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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 5 — Feb. 28, 2011
  • pp: 3937–3944
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Real-time ultrawide-band group delay profile monitoring through low-noise incoherent temporal interferometry

Yongwoo Park, Antonio Malacarne, and José Azaña  »View Author Affiliations


Optics Express, Vol. 19, Issue 5, pp. 3937-3944 (2011)
http://dx.doi.org/10.1364/OE.19.003937


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Abstract

A simple, highly accurate measurement technique for real-time monitoring of the group delay (GD) profiles of photonic dispersive devices over ultra-broad spectral bandwidths (e.g. an entire communication wavelength band) is demonstrated. The technique is based on time-domain self-interference of an incoherent light pulse after linear propagation through the device under test, providing a measurement wavelength range as wide as the source spectral bandwidth. Significant enhancement in the signal-to-noise ratio of the self-interference signal has been observed by use of a relatively low-noise incoherent light source as compared with the theoretical estimate for a white-noise light source. This fact combined with the use of balanced photo-detection has allowed us to significantly reduce the number of profiles that need to be averaged to reach a targeted GD measurement accuracy, thus achieving reconstruction of the device GD profile in real time. We report highly-accurate monitoring of (i) the group-delay ripple (GDR) profile of a 10-m long chirped fiber Bragg grating over the full C band (~42 nm), and (ii) the group velocity dispersion (GVD) and dispersion slope (DS) profiles of a ~2-km long dispersion compensating fiber module over an ~72-nm wavelength range, both captured at a 15 frames/s video rate update, with demonstrated standard deviations in the captured GD profiles as low as ~1.6 ps.

© 2011 OSA

1. Introduction

In this context, higher-order group delay (GD) profiles beyond the GVD (including the DS for DCFs, and the GDR for CFBGs) need to be characterized, preferably in real-time, over the full operational wavelength range. In general, real-time measurement of the group-delay characteristics of dispersive devices over their operational spectral bandwidth is highly desired for monitoring both the fabrication and the performance of DC modules.

2. Operation principle

3. Experiments

Figure 1 shows the experimental setup. An SLD amplified through a semiconductor optical amplifier (SOA) implements the low-noise incoherent input broadband light source with a FWHM bandwidth of about 80 nm and an output power of 35 mW. To avoid any back reflection from and to the SOA, we used two isolators (ISO). Because of the polarization dependence of both the SOA (polarization dependence gain, PDG ≈ 1.5 dB) and the Mach-Zehnder intensity modulator (MZM), we placed a polarization controller (PC) for optimizing the input signal polarization state at the input of each of them. The RF modulation pulses, which had a time width of ~250 ps and a repetition rate of 50 kHz, were generated from an arbitrary waveform generator (AWG).

In the first example reported here, the dispersive element under test was a CFBG (Proximion Inc., insertion loss of 6 dB) with a first-order dispersion of ~2040 ps/nm and a full-width reflection bandwidth of ~42 nm, i.e. covering the entire C band. Following the dispersion-induced FTM, a simple Mach-Zehnder interferometer (MZI) was employed to split the dispersed signal into two identical copies, which were then recombined with a relative time delay Δτ = 25 ps. The two output arms of the MZI were employed to acquire the interferogram through a high-speed (20 GHz) dual-balanced photo-receiver, mounted on an 8 GS/s fast digitizer able to collect 200 waveforms per update, each waveform composed by 400 points. In Fig. 2(a)
Fig. 2 Zoom of the interference pattern after balanced photo-detection (a), corresponding baseband spectrum (b), 47 times averaged simulated spectrum mapped into the time domain at the dispersive medium output (c), simulated input optical pulse (d), experimental spectrum mapped into time domain at the dispersive medium output measured by a sampling oscilloscope with no averaging (e), experimentally measured input optical pulse (f).
the interference pattern, averaged 200 times, is reported together with its corresponding baseband Fourier spectrum, shown as an inset in Fig. 2(b) (SNR = 30 dB).

Figure 2(e) shows the experimentally measured time-domain mapped spectrum after FTM, acquired in sampling mode with no averaging. Considering a completely incoherent input spectrum (white light noise) in a simulation, to reach the same SNR as that of the experimentally measured time-domain spectrum (7.75 dB), the simulated time-mapped spectrum needed to be averaged 47 times, as shown in Fig. 2(c). The inset of Fig. 2(d) reports the simulated optical pulse after modulation with a SNR = 0 dB, whereas in Fig. 2(f) the actually measured optical pulse after modulation is reported, showing a much higher SNR. In view of this performance, we have referred to our light source as low-noise incoherent source.

Figure 3(a)
Fig. 3 GDR profiles of the CFBG measured by (a) the proposed method with 15-repeated measurements (the azure curve shows the 15 overlapped plots while the averaged data line is shown in blue), and (b) optical vector analyzer. A single frame of the real-time video rate (15 frames/sec) update reporting the interference pattern (up) and the corresponding derivative of the time-domain phase profile, omitting its first and second-order terms (down), (c). Real-time measurement example (Media 1).
shows the GDR profile of the CFBG, measured by the proposed method over a 42 nm bandwidth, compared with the measurement performed by a commercial optical vector analyzer [10

10. B. Soller, D. Gifford, M. Wolfe, and M. Froggatt, “High resolution optical frequency domain reflectometry for characterization of components and assemblies,” Opt. Express 13(2), 666–674 (2005). [CrossRef] [PubMed]

] (OVA, LUNA Technologies Inc.), reported in Fig. 3(b). The GDR curve has been obtained by subtracting the second-order polynomial fitting curve from the measured group-delay profile. In Fig. 3(a) the azure zone shows the overlapped plots from 15 different consecutive measurements whereas the blue solid curve is the 15-times progressive averaged data line. The two measurements (Fig. 3(a) and (b)) present an excellent agreement. Figure 3(c) reports a frame from a movie at 15 frames/s video rate (Media 1) monitoring the interference pattern (up) and the corresponding derivative of the temporal phase profile (down) omitting its first and second-order terms.

As a second example of measurement, a DCM (PureForm DCM, −368 ps/nm @ 1566 nm, insertion loss of 5.5 dB, Avanex) was tested using the same measurement setup described above by simply replacing the CFBG and the circulator with the DCM. Measured GD profiles could be fitted accurately with up to 4th-order polynomials characterizing the higher-order terms of the GD profile of the DCM thanks to the wide-wavelength range (> 70 nm) covered by the measurement. Figure 4
Fig. 4 GD profiles of the DCM measured by the proposed method with 10-repeated consecutive measurements (in azure the 10 overlapped plots; in blue their averaged data line). (a) measured GD profiles and (b) GD profiles subtracted with a linear curve fit. (c) GD profiles subtracted with the second order fit. (d) GD profiles subtracted with the third order fit. (e) measured dispersion parameter (brown color solid curve) and dispersion slope profiles (blue color solid curve) and dispersion parameter given in the specification (solid dots).
shows the measured GD profiles with respect to a baseband frequency centered at 193.84 THz (1547.7 nm in wavelength). Fast updated GD profiles are shown in Fig. 4(a)-(d); notice that in all these figures, the 10 overlapped plots shown in azure were progressively updated in real-time at a specific rate of 15 frames/s (an example of real-time movie is shown in Fig. 5
Fig. 5 Real-time GD measurement example for the DCM module. (Media 2)
and Media 2).

The average plot of the 10 recovered profiles is overlapped in each figure using a blue line. Figure 4(a) shows the total measured GD profile. The second-order GD profile is dominantly observed when subtracting a first-order polynomial fitting from the total measured GD curve, e.g. C1f+C0, with f being the baseband frequency and C1=2738.4 [2π ps2], see results shown in Fig. 4(b). The coefficient of the first-order polynomial fitting determines the GVD parameter, D, of the DCM; in particular D=c/(λ02C1), where c is the speed of light in vacuum and λ 0 is the central wavelength. In our calculation, the total (multiplied by the fiber length) GVD parameter was determined to be – 342.96 ps/nm at 1547.7 nm, which agrees very well with the value estimated from the product data specifications: - 343.2 ps/nm. Higher-order dispersion terms can be extracted following a similar strategy, i.e. by subtracting the corresponding higher-order polynomial fittings from the total measured GD profile, see results in Figs. 4(c) and (d). Figure 4(e) shows the total (multiplied by the fiber length) GVD in ps/nm and DS in ps/nm2 as a function of wavelength. These curves were extracted by differentiating the third-order polynomial equation (the equation shown in Fig. 4(d)) with respect to wavelength. There is a good agreement with the dispersion data from the fiber specifications, shown as solid dots in Fig. 4(e). Figure 5 shows a frame from a movie at 15 frames/s video rate (Media 2) demonstrating real-time acquisition of the higher-orders GD curve (orders higher than the second one, corresponding to the case reported in Fig. 4(c)).

4. Performance

The accuracy of the GD measurement has been evaluated by comparing the standard deviation (SD) of the GD profiles measured (i) by use of our dual-balanced detection, and (ii) by use of single-ended detection. In these measurements the same DCM was used as the dispersive medium in order to investigate the dependence of the SD with the source spectral power density. Figure 6
Fig. 6 Comparison of the standard deviation (SD) obtained in GD measurements based on the use of (i) balanced detection and (ii) single-ended detection. The SDs are plotted with respect to the baseband frequency centered at 193.84 THz (1547.7 nm).
shows the SDs of 15 consecutive GD profile measurements for the case of the balanced detection and the single-ended detection, respectively. A minimum SD of ~1.6 ps can be found around the peak of the power spectrum density for our GD measurements using dual-balanced detection whilst a 2.5-ps SD was the best obtained result using the single-ended detection. This GD measurement accuracy improvement is mainly due to the fact that the balanced detection provides a 3-dB improvement on the interference signal power as compared with the single-ended detection case. This 3-dB gain translates into a nearly doubled accuracy level in the GD measurements. This evidences the dependence of the GD SD with the SNR in the interferogram measurement. Indeed, in line with this observation, our results also show that the SD curves reach a minimum, i.e. maximum accuracy, towards the frequency at which the energy spectral density of the source is maximized (source spectrum peak).

Concerning the wavelength resolution of the method, similarly to the technique based on a coherent pulse light source [8

8. C. Dorrer, “Chromatic dispersion characterization by direct instantaneous frequency measurement,” Opt. Lett. 29(2), 204–206 (2004). [CrossRef] [PubMed]

], [9

9. T.-J. Ahn, Y. Park, and J. Azaña, “Fast and accurate group delay ripple measurement technique for ultralong chirped fiber Bragg gratings,” Opt. Lett. 32(18), 2674–2676 (2007). [CrossRef] [PubMed]

], this is inversely proportional to the dispersion of the medium under test, estimated to be ~42 pm for the example on the CFBG, whereas the optical vector analyzer (OVA) resolution was about 3 pm. If needed, the wavelength resolution in our system could be improved by use of an additional well-characterized linear dispersive medium connected in series with the device under test [9

9. T.-J. Ahn, Y. Park, and J. Azaña, “Fast and accurate group delay ripple measurement technique for ultralong chirped fiber Bragg gratings,” Opt. Lett. 32(18), 2674–2676 (2007). [CrossRef] [PubMed]

].

5. Conclusions

We have demonstrated a novel, highly accurate method for real-time group-delay monitoring of photonic dispersive devices over ultra-broad spectral bandwidths based upon time-domain interferometry using a simple and practical low-noise incoherent light source. Two critical elements are required for real-time operation: (i) a relatively low-noise broadband light source with a spectral bandwidth covering the desired measurement wavelength range; and (ii) a balanced high-speed photo-detection scheme. Real-time monitoring of the group-delay profiles (including accurate reconstruction of the higher-order GD terms and GDR profiles) of a 10-m long CFBG over the full C band (~42 nm) and a ~2-km long DCM over a wavelength range of ~72 nm have been experimentally demonstrated at a 15 frames/s video rate. A remarkable measurement standard deviation of ~1.6 ps has been achieved in these measurements. This method should prove particularly useful for real-time monitoring of the group-delay characteristics of broadband dispersion-compensation devices (e.g. CFBGs, DCMs, or tunable dispersion-compensation technologies) during fabrication or as they are operated within a given system, e.g. to be used in optimization feedback loops.Yongwoo Park is now with Automated Precision Inc. 15000 Johns Hopkins Dr.Rockville, MD USA 20850, kjywpark@gmail.com

References and links

1.

S. Ramachandran, Fiber based dispersion compensation: Introduction and overview, Springer Science+Business media, LLC (2007)

2.

M. Wandel, and P. Kristensen, “Fiber designs for high figure of merit and high slope dispersion compensating fibers,” in Fiber based dispersion compensation, edited by S. Ramachandran, Springer Science+Business media, LLC (2007)

3.

C. Lin, H. Kogelnik, and L. G. Cohen, “Optical-pulse equalization of low-dispersion transmission in single-mode fibers in the 1.3–1.7-µm spectral region,” Opt. Lett. 5(11), 476–478 (1980). [CrossRef] [PubMed]

4.

F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,” Opt. Lett. 12(10), 847–849 (1987). [CrossRef] [PubMed]

5.

C. R. Doerr, L. W. Stulz, S. Chandrasekhar, and R. Pafchek, “Colorless tunable dispersion compensator with 400-ps/nm range integrated with a tunable noise filter,” IEEE Photon. Technol. Lett. 15(9), 1258–1260 (2003). [CrossRef]

6.

J. C. Cartledge, “Effect of modulator chirp and sinusoidal group delay ripple on the performance of systems using dispersion compensating gratings,” J. Lightwave Technol. 20(11), 1918–1923 (2002). [CrossRef]

7.

M. Sumetsky, P. I. Reyes, P. S. Westbrook, N. M. Litchinitser, B. J. Eggleton, Y. Li, R. Deshmukh, and C. Soccolich, “Group-delay ripple correction in chirped fiber Bragg gratings,” Opt. Lett. 28(10), 777–779 (2003). [CrossRef] [PubMed]

8.

C. Dorrer, “Chromatic dispersion characterization by direct instantaneous frequency measurement,” Opt. Lett. 29(2), 204–206 (2004). [CrossRef] [PubMed]

9.

T.-J. Ahn, Y. Park, and J. Azaña, “Fast and accurate group delay ripple measurement technique for ultralong chirped fiber Bragg gratings,” Opt. Lett. 32(18), 2674–2676 (2007). [CrossRef] [PubMed]

10.

B. Soller, D. Gifford, M. Wolfe, and M. Froggatt, “High resolution optical frequency domain reflectometry for characterization of components and assemblies,” Opt. Express 13(2), 666–674 (2005). [CrossRef] [PubMed]

11.

C. Dorrer, “Temporal van Cittert-Zernike theorem and its application to the measurement of chromatic dispersion,” J. Opt. Soc. Am. B 21(8), 1417–1423 (2004). [CrossRef]

12.

C. Dorrer, “Statistical analysis of incoherent pulse shaping,” Opt. Express 17(5), 3341–3352 (2009). [CrossRef] [PubMed]

13.

S. Shin, U. Sharma, H. Tu, W. Jung, and S. A. Boppart, “Characterization and Analysis of Relative Intensity Noise in Broadband Optical Sources for Optical Coherence Tomography,” IEEE Photon. Technol. Lett. 22(14), 1057–1059 (2010). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2340) Fiber optics and optical communications : Fiber optics components
(230.1480) Optical devices : Bragg reflectors
(230.2035) Optical devices : Dispersion compensation devices
(060.3735) Fiber optics and optical communications : Fiber Bragg gratings

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 13, 2010
Revised Manuscript: January 27, 2011
Manuscript Accepted: February 2, 2011
Published: February 14, 2011

Citation
Yongwoo Park, Antonio Malacarne, and José Azaña, "Real-time ultrawide-band group delay profile monitoring through low-noise incoherent temporal interferometry," Opt. Express 19, 3937-3944 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-5-3937


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References

  1. S. Ramachandran, Fiber based dispersion compensation: Introduction and overview, Springer Science+Business media, LLC (2007)
  2. M. Wandel, and P. Kristensen, “Fiber designs for high figure of merit and high slope dispersion compensating fibers,” in Fiber based dispersion compensation, edited by S. Ramachandran, Springer Science+Business media, LLC (2007)
  3. C. Lin, H. Kogelnik, and L. G. Cohen, “Optical-pulse equalization of low-dispersion transmission in single-mode fibers in the 1.3–1.7-µm spectral region,” Opt. Lett. 5(11), 476–478 (1980). [CrossRef] [PubMed]
  4. F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,” Opt. Lett. 12(10), 847–849 (1987). [CrossRef] [PubMed]
  5. C. R. Doerr, L. W. Stulz, S. Chandrasekhar, and R. Pafchek, “Colorless tunable dispersion compensator with 400-ps/nm range integrated with a tunable noise filter,” IEEE Photon. Technol. Lett. 15(9), 1258–1260 (2003). [CrossRef]
  6. J. C. Cartledge, “Effect of modulator chirp and sinusoidal group delay ripple on the performance of systems using dispersion compensating gratings,” J. Lightwave Technol. 20(11), 1918–1923 (2002). [CrossRef]
  7. M. Sumetsky, P. I. Reyes, P. S. Westbrook, N. M. Litchinitser, B. J. Eggleton, Y. Li, R. Deshmukh, and C. Soccolich, “Group-delay ripple correction in chirped fiber Bragg gratings,” Opt. Lett. 28(10), 777–779 (2003). [CrossRef] [PubMed]
  8. C. Dorrer, “Chromatic dispersion characterization by direct instantaneous frequency measurement,” Opt. Lett. 29(2), 204–206 (2004). [CrossRef] [PubMed]
  9. T.-J. Ahn, Y. Park, and J. Azaña, “Fast and accurate group delay ripple measurement technique for ultralong chirped fiber Bragg gratings,” Opt. Lett. 32(18), 2674–2676 (2007). [CrossRef] [PubMed]
  10. B. Soller, D. Gifford, M. Wolfe, and M. Froggatt, “High resolution optical frequency domain reflectometry for characterization of components and assemblies,” Opt. Express 13(2), 666–674 (2005). [CrossRef] [PubMed]
  11. C. Dorrer, “Temporal van Cittert-Zernike theorem and its application to the measurement of chromatic dispersion,” J. Opt. Soc. Am. B 21(8), 1417–1423 (2004). [CrossRef]
  12. C. Dorrer, “Statistical analysis of incoherent pulse shaping,” Opt. Express 17(5), 3341–3352 (2009). [CrossRef] [PubMed]
  13. S. Shin, U. Sharma, H. Tu, W. Jung, and S. A. Boppart, “Characterization and Analysis of Relative Intensity Noise in Broadband Optical Sources for Optical Coherence Tomography,” IEEE Photon. Technol. Lett. 22(14), 1057–1059 (2010). [CrossRef]

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