Quantum Cascade Laser Function. Quantum cascade lasers are a fairly new technology that is promising for it’s many incredible properties. High powered, wide wavelength range, and room temperature operation in pulsed mode.
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Quantum cascade lasers are a fairly new technology that is promising for it’s many incredible properties.
High powered, wide wavelength range, and room temperature operation in pulsed mode.
Pulsed mode: current is sent in a nanosecond bursts, emitting radiation in pulses. Peak power range: several Watts
Continuous mode: a constant bias is applied to the cascade laser. Peak power range: tens of milliwatts.
The structure of the QCL: an array of quantum wells gradually dropping in height.
Two regions: active well and injector region
Structure: A gradually slanted array of quantum wells in the conduction region of a semiconductor material.
When a bias is applied to the falling array or “cascade,” a beam is emitted.
Electrons tunnel through one injector region and are confined in a quantum well. This confinement forces the electrons to obey wave mechanics, with a quantized vertical motion. The electron drops in energy level emitting a photon, it then tunnels through the thin injector region to the next quantum where it again is confined and drops in energy level. (Figure 1).
The electron continues this quantum mechanics-described movement, releasing photons as it moves through the lattice of quantum wells .
Detailed Cascading Scheme
▪ Once the array of quantum wells is formed, the power is based on step number and wavelength is based on size of quantum Wells.
See the diagram to the right of a MBE system.
Distributed feedback (DFB) QC laser
Distributed feedback (DFB) QC lasers Designed For spectroscopy on gases, single-mode, narrow linewidth lasers with well-defined, precise tunability are required. On order to achieve these goals, scientists fabricated quantum cascade lasers with a periodic waveguide structure built in the cavity.
Surface gratin…. This periodic variation of the refractive index or the gain leads to a certain amount of coupling between the back- and forth-traveling waves. The coupling becomes strongest if the periodicity is a integer multiple of half the laser wavelength in the cavity, according to the following formula: L = l/2neff
Here, L is the grating periodicity, l the laser wavelength in vacuum, and Neff the effective refractive index of the waveguide. Because feedback occurs along the whole cavity and not only on the mirrors, these devices are called distributed feedback lasers.
cross-section through the laser waveguide
SEM picture of a QC DFB laser
Gas Sensing – Uses the QCL for direct absorption spectroscopy in the mid-IR region. Able to detect trace amounts of gas in real time. Environmental, Industrial, Military/Defense, and Health Sciences are only a select few of the many possible applications.
Spectroscopy – The study of spectra by use of spectroscope, the QCL is excellent as a spectroscopic tool
Nonlinear Light – The incorporation of Raman Scattering to QCLs yields a very innovative nonlinear light technology.
“Ultra short” current pulses sent to the laser, laser is tuned to the spectroscopic transition with an additional current or a temperature ramp
Spectral resolution is limited by frequency chirps generated by this pulsation process
Tuning range: 1 – 2 cm^(-1) [wavenumber]
Repetition rates: 10 Hz – 1 KHz
Room temperature operation
Intrapulse: Higher Resolution & Repetition Rate
“ultra short” current pulses sent through laser, again generating frequency chirps
Chirp utilized to sweep swiftly through the frequencies of interest
Frequency downchirps 4 – 6 cm^-1 wide are generated by microsecond long current pulses several amps above lasing threshold
The downchirps are produced by the subsequent heating within the laser
High Resolution: .01 cm^-1 [wavenumber]
Repetition rates: up to 100 kHzSpectroscopy: Inter- and Intra-