Massive-scale spatial multiplexing of multimode VCSELs with a 3D-printed photonic lantern

Domain: chemistry
Published: 2026-03-09
Editorial outcome: major_revision

Abstract
Incoherent beam combining is widely used in high-power laser systems due to its simplicity, stability, and scalability, as it avoids frequency, phase, and polarization locking. However, efficiently coupling light from large multimode VCSEL arrays into multimode fibers with matched modal capacity, while preserving brightness, remains challenging. This work presents a 3D-printed microscale photonic lantern (PL) for efficient incoherent combining of multimode sources, enabling direct multiplexing of many few-mode VCSELs into MMFs with matched modal capacity. Unlike conventional PLs designed for single-mode inputs, the proposed PL supports few-mode sources. We demonstrate PLs multiplexing 7, 19, and 37 six-mode VCSELs, fabricated directly on the laser apertures, achieving coupling losses as low as −0.6 dB (19–MM PL) and −0.8 dB (37-MM PL) into a 50 μm, NA = 0.22 step-index MMF. Comprehensive experiments demonstrate efficient power delivery, preserved brightness, and relaxed alignment, highlighting 3D-printed PLs as compact and scalable solutions for high-power laser systems and optical communications.

Introduction
Coherent and incoherent beam combining are techniques for improving laser system power delivery (or performance). Incoherent beam combining offers several advantages, as it eliminates the need for frequency, phase and polarization locking of individual lasers, allowing for scalable, compact, and reliable systems1. These systems are predominantly employed in free-space optics arrangements1–3 or fiber tapers4,5.

Two-dimensional (2D) VCSEL arrays are widely used as incoherent sources due to their wafer level fabrication ease. Coupling these independent sources into a multimode fiber (MMF) enables beam delivery for applications such as high-power fiber laser pumping and optical communications6,7. Typically, VCSEL sources are combined using a relay lens system that includes a micro-lens array (MLA) and a condenser lens to focus the light into the MMF (see Discussion). The MMF is generally a highly multimode fiber with a large core diameter (greater than 50 μm) having a high numerical aperture (NA > 0.22). These fibers support thousands of guided modes, enabling efficient light coupling.

However, beam brightness is a key factor for optical systems (see Discussion section). Brightness defined by the beam’s power, devide by its area and divergence8, is a physical property that cannot be increased beyond the source measure. Coupling light into a large-core high-NA multimode fiber increases the beam area, and the excitation of extensive modal content results in greater divergence, thereby diminishing the delivered brightness. A brightness-efficient approach involves coupling the energy into a fiber that supports the same number of spatial modes as the source, which corresponds to the maximum degrees of freedom of the system9.

Spatial multiplexers have wide-ranging applications in various light-related fields, such as high-capacity mode-division multiplexing (MDM) for fiber communication networks10–12, free-space optical communication13,14, coherent power combining15,16, adaptive optics17,18, and wavefront sensing19,20. Spatial mode multiplexers (and demultiplexers) convert individual, distinct sources (modes) into spatially overlapping and orthogonal modes, ideally with low losses and in compact form.

One prominent spatial mode multiplexer is the multi-plane light conversion (MPLC) multiplexer, which facilitates efficient spatial mode transformations through a series of discrete phase masks or refractive planes21–23. Incoherent and coherent laser beam combining has been demonstrated using MPLC24,25 yet for single mode source arrays. Another spatial multiplexer is the Photonic Lantern (PL), which consists of an adiabatic and continuous spatial transition from a multi-mode optical waveguide to a finite set of single-mode (SM) waveguides, converting between matching mode and waveguide counts26. Both MPLC and PL devices ideally convert from the multi-mode domain to the single-mode array domain and vice versa with minimal loss, making them key enablers of space division multiplexing (SDM)27. Photonic lanterns can be fabricated from single-mode fibers that merge into one28–30, through waveguide inscription in glass via direct laser writing31,32, often resulting in millimeter-scale devices, or via 3D nano-printing with polymer core/air cladding waveguides33–36, which enables the creation of micrometer scale PL devices using 3D nano-printing technology and high-index contrast waveguides (polymer vs. air). Moreover, 3D nano-printing technology allows fabrication on versatile surfaces and sources, making it an excellent platform for VCSEL integration37,38 and a wide range of microscale photonics systems.

Traditional PL and MPLC spatial multiplexers are typically designed to receive single-mode (SM) fibers or waveguides as inputs, with these designs being incompatible for beam combining from multiple multi-mode (MM) laser or VCSEL sources. In this manuscript, we introduce a novel 3D-printed micro-scale photonic lantern (PL) designed to multiplex multiple (7, 19, and 37) 6-mode beams into a multi-mode fiber (MMF) that supports the corresponding number of modes (i.e., 7 × 6 = 42, 19 × 6 = 114, and 37 × 6 = 222). We refer to this device as an N-MM PL, where N is the number of inputs. The manuscript presents the design, fabrication, and characterization of three scales of MM PL devices (7, 19, and 37 inputs). These devices were fabricated directly on a commercial multi-mode incoherent VCSEL array. The ability to 3D-print the PL device at high precision and with micrometer scale allows for efficient and compact integration, with direct coupling between VCSEL sources and the PL and finally with the collecting fiber. In addition to comprehensive characterization, including efficiency, modal content and beam quality assessment of the PL, we directly couple to a 50 μm core diameter, NA = 0.22 MMF, achieving coupling losses of −0.8 dB.

Results
Designing a multi-mode photonic lantern

The design of a PL supporting array count of MM sources, specifically, the design of a 7-MM PL that multiplexes seven 6-mode sources into a 42-mode waveguide, a 19-MM PL that combines nineteen 6-mode sources into a 114-mode waveguide, and a 37-MM PL that integrates thirty-seven 6-mode sources into a 222-mode waveguide is challenging due to scale and computational complexity. The incoherent MM VCSEL source used in this work (Vixar, model V0013239), lases at 940 nm has a source pitch of 44 μm and 10 μm mode field diameter for the fundamental mode. In this section we outline the methodology and the computational speed-ups we incorporated for their design. Figure 1a illustrates the targeted output fiber modes for these devices.

The primary objective for MM-PL devices is to maximize transmission power while ensuring power confinement within the designated mode sets with no requirments over mode mixing. This approach minimizes the excitation of higher-order modes, reducing the M2 value of the output beam. Consequently, brightness is preserved, and coupling efficiency into a multimode fiber (MMF) having the same mode count is improved. Achieving power confinement within the target modes (42, 114, or 222) requires an adiabatic transition between the inputs (VCSEL sources) and the multimode waveguide. An ideal adiabatic transition would eliminate losses during this process, allowing the PL device to be represented as a unitary matrix.

PL multiplexers can be classified as either mode-preserving or mode-mixing devices. In the mode-mixing case, injecting light into any input of the PL results in a linear superposition of the supported modes at the multimode output. Conversely, mode-preserving PLs are designed to target specific modes or mode groups from each input40. In this study, we focus on the mode-mixing PL, as our goal is to efficiently transfer power from the MM VCSEL sources to the MMF.

The PL device design relies on algorithmic convergence and numerical FDTD simulations to model light propagation within the guiding structure. These simulations are computationally intensive due to the MM-PL volume and source count, with individual runs taking anywhere from minutes to hours depending on mesh point density. Therefore, significant effort has been dedicated to optimizing computational efficiency and speed.

To evaluate the PL’s performance, the coupling matrix is computed, representing the connectivity between each input source mode and each output mode. From the coupling matrix, insertion loss (IL) and mode-dependent loss can be derived using singular value decomposition (SVD). Calculating the full coupling matrix requires 2 × N FDTD simulations, where N is the number of modes and the factor 2 accounts for polarization. This linear increase in computation time with the number of modes makes optimization challenging. To address this, an IL estimator was developed that significantly reduces the number of required simulations by leveraging the PL’s symmetry relative to the central waveguide and the fact that the PL is designed for mode mixing. Figure 1e depicts the PL input waveguide arrangements for the 7, 19 and 37-waveguide configurations, where circles labeled “A” to “D” represent symmetrical waveguide groups. The 7-source PL utilizes groups “A” and “B”, while the 19-source PL comprises groups “A” “B”, and “C” and the 37-source PL uses all four waveguide groups. The IL estimator utilizes a single simulation per symmetry group, significantly reducing computational requirements. For instance, the 7-MM PL requires only two simulations per algorithm step, the 19-MM PL requires three, and the 37-MM PL requires four. This method ensures that computation time does not increase linearly with the number of modes.

Our IL estimator is defined by the weighted sum:

IbL = 1/N ∑_{i=1}^{S} M_i P_i ≤ IL ≤ 1 (1)

where N is the total number of PL modes, P_i represents the total power transmission from a selected input source from the i-th simulation to the output, S denotes the number of symmetry groups, and M_i is the number of modes in each symmetry group where N = ∑_{i=1}^{S} M_i. To address the more complex scenario of multimode inputs, which demands another simulation multiplier to account for each source mode, the estimator function is calculated based on the highest-order input source spatial mode only, which always experiences the lowest efficiency. This strategy reduces the required simulations per waveguide group from 2 × 6 to just one. The rationale is that if the structure can efficiently support the highest-order mode—associated with the highest transverse momentum and therefore is the most challenging to couple adiabatically—it is expected to perform better for lower-order input modes. As a result, the IL estimator consistently underestimates the actual IL and its maximization converges to well performing designs (maxmin optimization). Further details are provided in the supplementary materials section 1. This assumption is particularly valid for adiabatic devices but may not apply to other types of multiplexers, such as MPLC, which rely on discrete phase masks.

Using the IL estimator as the objective function, we utilized a genetic algorithm35 (GA) to optimize the designs of the 7-MM PL, 19-MM PL, and 37-MM PL. Each PL leg of groups A-D was defined by a set of 9 parameters, including 8 parameters that determine the waveguide curve shape and the waveguide tapering rate35. The the PL length is another parameter defined equally for all waveguide groups. Each unique parameter set corresponds to a distinct design. The GA aimed to identify the parameter set that maximized the IL estimator. Figure 1(f–h) illustrates the GA scores at each iteration for the three devices, demonstrating convergence to well performing designs.

After selecting the best design from the GA optimization, we evaluated its real IL by evaluating all input modes and sources. For the 7-MM PL, the best IL estimator was −0.8 dB, corresponding to the estimated loss using the highest-order spatial mode only. Since the estimator provides a lower bound for the insertion loss, the total efficiency is expected to be higher. Calculating the complete coupling matrix for the optimized device, an IL of −0.45 dB was obtained. Similarly, for the 19-MM PL, the best IL estimator was −1.55 dB, with an actual IL of −0.86 dB. For the 37-MM PL, the best IL estimator was −2 dB, with an actual IL of −1.4 dB, confirming the design methodology we adopted.

Both the 19-MM and 37-MM PLs, are designed to interface with a 0.22 NA, 50 μm step-index (Thorlabs model FG050UGA) multimode fiber, have total lengths of 425 μm and 470 μm, respectively, and terminate with an expansion taper of 52 μm diameter cross-section (Fig. 1c,d) which best matches the MM fiber according to simulations. The 7-MM PL has a total length of 345 μm only and terminates with a 10 μm diameter cross-section to expand the output beam and reduce the required NA of the measuring optical system (Fig. 1b).

PL on VCSEL 3D-printing

We utilized direct laser writing with a Nanoscribeⓒ Photonic Professional GT2 3D Printer for the fabrication of the PL devices. Tightly focused beam of the pulsed laser with the center wavelength of 780 nm was used to polymerize IP-S resin through a two-photon polymerization process. The writing was done using a 25× objective. The system is calibrated to deliver 50 mW (CW equivalent) at 100% laser power. This fabrication technology enables fine resolution feature definition down to 200 nm. Fig. 2 shows SEM images of the three PL devices (a: 7-MM, b: 19-MM and c: 37-MM) directly printed on a MM-VCSEL array corners.

Transfer matrix analysis of a 7 source-six mode input PL

To accurately assess the performance and modal behavior of the fabricated MM-PL, the coupling matrix must be experimentally measured. This is normally done (in the SM input case) by exciting one input source at a time. Using the MM VCSEL array as the source, it is not possible to individually excite one VCSEL source in our device, let alone controllably radiate by a mode and polarization basis. Hence, for characterization purposes, we fabricated the PL on a glass substrate and operate it in reverse, serving as a mode demultiplexer. We excited the PL from the 10 μm output side with 42 LP modes, generated with a phase spatial light modulator (SLM) that converts an incident collimated Gaussian beam to each mode after two reflections from different SLM regions with applied phase patterns to generate each target mode. The PL demultiplexes each input mode to orthogonal states spanning the seven PL waveguides and six modes (as the PL mode mixes). Hence we must record the entire complex field emerging cumulatively from all PL legs, which we do with a digital holography (DH) setup that records the interferogram of light emerging from the PL legs with a reference collimated beam from the same input laser source, (setup in Fig. 3e). For this free-space coupling measurement, the PL was slightly modified for operation at 1550 nm wavelength, which better suits our communication-focused lab equipment. To reduce background light, we also gold-coated the glass substrate, leaving only an array of seven apertures matching the input waveguides’ position and size for PL printing. We recorded the interferograms from the seven 6-mode waveguides, extracted the complex electric field and assessed the PL transfer matrix in reverse, functioning as a demultiplexer. From these complex field measurements, we derived the modal content of each waveguide and relative phase and constructed the complete coupling matrix with the proper phase relationship between matrix elements, with each measurement contributing a row (Fig. 3b: The absolute squared matrix and c: The phase values). Fig. 3a illustrates the generated input LP mode, the measured output electric field, and its intensity for each measurement. Figure 3d presents the singular values of the measured transfer matrix. As illustrated, 7 out of the 42 singular values have power levels below −5 dB, indicating that the majority of the system’s modes are functioning effectively. The mode-dependent loss (MDL) is calculated as −5.1 dB using the standard deviation of the singular values (−40 dB when assessed using the min-max metric). Based on the average of the singular values, the insertion loss (IL) is determined to be −1.6 dB, reflecting the fraction of transmitted optical power confined within the 42 target modes.

Beam quality measurements

Understanding the modal content and beam quality of PL devices is essential for a complete analysis of the VCSEL+PL system. To achieve this, we employed a self-interference method to measure the complex electric field of the emitted light from VCSELs or PL devices41. The experimental setup is depicted in Fig. 4a.

The process begins by splitting the VCSEL beam using a beam splitter (BS) and recombining it on a camera sensor with another BS. The optical path lengths of the two beams were carefully matched to ensure the temporal coherence of the VCSEL source. The reference path includes spatial filtering to create a broad, Gaussian-like beam with flat phase fronts, which is then magnified to fully overlap with the second, multimode (MM) beam. The second BS is slightly tilted, introducing a small angle (~3°) between the two beams for fringe formation.

Subsequently, we apply an off-axis digital holography technique to reconstruct the electric field of the VCSEL beam. This method allows each lasing source to interfere with itself, on account of self-coherence, enabling wavefront reconstruction42. Finally, we perform a modal decomposition using the Laguerre-Gaussian (LG) mode basis. This decomposition identifies the spatial modes present in the VCSEL beam and determines the beam quality factor (M2) using the formula43:

M^2 = ∑_{p=0}^{1} ∑_{l=-p}^{p} (2p+l+1) |c_{pl}|^2 (2)

Where, l, p are the LG modes indexing, and C_pl is the coupling coefficient of l, p mode to the measured electric field. Additionally, we estimated the coupling efficiency to a multimode fiber (MMF) by projecting the measured fields onto a step-index MMF modal basis. For each measured PL (7, 19, or 37), the simulated MMF was configured to support the same number of modes as those considered in the PL design (7 × 6 = 42, 19 × 6 = 114, and 37 × 6 = 222). This approach allows us to emulate the performance of ideal, mode count matched fiber coupling35. A characterization of the modal content and beam quality of a single VCSEL source is shown in the supplementary materials section 3. From the measurements, most of the power is concentrated in the first six LG modes, with a small fraction extending up to the 12th mode at higher injection currents. The VCSEL source M2 factor varies between 2.25 and 4.1, with lower values at lower current (see supplementary).

Beam quality measurement of a 7-MM PL and 42-mode fiber coupling estimation. The M2 and modal analysis were performed on the 7-MM PL printed on a VCSEL array using the self interference digital holography technique. Figure 4b shows the electric fields and intensity profiles of the PL output at various injection current levels. It also includes a plot of the measured M2 values and calculated fiber coupling efficiency for each current. Figure 4c shows the fiber coupling values range from −1 dB to −1.5 dB, closely matching the insertion loss (IL) estimated via digital holography, which was −1.6 dB for all 42 designed modes. The M2 values were found to range between 7.6 and 8.6.

Beam quality measurement of a 19-MM PL and 114-mode fiber coupling estimation. The M2 and field analysis were subsequently conducted on the 19-MM PL printed on a VCSEL array. Figure 4b illustrates the electric fields and intensity profiles of the PL output at different injection currents. Figure 4c shows the fiber coupling values, representing the power contained within the 114 target modes, range from −1.2 dB to −1.6 dB. The M2 values were observed to vary between 12.3 and 13.42.

Beam quality measurement of a 37-MM PL and 222-mode fiber coupling estimation. The M2 and field analysis were subsequently conducted on the 37-MM PL printed on a VCSEL array. Figure 4b illustrates the electric fields and intensity profiles of the PL output at different injection currents. Figure 4c shows the fiber coupling values, representing the power contained within the 114 target modes, range from −1 to −2.2 dB. The M2 values were observed to vary between 14.9 and 17.

Power transmission and fiber coupling

To assess the transmission efficiency of the MM-PL devices, we performed direct optical power measurements by imaging the PL output onto a large photodetector, using a spatial filter to suppress background light from adjacent VCSELs on the chip that are not attached to the PL. Additionally, the 19-MM PL and 37-MM PL were directly coupled to a step-index MMF with a 50 μm core diameter and NA = 0.22. The PL-to-fiber alignment was performed actively by measuring the fiber output power while precisely adjusting the VCSEL chip and PL using a piezo-actuated x, y, z, and tip/tilt stages. The PL was brought into direct contact with the fiber. Microscope images of the butt-coupled VCSEL+PL+MMF system are shown in Fig. 5c, f. Since the fiber supports approximately 677 spatial modes, efficient coupling is expected.

7-MM PL Power transmission. A measurement of the 7-MM PL device was conducted. Figure 5a presents the optical power measurement of the PL, alongside the power from a single VCSEL source. An additional curve, scaling by 7× the single VCSEL power, illustrates the maximum potential power at the PL output, assuming all sources emit at approximately the same power. This assumption is valid as the VCSELs were operated with injection current above the threshold lasing current, and the reference VCSEL was measured on the same sources used for the PL (Prior to its fabrication). Furthermore, the reference VCSEL measurement was performed with a source filled with a 3D-printed photopolymer pillar to match lasing conditions to the PL itself. Figure 5b shows the PL power loss relative to the measurement of seven single VCSEL sources, with losses ranging from −1.5 dB to −0.8 dB as a function of current.

19-MM PL Power transmission and MM fiber coupling. A similar measurement was conducted for the 19-MM PL device. A microscope image of the VCSEL+19-MM PL+MM fiber is shown in Fig. 5f. Figure 5d shows the optical power measurement, while Fig. 5e presents the corresponding loss curves. The power loss for the 19-MM PL was found to range from −1.5 dB to −1.7 dB, following the same procedure used for the 7-MM PL. The figures also includes coupling results to a multimode fiber (MMF), performed with Thorlabs FG050LGA 50 μm core diameter step-index MMF (NA = 0.22). The transmitted power through the fiber is shown in Fig. 5d, and the loss is depicted in Fig. 5e, ranging from −2.1 dB to −2.3 dB. The fiber coupling efficiency (green curve in Fig. 5e) was calculated by subtracting the PL transmission curve from the fiber transmission curve, yielding a range of −0.8 dB to −0.5 dB, indicating efficient power transfer from the PL to the fiber.

37-MM PL Power transmission and MM fiber coupling. A similar measurement was also performed for the 37-MM PL device. A microscope image of the VCSEL+37-MM PL+MM fiber is shown in Fig. 5f. Figure 5g shows the optical power measurement, while the loss curves are presented in Fig. 5h. The power loss for the 37-MM PL was measured to range between −3.5 dB and −1.8 dB. The transmitted power through the fiber is shown in Fig. 5g, while the loss is depicted in Fig. 5h, ranging from −4 dB to −2.5 dB. The measured fiber coupling efficiency (green curve in Fig. 5h), which was calculated by subtracting the fiber transmission curve from the PL transmission curve, is around an average of −0.7 dB, demonstrating effective coupling and efficient power transfer from the 37-MM PL to the fiber.

Long duration operation measurement

To assess potential damage formation from long duration operation, long duration optical power measurements were performed at the PL output utilizing the 19-MM PL. The system was characterized at six distinct driving currents; each measurement duration was 1 h and 10 min, resulting in a total operational time of 7 h. To ensure stable lasing of the VCSEL sources, a backside thermal stabilization system was implemented. This setup included a cooling fan, a thermoelectric cooler (TEC) controller, a temperature sensor, and a PID (Thorlab’s TED4015) control loop (see Fig. 6a). The VCSEL surface temperature was stabilized at 25°. Fig. 6b illustrates the temperature deviation over the measurement period, showing a stability of approximately ±0.02°. The measured optical power versus time for different injection current levels is presented in Fig. 6c. The output power remained stable over time, with minor fluctuations attributed to intrinsic laser noise and potential mechanical instabilities within the laboratory setup. The measurement results for each current level-including mean power (μ), standard deviation (σ), and power stability—are summarized in Table 1. The stability metric is calculated as 20log10(σ/μ) and describes the fluctuation strength relative to the average power. For all tested current values, the stability was observed to be less than −50 dB. These results demonstrate a robust and stable PL-VCSEL system capable of continuous operation across a wide range of driving currents over extended periods.

Methods
Simulation tools

The electromagnetic simulations carried out using Ansys Lumerical solvers encompass the utilization of FDTD, EME, and FDE algorithms. The optimization and design process involves utilizing a Python API that has been developed in house for the Lumerical solver.

Digital holography and SLM mode generation

To measure the coupling matrix of the PL at a single wavelength (1.55 μm), we employed off-axis digital holography to capture the complex electric field at the device’s output. The experimental setup included the following equipment: a Yenista Optics model−1560 ECL (External Cavity Laser) source, Thorlabs MPC320 polarization controllers, and a Thorlabs TC25FC-1550 fiber collimator for collimating the reference beam and the signal beam before the SLM. To image the PL output beams onto the camera plane, we used a ×20 Mitutoyo T1.1 LCD Plan Apo NIR (Near-Infrared) infinity-corrected objective paired with a 200 mm tube lens. The imaging system incorporated an Allied Vision Goldeye G-033 TEC less InGaAs camera for capturing the beam. Mode patterns were generated using a Holoeye PLUTO-2.1 TELCO SLM combined with a λ/2 waveplate to ensure proper polarization alignment on the SLM. The SLM phase patterns were optimized using a gradient descent algorithm (see Supplementary Information section 2) to maximize the quality of the generated LP modes. A f = 3 mm aspheric lens was used to focus the collimated light after passing through the SLM onto the PL input.

Beam quality measurement

The experimental setup is shown in Fig. 4a. A 60×/0.85 Plan infinity-corrected microscope objective was used to collect light from the PL or VCSEL source. Two identical 4f systems with a magnification ratio of 1:5 (f = 10 mm, f = 50 mm) were employed to magnify the reference beam and demagnify the PL beam, ensuring uniform illumination of the reference beam on the measured beam. A pinhole with a 75 μm diameter was selected to produce a clean Gaussian beam with sufficient power. Adjustable neutral density (ND) filters, ranging from 0.1 to 4, were placed in the signal path to balance the intensities of the signal and reference beams. The setup utilized an iDS UI-148xLE CMOS camera for detection and a Thorlabs LDC4020 laser diode controller to operate the VCSELs.

Power transmission measurement

The setup incorporates an imaging system consisting of a 60×/0.85 Plan infinity-corrected microscope objective and an f = 180 mm tube lens to focus the measured light onto a 1 mm2 HP-8153A optical power meter. To eliminate stray light from neighboring VCSEL sources, an adjustable iris was placed after the microscope objective. The imaged light was split by a beam splitter, directing it simultaneously to a camera and the power meter. The camera served to verify that the correct VCSEL or PL light was being accurately imaged onto the power detector.

3D nano-printing on a VCSEL array

The process of 3D nanoprinting on VCSELs is highly intricate and requires meticulous surface preparation. Initially, the VCSEL was submerged in acetone and subjected to ultrasonic cleaning in a 55°C bath, just below acetone’s boiling point, for 1 h. This step removed contaminants, reducing the risk of issues such as explosions caused by residual particles. To enhance adhesion, a three-step process was employed: the VCSEL surface was silanized by immersing it in 30 mL of ethanol mixed with three drops of 3-(Trimethoxysilyl)propyl methacrylate for 30 min, introducing coupling agents and functionalizing the surface. A drop of material was then applied to the VCSEL and placed in a desiccator where air was evacuated for 20 min, ensuring the IP-S photopolymer filled the apertures without bubbles. A 2 μm-thick circular skirt, raised 3 μm above the waveguide bottom—aligned with the VCSEL’s gold contact surface—was added to increase the contact area between the polymer layer (PL) and the VCSEL top electrode without introducing additional optical losses.

Before printing, the VCSEL was carefully aligned relative to the printing direction to ensure precise coupling between the VCSEL and the waveguides. Each leg of the PL was positioned over its corresponding aperture, with alignment fine-tuned by activating the VCSEL and ensuring its beam center matched the waveguide center. The reflective nature of VCSEL surface materials, such as gold and GaAs, required a reduced laser power for the initial layers. The first 5 μm were printed near the polymerization threshold at 10% of maximum laser power, while subsequent layers were printed at 32% power with a Galvo scanning speed of 10,000 μm/s. A 25× objective lens and IP-S resin were used, and the structure was sliced into 100 nm increments in both vertical and horizontal planes to ensure a smooth and high-quality PL surface. The PL’s fine features, with widths as small as 2 μm, were reinforced against deformation during development by applying a 60-degree offset to the hatching angle.

The heights of the PLs were set to 345 μm, 425 μm, and 470 μm for the 7-MM, 19-MM, and 37-MM configurations, respectively. Since the piezo stage’s maximum longitudinal range is 300 μm, the structures were divided and stitched at 295 μm. Alignment of the upper and lower sections was performed atop the bottom part before continuing the print. To mitigate power losses from uncovered VCSEL apertures, unused apertures were sealed with 15 μm-high pillars, ensuring uniform lasing power distribution across the array.

After printing, the structures were cleaned in propylene glycol methyl ether acetate (PGMEA) for 20 min, rinsed in isopropanol (IPA) for 2 min, and dried in Novec 7100 for 1 min. Additional curing of potentially non-polymerized material in the apertures beneath the circular plate was completed using strong UV light to ensure full polymerization.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reports the integration of photonic lanterns on the surface of a VCSEL array to achieve VCSEL mode multiplexing. Photonic lanterns are a well-established research topic, and their implementation via laser direct writing has already been reported by the authors or other groups (Optics Express, 2011, 19(6): 5698–5705; Photonics Research, 2025, 13(8): 2088–2095; Light: Science & Applications, 2024, 13(1): 126). The claimed novelty of this work therefore appears limited to the integration of photonic lanterns directly on the surface of a VCSEL array and their design for multimode VCSELs. However, related work on 3D-printed photonic structures on VCSEL array surfaces has also been reported (Nano Letters, 2023, 23(19): 9096–9104). In addition, the results presented here do not demonstrate any unexpected or significant advances for photonic lanterns designed for multimode VCSELs.\nThe authors suggest potential applications in optical communications. However, optical communications require addressable VCSEL arrays, whereas the VCSEL array demonstrated here operates with a common p-electrode and identical modes, making it unable for such applications. Regarding beam combining, the demonstrated photonic lanterns exhibit energy losses exceeding 30% (Fig. 5g), which is a poor experimental outcome. Furthermore, compared to the relatively complex and less reliable method of laser-printing photonic lanterns on VCSELs for fiber coupling, recent work has shown the direct monolithic integration of VCSELs onto the end face of optical fibers (Nanophotonics, 2025, “Monolithic integration of one VCSEL on a single-mode fiber”), which offers greater reliability and stability. For these reasons, the manuscript does not meet the requirements of Nature Communications in terms of novelty and broad impact. Instead, it may be more suitable for a specialized journal focusing on micro/nano fabrication and device integration.\nIn addition to the above major concerns, I have the following specific comments:\n1. There is an editorial error in the Abstract: the statement “achieving coupling efficiency as low as −0.6 dB” should read “coupling losses as low as −0.6 dB.”\n2. The photonic lantern should be implemented on addressable VCSEL arrays, where each VCSEL supports distinct spatial modes, to demonstrate potential for communications applications.\n3. The study lacks direct comparison with established coupling schemes such as microlens arrays plus multimode fibers, particularly in terms of long-term stability and scalability.\n4. Issues such as thermal effects, VCSEL array uniformity, and long-term device reliability are not addressed, leaving significant doubts about the practicality of the proposed approach.", "reviewer_2": "This manuscript reports the use of 3D nanolithography to fabricate free standing photonic lantern structures on top of an array of VCSELs for incoherent mode combination. In contrast to most other photonic lanterns, the photonic lanterns reported by the team adiabatically couple N multimode sources, each emitting M modes, to a multimode waveguide supporting an appropriate number of modes, such that the brightness etc of the VCSEL array is not degraded.\nThe authors also show that this device can be used to couple the VCSEL array to a multimode fibre with an efficiency significantly greater than the efficiency that could be achieved using a microlens array and a lens (see Fig. 6).\nOf course, the final goal of this work would be to scale the incoherent combination to arrays consisting of hundreds of VCSELs, but I do have doubts on the power handling capabilities of these 3D nanofabricated lanterns.\nDespite these concerns about the future real-world impact of the technology, which perhaps the authors could address in the revised manuscript, the paper is extremely well presented, interesting and technically impressive.\nI see no reason not to recommend publication essentially as is."}, "author_response": "Thank you very much for handling our paper and securing the evaluations by the expert reviewers. We thank the reviewers for their comments and suggestions. We took all their comments and suggestions into consideration and revised the manuscript accordingly. Below is a detailed response to each of the received comments.\nReviewer #1 (Remarks to the Author):\nThis paper reports the integration of photonic lanterns on the surface of a VCSEL array to achieve VCSEL mode multiplexing. Photonic lanterns are a well-established research topic, and their implementation via laser direct writing has already been reported by the authors or other groups (Optics Express, 2011, 19(6): 5698–5705; Photonics Research, 2025, 13(8): 2088– 2095; Light: Science & Applications, 2024, 13(1): 126). The claimed novelty of this work therefore appears limited to the integration of photonic lanterns directly on the surface of a VCSEL array and their design for multimode VCSELs. However, related work on 3D-printed photonic structures on VCSEL array surfaces has also been reported (Nano Letters, 2023, 23(19): 9096– 9104). In addition, the results presented here do not demonstrate any unexpected or significant advances for photonic lanterns designed for multimode VCSELs.\nOur response: We thank the reviewer for this insightful comment. We acknowledge that the novelty of our work may not have been sufficiently emphasized in the manuscript and have clarified this in the revised version. Below, we outline the key distinctions between our study and the referenced works.\nThe work reported in Optics Express (2011, 19(6): 5698–5705) demonstrated photonic lanterns fabricated by laser direct writing inside a glass substrate, designed for multiplexing single-mode inputs into a few-mode fiber. These devices were based on low-index-contrast waveguides, resulting in device lengths on the order of tens of millimeters, whereas our photonic lanterns are sub-millimeter-scale (micron-scale). Integrating such glass-embedded lanterns with a VCSEL array would require additional relay optics and interconnects, leading to a significantly bulkier and more complex system than our directly integrated approach.\nThe studies published in Photonics Research (2025, 13(8): 2088–2095) and Light: Science & Applications (2024, 13(1): 126)—both from our group— reported 3D-printed photonic lanterns designed for single-mode inputs and directly printed on multi-core fibers. These works represented an important step forward from the 2011 demonstration toward miniaturized lanterns, but they still dealt exclusively with single-mode excitation.\nIn contrast, the current work introduces several distinct innovations:\n1. Massive-scale space-division multiplexing (SDM)–based incoherent beam combining scheme for multimode laser sources. Unlike other approaches for power combining—such as those based on multi-plane light conversion (e.g., Cailabs)—our method supports integration with higher-order mode inputs, as written in the introduction “Traditional PL and MPLC spatial multiplexers are typically designed to receive single- mode (SM) fibers or waveguides as inputs, with these designs being incompatible for beam combining from multiple multi-mode (MM) laser or VCSEL sources.” Optimizing an MPLC to this task is much more complex, as explained shortly in the design section “As a result, the IL estimator consistently underestimates the actual IL and its maximization converges to well performing designs (maxmin optimization). Further details are provided in the supplementary materials. This assumption is particularly valid for adiabatic devices but may not apply to other types of multiplexers, such as MPLC, which rely on discrete phase masks.” Utilizing SDM for incoherent combining theoretically enables brightness preservation and efficient power delivery into a multimode fiber (MMF).\nDirect integration with a large-scale VCSEL source array: We developed and optimized a dedicated fabrication process for 3D printing directly on VCSEL surfaces, as detailed in the Methods section and Supplementary Materials. This integration enables precise optical interfacing between the source and the multimode fiber, minimizing coupling losses and system volume. While Nano Letters (2023, 23(19): 9096–9104) demonstrated 3D printing on a single VCSEL, our work extends this concept to VCSEL arrays, involving larger device scales and additional fabrication challenges. Moreover, our design allows direct optical power delivery to a multimode fiber without requiring any intermediary optical components. Since the work from Nano Letters is relevant, we have added the following sentence to the Introduction, including a citation of the study:\n“Moreover, 3D nano-printing technology allows fabrication on versatile surfaces and sources, making it an excellent platform for VCSEL integration [37,38] and a wide range of microscale photonic systems.”\n[37] Nano Letters (2023, 23(19): 9096–9104)\n[38] Dana, Y., Shukhin, ECOC 2024\n2. All previously reported photonic lanterns were designed to interface single-mode (SM) input sources and multiplex them into a few-mode fiber (FMF) (e.g., six SM waveguides combined into a six-mode fiber). In contrast, our work introduces a new design framework capable of handling M sources, each supporting N modes, and multiplexing them into an N×M-mode fiber (with an additional factor of two accounting for polarization modes). We developed a dedicated design flow based on FDTD simulations, enabling efficient optimization for systems with a large number of modes. A key element of this process is an optimization merit function that dramatically reduces the computational load: for instance, evaluating the merit function of the presented 37-MM PL requires only four FDTD simulations, compared to 37×6×2 = 444 simulations needed for a full insertion loss analysis. The optimization framework is described in detail in the Design section and the Supplementary Material. Furthermore, we provide extensive experimental validation, including a thorough characterization of three devices—both with incoherent VCSEL sources and in a controlled setup for the 7-MM PL, where its modal behavior is analyzed in detail in the section “Transfer matrix analysis of a seven-source, six-mode input PL.”\n3. Compact, scalable, and versatile architecture: The presented sub- millimeter photonic lanterns are highly compact and mechanically stable, allowing straightforward integration with VCSEL arrays, photonic integrated circuits, and multicore fibers.\nIn summary, this work represents, to the best of our knowledge, the first demonstration of a photonic lantern designed, fabricated, and characterized for multimode VCSEL sources, directly integrated at the microscale. The resulting platform enables incoherent optical power combining and efficient coupling to multimode fibers without the need for bulky optical components, marking a significant step toward compact and scalable light delivery systems.\nTo highlight the novelty of our work, we have added the following statement to the abstract: “Compared to previously reported PL devices that were designed for single-mode input sources, we demonstrate a design scheme capable of handling few-mode input sources.” Furthermore, the text added to the Conclusions section, as mentioned in our response to Comment 3, further reinforces this point.\nThe authors suggest potential applications in optical communications. However, optical communications require addressable VCSEL arrays, whereas the VCSEL array demonstrated here operates with a common p- electrode and identical modes, making it unable for such applications.\nResponse: We thank the reviewer for this important comment and refer to our response to (2.) for further details.\nRegarding beam combining, the demonstrated photonic lanterns exhibit energy losses exceeding 30% (Fig. 5g), which is a poor experimental outcome. Furthermore, compared to the relatively complex and less reliable method of laser-printing photonic lanterns on VCSELs for fiber coupling, recent work has shown the direct monolithic integration of VCSELs onto the end face of optical fibers (Nanophotonics, 2025, “Monolithic integration of one VCSEL on a single-mode fiber”), which offers greater reliability and stability.\nOur response:\nWe thank the reviewer for the valuable comment regarding the transmission loss of the 3D-printed photonic lantern (PL). Before addressing the specific loss mechanisms, it is important to note that simulations under ideal conditions (perfect fabrication and ideal source profiles) indicate that the PL- based incoherent combining approach offers higher potential efficiency compared to a conventional lens system—showing simulated losses of −1.4 dB for the PL versus −3.5 dB for the lens system.\nIn the manuscript, we analyzed the case of coupling 𝑁×𝑀 source modes into an 𝑁×𝑀 mode fiber. This configuration is conceptually analogous to coupling a single-mode (SM) source into an SM fiber—an inherently sensitive process, as also discussed in Nano Letters (2023, 23(19): 9096–9104). However, it is substantially more challenging than coupling a VCSEL array supporting 𝑁 ×𝑀 modes into a multimode fiber with a significantly higher number of modes, as is typical in most VCSEL–MMF systems. As noted in the manuscript, such mismatched coupling can lead to beam brightness degradation, which represents a key performance limitation for many applications.\nRegarding the loss mechanisms, there are two main contributing factors:\n1. Fabrication imperfections — Misalignments between sources, waveguide deformation, material shrinkage, and surface roughness can all introduce optical losses. In our current process, we primarily addressed waveguide shrinkage and source alignment; however, surface roughness and waveguide deformation were not yet optimized and required further investigation.\n2. VCSEL source behavior — Analysis of the VCSEL array (presented in the Supplementary Materials) revealed the presence of higher-order spatial modes beyond the expected six modes at elevated driving currents. Since the PL is optimized for a specific mode set, excitation of unaccounted higher-order modes results in additional coupling losses.\nRegarding the work reported in Nanophotonics (2025), they propose a promising solution for integrating a single VCSEL source with a single fiber, which is both robust and reliable. However, handling multiple VCSEL sources and combining them into a single fiber requires an additional combining mechanism, which could involve relay optics or waveguides. Therefore, the direct bonding approach suggested in Nanophotonics (2025) is not suitable for such multi-source applications. Consequently, it is challenging to directly compare the performance of the two approaches. In their work, they report coupling efficiencies to a single-mode fiber (SMF) below 45%. In comparison, our work demonstrates coupling to a few-mode fiber (FMF) with the same number of modes as the source modes, yielding a combined efficiency of −3.8 dB (i.e., 41.7%) based on holographic mode analysis and power loss measurements. This comparison is not entirely equivalent, as our analysis assumes ideal coupling conditions while also accounting for sources supporting higher-order modes beyond the design specification.\nFor these reasons, the manuscript does not meet the requirements of Nature Communications in terms of novelty and broad impact. Instead, it may be more suitable for a specialized journal focusing on micro/nano fabrication and device integration.\nOur response:\nWe appreciate the reviewer’s opinion regarding the novelty and impact of our work. On the novelty side, the presented manuscript introduces a new approach for incoherent multimode beam combining based on space-division multiplexing (SDM), enabling a highly compact and small-footprint system that efficiently delivers optical power from multiple sources into a receiving waveguide or multimode fiber (MMF). The system eliminates the need for bulky optical systems and their associated alignment challenges, requiring only a single alignment step between the PL output facet and the MMF—an issue that can be readily resolved in practical implementations using a dedicated mechanical connector, similar to standard fiber interconnects.\nThe manuscript also provides a theoretical comparison between SDM and conventional relay optics, highlighting the physical justification for using SDM- based architectures to preserving beam brightness. We further demonstrate a microscale 3D-printed photonic lantern (in the sub-mm regime, compared to traditional multi-millimeter-scale PLs) featuring a design scheme capable of efficiently coupling few-mode sources, unlike conventional PLs designed solely for single-mode inputs.\nWe believe that this work will be of broad interest to the readers of Nature communications, both for its novel device concept based on 3D printing of diminutive PL devices and for advancing beam combining techniques. Coherent and incoherent VCSEL arrays are finding diverse uses in sensing, communications, and illumination and pumping applications, and shaping and modifying the output radiation enables new applications and greater system efficiency. In addition, we report a detailed characterization of three PL multiplexers integrated with multimode VCSEL arrays, employing advanced diagnostic tools such as holographic measurements and high-fidelity spatial beam generation. These methodologies are widely applicable to a range of photonic devices and systems, further enhancing the relevance, reader interest and potential impact of the study.\nIn addition to the above major concerns, I have the following specific comments:\n1. There is an editorial error in the Abstract: the statement “achieving coupling efficiency as low as −0.6 dB” should read “coupling losses as low as −0.6 dB.”\nOur response: We thank the Reviewer for his comment but we disagree with this observation. Using common definitions, efficiencies are bound by one and presented in negative dB values, whereas losses are expressed in positive dB losses and are unbounded.\n2. The photonic lantern should be implemented on addressable VCSEL arrays, where each VCSEL supports distinct spatial modes, to demonstrate potential for communications applications.\nOur response: We agree with the reviewer’s insightful observation on the communication application. Indeed, the specific VCSEL array used in our work is not optimized for communication platforms. In our study, we considered two relevant communication schemes. The first, and currently more practical, is an intensity modulation/direct detection (IM/DD) approach, where efficient power coupling into a multimode fiber (MMF) is essential. Our photonic lantern– VCSEL system demonstrates high coupling efficiency without requiring relay optics alignment, enabling compact and robust short-reach communication links.\nThe second, more advanced case involves mode-division multiplexing (MDM), which would require extensive MIMO signal processing to compensate for modal mixing, as our photonic lantern is not designed for modal selectivity. In either case, while the specific VCSEL array we employed is not ideal for communication applications, we have demonstrated successful integration with a general VCSEL array architecture. Our approach can readily be mapped to other VCSEL arrays that are designed and optimized for communication purposes, as the novelty of our work lies in the 3D-integrated photonic lantern, not in the VCSEL source itself. To make this point clearer to the readers we added the following in the conclusions section: “Since this work primarily focuses on power delivery applications—namely, efficient optical power transmission from a VCSEL array to a multimode fiber—other use cases, such as short-reach optical fiber communication, can be realized by employing a VCSEL array designed for communication, as the photonic lantern can be readily optimized for different VCSEL platforms.”\n3. The study lacks direct comparison with established coupling schemes such as microlens arrays plus multimode fibers, particularly in terms of long-term stability and scalability.\nOur response: We thank the reviewer for this important observation. In terms of performance, our study includes a comparison between the proposed photonic lantern (PL) approach and a theoretical model of a microlens-array– to–MMF coupling scheme (exactly as the reviewer suggested). The goal of this comparison is to illustrate the fundamental coupling limit of a lens-based system that connects an equal number of input sources to a multimode fiber supporting the same number of modes—whereas, in theory, space-division multiplexing (SDM) represents the ideal solution for such a task.\nWe chose to compare our experimental PL results with an idealized simulation of a microlens-based system rather than an experimental one, because designing and realizing a dedicated microlens system optimized for our specific VCSEL array would require substantial additional effort to ensure a fair and meaningful comparison. As presented in the paper, the ideal microlens simulation closely matches the experimental performance of our PL device, even though the simulation does not account for aberrations, misalignments, tilts, or fabrication imperfections.\nAs stated in the conclusion section:\n:” PL-based system. Using the beam quality measurements, which calculate the estimated fiber coupling, we can determine the fraction of transmitted light remaining in the target modes. This, combined with the power transmission measurements that evaluate the PL’s total optical transmission, provides an overall assessment of the system’s efficiency. Focusing on the 37-MM PL device, the average estimated fiber coupling (from Figure 4-c) is L = Coupling −1.6 dB , and the average power transmission (from Figure 5-h) is L = Power −2.2 dB . Therefore, the estimated total power coupling of the experimental system is L = L + L = −3.8 dB , which closely matches the Total Power Coupling results from the simulated lens system despite the simulation assuming ideal conditions without alignment errors, aberrations, or other practical imperfections.”\nRegarding scalability, while lens-based systems can indeed support large VCSEL arrays in principle, the PL platform provides two clear scaling pathways:\n1. Higher-order mode optimization: Our established PL optimization framework can be extended to support a larger number of modes and to interface efficiently with multi-source VCSEL arrays.\n2. Modular integration: The demonstrated 37-MM PL can serve as a building block for larger systems. By interconnecting multiple PL units, larger arrays can be realized. Modern 3D nanoprinting tools (e.g., Nanoscribe) can now fabricate structures spanning several millimeters, enabling single-shot fabrication of large-scale VCSEL–PL assemblies.\nAs part of our ongoing work, we have demonstrated in our lab a 3×37-MM PL prototype (integrating 111 VCSEL sources). Although this work is still preliminary and optimization is ongoing, SEM images of the fabricated structures on glass substrates (Fig. 1 a,b) confirm the feasibility of such scaling.\na) b)\nFigure.1- a) Top, b) side views SEM images of 3x 37 MM PL prototype, printed on a glass substrate.\nTo emphasis the scalability options we have added the following to the conclusions and discussions: “The 3D nano printer we use is limited to a writing transverse area of 400 × 400 [μm] with the 25× objective and galvo scanning mirrors. Without this constraint, it would be possible to cover a large VCSEL array with hundreds of sources using the same design methodology. To scale the number of integrated VCSEL sources, two complementary approaches can be employed: first, by extending the photonic lantern optimization framework to higher-order modes, allowing efficient coupling of a larger number of inputs; and second, through modular integration, where the demonstrated 37-MM PL serves as a repetitive building block that can be interconnected to form larger assemblies. With modern 3D nanoprinting systems capable of multi-millimeter writing fields, such scaling can be achieved in a single fabrication step, enabling dense and compact integration of large VCSEL arrays with the photonic lantern.”\nRegarding the stability of the photonic lantern (PL), we understand that the reviewer’s concern likely refers to the mechanical robustness of the 3D- printed structure. Based on our experience, the fabricated devices are mechanically stable and remain intact for extended periods—several months (at least) in our case—within optical setups and standard laboratory environments. Notably, one of our six-mode PL devices was shipped to Nokia Bell Labs (USA) and remained fully functional during subsequent testing.\nThe multimode PL (MM-PL) devices described in the manuscript feature wide waveguide structures with multiple supporting “legs”, which further enhance their structural stability and mechanical strength. Moreover, for product-oriented implementations, the PL can be encapsulated within a protective housing to protect against environmental factors such as dust, accidental contact, and other potential mechanical disturbances.\n4. Issues such as thermal effects, VCSEL array uniformity, and long-term device reliability are not addressed, leaving significant doubts about the practicality of the proposed approach.\nOur response: We thank the reviewer for this important comment. In our approach, incoherent beam combining is employed; therefore, phase stability among the VCSEL emitters is not a critical factor and no active thermal control was required or was employed. For this reason, thermal effects were not treated experimentally or in simulation.\nIt is important to note that the photonic lantern (PL) is a passive optical component, designed to efficiently multiplex a defined set of input modes into corresponding output modes. As long as these modes remain within the supported modal set, the PL operates effectively, and each VCSEL out of the incoherent source array, functions as an independent optical source without mutual interference within the lantern.\nWith respect to VCSEL output uniformity and thermal management, such considerations would indeed be addressed in a product implementation. However, the current study focuses primarily on the 3D integration novelty and design methodology fabrication and characterization of the PL–VCSEL interface, rather than on product engineering to customer specification.\nRegarding reliability, we can provide information only on the 3D-printed PL structures (see also our previous response). To clarify this for the reader, we have added the following text:\n“The 3D-printed PL structures are mechanically robust and remain stable over extended periods in standard laboratory conditions. For practical implementations, the device can be encapsulated to protect against dust, physical contact, and other environmental factors.”\nReviewer #2 (Remarks to the Author):\nThis manuscript reports the use of 3D nanolithography to fabricate free standing photonic lantern structures on top of an array of VCSELs for incoherent mode combination. In contrast to most other photonic lanterns, the photonic lanterns reported by the team adiabatically couple N multimode sources, each emitting M modes, to a multimode waveguide supporting an appropriate number of modes, such that the brightness etc of the VCSEL array is not degraded.\nThe authors also show that this device can be used to couple the VCSEL array to a multimode fibre with an efficiency significantly greater than the efficiency that could be achieved using a microlens array and a lens (see Fig. 6).\nOf course, the final goal of this work would be to scale the incoherent combination to arrays consisting of hundreds of VCSELs, but I do have doubts on the power handling capabilities of these 3D nanofabricated lanterns.\nDespite these concerns about the future real-world impact of the technology, which perhaps the authors could address in the revised manuscript, the paper is extremely well presented, interesting and technically impressive.\nI see no reason not to recommend publication essentially as is.\nOur response: We greatly appreciate the reviewer’s thoughtful comments and believe that the key messages and concepts of our work have been clearly conveyed in the manuscript. Regarding the concern about power handling, the reviewer raises an excellent point. Our current research focuses on the power regime ranging from milliwatts to a few watts. However, we are also pursuing a complementary research direction involving the fabrication of 3D devices (using the same Nanoscribe system) based on a silica-rich sol-gel material, which has proven to be significantly more robust for high-power applications compared to the current IP-S polymer.\nThis work was recently published in Advanced Optical Materials (“Silica-Rich Sol-Gel Ink for Two-Photon Direct Laser Writing of Microscale Structures and Optical Elements”) and includes 3D-printed photonic lanterns designed by our group. The newly developed silica-rich sol-gel ink (TPP-FSG) demonstrates exceptional optical power handling, far exceeding that of standard commercial photoresists:\n• Silica-rich sol-gel ink (TPP-FSG):\no Withstood continuous-wave (CW) laser irradiance above 7 × 10⁵ W·cm⁻² without structural deformation or optical loss.\no Showed no damage or mode distortion even at 3 W input power (1064 nm).\no Maintained >90% transmission and a refractive index close to fused silica.\n• Commercial photoresist (IP-S, Nanoscribe):\no Exhibited beam distortion and irreversible damage under similar irradiance levels.\no Became unstable at approximately 1 W input power.\no Demonstrated at least two orders of magnitude lower power tolerance than the silica-rich sol-gel ink.\nIndicates at least two orders of magnitude lower power tolerance than the silica-rich sol-gel ink.\nWe added the following to the conclusions section to emphasize this point :” For improved robustness and higher power handling capability, it is also possible to employ alternative inorganic materials such as silica-rich sol-gel, which has been shown to withstand significantly higher optical power levels compared to standard photoresists like IP-S [45].”\n[45] Bin Nun, M. et al. Silica-rich sol-gel ink for two-photon direct laser writing of microscale structures and optical elements.\nAdv. Opt. Mater. 12, 2400313 (2024)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I now agree that this study represents a nontrivial extension beyond prior single-mode photonic lantern demonstrations, and that the overall design framework is technically sound. However, while the underlying concept and device realization are convincing, additional experimental data are required to improve the completeness and robustness of the study.\n1. The authors should provide basic characteristics of the VCSELs used in this work. It would also be helpful to include complete L–I–V curves and near-field intensity patterns for representative emitters across the array (for example, emitters at the center and near the edge) measured at different injection currents, rather than limiting the characterization to a single VCSEL as shown in Supplementary Figure 3.These measurements would clarify how the modal content and unit-to-unit uniformity of the VCSEL array evolve with current, which is directly relevant to the assumptions used in the photonic lantern design. In addition, some information, like the source and type of the VCSEL chips should be provided.\n2. In the previous round of comments, I raised the issue of device reliability, which remains important for understanding long-term performance. During continuous operation, heat generated by the VCSELs could affect the coupling between the VCSEL array and the photonic lantern, leading to changes in insertion loss. One useful characterization would be to operate the VCSELs at a stabilized injection current and monitor the output power of the photonic lantern over an extended period, in order to quantify any performance drift over time.", "reviewer_2": "Despite the comments made by the other reviewer, which were justified to some extent, I believe the authors have addressed any remaining concerns and I am happy to support publication.\nThe paper is interesting, novel and will be of interest to the community."}, "author_response": "Response to the reviewer NCOMMS-25-56123B\nWe sincerely thank the reviewers for their constructive comments. In this second revision, we have addressed all feedback and updated the manuscript accordingly. A detailed response to each comment is provided below.\nReviewer #1 (Remarks to the Author):\nI now agree that this study represents a nontrivial extension beyond prior single- mode photonic lantern demonstrations, and that the overall design framework is technically sound. However, while the underlying concept and device realization are convincing, additional experimental data are required to improve the completeness and robustness of the study.\nOur response: Thank you for the comment. We are glad the novelty of our results are now clearer.\n1. The authors should provide basic characteristics of the VCSELs used in this work. It would also be helpful to include complete L–I–V curves and near- field intensity patterns for representative emitters across the array (for example, emitters at the center and near the edge) measured at different injection currents, rather than limiting the characterization to a single VCSEL as shown in Supplementary Figure 3.These measurements would clarify how the modal content and unit-to-unit uniformity of the VCSEL array evolve with current, which is directly relevant to the assumptions used in the photonic lantern design.\nOur response: We thank the reviewer for this valuable comment, which helps to further validate our conclusions. To address the near-field behavior, we measured the beam quality factor (𝑀2) of multiple neighboring VCSEL sources, providing insight into the beam quality and multimode characteristics of the emitters. This measurement is described in the Supplementary Materials, in the section “Multiple VCSEL Sources 𝑀2 Measurement”, and the results are shown in Fig. 1, consistent with those presented in the manuscript.\nIn addition, we measured the optical power emission of representative individual VCSEL sources as a function of drive current and compared the results for empty cavities and polymer-filled cavities, which is directly relevant to the 3D-printed structure configuration. These results are presented in the Supplementary Materials in the section “VCSEL Sources Optical Power Measurements” and are illustrated in Fig. 2. Key characteristics of the VCSEL chip, including physical dimensions, are also provided.\nFigure 1: M2 measurements of multiple beams.\nFigure 2: Optical power measurements of VCSEL sources.\nIn addition, some information, like the source and type of the VCSEL chips should be provided.\nOur response: VCSEL model (VIXAR V00132) is specified in both the manuscript and the Supplementary Materials, where additional details can be found. In this work, the VCSEL array underwent extensive modifications, including deep dicing to isolate sections of the array, custom packaging, and 3D printing (Detailed in the supplementary materials “VCSEL integration and 3D-printing process flow”. These processes significantly affect VCSEL performance; therefore, we performed a thorough in-house characterization of the sources rather than relying solely on vendor datasheet specifications.\n2. In the previous round of comments, I raised the issue of device reliability, which remains important for understanding long- term performance. During continuous operation, heat generated by the VCSELs could affect the coupling between the VCSEL array and the photonic lantern, leading to changes in insertion loss. One useful characterization would be to operate the VCSELs at a stabilized injection current and monitor the output power of the photonic lantern over an extended period, in order to quantify any performance drift over time.\nOur response: We thank the reviewer for this important point. We performed a long-term operation measurement of the photonic lantern (PL) device by monitoring the output power over several hours. The PL output power was measured at six different drive currents, with each current applied for 1 hour and 10 minutes, resulting in a total operation time of 7 hours. In addition, a temperature stabilization system was implemented to ensure stable VCSEL operation.\nThe results show no observable performance degradation over time and very low power fluctuations (below -50 dB). The corresponding analysis and results are presented in the section “Continuous Operation Measurement”, and the measurement data are shown in Fig. 2. This experiment provides an additional perspective on the system’s performance and long-term stability. Furthermore, we included the same characterization for the VCSEL array without the PL device in the supplementary materials, demonstrating comparable levels of power fluctuation.\nFigure 3: Figures containing the results from the manuscript.\nReviewer #2 (Remarks to the Author):\nOur response: We sincerely thank the reviewer for their positive assessment and support for publication. We are grateful for the recognition of the novelty and interest of our work to the community, and we appreciate the constructive feedback provided throughout the review process."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed my comments, and I recommend the manuscript for publication."}, "author_response": "Our response: We sincerely thank the reviewer for their positive assessment and support for publication. We are grateful for the recognition of the novelty and interest of our work to the community, and we appreciate the constructive feedback provided throughout the review process."}]

Decision letter