Abstract
First phase
A step-tunable external cavity laser with two Fabry-Pérot
etalon filters is demonstrated. The angle of one etalon induces
a step-tuning by 100 GHz. And the possibility that a steptuning is induced by the variation of a refractive index is
shown.
Second phase
I propose a new external cavity laser which can be step-tuned
based on the Vernier effect between a Fabry-Pérot etalon and
the longitudinal mode of an external cavity.
Widely Tunable External Cavity Lasers
Based on the Vernier Effect
M.Kinoshita
Outline
1. Introduction
2. Principle
External cavity laser
Vernier effect
3. Experiments
4. Summary
Introduction
optical transmission networks
Wavelength Division Multiplexing (WDM) which let
us have large transmission capacities is very important
system for the next generation communication. This
system needs widely tunable lasers in order to become
more efficient.
semiconductor laser
l1
semiconductor laser
l2
semiconductor laser
…
at present …
Fixed wavelength lasers are
used on the each channels.
l3
semiconductor laser
ln
multiplexer
～
～
The space of each channels is
usually 100 GHz (=0.8 nm).
Purpose
The realization of the step-tuning of the semiconductor laser’s frequency
spectral image
100 GHz
100 GHz
Intensity (a.u.)
100 GHz
・・・
0
detuning (GHz)
100
detuning (GHz)
200
detuning (GHz)
300
detuning (GHz)
We expect that laser frequency is step-tuned by 100 GHz.
Sampled Grating DBR laser based on Vernier effect
Sampled
Grating 1
Sampled
Grating 2
Phase
Gain
1.2
1
1.2
1
1
R1
T4  
0.5
T43  
R2
0.5
T2  
0.5
0
 0.1
193.6
0
193.5
 0.1
193.6
193.5
193.8
194

194.2
194.4
194.5
193.8
194

194.2
Beat
194.4
0
194.5
193.6
193.8
194
194.2
194.4

This monolithic array is complicated and requires a
high processing technique, although it is compact.
In this study
We use the external cavity lasers because of its simplicity,
expandability, and thermal stability.
External Cavity Diode Laser
Usually …
In the case of external cavity lasers …
AR coating
Both side facets act as
Fabry-Pérot resonator.
The facets have AR-coating.
And we made a resonator outside.
Depending on the form of the external cavity, various tuning can be
achieved. For example, single mode tuning, continuous tuning, and
widely step-tuning can be done.
External Ring-Cavity Laser
mirror
PBS
isolator
Laser Diode
Specification
cavity length
feedback ratio
output power
linewidth
100 mm
386 mm
60 %
1.7 mW (at 70 mA)
50 kHz
Fabry-Pérot etalon
transmittance
T   
1  A  R 2
1  R 2  4 R sin 2  2nL

c
the velocity
of light c

cos   
 frequency 
Free Spectral Range
finesse
transmittance

sin   n sin  
FSR
FWHM
1
L

c
FSR 
2nL cos  
f
reflectance R
loss A
refractive index n
transmittance
1
FSR
FWHM
0.5
T 1(  )
0
0
193
frequency 

194
Vernier effect
1 individual
Two etalons have
slightly different FSR
each other
transmittance
1
transmittance
T 1(  )
T 2(  )
0
0

194

194

194

194
193
frequency
193
frequency
1 beat transmittance
transmittance
1
T(  )
0
0
1 individual
transmittance
revolve
one etalon
1
transmittance
T 1(  )
T 2(  )
0
transmittance
0
193
1 beat
1
transmittance
frequency
T(  )
0
0
193
frequency
Transmission spectra of the etalon filters
FSR=95 GHz, finesse=5.1
0.1
transmittance
transmittance
1
195.8
196
196.2
196.4
0
196.6
195.8
frequency (THz)
196
196.2
196.4
196.6
frequency (THz)
beat
0.1
transmittance
0
FSR=100 GHz, finesse=36
0
resolution：6.4 GHz
195.8
196
196.2
196.4
frequency (THz)
196.6
The first phase
Step-tuning using two etalon filters
etalons
angle
6~6.5 deg
0 deg
FSR
95.0GHz
100GHz
Finesse
5.1
36

l/2 plate
optical isolator
mirror
polarizing beam splitter
(PBS)
lens
LD
laser diode
spectrum
analyzer
Experimental result
step-tuning by the angle of the etalon filter
16 ch
intensity (a.u.)
100GHz
 (deg)
6.1
6.2
6.3
6.4
195
195.5
196
196.5
frequency (THz)
197
6.5
197.5
Analysis
We calculate the dependence of the laser frequency
(= the peak of two etalon’s beat) on the etalon’s angle.
The calculated beat spectrum
transmittance
1
1
100GHz
T43 
0.5

D
0
0
195
transmittance
1

Frequency
197.5
1
100GHz
T43 
0.5
0

0
195

Frequency
197.5
The dependence of laser frequency on the etalon’s angle
laser frequency (THz)
197.5
197
196.5
experiment
実験値
calculation
計算値
196
195.5
195
5.9
6.0
6.1
6.2
6.3
angle of etalon (deg)
6.4
6.5
Problem
The loss of the etalon filters increases threshold current
and reduces the maximum output power.
with two etalons
2.0
0.2
1.5
0.15
power (mW)
power (mW)
without etalons
1.0
0.1
0.05
0.5
threshold
threshold
0
0
10
20
30
40
50
60
current (mA)
70
80
90
0
0
10
20
30
40
50
60
current (mA)
70
80
90
We tried to induce the step-tuning by
the variation of a refractive index n,
not the angle of the etalon.
So far
c
2nL cos  
refractive index
FSR 
slow
1.2
1
T43  
0.5
0
 0.1
193.6
193.8
194
194.2
194.4

193.5
194.5
1.2
1
1
T4  
We used the mechanical control
which has a slow reaction velocity.
T2  
0.5
0.5
0
0
 0.1
193.6
193.8
194
194.2
194.4

193.5
193.6
193.8
194
194.2
194.4

194.5
Next
fast
1.2
1
T43  
0.5
0
 0.1
193.6
193.5
1.2
1
1
T4  
T2  
0.5
0.5
0
0
 0.1
193.6
193.5
193.8
194

194.2
194.4
193.6
194.5
193.8
194

194.2
194.4
193.8
194

194.2
194.4
194.5
We are going to use the electrical
control which has a fast reaction
velocity.
1.3 or 1.46 mm wavelength semiconductor laser chips are used
as the etalon filters with the variability of a refractive index.
Because …
We would expect that the peak of the transmission can be
shifted of dozens GHz by the carrier plasma effect.
Both side facets act as Fabry-Pérot resonator from the
beginning.
V
Laser chips
1.46 mm laser chips
1.3 mm laser chips
300 mm
100 mm
300 mm
100 mm
Variation of the longitudinal mode by the injected current
Transmission of the 1.46 mm laser chip
194.35
194.35
Variation of the peak frequency
194.2
194.2
frequency (THz)
transmission (a.u.)
current
194.3
194.3
194.25
194.25
194.25
194.25
194.3
194.3
frequency (THz)
194.35
194.35
194.2
194.2
0
injected current (mA)
1
Problem
individual transmittance
1
1
transmittance
1
0.8
T3 
T4 


25 GHz
0.6
0.4
0.2
0
0
0
193.4
193.45
193.5
frequency
beat transmittance between two etalons
1
193.6
193.62
1
transmittance
1
193.55

193.4
25 GHz
0.8
0.6
T3 
T4  
0.4
0.2
 4
1.36210
0
0
193.4
193.4
193.45
193.5
frequency
193.55

longitudinal mode
193.6
193.62
1
1
1
1 GHz
transmittance
0.8
0.6
We should consider the longitudinal
mode of an external cavity as well as
beat of two etalons.
T3  T4  f  
0.4
0.2
5
7.54310
00
193.503
193.5024
193.504
frequency
193.505
193.506
193.507

193.508
193.509
193.51
193.5104
The second phase
Vernier effect between a Fabry-Pérot etalon
and the longitudinal mode of an external cavity
external mirror
lens
etalon
Gain
Phase
AR
HR
cavity’s mode
etalon’s mode
0.8
1
0.6
×
0.4
0.2
0.6
0.6
T1(  )
f (  )( T1(  ) )
0
193.4
193.4
0
193.45
193.5
193.55
193.6
frequency

193.65
193.7
2
0.4
0.4
0.2
0.2
4
beat
1
0.8
0.8
f( )
8.15910
1
transmittance
transmittance
1
transmittance
1
1
0
193.4
193.4
0
193.45
193.5
193.55
193.6
frequency

193.65
193.7
0
193.4
193.4
193.45
193.5
193.55
193.6
frequency

193.65
193.7
Simulation of the lasing spectra
using the transfer matrix
Transfer matrix
L
Er+ ＝ tEf+‐rEr-
Ef+
t
r
Ef+
Er-
Ef- = rEf+ + tE-
M
 1
 Er    t

  
 Er     r
 t
Er+ ＝ tEf+exp(-ikL)
ErEf- = Er-exp(-ikL)
r
  E 
t  f  
1  E f  

t 
P
0
 Er    exp( ikL)
 E f  


  

0
exp( ikL)  E f  
 Er   
P
M
Transfer matrix of a reflector
Transfer matrix of space
Result of the simulation
The calculated lasing spectrum
1 10
5
5
10
1 10
4
SMSR > 30 dB
1 10

intensity (a.u.)
3
Eout (  )
100
10
1
2
0.1
0.01
1 10
3
1 10
4
1 10
5
6
8.66910 1 10 6
14
14
14
14
14
14
14
14
14
14
14
1.94 10 1.942 101.944 101.946 101.948 10 1.95 10 1.952 101.954 101.956 101.958 10 1.96 10
12
12

19410
19610
frequency (Hz)
Lasing frequency is shifted to the next channel by variation
of the refractive index about 10-4.
Summary
We have realized the external cavity laser with two
etalon filters tuned in step of 100 GHz from 1522.8 nm
to 1534.5 nm.
The longitudinal mode of the 1.46 mm wavelength laser
chip was shifted over its FSR by the injected current’s
variation of 1 mA. It suggest that the step-tuning induced
by the variation of the refractive index can be achieved.
The spectrum of the step-tunable laser based on the Vernier
effect between a Fabry-Pérot etalon and the longitudinal
mode of an external cavity was simulated.