CORM 2015 Exploring optical radiation pressure as a convenient measure of high-power Paul A. Williams Joshua A. Hadler Brian Simonds Gordon A. Shaw Nathan A. Tomlin Ari Feldman Amanda Higley Marla L. Dowell John H. Lehman laser emission Daniel King (Naval Surface Warfare Center) Robert Lee Frank C. Maring (Scientech, Inc.)
High-power laser radiometry: Limitations How do you ACCURATELY measure the power in a laser which is designed to DESTROY things? Primary standards: High power laser radiometry 1-100 kw Fundamentally-calibrated Directly traceable to the SI Calorimeters & power meters 78 kw Traditional approach Big Heavy Slow Exclusive Radiation pressure Small Light Fast Simultaneous use
Outline: 1. Traditional approach to high-power laser radiometry 2. Advantages of Radiation pressure radiometry 3. Prototype results 4. Further applications
Traditional approach: measure optical power as heat Simple concept Traditional approach: Energy meter Absorption-based Energy DT Incident Energy T Calorimeter: Energy range: < 300 kj Response time: minutes Size: cubic meters Weight: 100 s of pounds Uncertainty: 1 % Current approach: Power meter Absorption-based Power T 2 -T 1 T 1 Incident Power T 2 Flowing water power meter: Power range: 1-100 kw Response time: tens of seconds Size: cubic meters Weight: hundreds of pounds Uncertainty: 1 %
Traditional approach: improvements Traditional high-accuracy laser power meters absorb incident light. (+) Allows high accuracy ~ 1% (-) Requires large thermal mass (big, heavy, slow, exclusive) Traditional calorimetry ~100 % absorption power~ T Pickoff beamsplitter: Few % absorption (poor accuracy) beam splitter Ideal technique: Measures entire beam Does not require absorption of beam power~ T/b.s. ratio
Radiation pressure: Light has momentum. A photon s impact causes a force. p = h/l (Comet tail) (Optical tweezers) F An absorbed photon imparts a force F (Spacecraft) 2F A reflected photon imparts a force 2F
Radiation pressure measures optical power Concept: A precision scale with a mirror attached can measure the radiation force of light. Minimal absorption, power-scalable, no thermal recovery time. Traceable to the kilogram q F (2 P / c) r cosq F = Force (Newtons) P = optical power (Watts) c = speed of light (m/s) r = R+(1-R)a/2 reflectivity q = angle of incidence P F ( c / 2r cos q ) Calibration done with a standard mass. Horizontal force operation: Ease of use
What kind of scale sensitivity is required Conversion factor (for normal incidence and perfectly reflecting mirror): k 9 6.67 10 N/W 670 mg / kw Laser power Application Equivalent mass Object 10 W marking 6.7 microgram eyelash 1 kw welding/cutting 670 microgram grain of sand 100 kw Research / Defense 67 milligrams two staples Goal: Power range: 1 kw 100 kw Accuracy: 1-5 % Speed: few seconds (or less) Size: <0.5 m 3 Weight: < 10 kg Commercial ( off the shelf ) scale 10 mg readability 4 s time constant Direct-loading allows horizontal or vertical operation). Replace pan with a mirror
Mass (g) Early Proof of concept work ( low power) Yb fiber laser 500 W @ 1.07 mm Squarewave Gen. (0.1 Hz) 2q 24.1010 24.1000 Scale: Scientech, Inc. Readability 10 mg (100 nn or 15 W) Air current / acoustic noise 24.0990 Thermal effect 200 400 Time (s) 600
Shaft temperature (C) 2 nd generation prototype (thermal / acoustic) Enclosed housing A.R. coated windows Insulate mirror back Secondary heat barrier 76 mm dia. Fused silica (no insulation) 76 mm dia. Fused silica (WITH insulation) 28.95 28.90 28.85 28.80 28.75 28.70-200 0 200 400 600 Highreflectivity mirror Time (s)
Mass (g) 2 nd generation prototype Mass (g) Fiber laser (1.07 mm) Calibrated thermopile (+/- 1 %) NIST multi-kw power meter calibration lab.: 10 kw fiber laser Less noise but thermal effects persist 5 kw CW injection 3.123 5 kw modulated injection 3.2520 3.122 3.121 3.2510 3.2500 3.2490 3.120 3.119 0 200 400 600 800 1000 1200 28.82 28.80 28.78 28.76 28.74 28.72 Shaft temp (C) 0 100 200 300 400 Time (s) Time (s)
Preliminary (best) results: slope removed Measured power (W) Measured Power (W) Measured power (W) Without shielding Modulate (5 kw @ 50 mhz): ~1 % discrepancy 2000 Last year modulation 0 ( 500 W) discrepancy ~ 7 % 1000-1000 -2000 0 50 100 150 200 250 Time (s) Constant power (1 kw): 8% discrepancy Constant power (5 kw): 1% discrepancy 1000 800 600 400 200 0 0 Radiation pressure Thermopile 100 200 Time (s) 300 400 5000 4000 3000 2000 1000 0-100 0 100 Time (s) Radiation pressure Thermopile 200
Where is this linear drift coming from? Radiation force, F P cos(q) 2q Radiometric force, F P (requires thin plate, vacuum) hot cold Radiation force depends on incident angle of light. What is this linear drift? Not radiometric (Crookes radiometer) Simple heating of scale Convection within the chamber
Finite element modelling of convection Heat Input = (1-R)*P 0 Forces: 1) Radiation Pressure: F Rad = 2 R P 0 cos 2 (θ) c 2) Buoyancy Force: (drives convection) F B = g(ρ i ρ 0 ) Boundary Conditions: Hermetic Sealed Box Thermal convection losses Thermal radiation losses P 0 P 0 *R
Simulation results Velocity (m/s) Temperature (C) Heating of mirror drives natural convection and creates a pressure differential Total Force on mirror found by integrating pressure on all surfaces of mirror and summing Temperature rise of scale roughly matches that measured with IR camera 60 s. illumination \ 60 s. OFF P = 500 W, R = 99.6%
Compare simulation to experiment 1.2 1.0 Simulation Data Simulation plus f(t) 0.8 Mass (mg) 0.6 0.4 0.2 0.0-0.2 0 100 200 300 400 500 600 Time (s) Black curve: radiation pressure + CONVECTION. Dashed curve: adds a slow function to imitate scale heating.
High power capability? Mirror: (High reflectance at 10.6 mm, Lightweight) Silicon wafer (20 cm dia., Au/dielectric coating) 99.8 % reflectance 100 kw CO 2 laser, (Wright-Patterson AFB) 10 cm beam dia., 1.2 kw/cm 2 CO 2 laser Early results no thermal insulation, minimal acoustic shielding. (3-second shot 92 kw) Calibrated (6%) calorimeter
Other applications Fundamental comparison: kilogram and optical Watt: Mass calibrates high optical powers / Optical power calibrates small masses Mass-optical-power comparison with NIST electrostatic force balance In situ calibration for manufacturing lasers (100 5 kw, 10-100 Hz repetition rates) Need: lower noise, increased bandwidth (next-generation force measurement)
Conclusions: Radiation pressure is a promising technique for highpower laser radiometry (size, weight, portability, response time) Must understand/reduce thermal drift Higher mirror reflectance Non-air chamber environment Perform high accuracy comparison with existing technology High-speed applications ~ 100 Hz will require new forcemeasurement approach