[Jambhulkar*, 4.(7): July, 2015] ISSN: (I2OR), Publication Impact Factor: 3.785

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1 [Jabhulkar*, 4.(7): July, 25] ISSN: IJESRT INTERNATIONAL JOURNAL OF ENGINEERING SCIENCES & RESEARCH TECHNOLOGY A REVIEW: OPTICAL FREQUENCY COMB GENERATOR (OFCG) Sneha Jabhulkar*, P. B. Borole Electrical Departent, VJTI Collage, India ABSTRACT With the recent breakthrough in the field of optical frequency easureent and synthesis, it is now possible to accurately easure the frequency of optical frequency signals, generate arbitrary wavefor light pulses and perfor accurate optical frequency synthesis. Generating precise optical frequencies with a functional power is necessary in any fields of science and technology. This paper reviews different ethods for generating the optical frequency cob generator (OFCG). This article highlights the different echaniss integral to the operation of the pulsed laser and the tailored optical fiber. It also presents the techniques that enable the proised frequency synthesis and easureent. Three different techniques discussed in this paper which is ) EO Phase Modulator, 2) Mode locked Laser and 3) Fiber Ring based Cob generator. KEYWORDS: Optical frequency cob generator, Electro-Optic phase odulation, Mode locked laser, Fiber ring based optical frequency cob generator. INTRODUCTION Researchers working with the optical part of the electroagnetic spectru (4-5 Hz) have wished to have coherent instruentation that can perfor siilar operations to the instruents available for the radio-frequency and icrowave parts of the spectru ( Hz - Hz). The difference between two laser frequencies can be easured easily by superiposing the two laser beas on a photo detector and onitoring a beat signal. It is possible to directly count the frequency differences up to the order of GHz using coercial fast photodiodes and icrowave frequency counters [7]. As the frequency gap between the endpoint of a haronic laser frequency chain and an unknown optical frequency to be easured can easily range up to the hundreds of THz, there has been a greater interest in easuring the differences of uch larger optical frequencies. Optical frequency cob generator (OFCG) is yet another copact setup to easure frequency gaps on the order of a few THz based on very efficient creation of side bands in a large index electro optic phase odulator.further side bands are created and the side bands are in resonance if the odulation frequency atches the free spectral range of the optical resonator. Figure : A typical situation in frequency etrology, a well-known reference frequency and an unknown frequency tens or hundreds of THz apart. Optical frequency cobs is a unique technique for ultra-precise easureents of tie and frequency. However the challenge is ainly due to requireents of very lager cob frequency spacing, nonlinear frequency conversion and broadening with sufficient power. Optical Frequency Cob Generator (OFCG) having ultiple applications such as Wavelength Division Multiplexing (WDM), spectroscopy and Optical arbitrary wavefor generation. We get a broad and aplitude flattened cob with an equal spacing in Optical arbitrary wavefor generation.j.l. Hall at National Institute of Standard and Technology and T.W. Hansch at Max-Plank Institute were awarded with the Nobel Prize in Physics for their contribution to the developent of laser-based spectroscopy, including the Optical frequency cob technique. [83]

2 [Jabhulkar*, 4.(7): July, 25] ISSN: RELATED WORK Photonics is widely seen as a rapidly expanding interdisciplinary field having nuerous applications fro consuer products to defence and space hardware developent. Fiber optic counication is one such application which enabled developent of a variety of optical coponents and technologies. The Optical frequency cob generator (OFCG)produces a spectru of evenly spaced discrete cob lines, centred at optical carrier frequency. These cob lines are spaced according to f(n) = fo + n.fr wheren is an integer, fr is the cob spacing and fois the carrier frequency. The optical frequency cob generator (OFCG) has proising applications in optical etrology, frequency chain generation, optical atoic clocks, wavelength/ frequency division ultiplexing in optical counication [2] and high precision spectroscopy [3]. Frequency cob source can be generated by using different echaniss such as odulation of a continuous wave laser, stabilization of the pulse train generated by ode locked laser and using ring resonator approach et The ode-locked laser based OFCG has fixed cob spacing due to its cavity length with stability depending upon ode locking condition. The OFCG with adjustable central frequency and cob spacing can be ade by odulating continuous wave laser with a RF source schee generates few cob lines having unequal aplitude, which can be ade flat by cascading few odulation stages or using a feedback loop with optical aplification. DIFFERENT TYPES OF ALGORITHM EO Phase Modulator based cob Generator Optical Frequency cob generator can be generated by using phase odulation and Electro Optic (EO) phase odulation.basic principle of an Electro-Optic odulator is change in refractive index or change in birefringence based on electrically induced field. It is depend on the device configuration of an Electro-Optic odulator, the following properties of light wave varied in controlled ways: phase, polarization, aplitude, frequency, or direction of propagation [2].A light wave can be odulated with respective to the change in the index of refraction of electrooptic aterial. The applied voltage around the electro-optic aterial causes the change in refractive index. The depth of odulation depends on the odulating signal paraeters aplitude (voltage) and frequency. The scheatic explains the generation of optical frequency cob by using electro-optic phase odulator. Figure 2: Scheatic of Electro-Optic phase odulation Let us assue input optical signal electrical field is E E cos( t ) ix i () The output of the odulated optical signal at the end of the odulator represented as E( Ei cos( t ) (2) Where, 2 x ( n nx ) L (3) Φ represents the total odulated phase shift, Φ is initial phase and ΔΦx is the change in phase due to odulation. The induced phase shift ΔΦx can be written as in ters of electric field of odulating signal 3 x ( ) n x rle (4) Where L is the length of the electro-optic aterial of width d and E is the electric field generated around the electrooptic aterial due to applied odulating signal of voltage V. The electric field of odulating signal can be written as E E sin( t E V ) & d (5) [84]

3 [Jabhulkar*, 4.(7): July, 25] ISSN: V x ( ) V Induced phase (6) Where V is applied voltage and Vπ is half wave voltage of a given odulator configuration. Total odulated phase is ( ) nxl sin( (7) δis called depth of odulation index. By neglecting the constant phase ter and applying the identity, the output of odulated light wave becoes E ( E ( J ( )cos( J ( )cos( ) t J 2 i J ( )cos( ) t J ( )cos( ) ) 2 ( )cos( ) t (8) Figure 3:Spectru of the coposed carrier with an aplitude J(δ) and a set of sidebands spaced syetrically on either side of the carrier at frequency separation ω, 2ω,3ω, 4ω.etc The spectru is coposed of a carrier with an aplitude J(δ) and a set of sidebands spaced syetrically on both side of the carrier at frequency separation ω, 2ω,3ω, 4ω.et The function Jn(δ) is called Bessel function of first kind and order n. The aplitude of the spectral coponents depend on Bessel function Jn(δ). The difference between the two closest frequencies at which odulation index (δ) falls to 5 percentage of the axiu value or the spectru within 3dB is called Bandwidth of the cob spectru. The nubers of spectral lines are linearly related with RF voltage deriving the phase odulator (δ α V). RF power liits the nuber of lines generated by a single phase odulator. The bandwidth of cobs however is liited. Coercially available phase odulator having low Vpi ~ 3V and RF signal power liits W (RF voltage ~ 7V). It liits the nuber lines 2 within 3 db bandwidth. Carson s rule also explaining the bandwidth of the odulation is equal to 2(δ+) and odulation index is proportional to RF voltage.the bandwidth of a cob spectru was liited due to the liitation of low Vπ and power of the RF signal. To generate large bandwidth, we would have to cascade phase odulators in series, but this process was expensive and inefficient. By using feedback echanis we can generate broadband cob signal, it was cost effective (inexpensive) and efficient also []. Fetosecond Frequency Cob Generation In 99 the discovery of Kerr-lens ode-locking in Wilson Sibbett s laboratory at the University of St. Andrews [7] led to a reliable and siple ethod for producing pulses of laser radiation in the near infrared with a duration of just a few tens of fetoseconds: in other words each light pulses has just a few cycle of light wave. This rearkable advance has been the enabling technology for a whole raft of new types of easureents in an exceptionally wide range of fields. In the first flash lap puped Nd:glass and Nd:YAG ode locked laser appeared in the id-sixties with less than pico second in duration and deonstrated one of the ost powerful interference phenoena of the nature. CW operation of dye lasers with broad bandwidth triggered the second generation of ode locked lasers. Optical pulses shorter than pico second could be produced and iproveents in the cavity design allowed breaking of the -feto second barrier. In 984Intracavity dispersion control by eans of Brewster angled pris pairs was the next ajor breakthrough [8]. The developent of new solid state laser aterials led to the eergence of third generation laser sources with the discovery of self-ode locking in Ti:sapphire laser, the explanation as perthe Kerr-lens ode-locking and developent of the design to produce -feto second pulses. Recently pulses shorter than 6 fetosecond have been created directly fro a Ti:sapphire laser oscillator with the help of special dispersion-copensating irrors. To understand the ode structure of a fetosecond frequency cob and the techniques applied for its stabilization one can look at the idealized case of a pulse circulating in a laser cavity with length L as a carrier wave at fc that is [85]

4 [Jabhulkar*, 4.(7): July, 25] ISSN: subject to strong aplitude odulation described by envelope function A(.The function defines the pulse repetition tie T=fr - by deanding A(= A(t T ) where T is calculated fro the cavity ean group velocity: T = 2L/ v g. The pulses however are not necessarily identical. This is because the pulse envelope A( propagates with v g while the carrier wave travels with its phase velocity. As a result the carrier shifts with respect to the pulse envelope after each round trip by a phase angle as shown in Figure.4. Unlike the envelope function, which provides us with a ore rigorous definition of the pulse repetition tie T=fr -, the electric field is, in general, not expected to be periodic in tie. Because of the periodicity of the envelope function the electric field at a given place can be written as 2 f t 2 E( A( e c c q A e q i( f qf ) t c r (9) As the envelope function A( is strictly periodic it has been written as a Fourier series qfrt A( Aqe q () Where A q are Fourier coponents of A(.Equation 9 shows that, under the assuption of a periodic pulse envelope, the resulting spectru represents a cob of laser frequencies separated by the pulse repetition frequency f r. Since f c is not necessarily an integer ultiple of frthe odes are shifted fro being exact haronics of the pulse repetition frequency by an offset f o <f r : f n nf r f o () with a large ( 6 ) integer n. This equation aps two radio frequencies frandfo onto the optical frequencies fn. While fris readily easurable and usually lies between a few MHz and a few GHz depending on the length of the laser resonator, f o is not easy to access unless the frequency cob contains ore than an optical octave.the intuitive picture given here can even cope with a frequency chirp, i.e. a carrier frequency that varies across the pulse. In this case the envelope function becoes coplex in value and the cob structure derived above stays valid provided the chirp is the sae for all the pulses. Under this assuption, which is reasonable for a stationary pulse train, A( reains a periodic function. In the tie doain the frequency offset is obvious because the group velocity differs fro the phase velocity inside the cavity and therefore the carrier wave does not repeat itself after one round trip but appears phase shifted by Δϕ as shown in Figure.4. The offset frequency is then calculated fro f o = ϕ /(2πT). Figure4: Pulse train eitted by a ode locked laser and the corresponding spectru. One ight argue that no laser has line width zero and that one should treat the carrier not as a ideal single frequency wave fc but as a source with general line width function C(. Even if no technical noise would be present, there would still be soe sort of fundaental Schawlow-Townes liit connected with the line width of each ode. As long as we still have the periodicity of A( Equation. 3.2.reads then as E( A( C( (2) Fourier transforing E( brings us into the frequency doain and back (ω=2πf): jt jt E( ) E( e dt, E( E( ) e d (3) [86]

5 [Jabhulkar*, 4.(7): July, 25] ISSN: With the help of the convolution theore j ( ) ( ) ( ( ) ( )) t A t C t A t C t e d( ) We get E( ) ( A( ) C( )) A( ') C( ') d' The Fourier transfors of A( and C( are given by it C( ) C( e dt (7) So therefore ( ) 2 A A n ( nr ) n (4) (6) (5) E( ) AnC( nr ) n (8) This su represents a periodic spectru in frequency space with periodicity fr=/t. The ode shape function is duplicated by the strong aplitude odulation induced by Kerr lens ode locking. Assuing the siplified case of a carrier wave C( = e -2πfct brings us back to equation.9. A chirp of the pulse ay be hidden in the coplex Fourier coponents An Note that the only assuption necessary to create a precisely equidistant cob is the periodicity of the envelope function. In the tie doain, the output of a ode-locked fetosecond laser ay be considered as a continuous carrier wave that is strongly aplitude odulated by a periodic pulse envelope function. The beat note between the carrier wave and the CW oscillator is observed in a stroboscopic sapling schee, if such a pulse train and the light fro a CW laser are cobined on a photo detector. The detector signal will therefore reveal a slow odulation at the beat frequency odulo the sapling rate or pulse repetition frequency. The iportant fact to learn fro this section is that such a fetosecond frequency cob has two degrees of freedo which are the repetition frequency fr and the offset frequency f o <.f r Depending on the application one or both degrees of freedo have to be stabilized Furtherore the fast aplitude odulation of the Kerr lens keeps the interode spacing constant even across a vast spectru of odes.the ode-locked laser based OFCG fixed cob spacing due to its cavity length with stability depending upon ode locking condition. As the spectral width of these pulsed lasers scales inversely with the pulse duration the advent of fetosecond lasers has opened the possibility to directly access THz frequency gaps. Fiber Ring Based OFCG Configuration: The fiber ring based OFCG was first proposed by Ho and Kahn. Pioneering work was carried out by Seeds' group at UCL, who successfully deonstrated a THz cob span fro a fiber ring based OFCG [5]. The operation of the fiber ring based OFCG is based on using a resonant cavity to enhance the phase odulation of the light to help spread the sideband spectru widely. Due to the ultiple passes through the electro optical odulator (EOM), pulse trains are fored within the cavity and an optical cob is produced as a result. Due to its exceptionally long cavity copared to other types of OFCG, atching the cavity length such that its resonances coincide with both the reference light wave frequency and the RF reference presents a ajor challenge. Without this atching, the cob generation will be neither efficient nor stable. The UCL group has adopted a wide line width laser source, operating the OFCG in ultiple cavity odes to help stabilize the cob; thus, each of the cob lines is coposed of several cavity odes. The alternative approach is actively tracking the reference laser frequency drift by adjusting the cavity length [6],keeping the cob in stable single ode operation. Fiber ring based OFCG that consists of an optical aplifier, an optical phase odulator, fiber delay lines and optical couplers, as shown in Figure 5.Isolators are used to ensure that the light propagates only in one direction, and to avoid unwanted reflections. Thephase odulator is polarization dependent andpackaged with PM fiber, so a polarization controller is required before it. An additional polarizer can be usedto suppress the unwanted polarization and to ensure that the output cob has a stable polarization. With the optical delay line in the cavity, the cavity length of the OFCG can be adjusted and controlled so that the free spectral range of the cavity is alost equal to 5 MHz. All these optical coponents are placed within anenclosed box to isolate the fro abient acoustic noise and theral drift. [87]

6 [Jabhulkar*, 4.(7): July, 25] ISSN: Figure 5: Fiber ring based OFCG A cavity length servo loop is used to hold one of the cavity odes coincident with the reference laser frequency. This is done by the serving aplitude of st haronic beat signal to a constant value. A hoodyne detection ethod is used to easure the aplitude of the stsideband beat signal. Due to the high theral sensitivity of the ring cavity, this part is kept in a separate box fro the reaining optical coponents. A stable narrow line width fiber laser is used as the reference light, a dual coupler configuration is adopted in this design to avoid the destructive interference between the reference light wave and the central line of the cob. Analysis shows that the output cob is then a copy of the light wave circulating inside the cavity, and the central cob line is always aintained at a certain power level, higher than the rest of the cob lines, and is less sensitive to the odulation depth, cavity gain and optical power level. Due to the optical aplifier inside the cavity, there is no requireent for a high power reference laser. The long ring cavity does pick up-ore acoustic noise than shorter cavities, and presents a tighter requireent for the stability of the reference laser frequency drift. Without the tracking loop closed, the OFCG cannot work stably. However, when the loop is closed, the ring length is autoatically adjusted to adapt to the wavelength of the reference laser, and to track its drift. The long cavity of the OFCG also akes possible the fast switching of the cob line spacing by the RF reference frequency, in steps of the free spectral range. Due to the electrical liits of the cavity length servo loop, our cob generator will allow the RF odulation frequency (cob line spacing) to operate fro 8 GHz to 2.5GHz. By balancing the appropriate path lengths, the phase relationship between the beat signal and the LO signal in the hoodyne detection part can be aintained roughly atched over a wide frequency range for good detection efficiency. In this case, there is no need to adjust the phase shifter to achieve phase atching. This odel show that the build-up tie of the cob in the OFCG is typically less than ilisec; the fiber stretcher will then aintain its position during the frequency switching, and hence the cavity length. Therefore, both resonance conditions will still hold to aintain the stable operation of the OFCG. The result is that the OFCG can be fast tuned with a frequency step of 5 MHz in less than. sec, which is liited by the updating speed of the optical spectru analyzer. Fine tuning with the range of khz to 5 MHz needs the ring cavity length to be adjusted. This is liited by the speed and accuracy of the optical delay line. Cobined with the fast tuning at 5 MHz steps, it has been possible to construct an OFCG to cover the whole operational frequency range. CONCLUSION The above all three ethods were discussed in details and based upon these three different paraeters were copared. It can be inferred that all ethods have their own boons and bans. As seen fro the table we conclude that using EO Phase odulation will get here frequency and aplitude of odulating RF signal are varied to investigate its effect on cob spectra. It is observed that the technique can be used to generate a broad spectru of cob lines under ideal conditions. However, the 3 volt Vπof electro-optic phase odulator and upper liitation on RF signal voltage restrict its usage practically to cob lines generation. [88]

7 [Jabhulkar*, 4.(7): July, 25] ISSN: Table. Coparision of different paraetersfor generation of Optical Frequency Cob Generator Paraeters EO Phase Modulation Fiber Ring Based Fetosecond OFCG Spacing Between the cob lines Directly proportion to the RF frequency. Path lengths, the phase relationship between the beat signal and the LO signal. Constant due to fast aplitude odulation of the Kerr lens. Cavity Length Not dependent on cavity length. Optical delay line in the cavity it is kept to 5 MHz(Tunable Length) Kerr lens Mode locking condition(fixed Length) No. of cob lines dependent on Linearly related with RF voltage Not Applicable Repetition rate inside the cavity REFERENCES [] Isao M., Takahide S., Naokatsu Y., Hideyuki S., Tetsuya K. and Iwao H., THz-Bandwidth Optical Cob Generation using Mach-Zehnder-Modulator-Based Flat Cob Generator with Optical Feedback Loop, National Institute of Inforation and Counications Technology, 4-2- Nukui-Kitaachi, Koganei, Tokyo , Japan, OSA/OFC/NFOEC 2 [2] Theresa A. Maldonado, Optical Eleents, Chapter-3, Electro-Optic Modulators, University of Texas, Arlington. [3] Bahaa E. A. Saleh, Malvin Carl Teich, Fundaentals of Photonics, Chapter-8, Electro-Optics, John Wiley & Sons, In ISBNs: ; [4] K. P. Ho and J. M. Kahn, Optical fiequency cob generator using phase odulation in aplified circulating loop, IEEEPhotonic Technology Letter, pp , 993. [5] S. Bennett, B. Cai, E. Burr,. Gough, and A. J. Seeds,.8-THz bandwidth, o-~equencyerror, huble optical cob generator for DWDM applications, IEEE Photonic Technology Mer, pp ,999. [6] P. Shen, P. A Davies, W. P. Shillue, L. R D Addario, I. M. Payne, Millietre-wave generation using an optical cob generator with optical phase-locked loops, ZnrenrionalTopicalMeeting on Microwe Photonics, Technical digest, pp. -4,22 [7] Andre Luiten, Accurate optical frequency easureent and synthesis, Frequency Standards and Metrology Group, Physics Departent, University of Western Australia, Nedlands 697 WA, Australia [8] Fork, R. L., Martinez, O. E. and Gordon, J. P., Negative Dispersion Using Pairs of Priss. Optics Letters, 9, 5-52 (984). [89]