1. Introduction
Research on doped fiber lasers using gain media dates back to the 1960s, when Snitzer reported in 1963 the creation of a fiber laser with neodymium (Nd 3+ ) ions doped in a glass matrix.
Since the 1970s, significant progress has been made in fiber preparation technology and the exploration of pump and resonant cavity structures for fiber lasers.
In the mid-1980s, an advance in doped fiber (Er 3+ ) at the University of Southampton in the United Kingdom greatly improved the practicality of fiber lasers, showing very promising application prospects.
Compared with traditional solid and gaseous lasers, fiber lasers have many unique advantages such as high beam quality, small size, light weight, maintenance-free, air-cooled, easy to operate, low operating cost and long use term in industrial environments. .
They also offer high processing accuracy, fast speed, long service life, energy savings and excellent flexibility for intelligence and automation. Therefore, they have replaced traditional YAG and CO2 lasers in many fields.
The output wavelength range of fiber lasers is between 400-3400 nm, applicable in various fields such as optical data storage, optical communication, sensor technology, spectroscopy and medical applications.
At present, rapid development is observed in doped fiber lasers, fiber Bragg grating lasers, tunable narrow linewidth fiber lasers and high-power dual fiber lasers.
2. Basic Structure and Working Principle of Fiber Lasers
2.1 Basic Structure of Fiber Lasers
The fiber laser mainly consists of three parts: the gain medium that can generate photons, the optical resonant cavity that allows photon feedback and resonant amplification in the gain medium, and the pump source that can excite the laser medium.
The basic structure of the fiber laser is shown in Figure 2.1.
The gain medium is a fiber core doped with rare earth ions. The doped fiber is placed between two mirrors with selected reflectivity. The pump light is coupled to the fiber from the left mirror of the fiber laser and emits laser light through an optical collimation system and filter.
Theoretically, the pump source and gain fiber are the essential components of the fiber laser, and the resonant cavity is not indispensable. Resonant cavity mode selection and gain medium stretching are not necessary in fiber lasers because the fiber itself can be very long, thus achieving very high single-pass gain, and the waveguide effect of the fiber can Play a mode selection function.
However, in practical applications, people generally prefer to use shorter fibers; therefore, in most cases, a resonant cavity is used to introduce feedback.
Due to the waveguide structure of fiber lasers, they can accommodate strong pumping and have high gain (single-pass gain of up to 50dB). The rare earth elements in the glass matrix have a wide linewidth and tuning range (Yb 3+ is 125 nm, Tm 3+ >300nm).
Specific features are as follows:
1) The fiber serves as a waveguide medium, offering high coupling efficiency, small core diameter and ease of forming high power density within the fiber. It can conveniently connect to current fiber optic communication systems. The resulting lasers have high conversion efficiency, low laser threshold, excellent beam quality, and narrow linewidth.
2) Given the high “surface area/volume” ratio of the fiber, it has good heat dissipation. Ambient temperatures can range from -20 to 70°C, eliminating the need for a large water cooling system and requiring only simple air cooling.
3) The fiber laser can operate under harsh conditions such as high impact, high vibration, high temperature and dusty conditions.
4) Due to the excellent flexibility of the fiber, the laser can be designed to be quite small and flexible, with a compact shape and small volume, facilitating system integration and offering a high performance-to-price ratio.
5) The fiber laser has many adjustable parameters and selectivity, allowing it to cover a wide adjustment range, excellent monochromaticity and high stability. It has a long pump life, with an average failure-free operating time of 10kh or even more than 100kh.
2.2 Fiber laser working principle
Currently developed fiber lasers mainly use fibers doped with rare earth elements as the gain medium.
The working principle of the fiber laser is that the light from the pump falls on the doped fiber through the front reflector (or front grid), and the rare earth ions that have absorbed the photon energy will undergo energy level transitions, achieving “inversion of the number of particles”.
The inverted particles will return to the ground state in the form of radiation after relaxation, simultaneously releasing energy in the form of photons and emitting the laser through the back reflector (back grid).
Fiber amplifier doped with rare earth elements has promoted the development of fiber lasers, because fiber amplifiers can form fiber lasers through appropriate feedback mechanisms.
When the pump light passes through the rare earth ions in the fiber, it will be absorbed by the rare earth ions. At this time, the rare earth atoms that absorb photon energy will be excited to a higher laser energy level, thereby achieving ion number reversal.
The inverted ionic number will transition from the high energy level to the ground state in the form of radiation and release energy, completing the stimulated radiation. The mode of radiation from the excited state to the ground state has two types: spontaneous radiation and stimulated radiation.
Among them, stimulated radiation is radiation of the same frequency and phase, which can form a very coherent laser. Laser emission is a physical process where stimulated radiation far exceeds spontaneous radiation.
For this process to continue, ionic number inversion must be formed. Therefore, the energy levels involved in the process must exceed two, and there must also be a pump source to provide energy.
The fiber laser can actually be called a wavelength converter, through which the pump wavelength light can be converted into the required laser wavelength light.
For example, an erbium-doped fiber laser pumps out 980 nm light and produces 1550 nm laser. The laser output can be continuous or pulsed.
Fiber lasers have two laser states, three-level laser and four-level laser. The principles of three- and four-level laser are shown in Figure 2.2.
The pump (high energy, short wavelength photon) causes the electron to transition from the ground state to the high energy state E4 4 or E3 3 then transitions to the higher laser level E4 3 or E3 2 through non-radiative transitions.
When the electron transitions from the upper laser level to the lower energy level E4 2 or E3 1 the laser process will occur.
3. Types of fiber optic lasers
There are several types of fiber optic lasers that can be divided into different categories as shown in Table 3.1. The following sections will provide an introduction to several types of these lasers.
Table 3.1 Classification of Optical Fiber Lasers
| Classification by Resonator Structure | FP Cavity, Ring Cavity, Loop Reflective Fiber Resonator and “8” Shaped Cavity, DBR Fiber Laser, DFB Fiber Laser |
| Classification by Fiber Structure | Single cladding fiber laser, double cladding fiber laser |
| Classification by means of gain | Rare earth doped fiber laser, nonlinear effect fiber laser, single crystal fiber laser, plastic fiber laser |
| Classification by working mechanism | Upconversion fiber laser, downconversion fiber laser |
| Classification by Doping Elements | Erbium (Er 3+ ), Neodymium (Nd 3+ ), Praseodymium (Pr 3+ ), Thulium (Tm 3+ ), Ytterbium (Yb 3+ ), Holmium (Ho 3+ ) and 15 other types |
| Classification by output wavelength | S-band (1280-1350 nm), C-band (1528-1565 nm), L-band (1561-1620 nm) |
| Output Laser Classification | Pulsed laser, continuous wave laser |
3.1 Rare earth doped fiber lasers
Rare earth elements comprise 15 elements, positioned in the fifth row of the periodic table.
Currently, maturely developed rare earth ions incorporated into active fibers include Er 3+ Nd 3+ Pr 3+ Tm 3+ and Yb 3+ .
In recent years, double cladding doped fiber lasers using cladding pumping technology have significantly increased the output power, becoming another hot spot of research in the field of lasers.
This type of fiber structure, as shown in Figure 3.1, is composed of an outer cladding, an inner cladding and a doped core.
The refractive index of the outer cladding is lower than that of the inner cladding, which in turn is lower than the refractive index of the fiber core, thus forming a double-layer waveguide structure.
Double-doped fiber is a key component in the construction of fiber lasers. Its main functions in a fiber laser include:
1) Convert the pump light power into the laser working medium;
2) Collaborate with other devices to form a laser resonator.
Its working principle mainly involves injecting the pump light into the fiber laterally or from the end face. Because the refractive index of the outer cladding is much lower than that of the inner fiber cladding, the inner cladding can transmit multimode pump light.
The cross-sectional dimension of the inner shell is larger than the core. Thus, for the laser wavelength generated, the inner cladding and rare earth doped core form a perfect single-mode waveguide, while it and the outer cladding form a multi-mode waveguide to transmit the pump light power.
This allows the high power multimode pump light to be coupled to the inner casing. Light from the multimode pump is absorbed several times as it travels along the fiber, crossing the core. Due to the excitation of rare earth ions in the core, a high-power signal laser output is produced.
The operating principle is illustrated in Figure 3.1.
3.2 Bragg fiber grating laser
The increasing maturity of UV-written fiber Bragg grating technology in the 1990s led to increased attention to fiber Bragg grating lasers, particularly Distributed Bragg Reflector (DBR) and Distributed Feedback (DFB) fiber grating lasers. .
The main difference between the two is that the DFB fiber laser uses only one grating to achieve optical feedback and wavelength selection, thus offering better stability and avoiding loss of fusion between the Er-doped fiber and the grating.
However, although the grid can be directly written on the Er-doped fiber using UV, the practical fabrication of DEB fiber laser is not easy due to the low Ge content in the fiber core and low photosensitivity.
In contrast, the DBR fiber laser can be fabricated more easily by fusing a Ge-doped fiber grating at both ends of the Er-doped fiber to form a resonant cavity.
DBR and DFB fiber grating lasers face several problems such as low pump absorption efficiency due to short resonant cavities, broader spectral lines than ring lasers, and mode hopping.
Continuous efforts are being made to resolve these issues. Proposed improvements include the use of Er:Yb co-doped fiber as a gain medium, the adoption of an intracavitary pumping method, and the integration of the oscillator and power amplifier.
3.3 Ultrashort pulse fiber lasers
Ultrashort-pulse lasers are currently a hot topic of research in fiber lasers, mainly utilizing passive mode-locking techniques.
Similar to solid-state lasers, fiber lasers generate short-pulse laser outputs based on the mode-locking principle. When a fiber laser operates in a large number of longitudinal modes within the gain bandwidth, mode locking is achieved when each longitudinal mode phase is synchronized and the phase difference between any two adjacent longitudinal modes is constant.
The single pulse circulating in the resonant cavity emits energy through the output coupler. Fiber lasers are divided into active mode-locked fiber lasers and passive mode-locked fiber lasers.
The active mode-locked modulation capability limits the pulse width of the mode-locked pulse, which is generally on the order of picoseconds. Passive mode-locked fiber lasers utilize the nonlinear optical effects of the fiber or other optical components to achieve mode locking.
The structure of the laser is simple and can achieve automatic startup mode locking under certain conditions without any modulation components. The use of passive mode-locked fiber lasers can generate ultrashort pulses on the order of femtoseconds.
Ultrashort-pulse lasers have been used in ultrafast light sources, resulting in a variety of time-resolved spectroscopy and pumping techniques. Ultrashort pulse generation technology is the key to achieving ultra-high-speed optical time division multiplexing (OTDM). Ultrashort pulse fiber lasers are widespread in various fields such as materials, biology, medicine, chemistry and military.
4. Future Perspectives
Lasers are the core of laser technology, and the future development direction of fiber lasers will be to further improve the performance of fiber lasers, such as further increasing the output power and improving the beam quality; expanding new laser wavelengths, expanding the adjustable range of lasers; narrowing the laser spectrum; development of ultrashort pulses (ps and fs levels) of high brightness lasers; and conduct research on general miniaturization, practicality and intelligence.
In recent years, development has mainly focused on three aspects:
(1) improve the performance of fiber Bragg gratings, allowing them to be well applied in fiber lasers;
(2) fiber lasers with narrower pulse and spectral linewidths, higher output power, wider tuning range, etc.;
(3) make fiber lasers more practical.
Industrial Applications: The most notable application of fiber lasers in industry is materials processing. With their ever-increasing power, fiber lasers began to be used on a large scale for industrial cutting.
Fiber lasers are ideal for cutting, processing and handling metallic and non-metallic materials. They can be used for laser product calibration, precision cutting, laser engraving, laser welding, precision drilling, laser detection, microbending, laser measurement and other technical aspects.
Telecommunications Applications: To meet today's high-capacity communication requirements, the application of fiber lasers has become an emerging technology in communication.
Future communication technology will gradually transition from electrical to optical communication. Fiber lasers can not only generate continuous laser output, but also produce ultrashort picosecond (ps) or even femtosecond (fs) laser pulses.
Fiber lasers have made great strides in lowering thresholds, broadening wavelength ranges, and tunable wavelength capabilities. Soliton communication, a practical technology, can achieve a transmission distance of millions of kilometers, a transmission rate of 20 Gb/s and a bit error rate of less than 10-13, achieving high-speed and high-quality signal transmission .
Military Applications: With the continued increase in the power of fiber lasers, their application in the military is becoming increasingly widespread.
To achieve the purpose of directed energy weapons, multiple fiber lasers are combined into a coherent array structure, which can increase the power of fiber lasers.
At the United States Air Force Research Laboratory, research is currently being carried out on 100 kW fiber lasers to meet military application objectives.
