2007 Issue 2 - Raytheon

raytheon

2007 Issue 2 - Raytheon

onTechnology

Fiber Communication Technology

Benefits Eye-Safe Laser Development

Over the past couple of decades, an

enormous amount of effort and investment

has been made in the area of fiber optic

communications technology and equipment.

This area is now providing a great

deal of benefit toward the development of

the next generation of highly efficient and

compact eye-safe laser sources.

Existing Eye-Safe Tactical Laser

Technology

Most existing eye-safe tactical laser systems

start out with a non-eye-safe Nd:YAG laser

source that transmits at 1.064 micron. The

non-eye-safe wavelength is then converted

to an eye-safe wavelength using a Raman

cell (1.54 micron) or an Optical Parametric

Oscillator (OPO) that emits at 1.57 micron.

These conversion techniques are effective but

are inefficient, add weight and consume

additional space. Wall plug efficiency of a

system like this is generally around 8 percent.

Adapting Fiber Communication

Hardware for Tactical Eye-safe Lasers

Erbium-doped fiber amplifiers (EDFA) have

become a main component of the telecommunications

industry. A typical EDFA in

telecommunications is used to amplify a

signal to be transmitted over extended

distances. Telecom uses erbium-doped

silica fibers that transmit in the Near

Infrared (NIR), because they have less

attenuation and dispersion than visible.

Also EDFAs intrinsically transmit in this

same NIR region.

Signal

Source

Diode Pump Source

• 1480 nm

• 980nm

Amplified

Output

• 1538 nm, C-Band

• 1617 nm, L-Band

Figure 1. Simple Er-doped fiber amplifier

26 2007 ISSUE 2 RAYTHEON TECHNOLOGY TODAY

Corner Cube

Er:YAG Laser Rod

Q-Switch

Assembly Pump Diodes

1480 nm

Figure 2. Direct eye-safe 1.617 um laser resonator

The laser diode pump sources with output

wavelengths of either 1480 or 980

nanometers are used to optically pump the

erbium-doped fiber and provide a significant

(30 dB) signal gain out of the overall

amplifier at 1538 or 1617 nanometers. Both

of these diode sources are widely available

today because of the investments made by

the telecommunications industry. The 1538

nanometer output is within the C-Band or

conventional band (C-Band: 1530-1570 nm)

of operation. The 1617 nm output is within

the L-Band, or long band (L-Band: 1570-

1620 nm) of operation.

980 Nanometer GaAs vs. 1480

Nanometer InP Pump Diodes

The erbium-doped silica fibers have an

extremely long absorption length enabling

practical 980 nm pumping with the higher

maturity GaAs-based diode sources.

Although the quantum efficiency of 980 nm

pumped EDFAs is only ~65 percent, the

overall efficiency of the 980 nm pumped

EDFA is still respectable due to the 100 percent

pump absorption within the long fiber

gain length and high wall-plug efficiencies

(>50 percent) of the 980 nm diodes.

In a bulk Er-doped crystal (e.g. YAG), 980

nm pumping is not practical unless a high

co-doping of Yb is utilized. In crystal hosts,

however, the Yb-Er energy transfer is poor,

therefore rendering this type of pump

implementation impractical for efficient

pumping of bulk lasers. Er-doped crystals

are preferentially pumped at the resonant

state near 1500 nm (1480nm) where the

absorption is much stronger and broader

than at 980 nm. This enables efficient pump

absorption and, therefore, the overall high

efficiency operation of short bulk Er:crystal

E O / L A S E R S

Pump Resonator

Output Coupler

1.617 Microm Output

1.617 um Energy

gain geometries. Pumping an Er:YAG 1617

nm laser with a 1480 nm source correlates

to a very high 92 percent quantum efficiency.

The high quantum efficiency and broad

absorption range of erbium at 1480 nm

makes these diodes an ideal choice for

pumping bulk solid state laser sources for

tactical laser sensor applications.

Although less mature as compared to the

GaAs-based 980nm diodes, the 1480 nm

InP–based diodes are gaining in wall-plug

efficiency and power levels that rival the

980 nm devices. Kilowatt class multi-bar

stack diode arrays are currently available

from a number of suppliers. These larger

packages can be incorporated into solid

state laser transmitters that utilize a much

larger laser gain medium than an erbiumdoped

silica fiber. Gain medium such as a

rod or slab made of Er:YAG are ideal for

these applications. Figure 2 shows a simplified

block diagram of a direct eye-safe

1.617 micron laser. Wall plug efficiency of a

system like this is 35 percent or greater.

Recent development work in this area has

yielded impressive results. An experiment

using a 30 mm long Er:YAG laser rod and

1480 nm pump sources demonstrated 7

watts of average output power at three different

pulse rates (3 kHz, 4 kHz and 5 kHz).

Utilizing this technology, eye-safe laser systems

can be created with significant pulsed

output energies and a variety of pulse formats

that are capable of filling numerous

current and future sensor needs. •

Douglas A. Anderson

daanderson@raytheon.com

Kalin Spariosu

kalin_spariosu@raytheon.com

Y E S T E R D A Y … T O D A Y … T O M O R R O W

More magazines by this user
Similar magazines