Polarization Switching Point Research Articles

Distribution of the built-in field extracted from switching dynamics in HfO2-based ferroelectric capacitor

In this work, we proposed a method to extract the distribution of the built-in field (Eb) from the switching dynamics of the TiN/HfZrO/TiN capacitor. The relationship between reversal polarization and the distribution of Eb is established based on the classic inhomogeneous field mechanism model. Both positive and negative write pulses with different amplitudes and durations are applied to obtain the distribution parameters of Eb. The distribution of Eb is fitted by a Gaussian-type distribution, and the mean value and standard deviation are about −0.02 MV/cm and 0.28 MV/cm, respectively. This work provides an effective approach to analyze Eb directly from the electrical measurement and helps optimize the device design from the polarization switching point of view.

Polarization Instabilities in Vertical-Cavity Surface-Emitting Lasers

We report on experimental investigation of short-period InGaAs/InGaAlAs superlattice vertical cavity surface emitting lasers characteristics (VCSEL), including light-current-voltage characteristics, optical and radiofrequency spectra and polarization mode hopping between orthogonal modes. The observed polarization switching features is similar to what is observed in quantum well VCSEL. Future investigations will consider polarization-resolved optical and radiofrequency spectra, total intensity noise analysis of VCSEL biased near the polarization switching point.

Polarization switching mechanism in surface-emitting semiconductor lasers

Within the scope of the earlier proposed approach to the description of polarization dependences in VCSEL based on analysis of the experimental and theoretical data, an explicit expression for the function describing the relationship between the amplification anisotropy and the current density is suggested. The conducted numerical calculations support the peculiarities of the polarization switching (PS) process and enable one to explain the effect of the PS point anomalous shift when the current variation rate is high. A special attention is given to a fairly clear physical interpretation of the formation of polarization radiation in VCSEL.

Polarization switching characteristics of polarization maintaining optical feedback and orthogonal optical injection of 1550 nm-VCSEL

Based on the spin flip model, four schemes for the polarization switching of the 1550 nm vertical cavity surface emitting laser are studied by using numerical simulation, which are free running laser orthogonal injection free running laser, free running laser orthogonal injection polarization maintaining optical feedback laser, polarization maintaining optical feedback laser orthogonal injection free running laser, and polarization maintaining optical feedback laser orthogonal injection polarization maintaining optical laser. We can draw three conclusions from the numerical results. Firstly, changing the feedback strength can make the polarization switching point of the injection intensity in the different regular movements when the normalized injection current is small. The injection intensity of the polarization switching point increases with the increase of the feedback strength for the free running laser orthogonal injection polarization maintaining optical feedback laser; the injection intensity of the polarization switching point decreases with the increase of the feedback strength for the polarization maintaining optical feedback laser orthogonal injection free running laser; the injection intensity of the polarization switching point is nonlinear and fluctuates with the increase of the feedback strength for the polarization maintaining optical feedback laser orthogonal injection polarization maintaining optical laser. The reason is that the non dominant X polarization component cannot go up when the normalized injection current is small, then, as the feedback intensity increases, the difference between the two polarization components will be increased. Secondly, when the normalized injection current is large, changing the feedback intensity can make the polarization switching point of the injection intensity in the irregular movement. The reason is that the non dominant X polarization component can go up when the normalized injection current increases up to a certain value, which can form the significant nonlinear wave together with the dominant Y polarization component. Thirdly, changing the frequency detuning can make the polarization switching point of the injection intensity in the same regular movement. The injection strength required for the occurrence of polarization switching point first decreases and then increases for the four schemes, when the frequency detuning is from approximately -60 GHz to the minimum, presenting the symmetrical distribution of V type with -60 GHz as the axis. The same regular movement of the polarization switching point of the injection intensity is not changed with the change of the normalized injection current.

Polarization-resolved characterization of long-wavelength vertical-cavity surface-emitting laser parameters

A measurement of the parameters characterizing polarization behavior of a 1550 nm single transverse mode vertical-cavity surface-emitting laser (VCSEL) is presented from the analysis of CW high-resolution optical spectra. Current-induced polarization switching (PS) is observed in our device. A minimum in the absolute value of the effective dichroism is observed at the PS point. The value of the effective birefringence has a discontinuity at the current at which PS is observed. Birefringence oscillations with the bias current are also observed. PS characteristics are used for the extraction of the parameters describing the polarization behavior of the VCSEL. Obtained values of the spontaneous emission fraction and spin-flip rate are larger than those typically used in theoretical modeling of these devices.

Fast, electrically controlled polarization modulation of multimode vertical-cavity surface-emitting lasers by RF frequency modulation

We report on a study of polarization properties of asymmetric, multimode vertical-cavity surface-emitting lasers (VCSEL) subjected to electrical RF modulation. When subjected to RF modulation, complex frequency-dependent polarization properties, especially near the polarization switching point are revealed. We propose a scheme of rapidly switching the two RF frequencies modulating the VCSEL, in order to achieve fast polarization modulation in these VCSEL. Polarization modulation up to 300 MHz by modulating the RF frequency and up to 1.5 GHz with RF power modulation has been demonstrated; the fastest reported electrically controlled polarization modulation for multimode VCSELs.

Impact of optical feedback on current-induced polarization behavior of 1550 nm vertical-cavity surface-emitting lasers

Polarization switching (PS) between two orthogonal linearly polarized fundamental modes is experimentally observed in commercial free-running 1550 nm vertical-cavity surface-emitting lasers (VCSELs) (Raycan). The characteristics of this PS are strongly modified after introducing a polarization-preserved (PP) or polarization-orthogonal (PO) optical feedback. Under the case that the external cavity is approximately 30 cm, the PP optical feedback results in the PS point shifting toward a lower injection current, and the region within which the two polarization modes coexist is enlarged with the increase of the PP feedback strength. Under too-strong PP feedback levels, the PS disappears. The impact of PO optical feedback on VCSEL polarization behavior is quite similar to that of PP optical feedback, but larger feedback strength is needed to obtain similar results.

100-Nanosecond-Level Square-Pulse Generation in a Ring Cavity with a Tellurite Single-Mode Fiber

We demonstrate the square-pulse generation in a ring cavity with a 207 m tellurite single-mode fiber. The stable square pulse with the wide full width at half maximum (FWHM) of 118 ns is formed by the nonlinear polarization switching in the cavity. The high nonlinearity and birefringence of the tellurite fiber play the key role during the square-pulse formation and the net normal dispersion is advantageous for stabilizing the square pulse. When the linear phase delay bias is set close to the polarization switching point, the third-order harmonic pulses and multiple packets are observed under different pump powers.

Theoretical investigations on the polarization performances of current-modulated VCSELs subject to weak optical feedback

Based on the framework of the spin-flip model (SFM) for GHz level modulation frequency, the polarization performances of current-modulated vertical-cavity surface-emitting lasers (VCSELs) subject to weak optical feedback have been theoretically investigated. The results show that, for type 1 polarization switching (PS), the influences of optical feedback intensity and the current modulation amplitude on the high-frequency modulation response is not obvious. For low-frequency modulation, optical feedback will result in the output of a VCSEL having rich dynamics. A new style PS can be obtained under large-signal modulation through adjusting the modulation frequency. For type 2 PS the situation is more complex, and optical feedback increases the probability of polarization conversion and competition, while also affecting the PS point. The modulation amplitude also affects the output dynamics of the VCSEL.

Modeling thermal effects and polarization competition in vertical-cavity surface-emitting lasers

We analyze the influence of thermal effects on the polarization-resolved light-current (LI) characteristics of verticalcavity surface-emitting lasers (VCSELs). We use a model that is an extension of the spin-flip model incorporating material gain that is frequency and temperature dependent, and a rate equation for the temperature of the active region, which takes into account decay to a fixed substrate temperature, Joule heating and nonradiative recombination heating. The model also incorporates the red shift for increasing temperature of the gain curve and of the cavity resonance. The temperature sensitivity of the lasing threshold current is found to be in good qualitative agreement with observations and with previous reports based on detailed microscopic models. The temperature dependence of the polarization switching point, when the dominant polarization turn off and the orthogonal polarization emerges, is characterized in terms of various model parameters, such as the room-temperature gain-cavity offset, the subtracte temperature, and the size of the active region.

Polarization Dynamics of Current-Modulated Vertical-Cavity Surface-Emitting Lasers

We study the dynamics of vertical-cavity surface-emitting lasers (VCSELs) with direct current modulation in the framework of a model for index-guided VCSELs that takes into account two orthogonal linear polarizations. We analyze the effect of current modulation near the polarization switching (PS) of type I, from the high to the low frequency polarization, and near the PS of type II, from the low to the high frequency polarization. We find that the oscillations of the total power are as those of a single-mode laser, unaffected by the underlaying polarization coexistence or polarization competition. We also study the small-signal modulation response in the Fourier domain, for modulation dc values near the PS point. Close to type I PS the response of the total power as well as the response of the orthogonal polarizations has the same functional dependency on the modulation frequency, and can be fitted by the response function of a single-mode laser. Close to type II PS, polarization competition is a significant process at low modulation frequencies. The polarization-resolved modulation response displays features at low frequencies that are not present in the response of the total power, which is as that of a single-mode laser. The dynamics becomes increasingly complex as the modulation amplitude grows, and there is multistability of solutions.

Impact of orthogonal optical feedback on the polarization switching of vertical-cavity surface-emitting lasers

We study experimentally and numerically the influence of orthogonal optical feedback on the polarization-resolved light versus bias current characteristic (L-I curve) of vertical-cavity surface-emitting lasers (VCSELs). The feedback scheme is such that only one linear polarization is selected to be fed back into the laser while the orthogonal polarization is completely suppressed before the output is rotated 90° and reinjected into the laser. We experimentally demonstrate that weak feedback levels modify the polarization switching point only slightly, but as the feedback increases the otherwise depressed mode grows and the hysteresis is suppressed. While polarization-preserved and X-orthogonal feedback have similar effects (X indicates the direction of the polarization selected at threshold), Y-orthogonal feedback strongly modifies the shape of the L-I curve, even suppressing the polarization-switching for strong enough feedback. Numerical simulations of the spin-flip model show good qualitative agreement with the observations. We also analyze the influence of various parameters, such as linear birefringence, dichroism, and the spin-flip relaxation rate.

Frequency response of polarization switching in vertical-cavity surface-emitting lasers

We present an experimental study of the current-driven polarization modulation properties of VCSELs. In some VCSELs, abrupt polarization switching (PS) between two polarization modes is observed at a particular value of the pump current. We investigate the dynamics and the associated dominating time scales of PS as these features are strongly linked with the underlying physical mechanism causing the PS. To this end, we measure both for gain- and index-guided VCSELs the critical modulation amplitude necessary to steadily force PS back and forth across the PS point as a function of the modulation frequency. This yields the current-driven polarization modulation frequency response, which we compare with the thermal frequency response of the studied devices. The dynamic behavior turns out to be strikingly different for the different VCSEL types. Thermal effects only play a minor role in the PS in our index-guided VCSELs, while they really seem to lie at the origin of PS in the gain-guided VCSELs. By implementing this in a rate-equation based theoretical model, we are able to explain the peculiarities of the measured response curves and to reproduce the experimental findings.