Higher erbium concentrations raise the saturated output power levels and higher ytterbium concentrations increase the absorption per unit length. The Er:Yb doped phosphate fiber is placed in a resonant cavity that provides the feedback necessary to sustain laser operation. Pumping of the Er:Yb doped phosphate glass populates the erbium metastable state creating a population inversion. Spontaneous emission catalyzes the stimulated emission of the erbium ions in the metastable state.
In general, laser emission occurs when the total gain produced by stimulated emission overcomes the losses in the cavity. The gain profile of erbium and the geometry of the resonant cavity create preferential feedback so that laser emission only occurs at one or more discrete wavelengths.
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An ultra-short cavity together with a wavelength selective grating limits laser emission to a single wavelength. More specifically, the process of amplifying signals in the cavity to produce laser emission is explained in reference to energy levels shown in FIG. Higher erbium doping levels allows more absorption of the pump light and ultimately higher gain and higher output power levels.
More specifically, the enhancement process is explained in reference to energy levels shown in FIG. Ytterbium ions exhibit not only a large absorption cross-section but also a broad absorption band between and nm. Cooperative upconversion effects in phosphate glass at high levels of erbium doping concentration are significantly smaller than in silica glass.
The phosphate glass host provides for very efficient energy transfer. As can be seen from FIG. The modifier modifies the phosphate's glass network, thereby reducing its melting temperature and creating additional dopant sites. The intermediator bridges some of the bonds in the network thereby increasing the network's strength and chemical durability without raising the melting temperature appreciably. The multi-component glasses of interest thus have a much lower softening temperature than silica SiO 2 , which greatly simplifies processing.
The modified network provides many more sites for hosting high concentrations of rare-earth dopants such as Er and Yb without clustering problems. The fiber core is then doped with high concentrations of rare-earth dopants such as erbium or co-doped with, for example, erbium and ytterbium.
The cladding layer s are typically undoped. Doping levels are 0. Typical values are 0. Initial experiments have demonstrated that this class of phosphate glass supports continuous single-mode lasing without self-pulsation at high doping concentrations. Numerous glass compositions were characterized for thermal properties crystallization, expansion coefficient, transition and softening temperatures, core-to-clad similarity , chemical durability, ability to host high Er:Yb doping concentrations without quenching, spectroscopic properties maximum emission cross-section at 1.
Characterization provided the following results:.https://rikonn.biz/wp-content/2020-09-18/come-attivare-la-rete-dati-cellulare-iphone-6-plus.php
What are Rare-Earth Doped Fibers? – Fosco Connect
When configured for single-mode core-pumping, the doped phosphate glass fiber contains the following ingredients by weight percentages: P 2 O 5 from 30 to 80 percent, Yb 2 O 3 from 0. When configured for multi-mode clad-pumping, the doped phosphate glass fiber contains the following ingredients by weight percentages: P 2 O 5 from 30 to 80 percent, Er 2 O 3 from 0. In one preferred embodiment, a fiber for multi-mode clad-pumping comprises a cladding that includes. The following disclosure describes in more detail the characterization of phosphate glasses suitable for utilization as core or cladding glasses in the phosphate glass fibers of the present invention.
Compounds with less than 10 ppm of iron or copper were used as the starting chemicals for glass preparation. The liquid was then held at this temperature for thirty minutes. The liquid was cast into an aluminum mold. A variety of samples from these specimens were prepared for the measurements. The refractive index of glass was measured with a prism coupler e.
The thermal expansion coefficient, glass transition temperature, and softening temperature of each sample were measured on a dilatometer. The absorption spectra were recorded on a spectrophotometer e. Emission spectra were measured on samples with a thickness of 0. The emission spectrum measurement at 1. Absolute emission cross section were calculated using the McCumber theory.
According to the McCumber theory, the absorption and emission cross sections are related by equation 1 if the time required to establish a thermal distribution within each manifold is short compared with the lifetime of that manifold. The absorption cross section was determined from the absorption coefficient.
The peak absorption cross section for a 63 P 2 O 5. The peak stimulated emission cross section is calculated to be 0. The absorption and fluorescence spectra, 28 and 30 respectively, of erbium doped phosphate glass bulk are illustrated in FIG. Next, undoped glasses no erbium or ytterbium ions were fabricated as cladding glasses. In order to insure a low stress and birefringence in the fiber, similar thermal characteristics between the core glass and the cladding glass are required. To form waveguides in the core glass with good confinement, a slightly lower refractive index of the cladding glass is also required.
In addition, glass transition temperatures and softening temperatures of these cladding glasses are also very close to that of the core glass see Table 3 below. Small differences in thermal expansion coefficient, glass transition temperature, and softening temperature between the core and cladding glasses ensure high quality of the fiber.
A good refractive index match with the core glass is another key parameter for the cladding glass besides the thermal behavior match. The refractive index of the cladding and core glasses is listed in the Table 4 see below. The refractive index at 1. With such values of numerical aperture, erbium doped optical fibers with a mode profile perfectly matched with standard telecommunication fiber can be fabricated.
Finally, an ytterbium-doped glass composition of 63 P 2 O 5. This glass composition contains 3.
The refractive index of this glass composition was measured with a prism coupler at The absorption and emission cross sections were determined to be 0. The thermal expansion coefficient, glass transition temperature, and softening temperature of the ytterbium-doped core glass were measured to be 8. The cladding glass was designed to match the thermal performance of the ytterbium-doped core glass to ensure low stress in the fiber. The high doping concentrations possible, the low cooperative upconversion rate, the high transfer efficiencies, and the high saturated output power levels make doped phosphate glass ideal for high power single-mode lasers.
To achieve a low cost solution, fiber is superior to waveguides. A single preform can be drawn and diced into fibers for thousands of devices whereas a similar glass ingot provides only a few waveguides. Fiber manufacturing includes first the manufacture of the glass preform and second the drawing of the fiber from the preform. A phosphate glass ingot not shown in FIG. Once the ingot is formed, the ingot is cored to produce in step 54 a first core glass rod The barrel of the first core glass rod 56 is polished in step In step 60 , a cladding glass tube 62 i.
Both the inside and outside surfaces of the cladding glass tube are polished. In step 64 , the first core glass rod 56 is placed inside the first cladding glass tube 62 , and the assembly is drawn to form a second core glass rod 66 step The second core glass rod 66 is placed, in step 68 , inside a second cladding glass tube 70 , and the assembly 72 is drawn into a single mode fiber 74 step No plastic coating was applied to the fiber. The resonant cavity is formed between a Fiber Bragg Grating FBG on a polarization maintaining PM fiber , which is optically coupled to one end of the active fiber , and a second FBG at the other end of active fiber FBG provides single polarization output of the fiber laser.
FBG may be replaced with a dielectric mirror. As shown in FIGS. Either the wavelength selective FEG or the active fiber can be temperature controlled to spectrally overlap a certain cavity mode with the maximum of the cavity reflector.
Alternately, a dielectric mirror, either external or realized by direct coating of the fiber end facet can be used. A PM WDM is coupled to the free end of PM fiber to bring the pump signal into the cavity and to output the single-mode signal to a silica telecom fiber not shown through a PM Isolator optional to suppress any back reflection.
The resonant cavity is formed between a Fiber Bragg Grating FBG at the input side of active fiber and a wavelength selective FBG on a polarization maintaining PM fiber , which is optically coupled to the output side of active fiber FBG may be replaced with a broadband dielectric mirror. A PM Isolator optional may be coupled to the free end of PM fiber to suppress any back reflection. To pump the laser, pump radiation at or near nm must be coupled into the fiber and absorbed in the fiber core.
The conventional approach is to align a single-mode pump laser with the fiber core and couple the pump radiation directly into the core. The advantage of this approach is that substantially all of the pump light is coupled into the core. In the present invention, the higher ytterbium concentrations translate directly to higher absorption per unit length, hence higher output powers in the short cavity lengths required for single-mode lasers.
The gain profile for erbium is shown in FIG.
Within the C band of nm, the peak in the gain curve occurs at about nm and the minimum gain is at the far tail of nm. Using a mW single-mode pump, the output power exceeds 50 mW in the worst case. In other words, 50 mW over the full C band is achievable. The drawback to single-mode core pumping is the cost of the pump laser and the cost associated with the precision active alignment required to couple the pump radiation into a few micron core.
More recently, pump schemes have been proposed for coupling pump radiation from a multi-mode pump laser into the cladding surrounding the core in a silica fiber. Based on a geometric optical model in which the incident pump field is represented as an ensemble of rays of varying position and incident angle over the cladding entrance, those ray trajectories that cross or overlap the core as they propagate down the fiber correspond to rays that are absorbed, whereas those rays that avoid overlapping the core upon propagation are assumed to suffer no absorption: The overlapping rays are associated with the low order wave optical modes of the fiber with intersect the core, and the non-overlapping rays are associated with higher order modes that are localized in the cladding and intersect the core negligibly.
Rare Earth-Doped Fiber Lasers and Amplifiers
The fraction of the pump power that can be absorbed by the core is then approximated as the ratio of those rays that cross or overlap the core to the total number of rays. To fully absorb the available pump power in the relatively lightly doped core, the fiber must be quite long, on the order of several meters. Kouznetsov, J. Full Name Comment goes here.
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Embeds 0 No embeds. No notes for slide. Rare earth doped fibers 1. This term paper reviews what are earth dopedfibers, their common types, wave guide designs and TABLE 1characterization. It also describes the fabrication of these fibers The most common laser-active ions and host glasses and also typicalthrough various processes and gives a brief introduction on earth emission wavelength ranges of rare-earth-doped fibersdoped fiber amplifiers I.
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