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HIGH-EFFICIENCY INTERBAND CASCADE
LASERS
2nd International Conference on Lasers, Optics, and Photonics
Philadelphia, PA, Sept. 8, 2014
Charles D. Merritt, William W. Bewley, Chul Soo Kim, Chadwick L. Canedy,
Joshua Abell, Igor Vurgaftman, & Jerry R. Meyer
Naval Research Lab, Washington DC 20375 [MWIR_lasers@nrl.navy.mil]
Mijin Kim
Sotera Defense Solutions, Crofton MD 21114
3 DISTINCT WAYS TO PROVIDE CARRIERS
FOR POPULATION INVERSION
n
p
(1) Conventional
Diode Laser
(2) Quantum
Cascade Laser
n
n
n
n
n
p
n
p
(3) Interband
Cascade Laser
n
p
n
Hybrid of conventional diode (Interband active
transitions) & QCL (Cascaded stages)
THE INTERBAND CASCADE LASER
"W"
2.5
Active
Electron
Injector
Hole
Injector
InAs
AlSb
2.0
CB
Energy (eV)
1.5
InAs
1.0
GaSb
0.5
0.0
-0.5
GaInSb
-1.0
0
Semimetallic
Interface
100
200
VB
300
400
500
Distance (Å)
1st Proposed: R. Q. Yang (1994)
Design Improvements: NRL (1996)
1st Experimental Demo: U. Houston & Sandia (1997)
Further Development: ARL, Maxion, JPL, U. Oklahoma, U. Würzburg, Nanoplus,
U. St. Andrews
RT cw operation: NRL (2008)
REBALANCING EFFECT ON THRESHOLD
Heavily n-Doped
-3
18
Injector QWs (5 x 10 cm )
2.5
10000
2.0
W-Diode
CB
ICLs
150 m x 2 mm
T = 300 K
 = 2.9-4.0 m
1.5
2
Pulsed jth (A/cm )
19
-3
Energy (eV), n, p (10 cm )
[Vurgaftman et al., Nature Com. 2, 585 (2011); U.S.
Patent Application 13/422,309 (2012); International
Patent Application PCT/US12/29396 (2012)]
1.0
0.5
n
0.0
p
-0.5
10-Stage
Best 30-40
Stage QCL
60-Stage QCL
1000
Gen3
VB
RT CW
-1.0
0
100
200
300
400
500
600
Distance (Å)
Major performance breakthrough with
“carrier rebalancing”, via heavy n-doping of
electron injectors, to roughly equalize
electron & hole populations in active region
Gen2
Gen1
5-Stage (Gen0)
100
2005
2007
Carrier
Rebalancing
2009
2011
2013
Year
Dramatic threshold reduction
compared to all previous ICLs
ICL SPECTRAL RANGE & LOW DRIVE POWER
Threshold power density:
Pth = Vth x jth
[Vurgaftman et al., Nature Com. 2, 585 (2011)]
100
12
CW Output Power (mW)
2
Pulsed Pth (kW/cm )
50
QCLs
20
10
T = 300 K
5
2
NRL 5-Stage Gen3 ICLs
1
0.5
0.2
3.5
4.0
4.5
5.0
5.5
Wavelength (m)
Power density thresholds 30x lower
than record QCL results
CW operation to T = 48 °C @  = 5.7 m
HR/Uncoated
T = 25 °C
8
6
w = 5 m
8 m
11 m
4
2
0
3.0
Lcav = 0.5 mm
10
0
20
80
60
40
Input Power (mW)
100
T = 25 °C: Input for lasing < 30 mW
Best QCL result:  700 mW
Critical for battery-operated,
hand-held, solar-powered, etc.
DFB ICLs
Grown, processed, & tested @ NRL: Grating
etched into deposited Ge [Kim et al., APL 101,
Normalized Spectral Intensity
061104 (2012)]
I = 170 mA
1
Grown @ NRL; Processed & tested @ JPL:
2nd-order side grating yields single mode
o
o
65 C
45 C
[Forouhar, et al., to be published]
o
o
35 C
o
o
55 C
T = 25 C
75 C
0.1
0.01
[von Edlinger et al., PTL 26, 480 (2014)]
1E-3
3.78
3.79
3.80
Wavelength ( m )
3.81
Single-mode tuning with temp: 21.5 nm
with current: 10 nm
3
o
Voltage (V)
40 C
 = 531 nm
7.4 m x 2 mm
HR/AR
2
50
40
30
o
60 C
20
1
Power (mW)
o
40 C
o
80 C
10
o
75 C
0
o
80 C
0
100
200
300
400
0
Current (mA)
Poutcw = 27 mW @ T = 40 ºC, 1 mW @ 80 ºC
Threshold drive power = 280 mW @ 40 ºC
Poutcw = 18 mW @ T = 46 ºC
Threshold drive power < 400 mW @ 36 °C
Lifetime testing: > 10,000 hrs. cw operation
@ 40 °C with negligible degradation
EPI-DOWN MOUNTING: HIGHER Tmaxcw
[Bewley et al., Opt. Expr. 20, 20894 (2012); U.S. Provisional Patent Application 61611800 (2012)]
400
118C
Type-II Mid-IR Lasers
CW Tmax (K)
350
Gen1
Gen0
300
JPL/OU
JPL
Maxion
/UMD
With NRL carrier
rebalancing
50
0
Maxion
NRL ICLs
Maxion
200
Maxion/NRL
Room Temperature
250
Würzburg
100
°C
Gen3
-50
W Diode (NRL MBE)
W Diode (Sarnoff MBE)
2000
2004
2008
Year
2012
-100
CORRUGATED-SIDEWALL ICLs
Lateral Mode Profiles
3
[Bewley et al., Opt. Expr. 20, 20894 (2012)]
2
Ridge
40 °C
60 °C
2
80 °C
50
100 °C
0.2
0.4
0.6
0.8
105 °C
1.0
1.2
Current (A)
Pmaxcw > 200 mW @ T = 25 ºC (M2 = 1.6)
I = 400 mA (M = 1.3)
800 (1.5)
1200 (1.6)
Normalized CW Far Field
Output Power (mW)
5-Stage ICL
Corrugated SW
Epi-Down (HR/AR)
150
18.2 m x 4 mm
 = 3.7-3.9 m
100
0
0.0
1
T = 25 °C
200
-40
-20
0
Angle (°)
Corrugated SW
18.2 m x 4 mm
HR/AR
T = 25 °C
20
Wider ridge (25.1 m): 305 mW (M2 = 2.2); WPE = 6.6% @ Pmax
40
TAPERED ICLs
[Bewley et al., APL 103, 111111 (2013)]
3
200
Device A
o
T = 25 C
1
0
0.0
0.5
1.0
1.5
Current (A)
100
2.0
0
Tapered ridge with
no straight section
CW Farfield Intensity (Normalized)
300
2
CW Power (mW)
Voltage (V)
400
T120606 HR/AR
0mm x 5m + 4mm @ HA=0.42°
I(mA)
500
750
1000
1250
1500
1750
2000
CW
T = 25 °C
-15
-10
-5
0
Angle (°)
Pmaxcw (25 ºC) = 403 mW (M2 = 2.3), WPE = 7.0% @ Pmax
5
Pout(mW)
63
138
207
265
310
342
359
10
15
TO FURTHER ENHANCE POWER &
BRIGHTNESS: INCREASE EFFICIENCY
Ext Diff QE/Stage (2 Facets, %)
As of October 2012:
40
ICLs
150 m x 2 mm
T = 300 K
Gen3
30
Gen1
20
Gen0
10
0
Gen2
3.0
3.5
4.0
4.5
5.0
5.5
Wavelength (m)
Try varying other parameters in rich ICL design space, e.g.:
• Thicker n-GaSb separate confinement layers (SCLs),
for lower mode overlap with active & clad, hence reduced loss
• More active stages (e.g., 7 vs. 5), for higher slope efficiency & gain
Top Contact
Top Clad
SCL
Active QWs
SCL
Bottom Clad
Substrate
Diff Slope Eff (mW/A per Facet)
FIRST 5 STAGES WITH THICKER SCLs
5-Stage ICLs
 = 3.2-3.8 m
150 m x 2 mm
Pulsed
600
400
5-Stage
SCL = 500 nm
200
SCL = 700-800 nm
0
300
320
340
360
Temperature (K)
380
Thick SCLs increase efficiency at 300 K, but fail to provide enough gain at high T
7 STAGES
Diff Slope Eff (mW/A per Facet)
[Bewley et al., Opt. Expr. 22, 7702 (2014)]
600
7-Stage
5- & 7-Stage ICLs
 = 3.2-3.8 m
150 m x 2 mm
Pulsed
400
5-Stage
200
0
300
320
340
360
Temperature (K)
380
Thick SCLs increase advantage at 300 K, while retaining sufficient gain at high T
Even better news: Slope7/Slope5 > 7/5 indicates lower loss!
EXTERNAL DIFFERENTIAL QUANTUM EFFICIENCY
Result is significantly higher EDQE:
Ext Diff QE/Stage (2 Facets, %)
60
50
Gen3B (7-Stage/Thick SCL)
40
Gen3
NRL ICLs
150 m x 2 mm
T = 300 K
Pulsed
30
Gen2
20
10
Gen1
Gen0
0
3.0
3.5
4.0
4.5
5.0
5.5
Wavelength (m)
7-stage ICLs with thick SCLs (Gen3B) exhibit higher EDQE & lower loss at all 
CW POWER & FAR FIELD PROFILE
5
300
3
200
100
0
0
200
7-Stage ICL
CH4 Etch
2
Epi-Down, HR/AR
22.0 m x 3 mm
o
T = 25 C
1
400
600
0
800
7-Stage ICL, CH4 Etch
Farfield Intensity (Arb. Units)
4
Voltage ( V )
Power (mW)
400
Epi-Down, HR/AR
22.0 m x 3 mm
o
T = 25 C
I = 750 mA
2
(M = 2.6)
500
(2.3)
-40
Current (mA)
-20
0
Angle (°)
Pmaxcw = 384 mW in high-quality beam (M2 = 2.6)
WPE = 12.4%
20
40
[Bewley et al., Opt. Expr. 22, 7702 (2014)]
CW Wallplug Efficiency (%)
WIDER RIDGE (w = 32 m)
15
7-Stage ICL (32.4 m x 3 mm)
10
5-Stage ICL (25.2 m x 4 mm)
5
T = 25 oC
Corrugated Sidewalls
0
0
500
1000
1500
Current (mA)
T = 15 C
o
25 C
Epi-Down, HR/AR
32.4 m x 3 mm
o
35 C
400
o
45 C
o
55 C
200
o
65 C
Wallplug Efficiency (%)
Power (mW)
600
7-Stage ICL, 32.4 m x 3 mm
o
7-Stage ICL
CH4 Etch
15
o
T = 15 C
o
25 C
o
35 C
10
o
45 C
o
55 C
5
o
65 C
o
70 C
o
70 C
0
0
0
200
400
600
800
Current (mA)
1000 1200 1400
0
200
400
600
800
1000 1200 1400
Current (mA)
Pmaxcw up to 592 mW (WPE = 10.1%, M2 = 3.7) @ 25 C; 696 mW (11.7%) @ 15 C
LATEST RESULTS: 10 STAGES ( = 3.45 m)
500
400
w = 22 m
300
200
5
100
0
0.0
0.2
0.4
0.6
Current (A)
0.8
1.0
0
Farfield Intensity (a.u.)
10
T = 25 oC
10-Stage ICLs
Epi-Down, HR/AR Facets
Lcav = 4.5 mm
CW Output Power (mW)
Wallplug Efficiency (%)
15
10-Stage ICL
22 m x 4.5 mm
I = 0.960 A
2
(485 mW, M = 2.5)
I = 0.76 A
2
(404 mW, M = 2.2)
I = 0.56 A
2
(289 mW, M = 1.9)
-30
-20
-10
0
10
Angle (degrees)
Also: Pmaxcw = 464 mW (WPE = 11%, M2 = 1.9) @ 25 C
20
30
RECORD ICL WALLPLUG EFFICIENCIES
20
T = 25 oC
10-stage ICL
Lcav = 1 mm
Epi Down, HR/AR
0
0.00
0.05
0.10
Current ( A )
0.15
CW WPE (%)
CW WPE (%)
15
5
200
15
150
15 m
w = 12 m
10
20
0.20
10
o
T = 25 C
7-Stage ICLs
Lcav = 1 mm
5
0
0.00
w = 12 m
w = 15 m
0.05
0.10
0.15
Current (A)
CW WPEs for 4 devices from 2 wafers (7-Stage & 10-Stage): ≈18%
100
50
0
0.20
CW Output Power (mW)
With much shorter 1 mm cavity:
SUMMARY: CW POWER
& BRIGHTNESS
Year
Stages

(m)
ai
-1
(cm )
Ridge
Mount
Lcav
(mm)
width
(m)
Pmax25C
(mW)
WPE(Pmax)
(%)
M2
2008
5
3.75
12.2
Straight
Epi-Up
3
9
10
0.7
≈2
5
2009
5
3.67
6.6
"
"
3
10
59
3.1
≈2
30
2011
5
3.57
6.9
"
"
3
11
158
9.9
3
53
2012
5
3.66
4.5
Straight
Epi-Down
4
11
198
7.1
1.8
110
Corrug.
"
"
25
305
6.5
2.2
139
Brightness
2
(Pmax/M )
2013
5
3.72
5.2
Tapered
"
4
5 - 63
403
7.0
2.3
175
2014
7
3.45
3.0
Corrug.
"
3
28
522
10.3
3.1
168
10
3.45
3.4
Corrug.
"
4.5
18
464
11.2
1.9
245
7
3.11
3.3
Corrug.
"
4.5
18
326
6.9
1.3
243
QCL vs. ICL
•
•
•
QCLs much more widely studied & matured, provide very high cw output powers
ICLs provide much lower power dissipation, possibility for vertical emission, &
minimal beam steering – also less mature, so more room for improvement
 = 3-4 m: ICLs generally preferred
– QCLs now produce Pmaxcw > 1 W @ RT, but higher threshold, lower efficiency,
& questionable yield thus far
•
 = 4-6 m: QCL sweet spot for high power (Up to 5 W cw demonstrated)
– But ICL still preferred in applications requiring low power from ultra-compact
battery-operated package (e.g. laser spectroscopy)
•
 = 2.5-15 m LEDs: Only ICLs suitable for top emission
•
 = 6-150 m Lasers: QCLs preferred (so far, high loss in ICLs)
•
QCLs & ICLs can complement each other throughout mid-IR
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