Circular cladding waveguides were realized in a 5.0-mm long, 1.1-at.% Nd:YAG ceramic by direct femtosecond-laser writing using a scheme in which the laser medium is moved on a helical trajectory along its axis and parallel to the writing direction. Laser emission was obtained under the pump with a fiber-coupled diode laser. A 100-μm diameter waveguide delivered laser pulses at 1.06 μm with 3.4-mJ energy for the pump with pulses of 13.1-mJ energy, at 0.30 slope efficiency; laser pulses at 1.3 μm with 1.2-mJ energy were obtained from the same device. Comparison with a waveguide of the same dimension that was inscribed by the classical translation method of the laser medium is made. Efficient integrated lasers based on cladding waveguides that are pumped by fiber-coupled diode lasers could be realized by this writing method.
© 2014 Optical Society of America
Since the first realization of waveguides in glasses by direct writing with femtosecond (fs)-laser pulses , micro-structuring have been performed in various materials. It was shown that two- or three-dimensional optical devices can be shaped using this inscribing technique , aiming a large range of applications. Among these devices the waveguide lasers  show interesting features, like compactness, low emission threshold and output performances close of those yielded by the bulk material, which makes them very attractive in optoelectronics. The writing process depends mainly of the material type and of the fs-laser pulse characteristics. In many glasses the irradiated material melts during the writing process and then it re-solidifies. A track with a higher refractive index compared with that of the bulk medium results, this track being used itself for light propagation.
On the other hand, in the case of a lot of laser crystals the writing process alters the medium inside the track (where a lower refractive index is obtained in comparison with that of the bulk) and causes a stress-induced refractive index increase in the adjacent regions. The light is guided in between two such tracks (or walls) . Buried waveguides of two-wall type have been inscribed in various laser active media, such as Nd:Y3Al5O12 (Nd:YAG) [5,6], Yb-doped monoclinic potassium double tungstates , Yb:YAG [8–10], Nd:YVO4  and Nd:GdVO4 , or Pr:SrAl12O19 . Efficient laser emission was reported from these waveguides pumped with tunable Ti:sapphire lasers [5,6,8,10–13] or with diode lasers [7,9].
A step forward toward realization of a compact waveguide laser was the demonstration of the depressed-cladding waveguide . Using this technique structures with tubular shapes can be fabricated by writing many parallel tracks around a defined contour. Thus, waveguides having rectangular and elliptical cross-sections were realized in Nd:YAG single crystals [14,15], with circular, hexagonal and trapezoidal shapes in Nd:YAG ceramic , circular in Tm:YAG ceramic  or ZnS , rhombic in Pr:YLiF4 , or of circular double-cladding shapes in Nd:YAG . Because the waveguide dimensions can be increased the pump with diode laser becomes feasible [15,19]. Using the pump with a fiber-coupled diode laser we have reported recently laser emission at 1.06 μm and 1.3 μm from two-wall type and cladding (circular and rectangular shaped) waveguides that were inscribed in Nd:YAG single crystal [21,22] or from circular waveguides that were written in Nd:YAG ceramic media ; laser emission on the quasi-three-level 4F3/2→4I9/2 transition at 946 nm was also observed .
The cladding waveguides mentioned above were obtained with a technique similar to that proposed in . In this arrangement the fs-laser beam employed for writing and the laser crystal direction used for lasing were perpendicular to each other. The laser crystal was translated and once the writing was made along the entire medium length the fs-laser focusing position was moved to a new location. Many tracks are inscribed around the waveguide shape, but there is always a space of unmodified refractive index left between any consecutive tracks.
In this work we are using a scheme in which the laser crystal is moved along a helical trajectory during the writing process , eliminating the regions with unchanged refractive index. Furthermore, a geometry in which the fs-laser beam is parallel to the crystal axis used for lasing is considered. We have applied this arrangement to inscribe, in Nd:YAG ceramic, circular waveguides with well defined walls. Efficient laser emission at 1.06 μm and 1.3 μm is obtained under the pump with a fiber-coupled laser diode. Thus, a waveguide with 100-μm diameter that was realized in a 5.0-mm long, 1.1-at.% Nd:YAG ceramic yielded laser pulses of 3.4-mJ energy at 1.06 μm and of 1.2-mJ energy at 1.3 μm, with overall optical-to-optical efficiency of 0.26 and 0.09, respectively. To the best of the authors’ knowledge this is the first time when helical movement is applied for writing waveguides in a laser medium.
2. Waveguide fabrication
The laser medium was a 5.0-mm thick, 1.1-at.% Nd:YAG ceramic (Baikowski Co. Ltd., Japan). For inscribing we used a chirped pulsed amplified laser system (Clark CPA-2101) that yielded pulses at 775 nm with 200-fs duration and energy up to 0.6 mJ, at 2-kHz repetition rate [21–23]. The fs-laser pulse energy was controlled by a combination of a half-wave plate, a polarizer and calibrated neutral filters; the beam was then focused at a certain depth below the Nd:YAG surface with a microscope objective or through a lens (as shown in Fig. 1).
The scheme proposed in  is illustrated in Fig. 1(a). In this geometry the Nd:YAG ceramic is moved transversally (with speed v1) to the fs-laser beam, on direction Ox starting from surface S1. Once surface S2 is reached the fs-laser focusing point is changed to a new location (in the Oyz plane) and the writing continues with a new translation. It is worthwhile mentioning that tracks have to be inscribed following an algorithm; for example we used the (1, 2, …, n-1, n, n + 1, …, m) orders and in this way the overlap between the fs-laser beam and any already inscribed track was avoided. Using this writing method an unmodified bulk material that is surrounded by many tracks with decreased refractive index in the adjacent boundaries is obtained; waveguiding is realized in the region surrounded by the tracks. In order to avoid the medium fracture, the tracks are inscribed at a distance of few μm between; an unmodified material will therefore remain between the tracks (as illustrated in the inset of Fig. 1(a)). These regions with unchanged refractive index can increase the waveguide propagation losses, decreasing thus the laser emission performances.
A better overlap between the inscribed marks that build the waveguide walls can be achieved by moving the Nd:YAG medium on a helical trajectory. As a first choice, the medium motion can be perpendicular to the fs-laser beam (Fig. 1(b)). The right selection of the rotation velocity (in the Oyz plane) and of the speed translation (v2 on direction Ox) can deliver a smooth aspect of the walls (inset of Fig. 1(b)). Still the wall is not rounding, as the shape of the inscribed marks depends on the characteristics of the focusing optics.
As an alternative solution the Nd:YAG is 90° rotated on the motorized stage and the writing is made parallel to the direction on which laser emission will be obtained, as shown in Fig. 1(c). In this case the medium is moved circularly in the Oxy plane and translation is performed on direction Oz (with speed v3). This writing method can provide waveguides with circular walls (inset of Fig. 1(c)). Moreover, because the typical depth of an inscribed mark has few tens of μm, the translation speed is increased in comparison with the arrangement from Fig. 1(b). We comment that the helical movement can be replaced by a sequence of a circular trajectory in the Oyz plane followed by a step translation on direction Oz. These arrangements require a carefully choice of the focusing optics such as to realize inscribing on a medium with length ℓ sufficient for efficient absorption of the pump beam. Additional adjustments of the fs-laser beam energy may be necessary in the writing process as the focus point moves on a considerable depth below surface S2 of the Nd:YAG medium.
For the lasing experiments we inscribed three circular (with diameters of 50, 80 and 100 μm) cladding waveguides in the Nd:YAG ceramic by using helical movement of the medium. A 10× microscope objective with a numerical aperture (NA) of 0.30 was employed to focus the fs-laser beam to a diameter (in air) of ~12 μm. A video camera was used to visualize the process and thus to choose suitable writing parameters for a good overlap between the traces inscribed at each helix. For example, in the case of the 100-μm diameter structure a complete circle in the Oxy plane was done in 0.84 sec. The fs-laser pulse energy was set at 15 μJ. The depth of the track inscribed in Nd:YAG on Oz direction was measured, and based on this evaluation the pitch of the helical trajectory was fixed at 40 μm. Thus, the time necessary for writing this waveguide was ~105 sec. The Nd:YAG end faces (S1 and S2) were polished after inscribing, and the final length of the laser medium was 4.7 mm.
Cross-section views of some circular waveguides are presented in Fig. 2. The waveguides realized by the Nd:YAG helical movement will be denoted by DWG-1 (with diameter ϕ = 100μm in Fig. 2(a)), by DWG-2 (ϕ = 80 μm, not shown) and by DWG-3 (with ϕ = 50 μm in Fig. 2(b)). For comparison a fourth waveguide with ϕ = 100 μm (indicated in Fig. 2(c) by DWG-4) was obtained by the classical method. In this writing the fs-laser beam, of ~1.5-μJ energy, was focused with an achromatic lens of 7.5-mm focal length; the beam-waist diameter in air was ~5 μm. The waveguide was centered 500-μm below the Nd:YAG surface and consisted of 38 parallel lines that were inscribed on Ox direction at 50 μm/s speed of the translation stage. The writing time was about 1 h. In addition, photos of the waveguides walls taken along the writing direction are shown in Fig. 2(d) for the DWG-1 waveguide and in Fig. 2(e) for waveguide DWG-4. Continuous boundaries were realized by the helical movement of the Nd:YAG; on the other hand, it is clearly seen that the DWG-4 waveguide contour consists of the sum of the inscribed tracks with some regions of unmodified medium left between.
The propagation losses were determined by coupling a HeNe linearly-polarized laser beam into each waveguide and measuring the power of the transmitted light; the HeNe beam coupling efficiency was unity. The propagation losses (at 632.8 nm) were in the range of 1.1 to 1.2 dB/cm for all the DWG-1, DWG-2 and DWG-3 waveguides. Obvious differences between losses depending on the polarization status of the HeNe beam were not observed. In the case of the DWG-4 waveguide, the losses were 1.6 dB/cm for TM polarization (parallel to the writing direction) and a little higher (~1.9 dB/cm) for TE polarization. This increase could be attributed to some leakage of the TE polarized light through the Nd:YAG regions with unmodified refractive index. Thus, helical movement of the laser crystal during inscribing seems to provide waveguides with low propagation losses compared with those of similar structures realized by classical translation of the medium. Also, in general these losses are comparable or smaller than those reported for the various cladding waveguides inscribed in Nd:YAG ceramics (0.8 to 1.4 dB/cm in  and 1.0 to 1.8 dB/cm in ) or Nd:YAG single crystals (1.7 to 2.0 dB/cm in  and 1.3 to 2.2 dB/cm in ).
3. Laser emission experiments. Results and discussion
The experimental set-up used for laser emission was similar to that we have employed in our previous works [21–23]. The resonator was linear, with the mirrors (both plane) positioned very close to the Nd:YAG surfaces. The rear high-reflectivity (HR) mirror of the resonator was coated HR (R> 0.998) at the lasing wavelength (λem) of 1.06 μm or 1.3 μm and with high transmission, HT (T> 0.98) at the pump wavelength (λp) of 807 nm. Various output coupling mirrors (OCM) with a defined T at λem were used. Furthermore, in the case of lasing at 1.3 μm the OCM had a specified T at this wavelength, but it was also coated HT (T> 0.995) at 1.06μm in order to suppress emission at this high-gain line. The pump (at λp) was made with a fiber-coupled diode laser (LIMO Co., Germany) that was operated both in continuous-wave (cw) mode and in quasi-cw (pump pulse duration of 1 ms at 10 Hz repetition rate) regime. The fiber end (100-μm diameter, NA = 0.22) was imaged into Nd:YAG using an achromatic collimating lens of 50-mm focal length and an achromatic focusing lens of 30-mm focal length. The uncoated Nd:YAG ceramic was placed on a metallic plate with no cooling.
Figure 3 shows images of Nd:YAG surface S2 when the pump beam was incident on side S1 in the bulk material (Fig. 3(a)) or it was coupled into the waveguides. A good guiding is obvious in the waveguides made by the medium helical movement (Fig. 3(b) and Fig. 3(c)), while leakage of the pump beam through the unmodified material left between the tracks is observed in the case of the waveguide inscribed by the classical writing method (Fig. 3(d)).
Characteristics of the laser emission at λem = 1.06 μm that was obtained from the 100-μm diameter DWG-1 waveguide in quasi-cw pumping regime are given in Fig. 4(a).With an OCM of T = 0.01 this waveguide yielded laser pulses with maximum energy Ep = 1.1 mJ for the pump with pulses of energy Epump = 13.1 mJ; the slope efficiency was ηs = 0.09. The best performances were recorded when the OCM had T = 0.10: The energy Ep increased at 3.4 mJ (the overall optical-to-optical efficiency ηo was ~0.26) and the slope efficiency improved to ηs = 0.30. The near-field distribution (recorded with a Spiricon camera, model SP620U, 190-1100 nm spectral range) is shown in Fig. 4(b). The beam was stable in time and its transverse distribution was highly multimode with a M2 factor (measured by the 10%-90% knife-edge method) of ~27. We mention that the Nd:YAG bulk delivered laser pulses with Ep = 5.5 mJ (ηo = 0.42) at slope ηs = 0.45. The beam transverse mode (its near-field distribution is shown in Fig. 4(c)) has M2~5.0. The pump-beam absorption efficiency in bulk was measured to be ηa~0.80. A comparison between laser performances obtained in bulk and in the waveguide should be made carefully, because the fraction of the pump power that is coupled and then absorbed into the waveguide, or the waveguide losses cannot be determined exactly. Nevertheless, by using an integral overlap between the pump beam shape and the waveguide input surface the pump beam coupling efficiency (ηc) was calculated close to unity. Thus, the lower performances of waveguide were due mainly to its larger propagation losses compared with those of the bulk Nd:YAG; these losses influence the fraction of the pump light absorbed in the waveguide, as well as the laser emission performances.
Figure 5(a) compares the laser pulse energy Ep at λem = 1.06 μm delivered by all the waveguides, with an OCM of T = 0.10. The DWG-2 and DWG-3 waveguides yielded pulses with Ep = 3.5 mJ (ηo~0.27) and 4.1 mJ (ηo~0.31) at slope ηs of 0.31 and 0.36, respectively. Although less pump light was coupled into DWG-3 (according to the calculus ηc~0.70) a better overlap between the pump volume and the laser beam could compensate the decrease of ηc. The waveguide DWG-4 delivered laser pulses at 1.06 μm with Ep = 2.2 mJ (ηo~0.17) at slope ηs = 0.20; the laser beam M2 factor was ~20.1. The laser pulse energy at 1.3 μm is presented in Fig. 5(b) for an OCM with T = 0.03 at this wavelength. Pulses with energy Ep = 1.2 mJ (ηo~0.09) at slope ηs = 0.12 were obtained from the DWG-1 waveguide. Again, the DWG-4 waveguide yielded lower performances, Ep = 0.82 mJ with optical efficiency ηo~0.06, while the slope ηs decreased at 0.10.
The waveguides were pumped also in cw regime. Figure 6 shows the laser performances at 1.06 μm that were measured with an OCM of T = 0.05 at this λem. Output power Pout = 0.48 W was obtained from DWG-1 for the pump with 3.7 W at 807 nm; the slope was ηs = 0.24. A slightly increased power of 0.51 W was yielded by the DWG-2 waveguide. The highest power recorded from the DWG-4 waveguide was Pout = 0.37 W (at ηo~0.10) with slope ηs = 0.19.
All the waveguides realized by the helical movement showed emission at 1.3 μm, although with low performances (in the case of DWG-1 waveguide, Pout was below 0.15 W for the pump with 3.7 W at 807 nm). On the other hand, DWG-4 did not lase at 1.3 μm. Thermal effects are influencing these results [23,25]; cooling of the laser medium and Nd:YAG media with different doping level will be considered in future works in order to improve the 1.3-μm emission from such waveguides.
In summary, we have reported the first realization of circular cladding waveguides by helical movement of the laser medium during the direct fs-laser writing process, the direction of translation and the fs-laser beam being parallel. Laser pulses with 3.4-mJ energy at 1.06 μm and with 1.2-mJ energy at 1.3 μm under the pump with pulses of 13.1-mJ energy at 807 nm were obtained from a 100-μm diameter circular waveguide that was inscribed in a 1.1-at.% Nd:YAG ceramic. The same waveguide yielded 0.48-W cw output power at 1.06 μm. Pulses at 1.06 μm with energy up to 4.1 mJ (overall optical-to-optical efficiency of 0.31) were obtained from a 50-μm diameter circular waveguide. For comparison, a circular waveguide with 100-μm diameter was inscribed by the classical translation method in the same medium. This device outputted laser pulses with maximum energy of 2.2 mJ at 1.06 μm and of 0.82 mJ at 1.3 μm. Optimization of the new inscribing procedure is still necessary and should include the choice and correlation of the focusing optics, of the fs-laser pulse energy and of the helical trajectory parameters. Nevertheless, the results of this work show that the helical movement of the laser medium during fs-laser writing could be a step forward towards realization of efficient integrated lasers consisting of cladding waveguides pumped by diode lasers.
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 and partially supported by the EC initiative LASERLAB-EUROPE (contract no. 284464) - WP33 - European Research Objectives on Lasers for Industry, Technology and Energy (EURO-LITE).
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