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.
At the beginning, I’d like to talk about the abstract of my presentation.
At the first phase, a step-tunable external cavity lasers with two Fabry-Perot etalon filters is demonstrated. The angle of one etalon induces a step-tuning by one hundred GHz. And the possibility that a step-tuning is induced by the variation of a refractive index is shown.
At the second phase, I propose a new external cavity laser which can be step-tuned based on the Vernier effect between a Fabry-Perot etalon and the longitudinal mode of an external cavity.
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 Lasers Based on the Vernier Effect
M.Kinoshita
The title of my presentation is “widely tunable external cavity lasers based on the Vernier effect”. And I show you the outline of my presentation. I’d like to start with the introduction, then talk about principles of the external cavity laser and Vernier effect, experiments, and summary.
Outline
1. Introduction
2. Principle
External cavity laser
Vernier effect
Experiments
Summary

3. optical transmission networks
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.
In the optical transmission networks, Wavelength Division Multiplexing, we call it WDM, which let us have large transmission capacities is very important system for the next generation communication. This system has many channels. The space of each channels is usually one hundred GHz in frequency, or o point eight nm in wavelength. At present, fixed wavelength lasers are used on the each channels.
So, 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 laser’s frequency
spectral image
100 GHz
100 GHz
100 GHz
The purpose of this study is the realization of the step-tuning of the semiconductor laser’s frequency. This figures show spectral images. Like this, we expect laser frequency is step-tuned by one hundred GHz.
Intensity (a.u.)
・・・
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
Gain
Phase R1 R2
Recently, sampled grating or super structure grating DBR lasers based on Vernier effect has attracted attention as widely tunable light source. But this monolithic array is complicated and requires a high processing technique, although it is compact. So, in this study, we use the external cavity lasers because of its simplicity, expandability, and thermal stability.
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
In the usual semiconductor lasers, both facets act as Fabry-Perot resonator. On the other hand, In the case of external cavity lasers, the facets have AR-coating. And we made a resonator outside. Hereby 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.
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 長所 効率が良い lens 扱い易い mirror laser diode 短所（本実験における） 戻り光に弱い
　効率が良い
　扱い易い
短所（本実験における）
　戻り光に弱い
　光が往復している
lens
mirror
laser diode
etalon LD
AR coating
ring cavity 長所
　空間的ホールバーニング
　がない
　戻り光を防ぐことができる
　光は一方通行
optical isolator LD

8. External Ring-Cavity Laser
mirror
PBS
isolator
This slide shows the picture of the external ring-cavity laser which is used in this experiment. A laser diode is inside this box. This ring-cavity consists of three mirrors, one polarizing beam splitter and optical isolator. The specification of this ring laser is shown here. The cavity length is three hundred and eighty-six mm, the ratio of the feedback power to the output power is sixty %, output power is one point seven mW at seventy mA injection current, and the linewidth of the lasing spectrum is about fifty kHz.
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 Free Spectral Range finesse
reflectance R
the velocity
of light c
loss A L
frequency n
Free Spectral Range
We use the Fabry-Perot etalons. The transmittance, free spectral range, and finesse of the etalon is shown in this slide. And this graph shows the dependence of etalon's transmittance on the frequency according to this equation.
finesse
refractive index n
transmittance 1
FSR
FWHM
transmittance
0.5
frequency n

10. slightly different FSR
Vernier effect
individual transmittance 1
Two etalons have
slightly different FSR
each other
transmittance
beat transmittance
frequency 1
transmittance
revolve one etalon
This slide describes the Vernier effect. We use two etalons that have slightly different FSR each other. This graph shows individual etalon’s transmittance, and this graph shows the beat transmittance between two etalons. If one etalon is revolved, the frequency of beat peak is shifted to the next channel. Because, one etalon’s FSR is changed. In case these etalons are inside the external cavity, laser frequency is equal to the peak frequency of this beat. Therefore, we can control laser frequency by the etalon’s angle.
frequency
individual transmittance 1
transmittance
frequency
beat transmittance 1
transmittance
frequency

11. Transmission spectra of the etalon filters
FSR=95 GHz, finesse=5.1
FSR=100 GHz, finesse=36 1
0.1
transmittance
transmittance
195.8
This slide shows the transmission spectra of the etalon filters. One etalon’s FSR is ninety-five GHz, another one is a hundred GHz. And we observed the beat spectrum between two etalon’s transmittance.
196
196.2
196.4
196.6
195.8
196
196.2
196.4
196.6
frequency (THz)
frequency (THz)
beat
0.1
transmittance
resolution：6.4 GHz
195.8
196
196.2
196.4
196.6
frequency (THz)

12. polarizing beam splitter
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
polarizing beam splitter
(PBS)
In the first phase, I demonstrate step-tuning using two etalon filters. This slide illustrates external ring-cavity laser with two etalon filters. We measured the dependence of the laser spectra on the etalon’s angle q using a spectrum analyzer.
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.)
This is the experimental results of the dependence of the laser spectra on the etalon’s angle. We achieved the step-tuning by the angle of the etalon filter. Each peak is separated by one hundred GHz in frequency. And total tunable range is twelve nm in wavelength.
6.1
6.2
6.3
6.4
6.5
195
195.5
196
196.5
197
197.5
frequency (THz)

14. 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 1
100GHz
transmittance
0.5
Here, we calculated the dependence of the laser frequency which is equal to peak of two etalon’s beat on the variation of q. This figures show the calculated beat spectrum. Dq
Frequency 1
100GHz
transmittance
0.5
Frequency

15. The dependence of laser frequency on the etalon’s angle
197.5
197
196.5
laser frequency (THz)
This figure shows the dependence of laser frequency on the etalon’s angle. The red open circles mean calculated results. The blue closed circles mean experimental results. The calculated result is consistent with the experimental result. So, it must be reasonable.
experiment
calculation
196
195.5
195
5.9
6.0
6.1
6.2
6.3
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
2.0
0.2
1.5
0.15
The problem of this laser is that the loss of the etalon filters increases threshold current and reduces the maximum output power. The left figure shows dependence of the output power on the injection current without etalon filters. Right figure shows dependence of the output power on the injection current with two etalon filters. In right figure, threshold current is higher and maximum output power is lower than left one.
power (mW)
1.0
power (mW)
0.1
0.5
0.05
threshold
threshold 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90
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
Then, we tried to induce the step-tuning by the variation of a refractive index n, not the angle of the etalon. So far, we used the mechanical control such as etalon’s angle which has a slow reaction velocity. While we are going to use the electrical control which has a fast reaction velocity.
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.
One point three or one point four six 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 dozen GHz by the carrier plasma effect. And both side facets act as Fabry-Perot resonator from the beginning.
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 300 mm 100 mm
This slide shows the pictures of the laser chips. According to these pictures, laser chips have a waveguide and anode.
300 mm
100 mm
300 mm
100 mm

20. Variation of the longitudinal mode by the injected current
Transmission of the 1.46 mm laser chip
Variation of the peak frequency
194.35
current
194.3
transmission (a.u.)
The left figure shows the dependence of the transmission spectra of the one point four six mm laser chip on the injected current. And the right figure shows the dependence of the each peak frequency on the injected current. According to this figure, the total variation of the peak frequency is wider than its FSR. It suggest that the step-tuning induced by the variation of the refractive index can be achieved. (But we have not achieved it yet.)
frequency (THz)
194.25
194.2
194.2
194.25
194.3
194.35 1
frequency (THz)
injected current　(mA)

21. Problem
individual transmittance 1
25 GHz
transmittance
frequency
beat transmittance between two etalons 1
In fact, we should consider the longitudinal mode of an external cavity as well as beat of two etalons. We expect that the external cavity’s FSR is shorter than one GHz. It is very complicated to consider the beat between these three components.
25 GHz
transmittance
frequency
longitudinal mode 1
1 GHz
We should consider the longitudinal mode of an external cavity as well as beat of two etalons.
transmittance
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
To resolve this problem, we propose a new external cavity laser which can be step-tuned based on Vernier effect between a Fabry-Perot etalon and the longitudinal mode of an external cavity. This laser consists of a two-section Fabry-Perot semiconductor laser with AR and HR coating, lens, etalon filter, and external mirror. We expect the injection current to the phase section induce the step-tuning. HR AR
cavity’s mode
etalon’s mode
beat ×
transmittance
transmittance
transmittance
frequency
frequency
frequency

23. Simulation of the lasing spectra using the transfer matrix
Ef+
Ef- = Er-exp(-ikL)
Er+ ＝ tEf+exp(-ikL)
Er- L M
Ef+
Ef- = rEf+ + tE-
Er+ ＝ tEf+‐rEr-
Er- r t
We simulated the lasing spectra using the transfer matrix. Transfer matrix is the method for describing the relation between the left-side electric field and right-side one. For example, this is transfer matrix of a reflector M. And this is transfer matrix of space P. The lasing spectra are calculated by applying these matrixes to the components of the external cavity laser. P M
Transfer matrix of a reflector
Transfer matrix of space

24. Result of the simulation
The calculated lasing spectrum
SMSR > 30 dB
intensity (a.u.)
This figure shows the calculated lasing spectrum. According to this simulation, the side mode suppression ratio is better than thirty dB and the lasing frequency is shifted to the next channel by variation of the refractive index about ten to the minus four. We can design the new external cavity lasers using this method.
frequency (Hz)
Lasing frequency is shifted to the next channel by variation of the refractive index about 10-4.

26. Summary
We have realized the external cavity laser with two etalon filters tuned in step of 100 GHz from nm to 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.
We have realized the external cavity laser with two etalon filters tuned in step of one hundred GHz from one thousand five hundred and twenty-two point eight nm to one thousand five hundred and thirty-four point five nm.
The longitudinal mode of the one point four six mm wavelength laser chip was shifted over its FSR by the injected current’s variation of one mA. It suggest that step-tuning induced by the variation of the refractive index can be achieved.
The spectrum of the step-tunable laser on the Vernier effect between a Fabry-Perot etalon and the longitudinal mode of an external cavity was simulated.
Thank you for your kind attention.
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. 今後の展望 一方のエタロンをレーザーチップに置き換え、そのFSRを電気的に制御することで、高速での変調を実現させる。
共振器の縦モードとエタロンによるバーニア効果を利用した外部共振器型半導体レーザーの設計。
変調可能な周波数チャンネル数を増やす。
（30～40chくらいが目標）