The 980/1550nm wavelength division multiplexer (WDM) is a key component of erbium-doped fiber lasers and amplifiers. 980/1550nm WDM is mostly made of single-mode fiber (SMF) and made by winding fusion tapering method. With the development of optical fiber communication and sensing technology and the successful development of polarization-maintaining fibers, PMF circulators and isolators, more and more systems use PMF and polarization-maintaining devices to package the polarization characteristics of the optical transmission in the subsystem.
In order to realize the polarization stable output of the fiber laser and the emitting atmosphere. As the transmission device in the system, WDM must have high-performance polarization maintaining characteristics at the 1550nm port due to the FBT type. In addition, the FBT type coupler has the characteristics of low loss, good temperature stability, sturdy structure and simple manufacturing process. Therefore, the FBT type 980/1550nm PMF WDM has become a requirement for the development of PMF lasers and amplifiers.
The output of the 980nm pump light source is mostly unpolarized light. In order to match the output fiber of the light source, the 980nm port uses HI1060 SMF, and the 1550nm port uses lateral easy coupling and matching PMF. In order to prevent the stress zone of the PMF from affecting the energy coupling between the fibers, the fast axis of the PMF is adjusted to be consistent with the core connections of the two fibers before the FBT of the two fibers. The laser beam propagates in the fiber cone whose diameter changes from large to small, and the mode field radius of the core inverted mode changes from small to large. When the normalized frequency of the fiber core drops to a certain level, the confinement effect of the fiber core on the guided mode transmission is greatly reduced. At this point, a large part of the energy of the optical field is transmitted in the cladding, and it interacts with air or other refractions. A medium with a lower refractive index than the original fiber cladding forms an irregular waveguide. When the waveguide diameter changes, coupling occurs between modes, and the optical power is distributed according to the coupling coefficient and coupling length at the second cone and coupled out to become the additional loss of the device.
Since the interval between 980nm and 1550nm is larger, their coupling coefficients are also quite different, so it is easier to implement the wavelength division multiplexing of the coupler. Appropriate selection of flame temperature and stretching speed, FBT obtains a certain coupling mechanism, 1550nm light is energy-coupled between fibers, when light energy is exchanged and re-coupled back to PMF, 980nm light is almost completely coupled into SMF, Devices with this feature can be used as 980/1550nm WDM.
The 980/1550nm wavelength division multiplexer WDM has 0.2db insertion loss, 32db isolation, and 22.8db extinction ratio at 1550nm wavelength. It ensures the polarization characteristics and low loss of the full polarization maintaining fiber system: at 980nm wavelength Insertion loss of 0.2db, isolation of 14.8db. The development of PMF WDM successfully solved the problem of polarization stability of PMF lasers and amplifiers, and played an important role in the development of lasers and amplifiers in Guanghe County.
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