Ultrafast laser fabrication of Bragg waveguides in chalcogenide glass

Abstract

Bragg waveguides are fundamental components in photonic integrated circuits and are particularly interesting for mid-IR applications in high index, highly nonlinear materials. In this work, we present Bragg waveguides fabricated in bulk chalcogenide glass using an ultrafast laser. Waveguides with near circularly symmetric cross sections and low propagation loss are obtained through spatial and temporal beam shaping. Using a single-pass technique, the waveguide and Bragg structure are formed at the same time. First through sixth order gratings with strengths of up to 25 dB are realized, and performance is evaluated based on the modulation duty cycle of the writing beam. Chalcogenide (ChG) glasses are known for their excellent mid-IR transparency and large nonlinear refractive indices of up to 1000 times that of fused silica [1,2]. These unique traits make them attractive in a wide range of applications. In particular, ChG glasses are suitable substrate materials for fabricating photonic integrated circuits (PICs), due to inherently large χ 3 nonlinearities with ultrafast response times and low two-photon absorption at telecom wavelengths. These properties become especially important in applications such as high-data-rate signal processing [3][4][5]. In such PIC devices, Bragg waveguides are often employed as basic building blocks, enabling operations such as 2R regeneration [6,7], and all-optical switching [8]. Fabrication of these structures generally relies on thin film deposition and various photolithography schemes. However, thin-film processes incur restrictions when building multilayer structures for complex optical interconnects by limiting devices to a planar geometry. Ultrafast laser writing offers great design flexibility with its capability for making three-dimensional structures [9,10] and is a proven technique to form high-quality waveguides in bulk optical substrates. Extensions of this technology have also been applied to the fabrication of Bragg waveguides in fused silica using several approaches, such as the point-by-point technique [11][12][13], and the multiscan technique [14]. Applying these methods to ChG glasses, however, presents a unique design challenge. The high refractive index of these materials induces spherical aberrations at the focus of the writing beam, which are further compounded by nonlinear effects brought about by high-intensity laser pulses. These interactions often lead to self-focusing and filamentation, producing highly asymmetrical waveguides with an elongated cross section and multiple guiding regions [15]. Mitigation of these effects is essential for fabricating high-quality waveguides in ChG glasses.Recently, it has been demonstrated that temporal [16] and spatial beam shaping [17] may be used to control the size and shape of the cross section of a laser-written waveguide. These techniques, combined with adjustment of other processing parameters such as writing speed, pulse energy, and repetition rate, have produced nearly symmetric waveguides with propagation losses ...

Single-mode sapphire fiber Bragg grating

“…A new approach was therefore needed to fabricate WBGs suitable for sapphire fibers requiring high temperature stability. We developed a three-step process as follows: The WBG fabrication method is similar to one employed for bulk glass material [30], where the refractive index modulation is formed by utilizing the intrinsic shape of the Gaussian beam along the beam propagation direction. Figure 6(c) shows the top-view of a fabricated WBG structure.…”

Fabricating waveguide Bragg gratings (WBGs) in bulk materials using ultrashort laser pulses

“…For comparison, Table 1 lists κ values for The duty cycle was fixed at 35%, whereas the writing pulse energy was varied. [21] PbP 1551 25 -Boro-aluminosilicate (Eagle2000) [22] PbP 1551 145 0.6 Boro-aluminosilicate (Eagle2000) [23] PbP 1550 472 0.5 Soda-lime [27] Two-step PbP 1577 14 -Fused silica [26] Modulated burst 1548 111 1.5 Er:Yb codoped phosphate [30] Modulated burst 1537 242 -Yb-doped phosphate [42] Modulated burst 1535 221 -Boro-aluminosilicate (Eagle2000) [31] Modulated burst 1551 713 -Fused silica [43] Modulated burst 1546 36 0.7 Boro-aluminosilicate (Eagle2000) [24] PbP 799 65 -Boro-aluminosilicate (Eagle2000) [28] Modulated burst 1552 157 -ZBLAN [29] PbP überstructure 1550 324 -Boro-aluminosilicate (Eagle2000) [44] Multiscan modulated burst 1563 177 -LiNbO 3 [45] Modulated burst stressors 1540 45 -Boro-aluminosilicate (Eagle2000) [41] PbP 648 136 -Boro-aluminosilicate (Eagle2000) [41] PbP 698 183 0.8 Boro-aluminosilicate (Eagle2000) [41] PbP 748 187 -Boro-aluminosilicate (Eagle2000) [41] PbP 798 163 -Fused silica [46] Modulated burst 1550 210 2.2 Fused silica [47] Modulated burst 1549 280 < 1 Chalcogenide (GLS) [48] Modulated burst 1551 179 -LiNbO 3 [49] Multiscan modulated burst (depressed clad) 1558 1230 1.52-3.51 Boro-aluminosilicate (Eagle2000) [50] Two-step PbP 1545 120 -Fused silica [25] PbP (waveguide bundle) 840 126 -Fused silica [25] PbP (waveguide bundle) 1550 220 1.6 Borosilicate (AF45) [51] Modulated …”

“…Chalcogenides being transparent up to 5 mm are a potential host for this application. Through spatial and temporal beam shaping, McMillen et al reported WBGs with circular cross-sections in GLS chalcogenide glass [48]. The strongest WBGs were written with a 25% duty cycle, were single mode at 1550 nm, and had grating depths of up to 25 dB.…”

Comparative analysis of ways for femtosecond laser formation of optical waveguides

“…Прямая фемтосекундная запись является перспективной безмасочной и ши-роко распространенной технологией создания оптических волноводов в объеме стекол и кри-сталлов [1][2][3][4][5][6][7]. Оптические стекла и кристаллы могут быть разделены на три группы на осно-ве знака постоянно изменяющегося показателя преломления (ПП): 1) материалы, ПП которых увеличивается при фемтосекундной микрообработке; 2) материалы, ПП которых понижается;…”