Session 3 Light Sourcesand other Components

2

N- and P- Type Semiconductors

The P- has a surplus of holes.

The N- has a surplus of negative electrons.

3

P-N Junction One of the crucial keys to

solid state electronics is the nature of the P-N junction.

When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Specifically, current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode.

This non-reversing behavior arises from the nature of the charge transport process in the two types of materials.

4

PN junction At the junction,

electrons fill holes so that there are no free holes or electrons there.

A barrier is formed at the depletion region with an electrostatic field of 0.6V for Si.

5

Forward Biased P-N Junction

if the voltage is high enough then the barrier will be overcome and current will flow through the junction.

the negative terminal pushes negative electrons towards the junction.

the positive terminal pushes holes towards the junction.

6

LEDs When the applied forward voltage on the

diode, the LED drives the electrons and holes into the active region between the n-type and p-type material, the energy can be converted into infrared or visible photons.

This implies that the electron-hole pair drops into a more stable bound state, releasing energy on the order of electron volts by emission of a photon.

The red extreme of the visible spectrum, 700 nm, requires an energy release of 1.77 eV to provide the quantum energy of the photon. At the other extreme, 400 nm in the violet, 3.1 eV is required.

7

LED Radiation Patterns An LED is a directional light

source, with the maximum emitted power in the direction perpendicular to the emitting surface.

The typical radiation pattern shows that most of the energy is emitted within 20° of the direction of maximum light.

Some packages for LEDs include plastic lenses to spread the light for a greater angle of visibility.

8

Light-emitting Diode (LED) Datacom through air & multimode fiber

Very inexpensive (laptops, airplanes, lans)

Key characteristics Most common for 780, 850, 1300 nm Total power up to a few W Spectral width 30 to 100 nm Coherence length 0.01 to 0.1 mm Little or not polarized Large NA ( poor coupling into fiber)

P-3 dB

Ppeak

BW

9

Lasers Laser is an acronym for light

amplification by the stimulated emission of radiation

Laser characteristics: Nearly monochromatic: the light

emitted has a narrow band of wavelengths

Coherent: the light wavelength are in phase, rising and falling thought the sine-wave cycle at he same time

Highly directional: the light is emitted in a a highly directional pattern with little divergence.

10

Three basic elements of a laser

A typical laser consists of three things:• a Pump, a Gain Medium, and a Cavity.

The pump would send energy into the gain medium and this would excite the electrons and holes within it.

This process then gets amplified within the cavity and lasing takes place.

11

A semiconductor laser diode Pump - by applying a potential difference V Gain medium - modified pn-junction or MQW cavity – the cleaved surfaces + coating A feedback circuit is also implemented in

order to control the amount of current sent to the laser diode.

12

Properties of LDs Here is a list of the most important

properties of LDs three general categories: Electrical,

Optical, and Temperature.Electrical Optical• Laser threshold ● Light output power

• Operating current ● Slope efficiency• Operating Voltage ● Beam Divergence

● Peak wavelengthTemperature• operating temperature• wavelength shift

13

Fabry-Perot (FP) Laser Multiple longitudinal mode (MLM) spectrum “Classic” semiconductor laser

First fiberoptic links (850 or 1300 nm) Today: short & medium range links

Key characteristics Most common for 850 or 1310 nm Total power up to a few mw Spectral width 3 to 20 nm Mode spacing 0.7 to 2 nm Highly polarized Coherence length 1 to 100 mm Small NA ( good coupling into fiber)

Ppeak

P

I

Threshold

14

Distributed Feedback (DFB) Laser

Single longitudinal mode (SLM) spectrum High performance telecommunication laser

Most expensive (difficult to manufacture) Long-haul links & DWDM systems

Key characteristics Mostly around 1550 nm Total power 3 to 50 mw Spectral width 10 to 100 MHz (0.08 to 0.8

pm) Sidemode suppression ratio (SMSR): > 50

dB Coherence length 1 to 100 m Small NA ( good coupling into fiber)

P peak

SMSR

15

Source Characteristics Characteristic LED Laser Output lower

higher Speed slower faster Output pattern (NA) higher lower Spectral width wide

narrow Single-mode compatibility no

yes Ease of use easier harder Lifetime longer long Cost lower higher

16

Output Power Output power is the optical power

emitted at a specified drive current.

Output power (mW)

Drive Current (mA)

LED

Laser

17

Spectral width

Wavelength (nm)

Relative Output

Laser : 0.1 to 5 nm

LED : 40 nm

18

How they look like Semiconductor

laser diodes come in many shapes and sizes.

Package: TO cans; fiber pigtail; hermetic seal

19

Fiber Optic Detectors

They convert optical signals back into electrical impulses that are used by the receiving end of the fiber optic data, video, or audio link.

Detectors perform the opposite function of light emitters.

The most common detector is the semiconductor photodiode, which produces current in response to incident light.

1 Photodiode; 2 PIN photodiode; and 3

APD

20

Detectors for optical communications

PN photodiodes Electron-hole pairs are created in the

depletion region in proportion to the optical power

Electrons and holes are swept out by the electric field, leading to a current

PIN photodiodes Electric field is concentrated in a thin

intrinsic (i) layer Avalanche photodiodes

Like pin photodiodes, but have an additional layer in which an average of M secondary electron-hole pairs are generated through impact ionization for each primary pair

21

Material Aspects

Silicon (Si) Least expensive

Germanium (Ge) “Classic” detector

Indium gallium arsenide (InGaAs) Highest speed

Wavelength nm

500 1000 1500

Silicon

Germanium

InGaAs

Quantum Efficiency = 1

0.1

0.5

1.0

Responsivity (A/W)

22

Detector Materials and Wavelength

Material Bandgap Wavelength Peak Respossivity

Si 1.17eV 300-1100nm 800nm 0.5A/W

Ge 0.775 500-1800 1550 0.7

InGaAs 0.75-1.24 1000-1700 1700 1.1

semiconductor detectors for optical communications

23

Characteristics of PN photodiodes

Reverse-biased The active detection area (depletion area)

is small; many electron-hole pairs recombine before

they can create a current in the external circuit.

Unsuitable for most fiber-optic communication Low gain - fairly high optical power is

needed to generate appreciable current The slow response - limits operations to

the kHz range.

24

Simple PN photodiode circuit How to connect a PN photodiode?

25

PIN photodiode The name comes from the layering of

these materials positive, intrinsic, negative — PIN

Basic idea: Sandwiching a thin layer of a different

semiconductor material (of intrinsic conductivity) between the outer p and n layers

Choosing the outer p and n layers to be transparent to light in the working wavelength range

26

PIN photodiode In the PIN photodiode, the depleted region is

made as large as possible. A lightly doped

intrinsic layer separates the more heavily doped p-types and n-types.

27

Avalanche photodiode (APD) operates as the

primary carriers, the free electrons and holes created by absorbed photons, accelerate, gaining several electron Volts of kinetic energy.

A collision of these fast carriers with neutral atoms causes the accelerated carriers to use some of their own energy to help the bound electrons break out of the valence shell.

28

Avalanche photodiode Electron-hole pairs created by absorption of

photons are accelerated to energies at which more pairs are created, then the new pairs are accelerated and create more pairs, in an “avanlanche”

Avalanche multiplication creates excess noise

Much better signal-to-noise ratio than with external amplification

APDs require high-voltage power supplies for their operation. The voltage can range from 30 or 70 Volts for InGaAs APDs to over 300 Volts for Si APDs. This adds circuit complexity.

APDs are very temperature sensitive, further complicating circuit requirements.

29

APD vs PIN In general, APDs are only useful for

digital systems because they possess very poor linearity.

Because of the added circuit complexity and the high voltages that the parts are subjected to, APDs are always less reliable than PIN detectors.

At lower data rates, PIN detector-based receivers can almost match the performance of APD-based receivers, makes PIN detectors the first choice for most deployed low-speed systems.

At multigigabit data rates, however, APDs rule supreme.

30

Comparison of PIN and APD

Parameter PIN Photodiodes APDsMaterials Si, Ge, InGaAs Si, Ge,

InGaAsBandwidth DC to 40+ GHz DC to 40+

GHz Wavelength 0.6 to 1.8 µm 0.6 to 1.8 µmEfficiency 0.5 to 1.0 A/W 0.5 to 100

A/WCircuitry none HV, Temp StaCost (Fiber Ready)$1 to $500 $100 to

$2,000

31

Detector Characteristics Respossivity is defined as the ratio of

the photocurrent to the optical power, Pin: R = Ip/Pin (units: A/W)

32

Quantum Efficiency Quantum Efficiency is the Ratio of primary

electron-hole pairs created by incident photons to the photons incident on the detector material.

h = (# of emitted electrons)/(# of incident photons)

A quantum efficiency of 70% means seven out of ten incident photons create a carrier.

33

Dark current The induced current that exists in a

reversed biased photodiode in the absence of incident optical power.

34

Minimum detector power Determines the lowest level of incident

optical power that the detector can handle.

The noise floor of a PIN diode tells the minimum detectable power. Noise floor = dark noise/responsivity

R = 0.5 mA/mW, and a dark current of 2nA. The noise floor = 2nA/(0.5mA/mW) = 4nW

35

Response Time Response Time is the time needed for the

photodiode to respond to optical inputs and produce and external current.

The response time relates to its usable bandwidth.

36

Response time BW = 0.35/tr,

The RC time constant of a detector also limits the bandwidth. BW = 1/(2pRLCd), RL is the load

resistance and Cd is the diode capacitance

37

Bias Voltage 5V for PIN PD to ~ 100V for APDs. Affects operation. dark current, responsivity, response

time increase with the bias voltage. Temperature sensitive.

38

Integrated detector/preamplifier

A detector package containing a PIN photodiode and transimpedance amplifier

The output is voltage (V/W) Integrated package

39

What are transmitters and receivers?

Transmitter: A device that includes a source and driving electronics. It functions as an electrical-to-optical converter

Receiver: A terminal device that includes a detector and signal processing electronics. It functions as an optical-to-electrical converter

40

Basic transmitter concepts

41

LED based transmitter The most common

devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD).

LED Driver

Input BufferInput

Bias LEDs are widely used for short to moderate

transmission distances because they are much more economical, and stable in terms of light output versus ambient operating temperature.

LDs are used for long transmission distances. can couple many times more power to the fiber than LEDs but are unstable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability.

42

LD-based transmitter Not on and off but is simple modulated

between high and low levels above the threshold current.

Power monitor to compensate temperature changes

Modulator Input BufferInput

Ref Gen Bias Current

Signalconditioner

Duty CycleCompensation

43

Basics receiver concepts

44

Basic receiver concepts Sensitivity: the lowest power that is

detectable. Determined by the noise floor - SNR or BER of the system Detector used in dB or W unit

Dynamic range: the difference between the minimum and maximum acceptable power levels.

45

Transceivers Transceiver: transmitter + receiver

46

Optical Connectors Optical connectors are the means by

which fiber optic cable is usually connected to peripheral equipment and to other fibers.

These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices to tolerances of a few ten thousandths of an inch.

47

SC connector Snap-in Single-Fiber Connector A square cross section allows high packing

density on patch panels Used in premise cabling, ATM, fiber-

channel, and low-cost FDDI. Available in simplex and duplex

configurations

48

ST connector The most widely used type of connector for

data communications. A bayonet-style “twist and lock” coupling

mechanism allows for quick connects and disconnects, and a spring-loaded 2.5 mm diameter ferrule for constant contact between mating fibers.

49

LC connector Small Form Factor Connector Similar to SC connector but designed to

reduce system costs and connector density.

50

FC Connector

Twisted-on Single-Fiber Connector Similar to the ST connector and used

primarily in the telecommunications industry.

A threaded coupling and tunable keying allows ferrule to be rotated to minimize coupling loss.

51

Patchcords

“Jumper cables” to connect devices and instruments

“Adapter cables” to connect interfaces using different connector styles

Insertion loss is dominated by the connector losses (2 m fiber has almost no attenuation)

Often yellow sheath used for single-mode fiber, orange sheath for multimode

52

Types of Optical Connectors Connector Insertion

Loss Repeatability Fiber Type Applications

  FC 0.50-1.00 dB 0.20 dB SM, MM Datacom, Telecomm

                           FDDI 0.20-0.70 dB 0.20 dB SM, MM Fiber Optic

Network                          

LC

0.15 db (SM)0.10 dB (MM) 0.2 dB SM, MM High Density

Interconnection

                    MT Array 0.30-1.00 dB 0.25 dB SM, MM High Density

Interconnection

                            SC

0.20-0.45 dB 0.10 dB SM, MM Datacom

             SC Duplex 0.20-0.45 dB 0.10 dB SM, MM Datacom

             

ST

0.40 dB (SM)0.50 dB (MM)

0.40 dB (SM)0.20 dB (MM) SM, MM

Inter-/Intra-Building,

Security, Navy

53

Couplers and Splitters Some of the most common applications

for couplers and splitters include: Local monitoring Distributing. An 8-port coupler allows a

single transmitter to drive eight receivers.

Making a linear, tapped fiber optic bus.

54

Couplers Split optical signals into multiple paths

or combine multiple signals on one path.

The number of input and output ports, expressed as an N x M configuration, characterizes a coupler.

The letter N represents the number of input fibers, and M represents the number of output fibers.

Fused couplers can be made in any configuration, but they commonly use multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).

Passive and bidirectional

55

Splitters The simplest couplers are fiber optic

splitters. These devices possess at least three ports

but may have more than 32 for complex devices.

The coupler manufacturer determines the ratio of the distribution of light between the two output legs. Popular splitting ratios include 50%-50%, 90%-10%, 95%-5% and 99%-1%; however, almost any custom value can be achieved.

For example, using a 90%-10% splitter with a 50 µW light source, the outputs would equal 45 µW and 5 µW.

However, excess loss hinders that performance. All couplers and splitters share this parameter.

56

Calculation Through Port Loss Lossthr = 10log10(P2/P1) TAP LossLosstap = 10log10(P3/P1) Excess LossLossE = 10log10[(P2+P3)/P1]

P1: inputP2: throughput

99%

P3: tap port 1%

Output 50%Output 50%

inputSplitting ratio

Throughput loss Tap loss

1:1 3 dB 3 dB2:16:110:1

57

Wavelength-Independent Couplers

Wavelength-Independent coupler (WIC) types: couple light from each fiber to all the

fibers at the other side 50% / 50% (3 dB) most common 4 port

type 1%, 5% or 10% taps (often 3 port

devices)

Excess Loss (EL): Measure of power “wasted” in the

component

EL = -10 • log10

Pout

Pin

58

Wavelength-Dependent Couplers

Wavelength-division multiplexers (WDM) types: 3 port devices (4th port terminated) 1310 / 1550 nm (“classic” WDM

technology) 1480 / 1550 nm and 980 / 1550 nm for

pumping optical amplifiers (see later) 1550 / 1625 nm for network monitoring

Insertion and rejection: Low loss (< 1 dB) for path wavelength High loss (20 to 50 dB) for other

wavelength

Common l1

l2

59

Wavelength-division Multiplexing

60

Dense Wavelength-Division Multiplexing (DWDM)

Monitor Points

Dem

ulti

plex

er

l2

ln

l1

ln-1

Wavelength Converter

NT

NT

l2

ln

l1

ln-1

Mul

tipl

exer

Wavelength Converter

NT

NT

NT

NT

NT

NT

Net

wor

k Te

rmin

als

61

DWDM Spectrum

1565 nm

RL +0.00 dBm5.0 dB/DIV

1545 nm

AmplifiedSpontaneousEmission (ASE)

Channels: 16Spacing: 0.8 nm

62

WDM Standards ITU grid

“Optical Interfaces for Multichannel Systems

with Optical Amplifiers” Wavelength range 1532 to 1563 nm 100 GHz (0.8 nm) channel spacing 193.1 THz (1552.51 nm) reference

Eg l = 1549.32 nm, Optical Frequency = 193.5 THz

1550.12 193.4 1550.92 193.3 1551.72 193.2

63

WDM Standards

Current state of the art is 80 wavelengths on one fiber in 1550 nm range (36-40 is more common).

ISO has a standard for 100 GHz (approx. 1 nm) spacing. New standards: 50, 25 GHz.

All wavelengths can be amplified by one EDFA but available output power is divided by number of wavelengths.

64

WDM Spectral bands (proposal)

65

Fiber Bragg Gratings (FBG) Single-mode fiber with “modulated”

refractive index Refractive index changed using high

power UV radiation Regular interval pattern: reflective at one

wavelength Notch filter, add / drop multiplexer (see

later) Increasing intervals: “chirped” FBG

Compensation for chromatic dispersion

l

66

Description

1. A short length of regular optical fiber has been modified.

2. for being exposed to ultraviolet (UV) radiation in a regular pattern

3 the refractive index of the fiber core is altered in a periodic pattern too.

lB

Reflection in phase

lB

lB

67

FBG is a very selective spatial reflector

The Bragg Condition is lB = 2Lneff

lB is reflected wavelength; L is the grating periodicity, typically L = 0.5m; neff is the effective index.

68

Optical Amplifiers

Most common type is erbium-doped fiber amplifier (EDFA).

Pump laser used to add power to the optical signal in the fiber.

No electrical parts (except laser power supply).

No need to convert between optical and electrical signals.

69

EDFA

70

Commercial EDFA

71

Erbium Energy States

Non-radiating

Transitions

Radiating Transition triggered by photon

Pump Laser Provides Energy

Not used

72

Properties of Erbium Amp When erbium is excited by photons at 800

nm or 980 nm, it has a non-radiative decay (energy drops without producing light) to a state where it can stay excited for relatively long periods of time - on the order of 10ms.

Erbium can also be excited by photons at 1480nm, but this is typically undesirable as it is too close to the signal wavelength.

When a photon at about 1550 nm interacts with an atom with an electron in the excited state, that electron returns to the valence band, emitting a photon of the same wavelength.

Result is high gain (up to 40 dB) and power output (up to 20dBm).

73

Typical EDFA SpecificationsFlat Gain EDFA EDFA-

FG13 EDFA-FG15

EDFA-FG18

Wavelength Range nm 1530-1560 1530-1560 1532-1560

Saturation Power dBm

13 15 18

Gain Value dB >20 >22 >25

Optimum Gain Flatness (1530-1560nm)

dB <1 <1 <1

Noise Figure dB <6.0 <6.5 <6.0

Min. Isolation at Input and Output

dB 30 30 30

Connectors FC/APC FC/APC FC/APC