Generation of Coherent Light by a Moving Medium

We show that steady nonuniform motion of a medium through an optical resonator can yield light amplification at the resonator frequency. High gain can be achieved if at the generated frequency the medium refractive index is close to zero or the medium has a very strong frequency dispersion. We also discuss an analogy between light amplification by a moving medium and the generation of sound waves when gas flows along a tube with acoustically closed-open boundaries.

Figure 1

Dielectric medium flows in a tube with mirrors at the edges. The system is a resonator for electromagnetic field. The orifice in the middle part of the cavity is sealed with a porous metallic or dielectric material with the medium flowing through.

Figure 2

Electric field at the cavity center as a function of time for the fundamental cavity mode obtained by numerical solution of Eq. (1) with refractive index $n=1.8$ and velocity profile $V(z)=V_0\sin (2\pi z/L)$ for $V_0=0.05c$ (a) and $V_0=-0.05c$ (b). (c) Gain per unit time $G$ as a function of $V_0$ obtained by numerical solution of Eq. (1) (dots) and using analytical formula (9) for $m=0$ (solid line).

Figure 3

Geometry of the closed-open tube with gas flowing along the tube axis. Porous plate at the end provides acoustically closed boundary condition.

Figure 4

Gas density at the closed end of the tube as a function of time obtained by numerical solution of Eqs. (13)–(15) with initial and boundary conditions (16), (17) for flow velocity $V_0=0.03u_s$ (a) and $V_0=-0.03u_s$ (b).