Optics, Lasers, and Remote Sensing Department 1128

Microlasers and Nonlinear Optics

R. Schmitt, D. Armstrong, A. Smith, B. Do, and Greg Hebner

Sandia National LaboratoriesLaser, Remote Sensing, Plasma Physics and Complex Systems

Department 1128

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Optics, Lasers, and Remote Sensing Department 1128

Emerging and enabling technologies require investment

• Long history of atomic and molecular physics– Spectroscopy, optical surface diagnostics

• Fiber lasers– Emerging compact high power sources.

• Frequency extension, nonlinear optics– Generate wavelength(s) matched to the mission

requirements.– Recent advances in nonlinear optics understanding are

enabling new system designs.

• Exploiting the THz region of the EM spectra– New source and detector technology is opening up the THz

spectral region.– High chemical specificity, unique and not widely published

spectral signatures, low probability of intercept, broad area imaging using SAR like processing.

• Compact, robust sources– Micro lasers, high power in a small size.– Solid-state and semiconductor laser systems

• UV solid state lasers are relatively well developed at 400 nm with 290 nm sources on the horizon. Wavelengths are a good fit to some remote sensing opportunities.

0.6 THz image

1.5 1 0.5 0 0.5 1 1.51.5

1

0.5

0

0.5

1

1.5

Optics, Lasers, and Remote Sensing Department 1128

Basic design for passively Q-switched microlaser

• fiber-coupled diode laser pump (electrical isolation)• passively Q-switched (electrical isolation)• Cr:Nd:GSGG active material (rad hard)• Cr4+:YAG Q-switch (rad hard)• simple cavity with mirror coatings directly on crystal faces• crystals bonded together to form rugged, monolithic laser• thermal lensing and gain guiding stabilize flat-flat cavity

Cr:Nd:GSGG Cr4+:YAG 1.06 moutput pulse

lens808-nm pump light

Fiber

Optics, Lasers, and Remote Sensing Department 1128

The Cr:Nd:GSGG microlaser produces ~1.6-ns-wide pulses

• Eout = 57 μJ / pulse

• ~1.6 ns (FWHM) pulse width

• raw beam Imean ~ 150 MW/cm2

• after-pulse is common– ~15 - 25% total energy in

second pulse typical

– affected by details of pump focus, pump beam quality

• linearly polarized (>100:1)-5 0 5 10 15 200

50

100

150

time (ns)

Inte

nsity

(M

W/c

m2 )

1.6 ns FWHM

near field beam intensity (MW/cm2)

output energy is scalable from μJ to 100’s of μJ

Optics, Lasers, and Remote Sensing Department 1128

The Cr:Nd:GSGG microlaser produces excellent beam quality

• The near-field beam diameter (1/e2) is 150 μm (h) x 144 μm (v).

• The far-field divergence (1/e2 full angle) is 10.2 mRad (h) x 10.6 mRad (v).

• M2 ≈ 1.05 (fitted second moment beam diameter to propagation equation)

• Eout = 57 μJ / pulse Fpeak = 0.67 J / cm2 Imax ~ 300 MW/cm2 (raw beam)

near-field image

distance (mm)

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tan

ce (

mm

)

0.8 0.9 1

0.7

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far field image

divergence (mRad)

div

erg

ence

(m

Ra

d)

2 4 6 8 10 12

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80 100 120 140 160 180 200 220 240

seco

nd

mo

me

nt d

iam

ete

r (m

m)

distance (mm)

scan through focus

Optics, Lasers, and Remote Sensing Department 1128

laser 1 2

3

Waveguides written directly into bulk material can be used for optical interconnects

• Bulk optical waveguide elements provide functional “circuits” while fibers provides the “wires”.

• Waveguide structures are a building block for buried optical computing.

• All optical interconnects are a significant safety improvement.

– Alignment and materials are very robust but if broken, can be time consuming to repair.

• Waveguides in bulk material fail based upon well understood material properties (heat, water).

Phase contrast image of a waveguide written in borosilicate glass

Ring coupling structures have been manufactured using femtosecond laser machining.

Near field mode profile of a laser written waveguide is near Gaussian.

Writing custom optical elements directly into bulk glass

Optics, Lasers, and Remote Sensing Department 1128

Tunable high pulse energy UV: A difficult problemTunable high pulse energy UV: A difficult problem

• Typical method: SFG using Nd:YAG-pumped ns OPO signal + Nd:YAG 2

– 803 nm + 532 nm 320 nm

• Many problems to overcome:– High-energy ns OPO beam quality is poor

– Nanosecond OPO’s start late and back-convert– Q-switched Nd:YAG beam quality is poor

100/ cavitysignal2pump LD F

Signal

Depletedpump

F 200

Critical direction

OPO signal far-field fluence

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

Image rotating nonplanar ring “RISTRA” OPOImage rotating nonplanar ring “RISTRA” OPO

“RISTRA” cavity: Rotated Image Singly-Resonant Twisted RectAngle JOSA B 19, 1801–1814 (2002)

“RISTRA” cavity: Rotated Image Singly-Resonant Twisted RectAngle JOSA B 19, 1801–1814 (2002)

/2

Pump in

/2

Pump out

OPO: xz-cut KTP, = 58.4803(e) + 1576(o) 532(o)

10 × 10 × 15 mm3

SFG crystal Type-II BBO, = 48.2

803(e) + 532(o) 320(e)

Pulsed “self seed”

beam

UV out

Signal out

Mechanically robust. Long-term stability

No mirror adjustments

Mechanically robust. Long-term stability

No mirror adjustments

1.975"

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

Near- and far-field = 803 nm signal fluence profilesNear- and far-field = 803 nm signal fluence profiles

Fresnel # D2 / L > 450 for Signal = 803 nm Fresnel # D2 / L > 450 for Signal = 803 nm

Far field: Lens with effective f/# 77Far field: Lens with effective f/# 77Near field: Image of OPO output couplerNear field: Image of OPO output coupler

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

Pump depletion for seeded and unseeded oscillationPump depletion for seeded and unseeded oscillation

Self-seeded oscillation in two-crystal RISTRA~85% pump depletion

Self-seeded oscillation in two-crystal RISTRA~85% pump depletion

Free-running oscillation in two-crystal RISTRA ~37% pump depletion

Free-running oscillation in two-crystal RISTRA ~37% pump depletion

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

Are flat-top beam profiles important?Are flat-top beam profiles important?

2nd-order Gaussian pump0th-order Gaussian seed

~52% pump depletion

2nd-order Gaussian pump0th-order Gaussian seed

~52% pump depletion

Flat-top pump 100 mJ flat-top seed 10 mJ

Pump depletion = 93%

Flat-top pump 100 mJ flat-top seed 10 mJ

Pump depletion = 93%

Flat-top pump 100 mJGaussian seed 10 mJPump depletion = 92%

Flat-top pump 100 mJGaussian seed 10 mJPump depletion = 92%

Flat-top pumpFlat-top seed

~ 85 % pump depletion

Flat-top pumpFlat-top seed

~ 85 % pump depletion

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

Maximum extra-cavity UV energy ~ 190 mJMaximum extra-cavity UV energy ~ 190 mJ

Detector: Scientech 380101 absorberCalibration: 1 mJ/mV @ 10 HzTransmission loss: ~5%Efficiency: 1064 nm to 320 nm > 21%

Detector: Scientech 380101 absorberCalibration: 1 mJ/mV @ 10 HzTransmission loss: ~5%Efficiency: 1064 nm to 320 nm > 21%

Scientech 380101

Depleted SFG Pulses67 % depletion of 80350 % depletion of 532UV energy ~ 180 mJ

Depleted SFG Pulses67 % depletion of 80350 % depletion of 532UV energy ~ 180 mJ

SPIE 5887-3

Optics, Lasers, and Remote Sensing Department 1128

End