To be published in SPIE Vol. 4649, 2002
Beyond 850nm: Progress at Other Wavelengths and Implications from
the Standard
Jim A. Tatum*a, Mary K. Hibbs-Brennerb, James R. Biarda, Andrew Clarka, J. Allen Coxb, James K.
Guentera, Robert A. Hawthornea, Klein Johnsonb, Ralph H. Johnsona, Jin Kimb, Yue Liub, Fouad
Nusseibehb, and Gyoungwon Parkb
a
Honeywell VCSEL Products, 830 East Arapaho Road, Richardson, TX 75081
Honeywell VCSEL Products, 12001 State Highway 55, Plymouth, MN 55441
b
ABSTRACT
Born of necessity of application, the Vertical Cavity Surface Emitting Laser (VCSEL) is now found in nearly
all optical networking systems based on standards such as the IEEE 802.3z and ANSI X3.T11. Reliability continues to
be the hallmark of the technology, and the volume manufacturing aspects are now realized. While VCSELs satisfying
optical networking standards continue to provide the highest volume applications, the advantages of the technology are
beginning to enable novel optical equipment. This paper explores development of VCSELs at wavelengths from 650 to
850nm, and the commercial applications of these devices in both the data communications and optical sensing arenas.
VCSELs operating at longer wavelengths are also being developed, but are not at a stage of commercialization to be
discussed in this forum.
Keywords: VCSEL, data communications, optical sensors, reliability
1. INTRODUCTION
Events of the past year have led to a great deal of uncertainty in the photonics industry, and in the
telecommunications sector in particular. The previous enthusiasm for new photonics products may have been “irrational
exuberance” after all. The basic demand for bandwidth will not shrink, however, and renewed growth is to be expected
soon. As many authors have described, however, this new phase of sustainable growth— as distinguished from the just
past phase of “hypergrowth”— will require new behaviors from the photonics manufacturers. [5-8] In particular, the use
of building block components, maintaining standard configurations much more that has been typical previously, is
necessary to afford the full benefits of automation to photonics assembly. One such building block is the TO-style
component, which has been the workhorse of the industry, and which will likely continue to be for some time to come.
Its performance at high frequencies is inadequate for later generations of high-speed data communications, however, so a
new building block is necessary. Given the new economics, adherence to the developing packaging standard will be
even more important than it has been in the past.
When, as is sometimes the case, the application precludes the use of standard packaging, adherence to standard
processes is still desirable. However, every designer should ask the question: Could I possibly use a standard packaged
VCSEL? In some cases the savings in time, money, and quality can outweigh the effort required to allow the use of
standards. The purpose of this paper is to present developments in VCSELs at wavelengths operating from 650 to
850nm. The common thread bonding the seemingly disjointed sections is the paper is the application space enabled by
the technology. The commonly exploited traits of VCSELs include the optical and electrical characteristics, wafer level
processing and testing, reliability, packagability, and arrayability. Many of the aspects of VCSELs have been described
in the past, and are well understood, including the manufacturing aspects of growth, processing, and testing [1], and
reliability [2]. This paper will address now emerging applications that require advanced packaging of both singlet and
array devices. For example, results for VCSELs operating at 10GBd in a hybrid microwave package are presented, and
packaging of various VCSEL arrays is described.
*
Correspondence: [email protected], (972) 470-4572
To be published in SPIE Vol. 4649, 2002
2. 850NM VCSELS
Development of VCSELs in the 850nm range has focussed on higher speed, arrayability, and single mode operation.
The optical networking standards are actively pursuing a 10GBd solution (IEEE 802.3ae and ANSI 10GFC). The
principal applications of such high speed connectivity is local area network (LAN) equipment and computers inside
equipment rooms, and in the metropolitan area network (MAN) backbones. The networking standards accommodate
several technical approaches, including serial, parallel and coarse wavelength division multiplexing (CWDM) to achieve
the 10GBd aggregate data throughput. In addition to these three technology approaches, there are specifications for
lasers center wavelengths of 850nm, 1310nm, and 1550nm. The complication of so many technical and wavelength
choices was implemented to ensure the user the lowest cost approach for a wide range of connectivity implementations.
Here the development of a VCSEL capable of 10GBd serial communications will be described, as well as parallel optical
interconnections.
0
0
-5
-3
-10
-6
-15
-9
-20
-12
-25
-15
-30
|S12|
3
0
5
10
15
20
Frequency (GHz)
Figure 2 Measured S11 (blue line) and S12 (red line) for a packaged VCSEL.
|S11|
Figure 1 Test package for a VCSEL operating at 10GBd and the measured eye diagram using a k28.5
pattern. The Rise time is 24ps, fall time is 34ps, and total jitter (pk-pk) of 10ps. The VCSEL bias was
set to achieve a 6dB extinction ratio, and an average power of approximately 1mW.
To be published in SPIE Vol. 4649, 2002
Serial interconnections are generally the most cost effective to employ for longer distances, and parallel
interconnects for shorter distances. This is driven by the cost of the high-speed integrated circuits, as well as the cost of
the fiber and installation. Developing lasers and packages for serial 10GBd data communications is a complicated
procedure, and care must be taken to understand all aspects of the design. Among these are the VCSEL itself; the
thermal and mechanical issues of the package and optical connectivity; and the finally electrical interface to external
laser drivers. The VCSELs developed here are top P-side contact lasers on N-type substrates. Lateral oxidation is used
to form an aperture of ten microns in the p mirror, which controls both the optical and electrical properties. Figure 1
shows an example VCSEL package, and the corresponding eye diagram. Figure 2 is the measured S11 and S12
parameters from the same packaged VCSEL.
The results obtained with the non-optimized package bode well for the prospects of packaging VCSELs for
operation at 10GBd. The package parasitics can actually be designed to complement the VCSEL parasitics to achieve a
balanced load as a function of frequency. This is very important in 10GB operation because of the wide frequency band
requirements. For example, for IEEE 802.3ae applications, the coding mechanism is 64B/66B, which leads to maximum
run lengths of 33 bits. At 10GBd, that is a dynamic frequency range of 100MHz to more than 10GHz. In order to achieve
good jitter and pulse shape integrity, it is critical to maintain a stable load over the broadest frequency range possible.
Additionally, at the high frequency, group velocity dispersion in the electrical signal must be accounted for to ensure
optimal optical performance of the VCSEL. Finally, reliability of VCSELs operating at 10GBd has been a concern
because of the high current density required to achieve the required optical bandwidth. (Bandwidth is approximately
proportional to the square root of the current density.) The reliability of the VCSELs used to generate the results
presented here is under investigation, and preliminary results would indicate a mean time to failure at 70C ambient of
about 700,000 hours.
Many of the same engineering considerations are important in designing the receiver for 10GBd systems, and the
same design considerations have been employed in the receiver. Figure 3 is a plot of the measured differential output of
a detector and TIA in a package similar to that shown in Figure 1.
Another method of achieving high aggregate data throughput is use parallel optical interconnections. In this
application, each channel is operated at a lower data rate, typically less than 3.125GBd, with up to 12 channels operating
Figure 3 Differential outputs of a transimpedance amplifier (TIA) using an 850nm detector and a VCSEL source.
The photodiode used has capacitance less than 200fF, series resistance less than 20 ohms, and a total optical
bandwidth greater than 18GHz.
simultaneously. The primary problems with building a parallel optical interface involve the manufacturability aspects,
including reliability and component level burn in, electrical interface, and optical coupling. Reliability of array products
is generally estimated by simple exponential statistical extrapolation from reliability of individual components. That is to
say if single devices have an estimated reliability of Xhrs, then the array reliability is Xhrs divided by the number of
To be published in SPIE Vol. 4649, 2002
elements in the array. This is an overly pessimistic approach for Honeywell VCSELs because the reliability statistics are
lognormal. For reliability estimates of arrays based on lognormal statistics, instead of dividing the time to failure by the
number of units, one divides the failure rate by the number of units. This leads to nearly an order of magnitude
improvement in predicted array reliability. Figure 4 describes the estimated reliability of a 12-element array operating at
15mA average current and 40°C ambient.
Operating Hours
10,000,000
Time to 5%
failures of arrays
1,000,000
Time to 5%
failures of singlets
100,000
.05 .1 .2
.5
1
2
5
10
20
30
Cumulative Percentage Failing
Frequency
Figure 4 Reliability estimates for a 12 element array based on singlet data. The estimated reliability using
lognormal statistics is nearly an order of magnitude better than expected for exponential statistics.
Stabilized
-20.0%
-10.0%
0.0%
10.0%
20.0%
Percentage Change
Figure 5 Power output variation of stabilized (red line) and non stabilized (blue line) for VCSELs after 5
years of operation at 10mA and 40C.
To be published in SPIE Vol. 4649, 2002
Threshold
Slope Efficiency
10mA Power
Stabilized
µ
σ
0.55% 12.25%
-0.24% 1.75%
-1.07% 1.37%
Unstabilized
µ
σ
-11.1% 14.5%
4.4%
3.2%
7.85
4.58%
6
0.00%
5
-0.30%
4
-0.60%
3
-0.90%
2
-1.20%
1
-1.50%
0
-1.80%
0
5
10
Power Change (%/C)
Power (mW)
Another aspect of reliability is the short-term variation in optical parameters often observed in VCSELs. The shortterm stabilization affects the threshold current, slope efficiency, and the total optical power output, as well as the series
resistance. Today, singlet devices are burned in at a component level (TO package, for instance) to stabilize the optical
performance over time. One severe disadvantage for arrays to date has been the cost to package and burn in VCSELs in
a component form. Honeywell has invented a process that produces lasers already stable at the wafer level, thereby
eliminating the need to do component burn-in at later assembly stages. This stabilization reduces the parametric shifts in
optical performance and allows the transceiver designer to eliminate manufacturing steps. The parametric stabilization is
important to ensure the ability to meet both eye safety and communications standards over the life of the VCSEL array.
Figure 5 and the accompanying table demonstrates the change in optical power that can happen over time for both
stabilized and unstabilized VCSELs.
15
Current (mA)
Figure 6 Light output characteristic of a 12 element VCSEL array at 20 and 80°C.
Another area of concern for VCSEL array applications is the uniformity of device characteristics so that power
monitoring of individual channels can be eliminated. Figure 6 is a plot of light output as a function of current for a 12element VCSEL array at temperatures of 20 and 80C. The power change as a percentage per degree C is also calculated,
demonstrating that single power compensation techniques can be used for the entire VCSEL array.
Finally, the ability to package a VCSEL array into a subassembly for further integration into an optical transceiver is
also important. The design considerations here again are the optical and mechanical alignment of the array to the ribbon
connector, and the electrical parasitics introduced by the package. Figure 7 is a picture of a VCSEL array package that
incorporates a microlens element on a ceramic carrier.
Another area of productization for 850nm VCSELs is single mode operation. The operational characteristics and
structure of this VCSEL have been previously described [1]. While operation in a single longitudinal mode is often
guaranteed by design of the VCSEL, single transverse mode operation must be carefully engineered. Single mode
VCSELs are useful in several areas, including fiber optic communications on installed 9 micron core single mode fiber
To be published in SPIE Vol. 4649, 2002
Figure 7 VCSEL array on a ceramic submount with a microlens. Better than 80% coupling to a 50µm multimode
fiber array was achieved with less than 5% elemental variation was achieved with this package.
(In this case, we mean single mode at the telecom windows of 1300-1600nm) and optical sensing. For example, at
100MBd, and operating on single mode fiber, an optical link length of more than 2km can be easily obtained even at 850
nm, and is limited by either the attenuation or the modal dispersion of the fiber. Additionally, operation on multimode
optical fiber would allow similar link lengths, generally limited by the attenuation or modal bandwidth of the optical
fiber. One sensor application that has been described previously [3] now benefits from the packaging offered by a
VCSEL. In this application, the VCSEL light is self interfered from high order diffraction from a grating. The
interference is used as the measurement standard, and resolution well below 10nm is readily achievable. The VCSEL
was a natural choice for this application because of its good reliability, true single mode behavior and ability to
incorporate into a small, highly integrated package. The packaged VCSEL assembly (without optics) is shown in figure
8.
Integrated VCSEL
controller and detector
with encoder output
signals
VCSEL
Figure 8 Photograph of a VCSEL mounted chip on bard with a controller circuit and detector for optical encoder
applications. (Photo courtesy of MicroE).
Volume manufacturing aspects of single mode VCSELs are now being addressed, with very good uniformity over
time. As with multimode VCSELs, the emission wavelength can be precisely controlled, and the parametrics such as
threshold, slope efficiency and resistance are similarly repeatable. Figure 9 displays the normalized variation of several
device characteristics over a large number of VCSEL manufacturing wafers.
Because of the relatively small diameters (~4µm) Single mode VCSELs are generally limited in optical power by
thermal effects in the active region. One approach to increase the optical power in a single mode is to control the modal
behavior of the VCSEL by some means. By controlling the optical modes, larger oxide apertures are possible, which
reduces the thermally induced lensing in the VCSEL. Here we describe one such method that achieves several mW of
single mode power. Here, additional dielectrics are deposited and patterned on top of the VCSEL to enhance the mirror
reflectivity under the central region, and discourage lasing around the edges. The measured LI curve for a VCSEL with
To be published in SPIE Vol. 4649, 2002
1.5
Normalized Variation
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
W
av
el
en
gt
h
FW
HM
3m
A
FW
HM
5m
A
Po
w
er
Sl
op
e
Fo
rw
ar
d
Vo
lta
ge
Re
si
st
an
ce
Th
re
sh
ol
d
0.5
Figure 9 Normalized parametric values for a large number of single mode VCSEL wafers used in manufacturing.
and without a dielectric mode control structure is shown in Figure 9, with the point at which the VCSEL transitions to
multimode operation indicated. The results presented here are preliminary, and will be discussed in detail in later
publications.
8
4.0
6
Power (mW)
3.5
Dielectric
Mode Control
3.0
5
2.5
4
2.0
3
2
1.5
Multimode
Transition
Multimode
Transition
1.0
1
0.5
0
0.0
0
3
6
9
12
Forward Voltage (V)
7
15
Current (mA)
Figure 10 Light output characteristic of a VCSEL with and without the dielectric mode control structure.
To be published in SPIE Vol. 4649, 2002
3. 760-800NM VCSELS
VCSELs operating in the 760-800nm wavelength range have applications in laser printing [4], chemical sensing,
atomic clocks, and in wavelength multiplexed communications systems. Laser printing is a particularly attractive
application because both the print quality and speed can be increased while maintaining the use of low cost
electromechanical motors. Digital reproduction is another area where print engines are being used in place of other
copying means. Laser printing, as well as most sensing applications, require single mode VCSELs. Laser printing
applications also will require multielement VCSEL arrays, typically with very small spacing (<100µm) between emitting
elements. Therefore, it is beneficial to use VCSELs because of the reduced thermal, electrical, and optical crosstalk.
Building on the results presented for dielectric mode control, devices with more than 3mW of single mode power have
been realized. Figure 11 is a plot of the Light output and voltage characteristic for a typical 780nm VCSEL with a
dielectric mode control structure. The high threshold is a result of the mismatch between the quantum well gain and the
Fabry-Perot resonance of the laser cavity. The inset figure is the optical spectrum at 15mA showing good side mode
suppression (>26dB).
3.5
5
26dB
4
2.5
792.0
793.0
3
794.0
2
2
1.5
1
1
Light (mW)
Voltage (V)
3
0
0
5
10
15
20
Current (mA)
Figure 11 Light output characteristic of a VCSEL with and without the dielectric mode control structure.
10mA
12mA
14mA
16mA
18mA
-15
-10
-5
0
5
Divergence Angle (Deg)
10
15
Figure 12 Beam divergence form a single mode VCSEL at various current levels.
To be published in SPIE Vol. 4649, 2002
One other aspect of VCSELs for laser printing is the optical beam quality. The quality and size of the printed spot
depends on the ability to focus the laser beam to a small spot. Both the optical guiding effects of the oxide aperture and
the dielectric mode control structure have been carefully engineered to produce minimal changes in the beam divergence
as a function of current. Figure 12 is a plot of the measured far field divergence angle for multiple currents. The beam
profile remains guassian over the entire current range, and has minimal changes in the emission angle.
4. VISIBLE VCSELS (650-670NM)
The extension of VCSEL operation to visible wavelengths will benefit the areas of optical sensing where there is human
interaction required. Examples of this are bar code scanning, laser ranging, and proximity sensing, among others.
Development in VCSELs operating at visible wavelength has generally been limited to wavelengths longer than 670nm
because of the difficulty in confining carriers in the quantum wells. (This is caused by the low band edge offset between
4
Power (mW)
10C
3
20C
30C
2
40C
50C
1
60C
0
0
2
4
6
8 10 12
Current (mA)
14
16
18
Figure 13 Typical light output characteristic of a visible (670nm) VCSEL.
3mA
Normalized Intensity
4mA
5mA
6mA
8mA
10mA
-15
-10
-5
0
5
10
15
Divergence Angle (Deg)
Figure 14 Measured beam divergence angle for various currents in a 670nm VCSEL.
To be published in SPIE Vol. 4649, 2002
the Γ and L bands.) Additional complications in making visible wavelength VCSELs arise from the poor index contrast
in the mirror structure, and the increased thermal resistance of the material. The red VCSELs fabricated to date at
Honeywell are proton-isolated structures grown on off axis substrates. These VCSELs have demonstrated good threshold
and power performance, and reasonable performance over temperature. Figure 13 is a typical light output characteristic
of a visible wavelength (670nm) VCSEL.
Many visible wavelength applications rely on the narrow divergence and beam quality associated with visible VCSELs.
Figure 13 is a plot of the beam divergence as a function of current, and indicates the relatively mild dependence of the
beam divergence on current. At higher currents, a thermally induced lensing mechanism causes the VCSEL to deviate
from a Gaussian mode profile.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
J. K. Guenter et al., “Commercialization of Honeywell’s VCSEL Technology: Further Developments,” Proc. SPIE,
vol. 4286, 2001.
“850nm VCSEL Products Reliability Study,” Honeywell application note, available at www.honeywell.com/vcsel
B. Horowitz, “Diffractive Techniques Improve Encoder Performance,” Laser Focus World, October 1996.
R. L. Thorton, “Vertical-Cavity Lasers and Their Application to Laser Printing,” Proc. SPIE, vol. 3003, 1997.
E. Chen and D. Lu, “Enabling technologies: The ‘pick and shovel’of the photonics gold rush,” WDM Solutions,
September 2001
E. Chen and D. Lu, “Enabling technologies: Optical manufacturing emerges and evolves,” WDM Solutions,
September 2001
J. Lively, “Component manufacturers must reassess their priorities,” Compound Semiconductor, December 2001
S. Weiss, “The Automation Crisis,” Photonics Spectra, June 2001