Picosecond pulse amplification in AlGaAs flared amplifiers

Amplification of ultrashort pulses in a semiconductor flared amplifier is analyzed by using a two dimensional (2-D) time-domain BPM (beam propagation method) model including the effects of gain saturation, finite-gain bandwidth, self-phase modulation, index dispersion, carrier heating, and gain relaxation between successive pulses induced by carrier diffusion and recombination. The paper includes a presentation of the model and of the numerical resolution method, detailed analysis and discussion of the spatio-temporal pulse distortions in the case of picosecond and subpicosecond pulses and an evaluation of the device performance. The performance of the flared amplifier is evaluated considering the temporal length, the time-bandwidth product, the far field, the output energy as a function of the input energy, the injected current, and the repetition rate. While subpicosecond pulse amplification leads to strong distortions and to very low total gain, the flared amplifier is well adapted to the amplification of picosecond pulses at low-repetition rates     

Spectral hole-burning and carrier-heating dynamics in InGaAsquantum-dot amplifiers

The ultrafast gain and index dynamics in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers are measured at room temperature with femtosecond resolution. The role of spectral hole-burning (SHB) and carrier heating (CH) in the recovery of gain compression is investigated in detail. An ultrafast recovery of the spectral hole within ~100 fs is measured, comparable to bulk and quantum-well amplifiers, which is contradicting a carrier relaxation bottleneck in electrically pumped QD devices. The CH dynamics in the QD is quantitatively compared with results on an InGaAsP bulk amplifier. Reduced CH for both gain and refractive index dynamics of the QD devices is found, which is a promising prerequisite for high-speed applications. This reduction is attributed to reduced free-carrier absorption-induced heating caused by the small carrier density necessary to provide amplification in these low-dimensional systems     

High Repetition Rate Gigawatt Peak Power Fiber Laser Systems: Challenges, Design, and Experiment

We review the main challenges and give design guidelines for high-peak-power high-average-power fiber-based chirped-pulse amplification (CPA) systems. It is clearly pointed out that the lowest order fiber nonlinearity (NL), namely the self-phase modulation, limits the scalability of high-energy ultrashort pulse fiber amplifiers. Therefore, a distinguished difference arises between the consequences of accumulated nonlinear phase originating from the pulse envelope and initial weak modulations, resulting in a strong recommendation to operate an amplification system as linearly as possible in order to generate high-contrast pulses. Low-NL rare-earth-doped fibers, such as the recently available designs of photonic crystal fibers, are the key element for successful peak power scaling in fiber laser systems. In this paper, we present a detailed analysis and optimization of the extraction characteristics in connection with the accumulated nonlinear phase in such extreme fiber dimensions. Consequently, millijoule pulse energy femtosecond pulses at repetition rates in the 100 kHz range have already been demonstrated experimentally in a Yb-fiber-based CPA system that has even further scaling potential.      

Compensation of Gain Narrowing by Self-Phase Modulation in High-Energy Ultrafast Fiber Chirped-Pulse Amplifiers

We propose a method to compensate gain narrowing by use of the self-phase modulation effect in a femtosecond fiber chirped-pulse amplifier (CPA). An engineering rule is derived to determine the stretched pulse duration that allows this compensation. Simulations are carried out to validate this idea. A tradeoff is found between achievable pulse duration and output energy, but we show that this technique allows the generation of 100 fs pulses even for high-gain systems, with output energies of the order of 100 $mu$ J. An experimental proof of principle demonstrates the generation of 112 fs, 10 $mu$J pulses, the shortest high-energy pulse, to our knowledge, generated by a fiber CPA system.      

Few-optical-cycle laser pulses: from high peak power to frequencytunability

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of ~5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared     

Optical Signal Processing by Phase Modulation and Subsequent Spectral Filtering Aiming at Applications to Ultrafast Optical Communication Systems

This paper describes optical signal processing based on optical phase modulation and subsequent optical filtering, which is applicable to 160-Gb/s optical time-division multiplexed (OTDM) subsystems. Ultrafast phase modulation of an optical signal is done by self-phase modulation (SPM) and cross-phase modulation (XPM) when an optical pulse passes through a nonlinear optical fiber. Such phase modulation induces the spectral shift of the optical signal. Various types of optical signal processing are realized simply by filtering out the spectral-shifted component. Using SPM-based pulse reshaping in a 500-m-long silica-based highly nonlinear fiber (HNLF), we demonstrate highly stable generation of a 10-GHz 2-ps optical pulse train tunable over the entire C band. A phase-locked loop (PLL) can suppress the slow phase drift of the output pulse train induced by fluctuations of the nonlinear fiber length, enabling the application of the pulse generator to a 160-Gb/s OTDM transmitter. Based on XPM in a 2-m-long photonic crystal fiber, optical time-division demultiplexing of 160-Gb/s optical signals is demonstrated. The long-term stability is drastically improved as compared with the device composed of a conventional silica-based HNLF, because the short fiber length reduces the phase fluctuation between the signal and control pulses. Instead of nonlinear fibers, an electrooptic modulator such as a (LN) modulator also performs the phase modulation in a more practical manner. We propose and demonstrate an optoelectronic time-division demultiplexing scheme for a 160-Gb/s OTDM signal, which consists of an LN phase modulator driven by a 40-GHz electrical clock and an optical bandpass filter (BPF). We also demonstrate base-clock recovery from a 160-Gb/s optical signal with an optoelectronic PLL. The phase comparator is simply composed of an LN phase modulator and an optical BPF, ensuring the stable and reliable operation in the 160-Gb/s receiver.     

Recent Advances in Optical Processing Techniques Using Highly Nonlinear Bismuth Oxide Fiber

We report our recent studies on nonlinear processing of optical signals using a 35-cm highly nonlinear bismuth oxide fiber (Bi-NLF). Our findings are based on self-phase modulation, cross-phase modulation, and four-wave mixing in the Bi-NLF. We demonstrate applications of the nonlinear techniques in optical signal regeneration, tunable optical delay, stabilization of multiwavelength laser source, tunable optical pulse generation, microwave photonic carrier frequency multiplication, and all-optical wavelength conversion.     

Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

Nonlinear optical compression of Er3+-fiber amplified1.5-μm laser diode pulses

Femtosecond optical pulses of 110-200-fs width have been produced using a gain-switched distributed-feedback semiconductor laser followed successively by a linear compression stage and a nonlinear compression stage. Analysis is focused on this last stage where pulses with peak powers corresponding to 10and 12-order solitons are fed at the fiber input end. Experimental results are well described using both the modified nonlinear Schrodinger equation and an accurate intensity and phase model of the gain-switched laser diode. Experiments are shown to be correctly described only if both intrapulse stimulated Raman scattering and third-order dispersion are taken into account. Guidelines are then given to optimize the nonlinear fiber compression using laser diodes and fiber amplifiers. The influence of the third-order dispersion in the fiber compressor is first evaluated. Second, the nonlinear self-phase modulation induced in the fiber amplifier is studied. It is shown to be the main factor limiting any further pulse shortening with this technique     

Trends in chirped pulse optical parametric amplification

Since the proof-of-principle demonstration of optical parametric amplifier to efficiently amplify chirped pulses in 1992, optical parametric chirped pulse amplification (OPCPA) became a widely recognized and rapidly developing technique for high-power femtosecond pulse generation. In the meantime, we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers. These systems cover a broad class of femtosecond lasers, with output power ranging from a few gigawatts to hundreds of terawatts, with a potential of generating few-optical-cycle pulses at the petawatt power level. In this paper, we discuss the main issues of optical parametric chirped pulse amplification and overview recent progress in the field.     

Broadband amplification of 800-nm pulses with a combination of negatively and positively chirped pulse amplification

It is experimentally proved that successive amplification of negatively and positively chirped laser pulses (NPCPA) counteracts the gain narrowing effect typical in chirped pulse amplification (CPA) lasers. The scheme is robust and easy to adopt to even petawatt (PW) level high power laser systems. As a demonstration, a multi-terawatt (TW) Ti:sapphire laser system was modified to the NPCPA. The bandwidth of the 150 mJ output pulses exceeds 50 nm without any additional spectral correction, which is 30% broader than those currently available from conventional CPA lasers. Moreover, the NPCPA scheme gives an opportunity to increase an intensity temporal contrast without any compromise in pulse energy.     

Mechanism for self-synchronization of femtosecond pulses in atwo-color Ti:sapphire laser

An experimental and theoretical analysis of the nonlinear coupling mechanism between the two solitary pulses circulating in a two-color femtosecond laser is presented. Two operation regimes; synchronized; and nonsynchronized; and a hysteresis of the transition between the two regimes are clearly observed; while independent modelocking and tunability of the output pulse trains is found in both regimes. Pulses in the range from 15 to 100 fs are synchronized with a timing jitter below 2 fs. The combined effects of cross-phase modulation and negative group velocity dispersion are shown to be responsible for the strong pulse correlation in the synchronized regime. Our experimental observations are in agreement with numerical simulations, thus confirming the theoretical model     

某型导弹雷达微波信号发生器的设计与实现

介绍了针对某型导弹研制的宽频带、稳幅、大衰减输出、可实现自动测试的微波信号发生器。采用微控制器、闭环控制和脉冲调制放大等技术设计了宽带稳幅信号发生器;在PICC结构化高级语言平台上开发了一套具备自动匹配导弹测试流程的标准程序并设计了微波频率显示数据的加密算法。实验表明：该微波信号发生器动态范围大,测量误差小,保证了平坦的功率幅度特性,满足测试的特殊要求。     

Modeling of Semiconductor Optical Amplifiers

We present a tight-binding analysis of the polarization dependence of GaAs -strained semiconductors optical amplifiers. We explain how thin strained GaAs layers embedded in a lattice-matched InGaAsP/InGaAs quantum well can be used to achieve polarization insensitive optical amplification. We describe also the interaction between pulse propagation and gain compression within a pump-probe excitation in polarization insensitive MQW-SOA. Another important non-linear effect studied is Four Wave Mixing (FWM) on the pulse propagation in the active region of SOAs. Our model successfully predicts operation of optical data sampling using FWM interaction between a signal bit stream and an optical clock.     

Quasi-automatic phase-control technique for chirp compensation of pulses with over-one-octave bandwidth-generation of few- to mono-cycle optical pulses

This paper introduces our self-recognition type of the computer-controlled spectral phase compensator (SRCSC), which consists of a greatly accurate phase manipulator with a spatial light modulator (SLM), a highly sensitive phase characterizer using a modified spectral phase interferometry for direct electric field reconstruction (M-SPIDER), and a computer for phase analysis and SLM control operating in the immediate feedback (FB) mode. The application of the SRCSC to adaptive compensation of various kinds of complicated spectral phases such as nonlinear chirped pulses with a weak intensity, induced-phase modulated pulses, photonic-crystal-fiber (PCF) output pulses, and nonlinear chirped pulses exceeding a 500-rad phase variation over-one-octave bandwidth demonstrated that the SRCSC is significantly useful for compensation of arbitrary nonlinear chirp and hence enables us to generate quasi-monocycle transform-limited (TL) pulses with a 2.8-fs duration. To the best of our knowledge, this 1.5-cycle pulse is the shortest single pulse with a clean temporal profile in the visible to near-infrared region.     

Subpicosecond pulse compression in the VUV region by induced-phasemodulation in Xe

The feasibility of subpicosecond pulse compression in the VUV region using the induced phase modulation in Xe is theoretically studied by numerically solving equations for the amplitude and phase of a weak VUV signal pulse in the presence of an intense pump pulse at the visible wavelength. Substantial pulse compression can be realized with compact pulse compressors when the pump pulse width is decreased. When a 9.6 ps pump pulse at 403 nm with the peak intensity of 1 GW/cm² is assumed, the signal pulse at 157 nm is compressed from 12 ps to 260 fs by the pulse compressor, with a grating-mirror separation of <2.5 m. Influences on the pulse compression of the timing between two incident pulses and the dynamic change in nonlinear susceptibility are also described     

Polarization Independent All-Optical 3R Regeneration Based on the Kerr Effect in Highly Nonlinear Fiber and Offset Spectral Slicing

A detailed experimental characterization of a polarization-independent all-optical 3R regenerator is presented. The regenerator is comprised of a self-pulsating distributed feedback laser for clock recovery, cross-phase modulation in a highly nonlinear fiber (HNLF) and offset spectral slicing for retiming, and self-phase modulation in an HNLF and offset spectral slicing for reshaping. A key feature of the regenerator is that polarization-independent operation is achieved without additional complexity as compared to a conventional polarization-dependent implementation. The regenerator performance is assessed for important properties of the input signal, including the optical signal-to-noise ratio, waveform distortion due to residual group velocity dispersion and polarization mode dispersion, state-of-polarization, power, and wavelength. With careful attention to key design parameters, excellent performance is achieved.     

Ultrafast semiconductor-based fiber laser sources

In this paper, a novel ring laser platform is presented that uses a single active element, a semiconductor optical amplifier (SOA), to provide both gain and gain modulation in the optical cavity. Gain modulation is achieved by an externally introduced optical pulsed signal. This signal periodically saturates the amplifier gain and forces the ring laser to mode lock. Using this laser platform, we demonstrate picosecond pulsetrain generation at repetition rates up to 40 GHz, either in single or multiwavelength operation mode. In particular, using rational harmonic mode locking, 2.5-ps pulses were obtained up to a 40-GHz repetition rate, while output pulses and output power were constant over a 20-nm tuning range. In addition, a multiwavelength optical signal was obtained using the same laser platform with the addition of a Fabry-Pe/spl acute/rot filter for comb generation. Multiwavelength oscillation is possible due to the broad gain spectrum of the SOA used and its inhomogeneous line broadening. To this end, 48 oscillating wavelengths were obtained at the laser output, with 50-GHz line spacing. Combining both modes of operation, it was possible to mode lock the oscillating multiwavelength signal and to obtain at the output ten wavelength channels, simultaneously mode locked at a 30-GHz repetition rate. The mode-locked channels are temporarily synchronized and exhibit almost identical spectral and time characteristics.     

Design and construction of a TW-class 12-fs Ti:Sapphire chirped-pulse amplification system

We describe a design and a construction of a TW-class 12-fs Ti:sapphire chirped-pulse amplification system. We developed a broadband pulse stretcher, a broadband gain-narrowing compensator, broadband high-energy mirrors, high-energy dichroic chirped mirrors, a dispersion compensator, and a broadband pulse compressor for /spl sim/10-fs pulse generation. Utilizing these optical devices, we demonstrated a generation of 12-fs pulses from a 10-Hz-repetition-rate Ti:sapphire chirped-pulse multipass amplifier system and a 1-kHz-repetition-rate Ti:sapphire chirped-pulse regenerative amplifier system. Optimized designs of broadband Ti:sapphire amplifiers with multilayer gain-narrowing compensators and an adaptive dispersion compensator with a spatial light modulator contribute to the shorter pulse amplification.