uni s t.e.sale et al., hpsp 9, sapporo 2000, paper 27p15. gain-cavity alignment in efficient visible...

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T.E.Sale et al., HPSP 9, Sapporo 2000, paper 27P15.

Gain-Cavity Alignment in Efficient Visible (660nm) VCSELs Studied Using High

Pressure Techniques

T.E.Sale, S.J.Sweeney, G.Knowles, A.R.Adams, Department of Physics, University of Surrey,

Guildford, GU2 7XH, U.K.Tel: +44 1483 876813Fax: +44 1483 876781

e-mail: [email protected]

J.Woodhead & S.M.Pinches.Department of electronic & electrical engineering,

University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K.

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660nm Vertical-Cavity Surface-Emitting Lasers for polymer fibre applications are very sensitive to temperature and structural errors. Two devices with a 3meV wavelength separation were studied and significant differences in the output performance can be explained by the relative offset of the cavity mode and the peak of the gain spectrum. It also explains the different behaviour as a function of hydrostatic pressure. The temperature and pressure dependence can be used to shift the offset but self-heating means that the c.w. variation is not always as expected. Electron leakage also changes with both pressure and temperature, and in both cases its effect is dominant over

the gain-cavity offset in determining the threshold current variation.

Abstract:

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VCSELs operating close to 650nm are of use in low cost polymer optical fibre datacomms systems over short distance (<100m) at speeds up to 10Gb.s-1 per channel.

Here we investigate two ~660nm devices taken from different parts of a

large section of fabricated wafer. They differ only in the energy of the cavity resonance.

We use a liquid (Essence F) filled piston and cylinder system to apply hydrostatic pressure to the devices to change the offset between the QW gain peak and cavity (lasing) energy. Leakage currents via the X-minimum also increase as a consequence of reducing the Г-X splitting with pressure.

Very significant differences in the c.w. and pulsed L-I’s as a function of pressure can be readily explained by the small difference in resonant wavelength alone.

Introduction:

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X

kbarmeVdP

dE/0.7 kbarmeV

dP

dEX /1.2

kbarmeVdP

dEa /1.9

Energy, E

Electrons leak from Γ in active region through X states in p-DBR causing leakage current and loss of quantum efficiency via free carrier absorption, activation energy approx. 250meV, decreases at -9.1meV/kbar. Carrier densities in high 1018cm-3 readily obtained.

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Substrate

AlAs/Al0.5Ga0.5 As

InGaP/AlGaInPContact

Metalisation

Outline device structure1x8 Array used with POF

Double sided 1x8 VCSEL chip

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C.W. L-Is of devices A & B. in air, 0kbar 20°C.

Device A Ecav=1877.4meV Device B Ecav= 1874.8meV

Threshold current IthB<IthA due to better gain-cavity alignment.

Device A rolls over at a

higher current Imax.

Useful operating range (Ith- Imax) is similar for both, but higher electron leakage in A due to higher Imax, causes absorption in output mirror and reduced efficiency.

ImaxB

ImaxA

IthB IthA

PmaxB

PmaxA

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rcavcav

rcavcav TTT

EP

P

ETETPE

,0),(

)(,0),( maxmaxmaxmax r

ggrgg TT

T

EP

P

ETETPE

We need to consider the pressure and temperature dependence of the gain peak (Egmax) and cavity energy (Ecav). Over a suitable range they are described by:

Ecav Egmax

T

-0.11meV.°C-1 -0.43meV.°C-1

P 2.5meV.kbar-1 7.2meV.kbar-1

The partial derivatives are found by experiment:

The devices heat above ambient (Tr=20°C) by Ohmic heating at a rate 7.7°C.mA-1. Pressure is applied in a piston cylinder system.

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Device A. High pressure measurements in Essence F

Eqw<Ecav

Ith (pulsed)<Ith (c.w.) at 0 kbar since Ohmic heating worsens gain-cavity alignment.

Applied pressure improves initial alignment but Ith (pulsed) increases with pressure as electron leakage dominates the threshold current.

Ith (c.w.) increases even faster as leakage current heats device and further adds to leakage.

c.w. Ith & Imax in air. pulsed Ith in air

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Eqw>Ecav

Ith (c.w.) <Ith (pulsed) at 0kbar since d.c. current improves gain-cavity alignment.

Applied pressure improves alignment but Ith (pulsed) increases as leakage dominates the threshold.

At higher pressures Ith (c.w.) >Ith (pulsed) as gain-cavity crossover passed.

Device B. High pressure measurements in Essence F

c.w. Ith & Imax in air. pulsed Ith in air

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Ecav(0, 20°C))=1877.4meV (660.5nm)

Egmax (0,20°C) =1875meV

Ecav & Eqw cross at ~0.6kbar

For c.w. operation threshold Egmax & Ecav do not cross at any pressure due to current dependent heating which keeps the two energies at almost constant separation.

Energies are computed from experimental data using pressure, temperature and current coefficients described earlier.

Device A. Pressure dependence of gain, cavity and roll-over energies.

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Ecav(0, 20°C))=1874.8meV (661.4nm)

Pulsed Ecav & Eqw diverge with applied pressure.

For c.w. injection, Ohmic heating causes initial convergence of Ecav & Egmax

Ecav & Egmax are better aligned for c.w. operation in this device due to the very significant temperature rise (~15 °C) at threshold.Device B. Pressure dependence of gain,

cavity and roll-over energies.

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Conclusions: Very small differences (<3meV, or 1nm) in the gain-cavity offset in visible VCSELs have big effects on the output characteristics. So epitaxial growth control need to be very tight.

High pressure causes a blue shift in Ecav & Egmax, whilst temperature rise from Ohmic heating cause a red shift. Egmax moves around 3 times as fast as Ecav. Here we have used pressure to vary the offset between the two.

The relative alignment of Ecav & Egmax is evident in determining whether the threshold is smaller under pulsed or c.w. operation, and the peak output power.

Whatever the polarity the initial alignment of Ecav & Egmax, leakage dominates the temperature and pressure variation of threshold (see our poster 25P13 and talk 27D1 at this conference [1,2]). This indicates the problems associated with achieving shorter wavelength emission.

Further studies using higher power devices and wider variations in cavity energy are planned.

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[1] Assessing the performance of vertical-cavity surface-emitting lasers using highpressure and low temperature techniques, HPSP 9 Sapporo 2000. Submitted Phys. Stat.Sol.

[2] Qualifying the importance of indirect carrier leakage on visible Al(GaInP) lasersusing high pressures and low temperatures. S.J.Sweeney, G.Knowles, T.E.Sale &A.R.Adams, HPSP 9 Sapporo, 2000. Submitted Phys. Stat. Sol.

[3] A.R.Adams, M.Silver & J.Allam, High Pressure Semiconductor Physics II,Semiconductors & Semimentals, 55, Academic Press, London (1998).

[4] A.Onischenko, T.E.Sale, E.P.O'Reilly, A.R.Adams, S.M.Pinches, J.E.F.Frost &J.Woodhead, IEE Proc. J Optoelectron., 147, 1, pp15-21, 2000.

[5] S.M.Pinches, J.E.F.Frost, P.M.Martin, T.E.Sale P.N.Robson & J.Woodhead, IEEProc. J Optoelectron., 147, 1, pp11-14, 2000.

[6] T.E.Sale, Vertical Cavity Surface Emitting Lasers, Research Studies Press, Chichester(1995).

References:

uni s t.e.sale et al., hpsp 9, sapporo 2000, paper 27p15. gain-cavity alignment in efficient visible...

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