Showing papers on "Pulse repetition frequency published in 2018"
TL;DR: The beam monitoring system was able to measure in a reproducible way the total charge of electrons that exit the machine, as long as the electron pulse amplitude remains above a given threshold, which is suitable mainly for small animal irradiation.
Abstract: The Oriatron eRT6 is an experimental high dose-per-pulse linear accelerator (linac) which was designed to deliver an electron beam with variable dose-rates, ranging from a few Gy/min up to hundreds of Gy/s. It was built to study the radiobiological effects of high dose-per-pulse/dose-rate electron beam irradiation, in the context of preclinical and cognitive studies. In this work, we report on the commissioning and beam monitoring of the Oriatron eRT6 prototype linac.
The beam was characterized in different steps. The output stability was studied by performing repeated measurements over a period of 20 months. The relative output variations caused by changing beam parameters, such as the temporal electron pulse width, the pulse repetition frequency and the pulse amplitude were also analyzed. Finally, depth dose curves and field sizes were measured for two different beam settings, resulting in one beam with a conventional radiotherapy dose-rate and one with a much higher dose-rate. Measurements were performed with Gafchromic EBT3 films and with a PTW Advanced Markus ionization chamber. In addition, we developed a beam current monitoring system based on the signals from an induction torus positioned at the beam exit of the waveguide and from a graphite beam collimator.
The stability of the output over repeated measurements was found to be good, with a standard deviation smaller than 1%. However, non-negligible day-to-day variations of the beam output were observed. Those output variations showed different trends depending on the dose-rate. The analysis of the relative output variation as a function of various beam parameters showed that in a given configuration, the dose-rate could be reliably varied over three orders of magnitude. Interdependence effects on the output variation between the parameters were also observed. The beam energy and field size were found to be slightly dose-rate-dependent and suitable mainly for small animal irradiation. The beam monitoring system was able to measure in a reproducible way the total charge of electrons that exit the machine, as long as the electron pulse amplitude remains above a given threshold. Furthermore, we were able to relate the charge measured with the monitoring system to the absorbed dose in a solid water phantom.
The Oriatron eRT6 was successfully commissioned for preclinical use and is currently in full operation, with studies being performed on the radiobiological effects of high dose-per-pulse irradiation.
124 citations
TL;DR: A fully automatic approach for recognizing seven PRI modulation types using a convolutional neural network (CNN) is proposed in this letter and simulation results show that the CNN-based recognition method not only promotes performance but is also robust to the environment with lost and spurious pulses.
Abstract: In modern electronic warfare environments, there are multiple-radar transmitting signals. For an electronic support system, it is essential to recognize the modulation of pulse repetition intervals (PRIs), since it is directly related to the indication of radar emitters. However, PRI modulations are more difficult to recognize in modern electronical environments due to the high ratio of lost and spurious pulses. Therefore, a fully automatic approach for recognizing seven PRI modulation types using a convolutional neural network (CNN) is proposed in this letter. Simulation results show that our CNN-based recognition method not only promotes performance but is also robust to the environment with lost and spurious pulses. The recognition accuracy is 96.1% with 50% lost pulses and 20% spurious pulses in simulation scenario.
49 citations
TL;DR: Free-space experimental measurements based on notched radar waveforms are collected and synthetically combined with separately measured hopping interference under a variety of conditions to assess the efficacy of such an approach, including the impact of interference hopping during the radar CPI.
Abstract: Spectrum sensing and transmit notching is a form of cognitive radar that seeks to reduce mutual interference with other spectrum users in the same band. This concept is examined for the case where another spectrum user moves in frequency during the radar's CPI. The physical radar emission is based on a recent FM noise waveform possessing attributes that are inherently robust to sidelobes that otherwise arise for spectral notching. Due to increasing spectrum sharing with cellular communications, the interference considered takes the form of in-band OFDM signals that hop around the band. The interference is measured each PRI and a fast spectrum sensing algorithm determines where notches are required, thus facilitating a rapid response to dynamic interference. To demonstrate the practical feasibility and to understand the trade-space such a scheme entails, free-space experimental measurements based on notched radar waveforms are collected and synthetically combined with separately measured hopping interference under a variety of conditions to assess the efficacy of such an approach, including the impact of interference hopping during the radar CPI, latency in the spectrum sensing/waveform design process, notch tapering to reduce sidelobes, notch width modulation due to spectrum sensing, and the impact of digital up-sampling on notch depth.
45 citations
TL;DR: A novel approach based on the transmission of plane waves and the simultaneous reception of echoes from 16 distinct subapertures of a linear array probe, which produces eight lines distributed over a 2-D region is presented.
Abstract: Quantitative blood velocity measurements, as currently implemented in commercial ultrasound scanners, are based on pulsed-wave (PW) spectral Doppler and are limited to detect the axial component of the velocity in a single sample volume. On the other hand, vector Doppler methods produce angle-independent estimates by, e.g., combining the frequency shifts measured from different directions. Moreover, thanks to the transmission of plane waves, the investigation of a 2-D region is possible with high temporal resolution, but, unfortunately, the clinical use of these methods is hampered by the massive calculation power required for their real-time execution. In this paper, we present a novel approach based on the transmission of plane waves and the simultaneous reception of echoes from 16 distinct subapertures of a linear array probe, which produces eight lines distributed over a 2-D region. The method was implemented on the ULAO-OP 256 research scanner and tested both in phantom and in vivo . A continuous real-time refresh rate of 36 Hz was achieved in duplex combination with a standard B-mode at pulse repetition frequency of 8 kHz. Accuracies of −11% on velocity and of 2°on angle measurements have been obtained in phantom experiments. Accompanying movies show how the method improves the quantitative measurements of blood velocities and details the flow configurations in the carotid artery of a volunteer.
43 citations
01 Oct 2018
TL;DR: A new signaling scheme for Dual-Function Radar Communications (DFRC) that enables frequency-hopped multiple-input-multiple-output (MIMO) orthogonal radar wave-forms to carry communication symbols is proposed and is shown to have better spectral efficiency.
Abstract: Intensifying competition over the frequency spectrum has driven the research effort into strategies for coexistence between radar and communications. Strategies for achieving this range from spectrum sharing using cognitive radio techniques to co-design where the radar and communications systems are re-imagined to ensure they do not interfere with each other. In this paper we propose a new signaling scheme for Dual-Function Radar Communications (DFRC) that enables frequency-hopped multiple-input-multiple-output (MIMO) orthogonal radar wave-forms to carry communication symbols. The Frequency-hopping (FH) code is changed from subpulse to another such that the index of the selected code carries the desired symbol. Contrary to recent phase-shift keying (PSK)-based schemes which embed one PSK symbol within each subpulse, the proposed scheme does not suffer from phase-discontinuity and is shown to have better spectral efficiency. We show that the data rate that can be achieved using the proposed scheme is proportional to the size of the FH code, the number of transmit antennas, number of subpulses within a pulse repetition interval, and pulse repetition frequency. Simulations examples are provided to evaluate the performance of the proposed method and demonstrate the effectiveness of this information embedding scheme.
43 citations
TL;DR: It was shown that the PRF is an important parameter that could be used for flexible control of electroporation efficiency in the high frequency range and it was determined that a threshold PRF exists when the relaxation of the cell transmembrane potential is longer than the delay between the consequent pulses.
41 citations
TL;DR: This letter describes how to design an SAR system without the nadir interference constraint and how to remove (not only smear) thenadir echoes by means of waveform diversity on transmit and appropriate postprocessing.
Abstract: Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is, therefore, very attractive for the systematic observation of dynamic processes on the earth’s surface. However, as a consequence of the pulsed operation and side-looking geometry of SAR, nadir echoes may significantly affect the SAR image quality, if the pulse repetition frequency is not conveniently constrained in the design of the SAR system. As the nadir interference constraint typically limits both the swath width and the ambiguity performance of the SAR system, the investigation of novel concepts for nadir echo removal is of great interest. This letter describes how to design an SAR system without the nadir interference constraint and how to remove (not only smear) the nadir echoes by means of waveform diversity on transmit and appropriate postprocessing. The proposed technique yields improved image quality and can be exploited in a similar manner for range ambiguity suppression with remarkable benefits for the design of novel SAR systems.
40 citations
TL;DR: In this paper, the effects of discharge and flow parameters on ignition kernel development time were explored in flowing methane-air mixtures, where a nanosecond pulsed high frequency discharge in a pin-to-pin configuration was used as the ignition source.
40 citations
TL;DR: High-fidelity optoacoustic images can now be obtained using cost-efficient lasers with light intensity modulated at discrete frequencies, and the authors demonstrated imaging of structural details from fish and mouse tissue microvasculature, by superimposing structural details that only appear at certain frequencies.
Abstract: Optoacoustic (photoacoustic) sensing employs illumination of transient energy and is typically implemented in the time domain using nanosecond photon pulses. However, the generation of high-energy short photon pulses requires complex laser technology that imposes a low pulse repetition frequency (PRF) and limits the number of wavelengths that are concurrently available for spectral imaging. To avoid the limitations of working in the time domain, we have developed frequency-domain optoacoustic microscopy (FDOM), in which light intensity is modulated at multiple discrete frequencies. We integrated FDOM into a hybrid system with multiphoton microscopy, and we examine the relationship between image formation and modulation frequency, showcase high-fidelity images with increasing numbers of modulation frequencies from phantoms and in vivo, and identify a redundancy in optoacoustic measurements performed at multiple frequencies. We demonstrate that due to high repetition rates, FDOM achieves signal-to-noise ratios similar to those obtained by time-domain methods, using commonly available laser diodes. Moreover, we experimentally confirm various advantages of the frequency-domain implementation at discrete modulation frequencies, including concurrent illumination at two wavelengths that are carried out at different modulation frequencies as well as flow measurements in microfluidic chips and in vivo based on the optoacoustic Doppler effect. Furthermore, we discuss how FDOM redefines possibilities for optoacoustic imaging by capitalizing on the advantages of working in the frequency domain.
38 citations
TL;DR: Simulation results have proved that this improved SDIF algorithm using clustering algorithm and PRI transform has higher reliability and accuracy in the presence of PRI jitter and pulse missing.
Abstract: In the electronic countermeasure, the electronic support measure (ESM) receiver intercepts the interleaved radar pulse streams, which radiates from surrounding radar emitters. At present, deinterleaving algorithm based on time of arrival which is called pulse repetition interval (PRI) deinterleaving occupies an important part of the ESM. The sequential difference histogram (SDIF) is a well-known and effective PRI deinterleaving algorithm, but it has several disadvantages. When the PRI jitter and pulse missing exist in the pulse streams, the SDIF algorithm cannot estimate the real PRI value reliably, and then cannot separate the radar pulse trains corresponding to the real PRI value from the interleaved pulse streams. In order to overcome these disadvantages, an improved SDIF algorithm using clustering algorithm and PRI transform is proposed. Simulation results have proved that this improved algorithm has higher reliability and accuracy in the presence of PRI jitter and pulse missing.
37 citations
TL;DR: Time-modulated frequency diverse multiple-input–multiple-output (TMFD-MIMO) array for integrated radar and communications, where the communication information bits are embedded via spread sequence technique during each radar pulse, achieves satisfactory probability of target resolution and signal-to-interference-plus-noise ratio.
Abstract: In this letter, we explore time-modulated frequency diverse multiple-input–multiple-output (TMFD-MIMO) array for integrated radar and communications, where the communication information bits are embedded via spread sequence technique during each radar pulse. Orthogonal waveforms are adopted in the TMFD-MIMO array for radar functionality, and a switching time-modulation scheme is applied according to the information bits, i.e., binary phase-shift keying (BPSK) associated spreading sequences. Thus, the number of embedded information bits during each radar pulse equals the number of spread sequences. In doing so, the communication receiver interprets the bits associated with a particular waveform as binary information with prior knowledge of the spread sequences. When compared with existing radar-communications, the achievable data rate is then proportionally increased with the pulse repetition frequency, spreading sequence, and size of the BPSK constellation. Moreover, satisfactory probability of target resolution and signal-to-interference-plus-noise ratio for radar functionality and symbol error rate for communication functionality are achieved.
18 May 2018
TL;DR: This paper uses 512-point FFT based digital EW receiver, requires minimum pulse width of 750 ns and frequency separation between two simultaneous pulses is 2.63 MHz to extract the key parameters accurately.
Abstract: This paper brings out a unique FPGA based pulse detection and characterization approach for digital wideband ESM receiver targeted for EW applications. The proposed approach uses a high speed ADC and FPGA based architecture for sampling and digital signal processing of the received RADAR signals to extract the key parameters such as Frequency (F), Time of Arrival (TOA), Pulse Width (PW) and Pulse Repetition Interval (PRI). The proposed novel FFT based digital EW receiver is designed and verified by MATLAB and SIMULINK tools and VHDL code for same algorithm is generated using HDL Coder for FPGA implementation. The current paper uses 512-point FFT based digital EW receiver, requires minimum pulse width of 750 ns and frequency separation between two simultaneous pulses is 2.63 MHz to extract the key parameters accurately. The simulated and measured results for Pulse detection algorithm are presented.
TL;DR: In this paper, a sparse optimization method based on compressed sensing is proposed for high-resolution range-Doppler reconstruction from random frequency hopping and PRF-jittering pulses.
Abstract: Agility radar with the carrier frequency random hopping and the pulse repetition frequency (PRF) staggering from pulse to pulse achieves superior performance against the electromagnetic jamming. This novel scheme leads to the discontinuity of phase in a coherent processing interval, thus the fast Fourier transform-based method is no longer a valid way to estimate the velocity of a target. A novel sparse optimisation method based on compressed sensing is proposed for high-resolution range-Doppler reconstruction from random frequency hopping and PRF-jittering pulses. The performance of moving target detection of the proposed method for frequency agile and PRF-jittering radar is analysed by comparing it with parameters-fixed pulse Doppler radar. Both simulation and field experimental results demonstrate the effectiveness of the proposal.
TL;DR: Wavelength modulation spectroscopy (WMS) technique was used along with a custom-built FM demodulator simplifying the signal retrieval and acquisition of a photothermal gas sensor.
Abstract: A novel configuration of a photothermal gas sensor is demonstrated. Photothermal-induced gas refractive index (RI) modulation is probed by a simple, mode-locked (ML) ring cavity fiber laser, operating in the 1.55 µm wavelength region. The measured gas sample is placed in an open-path section of the ML laser and the RI variations directly translate to its optical path-length change, which is easily detectable as pulse repetition frequency deviations. Wavelength modulation spectroscopy (WMS) technique was used along with a custom-built FM demodulator simplifying the signal retrieval and acquisition. Normalized noise equivalent coefficient calculated for the sensor was 1 x 10-5 cm-1 W Hz-1/2.
TL;DR: A compact, highly efficient, diode-pumped, mechanically Q-switched Tm:YAP laser operating near 2 μm, based on a torsion spring resonant mirror scanner, had negligible optical loss and required very low electrical drive power.
Abstract: We describe a compact, highly efficient, diode-pumped, mechanically Q-switched Tm:YAP laser operating near 2 μm. The Q-switch, based on a torsion spring resonant mirror scanner, had negligible optical loss and required very low electrical drive power. At a 10 kHz pulse repetition frequency, the laser generated an average output power of 10.5 W at 1.94 μm, Q-switched pulse energy of 1.05 mJ, a pulse length of 31 ns, and a peak power of 34 kW. The Q-switched laser exhibited maximum optical and electrical efficiencies of 51% and 26%, respectively.
TL;DR: The results show that compact intra-cavity KTA-OPO derived by Nd:YVO4 self-Raman laser is an efficient method for 1.7 μm waveband laser generation, with potential applications in biological imaging, laser therapy, special materials processing, etc.
Abstract: An intra-cavity optical parametric oscillator (OPO) emitting at 1.7 μm derived by Nd:YVO4 self-Raman laser is demonstrated in this Letter, with a KTiOAsO4 (KTA) crystal used as nonlinear optical crystal. A laser diode end-pumped acousto-optic Q-switched Nd:YVO4 self-Raman laser at 1176 nm was employed as the pump source. At an incident pump power of 12.1 W and a pulse repetition frequency of 60 kHz, average output power up to 1.2 W signal light at 1742 nm was obtained, with diode-to-signal conversion efficiency of 10%. The pulse width was about 11 ns and spectral line width was less than 0.5 nm for the signal light. The results show that compact intra-cavity KTA-OPO derived by Nd:YVO4 self-Raman laser is an efficient method for 1.7 μm waveband laser generation, with potential applications in biological imaging, laser therapy, special materials processing, etc.
01 Jan 2018
TL;DR: This study, which evaluates emitter-by-emitter classification, appears to be a novel approach, based on a survey of current literature; previous work citing the use of CNNs in this domain has been limited to radar waveform recognition vice pulse-based specific emitter identification.
Abstract: In this paper, an application of convolutional neural networks (CNN) for rapid and accurate classification of electronic warfare emitters is investigated; a large data set with 58 separate emitter sources is used for training and testing. Data preprocessing creates 3-dimensional images with a feature space composed of pulse width (PW), radio frequency (RF), and pulse repetition interval (PRI), referenced with respect to time of arrival (TOA). The image representation has proven to be the most effective, consistently producing classification accuracies approaching 98.7%. This study, which evaluates emitter-by-emitter classification, appears to be a novel approach, based on a survey of current literature; previous work citing the use of CNNs in this domain has been limited to radar waveform recognition vice pulse-based specific emitter identification.
TL;DR: The simulation results show that with medium pulse repetition frequency radar waveforms used, the data transmission rate of the proposed JRC system is in the range of Mbits/s.
Abstract: A joint radar-communication (JRC) system with both radar sensing and communication abilities is proposed to improve spectrum utilisation efficiency. The transmitter of the JRC consists of multiple antenna subarrays transmitting orthogonal waveforms. If the communication receivers are in radar sidelobe directions, the communication data symbols are embedded in the magnitude ratio as well as the phase shift between transmit waveform pairs. In the case of the communication receivers in radar main lobe direction, the data symbols are embedded in the phase shift between transmit waveform pairs only to preserve optimum radar target detection performance. Novel symbol mapping constellation scheme is designed to achieve a high data transmission rate while maintaining a relatively low symbol error rate. The simulation results show that with medium pulse repetition frequency radar waveforms used, the data transmission rate of the proposed JRC system is in the range of Mbits/s.
TL;DR: A long-distance inverse synthetic aperture LADAR (ISAL) imaging experiment outdoors over 1 km for cooperative targets is demonstrated, which gets a two-dimensional high-resolution image with resolution exceeding 2.5 cm.
Abstract: A long-distance inverse synthetic aperture LADAR (ISAL) imaging experiment outdoors over 1 km for cooperative targets is demonstrated, which gets a two-dimensional high-resolution image with resolution exceeding 2.5 cm. The system utilizes an electro-optic in-phase and quadrature modulator to output a linear frequency-modulated continuous waveform (LFMCW) with a bandwidth of 6 GHz and pulse repetition frequency (PRF) of 16.7 KHz. For the problem of the coherence of the laser, the effects of the coherent processing interval (CPI) and time delay of the local oscillator (LO) on the coherence are discussed. The fiber delay line is set and the CPI is reduced to lower the requirement of the frequency stability of the laser source. The images are formed by two-dimensional Fourier transform and joint time-frequency transform methods, respectively. In this paper, we present the system structure, imaging processing, and the experiment result in detail. The experiment result validates the performance of our system for ISAL imaging.
TL;DR: The improvement of an ultrasonic pulse generator for a pipeline inspection gauge (PIG), which uses 64 transducers for inspecting distances up to 100 km with an axial resolution fixed at 3 mm and variable speeds between 0 and 2 m/s, is presented.
Abstract: This paper presents the improvement of an ultrasonic pulse generator for a pipeline inspection gauge (PIG), which uses 64 transducers for inspecting distances up to 100 km with an axial resolution fixed at 3 mm and variable speeds between 0 and 2 m/s. An ultrasonic pulse generator is composed of a high-voltage (HV) MOSFETs, driver logic and an HV power supply. We used a DC-HV DC converter device as the HV power supply because it reduces the size of the ultrasound system considerably. However, pipeline geometry and inspection effects such as hammer and shock cause a variable pulse repetition frequency (PRF), producing voltage drops, poor quality of the HV pulse generated, failures in the dimensioning of defects and damage to devices by over-voltage. Our improvement is to implement a control scheme to maintain the high quality of the HV regardless of the variable PRF. To achieve this, we characterized three transfer functions of the DC-HV DC converter, varying the connected load to 10%, 45% and 80%. For the characterization, we used the least squares technique, considering an autoregressive exogenous (ARX) model. Later, we compared three control schemes: (1) proportional-integral-derivative (PID) tuned by simultaneous optimization of several responses (SOSR), (2) PID tuned by a neural network (NN) and (3) PI tuned by the analytical design method (ADM). The metrics used to compare the control schemes were the recovery time, the maximum over-voltage and the excess energy when the shock and hammer effects happen to occur. Finally, to verify the improvement of the HV pulser, we compared the ultrasonic pulses generated for various frequencies and amplitudes using the pulse generator with and without the control scheme.
TL;DR: In this paper, a custom-built fiber laser capable of operating at a PRF of up to 2 MHz was combined with a fast laser scanning optical OR-PAM system based on a stationary fiber-optic ultrasound sensor.
Abstract: The imaging speed of optical resolution photoacoustic microscopy (OR-PAM) using pulsed excitation is fundamentally limited by the range ambiguity condition, which defines the maximum laser pulse repetition frequency (PRF). To operate at this theoretical upper limit and maximize acquisition speed, a custom-built fiber laser capable of operating at a PRF of up to 2 MHz was combined with a fast laser scanning optical OR-PAM system based on a stationary fiber-optic ultrasound sensor. A large area (10 mm × 10 mm) of the mouse ear was imaged within 8 s, when acquiring 16 million A-lines and operating the laser at a PRF of 2 MHz. This corresponds to a factor of four improvement in imaging speed compared to the fastest OR-PAM system previously reported. The ability to operate at high-imaging frame rates also allows the capture of hemodynamic events such as blood flow. It is considered that this system offers opportunities for high throughput imaging and visualizing dynamic physiological events using OR-PAM.
TL;DR: In this article, the authors presented an innovative design, named the "zigzag design", for the optimization of the compactness and of the rise time of a Marx generator using pulse forming network (PFN).
Abstract: Repetitive high-voltage square pulses are of great importance for producing long-pulse electron beams and high-power microwaves. One of possible technologies for the generation of such pulses is a Marx generator using pulse forming network (PFN) stages, often combined with a pulse sharpening technique to reduce the rise time to a few nanoseconds (peaking stage). This paper presents an innovative design, named the “zigzag design,” for the optimization of the compactness and of the rise time of 400-kV–85-ns PFN-Marx. Thanks to this design, the 16 stages of this generator, which delivers an open circuit output voltage of 720 kV, fit in a 650-mm length. For a slightly overmatched load ( $Z_{\mathrm {load}} = 100 ~\Omega$ ), the output voltage reaches 400 kV with a rise time as less as 5 ns. The inductance reduction associated with the innovative zigzag design, which allows this sharp rise time with no need for a peaking stage, is described. The 85-ns plateau duration of the pulse is given by the PFN construction of each stage, which is based on six ceramic capacitors (2.1 nF–45 kV) connected within a strip line. The 16 PFN stages are housed in a 360-mm diameter gas pressurized vessel. Burst mode operation for a duration of 10 s at a pulse repetition frequency of 100 Hz is reported, for a resistive load and for the electron beam diode of a X-band relativistic backward-wave oscillator (BWO). To reach further compactness, the BWO system is integrated on side of the generator vessel and a U-shaped gas pressurized line connects both systems through a compact conical vacuum insulator.
TL;DR: In the framework of space-borne CO2 lidar development, the transmitter is a critical unit and the development and the assessment of performances of a 2-μm single-frequency thulium fiber laser pumped Q-switched Ho:YLF laser is reported on.
Abstract: In the framework of space-borne CO2 lidar development, the transmitter is a critical unit. We report on the development and the assessment of performances of a 2-µm single-frequency thulium fiber laser pumped Q-switched Ho:YLF laser. To fulfill the requirements of space-based operation, a master oscillator power amplifier architecture has been chosen, and the oscillator works in double-pulse operation. The transmitter can generate a single-mode dual wavelength emission “ON” and “OFF” around the R30e line of the 20013?00001 band of CO212. It delivers a pair of OFF-ON pulses with 12 mJ and 42 mJ energy, respectively, at a pulse repetition frequency of 303.5 Hz. The pulse energy and central frequency stabilities are especially documented as well as pulse duration, polarization, overall efficiency, beam quality, pointing stability, and spectral purity. The possible limitations by light-induced damage or radiation-induced attenuation on the laser performances are also evaluated.
TL;DR: Tunable picosecond pulse generation was demonstrated in the 2074-2104 nm spectral range and mode-locked, near-transform-limited pulses as short as 170 fs were generated at 2093 nm.
Abstract: We report on the first demonstration, to the best of our knowledge, of a diode-pumped Tm:LuScO3 laser. Efficient and broadly tunable continuous wave operation in the 1973-2141 nm region and femtosecond mode-locking through the use of an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror are realized. When mode-locked, near-transform-limited pulses as short as 170 fs were generated at 2093 nm with an average output power of 113 mW and a pulse repetition frequency of 115.2 MHz. Tunable picosecond pulse generation was demonstrated in the 2074-2104 nm spectral range.
TL;DR: This paper proposes adapting for altimeters the Omega-Kappa SAR focusing algorithm that is performed in the two-dimensional wavenumber domain, accounting for the difference existing between SAR and altimeter from geometry and instrument point of view.
Abstract: Fully-focusing of radar altimeters is a recent concept that has been introduced to allow further improvement of along-track resolution in high pulse repetition frequency (PRF) radar altimeters. The straight potentiality of this new perspective reflects into a more accurate estimation of geophysical parameters in some applications such as sea-ice observation. However, as documented in a recent paper, such capability leaves unsolved the problem of the high computational effort required. In this paper, we face the problem of adapting for altimeters the Omega-Kappa SAR focusing algorithm that is performed in the two-dimensional wavenumber domain, accounting for the difference existing between SAR and altimeter from geometry (looking and swath width) and instrument (echoes are deramped onboard on receiving) point of view. Simulations and an application using in-orbit data show the effectiveness of the proposed approach and the highly reduced computational effort.
TL;DR: This paper reports on high-power stable nanosecond pulse generation at ~2.1 μm from an integrated Tm-Ho all-fiber master oscillator power amplifier (MOPA) system, which is designed to operate at 1980 and 2116 nm.
Abstract: In this paper, we report on high-power stable nanosecond pulse generation at ~2.1 μm from an integrated Tm–Ho all-fiber master oscillator power amplifier (MOPA) system. A total output power of 128.5 W is generated from the Tm–Ho hybrid MOPA, with an average power of 99.1 W from Ho emission at 2116 nm; the corresponding pulse repetition frequency and pulse width are 161 kHz and 322 ns, respectively, leading to a peak power of 1.91 kW. The Tm–Ho integrated master oscillator is designed to operate at 1980 and 2116 nm, where the former wavelength serves as the pump of the Ho-doped fiber. Stable laser pulses are generated from both the Tm and Ho oscillators owing to mutual modulation of emission from the two lasers. The prospects for further scaling in output power at ~2.1 μm using Tm-Ho integrated MOPA system are discussed.
TL;DR: A high average power actively Q-switched (AQS) orthogonal polarization dual-wavelength Nd:YLF laser at 1047 and 1053 nm is reported, which are the highest average output powers for both CW and AQS orthogonally polarized dual- Wavelength lasers based on laser crystals.
Abstract: We report a high average power actively Q-switched (AQS) orthogonal polarization dual-wavelength Nd:YLF laser at 1047 and 1053 nm The gain-to-loss balance of dual wavelengths was realized via an uncoated quartz etalon A maximum continuous wave (CW) output power of 142 W was obtained under the incident pump power of 417 W, corresponding to an optical-to-optical conversion efficiency of 341% and a slope efficiency of 383% Active Q-switching was accomplished by inserting an acousto-optic modulator in the cavity Under the incident pump power of 40 W, this setup delivered a maximum average output power of 10 W at the pulse repetition frequency (PRF) of 30 kHz and the largest pulse energy of 34 mJ at the PRF of 1 kHz, respectively To the best of our knowledge, these are the highest average output powers for both CW and AQS orthogonally polarized dual-wavelength lasers based on laser crystals
TL;DR: In this article, a wavelength-selectable visible emission from an acousto-optic Q-switched Nd:YVO4 self-Raman laser was demonstrated.
Abstract: A wavelength-selectable visible emission from an acousto-optic Q-switched Nd:YVO4 self-Raman laser was demonstrated. A BBO crystal was adopted as a nonlinear optical crystal to realize second harmonic generation or sum frequency generation in the self-Raman cavity by angle tuning, resulting in the output on demand of yellow, lime and green laser with quick switching. Under an incident pump power of 10.2 W and a pulse repetition frequency of 60 kHz, maximum output power of 2.05, 0.81 and 1.73 W were achieved for the 532, 559 and 588 nm lasers, respectively. This laser system can provide a compact and convenient method to achieve the wavelength-selectable pulse visible emission for application as biomedical diagnostics, visual displays, etc.
TL;DR: In this article, the authors demonstrated a continuous-wave and acousto-optical Q-switched Ho:YAP laser pumped by fiber-coupled laser diode at 1.91µm.
Abstract: We demonstrated a first continuous-wave and acousto-optical Q-switched Ho:YAP laser pumped by fiber-coupled laser diode at 1.91 µm. With a dual-end wing-pumping scheme, a maximum continuous-wave output power of 10.5 W at 2.1 µm and a slope efficiency of 53.2% were obtained when the incident LD power was 25.6 W, while diode-to-Ho conversion efficiency reached up to 41.0%. The Q-switched Ho:YAP laser was investigated for different pulse repetition frequencies from 3 to 20 kHz. The maximum average output power of 9.8 W and maximum pulse energy of 2.7 mJ were achieved at pulse repetition frequencies of 20 and 3 kHz, respectively. In addition, we demonstrated an efficient mid-infrared laser based on a diode-Ho-ZGP architecture for the first time. With a pulse repetition frequency of 3 kHz and an incident Ho pump power of 8.2 W, the average output power of 4.8 W and slope efficiency of 61.0% were reached in mid-infrared ZGP-OPO.
01 Oct 2018
TL;DR: An anti-jamming method is put forward by generating the LFM-PC signal using a set of orthogonal phased coded signal in different pulse repetition interval (PRI), which destroys the correlation of the jamming signal and the transmission signal.
Abstract: Repeater deception jamming is an effective SAR jamming technique. It has been the hot spot of Synthetic Aperture Radar (SAR) Electronic Counter Measures (ECM) research for its ability to generate false targets with low jamming power, which reduces the difficulty of hardware realization of the jammer. Linear format modulated (LFM) signal is the most commonly used radar signal. However it has poor anti-jamming performance for its simple form and high recognition. A new hybrid modulated signal-LFM-PC signal, using phase coded signal modulates LFM signal, is proposed. Then time-domain and frequency-domain characteristics, range resolution, Doppler tolerance, orthogonality and imaging algorithm adaptability of the signal are studied. We put forward an anti-jamming method by generating the LFM-PC signal using a set of orthogonal phased coded signal in different pulse repetition interval (PRI). Because the LFM-PC signal in different PRI is not known to the jammer, which destroys the correlation of the jamming signal and the transmission signal. Simulation results show that the proposed method has a good anti-jamming performance.