WO2025083527A1

WO2025083527A1 - Rare earth (re) dopants concentration measurements in active preforms

Info

Publication number: WO2025083527A1
Application number: PCT/IB2024/060047
Authority: WO
Inventors: Yoav Sintov
Original assignee: Israel Atomic Energy Commission
Current assignee: Israel Atomic Energy Commission
Priority date: 2023-10-16
Filing date: 2024-10-14
Publication date: 2025-04-24
Anticipated expiration: 2026-04-16

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 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] When a single RE component preform is analyzed, the present invention provides two possible alternatives for RE dopant analysis: [0006] First, if the RE dopant is highly absorptive, such as but not limited to Ytterbium (Yb), the RE dopant in the preform’s core is excited by two or more co-aligned light sources, operating at different wavelengths, each interacts with the RE dopant with different absorption characteristics. [0007] As a result, the emission pattern of the RE dopant 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 ADP and the overall RE dopant concentration. [0008] Second, if the RE dopant is has negligible absorbance across the core cross section, The RE dopant is excited by single wavelength. Since the preform absorption is negligible, the fluorescence pattern is proportional to the RE distribution across the core at the measurement location. Since the fluorescence emission intensity is proportional to the average RE concentration, measuring the emission intensity by a beam profiler allows identifying the average RE dopant concentration. This method, however, requires pre-calibration of the beam profiler with a preform sample with known RE concentration and core size. This preform sample is used as a reference for the investigated RE doped preform measurement. [0009] When two RE dopant components are incorporated in the preform, while one dopant is being used as a sensitizer for the other RE dopant, such as but not limited to Yb and Erbium (Er) co-dopants, where Yb is being used to store the pump energy and transfer it to the Er agent, the profile of each RE component is being monitored by exciting it with its characteristic negligible absorbance signal wavelength and monitor the emission pattern, which provides the cross sectional distribution of the relevant RE component. [0010] Since the use of two RE component is mainly to excite the low absorbance RE by the high absorbance RE, the high absorbance RE concentration and distribution can be identified by the above mentioned multiple-wavelength method. [0011] On the other hand, the low absorbance RE component concentration cannot be identified by the same method since its low absorption characteristics at each wavelength do not allow for a distribution difference, which is the essence of the multiple-wavelength method. [0012] In this case another embodiment of the present invention can allow for identifying the concentration of the low absorbance RE agent, by measuring the lifetime of the high absorbance RE. This method relies on the fact that the energy transfer rate from the high absorbance RE to the low absorbance RE is linearly dependent on the low absorbance RE concentration. Therefore, by measuring the fluorescence lifetime of the high absorbance RE, one can get the concentration of the low absorbance RE agent in the preform. [0013] One of the benefits of the invention is higher preform yield by eliminating the need to corrupt the measured preform. The following embodiments described below are of a setup and method for measuring RE concentration and distribution of single RE component, as well as two RE component preforms. BRIEF DESCRIPTION OF THE DRAWINGS [0014] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: [0015] 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. [0016] 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. [0017] Fig. 2A is a simplified illustration of a motorized preform stage and peripheral measurement setup, in accordance with another non-limiting embodiment of the present invention. [0018] Fig. 2B is a simplified illustration of the preform cross section and crossing diagnostic beam, in accordance with another non-limiting embodiment of the present invention. [0019] Fig. 3 is a simplified illustration of energy transfer from Yb3+ to Er3+, followed by non-radiative decay of erbium to the upper laser level, as known in the prior art. [0020] Fig. 4 is a simplified illustration of the transfer coefficient between ytterbium and erbium as a function of the Yb-concentration. [0021] Fig. 5A is a simplified illustration of a motorized preform stage and peripheral measurement setup, in accordance with yet another non-limiting embodiment of the present invention. [0022] Fig. 5B is a simplified illustration of the preform cross section and crossing diagnostic beam, in accordance with yet another non-limiting embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS [0023] 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. [0024] In one embodiment, there is a method and setup for measuring a high absorbance RE, such as but not limited to Yb, in a fiber preform core. This embodiment relies on two or more wavelengths, for which the RE absorption is different. [0025] For the purpose of measuring the RE concentration and ADP of a preform’s core, a motorized setup has been developed. The setup (Fig. 1A) 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 agent, crosses the preform and mildly excites the RE ions in the core (Fig. 1B). 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 profiler and provides the ADP, which is proportional to the RE concentration (n_RE) variations across the preform’s core. 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. [0026] Following the measurement at λ1 excitation wavelength, a spontaneous emission distribution at the same measured location under λ2 excitation wavelength also takes place. Since both λ1 and λ2 diode beams are co-aligned, scanning of the core, at both wavelength, is done at the exact same locations and angles of the preform. [0027] 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. I_sp-λ2 ^{^}^^{^}:

[0029] where a is the core radius and –a<r<a is the radial location where local fluorescence is measured (see Fig. 1B).

are the RE absorption cross sections at λ1 and λ2 wavelengths, respectively (^_λ1 ≪ ^_λ^). [0030] Since ^_^^^^^ ∝ I_sp-λ1^^^, eq. (1) can be written as follows:

[0032] 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 between the two, 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:

[0034] 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. [0035] 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 ^_^^^^^ . [0036] Reference is now made to Figs. 2A and 2B. In another embodiment, there is a method and setup for measuring a low absorbance RE, such as but not limited to Er, in a fiber preform core. This embodiment relies on one wavelength excitation, for which the RE absorption is negligible, but still provides sufficient fluorescence to be identified by a beam profiler. [0037] For the purpose of measuring the RE concentration and ADP of a preform’s core, a motorized setup has been developed. The setup (Fig. 2A) 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 λ, which is negligibly absorbed by the RE agent, 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-λ ^{^}^^{^}) at the shined location is detected by a beam profiler and provides the core RE dopant dimension

and ADP, which is proportional to the RE concentration (n_RE(r)) variations across the measured preform’s core. [0038] Following the measurement of the RE distribution at λ excitation wavelength, a power measurement by the beam profiler of the overall spontaneous emission at the same measured location ()_(^^ =

under λ excitation also takes place. The measured power provides information on the average RE concentration (^^_(^^), since it is proportional to the overall fluorescence power. [0039] In the following step, a preform sample of the same RE dopant type and host material and with a known concentration (^^_*^^) and core dimensions ('_*^^) is inserted at the same location of the measured preform. An additional power measurement of this sample ()_*^^) is taken by the beam profiler. By comparing the power measurement and RE core dimensions of the sample preform with the measured preform, the RE concentration of the measured preform can be found as follows:

[0041] Reference is now made to Figs. 5A and 5B. In yet another embodiment, the method and system relate to a preform containing two RE dopants, in which one dopant is being used as a sensitizing agent for the other RE dopant, such as but not limited to Yb and Er co-dopants. In Yb:Er co-doped preforms and fibers, Yb is being used to store the pump energy and transfer it to the Er agent, which is used as the lasing agent in the fiber produced from the preform. [0042] The profile of each RE component is being monitored by exciting it with its characteristic negligible absorption signal wavelength (for example: 1060nm for Yb and 1530nm for Er) and monitoring the emission pattern, which provides the cross sectional distribution of the relevant RE component. [0043] The high absorbance RE concentration and distribution is measured using the first embodiment described above. [0044] The low absorbance RE dopant concentration is measured by measuring the high absorbance RE fluorescence lifetime under pulsed excitation, as described in the following section. [0045] It is well known that the high absorbance dopant lifetime is dependent on the spontaneous fluorescence decay and on the transfer rate of its upper state population to the low absorbance ground state RE agent. In addition, the transfer rate is dependent on the concentration of the low absorbance RE. As a result, it is possible to deduce the low absorbance RE concentration by measuring the fluorescence decay of the high absorbance RE. [0046] As an example, but not limited to, in Yb:Er co-doped preforms, the energy transfer between the Yb dopant and the Er dopant is described in Fig.3. [0047] The Yb agent is being excited by a 975nm or 915nm diode laser light to its upper state and the upper state population is decayed to its lower state by spontaneous fluorescence decay as well as by energy transfer to the Er ⁴I_11/2 level and from there to the Er upper laser level (⁴I_13/2). [0048] Under pulsed excitation of the Yb:Er preform core, the following equation provides the Yb exponential decay rate (τ) after the excitation pulse ending:

[0050] where 8_*9:^is the spontaneous decay of the excited Yb ion dopants without the presence of Er (known to be 8_*9:^=1.4msec for typical phospho-silicate Yb doped preforms), ^^₆₇is the Yb average concentration, measured by using the method described in the first embodiment. 5_67^^^ ^{^}^^₆₇ ^{^} is the Yb-Er transfer constant, which is slightly dependent on the Yb ions concentration, as described in the work of Lester et.al (C. Lester, A. Bjarklev, T. Rasmussen, and P. G. Dinesen, “Modeling of Yb - sensitize Er -doped silica waveguide amplifiers,” J. Lightw.Technol., vol. 13, no. 5, pp.740–743, May 1995) and shown in Fig.4. [0051] By knowing the Yb spontaneous emission and the Yb-Er transfer rate, one can deduce the average Er concentration in the preform’s core, from the measured Yb upper state population decay time (τ) :

[0052] Figs. 5A and 5B describe a setup for measuring the concentration and distribution of Yb and Er in Yb:Er co-doped preforms. This example is also applicable to other two RE components preforms, in which the high absorbance component is used as a sensitizer for the other low absorbance RE component. [0053] The high absorbance Yb agent concentration and distribution are measured at each angle along the preform as described in the first embodiment of the present invention, utilizing high and negligible absorbance wavelengths such as 976nm and 1960nm, respectively. [0054] The low absorbance Er agent distribution is measured by recording the fluorescence distribution as a result of introducing 1530nm signal (but not limited to), which is not absorbed by the Yb dopant and negligibly absorbed in the Er agent in the preform’s core. As a result an energy transfer to the Yb ions from the Er ions occurs and the excited Yb ions fluoresce with a pattern measured by the Si beam profiler, which represents the Er distribution in the preform core at the measurement location. Alternative measurement method is a direct measurement of the Er fluorescence with an InGaAs based camera, which is sensitive to the Er fluorescence wavelength (~1550nm). This Er pattern result will be used later on to identify the absolute Er concentration cross sectional distribution. [0055] Following the Er and Yb distribution and Yb concentration measurements, the 975nm signal is pulsed and excites the Yb agent. [0056] By measuring the 1/e Yb lifetime, the measured average Yb concentration (^^₆₇^, and using Equation 5, one can get te Er average concentration (^^_^^) at the measurement location. For example for 0.5mol% Yb and 216microseconds measured lifetime, the Er concentration is 0.044mol% (2.2*10²⁵m^-3). [0057] From knowing the average Er concentration value at a certain location and angle along the preform ^^_^^ ^{^}^, ^{^}, one can deduce the Er concentration distribution at a certain location ^^, , ^^ in the core by normalizing the spontaneous emission pattern achieved from the beam profiler previous measurement to the measured average Er concentration (^^_^^ ^{^}^, ^{^}). [0058] The same measurement at various angles provides the ADP of both RE components at these angles and enables the user to form a 2D picture of the ADP across and along the preform.

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:

7. The method according to claim 1, wherein one of said light beams comprises a laser beam operating at a wavelength λ, which is negligibly absorbed by the RE dopant, and wherein said laser beam crosses said active optical fiber preform and mildly excites RE ions in the core, and wherein since RE absorption at said wavelength is negligible, the resultant spontaneous emission (I_sp-λ ^{^}^^{^}) at the shined location is detected by a beam profiler and provides the core RE dopant dimension (&_'^^) and ADP, which is proportional to the RE concentration (n_RE(r)) variations across the measured preform’s core. 8. The method according to claim 7, further comprising making a power measurement by said beam profiler of overall spontaneous emission at same measured location ((_'^^ =

^^) under excitation at said wavelength λ, wherein said power measurement provides information on average RE concentration (^^_'^^), since it is proportional to overall fluorescence power. 9. The method according to claim 7, further comprising inserting, at same location of the measured preform, a preform sample of same RE dopant type and host material and with a known concentration (^^_)^^) and core dimensions (&_)^^), and making an additional power measurement of said preform sample by said beam profiler, and comparing power measurement and RE core dimensions of said preform sample with the measured preform to calculate RE concentration of the measured preform as follows: ^^_*^^∙,_-^ ∙^ ^^ _{^ *^^} _'^^ = ,_*^^∙^_-^^ 10. The method according to claim 1, wherein said active optical fiber preform comprises two RE dopants, one of said dopants being used as a sensitizing agent for the other RE dopant. 11. The method according to claim 10, wherein said RE dopants comprise Yb and Er co-dopants, and said Yb dopant is used to store pump energy and transfer the pump energy to the Er dopant, and said Er dopant is used as a lasing agent in the fiber produced from t said active optical fiber preform. 12. The method according to claim 10, wherein one of said dopants is a relatively low absorption RE dopant and one of said dopants is a relatively high absorption RE dopant, and a concentration of said relatively low absorption RE dopant is measured by measuring fluorescence lifetime of said relatively high absorption RE dopant. 13. The method according to claim 10, wherein one of said dopants is a relatively low absorption RE dopant and one of said dopants is a relatively high absorption RE dopant, and a concentration of said relatively low absorption RE dopant is measured by shining the preform core by a relatively low absorption wavelength, which is not absorbed by said relatively high absorption RE dopant, and recording the fluorescence, which is proportional to ADP of said relatively low absorption RE dopant. 14. The method according to claim 10, wherein a concentration and ADP of said relatively high absorption RE dopant is measured by said two or more co-aligned light beams, 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.