SNR enhancement of Raman-based long-range distributed temperature sensors using cyclic Simplex codes

Abstract

Presented is a novel approach for enhancing the signal-to-noise ratio (SNR) of optical fibre distributed temperature sensors. The proposed method is based on cyclic codes and it can be implemented by highpower lasers, which are required for long-range applications but are unsuitable for conventional coding techniques. The expected performance improvement, in terms of temperature resolution, sensing range and measurement time, is confirmed by simulations.Introduction: Optical fibre distributed temperature sensors based on spontaneous Raman scattering (RDTS) have potential application in several industrial and scientific fields for reliable temperature monitoring over long distances (up to a few tens of kilometres) [1]. They are based on optical time-domain reflectometry (OTDR): an optical pulse is sent along a sensing fibre and the temperature-dependent backscattered Stokes (S) and anti-Stokes (AS) photons are acquired as a function of time. The collected samples are then processed to extract the temperature profile.The main issue to face in long-range RDTS design is the low signalto-noise ratio (SNR) due to the weakness of the backscattered S and AS traces, which generally exhibit optical power levels lower than a few nW. The energy of the launched pulse is indeed bounded by the targeted spatial resolution, which implies a small pulse width, and by the threshold for the onset of the fibre nonlinearities, which upper bounds the pulse peak level. To reduce the noise power introduced by the optical/electrical receiver, and reach a temperature resolution of a few degrees at the fibre-end, trace averaging is typically employed. However, the greater the number of averages, the longer the measurement time, which might become unsuitable for several applications requiring short response times.Coding techniques based on Simplex or Golay codes exploit a set of different sequences (i.e. codes) of short (about 10 ns) NRZ laser pulses to increase the launched energy without impairing the spatial resolution [2]. However, the required high repetition rate of the laser pulses -a hundred of MHz for a meter-scale spatial resolution -is not achievable by high peak power lasers, such as rare-earth doped fibre or passive Q-switched ones, which feature a maximum repetition rate of some hundreds of kHz [3].Here, a new coding technique tailored to high-power pulsed lasers is proposed [4]. The basic idea is to periodically sense the probing fibre with a multi-pulse pattern the repetition period of which is equal to the fibre round-trip time. This way, the pattern results as a code spread along the whole fibre, with a bit time inversely proportional to the code length. The pulse width can be kept in the order of 10 ns to guarantee a meter-scale spatial resolution and the peak power can be set close to the nonlinear effect threshold.

Advanced Pulse Coding Techniques for Distributed Optical Fiber Sensors

“…Unfortunately, this type of pulsed-laser technology is not compatible with the pulse modulation frequencies (up to 100 MHz for 1 m spatial resolution) required to implement standard pulse coding. Thus, recently a new method based on cyclic Simplex codes has been proposed to overcome this limitation [6]. Contrarily to standard coding methods, cyclic codes are based on a single quasi-periodic bit sequence [6]; and owning to its particular properties, bits do not need to be sent along the fiber in bursts, so that they can be individually launched into the fiber at a low repetition rate in order to fill the entire sensing length with many bits.…”

Raman-based distributed temperature sensor with 1 m spatial resolution over 26 km SMF using low-repetition-rate cyclic pulse coding

“…Unfortunately, this frequency is incompatible with the pulsed-laser technology typically used in RDTS systems, which is characterized by high peak power (several tens of watt), low repetition rates (a few hundred kilohertz) and very low duty cycles (typically 0.1%). To overcome this limitation, recently a new coding technique based on cyclic Simplex codes has been theoretically proposed [6]. In such a scheme, optical pulses are launched into the fiber at a low repetition rate, and the fiber is "filled" with a large number of optical pulses.…”

“…In the proposed cyclic coding technique, a suitable bit sequence is continuously generated from the pulsed laser light according to an M-bit binary pattern P ¼ fp 0 ; …; p M−1 g, where p j ¼ 0; 1 (with j ¼ 0; …; M − 1), with a proper repetition rate to fill the fiber with M bits (spaced in M consecutive intervals). In such a scheme, the detected trace y at a given sampling instant results from the sum of many contributions linked to the single-pulse fiber response x and the pulse pattern P [6], and it can be written as…”

“…where i and j are integer indexes associated with a given sampling instant T S [T S ¼ ΔT · ði þ jHÞ, where ΔT is the sampling period], and H is the number of sampling points within one interval (for theory details on cyclic coding please refer to [6]). Cyclic coding exploiting the Simplex pattern offers an SNR enhancement that is equal to ðLþ1Þ=ð2 p LÞ, where L is the code length.…”

“…Although a successful theoretical prediction of the scheme effectiveness has been reported in [6], so far there is no experimental evidence of the benefits and real sensing performance provided by this technique. In this work, the benefits provided by this novel coding method have been experimentally investigated, achieving what we believe is the first demonstration of a long-range RDTS with 1 m spatial resolution using SMFs.…”