Datasheet

CHAPTER 5. GENERAL QCL QUESTIONS 19

5.2.2 Why is such a large range obtainable?

This peculiar characteristic is due to the fact that in QC devices the emission wave-

length is determined by the geometry of the semiconductor layers that compose the

laser crystal.

More precisely, the laser transition is the transition of an electron inside

sub-bands from one upper quantum well level to a lower quantum well level. For more

details we would encourage the reader to consult semiconductor physics text book.

Using two different semiconductor materials (InGaAs and AlInAs), a series of poten-

tial wells and barriers for the electrons can be built. These wells and barriers are so thin

that the electrons are allowed only a discrete set of energy levels. This situation is very

similar to the orbitals of an electron around a nucleus in the case of an atom. In the

case of the QC structure, the positions of the permitted energy levels are determined

by the thicknesses of the wells and barriers.

It is thus possible, using only one material system (InGaAs/AlInAs grown on InP), to

deﬁne laser transitions with energies ranging over a wide span. The limits are set, on

the short wavelength range, by the potential difference between the wells and barriers,

and on the other side by intrinsic absorptions of the material.

In conclusion, a QC laser is a laser made with some sort of a speciﬁc composit

designed speciﬁcally for each wavelength but always composed of the very same ma-

terials.

5.2.3 How ”CW” does a QCL look like in pulsed mode?

For a slow detection system, a pulsed laser will appear CW. In order to deﬁne slow,

see ”Line-width limit”. A pulse will last between 10 and 100 ns and will be repeated at a

rate corresponding to 0...3% for usual DFBs and up to 10...20% for high power DFBs.

At 10 ns pulse length, the line-width will be close to minimal in pulsed operation, less

than 0.1/cm, and at 100 ns it will be larger, depending on the device.

5.2.4 What optical powers can be expected?

No standard output power ranges exist at this time, but Output power in the range of 1

or 2 mW can be expected for stock devices.

5.2.5 How precise should emission be speciﬁed?

Normally, speciﬁcation to 0.5/cm or 1nm should be sufﬁcient. As the laser tunes over

several linewidths, it is possible to temperature tune it to adjust its central wavelength.

It becomes critical only if extreme power is required or if the laser has to operate CW

at LN2 for the same reason.

In standard bipolar semiconductor lasers (e.g. 1.55µm telecom devices), the emission wavelength is closely related to an

intrinsic characteristic of the semiconductor material used, namely the band gap energy.