Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-10-18
2003-01-07
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S016000, C359S850000
Reexamination Certificate
active
06504650
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical systems and optical transformers, and more particularly to systems that re-orient axes of light beams, such as emitted by laser diodes, and combine the light beams in order to improve brightness of combined beams.
2. Description of Related Art
The light output from semiconductor diode lasers is often required to be focused into an optical fiber or similar small spot for applications including the optical pumping of solid-state laser crystals. For these applications and others, it may be desirable to combine the output beams from a plurality of diode lasers onto a single spot of dimensions similar to those obtained when focusing the light beam from a single diode laser. The ability to achieve this is limited by the so-called brightness of the laser diode source; for the purpose of this discussion, brightness refers to the amount of optical power emitted by a source per unit solid angle and per unit cross sectional area. It is well known from optical theory that, by combining the illumination from several individual and identical sources through the use of mirrors, lenses or other passive optical components, it is impossible to increase the brightness at a remote position beyond the brightness of an individual source. In practice, this optimal result is not achievable; the resultant remote illumination formed by the summation of light from the discrete sources is usually much less bright than any of the individual sources. The total optical power at the remote position may be nearly the sum of the individual sources, but the illumination may be spread out over a cross-sectional area much greater than the sum of the emitting areas or the light may be distributed into an excessively large solid angle. For example, the large angular distribution may preclude the coupling of the light from the combined sources into an optical fiber of small angular acceptance, even though the resultant source may be smaller in spatial extent than the input dimensions of the fiber.
The light emitted from a laser diode is characterized by having a brightness that varies greatly when measured in two distinct and perpendicular planes. Specifically, for a typical laser diode with an elliptically shaped output beam produced by an emitting region measuring several hundred microns by a few microns, and oriented such that the axis defining the wider dimension is aligned horizontally, the light emitted vertically may be considered to be many times brighter than that emitted in the horizontal direction. The concept of etendu, the product of the angular and spatial extent of a source in two perpendicular planes, and which is inversely proportional to brightness, is hereinafter used to describe the problem of optical beam combining. For purposes of illustration in
FIG. 1
, a system of coordinates
1
is defined, with the x-z plane arbitrarily defined as horizontal and the y direction defined as vertical, a typical laser diode
2
is considered, having an emission
3
with a height &Dgr;y of 1 &mgr;m, with an angular divergence &thgr;
yz
of 65 degrees in the vertical direction, a width &Dgr;x of 200 &mgr;m, and with an angular divergence &thgr;xz of 14 degrees in the horizontal direction. With the definition of numerical aperture N.A.=sin (2 angular divergence), the etendu in the vertical direction is then the product of the vertical beam width and the vertical N.A. and is then 0.54 &mgr;m*N.A. Similarly, the etendu in the horizontal direction is 24 &mgr;m*N.A.
Since the angular divergence of the laser diode emission is substantially different in the x-z and y-z planes, the illumination profile, or spatial extent of the diode laser emission changes rapidly with distance from the emitting surface of the diode. Said profile, described by ellipses
4
a-d
, denotes the extent of the laser diode emission where the illumination intensity has fallen to a value of one half that at the location of peak intensity. Such elliptical profiles are used throughout the figures in the following descriptions of prior and subject art to denote the illumination profile of the diode laser beam as it progresses through various transforming components.
Fan (U.S. Pat. No. 5,081,637) has shown that a nearly optimal way to combine the light from discrete sources with radically different etendu in two perpendicular axes is to physically orient the diodes in such a way that the low etendu axes of the diodes are aligned to be collinear with respect to each other, with the high etendu axes aligned perpendicular to the common low etendu axis. This embodiment is described in
FIGS. 2 and 3
. In
FIG. 2
, the light from an individual source
2
, propagating nominally in a z direction as defined by the coordinate axes
1
, is first collimated in the low etendu direction by a simple cylindrical lens
5
, forming a diode-lens combination
6
. As shown by the elliptical intensity profiles
7
a
and
7
b
before and after the influence of the cylindrical lens
5
, the effect of said lens is to substantially reduce the angular divergence of the diode laser beam in the y-z plane. In
FIG. 3
, a plurality of said diode-lens combinations
6
a-c
, three of which are shown, are then uniformly distributed and followed by an additional lens
8
that collects and focuses the light from the individual collimated sources to a remote image position
9
. With high-quality collimating lenses and accurate positioning of the components, it is possible to achieve a brightness at the remote image position
9
nearly equal to that of an individual source
3
a-c
while simultaneously increasing the total optical power incident at the remote image position.
In the embodiment of Fan (U.S. Pat. No. 5,081,637), the emitting surfaces of the individual diodes are generally coplanar and the directions of light propagation from the multiple sources are essentially parallel, while in the technique described by Streifer (U.S. Pat. No. 4,826,269), the individual sources may be oriented as if on the surface of a cylinder, with the individual propagation directions aligned to be coplanar but generally pointing toward a common point of intersection.
The objective of the subject invention is the efficient combining of the illumination produced by the individual laser sources within a laser diode array. Laser diode arrays consist of a plurality of discrete laser diode sources fabricated onto a common semiconductor wafer, and said arrays are powerful sources of optical radiation. Each discrete diode source has an elliptical intensity profile as described earlier in
FIG. 1
, and the plurality of said sources is aligned such that the high etendu axes of all sources are collinear and their emitting faces are coplanar. Referring to
FIG. 4
, the array
10
has a light output with a very high etendu in the axis
11
common to all sources
12
a-e
and a low etendu in the y-direction perpendicular to said common axis. The centers of each source region may be separated by a distance w that varies between 1.5 and 5 times the width &Dgr;x of a single source in the high etendu direction, and the array may consist of between 10 to 60 individual sources, distributed uniformly over a length of 0.5 to 2 centimeters. Because of the combined effects of the individual source spacings and orientations, the brightness of the array taken as a whole is much lower than the brightness of an individual source in the array. It is desired to combine the light from these discrete sources into a single spot, yet it can be see from the work of Fan (U.S. Pat. No. 5,081,637) that these individual sources are oriented improperly to allow for a straightforward combining of the light from the individual sources into a single spot while minimizing the etendu at the remote source. Specifically, the high-etendu axes
11
of the sources comprising the array are collinear, whereas the desired orientation is the collinear alignment of the low-etendu axes
13
a-e
. If the sources
12
a-e
comprising the array could be physic
Epps Georgia
Haynes Mark A.
Haynes Beffel & Wolfeld LLP
Seyrafi Saeed
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