WO2024261639A1 - Rare earth (re) dopants concentration measurements in active preforms based on side absorption by laser diodes at two or more wavelengths - Google Patents

Abstract

A method for measuring a rare-earth (RE) dopant concentration and active dopant profile (ADP) in an active optical fiber preform includes exciting an RE dopant in a core of an active optical fiber preform by two or more co-aligned light beams of different excitation wavelengths. Each light beam interacts with the RE dopant with different absorption characteristics and an emission pattern of the RE dopant in the core is different at each of the excitation wavelengths. Known RE absorption characteristics at each of the excitation wavelengths are used to translate the emission pattern at each of the excitation wavelengths into an active dopant profile (ADP) and an overall RE dopant concentration of the active optical fiber preform.

Description

RARE EARTH (RE) DOPANTS CONCENTRATION MEASUREMENTS IN ACTIVE PREFORMS BASED ON SIDE ABSORPTION BY LASER DIODES AT TWO OR MORE WAVELENGTHS FIELD OF THE INVENTION [0001] The present invention relates generally to rare-earth (RE) doped optical fibers, and particularly to a method and system for measuring the RE concentration and distribution in active optical fiber preforms. BACKGROUND OF THE INVENTION [0002] In rare-earth (RE) doped optical fibers used for fiber lasers and other applications, accurate control and characterization of the refractive-index profile (RIP) and the active dopant profile (ADP) are crucial for the fiber performance, both in terms of efficiency and overall performance. [0003] Although there are methods for monitoring the RE concentration in the fiber’s preform, these methods are destructive. In particular, measurements of the RE concentration and its longitudinal distribution along and across the preform are carried out by slicing the preform into thin discs and inspecting the RE concentration and its distribution in the disks, by x-ray diffraction (EDX (energy dispersive X-ray) or EPMA (electron probe microanalyzer)). Albeit these methods provide an accurate assessment of the RE dopant concentration, it renders the examined preform un- useful due to its destruction by slicing. SUMMARY OF THE INVENTION [0004] The present invention seeks to provide a method and system for measuring the RE concentration and distribution in active preforms. Unlike the prior art, the method of the invention is non-destructive. The method excites the RE dopants in the preform’s core by two or more co-aligned light beams of different wavelengths, wherein each light beam interacts with the RE dopant with different absorption characteristics. [0005] As a result, the emission pattern of the RE dopants in the preform’s core is different at each of the excitation wavelengths. Since the RE absorption characteristics at each of the excitation wavelengths is known, proper analysis allows for translating the emission pattern of the preform’s core at each of the wavelengths into the active dopant profile (ADP) and the overall RE dopant concentration. [0006] An advantage of the invention is higher preform yield by eliminating the need to corrupt the measured preform. The embodiment described below is of a setup and method for measuring RE concentration at two wavelengths. However, multiple wavelengths sources can also be employed. [0007] There is provided in accordance with a non-limiting embodiment of the present invention a method for measuring a rare-earth (RE) dopant concentration and active dopant profile (ADP) in an active optical fiber preform, including exciting an RE dopant in a core of an active optical fiber preform by two or more co-aligned light beams of different excitation wavelengths, wherein each of the light beam interacts with the RE dopant with different absorption characteristics and an emission pattern of the RE dopant in the core is different at each of the excitation wavelengths, and using known RE absorption characteristics at each of the excitation wavelengths to translate the emission pattern at each of the excitation wavelengths into an active dopant profile (ADP) and an overall RE dopant concentration of the active optical fiber preform. BRIEF DESCRIPTION OF THE DRAWINGS [0008] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: [0009] Fig. 1A is a simplified illustration of a motorized preform stage and peripheral measurement setup, in accordance with a non-limiting embodiment of the present invention. [0010] Fig. 1B is a simplified illustration of the preform cross section and crossing diagnostic beam, in accordance with a non-limiting embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS [0011] Reference is now made to Fig. 1A, which illustrates a motorized preform stage and peripheral measurement setup, in accordance with a non-limiting embodiment of the present invention. [0012] For the purpose of measuring the RE concentration and ADP of a preform’s core, a motorized setup has been developed. The setup allows for moving a scanning laser beam to certain points along the preform and at various angles. First, a laser diode beam operating at wavelength λ1, which is negligibly absorbed by the RE agents, crosses the preform and mildly excites the RE ions in the core. Since the amount of RE absorption at this wavelength is negligible, the resultant spontaneous emission (I_sp-λ1 ^{^}^^{^}) at the shined location is detected by a beam analyzer and provides the ADP which is proportional to the RE concentration (n_RE) variations across the preform. Same measurement at various angles provides the ADP at these angles and enables the user to form a 2D picture of the ADP across and along the preform. [0013] Following the measurement at λ1 excitation wavelength, a spontaneous emission distribution at the same measured location under λ2 excitation also takes place. Since both λ1 and λ2 diode beams are co-aligned, scanning of the core, at both wavelengths, is done at the exact same locations and angles of the preform. [0014] Under λ2 excitation, the beam is strongly attenuated due to a high RE ions absorption cross section at this wavelength. The amount of attenuation is proportional to the RE ions absorption from the beam entrance to the core (-a, see Fig. 1B), up to the point r at which the spontaneous emission is monitored, I.e.

[0016] where a is the core radius and –a<r<a is the radial location where local fluorescence is measured (see Fig. 1B).

and ^_λ2 are the RE absorption cross sections at λ1 and λ2 wavelengths, respectively (^_λ1 ≪ ^_λ^). [0017] Since ^_^^^^^ ∝ I_sp-λ1^^^, eq. (1) can be written as follows:

[0019] Therefore, since the RE absorption cross section at λ2 (^_λ2) is known, one can extract the integral RE concentration (_^ ^{^} ^_^ ^_^^ ^{^}^′^{^}^^′ ) out of eq. (2), by equating the normalized theoretical curve of the spontaneous emission at λ2 (according to eq. (2)) to the measured one. Once there is a match, the average RE concentration across the preform’s core (^^_^^^^, ^) (at the longitudinal location (z) and incident angle ( )) can be calculated from the resulting integral RE concentration value across the core: [0021] By accurately moving the preform up and down with the motorized stage (Fig. 1A), as well as rotating it, the RE ions concentration and their cross sectional distribution can be measured at various locations along the preform and at various angles, without harming the preform. [0022] From knowing the average RE concentration value at a certain location and angle along the preform ^^_^^^^, ^, one can deduce the RE concentration at a certain location ^{^}^, , ^^{^} in the core by normalizing the spontaneous emission pattern at λ1, I_sp-λ1^^^, which as mentioned before is proportional to ^_^^^^^ .

Claims

CLAIMS What is claimed is: 1. A method for measuring a rare-earth (RE) dopant concentration and active dopant profile (ADP) in an active optical fiber preform, comprising: exciting an RE dopant in a core of an active optical fiber preform by two or more co-aligned light beams of different excitation wavelengths, wherein each said light beam interacts with said RE dopant with different absorption characteristics and an emission pattern of said RE dopant in said core is different at each of said excitation wavelengths; and using known RE absorption characteristics at each of said excitation wavelengths to translate said emission pattern at each of said excitation wavelengths into an active dopant profile (ADP) and an overall RE dopant concentration of said active optical fiber preform. 2. The method according to claim 1, wherein said two or more co-aligned light beams comprise a first light beam and a second light beam, and said first light beam is negligibly absorbed by the RE dopant and said second light beam is strongly attenuated due to a high RE ions absorption cross section at the wavelength of said second light beam. 3. The method according to claim 2, wherein a resultant spontaneous emission at a shined location of said first light beam is detected by a beam analyzer and provides the ADP which is proportional to RE concentration variations across said preform. 4. The method according to claim 2, comprising shining said first beam at different angles on said core and determining the ADP at said different angles to form a 2D picture of the ADP across and along said preform. 5. The method according to claim 1, wherein an amount of attenuation of said second light beam up to a point r at which spontaneous emission of said second light beam is monitored, is I_sp-λ2 ^{^}^^{^} which is given by:

where a is core radius and –a<r<a is a radial location where local fluorescence is measured, and

and ^_λ2 are RE absorption cross sections at wavelengths (λ1^ and (λ2^ of said first and second light beams, respectively (^_λ1 ≪ ^_λ^); and ^_^^ ^{^}^^{^} ∝ I_sp-λ1 ^{^}^^{^}, so:

and since the RE absorption cross section at the second light beam wavelength λ2 (^_λ2) is known, further comprising extracting an integral RE concentration (_^ ^{^} ^_^ ^_^^^^′^^^′ ) out of eq. (2), by equating a normalized theoretical curve of spontaneous emission at λ2 (according to eq. (2)) to the measured spontaneous emission. 6. The method according to claim 5, further comprising determining an average RE concentration across said core (^^_^^^^, ^^) (at a longitudinal location (z) and an incident angle (^)) by: