Download External-cavity diode lasers provide absolute references for WDM

External-cavity diode lasers provide
absolute references for WDM testing
Successful WDM operation depends
he rapid growth in data traffic is
driving development of higher- on eliminating crosstalk between
capacity optical-fiber-transmis- the different wavelength channels. This
sion methods based on wave- requires that all the active and passive
length-division multiplexing system components operate within the
(WDM). In WDM, several lasers at dif- spectral window they were designed
for, without undue
ferent wavelengths
drift or chirp. At the
simultaneously
Michael Lang
manufacturing,
transmit separate
streams of data along a single fiber. At installation, and servicing/repair
the receiving end, the different wave- stages, this requirement creates a need
lengths are optically separated and indi- to check the wavelength performance
vidually detected. The transmission of key components such as lasers, filcapacity of an existing fiber link can, ters, and fiber gratings. Wavelength
therefore, be instantly increased by a meters, spectrometers, and spectrum
factor equal to the number of individual analyzers are important in this area.
wavelengths used. The rapid evolution However, while these tools may suffice
of WDM technology has created the for field testing of the latest WDM sysneed for laser sources with output that tems, more-accurate, absolute methods
is reliably locked to a known absolute are needed for use in quality control
wavelength (frequency) with
high precision.
Recently, the International
H12C14N
Telecommunications Union
(Arlington, VA) created a standard for 32-channel WDM
transmission that consists of 40
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wavelength channels, separated by only 100 GHz (0.8 nm),
H13C14N
eight of which are for overhead—signal management.
The close channel spacing is
necessary to ensure that all the
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channels fit under the highgain portion of the erbium12C H
2 2
doped fiber-amplifier gain
curve, centered around 1545
nm. Looking to the future,
higher-density WDM formats
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with 64 channels separated by
only 50 GHz (0.4 nm) are pre13
C2H2
dicted. And some researchers
are already experimenting
with channel spacings of less
than 2 GHz.
Absorbance
T
1.50
MICHAEL LANG is Product Manager
of Newport Corporation’s Tunable
Laser Diode products manufactured
in Boulder, Colorado.
e-mail: [email protected]
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1.52
1.53
1.54
1.55
1.56
and testing laboratories, as well as for
the research and development effort to
produce future generations of WDM
technology.
Absolute wavelength metrics
One method of providing an absolute
yet compact wavelength standard is to
lock the output of a diode laser to an
atomic or molecular spectral line.
Indeed, such absorption and emission
lines have long been used as absolute
wavelength standards in several fields.
Until recently, however, there were
almost no spectral standards in the
1550-nm spectral region. This absence
was partly because there are few strong
spectral absorptions in this region and
also because there was little commercial
demand for such absolute standards.
The explosive growth of WDM
technology has changed this situation.
In the past two years, tremendous advances have been made
in calibrating the spectra of both
acetylene (C2H2) and hydrogen
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cyanide (HCN), because these
molecules have a combination
of vibrational overtone absorption lines that span the entire
1550-nm erbium gain region
(see Fig. 1). The first atlas of
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acetylene lines has now been
published by the Tokyo Institute
of Technology,1 which claims
150-kHz absolute precision in
the frequency domain—traceable to the cesium atomic-clock
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frequency via an absorption line
of rubidium. Also, researchers at
NIST (Boulder, CO) have characterized the small, yet measurable, pressure shifts (up to 200torr pressure) of these lines.
1.57
Wavelength (µm)
FIGURE 1. Vibrational overtone bands of hydrogen cyanide (HCN) and
acetylene (C2H2) together span the erbium gain region.
Third-derivative locking
Unlike the single atomic lines
of neon and krypton, acetylene
and hydrogen cyanide each
Reprinted from the June 1998 edition of LASER FOCUS WORLD
Copyright 1998 by PennWell
Design and Applications
have a comb of spectral lines through- drifts in the absorption measurement. A
out the spectral region of interest. feedback loop actively adjusts the dc
Equally important, their absorption position of the tuning mirror to maintain
lines can be detected in simple, compact a zero third-derivative signal. Using this
cells with no need for noisy, delicate approach, the compact, portable source
discharge cells. For these reasons, Envi- has a long-term stability of ±1 MHz, or
ronment Optical Sensors Inc. (ESOI; one part in 108.
Boulder, CO) chose acetylene and
There are many rotational lines in the
hydrogen cyanide absorption lines as C 2 H 2 or HCN spectra to which the
the basis for a series of rugged reference WDM reference source can be stabisources for WDM work (see Fig. 2).
lized. In practice, the end user selects
The emitting element is a laser diode from this list of absolute wavelengths—
with a highly efficient (reflectivity just as RF engineers have selected the
<10–5) antireflection
coating on its front
facet. This diode
Tuning mirror
operates
in
a
Piezo translation
Littman-Metcalf
Collimated
Fiber
external
cavity,
laser diode
Out
where the position
of the tuning mirror
Diffraction grating
Beamsplitter
determines the output wavelength. The
Beamsplitter
collimated output of
this cavity is couInGaAs
Absorption cell
photodetector
pled into a FC/APC
InGaAs
(fiberconnecphotodetector
tor/angled physical
connector) fiber
coupler. A beam- FIGURE 2. WDM reference source from Environmental Optical Sensors
splitter, positioned Inc. consists of an external-cavity diode laser locked to an integral absorpbefore this coupler, tion cell containing either acetylene (C2H2) or hydrogen cyanide (HCN).
splits off 5% of the
collimated laser light, which allows a frequency of crystal reference oscillators
normalized (dual-channel) measure- for many years. The angle of the tuning
ment of the absorption in a cell contain- mirror is then permanently set at the
ing C2H2 or HCN, by a pair of indium factory. To further enhance the econogallium arsenide (InGaAs) photo- my of this approach, several active
heads (at different wavelengths) can be
detectors.
Both C 2 H 2 and
HCN have linewidths on the order
WDM
of 1 GHz, due to
reference
source
Doppler broadening.
To accurately lock to
Source
Fast InGaAs
line center, a techunder test
detector
Spectrum analyzer
nique called thirdor frequency counter
derivative locking is
used. A small (about
1 MHz) frequency FIGURE 3. Wavelength-division-multiplexing source is calibrated by meadither is applied to suring a heterodyne beat frequency using a high-speed InGaAs detector.
the laser diode by the
piezoelectric actuator that translates the used interchangeably with the same
tuning mirror. The detection electronics electronic controller.
then take the third derivative of the normalized absorption signal in the fre- Heterodyne source testing
quency domain. This third derivative These reference sources can be used to
has the twin advantages of being calibrate the accuracy of important
extremely steep as it passes through zero WDM diagnostic tools such as waveand being immune to any background length meters and high-resolution spec-
trometers, which are then used to check
the passive components such as fiber
gratings and receiver filters. These diagnostic tools also can measure the wavelength of the stabilized distributed-feedback diodes and fiber-grating controlled
laser-diode sources used in WDM. The
most accurate measurements of these
sources, however, are made by direct
measurement against the WDM reference source. The laser being measured
can then be traced back to the cesium
clock with ±1 MHz precision.
In this type of heterodyne measurement, the output of the WDM reference
source and the test laser are coupled
into a high-speed InGaAs photodetector
(see Fig. 3). The resulting beat frequency
is measured with a high-accuracy spectrum analyzer or frequency counter.
How fast must the photodetector be?
When answering this question, another
advantage of C2H2 and HCN becomes
apparent. The rotational line spacing in
both molecules is less than 30 GHz. It is
possible, therefore, to select a WDM reference source within roughly 15 GHz of
a given frequency. Detecting beat frequencies of 15 and even 30 GHz is
straightforward, because there are offthe-shelf InGaAs photodetectors capable of 50-GHz detection and custom
devices up to 100 GHz. Larger intervals
can be calibrated by “daisy-chaining”
the photodetectors. The WDM reference
source is used to calibrate one laser at
an interval of less than 50 GHz, which is
then used to calibrate another laser at an
additional interval of less than or equal
to 50 GHz, and so on. Passing along a
frequency standard from one source to
another in this type of linkage has long
been the standard method by which all
calibrated spectral lines are ultimately
referenced back to the cesium atomic
clock through intermediate calibrated
spectral features.
The overall backbone capacity of
fiberoptic telecommunications systems
is expected to double approximately
every 3.7 months. This growth rate
makes development of stabilized, accurate, absolute frequency references a
technological imperative.
❏
REFERENCE
1. K. Nakagawa et al., “Accurate optical frequency atlas of the 1.5 µm bands of acetylene,”
JOSA, B, vol. 13, 2708 (1996).
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