M. Breton

All-optical microwave frequency standard: a proposal

A simple method to produce a clock transition with purely optical means by modulated pumping is described. The field-independent ground state resonance of /sup 87/Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation) is observed. The 6.835 GHz resonance with a subharmonic modulation frequency of 1.139 GHz is detected. A high-contrast resonance peak is observed and a condition for zero light shift is found. The linewidth is 3 kHz (at 6.835 GHz) in this preliminary experiment, due to the small size of the light beam ( approximately 2 mm diameter) and the low buffer gas pressure (680 Pa) that was used. A theoretical model that explains the main features of the experiment is described. >

Calibrated Fabry-Perot etalon as an absolute frequency reference for OFDM communications

The frequency locking of a large number of lasers in OFDM applications requires an equally large number of stable references at predictable frequencies. The authors present a technique to lock a Fabry-Perot etalon on an atomic reference to provide multiple and evenly spaced absolute frequency references. Absolute etalon spacing is calibrated by matching two atomic references with two corresponding etalon modes. To maintain long-term stability in channel spacing, the etalon is locked to an atomic resonance using an electrical feedback loop to control mirror separation. This technique can be used for close spacing or coherent applications where channel separation and reproducibility is critical between transmitter and receiver.<<ETX>>

Multiwavelength sources using laser diodes frequency-locked to atomic resonances

It is shown that low-cost compact disk AlGaAs laser diodes make it possible to realize manifold optical sources with precise and stable wavelengths when locked to rubidium vapor resonances. Many optical frequency references can be obtained with the use of absorption cells containing a foreign gas or by placing cells in static magnetic fields. The same approach can be used with other wavelengths required by optical fiber communications. >

C 2 HD and 13 C 2 H 2 absorption lines near 1530 nm for semiconductor-laser frequency locking

Multiwavelength communications will require the establishment of absolute wavelengths for identification and routing. For this purpose we recorded high-resolution absorption spectra of C2HD and 13C2H2 between 1510 and 1550 nm, using a Fourier-transform spectrometer. Precise wavelength calibrations were performed by use of 87Rb lines and a commercial wavemeter. 50 lines of C2HD and 114 lines of 13C2H2 were calibrated and identified as rotation–vibration transitions. Lines P(14) of C2HD and R(7) of 13G2H2 were used to frequency stabilize two distributed-feedback lasers near 1529 nm. The square root of the Allan variance was 2 × 10−10 for τ > 10 s.

Toward the realization of a wavelength standard at 780 nm based on a laser diode frequency locked to rubidium vapor

The authors present the result of a study on the realization of an optical frequency standard based on a laser diode frequency-locked to Doppler resonance limited and sub-Doppler resonances of the /sup 87/Rb D/sub 2/-line (5S/sub 1/2/ from or to 5P/sub 3/2/, 780.027 nm). The characteristics of the signals obtained from the resonant phase-conjugate reflection, the nonlinear magnetic optical activity, and the saturated absorption are compared to the characteristics of the linear absorption signal. Improvement of laser diode frequency stability is discussed for these techniques. The experimental results indicate that the more complicated the system, the better defined is the reference line. >

Optically pumped rubidium as a frequency standard at 196 THz

The performance of 196.0-THz (1529-nm) DFB lasers frequency-locked to absorption lines of a rubidium vapor optically pumped at 384.2 THz (780.2 nm) is studied. The absorption profiles of the pumped vapor are measured under various conditions and compared with theoretical predictions. A bright resonance resulting from the cascade of two cycling transitions is characterized both experimentally and theoretically. The measured frequency stability of a DFB laser frequency-locked to this line reaches a level of 2/spl times/10/sup -10/ for an averaging time of 100 s when compared to a similar laser locked to an acetylene line. >

Second-harmonic generation of a 1560-nm distributed-feedback laser byuse of a KNbO 3 crystal for frequency locking to the 87 RbD 2 line at 780 nm

Second-harmonic generation of a semiconductor distributed-feedback laser at 1560 nm is described. It is produced by use of a 4.8-mm-long KNbO(3) crystal at room temperature. As much as 2.2 nW of power at 780 nm is obtained for 11.3 mW of fundamental power incident upon the crystal. This signal is used to interrogate a component of the linear absorption profile of the D(2) line in (87)Rb (780.241 nm) and to produce an error signal used to frequency lock the 1560-nm distributed-feedback laser. Such a system can therefore be a candidate for establishing an absolute wavelength standard at 1560 nm.

Frequency locking of a 1324 nm DFB laser to an optically pumped rubidium vapor

Some experimental results and a simple theoretical model on a dither-free frequency-locking technique of a DFB laser operating at 1324 nm are presented. A resonance observed through absorption in an /sup 87/Rb vapor optically pumped by a modulated 795-nm laser diode tuned to the D/sub 1/ line is used. This approach has the advantage of producing sub-Doppler resonances. Such a technique could be used to generate an unmodulated 1324-nm wavelength reference. >

Dither-free absolute frequency locking of a 1.3 mu m DFB laser on /sup 87/Rb

A technique for frequency locking a semiconductor laser in the 1.3 mu m region by observing the absorption in a /sup 87/Rb vapor optically pumped by another frequency-modulated semiconductor laser is presented. This technique offers well-defined strong atomic resonances and does not require frequency dithering of the 1.3 mu m laser to observe the discrimination pattern required for frequency locking. Dither-free absolute frequency locking of a DFB (distributed feedback) laser at 1324 nm was achieved using a pump laser diode operating at 795 nm (D/sub 1/-line). Applications include master oscillator of multiwavelength and coherent communication systems and reference source for an optical frequency synthesizer. Resonances in the 1.55 mu m region are also available through this approach.<<ETX>>

Absolute frequency control of a 1560 nm (192 THz) DFB laser locked to a rubidium absorption line using a second-harmonic-generated signal

We demonstrate second-harmonic generation from a DFB laser at 1560 nm in a type I critically phase-matched KNbO/sub 3/ crystal. We obtain 2.2 nW at 780 mm with 11.3 mW at 1560 nm incident on the crystal. The conversion efficiency is 17.2 pW/(mW)/sup 2/. The 780 nm beam is used to interrogate a resonance of the /sup 87/Rb-D/sub 2/ line at 780 nm and lock the laser frequency. To characterize the absolute frequency stability, two 1560 nm DFB lasers are respectively stabilized on a Doppler resonance of the /sup 87/Rb-D/sub 2/ line (780.246 nm) and of the /sup 85/Rb-D/sub 2/ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5/spl times/10/sup -9/ for averaging times between 3 and 100 s. To improve the precision of the frequency locking, we realize a setup to observe a saturated absorption profile. We use a 780 nm stabilized laser as a pump and the SHG signal as a probe. A saturated absorption profile is observed over the Doppler envelope. Work is under progress to use this saturated resonance for an improved frequency control. >