We report on the fabrication, by a 26 MHz stretched-cavity femtosecond Ti:sapphire oscillator, of optical waveguides in different glass substrates, and their optical characterization. Operation of these waveguides in the telecom range at 1.55 µm is demonstrated. Digital holography microscopy is used to measure their refractive index profile. The results evidence a strong dependence of the fabrication process on the glass matrix. ©2005 Optical Society of America
©2005 Optical Society of America
Optical waveguide writing with femtosecond laser pulses is rapidly becoming a valid alternative to standard fabrication techniques, due to its simplicity, flexibility and 3D structuring capabilities [1–15]. Two different regimes of material modification have been identified: the low repetition rate regime (<200 kHz) [1–11] and the high repetition rate regime (>1 MHz) [12–15]. In the former, energetic mildly focused pulses are employed and the single pulse is responsible for the modification of the whole focal volume; in the latter, using low-energy tightly focused pulses, thermal diffusion and cumulative effects give rise to a modified volume larger than the focal one.
Low repetition rate systems use amplified Ti:Sapphire lasers with energy of a few µJs, while high-repetition rate systems use stretched-cavity oscillators with energies of a few tens of nJs. High repetition rate systems offer several advantages, such as: (i) simpler and compact setup, avoiding amplification stages; (ii) greater processing speed, up to 20 mm/s; (iii) intrinsic symmetry of the waveguide transverse profile, determined by the isotropic heat diffusion. Up to now however this regime has been only partially explored, since waveguide writing has been demonstrated only in a few glass types (Corning 0211, 0215 and 7890) and there have been no reports on waveguides operating at the telecom wavelength of 1.5 micron [12–15].
Another important issue for waveguide characterization and for understanding the physical mechanisms underlying the fabrication process is an accurate measurement of their refractive index profile. The standard technique to measure refractive index profiles of the femtosecond laser written waveguides is refractive near field profilometry . An alternative technique, recently proposed, is based on the difference in etching speed in irradiated zones , but it is strongly dependent on the glass substrate and requires specific calibration.
In this paper we report on the fabrication and optical characterization of optical waveguides written with a stretched-cavity Ti:Sapphire oscillator. Operation of these waveguides at 1.5 micron wavelength is demonstrated in two commercial glasses: Corning 0211, and the previously unexplored Schott IOG10. We propose for the first time the use of the Digital Holography Microscopy (DHM) technique for an accurate refractive index profile measurement in this kind of structures. Although DHM requires the preparation of thin sample slices, it offers the advantages of high spatial resolution, high sensitivity and it provides an absolute value of refractive index change without the need of any calibration. In addition it enables the measurement of the refractive index profiles of birefringent structures if different holograms are recorded for different states of polarization of the probing light . The DHM measurements evidenced a strong dependence of the fabrication process on the type of the glass substrate.
2. Experimental setup
2.1 Waveguide writing setup
The writing setup (Fig. 1) is composed of a stretched-cavity Ti:Sapphire laser, a pulse pre-compression system, a focusing objective and a translation unit. The home-made Ti:Sapphire system, pumped by 5 W of a frequency-doubled Nd:YVO4 laser (Coherent Verdi), uses a 5-mm rod and a standard asymmetric cavity for Kerr-lens mode-locking . The cavity length is stretched by a 1:1 telescope composed of two R=2000 mm concave mirrors in a 4f arrangement. The telescope has a unitary ABCD matrix thus preserving, with respect to the standard cavity, the parameters required for stable mode-locking. The resulting repetition rate is 26 MHz; using an output coupler transmission of 30%, a pulse energy of 30 nJ at 800 nm with a 30-fs duration is obtained. Pump power higher than 5 W resulted in pulse breakup and instabilities due to excessive intra-cavity self-phase-modulation.
Due to the limited pulse energy of the laser, the short pulse duration must be preserved in order to achieve the peak intensity necessary for material modification. In particular, the large amount of glass inside the focusing oil-immersion 100X objective, used to write the waveguides, increases significantly the pulse duration due to second-order dispersion. For this reason an extra-cavity pre-compression stage is used to provide the negative dispersion necessary to exactly compensate for that introduced by the focusing objective. The precompression stage is formed by two SF10 prisms put at 41 cm distance with an end mirror for a standard two-pass configuration (Fig. 1). A collinear second-order autocorrelation of the pulse is performed using the 100X objective to focus the two delayed pulse replicas on the second harmonic crystal (100 µm thick KDP), in order to measure the pulse duration directly at the focus of the writing objective. Without any pre-compensation, the autocorrelation (Fig. 2(a)) evidences a strong chirp in the pulse (2500 fs2), resulting in a pulse duration of 220 fs. Using the pre-compressor previously described, it is possible to cancel the pulse chirp nearly completely and to obtain an autocorrelation in the focus of the 100X objective corresponding to a nearly transform-limited duration of 30 fs (Fig. 2(b)).
The beam after the pre-compressor is deviated in the vertical direction by a 45° mirror and then focused inside the sample by a 100X oil-immersion objective (Zeiss Plan-Apochromat). The writing beam completely filled the objective aperture, thus fully exploiting its 1.4 NA. The glass sample is mounted horizontally on high resolution micro-translation stages (Physik Instrumente – PI) allowing high-speed movements in the x and y directions. We found that this configuration has the lowest sensitivity to mechanical vibrations, which induce microbendings that are detrimental for the waveguide losses. For 3D structuring of the device also the control of the focus position in the z direction is important, but in order to keep the mounting stage of the sample low and compact we preferred to move the position of the microscope objective in that direction.
2.2 DHM setup
Digital Holography (DH) in a microscope configuration has been used for accurately measuring the refractive index profile of the waveguides. DH is an interferometric method useful for measuring slight changes in the optical path length (OPL) as experienced by a spatially coherent wave-front transmitted or reflected by a surface. The experimental set-up adopted in this work, schematically shown in Fig. 3, is based on a classical Mach-Zehnder configuration. A laser source emitting at a wavelength of λ=532 nm is split into two beams, the reference and the object beam respectively. The object beam is sent through the transparent sample. The wave front emerging from the sample is imaged by a microscope objective (MO) in a plane at a distance d in front of the CCD array plane. By DH it is possible to obtain the amplitude and phase map by reconstructing numerically the object wave-front back at distance d by using the scalar diffraction theory within the Fresnel approximation of the Rayleigh-Sommerfeld diffraction integral . The phase map gives information on the spatial distribution of the OPL. For measuring the refractive index profile of the waveguide, a thin slice of the sample was prepared. The thickness t of the slice was chosen to have OPL values such that OPL=|Δn|max t<λ, where |Δn|max is the maximum expected value in the refractive index change induced by the laser writing process and λ is the wavelength adopted in the DHM set-up. In this way the phase map has values ranging in [-π, π] without mod.2π ambiguity. Due to the high values of |Δn|max obtainable by the writing process (up to 10-2) the above condition imposes a quite severe constraint on the maximum thickness allowed for the sample (t≈50 µm).
The experimental procedure consists in a double exposure digital holographic interferometry in off-axis configuration . A first digital hologram of the diffraction pattern of the waveguide is recorded by DHM. A second digital hologram, the reference, is recorded by slightly translating the sample in a uniform region without the waveguide. The two holograms are numerically reconstructed at distance d on the exit face of the slice obtaining the complex wave fronts in that location. The phase maps are obtained by subtracting the reconstructed phase images of the two digital holograms.
The numerical reconstruction process of the digital holograms is based on the Fresnel Transformation Method (FTM) and consequently the spatial resolution is determined by the reconstruction pixel PR=λd/NΔξ, where the Δξ=6.7 µm is the pixel size of the CCD detector and N×N=512×512 is the number of pixels composing the digital holograms. For a reconstruction distance d=100 mm, the width of the reconstruction pixel is PR=15.5 µm, that scaled for the actual magnification M=90 gives a final value of 0.172 µm. The resolution can be further improved with FTM by a new approach described in .
3. Experimental results and discussion
3.1 Modification of various glass substrates
In this work several commercial glass substrates have been tested for waveguide writing with the stretched-cavity laser oscillator: fused silica from Foctek, QX phosphate glass doped with erbium and ytterbium from Kigre, IOG-10 alkali-zinc-silicate glass from Schott and 0211 zinc-borosilicate glass from Corning. Strong substrate dependence of the fabrication process has been found, as shown in the Differential Interference Contrast (DIC) microscope images (Fig. 4). The pulse energy was sufficient for modification on all the materials, as evidenced by a bright white spot in the focus of the writing beam due to the photo-induced plasma. On fused silica and phosphate glass irregular and void-like structures were formed for all available pulse energies and writing speeds. On the contrary, in IOG10 and 0211 uniform straight waveguides were obtained. It is worth noting that substrates that allow high quality waveguide writing with the kHz laser systems, such as fused silica  and QX phosphate glass [6,11], are not suitable for the MHz one. On the other hand, both the IOG10 and the 0211 are zinc-doped alkali-silicate glasses. These results indicate that waveguide writing at high repetition rate is a critical process that seems to be feasible only on a small category of glasses. Further investigation in order to understand the role of the glass composition is required.
Several straight waveguides have been inscribed in IOG10 and 0211 glass samples using different writing parameters, pulse energy and writing velocity. For IOG10, the best results in terms of homogeneity and guiding capabilities were achieved with 10–12 nJ and 0.5–1 mm/s, while for 0211 optimal parameters were 13–15 nJ and 3–7 mm/s.
3.2 Refractive index profile measurements
Figure 5 presents a refractive index measurement performed by the DHM technique on a waveguide fabricated on the IOG10 substrate with 10 nJ, 1 mm/s. The refractive index profile presents a depression at the center surrounded by an annular region of positive index change. Higher writing speed results in a lower absolute index change, but does not modify the profile shape. A completely different behavior is observed for the waveguides written in the 0211 glass. Figure 6 shows a waveguide written with 13 nJ, 7 mm/s in which the refractive index profile shows a positive peak in the center surrounded by an annular region of negative index change. Also for this glass, variations in the pulse energy and/or in the writing speed did not produce changes in the shape of the refractive index profile, which always presented a central positive peak.
A tentative explanation for such peculiar refractive index profiles could be based, according to , on localized melting and fast resolidification of the glass. The high repetition rate of the writing laser causes a cumulative deposition of energy in the irradiated volume. Then, by thermal diffusion, this energy melts a larger volume of glass. A rapid quenching of the molten material is then responsible for the non uniform refractive index distribution. If we assume that the density of the IOG10 glass increases when it is rapidly cooled, then we can understand the crater-shape of the refractive index profile. The cooling starts from the edges of the molten region, thus a higher density shell is created leading to a high refractive index region; as a consequence, due to mass conservation, the material at the centre is forced to solidify into a lower density, leading to a lower index core. The opposite behaviour, i.e., density decrease upon rapid quenching, could explain the central index change peak observed in the Corning glass. However, this simple model does not explain the ripples observed at the borders of the refractive index profiles.
3.3 Waveguide optical characterization
The waveguides fabricated on both substrates showed guided modes at 1.55 µm, which is important for telecom applications. Waveguides were butt-coupled with standard single mode telecom fibers. Figure 7 (left panel) shows the near field, acquired with a Vidicon camera, of the guided mode for the waveguide written in the IOG10 glass shown in Fig. 5. The mode profile displays a non-Gaussian shape with a dip in correspondence to the refractive index depression.
To validate the refractive index profile measurements presented in Section 3.2, we calculated the corresponding mode profiles with a beam propagation method (BeamPROP 4.0 – Rsoft), using as input data the measured refractive index profiles. For both glasses numerical simulations showed that the waveguides support only one guided mode at 1.55 µm. Figure 7 (right panel) shows the results of the simulations for the IOG10 waveguide. The calculated mode profile matches very well with the experimental one, correctly reproducing all its fine features. This result indicates that DHM is a valid technique for refractive index characterization with high spatial resolution of small waveguiding structures.
Similar procedure was adopted for the waveguide fabricated on 0211 glass. Figure 8 shows the experimental near field mode profile (left panel) compared to the calculated one (right panel). Also in this case the agreement is excellent. As expected from the refractive index profile, the guided mode, this time, shows a peak at the center, highlighting how the same process applied to different glass substrates can provide quite different guiding structures.
The insertion losses of the waveguides, butt-coupled to standard telecom fibers, were measured at 1.55 µm for both glass substrates. For the IOG10 glass we measured insertion losses of 3 dB for a 2.5-cm-long waveguide, including coupling and propagation losses. The estimated coupling losses from the overlap integral between the guided modes of fiber and waveguide are of 0.9 dB, resulting in propagation losses of about 0.5 dB/cm. The same measurements for the 1.8-cm-long waveguide on 0211 glass yielded insertion losses of 6 dB with estimated coupling losses of 0.5 dB, thus resulting in 2.8 dB/cm propagation losses. As expected from the shape of the guided modes (Figs. 7 and 8), waveguides on the 0211 glass, with a mode profile having a central positive peak, yield lower coupling losses than those fabricated on the IOG10 glass. On the other hand, the former waveguides have significantly higher propagation losses than the latter ones. This may be ascribed to two reasons: one is the higher writing speed (5 times faster) which causes stronger mechanical vibrations in the translation stage; the other could be the lower quality of the 0211 glass with respect to the IOG10. Indeed, while 0211 is normally used for microscope glass slides, IOG10 is an optical grade glass for integrated optics applications, thus highlighting the importance of having demonstrated waveguide writing on this glass.
In this work, we have reported a comprehensive characterization of waveguides written with a stretched cavity Ti:Sapphire oscillator. We showed that the cumulative regime obtained with MHz repetition rate lasers strongly depends on the glass substrate used. Two commercial glasses (0211 from Corning and IOG10 from Schott) have been successfully employed and waveguides working at 1.5 µm have been demonstrated. The latter substrate is particularly important since it is an optical grade one specifically developed for integrated optics applications. A new technique for measuring the refractive index profiles, based on digital holography microscopy has been introduced. The two glass substrates have shown refractive index profiles with opposite behaviors: IOG10 has a depressed refractive index region at the center surrounded by a positive index change cladding, while 0211 has a high index core surrounded by a negative index change cladding. A very good agreement is found between measured near field guided mode profiles and calculated ones based on the DHM measurements.
This research was funded by the European Union within contract G1ST-CT-2002-50266 (Development and Application of a Compact mode-locked laser Oscillator) and partially by the Ministero dell’Istruzione dell’Università e della Ricerca (MIUR) within the Fondo per gli Investimenti della Ricerca di Base project “Miniaturized systems for electronics and photonics” and partially by the project MIUR n.77 DD N.1105/2002 “Circuiti fotonici integrati per le telecomunicazioni ottiche e la sensoristica”.
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