A. Valle

Polarization Switching Regions of Optically Injected Long-Wavelength VCSELs

We report on an experimental study of the injected optical power required for polarization switching (PS) in a vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection. This power is found as a function of the frequency detuning between the injected light and suppressed linear polarization of the solitary VCSEL. Increasing the injected power, we found two types of PS when injecting light with wavelength close to the wavelength of the suppressed linear polarization and to that of the linear polarization of the solitary VCSEL. These two types of PS are, respectively: 1) single and double (with two minima of injected power necessary for PS) and 2) double PS (with two islands of PS and a separate region with a single minimum of injected power). Our experimental results confirm the theoretical predictions of Torre et al. Double PS is observed around the elliptically polarized injection locked states found by Lin et al. Different dynamics are found in the route to PS.

Transverse mode competition effects on the dynamics of gain-switched vertical-cavity surface-emitting lasers

An experimental analysis of the nonlinear dynamics of a multi-transverse-mode vertical-cavity surface-emitting laser (VCSEL) when subject to a high-frequency current modulation is performed. Regular periodic dynamics—with periods equal to the modulation period or twice the modulation period—and irregular pulsating dynamics are obtained. Our results show that the irregular pulsating dynamics in multimode VCSELs subject to large-signal current modulation is due to the competition between different transverse modes.

Diffusive turn-off transients in current modulated multitransverse mode VCSELs

Secondary pulsations are an example of diffusive turn-off transients that can limit the performance of VCSELs in optical communication systems. Secondary pulsations are firstly analysed by using a model where a modal expansion of the electric field is performed. The maximum power of the secondary pulsations and the time at which they appear fluctuate when the spontaneous emission noise is present. A linear relation between the two previous quantities for each individual turn-off event is found. In the single-mode regime, the averaged maximum power during turn-off transients increases when increasing the injection current. However, in the multi-mode regime, the strength of secondary pulsations decreases when increasing the current. Secondary pulsations are also analysed by using a spatio-temporal description of the VCSEL dynamics, where the modal profiles are determined from the distribution of injected carriers and the thermal lens. This model also incorporates polarization effects and a frequency-dependent susceptibility. In this model, the carrier-induced refractive index changes increase the strength of secondary pulsations as compared to that obtained with the modal expansion. It is also shown that the use of ring-shaped electrical contact enhances the strength of secondary pulsations, while it decreases when multi-transverse mode operation is present.

Enhancement of Chaos Bandwidth in VCSELs Induced by Simultaneous Orthogonal Optical Injection and Optical Feedback

In this paper, we present an experimental study of bandwidths of broadband chaotic signals in a single-mode vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection and optical feedback simultaneously. Bandwidths of the two linear polarizations and total power are measured for different values of frequency detuning and bias currents. When the VCSEL is subject to optical feedback only, both linear polarizations of the VCSEL are excited showing chaotic dynamics. Under simultaneous optical feedback and optical injection for small bias currents, we demonstrate that the orthogonal polarization can give a much larger value of the chaotic bandwidth than the total emitted power in agreement with Perez et al.

Experimental stability maps of a 1550nm-VCSEL subject to polarized optical injection

We report an experimental study of the injection locking properties and nonlinear dynamics of a 1550nm-Vertical Cavity Surface Emitting Laser (VCSEL) subject to parallel and to orthogonal polarized optical injection into the two orthogonal polarizations of the fundamental transverse mode of the device. A rich variety of nonlinear dynamics have been observed for both parallel and orthogonal optical injection outside the stable locking range, including limit cycle, period doubling, chaos and polarization switching. Additionally, for the case of orthogonal optical injection, we report a first experimental observation of the change in the shape of the stability map as the applied bias current is increased well above threshold.

Experimental study of relative intensity noise of multimode vertical-cavity surface-emitting lasers

We have performed an experimental investigation of the RIN spectra of multimode 1550 nm VCSELs. Several types of multi-transverse mode VCSELs have been considered. The first one emits in two transverse modes for large values of the bias current. Both modes are linearly polarized and have parallel polarizations. Two resonance peaks appear in the noise spectra of the individual modes and total power of the VCSEL in agreement with previous theoretical studies (A. Valle et al, IEEE J. Quantum Electron, vol. 40, 597, 2004). We have measured the frequencies corresponding to both peaks as a function of the bias current. We have obtained a similar dependence to the one found in the previous work. The second VCSEL type corresponds to a laser emitting at three different transverse modes for large values of the bias current. Similar behaviors are found while the bias current is small. Emission in three transverse modes with different polarizations is accompanied by the appearance of additional peaks in the noise spectra. It is suggested that the VCSEL polarization plays an important role in determining the multi-peaked structure of noise spectra. That is confirmed by measuring with another two-transverse mode VCSEL showing polarization instabilities in both transverse modes. It is shown that large values of the bias current applied to this VCSEL result in a complex dynamics characterized by the appearance of many additional peaks in the noise spectra.

Experimental study of transverse mode dynamics in vertical-cavity surface-emitting lasers under current modulation

In this work we report on an experimental investigation of the nonlinear dynamics of a 850 nm multitransverse mode vertical-cavity surface-emitting laser (VCSEL) when subject to high-frequency current modulation. Different frequencies and modulation amplitudes are applied to the VCSEL. Regular periodic dynamics - with periods equal to the modulation period or twice the modulation period - and irregular pulsating dynamics are obtained. Different dynamical behaviors are illustrated by using power time traces, radio-frequency spectra and bifurcation diagrams. Our results show that irregular pulsating dynamics in multimode VCSELs subject to large-signal current modulatton can be obtained due to the competition between different transverse modes.

Bias level dependence of turn-off oscillations in vertical-cavity surface-emitting lasers

Turn-off oscillations are dynamical phenomena that appear when modulating the current applied to vertical-cavity surface-emitting lasers (VCSELs). These phenomena consist of the generation of significant pulsed optical power, while the laser current remains at the lowest (bias) value during the modulation. We study the dependence of turn-off oscillations on the bias current for both single-mode and multitransverse-mode VCSELs. We show that as the bias current increases, the maximum power of these oscillations obtained with multimode lasers becomes similar to the one obtained with single-mode lasers. A criterion for obtaining the bias current above which turn-off oscillations are no longer dominated by spatial effects is given. We also show that the transverse spatial profile of the light emitted in multimode VCSELs during turn-off oscillations depends on the value of the bias current.

Theoretical and experimental study of polarization switching in long-wavelength VCSELs subject to parallel optical injection

We report a theoretical and experimental analysis of the polarization switching found in a single-transverse mode VCSEL when subject to parallel optical injection. We have found a novel situation in which injection locking of the parallel polarization and excitation of the free-running orthogonal polarization of the VCSEL are simultaneously obtained. Analytical expressions for the power of both linear polarizations in the previous steady state are determined. We show that considering two linear polarization modes in a model of a VCSEL subject to parallel optical injection leads to simpler expressions than those found for a VCSEL with only a single linear polarization. We show that the power emitted in both linear polarizations depend linearly on the injected power. The stability region of this solution is measured in the plane injected power versus frequency detuning.

Microwave signal generation using a 1550 nm VCSEL subject to dual-beam orthogonal optical injection

We experimentally and theoretically investigate microwave signal generation using a 1550nm VCSEL subject to two-frequency orthogonal optical injection. It is found that microwave linewidth is given by the sum of the linewidths of the two master lasers.