Optical Injection Scheme Research Articles

Controlling beam loading to produce large-charge high-quality electron beams by tuning the laser profile in laser wakefield acceleration

Controlling beam loading is essential to produce large-charge high-quality electron beams in laser wakefield acceleration. The bunch shape in the longitudinal direction is a key factor to optimize beam loading. However, how to generate an electron beam with an appropriate shape is always a difficult problem to solve. In the present paper, indirect manipulation on the bunch shape is realized by tuning the laser profile. This is because that the bunch shape relies on the initial transverse position of electrons, which can be controlled by the laser profile in the optical injection scheme. Based on this indirect manipulation method, we find that by reducing the focal spot size of the injection pulse and thus effectively inhibiting off-axis electron injection, the electron beam shape gradually changes from down-ramp to flat-top, which is more favorable for optimizing beam loading. Consequently, we use a cigar-like injection pulse to generate electron beams with large charge and low energy spread. In this manner, an electron beam with 0.1% energy spread, total charge and emittance can be produced.

Laser diodes with modulated optical injection: towards a simple signal processing unit?

The idea of using the dynamical behaviour of a semiconductor laser to perform a certain processing operation of an input signal has been around for quite a long time. While the unidirectional optical injection scheme seems well suited to such a target—with the injection serving as an optical carrier for the input signal—the impact of a modulation of the injection beam still requires thorough investigation. Here, we study the case of an optically injected laser with a simple single-tone modulation term added to the injection signal. We analyse the impact of amplitude modulation on the laser dynamics, and particularly focus on the evolution within the injection locking range. We highlight clear passband behaviour corresponding to the laser resonance at its relaxation oscillation frequency, and characterize its features for various parameter changes. Next, we report dramatic differences between amplitude and phase modulation as the latter quickly leads to a loss of the injection locking and to the emergence of chaotic dynamics in place from the passband response identified in the case of amplitude modulation. At last, we discuss the suitability of using laser diodes for signal filtering, as was recently proposed by others, and identify the main remaining issues that need to be overcome.

Stability of period-one (P1) oscillations generated by semiconductor lasers subject to optical injection or optical feedback.

We study the stability of period-one (P1) oscillations experimentally generated by semiconductor lasers subject to optical injection (OI) and by those subject to optical feedback (OF). With unique advantages of broad frequency tuning range and large sideband rejection ratio, P1 oscillations can be useful in applications such as photonic microwave generation, radio-over-fiber communication, and laser Doppler velocimeter. The stability of the P1 oscillations is critical for these applications, which can be affected by spontaneous emission and fluctuations in both temperature and injection current. Although linewidths of P1 oscillations generated by various schemes have been reported, the mechanisms and roles which each of the OI and the OF play have however not been investigated in detail. To characterize the stability of the P1 oscillations generated by the OI and the OF schemes, we measure the linewidths and linewidth reduction ratios (LRRs) of the P1 oscillations. The OF scheme has a narrowest linewidth of 0.21 ± 0.03 MHz compared to 4.7 ± 0.6 MHz in the OI scheme. In the OF scheme, a much larger region of LRRs higher than 90% is also found. The superior stability of the OF scheme is benefited by the fact that the P1 oscillations in the OF scheme are originated from the undamped relaxation oscillation of a single laser and can be phase-locked to one of its external cavity modes, whereas those in the OI scheme come from two independent lasers which bear no phase relation. Moreover, excess P1 linewidth broadening in the OI scheme caused by fluctuation in injection parameters associated with frequency jitter and relative intensity noise (RIN) is also minimized in the OF scheme.

Chaos generation in a semiconductor ring laser with an optical injection

Two chaos generation schemes are demonstrated numerically based on a semiconductor ring laser (SRL) with either a constant or a chaotic optical injection from an external distributed feedback (DFB) laser. Under a certain bias current, the chaotic output can be generated by the SRL with appropriate injection parameters. Parameter map is achieved numerically to show types of oscillation in the SRL. Bifurcation diagrams and Lyapunov exponent spectra roughly describe the routes to chaos for the SRL with disturbance. A bandwidth-enhanced chaos generation can be describes based on the chaotic optical injection scheme. The bandwidth of the bandwidth-enhanced chaos generated is about 12GHz, which achieves approximately a two-fold increase compared with the constant injection scheme.

Photonic Generation of Broadly Tunable Microwave Signals Utilizing a Dual-Beam Optically Injected Semiconductor Laser

We propose and study photonic generation of broadly tunable microwave signals utilizing a dual-beam optically injected semiconductor laser. By injecting a slave laser with two detuned master lasers at the stable locking states, microwave signals with frequencies corresponding to the frequency spacing of the master lasers can be generated. Without the need for a microwave reference source, the dual-beam optical injection scheme has the advantages of low cost and less system complexity. Moreover, without the limitations of period-doubling bifurcation and Hopf bifurcation, by utilizing the period-one oscillation state with a single-beam injection scheme, the microwave signals generated with the proposed scheme have a much broader tuning range. In this paper, optical and power spectra of the microwave signals generated with the dual-beam optical injection scheme are compared with those generated with the optical mixing, the single-beam injection, and the unlocked dual-beam injection schemes. Generation of tunable microwave signals up to 120 GHz is demonstrated, which is currently limited by the locking range of the slave laser determined by the frequency difference between the Hopf (higher frequency) and the saddle node (lower frequency) bifurcation curves.

Cold Optical Injection Producing Monoenergetic, Multi-GeV Electron Bunches

A cold optical injection mechanism for a laser-plasma accelerator is described. It relies on a short, circularly polarized, low-energy laser pulse counterpropagating to and colliding with a circularly polarized main pulse in a low density plasma. Contrary to previously published optical injection schemes, injection is not caused here by electron heating. Instead, the collision between the pulses creates a spatially periodic and time-independent beat force. This force can block the longitudinal electron motion, leading to their entry and injection into the propagating wake. In a specific setup, we compute after acceleration over 0.6 mm, a 60 MeV, 50 pC electron bunch with 0.7 MeV rms energy spread, proving the interest of this scheme to inject electron bunches with a narrow absolute energy spread. Acceleration to 3 GeV with a rms spread smaller than 1% is computed after propagation over 3.8 cm in a plasma channel.

Modeling of All-Optical-Signal Processing Devices Based on Two-Mode Injection-Locked Vertical-Cavity Surface-Emitting Laser

We proposed all-optical signal processing devices based on a two-mode injection locking scheme. The operating principle is based on an all-optical inverter using transverse mode switching of an injection-locked vertical-cavity surface-emitting laser (VCSEL). Injection-locking with CW assist light enables us to increase the operating speed and to stabilize the fixed polarization state. Also, it provides a nonlinear transfer function in input–output characteristics, which will be useful for signal re-shaping, re-amplification. The polarization state of an output signal can be strictly controlled by two-mode optical injection scheme. Thus, the polarization state of an inverted signal can be fixed for randomly polarized input light. In addition, all-optical 3R regeneration and NRZ–RZ format conversion can be achieved with an electrically driven clock signal. The proposed scheme gives rise to novel functions such as all-optical 3R regeneration with an optical re-polarization function, which is called 4R regeneration. In addition, NRZ–RZ format conversion can be supported. We carried out the modeling based on a multi-transverse-mode rate equation analysis, which includes two orthogonal polarization modes, exhibiting a potential for high-speed 4R regeneration and format conversion at 10 Gbps.

Improvement of electron beam quality in optical injection schemes using negative plasma density gradients

Enhanced electron trapping using plasma density down-ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector is examined in detail using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength leads to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength--e.g., approximately 5 fs), high charge ( > 20 pC at modest injection laser intensity approximately 10(17) W/cm(2)), with a relative energy spread of a few percent at a mean energy of approximately 25 MeV, and a normalized root-mean-square emittance of the order of 0.5 mm mrad.

Trapping and acceleration of nonideal injected electron bunches in laser Wakefield accelerators

Summary form only given. The standard regime for the laser wakefield accelerator (LWFA) usually requires external injection of MeV electrons. Ideally, the injected electron bunch should be injected into the proper phase of the accelerating wake, have a bunch length that is small compared with the plasma wavelength, and a low emittance and energy spread. There are several optical injection concepts that are theoretically capable of producing such precisely-timed (phased), high quality electron bunches, but the requirements are severe. This paper reports simulation studies of several nonideal injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided laser wakefield accelerator. A plasma channel is used to confine the laser pulse over a distance of many Rayleigh lengths. For the case of mono-energetic, unphased (long bunch) injection, the accelerated electrons have a large energy spread when the injection energy W/sub 0/ is high. However, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. Simulation results for the trapping fraction, final average energy and energy spread, and the final bunch length agree well with the predictions of a simple Hamiltonian model using an ideal sinusoidal wake moving at the group velocity of the laser pulse. This long bunch case may apply to injection with a conventional or photocathode RF gun. The case of a channel-guided LWFA with a broad injected energy spread has also been simulated. Although the trapping fraction is generally much smaller than in the monoenergetic case, some simulations exhibit final accelerated bunches with remarkably small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. The implications for planned LWFA experiments at NRL using various optical injection schemes will be discussed.

Approaching intraband relaxation rates in the high-speed modulation of semiconductor lasers

This paper uses a nonequilibrium semiconductor laser model to investigate high-modulation bandwidth operation in semiconductor lasers. In particular, limitations to /spl gsim/100GHz modulation response, which approaches the carrier-phonon scattering rate, are analyzed. It is found that plasma heating leads to a dynamic carrier population bottleneck, which limits scaling of modulation bandwidth. An optical injection scheme is proposed to verify this phenomenon experimentally.

Diverse waveform generation using semiconductor lasers for radar and microwave applications

The possibility of using semiconductor lasers to conveniently generate diverse microwave waveforms for radar and microwave applications is studied both numerically and experimentally. Such waveforms are generated from the dynamical states of semiconductor lasers in different perturbation schemes and varying operating conditions. Using an optical injection scheme, broad-band chaotic microwave waveforms and tunable narrow-band harmonic microwaves over a broad frequency range can be generated. Using an optoelectronic feedback scheme, chaotic pulsing, regular pulsing, frequency-locked pulsing, and quasi-periodic pulsing waveforms are generated. These optically generated microwave waveforms can be easily amplified and radiated out using microwave amplifiers and antennas. The power spectra, time series, and autocorrelation traces of such waveforms are studied. The peak-sidelobe level is calculated to quantitatively compare the correlation characteristics of these waveforms. A broad-band chaotic waveform with a clean single-spike /spl delta/-function-like correlation profile useful for radar and other applications that demand unambiguous correlation profile is demonstrated experimentally.

Synchronization of optical-feedback-induced chaos in semiconductor lasers by optical injection

We study the synchronization of feedback-induced chaos in semiconductor lasers by the optical injection scheme that consists of a transmitter laser with external optical feedback and a receiver laser with optical injection from the transmitter laser. The stable synchronization condition and the effects of the frequency detuning between the transmitter and the receiver lasers are investigated using numerical simulation. The results show that positive and negative frequency detuning exert different influences on the synchronization. A time lag between the waveforms of the transmitter and the receiver lasers in the case of a large frequency detuning is found and its experimental significance is commented on.