IEEE PHOTONICS TECHNOLOGY LETTERS 1 Error-Free...
6
IEEE Proof IEEE PHOTONICS TECHNOLOGY LETTERS 1 Error-Free Transmission Over 1-km OM4 Multimode Fiber at 25 Gb/s Using a Single Mode Photonic Crystal Vertical-Cavity Surface-Emitting Laser Meng Peun Tan, Member, IEEE, Stewart Thomas M. Fryslie, Student Member, IEEE , James A. Lott, Senior Member, IEEE , Nikolay N. Ledentsov, Dieter Bimberg, Fellow, IEEE , and Kent D. Choquette, Fellow, IEEE Abstract— With the separation of optical and current aper- 1 tures, photonic crystal vertical-cavity surface-emitting lasers can 2 reach a 3-dB small-signal modulation bandwidth of >18 GHz 3 while lasing in the fundamental mode. Because of reduced 4 chromatic dispersion, such devices enable error-free transmission 5 over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and 6 operating at a current density of 5.4 kA/cm 2 . This can potentially 7 lead to a laser source that is useful for rack-to-rack transmissions 8 in large data centers and potentially long device lifetime. 9 Index Terms—Direct modulation, multimode fiber, photonic 10 crystal, vertical-cavity surface-emitting lasers (VCSELs). 11 I. I NTRODUCTION 12 V ERTICAL-CAVITY surface-emitting lasers (VCSELs) 13 possess characteristics such as low power consumption, 14 circular beam output for efficient laser-fiber coupling, single 15 longitudinal mode lasing, and scalability in two-dimensional 16 arrays. Therefore they are suitable to be used in combination 17 with multimode fibers as the low-cost solutions for short- 18 haul optical data communication [1]. To satisfy the need 19 for rack-to-rack communications in large data centers which 20 require error-free transmission over long-distance fiber links 21 at high data rate, it is crucial to employ VCSELs with 22 single transverse mode or narrow spectral width for reduced 23 modal and chromatic dispersion so that signal integrity can be 24 maintained [2]. For conventional oxide-confined VCSELs [3], 25 the common practice is to reduce the oxide aperture diameter 26 so that the higher order modes are cut off. Such approach has 27 Manuscript received April 25, 2013; revised June 10, 2013; accepted July 3, 2013. Date of publication July 30, 2013. M. P. Tan, S. T. M. Fryslie, and K. D. Choquette are with the Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801 USA (e-mail: [email protected]; [email protected]; [email protected]). J. A. Lott was with VI Systems GmbH, D-10623 Berlin, Germany. He is now with the Institut für Festkörperphysik und Zentrum für Nanopho- tonik, Technische Universität Berlin, Berlin D-10632, Germany (e-mail: [email protected]). N. N. Ledentsov is with VI Systems GmbH, Berlin D-10623, Germany (e-mail: [email protected]). D. Bimberg is with the Institut für Festkörperphysik und Zentrum für Nanophotonik, Technische Universität Berlin, Berlin D-10632, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2275351 enabled error-free transmission at 17 Gb/s over 1-km OM3 28 MMF [4] and 25 Gb/s over 600-m OM3+ MMF [5]. For 29 the conventional oxide-confined VCSEL, the oxide aperture 30 in the mirror is also used for current confinement, so reduced 31 current aperture size implies increased operating current den- 32 sity, which is detrimental to device lifetime [6]. Moreover, 33 small aperture VCSELs exhibit series resistances in excess of 34 the maximum values allowed by standard drivers, and at very 35 small aperture sizes, a small composition nonuniformity of 36 the oxide layer across the wafer will reduce the yield of the 37 VCSELs with uniform parameters. Transverse mode control 38 such as a shallow surface relief can also reduce the number 39 of transverse modes and such VCSELs have demonstrated 40 25 Gb/s error-free transmission over 500-km OM3+ fiber [7], 41 though the current density exceeds the 10-kA/cm 2 industrial 42 benchmark for high-reliability operation [6]. 43 In this letter, photonic crystal VCSELs with separate optical 44 and current apertures can operate in the fundamental mode 45 and reach a maximum 3-dB bandwidth of > 18 GHz at a low 46 current density of ∼5 kA/cm 2 . Large signal modulation at 47 high data rate can still give open eyes and narrow spectral 48 width. Owing to these characteristics, error-free transmission 49 over 1-km OM4 MMF at a data rate of 25 Gb/s is obtained 50 with the VCSEL biased at 5.4 kA/cm 2 , which is in the regime 51 of high-reliability operation [6], and is consistent with prior 52 photonic crystal VCSEL lifetime testing [8]. 53 II. DEVICE DESIGN &FABRICATION 54 In this letter, the photonic crystal design and optical aperture 55 size determine the modal properties of the VCSELs [9], 56 whereas proton implantation is employed for current con- 57 finement; more details are provided in Ref. [10]. Note the 58 subthreshold spontaneous emission is observed to be within 59 the expected diameter of the implant aperture, which in turn 60 is smaller than the lasing optical mode within the photonic 61 crystal defect [10]. The photonic crystal design parameters 62 determined by photolithography are hole pitch a and hole 63 diameter b. With a fixed optical aperture size, the implant aper- 64 ture size can be determined independently but considerations 65 have to be made to reduce diffusion capacitance [9]. To ensure 66 low series resistance, the top contact of the VCSEL should 67 1041-1135 © 2013 IEEE
Transcript of IEEE PHOTONICS TECHNOLOGY LETTERS 1 Error-Free...
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Proof
IEEE PHOTONICS TECHNOLOGY LETTERS 1
Error-Free Transmission Over 1-kmOM4 Multimode Fiber at 25 Gb/s
Using a Single Mode Photonic CrystalVertical-Cavity Surface-Emitting Laser
Meng Peun Tan, Member, IEEE, Stewart Thomas M. Fryslie, Student Member, IEEE,James A. Lott, Senior Member, IEEE, Nikolay N. Ledentsov, Dieter Bimberg, Fellow, IEEE,
and Kent D. Choquette, Fellow, IEEE
Abstract— With the separation of optical and current aper-1
tures, photonic crystal vertical-cavity surface-emitting lasers can2
reach a 3-dB small-signal modulation bandwidth of >18 GHz3
while lasing in the fundamental mode. Because of reduced4
chromatic dispersion, such devices enable error-free transmission5
over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and6
operating at a current density of 5.4 kA/cm2. This can potentially7
lead to a laser source that is useful for rack-to-rack transmissions8
in large data centers and potentially long device lifetime.9
Index Terms— Direct modulation, multimode fiber, photonic10
crystal, vertical-cavity surface-emitting lasers (VCSELs).11
I. INTRODUCTION12
VERTICAL-CAVITY surface-emitting lasers (VCSELs)13
possess characteristics such as low power consumption,14
circular beam output for efficient laser-fiber coupling, single15
longitudinal mode lasing, and scalability in two-dimensional16
arrays. Therefore they are suitable to be used in combination17
with multimode fibers as the low-cost solutions for short-18
haul optical data communication [1]. To satisfy the need19
for rack-to-rack communications in large data centers which20
require error-free transmission over long-distance fiber links21
at high data rate, it is crucial to employ VCSELs with22
single transverse mode or narrow spectral width for reduced23
modal and chromatic dispersion so that signal integrity can be24
maintained [2]. For conventional oxide-confined VCSELs [3],25
the common practice is to reduce the oxide aperture diameter26
so that the higher order modes are cut off. Such approach has27
Manuscript received April 25, 2013; revised June 10, 2013; accepted July 3,2013. Date of publication July 30, 2013.
M. P. Tan, S. T. M. Fryslie, and K. D. Choquette are with the Departmentof Electrical and Computer Engineering, University of Illinois, Urbana,IL 61801 USA (e-mail: [email protected]; [email protected];[email protected]).
J. A. Lott was with VI Systems GmbH, D-10623 Berlin, Germany. He isnow with the Institut für Festkörperphysik und Zentrum für Nanopho-tonik, Technische Universität Berlin, Berlin D-10632, Germany (e-mail:[email protected]).
N. N. Ledentsov is with VI Systems GmbH, Berlin D-10623, Germany(e-mail: [email protected]).
D. Bimberg is with the Institut für Festkörperphysik und Zentrum fürNanophotonik, Technische Universität Berlin, Berlin D-10632, Germany(e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2013.2275351
enabled error-free transmission at 17 Gb/s over 1-km OM3 28
MMF [4] and 25 Gb/s over 600-m OM3+ MMF [5]. For 29
the conventional oxide-confined VCSEL, the oxide aperture 30
in the mirror is also used for current confinement, so reduced 31
current aperture size implies increased operating current den- 32
sity, which is detrimental to device lifetime [6]. Moreover, 33
small aperture VCSELs exhibit series resistances in excess of 34
the maximum values allowed by standard drivers, and at very 35
small aperture sizes, a small composition nonuniformity of 36
the oxide layer across the wafer will reduce the yield of the 37
VCSELs with uniform parameters. Transverse mode control 38
such as a shallow surface relief can also reduce the number 39
of transverse modes and such VCSELs have demonstrated 40
25 Gb/s error-free transmission over 500-km OM3+ fiber [7], 41
though the current density exceeds the 10-kA/cm2 industrial 42
benchmark for high-reliability operation [6]. 43
In this letter, photonic crystal VCSELs with separate optical 44
and current apertures can operate in the fundamental mode 45
and reach a maximum 3-dB bandwidth of >18 GHz at a low 46
current density of ∼5 kA/cm2. Large signal modulation at 47
high data rate can still give open eyes and narrow spectral 48
width. Owing to these characteristics, error-free transmission 49
over 1-km OM4 MMF at a data rate of 25 Gb/s is obtained 50
with the VCSEL biased at 5.4 kA/cm2, which is in the regime 51
of high-reliability operation [6], and is consistent with prior 52
photonic crystal VCSEL lifetime testing [8]. 53
II. DEVICE DESIGN & FABRICATION 54
In this letter, the photonic crystal design and optical aperture 55
size determine the modal properties of the VCSELs [9], 56
whereas proton implantation is employed for current con- 57
finement; more details are provided in Ref. [10]. Note the 58
subthreshold spontaneous emission is observed to be within 59
the expected diameter of the implant aperture, which in turn 60
is smaller than the lasing optical mode within the photonic 61
crystal defect [10]. The photonic crystal design parameters 62
determined by photolithography are hole pitch a and hole 63
diameter b. With a fixed optical aperture size, the implant aper- 64
ture size can be determined independently but considerations 65
have to be made to reduce diffusion capacitance [9]. To ensure 66
low series resistance, the top contact of the VCSEL should 67
1041-1135 © 2013 IEEE
IEEE
Proof
2 IEEE PHOTONICS TECHNOLOGY LETTERS
Fig. 1. Top-view images of photonic crystal VCSELs (left) sketch showingsome holes covered by contact (hatched region); and (right) scanning electronmicroscope image (Color online).
overlap the implant aperture [11]. By allowing the contact68
metal to be deposited in the air holes (see Fig. 1), the current69
aperture, optical aperture, and contact opening dimensions can70
all be separately optimized to simultaneously enable single71
mode emission and large current aperture for low operating72
current density.73
The VCSELs contain an active region with InGaAs quantum74
wells for enhanced differential gain [12]. Device fabrication75
begins with the photolithographic patterning and etch of the76
mesa and photonic crystal patterns to a SiO2 layer. The def-77
inition of implantation masks aligned to the photonic crystal78
patterns and proton implantation at 330 keV follows. Next79
the photonic crystal is etched approximately 70–90% through80
the top distributed Bragg reflector mirror [10]. Due to aspect81
ratio scaling of dry etching [13], the mesas are etched into82
the bottom mirror but the photonic crystal air holes etched83
simultaneously do not penetrate the active region so that exces-84
sive heating due to nonradiative surface recombination can be85
prevented. Subsequently, lower n- and upper p-contacts are86
deposited. Finally the devices are planarized with polyimide87
and coplanar ground-signal-ground contacts are deposited as88
shown in Fig. 1. Note that both optical and current apertures89
are defined with conventional photolithography, hence the90
VCSELs are manufacturable.91
III. EXPERIMENTAL RESULTS92
Fig. 2(a) shows the light-current-voltage characteristic of93
a photonic crystal VCSEL with b/a = 0.6 and a = 3 µm.94
Implanted VCSELs (without photonic crystal pattern) with95
the same nominal implant aperture size (11.8 µm) exhibit96
higher threshold current and discontinuity in the light output97
curve. This is due the lack of strong and stable index guiding98
and results in high diffraction loss [10]. The implant-only99
VCSELs are also highly multimode which results in broad100
RMS spectral width of >0.3 nm (calculated according to the101
IEEE 802.3 Standard). Using the photonic crystal for index102
confinement, as seen in Fig. 2(a) the threshold current is 1 mA,103
although the series resistance is higher [10]. This photonic104
crystal VCSEL operates in the fundamental Gaussian mode105
with a side-mode suppression ratio (SMSR) of more >40 dB106
as seen in Fig. 2(b); measuring several VCSELs gave SMSR107
varying from 30–50 dB. The calculated RMS spectral width108
of the single mode photonic crystal VCSEL is about an order109
of magnitude less than the implant-only VCSEL.110
Small signal modulation responses with various DC bias111
current to a different single mode photonic crystal VCSEL112
with the same design are shown in Fig. 3 (bias currents are113
(a)
(b)
Fig. 2. (a) LIV characteristic of a photonic crystal VCSEL with b/a = 0.6and a = 3 µm; (b) Lasing spectra of the VCSEL taken at 6.9 mA under DCbias (grey) and 25-Gb/s modulation (red), showing side-mode suppressionratio >40 dB and narrow spectral width for both cases (Color online).
Fig. 3. Small signal modulation responses with various DC bias current toa single mode photonic crystal VCSEL (b/a = 0.6, a = 0.3 µm). Maximum3-dB bandwidth is obtained at 5.9 mA (Color online).
2x, 3x,…, 6x threshold current). The maximum achieved 3-dB 114
bandwidth is 18.3 GHz obtained at an injection current of 115
roughly 6 times the threshold current. 116
Large signal modulation is also performed on the photonic 117
cyrstal VCSELs using a nonreturn to zero data pattern with 118
a 27–1 pseudorandom binary sequence. Bit error ratio (BER) 119
plots are generated when the photonic crystal VCSELs of the 120
same design are modulated at 25 Gb/s. Fig. 4(a) indicates that 121
operating with a DC bias of 6.9 mA under room temperature 122
and a Vpp of 0.6 V, a small power penalty of 1.3 dBm is 123
incurred when the transmission is extended from back-to-back 124
(BTB) to 500-m OM3+ MMF using the device in Fig. 2(b). 125
IEEE
Proof
TAN et al.: SINGLE MODE PHOTONIC CRYSTAL VCSEL 3
(a)
(b)
(c)
Fig. 4. (a) BER curves of a photonic crystal VCSEL (b/a = 0.6, a = 3 µm)biased at 6.9 mA under room temperature and modulated at 25 Gb/s withVpp = 0.6 V, indicating a power penalty of 1.3 dBm before and aftertransmitting through a 500-m OM3+ MMF; (b) Eye diagram of the modulatedoutput after transmitting through a 500-m OM3+ MMF indicating open eyes;(c) BER after transmission through 1-km OM4 MMF of a photonic crystalVCSEL with the same design biased at 5.9 mA (corresponding to operatingcurrent density of 5.4 kA/cm2) under room temperature and modulated at25 Gb/s with Vpp = 0.5 V (Color online).
In Fig. 4(b), the eye diagram after transmitting through 500-m126
OM3+ MMF modulated under the aforementioned conditions127
is displayed, showing open eyes. In addition, the VCSEL128
maintains a high SMSR of 40 dB and low spectral width under129
such high-speed modulation indicating low chirp, as evident130
in Fig. 2(b).131
The BER plot is also generated when a VCSEL of the132
same design as Fig. 2 is biased at 5.9 mA under room133
temperature with Vpp of 0.5 V. As evident in Fig. 4(c), error-134
free transmission (BER <10−12) over 1 km of OM4 MMF135
at a data rate of 25 Gb/s is achieved at an operating current 136
density of 5.4 kA/cm2, the latter which is half the industrial 137
benchmark for long lifetime operation [6]. 138
IV. CONCLUSION 139
In conclusion, we show that the separation of optical and 140
current apertures enables single mode emission, low operating 141
current density, and high modulation bandwidth simultane- 142
ously in photonic crystal implanted VCSELs. At the high data 143
rate of 25 Gb/s, the photonic crystal VCSELs maintain single 144
mode lasing and narrow spectral width, allowing error-free 145
transmission over 1-km OM4 multimode fiber operating at a 146
low current density of 5.4 kA/cm2. These devices could poten- 147
tially have long device lifetime and find application in rack- 148
to-rack data communications in data centers. Further design 149
optimization to allow greater output power may enable error- 150
free transmission over longer fiber links at higher data rates. 151
REFERENCES 152
[1] P. Pepeljugoski, et al., “Development of system specification for laser 153
optimized 50 µm multimode fiber for multigigabit short-wavelength 154
LANs,” J. Lightw. Technol., vol. 21, no. 5, pp. 1256–1275, May 2003. 155
[2] R. E. Freund, C.-A. Bunge, N. N. Ledentsov, D. Molin, and C. Caspar, 156
“High-speed transmission in multimode fibers,” J. Lightw. Technol., 157
vol. 28, no. 4, pp. 569–586, Feb. 15, 2010. 158
[3] K. D. Choquette, R. P. Schneider, Jr., K. Lear, and K. M. Geib, 159
“Low threshold voltage vertical-cavity lasers fabricated by selective 160
oxidation,” Electron. Lett., vol. 30, no. 24, pp. 2043–2044, Nov. 1994. 161
[4] P. Moser, et al., “99 fJ/(bit·km) energy to data-distance ratio at 17 Gb/s 162
across 1 km of multimode fiber with 850 nm single-mode VCSELs,” 163
IEEE Photon. Technol. Lett., vol. 24, no. 1, pp. 19–21, Jan. 1, 2012. 164
[5] P. Moser, et al., “Energy efficiency of directly modulated oxide-confined 165
high bit rate 850 nm VCSELs for optical interconnects,” IEEE J. Sel. 166
Topics Quantum Electron., vol. 19, no. 4, p. 1702212, Jul./Aug. 2013. 167
[6] B. M. Hawkins, R. A. Hawthorne, J. K. Guenter, J. A. Tatum, 168
and J. R. Biard, “Reliability of various size oxide aperture 169
VCSELs,” in Proc. 52nd Conf. Electron. Components Technol., May 170
2002, pp. 540–550. 171
[7] E. Haglund, Å. Haglund, P. Westbergh, J. S. Gustavsson, B. Kögel, and 172
A. Larsson, “25 Gb/s transmission over 500 m multimode fibre using 173
850 nm VCSEL with integrated mode filter,” Electron. Lett., vol. 48, 174
no. 9, pp. 517–519, Apr. 2012. 175
[8] A. M. Kasten, J. D. Sulkin, P. O. Leisher, D. K. McElfresh, D. Vacar, 176
and K. D. Choquette, “Manufacturable photonic crystal single mode 177
and fluidic vertical cavity surface emitting lasers,” IEEE J. Sel. Topics 178
Quantum Electron., vol. 14, no. 4, pp. 1123–1131, Jul./Aug. 2008. 179
[9] P. O. Leisher, J. D. Sulkin, and K. D. Choquette, “Parametric study 180
of proton-implanted photonic crystal vertical-cavity surface-emitting 181
lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 13, no. 5, 182
pp. 1290–1294, Sep./Oct. 2007. 183
[10] M. Tan, A. M. Kasten, J. D. Sulkin, and K. D. Choquette, “Planar 184
photonic crystal vertical cavity surface emitting lasers,” IEEE J. Sel. 185
Topics Quantum Electron., vol. 19, no. 4, p. 4900107, Jul./Aug. 2013. 186
[11] K. L. Lear, S. Kilcoyne, and S. Chalmers, “High power conversion 187
efficiencies and scaling for multimode vertical-cavity surface-emitting 188
lasers,” IEEE Photon. Technol. Lett., vol. 6, no. 7, pp. 778–780, 189
Jul. 1994. 190
[12] P. Westbergh, J. S. Gustavsson, Å. Haglund, M. Sköld, A. Joel, 191
and A. Larsson, “High-speed, low-current-density 850 nm VCSELs,” 192
IEEE J. Sel. Topics Quantum Electron., vol. 15, no. 3, pp. 694–703, 193
May/Jun. 2009. 194
[13] R. Gottscho and C. Jurgensen, “Microscopic uniformity in plasma 195
etching,” J. Vac. Sci. Technol. B, vol. 10, no. 5, pp. 2133–2143, 196
Sep./Oct. 1992. 197
IEEE
Proof
IEEE PHOTONICS TECHNOLOGY LETTERS 1
Error-Free Transmission Over 1-kmOM4 Multimode Fiber at 25 Gb/s
Using a Single Mode Photonic CrystalVertical-Cavity Surface-Emitting Laser
Meng Peun Tan, Member, IEEE, Stewart Thomas M. Fryslie, Student Member, IEEE,James A. Lott, Senior Member, IEEE, Nikolay N. Ledentsov, Dieter Bimberg, Fellow, IEEE,
and Kent D. Choquette, Fellow, IEEE
Abstract— With the separation of optical and current aper-1
tures, photonic crystal vertical-cavity surface-emitting lasers can2
reach a 3-dB small-signal modulation bandwidth of >18 GHz3
while lasing in the fundamental mode. Because of reduced4
chromatic dispersion, such devices enable error-free transmission5
over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and6
operating at a current density of 5.4 kA/cm2. This can potentially7
lead to a laser source that is useful for rack-to-rack transmissions8
in large data centers and potentially long device lifetime.9
Index Terms— Direct modulation, multimode fiber, photonic10
crystal, vertical-cavity surface-emitting lasers (VCSELs).11
I. INTRODUCTION12
VERTICAL-CAVITY surface-emitting lasers (VCSELs)13
possess characteristics such as low power consumption,14
circular beam output for efficient laser-fiber coupling, single15
longitudinal mode lasing, and scalability in two-dimensional16
arrays. Therefore they are suitable to be used in combination17
with multimode fibers as the low-cost solutions for short-18
haul optical data communication [1]. To satisfy the need19
for rack-to-rack communications in large data centers which20
require error-free transmission over long-distance fiber links21
at high data rate, it is crucial to employ VCSELs with22
single transverse mode or narrow spectral width for reduced23
modal and chromatic dispersion so that signal integrity can be24
maintained [2]. For conventional oxide-confined VCSELs [3],25
the common practice is to reduce the oxide aperture diameter26
so that the higher order modes are cut off. Such approach has27
Manuscript received April 25, 2013; revised June 10, 2013; accepted July 3,2013. Date of publication July 30, 2013.
M. P. Tan, S. T. M. Fryslie, and K. D. Choquette are with the Departmentof Electrical and Computer Engineering, University of Illinois, Urbana,IL 61801 USA (e-mail: [email protected]; [email protected];[email protected]).
J. A. Lott was with VI Systems GmbH, D-10623 Berlin, Germany. He isnow with the Institut für Festkörperphysik und Zentrum für Nanopho-tonik, Technische Universität Berlin, Berlin D-10632, Germany (e-mail:[email protected]).
N. N. Ledentsov is with VI Systems GmbH, Berlin D-10623, Germany(e-mail: [email protected]).
D. Bimberg is with the Institut für Festkörperphysik und Zentrum fürNanophotonik, Technische Universität Berlin, Berlin D-10632, Germany(e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2013.2275351
enabled error-free transmission at 17 Gb/s over 1-km OM3 28
MMF [4] and 25 Gb/s over 600-m OM3+ MMF [5]. For 29
the conventional oxide-confined VCSEL, the oxide aperture 30
in the mirror is also used for current confinement, so reduced 31
current aperture size implies increased operating current den- 32
sity, which is detrimental to device lifetime [6]. Moreover, 33
small aperture VCSELs exhibit series resistances in excess of 34
the maximum values allowed by standard drivers, and at very 35
small aperture sizes, a small composition nonuniformity of 36
the oxide layer across the wafer will reduce the yield of the 37
VCSELs with uniform parameters. Transverse mode control 38
such as a shallow surface relief can also reduce the number 39
of transverse modes and such VCSELs have demonstrated 40
25 Gb/s error-free transmission over 500-km OM3+ fiber [7], 41
though the current density exceeds the 10-kA/cm2 industrial 42
benchmark for high-reliability operation [6]. 43
In this letter, photonic crystal VCSELs with separate optical 44
and current apertures can operate in the fundamental mode 45
and reach a maximum 3-dB bandwidth of >18 GHz at a low 46
current density of ∼5 kA/cm2. Large signal modulation at 47
high data rate can still give open eyes and narrow spectral 48
width. Owing to these characteristics, error-free transmission 49
over 1-km OM4 MMF at a data rate of 25 Gb/s is obtained 50
with the VCSEL biased at 5.4 kA/cm2, which is in the regime 51
of high-reliability operation [6], and is consistent with prior 52
photonic crystal VCSEL lifetime testing [8]. 53
II. DEVICE DESIGN & FABRICATION 54
In this letter, the photonic crystal design and optical aperture 55
size determine the modal properties of the VCSELs [9], 56
whereas proton implantation is employed for current con- 57
finement; more details are provided in Ref. [10]. Note the 58
subthreshold spontaneous emission is observed to be within 59
the expected diameter of the implant aperture, which in turn 60
is smaller than the lasing optical mode within the photonic 61
crystal defect [10]. The photonic crystal design parameters 62
determined by photolithography are hole pitch a and hole 63
diameter b. With a fixed optical aperture size, the implant aper- 64
ture size can be determined independently but considerations 65
have to be made to reduce diffusion capacitance [9]. To ensure 66
low series resistance, the top contact of the VCSEL should 67
1041-1135 © 2013 IEEE
IEEE
Proof
2 IEEE PHOTONICS TECHNOLOGY LETTERS
Fig. 1. Top-view images of photonic crystal VCSELs (left) sketch showingsome holes covered by contact (hatched region); and (right) scanning electronmicroscope image (Color online).
overlap the implant aperture [11]. By allowing the contact68
metal to be deposited in the air holes (see Fig. 1), the current69
aperture, optical aperture, and contact opening dimensions can70
all be separately optimized to simultaneously enable single71
mode emission and large current aperture for low operating72
current density.73
The VCSELs contain an active region with InGaAs quantum74
wells for enhanced differential gain [12]. Device fabrication75
begins with the photolithographic patterning and etch of the76
mesa and photonic crystal patterns to a SiO2 layer. The def-77
inition of implantation masks aligned to the photonic crystal78
patterns and proton implantation at 330 keV follows. Next79
the photonic crystal is etched approximately 70–90% through80
the top distributed Bragg reflector mirror [10]. Due to aspect81
ratio scaling of dry etching [13], the mesas are etched into82
the bottom mirror but the photonic crystal air holes etched83
simultaneously do not penetrate the active region so that exces-84
sive heating due to nonradiative surface recombination can be85
prevented. Subsequently, lower n- and upper p-contacts are86
deposited. Finally the devices are planarized with polyimide87
and coplanar ground-signal-ground contacts are deposited as88
shown in Fig. 1. Note that both optical and current apertures89
are defined with conventional photolithography, hence the90
VCSELs are manufacturable.91
III. EXPERIMENTAL RESULTS92
Fig. 2(a) shows the light-current-voltage characteristic of93
a photonic crystal VCSEL with b/a = 0.6 and a = 3 µm.94
Implanted VCSELs (without photonic crystal pattern) with95
the same nominal implant aperture size (11.8 µm) exhibit96
higher threshold current and discontinuity in the light output97
curve. This is due the lack of strong and stable index guiding98
and results in high diffraction loss [10]. The implant-only99
VCSELs are also highly multimode which results in broad100
RMS spectral width of >0.3 nm (calculated according to the101
IEEE 802.3 Standard). Using the photonic crystal for index102
confinement, as seen in Fig. 2(a) the threshold current is 1 mA,103
although the series resistance is higher [10]. This photonic104
crystal VCSEL operates in the fundamental Gaussian mode105
with a side-mode suppression ratio (SMSR) of more >40 dB106
as seen in Fig. 2(b); measuring several VCSELs gave SMSR107
varying from 30–50 dB. The calculated RMS spectral width108
of the single mode photonic crystal VCSEL is about an order109
of magnitude less than the implant-only VCSEL.110
Small signal modulation responses with various DC bias111
current to a different single mode photonic crystal VCSEL112
with the same design are shown in Fig. 3 (bias currents are113
(a)
(b)
Fig. 2. (a) LIV characteristic of a photonic crystal VCSEL with b/a = 0.6and a = 3 µm; (b) Lasing spectra of the VCSEL taken at 6.9 mA under DCbias (grey) and 25-Gb/s modulation (red), showing side-mode suppressionratio >40 dB and narrow spectral width for both cases (Color online).
Fig. 3. Small signal modulation responses with various DC bias current toa single mode photonic crystal VCSEL (b/a = 0.6, a = 0.3 µm). Maximum3-dB bandwidth is obtained at 5.9 mA (Color online).
2x, 3x,…, 6x threshold current). The maximum achieved 3-dB 114
bandwidth is 18.3 GHz obtained at an injection current of 115
roughly 6 times the threshold current. 116
Large signal modulation is also performed on the photonic 117
cyrstal VCSELs using a nonreturn to zero data pattern with 118
a 27–1 pseudorandom binary sequence. Bit error ratio (BER) 119
plots are generated when the photonic crystal VCSELs of the 120
same design are modulated at 25 Gb/s. Fig. 4(a) indicates that 121
operating with a DC bias of 6.9 mA under room temperature 122
and a Vpp of 0.6 V, a small power penalty of 1.3 dBm is 123
incurred when the transmission is extended from back-to-back 124
(BTB) to 500-m OM3+ MMF using the device in Fig. 2(b). 125
IEEE
Proof
TAN et al.: SINGLE MODE PHOTONIC CRYSTAL VCSEL 3
(a)
(b)
(c)
Fig. 4. (a) BER curves of a photonic crystal VCSEL (b/a = 0.6, a = 3 µm)biased at 6.9 mA under room temperature and modulated at 25 Gb/s withVpp = 0.6 V, indicating a power penalty of 1.3 dBm before and aftertransmitting through a 500-m OM3+ MMF; (b) Eye diagram of the modulatedoutput after transmitting through a 500-m OM3+ MMF indicating open eyes;(c) BER after transmission through 1-km OM4 MMF of a photonic crystalVCSEL with the same design biased at 5.9 mA (corresponding to operatingcurrent density of 5.4 kA/cm2) under room temperature and modulated at25 Gb/s with Vpp = 0.5 V (Color online).
In Fig. 4(b), the eye diagram after transmitting through 500-m126
OM3+ MMF modulated under the aforementioned conditions127
is displayed, showing open eyes. In addition, the VCSEL128
maintains a high SMSR of 40 dB and low spectral width under129
such high-speed modulation indicating low chirp, as evident130
in Fig. 2(b).131
The BER plot is also generated when a VCSEL of the132
same design as Fig. 2 is biased at 5.9 mA under room133
temperature with Vpp of 0.5 V. As evident in Fig. 4(c), error-134
free transmission (BER <10−12) over 1 km of OM4 MMF135
at a data rate of 25 Gb/s is achieved at an operating current 136
density of 5.4 kA/cm2, the latter which is half the industrial 137
benchmark for long lifetime operation [6]. 138
IV. CONCLUSION 139
In conclusion, we show that the separation of optical and 140
current apertures enables single mode emission, low operating 141
current density, and high modulation bandwidth simultane- 142
ously in photonic crystal implanted VCSELs. At the high data 143
rate of 25 Gb/s, the photonic crystal VCSELs maintain single 144
mode lasing and narrow spectral width, allowing error-free 145
transmission over 1-km OM4 multimode fiber operating at a 146
low current density of 5.4 kA/cm2. These devices could poten- 147
tially have long device lifetime and find application in rack- 148
to-rack data communications in data centers. Further design 149
optimization to allow greater output power may enable error- 150
free transmission over longer fiber links at higher data rates. 151
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