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We present modal content measurements (S2) of two different negative curvature hollow-core photonic crystal fibers: a kagome fiber and an ice cream cone fiber. Their sensitivity towards mode matching, bending and polarization is analyzed. For the kagome fiber, a higher order mode suppression of 17dB under optimal conditions was achieved, and for the ice cream cone fiber there was a suppression of up to 42dB. Polarization turned out to be a critical parameter for good higher order mode suppression in both fibers.
© 2017 Optical Society of America
1. Introduction
In the last few years, hollow-core anti-resonant photonic crystal fibers have been attracting more and more attention due to their favorable properties such as large transmission windows, reduced modal overlap with the surrounding silica structure and very low dispersion compared to photonic bandgap PCFs. These features make them promising candidates for high energy [1] and high average power beam delivery [2]. They are designed [3] and used for ultrashort pulse delivery [4]. However, these applications are sensitive to higher order mode (HOM) content. Effort is made to obtain a more robust single-mode guidance of such fibers [5] and bending loss investigations have been done. Experiments comparing different fibers have also been presented, which typically check the modal content by transverse misalignment of the exciting laser beam [6]. Here we present modal investigations using the spatial and spectral resolved imaging method [7,8]. This allows quantitative characterization of the modal content. Two different negative curvature hollow-core fibers have been examined for different coupling conditions, bending radii and polarization states: a kagome-type and an ice-cream cone type fiber. This extends the work presented in [9], which considers similar fibers but lacks polarization investigations.
2. Setup
The experimental setup, depicted in Fig. 1, utilizes a narrow band tunable laser source to supply the measurement probe signal. Beam steering mirrors and a lens are used to focus the laser beam into the fiber under test, which is analyzed by a camera inside the S2 measurement system (FMA 100, Interfiber Analysis). Mode matching of the excitation beam with the fiber under test is realized with a lens. For subsequent analysis, a polarization beam splitter and a wave plate are added. The wavelength is tuned between 1010nm and 1045nm, leading to a resolution of >0.02ps/m. With this technique, the modal content of the two fibers was analyzed for different coupling conditions, bending diameters and polarization states. To this end the lenses listed in Table 1 were tested, as well as three bending diameters (12cm, 35cm and 93cm).
Fig. 1 Light of a tunable laser is reflected by two beam steering mirrors (M1, M2), passes a polarizer (POL1) and a half-wave plate (HWP) and is coupled into the fiber under test (FuT) by a lens. The output is analyzed using a polarizer (POL2) and a camera (CCD) inside the S2 measurement system.
Table 1. Lenses used for different coupling conditions
3. Kagome fiber
The first fiber to be analyzed was a 10m long kagome-type fiber, obtained from GLOphotonics (PMC-C-Yb-7C) with a mode field diameter of 40µm (see Fig. 2).
Fig. 2 Microscopic image of the kagome fiber (left) and SEM image of the ice cream cone fiber (right).
For this fiber a Gaussian-like mode profile does not necessarily imply proper mode matching. This becomes clear from Fig. 3, where S2-traces of different coupling lenses are shown for a bending diameter of D = 35cm. Several guided modes are present in the fiber, identifiable by the reconstructed mode field patterns that are also depicted in Fig. 3. The fraction of these HOM depends on the coupling lens. The best transmission of 91% was reached for f = 40mm, but even in that case several HOM are present.
Fig. 3 S2-traces of the kagome fiber for a bending diameter of D = 35cm under various coupling conditions, together with the reconstructed mode patterns.
The next step was to investigate the mode discrimination dependent on different bending radii, known as an effective method for HOM suppression in step index fibers. Although some kagome fibers were observed to be relatively insensitive to bending [10], here the modal content strongly depends on bending. The results can be found in Fig. 4. For a small bending diameter of D = 12cm, strong HOM discrimination is possible, but the transmitted power was observed to be very low (efficiency 12%), as suggested by [11]. For a large bending diameter of D = 93cm the HOM do not experience significantly higher loss than the fundamental mode over the propagation distance of 10m. A continuum of modes is observed whereas the noise level, reached for large delays, stays equally low. A moderate bending diameter of D = 35cm is a best compromise between efficiency (91%) and HOM suppression.
Fig. 4 S2 traces of the kagome-type fiber for different bending diameters under optimal coupling conditions.
As pointed out by [12], it is necessary to include polarization information to characterize a fiber’s output beam. To scrutinize the influence of polarization, linearly polarized light was launched in the fiber. By means of the half-wave plate the angle of the polarization axis can be changed. The fiber output is analyzed by a rotatable polarization filter. Measurements were performed for both excitation and analyzer polarization varying from 0° to 180° in steps of 20°. Changing the coupling polarization affects the modal content of the fiber. Although the intensity-delay curve, as shown in Fig. 5(a), is very similar compared to Fig. 5(b), which is taken at an polarization angle difference of 80°, the reconstructed mode pattern at delay = 0ps/m changes dramatically. This is illustrated in the insets of the Fig. 5(a-b) where the left ones show the mode pattern belonging to 0ps/m delay, the right ones show the first HOM peak. In Fig. 5(b) the fundamental mode is not predominant anymore and the former association of the peaks to the HOM is illegitimate [7]. Therefore, for mode suppression analysis, only the measurements that show proper LP01-content at 0ps/m delay are classified as valid and the other are disregarded. For quantification, a Gaussian-fit criterion is applied to each reconstructed 0ps delay mode. For measurements that pass this test, meaning the fitting error is below a certain threshold, the HOM suppression is calculated as the ratio of the integrated intensity in the 0ps/m delay peak and the integrated intensity of the strongest HOM peak. Results of a 6.5m long fiber piece are plotted in Fig. 5(c), that shows large areas without significant LP01-transmisson. The map also shows that any linear polarized coupling state does not stay linear, thus the bent hollow core fiber does not fully maintain the polarization state. The degree to which the fundamental mode is polarization maintaining seems to be dependent on the coupling polarization angle. One should note, that at angles of 20°/80° there is no analyzer angle for which the fundamental mode is strong enough. This can be explained because the fiber is not polarization maintaining so that the power of the mode spreads out to all polarization angles. For the preferred polarization states, the mode suppression reaches up to 17dB. One should also note that although the fiber has an azimuthal periodicity of 60°, Fig. 5(c) shows symmetry of 180°. This implies that bending is of critical influence and might imply sensitivity to the arrangement of the bent fiber. Measurements that keep one polarization angle fixed and rotate the other reproduce the symmetry of 180° for different fiber arrangements. This suggests that rearranging the fiber does not change the polarization map completely but simply adds an offset to both axes.
Fig. 5 S2-traces of the kagome-type fiber at a bending diameter of D = 35 cm for different coupling polarizations (a,b) together with the reconstructed mode patterns at a delay of 0ps/m (left insets) and of the first HOM peak (right insets). For different excitation and analyzer polarization angles, the mode suppression is calculated (c).
4. Ice cream cone fiber
The same process of mode matching, bending investigation and polarization analysis was applied to another fiber, an 8.4m long piece of an anti-resonant hollow-core “ice cream cone” fiber obtained from the University of Bath [13] with a MFD of 20µm (see Fig. 2). This fiber type is a promising candidate for strong HOM suppression [9]. Indeed it is close to single mode operation for all used coupling conditions, as shown in Fig. 6(a), where S2-traces for different mode matching are plotted for a bending diameter of D = 35cm. Although this fiber guides some intrinsic HOM, especially LP11- and LP21-like modes, their suppression of about 40dB or 60dB respectively, is very strong. The signals between 2ps/m and 3ps/m are too weak for proper mode identification. The LP21 mode is sensible to proper coupling and is less excited for larger beam spots. Nevertheless, the overall efficiency is at its optimum of 76% for f = 25mm. Using this lens also leads to less excitation of the LP11-mode.
Fig. 6 a) S2-traces of the ice cream cone fiber for a bending diameter of D = 35cm and various coupling conditions, including the reconstructed mode patterns. b) . S2 traces for different bending diameters under optimal coupling conditions.
Therefore the bending investigations were performed with the f = 25mm coupling lens. Tighter bending can decrease the already low HOM content of this fiber. Smaller bending diameters lead to even further increased HOM loss. Especially the LP21 mode can be completely suppressed, as shown in Fig. 6(b). Moreover the modal content at about 1ps/m can be strongly reduced, although this leads to an overall transmitted power decrease from 95% for D = 93cm to 59% for D = 12cm.
The polarization analysis is also performed on this fiber. The few mode operation of the fiber is mirrored in the polarization analysis, showing a fundamental LP01 mode at 0ps/m delay for all polarization states, in contrast to the kagome-type fiber. The mode suppression is plotted in Fig. 7. Therefore a Gaussian fit criterion is unnecessary and the polarization map could be filled completely. For a fixed excitation polarization, there are regions with strong HOM suppression and regions with less HOM suppression. An HOM suppression contrast ranging from 23dB to 42dB of the LP01 to the LP11 mode could be achieved.
Fig. 7 Mode suppression of the ice cream cone fiber for different excitation and analyzer polarization angles at bending diameter of D = 35 cm.
5. Conclusion
S2-measurements for two different negative curvature hollow-core fibers have been performed, enabling a quantitative analysis of the higher order mode content of these fibers. Especially polarization turned out to be crucial for close to single mode operation in both fibers. The kagome fiber reaches a HOM suppression of 17dB for optimized parameters of bending, coupling and polarization. The ice cream cone fiber showed stable few mode operation for all measured polarization parameters with a large HOM suppression of 23 dB for worst case and 42dB for optimal conditions. This fiber might be useful for further high power mode sensitive experiments.
Funding
The initial results have been funded within by the Fraunhofer and Max Planck cooperation program within the project “MEGAS”. Additional support was given by the funding under the innovation and transfer project “INNOspace” of Germany’s national program for space and Innovation of the BMWi within the grant “Eskalierung” (FKZ_50RP1510).
Acknowledgment
The authors would like to thank Fei Yu and Jonathan Knight for giving the opportunity to work with their fiber.
References and links
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