white paper laser sources for optical transceivers · 2020-02-18 · 3 white paper laser sources...
TRANSCRIPT
White Paper
Laser Sources For Optical TransceiversGiacomo Losio
ProLabs Head of Technology
September 2014
2 White Paper Laser Sources For Optical Transceivers
Giacomo LosioProLabs Head of Technology
September 2014
Optical transceivers use different semiconductor laser sources depending
mainly on the reach and bit rate that the device has to guarantee. In
this paper we will describe the most commonly used types and their
application. We will skip the physics of the laser and semiconductors;
we will focus only on the technological aspects of each device.
Edge-emitting lasers
A laser diode is electrically a p-i-n diode. The active region of the laser diode
is in the intrinsic (I) region, and the carriers, electrons and holes, are pumped
into it from the N and P regions respectively. The goal for a laser diode is
that all carriers recombine in the intrinsic region, and produce light. Thus,
laser diodes are fabricated using direct bandgap semiconductors. The active
layer often consists of quantum wells, which provide lower threshold current
and higher efficiency [1]. A quantum well laser is a laser diode in which the
active region of the device is so narrow that quantum confinement occurs.
Forward electrical bias across the laser diode causes the two species of
charge carrier – holes and electrons – to be “injected” from opposite sides of
the p-n junction into the depletion region. When an electron and a hole are
present in the same region, the result may be spontaneous. The difference
between the photon-emitting semiconductor laser and conventional
phonon-emitting (non-light-emitting) semiconductor junction diodes lies
in the use of a different type of semiconductor, one whose physical and
atomic structure confers the possibility for photon emission. These photon-
emitting semiconductors are the so-called “direct bandgap” semiconductors.
Suitable materials include. Gallium arsenide and indium phosphide,
In the absence of stimulated emission (e.g., lasing) conditions, electrons and
holes may coexist in proximity to one another, without recombining, for a
certain time before they recombine, then a photon with energy equal to the
recombination energy can cause recombination by stimulated emission. This
generates another photon of the same frequency, travelling in the same
direction. This means that stimulated emission causes gain in an optical wave
(of the correct wavelength) in the injection region, and the gain increases as
the number of electrons and holes injected across the junction increases.
Laser Sources For Optical Transceivers
3 White Paper Laser Sources For Optical Transceivers
The gain region is surrounded with an optical cavity to form a laser. In the simplest
form of laser diode, an optical waveguide is made on that crystal surface, such
that the light is confined to a relatively narrow line. The two ends of the crystal are
cleared to form perfectly smooth, parallel edges, forming a Fabry–Pérot resonator.
Photons emitted into a mode of the waveguide will travel along the waveguide
and be reflected several times from each end face before they are emitted. As a
light wave passes through the cavity, it is amplified by stimulated emission, but
light is also lost due to absorption and by incomplete reflection from the end
facets. Finally, if there is more amplification than loss, the diode begins to “lase”.
Some important properties of laser diodes are determined by the geometry of
the optical cavity. Generally, in the vertical direction, the light is contained in
a very thin layer, and the structure supports only a single optical mode in the
direction perpendicular to the layers. The wavelength emitted is a function of
the band-gap of the semiconductor and the modes of the optical cavity. The
width of the gain curve will determine the number of additional “side modes”
that may also lase, depending on the operating conditions. Single spatial
mode lasers that can support multiple longitudinal modes are called Fabry
Perot (FP) lasers. An FP laser will lase at multiple cavity modes within the gain
bandwidth of the gain medium. The number of lasing modes in an FP laser is
usually unstable, and can fluctuate due to changes in current or temperature.
Fig 1. Laser diode example
Single frequency
diode lasers are
either distributed
feedback (DFB) lasers
or distributed Bragg
reflector (DBR) lasers.
4 White Paper Laser Sources For Optical Transceivers
DBR and DFB lasers
The standard Fabry Perot Lasers are not wavelength selective. This leads to lasing
of many modes and allows for mode jumps. A possible method is to insert an
optical feedback in the device to eliminate other frequencies. Periodic gratings
incorporated within the lasers waveguide can be utilized as a means of optical
feedback. Devices incorporating the grating in the pumped region are termed
Distributed Feedback (DFB) lasers, while those incorporating the grating in the
passive region are termed Distributed Bragg Reflector (DBR) Laser. DFB and DBR
lasers oscillate in a single-longitudinal mode even under high-speed modulation, in
contrast to Fabry-Perot lasers, which exhibit multiple-longitudinal mode oscillation
when pulsed rapidly
The gratings or distributed Bragg reflectors (DBRs) are used for one or both cavity
mirrors. The grating consists of corrugations with a periodic structure. They are
used because of their frequency selectivity of single axial mode operation. The
period of grating is chosen as half of the average optical wavelength, which leads
to a constructive interference between the reflected beams. A DBR Laser can
be formed by replacing one or both of the discrete laser mirrors with a passive
grating reflector. Besides the single frequency property provided by the frequency-
selective grating mirrors, this laser can include wide tunability. Since the refractive
index depends on the carrier density, this can be exploited to vary the refractive
index electro optically on the sections by separate electrodes.
A distributed feedback laser (DFB) also uses grating mirrors, but the grating
is included in the gain region. Reflections from the ends are suppressed by
antireflection coatings. Thus, it is possible to make a laser from a single grating,
although it is desirable to have at least a fraction of a wavelength shift near
the center to facilitate lasing at the Bragg frequency. The pure DFB structure in
fact will lead to the oscillation of two symmetrical modes, but not at the Bragg
frequency. Adding a perturbation like a quarter wavelength shift leads to single
mode operation at Bragg frequency.
DFB Lasers are easier
to fabricate and show
fewer losses and
therefore have a lower
threshold current. The
DBR is widely tunable,
but relatively complex
since a lot of structure
must be created
along the surface of
the wafer. For this
reason DBR Lasers
are only formed when
their properties are
required. Both lasers
however work in single
mode.
DBR vs. DFB LasersFig 2.
DFB Facts
5 White Paper Laser Sources For Optical Transceivers
VCSEL
VCSEL has several advantages over the
production process of edge-emitting lasers.
Edge-emitters cannot be tested until the end
of the production. If the edge-emitter does
not work as per specification, the production
time and the processing materials have been
wasted. VCSELs however, can be tested at
intermediate steps to check for material quality
and processing issues. Additionally, because
VCSELs emit the beam perpendicular to the
active region of the laser as opposed to parallel
as with an edge emitter, tens of thousands of
VCSELs can be processed simultaneously. A 3”
wafer can yield approximately 15.000 VCSELs
but only about 4.000 edge emitting lasers of
comparable power. Furthermore, the yield can be
controlled to a more predictable outcome. Other
VCSEL advantages include higher reliability,
simple fiber coupling and packaging, all this
results in lower cost. The drawback of VCSELs
is that the longer the wavelength becomes,
the more complicated is the fabrication. As of
now they are not used in the 1550nm region.
There are many designs of VCSEL structure;
however they all have certain common aspects
in common. The cavity length of VCSELs is very
short typically 1-3 wavelengths of the emitted light.
As a result, in a single pass of the cavity, a photon
has a small chance of a triggering a stimulated
emission event at low carrier densities. Therefore,
VCSELs require highly reflective mirrors to be
efficient. In edge-emitting lasers, the reflectivity
of the facets is about 30%. For VCSELs, the
reflectivity required for low threshold currents
is greater than 99.9%. Such a high reflectivity
can’t be achieved by the use of metallic mirrors.
VCSELs make, use Distributed Bragg Reflectors
(DBRs). These are formed by laying down
alternating layers of semiconductor or dielectric
materials with a difference in refractive index.
VCSELs for wavelengths from 650 nm to 1300
nm are typically based on gallium arsenide
(GaAs) wafers with DBRs formed from GaAs and
aluminium gallium arsenide (AlxGa(1-x)As). The
refractive index of AlGaAs does vary relatively
strongly as the Al fraction is increased, minimizing
the number of layers required to form an efficient
Bragg mirror compared to other candidate
material systems. Furthermore, at high aluminium
concentrations, an oxide can be formed from
AlGaAs, and this oxide can be used to restrict the
current in a VCSEL, enabling very low threshold
currents. Since the DBR layers also carry the
current in the device, more layers increase the
resistance of the device therefore dissipation
of heat and growth may become a problem if
the device is poorly designed. The figure below
describes a realistic VCSEL implementation.
“VCSEL advantages include higher
reliability, simple fiber coupling and
packaging, all this results in lower cost.”
6 White Paper Laser Sources For Optical Transceivers
VCSEL device structure, bottom emission (from Wikipedia)Fig 3.
Description of laser typesFig 4.
Fig 3. Description
of VCSEL
implementiation.
Fig 4. In the tables we
report a comparison
of the laser types
used in optical
communications.
7 White Paper Laser Sources For Optical Transceivers
DWDM Application
The emission wavelength of a DFB laser can
be tuned acting on temperature, this fact has
two direct consequences: first for application
that requires transmission at a precise lambda,
thermal control has to be provided (for example a
thermoelectric cooler (TEC)), second changing the
temperature can lead to a device able to produce
different wavelengths. The second method give
the possibility to realize tunable lasers, setting
the working point at specific temperature and
– eventually - keeping it at a stable wavelength
over time using a wavelength locker. One widely
used structure comprises and Fabry-Perot etalon,
it includes a beam splitter, an etalon, a reference
photodiode, and an etalon photodiode.
The reference photodiode measures the laser
output directly (after splitter) and the other
measures the transmission through the etalon.
The coupling ratio of the splitter in the wavelength
locker is designed such that at each exact ITU
channel, the optical power levels falling on
the two photodiodes are equal. As the laser
frequency changes while the etalon detector
photocurrent varies periodically, the ratio of the
two etalon and reference photodetector currents
remains constant at the lock point. Therefore,
by monitoring the change in the ratio of the two
photocurrents, the wavelength of the laser can be
monitored and stabilized.
Lasers that are tunable over multiple wavelengths,
and progressively over the whole C-band appeared
at the beginning of the last decade. They soon
became widely used in DWDM systems since they
allowed the reduction of part numbers (before a
different line card or transceiver was needed for
every different wavelength) and together with
ROADM (reconfigurable add drop multiplexers)
became the core of the optical transport systems.
One of the first commercial full C-band tunable
lasers consisted in a selectable array of DFB
lasers that are combined in a multimode
interference coupler. The DFBs are powered one
at a time and each is manufactured with a slightly
different grating pitch to offset their output
wavelengths by about 3 or 4 nm. The chip is then
temperature tuned by some 30–40 C to access
the wavelengths between the discrete values of
the array elements. With N-DFB elements, then,
a wavelength range of up to about 4N nm can be
accessed, or with 8–10 elements the entire C-band
can be accessed [2].
Another example is an external-cavity laser. In this
case a “gain block” is coupled to external mode-
selection filtering and tuning elements via bulk
optical elements. The cavity phase adjustment,
necessary to properly align the mode with the
filter peak and the desired ITU grid wavelength,
can be included in one of several places e.g.
on the gain block or by fine tuning the mirror
position. In most external-cavity approaches the
mode selection filter is a diffraction grating that
can also double as a mirror. In this case, a retro-
reflecting mirror is translated as it is rotated.
DFB Laser array (source Fujitsu Laboratories)Fig 5.
8 White Paper Laser Sources For Optical Transceivers
Littman-Metcalf cavity (Source New Focus)Fig 6.
This combined motion changes the effective cavity length in proportion to the
change in center wavelength of the mode-selection filter to track the movement
of a single cavity mode. An obvious concern with these structures is their
manufacturability and reliability, given the need for assembling numerous micro-
optical parts and holding them in precise alignment.
More recent approaches that are well suited for monolithic integration are
variations of the DBR structure. In the SGDBR the wider tuning range filter is
provided by the product of the two differently spaced and independently tuned
reflection combs of the SGDBRs at each end of the cavity (front mirror and rear
mirror). Good side-mode suppression has been demonstrated, and tuning of over
40 nm is easily accomplished, but due to grating losses resulting from current
injection for tuning, the differential efficiency and chip output powers can be
somewhat limited. In the case of the SGDBR, this is easily addressed by the
incorporation of another gain section on the output side of the output mirror.
A variation of this concept is the Y-branch structure, where the combination of
two slightly different reflectors located in the two Y arms selects the wavelength
that can be emitted. Structures like this are suitable for monolithic integration of
a Mach-Zehnder modulator in a smaller footprint and low power way compared to
hybrid packaged or fiber-coupled devices. In addition, the chip can be tailored for
each channel across the wavelength band by adjusting the biases to the two legs of
the MZM. The compact size of devices like this made it possible their integration in
a transmitter whose small size was compatible with 10Gbit/s pluggable interfaces
(XFP and SFP+).
Fig 6. Explaination
of Litterman-Metcalf
cavity.
9 White Paper Laser Sources For Optical Transceivers
References
[1] Larry Coldren; Scott Corzine; Milan Mashanovitch (2012). Diode Lasers and Photonic Integrated Circuits (Second ed.). John Wiley and Sons.
[2] Coldren et al. “Tunable Semiconductor Lasers: A Tutorial” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004
Monolithycally integrated Transmitter (Agility)
Tunable laser technology comparison
Fig 7.
Fig 8.
Fig 8. Recap of the
different tunable laser
schemes.
Fig 7. Description
of Monolithycally
Intergrated
Transmitter.