A frequency-modulated continuous-wave (FMCW) generator with an ultra-large time-bandwidth product (TBWP) is crucial in modern radar systems. Herein we propose and experimentally demonstrate a programmable FMCW generator with an ultra-large TBWP based on two three-electrode distributed Bragg reflector (DBR) laser diodes (LDs) and one photodetector (PD). Thanks to the rapid response of the refractive index to the injection current in the passive phase control section, which is caused by the free-carrier plasma effect, the three-electrode DBR LD takes advantage of programmable and ultra-fast wavelength tuning. In the proposed FMCW generator, a programmable current source injects one three-electrode DBR LD to generate a broadband frequency-chirped optical signal. A constant current source injects the other one to perform a stable optical carrier. By beating the frequency-chirped optical signal and the optical carrier at the PD, a programmable FMCW is generated. Experimental results show that an ultra-wideband FMCW generator with full reconfigurability in modulation type, bandwidth, center frequency, chirp rate, and temporal duration is realized. A chirp rate over 10GHz/μs and a TBWP up to 2×10 ⁷ are experimentally demonstrated. Thanks to the monolithic integration of the critical devices, including two three-electrode DBR LDs and one PD, the demonstration promises breakthroughs in small-size and high-performance FMCW generators, facilitating applications in autonomous vehicles and space exploration systems.

A real-time eye diagram monitoring method for optical signals is proposed and experimentally demonstrated based on optical sampling. In the system, the optical signals under test are directly sampled by an optical sampling pulse train with a narrow pulse width and a high repetition frequency. The sampling pulse train is achieved in a Mach-Zehnder modulator (MZM), gated on-off by an electrical pulse. The sampled optical signals are then broadened and detected by a photodetector (PD). A low-speed electrical analog-to-digital converter (ADC) will then quantify the detected electrical signals. Combining with an algorithm based on the infinitesimal calculus, the quantified data is then used to achieve the eye diagram, according to which more time-domain parameters, such as period, time jitter, Q value, and bit error rate (BER) for the optical signals under test, are obtained. Thanks to the high repetition rate of the optical sampling pulse train, the eye diagram and the time-domain parameters of the optical signals are observed in real time. Experimental results show that a real time of about 350-μs eye diagram monitoring for a 2.5-Gb/s optical signal with a dynamic range from −10 to −22 dBm is achieved. In addition, time jitters are measured to range from 4.3 to 49.8 ps. Q values are estimated to range from 20.4 to 4.3, corresponding to BERs ranging from 2.3 × 10⁻⁹² to 8.5 × 10⁻⁶. The results are also verified by a commercial real-time oscilloscope.

A dual photonic-delay line cross-correlation method for phase noise measurement of microwave signal based on dense wavelength division multiplexing (DWDM) is proposed and experimentally demonstrated. The kilometer-long delay fiber is used together by the two mutual independent channels through the DWDM technology, resulting in that it is easier to control the kilometer-long fibers of the two channels are same. And it reduces the cost of the system by reducing the use of several kilometers of fiber and a Mach-Zendher modulator (MZM). Then, it uses variable optical fiber delay lines (VOLDs) to replace the electronic phase shifters, and a broader operation bandwidth can be obtained. A lower phase noise floor has been obtained by more averaging of the cross-correlation spectrum. In the experiments, the phase noise of a 10-GHz microwave signal has been measured accurately. A large operation bandwidth of 4-11 GHz is achieved and a measurement phase noise floor of -152.6 dBc/Hz at 10 kHz offset at 10 GHz in a 20 's delay measurement system is realized by averaging 100 times.

An active time-domain mode-locked (TDML) optoelectronic oscillator (OEO) is proposed and experimentally demonstrated based on intracavity polarization control. In the TDML OEO, the optoelectronic feedback loop gain is periodically controlled to above or below the OEO oscillation threshold using an electrical-controlled optical attenuator, which consists of two polarizers and one electronically controlled optical polarization controller (OPC). The loss of the optical attenuator is electrically controlled by adjusting the OPC to tune the polarization angle difference between the polarizer after the OPC and the linearly polarized light from the OPC. The active mode-locking is implemented when the period of loop loss matches the loop delay. The harmonic mode-locking is performed by increasing the loop-loss control frequency to ensure the time loop delay is an integer multiple of the loop loss period. Then, a microwave pulse is generated from the TDML OEO, where the pulse duty factor is tuned by tuning the ratio of the loop gains duration above the oscillation threshold and the loop gains duration below the OEO oscillation threshold. Simulation analysis and experimental demonstration are performed. In the experiment, microwave pulses with different duty factors ranging from 20% to 80% are generated at the modulation shape of a square. Microwave pulse generation with sinusoidal and triangular modulation shapes are also studied.

The generation of multi-format linearly chirped microwave waveforms (LCMW) based on dual-domain mode-locked optoelectronic oscillator (DDML OEO) is numerically and experimentally demonstrated. In the DDML OEO, the frequency domain mode locking (FDML) can be implemented using a frequency-scanning microwave photonic filter (MPF), which is realized based on a chirped laser diode (LD). The time domain mode locking (TDML) is realized by the period loop loss modulation via the intensity modulation of a dual-drive Mach–Zehnder modulator (DDMZM). The phase domain mode locking can be achieved using the phase modulation in the DDMZM. Controlling the OEO working at a time-frequency domain mode-locking (TFDML) state, pulsed LCMW or multi-band pulsed LCMW signals are generated when fundamental or ultra-high-order harmonic TDML is implemented. Controlling the OEO working as a phase-frequency mode locking (PFDML) state, a phase-coded LCMW signal is generated. In the experiment, a pulsed LCMW signal with a bandwidth of 8 GHz, a pulse width of 7.5 μs, and a repetition period of 12.3 μs is generated. A five-band pulsed LCMW signal is generated, and each band has a bandwidth of 1.3 GHz and a pulse repetition period of 0.41 ns. A phase-coded LCMW signal with a phase coding rate of about 50 Mb/s, a bandwidth of 2.3 GHz, and a repetition period of 12.3 μs is generated.

Optical phase locking is a critical technique in space coherent optical communication and active coherent laser beam combining. In a typical optical phase locking loop based on nonplanar ring oscillators, the pull-in range is normally less than 1 MHz, limited by loop delay and frequency tuning bandwidth of the laser source. Phase locking cannot be achieved at large initial frequency differences. In this work, a fast laser frequency acquisition method is demonstrated. The frequency difference between the signal and local lasers was measured via frequency dividing and period counting, and the frequency control signal was generated by a frequency discrimination and control module, to reduce the frequency difference to the pull-in range of the loop. Under the coordinating function of the loop filter and the frequency discrimination and control module, phase locking under a large initial frequency difference was achieved. The frequency acquisition range reached 164 MHz, and the acquisition and locking time was measured to be 440 ms. Additionally, the acquisition time was shortened with the decrease in initial frequency difference.