Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
2001-10-11
2004-07-20
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S636000
Reexamination Certificate
active
06765725
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a beam shaping apparatus for laser diodes and, move particularly, to a device used for combining a plurality of laser beams emitted from a plurality of laser diodes to produce a single laser beam with high brightness.
DESCRIPTION OF THE PRIOR ART
As is well known in the state of the art, a conventional “high power” laser diode output beam has a relatively low brightness (brightness is equal to the ratio between beam power and product of beam area and beam solid angle) for many applications. In part, this is because the light emitting area of high power laser diodes has a very asymmetrical, rectangular form with typical width of approximately 1 &mgr;m for most devices and a length of approximately 50-400 &mgr;m. The long (i.e., length) and short (ie., width) axes are generally referred to as slow and fast axis respectively. In addition, the laser diode beam divergence is very different for the slow axis (full width at half maximum FWHM~8 degrees) and for the fast axis (FWHM~40 degrees).
However, for many practical applications of laser diodes it is necessary to have a spatially symmetric output beam with uniform divergence and increased power (i.e., increased intensity or brightness).
One method of obtaining a symmetrical output beam is by coupling a laser diode into an optical fiber, i.e., by fiber pig-tailing. A beam with increased brightness can be achieved by coupling more than one laser diode into the same fiber. The smaller the fiber core diameter, the higher the brightness at the output of the fiber pigtail. However, the asymmetrical form and divergence of a laser diode output beam require the use of an optical coupling system between the laser diode and the input fiber end. Two basic approaches were previously implemented to attempt to overcome this problem.
The first approach relies on a pair of asymmetrical polymorphic or cylindrical lenses to compensate spatial and angular asymmetries of the laser diode output beam and then uses a spherical lens to focus the radiation into a fiber core (see, for example, U.S. Pat. No. 5,321,718 to Lang et al. and U.S. Pat. No. 5,369,661 to Yamaguchi, both of which are incorporated herein by reference in their entirety). This approach only collimates the output beam of the laser diode, leaving the rectangular form of the beam substantially unchanged.
The second approach uses an optical apparatus to deflect the laser diode output beam wave front in various ways (to make it more symmetrical) to enable focusing of the radiation into a fiber. This optical apparatus can be implemented on its own or in addition to an asymmetrical lens pair. The second approach may be used with a single laser diode or in conjunction with several separate laser diodes, which are coupled into the same optical fiber.
An example of a device using several separate laser diodes was discussed in U.S. Pat. No. 6,075,912 to Goodman, incorporated herein by reference in its entirety. In the approach suggested by Goodman, the beams of several diodes are brought into close proximity via reflection at one common mirror. However, the Goodman system leaves the beam of each individual laser diode essentially undisturbed, resulting in practical limitations for a minimum spot size of a resulting output beam.
Another implementation of the second approach (U.S. Pat. No. 4,828,357 to Arata et al., incorporated herein by reference in its entirety) includes an apparatus producing a high power laser beam including (see
FIGS. 1 and 2
herein) a plurality of lasers
101
and directing mirrors
102
, a plurality of reflecting mirrors
203
and a central focusing mirror
204
for focusing the resultant laser beam into one focal point. Note that in this approach the polarization of the pump beam is not preserved. Moreover, generally, a plurality of reflecting mirrors is employed, complicating the setup. Equally, the beam of each individual laser diode remains essentially undisturbed, which produces practical limitations for the minimum spot size of the resulting output beam.
Yet another variation of the second approach is disclosed in U.S. Pat. No. 5,263,036 to De Bernardi et al. (incorporated herein by reference in its entirety) in which an improved efficiency of combining laser beams is achieved through the use of suitably positioned dichroic mirrors
301
and laser diodes
302
(see, e.g.,
FIG. 3
herein). The limitation of this technique is again that the beam of each individual laser diode remains substantially undisturbed which results in practical limitations on the minimum achievable spot size of the resulting beam. Moreover, the disclosure addresses direct launching of pumping radiation only into an active multimode optical waveguide containing an active monomode region, a technique which complicates the pumping arrangement.
U.S. Pat. No. 5,877,898 to Holleman et al. (incorporated herein by reference in its entirety) as depicted in
FIG. 4
herein proposes to divide and recombine the collimated beams from several laser diodes to obtain a more symmetrical output beam. However, Holleman et al. rely on beam rotation to obtain an improved geometry of the output beam. As one skilled in the art would appreciate, beam rotation is very difficult to realize in practice. In this particular example several separate micro-optic elements are incorporated; i.e., first an optical beam is divided up into several smaller beams, then beam rotation is implemented for all of the individual beams and finally, the rotated beams are recombined before they are focused into an optical fiber. In the scheme by Holleman no provisions are made to obtain polarization sensitive operation. Moreover no explicit minimization of backreflections from the beam dividing and recombining optical elements is accomplished; i.e., the optical surfaces from the beam dividing and recombining elements are incorporated at an angle close to 90° with respect to the input beam.
Another variation of the second approach is discussed in U.S. Pat. No. 5,825,551 to Neilson et al. (incorporated herein by reference in its entirety). As shown in
FIG. 5
herein, the beam
501
from a single laser device is collimated and then sent through an optical ‘beam shaping’ apparatus
502
. The beam shaping apparatus contains two substantially parallel reflecting surfaces
503
,
504
to effectively rotate the extension of the laser device by 90° while not rotating the orientation of the divergence angles of the output beam from the laser device. However, no provisions are made to minimize the beam size of a single broad stripe laser diode or to minimize the beam size of more than one laser device. Moreover, no provisions are included to optimize the transmission through the ‘beam shaper’ depending on the polarization state of the light incident to the beam shaper.
Accordingly, a need exists for an improved method of producing the smallest cross section from a laser beam and system for addressing laser diode asymmetries in both single and multiple laser diode systems.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel integrated arrangement by which the radiation of a plurality of laser diode light emitting areas is so combined and shaped that the combined beam bundle has a reduced and, preferably the smallest possible cross section with reduced and preferably the least possible asymmetry and far-field divergence. The beam shaping arrangement is further configured to minimize its size and insertion loss depending on the polarization state of the input beam from the plurality of laser diode light emitting areas.
This objective is met in an arrangement in accordance with the invention, i.e., an arrangement for combining and shaping the radiation of a plurality of laser diode light emitting areas comprising at least one laser diode whose radiation has a cross section in the emission plane (x-y plane) with a longitudinal axis which is much greater than in the transverse axis. The invention may further include for each laser diode a collimator unit in the radiation direction. A
Bull Douglas
Fermann Martin E.
Kozlov Valery
Boston Laser Inc.
Fulbright & Jaworski LLP
Harrington Alicia M.
Mack Ricky
LandOfFree
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