Novel fiber design for flat gain Raman amplification using single pump and dispersion compensation in S band

This paper reports on a novel fiber design that has an inherently flattened effective Raman gain spectrum. Simulations show that gain-flattened broad-band Raman amplification, using a single pump, can be achieved in any wavelength band by suitably choosing the fiber parameters and the pump wavelength. The fiber also has a high negative dispersion coefficient-(380-515) ps/km/spl middot/nm over the operating range of wavelengths-and the shape of the dispersion curve is such that the total link dispersion can be not only compensated but also flattened. Hence, the designed fiber can serve as a lossless, broad-band, dispersion-flattening, and dispersion-compensating module for the S band, wherein lossless operation is achieved using inherently gain-flattened single-pump Raman amplification. The performance characteristics of such a module was modeled taking into account wavelength-dependent splice loss as well as background loss, and it has been shown through simulations that lossless operation with /spl plusmn/0.2-dB gain ripple is achievable over (1480-1511) nm using a single pump. Moreover, dispersion compensation for five spans of transmission in a 10-Gb/s system, over this 32-nm bandwidth in the S band, should be attainable using the proposed design.     

Dispersion compensation based on sampled fiber Bragg gratings fabricated with reconstruction equivalent-chirp method

Using uniform phase mask and conventional fabrication technology with submicrometer rather than nanometer precision, a pure third-order dispersion-compensating fiber Bragg grating (DCFBG) and a tunable slope DCFBG are demonstrated with high performance based on the combined reconstruction equivalent-chirp method and an error correction technique. The former DCFBG has a dispersion varying from /spl sim/1000 to /spl sim/-1000 ps/nm in the 100-GHz passband and group delay ripple is less than /spl plusmn/5 ps. The latter has a dispersion slope varying from -150 to 150 ps/nm/sup 2/ within the 3-nm passband, and its group delay ripple is less than /spl plusmn/10 ps.     

Compact fully fibre integrated source of 100 fs pulses at 1.1 /spl mu/m based on compression in holey fibre

A simple, compact, fully fibre integrated source of /spl sim/100 fs pulses at a wavelength of 1.1 /spl mu/m is reported. 4 ps pulses at 1063 nm from a modelocked fibre laser were amplified to 23 mW in a ytterbium-doped fibre amplifier and subsequently propagated through 62 m of holey fibre with a zero dispersion wavelength at 1040 nm. Soliton formation, breakup and self frequency shift resulted in the formation of /spl sim/100 fs pulses at 1.1 /spl mu/m. Wavelength tunability from 1113 to 1220 nm is demonstrated.     

Broad-band wavelength-tunable, single frequency, and single polarization bismuth oxide-based erbium-doped fiber laser

We demonstrate a broad-band wavelength-tunable Bismuth Oxide-based Erbium-doped fiber (Bi-EDF) laser covering both the conventional wavelength band and the long wavelength band. It features single frequency and single polarization oscillation; the former is obtained by making the cavity length short and incorporating an intracavity narrow-band fiber Fabry-Pe/spl acute/rot filter, and the latter is achieved with a sigma laser configuration using nonpolarization maintaining fibers. Tuning over 90 nm (1520-1610 nm) is achieved by utilizing the broad gain bandwidth of Bi-EDF amplifier.     

Design and optimization of photonic crystal fibers for broad-band dispersion compensation

Design of photonic crystal fibers (PCFs) for application of broad-band dispersion compensation is investigated by using an improved design model based on combination of a rigorous vector solver for modal properties and a scaling approach for dispersion characteristics. The newly designed PCF is shown to exhibit large normal dispersion up to -474.5 ps/nm/km, nearly five times of conventional dispersion compensating fibers, and compensate conventional single-mode fibers within /spl plusmn/0.05 ps/nm/km over a 236-nm wavelength range. Furthermore, the design model and methodology can be applied to design other dispersion-based devices such as dispersion flattened fibers and dispersion shifted fibers.     

MEMS tunable resonant leaky mode filters

A tunable double-grating resonant leaky mode microelectromechanical-type element is introduced. A significant level of tunability is demonstrated by adjusting mechanically the structural symmetry of the grating profile. Tuning is also possible by variation of the thickness of the resonant layer. For a particular example silicon-on-insulator structure treated, it is shown that the resonance wavelength can be shifted by /spl sim/300 nm with a horizontal movement of /spl sim/120 nm within the 1.4-1.7-/spl mu/m wavelength band. Additionally, the reflectance can be varied from /spl sim/10/sup -3/ to 1 at central wavelength of 1.55 /spl mu/m with a vertical movement of /spl sim/200 nm. These results demonstrate new possibilities in design of tunable optical devices.     

Multiwavelength pulse generation using an actively mode-locked erbium-doped fiber ring laser based on distributed dispersion cavity

A simple design of a stable multiwavelength pulse generator was demonstrated using a dispersion-tuned actively mode-locked erbium-doped fiber (EDF) ring laser with distributed dispersion cavity. The distributed dispersion cavity in the fiber laser successfully reduced the cross-gain saturation in EDF, and thus, enabled multiwavelength operation. Simultaneous generation of wavelength-tunable 10-GHz pulses up to four different wavelengths was achieved with the same wavelength space of 2.94 nm. The extinction ratio of all wavelengths was above 30 dB. In addition, smooth wavelength tuning was achieved over more than 41 nm when the laser was working at dual-wavelength mode. The super mode noise was /spl sim/60 dB below the signal level at both wavelengths. The laser state was found to be very stable.     

Midinfrared holmium fiber lasers

The use of the high-power Tm/sup 3+/-doped silica fiber laser as a pump source for Ho/sup 3+/-doped silica and Ho/sup 3+/-doped fluoride fiber lasers for the generation of 2.1-/spl mu/m radiation is demonstrated. The Ho/sup 3+/-doped silica fiber laser produced a maximum output power of 1.5 W at a slope efficiency of /spl sim/82%; one of the highest slope efficiencies measured for a fiber laser. In a nonoptimized but similar fiber laser arrangement, a Ho/sup 3+/-doped fluoride fiber laser produced an output power of 0.38 W at 2.08 /spl mu/m at a slope efficiency of /spl sim/50%. A Raman fiber laser operating at 1160 nm was also used to pump a Ho/sup 3+/-doped fluoride fiber laser operating at a wavelength of 2.86 /spl mu/m. An output power of 0.31W was produced at a slope efficiency of 10%. The energy transfer upconversion process that depopulates the lower laser level in this case operates at a higher efficiency when the pump wavelength is closer to the absorption peak of the /sup 5/I/sub 6/ energy level, however, this energy transfer process does not impede to a great extent the performance of the Ho/sup 3+/-doped fluoride fiber laser based on the /spl sim/2.1/spl mu/m laser transition.     

A novel design for dispersion compensating photonic crystal fiber Raman amplifier

This letter presents a novel design for dispersion compensating photonic crystal fiber (DCPCF) which shows inherently flattened high Raman gain of 19 dB (/spl plusmn/1.2-dB gain ripple) over 30-nm bandwidth. The proposed design module has been simulated through an efficient full-vectorial finite element method. The designed DCPCF has a high negative dispersion coefficient (-200 to -250 ps/nm/km) over C-band wavelength (1530-1568 nm). The proposed fiber module of 5.2-km length not only compensates the accumulated dispersion in conventional single-mode fiber (SMF-28) but also compensates for the dispersion slope. Hence, the designed DCPCF module acts as the gain-flattened Raman amplifier and dispersion compensator.     

Fabrication of Germanosilicate glass optical fibers containing Tm/sup 2+/ ions and their nonlinear optical properties

Germanosilicate glass optical fibers incorporated with the Tm/sup 2+/ ions were fabricated to enhance optical nonlinearity by providing a strong reduction environment based on the solution doping technique in the modified chemical vapor deposition (MCVD) process. The incorporation of the Tm/sup 2+/ ions into the fiber core was identified by the electron paramagnetic resonance (EPR) spectrum in the fiber preform, and the absorption and emission properties between 350 and 1600 nm of the Tm/sup 2+/ ions in optical fibers and the fiber preform. A strong broad absorption band due to the Tm/sup 2+/ ions appeared from 350 to /spl sim/900 nm, and a broad emission from /spl sim/600 to /spl sim/1050 nm and the other emission from /spl sim/1050 to /spl sim/1300 nm, which were not shown in the Tm/sup 3+/ ions, were found upon Ar-ion laser pumping at 515 nm. Both absorption and emission results confirm that the Tm/sup 2+/ ions in the germanosilicate glass have the 4f-5d energy band from 350 to /spl sim/900 nm and the 4f-4f energy level at /spl sim/1115 nm. Also, the resonant nonlinearity at /spl sim/1310 and /spl sim/1530 nm due to the Tm/sup 2+/ ions in the fiber was measured upon the 515 nm optical pumping by using a long-period fiber grating (LPG) pair method. The nonlinear refractive index n/sub 2/ at /spl sim/1310 and /spl sim/1530 nm was found to be /spl sim/4/spl times/10/sup -15/ m/sup 2//W, where 70% and 30% of the n/sub 2/ are attributed to the nonradiative transitions and the radiative transitions, respectively.     

High-nonlinearity dispersion-shifted lead-silicate holey fibers for efficient 1-/spl mu/m pumped supercontinuum generation

This paper reports on the recent progress in the design and fabrication of high-nonlinearity lead-silicate holey fibers (HFs). First, the fabrication of a fiber designed to offer close to the maximum possible nonlinearity per unit length in this glass type is described. A value of /spl gamma/=1860 W/sup -1//spl middot/km/sup -1/ at a wavelength of 1.55 /spl mu/m is achieved, which is believed to be a record for any fiber at this wavelength. Second, the design and fabrication of a fiber with a slightly reduced nonlinearity but with dispersion-shifted characteristics tailored to enhance broadband supercontinuum (SC) generation when pumped at a wavelength of 1.06 /spl mu/m-a wavelength readily generated using Yb-doped fiber lasers-are described. SC generation spanning more than 1000 nm is observed for modest pulse energies of /spl sim/ 100 pJ using a short length of this fiber. Finally, the results of numerical simulations of the SC process in the proposed fibers are presented, which are in good agreement with the experimental observations and highlight the importance of accurate control of the zero-dispersion wavelength (ZDW) when optimizing such fibers for SC performance.     

Electron beam direct-write tunable polymeric waveguide grating filter

We report a tunable electron beam direct-write polymeric waveguide Bragg grating filter based on a negative tone epoxy, The waveguide filter, with a 5-mm-long first-order grating, exhibits a transmission peak of -27 dB and a 3-dB bandwidth of /spl sim/0.8 nm, and there is an excellent agreement between experimental data and simulation results. The temperature response of the filter is also characterized. The rate of change of refractive index dn/dT is /spl sim/ -1.8 /spl times/ 10/sup -4///spl deg/1C at 1550-nm wavelength for both transverse electric and transverse magnetic polarizations, and the rate of change of peak wavelength d/spl lambda//dT is /spl sim/ -0.14 nm//spl deg/C. The tuning performance is comparable to other grating devices fabricated using multiple processing steps.     

Bismuth oxide nonlinear fibre-based 80 Gbit/s wavelength conversion and demultiplexing using cross-phase modulation and filtering scheme

Reported is the experimental demonstration of wavelength conversion and subsequent demultiplexing of an 80 Gbit/s OTDM signal using the cross-phase modulation and spectral filtering technique in short lengths of bismuth oxide-based nonlinear fibre with a nonlinearity of /spl gamma/=/spl sim/1100 W/sup -1/ /spl middot/ km/sup -1/. Error-free, overall operation is readily achieved over a wavelength tuning range of /spl sim/10 nm.     

A novel ultraflattened dispersion photonic Crystal fiber

A novel four-ring photonic crystal fiber (PCF) structure with two different air-hole sizes is proposed with nearly zero ultraflattened dispersion characteristics. Through optimizing only three geometrical parameters, two air-hole diameters, and one hole pitch, the ultraflattened zero dispersion PCF can be efficiently designed. As an example, a four-ring PCF with flattened dispersion of /spl plusmn/0.25 ps/km/nm from 1.295- to 1.725-/spl mu/m wavelength is numerically demonstrated. The corresponding design procedures for the novel PCF are also presented in this letter.     

Improved supercontinuum generation through UV processing of highly nonlinear fibers

We demonstrate that UV exposure of highly nonlinear, germano-silicate fibers can significantly broaden the infrared supercontinuum generated by femtosecond pulses in these fibers. Both simulations and measurements of the fiber chromatic dispersion show that UV-induced refractive index changes increase the waveguide dispersion by up to 5 ps/(nm-km) at 1570 nm and shift the dispersion zero by over 100 nm. We examine fibers with a range of UV exposure levels and show that the short wavelength edge of the supercontinuum can be continuously changed by more than 100 nm. We also show that the long wavelength edge is extended beyond that of the unexposed fiber. The resulting continuum spans from 0.85 to 2.6 /spl mu/m. Cutback measurements show that the supercontinuum in the exposed fiber is generated in as little as 1 cm of fiber. A nonlinear Schro/spl uml/dinger equation (NLSE) model of the supercontinuum generation in the nonlinear fiber shows that the short wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in refractive index of the fiber core.     

Highly efficient four-wave mixing in an optical fiber with intensity dependent phase matching

Wavelength conversion via four-wave mixing in an optical fiber is investigated under a pump intensity dependent phase-matching condition. To obtain a high conversion gain, we use a fiber with a small mode-field diameter (4.2 /spl mu/m) and a small dispersion slope (0.0307 ps/km/nm/sup 2/). When the signal wavelength is set so that it is 9.2 nm longer than the zero-dispersion wavelength of the fiber, we obtain a wavelength conversion gain of greater than 5 dB over a pump wavelength region of more than 8 nm.     

Large anomalous dispersion at short wavelength and modal properties of a photonic crystal fiber with large air holes

We investigated the dispersion and modal properties of a photonic crystal fiber (PCF) with larger air holes (LAH). The theoretical and numerical results show that large anomalous group-velocity dispersion at short wavelength down to around 700 /spl sim/ 800 nm can be achieved in a LAH-PCF with small pitch. Furthermore, the high-order modes usually are excited in a LAH-PCF when the optical field is launched from a traditional single-mode fiber, and all the excited modes will interfere when they propagate along the PCF. Finally, the properties of the excited modal spectrum and the multimode interference are determined by the normalized pitch and the normalized hole size of the PCF. All of these will provide references for the smoothing applications of the LAH-PCF.     

Dual-wavelength AlInGaAs-InP grating-outcoupled surface-emitting laser with an integrated two-dimensional photonic lattice outcoupler

Grating-outcoupled surface-emitting semiconductor lasers emitting two independent wavelengths separated by 9.5 nm from a common aperture in a monolithic crossed configuration is demonstrated. This configuration provides integrated wavelength multiplexing at /spl sim/1300 nm from a single aperture into a single fiber, simplifying packaging and reducing cost.     

Widely tunable long-period gratings fabricated in polymer-clad ion-exchanged glass waveguides

Long-period-grating filters were fabricated in polymer-clad ion-exchanged BK7 glass waveguides. The transmission spectra of the filters exhibited strong polarization dependence. A contrast as high as 25 dB at the resonance wavelength was obtained. The temperature sensitivity of the filters was measured to be /spl sim/9.0 nm//spl deg/C, which allows potential wavelength tuning over the entire S+C+L band of /spl sim/180 nm with a temperature control over a range of /spl sim/20/spl deg/C.     

High-gain low-noise mid-infrared quantum cascade optical preamplifier for receiver

Optical preamplifier (OPA) in the mid-infrared can boost weak signals in systems such as lidars or optical wireless links to improve the receiver sensitivity. Quantum-cascade-based traveling-wave OPA with off-normal Bragg-grating (BG) coupled surface-emitting configuration have been developed with a net small-signal gain up to 13 dB over a spectral range /spl sim/70 nm around 4.7 /spl mu/m, limited by the wafer gain bandwidth, and the linear dynamic range was >20 dB. Amplified spontaneous emission (ASE) was measured and the spontaneous emission factor was determined N/sub sp//spl sim/1, indicating near quantum-limit noise behavior. Surface-emitting BG devices yielded diffraction-limited output beam, and provided simultaneous wavelength dispersion and ASE filtering. The OPA was coupled with a detector to form a receiver that is polarization-selective, has a net gain 8-13 dB with a corresponding optical filter bandwidth 25-50 GHz. These results suggest that BG-coupled OPA receivers can be useful for multispectral/wavelength-division-multiplexing systems in free space applications.