Single-transverse Mode Vertical-cavity Surface-emitting Laser Research Articles

795-nm high-temperature and high-power operating vertical-cavity surface-emitting laser and application in atomic gyroscope

Single-transverse mode vertical-cavity surface-emitting lasers (VCSELs) are preferable optical sources for small low-power atomic sensors, including chip-scale atomic clocks, magnetometers, and gyroscopes.When VCSEL is used as the pump source of nuclear magnetic resonance gyroscope, it is required to have high single-mode output power. Oxide aperture diameter must be sufficiently small (< 4 µm) in a conventional oxide-confined VCSEL to support the fundamental mode alone. However, high series resistance (typically > 200 Ω for GaAs-based VCSEL) from the small aperture limits its output power and reliability due to excessive current-induced self-heating and high current density. It is a very attractive idea to achieve high power operation of an intrinsic single mode VCSEL based on a large oxide aperture by means of epitaxial structure design without introducing additional process steps. Transverse optical confinement in oxide-confined VCSELs crucially depends on the thickness of oxide layer and its position relative to standing wave. Modifying the structure reduces the overlap between the oxide layer and the standing wave as well as the difference in effective refractive index between core and cladding of the VCSEL, thereby reducing the number of transverse modes andincreasing the mode extension beyond oxide aperture. A 795-nm VCSEL is designed and fabricated based on this concept. A cavity structure of VCSEL with gain-cavity detuning of ~10.8 nm at room temperature is adopted in this paper. The effective refractive index and the standing wave distribution of the VCSEL are calculated, and the position of the oxide layer in the epitaxial structure of the VCSEL is optimized according to the standing wave distribution. Finally, the structure with low effective refractive index difference is obtained. The proposed device achieves high single-mode operation of 4.1 mW at 80 ℃, SMSR of 41.68 dB, and OPSR of 27.4 dB. The VCSEL is applied to a nuclear magnetic resonance gyroscope (NMRG) system as pump source due to its excellent device performance, and satisfactory test results are obtained. This paper presents a new method of designing single-mode high power VCSEL and its feasibility is also demonstrated through experimental results.

Long-Term Thermal Stability of Single-Mode VCSEL Under 96-Gbit/s OFDM Transmission

Thermal-dependent transmission performance and long-term stability of a single-transverse-mode (SM) vertical-cavity surface-emitting laser (VCSEL) with directly pre-leveled 16-quadrature amplitude modulation orthogonal frequency-division multiplexing (16-QAM OFDM) data over 100 m OM4 multi-mode-fiber (MMF) are demonstrated. Although narrowing the oxide-confined aperture to 3 μ m enables the SM VCSEL with low threshold current of 0.21 mA and high modulation bandwidth of 21.4 GHz, this technology also enlarges the insulating oxide area to accumulate the heat for raising core temperature. This induces a harsh environment as the core temperature persistently increases without dissipation even under the water-cooling control at 20 °C. As a result, the SM VCSEL can only guarantee a stable operation for 1.5 h under the encoding of QAM-OFDM data for transmission. After optimization, the SM VCSEL successfully supports a back-to-back transmission capacity as high as 96 Gbit/s. With the assistance of OFDM pre-leveling technique, the 96-Gbit/s QAM-OFDM data transmission is qualified over 100 m OM4 MMF with a receiving power penalty as low as 1.3 dB.

Polarization Dynamics in VCSEL-Based Gain Switching Optical Frequency Combs

In this paper, we report on the polarization dynamics of a 1550 nm vertical-cavity surface-emitting laser (VCSEL) under gain switching regime. We focus our analysis on the resulting optical frequency combs associated with the main and suppressed polarization modes. Measurement of the parameters characterizing the polarization behavior of our single-transverse mode VCSEL is performed from the analysis of high-resolution optical spectra. An extension of the spin-flip rate model to include nonlinear carrier recombination is developed. An analysis of polarization-resolved dynamics is performed by using the previous model with the measured VCSEL parameters. We show that two orthogonally polarized combs are generated that combine to produce a wider overall optical comb. We analyze the dependence of combs on the amplitude and frequency of the modulation. Our results are in agreement with experiments reported by Prior et al. We also analyze the dependence of the combs on the linear dichroism of the VCSEL We show that the change of this dichroism results in bistability between the two orthogonally polarized combs.

Transmission Over SSMF at 850 nm: Bimodal Propagation and Equalization

The combination of 850 nm vertical-cavity surface-emitting laser (VCSEL) with standard single-mode fiber (SSMF) presents an effective and low-cost interface to increase the reach provided by multi-mode fiber links. At 850 nm, SSMF propagates two modes, and in this study, it has been experimentally shown that the different commercially available SSMF's present dissimilar values of differential mode delay. To cope with this unequal behavior of modal dispersion, we propose a scheme based on bidirectional decision feedback equalization (BiDFE) to overcome limited performance of other solutions as mode filtering or classical equalizers. A single span SSMF cabling model, including a measurement-derived statistical characterization of optical connectors, is simulated to evaluate the reach provided by the equalizer attending to both the conditions of the fiber excitation and the characteristics of the VCSEL. A minimum 1.45 km link length at 10 Gb/s is achieved if a linear combining BiDFE (LC-BiDFE) equalizer is included in the receiver, whatever laser launching condition and employing a single-transverse mode VCSEL. If a multi-transverse mode VCSEL is used, the reach provided by LC-BiDFE is slightly reduced but assuring a minimum coverage of 1.15 km.

Measurement of the Intrinsic Parameters of Single-Mode VCSELs

A complete characterization of the working parameters of a long-wavelength vertical-cavity surface-emitting laser (VCSEL) is presented. A technique is described for extracting values for the parameters of semiconductor laser rate equations based on simple expressions for the laser power and linewidth as a function of the bias current. The differential carrier lifetime at threshold is estimated by using the linear relation between the laser linewidth and the bias current that is obtained for below threshold operation. High resolution CW optical spectrum measurements are performed to apply this technique for extraction of the parameters of a 1550-nm single-transverse mode VCSEL. Intensity noise spectrum analysis is used to complete our parameter extraction procedure and to compare with parameter values obtained from laser linewidth measurements.

Wavelength-induced polarization bistability in 1550 nm VCSELs subject to orthogonal optical injection

We report on a theoretical and experimental investigation of the bistable behavior of the polarization of long-wavelength single-transverse mode vertical-cavity surface-emitting lasers (VCSELs) when subject to orthogonal optical injection. We have studied the bistability properties of the polarization switching that appear when changing the master laser wavelength with a fixed master laser power. Two bistable regions are found for polarization switchings appearing at short and long wavelengths. The directions of the obtained hysteresis cycles are in agreement with the experimental results. The widths of both bistable regions are also found to be similar and nearly independent of the injected power. The width of the region where polarization switching takes place is shown to increase as the injected power is increased. A good overall qualitative agreement is found between our theoretical and experimental results.

Nonlinear dynamics of the polarization of multitransverse mode vertical-cavity surface-emitting lasers under current modulation

In this paper we report on a theoretical investigation of the nonlinear dynamics of the polarization of multitransverse mode vertical-cavity surface-emitting lasers (VCSELs) under current modulation. Special attention is given to the comparison with a previously studied case of single-transverse mode VCSEL emitting in two orthogonal polarizations. The consideration of spatial effects in VCSEL modifies the polarization dynamics that accompanies the period doubling route to chaos for large modulation amplitudes. Depending on the modulation parameters, the excitation of a higher order transverse mode may either induce chaotic pulsing in an otherwise regularly pulsating VCSEL, or induce a time-periodic pulsing dynamics in an otherwise chaotic VCSEL. Bifurcation diagrams obtained for different modulation frequencies, several values of the dichroism, and different transverse mode characteristics allow us to identify the different scenarios of polarization dynamics in a directly modulated VCSEL. Temporal analysis of carrier number radial profile reveals considerable changes for the multitransverse mode case only constituting the physical origin of the reported changes in the temporal and polarization dynamics.