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Abstract
In this work, quasi-static mode degradation in high power fiber amplifiers has been investigated experimentally. An increase of M2 from 1.3 to 2.6 with distortion of the beam profile is observed, which results in the signal spectra and backward light characterization departing from the traditional phenomena. The amplifier has been operated at the same input pump power of 705 W for nearly 2.2 hours to investigate the relationship between quasi-static mode degradation and photodarkening. The evolution of M2 factor/beam profile, mode correlation coefficient and output laser power at different working times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening and evolves on the scale of tens of minutes. A visible green light has been injected to photobleach the gain fiber for 19 hours, which reveals that the quasi-static mode degradation has been suppressed simultaneously. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to overwhelming advantages, including high conversion efficiency, convenient heat management, high beam quality and flexibility, ytterbium-doped fiber lasers with near diffraction limited beam quality have been widely used in scientific research, communication and industrial processing [1–4]. With the rapid development of double clad large mode area fiber and high brightness pump source technology, the average output power of Yb-doped fiber laser systems with near diffraction limited beam quality has been pushed as high as 20 kW [5]. Although the output power of Yb-doped fiber lasers has exceeded by several orders of magnitude in the past few decades, the stability of fiber lasers still faces challenges. It is shown that the power/efficiency of the fiber lasers decreases with operation time, which is associated with photodarkening (PD) effects [6]. The physical mechanism of PD has been investigated for several decades with effective methods being presented to mitigate PD effect, such as increasing the concentration of aluminium (Al) [7,8], phosphorus (P) [9–11] or germanium (Ge) [12], photobleaching with visible light [13,14], ultraviolet light irradiation [15] and heating the fiber [16]. On the other hand, the onset of dynamic mode instability (DMI) has become another major obstacle [1,17]. The typical characteristics of which are the sharp degradation of beam quality and fluctuation of beam profile with characteristic frequency from hundreds of Hz to several kHz [18–20], and is under intense research [21–23].
Recently, a new modal degradation mechanism, where photodarkening plays a key role, is built up to explain a quasi-static modal degradation, mentioned in a personal communication without technical detail [24]. As described in Ref. [24], the modal degradation is caused by photodarkening-induced heat load, which has a time scale of minutes to hours and been referred to as quasi-static degradation. Similar to the dynamic MI, the power coupling from fundamental mode (FM) to high order mode (HOM) is realized by a phase-shifted refractive index grating except that the phase shift results from the photodarkening-induced warm up. The photodarkening origin results to that the mode evolution is slow and typical temporal signal can hardly be observed. Although an experimental study of mode degradation in pulsed Yb-doped fiber amplifiers at a low average power of 3 W has been reported [25], there are few experimental results on quasi-static modal degradation in high power fiber amplifiers, where various nonlinear physical effects result in new experimental phenomena. In 2017, the QSMI in high power fiber lasers was reported in Ref. [26] without any technical details being given.
In this paper, quasi-static mode degradation in high power large mode area fiber amplifiers has been observed, which occurs on a time scale of several tens of minutes. The physical characterization of quasi-static mode degradation has been studied, which has all-around influence on laser performance, including beam quality, optical spectrum, and backward power. Photo-bleaching has been carried out to reverse the quasi-static mode degradation, which shows obviously improvement in beam quality and means that the photodarkening plays a significant role in this effect. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers, and the results are discussed in light of the current understanding of quasi-static mode degradation.
2. Experimental setup
The architecture of the monolithic fiber laser is shown in Fig. 1, which consists of a CW fiber laser oscillator and a one-stage fiber amplifier. The oscillator acting as seed is a linear cavity laser with a pair of home-made fiber Bragg gratings (FBGs) centered at 1064 nm. The reflectivity of the high-reflection (HR) FBG and the output-coupler (OC) FBG is 99.5% and 13%, respectively. The 3 dB reflection bandwidth of the HR-FBG and the OC-FBG is 0.312 nm and 0.18 nm, respectively. The gain medium utilized in the laser oscillator is double cladding ytterbium-doped fiber (YDF) with a core diameter of 10 µm and inner cladding diameter of 130 µm. A 50 W fiber-pigtailed 976 nm laser diodes (LDs) is used to pump the laser oscillator directly through the HR-FBG. A home-made cladding light stripper (CLS) is employed to dump the residual pump light and high-order signal mode propagating in the pump cladding [27]. An isolator (ISO) with additional multimode fiber is incorporated into the MOPA structure to block off and monitor the backward powers from the following main amplifier. After ISO, the seed laser, which delivers a power of 7.33 W, is coupled through a mode field adaptor (MFA) to the main amplifier for further power scaling. The input fiber of MFA is 10/130 µm while the output fiber is 30/250 µm. Then, the output laser from the MFA is launched into the main amplifier through the signal port of the (6 + 1) ×1 home-made signal/pump combiner. The silica active fiber has a core/cladding diameter of 30 µm/400 µm with core NA being 0.06, and the Yb3+ concentration is about 1000 ppm, which has been co-doped with Al/P. The gain fiber has been coiled into a racetrack groove with the minimum and maximum bend diameters of 15 cm and 17 cm, respectively. The cladding absorption coefficient of the active fiber is about 0.9 dB/m at 915 nm and the length of active fiber is 4 m to ensure the total absorption coefficient of more than 10 dB. The active fiber is pumped by six non-wavelength-stabilized 150 W 976 nm multimode laser diodes through the signal/pump combiner. CLS is performed in the similar way as is done in the laser oscillator. One homemade anti-reflection coated end cap with 2m-Germanium doped fiber (GDF) is spliced to deliver the output signal laser and avoid end-face reflection. All the fibers and devices with the exception of the end cap are fixed on a water-cooled heat sink to dissipate the waste heat during the laser operation.
Fig. 1. Experimental setup of a typical MOPA configuration based on 30/400 µm gain fiber (CLS: cladding light stripper, ISO: isolator, MFA: mode field adapter, QBH: quartz block head, PD: photo-detector, OSA: optical spectrum analyzer, PM: power meter).
To evaluate the laser performance, optical spectrum, output power, beam quality and temporal characteristics has been recorded. A photoelectric detector with the spectrum range of 900∼2000 nm is put in the center of the collimated beam to detect the temporal characteristics of output power. The detectable area is smaller than the beam size to ensure the fluctuation in temporary can be measured when the mode evolution occurs which is similar to that in Ref. [18,28]. The beam quality of output laser is measured by a M2 meter. According to the instruction of M2 meter, the spot size around the waist position is measured to figure out the size of waist and far field divergence angle, which has been used to calculate the M2 [29]. The spectrum of output laser is measured by an optical spectrum analyzer with the resolution of 0.02 nm and the output power is measured by an air-cooled power meter.
3. Experimental result
3.1 Characterization of quasi-static mode degradation
The output power was measured firstly in experiment. As shown in Fig. 2(a), the laser power rises gradually with the launched pump power increasing. One can also see in Fig. 2(a) that the slope of the power curve as well as the optical-to-optical efficiency exhibits notable change as the launched pump power increases, which is attributed to the wavelength shift of the diode laser. The emission wavelength of the LDs used in the experiment has a redshift with the increment of driven current or laser power due to the temperature change inside the LDs. The wavelength at lower power deviates from the narrow absorption peak of Yb3+ ions centered at ∼976 nm, leading to poor absorption and low efficiency of the laser. In this experiment, by properly setting the cooling temperature of the LDs, we ensure the pump wavelength shifts close to 976 nm at the maximum driven current to achieve high laser power. The maximum power emitted from the MOPA is 661W when a maximal 851 W pump power is injected into the cavity, with an optical-to-optical efficiency of 76.8%. As shown in Fig. 2(b), no sign of any other light except the signal and residual pump light has been observed. One can see from the residual pump light spectral that, with the increase of pump power, the center wavelength of pump light gradually drifts close to 976 nm. It is worth noting from the spectrums that the signal peak at 466 W and 661 W is slightly lower than that at 356 W shown in Fig. 2(c), which is misleading due to that higher signal laser power should result in higher measured signal peak. To further analyze the recorded spectral data, the 3dB linewidth is calculated and shown in Fig. 2(d). It shows that the linewidth of the fiber systems broadens near linearly as the lasing power increases, which is due to that the linewidth broadening in the MOPA is mainly caused by SPM [30,31]. However, when the lasing power reaches 407 W, which coincides with the power level where unusual spectral data happened, the broadening behavior deviates from the initial linear one, which indicates the trigger of additional physical effects.
Fig. 2. (a) The output power and corresponding optical efficiency; (b) the optical spectrum of output laser at different output power; (c) detail optical spectrum at 1064 nm; (d) center wavelength and 3 dB linewidth of output laser spectrum.
The power and spectrum of the backward light have been monitored from the multimode fiber port of the ISO, which is shown in Fig. 3. Figure 3(a) shows plot for the backward power versus the output signal power at 1064 nm. One can see that the backward power is nearly linear with the signal power until the signal power reaches 407 W, beyond which the backward power fluctuated around the linear trend. The backward spectrum is shown in Fig. 3(b), which reveals a 32 dB suppression of the backward ASE. The 3 dB linewidth of backward light is calculated and plotted in Fig. 3(c), which exhibits similar behavior as those of signal laser. Zooming in with the OSA at the backward spectrum in the vicinity of 1064 nm plotted in Fig. 3(d), no Stokes peak due to SBS has been observed at the maximal signal power.
Fig. 3. (a) The backward power; (b) the optical spectrum of backward laser at different output power; (c) center wavelength and 3 dB linewidth of backward laser spectrum; (d) detail optical spectrum at 1064 nm.
It’s worth noting that onset of mode instability can cause departure of spectrum and backward power behavior from the normal results [32]. Then we measured the beam profile and beam quality (M2 factor) of output signal laser at different output power levels. In experiment, it cost about three minutes to complete each point including output power, spectrum and M2 factor. The pump power versus time in the initial measurement is shown in Fig. 4(a). The pump power is increased from 386 W to 851 W. Each step in Fig. 4(a) is corresponding with each measuring point in Fig. 4(b). As described in Ref. [33,34], the mode correlation coefficient indicates how similar is reconstructed beam compared with the measured beam, which can be employed to quantitatively analyze the mode distortion caused by the presence of HOM. Then we calculated the mode correlation coefficient rcc between the theoretical fundamental mode intensity distribution ILP01 and the experimental intensity distribution Iexp by the simple formula:
Fig. 4. (a) Pump power and theoretical HOM fraction vs time in initial measurement; (b) beam quality (M2 factor) and modal correlation coefficient rcc evolution at different output power; (c) temporal trace and corresponding Fourier spectrum at different output power.
It is shown theoretically in [24] that the photodarkening results in quasi-static degradation, which agrees well with the experimental results. One can conclude that photodarkening results in the degradation of beam quality before reaching the MI threshold. To prove it, the amplifier has been operated at the same input pump power of 705 W (corresponding to the output power of 466 W in Fig. 4) for nearly 2.2 hours. The pump power vs time in experiment is shown in Fig. 5(a). As mentioned in Fig. 4(a), the steps with three minutes period represents the measurement period. After each measurement, the amplifier has been operated at the same pump power of 705 W, and the slight decrease of output power during the measurement has been submerged by the power fluctuation induced by environmental factors, such as the working of the cooling machine. So the measurement time in Fig. 5(b) has been ignored, and the time interval between measurements is 1hour, 0.5 hours, 0.5 hours and 0.2 hours respectively, which results to that the total length of operation time is regarded as 2.2 hours. We observed a slight reduction in output signal laser power as shown in Fig. 5(b), which is an obvious indication of photodarkening [6,35]. Due to that the signal laser power reduces as the operation time due to photodarkening, we choose the pump power as the reference parameters in the following study. The corresponding beam profile of output laser at different work time has been inserted in Fig. 5(b), which indicates a photodarkening-induced quasi-static degradation of beam quality on the scale of tens of minutes. One should also note that photodarkening increases the heat load and reduces the mode instability threshold [36–40]. Based on the time and frequency characteristics shown in Fig. 5(c), there is no sign of mode instability except for quasi-static degradation during the 2.2-hour operation.
Fig. 5. (a) Pump power vs time in measurement; (b) The output power and beam profile evolution at the same pump power of 705W; (c) temporal trace and corresponding Fourier spectrum at different working time at the same pump power of 705 W.
In addition, we measured the beam quality of output laser at different input pump power when the amplifier had worked for 1 hour, 1.5 hours and 2 hours, which is plotted in Fig. 6(a). It reveals that the M2 factor increases as the operation time increases at the pump power of 705 W and 743 W. In particular, the beam profile becomes worse and static mode degradation occurs with the increase of work hours. It is shown in [24] that the fraction of HOM increases as the operation time and is proportion to the signal power. This difference is due to that the M2 value cannot indicate accurately the mode content of the signal laser [41,42]. The beam profile of the laser is shown in Fig. 6(b). One can see that the beam profile at 664 W distorts obviously by the presence of HOM as the operation time increases to 2 hours while the measured M2 value is nearly the same. The calculated result of modal correlation coefficient rcc is shown in Fig. 6(c) which indicates that the mode degradation threshold decreases with the increase of working time. One can conclude from Fig. 6(c) that the fraction of HOM increases as the operation time increases, which agrees with the predication in [24].
Fig. 6. The beam quality evolution at different input pump power and work time: (a) M2 factor; (b) beam profile; (c) mode correlation coefficient rcc.
3.2 Suppressing of quasi-static mode degradation
In [24], a stimulated thermal Rayleigh scattering (STRS) process due to photodarkening has been proposed, which is similar to the mechanism of DMI. However, the origin of the phase shift proposed is the photodarkening-induced warm up of the fiber as it gradually darkens and absorbs more radiation, which gives rise to power coupling into a higher order mode with a very small frequency offset on a time of minutes to hours. Based on the theory mode described in [24], the output power of HOM and the couple strength as the function of time and propagation distance have been calculated, which is presented in Fig. 7. The calculation parameters of the fiber amplifier have been taken the same as those used in the experiment. One can see from Fig. 7(a) that, as the operation times increases, the photodarkening enhances, and more power coupling from FM to HOM due to photodarkening-induced refractive index grating, which reaches to the level of FM power on a time scale of tens of minutes, which can explain the beam quality degradation in Figs. 4–6. One can also see that the coupling strength is high at a point less than 1 meter from the input end. This is due to that the fiber amplifiers pumped in the forward direction, the excited state ion is high in the input port [43]. It is well known that the photodarkening depends on the Yb3+ ion concentration, particularly on the concentration of ions in the excited state [44], which means that the photodarkening is strong in the input port of the forward pump amplifier [43], and that the mode coupling mainly happens in the first 1m length as shown in Fig. 7(b).
Fig. 7. Simulation results: (a) HOM power and (b) coupling strength as the function of time and propagation distance.
Fig. 8. (a) The output power and corresponding optical efficiency at initial state(red symbol), worked for 2 hours(blank symbol), and after bleaching(blue symbol); the beam quality evolution at different input pump power before bleaching(BF), after bleaching(AF) and the initial state(IN); (b) M2 factor; (c) beam profile; (d) mode correlation coefficient rcc.
Due to that the active fiber used in the main amplifier has a core diameter of 30µm, bend diameter should be less than 8 cm to suppress HOM efficiently [45], which is detrimental for nonlinear effect suppression [46] and mechanical reliability of the fiber under high power operation [47]. The widely used mode control method by coiling the fiber should not be employed here. It is shown that photodarkening can be bleached effectively by visible light injection, such as green light [48]. A 20 mW green light source with the center wavelength of 521 nm has been injected from the input fiber of MFA to bleach the gain fiber for 19 hours. To compare the laser characteristics before bleaching (BF) and after bleaching (AF), we measured the power and beam quality of output laser after bleaching. As shown in Fig. 8(a), the output power and optical efficiency is improved after bleaching and nearly recovered to the initial level. After bleaching the gain fiber, the beam quality, beam profiles and the correlation coefficient rcc shown in Figs. 8(b)–8(d) have been significantly improved when the pump power exceeds 600 W, which means that the static mode degradation can be suppressed by green light bleaching, and methods to suppressing photodarkening can be employed to mitigate photodarkening-induced quasi-static degradation. It is worthy to note that the static mode degradation has not been mitigated completely by the green light, which is due to that there are different types of defect and requires different bleaching energy (e.g. different photon wavelengths) [49]. One can conclude that other methods that used to suppress photodarkening can also be employed to suppress quasi-static mode degradation [37].
4. Conclusions
To summarize, we have experimentally studied the quasi-static mode degradation in high power fiber amplifiers, which shows that the quasi-static mode degradation has all-around influence on laser performance, including beam quality, optical spectrum, and backward power. The evolution of M2 factor/beam profile and output laser power at different work times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening, and evolves on the scale of tens of minutes. Photobleaching with a green light has been carried out to reverse the quasi-static mode degradation, which shows obviously improvement in beam quality and means that the photodarkening plays a significant role in this effect while the static mode degradation has not been mitigated completely by the green light, which is due to that there are different types of defect and requires different bleaching energy.
Funding
National Natural Science Foundation of China (61905226).
Disclosures
The authors declare no conflicts of interest.
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