Surface emitting, low threshold (SELTH) laser diode

Two novel structures of a high-quantum efficiency, wavelength-tunable, surface-emitting, low threshold laser diode which optimally utilizes certain advantages of a distributed feedback (DFB) structure. The preferred embodiments combine a separate confinement heterostructure (SCH), surface-emitting distributed feedback laser diode structure with a multiple quantum well (MQW) active layer and an index-guiding buried heterostructure. A wave-length tuning section is included in the device structure for wavelength adjustment, and an arrangement of transparent electrodes useable for ohmic contacts to the device provide anti-reflection coatings for the emitting portion of the device. A first preferred embodiment is termed the SELTH laser diode (surface emitting, low threshold). A second, related preferred embodiment combines the SELTH laser diode structure with additional optical elements to provide a collimated output beam and is termed the COSELTH laser diode (collimated, surface emitting, low threshold).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a surface emitting laser diode, and 
more particularly to a low threshold, surface emitting distributed 
feedback laser diode. 
2. Description of the Prior Art 
The double heterostructure semiconductor laser diode provides optical 
waveguiding and carrier confinement in an active layer and is the basis 
for most modern laser diodes. One of the common methods of stimulating 
laser light emission is by the creation of a population inversion in the 
semiconductor with current injection in the gain media. If the applied 
current injection exceeds a threshold level, electron-hole pairs are 
stimulated and recombine to emit light with the direction and phase of the 
light in the waveguide of the resonant cavity. The electrical-to-optical 
conversion efficiency (differential quantum efficiency) for injection 
pumped lasers can be as high as 70% after the onset of threshold current. 
Thus, injection pumped semiconductor laser diodes are extremely attractive 
for a wide range of uses in optical and electroptical applications. 
However, the laser threshold current is a function of, among other things, 
temperature. The threshold current density in a cleaved cavity laser diode 
is given by: 
EQU J.sub.th =4.5.times.10.sup.3 d/g+(20d/gC)(a.sub.i +(1/L.sub.cavity)ln(1/R)) 
where: 
g=quantum efficiency 
C=confinement factor 
R=intensity reflectance 
d=active layer thickness 
a.sub.i =intrinsic absorption coefficient 
L.sub.cavity =laser cavity length 
Furthermore, the refractive index of the preferred semiconductive 
materials, such as AlGaAs or GaAs, is a function of temperature and 
injected current. The refractive index as a function of temperature is 
approximately given by: 
EQU dn(t)=4.times.10.sup.-4 dt 
and the dependence on current (i) is: 
EQU dn(t)=-(i*q.sup.2 /2m.sub.neff .times..sup.2)n 
where: 
q=electron charge 
m.sub.neff =electron effective mass 
x=radiation angular frequency 
n=average refractive index of the semiconductor. 
Accordingly, the refractive index increases with increasing temperature and 
decreases with increasing current. 
The lasing wavelength of a cleaved cavity laser diode is directly 
proportional to the mechanical length of the laser cavity, which in turn 
is also a function of temperature. The lasing light has to satisfy the 
condition: 
EQU mk/2=n.sub.GaAs L.sub.cavity 
where: 
L.sub.cavity =length of the resonator cavity 
n.sub.GaAs =refractive index of laser active region 
m=integer 
k=wavelength 
Hence, the laser output beam wavelength varies; the typical output 
characteristic of a laser diode according to temperature of the device 
consists of sections with a linear slope and discontinuous "mode hops" 
where the wavelength changes by one cavity mode spacing (C/2L). Changes in 
wavelength of the diode under current modulation also occur. Such 
instability in the output wavelength is quite undesirable and decreases 
the coherence of the laser light. Coherence is a necessary feature of the 
output beam in that the laser light is often made to interfere with 
itself, which is important in interferometers and coherent communications 
systems. 
Normally, light incident on an aperture is diffracted with the outermost 
angle being inversely proportional to the size of the aperture (i.e. 
smaller angles cause greater divergence). However, typical active layer 
thicknesses are on the order of 0.2 to 0.3 micrometer in a double 
heterostructure laser and are as small as 100 angstroms for quantum wells. 
As a result, typical half angle divergences H(fwhm) are 25 to 35 degrees 
perpendicular to the active layer. 
Because the light output of a typical laser diode comes from a 
non-symmetrical aperture, the output beam is undesirably non-symmetrical 
as well. Typically, the angular divergence perpendicular to the active 
layer is 2 to 5 times the divergence parallel to the active layer. 
Thus, two significant factors in the construction of a laser diode are the 
aperture and divergence of the emitted beam. Conventional laser diodes 
typically emit through an aperture of 0.2.times.5 micrometers, which is 
the result of the use of an extremely thin layer of semiconductive 
material, forming an active region, which is kept thin to eliminate the 
possibility of emission intensity distributions with higher order modes 
than the TEM.sub.00 mode. The width of the aperture is either determined 
by a current blocking oxide stripe or a refractive index waveguide 
fabricated into the laser. Thin (0.2-0.3 micrometer) active layer 
construction is often used because such construction can decrease the 
lasing threshold current density. 
Divergence in an edge-emitting laser diode is inversely proportional to the 
aperture size, and thus beam divergence is greatest in a direction 
perpendicular to the active layer. This causes the output from the laser 
to diverge at large angles, especially in edge-emitting laser diode. 
Typical edge-emitting laser diodes have half angle divergences of 34 
degrees perpendicular to the active layer and 8 degrees parallel to the 
active layer. These wide divergence angles necessitate the use of a 
collimating lens with a high numerical aperture to refract the light into 
a plane wave. For comparison, it should be noted that gas (for example, 
HeNe) and solid-state lasers have output beams which are already 
collimated with divergences on the order of milliradians. Laser diodes 
constructed to provide such a mode of emission (surface mode), but with 
low divergence output, are be potentially useful in a large number of 
applications. Such devices have not been easily achieved, however. 
The foregoing and other disadvantages of edge-emitting laser diodes 
therefore make them unsuitable for many applications. For example, in an 
area such as fiber optic communications, the foregoing disadvantages 
impede the use of edge emitting laser diodes, as the dispersion and 
absorption of glass fibers are minimized at certain wavelengths (1.3, 1.5 
lm) and therefore variations in wavelength cause phase delays and pulse 
broadening. Laser wavelength mode hops are also associated with 
undesirable intensity noise in optical data storage devices such as 
compact optical storage disks. 
When used in conjunction with an adjacent p/n junction, the wavelength of a 
laser diode can be tuned by changing the current in the adjacent diode. A 
surface emitting laser diode so constructed would allow for fiber optic 
communications with multiple beams of differing wavelength propagating in 
the same fiber. The beams could then be demultiplexed using a diffractive 
focusing feature which disperses the beam and focuses it onto an array of 
detectors, with one detector for each channel and each channel 
corresponding to a different wavelength. 
Accordingly, a wavelength-tunable, low current threshold, low divergence 
surface emitting laser diode would be extremely attractive for use in in 
coherent optical communications, position measuring devices based on 
interferometers with outputs similar to linear encoders, optical systems 
using holographic optical elements whose properties are wavelength 
dependent, and for illuminating compact holographic optical disk 
read/write heads, holographic-based laser deflectors (hologons), and laser 
lenses. 
Single or multiple versions of such a low threshold, low divergence surface 
emitting laser diode could also be formed along with other components such 
as GaAs MESFET's or photodiodes to form highly-useful integrated optical 
systems. The combination would also simplify the use of diffractive 
input/output couplers. 
SUMMARY OF THE INVENTION 
Two novel configurations of a high-quantum efficiency, wavelength-tunable, 
surface-emitting, low threshold laser diode have been devised which 
optimally utilize certain advantages of, among other things, what is known 
as a distributed feedback (DFB) structure. The preferred laser diode 
structures combine a separate confinement heterostructure (SCH), 
surface-emitting distributed feedback laser diode having a multiple 
quantum well (MQW) active layer with an index-guiding buried 
heterostructure. A phase modulating section is included in the device 
structure for wavelength tuning, and an arrangement of transparent 
electrodes useable for ohmic contacts to the device also provide 
anti-reflection coatings for the emitting portion of the device. The first 
configuration is termed the SELTH laser diode (surface emitting, low 
threshold). The second configuration is termed the COSELTH laser diode 
(collimated, surface emitting, low threshold). 
The invention, and its objects and advantages, will become more apparent in 
the detailed description of the preferred embodiments presented below.

DETAILED DESCRIPTION OF THE INVENTION 
Before describing the preferred embodiments of a laser diode constructed 
according to the invention, some basic concepts essential to an 
understanding of the contemplated laser diode structure will be discussed. 
Distributed Feedback Laser Structure 
Disadvantages in conventional laser diodes have been found to be eliminated 
or their effect reduced by use of a diode structure incorporating some 
type of periodic variation in the refractive index within the body of the 
semiconductor, to thus form a diffraction grating. These type of lasers 
include distributed feedback (DFB) lasers. 
In distributed feedback lasers, the injection current passes through an 
etched grating near the active layer where light emission takes place. 
Light interacts with the grating as it propagates further along the 
waveguide. DFB lasers can be made to emit light perpendicular to the 
active layer through the top surface of the laser, or along the the active 
layer, depending on the diffracting feature spacing and profile. 
Accordingly, I have devised a surface emitting laser diode structure that 
may be fabricated with an aperture of, for example, 300.times.200 
micrometers, which therefore creates a relatively low divergence output 
beam. A reduction to a divergence of 1 degree from the conventional 34 
degrees is feasible in the contemplated device. 
Refractive Index Step Waveguide Structure 
In my preferred embodiments of a laser diode constructed according to the 
invention, a refractive index step waveguide is provided parallel to an 
active layer. More specifically, a buried double heterostructure laser 
with separate confinement heretostructure is formed from a n-GaAS 
substrate having a n-Al.sub.x Ga.sub.(1-x) As layer is epitaxially grown 
on it, followed by a multiple quantum well active layer and a p-Al.sub.y 
Ga.sub.(1-y) As. A photoresist layer etch mask is formed on the wafer 
which covers a thin (approximately 3 micrometers) ridge. The wafer is 
etched down below the substrate and a mesa composed of the epitaxial 
layers is left standing. A burying layer of n-Al.sub.z Ga.sub.(1-z) as is 
then grown around the mesa. A SiO.sub.2 current blocking oxide mask is 
then formed and an ohmic contact is applied. 
In the contemplated double heterostructure, the active layer is surrounded 
on all sides by higher refractive index materials so the light is confined 
by total internal reflection. Hence, the structure is includable in the 
category of index guided laser structures and can be made to have almost 
no astigmatism. The contemplated structure offers a near square output 
beam intensity distribution (ratio of long to short beam axial 
length=1.0). The use of a refractive index step to define the emission 
aperture on all sides in a surface emitting DFB laser is a novel feature 
of the contemplated laser diodes. 
In the contemplated design of the distributed feedback section, the grating 
pitch can be chosen so that the light emerges perpendicular to the active 
layer. This provides a relatively large emission aperture and results in 
correspondingly less divergence. The emission aperture of the contemplated 
design can be as large as 300.times.200 micrometers. One might want to 
reduce the aperture to a 200.times.200 micrometers aperture to limit the 
amount of power to be dissipated in the body of the device. In either 
case, however, the aperture is much larger than the laser wavelength (0.78 
or 0.83 micrometers) and Fraunhofer diffraction theory can be used to 
calculate the far field intensity distribution resulting from passing 
through a rectangular slit. 
The surface emission from the contemplated design is, therefore, nearly 
collimated (approximately 1 degree divergence angle vs the 25 to 35 degree 
divergence from a edge emitting laser). 
Transparent Electrode Structure 
Another novel feature in the SELTH design is the use of of a transparent 
electrode material such as Indium Tin Oxide (ITO) or Cadmium Tin Oxide 
(CTO) to allow light to escape from the laser diode without being 
reflected by metal ohmic contact materials such as gold, nickel and their 
alloys. By using a transparent electrode one avoids energy losses by 
absorption of reflected light in the crystal. Also, the possibility of 
bright and dim bands appearing across the laser diode as a result of 
fabry-perot interference between the top and bottom of the laser diode are 
avoided. 
Furthermore, the electrode acts as a anti-reflection coating for light 
emerging normal to the crystal surface. Details on the contemplated ITO or 
CTO electrode structure are found in U.S. Pat. No. 4,495,514, issued to 
Lawrence et al. on Jan. 22, 1985, the disclosure of which is hereby 
incorporated by reference. 
The SELTH Laser Diode 
Turning now to the illustrations, and to FIG. 1 in particular, a preferred 
embodiment of a SELTH laser diode 10 includes a separate confinement 
heterostructure (SCH) having a surface emitting DFB laser section 12 with 
a multiple quantum well active layer (MQW). A buried heterostructure 14 is 
provided for index guiding. The SELTH includes a wavelength tuning section 
20 for wavelength tuning and a transparent electrode structure 22 for the 
ohmic contact which also acts as an anti-reflection coating. The SELTH 
device may be fabricated in a generally planar cubic structure, for 
example with dimension a=approximately 300 mm on a side. 
SELTH Laser Structure 
As shown in FIG. 2, the SELTH device 10 is composed of a n-GaAs substrate 
100 with a 2.0 micrometer thick layer 110 of N-Al.sub.0.3 Ga.sub.0.7 As 
followed by a 0.2 micrometer thick layer 120 of N-Al.sub.0.12 Ga.sub.0.88 
As. A multiple quantum well (MQW) active layer 130 consists of 10 layers 
of 80 angstrom thick Ga.sub.0.96 Al.sub.0.04 As quantum wells sandwiched 
between 11 barrier layers of 80 angstrom thick Al.sub.0.34 Ga.sub.0.66 As. 
The MQW active layer 130 is followed by a 0.2 micrometer thick 
P-Al.sub.0.12 Ga.sub.0.88 As layer 140 on which is formed a linear grating 
142. In the illustrated embodiment, the grating 142 includes first and 
second order gratings 142A, 142B. It is contemplated that for ease of 
fabrication or other reasons dependent upon the application of the SELTH 
device 10, the first order grating may be formed either partially or 
wholly into the tuning section 20. 
A P-Al.sub.0.3 Ga.sub.0.7 As layer 150 is formed above the layer 140 and 
grating 142. A p-GaAs layer 152 is grown. A mesa is etched, leaving the 
DFB and tuning section. An epitaxial burying heterostructure 160 is then 
selectively regrown around the plasma etched mesa formed of the foregoing 
layers. The burying heterostructure 160 thereby surrounds the DFB 12 and 
wavelength tuning 20 sections. 
Windows 170 of SiO.sub.2 are formed and a very thin contact layer 154 of 
Chromium (Cr) is formed to facilitate ohmic contact. The electrode 182 can 
be formed from gold-zinc (AuZn), AuCr, or Cr. A standard alloy is useable 
for electrode 182 as it need not be transparent. The ITO electrode 180 is 
used over the DFB section 12 in a thickness which is an odd integral 
multiple of k.sub.laser /4.sub.n to allow ohmic contact and act as a 
anti-reflection transmissive coating. A gold-zinc (AuZn) or chrome (Cr) 
contact 184 is formed on the ITO or CTO electrode 180 to facilitate a 
wirebond connection to the electrode 180. 
Fabrication Process for the SELTH Laser Diode 
The SELTH growth or fabrication process starts with the n-GaAs substrate 
100. Since the device structure includes a MQW active layer, either 
molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition 
(MOCVD) is preferred for at least part of the growth process. Since it is 
desirable to avoid meltback of the grating 142, liquid phase epitaxy (LPE) 
is not preferred for formation of the layers after the grating is formed 
on the P-Al.sub.0.12 Ga.sub.0.88 As. 
Preferably, MOCVD or MBE is used for the whole growth process. Also 
preferred are n type dopants of Si or Se and p type dopants of Zn or Mg. 
The SELTH device can be made to operate at 830 or 780 nm. For the purposes 
of clarity, a device construction for operation at 830 nm device operation 
will be presented. Variations in structure and fabrication to achieve 
other output wavelengths may be achieved as known in the art. 
The substrate 100 which will later receive a metallized layer 100A is 
inserted into an MOCVD growth apparatus and a 2.0 micrometer thick layer 
of N-Al.sub.0.3 Ga.sub.0.7 As is grown. This is followed by a 0.2 
micrometer thick layer of N-Al.sub.0.12 Ga.sub.0.88 As for the SCH. A MQW 
active layer composed of alternating layers of P-Al.sub.0.34 Ga.sub.0.66 
As and p-Ga.sub.0.96 Al.sub.0.04 As is grown using 11 P-Al.sub.0.34 
Ga.sub.0.66 As layers 80 angstroms thick and 10 layers of p-Ga.sub.0.96 
Al.sub.0.04 As 80 angstroms thick. The active layer is followed by the 0.2 
micrometer thick P-Al.sub.0.12 Ga.sub.0.88 As layer used as the other half 
of the SCH structure. The wafer is then removed from the MOCVD apparatus 
and cleaned. 
A photoresist layer is spin coated onto the wafer to form the desired 
grating 142. Shipley AZ 1350 positive photoresist has been used 
successfully. Preferably, first and second order linear gratings are 
imaged on the photoresist by a holographic exposure apparatus. In such an 
apparatus, a collimated laser beam is incident on a prism which bends each 
half of a laser beam to be incident on the wafer symmetrically. The 
preferred grating period is 0.2311 micrometer (for k=0.83 micrometer). The 
photoresist is then developed in a developer, rinsed in distilled water 
and dried. 
The photomask is transferred into the P-Al.sub.0.12 Ga.sub.0.88 As layer 
using reactive ion etching (RIE). To do so, the wafer is inserted into the 
RIE apparatus and a plasma using Cl as the reactive ion is used to etch 
the photomask pattern. 
The wafer is then reinserted into the MOCVD reactor. The 2.0 micrometer 
thick layer of P-Al.sub.0.3 Ga.sub.0.7 As is grown over the grating. This 
is followed by the 1.0 micrometer thick layer of p-GaAs used to facilitate 
the ohmic contact. The device is then removed from the MOCVD reactor. 
An SiO.sub.2 layer is grown over the wafer using a CVD process. A photomask 
is formed over the DFB and wavelength tuning sections and the structure is 
etched down to the n-GaAs substrate. The device is reinserted in the MOCVD 
apparatus and the epitaxial burying layer of N-Al.sub.0.4 Ga.sub.0.6 As is 
grown around the mesa. The epitaxial will not grow on the SiO.sub.2 layer 
and will only grow around the mesa. The growth is stopped when the burying 
layer is even with the mesa. The burying layer provides the index guiding 
feature and optically isolates the individual lasers from each other. The 
wafer is removed from the MOCVD apparatus and the SiO.sub.2 layer is 
removed. 
Another layer of SiO.sub.2 is formed and windows are opened over the DFB 
section to allow for ohmic contact thereto. A thin (20 angstroms) layer of 
"chromium" is vacuum deposited on the wafer to reduce the contact voltage 
associated with the ITO or CTO transparent electrodes. Then a layer of ITO 
is vacuum deposited with a thickness (around 1.5 micrometers) adjusted to 
be equal to an odd integral multiple of k.sub.laser /4n.sub.ITO so that it 
may act as an anti-reflection coating as well as an ohmic contact. The 
electrode is then annealed. 
Another set of windows is opened over the wavelength tuning section. The 
Au-Zn or Cr electrode is formed over the wavelength tuning section and in 
contact with the ITO or CTO electrode to allow for wirebonding to the 
device. 
The wafer is then removed form the vacuum deposition apparatus and tested 
for yield. According to known processes, the wafer is scribed and broken 
into individual lasers or small arrays of lasers (depending on the 
application). The devices are soldered to a heat sink and a wire bond is 
made to each (just outside the emission aperture). The packaging is 
completed and the laser diodes are ready for use. 
Advantages of The SELTH Laser Diode 
The contemplated SELTH device 10 incorporates a separate confinement 
heterostructure (SCH) which acts as a waveguide and eliminates 
nonradiative recombination currents at the active layer interface. The use 
of two layers with equal thickness and equal composition for the SCH 
provides for a symmetrical intensity distribution in the waveguide. By 
making these layers very thin the light in the waveguide is strongly 
coupled to the grating on the P-Al.sub.0.12 Ga.sub.0.88 As layer. 
The use of an MQW active layer allows the device to lase at a low threshold 
current and will reduce the dependence of the threshold current on 
temperature. It is contemplated that the threshold can be reduced below 1 
ma and thus the device may be able to be used as a thresholdless laser 
diode. The MQW active layer also decreases the heat dissipation margin 
necessary in the device due to the ohmic heat caused by the threshold 
current. The MQW structure increases the optical and carrier confinement 
in the active layer compared to bulk active layers. 
The use of a second order grating allows the light emission to be coupled 
out of the DFB section in a direction perpendicular to the active layer. 
This permits a symmetrical (square) output beam (150 micrometers by 150 
micrometers) with a low divergence (approximately 1 degree) to emerge from 
the top face of the laser. The grating feedback stabilizes the output 
wavelength with respect to temperature so that over the normal operating 
range of temperatures the SELTH laser diode is nearly athermal and does 
not mode hop to other wavelengths. This is critical for applications which 
require coherent, single frequency monochromatic light output. 
The surface emission configuration combined with the index guiding buried 
heterostructure will eliminate or greatly reduce the astigmatism often 
found in laser diodes. The light is emitted from the active layer, which 
is less than 0.2 micrometer thick, and the use of the index guiding 
feature tends to eliminate the possibility that light will appear to be 
emitted from different depths in the crystal. The low divergence coupled 
with elimination of astigmatism will allow the SELTH laser diode to be 
used in many applications without a collimating lens. 
By optimizing the grating profile, the light output from the DFB section 
can be made uniform. This achieved according to the known theory in case 
of critical coupling. Uniform output of the device is useful in laser 
printing and other imaging applications. 
Another advantage of the SELTH design is the achievement of a higher than 
average output power level. The light emission at the output surface is 
emitted from a much larger area and consequently is not limited by the 
optical damage thresholds found in, for example, cleaved facet lasers. For 
instance, a cleaved facet laser may have an output aperture of 5.times.0.2 
micrometers (aperture area=1.0 micrometers.sup.2) whereas a SELTH laser 
diode with an output aperture of 150.times.150 micrometer has a aperture 
area of 2.25.times.10.sup.4 micrometer.sup.2. The factor of a 
2.25.times.10.sup.4 difference in surface area will allow for a much 
higher damage threshold in the SELTH design if applied to the fabrication 
of a high power laser diode. 
The wavelength tuning section is extremely useful in suppressing device 
operation at two output wavelengths, and further allows for adjustment of 
the laser operating wavelength. The output wavelengths of random batches 
of commercially-available laser diodes typically vary about their nominal 
value by .+-.5 nm. This creates a need to utilize achromatized (wavelength 
insensitive) optics, or individually compensating for different laser 
wavelengths when making large quantities of devices using laser diodes. 
However, a SELTH laser diode may be made tunable over a range of 5 nm and 
may be produced and divided into 2 device batches. The devices may then be 
used in various systems with unachromatized optics (designed for one of 
two wavelengths) and still perform as expected. This capability of the 
device is especially important for use with holographic optical elements. 
The buried heterostructure provides excellent optical confinement of light 
in the waveguide. By continuing the etched grating as a first or second 
order grating beyond the current-driven region of the active layer, the 
grating can contribute to the optical isolation by scattering light out of 
the waveguide. This fact can be used to make one and two dimensional 
arrays of laser diodes which can be modulated separately from each other 
without optical or electrical cross talk. This can be very useful for 
applications such as multiple beam laser printers (which write 3 or 4 
images at once), optical computing (where a two dimensional array of laser 
diodes could be used for matrix manipulation) and multiple laser optical 
disk heads (one laser for writing on the disk, one for reading off the 
disk). If the devices are to be diced into individual lasers, the edges 
are rough sawn so as to prevent optical feedback from the facets. 
In sum, the SELTH laser diode offers very high quantum efficiencies (due to 
the MQW active layer), low power consumption (due to low threshold current 
and high quantum efficiency), smaller thermal dissipation requirements, a 
major decrease in package size relative to Nd:Yag, HeNe or Argon lasers, 
and potentially higher output power than available in single frequency 
commercial laser diodes. 
The COSELTH Laser Diode 
Turning now to FIG. 3, a second embodiment of a laser diode constructed 
according to the invention is shown. The COSELTH laser diode 200 is 
fabricated using a SELTH diode structure 10 having additional elements to 
provide an expanded and exactly collimated beam having a divergence 
limited only by diffraction. The COSELTH design also compensates for 
non-uniformities caused by imperfections in the laser structure or 
tendencies for the device to lase in filaments. 
The structure of laser diode 200 is the same as that of SELTH 10 with the 
exception that a diffractive lens 220 is incorporated above the Al.sub.0.3 
Ga.sub.0.7 As layer 150 or the p-GaAs layer 152. The preferred diffractive 
lens 220 is a surface relief lens of the type considered as a blazed, 
bleached, or binary lens, or may be a digital hologram or kinoform. The 
diffractive lens will provide a collimated wavefront diffracted by the 
linear grating adjacent to the active layer. The lens focuses the 
collimated wavefront to a point outside of the laser. 
Fabrication of the COSELTH Laser Diode 
The fabrication of the COSELTH laser diode 200 is the same as for SELTH 
laser diode 10 except that the wafer is removed from the MOCVD apparatus 
after the growth of the P-Al.sub.0.3 Ga.sub.0.7 As layer 150 and a series 
of binary masks are contacted printed and etched into the wafer to form a 
surface relief lens 220. The device is then reinserted into the MOCVD 
reactor and the p-GaAs layer 152 is grown. 
The diffractive lens is formed from a series of linearly ramped grating 
teeth etched into the P-Al.sub.0.3 Ga.sub.0.7 As layer (as shown in FIG. 
3) or on the electrode 180 by a multi-level photomasks as described by 
Logue and Chisholm in "General approaches to mask design for binary 
optics", SPIE 1052, Holographic Optics, pg. 19, 1989. In such diffractive 
lens technology, a series of binary chrome-on-glass masks are made and 
then contact printed onto photoresist on the wafer. Each layer is then 
etched before another level of the mask is contact printed and etched. 
Between 4 and 16 masks can be used to approximate the parabolic surface 
relief profile necessary for 100% diffraction efficiency. Normally a 
stepped linear ramp surface relief is arrived which approximates the 
parabolic profile. Sixteen mask level kinoforms can have up to 99% 
diffraction efficiency. 
A glass plate 230 is prepared with a metallized ring layer 224 for ohmic 
contacting coated on one side. An inner layer 222 is formed of an opaque 
material (metal or an absorbing layer) which has a small aperture 226 
aligned to the center of the metal contact ring layer 224. The aperture 
226 acts as a spatial filter if the thickness of the intervening glass is 
adjusted to be at the focal point of the diffractive lens. The diffractive 
lens 220 acts as an optical fourier transform lens for the incoming beam 
and the high frequency noise in the laser beam is obstructed by the 
aperture 226. 
The size of the aperture can be calculated by considering the equation for 
the diffraction limited spot size formed by a lens with a given focal 
length: 
EQU SS=c*k*fl./D.sub.beam 
where 
c=constant which depends on the desired degree of obscuration (typically 
1.5 to 2.5) 
fl.=focal length of the diffractive lens 
D.sub.beam =diameter of the beam on the diffractive lens (150 micrometers 
in this case) 
For example, if a focal length of 1 mm is chosen and c=2.0 the aperture 
diameter is 11.1 micrometers. 
The light exiting the aperture is then recollimated into a beam whose size 
is determined by the focal length and numerical aperture of the 
diffractive lens and a collimator lens. For instance, if the output from 
the diffractive lens has a numerical aperture of 0.5 and a F/1 collimator 
lens with a 5 mm focal length is used, the output beam is 5 mm in 
diameter. Shorter or longer focal lengths can be chosen with corresponding 
smaller and larger output beam diameters. If a zooming collimator lens 240 
is used, the output beam diameter can be adjustable. 
The collimator lens 240 can be made as a surface relief diffractive lens in 
the plate 230, or in plastic, or as a volume holographic lens using 
dichromated gelatin or photopolymer as the holographic medium. The use of 
a holographic or diffractive element as the collimator lens 240 makes the 
COSELTH 200 less expensive and easier to package. Various configurations 
using holographic and glass lenses for collimator lenses 240 are known in 
the art. 
The remainder of the process for the SELTH laser is completed until the 
wire bonding step, at which point the COSELTH laser is centered on the 
electrodes 180, 182 and aligned to a spatial filter aperture. It is 
unnecessary to apply a metal electrode to the COSELTH laser since it is 
contacted to the electrodes on the glass plate. This would allow the use 
of ITO or CTO for electrode 182 on the wavelength tuning section and 
eliminates a processing step. Finally the collimator lens is aligned to 
the output from the spatial filter and the aligned device is sealed in its 
package. 
It is contemplated that the COSELTH laser diode 200 can be approximated by 
aligning a SELTH laser 10 to a holographic or conventional glass lens 300H 
or 300L, respectively external to the SELTH laser 10 mounted on an 
apertured plate 300P as shown in FIGS. 4A and 4B. 
The COSELTH laser affords the advantages of SELTH laser and offers an 
expanded output beam with a smooth intensity profile. The collimation 
achieved with a COSELTH-based laser beam system can result in beam 
divergences on the order of milliradians. The COSELTH thus offers a stable 
output wavelength with high quantum efficiency. The contemplated device 
offers a collimated output beam having a smooth and uniform intensity 
distribution from a package size that can be as small as a cube 5 mm on a 
side. It is also inexpensive to produce compared to competing gas and 
solid state lasers. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.