Phase-locked Laser Arrays Research Articles

Phase-locked high-brightness interband cascade laser array with multimode interferometer couplers.

We report on the design, fabrication, and characterization of an interband cascade laser (ICL) array incorporating multimode interference (MMI) couplers for phase-locking emitters. The ICL array, emitting at 3-4 µm, was developed to address the challenges of heat dissipation and in-phase operation in mid-infrared lasers. The array features a 7.5-µm-wide ridge design for fundamental transverse mode propagation and employs MMI structures to realize an in-phase operation of multiple emitters. The far-field patterns, characterized by periodic and symmetrical interference fringes, confirm the coherent operation of the array and the efficacy of MMI couplers in achieving phase-locking. The single-ridge side of the array exhibits a single-lobe far-field profile, with higher-order transverse modes effectively suppressed, showcasing a nearly diffraction-limited beam quality (M2 ≈ 1.31) at high output powers (390 mW from the 1 × 4 array). The robust performance and scalable design of the ICL array, validated by experimental results and theoretical simulations, indicate its potential for high-power mid-infrared applications and as an optical phased array.

Directly Emitting a High-Power Phase-Locked Laser Array by an Internal Phase Sensing System

Directly Emitting a High-Power Phase-Locked Laser Array by an Internal Phase Sensing System

Atmospheric Turbulence Effects on the Performance of Orbital Angular Momentum Multiplexed Free-Space Optical Links Using Coherent Beam Combining

Atmospheric effects including absorption and scattering, and turbulence could introduce signal power loss and severe mode crosstalk for the orbital angular momentum (OAM)-based free-space optical communication (FSOC). Therefore, it is of great significance to simultaneously increase signal power and mitigate mode crosstalk. In this paper, for the OAM beam from a coherent laser array with a discrete vortex (CLA-DV) based on coherent beam combining, we investigate its propagation characteristics by employing theoretical derivation and the random phase screens simulation in atmospheric propagation, respectively. The probability density and OAM spectrum are given and compared for CLA-DV and Gaussian vortex beam. The results demonstrate that the Gaussian vortex beam exhibits smaller mode crosstalk under weak atmospheric turbulence conditions, while CLA-DV shows a good performance on crosstalk mitigation for strong atmospheric turbulence conditions in long-distance links. Furthermore, with a specially designed radial phase-locked Gaussian laser array composed of two orthogonal polarized coherent laser arrays carrying different OAM states, a scheme of optical communication system possessing simultaneously polarization-division multiplexing and OAM multiplexing is proposed. The normalized energy weight matrices of all 16 non-zeroth-order OAM modes are numerically calculated. To verify the feasibility of the proposed scheme, the performance of an eight-bit grayscale Lena image facing various atmosphere turbulences is evaluated. The quality of transmitted images becomes worse with the turbulence strength and transmission distance increase, which is confirmed by the trend of average optical signal error rates. This work will provide theoretical insight for improving the performance of OAM-based FSOC under scattering conditions.

Generation of perfect vectorial vortex beams by employing coherent beam combining.

Based on coherent beam combining, we propose a method for generating the perfect vectorial vortex beams (VVBs) with a specially designed radial phase-locked Gaussian laser array, which is composed of two discrete vortex arrays with right-handed (RH) and left-handed (LH) circularly polarized states and in turn adjacent to each other. The simulation results demonstrate that the VVBs with correct polarization order and topological Pancharatnam charge are successfully generated. The diameter and thickness of generated VVBs independent of the polarization orders and topological Pancharatnam charges further prove that the generated VVBs are perfect. Propagating in free space, the generated perfect VVBs can be stable for a certain distance, even with half-integer orbital angular momentum. In addition, constant phases φ0 between the RH and LH circularly polarized laser arrays has no effect on polarization order and topological Pancharatnam charge but makes polarization orientation to rotate φ0/2. Moreover, perfect VVBs with elliptically polarized states can be flexibly generated only by adjusting the intensity ratio between the RH and LH circularly polarized laser array, and such perfect VVBs are also stable on beam propagation. The proposed method could provide a valuable guidance for high power perfect VVBs in future applications.

Phase-locked terahertz quantum cascade laser array integrated with a Talbot cavity.

Increasing the power of a quantum cascade laser by widening laser ridges will lead to the degradation of the beam quality because of the operation of high-order transverse modes. We report on a phase-locked array scheme of terahertz quantum cascade laser (THz QCL) utilizing Talbot effect. By adjusting the absorbing boundary width of each ridge in the array, stable operation of the fundamental supermode is realized. A five-element array shows 4 times power amplification than that of a single ridge device. Due to the large power amplification efficiency, stable mode selection, and simple fabricating process, the phase-locked array scheme is very promising to further improve the performance of THz QCL.

Effects of High-${\rm{\beta }}$ on Phase-Locking Stability and Tunability in Laterally Coupled Lasers

Phase-locked laser arrays have been extensively investigated in terms of their stability and nonlinear dynamics. Specifically, enhancing the phase-locking stability allows laser arrays to generate high-power and steerable coherent optical beams for a plethora of applications, including remote sensing and optical communications. Compared to other coupling architectures, laterally coupled lasers are especially desirable since they allow for denser integration and simpler fabrication process. Here, we present the theoretical effects of varying the spontaneous emission factor <inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula>, an important parameter for micro- and nanoscale lasers, on the stability conditions of phase-locking for two laterally coupled semiconductor lasers. Through bifurcation analyses, we observe that increasing <inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> contributes to the expanding of the in-phase stability region under all scenarios considered, including complex coupling coefficients, varying pump rates, and frequency detuning. Moreover, the effect is more pronounced for <inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> approaching 1, thus underlining the significant advantages of implementing nanolasers with intrinsically high <inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> in phase-locked laser arrays for high-power generation. We also show that the steady-state phase differences can be widely tuned &#x2013; up to <inline-formula><tex-math notation="LaTeX">$\pi $</tex-math></inline-formula> radians &#x2013; by asymmetrically pumping high-<inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> coupled lasers. This demonstrates the potential of high-<inline-formula><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> nanolasers in building next-generation optical phased arrays requiring wide scanning angles with ultra-high resolution.

Iteration-free, simultaneous correction of piston and tilt distortions in large-scale coherent beam combination systems.

Coherent beam combination (CBC) holds promise for scaling the output power of the laser system while maintaining good beam quality. Owing to the thermal effect and mechanics instability, piston and tilt distortions always exist and affect the performance of the combined beam. To ensure the constructive interference in the far-field, dynamic correction of the distortions is highly required. Here, we propose an approach for the simultaneous correction of piston and tilt distortions in CBC systems. Based on the position and interval information of the near-field interference fringes, the theoretical expressions for the relative piston phase and tilt errors of each array element are derived, indicating that dynamic distortions in CBC systems can be directly calculated and then corrected by employing phase control servos. To demonstrate the feasibility of the proposed approach, Monte-Carlo Simulations have been carried out for different perturbative environments. Our results indicate that both piston phase and tilt errors can be calculated and compensated accurately (λ/25 and 0.11μrad) by the proposed approach even in 169 beamlets, which also has high tolerance for defocus errors. This work could provide valuable reference on the practical implementation of high-power, phase-locked fiber laser array systems.

Higher-dimensional supersymmetric microlaser arrays.

The nonlinear scaling of complexity with the increased number of components in integrated photonics is a major obstacle impeding large-scale, phase-locked laser arrays. Here, we develop a higher-dimensional supersymmetry formalism for precise mode control and nonlinear power scaling. Our supersymmetric microlaser arrays feature phase-locked coherence and synchronization of all of the evanescently coupled microring lasers-collectively oscillating in the fundamental transverse supermode-which enables high-radiance, small-divergence, and single-frequency laser emission with a two-orders-of-magnitude enhancement in energy density. We also demonstrate the feasibility of structuring high-radiance vortex laser beams, which enhance the laser performance by taking full advantage of spatial degrees of freedom of light. Our approach provides a route for designing large-scale integrated photonic systems in both classical and quantum regimes.

Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode

Plasmonic lasers suffer from low output power and divergent beams due to their subwavelength metallic cavities. We developed a phase-locking scheme for such lasers to significantly enhance their radiative efficiency and beam quality. An array of metallic microcavities is longitudinally coupled through traveling plasmon waves, which leads to radiation in a single spectral mode and a diffraction limited single-lobed beam in the surface normal direction. We implemented our scheme for terahertz plasmonic quantum-cascade lasers (QCLs) and measured peak output power in excess of 2 W for a single-mode 3.3 T H z QCL radiating in a narrow single-lobed beam, when operated at 58 K in a compact Stirling cooler. We thereby demonstrated an order of magnitude increase in power and thirty-times higher average intensity for monolithic single-mode terahertz QCLs compared to prior work. The number of photons radiated from the cavity outnumber those absorbed within its claddings and semiconductor medium, which constitutes &gt; --> 50 % radiative efficiency and is significantly greater than that achieved for previous single-mode mid-infrared or terahertz QCLs.

Semiconductor Laser Quasi-Array with Phase-Locked Single-Mode Emitting Channels

The construction of a semiconductor laser with a stripe waveguide is proposed, the geometry of which can make it possible to obtain radiation similar to that of a phase-locked laser array. The requirements for the parameters of the laser quasi-array and the technological feasibility of the proposed approach are discussed.

High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm.

We report a room-temperature eight-element phase-locked quantum cascade laser array emitting at 8 µm with a high continuous-wave power of 8.2 W and wall plug efficiency of 9.5%. The laser array operates primarily via the in-phase supermode and has single-mode emission with a side-mode suppression ratio of ~20 dB. The quantum cascade laser active region is based on a high differential gain (8.7 cm/kA) and low voltage defect (90 meV) design. A record high wall plug efficiency of 20.4% is achieved from a low loss buried ridge type single-element Fabry-Perot laser operating in pulsed mode at 20 °C.

Полупроводниковая лазерная квазирешетка с фазированными одномодовыми каналами излучения

We have proposed a semiconductor laser design with a stripe waveguide, which geometry can make it possible to obtain radiation similar to that of a phase-locked laser array. The requirements for the laser quasi-array parameters and the technological feasibility of the proposed approach are discussed.

Spectrally resolved modal characteristics of leaky-wave-coupled quantum cascade phase-locked laser arrays

The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.

Coherent emission from integrated Talbot-cavity quantum cascade lasers.

We report experimental realization of phase-locked quantum cascade laser (QCL) array using a monolithically integrated Talbot cavity. An array with six laser elements at a wavelength of ~4.8 μm shows a maximum peak power of ~4 W which is more than 5 times higher than that of a single ridge laser element and a slope efficiency of 1 W/A at room temperature. Operation of in-phase coherent supermode has been achieved over the whole dynamic range of the Talbot-cavity QCL. The structure was analysed using a straightforward theoretical model, showing quantitatively good agreement with the experimental results. The reduced thermal resistance makes the structure an attractive approach to achieve high beam quality continuous wave QCLs.

Phase-locked laser arrays through global antenna mutual coupling

Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold. Here, we show a conceptually novel phase-locking mechanism based on ‘antenna mutual coupling’ in which laser elements interact through far-field radiations with definite phase relations. This allows a long-range global coupling among the array elements to achieve a robust phase locking in two-dimensional laser arrays. The scheme is ideal for lasers with a deep subwavelength confined cavity, such as nanolasers, whose divergent beam patterns could be used to achieve a strong coupling among the elements in the array. We demonstrated experimentally such a scheme based on subwavelength short-cavity surface-emitting lasers at terahertz frequencies. More than 37 laser elements that span over ∼8 λo were phase locked to each other, and delivered up to 6.5 mW (in a pulsed operation) single-mode radiation at ∼3 THz, with a maximum 450 mW A–1 slope efficiency and a near-diffraction-limited beam divergence. Two-dimensional arrays of short-cavity surface-emitting THz quantum cascade lasers are phase-locked to each other via mutual coupling. A directive beam on the order of 10° divergence and a maximum slope efficiency of 450 mW A−1 is achieved.

High-power phase-locked quantum cascade laser array emitting at λ ∼ 4.6 μm

A phase-locked quantum cascade laser (QCL) array consisting of one hundred elements that were integrated in parallel was achieved at λ ∼ 4.6 μm. The proposed Fraunhofer’s multiple slits diffraction model predicted and explained the far-field pattern of the phase-locked laser array. A single-lobed far-field pattern, attributed to the emission of an in-phase-like supermode, is obtained near the threshold (Ith). Even at 1.5 Ith, greater than 73.3% of the laser output power is concentrated in a low-divergence beam with an optical power of up to 40 W.

Design of Resonant Leaky-Wave Coupled Phase-Locked Arrays of Mid-IR Quantum Cascade Lasers

A theoretical analysis of the optical and thermal characteristics of planarized phase-locked quantum cascade laser (QCL) arrays emitting at 4.8 μm was performed. Large aperture seven-element designs are established with a reasonably large process window for achieving single in-phase-mode operation. Full-wave optical simulations of these high-index contrast photonic crystal devices were coupled with 2-D heat flow analysis to investigate the influence of thermal lensing on modal behavior. Self-focusing under thermally induced index variations across the array are expected to ultimately limit the single-mode operational range. Anti-resonant reflective-optical waveguide terminations for three-element arrays can decrease the in-phase mode threshold-current density, improve discrimination against array modes composed of coupled first-order element modes, and improve the in-phase-mode near-field uniformity under CW operation.

Combining efficiency degraded by gain in passively phase-locked laser arrays: contribution of the spectral filtering.

Passive phase-locking of laser arrays is usually less efficient when the operating point is far above the laser threshold. We investigated the contribution due to the spectral filtering effect induced by the coupled cavities. Experiments carried out on a basic dual-arm laser as well as modeling show that some laser combining losses arise from the spectral filtering whose transmission windows have a non-uniform profile. A simple analytical model of the laser confirms this interpretation.

Broad area diode laser with on-chip transverse Bragg grating stabilized in an off-axis external cavity.

The emission characteristics of a novel, specially designed broad area diode laser (BAL) with on-chip transversal Bragg resonance (TBR) grating in lateral direction were investigated in an off-axis external cavity setup. The internal TBR grating defines a low loss transversal mode at a specific angle of incidence and a certain wavelength. By providing feedback at this specific angle with an external mirror, it is possible to select this low loss transverse mode and stabilize the BAL. Near diffraction limited emission with an almost single lobed far field pattern could be realized, in contrast to the double lobed far field pattern of similar setups using standard BALs or phase-locked diode laser arrays. Furthermore, we could achieve a narrow bandwidth emission with a simplified setup without external frequency selective elements.

The concept and the realization of the high-power laser diodes with multi-stripe-gain distribution

The slow axis far-field widening (far-field blooming) is one of the factors, which decrease the quality of the emitted beam of high-power laser diodes. This effect is connected with rising number of maintained lateral optical modes in the laser waveguide. In this paper the design of the gain distribution by the steering of current flow is presented as a way to restrict the far-field blooming. The numerical and experimental results are presented. The almost constant slow-axis width of the optical field distribution in the far-field is obtained up to four times above the threshold current. Full Text: PDF References J. Piprek, "Inverse Thermal Lens Effects on the Far-Field Blooming of Broad Area Laser Diodes", IEEE Photon. Technol. Lett., 25, 958 (2013) CrossRef J. Piprek, "Self-consistent far-field blooming analysis for high-power Fabry-Perot laser diodes", Proc. SPIE 8619, 861910 (2013) CrossRef P. Crump, S. Boeldicke, C. M. Schultz, H. Ekhteraei, H. Wenzel and G. Erbert, "Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers", Semicond. Sci. Technol., 27, 045001 (2012) CrossRef Y. Twu, K. L. Chen, A. Dienes, S. Wang and J. R. Whinnery, "High-performance index-guided phase-locked semiconductor laser arrays", Electron. Lett., 21, 324 (1985) CrossRef M. V. Maximov, Y. M. Shernyakov, I. I. Novikov, L. Ya. Karachinsky, N. Yu. Gordeev, U. Ben-Ami, D. Bortman-Abriv, A. Sharon, V. A. Shchukin, N. N. Ledentsov, T. Kettler, K. Posilovic, D. Bimberg, "High-Power Low-Beam Divergence Edge-Emitting Semiconductor Lasers with 1- and 2-D Photonic Bandgap Crystal Waveguide", IEEE J. Sel. Topics Quantum Electron., 14, 1113 (2008) CrossRef G. Sobczak, E. Dąbrowska, M. Teodorczyk, J. Kalbarczyk and A. Maląg, "Characterization of the optical beam emitted by high-power phase-locked arrays of diode lasers (P = 1 W CW)", Proc. SPIE 8902, 890217 (2013) CrossRef G. Sobczak, A. Maląg, "Optimization of phase-locked arrays geometry for high brightness laser systems", Opto-Electronics Review, 20, 134 (2012) CrossRef G. Sobczak, E. Dąbrowska, M. Teodorczyk, J. Kalbarczyk and A. Maląg, "Improving the beam quality of high-power laser diodes by introducing lateral periodicity into waveguides", Proc. SPIE 8702, 87020B (2013) CrossRef M. Spreemann, B. Eppich, F. Schnieder, H. Wenzel and G. Erbert, "Modal Behavior, Spatial Coherence, and Beam Quality of a High-Power Gain-Guided Laser Array", IEEE J. Quantum Electron., 46, 1619 (2010) CrossRef A. Maląg, E. Dąbrowska, M. Teodorczyk, G. Sobczak, A. Kozłowska and J. Kalbarczyk, "Asymmetric Heterostructure With Reduced Distance From Active Region to Heatsink for 810-nm Range High-Power Laser Diodes", IEEE J. Quantum Electron., 48, 465 (2012) CrossRef A. Kozłowska, A. Maląg, "Far-field emission characteristics of high-power laser diodes", Proc. SPIE 5120, 183 (2003) CrossRef W. Sun, R. Pathak, G. Campbell, H. Eppich, J. H. Jacob, A. Chin and J. Fryer, "Higher brightness laser diodes with smaller slow axis divergence", Proc. SPIE 8605, 86050D-1 (2013) CrossRef