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.

M.Kinoshita

Widely Tunable External Cavity LasersBased on the Vernier Effect

Outline

1. Introduction2. Principle

External cavity laserVernier effect

3. Experiments4. 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

semiconductor laser

semiconductor laser

semiconductor laser

n

…

multiplexer ～ ～

The space of each channels is usually 100 GHz (=0.8 nm).

Fixed wavelength lasers are used on the each channels.

at present …

The realization of the step-tuning of the semiconductor laser’s frequency

・・・

100 GHz 100 GHz 100 GHz

Inte

nsit

y (a

.u.)

detuning (GHz)0 100 200 300

detuning (GHz) detuning (GHz) detuning (GHz)

spectral image

Purpose

We expect that laser frequency is step-tuned by 100 GHz.

Sampled Grating DBR laser based on Vernier effect

GainSampled Grating 1

Sampled Grating 2

Phase

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T43

194.5193.5

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T4

194.5193.5

193.6 193.8 194 194.2 194.4

0

0.5

1

T2

R1 R2

Beat

This monolithic array is complicated and requires a high processing technique, although it is compact.

We use the external cavity lasers because of its simplicity, expandability, and thermal stability.

In this study

Usually …

Both side facets act as Fabry-Pérot resonator.

In the case of external cavity lasers …

The facets have AR-coating.And we made a resonator outside.

AR coating

External Cavity Diode Laser

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

Laser Diode 100 mm

cavity length 386 mmfeedback ratio 60 %output power 1.7 mW (at 70 mA)linewidth 50 kHz

Specification

mirror PBS

isolator

1

0

T1 ( )

194193

transmittance

0

0.5

1

tran

smitt

ance

frequency

FSR

FWHM

FSRf

cos2FSR

nL

cFree Spectral Range

finesse

FWHM

Fabry-Pérot etalon

cos2

sin41

1

22

2

cnL

RR

RAT

L

refractive index n

the velocity of light c

sinsin n

loss Areflectance R

transmittance

frequency

1

0

T1 ( )

T2 ( )

194193

1

0

T ( )

194193

1

0

T1 ( )

T2 ( )

194193

1

0

T ( )

194193

tran

smit

tanc

e

0

0

0

0

1

1

1

1

tran

smit

tanc

etr

ansm

itta

nce

tran

smit

tanc

efrequency

frequency

frequency

frequency

individual transmittance

beat transmittance

individual transmittance

beat transmittance

Two etalons have slightly different FSR

each other

revolve one etalon

Vernier effect

Transmission spectra of the etalon filters

1

0

0.1

0

0.1

0

tran

smit

tanc

e

tran

smit

tanc

e

tran

smit

tanc

e

196 196.2 196.4 196.6195.8 196 196.2 196.4 196.6195.8

196 196.2 196.4 196.6195.8frequency (THz)

frequency (THz) frequency (THz)

FSR=95 GHz, finesse=5.1 FSR=100 GHz, finesse=36

beat

resolution ： 6.4 GHz

The first phase

LD

optical isolator

etalons

plate

spectrumanalyzer

lens

mirror laser diode

polarizing beam splitter(PBS)

angle 6~6.5 deg 0 deg

FSR 95.0GHz 100GHz

Finesse 5.1 36

Step-tuning using two etalon filters

Experimental result

6.1

6.2

6.3

6.4

6.5

(deg)

195 195.5 196 196.5 197 197.5

inte

nsit

y (a

.u.)

frequency (THz)

step-tuning by the angle of the etalon filter

100GHz

16 ch

Analysis

1

0

T43

197.5195

1

0

T43

197.5195

tran

smit

tanc

etr

ansm

itta

nce

0

0.5

1

0

0.5

1

Frequency

100GHz

100GHz

We calculate the dependence of the laser frequency (= the peak of two etalon’s beat) on the etalon’s angle.

Frequency

The calculated beat spectrum

実験値計算値

5.9 6.0 6.1 6.2 6.3 6.4 6.5195

195.5

196

196.5

197

197.5

The dependence of laser frequency on the etalon’s angle

angle of etalon (deg)

lase

r fr

eque

ncy

(TH

z)

experiment

calculation

ProblemThe loss of the etalon filters increases threshold current

and reduces the maximum output power.

The loss of the etalon filters increases threshold currentand reduces the maximum output power.

0 10 20 30 40 500

90807060 0 10 20 30 40 50 90807060

0.5

1.0

2.0

1.5

0

0.05

0.1

0.2

0.15

current (mA)current (mA)

pow

er (

mW

)

pow

er (

mW

)

without etalons with two etalons

threshold threshold

We tried to induce the step-tuning by the variation of a refractive index n, not the angle of the etalon.

So far

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T43

194.5193.5

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T4

194.5193.5 193.6 193.8 194 194.2 194.4

0

0.5

1

T2

Next

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T43

194.5193.5

193.6 193.8 194 194.2 194.4

0

0.5

1

1.2

0.1

T4

194.5193.5 193.6 193.8 194 194.2 194.4

0

0.5

1

T2

cos2FSR

nL

c

refractive index

We used the mechanical control which has a slow reaction velocity.

We are going to use the electrical control which has a fast reaction velocity.

slow

fast

V

1.3 or 1.46 m wavelength semiconductor laser chips are used as the etalon filters with the variability of a refractive index.

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.

Because …

Laser chips

300 m

300 m

100 m

100 m

1.46 m laser chips 1.3 m laser chips

Variation of the longitudinal mode by the injected current

194.2

194.25

194.3

194.35

194.2 194.25 194.3 194.35

current

194.35

194.3

194.25

194.20 1

injected current 　 (mA)194.35194.3194.25194.2

frequency (THz)

freq

uenc

y (T

Hz)

tran

smis

sion

(a.

u.)

Transmission of the 1.46 m laser chip Variation of the peak frequency

Problem

193.4 193.45 193.5 193.55 193.60

0.2

0.4

0.6

0.8

11

0

T3 T4

193.62193.4

193.4 193.45 193.5 193.55 193.60

0.2

0.4

0.6

0.8

11

1.362 104

T3 T4

193.62193.4

193.503 193.504 193.505 193.506 193.507 193.508 193.509 193.510

0.2

0.4

0.6

0.8

11

7.543 105

T3 T4 f

193.5104193.5024

frequency

frequency

frequency

longitudinal mode

tran

smitt

ance

tran

smitt

ance

tran

smit

tanc

e0

0

0

1

1

1

25 GHz

25 GHz

1 GHz

individual transmittance

beat transmittance between two etalons

We should consider the longitudinal mode of an external cavity as well as beat of two etalons.

193.4 193.45 193.5 193.55 193.6 193.650

0.2

0.4

0.6

0.8

11

0

T1 ( )

193.7193.4

193.4 193.45 193.5 193.55 193.6 193.650

0.2

0.4

0.6

0.8

11

8.159 104

f ( )

193.7193.4

Gain Phase

193.4 193.45 193.5 193.55 193.6 193.650

0.2

0.4

0.6

0.8

11

0

f ( ) T1 ( )( )2

193.7193.4

×

cavity’s mode etalon’s mode beat

ARHR

frequency frequency frequency

tran

smit

tanc

e

tran

smit

tanc

e

tran

smit

tanc

e

The second phase

Vernier effect between a Fabry-Pérot etalon and the longitudinal mode of an external cavity

etalon

external mirrorlens

Simulation of the lasing spectra using the transfer matrix

Transfer　matrix　of　a　reflector

M

Ef+

Ef- = rEf+ + tE-

Er+ ＝ tEf+‐rEr-

Er-

r t

Transfer　matrix　of　space

P

Ef+

Ef- = Er-exp(-ikL)

Er+ ＝ tEf+exp(-ikL)

Er-

L

f

f

r

r

E

E

tt

rt

r

tE

E1

1

f

f

r

r

E

E

ikL

ikL

E

E

)exp(0

0)exp(

MP

Transfer matrix

1.94 10141.942 10

141.944 10

141.946 10

141.948 10

141.95 10

141.952 10

141.954 10

141.956 10

141.958 10

141.96 10

141 10

6

1 105

1 104

1 103

0.01

0.1

1

10

100

1 103

1 104

1 105

105

8.669 106

Eout ( ) 2

196 1012194 10

12

Result of the simulationThe calculated lasing spectrum

frequency (Hz)

inte

nsit

y (a

.u.)

SMSR > 30 dB

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 m 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.