Chirp Width Research Articles

Simple‐structure and cost‐effective FMCW radar test system using a PLL‐Gunn oscillator and fundamental mixer in the E‐band

The authors explain the development of a cost-effective frequency-modulated continuous wave (FMCW) radar test system in the E-band with a simple-structure, and demonstrate it for an automotive application using an FMCW radar chipset evaluation board and an FMCW radar. The system consists of a down-converter, based on a fundamental mixer with a phase-locked loop-Gunn oscillator, and a mixed domain oscilloscope or a conventional digital oscilloscope. It is able to evaluate an FMCW signal in the time domain, such as the chirp rate, chirp length, chirp width, linearity, and the relative power level.

Computational study of pulse propagation in photonic crystal semiconductor optical amplifier

A theoretical analysis of photonic crystal semiconductor optical amplifier (PC-SOA) is presented. The proposed model is based on rate and propagation equations. The analysis is carried out using two input signal regimes of constant and Gaussian waves. To the best of the authors’ knowledge, temporal and spatial variation of picosecond pulses have not been previously investigated in active PC medium. The effect of injection current, group index, and amplifier length variation on the output signal and amplifier gain is investigated. Based on the results, temporal peak shift, chirp, and spectral width increased in low current regime compared to conventional SOAs. It is also demonstrated that short length PC-SOA can be used as a preamplifier in photonic integrated circuits.

Propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium

We investigate the propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium. For certain chirp strength and chirp width, an incident 2π nonlinear chirped pulse will split into optical precursors and a stable self-induced transparency soliton. This is caused by the particular Fourier spectrum that includes not only central resonant frequency components but also high-frequency and low-frequency sidebands. Moreover, the effects of chirp parameters on the evolution of nonlinear chirped pulses are also discussed.

Analytical study of pulse amplification in silicon Raman amplifiers

The nonlinear process of stimulated Raman scattering is important for silicon photonics as it enables optical amplification and lasing. To understand the dynamics of silicon Raman amplifiers (SRAs), a numerical approach is generally employed, even though it provides little insight into the contribution of different SRA parameters to the signal amplification process. In this paper, we solve the coupled pump-signal equations analytically under realistic conditions, and derive an exact formula for the envelope of a signal pulse when picosecond optical pulses are amplified inside a SRA pumped by a continuous-wave laser beam. Our solution is valid for an arbitrary pulse shape and fully accounts for the Raman gain-dispersion effects, including temporal broadening and group-velocity reduction (a slow-light effect). It can be applied to any pumping scenario and leads to a simple analytic expression for the maximum optical delay produced by the Raman dispersion in a unidirectionally pumped SRA. We employ our analytical formulation to study the evolution of optical pulses with Gaussian, exponential, and Lorentzian shapes. The ability of a Gaussian pulse to maintain its shape through the amplifier makes it possible to realize soliton-like propagation of chirped Gaussian pulses in SRAs. We obtain analytical expressions for the required linear chirp and temporal width of a soliton-like pulse in terms of the net signal gain and the Raman-dispersion parameter. Our results are useful for optimizing the performance of SRAs and for engineering controllable signal delays.

Robust generation of superposition states

We describe experimental results of the generation of superposition states in a two-level system, $4s\phantom{\rule{0.2em}{0ex}}{S}_{1∕2}$ and $3d\phantom{\rule{0.2em}{0ex}}{D}_{5∕2}$ states, of a single calcium ion. The generation method is based on population transfer via rapid adiabatic passage (RAP), proposed by Vitanov and Shore [Phys. Rev. A 73, 053402 (2006)]. The coherence of the generated superposition state is evaluated with a Ramsey-like interferometry method and a fringe visibility of up to 0.93 is obtained. The state generation shows strong robustness with respect to the RAP pulse parameters, including frequency chirp width and Rabi frequency.

Characteristics of Gaussian pulses transmitting in the G.652 fiber with FBG dispersion compensation

AbstractThe compression characteristics of a Gaussian pulse with varied chirp and pulse width are obtained by theoretical analysis and experiments. The compensation effects are diverse, even with the same chirped optical fiber Bragg grating (FBG) dispersion compensator, and the optimal dispersion compensation length is related to the position of the FBG. At present, in most of transmission systems, the signal from the lasers are Gaussian pulses, thus an analysis of its broader characteristics, before and after it is reflected from the FBG, will be significant. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 480–484, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11095

Effects of chirping on the dissociation dynamics ofH2in a two-frequency laser field

We present the effects of frequency chirping of laser pulses on (1+1)-photon resonance-enhanced dissociation dynamics of ${\mathrm{H}}_{2}.$ The dissociation occurs via two closely spaced nonadiabatically coupled intermediate levels which are in one-photon resonance or near resonance with the initial level. Predissociating levels embedded into continua are considered. When the first laser field is sufficiently intense and suitably chirped, the dissociation probability is enhanced by adiabatic rapid passage through the avoided crossing arising from the frequency swept radiative interaction. The whole population of the ground level can be effectively transferred to the intermediate levels by this technique facilitating the dissociation process by the second field. We also report the effect of frequency detuning and chirp width on the dissociation probability. Widths of the two peaks of the dissociation line shape increase with an increase in chirp width, resulting in the possibility of control in the dissociation yield. When the first field is a laser pulse of low intensity and constant frequency and the second laser frequency is chirped, predissociating levels take important parts in the dissociation dynamics and we obtain a signature of the nonadiabatic effect of the first step on the second step of photodissociation dynamics. This feature is due to the presence of the predissociating levels and the nonadiabatic mixing of two intermediate levels. All these results can be explained in terms of the adiabatic dressed levels.

Temperature insensitive fiber optic accelerometer using a chirped Bragg grating

We report the development and demonstration of a fiber optic accelerometer. The principle of operation is based in the use of chirped Bragg gratings where the chirp width varies as the grating is physically deformed. The theory of operation is presented and experimentally compared with a conventional accelerometer's measurements for a vibrating beam.

Coherent saturation removal in time-domain optical memory by storage of frequency-chirped data pulses

We report a novel technique for storing digital optical data in coherent time-domain optical memory to circumvent the problem of coherent saturation. In this technique, the carrier frequency of a data stream to be stored is slowly chirped so its power is spread over the data bandwidth for maximum data storage. In a proof-of-concept experiment, a total of 500 bits of data were stored at a rate of 20 Mbits/s in a 45-MHz-wide channel within the inhomogeneous line of the (7)F(0)-(5)D(l) transition in Eu(3+):Y(2)SiO(5). The result suggests a minimum storage density of ~1.5 Gbits/cm(3) for this transition. Issues related to the storage of frequency-chirped data, such as the optimum chirp width and the saturation owing to peak-sidelobe interference, are also discussed.

High-speed 1.5- mu m compressively strained multi-quantum well self-aligned constricted mesa DFB lasers

A great improvement in the high-speed characteristics for compressively strained multi-quantum-well (MQW) distributed-feedback (DFB) lasers with self-aligned constricted mesa structures is described. Negative wavelength detuning is an important factor in making possible the extraction of potential advantages for the compressively strained MQW DFB lasers. A 17-GHz bandwidth, which is the highest among the 1.5- mu m MQW DFB lasers, is demonstrated. A wavelength chirp width of 0.42 nm at 10 Gb/s is obtained due to a reduced linewidth enhancement factor that has a magnitude of less than 2. Nonlinear damping K factor in a DFB laser with 45-nm negative detuning has drastically decreased to 0.13 ns, about half of that for unstrained MQW lasers. This is mainly due to an enhanced differential gain as large as 6.9*10/sup -12/ m/sup 3//s. The estimated intrinsic maximum bandwidth is 68 GHz. >

High-speed and ultralow-chirp 11.55 mu m multiquantum well lambda /4-shifted DFB lasers

A high-speed ultralow-chirp 1.55 mu m multiple-quantum-well (MQW) lambda /4-shifted distributed feedback (DFB) laser is demonstrated by using low capacitance structure and by the optimization of an NQW structure in an active layer. A 3-dB bandwidth of 14 GHz as an NQW-DFB laser was achieved, and a chirp width of 3.4 AA (20 dB down full width) was also achieved at 10 Gb/s direct modulation. In addition, kL dependence of chirp width is identified by the adiabatic chirping width is identified by the adiabatic chirping due to the spatial hole burning along a laser axis. From the systematic investigation of the nonlinear gain coefficient of NQW lasers as well as bulk lasers, it is suggested that the physical origin of nonlinear damping can be explained by the spectral hole burning theory. >

Linewidth enhancement factor and high temperature performance of 1.48 mu m strained InGaAs-InGaAsP multiquantum well laser

The linewidth enhancement factor alpha in strained InGaAs-InGaAsP multiquantum well (MQW) lasers emitting near 1.48 mu m has been experimentally determined. The measured alpha at the lasing wavelength is found to be 2 compared to a value of 5.5 and 3.5 typically observed in InGaAsP-InP regular double heterostructure (DH) lasers and MQW lasers, respectively. The small alpha shows that single wavelength strained MQW lasers have smaller chirp width under modulation and also smaller CW linewidth. The carrier lifetime at threshold in these strained MQW lasers does not decrease as rapidly with increasing temperature as is observed for regular InGaAsP-InP DH lasers. This indicates a smaller nonradiative carrier loss due to Auger recombination in strained quantum wells.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Linewidth enhancement factor in strained quantum well lasers

The linewidth enhancement factor α in an In0.2Ga0.8As/GaAs strained-layer multiple quantum well (MQW) laser has been determined from the spontaneous emission spectra below threshold. The measured α at the lasing wavelength is found to be 1.0 compared to a value of 5 typically observed for InGaAsP/InP double-heterostructure lasers. The smaller α shows that single wavelength strained MQW lasers may have smaller chirp width under modulation and also smaller cw linewidth.

Ultralow chirp and high-speed 1.55 mu m multiquantum well lambda /4-shifted DFB lasers

An ultra-low-chirp, high-speed, 1.55- mu m, multi-quantum-well (MQW), lambda /4-shifted, InGaAsP distributed-feedback (DFB) laser is demonstrated. A record chirp width of 0.4-0.5 nm (20-dB down full width) is achieved at 10-Gb/s direct modulation. This low-chirp single-longitudinal-mode operation is attained by an increase in the quantum size effect in the MQW active layer. >

Narrow spectral linewidth of MBE-grown GaInAs/AlInAs MQW lasers in the 1.55 mu m range

GaInAs-AlInAs multi-quantum-well (MQW) lasers fabricated by molecular-beam epitaxy (MBE) on InP substrates are considered. Room-temperature CW operation of ridge-type stripe MQW lasers was observed in the 1.55- mu m wavelength range. Spectral behavior of the MQW lasers was investigated from the standpoint of the linewidth and chirping characteristics. The minimum value of the spectral linewidth was as narrow as 2.5 MHz at an output power of 10 mW in a diode with a cavity length of 750 mu m. Small chirping characteristics were also confirmed, in which 1.5-AA chirp width was observed at 1-GHz direct modulation with a modulation depth of 67%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance characteristics of buried FACET quarter-wave shifted distributed feedback lasers

The performance characteristics of quarter-wave shifted distributed feedback lasers emitting near 1.3µm are described. The quarter-wave shifted grating is fabricated on a substrate using the double-exposure holographic technique. The low reflectivity required for this quarter-wave shifted DFB laser is obtained using buried facets at both ends of the laser. The lasers have threshold current of 30 mA, quantum efficiency of 0.18mW/mA/facet, bandwidth of 11.5GHz at 10mW and 10dB chirp width of 2.5 Å under 40 mA modulation current at 5Gbit/s.

Observation of chirp in a 1.5 μm DFB-LD using a streak camera

The transient wavelength shift of a 1.5 μm distributed-feedback laser diode (DFB-LD) modulated by a rectangular pulse is directly observed using a streak camera. As a result, chirp width increase with decreasing DC bias current is observed. When the DC bias current is set to less than the threshold current, the wavelength shifts briefly from the shorter wavelength to the equilibrium value during the optical pulse turn-on.

Frequency chirp under current modulaton in InGaAsP injection lasers

We have measured the frequency chirp in gain-guided, weakly index-guided, and strongly index-guided InGaAsP lasers under direct-current modulation. The measured chirp width is largest for gain-guided and smallest for weakly index-guided ridge waveguide lasers. The chirp width for 1.5-μm InGaAsP lasers is about a factor of 2 larger than that for 1.3-μm InGaAsP lasers of the same structure. The frequency chirping results from a modulaton of the carrier density which modulates the effective refractive index of the guided mode. The frequency chirping can introduce a limitation on the performance of single-frequency injection laser sources for high bit-rate digital transmission in fiber communication systems at 1.55 μm.