Researchers have demonstrated continuous-wave laser operation in a terbium-doped fluorozirconate glass fiber, generating coherent green light at a wavelength of 542 nanometers under optical pumping at 486 nanometers. Using a 4-meter length of double-clad ZBLAN fiber, the experimental system produced a maximum output power of 6.2 milliwatts with a slope efficiency of 1.2 percent relative to the launched pump power. When evaluated without taking into account the transmitted pump light, the slope efficiency reached 4.1 percent, and the efficiency relative to the actual absorbed pump power was estimated to match levels previously seen only in crystalline fluoride host materials. The demonstration provides a substantial experimental improvement over earlier terbium fiber laser systems and confirms the viability of fluoride glass fibers for visible-spectrum laser architectures.

Trivalent terbium ions offer a direct pathway to yellow and green optical transitions originating from the 5D4 upper laser level. However, utilizing terbium in solid-state systems has historically been constrained by weak resonant ground-state absorption at cyan-blue wavelengths near 488 nanometers, caused by the spin-forbidden 7F6 to 5D4 transition that exhibits absorption cross-sections on the order of 10 to the power of negative 22 square centimeters. In conventional bulk solid-state lasers, overcoming this weak absorption requires high doping concentrations and long crystal geometries, which severely impairs overlap with the low-brightness beams produced by commercial pump diodes. Fiber geometry circumvents this limitation by maintaining modal overlap between the inverted gain volume and the propagating laser mode across extended interaction lengths.

ZBLAN features a broad transparency window extending beyond silica, low maximum phonon energies that suppress non-radiative multiphonon decay, and a weak crystal field that reduces interconfigurational 4f-to-5d excited-state absorption. In addition, ZBLAN provides high rare-earth solubility and prevents ion clustering, enabling high-power visible laser operation without concentration quenching. Although the first continuous-wave terbium laser reported by Yamashita and colleagues operated in a fluoride fiber, that historic device yielded only 1.6 milliwatts of 542.8-nanometer output at a slope efficiency below 1 percent from 257 milliwatts of launched argon-ion pump power.

The fiber utilized in the present investigation was a step-index, double-clad ZBLAN structure supplied by FiberLabs Inc., incorporating an active core doped with 33,700 parts per million, or 3 weight percent, of terbium trifluoride. The concentric core measured 3.2 micrometers in diameter with a numerical aperture of 0.17, yielding a single-mode cut-off wavelength of approximately 1.4 micrometers and a normalized frequency V-parameter of roughly 6 at the 545-nanometer target emission band, making the core multimode at visible wavelengths. Surrounding the core was a circular inner cladding 123 micrometers in diameter with a numerical aperture of 0.5, composed of HBLAN wherein hafnium tetrafluoride replaces zirconium tetrafluoride to depress the refractive index. The outer cladding diameter was 200 micrometers, and the refractive index contrast between core and inner cladding was on the order of 0.01 at 633 nanometers, with nominal passive background losses specified by the manufacturer at 50 decibels per kilometer at 700 nanometers.

Spectroscopic characterization was performed by launching 15 milliwatts of 486-nanometer radiation from a frequency-doubled optically pumped semiconductor disk laser into the fiber core using an 8-millimeter focal length lens, focusing the 1.8-millimeter pump beam to a 2.8-micrometer waist. The resulting fluorescence spectrum was measured with an optical spectrum analyzer and converted into relative stimulated emission cross-sections via the Füchtbauer-Ladenburg relation by multiplying the intensity by the fifth power of the wavelength. Calibrating the peak value against prior Judd-Ofelt calculations yielded an emission cross-section of 0.73 times 10 to the power of negative 21 square centimeters at 542 nanometers. This compares to a higher peak cross-section of 1.91 times 10 to the power of negative 21 square centimeters in terbium-doped lithium yttrium fluoride crystals, an offset attributed to glass disorder and inhomogeneous line broadening rather than radiative lifetime disparities, as ZBLAN exhibits a calculated 3.92-millisecond radiative lifetime comparable to the 5-millisecond lifetime in YLF.

To evaluate pump propagation, 486-nanometer light was focused into the fiber cladding across sample lengths of 0.7 meters and 4.0 meters. Transmittance reached 92.5 percent across the 0.7-meter fiber and 69.1 percent across the 4.0-meter fiber, corresponding to a pump coupling efficiency of 98.4 percent and a pump attenuation coefficient of 0.0884 inverse meters. Assuming a homogeneous cladding-pump distribution and literature absorption cross-sections between 1.03 and 1.42 times 10 to the power of negative 22 square centimeters, standard geometric absorption modeling predicted an absorption efficiency of only 1.1 to 1.5 percent over 4 meters. However, reconciling that model with the measured 1.2 percent incident slope efficiency would demand an absorbed slope efficiency exceeding 80 percent, indicating that the true absorption efficiency had to be higher.

Independent transmission measurements conducted with a broadband white-light supercontinuum laser source clarified this absorption discrepancy. Comparing transmission values at spectral positions on and off the 486-nanometer absorption band yielded an estimated pump absorption efficiency of approximately 3.1 percent for the 4-meter fiber, corresponding to a realistic slope efficiency of 39 percent against absorbed pump power. The substantial pump transmission observed experimentally ruled out pure core confinement, indicating that pump light coupled into the central core partially leaked into the inner cladding, establishing an intermediate regime between pure core pumping and cladding pumping. Accounting for manufacturer baseline losses of 0.05 decibels per meter at 700 nanometers and Rayleigh scattering scaling inversely with the fourth power of wavelength, the actual absorption efficiency was bounded between 2 percent and 28 percent.

Laser cavity trials were arranged by butt-coupling a plane dielectric pump mirror with greater than 99 percent transmission at 486 nanometers and high reflectivity between 530 and 600 nanometers against the fiber input facet, while testing various flat output couplers at the distal facet. In the 0.7-meter fiber section, laser oscillation could not be initiated even when using a highly reflective mirror at the output. In contrast, the 4.0-meter fiber successfully attained continuous-wave laser operation at 542 nanometers for output coupler transmissions reaching as high as 78 percent. Photographic monitoring during operation revealed visible blue emission along the fiber length, confirming Rayleigh and pump scattering, though no photodarkening or power degradation was observed during the experimental tests.

Spectral measurements across variable pump powers documented clear amplified spontaneous emission and line narrowing before passing the laser threshold. When paired with an output coupler having 4 percent transmission in the green region, the laser exhibited its lowest threshold at 550 milliwatts of incident pump power. The higher threshold compared to early single-clad fiber experiments was attributed to the double-clad geometry, which dilutes pump absorption per unit length when pump energy travels through the cladding. Operating with an output coupler transmission of 14 percent yielded the optimum output power of 6.2 milliwatts at 1.2 watts of launched pump power, maintaining a slope efficiency of 1.2 percent. Changing the alignment and feedback conditions permitted spectral tuning of the output between 541 and 546 nanometers, whereas raising the output coupling transmission to 78 percent elevated the threshold to 1.1 watts and limited output power to 0.2 milliwatts.

Attempts to achieve laser emission on other terbium transitions, specifically the yellow transition at 585 nanometers and the red transition at 620 nanometers, yielded no observable laser oscillation despite employing cavity mirrors with high reflectivity at those wavelengths and high transmission at 545 nanometers. Analysis revealed that passive background losses in the ZBLAN fiber exceeded 0.3 decibels per meter, substantially exceeding the pump absorption itself.

Subject of Research: Continuous-wave green laser emission in terbium-doped ZBLAN fluoride fiber

Article Title: Green Tb3+:ZBLAN fiber laser

Article References: Badtke, M., Kalusniak, S., Hahn, O., Tanaka, H., & Kränkel, C. (2026). Green Tb3+:ZBLAN fiber laser. Applied Physics B, 132(8), Article 92. https://doi.org/10.1007/s00340-026-08703-3

Image Credits: AI Generated

DOI: 10.1007/s00340-026-08703-3

Keywords: fiber laser, terbium, ZBLAN glass, fluoride fiber, visible laser, green laser, spectroscopy, optical pumping, Green, ZBLAN, fiber, laser