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Causes of Optical Chirp and How to Reduce It

Jan 31st, 2018

Chirp is a sudden change of the center wavelength of a laser, caused by laser instability. A chirp is a signal frequency that increases over time, called “up-chirp,” or decreases, called “down-chirp.” In some sources, the term chirp is used interchangeably with sweep signal.

Chirp occurs when the rising edge of a pulse has a slightly different frequency than the falling edge. When pulses are generated at the transmitted end, intensity modulation causes phase modulation due to the carrier-induced change in the refractive index. That change is inherently due to the laser linewidth. The effect of chirp can be reduced by using external modulators.

When the modulator pulses the laser, a difference in the refractive index of the laser output can cause a chirp in a WDM (wavelength-division multiplexing) system. WDM combines various signals on laser beams at different infrared (IR) wavelengths for transmission on fiber optic media. Each laser is modulated by an independent set of signals. Modulation can be achieved by superimposing a serial data stream onto a carrier signal by changing one of the parameters of the carrier signal. In an optical network, data is modulated onto the light that a laser emits.

The term chirp is also used colloquially to refer to the frequency shift obtained with some laser sources when their intensity is changed. In an optical system with chromatic dispersion, that causes changes in propagation speed for different parts of an amplitude-modulated signal.

One example of optical chirp is a semiconductor laser diode. The refractive index of the semiconductor material depends on the current through the material. So as the laser is modulated the current density changes, which alters the optical length (physical length × refractive index) of the cavity, thereby shifting the center frequency of the laser diode.

Chirp signals are seen in many different applications ranging from radar, sonar, spread spectrum, optical communication, image processing, Doppler effect, FM modulation or as gravitation waves. Frequency chirp can limit the performance of a fiber optic system because when a semiconductor laser is directly modulated it is generally accompanied by phase modulation. The spectrum of an optical pulse is considerably broadened as a result of the frequency chirp. Using an external modulator such as a distributed feedback (DFB) laser will remove frequency chirp because it is on continuously; there is no turning on and off, which can cause the issue to occur. 

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