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Curr. Opt. Photon. 2023; 7(1): 33-37

Published online February 25, 2023 https://doi.org/10.3807/COPP.2023.7.1.33

Copyright © Optical Society of Korea.

Real-time Adaptive Polarization Control in a Non-PM Fiber Amplifier

Kyuhong Choi, Jinju Kim, Dal Yong Lee, Changsu Jun

Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 61005, Korea

Corresponding author: *changsu.jun@gist.ac.kr, ORCID 0000-0003-0585-0517

Received: November 17, 2022; Revised: January 5, 2023; Accepted: January 9, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Real-time adaptive control of laser output polarization is presented in a 10-W-level non-polarization-maintaining (non-PM) fiber amplifier. While the output polarization from a non-PM fiber amplifier tends to be irregular, depending on output power, time, and perturbation, closed-loop polarization control can maintain the polarization extinction ratio at higher than 20 dB. Real-time polarization control can attain the target linear polarization mostly within 1.4–25 ms and shows stability against external perturbations. This approach can satisfy both linear polarization and high output power in a non-PM amplifier, and facilitates optimization of laser performance and maintenance-free operation.

Keywords: Active polarization control, Linear polarization, Non-PM fiber amplifier

OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3425) Laser stabilization; (230.5440) Polarization-selective devices

Adaptive control of a laser’s output characteristics helps to enhance or stabilize the laser’s performance. Previous research demonstrated that spatial modes could be switched to or maintained in a stable single mode [1, 2], and stable mode locking or harmonic mode locking could be accommodated with the help of a proper closed-loop control system [3]. Active control of laser output polarization has also been studied, to match the output state of polarization among multiple lasers [4], or to obtain stable linear polarization in a non-polarization-maintaining (non-PM) fiber amplifier [58]. Especially for spectral or coherent beam combining for higher-power fiber lasers, linearly polarized output is preferred. The threshold powers of detrimental nonlinear phenomena and transverse mode instability, however, are lower in polarization-maintaining (PM) fiber amplifiers than in non-PM ones [9, 10]. This means that a non-PM amplifier is better for power scaling. Therefore, closed-loop control for linear polarization output in a non-PM fiber amplifier can be an attractive approach to satisfy both linear polarization and high output power. A few demonstrations of closed-loop polarization control in a non-PM fiber amplifier were introduced to scale up the output power of narrow-linewidth kW-level high-power fiber lasers; For example, about 1 kW [6], 1.43 kW [7], and 2 kW [8], with polarization extinction ratios (PERs) of 14.5 dB, 11.1 dB, and ~15 dB respectively. However, there were not sufficient results analyzing the difference before and after the closed-loop control of polarization, convergence time to the desired polarization state, and polarization stability over external perturbation.

In this study, we build a 10-W-level master oscillator power amplifier (MOPA) fiber laser, and polarization-stable single-mode output is obtained through closed-loop polarization control in a non-PM fiber amplifier. The output state of polarization from a non-PM fiber amplifier tends to be irregular, depending on output power or time. However, closed-loop polarization control can maintain the polarization extinction ratio at higher than 20 dB. The time to converge to the target vertical polarization from a random initial state of polarization is a few milliseconds on average, and stays stable against external perturbations.

2.1. Experimental Setup

Figure 1 is the schematic diagram of the MOPA fiber-laser system for closed-loop polarization control in a non-PM fiber amplifier. Only the 1,064-nm seed laser diode (LD) is based on a PM fiber; the rest of the fiber-optic components, including the amplifier, are based on a non-PM fiber. The seed laser beam goes through an electric polarization controller (PC) (POS-002; General Photonics, CA, USA), a 99:1 2 × 2 coupler, a mode-field adaptor (MFA), and an optical isolator before entering the amplifier section. The electric PC is the key component to adaptively control the state of polarization, which changes the state of polarization by rotating the Poincare-sphere angle (tracking speed 47π/s, feedback voltage 0.5–4.5 V, power supply 12 V). The 2 × 2 coupler is used to monitor the power and polarization of the seed part. The MFA is connected between 6/125-µm and 12/125-µm fibers with minimal loss, and the optical isolator blocks backward amplified spontaneous emission (ASE) from the amplifier, to prevent damage to the seed part’s components. The 12/125-µm double-clad Ytterbium-doped fiber (YDF) is pumped by a 976-nm pump LD via a pump combiner, and the residual pump beam is removed in the cladding power stripper (CPS). The output beam from the fiber end cap is collimated with a lens and then enters the monitoring part, after being reflected from a 1,064/976-nm dichroic mirror. The power meter is put between the dichroic mirror and the monitoring part, whenever output power needs to be measured. In the monitoring part, the beam splitter acts as an optical-power attenuator. A half-wave plate (HWP) is placed in front of a polarizing beam splitter (PBS) to measure the PER. For example, when the laser output has linear polarization with a certain angle to the normal, HWP rotation can match the polarization angle with an axis of the PBS, and then the PER can be measured accurately. By measuring the powers of both vertical and horizontal polarizations after passing the PBS, the PER is calculated, and one or two PD voltages are used as the input signal to the field-programmable gate array (FPGA) (cRIO-9045; National Instruments Corp., TX, USA). The closed-loop polarization control system consists of the monitoring part, FPGA, and electric PC. The electric PC has a response time of about 26 µs and is optimized in the direction to maximize the vertical polarization intensity (PD 1) that is reflected from the PBS and ranges between 0.5 and 4.5 V. The bandwidths of the PD and FPGA are 10 MHz and 40 MHz respectively. Control via a laptop central processing unit (CPU) requires milliseconds to communicate between the PD and electric PC, while the FPGA takes nanoseconds; Therefore, the FPGA is preferred for higher-speed closed-loop control.

Figure 1.Schematic diagram of the non-PM fiber amplifier with closed-loop polarization control. PM, polarization-maintaining; LD, laser diode; PC, polarization controller; MFA, mode-field adaptor; YDF, Ytterbium-doped fiber; CPS, cladding power stripper; HWP, half-wave plate; PBS, polarizing beam splitter; PD, photodetector; FPGA, field programmable gate array.

2.2. Polarization Behavior of a Non-PM Fiber Amplifier

Before running closed-loop control, the polarization behavior of the non-PM fiber amplifier is examined. Figure 2(a) is the PER as a function of output power for the non-PM fiber amplifier without closed-loop polarization control. The colored data in the orange box of Fig. 2(a) indicate different initial PERs from the seed laser, ranging from 3 to 33 dB. As the output power increases, the PER of the black data decreases, while those of the green and violet data largely increase. As can be seen, the output PER evolves irregularly, regardless of the initial state of polarization. However, the PER can reach higher than 20 dB, interestingly. In a separate experiment the PER evolution for 30 minutes is measured for three different output powers, as seen in Fig. 2(b). The case of 4 W (black squares) stays at a low PER of about 5 dB without drastic change, while the PER for 10 W increases up to about 24 dB over time, interestingly. It is noted that repeated experiments without external perturbation result in a similar tendency. The increase of PER with power or time in a non-PM amplifier is not common. Gain competition between two orthogonal polarization components may occasionally lead to higher PER due to unintended polarization-dependence in a non-PM amplifier, induced by external imperfections (twist, reflection, or nonlinearity) [1113], but more investigation is needed for a better understanding. The state of polarization in a non-PM amplifier may be fine-tuned based on these examinations, but external perturbation changes the PER or the angle of polarization easily, and stable polarization is not guaranteed in non-PM fiber amplifiers.

Figure 2.Polarization extinction ratio (PER) as a function of (a) output power and (b) time, in the non-PM fiber amplifier without closed-loop polarization control.

2.3. Closed-loop Polarization Control of a Non-PM Fiber Amplifier

To overcome the irregular and unstable state of polarization while taking advantage of a non-PM fiber amplifier, a closed-loop polarization control system is implemented with a MOPA fiber laser. The vertical polarization signal (PD 1) reflected from the PBS (see Fig. 1) is mainly used as a monitoring signal for the FPGA controller, and the electric PC is adaptively tuned to maximize the monitoring-signal voltage, or to reach a target value between 0.5 and 4.5 V. Vertical polarization with high PER can be acquired in this way. The horizontal polarization signal transmitted by the PBS in Fig. 1 is used auxiliarily in combination with the HWP when calculating the PER.

Figure 3(a) displays the results of high PER over 20 dB as a function of output power up to 12 W, in the case of closed-loop polarization control (red circles and orange triangles). For comparison, the results without closed-loop polarization control under the same experimental conditions are displayed together (violet diamonds, black squares, and blue triangles). Figure 3(b) shows that adaptive polarization control does not affect the single-mode operation. The target PER of this laser system can be set higher, such as at 25 dB, but then becomes susceptible to noise and highly unstable, because even a small perturbation of the e-PC at too high a PER can easily reduce the PER, and then frequent optimization is required, which wastes time, and leads to further instability. In this work, we set the target PER at 20 dB after examining the optimization parameters, response times of the components, and PD noise level.

Figure 3.The results of closed-loop polarization control. (a) Polarization extinction ratio (PER) as a function of output power in the non-PM fiber amplifier, with or without closed-loop polarization control. The green dashed line denotes a PER value of 20 dB. (b) The beam profiles without and with closed-loop polarization control.

The convergence time to the target polarization is also investigated. In the results of Figs. 2 and 3, the PER is accurately measured with the help of HWP rotation, and therefore it takes time to measure each data point. For high-speed measurement of polarization, the degree of linear polarization (DOLP), especially vertical polarization, is used as a metric, as seen on the y-axis of Fig. 4. DOLP is defined as the ratio of vertical polarization intensity to total intensity. Figure 4 shows an example of the locking time (4.24 milliseconds) to the target vertical polarization from a random initial state of polarization, in the non-PM fiber amplifier with closed-loop polarization control. Optimization of the learning rate and averaging is conducted. In the course of maximizing the vertical polarization intensity, the gradient-descent method [14] of iteration step size is used, which rotates the Poincare-sphere angle coarsely in steps of 21.12° in the beginning, followed by fine rotation down to 2.64° in the vicinity of the target vertical polarization. Five voltage data from the monitoring part are averaged to reduce the PD noise. In Fig. 4, the time to reach a DOLPvertical of 0.995 is measured as 4.24 ms, which corresponds to a PER of about 23 dB. The polarization-locking time is typically within 1.4–25 ms, but rarely can be up to 60 ms, depending on the initial conditions.

Figure 4.Polarization-locking time in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.

The stability of real-time polarization control is examined. Figure 5 shows the polarization stability over slow [Fig. 5(a)] and fast [Fig. 5(b)] external perturbations. The black data are the cases of manual HWP rotation without closed-loop control, and therefore DOLPvertical changes randomly depending on the HWP angle of rotation, while the red data are with closed-loop control. In the beginning, DOLPvertical is set higher than 0.995 (PER ~23.0 dB) in all cases, through closed-loop control. In Fig. 5(a) random HWP rotation, which simulates unwanted slow polarization rotation, lowers DOLPvertical to 0.95 (PER ~12.8 dB) in the case without closed-loop control, while closed-loop control keeps it higher than 0.99 (PER ~20.0 dB) regardless of external perturbation. Second, tapping the optical fibers or adding vibration induces much lower DOLPvertical down to 0.66, and DOLPvertical often does not return to the initial value even after perturbation ceases, as seen in Fig. 5(b). Closed-loop control, however, maintains DOLPvertical higher than 0.995, although during optimization it drops to 0.98 twice and recovers in 2.4 ms and 21.8 ms respectively. This result shows the immunity of the closed-loop polarization-control system to external perturbations.

Figure 5.Polarization stability over (a) slow and (b) fast external perturbations, in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.

In this experiment, PD noise hinders achieving higher PER and maintaining more stable closed-loop polarization control. Using a low-noise PD and further investigation of the controller and algorithm will enhance the PER and convergence time of this system.

As laser technology evolves, adaptive control for easy optimization of laser performance and maintenance-free operation will be strongly required. We have demonstrated the closed-loop control of the laser output’s polarization in a non-PM fiber amplifier. This system can satisfy both linear polarization and high output power in a non-PM amplifier. Based on a 10-W-level non-PM fiber amplifier, single-mode linear polarization with PER of over 20 dB was obtained through closed-loop polarization control. The polarization-locking time to a linear polarization with PER of 23 dB from a random initial polarization was typically within 1.4–25 ms, and the closed-loop polarization control system showed immunity to external perturbations. Study to attain better PER, speed, and stability with a simpler setup will be continued.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.

Gwangju Institute of Science and Technology (GK14720, GK15430, GK15320).

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  13. N. Moroney, L. Del Bino, S. Zhang, M. T. M. Woodley, L. Hill, T. Wildi, V. J. Wittwer, T. Südmeyer, G.-L. Oppo, M. R. Vanner, V. Brasch, T. Herr, and P. Del’Haye, “A Kerr polarization controller,” Nat. Commun. 13, 398 (2022).
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Article

Article

Curr. Opt. Photon. 2023; 7(1): 33-37

Published online February 25, 2023 https://doi.org/10.3807/COPP.2023.7.1.33

Copyright © Optical Society of Korea.

Real-time Adaptive Polarization Control in a Non-PM Fiber Amplifier

Kyuhong Choi, Jinju Kim, Dal Yong Lee, Changsu Jun

Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 61005, Korea

Correspondence to:*changsu.jun@gist.ac.kr, ORCID 0000-0003-0585-0517

Received: November 17, 2022; Revised: January 5, 2023; Accepted: January 9, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Real-time adaptive control of laser output polarization is presented in a 10-W-level non-polarization-maintaining (non-PM) fiber amplifier. While the output polarization from a non-PM fiber amplifier tends to be irregular, depending on output power, time, and perturbation, closed-loop polarization control can maintain the polarization extinction ratio at higher than 20 dB. Real-time polarization control can attain the target linear polarization mostly within 1.4–25 ms and shows stability against external perturbations. This approach can satisfy both linear polarization and high output power in a non-PM amplifier, and facilitates optimization of laser performance and maintenance-free operation.

Keywords: Active polarization control, Linear polarization, Non-PM fiber amplifier

I. INTRODUCTION

Adaptive control of a laser’s output characteristics helps to enhance or stabilize the laser’s performance. Previous research demonstrated that spatial modes could be switched to or maintained in a stable single mode [1, 2], and stable mode locking or harmonic mode locking could be accommodated with the help of a proper closed-loop control system [3]. Active control of laser output polarization has also been studied, to match the output state of polarization among multiple lasers [4], or to obtain stable linear polarization in a non-polarization-maintaining (non-PM) fiber amplifier [58]. Especially for spectral or coherent beam combining for higher-power fiber lasers, linearly polarized output is preferred. The threshold powers of detrimental nonlinear phenomena and transverse mode instability, however, are lower in polarization-maintaining (PM) fiber amplifiers than in non-PM ones [9, 10]. This means that a non-PM amplifier is better for power scaling. Therefore, closed-loop control for linear polarization output in a non-PM fiber amplifier can be an attractive approach to satisfy both linear polarization and high output power. A few demonstrations of closed-loop polarization control in a non-PM fiber amplifier were introduced to scale up the output power of narrow-linewidth kW-level high-power fiber lasers; For example, about 1 kW [6], 1.43 kW [7], and 2 kW [8], with polarization extinction ratios (PERs) of 14.5 dB, 11.1 dB, and ~15 dB respectively. However, there were not sufficient results analyzing the difference before and after the closed-loop control of polarization, convergence time to the desired polarization state, and polarization stability over external perturbation.

In this study, we build a 10-W-level master oscillator power amplifier (MOPA) fiber laser, and polarization-stable single-mode output is obtained through closed-loop polarization control in a non-PM fiber amplifier. The output state of polarization from a non-PM fiber amplifier tends to be irregular, depending on output power or time. However, closed-loop polarization control can maintain the polarization extinction ratio at higher than 20 dB. The time to converge to the target vertical polarization from a random initial state of polarization is a few milliseconds on average, and stays stable against external perturbations.

II. EXPERIMENT

2.1. Experimental Setup

Figure 1 is the schematic diagram of the MOPA fiber-laser system for closed-loop polarization control in a non-PM fiber amplifier. Only the 1,064-nm seed laser diode (LD) is based on a PM fiber; the rest of the fiber-optic components, including the amplifier, are based on a non-PM fiber. The seed laser beam goes through an electric polarization controller (PC) (POS-002; General Photonics, CA, USA), a 99:1 2 × 2 coupler, a mode-field adaptor (MFA), and an optical isolator before entering the amplifier section. The electric PC is the key component to adaptively control the state of polarization, which changes the state of polarization by rotating the Poincare-sphere angle (tracking speed 47π/s, feedback voltage 0.5–4.5 V, power supply 12 V). The 2 × 2 coupler is used to monitor the power and polarization of the seed part. The MFA is connected between 6/125-µm and 12/125-µm fibers with minimal loss, and the optical isolator blocks backward amplified spontaneous emission (ASE) from the amplifier, to prevent damage to the seed part’s components. The 12/125-µm double-clad Ytterbium-doped fiber (YDF) is pumped by a 976-nm pump LD via a pump combiner, and the residual pump beam is removed in the cladding power stripper (CPS). The output beam from the fiber end cap is collimated with a lens and then enters the monitoring part, after being reflected from a 1,064/976-nm dichroic mirror. The power meter is put between the dichroic mirror and the monitoring part, whenever output power needs to be measured. In the monitoring part, the beam splitter acts as an optical-power attenuator. A half-wave plate (HWP) is placed in front of a polarizing beam splitter (PBS) to measure the PER. For example, when the laser output has linear polarization with a certain angle to the normal, HWP rotation can match the polarization angle with an axis of the PBS, and then the PER can be measured accurately. By measuring the powers of both vertical and horizontal polarizations after passing the PBS, the PER is calculated, and one or two PD voltages are used as the input signal to the field-programmable gate array (FPGA) (cRIO-9045; National Instruments Corp., TX, USA). The closed-loop polarization control system consists of the monitoring part, FPGA, and electric PC. The electric PC has a response time of about 26 µs and is optimized in the direction to maximize the vertical polarization intensity (PD 1) that is reflected from the PBS and ranges between 0.5 and 4.5 V. The bandwidths of the PD and FPGA are 10 MHz and 40 MHz respectively. Control via a laptop central processing unit (CPU) requires milliseconds to communicate between the PD and electric PC, while the FPGA takes nanoseconds; Therefore, the FPGA is preferred for higher-speed closed-loop control.

Figure 1. Schematic diagram of the non-PM fiber amplifier with closed-loop polarization control. PM, polarization-maintaining; LD, laser diode; PC, polarization controller; MFA, mode-field adaptor; YDF, Ytterbium-doped fiber; CPS, cladding power stripper; HWP, half-wave plate; PBS, polarizing beam splitter; PD, photodetector; FPGA, field programmable gate array.

2.2. Polarization Behavior of a Non-PM Fiber Amplifier

Before running closed-loop control, the polarization behavior of the non-PM fiber amplifier is examined. Figure 2(a) is the PER as a function of output power for the non-PM fiber amplifier without closed-loop polarization control. The colored data in the orange box of Fig. 2(a) indicate different initial PERs from the seed laser, ranging from 3 to 33 dB. As the output power increases, the PER of the black data decreases, while those of the green and violet data largely increase. As can be seen, the output PER evolves irregularly, regardless of the initial state of polarization. However, the PER can reach higher than 20 dB, interestingly. In a separate experiment the PER evolution for 30 minutes is measured for three different output powers, as seen in Fig. 2(b). The case of 4 W (black squares) stays at a low PER of about 5 dB without drastic change, while the PER for 10 W increases up to about 24 dB over time, interestingly. It is noted that repeated experiments without external perturbation result in a similar tendency. The increase of PER with power or time in a non-PM amplifier is not common. Gain competition between two orthogonal polarization components may occasionally lead to higher PER due to unintended polarization-dependence in a non-PM amplifier, induced by external imperfections (twist, reflection, or nonlinearity) [1113], but more investigation is needed for a better understanding. The state of polarization in a non-PM amplifier may be fine-tuned based on these examinations, but external perturbation changes the PER or the angle of polarization easily, and stable polarization is not guaranteed in non-PM fiber amplifiers.

Figure 2. Polarization extinction ratio (PER) as a function of (a) output power and (b) time, in the non-PM fiber amplifier without closed-loop polarization control.

2.3. Closed-loop Polarization Control of a Non-PM Fiber Amplifier

To overcome the irregular and unstable state of polarization while taking advantage of a non-PM fiber amplifier, a closed-loop polarization control system is implemented with a MOPA fiber laser. The vertical polarization signal (PD 1) reflected from the PBS (see Fig. 1) is mainly used as a monitoring signal for the FPGA controller, and the electric PC is adaptively tuned to maximize the monitoring-signal voltage, or to reach a target value between 0.5 and 4.5 V. Vertical polarization with high PER can be acquired in this way. The horizontal polarization signal transmitted by the PBS in Fig. 1 is used auxiliarily in combination with the HWP when calculating the PER.

Figure 3(a) displays the results of high PER over 20 dB as a function of output power up to 12 W, in the case of closed-loop polarization control (red circles and orange triangles). For comparison, the results without closed-loop polarization control under the same experimental conditions are displayed together (violet diamonds, black squares, and blue triangles). Figure 3(b) shows that adaptive polarization control does not affect the single-mode operation. The target PER of this laser system can be set higher, such as at 25 dB, but then becomes susceptible to noise and highly unstable, because even a small perturbation of the e-PC at too high a PER can easily reduce the PER, and then frequent optimization is required, which wastes time, and leads to further instability. In this work, we set the target PER at 20 dB after examining the optimization parameters, response times of the components, and PD noise level.

Figure 3. The results of closed-loop polarization control. (a) Polarization extinction ratio (PER) as a function of output power in the non-PM fiber amplifier, with or without closed-loop polarization control. The green dashed line denotes a PER value of 20 dB. (b) The beam profiles without and with closed-loop polarization control.

The convergence time to the target polarization is also investigated. In the results of Figs. 2 and 3, the PER is accurately measured with the help of HWP rotation, and therefore it takes time to measure each data point. For high-speed measurement of polarization, the degree of linear polarization (DOLP), especially vertical polarization, is used as a metric, as seen on the y-axis of Fig. 4. DOLP is defined as the ratio of vertical polarization intensity to total intensity. Figure 4 shows an example of the locking time (4.24 milliseconds) to the target vertical polarization from a random initial state of polarization, in the non-PM fiber amplifier with closed-loop polarization control. Optimization of the learning rate and averaging is conducted. In the course of maximizing the vertical polarization intensity, the gradient-descent method [14] of iteration step size is used, which rotates the Poincare-sphere angle coarsely in steps of 21.12° in the beginning, followed by fine rotation down to 2.64° in the vicinity of the target vertical polarization. Five voltage data from the monitoring part are averaged to reduce the PD noise. In Fig. 4, the time to reach a DOLPvertical of 0.995 is measured as 4.24 ms, which corresponds to a PER of about 23 dB. The polarization-locking time is typically within 1.4–25 ms, but rarely can be up to 60 ms, depending on the initial conditions.

Figure 4. Polarization-locking time in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.

The stability of real-time polarization control is examined. Figure 5 shows the polarization stability over slow [Fig. 5(a)] and fast [Fig. 5(b)] external perturbations. The black data are the cases of manual HWP rotation without closed-loop control, and therefore DOLPvertical changes randomly depending on the HWP angle of rotation, while the red data are with closed-loop control. In the beginning, DOLPvertical is set higher than 0.995 (PER ~23.0 dB) in all cases, through closed-loop control. In Fig. 5(a) random HWP rotation, which simulates unwanted slow polarization rotation, lowers DOLPvertical to 0.95 (PER ~12.8 dB) in the case without closed-loop control, while closed-loop control keeps it higher than 0.99 (PER ~20.0 dB) regardless of external perturbation. Second, tapping the optical fibers or adding vibration induces much lower DOLPvertical down to 0.66, and DOLPvertical often does not return to the initial value even after perturbation ceases, as seen in Fig. 5(b). Closed-loop control, however, maintains DOLPvertical higher than 0.995, although during optimization it drops to 0.98 twice and recovers in 2.4 ms and 21.8 ms respectively. This result shows the immunity of the closed-loop polarization-control system to external perturbations.

Figure 5. Polarization stability over (a) slow and (b) fast external perturbations, in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.

In this experiment, PD noise hinders achieving higher PER and maintaining more stable closed-loop polarization control. Using a low-noise PD and further investigation of the controller and algorithm will enhance the PER and convergence time of this system.

III. CONCLUSION

As laser technology evolves, adaptive control for easy optimization of laser performance and maintenance-free operation will be strongly required. We have demonstrated the closed-loop control of the laser output’s polarization in a non-PM fiber amplifier. This system can satisfy both linear polarization and high output power in a non-PM amplifier. Based on a 10-W-level non-PM fiber amplifier, single-mode linear polarization with PER of over 20 dB was obtained through closed-loop polarization control. The polarization-locking time to a linear polarization with PER of 23 dB from a random initial polarization was typically within 1.4–25 ms, and the closed-loop polarization control system showed immunity to external perturbations. Study to attain better PER, speed, and stability with a simpler setup will be continued.

DISCLOSURES

The authors declare no conflicts of interest.

DATA AVAILABILITY

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.

FUNDING

Gwangju Institute of Science and Technology (GK14720, GK15430, GK15320).

Fig 1.

Figure 1.Schematic diagram of the non-PM fiber amplifier with closed-loop polarization control. PM, polarization-maintaining; LD, laser diode; PC, polarization controller; MFA, mode-field adaptor; YDF, Ytterbium-doped fiber; CPS, cladding power stripper; HWP, half-wave plate; PBS, polarizing beam splitter; PD, photodetector; FPGA, field programmable gate array.
Current Optics and Photonics 2023; 7: 33-37https://doi.org/10.3807/COPP.2023.7.1.33

Fig 2.

Figure 2.Polarization extinction ratio (PER) as a function of (a) output power and (b) time, in the non-PM fiber amplifier without closed-loop polarization control.
Current Optics and Photonics 2023; 7: 33-37https://doi.org/10.3807/COPP.2023.7.1.33

Fig 3.

Figure 3.The results of closed-loop polarization control. (a) Polarization extinction ratio (PER) as a function of output power in the non-PM fiber amplifier, with or without closed-loop polarization control. The green dashed line denotes a PER value of 20 dB. (b) The beam profiles without and with closed-loop polarization control.
Current Optics and Photonics 2023; 7: 33-37https://doi.org/10.3807/COPP.2023.7.1.33

Fig 4.

Figure 4.Polarization-locking time in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.
Current Optics and Photonics 2023; 7: 33-37https://doi.org/10.3807/COPP.2023.7.1.33

Fig 5.

Figure 5.Polarization stability over (a) slow and (b) fast external perturbations, in the non-PM fiber amplifier with closed-loop polarization control. DOLP, degree of linear polarization.
Current Optics and Photonics 2023; 7: 33-37https://doi.org/10.3807/COPP.2023.7.1.33

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Current Optics
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Wonshik Choi,
Editor-in-chief

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