34 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig Characterization of a Fluoride Fiber Laser with 3.9 m Emission Wavelength Jutta Schneider, Udo B. Unrau A ﬁber laser operating at 3.9 in the attenuation minimum of the 3 5 atmospheric window has been realized for the ﬁrst time. The Ho 3 -doped ﬂuoride ﬁber was pumped around 640 nm or 890 nm. More than 10 mW CW output power were measured while pumping at 890 nm. 1 Introduction Fluoride glasses such as ZBLA are the most promising candidates for mid-infrared ﬁber laser hosts above 2 m. Signiﬁcant properties are their low phonon energies and infrared transparency. The attenuation minimum of the atmosphere between 3.9 and 4.2 m offers numerous applications for light emitting devices. Mid-infrared ﬁber lasers have so far been realized up to 3.5 m only. CW laser operation in an Er3 - doped ZBLAN ﬁber at 3.45 m using the transition 4 F9 2 4I 9 2 was ﬁrst demonstrated in 1992 by . A superﬂuorescent ﬁber source at 3.9 m in Ho 3 was published in January 1995, . 2 Results and Discussion Several mechanisms within the Ho3 -ion had to be considered in order to enhance the properties of the transition 5 I5 5 I and to enable laser oscillation around 3.9 m. The pump wavelength around 6 640 nm was chosen because Ho3 has a strong absorption in this pump range, and a DCM dye laser was available as pump source. A laser cascade 3.9/1.2 m was chosen to deplete the lower laser level 5 I6 of the transition around 3.9 m to the ground state 5 I8 . A ﬂuorozirconate ﬁber fabricated by Le Verre was used for the experiments. The ﬁber was doped with 2000 ppm Ho3 . Fluorescence measurements were possible at room temperature with an InAs ﬁlter which was placed in front of the monochromator. These measurements led to a broad spectrum from 3.9 up to 4.1 m due to the poor spectral resolution of the monochromator which was necessary to obtain the required intensity. Further results were obtained using a Fabry-Perot resonator with an input mirror of high reﬂectivity at 3.9 m and 40% reﬂectivity at 1.2 m. The output coupler had a reﬂectivity of 96% for both laser wavelengths. During the laser measurements the ﬁber was cooled with liquid nitrogen. Spectral measurements of the ﬁber laser are presented in Fig. 1. Two mirrors were used to enable laser action. Below the laser threshold (lower curve) at approximately 310 mW launched pump power, the emission spectrum is a broad ﬂuorescence spectrum. Laser emission at 3.95 m sligthly above the threshold at 360 mW launched pump power is depicted in the upper curve. With increased pump power the spectrum is narrowed. Both spectra were measured using a spectral resolution of = 16 nm. The maximum output power for this pump range was 5 mW at 640 nm. Further measurements were made with pump wavelengths around 890 nm. This pump range enables a direct excitation of the upper laser level 5 I5 of the 3.9 m laser. The laser characteristic is shown in Fig. 2. The maximum CW laser output power was approximately 11 mW at 900 mW launched pump power (estimated launching efﬁciency: 50%). The laser at 3.95 m Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig 35 Fig. 1: Laser characteristics of ﬂuorescence and laser operation at 3.9 m using a pump wavelength around 640 nm Fig. 2: Laser characteristics of the laser at 3.9 m using a pump wavelength around 890 nm 36 Annual report 1995, Institut Hochfrequenztechnik, TU Braunschweig operates in cascade with simultaneous laser emission around 1.2 m. Pumping around 890 nm, the CW laser at 1.2 m had an output power of more than 70 mW. The output of laser at 3.9 m decreases with increasing temperature. Laser oscillation were measured up to 150 K, but the highest output powers were achieved at 77 K. References  H. "Room Temperature CW Fibre Laser at 3.5 m in Er3 –Doped ZBLAN Glass", Electron. Lett. 28, p. 1361–1362 (1992).  J. Schneider, "Superﬂuorescent ﬁber source at 3.9 m in the attenuation minimum of the atmospheric window 3–5 m", Int. J. of Infrared and Millimeterwaves 16 (1), p. 75/82 (1995).