1 An Introduction to Optical Communication Systems 1.1 INTRODUCTION Communication is a process in which messages, ideas and information can be exchanged between two individuals. From the early days when languages were developed, the methods people use to communicate have experienced a dramatic evolution. Nowadays, rapid transmission of information over long distances and instant access to various information sources have become conspicuous and important features of our society. The rapidly growing information era has been augmented by a global network of optical fibre [1]. By offering an enormous transmission bandwidth of about 10 14 Hz and a low signal attenuation, the low-cost, glass-based single-mode optical fibre (SMF) provides an ideal transmission medium. In order that information can be carried along the SMF, information at the transmitter side is first converted into a stream of coherent photons. Using a specially designed semiconductor junction diode with heavy doping concentration, semiconductor lasers have been used to provide the reliable optical source required in fibre- based lightwave communication. With its miniature size compatible to the SMF, the semiconductor laser diode has played a crucial role in the success of optical fibre communication systems. This chapter has been organised as follows: in section 1.2, the historical progress of optical communication is presented. Before exploring the characteristics of semiconductor lasers, various configurations of optical fibre-based communication systems are discussed in section 1.3. Depending on the type of detection method used, both direct and coherent detection schemes are discussed. Based upon the characteristics of coherent optical communication systems, the performance requirements of semiconductor lasers are presented at the end of the chapter. In particular, the significance of having an optical source that oscillates at a single frequency whilst having a narrow spectral linewidth is reviewed. 1.2 HISTORICAL PROGRESS In the early days of human civilisation, simple optical communication in terms of signal fires and smoke was used. In those days, only limited information could be transferred within line Distributed Feedback Laser Diodes and Optical Tunable Filters H. Ghafouri–Shiraz # 2003 John Wiley & Sons, Ltd ISBN: 0-470-85618-1 of sight distances. In addition the transmission quality was strongly restricted by atmospheric disturbances. This form of visual communication was extended and used in the form of flags and signal lamps until the early 1790s, when a French scientist, Claude Chappe [2] suggested a system of semaphore stations. Messages were first translated into a sequence of visual telegraphs. These were then transmitted between tall towers which could be as far as 32 km apart. These towers acted as regenerators or repeaters such that messages could be transmitted over a longer distance. However, this method was slow and costly since messages had to be verified between each tower. With the beginning of a modern understanding of electricity in the 19th century, scientists started to investigate how electricity might be used in long distance communication. The telegraph [3] and telephone [4] were two inventions best representing this early stage of the electrical communication era. During that period of time, optical communication in the atmosphere received less attention and the systems developed were slow and inefficient. The lack of suitable optical sources and transmission media were two factors that hindered the development of optical communication. It was not until the early 1960s when the invention of laser [5] once again stimulated interest in optical communication. A laser source provides a highly directional light source in which photons generated are in phase with one another. By modulating the laser, the coherent, low divergence laser beam enables the development of optical communication. Due to the atmospheric attenuation, however, laser use is restricted to short distance applications. Long distance communication employing laser sources became feasible after a breakthrough was reached in 1966 when Kao and Hockham [6] and Werts [7] discovered the use of glass-based optical waveguides. By trapping light along the central core of the cylindrical waveguide, light confined along the optical fibre could travel a longer distance as compared with atmospheric propagation. Despite the fact that the attenuation of the optical fibre used was so high, with virtually no practical application at that time, this new way of carrying optical signals received worldwide attention. With improvements in manufacturing techniques and intensive research, the attenuation of optical fibre continued to drop. Fibre loss of about 4.2 db km À1 was reported [8] for wavelengths around 1 mm, whilst low-loss fibre jointing techniques also became available. In order to build an optical communication system based on optical fibres, researchers in the 1960s started focusing on the development of other optical components including optical sources and detectors [9–11]. A new family of optical devices based on semiconductor junction diodes was developed. By converting electrical current directly into a stream of coherent photons, semiconductor lasers are considered to be reliable optical laser sources. Based on similar working principles, efficient photodetectors based on the junction diode were developed. By responding to optical power, rather than optical electromagnetic fields, optical signals received are converted back into electrical signals. In this early phase of development, semiconductor lasers used were restricted to pulse operation at a very low temperature. It was not until the 1970s that practical devices operating in continuous wave at room temperature became feasible [12]. The availability of both low-loss optical fibre and reliable semiconductor-based optical devices laid the cornerstone for modern lightwave communication systems. In the late 1970s, lightwave systems were operated at 0.8 mm [13]. Semiconductor lasers and detectors employed in these systems were fabricated using alluminium gallium arsenide alloy AlGaAs [14]. Optical fibres used had a large core of diameter between 50 and 400 mm whilst typical attenuation was about 4 dB km À1 . At the receiver side of the system, direct detection was 2 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS used in which optical signals were directly converted to baseband optical signals. The overall system performance was limited by the relatively larger attenuation and inter-modal dispersion of the optical fibre used. In order to reduce the cost associated with the installation and maintainence of electrical repeaters used in the lightwave communication systems, it was clear that the repeater spacing could be improved by extending the operating wavelength to a new region between 1.1 and 1.6 mm where the attenuation of the optical fibre was found to be smaller. Figure 1.1 shows the relation between the attenuation of a typical SMF and optical wavelength. For systems operating at a longer wavelength, semiconductor optical devices were fabricated using quantenary InGaAsP alloy. In order to avoid inter-modal competition associated with high-order oscillation modes inside the optical fibre, optical fibres having a smaller core diameter of about 8 mm were used. In this way, oscillation in an optical fibre was reduced to single mode. For systems operating in such a longer wavelength region, both wavelengths at 1.3 and 1.55 mm have received a lot of attention. For systems operating near 1.3 mm, it was found that the single-mode fibre used had minimum dispersion, and hence maximum bandwidth could be achieved. In the early 1980s, many systems were built using single- mode fibre at around 1.3 mm wavelength. An even lower fibre attenuation of about 0.2 dB km À1 is found at around 1.55 mm. However, the deployment of lightwave systems in the 1.55 mm region was delayed due to the intrinsic fibre dispersion which limits the maximum bit rate the system can support. The problem was later alleviated by adopting dispersion-shifted or dispersion-flattened fibre [15,16]. Alternatively, semiconductor lasers oscillating in single longitudinal modes were developed [17,18]. By limiting the spread of the laser spectrum, this type of laser is widely used in upgrading the 1.3 mm lightwave Figure 1.1 Attenuation of silica-based optical fibre with wavelength (after [44]). HISTORICAL PROGRESS 3 systems to 1.55 mm wavelengths in which conventional single-mode fibres were used. Since 1988, field trial tests for coherent lightwave communication systems have been carried out [19–21]. In order to improve the bit rate of the present lightwave system whilst utilising available fibre bandwidth in a better way, frequency division multiplexing (FDM) schemes [22] were implemented. Before information is converted into optical signals, electronic multiplexing is often applied in combining the signals. Such a system is normally referred to as coherent optical communication since heterodyne or homodyne detection is used at the receiver end. By mixing the incoming optical signal with an optical local oscillator, coherent detection employs a different technique as compared with the direct detection method. In the 1980s, the development of coherent optical communications was hindered due to poor spectral purity and frequency instability in semiconductor lasers. Due to advances in fabrication techniques, semiconductor lasers nowadays show improved performance. In long-haul optical fibre communication systems, fibre dispersion and intrinsic attenuation are two major obstacles that affect the system performance. In the 1990s, optical fibre communication systems continued to develop in order to tackle these obstacles. To circumvent the fibre dispersion, the non-linear optical soliton able to travel extremely long distances was proven both theoretically [23,24] and experimentally [25,26]. By using optical amplifiers [27,28] as pre-amplifiers, post-amplifiers and optical repeaters, one witnesses the deployment of these wideband amplifiers in optical communication networks. In the coming years, networks employing a densely spaced wavelength division multiplexing (WDM) scheme [29] are expected. As a result, more channels and hence more information will be transmitted over a single optical fibre link. There is no doubt that a new paradigm of communication comprising an optically transparent network is already on the way [30]. 1.3 OPTICAL FIBRE COMMUNICATION SYSTEMS By transferring information in the form of light along an optical fibre, a communication system based on optical fibres starts to grow rapidly. This system, like many other communication systems, consists of many different components. A simple block diagram as shown in Fig. 1.2 represents the various components required in an optical fibre communication system. At the transmitter side, information is encoded, modulated and is then converted into a stream of optical signals. At the receiver side, optical signals received are filtered and demodulated into the original information. For long distance applications, repeaters or regenerators have to be used to compensate the intrinsic attenuation of optical fibre. In order to maximise the amount of information that can be transferred over a single optical fibre link, various multiplexing schemes might also be applied. To ensure successful implementation of optical fibre communication links, careful planning and system consideration is necessary. Apart from the performance characteristics of every component used within the system, it is also necessary to consider interactions and compatibility between various components. Depending on the system requirements, the type of transmission (analogue or digital), required transmission bandwidth, cost and reliability, may vary from one system to another. According to the type of detection method used at the receiver end, it is common to categorise an optical fibre system into either a direct detection or a coherent detection scheme. 4 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS 1.3.1 Intensity Modulation with a Direct Detection Scheme Simply by varying the biasing current injected into a semiconductor laser diode at the transmitter, the so-called intensity modulation with direct detection (IM/DD) scheme was widely adopted. The expression ‘intensity modulation’ derives from the fact that the intensity of the light emitted at the transmitter side is linearly modulated with respect to the input signal for either digital or analogue systems. The expression ‘direct detection’ is used because the optical detector at the receiver end responds to optical power, rather than electromagnetic fields as compared to radio or microwave links. In other words, all optical signals received at the optical detector are demodulated into baseband electrical signals. Due to its simplicity and low cost, the IM/DD transmission scheme has had great success, in particular in point-to-point transmission systems. In order to explore the potential of the optical spectrum, however, coherent detection has to be used. 1.3.2 Coherent Detection Schemes Compared to the IM/DD transmission scheme, coherent optical communication [31–33] is characterised by mixing the incoming optical signal with the local oscillator so that the baseband signal (for homodyne detection) or an intermediate frequency (IF) signal (for heterodyne detection) is generated at the receiver. Since spatial coherence of the carriers and local oscillators is exploited, the expression ‘coherent’ is used to describe such a system configuration. The advantages of coherent detection have long been investigated and were recognised in the 1960s [34], but it was not until the late 1970s that single-mode transmission from an AlGaAs semiconductor laser was demonstrated [35,36]. With a Figure 1.2 Simple block diagram showing various components for optical fibre communication systems. OPTICAL FIBRE COMMUNICATION SYSTEMS 5 narrower spectral output, fibre-based lightwave systems employing coherent detection became feasible. Various digital modulation methods have been used in coherent optical communication, including the amplitude-shift keying (ASK), the frequency-shift keying (FSK) and the phase-shift keying (PSK) methods [37,38]. They differ from one another in the way digital messages can be transmitted by variations in amplitude, frequency and phase, respectively. For any digital transmission scheme and receiver architecture, a bit error rate (BER) in the region between 10 À9 and 10 À10 must be achieved at the receiver side for a satisfactory transmission. The coherent optical communication system using homodyne/heterodyne detection has several advantages over the IM/DD transmission scheme [39,40]. First of all, coherent detection can improve the receiver sensitivity by about 15 to 20 dB, depending on the modulation scheme adopted. As a result, spacing between repeaters is improved for long distance communication, whilst transmission rates can be increased in existing long distance links without reducing the repeater distance. Moreover, by using modulation like PSK or FSK, which are well known in communication theory, the receiver can push to reach the ideal quantum noise detection limit. In addition, by adopting densely spaced frequency- division multiplexing (FDM) or wavelength division multiplexing (WDM), a wider fibre bandwidth can be utilised. In practice, however, the coherent optical system has a stringent requirement for device performance. In Fig. 1.3, a general block diagram for the coherent optical communication system is shown. As illustrated in Fig. 1.3, two injection lasers are involved in the system. One acts as a transmitter and the other as a local oscillator. The laser transmitter which acts as an optical frequency oscillator can be used directly in the FSK transmission. An external modulator is optional for the ASK and the PSK transmission before the optical signals are launched into the single-mode fibre (SMF). Optical amplifiers like semiconductor laser amplifiers (SLA) or erbium-doped fibre amplifiers (EDFA) are used in long distance transmission for boosting the signal. Under the heterodyne receiver category with non-zero IF frequency, two different types of postdetection process have been adopted. The name heterodyne receiver with coherent postdetection processing (HE/CP) is usually given to one that has IF carrier recovered at the receiver. Similarly, heterodyne receiver with incoherent postdetection processing (HE/IP) describes the system that has no IF carrier recovered. Comparatively, the HE/IP receiver Figure 1.3 Schematic diagram for the coherent optical communication system. 6 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS configuration is the simplest as IF carrier reconstruction is unnecessary. However, it shows the weakest receiver sensitivity among the three receiver designs. The incoherent postdetection process could be used in conjunction with several modulation schemes such as ASK, FSK and differential phase-shift keying (DPSK). In the HE/CP receiver design, IF signals are recovered at the receiver stage for further signal processing. The coherent postdetection process can improve the receiver performance and so it is applicable to any modulation method. However, it is substantially more complicated than the incoherent method and stringent device performance is required. For zero IF frequency, the homodyne receiver has the best receiver sensitivity as data is recovered directly from the optical mixing process at the receiver. A narrower receiver bandwidth and only baseband electronic processing are required. These offer significant advantages to the homodyne receivers. In practice, however, the technologies required in achieving these advantages in the homodyne receiver are demanding. An effective synchronous demodulation process is essential in phase locking the local oscillator and the received optical signal. Phase jitters caused by phase noise and shot noise could impair the system performance easily. It has been evaluated [41] that the phase variance must be limited to within $10  to ensure a lower power penalty for a BER 10 À9 . This sets an upper limit on the permissible laser spectral linewidth and other laser performance characteristics. In the coming sections, we are going to discuss some fundamental device characteristics and their impact on system performance. 1.4 SYSTEM REQUIREMENTS FOR HIGH-SPEED OPTICAL COHERENT COMMUNICATION 1.4.1 Spectral Purity Requirements An ideal monochromatic laser source has been needed for some time. As a result, the spectral purity of the laser source has often been the first issue confronting users of semiconductor lasers in coherent optical communication. Due to the dispersive nature of the optical fibres used, digital pulses are broadened whilst propagating along the optical fibre. Such pulse spreading causes adjacent pulses to overlap so that errors occur as a result of inter-symbolic interference (ISI). Thus, apart from the power limitation due to the intrinsic fibre attenuation, the transmission distance is also limited by dispersion. The use of single-mode fibres has eliminated the severe inter-modal dispersion of multi- mode fibres. However, because of the finite spectral width of the optical sources, single- mode fibre is limited by chromatic dispersion (or intra-modal dispersion). Since the laser sources do not emit a single frequency but a band of frequency, each frequency component of the field propagates with a different time delay in the single-mode fibre, causing a broadening of the initial pulse width and hence intra-modal dispersion. The delay differences in single mode fibre may be caused by the dispersive properties of material through variation in the cladding refractive index (material dispersion) and also the guidance effects within the structure (waveguide dispersion). In order to minimise the effect of dispersion in single- mode fibre and hence improve the transmission distance, there are two different approaches. The first method involves the use of a dispersion-shifted or dispersion-flattened fibre. With a distinctive refractive index profile, these fibres can reduce the effect of dispersion at the SYSTEM REQUIREMENTS FOR HIGH-SPEED OPTICAL COHERENT COMMUNICATION 7 1.55 mm wavelength significantly. Another possible way involves the improvement of semiconductor laser sources. The ability to lase in single mode with a narrow linewidth can circumvent the effect of dispersion. In the rest of this section, the concept of single-mode operation, especially the possibility of a single longitudinal mode, will be discussed, whilst the impact and the control of spectral linewidth will be left for later sections. (a) Single mode along the transverse plane It was shown in the previous section that the coherent optical communication system requires semiconductor lasers that can emit at a monochromatic frequency in order to achieve the required system BER. As a result, it is necessary to achieve a single-mode oscillation in each of the orthogonal directions inside the laser cavity. To understand the transverse waveguiding problem of semiconductor lasers, one must start with electromagnetic wave theory the basis for the study of electromagnetic wave propagation is provided by Maxwell’s equations [42]. For a medium with zero conductivity the vector relationships may be written in terms of the electric field ~ E and magnetic field ~ H as r ~ E ¼Àj! ~ H ð1:1Þ r ~ H ¼ j!" ~ E ð1:2Þ where " and  are the permittivity and permeability of the medium. The above equations are expressed in the time harmonic form (with time variation term as e j!t ) and are true for source free and lossless media. By using the vector identity and taking the curl on both sides of eqn (1.1), one can arrive at the scalar wave equation for the electric field E such that r 2 ~ E ¼Àk 2 ~ E ¼À! 2 " ~ E ¼Àk 2 0 n 2 ðx; yÞ ~ E ð1:3Þ where k is the propagation constant in the medium with the refractive index distribution of n (x,y) and k 0 is the free space propagation constant. Similarly, by taking the curl on both sides of eqn (1.2), one ends up with the scalar wave equation for the magnetic field H. r 2 ~ H þ k 2 0 n 2 ðx; yÞ ~ H ¼ 0 ð1:4Þ Either eqn (1.3) or (1.4) can be used to determine the field components as they are related to one another by the Maxwell equations. Nevertheless, the scalar wave equation for the electric field is often used as the electric field is responsible for most physical processes and it is the principal field used by photodetectors. To determine the transverse modal field of the semiconductor laser, one must first find the thickness and the refractive indices of materials used in the fabrication process. Depending on the specific laser structure, it is quite possible to have three or four epitaxial layers lying on top of and below the active layer of the semiconductor laser. These laser structures may look complicated at first glance. In fact, their waveguiding properties can be explained with the use of a three-layer dielectric slab (or planar) waveguide. As shown in Fig. 1.4, the 8 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS asymmetric waveguide consists of three layers. The active layer, having refractive index n 1 and thickness d, is sandwiched between the substrate and the cladding of the waveguide. Without loss of generality, it is assumed that the refractive indices of the slab waveguide obey the following inequality n 1 > n 2 ! n 3 ð1:5Þ where the equal sign implies a symmetrical waveguiding structure. With such a planar structure, the field variation along the y-axis can be ignored and so @=@y ¼ 0. By separating the Maxwell equations into different field components, the following equations are obtained [42] @E y @z ¼ j!H x ð1:6aÞ @E x @z À @E z @x ¼Àj!H y ð1:6bÞ @E y @x ¼Àj!H z ð1:6cÞ À @H y @z ¼ j!"E x ð1:6dÞ @H x @z À @H z @x ¼ j!"E y ð1:6eÞ @H y @x ¼ j!"E z ð1:6fÞ The direction of wave propagation has always been assumed to be the longitudinal z direction. By inspecting the above equations carefully, one can separate the above equations into two groups. The first group includes E y , H x and H z from eqns (1.6a), (1.6c) and (1.6e). The results generated from these equations are referred to as the TE mode since the electric field is found along the transverse y-axis (normal to the propagation direction). The other group includes H y , E x and E z , which generates solutions for the TM mode. An inspection of the structure shows that either the TE or the TM mode is supported, but not both simultaneously. Since there is no physical boundary along the y direction, the continuity condition allows only H z or E z to exist. Figure 1.4 Schematic cross-section of a slab dielectric waveguide. Refractive indices of different regions are shown. SYSTEM REQUIREMENTS FOR HIGH-SPEED OPTICAL COHERENT COMMUNICATION 9 For a travelling wave propagating along the z direction, the electric field takes the form Eð ~ r; tÞ¼Eð ~ x; ~ yÞe Àjb z ~z ð1:7Þ where ~ r is the radial vector in space ðx; y; zÞ and the time harmonic term is omitted here for the sake of simplicity. b z is the propagation constant at a fixed angular frequency ! which can also be written as b z ¼ k 0 n eff ð1:8Þ with n eff being the effective refractive index. The electric field component E y for different layers in the slab dielectric can be obtained by substituting eqn (1.7) into eqn (1.3) and putting @=@y ¼ 0. This is @ 2 @x 2 E y ðxÞ¼Àðk 2 0 n 2 1 À b 2 z ÞE y ðxÞ¼Àh 2 E y ðxÞ; Àd x 0 ð1:9aÞ @ 2 @x 2 E y ðxÞ¼Àðk 2 0 n 2 2 À b 2 z ÞE y ðxÞ¼p 2 E y ðxÞ; 0 x < 1ð1:9bÞ @ 2 @x 2 E y ðxÞ¼Àðk 2 0 n 2 3 À b 2 z ÞE y ðxÞ¼q 2 E y ðxÞ; À1 < x Àd ð1:9cÞ where h, p and q are constants defined as h 2 ¼ k 2 0 n 2 1 À b 2 z p 2 ¼ b 2 z À k 2 0 n 2 2 q 2 ¼ b 2 z À k 2 0 n 2 3 ð1:10Þ Depending on the relative values of n 1 , n 2 , n 3 ,k 0 and b z , there are different regimes of propagation constant as shown in Fig. 1.5. Figure 1.5 Different types of modal solutions determined by the constants p, q and h. (after [44]). 10 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS [...]... anti-reflective coatings on the laser facets of DFB and DBR lasers The working principle of a uniform-grating DFB laser with perfectly anti-reflective facet coatings (sometimes called a conventional DFB laser) was first explained by Kogelnik and Shank in 1972 [50] using coupled wave theory, but it was not until 1975 [51] when the first DFB laser that operates at room temperature (300 K) under continuous wave... mechanism, one popular technique is to include an etched diffraction grating within the laser waveguide structure The grating-based single-frequency lasers are classified into two categories If the active layer and the grating extend along the whole length of the laser cavity, the device is known as a distributed feedback (DFB) laser If the grating or feedback sections are passive such that the gain region... DBR lasers, the Bragg grating sections are separated from the active section where major carrier recombination occurs In other words, the frequency at which the grating section reflects does not depend on the bias current and so non-linear influence on the guided refractive index (due to injected carriers) is rare However, the use of end passive gratings in DBR lasers implies extra etching processes during... plane is supported (b) Single longitudinal mode (SLM) In semiconductor lasers, electron movements occur between two energy bands that consist of a finite number of discrete energy levels Rather than a discrete energy transfer like the gaseous laser, semiconductor lasers are characterised by a wider gain spectrum In an inhomogeneously broadened laser, the gain spectrum may be found several times wider than... in DFB lasers, the grating period L must be carefully chosen to satisfy the Bragg condition L¼ mB 2ng ð1:26Þ where ng is the usual group refractive index and B is sometimes called the Bragg wavelength For a 1.55 mm InGaAsP laser, with a first-order grating ðm ¼ 1Þ, a typical value of ng is 3.4 and L is 0.23 mm Figure 1.11 shows the difference between the operation of FP and DFB lasers For DFB lasers,... single-frequency semiconductor lasers (such as DFB semiconductor lasers) is never ‘pure’ but always contaminated by a finite spectral width The finite spectral width is mainly due to the random phase of the spontaneous emission that couples into the lasing mode Consequently, the fluctuation in gain broadens the spectral linewidth as a result of lasing frequency shift [52] To measure the spread of the laser spectrum,... change, which induces extra linewidth broadening Note that this is not readily observed in a stable CW gas laser, since the time taken for the intensity to return to its initial steady state value is shorter than that of the semiconductor laser (of the order of 10À9s) Within such a short period of time, the above processes rarely happen in gas lasers and so the spectral linewidth of the gas laser is narrower... coupled cavity has also been used [49] Due to the interference with the external cavity, the overall modal loss becomes frequency dependent and hence limits the oscillation mode from the FP 16 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS Figure 1.9 (a) Possible gain profile of an inhomogeneously broadened laser; (b) the resulting intensity spectrum (after [44]) laser cavity On the other hand, both... fluctuations of field due to the spontaneous emission will contribute directly to phase noise 2 In semiconductor lasers, there is a second contribution to phase noise ðÁ00 Þ associated with the intensity change as described in mechanism 1 above According to the Kramer– Kroenig relation [56], any change of gain inside a semiconductor will alter the refractive index of the semiconductor Consequently,... oscillation frequency of DBR lasers, is difficult to control precisely Due to the tolerance inherited in fabrication, the relative grating phases of the two Bragg reflectors become unpredictable In general, DBR lasers are more complex, and effective measures are needed in tackling the problem of yield and reliability In order to suppress any possible FP mode in these grating structures, it is quite usual . semiconductor lasers oscillating in single longitudinal modes were developed [17,18]. By limiting the spread of the laser spectrum, this type of laser is. hindered due to poor spectral purity and frequency instability in semiconductor lasers. Due to advances in fabrication techniques, semiconductor lasers nowadays