Strong Optical Feedback Research Articles

Enhancing Modulation Bandwidth of Semiconductor Lasers beyond 50 GHz by Strong Optical Feedback for Use in Millimeter-Wave Radio over Fiber Links

We present the modeling on high-speed modulation of semiconductor lasers beyond 50 GHz over a frequency band of 2 GHz by using strong optical feedback (OFB). This ultra high modulation frequency is achieved by adjusting the external-cavity resonance frequency close to the modulation frequency and optimizing strong OFB conditions to achieve stable operation. We show that this optimized strong OFB results in about 20 dB improvement in the link gain of a 55.8 GHz radio over fiber (RoF) link. We also discuss the influence of environmental variations on the simulated modulation characteristics.

The frequency stabilization method of laser feedback interferometer based on external cavity modulation

Laser frequency stabilization is one of the foremost factors that influence the accuracy of measurement using a laser feedback interferometer, especially when the system is in the presence of strong optical feedback. In this paper, we use external cavity modulation to demonstrate a simple and effective method of frequency stabilization for laser feedback interferometer with strong optical feedback. The external cavity modulation is realized by oscillating a feedback mirror driven by a piezoelectric ceramic, the stable feedback optical field is obtained that has strong performance of anti-disturbance. Meanwhile, we stabilize the laser frequency by the thermal frequency-stabilizing. These lead to the realization of stable laser feedback interferometer. The relative uncertainty of the frequency stabilization is less than 1 part in 10(7).

On the feasibility of self-mixing interferometer sensing for detection of the surface electrocardiographic signal using a customized electro-optic phase modulator

Optical sensing offers an attractive option for detection of surface biopotentials in human subjects where electromagnetically noisy environments exist or safety requirements dictate a high degree of galvanic isolation. Such circumstances may be found in modern magnetic resonance imaging systems for example. The low signal amplitude and high source impedance of typical biopotentials have made optical transduction an uncommon sensing approach. We propose a solution consisting of an electro-optic phase modulator as a transducer, coupled to a vertical-cavity surface-emitting laser and the self-mixing signal detected via a photodiode. This configuration is physically evaluated with respect to synthesized surface electrocardiographic (EKG) signals of varying amplitudes and using differing optical feedback regimes. Optically detected EKG signals using strong optical feedback show the feasibility of this approach and indicate directions for optimization of the electro-optic transducer for improved signal-to-noise ratios. This may provide a new means of biopotential detection suited for environments characterized by harsh electromagnetic interference.

Intensity noise in ultra-high frequency modulated semiconductor laser with strong feedback and its influence on noise figure of RoF links

We characterize the intensity noise associated with the ultra-high frequency direct modulation of semiconductor lasers under strong optical feedback over a millimeter-wave frequency passband between 54.5 and 56.5 GHz. Enhancement of the modulation response over this millimeter wave band due to optical feedback is clearly shown by time-delay rate equation analysis fully involving the strong optical feedback effect. The contribution of the relative intensity noise of the laser to the noise figure of an ultra-high speed radio over fiber link is evaluated. We show that subjecting the laser diode to strong feedback improves the noise figure of a 55.8-GHz radio over fiber link nearly by 20 dB in the regime of small-signal modulation and 10 dB under large-signal modulation.

External-cavity diode laser in second-order Littrow configuration

In this Letter, we propose and demonstrate an external-cavity diode laser in second-order Littrow configuration. This topology utilizes a low-efficiency diffraction grating to establish a high-finesse external cavity, strong optical feedback, a high polarization discrimination, and a circular TEM00 output mode. In our proof-of-concept experiment, we realized a cavity with a finesse of 1855, being, to the best of our knowledge, the highest value ever reported for a three-port-grating-coupled cavity. With optical feedback, the laser threshold of the laser diode employed was reduced by a factor of 4.

Dynamics and noise of fiber-grating semiconductor lasers affected by the line-width enhancement factor

This paper numerically investigates the effect of the line-width enhancement factor on the dynamics and the intensity noise of InGaAs/InP lasers pumping in the wavelength of 980 nm at the strong optical feedback limit. The investigation is performed based on an improved time-delay rate equation model of a single-mode laser. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram, relative intensity noise and injection current. The simulation results indicate that the laser, under strong optical feedback, mainly operates in CW, pulsation or chaos operation depending on the line-width enhancement factor and injection current values. At larger values of the line-width enhancement factor and injection current the pulsing operation becomes more dominant over wider range of strong optical feedback. The corresponding relative intensity noise level is close to or slightly larger than the level of the solitary laser noise. The optical feedback noise is found to be as low as the quantum noise level when the laser is operated in pulsing region and at larger values of the line-width enhancement factor and injection current.

Numerical modeling of the output and operations of semiconductor lasers subject to strong optical feedback and its dependence on the linewidth‐enhancement factor

AbstractInfluence of the linewidth‐enhancement factor on the output and operations of InGaAs/InP pumping lasers emitting at a wavelength of 980 nm under strong optical feedback is investigated numerically. The investigations are performed based on intensive numerical integration of an improved time‐delay rate equations of semiconductor lasers over wide ranges of the linewidth‐enhancement factor and optical feedback strength. The results show that the semiconductor laser operates under strong optical feedback in continuous wave and pulsation at small values of the linewidth‐enhancement factor. Under large values of the linewidth‐enhancement factor, the laser happens to exhibit chaos and pulsation. We predict that semiconductor laser subjected to strong optical feedback exhibits much more stable pulsing operation under higher values of the linewidth‐enhancement factor, which indicates that the laser is locked at the external cavity frequency. Copyright © 2010 John Wiley & Sons, Ltd.

Chaos laser chips with delayed optical feedback using a passive ring waveguide

We report a novel chaos semiconductor laser chip in which a distributed feedback (DFB) laser, two semiconductor optical amplifiers (SOAs) and a photodiode (PD) are monolithically integrated with a passive ring waveguide. The ring-type structure with the two separate SOAs achieves stronger delayed optical feedback compared to previous chaos laser chips which use linear waveguide and facet-reflection. The integrated PD allows efficient detection of the optical signal with low optical loss. A rich variety of dynamical behaviors and optical signals can be selectively generated via injection currents to the two separate SOAs. In particular, the strong optical feedback makes possible the generation of strong broadband optical chaos, with very flat spectrum of ±6.5 dB up to 10 GHz. The stability and quality of the chaotic mode is demonstrated using strict statistical tests of randomness applied to long binary sequences extracted by sampling the optical intensity signal.

Measurement of micro- and nanovibrations and displacements using semiconductor laser autodynes

The results of theoretical and experimental studies of the possibility to use semiconductor quantum-well lasers, operating in the autodyne regime, for creating nanovibration and nanodisplacement meters are presented. The specific features of the autodyne signal formation under the conditions of strong and weak optical feedback, the influence of current modulation on the shape and spectrum of the autodyne signal, and the methods of measuring the parameters of micro- and nanomotions are described. The description of methods and instrumentation using laser autodynes for the analysis of nanovibration and nanodisplacement parameters of biological objects is presented.

Experimental characterization of DFB and FP chaotic lasers with strong incoherent optical feedback

We have experimentally investigated chaotic power oscillations in single-longitudinal mode DFB and multi-longitudinal mode FP lasers as a function of incoherent optical feedback strengths of up to 42%. We have demonstrated the existence of chaos in the output oscillations of both laser types using classical experimental tools such as RF spectrum, standard deviation, and maximum Lyapunov exponent, which all increase with increasing of feedback strength for both in single-longitudinal mode DFB lasers and multi-longitudinal mode FP lasers. It is also shown that power switching among longitudinal modes of multimode FP semiconductor laser is a considerable portion of the chaotic power oscillations for both strong and weak incoherent optical feedback.

Inducing single-mode oscillation in Fabry–Perot InGaAsP lasers by applying external optical feedback

The authors report on inducing single-mode oscillation in InGaAsP multimode lasers with Fabry–Perot (FP) structures by applying external optical feedback (OFB). The study is based on numerical solution of a time-delay multimode rate equation model taking into account both symmetric gain suppression and asymmetric gain suppression (AGS) of modal gain. The mode dynamics and modal operation are characterised over a wide range of OFB in terms of time trajectories of the most dominant modes and the corresponding time-average output spectrum. The obtained results show that strong OFB reduces AGS relative to the linear gain in the vicinity of the dominant modes. This effect moderates the contribution of AGS to the longitudinal mode competition in such a way to induce oscillation in single mode with the laser operating in continuous wave. In this region of strong OFB, the side-mode suppression ratio increases with strengthening OFB. Increasing the external cavity length was found to decrease the OFB range of single-mode oscillation and enhances chaotic dynamics of the laser.

Highly External Optical Feedback-Tolerant 1.49-$\mu$ m Single-Mode Lasers With Partially Corrugated Gratings

An InGaAsP single-mode laser diode (LD) highly tolerant to optical feedback was realized utilizing a partially corrugated grating with a window-mirror structure. The length and the coupling coefficient of the grating were properly chosen to enable moderate output power, low feedback sensitivity, and a side mode suppression ratio of 40 dB simultaneously. Under -16 dB external optical feedback, the relative intensity noise (RIN) was improved by 10 dB compared with that of a conventional DFB LD. The RIN was maintained at less than -120 dB/Hz under strong external optical feedback as high as -10 dB. These results show that the fabricated lasers are potentially useful for isolator-free optical modules.

Influence of radiation fluctuations on the magnitude of optoelectronic signal in semiconductor lasers with external optical feedback

Numerical modeling of dynamical processes in semiconductor lasers with external delayed optical feedback has been performed on the basis of the rate equations taking into account the quantum fluctuations of radiative transition rates. It has been shown that the nature of the optoelectronic signal as a function of pump current is determined by the magnitude of the optoelectronic coupling and corresponds to the behavior of the autocorrelation function of the radiation. It has been found that the radiation coherence decreases substantially with strong optical feedback. Experimental measurements confirming the theoretical results have been performed.

Optical feedback cavity ring-down technique for accurate measurement of ultra-high reflectivity

A novel cavity ring-down (CRD) technique, based on the optical feedback effect of Fabry-Perot diode lasers, is developed for accurate reflectivity measurement of highly reflective cavity mirrors. The strong optical feedback, including the direct reflection from the front cavity mirror of a linear ring-down cavity, is re-injected into the oscillator cavity of the diode laser, and large resonant peaks are observed in the ring-down cavity output signals. The diode laser is switched off by a threshold circuit when the amplitude of a resonant peak exceeds a pre-defined threshold. Exponentially decayed signals recorded immediately after switching off the laser are used to determine the cavity decay time. The ultra-high reflectivity of cavity mirrors is measured to be 99.99606% with a reproducibility of 0.00003%. Compared with the previous phase-shift CRD technique, the signal-to-noise ratio of the cavity output signals is improved with an enhancement factor of over 100.

A Novel Laser Feedback Interferometer for Metrology of Large Range Displacement

This paper presents a novel laser feedback interferometer for metrology of large range displacement. A Birefringence-Zeeman dual frequency laser is applied in the system. Due to its strong optical feedback effects, the intensity modulation curves of the two polarized lights will have a zero intensity part in a period. A quartz crystal is used in the external feedback cavity as a birefringent element. And a phase difference between the two lights can be adjusted to π/2. Thus, the intensity modulation curves in a period can be divided into four polarization zones with equal width. When the displacement of the object to be measured is produced, the four polarization zones will appear in sequence. According to the appearance order of the four zones, the direction of the displacement can be discriminated without ambiguity and the resolution of the system can be greatly increased. It is confirmed that the laser feedback interferometer has a resolution of 79.1nm and with a linearity of 0.01% over the whole measurement range of 15mm.

Terahertz quantum cascade lasers based on two-dimensional photonic crystal resonators

We demonstrate high spectral control from surface emitting THz Quantum Cascade Lasers based on a two-dimensional photonic crystal cavity. The perforated top metallic contact acts as an in plane resonator in a tight double-metal plasmonic waveguide providing a strong optical feedback without needing three-dimensional cavity features. The optical far-field patterns do not exhibit the expected symmetry and the shape of the cavity mode. The difference is attributed to a metal surface plasmon mediated light outcoupling mechanism also responsible for the relatively low extraction efficiency.

Numerical analysis of optical feedback phenomenon and intensity noise of fibre‐grating semiconductor lasers

AbstractThis paper demonstrates numerical analysis of the dynamics and intensity noise of fibre‐grating semiconductor lasers (FGSLs). The induced phenomenon of strong optical feedback (OFB) is analysed. The simulations are based on an improved time‐delay rate equations model of a single‐mode laser that takes into account the multiple round‐trips of the lasing field in the fibre cavity. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise (RIN). We explore influence of the fibre‐cavity length on the dynamics and RIN. The results show that when the fibre cavity is short, the regime of strong OFB is characterized by either continuous‐wave (CW) operation or periodic pulsation. The pulsation frequency is locked at the frequency separation of either the compound‐cavity modes or the external fibre‐cavity modes. The corresponding RIN level is close to or higher than the level of the solitary laser depending on pulse symmetry. When the fibre cavity is long, the laser exhibits unstable dynamics over wider range of OFB. Moreover, the strong‐OFB pulsation becomes beating quasi‐periodic at the relaxation oscillation frequency and the fibre‐cavity mode‐separation frequency. Copyright © 2007 John Wiley & Sons, Ltd.

Gain amplification and lasing properties of individual organic nanofibers

We study gain and lasing processes in individual self-assembled organic nanofibers grown on mica substrates. The gain-induced response of the nanofibers is found to depend sensitively on the fiber structure. In homogeneous fibers where no coherent optical feedback is present, high net optical gain (of up to 103cm−1) results in spectral narrowing at the material gain peaks. In the case of strong optical feedback, which occurs in long nanofibers with randomly distributed scattering centers, gain is in turn responsible for low-threshold coherent random laser action.

Strong optical feedback in birefringent dual frequency laser

Strong optical feedback in a birefringent dual frequency He–Ne laser with a high reflectivity feedback mirror has been investigated for the first time. The output characteristics of two orthogonally polarized modes are demonstrated in two different optical feedback cases: one is for both modes being fed back and the other is for only one of the modes being fed back. Strong mode competition can be observed between the two modes with strong optical feedback. And when one mode's intensity is near its maximum, the other mode is nearly extinguished. When both modes are fed back into the laser cavity, the mode competition is stronger than when only one mode is fed back. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition, which has been experimentally and theoretically demonstrated.

Performance analysis of very-small-aperture lasers

The fabrication of very-small-aperture lasers is demonstrated, and their performance is analyzed. Because of strong optical feedback caused by a gold film on the front facet of the laser, its behavior changes: The threshold current decreases, the density of light inside the laser diode and the redshift effect of the spectra are enhanced, and the laser diode's lifetime is shorter than that of common laser diodes with large driving current.