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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 20 — Sep. 28, 2009
  • pp: 18364–18370
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Deposition of carbon nanotubes around microfiber via evanascent light

Ken Kashiwagi and Shinji Yamashita  »View Author Affiliations


Optics Express, Vol. 17, Issue 20, pp. 18364-18370 (2009)
http://dx.doi.org/10.1364/OE.17.018364


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Abstract

Optical devices based on carbon nanotubes (CNTs) have been realized with several fabrication methods in different structures, such as free-space, fiber-end, waveguide, and fiber structures. Most of waveguide- and fiber-type devices utilize evanescent coupling between the guided light and CNT layers, and offer very high optical damage threshold and high third-order nonlinearity. However, the conventional fabrication methods require complicated processes and waste much of CNTs. In this work, we propose and demonstrate CNT deposition around microfibers induced by injecting light through the fibers. This method can area-selectively deposit desired number of CNTs around microfibers, and can be realized by a simple process and setup. We also demonstrate a passively mode-locked fiber laser using a CNT-deposited microfiber as a passive mode-locker.

© 2009 OSA

1. Introduction

In another aspect, thermal stability of the CNTs is very attractive to realize high power passively mode-locked fiber laser. Sprayed CNTs without any coating have low thermal transfer efficiency because air is filled among the CNTs. Heat due to light absorption elevates the CNTs temperature and damage or burn the CNTs. Therefore, high thermal resistance materials should be used to support CNTs and heat transfer design should be considered to realize high damage threshold saturable absorber. To achieve high damage threshold saturable abosorber, polycarbonate was used as a high thermal resistance material for embedding CNTs [10

10. F. Shohda, T. Shirato, M. Nakazawa, K. Komatsu, and T. Kaino, “A passively mode-locked femtosecond soliton fiber laser at 1.5 microm with a CNT-doped polycarbonate saturable absorber,” Opt. Express 16(26), 21191–21198 (2008). [CrossRef] [PubMed]

]. Nanoporous alumina was used as a heat sink to transfer the heat from CNTs [11

11. Y. Kurashima, Y. Yokota, I. Miyamoto, H. Kataura, and Y. Sakakibara, “Mode-locking nanoporous alumina membrane embedded with carbon nanotube saturable absorber,” Appl. Phys. Lett. 94(22), 223102 (2009). [CrossRef]

]. The other approach is to employ the evanescent coupling structure. The evanescent coupling structure device utilizes interaction between evanescent field of guided wave and CNTs located near the guiding structure. Effective interacting power is small, and the damage threshold of the device is high.

Recently, we proposed and demonstrated a method of CNT deposition onto optical fiber ends [12

12. K. Kashiwagi, S. Yamashita, H. Yaguchi, C. S. Goh, and S. Y. Set, “Novel Cost Effective Carbon Nanotubes Deposition Technique Using Optical Tweezer Effect,” in Proceedings of the SPIE Vol. 6478; Photonics Packaging, Integration and Interconnects VII, A. M. Earman and R. T. Chen, eds., pp. 6478–15 (SPIE, 2007).

,13

13. K. Kashiwagi, S. Yamashita, and S. Y. Set, “Optically manipulated deposition of carbon nanotubes onto optical fiber end,” Jpn. J. Appl. Phys. 46(40), L988–L990 (2007). [CrossRef]

]. We deposited CNTs only onto a core region of an optical fiber end by injecting a light beam from the end into a CNT-dispersed dimethylformamid (DMF) solution. Independent on our proposal, another group demonstrated optically driven CNT deposition [14

14. J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). [CrossRef] [PubMed]

]. In this report, they used ethanol for a solution. Using either of the method, the CNT deposited fibers had sufficient performance as saturable absorbers at 1.55 μm wavelength range. In the report [14

14. J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). [CrossRef] [PubMed]

], they reported that the technique can be also applied at 1 μm wavelength range and reported passively mode-locked fiber laser at 1.07 μm. Additional to the system, we employed optical reflectormetry into the system, and realized in situ monitoring of CNT layer so that the layer could be more uniform and the deposition system obtained higher repeatability [15

15. K. Kashiwagi, S. Yamashita, and S. Y. Set, “In-situ monitoring of optical deposition of carbon nanotubes onto fiber end,” Opt. Express 17(7), 5711–5715 (2009). [CrossRef] [PubMed]

]. However, the technique should be applicable not only to the deposition onto the fiber ends, but also along the core regions without cladding or with thin cladding via evanescent light.

In this work, we propose and demonstrate optically induced deposition of CNTs around microfibers. The deposition of CNTs around a microfiber was achieved only by injection light through the microfiber, which was immersed in a CNT-dispersed DMF droplet. The number of deposited CNTs was able to be controlled by adjusting the deposition time. We also demonstrate a passively mode-locked fiber laser using a CNT-deposited microfiber fabricated by the technique.

2. Deposition of carbon nanotubes around microfibers

First, we fabricated a microfiber using an experimental setup shown in Fig. 1
Fig. 1 Experimental setup for fabricating microfibers.
. A bare standard single mode fiber (SMF) was set on two fiber holders, which were fixed on two translation stages. The fiber ends were connected to an erbium doped fiber amplifier (EDFA) and a power meter, respectively. We fabricated a microfiber by stretching the fiber with heat produced by a flame. During the fabrication process, we monitored the insertion loss of the fiber using the power meter. The fiber was tapered down so that its taper waist diameter became ~6 μm.

Next, we deposited the CNTs around the microfiber using a setup shown in Fig. 2
Fig. 2 Experimental setup for CNT deposition around microfibers by light injection through the microfibers.
. The fabricated microfiber was immersed into a CNT-dispersed DMF droplet on a slide glass. Light from a laser diode at a wavelength of 1560 nm and at −10 dBm optical power was amplified up to 13 dBm by an EDFA and consequently injected into the microfiber. The output power was monitored by a power meter to detect start of CNT deposition, and consequently to control the deposition time. We used CNTs fabricated by high pressure carbon mono-oxide (HiPco) method. Absorption spectrum of the CNTs, which was deposited onto a quartz substrate by spray method, is shown in Fig. 3
Fig. 3 Absorption spectrum of the CNTs.
. The CNTs had three absorption peaks at around 1175, 1325, 1425 nm. The absorption band included the 1.55μm wavelength range and the saturable absorption can be expected at the wavelength for passive mode-locker.

Figure 4 (a)
Fig. 4 (a) Microscope image of CNT-deposited microfiber (waist ~6 μm), dotted circle indicates area where microscopic Raman spectrum was measured, (b) Microscopic Raman spectrum of CNT-deposited microfiber.
shows a microscope image of a CNT-deposited microfiber, whose waist diameter was ~6 μm. We found that the CNTs start to be deposited around the microfiber at the incident power of 13 dBm. To ensure the existence of CNTs, we performed microscopic Raman spectroscopy. The dotted circle in Fig. 4(a) was the area where microscopic Raman spectroscopy was performed. There are three major peaks in the Raman spectra of CNTs, G-band, D-band and radial breathing mode (RBM). RBM, which appears at around 250 cm−1, is the most characteristic peak of CNTs, and is a vibration mode of CNTs in their radial direction. The microscopic Raman spectrum in Fig. 4(b) confirms that the CNTs were certainly deposited around the microfiber. The optical deposition technique was confirmed to be applicable not only to deposition onto fiber ends but also to deposition around microfibers. We could detect the start of the CNT deposition by the drop of the output power due to scattering and absorption induced by the deposited CNTs. We stopped the light injection about 5 sec after the deposition started. The excess loss induced by the tapering was 0.2 dB. The CNT deposition increased insertion loss by 5.8 dB, and the total loss was 6 dB.

CNTs were deposited around the fiber in ~30 μm length as shown in Fig. 4(b), and the length was enough to achieve saturable absorption for passive mode-locking. Figure 5
Fig. 5 Saturable absorption characteristics of the CNT-deposited microfiber.
shows saturable absorption characteristics of the CNT deposited microfiber. The experimental setup was almost same as Fig. 2, except the DMF droplet on the slide glass. We changed the output power of the EDFA and measured the transmitted power by the power meter. The insertion loss decreased with increase of the input power, indicating that the absorption had been saturated. The loss decrease due to the saturation was over 1.2% and the value was sufficient for passive mode-locker.

We also measured polarization dependent loss (PDL) of the microfiber. It had small PDL less than 0.3 dB with input power both at 10 dBm (weaker power than saturable absorption threshold) and 17 dBm (higher power than saturable absorption threshold). These results indicate the CNTs uniformly were deposited around the fiber and canceled the polarization dependence of the each CNTs which had cylinder like structure. When we deposit the CNTs longer and thicker around the fiber, we could achieve higher absorption depth, but the insertion loss of the fiber was increased.

We did not observed any damages of the CNTs or the microfiber even we injected the high power light over 17 dBm. The damage threshold depends on the microfiber waist diameter. When the waist diameter of the microfiber was shorter than ~3 μm, the heat transfer from the CNTs were not enough and CNTs were damaged or the microfiber was broken by the heat caused by the light absorption.

We presume the similar phenomenon occurred both in the case of optical CNT deposition onto fiber ends and around microfibers. In the case of the CNT deposition onto fiber ends, swirl and convection caused by light injection to a CNT-dispersed solution might deposit CNTs (Fig. 6(a)
Fig. 6 Schematic of optical deposition of CNTs (a) onto fiber end (b) around microfiber.
). Under a microscope, we could see the convection when we started to incident the light into the solution and bundled CNTs were carried closer to the fiber end. On the contrary, in the present experiment, the light injection from air into the droplet thermally caused swirl and convection at the boundary (Fig. 6(b)). This convection could be seen under the microscope. The swirl and convection induced by the light injection might deposit the CNTs around the microfibers. This phenomenon occurs only at the light input side boundary, and enables us to area-selectively deposit them onto desired position.

Another possible mechanism is the optical tweezer effect, which can traps micro- and nano-sized objects by the optical intensity diversion in the solution. For the nano-sized objects, the objects can be treated as a point dipole. In electromagnetic field, Lorentz force is applied onto the object. Under optical power divergence or inhomogeneous electromagnetic field, the Lorentz force is stronger toward the higher optical intensity direction than the lower optical intensity diversion. The force, therefore, tends to trap the object at the region of strongest position. Evanescent field had the optical intensity diversion, and mode field mismatch between the air section and the DMF section caused the optical intensity diversion by scattering. The diversion might trap the CNTs by optical tweezer and deposited onto the microfiber.

However, the force of the optical tweezer might be too weak to trap bundled or entangled CNTs, since the optical power gradient of the evanescent field is not steep. We assume combination of the two mechanism described above deposited the CNTs around the core. The swirl and the convection carried the CNTs to the microfiber, and optical tweezer effect, consequently, trapped part of the CNTs which was closely flowed to the microfiber by the swirl and the convection.

3. Passive mode-locking of fiber laser incorporating CNT-deposited microfiber

We controlled the polarization state inside the laser cavity and gain of the EDFA. With the increase of injection current of the pump laser diode (LD) of the EDFA, the laser started to oscillate in passive mode-locking regime with the LD current at the 1307mA. Further increase of the LD current made the laser stable, and the shortest pulse width was achieved at 1419 mA. The average power output of the EDFA was 18.8 dBm, and the average power output of the laser was −1.8 dBm. Since the EDFA contained long fiber, laser cavity was long and the laser output was slightly unstable. When we inserted 10 to 30-m-long single mode fiber to manage the round trip dispersion of the laser cavity or raised the EDFA output, the laser started to harmonically oscillate, and soliton pulse output could not be achieved.

Figure 8
Fig. 8 Passively mode-locked fiber laser output (a) Optical spectrum of the fiber laser (resolution: 0.1 nm) (b) Autocorrelation trace of laser output (resolution: 50 fs,) corresponding pulse width: 1.14 ps (assuming Gaussian pulse) (c) Output pulse train of the fiber laser (repetition rate: 1.54 MHz)
shows the optical spectrum, the autocorrelation trace and the pulse train of the fiber laser output. The data was measured when we achieved the shortest pulse width and the highest pulse power. The laser had the center wavelength at 1565 nm and the 3-dB bandwidth of 3.7 nm. The full width half maximum (FWHM) of the autocorrelation trace was 1.61 ps, which correspond to inferred pulse duration of 1.14 ps (assuming Gaussian pulse profile). The time bandwidth product (TBP) of the pulse was 0.528. Comparing to an unchirped transform-limited value of Gaussian pulse (0.441), the result indicates the pulse had chirp. The chirp was due to the residual dispersion in the laser cavity. The repetition rate of the fiber laser was 1.54 MHz according to the time interval of the output pulse train. After the experiment, we measured the insertion loss of the microfiber and confirmed the loss did not change before and after the experiment. We expect that we can achieve much shorter and higher intensity pulse output with high stability by employing highly Er3+-doped EDF and designing the laser cavity parameters, such as length and round trip dispersion.

4. Conclusions

In this study, we proposed and demonstrated optically manipulated CNT-deposition around a microfiber. First, we fabricated a microfiber by stretching a bare SMF with heat. Deposition of CNTs around microfiber was achieved only by injection light into the microfiber. During the deposition, we performed in situ monitoring of the transmitted power to control the number of deposited CNTs. We achieved the CNT deposited microfiber with total insertion loss of 6 dB. The existence of CNTs was ensured by microscopic Raman spectroscopy. The microfiber had over 1.2% of saturable absorption depth at 17 dBm input power. The microfiber was used as a saturable absorber in a fiber ring laser, and provided passive mode-locking. The laser output had the bandwidth of 3.7 nm and the pulse duration of 1.14 ps at 1.54 MHz repetition rate.

In the whole experiment, we required only few drops of the CNT-dispersed DMF and CNT dissipation was small. Although the number of used CNTs was small, the CNTs were on the surface of the microfiber, and the CNT-layer and the evanescent field was effectively coupled with each other. Therefore, we were able to realize sufficient saturable absorption depth and passive mode-locking of the fiber laser. Our proposed technique possesses large advantages of the efficiency of CNT use and the simplicity of the device fabrication process compared to the alternative fabrication techniques. We believe our technique contribute to development of ultrafast photonic devices based on CNTs.

Acknowledgements

This work was supported by Strategic Information and Communications R&D Promotion Programme (SCOPE) of The Ministry of Internal Affairs and Communications (MIC), Japan.

References and links

1.

Y.-C. Chen, N. Raravikar, L. Schadler, P. Ajayan, Y.-P. Zhao, T.-M. Lu, G.-C. Wang, and X.-C. Zhang, “Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm,” Appl. Phys. Lett. 81(6), 975–977 (2002). [CrossRef]

2.

S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Laser Mode Locking Using a Saturable Absorber Incorporating Carbon Nanotubes,” J. Lightwave Technol. 22(1), 51–56 (2004). [CrossRef]

3.

Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: Applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys. 44(No. 4A), 1621–1625 (2005). [CrossRef]

4.

T. R. Schibli, K. Minoshima, H. Kataura, E. Itoga, N. Minami, S. Kazaoui, K. Miyashita, M. Tokumoto, and Y. Sakakibara, “Ultrashort pulse-generation by saturable absorber mirrors based on polymer-embedded carbon nanotubes,” Opt. Express 13(20), 8025–8031 (2005). [CrossRef] [PubMed]

5.

K. Kashiwagi, S. Yamashita, Y. Nasu, H. Yaguchi, C. S. Goh, and S. Y. Set, “Planar waveguide-type saturable absorber based on carbon nanotubes,” Appl. Phys. Lett. 89(8), 081125 (2006). [CrossRef]

6.

Y.-W. Song, S. Yamashita, C. S. Goh, and S. Y. Set, “Carbon nanotube mode lockers with enhanced nonlinearity via evanescent field interaction in D-shaped fibers,” Opt. Lett. 32(2), 148–150 (2007). [CrossRef]

7.

Y.-W. Song, K. Morimune, S. Y. Set, and S. Yamashita, “Polarization insensitive all-fiber mode-lockers functioned by carbon nanotubes deposited onto tapered fibers,” Appl. Phys. Lett. 90(2), 021101 (2007). [CrossRef]

8.

K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]

9.

Y.-W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]

10.

F. Shohda, T. Shirato, M. Nakazawa, K. Komatsu, and T. Kaino, “A passively mode-locked femtosecond soliton fiber laser at 1.5 microm with a CNT-doped polycarbonate saturable absorber,” Opt. Express 16(26), 21191–21198 (2008). [CrossRef] [PubMed]

11.

Y. Kurashima, Y. Yokota, I. Miyamoto, H. Kataura, and Y. Sakakibara, “Mode-locking nanoporous alumina membrane embedded with carbon nanotube saturable absorber,” Appl. Phys. Lett. 94(22), 223102 (2009). [CrossRef]

12.

K. Kashiwagi, S. Yamashita, H. Yaguchi, C. S. Goh, and S. Y. Set, “Novel Cost Effective Carbon Nanotubes Deposition Technique Using Optical Tweezer Effect,” in Proceedings of the SPIE Vol. 6478; Photonics Packaging, Integration and Interconnects VII, A. M. Earman and R. T. Chen, eds., pp. 6478–15 (SPIE, 2007).

13.

K. Kashiwagi, S. Yamashita, and S. Y. Set, “Optically manipulated deposition of carbon nanotubes onto optical fiber end,” Jpn. J. Appl. Phys. 46(40), L988–L990 (2007). [CrossRef]

14.

J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). [CrossRef] [PubMed]

15.

K. Kashiwagi, S. Yamashita, and S. Y. Set, “In-situ monitoring of optical deposition of carbon nanotubes onto fiber end,” Opt. Express 17(7), 5711–5715 (2009). [CrossRef] [PubMed]

OCIS Codes
(140.4050) Lasers and laser optics : Mode-locked lasers
(160.4330) Materials : Nonlinear optical materials
(220.4610) Optical design and fabrication : Optical fabrication
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 7, 2009
Revised Manuscript: September 20, 2009
Manuscript Accepted: September 22, 2009
Published: September 25, 2009

Citation
Ken Kashiwagi and Shinji Yamashita, "Deposition of carbon nanotubes around microfiber via evanascent light," Opt. Express 17, 18364-18370 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-20-18364


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References

  1. Y.-C. Chen, N. Raravikar, L. Schadler, P. Ajayan, Y.-P. Zhao, T.-M. Lu, G.-C. Wang, and X.-C. Zhang, “Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm,” Appl. Phys. Lett. 81(6), 975–977 (2002). [CrossRef]
  2. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Laser Mode Locking Using a Saturable Absorber Incorporating Carbon Nanotubes,” J. Lightwave Technol. 22(1), 51–56 (2004). [CrossRef]
  3. Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: Applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys. 44(No. 4A), 1621–1625 (2005). [CrossRef]
  4. T. R. Schibli, K. Minoshima, H. Kataura, E. Itoga, N. Minami, S. Kazaoui, K. Miyashita, M. Tokumoto, and Y. Sakakibara, “Ultrashort pulse-generation by saturable absorber mirrors based on polymer-embedded carbon nanotubes,” Opt. Express 13(20), 8025–8031 (2005). [CrossRef] [PubMed]
  5. K. Kashiwagi, S. Yamashita, Y. Nasu, H. Yaguchi, C. S. Goh, and S. Y. Set, “Planar waveguide-type saturable absorber based on carbon nanotubes,” Appl. Phys. Lett. 89(8), 081125 (2006). [CrossRef]
  6. Y.-W. Song, S. Yamashita, C. S. Goh, and S. Y. Set, “Carbon nanotube mode lockers with enhanced nonlinearity via evanescent field interaction in D-shaped fibers,” Opt. Lett. 32(2), 148–150 (2007). [CrossRef]
  7. Y.-W. Song, K. Morimune, S. Y. Set, and S. Yamashita, “Polarization insensitive all-fiber mode-lockers functioned by carbon nanotubes deposited onto tapered fibers,” Appl. Phys. Lett. 90(2), 021101 (2007). [CrossRef]
  8. K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]
  9. Y.-W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]
  10. F. Shohda, T. Shirato, M. Nakazawa, K. Komatsu, and T. Kaino, “A passively mode-locked femtosecond soliton fiber laser at 1.5 microm with a CNT-doped polycarbonate saturable absorber,” Opt. Express 16(26), 21191–21198 (2008). [CrossRef] [PubMed]
  11. Y. Kurashima, Y. Yokota, I. Miyamoto, H. Kataura, and Y. Sakakibara, “Mode-locking nanoporous alumina membrane embedded with carbon nanotube saturable absorber,” Appl. Phys. Lett. 94(22), 223102 (2009). [CrossRef]
  12. K. Kashiwagi, S. Yamashita, H. Yaguchi, C. S. Goh, and S. Y. Set, “Novel Cost Effective Carbon Nanotubes Deposition Technique Using Optical Tweezer Effect,” in Proceedings of the SPIE Vol. 6478; Photonics Packaging, Integration and Interconnects VII, A. M. Earman and R. T. Chen, eds., pp. 6478–15 (SPIE, 2007).
  13. K. Kashiwagi, S. Yamashita, and S. Y. Set, “Optically manipulated deposition of carbon nanotubes onto optical fiber end,” Jpn. J. Appl. Phys. 46(40), L988–L990 (2007). [CrossRef]
  14. J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). [CrossRef] [PubMed]
  15. K. Kashiwagi, S. Yamashita, and S. Y. Set, “In-situ monitoring of optical deposition of carbon nanotubes onto fiber end,” Opt. Express 17(7), 5711–5715 (2009). [CrossRef] [PubMed]

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