|Tunable Lasers and Fiber-Bragg-Grating
|Marg Wippich and Kathy Li Dessau
For years, the tunable laser remained confined to the laboratory
as a spectroscopist’s tool to investigate the spectral characteristics
of atoms and molecules. Today, it is being tested in many industrial
applications, including optical remote sensing, where laser-based
systems can provide improved performance over electronic means of
measuring strain, temperature, and pressure.
One reason for the movement of tunable lasers into uses beyond
the laboratory is the external-cavity diode laser (ECDL). Before
the ECDL, the workhorse of the tunable-laser market was the organic
liquid-dye laser—a cumbersome system that requires an expensive
pump laser and messy liquid dyes. In the late 1970s and early 1980s,
Michael Littman, Harold Metcalf, and Karen Liu pioneered broadly
tunable ECDLs, which were considerably less complex than their forerunners.
The gain medium of an ECDL consists of a laser diode chip in which
one of its end facets is coated with an antireflection layer. This
coating arrangement forces these diodes to act only as gain media.
The other components that make up the ECDL cavity include wavelength-
selective devices, such as diffraction gratings or etalons, and
other optical elements such as mirrors and lenses. However, ECDLs
did not evolve from quirky laboratory apparatuses to rugged and
reliable turnkey test-and-measurement instruments until the past
decade. Two conditions enabled tunable ECDLs to come of age: the
development of inexpensive, high-performance laser diodes and the
tremendous demand— driven by the dot-com/telecommunications
economic boom—for high-performance benchtop tunable lasers.
ECDLs became commercially available in the early 1990s, a few years
before the telecommunications boom. They were centered at 670 nm
and targeted at the spectroscopy market. These systems delivered
1 mW of continuous-wave output power and had 6-nm/s tuning rates
over 12-nm wavelength ranges. Their side-mode suppression ratio
(the ratio of the carrier to the nearest side mode expressed in
decibels) was 40 dB. Unlike their laboratory-built counterparts,
which required constant adjustment to maintain precise tuning, commercial
ECDLs were more rugged, and some achieved continuous tuning over
their entire tuning range without the wavelength hopping to a side
As telecommunications companies thrived in the late 1990s, ECDL
companies targeted the industry’s test-and-measurement market.
Wavelength-division-multiplexed (WDM) network component manufacturers
used ECDLs to test the spectral characteristics of their products,
whose spectral performance is essential to the overall performance
of WDM network systems. In a WDM network, a single fiber carries
many wavelength channels simultaneously (see
The Industrial Physicist, February/ March 2002, pp. 18–21). Thus, optical filters, wavelength
multiplexers, and demultiplexers must help direct the information
to its correct destination.
WDM components revolutionized the information
superhighway by dramatically increasing bandwidth capacity, which
led to next-generation networks and the maturation of the photonics
industry. Because companies needed these network components in
volumes and at narrower channel spacings, testing and measuring
these devices required ever-faster, lower-noise tunable lasers.
ECDLs were developed that could tune continuously at speeds of
to 100 nm/s over a wavelength range of 1520 to 1620 nm and with
sidemode suppression ratios as high as 70 dB, which resulted in
extremely low-noise performance. Because the lasers operated 24
hours a day, 7 days a week on the manufacturing floor—often
overseas—they had to be rugged, reliable, and stable and meet
strict shock and humidity standards.
Intense competition among laser
companies resulted in significant advances in the costeffective
manufacture of the lasers.
Because ECDLs reduced testing times
hours to seconds, they revolutionized the manufacture of passive
network components, which led to higher efficiencies and yields.
Moreover, many telecom companies realized that tunable lasers could
ultimately lead to increased network intelligence, functionality,
and efficiency. Technological advances made these devices compact
and so inexpensive that they would soon compete with the single-wavelength
distributed- feedback (DFB) lasers that are used in WDM networks.
Today, with the dot-com bubble burst and a bandwidth glut worldwide,
telecom spending has fallen dramatically. The industry’s cash
squeeze has affected all of its components, from giant corporations
to the smallest companies—many of which went out of business
or survived only by significant layoffs. In this new economic climate,
photonics companies are seeking opportunities in nontelecom markets—
including semiconductor, biotechnology and industrial-sensing applications—to
help them ride out the slump.
On the flip side, non-telecom markets
have taken advantage of photonics manufacturing companies’
expertise, product innovation, and business maturity that resulted
from the explosive growth and development of the telecom industry.
Today, ECDL modules are relatively low in cost and high in performance,
provide output power up to 10 mW, can be tuned across a wavelength
range of 400 to 1620 nm, and are compact enough to fit in the palm
of the hand. However, a lack of awareness about the technology
nontelecom companies has hampered its adoption in other arenas.
Often, decision-makers in nontelecom industries are unaware of
photonics can be applied in their areas.
Fiber Bragg gratings
provides one example of applying tunable lasers to a nontelecom
market. These lasers can serve as the optical source in fiber-Bragg-grating
(FBG) sensing systems to provide faster, more accurate, and more
sensitive systems. The progression to telecom tunable lasers in
FBG systems was natural because FBGs typically are centered in
1550-nm region. These devices provided dense wavelength-division
demultiplexing, dispersion compensation, and erbium-doped fiber
amplifier (EDFA) gain flattening for telecommunication networks.
Because the temperature and strain states of FBGs directly affect
their reflectivity spectrum, they can also be used for a variety
of sensing applications. As the fiber-optic analogue to conventional
electronic sensors, FBGs can serve as strain-gauge sensors to provide
structural engineers with measurements not previously possible.
Emerging applications include detecting changes in stress in buildings,
bridges, and airplane bodies; depth measurements in streams, rivers,
and reservoirs for flood control; and temperature and pressure
measurements in deep oil wells. The advantages of FBG sensors
- considerably improved accuracy, sensitivity, and immunity to
electromagnetic interference, radio-frequency interference, and
- the ability to be made into a compact, lightweight, rugged
device small enough to be embedded or laminated into structures
or substances to create smart materials that can operate in harsh
environments —such as underwater—where conventional
sensors cannot work
- the ability to be multiplexed and an inherent low transmission
loss at 1550 nm
- ease of installation and use
- potential low cost as a result of high-volume telecommunications
These features enable using many sensors on a single optical fiber
at arbitrary spacing. Using WDM technology, including tunable lasers,
one can interrogate each sensor independently and obtain a distributed
measurement over large structures. Because the gratings are multiplexed
on a single fiber, many sensors can be accessed with a single connection
to the optical source and detector. Conventional electronic straingauge
sensors require each sensor to have its lead wires attached and
routed to the sensor readout.
How it works
The physical principle behind the FBG sensor is that a change in
strain, stress, or temperature will alter the center of the
of the light reflected from an FBG. A fiber’s index of refraction
depends on the density of the dopants it contains. FBGs are made
by redistributing dopants to create areas that contain
greater or lesser amounts, using a technique called laser writing.
The FBG wavelength filter consists of a series of perturbations
in the index of refraction along the length of the doped optical
fiber. This index grating reflects a narrow spectrum that is directly
proportional to the period of the index modulation (L)
and the effective index of refraction (n).
|Figure 1. Multiple
fiber Bragg gratings may be written at discrete distances
(d) on a doped optical fiber by stripping away the protective
coatings, using the interference patterns of ultaviolet
beams, and recoating the fiber.
The wavelength at which the reflectivity peaks, called
the Bragg wavelength (lB),
is expressed by lB =
2 nL. Because temperature
and strain directly affect L and n,
any change in temperature and strain directly affects the lB.
In the 1,550-nm (C-Band) window, the main telecommunications
transmission frequency, a change in mechanical or thermal
strain on the FBG results in a wavelength/strain sensitivity
of 1.2 pm/µstrain (microstrain is a change
in dimension that is one millionth of the original) and a
wavelength/temperature sensitivity of 10 pm/°C. The gauge
lengths of FBGs are up to 5 mm, although lengths of up to
100 cm are being developed for civil-engineering applications.
FBGs are fabricated by writing an index grating directly
on a doped optical fiber. Two intense ultraviolet beams are
angled to form an interference pattern with the desired periodicity,
which is written on one side of a bare fiber after the external
coatings have been stripped away. The pattern’s intense
bright and dark bands cause local changes in the index of
refraction by the migration of the dopants in the fiber.
After the grating is written on the fiber, it is recoated
with polyamide (Figure 1).
Writing many sensors on a single optical fiber requires
careful consideration of each FBG’s specifications.
For example, the allowable strain range for any given FBG
sensor depends on the available optical bandwidth. When placing
many FBG sensors on a single fiber, each sensor must have
its own wavelength segment so that various signals do not
overlap. As the FBGs undergo strain, they shift in wavelength
within their allotted optical bandwidth range. This situation
is similar to wavelength-division multiplexing, in which
each channel is allotted a specific wavelength. In general,
there should be a 0.5-nm wavelength buffer between sensor
channels. The maximum change in wavelength of each sensor—and
thus, the distance between each channel— depends on
To measure wavelength shifts that result directly from changes
in temperature or tension, FBG sensor systems must consist of an
source that continuously interrogates the reflection spectrum and
a detection module that records the shifts in the peak reflectivity
versus wavelength. A system’s overall sensitivity depends
directly on the wavelength accuracy of the source and detection
module—the better the accuracy, the higher the sensitivity.
However, higher wavelength accuracy often results in slower update
rates, or scan frequencies, and less-frequent monitoring of sensors.
The system scan frequency is a combination of the speed
of the optical source, the bandwidth of the detectors, the
data acquisition rate, and the rate at which the analysis
of the wavelength shift can be performed. For applications
such as maintenance checks of an airplane’s structural
integrity, the slower—but more accurate—systems
are desirable. In other applications for which in situ monitoring
is required, high update rates are more important.
The simplest FBG sensor system uses a broadband light source,
such as an amplified- spontaneous-emission (ASE) white-light
source, with a tunable filter and a detector. Because detectors
are wavelength-insensitive, a tunable wavelength filter is
required to scan the wavelength range of the FBG sensors,
typically 40 nm, to determine the sensors’ Bragg wavelength.
The main advantage of these systems is their lower cost.
However, because the output power from the ASE white-light
source is low, only a limited number of in-line gratings
can be measured, and with a limited dynamic range. Moreover,
the requirement of an external wavelength filter limits the
accuracy and scan frequency of these systems.
A fiber laser provides higher output Laser diode chip High-reflectivity
coating Antireflecting coating Collimating lens Retroreflector
Spectrally narrowed laser output Diffraction grating Patented
pivot point provides mode-hop-free tuning power and, thus,
an increase in the number of FBG sensors that can be monitored
and the available dynamic range. In addition, a laser emits
only a narrow spectrum of optical radiation, which eliminates
the need for a tunable filter and thus allows faster update
rates (Figure 2, right).
|Figure 2. Two laser/detection
systems that measure the wavelength shifts in fiber Bragg
gratings resulting from strain, temperature, or pressure
changes—the fiber laser (a) and the ECDL tunable
ECDLs provide even higher output power than fiber lasers, which
increases the number of sensors and the dynamic range. The narrow
linewidths, fast sweep speeds, higher output power, and high side-mode
suppression ratios of ECDLs improve the accuracy and dynamic range
of FBG sensor systems. Moreover, incorporating a sweptwavelength
meter improves system accuracy. Although the laser-based FBG systems
have lower scan frequencies than those using fiber lasers, developments
in ECDL technology will soon provide ultrafast tuning rates of
to 10,000 nm/s, enabling both high-speed and highly accurate sensing
systems (Figure 3, below).
|Figure 3. In this ruggedized
Littman–Metcalf tunable laser design, the light diffracted
to the retroreflector returns to the external cavity diode
laser in a resonance effect that outputs a spectrally narrowed
Planes, floods, oil
Currently, ECDL-based FBG
sensing systems are being investigated for monitoring stress in
airplanes, flood control, and efficient oil-well recovery—just
to name a few applications. FBG sensors embedded throughout the
body of an aircraft can monitor structural stress. In flood control,
FBGs are placed in rivers upstream of populated areas, where they
serve as early warning systems that constantly monitor changes in
water pressure. If large changes occur, the systems provide enough
warning to allow safe evacuation of areas downstream.
used in oil recovery have the potential to provide geophysical
data enabling more efficient exploitation of oil fields, and continuous
real-time data on well-bore conditions. Currently, approximately
two-thirds of the oil in a field goes unrecovered because it is
not easy to access. As companies move to higher-cost and higher-
risk exploration areas, conditions in the well bore must be monitored
more closely, and FBG sensors will play an increasing role by providing
more reliable and less expensive methods of collecting well-bore
and geophysical data. With the development of robust and responsive
FBG sensors that can withstand extremely high temperatures and
environments, the main hurdle to adoption is the low awareness
by the industry that these telecom technologies have sensing applications.
FBG sensor systems have the potential for low-cost, robust, and
reliable monitoring in the harshest of conditions—where conventional
sensors cannot operate. They provide improved accuracy and sensitivity
over existing systems and are immune to electromagnetic interference.
And with the cost-effective manufacturing of FBGs and tunable
that resulted from the telecom era, FBG sensors using tunable ECDLs
will become cost-competitive with existing remote sensing systems.
Grattan. K. T. V.; Meggitt, B. T., Eds. Optical Fiber Sensor
Technology: Advanced Applications —Bragg Gratings and Distributed Sensors;
Kluwer Academic Publishers: Dordrecht, Netherlands, 2000; 385 pp.
Littman, M. G.; Metcalf, H. Appl. Opt. 1978, 17, 2224.
Littman, M. G. Opt. Lett. 1981, 6, 117.
Mark Wippich is
product marketing manager, tunables and Kathy
Li Dessau is a product manager for New Focus, Inc., headquartered
in San Jose, California.