Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-07-24
2004-03-09
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S027000, C385S036000, C372S006000
Reexamination Certificate
active
06704479
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for optically pumping double-clad fiber lasers and amplifiers using diode lasers, diode bars, or fiber-coupled diode sources. More particularly, the invention relates to an apparatus and method for optical side-pumping an optical fiber that includes an embedded side-mirror.
BACKGROUND OF THE INVENTION
Double-Clad Fiber Optical Sources
Rare-earth-doped fiber lasers and amplifiers are finding widespread use in applications requiring compact, rugged optical sources. In these sources, a rare-earth ion (e.g., Er
3+
or Yb
3+
) is doped into the fiber core and is optically excited (typically using a diode laser as a pump source); a signal beam propagating in the core experiences gain if a population inversion has been established by absorption of the pump beam by the rare-earth ions (and if the signal beam has a wavelength within the gain spectrum of the rare-earth dopant). The core often supports only the lowest-order transverse mode of the signal beam (a single-mode (SM) fiber), but in some cases can support more than one transverse mode (a multimode (MM) fiber).
In a conventional, single-clad fiber, the signal and pump beams both propagate in the fiber core, which is surrounded by a cladding whose refractive index is lower than that of the core (thereby serving to define the size and numerical aperture, NA, of the core). In double-clad (DC) fiber, a second cladding with a lower refractive index (typically a fluorinated polymer) surrounds the cladding; the first or “inner” cladding can thus guide light launched into it (much as the core guides light). In such a fiber, the signal beam is launched into the core (as in a single-clad fiber); the pump light, however, is launched into the much larger (and usually higher-NA) inner cladding. If the rare-earth dopant is confined to the core of the DC fiber, the pump light will be absorbed in the core, and signal light propagating in the core will experience gain, in a manner similar to that of a single-clad fiber. The advantage of DC fiber, however, is that it permits the use of pump sources that are relatively large (i.e., multimode), high-power, and inexpensive (in comparison with the single-mode pump sources capable of being launched into the core of single-clad fibers). The advent of DC fibers has allowed fiber sources to be scaled to average powers of >100 W.
Prior Art Coupling Schemes
Several techniques for launching pump light into DC fiber exist. These include end pumping, which is the most straightforward approach and which is often used in laboratory applications. The pump light is simply launched into the end of the fiber, typically using one or two lenses and possibly a mirror; optionally, pump light may be launched into both fiber ends. A major drawback of this technique is that one or both of the fiber ends are obstructed by the optics used to launch the pump light. Hence, coupling a signal beam into or out of the fiber requires a means to separate the pump and the signal (typically a dichroic mirror). In addition, this approach lacks scalability (the fiber has only two ends) and is difficult to implement in a compact and rugged manner; the simplicity is generally inferior to other techniques (larger parts count, more optical and mounting hardware). Finally, the fiber ends are easily damaged when high pump powers are used, e.g., if the fiber face is not kept very clean or if the pump beam becomes misaligned. In one implementation of end pumping (U.S. Pat. No. 5,185,758), multiple pump diodes, each with its own collimating lens, are arrayed along a focusing lens to provide more pump power (although not more brightness, because of the angular displacement of the beam from each diode impinging on the fiber face).
In another approach termed tapered, fused fiber bundles, which is described in U.S. Pat. No. 5,864,644, several diode lasers are coupled into individual multimode fibers; these fibers are bundled together, fused and drawn into a taper, and then fusion spliced to a DC fiber. Pump light from the diode lasers is thereby coupled into the inner cladding of the DC fiber. Optionally, the fiber bundle can include a single-mode fiber that is used to couple signal light into or out of the core of the DC fiber. This approach is stable and rugged (because the fibers are fused) and can have high efficiency (if the coupling efficiencies to the pigtails of the individual diode lasers are high). The approach is scalable, although it would be awkward to use with a diode bar. The problem of blocking the fiber end(s) is alleviated by employing the embodiment that incorporates a single-mode fiber into the bundle. Fabrication of a tapered, fused fiber bundle is a complex process, involving stripping the jacket (which exposes the delicate fibers), bundling the fibers in a close-packed formation, fusing (melting) the bundle, drawing the taper, and (usually) recoating with a low-index polymer. The shape and size of the fiber bundle must be customized for the given DC fiber being pumped.
In another approach termed V-groove side-pumping, described in U.S. Pat. No. 5,854,865 which is incorporated herein by reference, a V-shaped notch or groove is cut into the side of the DC fiber, and light from a pump diode (or fiber-coupled pump diode) is launched into the inner cladding by reflection from the facet of the V-groove. The depth of the V-groove is such that it penetrates the inner cladding but does not intersect the core. In the embodiment most commonly used, shown in
FIG. 1
a
, a pump source
1
is placed on the opposite side of a fiber
2
from a V-groove
3
, while a micro
4
is used to substantially focus the light onto a V-groove facet
5
, and the pump light is coupled into an inner cladding
6
via total internal reflection from the facet. An outer cladding and jacket
7
are stripped from fiber
2
prior to cutting V-groove
3
, and fiber
2
is mounted on a glass substrate
8
that transmits the pump light; the adhesive used for mounting fiber
2
to substrate
8
must have a refractive index less than or equal to that of the low-index fluorinated polymer of outer cladding
7
so that the light guiding properties of inner cladding
6
are not compromised. The angular acceptance of V-groove
3
can be increased by coating groove
3
for high reflectivity (HR), although this approach significantly increases the complexity of fabrication and subsequent servicing or repair.
Advantages of V-groove side pumping include high coupling efficiency and compact packaging; it is scalable (by cutting multiple V-grooves) and leaves the fiber ends unobstructed. In practice, however, this technique is alignment sensitive and thus presently lacks adequate long-term stability for many applications. The alignment sensitivity arises in part from the use of the lens (see below), which demagnifies the pump beam and thus increases its angular spread; moreover, available lenses do not allow the brightness of the pump source to be preserved. For a given combination of pump diode and DC fiber, a micro-lens has to be selected or fabricated to be compatible with the size and divergence of the diode and with the size and NA of the fiber inner cladding. For high-power applications, use of a diode bar as a pump source (rather than multiple diodes) would be desirable (see below). The prior art V-groove pumping technique, however, is not compatible with the use of diode bars because each element of the bar would require its own, individually aligned micro-lens, thus introducing prohibitive complexity; in principle, a lens array could be used, but present tolerances on the position and angle of the emitters on a diode bar are insufficient for this approach to be practical.
An alternative embodiment of V-groove side pumping that was recently introduced, in which the micro-lens is omitted, is described in
Proceedings of the Conference on Lasers and Electro-Optics
, OSA Technical Digest Series, paper CFC1 (Optical Society of America, Washington D.C., 2000), by L. Goldberg, J. Pinto, M. D
Karasek John J.
Legg L. George
Pak Sung
Sanghavi Hemang
The United States of America as represented by the Secretary of
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