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Abstract
Depressed cladding concepts such as double line depressed index and tubular low index cladding waveguide structures were fabricated in bulk photo-thermo-refractive glass using femtosecond laser pulses. Effects of the writing laser power, waveguide geometrical structures and heat treatment on the light guiding properties were investigated. The results show that depressed cladding tubular waveguide design exhibits better guiding performances than double line waveguides in PTR glass. For the tubular cladding waveguide, single and multimode fields can be obtained for a wide processing parameter window. The simulations agree well with the experiment results. Moreover, the normalized frequency can be tuned and the quality of the depressed cladding tubular waveguide can be improved by eliminating uneven stress distribution after heat treatment.
© 2017 Optical Society of America
1. Introduction
Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass, which is doped with silver (Ag), cerium (Ce), fluorine (F) and a range of various other elements. The composition creates conditions for structural evolution under the action of light and heat. Specifically, the refractive index can be tuned locally by a combination of laser irradiation, electronic excitation and heat treatment, opening thus opportunities for optical design and applications in photonics [1]. The photo-thermal changes are made typically under UV irradiation conditions, in the spectral range of photosensitivity. The photo-thermal-refractive process is based on linear excitation of Ce3+ (photo-sensitizer in PTR glass) and in the formation of Ag0 via a reduction mechanism. Then, the final refractive index modulation in PTR glass is caused by thermal precipitation of a crystalline phase of sodium fluoride (NaF) crystals [2]. In addition to the photo-thermal ability for highly-resistant index engineering, several physical properties of PTR glass indicate advantages and extended potential for applications. These include superior thermal and mechanical stability, high optical damage threshold and a wide transparency range from 350 to 2700 nm [2–4]. These characteristics make PTR particularly interesting for holographic printing, filtering and data storage applications [5, 6]. Consequently several practical applications in advanced laser systems engineering can be foreseen [7, 8]. Recently, in view of potential applications for bulk optical elements, interest has appeared in infrared photosensitivity using ultrashort laser pulses.
The use of ultrashort pulses with sub-bandgap photon energies involves several new features in the interaction, notably due to the direct pumping of free electrons. Generally, femtosecond (fs) high intensity laser pulses determine a high degree of energy localization in the volume of a transparent material, due to swift nonlinear absorption. Energy is released to the matrix via electronic excitation. The rapid relaxation creates structural, chemical, and thermo-mechanical transformations associated with refractive index changes. This offers undeniable advantages for increased-processing precision, becoming essential for three-dimensional (3D) structures fabrication in bulk optical materials [9–11]. The structural change carries firstly an optical function due to the design of the refractive index, with impact in creating primarily waveguiding structures. This optical function derives from density and electronic changes, with the former being determinant in standard optical glasses. The associated optical functionalities and the possibility to fabricate integrated optical systems remain today one of the main applications of laser photo inscription [12, 13].
The laser irradiation is a complex event and additional local thermo-mechanical effects can occur followed by laser exposure. The associated change in volume accompanying the refractive index change [14] can create mechanical effects in terms of tensile and compressive stresses that can be localized within or outside the laser-affected area [15]. The stress design can contribute to the definition of waveguiding zones, particularly in crystalline materials but as well in optical glasses [16]. In addition, the combination of optical and mechanical features determines new potential in glass micromechanics and polarization optics [17]. Thirdly, the fs laser pulse can locally modify the chemical evolution of the excited matter, leading to elemental separation, changes of valence states and a range of free-electron-intermediated chemical effects [18]. All these create a large potential for tuning local properties of an optical material in various geometries. Thus fs laser photo-inscription developed into a unique and powerful technique to modify refractive index, stress-states and chemical states of optical materials by tailoring locally electronic and matrix structure directly [19–22]. Consequently, fs laser induced modifications have been used in various applications, including integrated optics [13, 23], optomechanics [24], optofluidics [25]. All these features play potentially an important role in the PTR glass for the design of the refractive index, and we analyze here the possibility to create waveguide structures using fs laser irradiation. These waveguide structures could be the fundamental elements in integrated optical devices, such as directional couplers, beam-splitters, laser amplifiers, ring resonators and waveguide lasers [26, 27].
In the present work, depressed cladding concepts such as double line depressed index and tubular low index cladding waveguide structures were fabricated in PTR glass by perpendicular (transverse) and parallel (longitudinal) writing scheme. They rely on laser and heat-induced negative index changes in the cladding region and, to a lesser extent, potential stress action in central areas. The objectives of this work are to study the guiding characteristics of waveguides under different irradiation conditions, notably with different writing power, and to follow the effect of the geometrical arrangements of the structures in the PTR glass. Moreover, based on the special thermo-sensitive characteristics of PTR glass, the influences of heat treatment (HT) on the structure, stress distribution and effects on the near-field modes of different waveguide structures were investigated in detail.
2. Experimental procedure
The schematic drawing of the experimental setup used for the photo inscription different waveguide structures is shown in Fig. 1. Two writing geometries were used to produce different waveguide structures whereby samples were moved perpendicularly or parallel to the writing beam. In the perpendicular/transverse writing scheme, structures of arbitrary length and design were allowed to be written as shown in Fig. 1(a). Linear depressed index traces are thus fabricated and mode guiding will be investigated in the space within. Secondly, a depressed cladding waveguide with circular cross-sections was fabricated in the parallel writing scheme (Fig. 1(b)) with longitudinal scan. The longitudinal writing scheme allows for a better circular symmetry of the traces and of the overall design. A Ti: Sapphire regenerative amplifier ultrafast laser system (Phidia, Upteksolutions) was employed as the irradiation source for waveguides fabrication in PTR glass. The system working conditions are given as follows: center wavelength 800 nm, pulse duration 160 fs, maximum available pulse power 600 mW and repetition rate 50 kHz. A half wave plate in combination with a thin film linear polarizer enables a fine control of the incident power. After the thin film polarizer, the fs laser pulses were linearly polarized in the horizontal direction (Y-axis). The exposure time was controlled by an electromechanical shutter. PTR glass with composition of (69-73)SiO2–(11-15)Na2O–7(ZnO + Al2O3)–3(BaO + La2O3)–5NaF–1KBr (mol%) doped with 0.02SnO2–0.08Sb2O3–0.01AgNO3–0.02CeO2 (mol%) was used as a sample material to produce the embedded waveguides. The PTR glass sample was shaped and polished to highly optical quality into a parallelepiped with a dimension of 10 mm × 5 mm × 2 mm. During the fabrication process of waveguides, samples were mounted on 3D precision translation stage. The writing velocity was fixed at 200 μm/s and the fs laser was focused inside PTR glass through a 20 × microscope objective with numerical aperture of 0.42. All of the waveguide structures had the length of 5 mm.
Fig. 1 Schematic drawing of the experimental setup for waveguides fabrication: (a) perpendicular (transverse) writing; (b) parallel (longitudinal) writing. The white arrows mark the motion directions of the samples.
The interaction region was observed in real time in top-view geometry by an Olympus BX51 positive phase contrast microscope (PCM). An additional incoherent white light source and a horizontally polarized laser of 980 nm optical source were used to characterize the guiding properties. These were simultaneously projected into one end face of the waveguide using an aspheric lens with focal length of 18 mm. Another facet of the waveguide was imaged to a charge coupled device (CCD) by a 5 × microscope objective.
3. Results and discussion
3.1 Double line depressed index waveguide
During the writing process, pairs of parallel low index tracks with different separation distances and writing powers were designed in order to inspect the influence on the guided mode. Geometrically similar to stress waveguides, the double line waveguide is formed in the region between the two tracks due to the decrease of the refractive index in the tracks and possible increase of stress-induced refractive index in between the tracks with an action range of few microns. This geometry and the combination of factors can be optimized to support guiding. The physical mechanisms of the fs laser irradiation in PTR glass leading to the decrease of the refractive index in the focal regions are discussed below. Generally the interaction between the fs laser and PTR glass results in a series of complex changes in the glass, including nonlinear photoionization, structural transitions, photo-chemical processes, and fast thermal quenching. These interactions could result in subtle structural and chemical modifications of localized regions in PTR glass in the fs-exposed area, being related to increasing free electron density. Provided that a threshold was surpassed, this can lead to mechanical expansion and local density drops in the laser tracks, with stress on the sides. Such a mechanism will be driven by the local temperature increase of the glass matrix, mechanical expansion and rarefaction, ending up in a low density trace in the laser irradiation zone. Consequently, the refractive index decreases in the traces and compressive stress may appear outside the traces on a scale of few microns, originating from photo and thermal expansion. It is to be noted though that at very low doses, slight augmentation of the index can appear signaling structural glass changes. The nonlinear ionization releases equally free electrons that will play an active chemical role, particularly in changing the valence state of Ag. Altogether, a combination of thermomechanical and chemical effects are expected to define the index change inside and in the vicinity of the laser-affected area. We follow below the guiding characteristics of the structures in relation to topology and geometry.
The end-face images of the waveguides illuminated by incoherent white light are shown in Fig. 2(a) and the near-field mode distribution under 980 nm injection are shown in Fig. 2(b)-2(e). Different writing power (30, 40, 50 and 60 mW) and line spacing (20, 25, 30 and 35 μm) are used to inscribe traces in PTR glass in order to test the guiding properties. The near-field mode profiles indicate that the waveguides are single mode at lower writing power and shorter intervals. These guiding characteristics are shown on the left side of the dotted line. In these parameters regions, the index contrast stays moderate. For example, owing to the small refractive index change in both the tracks and inside the tracks at low writing power of 30 mW, the near-field patterns show that the waveguides are weak and the guiding performance is poor. With the further increase of the writing power (40 and 50 mW), the guiding achieves good performance at line separation of 25 and 30 μm. However, under the largest power of 60 mW used here and for the widest line spacing of 35 μm, the guiding mode changes to the higher mode. By comparing the experimental results, the best guiding performance of single mode and a relatively uniform near-field pattern could be obtained with double line separation of 30 μm and writing pulse power of 50 mW for PTR glass (Fig. 2(d3)). In addition, the total losses of the double line waveguide, including coupling losses and propagating loss are evaluated by coupling a 980 nm laser beam into the waveguide and measuring the transmitted power. The total losses are evaluated at 2 dB/cm.
Fig. 2 Optical white-light transmission microscope images of the double line depressed index waveguide end facet (a), and corresponding near-field mode images under 980 nm injection for the double line waveguide realized with different laser writing powers (b)-(e) and different trace spacing (1-4). The yellow dashed line represents the division between single mode and multimode guiding for double line waveguide.
After analyzing the optical characteristics upon laser excitation, a subsequent HT is performed. It is commonly assumed that the material characteristics of the PTR glass would be changed after HT due to the aggregation of Ag0 producing atomic silver nucleation centers, the aggregation of nano-sized fluoride crystals and a series of photochemistry changes [28]. Therefore, it is necessary to explore the effect of HT on the guiding properties of double line waveguide in PTR glass. Figure 3 indicates the PCM, optical white-light transmission and near-field mode images of the double line waveguide with a laser writing power of 50 mW and double line separation of 30 μm before and after HT. In the present case, the PTR glass undergoes HT for atomic silver nucleation with an annealing time of 5 h at 450-490 °C and for fluoride crystallization for 3 h at 520-550 °C. For positive PCM, the bright zones characterize the domains of relative refractive index decrease. As shown in Fig. 3(a), when the fs laser irradiates the PTR glass, the refractive index will decrease in the laser track regions. As indicated, we can relate this to thermo-mechanical rarefaction. However, shallower black areas are noted at both end-points of the laser track regions are observed in dotted boxes in Fig. 3(a1) and (a2). This may be connected to a gentle increase of refractive index in prefocus regions characterized by weaker fs laser irradiation. The whole ensemble is able to guide single mode light as seen in Fig. 3(c1).
Fig. 3 Analysis before (1) and after (2) HT. End face PCM images (a), optical white-light transmission microscope image of the double line waveguide end face (b) and corresponding near-field mode images under 980 nm injection for the double line waveguide with writing power of 50 mW and double line separation of 30 μm (c).
The subsequent HT makes index map evolve. Compared to Fig. 3(a1), it is obvious that the refractive index of the tips of the laser track regions distinctly decreases after HT (Fig. 3(a2)). This further evolution can be attributed to the accelerated growth of nano-sized fluoride crystals in fs laser exposure regions, which have a lower refractive index than the PTR matrix [28]. Besides, the morphology becomes non-uniform and irregular (Fig. 3(b2)) at end-points of the filament regions and just around the traces. This uniformity could increase the scattering loss and deteriorate the guiding ability because of the material inhomogeneity, interface roughness and the degree of light confinement. This is indeed the case and the damage of the waveguide structure influences the guiding ability and mode field uniformity (Fig. 3(c2)). In conclusion, double line waveguide with HT will produce strong asymmetry and structural damage under these experimental conditions.
3.2 Depressed cladding tubular waveguide
Depressed cladding tubular structures contain a chain of laser-written cylinders with a negative index change that surrounds a core of defined diameter. A parallel writing scheme was used. The laser confinement capacity of such structures is strongly dependent on the number of cladding traces. So increasing the number of traces can deliver smooth and circular cladding walls, avoiding light leakage. Here, the numbers of cladding traces are 40, 55, and 70 for waveguide diameter of 30, 40 and 50 μm, respectively. The cross-section views of depressed cladding tubular waveguide with different diameters are presented in Fig. 4(a). Figure 4(b)-(e) reveal the near-field mode profiles of the tubular low index cladding waveguide with writing power of 70, 90, 110 and 200 mW and diameter of 30, 40, and 50 μm by an end-face coupling scheme under 980 nm illumination. Firstly, the spatial confinement ability for light is enhanced with a higher contrast of the cladding and therefore with the increase of the writing power. Secondly, when the waveguide diameter is set at 30 μm, the tubular waveguide acquires a favorable single mode character for various writing powers. Thirdly, the waveguides could not only support single mode but they can also carry higher mode distributions for a waveguide diameter exceeding 40 μm. In addition, the superposed hybrid mode fields can be obtained at waveguide diameter of 50μm. These results indicate that the normalized frequency can be modulated by deliberate design of the diameters of the core and higher modes can be supported. Furthermore, tubular cladding waveguides support large mode area guiding properties at waveguide diameter of 50 μm within a wide range of writing powers. This is in principle beneficial to delivering high efficiency power and compacting high power oscillating laser devices in integrated optical circuits.
Fig. 4 Optical white-light transmission microscopy images of the depressed cladding waveguide end facets (a) and corresponding near-field mode images under 980 nm injection for waveguides fabricated with different writing powers and possessing different section diameter (b)-(e).
In order to investigate the guiding modes of a depressed cladding tubular waveguide with diameter of 50 μm and realized at a laser writing power of 110 mW, the near-field mode images are displayed upon injection with the 980nm laser diode. The excited mode depends on the injection position. The various guiding modes are shown in Fig. 5(a), in which the pattern distributions are TEM01, TEM11 and TEM21 modes, respectively. Moreover, the simulation results of power flow distribution are presented in Fig. 5(b), using the finite element method. According to the experimental conditions, the calculation details are given as follows: the core diameter is 50 μm and cladding thickness is 2 μm. The refractive index of PTR matrix is 1.49. Since the refractive index variation induced by fs laser in PTR glass is usually in an order of magnitude of 10−4, the decrement of the refractive index in the cladding is defined as 3.4 × 10−4. It can be extrapolated from the mode distribution that the simulations agree well with the experiment results.
Fig. 5 Near-field mode images depicting TEM01, TEM11 and TEM21 mode under 980nm laser radiation supported by the cladding waveguide (a) and corresponding finite element method simulation results of the supported mode of waveguide (b). The structure is written with a laser writing power of 110 mW and diameter of 50 μm.
For the waveguide surrounded by traces written in longitudinal scan, the overall cross section of depressed cladding tubular waveguide is uniform and regular, and the morphology of track stays consistent before and after HT (Fig. 6(a1) and (a2)). As demonstrated in Fig. 6(b), large mode area fields with symmetric Gaussian distribution can be supported with or without HT by the tubular cladding waveguide at writing power of 110 mW and diameter of 50 μm. The total losses of the depressed cladding tubular waveguide are 1.53 and 1.39 dB/cm before and after HT, respectively. Besides, the growth of nano-crystals could be beneficial to increasing the effective refractive index variation of the depressed cladding waveguide after HT, which can enhance the spatial light confinement and reduce losses. Figure 6(d) illustrates that the mode intensity diameter at 1/e2 decreases from 40.67 to 35.52 μm in horizontal direction after HT. In addition, stress accumulating within the structure could equally influence the guiding properties of waveguide. To verify this statement, crossed polarized microscopy is conducted to investigate the stress distribution of tubular waveguide delivering a polarization microscopy image of the end facet before and after HT. As shown in Fig. 6(c1), a birefringence domain inside and around the cladding ring can be found without HT, which could be attributed to the induced stress distribution. However, the stress distribution that is not uniform and very difficult to control in the experiment, could be eliminated after HT, as shown in Fig. 6(c2). In addition, in order to investigate the influence of HT time on the refractive index variation via the divergence angle of the depressed cladding waveguide controlled by the average size of NaF crystal, the divergence angles of the tubular waveguides are tested after repeated HT. The divergence angle is about 3.58° after first HT and increases to about 5.37° after second HT. This will translate into an increase of the guide numerical aperture (NA), the index contrast and thus the normalized frequency. By calculation, the NA increased from 0.063 to 0.094, the index contrast increased from 8.1 × 10−4 to 1.8 × 10−3 and the normalized frequency increased from 5.57 to 8.31 after second HT. After the third HT, the divergence angle does not increase due to the growth of NaF nano-crystals are limited by the content in the original glass. Based on the above experimental results, it is concluded that the tubular structure can serve as an effective depressed cladding waveguide with a strong ability to confine light. Compared to double line depressed index waveguide, the tubular waveguide possesses better symmetry, large effective mode areas and thermal stability. Moreover, HT is favorable to eliminating uneven stress and increasing refractive index variation, divergence angle and the normalized frequency, which can lead to an enhancement of large multimode field guiding properties of depressed cladding tubular waveguide. Therefore, depressed cladding tubular waveguide design is potentially useful to design high efficiency power optical waveguide concepts with large mode areas.
Fig. 6 Optical white-light transmission microscopy images of the tubular waveguide end face (a), corresponding near-field mode images under 980 nm injection for the waveguide (b), birefringence images of depressed cladding tubular waveguide under crossed polarized microscopy (c) and mode field intensity distribution in horizontal direction of the waveguide (d). The top row (1) indicates the waveguide without HT and the bottom row (2) shows waveguide after HT. The laser writing power is 110 mW and the tube diameter is 50 μm.
4. Conclusions
In this work, double line depressed index and tubular depressed cladding waveguide structures were successfully fabricated by fs laser irradiation in bulk PTR glass. The waveguide microstructures and guiding properties were investigated before and after HT. For double line waveguide, double line separation, writing power and HT have significant effects on the guiding properties. The best guiding performance is obtained with double line spacing of 30 μm and writing pulse power of 50 mW for PTR glass without HT. Tubular waveguides have good light confinement capability and guiding properties. Single mode guiding is observed for waveguide diameter less than 30 μm. Multimode fields can be obtained by controlling the waveguide diameter. The simulations agree well with the experiment results. Moreover, HT provides a facile route to eliminate uneven stress distribution, increase the effective refractive index variation and control the normalized frequency of the depressed cladding tubular waveguide. These are key parameters to define the waveguide quality and determine modal control within the waveguide. These results indicate that tubular cladding waveguide exhibits a better guiding performance than the double line waveguide in PTR glass. Such good guiding properties indicate that PTR glass can be an interesting material to fabricate depressed cladding waveguides, a design that supports large mode area guiding and show interest for extrapolating the guiding properties to larger wavelength transport. In our future work, the change of average crystal size with HT time will be studied, which could be in favor of controlling the normalized frequency of waveguide.
Funding
National Natural Science Foundation of China (61378019, 61223007, 61471301).
Acknowledgments
The corresponding author thanks Prof. Kuaisheng Zou in SooChow University for sample HT.
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