193 results about "Brillouin gain" patented technology

The invention discloses a distributed optical fiber sensing device based on a chaotic laser coherence method, and a measurement method of the distributed optical fiber sensing device. Chaotic laser light which is emitted from a chaotic laser is divided into detection light and reference light; the detection light is amplified by a light amplifier and then emitted into a sending optical fiber through an optical circulator, and a backward Brillouin scattering light signal is generated in the optical fiber; the Brillouin scattering light signal is amplified by the light amplifier, de-noised by a tunable light filter and then emitted into an optical fiber coupler; the optical length of the reference light is regulated by a variable light delay line, and interferes with the backward Brillouin scattering light signal which is generated by the detection light at different positions in the sensing optical fiber in the optical fiber coupler; an interference beat frequency signal is detected by a photoelectrical detector; and Brillouin gain spectra at different lengths are obtained through a data acquisition device and a signal processing device and then output to a display device, so strain or temperature sensing detection is realized.

The invention relates to distributed optical fiber sensing technology and provides a distributed optical fiber sensing device and a method based on a Brillouin Er-doped fiber laser. The distributed optical fiber sensing device and the method based on the Brillouin Er-doped fiber laser comprise a narrow linewidth pump laser, an optical fiber coupler, an optical pulse generator, an optical pulse amplifier, an optical fiber amplifier, an optical fiber circulator, sensing optical fibers, a Brillouin Er-doped fiber laser, an optical fiber isolator, an photoelectric detector and a signal collecting and processing device. Brillouin gain optical fibers of the Brillouin Er-doped fiber laser are wound on piezoelectric ceramic, and Brillouin frequency shift is changed through adjusting temperature of the Brillouin gain optical fibers and / or convergent-divergent of the piezoelectric ceramic. According to the distributed optical fiber sensing device and the method based on the Brillouin Er-doped fiber laser, pump light needed by a Brillouin optical time domain analysis meter (BOTDA) and detecting light which is provided with the Brillouin frequency shift and is tunable are obtained with the same light source, and no additional laser or high-speed modulator and microwave modulating source of the high-speed modulator is needed. The system cost is lowered, the system structure is simplified, and the system structure is enabled to be compact and convenient to pack.

The invention relates to a micro-tapered fiber for producing an ultra-narrow linewidth fiber laser. The micro-tapered fiber is produced by ordinary single-mode fibers through a tapering process. A plurality of tapering regions are formed on the ordinary single-mode fibers through multiple tapering operations, and the whole micro-tapered fiber is connected into the laser, so that the length of a resonant cavity of the laser is more than or equal to 80m; the tapering regions on the ordinary single-mode fibers can effectively suppress the Brillouin gain and ensure the stability of single-frequency operation of the laser, and meanwhile, the large-length resonant cavity can meet the requirement of vast accumulation of Rayleigh gain and improve the signal strength. The micro-tapered fiber provided by the invention has the beneficial technical effects that the single-frequency operation of the laser is ensured, and meanwhile, the accumulation of Rayleigh gain can be effectively realized, and the output signal strength is improved; the tuning range is unrelated to the matching of the pumping wavelength and the length of the laser cavity, and additional linewidth compression is not needed; and the structure is simple, the implementation is easy, and the fiber is a novel narrow-linewidth light source.

The invention discloses a coherent Brillouin optical time-domain analysis sensing system based on phase modulation probe light. In the system, an electro-optic intensity modulator is driven by an electric pulse signal to externally modulate light carrier waves to generate pump light, the electro-optic intensity modulator working at a carrier wave inhibition point is driven by a microwave signal to externally modulate light carrier waves to generate local light, the local light is externally modulated through an electro-optic phase modulator to generate symmetric side bands as the probe light, and the electro-optic phase modulator is driven by a frequency-adjustable radio-frequency signal. Two components of the probe light scan a Brillouin gain area and a loss area of the pump light respectively at the same time, and losses of the pump light can be dynamically compensated so that non-local effects can be reduced while amplitude modulation is carried out on the pump light by a Brillouin gain spectrum. Frequency beating is carried out on the probe light and the local light through a photoelectric detector, information of the Brillouin gain spectrum is loaded to GHz high-frequency carrier waves to avoid base band noise damage, and meanwhile the signal-to-noise ration of the whole system is improved through coherent detection.

The invention relates to the technology of distributed fiber sensing, specifically a Brillouin distributed fiber sensing device and method employing a chaotic correlation method for positioning, and solves problems that the prior art just can achieve the measurement of a single point on an optical fiber at each time, and cannot achieve the continuous measurement of long-distance temperature or strain. The sensing device and method enable a chaotic pump light signal to be divided into two ways, wherein one way still serves as a pump light signal and enter the sensing optical fiber, and the other way serves as a reference light signal. The device and method are directly implemented through adding one reference signal storage channel in the device. Moreover, the device and method obtain the Brillouin gain spectrum through recording the corresponding relation between a modulated frequency of a chaotic detection light modulation side band and the mean power of detection light. The device and method achieve the positioning of the temperature or strain of the sensing optical fiber through employing the correlation method of a pulse-modulation chaotic laser signal, are easier to measure the position of the temperature or strain of the optical fiber, and are longer in sensing distance.

The invention provides a high-bandwidth microwave photon filter based on a stimulated Brillouin scattering effect and a binary system phase shift keying technology, and belongs to the technical field of microwave photonics. The high-bandwidth microwave photon filter is composed of a laser, a coupler, a first phase modulator, an optical isolator, a vector network analysis meter, a high non-linear optical fiber, a strength modulator, an optical filter, a second phase modulator, an impulse code type generator, an Er-doped fiber amplifier, an optimal circulator and a photoelectric detector. The bandwidth of pumping signals is broadened through the binary system phase shift keying technology, the broadened pumping signals will generate a broadened stimulated Brillouin gain spectrum, and therefore the single-passband response microwave photon filter system with the broadened and adjustable bandwidth is obtained.

The invention discloses a dynamic distributed Brillouin optical fiber sensing device and method. The dynamic distributed Brillouin optical fiber sensing device comprises a narrow linewidth laser, a polarization-maintaining coupler, a coupler, a first electro-optic intensity modulator, a pulse signal generator, a frequency shifter, an optical amplifier, a polarization scrambler, an optical circulator, a polarization controller, a second electro-optic intensity modulator, a microwave signal source, a sensing optical fiber, a 3dB coupler, a balanced photoelectric detector and a data acquisition and processing module. Two technologies including a Brillouin gain spectrum dual-slope frequency point assisted method and a coherent detection technology are adopted at the same time. The coherent detection technology can increase the signal-noise ratio of a system, improve the measurement precision and increases the sensing distance; the Brillouin gain spectrum dual-slope frequency point assisted method solves a problem of influence of optical power fluctuation of a pumping pulse on the measurement precision in a conventional Brillouin gain spectrum slope frequency point assisted method. Therefore, the dynamic distributed Brillouin optical fiber sensing device and method can realize dynamic event measurement with relatively high measurement precision while guaranteeing that the Brillouin optical fiber sensing system has relatively long sensing distance.

The invention provides a long-distance distributed type large-measuring-range rapid response optical fiber dynamic strain sensing device. The device comprises a phase modulator, a multi-frequency signal generating module, an intensity modulator, a microwave switch, an electric pulse generating module, a microwave signal generating module and other components. The signal generating module comprising multi-frequency components is used for modulating the phase of detection continuous light, so that multi-light-frequency components are generated in the detection continuous light, and corresponding Brillouin gain spectrum amplitudes are adjusted through controlling the amplitudes of all the light-frequency components in the detection continuous light. A Brillouin gain spectrum of a needed spectrum width and a spectrum type is obtained through splicing, and the dynamic strain measuring range is greatly enlarged under the condition that the signal-to-noise ratio and the response speed of a system are not destroyed. The frequency difference between two light beams generating a Brillouin amplification effect is fixed in the middle of a beveled edge linear region of the spliced Brillouin gain spectrum, drifting of the spliced Brillouin gain spectrum is converted into power fluctuation of the detection light, and long-distance distributed type large-measuring-range high response speed quantitative measuring on optical fiber dynamic strain and optical fiber static strain is achieved.

The invention relates to a distributed optical fiber temperature strain sensor based on Brillouin optical amplification detection. The narrow-band Brillouin gain feature of a Brillouin gain optical fiber is used to detect the frequency shift of the Brillouin scattering light of a sensing optical fiber. The Brillouin scattering light and the frequency shift light input the Brillouin gain optical fiber from two ends. A narrow linewidth optical filter filters the stokes or anti-stokes sidebands of the Brillouin scattering light to perform photoelectric detection. An optical power control unit stably controls the power of the frequency shift light. When the frequency difference between the Brillouin scattering light and the frequency shift light equals to the Brillouin frequency shift of the Brillouin gain optical fiber, the strongest photoelectric detection signal is obtained, and detection of the Brillouin scattering light is achieved. The distributed optical fiber temperature strain sensor has the advantages that the frequency of photoelectric detection, circuit signal production and processing is lowered, technical difficulty and cost are lowered, flat frequency response feature is obtained by stable control of the frequency shift power, and precise detection of Brillouin scattering light frequency shift is facilitated.

A measurement precision is improved and a measurement range is extended by efficiently suppressing a noise level of integrated unnecessary components from non-correlation positions. Measuring means 33 detects the Brillouin gain of a probe light output from a measurement-target optical fiber FUT while sweeping a frequency difference between the pump light and the probe light, and measures the distribution of strains of the measurement-target optical fiber FUT. An optical intensity modulator 4 performs intensity modulation on output light in synchronization with frequency modulation performed on a light source 1. Accordingly, the spectrum distribution with respect to the frequency of light from the light source 1 can be adjusted arbitrarily, and a noise spectrum shape generated at a position other than a correlation peak position and spreading over a frequency axis can be adjusted, and the peak frequency of a Lorentz spectrum generated at the correlation peak position can be measured precisely. Moreover, a measurement range dm can be extended.

A silica-based optical fiber includes a core and a cladding that is formed on an outer circumference of the core. The core includes three or more layers including a layer doped with at least one of germanium and fluorine, and a concentration of the germanium or the fluorine in each of the layers is controlled in such a manner that a Brillouin gain spectral peak is spread into a plurality of peaks on a Brillouin gain spectrum. With this scheme, an optical fiber is provided, which has stable characteristics in the longitudinal direction, and which has a high SBS threshold so that generation of the SBS can be effectively suppressed.

The invention discloses a dynamic distributed Brillouin optical fiber sensing device and method, belongs to the optical field, and solves the problem of small strain measurement range in a dynamic sensing technology based on a Brillouin gain slope method. The dynamic distributed Brillouin optical fiber sensing device comprises a laser, a coupler, a to-be-measured polarization maintaining optical fiber, an erbium-doped fiber amplifier, an adjustable optical attenuator, a data acquisition module, a first polarization controller PC1, a second polarization controller PC2, a third polarization controller PC3, a fourth polarization controller PC4, a probe light intensity modulator IM1, a pump light intensity modulator IM2, an arbitrary waveform generator (AWG), a first circulator R1, a second circulator R2, a first signal amplifier Amp1 and a second signal amplifier Amp2. According to the invention, the intensity modulators can be driven by arbitrary waves, so that the frequency agility of probe light is realized, the fast frequency scan problem is solved, the distributed Brillouin dynamic sensing is achieved, and the strain of the to-be-measured polarization maintaining optical fiber is measured.

The invention relates to a microwave photon millimeter wave ultra-wideband signal generation method and device based on up-conversion, belonging to the technical field of microwave photonics. A lasersource, a polarization controller, an arbitrary waveform generator, a microwave signal source, a dual-parallel Mach-Zehnder modulator, an erbium-doped fib amplifier, an optical isolator, a first annulator, a single-mode optical fiber, a second annulator, a photodetector and a spectrum analyzer constitute the device. The invention is based on the up-conversion technology to generate the signal at 22GHz-29GHz millimeter wave UWB signal, the upconversion realized by SSB modulation can overcome the dispersion effect in single-mode fiber, so that the UWB signal can be transmitted over long distance. Single sideband modulation using secondary Brillouin scattering is simple and easy to implement. Moreover, the amplification of Brillouin gain and the attenuation of loss make the spectrum of the generated UWB signal satisfy the power spectral density mask specified by the Federal Communications Commission of the United States.

The invention relates to a stimulated Brillouin gain spectral linewidth narrowing processing, pulse coding and wavelet transform technology-based high-performance rapid Brillouin optical-time domain analysis type (BOTDA) strain measuring device and a data processing method and belongs to the distributed optical fiber sensing technical field. The measuring device is composed of a laser 1, a first optical coupler 2, a first modulator 3, a first microwave signal source 20, a first optical filter 4, a second modulator 5, a second microwave signal source 21, a direct-current power source 22, an optical amplifier 6, a second optical coupler 7, an optical isolator 8, a second modulator 9, a frequency synthesizer 14, a second optical filter 10, a fourth modulator 11, a pulse signal generator 15, a scrambler 12, an optical circulator 13, a sensing optical fiber 19, a photoelectric detector 16, a data acquisition card 17 and a computer 18. According to the measuring device and the data processing method of the invention, three pump signals are adopted to realize optical fiber stimulated Brillouin scattering gain spectral linewidth narrowing processing, and therefore, the precision of a BOTDA strain measurement system can be improved; pulse encoding and wavelet transform technology are combined to improve a signal-to-noise ratio, and therefore, measurement accuracy can be improved, and measurement time can be shortened.

The invention discloses a wide passband reconfigurable microwave quantum photon filtering device and filtering method based on stimulated brillouin scattering, relates to a microwave quantum photon filter based on the stimulated Brillouin scattering in optical fibers, and belongs to the optics and microwave crossing field. The wide passband reconfigurable microwave quantum photon filtering device and filtering method solves the problem that a passband of an existing microwave quantum photon filter is narrow. According to the quantum photon filtering device and filtering method, two lasers are adopted to respectively generate pump light and carrier wave light, two phase modulators are adopted to respectively conduct phase modulation on the pump light and the carrier wave light, two isolators are adopted to protect the lasers, the stimulated brillouin scattering occurs when first order lower edge frequency or first order upper edge frequency signals of the carrier wave in the optical fiber enter a stimulated Brillouin gain spectrum corresponding to the pump light, the width value of the edge frequency signals is amplified, a beat frequency signal is output through a detector, and then filtering of passband microwaves is realized. The quantum photon filtering device and filtering method is applied to microwave quantum photon filtering.

The invention provides a dynamic Brillouin optical time domain analysis system based on pump pulse frequency sweeping. The system divides laser signals output by a laser into two paths of signals. Onepath of signals is modulated into probe light by a microwave source, the high-frequency part of the probe light is removed by a first filter, and the filtered probe light is transmitted to a sensingfiber. For the other path of laser signals, a corresponding output waveform is generated by an arbitrary waveform generator according to preset rules, the path of laser signals is modulated into a pump pulse by using the output waveform, and part of energy in the pump pulse is transferred to the probe light based on stimulated Brillouin scattering. Then, the frequency difference between the pump pulse and the probe light is scanned to get the Brillouin gain spectrum of the sensing fiber, and the Brillouin frequency shift is obtained by fitting. Therefore, the measurement time of the Brillouinoptical time domain analysis system is reduced, and dynamic analysis of Brillouin optical time domain is realized.

The invention provides a brillouin single-longitudinal-mode frequency-shift fiber laser comprising a light source module, an optical fiber circulator, an optical amplifier, an optical fiber isolator, a first optical fiber coupler, a first frequency-shift fiber, a second frequency-shift fiber, and a second optical fiber coupler. According to the invention, a unidirectional active annular resonate cavity is constructed. On the basis of a technical scheme of a fiber composite cavity, a mode selection characteristic of the composite cavity is utilized, so that thus the cavity longitudinal mode interval is larger than the brillouin gain spectrum width; and thus only one cavity longitudinal mode in the brillouin gain spectrum range is capable of carrying out oscillation starting and forming laser emission, thereby realizing brillouin single-longitudinal-mode running frequency-shift fiber laser outputting with high efficiency and low cost. The brillouin single-frequency laser frequency follows pump seed light all the time; and the broadband frequency shift effect is stable. Single-longitudinal-mode running is realized based on the composite cavity. The laser has a simple structure without any complicated control requirement; and a strict application requirement on the broadband frequency shift technology by a spontaneous Brillouin scattering type-based distributed fiber temperature strain sensing system can be met fully.

The invention provides a stress monitoring method of a distributed optical fiber system. The method comprises the following steps of: dividing laser into two laser beams, modulating one laser beam with pulse or a random sequence signal into pulse light, and amplifying the pulse light; modulating the other laser beam and an OFDM (orthogonal frequency division multiplexing) signal into a light OFDM signal, and amplifying the light ODFM signal; taking the amplified pulse light as pump light, and the amplified light OFDM signal is taken as detection light; performing Brillouin gain on the two kinds of light in a single mode optical fiber; performing photovoltaic conversion on the pump light and the detection light after subjected to Brillouin gain so as to obtain an OFDM electrical signal; performing channel estimation on the OFDM electrical signal so as to obtain Brillouin frequency shift of each sub-carrier; and acquiring the stress value of each point distributed along an optical fiber axial direction according to the Brillouin frequency shift of each sub-carrier. By the method, the measuring time and precision can be improved; the dynamic range can be greatly improved; the stress monitoring can be performed on power grid system equipment in real time; and the reliability and instantaneity of a distributed sensing system can be improved.

The invention provides a seed injection BOTDR distributed optical fiber sensing system, relates to a seed injection Brillouin Optical Timedomain Reflectometer (BOTDR) technique, and belongs to the technical field of non-scanning type real-time measurement distributed optical fiber sensing. The seed injection BOTDR distributed optical fiber sensing system resolves the problems that an existing BOTDA system is low in signal to noise ratio, short in sensing distance and complex in structure, cannot carry out measurement in real time, and brings difficulty to fault detection. According to the seed injection BOTDR distributed optical fiber sensing system, wideband seed light covering the range of a Brillouin gain spectrum or a loss spectrum formed in sensor optical fibers through pump light is utilized to replace sweep frequency type probe laser in a traditional BOTDA system, so a frequency sweeping process is avoided, real-time sensing can be achieved, and fault detection can be completed under the circumstance of not increasing systematic complexity; the structure is simple, and compared with a BOTDR system based on a single-end method, outputting of sensing signals is more stable, sensing precision is high, and the signal to noise ratio is increased by more than 10dB within 50-80km. The seed injection BOTDR distributed optical fiber sensing system is suitable for engineering application of Brillouin optical fiber sensing.

The invention discloses a spectral analysis system and method based on the stimulated Brillouin effect and belongs to the technical field of spectral analysis. According to the method, special wavelength spectrum components of a to-be-detected signal are amplified on the basis of the stimulated Brillouin effect, the wavelength of a pump signal is changed in a push-broom mode, and spectral analysis of the to-be-detected signal is achieved; the spectrum resolution ratio is increased in the mode that a pair of attenuation spectra are superposed through stimulated Brillouin gain; by means of comprehensive utilization of polarization characteristics of interaction signals, a stimulated Brillouin gain amplification spectrum, a synchronous detection method, a method for improving the power of pump signals and the like, increase of the optical rejection ratio of the stimulated Brillouin gain amplification spectrum is achieved. The system and method can be used for ultrahigh-resolution spectrum testing of a new generation of optical network, optical devices and the like, and have the advantages that the spectrum resolution ratio is ultrahigh, the dynamic range is wide, precision is good, reliability is high and miniaturization is achieved.

A device and a method for measuring a brillouin gain spectrum in a non-scanning manner based on a pumping-detection method relate to a device and a method for measuring the brillouin gain spectrum and aim to solve the problems of low signal-to-noise ratio, long measurement time and complex device of the conventional device and method in brillouin gain spectrum measurement. In the device for measuring the brillouin gain spectrum in the non-scanning manner based on the pumping-detection method, the laser output end of an optical fiber laser is communicated with the first port of an optical fiber circulator; the second port of the optical fiber circulator is communicated with the first port of a polarization controller, and the second port of the polarization controller is communicated with one end of an optical fiber to be measured; the laser output end of an ASE (Amplified Spontaneous Emission) light source is communicated with the light input end of an optical fiber isolator; the light output end of the optical fiber isolator is communicated with the other end of the optical fiber to be measured; and the third port of the optical fiber circulator serves as the light output end of the device for measuring the brillouin gain spectrum in the non-scanning manner based on the pumping-detection method; and besides, the connection is optical fiber connection. The device and the method provided by the invention are suitable for measuring the brillouin gain spectrum.

The invention discloses a coherent brillouin light time domain analysis sensing system based on single-sideband modulation detection light. In the device, sound and light intensity modulator external modulation optical carrier is driven by an electric pulse signal to generate pump light, and local light and detection light are obtained through two steps: 1) driving electro-optics intensity modulator external modulation same-frequency optical carriers working at linear work points by a 10-12GHz tunable microwave signal to generate two sidebands, and 2) filtering the upper sideband and reserving the lower sideband (the detection light) and the optical carrier (the local light). After interactions with the pump light in optical fibers, the detection light and the local light load brillouin gain spectrum information to a 10-12GHz high-frequency carrier wave through the photoelectric detection beat frequency to prevent baseband noise damage, and improve the signal to noise ratio of the whole device by utilizing coherent detection. The brillouin gain spectrum information is demodulated by a logarithmic detector, and then temperature information and strain information are further solved through a Lorentz fit on a brillouin gain spectrum.

A distortion of a device under test (such as an optical fiber) can be precisely measured. There is provided a distortion measuring device including a signal processing unit 32 having a Brillouin scattered light spectrum recording unit 322a which records a spectrum of Brillouin scattered light generated in an optical fiber as a result of supplying incident light, a Rayleigh scattered light spectrum recording unit 322b which records a spectrum of Rayleigh scattered light generated in the optical fiber as a result of supplying the incident light, a deconvolution unit 324 which derives a Brillouin gain spectrum of the optical fiber based on the recorded spectrum of the Brillouin scattered light and the recorded spectrum of the incident light, a peak frequency deriving unit 326 which derives a peak frequency at which the derived Brillouin gain spectrum takes the maximum value, and a distortion deriving unit 328 which derives a distortion of the optical fiber based on the derived peak frequency.

A multi-wavelength Brillouin fiber laser based optical fiber temperature sensor which is formed according to an optical fiber Brillouin gain effect, an er-doped optical fiber amplifying effect, a multi-level Brillouin scattering temperature effect and a heterodyning beat frequency demodulation principle comprises a narrow line width single frequency laser, an optical branching device, a polarization controller, an opto-isolator, an optical circulator, a single-mode sensing fiber, an er-doped optical fiber amplifier, an er-doped optical fiber being not pumped, a high-speed photoelectric detector and a frequency analyzer. The multi-wavelength Brillouin fiber laser based optical fiber temperature sensor has the advantages of being many in the number of wavelengths, narrow in line width, fixed in wavelength interval and stable in output. The multi-wavelength Brillouin fiber laser based optical fiber temperature sensor achieves heterodyning best frequency demodulating detection of a high-order stokes wave and high-accuracy high-sensitivity temperature measuring with the single-mode optical fiber which provides gain for the fiber laser being served as a temperature sending detect unit.

The invention discloses a Brillouin distributed optical fiber sensor and a method of reducing a gain spectrum line width. Continuous light generated by a light generation unit passes through a first modulation amplification unit to obtain phase modulation signals; the phase modulation signals pass through a second modulation amplification unit to generate pumping light pulse signals; the pumping light pulse signals are injected to the beginning end of a sensing optical fiber through a circulator; continuous light of the other side path passes through a third modulation amplification unit to obtain bilateral band signals suppressing carriers; and light signals are filtered through a filter to obtain Stokes light or anti-Stokes light, and then the light signals are converted to electric signals through a photodetector. On the premise of not worsening the system spatial resolution, a narrower Brillouin gain spectrum, a higher temperature resolution and a longer sensing distance can be realized.

A system (100) for generating a millimeter wave signals from heterodyning wavelengths from a multi-wavelength signal generated by a Brillouin-Erbium fiber laser (101). The Brillouin-Erbium fiber laser (101) includes a source laser (105) that transmits a seed signal. An optical directional coupler (110) has a first input that receives the seed signal from the source laser and a first output. An Erbium doped fiber amplifier (115) has an input connected to the first output of the optical directional coupler and an output. The Erbium doped fiber amplifier amplifies the seed signal. A four port circulator (120) has a first port that receives the seed signal from the Erbium doped fiber amplifier. A Brillouin gain medium (135) is connected to a second port and a third port of the circulator. The seed signal propagates through the Brillouin gain medium in a first direction and a first order Stokes signal propagates through the Brillouin gain medium in a second direction to generate a second order Stokes signal propagating in the first direction. A fourth port of the circulator is connected to a second input of the optical directional coupler. The optical directional coupler receives the signal from the fourth port of the circulator and outputs a portion of the signal to a first output and a second portion to a second output of the optical directional coupler. Two wavelengths from the second output are then received by a photodiode (150) that heterodynes the signal to generate a millimeter wave signal.

A method of stimulated Brillouin scattering is proposed in which both slopes of the BGS spectrum are probed to achieve immunity to pump power variations. Suitably applied, the technique can maintain all the benefits of the known Slope-Assisted-BOTDA technique, while using the same setup but at the cost of halving the sampling speed. the method may include: obtaining a reference Brillouin gain spectrum (BGS) representative of steady state or average conditions in various locations of the optical fiber along its length; generating, based on the reference BGS, a probe wave being a complex waveform comprising a first and second set of waveforms, each set having waveforms tailored to match an opposite slope of the reference BGS, at the various locations; performing stimulated Brillouin scattering at the various locations by applying a pump pulse wave to the probe wave; and deducing the local Brillouin frequency shift (BFS) at the various locations.