The output of a freely oscillating laser will fluctuate in both
amplitude and frequency. Fluctuations of less than 0.1 Hz are com-
monly referred to as “drift”; faster fluctuations are termed “noise”
or, when talking about sudden frequency shifts, “jitter.”
The major sources of noise in a laser are fluctuations in the pumping source and changes in length or alignment caused by vibration, stress, and changes in temperature. For example, unfil- tered line ripple can cause output fluctuations of 5 to 10 percent or more.
Likewise, a 10-mrad change in alignment can cause a 10-percent variation in output power, and, depending upon the laser, a 1-mm change in length can cause amplitude fluctuations of up to 50 percent (or more) and frequency fluctuations of sev- eral gigahertz.
High-frequency noise (>1 MHz) is caused primarily by “mode beating.” Transverse Laguerre-Gaussian modes of adjacent order are separated by a calculable fraction of the longitudinal mode spacing, typically ~17 MHz in a 1-m resonator with long radius mir- rors. If multiple transverse modes oscillate simultaneously, hetero- dyne interference effects, or “beats,” will be observed at the difference frequencies. Likewise, mode beating can occur between longitudi- nal modes at frequencies of Mode beating can cause peak-to-peak power fluctuations of several percent. The only way to eliminate this noise component is to limit the laser output to a single transverse and single longitu- dinal mode.
Finally, when all other sources of noise have been eliminated, we are left with quantum noise, the noise generated by the sponta- neous emission of photons from the upper laser level in the lasing medium. In most applications, this is inconsequential.
The major sources of noise in a laser are fluctuations in the pumping source and changes in length or alignment caused by vibration, stress, and changes in temperature. For example, unfil- tered line ripple can cause output fluctuations of 5 to 10 percent or more.
Likewise, a 10-mrad change in alignment can cause a 10-percent variation in output power, and, depending upon the laser, a 1-mm change in length can cause amplitude fluctuations of up to 50 percent (or more) and frequency fluctuations of sev- eral gigahertz.
High-frequency noise (>1 MHz) is caused primarily by “mode beating.” Transverse Laguerre-Gaussian modes of adjacent order are separated by a calculable fraction of the longitudinal mode spacing, typically ~17 MHz in a 1-m resonator with long radius mir- rors. If multiple transverse modes oscillate simultaneously, hetero- dyne interference effects, or “beats,” will be observed at the difference frequencies. Likewise, mode beating can occur between longitudi- nal modes at frequencies of Mode beating can cause peak-to-peak power fluctuations of several percent. The only way to eliminate this noise component is to limit the laser output to a single transverse and single longitu- dinal mode.
Finally, when all other sources of noise have been eliminated, we are left with quantum noise, the noise generated by the sponta- neous emission of photons from the upper laser level in the lasing medium. In most applications, this is inconsequential.
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