Stable laser sources for the formation of ultracold molecules Bowden, William; Quentin, Damien; Sun, Jon-Paul
Atomic molecular and optical physicists require highly stable laser sources to cool molecules to ultracold temperatures. Last year, for our APSC 459 project we designed and characterized an interference filter-stabilized external-cavity diode laser (ECDL). This year’s project was primarily focused on stabilizing this laser via a lock either to a second ECDL or to a 5S₁/₂- 5D₅/₂ two photon transition in rubidium. A variety of methods and experimental techniques were used to characterize the performance of a range of electrical and optical components, and to investigate key design parameters in order to achieve a stable lock. In locking two ECDL’s together, the two beams were coupled in order to generate a beatnote. The beatnote was then split and sent through a long or short fiber, then mixed and filtered such that an error signal generated from the phase difference was sent to the proportional and integral (PI) control system. This control system generated a slow and fast response, from which we could control and adjust the laser’s frequency. To lock to the 5S₁/₂- 5D₅/₂ two photon transition, we built an oven to house the cell and various optical components and heating elements, coupled to a photomultiplier tube. A beam was sent through the cell and oven, and reflected back such that the two beams were coupled within the cell to make the atoms fluoresce. A lock-in amplifier was used to generate an error signal from the detected florescence in order to control the laser’s frequency. For the two laser locking experiment we achieved a lock on the order of 300 kHz. There are a few more techniques that we can investigate to tighten this lock, including switching to photodetector with less noise and a self-heterodyne beatnote measurement with a 100 km optical fibre and a 1550 nm laser diode. The rubidium locking technique was successful at achieving a lock on the order of 1.8 MHz. This lock can be reduced by addressing some of the broadening mechanisms responsible for widening the transition peaks of rubidium, as well as optimizing a few of our system parameters to produce an error signal with more gain.
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