U.S. patent application number 12/710156 was filed with the patent office on 2010-09-02 for direct modulated modified vertical cavity surface emitting lasers.
Invention is credited to Mary K. Brenner, Chen Chen, Kent D. Choquette, Klein L. Johnson.
Application Number | 20100220758 12/710156 |
Document ID | / |
Family ID | 42634243 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100220758 |
Kind Code |
A1 |
Brenner; Mary K. ; et
al. |
September 2, 2010 |
DIRECT MODULATED MODIFIED VERTICAL CAVITY SURFACE EMITTING
LASERS
Abstract
A laser system having separately electrically operable cavities
for emitting modulated narrow linewidth light with first, second
and third mirror structures separated by a first active region
between the first and the second and by a second active region
between the second and the third. The second mirror structure has
twenty of more periods of mirror pairs.
Inventors: |
Brenner; Mary K.; (Plymouth,
MN) ; Johnson; Klein L.; (Orono, MN) ;
Choquette; Kent D.; (Urbana, IL) ; Chen; Chen;
(Montreal, CA) |
Correspondence
Address: |
KINNEY & LANGE, P.A.
THE KINNEY & LANGE BUILDING, 312 SOUTH THIRD STREET
MINNEAPOLIS
MN
55415-1002
US
|
Family ID: |
42634243 |
Appl. No.: |
12/710156 |
Filed: |
February 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61208200 |
Feb 20, 2009 |
|
|
|
Current U.S.
Class: |
372/45.01 ;
372/46.01 |
Current CPC
Class: |
H01S 5/06226 20130101;
H01S 5/1021 20130101; H01S 5/18313 20130101; H01S 5/18302 20130101;
H01S 5/1833 20130101 |
Class at
Publication: |
372/45.01 ;
372/46.01 |
International
Class: |
H01S 5/34 20060101
H01S005/34; H01S 5/06 20060101 H01S005/06 |
Claims
1. A laser system having separately electrically operable cavities
for emitting modulated narrow linewidth light, said system
comprising: a compound semiconductor material substrate, at least
two first mirror pairs of semiconductor material layers in a first
mirror structure on said substrate of a first conductivity type, a
first active region on said first mirror structure with plural
quantum well structures, at least twenty second mirror pairs of
semiconductor material layers in a second mirror structure on said
first active region of a second conductivity type, a second active
region on said second mirror structure with plural quantum well
structures, at least two third mirror pairs of semiconductor
material layers in a third mirror structure on said second active
region of said first conductivity type, an intermediate electrical
interconnection at said second mirror structure, and a pair of
electrical interconnections separated by said substrate, said first
mirror structure, said first active region, said second mirror
structure, said second active region, and said third mirror
structure.
2. The system of claim 1 wherein said first and second active
regions are p-n semiconductor junctions further comprising a
biasing arrangement forward biasing each and a modulating voltage
source electrically connected between said intermediate electrical
interconnection and a selected one of said pair of electrical
interconnections.
3. The system of claim 2 wherein said modulating voltage source is
a first further comprising a second modulating voltage source
electrically connected between said intermediate electrical
interconnection and that remaining one of said pair of electrical
interconnections.
4. The system of claim 1 further comprising a selected one of said
intermediate electrical interconnection and said pair of electrical
interconnections also being connected to a bonding pad supported on
an electrical insulator with said electrical insulator also
supported by said substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims the benefit of Provisional
Application No. 61/208,200 filed on Feb. 20, 2009 for "DIRECT
MODULATED MODIFIED VERTICAL CAVITY SURFACE EMITTING LASERS" and
hereby incorporates herein by reference that application.
BACKGROUND
[0002] The present invention relates to a modulated intensity
output solid state laser and, more particularly, to a modulated
intensity output vertical cavity surface emitting solid state
laser.
[0003] Large numbers of closely spaced lateral circuit
interconnections, extending between various portions of individual
integrated circuit chips, between various integrated circuit chips
mounted on a printed circuit board, and between various printed
circuit boards mounted in a system, that can each transmit large
numbers of signal symbols with extreme rapidity are increasingly
needed. These interconnections are needed to move, between selected
locations, the large amounts of data generated by very fast signal
processors that appear on signal busses for transmitting signal
symbols representing such data, data that is to be received and
sent by those processors and by various related data receiving,
using, generating and transmitting devices.
[0004] As chip area and board mounted component density increases,
the numbers of unavoidable, but unwanted, electrical circuit
couplings, or parasitics, will most certainly increase
substantially. Dynamic power dissipation in on-chip and off-chip
circuits for operating circuit interconnections comprises the vast
majority of total power consumed. Dynamic dissipation scales
linearly with switching speed, and so power consumption per line in
electrical interconnections can be expected to soon outstrip that
of their optical interconnection counterparts where the power
dissipation is essentially independent of signal path length over
those interconnections. Hence, there will be transitions in the
future to optical interconnection based system architectures.
[0005] These optical interconnection arrangements will require low
cost, low power, directly modulated, high-reliability, single-chip
laser sources and source arrays operating at data rates in excess
of 17 Gbps, now, but capable of reaching 100 Gbps in the future, to
meet the demands of existing, and emerging future, serial chip and
board data communications requirements. Such required capabilities
for the laser sources lead to difficult requirements to be met by
those sources in terms of power dissipation, reliability, and
interconnection spatial densities.
[0006] Single lasers and one dimensional and two-dimensional laser
arrays are needed for fiber optic links, board-to-board and
chi-to-chip links. Each laser should dissipate less than 2 to 5
mW/laser. Reliability must be greater than 100,000 hours (10 years)
at a minimum. Device-to-device uniformity needs to be high
(variations being less than 5%), and device aging characteristics
must be sufficiently slow to eliminate any need for power
monitoring. Low device lasing thresholds and high modulation
efficiencies will be required to minimize electrical power drains
in the laser driver arrays. In addition, in the case of intra-chip
optical interconnects, thermal dissipations pose a particularly
challenging problem as the components may be expected to operate at
ambient temperatures in excess of 80 C. This not only will have a
significant impact on device intrinsic bandwidth, but on device
reliability as well.
[0007] Vertical cavity surface emitting lasers (VCSELs) have been
found to be suitable laser sources for short transmission distance
optical networks with 10 Gbps VCSELs being the laser devices with
the largest modulation rates commercially available today. VCSELs
thus are the dominant light emission source for short transmission
distance optical interconnection arrangements and local area
networks because of their large modulation rate capabilities, low
power consumption, spatially dense device array integration, and
low cost manufacturing of those devices when made in sufficiently
large numbers.
[0008] VCSEL sources that are directly modulated to correspondingly
vary the emitted light intensity at large modulation rates offer a
substantial decrease in cost over the typical alternative, a CW
laser operated in conjunction with an external adjacent
electro-absorption modulator. An important figure of merit for
modulation rates in lasers is the -3 dB small-signal modulation
bandwidth that is defined as the point at which the modulated
optical output, measured as a function of frequency, is reduced to
half of its low modulation rate value. A variety of methods have
been used to achieve greater modulation rates of light intensities
emitted by VCSELs. These have included use of metal contacts on
polymer layers as well as ion implantation to reduce device
capacitance to achieve small-signal modulation bandwidths of 16 to
20 GHz. Although state-of-art VCSELs in laboratories have been
demonstrated to provide modulation bandwidths of 40 Gbps, current
VCSEL technology makes achieving modulation bandwidths greater than
10 Gbps in practice very difficult because of reduced device
reliability if operated at the large current densities required to
do so. Therefore, VCSELs that can be reliably operated with greater
modulation bandwidths are desired.
SUMMARY
[0009] The present invention provides a laser system having
separately electrically operable cavities for emitting modulated
narrow linewidth light with the system having a compound
semiconductor material substrate with at least two first mirror
pairs of semiconductor material layers in a first mirror structure
on the substrate of a first conductivity type and a first active
region on that first mirror structure with plural quantum well
structures. There is at least twenty second mirror pairs of
semiconductor material layers in a second mirror structure on the
first active region of a second conductivity type and a second
active region on the second mirror structure with plural quantum
well structures with at least two third mirror pairs of
semiconductor material layers in a third mirror structure on the
second active region of said first conductivity type. An
intermediate electrical interconnection is provided at the second
mirror structure and a pair of electrical interconnections are
provided separated by the substrate, the first mirror structure,
the first active region, the second mirror structure, the second
active region, and the third mirror structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows (a) a diagrammatic cross section view of a
structure arrangement for a vertical cavity surface emitting laser,
and (b) a diagrammatic cross section view of a structure
arrangement for a composite resonator vertical cavity laser,
[0011] FIG. 2a shows a diagrammatic cross section view of a
composite resonator vertical cavity laser in a circuit arrangement
providing for combined current injection/electro-absorption
operation, and FIG. 2b shows a diagrammatic cross section view of a
composite resonator vertical cavity laser in a circuit arrangement
providing for push-pull operation,
[0012] FIG. 3a shows a graph displaying plots of responses of a
composite resonator vertical cavity laser operated using current,
electro-absorption and combined modulations with the
electro-absorption and current modulation in-phase, and FIG. 3b
shows a graph displaying plots of responses of a composite
resonator vertical cavity laser operated using current,
electro-absorption and combined modulations with the
electro-absorption and current modulation out of phase,
[0013] FIG. 4 shows a graph displaying plots of responses of a
composite resonator vertical cavity laser operated using combined
modulation with different optical standing wave overlap ratios,
.xi.1/.xi.2,
[0014] FIG. 5 shows graphs displaying plots of refractive index and
normalized optical field intensity profiles for the short optical
length cavity of a composite resonator vertical cavity laser along
the longitudinal direction thereof if (a) the top cavity is shorter
than the bottom cavity, and if (b) the top cavity is longer than
the bottom cavity with each graph having an inset thereby showing
the optical field intensity near the laser facet and the percentage
of light at that facet,
[0015] FIG. 6 shows graphs displaying plots of responses of a
composite resonator vertical cavity laser operated using push-pull
modulation for (a) different values of small signal cavities
currents amplitude difference A with currents phase difference
.theta.=.pi., and for (b) different values of small signal cavities
currents phase difference .theta. with current amplitude difference
A=1,
[0016] FIG. 7 shows a graph displaying plots of responses of a
composite resonator vertical cavity laser operated using push-pull
modulation with and without various device electrical equivalent
parasitics, and plots of responses of effectively a vertical cavity
surface emitting laser using standard current modulation with and
without various device electrical equivalent parasitics,
[0017] FIG. 8a shows a graph displaying plots of responses of a
vertical cavity surface emitting laser bonding pad interconnection
cutoff frequency f.sub.c resulting from interconnection parasitic
capacitance as a function of the laser series resistance at the
interconnection for an interconnection effected with 50 um bonding
pads supported on implanted GaAs or on various dielectric
thicknesses over implanted GaAs, and FIG. 8b shows a diagrammatic
cross section view of a structure arrangement for such
interconnection alternatives,
[0018] FIG. 9 shows a diagrammatic cross section view of a
structure arrangement for a composite resonator vertical cavity
laser with associated field amplitude distributions designated
.lamda..sub.S and .lamda..sub.L as the two longitudinal resonant
modes of the combined cavities,
[0019] FIG. 10 shows a table setting out an epitaxial layer
structure for a composite resonator vertical cavity laser for
emission at an example wavelength,
[0020] FIG. 11 shows a table setting out typical material
parameters for the device of FIG. 10,
[0021] FIG. 12 shows plots of long and short wavelength field
amplitude distributions versus relative cavity quantum well
refractive indices,
[0022] FIG. 13 shows a graph displaying plots of output coupling
efficiencies of long and short modes versus relative cavity quantum
well refractive indices,
[0023] FIG. 14 shows a graph displaying a plot of the extinction
ratio ER of a composite resonator vertical cavity laser between
on-state and off-state emitted light versus the number of periods
of alternating optical index material layers in the bottom,
intermediate and top mirrors therein,
[0024] FIG. 15 shows a table setting out typical device alternative
parameter selections for a composite resonator vertical cavity
laser,
[0025] FIG. 16 shows a table setting out further typical device
parameters for a composite resonator vertical cavity laser,
[0026] FIG. 17 shows a graph displaying plots of various time
domain responses of a composite resonator vertical cavity laser
with 44 intermediate mirror periods of alternating optical index
material layers operated with I.sub.bias=3.5 mA and I.sub.mod=1.5
mA and modulated with a 20 Gbps square wave,
[0027] FIG. 18 shows a graph displaying plots of various time
domain responses of a vertical cavity surface emitting laser
operated with I.sub.bias=3.5 mA and I.sub.mod=1.5 mA and modulated
with a 20 Gbps square wave,
[0028] FIG. 19 shows a graph displaying plots of various small
signal responses of a composite resonator vertical cavity laser
with 50 intermediate mirror periods of alternating optical index
material layers in solid lines, and plots of various small signal
responses of a vertical cavity surface emitting laser in dashed
lines showing a gentler 10 dB/decade roll-off for the composite
resonator vertical cavity laser which also has a 3 dB bandwidth
double that of the vertical cavity surface emitting laser for the
same operating electrical current,
[0029] FIG. 20 shows a graph displaying plots of various small
signal responses for different modulations depths Imod/Ibias of a
composite resonator vertical cavity laser with 50 intermediate
mirror periods of alternating optical index material layers, and a
plot of the small signal response of a vertical cavity surface
emitting laser that is independent of modulation depth,
[0030] FIG. 21 shows a graph displaying plots of 3 dB modulation
bandwidth as a function of modulation depth of a composite
resonator vertical cavity laser with 50 intermediate mirror periods
of alternating optical index material layers and a vertical cavity
surface emitting laser,
[0031] FIG. 22 shows a diagrammatic cross section view of a
composite resonator vertical cavity laser in another circuit
arrangement providing for push-pull modulation and suitable
operating bias current,
[0032] FIG. 23 shows a diagrammatic top view of a composite
resonator vertical cavity laser structure showing major structural
features and a "ground-signal-ground" contact layout, and
[0033] FIG. 24 shows an overlay of photolithographic mask layers
used in fabrication of a composite resonator vertical cavity laser
structure.
DETAILED DESCRIPTION
[0034] A multiple resonant cavity vertical cavity surface emitting
laser, 1, permitting very large rates of modulation in connection
with the coupling between those cavities, is shown on the right in
(b) of FIG. 1 in contrast to a conventional vertical cavity surface
emitting laser (VCSEL), 2, shown on the left in (a) of FIG. 1.
Multiple resonant cavity vertical cavity surface emitting laser 1,
or composite resonator vertical cavity laser (CRVCL) 1, in (b) of
FIG. 1 has two outer distributed Bragg reflector (DBR) mirrors, 3
and 4, (an upper outer mirror 3 and a lower outer mirror 4) and two
multi-quantum well (MQW) active regions, 5' and 5'', separated by
an intermediate DBR mirror, 6, between the two outer mirrors. MQW
active regions 5' and 5'' constitute the p-n semiconductor
junctions between upper mirror 3 and intermediate mirror 6, and
bottom mirror 4 and intermediate mirror 6, respectively. The
devices in FIG. 1 are formed as monolithic stacks of epitaxially
grown layers that are subjected to subsequent fabrication process
steps in completing those structures. The entire CRVCL structure
can be grown in a single growth layer sequence and fabricated in
semiconductor wafers in much the same way as conventional VCSEL 2
is fabricated. In FIG. 1, CRVCL 1 in (b) on the right is a
three-terminal device with a terminal on each of the outer ends
including an upper terminal, 7, and a lower terminal, 8, and an
intermediate terminal, 9, on a mesa at the location of intermediate
mirror 6 to thereby allow each active region between the mirrors to
be independently biased or modulated, or both.
[0035] CRVCL 1 in this structure has the capability to change the
photon density therein by varying the gain or absorption in an
upper cavity, 10, comprising mirrors 3 and 6 along with active
region 5' therebetween, or in a lower cavity, 11, comprising
mirrors 4 and 6 along with active region 5'' therebetween, while
maintaining constant the current injection into the remaining other
cavity (11 or 10, respectively), the further capability to detune
an optical cavity by current injection, and the yet further
capability to independently control carrier densities in both
cavities to thereby aid in achieving very large intensity
modulation rates. That is, the coupled cavities 10 and 11 in CRVCL
1, under appropriate biasing conditions, lead to an increase in the
small-signal bandwidth.
[0036] FIGS. 2(a) and 2(b) shows a cross section of a slightly
different CRVCL structure again having upper mirror 3, lower mirror
4, and intermediate mirror 6 as DBR mirror stacks, and two resonant
cavities 10 and 11 containing the active regions 5' and 5'',
respectively, which can be independently provided with injection
current, i.e. "pumped", or current biased or both. Two different
modulation processes for a CRVCL structure, or device, are
described below in which voltage waveforms of a voltage source as
an input signal source are impressed on the intensity waveforms of
the electromagnetic radiation emitted by the structure during
operation thereof. They are designated 1) simultaneous current and
electro-absorption modulation, or "combined modulation" illustrated
in FIG. 2(a), and 2) push-pull modulation, illustrated in FIG.
2(b). In each modulation process description, a small-signal model
based on rate-equation analysis is presented. In addition, a
large-signal model is presented to show the large-signal response
for the push-pull modulation process.
[0037] FIGS. 2(a) and 2(b) also show an equivalent circuit model
for how the CRVCL structure would effectively behave as an
electrical circuit component, and generally how such an effective
circuit component would be provided in an electrical circuit to
arrange for the desired kind of modulation to be applied to that
structure as indicated above for each figure. These figures assume
that the CRVCL structure would be grown with lower mirror 4 doped
n-type, intermediate mirror 6 doped p-type, and upper mirror 3
doped n-type, thereby forming an n-p-n structure. However, the
structure could also be grown p-n-p structure, with the upper and
lower mirrors doped p-type, and the intermediate mirror doped
n-type. In this latter situation, the operation algebraic sign of
the applied voltages would be reversed, and the direction of
current flows in the structure would also be reversed.
[0038] The modulating current in CRVCLs of FIG. 2 can be introduced
into one of cavities 10 or 11 while the current injection into the
remaining other cavity (11 or 10, respectively) is maintained
constant, and this results in the modulation of the total carrier
density, gain, and eventually the intensity of the electromagnetic
radiation output of the laser. However, the magnitude of the
relaxation oscillation frequency (ROF) due to interactions of
carriers and photons upon changes in the modulating current limits
the large signal modulation although steps can be taken to reduce
this magnitude. Alternatively, means to decouple this interaction
would lessen the relaxation oscillation effect to thereby provide
increased modulation bandwidth.
[0039] In the first variant of the present invention, one cavity is
forward biased, so that current is injected (in FIG. 2(a) active
region 5'' p-n junction in lower cavity 11 is shown forward biased
by constant value current source I.sub.bias). The modulated current
density in the forward-biased active region 5'' p-n junction can be
modulated either in-phase or out-of-phase with the
electro-absorption (EA) voltage modulation across the reverse
biased remaining junction (shown in FIG. 2(a) as active region 5'
p-n junction in upper cavity 11 being reverse bias by constant
value voltage source V1). Thus, in the device of FIG. 2(a) we show
the upper cavity active region 5' p-n junction as reverse biased,
and the lower cavity active region 5'' p-n junction as forward
biased, but that device would function similarly if the lower
cavity active region 5'' p-n junction were reverse biased and upper
cavity active region 5' p-n junction were forward biased. Variable
voltage modulation signal sources V1mod, in series with voltage
source V1, and V2mod, in series with current source I.sub.bias, may
or may not have equal voltage values. The modulating voltage V2mod
on the forward biased junction results in a modulation of the
current flow I.sub.mod through the junction such that the total
current I.sub.total=I.sub.bias+I.sub.mod.
[0040] The push-pull operating mode is illustrated in FIG. 2(b). In
this arrangement, where the resonator is balanced, large rate
resonance-free modulation is possible for a single longitudinal
mode due to carrier-induced index modulation of the output coupling
efficiency. In this case both active region 5' and 5'' p-n
junctions are forward biased. A current source in each loop of the
circuit maintains a constant total current in that loop,
I.sub.bias/2, and together, a constant total current into the
device, I.sub.bias. A modulation voltage is added to one or the
other junctions (in FIG. 2(b) shown to be in series with upper
active region 5' p-n junction) which results in a current
modulation of I.sub.mod. The result is a current flow of
I.sub.bias/2+I.sub.mod in the upper active region 5' p-n junction,
and a current modulation of I.sub.bias/2-I.sub.mod in the lower
active region 5'' p-n junction. In either variant illustrated by
FIGS. 2(a) and 2(b), the relaxation oscillation behavior can be
significantly modified to result in greater bandwidth.
[0041] In analyzing the small-signal response of a CRVCL under the
first modulation process, combined current and electro-absorption
modulation, a modified rate-equation model is used with two carrier
populations and a single longitudinal mode to describe the
modulation response. The assumption of a single longitudinal mode
simplifies the rate equations, and is also appropriate for much of
the operating range of the CRVCLs considered. Unlike the
conventional modulation of a laser in which a small-signal is
introduced through current modulation, the CRVCL undergoes the
modulation of absorption loss through application of a reverse bias
voltage to one of its cavities.
[0042] The rate equations for carrier and photon densities for
current/electro-absorption modulation are derived by assuming only
one optical mode is lasing, or
N 1 t = J qd - N 1 .tau. 1 - vg .xi. 1 S ( 1 ) N 2 t = - N 2 .tau.
2 + .eta. d .xi. 2 S ( 2 ) S t = ( .GAMMA. vg .xi. 1 - .xi. 2 .tau.
p ) S + .beta. R sp ( 3 ) ##EQU00001##
[0043] where N.sub.1 and N.sub.2 are the carrier densities in the
two active regions (5' and 5'') (l/cm.sup.3), .tau..sub.1 and
.tau..sub.2 are the carrier lifetimes in the current modulation
cavity (10 or 11) and electro-absorption modulation cavity (11 or
10), respectively, J is the injection current density (A/cm.sup.2),
q is the elementary charge (C), d is the gain region thickness
(cm), v is the group velocity of the optical mode in the material
(cm/s), g is the material gain (cm.sup.-1), .GAMMA. is the optical
confinement factor in the forward-biased cavity, .tau..sub.1p is
the photon lifetime, S is the photon density (l/cm.sup.3), .beta.
is the spontaneous emission factor, and R.sub.Sp is the spontaneous
emission rate per unit volume (l/cm.sup.3s). The quantities
.xi..sub.1 and .xi..sub.2 represent the fraction of the optical
standing wave overlapping with the current modulation cavity 10 or
11 and electro-absorption cavity 11 or 10, respectively. The
electro-absorption modulation cavity 11 or 10 under reverse-bias
behaves as a photodetector which converts the light emission from
the current modulation cavity 10 or 11 into a photocurrent in the
electro-absorption modulation cavity 11 or 10. This process is
accounted by assuming the photodetector efficiency .eta..sub.d in
equation (2).
[0044] As evident in equation (4), the total response of the device
under both current and electro-absorption modulation can be
considered as a superposition of the response under conventional
current modulation and electro-absorption modulation separately. If
the electro-absorption modulation is removed, i.e. m=0, the total
response will become that of a conventional laser under direct
current modulation,
s ( .omega. ) S 0 = .GAMMA..tau. p .omega. r 2 j ( .omega. ) qdS 0
D ( .omega. ) ( 6 ) ##EQU00002##
On the other hand, if the current modulation is removed by setting
j(.omega.)=0, then the total response will be in the same form as
the laser response under the electro-absorption modulation,
s ( .omega. ) S 0 = .xi. 2 m .tau. p ( - .omega. + 1 .tau. + vg ' S
0 ) ) / D ( .omega. ) . ( 7 ) ##EQU00003##
[0045] Comparing equations (6) and (7), the direct current
modulation produces a relatively flat modulation response yet with
somewhat limited 3-dB bandwidth, while the electro-absorption
modulation produces the opposite. As illustrated in FIG. 3(a),
combining both the modulation components in-phase enables a new
design freedom, so that a tradeoff between a flat response (a small
relaxation oscillation peak) and high modulation bandwidth can be
made for an improved bandwidth. Moreover, another option is to
combine both the modulation components out-of-phase such that the
overall modulation bandwidth can be enhanced as shown in FIG. 3(b).
This works better when the current and electro-absorption
modulation are relatively flat by themselves. The
electro-absorption modulation index m can be employed to tailor the
combined modulation response. By combining current and
electro-absorption modulation out-of-phase, a relatively flat
response with a 3 dB modulation bandwidth of .about.90 GHz can be
achieved (neglecting electrical parasitics).
[0046] Such a CRVCL allows detuning the cavity 10 and 11 lengths to
permit each cavity to have a different optical standing-wave
overlaps .xi..sub.1 and .xi..sub.2. FIG. 4 illustrates the combined
modulation response at a fixed photon density S.sub.0, while the
relative overlap of the longitudinal mode with each cavity is
varied. As evident from the figure, the cavity detuning is another
design freedom to produce an improved modulation response,
indicating that a 100 GHz 3-dB bandwidth and only .about.1 dB
relaxation oscillation peak can be achieved with an 80/20
distribution of the longitudinal mode overlap between the two
cavities. The current injection and voltage modulation create some
of this detuning, but it can also be built into the structure by
growing the two cavities with different cavity lengths, i.e.
adjusting the thickness of active region 5' relative to active
region 5''.
[0047] The second modulation process of a CRVCL, push-pull
modulation, provides another means to decouple the photon density
and current density and minimize the relaxation oscillation effect.
For push-pull modulation, the forward-bias injection current
through both the top 10 and bottom 11 cavities of a CRVCL will be
modulated simultaneously but maintained out-of-phase. As the
carrier density increases in one cavity, the carrier density in the
other cavity decreases by an equal amount, maintaining a constant
total carrier population. Hence, the net photon population
essentially decouples from changes in carrier population, which
results in the elimination of the relaxation oscillation peak.
[0048] Without losing generality, we will analyze a CRVCL in which
both the upper 10 (output) and lower 11 cavities are current
modulated. This is an extension of the analysis for the current
modulation in one cavity. Assuming that only one optical mode is
lasing, the rate equations can be written as:
N 1 t = J 1 qd - N 1 .tau. 1 - vg 1 .xi. 1 S ( 8 ) N 2 t = J 2 qd -
N 2 .tau. 2 - vg 2 .xi. 2 S ( 9 ) S t = ( .GAMMA. 1 vg 1 .xi. 1 +
.GAMMA. 2 vg 2 .xi. 2 - 1 .tau. p ) S + .beta. R sp ( 10 )
##EQU00004##
[0049] where for cavity m, N.sub.m is the carrier density
(l/cm.sup.3), J.sub.m is the injection current density
(A/cm.sup.2), .tau..sub.m is the carrier lifetime (s), g.sub.m is
the material gain (cm.sup.-1), .GAMMA..sub.m is the optical
confinement factor, and the other parameters are defined the same
as in equations (1) through (3).
[0050] The push-pull modulation response can be obtained by solving
the rate-equations (8) through (10), yielding
s ( .omega. ) S 0 = .GAMMA. 1 vg 1 ' ( j 1 ( .omega. ) qd - vg 10 S
0 .xi. 1 ( .omega. ) ) + .GAMMA. 2 vg 2 ' ( j 2 ( .omega. ) qd - vg
20 S 0 .xi. 2 ( .omega. ) ) .omega. - ( 1 .tau. 1 + vg 1 ' .xi. 10
S 0 ) .omega. - ( 1 .tau. 2 + vg 2 ' .xi. 20 S 0 ) .omega. 2 +
.omega. ( 1 .tau. 1 + vg 1 ' .xi. 10 S 0 ) - v 2 .GAMMA. 1 .xi. 10
g 10 g 1 ' S 0 - v 2 .GAMMA. 2 .xi. 20 g 20 g 2 ' S 0 .omega. - ( 1
.tau. 1 + vg 1 ' .xi. 10 S 0 ) .omega. - ( 1 .tau. 2 + vg 2 ' .xi.
20 S 0 ) ( 11 ) ##EQU00005##
where for cavity m, g.sub.m0 is the steady-state material gain
(l/cm), g.sub.m' is the differential gain (l/cm.sup.2), and
.xi..sub.m0 is the percentage of standing-wave overlap under
steady-state. However, by assuming identical cavity conditions,
equation (11) can be simplified to:
H ( .omega. ) = s ( .omega. ) j 1 ( .omega. ) = .omega. r 2
.GAMMA..tau. p qd ( 1 + A j .theta. ) [ ( .omega. 2 - .omega. r 2 )
2 + .omega. 2 .gamma. 2 ] 1 2 ( 12 ) ##EQU00006##
where A and .theta. account for the phase and amplitude difference
between the small-signal current components such that
j.sub.2(.omega.)=Aj.sub.1(.omega.)e.sup.4.theta. (13)
[0051] The response given by the equation (12) is similar to the
modulation response of a conventional laser, with additional
dependence on A and .theta..
[0052] In push-pull modulation, the external observability of the
modulation response of the two cavities must be determined, i.e.
whether external light modulation occurs which can be used to carry
information. Therefore push-pull modulation requires additional
analysis to understand the effects on the longitudinal modes.
[0053] Longitudinal mode modulation arises from the dynamic cavity
detuning under the (differential) current injection through both
cavities 10 and 11, leading to a change in the relative coupling
efficiencies of the top 3 and bottom 4 mirrors. Under the condition
of A=1 and .theta.=.pi., in push-pull modulation, the carrier
density always increases in one cavity 10 or 11 while it decreases
in the remaining other cavity 11 or 10, respectively. Owing to the
carrier-induced index change, the effective length for one cavity
will decrease, while for the other cavity the length will increase.
As a result, the optical mode will be either "pushed" towards the
CRVCL substrate 12 producing less output light, or "pulled" towards
the output facet 13 producing more output light as indicated in
FIG. 5. All this occurs while the photon lifetime and hence the
total photon population remains unchanged, i.e. the total number of
photons exiting the cavity through the top 3 and bottom 4 mirrors
remains constant. Since the longitudinal mode modulation is not
natively included in the rate equation analysis, it has to be
accounted for separately.
[0054] The longitudinal mode modulation is assumed to be 10% of the
available light at the output facet. This assumption will be
justified later. The equation (12) is modified to account for the
longitudinal mode modulation by adding an additional term:
s out ( .omega. ) = j 1 ( .omega. ) .omega. r 2 .GAMMA..tau. p qd (
1 + A .theta. ) [ ( .omega. 2 - .omega. r 2 ) 2 + .omega. 2 .gamma.
2 ] 1 2 0.15 % + 10 % 0.15 % S 0 ( 14 ) ##EQU00007##
where s.sub.out(.omega.) is the small-signal photon density at the
laser facet. The factor 0.15% represents the percentage of the
total internal intensity present at the output laser facet under
steady-state conditions.
[0055] FIG. 6 is plotted to illustrate the frequency dependence of
the push-pull modulation response and the dependence thereof on A
and .theta.. As indicated in the figure, an extraordinarily wide
and flat modulation bandwidth (>80 GHz) is possible under the
push-pull modulation scheme, and the behavior is markedly different
than the equivalent structure under "normal" direct current
modulation. The most pronounced feature is that the relaxation
oscillation vanishes for A=1 and .theta.=.pi. (black curves), which
corresponds to the small-signal current for both cavities having
the same amplitude and being exactly out of phase. The modulation
bandwidth is therefore limited only by the photon lifetime, if
electrical parasitic effects are neglected. As can also be seen in
the figure, the modulation response is reasonably tolerant to
deviations from the ideal push-pull modulation condition A=1 and
.theta.=.pi., which makes the operation of the device tolerant to
manufacturing variations.
[0056] The calculations so far assume the identical top 10 and
bottom 11 optical cavities. However, it can be expected that this
may be difficult to achieve in practice. For example, different
current densities J.sub.10 and J.sub.20 would likely result in
different values of .xi..sub.1/.xi..sub.2, g.sub.1'/g.sub.2' and
g.sub.10/g.sub.20. Therefore, the full form of the modulation
response given by equation (11) was invoked with the inclusion of
the longitudinal mode modulation terms. It was found that the
modulation response digresses from the ideal condition only
slightly for a relatively large change in .xi..sub.1/.xi..sub.2,
suggesting that a substantial fabrication tolerance exists.
[0057] Currently, intrinsic laser bandwidth and reliability at high
current densities have been the modulation rate limiting factors in
commercially available VCSEL-based transceivers, which generally
operate at 10 Gb/s and below. Accordingly, at such frequencies,
there is little need to be concerned about minimization of
electrical parasitics affecting devices used therein. Of course,
such parasitics are a fact of life and cannot simply be ignored,
certainly not at larger frequencies. FIG. 7 illustrates the modeled
push-pull modulation response of a CRVCL when both the upper 10 and
lower 11 cavities are limited to 20 GHz electrical bandwidth,
typically encountered in commercial devices, due to such parasitics
as diode forward resistance, diode junction capacitance, and
interconnection capacitances. The electrical parasitics introduce a
dominant pole to the push-pull modulation response, and thus
dominate the overall modulation bandwidth.
[0058] Therefore, the improvements in intrinsic laser bandwidth
afforded by push-pull modulation of the coupled cavities in a CRVCL
device can only be taken advantage of in conjunction with a
reduction in device parasitics. By far, the dominant parasitic
element in VCSELs is the device contact bonding pad capacitance
C.sub.bp, which together with the differential series resistance
R.sub.d of the diode form a low pass filter with a cutoff frequency
f.sub.c=1/2.pi.R.sub.dC.sub.bp. The contact bonding pad is the
metal area connected to terminals 7 and 9 which allow the device to
be connected to the outside world, such as a driver I.C, by a bond
wire or solder bump between the bonding pad and the IC. A
capacitance is associated with the interface between this bonding
pad metal and the semiconductor underneath. There is definitely
something to be gained from minimizing the series resistance, but
given the current advanced state-of-the-art in DBR design, it is
probably not realistic to expect results significantly better than
what is commonly achieved today. This is to say that R.sub.d will
almost certainly be in the range of 50 to 100 ohms for the 5 to 10
micron oxide aperture devices envisioned.
[0059] FIG. 8(a) shows a plot of f.sub.c vs R.sub.d for the
structure illustrated in FIG. 8(b) The structure in FIG. 8(b) shows
a bonding pad 14 which has a linear dimension of 50 .mu.m, on top
of a 1 to 3 .mu.m thick Cyclotene.TM. dielectric layer 15, which in
turn is on top of GaAs 16 which has an implanted region 17 that
extends to a depth of 3 .mu.m from the top surface of the GaAs. 50
.mu.m is used as it is the smallest diameter bonding pad that can
practically be used for a high manufacturing yield. As can be seen
in FIG. 8(a), the incremental gain in going from 80 ohms to 60 ohms
is relatively mirror when compared to the advantage of putting the
pad metal on thick dielectric. This is as opposed to putting it
directly on the implanted GaAs, as is commonly done. The data shows
that even for the limiting case of a 100 ohm device, f.sub.c>70
GHz can be obtained by putting the bonding pads on as little as 2
.mu.m of Cyclotene(BCB).
[0060] The foregoing analyses have illustrated the small-signal
modulation response of the CRVCL. However, use in communication
systems requires modulation using large amplitude signals.
Therefore, we will describe specific structures, and the
large-signal response of these structures. FIG. 9 shows a schematic
cross-section of the resonant structure of the coupled cavities in
a suitable CRVCL, and FIG. 10 shows the detailed structure for a
device as implemented to emit at a wavelength of 850 nm. However,
the CRVCL structure can be implemented to emit at any wavelength
that a more conventional structure emits at, ranging from UV (350
nm) to the IR (2500 nm). In the following description we assume the
notation Al.sub.xGa.sub.1-xAs where x defines the relative content
of aluminum to gallium in any given layer. For instance, x=1 refers
to AlAs and x=0 refers to GaAs. For simplicity, we will drop the
subscripts in the following description and refer to the value of x
to describe the composition. We will also refer to some thicknesses
in terms of optical thicknesses .lamda. of the emission wavelength.
For instance, for a device emitting at 850 nm, a 1.lamda. optical
thickness cavity in AlGaAs materials will be approximately 235 nm
in physical thickness, since the refractive index n is
approximately 3.6.
[0061] As shown in FIGS. 9 and 10, the resonator consists of a
lower DBR stack 4 with Nb periods of alternating low index 18 and
high index 19 quarter-wavelength thick layers with composition
graded layers 20 between the low and high index layers, an
intermediate central DBR 6 with Nc periods, and an upper top DBR 3
with Nt periods. The high index 19 and low index 18 materials in
the DBRs consist of x=0.15 and x=0.95 to 1.0 AlGaAs respectively.
These compositions are chosen to provide the maximum refractive
index contrast, while avoiding compositions that would absorb the
emitted light. Clearly, other compositions could be chosen as well.
The nominal x=0.15 composition could range from x=0.05 to x=0.30,
while the nominal x=0.95 AlGaAs could range from x=0.80 to 0.97 for
an oxide current aperture, or to x=1.0 for current confinement
provided by proton implantation or the etching of a mesa. For
emission at other wavelengths, similar considerations would guide
the choice of composition for the mirrors. Linearly graded
interfaces 20 are used to reduce electrical series resistance. The
grade could range to thicknesses as large as 200 nm.
[0062] At the interfaces between the mirrors are two active regions
5' and 5'' with each cavity total optical thickness equal to
1.lamda. when cold, each containing two symmetric 65% AlGaAs spacer
layers 21 and five 7 nm thick GaAs quantum wells 22 with eight
barriers with x=0.25 separating and surrounding the quantum wells
23. The number of quantum wells could be as few as one, and as many
as 7. Quantum well thickness could also vary from 5 nm to 10 nm,
and the barrier layer thickness could also vary. Space layer
compositions could vary from 30% to 70%. The substrate 12 is formed
of GaAs. FIG. 11 lists the material parameters.
[0063] Some additional layers are included in the structure to
provide control over the current flow through the device. Included
in the intermediate mirror 6 are two regions designated as
oxidation structures 24. The x=0.98 layer 32 is partially oxidixed
later in the process to confine the current to the center of the
device. Other layers around the x=0.98 layer are lower Al content
x=0.97 (33) and x=0.65 (34) and are designed to help control the
thickness of the oxidation layer. Contact layers 25 at the top
surface include a GaAs and AlGaAs layer with x=0.15 that are
heavily doped to provide a low resistance contact to metals
deposited on the top surface. The stop etch layer 26 and contact
layer 27 are used to provide the metal contact for terminal 9 that
contacts the intermediate mirror. The stop etch layer 26 is
designed to be very slow etching so that a mesa can be etched down
to this layer and stopped accurately. The contact layer 27 below it
is more heavily doped and lower aluminum content so that we can
make a low resistance contact to the structure.
[0064] Being a coupled-cavity resonator, the structure of FIG. 9
supports two longitudinal resonant modes, denoted .lamda..sub.L and
.lamda..sub.S, where .lamda..sub.L=.lamda..sub.0+.DELTA..lamda. and
.lamda..sub.S=.lamda..sub.0-.DELTA..lamda.. The splitting of the
resonant wavelengths is strictly determined by the number of
periods N.sub.c in the intermediate mirror 6. As Nc is increased,
the two cavities 10 and 11 become increasingly decoupled and the
two modes tend towards degeneracy.
[0065] As shown in FIG. 12, each mode is further characterized by
its electric field distribution within the composite cavity.
Characteristic of this type of structure, asymmetry of the field
profiles is only manifest in the presence of a difference in the
optical path lengths of the MQW active regions 5' and 5'',
regardless of the degree of coupling between the two cavities. For
identical cavities, the field profiles are symmetric and degenerate
for the two modes, i.e. the spatial distributions are equally split
between the two cavities. As the ratio of the relative optical
thicknesses of the two active regions 5' and 5'' departs from
unity, the peak modal e-field intensities migrate to one or the
other cavity.
[0066] Under cold (unpumped) conditions, the two optical cavities
10 and 11 are identical by design (.DELTA.n=0 in the figure). Any
change in the relative refractive indices of the quantum wells 22
(.DELTA.n=n.sub.qw1-n.sub.qw2) will cause the relative optical
thicknesses to change. In addition, the long and short mode field
profiles behave in opposite respect. For example, as the optical
path length of the active layer 5'' is decreased with respect to
that of the upper active layer 5', the peak field intensity of the
short wavelength mode shifts to the lower cavity 11 while that of
the long wavelength mode shifts to the upper cavity 10. As the
modal field distributions change, so too does the relative
efficiency with which the individual modes couple out of the
resonator through either the top 3 or bottom 4 mirror as indicated
in FIG. 13. This figure shows the ratio of the coupling out of the
top surface of the resonator to the bottom surface of the resonator
for both the long and short wavelength resonator modes,
.lamda..sub.L and .lamda..sub.S, respectively. This relative change
in coupling efficiency is what is exploited to impart amplitude
modulation.
[0067] The physical mechanism that changes the optical path lengths
in the two active regions 5' and 5'' is the change in refractive
index of the GaAs quantum wells 22 with injected carrier density.
This is the same mechanism that gives rise to transient and
adiabatic chirp in all semiconductor lasers, a dynamic shift in
laser frequency under modulation which ceases as the carrier
density in the laser cavity reaches equilibrium. The magnitude of
the effect is relatively small, with dn/dN being on the order of
1.2.times.10.sup.-20 cm.sup.-3. Given that the quantum wells 22
comprise approximately 15% of the 1.lamda. active region 5' or 5'',
this implies that a .about.0.05% shift in the optical thickness of
the spacers is achievable for a .DELTA.N of .about.1e18 cm.sup.-3
(a reasonable number). While small, this degree of cavity shift is
indeed adequate to achieve a significant (>3 dB) modulation in
the output power of the device as is seen in the following.
[0068] The dynamic state of this structure is modeled using rate
equations for the two carrier populations (upper 10 and lower 11
cavity) and two photon populations (long and short mode). The rate
equations are set out in somewhat unconventional form in being in
terms of the carrier densities N.sub.1 and N.sub.2 in the two
active regions 5' and 5'', and the total photon numbers S.sub.S and
S.sub.L in the two optical modes. This is because, in this
instance, the concept of expressing the mode equations in terms of
photon densities using an equivalent mode volume is not easily
accomplished due to the difficulty in defining mode volume in
connection with photons. The four coupled differential equations
are:
N 1 t = .eta. i I 1 qV 1 - N 1 .tau. e - BN 1 2 - v g g 0 .chi. 1
ln ( N 1 + N 0 N tr + N 0 ) ( .GAMMA. 1 S S S + .GAMMA. 1 L S L ) V
1 ( 16 ) N 2 t = .eta. i I 2 qV 2 - N 2 .tau. e - BN 2 2 - v g g 0
.chi. 2 ln ( N 2 + N 0 N tr + N 0 ) ( .GAMMA. 2 S S S + .GAMMA. 2 L
S L ) V 2 ( 17 ) S S t = v g g 0 [ ln ( N 1 + N 0 N tr + N 0 )
.GAMMA. 1 S .chi. 1 + ln ( N 2 + N 0 N tr + N 0 ) .GAMMA. 2 S .chi.
2 ] S S - S S .tau. p S + .beta. 2 .tau. e ( V 1 N 1 + V 2 N 2 ) (
18 ) S L t = v g g 0 [ ln ( N 1 + N 0 N tr + N 0 ) .GAMMA. 1 L
.chi. 1 + ln ( N 2 + N 0 N tr + N 0 ) .GAMMA. 2 L .chi. 2 ] S L - S
L .tau. p L + .beta. 2 .tau. e ( V 1 N 1 + V 2 N 2 ) ( 19 )
##EQU00008##
where,
[0069] N.sub.1, N.sub.2: carrier densities in first 5' and second
5'' active regions in l/cm.sup.3
[0070] S.sub.S, S.sub.L: photon number in short and long wavelength
modes
[0071] I.sub.1, I.sub.2: current injection in first 5' and second
5'' active regions in amps
[0072] V.sub.1, V.sub.2: volume of first 5' and second 5'' active
region in cm.sup.3
[0073] B: bimolecular recombination coefficient in cm.sup.3/s
[0074] .beta.: spontaneous emission factor
[0075] g.sub.0: gain coefficient in l/cm
[0076] N.sub.tr: transparency carrier density in l/cm.sup.3
[0077] N.sub.0: gain fitting parameter in l/cm.sup.3
[0078] .tau..sub.e: spontaneous carrier lifetime in seconds
[0079] .tau..sub.pL, .tau..sub.pS: photon lifetimes of long and
short wavelength modes in seconds
[0080] v.sub.g: group velocity in cm/s
[0081] .eta..sub.i: internal quantum efficiency
[0082] q: elementary charge in Coulombs
[0083] The .GAMMA.'s are the 2.times.2 matrix of confinement
factors of the two modes with the two active regions 5' and
5'',
.GAMMA. 1 , 2 S , L = .intg. V 1 , V 2 E S , L * E S , L V .intg. -
.infin. + .infin. E S , L 2 V ( 20 ) ##EQU00009##
For the gain function, we have used the logarithmic expression
g = g 0 ln ( N 1 + N 0 N tr + N 0 ) ( 21 ) ##EQU00010##
Here, g.sub.0 is the gain coefficient in cm.sup.-1, N.sub.tr is the
transparency carrier concentration, and N.sub.0 is a fitting
parameter to account for absorption under low injection
N<<N.sub.tr. The terms .chi..sub.1 and .chi..sub.2 account
for the gain suppression due to photon saturation
.chi. i = 1 + ( .GAMMA. i S S S + .GAMMA. i L S L ) V i P sat ( 22
) ##EQU00011##
where P.sub.sat is the photon saturation density in cm.sup.-3.
[0084] The total power emitted, taken from the top mirror 3 is
written as the sum of the power emitted from the short and long
wavelength modes
P = P S + P L where ( 23 ) P S = hv .eta. ext .tau. pS .theta. S
.theta. S + 1 S S and P L = hv .eta. ext .tau. pL .theta. L .theta.
L + 1 S L ( 24 ) ##EQU00012##
Here, .eta..sub.ext is the external quantum efficiency
.alpha..sub.m/(.alpha..sub.m+.alpha..sub.i). .theta..sub.S and
.theta..sub.L are functions that describe the relative power
splitting between the top 3 and bottom 4 mirror for each mode
.theta. = P top P bottom ( 25 ) ##EQU00013##
In all of the above, the .GAMMA.'s, .theta.'s, and .tau..sub.p's
are all dynamic functions of the carrier densities N.sub.1 and
N.sub.2.
[0085] Solving equations (16) through (19) and (23) in the time
domain requires expressions as functions of the carrier densities
N.sub.1 and N.sub.2 for the confinement factors .GAMMA..sub.1 and
.GAMMA..sub.2, the output coupling functions .theta..sub.S and
.theta..sub.L, and finally for the photon lifetimes .tau..sub.pS
and .tau..sub.pL. Rather than resorting to a first-principles
analytical treatment for the determination of these functional
relationships, a behavioral approach is followed in which the
parameters for the structure of FIG. 9 are first calculated using
the parameters of FIG. 11, and then they are fitted with an
appropriate functional form.
[0086] Associated with each mode is a photon lifetime, dependent
upon the distributed mirror losses due to material absorption
(.alpha..sub.i) and transmission through the mirrors
(.alpha..sub.m):
.tau. p = 1 v g ( .alpha. i + .alpha. m ) ( 26 ) ##EQU00014##
where v.sub.g is the group velocity of the mode in question. The
distributed mirror loss can be further separated into contributions
from the top 3 and bottom 4 mirrors
.alpha..sub.m=.alpha..sub.m.sup.t+.alpha..sub.m.sup.b (27a)
The emission from the device as observed through the top mirror 3
will be proportional to the ratio of the mirror losses
.theta. .varies. .alpha. m t .alpha. m b ( 27 b ) ##EQU00015##
Requiring that
.alpha..sub.m=.alpha..sub.m.sup.t+a.sub.m.sup.b=constant (28)
will assure that the modal threshold gain will remain constant, as
.tau..sub.p is constant as well. By keeping the modal threshold
gain constant, the carrier and photon populations will undergo a
minimum perturbation as the mode "sloshes back and forth" between
the upper 10 and lower 11 cavities.
[0087] The dependence of d.tau./dn, the derivative of the photon
lifetime with the index difference between the quantum wells in the
two active regions n.sub.qw1-n.sub.qw2 on the values of Nt, Nb and
Nc needs to be considered. The results show that for Nb-Nt=3, the
photon lifetime is invariant with .DELTA.n under all DBR
combinations.
[0088] This result merely states that, for .tau..sub.p to remain
constant, the reflectivities of the upper 3 and lower 4 DBRs should
be the same. The difference of 3 periods accounts for the fact that
the upper DBR 3 is terminated in air while the lower DBR 4 is
terminated in GaAs 12. Therefore, for the remainder of the modeling
exercise, we set Nb-Nt=3, so that,
.tau..sub.pL=.tau..sub.pS=.tau..sub.p (29)
[0089] The output coupling coefficients
.theta..sub.S(N.sub.1,N.sub.2) and .theta..sub.L(N.sub.1,N.sub.2)
are calculated as the ratios of the Poynting vector magnitudes of
the modal e-fields taken at the top 3 and bottom 4 mirrors
.theta. = S top S bottom ( 30 ) ##EQU00016##
The long and short wavelength modes are found to be symmetric in
.DELTA.n such that
.theta..sub.L(.DELTA.n)=.theta..sub.S(-.DELTA.n).
[0090] These output coupling coefficients have been found to be
quite well approximated by an exponential for small .DELTA.n, and
so the functions for the coupling coefficients have been chosen
as
.theta. S = .theta. ( .DELTA. n ) = K Q .DELTA. n = K Q n N ( N 1 -
N 2 ) ( 31 ) .theta. L = .theta. ( - .DELTA. n ) = K - Q .DELTA. n
= K Q n N ( N 2 - N 1 ) ( 32 ) ##EQU00017##
Equations (31) and (32) are substituted into (24) for the numerical
time domain simulation. For each different value of Nc examined,
the values for K and Q can be extracted. The values for K and Q are
tabulated in FIG. 15.
[0091] The maximum power deviation coupled through the top mirror 3
of the device through aperture 13 under modulation can be expressed
in the form of an extinction ratio
ER=.theta.(.DELTA.n)/.theta.(-.DELTA.n) (33)
This extinction ratio is an important parameter for communication
applications, as one wants a large contrast between the amount of
light emitted from the device in the on-state and the off-state.
Communication standards will often specify a minimum acceptable
extinction ratio. FIG. 14 represents the extinction ratio as a
function of Nc. Nt is varied from 14 to 30, Nb from 17 to 33, and
Nc from 0 to 33. The vertical bars that become more prominent at
higher values of Nc corresponds to the range of Nt and Nb that are
assumed in the simulation. This plot demonstrates that the
extinction ratio is essentially determined solely by the number of
intermediate DBR 6 pairs Nc, with very little dependence upon Nt or
Nb. Therefore, for a given .DELTA.n, a larger value of Nc will
yield an increase in modulation depth.
[0092] The confinement factors
.GAMMA..sub.1.sup.S=.GAMMA..sub.2.sup.L=.GAMMA..sub.1;
.GAMMA..sub.1.sup.L=.GAMMA..sub.2.sup.S=.GAMMA..sub.2 (34)
can be calculated using equation (20) for various values of Nc,
again with Nb-Nt=3. We find that for small .DELTA.n, the
confinement factors .GAMMA..sub.1 and .GAMMA..sub.2 are reasonably
well approximated as linear functions of .DELTA.n, and so express
them as
.GAMMA. 1 = .GAMMA. 0 - .GAMMA. n ( n qw 1 - n qw 2 ) = .GAMMA. 0 -
.GAMMA. n n N ( N 1 - N 2 ) ( 35 ) .GAMMA. 2 = .GAMMA. 0 + .GAMMA.
n ( n qw 1 - n qw 2 ) = .GAMMA. 0 + .GAMMA. n n N ( N 1 - N 2 ) (
36 ) ##EQU00018##
Equations (35) and (36) are substituted into equations (16) through
(19) for the numerical time domain simulation. For each different
value of Nc examined, we extracted values for .GAMMA..sub.0 and
d.GAMMA./dn.
[0093] Performance was evaluated for numerous cases as Nc was
varied from 25 to 50, all with Nt=17 and Nb=20. The values of Nc
correspond to an intermediate mirror reflectivity of greater than
99%. FIG. 15 lists the design choices examined, along with the
constants used in the behavioral models developed in the previous
section. A 5 um active region aperture 13 is used, although the
device concept tolerates a range of apertures.
[0094] Performance was evaluated by the measurement of eye diagrams
using symmetrical current modulation (i.e.
I.sub.1=I.sub.bias+/-I.sub.mod and I.sub.2=I.sub.bias-/+I.sub.mod)
in each cavity and long pattern length pseudorandom pulse
sequences. An eye diagram consists of overlapping the signals from
a long string of random "1"s and "0"s. An example is shown in the
inset picture in FIG. 17. One can see the "1" and "0" levels, as
well as the lines corresponding to the transitions between the two
states. A high quality signal has a large open area between the two
signal levels and the transitions. This corresponds to a condition
where there will be few errors transmitting the signal. Along with
FIG. 15, other parameters assumed in the rate equations analysis
are listed in FIG. 16.
[0095] FIG. 17 shows the time domain response of a CRVCL with Nc=44
operated with I.sub.bias=3.5 mA and I.sub.mod=1.5 mA modulated with
a 20 Gbps square wave. Plotted in the figure are the carrier
densities N.sub.1 and N.sub.2, along with their average carrier
density N.sub.ave. Emitted optical power originating from the long
and short wavelength mode is also shown. The long-wavelength mode
power is multiplied by a factor of 100 for clarity. In the inset is
the eye diagram generated with the pseudorandom bit sequence.
[0096] After the initial turn-on transient, the response settles
down as in a typical laser response. As expected, the modulation
present in the carrier densities is small, with the average being
almost flat. The output power is entirely dominated by the short
wavelength mode. The modulation of the long mode's confinement
factor, and hence its threshold gain, is 180 degrees out of phase
with the current modulation in the two active regions. Hence, the
long wavelength mode is almost completely suppressed and never
reaches threshold.
[0097] As shown in the insert, the eye diagram is very clean. There
is a complete lack of deterministic jitter and overshoot, in spite
of the relatively large extinction ratio and low I.sub.bias the
average current through the device. Note that the rise and fall
characteristics look far different from those of a typical
semiconductor laser.
[0098] In contrast, FIG. 18 shows the response of a conventional
VCSEL using the same physical parameters as in FIG. 17. Here, the
same bias conditions and photon lifetime were used, but the
confinement factor was multiplied by 2 to account for the single
active region.
[0099] In this case, there is a much larger modulation of the
carrier density, and there is severe eye diagram closure due to
deterministic jitter and overshoot. Certainly, the eye diagram
could be improved by increasing the bias current to unrealistic
levels, but, when driven at the same current density, the CRVCL has
far superior modulation characteristics.
[0100] We have found that Nc needs to be larger than 20 periods, or
equivalently, the intermediate mirror reflectivity needs to be
greater than 99%, for a minimum 2 dB extinction ratio. For other
emission wavelengths and materials systems, the intermediate mirror
reflectivity requirement of greater than 99% for good extinction
ratio remains the same. However, this requirement may result in a
different number of intermediate mirror pairs, depending upon the
refractive index differences of the materials used in the
mirror.
[0101] Small signal scattering parameter S.sub.21 response
characteristics were examined for a CRVCL with Nc=50 and a
conventional VCSEL with the same material parameters, aperture
size, and photon lifetime. S.sub.21 is a forward transmission
coefficient, and describes the ratio of an output signal (optical
in this case) to an input signal (electrical in this case) for the
CRVCL. The S21 curves for the CRVCL are the four solid lines, and
the S21 signal curves for the conventional VCSEL are the dotted
lines. FIG. 19 shows the results for bias currents of 2 to 5 mA and
0.1 mA modulation depth for both types of devices. For the two
structures, the relaxation oscillation frequency is essentially the
same, but the CRVCL shows much lower peaking and a gentler roll-off
of 10 dB/decade, leading to a substantially improved 3 dB
bandwidth. This result is consistent with the small signal analysis
provided above.
[0102] The CRVCL 3 dB bandwidth is shown to improve with modulation
depth. FIG. 20 shows the calculated small signal S.sub.21 response
for a CRVCL (Nc=50) for different values of modulation depth
I.sub.mod/I.sub.bias. For a modulation depth of 20%, the 3 dB
frequency exceeds 80 GHz. This is in contrast to the more typical
behavior of the conventional VCSEL (also shown) which is
independent of modulation depth as shown in FIG. 21. When compared
to the conventional VCSEL, there is a factor of four increase in
modulation bandwidth at a given current density.
[0103] FIG. 22 shows a more detailed cross section view of an
example of the CRVCL of the present invention. The device includes
two 5QW 22 1.lamda. active regions 5' and 5'', dual oxide apertures
29' and 29'' for current confinement in the upper and lower
cavities, a highly doped intracavity contact layer 27, and a GaInP
26 etch stop for the intracavity contact 9 etch.
[0104] Also shown in the figure is a bias and modulation
arrangement example for the push-pull mode of operation. Bias is
applied via a current source to the intracavity contact 9, whereas
the modulating voltage is applied to the upper cavity via a topside
contact 7. As the device behaves in a circuit sense essentially as
a back-to-back diode, modulating the device in such fashion
directly leads to the desired differential modulation of the cavity
currents. This arrangement allows for the minimization of the
bonding pad size, and avoids any parasitics which might otherwise
be a factor should modulation have been applied via the intracavity
contact. In any case, minimizing stray parasitics is important.
[0105] As shown in FIG. 23, which is a view of the device from the
topside of the chip, the example device employs a "bow tie" contact
structure. The upper terminal 7 metal contact comes from one side
of the device, while the intermediate terminal 9 metal contact
comes to the other side of the device. A ground 29-signal 30-ground
29 bond pad arrangement is used to enable wirebonding or on-chip
probing, or both, using coplanar waveguide probes. The ground 29
bond pads are attached to the lower terminal 9, while the signal
bond pad 30 is attached to the upper terminal 7 metal. The
ground-signal-ground (G-S-G) is a standard arrangement for the
application of very high frequency signals.
[0106] The epitaxial structure in the device example contains 17 to
20 top 3 n-doped DBR pairs, 20 to 23 bottom 4 n-doped DBR pairs,
and 38 to 50 intermediate 6 p-doped DBR pairs. All DBRs consist of
x=0.15/x=0.95 AlGaAs layers 18,19 with 20 nm linear compositional
and doping grades 20 to reduce the series resistance. The p DBRs
and n DBRs are doped .about.3e18 cm.sup.-3 and .about.2e18
cm.sup.-3 respectively, with the doping in the 6 pairs adjacent to
the cavities reduced by .about.50% to reduce free carrier
absorption. The 5 central mirror pairs in the low field region of
the intermediate DBR are highly doped (p>5e18 cm.sup.-3) for
improved current spreading, with one pair doped >2e19 cm.sup.-3
to facilitate the Ti/Au unalloyed ohmic contact 9, which is made to
a 12 nm thick GaAs layer 27 placed at a standing wave node to
minimize absorption. Adjacent to and immediately above the contact
layer is a 10 nm thick GalnP layer 26 for use as an etch stop
during the intracavity contact etch. A 15 nm thick GaAs ohmic
contact layer doped n>>8e18 cm.sup.-3 caps off the structure
25.
[0107] The active regions 5' and 5'' each have five undoped 7 nm
thick GaAs quantum wells 22 with 6 nm thick barriers 23 of undoped
x=0.25 AlGaAs between the quantum wells and 20 nm of the barrier
material on either side of the quantum well region. The remaining
cavity spacer material is of low doped (n,p<5e17 cm.sup.-3)
x=0.65 AlGaAs 21.
[0108] The oxide aperture layers 24 consist of 20 nm thick x=0.98
AlGaAs 32, bounded on one side by 12 nm of x=0.97 AlGaAs 33, and
x=0.65 AlGaAs 34 on the other. This design yields a slightly
tapered oxide "tip" in order to reduce diffraction losses.
[0109] The process flow requires 6 photolithographic steps,
represented by the individual layers in sequence:
[0110] a. Intracavity contact etch
[0111] b. Mesa/trench etch
[0112] c. Isolation implant
[0113] d. Intracavity contact ohmic metallization
[0114] e. Topside ohmic metal
[0115] f. Thick interconnect and bondpad metal
FIG. 24 shows the overlay of each of the mask layers used to define
each feature.
[0116] Referring to FIGS. 22 through 24, one possible fabrication
procedure involves first depositing a 1/4.lamda. SiN dielectric
layer to passivate the optical aperture and serve as contact
protection during the wet oxidation step. Next, a C12-based dry
etch using inductively coupled plasma reactive ion etching exposes
the intra-cavity contact etch area 35, followed by an HCL wet etch
of the GalnP etch stop to expose the GaAs contact layer. Another
1/4.lamda. SiN layer is then deposited to protect the contact layer
during the subsequent dry etch of the mesa trench area 36 to expose
the x=0.98 AlGaAs selective oxidation layers 32. A wet oxidation
step turns the x=0.98 layers into aluminum oxide (Al.sub.2O.sub.3).
This process is stopped before reaching the center of the mesa
(.about.10 .mu.m nominal lateral oxidation distance) to form two
oxide apertures 29' and 29''. In these apertures the aluminum oxide
is insulating, but the unoxidized region is still conducting, and
hence a guide for current is created. Following the oxidation, a
high-energy (>380 keV) multi-step proton implant through an 8 um
thick photoresist mask is used to electrically isolate the devices
from each other and intracavity anode from the cathodes. The
photoresist mask creates an unimplanted region 37 that protects the
contact regions 7 and 9, as well as the device emitting aperture
13. Implanting after the mesa etch ensures complete isolation of
the bottom DBR. The intracavity metal contacts 9 are then patterned
for liftoff, followed by a CF.sub.4 dry etch of the nitride and
deposition of the Ti/Pt/Au contact metal. After patterning and
deposition of the Ni/Ge/Au metallization of the topside ohmic
contacts 7, the top surface of the device is planarized with
Cyclotene(BCB), followed by deposition of 1.6 um thick Ti/Au metal
forming the interconnect metal and ground-signal-ground (GSG)
coplanar waveguide contacts 30 and 31. Finally, the wafer is
thinned to .about.6 mils, metallized with Ni/Ge/Au to form the
bottom terminal 8 metal contact and alloyed at >380C to activate
the ohmic contacts.
[0117] While the invention has been described with reference to an
exemplary embodiment(s), it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment(s) disclosed, but that the invention will
include all embodiments falling within the scope of the appended
claims.
* * * * *