Novel Low Phase Noise Low Amplitude Noise Semiconductor Laser · Importance of Low Laser Phase Noise FM Communication In frequency modulation (FM) formats phase or frequency fluctuations

Novel Low Phase NoiseLow Amplitude Noise Semiconductor Laser

Alex Rosiewicz, S. Coleman*,

J. Traynor, N. Kiner, T. Som

*current address: Physical Sciences Inc., Andover, MA

Outline

Outline

� Laser parameters

� Why low phase noise is important

� Existing approaches to low phase noise lasers

� Description of new design

� Performance of new design

� Comparison of new design performance with existing approaches

� Summary and conclusions

Laser Parameters

Amplitude Noise (RIN)

■ Dominates in direct detection scheme particularly in analog systems

Frequency or Phase Noise

■ Linewidth

Frequency Stability

■ Over time period of interest

Output Power

■ Fiber coupled

Tuning Range (Frequency)

■ Mode-hop free range

Importance of Low Laser Phase Noise

FM Communication

■ In frequency modulation (FM) formats phase or frequency fluctuations compete

with the signal. Reducing these fluctuations increases signal to noise ratio.

Interferometric Sensor Measurements

■ A laser signal may be used to interrogate path length changes in sensors such

as a fiber Bragg grating (FBG) or Mach-Zehnder interferometer (MZI) . If the

frequency or phase of the laser changes it appears as a change of the path

length thus degrading signal to noise ratio.

Existing Low Phase Noise Lasers

Solid state ring laser

■ Usually an Nd:YAG DPSS laser

■ A very long ring cavity is created

within the Nd:YAG stabilizing the

phase.

Fiber laser

■ The laser is created using pairs of

FBG’s at the ends of a long fiber. The

large optical path length is enabled by

the fiber.

Pump

Laser

FBG

Output

FBG

Existing Low Phase Noise Lasers

External cavity semiconductor laser

■ An FBG or VBG close to the laser chip

provides optical feedback stabilizing

the phase of the laser mode

DFB with electronic feedback

■ A small amount of power is tapped

from the output and passed through an

FBG which acts as an FM to AM

converter. The error signal is then

used to change laser drive current to

reduce FM fluctuations.

New Design Approach

In the new approach a standard DFB laser is stabilized by providing very low

level optical feedback to the chip from an external reflector.

DFB Laser

Fused

Splitter

Reflector

Isolator

Output

Design Detail

Use a PM fiber coupled DFB laser

■ Drive with low noise electronics

Split the output through a fused fiber coupler

Feedback leg provides reflection back to laser chip

■ Stable reflection from fiber end face

Output leg passes through external 55dB isolator

■ Ensures no feedback from output leg

Laser Operation

� The reflective element is currently formed by cleaving the fiber at a distance of

~4m from the chip

� The feedback to the laser chip from the reflector is in the range -30dB to -80dB

� The output leg runs through a secondary isolator (>55dB) to ensure controlled

feedback dominates laser behaviour

� The feedback acts as a low reflectivity external cavity that controls the laser

phase

• The external cavity does not need to be length controlled

� The free-spectral range (FSR) cavity modes of the external cavity are evident in

the laser noise spectrum

Mechanism of Operation

� The relatively slowly varying coherent feedback from the uncontrolled cavity

length shifts the absolute (center) oscillating frequency of the laser off the gain

peak established by the semiconductor laser and associated driver in the

presence of no feedback.

� The slow variation in length results in a slight destabilization of the laser

oscillating frequency to within 1 FSR of the secondary cavity.

� Thus the absolute laser line position drifts with ambient perturbations in

exchange for significant spectral narrowing. Since the FSR of the secondary

cavity is typically of the order of 15 MHz the change in line position is very low

compared to the oscillating laser frequency (eg. 193 THz).

EM750 Narrow Linewidth Laser

Based on Existing EM650 Module

Features

� Narrow Linewidth

� Low RIN

� High Power

� Tunable

� Pin-Compatible with EM650

� Custom and 50 GHz ITU Grid Wavelengths Available

Applications

� Sensors

� Spectroscopy

� Analog Communications� LIDAR

1kHz RBW 20min Average LinewidthCourtesy V. Urick (Naval Research Laboratories)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5-60

-50

-40

-30

-20

-10

0

Freq (MHz)

Norm

aliz

ed A

mplit

ude (

dB

)

4.6kHz Lorentzian

Measurement

Linewidth Reduction Range

The linewidth reduction mechanism is effective up to one FSR about the absolute

laser oscillating frequency.

■ ~18 MHz about 193.4THz in this case

Thereafter linewidth is the same as a free-running laser

Illustrated by measuring RIN through a short interferometer and fitting to the

measured data

■ See:

• R. W. Tkach and A. R. Chraplyvy, “Phase Noise and Linewidth in an InGaAsP DFB

Laser,” Journal of Lightwave Technology, LT-4, No. 11, Nov. 1986

Measured/Fit RIN Through MZI(fit assumes 1kHz Lorentzian)

Measured/Fit RIN Through MZI(fit assumes 200kHz Lorentzian)

Unlocked and Locked Laser

-5 -4 -3 -2 -1 0 1 2 3 4 5

x 106

-30

-25

-20

-15

-10

-5

0

Lorentzian Linewidth 216.7653kHz

Fourier Frequency (Hz)

Am

plit

ud

e (

dB

)

E0038410 Unlocked EM650

Measured

Lorentzian Fit

-5 -4 -3 -2 -1 0 1 2 3 4 5

x 106

-70

-60

-50

-40

-30

-20

-10

0

Lorentzian Linewidth 2.2546kHz

Fourier Frequency (Hz)

Am

plit

ud

e (

dB

)

E0038410 Locked

Measured

Lorentzian Fit

Absolute Frequency Cycling shown via Heterodyning

Locked and Unlocked Laser

(video screenshots)

1s 4s 6s

8s 8s 12s

“Snap” back

Phase Noise Spectra

EM750

Comparative Phase Noise of EM750, EM650 and NPRO

Comparative Phase Noise of EM750 and RIO

(improved low frequency measurement accuracy)

RIN Comparison

104

105

106

107

108

109

1010

-170

-165

-160

-155

-150

-145

-140

-135

-130

-125

-120

Frequency (Hz)

RIN

(dB

c/H

z)

EM750

- 0

Low

frequency

artifacts

removed

1E+04 1E+10

-120

-165

-120

-165

Narrow Linewidth Laser Comparison

Manufacturer /

Model

Linewidth RIN Output Power Tuning

80mW DFB w/

Lab Grade

Drivers

540kHz <-150 dB/Hz 80 mW ±100 GHz

EM4 EM650 170kHz EM4 typ. <-155 dB/Hz 80 mW ±100 GHz

EM750 5kHz (20 min) <-155 dB/Hz 30 mW – 70 mW ±100 GHz

JDSU NPRO <5kHz/ms (rated) <-125 dB/Hz 25 mW @ 1064nm

200 mW @ 1319nm±15 GHz

RIO Planex 3 kHz <-140 dB/Hz 10 mW ±3.7 GHz

NP Photonics

Rock

700-1kHz <-110 dB/Hz 25 - 125 mW ±30 GHz

Teraxion <5kHz <-130 (1kHz-10kHz) 80 mW ±10 GHz

NKT Koheras

Basik EIS 1.5um

<100Hz or

50kHz w/ “Low” RIN

(-140dBc/Hz)

<-100 @ Peak

<-135 @ 10MHz

40 mW ±62 GHz

Summary/Conclusion

� We have built and demonstrated a novel low phase noise laser (EM750)

� The laser is easily manufactured based upon an existing integrated DFB laser

(EM650)

� It has a combination of low phase noise, low RIN, wide tuning range and high

output power and is economical to produce

� The phase noise reduction is confined to the FSR of the external cavity

� As others have said “One Technique Does Not Fit All” but this new laser can

play an important role in the parameter space

THANK YOU

Alex Rosiewicz

Acknowledgements

Vince Urick, Joe Singley, John Diehl – NRL

■ For test, measurement, valuable discussion and insight