Transmission loss of a 125-μm diameter PANDA fiber with circularstress-applying parts

The transmission loss of a 125-μm diameter polarization-maintaining and absorption-reducing optical fiber (the PANDA fiber) with circular stress-applying parts (SAPs) has been investigated. The loss is calculated at a 1.55- μm wavelength for SAPs consisting of B₂O₃ doped silica glasses. The experimental measurements show the validity of the analysis. It is shown that, when the half distance between SAPs is more than 2.2 times the core radius, the additional transmission loss due to B₂O₃ absorption is less than 0.05 dB/km with normalized frequency of 2.2 to 2.4 and a B₂O₃ dopant concentration of 20 mol.%. As a result, it has been confirmed that transmission losses of 125-μm diameter PANDA fibers can be comparable to those of commercially available single-mode optical fibers     

Reproducible fabrication method for polarization preservingsingle-mode fiber couplers

The adaptation of the biconically-tapered-fused (BTF) fiber coupler fabrication approach to polarization preserving fiber couplers is reported. Two practical methods for identifying and aligning the birefringent axes of the fibers forming the coupler are described. The fabrication approaches are highly reproducible and the 3-dB couplers fabricated exhibit less than 1.0-dB insertion loss, controllable splitting ratio, and an average polarization extinction ratio of 15 to 20 dB     

Gain monitoring of erbium-doped fiber amplifiers by detectingspontaneous emission

Gain monitoring by detecting the spontaneous emission (SE) emitted by 0.98-μm laser diode (LD) pumped erbium-doped fiber amplifiers was investigated using three erbium-doped fibers (EDF's) with different absorption coefficients at 1.55 μm (α). Unfavorable 0.98-μm scattered light was observed and removed by an optical filter. The minimum gain accuracy was less than ±0.21 dB for EDF-b (α=1.5 dB/m) with launched pump powers (P_p) from 10 to 30 mW. The minimum gain reproducibility over a 9-h period was ± 0.015 dB for EDF-a (α=3.3 dB/m) at P_p=20 mW     

Surface plasmon polaritons in thin metal films and their role infiber optic polarizing devices

The polarization-selective evanescent field coupling of an optical wave in a 1.3-μm, fiber to surface, plasmon polaritons supported by a thin aluminium film was investigated theoretically and experimentally. Good agreement was observed between the theoretically predicted conditions for efficient coupling and the experimentally determined conditions for high TM/TE extinction ratio of the optical field. The use of silver and chrome in place of aluminum was investigated experimentally at 1.3 μm, and 0.63-μm devices using aluminium were also studied. The results led to the realization of readily manufacturable, high-extinction-ratio (>50 dB) low-loss (<0.5 dB) fiber optic polarizers and polarizing couplers in standard and highly birefringent fiber     

Improved coupling efficiency using Δn-controlled polymerwaveguides with two-dimensional spot-size transformation

A new type of polymer waveguide for two-dimensional spot-size transformation is reported, with which coupling efficiency of waveguides with 4- and 8-μm core diameters was reduced by ≈1.6 dB. The length needed for spot-size transformation was less than 1 mm along the direction of propagation. In this polymer waveguide, refractive index difference between core and cladding (Δn) is properly controlled along the direction of propagation while the core cross section is kept uniform     

Fabrication of fluoride glass single-mode fibers

Single-mode optical fibers are obtained using ZrF4-based fluoride glasses. The fibers are drawn from a preform and jacketing tube. The preform with cladding/core ratio of 5.1 is made by using a built-in casting method. The cutoff wavelength of the fiber is experimentally determined to be 2.7 μm by bending loss measurement. Minimum transmission loss of the obtained fiber is 160 dB/km at a wavelength of 3.28 μm. TheV-value at this wavelength is estimated to be 2.03 from the core diameter and the refractive index difference.     

一种用于高功率光纤放大器的侧面抽运耦合器

报道了一种应用于高功率光纤放大器的侧面抽运耦合器。采用熔融拉锥工艺以及最基本的2×1耦合方式, 实现了高耦合效率、高隔离度的光纤侧面耦合器的研制。通过对多种不同光纤组合的研究, 发现采用外径125 μm, 数值孔径为0.46的无源双包层光纤做信号传输光纤和抽运耦合光纤, 可获得高达74％的抽运耦合效率; 耦合器信号光通过率为95％; 信号输入端与抽运输入端的隔离度大于50 dB; 抽运输入端对输出端反向传输光的隔离度 为20 dB。采用该侧面耦合器, 实现了输出功率达1 W的窄线宽全光纤放大器。     

Theoretical investigation of light coupling phenomena inwavelength-flattened couplers

A study of light coupling phenomena in wavelength-flattened couplers by normal mode analysis is discussed. The effective index method and step approximation of the taper are used to analyze the mode coupling in the fused-taper region. It is shown that coupler parameters such as the dissimilarity of the two fibers, the degree of fusion, the minimum taper waist and the taper length should be optimized to get flat coupling characteristics. The optimum relationship among these coupler parameters is determined for fused-taper couplers employing standard telecommunication fibers     

A ±1.5-V, 4-MHz CMOS continuous-time filter with asingle-integrator based tuning

A 4-MHz, fifth-order elliptic low-pass Gm-C filter is described whose characteristics are tuned by an on-chip automatic tuning circuit. The tuning circuit uses only one integrator as the master of tuning instead of problematic voltage controlled oscillator (VCO) and voltage controlled filter (VCF). MOS transistors in linear operation region perform the voltage-to-current conversion in an operational transconductance amplifier, and thereby we achieved ±1.5 V operation. A prototype filter was implemented in a 0.8-μm double-poly, double-metal CMOS process. The filter exhibits the dynamic range of 57.6 dB and dissipates 10 mW with ±1.5-V supply. The stopband attenuation is better than 45.0 dB and the passband ripple is smaller than 1.0 dB     

Quasi planar spot-size transformer for efficient coupling between acleaved fibre and an InP/InGaAsP rib waveguide

A spot-size transformer structure is proposed for InP/InGaAsP double-heterostructure (DH) rib waveguide. Vertical expansion of a factor of 7 is achieved with 3 dB internal loss. Coupling from standard cleaved fibre (11 μm diameter mode) to a DH rib waveguide (5.8×0.8 μm large mode), is achieved with 4.5 dB polarisation independent loss. Coupling tolerances are highly relaxed. The fabrication method uses a thin cladding layer which ends up to a quasi-planar structure suitable with further integration process     

LD optical switch with polarization insensitivity over a widewavelength range

A double-heterostructure (DH) laser with TM mode lasing has been achieved with a narrow active-layer width, and a laser-diode optical switch (LDSW) module with less than a 0.35-dB gain difference between the TE and TM modes over a wide wavelength range has been constructed by introducing a square bulk active layer formed by dry etching and regrowth. The polarization-insensitive width of a 0.3-μm-thick DH laser is clarified to be between 0.30 and 0.35 μm, since the 0.30- and 0.35-μm-wide DH lasers lase in the TM mode and TE mode, respectively. The polarization-insensitive width of the fabricated 0.3-μm-thick LDSW is estimated to be about 0.32 μm for the fabricated LDSW with a trapezoidal active layer by measuring the single-pass gain and the gain difference between the TE and TM modes. This must be to within 0.01 μm. A 0.35-μm-wide, 300-μm-long LDSW module has lossless gain in the wavelength range of 1.31 to 1.36 μm at 20 mA. The gain difference between the TE and TM modes is as low as 0.35 dB, The rise and fall times are 1.0 and 0.55 ns, respectively. The bulk active-layer LDSW module is promising for use as a polarization-insensitive optical-gate switch in optical information systems     

Directional couplers using fluorinated polyimide waveguides

Optical properties of directional couplers using a fluorinated polyimide waveguide were demonstrated. At a wavelength of 1.3 μm, directional couplers with the refractive index difference of 0.3% between the core and cladding have a maximum power coupling ratio of more than 97 and 96% for TE and TM polarisations, respectively. The excess loss for a 3-dB coupler was as low as 0.3 dB. Moreover, the change in the coupling ratio of the 3-dB couplers remained within 0.5% between 25 and 100°C     

Tunable wavelength filter using a Bragg grating fiber thinned byplasma etching

We studied a tunable wavelength filter using a single-mode fiber Bragg grating with a cladding thinned by reactive plasma etching. Reflection wavelength shifts were demonstrated by controlling the effective refractive index or the period of the grating. When liquid paraffin was used for additional cladding on a thinned grating fiber of 19-μm diameter, the shift rate of 0.055 nm/°C was obtained. In the case of applying the tension to the grating region, the shift rate was 3.4 nm/100 V for the fiber of 41-μm diameter     

Proton implanted VCSEL's for 3 Gb/s data links

Transverse single-mode and multimode intensity modulated butt-coupled InGaAs vertical cavity surface emitting lasers (VCSEL)s are investigated as a light source for optical fiber communication systems. Data transmission at 3 Gb/s with a bit error rate (BER) of less than 10 ^-11 is reported for both 4.3 km of standard fiber, as well as 0.5 km of multimode graded-index fiber, 10-μm active diameter single-mode VCSELs are shown to have lower mode competition noise requiring 3 dB and 6 dB less power at the front end receiver at a BER of 10^-11 compared to 19-μm and 50-μm active diameter devices, respectively. In data transmission with multimode VCSELs, the dispersion penalty is lower than for single-mode sources since the noise at the receiver is mainly determined by transmitter-mode competition noise     

The effect of cladding layer thickness on large optical cavity650-nm lasers

The reduction in penetration of the optical mode into the cladding layers in large optical cavity (LOC) laser structures offers the possibility of reducing the cladding-layer thickness. This could be particularly beneficial in GaInP-AlGaInP high-power devices by reducing the thermal impedance and the electrical series resistance. We have designed and characterized 650-nm LOC lasers by modeling the optical loss due to incomplete confinement of the optical mode by the cladding layers and calculating the thermally activated leakage current. This indicated that the cladding thickness could be reduced to 0.5 μm without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1-μm-wide cladding layers. The measured optical mode loss of the 0.3-μm-wide cladding device was 36.2 cm^-1 compared with 12.4 and 11.3 cm^-1 for the 0.5- and 1-μm-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0-μm devices were similar over the temperature range investigated (120-320 K), whereas the 0.3-μm devices had significantly higher threshold current density. We show that this can be attributed to the higher optical loss and increased leakage current through the thin cladding layer. The intrinsic gain characteristics were the same in all the devices, irrespective of the cladding-layer thickness. The measured thermal impedance of 2-mm-long devices was reduced from 30.7 to 22.3 K/W by reducing the cladding thickness from 1 to 0.5 μm. Our results show that this can be achieved without detriment to the threshold characteristics     

Mode Converters for Coupling to High Aspect Ratio Silicon-on-Insulator Channel Waveguides

The design, simulation, and experimental performance of mode converters for coupling from single-mode silicon-on-insulator ridge waveguides to high aspect ratio channel waveguides are described. The converters consist of a two-level adiabatic taper structure. The final channel waveguide is 1.5 mum high by 0.8 mum wide. Simulations predict that for total coupler lengths longer than 20 mum, the coupling loss from the fundamental ridge mode to the slit mode is better than -0.2 dB. The couplers and waveguides were fabricated using a two-step self-aligned process. The measured coupling loss for fabricated mode converters is -0.4 dB     

A simple laterally tapered waveguide for low-loss coupling tosingle-mode fibers

Low-loss coupling between semiconductor photonic devices and single-mode fibers is achieved using a simple InP/InGaAsP tapered waveguide. The proposed simple structure has a small and nearly square guiding core at its output facet. In this structure, the output field has a non-Gaussian profile, but low-pass filter coupling can be achieved by optimizing the design of the guiding core sizes. The waveguide is composed of a laterally tapered InGaAsP guiding layer and an InP cladding region on an InP substrate, facilitating integration of the waveguide with active devices using conventional processes. The waveguide is shown to have a total insertion loss of 2.6 dB, including a coupling loss of 0.9 dB and large ±2.5-μm misalignment tolerance in lateral and vertical directions with single-mode filters     

Trapezoidal index single-mode fibers for the 1.55 µm band

An analytical study is made of single-mode fibers with a trapezoidal index of refraction profile for low dispersion operation in the 1.55-μm band using Ge doped silica core and pure silica cladding. It is found that certain trapezoid proportions give a fiber with a combination of less Ge doping, larger core diameter, faster decay of the field in the radial direction within the cladding, and smaller rate of change of dispersion with wavelength than is simultaneously possible with either the step index or triangular profiles.     

Optical time-domain reflectometry (OTDR) of diameter modulations insingle mode fibers

The influence of several types of diameter variations on the attenuation and on the optical time-domain reflectometry (OTDR) traces of specially prepared depressed cladding SM (single-mode) fibers is experimentally investigated. Depressed-cladding SM fibers have been prepared by the low-pressure plasma-induced chemical vapor deposition (PCVD) process. The core consists of GeO₂-doped SiO₂ and the cladding region of fluorine-doped silica. During drawing, the diameter of the fiber is controlled within ±0.3 μm for slow perturbations by adjusting the drawing speed (Anritsu M501A). Variations of the core diameter have been accomplished by changing the set point of the fiber diameter control. Core diameter changes give rise to steps or deviations in slope of the OTDR power trace of about 0.3 dB/μm. No indication of additional attenuation due to axial fiber parameter variations has been found up to core diameter changes of 12 μm/m, despite severe distortions of the OTDR traces     

1.4-μm-band gain characteristics of a Tm-Ho-doped ZBLYAN fiberamplifier pumped in the 0.8-μm band

The gain characteristics of a 1.4-μm-band thulium-holmium-doped ZBLYAN fiber amplifier are described. Signal gain is obtained over the whole 1.4-μm band for a pump power level of 73.5 mW. A maximum signal gain of 18 dB is achieved at a signal wavelength of 6.46 μm for a pump power of 150 mW at 0.79 μm. The noise figure is 5.6 dB in the signal wavelength region from 1.45-1.50 μm. From a comparison of the gain characteristics of thulium-holmium-doped ZBLYAN fibers and thulium-doped ZBLYAN fibers, it is proved that holmium ions play an effective role in increasing the gain and widening the gain spectrum