An ultrafast high-repetition-rate mode-locked fiber laser based on a semiconductor saturable absorption mirror is reported in this paper. The repetition rate is as high as 1 GHz. A piece of single-mode EDF is used as the gain medium and a linear cavity is built. The central wavelength of the mode-locked fiber laser is 1561 nm, the spectral width is 10.10 nm, the pulse interval is 1ns, the RF signal-to-noise ratio is greater than 68 dB, and the pulse width is 247 fs. The experimental results show that the stable high repetition rate mode-locked pulse can be obtained through this simple and compact structure, and the application of the mode-locked laser in optical sub-sampling is verified.
CMOS based Kerr soliton frequency comb has been demonstrated as the most promising multi-wavelength onchip light source. In this work, basic parameters of a Si3N4 micro-ring resonator (MRR) are characterized via the assistance of a fiber ring resonator and the calibration of a gas cell. Besides, by using of an auxiliary laser to suppress thermal dragging dynamics in dissipative soliton comb formation, a stable 100GHz Kerr soliton frequency comb generation is achieved.
In this paper, we illustrate a discrete Fourier processor based on dual optical frequency combs (OFCs). The simulation verified the DFT calculation with the frequency coverage of 0.2 GHz to 3 GHz and the resolution is 200 MHz. Meanwhile, demodulation of standard OFDM signal in time domain is simulated. This scheme avoids the problem of "electronic bottleneck" in current all-electronic processors. It is superior to the existing photon-assisted Fourier transform schemes in flexibility and accuracy, and has the characteristics of low power consumption, high throughput and high precision.
In this paper, we illustrate a radio-frequency (RF) channelization scheme based on dual optical frequency combs (OFCs). Modulated by two sets of cascaded electro-optic (EO) modulators, dual coherent OFCs with different free spectrum ranges (FSRs), namely 10 GHz and 11 GHz, are generated. Based on OFCs, a channelizer with six channels, 1 GHz channel spacing is experimentally demonstrated, covering frequency range from 1 GHz to 7 GHz. The input RF signal is impressed onto the signal comb by a Dual-Parallel Mach-Zander modulator (DPMZM). At the same time, balance detection methods and I/Q demodulation are used to achieve high-precision reception and the processing of RF signals. Selecting the corresponding channel with a waveshaper, the performance of six channels is shown and the spectra at different stages are also demonstrated.
Broadband radio frequency signal generation based on ultrafast optical pulse shaping, which is a typical microwave photonic technology, has been performed. The ultrafast optical pulse starts from our home-made mode-locked laser which has a repetition rate of ~171 MHz and spans from 1524 nm to 1593 nm. Following the MLL is an optical amplifier which makes up for the loss of the photonic link. A commercial pulse shaper is introduced to flatten and shape the spectrum of the optical pulse. However, it results in a additionally frequency cutoff. Only the C-band (5 THz optical bandwidth) is used efficiently which results in waste of spectrum resources. Then, the shaped pulses travel through a 5km long optical fiber realizing frequency-to-time mapping. The RF signal is acquired from the optical intensity profile by a high-speed photodetector at last. Several different kinds of wideband RF signals are generalized from the same hardware system, such as trigonometric and linear frequency-chirped waveform from 2 GHz up to 3 GHz, These results may be of interest to the radar and communication systems with ultraband RF signals.
Precise simulation of transient electrical behaviors of photodetectors under laser irradiation is becoming an increasingly concern. It not only can allow a detailed study and analysis of complex phenomena that cannot be carried out by experiments, but gives valuable information about the physical mechanisms which ultimately determine the response of the photodetectors. Finite difference numerical technique is adopted in the simulation to calculate the current response of photodetectors under pulsed laser irritation in this paper. To simulation the behaviors of photodetectors under pulsed laser irritation, the transport and trapping of carries and external circuit effects, including load resistance, junction capacitance, and parasitic capacitance, are considered. The basic equations governing the carrier behaviors are solved, including Poisson’s equation, the carrier motion equations, and the carrier continuity equations. The simulated transient carrier density and velocities are present, as well as corresponding transient electric field distributions. The behaviors of electrons and holes and its contributions to the external current response are analyzed. Then a general and brief image of the transient progress of photodetectors under pulsed laser irritation is established. How the carrier is induced, transported, and trapped and whether they make any significant contribution to the external current response are discussed. Besides, bias dependent response is also studied. Higher bias will improver the behaviors of photodetectors under pulsed laser irritation. The simulated results and theory analysis will show valuable clue for future research on the behaviors of photodetectors irradiated by pulsed laser.
In the experiments of photovoltaic detectors illuminated by CW lasers, some new mechanism has been
found, such as power saturation of photovoltage, hot carrier effect, as well as thermovoltage effect. To
investigate whether there is similar mechanism with pulsed laser irradiating, an 808nm femtosecond
pulsed laser is adopted. In the experiments, three photovoltaic infrared detectors are used, namely short
wavelength HgCdTe detector, medium wavelength HgCdTe detector and medium wavelength InSb
detector. Actually, the 808nm pulsed laser is spectral related laser for short wavelength HgCdTe
detector while spectral unrelated laser for medium wavelength HgCdTe and InSb detector. Under
various power densities, the detectors have a series of outputs. Power saturation of photovoltage is
observed. However, the characteristics of the outputs of these three detectors are quite different, even
between medium wavelength HgCdTe and InSb detector, which have the same packing method. There
are three major contributions in the paper. Firstly, explain the mechanism of power saturation of
photovoltage, mainly from hot carrier effect and the depressed ability of PN junction to separate
electrons and holes with the higher temperature induced by the laser. Secondly, compare the differences
between medium wavelength HgCdTe and InSb detector and give a qualitative analysis. Finally, the
difference of the outputs between short and medium wavelength HgCdTe detector is compared and
qualitatively analyzed, too, with the different mechanisms of interaction between infrared detectors and
spectral related or spectral unrelated laser. The experimental results and theory analysis will show
valuable clue for future research on photovoltaic detector irradiated by pulsed laser.
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