D.N. Payne

Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power

We have demonstrated a highly-efficient cladding-pumped ytterbium-doped fiber laser generating 1.36 kW of continuous-wave output power at 1.1 mum with 83% slope efficiency and near diffraction-limited beam quality. The laser was end-pumped through both fiber ends and showed no evidence of roll-over even at the highest output power, which was limited only by available pump power.

Er(3+):Yb(3+)-codoped fiber distributed-feedback laser.

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5 microm written directly into a 2-cm-long Er(3+)-doped fiber codoped with Yb(3+). We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

Birefringence and polarization mode-dispersion in spun single-mode fibers

A theoretical and experimental analysis of the polarization properties of twisted single-mode fibers is presented. It is shown that whereas a conventionally twisted fiber possesses considerable optical rotation, a fiber which has a permanent twist imparted by spinning the preform during fiber drawing exhibits almost no polarization anisotropy. It is thus possible to virtually eliminate the commonly observed fiber linear birefringence. As a consequence, fibers made in this way are ideally suited for use in the Faraday-effect current transducer. It is further shown that a permanent twist of a few turns/meter effectively eliminates polarization mode-dispersion. The technique therefore appears attractive for enhancing the bandwidth of very long unrepeatered telecommunication links.

Stable single-frequency traveling-wave fiber loop laser with integral saturable-absorber-based tracking narrow-band filter.

We demonstrate stable single-frequency and polarization operation of a traveling-wave, Er(3+)-doped fiber loop laser by incorporating an unpumped Er(+3)-doped fiber section butted against a narrow-band feedback reflector. The saturable absorber acts as a narrow bandpass filter that automatically tracks the lasing wavelength, thus ensuring single-frequency operation. Output powers up to 6.2 mW at 1535 nm were obtained for launched pump powers of 175 mW at 1064 nm. At this output, the relative intensity noise was less than -112 dBy/Hz at frequencies above 200 kHz and a laser linewidth of less than 0.95 kHz, whereas the lasing frequency was observed to drift slowly (~170 MHz/h) because of environmental effects.

Power Scaling of Single-Frequency Ytterbium-Doped Fiber Master-Oscillator Power-Amplifier Sources up to 500 W

We discuss continuous-wave single-frequency master- oscillator power-amplifier (MOPA) sources based on ytterbium- doped fibers (YDFs) with particular attention to their recent advances and our experimental results. This includes a 402-W plane-polarized MOPA source and a 511-W random-polarized MOPA source. In these MOPAs, the final-stage high-power amplifier operates with high efficiency of 70%-80%, and a high gain of over 20 dB in a near diffraction-limited beam. We report at least 7-dB enhancement of the stimulated Brillouin scattering (SBS) threshold for the 402-W polarization-maintaining (PM) YDF. In fact, we did not see any sign of SBS even at the highest output power. We eventually observed SBS appearance at around 500 W for the non-PM YDF. The observed SBS strengths were far weaker than expected in theory, unless we allowed for the Brillouin gain reduction from thermal Brillouin gain broadening induced by the quantum-defect heating.

Electric current sensors employing spun highly birefringent optical fibers

The theory of highly elliptically birefringent fibers fabricated by spinning a linearly birefringent fiber during the draw is described. These fibers are particularly interesting for application as Faraday-effect fiber current monitors, since, in contrast to conventional fibers, they can be wound in small multiturn coils while retaining their sensitivity. The fiber and its application in three optical schemes are modeled using Jone calculus and are also experimentally investigated. A simple optical configuration is proposed, combining the elliptically birefringent fiber and a broad-spectrum light source. An accurate, compact, and robust current monitor is obtained. The sensor is characterized by a measurement repeatability of +or-0.5%, a temperature drift of 0.05%/ degrees C and a sensitivity of 1 mA RMS/Hz/sup 1/2/. Further, the performance of this sensor with optimized fiber length for a given measurement bandwidth is predicted. >

Development of low- and high-birefringence optical fibers

The polarization properties of single-mode optical fibers are easily modified by environmental factors. While this can be exploited in a number of fiber sensor devices, it can be troublesome in applications where a stable output polarization-state is required. Fibers with both low and high birefringence have been developed to enhance or diminish their environmental sensitivity, and recent progress in each area is reviewed. Low-birefringence fibers are described which are made by spinning the preform during the draw. In addition, developments in high-birefringence fibers which maintain a polarization state over long lengths are summarized. The effect on performance of external factors such as bends, transverse pressure, and twists is analyzed. Consideration is also given to polarization mode-dispersion as a potential limiting factor in ultrahigh bandwidth systems.

Soliton pulse compression in dispersion-decreasing fiber

We investigate the adiabatic compression of picosecond and subpicosecond soliton pulses from all-fiber, passively mode-locked, erbium-doped fiber soliton lasers operating at 1550 nm in dispersion-decreasing fibers (DDFs). High-quality soliton compression from 630 down to 115 fs in a 100-m DDF and from 3.5 down to 230 fs in a 1.6-km DDF is obtained. The effects of third-order dispersion and Raman self-scattering on the compression process are observed and discussed.

Spectroscopic data of the 1.8-, 2.9-, and 4.3-μm transitions in dysprosium-doped gallium lanthanum sulfide glass

Infrared emission at 1.8, 2.9, and 4.3 microm is measured in dysprosium-doped gallium lanthanum sulfide (Ga:La:S) glass excited at 815 nm. Emission cross sections were calculated by Judd-Ofelt analysis, the Füchtbauer- Ladenburg equation, and the theory of McCumber. The sigmatau value for the 4.3-microm transition is ~4000 times larger in the Ga:La:S glass than in a dysprosium-doped LiYF(4) crystal, which has lased on this transition. The large sigmatau value and the recently reported ability of Ga:La:S glass to be fabricated into fiber form show the potential for an efficient, low-threshold mid-infrared fiber laser. The f luorescence peak at 4.3 microm coincides with the fundamental absorption of atmospheric carbon dioxide, making the glass a potential laser source for gas-sensing applications.

Mode conversion coefficients in optical fibers

A simple method has been devised for the experimental determination of mode conversion coefficients in multimode fibers and involves only the observation of the far-field output as the angle of incidence of a collimated input beam is changed. The normalized mode coupling coefficient in a liquid-core fiber is D = 3 x 10(-6) rad(2) m(-1) and increases by as much as a factor of 10 when transverse pressure is applied. Values some 2 orders of magnitude larger are found in glass-core fibers. There is good agreement between the theory presented and experiment.