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Coherent combination of two ytterbium fiber amplifier based on an active segmented mirror
Optics Communications 282 (2009) 1349–1353
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Coherent combination of two ytterbium fiber amplifier based on an active segmented mirror Ping Yang a,b,*, Ruofu Yang a,b, Feng Shen a, Xinyang Li a, Wenhan Jiang a a b
Institute of Optics and Electronics, P.O. Box 350, Shuangliu, Chengdu, Sichuan 610209, China Graduate School of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 2 December 2008 Accepted 8 December 2008
Keywords: Coherent combination Active segmented mirror PR Interference pattern
a b s t r a c t To control the phase noise of two ytterbium fiber amplifiers, a coherent combination system based on an active segmented mirror (ASM) has been established in our laboratory. The ASM is controlled by a feed back control loop on the basis of a Peak Rate (PR) algorithm which is realized on a DSP + FPGA hardware control board. Experimental results indicate that when the control loop is off, the far-field interference pattern is blurred and dynamic, while when it is on, the far-field beams interference pattern achieves clear and stable. At two different output powers, the contrasts of the interference stripes are improved from 7% to 19% and 8% to 28% respectively. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Coherent combination of fiber lasers is a very promising way to satisfy the high brightness requirement of many laser applications. Nowadays, there are many coherent combining schemes such as Self-Organized coherence in fiber laser arrays [1], adaptive optics [2], MOPA [3,4], etc. The combining efficiency of Self-Organized scheme is prominent when the number of fiber laser arrays is limited in a fewer; however, as the increase of arrays and power of every fiber laser, the combining efficiency decreases correspondingly. At present, MOPA method is very popular because of its simple and high real-time realization. However, with respect to MOPA method, the phase modulators (more often than not, the LiNbO3 crystal) cannot stand high power density and beams must be calibrated accurately to reduce the tilt aberrations. It is known that adaptive optics (AO) technique is a powerful technique for correcting dynamic and static phase aberrations in many laser systems [5,6]. Nowadays, it is also brought in many laser coherent combination systems [2]. As the key active element, an ASM can be used to correct tilt and piston aberrations of beams simultaneously; moreover, it can stand higher power by effective coating high reflectivity membrane on its surface. These two main advantages facilitate adaptive optics technique to be an important technique used for laser beam combination.
* Corresponding author. Address: Institute of Optics and Electronics, P.O. Box 350, Shuangliu, Chengdu, Sichuan 610209, China. Tel.: +86 028 85100686; fax: +86 028 85100433. E-mail address: [email protected] (P. Yang). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.12.019
In this paper, we investigate the possibility for coherent combination of two ytterbium fiber amplifiers using adaptive optics technique. An ASM with seven sub-mirrors is introduced to compensate the piston aberration and lock the phase of the two fiber amplifiers. In this paper, we first introduce the basic theory of beams coherent combination by adaptive optics technique, and then the PR algorithm used for controlling the ASM is described [7]. Finally, the experimental performances of two-beam coherent combination are demonstrated and analyzed. 2. The beam combination system based on AO Fig. 1 is the schematic of the experimental configuration of two ytterbium fiber amplifiers on the basis of AO technique. The seed laser oscillator is a commercial cw polarization maintaining double-clad ytterbium laser with a 1083 nm operation wavelength [8]. The bandwidth of the seed laser is about 20 KHz. The main advantage of such a narrow bandwidth is that the coherence length of the light is long enough for far-field interference. The beam from the polarized maintaining seed fiber laser is firstly divided into two beams by a fiber beam splitter (BS1), and then beams from the same seed laser pass through fiber amplifier 1 (Fiber Amp1) and fiber amplifier 2 (Fiber Amp2) respectively. After amplification, each beam is incident onto the center of a sub-mirror of the ASM. Since only two beams need to be controlled, thus the central two sub-mirror of the 7-element ASM are in practice used. To improve the coherent combination efficiency, two high reflectivity mirrors HR1–HR2 are employed to compress the distance between two beams to about 0.5 mm. The two beams
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P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 1. The schematic of two-beam combination based on AO.
are then divided into two parts by a beam splitter (BS2). The transmitting part is focused by Lens1 to form an interference pattern on the monitors for visual monitoring while the reflected part enters into a CCD (DALSA CA-D1-0064A) which has 64 64 pixels and can work with a maximum frequency of 2900 Hz. A PR algorithm, which is built in a DSP + FPGA control board, is adopted as the feed back algorithm. This algorithm can generate the updating control voltages to control the ASM for compensating two beams’ piston aberrations. Since each sub-mirror has three electrodes on its back, therefore, a D/A card is used to convert six digital voltages from control circuit to analog voltages which will at last be applied onto six electrodes of DM by a high-voltage amplifier (HVA). 3. The PR algorithm and its application in coherent combination Peak Ratio algorithm is firstly introduced to ascertain the piston aberrations of the segmented mirrors in the large aperture telescopes [7]. When piston aberrations are eliminated completely and with no other aberrations present, the two sub-peaks on the right and left of the largest peak of far-field interference pattern have the same intensity value. Therefore, we can use this characteristic to control the piston aberrations, that is, to make the two peaks reach the same intensity value. Fig. 2 shows schematic drawing of location the maximal right peak and maximal left peak. The implement principle of PR algorithm is as follows:
(iii) If the sub-largest peak is on the left of the largest peak, then max(Right Peak) of Eq. (1) is the largest value, while max(Left Peak) is the sub-largest one, well then PR is larger than 1. On the contrary, if the sub-largest peak is on the right of the largest peak, and then max(Left Peak) of Eq. (1) is the largest value while max(Right Peak) is the sub-largest one, then PR is in the range of (0, 1). Fig. 3 is the basic control flowchart of PR algorithm application in the beam combination system based on ASM. The executing course can be described as follows: Firstly, generating a voltage vector Vi randomly by computer and applied it on the actuators of ASM, then acquiring the corresponding far-field interference pattern, and then evaluating the PR value through executing steps (i) to (iii): if PR > =1, then reduce the voltage (Vi+1 = kVi Vf), else increase it (Vi+1 = kVi + Vf). k is the proportional coefficient while k1 and k2 are the integral coefficients (in our control program, k, k1 and k2 are fixed at 0.99, 0.000006 and 0.000004, respectively). PRmax and PRmin are the initial value and can be preset in advance. Renew Vi according to the flowchart of Fig. 3 until PR value reaches a stable extreme value. When PR value reaches an extreme (whether maximum or minimum is ok), it can be deemed that the piston aberration between two beams in the combination system is corrected as clear as possible.
(i) The first step is to sum the intensity of the far-field interference pattern along the y-axis, then searching the largest peak of the light stripes. (ii) Secondly, search two local maximum peaks at the left and right of the largest peak light stripes along the x-axis, then choose the intensity of the larger one (sub-largest) of the two local maximum to compare with the largest peak value. The formula for calculating the ratio is:
PR ¼ maxðRightPeakÞ= maxðLeftPeakÞ
ð1Þ
Fig. 2. Schematic drawing of location the maximal right peak and maximal left peak.
Fig. 3. The flowchart of beams combination controlled by PR algorithm.
P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 4. The far-field patterns of two beams at different output power: (a) 0.1 W (b) 0.59 W.
4. Experiments
I ¼ ðA21 þ A22 Þ½1 þ R cosðDuðtÞ
ð2Þ
where A1 and A2 are the amplitude of the two beams, and DuðtÞ is the phase of the interference beam, whereas R can be defined
R¼
2A1 A2 A21 þ A22
Fig. 6. The far-field patterns of two beams at the same output power: (a) 0.6 W (b) 0.6 W.
I ¼ 2I0 ½1 þ cos DuðtÞ:
The two ytterbium fiber amplifiers can operate with a maximum power of 1 W, and the output power can be adjusted continuously. It is known that the intensity distribution of two interfered beam can be written as:
ð3Þ
The ideal conditions for coherent combination can be described as: two beams propagate in the same direction and same axis; besides, two beams have the same amplitude, that is A1 ¼ A2 ¼ A0 ðR ¼ 1Þ. Now, let I0 ¼ A20 represent the far-field peak intensity of single beam, and then Eq. (2) can be rewritten as:
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ð4Þ
What we can know from the paragraphs described above is that it’s favorable to make two beams operate at same output power for improving the peak intensity of interference pattern. For demonstrating this phenomenon in experimental way, we control two beams at two cases: one case is to make two fiber amplifiers operate at different output power, whereas the other case is to make the output power of two fiber amplifiers be the same value. Experiments for two cases are accomplished, respectively. The stripe contrast is used to evaluate the performance of far-field interference pattern which can be defined as
r ¼ ðImax Imin Þ=ðImax þ Imin Þ
ð5Þ
where Imax is the peak intensity of the central stripe in the interference pattern, whereas Imin is the minimum intensity of the stripe close to the central stripe. The following paragraphs will give and analyze the open-loop and close-loop coherent combination results.
Fig. 5. The far-field interference patterns of two beams at different output powers (0.1 W and 0.59 W) with (a) the AO system off, (c) the AO system is on, while (b) and (d) are the line images through summing the interference pattern along the y-axis of (a) and (c) respectively.
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P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 7. The far-field interference patterns of two beams at the same output power with (a) the AO system off, (c) the AO system is on, while (b) and (d) are the line images through summing the interference pattern along the y-axis of plot (a) and plot (c) respectively.
Figs. 4 and 5 show the images on CCD camera of which Fig. 4 contains far-field intensity images of two beams (output powers are at 0.1 and 0.59 W respectively) taken separately while Fig. 5 contains the far-field interference patterns of two beams at different output powers. Similarly, Figs. 6 and 7 are images of which Fig. 6 contains far-field intensity images of two beams (output powers are both at 0.6 W) taken separately while Fig. 7 contains the far-field interference patterns of two beams at the same output power. The stripe contrast described in Eq. (5) is used to evaluate the far-filed interference pattern. Experimental results show that the stripe contrast of Fig. 5 is improved from 7% to 19% while the stripe contrast of Fig. 7 is improved from 8% to 28% when the AO system is on. To evaluate the piston correction performance, we also analyze the piston displacement VS the image in a series of continuous
frame number. Fig. 8 shows the contrastive result of which 1– 2900 frames (the first 1 s) are corresponding to the open-loop case while 2901–5800 frames (the second 1 s) correspond to the closeloop case. Fig. 8 demonstrates that the piston displacement has an increase trend as the time or frame number increase in the openloop case while it is reduced to a range about 150 to 150 nm in the case of close-loop. It means that the piston displacement is controlled at about 1/7k(k = 1083 nm) when the AO is on. We have also tested that the control bandwidth of the system is above 60 Hz, while the phase noise frequency of two fiber amplifier induced by piston aberration is below 50 Hz, therefore, the phase noise can be eliminated in real time. In fact, experimental results have proved that the interference pattern of two beams is very stable. 5. Conclusions
Fig. 8. The piston displacement VS the image frame number corresponding to the open loop and close loop case respectively.
Coherent combination of two ytterbium fiber amplifiers is experimentally investigated. Two beams from a fiber laser are amplified and then collimated and overlapped to form an interference pattern. The phase of two beams is locked by a digital feedback loop scheme. The scheme adopts an ASM to compensate the piston aberration which is the main factors to affect the far-field interference pattern of beams. The ASM is controlled by a PR algorithm which can correct the phase noise below 60 Hz. After the phase noise is corrected by AO system, the stripe contrast of interference pattern can be improved from 7% to 19% and from 8% to 28% respectively at two different cases. However, it should be noted that the stripe contrast is still not excellent even if the piston aberration is corrected by ASM. In fact, the tilt aberration can give the explanation, since the PR algorithm can only control the ASM to compensate the piston aberration between two beams, as a result, the tilt aberration of the two beams is still exist which will deteriorate the stripe contrast of far-filed interference pattern. Therefore, to exploit the tilt correction ability of ASM, some new control algorithm or scheme should be developed. Fortunately, a Hartmann-Shack (H-S) sensor,
P. Yang et al. / Optics Communications 282 (2009) 1349–1353
which is often used as the main detection element of a common AO system can be employed to detect the tilt aberration of two beams. As a result, the piston and tilt aberrations can both be corrected by the ASM which is controlled by PR algorithm in combination with the H-S sensor. It is obvious that the configuration of the new scheme will be different from the scheme used in this paper. This new scheme will soon be applied on a three 10-W level fiber amplifier coherent combination system in the near future. Acknowledgements We thank Professor Xu Xiaojun and his team for their generous help during the implementation of the experiments.
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References [1] Monica L. Minden, Hans W. Bruesselbach, Jeffrey L. Rogers, Metin S. Mangir, D.C. Jones, Gilmore J. Dunning, David L. Hammon, A.J. Solis, Lira Vaughan, SPIE 5335 (2004) 89. [2] D.R. Neal, M.E. Warren, J.K. Gruetzner, T.G. Smith, R.R. Rosenthal, SPIE 2201 (1994) 133. [3] T.M. Shay, Vincent Benham, J.T. Baker, Anthony D. Sanchez, Sgt. D. Pilkington, Lt. Douglas J. Nelson, Lt. Chunte A. Lu, SPIE (2007) 6451. [4] Stephen J. Brosnan, San Pedro, Donald G. Heflinger, Lee O. Heflinger, US Patent: 6366356, 2002. [5] A.C.F. Gonte, R. Dandliker, Opt. Eng. 41 (2002) 1073. [6] Ping Yang, Mingwu Ao, Yuan Liu, Bing Xu, Wenhan Jiang, Opt. Exp. 15 (2007) 17051. [7] G. Chanan, M. Troy, C. Ohara, SPIE 4003 (2000) 188. [8] Jing Hou, Rui Xiao, Zong-Fu Jiang, Xiang-Ai Cheng, Bai-Hong Shu, Jin-Bao Chen, Ze-Jin Liu, Chin. Phys. Lett. 22 (2005) 2273.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Coherent combination of two ytterbium fiber amplifier based on an active segmented mirror Ping Yang a,b,*, Ruofu Yang a,b, Feng Shen a, Xinyang Li a, Wenhan Jiang a a b
Institute of Optics and Electronics, P.O. Box 350, Shuangliu, Chengdu, Sichuan 610209, China Graduate School of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 2 December 2008 Accepted 8 December 2008
Keywords: Coherent combination Active segmented mirror PR Interference pattern
a b s t r a c t To control the phase noise of two ytterbium fiber amplifiers, a coherent combination system based on an active segmented mirror (ASM) has been established in our laboratory. The ASM is controlled by a feed back control loop on the basis of a Peak Rate (PR) algorithm which is realized on a DSP + FPGA hardware control board. Experimental results indicate that when the control loop is off, the far-field interference pattern is blurred and dynamic, while when it is on, the far-field beams interference pattern achieves clear and stable. At two different output powers, the contrasts of the interference stripes are improved from 7% to 19% and 8% to 28% respectively. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Coherent combination of fiber lasers is a very promising way to satisfy the high brightness requirement of many laser applications. Nowadays, there are many coherent combining schemes such as Self-Organized coherence in fiber laser arrays [1], adaptive optics [2], MOPA [3,4], etc. The combining efficiency of Self-Organized scheme is prominent when the number of fiber laser arrays is limited in a fewer; however, as the increase of arrays and power of every fiber laser, the combining efficiency decreases correspondingly. At present, MOPA method is very popular because of its simple and high real-time realization. However, with respect to MOPA method, the phase modulators (more often than not, the LiNbO3 crystal) cannot stand high power density and beams must be calibrated accurately to reduce the tilt aberrations. It is known that adaptive optics (AO) technique is a powerful technique for correcting dynamic and static phase aberrations in many laser systems [5,6]. Nowadays, it is also brought in many laser coherent combination systems [2]. As the key active element, an ASM can be used to correct tilt and piston aberrations of beams simultaneously; moreover, it can stand higher power by effective coating high reflectivity membrane on its surface. These two main advantages facilitate adaptive optics technique to be an important technique used for laser beam combination.
* Corresponding author. Address: Institute of Optics and Electronics, P.O. Box 350, Shuangliu, Chengdu, Sichuan 610209, China. Tel.: +86 028 85100686; fax: +86 028 85100433. E-mail address: [email protected] (P. Yang). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.12.019
In this paper, we investigate the possibility for coherent combination of two ytterbium fiber amplifiers using adaptive optics technique. An ASM with seven sub-mirrors is introduced to compensate the piston aberration and lock the phase of the two fiber amplifiers. In this paper, we first introduce the basic theory of beams coherent combination by adaptive optics technique, and then the PR algorithm used for controlling the ASM is described [7]. Finally, the experimental performances of two-beam coherent combination are demonstrated and analyzed. 2. The beam combination system based on AO Fig. 1 is the schematic of the experimental configuration of two ytterbium fiber amplifiers on the basis of AO technique. The seed laser oscillator is a commercial cw polarization maintaining double-clad ytterbium laser with a 1083 nm operation wavelength [8]. The bandwidth of the seed laser is about 20 KHz. The main advantage of such a narrow bandwidth is that the coherence length of the light is long enough for far-field interference. The beam from the polarized maintaining seed fiber laser is firstly divided into two beams by a fiber beam splitter (BS1), and then beams from the same seed laser pass through fiber amplifier 1 (Fiber Amp1) and fiber amplifier 2 (Fiber Amp2) respectively. After amplification, each beam is incident onto the center of a sub-mirror of the ASM. Since only two beams need to be controlled, thus the central two sub-mirror of the 7-element ASM are in practice used. To improve the coherent combination efficiency, two high reflectivity mirrors HR1–HR2 are employed to compress the distance between two beams to about 0.5 mm. The two beams
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P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 1. The schematic of two-beam combination based on AO.
are then divided into two parts by a beam splitter (BS2). The transmitting part is focused by Lens1 to form an interference pattern on the monitors for visual monitoring while the reflected part enters into a CCD (DALSA CA-D1-0064A) which has 64 64 pixels and can work with a maximum frequency of 2900 Hz. A PR algorithm, which is built in a DSP + FPGA control board, is adopted as the feed back algorithm. This algorithm can generate the updating control voltages to control the ASM for compensating two beams’ piston aberrations. Since each sub-mirror has three electrodes on its back, therefore, a D/A card is used to convert six digital voltages from control circuit to analog voltages which will at last be applied onto six electrodes of DM by a high-voltage amplifier (HVA). 3. The PR algorithm and its application in coherent combination Peak Ratio algorithm is firstly introduced to ascertain the piston aberrations of the segmented mirrors in the large aperture telescopes [7]. When piston aberrations are eliminated completely and with no other aberrations present, the two sub-peaks on the right and left of the largest peak of far-field interference pattern have the same intensity value. Therefore, we can use this characteristic to control the piston aberrations, that is, to make the two peaks reach the same intensity value. Fig. 2 shows schematic drawing of location the maximal right peak and maximal left peak. The implement principle of PR algorithm is as follows:
(iii) If the sub-largest peak is on the left of the largest peak, then max(Right Peak) of Eq. (1) is the largest value, while max(Left Peak) is the sub-largest one, well then PR is larger than 1. On the contrary, if the sub-largest peak is on the right of the largest peak, and then max(Left Peak) of Eq. (1) is the largest value while max(Right Peak) is the sub-largest one, then PR is in the range of (0, 1). Fig. 3 is the basic control flowchart of PR algorithm application in the beam combination system based on ASM. The executing course can be described as follows: Firstly, generating a voltage vector Vi randomly by computer and applied it on the actuators of ASM, then acquiring the corresponding far-field interference pattern, and then evaluating the PR value through executing steps (i) to (iii): if PR > =1, then reduce the voltage (Vi+1 = kVi Vf), else increase it (Vi+1 = kVi + Vf). k is the proportional coefficient while k1 and k2 are the integral coefficients (in our control program, k, k1 and k2 are fixed at 0.99, 0.000006 and 0.000004, respectively). PRmax and PRmin are the initial value and can be preset in advance. Renew Vi according to the flowchart of Fig. 3 until PR value reaches a stable extreme value. When PR value reaches an extreme (whether maximum or minimum is ok), it can be deemed that the piston aberration between two beams in the combination system is corrected as clear as possible.
(i) The first step is to sum the intensity of the far-field interference pattern along the y-axis, then searching the largest peak of the light stripes. (ii) Secondly, search two local maximum peaks at the left and right of the largest peak light stripes along the x-axis, then choose the intensity of the larger one (sub-largest) of the two local maximum to compare with the largest peak value. The formula for calculating the ratio is:
PR ¼ maxðRightPeakÞ= maxðLeftPeakÞ
ð1Þ
Fig. 2. Schematic drawing of location the maximal right peak and maximal left peak.
Fig. 3. The flowchart of beams combination controlled by PR algorithm.
P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 4. The far-field patterns of two beams at different output power: (a) 0.1 W (b) 0.59 W.
4. Experiments
I ¼ ðA21 þ A22 Þ½1 þ R cosðDuðtÞ
ð2Þ
where A1 and A2 are the amplitude of the two beams, and DuðtÞ is the phase of the interference beam, whereas R can be defined
R¼
2A1 A2 A21 þ A22
Fig. 6. The far-field patterns of two beams at the same output power: (a) 0.6 W (b) 0.6 W.
I ¼ 2I0 ½1 þ cos DuðtÞ:
The two ytterbium fiber amplifiers can operate with a maximum power of 1 W, and the output power can be adjusted continuously. It is known that the intensity distribution of two interfered beam can be written as:
ð3Þ
The ideal conditions for coherent combination can be described as: two beams propagate in the same direction and same axis; besides, two beams have the same amplitude, that is A1 ¼ A2 ¼ A0 ðR ¼ 1Þ. Now, let I0 ¼ A20 represent the far-field peak intensity of single beam, and then Eq. (2) can be rewritten as:
1351
ð4Þ
What we can know from the paragraphs described above is that it’s favorable to make two beams operate at same output power for improving the peak intensity of interference pattern. For demonstrating this phenomenon in experimental way, we control two beams at two cases: one case is to make two fiber amplifiers operate at different output power, whereas the other case is to make the output power of two fiber amplifiers be the same value. Experiments for two cases are accomplished, respectively. The stripe contrast is used to evaluate the performance of far-field interference pattern which can be defined as
r ¼ ðImax Imin Þ=ðImax þ Imin Þ
ð5Þ
where Imax is the peak intensity of the central stripe in the interference pattern, whereas Imin is the minimum intensity of the stripe close to the central stripe. The following paragraphs will give and analyze the open-loop and close-loop coherent combination results.
Fig. 5. The far-field interference patterns of two beams at different output powers (0.1 W and 0.59 W) with (a) the AO system off, (c) the AO system is on, while (b) and (d) are the line images through summing the interference pattern along the y-axis of (a) and (c) respectively.
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P. Yang et al. / Optics Communications 282 (2009) 1349–1353
Fig. 7. The far-field interference patterns of two beams at the same output power with (a) the AO system off, (c) the AO system is on, while (b) and (d) are the line images through summing the interference pattern along the y-axis of plot (a) and plot (c) respectively.
Figs. 4 and 5 show the images on CCD camera of which Fig. 4 contains far-field intensity images of two beams (output powers are at 0.1 and 0.59 W respectively) taken separately while Fig. 5 contains the far-field interference patterns of two beams at different output powers. Similarly, Figs. 6 and 7 are images of which Fig. 6 contains far-field intensity images of two beams (output powers are both at 0.6 W) taken separately while Fig. 7 contains the far-field interference patterns of two beams at the same output power. The stripe contrast described in Eq. (5) is used to evaluate the far-filed interference pattern. Experimental results show that the stripe contrast of Fig. 5 is improved from 7% to 19% while the stripe contrast of Fig. 7 is improved from 8% to 28% when the AO system is on. To evaluate the piston correction performance, we also analyze the piston displacement VS the image in a series of continuous
frame number. Fig. 8 shows the contrastive result of which 1– 2900 frames (the first 1 s) are corresponding to the open-loop case while 2901–5800 frames (the second 1 s) correspond to the closeloop case. Fig. 8 demonstrates that the piston displacement has an increase trend as the time or frame number increase in the openloop case while it is reduced to a range about 150 to 150 nm in the case of close-loop. It means that the piston displacement is controlled at about 1/7k(k = 1083 nm) when the AO is on. We have also tested that the control bandwidth of the system is above 60 Hz, while the phase noise frequency of two fiber amplifier induced by piston aberration is below 50 Hz, therefore, the phase noise can be eliminated in real time. In fact, experimental results have proved that the interference pattern of two beams is very stable. 5. Conclusions
Fig. 8. The piston displacement VS the image frame number corresponding to the open loop and close loop case respectively.
Coherent combination of two ytterbium fiber amplifiers is experimentally investigated. Two beams from a fiber laser are amplified and then collimated and overlapped to form an interference pattern. The phase of two beams is locked by a digital feedback loop scheme. The scheme adopts an ASM to compensate the piston aberration which is the main factors to affect the far-field interference pattern of beams. The ASM is controlled by a PR algorithm which can correct the phase noise below 60 Hz. After the phase noise is corrected by AO system, the stripe contrast of interference pattern can be improved from 7% to 19% and from 8% to 28% respectively at two different cases. However, it should be noted that the stripe contrast is still not excellent even if the piston aberration is corrected by ASM. In fact, the tilt aberration can give the explanation, since the PR algorithm can only control the ASM to compensate the piston aberration between two beams, as a result, the tilt aberration of the two beams is still exist which will deteriorate the stripe contrast of far-filed interference pattern. Therefore, to exploit the tilt correction ability of ASM, some new control algorithm or scheme should be developed. Fortunately, a Hartmann-Shack (H-S) sensor,
P. Yang et al. / Optics Communications 282 (2009) 1349–1353
which is often used as the main detection element of a common AO system can be employed to detect the tilt aberration of two beams. As a result, the piston and tilt aberrations can both be corrected by the ASM which is controlled by PR algorithm in combination with the H-S sensor. It is obvious that the configuration of the new scheme will be different from the scheme used in this paper. This new scheme will soon be applied on a three 10-W level fiber amplifier coherent combination system in the near future. Acknowledgements We thank Professor Xu Xiaojun and his team for their generous help during the implementation of the experiments.
1353
References [1] Monica L. Minden, Hans W. Bruesselbach, Jeffrey L. Rogers, Metin S. Mangir, D.C. Jones, Gilmore J. Dunning, David L. Hammon, A.J. Solis, Lira Vaughan, SPIE 5335 (2004) 89. [2] D.R. Neal, M.E. Warren, J.K. Gruetzner, T.G. Smith, R.R. Rosenthal, SPIE 2201 (1994) 133. [3] T.M. Shay, Vincent Benham, J.T. Baker, Anthony D. Sanchez, Sgt. D. Pilkington, Lt. Douglas J. Nelson, Lt. Chunte A. Lu, SPIE (2007) 6451. [4] Stephen J. Brosnan, San Pedro, Donald G. Heflinger, Lee O. Heflinger, US Patent: 6366356, 2002. [5] A.C.F. Gonte, R. Dandliker, Opt. Eng. 41 (2002) 1073. [6] Ping Yang, Mingwu Ao, Yuan Liu, Bing Xu, Wenhan Jiang, Opt. Exp. 15 (2007) 17051. [7] G. Chanan, M. Troy, C. Ohara, SPIE 4003 (2000) 188. [8] Jing Hou, Rui Xiao, Zong-Fu Jiang, Xiang-Ai Cheng, Bai-Hong Shu, Jin-Bao Chen, Ze-Jin Liu, Chin. Phys. Lett. 22 (2005) 2273.