We report on realization of buried waveguides in Nd:YAG ceramic media by direct femtosecond-laser writing technique and investigate the waveguides laser emission characteristics under the pump with fiber-coupled diode lasers. Laser pulses at 1.06 μm with energy of 2.8 mJ for the pump with pulses of 13.1-mJ energy and continuous-wave output power of 0.49 W with overall optical efficiency of 0.13 were obtained from a 100-μm diameter circular cladding waveguide realized in a 0.7-at.% Nd:YAG ceramic. A circular waveguide of 50-μm diameter yielded laser pulses at 1.3 μm with 1.2-mJ energy.
© 2014 Optical Society of America
The direct femtosecond (fs)-laser writing technique is now recognized as a powerful tool for realizing waveguides in various transparent optical materials [1,2]. Because of non-linear absorption processes, a focalized fs-laser pulse can produce modifications at micro or sub micrometric scales, thus inducing changes of the refractive index inside the material. There are a several techniques that can be used for inscribing waveguides . One of these methods is specific to glasses and LiNbO3. In this case melting and re-solidification of the irradiated volume provides a track with an increased index of refraction compared with that of the medium; the track is used itself for light propagation .
Another writing method can even damage the material inside the irradiated volume and by stress causes a decrease of the refractive index in the adjacent region; this time the light is guided in between two such tracks . Two-line (or two-wall) waveguides were realized in various laser materials, like Nd:Y3Al5O12 (Nd:YAG) single crystal  and ceramic , Nd-vanadates , Yb:YAG single crystal  or Pr:YLiF4 (Pr:YLF) . Efficient laser emission was realized from such waveguides. For example, an output power of 1.3 W at 1.06 μm was obtained from a Nd:YAG waveguide for the pump with 2.25 W at 808 nm , and the pump with 1.2 W at 941 nm yielded 0.8 W at 1.03 μm from an Yb:YAG waveguide . In the experiments a continuous-wave (cw) tunable Ti:Sapphire laser was used as the pump source.
A more complex procedure that implies writing of many tracks around a defined perimeter was proposed by Okhrimchuk et al. . This method allows obtaining of buried depressed cladding waveguides with different tubular shapes and sizes, thus enabling propagation of a randomly polarized beam (and not only of a linearly-polarized light like in the case of the two-wall waveguides). Waveguides with rectangular and nearly circular cross-sections were realized in Nd:YAG single crystals [10, 11], with circular, hexagonal and trapezoidal shapes in Nd:YAG ceramic , rhombic in Pr:YLF  crystal or circular in Tm:YAG ceramic . Furthermore, it was shown recently that this technique is promising for realizing integrated lasers consisting of double-cladding waveguides .
Laser emission under the pump with diode lasers of waveguides realized by fs-laser writing was reported in a few papers [9–11, 15]. Visible orange and deep-red laser lights with output powers of few tens of mW were achieved in Pr:YLF from a rhombic cladding waveguide under the pump at 444 nm with an array diode laser . A nearly-circular cladding waveguide realized in Nd:YAG single crystal yielded 180-mW output power at 1.06 μm using the pump at 808 nm with a fiber-coupled diode laser . Output power of 2.35 W at 1.03 μm was obtained from an Yb:YAG single crystal, two-wall waveguide pumped with a distributed-Bragg-reflector tapered diode laser . Our group has reported recently laser emission at 1.06 and 1.3 μm in buried waveguides realized in Nd:YAG single crystal, employing fiber-coupled diode lasers for pumping . Laser pulses with 1.4 mJ energy (Ep) at 1.06 μm and with Ep = 0.4 mJ at 1.3 μm were achieved from a circular waveguide with 110-μm diameter.
In this work we report our latest results on realization of buried two-wall type and circular cladding waveguide in Nd:YAG ceramic media by direct fs-laser writing method, and on laser emission characteristics obtained under the pump at 807 nm with fiber-coupled diode lasers. Laser pulses at 1.06 μm with Ep = 2.8 mJ and cw output power of 0.49 W were achieved from a circular cladding waveguide with 100-μm diameter that was inscribed in a 0.7-at.% Nd:YAG. The best result at 1.3 μm, i.e. laser pulses with Ep = 1.2 mJ, was obtained from a cladding waveguide with diameter of 50 μm. To the best of our knowledge these are the first results on laser emission obtained under the pump with fiber-coupled diode lasers from waveguides realized by direct fs-laser writing technique in Nd:YAG ceramic media.
2. Waveguides fabrication
The experimental set-up used for writing tracks in Nd:YAG ceramic media is shown in Fig. 1. A chirped pulsed amplified system (Clark CPA-2101) delivered laser pulses at 775 nm with duration of 200 fs, at 2-kHz repetition rate and energy up to 0.6 mJ. The fs-laser pulse energy was controlled by a combination of half-wave plate (λ/2), a polarizer (P) and calibrated neutral filters (F). An achromatic lens (L) with 7.5-mm focal length and numerical aperture NA = 0.3 was used to focus the beam to a diameter (in air) of ~5.0 μm. The laser media were two Nd:YAG ceramics (Baikowski Co. Ltd., Japan) with 0.7-at.% and 1.1-at.% Nd doping level. Each medium was placed on a motorized translation stage that allowed controllable movement on all directions. Tracks were inscribed on Ox direction at 50-μm/s speed of the translation stage, and the writing process was monitored with a video camera. The end faces of Nd:YAG were polished after writing; finally, each medium length was ℓ~7.8 mm.
Figure 2 presents images of the structures written in the two Nd:YAG ceramics. In the first attempts two lines (apart at distance w = 50 μm) were inscribed (Fig. 2(a)). The lines extent on the vertical Oz direction was 45 to 50 μm. Then, in order to increase the two-wall waveguide size on Oz we inscribed six lines, as shown in Fig. 2(b). Thus, two-wall structures with distances w = 50 μm (Fig. 2(b)) and 2w = 100 μm (Fig. 2(c)) were obtained; these waveguides will be indicated by WG-1 and WG-2, respectively. The fs-laser pulse energy used for writing was 2.0 μJ. Next, two circular cladding waveguides, first with diameter ϕ = 50 μm (Fig. 2(d)), denoted by DWG-1) and the second with diameter of 100 μm (Fig. 2(e)), denoted by DWG-2) were obtained by inscribing many parallel tracks separated by 5 to 6 μm at certain depths. For these writings the fs-laser pulse energy was decreased to 1.0 μJ. All structures were centered at the depth h = 500 μm below each Nd:YAG ceramic surface.
Waveguiding is possible between the parallel tracks of the WG-1 and WG-2 geometries or inside the circular DWG-1 and DWG-2 structures. The propagation losses of each configuration were evaluated by coupling (with efficiency close to unity) a polarized (along Oz axis) HeNe laser beam into every structure and by measuring the power of the transmitted light. The measurements concluded that, regardless of the Nd:YAG ceramic media, the propagation losses at 632.8 nm were around 0.5 dB/cm for the WG-1 waveguides and in the range of 0.6 to 0.7 dB/cm for the WG-2 waveguides. In the case of the circular structures losses were 1.0 to 1.2 dB/cm for DWG-1 and a little higher, 1.5 to 1.8 dB/cm for the DWG-2 waveguides. These numbers compare well with those reported for two-wall waveguides inscribed in Nd:YAG single crystals (1.6 dB/cm at 1063 nm)  and in Nd:YAG ceramic (0.6 dB/cm at 748 nm) , or with losses of the various depressed cladding waveguides realized in Nd:YAG ceramic (0.8 to 1.4 dB/cm at 632.8 nm) .
3. Laser emission results and discussion
For the laser experiments each uncoated Nd:YAG ceramic was positioned in a linear plane-plane resonator. The rear high-reflectivity mirror (HRM) was coated HR (reflectivity, R> 0.998) at the laser emission wavelength (λem) of 1.06 or 1.3 μm and with high transmission, HT (transmission, T> 0.98) at the pump wavelength (λp) of 807 nm. Various output coupling mirrors (OCM) with different T at λem were used. The mirrors were set very close of Nd:YAG, and each medium was placed on an aluminum plate without any additional cooling. The optical pumping was made at 807 nm with a fiber-coupled diode laser (LIMO Co., Germany) that was operated in quasi-cw mode (pump pulse duration of 1 ms at 10 Hz repetition rate), as well as in cw regime. The fiber end (100-μm diameter and NA = 0.22) was imaged into each Nd:YAG ceramic using a collimating lens of 50-mm focal length and a focusing lens of 30-mm focal length. Furthermore, a polarizer was placed between these lenses for the pump of the waveguides WG-1 and WG-2 with a linearly-polarized (parallel to the Oz axis) beam.
Figure 3 presents characteristics of the laser emission at 1.06 μm obtained in quasi-cw pumping regime from the waveguides realized in the 0.7-at.% Nd:YAG ceramic. The OCM transmission at this λem was T = 0.05. A maximum energy of the laser pulse Ep = 2.8 mJ was measured from the circular DWG-2 waveguide at the pump pulse energy (Epump) of 13.1 mJ (Fig. 3(a)), corresponding to an overall optical-to-optical efficiency (ηo) of 0.21. The slope efficiency with respect to the incident Epump was ηs = 0.23. On the other hand, for the pump in bulk (unmodified) 0.7-at.% Nd:YAG ceramic, the laser emitted pulses with Ep = 5.95 mJ (ηo~0.45) and slope ηs = 0.46. The pump beam absorption efficiency (ηa) in the bulk Nd:YAG was measured to be nearly 0.71, whereas the coupling efficiency of the pump beam into the DWG-2 waveguide was evaluated to be close to unity. Therefore, the lower performances obtained from waveguide DWG-2 were attributed mainly to the higher propagation losses in comparison with those of the bulk Nd:YAG ceramic (determined as 0.2 dB/cm at 632.8 nm).
The two-wall type WG-2 waveguide delivered a linearly-polarized beam with maximum energy Ep = 0.8 mJ for Epump = 4.8 mJ (i.e. ηo~0.17) and slope ηs = 0.22. Figure 3 shows also the laser beam near-field images that were recorded with a Spiricon camera (model SP620U, 190-1100 nm spectral range). In general, the beams were stable in time and present nearly symmetrical shapes. The laser beam M2 factor (measured by the 10%-90% knife-edge method) was 1.65 for bulk operation (Fig. 3(b)); for waveguides the laser beam quality degraded, having M2~10.1 for the circular DWG-2 waveguide (Fig. 3(c)) and M2 = 3.9 for the linear WG-2 waveguide (Fig. 3(d)).
The waveguides operated also in cw mode. Figure 4 presents the output power at 1.06 μm that was measured from the circular DWG-2 waveguides (2ϕ = 100 μm) inscribed in both Nd:YAG ceramic media. An output power of 0.49 W for 3.7-W pump power at 807 nm (ηo~0.13) and slope ηs = 0.25 was obtained from the DWG-2 waveguide of the 0.7-at.% Nd:YAG. The laser beam was symmetric (as shown in the inset of Fig. 4) with M2~3.2.
Table 1 summarizes the waveguides laser emission performances at 1.06 μm. The results obtained from both Nd:YAG media are similar, although the ηa in the bulk 1.1-at.% Nd:YAG is improved to ηa~0.84 (ηa = 0.71 for 0.7-at.% Nd:YAG). However, a Findlay-Clay analysis of the thresholds of emission (using several OCM with transmission between 0.01 and 0.10) concluded that the resonator residual losses Li were higher for the highly-doped Nd:YAG, i.e. Li~0.02-0.03, compared with Li~0.01 for the 0.7-at.% Nd:YAG. Thus, the use of a highly-doped 1.1-at.% Nd:YAG improves ηa but increases losses Li, which explains the very close results obtained in both Nd:YAG ceramics for emission at 1.06 μm.
For lasing at 1.3 μm the resonator was equipped with a HRM at this λem; the OCM had a specified T at 1.3 μm but also coating HT (T~0.995) at 1.06 μm in order to suppress emission at this high-gain line. Figure 5 shows the best laser emission characteristics obtained in quasi-cw regime with an OCM of T = 0.03. Pulses with energy Ep = 1.2 mJ (Epump = 13.0 mJ, ηo = 0.09) were measured from the circular DWG-1 waveguide realized in the 0.7-at.% Nd:YAG ceramic. The DWG-2 waveguide (2ϕ = 100 μm) inscribed in the 1.1-at.% Nd:YAG medium has an increased threshold of emission, yielding pulses with Ep = 0.75 mJ at slope ηs = 0.11.
Laser emission (of only few tens of mW power) was observed in cw mode, but it was unstable and disappeared in time. This behavior was attributed to thermal effects that in comparison with non-lasing regime are increased during emission at 1.3 μm in Nd:YAG with Nd doping below 1.14-at.% Nd . This finding was checked by mapping the temperature of each Nd:YAG output surface (operation in bulk) with a FLIR T620 thermal camera (−40°C to +150°C range, ±2°C accuracy). For the pump with 3.7-W cw power at 807 nm, the maximum temperature (Tmax) of the 0.7-at.% Nd:YAG under non-lasing was 78°C; Tmax decreased to 64°C for λem = 1.06 μm (output power, Pout = 1.40 W) and increased to 85°C for λem = 1.3 μm (Pout = 0.3 W). On the other hand, Tmax reached 104°C for non-lasing in the 1.1-at.% Nd:YAG, it decreased to 86°C for λem = 1.06 μm (Pout = 1.30 W) and increased only a little (to Tmax = 105°C) for λem = 1.3 μm (Pout = 0.7 W). Thus, while cooling of the Nd:YAG is necessary, a highly-doped Nd:YAG medium seems to be a better choice for improving the waveguides laser emission performances at 1.3 μm. These solutions will be investigated in further works.
In summary, laser emission at 1.06 and 1.3 μm was achieved, in buried waveguides that were inscribed in Nd:YAG ceramic media by fs-laser writing method, employing the pump with a fiber-coupled diode laser. A circular cladding waveguide of 100-μm diameter inscribed in a 0.7-at.% Nd:YAG delivered laser pulses at 1.06 μm with 2.8-mJ energy and 0.49-W cw power with overall optical-to-optical efficiency of 0.21 and 0.13, respectively. Laser pulses at 1.3 μm with 1.2-mJ energy were obtained from a 50-μm in diameter circular waveguide. Such devices show good potential for efficient integrated laser sources.
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS - UEFISCDI, project number PN-II-ID-PCE-2011-3-0363.
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