  We develop a model of the generation of coherent radio emission in the Crab
pulsar, magnetars and Fast Radio Bursts (FRBs). Emission is produced by a
reconnection-generated beam of particles via a variant of Free Electron Laser
(FEL) mechanism, operating in a weakly-turbulent, guide-field dominated plasma.
We first consider nonlinear Thomson scattering in a guide-field dominated
regime, and apply to model to explain emission bands observed in Crab pulsar
and in Fast Radio Bursts. We consider particle motion in a combined fields of
the electromagnetic wave and thee lectromagnetic (Alfvenic) wiggler. Charge
bunches, created via a ponderomotive force, Compton/Raman scatter the wiggler
field coherently. The model is both robust to the underlying plasma parameters
and succeeds in reproducing a number of subtle observed features: (i) emission
frequencies depend mostly on the length $\lambda_t$ of turbulence and the
Lorentz factor of the reconnection generated beam, $\omega \sim \gamma_b^2 (
c/\lambda_t) $ - it is independent of the absolute value of the underlying
magnetic field. (ii) The model explains both broadband emission and the
presence of emission stripes, including multiple stripes observed in the High
Frequency Interpulse of the Crab pulsar. (iii) The model reproduces correlated
polarization properties: presence of narrow emission bands in the spectrum
favors linear polarization, while broadband emission can have arbitrary
polarization. (iv) The mechanism is robust to the momentum spread of the
particle in the beam. We also discuss a model of wigglers as non-linear
force-free Alfven solitons (light darts).
