Research
"Raman and parametric mediated amplification and all optical processing for high speed fiber optics communication systems "
1-Scientific Background
1.1 Introduction
Growing demands for information capacity in optical systems and networks have stimulated the extension of operational wavelength regimes into previously dark regions of the fiber spectrum. After the C (1530-1565 nm) and L (1565-1600 nm) bands, the S (1480-1530 nm) and U (1625-1675 nm) bands are attracting interest as the next commercial transmission bands [1-2]. Operation over these extremely large optical bandwidths requires that several key components be developed. The most important ones are optical sources, wide band amplifiers and flexible all optical processing devices such as wavelength-converters which can also serve as all optical reshapers [3].
The increase in the number of channels and the data rates brought into focus the issues of inter channel crosstalk induced by Stimulated Raman Scattering (SRS) [4] and Four Wave Mixing (FWM) [5]. These potential impairments can judiciously be exploited to improve system and network performances when used for amplification and for all optical signal processing.
While Erbium Doped Fiber Amplifiers (EDFA) will continue to dominate fiber optics systems, other optical amplifiers are emerging. Most noticeable are Raman and Parametric fiber amplifiers, two amplifier types whose operational principles, based on SRS and FWM respectively, are well established in the field of non linear optics but whose detailed performances in fiber optics are not fully known, especially in the saturated regime.
In addition to sophisticated amplifiers, future systems and networks require a host of high speed all optical signal processing tools which allow to overcome the limitation of electronics and provide new functionalities. Many of these devices, such as wavelength converters, also serve as reshaping and noise suppressing components.
1.2 Non linearities in optical fibers
Even through the optical fiber is a remarkably linear transmission medium, it exhibits a slight degree of nonlinearity. Most of the nonlinear effects in optical fibers originate from the Kerr effect which refers to the intensity dependence of the reflective fiber index and is responsible for phenomena such Self Phase Modulation (SPM), Cross Phase Modulations (XPM) and FWM.
FWM is one of the nonlinear effects that will be analyzed extensively during this research. It amounts to the generation of signals at new frequencies due to the nonlinear interaction between two or more input signals. In quantum mechanical terms, FWM occurs when photons from one or two waves are annihilated and new photons are created at different frequencies such as the net energy and momentum are conserved. This interaction requires phase matching conditions between the interacting waves which is easily satisfied in low dispersion fibers. FWM has been extensively studied as the main impairment factor in WDM systems since it obviously causes inter – channel crosstalk [5]. The nonlinear effects governed by the Kerr effect are elastic in the sense that no energy is exchanged between the electromagnetic field and the dielectric medium.
A second class of nonlinear phenomenon results from inelastic scattering in which the optical field transfers a part of its energy to the nonlinear medium.. Two important nonlinear effects in optical fibers fall in this category; both of them are related to vibrational excitation modes of Silica. They are known as Raman Scattering (RS) and Brillouin Scattering (BS). The inelastic scattering effect of interest for this research is RS. In a simple quantum mechanical picture, a molecule, initially at the ground state, absorbs an incident photon and is then in a virtual (forbidden) energy state since in general, the photon energy does not correspond to a transition to an allowed molecule energy level. The molecule can return to a lower (allowed) energy state while emitting a frequency downshifted photon, called a Stokes photon and absorbing a phonon. This is the spontaneous Raman Scattering. There is abroad range of vibration states above the ground state due to the amorphous nature of glass. This leads to a relative (Stokes) frequency shift of more than 13 THz in optical fibers. For large intensities, beside the increase of spontaneous Raman scattering, another effect takes place: the Stimulated Raman Scattering (SRS). Here, when the molecule is in its virtual energy state, if a Stokes photon, created elsewhere hits the molecule, a so called induced emission results : a Stokes photon will be created having the same characteristics as the incident Stokes photon. SRS can lead to severe system degradations in dense WDM system by transferring energy from high frequency channels to low frequencies channels leading to significant crosstalk [4].
In optical fiber transmission, all the non linear phenomena take place simultaneously.
On the one hand they can have adverse effects and have to be overcome by sophisticated systems designs and on the other hand, if correctly used they can provide techniques for optical amplification and all optical signal processing as described in the present research.
1.3 Optical Amplifiers
Optical amplification is a crucial element in all optical communication links. The most important optical amplifier is the Erbium doped fiber amplifier which is considered to be the most important technological development of the 1990's as it led to high capacity high performance WDM systems which enabled the development of the internet as we know it. EDFAs enable simultaneous amplification of multiple channels and in some respect introduced to the optical communication world the first practical all optical processing principle. However, advances in WDM transmission systems have led to an almost complete usage of the ~ 30 nm available gain bandwidth in EDFA’s [6]. Also, there is an inherent gain variation across the EDFA gain bandwidth which impacts the performance of multi wavelength systems. Modified EDFAs with shifted or broader bandwidth [7] as well as flattened gain spectra have been demonstrated [8].
Prior to the invention of the EDFA, semiconductor optical amplifiers (SOAs) dominated the amplifier research scene. While SOAs are compact and efficient gain media, constructed from diode lasers whose facet feedback has been eliminated, they have an inherent gain nonlinearity [9] which prevents multi channel amplification and consequently SOAs were never deployed in transmission systems [10]. The nonlinearity can be explored for all optical signal processing applications. Numerous schemes such wavelength converter [11], OTDM demultiplexer [12] or 3R regenerators [13] have already been demonstrated and some are likely to find their use in future practical networks. SOAs have gain bandwidths which are similar to those of EDFA's (few tens of nm). The exact spectral placement of the gain can be tailored by the semiconductor composition but the bandwidth is limited. New generations of SOAs based on quantum dash or dot gain media [14] have wider bandwidths (up to 300 nm) and are more linear but many issue related to these are still open primarily issues of gain uniformity and noise.
Wide bandwidth amplifiers that can be used anywhere within the fiber bandwidth requires different approaches, ones where fundamental spectroscopic properties of the materials providing the gain do not play a role. This approach leads to the implementation of amplifiers based on nonlinear phenomenon where the exchange of energy is not mediated by stimulated emission. The most widely used nonlinear amplification mechanisms are Raman scattering and four wave mixing.
Raman amplifiers use stimulated Raman scattering to provide gain. The optical fiber itself becomes the amplifier (the complete transmission fiber makes a fully distributed amplifier [15], dispersion compensating or short highly nonlinear fibers make a discrete amplifier [16] but these are also distributed in nature as they have lengths from hundreds of meters to several kilometers). Since the Stokes shift in glass is more or less wavelength independent and equal to 13.2 THz, the choice of pump wavelength defines the gain wavelength and since the Raman gain is wide band (over 5 THz), multiple channels at any spectral location can be amplified and adequate multi pumping configuration can provide a broad flat gain spectrum [17].
Four wave mixing is used for the so called Optical Parametric Amplifier (OPA) which offers wide gain [18] and may be tailored to operate at any wavelength. An OPA is pumped with one or two intense pump waves and may provide gain over more than 200 nm.
A special brand of parametric amplifiers is the so called phase- sensitive amplifier which only amplifies components of the same phase as the signal [19]. This property has many applications for pulse reshaping [20] , soliton - soliton interactions and quantum noise suppression [21]. Another important application is the possibility of inline amplification with an ideal noise figure of 0 dB [22]. This should be compared we the quantum limited noise figure of 3 dB for standard phase insensitive amplifiers [23].
The main limitation of nonlinear fiber amplifiers (Raman and parametric amplifiers) relate to their efficiencies which in turn mean that they require high power pump signals. Improvements are possible once larger nonlinearities become available and indeed, highly nonlinear fibers are an active research topic wherever specialized-fiber technology is available and some special fibers are already available commercially [24].
Important issues not fully understood in nonlinear fiber amplifiers relate to their saturation properties and the appropriate noise characteristics under saturation. The noise in Raman amplifiers stems from the spontaneous Raman scattering and from noise transferred from the pump RIN [25]. The noise in parametric amplifiers stems from the pump and from vacuum field noise which represents the minimum noise possible in those amplifiers where the nonlinear energy transfer mechanism is not in itself a noisy process. In Raman amplifiers, pump and signal may propagate in the same or opposite directions and this defines the saturation and noise characteristics.
1.3.1 Optical noise in optical amplifiers
The noise accompanying optical amplification has a crucial impact on signal processing and detections capabilities.
The noise of an optical amplifier (EDFA, SOA, Raman or Parametric amplifier) operating in the linear gain regime is well understood and widely documented. The various amplifier types share some common fundamental noise properties such as the noise having a white, Gaussian nature. For amplifiers based on population inversion like EDFAs and SOAs, the noise spectral density is related to the gain in a simple manner: SN= nsphn(G-1), nsp = 1 for ideal inversion and nsp>1 for every practical amplifier. [26]
The noise of a Raman amplifier stems from spontaneous Raman scattering events [27] as well as Multi Pass Interference (MPI) noise [28] (from unintentional reflections or from Rayleigh scattering) while in parametric amplifiers the additive noise results from amplification of the input vacuum fluctuations .
An additional noise source stems from the amplifier pump [25]. While present in every amplifier type, pump noise is mainly significant in Raman and Parametric amplifiers where it transfers noise to the signals by crosstalk effect. Pump noise is significant in both the linear and nonlinear regimes as it translates directly to gain fluctuations and hence to uncertainties in the amplified signal and this is enhanced by the ultrafast response of the Raman and parametric amplification. In Raman amplifiers, the usual way to reduce such deleterious coupling is to make the pump and signal counter-propagating or use low noise pumps in co-propagation schemes. In OPA, the necessary dithering of the pumps, required to avoid Brillouin scattering, leads to spectral broadening of the idler which may translate to amplitude noise due to dispersion [29].
It can be shown that the minimum noise figure in the linear regime for any phase insensitive optical amplifier types is 3 dB [23].
In the saturation regime, the noise takes a different nature. Saturation effects in EDFA and SOAs have been studied before. Shtaif et al.[30] demonstrated that in SOAs saturation increases the spectral density of the noise due to a deterioration of the inversion quality. Moreover, the saturating signal propagating along the amplifier interacts nonlinearly with the noise causing spectral changes which result in an output noise that is no longer white and does not obey exact Gaussian statistics [31]. This has also been demonstrated in EDFA [32] and a recent publication by Inoue and Mukai [33] demonstrated the same spectral hole burning around signal and idler in the ASE spectrum of deeply gain saturated OPA. The noise properties of saturated Raman amplifiers have not been dealt with sufficiently in the literature and many issues, still unknown in this topic, are to be addressed within this research proposal.
Progress in WDM systems has raised the issue of cross connecting high speed WDM streams what highlights the potential role reserved for all optical devices, such as wavelength converters. The signals in such systems are degraded due to the introduction of optical noise originating from Amplified Spontaneous Emission (ASE) or from inter channel crosstalk.
All optical wavelength converters with reshaping and noise suppression capabilities will therefore be of outmost importance in future networks. These reshapers enable to improve the receiver sensitivity, extend transmission distance without needed of electronics repeaters while allowing flexible wavelength management..
An optical reshaping device should have a nonlinear function as close as possible to a step function in order to strongly reduce noise fluctuations on space and marks levels.
Several techniques for all optical wavelength conversion and reshaping, including acting as a hard limiter, have been investigated using optical fibers as well as nonlinear SOAs. These are based on nonlinearities such as saturated gain [34] [35] or absorption [36] and the Kerr effect [37]. Parametric processes such as FWM have also been used in numerous configurations [3] [37].
Issues not addressed sufficiently in the literature involve devices which operate over extremely wide optical spectral spans, for example wavelength conversion over 100 nm and more. These entitle a set of unique problems which will be addressed in the proposed research program. Furthermore, the nonlinear character of the devices impacts the nature of the noise, that is, it modifies the noise carried by the input signal and also changes the noise generated within the devices.
1.4.2 Optical pulse sources
The increasing demand on information capacity has initiated intense research efforts on the development of new optical sources with the capability of providing narrow optical pulses at high repetition bit rates. One of the most efficient methods to produce pulse sources is based on optical parametric amplifier (OPA).
The timing stability of such pulse sources is a key issue for optically time domain multiplexed communication and optical sampling systems. Several types of sources producing high quality pulse trains have been described in the literature. The most common scheme comprises a fiber or semiconductor mode-locked laser driven by an independent high quality oscillator [38]. Such actively mode locked lasers exhibit the lowest timing jitter reported to date. However, the low noise properties are mainly determined by a unique, ultra low phase noise, microwave drive oscillator which renders this solution impractical for most applications. An attractive alternative for low jitter pulse sources are various forms of self starting optoelectronic oscillators whose configuration encompasses optical pulse generation. Examples include a double fiber loop oscillator[39], mutually injection locked mode locked semiconductor laser - self oscillating photo transistor combinations [40] and an optoelectronic oscillator with an electro absorption modulator [41] whose inherent non linearity forces the pulse operation. These various self starting schemes yield very low jitter levels, of the order of tens of femtoseconds at repetition rates around 10 GHz, which are only slightly worse than the best actively driven pulse sources.
A multi-wavelength tunable pulse source has been demonstrated at 10 GHz using a pulsed pump [42]. However taking advantage of the nonlinear (exponential) dependence of the OPA gain on the pump power, it is possible replace the pulsed pump by a simpler, sinusoidally modulated pump [43]. Other techniques to generate multi-wavelength sources are based on spectral slicing of a super continuum generated spectrum [44].
2-Research achievements
Within my PhD work, several issues have been raised:
Noise properties of saturated fiber Raman amplifier in CW regime:
It has been demonstrated that a saturated fiber Raman amplifier a unique behaviour under saturation in that its noise reduces faster or slower than the gain according to the pumping direction unlike the case for an EDFA or a SOA.
An ultra wide band wavelength converter based on saturated Raman gain :
Conversion and regeneration over 1 THz have been demonstrated with excellent noise reduction and low errors at high data rates.
The dependence of detected clock frequency component on dispersion for chirped RZ signals:
A novel method has been developed to measure the chirp profile of RZ signal. This method is based on the analysis of the evolution of the detected clock power with the accumulated chromatic dispersion.
Multi-wavelength pulse sources based on saturated optical parametric amplifier:
Generation of short optical pulses by saturated high order FWM have been performed at 40 GHz repetition rate with few picoseconds pulse duration.
A self starting ultra low jitter optical pulse source based on mutually coupled optoelectronic oscillators with an intra cavity fiber parametric amplifier:
The system employs a phototransistor based microwave oscillator which is coupled to a fiber cavity optoelectronic oscillator with an intra-cavity fiber parametric amplifier. It self starts and exhibits 3 ps pulses at a rate of 10 GHz with an extremely low jitter of 29 – 40 fs.
A simple and novel pump modulation technique for multi-wavelength pulse sources based on saturated optical parametric amplifier:
Using Mach Zendher modulator under specific bias condition, 15 ps width pump pulses at 10 GHz repetition rate can be obtained and such pump pulses do not need phase modulation to eliminate Brillouin Scattering, which enables generation of multi wavelength pulse sources with 6 ps width and without spectrum broadening .
3-References
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