The hottest fiber laser and its technological prog

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Fiber lasers and technological progress

Abstract: as the most active laser light source device at present, fiber lasers are the frontier subject of laser technology. In this paper, the characteristics and basic principles of fiber lasers are discussed, and the recent progress of fiber lasers is summarized

I. Introduction

fiber laser is a technology developed on the basis of EDFA technology. As early as 1961, itzer and others of American optical company had made groundbreaking work in the field of fiber lasers, but due to the limitations of relevant conditions, their experimental progress was relatively slow. In the 1980s, Ole of southhampton University in England and others used the MCVD method to produce low loss erbium-doped fibers, which brought new prospects for fiber lasers

recently, with the wide application and development of optical fiber communication system, the research on the application of ultra fast optoelectronics, nonlinear optics, optical sensing and other fields has been paid more and more attention. Among them, the fiber laser based on fiber has made significant progress in reducing the threshold, oscillation wavelength range, wavelength tunable performance and so on. It is a new technology in the field of optical communication at present. It can be used in the existing communication system to support higher transmission speed, and is the foundation of the future high bit rate dense wavelength division multiplexing system and the future coherent optical communication. At present, fiber laser technology is one of the hot research technologies. In this paper, several new fiber laser technologies abroad in recent years are described

Second, the principle of fiber laser

the fiber amplifier developed by using the fiber doped with rare earth elements has brought revolutionary changes to the field of light wave technology. Since any optical amplifier can form a laser through an appropriate feedback mechanism, fiber lasers can be developed on the basis of fiber amplifiers. At present, the fiber laser developed mainly uses the fiber doped with rare earth elements as the gain medium. Because the fiber core in the fiber laser is very thin, it is easy to form high power density in the fiber under the action of pump light, resulting in the "particle number inversion" of the laser energy level of the laser working material. Therefore, when a positive feedback loop (forming a resonant cavity) is properly added, laser oscillation can be formed. In addition, because the fiber matrix has a wide fluorescence spectrum, fiber lasers can generally be made tunable, which is very suitable for WDM system applications

compared with semiconductor lasers, the advantages of fiber lasers are mainly reflected in: fiber lasers are waveguide structures, can accommodate strong pumping, have high gain, high conversion efficiency, low threshold, good output beam quality, narrow linewidth, simple structure, high reliability, etc., and are easy to realize coupling with optical fibers

we can classify fiber lasers from different angles. For example, according to the structure of the resonant cavity of fiber lasers, they can be divided into Fabry Perot cavity and ring cavity. It can also be divided into single wavelength and multi wavelength according to the number of output wavelengths. For the characteristics of different types of fiber lasers, the following points should be considered: (1) the lower the threshold, the better; (2) The linearity between output power and pump power is better; (3) Output polarization state; (4) Mode structure; (5) Energy conversion efficiency; (6) Laser operating wavelength, etc

Third, cladding pumped fiber laser technology

the emergence of double clad fiber is undoubtedly a breakthrough in the field of fiber, which makes the manufacture of high-power fiber lasers and high-power optical amplifiers a reality. Since e Snitzer first described cladding pumped fiber lasers in 1988, cladding pumping technology has been widely used in fiber lasers and fiber amplifiers, and has become the preferred way to make high-power fiber lasers. Figure 1 (a) shows a cross-sectional structure of a double clad fiber. It is not difficult to see that the technical basis of cladding pumping is the use of special doped fibers with two concentric cores. A fiber core is similar to the traditional single-mode fiber core, which is dedicated to transmitting signal light and realizing single-mode amplification of signal light. The large core is used to transmit multimode pump light of different modes (as shown in Figure 1 (b)). In this way, multiple multi-mode laser diodes are coupled to the clad fiber at the same time. When the pump light passes through the single-mode fiber core each time, the atoms of rare earth elements in the fiber core will be pumped to the upper energy level, and then spontaneous emission light will be generated through the transition. Through the frequency selection effect of the fiber grating set in the fiber, the spontaneous emission light of a specific wavelength can be oscillated and amplified to finally produce laser output. At present, this technology is called multi-mode parallel cladding pumping technology, and French keopsys company has formed a patent on this technology, which is called "V-groove technology"

The characteristics of multi-mode parallel cladding pumping technology determine that this kind of laser has the following outstanding performance

1. High power

a multimode pump diode module group can radiate 100 watts of optical power. Multiple multimode pump diodes are set in parallel, which allows the design of fiber lasers with very high power output

2. There is no need for thermoelectric cooler

this high-power wide area multimode diode can work at very high temperature, with simple air cooling and low cost

3. Wide pump wavelength range

the active clad fiber in the high-power fiber laser is doped with erbium/ytterbium rare earth elements, and has a wide and flat light wave absorption region (nm). Therefore, the pump diode does not need any type of wavelength stabilization device

4. High efficiency

the pump light crosses the single-mode fiber core for many times, so its utilization is high

5, high reliability

the stability of multi-mode pump diode is much higher than that of single-mode pump diode. Its geometric wide surface makes the optical power density on the cross-section of the laser very low, and the current density passing through the active surface is also very low. In this way, the reliable operation life of the pump diode exceeds 1million hours. At present, the technology of realizing cladding pumped fiber lasers can be summarized into three categories: linear cavity single end pumping, linear cavity double end pumping, all fiber ring cavity double clad fiber lasers. Double clad fiber lasers with different characteristics can be expanded from these three basic types

A document of OFC '2002 adopts the cavity structure shown in Figure 2 to realize a new cladding pumped fiber laser with an output power of 3.8W, a threshold of 1.7w and a tilt efficiency of 85%. In terms of product technology, IPG company of the United States has emerged as a new force. It has developed a 700W ytterbium doped double clad fiber laser and announced that it will launch a 2000W fiber laser

IV. Raman fiber laser technology

Raman optical amplification technology provides a new means to obtain power budget for long-distance transmission, and has become the focus of attention. For Raman amplification pump source, one method is to use multiple 14xxnm pumps ----- associate professor Ruan Shilun/doctor laser of Dalian University of technology to obtain Raman pump source through polarization multiplexing, but its cost is relatively high and its structure is complex. The second method is to use Raman fiber laser (RFL) to generate high-power laser with specific wavelength. At present, this technology has been developed to a considerable extent and has formed a commercial product (such as IPG in the United States, keyys in France and other companies can provide 5W Raman amplification pump module), which is considered to be a reasonable light source for Raman amplification and far pump EDFA amplification applications

4.1 linear cavity Raman fiber lasers

if divided from the output wavelength of linear cavity Raman fiber lasers, they can be divided into single wavelength and multi wavelength Raman fiber lasers. The structures of different linear Raman fiber lasers are basically similar, and Bragg gratings are used as the mirrors of their resonators. In terms of the active gain medium used by RFL, the doped fiber doped with GeO2 is usually used as the gain medium. The recent report is that the doped fiber doped with P2O5 is used as the gain medium. The difference between the two is that the stock shift obtained is different. Generally, the doped fiber doped with GeO2 is 440cm-1, while the doped fiber doped with P2O5 is 1330cm-1. Therefore, the number of Raman frequency conversion required to use P2O5 doped fiber is less, It can improve efficiency and reduce the complexity of RFL. Rukithoson et al. Reported an RFL experiment of 1480nm laser output using second-order Raman conversion in the ECOC '2001 conference. The pump wavelength is 1061nm[2], which reduces the first-order Raman up conversion compared with the RFL using GeO2 doped fiber. Another paper in ECOC '2001 reported the EDFA of +28dbm output using 1480nm single wavelength Raman fiber laser made of p-doped fiber [3]. In OFC '2001 conference, a paper reported the experiment of generating supercontinuum spectrum by pumping single-mode fiber with Raman fiber laser output from two-stage stocks as pump source [4]. It is composed of Raman fiber laser and supercontinuum (SC) cavity. See Figure 3 for the working principle of Raman fiber laser. Pumped by ytterbium doped fiber laser, praseodymium doped fiber is used as working material to output laser. The pump light is 1064nm, the output pulse is 1483.4nm laser (secondary stocks), and the output power is 2.22w

another recently emerged type, called multi wavelength Raman fiber laser (mwrfl), has attracted extensive attention. Among them, dual wavelength Raman fiber laser (2lrfl) and three wavelength Raman fiber laser (3lrfl) have been successfully demonstrated, and IPG and other products have begun to form

a reconfigurable three wavelength Raman fiber laser (3lrfl) reported by Alcatel at OFC '2002 conference is shown in Figure 4 [5], and the laser output with output wavelengths of 1427nm, 1455nm and 1480nm respectively is obtained, which can be used in c+l-band Raman amplifiers

in addition, by adjusting the output coupler, the output power of each wavelength can be adjusted in the range of 50mw-400mw. The main part of the 3lrfl is composed of 11 fiber gratings (FBGs) and 300 meters of p-doped fiber, and the yb3+ cladding pumped fiber laser with an output wavelength of 1117nm is used as the pump source. The internal stocks power migration is shown in Figure 5. Its basic principle is divided into the following three steps: first, under the action of 1117nm pump light, P2O5 is used to generate frequency shift, and the first-order stocks component of 1312nm is obtained; Then, under the action of first-order stocks, using the frequency shift of quartz fiber, the second-order stocks component of 1375nm is obtained; Finally, by using the frequency shift of quartz fiber again, 1427.0nm, 1455.0nm and 1480.0nm laser outputs are obtained at the same time. It should be pointed out that due to the distance between the Raman peaks, the interaction between different stocks can not be ignored. As shown by the dotted line in Figure 3, the stocks component of 1427.0nm pumps 1455.0nm and 1480.0nm and obtains gain. Similarly, the stocks component of 1312nm can obtain additional Raman gain at 1375nm, 1427nm, 1455nm and 1480nm

Fig. 5 Schematic diagram of stocks power transfer of three band Raman fiber laser

the structure is similar to that in Fig. 4. The other two papers of OFC '2002 reported the reconfigurable Raman fiber laser that generates four stages of stocks components under the action of pump light, and its output wavelengths are 1428nm, 1445nm and 1466nm [6] [7]. A paper in OFC '2001 reported a 3lrfl with output spectral lines of 0.8nm for 1427nm, 0.4nm for 1455nm and 1480nm [8]

4.2 ring cavity Raman fiber laser

ring cavity structure plays an important role in laser technology, and it is also another important way to build Raman fiber laser for normal tasks. A paper in OFC '2001 reported a dual wavelength circular Raman fiber laser (2lrfl) [9],

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