Instrumentation & Measurement Magazine 24-5 - 5

Linearity and Nonlinearity in
Hollow-Core Antiresonant Fiber
Sensors in the Terahertz Regime
Jakeya Sultana, Md. Saiful Islam, Md. Selim Habib, Mayank Kaushik,
Alex Dinovitser, Brian W.-H. Ng, and Derek Abbott
I
n nonlinear optics, a major challenge is to maximize the
interaction between the light from laser sources and lowdensity
media such as gases. An ultrafast laser beam can
be focused to form a highly intense spot over a small concentrated
area. The ultrafast laser beam has a short pulse width
of less than one picosecond and high peak power in the beam
profile (Fig. 1). The beam profile of the laser source describes
the energy density and distribution of light, and the beam profile
of a laser source is usually affected during the propagation
and collimation of the beam. An efficient nonlinear optical sensor
also requires high peak power at low energy (low average
power) over short duration of laser pulse, a high beam profile
and long interaction length. These requirements of a nonlinear
optical sensor with low attenuation constant can be achieved
in hollow-core photonic crystal fiber (HC-PCF). Gas-filled HCPCF
exhibits optical nonlinearity for an ultrashort temporal
and spectral broadening of NIR pulses [1], [2]. The nonlinearity
in HC-PCF can be achieved by tuning the gas pressure.
Gas-filled HC-PCF nonlinear media are low-cost, replenishable,
reconfigurable and exhibit sharp spectral lines [3]. Linear
and nonlinear responses are also found in other types of optical
fibers.
Fig. 1. (a) An ultrashort pulse with high peak power over a very short duration
of pulse width and low average power. The laser has a short pulse width of
less than one picosecond and high energy in the beam profile. (b) The graphical
representation for power and energy throughout laser pulses. Peak power
can be defined as the ratio of single pulse energy and the pulse width. Pulse
energy corresponds to the area under the pulse shape. The average power
denoted as the multiplication of single pulse energy and repetition time. The
short duration of the laser pulse prevents the sensor damage by lowering the
average power.
August 2021
For example, recently a single-mode fiber (SMF)/multimode
fiber (MMF)/SM quasi two-mode fiber was reported
for temperature measurements [4]. Similarly, an optical sensor
built as a cascade of SMF/HCF/HC-PCF filled with ethanol
can sense the refractive index and temperature of the liquid
ethanol simultaneously [5], or a cascade of SMF/HCF/No
core fiber (NCF)/PCF filled with liquid crystal/NCF/SMF
measures the electric field sensitivity with the strain (tension)
from the reflection and transmission spectrum [6]. For
the reported sensors [4]-[6], a nearly linear relation between
the peak wavelength and the temperature or refractive index
or electric field intensity was observed. A magnetic field sensor
based on a surface plasmon resonance (SPR) has also been
reported to detect magnetic fluids [7]. Using silver as a plasmonic
material to coat the fiber, the surface plasmon effect on
the fiber surface was observed. Due to the plasmonic effect,
this sensor shows nonlinear sensing characteristics.
Graphene has also been used to enhance the sensing performance.
Recent work on metal-graphene coated fiber-based
SPR sensors was theoretically analyzed for liquid analyte detection
[8]. The reported sensor shows an enhancement in
sensitivity when using graphene. Because of the nonlinearity
of graphene and metal, this sensor also exhibits nonlinear
sensing performance. A range of other nonlinear effects such
as supercontinuum generation [9], high-harmonic generation
[10], etc. have also been demonstrated in both resonant and antiresonant
modes [11]-[13]. However, research-based on such
nonlinearities in the terahertz regime has not been previously
addressed.
An open research problem is the lack of high power terahertz
laser sources that can create linear and nonlinear pulses
simultaneously for nonlinear applications such as terahertz
supercontinuum generation, terahertz high harmonic generation,
etc. In this article, we model and simulate a simple
hollow-core antiresonant terahertz waveguide, show the
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