Gain Switching Research Papers - Academia.edu (original) (raw)

In this paper, we summarize our recent results on nonlinear polarization-and transverse-mode dynamics of vertical-cavity surface-emitting lasers (VCSELs) induced by optical injection (OI) or current modulation. Due to the surface emission and cylindrical symmetry, VCSELs lack strong polarization anisotropy and may undergo polarization switching (PS). Furthermore, VCSELs may emit light in multiple transverse modes. This provides new features to the rich nonlinear dynamics induced in VCSELs by an external perturbation. We demonstrate for the case of orthogonal OI that new Hopf bifurcation on a two-polarization-mode solution delimits the injection locking (IL) region and that PS and IL of first-order transverse mode lead to a new resonance tongue for large positive detunings. Similarly, the underlying polarization-mode competition leads to chaotic-like behavior in case of gain switching and the presence of two transverse modes additionally reduces the possibility of regular dynamics.

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- Semiconductor Lasers, Vertical Cavity Surface Emitting Laser, Gain Switching, Bifurcation

In this paper, we propose a novel and cost effective system for optical millimeter-wave (mm-wave) generation and transmission of downstream data based on a gain switched laser (GSL). The GSL produces an optical comb spectrum that can be appropriately filtered to generate two optical sidebands spaced by more than 4 times the repetition rate of the GSL. These sidebands are modulated by baseband data and then transmitted via optical fiber to the remote antenna unit (RAU). At the RAU, the two sidebands are heterodyned using a photodetector to generate the electrical modulated mm-wave signal, before demodulation using self mixing. We demonstrate the distribution of 1.25 Gbit/s data OOK modulated onto a 60 GHz carrier, similar to that used in the IEEE 802.15.3c draft standard, over fiber lengths up to 62 km.

- by Dr Arshad Chowdhury
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- Optics, Medical Imaging, Fiber Optics, Telecommunications

We present a novel implementation of the “phase reconstruction using optical ultra fast differentiation” (PROUD) technique and apply it to characterize the time resolved chirp of a gain switched semiconductor laser. The optical temporal differentiator is a fiber based polarization interferometer. The method provides a fast and simple recovery of the instantaneous frequency from two temporal intensity measurements, obtained by changing the spectral response of the interferometer. Pulses with different shapes and durations of hundreds of picoseconds are fully characterized in amplitude and phase. The technique is validated by comparing the measured pulse spectra with the reconstructed spectra obtained from the intensity and the recovered phase.

- by Antonio Consoli
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- Optical Fiber Communications, Fiber Optics, Interferometry, Diode Lasers

Optical pulses with duration below 60 ps were generated by gain switching a vertical cavity surface emitting laser (VCSEL) emitting at 1.53 m. We used a radio frequency (RF) sinusoidal signal to modulate the device near threshold. Relaxation oscillations and pulse train generation were observed depending on the gain switching conditions. We have characterized the dependence of the pulse width and amplitude on the RF excitation frequency and amplitude for different bias currents. The optimal configuration of the current bias and RF modulating signal provided the shortest pulse duration of 57 ps at a frequency of 800 MHz.

We demonstrate the use of a miniature gain-switched Ti:Sapphire laser for efficient generation of visible supercontinuum light in a highly nonlinear microstructured optical fiber. This allows for a compact setup that puts out nanosecond pulses of light covering the spectral range from 410 to 1300 nm. The source offers a low-cost alternative to the mode-locked-laser-pumped systems in the generation of visible supercontinuum light.

- by Scott Buchter
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- Gain Switching, Nonlinear, Optical fiber, Microstructures

We report optoelectronk phase-tracking and electrooptic sampling of microwave signals from a free-running oscillator in a laser-diode-based system. A voltage-controlled oscillator (VCO) in a photoconductive harmonic mixer based phase-lock loop is utilized to gain switch a 0.8 pm laser diode for phase-tracking of the microwave signal at frequencies up to 20 GHz. Phase and amplitude of the microwave signal in a microstrip line is also measured using another 1.3-pm laser diode driven by the VCO.

- by Ci-Ling Pan
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- Sampling methods, Semiconductor Lasers, Gain Switching, Frequency

We propose and demonstrate a simple and low-cost uplink transmission scheme for coherent optical code-division multiple access (OCDMA) passive optical networks (PONs), using gain-switched Fabry-Pérot (GS-FP) lasers with external injection as local light sources at optical network units. Experimental results based on the two-user 1.25-Gb/s OCDMA system confirm the feasibility of this scheme. The system performance is compared with that of the OCDMA system using a conventional mode-locked laser (MLL). The auto-correlation peak to the maximum wing level (P/W) ratio is also given as the central wavelength of the GS-FP laser varies. The present low-cost scheme is highly preferable for OCDMA PON applications.

We built and characterized a high-energy, injection-seeded, single-longitudinal-mode pulsed titaniumdoped-sapphire laser with tuneable wavelength in the near-infrared and variable pulse temporal duration in the nanosecond regime. We show experimentally how the pulse duration can be easily varied either by changing the cavity length or by changing the pump energy. We successfully interpreted these results on the basis of a theoretical model which treats the operation of this type of laser as a gain-switching technique. Also, as far as the stabilization of the laser cavity, compared to the traditional techniques we employed a novel simplified solution involving the use of an avalanche photodiode.

- by Marco Tognetti
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- Titanium, Near Infrared, Avalanche Photodiodes, Gain Switching

We report on short optical pulse generation by gain-switching (GS) a low-cost commercial vertical-cavity surface-emitting laser emitting at 1.55 μm. The dependence of pulse characteristics on GS parameters is investigated and analyzed. Pulses with duration of 55 ps and time-bandwidth product between 0.91 and 2.2 are obtained at repetition rates between 1 and 3 GHz.

We present a laser source with several desirable characteristics, such as tunable wavelength in the near infrared, single longitudinal mode emission and variable pulse temporal duration in the nanosecond regime. The laser is based on an injection seeded Titanium doped sapphire ring cavity. Our experiments show how the pulse duration can be varied in a controllable fashion either by changing the cavity length or by changing the pump energy. We present a theoretical model which successfully reproduces the experimental results by treating the operation of this type of laser as a gain switching technique. As far as the stabilization of the laser cavity, we also present a novel solution involving the use of an avalanche photodiode.

- by Marco Tognetti
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- Laser Physics, Titanium, Near Infrared, Avalanche Photodiodes

The behaviour of a rf-excited waveguide CO 2 laser in the pulse regime is studied experimentally. The specific time sections of the pulse have been defined and described in detail. The output pulse evolution versus input power, pressure of the laser mixture, output coupling, repetition frequency and pulse width has been investigated. The spectral content of the pulse has been analysed. A model explaining the effect of laser tuning during the pulse duration is given. The ultrahigh pulse (UHP), defined as the pulse for which the bulge effect occurs, is predicted.

The side mode suppression ratio of self-seeded, gain-switched optical pulses is shown to be a vital parameter in wavelength division multiplexed communications systems. Experiments carried out on a 2-channel wavelength multiplexed set-up using tunable self-seeded gain-switched pulse sources at 2.5 GHz, have demonstrated the cross-channel interference effects which may be encountered if the side mode suppression ratio of one of the sources becomes degraded.

This Thesis describes the research work that has been carried out on the generation of optical pulses, with duration of tens of picoseconds, from semiconductor lasers. The work is focused on 1550 nm Vertical Cavity Surface Emitting Lasers (VCSEL), which are promising optical transmitters due to their advantageous characteristics in the context of fiber optical communications with directly modulated sources. The high bandwidth expected for future optical networks requires the accurate knowledge of the transmitter electrical properties and of the laser generated optical pulses which carry the binary information. This Thesis describes the various achievements obtained in the characterization of these devices and the generated pulses, as well as their applications to an optical communication environment. VCSELs emitting at 1550 nm, based on quantum wells and tunnel junction, have been characterized by static and dynamic impedance measurements and modulation response up to 10 GHz. The electrical parasitics and the equivalent circuit of the device have been modeled, taking into account the effects of capture and escape of carriers in quantum wells, and the laser intrinsic parameters have been extracted from the measurements. The VCSELs have been used for pulse generation at different repetition rates using the gain switching technique and the duration, peak amplitude, jitter and spectral width of the pulses have been measured as a function of the gain switching conditions. The shortest duration achieved has been 55 ps. The effect of optical injection on the pulses generated by gain switched VCSELs has been investigated, obtaining a jitter reduction over a wide range of injection parameters. An Optical Code Division Multiple Access (OCDMA) encoder based on optical delay lines has been designed and implemented, using the optical pulses generated by the gain switched devices. Finally, a novel implementation of the Phase Reconstruction using Optical Ultrafast Differentiation (PROUD) technique has been proposed and demonstrated for optical pulse characterization in amplitude and phase. The optical differentiator required in the PROUD technique has been realized with a birefringent interferometer based on a polarization maintaining fiber and PROUD has been applied to the measurement of the instantaneous frequency of pulses generated from a gain switched semiconductor laser. The linewidth enhancement factor of the laser has been extracted from the time resolved chirp and intensity measurements. The accuracy of the proposed method has been validated by the comparison between the independently measured and recovered (from temporal amplitude and phase) pulse spectra.

- by Antonio Consoli
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- Optical Fiber Communications, Optical Communication, Ocdma, Semiconductor Lasers
- by C.Z. Ning
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- Physics, Semiconductors, Modulation, Vertical Cavity Surface Emitting Laser

We report an experimental and theoretical investigation of the effect of optical injection on the characteristics of optical pulses generated by gain-switching a 1550 nm single transverse mode vertical-cavity surface-emitting laser (VCSEL). Under continuous wave operation the VCSEL emits in a linear polarization along the whole current range. The experimental analysis of the effect of external optical injection on the timing jitter, maximum power, and pulse width of optical pulses generated by gain-switching the single mode VCSEL is performed for several repetition rates and for different values of the detuning between the frequency of the optical injection and the VCSEL. Experimental results show that for 1 GHz repetition frequency, jitter reductions greater than 70 % can be obtained over a 47 GHz frequency detuning range with a slight increase of 22% in pulse width with respect to the solitary case. A clear anticorrelation between the maximum power and pulse width is also obtained. A theoretical study is also performed by using a model that incorporates both spatial dependence of carrier density and optical field profiles. The two polarization modes are also taken into account in the model. The theoretical results are in good agreement with the experimental results.

The current state of the art in commercial Wavelength Division Multiplexed (WDM) systems allows telecommunication operators install fixed WDM systems in which each channel can operate at a bit rate of 10 Gb/s, with a channel spacing of around 100 GHz. However, with the drive to develop long-haul transport networks exhibiting multi-Tb/s capacities, it is anticipated that these WDM systems will be upgraded by deploying higher wave counts (at 10 Gb/s) or higher capacities per wavelength. The latter seems to be the better alternative especially taking into account the numerous advantages such as lower overall cost for capacity and better terminal density. One of the factors that has been attracting a lot of attention, with the move to higher line rates, is the coding used at the transmitter. Most of the current systems, 2.5 -10 Gb/s, have tended to employ Non-Return-to-Zero (NRZ) coding. However, in order to achieve line rates of 40 Gb/s and higher, it may become necessary to use Return-to-Zero (RZ) coding. RZ (pulse) modulation formats offer a number of advantages over NRZ modulation schemes, especially in long haul transmission, which result in higher signalto-noise ratio and lower system bit error rate translating to better overall system performance. Hence, the development of picosecond optical pulse sources with excellent temporal and spectral properties is vital for future implementation of high capacity optical communications systems using OTDM and hybrid WDM/OTDM technologies.