All-optical Feedback Loop Research Articles

A Tunable Single-Mode All-Optical Microwave Oscillator by Using Period-One Oscillation in DFB-LD

A tunable all-optical microwave oscillator, which has no electrical microwave devices as well as photoelectric conversion and electro-optic modulation, is proposed and experimentally demonstrated. In this scheme, a distributed-feedback laser diode is utilized to be a light source and create a seed microwave signal from the period-one (P1) oscillation by a self-delayed optical feedback injection. A semiconductor optical amplifier-based all-optical feedback loop is introduced to perform optical–optical feedback modulation and mode selection. Meanwhile, a short fiber loop is coupled to further suppress the residual side modes. In the experiment, single-mode microwave photonic signals with good quality are obtained, whose frequencies can be tuned from 6.10 to 15.59 GHz by adjusting the optical feedback ratio. The measured single-sideband phase noise is −89 dBc/Hz at 10-kHz offset from a carrier of 10.98 GHz.

Passive Phase Locking of Three Nanosecond Fiber Amplifiers Using a Dammann Grating Spatial Filter

A passive coherent beam combination of three nanosecond Yb-doped fiber amplifiers by an all-optical feedback loop is realized by a Dammann grating intracavity spatial filter. By using this diffractive-optics-based spatial filtering technique, three tile-aperture laser beams are phase-locked with a peak power of 1.02 kW. The width of the combined pulses is 9.6 ns, and the repetition frequency is 2.208 MHz. The visibility of the far-field interference pattern is up to 82.9%. The results show that this approach can scale to larger arrays and higher powers.

Experiments and Perturbative Analysis of Dammann-Grating-Based Aperture Filling in a Passive Coherent Beam Combination

A Dammann-grating-based aperture filling technology is demonstrated in a passive coherent beam combination fiber amplifier array, which is phase-locked using an all-optical feedback loop. The maximum output power is 206 W. The combined efficiency of a three channels aperture filling is discussed and demonstrated to vary with the output power. Perturbative factors leading to the loss of the combined efficiency are theoretically simulated and the key factor is the residual phase error. Theoretical and experimental results show that this technology has good potential to achieve higher brightness in a single beam.

Stimulated Brillouin scattering, phase noise, and dynamical evolution in a passive fiber laser coherent beam combination

A theoretical model is proposed to study a passive fiber laser coherent beam combination (CBC) with an all-optical ring-cavity feedback loop. The dynamic evolution of a frequency filter is carefully studied and is shown to be sensitive to stimulated Brillouin scattering (SBS) and phase noise. SBS plays an important role in the process of mode selection or CBC because the linewidth of the longitudinal mode is narrowed continuously in the ring-cavity feedback. Numerical studies show that the SBS process is significantly influenced by phase noise, optical path length difference, and four-wave mixing. Our results indicate that SBS would be a remarkable problem in this type of CBC unless the phase noise causes a rapid mode jump in the frequency domain. Since the mode jump worsens the efficiency of the coherent combination or beam quality, a trade-off consideration is necessary for the SBS, phase noise, and beam quality. The maximal SBS-allowed CBC efficiency is obtained numerically for output laser power of 700 W–3.5 kW.

Dammann-grating-based passive phase locking by an all-optical feedback loop

A Dammann grating is used as a spatial filter for a passive coherent beam combination (CBC) of three Yb-doped fiber amplifiers with an all-optical feedback loop. Using this diffractive-optics-based spatial filtering technique, we demonstrate CBC with 20 W output power, and the visibility of the far-field interference pattern is up to 88.7%. Measurements suggest that this approach is robust with respect to laboratory environment perturbations, and it can scale to high powers and large arrays.

Passive coherent beam combining of four Yb-doped fiber amplifier chains with injection-locked seed source

An injection-locked fiber laser is introduced to the passive fiber laser coherent beam combination with all-optical feedback loop. A coherent beam combining system with two-dimensional four Yb-doped fiber amplifier chains is established, and the injection-locked fiber laser works as a switchable seed source. The 1064 nm output laser of the injection-locked fiber laser is extinguished automatically as the feedback injection power is high enough, and the injection-locked fiber laser acts as an amplifier for the feedback laser with 7.4 dB gains. We find that the phase-locked far-field interference pattern of our system with seed laser extinguished is stable, and the visibility is up to 91.5%, which is slightly higher than the prevalent method with auxiliary seed laser (88.2%).

复合环形腔被动相干合成技术重要特性研究

复合环形腔被动相干合成技术重要特性研究

Coherent beam combination of two nanosecond fiber amplifiers by an all-optical feedback loop

A passive coherent beam combination of two nanosecond amplifiers is realized by using an all-optical feedback loop. The width of the combined pulses is 9.7 ns, and the pulse repetition frequency is 2.023 MHz. With the least mismatch between pulse period and time of the cavity round trip in our loop, the visibility of far-field coherent patterns is more than 71%. The dynamics of passive phase locking is studied, and the established time of phase locking of two pulsed amplifiers is at least 10 cavity round trips in the all-optical feedback loop.

Polarization self-selection in a coherent beam combination system with an all-optical feedback loop

Polarization self-selection in passive phasing of four fiber amplifiers with an all-optical feedback loop is demonstrated. The polarization extinction ratio (PER) of the combined beam is increased, and the polarized direction is selected with the use of a polarization-maintaining (PM) isolator and some non-PM components. The best visibility of the interference patterns is observed at 95.2% and in the largest increment in the PER of the combined beam up to 7.4 dB. Results show that all PM components are unnecessary in the coherent beam combination with an all-optical feedback loop, whereas non-PM components have good potential to achieve high output power.

High Power Passive Phase Locking of Four Yb-Doped Fiber Amplifiers by an All-Optical Feedback Loop

We report the passive phase locking of four high power Yb-doped fiber amplifiers with ring cavity. The interference patterns at different output power are observed and the Strehl ratios are measured. The maximum coherent output power of the fiber array is up to 1062 W by multi-stage amplification. The stable beam profiles of various phase relationships are observed by controlling the position of the feedback fiber, in good agreement with the calculated results. By using master oscillator power-amplifier (MOPA) architecture and broadband operation of passively phased systems, higher power scaling with high beam quality appears to be feasible.

Power transients in time-division multiplexed discrete Raman fibre amplifier

In this paper, the effect of addition and/or dropping of wavelength-division multiplexed channels in an all-optical gain-clamped time-division multiplexed (TDM) pumped discrete Raman fibre amplifier (DRFA) has been investigated experimentally and analyzed by numerical simulation. Nineteen signals were amplified in a counter-directionally pumped DRFA consisting of 13 km long dispersion compensating fibre. Light of eight DFB lasers was square-wave modulated at 2 kHz to imitate channel addition/dropping and combined with signals from five 10 Gigabit Ethernet transmitters and with six continuous wave signals. Power transients of the surviving channels caused by cross-gain modulation of the DRFA were monitored at the output of the amplifier. All-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the TDM-pumped DRFA was based on numerical solution of coupled propagation equations for backward propagating pumps, signals, and both forward and backward propagating spectral components of amplified spontaneous emission powers.

Design and analysis of dynamic erbium-doped fiber amplifier gain-clamping systems with feedback control

We present an accurate and quantitative design scheme for erbium-doped fiber amplifier (EDFA) gain control systems. The proposed design scheme is demonstrated through the design of an eight-channel EDFA gain-clamping system with an all-optical self-tuning feedback loop. The effect of the time delay in the feedback loop and the selection of feedback wavelength are theoretically analyzed. Compared with existing electrical-optical EDFA gain schemes, the proposed design scheme is free from the feedback loop response speed limitation and does not need any predetermined information of the network traffic variation or operation point estimation. It can efficiently enhance the static and dynamic performance of the whole system. Furthermore, since the design can efficiently suppress the relaxation oscillation transient, it provides a cost-effective solution for the gain-clamping problem of a long EDFA chain by combining it with a fast-link control method.

Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop

We report the passive phase locking of an array of four fiber amplifiers in a unidirectional ring cavity. The feedback loop consists of a single-mode fiber that filters intracavity the far-field pattern of the four emitted beams. The pointing of the laser output can be managed by the intracavity filtering.

Surviving-Channel-Power Transients in Second-Order Pumped Lumped Raman Fiber Amplifier: Experimentation and Modeling

In this paper, the effect of addition and/or dropping of wavelength-division-multiplexed channels in an all- optical gain-clamped (AOGC) second-order (SO) pumped lumped Raman fiber amplifier (LRFA) has been investigated experimentally and analyzed by numerical simulation. Channel addition/ removal was simulated by transmitting ten signals through a counter-directionally pumped LRFA consisting of a 16-km-long dispersion-compensating fiber. The light from eight lasers was square-wave modulated at 500 Hz; power transients of the surviving channels caused by cross-gain modulation of the LRFA were monitored at the output of the amplifier. All-optical feedback loop was implemented in the form of a ring laser. Gain-clamping properties of the SO-pumped LRFA are compared with those of the first-order pumped LRFA having the same ON/OFF Raman gain. Theoretical analysis of the AOGC LRFA was based on numerical solution of coupled propagation equations for the backward propagating pump, signals, and both forward and backward propagating spectral components of amplified spontaneous emission powers.

Channel Addition–Removal Response in All-Optical Gain-Clamped Lumped Raman Fiber Amplifier

Application of all-optical gain-clamped (AOGC) lumped Raman fiber amplifier (RFA) for protection of surviving channels in multiwavelength networks is investigated experimentally and theoretically. Channel addition-removal was simulated by transmitting signals of two lasers through a counterdirectionally pumped RFA consisting of 16 km of dispersion compensating fiber. Light of one of the lasers was square-wave modulated at 500 Hz; power fluctuations of the other laser caused by cross-gain modulation of the RFA were monitored at the output of the amplifier with a digital oscilloscope. An all-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the AOGC lumped RFA is based on numerical solution of coupled propagation equations for forward and backward-propagating pumps, signals, and spectral components of amplified spontaneous emission powers.

Protection of surviving channels in all-optical gain-clamped lumped Raman fibre amplifier: modelling and experimentation

In this article we investigate, both theoretically and experimentally, suppression of transient effects in all-optical gain-clamped (AOGC) lumped Raman fibre amplifiers (RFA) caused by channel addition/removal. Theoretical analysis of the AOGC lumped RFA is based on numerical solution of coupled propagation equations for forward and backward propagating pumps, signals and spectral components of amplified spontaneous emission powers. In experimental verification, channel addition/removal was simulated by transmitting signals of two lasers through a counter-directionally pumped RFA consisting of 16 km of dispersion compensating fibre (DCF). Light of one of the lasers was square-wave modulated at 500 Hz, power fluctuations of the other laser caused by cross-gain modulation of the RFA were monitored at the output of the amplifier with a digital oscilloscope. All-optical feedback loop was implemented in the form of a ring laser.

All-optical gain control of in-line erbium-doped fiber amplifiers for hybrid analog/digital WDM systems

Automatic gain control using an all-optical feedback loop in in-line erbium-doped fiber amplifiers (EDFA's) used in hybrid analog/digital wavelength division multiplexing (WDM) systems was studied. It is found that the signal level variation for the digital channels can be maintained within a range /spl les/3-dB between the presence and dropout of the analog channel when the narrowband feedback is centered at the amplified spontaneous emission (ASE) peak (/spl sim/1532 nm) with loop loss ranging between 13-22 dB. Robust transmission at 2.5 Gb/s without measurable power penalty was obtained for the digital channels when the EDFA was saturated by either the analog or the control lasing signal.

All-optical versus electro-optical quantum-limited feedback.

All-optical feedback can be effected by putting the output of a source cavity through a Faraday isolator and into a second cavity which is coupled to the source cavity by a nonlinear crystal. If the driven cavity is heavily damped, then it can be adiabatically eliminated and a master equation or quantum Langevin equation derived for the first cavity alone. This is done for an input bath in an arbitrary state, and for an arbitrary nonlinear coupling. If the intercavity coupling involves only the intensity (or one quadrature) of the driven cavity, then the effect on the source cavity is identical to that which can be obtained from electro-optical feedback using direct (or homodyne) detection. If the coupling involves both quadratures, this equivalence no longer holds, and a coupling linear in the source amplitude can produce a nonclassical state in the source cavity. The analogous electro-optic scheme using heterodyne detection introduces extra noise which prevents the production of nonclassical light. Unlike the electro-optic case, the all-optical feedback loop has an output beam (reflected from the second cavity). We show that this may be squeezed, even if the source cavity remains in a classical state.

Gain control in erbium-doped fibre amplifiers by an all-optical feedback loop

An all-optical feedback loop is demonstrated which effectively suppresses slow, signal-induced gain fluctuations in Er-doped fibre amplifers (EDFAs). Gain control is achieved by operating the amplifier in a ring-laser configuration. The working principle is shown theoretically and experimentally by coupling a selected wavelength of the ASE at the output of the EDFA back to the input. Such a feedback loop, which can be fabricated out of all-fibre passive components, could be easily implemented in any system using EDFAs.