Biomedical Engineering Reference
In-Depth Information
must include consideration of the many noise sources. Some of these noise
sources include phase noise and noise introduced through the modulating
signal itself. This provides a foundation for reducing emission intensity or
modulation depth to maintain laser stability.
In the optical receiver, the noise current is the sum of quantum noise, ther-
mal noise, dark current noise, leakage current noise, and beat noise. Beat
noise is zero in a single-mode system, while the thermal noise is usually
dominant. Modulation levels must be selected to ensure a satisfactory distor-
tion level, with the tolerable level depending upon the nature of the informa-
tion being transmitted.
4.17.6 Instability due to Optical Feedback from Distant Reflectors
Instability can occur as a result of the finite phase and carrier number change
caused by fluctuations in spontaneous emission [36]. This instability only
occurs when the laser reaches a steady state that maximizes coherent feed-
back and laser light intensity. The instability caused by reflections vanishes
at strong feedback levels if the reflected energy is in phase with the emission
photon. The laser is nearly stable at threshold with moderate feedback but
unstable when operated well above threshold.
4.17.7 Stability with Moderate External Feedback
For the following discussion we assume a laser system operating in the pres-
ence of a moderate level of external feedback (−30 dB) from some component
many cavity lengths away from the laser active region. These represent the
typical conditions for applications such as video recording, coherent com-
munications, and sensing where external modulation is utilized [37]. The
effects of reflections on a CW laser are well documented [38]. Weak reflec-
tions cause line narrowing [39] and high levels of reflection cause line broad-
ening [40]. With moderate levels of optical feedback, the laser line becomes
significantly broadened to a linewidth on the order of 25 GHz. This compares
to a linewidth of from one to several Angstroms for a stable laser operating
in steady state with zero feedback. The effect of the excessive line broadening
(coherent collapse) is attributed to a lack of coherence between the field of the
continuously operating laser and the steady-state reflected field.
Under moderate feedback, a CW laser will exhibit periodic low-frequency
intensity fluctuation [41]. The dependence of the period of the fluctuation,
reflector separation, and laser current are related. The pulsation period is
related to the time required for a photon to make approximately ten round
trips in the external cavity and thus is related to cavity length. When the
laser is operating near threshold, the intensity of the pulsating light is ampli-
fied because of the many round trips in the external cavity. Between pulsa-
tions, the intensity increases and approaches a steady-state intensity, then
suddenly drops to zero and the process repeats.
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