Determining the refractive index and thickness of thin films, which can range from less than a nanometer to several microns in thickness, is crucial for characterizing them and enhancing the performance of sensors and devices utilizing such films. Ellipsometry, a well-established method with a wide array of available commercial solutions, is commonly employed for this purpose. However, ellipsometry calculates these parameters indirectly based on optical measurements and a known optical model of the thin film material, requiring prior knowledge.
A novel approach to ascertain the thickness and refractive index of thin films is proposed by the Sensors research group led by Prof. Ignacio R. Matías at the Public University of Navarra (Spain), in collaboration with the Advanced Photonic Components Laboratory of Prof. Jacques Albert at Carleton University (Canada). This approach relies on the wavelength shifts of multiple cladding mode resonances in tilted fiber Bragg gratings (TFBGs).
Optical fiber gratings are characterized by a periodic modulation of the refractive index along the core of an optical fiber, typically a single mode fiber with an 8 µm thick core and a 125 µm thick cladding. In the case of TFBGs, the grating period is around 500 nm, and the gratings are angled relative to the optical fiber axis.
The interaction between the light propagating through the core and the light backpropagating through the cladding, resulting from reflection by the gratings, leads to the emergence of cladding mode resonances in the optical spectrum. These resonances manifest at spectral intervals approximately 1 nm wide across a wavelength range of about 100 nm. The concurrent tracking of a substantial set of resonances, each offering a distinct measurement, facilitates the accurate determination of multiple parameters.
In a study published in Opto-Electronic Advances, the thickness and refractive index of a titanium dioxide (TiO2) thin film deposited on an optical fiber with a TFBG inscribed on it were simultaneously measured by analyzing the wavelength shift of eight resonances. This involved comparing the experimental wavelength shifts of these eight TFBG resonances during the deposition process with simulated shifts based on a range of thicknesses (T) and refractive index (n) values.
The solution for the thickness and refractive index of the deposited film was obtained by minimizing an error function computed for each (n, T) pair. The resulting values acquired through the TFBG (n = 2.25, final thickness of 185 nm) were within 4% of the validation measurements, which were conducted using a conventional ellipsometer and a scanning electron microscope.
This innovative approach offers a means to monitor the formation of nanoscale dielectric coatings on fibers in situ, especially for applications necessitating precise thicknesses and refractive indices, such as the optical fiber sensor field. Furthermore, the TFBG can serve as a process monitor for deposition on various substrates, ensuring uniform coatings even on dissimilarly shaped substrates.
In contrast to conventional methods that rely on co-located witness samples like ellipsometry or employ destructive measurements using coated fibers, this developed technique stands out. Therefore, this proposed method has the potential to surpass these limitations and set a new standard for measuring the thickness and refractive index of thin films deposited on optical fibers.