1. Single-wavelength EDFAs are divided into preamplifiers (PA), power amplifiers (BA), and line amplifiers (LA). What are the differences between them?
Answer: Erbium-doped fiber small-signal amplifiers (PAs) are specifically designed for amplifying weak optical signals in the range of -45dBm to -25dBm. Typical small-signal gains are as high as 35~45dB, while maintaining a low noise figure. They are commonly used before photodetectors to pre-amplify signals and improve their ability to detect weak light signals, hence the name preamplifier. For example, in the diagram below, with an input optical power of -35dBm, after amplification by a PA35 amplifier, the signal gains are provided by more than 35dB in the spectrum, and the signal-to-background noise ratio is still higher than 30dB. If a BA amplifier is used instead, although the total power can be amplified to the same level, the signal-to-background noise ratio will only be around 10dB or lower. The focus of PA amplifiers is to achieve a high gain coefficient.
Erbium-doped fiber line amplifiers (LA, In-Line Amplifiers) are optical power amplifiers specifically designed for line repeaters in fiber optic laser or fiber optic communication systems. They combine the advantages of power amplifiers (PA) and fiber optic amplifiers (BA), providing high-gain amplification for small signals and emitting high laser power. They feature high gain, high transmit power, and relatively low noise, and are used between fiber segments to increase repeater length or to compensate for branch losses in point-to-multipoint sections of optical access networks. Simply put, they can be considered a combination of PA and BA.
2. What are the ACC and APC modes of an EDFA?
Answer: ACC mode - Automatic Current Control: The EDFA pump current is set by the user and automatically locked by the EDFA, maintaining a constant pump current. Even if the input optical power fluctuates, the pump current will not respond, so the output power will also fluctuate. The EDFA does not interfere with this power fluctuation. ACC mode is available for all EDFA models. Small-signal EDFA amplifiers only have ACC mode.
APC Mode - Automatic Power Control: The user sets the signal optical output power of the EDFA, the PD automatically monitors and provides feedback on the output power, and the EDFA controls and adaptively adjusts the pump to stabilize the output signal. The advantage of APC mode is that when the input optical power fluctuates, the EDFA will minimize output power fluctuations, making it suitable for both power-type and line-type EDFAs.
The following diagram will help illustrate this:
3. Erbium-doped fiber amplifiers (EDFAs) are available in various types on the website, including single-wavelength EDFAs, multi-wavelength EDFAs, and pulsed EDFAs. What are the advantages of pulsed EDFAs compared to single-wavelength EDFAs?
Answer: Single-wavelength EDFA, multi-wavelength EDFA, and pulsed EDFA, while all based on the same principle—using a 980nm laser to pump erbium-doped fiber to generate optical gain in the C-band and L-band—are optimized and processed according to the type of input optical signal. Single-wavelength EDFA products are primarily designed for applications where only a single wavelength signal is input in the C-band or L-band at any given time, without considering simultaneous amplification of multiple wavelengths. Multi-wavelength EDFAs with gain flatness are designed for applications where multiple wavelength signals are input simultaneously in the C-band or L-band, requiring consideration of gain flatness for simultaneous amplification. Pulsed EDFAs are mainly for low repetition frequency (<1MHz) and narrow pulse width signals, as such pulse signals are prone to generating high pulses during amplification, exciting various nonlinear effects, leading to spectral degradation and pulse distortion. Therefore, the amplifier, while meeting power amplification requirements, minimizes EDFA distortion. 4. Optical nonlinear effects during amplification reduce pulse distortion and improve the signal-to-background ratio in the amplified signal spectrum.
5. Can a pulsed amplifier maintain the pulse shape when amplifying a square wave pulse?
Answer: Although pulsed amplifiers are optimized for pulsed laser signals, when amplifying wide-pulse square wave laser signals (pulse width > 50ns), due to the characteristics of the gain fiber, the leading edge of the signal pulse receives priority gain amplification, while the gain in the middle and tail of the pulse gradually decreases. Therefore, a square wave pulse with a flat top often exhibits a shape with an upward-curving leading edge and a gradually decreasing middle and tail after EDFA amplification. This phenomenon cannot be eliminated, and it becomes more pronounced with a wider pulse width (it can be understood as the upper-level ions consumed by the signal at the beginning of the pulse not having enough time to be replenished before the signal at the end of the pulse arrives, hence the gradual decrease in gain in the middle and rear of the pulse). As shown in the figure below, the pulse waveforms after amplification of a 500ns pulse and a 40us pulse show that the distortion is more severe after amplification of the 40us pulse.
5. How to choose a pulsed EDFA model? What is the price?
Answer: Pulsed laser amplifiers are quite specialized, with many parameter combinations. Each customer‘s required parameters are almost always different, so there is currently no standard price. The price depends on the customer‘s pre-amplification signal parameters and characteristics (signal wavelength/spectral signal-to-background ratio/pulse width/repetition rate/peak power), and their desired post-amplification pulse peak power and spectral signal-to-background ratio. For signals with pulse widths greater than 50ns, we also need to discuss with the user whether they can accept the distortion after pulse amplification (as mentioned above). We will then provide a quote after a detailed evaluation of the proposed solution.
What is a pump protector? How to choose one?
Answer: A pump protector is a fiber-coated filter device, typically placed between the pump laser and the user‘s WDM (Wave DM). It prevents a small portion of the signal light generated by the rare-earth fiber from returning to the pump via the WDM, thus preventing damage to the pump laser. For ease of use, our 980nm pump laser products can integrate the protector inside the chassis. This device allows bidirectional transmission of the pump wavelength (910~990nm) but blocks the signal light from either erbium-doped fiber (anti-reflection wavelength 1500~1600nm) or ytterbium-doped fiber (anti-reflection wavelength 1020~1120nm), thus protecting the pump laser. It‘s important to note that the pump protectors for erbium-doped and ytterbium-doped fibers are not interchangeable. Therefore, when purchasing a pump laser, you should confirm with us whether a protector is built into the pump module and the specific anti-reflection wavelength of the protector. Additionally, the protector does not provide unidirectional isolation for the 980nm pump wavelength; that is, 980nm laser light reflected along the output fiber can still return to the pump laser through the pump protector. If unidirectional isolation of the pump output is required, a 980nm isolator can be added. The usage of the pump protector is shown in the following diagram:
6. What is a Raman amplifier? How is it used?
Answer: A fiber optic Raman amplifier utilizes the Raman effect in silica fiber to provide gain to optical signals. It achieves high-gain, low-noise amplification of optical signals in the C or L bands, effectively compensating for signal attenuation in long-distance fiber optic transmission. It is commonly used in long-distance optical transmission systems, distributed fiber optic sensing, and dense wavelength division multiplexing (WDM) optical transmission systems. It‘s important to note that the gain of a Raman amplifier refers to the power difference between the pumped and de-pumped states, also known as switching gain. For signals with wavelengths around 1550nm, a Raman-pumped laser with wavelengths between 1450 and 1460nm is typically used. The signal light and the Raman pump light simultaneously enter the long-distance transmission fiber via WDM, forming a distributed fiber optic Raman amplifier. For signals with multiple wavelengths distributed in the C band, Raman pumping also needs to select multiple wavelengths from 1420 to 1470nm to achieve flat gain, as shown in the diagram below. The Raman amplifiers we provide are mainly Raman pump lasers. Users can provide their own wavelength division multiplexing (WDM) devices and gain fibers, or we can provide the complete set.
In our user system case, the Raman amplifier is shown below (schematic diagram).
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