Rare-Earth-Doped Soft Glass Optical Fibers for Coherent Wavelength Sources Above 2 Microns

These fiber lasers have potential uses in eye-safe LIDAR systems for defense applications.

Rare-earth-doped soft glass optical fibers were developed and characterized for fiber lasers emitting at wavelengths longer than 2 microns to allow efficient narrow line-width emission in the atmospheric window for coherent detection and LIDAR and DIAL sensing.

Optical micrograph of the Tm-Ho Optical Fiber.
The following glasses were prepared in order to fabricate a single-mode Tm-Ho doped optical fiber. Their composition is in mol%, and the rare-earth oxides were added in wt%.

  • Core glass: 75TeO2: 18ZnO: 7Na2O + 1wt% Tm2O3 + 0.4 wt% Ho2O3
  • 1st cladding glass: 73TeO2: 18ZnO: 9 Na2O
  • 2nd cladding glass: 72TeO2: 18ZnO: 10 Na2O

The minimum purity of the chemical precursors was 99+%. The onset melting temperature was 750 ̊C, and the duration of the process was two hours. The melt was cast in a brass mold preheated to 300 ̊C and annealed at Tg – 10 ̊C for two hours. Glass melting was carried out in a Pt crucible inside a chamber furnace. Core glass was melted twice; the first melting was carried out inside a glove-box in controlled atmosphere in order to minimize the OH content inside the glass. OH contamination is extremely detrimental, in particular when emission in the near infrared wavelength region is targeted. For this process, the core and 1st cladding glasses went through a second melting step that had to be carried out in laboratory atmosphere to ease the casting procedure.

The jacketing cladding tube was produced from the 2nd cladding glass by the rotational casting technique at a rotational speed of 3000 rpm. The core/clad structured rod produced by built-in-casting was stretched down to a diameter of 3 mm to fit into the jacketing tube.

The preform was drawn into fiber using a drawing tower developed in-house. The furnace consists in a graphite ring heated by an induction operating at 248 kHz and delivering 170 W to reach drawing temperature. The preform was fed into the furnace and drawn into fiber at a speed of 2.5 m/minute under a tension of 70 mN. About 150 m of fiber was manufactured.

In-line monitoring of fiber diameter determined a maximum diameter change of 3 μm in steady-state conditions. This measurement also allowed selecting the suitable fiber parts for the following experiments.

The fabricated optical fiber was first characterized by means of transmission optical microscopy in order to assess the morphology of the core and cladding shapes and features. In particular, the core-cladding interface quality is of great importance because it allows minimization of scattering centers that would result in higher attenuation loss of the overall fiber. The fiber was characterized in terms of attenuation loss at the wavelength of 980 and 1310 nm by butt-coupling a fiber pigtailed single-mode laser diode using the cut-back method. The output power of the fiber was collected with a power meter. Attenuation loss of ~2dB/m at 980 and of 1.8 dB/m at 1300 nm were measured.

The Tm-Ho tellurite optical fiber was excited in the core using a Q-photonics QFLD-795-100S single-mode fiber pigtailed laser diode emitting at 793 nm. The laser diode fiber (with a core radius of around 6 μm ) was butt-coupled to a 10-cm-long piece of Tm-Ho-doped tellurite optical fiber. The fiber was selected to be not too long to allow uniform pumping along the axis and not too short to allow for a sufficient intensity of amplified spontaneous emission. The excited fiber showed an evident green luminescence, which was ascribed to the up-conversion fluorescence process.

Fluorescence spectra in the visible wavelength region were collected using a multimode optical fiber butt-coupled to the Tm-Ho fiber end facet and then focused into a Hamamatsu photomultiplier tube through a Horiba Jobin Yvon iHR320 monochromator for wavelength selection. Luminescence spectra in the nearinfrared wavelength region were collected using a multimode optical fiber butt-coupled to the Tm-Ho fiber end facet, and then the ASE signal was focused into a single-channel PbSe photoconductive detector. Wavelength selection was carried out using a Horiba Jobin Yvon iHR320 monochromator. The laser diode used was a Q-photonics QFLD-795-100S and the spectrum was obtained with a pump power of 100 mW. Absorbed power was estimated to be around 50 mW.

This work was done by Daniel Milanese of Politecnico de Torino, Italy, for the Air Force Office of Scientific Research. AFOSR-0002

This Brief includes a Technical Support Package (TSP).
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Rare-Earth-Doped Soft Glass Optical Fibers for Coherent Wavelength Sources Above 2 Microns

(reference AFOSR-0002) is currently available for download from the TSP library.

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