white paper laser sources for optical transceivers · 2020-02-18 · 3 white paper laser sources...

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


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.


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


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.”


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.


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.


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.


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.

white paper laser sources for optical transceivers · 2020-02-18 · 3 white paper laser sources...

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