◆Characteristics and applications of fiber-doped amplifiers
An optical amplifier is a device that can directly amplify weak optical signals. It amplifies weak incident light based on the principles of stimulated emission or stimulated scattering, and its mechanism is exactly the same as that of a laser. In fact, structurally, an optical amplifier is a laser with little or no feedback. Optical gain is achieved when the optical medium experiences population inversion under the action of pump current or pump light, thus realizing optical amplification. This section introduces commonly used types of optical amplifiers and focuses on the principles and applications of erbium-doped fiber amplifiers.
Classification of optical amplifiers
Optical amplifiers can be broadly classified into three types based on their operating principles
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These types of optical amplifiers differ in their operating principles and excitation methods.
Working principle of ballasted fiber amplifier
EDFA structure
An erbium-doped fiber amplifier (EDFA) is a device that uses erbium-doped fiber as the gain medium to amplify signal light using pump light emitted from a laser diode. The structure of an erbium-doped fiber amplifier is shown in the figure.
A wavelength division multiplexer, also known as a multiplexer, combines pump light and signal light of 980/1550nm or 1480/1550nm wavelengths before feeding them into an erbium-doped fiber. It requires low insertion loss and insensitivity to light polarization.
An optical isolator ensures unidirectional light transmission, preventing light reflection back to the original device, as such reflection increases amplifier noise and reduces amplification efficiency.
The function of an optical filter is to filter out noise outside the operating bandwidth of an optical amplifier, thereby improving the signal-to-noise ratio of the system.
Erbium-doped fiber is the core component of an EDFA (Electrium-Doped Amplifier). It uses silica fiber as the matrix and dops the core with erbium ions, a solid-state laser working material. Within the several meters to tens of meters of erbium-doped fiber, light interacts with the matter and is amplified and enhanced.
The mode field diameter (MFD) of erbium-doped fiber is 3–6 μm, much smaller than that of conventional fiber (9–16 μm). This is to increase the energy density of the signal light and pump light, thereby improving their interaction efficiency. However, the reduced core diameter of erbium-doped fiber also leads to a mode field mismatch with conventional fiber, resulting in greater reflection and connection loss. The solution is to dope the fiber with a small amount of fluorine to lower the refractive index, thereby increasing the mode field diameter to achieve a level of match with conventional fiber. Additionally, during fusion splicing, the mode field diameter mismatch can be reduced by using transition fibers or lengthening the conventional fiber connector to reduce the core diameter.
To achieve more efficient amplification, during the fabrication of erbium-doped fibers, most ions are concentrated in the central region of the fiber core. This is because, in optical fibers, the light fields of the signal and pump light can be considered approximately Gaussian distributed, with the strongest light intensity along the fiber core axis. The presence of molybdenum ions in the paraxial region allows for greater interaction between light and matter, thereby improving energy conversion efficiency. Depending on the application of erbium-doped fiber amplifiers (EDFAs), various types of erbium-doped fibers are available for EDFA design. For example, the EDF-PAX-01 type is used for designing in-circuit amplifiers and preamplifiers, exhibiting a flat and wide gain bandwidth; the EDF-LAX-01 type can be used in in-circuit amplifiers, offering high power conversion efficiency and low noise; and the EDF-BAX-01 type provides high output power, etc.
Basic Parameters of Erbium-Doped Fibers
| Parameter | EDF-PAX-01 | EDF-LAX-01 | EDF-BAX-01 | EDF-HCX-01 |
|---|---|---|---|---|
| Numerical Aperture (NA) | 0.24 ± 0.02 | 0.24 ± 0.02 | 0.22 ± 0.02 | 0.24 ± 0.02 |
| Cut-off Wavelength (nm) | 953 ± 35 | 953 ± 35 | 920 ± 40 | 920 ± 40 |
| Peak Core Absorption @ 1530 nm (dB/m) | ≤ 1529.5 | 1530.5 ± 0.5 | 1531 ± 0.5 | 1530 ± 1 |
| Peak Attenuation @ 980 nm (dB/m) | 7 ± 2 | 7 ± 2 | 5 ± 2 | 8.5 ± 2 |
| Attenuation @ 980 nm (dB/m) | 5 ± 1.5 | 5 ± 1.5 | 3.55 ± 1.5 | 8.5 ± 2 |
| Background Loss @ 1200 nm (dB/km) | < 35 | < 15 | < 15 | < 15 |
| Saturated Power @ 1530 nm (mW) | 0.17 | 0.15 | 0.18 | 0.20 |
| Mode Field Diameter @ 1550 nm (μm) | 4.8 ~ 5.9 | 4.8 ~ 5.9 | 5.2 ~ 6.6 | 4.8 ~ 6 |
The pump source is another core component of the EDFA (Electro-Doped Fiber Optic Amplifier). It provides sufficient energy for optical signal amplification, a necessary condition for achieving population inversion of the activator particles. Since the pump source directly determines the performance of the EDFA, it is required to have high output power, good stability, and long lifespan. Practical EDFA pump sources are all laser diodes, with pump wavelengths of 980nm and 1480nm. The 980nm pump source is more commonly used due to its advantages of low noise, high pump efficiency, and power up to several hundred milliwatts.
The pump light and signal enter the optical fiber simultaneously. The pump light is strongest at the erbium-doped fiber inlet. As it propagates along the fiber, it gradually transfers energy to the signal light, increasing the signal strength while its own strength gradually decreases.
The output power and noise characteristics under different pumping methods are compared in the following figures. Figure a shows the relationship between the output optical signal power and the pump optical power; the differential conversion efficiencies of the three pumping methods are 61%, 76%, and 77%, respectively. Figure b shows the relationship between the noise figure and the amplifier's output optical power. As the output optical power increases, the particle inversion number decreases, resulting in an increase in the noise figure. Figure c shows the relationship between the noise figure and the length of the erbium-doped fiber. As can be seen from the figure, regardless of the length of the erbium-doped fiber, the EDFA with the co-directional pumping method has the lowest noise.
