Multimode Vertical-cavity Surface-emitting Lasers Research Articles

Spatially dependent noise model for vertical-cavity surface-emitting lasers

We present a comprehensive noise model for vertical-cavity surface-emitting lasers (VCSELs). The time-domain model accounts for the stochastic fluctuations in the local carrier density in the separate confinement heterostructure and quantum wells, and in the modal intensity and phase of both the internal and the out-coupled optical field. In this work, we consider these fluctuations to be caused by the temporal uncertainty of the processes that supply or consume carriers and photons, such as carrier diffusion and photons escaping the cavity, and the processes that create or annihilate carriers and photons, such as stimulated emission and absorption. The noise model is based on a deterministic quasithree-dimensional dynamic model that treats the carrier transport, heat generation and dissipation, and optical fields in the device. Langevin noise terms are derived and added to the rate equations in the numerical solution. The noise model is applied to simulate the noise characteristics of fundamental-mode stabilized VCSELs with a shallow, mode discriminating, surface relief. The relative intensity noise and frequency noise of the output are calculated. From the latter, the linewidth of the VCSEL can be estimated. The results are compared with those of conventional multimode VCSELs.

Optimal intensity noise reduction in multimode vertical-cavity surface-emitting lasers by polarization-selective attenuation.

We study in detail the effect of the anticorrelation of the polarization modes on the total intensity noise of a vertical-cavity surface-emitting laser. We show that small polarization-dependent losses can greatly improve the total intensity noise, and we determine and experimentally demonstrate the conditions necessary for optimal intensity noise reduction.

RF Transmission Over Multimode Fibers Using VCSELs—Comparing Standard and High-Bandwidth Multimode Fibers

Fiber-optic radio-frequency links have been assembled using oxide-confined vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers. Links with single and multimode VCSELs and with standard and high-bandwidth fibers have been evaluated and compared in the frequency range of 0.1-10 GHz. The best results were obtained for links with a multimode VCSEL and a high-bandwidth fiber. For a 500-m-long link, a spurious free dynamic range of 104 dB/spl middot/Hz/sup 2/3/ at 2 GHz and 100 dB/spl middot/Hz/sup 2/3/ at 5 GHz were obtained while allowing for a VCSEL-fiber misalignment of /spl plusmn/12 /spl mu/m. Corresponding numbers for the intrinsic link gain and noise figure are -29 and -33 dB, and 39 and 42 dB at 2 and 5 GHz, respectively. Inferior performance was observed for the standard fiber link due to a larger variation in modal group velocities. This paper also presents a detailed link analysis to identify performance limitations and to suggest modifications for improved performance.

Dynamic behavior of fundamental-mode stabilized VCSELs using a shallow surface relief

An extensive theoretical study was performed on the dynamic behavior of 850-nm-wavelength oxide-confined fundamental-mode stabilized vertical-cavity surface-emitting lasers (VCSELs), using a shallow surface relief. The surface relief is used to provide lower mirror loss for the fundamental mode, thus acting as a mode discriminator. In this way, single-mode operation at high power levels can be obtained. We utilized a comprehensive model that includes the detailed epitaxial layer structure and device geometry when calculating the optical fields and that accurately accounts for the dynamic effects of carrier density and temperature on the modal distributions. Modulation response, eye diagrams, bit error rate (BER), and relative intensity noise (RIN) were simulated and compared to the performance of VCSELs without a mode discriminator, i.e., conventional multimode VCSELs. The fundamental-mode stabilized VCSELs are associated with a higher out-coupling, which lowers the relaxation oscillation frequency and damping, and strong spatial hole burning, which induces a low-frequency roll-off in the modulation response and contributes to the damping of the relaxation oscillation at low bias. However, their dynamics is fully competitive with conventional multimode VCSELs at both 2.5 and 10 Gb/s although they exhibit a slightly higher eye closure. We only found a 0.5-dB power penalty in the BER. The RIN is enhanced, with a peak that is about 10-15 dB higher, caused by the lower damping of the relaxation oscillation. It should be noted that in the comparison we assume that all modes are equally captured from the multimode VCSEL. A mode-selective loss can severely degrade its performance.

Theoretical calculation of relative intensity noise of multimode vertical-cavity surface-emitting lasers

An analytical calculation of relative intensity noise spectra of weakly index-guided vertical-cavity surface-emitting lasers (VCSELs) in a multimode regime is performed. We obtain analytical expressions that incorporate the spatial dependence of electrical field and carrier density profiles. Expressions are derived for single-mode and two-transverse-mode operation. Our analysis shows that single-mode noise spectrum depends on the spatial mode profile through a modal effective volume. Two-mode noise spectra also incorporate the dependence on the degree of spatial overlapping between the modes. Two resonance peaks appear in the noise spectra of the individual modes and total power of the two-mode VCSEL. We analytically show that those peaks appear at frequencies that correspond to the relaxation oscillation frequencies of the multimode laser. Simple analytical expressions are also derived for noise spectra at zero frequency.

Single Transverse Mode Operation of a Self-Seeded Commercial Multimode VCSEL

This work presents a novel self-seeding configuration of the vertical-cavity surface-emitting laser (VCSEL) without classical wavelength selective elements. A multimode VCSEL shows stable single transverse mode operation in this configuration. The extremely simple setup consists merely of the commercial VCSEL, an iris, and a plane mirror, in which the reduction of relative intensity noise greater than 20 dB/Hz and the sidemode suppression ratio of 20 dB are achieved.

Determination of the optimum cluster parameters in a clustered free-space optical interconnect.

A model for a clustered free-space optical interconnect is developed and is used to determine the maximum array density that can be achieved, together with the optimal cluster parameters that maximize this density. This model includes misalignment tolerance and the impact of multimode vertical-cavity surface-emitting laser beams. We find that for short interconnect distances, the maximum channel density is limited by the speed of the relay lenses, but as the interconnect distance increases, geometric aberrations become the limiting factor. We also determine the interconnect distance below which a micro-channel relay is more suitable and the distance above which a single-lens solution is adequate.

Harmonic and intermodulation distortion in oxide-confined vertical-cavity surface-emitting lasers

The nonlinear distortion in single and multimode vertical-cavity surface-emitting lasers (VCSELs) under analog modulation was studied both numerically and experimentally. A quasi-three-dimensional time domain model was used to simulate the second-order harmonic and third-order intermodulation distortion in typical GaAs-based oxide-confined VCSELs emitting at 850 nm. The model is based on fundamental interrelated physical processes involving the optical fields and carrier and temperature distributions with no parameter fitting to experimental results. The simulations show that for low modulation frequencies (<2 GHz), the distortion is strongly affected by spatial hole burning, and at higher modulation frequencies, the distortion is dominated by the relaxation oscillation effects. At intermediate frequencies the two effects often cancel, resulting in a significantly lower distortion. Due to mode competition, the multimode VCSEL shows a more unpredictable behavior in the low frequency region. Moreover, in this region, the distortion of an individual mode can be much higher than the distortion of the total output power demonstrating the importance of avoiding mode-selective losses in multimode applications. The general trends observed in the simulations are well reproduced in the experiments.

Ultrafast simulations of multimode VCSEL using optimized waist paraxial eigenmodes

An ultrafast two-dimensional (2-D) multimode vertical-cavity surface-emitting laser (VCSEL) simulation tool suite is introduced using eigenmode expansion of the rate equations, utilizing a novel generic representation for laterally open VCSEL cavity eigenmodes by optimized-waist paraxial modes. By avoiding the need to resolve short spatial and fast temporal scales, the code runs much faster than finite difference time domain and beam propogation method codes, retaining the 2-D description for the nonuniform carrier density and intensity profiles, multimode excitation, and axial carrier density depletion (hole burning). Code validation results against experimental measurements, such as steady-state NF mode structure, threshold gain versus aperture diameter, and dynamic mode switching, are presented. The code yields the correct behavior regarding aperture placement, with lower thresholds for aperture at node. Fast (<1 b/s) execution time allows detailed dynamic simulation of driven response over long input streams for the statistical extraction of bit-error rate and relative intensity noise figures of merit.

Chip-level integrated diffractive optical microlenses for multimode vertical-cavity surface-emitting laser to fiber coupling

The standard assembly technologies for vertical-cavity surface-emitting laser (VCSEL) to fiber coupling systems involve the integration of discrete elements with demanding requirements for alignment effort and time. We present a method for the monolithic integration of diffractive microlenses on the chip level. This process is based on a UV-casting replication technique using ORMOCER® (a registered trademark of Fraunhofer-Gesellschaft) materials [hybrid organic-inorganic polymers (Streppel et al. 2001)] and offers the capability to be extended to a wafer-scale process. A mathematical description for the propagation of the laser modes through the system and the resulting fiber coupling efficiency is presented. We use a model for the source characteristics of the VCSEL based on a step-index fiber model for the simulation of the mode-field propagation. A model for the estimation of the diffraction efficiency of the lens is developed. Finally the simulations are compared with first experimental results of single replicated elements. Experimental coupling efficiencies for a multimode fiber [50/125, numerical aperture (NA) = 0.20] better than 0.7 over the entire operation range of the VCSEL are achieved. Losses below 0.5 dB (10%) are observed within lateral fiber displacement tolerances of m10 μm.

Tomographic amplitude and phase recovery of vertical-cavity surface-emitting lasers by use of the ambiguity function

The amplitude and the phase of a multimode vertical-cavity surface-emitting laser (VCSEL) are recovered by a simple tomographic procedure based on the ambiguity function. The results are of considerable importance for any field of optics in which VCSELs are employed.

Design of microchannel free-space optical interconnects based on vertical-cavity surface-emitting laser arrays.

We investigate the design of free-space optical interconnects (FSOIs) based on arrays of vertical-cavity surface-emitting lasers (VCSELs), microlenses, and photodetectors. We explain the effect of the modal structure of a multimode VCSEL beam on the performance of a FSOI with microchannel architecture. A Gaussian-beam diffraction model is used in combination with the experimentally obtained spectrally resolved VCSEL beam profiles to determine the optical channel crosstalk and the signal-to-noise ratio (SNR) in the system. The dependence of the SNR on the feature parameters of a FSOI is investigated. We found that the presence of higher-order modes reduces the SNR and the maximum feasible interconnect distance. We also found that the positioning of a VCSEL array relative to the transmitter microlens has a significant impact on the SNR and the maximum feasible interconnect distance. Our analysis shows that the departure from the traditional confocal system yields several advantages including the extended interconnect distance and/or improved SNR. The results show that FSOIs based on multimode VCSELs can be efficiently utilized in both chip-level and board-level interconnects.

10 Gb/s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications

TO-packaged GaAs vertical-cavity surface-emitting lasers (VCSELs) are being investigated for high-bit-rate data transmission over 103 cm of a 250×200 μm 2 core size polymer-based optical waveguide to be employed in optical backplanes. The bit-error rates for 5 and 10 Gb/s transmission over the waveguides incorporating 45° deflection mirrors are better than 10 −12. For the first time, the measured bandwidth-length-product for a polymer-based optical backplane exceeds 10 GHz× m .

Spatial investigation of transverse mode turn-on dynamics in VCSELs

Data transmission experiments with single-mode as well as multimode 850-nm vertical-cavity surface-emitting lasers (VCSELs) are carried out from a near-field point of view. Special attention is paid to important quantities like on/off-ratio and bit error rate (BER). A single-mode VCSEL oscillating on the fundamental LP/sub 01/ mode shows no change in eye opening, on/off-ratio, and BER at different lateral fiber coupling positions. In the case of a multimode VCSEL oscillating both on the LP/sub 01/ mode and LP/sub 11/ donut mode, we observe a significant lateral change in the on/off-ratio, which plays an important role in BER measurements.

Small and large signal performance and gain-switching of intra-cavity contacted, shallow implant apertured VCSELs

Results are presented on a lateral current injection vertical cavity surface emitting laser (VCSEL) structure, the implant apertured, index guided VCSEL (I 2-VCSEL). This approach was previously used for 980 nm emission and has now been adapted for 850 nm emission. The threshold current is as low as 0.8 mA for 7 μm diameter current apertures. Typical slope efficiencies of 0.45–0.5 mA/mW are obtained for output mirror reflectivity of 99.5%. Small-signal measurements of the modulation response indicate instrument-limited bandwidths in excess of 11 GHz. Large-signal measurements demonstrate 10 Gb/s operation with bit-error rates below 10 −13. In addition, high repetition rate pulses were generated by gain switching both single and multimode VCSELs. These Gaussian pulses had time bandwidth products below 2.5, pulse widths as low as 29.6 ps and timing jitter as low as 5.5 ps.

Nonlinear distortion reduction in transverse mode stabilized oxide-confined VCSELs

Fundamental mode operation has been achieved in a large area oxide-confined vertical-cavity surface-emitting laser (VCSEL) by etching a shallow surface relief in the top mirror. The nonlinear distortion during analog modulation has been measured and compared to the distortion in a planar multimode VCSEL. The distortion was quantified by conventional two-tone measurements from which the spurious free dynamic range was determined. Several decibels improvement in dynamic range was observed in the low frequency range (0.1-2 GHz) for the single-mode VCSEL. This is primarily due to a higher relaxation frequency and an absence of mode competition and mode partition noise in the single-mode VCSEL, indicating the advantages of using single-mode VCSELs for high-performance analog modulation.

Noise characteristics of oxide-confined vertical-cavity surface-emitting lasers

We demonstrate the possibility of generating intensity squeezed light with free-running or injected multimode vertical-cavity surface-emitting laser. Sub-shot-noise operation results from very strong anticorrelations between the transverse modes. The influence of the active media diameter on the amount of squeezing is analyzed.

Through-etched silicon carriers for passive alignment of optical fibers to surface-active optoelectronic components

Silicon carriers for passive alignment of optical fibers to surface-active optoelectronic components with micrometer precision are presented. The carriers are through-etched by deep reactive ion etching (DRIE) to make vertical fiber alignment holes that go through the chip. On the top surface of the carrier, high-precision electrodes and eutectic AuSn-solder bumps are deposited by evaporation and electroplating. Surface active opto-components, vertical cavity surface emitting lasers (VCSELs) or photodetectors are flip-chip mounted on the solder bumps, and are self-aligned to the carrier during the soldering process. Finally optical fibers are inserted and glued in the fiber holes, thereby aligning them to the opto-components. Alignment experiments with multi-mode VCSELs show a coupling efficiency of >90%.

Spatial quantum noise of semiconductor lasers

The transverse distribution of intensity noise in the far field of semiconductor lasers has been experimentally studied. For a single-mode edge-emitting laser, it has been found that a large amount of noise is present in higher-order nonlasing transverse modes parallel to the diode junction. In the case of a spatially multimode vertical-cavity surface-emitting laser, each mode exhibits a large noise, but these noises show strong anticorrelations.

AlGaAs vertical-cavity surface-emitting laser responses to 4.5-MeV proton irradiation

We have irradiated single- and multimode AlGaAs vertical-cavity surface-emitting laser (VCSEL) arrays operating at a nominal wavelength of 780 nm with 4.5-MeV protons and doses ranging from 10 to 30 Mrad in the active region. We observed a peak power reduction of about 2% per Mrad in the 14-μm aperture, multimode VCSELs. Single-mode VCSELs having an aperture of 6 μm exhibited a smaller peak power reduction of 0.4%-1% per Mrad. A slight shift in the current threshold was observed only for the multimode VCSELs at dose levels above 10 Mrad. First results indicate a reduced VCSEL peak laser power output that is dominated by a temperature shift caused by the radiation induced increase in resistive heating. In contrast, the power reduction in edge-emitting lasers is dominated by the enhanced radiation induced nonradiative recombination rate. The VCSEL irradiation was performed with a focused ion micro beam that was rastered over the device surface, ensuring a very uniform exposure of a single device in the array.