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IEEE/LEOS Summer Topical Meetings

An absolute frequency reference at 1556 nm for WDM communication systems

M. Poulin, Christine Latrasse, Normand Cyr, M. Têtu

Published Nov 22, 2002• 0 citations
Publisher

Abstract

Standardization of channel frequency allocation and calibration of the frequency scale at 1.55 /spl mu/m will require some frequency standards in this range. A promising avenue consists of making use of the two-photon transitions of rubidium at 778 nm to frequency-lock a 1556 nm laser. These lines have been recently measured by direct comparison to the Cs frequency standard with an uncertainty of 2 kHz. The method which we develop consists in using the second harmonic of the 1556 nm laser to injection-lock a low-cost laser diode operating at 778 nm which is then used to observe the transitions in rubidium.

DOI
10.1109/leosst.1997.619147

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Summary form only given. Summary from only given. Frequency-stabilized sources near 1550 nm are required as frequency references. Owing to recent progress in optical amplifiers and in waveguide frequency doublers, we have now obtained sufficiently powerful second harmonic signal for direct detection and locking to the rubidium transition, using a single 1556 nm source.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Nov 11, 2002•0 citations
Standardization of channel frequency allocation and calibration of the frequency scale at 1.55 /spl mu/m will require some frequency standards in this range. A promising avenue consists of making use of the two-photon transitions of rubidium at 778 nm to frequency-lock a 1556 nm laser. These lines have been recently measured by direct comparison to the Cs frequency standard with an uncertainty of 2 kHz. The method which we develop consists in using the second harmonic of the 1556 nm laser to injection-lock a low-cost laser diode operating at 778 nm which is then used to observe the transitions in rubidium.
This is a progress report on the realization of a compact and transportable frequency standard at 1556 nm based on a two-photon transition in rubidium at 778 nm. These hyperfine transitions present great metrological interest. They have a narrow theoretical linewidth of 150 kHz when observed with a 1556 nm laser, and their absolute frequency is known with an uncertainty of 5.2 X 10-12. In this experiment, we use a high power 1556 nm DFB laser and reduce its linewidth to the 10 kHz level using optical feedback from a confocal cavity. We generate its second harmonic in a periodically poled LiNbO3 crystal and use this signal to injection-lock a Fabry-Perot laser emitting 42 mW at 778 nm. The slave laser is used to observe the Doppler-free two- photon transitions: two counter-propagating beams excite rubidium atoms which emit a blue fluorescence on resonance. This 420 nm light is detected on the side of the Rb cell with a photomultiplier. Such an optical frequency standard at 1556 nm, standing in the multiwavelength telecommunications systems window, becomes an attractive source for absolute frequency calibration of WDM components, optical spectrum analyzers and wavemeters.
We have developed frequency standards at 192.6 THz (1556.2 nm) based on the two-photon transitions in rubidium at 385.2 THz (778.1 nm). These standards use a high power DFB laser at 1556.2 nm and second harmonic generation (SHG) in a periodically poled lithium niobate (PPLN) crystal. The linewidth of the DFB is reduced to the kHz level using optical feedback from a confocal cavity. The SH light is used to injection-lock a 778.1 nm laser diode which allows to observe and lock the 1556.2 nm laser to the (5S/sub 1/2/,F/sub g/=2-5D/sub 5/2/,F/sub e/=4) two-photon transition /sup 87/Rb. Allan variance measurements between identical standards show a beat stability 2.5/spl times/10/sup -13///spl tau//sup 1/2/ for observation times between 100 ms and 10 s and a level of 5.8/spl times/10/sup -14/ for 100 s is obtained. These standards have been moved recently to the National Research Council, Ottawa, Canada in order to measure their absolute frequency.
Improvements in the power level of sources near 1550 nm and in the efficiency of waveguide frequency doublers enabled us to lock a frequency-doubled source directly to the 5S(1/2) ? 5D(5/2) two-photon transitions near 778 nm. We obtained a sufficiently powerful second-harmonic signal, exceeding 2 mW, by doubling an external-cavity diode laser that was amplified by an erbium-doped fiber amplifier in a periodically poled LiNbO(3) channel waveguide. Our experimental scheme can be used for realizing compact, high-performance frequency standards near 1550 nm for fiber-optic communication and sensing applications.
To transfer and expand in the optical telecommunication range the accurate 5S1/2(F = 3) – 5D5/2 (F = 5) rubidium frequency reference at 778 nm (385.285 THz), we employ a second harmonic generation (SHG) process coupled to an optical frequency comb (OFC) both based on the use of a low power and narrow linewidth laser diode operating around 1556 nm. The Rb optical frequency reference is known with an accuracy of 2.6 × 10−12 and exhibits a frequency stability of 4 × 10−13τ−1/2, with a Flicker plateau of 2 × 10−14 for τ > 200 s. With 20 mW of laser power at 1556 nm, we perform a SHG process in a periodically poled lithium niobate (PPLN) crystal and we obtain 6.5 μW of harmonic radiation at 778 nm, used to phase lock the IR laser against a Rb optical frequency standard (OFS). This Rb-referenced fundamental radiation at 1556 nm is duplicated as 34 lines spaced by 10 GHz using an electro-optic phase modulator (EOM). This preliminary result is mainly limited by unexpected high optical losses in the Fabry Perot cavity, which comprises the EOM.