Technology Description - Alpes Lasers

General device characteristics

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How do I drive the device?

As for any semiconductor laser, the performance of the device depends on the temperature. In general, unipolar lasers need (negative) operating voltage around 10 V with (peak-) currents between 1 and 5 A, depending on the temperature and the device. Around room temperature, that is the temperature range (-40..+70 °C) that can be reached by Peltier elements, unipolar lasers operate only in pulsed mode because of the large amount of heat dissipated in the device. In general, pulse length around 100 ns are suitable for Fabry Pérot devices. Alpes Lasers sells electronic drivers dedicated to unipolar lasers.

Electrical behaviour and I-V characteristics

Quantum cascade lasers exhibit I-V curves that are diode like characteristics for short wavelength devices (l = 5 µm) to almost ohmic behaviour for l = 11 µm. In any case the differential resistance at threshold is a few ohms. Long wavelength devices often exhibit a maximum current above which, if driven harder, the voltage increases abruptly while the optical power drops to zero. This process which occurs only in unipolar lasers, is usually non-destructive and reversible if the device is not driven too hard above its maximum current.

Room temperature I-V curves of unipolar lasers (measured in pulsed mode). The device operating at l = 10 µm has a maximum operation current (because of the appearance of Negative Differential Resistance or NDR) of 3.2 A.

Electrical model:

In a simplified way, the device can be modelled, for electronic purpose, by a combination of two resistors and two capacitors. As shown by the above I-V curves, R1 increases from 10 to 20 Ohms at low biases to 1-3 Ohms at the operating point. C1 is a 100 pF capacitor (essentially bias independent) between the cathode and the anode coming from the bonding pads. C2 depends on your mounting of the laser typically in the Laboratory Laser Housing, C1<100 pF

Temperature dependence of the laser characteristic

The threshold current and slope efficiency are temperature dependent, although this dependence is much weaker than the one observed in interband devices at similar wavelengths. Shown below are a set of power versus current curves taken from a device l = 10 µm at various temperatures. In general, the device has a maximum operation temperature which, depending on the design and wavelength, can be between 300K to a maximum of 400K. As maximum power and sometimes slope efficiencies both increase with decreasing temperature, it is usually advisable to cool the device with a Peltier element. Alpes Lasers sells a special Peltier cooled housing dedicated to driving unipolar lasers. Peak power between 20 and 100 mW which is equal to average powers between 2 and 10 mW are obtained typically.

Peak and average power (at a duty cycle of 1.5\%) for a unipolar laser as a function of temperature.

Typically, because of excess heat due to the driving current, unipolar lasers must be driven by current bursts with typically 10 ns rise time and a pulse-length of 100 ns. Some unipolar lasers may also operate in continuous wave (c.w.) at cryogenic temperatures, with a maximum operating temperature of 50 to 100 K depending on the design. Alpes Lasers specify c.w. operation on special request.

Spectral characterisics

Under pulsed operation, the spectra of these lasers are multimode, the spectral width of the emission being of about one to fifty nanometer (1-30 cm-1, typically 10 cm-1) depending on the device design and operating point. Although it is not a property common to all unipolar laser designs, our long-wavelength devices will blue shift with increasing current, as shown on the figure below.

a) spectra of a long wavelength laser based on a diagonal transition b) spectrum of a short wavelength laser based on a vertical transition

Electrical tuning

By driving the device with two different electrodes, wavelength and output power can be independently adjusted. Tuning ranges as large as 40 cm-1 at a peak power of 5 mW and a temperature of -10 °C have been obtained by Alpes Lasers. See literature for more details on this technique.

Distributed Feedback Laser (DFB)

In a Distributed Feedback Laser, a grating is etched into the active region to force the operation of the laser at very specific wavelength determined by the grating periodicity. As a consequence, the laser is single frequency which may be adjusted slightly by changing the temperature of the active region with a tuning rate of 1/n Dn/DT = 6x10-5K-1.

Scanning Micrograph image of a Distributed Feedback Unipolar Laser (DFB-UL). The grating selecting the emission wavelength is well visible on the surface.

Emission spectra versus temperature for a DFB-UL. The device is driven at its maximum current. It must be stressed that because of this tuning effect, when operated in pulsed mode close to room temperature, the linewidth of emission is a strong function of quality of electronics driving the laser. The latter should optimally deliver short pulses (best 1-10 ns to obtain the narrowest lines) with an excellent amplitude stability. The laser will drift at an approximate rate of a fraction of kelvin per nanosecond during the pulse [see literature].

Beam Properties

Polarization

Because the intersubband transition exhibit a quantum mechanical selection rule, the emission from a unipolar laser is always polarized linearly with the electric field perpendicular to the layers (and the copper sub mount).

Beam divergence

The unipolar laser is designed around a tightly confined waveguide. For this reason, the beam diffracts strongly at the output facet and has a (full) divergence angle of about 60 degrees perpendicular to the layer and 20 degrees parallel to the layers (see figures below). A f#1 optics will typically collect about 70\% of the emitted output power. Be careful that the collected output power will decrease with the square of the f-number of the collection optics. The mode is usually very close to a Gaussian 0,0 mode.

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