# Demonstration of High-Speed Optical Transmission at 2 \(\upmu\)m in Titanium Dioxide Waveguides
## 3 Validation of the Transmission of a 10 Gbit/s Signal  
In order to demonstrate the suitability of our TiO\({}_{2}\) devices for 2-\(\upmu\)m optical communications, we implemented the experimental setup detailed in Figure 5, based on 2-\(\upmu\)m commercially available devices. The transmitter (TX) was based on a laser diode centered at 1980 nm and was intensity modulated by means of a commercial Niobate-Lithium modulator (model MX2000-LN-10, iXblue Photonics, Besancon, France). The Non-Return-to-Zero On-Off-Keying signal under test was a 2\({}^{31}\)-1 pseudorandom bit sequence (PRBS) at 10 Gbit/s. Since the SM waveguide is polarization-sensitive, a polarization controller was used after the intensity modulator. Then, a thulium-doped fiber amplifier (TDFA) was used before a 90/10 coupler, allowing us to monitor the power injected into the waveguide. Note that the variable optical attenuator is usually implemented just in front of the receiver, but due to limited sensitivity of the available power meters working at 2 \(\upmu\)m, it was more convenient to insert it before the waveguide. The receiver (RX) was based on a second TDFA. This TDFA was set to work with a constant gain instead of a constant output power, as usually used in C-band. Therefore, a variable optical  
Figure 4: Total insertion losses as a function of the injected wavelength for an MM waveguide (grey curve) and an SM waveguide (black curve) obtained with the amplified spontaneous emission (ASE) source (for \(w_{1}\) = 630 nm). The oscillations of short period on the grey curve are due to a Perot–Fabry effect. The grey dashed line corresponds to the central wavelength of the laser source used in the next section. Figure 5: Experimental setup for a 10-Gbit/s 2-\(\upmu\)m optical transmission. CW: continuous wave; IM: intensity modulator; PC: polarization controller; PRBS: pseudorandom binary sequence; TDFA: thulium-doped fiber amplifier; VOA: variable optical attenuator; PM: power meter; OSA: optical spectrum analyzer; PD: photodiode; BERT: bit error rate tester (model MU181040A, Anritsu Corporation, Atsugi-shi, Japan). attenuator was implemented at its output to ensure that the photodiode operated at a constant power level. An optical bandpass filter (0.64 nm bandwidth) (model OETFG-100, O/E Land Inc., LaSalle, QC, Canada) was also inserted at the output of the system, in order to limit the accumulation of amplified spontaneous emission from the TDFAs. The signal was finally analyzed both in the spectral and temporal domains. An optical spectrum analyzer was used to measure the optical signal-to-noise ratio (OSNR) of the received signal (using the usual 0.1 nm noise bandwidth), whereas a photodiode (12.5 GHz electrical bandwidth) (model 818-BB-51F, Newport corporation, Irvine, CA, USA) enabled bit error rate measurements in addition to recording of the output eye diagram on a high-speed sampling oscilloscope. Figure 6 summarizes the results obtained at 1980 nm. Error-free operation can be achieved for back-to-back measurements as well as in the presence of the TiO\({}_{2}\) waveguides. Examples of the corresponding eye-diagrams are provided in panels (a): in both configurations, a widely open eye can be recorded and the insertion of the waveguides under test does not induce any visible degradations of the transmission. The quality of the transmission through the TiO\({}_{2}\) waveguide was more quantitatively evaluated through systematic measurements of the Bit-Error-Rate according to the OSNR on the receiver. The results obtained for the various configurations are summarized in Figure 6b. From a general point of view, the global trends are very similar, with very moderate penalty (\(\sim\)1.5 dB) obtained after transmission through the waveguides and compared to the back-to-back configuration for BER lower than \(10^{-8}\). Moreover, no significant difference was observed between the SM and MM waveguides.