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.     

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.     

Influence of Optical Feedback from Birefringence External Cavity on Intensity Tuning and Polarization of Laser

The characteristics of intensity tuning and polarization of He-Ne laser with optical feedback are studied. When the internal cavity length of the laser with birefringence optical feedback is tuned, not only does output intensity vary with laser frequency, but also the polarization periodically hops between two orthogonal directions. If the phase difference of birefringence is π/2, two polarization states Mternately oscillate and have equal bandwidths within the longitudinal mode spacing. The times of polarization flipping in the longitudinal mode bandwidth is proportional to the ratio of external cavity length to internal cavity length. The experimental results are explained, and the potential uses are also discussed.     

Low frequency fluctuation with two external cavity reflectors

The dynamics of a semiconductor laser with two optical feedbacks is studied in this paper. A new set of nonlinear rate equations that can describe external cavity semiconductor lasers with any amount of two optical feedbacks is proposed. It is found that when the laser is biased above the threshold and subjected to one feedback, the other feedback can induce low-frequency fluctuations.     

Single mode rate equations for two sections self—pulsating DFB laser

We Propose a set of single mode rate equations for multi-section distributed feedback(DFB) lasers. On the basis of the rate equations, the self-pulsation in the two-section DFB lasers with asymmetrical injection current was explained. It was found that the dynamic distribution of the power in two sections play key role in the self-pulsation running.     

A Step—by—Step Tunable Compound External Cavity Semiconductor Laser with Two—Side Feedback Coupling

A kind of compound external cavity semiconductor lasers(CECSL) is presented,by which laser with a discretely tunable wavelength can be obtained to meet the potential demands of the wavelength division multiplexing system.By the introduction of two-side feedback coupling in the CECSL,the CeCSL can operate more efficiently,and its side-mode suppression ratio is high up to 50dB.     

Optical Feedback Characteristics in He-Ne Dual Frequency Lasers

Optical feedback characteristics in He-Ne dual frequency lasers are studied systematically in different feedback power ratios with a variable attenuator. Feedback power ratios vary from 0.010 up to 0.998. Five distinct regimes of self-interference effects are found and defined as regimes Ⅰ, Ⅱ,Ⅲ,Ⅳand Ⅴ. Accordingly, five optical feedback levels have been put forward in He-Ne dual frequency lasers. Strong mode competitions are observed in regimes Ⅲ and Ⅳ. In regime Ⅴ, multiple feedback effects are investigated. The basic theoretical analysis is also presented. Our results can advance the research of self-mixing interferometer and displacement sensor of He-Ne orthogonally polarized dual frequency lasers.     

Optical microwave generation using two parallel DFB lasers integrated with Y-branch waveguide coupler

A new device of two parallel distributed feedback (DFB) lasers integrated monolithically with Y-branch waveguide coupler was fabricated by means of quantum well intermixing. Optical microwave signal was generated in the Y-branch waveguide coupler through frequency beating of the two laser modes coming from two DFB laser in parallel, which had a small difference in frequency. Continuous rapid tuning of optical microwave signal from 13 to 42GHz were realized by adjusting independently the driving currents injected into the two DFB lasers.     

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.     

Tuning Characteristics of Frequency Difference for Zeeman—Birefringence He—Ne Dual Frequency Laser

Characteristics of frequency difference tuning of Zeeman-birefringence He-Ne dual frequency lasers (ZBDFLs) are explored.We design an automatic system of tuning cavity and power detection,which can tune the laser cavity and record the tuning curves of light power and frequency difference simultaneously.A synthetic phenomenon by Zeenman effect,mode pulling effect and birefringence effect is verified to exist in ZBDFLs.By analysing the tuning behaviour,this synthetic phenomenon is discovered and qualitatively explained for the first time.     

Intensity modulation in single-mode microchip Nd：YAG lasers with asymmetric external cavity

Intensity modulation induced by the asymmetric external cavity in single-mode microchip Nd:YAG lasers is presented. Two kinds of experimental results are discussed based on multiple feedback effects. In one case, the intensity modulation curve is a normal sine wave, whose fringe frequency is four times higher than that of a conventional optical feedback system, caused by multiple feedback effects. In the other case, the intensity modulation curve is the overlapping of the above quadruple-frequency signal and conventional optical feedback signal, which is determined by the additional phase difference induced by the asymmetric external cavity. The theoretical analyses are in good agreement with the experimental results. The quadruple-frequency modulation of the laser output intensity can greatly increase the resolution of displacement measurement of an optical feedback system.     

Intensity Tuning in Single Mode Microchip Nd：YAG Laser with External Cavity

We investigate the characteristics of intensity tuning in a single mode microchip Nd：YAG laser with an external cavity. The undulation of laser intensity in a period of A/2 change of the internal cavity length is observed. Two different optical feedback cases are performed. One is an external cavity reflector perfectly aligned and the other is an external cavity reflector tilted. However, the fluctuation frequency of laser intensity in a period of A/2 change of the internal cavity length in these two cases is found to be determined by the ratio of external cavity length to internal cavity length. Meanwhile, for the tilted external cavity, the fluctuation frequency is also related to multiple feedbacks in the tilted external cavity.     

Coherent combining of tunable erbium-doped fiber lasers by a single-mode fiber feedback loop

We experimentally demonstrate the coherent combining of two tunable erbium-doped fiber lasers by using a single-mode fiber feedback loop configuration. A single-mode fiber is arranged in the feedback loop to filter the far-field pattern, and the energy of desired in-phase mode is collected and injected into the resonators of two component fiber lasers. The coherently combined laser is tunable over a wide spectrum ranging from 1536 to 1569 nm, which means that the combining scheme is compatible with wavelength tuning. The effects and necessity of whether adopting polarization controlling measures or not in component lasers are investigated in detail. The results indicate that adding polarization controlling can improve the array＇s coherence, whereas it will decrease the output power and efficiency simultaneously.     

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

This paper investigates the intensity tuning characteristics of a double longitudinal modes He--Ne 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.     

Wavelength stabilization of a 980-nm semiconductor laser module stabilized with high-power uncooled dual FBG

An optimized dual fiber Bragg grating (FBG) is proposed for 980-nm semiconductor lasers without thermoelectric coolers to restrict temperature-induced wavelength shift.The mathematical model of the temperature-induced wavelength shift of the laser with the dual FBG is built using the external cavity feedback rate equations.The external cavity parameters are optimized for achieving the stability mode-locking laser output.The spectral characteristics of the dual FBG stabilized laser are measured to range from 0 to 70 °C.The side mode suppression ratio (SMSR) is more than 45 dB,while the full-width at half-maximum (FWHM) is less than 1 nm.The peak wavelength shift is less than 0.1 nm.The dual FBG wavelength shift proportional coefficient is between 0.1086 and 0.4342.     

Self-mixing interferometry based on nanometer fringes and polarization flipping

Self-mixing interferometry (SMI) based on nanometer fringes and polarization flipping is realized. The interferometer comprises a single-mode He-Ne laser and a high-amplitude reflectivity feedback mirror. The nanometer fringes are obtained by tilting the external feedback mirror. The fringe density is 35 times higher than that derived with conventional two-beam interference, and each fringe corresponds to a λ/70 displacement in external cavity length. Moreover, polarization flipping occurs when the external feedback mirror moves in the opposite direction. Such movement can be used to easily distinguish displacement direction. Experimental results show an optical resolution of displacement measurement of 9.04 nm with a range of 100 μm. The proposed SMI presents promising application prospects in precisely measuring displacement and calibrating other micro-displacement sensors because of its optical wavelength traceability.     

Four distributed feedback laser array integrated with multimode-interference and semiconductor optical amplifier

Monolithic integration of four 1.55-μm-range InGaAsP/InP distributed feedback (DFB) lasers using varied ridge width with a 4×1-multimode-interference (MMI) optical combiner and a semiconductor optical amplifier (SOA) is demonstrated. The average output power and the threshold current are 1.8 mW and 35 mA, respectively, when the injection current of the SOA is 100 mA, with a side mode suppression ratio (SMSR) exceeding 40 dB. The four channels have a 1-nm average channel spacing and can operate separately or simultaneously.     

Theoretical study of laser diodes with double optical feedbacks

A new set of nonlinear rate equations that can describe the external cavity semiconductor laser with two optical feedbacks is proposed. The dynamics of the semiconductor laser with two optical feedbacks are studied. It is found that when lasers are biased above the threshold and operate in regime V, another feedback can induce low frequency fluctuations.     

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.     

Fabrication of widely tunable ridge waveguide DBR lasers for WDM-PON

We report the fabrication of widely tunable ridge waveguide distributed Bragg reflector(DBR) lasers with InGaAsP butt-joint as grating material. The shape of the butt–joint interface is found to have significant effect on the properties of the lasers. It is shown that irregular mode jumps during wavelength tuning can be avoided by a V-shaped butt–joint interface. From the fabricated device, 23 channels with 0.8 nm spacing and greater than 35 dB side mode suppression ratios are obtained. The different tuning characteristics of the ridge waveguide and the previously reported buried ridge stripe DBR lasers are discussed. Combined with the wide tuning range and the simple structure, the ridge waveguide DBR lasers are promising for use in wavelength division multiplexing passive optical networks(WDM-PONs).