APPLICATION NOTE. Automated Gain Flattening. 1. Experimental Setup. Scope and Overview

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1 APPLICATION NOTE Automated Gai Flatteig Scope ad Overview A flat optical power spectrum is essetial for optical telecommuicatio sigals. This stems from a eed to balace the chael powers across large distaces. E.g. i log-haul fibre etworks that require may optical amplifiers i order to maitai power across large trasmissio distaces. Commoly used amplifiers such as Erbium Doped Fibre Amplifiers (EFDA) ad Rama amplifiers provide optical amplificatio that varies with wavelegth. Oe solutio is to flatte out the powers across the trasmissio widow by atteuatig higher power chaels comparatively to the lower power chaels. This is more geerally referred to as gai flatteig. There are may gai flatteig (or equalisatio) systems available, may of which are time-ivariat ad passive devices e.g. fibre Bragg gratigs or thi film based. They remai fixed after productio ad caot be altered i respose to spectral fluctuatios ad drifts that might occur over the etwork lifetime. Such chages ca occur due to drifts i temperature, chages i chael loadig, system recofiguratio, or the replacemet of optical compoets. Passive gai flatteig systems must be replaced uder these circumstaces. This applicatio ote describes how a WaveShaper 1 combied with a optical spectrum aalyser (OSA) or optical chael moitor (OCM) ca be used to create a robust automated gai flatteig system that adapts o-the-fly to chages i the optical power spectrum. The system is ot oly applicable to EDFA, but ay other optical amplifier or broadbad source e.g. Rama amplifiers or supercotiuum lasers. This system algorithm ca successively improve the level of flatess dow to as little as ±0.1 db 2 deviatio from the mea. The system is versatile ad applicable for use both i the field ad as part of a optical test bed. 1 Excludes the WaveShaper 100 Family. 2 This figure depeds o the accuracy of the OSA or OCM that is used i the AGF system. ±0.1 db was achieved usig the Yokogawa AQ6370 Optical Spectrum Aalyser. 1. Experimetal Setup This applicatio ote uses a WaveShaper 120S Gai Equalizer desiged specifically for gai flatteig i mid. Four key pieces of hardware are eeded for the automated gai flatteig system: the computer, the WaveShaper, the fibre tap, ad the OSA. A schematic of the experimetal setup is show i Figure 1. Optical Source Figure 1: Experimetal Setup Schematic for a AGF system. The optical source to be flatteed is coected to the WaveShaper commo port. The WaveShaper output is the coected to a fibre tap, sedig a small amout of optical sigal to a OSA/OCM. This fibre tap ca be part of a optical amplifier further dow the trasmissio lie. The OSA/OCM ca be ay device capable of providig the computer with a accurate readig of optical power as a fuctio of wavelegth. This applicatio ote uses the Yokogawa AQ6370 optical spectrum aalyser as part of the gai flatteig system, however it could be substituted with a suitable optical chael moitor (OCM) or aother OSA. The data from the OSA is set to the computer ad is used to perform the gai flatteig algorithm. Upload WS Profile Automated Gai Flatteig System WaveShaper Computer Measure OSA Power Spectrum Fibre Tap OSA/OCM Figure 2: Illustrates the algorithm feedback loop used i the AGF system. The automated gai flatteig (AGF) system uses a feedback loop show i Figure 2 to maitai a flat spectral profile 1% Apply AGF Algorithm Page 1

2 APPLICATION NOTE: Automated Gai Flatteig eve after spectral chages occur. Coceptually, this feedback loop allows the WaveShaper profile to be systematically modified based o the OSA measuremet. The key idea behid the AGF algorithm is to atteuate the power level at each frequecy dow to commo power level. I other words, additioal atteuatio is itroduced ito spectral regios of icreased power. The deviatio i measured power from the commo level is added to the WaveShaper atteuatio profile at each iteratio of the algorithm. This algorithm ca be expressed mathematically by Eq. 1: B A Meas( A), A = B mi( B ). + 1 Where A is the WaveShaper atteuatio profile i.e. the 2 d colum of the *.wsp file. Meas( A ) deotes the OSA measuremet of the atteuatio after applyig profile A. It is the egative of the measured power spectrum. The iitial WaveShaper profile A ca be set to zero atteuatio for all 0 frequecies. A schematic of this process showig the first two iteratios is illustrated i Figure 3. WSP Atteuatio [db] WSP Atteuatio [db] - A 0 Frequecy [THz] A 1 Frequecy [THz] Measured Atteuatio [db] Measured Atteuatio [db] Figure 3: Schematic showig the automated gai flatteig algorithm. The left figures show the WSP atteuatio profiles uploaded to the WaveShaper, while the right figures show the correspodig experimetal measuremets. I practice, the followig steps are take for the th iteratio: Frequecy [THz] Meas( A 1 ) Meas( A 0 ) Frequecy [THz] 2. The resultig power spectrum is measured usig the OSA. 3. The power spectrum data is trasferred to the computer, coverted to atteuatio ad the filtered. 4. This data is subtracted off A to give B. 5. To remove the possibility of egative atteuatio, the profile for the ext iteratio is equal to B mius a costat give by the miimum value of B. The WaveShaper is capable of producig filters cotaiig spectral features as small as 10 GHz. Features smaller tha 10 GHz are beyod the resolvig capabilities of the WaveShaper ad are filtered out by the iteral optics. As a result, spectral features smaller tha 10 GHz i the measured OSA data ca lead to istabilities ad divergig profiles. This is preveted by usig a appropriate filterig algorithm. 2. Measurig ad Filterig There are two mai regimes that a AGF system may ru uder: the first ecompasses the flatteig of a broadbad light source with a cotiuous power spectrum e.g. a ASE light source. The secod ivolves flatteig a light source cosistig of a series of optical comb lies of varyig peak power. The implemetatio of the flatteig algorithm varies slightly for each regime. Specifically the measuremet i.e. i Eq. 1, Meas( A ) is iterpreted differetly for each regime. 2.1 Flatteig a Cotiuous Power Spectrum May optical light sources e.g. ASE broadbad sources have a o-flat power spectrum, ad flatteig this light source is ofte essetial for may applicatios. For flatteig this type of source, the etire spectrum is measured with the OSA ad the filtered with a smoothig filter. I this regime, Meas from Eq. 1 icludes the filterig process. A illustratio of this smoothig filter is show i Figure A wsp profile with a atteuatio profile of A is uploaded to the WaveShaper. Page 2

3 APPLICATION NOTE: Automated Gai Flatteig OSA Trace Filtered Measuremet OSA Trace Iterpolated Measuremet Relative Atteuatio [db] 3 Relative Atteuatio [db] Frequecy [THz] Figure 4: Illustratig measurig ad filterig the power spectrum of a cotiuous light source. The black lie represets the data take from a OSA trace, while the red lie represets Meas( A ), the smoothed data used to calculate the ext iteratio. The OSA trace must be filtered i order to remove sharp features which would otherwise cause istabilities i the iterative system. It is sufficiet to suppress features smaller tha aroud 20 GHz. This applicatio ote uses a zero-phase Butterworth filter, however other filters such as a Savitzky-Golay, or Gaussia filter are suitable Flatteig a Existig Chael Pla Whe the optical sigal already cotais a set of modulated optical chaels, the measured power spectrum will cosist of a series of comb lies, each correspodig to a optical chael. If the same approach take whe usig a cotiuous light source is applied here, the algorithm would aim to flatte the comb lies themselves, rather tha the comb lie peak power. For this reaso, the approach take i Sectio 2.1 is ot suitable uder this regime. To perform spectral flatteig o a set of optical comb lies, the OSA measuremet, deoted by Meas, is calculated by measurig the peak power level at each comb lie peak ad liearly iterpolatig for power levels betwee peaks. This process is illustrated i Figure 5. Ulike i the previous regime, o filterig is required Frequecy [THz] 3. Algorithm Implemetatio This sectio provides iformatio about how to implemet the AFG system usig the WaveShaper s dedicated API. 3.1 Formattig Data As the format of the trace data supplied by the OSA may vary, the data must be iterpolated to the same resolutio used by the WSP profile strig, e.g. a 1 GHz grid. Furthermore, before beig used i Eq. 1, care should be take to make sure that the trace data is coverted to atteuatio, i.e. the egative of power i db. 3.2 Limitig the Atteuatio Rage It ca be useful to place a limit o the total amout of flatteig that ca occur. For example, if a light source varies i itesity by more tha 20 db, it might be useful to prevet the algorithm from atteuatio larger tha 20 db. To do this, simply coerce each evaluatio of A + 1 to values betwee 0 ad 20 db. 3.3 API Implemetatio The WaveShaper API allows the WaveShaper to be remotely cotrolled with may programmig laguages icludig C, C++, LabVIEW, MATLAB, Pytho, etc. Each laguage makes calls to a commo set of API commads. The basic program flow is illustrated i Figure 5. 3 The smoothig filter must be a zero-phase filter to prevet the data from misaligig. Page 3

4 APPLICATION NOTE: Automated Gai Flatteig Iitialisatio ws_create_waveshaper ws_ope_waveshaper Mai Algorithm Loop Shutdow ws_load_profile ws_close_waveshaper ws_delete_waveshaper Figure 5: Flow-diagram of the relevat API fuctios used i the AGF system. 3.4 Program Iitialisatio The followig fuctios are called whe the AGF applicatio first starts: ws_create_waveshaper The WaveShaper object is created usig the ws_create_waveshaper API fuctio. This fuctio loads all the ecessary calibratio files required to properly drive the WaveShaper. ws_ope_waveshaper Commuicatio is established usig the ws_ope_waveshaper fuctio. This fuctio requires the WaveShaper to be coected to your computer. 3.5 Mai Algorithm Loop The followig fuctio is called as part of the mai iterative gai flatteig loop: ws_load_profile WSP profile strigs are geerated by the algorithm ad applied to the WaveShaper usig the ws_load_profile fuctio. This fuctio is called oce every iteratio loop. 3.6 Shutdow Whe the AGF system is fiished operatio, there are some fuctios that must be called o shutdow: ws_close_waveshaper The ws_close_waveshaper fuctio is used to close commuicatio with the WaveShaper. ws_delete_waveshaper The ws_delete_waveshaper fuctio is called to fially delete the WaveShaper object created durig iitialisatio. 4. Results A gai automated system was implemeted to flatte the power spectrum of a broadbad ASE light source usig a C+L Bad WaveShaper 120S, with experimetal results show i Figure 6. I this experimet, the gai flatteig algorithm was operated i a frequecy rage of to The maximum atteuatio limit was set to 10 db. The regios betwee 186 THz to 188 THz ad 195 THz to 196 THz are ot flatteed because doig so requires atteuatig the whole profile beyod the 10 db limit. The results show that oly 3 4 iteratios were eeded before the profile coverged to withi 0.1 db. Relative Power [dbm] Figure 6: Experimetal results showig the automated gai flatteig of a broadbad ASE light source usig a C+L bad WaveShaper 120S. It shows the first 4 iteratios. 5. Further Applicatios Iteratio 0 Iteratio 1 Iteratio 2 Iteratio Frequecy [THz] The gai flatteig algorithm produces a WSP profile that is specifically tailored to flatte the power spectrum of a particular optical light source. It is possible to apply additioal WSP filters to carve out ew filter shapes while simultaeously couteractig the o-flat power spectrum of the light source. Page 4

5 APPLICATION NOTE: Automated Gai Flatteig 5.1 Carvig Comb Lies Optical test beds ofte require optical sources that emulate the power spectrum of real-life optical commuicatio systems. The WaveShaper ca be used to accurately reproduce the power spectrum of a modulated data sigal by carvig out comb lies. The shape of the comb lie filter could correspod to a particular modulatio format of a WDM system. Furthermore, the automated gai flatteig system allows the power of each chael to be equalised Methodology The automated gai flatteig system is first used to flatte a broadbad light source. The fial atteuatio profile geerated by the algorithm acts as a base flatteig profile that will the have the atteuatio of aother WSP profile added oto it. I this case, a series of comb lies desiged to emulate a DPSK chael pla with 50 chaels equally spaced at 50 GHz spacig are filtered out Results A filter profile featurig 21 Gaussia chaels with a chael spacig of 200 GHz. Each chael had a 3-dB badwidth of 40 GHz. This atteuatio profile of this filter was added to the atteuatio profile of a flatteed light source ad the measured with a Yokogawa AQ6370 OSA. The experimetal results are show i Figure 7. Relative Power [dbm] Frequecy [THz] Figure 7: Experimetal measuremet of Gaussia chaels spaced every 200 GHz. Each chael has a 3-dB badwidth of 40GHz. The peak power of each chael was foud to be equalised to withi 0.6 db Moffett Park Drive Suyvale, CA Tel.: Fax: waveshaper@fiisar.com Fiisar Corporatio. All rights reserved. Page 5 Fiisar is a registered trademark. WSPR 12/12