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MIOMD-X, September 5-9, 2010, Shanghai, China. P- 28. Diagnostic and characterization of VCSEL diodes based on GaSb substrate

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MIOMD-X,September 5-9,2010, Shanghai, China

P-28

Diagnostic and characterization of VCSEL diodes based on GaSb substrate

E. Hulicius1*, J. Pangrác1, J. Oswald1,T. Šimeček1, J. Vyskočil1, I. Matulková2, J. Cihelka2, Z. Zelinger2, S. Civiš2, Z. Chobola3* Corresponding author email: [email protected]

1Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i.,Prague, Czech Republic

2 J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i.,Prague, Czech Republic

3Brno University of Technology, Faculty of Civil Engineering, Brno, Czech Republic

Experimental

Abstract

Vertical-Cavity Surface-Emitting Laser (VCSEL) diodes are among the youngest members of the semiconductor laser diode family. The aim of our work focuses on the measurement of the properties (the spectral range of

A new type of VCSEL GaSb-based diodes for the 4250 cm−1 (around 2.3 μm) wavelength region have been developed at WSI of the TU München. The laser structures were prepared by MBE on n-type GaSb, with the n-contact at the bottom. The bottom Bragg epitaxial mirror consists of twenty-four couples of AlAsSb/GaSb layers. The active zone of these lasers consists of five 11-nm-thick Ga0.65In0.35As0.1Sb0.97 quantum wells surrounded by Al0.35Ga0.65As0.03Sb0.97 barriers. The device employs a p-GaSb/n-InAsSb-buried tunnel junction (5-8 μm in diameter) as a current aperture and replaces the high loss p-material by n-type material. The Ti/Pt/Au ring contacts are sputtered on InAsSb contact layer. The dielectric mirror from four couples of Si/SiO2 layers is placed at the top of the laser structure above the buried tunnel junction.

VCSEL structures and their preparation are described by our colleagues from WSI TU München here during MIOMD 10.

Fine tuning of these lasers can be realized by changing the temperature or the injection current. Lasers operate in the CW regime at room temperature with an output power around 50 μW. The threshold current is around 1.5 mA and its temperature dependence is relatively weak.

the laser emission, temperature, current tunability, far-field and lifetime) of experimental VCSEL diode lasers based on GaSb and operating in the wavelength infrared region around 2.3 μm.

These lasers were prepared in frame of EU project NEMIS at TU München.A high-resolution spectrometry was used for laser diagnostics.

AGEING

FAR FIELD MEASUREMENT

  • We have measured the far field patterns of the NEMIS lasers in order to provide certain additional insight into the modal behaviour.

  • There are two main shapes of the laser beam: Either a more or less circular symmetrical beam in the axis of the laser or a twofold distribution of the power in two directions both inclined from the axis of the laser some 11 degrees.

  • Measuring of the two beams revealed no difference in their spectra.

  • The spectra of both beams contained all the modes.

  • Far fields of some lasers exhibits this double beam (double mode) behaviour, but majority have nice simple ones.

Ageing test equipment

Ageing box with holders for 10 individual lasers on the headers under CW current provided by ten stabilized power sources and a temperature stabilized detector that can be regularly precisely attached to these lasers to monitor their relative optical power output was used.

For special purposes also two positions have been prepared for pulsed pumping by 1μs pulses with up to 100 kHz repetition frequency with pulsed detection using lock in amplifier. These positions enable to measure lasers with higher currents and worse heat dissipation.

Increase of output power of the green-marked laser A1244 can be explained by changes of its far field.

FF spectrum of laser A1244 (green-marked in the Ageing part,left Fig.) after 1500 and 9020 hours ageing, respectively.

ELECTRICAL NOISE LASER MEASUREMENT

HIGH RESOLUTION MODE SPECTRA MEASUREMENT

  • The last set of lasers is containing several devices, which exhibit greater side mode suppression ratio reaching even 20 dB, but even these lasers have one or few side modes of lower intensity.

  • A surprising feature of the side modes of some of the latest lasers is, that the side modes exhibit greater line width while the older lasers did not behave this way. We have not found any convincing explanation for the origin of these modes and their different behaviour in the complicated VCSEL optical resonator.

  • Most of the lasers, especially those prepared earlier exhibit multimode emission with relatively irregularly spaced modes as can be seen on the FTS spectra with the upper estimate of their line widths being some 150 MHz.

Plot of log I versus lin U and log I versus log U for different laser diodes in the forward bias direction.

  • We have observed short circuiting resistances for all samples at voltages below 0.3 V, being in the range 1×103 – 5×104.

  • For voltages exceeding 0.6 V we observed substantial voltage drop on the series resistance and contact resistance leading to the estimation of this resistance to be in the range 8 – 200 .

  • In the case of sample No. 3 we observed two PN junctions in series.

  • From the measurement of spectral voltage density SU on applied field in forward bias it was evident, that the contact quality is poor, especially for sample No.5. From the measurements at voltages in access of 0.5 V we observed in case of samples 2 and 5 also pulse noise, indicating instability in the tested structure.

The noisespectral density as a function of forward bias and load resistor RL=1k Ω for different laser diodes.

The authors gratefully acknowledge the financial support by the European Union via NEMIS (contract no. FP6-2005-IST-5-031845), research center LC-510, and the program MSM 6840770014 and K. Kashani, A. Bachmann and Prof. M.-C. Amann from the Walter Schottky Institut for providing the VCSEL samples and O. Petříček for ageing measurements. This work is the part of the research programs of the Grant Agency of the Academy of Sciences oftheCzech Republic (Grant No. A400400705), the Ministry of Education, Youth and Sports (COST MP0702 and COST 729) and the research program of Institute of PhysicsAS CR 10100521.


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