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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 10, Iss. 25 — Dec. 16, 2002
  • pp: 1503–1507
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Single-longitudinal-mode tunable WDM-channel-selectable fiber laser

Nathaniel J. C. Libatique, Li Wang, and Ravi K. Jain  »View Author Affiliations


Optics Express, Vol. 10, Issue 25, pp. 1503-1507 (2002)
http://dx.doi.org/10.1364/OE.10.001503


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Abstract

We report operation of a single-longitudinal-mode WDM-channel-selectable fiber laser. The use of a tunable fiber Bragg grating and a linewidth narrowing saturable absorption filter in conjunction with an intracavity etalon enabled single-frequency emission and discretely tunable WDM channel operation without the need for external wavelength locking modules. Side mode suppression ratios (SMSRs) > 50 dB have been demonstrated with ~ 3 dBm individual channel output powers for 8 channel (Δf = 50 GHz) operation of this WDM source.

© 2002 Optical Society of America

In this Letter, we report operation of a single longitudinal mode discretely tunable channel-selectable (Δf = 50 GHz) WDM laser. This is achieved by taking advantage of line narrowing intracavity filters based on saturable absorber fibers [9–10

9. M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, “Narrow-linewidth, singlemode erbium-doped fibre laser with intracavity wave mixing in saturable absorber,” Electron. Lett. 30, 648–649 (1994). [CrossRef]

] along with an intracavity glass etalon for channel pinning to the ITU grid [2

2. A. Bellemare, J.-F. Lemieux, M. Tetu, and S. Larochelle, “Er-doped fiber ring lasers step-tunable to exact multiples of 100 GHz (ITU-Grid) using periodic filters,” in Europ. Conf. on Opt. Commun. , (Madrid, Spain, 1998), 153–154.

]. Such narrow linewidth discretely tunable lasers have high potential for low noise WDM systems applications. In addition, the laser is constructed from readily available & off-the-shelf parts and components and as such are potentially manufacturable at relatively low set-up costs.

The fiber laser layout is based on a “hybrid” sigma-shaped fiber cavity, similar to that of Ref. [10

10. Y. W. Song, S. A. Havstad, D. Starodubov, Y. Xie, A. E. Willner, and J. Feinberg, “40-nm-wide tunable fiber ring laser with single-mode operation using a highly stretchable FBG,” IEEE Photon. Technol. Lett. , 13, 1167–1169 (2001). [CrossRef]

], obtained by coupling a ring cavity to a linear standing wave arm via a 3-port circulator, as depicted schematically in Fig. 1. A glass etalon (2 mm thick, 50 GHz free spectral range, 10 dB transmission peak-to-valley ratios) was inserted in the ring cavity to act as a frequency-periodic transmission filter referenced to the ITU-WDM 50 GHz grid. Specific channel tunability and mode selection were achieved by using a tunable fiber Bragg grating (3 dB linewidth < 0.05 nm) and a 5 m long Er:SiO2 saturable absorber (SA) fiber in the linear standing wave arm. A key feature of this linear arm is that the counterpropagating waves set up a spatial absorption modulation consisting of saturated (non-absorbing) and unsaturated (absorbing, lossy) regions along the entire length of the saturable absorber fiber, which can cause very large losses at neighboring longitudinal modes while allowing high transmission at the favored longitudinal mode to effectively narrow the linewidth of the FBG relatively efficiently. In order to have sufficient control of the intracavity power for optimized operation of this saturable absorber line narrowing filter, we placed two sections of 980 nm LD (laser diode)-pumped Er:SiO2 amplifiers in the ring cavity and adjusted the relative pump levels of these two sections appropriately as discussed below, to obtain single mode output operation while obtaining significant output power. Output coupling of laser power was achieved with the use of a 50/50 fused fiber coupler in the ring.

Fig. 1. Schematic of discretely tunable WDM laser source

Stepwise tuning of this wavelength-selectable WDM fiber laser is easily achieved by tuning the FBG, with output channel wavelengths determined by the Fabry-Perot etalon transmission peak spacing (Δf = 50 GHz). Nevertheless, achievement of single-mode operation of the laser source, at the target output power levels at these selected WDM channels, requires appropriate optimization of the saturable absorption. This was achieved by an appropriate choice of the length of the saturable absorber fiber and appropriate control of the pump powers for the two intracavity amplifiers (EDFAs). In our design we targeted an output power of ~ 3 dBm. The length of the Er:SiO2 saturable absorber fiber was then chosen such that the absorption saturation power Psat of the fiber was at least 3 dB lower than the 3 dBm target output power (Psat ≤ 0 dBm). Of the 3 fiber lengths available (Thorlabs EDF555: 1.5 m, 3.5 m, and 5 m lengths) that satisfied this condition, we chose the 5 m length, which had the highest saturated-to-unsaturated absorption loss differential of ~ 15 dB (vs. 8.5 dB & 4 dB for the 3.5 m & 1.5 m fibers respectively), as determined by absorption measurements at 1550 nm. Longer lengths of fiber can be used to increase this absorption differential (~30 dB estimated for ~10 m long fibers), however larger intracavity powers will be required to realize this. Further optimization of the saturable absorption was done by studying the output spectrum as a function of pump power with scanning Fabry-Perot interferometry (FPI) and self-heterodyne RF spectroscopy as described below.

Fig. 2. Laser output, as a function of LD#2 pump power Pp2, measured via Fabry-Perot interferometry with 20 MHz resolution (a) no saturable absorber (SA), Pp2 = 200 mW, highly multimode (b) with 5 m saturable absorber, Pp2 = 200 mW (c) with 5 m saturable absorber, Pp2 = 35 mW, stable singlemode output

Since the transmission linewidth (20 MHz) of the FPI was too large to resolve the laser mode spacing (~ 3 MHz), a self-heterodyne measurement (SHM) was used to confirm single frequency emission. Fig. 3a shows the self-heterodyne data when the saturable absorber was not spliced to the FBG and with Pp2 set to 200 mW. As expected (from the data in Fig. 2a) the emission in this case was multimode and extended over a broad spectral range (1 MHz – 400 MHz). With the saturable absorber connected to the FBG and for the same pump power (Pp2 = 200 mW), we observed (Fig. 3b, 1 MHz – 50 MHz range) significant narrowing (from ~ 400 MHz to 15 MHz) of the spectral range of the multimode emission. By comparing this data to that of Fig. 2b, it is clear that under these conditions, the laser has a tendency to mode hop from one longitudinal mode to another. As seen in Fig. 3c, single longitudinal mode operation was achieved for Pp2 set to 35 mW (as seen previously in Fig. 2c).

Fig. 3. Laser output, as a function of LD#2 pump power Pp2, measured via self-heterodyne measurements with 300 kHz resolution (a) no saturable absorber (SA), Pp2 = 200 mW, highly multimode (b) with 5 m saturable absorber, Pp2 = 200 mW, RF beating of 6 modes (c) with 5 m saturable absorber, Pp2 = 35 mW, stable singlemode output

Although not directly demonstrated in the self-heterodyne measurements described above, laser linewidths << 10 kHz are anticipated (due to the long laser cavity lengths [9

9. M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, “Narrow-linewidth, singlemode erbium-doped fibre laser with intracavity wave mixing in saturable absorber,” Electron. Lett. 30, 648–649 (1994). [CrossRef]

]) in the very near future.

Fig. 4. Tunable WDM outputs at 8 individually-selected channels (Δf = 50 GHz) 3.10 dBm ± 0.05 dBm outputs, > 50 dB SMSRs

References and Links

1.

T. Haber, K. Hsu, C. Miller, and Y. Bao, “Tunable EDF Laser precisely locked to the 50 GHz ITU frequency grid,” in Europ. Conf. on Opt. Commun., (Nice, France, 1999), Paper Mo B2.4.

2.

A. Bellemare, J.-F. Lemieux, M. Tetu, and S. Larochelle, “Er-doped fiber ring lasers step-tunable to exact multiples of 100 GHz (ITU-Grid) using periodic filters,” in Europ. Conf. on Opt. Commun. , (Madrid, Spain, 1998), 153–154.

3.

N. J. C. Libatique and R. K. Jain, “A Broadly Tunable Wavelength-Selectable WDM Source Using a Fiber Sagnac Loop Filter,” IEEE Photon. Technol. Lett. 13, 1283–1285 (2001). [CrossRef]

4.

M. Ibsen, S. Y. Set, G. S. Goh, and K. Kikuchi, “Broad-band continuously tunable all-fiber DFB lasers,” IEEE Photon. Technol. Lett. 14, 21–23 (2002). [CrossRef]

5.

D. M. Adams, C. Gamache, R. Finlay, M. Cyr, K. M. Burt, J. Evans, E. Jamroz, S. Wallace, I. Woods, L. Doran, P. Ayliffe, D. Goodchild, and C. Rogers, “Module packaged tunable laser and wavelength locker delivering 40 mW of fibre-coupled power on 34 channels,” Electron. Lett. 37, 691–693 (2001). [CrossRef]

6.

M. Mesh and Y. Weiss, “Device and method for monitoring and controlling laser wavelength,” Patent 6,233,262, ECI Telecom Ltd., May 15, 2001.

7.

G. H. Cross and E. E. Strachan, “Diode laser wavelength tracking using an integrated dual slab waveguide interferometer,” IEEE Photon. Technol. Lett. 14, 950–952 (2002). [CrossRef]

8.

M. A. Putnam, R.N. Brucato, M. A. Davis, D. G. Bellemore, and W. A. Helm, “Tunable optical structure featuring feedback control,” Patent # 6,310,990, Cidra Corporation, October 30, 2001.

9.

M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, “Narrow-linewidth, singlemode erbium-doped fibre laser with intracavity wave mixing in saturable absorber,” Electron. Lett. 30, 648–649 (1994). [CrossRef]

10.

Y. W. Song, S. A. Havstad, D. Starodubov, Y. Xie, A. E. Willner, and J. Feinberg, “40-nm-wide tunable fiber ring laser with single-mode operation using a highly stretchable FBG,” IEEE Photon. Technol. Lett. , 13, 1167–1169 (2001). [CrossRef]

11.

G. A. Ball and W. W. Morey, “Compression-tuned single-frequency Bragg grating fiber laser,” Opt. Lett. , 19, pp. 1979–1981 (1994). [CrossRef] [PubMed]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers
(140.3510) Lasers and laser optics : Lasers, fiber
(140.3570) Lasers and laser optics : Lasers, single-mode
(140.3600) Lasers and laser optics : Lasers, tunable

ToC Category:
Research Papers

History
Original Manuscript: November 11, 2002
Revised Manuscript: December 10, 2002
Published: December 16, 2002

Citation
Nathaniel Libatique, Li Wang, and Ravi Jain, "Single-longitudinal-mode tunable WDM-channel-selectable fiber laser," Opt. Express 10, 1503-1507 (2002)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-10-25-1503


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References

  1. T. Haber, K. Hsu, C. Miller, and Y. Bao, �??Tunable EDF Laser precisely locked to the 50 GHz ITU frequency grid,�?? in Europ. Conf. on Opt. Commun., (Nice, France, 1999), Paper Mo B2.4.
  2. A. Bellemare, J.-F. Lemieux, M. Tetu, and S. Larochelle, �??Er-doped fiber ring lasers step-tunable to exact multiples of 100 GHz (ITU-Grid) using periodic filters,�?? in Europ. Conf. on Opt. Commun., (Madrid, Spain, 1998), 153-154.
  3. N. J. C. Libatique and R. K. Jain, �??A Broadly Tunable Wavelength-Selectable WDM Source Using a Fiber Sagnac Loop Filter,�?? IEEE Photon. Technol. Lett. 13, 1283-1285 (2001). [CrossRef]
  4. M. Ibsen, S. Y. Set, G. S. Goh, and K. Kikuchi, �??Broad-band continuously tunable all-fiber DFB lasers,�?? IEEE Photon. Technol. Lett. 14, 21-23 (2002). [CrossRef]
  5. D. M. Adams, C. Gamache, R. Finlay, M. Cyr, K. M. Burt, J. Evans, E. Jamroz, S. Wallace, I. Woods, L. Doran, P. Ayliffe, D. Goodchild, and C. Rogers, �??Module packaged tunable laser and wavelength locker delivering 40 mW of fibre-coupled power on 34 channels,�?? Electron. Lett. 37, 691-693 (2001). [CrossRef]
  6. M. Mesh and Y. Weiss, �??Device and method for monitoring and controlling laser wavelength,�?? Patent 6,233,262, ECI Telecom Ltd., May 15, 2001.
  7. G. H. Cross and E. E. Strachan, �??Diode laser wavelength tracking using an integrated dual slab waveguide interferometer,�?? IEEE Photon. Technol. Lett. 14, 950-952 (2002). [CrossRef]
  8. M. A. Putnam, R.N. Brucato, M. A. Davis, D. G. Bellemore, and W. A. Helm, �??Tunable optical structure featuring feedback control,�?? Patent # 6,310,990, Cidra Corporation, October 30, 2001.
  9. M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, �??Narrow-linewidth, singlemode erbium-doped fibre laser with intracavity wave mixing in saturable absorber,�?? Electron. Lett. 30, 648-649 (1994). [CrossRef]
  10. Y. W. Song, S. A. Havstad, D. Starodubov, Y. Xie, A. E. Willner, and J. Feinberg, �??40-nm-wide tunable fiber ring laser with single-mode operation using a highly stretchable FBG,�?? IEEE Photon. Technol. Lett. 13, 1167- 1169 (2001). [CrossRef]
  11. G. A. Ball and W. W. Morey, �??Compression-tuned single-frequency Bragg grating fiber laser,�?? Opt. Lett. 19, 1979-1981 (1994). [CrossRef] [PubMed]

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