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The Differences between FP laser and DFB laser

Posted on 25th Sep 2017 @ 6:18 PM

FP Laser

FP (Fabry-perot) laser puts FP cavity to cavity, multi-longitudinal mode coherent light emitted semiconductor light emitting device. FP laser is mainly used for low-speed short-distance transmission, such as transmission distance is generally less than 20 kilometers, the rate is generally less than 1.25G, at the two wavelengths of the FP-1310nm / 1550nm.

Some manufacturers want to reduce the production cost, so they making Gigabit 40km module with FP devices so as to achieve the appropriate transmission distance, it will turn up the power of the emitted light on the market for a long time to make the product work devices premature aging, shorten life. The recommendation of the engineer's 1.25G 40km of dual-fiber modules, applications DFB device more reliable!

FP laser performance parameters:

1) Wavelength: center wavelength emitted by the laser spectrum.

2) Spectral width: multi-longitudinal-mode laser rms spectral width.

3) Threshold current: When the operating current of the device exceeds the threshold current laser emits coherent laser well.

4) output optical power: laser output power port optical power emitted
Typical parameters as following:

Parameter

Symbol

Unit

Conditions

Min.

Typical

Max.

Wavelength

λp

nm

IW=Ith+20mA

1290

1310

1330

1530

1550

1570

Spectral bandwidth

Δλ

nm

IW=Ith+20mA

 

3

5

Threshold Current

Ith

mA

 

5

20

 

Pigtail output power

Po

mW

IW, 1310nm

0.3

 

2.5

IW, 1550nm

0.3

 

2

Forward Voltage

vf

v

IW=Ith+20mA

 

1.1

1.5

DFB lasers

DFB lasers is based on FP lasers using grating-optical device consider the device has only one longitudinal mode output. DFB (Distributed Feedback Laser) generally use two wavelengths 1310nm, 1550nm, divided into refrigeration and no refrigeration, mainly for high-speed long-distance transmission, the transmission distance of 40 km.

DFB laser performance parameters:

1) Wavelength: center wavelength emitted by the laser spectrum.
2) Side-mode suppression ratio: the main mode laser operating with maximum side mode power ratio.

3) -20dB spectral width: from the highest point of the laser output spectrum at a spectral width is reduced 20dB.

4) Threshold current: When the operating current of the device exceeds the threshold current laser emits coherent laser well.

5) Output optical power: laser output power port optical power emitted

Typical parameters as the following:

Parameter

Symbol

Unit

Test Conditions

Min.

Typical

Max.

Wavelength

λp

nm

IW=Ith+20mA

 

1310

 

 

1550

 

Spectral bandwidth

Δλ

nm

IW=Ith+20mA

 

0.3

0.55

Threshold Current

Ith

mA

 

15

20

 

Pigtail output power

Po

mW

IW, 1310nm

0.3

 

2.5

IW, 1550nm

0.3

 

2

Forward Voltage

vf

v

IW=Ith+20mA

 

 

 

From the above tables we can see that the main difference between FP and DFB laser is the different spectral width, the DFB spectral width is generally narrower than the FB laser, which is a distributed single-longitudinal-mode negative feedback. The FP laser spectral width is relatively wide, is a multi-longitudinal mode lasers. Their operating wavelength, threshold current and forward voltage are also different.

In summary, FP and DFB lasers working principles are different; DFB laser is with higher production cost and suitble for long-distance transmission.

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