1. PHOTONICS CENTER 6/3/20031External Cavity Laser Diode Arrays - Thermal Effects Gregory Blasche Bennett Goldberg Boston University Physics Department M. Selim Ünlü Boston University Electrical & Computer Engineering http://ultra.bu.edu/ Thermal profile of high power laser diode arrays and implications in line- narrowing using external cavities

2. PHOTONICS CENTER 6/3/20032External Cavity Laser Diode Arrays - Thermal Effects Outline  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

3. PHOTONICS CENTER 6/3/20033External Cavity Laser Diode Arrays - Thermal Effects Motivation HP-MRI Human lung images courtesy of Mitch Albert Brigham and Womens Radiology Dept. “Distal Airways in Humans: Dynamic Hyperpolarized 3 He MR Imaging— Feasibility” Radiology 2003; 227:575–579 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. (Helium 3 can be generated at high pressure, Xenon 129 is generated at 1 atmosphere) High power laser diode arrays have linewidths greater than 2nm.

4. PHOTONICS CENTER 6/3/20034External Cavity Laser Diode Arrays - Thermal Effects 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 790795800 Wavelength (nm) Power/nm (arb. Units) Spectra for water-cooled LDA (different coolant directions)

5. PHOTONICS CENTER 6/3/20035External Cavity Laser Diode Arrays - Thermal Effects Air Cooling Have constant cooling along bar, but edges have less heating than middle. Inferred from 15W Fiber-coupled LDA (Optopower) Diode Peak Wavelength (nm) 5 10 15 20 25 795.5 795 794.5 796 796.5 797

6. PHOTONICS CENTER 6/3/20036External Cavity Laser Diode Arrays - Thermal Effects 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.

7. PHOTONICS CENTER 6/3/20037External Cavity Laser Diode Arrays - Thermal Effects Water Cooling Wavelength (nm) Power/nm (arb. units) 795800

8. PHOTONICS CENTER 6/3/20038External Cavity Laser Diode Arrays - Thermal Effects Water Cooling 1 11 22 33 44 Results for a 44-emitter 10W Laser Diode Array (similar for 50W) 01020304050 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Diode Peak Shift (nm)

9. PHOTONICS CENTER 6/3/20039External Cavity Laser Diode Arrays - Thermal Effects Additional Problems: Non-uniform Thresholds and IV Characteristics 7A 8A 9A 10A Current Laser Diode Array Imaging Cooling right side Cooling left side

10. PHOTONICS CENTER 6/3/200310External Cavity Laser Diode Arrays - Thermal Effects External Cavity Laser Diode Array  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.

11. PHOTONICS CENTER 6/3/200311External Cavity Laser Diode Arrays - Thermal Effects External Cavity Laser Diode Array – Littrow Cavity Scheme fcfc f1f1 f1f1 f2f2 f2f2 Top view Side view LDA Collimating Lens Imaging Lenses PBS λ/2 Waveplate Grating Monitor

12. PHOTONICS CENTER 6/3/200312External Cavity Laser Diode Arrays - Thermal Effects

13. PHOTONICS CENTER 6/3/200313External Cavity Laser Diode Arrays - Thermal Effects Off-resonance Feedback (Single Diode) 15° 25° 35° Detuning by 2nm gives ~30% loss of power in feedback mode. 790792794796798800 0 1 2 3 4 5 Power/nm (arb. units) Wavelength 790792794796798800 0 5 10 15 20 25 30 35 40 45 50 Power/nm (arb. units) Wavelength (nm)

14. PHOTONICS CENTER 6/3/200314External Cavity Laser Diode Arrays - Thermal Effects Off-Resonance Feedback 01020304050 0.5 0.6 0.7 0.8 0.9 1.0 0.00.51.01.52.02.5 0.5 0.6 0.7 0.8 0.9 1.0 λ Shift (nm) Relative Maximum Diode Relative Efficiency Net result is a 20%+/-10% loss of power in feedback mode due to thermal profile

15. PHOTONICS CENTER 6/3/200315External Cavity Laser Diode Arrays - Thermal Effects 794796 Linewidth as a Function of Grating Reflectivity 020406080 Waveplate Angle 1.0 0.8 0.6 0.4 0.2 Transmission Reflection Wavelength (nm) Power/nm (arb. units) 0°0° 45° 90° 795

16. PHOTONICS CENTER 6/3/200316External Cavity Laser Diode Arrays - Thermal Effects Power Current (A) Absorbed Power (W) Total Power (W) 0.4nm FB (tot) 0.4nm FB (abs) Free (tot) Free (abs) (note: laser goes up to 50A) 101214161820 0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 790795800 0.0 0.1 0.2 0.3 0.4 0.5 0.2nm fitted peak, 0.4nm fitted entire feedback. Note, not to scale.

17. PHOTONICS CENTER 6/3/200317External Cavity Laser Diode Arrays - Thermal Effects Aspheric Aberrations If use simple lenses, diodes off axis will have non-optimal coupling Bar Curvature Some diodes operate at incorrect wavelength due to being off-axis. Heat Expansion Laser must warm up before operating at correct temperature Additional Issues

18. PHOTONICS CENTER 6/3/200318External Cavity Laser Diode Arrays - Thermal Effects Conclusions  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