Berlin 2001 – wissenschaftliches Programm

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Q: Quantenoptik

Q 37: Optical Applications II

Q 37.3: Vortrag

Freitag, 6. April 2001, 18:00–18:15, H 3010

Frequency references for telecommunication wavelengths — •Gesine Grosche¹, Uwe Sterr¹, and Takayuki Kurosu² — ¹Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig — ²National Research Lab. of Metrology, Tsukuba, Japan

Optical fibre communications systems based on dense-wavelength-division-multiplexing (DWDM) - with projected channel spacings down to 25 GHz - require accurate frequency references in the 1.5 µm band with a relative uncertainty less than 10⁻⁸. We have investigated several techniques to stabilise a line-width narrowed DFB laser to Doppler-free molecular overtone transitions of acetylene, namely in the P-branch of the combination band ν₁+ν₃ of ¹³C₂H₂. These ro-vibronic transitions are attractive as references because of their low sensitivity to external perturbations and narrow natural linewidths (Δ ν ≈ 1 kHz). Placing the absorber inside a build-up cavity, such a weak transition could be saturated at a low pressure of a few Pa, minimising pressure shifts. The DFB laser (output power 5 mW) was locked to this cavity using the Pound-Drever-Hall technique. To stabilise the laser to the absorption line, the cavity length was modulated and the derivative of the molecular absorption could be detected using a lock-in amplifier. In transmission, the cavity converts laser frequency noise into intensity noise, thus limiting the signal-to-noise ratio. Drastic improvement may be achieved by either detecting the sum of transmitted and reflected signal or by applying a phase modulation at a frequency equal to the free-spectral-range of the cavity and using the side-bands as reference for the heterodyne detection of the phase shift that is introduced by the molecular absorption.