Double-clad fibers (DCF) have many advantages in fibered confocal microscopes as they allow for coherent illumination through their core and partially coherent detection through their inner cladding. We report a double-clad fiber coupler (DCFC) made from small inner cladding DCF that preserves optical sectioning in confocal microscopy while increasing collection efficiency and reducing coherent effects. Due to the small inner cladding, previously demonstrated fabrication methods could not be translated to this coupler’s fabrication. To make such a coupler possible, we introduce in this article three new design concepts. The resulting DCFC fabricated using two custom fibers and a modified fusion-tapering technique achieves high multimodal extraction (≥70 %) and high single mode transmission (≥80 %). Its application to reflectance confocal microscopy showed a 30-fold increase in detected signal intensity, a 4-fold speckle contrast reduction with a penalty in axial resolution of a factor 2. This coupler paves the way towards more efficient confocal microscopes for clinical applications.
© 2015 Optical Society of America
Partially coherent detection presents advantages in imaging such as improvement in signal collection efficiency and reduction of coherence effects . It may be achieved through coherent illumination of a sample combined to a less spatially coherent detection scheme [2,3]. Systems exploiting partially coherent detection can be fiber-based using a single mode fiber for coherent illumination and a multimode fiber for the detection of several spatial modes lowering the spatial coherence of the detected signal.
Confocal microscopy (CM) is an example of an imaging system exploiting partial coherence. In bench-top implementations, it was shown that using a larger detection pinhole [4, 5] yields images with higher contrast, lower speckle noise with a minimal penalty in optical sectioning. Fiber-based confocal microscopes have the potential to translate the benefits of confocal microscopy to clinical applications such as dermatology [6, 7] and ophthalmology [8–10]. Using fiber bundles or scanned single fibers, confocal microscopy may be incorporated into endoscopes for gastroenterology [11–15], laryngology  and pneumology [17, 18] applications.
Single-fiber confocal microscopes may benefit from partially coherent detection when replacing the single mode fiber with a double-clad fiber (DCF) [19, 20]. DCFs provide both coherent and partially coherent guiding regions through concentric areas having different refractive indices: a single mode core (for coherent illumination), an inner cladding (for partially coherent detection) and an outer cladding.
In free-space confocal microscopy, the ideal pinhole size for biological sample imaging was found to be approximately vp = 5 in normalized coordinates [4,5]. The ideal detection pinhole is thus larger than the illumination pinhole, resulting in an increase of detected signal and lowered speckle contrast with a moderate penalty in resolution. This dual-pinhole configuration can be adapted to fiber-based confocal microscopy with a double-clad fiber with the appropriate geometry. This optimal pinhole size of vp = 5 translates to an inner-cladding diameter to mode-field diameter (MFD) ratio of ≈ 5 , the core serving as the illumination pinhole and the inner cladding serving as the detection pinhole. The first obstacle to integration of DCFs to confocal microscopes and endomicroscopes is the limited availability of fibers with these optimal dimensions, i.e. a ratio of 5 between the inner cladding diameter and the MFD.
The second obstacle is the separation of the coherent signal (propagating in the core) and the partially coherent signal (propagating in the inner cladding). Double-clad fiber couplers (DCFCs) are devices designed to separate such signals and were recently demonstrated in the context of endoscopy and optical coherence tomography coupled to fluorescence [22, 23]. Previous DCFCs were however fabricated with DCFs operating at 1300 nm with ratios of inner cladding to MFD greater than 10, which is not suitable for confocal microscopy. DCFs appropriate for confocal microscopy (inner cladding to MFD ratio smaller or equal to 5) will be designated throughout this paper as small-ratio DCFs.
To the best of our knowledge, no DCFC based on small-ratio DCFs has been demonstrated. In addition to the unavailability of small-ratio DCFs, the main hurdle for signal separation within the DCFC is the relative dimensions of the cladding areas. In the context of large-ratio DCFs, the multimode signal is easily accessible through a small and manageable outer cladding (typically 10 to 15 microns thick). Conventional fabrication techniques can be applied to yield good performances through a repeatable fabrication process. With small-ratio DCFs, the outer cladding is a much greater obstacle to overcome, as it ranges from 30 to 50 microns in thickness surrounding the inner cladding. Current DCFC fabrication techniques including twisting , polishing  or fusion-tapering [26–28] cannot be translated to small-ratio DCFCs (sDCFCs) as this would result in coupling a large fraction of light to the outer cladding modes. Theses modes are lost at the output of the sDCFC, thus resulting in a component with very high losses.
We introduce here three new design concepts to enable the fabrication of an sDCFC: an adiabatic single mode capture, a complete multimode expulsion and a double asymmetry in the coupler’s structure. Single mode capture is defined as the adiabatic transition in a tapered fiber structure allowing for the surrounding inner cladding of the fiber to behave as a single mode waveguide under specific conditions. Complete multimode expulsion is defined as the migration of all supported modes of a specific guiding area in the fiber to a surrounding guiding area. The last concept, double asymmetry, is linked to the fabrication process in which the tapered DCF is longitudinally displaced (longitudinal asymmetry) prior to fusion with a fiber having a different diameter (transverse asymmetry)  to form a doubly-asymmetric structure.
In this paper, we report the first DCFC made from small-ratio DCF, which also exploits a novel fabrication technique specifically tailored to sDCFC. We describe the underlying coupling mechanisms at play for efficient separation of illumination and sample signals within the sDCFC; namely the adiabatic single mode capture and the complete multimode expulsion. We then present designs for the custom fibers and for the sDCFC, a description of the fabrication process including double asymmetry and a characterization of the coupler performances both in single- and multimode regimes. To further demonstrate the new capabilities of the device, we inserted the sDCFC into a laser scanning confocal microscope operating at 785nm and compared its imaging performances to that of standard fiber coupler. Imaging performances were tested on a resolution target as well as on a freshly excised piece of biological tissue.
2. Small-ratio double-clad fiber coupler
The small-ratio double-clad fiber coupler is an all-fiber four-port device built with two branches on each side of the coupler. In the context of CM with partially coherent detection, only three ports are used. Figure 1 shows a schematic diagram of the sDCFC. Excitation light is launched in the core of the DCF at Port 1 through a splice with a mode-matched single mode fiber. This allows for coherent illumination of a sample through dedicated optics placed at Port 2. Backscattered light from the sample is collected by the core and by the inner cladding of the DCF at Port 2. Core signal returns quasi-losslessly to Port 1 whereas the partially coherent signal propagating in the inner cladding is redirected through the multimode fiber (MMF) to Port 3. For ideal illumination and imaging performances, an sDCFC should allow for quasi-lossless transmission of core light from Port 1 to Port 2 (and back) while transferring all inner cladding light from Port 2 to Port 3.
Two fibers, an sDCF and an MMF, were designed to meet the stringent requirements imposed by CM. The sDCF’s single mode core (diameter 4.5 μm, numerical aperture NA=0.12, cutoff wavelength 610 nm) permits efficient coherent illumination from 610 nm to 1000 nm. In addition, the core is mode-matched (MFD ≈ 5 μm) with a commercially available single mode fiber at 785 nm (780-HP, Nufern, USA). The sDCF’s inner cladding has a diameter of 26 μm, to embody the condition on the optimal inner cladding of ≈ 5 × MFD. At this wavelength, the inner cladding supports approximately 150 modes. The multimode fiber used in the coupler is a highly multimode coreless pure silica fiber (diameter 125 μm, 0.5 NA), which matches the refractive index of the DCF’s outer cladding. Guiding within the coreless fiber relies on a fluoroacrylate cladding, also serving as a protective jacket. This silica-based fiber has a standard outer diameter of 125 μm.
Both fibers were custom-drawn at Université Laval (Canada) and validated according to our specific fiber design. The combination of these two specific fibers allows for adiabatic single mode capture and multimode expulsion. The coupler is fabricated using the fusion-tapering technique described in [26, 28] with modifications to accomodate the new design concepts.
2.2. Modal propagation through the coupler
Figure 2(a) is a schematic diagram of the resulting DCF taper composed of two transition sections (TS) and one constant section (CS). Representations of the fundamental (red) and inner cladding (blue) modes are also shown in the unmodified DCF section and in the CS. Figure 2(b) plots the evolution of the modes’ effective indices (neff) in the intact DCF and in the constant section of the taper. The various modes can either be guided by or radiated out of the structure, each mode having its own effective refractive index. The only guided mode within the core is the fundamental mode with its neff between the refractive index of the core nco and that of the inner cladding nic. The major part of its electric field is confined within the core. An inner cladding mode is a mode whose neff is comprised between nic and noc, the refractive index of the outer cladding. This type of guided mode has an electric field mostly confined within the inner cladding area. Radiated modes have an neff ≤ nair and are not detected after a short propagation length since they propagate mostly in the jacket or in air.
In order to achieve adiabatic single mode capture, the slopes of the tapered region were chosen to provide an adiabatic transition of the fundamental mode and a short taper length [29, 30]. A structure is said to be adiabatic if the coupling between the fundamental mode and the higher order modes is negligible . The geometry of the single mode core along the structure is shown in red whereas the inner cladding is shown in blue. On the left hand of Fig. 2(a), representative power distributions in both areas are shown as they enter the taper. The fundamental mode adapts to the structural change as it travels down the taper until the structure reaches approximately 40 % of its initial diameter. At this point, the fundamental mode diffracts out of the core and spreads into the inner cladding. In the CS, it becomes an inner cladding mode. The reduction in fiber dimensions is chosen such that the inner cladding behaves as a single mode waveguide, thus enabling adiabatic single mode capture. In this configuration, the fundamental mode is confined inside the DCF along the whole structure. As the fundamental mode propagates into the second transition section, it gradually transforms back into a core mode, since an adiabatic transition is a reversible process. As the core increases in size in the second TS, the fundamental mode couples back into the core.
The taper also allows for the multimode expulsion. The structure changes inner cladding modes into outer cladding modes, as their neff in CS are comprised between noc and nair. When launched in the structure, the modes are confined in the inner cladding. As they progress in the transition section (TS), the modes gradually leak to the outer cladding. The cladding modes are guided by the outer cladding-air interface. This means that all cladding modes extend to the interface and may radiate out or couple into another fiber, achieving complete multimode expulsion.
The MMF is a coreless fiber with a refractive index equal to or higher than that of the DCF’s outer cladding. This way, the coupler can efficiently extract the multimode signal and fully benefit from the complete multimode expulsion. At the same time, it leaves the core signal (in the fundamental mode) quasi-unperturbed, achieving adiabatic single mode capture. The third design concept, the double asymmetry, is implemented by fusing the two fibers in one of the TS, as seen on Fig. 1, which enables efficient extraction of the MM signal. The second transition section being unfused, the signal remains in the MMF. Therefore, the device functions as designed: it transmits the single mode signal in the DCF throughout the coupler and it extracts the multimode signal in the inner cladding of the DCF into a second fiber for detection.
2.3. Model for predicting performances
A theoretical prediction of the power transfer in DCFCs was suggested in , where the etendue ratio of the two regions guiding the cladding modes of the fibers dictates the multimode signal extraction. Since optical etendue, which can be expressed according to the number of supported modes, is a metric that can only grow in a system, similar to entropy, the coupling process tends to send more power in the fiber with a higher etendue. Therefore, we prefer a combination of fibers that support a very different number of modes. With this specific fiber combination, a theoretical multimode power transfer of 98 % is predicted.
2.4. Fabrication and characterization
The fusion-tapering technique is modified to implement a double-asymmetry in the coupler. A first asymmetry is introduced in the transverse cross-sections to enable a high multimode signal extraction. The second asymmetry is introduced in the longitudinal axis of the coupler, as the fibers are fused in the transition section and not symmetrically distributed with respect to the middle of the constant section. The new fabrication technique requires to pre-taper the DCF prior to the fusion step. In order to handle the fragile tapered structure and bring into contact the two fibers of the device, the taper is twisted around the MMF before the fusion step.
A 2 m segment of sDCF is spliced at both ends with single mode fiber (780-HP, Nufern, USA) and then connected to a laser diode and optical spectrum analyzer (OSA, AQ6317, Ando Electric, Japan) to monitor single mode response during fabrication. The DCF is stripped cleaned of its protective polymer jacket in the middle portion of the DCF over a 30 mm span and cleaned with acetone. The fiber is then fixed mechanically on a custom-made fusion-tapering setup. A micro-torch is used to heat up the fiber to temperatures around 1550 °C and is fuelled with propane and oxygen.
For the tapering part of the process, the micro-torch heats the DCF while traveling steadily over 8 mm. The heating length has been selected to ensure that the adiabaticity criterion is respected for the fundamental mode along the whole structure. The motors pull on the fibers at a speed of 0.1 mm s−1. The final size of the structure is determined by the tapering time. The DCF is tapered down until an outside diameter of 20 μm is reached. The single mode transmission is not measurably degraded after tapering,which confirms that the adiabatic single mode capture is achieved.
A fiber segment of coreless fiber is stripped over 60 mm and cleaned. The MMF is then installed on the fusion-tapering setup next to the tapered DCF. Both fibers are placed in close contact mechanically onto blocks and the twist is manually performed. Both fibers are then fused in the TS and CS until they coalesce into a nearly circular cross-section. The micro-torch travels along the fibers to heat the structure over a length of 4 mm to 8 mm. The final single mode transmission spectrum is inspected and low losses are measured. The sDCFC is finally packaged in a stainless tube and sealed at both ends.
Further characterization is performed as described previously . Single mode transmission is characterized from Port 1 to Port 2 and from Port 2 to Port 1. Multimode transmission is characterized from Port 2 to Port 3. The single mode and multimode responses are shown in Fig. 3. Multimode characterization is performed over the 600 nm to 800 nm range with a broadband source, a tunable narrow-band filter and a calibrated powermeter. The single mode performance is characterized with a single mode broadband source and an optical spectrum analyzer. Single mode transmission is ≥80 % over the entire spectral band. Multimode transmission is 70 % at 600 nm and reaches 80 % at 800 nm.
3. Application to confocal microscopy
In this section, we present results from confocal imaging using the sDCFC. Imaging performances of this coupler were characterized using a standard resolution target as well as freshly excised swine tissues representative of turbid biological samples. Images were also compared to that obtained with a similar setup using single mode fiber coupler.
3.1. Imaging setup
Figure 4 shows a schematic diagram of the sDCFC-based laser scanning confocal microscopy setup. A fiber pigtailed laser diode (LP785-SF100, 785 nm, 100 mW, Thorlabs, USA) is spliced to Port 1 of the sDCFC. Light propagates into the core of the DCF and is launched into the imaging system at Port 2 of the sDCFC. Light from Port 2 is collimated (CFC-11-X-B, Thorlabs, USA) and scanned on the sample with dual-axis galvanometer-mounted mirrors (GVSM002, Thorlabs, USA). The beam is expanded (3× magnification) with a telecentric telescope and focused on the sample with a microscope objective (LUMFLN 60XW, NA= 1, water, Olympus, Japan). Light backscattered by the sample couples partly into the core of the DCF and partly into the inner cladding. The inner cladding of the sDCFC acts as a detection spatial filter, thus removing the need for a pinhole in the setup.
The multimode signal is sent to Port 3 of the sDCFC and is detected with a photodiode (PDA36A, Thorlabs, USA). The unused branch of the coupler (Port 4) is connected to a beam dump to prevent noise coming from back reflections. A single mode power splitter (FC780-50B-APC, 50:50, Thorlabs, USA) can replace the sDCFC in the setup for fully coherent imaging in reflectance for comparison with the sDCFC.
Imaging performances (resolution, signal intensity, speckle contrast) were characterized in partially coherent detection (PCD) with the sDCFC and in coherent detection (CD) by replacing the sDCFC by a single mode power splitter. Resolutions were measured with a USAF1951 resolution target (Edmund Optics, USA) mounted on a translation stage (M-112.1, Physik Instrumente, Germany). Signal intensity as a function of mirror axial position in CD and PCD channels were recorded. Axial resolution (full-width half-maximum of axial response) in CD and PCD were 2.0 μm and 4.0 μm respectively, compared to theoretical values of 1.2 μm and 2.6 μm respectively.
Lateral resolution was measured with the 10 % to 90 % edge criterion. CD and PCD lateral resolutions of 0.40 μm and 0.65 μm respectively were measured, whereas the theoretical values are 0.35 μm and 0.49 μm. Signal collection and speckle contrast values were measured on a diffusive sample (20 % intralipid solution). The detected PCD signal with the sDCFC was 31 times higher than in the CD configuration with the SM 50:50 coupler. Following , we define speckle contrast as C = σ/I, where σ is the pixel intensity standard deviation of a featureless field of view within an image of intralipid, while I is the mean pixel intensity within the same field of view. Coherent imaging yielded an almost fully developed speckle pattern with C = 0.89 ± 0.03 (C = 1 for fully developed speckle pattern) while this value dropped to C = 0.23 ± 0.03 for the partially coherent channel.
Figure 5 compares images of biological tissues acquired sequentially by using a 50:50 fiber coupler (left column) and an sDCFC (right column). The sample consists of freshly excised swine thyroid tissue and was imaged at 1 frame/second. Image brightness and contrast were adjusted separately to allow for comparison despite differing intensities. Insets illustrate the effect of speckle reduction in imaging of biological tissue. The top inset (magnified in Fig. 5(c) (CD) and 5(d) (PCD) respectively) highlights a structure (arrow) which is the product of speckle noise as its texture changes in the PCD image. Structures shown by arrows in the bottom inset (magnified in Fig 5(e) (CD) and 5(f) (PCD)) are not speckle grains but real cellular features which are still present when speckle contrast is decreased.
To demonstrate the improvement in signal-to-noise ratio when using the sDCFC, swine muscle tissue was imaged at increasing depths. A movie shows the confocal images as depth increases from 15 μm to 105 μm ( Media 1). Still frames from the movies are presented in Fig. 6, where (a) shows CD and (b) shows PCD images taken at a depth of 72 μm. Contrast and brightness were adjusted independently for each image to better highlight its content.
4. Discussion and conclusion
We demonstrated the first small-ratio double-clad fiber coupler operating at 785 nm dedicated to partially coherent detection. The fabrication of this device is possible due to the combination of three new design concepts: an adiabatic single mode capture, a complete multimode expulsion and a double asymmetry in the coupler’s structure. The reported device shows achromatic transmission of the core signal and high multimode signal extraction. Fabrication of this coupler can be further improved, at the mechanical twist and fusion steps. Greater multimode transmission could be obtained with a stronger fusion of the fibers, at the cost of larger single mode losses. Minor spectral features as well as the lower than expected transmission can be explained by the presence of residual stress in the fibers.
Further development should help us bridge the gap between theoretical predictions and our experimental results. The addition of three design concepts makes the coupler design scalable to different wavelength ranges with appropriate double-clad fibers. Coherent illumination combined with both efficient single mode and mutimode detections is possible, paving the way for more applications.
Image improvements observed on freshly excised swine thyroid tissues include increased signal-to-noise ratio and reduced speckle contrast. The obtained speckle contrast of 0.23 is similar to results previously obtained by [5, 33] and significantly better than that provided by the single-mode scheme at 0.89. The optimal inner cladding diameter to MFD ratio also resulted in a two-fold penalty in resolution, from 2.0 μm to 4.0 μm. This penalty is modest and still allows for discrimination of very fine features such as muscle striation (Fig. 6). In Media 1, we further notice slower degradation of signal to background ratio in PCD as depth increases. This exemplifies the usefulness of this coupler for imaging of turbid tissues. Advantages in signal intensity and speckle contrast reduction coming from having a larger inner cladding are however rapidly lost when increasing from a small ratio (5) to a larger ratio (10). Multiply scattered background and out of focus light contributions dramatically reduce contrast and signal to noise ratio when imaging a scattering sample such as biological tissue.
The sDCFC was used at 785 nm in reflectance, but can be used at other wavelengths ranging from 610 nm to 1000 nm in reflectance and/or combined with fluorescence confocal microscopy. It can also be integrated into an endoscope for confocal endomicroscopy by coupling the imaging branch of the coupler to a miniaturized scanning mechanism such as a piezoelectric scanner , a micro-electro-mechanical-system , or using spectral encoding [20, 36] through a double-clad fiber rotating joint . A DCF-based endomicroscope would benefit from ease of alignment, improved stability over time and reduced preparation time between imaging sessions . Another unique feature is the ability to separate both coherent and partially coherent signals, paving the way for applications combining speckle and phase detection to high throughput partially coherent detection.
A double-clad fiber with a small-ratio used in combination with a sDCFC offers an efficient and robust way to improve image quality. It also allows for auto-alignment of the illumination and detection channels by the structure of the fiber. This coupler was demonstrated in confocal microscopy setups as a more stable, robust and environment insensitive alternative to traditional free-space beamsplitter or dichroic mirror assemblies. While fiber bundles and two fibers  approaches have distinct advantages, the DCF approach yields a small footprint and can be used in both reflectance and fluorescence. This sDCFC paves the way towards more efficient confocal microscopy and endomicroscopy for clinical applications.
The authors would like to gratefully acknowledge Dr. Amber M. Beckley for fruitful discussions. The authors also thank Pr. Younes Messaddeq and his team (COPL, Université Laval, Quebec City, Canada) for drawing the custom fibers. Mrs. Madore, Mr. De Montigny and Mr. Ouellette are Natural Sciences and Engineering Research Council of Canada (NSERC) scholars. This work was supported by an NSERC Idea to Innovation grant.
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