- Multiwavelength Lasers Simplify WDM Networks and Applications
Devices offer optical combs with channel spacing ranging from greater than 100 GHz to 3.125 GHz, thus reducing laser requirements.
Mode-locked lasers are common tools for producing short pulses in the time domain, including
telecommunications applications at multigigahertz repetition frequencies that require
tunability in the C-band. Now they also can work as multiwavelength sources in wavelength
division multiplexing (WDM) applications.
Both cost-effectiveness and performance are fundamental
requirements of today’s WDM systems, which are built using multiple wavelengths
at precise locations on the ITU grid. Because mode-locked lasers produce a comb
of high-quality channels separated precisely by the pulse repetition frequency,
one source can replace many of the distributed feedback lasers currently used. Channel
spacing can range from greater than 100 GHz to 3.125 GHz.
This single-source solution for WDM
system architectures can reduce costs and enable applications in metro and access
networks, test and measurement instrumentation, and portable field test equipment.
New applications, such as supercontinuum generation, frequency metrology and hyperfine
distributed WDM, also could benefit from the lasers’ spectral and temporal
The output of mode-locked lasers in the time domain
is a continuous train of quality pulses, which in this example exhibits a 25-GHz
repetition rate, a 40-ps period and a pulse width of approximately 4 ps (Figure
1). In general, a laser supports modes at frequencies separated by a free spectral
range of c/2L, where L is the cavity length. Often a laser has multiple modes, with
mode phases varying randomly with time. This causes the intensity of the laser to
fluctuate randomly and can lead to intermode interference and mode competition that
reduces its stability and coherence. Stable and coherent CW lasers usually have
only one mode that lases.
Figure 1. Each 25-GHz optical
pulse from a mode-locked erbium-glass laser at 1535 nm has a width of 4 ps FWHM,
40-ps repetition period, extinction ratio more than 30 dB and average power exceeding
10 dBm (left). In the wavelength domain spectrum of the same laser measured with
an optical spectrum analyzer with 0.01-nm resolution (right), the noise floor is
flat between the channels. The rise in the center is caused by the analyzer’s
Mode-locking produces stable and coherent
pulsed lasers by forcing the phases of the modes to maintain constant values relative
to one another. These modes then combine coherently. Fundamental mode-locking results
in a periodic train of optical pulses with a period that is the inverse of the free
The pulsation period is the interval
between two successive arrivals of the pulse at the cavity’s end mirrors.
There is a fixed relationship between the frequency spacing of the modes and the
pulse repetition frequency. In other words, the Fourier transform of a comb of pulses
in time is a comb of frequencies or wavelengths. This capability is key to making
a mode-locked laser a multi-wavelength source.
Mode-locking occurs when laser losses
are modulated at a frequency equal to the intermode frequency spacing. One way to
explain this is to imagine a shutter in the laser cavity that opens only periodically
for short intervals. The laser can operate only when the pulse coincides exactly
with the time the shutter is open. A pulse that operates in this cavity would require
that its modes be phase-locked, and the shutter would trim off any intensity tails
that grow on the pulses as the mode phases try to wander from their ideal mode-locked
values. Thus, a fast shutter in the cavity has the effect of continuously restoring
the mode-locked condition.
Mode-locked lasers operate at repetition
frequencies and pulse widths that require much higher performance than a mechanical
shutter can offer. There are two basic ways to modulate the losses in the laser
cavity to achieve mode-locking. Actively mode-locked lasers usually employ an electro-optic
modulator driven by a radio-frequency signal at the repetition frequency of the
cavity. Passively mode-locked lasers, on the other hand, employ devices called saturable
absorbers to spontaneously lock the modes with fast material response times, without
the use of an external drive signal.
Fiber, semiconductor and erbium-glass
lasers are among the mode-locked devices used at telecommunications wavelengths.
Fiber lasers are usually actively mode-locked at a harmonic of the final repetition
frequency. Their cavities are long because a long fiber is required to obtain sufficient
gain. They tend to be relatively large and complex, but offer flexibility in parameter
adjustment and high output powers. Semiconductor lasers are also actively mode-locked,
in most cases. These small devices, which tend to have relatively low power and
stability, are still a developing technology in research laboratories.
The passively mode-locked erbium-glass
laser, on the other hand, is a simple high-performance platform (Figure 2). The
cavity comprises the gain glass, laser mirrors, a saturable absorber and a tunable
filter. The cavity is short for 25-GHz lasers at approximately 6 mm, allowing a
compact device that also offers high output power (see inset). In this context,
passive mode-locking means that the CW pump laser is focused into the cavity at
980 nm and that picosecond pulses emit from the cavity at 1550 nm, with no other
inputs or signals required.
Figure 2. This erbium-glass multiwavelength laser focuses a 980-nm
CW pump into the erbium gain glass. A saturable absorber provides passive mode-locking,
so no active signal is required. The cavity length for the 25-GHz laser is 6 mm.
The erbium-glass device takes advantage
of the maturity of components used in erbium-doped fiber amplifier products, and
it is optically pumped with an industry-standard 980-nm diode. These pumps are becoming
cheaper and more robust even as they achieve higher output powers and stability.
The current average output power of the multiwavelength laser across the C-band
is 10 dBm.
This device has a saturable absorber
combined with a reflective substrate to create a semiconductor saturable absorbing
mirror with reflectivity that increases with optical intensity. It is an ultrafast
optical switch that acts like an intracavity shutter to produce the mode-locked
spectrum. This has the effect of accumulating all the lasing photons inside the
cavity in a very short time with a very high optical fluence. The mirror also has
response time on the order of femtoseconds for pulse formation and picoseconds when
it is time to initiate self-start of the laser. The proprietary component is made
with fundamental semiconductor techniques.
The erbium-glass laser is tunable through
the C-band so that the comb of wavelengths can be set to cover any section of grid
channels from 1530 to 1565 nm. Locking to the ITU grid requires the multiwavelength
comb to be shifted in frequency to coincide exactly with the known reference grid,
where it is then locked. The maximum frequency shift needed would be the comb spacing,
which is equal to the free spectral range of the mode-locked laser. A shift of one
free spectral range in the laser requires a cavity length change of one wavelength,
which is 1.5 μm. Filtering out one channel of the comb’s edge then allows
ITU grid locking with minor cavity adjustment.
WDM channel generation
By combining the erbium-glass multiwavelength
laser with other available telecommunications components, it is possible to make
a multichannel WDM source (Figures 3a and b). The laser is connected to a dynamic
gain equalizer and an erbium-doped fiber amplifier to produce a flattened 32-channel
distributed WDM wavelength comb with channel linewidth on the order of 1 MHz.
Figure 3a. In this multiwavelength platform
setup, a dynamic gain equalizer flattens and filters the laser’s spectrum.
An erbium-doped fiber amplifier increases channel power. Using one channel, one
wavelength locker and a cavity adjustment of less than 1 μm, the entire wavelength
spectrum can be locked to the ITU grid.
In this application, engineers set
the 25-GHz comb-generating laser to a center wavelength of 1535 nm and an average
power of 12 dBm. With this device, the optical signal-to-noise ratio for the modes
in the center of the output spectrum is typically greater than 60 dB. Numerous locked
modes extend in each direction from the center of the spectrum, with decreasing
power and signal to noise. Thus, the number of usable channels from the multiwavelength
laser can be defined using comparable signal-to-noise requirements of current WDM
Figure 3b. The
multiwavelength laser platform produced this 32-channel WDM channel grid. Signal
to noise is greater than 30 dB, and the channels are separated by exactly 25 GHz
on the ITU grid. Channel flatness is less than 0.4 dB.
Because the laser is fundamentally
mode-locked, there are no side modes between the channels, but the side-mode-suppression
ratio of a typical distributed feedback laser can be used as a threshold for the
signal-to-noise requirements of the channels from the multiwavelength laser. Typical
suppression ratios for WDM laser sources are around 35 dB. More than 32 modes have
ratios greater than 35 dB in the multiwavelength spectrum, so this test can be run
using 32 channels.
The dynamic gain equalizer allows flattening the
comb of 32 channels and attenuating the modes outside the desired comb bandwidth.
The erbium-doped fiber amplifier takes the channels to power levels consistent with
WDM applications. In one test, channel powers were demonstrated up to levels of
It also is possible to set the profile
of the equalizer to account for the amplifier’s gain profile. This allows
optimization of the system for channel count, signal to noise and power. The optical
spectrum analyzer used to capture the DWDM spectrum has 0.01-nm resolution.
The gain equalizer in this example
is a product from Silicon Light Machines of Sunnyvale, Calif., that has high enough
resolution to support any channel spacing throughout the C-band. The device acts
as an addressable diffraction grating with numerous narrow ribbons of individual
microelectromechanical systems in a long row.
The relative power accuracy and spectral
power ripple are ±1 dB. The dynamic range is greater than 15 dB. The test setup
has a standard erbium-doped fiber amplifier with a saturated output power of 27
Besides providing a platform to test
WDM components, the mode-locked source can be used to demonstrate production of
a supercontinuum spectrum. Scientists have used highly nonlinear fibers with decreasing
dispersion profiles to extend multiwavelength combs to cover up to 300 nm of optical
bandwidth. The high peak power of the picosecond pulses interacts with the nonlinear
fiber to produce the supercontinuum. Pulses from the 25-GHz erbium-glass laser are
a good fit with the requirement of supercontinuum generation.
This capability can open up many new applications
by generating more than 1000 high-quality optical carriers for distributed WDM,
enabling multiwavelength short pulses for optical time division multiplexing and
WDM and producing precision optical frequency grids for frequency metrology.
Another advanced application is hyperfine
distributed WDM, which transmits slower data rates on very densely spaced channels
as close as 3.125 GHz. The slower data rates simplify the electronics, avoid added
time division multiplexing and eliminate the serious dispersion problems suffered
by higher-speed signals, particularly at 40 GHz. Multiwavelength lasers are uniquely
suited to this application because of their ability to generate many channels with
a single source at very high densities.
In essence, a variety of practical
solutions to current and near-term challenges are possible with the multiwavelength
platform. WDM systems must compete in an increasingly demanding environment in terms
of cost, size, power consumption and complexity.
A multiwavelength platform allows new
and more efficient architectures to be developed and tailored for specific applications.
Meet the author
Michael Brownell is vice president of product development at GigaTera in Dietikon, Switzerland.
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