Fig. 1. Principle of TDM pulsed OEO to realize PT symmetry. (a) Scheme diagram of the PT-symmetry pulsed OEO. (b) Frequency splitting of the PT-symmetric supermodes, where fS is the longitudinal mode spacing and κEP is the exception point.

Fig. 2. Detailed experimental setup of the pulsed OEO. (a) Proposed PT symmetry in time-multiplexed delay systems within a single oscillator. MZM, Mach–Zehnder modulator; WDM, wavelength-division multiplexer; SMF, single-mode fiber; EA, electronic amplifier; BPF, bandpass filter; PD, photonic detector. (b) Microwave pulses (black line) in the cavity and optical pulses generated by the light source. (c) Coupled pulses after the coupling part and the microwave pulses received by the PD (black lines).

Fig. 3. Spectrum of pulse OEOs with a span of 20 MHz: (a) PT-broken state; (b) PT-symmetry state.

Fig. 4. Experiment of PT-symmetry pulsed OEOs. (a) Experimentally and theoretically obtained frequency detuning of the pulses, function of the coupling strength κ/κEP. (b)–(d) Spectrum of three groups of PT-symmetric microwave pulses with the coupled strength marked in (a) with a span of 160 MHz and a frequency resolution of 1 kHz.

Fig. 5. Experiment of even gain pulsed OEOs. (a) Experimentally and theoretically obtained frequency detuning of the pulses, function of the coupling strength κ/κEP. (b)–(d) Spectrum of three groups of even gain microwave pulses with the coupled strength marked in (a) with a span of 160 MHz and a frequency resolution of 1 kHz.

Fig. 6. Numerical simulation of the disappearance of PT symmetry with phase perturbation. (a) Real parts of the eigenvalues for PT symmetry with different δθ. (b) Imaginary parts of the eigenvalues for PT symmetry with δθ.

Fig. 7. Coupling in the proposed pulsed OEO.