Dispersion (ps/km.nm) Dispersion (ps/km.nm)

(1)

Nonlinear Optics Lab

. .

HanyangHanyang Univ.Univ.

광통신

(Optical Communication)

(2)

. .

광통신의 발전

- 봉화, 횃불, 등대

- 1870, J. Tyndall : 자유낙하 하는 물줄기에 빛이 가두어짐을 발견 - 1880, A.G. Bell : 태양광으로 213m 소리 전송

- 1962, T. Maiman : 반도체레이저 개발

- 1966, 영국 스탠더드통신연, K.C. Kao, Hockham : 광섬유 원거리 전송 제안 - 1970 초, 미국 코닝사, R.D. Maurer (20dB/km)

- 1979, 미국 벨연구소, 1.55 μm LD 개발, 저손실 광섬유 (0.2dB/km) - 1988, 태평양 횡단 해저 광케이블 설치

(3)

. .

광통신의 장점

- Compact : 전선에 비해 가볍고 작음

- Low Cost : 같은 전송능력에서 전선보다 매우 저렴.

- No electrical connection (ground loop) - No EMI (electromagnetic interference)

- Transmission capacity : 10Gbps 상용, 100Gbps 연구중

- Long repeater spacing : 40km 상용, 120 km 가능 (전선 : 12km) - Better security : tapping 불가

(4)

. .

광통신의 원리

core (n₁)

cladding (n₂) (n₁> n₂) TIR(total internal reflection, 내부전반사)

core (n₁)

cladding (n₂) θ₁

θ₂

* critical angle (θ₂=π/2) : θ_c1= sin^-1(n₂/n₁) θ>θ_c1

<광섬유(optical fiber) 기본 구조>

<실제 광섬유 구조>

(5)

. .

광통신의 제한점

- Optoelectric conversion efficiency Æ energy loss

- Difficulty in joining fiber cable : fusion splice, connector Æ transmission loss - Cable bending loss

- Relatively expensive components

- Difficulty in controlling optical signal

exceed the critical angle condition : bending loss

core

(6)

. .

광통신 시스템의 개요

signal in

signal out transmitter

(laser/LED)

repeater

(photodiode/ laser/LED)

coupler receiver

(photodiode) splice

optical fiber

Main issues

- High quality fiber (low loss/dispersion)

- Efficient laser/LED (low power consumption /narrow single mode) - High speed switching (>100Gb rate)

…

(7)

. .

광섬유 손실

(8)

. .

NA(numerical aperture) of optical fiber : light accepting capability

core (n₁)

cladding (n₂)

θ₁

θ₂ n₀

θ₀

NA = n₀sinθ₀= n₁sinθ₁ θ₂= π/2 − θ₁ = sin^-1(n₂/n₁)

2 2 2

1

n

NA = −

Î

(9)

. .

광섬유의 분산 특성

1. Modal dispersion

: multimode fiber(다중모드 광섬유)에서 고차 모드일수록 군속도가 느림.

Î optical pulse의 broadening

high order mode low order mode

1 ) ( 1

max , min

, g

g v

L v −

= Δτ

(10)

. .

Modal dispersion의 제거 - Single mode fiber 사용 - Graded-index fiber 사용

Higher order mode의 군속도가 빨라져, 펄스 확대가 줄게 됨.

(11)

. .

2. Intramodal dispersion : Material dispersion & waveguide dispersion

- Material dispersion : 단일 모드라도 일정 주파수폭을 가지므로, v_g =v_g(ω) : fiber material property

- Waveguide dispersion : Core와 cladding에서 광에너지 전파속도가 다르기 때문 : fiber geometrical (& doping) property

c n n

n c

d d

v_g _m _w⎥ +

⎦

⎢ ⎤

⎣

⎡ ⎟

⎠

⎜ ⎞

⎝

⎛

∂ + ∂

⎟⎠

⎜ ⎞

⎝

⎛

∂

− ∂

=

ω ω

λ ω

β

1

⎥ ⎦

⎢ ⎤

⎣

⎡ ⎟

⎠

⎜ ⎞

⎝

⎛

∂ + ∂

⎟ ⎠

⎜ ⎞

⎝

⎛

∂

− ∂

=

w m

n n

D c

ω ω

λ

: group velocity dispersion

(12)

. .

Intramodal dispersion의 제거 - Phase conjugation에 의한 보상 - Material, waveguide dispersion의 상쇄

fiber1 fiber1

phase conjugator

EDFA1 EDFA2

DSF

Pump LD

(13)

. .

0 10 20

-20 -10

Dispersion (ps/km.nm)

1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (μm)

waveguide dispersion material dispersion

<Material, waveguide dispersion의 상쇄>

0 10 20

-20 -10

Dispersion (ps/km.nm)

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Wavelength (μm)

dispersion shifted dispersion flattened

(14)

. .

광결정 (photonic crystals)

광자밴드갭(photonic bandgap)을 이용한 신개념 광섬유

(15)

. .

광결정섬유 (photonic crystal fibers)와 도파로(waveguide)

(16)

. .

광통신용 레이저 광원

+

- p

n

LED(light emitting diode)

- forward-biased heavily doped p-n junction - pumping by injection current

=> Spontaneous emission

LD(laser diode)

- optical feedback by cleaved surface

=> Stimulated emission

LD(laser diode)

- small size, high efficiency, integrability with electronic components - ease of pumping and modulation by current injection

- spectral linewidth is larger than that of other lasers

(17)

. .

LED(light emitting diode)

- Display

- Traffic signal lamp - Memory

- Laser disk - Lighting

…

- GaN series Î Blue color

- OLED(organic LED) Î high efficiency Î

(18)

. .

<Characteristic optical properties of LDs> +

- elliptical beam

short cavity length Output power(P) & oscillation frequency(ν)

sensitively depend on temperature,

injection current and also feedback light.

polar

ization (TE)

LD(laser diode)

(19)

. .

Differential responsibility of laser

0

24 . 1 η_d λ

o

d di

R = dP =

Overall efficiency

eV h i i_t

d

η ν

η _⎟

⎠

⎜ ⎞

⎝⎛ −

= 1

(above threshold)

Output power

(20)

. .

Oscillation frequency

- Laser oscillation condition : γ₀(ν) >α_r

- Phase condition : ν_n ≈ qν_F, (q =1,2,...)

- FWHM linewidth : δν ≈ν^FF ^where, : finesse of resonator

r r

= − 1

2 /

π 1

F ^where, : mode spacing

c nd

F

= 2 ν

Fig. 14.2-4

(21)

. .

example)

* Since the mode closest to the gain maximum Increases in power while the side peaks saturate, the number of modes decreases as the injection current increases.

InGaAsP laser(n = 3.5), d = 400 μm => ν_F~ 107 GHz λ₀~ 1.3 μm => λ_F= λ₀² ν_F/ ν = λ₀²/2nd ~ 0.6 nm

Gain width ~ 7 nm => ~ 11 modes oscillation

InGaAsP laser, 1.3 μm / 1.5 μm

10Gbps DFB-LD/module

(22)

. .

Internal feedback laser

- DBR(distributed Bragg reflector) - DFB(distributed feedback)

<DBR structure> <DFB structure>

(23)

. .

Surface emitting laser

SEM image of a VCSEL

(24)

. .

Kwangju Institute of Science & Technology(K-JIST) Optoelectronics Lab.

Fabrication of oxide-confined VCSEL

Light output

QW

Polyimide(PI2555)

n- DBR

n- metal(Ni/Au/Ge/Ni/Au) Substrate

p- DBR P-metal(Ti/Pt/Au)

Oxide

aperture Output window

Pad metal

9Mesa diameter : 40μm

9Current aperture diameter : 5 μm

9Output window diameter : 20 μm

9Bonding pad : 200 X 120 μm²

Surface emitting laser (광주과기원)

(25)

. .

Surface emitting laser (과기원 – science 게재)

Dispersion (ps/km.nm) Dispersion (ps/km.nm)

. .

광통신

(Optical Communication)

. .

광통신의 발전

. .

광통신의 장점

. .

광통신의 원리

. .

광통신의 제한점

. .

광통신 시스템의 개요

. .

광섬유 손실

. .

n

n

NA = −

. .

광섬유의 분산 특성

. .

. .

ω ω

λ ω

β

⎥ ⎦

⎢ ⎤

⎣

⎡ ⎟

⎠

⎜ ⎞

⎝

⎛

∂ + ∂

⎟ ⎠

⎜ ⎞

⎝

⎛

∂

− ∂

=

n n

D c

ω ω

λ

. .

. .

. .

광자밴드갭(photonic bandgap)을 이용한 신개념 광섬유

. .

. .

광통신용 레이저 광원

. .

. .

LD(laser diode)

. .

. .

. .

. .

. .

. .

Fabrication of oxide-confined VCSEL

. .

광결정을 이용한 고효율 레이저 광원