1. Introduction

Integrated optical elements continuously attract considerable interest in applied and basic research [1]. The fabrication of photonic structures in planar transparent standard polymers [2,3] or organic-inorganic hybrid polymers [4–6] is commonly based on UV irradiation of the substrate. Using phase masks or interference effects, integrated optical elements such as Bragg gratings can be generated [7–9] and can be, mainly but not solely, used in sensor applications [3,10]. While these processes are associated with a very high performance, they have a limited freedom of design and are generally restricted to the substrate surface. An efficient possibility to create internal structures, in arbitrary depth below the surface, e.g. hollow microfluidic channel architectures in transparent materials, is the application of ultrashort laser pulses enabling a localized and distinctive energy deposition inside of the material [11,12]. By using focused femtosecond laser pulses to produce photonic structures, restrictions to the surface can be overcome as already demonstrated for planar glass substrates and fibers showing optical waveguides, couplers, interferometers, and grating structures [13–15] inside bulk material. Based on nonlinear absorption of the laser pulse energy, a local change of the refractive index inside each focal voxel can be achieved. Especially Bragg gratings profit from an increased flexibility as point-by-point inscription allowing simultaneous creation of waveguide and Bragg gratings of almost any desired period [16] and structure, such as chirped Bragg gratings [17], superstructure Bragg gratings [18], phase-shifted Bragg gratings [19], or long period Bragg gratings [20] using only a single setup without the necessity to apply various cost-intensive phase masks [15]. This greater flexibility in conjunction with value-added total cost of ownership qualifies femtosecond lasers as a powerful tool in photonic device manufacturing.

In transparent standard polymers, e.g. polymethylmethacrylate (PMMA), there are various reports on refractive index shifts in both positive [21–23] and negative [24–26] direction induced by ultrashort laser pulses depending on the applied processing conditions e.g. laser pulse repetition rate. According to the state of the art, however, photonic structures fabricated by femtosecond laser irradiation in planar transparent polymer substrates are still limited to the generation of simple configurations such as internal waveguides and couplers in PMMA [21–23,27,28].

Amongst others, low-cost polymer planar waveguide-based devices fabricated by UV irradiation have already been applied for refractive index sensing, lab-on-a-chip applications, highly sensitive temperature measurements, and deformation sensing [29–31]. Especially cyclic olefin copolymers (COCs), offering a low water absorption of less than 0.01 %, a high glass transition temperature, and a high resistance against common solvents are highly suitable to be used in a wide range of sensor applications [32]. In this study, we present a method to combine both, direct writing of waveguides and Bragg gratings in COCs based on femtosecond laser irradiation using an adaptive beam shaping approach. The generated structures are characterized according to their refractive index profile and their optical performance in terms of waveguide attenuation and Bragg grating reflectivity. To the best of our knowledge, this is the first report on femtosecond laser inscribed optical waveguides and Bragg gratings inside COCs bulk material.

2. Experimental

2.1 Laser system

 

Fig. 1. Schematic illustration of the setup for femtosecond laser inscription of photonic structures inside bulk polymer including a spatial light modulator (SLM) and a 4f telescope with an additional pinhole in front of the focusing objective (a) and schematic illustration of an internal Bragg grating connected to waveguides on both ends (b).

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We employ an ultrashort pulse laser (Light Conversion, Pharos-10-600) with a fundamental wavelength of 1028 nm featuring an adjustable pulse duration from 220 fs to 15 ps and variable repetition rates up to 610 kHz. By using a second harmonic generation module, a laser wavelength of 514 nm can be emitted. The linear polarized laser beam is shaped using a spatial light modulator (SLM; Holoeye, Pluto VIS 21) with a resolution of 1920x1080 pixels and a pixel size of 8 µm. A 4f setup (f=300 mm) is used to image the SLM pattern at the focusing objective. At the focal plane of the telescope a pinhole is inserted to block the zeroth order of a blazed grating displayed on the SLM, as depicted in Fig. 1. The first diffracted order passes unaffected through the pinhole and is focused by an air objective with a NA of 0.5 (Zeiss, EC Epiplan-Neofluar) into the material. The phase function displayed on the SLM is calculated to enable both, a shaping of the beam to achieve a thin disk upright standing energy distribution [33–35] and to correct the spherical aberration lens effect caused by the refractive index mismatch between air and processed transparent polymer [36]. Beam measurements on the resulting focal volume (voxel) using a coaxial CCD camera (Imagingsource, DMK 27AUJ003) are depicted in Fig. 2. The energy distribution of the focal voxel is characterized by a horizontal size of 1.5 µm in x direction, 25 µm in y direction (both measured at 1/e$^2$) and a focal height of 7.5 µm (FWHM). During processing, samples are positioned using linear translation stages (Aerotech, ANT130-XY). The focal position is controlled by a nanopositioning z stage (Aerotech, ANT95-50-L-Z).

 

Fig. 2. Measured normalized beam profile of the elliptical focal voxel in (a) x-y cross-section at the focal position, (b) y-z cross-section, (c) x-z cross-section using an ellipticity of 15 and an effective NA of 0.24 on the major axis.

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2.2 Material and methods

Photonic structures are generated according to the scheme shown in Fig. 1 and are consisting of a centered Bragg grating with a length of 5 mm which is directly coupled to laser inscribed waveguides on both sides. Thereby, all respective structures are created in a depth of 500 µm below the surface. Commercially available injection molded bulk COC sheets (type TOPAS 6017) with a thickness of 1.5 mm and a glass transition temperature of $175^\circ {\mbox {C}}$ are used. The refractive index $n$ of the material at the laser wavelength of 514 nm is 1.5362 at room temperature measured by Abbe refractometry [37]. The polymer sample is cut using the same ultrashort pulse laser in combination with a precision cutting head (FineCutter, Precitec) as already demonstrated by the authors [3]. Employing a laser pulse repetition rate of 250 kHz and a laser output power of 8.5 W at a wavelength of 1028 nm enables a cutting speed of 1 mm/s. The cutting process is supported by a coaxial nitrogen stream with a pressure of 6 bar enabling burr-free surfaces.

2.3 Characterization

The spatial distribution of the laser induced shift of the refractive index at 633 nm is measured by using a Mach-Zehnder interferometer microscope, a sophisticated discussion of the measurement setup is given by Hessler et al. [37,38]. Optical micrographs are obtained by using a reflection and transmission light microscope (Nikon, 70 Eclipse LVDIA-N). The attenuation of the waveguide is determined by reducing the size of a processed COC substrate using an advanced micromilling machine (Minitech Machinery, Minimill 4) and performing a measurement of the transmitted power at each length. To facilitate attenuation measurements, the optical waveguide is coupled to a single mode fiber. The output signal is collected by a multi-mode fiber. The attenuation coefficient is obtained for the structure under test by best fit straight line. In addition, generated structures are analyzed by recording the reflected and transmitted spectra of the Bragg grating using a source and detector interrogation system (Micron Optics, si155 HYPERION) that operates in a wavelength range from 1460 to 1620 nm. Thereby, the spectral position of the reflected Bragg wavelength, the full-width at half-maximum bandwidth (FWHM) and the reflected power are examined.

3. Results and discussion

The fabrication of photonic structures inside of bulk COC substrates is performed in a single processing step. Substrates are irradiated using a laser pulse energy between 280 nJ and 336 nJ at a wavelength of 514 nm, a laser pulse repetition rate of 25 kHz and a pulse duration of 450 fs. The spatial distribution of the laser induced refractive index modification generated in a depth of 500 µm with a pulse spacing of 600 nm (corresponding writing speed = 15 mm/s) is illustrated in Fig. 3(a) measured by using a Mach-Zehnder interferometer at a wavelength of 633 nm. Size and shape of the symmetric circular modification is determined by the intensity distribution of the focal voxel shown in Fig. 1. The strength of the internal refractive index shift is controlled by the applied laser pulse energy. At a laser pulse energy of 336 nJ a maximum positive refractive index increase of $2\cdot 10^{-4}$ RIU is induced inside the bulk polymer. By performing a tempering process step (2 h at 120 °C) the maximum refractive index shift can be increased up to $4\cdot 10^{-4}$ RIU. Femtosecond laser induced refractive index shifts in COC were measured to be lower as compared to the strengths of modifications induced by using deep UV-irradiation at the surface of COC [37] ($1.86\cdot 10^{-3}$ at 633 nm) and to femtosecond laser induced internal structures generated in PMMA substrates [22] ($4.6\cdot 10^{-4}$ at 633 nm). The maximum strength of femtosecond laser induced internal modification is limited by both, a degradation of the material resulting in optical defects and by the appearance of beam filamentation due to the nonlinear refractive index of the polymer substrate at higher laser pulse energies. In addition to this, internal laser induced modifications are observable using a transmission white light microscope. In Fig. 3(b) the lateral dimensions of the horizontal modification are clearly visible and correspond well to the spatial dimensions of the refractive index shifts measured by using Mach-Zehnder interferometry.

 

Fig. 3. Refractive index profile of laser inscribed waveguides at a wavelength 633 nm (a), top view white light transmission microscope image of a waveguide modification 500 µm below the surface (b).

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The generated structures exhibit a positive refractive index shift and are appropriate to be used as Type I waveguides [39,40]. The output profile at a wavelength of 633 nm of a waveguide inscribed with a laser pulse energy of 336 nJ is given in Fig. 4(a). Size and shape of the measured intensity distribution correspond well with the simulation of the output profile of the integrated waveguide illustrated in Fig. 4(c). The simulation is based on the measured refractive index profile given in Fig. 4(b) using the beam propagation method (Synopsys, Rsoft). The resulting circular gaussian intensity distributions are comparable to femtosecond laser inscribed waveguides in other transparent materials [21,41].

 

Fig. 4. Measured intensity distribution at the output of an internal waveguide at a wavelength of 633 nm (a), femtosecond laser induced refractive index profile measured by Mach-Zehnder interferometry (b) and corresponding simulation of the intensity distribution using RSoft at a wavelength of 633 nm (c).

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The attenuation of femtosecond laser inscribed waveguides is measured by using the cut-back method in a wavelength range from 1545 to 1555 nm. In Fig. 5 the graph represents the transmitted optical power of an integrated waveguide at decreasing sample length. Based on linear regression, the integrated waveguide exhibits an attenuation of 3.2 dB/cm. In comparison to results of UV-laser induced waveguides at the surface of planar COC substrates [42] (1.2 dB/cm at 1550 nm) the attenuation coefficient of the internal waveguide is higher. However, compared to femtosecond laser direct written waveguides in planar PMMA substrates [21] (4.2 dB/cm at 633 nm) here, a lower attenuation coefficient is achieved although studied polymers exhibit a remarkable lower absorption at 633 nm.

 

Fig. 5. Transmitted optical power in a wavelength range of 1545 to 1555 nm of a waveguide measured during the cut-back method.

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Besides the generation of optical waveguides, femtosecond laser direct writing enables the creation of functional elements such as Bragg gratings inside of the transparent material. The inscription of the corresponding periodic refractive index modulation is carried out by a mask-less point-by-point writing exposure. As depicted in Fig. 6(b), the elliptical focal spots enables both, a wide grating perpendicular to the waveguide and a small period along the writing direction. Each period visible is induced by a single laser pulse. The spectral position of the characteristic Bragg wavelength $\lambda _B$ can be calculated according to Eq. (1) with ${n}_{eff}$ being the effective refractive index, $\Lambda$ denoting the period of the grating structure and $m$ describing the order of the Bragg reflection.

 

Fig. 6. Transmission and reflection spectra of a waveguide coupled optical grating with a period of 1040 nm (a), white light transmission microscope image of a waveguide coupled optical grating fabricated by femtosecond laser direct writing in cyclic olefin copolymers (b).

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4. Conclusion

It was demonstrated that femtosecond laser direct writing is suitable to create photonic structures inside of transparent planar COC substrates. By the application of ultrashort laser pulses at a wavelength of 514 nm a maximum positive refractive index shift of $4\cdot 10^{-4}$ RIU measured by Mach-Zehnder interferometry is obtained. Depending on the processing conditions internal modifications can serve for both, optical waveguides and Bragg gratings. The results show a reflected wavelength of the waveguide coupled Bragg grating of about 1582 nm corresponding to the calculated and measured pulse-to-pulse distances. The high degree of freedom according to the general layout and the Bragg grating characteristics offers outstanding potential for the integration of photonic structures inside of planar polymer substrates.