Development of a 638.6nm External Cavity Diode Laser for Scalable Ion-Trap Quantum Computing

Atomic ion traps offer a promising architecture for scalable quantum information processing. External cavity diode lasers yield stable output frequency and narrow spectral bandwidth by exploiting the sensitivity of diode lasers to external optical feedback. This project aims to design and develop a high-coherence external cavity diode laser used to depopulate Yb171+ from the undesirable 2F7/2 orbital. The system uses frequency-selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, and frequency tunable light at 638.6nm. A piezo-electric actuator is applied to the grating to fine-tune the output frequency, while flexures are utilized for the alignment of the beam. A low-noise current source and a PID temperature controller for thermal stability are developed as a part of the system. The complete system can be easily tuned to other lasing frequencies, and will offer great benefits in laser cooling and spectroscopy of neutral atoms. Figure 2-1: Hyperfine energy levels of the ion [3] Figure 2-2: Simple two-level quantum system Figure 2-3: A four-level system used for population inversion [6] Figure 2-4: Experimental setup of a generic laser [6] Figure 2-5: Semiconductor pn-junction laser diode schematics [8] Figure 3-1: Diffraction grating Figure 3-2: External cavity diode laser schematics Figure 3-3: Optical feedback as a paraxial approximation Figure 3-4: Intensity pattern for a cavity with different values of reflectivity Figure 3-5: Intensity pattern for a cavity with different values of reflectivity. Figure 3-6: External cavity diode laser gain curves [10] Figure 3-7: Cylinder design for optical feedback alignment Figure 3-8: Grating mount Figure 3-9: Setup for the pivot point problem Figure 3-10: Overall laser system block diagram Table 4-1: Properties of different candidate materials Figure 4-2: Design of the 638.6nm external cavity diode laser Table 4-3: List of parameters Figure 4-4: Gain curves for 638.6nm external cavity diode laser Figure 4-5: IP curve for the 650nm Digikey laser diode without optical feedback-4-Figure 4-6: IP curve for the 650nm Digikey laser diode with optimized optical feedback-5