# Integrated Photonics on Glass: A Review of the Ion-Exchange Technology Achivements
## 4 Sensors  
Integrated photonics is intrinsically interesting for the realization of optical sensors because it provides compact and reliable self-aligned devices that can be easily deported when pigtailed to optical fibers. Glass is a material that is chemically inert, bio-compatible, and mechanically stable. Therefore, making optical sensors on glass wafers or integrating optical glass chips into complex set-ups have encountered a huge interest. We will detail here a selection of ion-exchanged based glass sensors as examples of possible applications. Although AWGs used in telecom are actually integrated spectrometers, they are not well adapted to the rapid measurement of full spectra. For this reason, a Stationary-Wave Integrated Fourier-Transform Spectrometer (SWIFTS) has been proposed and developed [93]. It is a static Fourier Spectrometer that measures directly the intensity of a standing wave with nanoprobe placed on a waveguide. In the instrument reported by [PERSON] et al. [94], the waveguide is made by a silver ion-exchange on a silicate glass and the nanoprobes are gold nanodots. The interaction of gold nano-antennas with an ion-exchanged waveguide has been studied by [PERSON] et al. [95]. This spectrometer has a spectral measurement range that starts at 630 nm and ends at 1080 nm with a spectral resolution better than 14 pm. SWIFTS interferometers are currently integrated in the product line commercialized by Resolution Spectra Systems [96]. Displacement sensors allow measuring accurately the change of position of an object through interferometry. [PERSON] et al. [97] implemented a double Michelson interferometer on a glass substrate using potassium ion-exchange; the device provided two de-phased outputs in order to give access not only to the distance of the displacement but also its direction. However, having only two interferometric signals is not sufficient to prevent the measure from being affected by unexpected signal variations. For this reason, [PERSON] et al. [98] proposed a new design for the interferometric head that provided four quadrature phase shifted outputs. The device made by potassium ion-exchange demonstrated a measurement accuracy of 79 nm over a measurement range of several meters when used with an HeNe laser as a source. After technological improvements and the use of a silver-sodium ion-exchange on GO14 glass, an evolution of this sensor is now commercialized by TeemPhotonics and presents a resolution of 10 pm for a 1530 nm-1560 nm operating wavelength range [48]. Measuring speed is also something that can be of major importance, specifically in the case of aircrafts where their True Air Speed (TAS), which is their speed with respect to the air surrounding them, conditions their lift. Airborne LIDARs have hence been developed as a backup to Pitot gauges in order to increase the safety of flight by providing a redundant accurate measurement of the aircraft TAS. The operation principle of an airborne LIDAR is based on the Doppler frequency shift measured on a laser signal reflected on the dust particles of the atmosphere. This shift being quite low and presenting a low amplitude when compared to the emitted signal, a laser source that presents a narrow linewidth, a low Relative Intensity Noise and that is resilient to mechanical vibrations is required. [PERSON] et al. [99] realized such a laser source on an Er/Yb doped phosphate glass with silver ion-exchanged waveguides and a DFB structure. This laser presented a fiber coupled output power of 2.5 mW, a linewidth of 2.5 kHz, and a RIN that was 6 dB lower than the specification limit. The device has then been successfully implemented in the LIDAR set-up and validated in flight [100]. Astrophysical research programs rely on telescopes with always higher resolution to detect exoplanets, young star accretion disks, etc. Optical long baseline instruments, which interferometrically combine the signal collected by different telescope have been developed for this purpose.