Fibre Bragg Grating

For Scientific Research & Industry Modernisation.

Fiber Bragg grating (FBG) is a key optical element in all-optical networks due to several of its characteristics. It has low additional loss and small size. It makes good coupling with optical fiber and integrates well with other fiber devices. FBG technology can provide effective solutions for light source, light amplification, dispersion compensation, optical terminal multiplexer (OTM), optical cross-connector (OXC) and other key components in all-optical communication network system.

FBGs are extensively used for sensing strain and temperature changes in industries such as mining, aircraft manufacturing, structural civil engineering and power transmission lines. The wide-spread applications of FBGs as sensors stems from their ability to perform read-out tasks in harsh environments. They possess several qualities that make them more attractive than metal strain gauge counterparts.

Traditional method to fabricate FBGs is to use UV light illumination combined with a phase mask, despite of the high throughput, the method lacks of flexibility and the fibre must be stripped to remove the polymer coating prior to UV (< 250 nm) irradiation, making it susceptible to fractures and forcing a recoating step after the formation of the grating. Optimal results are obtained only when photosensitive fibres are used.

FBGs manufactured by femtosecond laser technology possess significant advantages over conventional FBGs fabricated by ultraviolet lasers. This is due to the large refractive index difference introduced by a femtosecond laser, which can be up to 10-3 (Δn ∽ 10-3). Femtosecond laser manufactured FBGs offer two major advantages:

  1. Achieving a high reflectivity with a short grating length
  2. Femtosecond laser manufactured FBGs can work in a high-temperature environment
  3. Capability and high flexibility of processing different types of FBGs, e.g. FBGs with ultra-narrow bandwidth, ultra-wide bandwidth, apodized FBGs, chirped FBGs etc.

Industry Challenge

However, currently the cost of femtosecond laser processed FBG products is significantly greater than that of traditional UV laser processed FBG products. Meanwhile, the performance parameters of current femtosecond FBG products cannot be well-controlled with precision and stability. Additionally, the yield rate is low. Such issues limit the broad applications of femtosecond FBG. 

To elaborate, the problems arise from the following three challenges of femtosecond laser FBG processing :

During femtosecond laser processing, the accurate control of the femtosecond laser power stability as well as the precise synchronization of exposure time and speed of the displacement stage, are the rigorous requirements for precise processing of FBG with fine quantitative measures of identical performance index.

Solution Overview

Automatic fabrication position finding

Fig. The software automatically finds the fibre core. The above case shows that the focus is already on the core

One of the largest challenges of FBG fabrication using a femtosecond laser direct laser writing system is to find the position of the fibre core and during the fabrication to ensure the laser focal spot, thus the fabricate FBG, is consistently on the fibre core. In most cases, due to the tilt or the non-straight nature of the fibre, the fabrication becomes out of focus during fabrication, which leads to the high failure ratio of the FBG fabrication in most cases, even when the initiation fabrication is successful. The fabrication process needs to be attended and corrected in the real time, which is impossible to realise using manual fabrication method with the current market available equipment solutions. Innofocus FBG fabrication uses advance imaging recognition system to direct and automatically identify the position of the core during the entire process of the fabrication.

Automatic fabrication position correction

In the above-mentioned cases, we assume the fibre is perfectly aligned with the scanning stage without any bending or tilt. Under those circumstances, during the whole fabrication process, it needs to find fabrication position only once. However, more often this ideal situation cannot be achieved, the tilting and bending of the fibre will strongly affect the fabrication position and reduce the quality of the FBGs.

With the Innofocus automatic position finding software, the inaccuracy caused by the tilting and bending of the fibre can be completely avoided. The software automatically measures the angle between the fibre and the writing axis (x-axis in this case) with accuracy up to 0.001°. By doing this, the software will automatically adjust the fabrication path to align with the fibre core, which does not require further adjustment of the mounting. Thus high repeatability and yield can be achieved.

Fig. For tilted and bending fibres, Innofocus® system enables the focus to be always on the core

Robustness of the software recognition process

For the high quality FBG fabrication, it requires high robustness of the software to deal with any situation. For example the dusts on the fibre can affect the recognition process. The robustness of the Innofocus software was tested by intentionally using a fibre with some dusts. The result shows that each dust displays a diffraction pattern, and such dusts will not affect the automatically position finding process. Thus, the robustness of the software is confirmed.

Customer Value

High degree of automation with positioning accuracy above 50 nm

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Industry Applications

All-Optical Communications, Sensing Mining, Aircraft Manufacturing, Structural civil engineering, Strain and temperature variation measurement in industries such as power transmission lines, High temperature and high voltage, Smart grids

Ultra-fast FBG device integration solution for in-situ inspection of various batteries (including, but not limited to, lithium, zinc-ion, and gallium nitride batteries, etc.) under operating conditions.

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Optical Communications

Femtosecond laser processing with the NanoPrint 3D Intelligent Nano-fabrication system allows direct writing of micro-, sub-micron and even nano-scale 3D micro and nano structures in transparent media, with the advantages of maskless, flexible structure, simple design and fast processing speed. By combining with different optical materials, it can achieve a wide range of applications in the field of all-optical communication, especially in the fabrication of diffractive optics, integrated optics, on-chip optics, silicon photonics, nano-optics, and quantum optics, which stand out among the many micro and nano fabrication technologies and become increasingly important in enabling technology.

 

Three-dimensional micro-nano structures can be designed to enhance the interaction between the local optical field and matter, thus giving rise to a variety of linear and non-linear optical phenomena and shortening the scale of action, thus effectively achieving miniaturization, integration and low energy consumption of devices. For example, Nanoprint 3D Intelligent Laser Nano-Fabrication system can realize various miniature diffractive optical elements including micro-lens, integrated grating, and waveband sheet, which can play a great role in imaging, wavelength selection and dispersion compensation.

 

In addition, the NanoPrint system’s unique high-power femtosecond laser enables interaction with different materials such as glass, silicon, sulfur-based glass, and lithium niobate crystals. These materials can effectively introduce nonlinear optical interactions for wavelength conversion, optical switching, nonlinear tuning, etc. Nonlinear interactions in nanoscale waveguides can be exploited to generate effective sources of entangled photons for quantum optics. Femtosecond lasers can introduce ultra-high refractive index changes in optical fibers, bulk glass, and two-dimensional materials to form high-quality optical waveguides, ultra-thin devices, and complex three-dimensional integrated optical systems such as optical connectors and on-chip integration, which are essential enabling components for ultrafast, ultra-high-capacity, and quantum communications

Sensor

In today’s society where everything is smart, sensors are playing an increasingly important role in our daily lives and in the Internet of Things. Sensors monitor our health (e.g. heartbeat), air quality, home security, and are widely used in the Industrial Internet of Things (IIoT) to monitor production processes. Our lives are surrounded by smartphones, wearables, and other smart devices, all of which are inseparable from sensors.

 

Widely used sensors in daily life include thermometers, pressure sensors, light sensors, accelerometers, gyroscopes, motion sensors, gas sensors and so on. Their design and manufacturing often rely on traditional electromechanical processing, which has a single function, large size and high energy consumption, and is not suitable for the new sensor miniaturization, integration, integration, high precision and low energy requirements. Therefore, how to innovate fabrication methods and improve the fabrication process has become a difficult problem in the field of sensing.

 

The NanoPrint 3D Intelligent Laser Nano-fabrication System can write flexible design and versatile 3D micro-nano structures in a variety of materials including polymers, 2D materials, metals, semiconductors, crystals, optical fibers, and other materials to form sensing mechanisms. Its writing method is flexible and simple to fabricate, and no vacuum or mask is required. More importantly, the written structure is small and highly integrated, and can form an immersion sensing with the substrate or environment. It is also possible to introduce in-situ high-precision sensors directly at the sites that need to be measured in sensing, without damaging the original environment and overall appearance.

 

In particular, graphene micro and nano sensors realized with NanoPrint system have been playing an important role in artificial skin, intelligent robots, health care, early diagnosis of diseases, wearable devices, etc. in recent years. In addition, FBG fabricated by the unique patented technology with NanoPrint system has the characteristics of easy operation, fast writing speed, stable process, flexible formation of a variety of special grating, and high raster rate. It solves the problems of expensiveness, complex process and single grating structure with the traditional fiber grating fabrication equipment. The formed FBG has large change in refractive index, high temperature resistance, high performance index and good stability, has replaced the traditional fiber grating and plays an irreplaceable role in the high temperature and high pressure environment.