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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