THE CATHOLIC UNIVERSITY OF AMERICA Technique for Reducing Laser Beam Divergence of Intra-Cavity Nonlinear Conversion A DISSERTATION Submitted to the Faculty of the Department of Electrical Engineering and Computer Sciences School of Engineering Of The Catholic University of America In Partial Fulfillment of the Requirements For the Degree Doctor of Engineering By Alan D. Hays Washington, D.C. 2014 Technique for Reducing Laser Beam Divergence of Intra-Cavity Nonlinear Conversion Alan D. Hays, PhD Director: Scott Mathews, PhD During the past few years the Monoblock laser has become the laser-of-choice for laser range-finders. It’s eye-safe 1570 nm emission, high pulse energy, simple construction, and high efficiency, when pumped by a laser-diode stack, provide advantages that are not available with other laser types. Although the relative divergence of the Monoblock output beam is large, it can be reduced to the required <1 mR using a telescope with a large magnification. This solution, however, is not acceptable for applications where the laser and telescope size must be kept to a minimum. A simple and compact technique for achieving significant reduction in the Monoblock beam divergence using a partial reflector that is placed a short distance from the optical parametric oscillator (OPO) has been developed. Using an ultra-compact 38 mm Monoblock with a 10 mm long KTP OPO, we achieved a beam divergence of <4 mR, corresponding to a >2.5 X reduction from the unmodified laser. Modeling and experimental results are presented detailing the theory and performance for this novel technique. This dissertation by Alan D. Hays fulfills the dissertation requirement for the doctoral degree in Electrical Engineering approved by Dr. Scott A. Mathews, as Director, and by Dr. Jessica Ramella-Roman, and Dr. Lew Goldberg as Readers. _________________________________ Dr. Scott A. Mathews, Director _________________________________ Dr. Jessica Ramella-Roman, Reader _________________________________ Dr. Lew Goldberg, Reader ii Table of Contents 1. Introduction .................................................................................................................................... 1 2. Monoblock ..................................................................................................................................... 4 2.1 Optical Parametric Oscillators ............................................................................................. 13 2.2 Diode-Pumping of Solid-State Laser ................................................................................... 17 3. Etalon Theory ............................................................................................................................... 22 3.1 Composite Output Coupler ........................................................................................................ 27 4. Techniques for Transverse Mode Discrimination ........................................................................ 35 4.1 Variable Reflectivity Mirrors ..................................................................................................... 35 4.2 Polarization Dependent Output Couplers ................................................................................... 38 5. Results and Observations ............................................................................................................. 41 5.2 Experimental Results ................................................................................................................. 47 6. Feedback mirror applied to Nd:YAG laser .................................................................................. 63 7. Conclusions and Future Work ...................................................................................................... 70 8. Bibliography ................................................................................................................................ 73 iii List of Figures Figure 1 - Photograph of Monoblock assembly ............................................................................................ 5 Figure 2 - Schematic of Monoblock detailing optical components and surface coatings ............................. 5 Figure 3 CAD model of assembly and test fixture ........................................................................................ 6 Figure 4 - SMC Output energy distribution for lot shipped ........................................................................ 11 Figure 5 - SMC Divergence distribution for lot shipped ............................................................................ 11 Figure 6 - Tuning curves for x-cut KTP OPO [19], units shown with labels versus angle ........................ 15 Figure 7 - Absorption spectrum of Nd:YAG [24] ....................................................................................... 18 Figure 8 - Absorption convolution with single and three color pumps ....................................................... 20 Figure 9 - Laser diode array specifications, [27] ........................................................................................ 21 Figure 10 - Laser diode array spectrum and I-P curve ................................................................................ 21 Figure 11 - Schematic of etalon .................................................................................................................. 23 Figure 12 - Transmission through etalon versus phase R=0.7 .................................................................... 24 Figure 13 - Transmission of divergent light through etalon ....................................................................... 25 Figure 14 - Schematic of OPO and Feedback Mirror ................................................................................. 28 Figure 15 - Effective reflectivity of composite mirror versus angle, in radians. OPO mirror .................... 29 reflectivities of 60% and feedback mirror of 40%, length 10 mm .............................................................. 29 Figure 16 - Minimum reflection node versus composite mirror length ...................................................... 31 Figure 17 - Angular dependence of the effective reflectivity for three composite mirror .......................... 32 Figure 18 - Angular dependence of the effective reflectivity for feedback mirror reflectivites ................. 33 Figure 19 - Normal incident maximum reflection of composite mirror ..................................................... 34 Figure 20 - Variable Reflectivity profiles for n = 2, 4, 8 ............................................................................ 36 Figure 21 - Transmission vs radius for birefrinegent lens, radial units in mm ........................................... 39 iv Figure 22 - Schematic of experimental arrangement .................................................................................. 41 Figure 23 - Experimental arrangement showing laser diode array, Monoblock assembly, ........................ 42 feedback mirror and interference filter to pass only 1.57 µm energy. ........................................................ 42 Figure 24 - Laser diode pump current pulse (200µsec), 1.064 µm fluorescence build up, and Q .............. 44 switched output ........................................................................................................................................... 44 Figure 25 - Far field beam divergence using 40% R .................................................................................. 48 Figure 26 - Far field beam divergence using 60% R .................................................................................. 49 Figure 27 - Monoblock output energies versus feedback mirror position .................................................. 50 Figure 28 – Laser output energy showing pulse to pulse stability .............................................................. 51 Figure 29 - Beam divergence using convex feedback mirrors .................................................................... 53 Figure 30 - Output energy for convex feedback mirrors............................................................................. 53 Figure 31 - Beam divergence as a function of feedback mirror misalignment ........................................... 54 Figure 32 - Temporal pulselength of the Monoblock output. Peak intensities were normalized for both pulses. (normalized intensity, a.u.) ............................................................................................................. 56 Figure 33 - Monoblock emission spectra without feedback mirror ............................................................ 57 Figure 34 - Monoblock emission spectra with feedback mirror ................................................................. 57 Figure 35 - Far field image with no feedback mirror .................................................................................. 60 Figure 36 - Far Field image with feedback mirror at 5 mm ........................................................................ 61 Figure 37 - Far field image with feedback mirror at 50 mm ....................................................................... 62 Figure 38 - Divergence for 1.064 µm long pulse laser. .............................................................................. 64 Figure 39 - 1.064 µm far field divergence without feedback mirror. ........................................................