Result and Discussion

Fig. 4-6 shows the typical light output and voltage versus current (LIV) curves of the SC-MQWs InGaAsP/InGaP VCSEL at room temperature and 85^oC under CW opteration.

These VCSELs exhibit kink-free current-light output performance with threshold currents

~0.4 mA, and slope efficiencies ~ 0.6 mW/mA. The threshold current change with temperature is less than 0.2 mA and the slope efficiency drops by less than ~30% when the substrate temperature is raised from room temperature to 85^oC. This is superior to the properties of GaAs/AlGaAs VCSELs with similar size [14]. The resistance of our VCSELs is

~95 Ohm and capacitance is ~0.1 pF. As a result, the devices are limited by the parasitics to a frequency response of approximately 15 GHz. The lateral mode characteristics is an important feature since it strongly affects the transmission properties. Fig. 4-7 shows the emission spectrum of a VCSEL at an operating current of 6 mA. This spectrum was recorded with an optical spectrum analyzer (Advantest 8381A) with spectral resolution of 0.1 nm. Two dominant modes were observed at 844.2 nm and 843.7 nm. The root-mean-squared (RMS) spectral linewidths at 2, 6, 8 mA are 0.15, 0.37, and 0.4 nm respectively, which can fulfill the requirement (<= 0.45 nm) of 10Gbps data transmission.[15]

The small signal response of VCSELs as a function of bias current was measured using a calibrated vector network analyzer (Agilent 8720ES) with on-wafer probing and a 50 µm multimode optical fiber connected to a New Focus 25GHz photodetector. Fig. 4-8 shows the measured (dashed lines) and fitted (solid lines) small-signal frequency response of a 5 µm VCSEL at different bias current levels. The modulation frequency is increased with increasing bias current until flattening at a bias of approximately 5mA. With only 3mA (5mA) of bias current, the maximum 3dB modulation frequency response is measured to be ~13 (14.5) GHz at 25 °C and is suitable for 12.5 Gb/s operation. The measured data were fit to a general 3-pole modulation transfer function [16, 17]

where f is modulation frequency, fp is parasitic roll off frequency, fr is resonant frequency and γ is damping rate.

Fig. 4-9 shows the 3dB modulation frequency as a function the square root of the difference in current above threshold. The relaxation resonance frequency is found saturate for driving currents above 3mA. By fitting the lowest current points in Fig. 4-8, we obtain a modulation current efficiency of 11.6 GHz/(mA)^1/2[18]. This is higher than GaAs/AlGaAs VCSELs with similar size [6, 19] and is comparable with oxide confined VCSELs with InGaAs based quantum wells [7, 16]. Plotting the damping rate γ versus fr2 reveals a K-factor of 0.3 ns. Neglecting heating effects and external parasitics, the intrinsic bandwidth was found to be 29.6GHz using the relation fmax = . ^2(2π/K)

To measure the high-speed VCSEL under large signal modulation, microwave and

lightwave probes were used in conjunction with a 12.5-Gb/s pattern generator and a 12-GHz photoreceiver. The eye diagrams were taken for back-to-back (BTB) transmission on SC-MQWs InGaAsP/InGaP VCSEL. As shown in Fig. 4-10(a), the room temperature eye diagram of our VCSEL biased at 4 mA with data up to 12.5 Gb/s and 6dB extinction ratio has a clear open eye pattern indicating good performance of the VCSELs. The rise time Tr is 28 ps and fall time Tf is 41 ps with jitter (p-p)=20 ps. The VCSELs also show superior performance at high temperature. Fig. 4-10 (b) demonstrated the high speed performance of our VCSELs (biased at 5mA) with reasonably open eye-diagrams at 12.5 Gb/s and 6dB extinction ratio at 85°C. This further confirms the superior performance of our VCSELs.

To guarantee the device reliability is always a tough work but a natural task for the components supplier in the data communication markets. We have accumulated life test data up to 1000 hours at 70^oC/8mA with exceptional reliability. As shown in Fig. 4-11, the light output is plotted versus time scale for SC-MQW VCSEL chips under the high temperature operation lifetime (HTOL) test at 70^oC/8mA. None of them shows the abnormal behavior.

References

[1] J. A. Tatum, A. Clark, J. K. Guenter, R. A. Hawthorne, and R. H. Johnson,

“Commercialization of Honeywell’s VCSEL technology,” Vertical-Cavity Surface-Emitting Lasers IV, K. D. Choquette and C. Lei, editors, Proceedings of the SPIE, vol. 3946, pp. 2-13, SPIE, Bellingham, WA, 2000..

[2] F. H. Peters and M. H. MacDougal, “High-Speed High-Temperature Operation of Vertical-Cavity Surface-Emitting Lasers” IEEE Photon. Technol. Lett., vol. 13, No 7, p.

645–647, July 2001.

[3] F. H. Peters, D. J. Welch, V. Jayaraman, M. H. MacDougal, J. D. Tagle, T. A. Goodwin, J.

E. Schramm, T. D. Lowes, S. P. Kilcoyne, K. R. Nary, J. S. Bergey, and W. Carpenter, “10 Gb/s VCSEL-based data links,” Photonics West, San Jose, CA, Tech. Rep. OE 3946–26, 2000.

[4] C.W. Wilmsen, H. Temkin, and L.A. Coldren, eds., Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, Cambridge University Press, 1999.

[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 Dig. IEEE/LEOS Summer Topical Meetings, 1997, pp. 53–54.

[6] J. A. Lehman, R. A. Morgan, M. K. Hibbs-Brenner, and D. Carlson, “High-frequency modulation characteristics of hybrid dielectric/AlGaAs mirror singlemode VCSELs,”

Electron. Lett., vol. 31, pp. 1251-1252, 1995.

[7] 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, 1996.

[8] L. J. Mawst, S. Rusli, A. Al-Muhanna, and J. K. Wade, “Short-wavelength (0.7 µm < λ

<0.78 µm) high-power InGaAsP-active diode lasers,” IEEE J. Select. Topics Quantum Electron., vol. 5, pp. 785–791, (1999).

[9] N. Tansu, D. Zhou, and L. J. Mawst, “Low Temperature Sensitive, Compressively-Strained InGaAsP Active (λ=0.78-0.85 µm) Region Diode Lasers,” IEEE Photon. Technol. Lett., Vol.12(6), pp.603-605 , 2000.

[10] T. E. Sale, C. Amamo,Y. Ohiso, and T.Kurokawa, “Using strained lasers (AlGa) In As P system materials to improve the performance of 850 nm surface- and edge-emitting lasers,” Appl. Phys. Lett., vol. 71, p. 1002–1004, (1997).

[11] N. Tansu, L.J Mawst, “Compressively-strained InGaAsP-active (λ=0.85µm) VCSELs”

IEEE Lasers and Electro-Optics Society 2000 Annual Meeting. LEOS 2000. vol. 2, p. 724 -725 (2000).

[12] H. C. Kuo, Y. S. Chang, F. I. Lai, T. H. Hsueh, “High speed modulation of 850 nm InGaAsP/InGaP strain-compensated VCSELs. Electon. Lett. Vol 39, p. 1051-10523 (2003)

[13] S. L. Chuang, "Efficient band-structure calculation of strained quantum-wells", Phys.

Rev. B, 43, 9649-9661 (1991).

[14] David J. Bossert, Doug Collins, Ian Aeby, J. Bridget Clevenger, “Production of high-speed oxide confined VCSEL arrays for datacom application” Photonics West, San Jose, CA, p.p 142 Proc. SPIE Vol. 4649 (2002)

[15] http://www.ieee802.org/

[16] Thibeault BJ, Bertilsson K, Hegblom ER, “High-speed characteristics of low-optical loss oxide-aperture vertical-cavity laser,” IEEE Photon. Technol. Lett., vol. 9, pp. 11-13, Jan.

1997

[17] L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Integrated Ciurcuits.

New York: Wiley, 1995, pp. 201-204

[18] T. R. Chen, B. Zhao, L. Eng, Y. H. Zhuang, “Very high modulation efficiency of ultralow threshold current single quantum well InGaAs lasers”, Electon. Lett., Vol. 29, p.

1525-1526 (1993)

[19] Sven Eitel, Stephan Hunziker, Dominique Vez, “Multimode VCSELs for high bit-rate and transparent low-cost fiber-optic links”, p183 Proc. SPIE Vol. 4649 (2002)

In

_0.4

Ga

_0.6

P

(Δa /a = - 0.5%) 100 Å

In

_0.18

Ga

_0.82

As

_0.8

P

_0.2

(Δa /a = 0.6%) 80Å

Al

_0.6

Ga

_0.4

As

In

_0.4

Ga

_0.6

P

(Δa /a = - 0.5%) 100 Å

In

_0.18

Ga

_0.82

As

_0.8

P

_0.2

(Δa /a = 0.6%) 80Å

Al

_0.6

Ga

_0.4

As

FIG. 4-1 Schematic energy bandgap diagram for In0.18Ga0.82As0.8P0.2/In0.4Ga0.6P active region.

Fig. 4-2(a)

0 200 400 600 800 1000

0.82 0.825 0.83 0.835 0.84 0.845 0.85 0.855 0.86 InGaAsP

GaAs

G ain (1 /c m)

Wavelength (µm)

Fig. 4-2(b)

0 500 1000 1500 2000 2500

1 1.5 2 2.5 3 3.5 4

InGaAsP GaAs

Ga in ( 1/ cm )

Carrier concentration (10

¹⁸

/cm

³

)

FIG. 4-2 Material gain spectrum and material gain as a function of carrier density of In0.18Ga0.82As0.8P0.2/In0.4Ga0.6P MQW and GaAs/Al0.26Ga0.74As MQW

N-contact

FIG. 4-3(a) Schematic cross section of high speed VCSEL structure.

ICP etching (BCl

ICP etching (BCl_{3 3} , Cl, Cl₂₂))

Step2. Selective oxidation Selective oxidation Step3. Wet etching Step3. Wet etching

n-DBR

Step2. Selective oxidation Selective oxidation Step3. Wet etching Step3. Wet etching

n-DBR

Step4. SiNSiNfilm and n-film and n-contactcontact metal deposition

metal deposition

Step6. p

Step6. p--contact (Ti/Pt/Au)contact (Ti/Pt/Au) metal deposition

Step4. SiNSiNfilm and n-film and n-contactcontact metal deposition

metal deposition

Step6. p

Step6. p--contact (Ti/Pt/Au)contact (Ti/Pt/Au) metal deposition

FIG. 4-3(b) Process steps of high speed VCSELs.

FIG. 4-4 PL spectra of SC-MQW with different growth interruption times.

FIG. 4-5 SEM picture of the finished VCSEL

0 2 4 6 8

0 1 2 3 4 5

25C

85C

P o w e r (m W )

Current (mA)

0 1 2 3 4

R=95 Ohm

Voltage (V)

FIG. 4-6 SC-MQWs InGaAsP/InGaP VCSEL light output and voltage versus current (LIV) curves at room temperature and 85°C.

840.0 841.5 843.0 844.5 846.0 847.5 -60

-50 -40 -30 -20 -10 0

In ten s it y (d Bm )

wavelength(nm)

FIG. 4-7 Optical spectrum at 6 mA of the VCSEL

-50 -40 -30 -20 -10 0 10 20

0 3 6 9 12 15 18

Modu lat ion R e sponse (d B )

Frequency (GHz)

1 mA 1.4 mA 2 mA

3 mA

5 mA

FIG. 4-8 Small-signal modulation responses of a 5 µm diameter VCSEL at different bias current levels.

0.0 0.5 1.0 1.5 2.0 2.5

0 2 4 6 8 10 12 14 16

MCEF=11.6 GHz/mA

^1/2 f3dB (GHz)

(I-I_th)^1/2 (mA^1/2)

Fig. 4-9 Resonant frequency as a function of square root of current above threshold current.

Fig. 4-10(a)

Fig. 4-10(b)

FIG. 4-10 (a) 25°C (b) 85°C eye diagram of SC-VCSEL up to 12.5 Gb/s with 6dB extinction ratio. The scale in the fig. is 15 ps/div.

0.0 1.0 2.0 3.0 4.0

0 200 400 600 800 1000

Time (hour)

Po w e r @ 8 m A ( m W)

FIG. 4-11 HTOL (70^oC/8mA) performance of strain compensated VCSEL

CHAPTER 5 Reducing capacitance of Oxide-Confind VCSEL by

在文檔中 面射型雷射高頻特性之研究 (頁 56-69)