Fiber laser and technology progress

Abstract: As the most active laser light source device, fiber laser is a frontier subject of laser technology. This article discusses the characteristics and basic principles of fiber lasers, and summarizes the recent developments in fiber lasers.

1. Introduction Fiber laser is a technology developed on the basis of EDFA technology. As early as 1961, E. Snitzer and others of the American Optical Company carried out pioneering work in the field of fiber lasers, but due to the limitations of relevant conditions, the progress of their experiments was relatively slow. In the 1980s, S.B. Poole of Southhampton University in the United Kingdom used MCVD to make low-loss erbium-doped fibers, which brought new prospects for fiber lasers.

   Recently, with the widespread application and development of optical fiber communication systems, research on applications in various fields such as ultra-fast optoelectronics, nonlinear optics, and optical sensing has received increasing attention. Among them, fiber lasers based on optical fibers have made significant progress in reducing threshold, oscillation wavelength range, and wavelength tunable performance. It is an emerging technology in the field of optical communication. It can be used in existing communication systems to make Its support for higher transmission speeds is the basis for future high-bit-rate dense wavelength division multiplexing systems and future coherent optical communications. At present, fiber laser technology is one of the hot research topics. This article elaborates on several new types of fiber laser technologies abroad in recent years.

Second, the principle of fiber laser

   The optical fiber amplifier developed by the optical 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. The currently developed fiber lasers mainly use rare-earth-doped fibers as gain media. Because the fiber core in the fiber laser is very thin, it is easy to form a high power density in the fiber under the action of the pump light, which causes the "population inversion" of the laser energy level of the laser working substance. Therefore, when a positive feedback loop (to form a resonant cavity) is properly added, laser oscillation can be formed. In addition, because the fiber matrix has a very 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 have a waveguide structure, can tolerate strong pumping, have high gain, high conversion efficiency, low threshold, good output beam quality, narrow line width, simple structure, High reliability, easy to realize coupling with optical fiber.

   We can classify fiber lasers from different angles. For example, according to the structure of fiber laser resonant cavity, it 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 threshold should be as low as possible; (2) The linearity of output power and pumping optical power should be better; (3) Output polarization state; (4) Mode structure ; (5) Energy conversion efficiency; (6) Laser working wavelength, etc.

3. Cladding-pumped fiber laser technology

   The emergence of double-clad fiber is undoubtedly a major breakthrough in the field of optical fiber. It makes the production 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 first choice for the production of 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 a special doped fiber 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 different modes of multi-mode pump light (as shown in Figure 1(b)). In this way, multiple multi-mode laser diodes are simultaneously coupled to the cladding fiber, and each time the pump light traverses the single-mode fiber core, it will pump the rare-earth element atoms in the core to the upper energy level. Then the spontaneous emission light is generated through the transition, and through the frequency selection effect of the fiber grating arranged in the optical fiber, the spontaneous emission light of a specific wavelength can be oscillated and amplified to finally generate laser output. At present, this technology is called multi-mode parallel cladding pumped technology. The French company Keopsys has formed a patent on this technology, called "V-Groove Technologe".

The technical characteristics of multi-mode parallel cladding pumping determine the outstanding performance of this type of laser in the following aspects.

1. High power

   A multi-mode pump diode module group can radiate 100 watts of optical power, and multiple multi-mode pump diodes are arranged in parallel to allow the design of high-power fiber lasers.

2. No need for thermoelectric cooler

   This kind of high-power wide-face multi-mode diode can work at very high temperature, only needs simple air cooling, and low cost.

3. A wide range of pump wavelengths

   The active cladding 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 (930-970nm). Therefore, the pump diode does not require any type of wavelength stabilization device

4. High efficiency

   The pump light traverses the core of the single-mode fiber multiple times, so its utilization rate is high.

5. High reliability

   The stability of multimode pump diodes is much higher than that of single mode pump diodes. Its geometrically wide surface makes the optical power density on the cross section of the laser very low and the current density through the active surface is also very low. As a result, the reliable operating life of the pump diode exceeds 1 million hours. The current technology for cladding-pumped fiber lasers can be summarized into three categories: linear cavity single-ended pumping, linear cavity double-ended pumping, and all-fiber ring cavity double-clad fiber lasers. Double-clad fiber lasers with different characteristics It can be expanded from the three basic types. A document of OFC’2002 uses a cavity structure as 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 up to 85% [1]. In terms of product technology, IPG Corporation of the United States has developed a 700W ytterbium-doped double-clad fiber laser and announced that it will launch a 2000W fiber laser.

4. Raman fiber laser technology

   Raman optical amplification technology provides a new means of obtaining power budget for long-distance transmission and has become the focus of attention. For the Raman amplifying pump source, one of the methods is to use multiple 14XXnm pump lasers to obtain the Raman pump source through polarization multiplexing, but its cost is relatively high and the structure is complicated. The second method is to use Raman fiber lasers (RFL) to generate high-power lasers with specific wavelengths. At present, this technology has been developed to a considerable extent and has formed commercial products (such as the United States IPG, French Keosys and other companies can provide 5W Raman Amplified pump module), and is considered to be a reasonable light source for Raman amplification and remote pump EDFA amplification applications.

4.1 Linear cavity Raman fiber laser

   If divided from the output wavelength of the linear cavity Raman fiber laser, it can be divided into two categories: single-wavelength and multi-wavelength Raman fiber lasers. The structures of different linear Raman fiber lasers are basically similar, and all use Bragg gratings as their resonator mirrors. As far as the active gain medium used in RFL is concerned, GeO2 doped fiber is usually used as the gain medium. A recent report is to use P2O5 doped fiber as the gain medium. The difference between the two is the stock offset obtained. Different, generally, GeO2 doped fiber is 440cm-1, and P2O5 doped fiber is 1330cm-1, so the number of Raman frequency conversions required for P2O5 doped fiber is less, which can improve efficiency and reduce The complexity of RFL. N.Kurukithoson et al. reported in the ECOC'2001 conference an RFL experiment using a two-stage Raman transform to obtain a 1480nm laser output. The pump wavelength is 1061nm [2], which is compared with the RFL using GeO2 doped fiber , Reducing the first-level Raman up-conversion. Another paper of ECOC’2001 reported that a 1480nm single-wavelength Raman fiber laser made of P-doped fiber achieved +28dBm output EDFA[3]. A paper in the OFC’2001 conference reported an experiment in which the Raman fiber laser output by the secondary Stocks was used as the pump source to excite a single-mode fiber to generate a supercontinuum [4]. It consists of a Raman fiber laser and a supercontinuum (SC) cavity. The working principle diagram of the Raman fiber laser is shown in Figure 3. Under the pump of ytterbium-doped fiber laser, the praseodymium-doped fiber is used as the 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 type of multi-wavelength Raman fiber laser (MWRFL) that emerged recently has attracted widespread attention. Among them, dual-wavelength Raman fiber laser (2lRFL) and three-wavelength Raman fiber laser (3lRFL) have been successfully demonstrated, IPG etc. Products have begun to form.

   A reconfigurable three-wavelength Raman fiber laser (3lRFL) reported by Alcatel at the OFC'2002 conference is shown in Figure 4 [5]. The laser output with output wavelengths of 1427nm, 1455nm and 1480nm was obtained, which can be used for C+L band Raman amplifier. In addition, by adjusting the output coupler, the output power of each wavelength can be adjusted within the range of 50mW-400mW. The main part of the entire 3lRFL is composed of 11 fiber gratings (FBG) and 300 meters of P-doped fiber, and uses a Yb3+ cladding pumped fiber laser with an output wavelength of 1117nm as the pump source. The internal Stocks power migration is shown in Figure 5. The basic principle is divided into the following three steps: First, use P2O5 to generate frequency shift under the action of 1117nm pump light to obtain the first-level Stocks component of 1312nm; then, under the action of first-level Stocks, use the frequency shift of quartz fiber , Get the secondary Stocks component of 1375nm; finally, by using the frequency shift of the quartz fiber again, simultaneously obtain the laser output of 1427.0nm, 1455.0nm and 1480.0nm. It should be pointed out that because the Raman peaks are far apart, the interaction between different Stocks cannot be ignored. As shown by the dotted line in Figure 3, the Stocks component of 1427.0nm pumps 1455.0nm and 1480.0nm and gains gains. Similarly, the Stocks component of 1312nm can make 1375nm, 1427nm, 1455nm and 1480nm obtain additional Raman gain.

   Using a structure similar to Figure 4, two other papers of OFC'2002 reported that a reconfigurable Raman fiber laser that generates a four-level Stocks component under the action of pump light has output wavelengths of 1428nm, 1445nm and 1466nm[6 ][7]. An OFC’2001 paper reported a 3lRFL, the output spectral lines were respectively: 1427nm spectral width of 0.8nm, 1455nm and 1480nm spectral width of 0.4nm[8].

4.2 Ring cavity Raman fiber laser

   The ring cavity structure has an important position and role in laser technology, and it is also another important way to construct Raman fiber lasers. A paper in OFC’2001 reported a dual-wavelength circular Raman fiber laser (2lRFL) [9], the structure of which is shown in Figure 6. In the figure, except for the fiber grating 1480A, which has a reflectance of 90%, the reflectances of other fiber gratings are greater than 99%. Raman fibers A and B are dispersion compensation fibers (DCF) with lengths of 120 meters and 220 meters, respectively. Under the action of the Nd:YLF laser with a working wavelength of 1313nm as the pump source, the secondary Stocks wavelengths of the laser are 1480nm and 1500nm. The reported data shows that the fiber laser can obtain a laser output greater than 400mW under a pump of 3.2W. In addition, by adjusting the reflectivity of the fiber grating 1480B, the power of the output wavelength can be controlled and adjusted. This feature enables this type of fiber laser to be better used in Raman amplification with flat gain.

5. New fiber laser technology

   The early development of lasers mainly focused on the study of short pulse output and the expansion of the tunable wavelength range. Today, the rapid development and increasing progress of dense wavelength division multiplexing (DWDM) and optical time division multiplexing technologies accelerate and stimulate the progress of multi-wavelength fiber laser technology, super continuous fiber lasers, and so on. At the same time, the emergence of multi-wavelength fiber lasers and super-continuous fiber lasers provides an ideal solution for low-cost realization of Tb/s DWDM or OTDM transmission. In terms of the technical approach to its realization, technologies such as spontaneous emission amplified by EDFA, femtosecond pulse technology, and super-luminescent diodes have all been reported.

5.1 Multi-wavelength fiber laser

   Literature [10] proposed a multi-wavelength fiber laser based on a semiconductor optical amplifier (SOA) as shown in Figure 7. In the figure, the length of SOA1 is 500mm, and the small signal gain provided at 1522nm is 23dB. The length of SOA2 is 250mm, which can provide 10.5dB small signal gain at 1530nm. Both SOAs are of InGaAsP/InP roof waveguide type. The free spectral range (FSR) of the fiber F-P cavity is 47.75GHz, the fineness is 8.1, and the loss is 12dB. The polarization controllers PC1 and PC2 are used to compensate the polarization-related gain errors of SOA1 and SOA2 to the TE axis and the TM axis, respectively. This structure realizes a multi-wavelength DWDM light source with a wavelength interval of 50GHz and 50 channels in the range of 1554nm-1574nm. The maximum optical power difference between the 50 channels is less than 1.6dB, the extinction ratio is greater than 15dB, and the linewidth of the laser is less than 5GHz.

   In order to obtain a flat power output spectrum, literature [11] proposed a correction scheme as shown in Figure 8. In the picture, FRM is a Faraday rotating mirror, and VOA is a tunable optical attenuator. Due to the introduction of the optical feedback arm, an intuitive feature is that it can perform feedback monitoring on the laser output. In addition, the correction structure can also provide a greater degree of improvement in the output light performance of the laser. According to reports, this structure realizes a multi-wavelength DWDM light source with a channel spacing of 50GHz and 52 channels in the wavelength range of 1554.7—1574.7nm, and the maximum optical power difference between channels is less than 0.3dB, and the extinction ratio reaches