High numerical aperture large-core photonic crystal fiber for a broadband infrared transmission

Infrared Physics & Technology 79 (2016) 10–16

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High numerical aperture large-core photonic crystal fiber for a broadband infrared transmission J. Pniewski a,⇑, G. Stepniewski a,b, R. Kasztelanic b, B. Siwicki a,b, D. Pierscinska c, K. Pierscinski c, D. Pysz b, K. Borzycki d, R. Stepien b, M. Bugajski c, R. Buczynski a,b a

Faculty of Physics, University of Warsaw, Pasteura 7, 02-093 Warszawa, Poland Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warszawa, Poland Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warszawa, Poland d National Institute of Telecommunications, Szachowa 1, 04-894 Warszawa, Poland b c

h i g h l i g h t s
A large mode area photonic crystal fiber made of a heavy metal oxide glass is proposed.
The fiber was made using well-known stack-and-draw technique.
The fiber guides light from visible to mid-infrared region of wavelengths.
The fiber can be pigtailed to quantum cascade lasers without additional optics.

a r t i c l e

i n f o

Article history: Received 1 June 2016 Revised 7 September 2016 Accepted 9 September 2016 Available online 13 September 2016 Keywords: Microstructured fiber Photonic crystal fiber Soft glass Large mode area fiber Mid infrared transmission

a b s t r a c t In this paper we present a large mode area photonic crystal fiber made of the heavy metal oxide glass CS-740, dedicated for a broadband light guidance in the visible, near- and mid-infrared regions of wavelengths from 0.4 to 4.7 lm. The fiber is effectively multi-mode in the considered wavelength range. It is composed of a ring of air-holes surrounding the core, with a high linear filling factor of 0.97. The fiber was made using a standard stack-and-draw technique. Each hole has a size of approx. 2.5  3.0 lm and diameter of core is 80 lm. Fiber attenuation is below 3 dB/m in the 0.9–1.7 lm wavelength range, while at 4.4 lm (mid-IR) it is approx. 5 dB/cm. Bending loss at the 1.55 lm wavelength is 0.45 dB per loop of 8 mm radius. Fiber numerical aperture is 0.53 at 1.55 lm. The effective mode area of the fundamental mode is approx. 2400 lm2 in the wavelength range of 0.8–1.7 lm. We present a proof-of-concept demonstration that our large core photonic crystal fiber is able to efficiently collect light directly from a mid-IR quantum cascade laser without use of additional optics and can be used for pigtailing mid-IR sources and detectors. Ó 2016 Published by Elsevier B.V.

1. Introduction During the last years large mode area photonic crystal fibers (LMA PCFs) have been generating broad interest due to their exceptional guiding properties, especially single-mode guidance in extremely large structures, low nonlinearity, strong birefringence and dispersion management ability [1,2]. These unique properties result from microstructured cladding and proper choice of glass material. High power delivery combined with possibility to collect light over large areas offered by LMA PCFs are desirable in

⇑ Corresponding author. E-mail address: [email protected] (J. Pniewski). http://dx.doi.org/10.1016/j.infrared.2016.09.002 1350-4495/Ó 2016 Published by Elsevier B.V.

various applications, such as optical coherence tomography, infrared spectroscopy or airborne light detection and ranging (LIDAR). In conventional optical fibers the mode area is limited by the accuracy of controlling low concentration of dopants in the fiber core. However, single mode performance with the mode area of 400 lm2 at 1.550 lm has been reported [3]. Large mode area can also be achieved in PCF fibers, which do not need doping and do not have a cut-off wavelength [4]. In this case, the number of guided modes is determined by geometrical properties of a photonic cladding, such as the lattice constant and the linear filling factor. Several successful approaches for the design and fabrication of LMA PCFs have been reported [5–9]. Recently, ultra large core PCFs have been proposed, having mode area of 1454 lm2 [10].

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Fig. 1. Schematic of the proposed ideal LMA PCF.

Majority of LMA PCFs are made of silica glass with an air photonic crystal cladding, which limits their applications to the visible and near-infrared parts of the light spectrum [10,11]. Moreover, it is difficult to enlarge the effective mode area for broadband applications, due to trade-off between fiber cut-off wavelength and bending losses. A number of doped fibers based on silica were also demonstrated [12].

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Increasing number of mid-IR sources and the demands of modern spectroscopy and sensing create a need for the development of new LMA PCFs, which are capable to transmit light also in the midIR region, especially at wavelengths above 2 lm, and exhibit low bending losses, with good mechanical properties and high resistance to the laser damage [13]. Standard photonic solutions cannot be used due to limited transmittance of most materials in this range of wavelengths. New light sources are developed, such as quantum cascade lasers (QCLs). The concept of QCLs has its origin in a theoretical paper published in 1971 by Kazarinov and Suris [14]. The first QCL was successfully demonstrated in 1994, by the pioneering work of Federico Capasso’s group [15] at Bell Labs (USA), and further developed into the promising devices of today. The physics behind optical transitions of a QCL differs from that of a diode laser. In a QCL the lasing transition occurs between states within a conduction band of a given coupled quantum well system. In contrast, in a diode laser the transitions occur between the conduction band and the valence band of a semiconductor material. One of the advantages of this device is that the electron responsible for the emission of the photon tunnels into the next segment of quantum wells and as a result, a single electron can generate multiple photons, thereby making such devices extremely efficient. The tunneling from one segment to the next is where the term ‘‘quantum cascade” comes from. Furthermore, the depths of wells can be engineered by controlling layer thicknesses during the growth process and hence the wavelength of the lasing transition is

Fig. 2. (a) Refractive index of the CS-740 glass, transmittance of the CS-740 glass in the range of (b) 0.35–3.3 lm and (c) 2.5–5.5 lm for a 2 mm–thick sample. Across the range of 0.7–2 lm the transmittance is above 80%. The measurements were taken using (b) Varian Cary 500 and (c) Bruker IFS 113V spectrophotometers. The Fresnel reflection loss is encompassed in the transmittance values.

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Fig. 3. SEM images of the developed fiber.

Fig. 4. The characteristics of the fiber modelled numerically on the basis of SEM images: (a) effective refractive index; (b) effective chromatic dispersion of the fundamental mode and material dispersion of the glass. ZDW of the effective dispersion is 2.223 lm, while of the material dispersion is 2.175 lm.

dependent on the geometrical structure of the device. This gives a lot of freedom in structure design. By carefully designing the quantum wells, lasing has been achieved at wavelengths as short as 2.75 lm [16] and as long as 161 lm (1.9 THz) [17]. The long wavelength devices still require cryogenic cooling, but at room temperature operation has been observed to at least 16 lm [18]. Despite significant improvements and development of mid-IR sources, there is still a problem of delivery of light to experimental setups due to a lack of efficient fibers and coupling optics [19,20]. Therefore, the solution proposed in this paper can potentially bridge the gap between the source and the detector in the midIR range. To meet the above mentioned requirements, various soft glasses, such as chalcogenide, fluoride, tellurite or heavy metal oxide glasses, can be used as an alternative to fused silica. The chalcogenide glasses offer transparency in the range of 1–16 lm, however they have a tendency towards crystallization during thermal processing [21–23]. Heavy metal oxide, fluoride and tellurite glasses offer transmission in the range of 0.4–5 lm, 0.4–6 lm, 0.5–5 lm, respectively [24–26]. In contrast, the transmission window of silica is limited to <3 lm. Furthermore, the threshold of nonlinear effects in soft glasses is usually higher than in fused silica, and lower nonlinearity can be expected in such fibers. Large mode area is possible in these fibers, in the thousands of lm2 range [25]. There is a strong demand to develop pigtailed mid-IR sources and detectors, potentially easy to handle. This task can be

accomplished with LMA PCFs having a large core area and a high numerical aperture, allowing to collect light directly from the source, in the on-chip configuration. Such fibers are usually fewmode fibers (FMF) with large effective area. Some concepts of FMFs were already published, mostly for telecommunication applications, with the effective mode area of several hundreds of lm2 [27]. Certain few-mode multicore fibers for long-haul transmission at telecom wavelengths were also presented [28–30]. A combindex fiber intended for high-power fiber lasers (1064 nm) was proposed, which generates beam of good quality and exhibits large mode area, complemented by high tolerance to fabrication errors [31]. A fiber with a large number of cores (36  3) was also proposed for space/mode division multiplexed transmission [32]. We selected a PCF design with a single ring of air-holes. This type of photonic crystal fiber offers large mode area up to thousands of lm2 and very high NA up to 0.95 [33]. Usually similar design a double cladding is used for fiber lasers as [34]. We adopt this concept to obtain an LMA fiber, while keeping limited total diameter of the fiber. Similar performance can be obtained with a regular lattice photonic cladding, but in this case fiber diameter is much larger due to large thickness of the photonic cladding, which severely restricts minimum fiber bending radius. In this paper we present for the first time a LMA PCF made of a heavy metal oxide glass CS-740, dedicated to broadband light guidance in the near- and mid-infrared regions of the optical spectrum. The fiber was made using the stack-and-draw process. Use of a soft glass has advantages over other solutions. ZBLAN fibers may be

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Fig. 5. Numerically calculated intensity distributions of representative modes from four groups of modes guided in the fiber, for the wavelength of 800 nm: (a) LP01; (b) LP02; (c) LP11; (d) LP21. The structure of the LMA PCF is overlapped on the image and shown in white.

Table 1 Attenuation and effective mode area for the guided modes at 0.8 lm and 4.7 lm. The effective mode area is estimated with uncertainty of 100 lm2. Mode

0.8 lm Loss [dB/m]

LP01 LP11 LP21 LP02

11

2  10 2  1010 2  108 2  105

4.7 lm Aeff [lm2] 2400 2500 2200 1800

Loss [dB/m] 8

6  10 4  107 2  105 2  104

Aeff [lm2] 2400 3300 2300 1900

Fig. 6. The setup for the attenuation measurements in the near-IR range.

difficult to work with and are not mechanically durable [35], while chalcogenide glasses are toxic and expensive. Heavy metal oxide glasses are a kind of compromise choice. They are relatively cheap to make, the processing is easy and allows hot embossing, while fibers are mechanically durable and have good transmission

properties. The developed fiber was analyzed numerically and characterized in the near- and mid-IR ranges. Thanks to a large core and numerical aperture it is dedicated for light delivery and collection in the broad wavelength range from 0.5 to 5 lm in pigtailed optical devices, especially in on-chip configurations.

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2. Development and analysis of the LMA PCF The proposed ideal fiber has a large core surrounded by a ring of air-holes of 3.2 lm diameter, shown schematically in Fig. 1. The distance between centers of adjacent holes is 3.33 lm, which implies a high linear filling factor of approx. 0.97. Core diameter is 80 lm. To make the fiber we have used the standard stack-and-draw technique, commonly employed for making soft glass PCFs [36]. The fiber is made of in-house developed heavy metal oxide glass CS-740 [37]. This glass offers a high transmission in the broad wavelength range of 0.5–5 lm and does not exhibit selfdarkening, unlike common ZBLAN fibers. Spectral dependence of bulk refractive index of the CS-740 glass, shown in Fig. 2a, is described using the following Sellmeier relation

" nðkÞ ¼ 1 þ

B1 k2 k2  C 21

þ

B 2 k2 k2  C 22

þ

B3 k2 k2  C 23

#12 ;

ð1Þ

with coefficients B1 = 2.1598, B2 = 0.4035, B3 = 1.7698, C1 = 0.1291 C2 = 0.2669, C3 = 15.8114. Spectral transmittance characteristic of the same glass is shown in Fig. 2b and c. Fig. 3 shows the cross-section of the developed fiber, as seen by a scanning electron microscope (SEM). The holes are no longer circular but elongated in the radial direction. Diameter of core is 72 lm, while the diameter of holes is approx. 3 lm in the radial direction and approx. 2 lm in the circumferential direction. Cladding diameter is approx. 120 lm.

The modal and dispersion properties of this fiber were modelled numerically using the finite difference method [38] based on the SEM image of fiber cross-section. This way, we use the real refractive index distribution of the background glass in our calculations, while including all imperfections which occurred during fiber manufacturing. Fig. 4a depicts the effective refractive index of the fundamental mode, and Fig. 4b presents the chromatic dispersion of the fundamental mode complemented by material dispersion in the wavelength range of 0.5–5 lm. The fundamental mode area of the PCF is 2398 lm2 at the 0.8 lm wavelength and 2406 lm2 at 1.5 lm. The dispersion of the fundamental mode is close to zero across a broad spectral range and does not exceed ±100 ps nm1 km1 in the range of 1.45–4.5 lm. Due to a relatively large core, the LMA PCF is a multi-mode fiber and the intensity distributions of four modes are shown in Fig. 5. The effective refractive index and chromatic dispersion characteristics of the higher order guided modes are similar to those of the fundamental mode. The attenuation of the modes and the effective mode area for modes shown in Fig. 5, at wavelengths between 0.8 and 4.7 lm are shown in Table 1. The attenuation was calculated on the basis of a SEM image, which shows actual manufacturing flaws, and is higher for high order modes in comparison to an ‘‘ideal” fiber free of any flaws.

3. Characterization of LMA PCF The fiber was characterized using of a broadband setup with supercontinuum light source (Koheras SuperK Compact). We have measured the attenuation at wavelengths in the range of 0.730–1.7 lm as well as bending losses and numerical aperture (NA) at 1.550 lm. 3.1. Spectral attenuation

Fig. 7. Attenuation spectrum of LMA PCF measured in the 0.73–1.7 lm range.

PCF attenuation was measured using the cut-back technique. The experimental set-up for the measurement in the range of 0.73–1.7 lm is shown in Fig. 6. The light was delivered to the fiber under test using a telecom single mode fiber (SMF). The beam was collimated by objectives L1 and L2 and coupled into the fiber under test. The power coupling efficiency between SMF and the PCF tested was 68.75%. The output beam was measured with an optical spectrum analyzer (OSA). Spectral characteristic of fiber attenuation is shown in Fig. 7. For the wavelength range of 1–1.4 lm, the attenuation was below 2 dB/m. At longer wavelengths it increases to approx. 2.5 dB/m, and at shorter wavelengths rises to approx. 8 dB/m at 0.73 lm. The oscillations seen in the

Fig. 8. Test setup for characterization of fiber bending loss.

Fig. 9. The setup for fiber NA measurement.

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propagation of highly attenuated modes can be neglected. NA measured was approx. 0.53. 4. Proof-of-concept QCL pigtailed with the LMA PCF

Fig. 10. Schematic of the experimental setup for the coupling efficiency measurement in mid-IR. The distance l varies in the range of 0–100 lm.

Fig. 11. Spectral characteristics of the QCL for two pulse power values.

attenuation characteristics are mostly caused by the reflections introduced in L3 microscope objective.

To verify the suitability of using our LMA PCF for pigtailing quantum cascade lasers emitting in the mid-IR range we have arranged a proof-of-concept setup, a schematic of which is shown in Fig. 10. A strain-compensated quantum cascade laser (QCL), designed for emission at 4.4 lm wavelength, operating in pulsed regime and at a temperature of 20 °C was used as a source of light. The active area of the source is approx. 10  7 lm, while the NA is 0.62 in the direction of the growth of the crystal and 0.95 in the perpendicular direction. Pulse duration and repetition rate were 200 ns and 5 kHz, respectively. Two pulse power values were used: 467 mW and 933 mW. Emission spectra of this source is shown in Fig. 11. The source is located directly at one end of the fiber and almost touches its surface, the gap being less than 100 lm in a butt-coupling configuration. The microscopic image of those two elements of the setup and the schematic of the QCL are shown in Fig. 12. Input and output power were measured with an MCT TE cooled detector. To verify coupling efficiency we used only a short length of fiber (2 cm) to minimize influence of fiber attenuation on output power. We have measured 270 mW and 550 mW at fiber output in case of 467 mW and 933 mW total optical output of QCL. This corresponds to coupling efficiency of approx. 58%. The demonstration confirmed that the newly made fiber is well suited for pigtailing of QCLs. The butt-coupling from QCL to LMA PCF is possible and effective. The CS-740 glass allows to use only few centimeters-long fibers for applications at the wavelength of 4.4 lm, since the bulk glass attenuation is approx. 5 dB/cm. This glass works best for wavelengths below approx. 2.8 lm. At longer wavelengths, ZBLAN or tellurite glasses should be used, as those exhibit higher transmittance in the mid-IR above 3 lm.

3.2. Bending losses The setup used for measurements of bending loss at a 1.55 lm wavelength is shown in Fig. 8. The light source (SRC) employed was of ASE (Amplified Spontaneous Emission) type. The fiber under test was wrapped around a mandrel having a controlled radius R. Maximum bending losses in respect to straight fiber (R = 1) reach 0.45 dB/loop for the bending radius R = 8 mm. The fiber was mechanically resistant and had low sensitivity to bending. 3.3. Numerical aperture The setup for NA measurement is shown Fig. 9. Light from a source operating at 1.55 lm was coupled into the fiber using a microscope objective with magnification 60 and NA = 0.85. Light intensity measurement is conducted using an InGaAs detector, mounted on a precise XYZ translation stage. The radius of the output light beam was measured at 5% of maximum intensity, and fiber sample was 50 cm long. At this length, any effects related to

5. Conclusions In this paper, for the first time, a large mode area photonic crystal fiber (LMA PCF) made of heavy metal oxide glass CS-740 was presented, designed for a broadband light guidance in the nearand mid-infrared regions. The fiber was modelled and then made using the stack-and-draw process. Fiber attenuation measured in the wavelength range 1–1.4 lm was below 2 dB/m, while its bending loss was 0.45 dB per loop of 8 mm radius and the NA was 0.53 at 1.55 lm. The proof-of-concept experiment demonstrated that the new fiber is suitable for making pigtails to quantum cascade lasers and MCT detectors. The fiber can be used in butt-coupling configurations allowing to couple light without any optics, which is simple, inexpensive and suitable for on-chip setups. Low chromatic dispersion of the fiber allows for transmission of short pulses at infrared wavelengths for applications in dynamic chemical spectroscopy.

Fig. 12. (a) Microscopic image of QCL and the end of fiber during movement towards the source. The outer diameter of the fiber is 120 lm. (b) Structure of QCL used.

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Acknowledgements This work is supported by the TEAM/2012-9/1 project within the Foundation for Polish Science Team Programme, co-financed by the European Regional Development Fund, Operational Program Innovative Economy 2007–2013, by the National Science Centre – Poland research Grant No. 2011/03/B/ST3/03337, and by the COST Action MP1204 TERA-MIR Radiation: Materials, Generation, Detection and Applications. References [1] J.C. Knight, T.A. Birks, R.F. Cregan, P.St.J. Russell, J.-P. de Sandro, Large mode area photonic crystal fiber, Electron. Lett. 34 (13) (1998) 1347–1348. [2] J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, R. Iliew, F. Lederer, J. Broeng, G. Vienne, A. Petersson, C. Jakobsen, High-power air-clad largemode-area photonic crystal fiber laser, Opt. Express 11 (7) (2003) 818–823. [3] J.C. Baggett, T.M. Monro, K. Furusawa, D.J. Richardson, Comparative study of large-mode holey and conventional fibers, Opt. Lett. 26 (14) (2001) 1045– 1047. [4] M.D. Nielsen, J.R. Folkenberg, N.A. Mortensen, A. Bjarklev, Bandwidth comparison of photonic crystal fibers and conventional single-mode fibers, Opt. Express 12 (3) (2004) 430–435. [5] T. Ritari, T. Niemi, H. Ludvigsen, M. Wegmuller, N. Gisin, J.R. Folkenberg, A. Petterson, Polarization mode dispersion of large mode-area photonic crystal fibers, Opt. Commun. 226 (1–6) (2003) 233–239. [6] J. Folkenberg, M. Nielsen, N. Mortensen, C. Jakobsen, H. Simonsen, Polarization maintaining large mode area photonic crystal fiber, Opt. Express 12 (5) (2004) 956–960. [7] M.-Y. Chen, Polarization and leakage properties of large-mode-area microstructured-core optical fibers, Opt. Express 15 (19) (2007) 12498–12507. [8] X. Tan, Y. Geng, E. Li, W. Wang, P. Wang, J. Yao, Characterization of bent largemode-area photonic crystal fiber, J. Opt. A: Pure Appl. Opt. 10 (8) (2008) 085303. [9] M. Napierała, T. Nasilowski, E. Beres´-Pawlik, P. Mergo, F. Berghmans, H. Thienpont, Large-mode area photonic crystal fiber with double lattice constant structure and low bending loss, Opt. Express 19 (23) (2011) 22628–22636. [10] T. Matsui, T. Sakamoto, K. Tsujikawa, S. Tomita, M. Tsubokawa, Single-mode photonic crystal fiber design with ultralarge effective area and low bending loss for ultrahigh speed WDM transmission, J. Lightwave Technol. 29 (4) (2011) 511–515. [11] W. Li, Q. Zhou, L. Zhang, S. Wang, M. Wang, C. Yu, S. Feng, D. Chen, L. Hu, Wattlevel Yb-doped silica glass fiber laser with a core made by sol–gel method, Chin. Opt. Lett. 11 (9) (2013) 91601. [12] L. Fu, H.A. McKay, L. Dong, Extremely large mode area optical fibers formed by thermal stress, Opt. Express 17 (14) (2009) 11782–11793. [13] R. Waynant, I. Ilev, I. Gannot, Mid-infrared laser applications in medicine and biology, Philos. Trans. R. Soc. A 359 (1780) (2001) 635–644. [14] R.F. Kazarinov, R.A. Suris, Possibility of the amplification of electromagnetic waves in a semiconductor with a superlattice, Sov. Phys.—Semicond. 5 (4) (1971) 707. [15] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Quantum cascade laser, Science 264 (1994) 553. [16] J. Devonson, O. Cathabard, A.N. Baranov, InAs/AlSb quantum cascade lasers emitting at 2.75–2.95 lm, Appl. Phys. Lett. 91 (25) (2007) 251102. [17] J. Devenson, R. Teissier, A.N. Baranov, InAs/AlSb quantum cascade lasers emitting below 3 lm, Appl. Phys. Lett. 90 (11) (2007) 111118. [18] S. Kumar, B.S. Williams, Q. Hu, J.L. Reno, 1.9 THz quantum-cascade lasers with one-well injector, Appl. Phys. Lett. 88 (12) (2006) 121123.

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