<title>Three-dimensional planar-integrated optics: a comparative view with free-space optics</title>

Recently there have been many advances in vertical-cavity surface-emitting (VC-SE) lasers. VC-SE lasers are promising because they facilitate simple fabrication of high-density, low-threshold, and cost-effective two dimensional (2-D) laser arrays. Monolithic 2-D multiple-wavelength laser arrays are potentially useful for applications in optical communications and interconnects using wavelength-division multiplexing (WDM).1

Vertical cavity surface emitting laser (VCSEL) is an important laser source for their evident plentiful applications in optical communication. The present investigation reports a comparison of the modeling and optimization of long wavelengths 1310 nm and 1550 nm high speed short cavity VCSEL for continuous wave operation at various temperature (283-3230K) for various diameters. The continuous wave lasing is demonstrated for the device diameter from 2 to 5 μm with threshold current of 1.07-1.33 mA with threshold power consumption of 1.86-2.57 mW for 1310 nm and threshold current of 0.94-1.24 mA and threshold power consumption 1.67-2.1 mW for 1550 nm VCSEL. The results demonstrate that the threshold current, peak emitted power and power consumption increases with the increase in device diameter. The results confirm that VCSELs with 2 μm diameter is most suitable to achieve energy-efficient operation. Although rollover current increases with the diameter, but, due to the advantage of lower threshold current and power consumption, VCSEL having smaller diameter is best suited. The power conversion efficiency for proposed long wavelength VCSELs is approximately 50% which is extremely useful for low power applications. The proposed VCSELs are suitable for very short reach (<2 m) optical interconnects such as chip-to-chip and board-to-board communication in high performance computers.

Wavelength multiplexing and tuning capabilities are highly important for applications in optical interconnects and optical communications using wavelength division multiplexing (WDM). We review novel methods and experimental results to achieve these functions in vertical cavity surface-emitting lasers (VCSEL), we believe for the first time. By implementing spatially chirped layers in the laser cavities and obliquely aligning the array axes to the thickness chirp direction, we obtained 7 × 20 VCSEL array emitting 140 unique, non-redundant, controllable, uniformly separated single-mode wavelengths. A large total wavelength span of 43 nm is obtained without compromising the performance of the lasers. Moreover, our techniques are generic and can be readily extended to other wavelength regimes. In addition, we describe our work on wavelength-tunable VCSELs with a 1.8-nm (~540 GHz) continuous tuning and no mode hopping. The laser output power remains nearly constant throughout the tuning range. The wavelength tuning speed is ~400 ns. The multiple wavelength tunable laser arrays are promising for achieving ultrahigh bandwidth (hundreds Gbit/sec to Tbit/sec), high redundancy and reliability, reconfigurable optical networks.

Over the past several years, high-speed vertical-cavity surface-emitting lasers (VCSELs) have been the subject of intensive worldwide research due to their applications in optical interconnects and optical data networks. The performance of VCSELs, especially with respect to their high-speed characteristics, has made significant progress. In this chapter, we first present the basic theory for current-modulated VCSELs using rate equations and small-signal analysis. Factors that affect the modulation bandwidth, including the intrinsic laser responses and extrinsic parasitics, are identified. Once these limitations are known, we discuss various designs that have been implemented in VCSELs to specifically address them, followed by a review of the current high-speed VCSEL performance based on these designs at several different wavelengths, including 850 nm, 980 nm, \(1.1\;\upmu\hbox{m},\) and \(1.3\mbox{--}1.6\;\upmu\hbox{m}.\) Finally, we consider new modulation schemes based on loss modulation in coupled-cavity VCSELs, which has the potential to reach even higher speeds.

Recent progresses in research on bistable laser diodes and their applications in optical communications and photonic switching are reviewed. In addition to the conventional absorptive and dispersive bistable laser diodes, bistability in two-mode lasers via gain saturation has recently attracted attention, because of its ultra high speed. On the other hand, bistable laser diodes with saturable absorbers are mainly used in the system applications because of their stable operations at present. This paper presents the theoretical analysis of the two-mode bistable laser diodes, the stripe lasers and the vertical-cavity surface-emitting lasers (VCSELs) as the two major representatives of bistable lasers, and the profound discussion of their possible applications.

High-speed and “green” ~850 nm vertical-cavity surface-emitting lasers (VCSELs) have lately attracted lots of attention due to their suitability for applications in optical interconnects (OIs). To further enhance the speed and its maximum allowable linking distance of VCSELs are two major trends to meet the requirement of OI in next generation data centers. Recently, by use of the advanced 850 nm VCSEL technique, data rate as high as 64 Gbit/sec over 57m and 20 Gbit/sec over 2km MMF transmission have been demonstrated, respectively. Here, we will review our recent work about 850 nm Zn-diffusion VCSELs with oxide-relief apertures to further enhance the above-mentioned performances. By using Zn-diffusion, we can not only reduce the device resistance but also manipulate the number of optical modes to benefit transmission. Combing such device, which has excellent single-mode (SMSR >30 dB) and high-power (~7mW) performance, with advanced modulation format (OFDM), record-high bit-rate-distance-product through MMF (2.3 km×28 Gbit/sec) has been demonstrated. Furthermore, by selective etching away the oxide aperture inside Zn-diffusion VCSEL, significant enhancement of device speed, D-factor, and reliability can be observed. With such unique VCSEL structure, >40 Gbit/sec energy-efficient transmission over 100m MMF under extremely low-driving current density (<10kA/cm2) has been successfully demonstrated.

Dynamic beam switching of vertical-cavity surface-emitting lasers (VCSELs) have been demonstrated for the first time using integrated optical beam routers. The output beam of the VCSEL is switched electrically between two different emission angles. High-speed switching up to 2 GHz has been achieved with an 8 μm×8 μm device. Such beam-steering devices are particularly suitable for optical interconnect and photonic switching applications. By integrating the beam routers with VCSELs, the complexity and size of the switching nodes are significantly reduced.

Refractive index change using quantum well intermixing (QWI) was investigated for lateral light confinement of vertical cavity surface emitting lasers (VCSELs). Refractive index change of a GaInAs/GaAs quantum well by the QWI was numerically analyzed from calculated absorption spectrum. 1% of refractive index change is expected by control of the QWI condition. A QWI-VCSEL was fabricated for verification of the QWI technique as the VCSEL fabrication process. Though the clear effect of the light confinement was not confirmed, the QWI-VCSEL showed similar or better lasing characteristics in comparison with a standard VCSEL. The QWI technique is available for light confinement of VCSEL as well as various photonic devices.

We demonstrate the characteristics of 2 mm to 1 cm long vertical-cavity surface-emitting laser (VCSEL) amplifiers that emitted continuous wave (CW). VCSELs have various applications in different fields, including datacom, optical interconnects, printers, and sensors. However, VCSEL technology has not been applied in laser material processing that needs a light source with high power density. This is because single-mode (SM) VCSELs have fundamental difficulty in producing beams having an output power of more than several milliwatts. On the contrary, the application range of VCSEL amplifiers is expected to be extended into fields that simultaneously require lasers with high power and high beam quality because they have the potential to attain both characteristics. In this paper, we demonstrate the CW operation of high-power SM 2 mm to 1 cm long VCSEL amplifiers that had an output power of several watts at 20 °C. By optimizing the top-mirror reflectivity and seed-laser wavelength, the output power was linearly proportional to the amplifier length when the growth of amplified spontaneous emission (ASE) was suppressed. The I–L characteristics for different amplifier lengths were measured. All devices emitted narrow beams with divergence angles of approximately 0.1°. The maximum SM output power of the devices with lengths of 2 mm, 6 mm, and 1 cm were 0.62, 2.13, and 3.25 W, respectively. This result shows the possibility to achieve high power and high beam quality for direct semiconductor laser processing.

High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode filter is employed. Fabricated VCSELs are applied in various interconnect schemes. InGaAs quantum-well based VCSELs at 935 nm emission wavelength are investigated for use in perfluorinated graded-index plastic-optical fiber (GI-POF) links. We obtain a 7 Gb/s pseudo random bit sequence (PRBS) nonreturn-to-zero (NRZ) data transmission over 80 m long 155 /spl mu/m diameter GI-POF. We investigate data transmission over standard 1300 nm, 9 /spl mu/m core diameter single-mode fiber with selectively oxidized single-mode GaAs and InGaAs VCSELs. We achieve biased 3 Gb/s and bias-free 1 Gb/s pseudo-random data transmission over 4.3 km at 830 nm emission wavelength where a simple fiber mode filter is used to suppress intermodal dispersion caused by the second order fiber mode. For the first time, we demonstrate 12.5 Gb/s data rate transmission of PRBS signals over 100 m graded-index multimode fiber or 1 km single-mode fiber using high performance single-mode GaAs VCSELs of 12.3 GHz modulation bandwidth emitting at /spl lambda/=850 nm.

Central to the photonic revolution is the development of miniature light sources such as the vertical-cavity surface-emitting laser (VCSEL). Today, VCSEL manufacturing has been established to serve data and telecommunication applications. Moreover, recent progress in new materials, fabrication technologies, and microcavity physics has enabled a new generation of high performance VCSELs. We will overview the present and near-term commercial VCSELs as well as discuss recent research, including new device structures such as 2-dimensional VCSEL arrays, composite resonators, and photonic crystal devices.

Semiconductor nanowire (NW) lasers provide significant potential to create a new generation of lasers and on-chip coherent light sources by virtue of their ability to operate as single mode optical waveguides at the nanoscale. Due to their unique geometry, a major benefit lies also in the feasibility for direct integration on silicon (Si), enabling III–V-on-Si NW lasers that could fuel applications in optical interconnects and data communication. In this review, we describe the state-of-the-art and recent progress in GaAs–AlGaAs based NW lasers emitting in the near infrared (NIR) spectral region, with a specific emphasis on integration on a Si platform. First, we explore design rules for the photonic properties in GaAs NW waveguides based on finite difference time domain calculations. The lasing characteristics of GaAs–AlGaAs core–shell NW lasers are then investigated under various different optical pumping schemes ranging from pulsed to continuous wave excitation. We further review recent activities on the realization of low-dimensional quantum heterostructures inside NW cavities as a means to tune lasing wavelength, gain and threshold properties. Ultimately, we describe schemes for monolithic integration of GaAs-based NW lasers directly on Si and show how such vertical nanocavity lasers are excellent candidates for low-threshold lasing, high spontaneous emission coupling (high β-factor lasers), and ultrafast emission characteristics.

Vertical cavity surface emitting lasers (VCSELs) have long been predicted as low-cost enabling laser sources for many applications including optical communications, sensing and imaging. Traditional VCSELs use distributed Bragg reflectors (DBRs) as mirrors, which consist of many tens layers of epitaxy layers with alternating refractive indecies. Snice 2004, we develeoped a new class of planar optics has emerged using near-wavelength dielectric structures, known as high contrast metastructures (HCM). Many extraordinary properties can be designed top-down based for integrated optics on a silicon or GaAs substrate. The one-dimensional version, a single layer high index contrast near-wavelength gratings (HCG), has been used to replace the hundred-layered DBR in a VCSEL structure. In this talk, I will review recent results using HCG/HCM in device applications of VCSELs, beam-steering optical phased array, biosensor, 4-wave generation, and spatial light modulator. I will discuss inventions and advances in VCSELs that have led to recent global deployment of commercial applications including 3D sensing, LIDAR and optical coherent tomography applications. I will also discuss future prospects for advanced applications.

This letter presents a feedforward optical power control scheme for automotive 150-Mb/s vertical-cavity surface-emitting laser (VCSEL) transmitters. The bias current exhibits a second-order dependence upon temperature, while the modulation current is provided with a piecewise linear compensation of temperature variations. The resulting VCSEL modulation performance was analyzed in detail and experimentally validated. Over the harsh automotive temperature range from -40°C to 105°C, a high extinction ratio was maintained despite very tight tolerances on pulsewidth distortion (PWD); the variation of the emitted optical power was measured to be less than 1.2 dB, while maintaining an extinction ratio better than 12 dB and a PWD less than ±2%.

Harnessing the orbital angular momentum (OAM) of light is an appealing approach to developing photonic technologies for future applications in optical communications and high-dimensional quantum key distribution (QKD) systems. An outstanding challenge to the widespread uptake of the OAM resource is its efficient generation. In this work we design a new device that can directly emit an OAM-carrying light beam from a low-cost semiconductor laser. By fabricating micro-scale spiral phase plates within the aperture of a vertical-cavity surface-emitting laser (VCSEL), the linearly polarized Gaussian beam emitted by the VCSEL is converted into a beam carrying specific OAM modes and their superposition states, with high efficiency and high beam quality. This new approach to OAM generation may be particularly useful in the field of OAM-based optical and quantum communications, especially for short-reach data interconnects and QKD. (C) 2015 Optical Society of America