Page 190 - Photonics Essentials an introduction with experiments

Direct Modulation of Laser Diodes

184 Advanced Topics

caused by the current step and the emission of light is shown schemat-
ically in Fig. 8.1. What happens to the carriers during this time? They
are filling hole and electron states near the band gap. Carriers that
continue to arrive must seek unoccupied states at higher energy. We
call this effect band-filling. Electrons and holes in these higher energy
states will have a shorter lifetime and thus a higher recombination rate
than electrons and holes near the band edge. Spontaneous optical re-
combination will be dominated by these higher energy states, seeding
stimulated emission at photon energies above the band gap energy.
The onset of stimulated emission will deplete this excess carrier con-
centration, proceeding from the higher energy states to the band edge
states in an orderly progression. The energy of the emitted photons re-
flects this process, starting at higher energy and progressing toward
the band gap energy. This effect is called wavelength chirp. The degree
of chirp increases as the laser is driven over the threshold. If the wave-
length shift is large enough to modulate the laser emission wavelength
by a nanometer, then significant crosstalk interference between adja-
cent channels will occur in today’s wavelength division multiplexing
communications systems.
A meaningful physical model of chirp will require detailed knowl-
edge of the semiconductor band structure, and the procedure needed
to calculate the chirp effect is too complicated for presentation here.
There are possible remedies.
In order to minimize turn-on delay and, as we will see shortly, in or-
der minimize the effects of relaxation oscillations, you would like to
drive the laser well above threshold. This is not good news as far as
chirp is concerned. One approach that has been used with some suc-
cess is wavelength stabilization. In the lasers we have discussed so
far, the output wavelength is determined by the process of stimulated
emission, which chooses the wavelength where the gain is maximum.
To force the laser operation to occur at one specific wavelength, an ad-
ditional optical resonator having only one mode in the entire laser
gain spectrum can be imposed on the laser structure. This is achieved
by cutting a periodic grating into the laser, close to the gain region.
The grating acts like a narrow-band optical interference filter. The de-
vice is called a distributed Bragg reflector laser. The presence of this
grating significantly extends the region of laser drive current over
which single-wavelength, chirp-free emission can be obtained under
pulsed operation.

Part 2. After the laser has reached threshold: d < t < T 0
where T 0 is the bit period
Now the laser is “on.” The drive current density is constant, and the
equations for the photon and carrier densities are

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