Tunable fiber laser yields 11-nm range

Researchers at E-TEK Dynamics (San Jose, CA) have designed a tunable high-power fiber laser for use in reconfigurable, high-speed dense wavelength-division-multiplexing (DWDM) networks, according to a technical presentation at the 1999 Conference on Lasers and Electro-Optics (CLEO; Baltimore, MD). The miniature transmitter design provides an 11-nm tuning range and 62 mW of output power, while maintaining relative intensity noise below 165 dB/Hz and a measured system intensity of -32 dBm, accordi

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Researchers at E-TEK Dynamics (San Jose, CA) have designed a tunable high-power fiber laser for use in reconfigurable, high-speed dense wavelength-division-multiplexing (DWDM) networks, according to a technical presentation at the 1999 Conference on Lasers and Electro-Optics (CLEO; Baltimore, MD). The miniature transmitter design provides an 11-nm tuning range and 62 mW of output power, while maintaining relative intensity noise below 165 dB/Hz and a measured system intensity of -32 dBm, according to E-Tek founder, J. J. Pan.

The heart of the device is an interactive fiber laser (IFL) constructed of a distributed feedback (DFB) fiber laser sandwiched between the highly reflective and optical-coupling fiber Bragg gratings (FBGs) of a distributed Bragg reflector fiber laser. The IFL is then incorporated into a tunable fiber laser with a 980-nm laser diode pump and a combination thermal-mechanical tuning system (see figure on p. 22). Actual data transmission is accomplished through signal-intensity modulation by a semiconductor or titanium-doped lithium niobate (Ti:LiNbO3) optical modulator.

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To boost fiber laser efficiency and avoid short-wavelength loss at the 980-nm pump frequency caused by hydrogen loading, the DFB-FBG sandwich was built on an erbium-ytterbium-codoped fiber that is photosensitive in the ultraviolet and allowed strong FBGs to be written without hydrogen loading, Pan said. The reason for making the IFL sandwich in the first place was to make the most out of the low-noise and stable-frequency characteristics of the DFB laser and to boost the output power through the coupling FBG. These criteria were achieved by monitoring output power during ultraviolet laser writing of the coupling FBG, which allowed the researchers to optimize its reflectivity.

The tuning requirements also presented obstacles, Pan said. Direct thermal tuning techniques could not provide the desired wavelength range, and mechanical methods presented reliability concerns. The researchers solved this with a thermomechanical system of high-thermal-expansion Teflon plates in a V-groove configuration, in which thermal control is provided by a TE-cooler. The miniaturized, precision system measures 9 ? 9 ? 60 mm3.

"For the Teflon tuning fixture with a thermal coefficient of 140 ? 10-6/°C, the fiber laser has a temperature tuning coefficient of 0.16 nm/°C," Pan said. "A tuning range of 11.2 nm is obtained with a good wavelength control accuracy and repeatability for the TE-cooler temperature range of -25°C to +55°C."

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