Double-Clad Bismuth-Doped Fiber with a Rectangular Inner Cladding for Laser Application

Abstract

In this paper, we report the latest research results on the fabrication of double-clad Bi-doped germanosilicate core fibers with a rectangular inner cladding design for improved laser performance in the near-IR spectral region. Detailed comparative analysis of the absorption characteristics of the Bi-doped fibers with a circular- and rectangular-shaped inner cladding was performed. A series of cladding-pumped, Bi-doped fiber lasers emitting near 1.46 µm was developed using the semiconductor’s multi-mode fiber-coupled laser diodes at $\lambda$ = 808 nm. The peculiarities of the laser parameters of the fabricated active fibers with the double-clad design were thoroughly studied by analyzing the dependencies of the slope efficiency of the lasers, namely the pump power, active fiber length and core-to-inner-cladding area ratio. The obtained results show that the rectangular design provided enhanced cladding absorption and improvements in laser performance. In particular, we achieved maximal slope efficiencies of 5.5% and 4.3% for the absorbed pump power introduced into the inner cladding with cross-section areas of 80 × 80 µm${}^2$ and 125 × 125 µm${}^2$, respectively. Multi-wavelength lasing operation in a free-running cavity due to a few modes’ propagation regimes was found using the Bi-doped fiber with an 80 × 80 µm${}^2$ inner cladding.

1. Introduction

High-power fiber lasers are contemporary optical devices capable of providing output radiation at wide wavelength ranges in the near-IR spectral region, where they have many practical applications, such as material processing, remote sensing, space applications and lithography [1]. Significant progress in output power scaling for fiber lasers has been achieved mainly due to the development of active fibers with a double-clad structure [2]. As a result, it was possible to create relatively low-cost efficient devices with high-level output powers [3]. The structure of such a fiber consists of an active ion-doped core surrounded by a silica inner cladding. An outer polymer cladding with a lower refractive index than that of silica glass provides low-loss propagation of pump light in the inner cladding (i.e., the pump is restricted to the inner cladding), which partly overlaps with the single-mode core, where it can be absorbed by the laser-active ions. In this case, it is not necessary to provide signal-mode sources for pumping, and low-brightness multi-mode laser diodes (LDs) can be used. However, the cladding pumping approach is efficient when the pump light’s overlap with the active core is significant (ideally, complete absorption of the pump light). Therefore, it is important to design the inner cladding in such a way as to avoid the occurrence of transverse modes with poor overlap with the core area [4,5]. For this purpose, fibers with non-cylindrically symmetric shapes of the inner cladding (e.g., “D-shaped”, “rectangular”, “hexagonal” inner cladding or “off-center” core) should be created [6,7,8]. These constructions applied to cladding-pumped lasers based on fibers doped with rare-earth (RE) ions are widely used [9].

Bismuth-doped fibers (BDFs) are characterized by distinctive optical properties, depending significantly on the local structure of the glass host, in contrast to the RE-doped fibers. These fibers allow one to provide amplification and lasing in the near-IR regions, where the “traditional” RE-doped fibers are inefficient. To date, the total spectral region of operation of bismuth-doped fiber lasers (BDFLs) covers the wavelength ranges from 1150 to 1775 nm [10,11,12,13], where the output power of these lasers can reach up to 22 W. However, the above-mentioned output characteristics of BDFLs were achieved with the core pumping configuration, while a cladding-pumped approach came into play only recently. Previously, the cladding-pumped BDFLs were developed using germanosilicate and phosphosilicate glass as the core material [14,15]. The cladding-pumped lasers work by utilizing a classic three-level scheme, with the laser transitions belonging to bismuth active centers associated with silicon (BACs-Si) and phosphorus (BACs-P), depending on the lasing wavelength. Nevertheless, the slope efficiency of such lasers was reported to be very low, being ≈0.5–1% with respect to the absorbed pump power at a wavelength of 808 nm. It should be noted that the developed double-clad Bi-doped fibers, which were used as active media for pioneering cladding-pumped BDFLs, had a cylindrically symmetric structure. This is the reason why they did not allow one to obtain better absorption characteristics.

In this paper, we report the latest results on fabrication and perform measurements of the optical characteristics of the double-clad bismuth-doped GeO${}_2$–SiO${}_2$ glass core fibers with a rectangular-shaped inner silica cladding having cross-section areas of 80 × 80 µm${}^2$ and 125 × 125 µm${}^2$. The main attention is paid to the output characteristics of the BDFLs based on the developed active fibers. The slope efficiency of the BDFLs as a function of the pump power and active fiber length is measured for a variety of parameters for the laser cavities. We compare the obtained results with those related to BDFs with a circular geometry for the inner cladding. The main benefits of the double-clad BDFs with a non-circular shape for the inner cladding are presented.

2. Materials and Methods

To fabricate all the tested fiber samples, a preform manufactured by the modified chemical vapor deposition (MCVD) technique was used. A detailed description of the Bi-doped preform fabrication process and the refractive index profile (RIP) of the fiber preform can be found in [14,16]. A negligible variation in chemical composition, the refractive index profile and other relevant parameters along the preform length allowed us to divide the preform into three similar pieces of a few tens of centimeters each. Using one of the pieces, a single-mode BDF with a circular design for the inner cladding (F1) was fabricated. The other two pieces served as starting samples for the fabrication of BDFs with the non-circular cladding structure. For this purpose, they were jacketed with a silica tube (Heraeus F300) to achieve the required core-to-clad ratio. Then, the conventional technique that enabled shaping of the silica preform surface by faceting and subsequent polishing to obtain a desired structure (in this case, it was a rectangular cladding) was performed. The BDFs with inner cladding cross-section areas of 125 × 125 µm${}^2$ (F2) and 80 × 80 µm${}^2$ (F3) were drawn from the preforms. All the investigated fibers had a polymer coating with a refractive index of 1.396, which was sufficient to provide light guidance in the inner silica cladding (NA ≈ 0.42). It should be noted that in contrast to the F1 and F2 fibers with cut-off wavelengths ${\lambda }_c$ ≈ 1.4–1.45 µm, F3 was a few-mode fiber in a spectral range of 1.3–1.5 µm (${\lambda }_c$ ≈ 1.8 µm); that is, the single-mode propagation regime was traded off for an increased core-to-clad ratio. The images of the fiber ends with respect to the investigated fiber samples obtained by a JSM-5910LV scanning electron microscope (JEOL) with an AZtecENERGY analytical system (shared research facilities GPI RAS) are shown in Figure 1. It can be seen that the BDFs had a structured core consisting of two layers with different Ge contents (quasi-gradient RIP) [16]. This RIP was fabricated deliberately in order to concentrate the fundamental mode in the central part and thereby reduce the optical losses that could occur at the core–clad interface. The fiber core diameter was close to 8 µm for F1 and F2, whereas the fiber core for F3 was almost 11.5 µm.

The small-signal cladding absorption spectra of the tested fibers were measured with the conventional cut-back technique, which is based on a comparison of the intensities of the transmitted signals through long and short pieces of a fiber sample. In this research, the probe signal from a halogen lamp was launched into the inner silica cladding of the fiber under testing through a multi-mode optical fiber with a core diameter of 105 µm. The signal transmitted through an active fiber was detected with an Ocean Optics QE65000 spectrometer with multi-mode optical input. For these measurements, fiber samples with an initial length of longer than 200 m were used. Figure 1a shows the measured cladding absorption spectra of the tested fibers. The obtained spectra of all the investigated fibers were similar. One can see a distinctive band peaking at a wavelength of 820 nm and a tail of a short-wavelength band, which were attributed to BACs-Si. The band peaking at around 750 nm was probably related to the polymer coating, but it certainly did not originate from the BACs. Expectedly, among the studied fibers, the F3 fiber with the largest core-to-clad ratio was characterized by the highest absorption value at 820 nm of ≈60 dB/km, which was almost 2 times greater than that of both F1 (30 dB/km) and F2 (≈36 dB/km). The small difference in the absorption values of the F1 and F2 samples can be explained by the fact that the drawing process of the corresponding preforms led to changes in the inner cladding shape of the fibers, which became closer to a circular design due to a high enough drawing temperature. In the current research, there was no target to keep the inner cladding strictly rectangular, and the main focus was on maintaining the same drawing conditions for all the investigated fibers.

Indeed, most efforts were focused on laser experiments using the drawn fibers. The cladding-pumped BDFL designed for this purpose had a linear configuration with a Fabry–Perot cavity formed by a BDF and a pair of mirrors (M1 and M2) with respect to the experimental set-up, which is schematically shown in Figure 1c. A pair of fiber-coupled multimode laser diodes (LDs) operating at a wavelength of 808 nm was used as a pump source. As one can see in Figure 1, the pump wavelength fell almost perfectly into the maximum of the absorption band of the BACs-Si. The pump light was launched into the inner cladding of the active fiber via two ports of a fiber-optic combiner (2 + 1) × 1 (Pump and Signal Combiner from OptoLink Corp.). The length of the active fiber was varied from 100 to 500 m to secure absorption of the pump power.

To eliminate the unabsorbed pump power propagating through the cladding, we placed a cladding mode stripper after the active fiber. The BDFL cavity was formed with mirrors M1 and M2, which were a ≈100% reflection fiber Bragg grating (FBG) at a wavelength of 1460 nm ($\Delta \lambda$ ≈ 1 nm) and a perpendicularly cleaved bare fiber end providing ≈4% back reflection, respectively. The performance of the BDFL operating in a free-running mode, when only the cleaved fiber ends acted as the cavity mirrors, was also investigated.

3. Results and Discussion

Initially, the output characteristics of the continuous-wave BDFLs based on the 200-m-long active fibers were studied. A typical output emission spectrum of the cladding-pumped BDFLs measured with an HP 70950B spectrometer is depicted in Figure 2a. One can observe both the amplified spontaneous emission bands and the laser and pump lines at 1.46 and 0.8 µm, respectively. The laser line measured with a higher resolution is presented in the inset of Figure 2a. It should be noted that the laser based on F3 was stably operated in the single-mode regime at the corresponding wavelength, even though the fiber was capable of guiding a few transverse modes. This can be explained by the mode selection regime secured by the used FBG written in a single-mode core of a fiber similar to SMF-28. The dependencies of the BDFL output power on the absorbed pump power are depicted in Figure 2b. The obtained curves for the F2 and F3 samples did not follow a linear function. The monotonic decrease in an increment of the output power of the BDFLs took place with the pump power’s increase (partial saturation of the output power). This trend was less prominent in the case of BDFLs based on the F1 sample (see Figure 2b). At relatively low powers, the slope efficiency of the BDFLs can reach up to ≈4.3% for F3 and 2.6% for F2. With the increasing pump power, the slope efficiency became noticeably lower (almost 1% for both the F2 and F3 fibers). The observed behavior can be explained by a longer relaxation time (i.e., a lower relaxation rate) inherent to the BACs-Si non-radiative transitions between the pump level and the metastable laser level. This effect became pronounced when the pumping rate was greater than or comparable to the relaxation rate. The output power saturation effect in the cladding-pumped, Bi-doped fiber lasers was studied in detail by direct experiments and numerical simulation in [17]. This is why the samples with enhanced cladding absorption experienced pump saturation the most; for these fibers, the critical absorbed power could be achieved notably earlier.

It is vital to investigate the performance characteristics of the developed BDFLs regarding the length of the active fiber. This is why the subsequent research was focused on the study of BDFLs based on the F3 sample spliced with the FBG, where noticeable progress was expected. For this purpose, in addition to the initial fiber length of 200 m, fiber pieces with lengths of 100 and 300 m were used in the laser experiments. The obtained results are depicted in Figure 3. As one might expect, the variation in the active fiber length significantly affected the output characteristics of the BDFLs. The saturation effect became noticeable even at low pump powers for the BDFL with the shortest length of active fiber. This was clearer from the analysis of the dependence of the slope efficiencies of the BDFLs on the absorbed pump power (see Figure 3b). These curves were the result of numerical differentiation of the experimental data presented in Figure 3a. As one can see, there was a monotonic decrease in the slope efficiency of the BDFLs with the increase in pump power, and this trend was similar for different fiber lengths despite starting at different initial points. Upon a further increase in pump power, the changes in the slope efficiencies of the BDFLs became much smaller. Nevertheless, we found that the maximum slope efficiency and output power of such lasers with a relatively low threshold of 1–2 W could become up to ≈5% and 270 mW, respectively.

It is well known that BACs-Si can be formed into silica-based glass matrices regardless of any index-raising co-doping elements. For instance, this type of BAC coexists with either BACs-P in a phosphosilicate glass host or BACs-Ge in germanosilicate glass hosts. It is reasonable to assume that the situation with the saturation effect can be partially improved by means of modification of the local environment of BACs-Si, since it can lead to an increase in the relaxation rate. To confirm this suggestion, we estimated the BACs-Si relaxation time between the pumping level and metastable laser level in different glass matrices using the experimental data regarding the build-up dynamics of the characteristic luminescence peaking at 1430 nm, which originated from the laser transition of BACs-Si.

In fact, the relaxation time of BACs-Si caould be reduced by more than 2–3 times with the addition of P or Ge content in the silica glass host. Numerically, it was determined that with shorter relaxation times, the dependencies of the output power on the absorbed pump power became much closer to a linear function, allowing one to obtain higher output powers with respect to the investigation performed in [17].

Further research was aimed at studying the BDFL operation in free-running mode for clearer comparison of the laser performance. Using fiber sample F3 as the active medium, multi-wavelength lasing was investigated. A spectral range of laser oscillation was determined by the BDF length because, as is typically the case, the maximum gain shifted toward longer wavelengths with the increasing fiber length due to the reabsorption effect. This is why the BDFLs with shorter wavelengths had shorter cavities, and vice versa. It should be noted that this lasing regime was unstable, which manifested itself in the change in intensity of the observed laser lines over time. Figure 4a shows the obtained laser lines with a complex shape corresponding to BDFLs with different lengths for the active fiber. It can be seen that the shortest 100-m cavity BDFL operated at two wavelengths of 1440 nm and 1441 nm, while the use of a 200-m segment of the active fiber under the same conditions led to laser generation simultaneously at three wavelengths of 1455, 1457 and 1459 nm. The largest shift in the laser wavelength of up to 1473 nm was observed in the BDFL cavity with a 300-m-long active fiber. However, the certain shapes of the individual laser lines should not be estimated in fine-grain detail, since some of them might have been measurement artifacts. We suggest that the appearance of two laser wavelengths could be explained by sufficient gain in the cavity for two degenerate mode groups, including LP${}_{01}$ and LP${}_{11}$, due to the strong overlap of each group with an active core (Figure 4b). While the presence of a third wavelength might be attributed to the intermodal coupling between LP${}_{01}$ and LP${}_{11}$ [18]. An accurate description and profound analysis of the multiwavelength lasing origin requires further research, as this is beyond the scope of this paper.

The mode composition propagating in the F3 fiber core was determined by numerical calculation using OptiFiber Software while taking into account the refractive index profile of the developed fiber [16]. It was found that only two mode groups could be supported in the wavelength range from 1440 nm to 1470 nm, thus confirming our assumptions. As far as the efficiency of the BDFLs operating in the free-running regime was concerned, their slope efficiency was determined using the experimentally obtained dependencies of the total power at the BDFL output on the absorbed pump power (as depicted in Figure 5). As can be observed in the inset of Figure 5 (where data corresponding to the BDFL operation with up to 500-m-long cavities are also included), the free-running lasers had a slightly lower slope efficiency than that for the cavity formed with FBGs. The slope efficiency grew with the increase of the active fiber length up to 500 m, and its maximum value was less than 4.5%. This was also typical for the BDFLs based on the tested fibers (F1 and F2).

4. Conclusions

To summarize the obtained results, it is stated that the double-clad Bi-doped germanosilicate glass core fibers with a rectangular inner cladding can successfully be used to produce lasing oscillation in the near-IR spectral region. We found that a non-circular inner cladding has noticeable benefits, facilitating higher pump absorption and slope efficiencies. This conclusion was made after the performed thorough study of a series of Bi-doped fiber lasers pumped into the inner silica cladding using semiconductor multi-mode, fiber-coupled laser diodes at a wavelength of 808 nm. The slope efficiency of the developed lasers can vary in the range of 3–5% with respect to the absorbed pump power, which is almost an order of magnitude greater than those of similar Bi-doped fiber lasers with a circular geometry for the inner cladding. It was found that a monotonic decrease in the slope efficiency and the output power saturation effect are main challenges for cladding-pumped lasers based on Bi-doped germanosilicate fibers. Moreover, it was observed that the active fiber with an increased core-to-cladding ratio produced enough gain to yield multiwavelength lasing in a free-running cavity.