Abstract

1
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 step-tuning 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.
2
Widely Tunable External Cavity LasersBased on
the Vernier Effect
M.Kinoshita
Outline

• 1. Introduction
• 2. Principle
• External cavity laser
• Vernier effect
• Experiments
• Summary

3
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
The space of each channels is usually 100 GHz
(0.8 nm).
semiconductor laser l2


semiconductor laser l3
multiplexer
at present

Fixed wavelength lasers are used on the each
channels.
semiconductor laser ln
4
Purpose
The realization of the step-tuning of the
semiconductor lasers frequency
spectral image
100 GHz
100 GHz
100 GHz
Intensity (a.u.)

0
100
200
300
detuning (GHz)
detuning (GHz)
detuning (GHz)
detuning (GHz)
We expect that laser frequency is step-tuned by
100 GHz.
5
Sampled Grating DBR laser based on Vernier effect
Sampled Grating 1
Sampled Grating 2
Gain
Phase
R1
R2
Beat
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.
6
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.
7
linear cavity
?? ????? ???? ??(???????) ?????? ????????
lens
mirror
laser diode
LD
AR coating
ring cavity
?? ??????????? ??? ???????????? ??????
optical isolator
LD
8
External Ring-Cavity Laser
PBS
mirror
isolator
100 mm
Laser Diode
Specification
cavity length 386 mm feedback ratio 60 output
power 1.7 mW (at 70 mA) linewidth 50 kHz
9
Fabry-Pérot etalon
transmittance
reflectance R
the velocity of light c
loss A
L
frequency n
Free Spectral Range
finesse
refractive index n
transmittance
1
FSR
FWHM
transmittance
0.5
0
frequency n
10
Vernier effect
individual transmittance
1
Two etalons have slightly different FSR each
other
transmittance
0
frequency
beat transmittance
1
transmittance
revolve one etalon
0
frequency
individual transmittance
1
transmittance
0
frequency
beat transmittance
1
transmittance
0
frequency
11
Transmission spectra of the etalon filters
FSR95 GHz, finesse5.1
FSR100 GHz, finesse36
1
0.1
transmittance
transmittance
0
0
196
196.2
196.4
196.6
195.8
196
196.2
196.4
196.6
195.8
frequency (THz)
frequency (THz)
beat
0.1
transmittance
resolution6.4 GHz
0
196
196.2
196.4
196.6
195.8
frequency (THz)
12
The first phase
Step-tuning using two etalon filters
etalons
angle 66.5 deg 0 deg
FSR 95.0GHz 100GHz
Finesse 5.1 36
polarizing beam splitter (PBS)
l/2 plate
optical isolator
lens
LD
mirror
laser diode
spectrum analyzer
13
Experimental result
step-tuning by the angle of the etalon filter
16 ch
100GHz
q (deg)
intensity (a.u.)
6.1
6.2
6.3
6.4
6.5
196
196.5
195
195.5
197
197.5
frequency (THz)
14
Analysis
We calculate the dependence of the laser
frequency ( the peak of two etalons beat) on
the etalons angle.
The calculated beat spectrum
1
100GHz
transmittance
0.5
Dq
0
Frequency
1
100GHz
0.5
transmittance
0
Frequency
15
The dependence of laser frequency on the etalons
angle
197.5
197
196.5
laser frequency (THz)
experiment
calculation
196
195.5
195
5.9
6.0
6.1
6.3
6.2
6.4
6.5
angle of etalon (deg)
16
Problem
The loss of the etalon filters increases
threshold current and reduces the maximum output
power.
without etalons
with two etalons
0.2
2.0
0.15
1.5
0.1
1.0
power (mW)
power (mW)
0.05
0.5
threshold
threshold
0
0
0
10
20
30
40
50
90
80
70
60
0
90
80
60
10
20
30
40
50
70
current (mA)
current (mA)
17
We tried to induce the step-tuning by the
variation of a refractive index n, not the angle
of the etalon.
refractive index
So far
slow
We used the mechanical control which has a slow
reaction velocity.
Next
fast
We are going to use the electrical control which
has a fast reaction velocity.
18
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
19
Laser chips
1.46 mm laser chips
1.3 mm laser chips
100 mm
300 mm
100 mm
300 mm
20
Variation of the longitudinal mode by the
injected current
Variation of the peak frequency
Transmission of the 1.46 mm laser chip
194.35
current
194.3
frequency (THz)
transmission (a.u.)
194.25
194.2
194.35
194.2
194.3
194.25
0
1
injected current (mA)
frequency (THz)
21
Problem
individual transmittance
1
25 GHz
transmittance
0
frequency
beat transmittance between two etalons
1
25 GHz
transmittance
0
longitudinal mode
frequency
1
1 GHz
We should consider the longitudinal mode of an
external cavity as well as beat of two etalons.
transmittance
0
frequency
22

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
cavitys mode
etalons mode
beat

transmittance
transmittance
transmittance
frequency
frequency
frequency
23
Simulation of the lasing spectra using the
transfer matrix
Transfer matrix
P
M
Transfer matrix of a reflector
Transfer matrix of space
24
Result of the simulation
The calculated lasing spectrum
SMSR gt 30 dB
intensity (a.u.)
frequency (Hz)
Lasing frequency is shifted to the next channel
by variation of the refractive index about 10-4.
25
(No Transcript)
26
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 currents 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.
27
?????
???????????????????????FSR????????????????????????
??
????????????????????????????????????????????
??????????????????? (3040ch??????)