Christine Latrasse

Absolute frequency measurement of a laser at 1556 nm locked to the 5S1/2-5D5/2 two-photon transition in 87Rb

Abstract The frequency of a diode laser system at 193 THz (1556 nm), which is frequency doubled and locked to a two-photon transition in rubidium at 385 THz (778 nm), has been measured with a Cs-based frequency chain and a single Sr + ion standard at 445 THz. The output frequency of the diode laser system was measured to be 192 642 283 183 700 ± 500 Hz. After applying corrections for systematic offsets in the rubidium spectrum, the frequency of the 87 Rb 5S 1/2 ( F g =2)−5D 5/2 ( F e =4) two-photon transition is found to be 385 284 566 370.4 ± 2 kHz.

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.

A high-coherence high-stability laser for the photonic local oscillator distribution of the Atacama Large Millimeter Array

We present the architecture and the characterization results of a Master Laser prototype that can be used to distribute a phase-coherent millimeter wave reference within the Atacama Large Millimeter Array. This source is obtained by frequency-locking a 1556-nm narrow linewidth DFB fiber laser to a two-photon transition in rubidium 85 at 778 nm after second harmonic generation in a non-linear waveguide crystal. The prototype yielded an absolute wavelength of 1556.210 843 nm, a stability of 2x10-12 at tau = 1 s, a linewidth of 2 kHz over 1 ms, a coherence of 40% at 50 km over 1 ms, and a RIN below -145 dBc/Hz for f>10 MHz. Using this laser, the transmission of an 18.6 GHz reference over 10 km of fiber was achieved with a residual phase fluctuations lower than 0.22 degrees RMS (33 fs RMS) over 10 s.

Frequency stability of an optical frequency standard at 192.6 THz based on a two-photon transition of rubidium atoms

Abstract We have developed two frequency standards at 192.6 THz (1556.2 nm) based on a two-photon transition 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 the (5S1/2, Fg=2−5D5/2, Fe=4) two-photon transition in 87 Rb and lock the 1556.2 nm laser. Allan variance measurements between two identical standards show a beat stability of 2.5×10−13/τ1/2 for observation times between 100 ms and 10 s and a level of 5.8×10−14 for 100 s. Systematic effects shifting the locked frequency of the standards from that of the Rb transition are detailed and some experimental measurements are presented. Finally, absolute frequency measurements were performed at the INMS/CNRC in Ottawa allowing the determination of the absolute frequency of the standard to be 192 642 283 183.7±0.5 kHz.

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.

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.

Ultra-narrowband fiber Bragg gratings for laser linewidth reduction and RF filtering

We review the improved performances of a narrow linewidth laser using negative electrical feedback obtained through advances on narrowband FBG filters. Noteworthy, the tolerance of the laser to vibrations is significantly improved. As an extension of this work, these narrow filters are proposed for filtering optical signals in RF photonics systems.

Use of a sampled Bragg grating as an in-fiber optical resonator for the realization of a referencing optical frequency scale for WDM communications

Standardization for absolute laser frequency assignment in projected dense wavelength-division multiplexing (DWDM) networks has become a very important issue. Recently, we have proposed a referencing scale of standard optical frequencies that are exact multiples of 100 GHz. Such a scale is independent of the atomic or molecular reference used to set the absolute value and is uniformly applicable to all spectral ranges. Here, we propose the use of sampled Bragg gratings to realize the optical resonator. This resonator is adaptable, robust, potentially cost efficient and readily compatible with the fiber network.