# Presentation: 102260

## Microlasers and Nonlinear Optics

**Microlasers and Nonlinear Optics**

**R. Schmitt, D. Armstrong, A. Smith,    B. Do, and **_**Greg Hebner**_
- Sandia National Laboratories
- Laser, 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.

## 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.

## 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 m
- output pulse

- lens

- 808-nm pump light

- Fiber

## 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)

- near field beam intensity (MW/cm2)

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

## 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)

- scan through focus

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

**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

## Tunable 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

- SPIE 5887-3

## Image rotating nonplanar ring “RISTRA” OPO

- “RISTRA” cavity: _R_otated _I_mage _S_ingly-_R_esonant _T_wisted _R_ect_A_ngle JOSA **B** 19, 1801–1814 (2002)

- /2

- Pump in

- **/**2

- Pump out

- OPO: *xz*-cut KTP, ** = 58.4803(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

- SPIE 5887-3

## Near- and far-field  = 803 nm signal fluence profiles

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

- Fresnel **# ****** *D*2 **/ ***L* **>** 450  for Signal = 803 nm

- Far field: Lens with effective f**/#**  77

- Near field: Image of OPO output coupler

- SPIE 5887-3

**Notes:**
Boasting about the RISTRA’s beam quality. It’s very important for high-efficiency UV generation.

## Pump depletion for seeded and unseeded oscillation

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

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

- SPIE 5887-3

## Are flat-top beam profiles important?

- 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 mJGaussian seed 10 mJPump depletion = 92%

- Flat-top pumpFlat-top seed**~** 85 % pump depletion

- SPIE 5887-3

## Maximum extra-cavity UV energy ~ 190 mJ

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

- SPIE 5887-3

**Notes:**
Maximum UV generation. Specs for Scientech head are given in the box. The 803 and 532 nm pulses are shown to emphasize that we can do better because the depletion (532 mixing with 803 to make 320) is not equal. We don’t yet have “photon balance,” where UV generation would approach its maximum.

## End