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參數(shù)資料
| 型號(hào)： | GD16076A-50SLP |
| 英文描述： | Laser Diode/LED Driver |
| 中文描述： | 激光二極管/ LED驅(qū)動(dòng)器 |
| 文件頁(yè)數(shù)： | 3/9頁(yè) |
| 文件大?。?/td> | 332K |
| 代理商： | GD16076A-50SLP |
Figure 2.
Optical Modulator with Segmented Transmission Line.
The incoming light to the modulator is an
unmodulated continuous light wave, and
therefore does not suffer from chirp. The
signal treatment within the modulator is
linear, see below. Consequently the
modulated light signal does not suffer
from chirp.
High speed optical communication
systems using a Mach Zender Interfer-
ometer therefore has better system per-
formance than systems using direct
current modulation.
Characterisation of
The Modulator
When the light enters the modulator it is
split into two branches, as shown in
Figure 2. On the output it is combined
again.
The propagation delay of the light wave
can be adjusted through one (or both) of
the branches by applying a voltage to the
substrate near the optical wave-guide.
This is because the refractive index of
the material changes proportional to the
voltage applied to the substrate and be-
cause the velocity of light is proportional
to the refractive index. Changing the re-
fractive index in one branch therefore
gives a tuneable delay variation between
the two branches. Thereby the light can
be combined in phase, making the light
pass through to the output without atten-
uation, or in counter phase, thereby turn-
ing off the light.
Typically the electrical data signal for a
high-speed modulator is connected into
the modulator via a transmission line,
traveling along one of the optical
branches. On the output the transmission
line is terminated in order to obtain a
good input impedance.
An equivalent diagram for the modulator
has been derived. The diagram was
made from the physical components of
the modulator (input pin, bonding wires
and transmission line characteristics),
and fitting the component values to mea-
surements.
At low modulation rates the relation be-
tween the voltage applied to the modula-
tor and the relative light p on the output
can be described as:
+
+
(
cos( (
1
2
Where:
V
ACT
is the voltage applied to the active
region
V
OFF
is an offset voltage (material de-
pendent)
V
is the voltage difference between the
applied voltages that causes fully on and
fully off light on the output respectively.
p
V
V
V
ACT
OFF
=
)/
))
(1)
From (1) some important features of the
modulator can bee derived. When
(V
ACT
+ V
OFF
)/V
= 1 then p = 0.
If (V
ACT
+ V
OFF
)/V
becomes slightly
larger than 1 or smaller than 1, p still ap-
proximates 0 very closely. 20 % over (un-
der) shoot causes p to be only 0.1. This
is due to the sine transformation in (1).
Therefore the modulator effectively acts
as a pulse shaper on the voltage V
ACT
defined above and attenuates any small
over and/or undershoot in the electrical
signal.
However if the over and/or undershoot in
the electrical signal becomes larger than
approximately 1/3 V
there will be no lim-
iting effect. Instead two pulses will be
created on the optical output. An over-
shoot of 50 % causes p to be 0.5. I.e. in-
stead of only one optical output pulse a
second pulse has been created. There-
fore it is important to ensure that the ring-
ing on the electrical signal is less than
approximately 20%.
The above formula works well at low
modulation rates. However at high speed
data rates the formula does not describe
the function precisely, because the volt-
age actually travels as a wave along the
active part.
Assuming first that the velocity of light is
much higher than the electrical signals
propagation velocity, the voltage that any
light wave actually sees, will be the aver-
age of the voltages along the transmis-
sion lines, taking into account that the
transmission line represents a loss. Now
this voltage can be used as V
ACT
in the
above formula. In reality the velocity of
light v
l
is approximately c/2.2 for LiNbO
3
,
whereas the electrical signal’s propaga-
tion velocity is approximately
c
c
.
113
34
.
=
where c is the velocity of light in open air.
This means that instead of just averaging
the voltage across the active area, the
voltage that the optical wave actually
sees is a function of time.
This was modeled by splitting up the ac-
tive area into 10 parts, see Figure 2. The
optical wave present at L
n
at time t
n
was
present at L
n+1
at time t
n
- d
t
/ v
l
, where d
t
is the distance from L
n
to Ln+1. The ef-
fective voltage V
ACT
exposed to the light
wave entering the active area at time t -
causing the optical refractive index of the
LiNbO
3
to change, and thereby changing
the velocity of the light - therefore can be
expressed as:
V
N
V t
n
ndt V
ACT
I
n
N
=
+
=
∑
1
1
0
(
/
)
(2)
where V
n
(t) is the voltage at L
n
.
The above formulas (1) and (2) were
used together with the electrical equiva-
lent diagram for the modulator to make
SPICE simulations of the behaviour of
GD16076A connected to an optical
modulator as shown in Figure 1.
Data Sheet Rev. 07
GD16076A
Page 3
VCC
Optical Wave Guides
in LINbO3
Active Area
d
t
V
; L
n+1
n+1
L
n
V
n
V
n
v
sumn
Delay = nd / v
I
+
-
Electrical
Signal Input
Optical Input
Segmented
Transmission
Line
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