A random distributed Rayleigh feedback fiber laser based on a double-cladding weakly ytterbium-doped fiber includes: a pump laser source, a pump combiner, a cladding power stripper, and a double-cladding weakly ytterbium-doped fiber for simultaneously achieving distributed active gain and random distributed Rayleigh feedback. An output end of the pump combiner is connected with one end of the double-cladding weakly ytterbium-doped fiber, the other end of the double-cladding weakly ytterbium-doped fiber is connected with an input end of the cladding power stripper, and a concentration of ytterbium ions in the double-cladding weakly ytterbium-doped fiber is in a range of 0.510.sup.23 to 110.sup.25/m.sup.3. The laser provided by the present invention solves the problem that the existing random fiber lasers cannot simultaneously utilize distributed active gain and random distributed Rayleigh feedback with a single type of fiber.

An optical communication apparatus includes: a light-receiving device that receives an optical signal transmitted from another optical communication apparatus through an optical fiber and converts the optical signal into an electrical signal; a first measurement circuit that measures an average power and a modulation power of the optical signal based on the electrical signal; a light-emitting device that transmits the optical signal to the another optical communication apparatus by emitting light in accordance with a driving current; a driver that causes the light-emitting device to transmit the optical signal according to a transmission signal by controlling the driving current based on the transmission signal; and a processor that adjusts the driving current based on the average power and the modulation power.

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.

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 fiber amplifier has a first amplification stage and a second amplification stage. The first amplification stage comprises a first gain fiber that is configured to receive, at its input end, a signal light and a pump light. The first gain fiber uses a portion of the pump light to provide first-stage amplification to the signal light. The second amplification stage comprises a second gain fiber that is configured to receive, at its input end, the first-stage-amplified signal light and residual pump light. The second gain fiber uses the residual pump light to provide second-stage amplification of the first-stage-amplified signal light and to provide, at its output end, the second-stage amplified signal light. The first amplification stage may include a single-mode gain fiber, and the second amplification stage may include a higher-order-mode gain fiber, and the first amplification stage may be configured to provide single-mode amplification of a sub-threshold input to satisfy the low-ASE threshold of the second amplification stage.

An optical amplifier repeater system includes an optical fiber propagating a light beam in a plurality of propagation modes and an optical amplifier repeater amplifying the light beam propagated through the optical fiber. The optical amplifier repeater includes an optical demultiplexer demultiplexing the light beam in the plurality of propagation modes propagated through the optical fiber into a plurality of single-mode light beams, an optical amplifier amplifying, by simultaneous pumping, intensities of the plurality of single-mode light beams using a light beam generated by one pumping light source, an optical multiplexer multiplexing the plurality of single-mode light beams amplified by the optical amplifier into a light beam in the plurality of propagation modes, and an optical intensity adjusting unit adjusting the intensity of each of the plurality of single-mode light beams at least one of before or after the amplification by the optical amplifier. The optical intensity adjusting unit performs the adjustment by amplifying or attenuating the optical intensity of each of the plurality of single-mode light beams in an individual optical path through which the single-mode light beam is propagated.

A random distributed Rayleigh feedback fiber laser based on a double-cladding weakly ytterbium-doped fiber includes: a pump laser source, a pump combiner, a cladding power stripper, and a double-cladding weakly ytterbium-doped fiber for simultaneously achieving distributed active gain and random distributed Rayleigh feedback. An output end of the pump combiner is connected with one end of the double-cladding weakly ytterbium-doped fiber, the other end of the double-cladding weakly ytterbium-doped fiber is connected with an input end of the cladding power stripper, and a concentration of ytterbium ions in the double-cladding weakly ytterbium-doped fiber is in a range of 0.510.sup.23 to 110.sup.27/m.sup.3. The laser provided by the present invention solves the problem that the existing random fiber lasers cannot simultaneously utilize distributed active gain and random distributed Rayleigh feedback with a single type of fiber.

An optical communication apparatus includes: a light-receiving device that receives an optical signal transmitted from another optical communication apparatus through an optical fiber and converts the optical signal into an electrical signal; a first measurement circuit that measures an average power and a modulation power of the optical signal based on the electrical signal; a light-emitting device that transmits the optical signal to the another optical communication apparatus by emitting light in accordance with a driving current; a driver that causes the light-emitting device to transmit the optical signal according to a transmission signal by controlling the driving current based on the transmission signal; and a processor that adjusts the driving current based on the average power and the modulation power.

A light amplifier according to an aspect of the present invention includes: a seed light source configured to generate a pulsing seed light; an excitation light source configured to generate excitation light; a light amplifying fiber configured to amplify the seed light by the excitation light and output the amplified light; and a control unit configured to control the seed light source and the excitation light source. The control unit has a mode to control the excitation light's power such that as a set value of a pulse width of the amplified light increases, the amplified light's peak energy increases within a threshold value at a minimum set value of the pulse width.