  Counting the microwave photons emitted by an ensemble of electron spins when
they relax radiatively has recently been proposed as a sensitive method for
electron paramagnetic resonance (EPR) spectroscopy, enabled by the development
of operational Single Microwave Photon Detectors (SMPD) at millikelvin
temperature. Here, we report the detection of spin echoes in the spin
fluorescence signal. The echo manifests itself as a coherent modulation of the
number of photons spontaneously emitted after a $\pi/2_X - \tau - \pi_Y - \tau
- \pi/2_\Phi $ sequence, dependent on the relative phase $\Phi$. We demonstrate
experimentally this detection method using an ensemble of $\mathrm{Er}^{3+}$
ion spins in a scheelite crystal of $\mathrm{CaWO}_4$. We use
fluorescence-detected echoes to measure the erbium spin coherence time, as well
as the echo envelope modulation due to the coupling to the $^{183}\mathrm{W}$
nuclear spins surrounding each ion. We finally compare the signal-to-noise
ratio of inductively-detected and fluorescence-detected echoes, and show that
it is larger with the fluorescence method.
