A bidirectional page link using optical circulators


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Introduction
The standard fiber cables in use today, shows variations in the attenuation characteristics. The minimum attenuation occurs at wavelengths around 1310 nm and 1550 nm. Any one of the above wavelengths is used for transmitting the signal using the laser diodes through the optical fiber. While constructing the bidirectional link, any one of the above wavelengths is considered and used for the bidirectional communication. The key component of bidirectional page link is the circulator which is a 3-port, non reciprocal, passive device. The bidirectional page link using circulators introduces the noise of approximately 2.0 dB, while that using the 3 dB beam-splitters imposes more than 6 dB of signal noise. Thus, the probable solution for constructing the bi-directional page link is by the use of the circulators. The experimenta l observations along with the total page link performance or the total page link loss budget is estimated . The page link is also tested for the leakage loss, back reflection, the isolation introduced due to the simultaneous transmission of the two signals.



2.Important definitions and terminologies used

2.1 Intrinsic gain

The analog fiber optic page link has a conversion loss given by equation1. Due to the parallel combination of Rload and the output impedance only half of the detected photodiode current is available and hence the factor ½ is used in equation 1,2 and 4. The page link has a conversion loss of –33.45 dB and with an optical loss of 1 dB in the page link the total RF loss will be 35.45 dB. The conversion loss is also referred as intrinsic gain of a link. With better slope efficiency and reactive impedance matching at the laser and photodiode the loss can be reduced. VCSEL laser has higher slope efficiency compared to FP and DFB lasers and help in improving the conversion loss of the link. The intrinsic gain increases with an increase in optical power of an externally modulated link. A directly modulated page link does not shown any improvement in gain with increase in optical power.



2.2 Noise performance of the link
The total noise performance of an analog fiber optic page link depends on the individual noise contribution of various components in the link. Three major noise sources in an analog page link are Laser noise (RIN noise) generated at the laser, shot noise at the photodiode detection and thermal noise at the receiver circuit.

a. Laser noise
The laser noise arises from the random fluctuations in the intensity of the optical signal generated at the laser diode. The laser noise is measured directly at the transmitter and is referred to as relative intensity noise (RIN) in the laser diode specification. The RIN is the ratio of mean square amplitude of the noise fluctuations per unit bandwidth (<P2>) to the square of the DC optical power (Po2). The laser noise power Plaser in an analog fiber optic page link for direct detection is given by equation 2.