WB2
11:00 – 11:15
*
th
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 VHG
3,4,5
. 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 vendors
4,5
, 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: paul.leisher@nlight.net
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
4
2
0
14
12
10
8
CW, 25ºC
200 μm x 3.0 mm
2.4
2.0
CW, 25ºC
6 Amps
1.6
1.2
2.7 nm
FWHM
0.8
0.6 nm
FWHM
Unlocked
Locked
0.4
0 2 4 6 8 10 12 14
Current (A)
0.0
(a)
865 875 885
Wavelength (nm)
895
75
60
CW, 15ºC
16-emitter array
25
20
CW, 15ºC
12 Amps
45
30
15
10
11.7 nm
FWHM
0.2 nm
FWHM
15
0
Unlocked
Locked
5
0 2 4 6 8 10 12 14
Current (A)
0
1490 1510 1530 1550
Wavelength (nm)
(c)
12
10
8
6
CW, 25ºC
95 μm x 3.8 mm
4
2
0
Unlocked
Locked
0.8
0.4
0 2 4 6 8 10 12 14
Current (A)
0.0
2.4
2.0
1.6
1.2
2.0
1.6
1.2
0.8
0.4
0.0
0
CW, 25ºC
100 μm x 3.0 mm
1.4
1.2
1.0
Unlocked
Locked
3 6 9 12 15
Current (A)
0.8
0.6
0.4
0.2
0.0
CW, 25ºC
8 Amps
3.4 nm
FWHM
0.4 nm
FWHM
(b)
955 965 975
Wavelength (nm)
985
CW, 15ºC
5 Amps
10.8 nm
FWHM
2.5 nm
FWHM
(d)
1835 1855 1875 1895
Wavelength (nm)
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 nm
6
, and (d) 1863 nm. In all cases, complete locking to the grating wavelength is achieved with virtually no change to the LIV characteristics.
Ȝ
Peak
(nm) ǻȜ
FWHM
(nm)
885 nm 0.6
P
Rated
(W)
12
Ș
WP,Peak
(%) Power Penalty (%)
64% ~ 1%
Motivation
Nd-doped DPSS
976 nm
1532 nm
1863 nm
0.4
0.2
2.5
Á
10
65
‚
1.8
62%
37%
15%
~ 1%
~ 0%
~ 0%
Yb-doped DPSS
Er-doped DPSS
Ho-doped DPSS
‚
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).
2
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).
147