# Optical Coupling Structures of Fiber-Optic Mach-Zehnder Interferometers Using CO2 Laser Irradiation

Authors: Chien-Hsing Chen; Chih-Yu Hsu; Pei-Hsing Huang; Jian-Neng Wang; Wei-Te Wu
Journal: International Journal of Antennas and Propagation
 (2014)

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## Abstract

The Mach-Zehnder interferometer (MZI) can be used to test changes in the refractive index of sucrose solutions at different concentrations. However, the popularity of this measurement tool is limited by its substantial size and portability. Therefore, the MZI was integrated with a small fiber-optic waveguide component to develop an interferometer with fiber-optic characteristics, specifically a fiber-optic Mach-Zehnder interferometer (FO-MZI). Optical fiber must be processed to fabricate two optical coupling structures. The two optical coupling structures are a duplicate of the beam splitter, an optical component of the interferometer. Therefore, when the sensor length and the two optical coupling structures vary, the time or path for optical transmission in the sensor changes, thereby influencing the back-end interference signals. The researchers successfully developed an asymmetrical FO-MZI with sensing abilities. The spacing value between the troughs of the sensor length and interference signal exhibited an inverse relationship. In addition, image analysis was employed to examine the size-matching relationship between various sensor lengths and the coupling and decoupling structure. Furthermore, the spectral wavelength shift results measured using a refractive index sensor indicate that FO-MZIs with a sensor length of 38 mm exhibited excellent sensitivity, measuring 59.7 nm/RIU.


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## Body

## 1. Introduction

Recently, several types of optical waveguide sensors or optical fiber-based refractometer have been proposed, including long period fiber gratings, Fabry-Perot interferometers, and Mach-Zehnder interferometers (MZI). The operation principle of the MZI by the first coupling point induces coupling of a fraction of the incident light propagating in the core mode to the cladding mode, and the second one performs the opposite function, generating the interference of light propagating in different optical paths. The fiber-optic Mach-Zehnder interferometer (FO-MZI) is a miniaturized and economical sensor component with label-free, real-time detection abilities. The FO-MZI does not require surface modification to induce surface plasmon resonance (SPR) [1] or particle plasmon resonance (PPR) [2]. The FO-MZI has been successfully employed for measuring refractive indices and temperatures and exhibits considerable future market potential [3]. The advantage of this type of sensor is relatively simple in construction, compact, low cost, ease of use, immunity to electromagnetic interference, and high sensitivity to the external refractive index (RI).This study proposes a coupling and decoupling structure (which is a duplicate of the beam splitter in traditional interferometers) for the FO-MZI; thus, the structural characteristics are identical to those of a beam splitter, inducing coupling and decoupling during fiber-optic transmissions [4], as shown in Figure 1. Previous studies have examined the FO-MZI; however, few have investigated the influence that sensor length (L) variations have on interference signals. The optical coupling and decoupling structure does not need to be linked to an optical source or spectrometer. Interference signals are obtained by satisfying the size-matching criterion for the specific structure.Optical transmission path.
(a)
The fiber-optic Mach-Zehnder interferometer
(b)
The traditional Mach-Zehnder interferometer


## 2. Experimental

### 2.1. Fabrication of FO-MZI Using a CO2 Laser

This study adopted a CO2 laser-processing method, which is simple, rapid, and flexible for correcting basic parameters. We employed a single-mode communication optical fiber as the material for producing the FO-MZI because intermodal dispersion does not exist in single modes; therefore, single modes can be applied to long-distance transmissions. Furthermore, the small size of the single-mode communication optical fiber is ideal (see Figure 2) [5].Figure 2
Single-mode optical fiber.The production process equipment for the CO2 laser is shown in Figure 3. The researchers used a crimping tool to remove the fiber cladding layer covering the area required to develop the coupling and decoupling structure. The optical fiber was subsequently placed on a CO2 laser-processing platform, and an amplified spontaneous emission CL waveband optical source was steered onto the left end of the optical fiber [3]. An optical spectrum analyzer was connected to the right end of the optical fiber. Suitable processing parameters for the CO2 laser were adjusted using a computer, and a heat removal method was employed, using the CO2 laser to remove the specific target area. Thus, the coupling and decoupling structure was established, satisfying the criterion for forming interference.Figure 3
Schematic of FO-MZI production process with CO2 laser.


### 2.2. Principle of FO-MZI

A schematic of FO-MZI is shown in Figure1(a), in which sensing length L denotes the distance between the two coupling points. The beam splitters of the MZI were made by forming two coupling regions at two positions separated by a distance L in the fiber. An incident light propagating as a core mode is split into the core mode and the cladding modes after passing through the first coupling region. The light of the cladding modes is recombined with the core mode after the second coupling region, resulting in an interference pattern if the phase matching condition is properly fulfilled. This section of single-mode fiber with a pair of coupling regions functions as a MZI interferometer. The interference fringe pattern can be finely tuned by adjusting the loss strength of the two coupling regions and their separation.


### 2.3. Sensor Length Control

This study covered the followingLs for the FO-MZI: 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm. Each length of FO-MZI samples comprised 10 sensors. Optical sources and spectrometers were used for real-time monitoring to observe the variations in interference signals for FO-MZIs with different lengths.


### 2.4. Measurements for the Coupling and Decoupling Structures

In this study, the FO-MZI produced using a CO2 laser was asymmetrical (see Figure 1). In other words, the coupling and decoupling structure was only on one side of the optical fiber. The removed area was small, the structure was simple, and the duration was short. Therefore, the occurrence of production errors was reduced. Because of its excellent mechanical strength, the optical fiber was not prone to breaking. In addition, the radially symmetric exterior of the optical fiber was not welded [7] or etched [8].The heat accumulation and thermal diffusion effects of the material used for CO2 laser-processing affect a portion of the material. Therefore, this section presents the use of a noncontact image-measuring instrument for measuring the size-matching relationship between FO-MZIs with different Ls to establish a database as a reference for future production processes. However, general detection platforms cannot be used to fixate the FO-MZI accurately, leading to measurement and determination errors, thereby reducing the reliability of the acquired size. To eliminate measurement errors, the fixture shown in Figure 4 was designed. At the center of this fixture, an auxiliary line was hypothesized. The center line divides two equal parts (see Figure 5). The optical coupling and decoupling structure was adjusted to satisfy the criterion of the center line, establish a side view of the structure, and achieve a vertical charged-coupled device lens for correction.Figure 4
Fixture for the image-measuring instrument: (a) spring struts, (b) silicone gasket, (c) detection platform, and (d) scale turntable.Figure 5
Image measurement technique.


### 2.5. Refractive Index Detection

As shown in the experiment framework presented in Figure6, we placed the FO-MZI on a detection platform and connected each end of the sensor to an optical source and spectrometer to detect variations in the interference signals. The test solution was then introduced into a butterfly needle catheter using a syringe. Deionized (DI) water with a refractive index of 1.333 RIU was injected, and sucrose solutions between 1.343 RIU and 1.373 RIU were separately injected into the microfluidic channel of the chip. The injected solutions interacted with the FO-MZI in the microfluidic channel, as shown in Figure 7. A shift in the interference signal was observed, and the FO-MZI having the optimal L, which exhibited excellent sensitivity, was subsequently identified.Figure 6
Schematic of refractive index detection using FO-MZI sensors.Figure 7
Structures of the FO-MZI sensor for refractive index detection.


## 2.1. Fabrication of FO-MZI Using a CO2 Laser

This study adopted a CO2 laser-processing method, which is simple, rapid, and flexible for correcting basic parameters. We employed a single-mode communication optical fiber as the material for producing the FO-MZI because intermodal dispersion does not exist in single modes; therefore, single modes can be applied to long-distance transmissions. Furthermore, the small size of the single-mode communication optical fiber is ideal (see Figure 2) [5].Figure 2
Single-mode optical fiber.The production process equipment for the CO2 laser is shown in Figure 3. The researchers used a crimping tool to remove the fiber cladding layer covering the area required to develop the coupling and decoupling structure. The optical fiber was subsequently placed on a CO2 laser-processing platform, and an amplified spontaneous emission CL waveband optical source was steered onto the left end of the optical fiber [3]. An optical spectrum analyzer was connected to the right end of the optical fiber. Suitable processing parameters for the CO2 laser were adjusted using a computer, and a heat removal method was employed, using the CO2 laser to remove the specific target area. Thus, the coupling and decoupling structure was established, satisfying the criterion for forming interference.Figure 3
Schematic of FO-MZI production process with CO2 laser.


## 2.2. Principle of FO-MZI

A schematic of FO-MZI is shown in Figure1(a), in which sensing length L denotes the distance between the two coupling points. The beam splitters of the MZI were made by forming two coupling regions at two positions separated by a distance L in the fiber. An incident light propagating as a core mode is split into the core mode and the cladding modes after passing through the first coupling region. The light of the cladding modes is recombined with the core mode after the second coupling region, resulting in an interference pattern if the phase matching condition is properly fulfilled. This section of single-mode fiber with a pair of coupling regions functions as a MZI interferometer. The interference fringe pattern can be finely tuned by adjusting the loss strength of the two coupling regions and their separation.


## 2.3. Sensor Length Control

This study covered the followingLs for the FO-MZI: 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm. Each length of FO-MZI samples comprised 10 sensors. Optical sources and spectrometers were used for real-time monitoring to observe the variations in interference signals for FO-MZIs with different lengths.


## 2.4. Measurements for the Coupling and Decoupling Structures

In this study, the FO-MZI produced using a CO2 laser was asymmetrical (see Figure 1). In other words, the coupling and decoupling structure was only on one side of the optical fiber. The removed area was small, the structure was simple, and the duration was short. Therefore, the occurrence of production errors was reduced. Because of its excellent mechanical strength, the optical fiber was not prone to breaking. In addition, the radially symmetric exterior of the optical fiber was not welded [7] or etched [8].The heat accumulation and thermal diffusion effects of the material used for CO2 laser-processing affect a portion of the material. Therefore, this section presents the use of a noncontact image-measuring instrument for measuring the size-matching relationship between FO-MZIs with different Ls to establish a database as a reference for future production processes. However, general detection platforms cannot be used to fixate the FO-MZI accurately, leading to measurement and determination errors, thereby reducing the reliability of the acquired size. To eliminate measurement errors, the fixture shown in Figure 4 was designed. At the center of this fixture, an auxiliary line was hypothesized. The center line divides two equal parts (see Figure 5). The optical coupling and decoupling structure was adjusted to satisfy the criterion of the center line, establish a side view of the structure, and achieve a vertical charged-coupled device lens for correction.Figure 4
Fixture for the image-measuring instrument: (a) spring struts, (b) silicone gasket, (c) detection platform, and (d) scale turntable.Figure 5
Image measurement technique.


## 2.5. Refractive Index Detection

As shown in the experiment framework presented in Figure6, we placed the FO-MZI on a detection platform and connected each end of the sensor to an optical source and spectrometer to detect variations in the interference signals. The test solution was then introduced into a butterfly needle catheter using a syringe. Deionized (DI) water with a refractive index of 1.333 RIU was injected, and sucrose solutions between 1.343 RIU and 1.373 RIU were separately injected into the microfluidic channel of the chip. The injected solutions interacted with the FO-MZI in the microfluidic channel, as shown in Figure 7. A shift in the interference signal was observed, and the FO-MZI having the optimal L, which exhibited excellent sensitivity, was subsequently identified.Figure 6
Schematic of refractive index detection using FO-MZI sensors.Figure 7
Structures of the FO-MZI sensor for refractive index detection.


## 3. Results and Discussion

### 3.1. The Relationship between Various Lengths and Interference Signal

The analysis results of the spacing value between troughs of variousLs are shown in Table 1. The results in Table 1 indicated that an inverse relationship existed between L and the average spacing value between troughs of various Ls. This inverse relationship results from variations in the interference signal caused by the direct influence that a specific L has on the time for transmitting split beams to the coupling structure [4, 9]. Consider
(1)Δλ×L=λ02Δneff,
where λ represents the center wavelength and Δneff=Δneffcore-Δneffcladding indicates the effective refractive index of various modes. Both values are fixed; therefore, when the L is increased, the spacing value Δλ between the troughs of the interference signal decreases. This finding verified the inverse relationship between the L and the spacing value between the troughs of the FO-MZI developed in this study, thereby validating the theoretical relationship. The fitting results were approximate to an inverse curve, which indicated that the basic requirements for the FO-MZI were fulfilled, as shown in Figure 8. The relationship between various Ls and the spacing values between troughs are shown in Figure 9. The average spacing is given by
(2)(Δλ1+Δλ2+⋯+Δλn)n,
where Δλn is the spacing value between the troughs of the interference signal and n is the number of spacing.Table 1
The relationship betweenL and interference signals.
The number of samples = 10
L (mm)
10
20
30
40
50
Average spacing (nm)= (Δλ1+Δλ2+⋯+Δλn)/n
45.9
26.6
16.8
13.8
9.9
Standard deviation (nm)
2.29
2.29
0.38
0.10
0.15Figure 8
The relationship between various lengths and the spacing value between the troughs.The relationship between various lengths and interference signals.
(a)
(b)
(c)
(d)
(e)


### 3.2. Optical Coupling and Decoupling Structures

The image measurement results of FO-MZIs with variousLs (see Table 2) show that the size matching of the structural width and depth varied according to the L, as shown in Figure 10. When the CO2 laser was used to produce a coupling and decoupling structure, an FO-MZI with interference signals can be established after satisfying the matching relationship criterion of the specific database. However, repeated processing resulted in greater removal of the material for the coupling and decoupling structure, directly affecting the depth of the structure.Table 2
The size matching of variousLs with the coupling and decoupling structure.
L (mm)
Width of removal (W) (μm)
Depth of removal (D) (μm)
Samples (sensors)
Mean
Standard deviation
Mean
Standard deviation
10
163.90
2.83
32.80
1.57
10
20
168.05
4.74
40.85
1.77
30
165.50
2.69
49.70
3.53
40
162.30
4.95
44.35
4.88
50
163.65
5.30
37.75
3.89Figure 10
The image measurement results of various lengths.


### 3.3. Refractive Index Measurement Results for Various Lengths

The experiments were repeated three times to measure the refractive index of variousLs according to spectral signal variations, as shown in Table 3. This study showed that the fluidic channel of general sensor chips is linear, whereas the FO-MZI is asymmetrical, as shown in Figure 11. Therefore, ensuring that the chip is placed precisely on the coupling and decoupling structure can be difficult during packaging. Other factors, such as the phase angle and test solution residue, can also influence the signal detection stability.Table 3
Refractive index measurement for various lengths (groove type).
Length(mm)
Number 01(nm/RIU)
Number 02(nm/RIU)
Number 03(nm/RIU)
Average(nm/RIU)
Standard deviation(nm/RIU)
10
17.3
16.3
17.0
16.87
0.51
20
12.2
9.6
10.4
10.73
1.33
30
19.5
19.5
15.6
18.20
2.25
40
54.4
54.4
54.4
54.40
0.00
50
38.4
38.2
38.4
38.33
0.11Figure 11
Influence of the linear fluidic channel chip.In response to this concern, this study proposed an improved groove chip, as shown in Figure12. Refractive index measurements of the linear and groove fluidic channel chips were obtained. The measurement results are shown in Table 4. Compared with the linear fluidic channel, the groove fluidic channel exhibited a higher linear regression value, indicating greater reliability. Furthermore, the sensing sensitivity of the groove fluidic channel chip rose from 45 nm/RIU to 54.4 nm/RIU. Therefore, the microfluidic channel of a sensor chips is another key factor that influences the sensing results. Subsequently, the L range decreased, as shown in Table 5, and an optimal sensitivity of 59.7 nm/RIU was detected at an L of 38 mm, as shown in Figure 13. Based on the results, groove or linear fluidic channel chip (shape), sensing length, and depth of removal are important fabrication parameters of the FO-MZI sensor. The sensor sensitivity of FO-MZI can be enhanced by these parameters which is based on the evanescent wave extends [10, 11].Table 4
Influence of the fluidic channel shape.
Length(mm)
Groove fluidicchannel chip
Linear fluidicchannel chip
Slope(nm/RIU)
R-square
Slope(nm/RIU)
R-square
10
17.3
0.98856
30.4
0.96057
20
12.2
0.99964
7.5
0.95356
30
19.5
0.97627
20.8
0.97674
40
54.4
0.97812
45.0
0.92771
50
38.4
0.97728
20.8
0.97674Table 5
Refractive index measurement results for the reducedL range.
Length (mm)
Slope (nm/RIU)
R-square
8
12.6
0.95677
13
34.1
0.89198
38
59.7
0.93973
43
38.6
0.91275Figure 12
Linear and groove fluidic channel chips.Figure 13
Refractive index measurement results for the reducedL range.


## 3.1. The Relationship between Various Lengths and Interference Signal

The analysis results of the spacing value between troughs of variousLs are shown in Table 1. The results in Table 1 indicated that an inverse relationship existed between L and the average spacing value between troughs of various Ls. This inverse relationship results from variations in the interference signal caused by the direct influence that a specific L has on the time for transmitting split beams to the coupling structure [4, 9]. Consider
(1)Δλ×L=λ02Δneff,
where λ represents the center wavelength and Δneff=Δneffcore-Δneffcladding indicates the effective refractive index of various modes. Both values are fixed; therefore, when the L is increased, the spacing value Δλ between the troughs of the interference signal decreases. This finding verified the inverse relationship between the L and the spacing value between the troughs of the FO-MZI developed in this study, thereby validating the theoretical relationship. The fitting results were approximate to an inverse curve, which indicated that the basic requirements for the FO-MZI were fulfilled, as shown in Figure 8. The relationship between various Ls and the spacing values between troughs are shown in Figure 9. The average spacing is given by
(2)(Δλ1+Δλ2+⋯+Δλn)n,
where Δλn is the spacing value between the troughs of the interference signal and n is the number of spacing.Table 1
The relationship betweenL and interference signals.
The number of samples = 10
L (mm)
10
20
30
40
50
Average spacing (nm)= (Δλ1+Δλ2+⋯+Δλn)/n
45.9
26.6
16.8
13.8
9.9
Standard deviation (nm)
2.29
2.29
0.38
0.10
0.15Figure 8
The relationship between various lengths and the spacing value between the troughs.The relationship between various lengths and interference signals.
(a)
(b)
(c)
(d)
(e)


## 3.2. Optical Coupling and Decoupling Structures

The image measurement results of FO-MZIs with variousLs (see Table 2) show that the size matching of the structural width and depth varied according to the L, as shown in Figure 10. When the CO2 laser was used to produce a coupling and decoupling structure, an FO-MZI with interference signals can be established after satisfying the matching relationship criterion of the specific database. However, repeated processing resulted in greater removal of the material for the coupling and decoupling structure, directly affecting the depth of the structure.Table 2
The size matching of variousLs with the coupling and decoupling structure.
L (mm)
Width of removal (W) (μm)
Depth of removal (D) (μm)
Samples (sensors)
Mean
Standard deviation
Mean
Standard deviation
10
163.90
2.83
32.80
1.57
10
20
168.05
4.74
40.85
1.77
30
165.50
2.69
49.70
3.53
40
162.30
4.95
44.35
4.88
50
163.65
5.30
37.75
3.89Figure 10
The image measurement results of various lengths.


## 3.3. Refractive Index Measurement Results for Various Lengths

The experiments were repeated three times to measure the refractive index of variousLs according to spectral signal variations, as shown in Table 3. This study showed that the fluidic channel of general sensor chips is linear, whereas the FO-MZI is asymmetrical, as shown in Figure 11. Therefore, ensuring that the chip is placed precisely on the coupling and decoupling structure can be difficult during packaging. Other factors, such as the phase angle and test solution residue, can also influence the signal detection stability.Table 3
Refractive index measurement for various lengths (groove type).
Length(mm)
Number 01(nm/RIU)
Number 02(nm/RIU)
Number 03(nm/RIU)
Average(nm/RIU)
Standard deviation(nm/RIU)
10
17.3
16.3
17.0
16.87
0.51
20
12.2
9.6
10.4
10.73
1.33
30
19.5
19.5
15.6
18.20
2.25
40
54.4
54.4
54.4
54.40
0.00
50
38.4
38.2
38.4
38.33
0.11Figure 11
Influence of the linear fluidic channel chip.In response to this concern, this study proposed an improved groove chip, as shown in Figure12. Refractive index measurements of the linear and groove fluidic channel chips were obtained. The measurement results are shown in Table 4. Compared with the linear fluidic channel, the groove fluidic channel exhibited a higher linear regression value, indicating greater reliability. Furthermore, the sensing sensitivity of the groove fluidic channel chip rose from 45 nm/RIU to 54.4 nm/RIU. Therefore, the microfluidic channel of a sensor chips is another key factor that influences the sensing results. Subsequently, the L range decreased, as shown in Table 5, and an optimal sensitivity of 59.7 nm/RIU was detected at an L of 38 mm, as shown in Figure 13. Based on the results, groove or linear fluidic channel chip (shape), sensing length, and depth of removal are important fabrication parameters of the FO-MZI sensor. The sensor sensitivity of FO-MZI can be enhanced by these parameters which is based on the evanescent wave extends [10, 11].Table 4
Influence of the fluidic channel shape.
Length(mm)
Groove fluidicchannel chip
Linear fluidicchannel chip
Slope(nm/RIU)
R-square
Slope(nm/RIU)
R-square
10
17.3
0.98856
30.4
0.96057
20
12.2
0.99964
7.5
0.95356
30
19.5
0.97627
20.8
0.97674
40
54.4
0.97812
45.0
0.92771
50
38.4
0.97728
20.8
0.97674Table 5
Refractive index measurement results for the reducedL range.
Length (mm)
Slope (nm/RIU)
R-square
8
12.6
0.95677
13
34.1
0.89198
38
59.7
0.93973
43
38.6
0.91275Figure 12
Linear and groove fluidic channel chips.Figure 13
Refractive index measurement results for the reducedL range.


## 4. Conclusion

This study employed a CO2 laser and successfully developed an asymmetric FO-MZI with a sensing function. The sensor length L between the two removals on the fiber surface and the spacing value between the troughs of the interference signal exhibited an inverse relationship. An image analysis method was proposed to examine the relationship between various Ls and the size matching of the coupling and decoupling structures. For future production of the FO-MZI using CO2 lasers, coupling and decoupling structures with interference can be established by referencing the database proposed in this study. Finally, the results of the refractive index measurements for the spectral wavelength shifts of the FO-MZIs with various Ls indicate that a superior sensitivity (59.7 nm/RIU) was exhibited at an L of 38 mm. The asymmetrical FO-MZI proposed in this study had significant advantages, including a simple structure and an easy production process. Among all interferometer types (with a coupling or decoupling structure), the refractive index measurement results of the FO-MZI exhibited higher sensitivity, as shown in Table 6. Proper selections of groove or linear fluidic channel chip (shape), sensing length, and depth of removal could yield even higher sensitivity. The unique sensing features are particularly suited for a wide variety of applications in smart structures, gain equalizer, telecommunications, and optical-sensor systems.Table 6
The sensitivity of various optical fiber interferometers.
Optical fiber interferometer type
Refractive index(RIU)
Optimal sensitivity value(nm/RIU)
Symmetrical [6]
1.333~1.361
26.27
Fabry-Perot [7]
1.315~1.362
29.00
Mismatch welding [7]
1.315~1.362
33.30
Asymmetrical
1.333~1.373
59.70



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*Source: 938693-2014-06-15.xml*