Note: Descriptions are shown in the official language in which they were submitted.
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PHOTONIC-CHIP-BASED OPTICAL SPECTRUM ANALYZER
FIELD OF THE INVENTION
[0001] The present invention relates to an optical spectrum analyzer
(OSA). More
particularly, the present invention relates to a photonic-chip-based OSA.
BACKGROUND OF THE INVENTION
[0002] Optical spectrum analyzers (OSAs) are used to measure optical
spectra in a
measurement wavelength (or frequency) range, typically, by measuring optical
power as a
function of wavelength (or frequency). Most OSAs use optical filters to
resolve each wavelength
in the measurement wavelength range. For example, a chip-scale OSA using a
Fabry-Perot filter
with a variable mirror spacing and a nanooptic filter array is described in
U.S. Patent No.
7,426,040 to Kim et al., filed on August 19, 2005. Many OSAs use tunable
optical filters that
can be tuned to resolve each wavelength in the measurement wavelength range.
[0003] In photonic chips, ring resonator systems with various
configurations may be used
as tunable optical filters. For example, double-ring resonator systems
suitable for use as tunable
optical filters for demultiplexing applications are described in "Theoretical
Analysis of Triple-
Coupler Ring-Based Optical Guided-Wave Resonator" by Barbarossa et al.,
Journal of
Lightwave Technology, 13, 148-157, 1995; in "Vernier Operation of Fiber Ring
and Loop
Resonators" by Ja, Fiber and Integrated Optics, 14, 225-244, 1995; and in S.
Suzuki, K. Oda,
and in "Integrated-Optic Double-Ring Resonators with a Wide Free Spectral
Range of 100 GHz"
by Hibino, Journal of Lightwave Technology, 8, 1766-1771, 1995. The use of two
cascaded
ring resonators as a sensor in a photonic chip has also been described in
"Experimental
characterization of a silicon photonic biosensor consisting of two cascaded
ring resonators based
on the Vernier-effect and introduction of a curve fitting method for an
improved detection limit"
by Claes et al., Optics Express, 18, pp. 22747-22761, 2010.
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SUMMARY OF THE INVENTION
[0004] Accordingly, an aspect of the present invention relates to an
optical spectrum
analyzer (OSA) for measuring an optical spectrum of an input optical signal in
a measurement
wavelength range, the OSA comprising: a modulator for modulating the input
optical signal by
applying a dither modulation to facilitate detection and noise rejection; an
integrated optical filter
that is sequentially tunable to selectively transmit each wavelength of the
modulated optical
signal in the measurement wavelength range; and a photodetector for
sequentially detecting each
wavelength of the modulated optical signal in the measurement wavelength range
to provide a
representative output electrical signal.
[0005] Another aspect of the present invention relates to a method of
measuring an
optical spectrum of an input optical signal in a measurement wavelength range,
the method
comprising: providing an OSA comprising: a modulator for modulating the input
optical signal
by applying a dither modulation to facilitate detection and noise rejection;
an integrated optical
filter that is sequentially tunable to selectively transmit each wavelength of
the modulated optical
signal in the measurement wavelength range; and a photodetector for
sequentially detecting each
wavelength of the modulated optical signal in the measurement wavelength range
to provide a
representative output electrical signal; modulating, by means of the
modulator, the input optical
signal by applying a dither modulation to facilitate detection and noise
rejection; sequentially
tuning the integrated optical filter to selectively transmit each wavelength
of the modulated
optical signal in the measurement wavelength range; and sequentially
detecting, by means of the
photodetector, each wavelength of the modulated optical signal in the
measurement wavelength
range to provide a representative output electrical signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Numerous exemplary embodiments of the present invention will now be
described in greater detail with reference to the accompanying drawings
wherein:
[0007] FIG. 1A is a schematic illustration of an optical spectrum analyzer
(OSA);
[0008] FIG. 1B is a schematic illustration of two cascaded tunable ring
resonators in the
OSA of FIG. 1A;
[0009] FIG. 2 is a schematic illustration of two coupled tunable ring
resonators;
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[0010] FIG. 3 is a plot of transmission spectra of a first tunable ring
resonator, a second
tunable ring resonator, and a ring resonator system;
[0011] FIG. 4 is a plot of transmission spectra of a tunable ring resonator
of as a function
of second voltage; and;
[0012] FIG. 5 is a plot of an output of an OSA when used to measure optical
spectra of
input optical signals from a tunable laser tuned to wavelengths of 1540 nm,
1550 nm, 1560 nm,
and 1570 nm, respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0013] We describe herein a photonic-chip-based optical spectrum analyzer
(OSA) for
measuring an optical spectrum of an input optical signal in a measurement
wavelength (or
frequency) range, typically by measuring optical power as a function of
wavelength (or
frequency) for the input optical signal. The input optical signal may be a
known or unknown
optical signal.
[0014] In some embodiments, the measurement wavelength range encompasses the C-
band, i.e., a wavelength range of about 1530 nm to about 1565 nm. Some
embodiments of the
OSA may be used for sensing or for optical channel monitoring. The OSA may
also be used
within an optical network.
[0015] With reference to FIG. 1, an exemplary embodiment of the OSA 100
comprises a
photonic chip, which includes an integrated modulator 110, an integrated
optical filter
comprising a ring resonator system 120, and an integrated photodetector 130.
Although the
integrated optical filter in the illustrated embodiment comprises a ring
resonator system 120, in
other embodiments, the integrated optical filter could be any suitable type of
integrated optical
filter that is tunable over the measurement wavelength range. In yet other
embodiments, the
integrated optical filter could be replaced by a non-tunable element that
provides different optical
paths for different wavelengths, such as a fixed-wavelength filter, an arrayed
waveguide grating
(AWG), or an echelle grating. Such a non-tunable element could be used
together with an array
of photodetectors.
[0016] In the illustrated embodiment, the modulator 110, the ring resonator
system 120,
and the photodetector 130 are all monolithically integrated on the photonic
chip. In other
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embodiments, an off-chip modulator and/or an off-chip photodetector could be
used. The
photonic chip may be fabricated using any suitable material system. Typically,
the photonic chip
is fabricated using a silicon-on-insulator (SOI) material system.
Alternatively, the photonic chip
could be fabricated using a silica-on-silicon material system, a silicon
nitride material system, a
silicon oxynitride material system, or a material system, for example.
[0017] The OSA 100 also comprises a signal generator 140, also known as a
pattern
generator, a lock-in amplifier 150, a voltage sweep module 160, and a clock
170. In some
embodiments, the voltage sweep module 160 and the clock 170 are implemented in
a controller,
e.g., a microcontroller or a computer.
[0018] In some embodiments, the signal generator 140 and/or lock-in
amplifier 150 can
be replaced by microelectronic chips, in which dither signals can be generated
in digital and
converted to analog through a digital-to-analog converter (DAC) at a certain
frequency, and the
same frequency can be extracted from the integrated photodetector 130 with an
analog-to-digital
converter (ADC) and digital filtering.
[0019] The input optical signal is launched into the integrated modulator
110, which
modulates the input optical signal by applying a dither modulation to
facilitate detection and
noise rejection, thereby improving the signal-to-noise ratio (SNR). In the
embodiment of FIG. 1,
the integrated modulator 110 is a Mach-Zehnder interferometer (MZI), which is
balanced to
allow wideband performance, so that the integrated modulator 110 is able to
modulate the input
optical signal over the entire measurement wavelength range. In general, the
integrated
modulator can be any kind of electro-optical modulator, provided that it
performs over the entire
measurement wavelength range. In some embodiments, the integrated modulator
can be an
electro-absorption modulator, a ring modulator, an amplitude modulator, or a
phase modulator.
If the integrated modulator is a phase modulator, a downstream detector that
has a phase-to-
amplitude converter may be required. In an exemplary embodiment, a phase
modulator may be
followed by a wavelength discriminator, followed by a differential delay line
or an unbalanced
MZI, followed by a pair of photodetectors.
[0020] The signal generator 140 simultaneously provides a modulation
electrical signal
to the integrated modulator 110 and to the lock-in amplifier 150. The
integrated modulator 110
modulates the input optical signal in response to the modulation electrical
signal, and the lock-in
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amplifier 150 uses the modulation electrical signal to extract the output
electrical signal from the
integrated photodetector 130 from noise, e.g., environmental noise.
[0021] The modulated optical signal then enters the ring resonator system
120, which
includes at least two tunable ring resonators. The tunable ring resonators
are, typically, formed
as waveguide loops that are circular, oval, or racetrack-shaped. The ring
resonator system 120
may also include at least two integrated heaters, which may be formed as
sections of doped
waveguide inside each tunable ring resonator, or as metal resistors on top of
each tunable ring
resonator.
[0022] In the embodiment of FIG. 1, the ring resonator system 120 includes
two
cascaded tunable ring resonators, a first tunable ring resonator 121 and a
second tunable ring
resonator 122. An input waveguide 123 is coupled to the first tunable ring
resonator 121, an
intermediate waveguide 124 is coupled to the first tunable ring resonator 121
and the second
tunable ring resonator 122, and an output waveguide 125 is coupled to the
second tunable ring
resonator 122. The first tunable ring resonator 121 and the second tunable
ring resonator 122 are
cascaded via the intermediate waveguide 124. The ring resonator system 120
also includes a
first integrated heater 126 for heating the first tunable ring resonator 121
in response to a first
voltage, and a second integrated heater 127 for heating the second tunable
ring resonator 122 in
response to a second voltage. An on-chip temperature sensor, such as an
integrated temperature
sensor of the type disclosed in U.S. Patent Application Publication No.
2016/0124251 to Zhang
et al., published on May 5, 2016, may be used to sense the temperature of each
tunable ring
resonator.
[0023] With reference to FIG. 2, in an alternative embodiment, the ring
resonator system
220 includes two coupled tunable ring resonators, a first tunable ring
resonator 221 and a second
tunable ring resonator 222. An input waveguide 223 is coupled to the first
tunable ring resonator
221, and an output waveguide 225 is coupled to the second tunable ring
resonator 222. The first
tunable ring resonator 221 and the second tunable ring resonator 222 are
directly coupled. The
ring resonator system 220 also includes a first integrated heater 226 for
heating the first tunable
ring resonator 221 in response to a first voltage, and a second integrated
heater 227 for heating
the second tunable ring resonator 222 in response to a second voltage.
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[0024] In other embodiments, the ring resonator system may include more
than two
tunable ring resonators in a cascaded or coupled configuration, each provided
with an integrated
heater.
[0025] With reference again to FIG. 1, the first tunable ring resonator
121 and the second
tunable ring resonator 122 serve as tunable optical filters. The transmission
spectrum of the first
tunable ring resonator 121 includes a first set of resonance peaks, which have
a first spectral
linewidth, i.e., a full width at half maximum (FWHM), and which are separated
by a first free
spectral range (FSR). Likewise, the transmission spectrum of the second
tunable ring resonator
122 includes a second set of resonance peaks, which have a second spectral
linewidth and which
are separated by a second FSR.
[00261 When a first voltage is applied to the first integrated heater 126
to heat the first
tunable ring resonator 121, the first set of resonance peaks shift
collectively, but the first FSR
does not change. When a second voltage is applied to the second integrated
heater 127 to heat
the second tunable ring resonator 122, the second set of resonance peaks shift
collectively, but
the second FSR does not change. By adjusting the first voltage applied to the
first integrated
heater 126, the first tunable ring resonator 121 can be tuned, and by
adjusting the second voltage
applied to the second integrated heater 127, the second tunable ring resonator
122 can be tuned.
Typically, two power supplies, e.g., direct current (DC) power supplies, are
used to apply the
first voltage to the first integrated heater 126 and the second voltage to the
second integrated
heater 127, respectively.
[0027] Usually, the FSR of a single tunable ring resonator is small,
resulting in a narrow
tunable range, e.g., a tunable range much narrower than the C-band. In order
to achieve a larger
FSR and a wider tunable range, e.g., a tunable range encompassing the entire C-
band, two or
more tunable ring resonators having4s1ight1y different radii may be cascaded
or coupled to
exploit the Vernier effect, as explained hereinbelow.
[0028] In the embodiment of FIG. 1, the first tunable ring resonator 121
and the second
tunable ring resonator 122 have different radii, e.g., 8 pm and 10 1.1m, and,
therefore, different
FSRs. Typically, on an SOI platform, the first tunable ring resonator 121 and
the second tunable
ring resonator 122 have radii of about 5 lam to about 20 p.m. When the input
optical signal is
launched into the ring resonator system 120, via the input waveguide 123, the
input optical signal
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is first filtered by the first tunable ring resonator 121. The output from the
first tunable ring
resonator 121 is then coupled into the second tunable ring resonator 122, via
the intermediate
waveguide 124 in the embodiment of FIG. 1, and filtered by the second tunable
ring resonator
122. The output from the second tunable ring resonator 122 is received via the
output waveguide
125 and detected by the integrated photodetector 130. When an absolute
difference between the
first FSR and the second FSR is large compared to the first linewidth and the
second linewidth,
the transmission spectrum of the ring resonator system 120 will include peaks
where the first and
second sets of resonance peaks coincide, but non-coincident peaks in the first
and second sets of
resonance peaks will be suppressed.
[0029] For example, with reference to FIG. 3, if the center peaks, a2 and
b2, in the
transmission spectrum 310 of the first tunable ring resonator and the
transmission spectrum 320
of the second tunable ring resonator are aligned, the ring resonator system
will output a
transmission spectrum 330 including the center peak, in which the non-aligned
peaks are
suppressed. Moreover, peak a2 does not necessarily have to be aligned with
peak b2, but can be
tuned to align with peak b3 or bl. Likewise, peaks al and a3 can also be
aligned with peaks bl,
b2, and b3. Accordingly, the tunable range of the ring resonator system may be
dramatically
increased by the Vernier effect.
[0030] With reference again to FIG. 1, because of the Vernier effect, the
ring resonator
system 120 has an extended FSR corresponding to a least common multiple of the
first FSR and
the second FSR, i.e., a smallest number that is a multiple of both the first
FSR and the second
FSR. The first FSR and the second FSR are selected to ensure that the least
common multiple of
the first FSR and the second FSR is greater than the measurement wavelength
range of the OSA
100, and to ensure that the absolute difference between the first FSR and the
second FSR is
greater than the first spectral linewidth and greater than the second spectral
linewidth. Typically,
the least common multiple of the first FSR and the second FSR is greater than
about 50 nm, and
the absolute difference between the first FSR and the second FSR is greater
than about 0.5 nm.
[0031] Accordingly, the transmission spectrum of the ring resonator system
includes only
one peak in the measurement wavelength range of the OSA 100 for a given pair
of values of the
first voltage and the second voltage. By cooperatively adjusting the first and
second voltages, by
means of the voltage sweep module 160, the peak can be shifted in wavelength
to scan over the
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measurement wavelength range. In other words, the ring resonator system 120
can be tuned to
resolve each wavelength in the measurement wavelength range.
[0032] Thus, when input light is launched into the ring resonator system
120, the ring
resonator system 120 is sequentially tunable to selectively transmit each
wavelength of the input
light in the measurement wavelength range of the OSA 100 by cooperatively
tuning the first
tunable ring resonator 121 and the second tunable ring resonator 122.
Typically, the first tunable
ring resonator 121 and the second tunable ring resonator 122 are pre-
calibrated by measuring
transmission spectra of the first tunable ring resonator 121 as a function of
the first voltage, and
by measuring transmission spectra of the second tunable ring resonator 122 as
a function of the
second voltage. An absolute wavelength standard or a laser of known wavelength
may be used
as a wavelength reference. Pairs of values of the first voltage and the second
voltage that result
in coincident resonance peaks at each wavelength in the measurement wavelength
range can be
identified. Thereby, pairs of values of the first voltage and the second
voltage that result in
selective transmission by the ring resonator system 120 at each wavelength in
the measurement
wavelength range can be predetermined.
[0033] For example, with respect to FIG. 4, a tunable laser was used to
measure
transmission spectra of an exemplary embodiment of a tunable ring resonator in
the
measurement wavelength range as a function of voltage. Transmission spectra
were collected
with voltage steps of 20 mV in a voltage range of 0 V to 9.3 V. The resonance
peaks collectively
shifted by a wavelength step of about 0.02 nm per voltage step for a total
wavelength shift of
about 12 nm over the voltage range.
[0034] With reference again to FIG. 1, once calibrated, the OSA 100 is
programmable for
real-time measurement of optical spectra. The voltage sweep module 160
sequentially adjusts
the first voltage and the second voltage to the predetermined pairs of values
of the first voltage
and the second voltage, and thereby tunes the ring resonator system 120 to
scan the measurement
wavelength range. The clock 170 synchronizes the integrated photodetector 130
with the voltage
sweep module 160, so that the integrated photodetector 130 sequentially
detects each wavelength
of the optical signal received from the ring resonator system 120 as the
measurement wavelength
range is scanned. The integrated photodetector 130 provides a representative
output electrical
signal for each wavelength, from which the optical spectrum can be re-formed.
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[0035] For example, with respect to FIG. 5, an exemplary embodiment of an
OSA was
used to separately measure optical spectra of input optical signals from a
tunable laser tuned to
wavelengths of 1540 nm, 1550 nm, 1560 nm, and 1570 nm, respectively. To
measure each
optical spectrum, the ring resonator system was sequentially tuned to
selectively transmit each
wavelength of the input optical signal in the measurement wavelength range,
with a wavelength
step of about 0.02 nm, by sequentially adjusting the first voltage and the
second voltage to
predetermined pairs of values of the first voltage and the second voltage.
Each optical spectrum
includes a single peak at the laser wavelength. The resolution of the OSA is
about 0.1 nm.
[0036] The present disclosure is not to be limited in scope by the
specific embodiments
described herein. Indeed, other various embodiments of and modifications to
the present
disclosure, in addition to those described herein, will be apparent to those
of ordinary skill in the
art from the foregoing description and accompanying drawings. Thus, such other
embodiments
and modifications are intended to fall within the scope of the present
disclosure. Further,
although the present disclosure has been described herein in the context of a
particular
implementation in a particular environment for a particular purpose, those of
ordinary skill in the
art will recognize that its usefulness is not limited thereto and that the
present disclosure may be
beneficially implemented in any number of environments for any number of
purposes.
