Imaging of guided waves using an all-fiber
reflection-based NSOM with self-compensation
of a phase drift
YI-ZHI SUN,
1,3
BIN-BIN WANG,
2
RAFAEL SALAS-MONTIEL,
2
SYLVAIN BLAIZE,
2
RENAUD BACHELOT,
2
LI-SHUANG FENG,
3
AND WEI DING
1,
*
1
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2
Laboratoire de Nanotechnologie et d’Instrumentation Optique (LNIO), ICD CNRS UMR 6281, Université de Technologie de Troyes, Troyes, France
3
School of Instrumentation Science and Opto-Electronics Engineering, Beihang University, Beijing 100191, China
*Corresponding author: wding@iphy.ac.cn
Received 2 July 2018; revised 23 August 2018; accepted 27 August 2018; posted 10 September 2018 (Doc. ID 336514); published 3 October 2018
A phase-resolved reflection-based near-field scanning optical
microscopy (NSOM) technique with an original all-fiber
configuration is presented. Our system consists of an intrin-
sically phase-stable common-path interferometer. The reflec-
tion from the waveguide input facet or from an integrated
fiber Bragg grating is used as the reference beam. This
arrangement effectively suppresses the phase drift caused
by environmental fluctuations. By raster scanning a silicon
atomic force microscope probe, we measure the complex
near fields of the propagating and stationary waves in silicon
nanowaveguides. Our robust, align-free, cost-effective, and
shot-noise-limited near-field imaging technique paves the
way for versatile optical characterizations of nanophotonic
structures on a chip.
© 2018 Optical Society of America
OCIS codes: (180.5810) Scanning microscopy; (130.2790) Guided
waves; (120.2920) Homodyning; (120.5050) Phase measurement.
https://doi.org/10.1364/OL.43.004863
Fully understanding, engineering, and employing light-matter
interactions at the nanoscale, where high spatial frequency
components of light fields undertake an essential role, form
the main theme of nano-optics research [1]. While the rapid
advances in nanofabrication and computing power have al-
lowed the investigation of ever finer nanophotonic structures,
the capability to accurately and conveniently probe their near
fields remains highly sought after [2]. Near-field scanning
optical microscopy (NSOM) is considered to be a powerful tool
that can provide deep sub-wavelength resolution [3]. However,
generally, it requires a sophisticated and expensive light collec-
tion system: low-transmittance metal-coated aperture probes in
aperture NSOM [4] or confocal-type microscopes in scattering
NSOM [5,6]. This requirement prevents to some extent the
vulgarization of this technique. Hopman et al. [7] and
Robinson et al. [8] proposed to replace the complicated light
collection system by a simple waveguide coupling setup. In the
so-called (all-fiber) transmission-based NSOM, a sharp metallic
or dielectric tip provides sub-10 nm level of spatial resolution
[9]. The large light extinction cross section of the probe [10]
ensures high collection efficiency. However, the transmission
configuration does not allow phase measurement [8].
To retrieve phase, one has to incorporate the NSOM into an
interferometer. In Refs. [11,12], a Mach–Zehnder interferom-
eter (MZI) configuration is adopted with homodyne detection.
Apart from this, heterodyne [13,14], pseudo-heterodyne [15],
and time-resolved [16] measurements have also been demon-
strated in free-space setups, and can be transferred to the all-
fiber scenario.
Nevertheless, the main issue of the phase measurement in
all-fiber systems is the poor mechanical and thermal stability
which causes strong phase drifts into the measured signal.
Whether Mach–Zehnder [17 ] or Michelson [18] fiber interfer-
ometers, whose sizes are at meter scale, are used, the phase dif-
ference of the two arms can be easily accumulated to a severe
extent [19]. Although many strategies can be used to minimize
environmental influences, e.g., enclosing the setup in a small
box, the phase drift problem still hampers the flexible use of
phase-resolved all-fiber NSOM.
In this Letter, we attempt to combine all the merits men-
tioned above in an all-fiber reflection-based NSOM. More pre-
cisely, we demonstrate that the reflection-based configuration
allows us to measure both the amplitude and phase of light
fields. The oscillation of the probe plays the role of a modulator,
and a common-path interferometer (CPI) setup self-compen-
sates for the phase drift induced by external environment. The
reference beam of the CPI can be generated from the back-
reflection at the waveguide input facet or from a spliced-in fiber
Bragg grating (FBG). The latter method provides a control to
the ratio between the reference beam and the NSOM signal
and, therefore, can tailor the interferometric amplification gain.
Using this convenient setup, we measure complex near fields of
the propagating and stationary waves in silicon nanowaveguides
at varied wavelengths and in three dimensions (3D).
First, we numerically calculate and compare the intensities
of optical waves in the transmission- and reflection-based
Letter
Vol. 43, No. 20 / 15 October 2018 / Optics Letters 4863
0146-9592/18/204863-04 Journal © 2018 Optical Society of America