U.S. patent application number 09/740446 was filed with the patent office on 2002-06-20 for vertical cavity surface emitting laser (vcsel).
Invention is credited to Cunningham, John E..
Application Number | 20020075929 09/740446 |
Document ID | / |
Family ID | 24976550 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020075929 |
Kind Code |
A1 |
Cunningham, John E. |
June 20, 2002 |
VERTICAL CAVITY SURFACE EMITTING LASER (VCSEL)
Abstract
A vertical cavity surface emitting laser emitting at about 1.3
microns while being optically pumped by a vertical cavity surface
emitting laser emitting at about 8.1 nanometers characterized in
that the resonant cavities of the two lasers are coupled to
increase the effectiveness of the pumping light to stimulate
emission from the gain medium of the long-wavelength medium. The
two lasers are formed in a multilayer stack of which all the layers
are epitaxial except for the layers at the top of the stack that
serve as a broad band output mirror and the upper bounding of the
long wavelength laser.
Inventors: |
Cunningham, John E.;
(Lincroft, NJ) |
Correspondence
Address: |
Arthur J. Torsiglieri
2 Linden Lane
Chatham
NJ
07928
US
|
Family ID: |
24976550 |
Appl. No.: |
09/740446 |
Filed: |
December 19, 2000 |
Current U.S.
Class: |
372/50.1 |
Current CPC
Class: |
H01S 5/34306 20130101;
B82Y 20/00 20130101; H01S 2302/00 20130101; H01S 5/0208 20130101;
H01S 5/041 20130101; H01S 5/18311 20130101; H01S 5/426 20130101;
H01S 5/18369 20130101 |
Class at
Publication: |
372/50 |
International
Class: |
H01S 005/00 |
Claims
What is claimed:
1. A vertical cavity surface emitting laser emitting light at a
first wavelength longer than a second wavelength of the light used
for optically pumping it comprising a substrate supporting a stack
of layers, the stack comprising in turn: a first section forming a
first mirror highly reflective of the light of the second
wavelength; a second section forming the cavity of a laser emitting
at the second wavelength and including the gain medium of the
laser; a third section forming a second mirror reflective of the
light of the second wavelength and partially transmissive of light
of said second wavelength, the first, second, and third sections
forming a vertical cavity surface emitting laser emitting at the
second wavelength; a fourth section forming a mirror highly
reflective of the first wavelength and transmissive to light of the
second wavelength; a fifth section forming the cavity of the laser
emitting at the first wavelength and including the gain medium of
such laser; and a sixth section forming a mirror reflective of the
first and second wavelengths and partially transmissive of light of
the first wavelength, characterized in that the third and sixth are
spaced to establish a standing wave of the second wavelength
therebetween and that the first, second, third, fourth, and fifth
sections are epitaxial with a substrate.
2. The vertical cavity surface emitting laser of claim 1 in which
the gain medium of the fifth section comprises a plurality of
quantum wells.
3. The vertical cavity surface emitting laser of claim 1 in which
the first wavelength is in the range between 0.9 micron and 1.7
microns and the second wavelength is in the range of between 6 and
16 nanometers.
4. The vertical cavity surface emitting laser of claim 1 in which
the first wavelength is in the range between 1.2 and 1.3 microns
and the second wavelength is in the range between and 9.0
nanometers.
5. The vertical cavity surface emitting laser of claim 4 in which
the substrate is gallium arsenide, the first section comprises a
distributed Bragg reflective mirror formed of alternate layers of
aluminum arsenide and aluminum gallium arsenide, the second section
comprises a p-i-n diode including quantum wells for providing the
gain medium, the third section comprises a distributed Bragg
reflective mirror formed of alternate layers of aluminum arsenide
and aluminum gallium arsenide that is more than one per cent
transmissive, the fourth section comprises a distributed Bragg
reflective mirror of aluminum arsenide and gallium arsenide, the
fifth section comprises layers of aluminum gallium arsenide and
interposed dual layers of gallium arsenide antimonide and gallium
arsenide, and the sixth section comprises alternately layers of
silicon oxide and a material chosen from the group consisting of
titanium oxide, tantalum oxide, zinc sulfide and zinc selenide.
6. The vertical cavity surface emitting laser of claim 4 in which
the substrate is gallium arsenide, the first section comprises
alternately layers of aluminum arsenide and aluminum gallium
arsenide; the second section comprises a doped layer of aluminum
gallium arsenide, alternating pairs of undoped gallium arsenide and
undoped aluminum gallium arsenide, and a layer of doped gallium
aluminum arsenide; the third section comprises alternately layers
of aluminum gallium arsenide and aluminum arsenide; the fourth
section comprises alternately layers of aluminum arsenide and
gallium arsenide; the fifth section comprises layers of aluminum
gallium arsenide interposed with dual layers of gallium arsenide
antimonide and gallium arsenide; and the sixth section comprises
alternately layers of silicon oxide and titanium oxide.
Description
FIELD OF THE INVENTION
[0001] This invention relates to semiconductor lasers and more
particularly to a vertical cavity surface emitting laser
(VCSEL).
BACKGROUND OF THE INVENTION
[0002] A VCSEL is a semiconductor laser that comprises a multi
layer semiconductive element that includes semiconductive layers
that serve as the gain medium sandwiched between reflective
mirrors, desirably distributed Bragg reflective layers, that form a
cavity resonant at the optical wavelength desired for the laser for
providing a standard wave, at a single fundamental mode, in a
direction vertical to the layers. Such light can be readily coupled
to an optical fiber for utilization. For efficiency, the mirrors
should have reflectivity in excess of about 99.5 percent. Such high
reflectivity mirrors are difficult to grow in the same epitaxial
process used to grow the grain medium in long-wavelength lasers,
for example at 1300 or 1500 nanometers, wavelengths important for
use in optical fiber transmission systems. Accordingly, VCSELs
designed for operation at long wavelengths typically have used for
the mirrors either evaporated layers of dielectric material or of
lattice mismatched semiconductors. Mirrors of this kind make
difficult the electrical injection of charge carriers into the gain
medium, the usual manner of creating the population inversion in
the gain medium necessary to achieve the desired stimulated
emission of radiation critical to laser operation.
[0003] To avoid this problem, there have been proposals to provide
the necessary population inversion in the gain medium by optical
pumping at a shorter wavelength. In such a VCSEL, there are
provided a pair of cavities. The first is tuned to the desired long
wavelength and includes a first medium suitable for being optically
pumped at a shorter wavelength to provide light at the longer
wavelength. The second cavity is tuned to the shorter wavelength
and includes a medium for lasing at the shorter wavelength by the
electrical injection of charge carriers, thereby providing light
for optically pumping the first gain medium.
[0004] Long-wavelength optically pumped lasers of this kind have
been described in U.S. Pat. No. 5,754,578 that issued on May 19,
1998. In a first arrangement described, in which the short
wavelength cavity is superposed on the long wavelength cavity, the
short wavelength radiation is emitted from the bottom surface of
the short wavelength laser and transmitted through the top surface
of the underlying long wavelength gain medium and the desired long
wavelength radiation typically exits from the bottom surface of the
long wavelength cavity.
[0005] In a first exemplary embodiment of this first arrangement,
all the layers used to provide the mirrors in both cavities are
made from the GaAs/AlGaAs system and the fabrication employs two
wafer fusion steps.
[0006] In a second exemplary embodiment of this first arrangement,
the bottom long wavelength mirror of the long wavelength VCSEL is
fabricated from either the InP/InGaAsP system or the InP/InGa AlAs
system and is grown in the same epitaxial step as the
long-wavelength gain medium. The upper mirror, which is grown in
the same epitaxial step as the short-wavelength VCSEL, is attached
to the long-wavelength gain medium by wafer fusion. The remaining
mirrors of the long-wavelength VCSEL and the short wavelength VCSEL
are fabricated from the GaAs/AlGa As system.
[0007] In a third exemplary embodiment of the first arrangement,
the long-wavelength mirror is attached to an upper GaAs/AlGaAs
mirror by metal bonding; and the other long-wavelength mirror is
either an epitaxially-grown InP/InGaAsP or InP/InGaAlAs mirror, a
wafer-fused GaAs/AlGaAs mirror or a metal-bonded GaAs/AlGaAs
mirror.
[0008] In a second arrangement, short wavelength light emitted from
the top surface of an underlying short wavelength VCSEL is
transmitted through the lower mirror of the long-wavelength VCSEL.
The lower mirror of the long-wavelength VCSEL is grown in the same
epitaxial step as the upper mirror of the short-wavelength gain
medium VCSEL and is fabricated from the GaAs/AlGaAs system.
[0009] In an exemplary embodiment of the second arrangement, all
the mirrors in the structure are from the GaAs/AlGaAs system,
except for the top mirror of the long wavelength VCSEL. The top
mirror of the long wavelength VCSEL is either (1) a wafer-fused
GaAs/AlGaAs mirror, (2) an epitaxially-grown InP/InGaAsP or
InP/InGaAlAs mirror, or (3) a sputtered or evaporated dielectric
mirror. Any of these three mirrors can be supplemented with a metal
reflector at the top of the stack to increase reflectivity.
[0010] It is characteristic of the various structures described in
such prior art that the long-wavelength cavity includes an outer
reflective boundary that is tuned to be highly reflective
selectively at the long wavelength that is to be emitted as the
output. There is no attempt made to establish in the cavity of the
long wavelength cavity resonant conditions at the shorter
wavelength so that a standing wave of the shorter wavelength is
established in the cavity of the long-wavelength laser.
[0011] In these structures, each cavity is essentially independent
of the other, and is designed to confine the electric field of the
emitted light essentially entirely within its boundaries and to
peak it within the gain medium included in its cavity. As a
consequence this tends to limit the efficiency of the short
wavelength light pumping process.
[0012] Additionally in the prior art structures, no attempt had
been made to form all but the upper reflective mirrors as layers
grown epitaxially on a monocrystaline substrate to make feasible a
relatively thick gain medium in the long-wavelength VCSEL.
SUMMARY OF THE INVENTION
[0013] The present invention seeks to improve a long-wavelength
VCSEL of the kind in which the long-wavelength VCSEL is optically
pumped by a short-wavelength VCSEL, by improving the efficiency of
the conversion of the short-wavelength wavelength pumping light to
long-wavelength output light by changes in the optical structure
that make possible efficient use of a thicker gain medium.
Basically this is done by a cavity design that effectively couples
together the cavities of the two VCSELs such that the electric
field of the pumping light emitted by the short-wavelength VCSEL
extends at significant strength beyond the cavity of the
short-wavelength VCSEL and into the gain medium of the
long-wavelength VCSEL.
[0014] This is done by stacking together on a common substrate the
long-wavelength laser supported over the short-wavelength laser and
designing the outer mirror of the long-wavelength laser cavity to
cooperate with the inner mirror of the short-wavelength mirror to
provide constructive buildup of the electric field of the
short-wavelength laser in the long-wavelength laser cavity. To this
end, the outer mirror of the long-wavelength laser cavity is a
broad band mirror that is highly reflective of both the short- and
the long-wavelength light involved in the device and is designed to
establish a standing wave of the short-wavelength light in the
cavity of the long-wavelength laser.
[0015] In an exemplary embodiment, a VCSEL designed to emit output
light of a long wavelength, typically 1.3 .mu.m, comprises a single
crystal substrate, on top of which are stacked a plurality of
monocrystalline semiconductive layers, advantageously all grown by
molecular beam epitaxy, topped by a dielectric mirror that forms an
essentially monolithic structure free of fused layers. These
multilayers form a first lower VCSEL, designed to use electrical
pumping and to lase at the shorter wavelength and to optically pump
the second upper laser that is to provide the longer-wavelength
light that is to be the output.
[0016] The multilayers of the stack that form the first VCSEL
typically comprise a first section that forms a mirror that is
highly reflective of the shorter wavelength light, a second section
that forms the cavity including the gain medium for such light, and
a third section that forms a mirror that is both sufficiently
reflective of the short-wavelength light to establish lasing in
such gain medium and sufficiently transmissive to permit laser
light of the short wavelength to penetrate into the second VCSEL.
The first and third sections serve as reflective boundaries of a
cavity resonant at the shorter wavelength, as in a conventional
VCSEL.
[0017] The layers of the stack that form the second VCSEL comprise
a fourth section that forms a mirror that is both sufficiently
reflective of the long wavelength light to support lasing at the
long wavelength and sufficiently transmissive of the
short-wavelength light to provide optical pumping of the second
VCSEL. The fifth section forms the cavity that includes the gain
medium of the long-wavelength laser and is advantageously wider
than it would normally be so that it can include a plurality of
quantum wells. The sixth section forms a mirror with the fourth
section boundaries of a cavity resonant at the long wavelength and
with the mirror of the first section boundaries of a cavity
resonant at the short wavelength. Accordingly the sixth section
needs to be highly reflective at both the short and long
wavelengths. It also needs to be sufficiently transmissive of the
long wavelength to provide a useful output.
[0018] In the preferred embodiment, the device is formed largely as
a stack of the first five sections as layers epitaxially grown on a
semi-insulating substrate, such as insulating monocrystalline GaAs.
On this substrate is first formed the outer mirror of the
short-wavelength VCSEL by a series of quarter short-wavelength
layers alternately of the AlAs and AlGaAs to form a distributed
Bragg reflector (DBR). Next there is formed the cavity of the
short-wavelength laser as a layer of GaAs in which is formed a
P-I-N diode. Next there is formed the second or inner mirror of the
short-wavelength VCSEL, also by a succession of one-quarter
wavelength short-wavelength layers, alternately of AlAs and AlGaAs
to form a DBR. This marks the end of the short-wavelength laser.
Next there follows a succession of one-quarter long-wavelength
layers alternately of AlGaAs and GaAs to form the inner DBR mirror
of the long-wavelength cavity followed by layers that form the
long-wavelength AlGaAs cavity, sufficiently wide to include at
least two layers of GaAsSb and GaAs that form the quantum wells
that serve as the gain medium for the long wavelength VCSEL.
Finally, as the outer mirror of the long-wavelength VCSEL there is
deposited a plurality of dielectric layers, alternately of
S.sub.iO.sub.2 and T.sub.iO.sub.2 to form a broad band mirror
highly reflective of both the short and long wavelength.
[0019] In particular, distance separating the inner mirror of the
short-wavelength VCSEL and the outer mirror of the long-wavelength
VCSEL is such as to establish a standing wave of the
short-wavelength therebetween.
[0020] The invention will be better understood from the following
more detailed description taken in conjunction with the
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0021] FIG. 1 shows the basic structure of long-wavelength VCSEL
pumped optically by a short-wavelength VCSEL in accordance with the
prior art.
[0022] FIG. 2 shows the electric field distribution within the
separate cavities of the two lasers of FIG. 1.
[0023] FIG. 3 shows the basic structure of a long-wavelength VCSEL
pumped optically by a short-wavelength VCSEL in accordance with an
exemplary embodiment of the invention.
[0024] FIG. 4 shows the electric field distribution with the two
coupled cavities of the VCSEL of FIG. 2.
DETAILED DESCRIPTION
[0025] In FIG. 1 there is shown a VCSEL laser 10 of the kind
described in the earlier mentioned U.S. Pat. No. 5,754,578.
[0026] It comprises a semi-insulating GaAs substrate 11 on which
has been grown a stack of layers that form a lower short-wavelength
VCSEL 12 that optically pumps an upper long-wavelength VCSEL 14.
The short-wavelength laser 12 includes a top (inner) DBR mirror 16,
a bottom (outer) mirror DBR 18, and bounding a cavity 19 including
its short-wavelength gain medium. In typical fashion to this end
the cavity includes p-type and n-type layers sandwiching a lightly
doped region to form a p-i-n diode. Typically to maintain laser
action, both mirrors 18 and 19 need to be highly reflective,
typically more than 99 per cent reflective. The long-wavelength
VCSEL 14 includes a top (outer) mirror 20, a bottom (inner) mirror
22, and a cavity 28 including the gain medium. The top
long-wavelength mirror 20 can be either (1) a wafer-fused or
metal-bonded GaAs/AlGaAs DBR mirror, (2) a DBR epitaxially-grown
InP/InGaAsP or InP/InGa AlAs mirror; or (3) a sputtered or
evaporated dielectric mirror. Any of these three mirror designs
optionally includes a metal reflector 24 at the top to the stack to
increase reflectivity. The bottom mirror of the long-wavelength
VCSEL 14 is advantageously grown in the same epitaxial growth step
as the short-wavelength VCSEL, generally by molecular beam
epitaxy.
[0027] A wafer-fused interface 26 is included between the long
wavelength gain cavity 28 and the bottom long-wavelength mirror 22
because of the lattice mismatched. The long wavelength gain cavity
28 typically consists of materials selected from the group
consisting of InP, InGaAsP, and InGaAlAs. In typical fashion the
gain medium includes a quantum well to provide the gain. Generally
the practice has been to insert at most three quantum wells. Th
concentration of the holes and electrons injected from opposite
sides of the junction falls off sharply with distance so that wide
intrinsic junction regions become inefficient.
[0028] Current confinement is obtained in the short-wavelength
VCSEL 12 by lateral oxidation using AlGaO layer 32. The
long-wavelength VCSEL 14 is index-guided by etching grooves 34
prior to the wafer fusion of the bottom long-wavelength 22 and the
long-wavelength cavity medium 28.
[0029] The short-wavelength VCSEL 12 is top-emitting, emitting most
of its power away from the substrate 10, while the long-wavelength
VCSEL 14 can be either top-emitting or bottom-emitting. In the
latter case, the VCSEL 12 and the substrate 11 need to be
transparent to such light. Both p-type and n-type conductive
contacts 36, 38 for electrical pumping of the short-wavelength
VCSEL 12 are made below the long-wavelength VCSEL 14.
[0030] When lasing, the electric field distribution that is created
along a vertical section between the top and bottom layers of the
stack is shown, for the long-wavelength light in FIG. 2A, and for
the short-wavelength light in FIG. 2B.
[0031] As is seen in FIG. 2A, the long-wavelength electric field
peaks sharply in the layers of the stack that correspond to the
relatively narrow long-wavelength cavity 28 and is essentially
confined within the layers 20 and 22 that form the two DBR mirrors
defining the reflective boundaries of the long-wavelength
cavity.
[0032] As is seen in FIG. 2B, the short-wavelength electric field
peaks sharply in the layers near the bottom of the stack that
correspond to the short-wavelength cavity and is essentially
confined by the layers that form the two mirrors 16 and 18 defining
the reflective boundaries of the short-wavelength cavity 12.
[0033] As is seen from the two plots, in the laser of FIG. 1 there
is essentially no coupling between the two cavities and the field
distribution in each VCSEL would remain essentially as shown even
if its two component VCSELs were physically separated.
[0034] FIG. 3A and FIG. 3B are comparable plots of the electric
fields desired for the long-wavelength light and short-wavelength
light, respectively, in a VCSEL laser in accordance with the
present invention, for example as shown in FIG. 4.
[0035] As is seen in FIG. 3A, the electric field for the
long-wavelength radiation peaks close to the top surface of the
stack in the dielectric mirror, but maintains a relatively high
uniform strength throughout the relatively longer long-wavelength
gain medium, and tapers off to a uniformly relatively low strength
through the short-wavelength VCSEL portion of the stack.
[0036] As seen in FIG. 3B, the short wavelength field maintains a
steady strength at a moderate level through the entire stack
portion including the long-wavelength gain medium for continually
interacting with the long-wavelength field and forms a peak in the
dielective mirror at the top of the stack, albeit at a lower level
than its peak in the gain medium region of the short wavelength
portion of the stack. This results because in the invention the
upper mirror of the short wavelength VCSEL used to provide the
pumping light can be more transmissive than in the prior art device
because part of the transmitted light is reflected back by the
upper mirror of the long wavelength VCSEL as a standing wave is
established between these two mirrors as will be discussed in more
detail subsequently.
[0037] In FIG. 4, there is shown a VCSEL in accordance with the
invention constructed to provide electric fields of the kind shown
in FIGS. 3A and 3B.
[0038] FIG. 4 shows a structure 110 that includes a long-wavelength
VCSEL 114 that is optically pumped up by a short-wavelength VCSEL
112 in accordance with the invention. It includes a substrate 111,
typically of high resistivity gallium arsenide, on which there has
been grown epitaxially a stack that includes the first five
sections of a stack similar in several respects to the stack shown
in the device of FIG. 1. In particular, it includes a first section
118 that forms a DBR outer mirror of the VCSEL 112 and is designed
to be highly reflective of the short-wavelength light produced by
the VCSEL 112. The DBR comprises quarter wavelength layers
alternately of low and high indices of refraction, for example,
AlAs and GaAs.
[0039] It further includes a second section 119 that forms the
cavity including the gain medium of the short-wavelength VCSEL 112
and includes p-type and n-type layers sandwiching a relatively
thicker high-resistivity layer to form a p-i-n diode. The gain
medium, for example, may be of AlGaAs, appropriately doped to form
a p-i-n diode and containing at least three 80 Angstroms GaAs
quantum wells.
[0040] The third section 120 is also a DBR mirror that needs to be
sufficiently reflective of the short wavelength light to support
laser operation but also can be more transmissive than typical of
prior art devices, for example, more than one per cent and as much
as two per cent transmissive, because part of the light will be
returned after reflection from the outer mirror of the long
wavelength VCSEL as a standing wave is established between these
two mirrors. This higher transmissivity will permit more pumping
light to be introduced into the long wavelength cavity where it can
be converted to long wavelength radiation. This also can comprise
alternate layers of aluminum arsenide and aluminum gallium
arsenide.
[0041] The fourth section 121 also includes a plurality of
quarter-wavelength layers of the longer wavelength of the VCSEL 114
alternately of high and low indices of refraction, for example AlGa
and GaAs, to form a DBR mirror that is highly reflective of light
of the longer wavelength and highly transmissive of the
short-wavelength light from VCSEL 112 that is to pump VCSEL
114.
[0042] The fifth section 122 comprises the cavity that includes the
gain medium 125 of the long-wavelength VCSEL 114. It is important
to grow this layer epitaxially without undue strain and that it be
sufficiently thick to accommodate at least several quantum wells to
permit extended interaction of the pumping light with the gain
medium. This section advantageously should be substantially thicker
than that of the cavity enclosing the gain medium that would be
used in prior art devices of the kind shown in FIG. 1, the latter
will typically include a cavity gain medium sufficiently thick and
to support without undue strain no more than two quantum wells.
This is the case because the relative inefficiency of the
conversion characteristics of such prior art device makes pointless
providing more quantum wells.
[0043] Because of the stronger field of the pumping light in the
gain medium of the long-wavelength VCSEL being described, it
becomes feasible to include a number of quantum wells, and
preferably at least six, in the gain medium 125. By using AlGaAs as
the cavity matrix for quantum wells of GaAsSb and GaAs, it becomes
feasible to introduce as many as six quantum wells in a cavity
thickness of two long wavelengths, about 7200 Angstroms, without
excessive strain or defects in the resulting monocrystalline
lattice. Alternatively, the gain medium could be introduced into
the AlGaAs matrix by quantum dots or layers of InGaAsN.
[0044] Finally, as a sixth section 126, there is deposited
alternate layers of S.sub.iO.sub.2 and T.sub.iO.sub.2 to form the
outer mirror 120 of the long-wavelength cavity. As discussed
previously, this mirror 126 needs to be highly reflective both of
the long and short wavelengths, as well as being sufficiently
transmissive of the long-wavelength light to provide useful output
power at the long-wavelength.
[0045] Moreover this mirror 120 needs to be appropriately spaced to
provide constructive interference with the inner mirror 116 of the
short-wavelength VCSEL 112 that there is established between these
two mirrors essentially a standing wave of the short-wavelength
light to increase the efficiency of the pumping light to generate
output light of the long wavelength. Optionally there may be
included a top metallic layer to increase the reflectivity at both
the long- and short-wavelengths.
[0046] In an examplary design the substrate monocrystalline gallium
arsenide, the first section consisted of 22 quarter wavelength
pairs of n-type aluminum arsenide and aluminum gallium arsenide,
respectively about 610 Angstroms and 711 Angstroms in thickness,
followed by a last quarter wavelength layer of aluminum arsenide.
The second section cavity consisted of an n-type layer of AlGaAs
followed by at least three quantum wells formed by alternating
pairs of 80 Angstroms thick undoped gallium arsenide and 50
Angstroms thick undoped aluminum gallium arsenide, and the p-type
doped layer 1000 Angstroms thick of aluminium gallium arsenide.
[0047] The third section was similar to the final section except
that there are only 18 periods to provide for transmission
therethrough of short-wavelength light to pump the long-wavelength
VCSEL.
[0048] The fourth section included 20 pairs of 1100 Angstrom thick
layers of aluminium arsenide and 900 Angstroms thick layer of
gallium arsenide.
[0049] The fifth section includes three layers of 1400 thick layers
of aluminium gallium arsenide, and two pairs of 60 Angstroms thick
gallium arsenide antimonide and 300 Angstrom thick gallium arsenide
interposed between both the first and second, and second and third
of the three layers.
[0050] Finally the sixth section of the stack includes eleven
pairs, alternately layers of 2280 Angstroms thick silicon dioxide
and 1470 Angstrom thick layers of titanium dioxide. Alternatively,
tantalum oxide zinc sulfide or zinc selenide may be substituted for
the titanium dioxide.
[0051] There are expected to be a variety of other materials
systems that should operate similarly to the particular material
system of the exemplary embodiment described. In particular, it
should be feasible to use materials that provide short wavelength
pumping light in the range of between six and sixteen nanometers
and long-wavelength output light of between 0.9 micron and 1.7
microns.
* * * * *