4. Laser Energy Transfer

4.1 Laser Energy Beaming

2

Graph1

10000

0.7

1.8

0.1

Power, W

Transmission distance, m

1km100m10m1m10cm1cm0.1cm

Electromagnetic Induction

Electric Resonance

Magnetic Resonance

Laser Energy Beaming

　　　Microwave Energy Beaming

Wireless TagActive Sensor

Electric wheelchairCleaning robot

Devices in wet area

Cordless phone

Y の値

1000

2.7

3.2

0.1

Sheet1

X の値Y の値

100001000

0.72.7

1.83.2

0.10.1

グラフのデータ範囲の大きさを変更するには、範囲の右下隅をドラッグしてください。

Features of Laser WPT

・No interference with communication radio waves used such as GPS. ・Small beam divergence and long transmission distance of the order of 100 km ~10,000 km. ・Lower technical maturity than microwave WPT. ・Harmful to eyes (retina) even at low power density because of its high coherence. (should be less than 1 mW at visible light 400 nm-700 nm)

3

Ground-based lasers

Ground-based lasers

・ Applications: laser space launchers, space vehicle propulsion, space infrastructures’ power source. ・Transmission through the atmosphere ・Output power: 100 MW-1 GW

4

Space-based lasers

Applications

Power supply to

Space station, space factory Satellite Aircraft Terrestrial facilities Propulsion

Spacecraft to the moon Spacecraft to Mars

Others

Space debris removal

・Transmission distance: order of10,000 km ・Transmitter and receiver apertures： Transmitter 1 m, Receiver 2 m (typical divergence 4×10-7rad@1μm) ・Output power：kW-class

5

Airborne Lasers

AL-1: A MW Airborne Laser(Air Force/TRW/ Boeing/ Lockheed-Martin) on a modified Boeing 747-400F

Application: missile defense system to destroy tactical ballistic missiles

6

4.2 Demonstration

7

Power transfer to a Lunar Rover and Rescue Robot.(Kinki University)

Rover model, 1997. 220W transmitted by a LD @ 805.8nm

Experiment in Hawaii Lanai island, 2003 Laser output 32W/CW, beam spot size was 80cm at 1.2km distance. Efficiency 9%.

Rescue Robot, 2005. 200W transmitted by a LD @ 806.1nm. 10m distance. Automatic tracking using corner cubes.

8

Transmission to Micro Aerial Vehicle (NASA Marshall Center)

Laser-Powered Aircraft (NASA Marshall center), 2003

9

A kite plane and auto-tracking/pointing system (Kinki University)

A kite plane with solar cells and Auto-tracking/pointing system, 2007

http://qube.phys.kindai.ac.jp/users/knobuki/homepage/pdf/kite.pdf

10

4.3 High-power Laser Oscillators （kW-GW level）

11

Chemical Gas-dynamic Lasers

1) CO2 Gas-dynamic Laser : λ=10.6μm.

2) HF Laser : λ=2.41～3.38μm. 2MW-output was achieved with MIRACL (Middle Infrared Advanced Chemical Laser.)

3) Chemical Oxygen Iodine Laser (COIL) : λ=1.315μm.

Energy is obtained from combustion and reverse population is created though a supersonic nozzle. Power reaches to MW levels and are used for cutting and drilling, and as directed-energy weapons.

12

CO2 Gas-Dynamic Lasers

CO2 Gas-Dynamic laser

Temperature

1400 K Pressure

17 atm

Chemical Composition

CO2 7.5%, N2 91.3%, H2O, 1.2%

Gas-Dynamic Lasers resemble a rocket engine. In the combustion chamber, free excited CO2 radicals are produced. The excited molecules then undergo stimulated emission in the optical resonator region of the laser.

13

HF/DF chemical laser

( )**2H+F HF F+4.24eVv→ +

( )*2F+H HF H+1.38eVv→ +

H2 /F2 or C2 H2 / NF3 is burned. This reaction produces free excited fluorine radicals. Just after the nozzle, hydrogen or deuterium gas is injected to the exhaust stream producing excited molecules of deuterium or hydrogen fluoride.

It radiates at the transitions between the vibration levels of 3→2, 2→1, 1→0. Oscillation wavelength ranges 2.41～

3.38μm.

MIRACL(Mid-Infrared Advanced Chemical Laser) : 2.2 MW DF (Deuterium Fluoride) laser

MIRACL (for space-based laser)

14

Chemical Oxygen-Iodine Lasers (COIL)

COIL for airborne laser

( )12 2 2 2 2Cl +H O +2KOH 2KCl 2H O+O a→ + ∆( ) ( ) ( )1 2 32 2 3 2 2I +3O a 2I P +3O∆ → Σ

( ) ( ) ( ) ( )2 1 2 33 2 2 1 2 2I P +O a I P +O∆ ↔ ΣResonant excitation

15

Free-electron Lasers

Free electron Laser

• An electron beam accelerated to relativistic speeds passes through an undulator, a periodic transverse magnetic field, which forces the electrons to assume a sinusoidal path, resulting in synchrotron radiation.

• Electron motion is in phase, so that the output light is coherent.

• The wavelength is tunable by adjusting electron energy or the magnetic field strength. λ=49 nm (X ray)～1 μm (far infrared)

16

Laser diode array

Laser diode（LD）array Advantage： low voltage, compact, low weight/power ratio, low thermal deformation, high beam quality, long lifetime (several 10,000 hours, 100 times), high reliability Disadvantage： difficulty in coherent coupling: low beam quality when arrayed. .

17

HAMAMATSU: High-power Laser Diode Bar Module, 1200W (CW)

Solid-state lasers

Rare earths or transition metals were doped in a optical crystal such as Nd:YAG, Ti: sapphire, Er:Fiber (erbium-doped optical fiber). Optically pumped by flush lamps. (YAG: Yttrium Aluminum Garnet ) Advantage：High power & coherent, high beam quality Disadvantage： lower efficiency, high heat load → LD-pumped solid-state laser ☞good wavelength matching(excitation and absorption): high efficiency & low heat load ☞Low threshold power density especially for new crystal Yb:YAG, etc.

18

LD pumped solid-state lasers

1) Nd:YAG rod laser a cylindrical crystal of ϕ6 mm×110 mm is surrounded by

190 optical fibers of 10 W output each; 1.9kW input in total.

☞Multi-longitudinal-mode at CW 450 W: light-light conversion of 40%, electricity-light conversion of 10% ☞ TEM00 mode at CW 450 W: M 2=23. 2) Zig-zag path slab laser ☞ M 2＝3.5@ 3.6 kW was achieved in U.S. PLM project

A rod or slab shape laser crystal is pumped by an LD array stack or an optical-fiber connected LD array

19

4.4 Solar-Pumped Lasers

20

Spectra matching in Solar-Pumped Solid-state Lasers

Nd:YAG absorption spectra

1) Additional absorption by Cr, Ti, Nd, Yb etc. 2) Long optical path of several meters by fiber-shape: High gain, low threshold, high light-light conversion efficiency.

21

22

Solar pumped laser systems Window

Tokyo Institute of Technology

Riken, Nd1%, Cr0.1%:YAlO4

Solar-Pumped Diode Lasers

Energy band diagram of Double hetero structure diode (Quantum well type) laser

GaAs laser threshold density: about 100 A/cm2. GaAs cell production current: 30 mA/cm2 by 1 sun. → 3000 sun (light-concentration) is necessary. Large heat load. Geometrical current enhancement: Annular confinement layer surrounding the active layer achieved 40times-high carrier accumulation capability. → +80 sun concentration

23

4.5 Optical Phased Array (OPA)

24

Optical Array Coherent coupling

(a) Injection Locking Master/Slave structure

(b) Coupled Oscillators No master. Synchronizing with neighbor lasers. Multimode appears at high power. Uniformity get worse with large aperture.

(c) External Cavities Forced mode-lock and phasing by a cavity. No controllers. The aperture is limited.

Mode locking & phasing

25

Injection Locking type LD array

Injection Locking type LD array

Master laser: Stable single mode laser. Slave laser: Amplifier. Power is easily enlarged by parallel

and serial connections.

26

Transmission efficiency of Optical Phased Array

Optical phased array

d

y

w0 x

n nd

z n

0

An OPA is one solution for high power applications:

It drastically reduces development cost because 1) existing laser technologies are applicable and 2) mass-production effect is expectable.

27

Far-Field Patterns of an Optical Array

Far-Field Patterns (Univ. Tokyo)

f = 1.0 (密)

f = 0.4 (疎)

source far-field pattern 0

1

2

MM

L2

Number of array elements (n x n)10x10 20x20 30x300

f=1.0f=0.8f=0.6f=0.4

π/2

Dependence of MML2 on n and f.

MML2 was preserved at π/2 that is identical to the diffraction-limited quality of a uniform profile beam. In other words, the receiver size is independent of n and d for given transmitter size nd.

28

Main-Lobe energy efficiency

Main-Lobe energy efficiency

0.4 0.6 0.8 10

0.5

1

Aperture fill factor (f)

Tran

smiss

ion

effic

ienc

y (η

ML)

z iα

iφiE0

Possible setting errors. Phase error, Pointing error, Intensity error.

Effect of phase error. Far-field patterns of a 10×10 array at z = 10zF. (a)σf /2π=0, (b) σf /2π=0.1.

29

4.6 Laser energy storage

30

Hydrogen Production System（JAXA）

Efficient hydrogen/ethanol production system using laser energy transmitted from in-orbit lasers 31

Magnesium Energy storage (Tokyo Institute of Technology, Mitsubishi Co.)

Reduction of Magnesia (MgO) by laser

2 2Mg+H O MgO+H +86kcal→

2 2 21H + O H O 57.8kcal2

→ +

An alternative fuel. portable, safe, large amount availability.

32

Magnesium Energy Cycle

Magnesium Energy Cycle

33

Summary of laser WPT

34

Ground-based, Space-based, and Airborne lasers will be used for various L-WPT applications.

High power Gas-dynamic lasers will be replaced by Solid lasers.

Solar-pumped solid lasers/diode lasers are attractive because of high energy conversion efficiency.

Laser energy can be stored in energetic materials such as Hydrogen and Mg.

4. Laser Energy Transfer4.1 Laser Energy BeamingFeatures of Laser WPTGround-based lasersSpace-based lasersAirborne Lasers 4.2 DemonstrationPower transfer to a Lunar Rover and Rescue Robot.(Kinki University)Transmission to Micro Aerial Vehicle�(NASA Marshall Center)A kite plane and auto-tracking/pointing system� (Kinki University)4.3 High-power Laser Oscillators�（kW-GW level）Chemical Gas-dynamic LasersCO2 Gas-Dynamic LasersHF/DF chemical laserChemical Oxygen-Iodine Lasers (COIL)Free-electron LasersLaser diode arraySolid-state lasersLD pumped solid-state lasers4.4 Solar-Pumped LasersSpectra matching in Solar-Pumped Solid-state LasersSolar pumped laser systemsSolar-Pumped Diode Lasers4.5 Optical Phased Array (OPA)Optical Array Coherent coupling Injection Locking type LD arrayTransmission efficiency of Optical Phased Array Far-Field Patterns of an Optical ArrayMain-Lobe energy efficiency4.6 Laser energy storageHydrogen Production System（JAXA）Magnesium Energy storage� (Tokyo Institute of Technology, Mitsubishi Co.)Magnesium Energy CycleSummary of laser WPT