rus
Issue #2/2021
R. R. Kashina, Yu. A. Konin, Yu. A. Velikotsky, A. R. Rakhmatullina, A. Yu. Petukhova, V. A. Shcherbakova, V. B. Romashova
Dependence of the Output Laser Radiation on Geometry of the Optical Fiber
Dependence of the Output Laser Radiation on Geometry of the Optical Fiber
DOI: 10.22184/1993-7296.FRos.2021.15.2.144.150
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The influence of the first fiber cladding geometry on the mode mixing function was investigated. The efficiency of the clad-core mode transfering is determined.
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The influence of the first fiber cladding geometry on the mode mixing function was investigated. The efficiency of the clad-core mode transfering is determined.
Теги: beam profile fiber laser fiber-optic components focuslight fujikura nufern optical fiber photonics sc “lls” standa ао «ллс» волоконно-оптические компоненты волоконный лазер оптическое волокно профиль пучка фотоника
Dependence of the Output Laser Radiation on Geometry of the Optical Fiber
R. R. Kashina 1, Yu. A. Konin 1, 2, Yu. A. Velikotsky 1, 3, A. R. Rakhmatullina 1, 3, A. Yu. Petukhova 1, 3, V. A. Shcherbakova 1, 3, V. B. Romashova 4
Perm Research and Production Instrument-Making
company PJSC, Perm
ITMO University, St. Petersburg
Perm Research Polytechnic University, Perm
SC “LLS”, St. Petersburg
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The influence of the first fiber cladding geometry on the mode mixing function was investigated. The efficiency of the clad-core mode transfering is determined.
Keywords: SC “LLS”, fiber-optic components, optical fiber, beam profile, fiber laser, FocusLight, Nufern, Fujikura, Standa, photonics
Received: 21.01.2021
Accepted: 15.02.2021
Introduction
A fiber laser or amplifier based on standard active single-mode fibers can generate a diffraction-quality laser output beam, but it is limited by the pump source and the percentage of laser conversion efficiency [1]. For laser applications, it is very important to have a clear and high quality output beam. However, the use of active multimode fibers often results in poor output beam quality.
The strongest progressive achievement in our time has become the appearance of double-clad optical fibers [2]. This technology makes it possible to design a fiber laser, the output of which has a power of more than 1 kW [3, 4].
The problem of obtaining a high quality output beam and high power conversion was solved with the advent of double-clad fibers. The whole point of the technology is that when using an optical fiber in a double cladding, the pump radiation does not immediately enter the fiber core, but first enters its cladding [5]. Then, the pumping power is transferred from the cladding to the core, which, in turn, is doped with ions of rare-earth elements. They absorb pump radiation photons and generate coherent radiation at the desired wavelength. It should be borne in mind that for high-power lasers it is impossible to inject high-power pump radiation directly into the fiber core.
The radius of the inner cladding is greater than the radius of the core. The section of the inner cladding can have not only circular geometry, but have one or several faces (Fig. 1).
The simplest fiber design has a round pump cladding and a centered core (see Fig. 1 – centered core). This design is easy to implement in the technological process and to dock with a number of passive components. However, in fibers of this type, there are propagation modes of the inner cladding (referring to spiral beams), which almost do not overlap with the core. Because of this, a significant part of the pump radiation is not completely absorbed [7, 8]. As a result, the gain and pump energy efficiency are reduced.
Poor core overlap modes can be avoided by using a modified design with lower symmetry. Examples are designs with an off-center or non-circular inner cladding (see Figure 1: e. g., elliptical inner cladding, D-shaped, or rectangular). Pump claddings are also often better suited to the shape of the pump beam, such as diode rods. However, when splicing them together, problems arise if the entire fiber (not just the cladding) is non-circular.
In addition to the physical characteristics of the fiber core, an important parameter is the ratio of the cross-sectional areas of the inner cladding and the core. This area ratio should not be too high. Otherwise, the effective absorption length of the pump becomes large, and the pump intensity in the core becomes small, which leads to low excitation levels, and this leads to a decrease in energy efficiency [9]. Usually, the ratio between the areas is maintained in the range of 100 to 1000. Pump sources with improved brightness allow the use of fibers with a smaller area ratio and, therefore, a shorter effective pump absorption length, which also reduces the effect of various types of nonlinearities.
Incomplete absorption of pump radiation can result from the appearance of cladding modes with weak core overlap. Even if strong mode mixing can be achieved with a suitable fiber design, the pump absorption is reduced due to the limited overlap of the pump light with the doped fiber core. Therefore, it is usually sought to provide a long active fiber. Although, in turn, this can be harmful, e. g., from the point of view of the influence of non-linear effects.
Furthermore, a high concentration of impurity ions can make it difficult for a laser or amplifier to operate at short wavelengths of radiation, and the increased contribution of fluorescence can reduce the energy conversion efficiency.
Other harmful effects are associated with the release of some of the radiation from the core into the pump cladding, which occurs, e. g., as a result of bending or when using a fiber Bragg grating. This light will remain in the pump cladding and will not escape (as is the case with other fibers) through the coating. A cladding-mode stripper (CPS) may be required to remove such radiation if it contributes to the noise output of the device.
Output beam profilometers can be used to analyze beam propagation in double-clad fiber. Many beam profilometers are based on digital cameras: for the visible and near infrared spectral regions, CMOS and CCD cameras are the most suitable for measuring complex beam shapes.
Different wavelength ranges require different types of detectors. Silicon detectors are used for visible and NIR wavelengths up to 1.0–1.1 µm, while InGaAs detectors can be used for IR wavelengths up to ≈1.7 µm.
The spatial resolution of the camera sensor is an important parameter. In silicon detectors, pixel sizes reach less than 10 μm, which makes it possible to measure beams with a diameter of up to 50 μm.
Most cameras are highly sensitive to light. The advantage of using them in problems of detecting weak signals becomes an obstacle in measuring the beam profile. Since the sensitivity turns out to be much greater than required for measurements.
In the profilometer, the registered beam profile can be shown on the monitor, possibly together with the measured parameters: the radius of the beam, the position of the beam, ellipticity, data on statistical values or noises. The software allows you to choose between different methods for determining the beam radius: by the criterion of reducing the beam power by a factor of e2 (criterion 1 / e2) or the 4σ method.
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The efficiency of the output radiation is determined.
EXPERIMENTAL PART
The study of radiation propagating along the optical fiber was carried out on a laboratory model for studying the beam profile. The laboratory model consists of a FocusLight pump laser diode with a wavelength of 976 nm, a Nufern MM105 / 125 delivery fiber spliced to the fiber under study, a Fujikura FH‑60-DC250 fiber holder, Standa 3PH pedestals, a Standa 20R6 rail slide, and a WinCamD-LCM camera, set of neutral filters ND1-ND5.
The study was carried out on active double-clad fibers doped with erbium and ytterbium ions, models MM-EYDF‑10 / 125-XP, and MM-EYDF‑10 / 125-XPH.
The profile of the diverging radiation beam was measured on the prototype to assess the efficiency of absorption of pump radiation. The profilometer camera recorded the cross section of the radiation beam. Its intensity profile can be described by the function I(x, y). The I(x, y) value of a particular pixel corresponded to the range [0–255]. The data received by the camera was transferred to a PC and interpreted using the DataRay v.8 software.
The beam quality can be assessed by various methods, e. g., the parameter M2 is used for Gaussian beams. But Gaussian beams propagate in single-mode fibers, and this method is incorrect for modes of operation with multimode fibers. To assess the pumping efficiency, we measured the level of the ratio of radiation propagating through the fiber core and propagating through its cladding.
RESULTS
For the first measurement, a fiber MM-EYDF‑10 / 125-XPH with a fiber length of 10 cm was chosen. It was supplied with radiation with a wavelength of 976 nm and a power of 500 mW. The beam profile is shown in Fig. 4.
The beam profile has an unusual shape. As can be seen from Fig. 4, the radiation propagating along the second light-guiding cladding has an octahedral shape. In addition, there is a ring of the cladding mode, which propagates along it; between the boundaries of the first and second claddings, there is an obvious drop in power. The radiation propagating along the first light-guiding cladding does not have a pronounced peak, but it is still present there (note in Fig. 5).
The shape of the radiation intensity on both the first and the second shell has a flat top. This indicates that a mode mixing function is implemented in the fiber under study, which helps to transfer energy from the passive light-guiding cladding to the active core of the fiber. The measured transfer rate was 48.6%. The presence of a cladding mode, as in Fig. 3, and the dip between the first and second light-guiding shells indicates an incomplete pumping of the radiation energy – this is a negative effect. This cladding mode will need to be filtered out using a mode stripper, e. g., during the further assembly of the laser.
For the second measurement, an MM-EYDF‑10 / 125-XP fiber with a length of 15 cm was chosen. It was supplied with radiation with a wavelength of 976 nm and a power of 500 mW. The beam profile is shown in Fig. 6.
The beam profile of this fiber has a Gaussian shape, more reminiscent of a Buddhist stupa. In this profile, no interface between the claddings is observed; the radiation from the core is pronounced. However, in the core itself, the beam profile is Gaussian and has a high power density, which can lead to nonlinear effects in the fiber material. The measured transfer rate was 64.8%. There is no clear mode mixing in the profile of this fiber, but it shows good conversion efficiency.
CONCLUSIONS
Several double-clad fibers were investigated. To assess their efficiency, a laboratory bench with a WinCamD-LCM beam profilometer was assembled. The embedded DataRay software made it possible to quickly and efficiently evaluate the beam profile and the degree of its transformation. The investigated fibers showed conversion rates of 49 and 65%, respectively.
ACKNOWLEDGEMENT
The authors of the article express their deep gratitude to SC “LLS” for the equipment provided.
AUTHORS
Velikotsky Yuri Andreevich, engineer researcher of PJSC PNPPK, master first year PNRPU, Perm
Rakhmatullina Alina Rimovna, design engineer of PJSC PNPPK, postgraduate student first year PNRPU, Perm.
Petukhova Alexandra Yurievna, design engineer of PJSC PNPPK, master first year PNRPU, Perm.
Shcherbakova Viktoria Aleksandrovna, engineer researcher PJSC PNPPK, postgraduate first year PNRPU welfare, Perm.
Romashova Vasilisa Borisovna, fiber systems engineer of SC “LLP”, St. Petersburg.
Kashina Rano Rustamovna, designer of PNPPK, Perm.
Konin Yuri Anatolievich, engineer researcher PNPPK, Perm , St. Petersburg.
R. R. Kashina 1, Yu. A. Konin 1, 2, Yu. A. Velikotsky 1, 3, A. R. Rakhmatullina 1, 3, A. Yu. Petukhova 1, 3, V. A. Shcherbakova 1, 3, V. B. Romashova 4
Perm Research and Production Instrument-Making
company PJSC, Perm
ITMO University, St. Petersburg
Perm Research Polytechnic University, Perm
SC “LLS”, St. Petersburg
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The influence of the first fiber cladding geometry on the mode mixing function was investigated. The efficiency of the clad-core mode transfering is determined.
Keywords: SC “LLS”, fiber-optic components, optical fiber, beam profile, fiber laser, FocusLight, Nufern, Fujikura, Standa, photonics
Received: 21.01.2021
Accepted: 15.02.2021
Introduction
A fiber laser or amplifier based on standard active single-mode fibers can generate a diffraction-quality laser output beam, but it is limited by the pump source and the percentage of laser conversion efficiency [1]. For laser applications, it is very important to have a clear and high quality output beam. However, the use of active multimode fibers often results in poor output beam quality.
The strongest progressive achievement in our time has become the appearance of double-clad optical fibers [2]. This technology makes it possible to design a fiber laser, the output of which has a power of more than 1 kW [3, 4].
The problem of obtaining a high quality output beam and high power conversion was solved with the advent of double-clad fibers. The whole point of the technology is that when using an optical fiber in a double cladding, the pump radiation does not immediately enter the fiber core, but first enters its cladding [5]. Then, the pumping power is transferred from the cladding to the core, which, in turn, is doped with ions of rare-earth elements. They absorb pump radiation photons and generate coherent radiation at the desired wavelength. It should be borne in mind that for high-power lasers it is impossible to inject high-power pump radiation directly into the fiber core.
The radius of the inner cladding is greater than the radius of the core. The section of the inner cladding can have not only circular geometry, but have one or several faces (Fig. 1).
The simplest fiber design has a round pump cladding and a centered core (see Fig. 1 – centered core). This design is easy to implement in the technological process and to dock with a number of passive components. However, in fibers of this type, there are propagation modes of the inner cladding (referring to spiral beams), which almost do not overlap with the core. Because of this, a significant part of the pump radiation is not completely absorbed [7, 8]. As a result, the gain and pump energy efficiency are reduced.
Poor core overlap modes can be avoided by using a modified design with lower symmetry. Examples are designs with an off-center or non-circular inner cladding (see Figure 1: e. g., elliptical inner cladding, D-shaped, or rectangular). Pump claddings are also often better suited to the shape of the pump beam, such as diode rods. However, when splicing them together, problems arise if the entire fiber (not just the cladding) is non-circular.
In addition to the physical characteristics of the fiber core, an important parameter is the ratio of the cross-sectional areas of the inner cladding and the core. This area ratio should not be too high. Otherwise, the effective absorption length of the pump becomes large, and the pump intensity in the core becomes small, which leads to low excitation levels, and this leads to a decrease in energy efficiency [9]. Usually, the ratio between the areas is maintained in the range of 100 to 1000. Pump sources with improved brightness allow the use of fibers with a smaller area ratio and, therefore, a shorter effective pump absorption length, which also reduces the effect of various types of nonlinearities.
Incomplete absorption of pump radiation can result from the appearance of cladding modes with weak core overlap. Even if strong mode mixing can be achieved with a suitable fiber design, the pump absorption is reduced due to the limited overlap of the pump light with the doped fiber core. Therefore, it is usually sought to provide a long active fiber. Although, in turn, this can be harmful, e. g., from the point of view of the influence of non-linear effects.
Furthermore, a high concentration of impurity ions can make it difficult for a laser or amplifier to operate at short wavelengths of radiation, and the increased contribution of fluorescence can reduce the energy conversion efficiency.
Other harmful effects are associated with the release of some of the radiation from the core into the pump cladding, which occurs, e. g., as a result of bending or when using a fiber Bragg grating. This light will remain in the pump cladding and will not escape (as is the case with other fibers) through the coating. A cladding-mode stripper (CPS) may be required to remove such radiation if it contributes to the noise output of the device.
Output beam profilometers can be used to analyze beam propagation in double-clad fiber. Many beam profilometers are based on digital cameras: for the visible and near infrared spectral regions, CMOS and CCD cameras are the most suitable for measuring complex beam shapes.
Different wavelength ranges require different types of detectors. Silicon detectors are used for visible and NIR wavelengths up to 1.0–1.1 µm, while InGaAs detectors can be used for IR wavelengths up to ≈1.7 µm.
The spatial resolution of the camera sensor is an important parameter. In silicon detectors, pixel sizes reach less than 10 μm, which makes it possible to measure beams with a diameter of up to 50 μm.
Most cameras are highly sensitive to light. The advantage of using them in problems of detecting weak signals becomes an obstacle in measuring the beam profile. Since the sensitivity turns out to be much greater than required for measurements.
In the profilometer, the registered beam profile can be shown on the monitor, possibly together with the measured parameters: the radius of the beam, the position of the beam, ellipticity, data on statistical values or noises. The software allows you to choose between different methods for determining the beam radius: by the criterion of reducing the beam power by a factor of e2 (criterion 1 / e2) or the 4σ method.
This research is devoted to characterizing the profile of a beam emerging from a double-clad optical fiber and having different cross-sectional geometries. The efficiency of the output radiation is determined.
EXPERIMENTAL PART
The study of radiation propagating along the optical fiber was carried out on a laboratory model for studying the beam profile. The laboratory model consists of a FocusLight pump laser diode with a wavelength of 976 nm, a Nufern MM105 / 125 delivery fiber spliced to the fiber under study, a Fujikura FH‑60-DC250 fiber holder, Standa 3PH pedestals, a Standa 20R6 rail slide, and a WinCamD-LCM camera, set of neutral filters ND1-ND5.
The study was carried out on active double-clad fibers doped with erbium and ytterbium ions, models MM-EYDF‑10 / 125-XP, and MM-EYDF‑10 / 125-XPH.
The profile of the diverging radiation beam was measured on the prototype to assess the efficiency of absorption of pump radiation. The profilometer camera recorded the cross section of the radiation beam. Its intensity profile can be described by the function I(x, y). The I(x, y) value of a particular pixel corresponded to the range [0–255]. The data received by the camera was transferred to a PC and interpreted using the DataRay v.8 software.
The beam quality can be assessed by various methods, e. g., the parameter M2 is used for Gaussian beams. But Gaussian beams propagate in single-mode fibers, and this method is incorrect for modes of operation with multimode fibers. To assess the pumping efficiency, we measured the level of the ratio of radiation propagating through the fiber core and propagating through its cladding.
RESULTS
For the first measurement, a fiber MM-EYDF‑10 / 125-XPH with a fiber length of 10 cm was chosen. It was supplied with radiation with a wavelength of 976 nm and a power of 500 mW. The beam profile is shown in Fig. 4.
The beam profile has an unusual shape. As can be seen from Fig. 4, the radiation propagating along the second light-guiding cladding has an octahedral shape. In addition, there is a ring of the cladding mode, which propagates along it; between the boundaries of the first and second claddings, there is an obvious drop in power. The radiation propagating along the first light-guiding cladding does not have a pronounced peak, but it is still present there (note in Fig. 5).
The shape of the radiation intensity on both the first and the second shell has a flat top. This indicates that a mode mixing function is implemented in the fiber under study, which helps to transfer energy from the passive light-guiding cladding to the active core of the fiber. The measured transfer rate was 48.6%. The presence of a cladding mode, as in Fig. 3, and the dip between the first and second light-guiding shells indicates an incomplete pumping of the radiation energy – this is a negative effect. This cladding mode will need to be filtered out using a mode stripper, e. g., during the further assembly of the laser.
For the second measurement, an MM-EYDF‑10 / 125-XP fiber with a length of 15 cm was chosen. It was supplied with radiation with a wavelength of 976 nm and a power of 500 mW. The beam profile is shown in Fig. 6.
The beam profile of this fiber has a Gaussian shape, more reminiscent of a Buddhist stupa. In this profile, no interface between the claddings is observed; the radiation from the core is pronounced. However, in the core itself, the beam profile is Gaussian and has a high power density, which can lead to nonlinear effects in the fiber material. The measured transfer rate was 64.8%. There is no clear mode mixing in the profile of this fiber, but it shows good conversion efficiency.
CONCLUSIONS
Several double-clad fibers were investigated. To assess their efficiency, a laboratory bench with a WinCamD-LCM beam profilometer was assembled. The embedded DataRay software made it possible to quickly and efficiently evaluate the beam profile and the degree of its transformation. The investigated fibers showed conversion rates of 49 and 65%, respectively.
ACKNOWLEDGEMENT
The authors of the article express their deep gratitude to SC “LLS” for the equipment provided.
AUTHORS
Velikotsky Yuri Andreevich, engineer researcher of PJSC PNPPK, master first year PNRPU, Perm
Rakhmatullina Alina Rimovna, design engineer of PJSC PNPPK, postgraduate student first year PNRPU, Perm.
Petukhova Alexandra Yurievna, design engineer of PJSC PNPPK, master first year PNRPU, Perm.
Shcherbakova Viktoria Aleksandrovna, engineer researcher PJSC PNPPK, postgraduate first year PNRPU welfare, Perm.
Romashova Vasilisa Borisovna, fiber systems engineer of SC “LLP”, St. Petersburg.
Kashina Rano Rustamovna, designer of PNPPK, Perm.
Konin Yuri Anatolievich, engineer researcher PNPPK, Perm , St. Petersburg.
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