Module of Fiber Coupled Diode Laser Based
on 808nm Single Emitter Combination
Abstract: Because of the good beam quality and heat dissipation of single emitter diode laser, it
is more resuitable to be used in the source of electro-optic countermeasure. Aim at the responer
curve of charge-coupled device (CCD) spectrum, 808nm single emitter is used as unitsource and
24 single emitters are divided into four groups. In order to increase the output power intensity,
space sombination and polarization combination are used in the experiment. Combined beam is
focused in an optical fiber through the focused lens group designed by ourself. All the single
emittwes are connected inseries. When the drive current is 8.5A, 162W output power is obtained
from a 300um fiber core with a numerical aperture of 0.22 at 808nm and coupling efficiency of
84%.
1 Introduction
Charge-coupled device (CCD), as a solid-state imaging device, is widely used in various
optoelectronic devices, and plays an important role in reconnaissance, monitoring, and
identification. The damage of CCD will cause the function of the optoelectronic device to be
weakened or even lost. Therefore, the interference of strong laser on CCD has become the
research focus at home and abroad. In the past, the semiconductor laser stacked array was used
as the photoelectric interference light source. The beam quality of the stacked array was poor,
requiring water cooling to dissipate heat, and the large system had limitations such as its
application in vehicle and airborne photoelectric interference light sources. The single-tube
semiconductor laser (LD) has the advantages of high beam quality and easy heat dissipation. The
fiber-coupled light based on the single-tube semiconductor laser combined beam has a wide
application space in photoelectric interference. Due to the high conversion efficiency and good
beam quality of 976nm single-tube semiconductor lasers, the research focus of single-tube
semiconductor lasers is currently focused on the 976nm band. The Fraunhofer Institute in
Germany uses a step mirror reflection method to combine two single-tube semiconductor lasers
with a wavelength of 975 nm into a fiber coupling module. The output power of a 105 μm fiber
reaches 100 W and the coupling efficiency reaches 80%. The American company Oclaro also uses
polarization beam combining technology to make a single tube semiconductor laser with a
wavelength of 980 nm into a fiber coupling module. The output power of 105 μm fiber is 100 W
and the coupling efficiency is 73%. Multiple fiber coupling modules are bundled by a fiber
combiner, and the power can reach thousands of watts. The single-tube semiconductor laser
beam combining technology with a wavelength of 808nm is relatively lagging. At present, there
are no reports in the literature. From the CCD spectral response curve used in the photoelectric
interference experiment, the response range of the silicon CCD is 200 ~ 1100nm. At the same
irradiance, the spectral responsivity at the wavelength of 976nm is only about 40% at 808nm.
Obviously, in order to improve the interference efficiency to the CCD device, it is an effective way
to apply the 808nm semiconductor laser to the photoelectric interference light source. In this

paper, 24 8W single-tube semiconductor lasers are divided into 4 groups, 6 in each group.
Through fast and slow axis collimation, spatial beam combining and polarization beam combining,
the beam is expanded and focused to achieve fiber-coupled output. All lasers are connected in
series. Under the current excitation of 8.5A, 162W laser is output through 300μm fiber, and the
coupling efficiency reaches 84%, which can be used in photoelectric interference experiments.
2 Basic principles
In semiconductor laser fiber coupling, the concept of optical parameter product (BPP, fBPP) is
usually used to evaluate beam quality. The optical parameter product is defined as
Where d0 / 2 is the beam waist radius, and θ0 is the far-field divergence half-angle. In
semiconductor laser fiber coupling, the size and divergence angle of the focused spot are
required to be smaller than the fiber core diameter and numerical aperture (NA). Because the
beam spot after semiconductor laser beams are square and the far-field distribution is also
square, the core diameter of the fiber Both NA and NA are symmetrically distributed, so the
beam quality of the focused beam axis must meet the following conditions:
Where fBPP-FA and fBPP-SA are the fast and slow beam quality of the semiconductor laser,
respectively, and fBPP-F is the optical parameter product of the fiber. Equation (2) gives the
optimal coupling relationship between the focused beam and the fiber.
Generally, the beam quality of semiconductor lasers varies greatly in the direction of the fast and
slow axes. Taking the 808nm semiconductor laser used in the experiment as an example, the
fast-axis emission size is 1μm, the slow-axis emission size is 200 μm, and the corresponding
divergence angle is 70°x 11° ( 95% energy), according to (1), it can be seen that the beam
quality in the fast axis direction is better, but the divergence angle is large, which is not conducive
to the superposition of the single-tube semiconductor laser in the fast axis direction. A fast axis
collimator (FAC) is required. ) Compress the fast axis divergence angle. Because the fast-axis
divergence angle of the semiconductor laser is too large, the FAC used in the experiment is an
aspherical cylindrical lens to reduce the aberrations generated during collimation. The slow-axis
divergence angle is relatively small, so the slow-axis collimator (SAC) Spherical cylindrical lenses
can be used. Figure 1 is a schematic diagram of the collimation of the fast and slow axes of a
single-tube semiconductor laser.

Table 1 shows the beam quality of the single-axis semiconductor laser before and after
collimation. It can be seen that the beam quality in the slow-axis direction is much worse than
that in the fast-axis direction. Therefore, it is necessary to superimpose the beams in the fast-axis
direction to equalize the beam quality in the slow-axis direction. Since the height of each FAC is
1.5 mm, the height difference between each two semiconductor lasers is 1.5 mm. Through
calculation, we can know that 12 semiconductor lasers can be superimposed in the fast axis
direction. In the experiment, a stepped array structure is used, and the height of each step is
1.5mm. Considering the mechanical structure and the optical path difference of the laser
transmission, 12 lasers are designed. The semiconductor laser is respectively welded on two step
heat sinks, and each step heat sink contains 6 lasers. In this way, the size of the fast and slow axis
is 9mmx5.6mm, the divergence angle is 3mradx8.8mrad, and the beam quality of the fast and
slow axis is respectively

The two ladder heat sinks are fixed at the same height, and one of the laser beams is increased by
9 mm in the fast axis direction by translating the prism, so that it is higher than the other laser
beam, and then the two beams are superposed in the fast axis direction by the reflective prism,
thereby achieving Space merges.
(5) Formula is the calculation formula of translational prism displacement. Where d is the
thickness of the translational prism, n is the refractive index of the material, I is the incident angle
of the beam, and I′ is the refraction angle of the beam. Figure 2 shows the principle of
horizontal beam shift. If a laser beam is incident at 45° with the inclined surface of the prism, if
the vertical translation is 9mm, the prism uses fused silica JGS1 optical glass, and the angle of the
prism is 45°. It can be calculated that the length of the prism should be 24.7mm and the height
dimension It is 20mm.

Figure 3 is a schematic diagram of the beam combining of a semiconductor laser. After space
combining, a 12-layer laser light source is formed on the fast axis. At this time, the beam quality
in the fast axis direction is
In order to ensure that the output power of the semiconductor laser fiber coupling module is
improved under the condition that the beam quality is unchanged, the two formed units are
combined by a polarization combining prism (PBS). Since the semiconductor lasers used in the
experiment are all P-polarized light, one of the units is first converted to S-polarized light through
a λ / 2 wave plate, and then passed through the PBS prism, the P-polarized light is transmitted,
and the S-polarized light is reflected. The power of the incident beam is doubled under the same
area on the polarization beam combining film of PBS, so the beam quality is theoretically
unchanged after polarization beam combining.
After polarization beam combining, the fast and slow beams are close in quality, but the fast-axis
divergence angle is 3mrad and the slow-axis divergence angle is 8.8mrad. The beam expanding
system expands the slow axis beam to make the fast and slow axis divergence angles equal, so
that a square spot can be obtained at the end face of the fiber after focusing. In the experiment,

an inverted Galileo telescopic structure was used to design a three-fold cylindrical beam
expander system, consisting of a plano-concave cylinder and a plano-convex cylinder, with
curvatures r1 = 11.33mm and r2 = 48. 72mm. After passing through the beam expansion system,
the slow-axis divergence angle is reduced by three times, which is approximately equal to the
fast-axis divergence angle. Using Zemax optical design software, a set of aspheric focusing lens
group is designed to focus the beam after combining. The focal length of the lens is 74mm, and
the transfer function is close to the diffraction limit, ensuring high coupling efficiency.
3 Analysis of experimental results
In the experiment, a single-tube semiconductor laser with a wavelength of 808 nm was used.
Each laser had a continuous output power of 8 W under a current excitation of 8.5 A, a slope
efficiency of 1.1 W / A, and an electro-optical conversion efficiency of 45%. Twenty-four lasers are
divided into four groups and welded to four step heat sinks. Each step heat sink has six lasers in
the fast axis direction. First, fast and slow axis collimation of all lasers is performed by FAC and
SAC. The CCD of OPHIR company is used to observe and measure the far-field beam collimation
of a single-tube semiconductor laser. The divergence angle is 4mrad, which exceeds the
experimental design value, which also causes the fast-axis beam quality to be greater than the
design value. This error is mainly due to the Gaussian distribution of the fast axis of a single-tube
semiconductor laser. After collimation, a part of the energy will still be distributed outside the
main light intensity range, resulting in a larger divergence angle. Because the slow axis collimator
cannot absolutely guarantee the verticality during processing, it also affects the collimation effect
of the fast axis. The two collimated lasers emitted by the lasers on the two step heat sinks are
spatially combined in the direction of the fast axis; the two spatially combined units are polarized
and combined; and finally, a self-designed beam expanding focusing system is used. Coupling into
a fiber with a core diameter of 300 μm and NA of 0.22.
Figures 5 and 6 show the focused spot and beam quality M2 factor measured with the Focus
Monitor beam quality analyzer, respectively. It can be seen from Figure 5 that the spot size of the
fast axis is larger than that of the slow axis, which is mainly caused by the fact that the divergence
angle of the fast axis exceeds the design value during the previous analysis.

Generally, the BPP value is calculatedby measuring the M2 factor. Figure 6 shows that M2 is 72.
From the formula (6), it is calculated that the beam quality in the fast axis direction is
18.5mm.mrad and the beam quality in the slow axis direction is 14.2mm.mrad. The slow-axis
beam quality is greater than the design value. On the one hand, because the slow-axis divergence
angle increases with the increase of the current, the beam quality becomes worse; on the other
hand, when the semiconductor laser is soldered, the low-temperature solder melts to generate
thermal stress to make the laser The position in the horizontal direction deviates from the ideal
position, so that SAC has difficulties in installation and commissioning, resulting in a small
directivity error in the slow axis direction of the beam after each laser collimation, but the
directivity error increases during the beam expansion process, resulting in light spots. As the size
becomes larger, the numerical aperture value of the focused beam is greater than the design
value during the focusing process, and this part of the energy is lost during the optical fiber
transmission process.
In the experiment, all the semiconductor lasers are connected in series, and the output power of

the fiber is measured at different operating currents at room temperature. The power-current
characteristic curve and coupling efficiency of the module are obtained based on the comparison
between the test data and the original power of the laser. As shown in Figure 7, when the
module operating current is 8.5A, the fiber output power is 162W, and the coupling efficiency
reaches 84%. The end faces of the high-energy fiber used in the experiment were not coated, and
Fresnel reflections were generated, and 4% of energy was lost on each of the two end faces.
4 Conclusion
The integration of multiple lasers is achieved using single-tube beam combining technology.
Multiple semiconductor lasers are combined and coupled into the fiber. All semiconductor lasers
in the module are connected in series. Under the current excitation of 8.5 A, the core diameter is
300 μm and NA is 0.22. The fiber output power is 162W, and the coupling efficiency is 84%. The
problem of fast-axis beam quality deviation is currently being solved to bring the fast-axis beam
quality close to the design value.