Optical gain, spectrum

Figure 3.31 Optical gain spectrum of a ZnO epilayer at excitation density of (a) ISOkWcm (b) 220kWcm, and (c) 300kWcm at RT. (After Ref [138].)...

I. Is it possible to observe a shift in coherent Raman scattering in the three-level system with A-type coupling We have done an experiment to obtain a femtosecond Raman gain spectrum in polydiacetylenes. The Raman spectrum is shifted to the red under increased pump (to i) intensity. By changing o>2> the amplification peak signal is to be shifted to lower frequency. If the optical Stark effect is observed, then, in principle, it should be possible to observe the effect of a high field on the coherent Raman spectrum (see Fig. 1). 

Unlike atomic or solid-state lasers, the lasing transitions in a semiconductor laser are transitions between continua of extended states rather than between localised states. The inversion criterion [4] then is that the electron and hole quasi-Fermi levels must be separated by more than the bandgap energies. The spectrum of the optical gain g is given by [5,6]... 

The onset of stimulated emission can be detected by a collapse of the broad emission spectrum to a narrow line. This line narrowing due to optical gain in a waveguide without resonator is commonly referred to as amplified spontaneous emission (ASE) [49], or travelling wave lasing, hi the case of additional optical resonators like gratings or microcavities, different modes with much narrower linewidths can be resolved. 

Gain spectrum The gain of an optical amplifier as a function of optical frequency or wavelength. 

Whether in the electrical or optical domain, amplification is never obtained for free—invariably the gain is associated with some level of added noise. In the case of an SOA, the fundamental origin of noise is spontaneous emission. As has already been discussed in Section II.A, with reference to recombination processes, electrons and holes spontaneously recombine resulting in the emission of a photon over timescales on the order of a nanosecond. Since the process is random it results in optical noise emitted over the complete gain spectrum of the SOA. Indeed, it was Einstein who first showed, on the basis of fundamental thermodynamic considerations, that in any inverted medium, spontaneous emission must occur at a rate that is proportional to the differential gain provided by the population inversion. In other words, spontaneous emission noise is an unavoidable fact of life. 

The mechanical or electro-optical modulation of CW lasers applied in these initial experiments was soon superceded by the use of injection-locked pulse-amplified pump lasers in combination with quasi-CW probe lasers [45,80]. When the probe laser has a higher frequency than the pump laser, one obtains a Raman loss or inverse Raman spectrum when the probe laser has the lower frequency, a stimulated Raman gain spectrum is observed by tuning one of the two lasers. Through the pulse amplification of the pump laser, its linewidth is increased over that of a CW laser to between 60 and 100 MHz (0.002 and 0.003 cm ). However, this was at first fully satisfactory, because the resolution in these experiments is largely determined by Doppler and residual pressure broadening. Moreover, the development of injection seeded Nd YAG lasers with smooth pulses has lowered the pump laser linewidth [81] to about 30 MHz. The construction of a seeded Nd YAG laser with longer pulses (35-45 ns) further reduced the linewidth to 10 MHz [82]. 

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