Small-signal frequency response of long-wavelength vertical

Small-Signal Frequency Response of
Long-Wavelength Vertical-Cavity Lasers
K. A. Black, Member, IEEE, E. S. Björlin, Student Member, IEEE, J. Piprek, Senior Member, IEEE,
E. L. Hu, Fellow, IEEE, and J. E. Bowers, Fellow, IEEE
Abstract—The small-signal frequency response of long-wavelength vertical-cavity lasers (LW-VCLs) is of interest in determining the ultimate bandwidth of these devices for use in
terrestrial networks. This letter analyzes two distinct device
structures of double wafer-bonded vertical-cavity lasers operating
at 1.5 m and compares their high-speed performance. We
demonstrate the highest modulation bandwidth and the highest
modulation current efficiency of an electrically pumped LW-VCL
of 7 and 4.1 GHz/ mA, respectively.
Index Terms—Frequency response, optical modulation, semiconductor lasers, vertical-cavity surface-emitting lasers, wafer
UCH of the current research activity surrounding longwavelength vertical-cavity lasers (LW-VCLs) is inspired
by the growing need for sources aimed at high capacity wavelength-division-multiplexed (WDM) networks. Until recently,
progress in the field has been slow as compared to shorter wavelength VCLs, mainly due to the lack of developed materials
systems capable of achieving both high mirror reflectivity and
long-wavelength emission. Extensive progress has been made
in the static performance and high temperature characteristics
through the uses of wafer bonding [1], antimonide-based structures [2], and GaInAsN [3]. However, perhaps the greatest attribute of the vertical-cavity structure is its small active volume,
which could potentially lead to large modulation bandwidths at
low drive currents. This is extremely important as one moves
toward OC-192 and 10-Gb Ethernet networks that demand inexpensive directly modulated sources.
To date, the best reported LW-VCL performance has been
demonstrated using wafer bonding. Previous transmission results at 1.5 m have demonstrated 2.5-Gb/s transmission over
200 km of single-mode fiber, as well as a small-signal modulation bandwidth of 4.7 GHz [4]. This letter investigates two
similar LW–VCL structures and their capacity to demonstrate
high modulation bandwidths. Both devices were made by wafer
bonding AlGaAs–GaAs-based mirrors onto an InP-based active
Manuscript received March 28, 2001; revised June 18, 2001.
K. A. Black was with the Electrical and Comuter Engineering Department,
University of California at Santa Barbara, Santa Barbara, CA 93106 USA. She
is now with Zaffire, San Jose, CA 95134 USA.
E. S. Björlin, J. Piprek, E. L. Hu, and J. E. Bowers are with the Electrical and
Comuter Engineering Department, University of California at Santa Barbara,
Santa Barbara, CA 93106 USA (e-mail: [email protected]).
Publisher Item Identifier S 1041-1135(01)08076-4.
The modulation bandwidth of a laser is governed by inherent
properties of the cavity and the materials used, drive current
relative to threshold, as well as external parasitics associated
with contact pads, current supplying leads, etc. The intrinsic
bandwidth of the laser cavity is limited by the relaxation resonance frequency of the cavity, thermal constrictions, and at
higher drive currents, by the damping rate.
The square of the relaxation oscillation frequency ( ) is proportional to the drive current less the threshold current, and inversely proportional to the active volume of the device. The low
threshold current and small active volume of VCLs, compared
to in-plane lasers, yields high relaxation resonance frequency at
relatively low drive currents, i.e., high modulation current efficiency factor (MCEF). Short wavelength VCLs have demonstrated modulation bandwidths of 21.5 GHz [5] and MCEF of
16.8 GHz/ mA [6]. Modulation bandwidths up to 30 GHz have
been achieved for in-plane lasers [7]. However, the MCEF was
in that case 1.2 GHz/ mA, clearly showing the advantage of
the vertical-cavity design.
The relaxation resonance oscillations of a laser cavity are
damped by the loss of carriers to leakage and spontaneous emission, as well as photon losses. The damping rate increases
with increased relaxation resonance frequency, eventually resulting in an over-damped system where the 3-dB rolloff occurs
before the relaxation resonance frequency. The dependence of
, where
the damping on can be described by
are constants. depends on carrier transport effects
and gain compression at higher photon densities. and can
be extracted from modulation response measurements, and the
intrinsic bandwidth can be calculated from these parameters.
As the drive current above threshold increases, the photon
density in the cavity, and thereby the rate of stimulated recombination increases. However, the laser cannot be driven
infinitely hard. Problematic to LW-VCLs is the early onset
of thermal rollover in the light output versus current versus
voltage ( – – ) characteristics, at which additional injected
current is dissipated as heat instead of generating more photons,
limiting the maximum achievable resonance frequency.
In practice, the modulation response of electrically pumped
LW-VCLs is usually limited by external parasitics [4], [8].
Contacts to the device add external capacitance, resulting in
a three-pole filter behavior instead of the intrinsic (two-pole)
RCL circuit behavior. The contact-pad capacitance can be
reduced by decreasing the contact-pad area [9] or by increasing
the distance between the p-contact and the ground [5].
1041–1135/01$10.00 © 2001 IEEE
Fig. 3. Small-signal modulation response of device A. A 3-dB bandwidth of
3 GHz is achieved. The modulation bandwidth is limited by external parasitics.
Fig. 1. Schematic of VCL structures with overlain equivalent circuit.
Fig. 2. Room temperature L–I –V characteristics.
The two analyzed device structures are shown in Fig. 1(a) and
(b). Exact detail concerning the device design and processing
can be found in [10]. Both devices were fabricated using the
same active region and mirror structures to eliminate inherent
device differences from growth. The only material difference is
in the top p-mirror doping close to the wafer bonded interface:
device A uses a Be-doped surface layer (30 nm) at 8 10 /cm ,
whereas device B has a lower Be doping of 3 10 /cm . Both
devices use oxide apertures for current confinement; the aperture opening is 7 m for device A and 4 m for device B. – –
characteristics of both devices are shown in Fig. 2. The threshold
currents of the devices are 2 and 1.1 mA, respectively, however
the series resistance and operating voltage of device B is considerably higher. This difference, we attribute to an increased
resistance across the wafer-bonded junction resulting from a
lower doping of the GaAs-based mirror at the surface. Although
this increase in operating voltage is most likely indicative of increased current spreading at the wafer bonded junction, it is estimated that the spreading will be no more than 1 m, effectively
making the active volumes of device A and B similar. A quantitative analysis of the exact active volume of these devices is
not necessary for the discussion of their frequency response, as
the resonance relaxation frequency is not the limiting factor in
either device.
Also shown in Fig. 1, there are equivalent circuits overlain
onto the structures. A resistance and capacitance in parallel ap-
proximate the active region; the top DBR is modeled as a series resistance. The series capacitance across the oxide aperture
has been neglected in this model, as in practice most currents,
will be transported through the aperture opening due to the high
parallel conductivity of the DBRs. Device A has a differential
resistance of 138 and device B has a differential resistance of
153 . The major structural difference between the two devices
is a reduction of the contact-pad capacitance in device B. In both
devices, current is injected through the top (p) mirror. In device
A, the p contact pad sits on the doped n-cladding, separated only
by 400 nm of SiN. In device B, the doped cladding layer was removed and the contact pad is separated from ground by over 500
m of undoped material (Fig. 1), thereby practically eliminating
the contact-pad capacitance. This structural difference does not
affect the dc performance of the devices.
All measurements were performed using a 20-GHz bandwidth HP8150 network component analyzer. A microwave frequency coplanar probe was used to contact the VCL contact
pads. The optical response of the lasers was collected using
a cleaved multimode fiber probe. Small-signal modulation response was measured for different drive currents. The measured
data were fit to a general three-pole modulation transfer function
modulation frequency;
parasitic rolloff frequency;
resonance frequency;
damping rate.
A. Device A
Fig. 3 shows the modulation response of device A. Measured
data is shown by dots; the curve fits are shown by solid lines.
(3 mA), a maximum 3-dB bandwidth of
At a dc bias of 1.5
3 GHz is achieved. At larger biases, the response is limited by an
initial low rolloff characteristic of external parasitic damping.
The parasitic rolloff frequency associated with this device is
found to be 1.2 GHz. Knowledge of the parasitic bandwidth allows one to extract the capacitance of the device from the RC
. The differential resistance of
time constant
this device is measured to be 138 ; the calculated capacitance
damping, the relation between
is given by
[12]. In our case,
occurs earlier
than suggested by the above relation, a deviation that we
attribute to damping. However, the linear dependence of
over the entire range of data suggests that this device is
not limited by damping. Both the 3-dB rolloff frequency and
relaxation oscillation frequency cease to increase with bias
current above 5 mA. This is precisely the current at which
thermal rollover in the – – characteristic occurs, suggesting
that Device B is thermally limited. The calculated thermal
limit to the maximum achievable bandwidth for this device,
neglecting damping, is 8 GHz.
Fig. 4. Small-signal response of device B. A modulation bandwidth of 7.0
GHz is demonstrated. Inset shows 3-dB bandwidth versus square root of current
above threshold. A modulation current efficiency of 4.1 GHz/mA1/2.
of the device is 2.6 pF. The maximum internal parasitic VCL capacitance is estimated to be less than 1 pF [11], indicating high
external parasitic capacitance.
reveals a -factor of
Plotting the damping rate versus
0.48 ns. The intrinsic bandwidth, including carrier transport and
gain compression, but neglecting heating effects and external
parasitics, can be calculated using the experimentally derived
-factor and the relation
. The intrinsic
bandwidth was found to be 18.4 GHz. This is a low value compared to those reported for short wavelength VCLs, due to the
large -factor of this device.
B. Device B
Device B was fabricated with contact pads optimized to reduce the external parasitic capacitance of the VCL. The modulation response of device B is shown in Fig. 4. A record LW-VCL
3-dB bandwidth of 7 GHz at 5.5-mA bias is demonstrated. Immediately apparent, is the reduction in external parasitic capacitance as compared with device A. Indeed, the total capacitance
calculated from the RC time constant was reduced to 0.92 pF, indicating that the contact-pad capacitance has been reduced significantly. The inset in Fig. 4 analyzes the modulation current
efficiency of device B. A linear fit at low biases above threshold
reveals a record LW-VCL MCEF of 4.1 GHz/ mA. The deviation from the fit at high bias is due to gain saturation and
heating of the active region. The -factor of this device is determined to be 0.65 ns, implying a maximum intrinsic bandwidth
of 13.8 GHz. The relatively large -factor of both device A and
B can be attributed to their high differential resistance and current spreading under the oxide aperture. The fact that device B
has a larger -factor than device A can be attributed to the lower
doping close to the bonded interface in this device. This further
supports our theory that reduced capacitance, not better carrier
transport or reduced damping, yields the improvement in device
performance between device sets A and B.
is proportional
Despite the large -factor of device B,
over the entire range of data collected. For low
We have demonstrated a record high LW-VCL small-signal
modulation bandwidth of 7.0 GHz through a reduction of external device parasitics. Further reductions in external parasitics
can be achieved through smaller diameter pillars and reduced
contact-pad area. In order to avoid possible charge build up at
the oxide aperture, an ion-implanted device might also be considered. To improve the frequency response even further, the
thermal performance of the double fused structure must be addressed.
[1] A. Karim, K. A. Black, P. Abraham, D. Lofgreen, Y.-J. Chiu, J. Piprek,
and J. E. Bowers, “Superlattice barrier 1528-nm vertical-cavity laser
with 85 C continuous-wave operation,” IEEE Photon. Technol. Lett.,
vol. 12, pp. 1438–1440, Nov. 2000.
[2] E. Hall, S. Nakagawa, G. Almuneau, J. K. Kim, and L. A. Coldren,
“Room-temperature, CW operation of lattice-matched long-wavelength
VCSELs,” Electron. Lett., vol. 36, pp. 1465–1467, Aug. 2000.
[3] K. D. Choquette, J. F. Klem, A. J. Fischer, O. Blum, A. A. Allerman,
I. J. Fritz, S. R. Kurtz, W. G. Breiland, R. Sieg, K. M. Geib, J. W.
Scott, and R. L. Naone, “Room-temperature continuous-wave InGaAsN
quantum-well vertical-cavity lasers emitting at 1.3 m,” Electron. Lett.,
vol. 36, pp. 1388–1390, Aug. 2000.
[4] S. Z. Zang, N. M. Margalit, T. E. Reynolds, and J. E. Bowers, “1.54-m
vertical-cavity surface-emitting laser transmission at 2.5 GB/s,” IEEE
Photon. Technol. Lett., vol. 9, pp. 374–376, Mar. 1997.
[5] K. L. Lear, M. Ochiai, V. M. Hietala, H. Q. Hou, B. E. Hammons, J.
J. Banas, and J. A. Nevers, “High-speed vertical-cavity surface-emitting lasers,” in 1997 Dig. IEEE/LEOS Summer Top. Meetings: VerticalCavity Lasers, Montreal, Canada, 1997, pp. 53–54.
[6] K. L. Lear, A. Mar, K. D. Choquette, S. P. Kilcoyne, R. P. Schneider,
Jr., and K. M. Geib, “High-frequency modulation of oxide-confined
vertical-cavity surface-emitting lasers,” Electron. Lett., vol. 32, pp.
457–458, Feb. 1996.
[7] O. Kjebon, R. Schatz, S. Lourdudoss, S. Nilsson, B. Stalnacke, and L.
Backbom, “30-GHz direct modulation bandwidth in detuned loaded InGaAsP DBR lasers at 1.55-m wavelength,” Electron. Lett., vol. 33, pp.
488–489, Mar. 1997.
[8] R. Stevens, S. Rapp, R. Schatz, E. Goobar, K. Strubel, and A.
Karlsson, “High-speed modulation characteristics of long-wavelength
vertical-cavity lasers based on an integrated InP Bragg reflector,” in
ECOC’98 Proc., Madrid, Spain, 1998.
[9] A. K. Dutta, H. Kosaka, K. Kurihara, Y. Sugimoto, and K. Kasahara,
“High-speed VCSEL of modulation bandwidth over 7.0 GHz and its
application to 100-m PCF datalink,” J. Lightwave Technol., vol. 16, pp.
870–875, May 1998.
[10] K. A. Black, P. Abraham, A. Keating, Y.-J. Chiu, E. L. Hu, and J. E.
Bowers, “Long-wavelength vertical-cavity lasers,” in IPRM’99, Davos,
Switzerland, 1999.
[11] J. Piprek, K. Takiguchi, A. Black, P. Abraham, A. Keating, V. Kaman,
S. Zhang, and J. E. Bowers, “Analog modulation of 1.55-m verticalcavity lasers,” SPIE Proc., vol. 3672, 1999.
[12] J. E. Bowers, “High-speed semiconductor laser design and performance,” Solid-State Electron., vol. 30, pp. 1–11, Jan. 1987.