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21) where τ is the recombination lifetime of carrier, τp is the photon lifetime in the cavity, V is the volume of gain medium, I is injection current, Nth is the threshold density of carrier, e is the electron charge, S is combination factors accounting for confinement factor, gain efficient, cavity volume, an other parameters. Under steady state condition. 23) where Ith = eV τ Nth is threshold injection current, one finds the photon density above the threshold is a linear function of injection current I, the total photon energy E in the laser cavity within an active volume V is written as E = Np V hν, where hν is a photon energy.

The fundamental configuration is shown in Fig. 5(c), in this way, two various operating schemes are involved, i): quasi-continuous tuning; ii): continuous tuning. i) Quasi-continuous tuning: this scheme is defined as continuous wavelength tuning with different longitudinal modes. The tuning method is shown in Fig. 5(c). When mode A comes to the point A , in similar way to the phase control, the Bragg wavelength is changed in such a way that the new mode B coincides in wavelength with the previous A .

1 Threshold condition Threshold current is the most important and basic parameter for laser diodes. Fig. 10 shows the structure of a general-purpose Fabry-Perot diode laser, which is modelling as a resonator containing plane optical waves travelling back and forth along the length of diode laser. Incident spontaneous emission light propagating to the reflection mirror is amplified by stimulated emission and comes back to initial position after a round trip inside the laser cavity. This process is subject to losses arising from light going through or diffracting at the reflection mirrors, and scattering or absorption within an active light-emitting medium.