Onset Of Coherence Collapse Research Articles

Analysis of the regimes of feedback effects in quantum dot laser

We investigated the optical feedback effects on the static and dynamic characteristics of 1.3 μm quantum-dot (QD) Fabry–Pérot laser under reflection from −40 dB up to −8 dB. The onset of coherence collapse is determined as −14 dB from the optical and electrical spectra. Although the degradation in small signal modulation is reported above this critical feedback level, transmission operation with available eye diagram under higher feedback is demonstrated. Under 10 Gb s−1 modulation, there is no obvious degradation in eye diagram regarding the eye shape and extinction ratio up to feedback ratio of −8 dB. The higher feedback tolerance of QD laser under large signal modulation is attributed to the impact of gain compression. This high-speed feedback-resistant operation also indicates that QD laser is a promising light source for isolator-free photonic integrated circuits.

Experimental characterization of the transition to coherence collapse in a semiconductor laser with optical feedback.

Semiconductor lasers with time-delayed optical feedback display a wide range of dynamical regimes, which have found various practical applications. They also provide excellent testbeds for data analysis tools for characterizing complex signals. Recently, several of us have analyzed experimental intensity time-traces and quantitatively identified the onset of different dynamical regimes, as the laser current increases. Specifically, we identified the onset of low-frequency fluctuations (LFFs), where the laser intensity displays abrupt dropouts, and the onset of coherence collapse (CC), where the intensity fluctuations are highly irregular. Here we map these regimes when both, the laser current and the feedback strength vary. We show that the shape of the distribution of intensity fluctuations (characterized by the standard deviation, the skewness, and the kurtosis) allows to distinguish among noise, LFFs and CC, and to quantitatively determine (in spite of the gradual nature of the transitions) the boundaries of the three regimes. Ordinal analysis of the inter-dropout time intervals consistently identifies the three regimes occurring in the same parameter regions as the analysis of the intensity distribution. Simulations of the well-known time-delayed Lang-Kobayashi model are in good qualitative agreement with the observations.

Optical Feedback Tolerance of Quantum-Dot- and Quantum-Dash-Based Semiconductor Lasers Operating at 1.55 $\mu$m

This paper reports on the tolerance of low-dimensional InAs/InP quantum-dash- and quantum-dot-based semiconductor lasers to optical feedback in the 1.55 mum window. For this purpose, the onset of coherence collapse (CC) is experimentally determined and systematically investigated as a function of different laser parameters, such as the injection current, differential gain, temperature, and photon lifetime. It is in particular found that for both material systems the onset of CC increases with the injection current in a similar way to bulk or quantum-well-based devices. Of most importance, we experimentally show that the differential gain plays a key role in the optical feedback tolerance. It is indeed shown to determine not only the range of the onset of CC but also the dependence of this threshold both on the temperature and laser cavity length. Increasing the operating temperature from 25degC to 85degC leads to a decrease of the onset of CC by a factor of only ~3 dB, well accounted for by the variation of the differential gain in this temperature range. We find no difference in the tolerance to external reflections of a truly 3-D confined quantum-dot-based laser and a quantum dash device of the same cavity length, which have similar differential gains. A tentative analysis of our data is finally carried out, based on existing models.

Tolerance to Optical Feedback of 10-Gb/s Quantum-Dash-Based Lasers Emitting at 1.51 $\mu$m

Tolerance to optical feedback is investigated on quantum-dash-based lasers emitting at 1.51 mum. The onset of coherence collapse regime is experimentally determined using three criteria: optical spectrum broadening, relative intensity noise increase, and bit-error-rate degradation. Measurements were first performed in static operation at different current values, using the first and second criteria. The onset of coherence collapse was found to increase from ~-41 to -27 dB with the bias current. Then tolerance to optical feedback was assessed in dynamic operation at 10 Gb/s, using the third criterion. In spite of a relatively high linewidth enhancement factor , (alphaH ~4.5) a -32-dB onset of coherence collapse corresponding to -24-dB maximum optical return loss tolerance was achieved at 10-Gb/s rate.

Transition from the lowest linewidth mode operation to coherence collapse in a semiconductor laser with feedback from a distant reflector

The influences of optical feedback from a distant reflector on single-mode semiconductor laser operation are analyzed theoretically. The stable operation in the lowest linewidth mode and the feedback parameter for the onset of coherence collapse are systematically derived. In the feedback regime for the lowest linewidth mode operation, the laser principally operates at the emission frequency of the solitary laser at the beginning of the operation with the feedback. It is shown exactly by asymptotic analysis of the equation for the field phase in the semiconductor cavity considering the time-lag of the feedback that the laser comes to operate in the lowest linewidth mode. In the lowest linewidth mode operation, the laser is stably phase-locked to the feedback. In the semiconductor cavity, the field amplitude and the field phase oscillate in almost the same phase in the fluctuation modes related to the relaxation oscillations, In the lowest linewidth mode operation, the phase oscillations enhance the amplitude oscillations through the feedback-action's dependence upon the phase difference between the field in the semiconductor cavity and the feedback. Thus, the damping of the relaxation oscillations decreases with further increased feedback and the transition to the coherence collapse occurs.

The onset of coherence collapse in DFB lasers

How the onset of coherence collapse in distributed-feedback (DFB) lasers varies with the laser output power has been measured. Theory predicts that as a laser's output power is increased, the amount of back-reflected light necessary to cause coherence collapse should also increase, yet two of the four lasers studied become dramatically more sensitive to back reflections at higher output powers. It is speculated that changes in the carrier density in these lasers cause this device-dependent behavior.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The onset of coherence collapse in DBR lasers

A study of the dependence of the onset of coherence collapse on laser output power is discussed. Three-section multiquantum-well-distributed-Bragg-reflector (MQW-DBR) lasers were used. The fraction of light reflected back into the lasing mode was varied, and the point at which the transition to coherence collapse occurred was measured. This feedback level varies approximately linearly with laser output power. For these lasers, when the output power is 1 mW, the transition to coherence collapse begins when the optical feedback into the lasing mode is below -40 dBm; when the feedback power is -35 dBm, the laser line is completely collapsed. >

Nonlinear injection locking dynamics and the onset of coherence collapse in external cavity lasers

A theoretical analysis is presented of the experimentally observed collapse of coherence in semiconductor lasers exposed to moderate optical feedback. A main point in the analysis is, that coherence collapse in the delayed-feedback (DFB) system shows up as bistability in a much simpler model which corresponds to an injection locking system. Based on this, the authors derive a simple analytical expression, valid for DFB as well as Fabry-Perot lasers, for the critical feedback level at which coherence collapse sets in. Simulations of the nonlinear injection locking model reveal the presence of complicated nonlinear dynamics with period-doubling bifurcations and coexisting attractors even inside what is normally denoted the stable locking range.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>