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Single-frequency fiber lasers of Koheras series with output power from 10mW to 15W emitting wave lengths from 1 to 1.5μm match many sensor systems criteria. Detection of aircraft wake vortex, piplines protection, liquid and gas transportation all need their unique characteristics, fine radiation line, low noise, small size and reliability
Теги: fiber-optical sensors single frequency fiber lasers волоконно-оптические датчики одночастотные волоконные лазеры
The Danish company NKT Photonics has been manufacturing low noise single frequency fiber lasers since 1997 for global research laboratories, optical sensing system integrators, and for the space and defense industry. Today the largest share of the fiber lasers is deployed in interferometric optical sensing systems for oil and gas exploitation, pipeline integrity monitoring, perimeter security and wind detection. The low phase noise (figure 1) and stable, mode-hop free single frequency laser operation and sub kHz linewidth are the key technical attributes that enable interrogation of optical fibers over tens of kilometers with high sensitivity and accuracy.
The fiber laser is a so-called DFB (Distributed Feed-Back) design, and is essentially a short and robust laser cavity. The high Q-value and the relatively long length of the DFB cavity combined with long radiative lifetimes of rare earth ions in silica provide for fundamentally low values of phase noise and spectral linewidth. Careful packaging design to reduce effects of vibrations and acoustic noise and use of low noise pump sources further reduce effects of technical noise. The end result is a laser source that combines ease of use, low power consumption, and compact size with values for phase noise and spectral linewidth that are unsurpassed in its class.
Fiber optical sensing systems are gaining ground these days because they offer several advantages over traditional technologies based on electrical transducers such as piezo electric hydrophones as they are passive i.e. no electronics, compact, lightweight, reliable, and can be multiplexed to interrogate very large sensor arrays, as well as enable longer sensing ranges with high dynamic range and sensitivity.
In an interferometic optical sensing system based on e.g. coherent Rayleigh backscatter detection, the optical fiber typically acts as a long continuous sensor that is extremely sensitive to acoustic perturbations from the surroundings. The small impacts cause a change of the optical path length in the fiber and when interrogated by coherent laser light and re-combined with an “un-impacted” reference light from the laser source itself (local oscillator in coherent detection scheme) on a photo-detector, an acoustic “fingerprint” is depicted by means of data processing to provide detailed information about an event at a specific location along the fiber. For perimeter surveillance the optical systems use sophisticated algorithms to discriminate background noise that can arise from such sources as from rain droplets or aircrafts, so that alarms are triggered only by relevant and potentially critical events.
In the oil industry and other industries such as water and waste water transportation over long range pipe lines, the integrity and health condition of the pipe lines is very critical. Due to their economic importance these pipe lines are also potential targets for intrusion, and furthermore mechanical cracks and fatigue can lead to enormous spills with severe economic and environmental consequences. Interferometric optical sensing systems offer preventive and accurate means to detect incidents or emerging mechanical failures so precautions can be taken at an early stage.
The increasing need for optimizing oil recovery, controlling the flow of oil through pipelines, monitoring the health condition of pipelines, and protecting critical assets has also created a demand for optical sensor systems and subsequently for low noise laser sources. Oil extraction has become more challenging today as existing reservoirs produce less oil. Presently only about one fourth of an oil well is exploited over time. However, with permanent reservoir monitoring it is possible to better locate and follow the movement of oil, something which helps to improve the exploitation of an existing well and benefit from the initial infrastructure investment around an oil reservoir. By way of example, in the Danish oil sector a single additional percentage of oil extraction from the underground corresponds to a gross value of 1b USD ¹. Low noise lasers are now being deployed at different oil fields to interrogate large fiber optic hydrophone arrays that pick up reflected acoustic signals from below the seafloor, generated by sound waves from large air guns, to help locate the oil (fig.2).
More sub-sea systems are planned for the coming years, and currently fiber optic land based geo-seismic systems with very high channel counts (towards 1 million channels) are also being investigated. For all these systems noise is a key performance parameter to obtaining solid data and clear images, and in this game the laser phase noise plays a central role. The continued development of fiber optic geo-seismic systems therefore has helped push the laser technology towards compact, fiber coupled, high reliability devices with un-surpassed low phase noise performance.
Koheras lasers also play an important role in a new generation of wind sensing Lidars (Light Detection and Ranging) for meteorology where the Doppler shift of light scattered by aerosols (Mie scattering) is used to indirectly measure wind velocity and turbulence by coherent homodyne detection. It requires first of all a single frequency, narrow linewidth source that can deliver up to 1 W of optical power, but also very low RIN (Relative Intensity Noise) to be able to detect the very weak back scatter from aerosols and particles carried by the wind. The laser emits light in an eye safe wavelength region at 1.5 µm not causing eye damage to humans or animals.
The use of laser anemometry will be important for future wind resource management as the accurate prediction of energy yield is vital to the success of wind farm projects. Wind data have traditionally been collected using expensive anemometry masts, which are expensive and further only perform point measurements. This creates great difficulty due to the numerous measurement points throughout the entire wind farm and in addition raising anemometry masts require building permits and dealing with health and safety aspects.
Wind Lidars are now being exploited as forward looking sensors to predict wind conditions ahead of wind turbines in operation, as opposed to traditional and widely used cup anemometers that are limited to instantaneous measurements at the turbine nacelle, or sonars which do predict the wind conditions ahead, however with low accuracy (fig.3). The ability to reliably predict the wind speed and direction is expected to be a very useful tool in the wind turbine industry to control the pitch and yaw for increased power optimization and even more importantly reduce load and fatigue of the large turbine structures. This could lead to new developments both in the construction design, farm site management, as well as operation and service lifetimes, and in the end reduced cost of ownership for the asset owners. The typical life time of wind turbines is around 20 years, but it is expected that it could be extended by 30% using wind Lidar feedback control to reduce the loads from wind turbulence [2}.
Other applications that can benefit from wind Lidars is aviation safety and airport wake vortex detection at landing fields. NKT Photonics has worked closely with the world leading research center in wind energy, Risø DTU National Laboratory for Sustainable Energy, and gained know-how of the laser requirements to make the fiber lasers suitable sources for wind Lidars.
Koheras lasers are also vital for important scientific projects around the world, for example for ESA’s Swarm mission scheduled for launch in November 2013 that will unravel one of the most mysterious aspects of our planet, the magnetic field [3] (fig.4). The Koheras laser was not originally intended to be space-born and installed on satellites when commercialized in the late 90’s, however this project is a good testimony of how the fiber laser is also capable of meeting the demanding requirements of the space industry and thus certainly live up to the high standards in the optical sensing industry.
The fiber laser is a so-called DFB (Distributed Feed-Back) design, and is essentially a short and robust laser cavity. The high Q-value and the relatively long length of the DFB cavity combined with long radiative lifetimes of rare earth ions in silica provide for fundamentally low values of phase noise and spectral linewidth. Careful packaging design to reduce effects of vibrations and acoustic noise and use of low noise pump sources further reduce effects of technical noise. The end result is a laser source that combines ease of use, low power consumption, and compact size with values for phase noise and spectral linewidth that are unsurpassed in its class.
Fiber optical sensing systems are gaining ground these days because they offer several advantages over traditional technologies based on electrical transducers such as piezo electric hydrophones as they are passive i.e. no electronics, compact, lightweight, reliable, and can be multiplexed to interrogate very large sensor arrays, as well as enable longer sensing ranges with high dynamic range and sensitivity.
In an interferometic optical sensing system based on e.g. coherent Rayleigh backscatter detection, the optical fiber typically acts as a long continuous sensor that is extremely sensitive to acoustic perturbations from the surroundings. The small impacts cause a change of the optical path length in the fiber and when interrogated by coherent laser light and re-combined with an “un-impacted” reference light from the laser source itself (local oscillator in coherent detection scheme) on a photo-detector, an acoustic “fingerprint” is depicted by means of data processing to provide detailed information about an event at a specific location along the fiber. For perimeter surveillance the optical systems use sophisticated algorithms to discriminate background noise that can arise from such sources as from rain droplets or aircrafts, so that alarms are triggered only by relevant and potentially critical events.
In the oil industry and other industries such as water and waste water transportation over long range pipe lines, the integrity and health condition of the pipe lines is very critical. Due to their economic importance these pipe lines are also potential targets for intrusion, and furthermore mechanical cracks and fatigue can lead to enormous spills with severe economic and environmental consequences. Interferometric optical sensing systems offer preventive and accurate means to detect incidents or emerging mechanical failures so precautions can be taken at an early stage.
The increasing need for optimizing oil recovery, controlling the flow of oil through pipelines, monitoring the health condition of pipelines, and protecting critical assets has also created a demand for optical sensor systems and subsequently for low noise laser sources. Oil extraction has become more challenging today as existing reservoirs produce less oil. Presently only about one fourth of an oil well is exploited over time. However, with permanent reservoir monitoring it is possible to better locate and follow the movement of oil, something which helps to improve the exploitation of an existing well and benefit from the initial infrastructure investment around an oil reservoir. By way of example, in the Danish oil sector a single additional percentage of oil extraction from the underground corresponds to a gross value of 1b USD ¹. Low noise lasers are now being deployed at different oil fields to interrogate large fiber optic hydrophone arrays that pick up reflected acoustic signals from below the seafloor, generated by sound waves from large air guns, to help locate the oil (fig.2).
More sub-sea systems are planned for the coming years, and currently fiber optic land based geo-seismic systems with very high channel counts (towards 1 million channels) are also being investigated. For all these systems noise is a key performance parameter to obtaining solid data and clear images, and in this game the laser phase noise plays a central role. The continued development of fiber optic geo-seismic systems therefore has helped push the laser technology towards compact, fiber coupled, high reliability devices with un-surpassed low phase noise performance.
Koheras lasers also play an important role in a new generation of wind sensing Lidars (Light Detection and Ranging) for meteorology where the Doppler shift of light scattered by aerosols (Mie scattering) is used to indirectly measure wind velocity and turbulence by coherent homodyne detection. It requires first of all a single frequency, narrow linewidth source that can deliver up to 1 W of optical power, but also very low RIN (Relative Intensity Noise) to be able to detect the very weak back scatter from aerosols and particles carried by the wind. The laser emits light in an eye safe wavelength region at 1.5 µm not causing eye damage to humans or animals.
The use of laser anemometry will be important for future wind resource management as the accurate prediction of energy yield is vital to the success of wind farm projects. Wind data have traditionally been collected using expensive anemometry masts, which are expensive and further only perform point measurements. This creates great difficulty due to the numerous measurement points throughout the entire wind farm and in addition raising anemometry masts require building permits and dealing with health and safety aspects.
Wind Lidars are now being exploited as forward looking sensors to predict wind conditions ahead of wind turbines in operation, as opposed to traditional and widely used cup anemometers that are limited to instantaneous measurements at the turbine nacelle, or sonars which do predict the wind conditions ahead, however with low accuracy (fig.3). The ability to reliably predict the wind speed and direction is expected to be a very useful tool in the wind turbine industry to control the pitch and yaw for increased power optimization and even more importantly reduce load and fatigue of the large turbine structures. This could lead to new developments both in the construction design, farm site management, as well as operation and service lifetimes, and in the end reduced cost of ownership for the asset owners. The typical life time of wind turbines is around 20 years, but it is expected that it could be extended by 30% using wind Lidar feedback control to reduce the loads from wind turbulence [2}.
Other applications that can benefit from wind Lidars is aviation safety and airport wake vortex detection at landing fields. NKT Photonics has worked closely with the world leading research center in wind energy, Risø DTU National Laboratory for Sustainable Energy, and gained know-how of the laser requirements to make the fiber lasers suitable sources for wind Lidars.
Koheras lasers are also vital for important scientific projects around the world, for example for ESA’s Swarm mission scheduled for launch in November 2013 that will unravel one of the most mysterious aspects of our planet, the magnetic field [3] (fig.4). The Koheras laser was not originally intended to be space-born and installed on satellites when commercialized in the late 90’s, however this project is a good testimony of how the fiber laser is also capable of meeting the demanding requirements of the space industry and thus certainly live up to the high standards in the optical sensing industry.
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