In this invited paper, we review the theoretical model and performance of an Interferometric Modulator with Phase modulating And Cavity-modulating Components (IMPACC). IMPACC has the highest reported SFDR (e.g., 132 dB-Hz) and offers additional advantages compared to other Mach-Zehnder Interferometer (MZI) based electro-optic modulators (e.g., MZI, ring-assisted MZI or RAMZI). The modulator is based on a unique combination of a RF driven phase-modulator (PM) and a ring resonator (RR) within a MZI interferometer. Both the PM and RR in the IMPACC are simultaneously driven by a RF signal of the same frequency, but not necessarily the same amplitude and phase. Here, we summarize the non-ideal and oftentimes detrimental effects such as: (1) RF bandwidth limitation due to free spectral range (FSR) of the RR, (2) RR waveguide loss, (3) deviation of RR coupling ratio from the ideal value, and (4) unbalanced MZ splitter/coupler on the performance of both IMPACC and RAMZI. We show that proper choice of RF power split ratio and RF phase for IMPACC compensate these negative effects and recover IMPACC’s ideal performance. Unlike RAMZI, this translates to higher device tolerance, added manufacturing flexibility, and superior modulator performance.
IMPACC (Interferometric Modulator with Phase-modulating and Cavity-modulating Components) is ultra-linear
optical consisting of a phase modulator and a ring resonator on different arms of a Mach Zehnder
interferometer (MZI). External control of the RF power split ratio from an input radio frequency (RF) signal
into the two separate arms of the interferometer has been shown to add (1) design flexibility, (2) the ability to
achieve high spurious free dynamic range (SFDR) of more than 130 dB, when compared to the single-ring
RAMZI (Resonator-assisted MZI) and (3) compensate parameter deviation due to manufacturing imperfection.
Our previous reports have assumed that the Optical power split ratio of the input optical signal into the two arm
of MZI is balanced with a 50:50 split ratio due to the optical splitter or optical coupler. Here, we investigate
three issues. First, we report the negative effect of unbalanced power of the input optical signal on the SFDR
performance of IMPACC. Second, we utilize the inherent compensate technique of IMPACC to counteract this
effect. Third, the power unbalanced effect is reported at high RF modulation frequency (23GHz) for three
different conditions of the ring resonator (RR) namely, critical coupling (CC), over coupling (OC), and under
coupling (UC). Lastly, we compare the performance of IMPACC to the single-ring RAMZI with traveling-wave
electrode design under sub-octave operations.
All-optical techniques for microwave and radio frequency (RF) signal processing has attracted considerable attention in
recent years. An important optical component in these all-optical signal processing techniques is the optical filter.
Tunable optical filters with a variety of transfer functions have been proposed. However, adjustability of the optical
filter transfer function is required to provide an extra degree of control. This adjustability of the shape of the transfer
function has not been addressed adequately in the literature. In this paper, we report on the theoretical basis for an all-fiber
based adjustable transfer function optical filter. In particular, we model the optical filter using FO-circuit transfer
matrices and Jones matrices to fully describe the state of polarization changes of the optical signals through the optical
filter. The filter is based on an all fiber Michelson Gires-Turnois interferometer (MGTI). The Gires-Turnois resonators
(GTRs) required for the formation of the MGTI are realized by pairs of fiber-loop mirrors (FLMs) in the two arms of the
Michelson interferometer. The optical reflectivity of the GTRs is control via adjustment of the polarization in the fiber
loop mirrors. We show that arbitrary transfer functions can be realized by adjusting the reflectivity of the FLMs as well
as the cavity length of the fiber based GTRs.
Almost all existing resonator-based linearized optical modulators suffer from an inherent RF
bandwidth narrowing that limit their applications and performance. Here, we show an ultra-linear optical
modulator, called IMPACC, (Interferometric Modulator with Phase-modulating And Cavity-modulating
Components) that has three unique RF bandwidth characteristics, which are not present in other resonator-based
linearized optical modulators. We demonstrate these unique characteristics by contrasting IMPACC's
performance to the Resonator-Assisted Mach Zehnder Interferometer (RAMZI) modulator. First, IMPACC can
enhance its linearized RF bandwidth up to ~ 20% of central RF frequency (compared to ~1% for RAMZI).
Second, unlike RAMZI, the RF bandwidth enhancement has a near-flat response characteristic across this
bandwidth window. Third, IMPACC can maintain an excellent spurious free dynamic range (SFDR) capability
(>130 dB at 1Hz bandwidth) across this window. The wide linear bandwidth capability is also demonstrated for
modulation frequencies up to 75GHz.
We demonstrate the superior RF spectrum characteristics of an ultra-linear modulator. This modulator
exhibits excellent linearity (Spurious Free Dynamic Range, SFDR ~ 133 dB at 1Hz bandwidth), wider
linearization bandwidth (an order of magnitude wider), and flatter RF modulation frequency dependence
characteristics (linearized 3dB bandwidth ~ 20% of central RF frequency) when compared with Resonator-
Assisted Mach Zehnder (RAMZ) modulators. The modulator is based on a traveling-wave electrode design, in
which a phase modulator (PM) is on the upper arm of a Mach Zehnder (MZ) interferometer and a ring resonator
(RR) is located on the lower arm. This modulator design is also shown to have flexible control of the power
split ratio of an RF input to both the PM and the RR, in addition to the RF phase bias between the RR and PM -
which enables better manufacturing tolerance.
Ultra-linear modulators are critical components for broadband access applications in analog fiber-optic
communication links, such as Radio-over-Fiber (RoF), Radio-over-Free Space (RoFS) and ultra-dense (cable
television) CATV. In our previous work, it was shown that an ultra-linear optical modulator - composed of a
phase modulator and ring resonator in a Mach-Zehnder (MZ) configuration, exhibited SFDR values > 130 dB.
In this paper, we introduce a new configuration based on a lumped electrode design, in which the phase
modulator (PM) is on one arm of the MZ and the ring resonator (RR) is located on the other arm. We also
carefully control and monitor the power split ratio of an RF input to both the PM and the RR, in addition to the
RF phase bias between the RR and PM. We report its unique and superior features such as superior linearity
(SFDR ~ 133 dB), modulation bandwidth extension (as much as 70%) over the ring resonator-assisted Mach
Zehnder design and high link gain. Lastly, we discuss the merits of the design flexibility, practicality and ease
of use of the new modulator.
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