High Power and Low Coherence Fibre-optic Source for Incoherent Photonic Signal Processing

Transcription

1 High Power and Low Coherence Fibre-optic Source for Incoherent Photonic Signal Processing Y u a n L i a n d R o b e r t A. M i n a s i a n School of Electrical and Information Engineering and APCRC University of Sydney, NSW 2006, Australia Abstract: A n e w s t r u c t u r e t h a t c a n g e n e r a t e f i b r e -o p t i c s o u r c e s o f o p t i m u m l i n e w i d t h f o r p h o t o n i c s i g n a l processor applications is presented. The structure is based on a two -s t a g e d o u b l e -p a s s t o p o l o g y. R e s u l t s o n a n 80 GHz linewidth source demonstrate high -power, high signal to sideband suppression ratio, and one of the h i g h e s t p u m p c o n v e r s i o n e f f i c i e n c i e s r e p o r t e d. 1. Introduction Photonic signal processing offers the potential of high -speed processing of signals directly in the optical domain to realise fibre systems with in-built signal conditioning. In order to avoid optical interference effects and obtain robust transfer functions, nearly all photonic signal processors operate in the incoherent regime [1] -[3]. This requires that the coherence time of the optical source is smaller than the minimum time delay in the processor. Hence, for high -speed processors that operate at multi -GHz fundamental frequencies, this requires an optical source with a relatively large linewidth appropriate to the processor frequency. However, optical source s with suitable linewidths are not readily available. Intrinsic laser linewidths are usually too small and can cause coherent interference effects in the processor. On the other hand, broadband sources such as ASE sources and SLD sources that have spectral bandwidths that are extremely wide, as needed for sensor applications, are unsuitable because of their low power density. Hence, spectrum-slicing approaches [4], [5] from such broadband sources are usually limited by the low source power generated. The object of this paper is to report a new structure that can generate optical sources of optimum linewidth for photonic signal processor applications, and which can simultaneously achieve a high power output. A significant advantage of this novel optical source is that it can achieve an extremely high conversion efficiency. Here we define conversion efficiency as the ratio of desired optical source power to the pump power input. The structure is based on a two- stage double-pass topology. We present an analysis and design to achieve the optimum conversion efficiency for the optical source. Experimental results are also presented which demonstrate the realisation of an 80 GHz linewidth optical source with a conversion efficiency of 34.5% and signal to sideband suppression ratio exceeding 36 db. 2. High power and low coherence fibre-optic source topology The structure of the new two -stage double-pass topology is shown in Fig 1. It is based on the double- pass configuration that is widely applied in the design of broadband sources [6]-[9], however the latter has several limitations. First, the output signal level that can be obtained is limited by the onset of lasing due to parasitic cavities formed by the unavoidable splicing reflections, particularly between the EDF and single mode fibre. Second, for a relatively narrowband source requirement, as required for photonic signal processing, the sideband suppression ratio that can be achieved is insufficient. To solve these problems, we propose the two - stage double- pass topology in Fig 1. In this scheme, the first stage essentially operates as a low coherence seed source and the second stage both enhances the seed and shapes the spectrum to achieve a high sideband suppression ratio. 1

2 The source of Fig 1 was designed to generate a high power low- coherence output having a linewidth of 80 GHz. Hence, the fibre Bragg gratings FBG 1 and FBG 2 have a reflection bandwidth of 0.7 nm n m p u m p l a s e r C o u p l e r WDM EDF 1 FBG 1 C i r c u l a t o r FBG 2 EDF 2 WDM I s o l a t o r OSA Fig. 1. Structure of the two -stage d o u b l e -pass low coherence fibre-o p t i c s o u r c e. An important objective of this work was to find a structure that can maximise the conversion efficiency from the pump power to the desired optical output power. Fig 1 shows that the power of the single pump is split between the two stages by the coupler with a ratio of R, where R = P 1 /P 2. To maximise the conversion efficiency, the optimum pump power split ration R must be found. Moreover, the optimum lengths of the erbium doped fibre lengths EDF 1 and EDF 2 must be determined. 3. Analysis and design We simulated the behaviour of the structure shown in Fig 1, using a full numerical model for the erbium-doped fibre, based on the rate equations. The parameters of the model were characterised by a range of measurements on the EDF. The model was then tested against several experiments and this verified that it could accurately predict the characteristics of the EDF. The optical source modelling also included the gratings FBG 1 and FBG 2, which were almost identical, and had a flat -top spectrum. Their measured reflection spectrum is shown in Fig 2. These had a 3 db bandwidth of 0.7 nm (with a 10 db bandwidth of 1.0 nm), and had a power reflectivity of 99%. The simulations were used to find the optimum ratio R of the pump power split between the stages, and the optimum lengths of doped fibre EDF 1 and EDF 2, to achieve high output power and high pump conversion efficiency. For a pump laser having a maximum output power of 100 mw at 1480 nm, it was found that the optimum lengths of EDF 1 was 26 m, and EDF 2 was 25 m, and the optimum coupling ratio R was 30%. 2

3 Reflectiion (db) Wavelength (nm) Fig 2. Measured reflection spectrum of gratings FBG 1 a n d F B G Experimental results. To verify the proof of principle for the new h igh -power low- coherence fibre-optic source, the structure of Fig 1 was experimentally set up. The erbium-doped fibre used was standard amplifier fibre type having the following parameters: attenuation (background): nm, Er peak: 3 db, cut -off: nm, numerical aperture: The pump comprised a 1480 nm laser having a maximum power of 100 mw. Measurements were taken for a range of fibre lengths for E D F 1 a n d E D F 2 and for a range of coupler ratios R. Fig. 3 shows the measured output results for the low coherence optical source with optimum lengths of 26 m for EDF 1, 25 m for EDF 2, and for a coupler ratio of R = 30% and pump power of 98 mw Pump=98 mw, EDF1=26 m, EDF2=25 m, R=30% Power (dbm) Wavelength (nm) Fig. 3. Measured optical output spectrum of the low coherence fibre opti c source. 3

4 The centre wavelength of the source is nm. The measured - 3 db bandwidth of the source is 0.7 nm or 80 GHz (and its 10 db bandwidth is 1.0 nm). It can be seen from Fig 3 that this source exhibits a high signal to sideband suppression ratio exceeding 36 db. Fig 4 shows a comparison between measured and predicted low coherence source output power versus pump power for different values of EDF length and coupling ratio. The experimental results for the optimum lengths EDF 1 = 26 m, EDF 2 = 25 m, and coupling ratio R = 30%, show that a high output power of 34 mw and a pump conversion efficiency (defined as the ratio of output optical power to the total pump power) of 34.5% have been achieved at a pump power of 98 mw. To our best knowledge, this is one of the highest pump conversion efficiencies reported EDF1=26 m, EDF2=25 m, R=30%, experiment EDF1=26 m, EDF2=25 m, R=30%, simulation EDF1=26 m, EDF2=15 m, R=50%, experiment EDF1=26 m, EDF2=15 m, R=50%, simulation Pump Power (mw) Fig. 4. Output power versus pump power for the low coherence fibre-o p t i c s o u r c e. 5. Conclusion A new structure that can generate optical sources of optimum linewidth for photonic signal processor applications has been presented. The structure is based on a two -stage double-pass topology. Advantages of this fibre-optic source include its ability to achieve an extremely high conversion efficiency, its high output power, and its high signal to sideband suppression ratio. In addition, arbitrary the source linewidths can be realised by changing the grating bandwidths. Experimental results demonstrated the realisation of an 80 GHz linewidth optical source with a conversion efficiency of 34.5% and signal to sideband suppression ratio exceeding 36 db. To our best knowledge, this is one of the highest pump conversion efficiencies reported. Such optical sources should find application in high- speed photonic signal processing at microwave and mmwave frequencies. References [ 1 ] R. A. Minasian, "Photonic signal processing of high- speed signals using fiber gratings," Optical Fiber Technology, p p , [ 2 ] K. Jackson, S. Newton, B. Moslehi, M. Tur, C. Cutler, J. Goodman, and H Shaw, Optical fibre delay-line signal processing, IEEE Microwave Theory Tech., p p ,

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