Multimode laser diode and side-coupled fiber package

Details Multimode laser diode and side-coupled fiber package Multimode laser diode and side-coupled fiber package

2002-09-25

2003-12-23

Epps, Georgia (Department: 2873)

Optical waveguides

With optical coupler

Input/output coupler

C359S341320, C372S108000

Reexamination Certificate

active

06668112

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to packaging optical fibers and more specifically to packaging a multimode laser diode that is side-coupled with a double-clad fiber.
2. Description of the Related Art
Optical fiber lasers and fiber amplifiers have become indispensable components in building optical networks for the transmission and amplification of optical signals. Optical amplification is achieved by pumping the fiber's doped core. Typically, the pump light is coupled directly into the doped core. This requires single spatial mode diffraction limited laser diodes with an emission aperture of a few microns. This process is expensive because of the sub-micron alignment and mechanical stability required to achieve efficient and stable diode to fiber coupling. A relatively current development is the use of double-clad active fibers that consist of a single mode fiber doped core, an inner cladding and an outer cladding. The large diameter and numerical aperture of the outer cladding make it possible to efficiently couple spatially incoherent emission from high power multimode diodes into the fiber. These broad area laser diodes are approximately ten times cheaper than pigtailed single mode laser diodes.
As shown in
FIG. 1
a
, a multimode diode laser
10
includes a chip
12
on a submount
14
that emits a laser beam
16
in a direction parallel to the diode's mounting surface
18
. As shown in
FIGS. 1
b
and
1
c
, the laser diode submount
14
can be mounted on a base plate
15
with diode mounting surface
18
parallel to the base plate surface
19
. In this configuration, the laser beam
16
is emitted in a direction parallel to the surfaces
18
and
19
, corresponding to the Z direction as described in
FIG. 1
a
. The laser beam
16
can be redirected by the use of a 45° mirror
25
or any other reflective surface as shown in
FIG. 1
c
. In this case, the laser beam
16
is now propagating in the direction Y perpendicular to the base plate surface
19
. In the configuration shown in
FIG. 1
d
, the chip
12
can be mounted on the side of a submount
14
, which has a chip mounting surface
18
that is perpendicular to the base plate surface
19
. The choice of diode configuration will be dictated by the application, package size limitations and type of coupler. Multimode diode laser
10
suitably comprises either a single broad emitting area or an array of single transverse mode lasers.
Because the active region
20
of chip
12
is more confined in the Y direction than in the X direction, the intensity distribution of beam
16
is more diffracted in the direction of high confinement, leading to an elliptical distribution with the slow axis
22
along the X direction and a fast axis
24
along the Y direction. When the chip
12
is mounted parallel to the base plate surface
19
as shown in
FIG. 1
b
, fast axis
24
is perpendicular to base plate surface
19
and active region
20
. When mirror
25
is used to redirect the beam as shown in
FIG. 1
c
, or when the laser chip is mounted on the end of submount
14
as shown in
FIG. 1
d
, fast axis
24
is parallel to base plate surface
19
.
Because the thickness of the semiconductor active region
20
is much smaller than its width, focusing along the fast axis is easier than along the slow axis. A number of different optical systems have been proposed that use some combination of one or more simple lenses, cylindrical lenses, microlens arrays, fibers and waveguides to reimage the sources so as to reduce one of the asymmetries inherent in laser bars or multimode laser diodes with broad area emitters. For example, the use of lenses is disclosed in U.S. Pat. No. 4,428,647 and the use of fibers in U.S. Pat. No. 4,763,975.
The multimode pump can be coupled into the fiber's inner cladding in a number of ways. The multimode pump shown in
FIG. 1
b
can be “butt” coupled to an open end of the fiber. The orientation of the fast-axis is not critical because the open end of the fiber is axially symmetric. However, this configuration does not provide access to both ends of the fiber. The same multimode pump can also be coupled to the fiber by using a special fused fiber coupler which allows pump light to be transferred from a multimode fiber into the fiber's inner cladding without disturbing the signal propagating in the core. However, such fused fiber couplers are inefficient, complex and expensive.
U.S. Pat. No. 5,854,865 to Goldberg teaches forming a groove in the side of the double-clad fiber to “side-couple” the multimode pump into the fiber. The laser diode is placed on the other side of the fiber so that the pump propagates laterally through the fiber and impinges on the side of the groove, which specularly reflects the pump along the horizontal axis of the fiber confined in the inner cladding. The formation of such grooves is a complex manufacturing process that weakens the fibers and requires relative large inner cladding diameters, which reduces coupling efficiency.
Packaging the multimode diodes, couplers and fibers is oftentimes the most challenging aspect of providing a commercially viable fiber laser or fiber amplifier. The package must provide (a) sufficient and cost-effective heat sinking, (b) efficiently couple the pump into the fiber, which means, in part, orienting the fast-axis parallel to the fiber at the coupler when the coupling aperture is not axially symmetric, (c) thermal isolation of the fiber from the diode, and (d) a simple, compact and reliable package.
International publication WO 01/48878A1 entitled “Hybrid Optical Coupling Component” discloses a configuration for packaging a multimode diode laser of the type shown in
FIG. 1
d
above with Goldberg's V-groove fiber coupler. A fiber
30
formed with the v-groove coupler
32
is mounted directly above diode
10
in a glass block
31
so that beam
16
emits perpendicular to the plane of the mounting surface with its fast-axis
24
oriented parallel to fiber
30
. The coupling aperture into the fiber is not axially symmetric so proper orientation of the fast-axis is critical to efficient coupling. Beam
16
passes through imaging lens
34
, propagates laterally through fiber
30
and is reflected off the v-groove horizontally along the fiber. This package provides adequate but sub-optimal heat sinking. The chip
12
is mounted flat on end surface
18
of sub-mount
14
to maximize surface area but the heat path from the chip
12
to end surface
18
of sub-mount
14
, and to surface
19
of base plate
15
is not straight. This makes the heat path longer, reducing the efficiency of the heat sinking. Furthermore, The package provides poor thermal isolation because fiber
30
is mounted directly on top of diode
10
. Heat and particularly fluctuations in temperature can change the characteristics of the gain fiber. In addition, placement of fiber
30
on top of the diode is a delicate and unreliable packaging challenge and also increases the package height, which is a critical parameter in many applications.
SUMMARY OF THE INVENTION
The present invention provides a simple, compact and reliable package for optical amplification that provides efficient coupling of the multimode pump to the double-clad fiber, sufficient heat sinking and good thermal isolation.
This is accomplished with a multimode laser diode mounted with its active region parallel to a mounting surface. The diode emits a beam parallel to the mounting surface with a fast-axis of the beam perpendicular to the diode's active region and the mounting surface. A beam rotator rotates the beam so that the beam's fast-axis is oriented parallel to the mounting surface. A multimode diode package of only the diode and rotator may be provided for integration with a side-coupled fiber.
The beam rotator comprises a pair of reflective surfaces such as provided by mirrors, a prism or a tilted v-shaped mirror that perform a pair of “off-plane bounces” to rotate the beam's fast axis. In an XYZ coordinate system, if the diode lie

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