**Hybrid Fiber Amplifier**

Inderpreet Kaur1 and Neena Gupta2

*1Rayat and Bahra Institute of Engineering, Mohali, 2PEC University of Technology (Formally Punjab Engineering College), Chandigarh India* 

#### **1. Introduction**

The advent of telecommunications in 1870s completely revolutionized the world of communications. Metallic cables consisting of twisted wire cables, co-axial cables were the media of choice for many years. These could be used efficiently up to frequencies of 10MHz but the system performance degraded beyond this range. However, with the increasing demand for telephone services, it was necessary to find an alternative medium for telephony to cope up with the high demand. The development of low loss optical fibers gave a solution to this problem and their use revolutionized the speed of telecommunication. Optical fibers have become an unavoidable part of any high speed communication system due to its high information carrying capacity, high bandwidth and extremely low loss. The transmission performance of the optical communication systems is limited by various effects such as attenuation, dispersion, non- linearity, scattering etc, which degrade the level of the signal. To compensate for all these limitations the signals have to be regenerated within the transmission link after some distance. While setting up the transmission link, it is to be ensured that the signal can be retrieved intelligibly at the receiving end. This can be done either by using optoelectronic repeaters or optical amplifiers. In optoelectronic repeaters the optical signal is first converted into an electric signal, then amplified in electric domain and finally converted back into optical signals. Regeneration by making use of repeater is a traditional way to compensate for loss and degradation along the transmission medium. Such regenerators become quite complex and expensive for dense wavelength division multiplexed (DWDM) lightwave systems. This process works well for moderate speed single wavelength operation but it can be fairly complex and expensive for high- speed multi- wavelength systems. Moreover these so called opto-electronic repeaters once installed into the system can not be upgraded to higher bit rates. Thus a great deal of effort has been spent to develop all optical amplifiers. These devices operate in the optical domain to boost the power level of the signals. In the history of optical fiber communication systems, the advent of optical amplifier was an important milestone. Optical amplifiers can amplify the optical signals directly without requiring its conversion to the electric domain. The development of optical amplifiers started in early eighties and their use for long haul communication systems became widespread during late nineties. Optical amplifiers provided flexibility while upgrading the installed transmission links to higher bit rates. This flexibility of the bit rates allows overcoming the electrical bottleneck of an electric repeater, which was unable to transmit at high bit rates. The opto-electronic repeaters provided with maximum of 40-80 Gbps bit rate.

Hybrid Fiber Amplifier 105

is the external pumping rate from the injection current density J(t) into an active layer having thickness d, τr is the combined time constant coming from carrier-recombination

is the net stimulated emission rate. Here, vg is the group velocity of incident light, Np is the photon density and g is the overall gain per unit length. The photon density Np is dependent on optical signal power, energy of photons, group velocity and dimensions of

> ( )( ) *<sup>s</sup> <sup>p</sup>*

In equation (4), Ps is the signal power, vg is group velocity, w and d are the width and thickness of active area of optical amplifier respectively. The difference between the structure of optical amplifiers and laser diodes is that there is no feedback system in optical amplifiers. So, for boosting an incoming signal optical amplifier requires a pump. The pump supplies energy to the electrons in an active medium, which in turn causes population inversion. An incoming signal photon triggers these excited electrons to drop to lower levels through a stimulated emission process, thereby producing an amplified signal. The amplifier is connected with the optical fiber through a fiber- to- amplifier coupler. The basic components of an optical amplifier are shown in the figure 2) [Keiser 2009;Mynbaev 2003].

The optical gain depends on the frequency/ wavelength of the signal. Let us consider a medium of two level systems for demonstrating the dependence of gain on frequency. The

1( )

 

*<sup>g</sup> <sup>g</sup> <sup>P</sup> <sup>T</sup>*

*sat*

(5)

*P*

gain coefficient of such a medium can be written as below [Agarwal 2003]:

( )

*h wd v* 

*<sup>P</sup> <sup>N</sup>*

*g*

*qd* (2)

<sup>2</sup> ( ) *R t gv N g p* (3)

(4)

1 ( ) ( ) *J t R t*

mechanisms and spontaneous emission, and

This photon density Np is given by equation (4),

Fig. 2. The Basic Structure of an Optical Amplifier

active area of optical amplifier.

where,

#### **1.1 DWDM systems**

To increase the transmission capacity of a single fiber, DWDM is used. DWDM is a technology, which combines large number of independent information carrying wavelengths onto the same fiber. A characteristic of DWDM is that the discrete wavelengths form an orthogonal set of carriers, which can be separated, routed and switched without interfering with each other. This isolation between channels holds as long as the total optical power intensity is kept sufficiently low to prevent non linear effects e.g. Stimulated Brillouin Scattering (SBS) and Four Wave Mixing processes (FWM) from degrading the link performance. The implementation of DWDM system requires a variety of passive and active devices to combine, distribute, isolate and amplify optical power at different wavelengths. Passive devices require no external control for their operation, so they are less flexible. The wavelength dependent performance of active devices can be controlled electronically, so they provide more flexibility to the network system. Optical amplifiers, tunable filters and tunable sources are integral part of any DWDM system. The key component of DWDM system is optical amplifier. In DWDM system, it is desirable to set a very narrow grid of optical carriers in order to allow more channels in the same optical bandwidth. This not only demands an optical amplifier with high gain but also very broad and flat gain profile to ensure a nearly identical amplification factor in every channel. Figure 1 shows the implementation of active as well as passive components in a typical DWDM system having post amplifier, in-line amplifier and preamplifier [Keiser 2009; Mynbaev 2003].

Fig. 1. Implementation of A DWDM System Having Various Types of Optical Amplifiers

#### **2. Review of an optical amplifier**

An optical amplifier works on the principle of stimulated emission. Optical amplifier increases the level of signal through this process. The mechanism for stimulated emission is same as that for lasers. The operation of laser diodes that are required for the fiber amplifier is similar to the external current injection method (which is used in semiconductor optical amplifiers, SOAs, discussed later). This method is the pumping method used to create population inversion needed for gain mechanism in fiber amplifiers. The sum of injection, stimulated emission and spontaneous recombination rates gives the rate equation that governs the carrier density N (t) in the excited state of both the amplifiers. This carrier density is given by equation (1) [Keiser 2009;Mynbaev 2003].

$$\frac{\partial N(t)}{\partial t} = R\_1(t) - R\_2(t) - \frac{N(t)}{\tau\_r} \tag{1}$$

where,

104 Optical Communications Systems

To increase the transmission capacity of a single fiber, DWDM is used. DWDM is a technology, which combines large number of independent information carrying wavelengths onto the same fiber. A characteristic of DWDM is that the discrete wavelengths form an orthogonal set of carriers, which can be separated, routed and switched without interfering with each other. This isolation between channels holds as long as the total optical power intensity is kept sufficiently low to prevent non linear effects e.g. Stimulated Brillouin Scattering (SBS) and Four Wave Mixing processes (FWM) from degrading the link performance. The implementation of DWDM system requires a variety of passive and active devices to combine, distribute, isolate and amplify optical power at different wavelengths. Passive devices require no external control for their operation, so they are less flexible. The wavelength dependent performance of active devices can be controlled electronically, so they provide more flexibility to the network system. Optical amplifiers, tunable filters and tunable sources are integral part of any DWDM system. The key component of DWDM system is optical amplifier. In DWDM system, it is desirable to set a very narrow grid of optical carriers in order to allow more channels in the same optical bandwidth. This not only demands an optical amplifier with high gain but also very broad and flat gain profile to ensure a nearly identical amplification factor in every channel. Figure 1 shows the implementation of active as well as passive components in a typical DWDM system having post amplifier, in-line amplifier

Fig. 1. Implementation of A DWDM System Having Various Types of Optical Amplifiers

An optical amplifier works on the principle of stimulated emission. Optical amplifier increases the level of signal through this process. The mechanism for stimulated emission is same as that for lasers. The operation of laser diodes that are required for the fiber amplifier is similar to the external current injection method (which is used in semiconductor optical amplifiers, SOAs, discussed later). This method is the pumping method used to create population inversion needed for gain mechanism in fiber amplifiers. The sum of injection, stimulated emission and spontaneous recombination rates gives the rate equation that governs the carrier density N (t) in the excited state of both the amplifiers. This carrier density is given by equation (1) [Keiser 2009;Mynbaev

> 1 2 ( ) ( ) () ()

*t*

*N t N t Rt Rt*

*r*

(1)

**1.1 DWDM systems** 

and preamplifier [Keiser 2009; Mynbaev 2003].

**2. Review of an optical amplifier** 

2003].

$$R\_1(t) = \frac{J(t)}{qd} \tag{2}$$

is the external pumping rate from the injection current density J(t) into an active layer having thickness d, τr is the combined time constant coming from carrier-recombination mechanisms and spontaneous emission, and

$$R\_2(t) \equiv \lg \upsilon\_g N\_p \tag{3}$$

is the net stimulated emission rate. Here, vg is the group velocity of incident light, Np is the photon density and g is the overall gain per unit length. The photon density Np is dependent on optical signal power, energy of photons, group velocity and dimensions of active area of optical amplifier.

This photon density Np is given by equation (4),

$$N\_p = \frac{P\_s}{(h\nu)(wd)v\_\mathcal{g}}\tag{4}$$

In equation (4), Ps is the signal power, vg is group velocity, w and d are the width and thickness of active area of optical amplifier respectively. The difference between the structure of optical amplifiers and laser diodes is that there is no feedback system in optical amplifiers. So, for boosting an incoming signal optical amplifier requires a pump. The pump supplies energy to the electrons in an active medium, which in turn causes population inversion. An incoming signal photon triggers these excited electrons to drop to lower levels through a stimulated emission process, thereby producing an amplified signal. The amplifier is connected with the optical fiber through a fiber- to- amplifier coupler. The basic components of an optical amplifier are shown in the figure 2) [Keiser 2009;Mynbaev 2003].

Fig. 2. The Basic Structure of an Optical Amplifier

The optical gain depends on the frequency/ wavelength of the signal. Let us consider a medium of two level systems for demonstrating the dependence of gain on frequency. The gain coefficient of such a medium can be written as below [Agarwal 2003]:

$$\log(\alpha) = \frac{\mathcal{S}\_0}{1 + \left(\alpha - \alpha\_0\right)^2 T^2 + \frac{P}{P\_{sat}}} \tag{5}$$

Hybrid Fiber Amplifier 107

**Pre-amplifier:** This is used before the photo detector at the receiver in order to strengthen the weak received signal. This increases the sensitivity of the detector effectively. This

**Post -amplifier:** This is used at the transmitting end, after the source and operates near the saturation region. The power launched into the fiber is enhanced and so the repeater span can become large. This serves to increase the transmission distance by 10- 100km depending on the amplifier gain and fiber loss. This configuration is shown in

The optical amplifiers which find widespread use in communication systems can be

The first two types, Fiber Raman Amplifier (FRA) and Erbium Doped Fiber Amplifier (EDFA) can be efficiently coupled to the transmission fiber by splicing with a minimum coupling loss. Of these two, EDFA requires lesser power for the pump source and the pump power requirements can be easily met by semiconductor laser diodes. Besides, the gain characteristics of EDFA are insensitive to polarization. Semiconductor Optical Amplifier (SOA) has the advantages of smaller size and lower power consumption. Its dimensional

configuration is shown in figure 3b.

Fig. 3b. Optical Amplifier as Preamplifier

Fig. 3c. Optical Amplifier as Post Amplifier

2. Erbium Doped Fiber Amplifier (EDFA) 3. Semiconductor Optical Amplifier (SOA)

**4. Types of optical amplifiers** 

classified into three categories:- 1. Fiber Raman Amplifier (FRA)

figure 3c.

Where, go is the peak value of the gain, ω is the optical frequency of the incident signal, ωo is the atomic transition frequency , P is the optical power of the signal being amplified, Psat is the saturation power and T is the dipole relaxation time. In the unsaturated region, P /Ps«1. So, the gain coefficient becomes

$$\log(\alpha o) = \frac{\mathcal{S}o}{1 + \left(o - o\_0\right)^2 T^2} \tag{6}$$

This equation shows that the gain reaches its maximum when the incident frequency coincides with the atomic transition frequency. Another term associated with optical amplifiers is amplification factor or amplifier gain (G) defined as:-

$$G = \frac{P\_{out}}{P\_{in}} \tag{7}$$

Where Pin and Pout are the input and output powers of the continuous wave signal being amplified.

#### **3. Applications of optical amplifiers**

Optical amplifiers have found many applications ranging from ultra long undersea links to short links in access networks[Keiser 2009; Mynbaev 2003; Olsson1989 & Agarwal 2003].


**In-line amplifier:** This is used as a repeater along the link at intermediate points. It can be used to compensate for transmission loss and increase the distance between regenerative repeaters, as shown in figure 3a.

Fig. 3a. Optical Amplifier as In-Line Amplifier

Where, go is the peak value of the gain, ω is the optical frequency of the incident signal, ωo is the atomic transition frequency , P is the optical power of the signal being amplified, Psat is the saturation power and T is the dipole relaxation time. In the unsaturated region, P /Ps«1.

> ( ) 1( ) *<sup>g</sup> <sup>g</sup> <sup>T</sup>*

> > *<sup>P</sup> <sup>G</sup>*

Where Pin and Pout are the input and output powers of the continuous wave signal being

Optical amplifiers have found many applications ranging from ultra long undersea links to short links in access networks[Keiser 2009; Mynbaev 2003; Olsson1989 & Agarwal

**In-line amplifier:** This is used as a repeater along the link at intermediate points. It can be used to compensate for transmission loss and increase the distance between regenerative

 

This equation shows that the gain reaches its maximum when the incident frequency coincides with the atomic transition frequency. Another term associated with optical

> *out in*

amplifiers is amplification factor or amplifier gain (G) defined as:-

0

2 2 0

(6)

*<sup>P</sup>* (7)

So, the gain coefficient becomes

**3. Applications of optical amplifiers** 

amplified.

2003].

 In-line amplifier Pre-amplifier Post -amplifier

repeaters, as shown in figure 3a.

Fig. 3a. Optical Amplifier as In-Line Amplifier

**Pre-amplifier:** This is used before the photo detector at the receiver in order to strengthen the weak received signal. This increases the sensitivity of the detector effectively. This configuration is shown in figure 3b.

Fig. 3b. Optical Amplifier as Preamplifier

**Post -amplifier:** This is used at the transmitting end, after the source and operates near the saturation region. The power launched into the fiber is enhanced and so the repeater span can become large. This serves to increase the transmission distance by 10- 100km depending on the amplifier gain and fiber loss. This configuration is shown in figure 3c.

Fig. 3c. Optical Amplifier as Post Amplifier
