Chapter 4 Result and Discussion
4.3 photonic crystal VCSELs DC characteristics
Oxide VCSEL and oxide photonic crystal VCSEL had same process structure, shown in Figure 4-8. Process structure was described in Chapter 3. Both VCSEL had the same oxide aperture as 12 µm and mesa as 50 µm.
The static characteristics of both VCSEL were shown in Figure 4-9.The threshold current is ~ 0.9 mA (0.5 mA) for oxide (photonic crystal) type VCSELs . The slope efficiency of oxide (photonic crystal) VCSELs ~ 0.239 mW/mA (0.254 mW/mA). The series resistance of oxide ( oxide photonic crystal) VCSELs ~ 145 Ω (191 Ω). The maximal output power exceeds 2.8(1.61) mW at room temperature.
The photonic crystal VCSEL with Λ=5 mm, a/Λ=0.4 exhibits single fundamental mode lasing on the low current range of operation with the side-mode suppression ratio (SMSR) of over 20 dB as shown in Figure 4-10 (a). But as a result of the oxide layer was influence on the index guide of photonic crystal. Therefore the photonic crystal VCSEL becomes multi-mode lasing over the high current range of operation as shown in Figure 4-10 (b). The near field images of the device also shown in the inset of Figure 4-10. Very strong emission can be seen in the central site, where the circular emission pattern is evidence of the PC confined mode as shown in Figure 4-10 (a).
The process structure Implant photonic crystal VCSEL as shown in Figure 4-11.
Process structure was described in Chapter 3. The oxide aperture of Implant photonic crystal VCSEL as 10 µm and mesa as 60 µm
The static characteristics of implant photonic crystal VCSEL were shown in Figure 4-12.The threshold current is ~ 1.25 mA for implant photonic crystal VCSELs .The slope efficiency of implant photonic crystal VCSELs ~ 0.18 W/A.The series resistance of implant photonic crystal VCSELs ~ 1.5K Ω. The maximal output power exceeds 1.24 mW at room temperature.
Lasing spectra of the implant photonic crystal VCSEL is shown in Figure 4-13(a), confirming singlemode operation within overall operation current. The PC-VCSEL reveals a side mode suppression ratio(SMSR) > 40 dB throughout the current range.
For comparison, lasing spectra of a proton-implanted VCSEL without photonic crystal holes hows multiple mode operation as the driving current increased above 4.25 mA Figure 4-13(b). As the driving current increases, even higher-order transverse modes
We present the far-field intensity distribution of implant VCSEL in Figure 4-14(a)~(c), The far-filed image shows a doughnut-liked pattern at injection current of 3.8mA (i.e. 1.1 Ith), which is illustrated in Figure 4-14(a). At higher injection current of 4.9 mA(i.e. 1.3 Ith), some radial structures appear at the periphery of doughnut-liked pattern, which is illustrated in Figure 4-14(b)~(c). It can be seen that multiple transverse modes could be observed when the drive current equals 3.8mA (i.e. 1.1 Ith). On the other hand, the intensity of higher order mode and the beam divergence angle both increase as the drive current is increased to 4.9 mA(i.e. 1.3 Ith).
The far-field intensity distribution of implant photonic crystal VCSEL in Figure 4-14(d)~(f), The far-filed image shows a doughnut-liked pattern at injection current of 2.8mA (i.e. 1.1 Ith), which is illustrated in Figure 4-14(a). At higher injection current of 3.5 mA(i.e. 1.3 Ith), some radial structures appear at the periphery of doughnut-liked pattern, which is illustrated in Figure 4-14(b)~(c). It can be seen that only fundamental modes could be observed when the drive current equals 2.8mA (i.e. 1.1 Ith). On the other hand, the beam divergence angle increases very small as the drive current is increased to 3.5 mA(i.e. 1.3 Ith). Figure 4-15 shows beam divergence angles as functions of VSCELs drive current. It was found that by incorporate with photonic crystal, the divergence angle was decrease. And the divergence angles of single mode VCSEL was not increase with current.
.
0 2 4 6 8 10 12 14
tapered oxide blunt oxide
tapered oxide blunt oxide
Figure 4-2 Typical LIV curve for tapered oxide and blunt oxide VCSELs
Figure 4-1 Tapered oxide and blunt oxide VCSEL structure
Substrate
Oxide layer Implant region Implant region
Tapered oxide layer
2 4 6 8 10 12 14 -15
-10 -5 0 5
Bias current 1.5 mA 2 mA 2.5 mA 3 mA 5 mA 7 mA 9 mA
Frequency response (dB) Frequency (Hz)
2 4 6 8 10 12 14
-10 0 10
Bias current
1.5mA 2.5mA 4mA 6mA 8mA 9mA
Frequency response(dB)
Frequency(GHz)
Figure 4-3 Modulation responses for both tapered oxide and Blunt oxide VCSELs
(a) Tapered oxide VCSEL
0.5 1.0 1.5 2.0 2.5 3.0 oxide and blunt oxide VCSELs
0 10 20 30 40 50 60 70 80 90 100
Damping rate (109 s-1 )
Resonant frequeny2 ( GHz2)
K= 0.15 ns
Figure 4-5 damping rate as a function of square of resonant frequency for tapered oxide and blunt oxide VCSELs
-2 0 2 4 6 8 10 12 14 16 18 20 22
Figure 4-6 Real and imaginary S11 parameter versus frequency from model and measured data (dash line is measured data, solid line is simulated data)
Bl B lu un nt t O Ox x id i de e
Figure 4-7 Eye-diagram for tapered oxide and blunt oxide VCSELs (a) Blunt oxide VCSEL
(b) Oxide-implant VCSEL
T T a a pe p e re r ed d O Ox xi id de e
B B lu l un nt t O Ox xi id de e
Figure 4-8 oxide VCSEL and oxide photonic crystal VCSEL structure
Oxide layer
Substrate n-DBR p-DBR active layer
Oxide layer Photonic crystal region p-contact
Substrate n-DBR p-DBR
active layer
n-contact
Figure 4-9 Typical LIV curve for oxide photonic crystal VCSELs
Figure 4-10 Spectral characteristics and near-field images for the photonic crystal VCSELs
830 835 840 845
0 10 20 30 40 50
dB intensity (a.u)
Wavelength(nm)
1mA
I=1.2mA
(a)
830 835 840 845
0 10 20 30 40 50
dB intensity (a.u)
Wavelength(nm) 1.5 mA
1.75mA
(b)
0 1 2 3 4 5 0
5 10 15 20
Current(mA)
Voltage(V)
Λ=5µm α=2.5µm mesa=60µm loop=3 1 point defect
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Power(mW)
Figure 4-12 Typical LIV curve for implant photonic crystal VCSELs Figure 4-11 Implant photonic crystal VCSEL
Substrate n-DBR p-DBR
active layer p-contact
n-contact
Photonic crystal region
Implant region
Figure 4-13 Spectral characteristics for the implant photonic crystal VCSELs and VCSEL without PC holes (a) PC-VCSEL (b)VCSEL
836 838 840 842 844
-70 -60 -50 -40 -30 -20 -10 0
1.5mA 2.5mA 4mA 5mA
Inte ns ity (dB )
Wavelength(nm) (a)
(b)
1.1 1.2 1.3
8.0 Implant VCSEL
Potonic crytal VCSEL
D ivergen ce an gle(
0)
I/I
thFigure 4-14 Far field patterns of the fimplant VCSELs observed under different drive currents are shown in (a)~(c) and the implant photonic crystal VCSEL are shown in (d)~(f)
Figure 4-15 The divergence angle for implant and photonic crystal VCSELs
0.25
Tapered oxide VCSEL
Table 4-1 Equivalent circuit elements at different bias current for Tapered oxide VCSEL
Chapter 5 Conclusion
We have performed experimental study of small signal modulation behavior of 850nm tapered oxide-implant and blunt oxide-implant VCSELs.
We have found that tapered oxides process gives VCSEL with excellent high speed performance without semi-isolating substrate. The damping rate from the modulation response was found to reduce two times in the tapered oxide VCSEL and therefore enhanced the maximal modulation bandwidth. With same oxide aperture size 5.5 µm, tapered oxide VCSEL shows better modulation bandwidth of 13.2 GHz while blunt oxide VCSEL has 9.5 GHz. A very clean eye was demonstrated from improved VCSEL with rising time of 26 ps, falling time of 40 ps and jitter of less than 20 ps, operating at 10Gb/s with 6mA bias and 6dB extinction ratio. We also build an equivalent circuit model to analyze the bandwidth limitation affected by VCSEL intrinsic impedance. The simulation results could make the modulation limitation clearly and help us to modify the VCSEL process for high speed operation.
In the second part of the thesis, we report a high power (>1 mW) singlemode proton-implanted photonic crystal vertical-cavity surface-emitting laser (PC-VCSEL) with high SMSR (> 40 dB) throughout the whole operation current range. This PC-VCSEL, with an aperture of about 10 µm, has ultra-low threshold current of about 1.25 mA. We analyze the L-I curve, emission spectra, near field pattern, divergence angles of photonic crystal VCSELs fabricated with oxide-confined and implant structure. The present results indicate that a VCSEL using proton implantation for
current confinement and photonic crystal for optical confinement is a reliable approach to achieve high-power singlemode operation of a VCSEL. This concept will be applied to a 1.3µm VCSEL and other commercial applications in the future.
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