Optical spectrometers are used in chemical and material analysis, biological sensing, optical communication system testing and light source characterization among others. This has in return triggered immense research in this field where majority of the work done so far focuses more on the spectral resolution and imaging quality leaving less covered on the polarization sensitivity. Spectrometers are generally sensitive to the polarization state of the measured light which directly indicates their measurement accuracy.
Researchers generally apply two main approaches to reduce the polarization sensitivity of the spectrometer. The first one is achieved by placing a polarization measurement instrument in front of the spectrometer thereby introducing extra attenuation to the light, resulting in a large and complex instrument. The second is achieved by adding a depolarizer fabricated using a quartz crystal in front of the incident slit. The latter is quite simple and more effective but it has been seen to cause the problem of double imaging, which in turn reduces the spectral resolution.
Yinxin Zhang and Yajun Pang at Tianjin University in collaboration with Huaidong Yang at Tsinghua University developed a new technique for reducing the polarization sensitivity of grating-based spectrometer by using an achromatic quarter-wave plate. They aimed at designing a grating-based spectrometer in a double-pass structure having the achromatic quarter-wave plate coated with reflective film on the rare facet which would be used to reflect the incident beam back for the second pass. Their research work is now published in the peer-reviewed journal, Optics Communications.
The researchers begun their empirical procedure by preparing the double-pass spectrometer system with a reflective achromatic quarter-wave plate. They then adopted an off-axis parabolic mirror as the collimating and focusing mirror simultaneously. A fiber array was then employed to input and output the signal light. They then collimated the light beam by the off-axis parabolic mirror and reflected it onto the grating. The light was then diffracted a second time by the grating and was focused by the off-axis parabolic mirror. The research team then applied a Jones matrix, using both the p and s components of the exiting and incident light respectively, to derive the polarization-insensitive system formulas and theoretical models. The authors of this paper were hereby able to observed that by rotating the polarization state of the second pass beam with an achromatic quarter-wave plate, the diffraction efficiencies of the grating for the s-polarized and p-polarized light become nearly the same.
The polarization-dependent loss of the designed spectrometer herein was less than 0.5 dB from the infrared optical communication wavelength range. Furthermore, it depicts the merits of miniaturization, simple structure and high resolution. This spectrometer was developed for spectrometric measurements in optical communication transmissions where the signal light is modulated in a polarization state with a high signal-to-noise ratio. In this case, the reduction of polarization sensitivity helps to avoid ambiguities in the recorded spectra. Moreover, this spectrometer has the potential to play an important role in a range of biological and physical studies in which polarized emission is of interest.
Yajun Pang, Yinxin Zhang, Huaidong Yang, Zhanhua Huang. Reduction of the polarization sensitivity of grating-based spectrometer using an achromatic quarter-wave plate. Optics Communications, volume 393 (2017) page 139–142.Go To Optics Communications