# Cryogenic Optical Packaging Using Photonic Wire Bonds

Becky Lin  Department of Electrical and Computer Engineering, The University
of British Columbia, V6T 1Z4, Vancouver, British Columbia, Canada  Stewart
Blusson Quantum Matter Institute, V6T 1Z4, Vancouver, British Columbia, Canada
Donald Witt  Department of Electrical and Computer Engineering, The University
of British Columbia, V6T 1Z4, Vancouver, British Columbia, Canada  Stewart
Blusson Quantum Matter Institute, V6T 1Z4, Vancouver, British Columbia, Canada
Jeff F. Young  Stewart Blusson Quantum Matter Institute, V6T 1Z4, Vancouver,
British Columbia, Canada  Department of Physics and Astronomy, The University
of British Columbia, V6T 1Z1, Vancouver, British Columbia, Canada  Lukas
Chrostowski  Department of Electrical and Computer Engineering, The University
of British Columbia, V6T 1Z4, Vancouver, British Columbia, Canada  Stewart
Blusson Quantum Matter Institute, V6T 1Z4, Vancouver, British Columbia, Canada

###### Abstract

We present the required techniques for the successful low loss packaging of
integrated photonic devices capable of operating down to 970 mK utilizing
photonic wire bonds. This scalable technique is shown to have an insertion
loss of less than 2 dB per connection between a SMF-28 single mode fibre and a
silicon photonic chip at these temperatures. This technique has shown
robustness to thermal cycling and is ultra-high vacuum compatible without the
need for any active alignment.

††journal: opticajournal

##  1 Introduction

In this era of emerging quantum technology, there is a growing demand for easy
and efficient optical connection methodologies for cryogenic photonic
circuits. Low temperature operation suppresses thermal noise and makes
available special material properties, such as superconductivity, for single
photon detectors [1, 2, 3], and single photon sources [4]. Efficient cryogenic
optical coupling is needed for practical implementations for all-optical
quantum computing [5, 6], hybrid quantum photonic integrated circuits [7, 8,
9], linking together nodes of the quantum internet [10], and quantum
communication [11]. Low insertion loss (IL) is especially important for
photonic connections that transport quantum information in the form of single
photons as photon loss can result in reduced photon pair generation rate [12,
13] and increased overhead for error correction [14].

In this paper, we use photonic wire bonding (PWB) to make reliable and
efficient optical connections between single mode fiber (SMF) and silicon
photonic circuits down to $970\text{\,}\mathrm{mK}$. PWB is a new emerging
optical integration technique that uses a polymer based free form structure to
connect two roughly aligned (> $30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ of
x-y tolerance, > $100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ of tolerance
along optical axis) optical ports [15]. This 3D optical bridge eliminates the
precise alignment required for other integration methods. The ability to write
adiabatic mode conversion tapers allows for efficient coupling between the
large area fiber mode (mode field diameter $\sim$
$10.4\text{\,}\mathrm{\SIUnitSymbolMicro m}$) to the small area waveguide mode
(mode field diameter $\sim$ $0.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$). The
resist (VanCore A) of the bond has low absorption in the O and C bands,
allowing for a large working bandwidth (see Supplement 1 for measurement of
resist transmission). These properties are attractive for quantum applications
that require the connection of color centers with different operating
wavelengths, allowing for the creation of hybrid integrated quantum photonic
circuits [9]. Its freeform nature also allows for maintenance and manipulation
of polarization [16]. Versatility has been demonstrated at room temperature by
the creation of fibre-to-chip [17], laser-to-chip [18], and chip-to-chip [19,
20] interfaces. Fig. 1 shows the best achieved IL of in-house bonded fiber to
surface taper (blue) [Fig. 2(a)] and fiber to edge coupler (red) [Fig. 2(b)]
at room temperature (RT), which are $1.3\text{\,}\mathrm{dB}$ and
$1.6\text{\,}\mathrm{dB}$ at $1550\text{\,}\mathrm{nm}$, respectively. The on-
chip surface tapers and edge couplers can be formed using standard fabrication
steps offered by silicon photonic foundries. Unlike other adiabatic tapers
that require coupling to specially made tapered fibers [21], PWBs can directly
bond to commercially available cleaved fibers and fiber arrays (FA). PWB
samples are also mechanically robust which makes them more immune to
vibrations from nearby pumps and compressors.

The main challenges of optical coupling between a fibre and a integrated
photonic circuit inside a cryogenic chamber stem from: (1) thermal induced
stresses and strains (2) inaccessibility of the closed testing setup, and (3)
the limitation of available cryogenic compatible materials. Thus, it can not
be assumed a device demonstrated to have good performance at room temperature
will remain so at low temperature. Thermal contraction during cool down can
introduce relative displacements that can jeopardize pre-aligned systems or
break pre-packaged samples [22]. However, without the camera and hardware
manipulators usually available for room temperature setups, it is difficult to
access and adjust samples that are loaded and sealed inside cryogenic chambers
[23].

Figure 1: Plot showing the transmission of best achieved in-house bonded
photonic wire bonds (PWBs) connecting between a fiber to silicon photonic
waveguide via surface tapers (blue) and edge couplers (red) at room
temperature. The transmission is calibrated for one PWB. Measurements are
optimized to TE polarization at $1550\text{\,}\mathrm{nm}$ using a
polarization paddle. TE calibrated transmission from fiber to surface taper
through one PWB at $5\text{\,}\mathrm{K}$ demonstrated in this work is shown
in yellow. The IL at $5\text{\,}\mathrm{K}$ for C band is estimated to be 2.0
$\pm$ $0.3\text{\,}\mathrm{dB}$.

In this context, the PWB technique offers some significant advantages. First,
the glue used no longer needs to be optically matched to, or physically in
contact with the coupling interface. This significantly reduces the constraint
on the glue used since optically clear glues, which are often UV activated,
are prone to crack at low temperatures [24]. It also avoids the problem of
spectral shifts caused by the glue’s changing refractive index during cool
down [25]. Second, no active and precise alignment is needed during assembly
[26] as this is achieved through image recognition by the PWB tool during the
bonding process. Third, assembly time and valuable real estate are saved when
multiple channels are assembled at once using a FA. By using FAs with up to 64
channels on a $127\text{\,}\mathrm{\SIUnitSymbolMicro m}$ pitch, access to
other important surface elements such as electronic pads can be greatly
improved.

In this paper, we show the assembly methodology for cryogenic optical
packaging using PWB and demonstrate coupling of light between fibers to
surface tapers down to $970\text{\,}\mathrm{mK}$ with an IL of
$2.0\text{\,}\mathrm{dB}$ per coupler at $1550\text{\,}\mathrm{nm}$. We
evaluate the performance of PWBs in ultra high vacuum (UHV) environment and
various input power levels. We use a micro ring resonator (MRR) to demonstrate
that we can successfully couple sufficient optical power to observe non-linear
behaviour at low temperatures.

Figure 2: (a) Adiabatic coupling via surface taper. The box around the surface
taper is etched silicon from a positive electron beam lithography process. The
white background is unetched silicon. (b) Mode matched coupling via edge
coupler. (c) The PWB consist of three sections: taper to evanescently couple
light to surface taper (I), uniform cross section waveguide connecting section
I and III (II), and a taper to adiabatically expand the mode to match to the
fiber core (III). (d) Generic workflow to assemble and connect optical fiber
to waveguide using a photonic wirebond written using two photon polarization.

Figure 3: (a) Image of successfully cooled sample. Roughly equal amounts of
cryogenic glue (Masterbond® EP29LPSPAO-1) are applied to opposite sides of
chip and FA to help equalize thermal induced stresses. The amount of glue is
minimized to reduce stresses inside the glue and between the glue and the
other components. (b) Side schematic view of working sample. The UV glue is
used to temporarily hold the chip and FA to the sub-mount while the cryogenic
glue cured over 12 hours and is removed cleanly by a scalpel afterwards. The
core of the SMF sits slightly (10 $\sim$
$20\text{\,}\mathrm{\SIUnitSymbolMicro m}$) below the silicon waveguide. The
distance between the SMF face and silicon waveguide taper tip is $\sim$
$270\text{\,}\mathrm{\SIUnitSymbolMicro m}$ to allow room for mode
transitioning taper and relaxed bond arc. (c) Both the cryogenic and temporary
UV glue are applied to the sides of the FA and chip only and not underneath
them.

##  2 Background

A PWB consists of three sections. Namely, a polymer taper on one end to
evanescently couple the mode to the tapered silicon waveguide (I in Fig.
2(c)), a polymer taper on the opposite end to adiabatically transition the
mode the fiber core (III in Fig. 2(c)) and a uniform polymer waveguide
connecting the two tapers together with a free form path (II in Fig. 2(c)).
The polymer resist has a refractive index of $\sim$ 1.531 at
$1550\text{\,}\mathrm{nm}$. The silicon waveguide has a cross-section design
width of $500\text{\,}\mathrm{nm}$ and height of $220\text{\,}\mathrm{nm}$
while the surface taper has a design length of
$65\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and tip width of
$130\text{\,}\mathrm{nm}$. The waveguides are not cladded and are thus exposed
to the environment.

An assembled sample consists of the photonic chip and fiber, or FA, glued onto
a common sub-mount. As photonic wire bonding allows for high alignment
tolerance, the assembly can be made by hand using tweezers under a
stereomicroscope. An overview of the generic photonic wire bonding process is
shown in Fig. 2(d). Namely, it consists of assembling the sample, applying the
polymer resist to the area of interest, using the Vanguard SONATA1000 tool to
identify the location of facets and write the bonds using the two-photon
polymerization process, developing the resist using solvents, and protecting
the PWB with a cladding resist (cladding is not applied in this work, see
section 3.3.3 for details).

##  3 Cryogenic Photonic Wire Bonding Process

###  3.1 Sample Assembly

The procedure for cryogenic photonic wire bonding adapts the room temperature
PWB process but with special considerations for successful cryogenic
operation. An assembled sample consists of a silicon photonic chip and an
8-channel single mode FA mounted on a gold-plated oxygen-free copper sub-
mount. Copper is chosen for its high thermal conductivity and the gold plating
maintains good thermal contact between the sub-mount and the gold-plated cold
stage of the cryostat [27]. The jacket of the FA is a thermal plastic (Hytrel)
that does not effect the vacuum. A cryogenic (and vacuum) compatible two-part
epoxy (Masterbond® EP29LPSPAO-1 Black [28]) is used instead of the UV glue
that is normally used to assemble room temperature devices.

Fig. 3(a) shows an image of a working sample and Fig. 3(b) shows its side
schematic view. The face of the FA is put $\sim$
$270\text{\,}\mathrm{\SIUnitSymbolMicro m}$ from the tip of the surface taper.
The heights of the fiber core and the silicon waveguide are
$715\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and
$728\text{\,}\mathrm{\SIUnitSymbolMicro m}$, respectively. This sets the fiber
core slightly below the waveguide. The core height is standard for a
$750\text{\,}\mathrm{\SIUnitSymbolMicro m}$ FA v-groove. The glue anchors for
the FA and the chip are roughly $1000\text{\,}\mathrm{\SIUnitSymbolMicro m}$
apart. The volume and position of the glue is roughly mirrored on both sides
of the FA and chip to help equalize any thermally induced stress. No glue is
applied underneath the FA or chip [Fig. 3(c)]. Glue is also applied to the
fiber ribbon as strain relief.

The samples assembled using the configuration described above did not break
when cooled to cryogenic temperatures. See Supplement 1 for the type of
assemblies that do not work.

###  3.2 Thermal Contraction Matching and Stress Management

The large amount of thermal contraction the sample undergoes while cooling to
cryogenic temperatures necessitates careful design considerations. Sub-
components of the sample, comprising the silicon photonic chip, copper sub-
mount, and polymer-based PWB, undergo different rates of thermal contraction
during cool down. Polymer has the highest thermal contraction ($\Delta$L/L =
$1.22\text{\,}\mathrm{\char 37\relax}$ from $293\text{\,}\mathrm{K}$ to
$4\text{\,}\mathrm{K}$) followed by copper ($\Delta$L/L =
$0.324\text{\,}\mathrm{\char 37\relax}$ from $293\text{\,}\mathrm{K}$ to
$4\text{\,}\mathrm{K}$) and silicon ($\Delta$L/L =
$0.022\text{\,}\mathrm{\char 37\relax}$ from $293\text{\,}\mathrm{K}$ to
$4\text{\,}\mathrm{K}$) [27, 29]. Here, we assume the PWB polymer is acrylate
based and has similar properties as methyl methacrylate (PMMA) [30] and use
total thermal linear contraction values ($\Delta$L/L $\equiv$
($L_{\text{$293\text{\,}\mathrm{K}$}}$-$L_{T}$)/$L_{\text{$293\text{\,}\mathrm{K}$}}$)
measured for PMMA as a reference for the PWB.

The contraction of the bond can be roughly matched to the contraction of the
copper sub-mount by adding a gap between the chip and FA. The estimated
contraction for a $270\text{\,}\mathrm{\SIUnitSymbolMicro m}$ polymer bond is
$3.3\text{\,}\mathrm{\SIUnitSymbolMicro m}$ which will require a copper length
of around $1020\text{\,}\mathrm{\SIUnitSymbolMicro m}$ to match. This distance
is set by choosing the corresponding anchoring points of the glue between the
chip and FA [Fig. 3(b)].

###  3.3 Cladding

The cladding (n $\approx 1.393$ at $1550\text{\,}\mathrm{nm}$) normally used
to protect the PWB and reduce the insertion loss by $\sim$
$0.2\text{\,}\mathrm{dB}$ at room temperature is not applied to cryogenic
samples, as it would out-gas in the cryostat. Attempts to mitigate the out-
gassing by encapsulating the cladding resist with UHV compatible epoxy failed,
likely due to stresses caused by the differing coefficients of expansion of
the additional cladding materials.

##  4 Experiment and Results

###  4.1 Experimental Setup

The photonically wire bonded devices were cooled to $5\text{\,}\mathrm{K}$
inside a custom closed cycle, UHV dry cryostat. Samples are cooled from
$300\text{\,}\mathrm{K}$ to $5\text{\,}\mathrm{K}$ within $\sim$ 4 hours and
remain at $5\text{\,}\mathrm{K}$ for > 17 hour before being warmed back up to
$300\text{\,}\mathrm{K}$. For both baking and repeatability cycle tests, the
sample is left inside the vacuum environment of cryostat. The devices were
also tested in a different custom dry cryostat (non-UHV) capable of cooling to
$970\text{\,}\mathrm{mK}$ to test their performance at even lower temperature.

The full optical path of the measurement is shown in Fig. 4. The devices are
optically measured using a Keysight 8164B mainframe equipped with an 81682A
tunable laser module and 81635A power sensor. The light passes from the
tunable laser module into a PS-155-A FiberPro polarization scrambler. The
light then passes into the cryostat through a UHV FC/APC fiber feedthrough
from SQS Vlaknova Optika. Then the light passes through the sample and out a
different port on the same feedthrough and into the 81635A power sensor.

The device is continuously measured over the full laser range during the whole
process to monitor the effect of each step.

Figure 4: Optical pathway of measurement.

###  4.2 Optical Performance of PWB

To measure the IL of the PWB and to demonstrate the coupling of light to
photonic devices at cryogenic temperatures, waveguide loopbacks and MRRs
structures are used [Fig. 5(a)]. As only one device can be measured during
each cooling cycle, the MRR device (circled in red) is prioritized as it can
provide information about the temperature and surface condition of the device
for comparison to the temperature and pressure sensors inside the cryostat.
For a consistent measurement, the system tracked the same device over the
cycle of cool down and warm up to examine the performance over temperature and
time.

Figure 5: (a) Optical input from one fiber is guided to the surface taper
through the PWB and coupled into the MRR. Light is coupled out through the
other fiber port. The cool down and warm up measurement for MRR B (circled in
red) is shown on the right: the calibrated transmission of two PWBs connected
to a MRR (b) before cool down at $300\text{\,}\mathrm{K}$ vs after cool down
at $5\text{\,}\mathrm{K}$ (c) before vs after maintaining at
$5\text{\,}\mathrm{K}$ for 17 hours (d) before cooling down at
$300\text{\,}\mathrm{K}$ vs after warming from $5\text{\,}\mathrm{K}$ at
$300\text{\,}\mathrm{K}$. The change in the temperature of the ring is evident
from the shift in resonance wavelength. The transmission remains approximately
the same through the cool down and warm up process.

The plots in Fig. 5 (b)-(d) show the measured transmission through the two
PWBs and MRR device before and after cooling. Prior to cooling, at
$300\text{\,}\mathrm{K}$, the transmission ranges from
$-4.5\text{\,}\mathrm{dB}$ at $1490\text{\,}\mathrm{nm}$ to
$-8.0\text{\,}\mathrm{dB}$ at $1640\text{\,}\mathrm{nm}$. After cooling to
$5\text{\,}\mathrm{K}$, the transmission at $1490\text{\,}\mathrm{nm}$ is
$-5.2\text{\,}\mathrm{dB}$ and $-8.0\text{\,}\mathrm{dB}$ at
$1640\text{\,}\mathrm{nm}$ [Fig. 5(b)]. Specifically, the transmission in the
C-band is $-5.3$ $\pm$ $0.2\text{\,}\mathrm{dB}$ which is relatively
wavelength independent compared to methods that use grating couplers.

The sample was left at $5\text{\,}\mathrm{K}$ for 17 hours to study the
stability. Figure 5(c) shows the transmission spectrum before and after
cooling for 17 hours. There is no change in the IL and the slight red shift in
the resonance is due to the deposition of condensates on the ring over time in
a unbaked vacuum system. No sign of degradation in the PWBs is observed.
Figure 5(d) compares the sample prior to cooling at $300\text{\,}\mathrm{K}$
to after warming up (from $5\text{\,}\mathrm{K}$) at $300\text{\,}\mathrm{K}$.
Overall, there is a slight increase of IL by less than
$0.2\text{\,}\mathrm{dB}$.

The IL of a single PWB is approximated by dividing the calibrated transmission
through two bonds by two. We assume the loss through the short silicon
waveguide ($430\text{\,}\mathrm{\SIUnitSymbolMicro m}$) and the two bends is
negligible. The resulting IL of the PWB at $5\text{\,}\mathrm{K}$ measured
using the polarization scrambler is then $2.7$ $\pm$ $0.1\text{\,}\mathrm{dB}$
at $1550\text{\,}\mathrm{nm}$. Accounting for the $0.7$ $\pm$
$0.2\text{\,}\mathrm{dB}$ of polarization dependent loss if the input is
optimized for TE mode (typically done for wavelength specific measurements),
the IL of the PWB would be $2.0$ $\pm$ $0.3\text{\,}\mathrm{dB}$ (yellow in
Fig. 1). The calibration procedure is described in more detail in Supplement
1.

The periodic shallow dips that are present in addition to the sharp resonance
peaks are likely the resonances of the TM mode introduced by the polarization
scrambler. The quality factor is expected to be lower for the TM mode
resonances as the coupling region of the ring was designed for TE coupling.
The TM resonances are more prominent at longer wavelengths because the mode at
shorter wavelengths is more contained inside the waveguide and is minimally
coupled to the ring.

The sample was also cooled to $970\text{\,}\mathrm{mK}$ using a separate (non-
UHV) dry cryostat. It survived temperature cylcing and performed the same at
$970\text{\,}\mathrm{mK}$ as it did at $5\text{\,}\mathrm{K}$.

Figure 6: (a) The system is baked at
$120\text{\,}\mathrm{\SIUnitSymbolCelsius}$ to reach UHV pressure of $\sim
9.2\times 10^{-9}$ mbar. A plot of resonance wavelength versus temperature
shows that the resonance remains the same over multiple cool down and warm up
cycle and agrees well with simulation (red). The simulation curve is shifted
by $0.1\text{\,}\mathrm{nm}$ to match to the measured wavelength at
$295\text{\,}\mathrm{K}$. The change in resonance wavelength is dominated by
the thermal optic effect of the silicon waveguide which is 22 times larger
than the buried oxide. (b) The transmission spectra of a silicon photonic MRR
operating at $5\text{\,}\mathrm{K}$ for different input powers. The blue curve
is used as a reference with no nonlinearity visible. The other curves have
been normalized by subtracting the laser input power setting so that they
overlapped for direct comparison. The input power at the device is noted on
the plot. The dashed lines indicate the response generated by scanning the
laser in the reverse direction. One can see the difference between the forward
and reverse scans for the red and the yellow spectra.

###  4.3 Multiple Cool Down Performance

The sample underwent three cool down cycles to test the performance
repeatability and the behaviour remained the same. Namely, the IL is observed
to degrade slightly during cool down, remain the same for several hours at
$5\text{\,}\mathrm{K}$, and return to the original performance after warming
up. Overall, it is observed a working sample can withstand the multiple cycles
of cooling and long cooling times over the several weeks of operation. See the
Supplement 1 for additional data in support of these claims.

###  4.4 Operation in Vacuum Conditions

Supplement 1 describes how gas adsorption gradually modifies the resonant
frequency of the MRR devices [31, 32] held at $5\text{\,}\mathrm{K}$ for
several hours. We therefore also tested the ability of the sample to survive
vacuum bake out prior to cooling. The system, which includes the cryogenic
chamber and sample, was heated to $120\text{\,}\mathrm{\SIUnitSymbolCelsius}$
for 166 hours to achieve a final pressure of $\sim 9.2\times 10^{-9}$ mbar. By
achieving UHV conditions before cooling to cryogenic temperatures, the effects
of undesirable gas molecule deposition were negligible over the course of our
experiments which acquired data over several days (including that shown in
Fig. 6(a)). However, the baking procedure increased the IL by approximately
$3\text{\,}\mathrm{dB}$ ($1.5\text{\,}\mathrm{dB}$ for each bond).

###  4.5 Power Limit at Different Pressures

Several potential applications for cryogenic photonic wire bonds depend on
nonlinear optical effects. These applications include photon pair sources [33]
and all optical switching [34]. In order to achieve these nonlinear optical
effects, one needs to have high optical power in the device. To examine the
suitability of photonic wire bonds in such cases, a series of power sweeps
were run to determine the damage threshold of the assemblies.

In normal atmospheric conditions, the power limit of the photonic wire bonds
exceeds the $13.5\text{\,}\mathrm{dB}$m maximum output power of our laser.
However, in a vacuum environment, whether the temperature of the sample is
$300\text{\,}\mathrm{K}$ or $5\text{\,}\mathrm{K}$, the threshold power was
found to be less than $6\text{\,}\mathrm{dB}$m when continuously sweeping the
laser wavelength over the standard scan range.

Optical images of the sample taken after such high power sweeps show the
middle section of the photonic wire bonds has evaporated (see Supplement 1).
The fact that the upper power limit of a photonic wire bond is the same
whether the sample is at $300\text{\,}\mathrm{K}$ or $5\text{\,}\mathrm{K}$
indicates that the likely cause is the lack of convection cooling in a vacuum
environment. This suggests the power limit may be higher in vapour or liquid
cryostats that can offer higher convection cooling for the insulating polymer
bonds.

###  4.6 Temperature Dependence of MRR Resonances

We track one of the resonances of the MRR over the $300\text{\,}\mathrm{K}$ \-
$5\text{\,}\mathrm{K}$ temperature range and compare it to simulations to help
verify the sample does in fact thermalize with the cold stage. Fig. 6(a) shows
a plot of resonance wavelength versus temperature in a UHV environment. The
resonance wavelength shift from $295\text{\,}\mathrm{K}$ or
$5\text{\,}\mathrm{K}$ over several cool down (blue coloured dots) and warm up
(orange coloured dots) cycles match well with the simulation (red line). The
simulation is produced using refractive index values for silicon and silicon
dioxide at different temperatures reported in [35, 36, 37]. See Supplement 1
for more details of the simulation method.

###  4.7 Cryogenic Ring Non-linearity

We also use the MRR to demonstrate packaging of a non-linear optical device
operating at cryogenic temperatures using PWB.

The nonlinear response was found to be significantly weaker at cryogenic
temperatures, as expected given the dramatically lower thermo optic
coefficient of silicon at low temperatures [35]. However, it was also found
that within the acceptable power limits of the photonic wire bonds, nonlinear
optical effects were still visible. An example of such a nonlinear response
can be seen in Fig. 6(b). The solid line indicates a forward sweep of the
measurement laser (from low to high wavelength). The dashed line indicates a
reverse sweep of the laser (from high to low wavelength). Though much reduced,
the thermo-optic effect may still be contributing to the observed redshift of
the resonances (in addition to the Kerr effect) at these low temperatures. A
nonlinear model will be used to quantify the relative contributions of Kerr,
thermo-optic, and free carrier effects on these low temperature, continuous
wave nonlinear results. Future pulsed laser excitation experiments will also
aid in separating the different nonlinear effects via differences in their
time constants [38].

##  5 Summary

The results presented here show that PWBs can be used for photonic packaging
from room temperature down to $970\text{\,}\mathrm{mK}$ with low loss, high
bandwidth, and mechanically robust performance. This method alleviates the
strict alignment challenge that occurs during assembling and cool down
experienced by other packaging methods. Additional features such as UHV,
vacuum bakeout compatibility and robustness to thermal cycling make PWB a
promising new photonic packaging technique for a wide range of applications
and operating conditions.

##  6 Discussion

Table 1: Comparison of Different Fixed Coupling Methodologies Used for
Cryogenic Silicon Photonic Integration At C-Band.

|  Tapered Fiber Glued to Waveguide [24] |  Angle Polished Fiber Glued to Grating Coupler [24, 25] |  Fiber Array Coupled to Edge Coupler [39] |  This Work
---|---|---|---|---
Demonstrated Temperature |  $7\text{\,}\mathrm{mK}$ |  $7\text{\,}\mathrm{mK}$ |  $4\text{\,}\mathrm{K}$ |  970
Coupling Efficiency Per Coupler |  $-5.2\text{\,}\mathrm{dB}$ |  $-6$ $\sim$ $-10\text{\,}\mathrm{dB}$ |  $-1.9\text{\,}\mathrm{dB}$ |  -2.7 (scrambled)
-2.0 at 1550 (TE optimized)
Coupling Bandwidth |  > $100\text{\,}\mathrm{nm}$ |  20 $\sim$ $55\text{\,}\mathrm{nm}$ |  > $20\text{\,}\mathrm{nm}$ |  > 100
Spectral Shift |  Minimal |  Large |  Minimal |  Minimal
Polarization Dependent Loss |  Minimal |  Large |  Minimal |  Minimal
Misalignment Tolerance |  High:$\sim$ $50\text{\,}\mathrm{\SIUnitSymbolMicro m}$ along optical axis |  Medium:$\sim$ $2\text{\,}\mathrm{\SIUnitSymbolMicro m}$ |  Low: $\sim$ $0.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ |  High: > 30 in all axis
Alignment Method |  Active |  Active |  Active |  Passive & Image Recognition
Scalability |  Single Fiber |  Single Fiber |  Medium to High |  Medium to High

###  6.1 Comparison of Cryogenic Coupling Techniques

We compare the method shown in this work to other cryogenic fixed coupling
methods that have been demonstrated for silicon photonics cooled to <
$10\text{\,}\mathrm{K}$, which is the temperature required for many
superconducting samples [Table. 6]. This work demonstrates one of the lowest
IL of these methods at $2.0\text{\,}\mathrm{dB}$ at $1550\text{\,}\mathrm{nm}$
which is approximately 3 to $7\text{\,}\mathrm{dB}$ less than the other
methods. We show the largest coupling bandwidth, > $100\text{\,}\mathrm{nm}$,
with potential to be larger in the O-band and near visible band. While
spectral shift is a prominent issue for other methods that use grating based
couplers during assembly and cool down, we do not observe any spectral shift.
We also observe minimal polarization dependent loss. The free-form capability
of the PWB also provides the largest misalignment tolerance of more than
$30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ in all three axis which is
significantly more than the 0.5 - $2\text{\,}\mathrm{\SIUnitSymbolMicro m}$
tolerance of the other methods using grating couplers or edge couplers. As a
result, the assembly process does not require any active alignment. Precise
bonding is achieved through image recognition in the PWB tool. This method can
be used for large scale integration using commercially available fiber arrays.

The method that uses a fiber array glued to grating couplers is not included
in the table as results have shown that configuration is prone to crack and
break when cooled to low temperatures due to the thermal stress from the large
quantity of glue needed to keep the FA intact [22, 40].

###  6.2 Thermal Conductivity

The thermal conductivity between the chip and the sub-mount is achieved
through the thermal conductive glue as the assembly process shown in this work
does not have any bonding agent applied underneath the chip. For chips with
larger heat load, a thin layer of thermal conductive paste, glue, or solder
may need to be applied between the chip and sub-mount to increase the thermal
conductivity.

###  6.3 Future Work and Improvements

We anticipate this technique to allow medium to high scaling beyond the
8-channel coupling demonstrated in this work. Commercial FAs can have more
than 64 channels which would be cumbersome to connect via FC/APC feedthroughs.
Luckily, this overhead may be mitigated by using MTP® connectors that are
compact and low loss.

In addition to coupling to on-chip surface tapers, PWBs can also be bonded to
edge couplers [Fig. 2(b)] which would be used for oxide cladded samples.

Aside from the fundamental absorption losses in the polymer resist, the IL may
be improved by new emerging 3D nano writing techniques that have improved
writing resolution and minimum feature size [41, 42]. This could reduce the
losses due to physical properties such as scattering from surface roughness
and back reflections from mode mismatch. The trajectory of the bond can also
be improved by optimizing the curvatures to minimize bending loss [43].

###  6.4 Future Applications

Beyond using PWB for fiber-to-chip coupling, it can be used for chip-to-chip
integration to achieve hybrid quantum photonic integrated circuits. Although
not all devices require cryogenic operation, it is shown that overall
performance of the system improves if they are also cooled down [44]. Surface
taper-to-surface taper coupling has been shown to have reliable and repeatable
IL of $0.7\text{\,}\mathrm{dB}$ [19].

Funding Refined Manufacturing Acceleration Process (ReMAP); Canada Foundation
for Innovation (CFI); B.C. Knowledge Development Fund (BCKDF); SiEPICfab
Consortium; Natural Sciences and Engineering Research Council of Canada
(NSERC); Mitacs Accelerate Program

Acknowledgments Thank you Matthias Lauermann and Matthew Mitchell for fruitful
discussions.

Disclosures BL: Dream Photonics (E), LC: Dream Photonics (I).

Data Availability Statement Data underlying the results presented in this
paper are not publicly available at this time but may be obtained from the
authors upon reasonable request.

Supplemental document See Supplement 1 for supporting content.

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