Normand Cyr

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. >

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.

An optical frequency scale in exact multiples of 100 GHz for standardization of multifrequency communications

Absolute laser frequency assignment in projected dense wavelength division multiplexing (DWDM) networks has become a very important issue for standardization purposes. Recently, a proposal was made to the International Telecommunication Union suggesting a set of standard wavelengths in the 1550 mn communications band. It recommended the use of a krypton line at 193.68625 THz as an absolute frequency reference and a set of 32 wavelengths evenly spaced by 100 GHz around that value. In this paper, we propose the use of an optical frequency scale with markers at exact multiples of 100 GHz for standardization. Our proposed scale is independent of the atomic or molecular species used for calibration (and thus accessible to any user), and moreover is uniformly applicable to all spectral regions. We show one way of implementing such a scale in the 1550 nm band through the use of an absolutely calibrated Fabry-Perot resonator set with a free spectral range of 100 GHz.

An absolute frequency reference at 192.6 THz (1556 nm) based on a two-photon absorption line of rubidium at 778 nm for WDM communication systems

A 192.6-THz (1556 nm) distributed-feedback (DFB) laser is frequency-locked on a two-photon transition of rubidium at 385.3 THz (778 nm) using second-harmonic (SH) generation. With 43 mW at 1556 nm, we obtain a SH power of 15 /spl mu/W using a KNbO/sub 3/ crystal placed in a ring cavity. Optical feedback from this cavity is used to reduce the DFB laser linewidth to the 10-kHz level and control its frequency. The SH signal is used to injection-lock a 778-nm Fabry-Perot laser in order to increase the interrogation power. With this scheme, we observe two-photon transitions in rubidium and lock the 1556-nm laser frequency.

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. >

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. >

Measuring the Link Distribution of PMD: Field Trial Using an RS-POTDR

A new POTDR measurement technique is used to investigate the spatial distribution of PMD in deployed fibers. Results help to identify high-PMD fiber sections that need to be replaced to enable 40Gbit/s transmission and beyond.

Optical frequency control for DWDM networks using sum-frequency generation in multilayer waveguides

Future high bit-rate DWDM networks will require an ensemble of laser sources whose frequencies can be set accurately. Considering the excellent short-term wavelength stability of todays DFB semiconductor lasers, network frequency management could be accomplished through periodic measurement and correction of essentially free-running sources. In this paper, we present experimental results on a novel technique for measuring and controlling the wavelength of a semiconductor laser using the non-linear properties of a multilayer waveguide. Using the described calibration procedure, absolute optical frequency control of a 1.3 /spl mu/m DFB laser within better than 2 GHz is demonstrated. Our technique allows full flexibility in channel frequency assignment, is already compatible with projected DWDM network wavelength control requirements and lends itself to monolithic optical integration.<<ETX>>