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Introduction to tunable fiber lasers
Tunable fiber lasers are enabling a broad range of applications today and in the near future. Several technologies show promise of being able to address some market segments. Tunable fiber lasers are valuable for a variety of different applications, like spectroscopy, photochemistry, optical communications, laser cooling, metrology, or medical treatments. These applications require tunable lasers as they provide the ability to specifically adjust their wavelength.
Characteristics of tunable fiber lasers
The distinctive feature of the tunable fiber laser is the wavelength of operation, which can be altered in a controlled manner. Only several types of fiber lasers allow continuous tuning over a significant range. Tunable fiber lasers are usually operating in a continuous way with a small emission bandwidth, although some Q-switched and mode-locked fiber lasers can also be wavelength-tuned. There are many types and categories of tunable fiber lasers, such as excimer fiber lasers, gas fiber lasers (CO2 lasers, He-Ne lasers, and more), dye fiber lasers (liquid and solid state), semiconductor crystal and diode lasers, and free electron lasers. Tunable fiber lasers find applications in spectroscopy, photochemistry, atomic vapor laser isotope separation, and optical communications.
Rare-earth-doped fiber lasers
Among fiber lasers, rare-earth-doped fiber lasers can be tuned over a wide range of wavelengths. For example, ytterbium fiber lasers are tunable over tens of nanometers. Tunable fiber lasers that offer wide tunability are Raman fiber lasers.
Applications of tunable fiber lasers
Tunable fiber laser systems are utilized in various applications.
- Spectroscopy. A high-frequency resolution of transmission recording is possible by using tunable lasers. Tunable fiber lasers are also used in LIDAR.
- Laser cooling. Some methods of laser cooling require tunable lasers that can be adjusted very precisely.
- Isotope separation. The process of isotope separation with the use of a tunable laser consists of adjusting the laser wavelength to atomic resonances first and tuning it to a particular isotope to ionize it and deflect it with an electric field.
- Optical fiber communications. Tunable fiber lasers are often used as a spare laser in case the main fixed wavelength laser breaks down. In this situation, a wavelength-tunable laser is tuned to the wavelength of a particular channel that has failed.
- Optical frequency metrology. In optical frequency metrology, the laser needs to be stabilized to a certain standard, e.g., an absorption cell, an optical reference cavity.
Challenges and pricing of tunable fiber lasers
Prices for fixed and tunable fiber lasers are not yet equivalent, however. Although some tunable types are priced like fixed-wavelength devices, they are tunable over only very narrow ranges, about 3-4 nm. Those fiber lasers that can be tuned across wide wavelength ranges remain at least two or three times as expensive as their fixed counterparts. The high price of tunable fiber lasers is explained by specific features: the increased complexity of manufacturing them, the extra testing required, and the newness of the technology, which has yet to reach true volume demand. As demand for tunable lasers rises, their prices will come down. Laser manufacturers claim the price premium for a widely tunable laser will drop to about 15-20 percent above that of a fixed laser anyway.
Future impact on optical networks
The significantly favorable changes in demand for tunable fiber lasers will occur in parallel with their application to make optical networks more flexible. Fiber optic networks based on different types of fiber optic devices are essentially fixed: the optical fibers are connected into pipes with huge capacity but little reconfigurability. It is almost impossible to change how that capacity is deployed in real time. In addition to this, there is a problem in choosing a wavelength for a channel: as traffic is routed through a network, certain wavelengths may already be in use across certain links. Tunable fiber lasers will ease the switch to alternative channels without swapping hardware or re-configuring network resources. The benefits gained from the use of tunable fiber lasers are in the time it takes to actually deliver different types of services. Undoubtedly, tunable fiber lasers can dramatically improve fiber optic networks’ efficiency and will play an important role in enabling future dynamically reconfigurable optical networks, along with optical switches and semiconductor optical amplifiers.