Angle selective side-pumping of fiber amplifiers and lasers

Details Angle selective side-pumping of fiber amplifiers and lasers Angle selective side-pumping of fiber amplifiers and lasers

2001-02-23

2003-03-04

Kim, Robert H. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S049000, C385S088000, C385S123000, C385S033000, C372S006000

Reexamination Certificate

active

06529657

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to fiber optics, and more particularly to high power fiber amplifiers and lasers and devices for optically pumping these in order to create a population inversion within an active medium of such amplifiers and/or lasers.
BACKGROUND OF THE INVENTION
Double cladding fiber structures have been demonstrated to be an effective approach of constructing high power fiber lasers and amplifiers. See L. Goldberg et al., “Highly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diode,” Optics Letters, v. 15, pp. 673-675, 1999. Since the typical inner cladding dimension of a double cladding fiber structure is 100-250 &mgr;m, non-diffraction emission from high power broad area laser diode pumps can be efficiently coupled into such fibers. A 100 &mgr;m wide broad stripe laser diode can generate an output power of 2-4 W at 810 nm, 915 nm or 980 nm with a long operating life. Larger pump powers required for building high power fiber amplifiers can be realized using multiple broad stripe pump diodes, coupled through a plurality of v-grooves in the double cladding fiber structure.
A broad stripe laser diode generates a beam that is diffraction limited in the plane perpendicular to the diode junction, and that emerges from a region approximately 1 &mgr;m wide at the diode facet. In this plane, diode emission exhibits a relatively large beam divergence of 30°-40° (full width at half-maximum of the intensity (“FWHM”)). On the other hand, in the plane parallel to the diode junction, the emission emerges from an extended region of 100 &mgr;m (equal to the width of the active stripe), diverges with an angle of 10°-12° (FWHM), and is characterized by low spatial coherence.
Another approach for increasing the available pump power is through the use of a fiber-coupled pump diode bar. Such diode bars are typically 1 cm wide and contain 10 to 40 of 100-200 &mgr;m wide emitting stripes, resulting in a total output power of 20-60 W. The emission of the 1 cm wide diode bar can be collected and coupled into a multimode delivery fiber with a typical numerical aperture of 0.1-0.3 and a diameter of 200 &mgr;m to 400 &mgr;m. The output of such a multimode fiber constitutes a low spatial coherence extended source, emitting a beam that cannot be focused into a diffraction-limited spot.
The v-groove technique, see U.S. Pat. No. 5,854,865 to Goldberg et al., entitled “Method and Apparatus For Side-Pumping An Optical Fiber,” for coupling pump light from a broad stripe laser diode into the inner cladding of a double cladding fiber is illustrated in
FIGS. 1
a
—
1
b
. As shown in
FIGS. 1
a
—
1
b
, a 90° v-groove extends nearly to the fiber core, the broad stripe laser diode is oriented so that its junction is perpendicular to the fiber axis, and the inner cladding is square shaped.
For the diode-fiber orientation of
FIGS. 1
a
—
1
b
, pump light emerging from the diode diverges as it propagates toward the v-groove so that the radiation emitted to the right of the junction impinges on the right facet (facet
1
) of the v-groove, while radiation emitted to the left impinges the left facet (facet
2
). Each of the facets reflects the impinging light and directs it along the fiber. The inner cladding captures and guides any pump light reflected by the v-groove facet if that light is launched at angles &thgr;
L
(measured outside of the fiber, relative to the fiber axis) equal or smaller than the critical angle &thgr;
c
, related to the numerical aperture of the cladding by NA=sin &thgr;
c
. For a typical double cladding fiber, the inner cladding numerical aperture is 0.4-0.5, corresponding to a maximum launch angle (measured outside of the fiber) of &thgr;
L
=24°-30°.
FIG. 1
c
illustrates angle space plots for the v-groove coupling. In these plots, a circle represents the acceptance angle of the inner cladding, since the inner cladding, regardless of pointing direction, captures rays incident at any angle below &thgr;c.
Light incident on v-groove facets at angles smaller than the critical angle for the glass-air interface of the facet undergoes total internal reflection (“TIR”). For silica glass with a refractive index of 1.46, the critical angle is 47° inside the fiber, measured relative to the glass surface. This means that inside the fiber all rays incident from the left side of the vertical or within 2 degrees on the right side of the vertical undergo TIR at the facet. From Snell's law, the steepest (relative to the v-groove facet) angle of incidence outside of the fiber is approximately 3 degrees. Therefore, light which is incident on the right facet (facet
1
) in
FIG. 1
a
undergoes TIR for angles &thgr;y ranging from −3° (rays traveling to the left of the vertical) to all positive angles (rays traveling to the right of the vertical). Similarly, light incident on the left facet undergoes TIR for all angles &thgr;
y
ranging from +3° to all negative angles. Since diode emission in the yz plane in
FIG. 1
a
is effectively bisected by the vertical line projected from the v-groove apex to the diode junction, pump light impinges on the right facet only at positive angles, while it impinges on the left facet only at negative angles, as shown.
Why the pump light impinges on the right facet only at positive angles, while it impinges on the left facet only at negative angles is explained by
FIG. 1
c
. In
FIG. 1
c
, the shaded area above the &thgr;
x
-axis depicts the angular coverage of facet
1
, showing that all positive diode emission angles &thgr;
y
are TIR reflected by facet
1
and are captured by the fiber. Similarly, the shaded area below the &thgr;
x
-axis of
FIG. 1
c
shows that all negative diode emission angles &thgr;
y
are TIR reflected by facet
2
and are captured by the fiber. Therefore, all light emitted by the diode undergoes TIR at the v-groove facets. The combined angular coverage provided by the two facets allows efficient coupling of the light from the broad stripe laser into the inner cladding of the double cladding fiber. This is only possible because in the yz plane, the diode emission originates from a very small area (approximately 1 &mgr;m) so that a vertical line through the v-groove apex and the diode junction effectively bisects the diode emission's angular divergence.
Unfortunately, the above-described method of coupling pump light into a double cladding fiber does not function well when an extended pump source, such as a multimode fiber is used, as shown in
FIG. 2
a
. Here the light incident on facet
1
or facet
2
contains the full angular distribution of the pump fiber output. As shown in
FIG. 2
b
, the angular distribution is uniform in all directions and extends up to the critical angle of the pump delivery fiber. Since TIR reflection at the v-groove facet occurs only for one half of this distribution (positive angles for facet
1
, and negative angles for facet
2
), a substantial fraction of the incident power will be transmitted through the facet-air interface.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for constructing high power fiber amplifiers and lasers. Embodiments of the present invention provide a means for efficient pumping of fiber amplifiers and lasers that make use of double cladding fibers. Embodiments of the present invention are designed to work in conjunction with a v-groove coupling technique for injecting the pump light from an extended pump source into the inner cladding of a double cladding fiber.
In v-groove coupling, when pump light is incident on v-groove facets over a limited angular range, the pump light is reflected by TIR at the facet-to-air interface and directed into the fiber. An embodiment of present invention comprises an apparatus and method for efficiently v-groove coupling a pump light emerging from an extended pump source, such as a multimode fiber, into a double cladding fiber. This apparatus and method preferably utilize an optical system that separates the pump beam into two beams, each with approxi

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