LASER DRIVER FOR HIGH POWER ERBIUM DOPED FIBER AMPLIFIER (EDFA)

Report On
LASER DRIVER FOR HIGH POWER ERBIUM DOPED FIBER
AMPLIFIER (EDFA)
By,
Kaushik K Naik
naikkaushik93@gmail.com

Laser Driver for High Power Erbium Doped Fiber Amplifier (EDFA)
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Table of Contents
1. Introduction...............................................................................................................................................................3
2. Principle of EDFA....................................................................................................................................................4
3. Design Equations....................................................................................................................................................6
3.1. Pump Power Calculations ..............................................................................7
3.2. Length of EDFA Calculation ............................................................................8
3.3. Current Calculation ........................................................................................8
4. Design of Laser Driver.......................................................................................................................................11
4.1. Current Sensing Circuit ................................................................................17
4.2. Temperature Sensing Circuit........................................................................18
5. Simulation Results...............................................................................................................................................20
5.1. Simulation of Laser Driver............................................................................20
5.2. Simulation of Current Sensing Circuit...........................................................22
5.3. Simulation of Temperature Sensing Circuit..................................................24
6. References ................................................................................................................................................................27

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1. Introduction
In-line amplification of signal in an optical fiber can be achieved using optical fiber amplifiers. The
amplification is mainly obtained because of the core of optical fiber been doped with rare earth ions.
Erbium (Er3+
) is a rare earth material and this is preferred because of its high gain amplification property
and low coupling losses. The block representation of an EDFA is shown in Figure 1.
Figure 1: Block representation of an EDFA
A weak power input signal from seed laser is mixed with a high powered light beam from a pump laser
using Wavelength Coupler(WC). This mixed signal is passed through EDFA. The erbium ions are excited to
higher energy state because of this high powered light signal. Thus creating the condition of population
inversion. Once population inversion is achieved, the higher energy level contains a large number of
electrons which undergo stimulated emission and jumps back to the lower energy level. Since the erbium
ions gives energy in the form of photons in the same phase and direction of input signal, the signal gets
amplified. Thus amplified signal and residual light signals are separated in a Wavelength De-Coupler
(WDC).

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2. Principle of EDFA
EDFA has two pumping bands: 980nm and 1480nm. A 980nm pumping band is used where low noise
performance and greater separation from signal wavelength is required and 1480nm pumping band is
used for high power amplifiers but has high noise.
The pump signal and input signal interaction is independent of the direction of pumping. If the pump
signal id in the direction of input signal or in the opposite side of input signal, the EDFA would have the
same characteristics of performance.
Thus there are two configurations of pumping light to EDFA:
1. Co-directional pumping: has better noise performance.
2. Counter-directional pumping: has better isolation between pump and signal photons.

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Figure 2: (a) Energy level diagram of Erbium doped region of the core (b) All ions in ground state (c)
Absorption of pump photon and non-radiative relaxation to state m (d) Spontaneous Emission (e)
Stimulated Emission

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The energy level diagram of Erbium doped region of the core is as shown in Figure 2. There are 3 groups
of energy levels denoted by u, m and g representing the upper, metastable and ground states
respectively. The ions will be in the ground state in the absence of any radiation. When a beam of light
with particular frequency is incident on the system, the ions in the ground state will get excited to the
higher levels. This incident beam of light is called pump radiation. If the wavelength is chosen to be
980nm, the ions will get excited to the upper level u. The lifetime of ions in the upper level is
approximately 1us. The ions decay to the metastable level m by releasing heat (non radiative decay). The
transition of ions from the metastable level to ground state occurs either by spontaneous emission or
stimulated emission. The energy difference between metastable and ground level is 1550nm. Thus EDFA
operates in 1550nm wavelength window.
The spontaneous emission is highly incoherent and occurs naturally and stimulated emission is coherent
and occurs when stimulated by an incident photon. The stimulated emission is of utmost important for
amplification of signal. The spontaneous emission can be reduced to a level but can’t be eliminated
completely and these are the main sources of noise component in the amplified output.
3. Design Equations
The required specifications for signal amplification are given in Table 1.
Parameter Value
Input signal power (𝑃𝑆𝑖𝑛𝑝𝑢𝑡
) 10mW
Output signal power (𝑃𝑆 𝑜𝑢𝑡𝑝𝑢𝑡
) 5W

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Input signal wavelength (𝜆 𝑆) 1550nm
Pump source pumping
wavelength (𝜆 𝑃)
980nm
Table 1: Required specifications for signal amplification
The gain of amplifier is given in logarithmic scale as in Equation below.
𝐺 = 10𝑙𝑜𝑔
𝑃𝑆 𝑜𝑢𝑡𝑝𝑢𝑡
𝑃𝑆 𝑖𝑛𝑝𝑢𝑡
= 10𝑙𝑜𝑔
5
0.01
= 26.97𝑑𝐵
The two main important parameters to be considered when designing an EDFA driver are:
 Pump power.
 Length of EDFA.
3.1. Pump Power Calculations
If the power from the pump source is insufficient, the required population inversion is not created for
signal amplification. This situation is expressed mathematically based on the conservation of photons
during the entire amplification process.
≤ 𝑃𝑆𝑖𝑛𝑝𝑢𝑡
+
𝜆 𝑃
𝜆 𝑆
𝑃𝑝𝑢𝑚𝑝
The total pump power which contributes to the output signal amplification process is determined by the
power conversion efficiency (PCE).

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𝑃𝐶𝐸 =
− 𝑃𝑆 𝑖𝑛𝑝𝑢𝑡
𝑃𝑝𝑢𝑚𝑝
=
𝜆 𝑃
𝜆 𝑆
≤ 1
𝑃𝐶𝐸 =
𝜆 𝑃
𝜆 𝑆
=
980𝑛𝑚
1550𝑛𝑚
= 0.6322 = 63.22%
Therefore, the required pump power for amplification is calculated as,
𝑃𝑝𝑢𝑚𝑝 =
− 𝑃𝑆 𝑖𝑛𝑝𝑢𝑡
𝑃𝐶𝐸
=
5 − 0.01
0.6322
= 7.89𝑊
3.2. Length of EDFA Calculation
The gain of EDFA is also a function of the length of the Erbium Doped Fiber. If L is the length of fiber
(m), ρ is the concentration of Erbium ions (m-3
) and σe is the signal emission cross section of EDF, then
gain of EDFA is given by,
𝐺 =
𝑃 𝑆 𝑜𝑢𝑡𝑝𝑢𝑡
𝑃 𝑆 𝑖𝑛𝑝𝑢𝑡
= 𝑒 𝜌 𝜎 𝑒 𝐿
From above equation the length can be given as,
𝐿 =
𝑙𝑛
𝑃𝑆 𝑖𝑛𝑝𝑢𝑡
𝜌𝜎𝑒
3.3. Current Calculation
The energy of photon is given by the equation,

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𝐸 = 𝑛𝑕
𝑐
𝜆
Where, 𝑕 is the Plank's constant = 6.626 × 10-34
(Joule-sec)
𝑐 is speed of light = 2.998 × 108
(mtr / sec)
The energy of single photon (n = 1) is given by,
𝐸 =
6.626 × 10−34
× 2.998 × 108
𝜆
=
1.99 × 10−25
𝜆
For a wavelength of 980nm, the energy of photon is calculated as,
𝐸 =
1.99 × 10−25
𝜆
=
1.99 × 10−25
980 × 10−9
= 2.03 × 10−19
𝐽𝑜𝑢𝑙𝑒𝑠
The energy is related to power as,
𝐸 = 𝑃 × 𝑡
Where, P is Power (Watts)
t is time (sec)
Current is nothing but coulomb / sec, i.e.,
𝐼 =
𝑞
𝑡
Where, q is charge (coulomb)
t is time (sec).

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Substituting for t from above equation,
𝐸 = 𝑃 ×
𝑞
𝐼
Charge of 1 electron, 𝑞 = 1.602 × 10−19
𝐶.
From above equation, the current required to be flown to obtain a required energy with 7.89W power is
calculated as,
𝐼 =
𝑃 × 𝑞
𝐸
=
7.89 × 1.602 × 10−19
2.03 × 10−19
= 6.23𝐴
Laser diode of PART No. QSP-975-10 is selected based on the power rating, current rating and
wavelength. The important specifications of this laser diode is given in Table 2.
Parameters Value
Optical power (𝑃𝑓) 10 W
Wavelength (𝜆 𝑐) 974 – 980
nm
Forward current (𝐼𝑓) 11 A
Forward voltage (𝑉𝑓) 2 V
Threshold current (𝐼𝑡𝑕 ) 0.6 A
Slope efficiency (𝜂 𝑠𝑙𝑜𝑝𝑒 ) 0.9 W/A
Table 2: Laser diode specifications

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The slope efficiency parameter of a laser diode tells the efficiency of pump power obtained as optical
power. The slope efficiency is defined as the slope of the curve obtained by plotting the laser output versus
the pump power.
The slope efficiency is given by,
𝜂 𝑠𝑙𝑜𝑝𝑒 =
𝑃𝑜𝑝𝑡𝑖𝑐𝑎𝑙
𝐼 − 𝐼𝑡𝑕
Where, 𝐼 is the required current to be flown through the laser.
𝐼𝑡𝑕 is the threshold current of laser diode (datasheet).
From above equation, the optical power output from this laser diode can be calculated as,
𝑃𝑜𝑝𝑡𝑖𝑐𝑎𝑙 = 𝜂 𝑠𝑙𝑜𝑝𝑒 (𝐼 − 𝐼𝑡𝑕 ) = 0.9(6.23 − 0.6 = 5.07𝑊
But for our application optical power of 7.89W is required. So three laser diodes are used to meet the
requirement and also the current requirement also reduces and calculations are explained in subsequent
section.
4. Design of Laser Driver
The Pump Laser driver circuit is as shown in Figure 3.

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Figure 3: Pump Laser Diriver
 An optical power of 7.89W from the laser pump diode needs to be fed to the incoming input seed laser.
 For a laser diode QSP-975-10, a current of 9.36A need to be flown to obtain an optical power output of
7.89W.
 Thus three laser diodes are used so that the current requirement reduces. Thus an optical power output
required from a single laser diode is,
𝑃𝑜𝑝(𝑒𝑎𝑐 𝑕) =
𝑃𝑜𝑝
3
=
7.89
3
= 2.63 𝑊
Thus current to be flown for an optical power of 2.63W is,
𝜂 𝑠𝑙𝑜𝑝𝑒 =
𝑃𝑜𝑝
𝐼 − 𝐼𝑡𝑕

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𝐼 =
𝑃𝑜𝑝
𝜂 𝑠𝑙𝑜𝑝𝑒
+ 𝐼𝑡𝑕 =
2.63
0.9
+ 0.6 = 3.52 𝐴
Thus a current capacity upto 4A will be provided. The sense voltage for a current of 4A is,
𝑉𝑠𝑒𝑛 𝑠𝑒 = 𝐼𝑠𝑒𝑛𝑠𝑒 × 𝑅 𝑠𝑒𝑛𝑠𝑒 = 4 × 0.033 = 0.133 𝑉
Voltage at point C,
𝑉𝐶 = 𝑉𝑠𝑒𝑛𝑠𝑒 × 1.185 = 0.133 × 1.185 = 0.16 𝑉
Voltage at point B and C should be equal.
For a maximum firing pulse voltage 𝑉𝐴 of 5V, the resistor SEL should be chosen to be,
𝑆𝐸𝑙 =
𝑉𝐵 × 10𝑘
𝑉𝐴 − 𝑉𝐵
=
0.12 × 10𝑘
5 − 0.12
= 326 Ω
A standard resistor value of 330 Ω is selected.
 For a better reliability two such circuits are used. Thus the current requirement will get reduced to half.
i.e., I=1.76A.
 Whenever any one of the circuit fails due to some fault, the other circuit can be used in full load. Thus a
variable current setting from 1.5A to 4A should be provided.
 This purpose is achieved by varying the firing pulse voltage 𝑉𝐴 as per the current requirement. The firing
pulse voltage can be varied by connecting it to 8 bit DAC. The 8-bits corresponding to particular current
is calculated and tabulated as below:
I (A)
Vsense
(V)
VC = VB (V) VA(V) DEC
DAC I/p
Bits
1.5 0.04995 0.0592 1.85 168 10101000

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1.6 0.05328 0.0631 1.98 171 10101011
1.7 0.05661 0.0671 2.10 174 10101110
1.8 0.05994 0.0710 2.22 178 10110001
1.9 0.06327 0.0750 2.35 181 10110100
2 0.0666 0.0789 2.47 184 10110111
2.1 0.06993 0.0829 2.59 187 10111010
2.2 0.07326 0.0868 2.72 190 10111101
2.3 0.07659 0.0908 2.84 193 11000000
2.4 0.07992 0.0947 2.96 196 11000011
2.5 0.08325 0.0987 3.09 199 11000110
2.6 0.08658 0.1026 3.21 202 11001001
2.7 0.08991 0.1065 3.34 205 11001100
2.8 0.09324 0.1105 3.46 208 11001111
2.9 0.09657 0.1144 3.58 211 11010010
3 0.0999 0.1184 3.71 214 11010101
3.1 0.10323 0.1223 3.83 217 11011000
3.2 0.10656 0.1263 3.95 220 11011100
3.3 0.10989 0.1302 4.08 223 11011111
3.4 0.11322 0.1342 4.20 226 11100010
3.5 0.11655 0.1381 4.32 229 11100101
3.6 0.11988 0.1421 4.45 232 11101000
3.7 0.12321 0.1460 4.57 235 11101011
3.8 0.12654 0.1499 4.69 238 11101110
3.9 0.12987 0.1539 4.82 241 11110001
4 0.1332 0.1578 4.94 244 11110100
Decimal value for required 𝑉𝐴 is calculated using the formula below. Then this decimal value is
converted to binary.

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DEC =
256
𝐼 𝑅𝐸𝐹
𝑉𝐴
5𝑘
+
5
5𝑘
=
256
(5/2.4𝑘)
𝑉𝐴
5𝑘
+ (1 × 10−3
)
 The table below gives a current setting, optical pump output and the EDFA output for a particular 8-
bit input.
DAC i/p
Bits
I (A)
Optical
Pumping per
LDD (W)
O/P of
EDFA (W)
10101000 1.5 2.43 1.55
10101011 1.6 2.70 1.72
10101110 1.7 2.97 1.89
10110001 1.8 3.24 2.06
10110100 1.9 3.51 2.23
10110111 2 3.78 2.40
10111010 2.1 4.05 2.57
10111101 2.2 4.32 2.74
11000000 2.3 4.59 2.91
11000011 2.4 4.86 3.08
11000110 2.5 5.13 3.25
11001001 2.6 5.40 3.42
11001100 2.7 5.67 3.59
11001111 2.8 5.94 3.77
11010010 2.9 6.21 3.94
11010101 3 6.48 4.11
11011000 3.1 6.75 4.28
11011100 3.2 7.02 4.45
11011111 3.3 7.29 4.62

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11100010 3.4 7.56 4.79
11100101 3.5 7.83 4.96
11101000 3.6 8.10 5.13
11101011 3.7 8.37 5.30
11101110 3.8 8.64 5.47
11110001 3.9 8.91 5.64
11110100 4 9.18 5.81
Optical pump power/LDD = 3 × 𝜂 𝑠𝑙𝑜𝑝𝑒 𝐼 − 𝐼𝑡𝑕 = 3 × 0.9 𝐼 − 0.6
EDFA output power/LDD = ( 𝑃𝐶𝐸 × 𝑃𝑝𝑢𝑚𝑝 ) + 𝑃𝑖𝑛 = ( 0.6322 × 𝑃𝑝𝑢𝑚𝑝 ) + 0.01
 If two LDDs are used, a current of 2.1A needs to be flown through both the LDDs (bits setting as per
column coloured red) and a current of 3.6A needs to be flown through one of the LDDs in case of
other gets fault (bits setting as per column coloured blue) to obtain an EDFA output of 5W.

17 | P a g e
4.1. Current Sensing Circuit
Figure 4: Current Sensing Circuit
 The current sense resistance is constant. So the voltage 𝑉𝑠𝑒𝑛𝑠𝑒 is monitored. The change in this voltage
is directly proportional to change in current related as:
𝑉𝐶𝑈𝑅 =
100𝑘
3.3𝑘
𝑉 𝑠𝑒𝑛𝑠𝑒
I (A) Vsense (V)
Vcur
(V)
I (A) Vsense (V)
Vcur
(V)
1.5 0.0500 1.51 2.8 0.0932 2.83
1.6 0.0533 1.61 2.9 0.0966 2.93
1.7 0.0566 1.72 3 0.0999 3.03
1.8 0.0599 1.82 3.1 0.1032 3.13

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1.9 0.0633 1.92 3.2 0.1066 3.23
2 0.0666 2.02 3.3 0.1099 3.33
2.1 0.0699 2.12 3.4 0.1132 3.43
2.2 0.0733 2.22 3.5 0.1166 3.53
2.3 0.0766 2.32 3.6 0.1199 3.63
2.4 0.0799 2.42 3.7 0.1232 3.73
2.5 0.0833 2.52 3.8 0.1265 3.83
2.6 0.0866 2.62 3.9 0.1299 3.94
2.7 0.0899 2.72 4 0.1332 4.04
4.2. Temperature Sensing Circuit

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Figure 5: Temperature Sensing Circuit
 A constant current (0.2mA) is supplied through the Thermistor. The resistance of thermistor varies
corresponding to a particular temperature and this is given in the datasheet of thermistor. Thus the
voltage across the thermistor is sensed and is directly proportional to the temperature and is related
as:
𝑉𝑇𝐸𝑀𝑃 = 0.2𝑚𝐴 × 𝑅 𝑇𝑕𝑒𝑟𝑚𝑖𝑠𝑡𝑜𝑟
Temp R Thermistor VTEMP
0 16312 3.26
5 12691 2.54
10 9948 1.99
15 7856 1.57
20 6246 1.25
25 5000 1.00
30 4028 0.81
35 3265 0.65
40 2662 0.53
45 2183 0.44
50 1799 0.36
55 1491 0.30
60 1242 0.25
65 1039 0.21
70 873.8 0.17
75 738 0.15
80 625.9 0.13

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5. Simulation Results
The validation of the designed circuits in the section 4 is by simulating the circuits in OrCAD Capture tool.
5.1. Simulation of Laser Driver
Simulation of the circuit as in Figure 3 is conducted. The Circuit is as shown in Figure 6.
Figure 6: Simulation of Laser Driver
Here resistor R31 is used to simulate a voltage drop of 6V (3 laser diodes of 2V each). V1 represents the
voltage 𝑉𝐴. By varying 𝑉𝐴 current flowing through R8 can be varied. Figure 7 shows the simulation results
for V1=2.59V and Figure 8 shows the simulation results for V1=4.45V.
Vsense
U1A
OP484
-
2
+
3 V-
11
OUT
1
V+
4
R3
10k
R4
330
R5
10k
R6
1k
R7
100k
R8
0.033
0
0
C1
1n
0
8VDC5VDC
C5
10u
C2
100n
M3
M2N6770
0
R31
1.5
V1
4.45
V2
5Vdc
V3
8Vdc

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Figure 7: Simulation results for V1=2.59V

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Figure 8: Simulation results for V1=4.45V
5.2. Simulation of Current Sensing Circuit
Simulation of the circuit as in Figure 4 is conducted. The Circuit is as shown in Figure 9.

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Figure 9: Simulation of current sensing circuit
For a current of 3.6A, 𝑉𝑠𝑒𝑛𝑠𝑒 = 119𝑚𝑉. Thus simulation result for this condition is as shown in Figure 10.
From the simulation result it is clearly seen that the for a current of 3.6A, 𝑉𝑐𝑢𝑟 = 3.6𝑉.
U1B
OP484
-
6
+
5
V-
11
OUT
7
V+
4
R9
1k
Vsense
C3
100n
0
R10
100k
R11
3.3k
Vcur
0
5VDC
0

24 | P a g e
Figure 10: Simulation result for I=3.6A
5.3. Simulation of Temperature Sensing Circuit
Simulation of the circuit as in Figure 5 is conducted. The Circuit is as shown in Figure 11. In this circuit R15
is the Thermistor. The value of thermistor resistance for 30o
C is 4028Ω. Thus simulation result for this
case is as shown in Figure 12. It is seen that as per the design calculation the output 𝑉𝑡𝑒𝑚 𝑝 should be
equal to 0.81V and the desired result is obtained.

25 | P a g e
Figure 11: Simulation of temperature sensing circuit
U1C
OP484
-
9
+
10
V-
11
OUT
8
V+
4
R12
680
R13
1.8k
R14
18k
R15
4028
M4
BSS83P/L1/INF
U2C
OP484
-
9
+
10 V-
11
OUT
8
V+
4
5VDC
5VDC
5VDC
5VDC
0
0
0
0
Vtemp

26 | P a g e
Figure 12: Simulation result for 30o
C

27 | P a g e
6. References
[1] R.K.Shevgaonkar, “Erbium Doped Fiber Amplifier”, Lecture 27, FIBER OPTICS, Indian Institute of
Technology, Bombay.
[2] M.A. Mahdi , F.R.M. Adikan and H. Ahmad, “Single-mode pumping scheme for EDFA with high power
conversion efficiency using 980 nm Ti:S laser”, Microwave and Optical Technology Letters ,vol. 48, no. 1,
pp. 71-74, 2006.
[3] Rick Downs, “An Optical Amplifier Pump Laser Reference Design Based on the AMC7820”, SBAA072A
Application Report, Texas Instruments.
[4] Neil Albaugh, “Optoelectronics Circuit Collection”, SBEA001 Application Report, Texas Instruments.

LASER DRIVER FOR HIGH POWER ERBIUM DOPED FIBER AMPLIFIER (EDFA)

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