We report on high brightness diode laser at 1.5 μm with wavelength stabilized output. 22W
are delivered from a uncoated 100μm fiber with 0.15 NA at 1532nm with a bandwidth of 2 nm.
InP diode lasers emitting at 1.5 μm show much lower power than GaAs based diodes emitting
around 900 nm due to the low electro-optical efficiency of 1.5 μm diodes of about 35%,
compared to about 65% of GaAs diodes.
Single emitters allow the highest power from given size broad area emitter due to optimized
cooling. Up to 6W (15W) are available from a 95 μm broad area single emitter at 1.5 μm (9xx
nm). At 1.5 μm the maximum power is typically limited by thermal roll over and efficient heat
dissipation from the diode is essential for power scaling. Optical stacking is deployed for
power scaling thus symmetrizing the beam quality in fast and slow axis for efficient fiber
coupling. Typically, 65% efficiency for optical stacking and fiber coupling are achieved
resulting in more than 22W from a 100 μm fiber of 0.15 NA.
Resonant pumping of Er lasers requires a 2nm linewidth centered at 1530nm. The free
running diodes show about 10nm linewidth (96% power content) and about 2.5nm/A tuning
coefficient with varying drive current depending on heatsinking. Frequency stabilization is
achieved with external Volume Bragg Gratings. More than 85% power is confined within a
2nm bandwidth up to 8A drive current resulting in 17W from the uncoated 100 μm fiber. The
diodes are emitting at 1546nm at 8A without VBG and 20W from the fiber are possible with
the proper lower wavelength diode.
Multiple Single Emitter (MSE) modules allow highest power and highest brightness fiber coupled diode lasers
based on standard broad area diodes. 12 single emitters, each rated at 11W, can be stacked in fast axis and
yield more than 100W in a fully collimated beam with a beam quality of 7mm*mrad in both axes. Optical
transfer efficiencies of >88% from diode facet to after the fiber are achieved resulting in efficient and compact
fiber coupled modules. Volume Bragg Gratings (VBG) stabilize the wavelength over a tuning range of >10nm
and narrow the linewidth of individual diodes to less than 2nm. The brightness is scaled by polarization
multiplexing and optical stacking is deployed for larger fibers resulting in 700W delivered from a 200μm fiber,
0.2 NA. Wall plug efficiencies of 35% are achieved.
The challenge of MSE fiber coupled diode lasers lies in high precision, high yield manufacturing and not so
much the optical design of the device, since only collimating lenses and a focusing optic are used. However,
a large number of individual components must be handled and consistently aligned with high precision. The
100W module comprises 12 single emitters and the 700W/200μm/0.2 laser comprises 120 single emitters
with 85% optical fill factor. Pointing tolerances and collimation errors of all emitters cannot exceed 10% of the
spot size to realize the benefits of highest brightness from single emitters compared to bars.
The two major assembly processes of MSE fiber coupled diode lasers are the precision diode reflow process
and the accurate 5 axis alignment of the fast axis collimation lens (FAC). The reflow process enables
positioning of 12 single emitter diodes on submounts within +/-5μm on a common heatsink. Special image
processing software performs automated precision alignment and fixation of the FAC with a consistent
accuracy of better 0.3um and 0.12mrad. It is also deployed for automated alignment of the external VBG.
Wavelength stabilization in an external resonator aims to maximize the locking range and to minimize the
drop of output power as well as linewidth. Front facet reflectivity of the diode laser, reflectivity of the volume
Bragg grating (VBG) and different resonator designs are investigated.
Commercial high power fiber coupled diode lasers reach power levels of 200W from a 0.2mm fiber, NA=0.2.
2D fiber coupled single emitter (SE) arrays are described delivering 500W from a 0.2mm fiber.
The beam quality of standard 90μm single emitter (SE) is 6mm*mrad (slow axis) and 0.7mm*mrad (fast axis)
including errors from fast axis lensing. 3 SEs (24) can be arranged in slow axis (fast axis) to fill the aperture
for coupling into a 0.2mm fiber, NA=0.2. For high efficiency, beam shaping optics are avoided. A lens array
for slow axis collimation and a focusing optic complete the fiber coupled module. 44 SEs' are arranged as a
2D array, polarization multiplexed and coupled into a 0.2mm fiber, NA=0.2. 62% optical to optical and 75%
coupling efficiency are achieved, close to the modeled coupling efficiency of 80%. Alignment tolerances in
the system do account for additional losses. Precise manufacturing processes are essential. The SEs on
submounts are soldered in one reflow process to a common heatsink and FAC-lensing station automatically
aligns the lens based on image processing ensuring minimum total lensing errors (focusing and pointing) of
each SE to <15% of total spot size.
Tighter tolerances during SE mounting, improved fast axis collimation and a redesigned coupling optic will
increase the coupling efficiency to 80% resulting in 410W linear polarized output from the 0.2mm fiber,
NA=0.2. Polarization (800W) and dense wavelength multiplexing (1.4kW) open the door to kilowatt level.