Bistable Laser Research Articles

Polarization-dependent coupling between a polarization-independent high-index-contrast subwavelength grating and waveguides

We investigated the optical coupling between a polarization-independent high-index-contrast subwavelength grating (HCG) and two orthogonal in-plane waveguides. We fabricated the HCG with waveguides on a silicon-on-insulator substrate and demonstrated that a waveguide with a strong output is switched by changing the polarization of light injected into the HCG. The light coupled more strongly to the waveguide in the direction perpendicular to the polarization of the incident light than to that in the parallel direction. If this waveguide-coupled HCG is incorporated into a polarization bistable vertical-cavity surface-emitting laser (VCSEL), the output waveguide can be switched by changing the lasing polarization of the VCSEL.

Bistable random laser that uses a phase transition of polyethylene glycol

Polyethylene glycol (PEG) is transparent in the liquid phase and turns to a translucent solid through a phase transition at around room temperature. A PEG solution of rhodamine 6G was excited by a laser pulse (527 nm wavelength, 10 ns duration, 190 μJ) to measure fluorescence spectra during the phase transition process. Whereas the fluorescence peak was weak and broad at 60 °C (spontaneous emission), a strong, narrow peak appeared in the temperature range below 50 °C, since a stimulated emission (amplified spontaneous emission) took place due to the scattering in the solid PEG. This laser emission control was repeatable by reversing an electric voltage (−12 or +12 V) that drove a Peltier element. Interestingly, the sample exhibited a strong stimulated emission at 52 °C during the heating process, although no strong emission was visible at the same temperature during the cooling process (bistability). The fluorescence peak wavelength was tunable between 566 and 572 nm by changing the cooling rate.

Optical-cell model based on the lasing competition of mode structures with different Q-factors in high-power semiconductor lasers

A model describing the operation of a completely optical cell, based on the competition of lasing of Fabry-Perot cavity modes and the high-Q closed mode in high-power semiconductor lasers is proposed. Based on rate equations, the conditions of lasing switching between Fabry-Perot modes for ground and excited lasing levels and the closed mode are considered in the case of increasing internal optical loss under conditions of high current pump levels. The optical-cell operation conditions in the mode of a high-power laser radiation switch (reversible mode-structure switching) and in the mode of a memory cell with bistable irreversible lasing switching between mode structures with various Q-factors are considered.

All-optical 2-bit header recognition and packet switching using polarization bistable VCSELs.

We propose and evaluate an all-optical 2-bit header recognition and packet switching method using two 1.55-µm polarization bistable vertical-cavity surface-emitting lasers (VCSELs) and three optical switches. Polarization bistable VCSELs acted as flip-flop devices by using AND-gate operations of the header and set pulses, together with the reset pulses. Optical packets including 40-Gb/s non-return-to-zero pseudo-random bit-sequence payloads were successfully sent to one of four ports according to the state of two bits in the headers with a 4-bit 500-Mb/s return-to-zero format. The input pulse powers were 17.2 to 31.8 dB lower than the VCSEL output power. We also examined an extension of this method to multi-bit header recognition and packet switching.

Highly controllable optical bistability effect in a 2 μm Tm:YAG ceramic laser at room temperature.

A highly controllable optical bistability in a Tm:YAG ceramic laser system is reported, which is attributed to the thermal-induced change in the stability of the resonator. The width of the bistable region can be tuned in a large scale from 0.8 W to 6.3 W. At nearly semi-confocal cavity configuration, a second lasing condition was observed in the bistable region with a modulated shape of the laser beam and a broadened laser spectrum. The influence of the temperature on the bistable laser operation was also discussed in detail. To our knowledge, this is the first report on the optical bistability effect in Tm:YAG ceramic lasers.

Phase space of tristability in dual injection-locked Fabry-Perot laser diodes

By using the model of an optically bistable injection-locked semiconductor Fabry-Perot laser diode, we theoretically investigate the case of dual injection-locking, in which the two light signals are simultaneously externally injected into the cavity of a slave laser. We show that dual injection-locking leads to formation of new stationary points, i.e., to optical tristability of the slave laser. We analyze the range of injection power and frequency detuning for which this tristability occurs.

Vibrational higher-order resonances in an overdamped bistable system with biharmonic excitation.

Experimental evidence of vibrational higher-order resonances in a bistable vertical-cavity surface-emitting laser driven by two harmonic signals with very different frequencies is reported. The phenomenon shows up in a parameter space (the dc current, the amplitude of the high-frequency signal) as well-defined structures with multiple local maxima at higher harmonics of the low-frequency signal. Such structures appear due to a strong suppression of higher harmonics for certain values of the high-frequency amplitude and the dc current. Complexity of the structures and the total number of the local maxima depend on the harmonic order k. The behavior of nonlinear distortion factor is also studied. The experimental results are in a good agreement with the numerical results which were obtained in the model of the bistable overdamped oscillator with biharmonic excitation.

Comparison of switching times in optically bistable injection-locked semiconductor lasers

In this paper, on the basis of the derived analytical model, we analyze switching between stable states of the optically bistable injection-locked semiconductor Fabry–Perot laser. We investigate three different mechanisms of switching: injection power, frequency detuning and bias current variation, and compare switching times obtained for these switching mechanisms.

Bit Error Rate Measurements of All-Optical Flip-Flop Operations of a 1.55-μm Polarization Bistable VCSEL

We measured the bit error rate (BER) of all-optical flip-flop operations using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL). Polarization bistable flip-flop operations were obtained by injecting optical set and reset pulses with independently adjusted wavelengths toward the two VCSEL lasing modes, whose polarizations are orthogonal to each other. Acquired waveforms showed clearly opened eyes, and BERs less than 1 × 10−9 were measured up to 1 Gb/s although the optical set and reset pulse power was much lower than the VCSEL output power. We also demonstrated all-optical flip-flop operations by injecting optical set and reset pulses with the same wavelength, which were generated by a single laser diode. In this case, we obtained polarization bistable flip-flop operations up to 500 Mb/s with almost the same BER values that were obtained when the wavelengths of the set and reset pulses were adjusted independently. The optical input pulse powers were higher than for the case using input pulses with two different wavelengths, but still lower than the VCSEL output power.

Front Pinning and Localized States Analogues in Long-Delayed Bistable Systems

Localized structures have been observed in many spatially extended systems of either biological, chemical, or physical nature. Here, we study experimentally front pinning and dissipative localized structures in a delayed optical system based on a bistable semiconductor laser with optoelectronic feedback. We observe that many of the concepts known to apply to spatially localized structures also apply in this context, with specificities related to the lack of reversibility symmetry. Numerical simulations based on purely prototypical modeling reproduce very well the experimental findings, which indicates that the results do not depend on the specific physical system under consideration, but are, on the contrary, very generic features of time delayed systems.

Nucleation in bistable dynamical systems with long delay

In an asymmetric bistable dynamical system with delayed feedback, one of the stable states is usually "stronger" than the other one: The system relaxes to it not only from close initial conditions, but also from oscillatory initial configurations which contain epochs of stay near both attractors. However, if the initial nucleus of the stronger phase is shorter than a certain critical value, it shrinks, and the weaker state is established instead. We observe this effect in a paradigmatic model and in an experiment based on a bistable semiconductor laser and characterize it in terms of scaling laws governing its asymptotic properties.

All-Optical Directional Switching in Bistable Semiconductor-Ring Lasers

We investigate the operation of directionally bistable semiconductor-ring lasers as all-optical flip-flops. We demonstrate fast switching between the two lasing directions by injection of optical pulses acting as set and reset control signals with switching times as fast as 20 ps, delay times as short as 60 ps, and switching energy of 150 fJ.

Spatial Gain Overlap-Controlled Hysteresis in Multimode Interference-Based Bistable Laser Diodes

Multimode interferometer bistable laser diodes (MMI-BLDs) appeared recently as a potential candidate for implementing all-optical flip-flops. It was reported that in such MMI-BLDs, cross-gain saturation effect and saturable absorber together can control the hysteresis window characteristics; however, there was no clear description on how to analyze and estimate the effect of cross-gain saturation on hysteresis characteristics. In this paper, we present a model that can explain the experimental results reported for MMI-BLDs and predict the hysteresis behavior for such devices. The proposed simulation model can work for both symmetric and asymmetric configurations of MMI-BLDs as it considers the spatial gain overlap between the co-propagating modes inside the MMI-BLDs. This spatial overlap accounts for the discrepancies in hysteresis characteristics, i.e., hysteresis width and threshold currents, between both configurations. With the proposed model, as the asymmetric configuration provides variable spatial cross-gain overlap between the fundamental and first-order modes depending on device dimensions, hysteresis window characteristics vary accordingly. To confirm the spatial gain overlap role, we present the simulation results obtained by the proposed model for several asymmetric MMI-BLDs of different spatial gain overlap with fixed saturable absorber effect. The results obtained show that the higher the spatial gain overlap is, the lower the threshold currents of the hysteresis window are for the same hysteresis window width, which is determined mainly by the saturable absorption effect. The obtained simulation results match with the experimental results reported for the asymmetric MMI-BLD configuration.

Multivalued Stability Map of an Injection-Locked Semiconductor Laser

We present a novel and detailed analysis of the locking range and the stability map for side-mode injection-locked in-plane semiconductor multimode lasers. By including the usually neglected unlocked modes in our model, we predict a multivalued locking range and stability map for this type of lasers. We also explain and relate, the previously noticed slave laser bistability phenomenon with the multivalued character of the locking range and stability map, and experimentally validate our findings. Moreover, we find that the unstable operating region, commonly found in literature by stability analysis of the injection-locked mode alone, is actually much smaller.

All-Optical Header Recognition and Packet Switching Using Polarization Bistable VCSEL

We demonstrate an optical header processing system using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL) that operates as an all-optical AND gate and a holding element. Depending on the state of 1 bit in the 4-bit header, the VCSEL held one of two-orthogonal polarization states. The light of the VCSEL output through a polarizer switched and held the output state of an optical switch. This enables the payload to be sent to the destination determined by the header. The characteristics of the polarization bistable VCSEL enable low-power consumption and lower latency operation. The system also has the potential to process multibit optical headers involved in ultrafast optical packets.

Enhancement of response of a bistable VCSEL to modulated orthogonal optical feedback by vibrational resonance

It is experimentally demonstrated that the response of a bistable vertical-cavity surface-emitting laser at a selected polarization to the effect of the modulated optical feedback at the orthogonal polarization can be considerably enhanced by the additional periodic current modulation via vibrational resonance. It shows up as a nonmonotonic dependence of the response at the frequency of the modulated optical feedback as a function of the amplitude of the current modulation. In such conditions the laser response can be amplified more than 80 times for a weak optical feedback. At the optimal amplitude of the current modulation a complete synchronization of optical switchings between polarization states with modulated optical feedback is observed. The effect of asymmetry of a bistable quasi-potential is also experimentally demonstrated.

Bistable Micro-Ring Lasers With Compact Footprint and High Output Efficiency

Micro-ring lasers, with a minimum bend radius of 10 $\mu{\rm m}$ , are presented that exhibit directional bistability and output powers up to 1.5 mW. In order to achieve devices with low threshold currents, mW output powers and directional bistability, the confilicting design constraints on the device are highlighted. Optimized adiabatic waveguide bends, bi-level evanescent couplers, and deeply etched ridge waveguide designs are presented in order to meet the stringent device design requirements. The optimized micro-ring laser devices exhibit directional extinction ratios of 25 dB and side-mode suppression ratios ${>}{\rm 25}~{\rm dB}$ .

Frequency, clockwise to counter clockwise and even to odd lasing mode switching based on optical feedback mechanism

Analyzing modes in a micro-ring waveguide coupled with a feedback resonator reveals that with appropriate parameters, the quality factors of two neighboring ring modes vary periodically and considerably. The effect occurs upon changing the length of the feedback waveguide, whereas the two resonant frequencies remain almost invariant with length change. On the basis of this property, four methods are theoretically demonstrated to realize laser switching: two for frequency switching, one for clockwise to counterclockwise mode switching, and one for even to odd mode switching. The analysis provides methods for bistable lasing and optical logic device design.

All-Optical Flip-Flop Operation at 1-mA Bias Current in Polarization Bistable Vertical-Cavity Surface-Emitting Lasers With an Oxide Confinement Structure

Polarization bistable vertical-cavity surface-emitting lasers (VCSELs) with an oxidation confinement structure were fabricated for low power consumption operation. Threshold current of the VCSEL was reduced to 0.22 mA. The VCSEL operated in a single longitudinal and transverse mode and in one of two orthogonal linear polarization modes. All-optical flip-flop operation based on polarization bistability was achieved at a record low bias current of 1.15 mA, which is about one tenth of our previously developed polarization bistable VCSEL without a current confinement structure.

All-optical flip-flop operation based on asymmetric active-multimode interferometer bi-stable laser diodes

We demonstrate fast and low energy all optical flip-flop devices based on asymmetric active-multimode interferometer using high-mesa waveguide structure. The implemented devices showed high speed all-optical flip-flop operation with 25 ps long pulses. The rising and falling times of the output signal were 121 ps and 25 ps, respectively. The required set and reset pulse energies were only 7.1 fJ and 3.4 fJ, respectively.