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Laser diode rate equations
The semiconductor laser multimode rate equations relate photon and carrier (electron) numbers or densities, to device parameters such as carrier and photon lifetime, optical gain, injection current and other material parameters.
From these equations, it is possible to derive a set of steady state and small signal equations to help in further understanding the static and dynamic characteristics of semiconductor lasers.
Note that these equations can be cast in many different forms, this is one example of them.
The Multimode Rate Equations
N and P are carrier and photon densities respectively. I is the injected current and e the electronic charge.τn and τp are carrier and photon lifetimes respectively. M is the number of modes modelled. V is the active region volume which can be omitted if the equations are to be interpreted in terms of carrier and photon numbers as opposed to densities. Γ is the mode confinement factor which describes how much of the mode is confined in the active region. μ is the mode number.βmu is the spontaneous emission factor.
Gμ is the gain of the μth mode and can be modelled by a parabolic dependence of gain on wavelength as follows:
where: α is the gain coefficient and ε is the gain compression factor (see below). λmu is the wavelength of the μth mode, δλg is the full width at half maximum (FWHM) of the gain curve, the centre of which is given by
where λ0 is the centre wavelength for N = Nth and k is the spectral shift constant (see below). Nth is the carrier density at threshold and is given by
where Ntr is the carrier density at transparency.
βμ is given by
β0 is the spontaneous emission factor, λs is the centre wavelength for spontaneous emission and δλs is the spontaneous emission FWHM. Finally, λmu is the wavelength of the μth mode and is given by
where δλ is the mode spacing.
Explanation of Terms
is the increase in carriers due to current injection
is the decrease in carriers due to spontaneous emission
is the decrease in carriers due to stimulated emission
is the increase in photons due to stimulated emission
is the decrease in photons as a result of cavity losses
is the increase in carriers as a result of spontaneous emission.
The gain term, G, cannot be independent of the high power densities found in semiconductor laser diodes. There are several phenomena which cause the gain to 'compress' which are dependent upon optical power. The two main phenomena are spatial hole burning and spectral hole burning.
Spatial hole burning occurs as a result of the standing wave nature of the optical modes. Increased lasing power results in decreased carrier diffusion efficiency which means that the stimulated recombination time becomes shorter relative to the carrier diffusion time. Carriers are therefore depleted faster at the crest of the wave causing a decrease in the modal gain.
Spectral hole burning is related to the gain profile broadening mechanisms such as short intraband scattering which is related to power density.
To account for gain compression due to the high power densities in semiconductor lasers, the gain equation is modified such that it becomes related to the inverse of the optical power. Hence, the following term in the denominator of the gain equation :
Dynamic wavelength shift in semiconductor lasers occurs as a result of the change in refractive index in the active region during intensity modulation. It is possible to evaluate the shift in wavelength by determining the refractive index change of the active region as a result of carrier injection. A complete analysis of spectral shift during direct modulation found that the refractive index of the active region varies proportionally to carrier density and hence the wavelength varies proportionally to injected current.
Experimentally, a good fit for the shift in wavelength is given by:
where I0 is the injected current and Ith is the lasing threshold current.
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