The need for obtaining short-wavelength operation sources is constantly increasing. These types of light sources are already highly used in an ample range of disciplines and applications such as interferometry, laser microscopy, biomedicine, photolithography, optical data storage, pump sources, laser projection displays, among others. As a general rule, shorter-wavelength sources focus on smaller spots and resolve finer structures in imaging applications.

These sources are often obtained from frequency doubling or tripling lasers emitting around 600–1100 nm. The most common lasers used for this purpose are Nd:Y3Al5O12 (Nd:YAG) (946 nm to obtain 473 nm and 1064 nm to get 355 nm)^[1], Nd:YVO4 (914 nm to obtain 457 nm and 1064 nm to obtain 532 nm)^[2], Nd:GdAl3(BO3)4 (Nd:GAB) (1062 nm to get 531 nm)^[3], and Nd:YAlO3 (930 nm to get 465 nm)^[4]. Nevertheless, these kinds of lasers are usually bulky and hence usually require external cooling systems. Their short absorption lengths translate into low efficiencies. All of these qualities together make them expensive with relatively complicated operation and maintenance.

As an alternative to the bulk solid-state laser systems, the up-conversion fiber lasers doped with rare-earth ions are excellent candidates. They have many properties that make them suitable for generating short wavelengths in the visible (VIS) and the ultraviolet (UV) regions. At moderated powers (up to some watts), they do not usually present thermal issues and have superior conversion efficiencies. Among the rare-earth ions, thulium (Tm3+) as a dopant in ZrF4–BaF2–LaF3–AIF3–NaF (ZBLAN) glass is capable to convert low-energy infrared (IR) photons into high energy photons (in the VIS and UV regions); the reason is that fluorozirconate glasses based on ZBLAN have more metastable levels compared to silica glasses, expanding in this way the number of radiative transitions for most rare-earth dopants.

Since the first report of VIS up-conversion lasing in fluoride glass fibers^[5,6], short-wavelength laser operation in Tm3+:ZBLAN has been demonstrated by many groups in the past, usually obtaining 480 nm pumped by IR laser sources (generally at 1064 nm with Nd:YAG lasers)^[7–10]. Also, wavelengths in the UV region (284 nm, 293 nm, 351 nm, and 360 nm) have been achieved by pumping with solid-state lasers such as Nd:YAG at 1064 nm^[11], with an Ar+ laser at 485 nm and a dye laser at 585 nm^[12]. However, these systems often require absorption of three photons, which often have a tradeoff between ground-state absorption (GSA) and excited-state absorption (ESA) in addition to having small absorption [less than 5 dB/(km·ppm) (ppm, parts per million) at 1064 nm and 10 dB/(km·ppm) for 485 nm]. They also present a factor that may reduce the efficiency of their systems; when being pumped with IR sources, it may produce a photodarkening effect^[13,14].

An alternative arrangement for obtaining short-wavelength emission in Tm3:ZBLAN is pumping with a dual-wavelength configuration^[15,16]. Since the strongest absorption band of thulium-doped fluorozirconate is centered at around 690 nm (H36→F33 transition), we proposed to use this wavelength as GSA^[17]. Using this wavelength, we can expect an optimal absorption [≥35dB/(km·ppm)]^[18]; this represents a factor of 3.5 to 7 better than that of IR and some other VIS (485 nm or 585 nm) pump sources. Additionally, using ∼645 nm (F34→D12) as ESA, it is then possible to obtain a variety of radiative transitions in the UV (360nmfromD12→H36) and in the blue-VIS region (450nmfromD12→H34 and 475nmfromG14→H36). An additional advantage is that these kinds of pump sources are already available from AlGaInP laser diodes (LDs), which are reasonably priced, and have an extraordinarily large life span, in addition to being highly efficient and compact. Moreover, it has been amply demonstrated that excitation with these wavelengths (red-VIS region) mitigates the undesirable photodarkening effect^[19–21], usually produced by IR pumping.

A tunable AlGaInP-based LD operating at 687 nm was used as the pump source to excite Tm3+ ions from the H36 ground state to the F33 excited state. A second tunable LD operating at 645 nm was used to excite the F34 energy level (ESA) to the D12 upper level, and these wavelengths, 687 nm and 645 nm, were selected due to their optimal absorption. Radiative transitions from here take place while covering some UV and VIS bands (∼360 nm from D12→H36 transition, 450 nm from D12→H34, and 475 nm from G14→H36). The partial energy-level diagram of Tm3+ when doped in fluoride glass is illustrated in Fig. 1.

The experimental configuration (see Fig. 2) consisted of a quasi-single-mode Tm3+-doped fluorozirconate ZBLAN optical fiber doped with a concentration of 4500 ppm (7.1063×1025ions/m3); its core diameter of 2.5±0.3μm and NA of 0.26±0.02 resulted in a cutoff wavelength for mono-mode operation at 849 nm. The pumps, 680–695 nm (GSA) and 640–650 nm (ESA), were provided by temperature-tunable LDs. These were combined into a single beam using a folding mirror (99% reflectance). The resulting beam was coupled into the fiber by a 10× microscope objective (MO) lens. The calculated coupled power was estimated to be 85% for the ESA pump (centered at 645 nm), i.e., 76 mW; it was 30% for the GSA pump (centered at 687 nm), i.e., 15 mW. As described in the next section, lateral fluorescence at some points along the fiber (see fiber pigtail 1 in Fig. 2), as well as amplified spontaneous emission (ASE) at the output (fiber pigtail 2), was analyzed using an Ocean Optics^® USB-SD2000 dual-fiber optical spectrum analyzer (OSA), with a wavelength range of 200–850 nm and 1 nm resolution for different fiber lengths. For the ASE spectral acquisition, a blue passing filter was used to reject the near-IR (NIR) residual pump when acquiring spectra in the UV-VIS region.

We started analyzing the locally generated lateral fluorescence at some points along the fiber in a dual-pump configuration. This allowed us to study the fluorescence evolution along the fiber and, to some extent, determine ideal fiber lengths for optimum amplification of the generated signals.

Figure 3 shows the corresponding spectra at different points along the fiber (13, 26, 35, and 46 cm from the pumping fiber end); the total fiber length was 50 cm.

Since the distance between the test fiber and the OSA’s collecting-fiber end is difficult to maintain constant at all analyzing points, the fluorescence peak at 650 nm is taken as a reference to normalize emitted signals (including the locally scattered ones from the pumps); in this way, the experiment was self-calibrated, and, as a consequence, the relative intensities obtained do not depend on this distance.

We observed that at closer points (13 cm and 26 cm) the signal’s relative intensity in the UV-VIS regions (350 nm from I16→H34, 360 nm from D12→H36, 450 nm from D12→H34, and 475 nm from G14→H36) dominates over the highly competitive NIR at 806 nm (observed at the red 26 cm and the black 13 cm lines). In particular, the 26 cm point presents the lower scattered pump signals (GSA and ESA) and the lowest NIR-generated signal. Hence, a fiber of this length produces the best UV-VIS emission signals over the NIR ones. Although the 13 cm point might also be convenient for UV signals, it is more seriously attenuated by the ZBLAN glass (approximately 1–2 dB/m)^[22]. This result is reaffirmed by the fact that, although more distant points (blue 35 cm and green 46 cm) maintain similar signal levels, the NIR signal (806 nm from F34→H36) is more highly generated by these lengths. This allows us to affirm that the shorter-wavelength signals, 360 nm from the ground state to D12 and 450 nm from the ground state to G14, produced closer to the pumping end have been reabsorbed, finally populate F34, and hence intensify the NIR signals. This reabsorption derives from the overlap of different spectra (see Fig. 1): GSA (H36) and ESA produced on the three metastable levels above the ground state (H34∼6ms,H35∼5ms, and F34∼1.4ms)^[23]; what is also important here is the fact that longer fibers tend to attenuate the blue-shifted signals and amplify the red-shifted ones produced by a given transition, as in three-level pumping schemes.

Regarding the non-generated but locally scattered pump signals, it is important to mention that at 13 cm they are still strong, revealing that such fiber lengths would still present a high residual pump power and hence are not efficient enough. Farther away from this point, the pump signals are remarkably well absorbed. At this stage of our experiments, we may affirm that under these conditions shorter-wavelength transitions dominate for shorter fibers and do not have to strongly compete with the emitting transition of Tm3+ at NIR (806 nm). At the same time, longer fibers reabsorb the generated UV-VIS signals to enhance the NIR transition.

Let us now analyze and compare the single and double-pump configurations at the location that presented the best UV-VIS generation, the point at 26 cm; Fig. 4 displays the corresponding spectra. For single GSA pump (687 nm), no UV-VIS emission occurs [see Fig. 4(a)], and the NIR signals are remarkably intense [Fig. 4(b)]. This scheme was investigated in Ref. [17], where IR lasing at 806 nm was obtained; hence, this pump seems an appropriate option within the context of our experiments. For the single ESA pump (645 nm), all the signals are produced, although they are kind of weak, meaning that 645 nm signal is not quite resonant with the ground state, and thus the ESA pump absorption is not optimum. Nevertheless, by observing the vertical scales, one may observe that the UV-VIS emissions dominate over NIR, giving this scheme chances to be used to pursue UV-VIS light amplification. On the other hand, this result becomes highly improved by exciting with both pumps. Only 15 mW of the GSA pump is added to the 76 mW ESA pump to produce such results, in which all the signals are (at least) threefold intensified, whereas the relative intensity is almost maintained. Apart from making the system more efficient to generate the UV-VIS signals, both pumps are co-dependent, as they increase the absorption of each other; this fact will be clear when analyzing the ASE below.

Before discussing the following results, it is important to mention the differences between the lateral fluorescence and the ASE. Lateral fluorescence is a sample of the local fluorescence (emitted in all directions) excited by pump and ASE signals arriving from the preceding points; this fluorescence also contains Rayleigh-scattered portions of the exciting signals. On the other side, ASE refers exclusively to the coupled fluorescence signals that are able to sum up and reach the end of the fiber (i.e., low attenuated or amplified) plus the residual pump. The information given for both is complimentary, as the lateral gives approximate qualitative information of where to cut the fiber, whereas ASE gives a more realistic scenario on what components are being amplified and also the degree of pump absorption; for systems with a high degree of modeling complexity, like the one investigated here, such empirical estimations of optimal length are highly valuable for deciding on the cavity length of a fiber laser or amplifier length.

In Fig. 5, the ASE spectra for 17 cm doped fiber are presented. This length was chosen because it is between 13 and 26 cm, which presented the best UV-VIS lateral fluorescence. The use of a blue-pass filter was necessary to block residual pump when recording the UV-blue signals [Fig. 5(a)]. Coupling-pump and signal-collecting conditions were fixed, only blocking or passing the pump signals. Compared with points at 13 and 26 cm of Fig. 3(a), we may observe that, although locally produced, the coupled 350 nm signal (I16→H34) is not amplified; the same affirmation might valid for 475 nm (G14→H36) and 510 nm (D12→H35); the NIR signal is also negligible (compared to residual pump).

The main fact for this optimum fiber length is with single ESA pump and with double pump, the level D12 becomes highly populated, and, as a consequence, the UV 360 nm and blue 450 nm signals are amplified. The latter is with a tremendously superior efficiency, as the pumps contribute to enhancing their mutual absorption, closing, in this way, a virtual cycle that benefits the population of the D12 metastable level. The reciprocally enhanced pump absorption mechanism, the light-controlling-light, was previously reported by one of the authors in Ref. [24], in which UV-VIS signals become strongly benefited and are one of our main interests, as shorter fibers are required for amplification.

For the longer fiber, 50 cm, in Fig. 6(a) [consider the vertical axis values as independent from Fig. 5(a)], the single ESA pump resulted in the best option for UV-blue generation with negligible NIR competition and still high residual pump power. An opposite behavior, good NIR generation and negligible UV blue signal, was the result of the single GSA pump (687 nm); we observe that the NIR band appears to be centered more than 15 nm above the one of lateral fluorescence; this shifting reveals that the blue-shifted components are reabsorbed as they travel along the fiber, generating longer wavelengths, and the longer the fiber the longer the shifting (three-level pumping scheme). With both pumps, the difference is minimal, and the NIR signal remains the same, compared to the ESA-pump configuration, whereas UV blue signal lowers a little bit. However, with this fiber length, the light-controlling-light behavior becomes very clear; when 645 nm signal is passing through the fiber [intensity of 2750 in Fig. 6(b)] and when just 15 mW of the 687 nm signal is co-propagated, the transmitted 645 nm signal drops down to less than 1000 (36% transmitted). The coupling pump conditions might have (slightly) varied and surely the collecting-signal conditions as well. In this sense, an implementation of a desirable cutback technique giving the emission strength per unit of fiber length versus wavelength was not possible. This represents a challenge due to the fragility of the ZBLAN fibers as well as the fact that they are very short.

In conclusion, the double-line pumping comprising GSA (687 nm) together with the ESA (645 nm) in Tm3+:ZBLAN fiber systems generating UV-VIS signals at 350 nm, 360 nm, and 450 nm has been investigated and compared to single-line pumping. The analysis of lateral emission along the fiber together with ASE at its end gives complementary information that allows us to estimate the amplification region within the doped fibers and, hence, the optimal length of an amplifier or laser. Apart from considering the advantages and disadvantages of the different configurations, the most relevant results rely on the great co-dependency between the pumps, as they enhance the absorption of each other, clearly resulting in a valid proposal for pumping amplifiers or lasers. To obtain strong UV-VIS emission signals, short fibers (∼20 cm) are required; whereas under the same pumping scheme, longer fibers (>40cm) reabsorb such signals and favor the IR amplification bands. This work represents a proposal for producing more efficient short-wavelength fiber lasers.