Phone numbersMid-IR Physics and Devices :: Broadband QCLs
The aim of this new project is to construct an Optical Coherence Tomography system which operates at λ~6-12µm, demonstrating real-time imaging with a spatial resolution better than 100µm on engineered tissues grown in bioreactors.
Broadband QCLs
Broadband laser emission is achieved by extending the idea of a conventional QC laser. Since the emission wavelength of QCL can be tailored by means of band structure engineering, a net broadband gain can be achieved over a target wavelength range by incorporating a series of active regions with different emission wavelengths.
An example of our broadband QCL design is shown in figure 1 and 2. It is designed to emit between 6 and 8µm and contains 11 different active regions of the so called 'three well vertical transition' type, each repeated either 3 or 4 times to give 36 active stages in total. The material system used is InGaAs/AlInAs latticed matched to InP and is grown by MBE.
Figure 1: Our designs here at Sheffield were grown by MBE using the AlInAs/InGaAs materials system, lattice matched to an InP substrate. The design incorporated 36 different active stages designed to emit at 11 different wavelengths between the range of λ~6-8µm.
Figure 2: Conduction band profile of a broadband QCL showing the active regions for 6.9µm and 7.09µm.
A talk on broadband QCLs and superluminescence LEDs was presented at the seventh international conference on Mid-Infrared Optoelectronics: Materials and Devices (MIOMD VII), Lancaster UK, September 2005.
Sample Performance
Several broadband QCLs designed to operate between 6-8µm have been fabricated and characterised. Peak power levels in the order of several hundred milliwatts and typical pulsed threshold current densities (Jth) in the order of 1.4kAcm-2 at cryogenic temperatures have been measured.
FTIR spectroscopic measurements (figure 3) and calibrated L-I characteristics (figure 4) are shown below for a broadband QCL device operating at 4.8A over the range of 6-8µm. The emission spectra shows continuous broadband lasing over the range 6.0-8.0µm. We show values of Jth of ~1.35kAcm-2 (10K) and ~5.1kAcm-2 (300k) with a characteristic temperature (T0) ~198K.
Figure 3: Emission spectra of broadband QCL, lasers were driven at 15kHz with 100ns long current pulse. Continuous lasing is observed across the 6 to 8µm range.
Figure 4: Calibrated L-I characteristics of a broadband QCL showing lasing up to a temperature of 320K
Figure 5 exhibits the coherence length measurements using a Michelson Interferometer. The estimated coherence length at a current density of 6.5kAcm-2 is ~25µm. A requirement of OCT is a short coherence length and shorter coherence lengths will be achieved in future by making QCLs that emit over a broader wavelength range.
Figure 5: Coherence length measurement in air, laser driven at 15 kHz with 100ns long current pulse.
Work is also underway to make the emission more continuous over the designed wavelength range and to extend the operating wavelength into the 8-10µm range; however there are still some unanswered questions about the factors that determine the degree of continuity in the laser ridge emission.
This research involves: John Cockburn, Evgeny Zibik, Wing Ng and Michael Soulby and is a collaborative project with Prof. J C Hebden, Prof. D Delpy and colleagues at UCL, funded by EPSRC.
