Two-dimensional multiple wavelength tunable surface-emitting laser arrays

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.

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

Reconfigurable high-speed optical fibre networks: Optical wavelength conversion and switching using VCSELs to eliminate channel collisions

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.

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.

This paper reports on the viability, effectiveness, versatility, and the utility of the concept of the planar integrated optical interconnection scheme with respect to the concept of the free-space interconnection scheme in realizing multiple integration of various micro/nano- photonic devices and components for applications in optical interconnection, optical circuits, optical switching, optical communication and information processing. Several planar optics schemes to detect parallel optical packet addresses in WDM switching networks, to perform a space- variant processing such as fractional correlation, and to construct multistage interconnection networks, have been successfully demonstrated in the 3D glass blocks. Using a gradient-index (GRIN) substrate as a signal propagation medium in the planar optics is a unique advantage, when compared to the free-space optics. We have demonstrated the GRIN-substrate concept by using six 1/4-pitch GRIN rod lenses and a vertical cavity surface emitting laser (VCSEL). The GRIN planar optics can be further extended to the use of 2D array of VCSEL microlasers and modulators in making massively parallel interconnects. A critical comparison between the planar integrated optics scheme and the free- space integrated scheme is given in terms of physics, engineering and technological concept.

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.

Electrically-pumped continuous tunable vertical cavity surface emitting lasers (VCSEL) emitting in the 1530-1620 nm wavelength regime is reported for the first time. The VCSELs exhibit a continuous, repeatable and hysterisis-free wavelength-tuning characteristic. The VCSELs are directly modulated at 2.5 Gbps and has > 45 dB side mode suppression ratio (SMSR) over the entire tuning range. Wavelength locking is achieved in /spl sim/200 usec by a simple universal locker.

With an intracavity phase tuning technique, the authors demonstrate &amp;lt;2 nm precision in wavelength setting of vertical-cavity surface-emitting laser (VCSEL) microresonators in the wavelength range 1040–1070 nm using an Ar ion-beam etching process with subnanometer precision in etch depth. This postgrowth fabrication technique for controlling the wavelength of individual VCSELs is promising for the development of monolithic multiwavelength VCSEL arrays with precisely defined wavelengths for wavelength division multiplexed (WDM) optical interconnects and the assembly of compact and high-capacity WDM transmitters.

For use in wavelength division multiplexing (WDM) with high-speed wavelength routing functions, the fast wavelength switching of tunable lasers is a key function. A tunable MEMS vertical cavity surface emitting laser (VCSEL) is a good candidate as a light source for this purpose. The cantilever in MEMS VCSELs has a high mechanical resonance frequency thanks to its small size, but the switching time is limited by the ringing of the cantilever structure. In this paper, we analyzed the mechanical behavior of a cantilever MEMS mirror and demonstrated ringing-free operation with an engineered voltage signal. The applied voltage waveform was optimized in a two-step format and we experimentally obtained ringing free wavelength switching. We measured the transient response of the wavelength by inserting a tunable filter, exhibiting the settling time of less than 2.5 µs, which corresponds to a half period of the cantilever resonance frequency.

Multiple-wavelength laser arrays at 1.55 μm are key components of wavelength division multiplexing (WDM) systems for increased bandwidth. Vertical cavity surface-emitting lasers (VCSELs) grown on GaAs substrates outperform their InP counterparts in several points. We summarize the current challenges to realize continuous-wave (CW) GaInNAsSb VCSELs on GaAs with 1.55 μm emission wavelength and explain the work in progress to realize CW GaInNAsSb VCSELs. Finally, we detail two techniques to realize GaInNAsSb multiple-wavelength VCSEL arrays at 1.55 μm. The first technique involves the incorporation of a photonic crystal into the upper mirror. Simulation results for GaAs-based VCSEL arrays at 1.55 μm are shown. The second technique uses non-uniform molecular beam epitaxy (MBE). We have successfully demonstrated 1x6 resonant cavity light-emitting diode arrays at 850 nm using this technique, with wavelength spacing of 0.4 nm between devices and present these results.

New generation of datacom standards require vertical cavity surface emitting lasers (VCSELs) operating at data rates beyond 50 Gbaud in on-off-keying (NRZ) and pulse amplitude modulation (PAM4) formats. New applications in Ethernet also demand extended wavelength range of the devices to enable wavelength division multiplexing (WDM) in multimode fiber. We report on 850 nm and 910 nm single mode industrial technology VCSELs capable for error free NRZ data transmission at 60 Gb/s over 800 m and 400 m of OM5 multimode fiber (MMF), respectively. Both types of VCSELs are also suitable for PAM4 transmission up to 60 Gbaud.

High performance GaAs- and InGaAs-based selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays emitting in the 780 to 980 nm wavelength regime are investigated as light sources for high bit rate optical interconnects. Solid source molecular beam epitaxy (MBE) with carbon as p-type dopant is used for crystal growth. Single-mode devices show record-high output powers up to 4.8 mW with a butt-coupling efficiency of more than 80% into single-mode fiber. Linear 1/spl times/8 and 1/spl times/16 as well as two-dimensional 4/spl times/8 VCSEL arrays have been employed as transmitters in optical fiber links. Non-return-to-zero (NRZ) pseudo-random bit sequence (PRBS) signals with 2/sup 31/-1 wordlength at total data rates up to 24 Gb/s (8/spl times/3 Gb/s) have been transmitted over 500 m of multimode fiber. Bias-free data transmission of 2.5 Gb/s over 500 m of multimode fiber is demonstrated. Polymer optical fiber (POF) was used for short distance data transmission. A visible, red emitting VCSEL (/spl lambda/=780 nm) is employed to transmit 2.5 Gb/s over 1 m of small core diameter (120 /spl mu/m) POF. For all data transmission experiments bit-error rates (BERs) of better than 10/sup -11/ were observed for back-to-back testing as well as for fiber transmission.

We report on new dynamics in vertical-cavity surface-emitting lasers (VCSELs), which is induced by a polarization preserving optical feedback. Our solitary VCSEL, i.e. without feedback, emits in a single linearly polarized (LP) and fundamental transverse mode. Optical feedback induces multiple polarization switchings in the L-I curve. When the injection current is set close to one of the multiple polarization switching points, the laser system exhibits a random anticorrelated hopping of the two orthogonal LP fundamental modes. Of particular interest is the observation of antiphase oscillations in the two LP modes that occur with a period corresponding to the delay time, and which complement the slow mode-hopping. Moreover, the LP-resolved optical spectra show random frequency hops between ECMs. The laser system is therefore not only bistable between the LP modes, but also multistable between ECMs. A theoretical rate equation approach yields a good qualitative agreement with our experiment. We report similar delay-periodic oscillations complementing a mode hopping in a scalar delay differential equation modeling the motion of a particle in a double well potential. Such a model was recently investigated by Tsimring and Pikovsky, in a very general context. Our results in VCSELs yield the first experimental evidence of delay-periodic oscillations in a bistable physical system with noise, and are therefore thought to be of general interest for many other processes in biology, medicine, etc. that exhibit interaction of noise, delay and bistability.

We study the temperature effects on injection-locked vertical-cavity surface-emitting lasers (VCSELs) for use in wavelength-division-multiplexed (WDM) transmitters. We show that injection locking can be used to lock an uncooled VCSEL to the WDM grid while improving its analog performance.