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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 14 — Jul. 5, 2010
  • pp: 15201–15206
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Brillouin induced self-heterodyne technique for narrow line width measurement

P. Sevillano, J. Subías, C. Heras, J. Pelayo, and F. Villuendas  »View Author Affiliations


Optics Express, Vol. 18, Issue 14, pp. 15201-15206 (2010)
http://dx.doi.org/10.1364/OE.18.015201


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Abstract

A novel technique is introduced for jitter-insensitive sub-KHz resolution linewidth characterization technique in ultra-narrow lasers for optical communication applications. The technique is based on self-heterodyne detection induced by Stimulated Brillouin Scattering (SBS). Non linear SBS drives the heterodyne mixing through optical frequency locking of a narrow tunable laser source and the signal under test, which is modulated in the low frequency range. Due to SBS nature, jitter variations in the optical frequency do not affect the correlation spectra measured with resolution figures up to 300 Hz, without the need for optical delay line as in conventional homodyne correlation techniques.

© 2010 OSA

1. Introduction

Traditional linewidth measurement optical techniques are based on interferometric and filtering processes [8

8. R. Hui, and M. O'Sullivan, Fiber Optic Measurement Techniques (Academic Press, 2009), Chap. 2.

]. Fabry-Perot interferometer-based and diffraction gratings have been widely used in commercial optical spectrum analyzers. The main disadvantage of these techniques is the trade-off between features, making it impossible to reach high performance values for all the features in the same setup. F-P interferometers go up to 1 pm with narrow spectral range but limited dynamic range (MicronOptics FFP-TF2); on the other hand, gratings offer higher dynamic and spectral range but 10 pm limited resolution (Yokogawa AQ6319). Other techniques are based on converting frequency fluctuations into intensity fluctuations via Mach-Zehnder interferometer. These techniques basically involve self-heterodyne and homodyne detection [9

9. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1-3), 180–186 (1998). [CrossRef]

,10

10. P. Horak and W. H. Loh, “On the delayed self-heterodyne interferometric technique for determining the linewidth of fiber lasers,” Opt. Express 14(9), 3923–3928 (2006). [CrossRef] [PubMed]

]. In both cases the beating of the signal with either a modulated or unmodulated replica of itself after traveling through a delay line provides very high resolution, up to kHz, but with technical difficulties due to the delay line. As this problem worsens with narrower lasers, tedious interpretation for delayed times below coherence time becomes necessary [9

9. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1-3), 180–186 (1998). [CrossRef]

]. Later works were based on heterodyne interference with sweeping local oscillator [11

11. J. Debeau, B. Kowalski, and R. Boittin, “Simple method for the complete characterization of an optical pulse,” Opt. Lett. 23(22), 1784–1786 (1998). [CrossRef]

,12

12. B. Szafraniec, and D. M. Baney, “Swept Coherent Spectrum Analysis of the Complex Optical Field” in 1st IEEE Conference on Lightwave Technologies in Instrumentation and Measurement, IEEE ed. (Palisades, NY, Oct. 2004) pp. 68–72.

]. In the framework of narrow optical filtering spectroscopic methods, a high-resolution optical spectrum analyzer was developed several years ago based on the narrow width, ≈10 MHz, of the SBS gain curve [13

13. J. M. S. Domingo, J. Pelayo, F. Villuendas, C. D. Heras, and E. Pellejer, “Very High Resolution Optical Spectrometry by Stimulated Brillouin Scattering,” IEEE Photon. Technol. Lett. 17(4), 855–857 (2005). [CrossRef]

]. This technique, with wavelength sweeping along wide range, offers a high resolution (up to 0.08 pm), over full C-band, and high dynamic range (80 dB) [14], being the last developed alternative in optical filtering methods but still below self-beating methods resolution.

Stimulated Brillouin Scattering has been recently used in the framework of high resolution spectral analysis based on the heterodyning process of the signal under test and the Stokes wave generated by itself in a Brillouin ring laser [15

15. F. Mihélic, D. Bacquet, J. Zemmouri, and P. Szriftgiser, “Ultrahigh resolution spectral analysis based on a Brillouin fiber laser,” Opt. Lett. 35(3), 432–434 (2010). [CrossRef] [PubMed]

]. That system can achieve resolutions reaching the kilohertz level by means of the narrow Brillouin self-stimulated laser used as local oscillator after a vast number of recirculation in the ring. This beating is fixed at the Brillouin shift frequency, ≈11GHz, which requires fast response detection. Besides, as in others heterodyne techniques, power spectral density cannot obviate the contribution of sudden jumps in the signal under test emission frequency, thus obtaining and overestimated effective linewidth instead of natural linewidth.

In comparison with other already known techniques, Brillouin induced self-heterodyning offers some advantages: it is jitter free as both contributions to detection come from the same source at the same time, there is no need of high frequency modulation such as in homodyne technique, and delay line is no longer a requirement as in practically all self-interferometric techniques. As a result of the spectral width of the Brillouin Scattering gain curve, ≈10 MHz, this method is suitable for lasers in which Brillouin scattering is efficient, i.e. are not wider than Brillouin gain width, and frequency stable enough so random phase jumps will not shift laser wavelength out of gain curve, e.g. C-Band communication lasers. This technique can achieve resolution up to hundreds of hertz limited by the acoustic properties in the Brillouin scattering phenomena.

2. Experimental setup

Phase modulation could also be used instead of amplitude modulation, obtaining the same result when side band stimulates Brillouin scattering. Regarding modulation frequencies, values above the 10 MHz Brillouin gain bandwidth is the only requisite, so standard signal generators can be used and no very fast response electronics is necessary for detection. The AC nature of the signal detected, with electrical spectrum centered in the SUT modulation frequency, does not require the measurement of DC or near zero frequency components contrariwise homodyne interferometry.

The signal tracked in the photodiode contains the beat between carrier and Brillouin backscattered wave stimulated by either of the modulation bands. As the phase noise of pump laser is incorporated into the Brillouin-backscattered wave [16

16. A. Debut, S. Randoux, and J. Zemmouri, “Experimental and theoretical study of linewidth narrowing in Brillouin fiber ring lasers,” J. Opt. Soc. Am. B 18(4), 556–567 (2001). [CrossRef]

], and SUT and pump originate from lasers devices with uncorrelated phase variations, this technique has the characteristics of a heterodyne one, so that delay line required in conventional homodyne and self-heterodyne detection for coherence loss is no longer necessary. The presence of this line has been reported as the main drawback of these techniques because the narrower the laser spectrum the longer the delay line required, so loss became a concern and signal interpretation is not straightforward [9

9. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1-3), 180–186 (1998). [CrossRef]

]. On the other hand, heterodyne techniques suffer a lack of effective resolution due to relative frequency jitter between SUT and probe laser, which is not present in the Brillouin induced self-heterodyne method because of the properties of Brillouin backscattering generation. Considering the stimulation mode case, Stokes backscattered wave and stimulus are frequency locked so sudden relative frequency variations between pump and stimulus can be neglected as far as the stimulus lays in the Brillouin Stokes wave gain width, i.e. 10MHz [2

2. J. Subías, C. Heras, J. Pelayo, and F. Villuendas, “All in fiber optical frequency metrology by selective Brillouin amplification of single peak in an optical comb,” Opt. Express 17(8), 6753–6758 (2009). [CrossRef] [PubMed]

]. This requirement is easily satisfied in the experimental conditions for which this measurement technique is considered for, and consequently the measurement can be thought as jitter free.

3. Results and discussion

Before presenting any measurement with the proposed technique, in order to get some data to compare, we characterize the source 1 laser (LS1), commercial tunable LS model (Agilent 8164B), by a conventional heterodyne technique with a different unit of same tunable LS model as local oscillator. The measured spectrum is shown in Fig. 3
Fig. 3 Spectrum of LS1 taken by heterodyning with a laser of the same model as local oscillator. Measure was taken in a 4ms swept time and averaged over 10 measures. Spectrum is centered at the beat frequency between probe and signal.
. Lorentzian shape fitting gives a linewidth of 245 kHz.

While measurement was taken, excursions in a narrow frequency range of emission wavelength in both lasers were observed, for this reason several measurements were taken with a short swept time in the ESA and then averaged after tracking the beat note. Taking this into account the spectra we measured describes the statistical properties of slow frequency fluctuations in both lasers instead of an intrinsic instantaneous linewidth characterizing the ultimate coherence of the laser emission.

Now with the already mentioned setup LS1 was measured using a different unit of the same laser model as a pump source, wavelength was settled to λSUT = 1550 nm. The spectrum obtained is shown in Fig. 4
Fig. 4 Heterodyning Spectrum of the LS1 laser measured by the non-linear mixing technique, 10 kHz span centered in the modulation frequency. Resolution bandwidth was set to 10Hz, swept time to 2.5s and values averaged over 20 measures. Electric modulation spectrum is plotted in grey color.
. Signal was modulated with ωm≈600 MHz keeping the bias voltage for the MZ modulator at quadrature point. Pump power after amplification was around 20 dBm. ESA was set to 10 Hz resolution. Span was adjusted to 10 kHz.

Gray plot in Fig. 4 represents the spectrum signal measured by the detector when the pump is off, in that case no amplification of sideband is present and, as shown, the electric spectrum presents just the pure tone frequency at ωm, for the amplitude modulation mode.

In both figures the pattern displays the jitter-free natural linewidth of the laser. Because of partial transmission of pump phase noise to the Brillouin wave, complete evaluation of the exact relationship between measured spectra and linewidth caused exclusively by phase noise in the SUT will require a further thorough characterization of this cross-phase mixing phenomenon. Resolution is determined by the spatial period of the slower oscillation that one acoustic wave can undertake along the effective interaction length with the light in the Brillouin scattering effect. This spatial period can be at most of the order of one coherence length of the Brillouin effect, in order to be transferred to the scattered optical wave. Thus, a Brillouin effective bandwidth of the order of 10 MHz (coherence length in the order of 20 m [17

17. P. C. Wait and T. P. Newson, “Measurement of Brillouin scattering coherence length as a function of pump power to determine Brillouin linewidth,” Opt. Commun. 117(1-2), 142–146 (1995). [CrossRef]

]), and a sound speed of about 6 km/s in the fiber, yields to a frequency resolution around 300 Hz.

4. Summary

In summary we have proposed a new technique for linewidth characterization of narrow lasers emission lines based on self-heterodyning but getting rid of the main drawbacks of conventional methods, and so, obtaining interesting features. The need of an optical delay line is overcome thanks to non-linear Brillouin effect. Properties of stimulated backscattering result as well in a jitter insensitive technique. Low-frequency modulation of the SUT is used; hence no DC noise in detection is present opposite to homodyne detection. The ultimate resolution of the technique is limited by the coherence length of the Brillouin phenomena with results up to 300 Hz. This set of advantages makes this technique a choice for linewidth measurements in communications band for ultra-narrow lasers, with better achievable resolution than filtering techniques but simpler implementation than interferometric ones. In the latest class of high-resolution filtering devices, stimulated Brillouin scattering is used as a selective filter at a wavelength that can be swept over a wide range, scanning the measured spectrum. In this case, the proposed technique allows the explorations of narrow lines in the measured spectrum using the same kind of interaction to implement self-heterodyne interferometry, and going further of the 10 MHz resolution limit of the primary filtering.

Acknowledgements.

This work was supported by the Spanish Secretaría de Estado de Investigación (MICINN) under project FIS2007-64443.

References and Links

1.

E. Ip, A. P. Lau, D. J. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express 16(2), 753–791 (2008). [CrossRef] [PubMed]

2.

J. Subías, C. Heras, J. Pelayo, and F. Villuendas, “All in fiber optical frequency metrology by selective Brillouin amplification of single peak in an optical comb,” Opt. Express 17(8), 6753–6758 (2009). [CrossRef] [PubMed]

3.

D. Guyomarc’h, G. Hagel, C. Zumsteg, and M. Knoop, “Some aspects of simulation and realization of an optical reference cavity,” Phys. Rev. A 80, 063820 (2009).

4.

Y. Yamamoto and T. Kimura, “Coherent optical fiber transmission system,” IEEE J. Quantum Electron. 17(6), 919–935 (1981). [CrossRef]

5.

T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noé, “Coherent optical communication: towards realtime systems at 40 Gbit/s and beyond,” Opt. Express 16(2), 866–872 (2008). [CrossRef] [PubMed]

6.

A. J. Lowery, “Amplified-spontaneous noise limit of optical OFDM lightwave systems,” Opt. Express 16(2), 860–865 (2008). [CrossRef] [PubMed]

7.

V. Ferrero and S. Camatel, “Optical Phase locking techniques: an overview and a novel method based on single side sub-carrier modulation,” Opt. Express 16(2), 818–828 (2008). [CrossRef] [PubMed]

8.

R. Hui, and M. O'Sullivan, Fiber Optic Measurement Techniques (Academic Press, 2009), Chap. 2.

9.

H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1-3), 180–186 (1998). [CrossRef]

10.

P. Horak and W. H. Loh, “On the delayed self-heterodyne interferometric technique for determining the linewidth of fiber lasers,” Opt. Express 14(9), 3923–3928 (2006). [CrossRef] [PubMed]

11.

J. Debeau, B. Kowalski, and R. Boittin, “Simple method for the complete characterization of an optical pulse,” Opt. Lett. 23(22), 1784–1786 (1998). [CrossRef]

12.

B. Szafraniec, and D. M. Baney, “Swept Coherent Spectrum Analysis of the Complex Optical Field” in 1st IEEE Conference on Lightwave Technologies in Instrumentation and Measurement, IEEE ed. (Palisades, NY, Oct. 2004) pp. 68–72.

13.

J. M. S. Domingo, J. Pelayo, F. Villuendas, C. D. Heras, and E. Pellejer, “Very High Resolution Optical Spectrometry by Stimulated Brillouin Scattering,” IEEE Photon. Technol. Lett. 17(4), 855–857 (2005). [CrossRef]

14.

http://www.aragonphotonics.com/index.php

15.

F. Mihélic, D. Bacquet, J. Zemmouri, and P. Szriftgiser, “Ultrahigh resolution spectral analysis based on a Brillouin fiber laser,” Opt. Lett. 35(3), 432–434 (2010). [CrossRef] [PubMed]

16.

A. Debut, S. Randoux, and J. Zemmouri, “Experimental and theoretical study of linewidth narrowing in Brillouin fiber ring lasers,” J. Opt. Soc. Am. B 18(4), 556–567 (2001). [CrossRef]

17.

P. C. Wait and T. P. Newson, “Measurement of Brillouin scattering coherence length as a function of pump power to determine Brillouin linewidth,” Opt. Commun. 117(1-2), 142–146 (1995). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(070.4790) Fourier optics and signal processing : Spectrum analysis
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(290.5900) Scattering : Scattering, stimulated Brillouin

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 17, 2010
Revised Manuscript: April 21, 2010
Manuscript Accepted: May 5, 2010
Published: July 1, 2010

Citation
P. Sevillano, J. Subías, C. Heras, J. Pelayo, and F. Villuendas, "Brillouin induced self-heterodyne technique for narrow line width measurement," Opt. Express 18, 15201-15206 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-14-15201


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References

  1. E. Ip, A. P. Lau, D. J. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express 16(2), 753–791 (2008). [CrossRef] [PubMed]
  2. J. Subías, C. Heras, J. Pelayo, and F. Villuendas, “All in fiber optical frequency metrology by selective Brillouin amplification of single peak in an optical comb,” Opt. Express 17(8), 6753–6758 (2009). [CrossRef] [PubMed]
  3. D. Guyomarc’h, G. Hagel, C. Zumsteg, and M. Knoop, “Some aspects of simulation and realization of an optical reference cavity,” Phys. Rev. A 80, 063820 (2009).
  4. Y. Yamamoto and T. Kimura, “Coherent optical fiber transmission system,” IEEE J. Quantum Electron. 17(6), 919–935 (1981). [CrossRef]
  5. T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noé, “Coherent optical communication: towards realtime systems at 40 Gbit/s and beyond,” Opt. Express 16(2), 866–872 (2008). [CrossRef] [PubMed]
  6. A. J. Lowery, “Amplified-spontaneous noise limit of optical OFDM lightwave systems,” Opt. Express 16(2), 860–865 (2008). [CrossRef] [PubMed]
  7. V. Ferrero and S. Camatel, “Optical Phase locking techniques: an overview and a novel method based on single side sub-carrier modulation,” Opt. Express 16(2), 818–828 (2008). [CrossRef] [PubMed]
  8. R. Hui and M. O'Sullivan, Fiber Optic Measurement Techniques (Academic Press, 2009), Chap. 2.
  9. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1-3), 180–186 (1998). [CrossRef]
  10. P. Horak and W. H. Loh, “On the delayed self-heterodyne interferometric technique for determining the linewidth of fiber lasers,” Opt. Express 14(9), 3923–3928 (2006). [CrossRef] [PubMed]
  11. J. Debeau, B. Kowalski, and R. Boittin, “Simple method for the complete characterization of an optical pulse,” Opt. Lett. 23(22), 1784–1786 (1998). [CrossRef]
  12. B. Szafraniec and D. M. Baney, “Swept Coherent Spectrum Analysis of the Complex Optical Field”, in 1st IEEE Conference on Lightwave Technologies in Instrumentation and Measurement, IEEE ed. (Palisades, NY, Oct. 2004) pp. 68–72.
  13. J. M. S. Domingo, J. Pelayo, F. Villuendas, C. D. Heras, and E. Pellejer, “Very High Resolution Optical Spectrometry by Stimulated Brillouin Scattering,” IEEE Photon. Technol. Lett. 17(4), 855–857 (2005). [CrossRef]
  14. http://www.aragonphotonics.com/index.php
  15. F. Mihélic, D. Bacquet, J. Zemmouri, and P. Szriftgiser, “Ultrahigh resolution spectral analysis based on a Brillouin fiber laser,” Opt. Lett. 35(3), 432–434 (2010). [CrossRef] [PubMed]
  16. A. Debut, S. Randoux, and J. Zemmouri, “Experimental and theoretical study of linewidth narrowing in Brillouin fiber ring lasers,” J. Opt. Soc. Am. B 18(4), 556–567 (2001). [CrossRef]
  17. P. C. Wait and T. P. Newson, “Measurement of Brillouin scattering coherence length as a function of pump power to determine Brillouin linewidth,” Opt. Commun. 117(1-2), 142–146 (1995). [CrossRef]

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