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White light supercontinuum light source without blue light defect

changing the air filling rate in the photonic crystal fiber used in white light supercontinuum light source can eliminate the traditional "blue light defect" in this kind of light source, so as to produce real white light supercontinuum light

in the past decade, the emergence of highly nonlinear photonic crystal fibers (PCF) and compact, high-power ultrafast fiber lasers has made the output power of white light laser sources reach 10W and the spectral power density reach 10MW/nm

although these commercial optical fiber supercontinuum light sources have shown some application potential in industry and biomedicine, they all have common defects in the blue light region of the spectrum. The spectrum of the system with the highest index has reached 450nm, but there are still certain limitations if the optical fiber supercontinuum laser is used to replace the lighting source

however, the nonlinear PCF recently developed by researchers at the University of bath can make up for the blue light defect in supercontinuum fiber lasers and enhance the output of the blue light band (see Figure 1). At present, the technology is being commercially promoted by fianium

Figure 1 Through a prism dispersion, the comparison results of the new (upper spectrum) and traditional (lower spectrum) supercontinuum light sources in the visible spectrum region show that the use of the latest photonic crystal fiber has achieved the expansion of the blue and UV spectra

white laser

the temporal coherence of the laser corresponds to its limited spectral width, which is fundamentally caused by the limited gain bandwidth of all materials. Even the widest gain curve width is only 10% - 20% of its central frequency

however, many applications require a wider spectral width without brightness or power loss. One way to generate this ultra wide spectrum is to use the nonlinear response of materials at high optical power. White laser light source based on this technology appeared almost when laser was invented. [1] Most early structures needed large and bulky pump laser systems; Many experiments use block materials as nonlinear media. However, the nonlinear process of producing broadband light will also degrade the output beam quality due to optical filaments and breakdown

the rapid development of photonic crystals and microstructure fibers has brought breakthroughs to the industry, in which light is conducted in the quartz core surrounded by tiny air holes. Using titanium doped sapphire laser as the pump source of graphene, which is called "black gold", broadband supercontinuum light with high beam quality in the visible band is produced in these fibers. [2] This makes the technology enter a period of rapid development. While trying to understand the essential characteristics of these processes, researchers optimize the system (the coupling of pump laser and nonlinear fiber) to produce a broadband, flat, low-noise spectrum, and use cheap pump elements. Some potential applications have attracted researchers' interest, which may benefit from new light sources that are essentially different from the previous ones in many aspects

optical fiber supercontinuum spectrum phenomenon

how to make a narrow-band laser beam simply pass through a section of optical fiber made of quartz and transform it into a rainbow like color covering from visible light to near-infrared light

the details of this process are complex and vary with wavelength, pulse width, and the fiber used. However, all of these are usually due to Kerr and Raman nonlinear effects, as well as group velocity dispersion (GVD). If GVD is normal dispersion, Kerr and dispersion terms will make the pulse disperse at the same time. Therefore, after passing through a very short length of fiber, the peak power of the pulse will decrease, and the nonlinear response of the fiber will be significantly reduced. Therefore, in this case, the spectrum will be weakened and broadened

however, if the group velocity dispersion is abnormal dispersion (that is, the group refractive index increases with the increase of wavelength), the GVD effect and Kerr nonlinear effect will cancel each other, and the pulse will travel a long distance in the form of optical solitons in the optical fiber without chromatic dispersion. Transmission with high peak power for a long distance will cause a series of optical effects, which will further broaden the spectrum and eventually produce an effective supercontinuum spectrum. This is due to the high efficiency of photonic crystal fiber to a certain extent: Zero GVD can be achieved in a wide wavelength range through control, and abnormal dispersion (such as 1064nm and 800nm) can be achieved at common pump laser wavelengths, which is difficult to achieve in traditional fiber technology

group refractive index matching: overcome the blue light defect

we now know that the blue light edge of supercontinuum can be set by a relatively simple condition (see Figure 2). [3] The pump light of the fiber is close to the zero dispersion wavelength, but on the abnormal dispersion side. The pump pulse generates solitons, which experience Raman self frequency shift in the process of their transmission, and therefore move towards the long wave direction. The pump pulse simultaneously generates short wave radiation in the normal dispersion region. Because the group refractive index curve and the pump light approach the lowest value of the curve, the long wave and short wave on both sides of the pump wavelength have the same group velocity. Results in the transmission process, they interact through four wave mixing. This interaction, combined with the Raman self frequency shift of infrared solitons, leads to continuous short wave radiation shifted towards blue light (see Figure 3)

Figure 2 The matching of group refractive index is very important for enhancing the blue light band. The wavelength of the pump pulse in the quasi threshold fiber coupled to the optical decline market is in the anomalous dispersion region, but close to the zero dispersion point. It will generate solitons that will shift from the frequency to the long band, and also produce short wave radiation. Because long wave and short wave have the same group refractive index, they transmit simultaneously in the optical fiber and continue to interact through four wave mixing. This will extend the supercontinuum spectrum to the short wave until the end of the soliton, for example, because of the high loss spectrum of the fiber

finally, the frequency shift of the infrared soliton is no longer, which may be due to the fact that it has moved to the high loss region of the fiber. In most PCFs, the frequency shift of infrared solitons stops at 2.5 μ M wavelength, mainly due to Oh (hydrogen oxygen bond) in the fiber core. When the infrared pulse is absorbed or scattered, there is no mechanism to drive the short wave radiation to expand to shorter wavelengths. As a result, for a given fiber, there is the shortest wavelength due to this mechanism. The shortest wavelength has the same group refractive index as the severely attenuated long wave soliton

most commercial high brightness supercontinuum lasers rely on ultrafast fiber lasers operating at 1064nm to pump PCF. As a result, the PCF used in these systems is designed to have zero GVD at a wavelength very close to 1064nm. Such an optical fiber has become the first choice for people to use 1064nm pump to generate supercontinuum spectrum (see Figure 3, small picture on the left). The "endless single mode" (ESM) PCF, which is often mentioned, can provide zero GVD in the range of 1030 ~ 1060nm. At the same time, it also produces a waveguide geometry, which can upload guided fundamental modes throughout the supercontinuum spectrum

Figure 3 Massive quartz, endless single-mode fiber (ESM) used in previous supercontinuum light sources, and 5 μ Comparison of the actual group refractive index curve of the plane of the pressing plate measured by quartz fiber (approximate PCF of large air hole) surrounded by an air hole of M. The different characteristics of these three curves at long waves produce different short waves. In general, the largest air hole in the PCF will produce a shorter wavelength. For two of these cases, the vertical dotted line indicates the long wave absorption edge (assumed here is 2.5 μ m) And the shortest group refractive index matching wavelength

based on our new understanding of supercontinuum generation in PCF, ESM fiber is not an optimal design, because the shape of GVD curve limits the short wave edge of supercontinuum to 450 ~ 500nm. However, by changing the design of the optical fiber used (see Figure 3, small figure on the right), a supercontinuum with a shorter wavelength can be generated

this new understanding has changed the design goal of PCF. It is no longer to optimize the zero GVD wavelength and design according to the endless single-mode fiber. The goal of the new method is to optimize the coverage of the entire spectrum from ultraviolet to infrared over 3 μ GVD curve of M. The fiber core is required to be within 5 μ M, which requires the zero GVD wavelength to be reasonably close to the pump wavelength of 1.06 μ Determined by M. However, the air filling rate is a free parameter. The highest air filling rate can produce the shortest wavelength, which is closest to the thin quartz sheet exposed to the air

the design results are exciting. Compared with the previous results of using commercial ESM nonlinear fiber, the supercontinuum spectrum produced by using the newly designed fiber is 50 ~ 100nm more in the blue and ultraviolet spectrum

turnkey solution

for pump laser sources that produce efficient and stable supercontinuum spectra, people naturally choose high-power ultrafast fiber lasers, which can provide terahertz pulse repetition rate and output power of tens of watts. The structure of all optical fiber means that the pump light and PCF can be connected through the fusion of optical fiber, so that the system has good stability. In addition, a high-efficiency ytterbium doped optical fiber amplifier is developed to make it suitable for pumping nonlinear PCF. The working band of these light sources is 1 ~ 1.1 μ m。

the combination of high-power ultrafast fiber laser and nonlinear PCF made the University of bath transfer the technology to fianium. The photonic crystal fiber designed based on the technology of Bath University is pulled by a third-party PCF manufacturer and integrated into a turnkey commercial supercontinuum fiber laser source (see Figure 4)

Figure 4 SC400 blue light enhanced supercontinuum product developed by fianium company. The research results of this product are provided by Bath University

the new system can cover a wide range of wavebands from less than 400nm (it is reported that it can reach 380nm) to nearly 2400nm, with a power spectral density of several milliwatts per nanometer and diffraction limited output

from 450nm to UV band is very important for many fluorescent imaging applications. For applications that traditionally use arc lamps as lighting sources, turnkey blue enhanced supercontinuum light sources can provide a considerable spectrum, but the brightness can be increased by several orders of magnitude

challenge of ultraviolet light

improving the power of supercontinuum light source and expanding the supercontinuum spectrum to 300nm ultraviolet band are the next engineering goals of Bath University and fianium company. It is still a challenging goal to effectively convert pump light to UV band. The design and material research of PCF with high-energy ultraviolet photons will be a challenge for the development of supercontinuum fiber lasers in the next 12 months


1 R.R. Alfano et al., Phys. Rev. Lett. 24, 854, (1970).

2. J.K. Ranka et al., Optics Lett. 25(1) 25, (2000).

3. J.M. Stone et al., Optics Express 16(4) 2670, (2008).

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