We report visible continuous-wave laser emission at 636 nm from a praseodymium-doped fluorozirconate glass guided-wave chip laser. This ultra-fast laser inscribed gain chip is demonstrated to be a compact and integrated laser module. The laser module, pumped by 442 nm GaN laser diodes, generates lasing output with a beam quality of . To the best of our knowledge, this is the first visible laser emission from a glass-based waveguide chip laser.
© 2017 Optical Society of America
The application space for visible lasers is evolving rapidly due to their use in various application fields, such as full color laser displays [1,2], laser imaging [3,4], on-chip wavelength-division multiplexing [5,6], and biological and chemical sensing [7,8]. Although frequency doubling is a common method [9,10] to achieve visible laser emission, diode-based laser technology  is progressively exploring new avenues to generate direct laser emission across the entire visible spectrum, with blue (InGaN) and red (AlGaInP) laser diodes being the most mature technology  to date.
Visible diode lasers are currently the preferred choice for continuous-wave applications due to their low-cost and ready availability, albeit with astigmatic and non-circular beam profiles. Commonly available infrared (IR) solid-state -switched lasers, such as (0.95, 1.1, and 1.3 μm), can be frequency doubled  to produce discrete wavelengths. Alternatively, tunable visible output can be produced by a more complex system based on optical parametric oscillators pumped by frequency tripled solid-state lasers. Thus, visible laser applications that require high peak power and short-pulse operation (e.g., ) are typically complex to operate with concomitant higher costs.
A less complex alternative for high beam quality visible laser emission is direct generation using the optically pumped rare-earth (RE) ion praseodymium () in a suitable host. Trivalent can generate laser emission in the red, orange, green, and blue spectral domains . Direct generation in a RE ion has the advantage of energy storage and broad gain bandwidths, thus allowing tunable, -switched, and mode-locked laser operation in the visible spectral domain [14–16].
Fluoride-based materials are preferred over other host materials, as they possess lower phonon energies and provide higher luminescent intensity without multiphoton relaxation . To date, the only visible laser emission reported is in -doped fluoride crystals [13,18,19] and fluorozirconate glass fibers . Crystalline hosts are typically operated in a more traditional solid-state resonator or microchip configuration . Fluorozirconate glasses, such as zirconium barium lithium aluminum sodium fluoride (ZBLAN), are transparent from the UV well into the mid-IR spectral domain, and a low phonon energy enhances the excited-state lifetimes . In 2012, an ion-exchanged glass waveguide was reported  that showed the amplification of laser light in the visible domain, but visible laser operation from the glass waveguide was not reported, owing to the higher round-trip losses in the ion-exchanged waveguides and lower pump power.
Recently, we have achieved efficient laser operation of waveguide-based ZBLAN chip lasers in the near IR to mid-IR spectral domain. These chip lasers are based on the ultra-fast laser inscription (ULI) of depressed-cladding waveguides into bulk RE ion-doped ZBLAN chips [24–27]. In contrast to laser emission in the IR domain, visible laser operation of is relatively challenging due to (1) a short upper-state lifetime (), (2) narrow absorption band of at [Fig. 1(B)], and (3) multiple terminal laser levels  at 479 nm (), 523 nm (), 635 nm (), and 716 nm (), as shown in Fig. 1(A).
In this Letter, we report the first, to the best of our knowledge, planar, RE-doped waveguide laser operating in the visible region, specifically a -doped guided-wave glass chip laser. This monolithic ULI waveguide laser, with a relatively large mode area (40 μm diameter mode; area ), produces visible laser emission at 636 nm with average power and a slightly multi-mode transverse profile with value of .
The glass chips are based on ULI depressed-cladding waveguides [27,29–31], which are directly inscribed in the 0.5 mol. % glass substrate  by a commercially available frequency doubled ultra-fast laser system (, pulse width , IMRA, DE0210). A bright-field microscope image of the end-face of one of the waveguides is shown in Fig. 2(B). The glass chip () contained 12 waveguides with diameters ranging from 30 to 45 μm. The waveguides were inscribed below the surface of the chip. A broadband visible anti-reflection (AR) coating was applied to the end-faces of the chip to reduce the Fresnel loss (refractive index ). This broadband AR coating has three layers, namely (first layer), (second layer), and (third and final layer), respectively. The final layer is sensitive to temperature and abrasion and showed visual evidence of degradation after the chips were diced. (Large AR coated glass “slabs” were diced into chips. Wax bonding, , was used during the dicing process, leading to coating damage.)
The experimental setup is shown in Fig. 2. The chip was pumped by two InGaAs diodes (Nichia NDB4116, , ), which were polarization combined by using a wave plate (AHWP05M-600, Thorlabs) and a polarization cube beam splitter (PBS121, Thorlabs). The pump laser diodes (PLDs) have an elliptical beam profile that was circularized by using a combination of an aspheric lens () and an anamorphic prism pair (PS879-A, Thorlabs).
The combined pump beams were coupled into the waveguide by a plano-convex lens () through an input coupler (, ). To achieve lasing operation at 636 nm, two different output couplers (OCs) were used, namely , and , . The ½ in. diameter input couplers and OCs are directly end-coupled to the chip and held in place by a UV curable adhesive (Norland Optical Adhesive 65, USA), as shown in Fig. 2(C). The laser output is collimated by using a plano-convex lens (). A long pass filter (FGL495S, Thorlabs) absorbs the residual 442 nm pump laser from the red lasing output. A near-field image of the collimated beam emitted from the waveguide laser is shown in Fig. 2(A). The collimated laser output at 636 nm is measured using a thermal absorbing power meter (PM100D, sensor S302C, Thorlabs).
As shown in Fig. 3, the lasing emission from the waveguide chip laser (waveguide diameter ) is measured for a range of incident pump powers for two different OCs, namely with transmission of 5% and 3% at 635 nm. Laser thresholds () of 55 and 52 mW were achieved for 5% and 3% OC with slope efficiencies () of 10% and 7%, respectively. The observed slope efficiencies of up to 10% at 636 nm lasing output are lower than fiber-based lasers [13,18,20], where slope efficiencies of have been achieved. We can attribute our lower lasing performance to (1) thermally damaged AR coatings ( reflection loss per surface at 633 nm), (2) relatively higher waveguide losses ( measured at 633 nm) in the laser-etched waveguides as compared to fiber-based (, ) and fluoride-crystal-based ( total internal loss, ) visible lasers, and (3) the un-optimized length of the waveguide as the gain medium . Further investigations to achieve higher lasing efficiency with improved AR coatings, different chip lengths, improved laser-etched waveguides, and higher pump powers are under consideration.
The spectrum of the lasing output, as shown in Fig. 4(A) (solid black line), is measured by an optical spectrum analyzer (OSA, Ando AQ6315E) and referenced to a HeNe laser [Fig. 4(A), red dotted line]. This free-running waveguide laser module generates 636 nm (solid line) emission with a measured spectral full width at half-maximum (FWHM) of (time averaged, characteristic of many longitudinal modes reaching threshold). For reference, the measured spectral output of a HeNe laser (dotted line) is also shown with a spectral FWHM of , which signifies the resolution of the OSA. To quantify the beam quality of the chip laser, a measurement is conducted by focusing the collimated output to a waist by a 300 mm focal length lens, and the beam profile is recorded by using a silicon CCD camera (PS904, Ophir Photonics). The second-moment beam diameter () is determined at different points before, near, and after the focus by fitting a Gaussian function to the recorded beam profiles. The recorded beam profile data is shown in Figs. 4(B) and 4(C) as a function of travel along the beam propagation direction as it goes through the focal point. Negative and positive translations represent the points before and after the focus, respectively. For validation purposes, a similar beam characterization setup is used to quantify the beam quality from a standard HeNe laser (Thorlabs, model # HNLS008R-EC). The only difference being that a 150 mm focal length lens was used in this case. Figures 4(B) and 4(C) show the beam profiles for the and the HeNe laser, respectively. To determine the value, a hyperbolic line-shape (dotted line), as per Eq. (1), was fitted to the experimentally measured data and the values for horizontal (black circle) and vertical (red hexagon) beam profiles that were calculated by using Eqs. (2) and (3):1.
This waveguide-based monolithic module provides a good alternative for varied industrial and scientific applications, including laser imaging, biomedical, and sensing applications. Recently, mode-locked pulses of were reported  for -doped crystalline material. The available gain bandwidth of up to 1.8 nm in  makes it a suitable candidate for solid-state, multi-gigahertz (GHz) repetition rate, passively mode-locked, monolithic devices. Due to the reasonable upper-state lifetime of and short cavity lengths, -switched operations with nanosecond (ns) pulse widths can also be realized. We aim to increase the lasing efficiency of this waveguide-based laser module by using improved AR coatings, longer chip lengths, and improved coupling of the PLDs into the chip by designing bespoke optics.
In conclusion, we report the first compact and monolithic waveguide chip laser that emits laser radiation at 636 nm with power and a value of . Improved laser performance may also realize laser operation of the lower gain 527 nm green transition, thus allowing the chip laser to simultaneously emit blue (un-depleted pump), green, and red laser emissions from the same waveguide.
South Australian Government Innovation Grant (IVP137).
Funding was provided by a South Australian Government Innovation Grant in collaboration with SMR Australia. T. M. Monro acknowledges the support of an ARC Georgina Sweet Laureate Fellowship. The authors also appreciate early contributions to this glass composition by Ms. Jun Shi (HUST).
1. J. J. Wierer, J. Y. Tsao, and D. S. Sizov, Laser Photon. Rev. 7, 963 (2013). [CrossRef]
2. K. V. Chellappan, E. Erden, and H. Urey, Appl. Opt. 49, F79 (2010). [CrossRef]
3. J. Zhao, H. Jiang, and J. Di, Opt. Express 16, 2514 (2008). [CrossRef]
4. A. Kotani, M. A. Witek, J. K. Osiri, H. Wang, R. Sinville, H. Pincas, F. Barany, and S. A. Soper, Anal. Methods 4, 58 (2012). [CrossRef]
5. W.-Y. Lin, C.-Y. Chen, H.-H. Lu, C.-H. Chang, Y.-P. Lin, H.-C. Lin, and H.-W. Wu, Opt. Express 20, 9919 (2012). [CrossRef]
6. Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, Opt. Express 21, 1203 (2013). [CrossRef]
7. S. K. Tang, Z. Li, A. R. Abate, J. J. Agresti, D. A. Weitz, D. Psaltis, and G. M. Whitesides, Lab Chip 9, 2767 (2009). [CrossRef]
8. M. Pascu, N. Moise, and A. Staicu, J. Mol. Struct. 598, 57 (2001). [CrossRef]
9. V. Gaebler, B. Liu, H. J. Eichler, Z. Zhang, and D. Shen, Opt. Lett. 25, 1343 (2000). [CrossRef]
10. C. Czeranowsky, E. Heumann, and G. Huber, Opt. Lett. 28, 432 (2003). [CrossRef]
11. M. A. Haase, J. Qiu, J. M. DePuydt, and H. Cheng, Appl. Phys. Lett. 59, 1272 (1991). [CrossRef]
12. R. D. Dupuis and M. R. Krames, J. Lightwave Technol. 26, 1154 (2008). [CrossRef]
13. T. Sandrock, T. Danger, E. Heumann, G. Huber, and B. H. T. Chai, Appl. Phys. B 58, 149 (1994). [CrossRef]
14. M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, Opt. Lett. 39, 6939 (2014). [CrossRef]
15. Y. Zhang, H. Yu, H. Zhang, A. Di Lieto, M. Tonelli, and J. Wang, Opt. Lett. 41, 2692 (2016). [CrossRef]
16. Y. Zhang, H. Yu, R. Zhang, G. Zhao, H. Zhang, Y. Chen, L. Mei, M. Tonelli, and J. Wang, Opt. Lett. 42, 547 (2017). [CrossRef]
17. X. Zhu and N. Peyghambarian, Adv. Optoelectron. 2010, 1 (2010). [CrossRef]
18. F. Cornacchia, A. Richter, E. Heumann, G. Huber, D. Parisi, and M. Tonelli, Opt. Express 15, 992 (2007). [CrossRef]
19. B. Xu, P. Camy, J.-L. Doualan, Z. Cai, and R. Moncorgé, Opt. Express 19, 1191 (2011). [CrossRef]
20. H. Okamoto, K. Kasuga, I. Hara, and Y. Kubota, Electron. Lett. 44, 1346 (2008). [CrossRef]
21. T. Taira, A. Mukai, Y. Nozawa, and T. Kobayashi, Opt. Lett. 16, 1955 (1991). [CrossRef]
22. V. Nazabal, M. Poulain, M. Olivier, P. Pirasteh, P. Camy, J.-L. Doualan, S. Guy, T. Djouama, A. Boutarfaia, and J.-L. Adam, J. Fluorine Chem. 134, 18 (2012). [CrossRef]
23. M. Olivier, J.-L. Doualan, P. Camy, H. Lhermite, P. Pirasteh, J. Coulon, A. Braud, J.-L. Adam, and V. Nazabal, Opt. Express 20, 25064 (2012). [CrossRef]
24. D. G. Lancaster, V. J. Stevens, V. Michaud-Belleau, S. Gross, A. Fuerbach, and T. M. Monro, Opt. Express 23, 32664 (2015). [CrossRef]
25. D. G. Lancaster, S. Gross, M. J. Withford, and T. M. Monro, Opt. Express 22, 25286 (2014). [CrossRef]
26. G. Palmer, S. Gross, A. Fuerbach, D. G. Lancaster, and M. J. Withford, Opt. Express 21, 17413 (2013). [CrossRef]
27. D. G. Lancaster, S. Gross, H. Ebendorff-Heidepriem, A. Fuerbach, M. J. Withford, and T. M. Monro, Opt. Lett. 37, 996 (2012). [CrossRef]
28. M. B. M. Panah and M. Zavvari, Opt. Quantum Electron. 48, 1 (2016). [CrossRef]
29. S. Gross, D. G. Lancaster, H. Ebendorff-Heidepriem, T. M. Monro, A. Fuerbach, and M. J. Withford, Opt. Mater. Express 3, 574 (2013). [CrossRef]
30. D. G. Lancaster, S. Gross, A. Fuerbach, H. E. Heidepriem, T. M. Monro, and M. J. Withford, Opt. Express 20, 27503 (2012). [CrossRef]
31. S. Gross, M. Ams, G. Palmer, C. T. Miese, R. J. Williams, G. D. Marshall, A. Fuerbach, M. J. Withford, D. G. Lancaster, and H. Ebendorff-Heidepriem, Int. J. Appl. Glass Sci. 3, 332 (2012). [CrossRef]
32. K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao, Appl. Phys. Lett. 71, 3329 (1997). [CrossRef]
33. H. Okamoto, K. Kasuga, and Y. Kubota, Opt. Lett. 36, 1470 (2011). [CrossRef]