Optical-feedback-induced chaos and its control in multimodesemiconductor lasers

The effects of optical feedback in multilongitudinal mode semiconductor lasers are studied through computer simulations. Two separate regimes are found based on the length of the external cavity. For long external cavities (external-cavity mode spacing larger than the relaxation-oscillation frequency), the laser follows a quasi-periodic route to chaos as feedback is increased. For short external cavities, the laser can undergo both quasi-periodic and period doubling routes to chaos. When the laser output becomes chaotic, the relative-intensity noise is greatly increased (by more than 20 dB) from its solitary-laser value. Considerable attention is paid to the effects of optical feedback on the longitudinal-mode spectrum. The stabilization of the mode spectrum and the reduction of the feedback-induced noise through current modulation are studied and compared with experimental results. Current modulation eliminates feedback-induced chaos when the modulation frequency and depth are suitably optimized. This technique of chaos control has applications in optical data recording     

External optical feedback effects on intensity noise ofvertical-cavity surface-emitting lasers

The authors have experimentally evaluated the effect of external optical feedback on the relative intensity noise (RIN) of a vertical-cavity surface-emitting laser (VCSEL) diode. In the absence of optical feedback, the smallest RIN is found to be -135 dB/Hz. For optical feedback levels approaching -25 dB, the RIN is degraded by about 20 dB. The authors have measured feedback-induced power penalties in a 500-Mb/s intensity-modulation/direct-detection (IM/DD) link and determined that the penalty exceeds 1 dB if the optical feedback ratio is larger than -30 dB     

Noise analysis of semiconductor lasers within the coherencecollapse regime

The authors present a theoretical analysis of semiconductor laser noise with optical distant feedback. The results show good agreement with numerical simulations. With the analytical model developed, one can easily calculate the relative intensity noise (RIN), the frequency noise, and the spectral linewidth of the semiconductor laser diode. It was found that, because of the decreasing damping frequency, with increasing feedback, the maximum relative intensity noise at the relaxation frequency does not increase monotonically. The maximum RIN is given as the reciprocal value of the damping rate     

Nonlinear dynamics of a laser diode with optical feedback systemssubject to modulation

The nonlinear dynamic behavior of a direct frequency-modulated diode laser with strong optical feedback is examined and compared to a laser diode subject to electro-optically modulated, strong optical feedback. Direct modulation is achieved by sinusoidal modulation of the diode laser injection current. Electro-optic modulation is achieved by applying a sinusoidal voltage to an intracavity phase modulating element. The output state (characterized by the output power versus time, the intensity noise spectrum and the optical frequency spectrum) for both types of modulation is dependent on the ratio of the modulation frequency to the external cavity resonant frequency, and the modulation power. A number of distinct states are observed: conventional amplitude modulation (with FM spectra); multimode, low-noise amplitude modulation; multimode, high-noise amplitude modulation; periodic limit-cycle operation; quasi-periodicity; chaos; low-frequency fluctuations; and mode-locking. There are significant differences between the direct and electro-optic frequency-modulation cases. The onset of the dynamic instability is characterized as a noisy period-one oscillation for direct modulation and a low-frequency fluctuation for intracavity electro-optic modulation. Phase portraits produced experimentally with the use of a digital phosphor oscilloscope are shown to agree well with those constructed from output power versus time data. This represents an experimental method for examining the dynamics phase portraits in real-time     

Relative intensity noise for laser diodes with arbitrary amounts ofoptical feedback

A unified study of the noise characteristics of semiconductor lasers with optical feedback and short external cavity length is presented. A new set of nonlinear rate equations that can describe a laser diode with any amount of optical feedback is proposed. The relative intensity noise (RIN) is calculated by using a numerical solution of these equations. This paper concentrates mainly on the moderate and strong feedback regimes. The spectral phenomena observed during the transition from the weak feedback to the “coherence collapse” regime and then to the strong feedback regime are studied and explained. The effect of the variation of some of the laser diode parameters on the RIN characteristics is also investigated     

Dynamics and chaos stabilization of semiconductor lasers withoptical feedback from an interferometer

This paper studies dynamical behaviors and noise properties of semiconductor lasers with optical feedback from an interferometer, i.e., two external cavities. Bifurcation diagrams versus external-cavity length and reflectivity reveal the existence of stable regions for fixed states and limit cycles. The steady-state analysis for a laser diode with two external feedbacks is performed, which shows that oscillation modes are locked at minimum excess gain modes. Oscillation frequencies of limit cycles show discrete transitions with the variation of ratios between two external-cavity lengths and reflectivities. It is proposed that such a model might be applied to stabilization of the feedback-induced chaos in external-cavity semiconductor lasers since the stable region is quite robust for wide parameter ranges. Intensity and phase noises are examined and the results indicate that the low-frequency relative intensity noise level and the linewidth can be greatly improved when the laser output is a fixed state or a limit cycle     

Stability analysis for a semiconductor laser in an external cavity

A detailed theoretical analysis of stability is presented for a semiconductor laser in an external cavity. The limits of stable operation are determined as a function of the external cavity parameters and the linewidth enhancement factor α. Instability is related to jumps of the laser frequency between external cavity modes (frequency bistability) or to feedback-induced intensity pulsations due to the carrier density dependence of the refractive index. The limit of bistability is derived from the steady-state solutions of the rate equations and the intensity pulsation limit is obtained from a small-signal analysis. This analysis also gives the location of zeros in the system determinant and the resulting FM noise spectrum. For practical applications we emphasize the determination of the stable tuning range for the phase in the external cavity and the classification of the possible types of instability for various feedback levels.     

Oscillation frequency, linewidth reduction and frequency modulation characteristics for a diode laser with external grating feedback

Simple analytical expressions are given for the oscillation frequency, linewidth, FM noise spectrum and frequency modulation characteristics of a diode laser with external grating feedback. The analytical expressions are found to be in good agreement with experimental results and to be useful for quantitative predictions and optimisations of the external feedback controlled diode laser performance.     

Modal noise reduction of laser diodes with specific thickness coating

A low facet reflectivity laser diode is able to oscillate in multilongitudinal modes, and has a high relative intensity noise (RIN) and reduces mode hopping noise. By introducing the electron-cyclotron-resonance (ECR) plasma deposition method, a good facet coating film has been obtained. We have demonstrated that a specific coated laser has very low modal noise characteristics even under direct analogue intensity modulation.     

An integrated equivalent circuit model for relative intensity noise and frequency noise spectrum of a multimode semiconductor laser

Relative intensity noise (RIN) and the frequency/phase noise spectrum (FNS) equivalent circuit of a multimode semiconductor laser diode are derived from multimode rate equations with the inclusion of noise Langevin sources. FNS is an important parameter in optical communication systems, and its circuit model is presented, for the first time, in this paper. Both circuit models for RIN and FNS are integrated in one circuit. RIN and FNS are calculated as functions of frequency, output power, and mode number. It is shown that the RIN of the main mode is increased in the multimode lasers with higher mode numbers. Furthermore, we show that RIN and FNS are enhanced for higher output power. The dependency of a multimode laser diode linewidth on output power is also analyzed using the model.     

Intensity noise of semiconductor laser in presence of arbitrary optical feedback

The relative intensity noise (RIN) Fourier spectrum of a semiconductor laser with arbitrary optical feedback has been derived. The authors calculations show that intensity noise can be highly suppressed at strong feedback, and that the phase of the feedback field plays an important role in the suppression of intensity noise.<>     

Intensity noise suppression and modulation characteristics of a laser diode coupled to an external cavity

Static and dynamic properties of a GaAlAs laser diode with an external cavity (1.5-50 cm) have been studied experimentally, it is found that the optical feedback induced intensity noise is strikingly suppressed by 30 dB when a single frequency oscillation is realized with good phase matching. The threshold of modulation depth, over which intensity noise increases abruptly, has been measured as a function of the modulation frequency in a range of 10-400 MHz. It has the maximum at about 40-70 MHz and increases with the decrease of the external cavity length. On the basis of the compound cavity model the phase and gain conditions for the single frequency oscillation are discussed and the threshold modulation depth is calculated, which is shown to be in good agreement with the experimental results.     

RIN transfer suppression technique for dual-order Raman pumping schemes

An optical technique is demonstrated for suppressing the relative intensity noise (RIN) transfer from a high-power second-order pump laser to the signal radiation in a copropagating dual-order Raman pumping scheme. RIN transfer suppression is accomplished by intensity-modulating a low-power semiconductor laser diode first-order pump. Measurements are presented demonstrating suppression over the entire electronic bandwidth and -20-dB suppression spanning a 23-nm optical bandwidth. This technique may facilitate the use of high-power fiber lasers in copropagating dual-order distributed Raman amplifiers.     

Performance improvements in high-frequency modulation of a laser diode under enhanced optical feedback

Improvement in the high-frequency modulation characteristics of a Fabry-Perot laser diode has been observed when the device is driven in an external cavity that provides an amplified optical feedback. At a driving frequency of 10 GHz, the weak sinusoidal waveform in the original output is turned into a train of narrow pulses with a width of 23 ps. Under an optimized feedback level, an improvement of 15.4 dB in the electrical modulation efficiency and 21.2 dB in the sidemode suppression ratio are observed. The output pulses also exhibit a reduced chirp with a minimized total root-mean square noise.     

Relative intensity noise measurements of a widely tunablesampled-grating DBR laser

The intensity noise of a sampled-grating distributed Bragg reflector laser with 50-nm tuning range and 45-dB side-mode suppression ratio has been measured. The resonance frequency, damping factor, and modified Schawlow-Townes linewidth are extracted from the noise spectra. At high output power, the relative intensity noise (RIN) of the laser is below the photodiode shot noise limit, which is -160 dB/Hz. The laser has uniform shot noise limited RIN properties along the whole tuning range. The maximum resonance frequency is 5.4 GHz at a bias current of 120 mA and the K factor is 0.58 ns     

FM subcarrier fiber optical transmission system design and itsapplication in next-generation wireless access

This paper presents a design method of optical frequency modulation (FM) subcarrier (with super carrier) transmission modem. The nonideal link characteristics, including laser chirping, fiber dispersion, voltage controlled oscillator (VCO) phase noise, relative intensity noise (RIN), and equivalent network model of laser diode of such a system that may bring about signal distortion are discussed first. We then propose a hierarchical methods to establish the system equivalent model. Finally this FM modem is applied to a GSM wireless system, in which multichannel signals are transmitted over fiber between radio port and basestation, and system performance is appraised by its dynamic range. It is found that optical FM subcarrier (with super carrier) transmission technique can improve the system dynamic range, compared with the intensity modulated direct detection (IMDD) method, and this is a economical and efficient method     

Relative intensity noise measurements of 5 μm quantum cascade laser and 1.55 μm semiconductor laser

The magnitude of the relative intensity noise (RIN) of a 5 mum distributed-feedback quantum cascade laser (DFB-QCL) was compared with a conventional 1.55 mum DFB laser diode (LD). The RIN for the DFB-QCL at a frequency of 1 MHz was 157 dBm at 10 mW light output, about 5 dB higher than that of the DFB-LD, which could almost be fully explained by the semi-classical noise model. The resonant tunnelling induced noise, which might be a cause of the RIN degradation, was not observed.     

Wavelength Selector External Cavity Laser Diode by Fiber Switch

A Fabry–Perot laser diode coupled to a single-mode fiber Bragg grating has the ability to emit a single longitudinal mode. In this paper a wavelength selector source based on a mechanical fiber switch used to change the Bragg grating in front of the laser diode is investigated. The external laser wavelength is changed. Critical coupling exists between the two fiber ends of the switch, which leads to the creation of a parasitic cavity inside the laser cavity. The aim of this work is to optimize the coupling ends of a single-mode fiber switch with respect to coupling efficiency and afterward measure the laser output power variation and spectrum to check their optical feedback characteristics on the relative intensity noise. Typical emission performance is a power of −2.5 dBm with a SMSR of 43 dB and a RIN of −146 dB/Hz.     

Small signal frequency response of a linear dispersive single-modefiber near zero first-order dispersion wavelength

A theory describing the propagation of signal and noise through a lossless linear dispersive singlemode fiber near zero first-order dispersion point is presented. Using the small signal analysis approach, this theory has been applied to modulation and noise characteristics of a laser diode, emitting near the fiber zero-dispersion wavelength, to obtain the small signal frequency response and the signal relative intensity noise (RIN) at the output of the fiber. It has been shown that, for very long broad-band transmission lines, dispersion effects may be no longer negligible also at zero-dispersion wavelength     

Noise analysis of injection-locked semiconductor injection lasers

The noise of injection-locked semiconductor lasers is analyzed by rate equations including the spontaneous emission noise. The side mode suppression and the relative intensity noise (RIN) of the locked laser (slave laser) are given for different wavelengths detuning between the master and slave laser and for different linewidth enhancement factors α. For large α, locking is difficult to achieve, whereas extremely low noise may be obtained for injection-locked lasers with a low linewidth enhancement factor.