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Highly-efficient high-power fiber lasers operating at wavelength below 1020nm are critical for tandem-pumping in >10kW fiber lasers to provide high pump brightness and low thermal loading. Using an ytterbium-doped-phosphosilicate double-clad leakage-channel fiber with ~50µm core and ~420µm cladding, we have achieved ~70% optical-to-optical efficiency at 1018nm. The much larger cladding than those in previous reports demonstrates the much lower required pump brightness, a key for efficient kW operation. The demonstrated 1018nm fiber laser has ASE suppression of ~41dB. This is higher than previous reports and further demonstrates the advantages of the fiber used. Limiting factors to efficiency are also systematically studied.
© 2015 Optical Society of America
1. Introduction
Many advantages of ytterbium-doped fiber lasers such as excellent efficiency, low quantum defect, and readily available high-power pumps have made them an ideal choice for high-power fiber lasers and amplifiers [1]. In the past decade, power scaling of ytterbium-doped fiber lasers have been extensively studied. Large mode area fibers, such as photonic crystal fiber (PCF), photonic bandgap fiber (PBF), and leakage channel fiber (LCF), have been developed for mitigating nonlinear effects in the course of power scaling [2–5].
To date, most high-power ytterbium-doped fiber lasers operate at 1030nm-1200nm. However, many applications such as spectroscopy and laser cooling require shorter operating wavelengths in order to be frequency doubled or quadrupled to the desired wavelengths [6]. Most importantly, multimode high-power fiber lasers operating below 1020nm can be used in a tandem pumping scheme as pumps to reduce quantum defect heating and provide high pump brightness in ytterbium-doped fiber lasers operating at ~10kW. In this high-power regime, single-mode operation becomes much more challenging due to mode instability driven by quantum defect heating [7–9]. Thermal management also becomes much more challenging due to the much higher heat load. Multimode 1018nm fiber lasers are key components in the tandem pumping scheme to provide high brightness pumps and much lower quantum defects in IPG 10kW fiber lasers [10].
However, it is difficult to realize stable and efficient 1000-1018nm ytterbium-doped fiber lasers in conventional ytterbium-doped aluminosilicate host as it requires a large population inversion to reach gain threshold. Stable operation can only be realized by shortening the fiber, which leads to poor pump absorption. One additional challenge is parasitic lasing at the ASE peak of ~1030nm. This can be mitigated by using a fiber Bragg grating to some extent. The relative gain of 1030nm can be calculated using the simple model proposed by Nilsson et al, which is dependent on the pump absorption and clad/core area ratio [11]. The first key factor for high efficiency for a given launched pump power is low inversion required for reaching the lasing threshold. A second key factor is a small cladding-to-core ratio. This effectively lowers signal intensity in a relatively larger core if the cladding is kept the same, allowing a lower pump intensity to maintain a given inversion. Addressing these two factors effectively can further minimize pump power exiting the fiber.
For tandem pumping, high efficiency is clearly critical due to the very high power involved at multiple kW levels. If high efficiency is not critical, ytterbium-doped fiber lasers can be operated from 976nm-1120nm as demonstrated by Royon et al [12]. Recently, a number of high-power 1018nm ytterbium-doped fiber lasers have been demonstrated. A 85W ytterbium-doped 1018nm fiber laser with 15µm core and 130µm cladding was reported in [13]. The highest reported power is 309W at 1018nm with an efficiency against the launched pump power of 71% using a double-clad fiber with a 30µm core diameter and 250µm cladding diameter [14].
Phosphosilicate host is known for reaching gain threshold at lower inversion than that required for conventional aluminosilicate host for lasing wavelengths below 1020nm due to its high emission cross section at shorter wavelength [15]. Using an ytterbium-doped phosphosilicate leakage channel fiber with ~50µm core diameter and ~420µm cladding diameter, we have achieved 70% slope efficiency with respect to the launched pump power at 1018nm. The efficiency is similar to what is reported previously for ytterbium-doped fiber laser operating at 1018nm [13,14].
The cladding diameter of ~420µm in our demonstration is, however, much larger than the 250µm and 130µm reported respectively in [13, 14]. The much smaller cladding diameters in [13,14] were critical for the much higher pump brightness required to achieve the reported 71% slope efficiency. The much larger cladding diameter in our work demonstrates that much lower pump brightness is required in our fiber. Tandem pumping is only required currently for 10kW single-mode fiber lasers to lower the quantum defect of the output amplifier. The 1018nm fiber laser pumps need to have high enough power to be useful for this application. Currently commercially available pump power in 200µm-core 0.22NA fiber is 500W, while it is 6kW (Laserline) for 400µm-core 0.22NA fiber. This work therefor provides the technical basis for >3kW 1018nm fiber lasers, which will be a key for scaling single-mode fiber lasers to beyond 10kW.
Besides, our 1018nm laser has an ASE suppression of 41dB, higher than the ~40dB reported in [13] and ~25dB reported [14]. The laser intensity is over ~60dB relative to the ASE peak. The superior performance reported in this work clearly demonstrates, for the first time, the benefits of phosphosilicate host for the short-wavelength operation of the ytterbium-doped fiber lasers.
Furthermore, a number of ytterbium-doped fiber lasers operating between 1008nm and 1020nm were constructed, optimized and carefully characterized in order to understand the limit of laser efficiency. For comparison, free-running fiber lasers at 1030nm were also constructed. The performance of the free-running fiber lasers was carefully characterized for various ytterbium-doped fiber lengths. This study allows us to gain significant insights to what are limiting the laser efficiency.
We have found the laser efficiency decreases sharply below 1018nm. The cause of the efficiency decrease at shorter lasing wavelengths is primarily attributed to the shorter fiber lengths used in order to achieve the higher inversions required for the shorter lasing wavelengths. A reduction in efficiency at very high inversion levels is also observed. This may be a result of an ytterbium cooperative up-conversion process. This, however, does not affect the efficiency of the fiber lasers in the wavelength range studied. To our knowledge, this is the first time a study of this nature has been conducted to understand the efficiency limits of ytterbium-doped fiber lasers operating below 1020nm.
2. Experiments
The configuration of the fiber laser system shown in the Fig. 1 consists of a section of Yb-doped phosphosilicate LCF and a FBG with high reflectivity. The Yb-doped LCF used in this system has been previously reported in [4]. The fiber core is 52μm at its smallest dimension (flat-to-flat) and 60μm at its largest dimension (corner-to-corner). The doped area is 30μm in diameter and is made of a highly uniform Yb-doped phosphosilicate glass with an index very slightly below that of silica glass by 2 × 10−4. The two-layers of features in the cladding are made from fluorine-doped silica glass with a refractive index of 0.0155 below that of silica. The cladding diameter is ~420μm and is coated with a low-index polymer coating (n = 1.375) to guide the pump light with a NA of 0.46. Pump absorption at 975 nm was measured to be 1.05 dB/m. The output mode profile is flat-top instead of Gaussian due to the slight refractive index depression in the core. This provides an effective mode area of ~1900µm2 at 1050nm. For high efficiency, it is critical to minimize the intra-cavity splice loss. The photosensitive fiber for the fiber Bragg gratings was specially made at Clemson and has a similar core/cladding dimension of 50μm/400μm as the LCF to minimize splicing loss. A series of FBGs with different reflecting wavelengths were written using an interferometer and a frequency-quadrupled YAG laser in our laboratory at Clemson. It is worth mentioning that the fabrication process is kept constant to ensure the reflectivity for various wavelengths is similar. Due to the inherent nature of the multimode photosensitive fiber, one cannot accurately obtain the reflectivity of the FBG. Figure 2 shows the relative transmission spectrum of a 1018nm FBG. Nearly 20dB of relative transmission loss is achieved at peak of the reflectivity. This measurement is strongly dependent on the excitation of modes in this multimode fiber. It indicates >99% reflectivity for a subset of modes. One end of the FBG is angle cleaved to suppress the ASE while the other end is spliced to the Yb-doped LCF. The Yb-doped LCF is coiled at diameter of 80cm and pumped by 976nm laser diode. Emitted laser lights are recorded at both ends of the fiber.
To begin with, a 8m Yb-doped LCF and 1018nm FBG is used. However, the ASE is very strong and ultimately lead to spurious oscillation at 1030nm when the pump power exceeds the threshold even though the FBG was angle cleaved. This is because the signal gain at 1018nm was much lower than that at 1030nm at the threshold in the long fiber length used. The fiber was then gradually cut back by 30-40cm each time so that the net gain at 1018nm can ultimately exceed the net gain near 1030nm. Once the fiber laser is operating stably at the desired wavelength and the efficiency is recorded. The FBG is then replaced with a slightly shorter wavelength FBG and the cut-back process was repeated. Figure 3(a) shows the spectra of the output of all the fiber lasers tested. The laser wavelength ranges from 1008nm to 1020nm. All the spectra are captured at the highest pump power. Over 50dB of difference between signal peak and ASE peak has been achieved for all the wavelengths tested. In particular, as for the 1018nm fiber laser, the laser peak is over 60dB higher than the ASE peak. By integrating the spectrum, the ASE suppression of 1018nm fiber lasers is calculated to be 41dB. Figure 3(b) shows the output power of the corresponding fiber lasers versus the launched pump power. The highest output power achieved is 52W at 1020nm when the pump power launched into the fiber reached 82W. The output power is limited by the available pump power. The efficiency is then calculated as 73% with respect to the launched power in this case. The slope efficiency then start to decrease as lasing wavelength becomes shorter.
Fig. 3 (a) Optical spectra at the laser output, wavelengths ranges from 1008nm to 1020nm. (b) Output powers versus the launched pump powers at various lasing wavelengths.
In this experiment, the fiber length is cut back a small section of length at a time to suppress the ASE until the optimum fiber length is reached for the stable laser operation at the expenses of inadequate pump absorption. As the length of the fiber gets shorter, in order to achieve the same total gain (in our case, round-trip loss is more or less fixed), a higher average inversion over the fiber length is required to reach the gain threshold. At higher inversions, the ratio of gain at the shorter lasing wavelength over the ASE peak increases. With the help of FBG, the threshold eventually reaches first at the shorter lasing wavelength and the fiber laser will then operate stably. We have conducted this cut-back procedure for several lasing wavelengths between 1008nm and 1020nm. The efficiency of the fiber lasers with respect to the launched pump power and the ytterbium-doped fiber lengths used are shown in Fig. 4. It can be seen clearly that the slope efficiency decreases sharply when the operating wavelength is below 1018nm. The decrease of the slope efficiency at shorter lasing wavelengths correlates well with the shorter fiber lengths used at various wavelength. Based on the results in Fig. 4 alone, we cannot be sure if the poorer efficiency at the shorter lasing wavelength is due to inadequate pump absorption as a result of the shorter fiber length used or some up-conversion processes at higher inversions or a combination of both.
Fig. 4 Launched efficiency (black circle) and fiber length (red triangle) as a function of wavelength.
To further understand the mechanisms responsible for the poorer efficiency at shorter lasing wavelengths, we need to characterize slope efficiency with respect to both launched and absorbed pump powers at various inversion levels. It is hard to get an accurate measurement of the unabsorbed pump powers in order to determine the slope efficiency with respect to the absorbed pump powers in the case where a FBG is spliced to the Yb-doped fibers. We therefore decided to do the cut-back measurement in a free-running fiber laser without the FBG. Another ~6m Yb-doped LCF was used for the 1030nm fiber laser measurement. Two facets of the fiber were perpendicularly cleaved to serve as two reflectors at ~4% reflectivity thus the laser would naturally emit at ~1030nm. The experimental configuration is similar to that used in [2]. The fiber was repeatedly cut back 40-50cm every time followed by laser efficiency test at each fiber length.
The slope efficiency with respect both to the launched and absorbed power of the 1030nm fiber laser versus the inverse of fiber length is plotted in Fig. 5. The free-running fiber laser measurements were done twice and both results are shown as the 1st measurement and the 2nd measurement. The inverse of fiber length serves as a good surrogate for the average inversion in case of constant round-trip loss. For comparison, the efficiency with respect to the launched pump power of the LCF-FBG fiber lasers is also presented. It can be clearly seen that the efficiency with respect to the launched pump power overlaps fairly well in all cases including the free-running fiber laser and the LCF-FBG fiber laser. The LCF-FBG fiber laser has a lower round-trip loss due to the much higher reflectivity of the FBG. The free-running fiber laser therefore operates at a much higher threshold gain and therefore a much higher average inversion for the same fiber length. Despite the difference in inversion levels among the two types of fiber lasers, their efficiency with respect to the launched pump power versus inverse fiber length overlaps very well. This is a strong indication that the launched efficiency is primarily limited the poor pump absorption as a result of the shorter fiber length and is not related to the higher inversion. A further evidence is that the slope efficiency versus the absorbed pump power for the free-running laser at fiber length longer than 3m, i.e. 1/L<0.33, is between 92.4% to 93.5%, very close to the quantum limit of 94.7%. The average inversion of the free-running fiber laser with 3m-long fiber is estimated to be ~40% based on the round-trip loss and absorption/emission cross sections. This is higher than the average inversion of any of the fiber lasers with FBGs. The highest of which is ~40% for the LCF-FBG fiber laser with a lasing wavelength of 1008nm.
Another detail worth noting is that the absorbed efficiency of the free-running 1030nm fiber laser starts to decrease when the fiber is cut back to shorter than 3m, i.e. 1/L>0.33. This suggests that other loss mechanism is introduced at higher population inversion. This additional loss mechanism may be related to the cooperative luminescence [16] as the characteristic green fluorescence indeed becomes more visible at shorter fiber length. The difference between the measured absorbed efficiency and the quantum limited efficiency is plotted in Fig. 6 versus the estimated average inversion. A quadratic fit is expected if the cooperative up-conversion process is expected to be responsible for the deviation from the quantum limit. The data at higher inversion can indeed be reasonably fitted with a quadratic curve. At lower inversion, the measured data deviates from a quadratic fit. This poor fit at lower inversion may be due to the fact that other losses such as fiber background loss and measurement errors play a more significant role in this regime.
Fig. 6 Deviation of measured absorbed efficiency from quantum efficiency as a function of average inversion. Solid red line is the quadratic fit for the measured data.
Figure 7 shows the net gain cross-section for the phosphosilicate host at various population inversion levels. This is obtained using the absorption and emission cross sections reported in [15]. The emission peaks of ytterbium-doped phosphosilicate fibers are at shorter wavelengths comparing to that of ytterbium-doped aluminosilicate fibers, due to a narrower Stark split [15]. Compared with aluminosilicate fiber, the phosphosilicate fiber has higher gain between 1000nm and 1020nm for the same inversion [15]. As it can been seen that even with 20% population inversion, the net gain is positive in the 1000-1030nm range.
3. Conclusion
In conclusion, we have demonstrated that ytterbium-doped phosphosilicate host is much superior for efficient lasing operation at wavelengths between 1008 and 1020nm. Highly efficient 1018nm fiber laser with 70% efficiency with respect to the launched pump power is demonstrated in a 50µm-core Yb-doped LCF with 420µm cladding diameter, demonstrating the low brightness pump required for such efficiency. This large cladding diameter will allow the use of much higher power, lower brightness pump diodes, which is essential for tandem pumping of single-mode fiber lasers at higher powers. The demonstrated 1018nm fiber laser has ~41dB ASE suppression, demonstrating the high stability of the laser. We have conducted further study to show that the poorer slope efficiency with respect to the launched pump power at shorter operating wavelengths is dominated by poorer pump absorption due to the shorter fiber length required to achieve stable operation. It is also found that there is an additional loss mechanism at higher population inversions, possibly due to the cooperative up-conversion.
The fiber lasers operate in multimode in this work due to the poor high-order-mode suppression in the LCF used. This is due to the lower refractive index of the ytterbium-doped glass, which modified the operation of the LCF [4]. Single-mode operation is possible with more optimized fibers such as those used in [2]. Since single-mode operation is not necessary for tandem pumping, which is the focus of this work, we did not attempt it in this work. This work, however, demonstrates that efficient high-power single-mode ytterbium-doped fiber lasers at 1018nm is feasibly with single-mode photonic bandgap fibers [2] or more optimized leakage channel fibers [3]. These fiber lasers have a very low quantum defect of 4.1% when pumped at 976nm, providing a path for the mitigation of thermal effects without the complexity of tandem pumping for multi-kilowatt fiber lasers.
Acknowledgment
This material is based upon work supported in part by the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number W911NF-12-1-0332 through a Joint Technology Office MRI program.
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