Mode Suppression Ratio Research Articles

Fourier-method beam analysis for coherent vertical cavity surface emitting laser arrays.

Effective engineering and exploitation of coherently coupled vertical cavity surface emitting laser arrays will benefit from simple and fast characterization of the optical coupling and coherence. We propose a Fourier method of analyzing beam profiles as an alternative to the prior beam visibility analysis and show that the mode suppression ratio and phase between a pair of supermodes can be extracted. Our analysis enables fast quantitative determination of the array coherence and supermode characteristics.

Wideband balanced networks with wideband common mode suppression and arbitrary power-dividing ratio

ABSTRACTTwo novel wideband balanced networks with wideband common mode suppression and arbitrary power-dividing ratio are proposed in this paper. The differential/common mode equivalent circuits of the balanced networks can be easily reduced based on the matrix transformation. Microstrip-slotline 180° phase inverters loaded with four isolation resistors are used to realize high isolation for the power division output ports. Two planar wideband equal and unequal power division balanced networks with bandwidths of 33% of (1:1, equal) and 39% (1:2.22, unequal) for the differential mode and over 120% common mode suppression (0.96–3.6 GHz, |Scc21, 31| < –10 dB) are designed and fabricated.

Wavelength bistability based on optical injection in a novel tunable dual mode laser.

A novel tunable dual mode laser is proposed based on a double-layer Bragg grating structure, named double-layer grating distributed Bragg reflector (DLG-DBR) laser. With controlling the gain saturation condition, different oscillating states can be observed. The nonlinear dynamics characteristics including bifurcation of the laser under optical injection with two different start-up sequences are analyzed with numerical simulations. Furthermore, we investigate the lasing wavelength bistability of the laser combined with its tuning characteristics theoretically. Optical memory operation based on the external optical injection locking bistability of the dual mode device is proposed. The memory operation can be achieved in the whole wavelength window with a 10 nm tuning range. A mode suppression ratio (MSR) of more than 45 dB is achieved between the two bistable states.

Influence of the Reflectivity Distribution of Fiber Grating on the Output Characteristics of Fiber Grating External Cavity Diode Lasers

The dot function approximation of reflectivity distribution of a fiber grating (i. e., supposed that the fiber grating supplies feedback only in bragg reflection wavelength and the reflectivity at other wavelengths is regarded as zero), which is usually used to analyze the output characteristics of a fiber grating external cavity diode laser (FGECDL), has been proved to be unreasonable from physical facts. As a result, a theoretical model including the reflectivity distribution of fiber grating applicable to FGECDLs is presented. Taking the mode suppression ratio (MSR) of FGECDLs as a example for comparison, the results show that MSR obtained by the model is obviously lower than that obtained by the dot function approximation.

Mode Suppression of Single-mode Micromachined Fabry-Perot Semiconductor Lasers

Single-mode can be achieved on the micromachined F-P semiconductor lasers (MLDs) when the micromachined defects were properly positioned inside the diode cavity. In this work, an analytical expression of the output spectrum from the micromachined F-P diode lasers has been deduced. Based on this expression, a general discussion has been made on the mode-suppression-ratio (MSR) of the MLDs.

Design of widely tunable semiconductor lasers and the concept of binary superimposed gratings (BSG's)

We propose a new concept of binary superimposed gratings (BSG's) as multiwavelength grating reflectors and show it as an effective structural improvement over the existing designs, to both the fabrication process and device performance. We also present a study of key design issues for widely tunable lasers based on grating mirrors with a comb-like reflection spectrum and summarize simple design rules for the grating part of the laser, based on analytical and numerical analysis. The binary supergrating consists of elements of equal size whose refractive index is allowed to be one of two possible values, which are sequenced according to a binary optics formalism to effect a spatial superposition of multiple sets of single-frequency gratings, and its implementation is well within the standard e-beam lithography limits. The calculations of lasing frequency, mode, and side-mode suppression ratio of the tunable laser are formulated and presented along with numerical examples.

Analysis of metal-interlaced-grating vertical-cavity surface-emitting lasers using the modal method by modal expansion

It is analyzed by the modal method by modal expansion (MMME) that the strong polarization discrimination of a vertical-cavity surface-emitting laser (VCSEL) can be effectively obtained by using the metal-interlaced grating on the topmost layer of the VCSEL. The MMME is used to overcome the problems coming from the large permittivity discontinuities at GaAs-Al-air-Al interfaces. The analyses predict the correct direction of dominant polarization and the mode-suppression ratio. The calculated transmission efficiencies explain the characteristics of the VCSEL's below threshold. It is found that the difference in phase and amplitude between the two orthogonal polarization generated by the grating plays important roles for the selection of a dominant polarization through mirror reflectivities and mirror losses. In addition, the influence of various thin metal layers on polarization stabilization is demonstrated.

Influence of Facet Reflectivities on the Output Characteristics of Laser Diodes

Starting from the analytical solutions to the multimode rate equations, the influence of facet reflectivities on the output characteristics of laser diodes have been studied. For laser diodes biased above the threshold, the output spectra and the mode suppression ratios under different facet reflectivities have been simulated numerically. For laser diodes biased below threshold, with the emphasis on the superluminescent diodes (SLDs), the dependences of such important operation parameters as output spectral width (FWHM) and output power on the facet reflectivities have been investigated.

Studying Semiconductor Lasers with Ray Trace Method

Tracing the amplified spontaneous emission light inside the cavity of the semiconductor laser, the expression of the photon density of various diode modes inside the cavity has been derived based on the mean carrier density approximation. After summing up the photon densities, an analytical expression for the deficit ΔN of the carrier density from the conventional nominal threshold carrier density N, has been deduced. As a result, both the mode spectra and the mode suppression ratio (MSR) can be expressed analytically, irrespecting whether the peak wavelength of the gain is coincident with one of the resonant modes of the diode or not

Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings

The authors have recently demonstrated 57 nm of tuning in a monolithic semiconductor laser using conventional distributed-Bragg-reflector (DBR) technology with grating elements removed in a periodic fashion. They describe the theory and design of these so-called sampled-grating tunable lasers. They calculate sample-grating reflectivity, and present normalized design curves which quantify tradeoffs involved in a sampled-grating DBR laser with two mismatched sampled-grating mirrors. These results are applied to a design example in the InP-InGaAsP system. The design provides 70-nm tuning while maintaining >30-dB mode suppression ratio (MSR), with fractional index change Delta mu / mu <0.2% in the mirrors, and only 1 mm of total sampled-grating length.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the spectral properties of widely tunable lasers using grating-assisted codirectional coupling

The analysis of mode-suppression-ratio (MSR) and wavelength tunability of widely tunable semiconductor lasers using grating-assisted codirectional-coupler (GACC) filters is considered. A tradeoff is found between tuning range and spectral purity when using this type of filter as an intracavity mode selector. It is shown that in order to improve the tuning range by a factor of 10 compared to a conventional tunable laser using distributed feedback reflectors (DBRs), one has to tolerate a MSR around 23 dB at 1 mW output optical power/facet for an ideal cavity design. Limitations imposed by the materials are illustrated with design curves based on a vertical-twin-guide structure in both GaAs and InP systems for lasing wavelengths at 0.98 and 1.55 mu m, respectively. >

Singlemode surface emitting laser using partial mirror disordering

A vertical cavity surface emitting semiconductor laser is described which uses zinc diffusion and partial disordering of the epitaxial output mirror to provide waveguiding for lateral mode control. Mode suppression ratios against higher order transverse modes as high as 36 dB have been observed.

Importance of self-induced carrier-density modulation in semiconductor lasers

Semiconductor lasers have a built-in mechanism for modulating the carrier density at multiples of the longitudinal-mode spacing. This mechanism is believed to be relatively unimportant for solitary laser diodes since the mode spacing typically exceeds 50 GHz. A general numerical model capable of including self-saturation, cross saturation, and four-wave mixing occurring due to both interband and intraband effects is presented, and the self-induced carrier-density modulation is shown to play an important role in solitary laser diodes. In particular, it can severely degrade the gain margin and the mode-suppression ratio in single-mode semiconductor lasers when the operating current is increased. Degradation depends on the linewidth enhancement factor and the laser length and can be especially severe when the cavity length exceeds 1 mm. >

Correlation of relative intensity noise with linewidth floors in narrow linewidth DFB lasers

The relative intensity noise of narrow linewidth DFB lasers in the vicinity of linewidth floors was measured. A strong correlation between linewidth rebroadening and a corresponding increase in the relative intensity noise for frequencies below 3 GHz was observed for devices with mode suppression ratios in excess of 35 dB.

Mode-partition noise and intensity correlation in a two-mode semiconductor laser.

The intensity-noise characteristics of a nearly single-longitudinal-mode semiconductor laser are analyzed by adopting a two-mode model. An approximate solution of the resulting Langevin rate equations is used to obtain an analytic expression for the enhancement of the main-mode intensity noise caused by mode partition. We also obtain analytic expressions for the autocorrelation and cross-correlation functions which are used to discuss noise variances in the two modes. In particular, the cross-correlation function is related to the mode-partition coefficient, a measure of anticorrelation between the two modes that is useful to analyze the system performance. Our results show explicitly how the intensity correlation is affected by the mode-suppression ratio and other device parameters.