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Single-frequency fiber lasers using rare-earth-doped silica

Single-frequency fiber lasers using rare-earth-doped silica

Commercial silica fibers doped with ytterbium and erbium achieve sufficient gain to enable fabrication of distributed Bragg reflector single-frequency fiber lasers.
25 June 2015 Wei Shi
A single-frequency laser operates with only one longitudinal mode, and thus can emit quasi-monochromatic radiation with a very narrow linewidth and low noise. Such lasers, both in continuous waveform and pulsed operations, are of interest for interferometric sensing, coherent light detection and ranging, laser spectroscopy, and laser nonlinear frequency conversions.
The distributed Bragg reflector (DBR) fiber laser is based on a short cavity combined with a narrowband fiber Bragg grating (or FBG, an optical fiber that reflects particular wavelengths of light and transmits the rest: see Figure 1). We can fabricate the cavity by cleaving the high-reflection (HR) and partial reflection (PR) FBGs very close to the index modulation region and directly splicing to a centimeter's length a glass fiber highly doped with rare earth elements. This laser provides robust single-frequency operation with watt-level output power1, 2 and at sub-kilohertz linewidth.3 Its most significant component is the rare-earth-doped glass fiber, which enables high gain (usually several dB/cm) per unit length. Developers have used rare-earth-doped soft glass fiber, including phosphate glass1, 2,4 and germanate glass fibers5–7 for DBR single-frequency fiber lasers operating in the 1, 1.5, and 2μm spectral regions. However, fusion splicing between these soft glass active fibers with the passive silica fiber is challenging, and is usually undertaken only by specially trained and experienced technicians using expensive and complex fiber fusion splicers.

Figure 1. The schematic of a distributed Bragg reflector (DBR) single-frequency fiber laser. HR/PR FBG: High-reflection/partial-reflection fiber Bragg grating.
By contrast, splicing rare-earth-doped silica fibers is relatively easy, with ultra-low loss (usually <0.02dB). Since the 1990s, developers have used active silica fiber to build single-frequency fiber lasers.8 However, the low pump absorption limited the output power to hundreds of microwatts. With the development of glass materials and fibers, some commercial rare-earth-doped silica fibers enabled very efficient absorption for the pump, producing very high gain per unit length.
Recently, we developed DBR single-frequency fiber lasers with tens of milliwatts output power using a commercial rare-earth-doped silica fiber.9 Figure 2 shows the spectrum and the output power under different pump levels for the DBR single-frequency laser operating at 978nm. We pumped a 2cm-long silica fiber highly doped with ytterbium (Yb3+) by a single-mode 915nm laser diode to provide gain in the cavity. The center wavelength of this laser is ∼978.01nm, and we achieved a >45dB signal-to-noise ratio. We verified the single-frequency performance of the laser using a fiber-based Fabry-Perot (F-P) interferometer with a free spectral range of ∼2GHz and finesse of ∼157: see Figure 2(a). We achieved more than 25mW output when the launched pump power was ∼150mW: see Figure 2(b).

Figure 2. (a) Spectrum of the 978nm DBR single-frequency fiber laser. Left inset: Fabry-Perot (F-P) interference pattern. Right inset: Zoom-in spectrum of the laser measured using an optical spectrum analyzer with 0.02nm resolution. (b) Output laser power of the 978nm DBR single-frequency fiber laser vs. the launched pump power in milliwatts (mW). dBm: Decibel-milliwatts.
We have also developed a DBR single-frequency fiber laser operating in the 1.5μm region (see Figure 3). We fabricated the laser cavity by cleaving the HR (>99.9%) and the PR (=77.3%) FBGs very close to the index modulation region and directly splicing to a 2cm-length erbium/Yb co-doped silica fiber. This laser produced ∼10mW output power.

Figure 3. Spectrum of the 1550nm DBR single-frequency fiber laser.
In summary, we have shown how state-of-the-art commercial rare-earth-doped silica fibers can provide enough gain per length to facilitate fabrication of monolithic, robust DBR single frequency fiber lasers. Our next steps will be to commercialize these lasers for applications in light detecting and ranging and remote sensing.
This work was supported by the National Natural Science Foundation of China (61335013) and the National High Technology Research and Development Program (2014AA041901).
Wei Shi
Tianjin University
Tianjin, China