1 C-band Four Channel Optical
De-Multiplexer
The rapid traffic growth within the local area networks has
tremendously increased the deployment of Wavelength Division
Multiplexing (WDM) optical links in short reach applications. With the
increasing market size in data centre applications, optical
de-multiplexers are being widely used as a key element in WDM
transceivers. Optical de-multiplexers can be implemented using various
ways but the Mach-Zehnder Interferometer (MZI) hs been the most
preferred choice due to the ease of fabrication in the CMOS platform,
the size and the insertion loss. Here, we present a four channel optical
de-multiplexer filter which is designed using the building blocks
developed by VLC Photonics, based on the silicon nitride (SiN)
technology, established and running at CNM (Centro Nacional De
Microelectronica). SiN is a common material in CMOS fabs which provides
moderate index contrast and relatively low thermal oxide (TO)
coefficient, thus enables the reduction of process sensitivity and
temperature dependence.
2 Theory
MZI based optical filters are widely used in the transceivers that
separate an incoming spectrum into two complementary set of periodic
spectra as even and odd channels. In this article we showcase a 1×4
optical de-multiplexer filter, shown in figure 1, which comprises of two
stages. Stage 1 consists of an asymmetric Mach-Zehnder Interferometer
(MZI), made of two 50/50 splitters and the waveguides forming the delay
length ((Delta L)). The light is
injected into this stage. Similarly, stage 2 has two arms, each
consisting of asymmetric MZI. The delay lengths in both the arms of
stage 2 is half the delay length ((Delta
L)) in the stage 1. The even and odd frequency channels are
separated in stage 1. The odd frequency channels are sent to the upper
arm of the stage 2 and the even frequency channels are sent to the lower
arm, where they are further separated out. The delay length ((Delta L)) of the stage 1 MZI is set based
on the targeted channel spacing, also known as Free Spectral Range
(FSR), of 200 GHz in the C-band, using following relation: –
[begin{equation}
Delta L = frac{lambda^2}{2 n_g (deltalambda)}
end{equation}]
where,
(lambda) is the central
wavelength of operation
(n_g) = group index of the
waveguide forming the delay length
(deltalambda) = Channel spacing
or FSR
3 Design
The circuit of a 1×4 channel de-multiplexer is build in the
simulation tool, known as S-edit from Siemens Tanner. The circuit can be
seen in the figure 2, which comprises of the following devices from the
VLC-CNM PDK: cnmMMI1x2DEBB_TE (1x2_50/50_MMI), cnmMMI2x2BB_DE
(2x2_50/50_MMI) and cnmWaveguideDE (deep etched waveguide). These
devices are connected to the other opto-electronic devices from the
OptiSPICE library to complete the circuit.
Four lasers (L1, L2, L3, L4) from the OptiSPICE library send four
different frequency channels (193.451 THz, 193.652 THz, 193.85 THz,
194.052 THz), respectively, into the stage 1 of the 1×4 de-multiplexer
optical filter. The MZI of stage 1 is build using the devices:
cnmMMI1x2DEBB_TE, cnmMMI2x2BB_DE and cnmWaveguideDE, where the length of
the upper waveguide is set to 497.518 um and for the lower waveguide is
set to 100 um. This is further connected to the MZIs of the two arms of
stage 2, which are also built using the same devices as MZI of stage 1.
The length of the upper waveguide, of the MZI of upper arm, is set to
301.901 um and the lower waveguide is set to 100 um. Similarly, the
length of the upper waveguide, of the MZI of the lower arm, is set to
305.04 um and the lower waveguide is set to 100 um. The waveguide
lengths of the MZIs in the circuit are set to match the delay lengths
((Delta L)), as shown in the figure
1.
Lasers are connected to the bit voltage source, each sending in
random bit pattern which are combined using the device cnmMMI1x2DEBB_TE
before entering de-multiplexer filter. Each output of the de-multiplexer
filter is connected to the photodiode to detect the signal. This signal
passes through a lowpass electrical filter to evict any unwanted noise,
which is further measured using the probe. The probe N_5 and N_6 are
connected at the output of the upper arm whereas, the probes N_7 and N_8
are connected to the output of the lower arm.
4 Simulation and Results
A time domain transient analysis is carried out for this circuit with
the stop time of 1 ns and the time-step of 0.1 ps. The rise and fall
time of the bit source is 10 ps with the pulse width of 100 ps. The
vbit_1 (bit pattern = 0101010101) is connected to laser1, vbit_2 (bit
pattern = 00011011100) is connected to laser2, vbit_3 (bit pattern =
1110000010) is connected to laser3 and vbit_4 (bit pattern = 0110001111)
is connected to laser4. The waveform viewer showing four different
frequency channels, measured at the output using the photodetectors, can
be seen in figure 3.
The transmission plot above shows the odd frequency channels coming
from laser1 and laser3 are measured using the probes N_5 and N_6,
respectively, at the output of the upper arm. Similarly, the even
frequency channels coming from the laser2 and laser4 are measured using
the probes N_7 and N_8, respectively, at the output of the lower
arm.
The AC analysis was also carried out for the filter where the
frequency was swept from 192.4 THz to 193.4 THz and the output
transmission response is shown in figure 4. The measured FSR of the
filter is 200 GHz (1.6 nm) in the C-band (near 1550 nm). The crosstalk
with the neighboring channel is measured to be around ~ 20dB and the
3-dB bandwidth is approximately 178.71 GHz (1.432 nm). Thus, optical
de-multiplexer filter with the desired FSR is designed and simulated.
Which obtains the split odd frequency channels in the upper arm of the
stage2 and even frequency channels in the lower arm of stage2.