Injection-locking Technique Research Articles

Integrated and DC-powered superconducting microcomb

Frequency combs, specialized laser sources emitting multiple equidistant frequency lines, have revolutionized science and technology with unprecedented precision and versatility. Recently, integrated frequency combs are emerging as scalable solutions for on-chip photonics. Here, we demonstrate a fully integrated superconducting microcomb that is easy to manufacture, simple to operate, and consumes ultra-low power. Our turnkey apparatus comprises a basic nonlinear superconducting device, a Josephson junction, directly coupled to a superconducting microstrip resonator. We showcase coherent comb generation through self-started mode-locking. Therefore, comb emission is initiated solely by activating a DC bias source, with power consumption as low as tens of picowatts. The resulting comb spectrum resides in the microwave domain and spans multiple octaves. The linewidths of all comb lines can be narrowed down to 1 Hz through a unique coherent injection-locking technique. Our work represents a critical step towards fully integrated microwave photonics and offers the potential for integrated quantum processors.

Phase-noise-cancelled FMCW Lidar based on CS-DSB modulation and injection-locking technique

Phase-noise-cancelled FMCW Lidar based on CS-DSB modulation and injection-locking technique

A 223–276-GHz Cascadable FMCW Transceiver in 130-nm SiGe BiCMOS for Scalable MIMO Radar Arrays

This article reports a 223–276-GHz monostatic frequency-modulated continuous-wave (FMCW) radar transceiver (TRX) chip. The TRX is suitable for building massive multiple-input multiple-output (MIMO) radar arrays in the daisy-chain topology because of its cascadable feature based on the injection-locking feedthrough technique. The TRX is based on 18 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> frequency multiplication chain that can be fed by either an external signal generator (SG) or its internal voltage-controlled oscillator (VCO). The modulation bandwidth (BW) is about 53 GHz when the internal VCO is activated, resulting in a range resolution of better than 3 mm. The peak output power is 3.1 dBm at 240 GHz with a 3-dB BW of 41 GHz. Its receiver (RX) channel is based on the in-phase/quadrature architecture enabling complex signal sampling. The minimum single-sideband (SSB) noise figure (NF) is 18.7 dB at 255 GHz, and the average value is 20.7 dB. The TRX chip includes an on-chip antenna coupled with a silicon lens, offering a peak effective isotropic radiated power (EIRP) of 27 dBm at 240 GHz. The TRX consumes a dc current of 234 and 270.1 mA from a single supply voltage of 3.3 V for external and internal VCO operations, respectively. The chip occupies a total area of 2.7 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (2.16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 1.26 mm).

Deep Time-Delay Reservoir Computing With Cascading Injection-Locked Lasers

Reservoir computing is a simplified recurrent neural network, which requires training only at the output layer and hence significantly reduces the training cost. Time-delay reservoir computing (TDRC) introduces a large number of virtual neurons based on a single physical neuron and a feedback loop, which is friendly for hardware implementations. This work proposes a scheme for implementing the deep TDRC architecture based on cascading injection-locked semiconductor lasers. In each layer, the reservoir consists of a quantum dot laser and an optical feedback loop. The output of each reservoir layer is fed into the subsequent one through the optical injection-locking technique. This all-optical approach (for reservoir layers) has the merit of high scalability without any depth limitation. Theoretical analysis shows that the deep TDRC improves the performance on multiple benchmark tasks, including the memory capacity, the prediction of chaos, the nonlinear channel equalization, and the recognition of spoken digits.

Recent Advances in Optical Injection Locking for Visible Light Communication Applications

The introduction of visible light communication (VLC) technology could increase the capacity of existing wireless communication systems towards 6G networks. In practice, VLC can make good use of lighting system infrastructures to transmit data using light fidelity (Li-Fi). The use of semiconductor light sources, including light-emitting diodes (LEDs) and laser diodes (LDs) are essential to VLC technology because these devices are energy-efficient and have long lifespans. To achieve high-speed VLC links, various technologies have been utilized, including injection locking. Optical injection locking (OIL) is an optical frequency and phase synchronization technique that has been implemented in semiconductor laser systems for performance enhancement. High-performance optoelectronic devices with narrow linewidth, wide tunable emission, large modulation bandwidth and high data transmission rates are desired for advanced VLC. Thus, the features of OIL could be promising for building high-performance VLC systems. In this paper, we present a comprehensive review of the implementation of the injection-locking technique in optical communication systems. The enhancement of characteristics through OIL is elucidated. The applications of OIL in VLC systems are discussed. The prospects of OIL for future VLC systems are evaluated.

Injection-Locking Techniques for CMOS Millimeter-Wave and Terahertz Signal Generation

This brief presents a comprehensive tutorial on the recent advances of injection-locking techniques used for millimeter-wave (mm-Wave) and terahertz (THz) signal generation systems. Injection locking has the potential to significantly improve the system performance in terms of low phase noise and low power consumption. The focus will be on the state-of-the-art injection-locking techniques to effectively widen the locking range and reduce the phase noise, including injection-locked frequency dividers (ILFDs), injection-locked frequency multipliers (ILFMs), and sub-harmonic injection-locked phase-locked loops (IL-PLLs).

Improved Performance Tradeoffs in Harmonic Injection-Locked ULP TX for Sub-GHz Radios

This article presents a digital-intensive transmitter (TX) architecture for ultralow-power (ULP) wireless communication telemetry applications. A Type-I correction loop is proposed to adjust the frequency of the voltage-controlled oscillator (VCO) and enable the use of harmonically injection-locked (IL) technique without significant spurious tones. Two TXs implementations with different loop filters in the Type-I phase-locked loop (PLL) are demonstrated to suppress the voltage ripple at the VCO input. The proposed implementation not only minimizes the spurs from the correction loop but also overcomes the performance tradeoff between spur suppression, phase margin, and maximum achievable bandwidth. Prototyped in the standard 180-nm CMOS process, the two proposed TXs achieve an energy efficiency of 66.97 and 12.5 pJ/bit, respectively, and occupy an active silicon area of 0.0413 and 0.0435 mm2, respectively, while delivering the same output power of −14 dBm with >60-dB in-band spur suppression.

10 Tbit/s QAM Quantum Noise Stream Cipher Coherent Transmission Over 160 Km

We describe in detail our recent demonstration of a 10 Tbit/s secure physical layer transmission that we achieved by using digital coherent QAM quantum noise stream cipher (QNSC) and injection-locked WDM techniques. We used an FPGA-based transmitter and receiver to demonstrate a 165 channel polarization-multiplexed WDM 5 Gbaud 128 QAM/QNSC (70 Gbit/s) on-line transmission over 160 km with a spectral efficiency of 6 bit/s/Hz. In the present system, the original 128 QAM data were encrypted in a 1024 × 1024 QAM format using basis information. The encrypted signal was then masked by a large ASE noise, which reduced the detection “success” probability for an eavesdropper to 0.13% for each symbol. Furthermore, the multiplicity of the original QAM data and the seed keys used to generate the basis information were arbitrarily changed with time, which makes the decryption much more difficult.

Highly Efficient Microwave Power System of Magnetrons Utilizing Frequency-Searching Injection-Locking Technique With No Phase Shifter

A microwave power-combining technique based on magnetrons has been widely researched to solve the urgent demands for high-power, low-cost microwave sources in microwave industrial applications. To achieve high resultant efficiency, conventionally, injection locking with static frequency is utilized to drag the output frequency of each magnetron source to the exact same frequency, and a phase shifter is required to adjust the output phase of each combining source. In this article, we propose a novel phase shifterless microwave power system with two magnetrons utilizing a frequency-searching injection-locking technique. Coaxial cables with different physical lengths are used to realize phase adjustment under different injection frequencies. By sweeping the injection frequency in a small band, the phase difference between combining signals can be adjusted. Thus, high resultant efficiency can be obtained. When the injection frequency is swept, the phase difference between the output and injection signals has been analyzed theoretically, the phase difference between the different-length coaxial injection cables has also been analyzed. Furthermore, the relationship between the sweeping frequency bandwidth and requirements of the minimum length difference of the coaxial injection cables has been derived and analyzed. Experimental results show that a high resultant efficiency of 94.6% can be obtained with the proposed phase shifterless system. The system could maintain high resultant efficiency even when the free-running frequency and power of one magnetron are changed by adjusting its anode current. The experiments reveal that the proposed method works for different combining sources.

Spur Minimization Techniques for Ultra-Low-Power Injection-Locked Transmitters

Frequency multiplying wireless transmitters (TX) employing harmonic injection-locked technique benefit from high energy efficiency and less hardware complexity but largely suffer from reference spurs that violate the TX spectral specifications. This work proposes a self-aligned phase-locked loop (PLL) in conjunction with harmonic injection locking technique to achieve significantly improved spur performance under ultra-low-power (ULP) operations for sub-GHz IoT TXs. The on-chip type-I PLL calibrates the phase error in the ring oscillator (RO) in real-time and avoids large spurs induced from the frequency deviation in the harmonic injection locking. A compact zero power consumption twin-T notch filter for spur suppression is implemented within the PLL loop to tackle the rail-to-rail voltage jump that comes from the output of the phase detector (PD). Designed and fabricated in the TSMC 180 nm CMOS process, the proposed frequency multiplying TX occupies an active area of only 0.0413 mm2. The lowest power consumption with >3X improved energy-efficiency is observed while consistently achieving >62 dB spur suppression with −14 dBm output at 915 MHz. The TX supports OOK modulation with an average power consumption of $200.9~\mu \text{W}$ only at 3 Mb/s data rate achieving a normalized 66.97 pJ/bit energy efficiency.

Modulation Bandwidth Enhancement of Monolithically Integrated Mutually Coupled Distributed Feedback Laser

Modulation bandwidth enhancement of directly modulated semiconductor lasers (DMLs) has attracted broad interest to accommodate the tremendously growing demand for network traffic. In this paper, a monolithically integrated mutually coupled (IMC) laser for the O-band is demonstrated both numerically and experimentally. The direct modulation bandwidth was enhanced utilizing a photon–photon resonance (PPR) effect based on the mutual injection-locking technique. The IMC laser consisted of two distributed feedback (DFB) laser sections with a semiconductor optical amplifier (SOA) section in between. The relationship between the PPR frequency and SOA length was analyzed numerically to achieve a flat modulation response by optimizing the SOA length. Then, an enhanced 3-dB bandwidth of 38.7 GHz was realized experimentally, a nearly threefold enhancement over the modulation bandwidth of a solitary DFB laser at the same bias. Moreover, clear open eyes up to 40 Gb/s transmission over a 25-km single-mode fiber were achieved. Although the dynamic extinction ratio of the eye diagram was 1.1 dB, it can be further improved by increasing the mutual injection locking range of the IMC laser.

A Low-Spur and Intrinsically Aligned IL-PLL With Self-Feedback Injection Locked RO and Pseudo-Random Injection Locked Technique

A novel injection-locked phase locked loop (IL-PLL) with low spur and low phase noise is presented in this paper, in which self-feedback injection locked ring oscillator (SFIL-RO) and pseudo-random injection locked technique (PRILT) are presented. SFIL-RO, of which the oscillation signal is feedback to the input of the pulse generator to generate injection pulse, is proposed to achieve inherently aligned injection timing to decrease the injection spur. Meanwhile, it avoids the risk of suffering from pseudo-locking phenomenon, enhancing the immunity to the PVT variation. The periodicity of conventional injection behavior is broken in proposed PRILT, which essentially reduces the spur. The actual output of the IL-PLL with PRILT is the pseudo-random edge combination of two SFIL-ROs. Thus, the equivalent injection pulse is aperiodic. Calibration circuit is included to cope with the mismatch between two SFIL-ROs. To demonstrate the effectiveness of the proposed SFIL-RO and PRILT on optimizing spur, an IL-PLL was fabricated in SMIC 180 nm CMOS process. Measurement results show the best spur of −67.2 dBc and the RMS jitter of 124 fs are achieved when SFIL-RO and PRILT are activated. The core area was 0.142 mm2 and the power consumption is 6.1 mW from 1.8 V supply voltage.

An Active Phased Array Radar Transmitter Design

This paper presents an active phased array radar transmitter design using Injection-locking oscillators. The required phase relation for each array element is obtained by electronic tuning through injection-locking technique, and the beam-scanning capability is demonstrated, so a shifter less beam scanning method is proposed.

Phase-Dispersion Spectroscopy With High Spectral Resolution Using a Wideband Ultra-Linearly Swept Optical Source

We propose a phase-dispersion spectroscopy with high spectral resolution by developing a wideband ultra-linearly swept optical source (ULSOS). Highly precise optical frequency sweeping is achieved by externally modulating a narrow-linewidth fiber laser with a linearly swept radio-frequency signal. By using the injection-locking technique and high-order four-wave-mixing process, the sweeping span is enlarged to be 98 GHz, while the sweeping nonlinear error is as low as 136 kHz. Benefiting from the high-performance of the ULSOS, a spectrometer with an ultrahigh spectral resolution (136 kHz, determined by the sweeping nonlinear error of the ULSOS) is developed. Besides, an unbalanced Mach–Zehnder interferometer is used in the system with sample under test (SUT) incorporated inside it, and a phase extraction algorithm is employed to characterize the phase-dispersion features of the SUT. Moreover, a sweeping nonlinearity compensation technique is utilized to remove the noise in the measured transmission phase caused by the sweeping nonlinear error of the ULSOS. As a proof of the concept, the transmission intensity and phase spectra of HCN gas and fiber resonators with MHz-level bandwidth are characterized. The proposed technique opens a new possibility for spectroscopy with sub-MHz spectral resolution and the capability of dispersion measurement, without the usage of costly mode-locked laser.

The Improved Optical Heterodyne Scheme for RoF Systems Based on Optical Carrier Suppression and Optical Injection Locking Techniques

Abstract In this paper, we proposed an improved optical heterodyne scheme based on optical carrier suppression (OCS) and optical injection locking techniques (OIL) for prospective applications in radio-over-fiber (RoF) systems to generate high-quality millimeter waves. In our scheme, the 60-Ghz millimeter wave signal is generated by heterodyning the outputs of two vertical cavity surface emitting lasers (VCSELs) that are optically injection locked by the two modulation sidebands provided by a 30-GHz externally modulated master tunable laser with suppressed center wavelength. The combined OCS and OIL heterodyne method are not only able to eliminate the phase noise caused by the instability of laser frequencies, but also can further suppress the center wavelength, thus improving the heterodyne performance. We evaluate the transmission quality of the proposed RoF system from the center station to base stations through simulation, and results prove our proposed OCS-OIL-heterodyne scheme to have error-free propagation within a distance of 40 km, better than the OCS-heterodyne scheme.

Femtosecond tuning dynamics of organic amplifiers based on injection into DFB resonators of slant gratings

We report regenerative amplification of femtosecond laser pulses locked by injection of supercontinuum spectrum into a distributed feedback (DFB) microcavity constructed by slant gratings. Destroying the symmetry of the DFB resonance modes about the normal injection, the slant grating not only enabled concentration of the energy amplification on single modes, but also facilitate continuously tuning of the amplification in a broad band. The injection has locked efficiently the amplification spectrum, the transverse mode and the direction of the output beam, as well as the pulse length. In-plane amplification using a thin-film DFB device and injection locking supplies an important approach to produce high-quality ultrashort-pulsed laser beam from organic semiconductors. Advantages of injection-locked amplification techniques are also largely extended.

Two-way lightwave transmission system with a centralized-light-source and VCSEL-based upstream wavelength selector

A lightwave transmission system employing a centralized-light-source and vertical-cavity surface-emitting laser (VCSEL)-based upstream wavelength selector in two-way transmission is proposed and successfully demonstrated. The transmission performances of cable television (CATV), millimeter-wave (MMW), and baseband (BB) signals are investigated in a bidirectional way, with the assistance of the centralized-light-source and VCSEL-based upstream wavelength selector on the transmitter side. Extra optical components, such as a reflective semiconductor optical amplifier (RSOA), Fabry-Perot laser diode (FP LD), and phase modulator (PM), and the elaborated injection-locked technique are not required for upstream modulation, which largely reduce the cost and complication of two-way lightwave transmission systems. Excellent transmission performances of the carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple-beat (CTB), and bit error rate (BER) are obtained over a 25-km single-mode fiber (SMF) transmission with 100-m free-space link/5-m RF wireless transmission. This proposed two-way lightwave transmission system shows a convenient and economic eminent structure.

Phase noise reduction by optical phase-locked loop for a coherent bichromatic laser based on the injection-locking technique.

We present a coherent bichromatic laser system with low phase noise. An optical injection process is used to generate coherent laser beams with a frequency difference of 9.192 631 77 GHz using an electro-optical modulator. An optical phase-locked loop is then applied to reduce the phase noise. The phase noise of the beat note is -41, -81, -98, -83, and -95 dBrad2/Hz at the offset frequencies of 1 Hz, 100 Hz, 1 kHz, 10 kHz, and 1 MHz, respectively. Compared to a system that uses optical injection alone, the phase noise is reduced by up to 20-30 dB in the low-frequency range, and the intermodulation effect on the continuous atomic clock is reduced by an order of magnitude. This configuration can adjust the intensities and polarizations of the laser beams independently and reduce the phase noise caused by environmental disturbances and optical injection, which may be useful for application to atomic coherence experiments.

A C-Band FMCW SAR Transmitter With 2-GHz Bandwidth Using Injection-Locking and Synthetic Bandwidth Techniques

A synthetic bandwidth architecture utilizing an injection-locked oscillator to enhance the bandwidth of a linear frequency-modulated continuous wave transmitter is presented. A 500-MHz narrow bandwidth chirp from a direct digital synthesizer is upconverted to four adjacent carrier frequencies with 500-MHz spacing to obtain an effective synthetic bandwidth of 2 GHz at the C-band (43%). The four adjacent frequencies are obtained through the injection-locking technique to achieve fast frequency switching. This effectively results in an improved range resolution of 7.5 cm. Single-sideband mixing is employed to reduce in-band spurs and to avoid the use of switched filters. The architecture demonstrates ranging capability with a standard deviation of about 0.8 mm ( $1.36 \sigma_{\mathrm {CRLB}}$ , 26-dB SNR). Synthetic aperture radar images obtained from the proposed transmitter show sidelobe levels of less than −27 and −36 dBc in range and azimuth, respectively.

Optical frequency comb active filtering and amplification for second cooling laser of strontium optical clock

In this paper, we propose an optical frequency comb active filtering and amplification method combined with injection-locking technique to select and amplify a single mode from a femtosecond mode-locked laser. The key concept is to optically inject an optical frequency comb into a single mode grating external cavity semiconductor laser. The optical frequency comb based on a femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser. The center wavelength of the optical frequency comb is 689 nm with a 10 nm spectral width. A single mode grating external cavity semiconductor laser with a grating of 1800 lines/mm is used as a slave laser, and the external-cavity length from the diode surface to the grating is approximately 50 mm. The master laser is injected into the slave laser, and in order to select a single comb mode, we adjust the power of the master laser to control the locking range of the slave laser whose linewidth is smaller than the optical frequency comb repetition rate (250 MHz). While the operating current of the slave laser is set to be 55 mA and a seeding power is adopted to be 240 W, a single longitudinal mode is selected and amplified from 2.5104 longitudinal modes of the femtosecond optical comb despite the low power of the single mode. By tuning the optical frequency comb repetition frequency, the single longitudinal mode follows the teeth of the femtosecond optical comb, indicating the success in the optical frequency comb active filtering and amplification. The locking range is measured to be about 20 MHz. Meanwhile, the repetition frequency of the optical frequency comb is locked to a narrow linewidth 698 nm laser system (Hz level), thus the slave laser inherits the spectral characteristics of the 698 nm laser system. The linewidth is measured to be 280 Hz which is limited by the test beating laser. Then a continuous-wave narrow linewidth 689 nm laser source with a power of 12 mW and a side-mode suppression ratio of 100 is achieved. This narrow linewidth laser is used as a second-stage cooling laser source in the 88Sr optical clock, the cold atoms with a temperature of 3 K and a number of 5106 are obtained. This method can also be used to obtain other laser sources for atomic optical clock, and thus enabling the integrating and miniaturizing of a clock system.