Wavelength-tunable Vertical-cavity Surface-emitting Lasers Research Articles

High Fidelity MZI-BCG Sensor With Homodyne Demodulation for Unobtrusive HR and BP Monitoring

An all-optical active homodyne detection aided fiber-optic Mach-Zehnder interferometer (MZI) vital signs monitoring system is proposed and investigated. The active homodyne detection is mainly realized by employing a wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) and automatic closed-loop control system (CLCS). It can effectively maintain the MZI sensor to operate around the quadrature point and successfully eliminate the fading effect of output signal induced by low frequency drift. Therefore, high fidelity and high stableness of the ballistocardiograph (BCG) signal is guaranteed. The results show that according to the Bland-Altman and the confidence ellipse analysis, the heart rate (HR) monitoring performance reveals high accuracy and good consistence. What’ more, a multiple linear regression-based blood pressure (BP) estimation model is presented, which shows that the BP is highly correlated with I-J-K complex oriented features, including IJ interval, JK interval and JK amplitude. It indeed offers a potential approach to achieve cuff-less BP estimation merely based on the high-fidelity BCG signal. In conclusion, the proposed MZI-BCG monitoring system is simple to use, and it shows great potential in future unobtrusive cardiac monitoring both in home and hospital.

Low-threshold wavelength-tunable ultraviolet vertical-cavity surface-emitting lasers from 376 to 409 nm

Optically pumped wavelength-tunable vertical-cavity surface-emitting lasers (VCSELs) operating in the ultraviolet A (UVA) spectrum were demonstrated. The VCSELs feature double dielectric distributed brag reflectors and a wedge-shaped cavity fabricated using the substrate transfer technique and laser lift off, resulting in a graded cavity length in one device. A resonant period gain structure is used in the InGaN/GaN multi-quantum well active region to enhance the coupling between the cavity mode field and the active layers. The optical field inside the cavity is modulated by the cavity length; thus, tunable lasing at different wavelengths is realized at different points of a single VCSEL chip. The lasing wavelength extends from 376 to 409 nm, covering most of the UVA band below the band gap of GaN. The threshold pumping power density of the UVA VCSELs at different wavelengths ranges from 383 to 466 kW/cm2, which is among the lowest values for ultraviolet (UV) VCSELs. This study is promising for the development of small-footprint, power-efficient UV light sources.

MEMS-VCSEL as a tunable light source for OCT imaging of long working distance

We present a wavelength tunable vertical-cavity surface-emitting laser (VCSEL) constructed by die-bonding a half-cavity InGaAs laser diode (LD) chip onto a silicon-on-insulator chip with a microelectromechanical system electrostatic diaphragm mirror that functions as a Fabry–Perot interferometer. As a result of the short cavity length, the integrated tunable LD has single-mode lasing characteristics with an extremely large coherence length of 150 m or more. The developed wavelength tunable LD is used to perform swept-source optical coherence tomography with a large scan depth, which is applicable to ophthalmic observation for the diagnosis of pathologic nearsightedness based on the measurement of the axial length of an eye.

Wavelength-Swept VCSELs

The ability to continuously tune the wavelength of a laser is of critical importance and is a fundamental building block for many optical systems, including wavelength division multiplexed optical fiber communication systems. Vertical cavity surface emitting laser (VCSEL) structure offers a unique advantage to engineer the lasing wavelength because of its ultrashort cavity length, inherently supporting a single Fabry–Perot mode. Hence, a continuous change in VCSEL cavity thickness leads to a continuous sweep of VCSEL wavelength. Wavelength-tunable VCSELs have been a subject of intense interest for the last two decades. Incorporating part or entire top mirror of a VCSEL in an optical microelectromechanical structure (MEMS), continuous wavelength sweeps have been reported. The monolithic integration brings together the best of both technologies and leads to an unprecedented performance in the continuously swept wavelength range. In addition, with the advances of ultrathin high contrast gratings and metastructures (HCG/HCM), the wavelength tuning speed of MEMS-VCSELs has been increased to 1–10 MHz range. Such lasers, now referred as wavelength-swept lasers, are enabling new applications in optical coherent tomography and light detection and ranging systems. In this paper, we review various structures and recent progress of MEMS-VCSELs. We summarize some of the early breakthroughs in designs and properties of micromechanical tunable VCSELs, including advances in HCG/HCM-based MEMS-VCSELs emitting at 850, 1060, and 1550 nm wavelength regimes. In addition, we report a brand new design leading to a record high Δλ/λo = 6.9% tuning ratio with 600 kHz speed at center wavelength of 1060 nm.

Characteristics of Wavelength Tunable Vertical-Cavity Surface-Emitting Laser Using Mesa Heater for Detecting Application

We fabricated mesa heater wavelength tunable vertical-cavity surface-emitting lasers (VCSELs) and observed the dependence of their wavelength tuning characteristics on the tuning current and measurement time. The mesa heater tunable VCSEL shows an emission wavelength of 787.5 nm and a line-width as narrow as 0.2 nm without any tuning current. A total tuning range of 5.3 nm was observed in polarized single mode operation mode over the whole tuning range. The tuning efficiency was 0.059 nm/mA and the side mode suppression ratio was larger than 30 dB over the whole tuning range. The concept of tunable VCSELs is suitable for the detecting system as compact label-free biosensor.

Micromechanical tunable vertical-cavity surface-emitting lasers

We report the study on a short wavelength-tunable vertical-cavity surface-emitting laser utilizing a monolithically integrated bridge tuning microelectromechanical system. A deformable-bridge top mirror suspended above an active region is utilized. Applied bridge-substrate bias produces an electrostatic force which reduces the spacing of air-gap and tunes the resonant wavelength toward a shorter wavelength (blue-shift). Good laser characteristics are obtained: such as continuous tuning ranges over 11 nm near 940 nm for 0–9 V tuning bias, the peak output power near 1 mW and the full-width-half-maximum limited to approximately 3.2–6.8 nm. A detailed simulation of the micromechanical and optical characteristics of these devices is performed, and the ratio of bridge displacement to wavelength shift has been found to be 3:1.

Long Wavelength-Tunable VCSELs With Optimized MEMS Bridge Tuning Structure

In this letter, we present the first demonstration of a long wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) using a monolithically integrated bridge tuning microelectromechanical system structure. The use of such a tuning structure allows for better wavelength tuning repeatability and wider tuning range as compared to cantilever-based tunable VCSELs. The devices show tuning ranges over 22 nm with peak output power of 1.3 mW in continuous-wave operation at 15/spl deg/C. The bridge structure has a mechanical resonant frequency of 80 kHz. The transmission test through 300-km single-mode fiber (SMF-28) using the tunable VCSEL under mechanical vibration below 10 KHz and 10-G acceleration shows little bit-error-rate degradation.

Long wavelength MEMS tunable VCSEL with InP-InAlGaAs bottom DBR

In this letter, we present the first demonstration of a long wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) using an InP-InAlGaAs distributed Bragg reflector (DBR). The use of such a DBR improves the thermal resistance of the VCSEL while keeping the growth process simple. The devices show operation to temperatures greater than 75/spl deg/C, and single-mode devices emit powers as high as 0.9 mW at room temperature. The tuning range is as high as 17 nm.

Transmission performance of a 1.5-μm 2.5-Gb/s directly modulated tunable VCSEL

We present experimental and theoretical transmission performance results for a novel 1.5-/spl mu/m 2.5-Gb/s directly modulated wavelength tunable vertical-cavity surface-emitting laser over different fiber types. Uncompensated reach of 100-800 km is demonstrated depending on the fiber choice.

Tunable VCSEL

Vertical-cavity surface-emitting lasers (VCSELs) are now key optical sources in optical communications. Their main application is currently in local area networks using multimode optical fibers. VCSELs are also being rapidly commercialized for single-mode fiber metropolitan area and wide area network applications. The advantages of VCSEL include simpler fiber coupling, easier packaging and testing, and the ability to be fabricated in arrays. In addition, VCSELs have an inherent single-wavelength structure that is well suited for wavelength engineering. All these advantages promise to lead to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. The author reviews the advances on wavelength-tunable VCSELs. She summarizes some of the early breakthroughs in wavelength engineering of VCSELs and then concentrates on the designs and properties of micromechanical tunable VCSEL.

All-optical crossbar switch using wavelength division multiplexing and vertical-cavity surface-emitting lasers

A design for an all-optical crossbar network utilizing wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) technology and a combination of free-space optics and compact optical waveguides is presented. Polymer waveguides route the optical signals from a spatially distributed array of processors to a central free-space optical crossbar, producing a passive, all-optical, fully connected crossbar network directly from processor to processor. The analyzed network could, relatively inexpensively, connect local clusters of tightly integrated processors. In addition, it is also believed that such a network could be extended, with wavelength reuse, to connect much larger numbers of processors in a multicluster network.

Micromachined widely tunable vertical cavity laser diodes

Wavelength tunable vertical-cavity surface-emitting lasers (VCSELs) are potentially useful for future optical communications. Traditionally, the emission wavelength of a vertical cavity laser was tuned by modulating the active region temperature. However, thermal tuning is slow, and the realized tuning range is quite limited. Micromachined tunable VCSELs (Mi-T-VCSELs) combine the traditional vertical cavity laser structure with a monolithically micromachined deformable membrane, enabling continuous wavelength tuning without mode hopping. In addition to a large wavelength tuning range, this technique does not suffer from the shortcomings of the thermal tuning technique. This paper presents the background theory, processing sequence, and experimental results for Mi-T-VCSELs.

10.1 nm range continuous wavelength-tunable vertical-cavity surface-emitting lasers

A record wide continuous wavelength tuning range of 10.1 nm is demonstrated in vertical-cavity surface-emitting lasers (VCSELs) with monolithically integrated thin-film metal heaters. Single longitudinal mode and single transverse mode are maintained throughout the tuning range. The thin-film heater can stand higher temperatures over a wider tuning range. It is also separated and electrically isolated from the laser, and can be applied to most existing VCSEL structures.