Equivalent circuit design and modeling

We established an equivalent circuit based on VCSEL structure to investigate the limitation of the modulation response caused by parasitic effect. Figure 5.8 (a) shows an equivalent circuit in the VCSEL structure and Figure 5.8 (b) shows an equivalent circuit for AC analysis, which neglects the ideal diode appeared in the equivalent circuit of the VCSEL structure. The measured data is obtained for the simulation and the fitting of the

parameters from the RF measurement system described in chapter 4. We used the build in function of the Agilent IC-CAP 2002 software [8] to simulate and fit the measured data for extracting the value of each component in equivalent circuit. Figure 5.9 shows a couple results after best fitting the simulated data with the measured data, which is reflection coefficient (S11) of the equivalent circuit. The best fitting can be obtained after performing the optimization function of ICCAP 2002. The value of the components can be extracted in the equivalent circuit after getting the best fitting. Table 5.4 lists the extracted values of the components for the oxide-confined VCSEL with different current bias. Table 5.5 lists the extracted values of the components for oxide-implant VCSEL with different current bias. We found that the parasitic capacitance (Cp) of the oxide-confined VCSEL can be reduced effectively after performing the proton implant process. The amount of the capacitance is reduced from 1.904 pF of the oxide-confined VCSEL to 0.36pF of the oxide-implant VCSEL. This result demonstrates that the modulation response can be improved by confining current flow, but the resistance of the DBR mirrors (Rm) increases due to the diameter reduction of the oxide aperture. This is a trade-off that needs to be optimized between the parasitic resistance (Cp) and the modulation bandwidth. In our experiment, we demonstrated the oxide-implant VCSEL is better than the oxide-confined VCSEL for modulation bandwidth.

Parameter

Oxide-confined 2.3 91 0.337

Oxide-implant 1.4 108 0. 484

Table 5.1 The static characteristics of the oxide-confined VCSEL and the oxide-implant VCSEL

Table 5.2 The static parameters for different oxide aperture size

Parameter

Table 5.3 The static parameters for different oxide aperture size

Bias

Table 5.4 The extracted values of the components for the oxide-confined VCSEL with different current bias

Table 5.5 The extracted values of the components for oxide-implant VCSEL with different current bias

Figure 5.1 The VCSEL structure of the oxide-confined and the oxide-implant

(a) Oxide-confined VCSEL (b) Oxide-implant VCSEL Figure 5.2 The L-I-V curves of the oxide-confined and the oxide-implant VCSEL

(a) Oxide-confined (b) Oxide-implant

Figure 5.3 Modulation responses of the oxide-confined and the oxide-implant VCSEL

Figure 5.4 The (I-Ith)^1/2 versus the resonance frequency (f_r) and 3dB frequency (f_3dB) for the oxide-confined and oxide-implant VCSEL

(a) The eye diagram of the oxide-confined VCSEL

(b) The eye diagram of the oxide-implant VCSEL

Figure 5.5 The eye diagrams measurement of oxide-confined and oxide-implant VCSEL

(a) VCSEL of the 6 µm oxide aperture

(a) VCSEL of the 7 µm oxide aperture

(a) VCSEL of the 8 µm oxide aperture

Figure 5.6 the LIV curve and small signal modulation response of the 6, 7, and 8µm oxide aperture VCSEL

Figure 5.7 The resonance frequency and the 3dB frequency of the 6, 7, and 8µm oxide aperture VCSEL.

Circuit component definition Rm: mirror resistance Ra: active region resistance Ca: active region capacitance Rp: shunt resistance under pad Cp: under pad capacitance L : bonding wire

(a) An equivalent circuit in the VCSEL

structure.

(b) An equivalent circuit for AC analysis

Figure 5.8 The equivalent circuit of VCSEL

(a) The fitting results of the real and the imaginary part

(b) The fitting results of the magnitude

(c) The fitting results of the Smith Chart.

Figure 5.9 The fitting results of the oxide-confined VCSEL, which works at the bias currtnt of 4mA

Reference

[1] 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. Letter, pp.457-458, February 1996.

[2] C. Carlsson, H. Martinsson, R. Schatz, J. Halonen, and A Larsson, “Analog modulation properties of oxide confined VCSELs at microwave frequencies,” IEEE Journal of Lightwave Technology, vol.20, pp.1740-1749, September 2002.

[3] C. H. Chang, L. Chrostowski, and J. Chang-Hasnain, “Parasitics and design considerations on oxide-implant VCSELs,” IEEE Photonics Technology Letters, vol.13, December 2001.

[4] A. K. Dutta Dutta, H. Kosaka, K. Kurihara, Y. Sugimasa, and K. Kasahara,

"High-speed VCSEL of modulation bandwidth over 7.0 GHz and its application to 100 m PCF datalink," IEEE Journal of Lightwave Technology, vol.16, pp.870-8755, May 1998.

[5] A. Larsson, C. Carlsson, J. Halonen, and R. Schatz, “Microwave modulation characteristics of 840nm oxide confined / proton implanted VCSELs,” Microwave Photonics technical report, October 2001.

[6] A. Larsson, C. Carlsson, A. Haglund, J. Halonen, and R. Schatz, “Microwave modulation characteristics of BCB-planarized oxide confined 850nm VCSELs,”

Microwave Photonics technical report, June 2002.

[7] A. N. AL-Omari, and K. L. Lear, “Polyimide-planarized vertical cavity surface emitting lasers with 17.0GHz bandwidth,” IEEE Photonics Technology Letter, vol.16, pp.969-971, April 2004.

[8] “Agilent 85190A IC-CAP 2002 User’s Guide”, Agilent Technologies.

Chapter 6 Conclusion

We investigated the high speed performance of the oxide-confined VCSEL in this thesis. We found the parasitic capacitance of the oxide-confined VCSEL can be reduced by employment of the proton implant process. We demonstrated the parasitic capacitance of the oxide-confined VCSEL can be reduced effectively by using an equivalent circuit. The small signal modulation bandwidth of the oxide-confined VCSEL can be improved from 2.3 GHz to 9 GHz after using proton implantation process. The eye diagram of the oxide-confined VCSEL, which operating at bias current of 6mA, at 10Gps bias and 6dB extinction ratio showed a very clean eye with a jitter of less than 20 ps.

To investigate the extrinsic bandwidth limitation of the oxide-confined VCSELs, an equivalent circuit instead of the oxide-confined VCSEL impedance was introduced.

The extrinsic bandwidth can be obtained by combining the bandwidth of the equivalent circuit with the measured data of the probe station. We analyzed the difference of the parasitic components between the oxide-confined and oxide-implant VCSEL. The limitation factor of the modulation bandwidth can be found out through the extraction of each component value in the equivalent circuit by adopting Integrated Circuit Characterization and Analysis Program (IC-CAP). We found the bondpad capacitance of the oxide-confined VCSEL can be reduced from 1.854 pF to 0.277 pF after proton implantation process. This extraction method, the use of the IC-CAP, was proved that is very useful to characterize the high speed performance of VCSELs and this extraction method also can be applied to most diode based optoelectronics devices.