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 2×10^-10 for an averaging time of 100 s when compared to a similar laser locked to an acetylene line     

Microwave signals generated by optical heterodyne betweeninjection-locked semiconductor lasers

Single-mode-laser rate equations with added Langevin noise sources are used to study injection-locked semiconductor lasers. Two slave lasers are frequency-locked on the same or different sidebands of a current-modulated master laser. The optical heterodyne between the two secondary lasers is characterized. It is demonstrated that the frequency stability of the source modulating the master laser is preserved on the sidebands and partially transferred to the slaves. A linear model is first investigated. Static operation conditions and small-signal behavior are then calculated. Direct simulation of the rate equations for each laser is next achieved. This highlights the validity domain and limitations of the linear model. A more complete set of results-such as laser and heterodyne spectra-is also obtained. It is moreover shown that synchronization of the slave laser diodes by optical injection-locking leads to strongly correlated, while not identical, laser fields. Finally, simulation results are compared to experimental data     

Absolute frequency control of a 1560 nm (192 THz) DFB laser lockedto 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 KNbO₃ 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)². The 780 nm beam is used to interrogate a resonance of the ⁸⁷Rb-D₂ 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 ⁸⁷Rb-D₂ line (780.246 nm) and of the ⁸⁵Rb-D₂ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5×10^-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     

Topology optimisation of slow light coupling to photonic crystal waveguides

The slow light coupling efficiency in photonic crystal waveguides is enhanced by using the topology optimisation method. As much as 5 dB improvement in transmission can be achieved in the proximity of the spectrum cutoff. Moreover, the resemblance of the resulting two optimised spectra from different initial configurations indicates the possible convergence of the optimisations.     

Frequency stability characterization from the filtered signal of aprecision oscillator in the presence of additive noise

The authors present a generalized theory to express the frequency stability characterization of a precision oscillator when its signal, perturbed by additive noise, is filtered. The general expressions for the power spectral density of the amplitude and phase fluctuations of the filtered signal are calculated as functions of the oscillator amplitude and phase fluctuations, the additive noise, and the filter characteristics. The results obtained for the phase fluctuations of the filtered signal are used to characterize the frequency stability of the oscillator. The contribution of white additive noise to the generalized Allan variance is expressed in terms of a parameter, the equivalent bandwidth. The contributions of other types of noise are also calculated. For the first-order low-pass filter, the contributions of all types of additive, amplitude, phase, and frequency noise are given. Experimental results show excellent agreement with the theoretical predictions     

Frequency locking of semiconductor laser using PSK modulated signal

A random PSK modulated signal generated with an optical waveguide phase modulator is used to obtain an error signal and lock the frequency of an AlGaAs semiconductor laser to the /sup 7/Rb D/sub 2/ line. The measured error signal is similar to that obtained with sinusoidal phase modulation under the same conditions. The technique could be applied to longer wavelength lasers and other types of laser that cannot be directly modulated such as fibre lasers.<>     

Progress in the realization of a frequency standard at 192.1 THz(1560.5 nm) using 87Rb D2-line and second harmonicgeneration

Second harmonic generation of a 192.1 THz semiconductor distributed feedback (DFB) laser is achieved using a KNbO₃ crystal in a resonant ring cavity. Optical feedback from this cavity is used to stabilize the laser frequency and reduce its linewidth. A second harmonic power of 5.5 μW is generated with 38 mW incident on the cavity. We use the second harmonic signal to observe saturated absorption lines and orientation signals in rubidium vapor. Injection-locking of a 780 nm Fabry-Perot laser using the second harmonic signal is also demonstrated. With this scheme, we observe saturated absorption lines in rubidium     

Use of laser diodes locked to atomic transitions in multiwavelengthcoherent communications

The possibility of building multiwavelength coherent communication systems based on low-cost semiconductor lasers in the 0.8 μm range locked to rubidium linear absorption is investigated. The values of frequency offset between the resonances of both the D₂ (780 nm) and D₁ (794.7 nm) lines are considered. A simple two-carrier FDM set-up using three resonances of the D₂ line is presented and the optical frequency offsets are measured     

Simultaneous absolute frequency control of laser transmitters inboth 1.3 and 1.55 μm bands for multiwavelength communication systems

Absolute frequency control will be an essential part of future dense WDM systems. In this paper, we demonstrate two promising techniques that allow the absolute frequency control of an ensemble of laser transmitters operating in both the 1.3 and 1.55 μm bands. First, a Michelson interferometer is absolutely calibrated by means of a frequency-stabilized master DFB laser. This interferometer provides an ensemble of evenly-spaced absolute frequency references that covers both the 1.3 and 1.55 μm regions. Lasers are frequency-locked to transmission nulls of this interferometer with a precision of a few hundred MHz. The second technique allows full flexibility in channel frequency assignment and relies on frequency offset control of an ensemble of laser sources relative to a master reference laser. The frequency comparator is based on a surface-emitting nonlinear semiconductor multilayer waveguide. This technique provides simultaneous frequency measurement and control of lasers in both bands with a precision of a few GHz