Photosensitization of standard fibers with deep UV laser radiation

A strong and permanent ultraviolet photosensitivity enhancement was locked into standard optical fiber pre-treated with above-bandgap 157-nm F/sub 2/-laser radiation. Below-bandgap 248-nm irradiation of the pre-treated fiber yielded fiber Bragg gratings (FBGs) with an index contrast of 6.5 /spl times/ 10/sup -4/, a value four-fold larger than that available in 16% Ge-doped photosensitive fiber. The F/sub 2/-laser photosensitivity locking was 300 times more effective than with KrF-laser pre-treatment in terms of the accumulated fluence required to lock in a similar refractive index change. Accelerated FBG aging tests equivalent to 25 years at 70/spl deg/C revealed a less than 2% reflectivity decay in both 157- and 248-nm pre-treated fibers, which is sufficient to eliminate the postannealing step typically required in hydrogen-loaded fibers. The large photosensitization and the superior thermal stability of both 248- and 157-nm pre-treated fibers are associated with a new Si-H bond absorption feature at 1387 nm, which appears only during the FBG formation step.     

Photosensitivity and application with 157-nm F/sub 2/ laser radiation in planar lightwave circuits

Photosensitivity studies of germanosilica planar waveguides were carried out with short-wavelength 157-nm light from an F/sub 2/ laser. More than a 5/spl times/10/sup -3/ refractive-index change was induced in a nonuniform index profile concentrated near the cladding-core interface and confirmed by an atomic force microscopy in 157-nm radiated fiber. This profile geometry narrows with the laser exposure to offer practical application in trimming phase errors and controlling birefringence in frequency domain modulators where a 1.7/spl times/10/sup -3/ effective index change and a 5/spl times/10/sup -4/ birefringence change were induced, respectively. The 157-nm photosensitivity response is more than 15 times stronger than that by a 248-nm KrF laser and more than twofold stronger than that by a 193-nm ArF laser.     

Limitation of the Kirchhoff boundary conditions for aerial imagesimulation in 157-nm optical lithography

The aerial images of half-wavelength features with 0° and 180° phases obtained by using the Kirchhoff boundary conditions are compared with those obtained by using rigorous electromagnetic field computation for 248-nm lithography and 157-nm lithography. The discrepancies between the aerial images computed by the two methods are large at both wavelengths, but they are much larger for TM polarization at the wavelength λ=157 nm. These discrepancies are due to diffraction effects in the aperture regions, which are more pronounced at λ=157 nm because of the larger ratio of the thickness of the chromium absorber to the wavelength required at λ=157 nm for a given attenuation factor. This shows that diffraction effects in the aperture regions must be included when simulating aerial images in 157-nm lithography     

Vacuum-ultraviolet laser-induced refractive-index change and birefringence in standard optical fibers

Strong photosensitivity responses with the use of above-bandgap 157-nm F/sub 2/-laser radiation are reported for standard germanosilicate fiber. A large 1.3/spl times/10/sup -3/ effective index change at 1.55 /spl mu/m was inferred by trimming strong and weak Bragg gratings in hydrogen-free fiber. The F/sub 2/-laser fluence-processing window of <50 mJ/cm/sup 2/ is much lower than with traditional ultraviolet (UV) lasers. For hydrogen-soaked fiber, highly asymmetric refractive-index profiles were noted by atomic force microscopy and microreflection microscopy, yielding a peak index change of >0.01 across a small 1-/spl mu/m penetration depth at the fiber core. The index asymmetry appears to underlie the large >5/spl times/10/sup -5/ value of laser-induced birefringence.     

Performance of telecommunication-grade multimode fiber at 780 nm

Presents the physics of graded-index multimode fiber and test results that demonstrate 780-nm compact disc (CD) lasers operating at the high speeds (266 Mb/s) and moderate distances (~2 km) for data communications. The test results include fiber characterizations of bandwidth and attenuation, and system performance testing of bit error rate measurements and eye closure characterization. The predictability of 780-nm performance from existing characterization at the traditional wavelengths of 850 and 1300 nm is sufficiently precise to permit the conservativeness necessary for standardization. Telecommunication-grade 50-μm fiber is ideally suited to this application because commercial pressures have caused 62.5-μm fiber to be optimized for 1300-nm light emitting diode (LED) operation     

Gain characteristics of quantum dot fiber amplifier based on asymmetric tapered fiber coupler

We theoretically analyzed the gain characteristics of an integrated semiconductor quantum dot (QD) fiber amplifier (SQDFA) by using a 2 × 2 tapered fiber coupler with a PbS QD-coated layer. The asymmetric structure of the fiber coupler is designed to have a maximum working bandwidth around 1550-nm band and provide a desired optical power ratio of the output signals. By using 600 mW of 980-nm pump, 10 dB gain of a 1550-nm signal is estimated with the gain efficiency of 4.5 dB/cm.     

Ultrawide 75-nm 3-dB gain-band optical amplification witherbium-doped fluoride fiber amplifiers and distributed Raman amplifiers

An extremely large 3-dB gain-bandwidth of 75 nm (1531-1606 nm) is achieved with a partially gain-flattened erbium-doped fluoride fiber amplifier and a distributed Raman amplifier. The Raman amplifier consists of a 85-km dispersion-shifted fiber (transmission fiber) and a practical 1505-nm Fabry-Perot laser diode pump. 9×2.5 Gb/s wavelength-division-multiplexing (WDM) transmission is successfully demonstrated using two 75-nm gain-band amplifiers as in-line and preamplifiers     

Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK,and DPSK lightwave systems

A comparison is presented of the performance of amplitude-shift-keying (ASK), frequency-shift keying (FSK), and differential-phase-shift-keying (DPSK) lightwave systems which operate at 10 Gb/s with directly modulated 1550-nm distributed feedback (DFB) laser transmitters and conventional 1310-nm dispersion-optimized fiber. Computer modeling techniques were used to accurately simulate the amplitude modulation response and the frequency modulation response of DBF lasers. The system performance is evaluated from simulated eye patterns for both direct and heterodyne detection. With the narrow-optical spectral widths of these signal formats, fiber chromatic dispersion limits up to 70 km were obtained for transmission at 1550-nm using conventional 1310-nm optimized fiber     

Application of Raman-distributed amplification to WDM transmissionsystems using 1.55-μm dispersion-shifted fiber

A numerical design method for wavelength division multiplexing (WDM) transmission systems employing distributed Raman amplification (DRA) is proposed. This method evaluates fiber nonlinear effects by considering the equivalent fiber loss with DRA. The method is used to evaluate the performance of WDM transmission systems in which DRA is employed in a 1.55-μm dispersion-shifted fiber (DSF) transmission line. Transmission limit and the optimum fiber input powers for 1550-nm band (C-band) and 1580-nm band (L-band) transmission are investigated. Results show that bidirectional pumping is the best approach to extending transmission distance. Furthermore, the transport limits of optical transport networks that use DRA and optical add/drop multiplexers are analyzed     

1.5-2.0-/spl mu/m multiwatt continuum generation in dispersion-shifted fiber by use of high-power continuous-wave fiber source

We demonstrated the generation of multiwatt single-mode infrared continuum by using highly nonlinear dispersion-shifted optical fiber pumped by a continuous-wave erbium fiber source. A smooth 1000-nm at -20-dB-wide continuum is demonstrated with a remarkable 450-nm full-width at half-maximum bandwidth. A maximum spectral power density of 4.5 mW/nm is obtained.     

Simultaneous operation of a Raman fiber amplifier and laser pumped by a dual-wavelength Nd/sup 3+/-doped fiber laser

This paper presents an experimental study on the dynamics of Raman fiber lasers that use highly GeO/sub 2/-doped fibers as an active medium and a dual-wavelength (1060 and 1090 nm) Nd/sup 3+/-doped fiber laser as a pump source. The 1090-nm pump wavelength is located within the SiO/sub 2/ Raman gain spectrum relating to the 1060-nm pump wavelength, and competition is observed between Raman amplification of the 1090-nm emission with the 1060-nm emission used as the pump source and Raman lasing, which is independent of the 1090-nm amplification and which is also uses the 1060-nm emission as the pump source. Several pump configurations have been demonstrated to generate specific Stokes emissions generated through Raman lasing or amplification. Changing the gain-to-loss ratio by introducing intracavity loss of Raman emissions or increasing the Raman fiber length within each configuration can force either Raman amplification or lasing to dominate. The maximum slope efficiency as a function of the launched pump power was /spl sim/55% with a total output power of 1.6 W produced. A red shift of both the pump and the Stokes wavelengths is experimentally observed when the launched diode pump power is scaled up.     

All-optical 1300-nm to 1550-nm wavelength conversion using cross-phase modulation in a semiconductor optical amplifier

All-optical 1300-nm to 1550-nm wavelength converters may be important components in lightwave networks which use both the 1300-nm and the 1550-nm low-loss transmission windows of silica optical fiber. We describe a new all-optical 1300-nm to 1550-nm wavelength converter, based on cross-phase modulation in a 1300-nm semiconductor optical amplifier. We demonstrate operation of the wavelength converter at 1.25 Gb/s, and present bit-error rate measurements. The wavelength converter demonstrated here potentially operates at high speed, with low input power and low polarization-sensitivity.     

980-nm pump-band wavelengths for long-wavelength-band erbium-dopedfiber amplifiers

The impact of pump wavelength and input signal power on the output signal and backward spontaneous emission noise power of L-band erbium-doped fiber amplifiers is examined. It is shown experimentally that tuning the pump wavelength ±30-nm away from the 980-nm absorption peak provides 3-5-dB improvement in pump-to-signal conversion     

Simultaneous distribution of multichannel analog and digital videochannels to multiple terminals using high-density WDM and a broad-bandin-line erbium-doped fiber amplifier

The simultaneous amplification of 16 distributed feedback (DFB) lasers over a 34-nm spectral range in an erbium-doped fiber amplifier is reported. Sixteen DFB lasers spaced at approximately 2-nm intervals and covering 34 nm in the 1.55-μm band were modulated with a total of 100 studio-quality analog FM-TV channels and six 622-Mb/s channels     

Cascaded Two-Wavelength Lasers and Their Effects on C-Band Amplification Performance for Er3+-Doped Fluoride Fiber

In this paper, we report cascaded two-wavelength 853-nm (⁴ S_3/2rarr⁴I_13/2 transition) and 1533-nm (⁴I_13/2rarr⁴I_15/2 transition) lasing from Er³⁺-doped fluoride fiber pumped at 974 nm. The cavity for cascaded two-wavelength lasing is composed of two fiber ends with 4% Fresnel reflection. Its optical-to-optical efficiency is up to 26.6%. Its effects on C-band fiber amplifiers and green upconversion fiber lasers are discussed. A new way to get high efficiency and low noise C-band amplifier is suggested, i.e., a fluoride-based Er³⁺-doped fiber amplifier including 853-nm lasing cavity. Our simulated results show that such a new amplifier can enhance the signal gain greatly and break the limit of the saturated gain intensity for a normal amplifier     

Calculation of the noise figure of erbium-doped fiber amplifiersusing small signal attenuations and saturation powers

A theory allowing the noise figure of an erbium-doped fiber amplifier to be computed with a reduced set of fiber parameters is evaluated. The approach is accurate when the amplified spontaneous emission has no significant influence on the gain. Then four parameters, saturation power and absorption coefficient for pump and signal wavelength, are sufficient for 980-nm pumped fiber amplifier. This theory is used to investigate the noise characteristics of a 980-nm pumped booster amplifier within the whole gain spectrum     

Spectral dependence of the small-signal gain around 1.5 μm inerbium doped silica fiber amplifiers

A simple theoretical analysis of small-signal Er³⁺-doped silica fiber amplifier is presented, comparing the efficiency of the 800-, 980-, and 1500-nm pump bands, and demonstrating that wide bandwidth and high gains can be achieved simultaneously by a suitable choice of pump power and fiber length. The comparison shows that the 980- and 1500-nm pump bands have much the same efficiency in terms of dB/mW, and that the 800-nm pumped amplifier is able to produce high gains but at nearly an order of magnitude higher pump powers     

Tunable multiwavelength Brillouin-erbium fiber laser with a polarization-maintaining fiber Sagnac loop filter

We demonstrate the generation of 12-wavelength comb with 14.5-nm tunable range in the Brillouin-Erbium fiber laser. This is achieved through tuning the gain profile of Erbium-doped fiber by a Sagnac loop filter with 18-cm polarization-maintaining fiber, in conjunction with modifying the wavelength of Brillouin pump. The experiment also demonstrates that the tunable range is closely equal to the 3-dB bandwidth of Sagnac loop filter due to its periodic transmission profile. Meanwhile, the effect of the 980-nm pump on the tunable multiwavelength generation is investigated. The power of 980-nm pump has a great effect on the wavelength number and the output power of the generated multiwavelength comb, but little on the tunable range. This technique may be applicable to multiwavelength operation lasers with a large tunable range.     

A novel hybrid silica wide-band amplifier covering S+C+L bands with 105-nm bandwidth

We report a novel hybrid optical amplifier covering S+C+L bands with 105-nm total bandwidth using a silica fiber. The principle of amplification is based on the stimulated radiative transition of Er-ions for C-band and on the stimulated Raman scattering for S- and L-band, respectively. In this letter, we analyze the amplification characteristics for two types of active fiber mediums through numerical simulation. One is a silica fiber configured with Er-doped cladding and Ge-doped core and the other is a medium consisting of Er-doped fiber and dispersion-compensating fiber. By optimizing parameters such as fiber length and pump power, we newly achieve wide-band amplification with 105-nm bandwidth showing a flat gain characteristic over the entire S+C+L bands.     

Monolithic Fiber Mirror and Photonic Crystal Technology for High Repetition Rate All-Fiber Soliton Lasers

In this letter, we discuss the technology of a thin-film coating made with electron beam evaporation on a single-mode fiber end facet. A dichroic mirror made of ZrO₂ and SiO₂ was found to provide the necessary selectivity for 980-nm pump and 1040-nm signal wavelengths and enabled us to build a short-cavity mode-locked ytterbium fiber laser. Combined with a photonic crystal fiber dispersion compensator, it allows the realization of a 572-fs soliton all-fiber laser with a fundamental repetition rate of 571 MHz.