An optical signal processing method and apparatus. The method includes: obtaining a first sending signal, where the first sending signal is a signal that is sent by a first transmitter to a second receiver through a first optical fiber; determining estimation information of a backward optical signal based on the first sending signal; the backward optical signal is generated during transmission of the first sending signal, the backward optical signal is transmitted through at least one fiber section in the first optical fiber, and a transmission direction of the backward optical signal is opposite to a transmission direction of the first sending signal; and obtaining a second sending signal based on the estimation information of the backward optical signal. According to the embodiments, impact of the backward optical signal on effective signal transmission can be reduced, and a signal-to-noise ratio can be improved.

A multi-stage optical amplifier has an input port for receiving an optical signal and a relatively short erbium doped optical fiber is coupled to the input port. Complex costly pump feedback is not required as a constant non-varying saturation pump is configured to provide non varying output power pump light of a predetermined wavelength suitable for excitation and full saturation of the erbium ions such that a full population inversion occurs. The length of the short erbium doped fiber and rare earth doping concentration of the erbium doped fiber is such that when pumped by said pump provides amplification of the optical signal of less than 15 dB. Locating a gain flattening filter after the short erbium doped optical fiber provides a relatively flat amplified output signal. Multi-stages of similar short erbium doped fibers pumped and saturated by the same pump signal economically provide increased amplification of the signal and filters after each state flatten the gain.

An erbium doped block of glass has input port and reflective end faces arranged such that a signal is launched into the block and is amplified as it traverses the block following a zig-zag path. A laser diode pump is focused to excite erbium ions within the block thereby amplifying the input signal light traversing the block numerous times. A gain flattening filter flattens the gain of the signal being amplified numerous times as the filter is within the path upon each pass across the block.

A multipass fiber amplifier comprises a micro-optic-module polarization separating device including a first ASE blocking device, a micro-optic-module 90 polarization rotating reflector including a second ASE blocking device, a pump source for providing pump light; a micro-optic-module wavelength-division multiplexer (WDM) for combining the pump light and the laser beam; and a gain fiber having a first end and a second end for amplifying the laser beam using the pump light, where the first ASE blocking device is coupled to the first end of the gain fiber and the second ASE blocking device is coupled to the second end of the gain fiber.

A multipass fiber amplifier comprises a micro-optic-module polarization separating device including a first ASE blocking device, a micro-optic-module 90 polarization rotating reflector including a second ASE blocking device, a pump source for providing pump light; a micro-optic-module wavelength-division multiplexer (WDM) for combining the pump light and the laser beam; and a gain fiber having a first end and a second end for amplifying the laser beam using the pump light, where the first ASE blocking device is coupled to the first end of the gain fiber and the second ASE blocking device is coupled to the second end of the gain fiber.

Techniques for extending distributed acoustic sensing (DAS) range in undersea optical cables are provided. For example, DAS range can be extended by transmitting and amplifying a DAS signal along multiple spans of a first optical fiber, routing or bypassing the DAS signal from the first optical fiber to a second optical fiber different from the first fiber via a high-loss loopback architecture, and returning and amplifying the DAS signal along the same multiple spans back to a DAS device. The DAS device may then receive and process the DAS signal to detect any changes in the DAS environment. The loopback configuration may be based on different types of loopback architecture.