# Presentation: 928249

## ChallengesSolar Interference

- Direct or reflected sunlight can saturate receivers or tracking sensors
- Primarily affects East/West links
- Duration and frequency of outages dependent upon global location of link
- Outages typically on the order of minutes
- Outage frequency and duration wavelength dependent

- Sunlight

## ChallengesObstructions to Line of Sight

## Transceiver DesignBasic Design Transceiver Trades

**Design Constraints**
- Price Point
- Range and Availability
- Data Rate
**Major Technical Choices**
- Wavelength
- Wide Divergence Vs. Automatic Pointing and Tracking
- Single-Transmitter Vs. Multi-Transmitter
- Direct Coupling Vs. Fiber Coupling

## Transceiver DesignChoosing a Wavelength

- X-Ray

| Typical FSO Wavelengths |  |
| --- | --- |
| 785-850 nm | ~380 THz |
| 1550 nm | ~190 THz |
| 10000nm (10 micron) | ~30 THz |

- FSO wavelengths selected for low atmospheric molecular absorption

## Transceiver DesignChoosing a Wavelength

| Wavelength
(nm) | Advantages | Disadvantages |
| --- | --- | --- |
| 785-850 | Low Cost
High Sensitivity/Large Area Silicon Detectors | Relatively Easy to Detect
Close to Visible Wavelengths |
| 1550 | Higher Power Vs. Eye-safety
Same Components as Fiber
Compatible w/ EDFAs | High Cost
Small InGaAs Detectors |
| 10000 | Better Fog Penetration????
Higher Power Vs. Eye-safety | Doesn’t Penetrate Glass
Not Compatible w/ Fiber Components
Limited Sources and Detectors |

## Transceiver DesignDivergence Vs. Pointing and Tracking

- Wide Divergence
- Easier to keep on target
- Low cost
- Narrow Divergence
- Stronger signal at the receiver
- Longer range
- Higher availability
- Requires automatic pointing and tracking for building motion
- Security Considerations
- Wider beams are easier to intercept

## Transceiver DesignField of View (FOV)

- FOV is the receiver’s equivalent of divergence
- If light doesn’t originate within the defined cone, the receiver won’t see it (e.g., Laser 2)
- Smaller FOV means less noise entering system
- Small FOV makes interference highly unlikely
- High data rate detectors typically have small FOVs
- FOV also drives the need for pointing and tracking

- Laser 1

- Receiver

- Field of View Cone

- Laser 2

## Transceiver DesignAutomatic Pointing & Tracking Systems

**Transceiver Design****Automatic Pointing & Tracking Systems**

- Steering Mirror

- Compensates for base motion
- Adjusts to maintain alignment of receiver and transmitter
- Essential for narrow divergence/FOV systems

- Receiving Optics

- GimbalMotors

## Transceiver DesignSingle Transmitter Vs. Multi-Transmitter

-    Spot Size = X

-      Spot Size = X+Y

-    Distance Between Centers = Y

## Transceiver DesignSingle Transmitter Vs. Multi-Transmitter

- At long range, the multiple beams merge into one
- No inherent power or alignment advantages
- Does offer significant advantages for overcoming the effects of scintillation
- Typically not important for high availability fog limited installations

## Transceiver DesignDirect Coupling Vs. Fiber Coupling

- Fiber

- Detector

**Direct Coupling**
- Light is focused directly onto the detector
- Allows for a large FOV
- Electronics closely coupled to optics
- Low cost

**Fiber Coupling**
- Light is focused onto the end of a fiber
- Eventually the fiber connects to a detector
- May be after several FSO links
- Fiber requires a small FOV
- Pointing and tracking required
- Not required for data rates <2.5 Gbps
- The Ultimate vision of FSO** – *****Fiber through the air***