A broad-band waveguide grating coupler with a subwavelength grating mirror

A new diffractive device for light coupling between a planar optical waveguide and free space is proposed. The device utilizes a second-order waveguide grating to diffract the fundamental waveguide mode into two free propagating beams and a subwavelength grating (SWG) mirror to combine the two free propagating beams into a single beam. The finite-difference time-domain (FDTD) simulations show that the SWG mirror improves the coupling efficiency of the waveguide fundamental mode into the single out-coupled beam from about 30% to 92%. A high efficiency (>90%) is predicted for a broad wavelength range of 1520-1580nm. The proposed device is compact (/spl sim/80 /spl mu/m in length) and it eliminates the need for blazing the waveguide grating.     

Polarization Control in Buried Tunnel Junction VCSELs Using a Birefringent Semiconductor/Dielectric Subwavelength Grating

We present long-wavelength InGaAlAs-InP vertical-cavity surface-emitting lasers (VCSELs) with a buried tunnel junction (BTJ) and a well-defined polarization accomplished by a semiconductor/dielectric subwavelength grating (SWG). The grating is incorporated in the inner VCSEL cavity and exhibits an effective birefringence for polarizations parallel and perpendicular to the grating grooves. Theoretical modeling leads to distinct design rules for VCSELs with grating structure that deviate from the conventional design without grating. Due to the large polarization-mode separation, lasing activity is enabled only for one polarization mode. BTJ-VCSELs with properly chosen SWG parameters show predictable and full polarization stability. In addition, the relevant device parameters such as threshold current are comparable to conventional BTJ-VCSELs.     

Fabrication of biomimic GaAs subwavelength grating structures for broadband and angular-independent antireflection

We combine interferometric lithography and inductively coupled plasma etching to fabricate GaAs subwavelength grating (SWG) which mimics the moth eye structures. Through the modification of morphology parameters, including profile, height and packing fraction, tapered, high-aspect-ratio and closely-packed GaAs SWGs are obtained. The measurement of spectral reflectance of the fabricated SWGs shows that reflection has been dramatically reduced compared to the polished GaAs surface. Particularly, the optimized SWG structures exhibit an average reflection below 5% in the wavelengths ranging from 350 to 900 nm. Furthermore, the angular-independent property is demonstrated by measuring the reflectance versus varying angles of incidence at 532 and 632.8 nm wavelengths.     

High efficient light-emitting diodes with antireflection subwavelength gratings

A two-dimensional subwavelength grating (SWG) has been fabricated on a GaAlAs light-emitting diode (LED). The SWG is patterned by electron beam lithography and etched by fast atom beam with Cl/sub 2/ and SF/sub 6/ gases. The fabricated grating has 200 nm period and the tapered grating shape with aspect ratio of 1.38 to prevent reflection in the spectral region including 850 nm light emission. The emission is increased by 21.6% at the normal emission angle. The total emittance is increased by 60% with the SWG in comparison with that of the flat surface.     

A novel ultracompact two-mode-interference wavelength division multiplexer for 1.5-/spl mu/m operation

A novel ultracompact 2/spl times/2 wavelength division multiplexer (WDM) for 1.55-/spl mu/m operation based on highly dispersive two-mode interference (TMI) was designed, theoretically modeled, and verified using a finite-difference-time-domain (FDTD) method. A two-moded waveguide assisted with a dispersive tooth-shaped grating provided a mode-dependent reflection band of central wavelength at 1.55 /spl mu/m. The wavelengths of 1538 and 1572 nm that were at the band edges and had the lowest reflection losses and relatively high dispersion were selected for wavelength multiplexing. The result showed that the wavelengths were separated by grating dispersion in a coupler length of 75 /spl mu/m which was much shorter than the required length of 1.1 mm in a regular TMI multiplexer of no grating. Insertion loss of about 1.7 dB and channel contrast of about 12 dB were achieved.     

High-Throughput Continuous Beam Steering Using Rotating Polarization Gratings

A new beam steering concept comprising independently rotating, inline polarization gratings (PGs) is experimentally demonstrated. The approach, which we term Risley gratings, achieves high steering throughput within a large field-of-regard (FOR) in a fashion similar to Risley prisms, composed of wedged prisms. However, because PGs are patterned in thin liquid crystal layers, they enable a system with far less thickness, weight, and beam walk-off. Furthermore, large apertures are feasible and wavelengths from visible to infrared can be chosen. Any direction within a solid angle defined by twice the diffraction angle of each PG can be addressed mechanically. Here we demonstrate a Risley grating system with a 62 $^{circ}$ FOR and 89%–92% transmittance at 1550-nm wavelength, using two PGs with 6- $muhbox{m}$ grating period.      

Dispersion in nonlinear figure of merit of As/sub 2/Se/sub 3/ chalcogenide fibre

Wavelength-dependent measurements of the nonlinear figure of merit (FOM) in a highly nonlinear As/sub 2/Se/sub 3/ chalcogenide fibre, from 1415 to 1554 nm, are presented. It is found that the FOM decreases monotonically towards shorter wavelengths, as the material half-bandgap is approached. The FOM > 1 for wavelengths above 1450 nm suggests applications for nonlinear optics in the C-band.     

Dark current analysis and characterization ofInxGa1-xAs/InAsyP1-y gradedphotodiodes with x>0.53 for response to longer wavelengths(>1.7 μm)

The dark current properties of In_xGa_1-xAs photodiodes, where x is varied from 0.53 to 0.82 for extending the long wavelength cutoff from 1.7 to 2.6 μm, are described. Detailed analyses of optoelectrical parameters of In_0.82Ga _0.1As photodiodes are presented. Dark current, which is a critical parameter and limits the operation of the photodiode, is analyzed and compared with the experimental values. Typical characteristics of photodiodes with cutoff wavelengths of 1.7 μm (x=0.53), 2.2 μm (x=0.72), and 2.6 μm (x=0.82) are presented. The typical and best values of the dark currents obtained are presented     

Demonstration of 9×200 Mbit/s wavelength division multiplexedtransmitter

A multifrequency laser capable of generating 200 Mbit/s data rates simultaneously at nine different wavelengths is demonstrated. The laser is based on the monolithic integration of a waveguide grating router with semiconductor optical amplifiers and thus provides automatic alignment of the optical channel wavelengths. This multifrequency laser may have important applications for local access WDM networks     

Diplexer With Integrated Filters and Photodetector in Ge–Si Using $Gamma-{X}$ and $Gamma-{M}$ Directions in a Grating Coupler

We demonstrate a central-office-type diplexer in which the filter and photodetector are monolithically integrated on a silicon-on-insulator substrate. The photonic integrated circuit receives a 1577-nm signal from an external laser and sends it to the fiber link using a two-dimensional grating coupler. The same grating coupler receives a 1270-nm signal from the fiber link and sends it to a monolithically integrated germanium photodetector using a polarization-diversity scheme to achieve polarization independence. The grating coupler is novel in that both the $Gamma-{X}$ and $Gamma-{M}$ directions are employed. This allows the grating coupler to couple both the 1577- and 1270-nm wavelengths with a small fiber tilt angle and hence have low polarization-dependent loss.      

Packet-by-packet wavelength-routing interconnect technique for 5Tbit/s switching system

A packet-by-packet wavelength-routing interconnect technique for a 5 Tbit/s switching system with a three-stage architecture has been demonstrated. The technique uses an optical wavelength division multiplexing (WDM) link and dynamic bandwidth-sharing among wavelengths. The inter-stage, electro-optical interconnection subsystem was fabricated using very compact 2.5 Gbit/s, eight-wavelength WDM transmitters/receivers and an arrayed-waveguide grating router     

All-active tapered 1.55-/spl mu/m InGaAsP BH-DFB laser with continuously chirped grating

We demonstrate the first realization of all-active tapered index coupled 1.55-/spl mu/m InGaAsP buried-heterostructure distributed feedback lasers involving chirped gratings. The variation of the effective refractive index along the tapered active stripe is compensated using an optimized continuously chirped grating. The grating has been formed using a novel direct-write electron-beam lithography technique. Lasers with an antireflection/cleaved cavity show stable single-mode operation and high optical output power up to 60 mW. The yield of lasers with a sidemode suppression ration > 40 dB is more than 70%. The -3-dB farfield angles (full-width at half-maximum) amount to 14/spl deg/ and 20/spl deg/ in lateral and vertical direction, respectively.     

Single-mode behavior of a circular grating for potentialdisk-shaped DFB lasers

The reflectivity and the resonance condition of a circular grating for a disk-shaped distributed-feedback (DFB) laser are calculated. The periodicity and the position of the grating are so chosen that all of the reflections from each refractive index step are superimposed in-phase so as to be consistent with the resonant behavior of the fundamental mode wave. The calculated wave impedance (E/H ) is almost purely imaginary in the central region of the grating for the higher modes     

Grating degeneration technique and sandwich structures forholographic N×N passive star couplers

Design for an efficient N×N passive integrated optical star coupler are proposed. The concepts of grating degeneration and sandwich structures are presented. The techniques and their combinations can significantly reduce the number of gratings required by the coupler and thus make it possible to build holographic N×N star couplers with recording materials of a moderate dynamic range     

A WDM cross-connected star topology for multihop lightwave networks

The authors propose a star topology for multihop lightwave networks in which the conventional N×N passive star coupler is replaced by fixed wavelength division multiplexed (WDM) cross connects. The proposed topology overcomes three major limitations of the conventional star topology. First, it reduces the number of wavelengths needed in a (p,k) ShuffleNet from kp ^k+1 wavelengths in the conventional topology to p wavelengths in the proposed one. Second, the signal power loss due to the 1/N power splitting at the star coupler no longer exists in the WDM cross connects and, therefore, the restriction on the supported number of users by the star network is alleviated. Third, it completely eliminates the need for wavelength filtering at the input to the receivers as is the case in the conventional star topology     

InP-based 1.3-1.6-/spl mu/m VCSELs with selectively etched tunnel-junction apertures on a wavelength flexible platform

In this letter, the authors demonstrate a wavelength flexible platform for the production of long-wavelength vertical-cavity surface-emitting lasers which provide full wavelength coverage from 1.3-1.6 /spl mu/m. All-epitaxial InP-based devices with AsSb-based distributed Bragg reflectors were achieved through a common design, process, and growth technology at both the important telecommunications wavelengths of 1.3 and 1.5 /spl mu/m. Thin selectively etched tunnel junctions were implemented as low-loss apertures and offer scalability to small device dimensions. Devices showed low threshold currents (<2 mA), near single-mode (SMSR>20 dB) operation, and high differential efficiency (>40% at 1.3 /spl mu/m and >25% at 1.5 /spl mu/m).     

Tunable Super-Structured Fiber Bragg Gratings with Perfect Sequences Based on m-Sequence

Recently, a tunable fiber Bragg grating (FBG) was developed by using stress-responsive colloidal crystals. In this paper, we have simulated the application of these nanoparticles into the super-structured fiber Bragg grating (SSFBG) written with perfect sequences derived from a short maximal-length sequence. A tunable SSFBG will be available to overcome the prohibitive temperature variation of the optical codecs. Nevertheless, we presented a method to implement coherent time spreading optical code-division multiple-access (OCDMA) where a unique code (or perfect sequence) can be reused and mixed with different wavelengths to obtain a tunable wavelength-division multiplexing (WDM) system. In order to maximize the binary throughput, we have selected a unique short maximal-length sequence composed of 7 chips that can be tuned with 7 different optical wavelengths. We found thousands of different tunable combinations that presented power contrast ratios (P/C) higher than 12 dB. When a WDM-OCDMA system used 2 different combinations simultaneously, the perfect binary detection with error correction codes was achieved successfully. The tunable SSFBG with colloidal crystals will be a simple and good alternative choice for fiber-to-the-home (FTTH) communications.     

Reduction in reflection-induced chirp from photonic integrated sources

Previously electroabsorption modulated lasers (EMLs) have been demonstrated using source lasers with grating strengths of two or less (k/sub r/L /spl les/ 2.0). Spatial holeburning was believed to limit the magnitude of the optimal grating strength. However, those standard-grating-strength lasers are sensitive to reflections from modulator output facet, integrated amplifiers, combiners, fiber tip, etc. and are a barrier to making integrated modules for long-span transmission. EMLs having record-strength (K/sub r/L > 6) gratings are fabricated. Output power >9.5 dBm, sidemode suppression ratio beyond 50 dB, single-mode yield >60% at high power, and power penalty <0.3 dB at 10 Gb/s for 75 km of fiber are demonstrated.     

Wavelength and power stabilisation of uncooled 975 nm thick quantum well lasers over 110/spl deg/C temperature range

The use of a thick quantum well enables coherence-collapse operation of a Bragg grating stabilised laser diode over 110/spl deg/C temperature range, yielding 975 nm wavelength emission with >40 dB sidemode suppression ratio, <1% power variation, and fibre-output power as high as 293 mW at 15/spl deg/C and 168 mW at 125/spl deg/C.     

Monolithic integration of low-threshold-current 1.3 ?m GaInAsP/InP DFB lasers

Two 1.3 ?m GaInAsP/InP DFB lasers with low threshold currents (28 and 29 mA) were successfully integrated. Both DFB lasers operated continuously at temperatures of up to 68°C. The 12 ? separation in wavelength between the two lasers was produced by a 2 ? difference of the grating periods. A thermal interaction between the two lasers was estimated from the shift in their wavelengths.