Berlin 2001 – scientific programme
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Q: Quantenoptik
Q 37: Optical Applications II
Q 37.3: Talk
Friday, April 6, 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.