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Depressed cladding waveguides have been realized in Nd:YVO4 employing direct writing technique with a femtosecond-laser beam. It was shown that the output performances of such laser devices are improved by the reduction of the quantum defect between the pump wavelength and the laser wavelength. Thus, under the classical pump at 808 nm (i.e. into the 4F5/2 level), a 100-μm diameter circular waveguide inscribed in a 0.7-at.% Nd:YVO4 outputted 1.06-μm laser pulses with 3.0-mJ energy, at 0.30 optical efficiency and slope efficiency of 0.32. The pump at 880 nm (i.e. directly into the 4F3/2 emitting level) increased the pulse energy at 3.8 mJ and improved both optical efficiency and slope efficiency at 0.36 and 0.39, respectively. The same waveguide yielded continuous-wave 1.5-W output power at 1.06 μm under the pump at 880 nm. Laser emission at 1.34 μm was also improved using the pump into the 4F3/2 emitting level of Nd:YVO4.
© 2014 Optical Society of America
1. Introduction
The inscribing process of various structures in amorphous or crystalline materials is nowadays recognized as a suitable and powerful tool for fabrication of various miniature components for integrated optical devices. Using this technique, localized (at micrometer scale) changes of the refractive index are induced by a femtosecond (fs)-laser beam [1, 2]. Of great interest for optoelectronics are the waveguide lasers. In general, such a device possesses a low threshold of emission due to a small size of the pump beam, whereas the high overlap between the pump beam and the laser beam along the entire medium length leads to good output performance.
Waveguides were inscribed by fs-laser writing in many glasses, laser crystals or nonlinear media [3]. Among them the depressed cladding waveguides, which were proposed and for the first time realized by Okhrimchuk et al [4], consist of a region of unmodified medium that is surrounded by a large number of inscribed tracks with lower refractive index. Contrary to the ‘double-line’ waveguides, which are limited at 10 μm to 20 μm separation between tracks and that allow propagation of linearly-polarized beams [5–9], a depressed cladding waveguide can be realized with a large cross section. Furthermore, the waveguide core can be shaped to allow the pump with array or fiber-coupled diode lasers. Some examples are the circular, rectangular, trapezoidal or hexagonal waveguides written in Nd:YAG [4, 10, 11], circular in the mid-infrared emitting Tm [12, 13] and polycrystalline Cr:ZnS [14, 15] lasers, or rhombic in Pr:YLiF4 with emission into the visible spectrum [16]. Recently, we have used the pump with a fiber-coupled diode laser to achieve efficient laser emission from circular depressed cladding waveguides fabricated in Nd:YAG by the fs-laser writing method [17–19].
Nd-vanadate crystals have suitable spectroscopic characteristics (like high absorption and emission cross sections) and good thermal properties that recommend these media for efficient, miniature lasers [20]. Until now, circular depressed cladding waveguides were inscribed in Nd:YVO4 [21] and circular depressed double-cladding waveguides have been realized in Nd:GdVO4 [22]. Furthermore, efficient 1.06-μm laser emission with slope efficiency (versus the absorbed pump power) higher that 50% and output power of few-hundreds of mW was achieved from these waveguides using the pump at 808 nm with tunable, linearly-polarized Ti:sapphire lasers [21, 22].
In this work we report on realization of depressed cladding waveguides in Nd:YVO4 by the direct fs-laser writing technique and obtain efficient laser emission at 1.06 μm and 1.34 μm under the pump with a fiber-coupled diode laser. Thus, using classical pump at 808 nm (i.e. into the highly absorbing 4F5/2 emitting level), a 100-μm in diameter cladding waveguide that was inscribed in a 4.8-mm long, 0.7-at.% Nd:YVO4 delivered laser pulses with 3.0-mJ energy (Ep); with respect to the absorbed pump pulse energy, the optical-to-optical efficiency (ηoa) and the slope efficiency (ηsa) were 0.30 and 0.32, respectively. The waveguide outputted 0.9-W continuous-wave (cw) power at 1.06 μm with efficiency ηoa = 0.14 and slope ηsa = 0.19.
In order to increase the waveguide performances we have used the pump at 880 nm, directly into the 4F3/2 emitting level [23–27]. Thus, by decreasing the quantum defect between the pump wavelength and the laser wavelength and by making use of the Nd:YVO4 high absorption efficiency at 880 nm, which is about 70% of that corresponding to the 808-nm absorption [20, 25, 26], systematic improvements of the laser emission characteristics have been obtained. For the pump at 880 nm the same waveguide outputted laser pulses with Ep = 3.8 mJ at increased optical efficiency ηoa = 0.36 and slope ηsa = 0.39; furthermore, the cw output power increased at 1.5 W (with ηoa = 0.27 and ηsa = 0.28). Improvements of the laser emission performances at 1.34 μm were also obtained for the pump at 880 nm in comparison with the 808-nm pump. The measurements of the Nd:YVO4 temperature prove that less heat is generated in the laser crystal for the pump directly into the 4F3/2 emitting level. This is the first time when the pump at 880 nm is applied to fs-laser inscribed waveguides and could be a good approach for fabricating efficient integrated waveguide lasers pumped by diode lasers.
2. Waveguides fabrication and characterization: experimental conditions
For inscribing tracks in Nd:YVO4 we used the same facility, as the one in our previous reports [17–19]. Thus, the laser beam at 775 nm with 200-fs duration, 2-kHz repetition rate and energy up to 0.6 mJ was delivered by a Clark CPA-2101 chirped-pulsed amplified system. A combination of half-wave plate, a polarizer and a neutral filter was used to vary the fs-laser beam energy. Focusing into the laser medium was made with a 20× microscope objective of numerical aperture NA = 0.30; the beam diameter at the waist location (in air) was ~5.0 μm.
The laser crystals were three a-cut Nd:YVO4 media with 0.5-at.%, 0.7-at.% and 1.0-at.% Nd doping; the thickness t (Oz axis) and width w (Ox axis) were 3.0 mm and 6.0 mm, respectively. Each crystal was positioned on a XYZ motorized stage that was translated along axis Oy (corresponding to the crystal length ℓ) at a speed of 50 μm/s. Circular cladding waveguides of 100-μm diameter were obtained by inscribing parallel tracks (that were positioned ~5 μm apart each other) around circular contours. A square waveguide (80 μm × 80 μm) was also realized in the 0.5-at.% Nd:YVO4 crystal, to prove the writing method versatility. The waveguides were centered 500-μm below each w × ℓ medium surface. Through successive attempts (and by monitoring the writing process with a video camera), tracks were obtained by keeping the fs-laser pulse energy slightly below 0.3 μJ. The lateral sides of each Nd:YVO4 crystal were polished after the writing process. Thus, the final length of the 0.5-at.%, 0.7 at.% and 1.0-at.% Nd:YVO4 crystals was 7.2 mm, 4.8 mm and 3.6 mm, respectively.
The depressed circular waveguides will be denoted by CWG-1 (0.5-at.% Nd:YVO4), CWG-2 (0.7-at.% Nd:YVO4) and CWG-3 (1.0-at.% Nd:YVO4), whereas SWG (0.7-at.% Nd:YVO4) will stand for the square-shaped waveguide. Microscope photos of waveguides CWG-1 and SWG are shown in Fig. 1(a) and Fig. 1(b), respectively. It can be observed that the inscribed tracks are clear with no visible cracks. The propagation losses were evaluated by coupling a 632.8-nm HeNe laser into each waveguide and by measuring the power of the transmitted light. After extracting the coupling efficiency (that was evaluated to unity) and the Fresnel losses, we concluded that propagation losses for the HeNe beam polarized parallel to the inscribed tracks (axis Oz in Fig. 1(a)) were 2.4 dB/cm for CWG-1, in the 1.5 to 1.7 dB/cm range for CWG-2 and CWG-3, whereas the square SWG waveguide has a little higher losses (3.4 dB/cm). An increase of the losses, between 5.5 to 6.0 dB/cm for the circular waveguides and nearly 6.3 dB/cm for the square-shape waveguide, was observed when the HeNe beam was polarized perpendicular to the inscribed tracks; this could be attributed to some leakage of the light through the crystal left unmodified between the tracks [18].
Fig. 1 Microscope photos of the depressed cladding waveguides inscribed in the 0.5-at.% Nd:YVO4 crystal are shown: (a) CWG-1, circular with diameter of 100 μm and (b) SWG, 80 μm × 80 μm square; the white dashed lines indicate the waveguides’ boundary. Fluorescence images of the waveguides are presented: (c) CWG-1, (d) CWG-2, inscribed in the 0.7-at.% Nd:YVO4, (e) CWG-3, written in the 1.0-at.% Nd:YVO4, and (f) the waveguide SWG.
The optical pump for the laser emission experiments was made with fiber-coupled diode lasers (LIMO Co., Germany) at 808 nm and at 880 nm (with no polarization control of the pump beam). Each fiber end (both with diameter of 100 μm and NA = 0.22) was imaged into a waveguide through a collimating lens of 50-mm focal length and a 30-mm focal length focusing lens. The diodes were operated in quasi-cw regime (1.0-ms pump pulse duration and up to 100-Hz repetition rate), as well as in cw mode. The resonator was linear and consisted of two plane mirrors that were positioned very close to each Nd:YVO4 crystal sides. The rear mirror (the one facing the pump line) was coated high reflectivity HR (reflectivity, R> 0.998) at the laser emission wavelength (λem) of 1.06 μm or 1.34 μm and with high transmission, HT (transmission, T> 0.98) at the pump wavelengths (λp) of 808 nm and 880 nm. For the emission at 1.06 μm several out-coupling mirrors (OCM) with T between 0.01 and 0.10 were used. On the other hand, in the case of lasing at 1.34 μm the OCM had a specified T (between 0.02 and 0.07) at this wavelength, and it was also coated HT (T> 0.95) at 1.06 μm in order to suppress emission at this high-gain line. In addition, a spectrometer was used to check the absence of the 1.06-μm line during emission at 1.34 μm. Fluorescence images of the waveguides (which were recorded with a 190-1100 nm spectral range Spiricon camera, model SP620U) are given in Fig. 1(c) for waveguide CWG-1, in Fig. 1(d) for waveguide CWG-2, in Fig. 1(e) for waveguide CWG-3 and in Fig. 1(f) for waveguide SWG. Good confining of the laser beam in the waveguides can be observed.
3. Laser emission results and discussion
Figure 2 presents the laser emission performances at 1.06 μm obtained from the CWG-2 waveguide under quasi-cw pumping (100-Hz repetition rate). We mention that the pump beam absorption efficiency was determined by measuring the incident and the transmitted energy of the pump pulse after each waveguide and extracting the Fresnel losses at the incident surface of a Nd:YVO4 laser crystal. Besides, these measurements were performed in nonlasing condition; therefore, while the diode current was varied a neutral filter was placed between the coupling lenses in order to keep the pump beam intensity low, such as to avoid the saturation effects of the absorption [28]. The filter was removed when lasing was investigated. Furthermore, in order to compare the laser performances at similar absorption, in all experiments the maximum energy of a pump pulse was limited to 11.5 mJ for the pump at 808 nm and to 17.0 mJ for the 808-nm pumping. Pulses with maximum energy Ep = 3.0 mJ were measured under the pump at 808 nm (OCM with T = 0.05); the optical-to-optical efficiency and the slope efficiency with respect to the absorbed energy of the pump pulse were ηoa = 0.30 and ηsa = 0.32, respectively. The change of λp to 880 nm improved the laser pulse characteristics: Ep increased to 3.8 mJ (with ηoa = 0.36), whereas ηsa amounted to 0.39. Insets of Fig. 2 show the laser beam near-field distributions at the maximum Ep. A measurement of M2 factor (which was done by the 10%-90% knife-edge method) concluded that the laser beam had M2 = 9.8 for the pump at 808 nm and a higher value, M2~15.0 for the pump at 880 nm. This behavior is different from that observed in our previous works [25–27], where a change of λp from 808 nm to 880 nm improved the laser beam quality. However, one should consider that in the present experiments the pump is made in a waveguide structure and not in the bulk material. Indeed, when the pump was performed in bulk at 808 nm, the 0.7-at.% Nd:YVO4 crystal yielded pulses at 1.06 μm with energy Ep = 5.5 mJ (at ηoa = 0.62) and slope ηsa = 0.64. The laser beam M2 factor was 4.6. The change of λp to 880 nm increased Ep to 6.4 mJ (ηoa = 0.72); the slope rose to ηsa = 0.74 and the laser beam M2 improved to 4.2.
Fig. 2 Laser pulse energy at 1.06 μm obtained from waveguide CWG-2 (0.7-at.% Nd:YVO4) under the pump at 808 nm and at 880 nm. Insets are the laser beam near-field distributions (2D maps) at the indicated points. T is the OCM transmission at 1.06 μm.
Performances of laser emission at 1.34 μm yielded by waveguide CWG-1 (0.5-at.% Nd:YVO4) are shown in Fig. 3. Under the classical pump at 808 nm this waveguide yielded pulses with energy Ep = 1.5 mJ at optical efficiency ηoa = 0.14 and slope ηsa = 0.19. The laser beam M2 factor was ~5.9. For the pump directly into the emitting level the pulse energy increased to Ep = 1.8 mJ (with ηoa = 0.18) and the slope improved to ηsa = 0.23; the laser beam quality was characterized by M2~9.2.
Fig. 3 Quasi-cw mode operation at 1.34 μm of the CWG-1 waveguide (0.5-at.% Nd:YVO4) under the pump at 808 nm and at 880 nm. T is the OCM transmission at 1.34 μm.
The emission performances recorded under quasi-cw pumping are summarized in Table 1. It can be observed that for lasing at 1.06 μm the depressed circular waveguides outputted pulses with quite similar characteristics. Thus, under the pump at 808 nm the pulse energy Ep was in the range of 2.8 mJ (CWG-1) to 3.3 mJ (CWG-3) at optical efficiency ηo of 0.26 (CWG-1) to 0.32 (CWG-3). A systematic increase of Ep and improvements of ηo and of the slope efficiency ηsa were obtained by changing λp to 880 nm. It was observed that lower performances were obtained from the square SWG waveguide; because the coupling efficiency of the pump beam in all waveguides was evaluated to unity, this behavior was attributed to a smaller overlap between the pump beam and the laser beam in SWG in comparison with the circular waveguides. Also, it can be seen that the pump at 880 nm directly into the 4F3/2 emitting level improved the emission parameters at 1.34 μm, in comparison with classical pump at 808 nm.
Table 1. Characteristics of laser emission at 1.06 μm (OCM with T = 0.05) and at 1.34 μm (OCM with T = 0.04) obtained under the pump at 808 nm and at 880 nm, quasi-cw mode operation
It is known that quasi-cw pumping reduces significantly the laser crystal thermal load and thus allows lasing with good performances. In the next experiments the waveguides emission characteristics were investigated in cw-pumping regime. For the pump at 880 nm the CWG-2 waveguide yielded 1.5-W output power (Pout) with efficiency ηoa = 0.27 (absorbed pump power, Pabs = 5.5 W); the slope efficiency was ηsa = 0.28 (as shown in Fig. 4). On the other hand, this waveguide delivered Pout = 0.9 W at 1.06 μm for Pabs = 5.2 W at 808 nm (ηoa~0.17); signs of power saturation were evident for Pabs in excess of 5.5 W at 808 nm, most probably due to stronger thermal effects induced in the waveguide at this wavelength λp. Table 2 summarizes the best results measured in cw mode operation at 1.06 μm from all the waveguides. Watt-level emission at 1.06 μm was available also from CWG-1 (Pout = 1.44 W) and CWG-3 (Pout = 1.21 W) under the pump at 880 nm. During these experiments the pump power at 808 nm and at 880 nm was limited to ~5.8 W and 8.0 W, respectively.
Fig. 4 Cw operation at 1.06 μm recorded from the CWG-2 waveguide, OCM with T = 0.05. The near-field distribution (2D plot) at the maximum output power is shown for the pump at 880 nm.
Table 2. Performances of cw laser emission at 1.06 μm, OCM with T = 0.05
We mention that cw laser emission at 1.34 μm was obtained from all the circular waveguides, but of low level. For example, CWG-1 delivered Pout = 0.2 W for Pabs = 4.3 W at 880 nm; under similar Pabs at 808 nm the power Pout was limited to 0.15 W and showed time fluctuations. Increased thermal effects for the 1.34-μm emission in comparison with lasing at 1.06 μm are believed to be responsible for these results, like in the case of Nd:YAG [29, 30].
It is known that in the case of laser emission at 1.06 μm a change of λp from 808 nm to 880 nm increases the quantum defect ratio between the pump wavelength and the laser wavelength (ηqd = λp/λem) by ~8.8% (i.e. from ηqd~0.76 for λp = 808 nm to ηqd~0.827 for λp = 880 nm). In conditions of efficient emission this could induce a decrease of the generated heat from 0.24 (for λp = 808 nm) to ~0.173 (for λp = 880 nm), i.e. by ~28% [25, 26]. A proof of the heat generation reduction is the laser medium temperature under the two wavelengths of pumping. We comment that during the previous lasing experiments each laser crystal was wrapped in indium foil and placed in contact with a copper block. For the next measurements the Nd:YVO4 upper cover (the indium foil and the copper) was removed and the crystal surface temperature was measured with a FLIR T620 thermal camera (−40°C to + 150°C range, ± 2°C accuracy). From these data the temperature of each Nd:YVO4 crystal surface positioned right above the waveguide was read. Although this is not the exact temperature in the waveguide (because the waveguide was positioned 500-μm below the crystal surface, and because of modified cooling conditions), the data suggest general behavior of the heat generated in the crystal under pumping at 808 nm and 880 nm, in lasing as well in the nonlasing conditions.
Figure 5 shows the maximum temperature of the 0.7-at.% Nd:YVO4 upper surface for Pabs = 5.0 W at 808 nm (Fig. 5(a)) and at 880 nm (Fig. 5(b)). For the pump at 808 nm the temperature rose to ~128°C in nonlasing regime (this peak was obtained ~0.5 mm inside the waveguide, corresponding to the optimum focusing position of the pump beam). Once the lasing was allowed, the maximum temperature (Tmax) decreased to ~108°C (Fig. 5(a)); the laser output power was ~0.9 W (Fig. 4). On the other hand, under the pump at 880 nm and nonlasing Tmax was ~100°C; under lasing (with output power of ~1.3 W, Fig. 4) Tmax was reduced to ~90°C (Fig. 5(b)). It should be also noted that the temperature distributions are different, showing better uniformity under the pump at 880 nm; this is a consequence of a lower absorption coefficient at this pump wavelength in comparison with that at 808 nm. Similar behavior was observed for the other cladding waveguides. Evaluation of the temperatures corresponding to the exact experimental conditions can be performed from these data, a subject that will be considered in future. Other investigations could consider realizing of waveguides with decreased propagation losses, by improving the present writing techniques or by using the helical movement method [18]. It is also worthwhile to mention that the absorption efficiency was nearly 0.94 for the pump at 808 nm (for the waveguide CWG-1 that was inscribed in the 7.2-mm long, 0.5-at.% Nd:YVO4 crystal) and about 0.65 for the pump at 880 nm of the same waveguide; the design of waveguides with higher absorption at 880 nm will be also considered in further works.
Fig. 5 Maximum temperature of the 0.7-at.% Nd:YVO4 crystal upper surface that was measured along the waveguide CWG-2 for an absorbed pump power of 5 W at (a) 808 nm and (b) 880 nm, nonlasing and lasing at 1.06 μm. Insets are the temperature maps of the laser crystal surface. The white dashed lines show the waveguide position.
4. Conclusions
In summary, we report on realization of depressed cladding waveguides in Nd:YVO4 by the direct writing technique with a fs-laser beam and have obtained laser emission from these waveguides under the pump with fiber-coupled diode lasers. Employing the classical pump at 808 nm (i.e. into the highly-absorbing 4F5/2 level), laser pulses at 1.06 μm with 3.0-mJ energy at optical efficiency of 0.30 and 0.32 slope efficiency have been measured from a circular waveguide of 100-μm diameter that was inscribed in a 0.7-at.% Nd:YVO4 crystal. It has been proved that the pump directly into the 4F3/2 emitting level is an effective method for improving the emission performances of such a laser device. Thus under the pump at 880 nm the same waveguide yielded laser pulses with increased energy of 3.8 mJ, at higher optical efficiency and slope efficiency of 0.36 and 0.39, respectively. Cw output power of 1.5 W at 1.06 μm was outputted by this waveguide for the pump at 880 nm, in comparison with the 0.9-W output power that was achieved for the 808-nm pump. A similar waveguide inscribed in a 0.5-at.% Nd:YVO4 crystal yielded laser pulses at 1.34 μm with 1.5-mJ energy (at 0.14 optical efficiency) and slope efficiency of 0.19, whereas the pump at 880 nm improved the pulse energy at 1.8 mJ (with optical efficiency of 0.18) and increased the slope to 0.23. This is the first report on diode-pumped laser emission in depressed cladding waveguides that were realized in Nd:YVO4 by the fs-laser beam writing. Furthermore, the results of this work suggest that the pump with diode lasers directly into the emitting level could be a good solution for realization of efficient waveguide lasers that are inscribed in Nd-vanadate laser media.
Note: While the manuscript was in the peer-review process, results on fabrication and laser performances of a depressed circular waveguide that was inscribed in Nd:GdVO4 were reported [31]. Cw emission and Q-switch operation by graphene saturable absorber were achieved at 1.06 μm employing the pump at 808 nm with a Ti:sapphire laser.
Acknowledgments
This work was financed by a grant of the Romanian National Authority for Scientific Research, CNCS - UEFISCDI, project number PN-II-ID-PCE-2011-3-0363. The authors would like to thank Mr. F. Voicu for polishing the Nd:YVO4 laser crystals and Dr. T. Dascalu for various discussions during the experiments.
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