Instrumentation & Measurement Magazine 24-5 - 9

Fig. 6. (a) Schematic of the experimental setup to achieve broadband terahertz generation and detection. CM: chirped mirror; BS: beam splitter; TS: translational
stage; L: lens, GaP: <110>-oriented 220 μm -thick gallium phosphide crystal; Ge: germanium wafer; Si: silicon wafer; λ/4: quarter-wave plate; WP: Wollaston
prism; PD: photodetector. (b) Temporal amplitude recorded with terahertz lock-in amplifier prepared in the HC-PCF at different Ar pressures. (c) Corresponding THz
spectral amplitude [16], used with permission under a Creative Commons Attribution (CC BY) license.
Note that, the dispersion properties of kagomé cladded
PCF in Fig. 5b and the capillary cladded ARF in Fig. 2a are similar;
however, the transmission losses of these two are different.
At the same core diameter, the kagomé cladded fiber exhibits
significantly lower loss than capillary cladded fiber. However,
considering fabrication feasibility, capillary cladded fiber is
simpler than the kagomé fiber. Therefore, it is more practical
to use capillary cladded fiber for any nonlinear applications.
Ways to Create Nonlinearities in HC-ARF
In a hollow core terahertz fiber, the possible nonlinearities can
be obtained following the experimental setup proposed by Cui
et.al [16]. In a gas-filled environment, the HC-ARF can be used
to broaden the terahertz spectrum. The proposed terahertz
system setup can be used with a stable MHz laser that can deliver
pulses of sub-microjoule energy and duration of a few
hundred femtoseconds.
A Yb:KGW ultrafast laser amplifier propagates near-infrared
(NIR) pulses as an optical source shown in Fig. 6a. The
ARF in a gas-filled environment can be used to compress the
NIR pulses for efficient broadband terahertz spectrum by
varying the gas pressure. Here, the linear and nonlinear properties
of the HC-ARF can be controlled by controlling the gas
pressure that also permits the optimization of pulse spectral
broadening.
To compensate for the positive chirp induced from selfphase
modulation (SPM), a pair of identical chirped mirrors
August 2021
can be placed after the HC-ARF. A beam splitter divides the
NIR pulses into linear pump pulse and probe pulse where two
nonlinear gallium phosphide (GaP) crystals are used for terahertz
generation and detection, respectively, based on optical
rectification. The emitted terahertz pulses from GaP are collected
and focused on the identical detection crystal (GaP)
using parabolic mirrors. The terahertz pulse in detection crystal
and the pump pulse together allow coherent detection.
When the terahertz electric field penetrates along the one axis
of the nonlinear detection crystal, the variations in crystal refractive
index create birefringence in the terahertz electric
field, which is assumed linearly polarized at the beginning. A
quarter-wave plate has two perpendicular axes and decomposes
the electric field as Ex
and Ey
. When the quarter-wave
plate is not oriented at 45°, to the elliptically polarized electric
field, it adds π/2 dephasing for both components. A suitably
oriented Wollaston prism (WS: two triangular prisms interfaced
together) provides the intensities for both orthogonal
components. The intensity mismatch is measured using two
photodetectors. This detection procedure, known as electrooptic
sampling, has been applied for coherent pulse detection
to accurately recover the phase difference and amplitude of
two orthogonal components. The nonlinear crystal modifies
the polarization state of the terahertz electric field, and by resolving
the change in polarization, it is possible to directly
map the terahertz electric field. As the gas pressure increases
from 0 to 10 bar, the peak terahertz amplitude increases due
IEEE Instrumentation & Measurement Magazine
9

Instrumentation & Measurement Magazine 24-5

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 24-5