DIODE PUMPED ALKALI LASERS (DPAL) AND OTHER Matthew …

AFRL-RD-PS- TP-2018-0021

AFRL-RD-PS- TP-2018-0021

DIODE PUMPED ALKALI LASERS (DPAL) AND OTHER GAS LASERS

Matthew Cooper

29 November 2018

Technical Paper

AIR FORCE RESEARCH LABORATORY Directed Energy Directorate 3550 Aberdeen Ave SE AIR FORCE MATERIEL COMMAND KIRTLAND AIR FORCE BASE, NM 87117-5776

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Using government drawings, specifications, or other data included in this document for any purpose other than government procurement does not in any way obligate the U.S. Government. The fact that the government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them.

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AFRL-RD-PS-TP-2018-0021 HAS BEEN REVIEWED AND IS APPROVED FOR PUBLICATION IN ACCORDANCE WITH ASSIGNED DISTRIBUTION STATEMENT.

CAPTAIN MATTHEW COOPER , DR-IChief, Laser Division

This report is published in the interest of scientific and technical information exchange, and its publication does not constitute the Government’s approval or disapproval of its ideas or findings.

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Oct 2016-Oct 2018 4. TITLE AND SUBTITLEDIODE PUMPED ALKALI LASERS (DPAL) AND OTHER GAS LASERS

5a. CONTRACT NUMBER

In-House 5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)Matthew CooperGreg A. Pitz

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5f. WORK UNIT NUMBER

D058 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

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AFRL/RDLT

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13. SUPPLEMENTARY NOTES

14. ABSTRACTThe Next generation program has achieved the highest power level for a potassium DPAL. This was achieved through a vigorous experimental andmodeling effort. They have also looked into alternate lasing schemes such as the first pulsed DPAL. Additionally, the program studied a newconcept for pumping an alkali laser using an erbium doped ZBLAN fiber.

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NUMBER(S)AFRL-RD-PS- TP-2018-0021

12. DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release: distribution unlimited.

OPS-18-22110. AFMC-2019-0391

Air Force Research Laboratory3550 Aberdeen Ave, SEKirtland AFB, NM 87117-5776

Gas Lasers, Diode Pumped Alkali Lasers, Cavity Dumped DPAL, Fiber Pumped Alkali Laser, Alkali Laser Modeling, CFD Modeling

SAR

2018 Interim Report for Case File D058

(Diode Pumped Alkali Lasers)P M : C a p t M a t t h e w A C o o p e r

P I : D r . G r e g A . P i t z ,

Approved for public release: distribution unlimited.

OutlineI. DPAL Refresher

II. Scaling of flowing DPAL system at AFRL

III. Cavity dumped DPAL

IV. Fiber Pumped Alkali Laser


Diode Pumped Alkali Lasers (DPAL)

n2P3/2

n2S1/2

n2P1/2

λlaser

Spin-Mixing

λpumps

AlkaliAtom

Pump D2wavelength

(nm)

Lasing D1wavelength

(nm)Spin-Orbit

Splitting (cm-1)

K 766.70 770.11 57.7

Rb 780.25 794.98 237

Cs 852.35 894.59 554

Positive Attributes of DPALs• Excitation wavelength accessible

by diode laser• Small quantum defect (~0.5 – 5%)• Gas phase lasing media

Lots of low power, poor BQ diodes in…bright intense beam out


Presenter

Presentation Notes

The gas laser program has many facets, but its bread and butter is the diode pumped alkali lasers. This electric hybrid laser utilizes all the benefits of a solid state laser (being electrically driven) and all the advantages of a chemical laser, ie the ability to flow the gain medium. DPALs does this by employing diode pumps, much like fibers do, to pump an alkali along its D2 transition. This transition is spectrally matched to the pump by either large amounts of pressure in the gas or by narrow banding the diodes. Our approach ulitzes ultra narrow banded diodes and relatively low pressures. The populations is then transferred to the P1/2 state through collsions with the buffer gas. If we were using high pressures helium would be sufficient for the task, but we aren’t so we use hydrocarbons, such as methane to do the spin mixing. It then will lase along the D1 transition. Lasing wavlength differs for each alkali, 895 for Cs, 795 for Rb, and 770 for K.

Current FDPAL Setup

Laser head w/Brewster windows

Gas flow direction

Same simple flow design from 2009• Decouples the thermal management system

from the lasing head• Reduce thermal gradients in the lasing region

•Improved BQ•Added mixing nozzle to add in hydrocarbon postalkali bed to avoid breakdown

Continued exploration of Open Loop• Moving through open loop trade space for

expanded model validation• Pressures, flow rates, gas mixtures, alkali

concentrations, etc.


Presenter

Presentation Notes

Describe why AFRL is pursuing a flowing gain medium as opposed to a static cell with micro-coolers

Flowing DPAL at AFRL

Curtain feeds

Curtain channel

Gas Flow

Current Laser head Design:

• Triply Transverse Design• Reduce intensity on windows• Added Brewster windows on

laser side

• Curtain flows:• Helium gas flow• Both pump, lasing windows


Presenter

Presentation Notes

In the past year or so we have had four major areas of advancement. We have upgraded the test bed in order to utilize a pletherera of various diodes from HELJTO, MDA, and the RD UFR process. We have improve our diode coupling improving O:O effeciency out our system. We have bench mark our models with higher and higher powers by employing numerous and various diodes into out system. 3D Rendering is already in public domain

DPAL 8 Level Model

• Finite volume CFD with thermal transport• Laser kinetics• Radiation transport• Spectral Resolved Ray-Optic Pumping• Ray Optic or Wave Optic Laser Model

• Numerous rates missing, usinganalogous rates from other alkali as basisfor assumptions.

Process Formula Description

Alkali number density decrease in high temperature regions

Results from fluid dynamics

Convective flow Results from inclusion of buoyancy in fluid dynamics

Pressure broadening and shifting of the absorption lineshape

including hyperfine structure

Pressure dependent variations in absorption lineshape

Radiative decay A* → A + hν

Quenching A* + M → A + M

Off-resonant absorption* A*+ hν → A**

Penning ionization A** + A* → A+ + e- + A

Radiative recombination** A+ + e- → A** + hν

Three-body recombination** A+ + 2e- → A** + e-

A+ + e- + M→ A** + M

Thermal aberration Change to resonant mode structure

Dissociative recombination** A+ + A + M→ A2+ + M, then

A2+ + e- → A** + A

Multi-photon ionization* A*+ 2hν → A+

Alkali-hydrocarbon reaction A** + HC → AH + products

Electron impact ionization* e- + A* →(e-)*, then(e-)*+A**→A+ +2 e-Approved for public release: distribution unlimited.

Presenter

Presentation Notes

AFRL’s DPAL model consists of 8-10 levels (depending on ionization is included). It can include numerous processes depending on output fidelity needed and computational expense. All modeling includes finite volume CFD, thermal transport, radiation transport. We use a spectrally resoved ray optic pumping model to simulate the pumping architecture. Additionally, we can use either a ray Optic or wave optic laser model for the output beam. ASSUMING CONSTANT INTENSITY Over simplified

Down Laser Axis

Down Pump Axis

Pump beam does not span alkali flow because the flow thermally expanded

Gain (During Steady-State Lasing)

High GainLow GainAbsorption

High GainLow GainAbsorption


Presenter

Presentation Notes

Gain with in the cavity from two different perspectives.

Down Laser Axis

Down Pump Axis

Location of downstream thermo-couple

Temperature

HighLow

HighLow


Presenter

Presentation Notes

This is an example of the temperature rise within the DPAL Laser medium. We have two different perspectives dwont eh laser and pump axis.

Down Laser Axis

Down Pump Axis

Velocity

FastSlow

FastSlow


Presenter

Presentation Notes

The flow includes the helium curtains all the way around gain medium as well as the the purge in the Brewster arms. The temperature rise within the medium does cause an acceleration of the gas flow.

Down Laser Axis

Down Pump Axis

Index of Refraction


Power (W) Percent of Pump

Pump Power 5525 N/ALaser Power 4395 79.5%

Unabsorbed Pump Power 23 0.4%Optical Loss (at windows & mirrors) 160 2.9%

Fluorescence 448 8.1%Heat 499 9.0%

Optimized SimulationPower and Efficiency Results

Peak Temperature Rise: 333 oC


Presenter

Presentation Notes

Predictions from laser model, based on simplified 3 Level laser model with CFD and Thermal transport.

Results- 13 Modules

- 11 with 425W of max power- 2 with 360W of max power

- Slope efficiency of 66%- Predicted 72%- wrt absorbed power 83%

- O:O efficiency of 58%- Predicted 68%- wrt absorbed power 74%

- 3kW Out - Highest published power to ourknowledge for a diode pumped alkali laser


Presenter

Presentation Notes

These are the results from testing.

Peak

Pow

er, P

pk(W

)

Period, T (μs)

Cavity Dumped DPAL• Dr. Masamori Endo suggested Cavity dumping is a viable energy storage method for

alkali lasers• Alkali lifetimes too short for Q-switching

• AFRL produced World’s First Cavity Dumped Alkali Laser• Characterizing to assess potential at small-scale (10’s mW)• Characterizing a gas phase rotator to be employed with FDPAL

• Initial concept review being perform mid-December

Theory Experiment


Presenter

Presentation Notes

Due to the short lifetime of the alkalis laser cannot store energy with in the gain medium. However, Dr. Endo Masamori proposed using a cavity dumped approach to pulse a DPAL system. We demonstrated this concept at the mW level using a pockels cell to rotate the polarization to dump the cavity. AFRL is also developing a gas phase rotator to assess the potential for scaling.

Gas Phase Faraday Rotator Characterization• Solid-state rotation, limits scalability

• Able to map spectral-dependency of FR using alkali metal vaporlasing cell

• Identified coil temperature issues for high current required forpolarization rotation

• Completed design of new cooled coil to allow pulsed operation of highcurrent to drive magnetic field

• Planned cavity dump demonstration using new coil later this month


Presenter

Presentation Notes

Gas Phase Rotator is possible using alkali and anomalous dispersion. We have characterize a simple cell within a coil for the purpose of designing a pulsed switch for integration into Flowing DPAL.

• Goal: create a fiber capable of pumpingan alkali

• Purpose: provides a narrow bandsource for atomic transition whileproviding a brightness converter in lieuof beam combination

Results- Max power out 172mW

- Efficiency of 27% wrtpower absorbed

- Demonstrated firstFiber-Pumped AlkaliLaser

Possible Future Work:- Hyperfine Lasing within

alkali

- Fiber study to improveefficiency of Er-ZBLAN

Ener

gy (e

V)

Ener

gy (c

m-1

)

Pow

er O

ut, P

o(m

W)

Power Absorbed, PAbs (mW)

Inte

nsity

, I (a

rb)

Fiber Pumped Alkali Laser


Summary• Demonstrated high power DPAL approaching 5kW

• Developed high power validated numeric DPAL Model

• Demonstrated first cavity dumped alkali laser

• Demonstrated first fiber pumped alkali laser


Presenter

Presentation Notes

Summary of work at AFRL

DISTRIBUTION LIST

1 cy

1 cy

DTIC/OCP 8725 John J. Kingman Rd, Suite 0944 Ft Belvoir, VA 22060-6218

AFRL/RVIL Kirtland AFB, NM 87117-5776

Matthew Cooper Official Record Copy AFRL/RDLTD 1 cy


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