The Difference Between FP And DFB Laser--ETU-LINK

The Difference Between FP And DFB Laser

May 06. 2016

Recently many customers and engineering partners often ask: What are FP lasers and DFB lasers? What’s the real difference between FP and DFB lasers in optical modules? To help you explain clearly to customers and quickly choose the right laser for your project, we’ve made this detailed professional summary about FP laser vs DFB laser.

I. What Is an FP laser?

FP (Fabry-perot) laser is mainly used for low data rate short-distance transmission, the transmission distance is generally within 20 km,the rate is generally within 1.25G. FP is divided into two wavelengths, 1310nm/1550nm.

Some customer in order to decrease the cost, so chose the FP laser to make the 1.25G 40km module. Reason by make the module reach the 40km transmission distance, the engineer must adjust the output power rather bigger to meet the distance, but it will make the laser ageing fastly cause more issue.

What Are the Key Performance Parameters of FP Lasers?

The performance parameters of FP laser:

1) Working wavelength: Emitted the center wavelength of spectrum by laser.

2) The width of spectral: RMS spectral width of multi-longitudinal mode laser.

3) Threshold current: The laser send out a very good laser when the working current of the device exceeds the threshold current.

4) Output power: The optical power emitted by the output port of the laser.

The typical parameters are shown in the following table:

Parameters	Symbol	Unit	Test Condition	Minimum	Typical	Maximum
Working Wavelength	λp	nm	Iw=Ith+20mA	1290	1310	1330
Working Wavelength	λp	nm	Iw=Ith+20mA	1530	1550	1570
Spectral Width	Δλ	nm	Iw=Ith+20mA	—	3	5
Threshold Current	Ith	mA	—	5	20	—
Output Power of Pigtail	Po	mW	Iw,1310nm	0.3	—	2.5
Output Power of Pigtail	Po	mW	Iw,1550nm	0.3	—	2
Forward Voltage	Vf	V	Iw=Ith+20mA	—	1.1	1.5

FP lasers belong to multi-longitudinal mode lasers, with low cost and simple structure. They are the most commonly used laser type in low-speed short-range optical transceivers.

II. What Is a DFB laser?

DFB ( Distributed Feedback Laser) laser use the grating filter device to make the device have only one longitudinal-mode output based on the FP laser.DFB generally also use two wavelengths 1310nm and 1550nm, divides into refrigeration and No-refrigeration, mainly used for high data rate long-distance transmission.The transmission distance is generally above 40 kilometers, the rate is above 1.25G.

What Are the Key Performance Parameters of DFB Lasers?

The performance parameters of DFB laser:

1) Working wavelength: Emitted the center wavelength of spectrum by laser.

2) The suppression ratio of side mode: The power ratio of the main mode and the maximum side mode of the laser.

3) Decreased the spectral width of 20dB: Decreased the spectral width of 20dB by the highest point of the laser output spectrum.

4) Threshold current: The laser send out a very good laser when the working current of the device exceeds the threshold current.

5) Output power: The optical power emitted by the output port of the laser.

The typical parameters are shown in the following table:

Parameters	Symbol	Unit	Test Condition	Minimum	Typical	Maximum
Working Wavelength	λp	nm	Iw=Ith+20mA	—	1310	—
Working Wavelength	λp	nm	Iw=Ith+20mA	—	1550	—
Spectral Width	Δλ	nm	Iw=Ith+20mA	—	0.3	0.55
Threshold Current	Ith	mA	—	15	20	—
Output Power of Pigtail	Po	mW	Iw,1310nm	0.3	—	2.5
Output Power of Pigtail	Po	mW	Iw,1550nm	0.3	—	2
Forward Voltage	Vf	V	Iw=Ith+20mA	—	—	—

DFB lasers are single longitudinal mode lasers with built-in grating structure. They feature narrow linewidth, high side mode suppression ratio, low dispersion, and stable transmission, making them ideal for high-speed and long-distance optical transmission.

III. What Are the Main Differences Between FP and DFB Lasers?

The main difference can be got by the form between the FP and DFB laser is that the spectral width is different. The spectral width of the DFB laser is generally narrow, is a single longitudinal mode with distributed negative feedback. While the spectral width of the FP laser is relatively wide, it is a multi-longitudinal mode laser. The working wavelength, threshold current and voltage are also different.

IV. FAQ: Common Questions About FP vs DFB Lasers

(1). Can FP Lasers Be Used for 40km or Longer Transmission?
Not recommended. Although increasing power can barely reach 40km, it accelerates aging, reduces stability, and increases failure risk. For 40km+, DFB laser is the standard and reliable solution.

(2). Which Is Better: FP Laser or DFB Laser?
No absolute better—choose by scenario. Short-distance & cost-sensitive → FP; long-distance & high-speed → DFB. DFB offers better performance but higher cost; FP is cost-effective for short-range.

(3). Are DFB Lasers Only Used in 1550nm?
No. DFB lasers are widely used in both 1310nm and 1550nm, as well as CWDM/DWDM wavelength plans. They support cooling and uncooled versions for industrial and telecom environments.

(4). What Types of Optical Modules Use FP/DFB Lasers?
- FP laser: 1.25G SFP 20km, 100M Ethernet, PON ONU, cheap short-range modules
- DFB laser: 1.25G 40km/80km, 10G SFP+, 25G SFP28, 100G QSFP28, CWDM/DWDM modules, base station fronthaul

Thanks for your reading! Hope these shares will give you some understanding of FP and DFB lasers! Welcome to discuss with the industry’s elite together, also hope to provide your valuable views and suggestion, creating progress together with you！

Last updated: May 11, 2026

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