Fiber length dependence of phase change induced by laser-diode pumping in Yb/sup 3+/-Al/sup 3+/ co-doped optical fibers

We investigated the fiber length dependence of the phase change near 1580 nm induced by 980-nm pump beam in Yb/sup 3+/-Al/sup 3+/ co-doped optical glass fibers by use of a long-period fiber grating pair. As the pumping power increased, the phase change linearly increased and then was gradually saturated. Both the saturated phase change and the saturation pump power increased with the increase of the fiber length. The saturated phase change in Yb/sup 3+/-Al/sup 3+/ co-doped fibers of 16.7, 33.2, and 43.5 cm was found to be /spl sim/0.5, /spl sim/0.8, and /spl sim/1.0 rad, respectively. The saturation of the phase change is attributed to the saturation of the absorption from the finite ytterbium ions doped in the fibers.     

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.     

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.     

1.49-/spl mu/m-band gain-shifted thulium-doped fiber amplifier for WDM transmission systems

This paper describes in detail the amplification characteristics of gain-shifted thulium-doped fiber amplifiers (GS-TDFAs) operating in the 1480to 1510-nm wavelength region (1.49-/spl mu/m S-band) for use in wavelength-division-multiplexing (WDM) systems. Gain shifting of a TDFA, which normally has a gain band at 1.47 /spl mu/m (S/sup +/-band), is achieved by two types of dual-wavelength pumping: (1) 1.05 and 1.56 /spl mu/m or (2) 1.4 and 1.56 /spl mu/m. The main pump source at 1.05 or 1.4 /spl mu/m creates population inversion between /sup 3/F/sub 4/ (upper laser level) and /sup 3/H/sub 4/ (lower laser level), while the auxiliary pump source at 1.56 /spl mu/m reduces the average fractional inversion down to approximately 0.4, which is a desired level for gain shifting. We show experimentally that the former provides a low internal noise figure (<4 dB) due to high fractional inversion at the input end of a thulium fiber, while the latter provides a very high optical efficiency but a higher internal noise figure (/spl sim/5 dB) due to the lower fractional inversion at the input end. These characteristics were verified by numerical simulation based on a comprehensive rate equation modeling. We demonstrated a 1.4- and 1.56-/spl mu/m laser-diode-pumped GS-TDFA with an optical efficiency of 29.3% and high output power of +21.5 dBm. Gain flatness and tilt control were also investigated. These results strongly confirm the feasibility of using GS-TDFAs in practical ultralarge-capacity WDM networks.     

A novel bidirectional optical coupling module for subscribers

To overcome drawbacks and limitations of planar lightwave circuit based modules for bidirectional communications, such as the demand for several chips and in consequence more packaging efforts, we have recently developed a novel optical coupling technique using our unique 155 Mbps bidirectional laser chip. Since the chip is structured with a pin-photodiode monolithically integrated on a laser diode's waveguide, the optical coupling requires only the alignment of the chip with a fiber. To optically couple the laser diode and photodiode simultaneously with a single fiber, we have designed an unusual coupling structure using a fiber having a cleaved surface whose normal is 35/spl deg/ angled to the fiber core axis, and using an index-controlling medium with a refractive index of /spl sim/1.3. The bidirectional chip is flip-chip bonded and the fiber is passively aligned using a V-groove on the same substrate of 2.5/spl times/1.3 mm/sup 2/ in size. Even with this extremely small and simple scheme for bidirectional optical coupling, we could obtain an optical output power of -7/spl sim/-10 dBm and a responsivity of <-30 dBm, which are satisfactory to the STM-1 level telecommunications specifications.     

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.     

Multiwavelength diode-cladding-pumped Nd/sup 3+/-doped germano-aluminosilicate fiber laser

Simultaneous multiwavelength oscillation at 1060 and 1090 nm has been produced from a free-running Nd/sup 3+/-doped multicomponent silica fiber laser. As a result of co-doping with both Al/sub 2/O/sub 3/ and GeO/sub 2/, the Nd/sup 3+/ ions are situated at separate sites relating to either Al/sup 3+/-rich or Ge/sup 4+/-rich regions of the germano-aluminosilicate glass. The slope efficiency of the combined /spl sim/1-/spl mu/m output was /spl sim/52% (56%) with respect to the launched (absorbed) pump power. The 1060-nm emission reaches threshold first because of the greater number of Nd/sup 3+/ ions that are located at Al/sup 3+/-rich sites. On chopping the pump light the relaxation oscillations relating to the 1090-nm emission are antiphase with the oscillations observed with the 1060-nm emission. A degree of spectral overlap exists between the fluorescence emitted from Nd/sup 3+/ ions located at each site. Power equalization (to /spl sim/1 W each) of the 1060- and 1090-nm emissions was carried out by way of Raman amplification that occurred either internally or externally to the Nd/sup 3+/-doped silica fiber laser.     

A 43-W C-band tunable narrow-linewidth erbium-ytterbium codoped large-core fiber laser

The authors report a widely tunable and highly efficient cladding-pumped erbium-ytterbium codoped large-core fiber laser, generating up to 43 W of output power at /spl sim/1.5 /spl mu/m with a narrow linewidth (0.16-nm full-width at half-maximum) and a good beam quality (M/sup 2/<1.7). By use of a tunable narrow-band fiber Bragg grating, the laser wavelength was tuned from 1532 to 1567 nm, limited upwards by the tuning range of the fiber grating. The overall slope efficiency was 32% with respect to launched pump power.     

High-power nearly diffraction-limited AlGaAs-InGaAs semiconductor slab-coupled optical waveguide laser

Beam-quality measurements on the output of a 915-nm AlGaAs-InGaAs-GaAs slab-coupled optical waveguide laser (SCOWL) are reported. This device had a nearly circular mode (3.8 /spl mu/m by 3.4 /spl mu/m 1/e/sup 2/ widths in the near-field) and was capable of a single-ended continuous-wave output power of greater than 1 W. Measurements of M/sup 2/ indicate that the SCOWL output beam is nearly diffraction-limited in both directions with M/sub x//sup 2/ /spl sim/ M/sub y//sup 2/ /spl sim/ 1.1 over the entire range of output powers measured.     

Efficient amplification using the /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 9/2/ transition in Nd-doped silica fiber

Amplification characteristics of the three-level /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 9/2/ transition in Nd-doped silica glass fiber are investigated under strong signal saturation and high pump power (150 mW). Aluminum codoped Nd-silica fibers exhibit strong superfluorescent behavior in the four-level /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 11/2/ transition which limits the optical conversion efficiency into the three-level transition. Ge-doped silica fibers do not exhibit this limitation and can efficiently amplify in the three-level transition with current laser-diode pump technology.     

210 mW 2.84 /spl mu/m Ho/sup 3+/, Pr/sup 3+/-doped fluoride fibre laser

210 mW of unsaturated continuous-wave output from a Ho/sup 3+/, Pr/sup 3+/-doped fluoride fibre laser is demonstrated using the /spl sim/1100 nm output from a free-running diode-cladding-pumped Yb/sup 3+/-doped silica fibre laser as the pump source. A slope efficiency of 3.2% and an output wavelength of 2.84 /spl mu/m are obtained.     

Nanosecond pulse generation in a passively Q-switched Yb-doped fiber laser by Cr/sup 4+/:YAG saturable absorber

We explored efficient nanosecond pulse generation in a passively Q-switched cladding-pumped Yb-doped fiber laser by use of an external-cavity configuration containing a Cr/sup 4+/:YAG saturable absorber crystal. By exploiting passive Q-switched regime and stimulated Brillouin scattering, pulses of durations as short as 2.7 ns, corresponding to a peak power of /spl sim/9 kW, have been achieved. The laser was tunable over 70 nm and green output power generation was obtained by external frequency doubling in a KTP crystal.     

Multi-watt, high efficiency, diffraction-limited Nd:YAG planar waveguide laser

An efficient, longitudinally diode-pumped, diffraction-limited, Nd:YAG double-clad planar waveguide laser was operated on four transitions of the Nd/sup 3+/ ion. Optimized output powers of 4.3, 3.5, and 2.7 W were obtained for absorbed pump powers of /spl sim/7 W, for the transitions at the lasing wavelengths of 1.064 /spl mu/m, 946 nm, and 1.3 /spl mu/m, respectively. Operation of the weak /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 5/2/ transition, lasing at 1.833 /spl mu/m, was demonstrated at an absorbed pump power threshold of 300 mW and an output power of 400 mW, with a nonoptimized output coupling. Diffraction-limited performance was obtained in both the guided and nonguided axes.     

Multiple-Stokes stimulated Brillouin scattering generation in pulsed high-power double-cladding Er/sup 3+/-Yb/sup 3+/-codoped fiber amplifier

Cascaded Stokes waves generation due to stimulated Brillouin scattering (SBS) of coherent optical pulses in a double-cladding Er/sup 3+/-Yb/sup 3+/ codoped fiber amplifier is reported. The highest attainable output power strongly depends on the amplifier pumping arrangement. A maximum of 40-W peak power has been obtained in counterpumping configuration. The highest energy extracted from the single-mode fiber amplifier in 1-/spl mu/s pulses is limited SBS to 15 /spl mu/J. Theses results have been theoretically confirmed using coupled-waves SBS model.     

All-optical wavelength conversions based on MgO-doped LiNbO/sub 3/ QPM waveguides using an EDFA as a pump source

We propose and demonstrate cascaded /spl chi//sup (2)/ wavelength conversion in an MgO-doped LiNbO/sub 3/ quasi-phase-matched waveguide at reduced pump cost by using only an erbium-doped fiber amplifier (EDFA) as the exclusive pump source in a fiber-ring resonator. A fiber Bragg grating in conjunction with an optical circulator isolates the pump light and confines it to the resonator. A conversion efficiency of -30 dB was obtained with a 3-dB gain-saturation output power of only +12 dBm from the EDFA. It is anticipated that -14-dB conversion efficiency is possible, if an EDFA with a 3-dB gain-saturation output power of +20 dBm is used.     

Generation of 9.3-W multimode and 4-W single-mode output from 7-cm short fiber lasers

We generate 9.3-W continuous-wave 1535-nm multimode output from a 7.0-cm short-length Er-Yb codoped phosphate fiber laser. A slope efficiency of 29% is obtained at pump powers below 27 W. Very high output power per unit fiber length of 1.33 W/cm is achieved. From another 7.1-cm Er-Yb codoped fiber laser, 4.0-W single-transverse-mode output with M/sup 2//spl ap/1.1 is generated.     

FROG measured high-power 185-fs pulses generated by down-chirping of the dispersion-managed breathing-mode semiconductor mode-locked laser

A dispersion-managed breathing-mode mode-locked semiconductor ring laser generates linear down-chirped pulses which are dispersion compensated to duration as short as 185 fs and characterized by second-harmonic generation frequency-resolved optical gating. Down-chirping when compared to up-chirping allows broader mode-locked spectra and shorter pulse generation owing to the temporal and spectral semiconductor gain dynamics. The measured average output power is 14 mW at 323-MHz pulse repetition rate, implying a peak power of /spl sim/230 W, and a focused intensity of /spl sim/4.6 GW/cm/sup 2/. To our knowledge, this is the highest peak power and the shortest pulse generation from an electrically pumped all-semiconductor system, obtained only by linear chirp compensation.     

Solid-state laser pumping of 1.5-μm optical amplifiers andsources for lightwave video transmission

An optical power amplifier and a laser source are demonstrated at 1.5 μm. A diode-pumped Nd:YAG laser is used as the pumping source for an Er/Yb co-doped gain medium. The power scaling advantages of this approach are demonstrated. Up to +21 dBm of output power is obtained from the Er/Yb amplifier and up to +19 dBm is obtained from the laser source. The Er/Yb power amplifier was deployed in a 42-channel AM link with 40 km of fiber, and an optical loss budget of 18 dBm was demonstrated     

Singly Ho/sup 3+/-doped fluoride fibre laser operating at 2.92 /spl mu/m

A singly Ho/sup 3+/-doped fluoride fibre laser that uses energy transfer upconversion to maintain the population inversion has produced 340 mW at 2.92 /spl mu/m. The /spl sim/1100 nm output from a diode-cladding-pumped Yb/sup 3+/-doped silica fibre laser was used as the pump source and a maximum slope efficiency of 5% was obtained.     

1.47 /spl mu/m Tm/sup 3+/:ZBLAN fibre laser pumped at 1.064 /spl mu/m

A large core area (1257 /spl mu/m/sup 2/) Tm/sup 3+/-doped ZBLAN fibre laser operated at 1.47 /spl mu/m is demonstrated. The pump source is a Nd:YAG laser operated at 1.064 /spl mu/m. A laser output power of 1.56 W continuous wave was obtained for 5.2 W of launched pump power. The slope efficiency with respect to the launched pump power was measured to be 33%.