Aperiodic phase masks for inscribing complex multi-notch OH-emission filters for astronomy
aa r X i v : . [ a s t r o - ph . I M ] D ec Letter Optics Letters 1
Aperiodic phase masks for inscribing complexmulti-notch OH-emission filters for astronomy K ALAGA M ADHAV , Z
IYANG Z HANG , AND M ARTIN
M R
OTH innoFSPEC, Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, Potsdam, Germany, 14482 * Corresponding author: [email protected] December 24, 2018
We demonstrate for the first time, a new type of aperi-odic phase mask (APM) for fabricating multi-channelaperiodic fiber Bragg gratings. The mask is made ofindividual diffraction phase gratings with discrete un-equal phase-steps incorporated at periodic locations.The diffraction at the discrete phase-steps in the phasemask produces corresponding half phase-steps at peri-odic locations along the fiber. The accumulated phase,along with index modulation, generates the desiredmultinotch reflection spectrum. Complex fiber Bragggrating filters fabricated using APM, in a standardphase mask based fabrication setup, can be used tosimultaneously suppress multiple aperiodic OH emis-sion wavelengths in near infrared (NIR) existing in up-per atmosphere, and increase the sensitivity of groundbased telescopes. © 2018 Optical Society of America
OCIS codes: (060.3735) Fiber Bragg gratings; (050.5080) Phase shift;(060.2340) Fiber optics components; (120.2440) Filters; (230.1480) Braggreflectors http://dx.doi.org/10.1364/ao.XX.XXXXXX
1. INTRODUCTION
Observation at near infrared wavelengths (NIR) between 0.9to 2.5 µ m are critical for modern astrophysics, as they provideaccess to objects heavily obscured by dust extinction, e.g. thesupermassive black hole at the galactic center, and star form-ing regions, to cool objects like AGB stars, to the high redshiftuniverse, etc. – to name but a few. Furthermore, the availabil-ity of high sensitivity large format image sensors and the ad-vent of adaptive optics have made the NIR an extremely attrac-tive wavelength range such that the new generation of largeground-based telescopes like the ELT, TMT, or GMT must beconsidered mainly NIR facilities. However, observations offaint objects in the NIR from the ground are overwhelmed bya sky background emission line spectrum that is typically 1000times brighter than the NIR light from the objects of interest.The emission occurs due to de-excitation of atmospheric hy-droxyl (OH) molecules in a cold layer of 6-10 km thickness ataltitudes of 90km . At the central wavelengths of these emis-sion lines, the signal of faint objects is heavily affected by the photon shot noise and strong residuals associated with the OHlines, so no reliable data can be recorded. Instead, one has toresort to the interline continuum, a technique also known asOH avoidance [1]. However, even when resorting to observa-tions between the bright OH lines, it has been discovered thatthe faint extended wings, that are due to scattered light occur-ring inevitably within the spectrograph, are still bright enoughto affect the detection limit of faint objects in the continuum.Therefore, it has been considered to filter out the OH lines athigh dispersion, however, first concepts have in reality not pro-vided convincing results (e.g. [2–4]). A radically new idea wasproposed by [5] which consists of a filter placed in front of theoptical system before the light enters the spectrograph, thus giv-ing nowhere an opportunity to create scattered light [6, 8]. Inorder to suppress or filter out the OH emission lines, an op-tical filter capable of > > ∼
100 lines has been previously demonstrated in theGNOSIS experiment [7, 9].However, fabricating such filters, with good reproducibil-ity is not a trivial task and requires accurate control of the in-tensity, phase and exposure length of a complex interferencepattern over a moving photosensitive optical fiber. Simpleor complex gratings can be fabricated through point-by-point(PbP) [10] or line-by-line (LbL) [11] inscription process usingfemtosecond lasers and de-phasing methods [12]. Ultra-longgratings have been fabricated using electro-optic modulators(EOMs) [13] in push-pull configuration, and complex OH filtershave been fabricated using acousto-optic modulators (AOMs)[14]. E- or A-OMS fabrication techniques generate a “running-interference”pattern, similar to a rack-and-pinion, that is syn-chronised with the velocity of optical fiber, requiring precisecontrol on the intensity, focus spot size, and velocity over along length in real time. Phase mask offers a convenience thatpreviously mentioned methods do not offer. By transferringthe complexity of fabrication, such as in femtosecond, EOM orAOM techniques, to the one-time manufactured complexity inthe phase mask, the convenience is preserved. Requiring nomoving parts, or stringent alignment, the complex phase maskcan be used off-the-shelf in a standard UV based FBG inscrip-tion setup. In this paper, we introduce for the first time the de- etter Optics Letters 2 sign of an aperiodic phase mask to inscribe multi-channel ape-riodic filters in hydrogenated or doped photosensitive fibers, inorder to suppress the night-sky OH emission.
2. APERIODIC BRAGG GRATING
The index modulation ∆ n g and phase φ g , of the complex grat-ing can be reconstructed from the desired reflection spectrum | r | , by using layer peeling method described in [21, 22]. For thedesign of APM, we selected OH sky emission lines in H-bandranging from 1400nm to 1700nm [23]. For compatibility withfuture tests on PRAXIS [24] system, that uses the existing GNO-SIS filters, the full-width half maximum (FWHM), transmission,and wavelength of OH lines are selected as given in Tables.(1,2)in [8]. The aperiodic filter is defined by [15], | r (cid:0) λ (cid:1) | = p R i N ∑ i = exp " − ( π n e f f p i (cid:16) λ − λ i (cid:17)) q i × exp " i π n e f f λ − λ ! g i (1) where, R i is the desired reflectivity of individual OH-emissionline filter, N is the number of filters, ( p i , q i ) define the shape ofthe individual filters, n e f f is the effective index, λ is the seedgrating, which also defines the APM’s pitch Λ m , and g i is theindividual channel’s group delay. Since there exists an upperlimit to the index change achievable in a fiber, g i can be opti-mised to reduce the maximum index modulation required. Theright choice of g i , or “de-phasing”, [12, 15] effectively spreadsthe individual gratings over the length of the grating, insteadof crowding them in the same location spatially. We now de-scribe the steps required to design a mask that can be used ina standard UV inscription setup to fabricate a grating with areflection spectrum defined by eq.(1).
3. DESIGN OF APERIODIC PHASE MASK
It is well known [16–18] that when a φ m -shifted phase mask isused in side-writing technique for inscribing FBGs, the phaseshift in the mask is split into two half-phase shifts ( φ m ), sepa-rated by ∆ z = y tan θ in the fiber, as shown in Fig.1, where, y is the distance between the mask and the fiber core, and θ is theangle of diffraction of ± Fig. 1.
Schematic showing the propagation of phase maskphase to fiber grating phase.The cumulative phase of the light propagating through thefiber gives the desired phase φ g in the grating. For example, ifa π -shifted phase mask is used to fabricate the grating, alongthe grating length there will be two locations with φ g = π /2phase. In the transmission spectrum, we would see the charac-teristic single narrow transmission window at the filter center. With increasing φ m , the narrow transmission window withinthe Bragg stopband shifts to longer wavelength [19]. To achievethe desired phase φ g in the grating, the width of the groove, orphase-step, δ m , in the phase mask is given by, δ m = Λ m π (cid:16) π + φ g (cid:17) (2) For example, if φ g = − π for the standard π − shifted FBG, wewill require a phase mask with δ m = Λ m at the center of themask of length L . If we use this mask for fabrication, in the fiberthe phase will be split, φ g = (cid:0) − π /2, − π /2 (cid:1) . By tuning δ m , orequivalently φ m , we can inscribe a desired φ g in the grating atselected locations. Also, when δ m = Λ m /2, then we get φ g = φ g ). We use layer peeling (LP)technique to first derive the grating’s complex coupling coeffi-cient, κ , from which we can extract φ g . By knowing φ g , we canthen calculate the phase-steps δ m of the APM using eq.(2). Fig. 2.
Representative 2D model of a section of the APM show-ing two Λ m /4 shifted grooves separated by ∆ z correspondingto ( − π , + π ) phase. ρ = λ uv /2 ( n uv − ) is the groove depthof the phase mask, defined by the wavelength λ uv of the laserused for fabrication, and the refractive index n uv of the maskmaterial.
4. SIMULATION AND DISCUSSION
Fig.3 shows the FDTD simulation for phase steps at two lo-cations on a phase mask of 60 µ m length, separated by 20 µ m ,where mask period Λ m = µ m . The two phase steps δ m = n Λ m , Λ m o corresponding to mask phases φ m = (cid:8) − π , + π (cid:9) ,respectively, result in four half-phase regions in the fiber.For designing the APM specifically made to fabricate anaperiodic grating, we first define the filter characteristics, suchas, transmission, FWHM and channel center. We chose N =
37 OH-emission lines between 1500nm to 1600nm, and L = ∆ n and φ g , as shownin Fig.4. For covering the filter’s bandwidth ( β ), we will requirea φ g , or δ m discretization interval or layer thickness in layer peel-ing [22], ∆ z = π / β = µ m . We use eq.(2) to calculate theAPM’s groove width δ m from φ g . Since φ g = [ − π , + π ] rad, wehave δ m = [ ] nm. δ m can be incorporated in the mask as a nonlinear chirp,where the groove width δ m , varies continuously over the lengthof the mask. To achieve accurate continuously varying δ m using etter Optics Letters 3 Fig. 3.
Near field of phase mask with two phase steps of − π and + π . ∆ n -1000-500050010001500 P ha s e φ (r ad / π ) λ ( µ m) T r an s m i ss i on ( d B ) δ m ( n m ) Fig. 4.
Transmission spectrum, index modulation, phase andAPM groove width for fabricating an aperiodic filter. e-beam process would be challenging. Alternatively, we designthe mask with a global mask pitch of Λ m = λ = ∆ z along the seed phase mask, we in-corporate grooves of width δ m defined by eq.(2). An exampleof a mask segment with two δ m is shown in Fig.2.
5. CONCLUSION
We have shown the steps involved in transferring the spatialstructure of an aperiodic fiber Bragg grating to the correspond-ing structure in an aperiodic phase mask. Fabrication of themask with the desired groove accuracy at periodic intervals, oras a continuous chirp is not a trivial task. However with recentadvances in e-beam processes, the accuracy required to repro-duce δ m is a reality. Fabrication of APMs based on the methoddescribed in this paper is currently ongoing. With APMs, thecomplexity of alignment in fabrication setups such as EOM,AOM or femtosecond laser, is now transferred to the fabricatedcomplexity in the mask, facilitating the use of standard phasemask fabrication to inscribe complex gratings. ACKNOWLEDGEMENTS
This work is supported by the BMBF project “Meta-ZiK As-trooptics” (grant no. 03Z22A511).