**2. Evaluation of compatibility and co-operation on fiber-optic FBG sensor and single channel 10 Gbit/s NRZ-OOK transmission system**

In this section, FBG temperature sensor integration and co-operation with operating fiber-optical transmission system are experimentally evaluated in the laboratory environment. The transmitter part of the experimental setup (see **Figure 1**) includes a broadband light source - ASE source, which is necessary to provide the operation of the deployed FBG sensor.

The measured output spectrum of the ASE light source is shown in **Figure 2**. The maximal peak power of around −10 dBm is located in wavelength bands of 1532–1534 and 1550–1560 nm. The high output power of the ASE light source and FBG sensor reflectivity is essential when monitoring distance (between the OSSI unit and FBG sensor) is long. Broad spectral band and fixed output power of the light source spectrum are crucial for multiplexing many sensors.

The output of the ASE light source is connected with an optical bandpass filter (OBPF). An OBPF (wavelength range: 1530 to 1610 nm (C&L Band), crosstalk >50 dB, bandwidth: 0.2 to 10.0 nm) is used to filtered spectral band for FBG temperature sensor. The spectral band is calculated based on FBG sensor defined operating

#### **Figure 1.**

*Experimental setup of single–channel 10 Gbit/s transmission system with integrated optical FBG sensor.*

**69**

**Figure 3.**

26°C ( λ

**Table 1.**

**Figure 2.**

*Measured ASE output spectrum.*

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems*

temperature band (−20 to +40°C). Temperature change by one degree causes a wavelength shift of 10.174 pm, taking into account the FBG reference temperature of

Central wavelength and frequency values for FBG temperature sensors are shown in **Table 1**. The wavelength band from 1565.05 to 1565.66 nm are set as a bandwidth of OBPF. The measured spectral curve of the OBPF passband is shown in **Figure 3**. The output of OBPF is connected to one of the optical coupler ports. For the generation of FOTS data channel signal, the tunable laser diode (LD) with +9 (fiber length 20 km) and 12 dBm (fiber length 40 km) output power, 100 kHz linewidth, 50 dB sidemode suppression ratio (SMSR) is used. LD output is connected with Mach-Zehnder modulator (MZM) with polarization-maintaining PANDA type

**Temperature Wavelength [nm] Frequency [THz]** −20 °C 1565.66 191.48 26°C 1565.19 191.54 40°C 1565.05 191.55

*FBG temperature sensor central wavelength and frequency values.*

*Measured spectral curve of the OBPF filter passband.*

ficient and the thermal-optic coefficient of common single-mode fiber.

*ref* = 1565.191 *nm* ). Wavelength shift depends on thermal-expansion coef-

*DOI: http://dx.doi.org/10.5772/intechopen.94289*

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems DOI: http://dx.doi.org/10.5772/intechopen.94289*

**Figure 2.** *Measured ASE output spectrum.*

*Application of Optical Fiber in Engineering*

sors can be longer than 40 km.

spontaneous emission (ASE) light source.

operation of the deployed FBG sensor.

Section 2 of this article.

It is necessary to analyze sensor influence on deployed and operating fiber optical communications systems data channels before FBG sensors integration in this fiber optical network infrastructure. Optical sensors signal interrogation (OSSI) units maximal monitoring distance between monitoring equipment and FBG sen-

First, in this paper optical sensor and single–channel 10 Gbit/s transmission system compatibility and co-operation were experimentally evaluated in the fiber-optical transmission system (FOTS) laboratory of Riga Technical University, Communication Technologies Research Center (RTU SSTIC), as described in

Further, we have also demonstrated the collaboration with 32-channel spectrum-sliced wavelength-division-multiplexing passive optical network (SS-WDM PON) data channels and FBG sensor network in the simulation environment, as described in Section 3 of this article. Results showed that the optical transmission system with SS-WDM PON data and FBG sensor channels is an energy and costefficient solution, because its transmitter part is realized using a single amplified

**2. Evaluation of compatibility and co-operation on fiber-optic FBG sensor and single channel 10 Gbit/s NRZ-OOK transmission system**

In this section, FBG temperature sensor integration and co-operation with operating fiber-optical transmission system are experimentally evaluated in the laboratory environment. The transmitter part of the experimental setup (see **Figure 1**) includes a broadband light source - ASE source, which is necessary to provide the

The measured output spectrum of the ASE light source is shown in **Figure 2**. The maximal peak power of around −10 dBm is located in wavelength bands of 1532–1534 and 1550–1560 nm. The high output power of the ASE light source and FBG sensor reflectivity is essential when monitoring distance (between the OSSI unit and FBG sensor) is long. Broad spectral band and fixed output power of the

The output of the ASE light source is connected with an optical bandpass filter (OBPF). An OBPF (wavelength range: 1530 to 1610 nm (C&L Band), crosstalk >50 dB, bandwidth: 0.2 to 10.0 nm) is used to filtered spectral band for FBG temperature sensor. The spectral band is calculated based on FBG sensor defined operating

*Experimental setup of single–channel 10 Gbit/s transmission system with integrated optical FBG sensor.*

light source spectrum are crucial for multiplexing many sensors.

**68**

**Figure 1.**

temperature band (−20 to +40°C). Temperature change by one degree causes a wavelength shift of 10.174 pm, taking into account the FBG reference temperature of 26°C ( λ*ref* = 1565.191 *nm* ). Wavelength shift depends on thermal-expansion coefficient and the thermal-optic coefficient of common single-mode fiber.

Central wavelength and frequency values for FBG temperature sensors are shown in **Table 1**. The wavelength band from 1565.05 to 1565.66 nm are set as a bandwidth of OBPF. The measured spectral curve of the OBPF passband is shown in **Figure 3**. The output of OBPF is connected to one of the optical coupler ports.

For the generation of FOTS data channel signal, the tunable laser diode (LD) with +9 (fiber length 20 km) and 12 dBm (fiber length 40 km) output power, 100 kHz linewidth, 50 dB sidemode suppression ratio (SMSR) is used. LD output is connected with Mach-Zehnder modulator (MZM) with polarization-maintaining PANDA type


#### **Table 1.**

*FBG temperature sensor central wavelength and frequency values.*

**Figure 3.** *Measured spectral curve of the OBPF filter passband.*

fiber. Electrical data signals are generated by the pattern generator (PPG) (Anritsu, operating bitrate 10 Gbit/s, PRBS 215–1, signal purity −75 dBc/Hz). PPG data output and electrical RF input of the MZM is connected with proper RF cable. MZM optical output is connected with one of the optical power coupler (OPC) ports. OPC couples signals for FBG sensor and FOTS data channels.

OPC output is connected with the optical circulator (OC) port (1), necessary for separation of the sensor systems optical signal flows (transmitted and reflected). Please see the measured optical circulator insertion loss values in **Table 2**.

The optical circulator port (2) is connected with the optical fiber line and FBG sensor. 20 and 40 km long single-mode optical fiber (SMF-28) spools with insertion loss 4.3 and 8.3 dB are used in these experiments. The optical fiber output is connected with the FBG temperature sensor. FBG sensor structure and operation principle are shown in **Figure 4**.

FBG sensor technology is based on periodical reflection index changes in the fiber core [9–12]. FBG sensor reflects one part of the signal, but another part is transmitted further through the optical fiber. If the object's temperature changes, it shifts transmitted and reflected Bragg wavelength ( λ*<sup>B</sup>* ), also known as signal central wavelength (see in **Figure 5**). OSA1 and OSA2 are used for the analysis of the FBG temperature sensor reflected and transmitted signals.

Bragg wavelength ( λ*<sup>B</sup>* ) can be described by the following formula (1):

$$
\mathcal{A}\_{\mathfrak{B}} = \mathfrak{Z} \cdot \mathfrak{n}\_{\mathfrak{eff}} \cdot \wedge \tag{1}
$$

Where:

Λ − grating period, nm;

*neff* − effective group reflection index;

λ*<sup>B</sup>* − Bragg wavelength, nm [10].

FBG sensor temperature is calculated, based on the formula (2):

$$\mathbf{t} = \mathbf{t}\_{ref} + \left(\frac{-\mathcal{\lambda}\_{ref} + \mathcal{\lambda}\_{maa}}{\Delta \mathcal{\lambda}\_{oo}}\right) \tag{2}$$

**71**

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems*

As we can see in formula 2, and the measured graph (**Figure 6**), the temperature

*ref* ) 1565.191 nm

*coe* ) 10.174 pm

FBG output is connected with 40-channel (100 GHz channel spacing) arrayed waveguide grating (AWG) flat-top filter with operating wavelength band 1530.334– 1561.419 nm (192–195.9 THz). The AWG is used to test the system with different spacing between data and sensor channels. The AWG (with 54 GHz 3-dB and 132 GHz 20-dB bandwidth) 29th and 40th channels are used in experiments, and its

AWG output is connected with optical power splitter (20:80%), where 20% are used for signal power monitoring, but 80% are transmitted to photodiode (PD) (with sensitivity (1e-10 BER) = −20 dBm, operation wavelength range = 1280 nm–1580 nm and maximum output voltage = 350 mVp-p) that

versus wavelength relationship has linear nature.

*Parameters of experimentally used FBG temperature sensor.*

λ

λ

*Measured reflected FBG sensor signal spectrum at different temperatures.*

**FBG sensor parameter Value** Reference temperature ( *ref t* ) 26°C

Sensor size 3 × 3 × 23 mm Operation temperature band −40 °C to +80°C

parameters are listed in **Table 4**.

*DOI: http://dx.doi.org/10.5772/intechopen.94289*

**Figure 4.**

**Figure 5.**

**Table 3.**

Reference wavelength (

Wavelength coefficient ( ∆

*FBG structure and operation principle.*

Where:

*ref t* - reference temperature (defined in the sensor specification), o C

λ*ref* – reference wavelength (defined in the sensor specification), nm

λ*mea* - measured wavelength value, nm

∆λ*coe* – wavelength coefficient, describing wavelength shift, when temperature is changed per 1°C (defined in sensor specification), nm

Temperature sensor parameters used in experiments are listed in **Table 3**.


**Table 2.**

*The insertion loss of experimentally used optical circulator.*

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems DOI: http://dx.doi.org/10.5772/intechopen.94289*

**Figure 4.**

*Application of Optical Fiber in Engineering*

principle are shown in **Figure 4**.

Bragg wavelength (

Λ − grating period, nm;

Where:

Where:

λ

λ

∆λ

λ

signals for FBG sensor and FOTS data channels.

shifts transmitted and reflected Bragg wavelength (

λ

*neff* − effective group reflection index;

*mea* - measured wavelength value, nm

is changed per 1°C (defined in sensor specification), nm

1 2 → 2 2 3 → 1 1 3 → 60

*The insertion loss of experimentally used optical circulator.*

**Direction Measured insertion loss (dB)**

*<sup>B</sup>* − Bragg wavelength, nm [10].

FBG temperature sensor reflected and transmitted signals.

fiber. Electrical data signals are generated by the pattern generator (PPG) (Anritsu, operating bitrate 10 Gbit/s, PRBS 215–1, signal purity −75 dBc/Hz). PPG data output and electrical RF input of the MZM is connected with proper RF cable. MZM optical output is connected with one of the optical power coupler (OPC) ports. OPC couples

OPC output is connected with the optical circulator (OC) port (1), necessary for separation of the sensor systems optical signal flows (transmitted and reflected).

The optical circulator port (2) is connected with the optical fiber line and FBG sensor. 20 and 40 km long single-mode optical fiber (SMF-28) spools with insertion loss 4.3 and 8.3 dB are used in these experiments. The optical fiber output is connected with the FBG temperature sensor. FBG sensor structure and operation

FBG sensor technology is based on periodical reflection index changes in the fiber core [9–12]. FBG sensor reflects one part of the signal, but another part is transmitted further through the optical fiber. If the object's temperature changes, it

central wavelength (see in **Figure 5**). OSA1 and OSA2 are used for the analysis of the

λ

FBG sensor temperature is calculated, based on the formula (2):

*ref*

*ref t* - reference temperature (defined in the sensor specification), o

*ref* – reference wavelength (defined in the sensor specification), nm

Temperature sensor parameters used in experiments are listed in **Table 3**.

*t t*

λ

*B eff* = ⋅ ⋅∧ 2 *n* (1)

<sup>∆</sup> (2)

C

*<sup>B</sup>* ) can be described by the following formula (1):

*ref mea*

λ λ

*coe*

λ

*coe* – wavelength coefficient, describing wavelength shift, when temperature

 − + = + *<sup>B</sup>* ), also known as signal

Please see the measured optical circulator insertion loss values in **Table 2**.

**70**

**Table 2.**

*FBG structure and operation principle.*

#### **Figure 5.**

*Measured reflected FBG sensor signal spectrum at different temperatures.*


#### **Table 3.**

*Parameters of experimentally used FBG temperature sensor.*

As we can see in formula 2, and the measured graph (**Figure 6**), the temperature versus wavelength relationship has linear nature.

FBG output is connected with 40-channel (100 GHz channel spacing) arrayed waveguide grating (AWG) flat-top filter with operating wavelength band 1530.334– 1561.419 nm (192–195.9 THz). The AWG is used to test the system with different spacing between data and sensor channels. The AWG (with 54 GHz 3-dB and 132 GHz 20-dB bandwidth) 29th and 40th channels are used in experiments, and its parameters are listed in **Table 4**.

AWG output is connected with optical power splitter (20:80%), where 20% are used for signal power monitoring, but 80% are transmitted to photodiode (PD) (with sensitivity (1e-10 BER) = −20 dBm, operation wavelength range = 1280 nm–1580 nm and maximum output voltage = 350 mVp-p) that

**Figure 6.**

*Temperature versus wavelength relationship for experimentally used FBG temperature sensor.*


#### **Table 4.**

*AWG filter parameters 29th and 40th channel.*

converts optical signal to electrical signal. PD output is fed through an RF cable to the eye diagram analyzer (EDA) for data signal quality analysis. For synchronization, the PPG clock signal is transmitted with RF cable to EDA.

Please see the measured FOTS data channel and FBG temperature sensor reflected spectrum in **Figures 7** and **8**, respectively. FBG temperature sensor reflected signal central wavelength is 1565.1279 nm, and the temperature (calculated with formula (2)) is 19.7°C.

**73**

**Figure 9.**

*temperature sensor.*

**Figure 8.**

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems*

Experimentally measured eye diagrams for NRZ-OOK modulated 10 Gbit/s FOTS (after 20 and 40 km long transmission) with and without integrated FBG temperature sensor are shown in **Figure 9**. As we can see in **Figure 9**, the data channel eye diagrams' quality is not degraded by the FBG sensor. Dispersion influence can be observed in eye diagrams (c, d) after 40 km signal transmission, which

*Comparison of NRZ-OOK modulated 10 Gbit/s FOTS experimental eye diagrams. (a) 20 km FOTS. (b) 20 km FOTS with integrated FBG temperature sensor. (c) 40 km FOTS. (d) 40 km FOTS with integrated FBG* 

*DOI: http://dx.doi.org/10.5772/intechopen.94289*

*Measured reflected spectrum of FBG temperature sensor.*

**Figure 7.** *Measured spectrum of transmission data channel.*

*Fiber Bragg Grating Sensors Integration in Fiber Optical Systems DOI: http://dx.doi.org/10.5772/intechopen.94289*

*Application of Optical Fiber in Engineering*

**Figure 6.**

**Table 4.**

converts optical signal to electrical signal. PD output is fed through an RF cable to the eye diagram analyzer (EDA) for data signal quality analysis. For synchroniza-

**No. of AWG Channel Central wavelength [nm] Frequency [THz] Attenuation [dB]**

29 1552.524 193.1 3.91 4.05 40 1561.419 192.0 4.28 4.41

**min max**

Please see the measured FOTS data channel and FBG temperature sensor reflected spectrum in **Figures 7** and **8**, respectively. FBG temperature sensor reflected signal central wavelength is 1565.1279 nm, and the temperature (calcu-

tion, the PPG clock signal is transmitted with RF cable to EDA.

*Temperature versus wavelength relationship for experimentally used FBG temperature sensor.*

lated with formula (2)) is 19.7°C.

*Measured spectrum of transmission data channel.*

*AWG filter parameters 29th and 40th channel.*

**72**

**Figure 7.**

**Figure 8.** *Measured reflected spectrum of FBG temperature sensor.*

Experimentally measured eye diagrams for NRZ-OOK modulated 10 Gbit/s FOTS (after 20 and 40 km long transmission) with and without integrated FBG temperature sensor are shown in **Figure 9**. As we can see in **Figure 9**, the data channel eye diagrams' quality is not degraded by the FBG sensor. Dispersion influence can be observed in eye diagrams (c, d) after 40 km signal transmission, which

#### **Figure 9.**

*Comparison of NRZ-OOK modulated 10 Gbit/s FOTS experimental eye diagrams. (a) 20 km FOTS. (b) 20 km FOTS with integrated FBG temperature sensor. (c) 40 km FOTS. (d) 40 km FOTS with integrated FBG temperature sensor.*

can be prevented with chromatic dispersion (CD) compensation, such as dispersion compensation fiber (DCF).
