8. Disadvantages of old LASERs
• Large area
• Low output
• Low life
• Not optically integrable
9. Optical feedback and laser oscillation
• Light amplification in the laser occurs when a photon colliding with an atom in the
excited energy state causes the stimulated emission of a second photon and then
both these photons release two more.
• This is accomplished by placing or forming mirrors (plane or curved) at either end
of the amplifying medium, as illustrated in Figure 6.4.
• The optical cavity formed is more analogous to an oscillator than an amplifier as it
provides positive feedback of the photons by reflection at the mirrors at either end
of the cavity.
• Furthermore, if one mirror is made partially transmitting, useful radiation may
escape from the cavity
27. Quantum well LASER
• Hence, quantum-well lasers exhibit an inherent
advantage over conventional DH devices in that they
allow high gain at low carrier density, thus providing
the possibility of significantly lower threshold currents.
• Both single-quantum-well (SQW), corresponding to a
single active region, and multiquantum-well (MQW),
corresponding to multiple active regions, lasers are
utilized Better confinement of the optical mode is
obtained in MQW lasers in comparison with SQW
lasers, resulting in a lower threshold current density for
these devices.
28. Quantum dot LASER
• More recently, quantum-well lasers have been
developed in which the device contains a
single discrete atomic structure or so-called
quantum dot (QD) [Ref. 35].
• Quantum dots are small elements that contain
a tiny droplet of free electrons forming a
quantum-well structure. Hence a QD laser is
also referred to as a dot-in-a-well device
30. Quantum dot LASER
• They are fabricated using semiconductor crystalline materials and
have typical dimensions between nanometers and a few microns.
The size and shape of these structures and therefore the number of
electrons they contain may be precisely controlled such that a QD
can have anything from a single electron to several thousand
electrons.
• Theoretical treatment of QDs indicates that they do not suffer from
thermal broadening and their threshold current is also temperature
insensitive .
• If the conventional injection laser diode is regarded as three
dimensional and a quantum well (i.e. an SQW where an array of
SQWs forms an MQW structure) is confined to two dimensions,
then the QD structure can be considered to be zero dimensional.
• It should be noted, however, that the single dimensional structure
forms a quantum wire or dash.
36. Distributed Bragg reflector(DBR)
• In the DBR laser the grating is etched only near
the cavity ends and hence distributed feedback
does not occur in the central active region.
• The unpumped corrugated end regions effectively
act as mirrors whose reflectivity results from the
distributed feedback mechanism which is
therefore dependent on wavelength.
• DBR is more stable frequency output as compare
to DFB
• Fabrication of DBR is difficult.
42. Einstein relation
The population of the two energy levels of such a system is described by
Boltzmann statistics which give:
where N1 and N2 represent the density of atoms in energy levels E1 and E2,
respectively, with g1 and g2 being the corresponding degeneracies† of the levels, K is
Boltzmann’s constant and T is the absolute temperature
Absorption
Absorption =spontaneous+ stimulated
emission
Spontaneous emission
N2A21
