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According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of self-correlation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths, the measurement of intermediate frequency is accurate without the influence of noise as the delay d between the received signal and locally generated signal is less than the coherence time c of laser source. If the delay d is greater than the coherence time c, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay d is greater than the coherence time c. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1 km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked, which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.
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Keywords:
- heterodyne detection /
- laser applications /
- lasers
[1] Kingston R 1977 Optics News 3 27
[2] Cohen S C 1975 Appl. Opt. 14 1953
[3] Li Y C, Wang C H, Qu Y, Gao L, et al. 2011 Chin. Phys. B 20 014208
[4] Luo Y, Feng G Y, Liu J, et al. 2014 Chinese J. Lasers 183 (in Chinese) [罗韵, 冯国英, 刘建 等 2014 中国激光 183]
[5] Swanson E A, Carter G M, Bernays D J, et al. 1989 Appl. Opt. 28 3918
[6] Mosley D E, Matson C L, Czyzak S R 1998 SPIE Conference on Laser Radar Technology and Apglications III (Florida: Oriando) 243
[7] Zhou B K, Gao Y Z, Chen T R, et al. 2009 Principles of Lasers (6th Ed.) (National Defense Industry Press) (in Chinese) [周炳琨, 高以智, 陈倜嵘 等 2009 激光原理(第六版) (国防工业出版社)]
[8] Mercer L B 1991 J. Lightwave Technol. 9 485
[9] Richter L, Mandelberg H, Kruger M, et al. 1986 IEEE J. Quantum Electron. 22 2070
[10] Gallion P B, Debarge G 1984 IEEE J. Quantum Electron. 20 343
[11] Cai L Z 2007 Optics (3rd Ed) (Beijing: Science Press) (in Chinese) [蔡履中 2007 光学 (第三版) (北京: 科学出版社)]
[12] Hao Y Q, Ye Q, Pan Z Q, et al. 2013 Chin. Phys. B 22 074214
[13] An Y Y, Liu J F, Li Q H, et al. 2007 Optoelectronic Technology (2nd Ed.) (Beijing: Publishing House of Electronics Inducstry) (in Chinese) [安毓英, 刘继芳, 李庆辉 等 2007 光电子技术 (第二版) (北京: 电子工业出版社)]
[14] Wang Y D, Wang J 2011 Fundamentals of Random Signal Analysis (3rd Ed) (Publishing House of Electronics Inducstry) (in Chinese) [王永德, 王军 2011 随机信号分析基础 (第三版) (北京: 电子工业出版社)]
[15] Rowe H E 1965 Signal and Noise in Communication Systems (Princeton, NJ: van Nostrand)
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[1] Kingston R 1977 Optics News 3 27
[2] Cohen S C 1975 Appl. Opt. 14 1953
[3] Li Y C, Wang C H, Qu Y, Gao L, et al. 2011 Chin. Phys. B 20 014208
[4] Luo Y, Feng G Y, Liu J, et al. 2014 Chinese J. Lasers 183 (in Chinese) [罗韵, 冯国英, 刘建 等 2014 中国激光 183]
[5] Swanson E A, Carter G M, Bernays D J, et al. 1989 Appl. Opt. 28 3918
[6] Mosley D E, Matson C L, Czyzak S R 1998 SPIE Conference on Laser Radar Technology and Apglications III (Florida: Oriando) 243
[7] Zhou B K, Gao Y Z, Chen T R, et al. 2009 Principles of Lasers (6th Ed.) (National Defense Industry Press) (in Chinese) [周炳琨, 高以智, 陈倜嵘 等 2009 激光原理(第六版) (国防工业出版社)]
[8] Mercer L B 1991 J. Lightwave Technol. 9 485
[9] Richter L, Mandelberg H, Kruger M, et al. 1986 IEEE J. Quantum Electron. 22 2070
[10] Gallion P B, Debarge G 1984 IEEE J. Quantum Electron. 20 343
[11] Cai L Z 2007 Optics (3rd Ed) (Beijing: Science Press) (in Chinese) [蔡履中 2007 光学 (第三版) (北京: 科学出版社)]
[12] Hao Y Q, Ye Q, Pan Z Q, et al. 2013 Chin. Phys. B 22 074214
[13] An Y Y, Liu J F, Li Q H, et al. 2007 Optoelectronic Technology (2nd Ed.) (Beijing: Publishing House of Electronics Inducstry) (in Chinese) [安毓英, 刘继芳, 李庆辉 等 2007 光电子技术 (第二版) (北京: 电子工业出版社)]
[14] Wang Y D, Wang J 2011 Fundamentals of Random Signal Analysis (3rd Ed) (Publishing House of Electronics Inducstry) (in Chinese) [王永德, 王军 2011 随机信号分析基础 (第三版) (北京: 电子工业出版社)]
[15] Rowe H E 1965 Signal and Noise in Communication Systems (Princeton, NJ: van Nostrand)
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