(519) 661-3324 (Office), (519) 661-2188 (Lab)
rosner at uwo.ca (Please change “at” to “@”)
· high-precision measurement of laser wavelengths using Fabry-Pérot and Michelson interferometry
Our UWO research group currently consists of
Our collaborator in high-precision wavelength measurement is Dr. Alan Madej of the National Research Council of Canada.
Technical details of our work can be found in our publications.
A major thrust of our work is measurements on atoms and molecules of astrophysical interest. Comets, stars, and interstellar clouds absorb and emit electromagnetic radiation (e.g. light, xrays, radio waves) which are detected with terrestrial or orbiting telescopes. This radiation contains information about the composition and dynamics of the source, but extracting the information requires accurate knowledge of basic properties of the atoms and molecules therein. The wavelengths of the radiation tell us what the source is made of and how the source is moving as seen from Earth; the intensity of the radiation is related to the abundance of its different components. We make high-precision measurements of a quantity known as the lifetime of an excited state, which is related to the rate of light emission. We also measure branching ratios, which, combined with the lifetime value, leads to the intrinsic probability for absorption, usually tabulated as an oscillator strength. This quantity is used in the calculation of abundance of a particular species in an astrophysical source from the observed light intensity emanating from that source.
In a typical experiment, we produce a beam of the atom or molecule of interest in an electrically charged state known as an ion. The charge allows us to accelerate the ion, making a narrow, fast ion beam, which is probed further downstream with a laser. In order to learn about the allowed energy states and structure of the ion, the laser wavelength is varied through different colours, and the resulting laser-induced fluorescence emitted by the ion is observed with a very sensitive light detector. To measure the lifetime of a particular energy state, the laser wavelength is fixed at a colour that will excite that state, and the laser-induced fluorescence is viewed as a function of distance downstream. The longer the lifetime, the farther the light detector can be moved downstream before the ions are essentially done emitting light. To measure branching ratios, again we fix the laser wavelength so that a particular energy level is excited by the light, and we observe the intensity of all the light emitted by the atom over a range of different wavelengths. The relative intensity of the emissions at different wavelengths leads to the branching ratios.
Another of our interests is studying the structure and properties of simple atoms, i.e. those with only one or two electrons, has always been an important test of our understanding of the physical world. Some of the most stringent tests of the theory known as quantum mechanics, which describes the atomic and sub-atomic world, are made on hydrogenic (one-electron) and heliumlike (two-electron) atoms. In a typical experiment the wavelengths of light absorbed or emitted by these atoms are measured to the highest possible accuracy and compared with theoretical predictions. At the level of precision that such an experiment can now achieve, it is very important to incorporate the effects of relativity. Although theorists have known for some time how to combine quantum mechanics and relativity for one-electron atoms, the extension to two-electron atoms raises some fundamental new problems and currently poses an important challenge, since there are some disagreements between theory and experiment. Our group at UWO has made measurements in heliumlike lithium and beryllium, and we plan to repeat the beryllium work at higher precision.
Our photonics work involves the high-precision measurement of laser wavelengths using interferometry. Accurate knowledge of laser carrier frequencies is very important in communication using infrared light. Information is carried on different carriers (wavelengths ~1.5 mm) whose frequencies are spaced by only several GHz, and must be stabilized accurately. Although light frequencies may now be measured directly to precisions of 1 part in 1015, the equipment to do this is very expensive. On the other hand, light wavelengths can be measured to 1 part in 109 and better using interferometers whose dimensions are referenced to laser standards. Such accuracy is sufficient for the communications industry. Our device uses a Fabry-Pérot etalon in combination with a Michelson moving-mirror wavemeter to compare an unknown laser wavelength with a known standard (an iodine-stabilized He-Ne laser). It is intrinsically broadband and relatively insensitive to alignment differences in the two lasers.
Our research work provides valuable training in such skills as
· laser operation
· optical design
· high vacuum systems
· ion source use and development
· automated data acquisition
· computer modeling
· data analysis.