We report the realization of a thulium doped fiber amplifier designed for optical communications providing high gain (>35dB) and low noise figure (<;6dB) over 1910nm-2020nm with a maximum saturated output power of more than 1W.

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Thulium-doped fiber amplifier for optical communications at 2µm

This paper is a tutorial reviewing research and development performed over the last few years to extend the reach of passive optical networks using technology such as optical amplifiers.

Long-Reach Passive Optical Networks

A source of pulses of coherent radiation at a wavelength of approximately 1 μm, comprises a pump source for producing pump light; a laser cavity comprising an Yb3+-doped gain medium arranged to receive the pump light, the laser cavity being modelocked to generate laser pulses at a defined repetition rate; a pulse detector arranged to generate a pulse selection signal indicative of the repetition rate; a pulse selector arranged to reduce the repetition rate of the laser pulses responsive to the pulse selection signal from the pulse detector by passing only selected ones of the laser pulses; and at least one optical amplifier for amplifying the laser pulses of reduced repetition rate. The at least one optical amplifier can be configured for chirped or parabolic pulse amplification.

Pulsed light sources

Continuous-wave silica-based erbium-doped fibre lasers

The first demonstration of a hollow core photonic bandgap fiber (HC-PBGF) suitable for high-rate data transmission in the 2 µm waveband is presented. The fiber has a record low loss for this wavelength region (4.5 dB/km at 1980 nm) and a >150 nm wide surface-mode-free transmission window at the center of the bandgap. Detailed analysis of the optical modes and their propagation along the fiber, carried out using a time-of-flight technique in conjunction with spatially and spectrally resolved (S2) imaging, provides clear evidence that the HC-PBGF can be operated as quasi-single mode even though it supports up to four mode groups. Through the use of a custom built Thulium doped fiber amplifier with gain bandwidth closely matched to the fiber's low loss window, error-free 8 Gbit/s transmission in an optically amplified data channel at 2008nm over 290m of 19 cell HC-PBGF is reported.

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Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth hollow core photonic bandgap fiber

A novel structure, which utilizes detrimental backward amplified spontaneous emission as a secondary pump source is suggested for a silica-based fiber amplifier, operating at a wavelength range from 1570 to 1610 nm. By using the secondary pumping effect from the strong, wasted 1550-nm band amplified spontaneous emission power in the unpumped section of the erbium-doped fiber, it was possible to achieve a considerable improvement in power conversion efficiency, increasing small-signal gain by more than 4 dB. The suggested pump structure was also shown to be useful in overall conversion efficiency improvement for L-band EDFA's, regardless of pump wavelength choice.

Enhancement of power conversion efficiency for an L-band EDFA with a secondary pumping effect in the unpumped EDF section

A flat-gain output of 13.5 (15.5) dBm is obtained over 30 nm in the L-band with only a 97 (130)-mW pumping of a 980-nm laser diode. Such highly efficient flat-gain L-band erbium-doped fiber amplifiers are achieved by simply adding a regular fiber Bragg grating. An efficient and simple gain-clamping operation is also demonstrated.

Demonstration of highly efficient flat-gain L-band erbium-doped fiber amplifiers by incorporating a fiber Bragg grating

We demonstrate a novel means of gain flatness monitoring and control of a multichannel gain flattened erbium-doped fiber amplifier (EDEA), which does not require a gain dependent reference level for the feedback loop. Using the difference of amplified spontaneous emission/probe powers extracted from the edges of the gain-flattened bandwidth, gain flatness monitoring, equalization, and transient gain suppression can be achieved regardless of the steady-state gain value of an EDFA. This approach eliminates the need of elaborate measurements for the determination of control parameters for each application, making it possible to use the same circuitry for different amplifiers. Measurements show flatness control within 0.03 dB/10 nm, at the same time the suppression of time-dependent gain excursion to below 0.2 dB.

Reference level free multichannel gain equalization and transient gain suppression of EDFA with differential ASE power monitoring

Channel power oscillations in an optically amplified wavelength-division-multiplexed (WDM) ring network have been analyzed to reveal the oscillation dynamics and governing parameters of the chaotic behavior. The WDM ring round-trip frequency determined by the closed ring network span length and the speed of the channel power equalizer were found to be the two main factors defining the network stability. Spectral analysis shows that the formerly unresolved chaotic ring network behavior can be understood in terms of mode-locking-like instability which takes a closed WDM ring network as a loss-modulated laser.

Analysis on the channel power oscillation in the closed WDM ring network with the channel power equalizer

An efficient and tunable 1580-nm-band erbium-doped fiber ring laser is achieved by injection of amplified spontaneous emission in a conventional band by a fiber Bragg grating into the gain medium. The insertion of the fiber Bragg grating reduces the lasing threshold to 25%. The tuning range is 1560-1610 nm; the side-mode suppression ratio is better than 65 dB.

Seong Joon Ahn

Papers

Enhancement of power conversion efficiency for an L-band EDFA with a secondary pumping effect in the unpumped EDF section

Demonstration of highly efficient flat-gain L-band erbium-doped fiber amplifiers by incorporating a fiber Bragg grating

Reference level free multichannel gain equalization and transient gain suppression of EDFA with differential ASE power monitoring

Analysis on the channel power oscillation in the closed WDM ring network with the channel power equalizer

Incorporation of a fiber Bragg grating to improve the efficiency of a 1580-nm-band tunable fiber ring laser.