Distributed Feedback Fiber Laser Research Articles

Large dynamic-range fiber Bragg grating sensor system for acoustic emission detection.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector. The 0.2 nm linewidth FBG sensor can detect the impact signal with a frequency of up to 300 kHz with a maximum of 29.17 µɛ, which is comparable with the detecting result of the piezoelectric transducer sensor. Moreover, the directional response of the FBG sensor is maximized when the direction of acoustic wave propagation is parallel to the optical fiber. The relation between the sensitivity and the FBG spectrum linewidth is presented, and the detectable strain range versus different FBG linewidths is also discussed.

Switchable and tunable multi-wavelength erbium-doped random distributed feedback fiber laser based on a compound filter

A switchable and tunable multi-wavelength random distributed feedback fiber laser (RDFBFL) is realized based on the compound filter which is comprised of a dual-pass Mach-Zehnder interferometer (MZI) and a Sagnac loop mirror. The lasing threshold of the proposed RDFBFL is suppressed to 50 mW by the pumped erbium-doped fiber (EDF), and the pump efficiency is 1%. By simply adjusting the angle of polarization controller (PC) in the Sagnac loop, 2, 3, 5 and 6 lasing channels can be obtained under the pump power of 200 mW, and the central wavelengths of the 3 lasing channels can be simultaneously tuned with a fixed channel separation. Moreover, the power amplitudes and side mode suppression ratios (SMSRs) of lasing channels are maintained around − 10 dBm and 34 dB, respectively. The proposed RDFBFL has the advantages of ideal tunability and low threshold, which can be widely used in imaging systems and long-distance sensing.

Optimal defect position in a DFB fiber laser

Fiber lasers with compact cavity have numerous potential applications in sensing, communications, and medicine. Distributed feedback (DFB) rare-earth doped fiber lasers based on Bragg gratings with a phase shift are the most promising in this aspect. In this paper, we theoretically study such lasers and carry out a complex-frequency analysis of the DFB cavity modes. Our approach is based on the study of poles of open cavity response function and on the laser rate equations. An optimal defect position in the Bragg grating, which maximizes an output power towards one side, was found with this approach. We show that the optimal defect position depends on the pump power. At the pump level close to the lasing threshold, the defect should preferably appear in the middle of the grating to maximize the one-side output power. At higher pumping, the optimal position of the defect becomes asymmetric. We have found specific variables, which allow for determination of optimal defect position for a large variety of DFB laser configurations.

Coherent laser detection of the femtowatt-level frequency-shifted optical feedback based on a DFB fiber laser.

The theoretical basis and experimental realization of an all-fiber self-mixing laser Doppler velocimetry based on frequency-shifted feedback in a distributed feedback (DFB) fiber laser are presented, which employs a pair of fiber-coupled acousto-optic modulators to adjust the modulation intensity and frequency of the laser self-mixing effect. Moreover, the minimum optical feedback intensity for the velocity signal successfully measured by the interferometer is 5.12 fW, corresponding to 0.16 photons per Doppler cycle. The results demonstrate that the proposed scheme can adapt to the non-contact measurement requirements of the wide-range speed and weak feedback level in the complex environment.

All-fiber 1.55 µm erbium-doped distributed-feedback laser with single-polarization, single-frequency output by femtosecond laser line-by-line direct-writing

We report an all-fiber single-polarization, single-frequency distributed-feedback (DFB) laser by a novel femtosecond laser line-by-line (LbL) direct-writing method. The phase-shifted fiber Bragg grating with a length of 26 mm is written in a single-mode non-polarization-maintaining (PM) Er-doped silica fiber. Single-polarization, single-frequency 1.55 µm laser oscillation is observed in the all-fiber DFB cavity configuration, with a pump threshold of 16 mW. The maximum laser output power from one port of the laser approaches 3.9 mW, and the slope efficiency of the DFB laser is 0.7% under the pump power of 640 mW. The FWHM (Full Width at Half Maximum) linewidth of the DFB fiber laser output is measured to be <1 kHz. High polarization extinction ratio of >30 dB has been observed by measuring the powers in the two orthogonal polarization directions of the DFB laser output. Single polarization mode laser oscillation has been verified. It is promising to use the femtosecond LbL writing method to achieve high-performance all-fiber single-polarization, single-frequency DFB fiber laser sources without relying on PM fiber elements.

Tunable multi-wavelength random distributed feedback fiber laser based on dual-pass MZI

A tunable half-open cavity multi-wavelength erbium-doped random distributed feedback (RDFB) fiber laser is proposed and experimentally demonstrated. The backward Rayleigh scattering generated by inhomogeneous reflective in the 25-km-long single mode fiber is amplified by the 8-m-long pumped erbium-doped fiber (EDF). The dual-pass Mach–Zehnder interferometer (MZI) acts as the reflection mirror in the half-open cavity. Due to the EDF gain and the half-open structure, the laser threshold is suppressed to 40 mW. The number of laser channel increases linearly with the increasing pump power. Under the pump power of 350 mW, there are seven lasing channels in the spectrum. One arm of the MZI is circled on piezoelectric transducer (PZT) and tunable multi-wavelength lasing operation is realized by adjusting the DC voltage on PZT. When the pump power and the driving voltage attached on PZT are 250 mW and 5 V, the wavelengths of six channels are blue-shifted 0.7 nm. The proposed tunable RDFB fiber laser has the advantages of low cost, simple structure and low threshold, which can be widely used in remote sensing and biomedical imaging.

Tunable and Switchable Multi-Wavelength Random Distributed Feedback Fiber Laser Based on Smf and Cascaded Sagnac Loops Filter

Tunable and Switchable Multi-Wavelength Random Distributed Feedback Fiber Laser Based on Smf and Cascaded Sagnac Loops Filter

Real-Time Correction Homodyne Symmetry Algorithm to Demodulate DFB Fiber Laser Hydrophone

Real-Time Correction Homodyne Symmetry Algorithm to Demodulate DFB Fiber Laser Hydrophone

Switchable multi-wavelength erbium-doped random distributed feedback fiber laser incorporating two-stage Sagnac filter

A low-threshold and simple structure switchable multi-wavelength fiber laser based on random distributed feedback (RDFB) is presented. The experiment setup incorporates a two-stage Sagnac fiber loop mirror, an 8 m long erbium-doped fiber (EDF), and a 25 km long single-mode fiber (SMF). The SMF provides RDFB via Rayleigh scattering. Due to the implementation of the half-open cavity structure and the gain generated by the EDF, the laser threshold can be significantly suppressed to 60 mW, and the slope efficiency of output lasing power against pump power is ∼0.9%. By adjusting the polarization controllers of the proposed two-stage Sagnac loop mirror, single-, double-, triple-, and quadruple-wavelength lasing channels in the range of 1527–1534 nm are achieved. In the case of quadruple-wavelength channel lasing, the optical signal noise ratio is up to 33 dB under the 180 mW pump power. The proposed random fiber laser can be applied in distributed sensing systems and biomedical imaging.

Advanced distributed feedback lasers based on composite fiber heavily doped with erbium ions

Specially designed composite heavily Er3+-doped fiber in combination with unique point-by-point inscription technology by femtosecond pulses at 1,026 nm enables formation of distributed-feedback (DFB) laser with ultra-short cavity length of 5.3 mm whose parameters are comparable and even better than those for conventional Er3+-doped fiber DFB lasers having much longer cavity. The composite fiber was fabricated by melting rare-earth doped phosphate glass in silica tube. The ultra-short DFB laser generates single-polarization single-frequency radiation at 1,550 nm with narrow linewidth (3.5 kHz) and 0.5 mW output power at 600 mW 980-nm pumping. The same fiber with conventional CW UV (244 nm) inscription technology using phase mask enables fabrication of 40-mm long DFB laser with > 18 mW output power at 3.3% pump conversion, which is a record efficiency for Er3+-doped fiber DFB lasers. The developed technologies form an advanced platform for Er3+-doped fiber DFB lasers operating around 1.55 µm with excellent output characteristics and unique practical features, in particular, the ultra-short DFB lasers are attractive for sensing applications.

High resolution temperature sensor based on frequency beating between twin DFB fiber lasers.

We present a high resolution temperature sensor using the beat frequency between the longitudinal modes of twin single-mode distributed feedback fiber lasers. The lasers are made by femtosecond inscription of π-shifted fiber Bragg gratings in a thulium-doped fiber. Combining the light from two single frequency fiber lasers on a photodetector produces a rf beat frequency signal which is dependent on temperature. Experimental results show a sensitivity of 1900 MHz/°C, leading to a precision of 0.0007 °C.

Ultralow noise DFB fiber laser with self-feedback mechanics utilizing the inherent photothermal effect

Single frequency laser sources with low frequency noise are now at the heart of precision high-end science, from the most precise optical atomic clocks to gravitational-wave detection, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference cavity. Despite the tremendous progress, these laser systems are relatively high in terms of complexity and cost, essentially suitable for the laboratory environment. Nevertheless, more and more commercial applications also demand laser sources with low noise to upgrade their performance, such as fiber optic sensing and LiDAR, which require reduced complexity and good robustness to environmental perturbations. Here, we describe an ultralow noise DFB fiber laser with self-feedback mechanics that utilizes the inherent photothermal effect through the regulation of the thermal expansion coefficient of laser cavity. Over 20 dB of frequency noise reduction below several tens of kilohertz Fourier frequency is achieved, limited by the fundamental thermal noise, which is, to date, one of the best results for a free-running DFB fiber laser. The outcome of this work offers promising prospects for versatile applications due to its ultralow frequency noise, simplicity, low cost, and environmental robustness.

Experimental Study on Brillouin Erbium Fiber Laser: Configuration and Characteristics

We carry out the experimental study on Brillouin erbium fiber laser in two different structure configurations. In the first configuration, Brillouin pump laser directly stimulates the Brillouin scattering light, whilst in the second it gets pre-amplification before stimulating the Brillouin scattering light in silica optical fiber. Employing distributed-feedback fiber laser as the Brillouin pump laser, we get the Brillouin erbium fiber lasers having subhundred Hertz linewidth. The first configuration has a higher pump threshold and a higher maximum output power, which is over 20 mW for a 250 mW 980 nm pump. The second configuration with pre-amplification for Brillouin pump laser has a lower pump threshold and a smaller maximum output power.

Delay-induced instability in phase-locked dual-polarization distributed-feedback fiber lasers

We study experimentally and theoretically a dual-polarization fiber laser submitted to time-delayed frequency-shifted optical feedback. In addition to the usual frequency-locking regime that is expected when the frequency shift is close to the polarization beat frequency, we observe at high pumping rates a dynamical pulsing regime inside the locking range. This regime is experimentally evidenced in a full-fibered experiment based on an erbium-doped distributed-feedback fiber laser in which polarization beat frequency is about 1 GHz. A rate-equation model including the frequency-shifted feedback term reproduces well the experimental bifurcation maps, provided that both the time delay and a phase-amplitude coupling parameter ($\ensuremath{\alpha}$ factor) are taken into account. The impact on microwave-photonics applications is discussed.

Poisson distribution of extreme events in radiation of random distributed feedback fiber laser.

In the present Letter, we experimentally investigate extreme events in the time dynamics of the random distributed feedback fiber laser. We find that the probability of extreme events depends on the wavelength of the generated light. On spectrum tails, we register extreme events with intensity up to 50 times higher than the average generation power. Analysis of return times between successive rogue waves reveals their exponential distribution. Further investigation proves that the appearance of extreme waves in laser radiation obeys Poisson law. Characteristic radiation time varies from nanoseconds to tens of microseconds for most intense waves.

A Comparative Study of Thermal Impact on Erbium Doped Distributed Feedback Fiber Laser Output Power

The temperature dependency of erbium doped distributed feedback fiber laser output performance is experimentally and theoretically investigated with two major pump schemes. The output power declined linearly as temperature increased from 30 °C to 120 °C for 980 nm and 1480 nm pumped test laser while the later exhibited stronger heat susceptibility. Further experiment and analysis identified the temperature induced absorption and emission cross section variation at pump and signal wavelength together with population inversion rate difference are the originations of the thermal sensitivity discrepancy between the two pump setups. Associated theoretical simulation concludes the erbium doped distributed feedback fiber laser thermal output sensitivity is dependent on its gain dynamics and grating structure simultaneously.

Dual-wavelength random distributed feedback fiber laser with wavelength, linewidth, and power ratio tunability.

Owing to the special power distribution property, a random distributed feedback Raman fiber laser can achieve a high power spectrally flexible output with a low power spectrally tuning device. Here, an all-fiberized linearly polarized dual-wavelength random distributed feedback Raman laser with wavelength, linewidth, and power ratio tunability is demonstrated. By adopting two watt-level bandwidth adjustable optical filters, a spectrum-manipulable dual-wavelength output with nearly a 10 W output power is achieved. The wavelength separation can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the output can be doubled by increasing the bandwidth of the optical filter. The power ratio of each laser line can be tuned from 0 to nearly 100% with the help of two variable optical attenuators. A maximum output power of 9.46 W is realized, with a polarization extinction ratio up to 20.5 dB. The proposed dual-wavelength fiber laser can be employed as a pump source in frequency tunable, bandwidth adjustable terahertz microwave generation, and mid-infrared optical parametric oscillators.

Seed-injected, actively Q-switched fiber ring laser using an AOM of zero-order transmission

Using an acousto-optic modulator of zero-order transmission as the Q-switch, single-frequency laser pulses were obtained in a ring cavity fiber laser with the seed injection technique. The acousto-optic modulator avoided the frequency-shift resulting from the RF driver signal in the first-order transmission. As the seed laser, a distributed-feedback fiber laser based on the π phase-shift fiber Bragg grating was directly inserted into the ring cavity. It decreased the cavity loss and improved the injection efficiency, compared with the common injection by an optical fiber coupler. With the seed laser power over 150 μW, the single-frequency laser pulses with the wavelength of 1550.12 nm were achieved and verified by a scanning Fabry–Perot interferometer. Under the pump power of 450 mW, the pulse duration ranged from 90 ns to 350 ns with the repetition frequency from 10 kHz to 50 kHz. The measured linewidth demonstrated that the pulse laser was almost transform limited: 1.9 MHz for a train of pulses of 50 kHz repetition frequency, 350 ns pulse duration and 36.2 mW maximum average power.

Dual-wavelength random fiber laser incorporating micro-air cavity

A dual-wavelength random distributed feedback fiber laser with external micro-air cavity (MAC) is experimentally demonstrated. The structure that comprises of a fixed 200 μm air-gap distance between two flat-angle fiber-end facets behaves similarly to the role of reflectors. The generation of dual-wavelength lasing at 1552.5 and 1557.7 nm was achieved. The accomplishment is associated to the filtering effect of MAC superimposed with Raman gain spectrum. Equal peak power of approximately −12.7 dBm was attained at maximum 2.0 W pump power. In this case, the respective optical signal-to-noise ratios were 18.79 and 18.73 dB. Excellent stability of the spectral width was realized over 60 min observation time at room temperature, which denotes the practicality of this device in addition to the relatively simpler architecture.

Non-intrusive flow measurement based on a distributed feedback fiber laser

We propose a new non-intrusive flow measurement method using the distributed feedback fiber laser (DFB-FL) as a sensor to monitor flow in the pipe. The relationship between the wavelength of the DFB-FL and the liquid flow rate in the pipeline is derived. Under the guidance of this theory, the design and test of the flow sensor is completed. The response curve is relatively flat in the frequency range of 10 Hz to 500 Hz, and the response of the flow sensor has high linearity. The flow from 0.6 m3/h to 25.5 m3/h is accurately measured under the energy analysis method in different frequency intervals. A minimum flow rate of 0.046 m/s is achieved. The experimental results demonstrate the feasibility of the new non-intrusive flow measurement method based on the DFB-FL and accurate measurement of small flow rates.