The optical characteristics of graphene oxide (GO) were explored to design and fabricate a GO-based optical humidity sensor. GO film was coated onto a SU8 polymer channel waveguide using the drop-casting technique. The proposed sensor shows a high TE-mode absorption at 1550 nm. Due to the dependence of the dielectric properties of the GO film on water content, this high TE-mode absorption decreases when the ambient relative humidity increases. The proposed sensor shows a rapid response (<1 s) to periodically interrupted humid air flow. The transmission of the proposed sensor shows a linear response of 0.553 dB/% RH in the range of 60% to 100% RH.

We have demonstrated a broadband waveguide polariser with high extinction ratio on a polymer optical waveguide coated with graphene oxide via the drop-casting method. The highest extinction ratio of nearly 40 dB is measured at 1590 nm, with a variation of 4.5 dB across a wavelength range from 1530 nm to 1630 nm, a ratio that is (to our knowledge) the highest reported for graphene-based waveguide polarisers to date. This result is achieved with a graphene oxide coating length along the propagation direction of only 1.3 mm and a bulk film thickness of 2.0 µm. The underlying principles of the strongly polarisation dependent propagation loss demonstrated have been studied and are attributed to the anisotropic complex dielectric function of graphene oxide bulk film.

An on-chip optical humidity sensor using Nano-anatase TiO₂ coating is presented here. The coating material was prepared so that the result is in solution form, making the fabrication process quick and simple. Then, the solution was effortlessly spin-coated on an SU8 straight channel waveguide. Investigating the sensitivity and performance (response time) of the device revealed a great linearity in the wide range (35% to 98%) of relative humidity (RH). In addition, a variation of more than 14 dB in transmitted optical power was observed, with a response time of only ~0.7 s. The effect of coating concentration and UV treatment was examined on the performance and repeatability of the sensor. Interesting observations were found, and the attributed mechanisms were described. In addition, the proposed sensor was extensively compared with other state-of-the-art proposed counterparts from the literature and remarkable advantages were found. Since a high sensitivity of ~0.21 dB/%RH and high dynamic performances were demonstrated, this sensor is proposed for use in biomedical applications.

Increased absorption of transverse-magnetic (TM) - polarised light by a graphene-oxide (GO) coated polymer waveguide has been observed in the presence of transverse-electric (TE) - polarised light. The GO-coated waveguide exhibits very strong photo-absorption of TE-polarised light - and acts as a TM-pass waveguide polariser. The absorbed TE-polarised light causes a significant temperature increase in the GO film and induces thermal reduction of the GO, resulting in an increase in optical-frequency conductivity and consequently increased optical propagation loss. This behaviour in a GO-coated waveguide gives the action of an inverted optical switch/modulator. By varying the incident TE-polarised light power, a maximum modulation efficiency of 72% was measured, with application of an incident optical power level of 57 mW. The GO-coated waveguide was able to respond clearly to modulated TE-polarised light with a pulse duration of as little as 100 μs. In addition, no wavelength dependence was observed in the response of either the modulation (TE-polarised light) or the signal (TM-polarised light).

A Ti ₃ AlC ₂ MAX phase-based saturable absorber (SA) is successfully demonstrated to induce passive Q-switching in an erbium-doped fiber laser (EDFL) at the 1.5 μm region. The Ti ₃ AlC ₂ -PVA film is fabricated using the solution casting method and incorporated into the laser cavity which uses a 0.89 m long erbium doped fiber as the gain medium. Stable and self-starting Q-switched laser pulses are observed above a threshold pump power of 87.81 mW at a central wavelength of 1567.4 nm. The repetition rate is observed to increase from 24.07 kHz to 46.3 kHz, while the corresponding pulse width decreases from 5.0 μs to 0.99 μs over a pump power range of 108.9 mW to 244.5 mW. At the maximum pump power of 244.5 mW, the highest pulse energy of 102.7 nJ and the maximum output power of 4.75 mW are obtained. These findings demonstrated the feasibility of the Ti ₃ AlC ₂ MAX phase as an SA material with significant promise for photonic applications. This is also, to the best knowledge of the authors, the first report of a Ti ₃ AlC ₂ MAX phase-based SA for inducing passive Q-switching in an EDFL.