Definition of a laser threshold

Gunnar Björk, Anders Karlsson, and Yoshihisa Yamamoto
Phys. Rev. A 50, 1675 – Published 1 August 1994
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Abstract

We propose that the threshold of a laser is more appropriately described by the pump power (or current) needed to bring the mean cavity photon number to unity, rather than the conventional ‘‘definition’’ that it is the pump power at which the optical gain equals the cavity loss. In general the two definitions agree to within a factor of 2, but in a class of microcavity lasers with high spontaneous emission coupling efficiency and high absorption loss, the definitions may differ by several orders of magnitude. We show that in this regime the laser undergoes a transition from a linear (amplifier) behavior to a nonlinear (oscillatory) behavior at our proposed threshold pump rate. The photon recycling resulting from the high spontaneous emission coupling efficiency and high absorption may in this case result in lasing without population inversion, and coherent light is generated via ‘‘loss saturation’’ instead of gain saturation. This mechanism for lasing without inversion is very different from lasing without inversion using a radiation trapped state.

  • Received 31 January 1994

DOI:https://doi.org/10.1103/PhysRevA.50.1675

©1994 American Physical Society

Authors & Affiliations

Gunnar Björk, Anders Karlsson, and Yoshihisa Yamamoto

  • Department of Electronics, Royal Institute of Technology, Electrum 229, S-164 40 Kista, Sweden
  • ERATO Quantum Fluctuation Project, Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305
  • NTT Basic Research Laboratories, Atsugi-shi, Kanagawa 243, Japan

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Vol. 50, Iss. 2 — August 1994

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