Last updated on 01/19/2023
High-power, diffraction-limited light source around 800 nm has long been relied on Titanium-Sapphire laser (Ti:Sapphire laser), which provides watt-class output power with a very broad tuning range. The use of the laser is, however, limited in lab environments only, as it needs to be installed on a large anti-vibration optical table placed in temperature controlled environment.
A high-power tapered semiconductor optical amplifier, seeded by a low-power seed laser, offers small footprint and watt-class output power. The long-term stability of beam quality, however, remains an issue.
In this post, we demonstrate a Thulium (Tm) doped ZBLAN fiber amplifier system for creating a high-power single-mode light source at 810 nm. Stable, diffraction-limited beam quality is ensured by single-mode fiber design. The light amplification originates from the 3H4→3H6 energy transition of Tm ions (see TDFA) doped in ZBLAN glass. Low phonon energy of ZBLAN glass, compared to that of silica glass, enables efficient light amplification around 810 nm.
The fiber amplifiers presented in this post are available for demo upon request. Though it is a proof-of-concept model and has not been released as our official product, most functions have been implemented for evaluation. Please contact us and show your interest, if you would like to test them for your project.
We first demonstrate amplification of narrow-linewidth seed laser operating at 813 nm. This wavelength (813.4 nm) is known as the magic wavelength for Strontium. A high-power narrow-linewidth light source with diffraction-limited beam quality has been of great interest for the creation of a Strontium optical lattice clock.
The setup of our experiment is schematically shown in Fig. 1. An External-Cavity Laser Diode (ECLD), tuned to 813 nm and generating 20-mW output power, comprised a seed laser. The output power from the seed laser was amplified by a dual-stage fiber amplifier system. The light was amplified to 70 mW at the first stage, and is further amplified to 300 mW at the second stage.
Figure 1: Schematic of dual-stage amplifier
Figure 2 shows normalized output spectra out of the ECLD and each of the two amplifier stages. The figure shows that the SNR (Signal-to-Noise Ratio) is about 50 dB after amplification, surpassing that of a tapered semiconductor optical amplifier.
Figure 2: Normalized output spectra
The temporal variation of output power from the second amplifier is shown in Fig. 3. The dual-stage amplifier operated over 120 hours and the output power stability was less than 15%. The amplifier was operated in ACC mode, and thus the output power was affected by room temperature change (22 degC to 29 degC within one day). The output power stability would be improved by implementation of ALC mode.
Figure 3: Temporal variation of output power from second amplifier
We next demonstrate amplification of a small-power ASE light source exceeding 200 mW. A fiber ASE source is a versatile light source, offering broad bandwidth and inherently low Degree-of-Polarization (DOP). Such a high-power broadband light source will be useful when your application requires low coherence and polarization insensitivity.
A picture of the amplifier system is shown in Fig. 4. This system is indeed the same as the one used for the previous narrow-linewidth amplification, except that there is no seed laser. When the first fiber amplifier is operated without input signal, it generates broadband ASE centered at 805 nm (see our previous post). The second amplifier amplifies the ASE light to more than 200 mW.
Figure 4: High-power ASE light source system (dimension 88 x 440 x 350 mm each)
The output power and the spectrum of the amplified ASE light source are shown in Figs. 5 and 6. After one-hour warm-up time, the output power stability was less than ±5% for 50 hours. Since the room temperature change was much smaller than in the case of the narrow-linewidth amplification, the output power stability was more stable.
Figure 5: Temporal variation of the high power ASE light source at 805 nm
Figure 6: Output spectrum of high power 805-nm ASE light source.
Optical fiber amplifiers are very versatile platform for the creation of high-power light sources, as it can be combined with essentially any type of seed laser (e.g. narrow-linewidth, broadband, continuous-wave, pulsed…). By choosing a proper seed laser, it can be applied to various applications such as:
Currently the maximum output power from our amplifier system is 300 mW. We aim to produce more than one watt out of this platform by adding the third stage power amplifier. With an output power more than one watt, this platform can be used for a wide range of applications in biomedical and quantum science.
Furthermore, development of Polarization-Maintaining fiber amplifier is planned to pave the way for new applications that require linearly polarized output.