# Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Theoretical Background

#### 2.1. Amplitude-Modulated Incoherent Frequency-Domain Reflectometry

#### 2.2. I-OFDR Using Homodyne Electro-Optic Downconversion

## 3. Experimental System and Signal Processing

## 4. Results

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

- Kapron, F.P.; Kneller, D.G.; Garel-Jones, P.M. Aspects of optical frequency-domain reflectometry. In Technical Digest of the International Conference on Integrated Optics and Optical Fiber Communication; Optical Society of America: San Francisco, CA, USA, 1981. [Google Scholar]
- Ghafoori-Shiraz, H.; Okoshi, T. Fault location in optical fibers using optical frequency domain reflectometry. J. Light. Technol.
**1986**, 4, 316–322. [Google Scholar] [CrossRef] - Nakayama, J.; Iizuka, K.; Nielsen, J. Optical fiber fault locator by the step frequency method. Appl. Opt.
**1987**, 1987 26, 440–443. [Google Scholar] [CrossRef] - Schlemmer, B. A simple and very effective method with improved sensitivity for fault location in optical fibers. IEEE Photonics Technol. Lett.
**1991**, 3, 1037–1039. [Google Scholar] [CrossRef] - Dolfi, D.W.; Nazarathy, M.; Newton, S.A. 5-mm-resolution optical-frequency-domain reflectometry using a coded phase-reversal modulator. Opt. Lett.
**1988**, 13, 678. [Google Scholar] [CrossRef] - Nazarathy, M.; Dolfi, D.W. Optical frequency domain reflectometry with high sensitivity and resolution using synchronous detection with coded modulators. Electron. Lett.
**1989**, 25, 160–162. [Google Scholar] - Derickson, D. Fiber Optics Test and Measurement; Prentice Hall: Upper Saddle River, NJ, USA, 1998. [Google Scholar]
- Keysight Technologies. Time-Domain Analysis Using A Network Analyzer. Available online: Http://literature.cdn.keysight.com/litweb/pdf/5989-5723EN.pdf (accessed on 4 May 2019).
- Hiebel, M. Fundamentals of Vector Network Analysis; Rohde & Schwarz: Munich, Germany, 2005. [Google Scholar]
- Ricchiuti, A.L.; Barrera, D.; Sales, S.; Thévenaz, L.; Capmany, J. Long fiber Bragg grating sensor interrogation using discrete-time microwave photonic filtering techniques. Opt. Express
**2013**, 21, 28175–28181. [Google Scholar] [CrossRef] [PubMed] - Werzinger, S.; Bergdolt, S.; Engelbrecht, R.; Thiel, T.; Schmauss, B. Quasi-distributed fiber Bragg grating sensing using steeped incoherent optical frequency domain reflectometry. J. Light. Technol.
**2016**, 34, 5270–5277. [Google Scholar] [CrossRef] - Wei, T.; Huang, J.; Han, Q.; Xiao, H. Optical fiber sensor based on a radio frequency Mach-Zehnder interferometer. Opt. Lett.
**2012**, 37, 647–649. [Google Scholar] [CrossRef] - Huang, J.; Lan, X.; Luo, M.; Xiao, H. Spatially continuous distributed fiber optic sensing using carrier based microwave interferometry. Opt. Express.
**2014**, 22, 18758–18767. [Google Scholar] [CrossRef] [PubMed] - Huang, J.; Lan, X.; Song, Y.; Li, Y.; Hua, L.; Xiao, H. Microwave interrogated sapphire fiber Michelson interferometer for high temperature sensing. IEEE Photonics Technol. Lett.
**2015**, 27, 1398–1401. [Google Scholar] [CrossRef] - Xu, Z.; Shu, X.; Fu, H. Sensitivity enhanced fiber sensor based on a fiber ring microwave photonic filter with the Vernier effect. Opt. Express
**2017**, 25, 21560–21566. [Google Scholar] [CrossRef] - Cheng, B.; Hua, L.; Zhang, Q.; Lei, J.; Xiao, H. Microwave-assisted frequency domain measurement of fiber-loop ring-down system. Opt. Lett.
**2017**, 42, 1209–1212. [Google Scholar] [CrossRef] - Liehr, S.; Nöther, N.; Krebber, K. Incoherent optical frequency domain reflectometry and distributed strain detection in polymer optical fibers. Meas. Sci. Technol.
**2009**, 21, 017001. [Google Scholar] [CrossRef] - Garus, D.; Gogolla, T.; Krebber, K.; Schliep, F. Brillouin optical-fiber frequency-domain analysis for distributed temperature and strain sensing. J. Light. Technol.
**1997**, 15, 654–662. [Google Scholar] [CrossRef] - Karamehmedović, E.; Glombitza, U. Fiber-optic distributed temperature sensor using incoherent optical frequency domain reflectometry. Proc. SPIE
**2004**, 5363, 107–115. [Google Scholar] - Köppel, M.; Werzinger, S.; Ringel, T.; Bechtold, P.; Thiel, T.; Engelbrecht, R.; Bosselmann, T.; Schmauss, B. Combined distributed Raman and Bragg fiber temperature sensing using incoherent optical frequency domain reflectometry. J. Sens. Sens. Syst.
**2018**, 7, 91–100. [Google Scholar] [CrossRef] - Hervás, J.; Ricchiuti, A.L.; Li, W.; Zhu, N.H.; Fernández-Pousa, C.R.; Sales, S.; Li, M.; Capmany, J. Microwave Photonics for optical sensors. IEEE J. Sel. Top. Quantum Electron.
**2017**, 23, 327–339. [Google Scholar] [CrossRef] - Xi, L.; Cheng, R.; Li, W.; Liu, D. Identical FBG-based quasi-distributed sensing by monitoring the microwave responses. IEEE Photonics Technol. Lett.
**2015**, 27, 323–325. [Google Scholar] [CrossRef] - Werzinger, S.; Härteis, L.-S.; Koeppel, M.; Schmauss, B. Time and wavelength division multiplexing of fiber Bragg gratings with bidirectional electro-optical frequency conversion. In Proceedings of the 26th International Conference on Optical Fiber Sensors, Optical Society of America, Lausanne, Switzerland, 24–28 of September 2018. paper ThE19. [Google Scholar]
- Mao, W.; Kim, H.H.; Pan, J.K. A novel fiber Bragg grating sensing interrogation method using bidirectional modulation of a Mach-Zehnder electro-optical modulator. In Proceedings of the OECC 2010 Technical Digest, Sapporo, Japan, 5–9 July 2010; pp. 314–315. [Google Scholar]
- Hervás, J.; Fernández-Pousa, C.R.; Barrera, D.; Pastor, D.; Sales, S.; Capmany, J. An interrogation technique of FBG cascade sensors using wavelength to radio-frequency delay mapping. J. Light. Technol.
**2015**, 33, 2222–2227. [Google Scholar] [CrossRef] - Clement, J.; Torregrosa, G.; Hervás, J.; Barrera, D.; Sales, S.; Fernández-Pousa, C.R. Interrogation of a sensor array of identical weak FBGs using dispersive incoherent OFDR. IEEE Photonics Technol. Lett.
**2016**, 28, 1154–1156. [Google Scholar] [CrossRef] - Clement, J.; Torregrosa, G.; Maestre, H.; Fernández-Pousa, C.R. Remote picometer fiber Bragg grating demodulation using a dual-wavelength source. Appl. Opt.
**2016**, 55, 6523–6529. [Google Scholar] [CrossRef] [PubMed] - Cheng, R.; Xia, L.; Ran, Y.; Rohollahnejad, J.; Zhou, J.; Wen, J. Interrogation of ultrashort Bragg grating sensors using shifted gaussian filters. IEEE Photonics Technol. Lett.
**2015**, 27, 1833–1836. [Google Scholar] [CrossRef] - Werzinger, S.; Gottinger, M.; Gussner, S.; Bergdolt, S.; Engelbrecht, R.; Schmauss, B. Model-based compressed sensing of fiber Bragg grating arrays in the frequency domain. Proc. SPIE
**2017**, 10323, 103236H. [Google Scholar] - Clement, J.; Hervás, J.; Madrigal, J.; Maestre, H.; Torregrosa, G.; Fernández-Pousa, C.R.; Sales, S. Fast incoherent OFDR interrogation of FBG arrays using sparse radio frequency responses. J. Light. Technol.
**2018**, 36, 4393–4400. [Google Scholar] [CrossRef] - Urban, P.J.; Amaral, G.C.; von der Weid, J.P. Fiber monitoring using a sub-carrier band in a sub-carrier multiplexed radio-over-fiber transmission system for applications in analog mobile fronthaul. J. Light. Technol.
**2016**, 34, 3118–3125. [Google Scholar] [CrossRef] - Amaral, G.C.; Baldivieso, A.; Dias Garcia, J.; Villafani, D.C.; Leibel, R.C.; Herrera, L.E.Y.; Urban, P.J.; von der Weid, J.P. A low-frequency tone sweep method for in-service fault location in sub-carrier multiplexed optical fiber networks. J. Light. Technol.
**2017**, 35, 2017–2025. [Google Scholar] [CrossRef] - Ye, F.; Zhang, Y.; Qian, L. Frequency-shifted interferometry – a versatile fiber-optic sensing technique. Sensors
**2014**, 14, 10977–11000. [Google Scholar] [CrossRef] - Xiong, Z.; Zhou, C.; Guo, H.; Fan, D.; Ou, Y.; Liu, Y.; Sun, C.; Qian, L. High spatial resolution multiplexing of fiber Bragg gratings using single-arm frequency-shifted interferometry. Proc. SPIE
**2017**, 10323, 103233S. [Google Scholar] - Clement, J.; Maestre, H.; Torregrosa, G.; Fernández-Pousa, C.R. Incoherent optical frequency domain reflectometry using balanced frequency-shifter interferometry in a downconverted phase-modulated link. In Proceedings of the 2018 International Topical Meeting on Microwave Photonics (MWP), Toulouse, France, 22–25 October 2018; pp. 1–4. [Google Scholar]
- Porte, H.; Mottet, A. Band pass & low-voltage symmetrical electro-optic modulator for absolute distance metrology. In Proceedings of the 2018 International Topical Meeting on Microwave Photonics (MWP), Toulouse, France, 22–25 October 2018; pp. 1–4. [Google Scholar]
- Qi, B.; Qian, L.; Tausz, A.; Lo, H.-K. Frequency-shifted Mach-Zehnder interferometer for locating multiple weak reflections along a fiber link. IEEE Photonics Technol. Lett.
**2006**, 18, 295–297. [Google Scholar] - Proakis, J.G.; Manolakis, D.G. Digital Signal Processing; Pearson: Upper Saddle River, NJ, USA, 1996. [Google Scholar]
- Anderson, D.R.; Johnson, L.; Bell, F.G. Troubleshooting Optical-Fiber Networks; Elsevier: San Diego, CA, USA, 2004. [Google Scholar]
- Kaiser, J.F.; Schafer, R.W. On the use of the I
_{0}-sinh window for spectrum analysis. IEEE Trans. Acoust. Speech Signal Process.**1980**, 28, 105–107. [Google Scholar] [CrossRef] - Shadaram, M.; Kuriger, W.L. Using the optical frequency domain technique for the analysis of discrete and distributed reflections in an optical fiber. Appl. Opt.
**1984**, 23, 1092–1096. [Google Scholar] [CrossRef] [PubMed] - Liehr, S. Fiber Optic Sensing Techniques Based On Incoherent Optical Frequency Domain Reflectometry. Ph.D. Thesis, BAM, Berlin, Germany, 2015. [Google Scholar]
- Ibrahim, S.K.; Van Roosbroeck, J.; O’Dowd, J.A.; Van Hoe, B.; Lindner, E.; Vlekken, J.; Farnan, M.; Karabacak, D.M.; Singer, J.M. Interrogation and mitigation of polarization effects for standard and birefringent FBGS. Proc. SPIE
**2016**, 9852, 98520H. [Google Scholar] - Ibrahim, S.K.; O’Dowd, J.A.; Bessler, V.; Karabacak, D.M.; Singer, J.M. Optimization of fiber Bragg grating parameters for sensing applications. Proc. SPIE
**2017**, 10208, 102080P. [Google Scholar] - Francisangelis, C.; Floridia, C.; Simões, G.C.C.P.; Schmmidt, F.; Fruett, F. On-field distributed first-order PMD measurement based on pOTDR and optical pulse width sweep. Opt. Express
**2015**, 23, 12582–12594. [Google Scholar] [CrossRef]

**Figure 1.**Schemes of stepped-frequency I-OFDR systems based on (

**a**) vector network analysis; and (

**b**) electro-optic downconversion. Photonic components are represented in green; RF and electronic components in blue.

**Figure 2.**Backscattering transfer function $10log\phantom{\rule{0.166667em}{0ex}}|{H}_{s}\left(f\right)|$ of SMF at 1.55 $\mathsf{\mu}$m for different fiber lengths: 30 km (blue), 3 km (orange), and 300 m (yellow).

**Figure 3.**System setup. ADC, analog-to-digital converter; AMP, RF amplifier; LF, low-frequency signal generator; FUT, fiber under test; MZM, Mach–Zehnder modulator; PBBS, polarized broadband source; PC, polarization controller; PD, photodiode; RF, stepped RF source. Dashed lines stand for TTL signaling indicating RF steps (Steps) and reference tone (Ref).

**Figure 4.**Interferogram constellations ${A}_{c}\left[k\right]exp(-j\theta )$ after in-phase demodulation and rotation of: (

**a**) a flat fiber end; (

**b**) a 10-km SMF Rayleigh backscattering; and (

**c**) system’s noise level when the RF signal is turned off but the optical source is on (blue), and when both RF and optical sources are off (orange), also for a 10-km SMF spool. The number of points in each constellation, or number of I-OFDR frequency steps ${f}_{k}$, is 5000.

**Figure 5.**(

**a**) Spectral interferogram ${H}_{n}\left[k\right]$ used for normalization and calibration, where the FUT is the reflection in a flat fiber end located 1 m after the circulator. Inset: Zoom over the 9-GHz region. Points are ${H}_{n}\left[k\right]$ samples; (

**b**) Normalized and calibrated interferogram $H\left[k\right]$, where the flat fiber end is situated 3 m after the calibration plane. Notice the constant level in a 10-GHz bandwidth and the trimming of $H\left[k\right]$.

**Figure 6.**(

**a**) System measurements limited by dynamic range (attenuated PC-air reflection, red) and sensitivity, (APC-air reflection, blue); and (

**b**) peak (orange) and rms noise floor (blue) levels for progressively attenuated PC-air reflections.

**Figure 7.**(

**a**) FBG array measurement with orthogonally polarized light; (

**b**) addition of traces on (

**a**)) with (orange) and without (blue) CFBG right before down-converting MZM; and (

**c**) FBG array reflectivity spectrum with resolution bandwidth 0.5 nm (blue) and 0.06 nm (orange).

**Figure 8.**Zoom view of the two rightmost reflective peaks of Figure 7c, corresponding to the FBG at ∼1554 nm, with (blue) and without (orange) dispersive element. The window used here is rectangular.

**Figure 9.**(

**a**) Reflectometric traces, in $10log$ scale, of a 10-km spool of SMF using I-OFDR (blue) and a commercial OTDR (orange); (

**b**) I-OFDR traces for 10-km SMF with $B=20\mathrm{MHz}$ (blue) and $B=5\mathrm{MHz}$ (orange); (

**c**) I-OFDR (blue) and commercial OTDR (orange) measurements for the 10-km SMF coil followed by a 3-dB attenuator and 1.6 km of SMF; and (

**d**) I-OFDR traces for 1.6-km SMF ended in PC (blue) and APC (orange) connectors. The acquisition time was 30 s at a pulse width of 1 $\mathsf{\mu}$s for the OTDR and ≲2 min for the I-OFDR.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Clement, J.; Maestre, H.; Torregrosa, G.; Fernández-Pousa, C.R. Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation. *Sensors* **2019**, *19*, 2075.
https://doi.org/10.3390/s19092075

**AMA Style**

Clement J, Maestre H, Torregrosa G, Fernández-Pousa CR. Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation. *Sensors*. 2019; 19(9):2075.
https://doi.org/10.3390/s19092075

**Chicago/Turabian Style**

Clement, Juan, Haroldo Maestre, Germán Torregrosa, and Carlos R. Fernández-Pousa. 2019. "Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation" *Sensors* 19, no. 9: 2075.
https://doi.org/10.3390/s19092075