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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 12, JUNE 15, 2012
Coherent Polarization Beam Combining of Four High-Power Fiber Amplifiers Using Single-Frequency Dithering Technique Pengfei Ma, Pu Zhou, Yanxing Ma, Rongtao Su, and Zejin Liu
Abstract— We demonstrate a coherent polarization beam combining (CPBC) of four all polarization-maintained high-power fiber amplifiers with a total output power of 60 W, using the single-frequency dithering technique. When the control system is in the closed loop, the intensity profile is steady and the phase noise can be suppressed effectively. The experimental results show that the combining efficiency of the whole system is as high as 90% in the circumstance of imbalance power ratios of the four beams. The technique used in this letter can be straightforwardly scaled to CPBC of a large array of high-power fiber amplifiers. Index Terms— Coherent beam combining, coherent polarization beam combining, fiber amplifiers, polarization combining.
I. I NTRODUCTION
F
IBER lasers and amplifiers have attracted considerable attention due to their great potential in scaling to high power level with diffraction-limited beam quality. However, several obstacles, including the availability of high brightness pump diodes, fiber damage, thermal effects and non-linear effects, will ultimately restrict the enhancement of the output power of a single fiber laser [1-2]. Coherent beam combination (CBC) provides a feasible approach to overcome the limitations above-mentioned [3-14]. In the common CBC configuration, all the emitters are tiled into an array, which induces a portion of power encircled into the sidelobes in the far-field pattern [14], thus inevitably degrades the beam quality and power concentration [5-14]. As one of the CBC structure, coherent polarization beam combining (CPBC) overcome the deficiency of sidelobes due to that all the beams are coaxially combined [15,16]. In this letter, we scale the CPBC system to combine four channel of all-polarization-maintained fiber amplifiers and obtain 60 watt output power. It is to be noted that if N channels of laser beams is to be combined by heterodyne detection phase control technique in previous studies, N channels of detector modules and control modules should be employed. Different from previous demonstrations,
Manuscript received February 9, 2012; revised March 27, 2012; accepted March 28, 2012. Date of publication April 13, 2012; date of current version May 9, 2012. The authors are with the College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2194139
Fig. 1. Polarization state of combined beam. (a) Undefined phase. (b) Phase locked.
the single-frequency dithering technique is employed to phase control the CPBC system, which is more compact compared with previous studies. The system performs steady in the closed loop and the combining efficiency can be as high as 90%. In principle, the implementation of multi-channel CPBC system can be illustrated as follows. When two orthogonally linear-polarized beams with undefined phase correlation are injected into a polarization beam combiner (PBC), the combined beam is not a pure linear-polarized one, thus limit the further extendibility of the polarization combination system, as shown in Fig. 1(a). However, when the phase difference between the two orthogonal polarizations is locked and set to δ = nπ, where n is an integer, the combined beam is a new pure linear-polarized one (see Fig. 1(b)), which can be further combined with a linear polarized beam, so multi-channel beams can be combined through phase-locking straightforwardly [15]. Our experimental setup is shown in Fig. 2. The seed laser is a linear-polarized single-frequency with a central wavelength of 1064.4 nm. The laser power from the seed laser through the isolator (ISO) is about 30mW and amplified to 120 mW output power via an amplifier (AMP) before split into four channels and coupled to four phase modulators (PMs). PMs are LiNbO3 phase modulators with more than 100 MHz modulating bandwidth and which work at 1060 nm wavelength. The output power from the output of each phase modulator is more than 20 mW, the loss of laser power is induced by the insertion loss of the modulator. Then each fiber channel is coupled to a 2-stage all-fiber polarization-maintained amplifier made by ourselves. The active fiber in the first stage is singleclad Yb-doped fiber that had a core diameter of 6um and a cladding diameter of 125 μm. After pumped by a 480 mW single-mode fiber pigtailed laser diode with a 974 nm central wavelength, the laser power in each channel can be boosted to
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MA et al.: CPBC OF FOUR HIGH-POWER FIBER AMPLIFIERS
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Fig. 3.
Fig. 2. Experimental setup of CPBC of four channel fiber amplifiers. Seed: seed lasers. ISO: isolator. AMP: amplifier. PM: phase modulator. C1– C4: collimators. HWP: half wavelength plate. PBC1-3: polarization beam combiners. P: polarizer. M1: all-reflectance mirrors. M2: high-reflectance mirror. PD: photo detector. SP: signal processing.
be more than 150 mW. The active fiber in the second stage is double-clad Yb-doped fiber, which had a 10 μm core diameter and 125 μm inner cladding diameter. The NA of the core and the inner cladding of the double-clad fiber is 0.08 and 0.46, respectively. The laser beam is strictly single mode due to the reason that the double-clad fiber has a V number of 2.36. Two laser diodes with 975 nm central wavelength, 105 μm pigtailed fiber are used to pump the active double-clad fiber in each channel via a (2+1)×1 pump combiner. The input port of the pump combiner has a fiber type of single-mode fiber with 6μm core diameter, and the output port of the pump combiner has a fiber type of 10/125 μm core/inner cladding diameter un-doped double clad fiber, which perfectly matches the active fiber. A section of passive fiber is spliced after the active fiber for output power delivery. The spliced region is covered in high-index gel to strip out the residual pump laser. Then the passive fiber is fused to the collimator with an embedded isolator in each channel to prevent backscattering light and send the laser beam into free-space. The output power of each pump diode in the second amplification stage is about 20 watt, and the output power of each amplifier is about 25 watt. Nevertheless, due to the difference in insertion loss, the power of the four channels is 16.7W, 18.7W, 11.4W, 19.7W respectively. By rotating the half wavelength plates (HWPs) and adjusting the four beams coaxially, they can be combined by the polarization beam combiners (PBCs). M1 is an all- reflectance mirror and M2 is a high- reflectance mirror. After M2 , a small portion of the beam is sent to a self-made pinhole with a 50μm radius, and a photo detector is located immediately behind the pinhole. Another part of the beam after M2 is sent to an infrared camera, which is used to profile of the combined beam. The photo detector is an InGaAs amplifier detector produced with a 700–1800nm response wavelength and 8.5MHz bandwidth when the gain is at 10 dB. The single-frequency dithering algorithm is performed based on the homemade field programmable gate array (FPGA), and the residual phase error is less than λ/20. The mathematical principle and process of single frequency dithering technique can be found in Ref. [13] and it is not repeated here to save
Intensity profiles with the system. (a) Open loop. (b) Closed loop.
place. The metric function of the single-frequency dithering algorithm is the output power of the system, which can be also reflected by the amplitude of the voltage signal transformed by the photo detector. The voltage signal transformed by the photo detector contains the information of phase error between different channels, which can be used to generate the phase control signal in the homemade control module by modulation and demodulation technique. Comparing with the heterodyne detection phase control technique, the single frequency dithering technique bases on the time division multiplexing principle, only one modulation frequency and one phase control module are required in the system, which is more compact and efficient than previous demonstrated modules. It is to be noted that the tip-tilt error among each laser beam should be carefully pre-compensated by carefully tuning the four collimators [6]. In the experiment, when the CPBC system is in the open loop, some of the laser power leaks through PBC3, the intensity profile at the camera is changing all the time and the combined output power is unsteady due to the undefined phase difference among each beam. Fig. 3(a) plots three snapshots of the intensity pattern when the system is in the open-loop. When the single-frequency dithering algorithm is implemented and the whole system is in the closed loop, the intensity profile and the combined output power is steady. The intensity profile in closed loop is shown in Fig. 3(b). The combined output power measured at the output port of M2 is 60W. The combining efficiency is calculated to be 90%, where the combining efficiency (η) defined by the formula η = Pout /Pin , Pout is the combined output power, Pin is the total power after the collimators. In the practical system, three major factors, overlap error (the beams cannot be superposed entirely due to the precision of the mechanical tuning devices), tilt error, and residual phase error, influence the combining efficiency of the system. The schematic of a CPBC unit with overlap error, tilt error, and phase error is shown in Fig. 4, where d is the magnitude of the central distance of the two beams in the unit, θ1 and θ2 represent the tilt angles, as denoted in Fig. 4. Due to the influence of the residual phase error, the output beam is not purely linear-polarized. The effects of those factors on CPBC can be estimated by the model proposed in Ref [17], and we will now compute and discuss them briefly. Assuming that w is the beam waist of the fiber laser beam throughout the collimator, the detailed analysis reveals that to ensure the combining efficiency of a four-channel CPBC system to be more than 90%, the overlap error d/w in each CPBC unit should be controlled to be less than 0.47, the tilt error
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 12, JUNE 15, 2012
Fig. 4. Schematic of a CPBC unit with overlap error, tilt error, and residual phase error.
is 60 W and the combining efficiency of the whole system can be as high as 90%. Through study by the time series signal and spectral density of energy collected by the pinhole, the phase noise is suppressed effectively below 200 Hz and the whole CPBC system can be implemented with excellent stability. According to the measuring result [18] that the phase fluctuating frequency of a large-mode-area ytterbium-doped fiber amplifier at 260 W output power in a relatively quiet laboratory environment is well below several hundred Hz, we believe CPBC of fiber amplifiers using single-frequency dithering technique has the potential to be scaled to a high output power. R EFERENCES
(a)
(b) Fig. 5. Time series signals and spectral density of energy encircled in the pinhole in open loop and closed loop. (a) Time series signals. (b) Spectral density.
of each beam in the CPBC unit should be less than π/20rad, and the phase error in each CPBC unit should be limited to be within λ/14 RMS. Further enhancing the combining efficiency of the CPBC system should be concentrated on high precision adjustment with micron-level precision, suppressing the noise of photo-detector and phase-locking circuit, and boosting the control band of the single-frequency dithering technique. The fidelity of CPBC and the phase noise suppression can be further studied by the time series signal and spectral density of energy collected by the pinhole of the PD, which is shown in Fig. 5. When the control system is open, the normalized energy collected in the pinhole fluctuates randomly. When the control system is closed, the normalized power in the pinhole can be locked stably (see Fig. 5(a)), and the spectral density of power is about 20 dB lower than in open loop below 200 Hz (see Fig. 5(b)), which indicates an effective suppression of the phase noise. II. C ONCLUSION In summary, we have presented CPBC of four channel amplified fiber lasers based on single frequency dithering technique. When the system is in closed loop, the output power
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