InGaAsPN-InP-based photodetectors for long wavelength(λ>1.65 μm) applications

We demonstrate InGaAsPN p-i-n photodetectors lattice-matched to InP substrates with cutoff wavelengths larger than 1.65 μm. The narrow bandgap InGaAsPN absorption layers were grown by gas source molecular beam epitaxy using an RF plasma nitrogen source. Optical absorption spectra reveal that InGaAsPN with 5% P and 2.8% N has a cutoff wavelength λ_CO=1.90 μm Background doping in the absorption layer for a detector with 1.5% N and 5% P is reduced from (1.5±0.5)×10¹⁷ cm^-3 for the as-grown device, to (5±0.5)×10¹⁶ cm^-3 for a thermally annealed device. The unintentional high background doping is due to N-H bond formation or local strain induced defects. Spectral response measurements indicate that λ_CO=1.85 μm is achieved for detectors annealed at 800°C with 2% N and 5% P in the InGaAsPN absorption layer, suggesting that annealed InGaAsPN alloys are promising for use in detectors with response in the near and mid-IR wavelength spectral range     

Fabrication of planar optical waveguides in LiB3O5 by 2 MeV He+ ion implantation

The fabrication of planar optical waveguides in LiB₃O ₅ is discussed. Using 2-MeV ⁴He⁺ implantation with a dose of 1.5×10¹⁶ ions/cm² at 300 K, the refractive indexes of a 0.2-μm-thick layer 5.1 μm below the crystal surface are reduced to form optical barrier guides. For this ion dose the maximum change from the bulk values of refractive index at a wavelength of 0.488 μm are 1.5%, 5.25%, and 4% for n_x, n_y, and n_z, respectively. The refractive indexes of the guiding region change by less than 0.02% from the bulk values. The dose dependence of the optical barrier height has been measured. A threshold ion dose of about 0.75×10¹⁶ ions/cm² is required to form a refractive index barrier and ion doses higher than about 2.5×10¹⁶ ions/cm². saturate the refractive index decrease. Waveguide propagation losses for annealed single energy implants of dose 1.5×10¹⁶ ions/cm² are dominated by tunneling and are estimated to be ~8.9 dB/cm for the z-cut waveguides used. Multiple energy implants broaden the optical barrier, and losses of <4 dB/cm have been observed     

Fabrication of buried channel waveguides in fused silica usingfocused MeV proton beam irradiation

Low-loss channel waveguides have been fabricated in fused silica using a beam of MeV protons focused down to a spot size of several microns. By using a combination of beam and sample scanning, single- and multimode graded index waveguides with lateral dimensions down to approximately 5 μm×5 μm have been fabricated using ion doses in the range (3×10¹⁴)-(6×10¹⁶) ions/cm ². Typical beam currents in the range 100 pA-10 nA were used. Optical mode profiles have been measured at 670 nm and propagation losses of the order of 3 dB/cm measured in unannealed samples. Annealing the substrate for 1 h at 500°C reduced these losses to below 0.5 dB/cm     

A TM Y-branch wavelength multi/demultiplexer by K+ andAg+ ion-exchange for λ=1.31 and 1.55 μm

A Y-branch multi/demultiplexer by K⁺ and Ag⁺ ion-exchange was optimally designed, using the 2-D beam propagation method (BPM) and successfully fabricated for λ=1.31 and 1.55 μm. The measured extinction ratios of our preliminary devices are about 20 dB     

Low-loss polymeric optical waveguides fabricated with deuteratedpolyfluoromethacrylate

Deuterated polyfluoromethacrylate which has high transparency, low birefringence and good processability was newly synthesized for use as optical waveguide materials, and both single-mode and multimode optical waveguides were fabricated using the polymer. The propagation loss and waveguide birefringence of the single-mode waveguides were as low as 0.10 dB/cm and -5.5×10^-6 at 1.31 μm, respectively. The propagation losses of the multimode waveguides were less than 0.02 dB/cm at both 0.68 and 0.83 μm, and 0.07 dB/cm at 1.31 μm     

GaAs/AlGaAs quantum-well, long-wavelength infra-red (LWIR) detectorwith a detectivity comparable to HgCdTe

The authors achieved the first high responsitivity R_v=30000 V/W, high detectivity D*=1×10¹⁰ cm √(Hz)/W GaAs/AlGaAs multiquantum-well superlattice detector which is sensitive in the long wavelength infra-red (LWIR) spectral region. This detector operates at λ=8.3 μm and at a temperature of T=77 K     

Proton-exchanged optical waveguides in Z-cut LiNbO3 using phosphoric acid

Proton-exchanged Z-cut LiNbO₃ planar waveguides formed using phosphoric acid were characterized optically. The refractive index profile and the diffusion parameters were studied systematically. These waveguides have propagation losses of less than 1 dB/cm and exhibit properties that are different from those obtained using benzoic acid. The index profile is not a simple step function and can be modeled accurately by a polynomial expression. A maximum surface index increase of 0.145 was measured at a 0.633-μm wavelength. The diffusion constant D₀ and the activation energy Q for the proton-exchange process using this acid were found to be 6.43×10⁸ μm²/h and 82.91 kJ/mol, respectively. The annealing properties of these waveguides were also established, and the effects of annealing on surface index change and waveguide depth increase were found to follow a power-law relationship     

High-performance long-wavelength (λ~1.3 μm) InGaAsPNquantum-well lasers

We demonstrate high-performance InGaAsPN quantum well based long-wavelength lasers grown on GaAs substrates, nitrogen containing lasers emitting in the λ=1.2- to 1.3-μm wavelength range were grown by gas source molecular beam epitaxy using a RF plasma nitrogen source. Under pulsed excitation, lasers emitting at λ=1.295 μm exhibited a record low threshold current density (J_TH) of 2. 5 kA/cm². Lasers grown with less nitrogen in the quantum well exhibited significantly lower threshold current densities of J_TH =1.9 kA/cm² at λ=1.27 μm and J_TH=1.27 kA/cm² at λ=1.2 μm. We also report a slope efficiency of 0.4 W/A and an output power of 450 mW under pulsed operation for nitrogen containing lasers emitting at 1.2 μm     

High contrast GaInAs:Fe photoconductive optical AND gate fortime-division demultiplexing

A high-contrast, three port optical AND gate based on the photoconductive effect in Ga_0.47In_0.53As:Fe and operating in the λ=1.3-5 μm wavelength range is demonstrated. A 250:1 optical power contrast ratio (or 48 dB in electrical power after detection) is obtained in an optical-to-optical time division demultiplexing of a 100 MHz pulse train by a 6.25 MHz clock, both at λ=1.3 μm, with the demultiplexed output pulses at λ=1.5 μm     

Fabrication of wavelength-insensitive 8×8 star coupler

The fabrication and performance of a wavelength-insensitive 8×8 star coupler is described. The integrated-optic star coupler consists of two fan-shaped channel waveguide arrays facing each other and a slab waveguide region located in between. The wavelength-insensitive light splitting was realized by properly tailoring the mode coupling in the input array region. The coupler exhibits low-average excess losses of α=1.4 dB (standard deviation σ=0.40 dB) at λ=1.3 μm and α=1.7 dB (σ=0.44 dB) at λ= 1.55 μm, respectively     

Optical waveguides in SIMOX structures

Propagation characteristics determined experimentally and theoretically for planar optical waveguides formed in separation by implantation of oxygen (SIMOX) structures are discussed. All samples were found to support both TE and TM modes at both 1.15 μm and 1.523 μm with a lowest propagation loss of 8 dB/cm. This loss was measured at a wavelength of 1.15 μm for the TE₀ mode of a planar waveguide with a 2.0-μm-thick Si guiding layer     

High T0 long-wavelength InGaAsN quantum-well lasersgrown by GSMBE using a solid arsenic source

We demonstrate high performance, λ=1.3- and 1.4-μm wavelength InGaAsN-GaAs-InGaP quantum-well (QW) lasers grown lattice-matched to GaAs substrates by gas source molecular beam epitaxy (GSMBE) using a solid As source. Threshold current densities of 1.15 and 1.85 kA/cm² at λ=1.3 and 1.4 μm, respectively, were obtained for the lasers with a 7-μm ridge width and a 3-mm-long cavity. Internal quantum efficiencies of 82% and 52% were obtained for λ=1.3 and 1.4 μm emission, respectively, indicating that nonradiative processes are significantly reduced in the quantum well at λ=1.3 μm due to reduced N-H complex formation. These Fabry-Perot lasers also show high characteristic temperatures of T₀ =122 K and 100 K at λ=1.3 and 1.4 μm, respectively, as well as a low emission wavelength temperature dependence of (0.39±0.01) nm/°C over a temperature range of from 10°C to 60°C     

High-performance undoped InP/n-In0.53Ga0.47AsMSM photodetectors grown by LP-MOVPE

A new high-performance undoped In_0.53Ga_0.47As metal-semiconductor-metal photodetector (MSM-PD) with an undoped InP barrier-enhancement layer is reported. The layers were grown by the low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) technique. The main features of this device include: a very low dark current of less than 60 nA, (100×100) μm², at 1.5 V; a short risetime of 30 ps at 6 V; and a high responsivity of 0.42 A/W for λ=1.3 μm     

Low-loss proton-exchanged waveguide in MgO-doped LiNbO3fabricated with pyrophosphoric acid using Ta2O5protective mask

The authors describe low-loss proton-exchanged channel waveguides in MgO-doped LiNbO₃. The authors demonstrate the application of a Ta₂O₅ film for the protective mask material in proton-exchanging instead of a Ta film in order to reduce the propagation loss. A Ta₂O₅ sputtered film was applied as a protective mask with pyrophosphoric acid. The propagation loss of the waveguide, measured with laser diode light (λ=0.83 μm) was 0.5 dB/cm. It is shown that the use of a Ta₂O₅ mask reduces the propagation loss compared with the use of a Ta mask (1.5 dB/cm)     

Second harmonic generation and sum frequency mixing of dualwavelength Nd:YAlO3 laser in flux grown KTiOPO4crystal

On the basis of accurately measured refractive indexes, the authors have obtained the Sellmeier's equations for flux grown KTiOPO ₄ (KTP) crystal and used them to calculate the phase matched angles (&thetas;_m, φ_m) and effective nonlinear coefficients (d_eff) for type I and III second harmonic generation (SHG) and sum frequency mixing (SFM) of radiations at 1.0795 and 1.3414 μm. The optimum phase matching conditions for 1.0795 and 1.3414 μm SHG are that &thetas;_m=86.88 and 58.88°, respectively, in an XZ plane (φ=0) and for SMF of 1.0795 and 1.3414 μm in the same plane 76.02°. The corresponding d_eff values calculated from &thetas; _ms are 18.07×10^-9 and 17.42×10^-9 esu     

Single-mode Nd3+-doped graded-index polymer waveguideamplifier

The first graded index (GRIN) polymer waveguide amplifier working at 1.06 μm wavelength has been realized, using a Nd³⁺: photolime gel material combination. Throughput intensity of 3.8 mW at 1.06 μm, corresponding to 8.5 dB gain, was observed when employing a 40 mW pumping laser beam operating at 790 nm. The gain medium is a 2.2 cm waveguide active region with Nd³⁺ concentration of 1.03×10²⁰/cm³. The GRIN characteristic of the photolime gel thin film provides a universal means for implementing polymer-based photonic integrated circuits (PICs) on any substrate of interest     

Singlemode channel waveguides and electrooptic modulators in KTiOPO4 for the short visible wavelength region

Single-mode channel waveguides at short visible wavelengths have been fabricated in KTiOPO₄ by Rb&rlhar2;K ion exchange in mixed melts of RbNO₃-KNO₃-Ba(NO₃)₂ . The technological parameters have been chosen by means of theoretical WKB- and “effective index” calculations concerning the singlemode region of the effective channel waveguide index N₀₀ at the given wavelength. Great diffusion anisotropy and small dispersion of the surface refractive index change guarantee singlemode operation of the very same channel waveguide from the blue up to the red. Typical attenuation of about 2.0 dB/cm for TM- and 1.5 dB/cm for TE polarization was obtained at λ=0.5145 μm. Light-induced refractive index changes (photorefractive effect) have been determined as a function of time, wavelength, guided optical mode intensity and temperature. The light-induced effects in Rb&rlhar2;K ion-exchanged channel waveguides in KTiOPO₄ are about two orders of magnitude smaller than those in annealed proton-exchanged channel waveguides in LiNbO₃. Electrooptic phase modulators have been successfully investigated concerning dynamic V_π measurements, the electric-optical field overlap and dc-drift phenomena. Design, fabrication and experimental results of integrated-optic Mach-Zehnder-interferometer modulators for short visible wavelengths are presented     

InGaAlAs-InP quantum-well infrared photodetectors for 8-20-μmwavelengths

We demonstrate the first long-wavelength quantum-well infrared photodetectors using the lattice-matched n-doped InGaAlAs-InP materials system. Samples with AlAs mole fractions of 0.0, 0.1, and 0.15 result in cutoff wavelengths of 8.5, 13.3, and 19.4 μm, respectively, a 45° facet coupled illumination responsivity of R=0.37 μm and detectivity of D_λ*=3×10⁸ cm·√(Hz)· at T=77 K, for a cutoff wavelength λ_c=13.3 μm have been achieved. Based on the measured intersubband photoresponse wavelength, a null conduction band offset is expected for In_0.52Ga_0.21Al_0.27 As-InP heterojunctions     

A dual wavelength (1.32-1.56 μm) directional couplerdemultiplexer by ion exchange in glass

A 1.32-1.56-μm demultiplexer using a symmetrical directional coupler consisting of Ag⁺-Na⁺ exchanged channel waveguides in BK7 glass with a total insertion loss of 1.25 dB for a 14.5-mm-long device and a propagation loss of 0.15 dB/cm is demonstrated. The crosstalk at 1.315 and 1.561 μm and the cross power ratio at 1.315 μm were measured to be less than -40 dB for the TE mode     

Eigenvalues of (↓2)H and convergence of the cascade algorithm

This paper is about the eigenvalues and eigenvectors of (↓2)H. The ordinary FIR filter H is a convolution with a vector h=(h(O),...,h(N)), which is the impulse response. The operator (↓2) downsamples the output y=h*x, keeping the even-numbered components y(2n). Where H is represented by a constant-diagonal matrix, this is a Toeplitz matrix with h(k) on its kth diagonal, the odd-numbered rows are removed in (↓2)H. The result is a double shift between rows, yielding a block Toeplitz matrix with 1×2 blocks. Iteration of the filter is governed by the eigenvalues. If the transfer function H(z)=Σh(k)z^-k has a zero of order p at z=-1, corresponding to ω=π, then (↓2)H has p special eigenvalues ½,¼...,(½)^p. We show how each additional “zero at π” divides all eigenvalues by 2 and creates a new eigenvector for λ=½. This eigenvector solves the dilation equation φ(t)=2Σh(k)φ(2t-k) at the integers t=n. The left eigenvectors show how 1,t,...,t^p-1 can be produced as combinations of φ(t-k). The dilation equation is solved by the cascade algorithm, which is an infinite iteration of M=(↓2)2H. Convergence in L² is governed by the eigenvalues of T=(↓2)2HH^T corresponding to the response 2H(z)H(z^-1 ). We find a simple proof of the necessary and sufficient condition for convergence