A photonic-assisted microwave frequency measurement (MFM) method based on frequency-to-time mapping (FTTM) and frequency-to-power mapping (FTPM) is proposed and experimentally demonstrated. The FTTM is used to perform the MFM in a broadband. Utilizing the results of the coarse measurements with FTTM as a guide, the microwave signal is downconverted into an intermediate frequency (IF) signal to further achieve the fine measurement using the FTPM based on an integrated Fano resonator and feedforward neural network (FNN). In the FTPM, the IF signal is injected into the Fano resonator-based microwave photonic link, where the carrier suppressed-single sideband modulation (CS-SSB) is achieved with the assistance of a dual-parallel Mach-Zehnder modulator (DPMZM), to map the frequency information on the constructed amplitude comparison function (ACF), denoted by the relation between the IF signal and the output splitting ratio (OSR) of the two output ports of the Fano resonator. Thanks to the high slope of the ACF, a high-sensitivity MFM is implemented in the FTPM. The measured frequency is the difference between the local oscillator (LO) frequency and the result of the FTPM. Further, the FNN is used to enhance the MFM accuracy based on the results of the FTPM. In the experiment, +/- 5.8 MHz and +/- 0.59 MHz measurement errors are demonstrated before and after the FNN operation within a range from 2 to 20 GHz, corresponding to relative errors of +/- 0.032% and +/- 0.003%, respectively.

An ultra-narrow-linewidth laser is a core device in fields such as optical atomic clocks, quantum communications, and microwave photonic oscillators. This paper reports an ultranarrow-linewidth self-injection locked semiconductor laser, which is realized through optical feedback from a high-Q (258 million) Fabry-Perot (FP) cavity constructed with three mirrors, generating an output power of 12 mW. Employing a delay self-heterodyne method based on a signal source analyzer, the phase noise of the laser is -129 dBc/Hz at 100 kHz offset frequency, with an intrinsic linewidth of 3 mHz. This is the shortest, to our knowledge, intrinsic linewidth obtained in a self-injection-locked laser. Additionally, the estimated integral linewidth is 11.8 Hz, validating its extremely low noise level and outstanding narrow-linewidth performance. This study provides strong technical support for the extra- laboratory applications of narrow-linewidth semiconductor lasers, which is of great significance to the development of various coherent optical systems. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

A controllable frequency-hopping (FH) optoelectronic oscillator (OEO) based on active time-domain mode-locking (TDML) is proposed and experimentally demonstrated. In the proposed FH OEO, a dual-passband microwave photonic filter (MPF) based on phase-modulation-to-intensity-modulation (PM-IM) conversion is implemented using two laser diodes (LDs), a phase modulator, a micro-disk resonator, and a photodiode. Using two synchronized electrical control signals, the two LDs are intensity modulated to achieve the controllable two sub-passbands of the dual-passband MPF. Setting the frequencies of the two control signals equal to an integral multiple of the OEO free spectral range to achieve the TDML, a controllable FH signal generation scheme is achieved in the proposed FH OEO without the mode competition effect. In the experiment, a binary FH signal with controllable time duration, carrier frequency, and repetition period of the sub-signals is generated. The two sub-signals of the generated FH signal with an FH speed of about 21 ns, a time duration from 1% to 99% of 14.75 μs, a frequency range from 6 GHz to 18 GHz, and a repetition period from 14.75 μs to 147.5 ns are successfully demonstrated.

An active time-domain mode-locked (TDML) optoelectronic oscillator (OEO) based on direct intensity modulation of a semiconductor laser is proposed and experimentally demonstrated. The proposed TDML-OEO has the same structure as a conventional single-loop OEO, and the loop gain is modulated periodically by direct intensity modulating a distributed feedback (DFB) laser diode (LD). Matching the loop gain modulation frequency to the OEO free spectral range (FSR), TDML is achieved for OEO. The proposed TDML-OEO features flexible pulse duty cycle tunability and higher order harmonic mode locking. In the experiment, microwave pulses with different duty cycles ranging from 2% to 96% are generated. A 50th-order harmonic mode locking for the TDML-OEO is implemented.

After publication of [Opt. Express33(21), 44226 (2025)10.1364/OE.575241], the authors noticed that Fig. 9 in [1] was incorrectly placed as Fig. 8. This erratum corrects Fig. 9 in [1] based on the "Author Response" file archived in the Prism system. All the conclusions remain unchanged after the correction.

A linearity improvement method for frequency-modulated distributed feedback laser diodes (DFB-LD) is proposed and demonstrated based on a pre-distortion signal and an electro-optical phase-locked loop (EO-PLL). The pre-distortion signal is used to reduce the deterministic frequency errors. The EO-PLL is further used to suppress the stochastic frequency noise and enhance the coherence of the DFB-LD. In the EO-PLL, the DFB-LD output is transmitted through a Mach-Zehnder interferometer (MZI) and detected by a photodetector (PD) to get a beat note signal, which denotes the nonlinearity of the chirp. A mixing signal, achieved by mixing the beat note signal with a fixed frequency reference signal, is then filtered by a proportional integral filter (PIF) and feedback to the DFB-LD to reduce the stochastic frequency noise in the chirp. The EO-PLL bandwidth can be adjusted by tuning the PIF response. Consequently, a linear chirp optical signal with an enhanced linearity is generated from the DFB-LD. In the experiment, 788- and 321-time linearity improvements are implemented when the loop bandwidths are about 100 and 550 kHz, respectively. Correspondingly, residual frequency errors of 1.65 and 4.85 MHz at 52- and 450-THz/s chirp rates are obtained.

Conventional resonators such as microring resonators (MRRs) or microdisk resonators (MDRs) in large sizes can achieve a high quality factor (Q factor) but also introduce the trouble of multi-mode. Here, a silicon nitride elliptic MDR is designed and experimentally verified to suppress higher order modes while achieving a high Q factor. The resonator consists of an elliptic microdisk and an optimized bent waveguide. The bending loss in the waveguide is used to break the coupling conditions of the modes, thus reducing the coupling efficiency of the higher-order modes and obtaining a few-mode MDR transmission spectrum. In the experiment, the elliptic MDR with only two radial modes is successfully obtained. The TE1-mode 3-dB bandwidth, Q factor, and extinction ratio (ER) are 123 MHz, 1.57 × 106, and 14.8 dB, respectively. The suppression ratio (SR) between modes TE1 and TE2 is 12 dB. The proposed few-mode elliptic MDR has a broad application prospect in integrated microwave photonic systems.

A reconfigurable mode-locked optoelectronic oscillator (OEO) based on a directly modulated distributed feedback laser diode (DFB-LD) is proposed and experimentally demonstrated. In the mode-locked OEO, flexible switching of frequency domain mode-locking (FDML), time domain modelocking (TDML), and time-frequency domain mode-locking (TFDML) states is achieved based on the controlled intensity and frequency modulation of the DFB-LD. In the FDML state, the DFB-LD is frequency-modulated by a triangular wave signal to implement a frequency-scanning microwave photonic filter (MPF). Controlling the bias current of the DFB-LD to make the minimum gain of the loop greater than one and the frequency of the MPF equal to an integer multiple of the free spectral range (FSR), the FDML OEO is achieved. In the TDML state, the DFBLD is intensity-modulated by a square wave signal to achieve an on-off keying MPF. The TDML OEO is realized when the frequency of the MPF is equal to an integer multiple of the FSR. In the TFDML state, the DFB-LD is both intensity and frequency modulated by a triangular wave signal to construct an intensitymodulated frequency-scanning MPF with a frequency equal to an integer multiple of the FSR. By controlling the bias current of the DFB-LD to make the maximum gain within the loop greater than one and the minimum gain less than one, the TFDML OEO is achieved. The proposed approach is demonstrated experimentally. In the experiment, the linearly chirped microwave waveform (LCMW) signals, microwave pulse signals, and pulsed LCMW signals with powerful tunability are generated in the FDML, TDML, and TFDML states, respectively.

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 mu s, and a repetition period of 12.3 mu s 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 mu s is generated.

The fast-growing integrated ultrafast tuning rate semiconductor lasers have advanced the fields of light detection and ranging (LiDAR) and microwave photonics (MWP). Recent advances have demonstrated the exahertz-per-second fast tuning laser and the success of the LiDAR and MWP radar driven by tens of petahertz-per-second fast tuning lasers. However, a trade-off between the tuning rate and tuning range for the state-of-the-art fast-tuning lasers is still a must, which limits the detection resolution and speed of LiDAR or MWP radar systems. Here, we upset such a trade-off consideration in the same fast-tuning laser, reporting a laser with a record-high wavelength tuning rate of 4.817 exahertz per second at a continuous tuning range of over 20 gigahertz based on indium phosphide (InP) integrated circuits. Using the reported laser unit, we perform a proof-of-concept frequency-modulated continuous wave (FMCW) LiDAR with a high resolution of around 0.5 cm. Moreover, the demonstrated fast-tuning laser has the advantages of high linearity, small size, lightweight, and field deployment, providing a fast-tuning laser that surpasses existing ones for atomic physics, LiDAR, and MWP applications.

A photonic-assisted microwave frequency measurement (MFM) method based on optical heterodyne detection is proposed and experimentally demonstrated. In the proposed MFM system, a linearly chirped optical waveform (LCOW) from a three-electrode distributed Bragg reflector laser diode (DBR-LD) and a multi-wavelength signal from a Mach-Zehnder modulator (MZM), where the signal under test (SUT) is modulated on an optical carrier from a distributed feedback laser diode (DFB-LD), are heterodyne detected by the photodetector (PD). A bandpass filter then filters the detected signal, and the envelope is detected by an oscilloscope. Then, frequency-to-time mapping is realized, and the signal frequency is measured. Thanks to the fast tuning rate and large tuning range of the DBR-LD, the proposed MFM system has a high measurement speed and a broad instantaneous measurement bandwidth. In the experimental demonstration, a measurement error below 39.1 MHz is achieved at an instantaneous bandwidth of 20 GHz and a measurement speed of 1.12 GHz/µs. The MFM of a frequency-hopping signal is also experimentally demonstrated. The successful demonstration of the MFM system with a simple structure provides a new optical solution for realizing broadband and fast microwave frequency measurements.

A photonic-assisted tunable multi-band linearly frequency-modulated (LFM) waveform generator based on optical heterodyne detection is proposed and experimentally demonstrated. The multi-band LFM waveform is generated from a photodetector (PD), where a broadband LFM optical pulse and a multi-wavelength optical signal with the same magnitude at different wavelengths are heterodyne-detected. The LFM optical pulse is generated from a three-electrode distributed Bragg reflector laser diode (DBR-LD). The multi-wavelength optical signal is produced through a Mach-Zehnder modulator (MZM), in which a light wave from an optical fiber laser (OFL) is nonlinearly intensity-modulated by a radio frequency (RF) signal. By tuning the wavelength of the LFM optical pulse lower than that of the multiwavelength optical signal, a multi-band LFM waveform is generated at the PD. The center frequency of the generated LFM waveform can be adjusted by manipulating the wavelength of the OFL. The versatility in the bandwidth and temporal duration of the LFM waveform is attained through the precise tuning of the LFM optical pulse. The tuning of the center frequency difference between frequency bands is performed by altering the frequency of the RF signal. Experimental results have corroborated the realization of a tunable multi-band LFM waveform generator. The bandwidth and temporal duration of each band and the center frequency difference between frequency bands have tuning ranges from 0.7 to 6.7 GHz, 1 to 50 mu s, and 2 to 6.7 GHz, respectively. The demonstration shows new avenues to implement high-performance multi-band LFM waveform generators for applications in multi-functional modern radar systems.

A microwave photonic frequency-doubling phase shifter with a broad bandwidth and large tuning range is proposed in this paper. Frequency doubling and phase shifting are realized by processing the input microwave signal in the optical domain at a dual-drive dual-parallel Mach–Zehnder modulator (DD-DPMZM) and a dual-parallel Mach–Zehnder modulator (DPMZM). The input signal is split into two branches through a 90-degree hybrid splitter. One signal is sent to the DD-DPMZM to achieve a phase-shifted carrier-suppressed up-sideband by tuning the bias voltage, and the other is sent to the DPMZM to realize a carrier-suppressed down-sideband. By beating the phase-shifted up-sideband and the down-sideband at a photodetector (PD), the input signal is frequency doubled and phase shifted. The proposed frequency-doubling phase shifter is simulated. The results show that the frequency-doubled signal has a phase-tuning range from 0 to 360 degrees. In addition, the influence of the amplitude and phase unbalance of the 90-degree hybrid splitter on the magnitude variation and phase deviation of the frequency-doubling phase shifter is studied.

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 frequency-modulated interrupted continuous waveform (FMICW) generator with an ultra-large bandwidth based on optical heterodyne detection is proposed and experimentally demonstrated. The FMICW generator is implemented through the heterodyne detection of a stable optical carrier and a gate-function-modulated frequency-chirped optical pulse. A stable sub-kHz optical fiber laser operates the optical carrier. The optical pulse is generated from a three-electrode distributed Bragg reflector (DBR) laser diode (LD), which is gate-function modulated by a current source via the passive phase control section. By beating the optical carrier and the optical pulse at a photodetector (PD), an FMICW is generated. The bandwidth of the FMICW is over 20 GHz, and the temporal duration is around 200 $\mu \text{s}$ . The compression ratio is obtained as large as $3.78\times 10 ^{6}$ . Thanks to the tunability of the gate-function-modulated frequency-chirped optical pulse, the generated FMICW is fully tunable in terms of gate-function period and duty ratio.

A photonic-assisted wideband frequency downconversion with simultaneous self-interference cancellation (SIC) and image rejection (IR) based on a dual-polarization Mach-Zehnder modulator (DPol-MZM) is proposed. The SIC is achieved by combining the DPol-MZM, a polarization controller, and a polarizer. The IR is performed by using a wavelength division multiplexer, two photodetectors, and an electrical 90-degree hybrid coupler. Because the received and assisted signals are injected into the system independently of each other, the system has strong robustness. The performance of the system is demonstrated through simulation experiments. The SIC depth higher than 35 dB and the image rejection ratio larger than 55 dB within 1 GHz bandwidth are achieved. Besides, the recovery 16-QAM signal has a clear constellation diagram and an EVM of 8.4%.

A novel multi-beam photonic receiver with a variable beam count has been proposed. The reconfigurable characteristic of the system is enabled with the variable splitters and optical switch arrays. According to the number of beams it is due to receive, the splitter with suitable ports is chosen to split by controlling the optical switch in order to configure the optical path. After splitting, the multi-wavelength optical carriers are sent to the wavelength division multiplexer to combine and complete the multi-beamforming process. Two structures have been designed to realize the reconfigurable function, while the optimized structure is proved by using several benchmarks for comparison. The operation step of the reconfigurable multi-beam receiver is then given. The first simulations validate that the proposed architecture can dynamically receive the multi-beam with a variable number by reconfiguring the state of the optical switch. The second simulation of the influence on the received beam pattern was performed under a different amplitude and phase inconsistency random values. The results show that the amplitude and phase inconsistency will decline the quality of the multi-beam. When the amplitude and phase inconsistency are within limits, the difference in the optical path within the outputs must be balanced and compensated during the proposed system's fabrication and deployment.

A time jitter analysis method for an optical signal based on gated on-off optical sampling and dual-Dirac modeling is proposed and demonstrated experimentally. The optical signal under test is firstly sampled by an optical sampling pulse train generated through the gating on-off modulation of a Mach–Zehnder modulator (MZM). The sampled pulse is then broadened using optical true-time delay and electrical low-pass filtering to reduce its bandwidth to match the sample rate of a low-speed electrical analog-to-digital converter (ADC), which is used to quantify the sampled pulse. An eye diagram is obtained from the quantified data and used to plot a time jitter histogram. Finally, the dual-Dirac model is introduced to analyze the time jitter histogram to obtain the total jitter (TJ), including the deterministic jitter (DJ) and random jitter (RJ). In the experiment, a 19.05 ps TJ, including a 13.20 ps DJ and a 5.85 ps RJ, is measured for a 2.5 GHz optical signal using the proposed time jitter analysis method. The results agree well with those measured with a commercial real-time oscilloscope.

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.