Narrow Linewidth Laser Technology Part Two

(3) Solid state laser

In 1960, the world's first ruby laser was a solid-state laser, characterized by a high output energy and a wider wavelength coverage. The unique spatial structure of solid-state laser makes it more flexible in the design of narrow linewidth output. At present, the main methods implemented include short cavity method, one-way ring cavity method, intracavity standard method, torsion pendulum mode cavity method, volume Bragg grating method and seed injection method.

Figure 7 shows the structure of several typical single-longitudinal mode solid-state lasers.

Figure 7(a) shows the working principle of single longitudinal mode selection based on the in-cavity FP standard, that is, the narrow linewidth transmission spectrum of the standard is used to increase the loss of other longitudinal modes, so that other longitudinal modes are filtered out in the mode competition process due to their small transmittance, so as to achieve single longitudinal mode operation. In addition, a certain range of wavelength tuning output can be obtained by controlling the Angle and temperature of the FP standard and changing the longitudinal mode interval. FIG. 7(b) and (c) show the non-planar ring oscillator (NPRO) and the torsional pendulum mode cavity method used to obtain a single longitudinal mode output. The working principle is to make the beam propagate in a single direction in the resonator, effectively eliminate the uneven spatial distribution of the number of reversed particles in the ordinary standing wave cavity, and thus avoid the influence of the spatial hole burning effect to achieve a single longitudinal mode output. The principle of bulk Bragg grating (VBG) mode selection is similar to that of semiconductor and fiber narrow line-width lasers mentioned earlier, that is, by using VBG as a filter element, based on its good spectral selectivity and Angle selectivity, the oscillator oscillates at a specific wavelength or band to achieve the role of longitudinal mode selection, as shown in Figure 7(d).
At the same time, several longitudinal mode selection methods can be combined according to needs to improve the longitudinal mode selection accuracy, further narrow the linewidth, or increase the mode competition intensity by introducing nonlinear frequency transformation and other means, and expand the output wavelength of the laser while operating in a narrow linewidth, which is difficult to do for semiconductor laser and fiber lasers.

(4) Brillouin laser

Brillouin laser is based on stimulated Brillouin scattering (SBS) effect to obtain low noise, narrow linewidth output technology, its principle is through the photon and the internal acoustic field interaction to produce a certain frequency shift of Stokes photons, and is continuously amplified within the gain bandwidth.

Figure 8 shows the level diagram of SBS conversion and the basic structure of the Brillouin laser.

Due to the low vibration frequency of the acoustic field, the Brillouin frequency shift of the material is usually only 0.1-2 cm-1, so with 1064 nm laser as the pump light, the Stokes wavelength generated is often only about 1064.01 nm, but this also means that its quantum conversion efficiency is extremely high (up to 99.99% in theory). In addition, because the Brillouin gain linewidth of the medium is usually only of the order of MHZ-ghz (the Brillouin gain linewidth of some solid media is only about 10 MHz), it is far less than the gain linewidth of the laser working substance of the order of 100 GHz, so, The Stokes excited in Brillouin laser can show obvious spectrum narrowing phenomenon after multiple amplification in the cavity, and its output line width is several orders of magnitude narrower than the pump line width. At present, Brillouin laser has become a research hotspot in photonics field, and there have been many reports on the Hz and sub-Hz order of extremely narrow linewidth output.

In recent years, Brillouin devices with waveguide structure have emerged in the field of microwave photonics, and are developing rapidly in the direction of miniaturization, high integration and higher resolution. In addition, the space-running Brillouin laser based on new crystal materials such as diamond has also entered people's vision in the past two years, its innovative breakthrough in the power of the waveguide structure and the cascade SBS bottleneck, the power of the Brillouin laser to 10 W magnitude, laying the foundation for expanding its application.
General junction
With the continuous exploration of cutting-edge knowledge, narrow linewidth lasers have become an indispensable tool in scientific research with their excellent performance, such as the laser interferometer LIGO for gravitational wave detection, which uses a single-frequency narrow linewidth laser with a wavelength of 1064 nm as a seed source, and the linewidth of the seed light is within 5 kHz. In addition, narrow-width lasers with wavelength tunable and no mode jump also show great application potential, especially in coherent communications, which can perfectly meet the needs of wavelength division multiplexing (WDM) or frequency division multiplexing (FDM) for wavelength (or frequency) tunability, and is expected to become the core device of the next generation of mobile communication technology.
In the future, the innovation of laser materials and processing technology will further promote the compression of laser linewidth, the improvement of frequency stability, the expansion of wavelength range and the improvement of power, paving the way for human exploration of the unknown world.