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Receiver Optik - Photo DiodaPIN (p-layer, Instrinsic layer, n-layer)
Highly linear, low dark current
Detektor diikuti dengan penguat transimpedansi
APD (Avalanche Photo Diode)
Gain sampai 100x (thd noise elektrik receiver)
Sangat tergantung dari temperatur
Karakteristik Utama
Efisiensi Kuantum
Dark Current
Responsivitas dan ketergantungan panjang
gelombang
3
Jenis detektor optik berupa dioda :
1. Dioda PIN (P Intrinsic N)
2. Dioda APD (Avalanche Photo Diode)
Prinsip kerja dioda PIN :
- Mengubah energi optik (photon) yang diterima menjadi arus keluaran berdasarkan photo voltaic effect.
- Memerlukan bias mundur (terbalik).
Karakteristik detektor optik :
1. Responsitivity (R)
2. Efesiensi kuantum ()
3. Kecepatan respon (rise time)
4. Daya optik minimum (MRP: Minimum Required Power)
Receiver Optik
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1. Responsitivity ( R )
Ip = Arus photo detektor
Po = Daya optik diterima
= Efisiensi kuantum
e = Muatan elektron
h = konstanta plank
f = frekuensi
2. Efisiensi kuantum ( )
R dalam A /W
: panjang gelombang dalam m
Parameter Detektor Optik – Dioda PIN
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3. Kecepatan respon
Ditentukan oleh rise time dari detektor tersebut (kapasitansi) dan waktu hidup pembawa muatan
4. Minimum Required Power (MRP)
Daya minimum diperlukan pada BER (Bit Error Rate) tertentu.
Parameter Detektor Optik – Dioda PIN
6
Prinsip kerja dioda APD :
• APD bekerja pada reverse bias yang besar
• Pada medan listrik yang tinggi terjadi avalanche effect yang menghasilkan impact ionization berantai dan terjadi multiplikasi avalanche
• Terjadi penguatan atau multiplikasi arus
Karakteristik dioda APD :
Responsitivity = RAPD = RPiN . M
M = Faktor multiplikasi APD.
M berharga antara 10 - 250.
Panjanq qelombang operasi
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Receiver: Sensitivitas dan Bandwidth
ModulasiSensitivitas tinggi memerlukan detektor yang besar/dalam
Mendeteksi tiap foton = meningkatkan efisiensi spektrum
Bandwidth yang lebar memerlukan detektor yang kecil/dangkal
Perlu untuk menditeksi secara cepat untuk mengakomodasi pulsa yang pendek
Semakin besar detektor semakin panjang waktu relaksasi dari proses deteksi
Tradeoff antara sensitivitas dan bandwidth: bandwidth yang lebih lebar, sensitivitas yang
lebih rendah
pin Photodetector
The high electric field present in the depletion region causes photo-generated carriers to
Separate and be collected across the reverse –biased junction. This give rise to a current
Flow in an external circuit, known as photocurrent.
w
Photocurrent
• Optical power absorbed, in the depletion region can be written in terms
of incident optical power, :
• Absorption coefficient strongly depends on wavelength. The upper
wavelength cutoff for any semiconductor can be determined by its energy
gap as follows:
)1()( )(
0
xsePxP [6-1]
)( s
)(xP
0P
(eV)
24.1)m(
g
cE
[6-2]
Responsivity
• Arus yg mengalir:
• Quantum Efficiency:
• Responsivity:
)1)(1( )(
0 f
w
p RePh
qI s
[6-3]
hP
qIP
/
/
photonsincident of #
pairs atedphotogener hole-electron of #
0
[6-4]
[A/W] 0
h
q
P
IP [6-5]
Avalanche Photodiode (APD)
APDs internally multiply the
primary photocurrent before it
enters to following circuitry.
In order to carrier multiplication
take place, the photogenerated
carriers must traverse along a
high field region. In this region,
photogenerated electrons and
holes gain enough energy to
ionize bound electrons in VB
upon colliding with them. This
multiplication is known as
impact ionization. The newly
created carriers in the presence of
high electric field result in more
ionization called avalanche
effect.
Reach-Through APD structure (RAPD)
showing the electric fields in depletion
region and multiplication region.
Optical radiation
Responsivity padaAPD
• Gain arus pada photodiode(M):
• dimana rata2 nilai output arus dan arus utama
• The responsivity pada APD adalah:
p
M
I
IM [6-6]
MI PI
MMh
q0APD
[6-7]
Persamaan sinyal pada photodetektor
• Optical power signal P(t) :
• Where s(t) adalah sinyal informasi and m adalah indek modulasi. Shg arus
yang mengalir adalah (untukpin photodiode M=1):
• RMS arus adalah:
)](1[)( 0 tmsPtP [6-8]
]current AC)[(] valueDC[)(ph tiItMPh
qi pP
[6-9]
signal sinusoidalfor 2
2222
2222
Ppp
sps
Imi
Mii
[6-9]
[6-10]
Noise pada Photodetektor
• The principal noises associated with photodetectors are :
1- Quantum (Shot) noise: arises from statistical nature of the production
and collection of photo-generated electrons upon optical illumination. It has
been shown that the statistics follow a Poisson process.
2- Dark current noise: is the current that continues to flow through the
bias circuit in the absence of the light. This is the combination of bulk
dark current, which is due to thermally generated e and h in the pn
junction, and the surface dark current, due to surface defects, bias voltage
and surface area.
• In order to calculate the total noise presented in photodetector, we should
sum up the root mean square of each noise current by assuming that those
are uncorrelated.
• Total photodetector noise current=quantum noise current +bulk dark
current noise + surface current noise
Photodetector Response Time
• The response time of a photodetector with its output circuit depends mainly
on the following three factors:
1- The transit time of the photocarriers in the depletion region. The transit
time depends on the carrier drift velocity and the depletion layer
width w, and is given by:
2- Diffusion time of photocarriers outside depletion region.
3- RC time constant of the circuit. The circuit after the photodetector acts
like RC low pass filter with a passband given by:
dt dv
d
dv
wt [6-18]
TT CRB
2
1 [6-19]
daTLsT CCCRRR and ||
Respon photodiode thd pulsa optik
Typical response time of the
photodiode that is not fully depleted
Variasi respon optik photodetector:
Trade-off between quantum efficiency & response time
• To achieve a high quantum
efficiency, the depletion layer
width must be larger than
(the inverse of the absorption
coefficient), so that most of the
light will be absorbed. At the
same time with large width, the
capacitance is small and RC
time constant getting smaller,
leading to faster response, but
wide width results in larger
transit time in the depletion
region. Therefore there is a
trade-off between width and
QE. It is shown that the best
is:
s/1
ss w /2/1
struktur InGaAs pada APD
• Separate-absorption-and multiplication (SAM) APD
• InGaAs APD superlattice structure (The multiplication region is composed
of several layers of InAlGaAs quantum wells separated by InAlAs barrier
layers.
Metal contact
InP multiplication layer
INGaAs Absorption layer
InP buffer layer
InP substrate
light