Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-01-20
2002-11-05
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S027000, C385S127000, C372S006000
Reexamination Certificate
active
06477295
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to optical pump coupling of pump light, such as from a light source, to an optical guide, such as an optic fiber, and more particularly to highly efficient coupling of pump light from a multimode fiber into the inner cladding of a double clad fiber (DCF).
BACKGROUND OF THE INVENTION
The use of double clad fibers (DCFs) have increasingly become important in optical applications requiring higher powers of amplified light or high power fiber lasers. The advent of DCFs has been known in the art for many years starting, for example, with U.S. Pat. No. 3,808,549 to Maurer, issued Apr. 30, 1974. In this patent, a double clad fiber is shown with an inner cladding being pumped with multiple LED sources, although it is clear from the disclosure that these pump sources may also be semiconductor laser sources, such as AlGaAs laser diodes pumping a neodymium doped core of the DCF. Thus, this patent represents one of the earliest disclosures of the concept relating to end pumping of a DCF with pump light from plural semiconductor laser sources.
As is well known in the art, DCFs have a single mode core with a refractive index n
1
and having a diameter of several microns, such as approximately 6 &mgr;m, or a multimode core with a refractive index n
1
and having a diameter such as 10 &mgr;m or 12 &mgr;m, either of which may be doped with a rare earth active material. A single mode core provides for single mode propagation of a light signal beam in optical communication systems. A first or inner cladding with a refractive index n
2
, which may be of several tens or hundreds of microns, such as approximately 200 &mgr;m, (sometimes referred to as a pump core and comprising, for example, silica glass, fluoride glass or ZBLAN) surrounds the fiber core and receives pump light from one or more pump light sources for multimode propagation of the pump light. A second or outer cladding with a refractive index n
3
, wherein n
1
>n
2
>n
3
, surrounds the inner cladding, which may be comprise of a polymer, and confines the pump light to the inner cladding.
Many advantages arise from the monolithic nature of a DCF laser system in that all components of the system are linked by fiber, rendering them immune to mechanical misalignments or contamination of the optical path, such as from noise and optical losses. The critical links in these systems are at the ends of the fiber where the pump source light or injection source light are coupled into the fiber, and at the transmitter head, where the light is launched into a transmitter telescope to converge the light, via an optical guide into the fiber, such as illustrated in U.S. Pat. No. Re. 33,722 to Scifres et al. At the pump end of such a system, one technology currently employed for coupling light into the fiber is to employ a fiber coupled laser diode bar with the individual outputs of the individual laser emitters of the bar are individually coupled to a fiber and the individual fibers are bundled into a larger aperture, such as through stacking as illustrated in U.S. Pat. No. 5,268,978 to Po et al. and in U.S. Pat. No. Re. 33,722 to Scifres et al., or are fused together as, into a multimode fiber as illustrated also in U.S. Pat. No. Re. 33,722 to Scifres et al. Moreover, their combined output may then be coupled into a multimode fiber, or a telescoped output or fused array of fibers that is a multimode output that is imaged onto the inner cladding of the DCF.
Other current technology for coupling pump light beam and a signal light beam into either a single mode fiber or a double clad fiber, functioning as a fiber amplifier or a fiber laser, comprises free-space optics a dichroic beamsplitter and lens system, as illustrated in FIG.
1
. In this scheme, a signal to be amplified from single mode fiber
18
is coupled into a DCF amplifier
19
by a conventional lens system
15
A and
15
C. The signal beam
18
A is transparent to and passing through dichroic mirror
17
into the core of DCF amplifier
19
. Pump source
11
comprises a laser diode array or bar
12
where the output emission from the plural emitters are each respectively coupled into a fiber
13
. Fibers
13
are bundled at their output ends and the bundled fibers are coupled into a single output, such as optical medium
14
, which may be, for example, a multimode fiber. In order to achieve higher pumping powers, multimode laser sources with their combined multimode outputs into a single beam is preferred over other sources. The combined output of pump light
14
A is collimated by lens
15
B and is reflected by dichroic mirror
17
into the inner cladding of DCF amplifier
19
.
While this standard coupling scheme can be made small and self-contained, it presents a potential mechanical weakness in the overall optical system and has relatively high optical losses for the signal beam propagating in the fiber because it is coupled out and then back into the transmission fiber. Taking the case of a DCF as amplifier
19
, the coupling of pump light
14
A into the inner cladding of the DCF is relatively tolerant since it constitutes a multimode coupling. However, the coupling of signal light beam
18
A into the inner core of the DCF is more sensitive to misalignment, since it constitutes a single mode coupling. This is a critical factor. As a general principle, any diffraction limited laser system is a single spatial mode system, whether in fiber or in free space; and the alignment tolerances on single mode systems are severe. The advanced functionality of modern laser systems, particularly multi-stage systems, require that at each stage, the signal is mode matched to the next stage. Fiber laser systems are no exception. However, a fiber laser offers an opportunity to employ commercial fusion splicing since single mode fibers can be spliced together with negligible insertion loss, typically about 0.1 dB to about 0.25 dB. Once spliced, a single mode fiber connection is permanent, provides low loss and is immune to misalignment and contamination.
Breaking the continuity of an injection source single mode fiber
18
in order to inject the multimode pump light into amplifier
19
unnecessarily introduces a free-space single mode coupling into the optical system. In other words, in order to achieve end pumping of DCF amplifier
19
, a spaced separation of single mode fiber
18
apart from the end of the DCF is necessary to launch the pump light into the inner cladding of DCF amplifier
19
. Consequently, it would be highly desirable to develop a scheme to couple pump light such as from a multimode fiber into a double clad fiber without interrupting the light path of signal beam
18
A into the single mode core.
This is not to say that others have not tried to achieve a similar function of coupling a single mode signal source into the core of a DCF and a multimode pump source into the cladding of a DCF without significant loss. U.S. Pat. No. 5,170,458 to Aoyagi et al. suggests such an optical coupler at
46
in
FIG. 3
of the patent involving end pumping of a DCF. In
FIG. 3
of that patent, coupler
46
is shown as a box with two inputs, one for a signal light
6
via single mode fiber
21
and another for pump light
38
from a semiconductor source
18
via multimode fiber
48
. The single output from coupler
46
indicates direct coupling of signal light
6
to a core
52
of double clad fiber
50
and direct coupling of the pump light
38
into inner cladding
54
of fiber
50
. In the disclosure, it is indicated that coupler
46
is “of a well-known type”, but those skilled in this art, particularly at the time of this disclosure in 1990, were not readily familiar with a single mode/multimode fiber coupled input to a double clad fiber, at least one that had high optical coupling efficiency and was readily available for successful commercial applications. Thus, there is no disclosure in this patent as to how such a coupler should be designed to provide for high efficient light coupling without introducing free-space single
Booth Ian J.
Giltner David M.
Lang Robert J.
Sanders Steven
Waarts Robert G.
Bovernick Rodney
JDS Uniphase Corporation
Kang Juliana K.
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