the performance features of our new OFFS-CRDS spectrometer are unprecedented. It combines subkilohertz frequency stability and resolution with terahertz-wide tunability as well as absorption detection limits at the
10-13 cm-1 level. The experimental setup as well as a characterization of its performance have been already published[8],[9],[10].









[8]: poster presentation at HRMS Budapest

  • Author(s): J. Burkart, S.Kassi
  • Published: 26 August 2013.