Generation of a 100-GHz optical pulse train by pulse repetition-rate multiplication using superimposed fiber Bragg gratings

We demonstrate the use of superimposed fiber Bragg gratings (FBGs) operating in reflection as amplitude and phase filtering stages for multiplying the repetition rate of a given optical pulse sequence. In particular, we use a 1-cm-long structure of two superimposed linearly chirped FBGs to generate a continuous optical pulse train with a repetition rate of 100 GHz (duty cycle /spl ap/50%) at a wavelength of 1.55 /spl mu/m from a 10-GHz mode-locked fiber laser.     

Er-Yb femtosecond ring fiber oscillator with 1.1-W average power and GHz repetition rates

We report an all-fiber passively mode-locked femtosecond laser oscillator based on the heavily doped Er-Yb phosphate-glass active fiber. Only 20 cm of the gain fiber is sufficient to produce as much as 1.1 W of average output power at 1.5 /spl mu/m directly from the oscillator. The laser can be harmonically mode-locked at repetition rates ranging from 1.7 to 7.2 GHz by adjusting the polarization bias in the cavity. The pulsewidth varies from 300 to 570 fs at the lowest and the highest repetition rate, respectively, and the maximum peak pulse power exceeds 1 kW.     

Analysis and optimization of Q-switched operation of a Tm/sup 3+/-doped silica fiber laser operating at 2 /spl mu/m

Analysis and operation of a Q-switched Tm/sup 3+/-doped silica fiber laser in the wavelength region of 2 /spl mu/m is described when pumped with a Nd:YAG laser operating at 1.319 /spl mu/m. A large core of 17-/spl mu/m diameter was used to increase the laser gain volume, allowing high pump-power absorption and an output of high pulse energy and peak power. An acoustooptic modulator was used as Q-switching element and operated at repetition rates up to 30 kHz. A maximum peak output power of greater than 4 kW and a pulse duration at full-width at half-maximum of 150 ns has been obtained. This is the first report of high peak-power operation of the thulium-doped silica fiber laser.     

Diode-pumped passively mode-locked Nd:YVO/sub 4/ lasers with 40-GHz repetition rate

We present two different diode-pumped passively mode-locked Nd:YVO/sub 4/ lasers with a repetition rate of 40 GHz. This is the highest repetition rate demonstrated so far with diode-pumped 1-/spl mu/m solid-state lasers. The first laser design allows short pulses of 2.7-ps duration whereas the second laser design is optimized for high average output power of up to 288 mW. We compare both design approaches and show that there is a tradeoff between output power and pulse duration.     

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.     

Passive mode locking in a diode-pumped Nd:GdVO/sub 4/ laser with a semiconductor saturable absorber mirror

We demonstrate Q-switched and CW passive mode locking in a laser-diode-pumped Nd:GdVO/sub 4/ laser with a semiconductor saturable absorber mirror. The repetition rate of the Q-switched envelope increased from 23.1 to 260 kHz as the pump power increased from 1.75 to 13.0 W. At a pump power of 13.7 W, the Q-switched mode locking changed to CW mode locking. The maximum average output power of 4.9 W with a 140-MHz repetition rate was obtained at a pump power of 17.9 W and the single mode-locked pulse energy was 0.035 /spl mu/J. The CW mode-locked pulse duration was measured to be /spl sim/11.5 ps.     

Semiconductor light sources for 40-Gb/s transmission systems

The status and prospects of semiconductor light sources for 40-Gb/s transmission systems are reviewed in regard to the following three topics: direct modulation, external modulation, and pulse sources for return-to-zero format. Included are discussions on direct modulation of a 1.3-/spl mu/m distributed feedback laser for 40-Gb/s very-short-reach optical links, progress made in developing external modulators based on electroabsorption of multiple quantum wells, and mode-locked lasers for carrier-suppressed return-to-zero modulation format.     

35-dB gain Tm-doped ZBLYAN fiber amplifier operating at 1.65 /spl mu/m

We demonstrate the first high gain rare-earth-doped fiber amplifier operating at 1.65 /spl mu/m. It consists of ZBLYAN fiber with a Tm/sup 3+/-doped core and Tb/sup 3+/-doped cladding, pumped by 1.22 /spl mu/m laser diodes. It is possible to achieve efficient amplification with Tm/sup 3+/ ions if their amplified spontaneous emission (ASE) in the 1.75 to 2.0 /spl mu/m wavelength region is suppressed by doping Tb/sup 3+/ ions in the cladding. A two-stage-type fiber amplifier is constructed and a signal gain of 35 dB is achieved for a pump power of 140 mW. A gain over 25 dB is realized in the 1.65 /spl mu/m to 1.67 /spl mu/m wavelength region.     

Germania-based core optical fibers

Germania-glass-based core silica glass cladding single-mode fibers (/spl Delta/n up to 0.143) with a minimum loss of 20 dB/km at 1.9 /spl mu/m were fabricated by the modified chemical vapor deposition (MCVD) method. The fibers exhibit strong photorefractivity with the type-IIa-induced refractive-index modulation of 2/spl times/10/sup -3/. The Raman gain of 300 to 59 dB/(km/spl middot/W) was determined at 1.07 to 1.6 /spl mu/m, respectively, in a 75 mol.% GeO/sub 2/ core fiber. Only 3 m of such fibers are enough for the creation of a 10-W Raman laser at 1.12 /spl mu/m with a 13-W pump at 1.07 /spl mu/m. Raman generation in optical fiber at a wavelength of 2.2 /spl mu/m was obtained for the first time.     

1.50 /spl mu/m CW operation of GaInNAs/GaAs laser diodes grown by MOCVD

Room-temperature continuous-wave operation of a 1.5 /spl mu/m range GaInNAs laser grown by metal organic chemical vapour deposition is reported. The lasing wavelength of the GaInNAs/GaAs double quantum well laser was 1.50 /spl mu/m and the threshold current was 245 mA. The characteristic temperature between 10 and 50/spl deg/C was about 119 K under pulse operation.     

Continuous-fiber modulator with high-bandwidth coplanar strip electrodes

Operation of a continuous-fiber modulator based on coupling from a fiber side-polished beyond cut-off to a multimode planar waveguide has been demonstrated for the first time at gigahertz frequencies. The bandwidth of the modulator electrode structure was /spl sim/4 GHz while the optical insertion loss was measured at <0.5 dB. The device was used to produce mode-locked pulse trains in an erbium fiber laser at repetition rates of /spl sim/3 GHz.     

Wavelength-tunable short-pulse diode-laser fiber-amplifier system around 1.06 /spl mu/m

We report the first actively mode-locked diode-laser fiber-amplifier system with wide wavelength tuning around 1.06 /spl mu/m. Tuning is achieved over 45 nm via a feedback grating. Pulses with a repetition rate of 1.4 GHz and a pulse duration of 30 ps are generated with up to 9.5 W of average output power.     

Neodymium-doped cladding-pumped aluminosilicate fiber laser tunable in the 0.9-/spl mu/m wavelength range

A tunable high-power cladding-pumped neodymium-doped aluminosilicate fiber laser is demonstrated. The maximum power reached was 2.4 W with a slope efficiency of 41% and a threshold pump power of 1.68 W, both with respect to launched pump power, when cladding pumped by two 808-nm diode pump sources at both fiber ends. The dependence of the tuning range on the fiber length is investigated. The tuning range changed from 922 to 942 nm for a 25-m-long fiber to 908-938 nm with a 14-m-long fiber, because of reabsorption effects. The output linewidth was 0.26 nm in a diffraction-limited beam. Operation on the challenging 0.9-/spl mu/m three-level transition in neodymium-doped double-clad fiber laser was facilitated by a W-type core refractive index profile. This filtered out the unwanted and competing strong transition at 1.06 /spl mu/m while guidance of 0.9 /spl mu/m remained intact.     

1.5 /spl mu/m GaInNAs semiconductor saturable absorber for passively modelocked solid-state lasers

Fully self-starting and passively modelocking of a 1.5 /spl mu/m solid-state laser with a GaInNAs semiconductor saturable absorber mirror (SESAM) has been demonstrated for the first time. A saturation fluence of 20 /spl mu/J/cm/sup 2/, a modulation depth of 0.39% and a fast temporal decay of 18 ps were measured. These well-suited nonlinear optical SESAM parameters allowed for self-starting and passive modelocking of a diode-pumped Er:Yb:glass laser at 1.534 /spl mu/m with a pulse duration of 5 ps at 61 MHz.     

Optical generation of amplitude-equalized pulses from a rational harmonic mode-locked fiber laser incorporating an SOA loop modulator

Rational harmonic active and passive mode locking has been demonstrated in an erbium-doped fiber laser using an all-optical approach. Based on the modulation and the self-switching effects of a semiconductor optical amplifier nonlinear loop mirror, hybrid mode locking and pulse-amplitude equalization are simultaneously achieved in the laser. In our setup, amplitude-equalized pulse trains up to about 12 GHz are obtained at 1.55 /spl mu/m using 3-GHz optical driving pulses.     

Phase-plane analysis of rational harmonic mode-locking in an erbium-doped fiber ring laser

We investigate the system performance of rational harmonic mode-locking in an erbium-doped fiber ring laser using the phase-plane technique of the nonlinear control engineering. Contributions from harmonic distortion, a Gaussian-like modulating signal, and its duty cycle to the system behavior are studied. We also demonstrate 660/spl times/ and 1230/spl times/ repetition rate multiplications on a 100-MHz pulse train generated from an active harmonically mode-locked fiber ring laser, and we hence achieve 66- and 123-GHz pulse operations by using the above-mentioned technique. It has been found out that the maximum obtainable rational harmonic order is limited by the harmonic distortion of the system as well as the pulse width of the generated signal, which in turn is determined by the duty cycle of the modulating signal. Furthermore, the rational harmonic order increases the complexity of the pulse formation process and hence challenges its stability.     

80-ps and 4-ns pulse-pumped gains in a GaP-AlGaP semiconductor Raman amplifier

The characteristic of 80-ps mode-locked (ML) pulse-pumped gain, which results in a decline that changes from a linear gradient to a square-root gradient at introduced pump densities exceeding 10 dB, makes it difficult to develop pulse-pumped gains for high efficiency amplification. To overcome this disadvantage with pumping, we compared an 80-ps ML pulse and 4-ns Q-switched pulse in a straight waveguide. The amplification of the 4-ns pulse was linear and had a maximum gain of 23.3 dB at an introduced pump density of 1.4 W//spl mu/m/sup 2/ in a straight waveguide. The gain was more efficient than with the 80-ps pulse, which was limited by the optical damage threshold of the input antireflective coating (1.6 W//spl mu/m/sup 2/). These high-gain operations should enable semiconductor Raman amplifiers to be used for detecting signals from chemical or biological materials, in addition to infrared light frequency selective amplification with wavelength-division multiplexing in optical communications.     

Analysis of heat generation and thermal lensing in erbium 3-/spl mu/m lasers

The influence of energy-transfer upconversion (ETU) between neighboring ions in the upper and lower laser levels of erbium 3-/spl mu/m continuous-wave lasers on heat generation and thermal lensing is investigated. It is shown that the multiphonon relaxations following each ETU process generate significant heat dissipation in the crystal. This undesired effect is an unavoidable consequence of the efficient energy recycling by ETU in erbium 3-/spl mu/m crystal lasers, but is further enhanced under nonlasing conditions. Similar mechanisms may affect future erbium 3-/spl mu/m fiber lasers. In a three-dimensional finite-element calculation, excitation densities, upconversion rates, heat generation, temperature profiles, and thermal lensing are calculated for a LiYF/sub 4/:Er/sup 3+/ 3-/spl mu/m laser. In the chosen example, the fraction of the absorbed pump power converted to heat is 40% under lasing and 72% under nonlasing conditions. The heat generation in a LiYF/sub 4/:Er/sup 3+/ 3-/spl mu/m laser is 1.7 and the thermal-lens power up to 2.2 times larger than in a LiYF/sub 4/:Nd/sup 3+/ 1-/spl mu/m laser under equivalent pump conditions, thus, also putting a higher risk of rod fracture on the erbium system.     

Injection-seeded Q-switched fiber ring laser

We present a narrow linewidth, injection-seeded Q-switched Er fiber ring oscillator, that provides over 600 /spl mu/W of average output power at 500 Hz, with 1.2 /spl mu/J per pulse, before the output appears to be significantly affected by stimulated Brillouin scattering. This laser configuration provides multiple advantages in LIDAR systems because it offers the possibility of broad and rapid tunability.     

1300-nm pulsed fiber lasers mode-locked by purified carbon nanotubes

For the first time, we demonstrate a novel passively mode-locked fiber laser operating at 1300 nm using purified single-walled carbon nanotubes (CNTs) as a saturable absorber. The saturable absorber incorporates diameter-controlled CNTs with peak absorption /spl sim/1300 nm, guaranteeing mode-locking over the same wavelength region. The ring laser uses praseodymium-doped fiber as a gain medium. The pulse repetition rate is 3.18 MHz, and the spectral half-width is 0.15 nm. Dual-wavelength mode-locking is also demonstrated with a channel spacing of 1.1 nm.