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Chapter 5: Transmitter phase noise control subsystem
The dynamically tunable mm-wave optical transmitter (DTMOT) is conceptually
an ideal voltage controlled oscillator with high-speed and very large tuning range.
However, its free running phase noise is high because the heterodyning process directly
converts the laser phase noise into the mm-wave subcarrier domain. In order to generate a
low noise subcarrier signal, a phase noise control subsystem must be introduced. In this
chapter, we’ll discuss two distinctive yet conceptually related subsystems designed for
the dynamic tunable transmitter phase noise control, namely (1) digital frequency
synthesizer (section ), and (2) delay line optical frequency locking loop (OFLL)
(section ). The emphasis is placed on the OFLL subsystem due to its superior
performance at higher frequencies. Finally, we will present the experimental results
regarding noise suppression (section ).
Digital frequency synthesizer
We first discuss the digital frequency synthesizer phase noise control subsystem.
A simplified synthesizer diagram is depicted in Fig. .
122
DTMOT Loop Filter
PD Prescaler N Phase
Counter Detector
Modulation
Signal Optional 2nd
reference: f
ref2 Reference R
Output fref1 Counter
Figure : The digital frequency synthesizer
Except for the fact that the voltage-controlled oscillator is replaced by the
DTMOT, the digital frequency synthesizer phase noise control subsystem is denticali to a
conventional digital synthesizer [52], the output of which is:
P× N
f = × f + f ()
out R ref 1 ref 2
where P is the prescaler dividing number, and N and R are the counting numbers of the N
counter and R counters, respectively. In the following, we’ll briefly discuss the issues that
are pertinent to the digital synthesizer phase noise control system.
Synthesizer subsystem phase noise
The noise sources of the output