A Penalty-Free Approach to Wavelength Stabilization of Paul Leisher

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A Penalty-Free Approach to Wavelength Stabilization of
High Power Diode Lasers from 900 nm to 1900 nm
Paul Leisher* and Rob Martinsen
nLight Corporation; 5408 NE 88th St., Bldg E; Vancouver, WA 98665; USA
Several laser gain materials (such as Nd, Yb, Er, and Ho) exist which can be pumped with low quantum defect with
diode pumps operating at wavelengths close to the laser transition 1,2. The corresponding absorption features can be
narrow, placing ever-tighter constraints on the spectral performance of the diode pump. This is because efficient
laser systems requiring uniform absorption of the pump light need the diode source to be well-matched (in terms of
spectral width and spectral position) to the absorption feature.
Standard high-power broad area (multimode) semiconductor diode lasers do not employ wavelength-selective
feedback mechanisms, and as a result, lase on all modes which experience sufficient round-trip gain within the
spectral gain bandwidth of the active region. This fixes the free-running laser spectral bandwidth and wavelength
drift with temperature for a given material system. Wavelength stabilization of diode lasers is typically achieved
through wavelength-selective feedback which locks the laser to one (or a few) longitudinal modes. The two most
common approaches utilized in the high power diode laser arena are internal buried distributed Bragg gratings and
external volume holographic gratings (VHGs)3-5.
VHGs are external optical components in which a periodic variation in index of refraction has been written into the
photorefractive glass, which thereby provides wavelength- (and angular-) selective feedback. This optic is then
carefully aligned to the output beam of the (typically lensed) diode, locking the wavelength of the laser. While the
approach is non-monolithic, it is highly manufacturable, leveraging the same well-developed microlensing
techniques which are utilized in virtually all commercial high power diode laser application. Further, the external
locking approach offers a few significant advantages over buried grating, including wavelength flexibility, improved
yield, improved temperature stability, and the use of laser epitaxy designs which are fully optimized for power and
efficiency (buried gratings require compromises to the vertical structure of the diode).
Previous work by other groups has shown a ~10% to ~20% penalty to the slope efficiency of the diode after external
wavelength locking by means of a VHG3,4,5. In this work, a method which effectively reduces this penalty to ~0% 6
is presented and applied to multiple high-power, high-efficiency diode lasers operating at wavelengths across the
800 nm to 1900nm band. The results demonstrate that this penalty-free locking method is wavelength-agnostic in
behavior.
Diode lasers of standard high-efficiency design at several wavelengths were fabricated using standard commercial
processes. Each emitter was cleaved to either 3.0 mm or 3.8 mm cavity length, and bonded junction-down using
AuSn solder to expansion matched-heatsinks. Microlenses provide collimation in the fast (growth) axis. Each laser
is tested and then a VHG locking optic is inserted before re-test. Figure 1 illustrates the power and voltage versus
current and lasing spectra of laser diode devices operating at 885 nm, 976 nm, 1532 nm, and 1863 nm. In each case,
WKHGHVLJQDFKLHYHVIXOOZDYHOHQJWKORFNLQJZLWKVHYHUDO¶VRIG%VLGHPRGHVXSSUHVVLRQUHVXOWV are shown on a
linear scale). Note in the case of the 1532 nm design, results are provided for a 16-emitter array, wherein each
emitter was also collimated in the slow axis, geometrically multiplexed, and locked with a single grating. This result
also demonstrates straightforward power scalability of the approach. The gratings utilized were provided by several
vendors4,5, with equivalent penalty-free results. Figure 2 summarizes, by wavelength, the achieved spectral
bandwidth, power, efficiency, and measured power penalty (with respect to the free-running diode). In all cases, the
observed power penalty was ~0% to ~1%. This result was enabled by a design methodology which minimizes
parasitic broadband reflections (to achieve good locking) while simultaneously providing for full retention of slope
efficiency.
*
Tel: 360.907.8347 Email: [email protected]
Portions of this work were supported by NASA, ONR, DARPA, and HEL-JTO.
978-1-4244-5684-0/10/$26.00 ©2010 IEEE
146
6
0.8
4
Unlocked
Locked
2
0
0.4
2.7 nm
FWHM
0.6 nm
FWHM
8
1.6
6
1.2
0.8
4
Unlocked
Locked
2
0.0
0
0 2 4 6 8 10 12 14
865
875
Current (A)
885
895
0.4
955
15
30
10
15
Unlocked
Locked
0
5
2.0
CW, 15ºC
12 Amps
1.6
Power (W)
45
Voltage (V)
Power (W)
20
11.7 nm
FWHM
0.2 nm
FWHM
Current (A)
1510
1530
1.2
1.0
1.2
0.8
0.8
0.6
0.4
Unlocked
Locked
0.0
1490
1.4
CW, 25ºC
100 μm x 3.0 mm
0.4
0
0 2 4 6 8 10 12 14
965
975
985
Wavelength (nm)
(b)
Intenisty (arb. units)
25
CW, 15ºC
60 16-emitter array
0.4 nm
FWHM
Current (A)
(a)
75
3.4 nm
FWHM
0.0
0 2 4 6 8 10 12 14
Wavelength (nm)
CW, 25ºC
8 Amps
Intenisty (arb. units)
1.2
2.0
1550
0
Wavelength (nm)
3
6
9
12
0.2
Intenisty (arb. units)
1.6
8
2.4
CW, 25ºC
95 μm x 3.8 mm
10
Voltage (V)
10
12
CW, 25ºC
6 Amps
Voltage (V)
2.0
Power (W)
2.4
CW, 25ºC
200 μm x 3.0 mm
Intenisty (arb. units)
Power (W)
12
Voltage (V)
14
CW, 15ºC
5 Amps
10.8 nm
FWHM
2.5 nm
FWHM
0.0
1835
15
Current (A)
(c)
1855
1875
1895
Wavelength (nm)
(d)
Fig. 1: At left, power and voltage versus current and at right, lasing spectrum measured before
and after VHG wavelength locking for various high-power laser diodes operating at (a) 885 nm,
(b) 976 nm, (c) 1532 nm6, and (d) 1863 nm. In all cases, complete locking to the grating
wavelength is achieved with virtually no change to the LIV characteristics.
ȜPeak (nm)
ǻȜFWHM (nm)
PRated (W)
ȘWP,Peak (%)
Power Penalty (%)
Motivation
885 nm
0.6
12
64%
~ 1%
Nd-doped DPSS
976 nm
0.4
10
62%
~ 1%
Yb-doped DPSS
1532 nm
0.2
‚
65
37%
~ 0%
Er-doped DPSS
1863 nm
Á
1.8
15%
~ 0%
Ho-doped DPSS
2.5
‚
Result taken from a 16-emitter array, with each emitter operating at 4W
Á
A broad-linewidth grating was intentionally used in order to meet the requirements of the application
Fig. 2: Table summarizing the performance characteristics (peak wavelength, of the wavelengthlocked diode, as well as the measured power penalty associated with the external locking and the
application which motivated each device.
In summary, an external locking approach is presented which is both wavelength agnostic and believed to offer the
highest possible power and efficiency of any wavelength locking scheme for diode lasers, as it enables independent
optimization of the laser epitaxy for high power / high efficiency for unlocked operation combined with locking
through an external volumetric grating with virtually no penalty to power and conversion efficiency. The results
were achieved through a design approach which minimizes parasitic broadband reflections and ensures locked
operation with fully optimized slope efficiency.
1
M. Frede, et al., Optics Lett., vol. 31, pp. 3618-3619, (2006).
S. D. Setzler, et al., IEEE J. Sel. Top. Quant. Elect., vol. 11, no. 3, pp. 645-657, (2005).
3
B. Volodin, et al., Optics Lett., vol. 29, pp. 1891-1893, (2004).
4
A. Gourevitch, et al., Optics Lett., vol. 32, pp. 2611-2613, (2007).
5
G. Steckman, et al., IEEE J. Sel. Top. Quant. Elect., vol. 13, no. 3, pp. 672-678, (2007).
6
P. Leisher, et al., Proc. SPIE, vol. 7198, no. 38, (2009).
2
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