We report the ultrafast laser fabrication and mid-IR characterization (3.39
microns) of four-port evanescent field directional couplers. The couplers were
fabricated in a commercial gallium lanthanum sulphide glass substrate using
sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed
using optimal fabrication parameters were found to exhibit propagation losses
of 0.8 dB/cm. A series of couplers were inscribed with different interaction
lengths, and we demonstrate power splitting ratios of between 8% and 99% for
mid-IR light with a wavelength of 3.39 microns. These results clearly
demonstrate that ultrafast laser inscription can be used to fabricate high
quality evanescent field couplers for future applications in astronomical
interferometry.Comment: 4 pages, 4 figure

Arriola, Alexander

Choudhury, Debaditya

Labadie, Lucas

Mukherjee, Sebabrata

Thomson, Robert R

English

arXiv

Alexander Arriola

Sebabrata Mukherjee

Debaditya Choudhury

Lucas Labadie

Robert R. Thomson

Crossref

Optics Letters

Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry

We report the ultrafast laser fabrication and mid-IR characterization (3.39 μm) of four-port evanescent field directional couplers. The couplers were fabricated in a commercial gallium lanthanum sulfide glass substrate using sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed using optimal fabrication parameters were found to exhibit propagation losses of ∼0.8  dB⋅cm−1. A series of couplers were inscribed with different interaction lengths, and we demonstrate power-splitting ratios of between 8% and 99% for mid-IR light with a wavelength of 3.39 μm. These results clearly demonstrate that ultrafast laser inscription can be used to fabricate high-quality evanescent field couplers for future applications in astronomical interferometry

Heriot Watt Pure

    Heriot-Watt University Research Gateway                                                          Heriot-Watt UniversityUltrafast laser inscription of mid-IR directional couplers for stellar interferometryArriola, Alexander; Mukherjee, Sebabrata; Choudhury, Debaditya; Labadie, Lucas; Thomson,Robert RPublished in:Optics LettersDOI:10.1364/OL.39.004820Publication date:2014Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Arriola, A., Mukherjee, S., Choudhury, D., Labadie, L., & Thomson, R. R. (2014). Ultrafast laser inscription ofmid-IR directional couplers for stellar interferometry. Optics Letters, 39(16), 4820-4822. 10.1364/OL.39.004820General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Ultrafast laser inscription of mid-IR directionalcouplers for stellar interferometryAlexander Arriola,1,* Sebabrata Mukherjee,1 Debaditya Choudhury,1 Lucas Labadie,2 and Robert R. Thomson11Institute of Photonics and Quantum Sciences (IPaQS), School of Engineering and Physical Sciences,Heriot-Watt University, Edinburgh, EH14 4AS Scotland, UK2I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany*Corresponding author: a.arriola@hw.ac.ukReceived May 27, 2014; revised July 11, 2014; accepted July 16, 2014;posted July 17, 2014 (Doc. ID 212768); published August 11, 2014We report the ultrafast laser fabrication and mid-IR characterization (3.39 μm) of four-port evanescent field direc-tional couplers. The couplers were fabricated in a commercial gallium lanthanum sulfide glass substrate using sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed using optimal fabrication parameters werefound to exhibit propagation losses of ∼0.8 dB · cm−1. A series of couplers were inscribed with differentinteractionlengths,and we demonstrate power-splitting ratiosof between 8%and 99%for mid-IR light with a wave-length of 3.39 μm. These results clearly demonstrate that ultrafast laser inscription can be used to fabricatehigh-quality evanescent field couplers for future applications in astronomical interferometry. © 2014 Optical So-ciety of AmericaOCIS codes: (130.2755) Glass waveguides; (140.3390) Laser materials processing; (220.4000) Microstructure fabri-cation.http://dx.doi.org/10.1364/OL.39.004820Astronomical optical instruments that utilize photonicconcepts [1] such as integrated optical waveguides[2–7], Bragg gratings [8,9], and laser frequency combs[10,11] have the potential to revolutionize astronomyin a plethora of different areas. One such area is stellarinterferometry, which, by combining light gathered bymultiple widely spaced telescopes, enables imaging ofcelestial objects with extremely high spatial resolution(sub-milli-arcsecond [12–14]), well beyond that realisti-cally achievable using a single telescope.To image an object using a telescope interferometerarray is a highly nontrivial task. The collected light mustfirst be transported to a central location using vacuumdelay lines. A high-speed fringe tracker is also necessaryto correct for path length changes due to atmosphericconditions. Once at the central location, the transportedlight must be coherently combined using a beam-combin-ing instrument [15], which analyzes the mutual coher-ence between the light from each pair of telescopes inthe array. In 1996, Kern and Malbet [16] proposed thatintegrated optic (IO) waveguide technologies could pro-vide an inherently stable and scalable platform for beamcombination. Since then, IO beam combiners [6,17–20]have been utilized in a variety of leading interferometricbeam-combination instruments, such as the PIONIERbeam combination instrument for the VLTI [17], whichoperates in the astronomical H- and K-bands, between1.5 and 1.8 μm and 2.0 and 2.4 μm, respectively.One of the primary science drivers for the future devel-opment of stellar interferometry is exoplanetary science,since it should enable high-resolution direct imaging ofcircumstellar disks and Earth-like exoplanets. These coolobjects predominantly emit blackbody radiation in themid-infrared (mid-IR) [21], and, for this reason, themid-IR wavelength band (i.e., 2.0–12.0 μm) is a spectralregion of particular importance to modern astronomyand stellar interferometry in particular. There is, there-fore, a need to develop IO beam-combination technolo-gies for these longer mid-IR wavelengths.To date, prototype mid-IR IO beam combiners havebeen demonstrated using a variety of platforms, includ-ing Ti-indiffusion in LiNbO3 [22,23] and laser-writtenwaveguides on As2Se3 thin films [4]. Both of these tech-nologies are planar, however, which makes low crosstalkwaveguide crossovers extremely difficult. Furthermore,both of these technologies also result in optical wave-guides with asymmetric refractive index profiles, a po-tential problem for high-efficiency input coupling oflight from an optical fiber.With the drawbacks mentioned above in mind, werecently [24] used ultrafast laser inscription (ULI) [2]t ofabricate a prototype three-telescope mid-IR IO beam-combination component. In contrast with other laserwriting technologies, ULI relies on the nonlinear absorp-tion of focused sub-bandgap ultrashort pulses to locallymodify the refractive index of the substrate material. Bytranslating the material in three dimensions through thelaser focus, ULI enables the direct inscription of 3D op-tical waveguide circuits with zero crosstalk crossoversand highly symmetric refractive index profiles. In ourearlier proof-of-concept work, however, the beam com-bination was performed using Y junctions. These junc-tions are inherently lossy, and a useful IO beam-combiner must utilize evanescent field couplers ratherthan crude Y junctions. In this work, we report theULI fabrication and characterization of mid-IR evanes-cent field couplers for operation in the astronomicalL-band (3.39 μm).All structures reported in this Letter were fabricatedusing a Menlo BlueCut fiber laser, which supplied∼400 fs pulses of 1030 nm light at a pulse repetition fre-quency of 500 kHz. For our purposes, the polarization ofthe laser was adjusted to circular and focused approxi-mately 200 μm below the surface of the substrate usinga 0.4 NA lens. To translate the substrate [gallium lantha-num sulfide (GLS) from ChG-Southampton] through thelaser focus, we used a set of computer-controlled Aero-tech air-bearing XYZ translation stages (ABL series). For4820 OPTICS LETTERS / Vol. 39, No. 16 / August 15, 20140146-9592/14/164820-03$15.00/0 © 2014 Optical Society of Americathe remainder of this Letter, we will refer to the laserpropagation axis inside the sample as the z axis, theprimary waveguide axis as the x axis, and the y axisas the orthogonal axis to both the z and x axes.A ULI parameter investigation was performed, duringwhich single straight waveguides were inscribed usingpulse energies ranging from 80 to 15 nJ (in steps of 5%variation from the previous pulse energy), and transla-tion velocities of 2, 4, 8, and 16 mm∕s. The cross sectionof the inscribed waveguides was controlled using thewell-known multiscan waveguide shaping technique[25–27]. In this case, 21 scans with an interscan separa-tion of 0.3 μm along the y axis were used. After inscrip-tion, the end facets of the structures were ground andpolished to an optical quality finish. The guiding proper-ties of the inscribed structures were then investigated bycoupling vertically polarized 3.39 μm light from a He–Nelaser into the structures using free-space coupling, whileimaging the output of the structure onto a WinCam3Dmicrobolometer beam profiler. This camera is sensitivein the wavelength range of 2–16 μm. The optimal wave-guides within the parameter range were found to bethose fabricated using 70 nJ pulses (incident on thesample), a translation velocity of 4 mm∕s.As shown in Fig. 1, these waveguides were observedto support a single mode at 3.39 μm, with a mode fielddiameter ∼22.43  0.5 μm and ∼26.56  0.5 μm in the yaxis and z axis, respectively (Fig. 1). The propagationloss of these optimum waveguides was measuredusing the cut-back technique and found to be ∼0.80.05 dB · cm−1.Using the optimal ULI parameters identified above, aseries of evanescent field couplers were inscribed.Figure 2 shows a bright-field microscope image of the in-put facet of the two waveguides of the coupler fabricatedin GLS glass (a), together with the two output modes ofone of the couplers (b).Each of the couplers were formed from two single-mode waveguides, and each of these waveguidesconsisted of straight lead-in and lead-out sections, bendsthat were inscribed had a sine-squared form [yxA · sin22πx∕4Lc, with A  40 μm and Lc  3 mm],and a straight interaction region of length L (Fig. 3). Arepresents the change in the y axis for the curve, whileLc is the distance in the x axis for this lateral displace-ment to occur. The lead-in and lead-out sections of thetwo single-mode waveguides for each coupler wereseparated by 100 μm to avoid evanescent coupling inthese sections. For all couplers, the core-to-core separa-tion in the interaction region was set to 20 μm. Whilekeeping all other parameters constant, 32 couplers wereinscribed, using interaction lengths between 2 and 10 mmin steps of 250 μm.The power coupling ratios of each coupler were evalu-ated by measuring the Port 2/(Port 2 + Port3) outputpower ratio when coupling vertically polarized 3.39 μmlight into Port 1. These measurements were made usinga PbSe fixed-gain detector (1.5–4.8 μm) from Thorlabs.Figure 4 shows the through-port coupling ratio as a func-tion of interaction length, where it can be seen that themeasured coupling ratio varies between 8% and 99%, witha beat length of ∼5.75 mm. It also can be seen fromFig. 4 that the coupling ratio would be unlikely to reach100%, regardless of the sampling of the interaction length.This behavior is characteristic of differences in thepropagation constants of the two single-mode wave-guides that form the coupler [28], a possibility that couldbe due to a variety of reasons. For example, it is possiblethat the optical properties (e.g., mode index) of the firstwaveguide for each coupler are directly affected, but theinscription of the second waveguide (or indeed viceversa), or it is also plausible that temperature variationsFig. 1. Mode field profile at 3.39 μm of the single-modewaveguide.Fig. 2. (a) End-on bright-field microscope image of the twowaveguides of a coupler inscribed in GLS glass using themultiscan technique and (b) together with the two modes ofone of the couplers.Fig. 3. Schematic of the interference coupler structure, whered is the distance between the two waveguides along theinteraction length (L).August 15, 2014 / Vol. 39, No. 16 / OPTICS LETTERS 4821in the lab (∼  1°C) during the inscription process aresufficient to change the ULI parameters over the courseof the experiment.We observed that the couplers exhibited the same totalthroughput as a straight single-mode waveguide of thesame length, indicating that bend losses and radiationlosses in the couplers were not significant. Based on a∼0.8 dB∕cm  0.05 dB∕cm propagation loss, as mea-sured for the straight single-mode waveguides, we con-clude that the couplers would exhibit a maximum lossof ∼1.6 dB∕cm  0.1 dB∕cm. This, of course, assumeszero Fresnel reflections and input and output couplinglosses.In conclusion, we have demonstrated the ULI fabrica-tion of prototype four-port evanescent field couplers forthe mid-IR region. The devices were tested using mono-chromatic 3.39 μm light and were found to exhibit clearpower-coupling dependency on interaction length, with abeat length of 5.75 mm. The weak dependency of thepower-coupling ratio on interaction length will, in thefuture, enable precise fine tuning of the coupler charac-teristics for optimal operation in beam-combinationinstruments. We believe that this demonstration, whencombined with the 3D fabrication capability of ULI, willopen the way toward highly stable and scalable beam-combination instruments for mid-IR stellar interferom-etry and nulling interferometry.R.R.T. gratefully acknowledges funding from theSTFC, in the form of an STFC Advanced Fellowship(ST/H005595/1) and through the STFC Project Researchand Development (STFC-PRD) scheme (ST/K00235X/1),and also from the Royal Society (RG110551). We wish tothank the European Union for funding via the OPTICONResearch Infrastructure for Optical/IR astronomy(EU-FP7 226604). 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Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry

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