Laser intensity fluctuations when the photon number at threshold is small

Hideya Gamō, R. Grace, T. Walter

Ning Lu

We study the linewidth of a one-photon laser through the approach of the two-time correlation function. Our numerical calculations cover the regions of below, near, and above threshold. Well below threshold, the laser linewidth decreases linearly with increasing linear gain. Near and above threshold, in comparison with the approximate linewidth given by the phase-diffusion coefficient at the mean photon number (which is inversely proportional to the laser intensity---the Schawlow-Townes formula), we find that the laser intensity fluctuations increase the laser linewidth and that the Schawlow-Townes formula is valid only when the mean photon number is large and the photon-number variance is much smaller than the square of the mean photon number. Near threshold, the laser linewidth can be twice as large as that given by the phase-diffusion coefficient at the mean photon number (i.e., twice as large as that given by the Schawlow-Townes formula).

Ren Fang Chang, Robert W. Detenbeck, Victor Korenman +2 more

Holger Friedrich Hofmann, Ortwin G. Hess

The intensity fluctuations of laser light are derived from photon number rate equations. In the limit of short times, the photon statistics for small laser devices such as typical semiconductor laser diodes show thermal characteristics, even above threshold. In the limit of long-time averages represented by the low-frequency component of the noise, the same devices exhibit squeezing. It is shown that squeezing and thermal noise can coexist in the multimode output field of laser diodes. This result implies that the squeezed light generated by regularly pumped semiconductor laser diodes is qualitatively different from single-mode squeezed light. In particular, no entanglement between photons can be generated by use of this type of collective multimode squeezing.

Anders Smith, John A. Armstrong

H. J. Carmichael

Among the novel sources of nonclassical light that have recently been studied are a number of novel lasers. Conventional laser theory describes a classical stochastic field. I show that with a change in the standard operating conditions the conventional laser model produces nonclassical fields. Deviations from the classical description in standard laser theory are small by some inverse power of the threshold photon number. Operating conditions are designed to make stimulated emission dominant; thus laser cavities are good cavities that store photons and give a high threshold photon number. Low threshold photon numbers are achieved in bad cavities and most readily in a single-atom bad-cavity system. Such a system trades phase coherence for reduced intensity noise.