Ksendzov A, Lay O, Martin S, Sanghera J S, Busse L E, Kim W H, Pureza P C, Nguyen V Q, Aggarwal I D
Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive, Pasadena, California 91109, USA.
Appl Opt. 2007 Nov 10;46(32):7957-62. doi: 10.1364/ao.46.007957.
We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter Q(f) as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality Q(f) for this fiber at 10.5 microm wavelength is 1000+/-300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428+/-0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of Q(f).
我们提出了一种测量单模光纤模式滤波能力的技术。理想的模式滤波器会拒绝所有与滤波器基模没有重叠的输入场分量，并且不会衰减基模。我们将非理想模式滤波器的品质因数Q(f)定义为基模的透射率与和基模没有重叠的输入场的透射率之比。我们在美国海军研究实验室制造的一根20厘米长的中红外光纤上演示了该技术。这根光纤在10.5微米波长处的滤波器品质因数Q(f)为1000±300。基模中的吸收和散射损耗约为8分贝/米。包括菲涅尔反射在内，基模的总透射率为0.428±0.002。感兴趣的应用是使用消零干涉测量法寻找太阳系外类地行星。这需要高抑制比来抑制明亮恒星的光，以便微弱的行星变得可见。模式滤波器的使用通过降低对小波前误差的敏感度来提高抑制比（或者等效地，放宽对波前质量的要求）。我们从理论上表明，除了耦合损耗外，模式滤波器的使用会使双光束干涉仪中的抑制比提高Q(f)倍。
