The existing laser technology has always had a short board, which can only emit light of a single wavelength or narrow band. How to expand the frequency of the laser forming ultra broadband, super straight, ultraviolet, visible and infrared wavelengths of coherent white light laser, is still a human unfulfilled dream, it is a worldwide problem of science and technology. This is because the laser optics is composed of optical resonator, gain medium and pump source. The wavelength of the laser is determined by the energy level structure of the atoms, molecules or ions in the gain substance.Because the natural laser crystal material has a great limitation on the gain frequency range and gain bandwidth, the laser can't produce any wavelength of laser. Will the perfect white laser be created? What changes and developments could it bring to the application of lasers?
1. The sun light
It is known to all that all things grow by the sun, and the sunlight brings light and heat to the earth. The familiar sunlight is a kind of white light, and its spectrum covers ultraviolet - visible - near infrared - mid-infrared bands, as shown in figure 1, In the visible light band (400-700 nm), the radiation energy is strongest, covering seven colors of red, orange, yellow, green and blue, and continuous distribution and transition in the spectrum. Because the sun is white, rainbows are often seen in the sky after rain, or the sunlight passes through a glass prism through seven colors of light (figure 2). This is a common experience in everyday life.
One of the things that people are less familiar with is that sunlight is a completely incoherent light. In terms of spatial coherence, the sunlight cannot be straight and highly divergent. In terms of temporal coherence, there is no phase correlation and locking between different colors of sunlight. So sunlight can only be used to generate energy for heating, water heaters, solar cells and so on. But the use of modern science and technology, such as the use of sunlight to transmit information, seems to be out of the question. The spectral distribution of sunlight. The spectrum of solar radiation including the sun itself, the spectrum of sunlight entering the earth, and the spectrum of sunlight reaching the sea level due to the absorption of atmospheric water and carbon dioxide.
2. The advantages and application prospect of white light laser
White light laser light source, short wave laser light sources and continuous laser light compared with the ordinary white light, such as sunlight, incandescent lamp, white LED lamp, etc.), it has the advantages of high brightness, high peak power, wide frequency range etc. In the fields of scientific research, defense military, lighting, communication technology, information technology, industrial production, biomedicine, environmental detection, etc.it got a lot of attention.
White light, as a kind of new laser light source, it has a variety of advantages of good directionality, high energy density, super continuous spectrum, great bandwidth, the center of the flexible wavelength, high degree of coherence time and space. This will greatly expand the function and application scope of laser technology. The white light laser or the solar laser is the completely coherent light, not only the height of the laser beam, but also the very small region. Different color between amplitude and phase locking completely, by regulating the amplitude and phase, it can change the time of laser pulse shape follow one's inclinationsly, and produce very short pulse width (femtosecond and the femtosecond laser pulses. Such a solar laser will have the potential to realize the focus and convergence of light energy in space and time. It releases energy in small areas and very short periods of time to form extremely high instantaneous power density.