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Abstract
We report on the fabrication of channel waveguide by a femtosecond laser (fs-laser) micromachining system in a Pr:LiYF4 (Pr:YLF) crystal. The micro Raman (μ-Raman) spectra and scanning confocal fluorescence imaging investigations of the depressed cladding structure indicated that slight changes (with respect to widths of the emission lines and spectral positions) have been generated in the laser-modification region. In the meantime, the possible relation of these changes with the waveguide formation was analyzed. The microphotoluminescence (μ-PL) experiment manifests an excellent preservation of the fluorescence properties of the Pr3+ ions in the guiding area. π-polarized waveguide lasers at wavelengths of 605 nm and 720 nm were achieved with a pumping laser at a wavelength of 444.5 nm. The maximum output power of the lasers achieved was 66 mW and 47 mW with slope efficiencies of 9.5% and 6.3%.
© 2017 Optical Society of America
1. Introduction
Femtosecond laser inscription (FLI) is a well-known fast, flexible, clean and versatile fabrication technique for amount of applications, integrated photonics, optofluidics, and micromechanics as examples [1–4]. During the laser processing, FLI reduces heat diffusion a large extent to the surrounding regions. Meanwhile, the nonlinear absorption of the pulses in the dielectric materials combined the threshold effect enables outstanding spatial resolution, high flexibility, and precise focusing for the three-dimensional (3D) micro- and nanofabrication of transparent materials. By controlling machining parameters appropriately, varieties of phase and structural changes generate in a selective manner during the irradiation of focused femtosecond laser pulses into transparent dielectric materials, which paves a new avenue for the fabrication of integrated optical devices, in particular for the channel optical waveguides [5–8].
As the fundamental components in integrated photonics, optical waveguides confine the light propagation in small volumes and offer high optical intensities compared with the bulk materials [9]. These features benefit the waveguide lasers based on active media owing lower lasing threshold, considerable slope efficiency with respect to the bulk lasers. Furthermore, the channel and ridge waveguides confine laser propagation in two-dimensions have superior performances for construction more compact devices compared with the one-dimensional waveguides (i.e. planar or slab waveguides) [10,11].
Since the year 1996, when the pioneers, Davis et al., achieved fs-laser inscribed channel waveguides in glass first time, FLI has been demonstrated as a promising and quintessential technique for fabricating waveguides compared with the traditional techniques, such as thermal ion in-diffusion, ion implantation, and so on [12–16]. Previous investigations manifest that, depending on the diverse processing parameters, directly written waveguides (Type I waveguides, index increased in the irradiation region), dual-line and depressed cladding guiding configurations (Type II and III waveguides, index decreased in the irradiation regions) can be obtained in a wide range of transparent bulk materials [5, 7, 15, 16]. For instance, Type I waveguides has been achieved as beam splitters and quantum circuit components in transparent materials (e.g. in LiNbO3 and ZBLAN); Type II waveguides, which could be applied as miniature laser source, have been fabricated in Nd:GdVO4, Nd:GGG, Pr:YLF; Type III waveguides have been fulfilled in rare-earth doped YAG, KTiOPO4, Nd:YVO4, and Nd:GdVO4 with kinds of sectional geometries, which could be utilized as cw, Q-switched and mode-locked waveguide lasers, Y-branch splitters, second harmonic generation (SHG) [7, 15–27]. It is feasible that well-established geometry of Type III waveguides contributes to strong compatible devices for the integrated optical system, for example the reported “fiber-waveguide” devices [25, 26].
Currently, Pr3+-doped YLF crystal is a favorable solid-state laser gain media for approaching laser in the visible spectrum by directly pump [28–31]. RGB colors, including the blue (3P0→3H4), green (3P1→3H5), and red (3P0→3F2) lasers could be obtained through the emissions of trivalent praseodymium ions, which indicate that lasers operating from the Pr:YLF crystal are promising candidate for a new generation of color displays. In addition, orange (3P0→3H6) laser oscillations also generate in the emissions of excited Pr3+ ions [31]. Due to the undesirable features of the planar and dual-line waveguide lasers pumped by semiconductor lasers reported in the previous work, the investigation of the performances of the Type-III waveguide in Pr:YLF is of great significance for the development of integrated optical circuits [17,28].
In this work, we report on waveguides lasers at a wavelength of 605 and 720 nm generated in a depressed cladding waveguide pumped by a laser working at 444.5 nm. In the case of this channel waveguide lasers, the maximum output power of the waveguide lasers at 605 and 720 nm is around 66 and 47mW, with slope efficiencies of 9.5% and 6.3%, threshold of 298 and 253 mW, respectively. The μ-Raman and μ-PL was investigated for a further research of the formation of cladding structures in the Pr:YLF crystal.
2. Experiments in details
The optically polished Pr:YLF (doped by 0.8 at% Pr3+ ions) crystal used in this work was in size of 4 mm × 3 mm × 7 mm. The depressed cladding configurations were produced by using an amplified Ti:sapphire laser system (Spitfire, Spectra Physics, generating linearly polarized 100 fs pulses at 800 nm, with 1 KHz repetition rate and 1 mJ maximum pulse energy) at the National University of Singapore, Singapore. The fs laser beam was focused by a microscope objective (Leica 40 × , N.A. = 0.65) and the pulse energy was adjusted to be around 0.6 μJ considering the optical elements in the experimental setup (e.g. a calibrated neutral density filter, a half-wave plate and a linear polarizer). In order to produce parallel and uniformed damage filaments, the crystal was scanned at a velocity of 1.5 mm/s on a motorized XYZ-stage. The separation between the adjacent tracks was set around 1 μm to fabricate compact guiding configurations and avoid leak of light. The schematic diagram of the waveguide is displayed in Fig. 1(a), and the top view and cross section view of microscope images of the guiding structure can be observed in Figs. 1(b) and 1(c).
Fig. 1 (a)Sketch map of the structure in Pr:YLF crystal, and microscope images of the femtosecond-laser micromachined channel waveguides with diameter of 30 μm (b)under top view and (c)cross-section view.
The confocal μ-Raman spectral of the guiding region, the filament and the bulk material were performed using a confocal microscope (Leica DM2700 M) attached to a 532 nm laser and a 50 × focusing objective. The peaks corresponding to phonon modes, and the full width half maximum (FWHM) of them which are assigned to the structure symmetry of the Pr:YLF are investigated. Meanwhile, for a further study of the fluorescence properties of the trivalent praseodymium ions in the guiding structures, the fluorescence images of the depressed cladding structures were achieved with a home-made fiber confocal microscope (PG2000-Prn, Morpho Inc.). The optical excitation was provided by a 500-nm cw fiber laser with a beam collimator. Mapping images were finally processed by using the software WSMP®.
The propagation loss at 632.8 nm is measured to be 0.6 dB/cm with the cut-back method (the length of the waveguide was polished to be 5 mm after the measurement). Afterwards, we estimate the refractive index change taking place at the damaged tracks in respect to bulk substrate with measurement of the numerical aperture of the waveguides, and the Δn is around −9 × 10−4.
We performed the waveguide laser experiments under an end-pumped arrangement as shown in Fig. 2. A fiber coupled laser diode operation at 444.5 nm with a maximum output power of 2 W was utilized as the pumping laser source. And then, a convex lens with focal length of 30 mm coupled the pump beam into the waveguide. Two dielectric mirrors (the input one with a high transmission 99% at 400-550 nm, and the output one with reflectivity >99% at 410-520 nm and transmission of 15% at 580-900 nm) were adhered to the two end faces to construct the Fabry-Perot oscillating cavity for the laser oscillation. The output lasers from the waveguide was collected by a microscope objective and characterized by a power meter and a spectrometer (USB 2000 + , Ocean Optics Inc.) after a filter. An visible-infrared CCD (300-1300 nm, WinCamD-UCD12, DataRay Inc.) was used to image the generated laser.
Fig. 2 Schematic diagram for the laser pumping setup. PL: pump laser; BC: Beam collimator; P1: polarizer; P2: half-wave plate; L1: coupling convex lens; Ci and Co: pump mirror and output mirror; L2: coupling lens; FI: filter; AP: aperture
3. Results and discussion
Figure 3 depicts the confocal μ-Raman spectral characterization of three crucial regions around the guiding area (e.g. the bulk material, the waveguide, and the filament) excited by a cw 532-nm laser. For Pr3+-doped YLF crystal, its expected Raman spectrum at room temperature is composed by delta function peaks which are assigned to totally symmetric stretching optical vibration modes of the reciprocal lattice. In the case of laser-written tracks, it is known that the strain/stress fields surrounding the damage tracks are not isotropic. Therefore, the energy blue/red shift and broadening of the modes perform a comparative structural analysis. Obviously, four phonon modes corresponding to the peaks of Raman shift around 258 (I), 321 (II), 373 (III), and 420 (IV) cm−1 are measured. It is clear that there is a strong reduction of the intensity took place in the filaments compared with those in the bulk materials and waveguide area, unequivocally manifest that the Pr:YLF network has been partially damaged and disordered at the fs-laser modification regions.
Fig. 3 μ-Raman spectral collected from the bulk material (green line), the waveguide (red line) and the filament (blue line) excited by a cw 532 nm laser.
Meanwhile, more detailed information about the peak positions and the FWHM is shown in the Table 1. As it can be observed from the table, broadening and a slight blue shift occur at these peaks, which suggest that the lattice has been partially damaged and disordered at the contour of the circular structure. However, the features of the Raman modes in the bulk crystal and the waveguide are nearly the same, which means that, the guiding volume is constituted by an almost original Pr:YLF network. We infer that the fs-laser inscription is accompanied by a small refractive index decrease only surrounding the focusing regions, and this fact approximately indicate that the waveguide fabrication does not induce a large propagation loss in the guiding configuration.
Table 1. Results of the Raman spectra of the bulk crystal, the waveguide and the filament
In order to gain a deeper knowledge of the fluorescence properties of Pr3+ ions in the waveguide cross sections, the room-temperature fluorescence emission spectra from 500 to 800 nm are obtained from the bulk substrate, the center of the guiding structure and the filament, respectively. As shown in Fig. 4, the emission spectra are related to the 3PJ→3H6 and 3PJ→3F2 transition of Pr3+ ions. In contrast to the blue line, the green line and the red line are virtually identical indicating that the fluorescence properties of Pr3+ ions at the waveguide’s volume have been preserved well, which is an outstanding feature since it means that the waveguide satisfies the basis requirement of low-threshold, high-gain integrated laser sources. Then, during the fs-laser inscription procedure, there is indeed a strong fluorescence quenching induced in the damage tracks as shown in Fig. 5. This is consistent with the μ-Raman spectral analysis, namely that the waveguide’s contour has been strongly modified with simultaneously damaged.
Fig. 4 Room-temperature fluorescence emission spectra related to the 3PJ→3H6 and 3PJ→3F2 transition of Pr3+ ions obtained from the fs-laser inscribed cladding waveguides in Pr:YLF crystal(red line), bulk of the sample (green line) and the filaments (blue line).
Fig. 5 2D spatial distribution of the emitted intensity of the Pr3+ emission lines obtained from the waveguide with a diameter of 30 μm cross section (a) and enlarged part of the damaged tracks (b). (c) and (d) correspond to the 3D spatial distribution, respectively.
Therefore, with the comprehensive exploration of structural and fluorescence properties of the inscribed structure, its potential use as integrated laser sources at the range of visible laser has been evaluated based on an end-face coupling system. Figures 6(a) and 6(b) show the π-polarized 2D and 3D laser modal profiles of the collected waveguide lasers, which is indicative of a fundamental mode laser generated in the pumping experiment. Simulated 2D and 3D modal profiles can be seen in Figs. 6(c) and 6(d), which is in good agreement of the experimental results. What should be pointed out here is that there is no waveguide laser generated when the pump laser is at σ polarization.
Fig. 6 Measured (a) 2D and (b) 3D waveguide laser modal distributions of circular waveguide under 444.5 nm optical pump, and the corresponding simulated 2D and 3D laser modes.
Figure 7(a) depicts the cw laser emission spectrum measured from the output end face of the Pr:YLF crystal cladding waveguide at room temperature pumped by π-polarization laser source. The sharp peaks of the emission line stay at 605 nm and 720 nm, and the FWHM of them are 1.8 nm and 1.4 nm, respectively. These laser oscillations are corresponding to the main transition line of the 3PJ→3H6 and 3PJ→3F4 transitions of Pr3+ ions. The dependence curves of the output waveguide laser power on the input power (launched power) from the guiding structure can be observed in Fig. 7(b). Based on the linear fit (solid lines) of the experiment data, we can determine the slope efficiencies of the lasers at 605 nm and 720 nm to be 9.5% and 6.3%, respectively. While, the maximum output power when the input power is at 1W is 66 mW and 47 mW, which is quite higher than the previous report on the Type II waveguides in Pr:YLF crystals. Although the diameter of the cladding waveguide is 30 μm, the laser performance enables the waveguide to be compatible with single mode fiber and contribute to set up a “fiber-waveguide-fiber” platform. Meanwhile, the depressed cladding waveguide with circular cross sections with lower propagation loss is a promising LD pumped compact laser sources for construction new integrated optical circuits.
Fig. 7 (a) The cw laser emission spectrum from the Pr:YLF crystal waveguide. The FWHM of the lasers at 605 and 720 nm are 1.8 and 1.4 nm, respectively. (b) The cw waveguide laser output power at 605 nm (red solid line) and 720 nm (green solid line) as a function of the input power.
4. Summary
In conclusion, we demonstrate cw laser operation at 605 nm and 720 nm in the Pr:YLF depressed cladding wavguide fabricated by FLI. The μ-Raman spectra and μ-PL spectra of the waveguide cross section have been analyzed to identify the main physical mechanisms at the basis of waveguide formation. Low refractive index barrier has been induced by the fs-laser modification with good fluorescence properties preservation of Pr3+ ions, which is paramount for light confinement within the configuration with low propagation loss. Nearly single-mode waveguide laser with slope efficiencies of 9.5% and 6.3%, and output power of 66 mW and 47 mW have been achieved with the end-face pumping system.
Funding
The work is supported by the National Natural Science Foundation of China (Grants No. 61575097 and 11704201), National Natural Science Foundation of Tianjin 17JCQNJC01600, the Fundamental Research Funds for the Central Universities and the Open Fund of the Key Laboratory of Optical Information Science & Technology (Nankai University). The authors also thank the support from the Specialized Research Fund for the Doctoral Program of Higher Education 20130121120043 and Natural Science Foundation of Fujian Province of China 2014J01251.
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