Microfiber Knot Resonators as Sensors

A Review

André D. Gomes and Orlando Frazão

INESC TEC and Department of Physics and Astronomy, Faculty of Sciences, University of Porto,

Rua do Campo Alegre 687, 4150-179 Porto, Portugal

Keywords: Microfiber Knot Resonator, Temperature Sensor, Refractive Index Sensor, Coated Microfiber Knot

Resonator.

Abstract: Microfiber knot resonators find application in many different fields of action, of which an important one is

the optical sensing. The large evanescent field of light can interact and sense the external medium, tuning the

resonance conditions of the structure. The resonant property of microfiber knot resonators can also provide,

in some cases, an enhancement in the sensing capability. Until nowadays, a wide variety of physical and

chemical parameters have been possible to measure with this device. New developments and improvements

are still being done in this field. A review on microfiber knot resonators as sensors is presented, with particular

emphasis on their application as temperature and refractive index sensors. The properties of these structures

are analyzed and different assembling configurations are presented. Important aspects in terms of the sensor

stability are discussed, as well as alternatives to increase the sensor robustness. In terms of new advances, an

overview on coated microfiber knot resonators is also presented. Finally, other microfiber knot configurations

are explored and discussed.

1 INTRODUCTION

Over the last few decades, optical fiber sensing

proved to be one of the most powerful and successful

application of both optical fibers and sensing

technology (Lou, Wang, and Tong, 2014). In fact,

with the developments and the advances of the

telecommunication industry, optical fibers begun to

be a subject of intense investigation. With this

outbreak, their application to various sensing fields

were widely explored until nowadays.

From the combination of optical fibers and

nanotechnology, the fabrication of micro and

nanofibers (MNF) by tapering a fiber to micrometric

or even nanometric size has led optical fiber sensing

to another level (Lou et al., 2014).

Until nowadays, new sensing configurations

based on MNFs were studied and developed. These

type of sensors come in many different forms and

normally the measured effects are sensed via intensity

or phase change of the transmitted light. One of the

configurations are the resonator-type MNF structures,

which use resonant structures such as loop resonators,

knot resonators or coil resonators.

Microfiber knot resonators (MKR) are one of the

configurations that had a huge impact, not only in the

field of sensing, but also in other fields such as, for

example, ultrafast optics, due to its high quality factor

(Q-factor) (Yi-ping Xu et al., 2015). These structures

are easy to fabricate and more stable compared, for

instance, with microfiber loop resonators. In the last

years, refractive index and temperature sensing using

MKR configurations were demonstrated.

Here we review the recent progress in microfiber

knot resonator as sensors, mainly for refractive index

and temperature sensing applications, as well as new

configurations of this structure. The sensor stability is

also discussed.

2 PROPERTIES OF MICROFIBER

KNOT RESONATORS

A microfiber knot resonator consists in tying a knot

in the taper waist region of a MNF, allowing coupling

and evanescent overlap between modes propagating

in adjacent turns. Light that enters the MKR will be

split in the knot region between the ring and the

Gomes A. and FrazÃ

co O.

Microﬁber Knot Resonators as Sensors - A Review.

DOI: 10.5220/0006264803560364

output. New light that enters the MKR will combine

with light going to the output of the structure, while

feeding at the same time the ring. To create the MKR,

first, the freestanding end of a MNF is assembled into

a large ring with few millimeters in diameter. Then,

the diameter of the ring is progressively reduced by

pulling the free end of the MNF until a microknot

with the desired dimensions is obtained (Figure 1).

This overlap of the fiber with itself allows no need for

a precise alignment (Xiao and Birks, 2011). The

small-size and low fabrication cost of these structures

have attracted much attention to produce them for

sensing purposes (G. Chen and Ding, 2013).

Figure 1: Schematic of a MKR. The structure is produced

using a taper.

2.1 Properties/Parameters

MKRs have interesting properties: high quality factor

and finesse, and a free spectral range (FSR) that

depends on the diameter of the ring. The schematic of

a high-Q MKR transmission spectrum is presented in

figure 2.

The quality factor (Q) is a useful quantity to

evaluate the losses of the resonator. The higher the Q-

factor, the longer the light stays in the ring. This factor

can be expressed as (Brambilla et al., 2009):













(1)

where 



is the resonance wavelength and 

is the full width at half-maximum. MKRs can achieve

Q factors up to 10

(De Freitas, Birks, and Rollings,

2015; X. Jiang, Tong, et al., 2006; Xiao and Birks,

2011).

The finesse is also a measure of the resonator

losses but it is independent of the resonator length. It

is defined as:







(2)

The FSR of a microfiber knot resonator, which is

the distance between two adjacent resonance

wavelengths 



and 



, is given by:

 















(3)

where 



is the effective refractive index of the

microfiber and  is the cavity length. In an MKR, the

cavity length is given by the perimeter of the ring.

This property is very useful since one can tune the

FSR of the sensor by adjusting the diameter of the

MKR.

Figure 2: Schematic of a MKR transmission spectrum.

2.2 Assembling Configurations

Most of the reported configurations assemble MKR

using a MNF with a freestanding end (like a pigtail)

and use a second microfiber (output fiber) to collect

the light transmitted out of the knot, as depicted in

figure 3. The microfiber is evanescently coupled to

the output fiber of the MKR, connected through Van

der Wall attraction. This solution presents some

limitations in terms of applications of the MKRs.

Despite some authors claim that MKRs assembled

from double-ended MNFs can easily break when

knotted (Brambilla et al., 2009; X. Jiang, Tong, et al.,

2006; X. D. Jiang, Chen, Vienne, and Tong, 2007),

Xiao et al. demonstrated the first MKR made from

double-ended tapered fibers, back in 2011 (Xiao and

Birks, 2011). The authors achieved MKRs with high

finesse (104) using a 128 µm-diameter MKR

assembled from a 1 µm-diameter fiber taper. A high

Q-factor of 97260 was also obtained for a

570 µm-diameter MKR assembled from the same

fiber taper. The minimum MKR diameter achieved

was 46 µm using the same fiber taper.

Also in 2011, the first all-fiber and optically

tunable MKR has been proposed (Chen et al., 2011).

The MKR was coated with a photoresponsive liquid

crystal mixture which presents an easily variable RI

that can be changed upon irradiation with UV light.

The change in the MKR effective refractive index

caused by the liquid crystal induces a change in the

resonance wavelengths. A 0.15 nm spectral shift was

observed when irradiating a 468 µm-diameter MKR

with 50mW/cm

of UV light. The MKR was

assembled from a 2.5 µm-diameter fiber taper. The

reversibility of the process is shown to be a great

advantage of this technique.

Figure 3: Schematic of a MKR using a second microfiber

as collecting fiber.

2.3 Stability

The stability of the MKR can be increased using

different methods or techniques. The first consists in

integrating the structure on a polished solid MgF

crystal substrate that helps to support the MKR. Since

the MgF

substrate presents low refractive index

(n=1.37), the MKR resonance is maintained (X.

Jiang, Yang, et al., 2006; Li and Ding, 2014; Y. Wu,

Rao, Chen, and Gong, 2009), despite a small change

in the resonance wavelengths due to a change in the

effective refractive index. This substrate also

provides good thermal conductivity which is helpful

for temperature measurements (Y. Wu, Rao, et al.,

2009).

Another common way to achieve long-term

stability is to coat the MKR with low refractive index

polymers (Li and Ding, 2014; Zeng, Wu, Hou, Bai,

and Yang, 2009), or simply use MKRs made out of

polymer microfibers (Y. Wu, Rao, et al., 2009; Y.

Wu, Zhang, Rao, and Gong, 2011; H. Yu et al., 2014).

Regarding this method, Vienne et al. performed an

analysis on the effect of host polymers in microfiber

knot resonators (Vienne, Li, and Tong, 2007). The

FSR and the maximum Q-factor and extinction ratio

did not change significantly after coating the MKR

with low-index polymer. However, the wavelength

region of high Q-factor and extinction ratio shifts

when the polymer coating is applied. Polymer

coatings allow to fix the knot structure, reducing the

probability of changing the MKR diameter. Li et al.

explored the optical degradation of a MKR over time

(Li and Ding, 2014). In fact, the transmission losses

of a bare MKR increased at the speed of around 0.24

dB/h, reaching 18 dB after 3 days, unlike a Teflon

coated MKR where no change was noticed for half a

month. This result shows that Teflon coating (n~1.31)

provides protection against degradation over time.

In 2011, a MKR was embedded in hydrophobic

aerogel (n~1.05) (Xiao and Birks, 2011). The losses

of the structure off-resonance (0.75 dB) proved to be

much lower than other encapsulants. Due to the low

refractive index, the aerogel avoids the reduction of

light confinment and change of dispersion when

compared with low-index polymers.

Robustness and resistance can also be gained

using large knots made out of larger diameter fibers

(Gomes and Frazão, 2016). However, the resonance

property can be lost due to the small evanescent field

of light.

3 SENSING WITH MICROFIBER

KNOT RESONATORS

Microfiber knot resonators can be used to sense a

wide variety of physical, chemical, and biochemical

parameters. A more detailed analysis of some

published results will now be made regarding mainly

the use of MKRs for temperature and refractive index

sensing.

3.1 Temperature Sensing

Temperature variations around a microfiber knot

resonator will change its length and refractive index,

resulting in a shift of the resonance wavelengths

(Zeng et al., 2009). By monitoring the resonance

wavelength variations one can have information

about the temperature variations around the sensor.

Using this principle of working, many MKRs have

been proposed for temperature sensing.

Wu et al. reported two temperature sensors based

on silica/polymer MKR (Y. Wu, Rao, et al., 2009).

Both sensors were placed in an MgF

crystal plate and

covered with an MgF

slab to make them more robust

and immune to environmental fluctuations, as shown

in figure 4. Moreover, the MgF

substrate ensures

good thermal conductivity and presents low refractive

index, as discussed back in section 2.3.

Figure 4: Schematic of the MKR temperature sensors

presented by Wu et al.

For the silica MKR, a 1.7 µm-diameter silica fiber

taper was used to create a 190 µm-diameter MKR.

The structure presents a Q-factor of 12000. A

temperature sensitivity of around 52 pm/ºC was

obtained between 30 ºC to 700 ºC. For the case of a

polymer MKR, a 2.1 µm-diameter PMMA fiber

(n~1.49) was used to produce a 98 µm-diameter

polymer MKR. Polymer microfibers have the

advantage of bending more easily and with smaller

radius of curvature than standard silica microfibers.

The structure presents a Q-factor of 8000, lower than

the silica MKR. However, in terms of temperature

sensing a sensitivity of 266 pm/ºC was achieved

between 20 ºC and 80 ºC. PMMA present higher

thermal expansion and thermal-optical coefficient

(TOC) than silica, therefore polymer MKRs exhibit

more temperature sensitivity than silica MKRs.

Despite have higher sensitivity, the measuring range

of the polymer MKR is lower than the silica MKR

due to the low melting point of the polymer.

Also in 2009, Zeng et al. presented a polymer

coated MKR for temperature sensing (Zeng et al.,

2009). The assembling of the sensor is similar to the

one presented in figure 4 but with some

modifications. Instead of an MgF

substrate, a glass

plate was adopted as substrate due to its good

adhesion ability to the polymer. A 20 µm thin layer

of low refractive index polymer (EFIRON UVF PC-

373, n=1.3759 at 852 nm) was coated on the surface

of the substrate to isolate the MKR from the high

refractive index of the glass plate. To fabricate the

MKR, a 1 µm-diameter fiber taper was used to

produce a 55 µm-diameter MKR. A second polymer

layer was coated above the MKR to make it immune

to environmental changes. The authors obtained

270 pm/ºC for the temperature sensitivity in the

heating process from 28 ºC to 140 ºC, and -280 pm/ºC

for the temperature sensitivity in the cooling process

from 135 ºC to 25 ºC. These values are similar to the

PMMA MKR temperature sensitivity demonstrated

by Wu et al. The reported resolution for this sensor is

0.5 ºC, although it can be increased if a higher

resolution spectrometer is used.

Later in 2014, a simple silica MKR was

theoretically an experimentally studied for seawater

temperature sensing by Yang et al. (Yang, Wang,

Wang, Liao, and Wang, 2014). Measuring seawater

temperature can be a little tricky since silica and

seawater show opposite thermal-optical coefficients.

So, when the TOC of silica is predominant the

resonant wavelengths with suffer a red shift, while in

the case of a predominance of the seawater TOC, a

blue shift is observed. Theoretically, there is a fiber

diameter for which the effect of silica TOC and

seawater TOC will cancel each other. This happens

for a 1.27 µm-diameter fiber taper. Bellow that

diameter the temperature sensitivity will be negative,

and above that value the temperature sensitivity will

be positive, saturating for fiber tapers with diameters

above 4 µm. Furthermore, the sensitivity will also

increase if the probing wavelength is increased. In

terms of experimental results, a maximum

temperature sensitivity of 22.81 pm/ºC was achieved

from 23 ºC to 33 ºC using a 473 µm-diameter MKR

produced from a 3.91 µm-diameter fiber taper. The

value was obtained with a probing wavelength of

1599.6 nm. The sensor presents a Q-factor of 3000

and a finesse of 11.69.

Still for temperature sensing, a configuration

based on a microfiber double-knot resonator was

developed for multi-point sensing by Wu et al. (Y.

Wu, Jia, Zhang, Rao, and Gong, 2012). The sensor

consists of producing two MKRs and then couple

both structures using the free-end of each MKR. A

schematic of the sensor is depicted in figure 5. The

two created knots, one with 506 µm-diameter and the

other with 500 µm-diameter, were fabricated from a

2.3 µm-diameter fiber taper. The distance between

the knots can be tuned by adjusting the length of the

coupling region.

Figure 5: Schematic of the microfiber double-knot

resonator in series.

If the diameter of both knots is the same (same

loop length), the interference spectrum shows a

regular resonance like a single MKR. On the other

hand, if now one of the diameters changes a new

phase matching condition will arise. In other words, a

second-order resonant peak will appear between the

primary resonant peaks. The authors demonstrate that

if the temperature changes locally on the first MKR,

only the primary resonant peak will shift while the

second-order resonant peak is constant. In the same

case, if the temperature of the second MKR changes,

the second-order resonant peak will shift while the

primary resonant peak is constant. With this, multi-

point temperature sensing is achieved in a very small

scale.

3.2 Refractive Index Sensing

Refractive index sensing using microfiber knot

resonators consists mainly of taking advantage of the

large evanescent field, which can interact with the

external environment. Variations in the external

medium refractive index will change the light

properties, which leads to a change in the MKR

resonance conditions. This effect is manifested as a

shift in the resonance wavelengths and in a change in

the FSR that are proportional to the external refractive

index variations.

Li et al. developed a Teflon coated MKR

Teflon

~1.31) for refractive index sensing (Li and

Ding, 2014). To apply the polymer a dip-coating

technique was used, whose process is outlined in

figure 6. To produce the sensor, a microfiber with an

initial diameter of 2.92 µm was used to create a

1.02 mm-diameter MKR. After the coating process,

the microfiber diameter increased to 3.95 µm,

allowing to estimate a value of 0.5 µm for the Teflon

thickness. The MKR diameter after coated was

reduced to around 0.45 mm due to tensions suffered

during the dip-coating process. A Q-factor of 20000

was obtained for the MKR structure before coating,

increasing to around 31000 after the Teflon coating.

A sensitivity of approximately 30.5 nm/RIU was

reported for this sensor in the refractive range of

1.3322 to 1.3412.

Figure 6: Illustration of the MKR dip-coat process in

Teflon.

Before that, Lim et al. proposed a MKR with

around 0.5 mm diameter in a Sagnac loop reflector

configuration (Lim et al., 2012). The Sagnac loop

reflector configuration allows the signal to be

collected through the incident path. The microfiber

structure was embedded in low refractive index

Teflon (n~1.31), except the MKR sensing region, to

obtain a balance between responsiveness and

robustness. A schematic of the experimental setup is

depicted in figure 7.

Figure 7: Schematic diagram of a MKR in a Sagnac loop

reflector.

For the sensor, a sensitivity of 30.49 nm/RIU was

obtained in a refractive index range from 1.334 to

1.348. This value of sensitivity is very similar to the

previously analysed Teflon coated MKR proposed by

Li et al. In terms of temperature response, the sensor

shows a sensitivity of 20.6 pm/ºC between 30 ºC and

130 ºC.

More recently, Yu et al. reported a polymer MKR

for refractive index sensing (H. Yu et al., 2014). A

microfiber of Poly(trimethylene terephthalate) (PTT)

was produced with 1.3 µm-diameter. PPT has strong

flexibility, large refractive index (n=1.638) and

presents good transparency in the visible and near-

infrared spectrum. A MKR with 85 µm-diameter was

created from the PPT microfiber and placed on an

MgF

-coated glass substrate. A Q-factor of 11000

was obtained. The sensor was analyzed under

different mixtures of water and glycerol at different

concentrations. The structure shows two different

linear regimes: a refractive index sensitivity of

46.5 nm/RIU for low concentration solutions

(1.35-1.38) with a resolution 4.310

-5

RIU, and a

refractive index sensitivity of 95.5 nm/RIU for high

concentration solutions (1.39-1.41) with a resolution

of 2.110

-5

RIU.

3.3 Other Sensing Applications

Microfiber knot resonators can also be applied for

sensing many other types of parameters. For example,

Wu et al. proposed the first MKR used for

acceleration sensing by optically interrogating the

vibration of a microelectromechanical system

(MEMS) (Y. Wu, Zeng, et al., 2009). They used a 1.1

µm-diameter fiber taper to create a 386 µm-diameter

MKR with a Q-factor of 8500. The MKR is placed at

the top surface of a MEMS cantilever. When

acceleration is applied to the system, the MKR will

experience strain, producing a shift in the resonant

wavelength. Monitoring the detected intensity, a

sensitivity of 654 mV/g was achieved in a dynamic

range of 20 g. In terms of wavelength shift, this

sensitivity is 29 pm/g. The resolution of this system

is 80 µg/

√



at 300 Hz.

In 2015, the first underwater acoustic sensor using

MKR was reported by Freitas et al. (De Freitas et al.,

2015). The authors use a 4 µm-diameter fiber taper to

produce an 800 µm-diameter MKR. The sensor

encapsulation of the sensor is made by placing the

MKR in a polytetrafluorethylene (PTFE) tray and

embedded in silicone rubber. A Q-factor of 41100

was achieved for this structure, being the highest Q-

factor obtained until that point for an encapsulated

MKR. Acoustic wavefields will change the external

pressure, causing variations in the MKR optical path

length, as well as changes in the fiber refractive index

due to elasto-optic effects. A normalized sensitivity,









⁄

, of -288 dB re/µPa was obtained using

acoustic frequencies from 25 Hz to 300 Hz.

Alternatively, the sensitivity can be expressed as

5.8310

-3

pm/Pa. For this system, a fast spectrometer

is needed to pick the transmission spectrum as it

changes in time.

MKRs can also be used for salinity sensing, which

is somehow similar to refractive index sensing. Liao

et al. presented a MKR for sensing the concentration

of NaCl in solution (Liao, Wang, Yang, Wang, and

Wang, 2015). Fiber tapers with diameters from 3.5 to

2.5 µm were used to make 855 µm-diameter MKRs.

A maximum sensitivity of 21.18 pm/% was achieved

in the range of 20.494% to 37.178%, with a probing

wavelength of 1600 nm.

A relative humidity sensor using MKRs was

proposed in 2014 (Gouveia, Pellegrini, dos Santos,

Raimundo, and Cordeiro, 2014). A 150 µm-diameter

MKR was produced from a 3 µm-diameter fiber

taper. The structure was coated with Nafion. Nafion

is a perfluorosulfonated-based polymer which

presents high hydrophilicity, chemical and thermal

stability, high conductivity, high adherence to silica,

and also mechanical toughness. The sensor shows

two regimes, one for low humidity (30 to 45%

relative humidity (RH)) where a maximum sensitivity

of (0.11±0.02) nm/% RH is obtained, and other for

higher-mid humidity (45 to 75% RH) where the

maximum sensitivity is (0.29±0.01) nm/%.

4 RECENT DEVELOPMENTS IN

COATED MICROFIBER KNOT

RESONATORS

More recently, the aim of the researchers has been to

incorporate other materials in the microfiber knot

resonators in order to achieve new devices with

enhanced sensitivity to the measured parameters. The

idea is to take advantage of the material and the MKR

resonant property to boost the sensor response.

In 2015, a palladium-coated MKR was presented

for enhanced hydrogen sensing (X. Wu, Gu, and

Zeng, 2015). Palladium (Pd) has been studied for

hydrogen sensing applications due to its highly

selective and reversible absorption of hydrogen

(Hübert, Boon-Brett, Black, and Banach, 2011; Silva,

Coelho, Frazão, Santos, and Malcata, 2012; Wadell,

Syrenova, and Langhammer, 2014). Since Pd

coatings are very thin (less than 1 µm), the interaction

between the confined light in optical fiber and the Pd

coating is insufficient. However, the proposed sensor

takes advantage of the MKR to enhance the

interaction of light with the Pd coating. In fact, due to

the recirculation of resonant light in the MKR, the

interaction of light with the Pd coating will

accumulate each time light travels another turn in the

MKR. This cumulative effect enhances the sensor

sensitivity while using just small interaction lengths

and a thin Pd film (~13 nm-thick), compared with

other existing optical fiber hydrogen sensors. The Pd

coating was performed using a plasma sputtering

device in a vacuum chamber and the hydrogen

concentration was monitored by measuring the shift

and the absorption at a resonant wavelength.

More recently, the deposition of graphene oxide

(GO) in MKR was studied for gas sensing, such as

and CO (C.-B. Yu et al., 2016). The refractive

index of GO is modified when gas molecules are

adsorbed on its surface. Hence, depending on the

concentration of gas molecules, the refractive index

of the GO film will change. The sensor is composed

of a 1.85 mm-diameter MKR, made out of a

5.1 µm-diameter fiber taper, placed over an MgF

structure, and covered with a GO sheet. To form the

GO film, a droplet of a 100 mg/L GO solution was

dropped onto the knot and heated up (~40 ºC) until it

was dried. A Q-factor of 78000 was achieved without

the GO sheet, which was reduced to 49000 after

depositing the GO. The sensing structure was used to

monitor carbon monoxide (CO) and ammonia (NH

)

concentrations. A CO sensitivity of ~0.17 pm/ppm

and a NH

sensitivity of ~0.35 pm/ppm were obtained

for concentrations bellow 150 ppm.

5 OTHER CONFIGURATIONS

In addition to the normal microfiber knot resonator

structure, new ways of producing different MKRs

were also explored in the last years. One of those

configurations is the reef knot microfiber resonator

demonstrated by Vienne et al. (Vienne et al., 2009).

The reef knot was fabricated using 2 microfibers, a

1.2 µm-diameter biconical taper and a 1.5

µm-diameter taper with a free end, as depicted in

figure 8.

Figure 8: Schematic of a reef knot microfiber resonator.

Table 1: Comparison between different reported configurations for temperature sensing.

Work Configuration Q-Factor Sensitivity Range

(Lim et al., 2012) MKR in Sagnac Loop Reflector X 20.6 pm/ºC 30ºC – 130ºC

(Yang et al., 2014) Simple Silica MKR (for Seawater) 3000 22.81 pm/ºC 23ºC – 33ºC

(Y. Wu, Rao, et al.,

2009)

Silica MKR with MgF

slab 12000 52 pm/ºC 30ºC – 700ºC

(Y. Wu, Rao, et al.,

2009)

Polymer (PMMA) MKR 8000 266 pm/ºC 20ºC – 80ºC

(Zeng et al., 2009) Polymer (EFIRON) MKR X 270 pm/ºC (Heating) 28ºC – 140ºC

--- --- X -280 pm/ºC (Cooling) 135ºC – 25ºC

Table 2: Comparison between different reported configurations for refractive index sensing.

Work Configuration Q-Factor Sensitivity Range

(Li and Ding, 2014) Teflon coated MKR 31000 30.5 nm/RIU 1.3322 – 1.3412

(Lim et al., 2012) MKR in Sagnac Loop Reflector X 30.49 nm/RIU 1.334 – 1.348

(H. Yu et al., 2014) Polymer (PTT) MKR 11000 46.5 nm/RIU 1.35 – 1.38

--- --- --- 95.5 nm/RIU 1.39 – 1.41

(Z. Xu et al., 2015) Cascaded MKR X 6523 nm/RIU 1.3320 – 1.3350

To produce the structure, the biconical taper is bent in

a “U” form and the free standing end of the second

taper is guided through the first taper, also in a “U”

shape, forming the reef knot. A non-circular reef knot

was created with 340 µm-diameter in the short axis

and 450 µm-diameter in the long axis. A Q-factor of

10000 was achieved for the “through” port and 3500

for the “drop” port. This device can be used as an add-

drop filter.

In 2015, a microfiber double-knot resonator with

a Sagnac loop reflector was proposed (Yiping Xu et

al., 2015). The difference between this double-knot

resonator and the one explored back in section 3.1 is

the location of the second knot.

Figure 9: Schematic of a microfiber double-knot resonator

in parallel.

In this case, the same microfiber is used to create

two knots, being the second knot is inside the first

one, as shown in figure 9. A Sagnac loop is created in

the end of the resonator so that light can experience

the double-knot structure twice, enhancing the

response, and also returning from the input port

allowing to monitor the sensor in reflection. The

transmission spectrum of a double-knot in parallel is

not the overlap of the transmission spectrum of each

knot independently, just as it happens with the

double-knot in series (Yiping Xu et al., 2014).

Xu et al. created an interesting small-size

refractometer for detecting slight refractive index

variations based on cascaded microfiber knot

resonators (CMKR) with Vernier effect (Z. Xu et al.,

2015). The Vernier effect is commonly used in

calipers and barometers to enhance the measurement

accuracy through the overlap between lines on two

scales with different periods. In this case, the setup

uses two MKR with millimeters of diameter

(1.178 mm and 1.230 mm) assembled from a

microfiber of 1.9 µm diameter. An illustration of the

used configuration is presented i\n figure 10.

Figure 10: Schematic of a cascaded microfiber resonator.

In the experiment, the ambient RI of the first

MKR is kept constant (1.3315), while the ambient RI

of the second MKR is slightly varying. The

wavelength shift due to the RI change was measured

and a sensitivity of 6523 nm/RIU was obtained. This

sensor allows a resolution of 1.53310

-7

RIU because

the measured wavelength shift is an absolute

parameter dependent on the relative optical intensity

variation. So, relative intensity noise in the light

source, thermal noise and shot noise in the photo-

detector of the spectrum analyzer do not affect the RI

detection. Furthermore, if the first MKR is embedded

in a low RI polymer, such as Teflon, it increases its

robustness and long-term stability, ensuring the first

MKR to be immune to ambient RI changes.

Recently, a Mach-Zehnder interferometer (MZI)

with a MKR was proposed for simultaneous

measurement of seawater temperature and salinity

(Liao, Wang, Wang, Yang, and Wang, 2016). Two

microfibers, the first with 2.8 µm-diameter and the

second one with 3.3 µm-diameter, were used to

produce the sensor. The thinner fiber was used to

create the MKR and the thicker fiber was used to form

the second arm of the interferometer, creating the

structure depicted in figure 11.

Figure 11: Schematic of a MZI with a knot resonator.

The transmission spectrum of the structure is a

combined response of the MZI and the MKR. A

temperature sensitivity of -112.33 pm/ºC and

13.96 pm/ºC was achieved for the interference peak

and the resonant peak respectively, from 13.7 ºC to

25 ºC. In terms of salinity, sensitivities of

208.63 pm/‰ and 16.21 pm/‰ were obtained for the

interference peak and the resonant peak, respectively,

between 25 ‰ and 37 ‰. Using these values, a matrix

method can be used to discriminate between

temperature and salinity.

6 CONCLUSIONS

So far, a great diversity of microfiber knot resonators

have been proposed and demonstrated for numerous

sensing applications. From these applications, the

temperature and refractive index sensing using

microfiber knot resonators were the subject of several

studies. A comparison between the different

microfiber knot resonator configurations discussed

previously is summarized in table 1 for temperature

sensing and in table 2 for refractive index sensing.

Microfiber knot resonators present many

characteristics (small size, fast-response, high-Q,

robustness, low cost, among others) which can be an

advantage over some conventional sensors.

Moreover, the ease of incorporating different

materials in microfiber knot resonator, such as

various low index polymers, aerogels, or even

materials like palladium and graphene oxide, reveals

great potential for new applications and for expanding

of the scope of these structures.

In the last years, the aim of the researchers has been

to explore and develop new microfiber knot resonator

configurations, as well as to incorporate materials in

microfiber knot resonators in order to achieve new

devices with enhanced sensitivity to the measured

parameters.

As a future outlook, we believe that the incorporation

of new materials in microfiber knot resonators is a

field that will continue to be developed. Another field

to be explored is the combination of microfiber knot

resonators with other well-known structures

(interferometers, Bragg gratings, Fabry-Perot

cavities) with the objective of obtain new sensors

with improved sensitivity and possibly being able to

discriminate between different physical parameters.

ACKNOWLEDGEMENTS

This work was supported by "Project "NanoSTIMA:

Macro-to-Nano Human Sensing: Towards Integrated

Multimodal Health Monitoring and

Analytics/NORTE-01-0145-FEDER-000016" is

financed by the North Portugal Regional Operational

Programme (NORTE 2020), under the PORTUGAL

2020 Partnership Agreement, and through the

European Regional Development Fund (ERDF).

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