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What Is an External-Cavity Tunable Laser? (Video)

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In this video, we will introduce what is an external-cavity laser.

The external-cavity laser (ECL) has a straightforward design. With the external-cavity method, the beam wavelength is tuned mechanically by adjusting the laser cavity. Other tuning methods use electric current or temperature changes that are applied to the semiconductor material.

This figure shows a diffraction grating-based External-Cavity Laser that is designed in the Littman-Metcalf cavity configuration.

The laser has a simple Fabry-Perot laser diode. The laser has a separately assembled gain medium and an external cavity. This external cavity has a diffraction grating and mirror integrated together.

Tuning is done by applying a voltage to the actuator of the MEMS (micro-electro-mechanical system). This rotates a mirror to allow a particular diffracted wavelength to couple back into the laser diode. The actual wavelength of the laser output is determined by combining the gain bandwidth of the diode, the grating dispersion, and the external-cavity mode structure.

The typical MEMS ECL has a 10 mW output and a 13 nm tuning range.

External-Cavity Lasers have many favorable characteristics for a switched optical network and optical add-drop multiplexers. They have continuous tuning across the bandwidth range, narrow line widths, high stability, low noise, no mode hops, comparatively high output power, insensitive to shock and other environmental changes.

Their disadvantages are their size and cost. But the use of MEMS technology has removed many of these disadvantages. 

These same devices can be repackaged to produce tunable receivers, polarization controllers, optical monitors, variable attenuators, optical switches, and tunable filters.

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