Vertical cavity surface emitting lasers with consistent slope efficiencies

A vertical cavity surface emitting laser (VCSEL) with variable tuning layer for adjusting the slope of the laser and method for manufacturing the same are disclosed. In practice, a VCSEL wafer is grown by any conventional technique, and fabricated into discrete lasers while maintained in wafer form. The initial lasers are then tested to determine characteristics, such as the slope efficiency distribution. A variable thickness dielectric layer is then deposited which is calculated to tune the slope efficiency distribution to within a target specification by altering the phase of the top facet reflectivity of the initial lasers. The resulting change in transmission directly changes the laser slope in a predictable fashion. The tuning step may be repeated, if necessary, to further refine the slope to the desired value. The method produces VCSELs with similar or consistent slopes from a plurality of wafers. Also disclosed are an optical subassembly and optical transceiver incorporating the improved VCSELs.

FIELD OF THE INVENTION 
This application relates generally to semiconductor lasers, and in 
particular to vertical cavity surface emitting lasers (VCSELs) with 
consistent slope efficiencies and a method of fabricating the same that 
allows the slope of the lasers to be predictably tuned during fabrication. 
BACKGROUND OF THE INVENTION 
Semiconductor lasers are widely used in applications such as optical 
communications. The edge emitting laser diode is a semiconductor laser 
that emits light from a plane which is a continuation of the p-n junction 
of the diode. Cleaved surfaces at the ends of the diode act as mirrors 
which together define an optical cavity. Optical feedback provided by the 
cleaved mirrors creates a resonance of the emitted light that results in 
lasing. 
The vertical cavity surface emitting laser (VCSEL) is another type of 
semiconductor laser in which the optical cavity is normal to the p-n 
junction of the semiconductor wafer from which it was fabricated. 
Ordinarily VCSELs are manufactured with many layers of semiconductor 
material deposited upon the substrate. The VCSEL includes highly 
reflective optical mirrors above and below the active layer which, in 
contrast to the edge emitting laser, enable laser output normal to the 
surface of the wafer. 
VCSELs are preferred over edge-emitting devices for a number of 
applications. Since they emit vertically and the beam is more symmetric 
and less divergent, coupling VCSELs to fiber or to other optical devices 
is easier in many cases. Typically a low-cost ball lens may be used rather 
than expensive aspheres. In addition, VCSELs are fabricated into completed 
lasers at the wafer level, so fabrication and testing are relatively 
inexpensive. These properties, combined with the small size of the VCSEL 
that allows high speed operation at low currents, make them desirable for 
lower-cost data communications transceivers. 
Because of their complexity, however, existing processes for manufacturing 
VCSELs do not always yield devices with consistent characteristics. The 
process involves hundreds of layers that depend on numerous parameters 
including, but not limited to, doping concentration, substrate 
temperature, material sources, and growth rate. These process parameters 
compound the manufacturing difficulty already well understood in the 
semiconductor field where fluctuations on the order of 50-100% are not 
uncommon. In the case of silicon technology, designers typically use 
ratios of values to minimize the effect of process variations. 
Unfortunately, in the case of discrete lasers, there is no suitable 
existing way to compensate for process variations within the device. The 
result is that the burden is placed on the higher level assemblies to 
compensate for device variations, adding complexity and cost. 
In the case of data communications, for example, the output power of the 
transmitter is ordinarily restricted to a specified range. In practice, 
either the total optical subassembly slope variation falls within 
specification, or the drive circuit must compensate by driving low slopes 
with higher currents and higher slopes with lower currents. The drawback 
with varying the drive currents, however, is that high speed performance 
varies, affecting the overall product consistency and yield. 
Accordingly, a process would be desirable that produces lasers with highly 
consistent slope efficiencies on a wafer to wafer basis. Slope efficiency, 
also referred to as external efficiency, or slope, generally refers to the 
product of the internal efficiency and the optical efficiency. The 
internal efficiency is the fraction of electrons that are converted to 
photons, and the optical efficiency is the fraction of photons that are 
transmitted out of the laser. Since internal efficiency is difficult to 
precisely control because of the complexity of semiconductor processes, 
those skilled in the art would prefer a process that enables the tuning of 
the slope efficiency of the laser by altering the optical efficiency, 
which is directly related to the transmission and reflectivity of the 
laser, to compensate for process variations in a relatively simple and 
cost effective manner. 
Some prior art lasers have been fabricated with a non-quarter wavelength 
layer of optically transparent material that had the side effect of 
changing the slope. An example of such a prior art VCSEL with a 
non-quarter wavelength layer has the specification shown in FIG. 12. 
However, the prior art process changed the slope of the laser in a fixed 
manner that generally did not take into account wafer to wafer variations. 
Therefore, any wafer to wafer variations upon application of the fixed 
layer led to the same variations in the final products. Those skilled in 
the art would prefer a process that enables predictable tuning during 
fabrication to achieve lasers having consistent slopes on a wafer to wafer 
basis. 
SUMMARY OF THE INVENTION 
There is therefore provided in a presently preferred embodiment of the 
present invention a VCSEL having a variable tuning layer for predictably 
adjusting the optical efficiency of the laser during fabrication to 
achieve lasers with substantially similar slopes on a wafer to wafer 
basis. 
To fabricate the VCSEL with variable tuning layer according to one 
embodiment of the present invention, a VCSEL wafer is grown by any of a 
variety of conventional techniques, and fabricated into discrete lasers or 
laser arrays while maintained in wafer form. The initial lasers are then 
tested by any conventional technique, preferably on a representative 
sample, to determine characteristics of the initial lasers, such as the 
slope efficiency distribution. A variable thickness dielectric layer is 
then deposited which is calculated to tune the slope efficiency 
distribution to within the target specification. The variable tuning layer 
changes the laser optical efficiency by altering the phase of the top 
facet reflectivity. The change in transmission by the altered reflectivity 
directly changes the laser slope in a predictable fashion. Once the 
variable tuning layer is deposited, vias are preferably etched for 
electrical contact to enable additional measurements to be performed. 
Based on the tests, the tuning step may be repeated, if necessary, to 
further refine the slope to the desired value. The process produces VCSELs 
with similar or consistent slopes from a plurality of wafers.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a VCSEL 2 with variable tuning layer according to an 
embodiment of the present invention generally includes a conventional 
VCSEL portion 5 and a variable tuning layer 10 having a thickness 
predetermined in an intermediate process step to achieve a laser with a 
desired slope efficiency. Advantageously, the present invention can be 
used with virtually any conventional VCSEL design, an exemplary embodiment 
of which is described herein. 
The exemplary conventional VCSEL portion 5 includes a substrate 12, a first 
or lower mirror 14, an optical cavity 16, and a second or upper mirror 18. 
The substrate 12 is made of gallium arsenide (GaAs) or any other suitable 
material. The first and second mirrors are comprised of multilayered 
distributed Bragg reflectors (DBRs), as is conventional in the art. In the 
exemplary embodiment, aluminum gallium arsenide (AlGaAs) with varying 
concentrations of aluminum and gallium are used to fabricate the mirrors. 
The optical thickness of each mirror layer is typically designed to be a 
quarter wavelength of the emitted light of the laser where the optical 
thickness is given by the product of the physical thickness and the index 
of refraction. 
The conventional optical cavity 16 (FIG. 1A) includes an active region 20 
surrounded by first and second cladding regions 22, 24. The first and 
second cladding regions are made of AlGaAs in the exemplary embodiment. In 
the active region, three quantum wells 26 made of GaAs are disposed 
adjacent barrier layers 28 made of Al.sub.0.25 Ga.sub.0.75 As. As is 
generally understood, the number of and materials forming the quantum 
wells and surrounding barrier layers can be varied depending on the 
design. 
The epitaxial structure is preferably formed into discrete lasers by a 
combination of current confinement and ohmic contacts. The contact 
metalization forming n-ohmic contact on the bottom of the substrate may 
be, for example, eutectic gold germanium deposited by electron beam 
evaporation or sputtering. The top contact metalization forming p-ohmic 
contact 32 may be, for example, gold with 2% beryllium added or a layered 
structure of titanium/platinum/gold, preferably deposited by electron beam 
evaporation. Current constriction is preferably provided by using proton 
implantation region 40 to convert the upper mirror DBR 18 to high 
resistivity in all areas except the active device, isolating the devices 
into individual VCSELs while in wafer form. Other techniques for current 
constriction, such as selective AlAs oxidation, are also applicable. A 
probe pad metalization 34 is preferably disposed onto the p ohmic contact 
32 to provide for wire bonding and electrical testing. 
According to the present invention, the variable tuning layer 10 is 
preferably disposed on the conventional VCSEL structure 5 to tune the 
slope efficiency and thereby compensate for manufacturing variations. The 
variable tuning layer may be made of any optically transparent, 
mechanically stable material. In a preferred embodiment, the variable 
tuning layer is formed of a dielectric layer of a silicon oxide or silicon 
nitride, whose thickness is chosen to center the slope efficiency 
distribution of the lasers on a wafer to compensate for wafer to wafer 
variation in the slope efficiency. 
The thickness of the variable tuning layer is preferably in the range from 
about zero to about one quarter wavelength, or multiples thereof, for 
yielding a final surface reflection that can be continuously varied from 
in phase to out of phase with the adjacent DBR. The term "surface 
reflection" is meant to have an ordinary meaning as known in the art, and 
is further meant to cover any reflections on surfaces (e.g., air, plastic, 
or a plurality of layers comprising an additional Bragg reflector), 
relating to a top layer and/or one or more intermediate layers. In 
practice, the phases of all reflections above the variable tuning layers 
are changed relative to the layers below the variable tuning layer. In the 
preferred embodiment, the tuning layer 10 has the effect of altering the 
top facet reflectivity of the VCSEL in a predictable manner, thereby 
adjusting the slope efficiency of the overall device, and enabling the 
production of a plurality of lasers having consistent slope 
characteristics from different wafers. 
Referring also to FIG. 2, the VCSEL with variable tuning layer 2 is 
preferably manufactured according to a process that includes the steps of 
fabricating 42 the initial VCSEL portion; measuring 44 a characteristic of 
the initial VCSEL portion 5, such as its resistance or slope efficiency; 
determining 46 the thickness of the variable tuning layer 10 based on the 
measured characteristic necessary to change the slope of the laser to a 
desired value; and depositing 47 a variable tuning layer 10 having the 
determined thickness to produce a laser with the desired slope. After 
measurement of the resulting slope 48, the determining and depositing 
steps can be repeated 49 if necessary to fine tune the lasers to the 
desired slope. Each of the steps is described in more detail hereinafter. 
As shown in FIGS. 3 and 4, in a presently preferred embodiment, the VCSEL 
with variable tuning layer is made by initially fabricating a wafer 50 of 
conventional VCSEL portions 5 leaving the surfaces of the VCSELs, which 
may include dielectric passivation layers, exposed. The various layers of 
the VCSELs are epitaxially deposited on the semiconductor substrate 
following techniques well known in the art. One such technique is 
described in U.S. Pat. No. 4,949,350, the contents of which are hereby 
incorporated by reference. To facilitate testing, a probe pad 34 is placed 
on the devices on the wafer to make a contact for electrical testing and 
subsequent wire bonding of the completed lasers. 
Once the conventional VCSEL portions 5 are fabricated, one or more 
characteristics of the initial lasers, such as resistance or slope 
efficiency, for example, is measured directly or indirectly by any 
conventional method. In the preferred embodiment, the measuring step is 
carried out as shown in FIG. 5 by placing the wafer 50 on a grounded chuck 
(not shown) of a conventional autoprober 54 which is preferably modified 
by any suitable technique to include the disposition of a broad area 
photodetector 56 above the probe tip 58. The probe tip is then moved into 
physical contact with probe pad 34 on the initial VCSEL portions 5, 
enabling electrical testing. 
The process of measuring the slope efficiency of the initial VCSELs 5 is 
preferably performed by determining the ratio of the change in laser 
optical output power produced by a change in the input bias current. This 
can be accomplished, for example, by stepping the applied bias current 
while measuring the optical output power with the photodetector to 
generate a current to light characteristic 60. In one method of 
calculation, the light characteristic is searched for the low current 
I.sub.min that produces a specified low level optical power P.sub.min. The 
high current I.sub.op is then calculated by adding a specified modulation 
current I.sub.mod to I.sub.min such that: 
EQU I.sub.op =I.sub.min +I.sub.mod. (I) 
The corresponding high level optical power P.sub.op is determined from the 
measured characteristic, and the slope efficiency .eta..sub.ext is 
calculated by 
EQU .eta..sub.ext =(P.sub.op -P.sub.min)/(I.sub.mod) (II) 
The low level power P.sub.min and modulation current I.sub.mod are 
preferably chosen to be representative of the conditions used in the 
higher level assemblies. Other conventional methods such as linear 
regression may be used to calculate slope efficiency as is known in the 
art. 
The measurement of slope efficiency is preferably made on a representative 
sample of VCSELs to capture the slope efficiency distribution for the 
wafer. For example, in a typical VCSEL layout, some 20,000 devices may be 
formed on a three inch wafer. A representative sample may be on the order 
of 200 devices, for example, spatially distributed on a regular grid over 
the wafer surface. 
Once the slope efficiency has been determined, the next step in the 
preferred embodiment is to modify the optical efficiency of the laser in 
order to achieve the desired slope efficiency. The slope efficiency 
.eta..sub.ext of a laser is the product of the internal efficiency 
.eta..sub.i and the optical efficiency .eta..sub.opt. 
EQU .eta..sub.ext =.eta..sub.i .eta..sub.opt (III) 
The internal efficiency .eta..sub.i is the fraction of electrons that are 
converted to photons while the optical efficiency .eta..sub.opt is the 
fraction of photons that are transmitted out of the laser. As shown in 
equation (III), adjusting the optical efficiency .eta..sub.opt so that the 
product is constant can compensate for variations in the internal 
efficiency. 
The optical efficiency .eta..sub.opt is calculated as the ratio of the 
transmission to the sum of the transmission and optical losses, 
EQU .eta..sub.opt =T/(T+L) (IV) 
where T is the transmission out of the cavity where the light is generated 
to the output facet, and L is the sum of all other losses including 
transmission out the other side of the laser. 
In practice, the transmission is modified by the variable tuning layer 
which alters the top facet reflectivity of the laser. Accordingly, the 
optical efficiency, and hence the slope, becomes adjusted. While the 
internal efficiency ordinarily varies in an unpredictable fashion, the 
change in transmission of the VCSEL as additional layers are deposited is 
highly predictable. Once the slope efficiency of the VCSEL has been 
measured, the internal efficiency for that wafer is essentially fixed, so 
the transmission can be tuned to compensate. 
The thickness of the variable tuning layer 10 to achieve the desired slope 
is preferably determined in the following manner. A ratio is first 
calculated between the measured slope efficiency to the desired value, and 
then a predetermined lookup table, described in more detail below, is 
referenced which relates the slope efficiency ratio to a tuning layer 
thickness. The desired values of slope efficiency for the VCSELs may be 
based, for example, on specifications for the VCSELs or specifications 
for, or tests conducted on, higher level assemblies. 
Referring to FIGS. 5 and 6, once the variable tuning layer 10 is deposited 
onto the initial VCSEL 5, via holes 62 are preferably etched to the probe 
pad 34 to provide a contact for further electrical testing. The 
representative sample lasers are preferably retested to confirm the 
effectiveness of the variable tuning layer. The tuning process may then be 
repeated, if needed, taking into account the tuning layer thickness 
already on the wafer. In practice, the tuning during the first quarter 
wavelength is monotonic, and therefore error in thickness is made on the 
low side to enable recovery from deviations by additional deposition 
rather than etching, although etching may be used if needed. Furthermore, 
the yield is preferably optimized by centering the wafer's distribution 
within a specification, so the above process is preferably applied to 
center the distribution and maximize yield. 
Referring to FIG. 8, in an alternate and presently preferred embodiment of 
the present invention, an additional process step is introduced after the 
testing step to coarsely tune the slope of the initial VCSELs 5 toward the 
desired range, and then fine tune the slope of the lasers to the desired 
range. This step is generally accomplished by disposing a matching layer 
100 and an additional Bragg stack 80 over the upper mirror to reduce the 
slope to an initial level, and then disposing an additional tuning, or 
dephasing, layer 86 over the Bragg stack to move the slope to the desired 
level. 
In the preferred embodiment, the matching layer is a one half wavelength 
silicon nitride layer 100, followed by four alternating pairs of one 
quarter wavelength silicon oxide 102, 104, 106, 108 and silicon nitride 
layers 103, 105, 107, 109, configured as the additional DBR 80. As is 
conventional in the art, the layer thicknesses are computed using the 
wavelength as measured in the material, so that the nitride layers with a 
higher index of refraction have a smaller absolute thickness than the 
oxide layers with a lower index of refraction. The thicknesses are 
preferably chosen to ensure that all reflections add completely in-phase 
relative to the original VCSEL upper mirror 18 reflection. The dephasing 
layer 86 is a variable-thickness oxide layer whose thickness is in the 
range of from about zero to about one quarter wavelength, or multiples 
thereof, to yield a final reflection which can be continuously varied from 
in phase to out of phase with the preceding reflections. As the thickness 
of the layer increases from zero, the reflection becomes progressively 
more out of phase and the total transmission out of the VCSEL is 
increased. 
Referring to FIG. 9, the preferred process for fabricating VCSELs with 
consistent slopes from a plurality of wafers is disclosed by graphical 
illustration. Through the measuring step, tests conducted on initial 
lasers from two different wafers prior to the tuning process show that the 
wafers have substantially different slope efficiency distributions 
centered as shown in curves 72 and 74. Both distributions are preferably 
greater than the desired efficiency 76, which is preferably set at the 
center of the specified distribution. The presently preferred process is 
to deposit the four period DBR 80 (FIG. 8) over the upper mirror to reduce 
the slope efficiencies for the wafers below the specified range as shown 
in curves 82 and 84, and then to deposit the wafer specific predetermined 
silicon dioxide tuning layer 86 (FIG. 8) to tune the slope efficiencies 
for the lasers on each of the wafers toward the desired value as shown in 
curves 88 and 90. As shown in FIG. 9, the tuning layer increases the 
transmission until it reaches an optical thickness of one quarter 
wavelength, and then the transmission is reduced to a minimum at a 
thickness of one half wavelength. The tuning is thus cyclical with layer 
thickness, oscillating with each half wavelength deposition. In another 
embodiment, one could start with two wafers as represented by curves 82, 
84 and then increase the transmission by applying either silicon oxide or 
silicon nitride tuning layers as shown in curves 88 and 90. 
In practice, a look up table such as in Table 1 is used in the preferred 
embodiment to determine the third DBR stack and thickness of the variable 
tuning layer to move the slope efficiency toward the center of the 
specification. As is shown in the "scaled" column, the tuning in the 
exemplary embodiment provides a 2.times. range (0.221/0.113) in the final 
slope efficiencies. 
TABLE 1 
______________________________________ 
Exemplary Lookup Table for an 850 nm VCSEL Including a 
Four Period Dielectric DBR and a Variable Oxide Tuning Layer 
(calculated up to a quarter wave optical thickness) 
VCL structure 
Oxide (D) Trans Loss .eta. opt 
scaled 
______________________________________ 
initial no mirror 0.256 0.3 0.461 1.000 
4 periods + 
0 0.017 0.3 0.052 0.113 
4 periods + 
200 0.017 0.3 0.053 0.115 
4 periods + 
400 0.018 0.3 0.056 0.122 
4 periods + 
600 0.020 0.3 0.063 0.136 
4 periods + 
800 0.023 0.3 0.071 0.155 
4 periods + 
1000 0.027 0.3 0.083 0.180 
4 periods + 
1200 0.032 0.3 0.096 0.209 
4 periods + 
1400 0.034 0.3 0.102 0.221 
______________________________________ 
The ratio of the center of the specified distribution to the median of the 
measured slope efficiency distribution is referred to in the "scaled" 
column of Table 1. The corresponding value for the oxide tuning layer 
thickness is then selected from the "oxide" column of Table 1. For 
example, if a slope efficiency distribution of an initial VCSEL wafer is 
centered on a value of 0.44 mW/mA, and the desired center for the 
distribution is 0.06 mW/mA, then the ratio is 0.06/0.44=0.136 and the 
oxide thickness to be deposited is preferably 600 angstroms, according to 
the exemplary table. 
In the preferred embodiment, the additional Bragg stack 80 and tuning layer 
86 are deposited using plasma enhanced chemical vapor deposition. As is 
conventional in the art, such optically transparent films can be routinely 
deposited in increments below 50 angstroms. In addition, an adhesion 
layer, such as titanium, is preferably deposited onto any exposed gold 
surfaces prior to dielectric deposition to enable good mechanical 
stability of the dielectric mirror and tuning layer. T he titanium layer, 
typically on the order of a 100 angstroms thick, may be deposited by any 
suitable method, such as by sputtering or electron beam evaporation. Once 
the dielectric mirror and tuning layer have been deposited, the film is 
preferably patterned and etched to create via holes (e.g., 62, FIGS. 6 and 
7) for electrical contact. The patterning and via etching may be 
accomplished using conventional photolithography techniques to mask the 
films and plasma etching using any suitable reactive gas such as CF.sub.4 
/O.sub.2. With the additional mirror and tuning layer complete, the lasers 
may be retested, if desired, by any suitable method to confirm that the 
process achieved the desired result. 
In the event the slope falls away from target, more material may be added 
if the slope needs to be increased, or material may be etched off if the 
slope needs to be decreased. For example, if the retest of a device were 
to produce a slope efficiency of 0.053 mW/mA instead of the desired 0.06 
mW/mA, an additional deposition may be used to increase the transmission 
further. The ratio of the specified slope efficiency to the measured value 
is calculated as described above, but the "scaled" column is preferably 
normalized to the value corresponding to the current tuning layer 
thickness, a value of 0.136 in the example of a 600 angstrom layer. 
Accordingly, the desired ratio would 0.06/0.053=1.13. This is achieved 
according to the table by adding an additional 200 angstroms, as the ratio 
of the scaled column entries for 800 and 600 angstroms is 
0.155/0.136=1.14, approximately the desired value. The process for tuning 
the slope efficiency is thus completed. 
The lookup table may be determined by calculation, empirical data, or any 
other suitable method. To determine the table empirically, any suitable 
procedure may be used. In practice of a presently preferred method, a 
conventional VCSEL wafer is processed to a testable level, and a 
representative sample of lasers is tested to determine the slope 
efficiency. Subsequently, a third mirror comprising any desired number of 
DBRs (including none) is deposited, followed by a partial deposition of 
the tuning layer. Vias are etched in the dielectric tuning layer to enable 
testing, and the same sample is retested. The procedure is preferably 
repeated until a complete quarter-wave thickness of tuning layer has been 
deposited. The data for the median device provides a table of slope 
efficiency vs tuning layer thickness for the device. Normalizing the slope 
efficiency data by the initial value produces the "scaled" column in Table 
1. 
Alternatively, to determine the table by calculation, the transmission from 
the cavity out of the VCSEL surface can be calculated using conventional 
transmission matrices, such as those generally described in Scott, J. W., 
"Design, Fabrication and Characterization of High-Speed Intra-Cavity 
Contacted Vertical-Cavity Lasers", University of California, Santa 
Barbara, Electrical and Computer Engineering Technical Report #95-06, June 
1995, the contents of which are hereby incorporated by reference, or by 
any other suitable technique known in the art. The calculation is applied 
to various tuning layer thicknesses, producing the data in the "T" column 
of Table 1. 
In the exemplary table set forth above, the power transmission T and round 
trip optical loss L are expressed in percent. The transmission is the 
fraction of power transmitted out of the cavity on a single reflection, 
while the optical loss represents the fractional power loss as a wave 
makes one complete round trip propagation within the cavity. The optical 
loss is a combination of internal losses that arise predominantly from 
free carrier absorption as well as transmission out the lower mirror DBR 
stack. In exemplary Table 1, the optical loss L is presumed constant wafer 
to wafer and generally remains constant for a given wafer. It can also be 
estimated using the transmission matrix formalism, or can be determined 
experimentally by correlating a set of experimental slope efficiency 
measurements with the theoretical prediction. Once the transmission T and 
optical loss L have been determined, the values for the optical efficiency 
.eta..sub.opt are calculated using equation IV. To produce the scaled 
data, the values of .eta..sub.opt are normalized to the initial 
.eta..sub.opt value. 
A second order effect that may be taken into account is that the 
transmission out of the lower mirror varies depending on the accuracy of 
the VCSEL growth relative to the design. These variations can usually be 
ignored, but may be important to consider if the growth thickness accuracy 
is highly variable, which may occur in some VCSEL manufacturing processes. 
In this case, a refinement of the described tuning process preferably 
includes modification of the optical loss values. The optical loss values 
to be used may be correlated with spectral measurements of the initial 
VCSEL or dynamic fits of optical loss value to agree with the change in 
slope observed upon the application of an intermediate dielectric 
deposition and test step. 
FIG. 10 illustrates the VCSEL with variable tuning layer 2 mounted into an 
optical subassembly (OSA) 110. The OSA enables application of DC biasing 
and AC modulation signals to the VCSEL. With the exception of the VCSEL 2, 
all of the parts of the OSA are conventional. The OSA generally comprises 
an electrical package 112 containing the VCSEL 2 and a power monitoring 
photodetector 114. The electrical package is preferably bonded to a 
precision molded plastic housing 116. The bonding process including 
conventional bonding material 117 preferably involves active alignment to 
optimize the coupling of the laser light into an optical fiber 120, as is 
conventional in the art. The OSA includes a ball lens 122 for coupling the 
light into the optical fiber. A ferule 134 provides alignment of the 
optical fiber. After the electrical package 112 and housing 116 are bonded 
together, the fiber is removed and the OSA 125 is complete. An exemplary 
optical subassembly is also described in U.S. patent application Ser. No. 
08/900,507, filed Jul. 25, 1997, now abandoned, the contents of which are 
hereby incorporated by reference. 
By obtaining a more accurate slope for the VCSEL 2, more toleration for 
mechanical variances in the OSA, and in the higher level assemblies is 
permissible. These mechanical variances may include, for example, 
variations in concentricity from fiber to fiber, sub optimal active 
alignment variations, shifts in mechanical position due to environmental 
changes such as temperature, and normal connector tolerances to allow 
insertion of a fiber into the housing. Allowing, increased mechanical 
variation reduces manufacturing complexity and increases yield, thereby 
resulting in lower overall product cost. Alternatively, the mechanical 
tolerances may be maintained at current levels to yield an OSA with more 
consistent performance characteristics. 
FIG. 11 illustrates in block diagram form an optical transceiver 130 
incorporating a VCSEL with variable tuning layer 2 fabricated according to 
the inventive method. With the exception of the VCSEL 2 all of the parts 
of the optical transceiver are conventional. The transceiver includes a 
transmitter portion 131 and a receiver portion 144. The transmitter 
portion provides an interface between a differential input 133 and an 
optical fiber output. In operation, a differential input signal is 
converted to a single ended signal by emitter coupled logic (ECL) line 
receiver 137 and an AC modulation signal is applied to the single ended 
signal in laser driver 138. A DC bias signal is then applied to the signal 
by DC laser bias signal generator 139 for application to the OSA 110. 
Start up circuitry 140 and reset circuitry 141 is preferably provided to 
control the transmission of data over the optical fiber. A laser fault 
indicator 145 provides a status indication of the transmitter portion 131. 
The receiver portion 144 takes an input from an optical fiber provided 
through a photodetector 145 and converts it to a differential output 
signal. The receiver pre amp signal is preferably low pass filtered in 
filter 147 to remove any high frequency noise present, amplified in 
amplifier 148 to regenerate the digital signal, and then transmitted off 
the board through the differential output 146. 
The use of VCSELs with highly consistent slopes in optical transceivers 
enhances the performance and reliability of the data communications 
system. This is because the total optical subassembly slope variation can 
be effectively tuned to fall within specification, so the drive circuit 
will not have to be used to compensate. Such a system will not suffer from 
changes in high speed performance, and will therefore have the desirable 
effect of generally improving overall product consistency and yield. 
Although a preferred embodiment of the present invention has been 
described, it should not be construed to limit the scope of the appended 
claims. Those skilled in the art will understand that various 
modifications may be made to the described embodiment. For example, the 
steps may be performed in different order than listed in the claims, and 
additional steps may be added to further tune the process. The variable 
tuning layer need not be the final layer on the VCSEL, but may include 
other layers over it, including a plurality of variable tuning layers 
separated by conventional or other layers. VCSELs may be constructed to 
operate at various wavelengths, such as but not limited to the 
telecommunications windows of 1200 to 1600 nanometers and 780 to 860 
nanometers as well as the visible wavelengths of 400 to 710 nanometers. 
Furthermore, the VCSEL with variable tuning layer may be used in any 
application of surface emitting lasers requiring substantially consistent 
slopes simply by appropriately adjusting the optical thickness.