Datasheet

CHAPTER 2. ELECTRICAL AND OPTICAL PROPERTIES 9
• maximum rise/fall time of 10ns (to prevent detrimental heating)
Alpes Lasers SA produces Starter Kits which provide at the same time driving, tem-
perature control and protection of the laser chip. For a CW QCL, some standard laser
drivers can provide the necessary conditions.
2.2.5 How do I drive a CW QCL?
A CW QCL is about as sensitive to electrical surges and instabilities as a conventional
bipolar laser diode (telecom NIR laser). It is necessary to use a good quality power
supply to ensure:
• current onset is formed by well controlled ramps without surges
• current and voltage compliance can be precisely set (1mV/1mA)
• current and voltage are stable within 0.1%
We recommend precision laser diode current sources such as the ILX Lightwave
LDX-3232.
2.3 Optical properties
2.3.1 Mode characteristics
The emitted mode is single lateral, and also single longitudinal for the DFB devices.
The divergence is 60deg FWHM in the vertical direction and 40deg FWHM in the hori-
zontal direction.
See the images on our website at (www.alpeslasers.ch/technology/Technology.htm).
Why do we observe such a far field?
The QCL is based on a mechanism (inter sub-band transitions) that exhibits a poor
efficiency: most of the electrons emit phonons instead of photons. The laser thus
heats a lot and thermal management at the microscopic scale of the waveguide is
important to allow operation. The waveguide is thus very small in the vertical direction
(i.e. perpendicular to the quantum wells plane) in order to optimize the overlap between
the optical mode and the gain region.
Why is the horizontal divergence not the same for all lasers?
Depending on the wavelength and parameter optimised in a laser, it requires a different
optimization of the width of the laser stripe. This results in a varying lateral confinement
on the beam, thus various lasers at different wavelengths may exhibit pretty different
lateral divergence. The lateral divergence is always smaller than the vertical diver-
gence.