F. Other Applications
In addition to the several important applications men-
tioned above, there are many other applications for inte-
grated ODLs in different areas. They can be used to build
an integrated optical gyroscope. For instance, the gyro-
scope in Ref. [
28] uses a 10-m-long single-layer Si
3
N
4
waveguide. The waveguide spacing is 50 μm, and the
entire ODL occupies an area smaller than 6.5 cm
2
.An
integrated tunable ODL loaded in a Mach–Zehnder
interferometer (MZI) works as a good receiver for
differential phase shift keying (DPSK) optical transmis-
sion systems
[29]
. The continuous control of the MZI
delay allows the optimization of the receiver performance
for the detection of DPSK signals at variable symbol
rates. Optoelectronic implementation of reservoir com-
puting needs a single nonlinear node and a delay line
as the key elements
[30]
. The bio-inspired approach is suf-
ficiently fast for real-time information processing, com-
parable to state-of-the-art digital implementations.
Other than PAAs and microwave photonic filters, the
ODLs have a rich use in processing microwave s ignals.
For instance, it can be employed in synthesizing of
reconfigurable radio-frequency arbitrary waveforms
[31]
.
The integrated on-chip optical delay elements offer an
approach to accurately manipulating individual radio-
frequency waveforms with higher speed and lower timing
jitter than electronic methods.
3. DESIGN CONSIDERATIONS
The design of an integrated ODL needs to take into con-
sideration the specific applications. It is quite challenging
to get optimal performances in all aspects. Usually, a com-
promise has to be made in order to reach a certain perfor-
mance. In general, the following several basic metrics need
to be considered in designing an ODL.
A. Delay Tuning Range
The delay tuning range represents the difference between
the minimum and the maximum delays that can be
achieved. The maximum delay is usually limited by wave-
guide loss, group delay dispersion, or chip size. The mini-
mum delay determines the minimum buffering capacity of
the ODL. A nanosecond tuning range is achievable in most
integration platforms, and this tuning range could already
satisfy a lot of applications.
B. Delay Tuning Resolution
The requirement for delay tuning resolution is application
dependent. For example, in phased array radar, the delay
tuning resolution is related to the angle scanning step dur-
ing beam steering. When used as an optical buffer in op-
tical communication systems, it is related to the data rate
of the system. A lot of ODL structures can provide con-
tinuous delay tuning, as long as the driving voltage is con-
tinuously varied. In practice, the delay fluctu ation due to
the drift of driving voltage and chip temperature may af-
fect the attainable resolution.
C. Optical Bandwidth
In multiple ODL structures, the optical delay and band-
width product is a constant value. A large delay is only
obtained with the sacrifice of its bandwidth. A high-speed
optical signal occupies a large bandwidth. The delay band-
width needs to be large enough to ensure that the signal
after ODL is not significantly distorted. Moreover, a
broadband ODL can support WDM transmission, provid-
ing the multi-channel parallel processing capacity.
D. Insertion Loss
Low loss is the basic requirement for an ODL in most ap-
plications. The loss mainly comes from the waveguide
propagation loss in passive ODLs. Tuning may bring
additional loss either from the material absorption loss
(e.g., free-carrier absorpti on) or from the structural
loss (e.g., optical switches) in actively tunable ODLs. If
the loss is too large, the optical signal must be boosted
by using an optical amplifier in subsequent transmission,
which increases the complexity and lowers the signal-
to-noise ratio.
E. Power Consumption
One of the advantages of integrated devices over tradi-
tional discrete devices lies in their low power consumption.
In optical routers or OBFNs, a large number of tunable
ODLs are required, so the power consumption of the delay
line will accumulate to a considerably large level, which in
turn will affect its stability due to thermal crosstalk. The
ODL therefore should be designed to have low tuning
power consumption.
F. Size and Weight
In photonic devices, the size determines its integration
density. The smaller the size, the better the ODL can
be integrated with other devices in convenience and flex-
ibility. The ODL device footprint is affected by multiple
factors like minimum bending radius, maximum delay, de-
lay tuning step, control electronics, and optical and ther-
mal isolation.
4. IMPLEMENTATION METHODS
The tunable ODL plays a key role in all of the above-
mentioned applications. Several schemes for realizing
the tunable ODL can be obtained by examining light
transmission in a dielectric waveguide. Assuming that
light enters a waveguide with a prop agation distance of
L, and the effective refractive index of the waveguide is
n
eff
ðωÞ, the phase change of the lightwave is given by
ϕðωÞ¼−
2π
λðωÞ
n
eff
ðωÞL: (1)
Thus, the grou p delay suffered by the lightwave is
t
g
¼ −
∂ϕðωÞ
∂ω
¼
L
c
n
eff
ðωÞþω
∂n
eff
ðωÞ
∂ω
: (2)
It can be seen from Eq. (
2) that there are three methods
to tune the group delay:
COL 16(10), 101301(2018) CHINESE OPTICS LETTERS October 10, 2018
101301-4