Noise characteristics of a single-mode laser diode subject to strong optical feedback

An experimental study has been performed of the relative intensity noise (RIN) of a semiconductor laser in optical feedback regimes I to V. At low bias current, a low RIN is observed with low feedback ratio, the RIN increased in the coherence collapse regime (regime IV) and decreased in regime V. The RIN in regime V is lower than that of the solitary laser. For higher bias current, a higher feedback ratio is needed for the semiconductor laser to transit from regime IV to V. The measurements are found to be in good qualitative and quantitative agreement with theoretical predictions.     

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     

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.     

Polarization-resolved relative intensity noise measurements of a vertical-cavity surface-emitting laser subjected to strong optical feedback

We experimentally measure the polarization-resolved relative intensity noise (RIN) in two orthogonal polarization states of a vertical-cavity surface-emitting laser (VCSEL) subjected to strong optical feedback. A comparison is made with the RIN of solitary VCSEL. Optical feedback is shown to enhance the RIN in one polarization direction while suppressing the RIN in the orthogonal polarization. The RIN in the dominant polarization mode exhibits complementary behavior to the RIN in the suppressed polarization mode.     

The dynamic properties and stability analysis for vertical-cavitysurface-emitting lasers

In this paper, a phenomenal analysis on the wavelength dependence of the gain temperature coefficient is presented. Based on this model, the relation between the dynamic properties and the lasing wavelength within the gain regime are investigated using the rate equations, including the wavelength dependence of both the differential gain and linewidth enhancement factor. Our calculation shows that when the lasing mode occurs in the negative damping rate regime, the relative intensity noise (RIN) is more than 43 dB higher, and the linewidth is broadened out more than 30 dB, which indicates laser being in a chaotic state. The stability of the vertical-cavity surface-emitting lasers with distant external optical feedback is analyzed numerically. The results for different cavity designs are compared. The effects of the nonlinear gain coefficient on RIN and spectral linewidth of the VCSEL's with external optical feedback is investigated     

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.     

Chaos control and noise suppression in external-cavitysemiconductor lasers

Feedback-induced chaos and intensity noise enhancement in a laser diode with external optical feedback are studied by computer simulations. The enhancement of relative intensity noise (RIN) that is often observed in experiments is considered as a result of the feedback-induced deterministic chaos and the intensity noise suppression is treated from the viewpoint of chaos control. Especially, the conventional noise suppressing technique known as a high-frequency injection modulation is turned into a problem of stabilizing chaos through parameter modulations. We developed an analytical method which allows to optimize the modulation frequency from the linear stability analysis of the dynamical model that describes the laser diode with external feedback. The robustness of the modulation with respect to the modulation frequency and depth is verified and the results suggest the feasibility of applying our method to actual noise suppression. The RIN in the low-frequency region (up to 100 MHz) is shown to be reduced to the solitary laser level when the feedback-induced chaos is effectively controlled with the optimized modulation frequency     

Intensity noise of semiconductor laser diodes in fiber optic analog video transmission

This paper presents an investigation of the effects of laser diode noise on analog video transmission in the HF and VHF bands, which resulted in the development of several designs of graded-index multimode fiber systems that can ignore reflection induced laser noise. The contents of the investigation include: 1) The evaluation of intrinsic laser noise of various laser structures and the evaluation of modulation effects on laser diode noise characteristics. It was found that the relative intensity noise (RIN) is less than -145 ∼ -150 [dB/Hz] when the modulation factor is less than 0.7 for index-guide mode stabilized lasers; 2) The quantitative evaluation of reflected laser beam effects on laser noise characteristics. The maximum laser-coupled reflected optical power that does not increase laser noise was determined as-65 ∼ -73 dB or less depending on the kind of laser structure; and 3) The evaluation of optical power reflected back into the laser in graded-index multimode fiber systems.     

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.<>     

Distributed feedback laser diode (DFB-LD) to single-mode fiber coupling module with optical isolator for high bit rate modulation

A distributed feedback laser diode (DFB-LD) module for single-mode fiber coupling is described in the 1.5-μm wavelength region. An optical isolator is constructed in the two lens coupling circuit to stabilize the DFB-LD characteristics. It consists of a DFB-LD as a polarizer, a yttrium iron garnet (YIG) plate and an analyzer. Coupling characteristics and attainable isolation of the proposed circuit have been studied theoretically and experimentally. The effect of reflected light on the DFB-LD characteristics such as relative intensity noise (RIN) and oscillation spectrum has been experimentally investigated taking into account the polarization of the reflected light. It has been clarified that the DFB-LD can function as an effective polarizer with an extinction ratio of more than 20 dB. The DFB-LD module was successfully fabricated to have stable spectrum characteristics and to sufficiently suppress the noise caused by reflections. It has been confirmed by a long haul transmission experiment that this module is applicable to large capacity optical fiber transmission systems.     

Optical feedback-induced noise in pigtailed laser diode modules

The sensitivity of commercially available pigtailed laser diode modules to optical feedback, in terms of laser relative intensity noise, is investigated. A pigtailed laser diode model which is based on multimode rate equations, Langevin operators, and optical feedback terms in used to predict that operation in the coherence collapse region can be avoided in high-speed, short-haul, optical data links by trading off optical launch power into the pigtail with decreased laser/fiber coupling efficiency. Experimental results for three pigtailed laser diode modules indicate that with a laser/fiber coupling efficiency of 4%, an external feedback level of between -15 and -17 dB can be tolerated before the laser enters the coherence collapse region. Good correlation between these results and predictions from the numerical model is obtained.<>     

RIN transfer analysis in pump depletion regime for Raman fibreamplifiers

A numerical analysis is presented that models the relative intensity noise transferred from a Raman pump to a signal laser in an optical fibre. The analysis is valid in both the pump-depletion and nondepletion regimes. The RIN requirements for a 0.1 dB Q-penalty in the pump depletion regime are similar to those in the nondepletion regime, exhibiting a RIN requirement reduction of only 3-4 dB/Hz     

Modeling high-speed optical transmission systems

Computer simulation of a 2.5-Gb/s intensity-modulated/direct-detected optical system with high path dispersion is described. This simulation, based on single-mode laser diode rate equations, includes transmit and receiver circuit filtering, receiver circuit noise, avalanche photodiode noise, fiber dispersion, and laser chirp. The rate equations are sufficiently general to model mode-offset distributed feedback laser diodes. The system power penalty, due to laser chirp and fiber dispersion, is calculated using Gaussian quadrature numerical integration. By simulating a population of some 12800 laser diodes and correlating the performance of each laser in a transmission system with its spectral characteristics, it is possible to deduce laser specifications that will assure satisfactory operation in long-span links. These results are used to study a laser diode specification under consideration by CCITT, and, by studying the simulated laser line shapes, some modifications to the CCITT specification are considered     

Integrated optical E-field sensors with a balanced detection scheme

Compact and nonperturbing integrated optical E-field sensors are used for electric field measurements. The theoretical sensitivity of an integrated optical sensor system is limited by the shot noise. In conventional sensors, however the minimum detectable electric field strength is limited by the relative intensity noise (RIN) of the laser diode. Achieving the theoretical sensitivity a new sensor system is presented. Using a balanced optical receiver both a signal gain and a reduction of the RIN at the optical receiver is obtained. The principle function and the potential of the balanced detection scheme in comparison with existing concepts as well as first measurements are presented, yielding a sensitivity of less than 300 μV/(m√Hz) with standard laser diodes and a receiving dipole length of 40 mm     

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.     

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     

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.     

Experimental and theoretical study of filtered optical feedback ina semiconductor laser

We report on the systematical investigation of the steady-state regime and the dynamical behavior of a semiconductor laser subject to delayed filtered optical feedback. We study a Fabry-Perot (FP) interferometer type of filter placed in the external feedback loop of a diode laser. The effects of the filter on the locking of the diode laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled laser and FP cavity system. We also present an experimental stability diagram that summarizes the dynamical behavior of the system     

Low-noise 0.78 μm-band laser diode unit containing Faradayrotator

A small, low-noise 0.78 μm-band laser diode unit, containing a Faraday rotator which suppresses optical feedback, was developed. Stable low-noise characteristics were realised within the 3 mW to 10 mW output power. Greater than 40 dB signal/noise ratio was obtained in an optical video disc with this unit