1. Single Mode Stabilization of Diode Lasers Ryan Courreges

2. Overview Motivation Introduction to Diode Lasers Theory Locking Methods Volume Holographic Method Future Plans Conclusion

3. Application in Solar Energy With a specially designed holographic optical element (HOE), a large portion of the expensive silicon photovoltaic (PV) cells in a solar panel can be replaced With this Holographic Planar Concentrator™ (HPC) technology, the solar modules produce more energy in all conditions when compared to a standard module with an equal area of silicon PV cells.

4. Application in Solar Energy

5. Introduction to Diode Lasers Inherently multi-mode – Large line width – Poor coherence length Why not gas or DPSS? – Cost – Size and packaging – Additional requirements for operation  Single mode diode laser necessary

6. Theory Why are diode lasers multi-mode? – Large bandwidth gain medium – Optical cavity supports many modes How is a diode laser made to operate in single mode? – Feedback Full Spectrum Wavelength Specific

7. Locking Methods Simple Extended Cavity Pros – Very easy to align – Inexpensive – Stabilizes mode hops Cons – Large linewidth unsuitable for many holographic applications

8. Locking Methods Littrow Pros – Wavelength tunable – Inexpensive – Narrow linewidth Cons – Beam alignment changes with changes in cavity alignment – Some power loss in secondary orders

9. Locking Methods Littman-Metcalf Pros – Wavelength tunable – Inexpensive – Narrow linewidth Cons – More difficult alignment than standard Littrow configuration – More power loss than standard Littrow configuration

10. Locking Methods Volume Holographic Grating Pros – Very narrow linewidth – Can be wavelength tunable – Very small package size Cons – Precise alignment is needed and must be maintained – VHGs are difficult to produce and expensive to purchase.

11. Volume Holographic Method Why would one choose to use a VHG instead of another method to mode-lock the laser given the downsides? – With proper temperature and current controls, a diode locked with a VHG can remain stable for long periods of time. – The size of the entire mode-locked laser unit can be made small, making for a versatile package. – Truly single mode operation is possible.

12. Volume Holographic Method Free Running Diode OutputPoorly Stabilized Diode Output

13. Volume Holographic Method Mode Locked Diode Laser

14. Volume Holographic Method Recall the issue of alignment – With the precision alignment required, keeping a laser locked long term can be difficult Small changes in temperature can effect alignment There are methods to improve this however – A self-aligning setup is possible – If a thinner holographic grating is used, the angular selectivity will be reduced.

15. Volume Holographic Grating Self-aligning MethodDirect Attachment to Diode Only a coarse alignment is needed for laser to be locked Wavelength of the laser can be changed by adjusting the angle of the grating Thin grating is less angularly and wavelength selective Grating will lock the diode provided the system is controlled to the proper temperature Linewidth larger than with thicker gratings

16. Plans for the Future Set up a high precision alignment station for further testing and development Run long term mode stability tests of locked laser diodes Design production level prototype modules Begin testing and development of holographic recording Implement laser system into full scale production line

17. Diode lasers are preferable to other laser types – Diodes in general are less expensive than gas and DPSS systems Most diode lasers are multi-mode and have short coherence lengths thus making them unsuitable for holography – With an extended cavity the linewidth of the diode can be significantly reduced and single mode operation is possible – With a smaller linewidth the diode will have a longer coherence length making the laser usable for recording holograms Conclusion

18. Acknowledgements I would like to thank Dr. Alex Cronin for all his support and guidance on this project. I would also like to thank everyone at Prism Solar Technologies for giving me the opportunity and means to pursue my project goals.