Unstable Resonator Semiconductor Lasers

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Unstable Resonator Semiconductor Lasers
Gregory Dente (GCD Associates) Michael Tilton,
Joseph Chavez, Donald Gianardi Jr. (Boeing
LTS) Andrew Ongstad, Ron Kaspi (AFRL/DELS)
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Outline

• Introduction
• - Beam Quality in High-Power
  Semiconductor Lasers
• - Mode-Media Interactions (Filament
  Formation)
• II. Suppressing Filaments
• Unstable Resonators for semiconductor lasers.
• Experimental results
• Summary Outlook

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I. Introduction Beam Quality in High Power SLs
lt 10 mm

- Narrow stripe gives good BQ but low power
(10-20 mW/?m of stripe)
300 - 3000 ?m
- Wide stripe gives poor BQ but high power
mode break-up with filament formation occurs
100 - 500 ?m
Carrier Variations Temp Variations
Gain Index Variations (antiguiding)
Intensity Variations
Mode-Media Interaction
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Gain / Index in Semiconductor Lasers
E
conduction

• Band to Band transition

momentum
valence (heavy hole)

• Saturable index at gain peak creates
  self-focusing

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II. The Tilted Amplifier Solution

• Several research groups have observed that
  tilted amplifiers
• can provide high-gain as well as good BQ

R0
R1
RWA
R0
R0
TWA
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600 mm x 1500 mm Amplifier Results 7o angle of
incidence (air)
lt .5 W
21 W
4 W
7
20
16
12
Experimental Farfield Diffraction Patterns
Output Power (W)
8
DL .165 deg
4
0
0
4
12
16
20
24
32
28
8
15 A
Current (A)
Goldberg L, Mehuys D, Surette MR, Hall
DC, High-power, near-diffraction-limited
large-area traveling-wave semiconductor
amplifiers, IEEE Journal of Quantum Electronics
,Vol 29, Issue 6, June 1993, pp. 2028 2043.
30 A
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Filament Channels
Avoid Counterpropagation
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Ways to avoid counterpropagation and
achieve angled extraction without filamentation
AR
AR
AR
TWA
Tapered MOPA DBR DFB MO
AR
AR
Tapered Amplifier
Unstable Resonators
Continuous Unstable Resonators

• We will now concentrate on unstable resonators.

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III. Unstable Resonators

• Invented by A. Siegman

- Large mode volumes - Near diffraction limited
performance (good mode discrimination) -
Insensitive to misalignments and aberrations in
the medium - Avoids exact counterpropagation and
suppresses filaments

• Applications to semiconductor lasers
• - Bogatov, et.al. (1980) - polished facet
  mirror
• - Craig, PhD Disseration (1985) - etched
  facet mirrors
• - Yariv, Salzman, et.al. (1985-87) - etched
  facet mirrors
• - Tilton, Dente, et.al. (1989-present) - ion
  milled, etched, lens train
• - Siegman, et.al. (1997) RIE Unstable
  etched facet mirrors

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Unstable Resonator Semiconductor Lasers
?nlt0
Regrown-Lens Train (AFRL, UNM)
Distributed Divergence
Continuous Etched Diverging Waveguide (AFRL, UNM)
n n0 n2x2
Ion-milled or etched facet
Half-Symmetric (AFRL, OGI, Stanford)
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High-Brightness from an Unstable Resonator
Mid-IR Semiconductor Laser

Unstable resonators provide regenerative
propagation of cylindrical waves originating from
sources located at locations V and V-. A round
trip carries a cylindrical wave from V to its
image at V- and back again to V.
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Analytic Solution for the Virtual Sources
Magnification
L
Ma
a
R
Optic Axis

V-
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IV. Experimental Results
Two methods for introducing the curved facet onto
the semiconductor laser were investigated
RIE Etching
Mechanical Polishing
Cavity Length (L) 3.5 mm Mirror Curvature (R)
10 mm Resonator Mag (M) 3.1
Cavity Length (L) 2 mm Mirror Curvature (R)
7.5 mm Resonator Mag (M) 2.7
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Improving lateral beam quality ICP etched curved
back facet (Mag 2.7, L 2 mm) Preliminary
qualitative measurements show greatly improved
lateral coherence
Re-image of degraded source at 2x threshold (
6-8 x DL )
Re-image of degraded source at 6x threshold (
6-8 x DL )
Fabry-Perot
Unstable
D
Virtual Source
Re-image of virtual source at 2x threshold. (
Diffraction Limited)
Re-image of virtual source at 6x threshold. (
Diffraction Limited)
Dmeasured 1.18 mm Dcalculated 1.15 mm
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Improving lateral beam quality Mechanically
Polished curved back facet (Mag 3.1, L 3.5
mm) Preliminary qualitative measurements show
greatly improved lateral coherence
Re-image of degraded source at 2x threshold (
6-8 x DL )
Re-image of degraded source at 6x threshold (
6-8 x DL )
Fabry-Perot
Unstable
D
Virtual Source
Re-image of virtual source at 2x threshold (
Diffraction Limited )
Re-image of virtual source at 6x threshold (
Diffraction Limited)
Dmeasured 1.84 mm Dcalculated 1.81 mm
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IV. Conclusions

• Filament formation problems have precluded
  high-power, good BQ, operation of
• broad-area semiconductor laser devices.

• Off-axis extraction, with angled
  counter-propagating fields, is an excellent way
  to
• ameliorate the filamentation tendencies.

TWA
RWA
Tapered MOPA
Tapered Amplifier
Unstable Resonators
Continuous Unstable Resonators
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Beam Propagation Method
Paraxial wave equation in the diode with
filamentation tendencies given by a-parameter
Carrier diffusion in the diode
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Theory
x
z
Pinc
F(x,z)
B(x,z)
Pout-R
Pout-T
Must self-consistently determine F(x,z) and B(x,z)
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17 Watts several buckets 5 Watts wide angle
Pinj 500 mW I 31 A AOI 7o Pout 23 W Prfl
4W
Reflected Field
4 Watts
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SEM Photograph of Etched Facet
Facet roughness
Air
4 mm GaSb cap layer
1.5 mm Active layer
Substrate
Copper Contaminant