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We demonstrate here a method for the online temperature measurement of the output-coupling fiber Bragg grating (FBG) in a high power fiber oscillator by a superimposed FBG, which is used as the output-coupling FBG and the temperature monitor simultaneously. The experimental results verify the feasibility of this method.

We measured the temperature of a single point using a uniform fiber Bragg grating (FBG) for the first time. A quasi- Gaussian temperature distribution pattern was achieved by heating at the midpoint of FBG, then the spectral characteristics, mainly including the central wavelength and amplitude of the resonance peaks were investigated in detail. Results show that the attenuation of each peaks accords well with the theoretical predictions, so the temperature of midpoint can be demodulated by the analysis of transmission spectra.

In this paper, we propose and demonstrate a novel method for the suppression of SBS in optical fibers using a tilted fiber Bragg grating (TFBG). We designed and fabricated a matched TFBG by using an excimer laser and phase mask in single mode fibers according to the operating wavelength of the fiber laser and the tiny frequency shift of SBS. Experimental results demonstrate that TFBGs can suppress the backward-propagating Stokes light caused by SBS to protect the whole system and improve the stable output power, which is very useful for power scaling of high-power narrow-bandwidth all-fiber laser in the future.

Here, the influence of rejection bandwidth of chirped and tilted fiber Bragg gratings on Raman suppression is studied in a 5 kilowatt-level direct LD pumped MOPA fiber laser. A better suppression could be achieved with a broader-band CTFBG, which is quite useful for further power scaling. © 2020 The Author(s)

We report here the effective suppression of stimulated Raman scattering (SRS) in a LD directly pumped 5 kW 1080 nm fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). The CTFBGs are carefully designed and fabricated to match the Raman gain spectrum, and experimental results show that the SRS suppression ratio is about 27.5 dB at 5 kW.

Long period fiber grating (LPFG) can be used as a band rejection component, providing a potential effective way for Stimulated Raman scattering (SRS) suppression in high power fiber laser. Here, an improved CO2 laser scanning system with a galvanometer is built up for fabrication of LPFGs, which greatly improves the speed and consistency of grating inscribing. The characteristics of the manufactured LPFG is measured in detail, then a direct laser diode pumped 5 kW-level fiber amplifier system is established to test the validity of the LPFGs for Raman suppression for the first time. Experimental results show that the SRS suppression ratio is about 14 dB at 5.1 kW with the LPFG being inserted between the seed and the amplifier stage, meanwhile the effective output signal laser power is increased. In contrast to the ultraviolet laser exposed band rejection components, such as chirped and tilted fiber Bragg grating (CTFBGs), this type of LPFG does not need photosensitive fibers, annealing treatment and has a much lower thermal effect, which is very useful for high-power fiber lasers applications.

A novel long-period fiber grating via periodically embedding no-core fiber (NCF) has been achieved. The 11-period SMF-NCF-SMF-long period fiber grating (SNS-LPG) can exhibit an extremely high loss resonant dip (>20 dB). Theoretical analysis shows good agreement with the measured transmission spectrum. We investigated both refractive index and temperature sensitivity of SNS-LPG. The fabricated sensor achieve an average wavelength sensitivity of 141.837 nm/RIU in the range of 1.333–1.400 RIU, and exhibit lower temperature sensitivity (−6.43 pm/ °C and 0.005 dB/ °C).

We propose the design of a high-power fiber oscillator with chirped and tilted fiber Bragg gratings (CTFBGs) to suppress the stimulated Raman scattering within the fiber cavity and in the delivery fibers. A high-power single-mode fiber oscillator system was constructed for demonstration. With a CTFBG inserted into the fiber cavity, obvious Raman suppression was observed and the output signal laser power increased by ∼7%. With a CTFBG inserted between the oscillator and the delivery fibers, a maximum Raman suppression ratio of >20 dB was achieved, which implies a longer delivery distance for the same laser power level. By further improving the performance of CTFBGs, including lower inversion loss at signal wavelength, wider rejection bandwidth and deeper rejection depth, meanwhile optimizing the positions of CTFBGs, better Raman suppression could be obtained, which is significant for the further power scaling and delivery distance increasing of high-power fiber oscillators.

A novel dual-use superimposed fiber Bragg grating (FBG) consisting of a normal FBG operating at 1.55 μm and a chirped FBG (CFBG) operating at 1.08 μm is fabricated and applied in a fiber laser oscillator. The 1.08 μm CFBG is used as a reflector of the fiber oscillator and the FBG is used as an in-core temperature on-line monitor of the laser simultaneously. According to the temperature characteristics of the FBG, the real-time inner temperature can be obtained by detecting the shift of the reflection spectrum. Experimental results have proved the effectiveness of the proposed temperature monitor, which can give the temperature inside a fiber laser comparing to the normal surface measurement techniques such as infrared (IR) camera. This paper provides a simple method to detect the local in-core temperature of fiber lasers, which is useful for investigations of high-power fiber lasers system.

The key remaining technological challenge to the realization of further power scaling for high-power fiber laser systems is overcoming the stimulated Raman scattering (SRS) effect. In past years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to be a simple and effective way to suppress SRS in high-power fiber amplifiers. However, the weak reflection at the Bragg wavelength could be strongly amplified, which not only limits the power and efficiency but also degrades the beam quality. We report here, for the first time to the best of our knowledge, the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg reflection wavelengths are fabricated and used for the comparison experiments. Test results show that the CTFBGs of longer Bragg wavelengths have a better suppression effect, in particular, at a higher power level. By further moving the Bragg wavelength of a CTFBG out of the Raman gain spectral range, a better suppression effect and a promotion in laser efficiency could be achieved, which is very useful for further power scaling.

A novel hybrid fiber Bragg grating (FBG) for simultaneous measurement temperature and strain is proposed. This sensor is fabricated by inscribing fusion splice joint between two types of fibers using excimer laser and phase mask. Due to the different characteristics of the two fibers, the two resonance peaks have different responses to temperature and strain, and both physical parameters can be demodulated by analyzing the central wavelength shifts. Practical measuring results show that the hybrid FBG proposed in this paper can precisely achieve simultaneous measurement of temperature and strain. This work provides a new scheme for multi-parameter simultaneous sensing.

We have demonstrated here, to the best of our knowledge, for the first time the suppression of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area (LMA) fibers according to the operating wavelength of the fiber laser oscillator. A maximum suppression ratio nearly 19 dB or 23 dB is achieved CTFBG insert before the OC grating or after the HR grating. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, a promotion in laser efficiency could be achieved. This work provides a novel idea for SRS suppression in a high-power all-fiber oscillator system, which is very useful for the output power increasing of fiber oscillators.

The average power of fiber lasers has been scaled deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting further power scaling. Here, we have demonstrated for the first time, to the best of our knowledge, the suppression of SRS in a half 10 kW tandem pumping fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). With specially self-designed and manufactured CTFBGs inserted between the seed laser and the amplifier stage, a maximum SRS suppression ratio of >15 dB in spectrum is observed with no reduction in laser efficiency. With one CTFBG, the effective output power is improved to 3.9 kW with a beam quality M2 factor of ∼1.7 from 2; with two CTFBGs, the effective laser power reaches 4.2 kW with an increasing ratio of 20% and an M2 factor of ∼1.8, and further power improvement is limited by the power and performance of the 1018 nm pump sources. This work provides an effective SRS suppression method for high-power all-fiber lasers, which is useful for further power scaling of these systems.