M. Tetu

Room temperature multifrequency erbium-doped fiber lasers anchored on the ITU frequency grid

Room temperature multifrequency erbium-doped fiber (EDF) lasers anchored on the ITU frequency grid and using frequency periodic filters are presented. The multifrequency operation is demonstrated both theoretically and experimentally by adding a frequency shifter in the ring cavity to prevent steady-state laser operation and single-frequency oscillation.

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. >

On hyperfine frequency shifts caused by buffer gases: Application to the optically pumped passive rubidium frequency standard

Experiments are reported on the effect of buffer gases on the hyperfine frequency of rubidium 87 as used in passive rubidium frequency standards. In some mixtures of buffer gases a strong quadratic dependence of the frequency on temperature is observed. The data are explained by means of a phenomenological approach and with the help of data published previously on pure buffer gases. A comment is made on an article which appears to lead to doubtful conclusions.

Performance of balanced detection in a coherent receiver.

The quality of balanced detection in a coherent receiver is analyzed theoretically and experimentally. The impact of the characteristics of the optics on the balanced detection is presented. A parameter that characterizes the performance of the balanced detection suitable for the whole optical front-end is propose.

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>>

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.

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.

A 100-GHz-tunable photonic millimeter wave synthesizer for the Atacama Large Millimeter Array radiotelescope

We propose a broadband photonic millimeter wave synthesizer based on the optical phase locking of telecom-grade DFB semiconductor lasers. A mathematical model of the OPLL shows that the frequency noise profile of these lasers is such that millimeter wave beat notes with residual phase noise levels of 2 x 10-5 rad2 can be achieved with loop path lengths as long as 60 cm. Experiments on a simplified proof-of-concept unit validate this model. The synthesizer is tunable over more than 100 GHz by changing the laser junction temperature. The photonic millimeter wave synthesizer will be used to generate and distribute the LO reference signal of the Atacama Large Millimeter Array radiotelescope.

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.