1. Introduction

Fiber lasers are widely used in many applications for the advantages of high conversion efficiency, good beam quality, compact structure and low maintenance operation. In the last decades, the output powers of monolithic fiber lasers have increased remarkably [1–11]. The record power of fiber lasers have reached over 100kW, which is realized by combining tens of kW-level single-mode fiber lasers into a multimode output. Further power scaling is promising by increasing the single-mode laser power and the number of beam combination. However, the power scaling of the single-mode fiber lasers is limited by the traditional fiber nonlinear effects and the newly observed transverse mode instability (TMI). Among fiber nonlinear effects, the stimulated Raman scattering (SRS) is the main limiting factor for continuous-wave broadband fiber lasers. Through the past years, large mode area (LMA) fibers are designed and employed to effectively mitigate the fiber nonlinear effects in high power operation. However, recent studies report that the transverse mode instability is observed and becomes a major limitation of laser power scaling in the LMA fibers. TMI refers to the fluctuations of normally stable high-quality beams emitted by high-power fiber lasers observed above a sharp average power threshold [12–20]. Different from the nonlinear effects, the detriment of TMI is that it degrades the laser beam quality and restricts the maximum average power of the fiber laser. Despite the relative short period since the observation, the TMI arouses the interest of researchers around the world and much progress has been made in understanding its physical origin.

The occurrence of TMI was observed in both the structures of fiber oscillators and amplifiers. To date, most studies and research of the TMI are focused on the fiber amplifiers [17–33], while the TMI in fiber laser oscillators are rarely reported [34,35]. For the fiber amplifiers, several theoretical models have been built to understand the physical origin of TMI and find techniques to mitigate the effect [13,15,16,28]. While for the fiber laser oscillators, only a simplified model [35,36] was put forward to analyze the threshold. Despite with the similar physical origin of TMI, the fiber laser oscillators have to handle with the bidirectional propagation of the signals, which makes the corresponding theoretical model more complicated. Recently, a coupled-mode model was generalized to simulate double-pass amplifiers [37]. It predicts a static thermo-optic instability in the double-pass fiber amplifiers, which is much different from the TMI in single-pass amplifier in origin and property. This model may help the researchers on the study of TMI in fiber laser oscillator as a reference.

The experimental reports and descriptions of the TMI in fiber laser oscillators are rare in the past years. In 2014, Hejaz et al. reported the techniques to mitigate TMI in an all-fiber laser oscillator by shifting pumping wavelength and increasing high-order modal loss [34]. The TMI threshold was characterized by the decline of the laser efficiency and the decrease of output power, but no detailed characteristic of the output laser around the TMI threshold was described. Several kW-level all-fiber laser oscillators were reported in the last years [2,3,5,11]. For the single-end pumped all-fiber laser oscillators, output power of 1.2kW and 1.5kW were achieved in 2012 and 2014, respectively [2,3]. For the bidirectional-pumped all-fiber laser oscillator, 2kW output power with near-diffractive limited beam quality was reported [5]. In the spatial configuration, single-mode fiber laser oscillator has reached 3kW by employing bulky optics [9]. The output powers of the fiber laser oscillators have exceeded the currently reported TMI threshold range of ~100W to several kW, but the experimental studies on the TMI in fiber laser oscillator are rarely reported.

In this manuscript, we report an experimental study of the TMI in an all-fiber laser oscillator with a maximum scaling power of 2.5kW. We constructed a bidirectional-pumped ytterbium-doped all-fiber laser oscillator operating at 1070nm and investigated the performance in co-pumping and bidirectional-pumping configuration respectively. In co-pumping scheme, the TMI is observed at ~1.6 kW and restricts further power scaling. The characteristics of TMI in the laser oscillator are described in details. By employing bidirectional-pumping, the output power is scaled up to 2.5kW without evidence of TMI. Experimental results validate that bidirectional-pumping shows a distinct advantage over the co-pumping scheme in mitigating TMI and scaling output power in fiber oscillators.

2. Experimental setup

The architecture of the all-fiber laser oscillator is sketched in Fig. 1. High-power wavelength-stabilized 976nm laser diodes (LDs) are employed as pumping, and tapered fused bundle (TFB) combiners are employed to combine the pumping light into the inner cladding of the double cladding fibers with a diameter of 400μm and a numerical aperture of 0.46. Two TFB combiners are utilized to constitute bidirectional pumping. The 7 × 1 TFB combiner is adopted as the co-pumping combiner, and the co-pumping light is launched into the laser cavity through the high reflective fiber Bragg grating (HR FBG). The (6 + 1) × 1 signal/pump combiner is incorporated in the laser cavity as the counter-pumping combiner, and the counter-pumping light is directly launched into the gain fiber. The signal port of the (6 + 1) × 1 signal/pump combiner is double cladding fiber with a core diameter /numerical aperture of 20μm/0.065. Both the FBGs are inscribed on the double cladding fiber with core/inner cladding diameter of 20/400μm. The HR FBG provides a reflectivity of ~99% at the center wavelength of ~1070nm with a 3dB bandwidth of 3nm, while the OC FBG has a reflectivity of ~10% at the center wavelength of ~1070nm with a 3dB bandwidth of 1nm. The gain fiber is double cladding ytterbium-doped fiber (YDF) with 20μm/0.065NA core and 400μm/0.46NA inner cladding. As the absorption coefficient of the adopted YDF at 976nm is ~1.5 dB∕m, the YDF length is set to be ~19m to ensure adequate absorption of pump powers. After the OC FBG, about 5m delivery fiber with core/inner cladding diameter of 25/400μm is spliced and an endcap is employed to eliminate facet reflection. Three parts of cladding light strippers (CLS) are utilized to fully dump residual pumps and signal propagating in the inner cladding. The output powers, optical spectra, temporal characteristics and beam quality of the fiber oscillator are measured and recorded in the experiment. All the components in the experiment, including YDF, LDs, TFB combiners, FBGs and CLSs, are placed on water-cooled heat sink to realize effective heat management and ensure the stability in high power operation.

 

Fig. 1 Experimental setup of the all-fiber laser oscillator pumped by 976nm LDs (CLS: cladding light stripper, CO: collimator, HR: high reflector, PD: photodetector, OSA: optical spectrum analyzer).

Download Full Size | PDF

3. Experimental results and discussion

In the experiment, we investigate the performances of the fiber laser oscillator with both co-pumping and bidirectional-pumping configuration. The co-pumping scheme is firstly utilized, and then bidirectional-pumping is utilized to further scale the output laser. The laser performance of the output laser with counter-pumping configuration is also investigated, but the achieved maximum output power is lower than the co-pumping scheme, for the limited power handling ability of the counter-pumping combiner. To be explicit, the counter-pumped laser oscillator performance is not given in this work.

3.1 Laser performance with co-pumping

In co-pumping scheme, the recorded output laser powers and optical efficiency dependence on the pumping powers are shown in Fig. 2(a). The optical efficiency of the laser oscillator is relatively lower at the beginning. As the output power increase, the optical efficiency reaches ~72%. However, an obvious decline in the optical efficiency is observed when the output power exceeds ~1.5kW. After reaching a maximum of 1.61kW, the output laser power shows a remarkable decrease. Meanwhile, the optical efficiency of the laser oscillator drops distinctly. An increase of the dumped light is also observed in the cladding light strippers regions. The optical spectra of the output laser at ~1.36kW and ~1.61kW operation are depicted in Fig. 2(b). No pumping light is observed during the power scaling, but the Raman stokes light is observed at ~1.36kW. At the operation of ~1.61kW, the Raman stokes light is ~28dB below the intensity of the signal laser. Despite the appearance of the Raman stokes light, the power proportion is small and the SRS is not the main reason to induce the output power decrease.

 

Fig. 2 (a) output laser power and optical efficiency dependence on the co-pumping power (b) optical spectra of the output laser at ~1.36kW and ~1.61kW.

Download Full Size | PDF

The output power decrease, we believe, is attributed to the occurrence of TMI in the fiber oscillator. To verify the onset of the TMI, the temporal characteristics of the output laser at different power levels are recorded using a photodectector with a bandwidth of 150MHz. To quantify the stability of the output laser, the standard deviation normalized to the mean value of the time domain signals are calculated [24,30,38], as shown in Fig. 3(a). When the laser output power reaches the roll-over point, the standard deviation values increase distinctly, which indicates a fluctuation in the time domain signals. To give more details and reveal the evolution, the time domain signals at the operation of ~1.36kW, ~1.61kW and ~1.56kW are depicted in Fig. 3(b), and the corresponding Fourier spectra obtained by performing Fourier transformation on the time domain signals are shown in Fig. 3(c). To avoid any misunderstanding caused by the output power decrease, the recorded output power levels are denoted as P1(~1.36kW), P2(~1.61kW) and P3(~1.56kW) respectively in Fig. 3(a). When the laser operates at P1 and P2, the time domain signals remain stable. However, distinct fluctuation is observed in the time trace, when the laser operates at P3. In the Fourier spectra, the frequency components of the fluctuation are revealed. The first notable frequency peak is only around ~20Hz, while the rest of the frequency components are mostly below 150Hz. From the temporal characteristics of the output laser, the occurrence of the TMI around the roll-over output power is verified.

 

Fig. 3 Performance of the laser oscillator with co-pumping, (a) the dependence of output laser power and standard deviation on the co-pumping powers (b) time domain signals of the output laser (c) Fourier spectra (d) beam profiles at three different power levels.

Download Full Size | PDF

The beam profiles of the output laser at the operation of P1, P2 and P3 are shown in Fig. 3(d). The measured output laser beam profiles show no evidence of obvious deterioration, despite the operation of P2 and P3 are around the TMI threshold. Typically, when the TMI occurs, there is energy coupling between the fundamental mode and the high order modes, which can be observed in the beam profiles of the output laser. In this experiment, modal coupling cannot be observed from the output beam profiles. This can be explained from the increased high order mode loss in the gain fiber induced by coiling. In the experiment, the gain fiber was coiled in circles with a minimum diameter of ~10cm. At the operation of ~1070nm, the gain fiber supports only the LP₀₁ and LP₁₁ modes. According to the classic fiber bending loss theory in Ref [39], the bending loss of the LP₁₁ mode at the coiling diameter of ~10cm is over 20dB/m. Owing to the bending loss of the fiber, when TMI occurs in the laser oscillator the coupled high-order modes experience higher modal loss and leak into the inner cladding of the fiber. The leaked power then gets dumped in the cladding light stripper regions, which causes the decrease of the laser efficiency and the output power. Since the high-order modes are stripped out, the output laser maintains good beam profile around the TMI threshold.

It should be noted that the frequencies of the periodic fluctuation in this work are several times lower than the dynamic TMI in single-pass fiber amplifiers, which are mostly in kHz level. To the best of our knowledge, there are only two reports of the static/quasi-static TMI up to date, which are both theoretical modeling published in this year [37,40]. The first is a modeling of quasi-static degradation induced by photodarkening effect in single-pass fiber amplifier [40]. It predicted a power transfer from the fundamental mode to a higher mode with low frequency offset. The other is a modeling of static thermo-optic instability in a double-pass fiber amplifier [37]. It predicted a static mode deformation, which is induced by the interaction of light propagating in either direction with thermo-optics index perturbations caused by light propagation in the opposition direction. In our work, the experimentally observed TMI in the laser oscillator provide similar quasi-static mode instability with low modulation frequency. Unfortunately, there is no suitable theoretical model for the fiber laser oscillators which can provide simulated details of the instability so far. Considering that the laser oscillators and the double-pass fiber amplifiers share the similarity of bidirectional signal propagation, we think the reported modeling of the static TMI in the double-pass fiber amplifiers can be taken as the reference.

3.2 Laser performance with bidirectional-pumping

Comparing with co-pumping, bidirectional-pumping can dissipate the heat load in the gain fiber. Since the TMI is correlated with the heat load, the bidirectional-pumping scheme can be employed to mitigate the TMI and further scale the output power. Moreover, the bidirectional-pumping enables that more pump power can be combined. Recent reports on the bidirectional pumped laser amplifiers show notable achievement in power scaling [10,41].

In the bidirectional-pumping scheme, the output powers and the corresponding standard deviation of the recorded time traces at different pumping powers are shown in Fig. 4(a). The output power increases almost linearly with the pump power with a slope efficiency of 74.5%. A maximum output of 2.5kW is achieved at the pumping power of ~3.46kW. No evidence of optical efficiency decline is observed in the power scaling process. The temporal signals are recorded and the standard deviations are calculated at different power levels. No remarkable increase of the standard deviation is observed. The inset in the Fig. 4(a) is the beam profile at ~2.5kW operation. The output laser beam quality is also measured, and the M² factors at ~2.5kW are around 1.3 in both axes. The temporal characteristic of the output laser at ~2.5kW is shown in Fig. 4(b). The inset is the corresponding Fourier spectra. As we can see, the time domain signal remains quite stable at the operation of ~2.5kW. In the Fourier spectra, there are no frequency components of TMI. So, it is confirmed that TMI doesn’t occur at the operation of ~2.5kW.

 

Fig. 4 Performance of the laser oscillator with bidirectional-pumping (a) the dependence of output power and standard deviation on the pump power (b) temporal characteristic at ~2.5kW.

Download Full Size | PDF

The optical spectra of the output laser are recorded with an optical spectra analyzer, as shown in Fig. 5. The spectral bandwidth of the output laser broadens gradually during the power scaling. At the operation of ~2.5kW, the full width half maximum (FWHM) spectral bandwidth is ~5.2nm. For three parts of cladding light strippers are utilized, no pumping light is observed in the spectra. In the bidirectional pumping scheme, the Raman stokes light is observed when the laser output power reaches ~1.9kW. At the operation of ~2.5kW, the intensity of Raman stokes light is ~20dB below the signal laser and the power ratio is less than 1%. For further power scaling of the all-fiber laser oscillator, the SRS has to be firstly mitigated. To be noted, in the co-pumping scheme, the Raman stokes light is observed at ~1.36kW, which is relatively lower than the ~1.9kW in the bidirectional pumping scheme. Bidirectional pumping also benefits from the advantage of a higher SRS threshold.

 

Fig. 5 Optical spectra of the output laser with bidirectional pumping.

Download Full Size | PDF

4. Conclusion

In summary, we have constructed a bidirectional-pumped ytterbium-doped all-fiber laser oscillator and investigated the laser performances with co-pumping and bidirectional pumping schemes respectively. When the laser oscillator was co-pumped, TMI was observed at ~1.6kW and limited further power scaling. Around the TMI threshold, the temporal characteristics were recorded and details of the evolution were provided. By employing bidirectional-pumping instead of co-pumping, the TMI was remarkably mitigated and the output power was further scaled to 2.5kW with a slope efficiency of 74.5%. At the output power of ~2.5kW, the beam quality(M²) was ~1.3, and the FWHM bandwidth was 5.2nm. The Raman stokes light was ~20dB below the signal and occupied less than 1% of the total power. For further power scaling of the laser oscillator, the SRS has to be mitigated.