Wavelength Chirp Research Articles

Transmission Distance Extension of Directly Modulated Tunable V-cavity Laser Using AWG Wavelength Detuning

Directly modulated tunable lasers are highly desirable for constructing dense-wavelength division multiplexing (DWDM) access networks, owing to their advantages of low cost, compact size and low power consumption. However, the transmission distance for 10 Gbps is usually limited to about 10 km in the telecom C-band in standard single-mode fiber (SMF) due to wavelength chirp. Here we propose and demonstrate a simple technique for extending the transmission distance by detuning the wavelength of a tunable V-cavity laser with respect to DWDM multiplexers. Experiment results show that 10 Gbps signal can be transmitted error-free (BER<10−12) over 20 km SMF with a wavelength-detuning with respect to a Gaussian-type arrayed waveguide grating (AWG) without dispersion compensation. The power penalty is only 1 dB compared to the back-to-back transmission.

Reliability analysis of free space optical communication link considering the effects of laser diode chirping

In this article, wavelength chirping of a semiconductor laser due to changes in current and temperature has been measured. Based on the range of variation, a suitable combined filter consisting of a short-pass filter and long-pass filter has been selected. The link budgeting was simulated using the specifications of the transceiver designed with Zemax software, and power and wavelength chirping experimental data. The effects of wavelength chirping and atmospheric visibility conditions on the free space optical communication link’s parameters are analyzed.

Effects of magnetic wiggler field and chirped laser pulse on the wakefield amplitude and electron energy gain in a wiggler-assisted laser wakefield accelerator

Wakefield generation and GeV electron acceleration in a plasma medium by an ultra-short and intense chirped laser pulse in the presence of a magnetic wiggler field are presented. To increase the wakefield amplitude and to maximize the acceleration gradient, we employ a helical magnetostatic wiggler. An analytical theory of wakefield generation and electron energy gain has been presented which includes the effects of the wiggler field. It was found that the wakefield spectrum and electron energy gain in the wiggler-assisted wakefield accelerator can be increased significantly compared to the non-wiggler situation ones. Numerical simulations reveal that for moderate wiggler magnetic field strengths, wakefield amplitude and electron energy gain have significant peaks in bubble-like structures. Besides, when the wiggler wavelength is clearly larger than the plasma wavelength, the wakefield amplitude and electron energy gain significantly enhance. In addition, it was found that the electron bunches can be enhanced by increasing the wiggler field strength or wiggler wavelength. The effect of the laser chirp parameter on wakefield and electron energy gain have also been investigated. It was concluded that in a wiggler-assisted laser wakefield accelerator, the electron energy and wakefield evolution can be tuned by the wiggler field strength, wiggler wavelength, and laser chirp parameter. This concept opens a path toward new generation of plasma accelerators based on wiggler structures.

Design and experimental research on cantilever accelerometer based on fiber Bragg grating

Currently, an acceleration sensor based on fiber Bragg grating (FBG) has been widely used. A cantilever FBG accelerometer is designed. The simulation of this structure was implemented by finite element software (ANSYS) to analyze its sensing performance parameters. And then the optimized structure was produced and calibration experiments were conducted. On the basis of simulation, optical fiber is embedded in the inner tank of the vibrating mass, and Bragg grating is suspended above the cantilever structure, which can effectively avoid the phenomenon of center wavelength chirp or broadening, and greatly improve the sensitivity of the sensor. The experimental results show that the FBG accelerometer exhibits a sensitivity of 75 pm/(m/s2) (100 Hz) and dynamic range of 60 dB. Its linearity error is <2.31% and repeatability error is <2.76%. And the resonant frequency is ∼125 Hz. The simulation results match the experimental results to demonstrate the good performance of FBG accelerometer, which is expected to be used in the actual project.

Electro-optical resonance modulation of vertical-cavity surface-emitting lasers

Optical and electrical investigations of vertical-cavity surface-emitting lasers (VCSEL) with a monolithically integrated electro-optical modulator (EOM) allow for a detailed physical understanding of this complex compound cavity laser system. The EOM VCSEL light output is investigated to identify optimal working points. An electro-optic resonance feature triggered by the quantum confined Stark effect is used to modulate individual VCSEL modes by more than 20 dB with an extremely small EOM voltage change of less than 100 mV. Spectral mode analysis reveals modulation of higher order modes and very low wavelength chirp of < 0.5 nm. Dynamic experiments and simulation predict an intrinsic bandwidth of the EOM VCSEL exceeding 50 GHz.

Rate Equation Analysis of High Speed Q-Modulated Semiconductor Laser

Dynamic response of a Q-modulated semiconductor laser is simulated based on a rate equation model. Numerical results from both small-signal and large-signal analyses show that the Q-modulation has much higher bandwidth limit and smaller wavelength chirp than the direct modulation. It is shown that a high-extinction-ratio, low-chirp modulation of 40 GHz RZ signal can be achieved and the effects of various parameters on the Q-modulation are discussed. In addition to high-speed communications, the Q-modulated laser is particularly suitable for microwave carrier generation in radio-over-fiber systems.

VCSEL transmission at 10 Gb/s for 20 km single mode fiber WDM-PON without dispersion compensation or injection locking

Vertical Cavity Surface Emitting Lasers (VCSELs) are extremely cost effective, energy efficient optical sources ideal for passive optical access networks. However, wavelength chirp and chromatic dispersion severely limit VCSEL performance at bit rates of 10 Gb/s and above. We experimentally show how off-center wavelength filtering of the VCSEL spectrum at an array waveguide grating can be used to mitigate the effect of chirp and the dispersion penalty. Transmission at 10 Gb/s VCSEL over 23.6 km of single mode fiber is experimentally demonstrated, with a dispersion penalty of only 2.9 dB. Simulated results are also presented which show that off-center wavelength filtering can extend the 10 Gb/s network reach from 11.7 km to 25.8 km for a 4 dB dispersion penalty. This allows for cheap and simple dispersion mitigation in next generation VCSEL-based optical access networks.

Measurement of gain and losses of a midinfrared quantum cascade laser by wavelength chirping spectroscopy

We present an optimized technique for the measurement of gain and losses of semiconductor lasers. We optically inject the beam of a distributed feedback laser (DFB) inside the cavity of the lasers under study. The DFB laser operates in a pulsed mode and shifts its emission wavelength as a function of time. This frequency chirp creates the Fabry–Pérot fringes of the transmitted intensity that contains all the information on the cavity losses. The setup has been validated by a quantitative study of the losses as a function of the injected current, for a quantum cascade laser emitting at 7.6 μm.

3.5-μm high-resolution gas sensing employing a LiNbO3 QPM-DFG waveguide module

Diode laser technology coupled with a wavelength-conversion unit to produce mid-infrared narrow bandwidth laser light applicable to trace-gas detection and with the potential for high-resolution spectroscopy is described. Quasi-phase-matched difference-frequency generation (QPM-DFG) in a compact and fibre-coupled periodically poled lithium niobate (PPLN) waveguide module mixing 1063 and 1525-nm radiations has been adopted for generating 34 μW of 3.5-μm wavelength laser light. Optical detection methods, including sensitive wavelength modulation spectroscopy and a rapid wavelength chirp technique, have been employed with a single-pass cell to investigate methane and formaldehyde absorption profiles around 2855 cm−1, as proof of principle experiments for high sensitivity and resolution spectroscopy on atmospherically important molecules.

Production of ultrashort FEL XUV pulses via a reverse undulator taper

We adapt the “reverse taper” scheme presented by Saldin et al. (Phys. Rev. ST Accel. Beams 9 (2006) 050702) for attosecond pulse production to the XUV/soft-X-ray regime. We find that GW-level pulses of a few femtosecond duration or shorter can be produced using electron beams of quite moderate parameters and undulators of 20-m length or shorter. The output pulse is significantly shifted in wavelength relative to the main background which permits a further increase in contrast ratio via simple monochromatization. Moreover, the output pulse has a natural wavelength chirp that allows further temporal compression, if wanted. Both positive and negative chirps can be produced depending upon the sign of the undulator taper.

High $Q$ Microwave Filter Using Incoherent, Continuous-Wave Supercontinuum and Dispersion-Profiled Fiber

We experimentally demonstrate the combined use of an ultrabroadband, incoherent, continuous-wave (CW) supercontinuum (SC) and a dispersion-profiled fiber to implement a photonic microwave filter with a -factor as high as 140. In our scheme based on spectrum slicing, such a high -factor can be readily achieved due to the following two reasons. The -factor restriction due to the limited wavelength tap number caused by narrow spectral bandwidth of conventional optical broadband sources can readily be solved owing to the ultrabroad bandwidth of the CW SC source. Second, the delay medium based on dispersion-profiled fiber compensates the nonuniform relative time delay between each wavelength filter tap caused by inherent wavelength chirping of conventional frequency domain multichannel comb filters. A fiber dispersion profile is provided for the nonuniform relative time delay compensation.

Detailed modulation response analyses on enhanced single-mode QWS-DFB lasers with distributed coupling coefficient

By using the coupled mode equations, the dynamic characteristics of λ/4 phase-shifted distributed feedback laser diode with distributed coupling coefficient (QWS-DCC-DFB) is analyzed. The numerical simulation indicates that, compared with purely QWS-DFB laser, the better dynamic single-mode can be achieved by QWS-DCC-DFB laser; Under the smaller biasing current, the modulation band-width is narrower, this difference shrinks for larger biasing current; For large-signal modulation, the distributed coupling coefficient is helpful to increase the output extinction ratio, but worsens the wavelength chirping. The good dynamic characteristic can be obtained by selecting system parameters reasonably. QWS-DCC-DFB laser would have potentials in optics communications because of better dynamic SMSR.

Wavelength chirping of laser-diodes and optimization of diode-pumped solid-state lasers

A spectrum-, temporal- and spatial-resolved physical model of LD pumping dynamics is set up, which includes the wavelength chirping and transient behavior of the power output of LD arrays. Based on this model, we investigate in detail the spectrum selective absorption characteristics from LD output by laser materials with different absorption bandwidths. Also the pumping excitation performances for typical three kinds of laser crystals, Yb:YAG, Yb:S-FAP, and Nd:YAG, are investigated specifically for the pulsed-energy-storage operation mode. Finally, we have optimized the initial central wavelength of LD arrays for the above three crystals and compared their sensitivity to the wavelength chirping of LD arrays.

Proposal for $Q$-Modulated Semiconductor Laser

A novel structure and mechanism for high-speed modulation of semiconductor lasers are proposed and analyzed. The modulator consists of an antiresonant cavity acting as a rear reflector of the laser. The change of the absorption coefficient in the modulator results in a change in the reflectivity of the rear reflector, and consequently the Q-factor, the lasing threshold, and the output power. An implementation structure and numerical results for a Q-modulated distributed feedback laser are presented. The monolithically integrated Q-modulated laser has potential advantages of high speed, low wavelength chirp, high extinction ratio, and high power efficiency

Analysis of the reduced thermal conductivity in InGaAs∕GaAs quantum dot lasers from chirp characteristics

The thermal conductivity of self-organized quantum dot (QD) active regions is estimated by measurements of wavelength chirp with injected current as a function of the current pulse duty cycle both below and above threshold. A simple model which separates out thermal and charge carrier chirps is used to estimate the thermal conductivity of the QD active region. With this model, the thermal conductivity of the InGaAs QD active region is estimated to be ∼0.1W∕mK, about two orders of magnitude less than that of the bulk material. This is consistent with theoretical predictions of the reduced thermal conductivity of QD regions.

Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp

The dynamic properties of distributed feedback lasers (DFBs) based on InAs/InGaAs quantum dots (QDs) are studied. The response function of QD DFBs under external modulation is measured, and the gain compression with photon density is identified to be the limiting factor of the modulation bandwidth. The enhancement of the gain compression by the gain saturation with the carrier density in QDs is also analysed for the first time. The linewidth of the QD DFBs is found to be more than one order of magnitude narrower than that of conventional quantum well (QW) DFBs at comparable output powers. The figure of merit for the narrow linewidth is compared between different semiconductor materials, including bulk, QWs and QDs. Linewidth re-broadening and the effects of gain offset are also investigated. Finally, the chirp of QD DFBs is studied by time-resolved-chirp measurements. The wavelength chirping of the QD DFBs under 2.5 Gbps modulation is characterized. The strong dependence of the linewidth enhancement factor on the photon density is explained by the enhancement of gain compression by the gain saturation with the carrier density, which is related to the inhomogeneous broadening and spectral hole burning in QDs.

Analysis of large-signal dynamic characteristics of 10-Gb/s tunable distributed bragg reflector lasers integrated with electroabsorption modulator and semiconductor optical amplifier based on the time-dependent transfer matrix method

We investigate large-signal dynamic characteristics of tunable distributed Bragg reflector (DBR) lasers with different structures. Because of high chirp and complex structures of tunable DBR lasers, it is difficult to accurately analyze large signal chirp and output pulse shapes with analytical models. Therefore, we apply a numerical model based on the time-dependent transfer matrix method to tunable DBR lasers integrated with electroabsorption modulator (EAM) and semiconductor optical amplifier (SOA). Using this model, we find a suitable α-parameter range in the Bragg grating section (α/sub Bragg/-parameter) that produces the tolerable wavelength chirp while maintaining a proper tuning range. To employ the tunable lasers in wavelength division multiplexed networks and improve flexibility of the networks, the tunable lasers should have high output power and low wavelength chirp. According to our simulation results, the EAM section had better be located in between SOA and DBR laser sections to obtain high output power and low wavelength chirp.

Effect of modulator chirp and sinusoidal group delay ripple on the performance of systems using dispersion compensating gratings

The implications that the nonideal characteristics of a dispersion compensating grating have on system performance are determined, for the most part, by the group delay ripple (GDR) of the grating response over the modulated signal bandwidth. Since the GDR typically exhibits an irregular variation with wavelength that has periodic features, it is convenient to use a sinusoidal variation to assess the implications on system performance. The portion of the grating bandwidth occupied by the modulated optical signal is determined by the carrier signal wavelength, bit rate, modulation format, and modulator chirp. The implications of modulator chirp on the performance of 10-Gb/s dispersion compensated systems are considered. Using a LiNbO/sub 3/ Mach-Zehnder modulator, an electroabsorption modulator, and a multiple quantum-well Mach-Zehnder modulator with distinct chirp properties, the results demonstrate that to accurately assess the implications of GDR, the properties of the modulator chirp must be considered. In particular, results for chirp-free optical signals underestimate the implications of the GDR on system performance.

Photoselective adaptive femtosecond quantum control in the liquid phase.

Coherent light sources can be used to manipulate the outcome of light-matter interactions by exploiting interference phenomena in the time and frequency domain. A powerful tool in this emerging field of 'quantum control' is the adaptive shaping of femtosecond laser pulses, resulting, for instance, in selective molecular excitation. The basis of this method is that the quantum system under investigation itself guides an automated search, via iteration loops, for coherent light fields best suited for achieving a control task designed by the experimenter. The method is therefore ideal for the control of complex experiments. To date, all demonstrations of this technique on molecular systems have focused on controlling the outcome of photo-induced reactions in identical molecules, and little attention has been paid to selectively controlling mixtures of different molecules. Here we report simultaneous but selective multi-photon excitation of two distinct electronically and structurally complex dye molecules in solution. Despite the failure of single parameter variations (wavelength, intensity, or linear chirp control), adaptive femtosecond pulse shaping can reveal complex laser fields to achieve chemically selective molecular excitation. Furthermore, our results prove that phase coherences of the solute molecule persist for more than 100 fs in the solvent environment.

Effect of carrier heating on chirp and modulation response of directly modulated semiconductor laser diodes

Abstract This paper examines the effect of carrier heating due to hot carrier injection in directly modulated semiconductor laser diodes. Using a single-mode rate equation model including an equation for the energy stored in the conduction band carriers, the changes in carrier temperature due to hot carrier injection from the barrier layer in quantum-well laser diodes are seen to cause a degradation in the device's sinusoidal modulation response. This degradation is coupled with an increase in the wavelength chirp of the light output. It is concluded that in order to minimize the detrimental effect of carrier heating on directly modulated quantum well laser diodes the barrier height should be kept as small as practical. Additionally, the appreciable change in device performance brought about by carrier heating should be accounted for when modelling such devices.