CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 OVERVIEW OF OPTICAL COMMUNICATION Optical fiber completely replaces coaxial cable and other low attenuation, free from electromagnetic interferences, comparatively less cost than copper cable are the main reasons for the deployment of optical fiber. The attenuation of optical fiber is 0.2 db/km at 1.55 µm wavelength. It has emerged as a major technology for both analog and digital transmission. Optical fiber link consist of optical transmitter, fiber and optical receiver. Single and multimode fibers are preferred for long and short communication links. Minimum dispersion and minimum attenuation are obtained at 1.3 µm and 1.55 µm wavelength respectively. Direct or external modulation is performed in the optical transmitter. RF signal or digital data directly modulates the laser drive current in direct optical modulation. This method of modulation is widely used in the fiber optic links due to simplicity and cost effectiveness. However, inherent non linear behavior, resonant oscillation and frequency chirp limit its applications. Electro optic or absorption modulators are used in the external optical modulation. Mach - Zehnder modulator is based on electro optic effect with the use of lithium niobate dielectric material. Electro absorption modulator is fabricated with semiconductor material. Franz-Keldysh effect

2 2 and Quantum -confined Stark effect are used to realize electro absorption modulators. 1.2 RADIO OVER FIBER (ROF) TECHNOLOGY In RoF technology, RF signals from remote antenna modulates the optical carrier either by directly varying the current in the laser diode or by external modulator. The applications of RoF technology are in the area of WLAN signal transmission, CATV, cellular mobile communication networks, phased array antennas and antenna remoting (Alwyn J Seeds 2002). It consists of optical transmitter, fiber and optical receiver. In mobile communication networks, this system reduces the number of equipments in the base station and overall system complexity. RoF transceiver can be located at tunnels, basements of large buildings where the direct wireless signals from base station antenna fails to provide the necessary coverage. The major limitation in the analog fiber optic link performance is low RF to optical conversion efficiency. The total link loss was reported around -20dB to -30dB due to low RF to optical conversion efficiency (Hamed Al-Rawesshidy 2002). In order to overcome this loss and to improve the signal to noise ratio (SNR), RF amplifiers are needed prior to the laser diode in the transmitter and after photo detector in the receiver. An alternate approach to increase the signal to noise ratio is by enhancing the modulation efficiency of laser diode. Gain lever effect has been realized in two section MQW laser diodes in order to improve the modulation efficiency and reported previously. 1.3 DEVICES FOR ROF TECHNOLOGY Laser diode, optical fiber, modulator and optical receiver are the essential components for an optical link. The RF performance of the optical

3 3 link are mainly depends on the characteristics of these components. The RF to RF link efficiency for the radio over fiber link is given by Seltzer et al (1995) eff L o d RF cl t (1.1) where 'cl' is the laser to fiber coupling loss, efficiency, 't o ' is the fiber attenuation and L is the laser diode slope d is the efficiency of the photo diode. The efficiency of the modulation device and photo diode has major impact on the overall link efficiency Laser Diodes Laser diode is an important optical source for fiber optic transmitters. In the radio over fiber link, the choice of the laser diode is based on its characteristics. High slope efficiency (W/A), modulation bandwidth, linearity are the required characteristics of the laser diode (Kawaguchi 1994). Similarly, the diode should have minimum noise and spectral width. Fluctuations in the intensity of the optical power occur in the laser diode due to temperature variation and spontaneous emission. This intensity noise is known as Relative Intensity Noise (RIN) and it is measured in db/hz. This affects the overall noise figure of the link along with thermal noise from electronic circuit. Fabry-Perot and Distributed Feed Back lasers are typically used in an optical fiber link. VCSEL laser diode is used in the 850 nm, low cost fiber link. Laser diodes are classified based on the structures such as Double Hetrostructure (DH), Quantum Well (QW), Quantum Dash and Quantum Dot (QD) devices (Coldren 2012). Gain of the RoF link mainly depends on the slope efficiency of the laser diode. Link gain is proportional to square of the laser diode slope

4 4 efficiency (Lau 2012). Gain lever laser diodes and bipolar cascade lasers have shown significant improvement in slope efficiency. In the case of cascaded laser diodes, device complexity and device noise limits its application potential. Hence, there has been a considerable focus on gain lever laser diodes. However the distortion in such devices may significantly affect the link performance. Gain lever effect has been realized in two section MQW laser diodes in order to improve the modulation efficiency and reported previously by a number of authors (Cox et al 2006). Recently, the performance of transistor laser is analysed for microwave photonic applications (Iezekeil 2014) External Modulator The light generated from the continuous wave (CW) laser diode can be externally modulated by either Mach-Zehnder or electro absorption modulator. The RF voltage is directly applied into electro optic material in the case of a Mach- Zehnder modulator. The refractive index of the material is changed with the application of RF signal. The characteristics of this external modulator are low half wave voltage, low insertion loss, high optical power handling and broad bandwidth. The linearity of the modulator is also important parameter which depends on the half wave voltage. This voltage allows the maximum output optical power from the modulator. The sensitivity is very low for lithium niobate electro optic material. Recently, silicon and polymer materials are also considered for electro optic modulators. Electro absorption modulator is based on semiconductor material and it can be easily integrated with other semiconductor devices like laser diode. Franz-Keldysh effect is used in bulk modulator and Quantum -confined Stark effect is used in the MQW. Franz-Keldysh effect is voltage controlled absorption of light. If the input optical energy is greater than or equal to the bandgap energy of the device, the light is absorbed and otherwise it is

5 5 transmitted. The ratio of output optical power to input optical power is depend on the applied voltage. The RF modulating signal is applied along with this voltage. Nowadays, electro absorption modulators are developed to handle high optical power (few hundred mw). This modulator is also provide improvement in the spur free dynamic range of the radio over fiber link Photo Detectors In an analog optical link, photo diode forms an important component. PIN or avalanche photodiode (APD) is generally used in the optical receiver. Gain-bandwidth product is high in the APD. But, optical power handled by the APD is too low. High performance radio over fiber link requires linear and highly efficient photo diodes. The bandwidth of the photo diode should be high for these applications. Responsivity and linearity are other desirable characteristics of the photodiode. High power handling capability for externally modulated radio over fiber link is also important. However, these characteristics cannot be simultaneously achieved in an optical detector. 1.4 GAIN LEVER EFFECT Gain lever effect is an approach by which modulation efficiency can be improved in two section MQW laser diodes. Two section laser diode can exhibit bistability, and self pulsation phenomenon, apart from gain lever effect. Optical gain lever effect was first analyzed by Vahala & Newkirk (1989) in two section quantum well laser, in which an external pump laser is used to modulate the carrier density. Gain lever effect in bulk and multiple quantum well laser was demonstrated by Seltzer et al (1995). For a laser diode, to exhibit gain lever effect, the active region has to be divided in to two unequal sections. RF current is given to the shorter section of laser diode and the longer section is dc biased at high gain level. The detailed rate equation

6 6 analysis of gain lever single quantum well GaAlAs laser diode was provided by Moore & Lau (1989). Improvement in modulation efficiency in gain levered long wavelength InGaAsP/InP lasers was reported at a frequency of 900MHz, by Seltzer et al (1995). Gain lever phenomenon results due to the non linear transfer characteristic of laser diode (Horri et al 2012). But it also introduces harmonic and inter modulation distortion at higher modulation frequencies. This is an undesirable phenomenon that restricts the wide spread use of such devices in analog fiber optic links, in spite of its improved modulation efficiency. Requirements for RF transmission through fiber include large bandwidth, efficient RF to optical conversion and minimum distortion (Kitayama et al 2014). Gain lever laser diode is a promising candidate due to its improved modulation efficiency. But, harmonic and intermodulation components produced in the optical output due to its inherent non linearity of the gain lever phenomenon limits its application potential. Bisection laser structures normally exhibit bistability and self pulsation. AM enhancement or gain lever are also observed in such devices under certain conditions (Pochet et al 2014). 1.5 DISTORTION REDUCTION SCHEMES In RoF technology, multiple radio signals modulate the light source. Harmonic and intermodulation products of input RF signals are generated in the optical output due to the nonlinearity of the optical modulators. The harmonic and intermodulation products generated for particular input may fall within another service (Roselli et al 2003). Feed forward and predistortion are conventional linearization techniques used in analog optical links (Lee et al 2006). Dual parallel linearization is also used to reduce the distortion. Feed forward scheme reduces the RIN in the laser diode. But, this scheme requires more number of optical components like,

7 7 laser diode, photo diode and optical coupler. The system complexity and cost are higher in this technique. The components for reducing intermodulation are added prior to the modulation device in predistortion schemes. The electronic components present in the predistortion circuits limits the bandwidth usage of the link. Volterra series based model of pre distorter can be used for linearization. Adaptive linearization scheme are also implemented for multi service RoF applications (Xavier et al 2002). In the case of RF and microwave amplifiers, second harmonic and difference frequency injection technique are used reduce IMD3 signals (Aitchison et al 2001). It is well known that if the signal due to intermodulation distortion falls in the main signal band it is difficult to reduce it. Among various distortion effects, third order intermodulation products (IMD3) fall on the main signal band and cannot be easily filtered out. Feed forward linearization technique and predistortion using electrical circuits, are conventionally used to reduce the distortion in the laser diodes (Pei et al 2013). Westbrook & Seltzer (1993) reports the intermodulation free dynamic range of InGaAs/InP gain lever lasers in 900 MHz analog fiber optic link, where the effect of short section bias current with the constant section ratio is analyzed. Recently, reduction in harmonic distortion (2HD) and third order intermodulation distortion (IMD3) in gain lever DBR laser diode, by optical injection locking method was reported by Sung & Wu (2008). In this method, a number of devices are used in optical transmitter thereby increasing its complexity. Further, a modified laser diode structure for improved modulation performance along with reduced non linear distortion was reported by Rana et al (2007), where the cavity structure is altered. Feedback harmonic injection is one method used in laser diodes and RF amplifiers, to reduce the magnitude of IMD3 components. In this linearization scheme,

8 8 IMD3 components, ), second harmonic components 1 2) and difference frequency component 1-2) are injected along with RF input signal. These harmonic components are applied from external RF sources or generated from a photodiode in the laser transmitter. Morthier et al (1991) analyzed the effect of nonlinearity on harmonic distortion in Fabry-Perot and Distributed Feedback Laser diodes. Chen et al (1999) investigated the characteristics of intermodulation distortion in DFB laser diodes with the effect of device nonlinearity and structures. Feedback harmonic injection, predistortion and feedforward linearization techniques are conventionally used to reduce intermodulation signals in single section laser diode (Hekkala et al 2012). Tapered cavity structure is another approach to reduce IMD3 and improve the modulation efficiency in two section gain lever laser diode. A novel linearization technique based on feedback harmonic injection to suppress the IMD3 components in multiple quantum well gain lever laser diode is proposed. We generate IMD3 components from second harmonic components, which are extracted from the longer section of bisection laser diode. The longer section voltage is calculated based on the analysis of Byrne & Keating (1989). This approach minimizes the number of components to generate the harmonic signals. However, RF components such as band pass filters, multipliers, RF amplifier and phase shifters are used in our proposed method. Reduction in IMD3 components using this scheme, are theoretically analyzed by solving laser diode rate equations under two tone injection. Based on this approach, a simultaneous IMD3 and IMD5 reduction scheme is also investigated.

9 9 6 SHORT OPTICAL PULSE AND MICROWAVE GENERATION SCHEMES Short Optical Pulse Generation Using Gain Lever Laser Diode Short optical pulses are used in optical fiber communication and optical signal processing applications (Kawaguchi & Carroll 1991). These optical pulses are also used in laser based radar systems, 3D imaging, vehicle safety devices, laser tomography and time imaging spectroscopy. Mode locking, Q switching and gain switching techniques are conventionally used in single section and bisection laser diodes to generate short optical pulses. Among these, gain switching technique is a popular scheme for short optical pulse generation. Gain switching for short pulse generation and its limiting factors are analyzed with rate equations, by Lau (1988). The effect of drive current amplitude, bias current and spontaneous emission factor are also predicted. Photon life time, differential gain and injected carrier density are found to be the important laser parameters that determines the pulse width. Gain switching in AlGaAs/GaAs multiple quantum well laser are analyzed by Ketterer et al (1988). Short optical pulse is observed in long wavelength MQW laser diode at n =2 quantum transition state by Nagarajan et al (1989). Large value of differential gain enhances the efficiency of optical pulse generation. Conditions for self pulsations and bistability in bisection laser diodes are thoroughly analyzed by Ueno & Lang (1985). Self pulsation phenomenon is also used for short optical pulse generation. Generation of short, optical pulses is also observed in bisection laser diodes along with optical bi stability. Bistability and pulsation in laser diodes are analyzed with inhomogeneous current injection by Harder et al (1982). The bistable characteristics of quantum well laser diode are experimentally analyzed by

10 10 Uenohara et al (1996). An equivalent circuit model of bisection laser diode is derived by Ganesh Madhan et al (1998) and the effect of self pulsation in bisection laser diode was used for short optical pulse generation. In this method, longer section (gain) section is excited with an electrical pulse to generate short optical pulses. The effect of absorber section reverse bias on optical pulse width is reported in bisection laser diode. Saito et al (2010), have analyzed the effect of absorber section bias on optical pulse generation in GaN-based laser diodes. Recently, short optical pulse generation is demonstrated in MQW with asymmetric wave guide structure. Passive mode locking of external cavity, bisection GaInN MQW laser diode is used in this case (Gee & Bowers 2001). An asymmetric wave guide laser diode with thick active layer is proposed for optical pulse generation (Ryvkin et al 2011). An 100 ps optical pulse is generated for visible light communications (Binh et al 2014). An efficient method of optical pulse generation in 1.3 µm, InGaAs/InGaAsP, MQW, bisectional laser diode utilizing gain lever effect is proposed. The effect of longer section optical confinement factor on optical pulse characteristics are theoretically analyzed. The effect of optical confinement factor variation is similar to that of asymmetric waveguide structures. In the existing optical pulse generation techniques based on bisectional laser diode, strong reverse bias is applied to absorber section. However, in the gain lever based scheme, low value of pulse current is injected into the shorter section. Hence, the electrical pulse energy required for short pulse generation is lower in our proposed method. The longer section of bisection laser diode is heavily biased. The two level rate equations are numerically solved for different values of longer section optical confinement factor at the fixed value of the shorter section optical confinement factor. From the simulation, short optical pulse of 88 ps width and peak optical power of 5.5 mw is obtained under the shorter section

11 11 electrical pulse current of 0.3 ma. The electrical to optical conversion efficiency is evaluated for different values of longer section confinement factor and the maximum efficiency is determined Microwave Generation Using Gain Lever Laser Diode RF and microwave sources are important components in high frequency communication systems. Frequency multipliers, Phase Locked Loop (PLL) are conventionally used to generate RF signals. Microwave signal generation from optical source are the most attractive approach in Radio over Fiber applications. These applications include wireless communication, RADAR, software defined radio and satellite communication (Lin et al 2009). Microwave signal generation by optical techniques has advantages of high speed, low cost, low power consumption and high dependability (Wang et al 1999). Direct and external modulation of semiconductor lasers, Optoelectronic oscillators, mode locked lasers, dual mode lasers, periodic oscillators and optical heterodyne with Optical Phase Locked Loop (OPLL) are the techniques used for microwave generation. The above mentioned schemes has its own advantages and drawbacks (Qi et al 2011). Direct modulation of laser diode is an simple approach for microwave signal generation. Relaxation oscillation frequency limits the modulation bandwidth of the laser diode and it further limits the generation frequency. Optical heterodyne or photo mixing is also used to generate high frequency microwave signal. Optical signals from two different light sources, high speed photodiode, OPLL and dual wavelength semiconductor lasers are used in this scheme. However, phase noise generated in this technique is a drawback.

12 12 External modulation scheme is also used for microwave generation. System complexity is higher for this approach. Microwave generation from passive and active mode locked semiconductor laser are widely reported. High speed modulator and reference source are required for active mode locking. Large phase noise results in passive mode locking. Microwave signal is also generated from passively mode locked, two section quantum dot laser diode (Lin et al 2009). 1.7 MODELLING OF TWO SECTION LASER DIODE Various models are used to simulate the characteristics of single and bisection laser diodes. The electro optical interactions in a laser diode are characterized by a set of differential equations, commonly referred as rate equations. These rate equations represent time varying average value of carrier and photon densities in the laser cavity. The solution of rate equations provides the output of the device for analog or digital signal input. The rate equations are conventionally solved by numerical method such as Runge - Kutta technique. Equivalent circuit models derived from rate equations for laser diode are also widely used to analyze the device characteristics (Tucker et al 1984). The rate equations are converted into equivalent circuit comprising of resistor, capacitor and inductors and controlled sources. Circuit simulator like PSPICE is used to solve the circuit model (Ganesh Madhan et al 1999). This model includes drive circuit interactions and parasitic effect. This approach is suitable for both small and large signal analysis. The analog optical fiber links are finding applications in CATV, wireless networks and antenna remoting. The solution of rate equations under RF and microwave signal input is required for these applications. Harmonic

13 13 balance model is another approach to solve the rate equations under high frequency signal input. This provides the analysis of harmonic and intermodulation distortion. 1.8 ORGANIZATION OF THE THESIS The work reported in the thesis is organized into six chapters. Chapter 2 deals about gain lever effect in bisection laser diode. This includes the rate equations description for gain lever effect and AM efficiency improvement under static and dynamic conditions. The gain lever is analyzed for 900 MHz for RoF applications. Chapter 3 deals with the optimization of gain lever and distortion with the effect of section lengths, bias current and optical confinement factor. The expression for second harmonic content in the optical output is derived. Chapter 4 deals with novel distortion reduction scheme for gain lever laser diode. Further, simultaneous reduction of IMD3 and IMD5 are also analyzed in this chapter. Chapter 5 includes the techniques for optical pulse and microwave generation using gain lever diode. Chapter 6 summarizes the conclusions of this work.