40-GHz picosecond-pulse second-harmonic generation in an MgO-doped PPLN waveguide

Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40-GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data.     

All-Optical Clock Recovery Using Erbium-Doped Fiber Laser Incorporating an Electroabsorption Modulator and a Linear Optical Amplifier

We demonstrated a 10-GHz all-optical clock recovery system using an erbium-doped fiber laser that incorporates an electroabsorption modulator and a linear optical amplifier. Stable pulses with peak power of 200 mW and pulsewidth of 6 ps are obtained. The output power and the pulsewidth of the recovered clock pulses are independent of the input data pattern. Stable optical clock can still be observed when the input data rate varies by more than 60% of the fundamental frequency without any optical tunable delay line inside the laser cavity. The scheme is essentially wavelength transparent for the whole C-band which recovers clock pulses from input data wavelength between 1525 and 1565 nm     

Active picosecond optical pulse compression in semiconductoroptical amplifiers

A novel technique is proposed to compress and amplify a weak picosecond optical pulse by utilizing a copropagating intense pump pulse in semiconductor optical amplifiers. It is shown that, simply by adjusting the time delay between the two pulses, a controlled compression of the pulsewidth by up to a factor of ~4 with pulse energy amplification of up to ~16 dB can be achieved simultaneously. Furthermore, a compressed pulse shape with strongly damped wings is also observed. The shortest achievable pulsewidth is demonstrated to be dependent on the frequency detuning and pump pulse properties     

Ultrahigh-speed photography of picosecond light pulses

A new technique for the display of picosecond light pulses is presented. Ultrashort (6 ps) green light pulses passing through a light-scattering medium are photographed from the side by a camera positioned behind a shutter of 10-ps framing time. The shutter is an ultrafast Kerr cell driven by infrared pulses 8 ps in duration. Color photographs show a bright spot on a dark background, revealing the unambiguous presence of well-isolated picosecond light pulses. The shape of the spot is the result of a convolution involving the three-dimensional shape of the green pulse and the time transmission function of the shutter, this function being dictated by the shape of the infrared pulse. The experiment indicates that a new technique for visualizing light pulses consists in simply observing their flight through a scattering medium from behind an ultrafast shutter having a framing time equal to the time resolution desired. The new technique has many advantages over the two-photon fluorescence display technique, such as higher sensitivity, wider spectral range, and easier interpretation. The ultrafast camera also can be used for the photographic measurement of ultrashort relaxation times in dielectrics and in fluorescent dyes.     

Self-phase modulation and spectral broadening of optical pulses insemiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems     

Compression of picosecond pulses, generated by an electroabsorptionmodulator, in negative dispersion fibres

Optical pulses generated by an electroabsorption modulator are experimentally compressed in various negative-dispersion fibres. Theoretical and experimental pulsewidth evolutions are shown to be in excellent agreement when the voltage-dependent transmission and chirp functions are taken into account     

Wavelength-tunable nearly transform-limited pulses generated by self-injection seeding of a laser diode at an arbitrary repetition rate

A scheme to generate tunable picosecond pulses at 1.55 /spl mu/m with an arbitrary repetition rate is developed. The self-injection seeded pulses are temporally compressed and are amplified to produce nearly transform-limited output. Throughout the tuning range of 13.16 mm, the pulsewidth and the time-bandwidth product are observed to be roughly constant at 5.9 ps and 0.6, respectively.     

Influence of probe depletion and cross-gain modulation on four-wavemixing of picosecond optical pulses in semiconductor optical amplifiers

The influence of probe depletion (PD) and cross-gain modulation (XGM) on four-wave mixing (FWM) of picosecond optical pulses in semiconductor optical amplifiers has been investigated numerically. The effects of PD and XGM become more significant for shorter input pulsewidth, higher pulse energy and smaller pump-probe frequency detuning. Results indicate that if the effects of PD and XGM are neglected the FWM efficiency can be seriously overestimated     

Electrical wavelength-tunable actively mode-locked fiber ring laserwith a linearly chirped fiber Bragg grating

Electrical wavelength-tunable picosecond laser pulses were demonstrated in an actively mode-locked fiber ring laser with a linearly chirped fiber Bragg grating (FBG). Continuous wavelength tuning with a range of 7.2 or 5.8 mm was achieved by changing the modulation frequency respectively around 2.48 or 6.3 GHz. A pulsewidth of about 20 ps was obtained in the entire tuning range, which was limited mainly by the reflection bandwidth of the FBG     

Highly flexible optical Nyquist pulses generation based on dual-parallel Mach–Zehnder modulator and intensity modulator

A new scheme for highly flexible optical Nyquist pulses generation is proposed by using a dual-parallel Mach–Zehnder modulator (DPMZM) and an intensity modulator (IM) without any external optical filter. The optical Nyquist pulses generator includes two stages. At the first stage, 2-, 3-, or 5-carrier light sources with two different frequency spacing are generated by utilizing a DPMZM. At the second stage, the light source used as seed light is then sent to IM, and two forms of optical frequency combs with spectral line flatness of less than 0.7 dB can be finally obtained by simply adjusting the DC bias and amplitude of radio frequency signal applied to IM. Ideal sinc-shaped Nyquist pulses are also generated in the time domain with bandwidth from 75 to 225 GHz and repetition rate from 40 to 80 ps. Simulation results show the high flexibility, effectiveness, and practicability of the proposed scheme, which can be applied to future high-capacity optical transmission systems.     

Observation of pulse compression in photonic Crystal coupled cavity waveguides

We demonstrate the compression of picosecond pulses using the large group velocity dispersion available from planar photonic crystal (PhC) coupled cavity waveguides (CCWs). A maximum pulsewidth reduction of 40%, from 1.91 to 1.17 ps, is achieved, in transmission through an 8-/spl mu/m-long planar PhC waveguide. The equivalent dispersion value is >10/sup 6/ times larger than that available from standard single-mode fiber, at the pulse center wavelength of 1536 nm. The device performance is analyzed with the aid of both two-dimensional (2-D) eigenmode expansion and 2-D finite-difference time-domain (FDTD) models. The models included the material dispersion of the GaAs/AlGaAs heterostructure, into which the PhCs are etched, and show remarkable agreement with experiment.     

Ultrafast operation of optically pumped resonant periodic gain GaAssurface emitting lasers

Generation and characterization of picosecond optical pulses from vertical-cavity resonant-periodic-gain GaAs-AlGaAs surface-emitting lasers optically pumped by picosecond dye-laser pulses is reported. The output pulseshape was obtained from the cross correlation of pump and signal pulses. Dependence of signal pulsewidth and pulse delay on pump power were investigated. The results are in good agreement with a single rate equation model of the pulse formation. A cavity lifetime of 8.3 ps, compared with a gain medium transit time of ~0.1 ps, is determined for this very high-Q structure     

The role of phonons and plasmons in describing the pulsewidth dependence of the transmission of ultrashort optical pulses through germanium

Recently, Elci et al. developed a first principles model that accounts for the generation and transient behavior of dense electron-hole plasmas produced in germanium by picosecond optical pulses. The agreement between this model and early experiments is substantial. However, recent transmission measurements, involving optical pulses of varying width, are in disagreement with initial predictions of this model. Here, we emphasize the dependence of the optical properties of the Ge plasma on the electron-phonon coupling constant, the broadening in the plasmon-assisted recombination, and the energy band structure, and we suggest adjustments to the original model that should produce agreement with these more recent experiments. The pulsewidth dependence in the transmission is shown to be sensitive to the relative strengths of the electron-phonon relaxation and the plasmon recombination during the temporal evolution of the hot electron-hole plasma.     

Multiwavelength picosecond optical pulse generation using anactively mode-locked multichannel grating cavity laser

A multichannel grating cavity (MGC) InGaAsP-InP ridge diode laser is found to be suitable for multiwavelength ultrashort optical pulse generation using active mode-locking techniques. By using different configurations of the MGC laser, dual wavelength picosecond pulses are successfully produced with good spectral quality, either simultaneously or with a programmable relative delay between the channels. For a channel separation of 2.2 nm, pulses with a duration of 60 ps and a spectral width of 11.6 GHz have been obtained. The experimental results are compared with those from a theoretical analysis of the multiwavelength mode-locking process using a set of modified coupled-cavity rate equations. The minimum achievable pulsewidths generated by the actively mode-locked MGC laser are shown to be limited by the resolution bandwidth of grating used. The dependence of pulsewidth on RF drive frequency detuning is examined and it is found that the ultrashort pulses can be obtained over a wide range of frequency detuning. The interchannel cross-talk originating from gain saturation and carrier depletion is also discussed     

Analysis of optical phase-conjugate characteristics of picosecondfour-wave mixing signals in semiconductor optical amplifiers

We have analyzed for the first time the optical phase-conjugate characteristics of picosecond four-wave mixing (FWM) signals in semiconductor optical amplifiers (SOAs) using the finite-difference beam propagation method (FD-BPM). We show that the optical phase-conjugate characteristics of the FWM signals are strongly dependent on input pump pulsewidths. As a typical example, we have demonstrated that SOAs act as an ideal phase-conjugator, within the confines of reversing the chirp of optical pulses, for a 10-ps input pump pulse and a ~2.2-ps linearly chirped input probe pulse. When the pulsewidth of pump pulse becomes short, the minimum compressed pulsewidth is obtained by using a fiber shorter in length than the input fiber, but having the same group velocity dispersion as the input fiber. For a much shorter pump pulse such as 1 ps, the short FWM signal can be obtained via the gating characteristics of the FWM. However, only a part of the phase information is copied to the FWM signal due to such gating characteristics. The phase information is also degraded due to the fast nonlinear effect in the SOA. Thus, the pulsewidth is not compressed by propagation through a dispersive medium     

Picosecond pulse response of broad-band guided-wave interferometric light modulators

Picosecond pulse operation of the guided-wave light modulator is reported in this paper. First, we analyze the time responses of phase and intensity modulators of the traveling-wave type, and the modulated output waveforms related to the modulating pulses are discussed. It is shown that there is an optimum interaction (or electrode) length of the modulator whereby the drive voltage is minimized for a desired output pulse width. The traveling-wave push-pull light intensity modulator of Ti-diffused LiNbO3waveguides was modulated by a pulse train of 1 GHz repetition frequency, and the modulated output was observed by the image tube streak camera modified for sinusoidal scan at the same frequency. The duration of the modulated light pulses was less than 45 ps, which shows good agreement with the predicted one.     

Fundamental limits of sub-ps pulse generation by active modelocking of semiconductor lasers: the spectral gain width and the facetreflectivities

The authors present a numerical simulation of active mode locking of a semiconductor laser amplifier (SCLA) in an external cavity, which includes the finite spectral gainwidth. This is shown to be essential for introducing a lower limit of the pulsewidth, although the gain linewidth is about a factor of 10-50 broader than the inverse pulsewidth. This numerical treatment consistently explains the experimental findings that even SCLA facet reflectivities as low as 10 ^-4 lead to trailing pulses with an intensity almost of the same order of magnitude as the leading one. This theory makes detailed predictions about the influence of the facet reflectivities, the spectral gainwidth, and the injection current on the picosecond pulse generation     

Interaction of Electromagnetic Transient Radiation with Biological Materials

The transmission characteristics of transient electromagnetic pulses in biological material are studied using a plane wave pulse incident normally on a semi-infinite layer model. With the dispersion properties of complex tissue dielectric constants taken into account, the steady state transfer function was examined as a function of frequency. Integral solutions for the transmitted field of a Gaussian pulse were obtained through Fourier transformation. The transmitted waveforms inside muscle were determined numerically for various depths pertinent to biological situations. The results suggest that incident pulse experiences severe reflection at the airtissue interface and, shorter pulses are transmitted more readily than longer ones. For an incident pulse of 50 kV/m and 1 us in pulsewidth, the transmitted amplitude is 221 V/m.     

Phase-engineered III-V MQW Mach-Zehnder modulators

We have optimized the negative frequency chirp while maintaining a high extinction ratio in an InP-InGaAsP multiple-quantum-well (MQW) Mach-Zehnder modulator. Our procedure is to elongate one modulator arm such that a /spl pi/ phase shift is produced at zero bias relative to the opposing arm. The resulting de extinction ratio is improved to /spl ap/16 dB from 10 dB relative to a symmetric design. The frequency chirp of the modulator at the rising edge is about -3 GHz enabling excellent transmission performance over a 95 km of conventional fiber.     

Fluctuation mechanism of ultrashort pulse generation by laser with saturable absorber

This paper presents a theoretical treatment of the fluctuation mechanism involved in the generation of picosecond laser pulses with saturable absorbers. The processes responsible for the shortening of the pulsewidth and for selection of the most intense pulse are treated. Some experimental results that confirmed the treatment are presented. The influence of inertia of saturable absorber and nonlinear losses (self-focusing and self-modulation) is discussed.