Tunable and broadband microwave frequency combs based on a semiconductor laser with incoherent optical feedback

Based on a semiconductor laser （SL） with incoherent optical feedback, a novel all-optical scheme for generating tunable and broadband microwave frequency combs （MFCs） is proposed and investigated numerically. The results show that, under suitable operation parameters, the SL with incoherent optical feedback can be driven to operate at a regular pulsing state, and the generated MFCs have bandwidths broader than 40 GHz within a 10 dB amplitude variation. For a fixed bias current, the line spacing （or repetition frequency） of the MFCs can be easily tuned by varying the feedback delay time and the feedback strength, and the tuning range of the line spacing increases with the increase in the bias current. The linewidth of the MFCs is sensitive to the variation of the feedback delay time and the feedback strength, and a linewidth of tens of KHz can be achieved through finely adjusting the feedback delay time and the feedback strength. In addition, mappings of amplitude variation, repetition frequency, and linewidth of MFCs in the parameter space of the feedback delay time and the feedback strength are presented.

Optical feedback characteristics in a dual-frequency laser during laser cavity tuning

The optical feedback characteristics in a Zeeman-birefringence dual-frequency laser are studied during the laser cavity tuning in three different kinds of optical feedback conditions： （i） only //-light is fed back; （ii） only ⊥-light is fed back; （iii） both lights are fed back. A compact displacement sensor is designed using the experimental result that there is a nearly 90 degrees phase delay between the two lights＇ cosine optical feedback signals when both lights are fed back into the laser cavity. The priority order that the two lights＇ intensity curves appear can be used for direction discrimination. The resolution of the displacement sensor is at least 79 rim, and the sensor can discriminate the target＇s moving direction easily.

Chaos synchronization in injection-locked semiconductor lasers with optical feedback

Based on the rate equations, we have investigated three types of chaos synchronizations in injection-locked semiconductor lasers with optical feedback. Numerical simulation shows that the synchronization can be realized by the symmetric or asymmetric laser systems. Also, the influence of parameter mismatches on chaos synchronization is investigated, and the results imply that these two lasers can achieve good synchronization, with smaller tolerance of parameter mismatch existing.

Intensity modulation characters of orthogonally polarized HeNe lasers with different optical feedback level

This paper demonstrates the intensity modulation characters of orthogonally polarized HeNe lasers with different optical feedback level generated by the variable reflectivity of external reflector. The modulation depths of the orthogonally polarized frequencies are increased when the optical feedback level becomes strong. It also observes that the modulation amplitudes are different for different external cavity length. Based on the vectorial extension of Lamb＇s semi-classical theory, it finds that the calculations are consistent with the experimental results.

Mode hopping in single-mode microchip Nd：YAG lasers induced by optical feedback

The mode hopping phenomenon induced by optical feedback in single-mode microchip Nd：YAG lasers is presented. With optical feedback, mode hopping strongly depends on two factors： the ratio of external cavity length to intra-cavity length, and initial gains of the two hopping modes, When external cavity length equals an integral multiple of intracavity length, there is almost no mode hopping. However, if the external cavity length does not equal an integral multiple of intra-cavity length, mode hopping occurs. The ratio of external cavity length to intra-cavity length determines the position of two-mode hopping, The initial gains of the two hopping modes determine the corresponding peak values and oscillating periods of them in the intensity modulation curves.

Anisotropic optical feedback of single frequency intra-cavity He-Ne laser

This paper presents the anisotropic optical feedback of a single frequency intra-cavity He-Ne laser. A novel phenomenon was discovered that the laser output an elliptical polarized frequency instead of the initial linear polarized one. Two intensities with a phase difference were detected, both of which were modulated in the form of cosine wave and a fringe shift corresponds to a λ/2 movement of the feedback mirror. The phase difference can be continuously modulated by the wave plate in the external cavity. Frequency stabilization was used to stabilize the laser frequency so as to enlarge the measuring range and improve the measurement precision. This anisotropic optical feedback system offers a potential displacement measurement technology with the function of subdivision of λ/2 and in-time direction judgment. The three-mirror Fabry Perot cavity model is used to present the experimental results. Given the lack of need of lasing adjustment, this full intra-cavity laser can significantly improve the simplicity and stability of the optical feedback system.

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.

Numerical investigation on photonic microwave generation by a sole excited-state emitting quantum dot laser with optical injection and optical feedback

Based on three-level exciton model,the enhanced photonic microwave signal generation by using a sole excited-state(ES)emitting quantum dot(QD)laser under both optical injection and optical feedback is numerically studied.Within the range of period-one(P1)dynamics caused by the optical injection,the variations of microwave frequency and microwave intensity with the parameters of frequency detuning and injection strength are demonstrated.It is found that the microwave frequency can be continuously tuned by adjusting the injection parameters,and the microwave intensity can be enhanced by changing the injection strength.Moreover,considering that the generated microwave has a wide linewidth,an optical feedback loop is further employed to compress the linewidth,and the effect of feedback parameters on the linewidth is investigated.It is found that with the increase of feedback strength or delay time,the linewidth is evidently decreased due to the locking effect.However,for the relatively large feedback strength or delay time,the linewidth compression effect becomes worse due to the gradually destroyed P1 dynamics.Besides,through optimizing the feedback parameters,the linewidth can be reduced by up to more than one order of magnitude for different microwave frequencies.

Optical feedback characteristics in a helium neon laser with a birefringent internal cavity

The output characteristics of optical feedback in a helium neon laser with a birefringent internal cavity are studied systematically in five different regions of the gain curve for the two orthogonally polarized modes. When the laser operates in the two end regions of the laser gain curve, one of the two orthogonally polarized modes will be a leading one in optical feedback. Strong mode competition can be observed. However, when the laser operates in the middle region of the laser gain curve, the two modes can oscillate equally with optical feedback. Besides the intensity of the two polarized lights, the total light intensity is also studied at the same time. M-shaped optical feedback curves are found. Particularly, when the average intensities of the two lights are comparable, the intensity modulation curve of the total light is doubled, which can be used to improve the resolution of an optical feedback system.

Sensitivity to external optical feedback of circular-side hexagonal resonator microcavity laser

The sensitivity to fault reflection is very important for larger dynamic range in fiber fault detection technique.Using time delay signature(TDS)of chaotic laser formed by optical feedback can solve the sensitivity limitation of photodetector in fiber fault detection.The TDS corresponds to the feedback position and the fault reflection can be detected by the laser diode.The sensitivity to feedback level of circular-side hexagonal resonator(CSHR)microcavity laser is numerically simulated and the feedback level boundaries of each output dynamic state are demonstrated.The peak level of TDS is utilized to analyze the sensitivity.The demonstration is presented in two aspects:the minimum feedback level when the TDS emerges and the variation degree of TDS level on feedback level changing.The results show that the CSHR microcavity laser can respond to the feedback level of 0.07%,corresponding to-63-dB feedback strength.Compared to conventional distributed feedback laser,the sensitivity improves almost 20 dB due to the shorter internal cavity length of CSHR microcavity laser.Moreover,1%feedback level changing will induce 1.001 variation on TDS level,and this variation degree can be influenced by other critical internal parameters(active region side length,damping rate,and linewidth enhancement factor).

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd：YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.

Analysis of gain distribution in cladding-pumped thulium-doped fiber laser and optical feedback inhibition problem in fiber-bulk laser system

The steady-state gain distribution in cladding pumped thulium-doped fiber laser（TDFL） is analytically and numerically solved based on the rate equations including loss coefficients and cross relaxation effect. With the gain curve, a problem, which is named optical feedback inhibition（OFI） and always occurs in tandem TDFL-Ho：YAG laser system, is analyzed quantitatively. The actual characteristics of output spectra and power basically prove the conclusion of theoretical analysis. Then a simple mirror-deflected L-shaped cavity is employed to restrain the external feedback and simplify the structure of fiber-bulk Ho：YAG laser. Finally, 25 W of 2097-nm laser power and 51.2% of optical-to-optical conversion efficiency are obtained, and the beam quality factor is less than 1.43 obtained by knife-edge method.

Intensity tuning characteristics of double-mode He-Ne laser with optical feedback

This paper investigates the intensity tuning characteristics of a double longitudinal modes HeiNe laser subjected to optical feedback. The intensity undulations of the total light and the two modes are observed for different external cavity length. Two modulations of the internal cavity length are performed. One is only for the internal cavity length being modulated and the other is for both the internal and the external cavity length being modulated. The undulation frequency of the total light is found to be determined by the ratio of external cavity length to internal cavity length in both modulations. When the external cavity length is integral times of the internal cavity length, the fringe frequency of the total light could be seven or even more times of that in conventional optical feedback. A simple theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.

External Optical Feedback Induced Noise in DFB Fiber Laser Sensor System

External optical feedback effects due to reflection, Rayleigh backscattering and coherent Rayleigh backseattering in fiber distributed feedback （DFB） fiber laser sensor system have been investigated. If the feedback intensity exceeds critical amount, excess noise would be induced in the demodulator. The maximum tolerable intensity back-reflection coefficient Rc and backscattering coefficient Sc into a fiber DFB laser with lead fiber length from I m to 37.5 km before the onset of instabilities are shown. Rc is found to decrease with increasing lead fiber length while Sc was relatively invariable with varying fiber length. The coherent Rayleigh backscattering （CRBS） would induce neglectable noise with a lead fiber exceeding 13.5 kin. To eliminate these noises, one or two isolators should be incorporated in the system.

Distortion of optical feedback signals in microchip Nd:YAG lasers subjected to external multi-beam interference feedback

This paper proposes a theoretical analysis for the characteristics of an external cavity Nd：YAG laser with feedback of multiple-beam interference, which is induced by the multi-reentrance of the light from the external Fabry-Perot cavity. The theoretical model considers the multiple beam interference of the external Fabry-Perot cavity. It is found that the optical feedback signals are distorted to pulse waveforms instead of the sinusoidal ones in conventional feedback. The experimental results are in good agreement with the theoretical analysis. The obtained theoretical and experimental results can advance the development of a laser feedback interferometer.

Time-Domain Traveling Wave Model of Single-Section Quantum Dot Mode-Locked Lasers with Optical Feedback

The optical feedback modulations of a passively mode-locked semiconductor laser with quantum dot structure were investigated in this study. The delay-induced dynamics of QD MLL under the condition of optical feedback have been studied by exploiting a time domain traveling wave(TDTW) model. The conventional TDTW model was improved to make it suitable for studying optical feedback system, especially the system with long external cavity structure. Combing the TDTW model with optical feedback and carrier rate equations, the pulse variation of a single section QD MLL was studied. This new model shows good agreement with the published experimental data. The roundtrip time and local light intensity modulation to the pulse evolution of QD MLL were studied. The results show that when the time for a light to go to and return from external cavity is equal to the integral times of the period of laser pulse without external feedback, resonance will be formed, and the period of pulse sequence is the shortest. The results also show that the stronger the local light intensity modulation is, the shorter the pulse period is.

Wideband chaos generation based on a dual-mode microsquare laser with optical feedback

We propose and demonstrate the generation of wideband chaos based on a dual-mode microsquare semiconductor laser with optical feedback. By adjusting the dual-mode intensity ratio and the feedback strength, wideband chaos covering more than 50 GHz in the RF spectrum is achieved. The standard and effective bandwidths of the chaotic signal are 31.3 GHz and 30.7 GHz with the flatness of 8.3 dB and 6.1 dB, respectively.

Absolute small-angle measurement based on optical feedback interferometry

We present a simple but effective method for small-angle measurement based on optical feedback interferometry （or laser self-mixing interferometry）. The absolute zero angle can be defined at the biggest fringe amplitude point, so this method can also achieve absolute angle measurement. In order to verify the method, we construct an angle measurement system. The Fourier-transform method is used to analysis the interference signal. Rotation angles are experimentally measured with a resolution of 10^-6 rad and a measurement range of approximately from -0.0007 to ＋0.0007 rad.

Low-frequency fluctuation in multimode semiconductor laser subject to optical feedback

Dynamics of a semiconductor laser subject to moderate optical feedback operating in the low-frequency fluctuation regime is numerically investigated. Multimode Lang-Kobayashi （LK） equations show that the low-frequency intensity dropout including the total intensity and sub-modes intensity is accompanied by sudden dropout simultaneously, which is in good agreement with experimental observation. The power fluctuation is quite annoying in practical applications, therefore it becomes important to study the mech- anism of power fluctuation. It is also shown that many factors, such as spontaneous emission noise and feedback parameter, may influence power fluctuation larger than previously expected.

Nonlinear dynamics of 1550 nm VCSELs subject to polarization-preserved optical feedback and orthogonal optical injection

Based on spin-flip model （SFM）, the nonlinear dynamics of 1550nm vertical-cavity surface-emitting lasers （VCSELs） subject to polarization-preserved optical feedback （PPOF） and orthogonal optical injection （OOI） are theoretically investigated. The results show that two linear polarization （LP） modes can be simultaneously stimulated and polarization switching （PS） can be observed, which is different from the case that only y LP mode exists in a VCSEL with PPOF. Under the joint action of PPOF and OOI, the two LP modes will exhibit rich dynamical states, such as period one, period two, multi- period and chaos oscillation. Different evolution routes to chaos can be also observed. Moreover, frequency detuning Af（Af=fm fs, wherefn andfs are the center frequencies of free-running master VCSEL and slave VCSEL, respectively） has an obvious influence on the PS. With the increase of the positive frequency detuning PS points shift toward larger injection strengths; meanwhile, a suitable negative frequency demning value makes the injection strength for PS be the lowest.