# Characterization and Novel Application of Power Over Fiber for Electronics
in a Harsh Environment

M. A. Arroyave  Fermi National Accelerator Laboratory, Batavia, IL, 60510, USA
B. Behera  South Dakota School of Mines and Technology, Rapid City, SD 57701,
USA  F. Cavanna  Fermi National Accelerator Laboratory, Batavia, IL, 60510,
USA  A. Feld  Fermi National Accelerator Laboratory, Batavia, IL, 60510, USA
F. Guo  Stony Brook University, SUNY, Stony Brook, New York 11794, USA  A.
Heindel  South Dakota School of Mines and Technology, Rapid City, SD 57701,
USA  C. K. Jung  Stony Brook University, SUNY, Stony Brook, New York 11794,
USA  K. Koch  Fermi National Accelerator Laboratory, Batavia, IL, 60510, USA
D. Leon Silverio  South Dakota School of Mines and Technology, Rapid City, SD
57701, USA  D. A. Martinez Caicedo  South Dakota School of Mines and
Technology, Rapid City, SD 57701, USA  C. McGrew  Stony Brook University,
SUNY, Stony Brook, New York 11794, USA  A. Paudel  Fermi National Accelerator
Laboratory, Batavia, IL, 60510, USA  W. Pellico  Fermi National Accelerator
Laboratory, Batavia, IL, 60510, USA  R. Rivera  Fermi National Accelerator
Laboratory, Batavia, IL, 60510, USA  J. Rodríguez Rondon  South Dakota School
of Mines and Technology, Rapid City, SD 57701, USA  S. Sacerdoti  Université
Paris Cité, CNRS, Astroparticule et Cosmologie, Paris, France  P. Shanahan
Fermi National Accelerator Laboratory, Batavia, IL, 60510, USA  W. Shi  Stony
Brook University, SUNY, Stony Brook, New York 11794, USA  D. Torres Muñoz
South Dakota School of Mines and Technology, Rapid City, SD 57701, USA  D.
Totani  University of California Santa Barbara, Santa Barbara, CA 93106, USA
C. Uy  South Dakota School of Mines and Technology, Rapid City, SD 57701, USA
C. Vermeulen  South Dakota School of Mines and Technology, Rapid City, SD
57701, USA  H. Vieira de Souza  Université Paris Cité, CNRS, Astroparticule et
Cosmologie, Paris, France

###### Abstract

Power-over-Fiber (PoF) technology has been used extensively in settings where
high voltages require isolation from ground. In a novel application of PoF,
power is provided to photon detector modules located on a surface at $\sim$
300 kV with respect to ground in the planned DUNE experiment. In cryogenic
environments, PoF offers a reliable means of power transmission, leveraging
optical fibers to transfer power with minimal system degradation. PoF
technology excels in maintaining low noise levels when delivering power to
sensitive electronic systems operating in extreme temperatures and high
voltage environments. This paper presents the R$\&$D effort of PoF in extreme
conditions and underscores its capacity to revolutionize power delivery and
management in critical applications, offering a dependable solution with low
noise, optimal efficiency, and superior isolation.

Keywords: Cryogenics, High voltage, Time Projection Chamber (TPC), Noble
liquid detectors, High-power lasers, Optical fibers, Electronic detector
readout concepts

ptFERMILAB-PUB-24-0265-AD-ETD-LBNF

00footnotetext: This manuscript has been authored by Fermi Research Alliance,
LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy,
Office of Science, Office of High Energy Physics. Additionally, this material
is based upon work supported by the U.S. Department of Energy, Office of
Science, Office of High Energy Physics under Award Number DE-SC0024450.
Moreover, this work is supported by subcontract No. 687391 between Fermi
Research Alliance, LLC and South Dakota School of Mines and Technology.

##  1 Introduction

The DUNE experiment will soon deploy large liquid argon (LAr) time projection
chambers (TPC) to detect neutrino interactions and other particle physics
phenomena [1, 2, 3, 4, 5]. In addition to the particle tracking provided by
the TPC, photon detectors in the cryostat will leverage the high scintillation
light yield of LAr to provide crucial timing and additional calorimetric
information. The development of the DUNE vertical drift (VD) TPC configuration
with its largely unobstructed geometry presents an opportunity to provide
excellent photo-detection coverage using $\sim$ 320 photo-detectors known as
X-ARAPUCAs [6] placed in the cathode plane (see Figure 1, left) and read them
out by DUNE VD cold electronics motherboard (DCEM) [5], provided that the
X-ARAPUCAs can be electrically isolated for safe operation on high-voltage
surfaces of the TPC. This requirement is met for the Photon Detector System
(PDS) of the DUNE VD detector using power-over-fiber (PoF) in a novel
application both in particle detectors for high energy physics and in a
cryogenic environment.

PoF transmits laser power over (non-conductive) lightweight, non-conductive
optical fiber to a remote photovoltaic receiver (also called an optical power
converter, OPC), allowing the operation of remote sensors or electronic
devices. Existing PoF technologies are commonly employed for voltage isolation
between source and receiver and embedded electronics in high voltage or high
noise environments [7, 8, 9, 10]. However, none of the commercially available
technologies are rated to operate in cryogenic environments such as liquid
argon (LAr) or liquid nitrogen (LN2).

Figure 1: Left: Schematic of the DUNE vertical drift concept featuring the
X-ARAPUCAs PDS located on the cathode plane. The figure is obtained and
adapted from the DUNE FD2 TDR [5]. Right: A cold readout electronic board,
powered by PoF, designed for a DUNE single photo-detector module prototype
located on a HV cathode [5].

In the PoF application shown in Figure 1 (right), multimode fibers with a core
radius of 62.5 $\mu$m are utilized to transmit the laser diode power. The
fibers have a black polyvinylidene fluoride (PVDF) jacket and are FC
terminated. Sheathing opacity prevents light from leaking out of the fiber and
minimizes background light noise to the PDS (Section 3.5). The electronics
power is obtained by stacking two gallium arsenide (GaAs) OPCs connected in
parallel on a cold readout board as shown in Figure 1 (right). The GaAs OPCs
convert the optical power to electrical power at an efficiency that depends
upon the load matching to laser power and to a lesser extent upon the
temperature. The GaAs OPC maximum efficiency can reach $\sim$ 50 - 55%, which
is significantly higher than the silicon OPC option that can reach between
$\sim$ 20 - 40% [5]. This innovative PoF solution provides three major
benefits: (1) voltage isolation, (2) noise immunity, and (3) spark-free
operation. A summary of the developed PoF system capability is shown in Table
1.

Parameter | Value
---|---
808nm Broadcom laser power per unit | tunable up to 2 W
Max OPC Efficiency (Optical-to-Electrical) |  $\sim$51% ($\sim$45% at room temperature)
Output voltage per OPC | 3-38 V 111Corresponding authors: $*$<EMAIL_ADDRESS>${\ddagger}$<EMAIL_ADDRESS>$\dagger$<EMAIL_ADDRESS>
Output current per OPC | up to 100 mA
Output power per OPC | up to 600 mW
Table 1: PoF system capability at LN2 temperature. 11footnotetext: The OPC
base voltage is determined by OPC design. The number of semiconductor
gaps/layers sets the base voltage.

The remainder of this paper is structured as follows. Section 2 summarizes the
performance of optical power converters (OPC) in warm (room temperature) and
cryogenic environments. Section 3 summarizes R&D of the optical fibers used in
the PoF system. Section 4 describes the laser transmitter unit operation
within the PoF system. Section 5 describes an example of the electrical power
delivered by an OPC within a PoF system. Section 6 describes the
characterization and mitigation of the light leakage from a PoF system
application alongside photodetectors, where photon background is a relevant
concern. Section 7 details the practical application of PoF technology within
the HV environment of the CERN Cold Box Test Facility. Finally, conclusions
are provided in Section 8.

##  2 Optical Power Converter (OPC)

OPCs are semiconductor devices used to convert optical power into an
electrical power. OPCs are typically categorized as either single junction or
multi junction devices, with the majority of modern OPCs falling into the
multi junction category. These multi junction designs, such as the vertical
epitaxial heterostructure architecture (VEHSA) design from Broadcom [11], are
composed of many semiconductor sub cell layers [12]. The material used for the
absorbing layer of these sub cells is primarily dependent on the wavelength of
the laser being used. Presently, 808 nm lasers are among the most widely
available commercially. At this wavelength, the most efficient OPCs utilize
GaAs absorbing layers. For higher spectral ranges, such as 960-990 nm,
Silicone is employed as the absorbing layer, while at 1500-1600 nm, InGaAs
lattice-matched to InP is used [12]. Similar to the wavelength used, the
maximum power capabilities of these devices are constantly evolving and can
vary significantly. Generally, optical power converters can be categorized
into three power levels: “regular-power” devices with input power below 1 W,
“medium-power” devices with input power below 6 W, and “high-power” devices
with input power below 50 W [12]. These OPC devices are typically tested and
certified for an operating temperature range of -40°C to 85°C, which is
significantly above the cryogenic range. This section describes the procedure
and results from the cryogenic characterization of GaAs Broadcom OPCs [11].

###  2.1 OPC Characterization at Room and Cryogenic Temperatures

The OPC characterization measures the current-voltage (I-V) and power
resistance (P-R) curves using different loads and input powers from the laser
box. The output power of the OPCs (P = I $\times$ V) was obtained using the
I-V curves. Later, the maximum efficiency of the OPC is obtained by
calculating the ratio between the maximum output power provided by the OPC
($P^{OPC}_{\text{max output}}$) and the input power provided by the laser box
($P^{laser}_{\text{input}}$), as described in Equation 1.

Figure 2: The test stand designed and built to characterize the OPCs at room
temperature (left) and in LN2 temperature (right).

$\text{Efficiency}=\frac{P^{OPC}_{\text{max
output}}(W)}{P^{Laser}_{\text{input}}(W)}$ (1)

Figure 2 shows the test stand designed and built to characterize the OPCs at
room and LN2 temperatures. “Room temperature” refers to the typical ambient
temperature of the environment, typically around 20 to 25°C, while “LN2”
refers to liquid nitrogen, with a temperature around -196°C. The setup
includes a single-channel laser box with a tunable input power up to $\sim$ 2
W connected to an optical fiber with a 62.5 $\mu$m core, which is then
connected to a single OPC. The output voltage of the OPC is measured using a
voltmeter across a range of loads controlled by a variable load board, which
enabled the load to be adjusted from 50 $\Omega$ to 140 $\Omega$ in 1 $\Omega$
increments. First, the test was conducted at room temperature (Figure 2,
left), and was later repeated at LN2 temperatures (Figure 2, right). During
the LN2 measurements, the OPCs were submerged $\sim$ 5 cm in LN2 to ensure
that the measurements were not affected by any power loss due to thermal
contraction in the fiber jacket, as described in Section 3.5.

Figure 3: I-V curves (upper panel) and P-R curves (lower panel) for 12 OPCs
(shaded color bands represent standard deviation) at room temperature (left
column) and cryogenic temperatures (right column).

Figure 3 displays the I-V curves (upper panel) and P-R curves (lower panel)
including the standard deviation (shaded color bands) obtained for the 12 OPCs
at room temperature (left column) and LN2 temperatures (right column). For
each OPC a total of six I-V and P-R curves were measured, with each colored
curve corresponding to a different input power provided by the laser box,
ranging from 0.5 W up to 1.0 W in steps of 100 mW. The I-V curves obtained at
room temperature and LN2 for the OPCs exhibited similar shapes, with the
measured OPC output voltage in LN2 being higher. The P-R curves showed a
slight upward shift, indicating that higher output powers on the OPCs are
achieved in LN2 when compared to room temperature measurements. Additionally,
the P-R curves showed that the output power of the OPCs decreases after the
maximum OPC output power is achieved as the input load is increased, both at
room temperature and in LN2 conditions.

Figure 4: The input power provided by the laser box (X-axis), maximum efficiency obtained in the OPCs (Y-axis in red), and the corresponding load where the maximum OPC output power was achieved (Y-axis in blue) for both room temperature (left) and LN2 measurements (right).  Input Power (W) | Environment Condition | Load (ohms) | Max Efficiency (%)
---|---|---|---
0.5 | room temperature | 126.35 $\pm$ 2.59 | 44.59 $\pm$ 1.19
LN2 | 125.52 $\pm$ 3.52 | 51.78 $\pm$ 1.51
0.6 | room temperature | 102.85 $\pm$ 2.55 | 45.42 $\pm$ 1.25
LN2 | 103.18 $\pm$ 2.90 | 51.56 $\pm$ 1.29
0.7 | room temperature | 87.18 $\pm$ 1.04 | 45.44 $\pm$ 0.80
LN2 | 88.77 $\pm$ 1.89 | 50.80 $\pm$ 1.11
0.8 | room temperature | 75.52 $\pm$ 1.50 | 46.01 $\pm$ 1.12
LN2 | 75.52 $\pm$ 1.71 | 51.66 $\pm$ 1.05
0.9 | room temperature | 67.68 $\pm$ 2.40 | 45.63 $\pm$ 1.86
LN2 | 68.35 $\pm$ 1.92 | 50.29 $\pm$ 1.20
1.0 | room temperature | 58.35 $\pm$ 1.48 | 46.15 $\pm$ 1.55
LN2 | 62.18 $\pm$ 1.44 | 49.31 $\pm$ 0.97
Table 2: The results obtained for the 12 OPCs maximum efficiency achieved for
the six input powers and the corresponding loads. A maximum efficiency of
$\sim$51% was achieved for the 12 OPCs measured in LN2, while $\sim$45% was
achieved for the room temperature measurements.

Figure 4 shows the input power provided by the laser box (X-axis), maximum
efficiency obtained in the OPCs (Y-axis in red), and the corresponding load
where the maximum OPC output power was achieved (Y-axis in blue). The error
bars in the maximum efficiency and the loads corresponds to the the standard
deviation calculated for the 12 OPCs measurements. The OPCs maximum efficiency
does not change significantly, and the load decreases as the input power
provided by the laser box increases for both room temperature (Figure 4, left)
and LN2 measurements (Figure 4, right). Table 2, summarizes the results
obtained for the 12 OPCs maximum efficiency achieved for the six input powers
and the corresponding loads. A maximum efficiency of $\sim$51% was achieved
for the 12 OPCs measured in LN2, while $\sim$45% was achieved for the room
temperature measurements.

Modifications to the OPC improved the cryogenic efficiency with a slight
negative impact to the warm efficiency. Additionally, efficiency is dependent
upon the matching of the laser light wavelength and the OPC semiconductor gap
energy. The cryogenic temperature will cause a physical change in the material
that would require a corresponding laser wavelength change to optimize
efficiency. However, price considerations often require use of off-the-shelf
lasers at the cost of efficiency.

###  2.2 Long Term Test at Cryogenic Temperatures

To assess the feasibility of using OPCs under cryogenic conditions for
extended time periods, a long-term test setup was designed, constructed, and
conducted. Figure 5 shows the long-term test setup which includes a 10-liter
Dewar and a scale to continuously monitor Dewar weight. An optical fiber was
used to connect the laser to the OPC submerged within the Dewar. The voltage
across the OPC was continuously monitored using a voltage sensor connected to
an Arduino Uno for data acquisition. Additionally, sensors were deployed to
monitor laser current and temperature to readout the laser’s performance and
overall test stability over extended durations. The sensitivity of each sensor
used in the long term test setup is determined by the resolution of the
analogue-to-digital conversion (ADC) levels within the Arduino Uno micro-
controller. The schematic illustration of each sensor’s connection to the
Arduino Uno is shown in Figure 6. Data from various sensors are captured using
an Arduino Uno, which operates with a 10-bit resolution providing 1024
discreet levels. Each sensor’s sensitivity is derived from the Arduino Uno’s
ADC precision, involving two discrete levels for each measurement. The voltage
sensor can measure up to 25 V and has a resolution of $\pm$50 mV. The current
sensor is a Hall effect sensor that can record up to 30 A and has a resolution
of $\pm$30 mA. Lastly, the temperature sensor is a 1 k$\Omega$ platinum
resistance temperature detector (RTD) that can measure between -200 ∘C and 200
∘C, and has a resolution of $\pm$0.8∘C. To safeguard against potential power
interruptions, an uninterruptible power supply (UPS) was employed to ensure
continuous operation throughout the months-long test period.

Figure 5: A schematic view of the long term test stand with an OPC submerged
in a 10L Dewar. Figure 6: The voltage of the OPC and the current and
temperature of the laser is monitored by various sensors using an Arduino Uno
microcontroller. Additionally, the data from each sensor was recorded using a
dedicated computer.

The monitoring of the output OPC voltage has shown a steady value of 6.74 V
with an input power of 0.8 W provided by the laser box over a testing period
of 4500 hours, equivalent to 6.1 months of data collection (Figure 7, left).
Additionally, a voltmeter was used to measure the voltage under an 80 $\Omega$
load to monitor the efficiency of the OPC, calculated using Equation 2, during
the last 155 days of the data collection. Figure 7 (right) illustrates that
the OPC efficiency has not changed significantly (less than 2.6%) over the
last 155 days of the data collection, with a mean efficiency of 50.04%.

$\text{Efficiency}(\text{at 80}\Omega)=\frac{P^{OPC}_{\text{output at
80}\Omega}(W)}{P^{Laser}_{input}(W)}=\frac{V^{2}/{80\Omega}}{0.8W}$ (2)

Figure 7: The OPC output voltage has remained stable at 6.74 V throughout the
4500-hour monitoring period, equivalent to 6.1 months (left), and the OPC
efficiency over the last 155 days (right) of data collection shows a mean
efficiency of 50.04%.

##  3 Optical Fibers

Optical fibers find wide applications in underwater power transmission,
telecommunications, rotor blade monitoring systems, current and voltage
sensors in high-voltage environments, and many other areas discussed in
literature [7]. Optical fibers are used in cryogenic applications for tasks
like detecting quenches in high-temperature superconductors [13], infrared
instruments for space [14], and in quantum computing [15] experiments
conducted at very low temperatures. There are two types of optical fibers
commonly used: single-mode [16] and multimode [17]. Several multimode fiber
configurations were studied, but eventually a focus was placed on fibers with
a core diameter of 62.5 $\mu$m. These fibers consist of a core, cladding,
coating, buffer, and outer jacket, as depicted in Figure 8. Table 3, details
the specific fiber configuration, featuring a large cladding, coating, buffer,
and a black PVDF jacket, which substantially minimizes light leakage along the
optical fiber and offers non-contamination at cryogenic temperatures. More
details about the PVDF jacket and its temperature-dependent properties are
discussed in Section 3.5.

Figure 8: A schematic representation detailing the structure of an optical fiber (left). The cross-sectional view (right) shows the core, cladding, and coating layers, as observed through Thorlabs’ optical fiber scope. The green lines outline scratches on the surface of the fiber end.  Item | Description
---|---
Numerical Aperture (NA) | 0.27
Index Profile | GI
Core Diameter | 62.5 $\pm$ 3 $\mu$m
Cladding Diameter | 200 $\pm$ 4 $\mu$m
Coating Diameter | 230 $\pm$ 10 $\mu$m
ETFE Buffer Diameter | 500 $\pm$ 50 $\mu$m
Black PVDF jacket | 1.5 $\pm$ 0.1 mm
Connector Type | FC/PC ceramic
Fiber Cable Length | 40 $\pm$ 0.2 m
Table 3: Characteristics of MH GoPower Company Limited [18] optical fibers
includes a cladding, coating, buffer, black PVDF jacket, connector type and
the cable length.

The process of down-selecting optical fibers suitable for PoF at cryogenic
temperatures took $\sim$ 3 years of research and testing across 5 types of
fibers. Initial efforts on fiber discovery for cryogenic PoF in high energy
physics (HEP) PDS looked at two aspects: Light leakage and physical
degradation. Fibers with non black jackets were quickly determined to not be
suitable. Jacket material was also explored extensively to understand
cryogenic viability, including PVC and silicone jacketed fibers being tested.
Table 4 provides a summary of the optical fiber types that underwent testing,
detailing their specific cladding and jacket materials.

Fiber Core ($\mu$m) | Length (cm) | Jacket Material/Coating Material
---|---|---
62.5 | 4000 | PVDF/Acrylate
62.5 | 2500 | without jacket/Acrylate
105 | 200 | ETFE/Polyimide
105 | 100 | PFA/Acrylate
105 | 100 | PFA/Polyimide
Table 4: List of different optical fibers tested with their varying
specifications.

###  3.1 Quality Assurance (QA/QC) Procedures

In order to ensure that optical fibers are suitable for their use in the PoF
technology, a three step quality assurance procedure was developed:

1\. Visual inspection: A visual inspection is needed to look for any visible
physical damage of the fiber, fiber ends (core and cladding) and jacket
surface. Since PoF technology uses a class-4 laser [19] as an optical power
source, damages like scratches, cracks, or chips can affect the light
transmission resulting in power loss. Scratches and cracks in the optical
fiber core not only impact power transmission, but it can also absorb a
significant amount of light, leading to hot spots in certain areas that could
potentially damage the fiber. Visual inspection makes it possible to detect
contaminants such as dust, dirt or oils that may accumulate on the fiber ends
during handling or cleaning. Figure 9 (left) illustrates an example of a fiber
end damaged by burns resulting from dust accumulation when the proper cleaning
procedure was not followed.

Figure 9: Left: Optical fiber end burned due to inadequate cleaning. Right:
Optical fiber end that passed successful visual inspection, no damage is
observed along the core and cladding. Optical fiber inspection was conducted
using a Thorlabs’ optical fiber scope.

2\. Light transmission: This test serves to check the light propagation along
the fiber as a function of the optical fiber’s length in room temperature.
Figure 10 (left) shows a basic light transmission setup using an LED of 810 nm
and a photo-diode power sensor. The light transmission test quantifies the
ratio of light power loss between a reference fiber and the fiber under
examination as shown in Equation 3. The ratio between the input power measured
at the reference fiber and the output power measured for each fiber under test
as shown in Figure 10 (right). A total of 87 fibers with a 62.5 $\mu$m core
and 40-meter length were tested obtaining a mean value for the ratio of 0.82
$\pm$ 0.04.

$\text{ratio}=\frac{\text{P}^{\text{Fiber to
test}}_{output}(nW)}{\text{P}^{\text{Reference fiber}}_{input}(nW)}$ (3)

Although this test verifies light transmission along the fiber, two of the
eighty seven fibers exhibited significant power loss when used to transmit
optical power O(100 mW) from a laser source, even though they were measured to
have a power ratio greater than 80%. Therefore, conducting an optical power
test is necessary to determine the suitability of the optical fibers for PoF
transmission.

Figure 10: Light transmission test stand (left) with results of 87 optical
fibers (right)

3\. Optical Power Performance: Since optical fibers are primarily used for
signal transmission, this does not guarantee their suitability for optical
power transmission on the order of hundreds of mWs. Conducting an optical
power performance test at room temperature involves monitoring the optical
power transmission over an extended period of time using an integrating sphere
photo-diode power sensor, as shown in Figure 11(a). For this study, two sets
of fibers consisting of 24 and 64 fibers (62.5$\mu$m core and 40 m length)
were tested for optical power performance under two laser input powers. Figure
11(b) shows the power performance test conducted over 24 fibers at an input
power of 0.4 W and Figure 11(c) displays the power performance of 64 fibers
for 25 minutes using a laser input power of 0.6 W. Each colored line in both
figures corresponds to the power performance of an individual fiber. A mean
power performance of 362 $\pm$ 7 mW at an input power of 0.4W and 553 $\pm$ 15
mW at 0.6W were measured, with a mean power loss of $\sim$38 mW and $\sim$47
mW, respectively.

(a) Optical power performance test stand

(b) Optical power performance test at 0.4 W for 24 fibers

(c) Optical power performance test at 0.6 W for 64 fibers

Figure 11: Power test for optical fibers to monitor optical power performance
using PoF technology at room temperature.

The described three-step quality assurance procedure made it possible to
measure a power loss of $\sim$ 10% with respect to a laser power input of 0.4
W and $\sim$ 9% with respect to a laser power input of 0.6 W at room
temperature. However, applications in harsh conditions such as cryogenic
temperatures present challenges not only for the optical fiber integrity, but
also for optical power transmission (see Sections 3.2, 3.3, 3.4 and 3.5).

###  3.2 Thermal Cycling Test

A thermal cycling test makes it possible to evaluate the optical fibers under
cryogenic conditions. This test subjects the optical fibers to controlled
thermal stress through repetitive cycles of warming and cooling using liquid
nitrogen. Figure 12 (left) shows the thermal stress test stand designed and
constructed for this study, where optical fibers underwent thermal cycling.

Figure 12: Left: A test stand for thermal cycling testing of a set of eight
optical fibers, each with a 62.5 $\mu$m core and a length of 40 m. The fibers
were wrapped to a spool of radius of $\sim$ 9 cm. Right: Example of the first
45 cycles recorded during the thermal cycling test for the set of 8 optical
fibers. Each thermal cycle had a duration of 20 minutes, distributed as
submerging the fibers in LN2 for 9 minutes, raising them for a period of 1
minute, holding the fibers at room temperature for 9 minutes, and then
lowering the fibers back into the LN2 for 1 minute, where a temperature
difference of $\sim$83oC was recorded.

Each cycle had a duration of 20 minutes, distributed as submerging the fibers
in LN2 for 9 minutes, raising them out of the LN2 for a period of 1 minute,
holding the fibers at room temperature for 9 minutes, and then lowering the
fibers back into the LN2 for 1 minute (see Figure 12, right). The temperature
profile of the cycle is monitored by a temperature sensor.

Two sets of optical fibers are evaluated under thermal cycling tests at
cryogenic conditions. One set with eight optical fibers undergoes a single
thermal cycling run of $\sim$ 129 cycles, and another set with five optical
fibers undergoes $\sim$ 72 cycles during three thermal cycling runs.

A total of eight optical fibers, each with a 62.5$\mu$m core and a length of
40 m were inserted into a black Polytetrafluoroethylene (PTFE) tube and
wrapped around a spool with a radius of $\sim$ 9 cm, as depicted in Figure 12
(left). The PTFE tube has a slit along its entire length, allowing the LN2 to
enter and circulate within. Light transmission tests were conducted both
before and after the thermal cycle test. Table 5 presents the ratio between
the input and output power (see Section 3.1) before and after conducting the
thermal cycling test. These results showed that the ratio before and after
conducting the 129 thermal cycles are similar and within the statistical
errors of the measurement.

| ratio = Poutput(nW)/Pinput(nW)
---|---
Before 129 cycles | 0.78 $\pm$ 0.02
After 129 cycles | 0.78 $\pm$ 0.01
Table 5: Light transmission tests conducted on a set of eight optical fibers
before and after 129 thermal cycles. Details of the light transmission test
are provided in Section 3.1.

A total of five new optical fibers with a core diameter of 62.5 $\mu$m and a
length of 40 m were also tested and encased within a protective polyester
sleeve and then inserted into a black PTFE tube. Each optical fiber underwent
three thermal cycling runs, each run consisting of $\sim$ 72 cycles, where the
fibers were spooled at a radius of $\sim$ 10 cm (see Figure 13, left). Light
transmission tests were conducted both before and after the thermal cycling
tests. Table 6, summarizes the light transmission results over the three
thermal cycling runs, and again no noticeable difference between the three
measurements was observed. In addition, an optical power performance test over
the five optical fibers was done before and after each thermal cycling run
using a laser input power of 0.8 W. Figure 13 (right) shows the results of the
optical power test measured for each fiber before and after each thermal
cycling run. No significant deviation was found in the optical fiber
performance after each test.

Figure 13: Left: Thermal cycling test stand for five optical fibers under three thermal cycle runs. Right: Optical power performance test for a set of five fibers before and after the three thermal cycles. Details of the optical power performance test procedure are provided in Section 3.1. | ratio = Poutput(nW)/Pinput(nW)
---|---
Before cycling | 0.83 $\pm$ 0.05
After first 72 cycling run | 0.79 $\pm$ 0.04
After second 72 cycling run | 0.78 $\pm$ 0.05
After third 72 cycling run | 0.80 $\pm$ 0.06
Table 6: Light transmission test conducted for five optical fibers before and
after three thermal cycling runs. Details of the light transmission test
procedure is provided in Section 3.1.

###  3.3 Power Loss at Cryogenic Temperatures

As described in Section 3.5, the optical fibers studied in this paper have a
Polyvinylidene fluoride (PVDF) black jacket. The PVDF jacket is a UV light
resistant plastic that offers chemical and radiation resistance, a high
relative melting point, high mechanical strength, non-flammability, very low
thermal conductivity, and physiological inertness [20]. The exposure of PVDF
to low temperatures [21] can induce compression forces along the fiber
(thermal contraction), contributing to microbending along its surface.
Microbending is caused when external forces are applied to the jacket surface
of an optical fiber [22]. The application of external forces to the jacket
surface results in deformation of the cladding and core, as shown in Figure 14
(left). While core-cladding deformation may not be visible to the naked eye,
it alters the angle of light reflection within the fiber. Consequently, the
transmitted light rays are redirected at angles that prevent further internal
reflection, causing the light rays to be lost within the cladding.

Figure 14: Left: The schematic view of applying external force to the jacket
surface resulting in the deformation of the cladding and core of a fiber.
Right: Test stand to study the power loss due to PVDF jacket compression as a
function of temperature, one optical fiber with a diameter of 62.5 $\mu$m and
a length of 40 m was tested inside a cryostat with $\sim$ 3 L of LN2. The
fiber was wrapped around a spool with a radius of $\sim$14.5 cm. Figure 15:
The optical power performance as a function of the temperature. An optical
power performance of 266.71 $\pm$ 2.00 mW was recorded at room temperature and
is depicted in the plot as a dashed red line. The blue line represents the
power performance after submerging the fiber in LN2. At the lowest temperature
recorded (-196 ∘C), a power loss of $\sim$ 39 mW was measured.

To study the power loss due to PVDF jacket compression as a function of
temperature, one optical fiber with a diameter of 62.5 $\mu$m and a length of
40 m was tested inside a cryostat with $\sim$ 3 L of LN2. The fiber was
wrapped around a spool with a radius of $\sim$14.5 cm, as illustrated in
Figure 14 (right). Initially, an optical power performance test was conducted
at room temperature. Subsequently, $\sim$ 27 m of the fiber was submerged in
LN2 until it completely evaporated, and the optical power was measured as a
function of temperature. Throughout the test, temperature variations were
monitored using a sensor attached to the fiber.

Utilizing an input power of 0.3W from a laser box, Figure 15 illustrates the
optical power performance as a function of the temperature. An optical power
performance of 266.71 $\pm$ 2.00 mW was recorded at room temperature and is
depicted in the plot as a dashed red line. The blue line represents the power
performance subsequent to submerging the fiber in LN2. The increase in
temperature observed occurred over the 10 hours that it took for the complete
evaporation of LN2 in the cryostat. At the lowest temperature recorded (-196
∘C) a power loss of $\sim$ 39 mW was measured. Taking into consideration that
$\sim$ 27 m of fiber was submerged in LN2, the optical fiber presented a power
loss of $\sim$ 1.44 mW/m.

###  3.4 Bending Radius Testing

In order to understand the performance of optical fibers used in a PoF system,
it is important to characterize the optical power loss as a function of the
fiber length for different operating conditions. Two main contributors to
optical power loss along the fiber are discussed here for room and cryogenic
temperatures.

First, optical power loss arises from cooling the fibers to cryogenic
temperatures. This occurs because the fibers are encased in an outermost
jacket layer of PVDF plastic, which contracts when exposed to cold
temperatures. This contraction around the fragile inner layers can lead to
microbending along the fiber, resulting in increased light reflection and
absorption during fiber transmission. More details about microbending can be
found in Section 3.3.

The second significant source of power loss occurs due to fiber bending. This
loss can be attributed to the operating principle of optical fibers,
specifically the concept of total internal reflection. The core and cladding
layers of the fiber have different indices of refraction, causing light to be
reflected at the fiber wall upon hitting it, a process that repeats along the
fiber’s length. When the fiber is bent, however, some light strikes the wall
at an angle less than the critical angle of the fiber/cladding boundary,
allowing it to be transmitted through rather than be reflected. The amount of
power loss due to this effect depends on the bending radius and the length of
fiber spooled at that radius.

Due to the limited availability of Dewar shapes and volumes in our laboratory,
it was not possible for the test presented in this paper to submerge straight
lengths of fiber. Instead, the thermal contraction and bending radius power
loss are characterized together. For this test, five 3D printed spools of
radius 7.5 cm were made to hold the fibers. The five fibers were wrapped
around the spools for varying number of loop counts, where these loop counts
correspond to different lengths of fiber being bent at the 7.5 cm bending
radius. For the 7.5 cm bending radius and each loop count, the power loss
measurement is taken at room temperature for the five optical fibers. Later,
the five fibers are then submerged in LN2, and again the optical power loss is
measured. Finally, the subtraction between the optical power loss at room
temperature and in LN2 is calculated, thereby giving the power loss due to
thermal contraction and bending of the fibers.

Figure 16: Left: The bending radius test setup at room temperature. Right: The
laser is connected to a reference fiber, which is also connected to the looped
test fiber (10, 20, 30, 40, or 50 loops) at a 7.5 cm radius.

To obtain the optical power loss measurements, a reference fiber was connected
to a laser box (see Figure 16, left), then each of the five optical fibers
power performance was measured separately using a power sensor (see Figure 16,
right). Figure 16 presents the power measurements taken at 10, 20, 30, 40, and
50 loops (corresponding to fiber lengths of 4.71, 9.42, 14.13, 18.84, and
23.55 meters respectively) at room temperature. The power loss at room
temperature was calculated by subtracting the power measurements of each fiber
from the value obtained with the reference fiber (see Equation 4). The
uncertainties for the optical power measurements ($\delta P$) are primarily
due to the laser power fluctuations (see Section 4).

$(P_{loss}\pm\delta P)_{room\;temperatures}=P_{\text{tested
fiber}}^{\text{room}}-P_{\text{reference fiber}}$ (4)

Subsequently, the fibers were then immersed in LN2, as shown in Figure 17. The
same procedure used for the room temperature measurements was followed for the
LN2 measurements, where power measurements were again taken at loop counts of
10, 20, 30, 40, and 50 (corresponding to fiber lengths of 4.71, 9.42, 14.13,
18.84, and 23.55 meters respectively). The power loss in LN2 was calculated by
subtracting the power measurements of each test fiber from the value obtained
with the respective reference fiber (see Equation 5).

$(P_{loss}\pm\delta P)_{LN2}=P_{\text{tested fiber}}^{LN2}-P_{\text{reference
fiber}}$ (5)

Figure 17: The bending radius test setup in LN2, which follows the same setup
as in room temperature, but now the test fiber at the respective loop count is
submerged in a dewar filled with LN2.

To find the power loss for the optical fibers due to being exposed to
cryogenic temperatures, it is necessary to first establish the baseline power
of the fiber at normal operating conditions, which is why the measurements at
room temperature were taken as well. In order to calculate the power loss due
to thermal contraction, the formula shown in Equation 6 was used, where the
$P_{loss}$ in LN2 and at room temperatures are subtracted for each power and
loop count.

$P_{loss}=(P_{loss}\pm\delta P)_{LN2}-(P_{loss}\pm\delta
P)_{room\;temperatures}$ (6)

Input Power | Power loss per meter | Power loss per meter
---|---|---
(mW) | (measurement) (mW/m) | (linear fit) (mW/m)
500 | 1.91 $\pm$ 0.64 | 2.16 $\pm$ 0.40
600 | 2.43 $\pm$ 0.66 | 2.57 $\pm$ 0.39
800 | 3.37 $\pm$ 0.80 | 3.58 $\pm$ 0.57
900 | 3.55 $\pm$ 0.82 | 4.67 $\pm$ 0.59
Table 7: Power loss per meter measurement for five optical fibers using four
input powers provided by the laser box. The five optical fibers were wrapped
around a bending radius of $\sim$ 7.5 cm.

Figure 18 (left) shows the power loss for input powers of 0.5 W, 0.6 W, 0.8 W,
and 0.9 W, with the X-axis representing the increasing length of fiber looped
at the 7.5 cm bending radius. The error bars in Figure 18 include the sum in
quadrature of the statistical uncertainty (limited by testing of five optical
fibers) and the systematic uncertainty associated with the laser power
fluctuations. The error on the optical fiber length submerged was $\sim$ 20
cm.

Figure 18: Power loss versus length plot for 7.5 cm bending radius for 10, 20,
30, 40 and 50 loops at powers 0.5 W, 0.6 W, 0.8 W, and 0.9 W (left), as well
as the results shown as input power vs linear fit slope (right).

The positive slopes present in the plot indicate that as the loop count
increases and more fiber is submerged in LN2, the power loss also increases at
a linear rate. In addition, a linear dependence of the optical power loss per
meter as a function of the power provided by the laser was measured to be 5.90
$\pm$ 1.57 $m^{-1}$ (see Figure 18, right). Table 7, summarizes the measured
power loss per meter and the corresponding linear fit parameters obtained for
0.5W, 0.6 W, 0.8 W, and 0.9 W laser powers used in the bending radius test as
shown in Figure 18.

###  3.5 Power Loss Measurement in an Optical Fiber Without Jacket

As demonstrated in Section 3.3, the fiber with a PVDF jacket experiences power
loss at cryogenic temperatures due to thermal contraction. A test was carried
out to measure the power loss of an optical fiber without a jacket. This test
measured the power loss exhibited in LN2 by two optical fibers: one with a
PVDF jacket (62.5 $\mu$m core diameter and 40 m length) and the other optical
fiber without the jacket (62.5 $\mu$m core diameter and 25 m length). Both
optical fibers were wrapped on a spool with radius $\sim$ 10 cm, as shown in
Figure 19 (left).

Figure 19: Left: Diagram showing the experimental setup for the fiber with and
without a jacket wrapped on a spool with a radius of $\sim$ 10 cm. Right: The
setup shows fiber 1, fiber with/without jacket (fiber 2) and fiber 3 connected
to the laser and power sensor.

The setup consisted of a reference fiber (Fiber 1) initially connected to a
laser box. Next, the fiber being tested with or without jacket (Fiber 2) was
connected, and finally, a third fiber (Fiber 3) was introduced in the setup.
All interconnections between fibers were established using FC-FC junctions as
illustrated in Figure 19 (right). A power performance in room and LN2
temperatures were recorded using a power sensor. For the LN2 measurements, 25
m of the jacketed fiber was introduced into the LN2 to allow for a direct
comparison with the 25 m of the fiber without a jacket.

To measure the power loss of the fibers with and without jacket under
cryogenic temperatures, Equation 6 was used, where the power loss is
calculated by subtracting the power loss at LN2 and room temperatures,
following a procedure similar to that outlined in Section 3.4. Table 8,
summarizes the power loss per meter calculated for both fibers with and
without jackets. The results show that the power loss per meter for fibers
with a PVDF jacket is approximately three times higher than that for fibers
without a jacket, for both 0.5 W and 0.8 W input powers.

Fiber type | Input Power (mW) | Power loss per meter (mW/m)
---|---|---
With Jacket | 500 | 1.74 $\pm$ 0.28
Without Jacket | 0.56 $\pm$ 0.21
With Jacket | 800 | 3.14 $\pm$ 0.51
Without Jacket | 1.06 $\pm$ 0.48
Table 8: Measurement of power loss per meter for optical fibers with and
without jacket in LN2 using 0.5W and 0.8W input powers provided by the laser
box.

###  3.6 Power Loss due to Polishing

The removal of imperfections such as epoxy and scratches from the optical
connector’s ferrule tip using lapping (polishing) film can have an impact in
the quality of fiber end surface. Poor polishing or overpolishing of the
optical fiber tip could result in a higher optical power loss. A preliminary
test to study the effect of hand-polishing on the power performance at room
temperature for a new fiber with a 62.5 $\mu$m core, 40 meters in length, and
with ceramic ferrule tips is performed. First, one end of the fiber being
tested (new fiber) is connected to a reference fiber using a FC-FC joint,
while the other end is connected to a power sensor (see Figure 20, left).

Figure 20: Left: Optical fiber end polishing was performed by hand using a
polishing disk and lapping film with 0.3 $\mu$m. Right: The ratio between
power performance measurements before and after hand-polishing.

Afterward, the fiber was disconnected from the reference fiber, hand-polished
with a 0.3 $\mu$m lapping film for less than one minute, and then connected
again. Finally, a power transmission test was performed before and after hand-
polishing. Figure 20 (right) shows the ratio between power performance
measurements before and after hand-polishing. Based on the power ratio, a
noticeable decrease in performance is observed after hand-polishing the
optical fibers. It is important to mention that hand-polishing can be affected
by the uniform pressure, speed, and time applied by the person performing the
polishing.

###  3.7 Optical Fiber Tensile Strength Test

Tensile strength refers to the maximum stress that a material can bear before
breaking when it is allowed to be stretched or pulled. Figure 21 show a test
setup built to assess the tensile strength of an optical fiber under varying
environmental conditions, including room and LN2 temperatures. The fiber was
suspended at a length of one meter using two anchor points, as shown in Figure
21. The two anchor points were secured in a customized support designed and
built for this test. The choice of one meter of optical fiber was made to
minimize the impact of power losses associated with exposure to LN2
temperatures, as described in Section 3.5.

Figure 21: Tensile strength test setup for room and LN2 conditions. The fiber
under test is secured with two anchor points to ensure $\sim$ 1 m length
during the strength test. Power sensor measure the optical power for varying
tensile strengths.

One anchor point of the fiber is secured at the top of the structure, and the
second anchor point is attached to a weight support. During the test multiple
weights were added while monitoring the optical power performance until the
optical fiber reaches its breaking point. Figure 22 (left) shows the results
of increasing weight on the optical power performance at room temperature. The
optical fiber breaking point was found to be approximately 5.5 kg for room
temperature. The fiber was inspected after the strength test with a digital
microscope, as shown in Figure 22 (right), revealing fractures in both the
cladding and core of the optical fiber.

Figure 22: Left: Tensile strength test results conducted at room temperature
with an input power of 0.8W using fibers with a 62.5 $\mu$m core. The colored
lines correspond to the power performance of a fiber during strength testing
with 3.5 kg, 4 kg, 4.5 kg, 5 kg and 5.5 kg compared with applying no weights.
Right: Fiber inspection after strength tests at room temperature.

Figure 23: Tensile strength test conducted in LN2 with an input power of 0.8W
using fibers with a 62.5 $\mu$m core. Upper left: Power performance a fiber
during strength testing with six different weights. Upper right: Power
performance of a fiber during strength testing with a $\sim$ 7 kg. Lower left:
Power performance of a fiber during strength testing with a $\sim$ 8 kg. Lower
right: Fiber inspection after strength tests in LN2.

A similar procedure to that conducted at room temperature was carried out for
LN2. The optical fiber was submerged to a depth of $\sim$ 1 m, as shown in
Figure 21. Figure 23 (upper left) shows the optical power performance of a
fiber when subject to tensile strengths using weights of 4 kg, 4.5 kg, 5 kg, 6
kg, and 7 kg and an input power of 0.8 W provided by the laser box. At 7 kg,
it was discovered that the jacket and cladding broke, and the core had a
fracture that reduced the power transmission (green line). A second test is
conducted using a different fiber and placing $\sim$ 7 kg to verify the fiber
fracture breaking point found in the first test. The optical power performance
is shown in Figure 23 (upper right) where the blue line represents the power
decreases over time until the fiber reached its breaking point at 30 min. A
third test was conducted using $\sim$ 8 kg and the fiber broke after $\sim$ 6
min as shown in Figure 23 (lower left).

The fibers used in each test were inspected with a digital microscope, as
shown in Figure 23 (lower right), revealing fractures in both the cladding and
core of the optical fiber. The volume of the LN2 displaced by the $\sim$ 7 kg
was $\sim$ 1.5 L, corresponding to approximately 1.2 kg of LN2. Considering
the buoyancy force, the apparent weight is equivalent to $\sim$ 5.8 kg.

##  4 PoF Laser

The Broadcom laser [19] used for the measurements reported in this paper
operates at 808 nm and can be directly powered from the power supply. The
maximum lasing efficiency is plateaued at 35% as shown in Figure 24. A
commercial regulating circuit is used in prototypes for fast turnaround. A
custom-designed control circuit is planned to be used for DUNE FD2 to simplify
the circuitry and meet DUNE safety requirements.

Figure 24: Lasing efficiency vs Supplied Power.

The PoF laser box contains a gallium arsenide (GaAs) 808-nm laser transmitter
module that transmits optical power through an optical fiber to the OPCs. This
high-power laser module has a 62.5$\mu$m pigtail optical fiber and FC/PC
ceramic ferrules. The laser-transmitter was designed for PoF applications by
Broadcom [19] and provides up to 2 W of power at room temperature. The laser
box is assembled using a commercial Broadcom laser controller. Laser
transmitters are widely used in multiple applications such as
telecommunications, space, imaging, and more [19]. Fluctuations in the output
power provided by the lasers needed to be studied in order to understand if
these variations could affect the stability of the OPC performance.

In order to study the laser-transmitter fluctuations, a test was conducted to
monitor the power performance of the laser over time using a sphere photo
diode sensor. Figure 26 (left) show the setup used to measure the laser power
performance at room temperature. The setup includes a laser box with a 62.5
$\mu$m pigtail connected to a reference fiber, which is then connected to the
power sensor. Figure 25 (left) and (right) displays the power performance of a
single GaAs laser transmitter during 240 minutes of operation, with input
powers of 0.8W and 0.5W, respectively. In both cases, the laser transmitter
stabilized after 25 minutes of operation.

Figure 25: Laser power performance is depicted for a laser transmitter with a
62.5 $\mu$m core and an input power of 0.8 W (left) and 0.5 W (right) over a
duration of 240 minutes, with power fluctuations of about 24 mW (left) and 13
mW (right) observed at room temperature.

Figure 26: Left: The laser power performance test stand includes a laser box
with a 62.5 $\mu$m pigtail connected to a reference fiber, which is then
connected to the power sensor. Right: The power fluctuations due to regular
laser operation as a function of different laser-transmitter powers with
colored lines representing different laser powers, where $\delta$ defines the
difference between upper and lower limit of power.

Figure 26 (right) shows the power fluctuations due to standard laser operation
as a function of different laser-transmitter powers, with colored lines
representing different laser powers. Other factors can contribute to
fluctuations in the laser output power, such as variations in laser
temperature and instability of the laser current. Section 2.2 describes the
setup built to monitor the operation of a single GaAs laser-transmitter.
Figure 27 presents continuous measurements of laser temperature (left) and
laser current (right) over a period of 6.1 months.

Figure 27: Long term measurements of laser temperature (left) and laser
current (right) over a period of 6.1 months. The laser temperature (green
color band) and laser current (pink color band) variations remained below
2.5°C and 225 mA, respectively.

##  5 Electrical Power Delivery

PoF technology enables electrical power supply to devices in harsh
environments such as high voltage and cryogenic conditions. Sections 2 and 3
present measurements of the optical-to-electrical power conversion efficiency
of the OPCs and power loss in optical fibers. These measurements could make it
possible to approximately estimate the maximum electrical power delivered by
an OPC in a PoF system when using a specific length of optical fibers (see
Table 4) and a certain number of OPCs, depending on the application’s
requirement for supplying power to electronic devices operating at room and
LN2 temperatures.

Figure 28: The measurement of the maximum output power delivered by an OPC (mW) for varying laser inputs at both room temperature and LN2 conditions, shown in the left and right figures, respectively. Additionally, a linear fit model is applied to the data. Input Power (mW) | Environment | Length (m) | Maximum Output
---|---|---|---
Condition | Power Delivered
| by the OPC (mW)
500 $\pm$ 12.54 | room temperature | 40.00 $\pm$ 0.2 | 205.78 $\pm$ 5.98
LN2 | 4.71 $\pm$ 0.2 | 234.94 $\pm$ 7.11
LN2 | 23.55 $\pm$ 0.2 | 214.67 $\pm$ 6.65
600 $\pm$ 12.63 | room temperature | 40.00 $\pm$ 0.2 | 250.82 $\pm$ 6.07
LN2 | 4.71 $\pm$ 0.2 | 280.37 $\pm$ 6.86
LN2 | 23.55 $\pm$ 0.2 | 255.33 $\pm$ 6.78
800 $\pm$ 13.97 | room temperature | 40.00 $\pm$ 0.2 | 340.32 $\pm$ 7.05
LN2 | 4.71 $\pm$ 0.2 | 374.64 $\pm$ 7.79
LN2 | 23.55 $\pm$ 0.2 | 340.11 $\pm$ 7.76
900 $\pm$ 21.87 | room temperature | 40.00 $\pm$ 0.2 | 376.75 $\pm$ 10.32
LN2 | 4.71 $\pm$ 0.2 | 407.81 $\pm$ 11.46
LN2 | 23.55 $\pm$ 0.2 | 366.95 $\pm$ 11.43
Table 9: Summary of the maximum output power delivered by the OPC at both room
and LN2 temperatures for four laser input powers provided by the laser box.

Figure 28 (left) and (right) show the maximum electrical output power
delivered by the OPC as a function of laser input powers of 0.5W, 0.6W, 0.8W,
and 0.9W at room and LN2 temperatures, respectively. For the LN2 measurements,
the maximum output power delivered by the OPC is measured for shorter (4.71 m)
and longer (23.55 m) optical fiber lengths submerged in LN2. For example, in a
scenario where a new PoF application in a cryogenic environment demands $\sim$
350 mW of electrical power (see dashed red line in Figure 28, right) one
option could be to require a laser input power of $\sim$ 850 mW with an
optical fiber length of $\sim$23 m. Table 9 summarizes the maximum output
power delivered by the OPC at both room and LN2 temperatures for a laser input
power of 0.5 W, 0.6 W, 0.8 W, and 0.9 W and different optical fiber lengths.

##  6 Mitigation of Light Leakage in PoF Systems for Photodetection
Applications in HEP

The 808 nm IR light escaping from the PoF system could change the SiPM signal
waveform baseline if there is no shielding protection in applications related
to photodetectors such as those in DUNE [5]. The PoF IR light noise sources
include the OPC, the optical fibers as well as the commonly used FC-FC fiber
connectors. The light leakage reduces the signal-to-noise ratio of the photon
detection system and the overall data quality. It is therefore critical to
understand the noise sources and develop mitigation solutions at the lab (see
Figure 29).

Figure 29: OPC (top left) light noise reaches a few photo-electrons per 10
$\mu$s when sub-$\mu$W laser power is delivered. The single PE amplitude is
defined as 20 mV above the waveform baseline as displayed in the horizontal
axis. A 10 cm long fiber (top right) light noise level is low. A 40m-long
spool of fiber (bottom left) light noise level is more than a few PE when PoF
is running at normal operation power. An FC-FC connector (bottom right) has a
few PE per 10$\mu$s noise level. For the bottom two plots, a 40m-long
62.5$\mu$m fiber and an extra FC-FC connector are added to the measurement
setup. In the legends, the powers specified outside (in) the brackets are the
actual measured power before (after) the FC-FC connector and the 40m-long
black fiber. Providing these two measured powers is for readers’ reference
since there is non-negligible power loss for a 40m-long fiber in LAr as
described in previous sections. In the bottom left plot, when measuring the
40m-long fiber spool, the SiPM board needs to be taken outside the sealed
black box because the fiber can not fit inside. As a result, the background
noise is higher than in this test as shown in the inset plot. Nevertheless,
the metric in the main plot is extra photo-electrons counts on top of this
background noise from the environment.

The light noise level is defined as the average number of single photoelectron
(PE) signals detected per 10 $\mu$s. As an example, the expected background
contribution from the Argon-39 (dominant) and other radiological background is
200 kHz in the DUNE photodetector application [5]. This translates to 2
PE/10$\mu$s and also sets the target noise level. Any mitigation solution
needs to reach a noise level below 2 PE per 10$\mu$s so that it will not be
more dominant than the radiological background. The noise level of each
component from the PoF system is subsequently characterized in the lab. A
Hamamatsu 4$\times$5-array SiPM with a breakdown voltage of 36V in LAr is used
to detect noise IR photons from the PoF system components. The SiPM signal is
read out at room temperature by a custom-designed analog readout board. A
Keysight oscilloscope is used for data acquisition at 250 Msps (4ns time
resolution), with a 60 MHz low pass filter applied. The SNR of this readout
system reaches 5.7 and is good for the characterization of light noise.

To measure the light leak from OPCs, a commercial GaAs OPC is fixed at 7 cm
away from the center of the 4$\times$5-array SiPM, with the OPC pins pointing
perpendicularly to the SiPM array. This distance is smaller than that in any
prototypes or DUNE FD2 where the OPCs are much further away from the
photosensors. The OPC-SiPM system is then placed in a well-sealed black box
that shields environmental light noise before being put in a Dewar filled with
LAr. The SiPM detects a few PE per 10$\mu$s noise from OPC with PoF operating
at sub-$\mu$W (red and green) as shown in Figure 29. At 1 $\mu$W laser power,
light leaks already start to produce too much current for SiPM. At normal PoF
operating power of hundreds of mW, an OPC light leak (Figure 30) produces too
much current in SiPM that the SiPM signal waveform baseline oscillates. The
SiPMs bias voltage dropped below breakdown and is not able to detect SPEs. In
this case, counting photoelectrons is not meaningful.

Figure 30: Light leakage (bright spots) from the pins of OPCs seen by an
infrared viewer when 400 mW of laser power is delivered using optical fibers.

The light noise from a 10 cm-long 62.5 $\mu$m black fiber is also studied. A
short straight fiber is run parallel to the 4$\times$5-array SiPM at a
distance of 3 cm, again smaller than the distance in any realistic prototypes
where fibers are much further away from photosensors. The fiber-SiPM system is
similarly placed in the black box inside a Dewar of LAr. The light noise from
this 10 cm long fiber is 10-2 PE with PoF running at its normal operating
power of hundreds of mW as shown in Figure 29. A further test of a 40m-long
spool of the same 62.5 $\mu$m black jacket fiber shows a much higher light
noise level as expected. The noise reaches a few PE already when PoF is
running at tens of mW (red and green curves in Figure 29). Starting at 59 mW
PoF power, light noise is too much for SiPM to work normally and the count of
PEs is not meaningful anymore.

Figure 31: Several investigated solutions to mitigate the light noise from the
PoF system.

A similar light noise test is performed on a typical FC-FC connector when the
PoF system is on. The FC-FC connector runs parallel to the SiPM array at a
distance of 3 cm inside the black box. Light noise from a typical rectangular
FC-FC connector is at a few PE per 10$\mu$s level as shown in Figure 29. Such
connectors only appear in small prototypes and will not be used in DUNE FD2.

In summary, the most critical light noise, beyond the single photoelectron
level, comes from the OPC output pins and the FC-FC connector that interfaces
the optical fiber and the OPCs. Investigations of several solutions (Figure
31) show that covering both OPC and the OPC FC-FC connector with adequate
electronic-grade silicone paste and a minimum of 1mm-thick copper tape can
easily bring light noise down to the level of $10^{-2}$ SPE per 10 $\mu$s,
fulfilling the DUNE photodetector application requirement. This measure could
be further strengthened with a much thicker metallic box to house the entire
readout including silicone potted PoF OPCs and FC-FC connectors. In addition,
all fibers can be shielded within black PTFE tubes to further reduce the light
noise. These measures are demonstrated to be effective in the DUNE
application.

##  7 Application of PoF Technology at CERN Cold Box Test Facility

The CERN Neutrino Platform’s cold box test facility [23] enables the
development and prototyping of next-generation neutrino detectors, such as the
DUNE prototypes (e.g., ProtoDUNE Vertical Drift [5]). The cold box test
facility at CERN has served as a platform for the successful integration of
PoF technology to power X-ARAPUCAs photodetectors located in high-voltage and
cryogenic environments, providing validation prior to their ProtoDUNE VD
deployment. The cold box cryostat has dimensions of (3 $\times$ 3 $\times$ 1)
m3 and is filled with LAr, where the photodetectors are situated on a cathode
surface (see Figure 32). The distance between the anode and cathode plane is
$\sim$ 25 cm, with high voltage (10 kV) applied [5], allowing for vertical
drift. The cold box tests used SiPM based X-ARAPUCAs photodetector modules
read-out by DCEM boards. The DCEM readout board is powered by PoF while the
SiPM electrical signals are converted to optical signals and transmitted
through a Signal-over-Fiber (SoF) laser diode through an optical fiber.

Figure 32: Left: Top view of the cold box facility at CERN, showing two
X-ARAPUCA photodetector prototypes installed on the cathode surface and
powered by PoF technology. Right: Photo-electron charge distribution for a
X-ARAPUCA photodetector located at the cathode surface and powered by PoF
technology. The distribution is fitted by a multi-Gaussian function, the
parameters presented in the figure are from the three first Gaussian. The
first set of parameters (Ab, $\mu_{b}$, and $\sigma_{b}$) corresponds to the
baseline Gaussian fit, while the second (A1, $\mu_{1}$, and $\sigma_{1}$) and
third (A2, $\mu_{2}$, and $\sigma_{2}$) sets correspond to the distribution of
one and two photo-electrons. The figures are obtained and adapted from the
DUNE FD2 TDR [5].

The PoF configuration of the X-ARAPUCA photodetector electronics utilizes two
GaAs OPCs similar to the ones described in Section 2, each supplying $\sim$
6.5V when operated in LAr. These units receive power through a 62.5$\mu$m core
optical fiber (see Table 4) and uses an 808 nm, 2-Watt laser transmitter
(Section 4). The analog signal is also transmitted using optical fibers.
Figure 32 (right) (taken from [5] ) shows the charge spectrum using LED pulses
from the calibration system, with a Signal-to-Noise ratio (SNR) of 5.9 [5].
This demonstration marked the first successful instance of powering
photodetectors at high voltage and in a cryogenic environment using PoF
technology in HEP.

##  8 Conclusions

In this paper, the first demonstration of PoF technology for cryogenic use and
its first ever application in a HEP detector system is presented. Detailed
characterization of the components of a PoF system, such as optical fibers and
OPCs, and their operation at both room and LN2 temperatures, is also provided.
A study of optical power loss at cryogenic temperatures for multiple optical
fiber lengths submerged in LN2 showed a linear dependence on the input power
and the amount of fiber submerged. It is also demonstrated that optical fibers
with jackets have about three times greater power loss than fibers without
jackets when submerged in LN2. The OPC’s optical-to-electrical power
conversion efficiency was measured to be about 51% in LN2 and 45% at room
temperature for a range of optical input powers ranging from 0.5 - 1 W. A
tensile strength test made it possible to estimate an approximate breaking
point of $\sim$ 5.5 kg for the studied PVDF optical fiber at room temperature
and $\sim$ 7 kg in LN2. A long-term test of the OPC operating in LN2 showed
that during $\sim$ 6.1 months of operation, the efficiency variation from
conversion of optical-to-electrical power is less than 2.6%. Laser
fluctuations has been studied under different input powers, and it was
observed that the laser transmitters power stabilizes after $\sim$ 25 min of
operation. During $\sim$ 6.1 months of the long term test operation, the laser
temperature and laser current variations remained below 2.5°C and 225 mA,
respectively. The results presented in this paper serve as a foundation for
future endeavors towards the use of PoF technology in other systems operating
in HV environments and cryogenic conditions, such as HEP dark matter
experiments, space exploration technologies, and other applications where low
noise, superior isolation, and optimal efficiency are needed. Further
investigations are underway to continue improving the PoF technology.

##  9 Acknowledgment

This manuscript has been authored by Fermi Research Alliance, LLC under
Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of
Science, Office of High Energy Physics. Dr. Shi would like to thank the
Visiting Scholars Award Program of the Universities Research Association for
supporting the light leakage study at Fermilab. The authors also thank
Broadcom and MH GoPower Company Limited for their participation in the R$\&$D
process to customize OPCs and fibers for operation in cryogenics.
Additionally, the authors would like thanks to Bora Kim and Minjoo Lawrence
Lee at UIUC for their fruitful discussions and studies aimed at improving the
efficiency of OPCs. This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of High Energy Physics under
Award Number DE-SC0024450. Moreover, this work is supported by subcontract No.
687391 between Fermi Research Alliance, LLC and South Dakota School of Mines
and Technology.

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