With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

Fiber Bragg Grating Sensors for the Oil Industry.

We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO2 laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP01 mode and LP11 mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

Mode converter based on the long-period fiber gratings written in the two-mode fiber

We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity acts as a mode converter from LP $_{01}$ mode to LP $_{11}$ (LP $_{21}$ ) mode with a broadband width. Linearly polarized OAM modes can be obtained by combining different vector modes HE $_{2,m}^{\rm {even}}$ (HE $_{2,m}^{\rm {odd}}$ ) and TE $_{0,m}$ (TM $_{0,m}$ ) when l = 1 or combining HE $_{l+1,m}^{\rm {even}}$ (HE $_{l+1,m}^{\rm {odd}}$ ) and EH $_{l-1,m}^{\rm {odd}}$ (EH $_{l-1,m}^{\rm {even}}$ ) when $l$ > 1 with a $\pi /2$ phase shift. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in a mode-locked fiber laser. The measured time duration of OVB pulses is 273 fs, 140 fs of OAM $_{\pm 1}$ and OAM $_{\pm 2}$ .

Generation of Femtosecond Optical Vortex Beams in All-Fiber Mode-Locked Fiber Laser Using Mode Selective Coupler

We fabricate mode converters by directly writing long-period gratings in a two-mode fiber with a CO2 laser. These mode converters allow the fundamental LP01 mode to be converted to any of the four higher order cylindrical vector modes or the LP11 modes. The transmission characteristics of these devices are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency >99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could find applications in mode-division-multiplexing systems and other applications that require cylindrical vector modes.

Temperature-Insensitive Mode Converters With CO 2 -Laser Written Long-Period Fiber Gratings

We propose and demonstrate an ultra-broadband mode converter based on a cascade chirped long-period fiber grating (CLPFG) written in a two-mode fiber. The mode converter can convert fundamental mode ( ${\rm{HE}}_{11}^{even}$ and ${\rm{HE}}_{11}^{odd}$ modes) into the first order cylindrical vector (CV) modes (TE01, TM01, ${\rm{HE}}_{21}^{even}$ and ${\rm{HE}}_{21}^{{odd}}$ modes). We design and analyze the mode conversion characteristics of this kind of grating in theory. The simulation results show a 10 dB bandwidth of 170 nm and 20 dB bandwidth of 145 nm can be achieved by optimizing the parameters of the CLPFG. In the terms of experiment, we achieve the broadband mode converter with 10 dB bandwidth of 170 nm from 1472 nm to 1642 nm.

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Ultra-Broadband Mode Converter Using Cascading Chirped Long-Period Fiber Grating

We propose a compact three-dimensional waveguide mode (de)multiplexer based on the configuration of two collocated asymmetrical directional couplers, formed with two dissimilar single-mode cores placed alongside a central rectangular three-mode core in the horizontal and vertical directions, respectively. The waveguides are designed to allow the LP 11a and LP11b modes of the central core to completely couple to the LP01 modes of the two side cores, respectively, with the LP01 mode staying in the central core, which is tapered down at one end to strip off any remaining LP11 modes. We fabricate the device with polymer materials by the conventional microfabrication process. A typical fabricated device, which has a total length of 9.0 mm, shows coupling ratios varying from 91% to 99% for the two LP11 modes, and crosstalks among the modes in the side cores varying from −23.2 to −14.6 dB in the wavelength range 1530–1570 nm (the C-band). The crosstalks in the central core are negligible. The insertion loss of the device with fiber leads is about 10 dB and the mode-dependent loss is about ±1 dB. The performance of the device is weakly sensitive to the polarization state of light and insensitive to temperature variations. This device can be used in broadband mode-division-multiplexing transmission systems based on few-mode fibers.

Compact Three-Dimensional Polymer Waveguide Mode Multiplexer

We demonstrate a passively mode-locked fiber laser that incorporates a two-mode fiber Bragg grating (FBG) for transverse-mode selection. By controlling the polarization state of light in the cavity, the laser can provide a continuous-wave output at the LP
 01
 mode, the LP
 11
 mode, or their mix, which correspond to different reflection wavelengths of the FBG. When the laser is mode-locked, it generates picosecond pulses at a fundamental repetition rate of 6.58 MHz for both the LP
 01
 and LP
 11
 modes with a signal-to-noise ratio higher than 56 dB. At a sufficiently high pump power, the laser can generate irregular multiple-pulse trains and harmonic mode-locked pulses. The laser could find applications in mode-division multiplexed systems.

Mode-locked fiber laser with transverse-mode selection based on a Two-mode FBG

We demonstrate a mode demultiplexer with two cascaded few-mode polymer waveguide directional couplers fabricated on the same substrate along the horizontal and vertical directions, respectively. The three waveguides that form the two couplers have the same core size. The horizontal and vertical couplers are designed to provide complete power transfer for the LP(11a) and LP(11b) modes, with the LP(01) mode staying in the central core that incorporates a biconical taper to suppress any remaining LP(11) modes. A typical fabricated demultiplexer, which is 18.5 mm long, shows a coupling ratio higher than ∼96% in the wavelength range of 1530-1570 nm for both couplers. The device shows negligible crosstalk to the LP(01)-mode channel, while the crosstalks to the LP(11)-mode channels are lower than -15.6 and -13.4  dB for the TE and TM polarizations, respectively. The device can be considered polarization insensitive. The propagation losses for the three modes are about 2.0  dB/cm. This device could find applications in mode-division-multiplexing systems.

Mode multiplexer based on integrated horizontal and vertical polymer waveguide couplers.

We experimentally demonstrate temperature-insensitive LP 01 -LP 11 mode converters with CO 2 -laser written long-period gratings in a two-mode fiber. Our 20-period grating can provide a conversion efficiency >99% over a 21.0-nm bandwidth at the wavelength 1562 nm.

Jiangli Dong

Papers

Temperature-Insensitive Mode Converters With CO 2 -Laser Written Long-Period Fiber Gratings

Compact Three-Dimensional Polymer Waveguide Mode Multiplexer

Mode-locked fiber laser with transverse-mode selection based on a Two-mode FBG

Mode multiplexer based on integrated horizontal and vertical polymer waveguide couplers.

Temperature-Insensitive LP 01 -P 11 Mode Converters Based on CO 2 -Laser Written Long-Period Fiber Gratings