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Thermal profile of high power laser diode arrays and implications in line-narrowing using external cavities

Thermal profile of high power laser diode arrays and implications in line-narrowing using external cavities. Gregory Blasche Bennett Goldberg Boston University Physics Department M. Selim Ü nl ü Boston University Electrical & Computer Engineering http://ultra.bu.edu/. Motivation

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Thermal profile of high power laser diode arrays and implications in line-narrowing using external cavities

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  1. Thermal profile of high power laser diode arrays and implications in line-narrowing using external cavities Gregory Blasche Bennett Goldberg Boston University Physics Department M. Selim Ünlü Boston University Electrical & Computer Engineering http://ultra.bu.edu/ External Cavity Laser Diode Arrays - Thermal Effects

  2. Motivation Thermal Profile of Laser Diode Arrays Origin of thermal profile Linewidth due to thermal profile External Cavity Laser Diode Array Implications of thermal profile Power and linewidth of external cavity Conclusions Outline External Cavity Laser Diode Arrays - Thermal Effects

  3. Motivation Hyperpolarized Noble Gas Magnetic Resonance Imaging Gases are generated via a spin-exchange with optically pumped Rubidium. Optical absorption linewidth of Rb is 0.2 nm/atm. (Helium3 can be generated at high pressure, Xenon129 is generated at 1 atmosphere) High power laser diode arrays have linewidths greater than 2nm. HP-MRI Human lung images courtesy of Mitch Albert Brigham and Womens Radiology Dept. “Distal Airways in Humans: Dynamic Hyperpolarized 3He MR Imaging— Feasibility” Radiology 2003; 227:575–579 External Cavity Laser Diode Arrays - Thermal Effects

  4. Thermal Profile of Laser Diode Arrays • FWHM linewidth of entire array is >2nm (At 50% operating current) • Linewidth of each diode is <2nm • Broadening is due to inhomogeneous heating/cooling of the laser bar Power/nm (arb. Units) 790 795 800 Wavelength (nm) Spectra for water-cooled LDA (different coolant directions) External Cavity Laser Diode Arrays - Thermal Effects

  5. Air Cooling 797 796.5 Peak Wavelength (nm) 796 795.5 Have constant cooling along bar, but edges have less heating than middle. Inferred from 15W Fiber-coupled LDA (Optopower) 795 794.5 5 10 15 20 25 Diode External Cavity Laser Diode Arrays - Thermal Effects

  6. Water Cooling For feedback, operate at much less current than free-running Diodes in the middle have a lower thermal coefficient than diodes on the edges. As well, the water heats up as it flows through the heat sink. External Cavity Laser Diode Arrays - Thermal Effects

  7. Water Cooling 2.5 2.0 1.5 1.0 44 0.5 Peak Shift (nm) 0.0 -0.5 33 -1.0 -1.5 22 -2.0 0 10 20 30 40 50 11 Diode 1 Results for a 44-emitter 10W Laser Diode Array (similar for 50W) External Cavity Laser Diode Arrays - Thermal Effects

  8. Additional Problems: Non-uniform Thresholds and IV Characteristics 7A 8A Cooling right side Cooling left side 9A Laser Diode Array Imaging 10A Current External Cavity Laser Diode Arrays - Thermal Effects

  9. Use a grating to form a Littrow external cavity. The thermal profile causes an effective detuning of the feedback. Detuning causes a loss of power in the desired feedback mode. External Cavity Laser Diode Array External Cavity Laser Diode Arrays - Thermal Effects

  10. External Cavity Laser Diode Array – Littrow Cavity Scheme fc Top view LDA PBS Grating λ/2 Waveplate Collimating Lens Side view f1 f1 f2 f2 Imaging Lenses Monitor External Cavity Laser Diode Arrays - Thermal Effects

  11. External Cavity Laser Diode Arrays - Thermal Effects

  12. Off-resonance Feedback(Single Diode) 5 50 45 4 40 35 3 30 25 Power/nm (arb. units) Power/nm (arb. units) 2 20 15 1 10 5 0 0 790 792 794 796 798 800 790 792 794 796 798 800 Wavelength Wavelength (nm) Detuning by 2nm gives ~30% loss of power in feedback mode. 15° 25° 35° External Cavity Laser Diode Arrays - Thermal Effects

  13. Off-Resonance Feedback 1.0 1.0 0.9 0.9 0.8 0.8 Relative Maximum Relative Efficiency 0.7 0.7 0.6 0.6 0.5 0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 10 20 30 40 50 Diode λ Shift (nm) Net result is a 20%+/-10% loss of power in feedback mode due to thermal profile External Cavity Laser Diode Arrays - Thermal Effects

  14. Power 20 5 18 0.5 16 4 0.4 14 12 3 0.3 10 0.2 Absorbed Power (W) Total Power (W) 8 2 0.1 6 0.0 4 1 790 795 800 2 0 0 0.2nm fitted peak, 0.4nm fitted entire feedback. Note, not to scale. 10 12 14 16 18 20 Current (A) 0.4nm FB (tot) 0.4nm FB (abs) Free (tot) Free (abs) (note: laser goes up to 50A) External Cavity Laser Diode Arrays - Thermal Effects

  15. Additional Issues Heat Expansion Laser must warm up before operating at correct temperature Bar Curvature Some diodes operate at incorrect wavelength due to being off-axis. Aspheric Aberrations If use simple lenses, diodes off axis will have non-optimal coupling External Cavity Laser Diode Arrays - Thermal Effects

  16. Have developed a >10W, 0.2nm wide line-narrowed laser diode array which should be capable of >20W, with 12W absorbed. Thermal effect limit the total power of the laser due to effective detuning Can tune the laser in frequency and linewidth Conclusions External Cavity Laser Diode Arrays - Thermal Effects

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