# Self-written high-efficiency single-mode optical link using a single near-infrared photopolymerization step

Nessim Jebali, Céline Molinaro, Julien Roul, Julie Jermann, Jean-Baptiste Doucet, Benjamin Reig, Serge Monneret, Olivier Haeberlé, Olivier Soppera, Véronique Bardinal

► To cite this version:

Nessim Jebali, Céline Molinaro, Julien Roul, Julie Jermann, Jean-Baptiste Doucet, et al. Self-written high-efficiency single-mode optical link using a single near-infrared photopolymerization step. Journal of Lightwave Technology, 2026, 44 (1), pp.194 - 200. 10.1109/JLT.2025.3628991. hal-05344409

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# Self-written high-efficiency single-mode optical link using a single near-infrared photopolymerization step

Nessim Jebali, Céline Molinaro, Julien Roul, Julie Jermann, Jean-Baptiste Doucet, Benjamin Reig, Serge Monneret, Olivier Haeberlé, Olivier Soppera and Véronique Bardinal

**Abstract**— We present a method for fabricating a self-written waveguide (SWW) between two optical fibers that are single-mode (SM) at 850 nm (780HP/ core diameter: 4.4 µm). The basic principle consists in exposing an acrylic photopolymer formulation sensitive in the near-infrared range (NIR) to a laser beam transmitted simultaneously from both fibers placed face to face, to build a continuous, flexible and self-aligned optical link. The specificity of the presented process (NIR-SM-SWW) lies in the use of a writing wavelength identical to that intended for single-mode propagation in the fibers. This enables the creation in a single step of a SWW directly adapted to the fundamental mode to be transmitted. A precise pre-positioning stage is used to optimize the process. For best photochemical conditions, a coupling efficiency as high as 82 % (-0.86 dB loss) is demonstrated for a 300 µm-long link. The effect of fiber-to-fiber axial and lateral distances is also investigated to estimate the propagation loss and misalignment tolerance, respectively. In addition, measurements performed by quantitative phase optical microscopy indicate a homogeneous index profile in the guide. Using these data, optical modeling is performed and compared to experiments, confirming that a high efficiency SM link is actually fabricated, without the need for further fabrication of an external cladding. This method could therefore be easily applied to the SM connection of a SM VCSEL (vertical-cavity surface-emitting laser) to a SM fiber, which is of major interest for the development of compact optical communications and instrumentation systems.

**Index Terms**— Near infrared photopolymerization, optical coupling, self-written waveguide, single mode fiber, self-alignment, optical interconnects.

## I. INTRODUCTION

Ensuring efficient connection between optical fibers and photonic devices, such as laser diodes or photodetectors, is crucial for the development of high-speed optical data transmission systems [1] and photonic integrated circuits on silicon [2][3][4] or lithium niobate platforms [5][6][7][8]. Minimizing coupling loss remains however a delicate task, due to disparities in the optical mode profiles of the various involved optical elements, and to low tolerances to initial misalignments, particularly in the case of single-mode components. To overcome these limitations, various methods

based on active or passive alignment strategies have been reported. Recently, in *situ* fabrication of 3D micro-optics by two-photon polymerization direct laser writing (2PP-DLW) has been proposed and successfully applied to several coupling configurations. Photonic wire bonding (PWB) has made it possible to realize continuous optical links from chip to chip, from fiber to chip, or from laser diode to chip [9]. Based also on the same 2PP-DLW technique, the fabrication of free-form microlens on each facet has been proposed to connect multimode optical fiber arrays to multimode VCSELs arrays or multimode fiber arrays to photodiode arrays, proving a faster process with relaxed tolerances [10]. However, the 2PP-DLW technique requires delicate positioning steps prior to writing and relies on a sequential process.

An alternative solution consists in exploiting one-photon polymerization in a photosensitive material placed between the elements to be coupled, leading to the creation of a continuous and self-aligned optical link. This self-written waveguide (SWW) approach was first proposed by S. Frisken et al. using a laser beam at 532 nm [11]. A large part of the published work since then concerns SWW fabrication using visible light between two standard fibers (SMF28, 8.2 µm core diameter) that are single mode at 1.55 µm [12][13]. Some works were also reported on SWW connections between optical elements having other core sizes, such as that of P. A. Mohammed et al. on the coupling of two fibers single mode at 980 nm (4.5 µm core size), using an incoherent visible light beam, leading to 3.5 dB optical loss measured at 1550 nm (for a fiber-to-fiber distance of 60 µm) [14]. H. Ozawa et al. reported the use of a green laser for SWW fabrication between a multimode fiber (50 µm core size) and a GaAs VCSEL diode at a distance of 500 µm, with resulting losses of 2.9 dB measured at the VCSEL’s operation wavelength (850 nm) [15]. The insertion losses are rather high in these works, as the writing wavelength is shorter than the desired operation wavelength, preventing from fabricating an optimal SM link [16]. Moreover, they rely on a unidirectional exposure, whereas bidirectional writing is known to be much more efficient to compensate initial misalignments, especially when the distance increases [17][18]. Consequently, the ideal configuration for fabricating a SM SWW is to expose the sensitive material from both sides and to use the same

This work was supported by Agence Nationale de la Recherche (ANR) (grant number: 20-CE24-0020 3D-BEAM-FLEX).

Corresponding author: Véronique Bardinal, LAAS-CNRS, Université de Toulouse, CNRS, 7 Avenue du Colonel Roche, 31400 Toulouse, France. (bardinal@laas.fr).

Nessim Jebali, Julien Roul, Jean-Baptiste Doucet, Benjamin Reig are with LAAS-CNRS, Université de Toulouse, CNRS, 7 Avenue du Colonel Roche, 31400 Toulouse, France.

Céline Molinaro, Julie Jermann and Olivier Soppera are with Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France Université de Strasbourg, F-67081 Strasbourg, France

Serge Monneret is with Aix Marseille Univ, CNRS, Centrale Med, Institut Fresnel, Marseille, France. Pr. Olivier Haeberlé is with Institut Recherche en Informatique, Mathématiques, Automatique et Signal (IRIMAS UR UHA 7499), Université de Haute-Alsace, IUT Mulhouse, 61 rue Albert Camus, 68093 Mulhouse Cedex, France.
wavelength at that the fibers and/or the optical devices are intended to be operated. In recent years, there has been renewed interest in NIR photopolymerizable systems, linked to the development of photopolymers, which are considered to be a green technology. The special properties of NIR also allow for deep polymerization or polymerization of filled materials thanks to the greater transparency of materials in NIR compared with visible or UV light. In this context, several research groups developed formulations suitable for a NIR-SWW process, whether at 760 nm [19] or 850 nm [20][21]. More recently, photopolymers sensitive beyond 1000 nm have been also proposed [22]. Kawasaki *et al.* reported NIR-SWW fabrication between two SMF-28 fibers distant from 100 µm with very low coupling loss (0.5 dB), but not measured at the writing wavelength (1310 nm) [23]. Based on the same material, a 1310 nm VCSEL-to-fiber connection has also been reported with no mention of the coupling efficiency [13]. Finally, it can be noted that in many reported works, the addition of a cladding around the SWW is often necessary, requiring a second polymerization step, either in a second material or in the same one that must include in this case two monomers [23][24].

In this work, we developed a NIR formulation that is sensitive at 850 nm and suitable for writing a SWW between two fibers single mode at 850 nm (780HP/ 4.4 µm core size) in a single step. To our knowledge, this is the first time a SWW between two fibers of such small optical core size is fabricated using the same wavelength as that targeted for single-mode propagation in the fibers. SWW fabrication was reported at 853 nm, but this was between two largely multimode optical fibers (50 GIF, 50 µm core size)[21]. The properties of our SWW are directly adapted to SM propagation at 850 nm with low loss and it is not necessary to fabricate an additional cladding. Our process will therefore be transferable to the fabrication of a SM link between a fiber and an active device having a fixed operating wavelength such as a VCSEL. In this article, we first describe the NIR sensitive photopolymer developed for writing waveguides having good opto-mechanical properties and with an optical power compatible with that typically emitted by a VCSEL device. We then present the optimization of the SWW process in SM fiber-to-fiber coupling configuration. We discuss in particular the influence of the writing dose on the waveguide diameter and coupling efficiency. Measurements led to determine the linear propagation loss, the alignment tolerance and the refractive index profile in the SWW are then presented. Exploiting these results and comparing experimental and theoretical coupling curves, we show that for optimized conditions, the SWW optical link is actually single mode at the working wavelength with no need of an external cladding.

## II. NIR PHOTOPOLYMER DEVELOPMENT

The photopolymer formulation suitable for the versatile manufacture of self-writing waveguides must meet the following requirements (i) high absorption at the source emission wavelength so that the reaction can be initiated within the available power range. The target power is typically 100 µW, easily achievable with NIR laser diodes. At the same time, this power must be at least 1.5 times higher than the

typical operating threshold of VCSELs (typically 50 µW) in order to be in a region of least divergence variation and to ensure better control of the diameter of the photo-inscribed waveguide, (ii) high efficiency: The mechanism for generating reactive species (in our case radicals) from excited states must be optimal to guarantee operation in this power range, (iii) photobleaching: the formed waveguide is intended to be used at the same wavelength as the writing, which requires the material to be perfectly transparent once polymerized to limit absorption losses, (iv) homogeneous material, smooth surface: this last point is important to limit scattering losses and allow good coupling. We also estimated that it was necessary for the polymer to adhere to the facets of the fibers and for the material to be flexible to ensure reliable, reproducible fabrication of the guides without breakage during characterization. These conditions will be necessary for practical applications.

<figure><img src="image_2.png" /><figcaption>**Fig.1:** Chemical formulae of the compounds used in this study: a) monomer, b) photoinitiator, c) iodonium salt d) absorption spectra of the formulation before and after photobleaching.</figcaption></figure>

To meet these requirements, a photopolymer formulation was developed and optimized on the basis of previous work [25] [26][27][28][29]. To obtain the conditions presented in (i) (ii) and (iii), different photoinitiator systems were evaluated. A Norrish II-type photoinitiator system has been proposed (Fig.1). It consists of a carbocyanine dye, IR140, combined with a co-initiator, N-phenylglycine (NPG), which generates radicals under NIR irradiation. As will be shown later, the addition of an iodonium salt greatly improves the efficiency of the photopolymerization reaction, allowing it to be carried out in the desired power range. The chosen monomer is Ebecryl 605 (E605), a commercial blend of two monomers shown in Fig. 1a. Acrylate functions are known to be reactive to radical
mechanisms. After polymerization, this monomer imparts
properties of adhesion to the surface of the optical fiber and
flexibility compatible with the intended application. This
choice of monomer meets criterion (iv) and the optical
properties of acrylates are well known [30][31][32][33]. With
this photopolymer and under the irradiation conditions chosen,
it should be noted that we did not observe any undesired micro
or nano-structuring phenomena with negative consequences on
the optical properties. The appearance of bubbles or
filamentation phenomena [34], which are known to cause
diffusion losses in the polymerized material if these defects are
present, were not observed. This underlines the interest of our
formulation for the intended application.

Fig.1d illustrates an important property of this formulation. It shows the absorption spectra of the formulation before and after NIR irradiation. The spectra have been recorded using a Lambda 950 spectrophotometer (Perkin-Elmer) by acquiring the transmission spectra of a drop of formulation sandwiched between two glass slides before and after NIR irradiation. It can be seen that the NIR dye (IR140) has an absorption maximum that allows polymerization to take place at around 800 nm. In particular, after irradiation we can see that the photoinitiator system has photobleached, with low absorption in the 400-900 nm spectral window. This characterization makes it a good candidate for a material for NIR optics. In addition, this property makes it possible to produce a long guide (several hundreds of µm) by front-end polymerization.

Fig.2 illustrates the role of the other two compounds, the
iodonium salt and the co-initiator (NPG). To illustrate their role,
polymerization tests are carried out at the end of an optical fiber
after deposition of a drop of formulation, as in the studies
[20][35]. It should be noted that, unlike the results presented in
section III, these experiments were carried out at the end of an
optical fiber single mode at 850 nm (780HP). Fig.2a
exemplifies the typical obtained results: images framed in red
correspond to conditions where no polymerization is observed;
in yellow, polymerization has taken place but the microtip is
broken during rinsing step; in green, a tip is clearly visible,
corresponding to effective polymerization. Fig.2b confirms, as
expected, that NPG is required to initiate polymerization of the
acrylate monomer from the excited states of IR140.
Concentrations of 1 and 2wt% correspond to efficient
polymerization, even for the shortest polymerization times. An
interesting feature, already observed for this dye under other
conditions, is the loss of polymerization efficiency at the
highest NPG concentrations. This behavior has been explained
by the phenomenon of annihilation between reactive species
[35] and shows that an optimum concentration range should be
used. Fig.2c shows the important role of the iodonium salt. It
significantly reduces the dose required to initiate
polymerization. In the absence of iodonium salt, it was not
possible to obtain polymerization at 0.1 mW, even with 50 s of
irradiation, whereas polymerization was effective at 5 s with
1 wt% iodonium salt. By adjusting these two parameters it is
therefore possible to adjust the photosensitivity of the
formulation. Finally, the reference formulation used in the
following section was selected with 0.2 wt% IR140, 2 wt%

NPG, 1 wt% iodonium in E605.

<figure><img src="image_3.png" /><figcaption>**Fig.2:** Demonstration of the role of NPG and iodonium salt on the photopolymerization process, using a polymerization configuration at the output of an optical fiber (0.1 mW, time varying between 1 and 50 s). a) Code for the frame colors: images framed in red correspond to conditions where no polymerization is observed; in yellow, polymerization has taken place but the microtip is broken during rinsing step; in green, a tip is clearly visible, corresponding to effective polymerization. b) Role of NPG, with 0.2 wt% IR140 and 2 wt% iodonium in E605. c) Role of iodonium, with 0.2 wt% IR140 and 4 wt% NPG in E605.</figcaption></figure>

III. FIBER-TO-FIBER COUPLING EXPERIMENTS

1) Experimental set-up

The experimental set-up used for the SM fibers coupling tests is shown in Fig.3. A single-mode laser beam emitting at 852 nm is propagated via a 2X2 coupler towards the two 780HP fibers placed face to face. A CCD camera and an ultra-zoom lens are used to control the distance between the fibers before fabrication, as well as the shape and dimensions of the SWW after development. Fiber 1 is mounted on a positioning stage with 6 degrees of freedom in x,y,z directions and 3 angles θx, θy, θz around x,y,z (Hexapod PI H-811.I2). This equipment allows displacements with a minimum accuracy of 200 nm along x and y and 80 nm along z, as well as angle control of 2.5 µrad along θx and θy and 5 µrad along θz. First, an automatic X-Y scan of fiber 1 position relatively to fiber 2 is performed to optimize the transmitted power through Port 4. After coupling optimization in air (Fig.3a), the formulation is inserted (Fig.3b) and the link is written by NIR irradiation from both sides
(Fig.3c). The coupled signal is observed in real time via Port 2. The SWW is then carefully rinsed in ethanol to remove the unpolymerized formulation. The result is a flexible polymer waveguide "optically welded" to the fiber facets (Fig.3d).

<figure><img src="image_4.png" /><figcaption>**Fig.3:** (Top) Schematic of the experimental set-up used to for fiber-fiber coupling experiments. (Bottom) Description of the steps of the NIR-SM-SWW process (images a to d).</figcaption></figure>

## 2) Optimization of photochemical conditions

Photofabrication conditions were optimized by varying the NIR exposure dose. Fig.4 shows the evolution of the guide diameter and the corresponding experimental coupling efficiency as a function of the exposure dose for a 300 µm fiber to fiber distance. As expected, the guide diameter was found to increase with the dose. High coupling efficiencies (>70%) were measured for writing doses comprised between 6 mJ and 15.5 mJ, corresponding to SWW diameters in the [5.8 µm-10.3 µm] range, which is higher than that of the core size of the fibers (4.4 µm). At high writing doses, coupling efficiency decreased, probably due to the guide diameter being too large to still efficiently ensure mode confinement. The best value was obtained for a dose of 12 mJ and a 7 µm diameter, with a maximal coupling efficiency of 82 % (0.86 dB), well above the state of the art [14]. It is 35 times higher than the coupling measured in the air at the same distance (2.3 %). A typical image acquired by scanning electron microscopy (SEM) of a SWW observed after being separated from one of the fibers is shown in Fig.5. It is worth noting that for diameters equal or higher than 7 µm, we observed that the coupling efficiency was increasing by only 1 or 2 % after rinsing step. Moreover, when adding a transparent liquid monomer around the SWW by way of a cladding, we observed no significant change in the coupling value. This indicates such waveguides are complete and do not need the addition of a cladding after the NIR writing and development steps.

<figure><img src="image_5.png" /><figcaption>**Fig.4:** Evolution of experimental coupling efficiency (black circles/dashed line) and SWW diameter (red squares/solid line) as a function of the NIR writing dose (fiber-to-fiber distance: 300 µm).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig.5:** Scanning electron microscope image of a typical SWW after separation from the second fiber. This guide has an average diameter of 10 µm.</figcaption></figure>

## 3) Coupling results vs fiber-to-fiber distance

Series of SWW links were fabricated between 780HP fibers using the above-described process under best conditions and for fiber distances ranging from 100 µm to 600 µm and the coupling efficiency η was measured at 850 nm in different configurations: air gap (as illustrated in Fig.3a), liquid monomer layer inserted in the gap (as in Fig.3b), and SWW after development (as in Fig.3d). This enabled us to calculate the evolution of the coupling enhancement factor brought by the SWW as a function of the fiber-to-fiber distance (Fig.6). We observed a very significant coupling gain enhancement with the distance. For 500 µm as an example, it is indeed 74 times higher compared to air, and 33 times higher compared to the monomer case. For comparison, the gain factor brought by the monomer insertion compared to the air gap is only 2.2, with no significant change is observed with the fiber-to-fiber distance. These results clearly show that an efficient waveguide is fabricated between the two single-mode fibers with the SWW. We also used these measurements made at several distances to estimate the linear propagation loss regardless of any other constant transmission loss. We found an average value of ~8.6 dB/cm. This can be accounted to residual absorption or scattering in the material and could be reduced by further optimizing the NIR resin composition. In particular, the concentration of NIR dye could be optimized to further reduce the residual losses by
absorption. Scattering losses due to non-well dissolved compounds could also account to the losses and could be also decreased after their identification. Nevertheless, this corresponds to only a loss of 10 % in transmission for the considered length range (500 µm).

<figure><img src="image_7.png" /></figure>

**Fig.6:** Factor of improvement of the coupling efficiency η brought by the SWW as a function of the fiber-to-fiber distance with respect to two cases with no SWW: an air gap (black squares) and a homogeneous layer of monomer (red circles). For comparison, the much lower improvement factor brought by the insertion of the monomer with respect to air is also plotted (blue triangles).

### 4) Coupling results vs lateral misalignment

Taking advantage of the high accuracy of our positioning stage, we also studied the variation of the coupling loss as a function of lateral misalignment between two 780HP fibers distant of 100 µm. Lateral misalignment was fixed before SWW fabrication by intentionally shifting fiber 1 with a step of 2 µm along the X or Y axis from the position corresponding to optimal coupling. As shown in Fig.7, for a lateral misalignment of ± 2 µm, the penalty loss remains lower than 1.7 dB, corresponding to a coupling efficiency higher than 67 %, which is remarkable considering the small size of the fibers' cores (4.4 µm). This is due to compensation effects occurring during bidirectional writing [17][18].

<figure><img src="image_8.png" /></figure>

**Fig.7:** 780HP fiber-to-fiber coupling loss measured as a function of the initial lateral misalignment (fiber-to-fiber distance 100 µm). Black triangles: air / blue circles: monomer layer/ red squares: SWW.

## IV. OPTICAL CHARACTERIZATION AND MODELING

### 1) Index profile measurement

We characterized the opto-geometric parameters of the guide by quantitative phase imaging using wavefront analysis [36], which gives an OPD (Optical Path Difference) map, i.e., the value (guide index - index of surrounding medium) x guide thickness at any point on the image. Knowing the refractive index of the microscope immersion oil (index of 1.518) used as the surrounding medium, this method can be used to find the local optical thickness of the guide. Fig.8 shows the OPD map of the guide (left) and a transverse OPD profile taken along the segment drawn on the map (right). If we try to optimize this profile according to a guide model that potentially has a radial distribution of the refractive index given by $n(r) = n_0 (1 - \Delta r/R)$, where r is the radial coordinate, R the local radius of the guide and $n_0$ its nuclear index, we obtain a value of $\Delta$ of 0.000 and a value of $n_0$ of 1.553. We can therefore assume that our self-written waveguide has a homogeneous profile and a constant refractive index equal to 1.553 at 850 nm.

<figure><img src="image_9.png" /></figure>

**Fig.8:** a) OPD map of the self-written waveguide b) experimental transverse profile of the OPD along the segment plotted on (a), in red, and profile model in blue based on a radial index distribution given by $n(r) = 1.553 (1 - \Delta r/R)$ with $\Delta = 0.000$ (magnification x150).

### 2) Analysis of the SWW modal behavior

To analyze the modal behavior of the 780HP-SWW-780HP link, we used an optical modeling tool (RP Fiber Calculator Pro) [37]. With this model, it is possible to calculate the number of propagated modes, the integral of overlapping modes and the expected final transmission ratio. We took into account the geometric and optical properties of the fibers and our experimental data acquired on the SWWs to calculate the theoretical transmission of all the modes excited in the SWW as a function of the guide diameter (Fig.9). Calculations have shown that only the LP01 and LP02 modes are excited in the SWW for the range of diameters considered. Other modes are negligible. The total transmission is plotted as a function of the SWW diameter in Fig. 10 (black open circles and dashed line). As seen, the maximum coupling efficiency is obtained for a diameter of 8 µm (92.3 %). The loss can be attributed to the differences in mode diameters and refractive indices of the fibers and SWW, resulting in imperfect modes overlap and Fresnel reflections, respectively.
<figure><img src="image_10.png" /><figcaption>**Fig.9:** Description of the simulated 780HP-SWW-780HP link and list of the modes that can be excited in each part. The transmissions calculated at 850 nm as a function of the SWW diameter are also indicated.</figcaption></figure>

Additional loss can come from the residual absorption in the SWW material. For comparison purposes, the coupling values measured on a series of 300 µm-long SWWs fabricated with various diameters are plotted in Fig.10. As seen, a similar behavior is observed. The maximum coupling efficiency is measured for diameters equal to 7 ± 0.5 µm, i.e. close to the optimal values given by the modeling (7.5 ± 0.5 µm). Taking into account the propagation loss measured for the considered distance (see section III.3), a quite good agreement between the theoretical and experimental values is found. We can conclude from this study that the SWW links with diameters from 6 µm to 10 µm are the best suited for getting a high efficiency single mode transmission at 850 nm.

<figure><img src="image_11.png" /><figcaption>**Fig.10:** Comparison of experimental and theoretical coupling efficiencies of the 780HP-SWW-780HP link as a function of SWW diameter. Red solid squares represent the experimental results. The dashed lines show the theoretical results (black open circles: total theoretical transmission, blue open triangles: total theoretical transmission taking into account the propagation loss in the SWW extracted from the experimental data of Fig.6 for the considered distance (300 µm).</figcaption></figure>

Finally, we calculated the total transmission of the SWW in case it is surrounded by a cladding, such as the one formed by unexposed formulation before the rinsing step (n=1.48) (Fig.11) and compared it to the curve obtained in the case of the SWW alone (air cladding, n=1). As seen, the simulation

confirms that if the diameter is equal to or greater than 7 µm, adding a cladding around the SWW does not improve coupling efficiency. This indicates that targeting a SWW diameter in the range [7-10 µm] corresponds to the best trade-off, as this not only leads to an efficient SM propagation at 850 nm, but also eliminates the need for further cladding fabrication.

<figure><img src="image_12.png" /><figcaption>**Fig.11:** Total theoretical transmission of the 780HP-SWW-780HP link calculated as a function of the waveguide diameter (without propagation loss) for a SWW with an infinite air cladding (n=1.0 /black open circles/solid line) or an infinite cladding with an index n=1.48 (red solid squares/dashed line).</figcaption></figure>

CONCLUSION

In conclusion, we have developed and optimized a NIR photopolymer formulation to achieve self-writing of a high-efficiency SM link between two 850 nm single-mode optical fibers. The development of this new NIR-SM-SWW process was made possible by the use of automated positioning equipment that guarantees a pre-alignment of the fibers with an accuracy better than 0.2 µm before writing. Best coupling results were obtained for SWWs having a diameter ranging from 6.5 to 10 µm, i.e., much larger than the fibers core (4.4 µm). A coupling efficiency as high as 82% could be measured for a distance of 300 µm, which corresponds to 0.86 dB loss and to an improvement factor of x35 compared with propagation in air and x16 compared with propagation in a monomer layer. This factor was found to increase sharply with the distance between the fibers, reaching x74 with respect to air case and x33 to monomer for 500 µm, clearly demonstrating that an effective guide was indeed fabricated. The linear propagation loss was also determined from this study (8.6 dB/cm). The process showed also a good tolerance to initial misalignments, with coupling efficiencies higher than 67 % for applied lateral offset of ± 2 µm. In addition, using quantitative phase microscopy, we found that the SWW had a homogeneous index profile, with a high refractive index of 1.553. These data were exploited to model the link transmission as a function of the guide diameter. The experiments were found to be in good agreement with the theoretical behavior, confirming the optimal performances of the created single mode connection were reached for SWW diameters between 7 µm and 10 µm and without the need for adding a cladding after NIR writing. Future work will consist in further reducing the propagation loss by adjusting the composition of the NIR resist and in simulating the optical field distribution in the link
to better explain our results. The NIR-SM-SWW process developed in this work makes it possible to achieve a compact link between two SM fibers without free-space propagation or the need for an active alignment during fabrication. By controlling the composition of the photopolymer, it is also possible to include an active material between the two fibers for optical pumping or sensing applications. This type of polymer-based junction can be also used to create couplers with a high degree of flexibility. Finally, this single step and low-cost process can be applied to the coupling of a SM VCSEL to a SM fiber using the same wavelength for NIR writing than that of the VCSEL, and thus fully exploiting the advantages of a bidirectional exposure.

## ACKNOWLEDGMENT

This work was supported by Agence Nationale de la Recherche (ANR) (grant number: 20-CE24-0020 3D-BEAM-FLEX), Université de Haute-Alsace (Grant Imothep) and LAAS-CNRS micro and nanotechnologies platform, member of the French RENATECH network.

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