Front Facet Reflectivity Research Articles

Numerical investigation of steering synchronization dynamics of laterally coupled laser array with monolithically-integrated external cold cavity.

This study numerically explores the synchronization dynamics of a one-dimension edge-emitting laser array monolithically-integrated with an external cold cavity comprehensively, aiming to achieve an in-phase mode optical field. By employing the optical feedback rate equation, the impact of mutual feedback coefficient and coupling coefficient on the synchronization process are investigated thoroughly. The proposed external cold cavity, designed according to the Talbot effect, could significantly diminish the reflectivity of front facet through separated electrode structure, therefore facilitating the phase-locking process. Consequently, the study uncovers the effective regime for establishing in-phase mode operation. Additionally, the numerical analysis also reveals the vivid synchronization dynamic from a chaotic state to partially-phase-locked, then completely-phase-locked, and ultimately periodic oscillation. Furthermore, the impact of practical fabrication tolerances on the synchronization process are explored as well. Based on the simulation results, our work could offer valuable insights for steering the on-chip optical field and developing novel laser arrays with high beam quality.

Experimental and theoretical studies into longitudinal spatial hole burning as a power limit in high-power diode lasers at 975 nm

Spatial-hole-burning as a limit to the continuous-wave (CW) output power of GaAs-based diode lasers is experimentally studied. For 90 μm stripe lasers with 6 mm resonator length and 0.8% front facet reflectivity, spontaneous emission (SE) intensity data show that the carrier density in the device center rises rapidly at the rear facet with bias and falls at the front, consistent with simulation. At the front, the carrier density at the edge of the laser stripe also rises rapidly with bias (lateral carrier accumulation, LCA), consistent with previous observations of increased local current flow. Devices with 20% front facet reflectivity for a flat longitudinal optical field profile show smaller variation in the local carrier density. Weak variation is seen in the carrier density outside the stripe; hence, current spreading is not a power limit. SE wavelength data show higher temperatures at the front with a twofold higher increase in temperature for 0.8% than for 20% front facet. The increased front temperature likely triggers lateral spatial-hole-burning and LCA in this region, limiting power. Finally, pulsed threshold current is more strongly temperature dependent for devices with 0.8% than 20% front facets, attributed to the higher rear facet carrier density. The temperature dependence of slope in pulsed is comparable for both devices at low bias but is more rapid for 0.8% at 20 A, likely due to non-clamping at the back. The temperature dependence of slope for CW is strong with 0.8% facets, likely due to the high temperature and LCA at the front but reduced for 20% facets.

Compact, Watt-class 785 nm dual-wavelength master oscillator power amplifiers

785 nm micro-integrated, dual-wavelength master oscillator power amplifiers with a footprint of 5 mm × 25 mm are presented. They are based on Y-branch distributed Bragg reflector ridge waveguide diode lasers and anti-reflection coated tapered amplifiers. In order to reduce the impact of potential optical feedback, devices with master oscillator front facet reflectivities of 5% and 30% as well as with an integrated miniaturized optical isolator have been realized. A comparison up to 1 W shows narrowband dual wavelength laser emission with a spectral distance of 0.6 nm (10 cm−1) and individual spectral widths <20 pm. As expected, a higher front facet reflectivity leads to a significant reduction of feedback related mode hops. Longitudinal modes corresponding to the master oscillator resonator length remain within spectral windows <0.15 nm (3 cm−1), suitable for applications such as Raman spectroscopy and especially shifted excitation Raman difference spectroscopy. Integrating a compact 30 dB optical isolator completely eliminates the observed optical feedback effects. Lateral beam propagation ratios of 1.2 (1/e2) enable easy beam shaping and fiber coupling. Outside of the experimental comparison, the developed MOPAs provide up to 2.7 W of optical output power available for applications.

Misestimate of the performance in VCSEL-based reservoir computing systems with optical information injection by high surface reflectivity.

In reservoir computing (RC) systems based on semiconductor lasers (SLs), the information that must be processed usually enters the reservoir through optical injection. Part of the injection information directly reflected by the front facet of the SLs is inevitably hybridized into the output of the SLs and contributes to the state of virtual nodes. For an RC system based on vertical-cavity surface-emitting lasers (VCSELs), the proportion of the reflected information coupled to the laser output is relatively huge due to the high surface reflectivity. Thus the influence of the directly reflected information will be much more obvious. Using a Santa Fe chaotic time series prediction task and waveform recognition task, we theoretically investigate the influence of high front facet reflectivity on the evaluation of the performance of a VCSEL-based RC system with optical information injection. The simulation results demonstrate that, after taking the directly reflected information into account, a lower error rate is obtained for each benchmark task. The physical mechanism to misestimate the RC performance has been studied through memory correlation and a statistical histogram of virtual node states.

THE INFLUENCE OF A RESIDUAL REFLECTIVITY AT THE FRONT FACET OF A MULTISECTION MASTER-OSCILLATOR POWER-AMPLIFIER

This paper reports on the theoretical end experimental investigation of the output characteristics of a multisection master-oscillator power-amplifier (MOPA). It consists of a master oscillator (MO) composed by a gain section surrounded by two DBR sections monolithically integrated with a power amplifier (PA). We use a traveling wave equation model to calculate the optical output power in dependence on the current injected into the PA. The experimental results can be well explained with our theoretical analysis. For a finite front facet reflectivity of the PA the system is acting as a compound cavity.

Improvement on Direct Modulation Responses and Stability by Partially Corrugated Gratings Based DFB Lasers With Passive Feedback

DFB lasers with ultrashort cavities or integrated DFB lasers with passive waveguide reflectors are frequently proposed to realize directly modulated lasers (DMLs) for carrying ultrahigh data rate signals. The former suffers from poor heatsink and laser cleavage yield, while the single-mode stability of the latter scheme is seldom addressed. The mode selection and device performance of a DFB laser is well known to be very sensitive to the facet reflection and external optical feedback. The DFB lasers with partial corrugated gratings and passive feedback (PCG-PFL) are proposed here to overcome the challenging issues for the aforementioned two schemes. With PCG structure, the DFB lasers can maintain high single-mode yield (SMY) even with a high-reflection-coating on the rear facet and strong reflection from the integrated passive section. This provides the robustness in applying the integrated passive reflector to reshape the modulation response or to enhance the 3-dB bandwidth by using the photon-photon resonance (PPR) effect. By designing the PCG-PFL to have 150-μm long laser section, 50-μm long passive section, and 30% front-facet reflectivity, it can have about 86% of SMY, reduced resonant peak in the intensity modulation response, and reduced waveform overshoot and undershoot for transmitting 56-Gbaud/s PAM-4 signals. By using a longer passive reflector, enhanced bandwidth can be achieved by the PPR effect.

Non-uniform longitudinal current density induced power saturation in GaAs-based high power diode lasers

The output power of modern 975 nm GaAs-based broad area diode lasers is limited by increasing carrier and photon losses at high bias. We use experiment and one-dimensional calculations on these devices to reveal that higher current densities (and hence higher local recombination rates and higher losses) arise near the front facet due to spatial hole burning and that the non-uniformity is strongly affected by laser geometry, which is more severe for longer resonators and less severe for higher front facet reflectivity. Specifically, we use devices with a segmented p-contact to directly measure the current distribution along the resonator and compare this with laser simulation. Devices with a 6 mm resonator show 29% more current at the front than back, twice as large as the 15% current non-uniformity in devices with a 3 mm resonator. In contrast, increased front facet reflectivity (20% rather than 0.8%) is shown to almost halve the current non-uniformity from 29% to 18% in devices with a 6 mm resonator and reduces power saturation. Although the magnitude of current non-uniformity in experiment and theory is broadly consistent, in experiment, an additional divergence is seen in current flow (and hence recombination rate) near the facets, and earlier power saturation occurs. We discuss the possible saturation mechanisms that are not included in the simulation.

Data erasure in Fabry–Perot Diode Lasers: effects of facet reflectivity

ABSTRACTIn this study, the effects of facet reflectivity on the data erasure ability of injection-locked Fabry Perot Laser Diode (FPLD), based on a remodulation method in wavelength division multiplexed passive optical network (WDM PON), were investigated. For this purpose, the feasible front facet reflectivity of FPLD was determined as between 0.2% and 7%, depending on the specified injected power interval. Firstly, the system carried out analysis without applying data at the user end to comprehensively demonstrate how much downstream data was completely erased in FPLD. Then, 2.5 Gb/s of non-return-to-zero (NRZ) data was applied and the results compared with the erased downstream signal without data. The results show that upstream transmission improved with an increase in facet reflectivity between 0.2% and 7%.

Investigation of red‐emitting distributed Bragg reflector lasers by means of numerical simulations

The authors report theoretical and experimental results on the properties of distributed Bragg reflector semiconductor lasers. Using the traveling wave equation model, they show that a proper choice of coupling coefficient and front facet reflectivity allows an optimisation of the laser operation, such that for wide range of injected current into the active region the laser emits a temporally stable output power. The numerical results are in a qualitative agreement with the measured characteristics.

Spectral Linewidth vs. Front Facet Reflectivity of 780 nm DFB Diode Lasers at High Optical Output Power

The influence of the front facet reflectivity on the spectral linewidth of high power DFB (distributed feedback) diode lasers emitting at 780 nm has been investigated theoretically and experimentally. Characterization of lasers at various front facet reflections showed substantial reduction of the linewidth. This behavior is in reasonable agreement with simulation results. A minimum linewidth of 8 kHz was achieved at an output power of 85 mW with the laser featuring a front facet reflectivity of 30%. The device with a front facet reflectivity of 5% reached the same linewidth value at an output power of 290 mW.

High-Efficiency Broad-Ridge Waveguide Lasers

Highly efficient single-spatial mode lasers with wattlevel output power in the 9xx-nm wavelength range are of great interest for a variety of applications. We experimentally investigate the influence of the mesa widths on the electrooptical properties of ridge-waveguide lasers, based on an extreme double asymmetric epitaxial structure. These properties are, for example, the conversion efficiency and the maximum power of single-lateral mode operation (kink power). At an output power of almost 1 W, single-lateral mode operation with the highest efficiency of 60% was achieved with a 10-μm ridge-width laser. The highest peak efficiency of 62% and the highest output power of 2.7 W with still 59% efficiency were yielded by a laser with a ridge width of 15 μm, at the expense of a slightly deteriorated beam quality due to the involvement of a higher order lateral mode. The kink power of the 10-μm-ridge laser can be further increased by decreasing the front facet reflectivity from 2.5% to 0.5%.

Optimisation of cavity parameters for lasers based on AlGaInAsP/InP solid solutions

We have studied the effect of laser cavity parameters on the light – current characteristics of lasers based on the AlGaInAs/GaInAsP/InP solid solution system that emit in the spectral range . It has been shown that optimisation of cavity parameters (chip length and front facet reflectivity) allows one to improve heat removal from the laser, without changing other laser characteristics. An increase in the maximum output optical power of the laser by has been demonstrated due to cavity design optimisation.

Ultrashort pulse generation by semiconductor mode-locked lasers at 760 nm.

We demonstrate the first semiconductor mode-locked lasers for ultrashort pulse generation at the 760 nm waveband. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, resulting in the generation of pulses at around 766 nm, with pulse durations down to ~4 ps, at pulse repetition rates of 19.4 GHz or 23.2 GHz (with different laser cavity lengths of 1.8 mm and 1.5 mm, respectively). The influence of the bias conditions on the mode-locking characteristics was investigated for these new lasers, revealing trends which can be ascribed to the interplay of dynamical processes in the saturable absorber and gain sections. It was also found that the front facet reflectivity played a key role in the stability of mode-locking and the occurrence of self-pulsations. These lasers hold significant promise as light sources for multi-photon biomedical imaging, as well as in other applications such as frequency conversion into the ultraviolet and radio-over-fibre communications.

Factors influencing brightness and beam quality of conventional and distributed Bragg reflector tapered laser diodes in absence of self‐heating

In this study, the authors examine some of the factors affecting the brightness and the beam quality of high-power tapered lasers. The large volume resonators required to achieve a high-power, high-brightness operation make the beam quality sensitive to carrier lensing and a multimode operation. These cause bleaching of the regions outside the ridge waveguide. The beam quality in the conventional and the distributed Bragg reflector tapered lasers is examined in the absence of the self-heating effects to investigate the effect of the carrier lensing effects. The influence of the front facet reflectivity and the taper angle on the beam quality is investigated. The beam quality was found to degrade with an increase in the front facet reflectivity and for the larger taper angles in the conventional tapered lasers, especially at low ridge waveguide currents. Finally, the performance of the conventional tapered lasers employing a beamspoiler was assessed. The beam quality was found to be comparable with that achieved in the DBR tapered lasers.

Modeling of Seeded Reflective Modulators for DWDM Systems

We propose a theoretical model of reflective modulators for a seeded dense wavelength-division multiplexing (DWDM) system based on amplified spontaneous emission light. A travelling-wave amplifier model is modified to take into account the finite front-facet reflectivity. The net gain and phase shift are updated at every round trip of the cavity to have complex gain for the injected field. With the proposed model, we calculate side-mode suppression ratio, gain, relative intensity noise suppression as a function of injection power, and wavelength detuning for antireflection-coated Fabry-Perot laser diodes. We also investigate the performance of the seeded DWDM system by using the model. The simulation results match well with the experimental results.

Investigation of Using Injection-Locked Fabry–Pérot Laser Diode With 10% Front-Facet Reflectivity for Short-Reach to Long-Reach Upstream PON Access

In this paper, we propose and investigate an injection-locked Fabry-Perot laser diode (FP-LD) with 10% front-facet reflectivity by using orthogonal frequency-division multiplexing-quadrature amplitude modulation (OFDM-QAM) in each optical network unit (ONU) for colorless operation in the carrier-distributed passive optical network (PON). Here, the 10-Gb/s 16-QAM OFDM upstream traffic can be generated via a 2.5-GHz-bandwidth injection-locked FP-LD for the short-reach (SR)-to-long-reach (LR) transmission. In this measurement, the CW injection power of -12 dBm for an injection-locked FP-LD is required to achieve 100-km single-mode fiber (SMF) transmission. Hence, error-free transmissions of 20-, 50-, 75-, and 100-km fiber lengths are achieved with the power penalties of 0.33, 0.86, 1.20, and 1.76 dB at the bit error rate (BER) of 3.8 × 10-3, respectively, for upstream traffic. Moreover, the relationship of CW injection power and signal-to-noise ratio (SNR) of each OFDM subcarrier in the injection-locked FP-LD has also been analyzed.

Performance of Long-Reach Passive Access Networks Using Injection-Locked Fabry–Perot Laser Diodes With Finite Front-Facet Reflectivities

In this study, we use the injection-locked Fabry-Perot laser diodes (FP-LDs) with front-facet reflectivities of 10% and 35% respectively, as colorless optical networking unit (ONU) in long-reach passive optical networks (LR-PONs). Here, the injection-locked FP-LD is directly modulated at 2.5 Gbit/s on-off keying (OOK) and 10 Gbit/s orthogonal-frequency-division-multiplexing (OFDM) modulations for upstream traffic transmission over 60 km of single-mode fiber (SMF). Different operating parameters for the injection-locked FP-LD to achieve higher data rate transmission are reported.

Capability Limitations for Slow Light Using Vertical-Cavity Surface-Emitting Laser Amplifier

An analytical expression for the delay-bandwidth product (DBP) of slow light in a vertical-cavity surface-emitting laser (VCSEL) is derived and experimentally confirmed. The expression and theoretical results show that the DBP has a maximum value, which is only dependent on the front facet reflectivity of the VCSEL. For a practical VCSEL with the front facet reflectivity exceeding 90%, the resulting maximum DBP is close to a fixed value of 0.64. The expression and the results are also verified by the measured DBP of a VCSEL with different bias currents.

Describing a Laser Diode Emulation Tool Using Single Emitter Simulation Results

This study describes and explores the use of a laser diode simulation tool at the single emitter level of operation and how they can be degraded. A test of the simulation tool is implemented to complement the by-emitter degradation analysis of high power laser diodes. The simulation tool is called <em>Speclase</em>, designed for the simulation of single emitters. Tests were performed using a 975 nm narrow-angle (<1º) tapered laser structure from Alcatel Thales III-V Lab with front and rear facet reflectivities of 3 and 90%, respectively. The tool worked for both the constant current and power modes of operation. Simulation results were obtained for both constant QW trap density, based on the maximum QW temperature and variable QW trap density generation due to local heating. Single emitter degradation results are obtained using the Arrhenius equation to compare the rate of degradation between the constant and variable QW trap densities.

Broadcast/multicast transmission of video signal in a WDM-PON using a low-cost injection-locked F-P LD with broadband ASE

A broadcast/multicast transmission method in a WDM-PON system using a low-cost Fabry-Perot laser diode with broadband ASE injection is proposed and experimentally demonstrated in this paper. Because of its low front-facet reflectivity (∼0.1%), the F-P LD exhibits a broadband lasing spectrum and reduced mode-partition noise. To demonstrate its performance, the bit error rate and packet loss rate were measured. The proposed optical source provides a cost-effective WDM-PON architecture for a broadcast/multicast service accommodating more than 32×622MBps. For the transmission of MPEG-2 TS packets, zero packet loss could be obtained at an optical power of -32dBm.