DOI: 10.1007/s00339-006-3560-x Appl. Phys. A 83, 347–350 (2006) Rapid communication Materials Science & Processing Applied Physics A g. matth ¨ aus ✉ j. burghoff m. will s. nolte a. t ¨ unnermann Thermal effects vs. gain in femtosecond laser written waveguides in neodymium doped fused silica Institute of Applied Physics, Friedrich-Schiller-University Jena, Max-Wien-Platz 1, 07743 Jena, Germany Received: 17 January 2006/Accepted: 19 January 2006 Published online: 4 March 2006 • © Springer-Verlag 2006 ABSTRACT The influence of thermal effects in gain measurements of fs laser-written waveguides in actively doped glass is reported for the first time. We show that these ef- fects strongly contribute to the signal enhancement (up to 50% observed). Thus, a new measurement scheme to distinguish between thermal induced signal increase and real amplification is proposed. PACS 42.82.-m; 42.82.Et; 42.65.Re 1 Introduction Modern communication sys- tems are based on integrated optical devices to control the properties of light in all-optical networks. Key elements within these networks are not only pas- sive waveguides, splitters, connectors and filters but also active elements to compensate for losses and to produce and amplify light. In the past 10 years a novel technique based on the use of ultrashort laser pulses for the direct writing of photonic structures within different glasses [1, 2], polymers [3] and also crystalline media [4, 5] has been demonstrated. Intense femtosec- ond laser pulses are focused into the bulk of a transparent solid, resulting in nonlinear absorption in the focal vol- ume, optical breakdown and permanent structural and refractive index changes. Depending on the type of material used these refractive index changes can be either positive or negative [6, 7] (note that there are also indications that the laser parameters have an additional im- pact on the sign of the refractive in- dex change [8]). In the case of posi- tive index changes, optical waveguides can be directly obtained with 3D flexi- ✉ Fax: +49 3641-657680, E-mail: matthaeus@iap.uni-jena.de bility by moving the sample with re- spect to the laser beam focus [9, 10], while in the case of negative refrac- tive index changes more complex setups have to be used [7]. Active devices can be simply obtained by using actively doped samples [11, 12]. Recently, the first waveguide laser [13] based on di- rectly written active waveguides was demonstrated. While thermal effects can be neg- lected in general concerning optical fibers, we will demonstrate for the first time to the best of our knowledge that these effects have an important impact on femtosecond written waveguides in active media. We present a detailed in- vestigation of pump-induced thermal in- fluences on gain measurements in laser written waveguides in doped materials. In the investigated samples up to 50% of the total signal increase can be attributed to thermal effects, yielding significantly lower gain values than expected from simple measurements. Thus, a modified setup for the gain measurement is used, which allows one to distinguish between thermally induced signal enhancement and optical gain. These measurements provide important data for the design of new stable and reliable active integrated optical devices based on the femtosec- ond direct writing technology. 2 Experiment and results Our waveguides were written with a commercial Ti:sapphire fs ampli- fier system (Spectra-Physics, Spitfire). It produces 50-fs pulses with a repetition rate of 1 kHz at a wavelength of 800 nm. To create the localized refractive in- dex changes, we focused pulses with an energy of 1 to 3 μ J into the doped laser glass (LG 680, Schott, see Table 1) approximately 200 μ m below the sur- face. The focusing was accomplished by a 20× microscope objective with a nu- merical aperture of NA = 0.35, which is corrected for a focal depth of 170 μ m. Optical waveguides with a length up to 2.6 cm were generated by moving the samples perpendicular to the laser beam axis at a velocity of 50–100 μ m/s by a computer controlled three-axis posi- tioning system (Aerotech, ABL9000). The third axis was used for controlling the focal depth inside the glass. The laser glass had a dopant concentration of about 3 wt. % Nd 2 O 3 and an absorption length (1/e) of 1.4 mm at 804 nm. Figure 1a displays the measured near-field intensity distribution of our fabricated active waveguides at a wave- length of 804 nm. With the help of the refracted near-field (RNF) method [14], we determined a refractive-index in- crease of ∼ 3 × 10 −4 , resulting in a nu- merical aperture of about 0.05. The propagation losses of the waveguides have been determined using the cut- back method yielding 0.8 dB/cm. Characterization of the optical am- plification properties has been perform- ed using the setup shown in Fig. 1b. We