# Presentation: 545618

## Free Space Optics (FSO)Technology Overview

- John Schuster
- Chief Technology Officer
- Terabeam Corporation

## Presentation Overview

**Why Free Space Optics?**
**Challenges**
**Transceiver Design**
**Predicting Availability**
**Eye Safety**
**Applications & Network Integration**
**The Future of FSO**

## Why Free Space Optics (FSO)?The “Last Mile” Bottleneck Problem

- Only about 5% of commercial buildings are lit with fiber

- Wide Area Networks between major cities are extremely fast
- Fiber based
- >2.5 Gbps

- Local Area Networks in buildings are also fast
- >100Mbps

- The connections in between are typically a lot slower
- 0.3-1.5 Mbps

## Why Free Space Optics?Why Not Just Bury More Fiber?

- Cost
- Rights of Way
- Permits
- Trenching
- Time

- With FSO, especially through the window, no permits, no digging, no fees

## Why Free Space Optics?How Fiber Optic Cable Works

**Why Free Space Optics?****How Fiber Optic Cable Works**

- Light Source

**Glass Fiber Strands**

- Detector

- NetworkDevice

- Pulses of light communicate the data
- “ON” = 1
- “OFF = 0
- Capable of more than 40 Gbps
- >7 CDs a second

- Light Source

- Detector

- NetworkDevice

## Why Free Space Optics?How FSO Works

**Why Free Space Optics?****How FSO Works**

- Anything that can be done in fiber can be done with FSO

## Beams only a few meters in diameter at a kilometer
Allows VERY close spacing of links without interference
No side lobes
Highly secure
Efficient use of energy
Ranges of 20m to more than 8km possible

- Allows VERY close spacing of links without interference
- No side lobes
- Highly secure
- Efficient use of energy
- Ranges of 20m to more than 8km possible

**Why Free Space Optics?****Very Narrow and Directional Beams**** **

## Why Free Space Optics?Deployment Behind Windows

- Rapid installations without trenching and permitting
- Direct connection to the end user
- Bypasses the building owner
- No roof rights
- No riser rights

## Why Free Space Optics?The FSO “Value Proposition”

- No interference
- Unlicensed
- Easy to install
- Through the window (or from the rooftop)
- No trenching, no permits
- Fiber-like data rates
- Many deployment options
- Fungible equipment

## Fundamental ConceptsSmall Angles - Divergence & Spot Size

| Divergence | Range | Spot Diameter |
| --- | --- | --- |
| 0.5 mrad | 1.0 km | 	~0.5 m (~20 in) |
| 2.0 mrad | 1.0 km | 	~2.0 m (~6.5 ft) |
| 4.0 mrad (~ 1⁄4 deg) | 1.0 km | 	~4.0 m (~13.0 ft) |

- 1° ≈ 17 mrad   →   1 mrad ≈ 0.0573°

## Fundamental ConceptsThe Decibel - dB

- A logarithmic ratio between two values
- In the optical world of Power in mW,
*	****dB=10*Log(power2/power1)***
- 3 dB = ratio of 2/1
- 6 dB = ratio of 4/1
- 10 dB = ratio of 10/1
- 20 dB = ratio of 100/1
- 50 dB= ratio of 100,000/1

- _Gain/Loss_

- _Multiplier_

- +30 db
- +20 db
- +10 db
- 0 db
- -10 db
- -20 db
- -30 db

- 1000
- 100
- 10
- 1
- .1
- .01
- .001

## ChallengesEnvironmental factors

**Challenges****Environmental factors**

**Sunlight**

**Building ****Motion**

**Alignment**

**Window**
**Attenuation**

**Fog**

**Each of these factors can “attenuate” (reduce) the signal.  However, there are ways to mitigate each environmental factor.**

**Scintillation**

**Range**

**Obstructions**

**Low Clouds**

## ChallengesAtmospheric Attenuation - FOG

- Absorption or scattering of optical signals due to airborne particles
- Primarily FOG but can be rain, snow, smoke, dust, etc.
- Can result in a complete outage
- FSO wavelengths and fog droplets are close to equal in size
- (Mie Scattering)
- Typical FSO systems work 2-3X further than the human eye can see
- High availability deployments require short links that can operate in the fog

## ChallengesLow Clouds, Rain, Snow and Dust

- Low Clouds
- Very similar to fog
- May accompany rain and snow
- Rain
- Drop sizes larger than fog and wavelength of light
- Extremely heavy rain (can’t see through it) can take a link down
- Water sheeting on windows
- Heavy Snow
- May cause ice build-up on windows
- Whiteout conditions
- Sand Storms
- Likely only in desert areas; rare in the urban core

## ChallengesScintillation

- Beam spreading and wandering due to propagation through air pockets of varying temperature, density, and index of refraction.
- Almost mutually exclusive with fog attenuation.
- Results in increased error rate but not complete outage.

**Challenges****Scintillation**

## ChallengesWindow Attenuation

- Tinted or insulated windows can have much greater attenuation
- Possible to trade high altitude rooftop weather losses vs. window attenuation

**Challenges****Window Attenuation**

- WAM

## ChallengesBuilding Motion

| Type | Cause(s) | Magnitude | Frequency |
| --- | --- | --- | --- |
| Tip/tilt | Thermal expansion | High | Once per day |
| Sway | Wind | Medium | Once every several seconds |
| Vibration | Equipment (e.g., HVAC), door slamming, etc. | Low | Many times per second |

## 
Automatic Pointing and Tracking
Allows narrow divergence beams for greater link margin
System is always optimally aligned for maximum link margin
Additional cost and complexity

Large Divergence and Field of View
Beam spread is larger than expected building motion
Reduces link margin due to reduced energy density
Low cost

**Automatic Pointing and Tracking**
- Allows narrow divergence beams for greater link margin
- System is always optimally aligned for maximum link margin
- Additional cost and complexity
**Large Divergence and Field of View**
- Beam spread is larger than expected building motion
- Reduces link margin due to reduced energy density
- Low cost

**Challenges****Compensating for Building Motion – Two Methods**

**0.2 – 1 mrad divergence**
**= 0.2 to 1 meter spread at 1 km**

## ChallengesBuilding Motion – Thermal Expansion

**Challenges****Building Motion – Thermal Expansion**

- Results from Seattle Deployment:
- 15% of buildings move more than 4 mrad
- 5% of buildings move more than 6 mrad
- 1% of buildings move more than 10 mrad