Single-mode Vertical-cavity Surface-emitting Lasers Research Articles

Polarization-stable single-mode multi-junction VCSELs with integrated anti-phase mode filter

In this paper, polarization-stable and spectrally single-mode operation of oxide-confined multi-junction vertical-cavity surface-emitting lasers (VCSELs) are achieved by using a non-circular integrated anti-phase transverse mode filter. A record-high single-mode output power is obtained by radially reducing effective mirror reflectivity at the surface areas where the higher order modes are expected to lase, leading to an increase in the threshold gain for these transverse modes, all the while minimal losses are introduced to the fundamental mode. The polarization control of such devices is achieved by using a non-circular mode filter in order to perturb the cylindrical symmetry of the cavity. An elliptically etched surface relief is shown to be effective in realizing a single-mode single-polarization device over a wide range of operating bias currents and temperatures. The continuous-wave light–current–voltage characteristics of 8-junction 940 nm VCSELs show single-mode output power >13 mW at room temperature, spectrally measured with a side-mode suppression ratio of >30 dB. This single-mode performance can be achieved depending on the ratio between the relief opening and the oxide aperture size. Additionally, the single-polarization behavior of such devices (with an extinction ratio of >15 dB) is shown to be dependent on the orientation of the elliptical mode filter, where the light is mainly polarized along the minor axis of the elliptical feature. Overall, combined with a multi-junction active region, the elliptical model filter is able to achieve highly efficient single-mode linearly polarized VCSELs.

Direct VCSEL interconnection with a hollow core fiber

The direct interconnection of a hollow-core fiber (HCF) to a laser diode or photodiode is essential to full exploitation of the unique characteristics of HCFs. In this paper, we investigate two specific methods for HCF interconnection (i.e., butt coupling and simple two-lens imaging) for coupling light from a single-mode vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm. Our research findings indicate that direct (lensless) butt coupling leads to poor coupling efficiency (∼10 %) due to the significant mode field diameter (MFD) mismatch between the waveguide and the HCF. However, the efficiency can be significantly improved (up to approximately 69 %) by using a graded-index fiber based all-fiber MFD converter. On the other hand, a two-lens system enables an extremely high coupling efficiency of ∼ 96 %.

Single-Mode-VCSEL With a Ring-Shaped Self-Aligned Recessed Metal Mode Filter

This article presents a single-mode vertical-cavity surface-emitting laser (VCSEL) using a novel ring-shaped self-aligned recessed metal (SARM) mode filter on the emitting window as a means of transverse mode control. The SARM structure acts as a spatial filter to suppress the higher-order transverse modes through selective recessed-etching of the top mirror and the self-aligned deposition of the p-type contact metals for cavity tuning and blocking the higher-order modes. The SARM mode filter could be a potentially high-yield alternative technique for the mass production of single-mode VCSELs, demonstrating a single-mode emission with 1.55-mW power and a 30.7-dB side mode suppression ratio.

Lasing properties and carrier dynamics of CsPbBr3 perovskite nanocrystal vertical-cavity surface-emitting laser

Abstract All-inorganic lead halide perovskite nanocrystals (NCs) have been widely investigated as highly promising optical gain materials due to their compelling electrical and optical properties. Although many efforts have been carried out, a deep understanding of perovskite NC vertical-cavity surface-emitting lasers (VCSELs) is elusive, which is very important in the development of photoelectronic integrated circuits. Along these lines, in this work, a low lasing threshold (22 μJ/cm2) single-mode VCSEL consisting of CsPbBr3 NCs film and two distributed Bragg reflectors was successfully constructed. The CsPbBr3 NCs were synthesized by using the supersaturated recrystallization method. Interestingly, benefiting from the strong coupling between the active layer and the optical field in the cavity, a single-mode lasing at 527 nm was demonstrated under femtosecond optical pumping. The carrier dynamics of the perovskite NC VCSEL was also thoroughly investigated by performing pump intensity-dependent time-resolved photoluminescence measurements. The typical gain-switching phenomenon was observed with an ultrafast decay of the laser pulse of ∼10 ps. Our work provides valuable insights for the implementation of the CsPbBr3 NC VCSEL for various optoelectronic applications.

Low Threshold Current and Polarization-Stabilized 795 nm Vertical-Cavity Surface-Emitting Lasers

Low threshold current and polarization-stabilized 795 nm vertical-cavity surface-emitting lasers (VCSELs) are fabricated by integrating a surface grating of high polarization selectivity and high reflectivity. The rigorous coupled-wave analysis method is used to design the surface grating. For the devices with a grating period of 500 nm, a grating depth of ~150 nm, and a diameter of the surface grating region of 5 μm, a threshold current of 0.4 mA and an orthogonal polarization suppression ratio (OPSR) of 19.56 dB are obtained. The emission wavelength of 795 nm of a single transverse mode VCSEL is achieved at a temperature of 85 °C under an injection current of 0.9 mA. In addition, experiments demonstrate that the threshold and output power also depended on the size of the grating region.

50 Gbit/s PAM4 Data Transmission Over 500-m Single-Mode Fiber Using an 880-nm Single Mode VCSEL

Error-free 50 Gbit/s PAM4 data transmission over 500-m single mode fiber was successfully realized using our oxide-confined 880-nm single-mode high-speed VCSELs. This transmission was accomplished because of the flat frequency response of the VCSEL across a wide bias span. Static characteristics and high-speed modulation properties at room temperature and high operation temperature were studied in detail. Data transmission experiments for the 880-nm VCSEL with 4- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> oxide-aperture diameter was performed. A 50-Gbps PAM4 module was packaged for data transmission. Error-free 50 Gbit/s PAM4 data transmission over 500-m single mode fiber was successfully realized. The proposed VCSEL is fit for the 400G DR8 module because of its 500-m single-channel 50 Gbit/s error-free transmission. Compared with commonly used DML, VCSELs are evidently advantageous in cost and power consumption.

Impact of Diffusion Profile on the Modulation Response of Single-Mode Disorder-Defined VCSELs

The impact of diffusion profile shaping through the use of tensilely and compressively strained diffusion masks on the modulation response of single-mode vertical-cavity surface-emitting lasers (VCSELs) using disorder-defined apertures is investigated. VCSELs designed for high-power single-fundamental-mode emission through the use apertures defined via impurity-induced disordering (IID) in conjunction with a standard oxide-confinement process are characterized to extract high-frequency optical cavity parameters across oxide aperture and diffusion mask strain variations. The 3-dB small-signal bandwidth is maintained for a 7.68 mW single-mode 850 nm VCSEL with an oxide aperture of 13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> using a tensilely strained diffusion mask relative to a non-disordered multimode device of the same oxide aperture. A large K-factor reduction is also observed, from 0.248 ns to 0.045 ns, indicating that damping and photon lifetimes within the cavity of VCSELs employing disorder-defined apertures are substantially reduced. Performance implications to data communication applications are discussed.

Ultra-narrow linewidth vertical-cavity surface-emitting laser based on external-cavity weak distributed feedback.

We demonstrate an ultra-narrow linewidth vertical-cavity surface-emitting laser (VCSEL) based on external-cavity weak distributed feedback from Rayleigh backscattering (RBS). A single longitudinal mode VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB. By adjusting the thermal resistance of the VCSEL from 4.5 kΩ to 7.0 kΩ, the laser wavelength can be tuned from 1543.324 nm to 1542.06 nm with a linear tuning slope of -0.506 nm/kΩ. In the tuning process, the linewidth fluctuates in the range of 553-419 Hz.

Angled Flip-Chip Integration of VCSELs on Silicon Photonic Integrated Circuits

An investigation of angled flip-chip integration of a singlemode 850 nm vertical-cavity surface-emitting laser (VCSEL) on a silicon nitride photonic integrated circuit (PIC) is presented. Using numerical FDTD simulations, we consider the conditions under which the VCSEL can be integrated at an angle over a grating coupler with high coupling efficiency and low optical feedback. With both coupling efficiency and feedback decreasing with increasing angle, there is a trade-off. With co-directional coupling, first-order diffraction loss sets in at a critical angle, which further reduces the coupling efficiency. No such critical angle exists for contra-directional coupling. We also experimentally demonstrate angled flip-chip integration of GaAs-based 850 nm single transverse and polarization mode VCSELs over grating couplers on a silicon-nitride PIC. At the output grating coupler, light is either collected by an optical fiber or converted to a photocurrent using a flip-chip integrated GaAs-based photodetector. The latter forms an on-PIC optical link. We measured an insertion loss of 21.9, 17.6 and 20.1 dB with a singlemode fiber, multimode fiber and photodetector over the output grating coupler, respectively.

Modeling of surface grating-loaded VCSEL with slowing light

We propose single-mode vertical cavity surface-emitting lasers (VCSEL) based on the surface grating with various cavity lengths ranging from 10 μm to 1 cm. We present the modeling of surface grating-loaded VCSEL based on the coupled-mode theory with slowing light. Thanks to the low effective index caused by the slow-wave propagation, a shallow surface grating of a few tens nm provides large optical feedback. A narrow stop band in the surface grating-loaded VCSEL waveguide appears because of the large waveguide dispersion of slow-waves. The numerical results on coupling coefficient, mode selectivity and threshold gain are given for single-mode, long-cavity and short-cavity grating-loaded VCSELs with different grating parameters. A wide range of grating coupling strength enables a short cavity of 10 μm and a long cavity of 1 cm with a single-mode operation. The proposed VCSEL structure may open up new VCSEL platforms for low-power consumption and high-power applications.

Coupling angle tolerance of the 850-nm single-mode VCSEL output collimated by lensed OM4-MMF or GI-SMF for a NRZ-OOK link.

By collimating the single-mode (SM) vertical-cavity surface-emitting laser (VCSEL) at 850 nm with either the OM4 multi-mode fiber (OM4-MMF) or the graded-index single-mode fiber (GI-SMF) with lensed end-face, the directly encoded non-return-to-zero on-off keying (NRZ-OOK) data transmission performance is characterized when tilting the coupling angle with respect to the surface normal of the SM-VCSEL. In comparison with the lensed OM4-MMF and lensed SMF coupling, the lensed OM4-MMF collimator shows a large coupling angle tolerance with the coupling efficiency only degraded by 5% when enlarging the tilted angle from 0° to 10°. In contrast, the lensed GI-SMF collimator attenuates the coupled SM-VCSEL output by more than 50% when tilting the coupling angle up to 10°. For the lensed OM4-MMF coupling, the receivable NRZ-OOK data rate in BtB and after 100-m OM4-MMF cases can achieve 50 Gbit/s with its corresponding BER degraded from 6.5 × 10-10 to 8.8 × 10-10 when enlarging its tilting angle ranged from 0° to 10°. By changing the collimator to the lensed SMF, the decoded BER significantly degrades from 5.8 × 10-5 to 1.2 × 10-1 when coupling and transmitting the NRZ-OOK data at 50 Gbit/s. Owing to the low coupling efficiency via the lensed SMF collimator, the error-free NRZ-OOK data rate under the lensed SMF coupling somewhat decreases to 35 Gbit/s in the BtB link and to 32 Gbit/s after the 100-m GI-SMF link with allowable coupling angle tilted from 0° to 4°. This work confirms the applicability of the lensed MMF or SMF collimator for coupling the SM-VCSEL output with a relatively large tolerance on the tilting angle with respect to the surface normal of the SM-VCSEL.

Wavelength tuning robustness optimization for a high-temperature single-mode VCSEL used in chip-scale atomic sensing systems.

In this paper, the wavelength current tuning characteristics of high-temperature-operation single-mode vertical-cavity surface-emitting lasers (VCSELs) for chip-scale atomic sensing systems are studied. Excellent wavelength current tuning robustness is helpful to improve the stability of atomic sensing systems. By optimizing the size of the oxide aperture combined with surface relief mode control technology, the single-mode VCSEL with an 8 µm oxide aperture can achieve 2.02 mW output power at 355 K, and the wavelength current tuning coefficient is ∼0.25nm/mA. This excellent wavelength current tuning robustness results from the low active current density and device heat generation due to the optimized oxide aperture size.

Ultra-compact VCSEL scanner for high power solid-state beam steering.

We demonstrate the lateral monolithic integration of a tunable first-order surface-grating loaded vertical-cavity surface-emitting laser (VCSEL) and slow-light waveguide with fan-beam steering and amplifier function. Shallow Bragg-grating formed on the surface of a VCSEL section enables the selection of a single slow-light mode, which can be coupled into the integrated long waveguide and amplified through pumping the amplifier above threshold. We obtained over 3W amplified slow-light power with single-mode operation and over 4W amplified quasi-single-mode power under pulsed current injection. To the best of our knowledge, this is the highest output power for single-mode VCSELs. Solid-state beam steering of the device is also demonstrated with 9° fan-beam steering range and 200 resolution points.

Two Polarization Comb Dynamics in VCSELs Subject to Optical Injection

Optical frequency comb technologies have received intense attention due to their numerous promising applications ranging from optical communications to optical comb spectroscopy. In this study, we experimentally demonstrate a new approach of broadband comb generation based on the polarization mode competition in single-mode VCSELs. More specifically, we analyze nonlinear dynamics and polarization properties in VCSELs when subject of optical injection from a frequency comb. When varying injection parameters (injection strength and detuning frequency) and comb properties (comb spacing), we unveil several bifurcation sequences enabling the excitation of free-running depressed polarization mode. Interestingly, for some injection parameters, the polarization mode competition induces a single or a two polarization comb with controllable properties (repetition rate and power per line). We also show that the performance of the two polarization combs depends crucially on the injection current and on the injected comb spacing. We explain our experimental findings by utilizing the spin-flip VCSEL model (SFM) supplemented with terms for parallel optical injection of frequency comb. We provide a comparison between parallel and orthogonal optical injection in the VCSEL when varying injection parameters and SFM parameters. We show that orthogonal comb dynamics can be observed in a wide range of parameters, as for example dichroism linear dichroism (γa=−0.1 ns−1 to γa=−0.8 ns−1), injection current (μ=2.29 to μ=5.29) and spin-flip relaxation rate (γs=50 ns−1 to γs=2300 ns−1).

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 (&lt; 4 µm) in a conventional oxide-confined VCSEL to support the fundamental mode alone. However, high series resistance (typically &gt; 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.

Review on Single-Mode Vertical-Cavity Surface-Emitting Lasers for High-Speed Data Transfer

Vertical-cavity surface-emitting lasers (VCSELs) are wide-spread laser sources for different applications in optical communication and sensing. The evolution of fabrication processes and new technological approaches allow to obtain high-Q single-mode VCSELs with data rates more than 100 Gbps. This review discusses basic designs and construction features of VCSELs that effect on their applications. The advances over the past 20 years for single-mode VCSELs of 850 nm, 1300 nm and 1550 nm wavelength ranges are presented.

High-Brightness, High-Speed, and Low-Noise VCSEL Arrays for Optical Wireless Communication

The development of high-speed and high-brightness vertical-cavity surface-emitting lasers (VCSELs), which can serve as an efficient light source for optical wireless communication (OWC), play a crucial role in growth of the next generation of wireless communication networks, e.g., 6 G and satellite communications. In this work, by optimizing the size of the Zn-diffusion and oxide-relief apertures in a high-speed 850 nm VCSEL, we obtain record-high brightness (2.9 MW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {cm}}^{-2}{\mathrm {sr}}^{-1}$ </tex-math></inline-formula> at 10 mW output) with single polarized and (quasi-) single-mode (SM) outputs under continuous wave (CW) operation. However, such high brightness output comes at the cost of spatial hole burning (SHB) effect and degraded quality of 25 Gbit/sec eye patterns. In addition, an SM VSCEL array structure is usually needed to further boost the total available optical power for long-reach OWC. Here, a novel (quasi-) SM VCSEL array structure is demonstrated which releases the trade-off between the performances of brightness and eye-pattern quality. Our demonstrated array has a special crisscross mesa connecting neighboring VCSEL units and an extra electroplated copper substrate integrated on the backside of the chip. Compared to the reference array without the copper substrate and connected active mesas, the demonstrated array exhibits a higher (quasi-) SM output power, narrower divergence angle, larger orthogonal polarization mode suppression ratio (OPSR), and flatter E-O response. This in turn leads to smaller jitter and less noise in the measured 12.5 Gbit/sec eye-patterns. The demonstrated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7\times 7$ </tex-math></inline-formula> array exhibits a maximum SM power of around 90 mW with a 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{e}^{2}$ </tex-math></inline-formula> divergence angle as narrow as 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> (FWHM: 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> ), single polarized output (10 dB OPSR), decent relative intensity noise performance (<−130 dB/Hz) and clear 12.5 Gbit/sec eye-opening. Such new device with remarkable static/dynamic performances has strong potential to further improve the product of the linking distance and data rate in the next generation of OWC channels.

High-power single-mode vertical-cavity surface-emitting lasers using strain-controlled disorder-defined apertures

Strain-engineered diffusion masks deposited via plasma-enhanced chemical vapor deposition are demonstrated to control the curvature of the zinc diffusion front and, hence, disordering front, in disorder-defined vertical-cavity surface-emitting lasers (VCSELs) for enhanced high-power single-mode operation. Tensilely strained silicon nitride diffusion masks are applied to limit the lateral undercut of the disordering front, thereby minimizing the interaction between the disordered region of the distributed Bragg reflector and the fundamental mode. This results in higher threshold modal gain and absorption losses from the disordered region for higher-order modes while enabling greater output powers for fundamental-mode operation in single-mode impurity-induced disordered VCSEL designs. Using this technique, 850 nm AlGaAs VCSELs are shown to operate in a single fundamental mode with record optical output powers in excess of 10 mW and side-mode suppression ratios greater than 35 dB. Electrical and optical performances of these devices are presented in addition to near-field images confirming single-fundamental-mode lasing.

The relative intensity noise of a vertical-cavity surface-emitting laser with a fiber Bragg grating external cavity

The research deals with the topical problem of stabilizing the optical parameters of a single-mode vertical-cavity surface-emitting laser. A solution to the problem is proposed by forming an external cavity based on a Bragg grating induced into an optical polarization-maintaining fiber. In this experimental research, the authors estimated the contribution of an external cavity to the relative intensity noise of a vertical-cavity surface-emitting laser. The stability of the laser generation with a varying phase of back reflections at various pump currents is investigated. The relative intensity noise of laser radiation in the pump current range of 1.1–6.3 mA is estimated using a photodetector with a bandwidth of 8.5 MHz. The phase change of the back reflections was carried out by shifting the optical fiber end with a 100 nm step. As a result of the research, it was obtained that the vertical-cavity surface-emitting laser with such an external cavity shows a stable emission in the current range of 1.8–3.2 mA. However, one can observe instability with constant switching between adjacent longitudinal modes outside this range. The most stable laser operation mode with an external cavity against a change in the phase of back reflections is registered at a pump current of 2.78 mA. Utilizing an external cavity with the fiber Bragg grating made it possible to reduce the relative intensity noise from 7.2∙10–10 1/Hz to 1.5∙10–11 1/Hz in the currents range 1.86–3.2 mA. The study can be useful in applications that require an optical radiation coherent source with a low relative intensity noise level (such as coherent optical communication or fiber-optic sensors).

Mode Division Multiplexed 850-nm Single-Mode VCSEL Transmission Using Standard Single-Mode Fiber

Mode division multiplexing (MDM) has attracted great attention in recent years to increase the network capacity. To apply MDM in data center applications, a low-cost and low power consumption system is desired. In this letter, we utilize 850-nm single-mode VCSEL as the laser source and report mode division multiplexed transmission over a standard single-mode fiber, which is two-mode at 850 nm. Due to the small spot size and low numerical aperture, single-mode VCSEL can be coupled into single-mode fibers with low loss. The mode multiplexer and de-multiplexer devices are designed and fabricated based on fiber mode selective couplers using Hi780 fiber as the 850-nm single-mode launch and receiving fiber and standard single-mode fiber as the few-mode transmission fiber. The MDM system supports two channels, corresponding to the LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode in the few-mode transmission fiber. Detailed characterizations of the multiplexer and de-multiplexer are conducted including the insertion loss and crosstalk measurements. System transmission at 25 Gb/s shows error free performance over 1-km standard single- mode fiber for both channels. The utilization of VCSEL and standard single-mode fiber can potentially provide a low-cost solution for MDM systems in data center applications.