We present highly efficient high power single longitudinal mode waveguide lasers. Single mode operation was achieved by combining a tapered waveguide Bragg grating with a Yb:YAG crystalline waveguide laser in a hybrid approach. Both structures were fabricated by fs-laser writing. We achieved 1.59 W of output power in a single longitudinal mode and 4.71 W of output power with a spectral bandwidth of 38 pm. The slope efficiency was 66% and the laser threshold 92 mW.
© 2017 Optical Society of America
Efficient high power compact single longitudinal mode lasers are unique tools for various applications requiring narrow band emission like spectroscopy, optical communication or range finding. However, lasers based on homogenously broadened gain media, such as rare earth doped crystalline laser materials, tend to oscillate on several longitudinal modes due to spatial hole burning . Spatial hole burning can be prevented in unidirectional ring oscillators . To obtain single longitudinal mode (SLM) operation in linear cavities the cavity length has to be decreased such that the longitudinal mode spacing is larger than the gain bandwidth. This is typically obtained in a microchip laser configuration . However, since Yb3+ doped materials exhibit a broad gain bandwidth, cavity lengths as short as a few hundred µm are necessary for SLM operation . Hence, also a very thin gain medium is required leading to reduced absorption efficiencies. For longer linear cavities an external form of frequency control is required (e.g. an etalon), increasing the complexity of the system.
Waveguide lasers are excellent alternatives for realizing very compact laser systems with low laser thresholds and only few mm cavity lengths. By implementing narrow band reflectors such as distributed Bragg reflectors (DBR) or distributed feedback (DFB) structures , SLM operation can be achieved.
Ultrafast laser inscription (ULI) is a proven technique used to fabricate waveguide lasers and Bragg grating structures in a wide range of materials including glasses [6–8] as well as crystalline laser gain media [9, 10]. To date, the output power of ULI waveguide lasers utilizing Bragg gratings is limited to the 100 mW range . SLM operation of such devices was either achieved with a hybrid approach [11, 12], in which the Bragg reflector is externally coupled to the waveguide laser, or with monolithic DFB devices . In this paper, to realize waveguide lasers with high output powers in SLM operation we coupled an external WBG to an Yb:YAG waveguide in a hybrid approach. This compact hybrid waveguide laser resulted in nearly two orders of magnitude increase in output power compared to our previous report . In particular, we demonstrate a SLM output power of 1.59 W.
2. Experimental setups
2.1 Waveguide and WBG fabrication
The waveguides as well as the WBGs were fabricated by the ULI technique. The Yb3+:Y3Al5O12 (Yb:YAG) waveguides were based on a stress induced refractive index change created by inscribing a parallel double track structure. The structure was fabricated by translating the sample perpendicular to the incident fs-laser beam. In comparison to a simple linear translation, a higher refractive index change and hence a stronger confinement of pump and laser modes can be achieved by superimposing the linear translation with a sine oscillation [9, 14]. Here an oscillation frequency of 70 Hz and amplitude of 3.5 µm were used for each of the tracks separated by 28 µm. The Yb:YAG chip exhibited a doping concentration of 7 at.% and was 6.7 mm long, resulting in a free-running longitudinal mode spacing of 43 pm (12.3 GHz).
First order WBGs at 1029.8 nm were fabricated in aluminoborosilicate glass (Schott AF45) with a mark-space ratio of 90%. The 12 mm long WBGs were preceded by a 3 mm waveguide taper to mode match to the Yb:YAG waveguide mode. The WBGs had a strength of 15 dB (coupling coefficient κ ≈200 m−1) resulting in a maximum reflectivity of 96%. Further details pertaining to Yb:YAG waveguide and WBG fabrication can be found in [6, 9, 11].
2.2 Laser setup
The laser setup is depicted in Fig. 1. An optically pumped semiconductor laser (OPS) was utilized as pump source. This device delivers up to 9 W of continuous wave output power at the pump wavelength of 969 nm with an M2 factor below 2.
The waveguide laser performance was investigated in three different configurations. The small volume of the active waveguide allows for very high inversion levels even at moderate pump powers, resulting in a high gain. Thus, lasing can be achieved without external mirrors by utilizing the Fresnel reflection at the end-facets of the waveguide. This configuration will be indicated as bare waveguide in the following. In this case the laser emits to both ends of the crystal. In the second configuration a highly reflective mirror (HR) is placed at the far side of the crystal leading to unidirectional laser output and lower total output coupling transmission. In the third configuration the HR mirror is replaced by the WBG, forcing single longitudinal mode emission. A dichroic mirror is used to separate pump and laser light at the input coupling side. Index matching fluid was not applied between the Yb:YAG chip and the HR mirror or the WBG chip, respectively. The HR mirror and the WBG were mounted on a piezo ring actuator and fixed on a mirror mount. Hence, a precise control of the air gap thickness between reflector and waveguide end-facet was possible.
For all input-output characteristics the pump power is defined as the measured power in front of the waveguide corrected by the Fresnel reflection losses at the incoupling facet. It is worth mentioning that we did not consider the coupling efficiency (typically >90%) and the fraction of absorbed pump power (typically >90%) for further corrections. In case of the bare waveguide the total output power (i.e. the added output power from both sides) is considered.
The emission spectrum is measured with an optical spectrum analyzer (OSA, Yokogawa AQ6370C) with a resolution of 34 pm at 1030 nm. Additionally, a plane-parallel scanning Fabry-Perot cavity with a free spectral range (FSR) of >25 GHz was used to analyze the longitudinal mode spectrum of the waveguide laser.
3. Results and discussions
Figure 2 shows the input-output power characteristics for the three different configurations. The bare waveguide exhibits the highest slope efficiency of 75% and a maximum output power of 5.21 W at a pump power of 7.24 W. Due to the low laser threshold of 153 mW and high slope efficiency a high optical-to-optical efficiency of 72% is achieved. The passive Yb:YAG waveguide allows for guiding of 2 – 3 transverse modes at the pump and laser wavelength. However, in case of waveguide laser operation only the fundamental mode is running due to the lower losses of this mode in comparison to higher order modes. For the configuration with a HR mirror the slope efficiency is only slightly reduced to 72% due to the somewhat lower resonator extraction efficiency. However, the laser threshold is reduced by a factor of about two in this case. This is a result of the lower total output-coupling transmission of 91.6%, corresponding to 2.48 logarithmic output-coupling losses, which are half of the logarithmic output-coupling losses of the bare waveguide. Since the pump source delivered a higher maximum pump power of 7.57 W during this experiment a slightly larger maximum output power of 5.31 W could be achieved.
We then investigated the laser performance utilizing the WBG instead of the HR mirror. The slope efficiency of the hybrid Yb:YAG/WBG waveguide laser was found to be 66% and the laser threshold 92 mW. The reduction in slope efficiency and increase in threshold in comparison to the configuration with the HR mirror is due to increased losses primarily resulting from the residual mode mismatch between the Yb:YAG waveguide and the tapered WBG. Under the assumption of a linear relation between laser threshold and logarithmic cavity losses the additional losses resulting from the insertion of the WBG can be estimated. This estimation leads to additional logarithmic losses between 0.50 and 0.64, corresponding to 39% - 48% loss, which is in good agreement with the expected effective reflectivity of the WBG .
For pump powers of up to 2.45 W the hybrid waveguide laser operated on a SLM. In comparison to the results previously achieved with a similar setup under single mode diode pumping , the efficiency as well as the output power of the system were strongly improved in particular due to a higher absorption efficiency of the employed pump source and more available pump power. Here the output power in SLM operation was increased by nearly two orders of magnitude to 1.59 W. Furthermore, this is an order of magnitude higher than previously reported for optically pumped SLM waveguide lasers. For pump powers exceeding 2.45 W the waveguide laser oscillated on 2 longitudinal modes.
Figure 3(a) shows the laser spectrum at maximum output power for the HR configuration (blue curve) and the bare waveguide (black curve), respectively. In both cases 19 longitudinal modes are oscillating resulting in a spectral width of more than 530 pm (FWHM). The number of longitudinal modes is reduced to 6 close to laser threshold but rapidly increases for higher pump powers. Due to the limited spectral resolution of the OSA, the longitudinal modes could not be fully resolved. However, from the peak to peak distance in the laser spectrum a mode spacing of 43 pm was determined. This is in excellent agreement with the expected mode spacing of a 6.7 mm Fabry-Perot laser cavity considering the refractive index of 1.815 of YAG at the laser wavelength. The central wavelength of 1030.39 nm of the bare waveguide laser is slightly red shifted by 0.21 nm in comparison to the HR configuration with a wavelength of 1030.18 nm. An inverse trend would be expected due to the higher inversion in the bare waveguide laser, but the air gap between waveguide end-facet and mirror counterbalanced this effect. By changing the air gap thickness the total wavelength dependent transmission characteristics of the mirror system consisting of the Yb:YAG end-facet, the air gap and the dielectric mirror, can be changed . Here the total transmission favored a slightly shorter wavelength. Additionally, a shift of the peak emission wavelength of approximately 90 pm is measured from 450 mW pump power to maximum pump power in both cases. This shift results from the temperature dependent emission cross sections of Yb:YAG, which exhibit a broadening and red shift for higher temperatures .
Figure 3(b) shows examples of laser spectra of the hybrid Yb:YAG/WBG waveguide laser for output powers of approximately 1 mW (close to laser threshold), 1.59 W, and 4.71 W, corresponding to 0.09 W, 2.45 W, and 7.23 W of pump power. A strong decrease in spectral width is achieved in comparison to the spectra in Fig. 3(a). For pump powers below 2.45 W SLM operation could be achieved when the WBG and Yb:YAG end-facets were in direct contact. For output powers of up to 1.59 W the spectral width of the laser output was determined to be below 34 pm (FWHM), limited by the spectral resolution of the OSA at this wavelength. Given the mode separation of more than 40 pm for the bare waveguide, this is a first indication for SLM operation. At higher power levels the spectral width increased with an additional side lobe indicating multi longitudinal mode operation. To minimize the spectral width for high pump powers a precise control of the air gap thickness with the piezo controlled mirror mount was necessary. Also in this case the peak emission wavelength exhibited a spectral red shift of 78 pm. This shift is due to thermal loading causing an increase in sample temperature and hence WBG reflection wavelength during the pumping process. In fs-laser written WBGs in glass this shift is in the order of 10 pm/°C . It should be noted that the SLM laser emission peaks at the shortest wavelength of the free running laser, which may compromise the laser efficiency.
Due to the 3 mm taper section as well as the penetration depth into the WBG the effective cavity length of the hybrid Yb:YAG/WBG is larger than 6.7 mm. This might result in a mode spacing below 30 pm, which is below the resolution limit of the OSA.
To confirm SLM operation the laser was thus analyzed with a scanning Fabry-Perot spectrometer. In Fig. 4 the Fabry-Perot spectra for output powers of 1.59 W and 4.71 W are depicted in green and orange, respectively. While at 4.71 W an additional peak within the FSR of the spectrometer indicates the presence of a second longitudinal mode, for an output power of 1.59 W SLM operation is confirmed by the side peak free green curve.
4. Summary and outlook
In summary we demonstrated a compact SLM Yb:YAG waveguide laser with 1.59 W output power and a slope efficiency of 66%. This output power level exceeds the output powers of previously reported SLM optically pumped waveguide lasers by more than an order of magnitude .
Higher output powers of 4.71 W were achieved with the oscillation of two longitudinal modes. We expect that SLM operation is possible even at this output power level with our hybrid approach by a further refinement of the grating parameters, an improved effective reflectivity by adapting the mode field diameters of the tapered WBG section to the Yb:YAG waveguide mode, as well as using shorter Yb:YAG waveguides. The direct inscription of DFB or DBR reflectors into Yb:YAG is also attractive to realizing a monolithic SLM waveguide laser. Recently, we demonstrated watt level SLM operation in a fs-laser inscribed Yb:YAG DFB waveguide laser .
(Deutsche Forschungsgemeinschaft) (CA 1380/1-1, EXC 1074); Australian National Fabrication Facility (OptoFab Node); Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems (CE110001018); Macquarie University’s outbound staff exchange program and the German Academic Exchange Service (DAAD) (57172988).
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