Alpes Lasers Cascade Laser Starter Kit Datasheet download pdf (Page 21)

CHAPTER 5. GENERAL QCL QUESTIONS 20

5.3 Tuning and linewidth

5.3.1 How does a DFB-QCL tune?

In QCLs there are no effects such as carrier density dependent index of refraction,

therefore no current tuning is observed. The only tuning mechanism is temperature

tuning of the index of refraction of the waveguide that changes the apparent optical

length of the wavelength selection grating. This of course will result in an observable

current tuning: the higher the current gets, the higher becomes the average temper-

ature of the active region of the QCL. But this apparent current tuning is based on

temperature tuning only.

5.3.2 How much can a DFB-QCL be tuned?

The operation range of the device is -30...+30C (dT=60K). The relative tuning is con-

stant for all wavelengths and is about 6E-5/K for wavelength and -6E-5/K for wavenum-

ber. This results in a tuning range of about 0.4% of peak emission wavelength or

wavenumber.

For a 1500/cm device, the total tuning is approximately given by

−6E − 5/K × 60K × 1500/cm i.e -5.4/cm.

Note: The relative tuning has a minus sign for wavenumbers and a positive sign for

wavelength. This is exactly opposite to how a lead-salt device would tune, for those

accustomed to this type of devices.

5.3.3 Why is the line-width of a DFB-QCL limited?

Like in every pulsed lasers, short enough pulses will lead to Fourier limited line width.

For intermediate pulse length, the limiting factor is the thermal tuning of the device.

The device heats up during the pulse and its emission wavelength follows and sweeps.

Optimum pulses of 5 to 15 ns will enable to get a minimal linewidth.

Some customers such as the company Aerodyne published data on the linewidth

reachable using our devices and electronics (Starter Kit).

Please have a look at (http://www.alpeslasers.ch/Conference-Papers/Workshop-Freiburg-

01.pdf) which describes measured line width in pulsed operation.

The standard measurement setup we use enables to verify that the laser is single

mode i.e. has a linewidth not exceeding 0.3/cm.

5.3.4 How and how much does a FP-QCL tune?

A FP-QCL tunes because of the shift in gain of the structure with temperature. This

tuning is about twice as fast as the index tuning of DFB-QCLs, i.e about 1.3E-4/K (in-

creasing temperature will result in increased wavelength).

On a Peltier cooler like the one included in the Alpes Lasers SA Starter Kit, the ob-

tainable temperature span is about 60K (-30...+30C). Therefore the central wavelength

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Alpes Lasers Cascade Laser Starter Kit Datasheet Page 21