Broadband quantum cascade lasers and superluminescence LEDs

1
Broadband quantum cascade lasers and
superluminescence LEDs

• Wing Ng1, Evgeny Zibik1, Luke Wilson1,
• Mark Hopkinson2, Kristian Groom2, John Cockburn1
• 1Dept of Physics and Astronomy, University of
  Sheffield, UK
• 2 National Centre for III-V Technologies,
  Sheffield, UK

2
Outline

• Introduction
• Sample design
• Laser characterisation
• Superluminescence QCLED
• Summary

3
Introduction

• Develop broadband mid-IR source for medical
  imaging applications.
• Species specific optical coherence tomography for
  tissue engineering
• In collaboration with the medical physics group
  at UCL, UK
• Need short coherence length to give higher
  resolution ? Broadband source
• Two approaches
• i) Multi-wavelengths QCL
• ii) Superluminescence QCLED

4
Broadband QCL

• Emission wavelengths can be tailored by
  band-structure engineering
• For intersubband transitions, emissions from
  different wavelengths cannot be absorbed by
  another active region

Interband transitions
Intersubband transitions
E
kx,y
5
Sample design

• QCL with 36 active regions designed to emit from
  6 µm to 8µm.
• AlInAs/InGaAs lattice matched to InP grown by MBE
• 11 different wavelength active regions within the
  single structure

6
Low temperature operation

• Jth 1.5kA/cm2 at 80K
• Very broad spontaneous emission spectrum

7
Room temperature operation

• Jth at room temperature (3.8kA/cm2) is very
  comparable with our single wavelength lasers
  (Jth2.9kA/cm2)
• At 2.2A, lasing peaks are observed at 6.9, 7.2,
  7.45 and 7.6µm.

8
Superluminescence QCLEDs
Wet etched facet

• Same material as used in making lasers
• Wet etched one facet and one mirror like cleaved
  facet
• The wet etched facet reduce the optical feedback
  and the device gives out amplified spontaneous
  emission

Cleaved facet
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Light-current (LI) characteristics
10K

• Superlinear characteristics in LI curves
• Longer devices gives higher output power
• Peak power 40µW for the 2.5mm device

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LI characteristics

• Peak power of 10µW at 150K
• Increasing average power as duty cycle increases

11
SLED spectra

• Emissions are observed from 6 to 8µm for the
  100ns 25kHz pulses and from 6 to 8.5µm for 300ns
  25kHz pulses
• Continuous emission from 5.7 to 8.5µm

12
Coherence length

• Information about the coherence length can be
  obtained from the interferogram
• Coherence length can also be estimated from the
  spectrum 20µm

Lc (2ln2/p)(?2/??)
13
Summary

• Broadband laser sample
• - Room temperature operation at 4 wavelengths
• - Emits continuously between 6 to 6.8µm at low
  temperature
• Single cleaved facet devices
• -Show superluminescence characteristics
• -Broad emission spectra from 6 to 8µm
• Coherence length was estimated to be 30µm for
  SLED