Diode lasers with optical feedback produce self-modulated square waves

In this paper, the influence of the rotating polarization-preserved optical feedback on the chaos synchronization of a vertical-cavity surface-emitting laser (VCSEL) is investigated experimentally. Two VCSELs' polarization modes (XP) and (YP) are gradually rotated and re-injected back into the VCSEL. The anti-phase dynamics synchronization of the two polarization modes is evaluated using the cross-correlation function. For a fixed optical feedback, a clear relationship is found between the cross-correlation coefficient and the polarization angle θp. It is shown that high-quality anti-phase polarization-resolved chaos synchronization is achieved at higher values of θp. The maximum value of the cross-correlation coefficient achieved is -0.99 with a zero time delay over a wide range of θp beyond 65° with a poor synchronization dynamic at θp less than 65°. Furthermore, it is observed that the antiphase irregular oscillation of the XP and YP modes changes with θp. VCSEL under the rotating polarization optical feedback can be a good candidate as a chaotic synchronization source for a secure communication system.

In this paper, the influence of variable-angle polarization-rotated optical feedback on the polarization switching properties of vertical-cavity surface-emitting lasers (VCSELs) is investigated theoretically and experimentally. The optical feedback (OF) level and the rotated polarization angle $\theta _{p}$ of OF are considered. Two configurations of OF are used to re-injected parallel and orthogonal polarization modes back into the VCSEL. For the parallel OF, it is shown that polarization switching (PS) of VCSELs’ modes can occur at a fixed feedback level and bias current with variable $\theta _{p}$ . However, in case of orthogonal optical feedback, PS occurs only at certain values of $\theta _{p}$ , particularly when the intensities of two polarization modes become comparable with each other. In the theoretical model for both OF conditions, there is good agreement between the experimental measurements and recent theoretical results reported in the literature.

We numerically investigate the performance of Chinese herbal medicine (CHM) recognition using a time delay reservoir computing (TDRC) system consist of a vertical-cavity surface-emitting laser (VCSEL) subjected to optical feedback. A database of 960 images from 12 CHM categories is used to evaluate the recognition performance, most of which contain many CHM samples and have different backgrounds. The CHM image feature vector is masked and optically injected into the VCSEL, and the VCSEL output in two polarization modes are sampled as the virtual node states for readout. After testing the CHM recognition performance of TDRC under injecting the same information into two polarization modes of the VCSEL, we further test the performance of parallel processing by injecting half of the image information into each of the two polarization modes. By concatenating the virtual node states collected from two modes for readout, the recognition speed is doubled. Meanwhile, the influence of key parameters on recognition performance are investigated and the relationship between system idle state and recognition performance is also examined. The results show that, under parallel processing and orthogonal optical feedback, the TDRC achieves the minimum recognition error of 1.7% at a reservoir processing rate of 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> images per second.

The characteristics of polarization self-modulation in a vertical-cavity surface-emitting laser (VCSEL) were studied for frequencies up to ≈9 GHz both experimentally and theoretically. Polarization self-modulation was obtained by rotating the linearly polarized output of the VCSEL by 90° and reinjecting it into the laser. Experimentally we simultaneously recorded time traces, optical and radio-frequency spectra. We found for increasing modulation frequencies that the output characteristics changed from square-wave to sinusoidal and the VCSEL system assumed new polarization eigenstates that are different from the free-running VCSEL eigenstates. We modeled polarization self-modulation as an interband process and found a good qualitative agreement between our experimental and numerical results.

We introduced the vertical cavity surface emitting laser (VCSEL) as the laser diode in tour external cavity system. Because VCSELs are now commercially available, and the External cavity diode laser (ECDL) systems using them are expected to improve their frequency stability, we have replaced a Fabry-Perot type laser diode with a VCSEL, and examined its oscillation-frequency stability. Therefore we were able to expect that the VCSELs with our double optical feedback system have good oscillation frequency stability. The obtained VCSEL's oscillation-frequency stability, i.e., the square root of Allan variance &sigma; was 4&times;10^-10, at an averaging time of &tau;=1 sec.

Using two orthogonal polarization mode outputs from a vertical-cavity surface emitting lasers (VCSEL) with double optical feedback as two chaotic carriers, a dual-channel chaos secure communication system is established, and the synchronization and communication performances of such a system are numerically investigated. The results show that under suitable operated condition, the time-delay signatures of two chaotic carriers originating from two linear polarization modes in the T-VCSLE with double optical feedback can be suppressed efficiently. Under the strong injection locking case, high-quality chaos synchronization between two corresponding modes of T-VCSEL and R-VCSEL can be realized by polarization-preserved optical injection from T-VCSEL to R-VCSEL. Moreover, the tolerance of the synchronization quality on the frequency detuning between T-VCSEL and R-VCSEL is enhanced with the increase of the injection strength. Adopting additive chaos modulation encryption scheme, two pieces of 500 Mbit/s encoded message can be hidden efficiently in the two chaotic carriers in the propagation process and can be successfully extracted at the receiver. Although Q factor decreases with the increase of message transmission rate, the values of Q factor for two channels are still larger than 6 for 6 Gbit/s message.

Vertical Cavity Surface Emitting Lasers (VCSELs) with isotropic optical feedback are studied especially in the so-called low frequency fluctuations regime. Correlation properties between linear polarizations are analyzed both analytically and numerically. The RF spectrum shows a double peak structure close to the external cavity frequency which can be predicted by our adapted Spin-Flip Model (SFM). We provide here numerical evidence of the interplay between modes and anti- modes which are solutions of the stability analysis of the VCSEL with feedback and demonstrate that this interplay is responsible for the double peak structure.

Based on the spin-flip model (SFM), we theoretically investigate the dynamics of a vertical-cavity surface-emitting laser (VCSEL) subject to optical injection and positive optoelectronic feedback. The results show that under the joint action of positive optoelectronic feedback and optical injection from a master VCSEL (M-VCSEL), two polarization modes of a slave VCSEL (S-VCSEL) will show many dynamic states such as period one, period two, multi-period and chaos, and the evolution routes of these states are different for two polarization modes. Mapping of dynamic region as a function of feedback strength f and injection strength is varied with frequency detuning between M-VCSEL and S-VCSEL (=m-s, where m ands are the free-running frequencies of M-VCSEL and S-VCSEL, respectively). Compared with the case for zero or negative frequency detuning, the region of chaotic state is expanded significantly under positive . For a fixed , the influences of f and on the chaotic bandwidth of S-VCSEL are discussed. Through selecting proper f and , chaotic bandwidth of S-VCSEL can be improved obviously.

Military aviators have a need for lightweight Helmet Mounted Display (HMD) systems capable of displaying Night Vision (NV) imagery (which has been collected at the O.665m-O.93Optm portion of the spectrum), Forward Looking Infrared (FLIR) imagery (which has been collected at the 3-5im and 8-l2im waveband portions of the spectrum) and/or Heads-Up Display (HUD) information. Present HMD systems have excessive head—supported weights and poor centers of gravity (CG), inducing fatigue and creating an unsafe ejection condition. These problems are created mostly by the weight of the optics and optics support structure attached to the side and front of the aviator's helmet. The optics are necessary to redirect the imagery output of a phosphor screen to the aviator's eyes with as much fidelity as possible. An alternative approach to present HMD systems involves the use of a microlaser based image output array. The micro—lasers would be an array of Vertical Cavity Surface Emitting Laser (VCSEL) diodes. VCSELs have circular output beams, as opposed to edge emitting laser diodes which have astigmatic output beams. The VCSEL circular output beam geometry lends itself to the use of micro-lenses. The micro-lenses allow for f/number modulation with a significantly reduced number of optical elements in the optical path during the redirection of the output image to the user's eyes. Reduced optical elements equate to reduced weight and better CG. In addition, this approach may allow for significantly greater fields-of-view (FOV), possibly in excess of 100 degrees. This paper addresses the attributes and drawbacks of current HMD systems (especially NV systems) and the attributes, drawbacks, and technical challenges associated with realization of a HMD utilizing VCSELs as a display illumination source.

Herein, we propose a 25 Gb/s (12.5 Gbaud/s) 4-level pulse amplitude modulation (PAM-4) vertical cavity surface emitting laser (VCSEL) diode driver in TSMC 90 nm CMOS technology. In the VCSEL driver, an equalizer was designed using an edge peak generator to enhance the rising and falling edges of the data signal. The VCSEL driver can combine two 12.5 Gb/s non-return-zero (NRZ) input signals as a 25 Gb/s PAM-4 output current signal to drive a VCSEL diode. For the VCSEL diode driver, the total modulation current is 9.2 mA, the power consumption is 87.47 mW, and the chip size is 0.49 mm×0.49 mm.

This paper provides an experimental investigation of the polarization stabilization at the output power of an 850 nm vertical-cavity surface-emitting laser (VCSEL) with optical feedback (OF) and a range of polarization angles, OF strength, and the bias current. The VCSEL's polarization stabilization is evaluated using the extinction ratio measurements of the optical output power of the polarization modes of the VCSEL. The results clearly show that rotation of the polarization angle and the OF level can radically change the polarization stabilization of the VCSEL. Both the polarization angle and OF introduce polarization switching (PS) and instability in the optical output power of the VCSEL. Consequently, these lead to performance degradation of the VCSEL in terms of the operating point and the modulation bandwidth. At a fixed OF level of −7 dB, polarization destabilization is first observed at 45° with the increasing level of polarization angle, whereas for the fixed polarization angles of 40° and 90°, polarization destabilization is observed at −14.5 dB and −14 dB, respectively, with the increasing level of orthogonal OF. We show that, with parallel OF, no PS is observed over the entire OF level. The results also indicate that the VCSEL with no polarization angle requires higher levels of OF in order to ensure PS compared with the case with the polarization angle.

We report the formation and growth characteristics of an interfacial misfit (IMF) array between AlSb and Si and their application to III-Sb based quantum well (QW) light-emitting devices including edge-emitting laser diodes and vertical-cavity surface emitting lasers (VCSELs) monolithically grown on a Si (001) substrate. A III-Sb epi-structure is grown monolithically on the Si substrate via a thin (=50 nm) AlSb nucleation layer. A 13% lattice mismatch between AlSb and Si is accommodated by using the IMF array. We demonstrate monolithic VCSELs grown on Si(001) substrates operating under room-temperature with optically-pumped conditions. A 3-mm pump spot size results in peak threshold excitation density of I th = 0.1 mJ/cm 2 and a multimode lasing spectrum peak at 1.62 μm. Moreover, broad-area edge-emitters consisting of GaSb/AlGaSb QWs are demonstrated under pulsed conditions at 77K with a threshold current density of ≅2 kA/cm 2 and a maximum peak output power of =20 mW for a 1 mm-long device. A use of 5° miscut Si substrates enables both IMF formation and suppression of an anti-phase domain, resulting in a drastic suppression of dislocation density over the III-Sb epi-layer and realization of electrically-injected laser diodes operating at 77 K. The current-voltage (I-V) characteristics indicate a diode turn-on of 0.7 V, which is consistent with a theoretical built-in potential of the laser diode. This device is characterized by a 9.1 Q forward resistance and a leakage current density of 0.7 A/cm 2 at -5 V and 46.9 A/cm 2 at -15 V. This IMF technique will enable the realization of III-Sb based electrically-injected VCSELs operating at the fiber-optic communication wavelength monolithically grown on a Si platform.

We report on new dynamics in vertical-cavity surface-emitting lasers (VCSELs), which is induced by a polarization preserving optical feedback. Our solitary VCSEL, i.e. without feedback, emits in a single linearly polarized (LP) and fundamental transverse mode. Optical feedback induces multiple polarization switchings in the L-I curve. When the injection current is set close to one of the multiple polarization switching points, the laser system exhibits a random anticorrelated hopping of the two orthogonal LP fundamental modes. Of particular interest is the observation of antiphase oscillations in the two LP modes that occur with a period corresponding to the delay time, and which complement the slow mode-hopping. Moreover, the LP-resolved optical spectra show random frequency hops between ECMs. The laser system is therefore not only bistable between the LP modes, but also multistable between ECMs. A theoretical rate equation approach yields a good qualitative agreement with our experiment. We report similar delay-periodic oscillations complementing a mode hopping in a scalar delay differential equation modeling the motion of a particle in a double well potential. Such a model was recently investigated by Tsimring and Pikovsky, in a very general context. Our results in VCSELs yield the first experimental evidence of delay-periodic oscillations in a bistable physical system with noise, and are therefore thought to be of general interest for many other processes in biology, medicine, etc. that exhibit interaction of noise, delay and bistability.

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.

Colloidal quantum‐dot (CQD) vertical‐cavity surface‐emitting lasers (VCSELs) enable a solution‐processable directional‐emitting coherent light source, which is desirable for various applications, including near‐eye display, sensing, and communication. However, it remains challenging to construct a controllable, high‐quality VCSEL cavity without damaging the photoluminescence of CQDs to achieve the desired lasing characteristics. Here, high‐quality CQDs with an engineered CdZnSe/ZnSe/ZnxCd1‐xS core/interlayer/graded shell structure are developed to ensure excellent stability and sub‐single‐exciton gain threshold, facilitating the achievement of high‐performance VCSEL. Subsequently, CQD VCSELs with distributed Bragg reflectors deposited on CQDs in situ by thermal evaporation are demonstrated. The proposed fabrication process not only enables precise control over the cavity structure but also a high cavity quality without compromising the optical properties of CQDs. Consequently, the developed CQD VCSEL exhibits a low lasing threshold of 58 µJ cm−2 and a high lasing quality factor up to 2395, setting a record for VCSELs based on CQDs or colloidal quantum‐wells. It also demonstrates stable operation for 300 h at room temperature, corresponding to 1.08 × 108 stable lasing pulses, placing it among the most stable nanocrystal lasers reported. This work presents an effective strategy for achieving high‐performance CQD‐based VCSEL, which is significant for the future development towards non‐epitaxial laser diodes.