Fresnel-assisted self-heterodyne detection
for Brillouin gain spectrum characterisationin polymer optical fibres
Y. Mizuno, N. Hayashi and K. Nakamura
It has been experimentally shown that by utilising the Fresnel-reﬂected
light as the reference light, Brillouin signals in polymer optical ﬁbres(POFs) can be observed with a higher signal-to-noise ratio (SNR)
than those measured with standard self-heterodyne detection. Thismethod is basically the same in setup as direct detection, leading toan additional advantage of system simplicity. Moreover, it has been
demonstrated that the Brillouin signal in a 1 cm-long POF can beobserved with a moderate SNR using this technique, indicating a
potential feasibility of POF-based distributed Brillouin measurementwith a millimetre-order spatial resolution.

Introduction: Brillouin scattering in optical ﬁbres has been applied tovarious devices and systems, including lasers, optical storages, slow
light generators and distributed strain and temperature sensors [1–3].
To improve their performance, Brillouin properties have been extensively studied not only for silica glass single-mode ﬁbres (SMFs), butalso for polymer optical ﬁbres (POFs) [4–6]. Instead of direct detection,
optical self-heterodyne detection is generally employed to observe therelatively small Brillouin signals in silica SMFs with a higher
signal-to-noise ratio (SNR) [1–3]. Accordingly, the same type of selfheterodyne detection has been used to observe and characterise theBrillouin signals in POFs [4–6], which are even smaller because of
the larger core diameters and multimode nature. As POF-based opticaldevices such as circulators and ampliﬁers are not widely available,
almost all the optical paths of the experimental setup for detecting theBrillouin signal in a POF basically need to be composed of silica
SMFs. Consequently, as long as a continuous wave (CW) is used aspump light, backscattered Stokes light inevitably includes aFresnel-reﬂected component generated at the interface between thesilica SMF (refractive index n ∼ 1.46) and the POF (n ∼ 1.35).
In this Letter, we experimentally show that, by exploiting theFresnel-reﬂected light as the reference light for heterodyne detection
and thus by removing an optical coupler for mixing the Stokes lightand the reference light, the Brillouin signal in a POF can be observedwith a higher SNR than in the previous reports using the standard selfheterodyne detection. This method, which we call Fresnel-assisted selfheterodyne detection, is similar in setup to direct detection, providing anadditional advantage of system simplicity, i.e. no need for preparing areference path including optical couplers, an ampliﬁer and a polarisation
controller (PC). We then demonstrate that the Brillouin signal in an
extremely short POF (1 cm) can be observed with a moderate SNR
using this technique.

scattering phenomena in the POF such as Rayleigh scattering is negligible. The Fresnel reﬂection at the open end of the POF (−16.5 dB =2.2%) was also found to have hardly any inﬂuence on the measurement
because of the high propagation loss and the low optical coupling efﬁciency at the POF-to-SMF interface, which was veriﬁed by the fact thatneither the reﬂectivity nor the Brillouin gain spectra (BGS) was changed
by immersing the POF end into water (n ∼ 1.33) to match the indices.
EDFA pump

1.55 mm
laser

POF
PC

StokesEDFA
reference
PCESAPDa1.55 mm
laser

EDFApump
POF
StokesPC
ESA

Fresnelreflection

PDb

Fig. 1 Schematic setups for detecting Brillouin signals in POFs EDFA:
erbium-doped ﬁbre ampliﬁer; ESA: electrical spectrum analyser; PC: polarisation controller; PD: photodiodea Standard self-heterodyne detectionb Fresnel-assisted self-heterodyne detection

Fig. 2 shows the BGS measured using the standard self-heterodynedetection (Fig. 1a) with reference powers of −6.0, 0 and 3.0 dBm,
and without the reference light (>50 dB bending loss was induced).
The BGS measured using Fresnel-assisted self-heterodyne detection(Fig. 1b) is also shown. The pump power was ﬁxed at 20 dBm, andthe polarisation state was optimised [11]. The Brillouin frequency
shift (BFS) was ∼2.84 GHz, which is in agreement with previous
reports [4, 5]. With the standard detection, by raising the reference
power, the SNR, i.e. the BGS height (difference in power between thenoise ﬂoor and the BGS peak) was reduced, which indicates that theFresnel-reﬂected power is higher than the optimal reference power inthis measurement. In contrast, when the coupler-free Fresnel-assisted
detection was used, the SNR was improved by ∼5 dB. This techniquewith a higher SNR is suitable for BGS characterisation in POFs, but it
cannot be directly applied to the Brillouin optical time-domain reﬂectometry (BOTDR) [2], in which not CW but optical pulses are used andthe Fresnel-reﬂected light is not detected at the same timing as theBrillouin-scattered light.
–60
ref. power, dBm–65
3.0
power, dBm

Theory: In general, compared with direct detection, optical heterodyne
detection enables high-SNR detection of low-power signals through theuse of a high-power reference light. The highest achievable SNR isreported to be limited by shot noise and/or photodiode (PD) saturation[7–9], and excessively high reference power results in a reduced SNR;
namely, there exists an optimal value for the reference power. It isalso known that the optical coupler used to mix the signal light andthe reference light should have a high transmission or, ideally, should
not be employed, so that most of the signal power is sent to the PD[7–9].

–70
0–75–6.0–80
Fresnel-assisted

Experiments: Perﬂuorinated graded-index POFs [10] with a numericalaperture of 0.185, a core refractive index of ∼1.35 and a propagation
loss of ∼250 dB/km at 1.55 μm were used as POFs under test. They
had a core diameter of 50 μm, a cladding diameter of 100 μm and anovercladding diameter of 500 μm. An experimental setup based onstandard self-heterodyne detection is schematically shown in Fig. 1a,in which two 3 dB optical couplers were employed along the reference
path, whereas that based on the Fresnel-assisted self-heterodyne detection is shown in Fig. 1b. In both cases, the POFs were butt-coupled
to the silica SMFs using ‘SC/FC’ adaptors [4]. In Fig. 1b, when a
50 m-long POF was used, irrespective of the incident pump power,
the reﬂectivity of the POF measured via a circulator was ∼−29 dB,
which agrees well with the Fresnel reﬂectivity at the POF-to-SMF interface (−28.1 dB = 0.15%), indicating that the inﬂuence of the other–85
2.5

2.7

2.9
frequency, GHzoff

3.1Fig. 2 BGS in 50 m-long POF measured with standard self-heterodyne
detection (reference powers of −6.0, 0 and 3.0 dBm, and without reference)and Fresnel-assisted self-heterodyne detection

Next, a minimal POF length required for Brillouin measurement wasinvestigated using this method. The initial POF length of 20 cm wasgradually reduced by cutting the section involving the open end, atwhich the Fresnel reﬂection was suppressed by angled cut and water
immersion. Fig. 3 shows the measured BGS dependence on the POF
length. The pump power was ﬁxed at 23 dBm. With decreasing POF

ELECTRONICS LETTERS 31st July 2014 Vol. 50 No. 16 pp. 1153–1155

length, the SNR deteriorated, but even when the POF was as short as1 cm, a BGS which was sufﬁciently clear to obtain the peak frequency(i.e. BFS) was observed. The BFS became slightly lower, because theentire length of the 1 cm-long POF was heated [5] (part of the optical
coupling loss at the POF-to-SMF interface is turned into heat). Thisresult suggests a potential feasibility of POF-based distributed
Brillouin measurement [12] with a millimetre-order spatial resolution.
Further increase of the pump power will improve the SNR, but weshould note that excessively high pump power may induce burning ordamage at the POF-to-SMF interface [6] as well as a ﬁbre fuse phenomenon [13].

and the SCAT Foundation. N. Hayashi acknowledges a Grant-in-Aidfor JSPS Fellows (no. 25007652).
© The Institution of Engineering and Technology 201418 March 2014
doi: 10.1049/el.2014.0963
One or more of the Figures in this Letter are available in colour online.
Y. Mizuno, N. Hayashi and K. Nakamura (Precision and Intelligence
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho,Midori-ku, Yokohama 226-8503, Japan)
E-mail: ymizuno@sonic.pi.titech.ac.jp
References–67

20
POF length, cmpower, dBm–69
105–71

3–73

–75
2.5

1

2.7

2.9
frequency, GHz

3.1Fig. 3 BGS in 20-, 10-, 5-, 3- and 1 cm-long POFs measured withFresnel-assisted self-heterodyne detection

Conclusion: Fresnel-assisted self-heterodyne detection was experimentally shown to provide a higher SNR of Brillouin measurement inPOFs than standard self-heterodyne detection. System simplicity is
another advantage. The Brillouin signal in a 1 cm-long POF was thenobserved with a moderate SNR, indicating a possible millimetre-order
spatial resolution in POF-based distributed Brillouin measurement.
Even though it cannot be directly applied to BOTDR, we believe that
this method will be a useful tool for characterising the Brillouin properties in POFs.
Acknowledgments: This work was partially supported by aGrant-in-Aid for Young Scientists (A) (no. 25709032) from the Japan
Society for the Promotion of Science (JSPS) and by research grants
from the General Sekiyu Foundation, the Iwatani Naoji Foundation

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