Influences of polarization-mode dispersion on soliton transmissionsystems

In soliton transmission systems with polarization-mode dispersion (PMD), random birefringence causes solitons to generate dispersive waves, which degrade soliton transmission systems in two aspects. First, the dispersive waves cause solitons to continuously lose energy, thus induce pulse broadening. Second, the dispersive waves interact with other soliton pulses and cause distortion of a sequence of soliton pulses. Both of these effects induce performance degradation of soliton transmission systems. We study these effects of PMD on both conventional and dispersion-managed (DM) soliton transmission systems. We show that, for conventional soliton systems, although single pulse has robustness to PMD, the interplay between the dispersive waves and solitons would seriously distort a sequence of pulses and make soliton systems worse than linear systems if all other transmission impairments are neglected. We also show that DM solitons are more robust to PMD than both conventional solitons and linear systems due to the enhanced nonlinearity and less sensitivity of DM solitons to perturbations. We further point out that soliton collision-induced polarization scattering causes additional timing jitter and system performance penalty in WDM soliton systems     

Ultrahigh-speed long-distance TDM and WDM soliton transmissiontechnologies

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management (DM) plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the DM soliton are compared with those of return-to-zero (RZ) and nonreturn-to-zero (NRZ) pulses. With a small dispersion swing, the system can still be described as an average soliton with a nonlinear Schrodinger equation (NLSE), whereas with a large dispersion swing, the soliton-like steady-state pulse becomes a chirped Gaussian pulse, in which the master equation is closer to a linear Schrodinger equation (LSE) with a parabolic potential well. An in-line modulation scheme up to 80 Gb/s per channel and its two-channel WDM transmission over 10000 km are described. A 640-Gb/s (40 Gb/s×16 channels) WDM soliton transmission over 1000 km is also reported with a DM single-mode fiber, without the use of in-line modulation. Finally, dark soliton transmission at 10 Gb/s over 1000 km is described as a different nonlinear pulse application     

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.     

Soliton mode-locking with saturable absorbers

We investigate ultrashort pulse generation based on the fundamental soliton generation that is stabilized by a saturable absorber. The case of an absorber with a recovery time much longer than the pulsewidth of the generated soliton is investigated in detail. Based on soliton perturbation theory we derive equations for the soliton variables and the continuum generated in a mode-locked laser. Analytic criteria for the transition from stable to unstable soliton generation are derived. The results demonstrate the possibility of ultrashort pulse generation by a slow saturable absorber only. The theoretical results are compared with experiments. We generate pulses as short as 13 fs using only semiconductor saturable absorbers     

EA-Modulator-Based Optical Time Division Multiplexing/Demultiplexing Techniques for 160-Gb/s Optical Signal Transmission

In this paper, 160-Gb/s optical-time-division-multiplexing (OTDM) techniques employing electroabsorption (EA)-modulator-based optical multiplexer are described. The optical multiplexer integrates four EA modulators with free-space optics and enables, stably, to generate an authentic 160-Gb/s OTDM signal. The optical multiplexer possesses a switching capability of modulation format, which originates in the thermo-optic effect in EA waveguide, so that it is possible to generate various phase-coded OTDM signals such as carrier-suppressed return-to-zero (CS-RZ) signal by tuning operation temperatures of the EA modulators. By employing the novel 160-Gb/s optical multiplexer, prototypes of 160-Gb/s OTDM transmitter and receiver were developed. EA modulators are also adopted to optical short pulse source at transmitter side, optical time division demultiplexer, and phase-locked loop (PLL) circuit for clock recovery at the receiver side. The 160-Gb/s system prototype exhibited a superior performance maintaining high stability, and its applicability to practical use is discussed, showing experimental results of 160-Gb/s 635 km field trial on Japan Gigabit Network II (JGN II) optical testbed     

Effect of Grating Parameters on Mode-Locked External Cavity Lasers

This paper deals with numerical solutions of actively mode-locked fiber grating semiconductor lasers using a time-domain solution of coupled wave equations and rate equations. Simulation of linearly chirped tanh apodized fiber Bragg grating (FBG) utilized in hybrid soliton pulse source (HSPS) shows an extreme increase in the mode-locking frequency range of HSPS. Our model predicts transform-limited pulses over a frequency range of 1.6 GHz (1.8–3.4 GHz) for this grating around a system operating frequency of 2.5 GHz, with a pulsewidth of 46 ps required for a practical soliton transmission system, whereas it ranges to about 1.3 GHz (2–3.3 GHz) for the linearly chirped raised-cosine flat top and 850 MHz (2.1–2.95 GHz) for linearly chirped Gaussian apodized. Furthermore, in this study, the effects of FBG parameters, such as peak reflectivity, grating length, grating chirp, and modulation index, on output of mode-locked HSPS are also described for the first time. The numerical results indicated that although pulsewidths decreased with the increase in grating chirp, shorter grating lengths gave shorter pulses, and the modulation index and peak reflectivity of the grating did not significantly affect the pulsewidths.      

Higher order chirp compensation of femtosecond mode-lockedsemiconductor lasers using optical fibers with different group-velocitydispersions

Higher order chirp compensation of optical short pulses by using two types of optical fibers with different group-velocity dispersions was theoretically and experimentally investigated in detail. By optimizing the lengths of two types of optical fibers, both second- and third-order dispersion of chirped optical pulses were found to be simultaneously compensated. Pulse-compression experiments with chirped optical pulses from a mode-locked laser diode demonstrated using this technique attained nearly transform-limited, 500-fs pulse generation     

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.     

A Silicon Modulator Enabling RF Over Fiber for 802.11 OFDM Signals

We investigate the linearity properties of silicon modulators and show that, contrary to the traditional lithium niobate Mach–Zehnder modulators (MZMs), the third-order intermodulation distortion (IMD3) for silicon modulators is a function of the modulator bias point. The bias point for silicon modulators can be chosen to reduce the IMD3 well below that of standard lithium niobate MZMs. Given the cost and integration advantages of the silicon photonics technology, silicon modulators offer significant advantages for emerging radio over fiber applications. As an example, we examine, for the first time to our knowledge, a silicon modulator for converting analog 802.11 RF signals to the optical domain, achieving an error vector magnitude of −30 dB.      

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated ≈30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses     

Actively mode-locked strained-InGaAsP multiquantum-well lasersintegrated with electroabsorption modulators and distributed Braggreflectors

This paper describes picosecond pulse generation at 20 Gb/s by monolithic mode-locked lasers integrated with electroabsorption modulators and distributed Bragg reflectors. The electroabsorption modulator using strained-InGaAsP multiquantum wells acts as a pulse shortening gate when a sinusoidal voltage is driven at a large reverse bias voltage. To obtain transform-limited picosecond pulses, the required spectral bandwidth of the distributed Bragg reflector is estimated. Pulse generation around 4 ps with a time-bandwidth product of 0.5 has been performed at a repetition rate of 20 GHz. Driving conditions of the modulator, such as bias voltage and modulation frequency, are investigated. It is shown that an increase in the intensity noise is the main factor limiting performance     

Nonlinear optics of a few-cycle optical pulse: slow-envelope approximation revisited

Using Maxwell-Bloch equations, we analyze the response of an ensemble of two-level atoms driven by a femtosecond optical pulse beyond the traditional approach of slowly varying amplitudes and phases. For optical pulses of a given duration, we show that the off-resonance optical field can evolve into a stable (spatio-) temporal soliton.     

Optimized pulse source employing an externally injected gain-switched laser diode in conjunction with a nonlinearly chirped grating

In this paper, we demonstrate the generation of transform-limited short optical pulses, which display excellent spectral and temporal qualities by employing a novel technology, based on an externally injected gain-switched laser in conjunction with a nonlinearly chirped grating. Using this technique, 3.5-ps optical pulses exhibiting a time-bandwidth product (TBP) of 0.45 are generated, which are suitable for use in high-speed 80 Gb/s optical time-division multiplexing (OTDM) communications systems. The numerical integration of a set of rate equations using suitable parameters for the devices used in the experiments were carried out to further confirm the feasibility of the proposed method for developing an optimized pulse source for high-speed photonic systems.     

High-Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy

It is now possible to generate terahertz (THz) pulses with sufficient energy and field amplitude to enable versatile applications in THz nonlinear optics and spectroscopy. In addition, THz waveform shaping at high intensities promises wide ranging new capabilities in THz coherent control. We review recent progress in generation of high-power THz phonon-polariton waves in lithium niobate that can be coupled into free space THz radiation. A ldquopolaritonicsrdquo toolset for control and processing of the THz waves is also reviewed briefly. Recent demonstrations of THz nonlinear optics and spectroscopy are then presented.     

Monolithic integration of multiple-quantum-well lasers andmodulators for high-speed transmission

Monolithically integrated single frequency lasers and electroabsorption modulators are attracting considerable interest as optical sources for long-haul and high-bit-rate fiber-optic links. Their frequency chirpless nature has indeed allowed nonreturn-to-zero (NRZ) transmission beyond the chromatic dispersion limit. They also offer a great potential as soliton pulse generators. This paper discusses the many integration schemes devised for their realization with particular emphasis on the identical active layer (IAL) approach. Recent progress in their high-speed applications is reported     

Toward single-cycle laser systems

Few-cycle pulse generation based on Ti:sapphire, Cr:forsterite, and Cr:YAG gain media is reviewed. The dynamics of these laser systems is well understood in terms of soliton and dispersion managed soliton formation stabilized by artificial saturable absorber action provided by Kerr-lens modelocking. These systems generate 5-, 14-, and 20-fs pulses with spectral coverages of 600-1150, 1100-1600, and 1200-1500 nm, respectively. The design of dispersion compensating laser optics providing high reflectivity and prismless operation over this bandwidth is discussed. A novel active synchronization scheme based on balanced optical cross correlation, the equivalent to balanced microwave detection, for synchronization of independently mode-locked lasers is introduced. Its use in synchronizing an octave-spanning Ti:sapphire laser and a 30-fs Cr:forsterite laser yields 300 attoseconds timing jitter measured from 10 mHz to 2.3 MHz. The spectral overlap between the two lasers is large enough to enable direct detection of the difference in the carrier-envelope offset frequency between the two lasers. These are the most important steps in the synthesis of single-cycle optical pulses with spectra spanning 600-1600 nm.     

Spectral narrowing and temporal expanding of femtosecond pulses by use of quadratic nonlinear processes

A nonlinear system capable of expanding pulse with spectral narrowing, an analogy to a spatial beam expander in linear optics, is studied theoretically and experimentally. The system, with features of high efficiency and maintaining near Fourier-transform limit (FTL), is constructed by using quadratic nonlinear processes with chirped pulses. The spectral and temporal characteristics of such a pulse expander are investigated analytically and computationally. It shows that group-velocity mismatch of nonlinear crystal plays a detrimental role, which leads to a deviate operation of pulse expander from its ideal case, e.g., temporal shortening, spectral broadening, and a deviation from the FTL of the expanded pulses. The criteria for designing a near aberration-free pulse expander are given based on these analyses. As a demonstration, we experimentally expand broadband 70-fs pulses from a Ti:sapphire regenerative amplifier to narrowband 60-ps longer pulses. The conversion efficiency from pump to idler of the nonlinear pulse expander is currently limited to a few percent and can be practically improved to 10% to 20%.     

Bright high-order harmonic generation from long gas jets toward coherent soft X-ray applications

We have performed the optimization of high-order harmonic brightness from long gas jets by using self-guided and chirped femtosecond laser pulses and analyzed their coherence properties. The characteristics of laser pulse propagation were analyzed both in theory and in experiments to understand the self-guiding process of laser pulses and chirp compensation mechanism. Highly efficient harmonic generation with low beam divergence and narrow bandwidth was achieved by applying these two techniques to the long gas jets. The coherence properties of the bright harmonics were examined using double-pinhole interference and spectral interference.     

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of ~10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks     

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.