Abstract

A perfect spatial overlap with multiple beams of different wavelengths is a prerequisite for multi–wavelength interferometry. Beam combination with the help of fibers seems to be an interesting method for this. We investigated three different types of fiber components, multi–mode wavelength division multiplexers (MM–WDMs), single–mode wavelength division multiplexers (SM–WDMs), and endlessly single–mode polarization maintaining photonic crystal fibers (PM–PCFs). All three seem potential candidates for a perfect spatial overlap of laser beams separated by an octave, i.e. fundamental and harmonic beams. We performed an experimental study on the impact of these components on polarization, wavefront, and coherence. These properties are essential for high–accuracy interferometry. As a test system, we used the fundamental and second harmonic beam of a Nd:YAG laser system at 1064 and 532 nm as a popular system in two–color interferometry for intrinsic refractive index compensation. The experimental results show the potential of PM–PCFs for this challenging application.

Highlights

  • L ARGE–SCALE distance measurement with a relative measurement uncertainty better than 10−6 is a current challenge in dimensional metrology

  • The wideband wavelength division multiplexing (WDM) transmission has been successfully achieved with photonics crystal fiber (PCF) for widespread applications, such as optical telecommunication [20]–[24], super– continuum source [25], fiber amplifier [26], physical sensor [27], and coupler fabrication [28], [29], for example

  • The results of the single–mode wavelength division multiplexers (SM–WDMs) depend very strongly on the specifics of the fiber guidance, e.g. on the existence of small curvatures. While this general tendency might not be completely surprising, we want to stress the high quality of the polarization of both colors transmitted through the polarization maintaining photonic crystal fibers (PM–PCFs) depicted in Fig. 2(f) and (g)

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Summary

INTRODUCTION

L ARGE–SCALE distance measurement with a relative measurement uncertainty better than 10−6 is a current challenge in dimensional metrology. The wideband wavelength division multiplexing (WDM) transmission has been successfully achieved with PCF for widespread applications, such as optical telecommunication [20]–[24], super– continuum source [25], fiber amplifier [26], physical sensor [27], and coupler fabrication [28], [29], for example To investigate these beam combination options for multi– wavelength interferometry, three commercial fiber components, an MM–WDM, an SM–WDM, and a combination scheme based on an endlessly single–mode polarization–maintaining photonic crystal fiber (PM–PCF) have been systematically investigated and compared to each other in this study.

EXPERIMENTAL SETUP AND COUPLING EFFICIENCIES
DEGREE AND STABILITY OF THE POLARIZATION
WAVEFRONT QUALITY
COHERENCE
Findings
CONCLUSION
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