Compressive strain tunes fiber laser
Eugene D. Jungbluth
Single-frequency lasers that are continuously tunable over a broad range of wavelengths are desirable for a variety of applications. The need for very high sensitivity in spectroscopic, sensor, and communication applications can be met by such narrow-linewidth sources. Scientists at United Technologies Research Center (East Hartford, CT) reported last month at OFC `95 (San Diego, CA) the development of a single-frequency erbium-doped Bragg-grating fiber laser that can be continuously tuned over a 32-nm range while maintaining uniform power of 3 mW.
In laser diodes, wavelength tuning can be accomplished in several ways. Semiconductor laser diodes with external grating mirrors can be tuned over a range of more than 10 nm but have limited practicality because of alignment sensitivities. Multisection distributed-Bragg-reflection diodes require several different current controls for tuning--thus adding to their complexity. Tunable twin-guide laser diodes have tuning ranges limited to 6-10 nm. Thermal effects may also introduce wavelength instability and modehopping in laser diodes.
According to one of the developers, Gary Ball, tunable fiber lasers are attractive because design and fabrication is simply engineered and continuous tuning occurs without modehopping. The wavelength of the fiber laser can be conveniently selected by using the photosensitivity of the fiber core to create gratings that determine the laser wavelength. Tuning a fiber laser over a range of wavelengths is normally accomplished by tensile stressing--literally mechanically stretching the fiber. Unfortunately, this technique results in a tuning range of only 10 nm.
In the 1550-nm region, tuning by tensile stressing develops a wavelength change of about 1.2 nm/millistrain. Unfortunately, the amount of strain that can be tolerated by a fiber is limited by its elasticity. The maximum strain a glass fiber can withstand before degradation and rupture is about 1%. Therefore, the maximum wavelength change caused by pulling a fiber end to end is only about 10 nm.
The developers, Ball and William Morey, said that these limitations are relieved by compressive straining because glass fiber is 23 times stronger under compression than under tension (see figure). Putting an unrestrained fiber under compression that is accurately aligned along the fiber`s axis is difficult to do without buckling. Ball explained that buckling was prevented by confining most of the fiber laser between floating precision-ground ceramic ferrules. The fiber was epoxied to a second set of ferrules placed outside the floating ferrules. One ferrule was rigidly fixed and the second was mounted to a stepper motor. Incremental movement of the motor provided the force to compress the fiber. The stepper motor had a linear resolution of 䕖 nm, corresponding to a wavelength resolution of ۬ pm.
The laser was compressively tuned over 32 nm, from about 1557 to 1525 nm, at which point the compression was about 2.5%. Ball said the tuning range was recently increased to about 40 nm. Three mW of constant, low-noise lasing power was generated in a system based on an integrated master oscillator power amplifier.1 The laser design included a short (2 cm) single-frequency fiber laser followed by a 15-m-long fiber amplifier with pumping from a 1480-nm laser diode. Active feedback was used to maintain constant power because of the nonuniform erbium lasing bandwidth. The output power reportedly can reach 10-20 mW if the pum¥power is increased to 120 mW.