Single mode operation with midIR hollow fibers in the range 5 . 1-10 . 5 μ m

Single mode beam delivery in the mid-infrared spectral range 5.1-10.5 μm employing flexible hollow glass waveguides of 15 cm and 50 cm lengths, with metallic/dielectric internal layers and a bore diameter of 200 μm were demonstrated. Three quantum cascade lasers were coupled with the hollow core fibers. For a fiber length of 15 cm, we measured losses down to 1.55 dB at 5.4 μm and 0.9 dB at 10.5 μm. The influence of the launch conditions in the fiber on the propagation losses and on the beam profile at the waveguide exit was analyzed. At 10.5 μm laser wavelength we found near perfect agreement between measured and theoretical losses, while at ~5 μm and ~6 μm wavelengths the losses were higher than expected. This discrepancy can be explained considering an additional scattering loss effect, which scales as 1/λ and is due to surface roughness of the metallic layer used to form the high-reflective internal layer structure of the hollow core waveguide. ©2015 Optical Society of America OCIS codes: (060.2390) Fiber optics, infrared; (140.5965) Semiconductor lasers, quantum cascade. References and links 1. B. Lendl and B. Mizaikoff, Optical Fibers for Mid-infrared Spectrometry, Handbook of Vibrational Spectroscopy Vol. 2 (John Wiley & Sons Ltd, 2002). 2. J. A. Harrington, “A review of IR transmitting, hollow waveguides,” Fiber and Integrated Opt. 19(3), 211–227 (2000). 3. V. Spagnolo, P. Patimisco, S. Borri, G. Scamarcio, B. E. Bernacki, and J. Kriesel, “Mid-infrared fiber-coupled QCL-QEPAS sensor,” Appl. Phys. B 112(1), 25–33 (2013). 4. V. Spagnolo, P. Patimisco, S. Borri, G. Scamarcio, B. E. Bernacki, and J. 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Theory 28(7), 704– 710 (1980). 20. M. Miyagi and S. Kawakami, “Design theory of dielectric-coated circular metallic waveguides for infrared transmission,” J. Lightwave Technol. 2(2), 116–126 (1984). 21. R. George and J. A. Harrington, “Infrared transmissive, hollow plastic waveguides with inner Ag-Agl coatings,” Appl. Opt. 44(30), 6449–6455 (2005). 22. Y. Matsuura, M. Saito, M. Miyagi, and A. Hongo, “Loss characteristics of circular hollow waveguides for incoherent infrared light,” J. Opt. Soc. Am. A 6(3), 423–427 (1989). 23. C. D. Rabii, D. J. Gibson, and J. A. Harrington, “Processing and characterization of silver films used to fabricate hollow glass waveguides,” Appl. Opt. 38(21), 4486–4493 (1999). 24. M. Miyagi, “Bending losses in hollow and dielectric tube leaky waveguides,” Appl. Opt. 20(7), 1221–1229 (1981).


Introduction
Hollow-core waveguides (HCWs) are excellent for the transmission of infrared laser radiation, as they offer good flexibility for easy handling and high output beam quality for precise medical applications.Potential uses include not only laser surgery, but also numerous applications in infrared spectroscopy, thermal imaging, sensing and infrared countermeasures [1][2][3][4].The HCWs structure is composed of a hollow glass capillary tube with a metallic/dielectric structure deposited inside the bore [5].The bore size, typically < 1 mm, determines the overall losses and mode quality of the HCW, whereas the thickness of the dielectric layer determines the spectral response [6].HCWs have extremely high coupling efficiency (e.g., > 95%), no back reflection, no cladding modes, and high energy/power handling capabilities [5,7,8].An additional important feature of hollow waveguides is that despite the relatively large bore size, they can be quasi single mode.This is the result of the strong dependence of loss on the fiber mode parameters [5,6,9].The single mode propagation and mode filtering capabilities have been found to depend strongly on the fiber diameter and the laser wavelength.Low-loss single mode propagation and effective mode filtering have been successfully demonstrated for HCWs with bore diameters of 300 µm with λ > 8 µm [3,4,10].Note also that, single mode operation has been achieved with solid core fibers for λ ≤ 5.5 µm (although the optical losses becomes very high for λ ˃ 4,6 μm) [11] and with photonic band-gap fibers made by chalcogenide glasses at 3.39 µm, 9.3 µm, and 10.6 µm [12].Empirically, hollow core fibers with bore sizes as large as 40 times the wavelength have been shown to provide a convenient, relatively low-loss means of delivering mid-IR laser beams with a single spatial mode.Consequently, HCWs with bore sizes up to 200 µm can operate in single mode when light at wavelength > 5 µm is guided inside, although until now this has been demonstrated only for λ ≥ 7.6 µm [13].
In this paper, we demonstrated HCW single mode performance at wavelengths down to λ = 5.1 µm.The fiber realized for this work has a bore diameter of 200 µm and lengths of 15 cm and 50 cm.Three commercial mid-IR external cavity quantum cascade laser (QCL) sources with spectral ranges of emission centered at 5.2 µm, 6 µm and 10 µm were employed.Different coupling conditions between the lasers beam and the waveguides entrance were investigated using mid-infrared ZnSe, Ge, or CaF 2 lenses with focal lengths in the range 25-76 mm.The mode quality of the output beam and the propagation losses are highly dependent on the beam waist of the coupling lens.The measured HCW losses are slightly higher than the calculated values.These differences are larger for 5-6 µm wavelengths (up to ~1.7 dB) and become almost negligible at 10.5 µm.The reason for these discrepancies is explained in terms of additional scattering losses due to the inner metallic surface roughness, which are inversely proportional to the wavelength squared [14,15].

Theory
Propagation losses in hollow waveguides are highly dependent on the launch conditions.In general, longer focal length lenses excite lower-order modes within the waveguides, decreasing the interaction of the light with the waveguide walls and minimizing the attenuation.In order to quantify this, we assume a perfect Gaussian beam coupled into the waveguide entrance.The spot size ω(z) can be shown to vary with propagation distance z along the optical axis as [16]: where ω 0 is the beam waist and R is the wave front radius.The parameter is the Rayleigh range.According to these expressions, only one parameter is needed to completely specify the waist ω 0 at the focal length of a lens, i.e., the laser beam spot size on the coupling lens.
The theoretical analysis of optical mode structure in dielectric hollow-core waveguide has been described in detail in [17][18][19].The fundamental modes are the HE nm hybrid modes, which have a small component of the electric field along the fiber or optic axis and correspond to skew rays (rays that do not cross the fiber axis; rather they travel in a corkscrew or helical paths down the waveguide).The HE 11 mode is the lowest order mode having a circularly-symmetric, Gaussian spatial profile.When a Gaussian beam is properly focused into a hollow waveguide along the optic axis, only the HE 1m modes are excited inside the waveguide.This is a result of the strong dependence of loss on the waveguide mode parameter.The losses of high order modes increase as the square of the mode parameter so even though the guides are multimode, in practice only the lowest order modes propagate.This is particularly true for small-bore (≤ 300 µm) waveguides.In order to theoretically estimate propagation losses L p of the HCWs we take into account both the power coupling efficiency η 1m of the Gaussian incident beam to each HE 1m waveguide mode and the attenuation coefficients α 1m of the HE 1m modes by using the following expression: where L is the length of the HCW.The power coupling efficiency of each HE 1m mode can be expressed as the normalized overlap integral between the input Gaussian mode with a beam waist ω 0 at the waveguide entrance, and the HE 1m waveguide modes, which can be approximated by zero-order Bessel functions.Hence, the power-coupling coefficient for the various modes is a function of 2ω 0 /d, i.e., ω 0 divided by the bore radius of the waveguide and so depends on how much the focused beam fills the hollow waveguide.A minimal loss condition is achieved when this ratio is about 0.64 [9].Hence, an appropriate focal length lens is needed to approximate as much as possible this ratio, in order to optimize the laser beam coupling into the HCW.For a HCW with a bore diameter d = 200 µm, optimal coupling occurs for ω 0 = 64 µm, which for λ = 5 µm corresponds to a numerical aperture NA = λ/(π•ω 0 ) = 0.025.Thus, relatively slow optics provide optimal coupling into the lowest order mode.When faster optics are used (i.e., a smaller beam size that under fills the HCW bore) there will be less coupling into the lowest order mode and more coupling into the higher order modes.On the opposite extreme, if the beam size is bigger than the HCW bore, the coupling efficiency will decrease due to clipping of the beam and the lowest order mode will be preferentially selected.For the attenuation coefficients α 1m at each HE 1m mode, we used the expression derived by Miyagi and Kawakami [20]: where n and k are the real and imaginary parts of the complex index of the metallic (typically Ag) layer, η 1m is the mth root of the zero-order Bessel function and a is the HCW bore radius.
For the wavelength dependence of n, k and n d we used relations reported in [21].

Experimental setup
In this work, we employed two HCWs with metallic (Ag)/dielectric (AgI) circular crosssection internal coatings, a bore size of d = 200 µm, and of 15 cm and 50 cm.Fabrication of the HCWs is accomplished using a wet chemistry process developed by Harrington et al. [5,6].It consists of depositing a reflective silver (Ag) layer followed by a dielectric silver iodide (AgI) layer inside a HCW tube.The glass tubing does not influence the optical properties of the HCW, but simply provides a smooth surface on which the coatings are applied.An external protective buffer on the outside of the capillary tube helps to shield the glass from scratching.By producing a hollow fiber with a specific dielectric thickness layer, the transmission spectrum of the waveguides can be tailored for different spectral wavelength ranges.The optimal dielectric film that minimizes propagation losses into the fiber for a wavelength of λ = 5 µm is 0.46 µm, assuming that the refractive index n d of silver iodine at this wavelength is 1.965 [21].The actual thickness of the AgI layer, calculated from the Fourier transform infrared spectrometer, is about 0.56 µm [9].Three commercial mid-IR external cavity quantum cascade laser (QCL) sources (Daylight Solutions Inc., San Diego, CA, USA, model #21052-MHF, #21062-MHF, #21106-MHF) were employed to investigated the attenuation of the hollow waveguides, the optical mode profile at the waveguide exit and the influence of the input launch conditions on the beam propagation through the fibers.The laser sources can work in the ranges 5.10-5.34µm (#21052-MHF); 5.92-6.27µm (#21062-MHF); 9.94-10.72µm (#21106-MHF).For each laser source, we study the fiber-output mode profile in all the operating spectral range.In the present work we show results obtained when operating the three QCL sources at the following wavelengths: λ a = 5.4 µm, λ b = 6.2 µm and λ c = 10.5 µm.Under these conditions, the emitted optical powers were 115 mW (λ a ), 80 mW (λ b ), and 56 mW (λ c ).The couplings between the QCLs and the hollow waveguide have been realized by using the experimental scheme shown in Fig. 1.A coupling lens with a diameter of 1/2" and attached to a translation mount was used to focus the collimated laser beam into the HCW entrance.The HCW was held in a kinematic mount, which allowed for tilt adjustments of the position of the HCW entrance with respect to the focused laser beam.In order to record the mode profile in the far field, a pyrocamera (Pyrocam III, Ophir Spiricon) with pixel sizes of 0.085 x 0.085 mm was mounted at distances ≥ 2.5 cm from the HCW output.For each QCL source, the output beam divergence was measured acquiring the far field profile at two different distances from the laser output and by measuring the radial distances at which the light intensity drops to 1/e 2 of its maximum central value.For the λ a -QCL a beam radius R 0 = 1.18 mm and a diffraction-limited beam divergence angle of ϑ A = 2.9 mrad were measured.In a similar way, we measured R 0 = 1.33 mm and ϑ B = 3.1 mrad for λ b -QCL and R 0 = 1.2 mm and ϑ C = 5.6 mrad for λ c -QCL.
To investigate the influence of the input launch conditions on the beam propagation through the HCW, we employed three different coupling lenses, with focal length f of 25 mm, 50 mm and 76 mm.In Table 1 are reported the 2ω 0 /d ratios calculated at λ a and λ b by using Eq. ( 1), where d = 200 µm.In each case, the alignment was optimized by maximizing the HCW output power, by adjusting the coupling lens position and the waveguide entrance tip/tilt mounting.Based on our calculations, the optimum coupling conditions can be obtained using a lens with f = 50 mm, both for λ a -QCL and λ b -QCL, with a focused beam collimated down to a ~70 µm spot radius at the HCW entrance.The 2-D far field acquisitions demonstrate that HCWs with bore sizes of 200 µm allow single-mode propagation of laser beam at λ = 5.4 µm, with both the 15 cm and 50 cm long fibers.In addition, even though the coupling conditions are significantly different, the modal purity is good, resulting in a beam shape matched to the hybrid HE 11 mode in almost all investigated configurations.We also verified that the single mode HCW operation is preserved during the entire laser source operating range, down to 5.1 µm.
The output divergence angle is the result of both the diffraction of light leaving the waveguide and the beam quality.It depends critically on both the nature of the optical mode propagating through the HCW, and the bore diameter.In principle, the output beam divergence can provide an indication of the number of high-order modes propagating in the HCW.The HE 11 mode will couple to free-space modes with a beam divergence θ given by [5]: From our data, for the best coupling conditions f = 50 mm, we calculate for λ a (λ b ) a divergence of 25.0 mrad (26.5 mrad) in good agreement with the expected theoretical value of 20.6 mrad (23.9 mrad), calculated by using Eq. ( 4) with u 11 = 2.4048.The quality of the output beam can be expressed by the ratio r of the measured beam divergence angle and the calculated one.We estimated r = 1.19 for λ a and r = 1.11 for λ b , indicating a good beam quality and the possibility to be re-focused to a tight spot.

Propagation losses
Absolute values of the losses of the excited mode in the HCW were determined by measuring the optical power at the waveguide entrance and the optical power at the output-end of the HCW by using a mid-IR power meter.(2) and ( 3), assuming that the HE 11 and higher-order modes up to m = 5 propagate.The experimental losses follow the trend predicted by the theoretical model of Miyagi [20].However, they result higher than the calculated α 1m values.This discrepancy between theoretical and experimental values increases for the longer HCWs.A possible explanation for the observation of losses higher than expected can be the presence of additional scattering losses, not predicted by theory and due to roughness of the HCW internal reflective coating layers.Matsuura et al. [22], using a ray-optics approach, demonstrated that these scattering losses decrease with wavelength as 1/λ 2 .To confirm this, we coupled the 200 μm-HCWs with the λ c -QCL emitting at 10.5 μm by using the same experimental setup reported in Fig. 1 and three different lenses with focal length f of 25 mm, 40 mm and 50 mm.In Table 2 are reported the calculated 2ω 0 /d ratios assuming a λ c -QCL beam diameter of 2.4 mm.The optimal coupling condition should be ensured by using the lens with f = 25 mm. Figure 5 Single-mode propagation has been obtained for all three lenses employed and in the entire QCL operating range.At 10.5 um we obtained a good agreement between the losses predicted by the theory and the experimental results, demonstrating that the contribution due to scattering losses results is almost negligible for this longer wavelength.In addition, this result confirms that our HCWs have a nearly ideal structure of the glass and perfect circular uniformity of the cross section [22,23].
In Fig. 6, the discrepancy α -α 1m between the measured losses and the theoretical values was plotted as a function of 1/λ 2 for a 50 cm-long HWC at λ a , λ b and λ c when 2ω 0 /d equals 0.72, 0.74 and 0.69 (see Table 1 and Table 2), i.e., the best achievable conditions in terms of output mode quality.
The results show an almost linear behavior, as predicted by Matsuura et al. [21], thereby supporting the notion that an additional contribution to the total losses originates from light scattering due to surface roughness of the inner HCW metallic layer.

Bending losses
An additional loss for HCWs is due to the waveguide bending.The dependence of such losses from HCW curving radius has been investigated for different metallic, and single-layer dielectric coated HCWs with bore size > 300 µm [14,15].In Fig. 7(a) we show the bending losses measured at λ = 5.2 µm, for the 200 µm-bore diameter HCW of length L = 50 cm and employing a coupling lens with f = 50 mm.and 7(c) show two representative measured profiles of the 50cm-long HCW bent to a 0.111 m radius and to a 0.302 m radius, respectively.For these measurements, the input and output ends of the guides are kept straight, and the bent portion of waveguide was kept constant.The data in Fig. 7(a) show that below a critical radius (R c = 1.15 m), the bending losses increase linearly with the reciprocal of the radius of curvature.The solid line in Fig. 7(a) is the best linear fit to the experimental data.The linear trend is characteristics of HCWs and the magnitude of this loss depends largely on the quality of the inner surface [24].

Conclusions
Up to now, mid-IR single mode operation was demonstrated at λ ≤ 5.5 μm with solid core fibers [11] or λ ≥ 7.6 μm with HCWs [13].In this work, we demonstrated single-mode propagation in the spectral range 5.1-10.5 μm using a HCW with bore size of 200 μm.In this way, single-mode operation now cover all the mid-IR spectral range.The waveguide optical properties depend on the laser beam launch conditions into the fiber, and propagation losses have been predicted by using fundamental waveguide theory.An additional contribution to the waveguide losses was measured and attributed to scattering losses, i.e. the interaction of light with the imperfect inner surface of the metallic layer in the hollow waveguide.Our study shows that these losses scale roughly as 1/λ 2 and become almost negligible at 10.5 µm.Bending losses have been measured as a function of the radius of fiber curvature and a linear dependence was observed below a critical radius.Our measurements demonstrate that it is possible to generate a single-mode HE 11 output profile even if the guide is bent up to 1/R ~10 m −1 .As a final remark, consider that the input beam quality determines the minimum fiber length providing single mode operation, so the achievement of single mode propagation with the 15-cm long fiber is also due to the excellent beam quality of the employed QCLs.

Fig. 1 . 4 .Fig. 2 .
Fig. 1.A schematic of the experimental The laser beam is focused into the HCW entrance using a coupling lens.The beam profile at the waveguide exit is acquired with an infrared pyrocamera.QCL -Quantum Cascade Laser; HCW -Hollow-Core Waveguide.4.Output beam profilesFigures 2(a) and 2(b) show the far field spatial intensity distributions of the λ a -QCL and λ c -QCL sources measured when shining the beams on to the detector sensitive area (without the presence of the HCW).The 2D profiles of the input beams resemble Gaussian beam power distributions.

Fig. 5 .
Fig. 5. (a) Far field spatial intensity distribution of λ c -QCL upon exiting a 15 cm long HCW by using the coupling lens with f = 25 mm.(b) Experimental losses (dots) as a function of 2ω 0 /d ratio.The solid line represents the theoretical losses trend calculated by using Eqs.(2) and (3) for λ c .

Fig. 6 .
Fig. 6.Differences between theoretical and experimental losses measured for λ a , λ b and λ c for a 50 cm-long HWC plotted as a function of 1/λ 2 .

Fig. 7 .
Fig. 7. (a) Bending losses (dots) for 50cm-long HCW measured by using λ a -QCL and the coupling lens with f = 50 mm.Solid line is the best linear fit of the data below the critical radius R c = 1.15 m.(b) Mode profile at the HCW exit bent at a radius of curvature of 0.111 m.(c) Mode profile at the HCW exit bent at a radius of curvature of 0.302 m.The corresponding data points are marked by arrows.

Figures
Figures 7(b) and 7(c) show two representative measured profiles of the 50cm-long HCW bent to a 0.111 m radius and to a 0.302 m radius, respectively.For these measurements, the input and output ends of the guides are kept straight, and the bent portion of waveguide was kept constant.The data in Fig.7(a) show that below a critical radius (R c = 1.15 m), the bending losses increase linearly with the reciprocal of the radius of curvature.The solid line in Fig.7(a) is the best linear fit to the experimental data.The linear trend is characteristics of HCWs and the magnitude of this loss depends largely on the quality of the inner surface[24].
Figures 7(b) and 7(c) show two representative measured profiles of the 50cm-long HCW bent to a 0.111 m radius and to a 0.302 m radius, respectively.For these measurements, the input and output ends of the guides are kept straight, and the bent portion of waveguide was kept constant.The data in Fig.7(a) show that below a critical radius (R c = 1.15 m), the bending losses increase linearly with the reciprocal of the radius of curvature.The solid line in Fig.7(a) is the best linear fit to the experimental data.The linear trend is characteristics of HCWs and the magnitude of this loss depends largely on the quality of the inner surface[24].