We present the method for coherence length measurement using coherence meter based on hybrid liquid crystal structures doped with gold nanoparticles. The results indicate that the method is able to determine the coherence length of coherent light sources with precision of 0.01？m at wavelength range from 200 to 800？nm for wide range of initial beam powers starting from 1？mW. Given the increasing use of laser technology in industry, military, or medicine, our research may open up a possible route for the development of improved techniques of coherent diagnostic light sources. 1. Introduction The coherence length of the laser light is the propagation distance from the emission source to the point in which electromagnetic wave maintains a specified degree of coherence. It is proportional to the output power divided by the square of the cavity round-trip time according to the Schawlow-Townes formula, but this limit is rarely achieved under the influence of various noise sources [1]. The coherence is also influenced by drift and environmental factors such as temperature drifts, humidity, or air pressure [2]. Since the commercial solutions for the measurement of the coherence length are not available, the vast majority of such measurements are made by Michelson interferometer [3]. The main drawbacks of this solution are the time-consuming preparations of the measurement system followed by the necessity of complicated manual modifications and complex calculations. Consequently, we still need faster and more efficient measurement technology for the coherence length of laser light. Since all the remarkable properties of laser light depend on the coherence, determining its length is essential especially when the coherence of the emitted light is specifically large or small. Lasers with large temporal and spatial coherence have found numerous applications, including interferometry, holography, and construction of certain types of optical sensors as well as, coherent beam combining technology, which is used in sectors such as metallurgy, medicine, military, and communication [4–7]. On the other hand, low temporal coherence combined with high spatial coherence is the key parameter of certain optical research techniques such as optical coherent tomography used for imaging of the retina and optic nerve structures [8, 9]. Presented coherence meter can cover all of the above examples as well as provide the answer to the question whether the tested laser maintains its optical properties over time. 2. Experiment The coherence meter presented in Figure 1(a) is based on the
References
[1]
W. Elsaesser and E. O. Goebel, “Multimode effects in the spectral linewidth of semiconductor lasers,” IEEE Journal of Quantum Electronics, vol. 21, no. 6, pp. 687–692, 1985.
[2]
S. Roling, B. Siemer, M. W？stmann et al., “Temporal and spatial coherence properties of free-electron-laser pulses in the extreme ultraviolet regime,” Physical Review Special Topics, vol. 14, no. 8, Article ID 080701, 2011.
[3]
P. G. Kwiat, W. A. Vareka, C. K. Hong, H. Nathel, and R. Y. Chiao, “Correlated two-photon interference in a dual-beam Michelson interferometer,” Physical Review A, vol. 41, no. 5, pp. 2910–2913, 1990.
[4]
R. F. Smith, J. Dunn, J. Nilsen et al., “Picosecond x-ray laser interferometry of dense plasmas,” Physical Review Letters, vol. 89, no. 6, Article ID 065004, 2002.
[5]
Q. Lou, J. Zhou, B. He, and H. Zhao, “Fiber lasers and their coherent beam combination,” Optics and Photonics News, vol. 19, no. 5, pp. 46–51, 2008.
[6]
W. M. Wang, K. T. V. Grattan, A. W. Palmer, and W. J. O. Boyle, “Self-mixing interference inside a single-mode diode laser for optical sensing applications,” Journal of Lightwave Technology, vol. 12, no. 9, pp. 1577–1587, 1994.
[7]
S. J. Augst, T. Y. Fan, and A. Sanchez, “Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers,” Optics Letters, vol. 29, no. 5, pp. 474–476, 2004.
[8]
E. Regara, J. A. Schaara, E. Montb, R. Virmanib, and P. W. Serruys, “Optical coherence tomography,” Cardiovascular Radiation Medicine, vol. 4, pp. 198–204, 2003.
[9]
A. F. Fercher and C. K. Hitzenberger, “Chapter 4 Optical coherence tomography,” Progress in Optics, vol. 44, pp. 215–302, 2002.
[10]
S. Bartkiewicz and A. Anczykowska, “Polish patent submission: method for coherence measurements of light sources and coherence meter,” MKP Classification: G01J9/00G01J9/02, Submission number: 387382, 2009.
[11]
A. Anczykowska, S. Bartkiewicz, M. Nyk, and J. My？liwiec, “Enhanced photorefractive effect in liquid crystal structures co-doped with semiconductor quantum dots and metallic nanoparticles,” Applied Physics Letters, vol. 99, no. 19, Article ID 191109, 2011.
[12]
A. Anczykowska, S. Bartkiewicz, M. Nyk, and J. Mysliwiec, “Study of semiconductor quantum dots influence on photorefractivity of liquid crystals,” Applied Physics Letters, vol. 101, Article ID 101107, 4 pages, 2012.
J. M. Khosrofian and B. A. Garetz, “Measurement of a Gaussian laser beam diameter through the direct inversion of knife-edge data,” Applied Optics, vol. 22, no. 1, pp. 3406–3410, 1983.
