High power fiber ribbon laser and amplifier

A single-mode fiber laser (66) that provides increased power. The fiber laser (66) includes a fiber ribbon (30) having a plurality of parallel waveguides (10). Each waveguide (10) includes a rectangular shaped core (12) that has a relatively thin dimension in one direction and a relatively wide dimension in an orthogonal direction. A step-index cladding (14, 16) is provided in the thin direction to limit the propagation of light in the core (12) to a single mode in that direction. Mode filters (18, 20) are provided adjacent the ends of the core (12) in the thickness direction and the various propagation modes in the core (12) enter the mode filters (18, 20). The mode filters (18, 20) guide the desirable single-mode back into the core (12) and reject the other modes away from the core (12). Light absorbing layers (22, 24) are provided adjacent the mode filters (18, 20) and opposite the core (12) to absorb the undesirable propagation modes of light. Therefore, the main propagation mode in the core (12) is a single low order mode, but the cross-sectional area of the core (12) is increased to provide more power. Each of the cores (12) in the ribbon (30) are optically pumped form the side by a bar (40) of diode arrays (42) positioned at strategic locations along the length of the ribbon (30) relative to the waveguides (10). Various transmission gratings (48) and/or reflection gratings (50) can be provided within a ribbon jacket (32) to launch the optical pump light down the waveguide (10). The ribbon (30) can be wrapped around a mandrel (68) and a cooling fluid can be introduced through the mandrel (68) to cool the ribbon (30) during laser operation.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to a single spatial mode laser and, more
 particularly, to a high power, single-mode, diode-pumped, fiber ribbon
 laser having a rectangular shaped core, where laser waveguides of the
 laser employ mode filters to remove higher-order propagation modes from
 the core.
 2. Discussion of the Related Art
 There exists a need in the art for an electrically driven laser that has a
 high average power, but does not employ chemical laser fuel or effluent.
 These types of lasers have many applications, including military
 applications against a variety of airborne threats, such as ballistic
 missiles, cruise missiles, and tactical aircraft. Diode-pumped,
 solid-state lasers employing an array of fiber laser waveguides or
 amplifiers is one known laser that satisfies this need. A practical
 technique is needed to make fiber lasers into rugged devices without
 complex optical components to produce useful and affordable laser systems.
 Typically, for applications of this type, the laser system must employ a
 fiber or core that generates a single-mode laser beam. This is because a
 single-mode laser beam generates the most intensity or power per unit area
 when the beam is focused. As the number of transverse modes of the laser
 beam increases, the size of the beam spot that can be focussed also
 increases as a result of beam diffraction. This reduces the beam power per
 unit area, which reduces its intensity.
 Diode-pumped, dual-clad ytterbium-doped glass fiber lasers have been used
 in the art to generate single-mode laser beams, generally having an output
 power up to 50 watts. The fibers in these lasers typically employ a round
 core having a diameter on the order of 5-8 microns to generate the
 single-mode laser beam. An inner cladding layer around the core traps the
 single-mode beam within the core, and an outer cladding layer reflects
 pump light across the core to be absorbed. A discussion of this type of
 fiber laser can be found in the proceedings for a conference on Advanced
 Solid-State Lasers, including DiGiovanni, David J., "High Power Fiber
 Lasers and Amplifiers," Fibers and Waveguide Devices, Feb. 3, 1999, pgs.
 282-284, and Nilsson, J. et al., "Widely tunable high-power diode-pumped
 double-clad Yb.sup.3+ -doped fiber laser," Fibers and Waveguide Devices,
 Feb. 3, 1999, pgs. 285-287.
 It is desirable in the art to increase the power output of single-mode
 fiber lasers for certain applications. The power output of the laser can
 be increased by increasing the length of the core and providing more
 optical pump light. However, the material of the core has power limits
 that if exceeded may damage the core material. More optical power can also
 be provided by making the core diameter larger. However, as the core
 diameter increases, the generation of higher-order modes begin to develop,
 and it becomes increasingly more difficult to limit the beam to a
 single-mode. Once a certain core size is reached, it is virtually
 impossible to limit the propagation modes to a single-mode. Further, as
 the size of the core and the power increases, the generation of heat in
 the core also increases. Cooling systems are known to reduce this heat,
 but larger diameter cores makes it more difficult to remove the heat from
 the center of the core. Therefore, a heat gradient may exist across the
 core, which causes the laser to decrease its performance.
 The core of a dual-clad fiber laser tends to have a length on the order of
 50 m. Therefore, the core is typically wrapped around a mandrel to
 decrease the size of the laser system and give the core structural
 rigidity. As the size of the core and associated cladding increases, it is
 more difficult to bend the core around the mandrel and still maintain
 mode-control, thus causing the mandrel to have to be larger. Also, the
 size of the core determines how tightly the core can be wrapped on the
 mandrel before stresses reduce or eliminate mode control caused by light
 leakage.
 What is needed is a single-mode fiber laser that has increased power over
 those fiber lasers known in the art without losing mode control and that
 allows effective laser cooling. It is therefore an object of the present
 invention to provide such a laser.
 SUMMARY OF THE INVENTION
 In accordance with the teachings of the present invention, a single-mode
 fiber laser is disclosed that provides increased power. The fiber laser
 includes a ribbon having a plurality of parallel waveguides that generate
 a sheet of optical light. Each waveguide includes a rectangular shaped
 core that has a relatively thin dimension in one direction and a
 relatively wide dimension in an orthogonal direction. Step-index cladding
 layers are provided in the thin dimension to limit the propagation of
 light in the core to a single-mode in that direction. Mode filters are
 provided adjacent the ends of the core in the wide dimension and the
 various propagation modes in the core enter the mode filters. The mode
 filters allow the desirable single-mode to propagate in the core with the
 least amount of loss, while higher order modes suffer greater losses.
 Light absorbing layers are provided adjacent the mode filters and opposite
 the core to absorb the undesirable propagation modes of the light.
 Therefore, the main propagation mode in the core is a single low order
 mode, but the cross-sectional area of the core is increased to provide
 more power.
 Each of the cores in the ribbon are optically pumped from the side by a bar
 of diode arrays positioned at strategic locations along the length of the
 ribbon relative to the waveguides. Various transmission gratings and/or
 reflection gratings can be provided within a ribbon jacket to launch the
 optical pump light down the waveguide in a manner that causes total
 internal reflection of the light to trap it within the waveguides as it
 crosses the core multiple times. The ribbon can be wrapped around a
 mandrel and a cooling fluid can be introduced through the mandrel to
 conductively cool the ribbon during laser operation. Multiple mandrels can
 be combined to provide multiple ribbons to increase the cross-sectional
 area of the resulting laser beam.
 Additional objects, advantages and features of the present invention will
 become apparent from the following description and appended claims, taken
 in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The following discussion of the preferred embodiments directed to a
 diodepumped fiber ribbon laser is merely exemplary in nature, and is in no
 way intended to limit the invention or its applications or uses.
 FIG. 1(a) is a top view and FIG. 1(b) is an end view of an optical
 waveguide 10 that can be used in a diode-pumped, single-mode, fiber laser
 system, as will be discussed below. The laser beam is amplified in a
 rectangular shaped core 12 or lasing medium. In one embodiment, the core
 12 is a ytterbium-doped glass, but can be any suitable light amplification
 medium as would be known in the art. The rectangular shape of the core 12
 defines a thin structure, where the narrow thickness dimension from the
 top to the bottom of the core 12 is on the order of 5-10 microns to allow
 propagation of only a single-mode beam in the thickness direction. The
 core 12 is bound on its upper and lower sides by step-index cladding
 layers 14 and 16 that provide propagation for the single optical
 propagation mode. In one embodiment, the cladding layers 14 and 16 are a
 doped glass having a different index of refraction than the core 12 to
 cause the beam travelling in the core 12 to be refracted and reflected
 back into the core 12. The core 12 is significantly wider in a width
 dimension that allows the cross-sectional area of the core 12 to be larger
 than the known cores in single-mode fiber lasers to increase the power
 output of the laser beam. The technique for limiting the propagation of a
 single-mode beam in the width direction will be discussed below.
 A first mode filter 18 is provided along one side of the core 12 and a
 second mode filter 20 is provided along the opposite side of the core 12
 in the width dimension. The various propagation modes that may be
 travelling through the core 12 expand out of the core 12 in the width
 dimension and enter the mode filters 18 and 20. The mode filters 18 and 20
 can comprise a series of doped glass layers that are optimized to interact
 with the various propagation modes of optical light in the core 12 to
 cause the desirable propagation mode to experience the least amount of
 losses, and the undesirable or high-order propagation modes to suffer
 greater losses. The various layers in the mode filters 18 and 20 include
 alternating regions of high and low refractive indexes to define grating
 layers to provide this function. The refractive index in the mode filters
 18 and 20 can be controlled by different dopant ions. The mode filters 18
 and 20 can be any suitable guide structure that performs the operation as
 discussed herein, such as a Bragg grating or photonic band gap materials.
 As is known in the art, photonic band gap materials provide interaction of
 different indexes of refraction to allow scattered optical beams to
 combine destructively and constructively depending on their phase
 relationship.
 A first absorber layer 22 is positioned adjacent the mode filter 18
 opposite the core 12, and a second absorber layer 24 is positioned
 adjacent the mode filter 20 opposite the core 12. The absorber layers 22
 and 24 would also be made of doped silica or glass where the dopant ions
 absorb the wavelengths of the laser light. The absorber layers 22 and 24
 do not provide discrimination of reflection angles, but absorb all of the
 laser wavelengths. The absorber layers 22 and 24 can be any optical glass
 absorber material suitable for the purposes described herein. By operation
 of the waveguide 10, the desirable lowest-order mode is allowed to
 propagate down the core 12, and the higher-order modes are caused to be
 absorbed by the absorber layers 22 and 24.
 The waveguide 10 is the basic building block of a fiber ribbon laser of the
 invention. FIG. 2(a) shows a top view and FIG. 2(b) shows an end view of a
 fiber ribbon 30 that includes a plurality of the waveguides 10 joined
 together in a parallel manner. One of the waveguides 10 is labeled with
 the reference numerals discussed above for FIG. 1, with the understanding
 that the other parallel waveguides 10 have the same components. As shown,
 the absorber layers 22 and 24 abut against a respective absorber layer 22
 or 24 of an adjacent waveguide 10. The absorber layers 22 and 24 between
 the cores 12 attenuate rejected modes and control coupling therebetween.
 In this example, six separate waveguides 10 are joined in parallel. As
 will be appreciated by those skilled in the art, this is by way of a
 non-limiting example in that other designs may incorporate more of less
 waveguides. The configuration of the waveguides 10 to form the fiber
 ribbon 30 is such that a sheet of parallel laser beams propagate out of
 the end of the ribbon 30, where each laser beam comprises the same
 single-mode beam. The organized sheet of laser beams can be effectively
 couple and aligned to output optics of the laser system in a much more
 effective manner than the jumbled, round fiber laser waveguides used in
 the prior art.
 An outer jacket 32 encloses the waveguides 10 to define the ribbon 30. The
 top layer of the jacket 32 is not shown in FIG. 2(a) to expose the
 waveguides 10 below. Edge reflectors 34 and 36 are provided in the outer
 jacket 32 to help confine the laser pump light, as will become apparent
 from the discussion below. The edge reflectors 34 and 36 can be any
 suitable reflective layer, such as a metal layer formed at the sides of
 the jacket 32.
 As is known in the art, pump light is required in these types of
 solid-state lasers to excite the atoms within the core 12 to amplify the
 laser light. Known diode-pumped lasers generally provide the pump light
 from an end of the core. In these systems, complex optics are sometimes
 required for providing coupling of both the pump light and the laser light
 from the same location. According to the present invention, the pump light
 is provided from a side location of the ribbon 30 to pump the laser. FIGS.
 3-5 are cross-sectional views through one of the waveguides 10 of the
 ribbon 30 in connection with a pumping source. The various embodiments
 shown in FIGS. 3-5 for introducing pump light into the ribbon 30 are
 intended to show that different applications for introducing the pump
 light can be used in different designs depending on the particular design
 requirements. In the FIGS. 3-5, the various layers of the waveguides 10
 are labeled in the same manner as discussed above.
 In FIG. 3, a diode bar 40 including several diode arrays 42 is provided,
 where each diode array 42 includes a plurality of diode emitters 44. Each
 diode array 42 extends across the ribbon 30 so that each emitter 44 in the
 array 42 aligns with the core 12 of a particular waveguide 10. In this
 example, four diode arrays 42 are provided in the diode bar 40 so that
 each waveguide 10 receives pump light from four of the emitters 44 to
 provide the necessary optical pumping for a particular application. Each
 diode emitter 44 generates a beam of pump light that is directed towards a
 collimating lens 46. In this embodiment, each collimating lens 46 extends
 across the ribbon 30 for each diode array 42.
 The pump light from the diode arrays 42 is collimated and impinges a blazed
 uniform transmission grating 48 provided in a section of the outer jacket
 32. The transmission grating 48 refracts the pump light from the diode
 arrays 42 so that it enters the particular waveguide 10 at a certain
 angle. The transmission grating 48 can be any suitable optical material
 that diffracts the pump light. For example, the transmission grating 48
 can be polished glass including etched grating lines. The angle of the
 pump light propagating across the waveguide 10 and the difference in the
 indices of refraction of the jacket 32 and the cladding layers 14 and 16
 provides total internal reflection to cause the pump light to be reflected
 back and forth down the waveguide 10 so that it continually crosses the
 core 12 multiple times as shown. The wavelength of the pump light from the
 diode arrays 42 is such that it excites the atoms in the core 12 so that
 most of the pump light is eventually absorbed by the core 12 to provide
 the laser pumping. The edge reflectors 34 and 36 help confine the pump
 light in the ribbon 30 that may happen to propagate out the sides of the
 ribbon 30.
 In the embodiment of FIG. 4, the bar 40 of the diode arrays 42 are
 provided, but the collimating lenses 46 are eliminated. In addition to the
 transmission grating 48, a reflection grating 50 is provided in an
 opposite wall of the jacket 32 to reflect the pump light within the
 waveguide 10. Because the collimating lenses 46 have been eliminated, the
 transmission grating 48 has a grating periodicity related to the
 wavelength of the pump light to cause it to be focussed into the waveguide
 10. In other words, the pump light is refracted through the transmission
 grating 48 to launch the pump light down the waveguide 10. The reflection
 grating 50 is a reflective surface instead of a transmission surface,
 where the etched lines in the grating 50 cause the pump light to be
 reflected at an angle desirable to capture the pump light within the
 waveguide 10 by total internal reflection.
 In FIG. 5, the bar 40 of diode arrays 42 is replaced with a bar 52 of diode
 arrays 54 including emitters 56. The collimating lens 46 are eliminated in
 this embodiment, and the diode arrays 54 are angled relative to the
 waveguide 10 so that the pump light enters the waveguide 10 through
 transmission gratings 58 in a desirable manner to provide optical pumping
 of the core 12. This embodiment provides a segmented chirped transmission
 grating application to launch the pump light down the waveguide 10 so that
 it is trapped therein.
 FIG. 6 shows a perspective view of a laser system 66 where the fiber ribbon
 30 is helically wrapped around a cylindrical mandrel 68. Because the fiber
 ribbon 30 is relatively thin in the thickness dimension, it can be
 relatively tightly wrapped around the mandrel 68 to reduce system size. A
 tube 70 extends through the mandrel 68 where the mandrel 68 acts as a heat
 sink to allow a cooling fluid to be pumped therethrough to cool the fiber
 30 during laser operation. Because the mandrel 68 has significant surface
 area and the fiber ribbon 30 is relatively thin, appropriate cooling of
 the ribbon 30 can be provided by the cooling fluid and the mandrel 68 to
 eliminate heat gradients for proper device performance.
 Coupling optics 72 are provided at an input end of the fiber ribbon 30 to
 couple the input laser beam to be amplified by the fiber ribbon 30.
 Coupling optics 74 are provided at an output end of the fiber 30 to couple
 the amplified laser beam to the output optics of the laser system 66. The
 optical pumping is provided by a bank 76 of diode bars 78. Each diode bar
 78 includes a plurality of diode arrays of the type as discussed above,
 where the diode arrays extend across the ribbon 30 so that a diode emitter
 is aligned with each core 12. In this embodiment, the diode bars 78 are
 provided in a row along the mandrel 68 at every other wrap location, so
 that optical pumping is provided every certain predetermined distance
 along the fiber ribbon 30. In other embodiments, the diode bars 78 can be
 provided at different locations around the mandrel 68 for different
 pumping and different applications. A cooling system 80, including a
 cooling tube 82, directs a cooling fluid to cool the diode arrays 78.
 The configuration of the laser system 66 discussed above provides a
 suitable length of the fiber ribbon 30 that is able to be tightly wrapped
 around the mandrel 68 to provide a compact laser assembly at an increased
 power over the prior art. To further increase the power, multiple laser
 assemblies 66 can be provided to stack the fibers. In FIG. 7, a
 perspective view an array 86 of laser systems 66 is shown where the optics
 74 couple the output from a plurality of ribbons 30, here six ribbons 30
 into a single beam, as shown.
 The foregoing discussion discloses and describes merely exemplary
 embodiments of present invention. One skilled in the art will readily
 recognize from such discussion and from the accompanying drawings and
 claims, that various changes, modifications and variations can be made
 therein without departing from the spirit and scope of the invention as
 defined in the following claims. Additional optics (not shown) can be
 provided to shape the output beam from the coupling optics 74 for a
 desirable beam shape.