Laser source technology for optical fiber sensing Part One

Optical fiber sensing technology is a kind of sensing technology developed along with optical fiber technology and optical fiber communication technology, and it has become one of the most active branches of photoelectric technology. Optical fiber sensing system is mainly composed of laser, transmission fiber, sensing element or modulation area, light detection and other parts. The parameters describing the characteristics of light wave include intensity, wavelength, phase, polarization state, etc. These parameters may be changed by external influences in optical fiber transmission. For example, when temperature, strain, pressure, current, displacement, vibration, rotation, bending and chemical quantity affect the optical path, these parameters change correspondingly. Optical fiber sensing is based on the relationship between these parameters and external factors to detect the corresponding physical quantities.

There are many types of laser source used in optical fiber sensing systems, which can be divided into two categories: coherent laser sources and incoherent light sources, incoherent light sources mainly include incandescent light and light-emitting diodes, and coherent light sources include solid lasers, liquid lasers, gas lasers, semiconductor laser and fiber laser. The following is mainly for the laser light source widely used in the field of fiber sensing in recent years: narrow line width single-frequency laser, single-wavelength sweep frequency laser and white laser.

1.1 Requirements for narrow linewidth laser light sources

Optical fiber sensing system can not be separated from the laser source, as the measured signal carrier light wave, laser light source itself performance, such as power stability, laser linewidth, phase noise and other parameters on the optical fiber sensing system detection distance, detection accuracy, sensitivity and noise characteristics play a decisive role. In recent years, with the development of long-distance ultra-high resolution optical fiber sensing systems, academia and industry have put forward more stringent requirements for the linewidth performance of laser miniaturization, mainly in: optical frequency domain reflection (OFDR) technology uses coherent detection technology to analyze the backrayleigh scattered signals of optical fibers in the frequency domain, with a wide coverage (thousands of meters). The advantages of high resolution (millimeter-level resolution) and high sensitivity (up to -100 dBm) have become one of the technologies with wide application prospects in distributed optical fiber measurement and sensing technology. The core of OFDR technology is to use tunable light source to achieve optical frequency tuning, so the performance of the laser source determines the key factors such as OFDR detection range, sensitivity and resolution. When the reflection point distance is close to the coherence length, the intensity of the beat signal will be exponentially attenuated by the coefficient τ/τc. For a Gaussian light source with a spectral shape, in order to ensure that the beat frequency has more than 90% visibility, the relationship between the line width of the light source and the maximum sensing length that the system can achieve is Lmax~0.04vg/f, which means that for a fiber with a length of 80 km, the line width of the light source is less than 100 Hz. In addition, the development of other applications also put forward higher requirements for the linewidth of the light source. For example, in the optical fiber hydrophone system, the linewidth of the light source determines the system noise and also determines the minimum measurable signal of the system. In Brillouin optical time domain reflector (BOTDR), the measurement resolution of temperature and stress is mainly determined by the linewidth of the light source. In a resonator fiber optic gyro, the coherence length of the light wave can be increased by reducing the line width of the light source, thereby improving the fineness and resonance depth of the resonator, reducing the line width of the resonator, and ensuring the measurement accuracy of the fiber optic gyro.

1.2 Requirements for sweep laser sources

Single wavelength sweep laser has flexible wavelength tuning performance, can replace multiple output fixed wavelength lasers, reduce the cost of system construction, is an indispensable part of optical fiber sensing system. For example, in trace gas fiber sensing, different kinds of gases have different gas absorption peaks. In order to ensure the light absorption efficiency when the measurement gas is sufficient and achieve higher measurement sensitivity, it is necessary to align the wavelength of the transmission light source with the absorption peak of the gas molecule. The type of gas that can be detected is essentially determined by the wavelength of the sensing light source. Therefore, narrow linewidth lasers with stable broadband tuning performance have higher measurement flexibility in such sensing systems. For example, in some distributed optical fiber sensing systems based on optical frequency domain reflection, the laser needs to be rapidly periodically swept to achieve high-precision coherent detection and demodulation of optical signals, so the modulation rate of the laser source has relatively high requirements, and the sweep speed of the adjustable laser is usually required to reach 10 pm/μs. In addition, the wavelength tunable narrow linewidth laser can also be widely used in liDAR, laser remote sensing and high-resolution spectral analysis and other sensing fields. In order to meet the requirements of high performance parameters of tuning bandwidth, tuning accuracy and tuning speed of single-wavelength lasers in the field of fiber sensing, the overall goal of studying tunable narrow-width fiber lasers in recent years is to achieve high-precision tuning in a larger wavelength range on the basis of pursuing ultra-narrow laser linewidth, ultra-low phase noise, and ultra-stable output frequency and power.

1.3 Demand for white laser light source

In the field of optical sensing, high-quality white light laser is of great significance to improve the performance of the system. The wider the spectrum coverage of white light laser, the more extensive its application in optical fiber sensing system. For example, when using fiber Bragg grating (FBG) to construct a sensor network, spectral analysis or tunable filter matching method could be used for demodulation. The former used a spectrometer to directly test each FBG resonant wavelength in the network. The latter uses a reference filter to track and calibrate the FBG in the sensing, both of which require a broadband light source as a test light source for the FBG. Because each FBG access network will have a certain insertion loss, and has a bandwidth of more than 0.1 nm, the simultaneous demodulation of multiple FBG requires a broadband light source with high power and high bandwidth. For example, when using long period fiber grating (LPFG) for sensing, since the bandwidth of a single loss peak is in the order of 10 nm, a broad spectrum light source with sufficient bandwidth and relatively flat spectrum is required to accurately characterize its resonant peak characteristics. In particular, acoustic fiber grating (AIFG) constructed by utilizing acousto-optical effect can achieve a tuning range of resonant wavelength up to 1000 nm by means of electrical tuning. Therefore, dynamic grating testing with such an ultra-wide tuning range poses a great challenge to the bandwidth range of a wide-spectrum light source. Similarly, in recent years, tilted Bragg fiber grating has also been widely used in the field of fiber sensing. Due to its multi-peak loss spectrum characteristics, the wavelength distribution range can usually reach 40 nm. Its sensing mechanism is usually to compare the relative movement among multiple transmission peaks, so it is necessary to measure its transmission spectrum completely. The bandwidth and power of the wide spectrum light source are required to be higher.

2. Research status at home and abroad

2.1 Narrow linewidth laser light source

2.1.1 Narrow linewidth semiconductor distributed feedback laser

In 2006, Cliche et al. reduced the MHz scale of semiconductor DFB laser (distributed feedback laser ) to kHz scale using electrical feedback method; In 2011, Kessler et al. used low temperature and high stability single crystal cavity combined with active feedback control to obtain ultra-narrow linewidth laser output of 40 MHz; In 2013, Peng et al obtained a semiconductor laser output with a linewidth of 15 kHz by using the method of external Fabry-Perot (FP) feedback adjustment. The electrical feedback method mainly used the Pond-Drever-Hall frequency stabilization feedback to make the laser linewidth of the light source be reduced. In 2010, Bernhardi et al. produced 1 cm of erbium-doped alumina FBG on a silicon oxide substrate to obtain a laser output with a line width of about 1.7 kHz. In the same year, Liang et al. used the self-injection feedback of backward Rayleigh scattering formed by a high-Q echo wall resonator for semiconductor laser line-width compression, as shown in Figure 1, and finally obtained a narrow line-width laser output of 160 Hz.


Fig. 1 (a) Diagram of semiconductor laser linewidth compression based on the self-injection Rayleigh scattering of external whispering gallery mode resonator;
(b) Frequency spectrum of the free running semiconductor laser with linewidth of 8 MHz;
(c) Frequency spectrum of the laser with linewidth compressed to 160 Hz
2.1.2 Narrow linewidth fiber laser

For linear cavity fiber lasers, the narrow linewidth laser output of single longitudinal mode is obtained by shortening the length of the resonator and increasing the longitudinal mode interval. In 2004, Spiegelberg et al. obtained a single longitudinal mode narrow linewidth laser output with a linewidth of 2 kHz by using DBR short cavity method. In 2007, Shen et al. used a 2 cm heavily erbium-doped silicon fiber to write FBG on a Bi-Ge co-doped photosensitive fiber, and fused it with an active fiber to form a compact linear cavity, making its laser output line width less than 1 kHz. In 2010, Yang et al. used a 2cm highly doped short linear cavity combined with a narrowband FBG filter to obtain a single longitudinal mode laser output with a line width of less than 2 kHz. In 2014, the team used a short linear cavity (virtual folded ring resonator) combined with an FBG-FP filter to obtain a laser output with a narrower line width, as shown in Figure 3. In 2012, Cai et al. used a 1.4cm short cavity structure to obtain a polarizing laser output with an output power greater than 114 mW, a central wavelength of 1540.3 nm, and a line width of 4.1 kHz. In 2013, Meng et al. used Brillouin scattering of erbium-doped fiber with a short ring cavity of a full-bias preserving device to obtain a single-longitudinal mode, low-phase noise laser output with an output power of 10 mW. In 2015, the team used a ring cavity composed of 45 cm erbium-doped fiber as the Brillouin scattering gain medium to obtain a low threshold and narrow linewidth laser output.

Fig. 2 (a) Schematic drawing of the SLC fiber laser;
(b) Lineshape of the heterodyne signal measured with 97.6 km fiber delay