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Laser written waveguides in crystalline materials can be used to make highly efficient, high gain lasers. The bi-directional emission from such lasers however is typically broadband with poor spectral control. Hybridizing a tapered, mode matched laser written Bragg grating with a broadband Yb:YAG crystalline waveguide laser, we demonstrate single longitudinal mode output from one end of the device. Careful control of the grating characteristics led to laser thresholds below 90 mW, slope efficiencies greater than 42% and output powers greater than 20 mW.
© 2015 Optical Society of America
1. Introduction
Ultrafast lasers are widely used nowadays for three-dimensional volume micro-structuring of dielectric materials since the technique was first demonstrated in 1996 [1]. The ultrafast laser inscription (ULI) technique has since been successfully utilized to create a multitude of optical devices in many materials [2,3]. Of particular note, miniaturized laser systems for integrated optical applications have been realized with waveguides in optically active media [4–9]. Waveguides offer a very good overlap between tightly confined pump and laser modes resulting in laser systems with low thresholds. Resonator mirrors can be coated directly onto the end-facets of the waveguide chip or by inscribing Bragg structures into the waveguide structure itself [10].
Rare earth doped dielectric crystalline materials offer many advantages over glass hosts, for example, superior thermo-mechanical properties and higher peak emission/absorption cross sections. Channel waveguides (based on a stress induced refractive index change) fabricated in doped crystals using ULI are therefore suitable for efficient waveguide lasers. In a crystalline system, laser gain is extremely high, allowing laser operation to be achieved by Fresnel reflections of sample end-facets rather than using external mirrors. For example, slope efficiencies >75% and output powers >5 W (measured by summing the output exiting from both ends of the waveguide) have been demonstrated in Yb:YAG with just Fresnel reflections creating a cavity [7]. The bi-directional emission from such lasers however is typically broadband with poor spectral control [11].
In this paper we report on the hybrid integration of an Yb:YAG waveguide laser coupled to a passive waveguide Bragg grating (WBG) fabricated in an aluminoborosilicate glass also using the ULI technique. This has the advantage of narrowing the laser emission as well as ensuring a uni-directional laser output. Highly efficient operation is obtained by tailoring the mode coupling between the integrated devices using tapered waveguides. This results in single longitudinal mode lasing with low pump thresholds. Cheap, rapid and 3D waveguide tapering of this sort is not easily realized with conventional fiber or CMOS technologies, ultimately positioning ULI as the competitive technology in this space. Single longitudinal mode operation together with the high signal-to-noise ratio of these laser sources lend well to applications encompassing high resolution sensing.
2. Waveguide lasers in crystalline Yb:YAG
A range of parallel linear tracks of varied separation was written 300 µm deep into an 8.6 mm long, 7% doped Yb:YAG crystal with a Clark-MXR CPA-2010 femtosecond laser (775 nm, 140 fs, 1 kHz) and a f = 3.1 mm lens (NA 0.68). The crystal was translated perpendicular to the incident laser pulses at a velocity of 25 µm/s. Since no beam shaping techniques were employed the tracks exhibit a cross section, which is elongated along the incident fs-laser beam. The linear translation was also superimposed with a sine oscillation (oscillation amplitude 1-2 µm, oscillation frequency 70 Hz) to produce an increased modified volume perpendicular to the long axis of the track cross section. This results in a larger stress-induced refractive index increase and thus stronger mode confinement between the parallel tracks in comparison to structures inscribed without the sine oscillation [12]. The combination of refractive index reduction of the tracks themselves and the stress induced positive refractive index change between the tracks leads to an index distribution which supports mode confinement around 1 µm. Further details pertaining to device fabrication can be found elsewhere [7].
The stress-induced channel waveguides were characterized in terms of their guided mode field diameters (MFDs) at the pump wavelength of 976 nm using a CCD camera imaging system. All waveguides exhibited elliptical modes with MFDs ranging from 10 µm (minor axis) to 26 µm (major axis) depending on both the writing pulse energy and separation of the damage line stressors. In all investigated waveguides the major elliptical axis was oriented parallel to the long axis of the track cross section, i.e. parallel to the incident fs-laser beam. The waveguide with the least elliptical mode (13 × 17 µm: obtained with stressors separated by 18 µm and a writing pulse energy of 750 nJ) was used for all waveguide laser experiments.
Lasing in the channel waveguide was obtained by pumping with the free space coupling scheme schematically shown in Fig. 1(a). Pumping of the Yb:YAG waveguide was performed at 976 nm, rather than at the absorption peaks near 940 nm or 969 nm due to pump diode availability. The absorption coefficient at this wavelength is 0.77 cm−1 compared with 8.01 cm−1 at 969 nm. Despite the low absorption (37%), laser emission was achieved with a resonator formed between the Fresnel reflection at the Yb:YAG pump input facet and a thin high reflecting (HR) mirror at the other facet. Laser emission was collected in a backwards direction through a dichroic mirror at 45° and interrogated using either a power meter or optical spectrum analyzer (Advantest Q8384, 10 pm resolution). The linearly polarized pump was aligned parallel to the long axis of the track cross sections. It has been previously shown that light with this polarization is guided with the lowest loss [11]. To minimize the MFD mismatch loss between the circular pump beam and elliptical laser mode in the Yb:YAG crystal the pump beam was expanded from 6.6 µm (PM 980 pump diode fiber mode) to 15 µm (averaging the 13 × 17 µm Yb:YAG mode). The coupling loss was calculated to be 0.27 dB by numerically evaluating the mode overlap integral of the two guided modes [13].
Fig. 1 (a) Free space coupling setup used to pump and characterize an Yb:YAG waveguide laser coupled to a high reflector (HR). [ L1: f = 6.24 mm aspheric lens (NA 0.40), L2: f = 15.29 mm aspheric lens (NA 0.16) ]. (b) Hybrid integration of the Yb:YAG waveguide laser with highly reflective waveguide Bragg gratings (WBGs). Cooperative Yb3+ fluorescence and fluorescence of Er3+ and Tm3+ impurities along the Yb:YAG channel waveguide can be seen.
To determine the absorbed pump power all of the following were taken into account: the Fresnel loss of the pump at the input (8%), the MFD mismatch of the pump/laser mode, the measured transmitted pump power and the partial reflection (7%) of the high reflector at 976 nm (resulting in a double pass of a part of the pump light). This results in a threshold absorbed pump power for lasing of 49 mW. The maximum output power was 52 mW and the slope efficiency 65%. For comparison, Calmano et al. obtained a lasing threshold around 140 mW with a slope efficiency of 79% using a 940 nm pump and a cavity formed with only the Fresnel reflections off the crystal waveguide end-facets [12]. In both the laser reported here and that previously reported by Calmano et al. the spectral output was highly multimode. In the laser described here the emission had a bandwidth of approximately 500 pm with 16 individual longitudinal laser modes [Fig. 2]. The longitudinal mode spacing was 34 pm representing a cavity length matching the crystal waveguide length.
Fig. 2 Broadband, multimode laser emission from a resonator formed between the Fresnel reflection at the Yb:YAG pump input facet and a thin high reflecting (HR) mirror at the other facet.
Clearly the gain in this laser is extremely high since even with a non-optimal pump wavelength and substantial intra-cavity losses (0.8-1.6 dB/cm) [7,11] as well as high output coupling, the laser threshold could easily be achieved. Lasing is however relatively broadband and the versatility would be much improved if the emission could be restricted to a single longitudinal mode. Single longitudinal mode operation could be achieved using etalons or gratings as is common for bulk laser systems, however in waveguide systems (for example fiber lasers) the most common approach is to attach a fiber Bragg grating (FBG). The advantage of this is that the system becomes monolithic and hence less affected by environmental changes. The use of FBGs to narrow the linewidth of the laser described here however is limited by the small MFDs typically found in single mode fibers. For example the MFD mismatch loss of a Hi1060 fiber mode and the Yb:YAG pump mode was measured to be 2.65 dB. Ultrafast laser written WBGs in glasses not only have the distinct advantage of unlimited choice of design resonant wavelengths but most importantly ULI has the ability to adjust the guided MFD to increase coupling efficiencies for integration either via tapering the index contrast and/or waveguide size [14,15] or by using specially designed multicore waveguide ensembles [16]. The next section outlines the WBGs used for hybrid integration in this study.
3. Waveguide Bragg gratings (WBGs) in AF45
WBGs were fabricated in an alkali free aluminoborosilicate glass (Schott AF45) using a regeneratively amplified Spectra-Physics Hurricane Ti:sapphire laser (800 nm, 120 fs, 1 kHz). AF45 is largely free of iron impurities and thus features a high optical transmission in the near-infrared [17], which makes it well applicable for waveguide laser integration. The laser writing beam was focused into the glass sample by a 20 × microscope objective (NA 0.45) at a depth of 170 µm below the surface of the sample. The sample was translated perpendicular to the incident writing beam at 25 µm/s. A 500 µm slit was also placed in the beam path in order to produce devices with a circular cross section [18]. In contrast to the Yb:YAG waveguides described above the waveguiding region of the WBGs is located within the fs-laser modified volume. The writing beam was circularly polarized to induce the maximum refractive index contrast [19] and square-wave modulated in intensity thereby creating a waveguide structure formed by segments of exposed glass with a desired period, i.e. a WBG [20]. By varying the modulation, WBGs with different duty cycles (mark/space ratios) and orders could be created in a one-step fashion. Two types of WBGs were fabricated; 1st order gratings with a large coupling coefficient κ and short length L, and 2nd order gratings with a small κ and longer L. Strong gratings were chosen in order to act as high reflectors thus enabling uni-directional laser output as well as to provide narrowband feedback to the Yb:YAG laser described above. After fabrication, the glass samples were ground and polished to expose the ends of the WBGs. All WBG characteristics are shown in Table 1.
Table 1. Characteristics of waveguide Bragg grating (WBG) structures fabricated in AF45 aluminoborosilicate glass using the ultrafast laser inscription technique.
1st order gratings were written as a function of pulse energy with a duty cycle of 50%. Grating depths were measured by fiber coupling a broadband 1 µm ASE source into the WBGs and collecting transmission spectra with an OSA. In a 5.3 mm long sample grating depths ranged from 26.5 dB to substantially greater than 33 dB for writing pulse energies between 950 nJ and 1150 nJ [Fig. 3]. For pulse energies < 900 nJ no resonance was detected whereas for pulse energies ≥ 1150 nJ out of band losses increased and reduced grating strengths. It was not possible to measure the maximum grating depth due to signal strength limitations of the ASE probe source. In this case κ was greater than 700/m. For such strong gratings the grating bandwidth has been shown to increase linearly with κ (increasing writing pulse energy) and remain almost constant with increasing length, L [21]. The grating bandwidth measured between the first zeros either side of the stop band ranged from 220 pm to > 300 pm. The peak reflection wavelength shifts to longer wavelengths for higher writing pulse energies due to the increased average refractive index change induced (increase in κ). Over this range of writing pulse energies the physical sizes of the waveguiding structures increased from 4.5 µm to 7.4 µm while the corresponding MFDs at 976 nm decreased from 14.8 µm to 9 µm.
Fig. 3 Transmission spectra of (right) 5.3 mm long, 50% duty cycle 1st order waveguide Bragg gratings (WBGs) with large coupling coefficients (κ ~700/m) and a (left) 12.3 mm long, 90% duty cycle 2nd order WBG with κ ~177/m.
In order to obtain WBGs with a small κ, and still moderately high reflectivity, 2nd order gratings with 90% duty cycle were fabricated using a pulse energy of 1100 nJ. The resulting 12.3 mm long WBG had a reduced grating coupling coefficient (κ ~177/m) and bandwidth (106 pm), whilst still being particularly strong having a grating depth of 13 dB [Fig. 3].
The propagation losses of the 1st and 2nd order gratings were measured by taking the ratio of transmitted power between two Hi1060 fibers with and without the WBGs in between and then subtracting the measured MFD mismatch losses from each facet. Index matching oil was used between coupling fibers and the sample to remove Fresnel reflections. The 1st order gratings exhibit propagation losses ranging from 1.6 to 4.1 dB/cm while the loss of the 2nd order grating was 1.1 dB/cm. Propagation loss has the effect of reducing the actual reflectivity of a WBG. Hence propagation loss together with the penetration depth of the laser field into the grating must also be taken into account when calculating the reflectivity of the WBG [22,23]. For the 1st order WBG used (26.5 dB, 5.3 mm long, 0.7 mm penetration depth) the peak reflectivity is 88%, while for the longer 2nd order grating (13 dB, 12.3 mm long, 3 mm penetration depth) the peak reflectivity is 71%.
While minimizing propagation losses are generally considered key to making high quality gratings, more substantial benefits are made in the integrated platform described here by minimizing the mode mismatch between such gratings and the Yb:YAG waveguide laser. For the 1st order WBGs this was achieved by appropriately decreasing the writing pulse energy, which in turn decreased physical structure sizes and increased the approximately circular MFDs. The lowest mode mismatch to the Yb:YAG laser mode occurred with the WBG written with 950 nJ. For the 2nd order WBG, approximate mode matching was achieved by writing a small waveguide taper section leading into the grating. In this case by varying the writing pulse energy from 850 nJ to 1100 nJ a MFD taper of 15 µm to 9 µm was obtained. The 9 µm mode matched the MFD of the WBG section and the 15 µm mode matched the averaged Yb:YAG laser mode of 13 × 17 µm. Utilizing these waveguide tapers not only resulted in a smaller mode mismatch loss but also meant the grating reflectivity remained strong. This can be seen by calculating the grating’s effective reflectivity which is the peak reflectivity taking into account MFD mismatch losses. The 1st order and 2nd order WBGs employed for laser experiments had an effective reflectivity of 63% and 62% respectively.
4. Hybrid integration of the Yb:YAG waveguide laser with AF45 WBGs
The Yb:YAG waveguide laser was integrated with the laser written grating structures by replacing the HR mirror shown in Fig. 1(a) with the AF45 WBG glass sample [Fig. 1(b)]. Independent control of both the pump alignment to the Yb:YAG crystal, and the Yb:YAG waveguide to the WBG structures was possible. Index matching oil or gel was not used as it was found to ‘burn’ with high pump powers. Initial experiments were performed with the 1st order WBGs as these had the highest peak reflectivity. All gratings had sufficient reflectivity to achieve uni-directional lasing with the results shown in Fig. 4(a).
Fig. 4 Hybrid integration of an Yb:YAG waveguide laser with 1st order waveguide Bragg grating high reflectors. (a) Uni-directional laser output with respect to pump power and (b) a typical optimized emission spectrum.
As expected the lowest lasing threshold of 94 mW absorbed pump power was achieved when the MFD of a WBG best matched that of the laser mode in the Yb:YAG crystal (14.8 µm c.f. 13 × 17 µm) resulting in the highest effective mirror reflectivity. As mentioned above, this was achieved with a writing pulse energy of 950 nJ. For this laser the slope efficiency was 47% and a maximum output power of 22 mW was achieved. The emission spectrum of the optimized output is shown in Fig. 4(b). The laser bandwidth was 130 pm and contained typically 4 modes, corresponding to the longitudinal laser modes of the resonator. It is interesting to note that the longitudinal mode spacing remained close to 34 pm as the strong 1st order WBG resulted in a shallow penetration depth and hence little change to the overall cavity length. Due to the low finesse of the Fabry-Perot etalon formed by the gap between the Yb:YAG and AF45 glass end-facets this mode selection cannot result from such an etalon effect.
In order to select only a single longitudinal mode clearly a narrower bandwidth grating is required. Narrow bandwidth gratings can be achieved by reducing the coupling coefficient, κ, and increasing the grating length, L [21]. For a singly resonant laser structure however, a long overall cavity length means smaller inter-mode spacing and so any advantages of the narrower grating may be lost. In principle the shortest overall length of gain medium and grating would be best. Using the ULI technique, narrow bandwidth gratings can be realised by writing high duty cycle, higher order gratings (to reduce κ) with a longer length.
To realise true single longitudinal mode lasing the Yb:YAG waveguide laser was integrated with the 2nd order WBG fabricated in the AF45 glass sample (a 3 mm waveguide taper before the WBG was used to mode match the crystal’s laser mode). Lasing was achieved at a threshold absorbed pump power of 86 mW. At maximum absorbed pump power the uni-directional output power amounted to 23 mW and the slope efficiency was 42%. This is an encouraging result as the threshold was actually lower than in the case of the experiments using the 1st order WBG described earlier. This indicates that the overall reflectivity of the 2nd order WBG was comparable to that of the 1st order WBG despite the grating strength being less than half. This is due to the grating’s effective reflectivity depending on the grating strength, the MFD mismatch, and propagation losses over the grating’s penetration depth. In the case of the 2nd order WBG with a tapered waveguide coupling section, the MFD mismatch was 0.4 dB compared to 0.7 dB resulting in an effective reflectivity of 62%, compared to 63% for the 1st order WBG respectively. The emission spectra of the 2nd order WBG coupled to the Yb:YAG waveguide laser is shown in Fig. 5. The laser operated primarily on a single line with an instrument limited linewidth of 14 pm FWHM, however with small changes to the alignment a 2nd mode would appear. To ultimately ensure stable single longitudinal mode operation the longitudinal mode spacing has to be greater than the coupled grating’s bandwidth. In fibre lasers this can be difficult due to the low gain/cm and hence long fibre lengths, however in the crystalline waveguide laser systems such as that used here very short gain sections should be possible (~2 mm) by pumping at the absorption peak near 940 nm or 969 nm. Work into gain length optimization and monolithic integration of WBGs is ongoing.
Fig. 5 Single longitudinal mode laser emission (at maximum output power of 23 mW) from the hybrid integration of an Yb:YAG waveguide laser with a 2nd order waveguide Bragg grating high reflector.
5. Conclusion
Table 2 summarizes our results on Yb:YAG and hybrid Yb:YAG/WBG waveguide lasers. By coupling highly reflective femtosecond laser written waveguide Bragg gratings to the Yb:YAG active medium we have been able to narrow the broadband emission of an Yb:YAG waveguide laser from 500 pm to 14 pm and achieve uni-directional output. A rough estimation of the power spectral density shows that we could increase this value from 0.1 mW/pm by more than an order of magnitude to 1.64 mW/pm with the hybrid device. Lasing was achieved primarily in a single longitudinal mode when implementing a 2nd order WBG. Effective grating reflectivities were improved by tapering the mode field diameter of the WBG to better match that of the Yb:YAG waveguide laser, an advantage of using the ultrafast laser inscription technique. Scaling to higher output powers is expected as both the Yb:YAG waveguide and AF45 WBGs are insensitive to typical lasing temperatures.
Table 2. Summary of Yb:YAG and hybrid Yb:YAG/WBG waveguide lasers
Acknowledgments
This research was conducted with the Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems (project number CE110001018) and the assistance of the LIEF programs. This work was performed in-part at the OptoFab node of the Australian National Fabrication Facility, utilising NCRIS and NSW state government funding. Moreover, we acknowledge funding by the Deutsche Forschungsgemeinschaft in the framework of the project CA 1380/1-1 and the excellence cluster 'The Hamburg Centre for Ultrafast Imaging - Structure, Dynamics and Control of Matter at the Atomic Scale'. S. Gross acknowledges funding from a Macquarie University Research Fellowship.
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