We present a novel concept of tailored GTE structure and show that such devices are very useful for the realization of DSC with almost arbitrary dispersion profile and also with tunability in dispersion slope

Bennion, Ian

Chisholm, Karen

Gillooly, Andrew

Mitchell, John

Shu, Xuewen

Sugden, Kate

English

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WK5 Tbnable dispersion slope compensator using novel tailored Gires-Tournois etalons Xuewen Shu, John Mitchell*, Andrew Gillnoly, Karen Chisholm*, Kate Sugden*, Ian Benninn Photonics Research Group. Aslm Universiv, Aston Triangle, Birminghom, 84 7ET. United Kingdom rshu  @asron.oc.uk * / - d i p  Photonics. Farodoy U'har$ Aslon Science Pork, Birmingham. 8 7  4BB. Uniled Kinngdorn Abstract: We present a novel concept of tailored Gires-Tournoise etalon structure and show that such devices are very useful for the realization of dispersion slope compensator with almost arbitrary diwersion motile and also with tunability in dispersion sloDe. 02WO Outical society of America OClS coder: (060.2340) Fiber optics components; (060.45 IO) Optical cammunications 1. Intmductinn Wavelength-division-multiplexing (WDM) is a key technology for expanding the capacity of optical fiber transmission systems. With increasing bit rate, chromatic dispersion compensation is more and more critical for such systems. However, mismatch between the dispersion slopes of the transmission and dispersion compensating fiber (DCF) results in only one wavelength that is perfectly compensated and different amounts of residual dispersion on all of the other channels. The impact of dispersion slope on system performance increases as the operating bandwidth increases and also as the transmission distance and bit rate increases. Moreover, the dispersion slope itself may also change due to optical path reconfiguration in network and environmental temperature variation. Thus, dispersion compensators with dispersion slope tunability are desirable for dispersion managed high-speed WDM system. Although there have been some reports of dispersion slope compensators @SCs) [1-3], most of them can only compensate dispersion slope with fixed value [ l ]  or can compensate variable slope but only for a single channel [2]. Only few schemes have shown performance for both multi-channel compensation capability and tunability [3]. Recently, we introduced all-in-fiber distributed Gires-Tournois etalons (DGTEs) for dispersion slope compensation [4]. This approach has promising features such as low loss, low group delay ripple and low cost. Although the reported DGTE-based DSC can compensate for fixed dispersion slope and also has addition tunability in dispersion [41, the dispersion slope cannot be adjusted after fabrication. In this paper, we extend the GTE concept to demonstrate a tailored GTE structure and show that such devices are very useful for the realization of DSC with almost arbitrary dispersion slope profiles and also tunability in dispersion slope. 2. Concept and characteristics of tailored Gires-Tournnis etalons A GT interferometer is a special configuration of a Fabry-Perot etalon, which consists of a partially reflective mirror (fust mirror) and a 100% reflective mirror (second). Recently, distributed GTEs have been developed that replace traditional mirrors with directly W-written Bragg grating in optical fiber [5 ] .  Both traditional GTEs and distributed GTEs have similar properties such as periodical oscillation structure in group delay and dispersion, which make them suitable for identical multi-channel dispersion control. In a GTE, the reflectivity of the first minodgrating mainly determines the dispersion properties (such as group delay amplitude, dispersion amplitude) of the structure. The concept proposed here involves a tailored GTE that is used to produce arbitrary group delay profiles by tailoring the reflectivity profile of the first mirrodgrating - the reflectivity of the second minodgrating is still kept close to 100% to achieve all-pass function. For bulk optical mirrors, it is difficult to tailor their reflection spectra and thus such structures would be difficult to achieve. However, by using fiber Bragg gratings in the GTE structure makes the realization of tailored GTE much easier since the reflection spectra of fiber Bragg gratings can be readily tailored during fabrication to have an almost arbitrary profiles. To obtain a target group delay profile, &A), (i.e. envelop of the group delay amplitude), we derived that the reflection spectrum of the fust mirror/grating of the GTE, R,(A), should be tailored to the following profile, Where r, the light speed in vacuum. 2nd l C  is the round trip time in the cavity, n is the mode effective index, d is the cavity length, c is We have fabricated both linearly and nonlinearly tailored DGTEs in the experiments. As examples, three of WK5 tnem are mown in kig.1. lne  measured group delay spectra, me envelopes 01 group aeiay amplituaes ana tnelr corresponding reflection spectra of the first grating recorded in fabrication are plotted in Fig la, Ih and IC, respectively. DGTE (0 and (In) that have FSR of about 50GHz are formed by two chirped fiber gratings (CFGs) with overlapped sttucture and DGTE (II) that has FSR of about 25GHz is formed by two CFGs with separated structure [ 5 ] .  It is clearly seen from Fig. l a  and Ib that DGTE (I) has linearly increased amplitude in group delay, while the DGTE (II) and (111) have positive quadratic and negative quadratic varied amplitude in group delay, respectively. Though the oscillation amplitudes of group delay vary channel by channel, for each channel, the group delay is quadratic and result in linear variation in dispersion. IW 11, I _  ,mm wm,m** 1mj w R w m 1 m /  w * s l m  1m1 Fig.1. Experimental exampler of t a i l d D G T E s .  (a) Measured p u p  delay rpecua, (b) the envelopes of p u p  delay amplitudes. (e) re0ection specva of the fist grating recaded in fabrication. 3. %able dispersion slope compensators based on tailored DGTEs We first demonstrate a simple tunable DSC realized with a single linearly tailored DGTE. The principle is schematically shown in Fig.2a The linearly tailored DGTE has the oscillation amplitude of the group delay increasing channel by channel, and thus resulting in the dispersion slope increasing channel by channel. Before tuning, each channel has been set at zero dispersion, therefor the dispersion slope over these channels is zero. L 4 C’ , . I  - 1% (545 (IYI Wavelength Inm) Fig.2. Tunable DSC with a single linearly tailored DGTE. (a) The principle. (b) measured dispersion values at different dispersion settings. Insset: measured gwp delay specmm of the tailored DGTE. When tuning, different channels will suffer different dispersion changes (ADl<ADkAD3), therefor dispersion slope over these channels will be generated. The dispersion slope increases as the tuning increases. A negative dispersion slope can be readily realized hy tuning the DGTE in the contrary direction shown in Fig.%. The measured group delay spectrum of the linearly tailored DGTE used in the experiment is shown in the inset of Fig. 2b. The different dispersion slope settings are realized by simply applying different strain to the DGTE. The measured dispersion values for dispersion slope settings of +23.8, +9.2, 0.0, -10.8 and -23.4pdnm2 over about 30 channels are shown in Fig.Zb, which clearly indicates the slope tunability with such a tailored GTE. The drawback of this scheme is that the slope for each channel (internal) is not compensated, therefore such a DSC can only he suitahle for the case of small signal bandwidth. This shortcoming can he overcome by using a pair of tailored DGTEs as described below. The principle of combining using two tailored DGTEs as a tunable DSC is schematically shown in Fig.3a. The two DGTEs both have linear envelopes of group delay amplitude over a certain number of channels. The envelopes are properly designed so that for each channel, the dispersion slopes of two DGTEs have same magnitude but different signs in a certain region. Therefor, for each channel, there is a region where dispersion WK5 h (a) Wavelength (nm) Fig.3 Tunable DSC with Iwo linearly tailored DCTEs. (a) The principle, (b) measured dispersion values at different dispersion settings. Inset: measured group delay speelra of the lwo tailored DGTEs. is constant when the two DGTEs are combined. The dispersion value for each channel is determined by the relative spectral shift between two DGTEs, which can be controlled either thermally or by strain. When the dispersion for each channel is set at zero dispersion, as shown in Fig.3a, the dispersion slope over these channels is also zero. When tuning the DGTEI towards shorter wavelength, the dispersions for all channels increase but the amounts of dispersion changes are different, as shown in Fig.3a, thus a dispersion slope is generated over these channels. The dispersion slope increases as the DGTEl is shifted further. The group delay spectra of the two linearly tailored DGTEs selected for the experiment are shown in the inset of the Rg.3b. The two DGTEs were combined using a four-port optical circulator. In the experiment, we simply controlled the relative spectral shift between them by applying strain. The measured results for dispersion slope settings of +20.2, +6.3, -0.7, - 9.4, and -20.2ps/nm2 for near 30 channels were plotted in Fig.3h, which clearly indicate the slope tunability. The slope tuning range of this DSC is from -22 to +22ps/nm2 and the usable bandwidth for each channel is about 45% of the FSR (SOGHz). To demonstrate even higher order dispersion compensation capability, we designed a DSC using two quadratically tailored DGTEs. The simulated group delay spectra for the two nonlinearly tailored DGTEs are shown in the inset of Fig.4. The principle of tuning is similar to Fig.3a. When there is a relative spectral shift between each other, the higher order slope (i.e. quadratic coefficients of the dispersion profile) will change. For the plotted dispersion curves in Fig.4, the quadratic coefficients vary between +6.3 and -1.1 and one clearly sees the change of dispersion profile over these channels. This example strongly indicates that tunable control of arbitrary dispersion profile may be realized by properly designed tailored DGTEs. 4. Conclusion 4 w J d : .  , . , . , 1544 lY6 I548 15W 1552 15cJ I556 Wavelength (nm) Fig.4. Design of tunable high+rda DSC with two quadratically tailored DGTG. Insee calculated group delay spccva of lhc two tailored DCTEr. We have introduced a novel concept of a tailored Gues-Tournois etalon. which is an all-pass filter with specially designed group delay profile achieved by tailoring the reflectivity of the fmt mimodgrating. Linearly and nonlinearly tailored DGTEs have been fabricated with W-written fiber Bragg gratings. We have successfully demonstrated the use of such devices as tunable dispersion slope compensators that can compensate for complex dispersion profiles with tunability. Due to the compact all-in-fiber design, such a DSC is an attractive solution for tunable dispersion slope compensation, having advantages such as low loss, low cost and low group delay ripples. References [tl K. T&iguchi, el. al. “lntpted optic dispersion slope equaliwr for Nx20Gbi~lsWDM transmission”. Electron Lett.. 37,701-703 (2001). [21 M lablo&, er al., “Adjustable coupled two-cavity allpass filta for dispersion slope compensatioa of optical fibres:’ Electron. Lett. 36, 511-512 (2WO). [3] Y.W. Song, et. al., ‘Tunable dispersion slope campeusatian for 4 0 G W s  WDM systems using broadband nonchamelized third-order chirped fiber Bragg gratings.” I. LightwaveTech.. 20,2259-2266 (2W2). 141 X. Shu, et. al.. “Design & realization of dispersion slope campewtors using distributed Gires-Tournois etatons,” ECOC‘Z003, paper We4.P.57. [ 5 ]  X. Shu, et. al., ‘Tunable disperdon compensator based on novd distributed Cues-Tournois d o n s ’ ’  OFC‘2W3, paper ThLI. 

Tunable dispersion slope compensator using novel tailored Gires-Tournois etalons

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Tunable dispersion slope compensator using novel tailored Gires-Tournois etalons

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