- Email: [email protected]
Yb codoped fiber and highly-nonlinear optical fiber
Optics Communications 266 (2006) 681–685 www.elsevier.com/locate/optcom
Experimental investigation of continuous-wave supercontinuum ring laser composed of clad-pumped Er/Yb codoped fiber and highly-nonlinear optical fiber Ju Han Lee
*,1,
Kazuhiro Katoh, Kazuro Kikuchi
Research Center for Advanced Science and Technology (RCAST), University of Tokyo, 4-6-1 Komaba, Meguro-Ku, Tokyo 153-8904, Japan Received 20 December 2005; received in revised form 2 May 2006; accepted 16 May 2006
Abstract We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470 nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.72.Ai; 42.65.k; 42.81.i; 42.55.Wd; 76.30.Kg Keywords: Supercontinuum; Optical fiber; Nonlinear optics; Optical fiber lasers; Erbium/Ytterbium
1. Introduction Optical fiber-based supercontinuum (SC) light sources have attracted huge attention as one of very promising photonic devices in recent years and are being used increasingly in various practical and commercial applications: for example, optical component characterization, optical coherence tomography (OCT) [1], metrology [2], and optical sensing. Typical supercontinuum sources have been based on a high-power pump laser whose output beam is injected into a nonlinear fiber, where the supercontinuum is generated [3]. Unlike the conventional schemes using a pulse laser *
Corresponding author. Tel.: +81 3 5452 5122; fax: +81 3 5452 5125. E-mail address: [email protected] (J.H. Lee). 1 Also with Photonics Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.05.040
as a pump beam, there was a novel proposition by Prabu et al. that a high power continuous-wave (CW) laser could also be used as a pump beam for SC generation [4]. In the CW SC case, output average power is usually extremely high (watt level) due to its tight requirement of high pump power for generation of nonlinear effects in optical fiber. Further investigation by various groups found that the physical mechanism for CW SC evolution in optical fiber is different from the pulse-mode SC evolution [5–7]. Both modulation instability (MI) and stimulated Raman scattering (SRS) were found to play key roles in transforming a narrow-band CW laser beam into a broadband spectral continuum [7]. It was also found that the center wavelength of a pump beam is also a key parameter to be considered since it is better to locate it at the anomalous dispersion regime near a zero dispersion wavelength of a HNL-DSF for efficient initiation of the MI process [8].
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Recently, we proposed a novel scheme of CW self-contained supercontinuum fiber ring laser that incorporated an erbium-doped fiber and a HNL-DSF inside a cavity [9]. Unlike previous CW SC schemes based on single propagation of a high power, CW laser beam through a highly nonlinear optical fiber, erbium gain inside a ring cavity converted a 1480-nm Raman pump beam into a seed light oscillation at 1562 nm and the oscillated light subsequently evolved into a SC of 250 nm with a relatively low output power by nonlinear effects in a HNL-DSF. Because of its relative simplicity, small size, and moderate level of output power, such a device might be more appropriate than conventional pulse-mode SC and watt-level CW SC sources for applications such as OCT and optical sensing. Note that watt-level CW SCs would be of no use for OCT applications because of critical damage to biological tissue unless an ultra-broadband optical attenuator is employed at the output end [10]. In particular, this is true for in vivo imaging of human retina. Another potential advantage of the SC fiber ring laser is that it does not require an expensive, high power, single mode laser whose center wavelength is close to a zero dispersion wavelength of a HNL-DSF as a pump beam. Owing to the photon conversion process from a low quality pump beam to a high quality seed light oscillation through rare earth-doped materials inside a cavity, low-cost multimode pump laser diodes (LDs) can readily be used for pumping the SC laser. It should be noticed that the high power pump lasers used for conventional CW SC schemes are either a complicated, cascaded Raman fiber laser [7,11] or a combined structure of a master laser and a rare earthdoped fiber-based power amplifier [12]. In this paper, we experimentally demonstrate a practical benefit of our proposed CW SC generating ring laser scheme in terms of low-cost device implementation and investigate its output intensity noise property relative to that of amplified spontaneous emission (ASE) light, as a continuing study from our previous paper of Ref. [9] where the basic concept of a SC ring laser was first demonstrated. First, we experimentally implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a HNL-DSF. The basic SC laser configuration used in this study is similar to that of Ref. [9]; however, this time we construct a ring cavity by use of a double clad Er/Yb fiber and low quality multimode pump LDs to underline the fact that our proposed scheme can be readily pumped by a low-cost 975-nm multimode beam without requiring any high power single-mode laser centered near a zero dispersion wavelength of a HNL-DSF. Using such a simple scheme a SC over a spectral bandwidth from 1280 nm to 1750 nm is readily achieved. Next, we investigate noise characteristics of the generated SC by measuring its relative-intensity-noise (RIN) and by comparing it with that of a pumped Er/Yb fiber ASE. A significantly high RIN level relative to that of the ASE is found and some discussions are presented regarding its origin.
2. Experimental results An experimental schematic for our SC laser is shown in Fig. 1(a). The commercially available, 8-m-long double clad Er/Yb fiber inside the cavity had a peak core erbium absorption of 35 dB/m at 1535 nm and a clad ytterbium absorption of 5 dB/m at 975 nm. The core NA was 0.17 and the cladding NA was 0.46. A 3-km-long HNL-DSF with a nonlinearity parameter c of 15.5 W/km was inserted into the cavity. The zero dispersion wavelength of the HNL-DSF was 1554 nm and the dispersion slope was 0.027 ps/nm2/km. In order to induce MI in the beginning stage of CW SC evolution, the zero dispersion wavelength of the HNL-DSF should be smaller than the peak wavelength of the seed light oscillation. We thus chose a HNL-DSF with a zero dispersion wavelength of 1554 nm as a proper nonlinear medium for our CW SC laser configuration considering the seed light oscillation wavelength of 1568 nm. Differently from the configuration in our preliminary experiment of Ref. [9] we chose to use 3-km length of HNL-DSF for this configuration to increase efficiency of the nonlinear process in the HNL-DSF due to insufficient optical power of the seed light oscillation. The fiber propagation loss was 1.3 dB/km. As an Er/Yb fiber pump source, we used two commercially available, multimode semiconductor LDs with a maximum output power of 4 W and the pump beams were coupled into the cavity by a passive pump/signal combiner. The laser output power was extracted from the ring cavity using an 80:20 fiber coupler, with which 80% power of the oscillated light was fed back into the Er/Yb fiber. An optical isolator was placed after the coupler within the cavity to ensure the directional light oscillation. Both the 80/20 coupler and the isolator had an operating wavelength band of 1520– 1580 nm. First, we measured output spectrum evolution as a function of the pump power of the SC laser, and the results are shown in the 3-dimensional (3D) plot of Fig. 2(a). Note that the displayed dark area at low power levels in the spectral range from 1700 nm to 1750 nm in Fig. 2(a) shows the receiver noise of our optical spectrum analyzer (OSA). A seed light oscillation with a broad linewidth was observed
975 nm Multimode Laser Diodes 975/1550 nm Pump Combiner
Isolator Output
8 m Er/Yb Codoped Double Clad Fiber
80% 20% Coupler 3 km HNL-DSF λ 0 = 1554 nm
Fig. 1. Experimental schematic for our Er/Yb fiber-based SC laser.
J.H. Lee et al. / Optics Communications 266 (2006) 681–685
(a)
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was clearly obtained at a pump power larger than 4.18 W. The measured 30-dB spectral bandwidth was larger than 470 nm. Note that the spectral bandwidth measurement was limited by the maximum measurable wavelength (1750 nm) of our OSA. The SC output included the transmission profiles of both the coupler and the isolator in some degree due to their non-flat responses over such an ultra-wide spectral band. The degree of polarization for the SC output was measured to be less than 3%, which is considered to be an almost depolarized level. In order to further investigate the output polarization property we inserted a polarization controller inside the ring cavity and observed the effect of polarization change of the seed light oscillation on the SC output by adjusting it. Neither significant output power change nor wavelength shift was found. For relative optical property comparison, ASE was generated from the pumped Er/Yb codoped fiber by removing the HNL-DSF as well as breaking the ring structure. In this case an isolator was located after the Er/Yb fiber to suppress undesirable internal lasing. A typical ASE spectrum of a pumped Er/Yb codoped fiber was obtained as expected. The significant difference of bandwidth between
150 Er/Yb Fiber SC Laser Er/Yb Fiber ASE
(b) Fig. 2. (a) Three-dimensional plot of measured spectral evolution of the optical output versus the pump power for the Er/Yb fiber-based SC laser. (b) Measured output spectra of both the Er/Yb fiber-based SC laser and the Er/Yb fiber ASE at a maximum pump power of 5.5 W.
Output Power (mW)
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at 1568 nm. The second spectral peak started growing at 1608 nm and its spectral separation from the seed light oscillation became larger with the increase in the pump power. The initial double-peak generation can be explained by both mode-hopping induced multimode operation of a long cavity fiber ring laser [13] and nonlinear effect induced suppression of cross gain saturation in erbium ions within a laser cavity that incorporated long length of HNL-DSF [14]. At a pump level of 0.73 W the third peak and the small fourth peak were observed due to four-wave mixing effect between the seed light oscillation and the second peak. Further increase in the pump power led to noisy Raman soliton formation due to MI and these solitons experienced self frequency shift owing to SRS [15]. A strong first-order Raman Stokes that was pumped by the second spectral peak, was generated 1730 nm with a 2.94 W pump power. Then, the output spectrum evolved into a spectral continuum at a 4.18-W pump level. High quality of a CW SC
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Fig. 3. (a) Measured output power versus pump power and (b) measured relative intensity noise (RIN) for both the Er/Yb fiber-based SC laser and the Er/Yb fiber ASE.
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the SC laser output and the ASE is clearly evident in the output optical spectra of Fig. 2(b), which were measured at a maximum pump power of 5.5 W. Second, we measured the output optical power versus pump power for both cases, i.e. the SC laser and the Er/Yb fiber ASE. The results are shown in Fig. 3(a). The SC laser exhibited abrupt decrease of the output power at a pump power of 4.18 W that corresponded to the point at which a spectral continuum was obtained, whilst the Er/Yb fiber ASE showed substantial increase of output power at a pump power larger than 2.94 W due to undesirable internal lasing. The maximum output power for the SC laser was 53.4 mW at a 5.5-W pump power. Third, we measured RIN of both output beams. The outputs were passed through a 15-nm bandpass filter centered at 1570 nm, and subsequently coupled onto a lownoise photodetector with a bandwidth of 150 MHz. The detected electrical signals were then ac-coupled into an electrical spectrum analyzer. Fig. 3(b) shows the measured RIN spectra. Interestingly, the SC laser output was found to have a much higher RIN level than the Er/Yb fiber ASE by 25 dB. The physical origin of such a high RIN in the SC laser needs to be further investigated in detail
although it is believed to be due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process [3,16]. According to previous detailed studies on coherence degradation and the corresponding nonlinear noise amplification process [3,16], higher order soliton evolution over background random noise that is caused by input signal noise or/and Raman scattering in an anomalous dispersion optical fiber, inevitably leads to a series of phenomena of MI-induced noise amplification, soliton pulse distortion, and temporal coherence degradation. Even though the previous studies were performed for the case of pulse-mode SC evolution, the findings can also be used for the explanation of the significant output RIN mechanism of our CW SC case due to the fact that the CW SC evolution in our SC laser or most of the demonstrated CW SC generating schemes is based on noisy Raman solitonic structure formation initiated by MI. Finally, we investigated the long-term stability of the output spectrum generated from our SC laser. The output spectrum was recorded every 1 h for 4 h whilst the pump power was set to be a maximum value of 5.5 W. Fig. 4(a) shows the measured output spectra and their close-up views are shown in Fig. 4(b). The resolution bandwidth of the OSA was 0.2 nm. No significant spectral fluctuations were observed although a linear increase of the output spectral power existed with a rate of 0.025 dB/h owing to temperature dependence of our fabricated pump LD driver circuit. It is clearly evident from this measurement that a stable SC laser was readily achieved. 3. Conclusion
(a)
(b) Fig. 4. (a) Measured optical spectra of the supercontinuum laser output recorded every 1 h for 4 h at a maximum pump power of 5.5 W. (b) Their close-up views.
We have experimentally performed a further study on our proposed CW SC ring laser scheme. This time, a clad pumped Er/Yb fiber-based SC laser was constructed for the demonstration of a practical benefit in terms of lowcost, compact, and self-contained device implementation. Using a simple fiber ring laser structure incorporating an Er/Yb fiber and a HNL-DSF, we readily achieved a stable SC laser output with a spectral bandwidth larger than 470 nm. A relatively low output power of 53.4 mW compared to that of conventional single pass CW SC sources was obtained; however, the power level is believed to be high enough for OCT or optical sensing applications. Relative-intensity-noise (RIN) property of the generated SC is then measured and compared with that of a pumped Er/Yb fiber amplified spontaneous emission (ASE) and the SC laser is found to have a much higher RIN than the fiber ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process. Further investigation is required to fully understand the high RIN mechanism and to figure out a proper method for the suppression of the excess intensity noise. Although the excess intensity noise problem still needs to be sorted out, we believe that such a simple, low-cost, and ultra-broad band SC source should find a
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range of applications in practical and commercial photonic systems such as OCT and optical sensing sensors. References [1] I. Hartl, X.D. Li, C. Chudoba, R.K. Ghanta, T.H. Ko, J.G. Fujimoto, J.K. Ranka, R.S. Windeler, Opt. Lett. 26 (2001) 608. [2] K. Kim, B.R. Washburn, G. Wilpers, C.W. Oates, L. Hollberg, N.R. Newbury, S.A. Diddams, J.W. Nicholson, M.F. Yan, Opt. Lett. 30 (2005) 932. [3] K.L. Corwin, N.R. Newbury, J.M. Dudley, S. Coen, S.A. Diddams, K. Weber, R.S. Windeler, Phys. Rev. Lett. 90 (2003) 113904. [4] M. Prabhu, N.S. Kim, K. Ueda, Jpn. J. Appl. Phys. 39 (2000) L291. [5] A.V. Avdokhin, S.V. Popov, J.R. Taylor, Opt. Lett. 28 (2003) 1353.
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[6] W. Zhang, Y. Wang, J. Peng, X. Liu, Opt. Comm. 231 (2004) 371. [7] A.K. Abeeluck, C. Headley, C.G. J;rgensen, Opt. Lett. 29 (2004) 2163. [8] A.K. Abeeluck, C. Headley, Opt. Lett. 30 (2005) 61. [9] J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599. [10] W. Drexler, J. Biomed. Opt. 9 (1) (2004) 47. [11] J.H. Lee, K. Kikuchi, Opt. Exp. 13 (2005) 4848. [12] P.A. Champert, V. Couderc, A. Barthelemy, IEEE Photon. Technol. Lett. 16 (2004) 2445. [13] F.D. Pasquale, IEEE J. Quantum Electron. 32 (1996) 326. [14] X. Liu, X. Yang, F. Lu, J. Ng, X. Zhou, C. Lu, Opt. Exp. 13 (2005) 142. [15] S.M. Kobtsev, S.V. Smirnov, Opt. Exp. 13 (2005) 6912. [16] M. Nakazawa, H. Kubota, K. Tamura, Opt. Lett. 24 (1999) 318.
Experimental investigation of continuous-wave supercontinuum ring laser composed of clad-pumped Er/Yb codoped fiber and highly-nonlinear optical fiber Ju Han Lee
*,1,
Kazuhiro Katoh, Kazuro Kikuchi
Research Center for Advanced Science and Technology (RCAST), University of Tokyo, 4-6-1 Komaba, Meguro-Ku, Tokyo 153-8904, Japan Received 20 December 2005; received in revised form 2 May 2006; accepted 16 May 2006
Abstract We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470 nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.72.Ai; 42.65.k; 42.81.i; 42.55.Wd; 76.30.Kg Keywords: Supercontinuum; Optical fiber; Nonlinear optics; Optical fiber lasers; Erbium/Ytterbium
1. Introduction Optical fiber-based supercontinuum (SC) light sources have attracted huge attention as one of very promising photonic devices in recent years and are being used increasingly in various practical and commercial applications: for example, optical component characterization, optical coherence tomography (OCT) [1], metrology [2], and optical sensing. Typical supercontinuum sources have been based on a high-power pump laser whose output beam is injected into a nonlinear fiber, where the supercontinuum is generated [3]. Unlike the conventional schemes using a pulse laser *
Corresponding author. Tel.: +81 3 5452 5122; fax: +81 3 5452 5125. E-mail address: [email protected] (J.H. Lee). 1 Also with Photonics Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.05.040
as a pump beam, there was a novel proposition by Prabu et al. that a high power continuous-wave (CW) laser could also be used as a pump beam for SC generation [4]. In the CW SC case, output average power is usually extremely high (watt level) due to its tight requirement of high pump power for generation of nonlinear effects in optical fiber. Further investigation by various groups found that the physical mechanism for CW SC evolution in optical fiber is different from the pulse-mode SC evolution [5–7]. Both modulation instability (MI) and stimulated Raman scattering (SRS) were found to play key roles in transforming a narrow-band CW laser beam into a broadband spectral continuum [7]. It was also found that the center wavelength of a pump beam is also a key parameter to be considered since it is better to locate it at the anomalous dispersion regime near a zero dispersion wavelength of a HNL-DSF for efficient initiation of the MI process [8].
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J.H. Lee et al. / Optics Communications 266 (2006) 681–685
Recently, we proposed a novel scheme of CW self-contained supercontinuum fiber ring laser that incorporated an erbium-doped fiber and a HNL-DSF inside a cavity [9]. Unlike previous CW SC schemes based on single propagation of a high power, CW laser beam through a highly nonlinear optical fiber, erbium gain inside a ring cavity converted a 1480-nm Raman pump beam into a seed light oscillation at 1562 nm and the oscillated light subsequently evolved into a SC of 250 nm with a relatively low output power by nonlinear effects in a HNL-DSF. Because of its relative simplicity, small size, and moderate level of output power, such a device might be more appropriate than conventional pulse-mode SC and watt-level CW SC sources for applications such as OCT and optical sensing. Note that watt-level CW SCs would be of no use for OCT applications because of critical damage to biological tissue unless an ultra-broadband optical attenuator is employed at the output end [10]. In particular, this is true for in vivo imaging of human retina. Another potential advantage of the SC fiber ring laser is that it does not require an expensive, high power, single mode laser whose center wavelength is close to a zero dispersion wavelength of a HNL-DSF as a pump beam. Owing to the photon conversion process from a low quality pump beam to a high quality seed light oscillation through rare earth-doped materials inside a cavity, low-cost multimode pump laser diodes (LDs) can readily be used for pumping the SC laser. It should be noticed that the high power pump lasers used for conventional CW SC schemes are either a complicated, cascaded Raman fiber laser [7,11] or a combined structure of a master laser and a rare earthdoped fiber-based power amplifier [12]. In this paper, we experimentally demonstrate a practical benefit of our proposed CW SC generating ring laser scheme in terms of low-cost device implementation and investigate its output intensity noise property relative to that of amplified spontaneous emission (ASE) light, as a continuing study from our previous paper of Ref. [9] where the basic concept of a SC ring laser was first demonstrated. First, we experimentally implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a HNL-DSF. The basic SC laser configuration used in this study is similar to that of Ref. [9]; however, this time we construct a ring cavity by use of a double clad Er/Yb fiber and low quality multimode pump LDs to underline the fact that our proposed scheme can be readily pumped by a low-cost 975-nm multimode beam without requiring any high power single-mode laser centered near a zero dispersion wavelength of a HNL-DSF. Using such a simple scheme a SC over a spectral bandwidth from 1280 nm to 1750 nm is readily achieved. Next, we investigate noise characteristics of the generated SC by measuring its relative-intensity-noise (RIN) and by comparing it with that of a pumped Er/Yb fiber ASE. A significantly high RIN level relative to that of the ASE is found and some discussions are presented regarding its origin.
2. Experimental results An experimental schematic for our SC laser is shown in Fig. 1(a). The commercially available, 8-m-long double clad Er/Yb fiber inside the cavity had a peak core erbium absorption of 35 dB/m at 1535 nm and a clad ytterbium absorption of 5 dB/m at 975 nm. The core NA was 0.17 and the cladding NA was 0.46. A 3-km-long HNL-DSF with a nonlinearity parameter c of 15.5 W/km was inserted into the cavity. The zero dispersion wavelength of the HNL-DSF was 1554 nm and the dispersion slope was 0.027 ps/nm2/km. In order to induce MI in the beginning stage of CW SC evolution, the zero dispersion wavelength of the HNL-DSF should be smaller than the peak wavelength of the seed light oscillation. We thus chose a HNL-DSF with a zero dispersion wavelength of 1554 nm as a proper nonlinear medium for our CW SC laser configuration considering the seed light oscillation wavelength of 1568 nm. Differently from the configuration in our preliminary experiment of Ref. [9] we chose to use 3-km length of HNL-DSF for this configuration to increase efficiency of the nonlinear process in the HNL-DSF due to insufficient optical power of the seed light oscillation. The fiber propagation loss was 1.3 dB/km. As an Er/Yb fiber pump source, we used two commercially available, multimode semiconductor LDs with a maximum output power of 4 W and the pump beams were coupled into the cavity by a passive pump/signal combiner. The laser output power was extracted from the ring cavity using an 80:20 fiber coupler, with which 80% power of the oscillated light was fed back into the Er/Yb fiber. An optical isolator was placed after the coupler within the cavity to ensure the directional light oscillation. Both the 80/20 coupler and the isolator had an operating wavelength band of 1520– 1580 nm. First, we measured output spectrum evolution as a function of the pump power of the SC laser, and the results are shown in the 3-dimensional (3D) plot of Fig. 2(a). Note that the displayed dark area at low power levels in the spectral range from 1700 nm to 1750 nm in Fig. 2(a) shows the receiver noise of our optical spectrum analyzer (OSA). A seed light oscillation with a broad linewidth was observed
975 nm Multimode Laser Diodes 975/1550 nm Pump Combiner
Isolator Output
8 m Er/Yb Codoped Double Clad Fiber
80% 20% Coupler 3 km HNL-DSF λ 0 = 1554 nm
Fig. 1. Experimental schematic for our Er/Yb fiber-based SC laser.
J.H. Lee et al. / Optics Communications 266 (2006) 681–685
(a)
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was clearly obtained at a pump power larger than 4.18 W. The measured 30-dB spectral bandwidth was larger than 470 nm. Note that the spectral bandwidth measurement was limited by the maximum measurable wavelength (1750 nm) of our OSA. The SC output included the transmission profiles of both the coupler and the isolator in some degree due to their non-flat responses over such an ultra-wide spectral band. The degree of polarization for the SC output was measured to be less than 3%, which is considered to be an almost depolarized level. In order to further investigate the output polarization property we inserted a polarization controller inside the ring cavity and observed the effect of polarization change of the seed light oscillation on the SC output by adjusting it. Neither significant output power change nor wavelength shift was found. For relative optical property comparison, ASE was generated from the pumped Er/Yb codoped fiber by removing the HNL-DSF as well as breaking the ring structure. In this case an isolator was located after the Er/Yb fiber to suppress undesirable internal lasing. A typical ASE spectrum of a pumped Er/Yb codoped fiber was obtained as expected. The significant difference of bandwidth between
150 Er/Yb Fiber SC Laser Er/Yb Fiber ASE
(b) Fig. 2. (a) Three-dimensional plot of measured spectral evolution of the optical output versus the pump power for the Er/Yb fiber-based SC laser. (b) Measured output spectra of both the Er/Yb fiber-based SC laser and the Er/Yb fiber ASE at a maximum pump power of 5.5 W.
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at 1568 nm. The second spectral peak started growing at 1608 nm and its spectral separation from the seed light oscillation became larger with the increase in the pump power. The initial double-peak generation can be explained by both mode-hopping induced multimode operation of a long cavity fiber ring laser [13] and nonlinear effect induced suppression of cross gain saturation in erbium ions within a laser cavity that incorporated long length of HNL-DSF [14]. At a pump level of 0.73 W the third peak and the small fourth peak were observed due to four-wave mixing effect between the seed light oscillation and the second peak. Further increase in the pump power led to noisy Raman soliton formation due to MI and these solitons experienced self frequency shift owing to SRS [15]. A strong first-order Raman Stokes that was pumped by the second spectral peak, was generated 1730 nm with a 2.94 W pump power. Then, the output spectrum evolved into a spectral continuum at a 4.18-W pump level. High quality of a CW SC
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Fig. 3. (a) Measured output power versus pump power and (b) measured relative intensity noise (RIN) for both the Er/Yb fiber-based SC laser and the Er/Yb fiber ASE.
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the SC laser output and the ASE is clearly evident in the output optical spectra of Fig. 2(b), which were measured at a maximum pump power of 5.5 W. Second, we measured the output optical power versus pump power for both cases, i.e. the SC laser and the Er/Yb fiber ASE. The results are shown in Fig. 3(a). The SC laser exhibited abrupt decrease of the output power at a pump power of 4.18 W that corresponded to the point at which a spectral continuum was obtained, whilst the Er/Yb fiber ASE showed substantial increase of output power at a pump power larger than 2.94 W due to undesirable internal lasing. The maximum output power for the SC laser was 53.4 mW at a 5.5-W pump power. Third, we measured RIN of both output beams. The outputs were passed through a 15-nm bandpass filter centered at 1570 nm, and subsequently coupled onto a lownoise photodetector with a bandwidth of 150 MHz. The detected electrical signals were then ac-coupled into an electrical spectrum analyzer. Fig. 3(b) shows the measured RIN spectra. Interestingly, the SC laser output was found to have a much higher RIN level than the Er/Yb fiber ASE by 25 dB. The physical origin of such a high RIN in the SC laser needs to be further investigated in detail
although it is believed to be due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process [3,16]. According to previous detailed studies on coherence degradation and the corresponding nonlinear noise amplification process [3,16], higher order soliton evolution over background random noise that is caused by input signal noise or/and Raman scattering in an anomalous dispersion optical fiber, inevitably leads to a series of phenomena of MI-induced noise amplification, soliton pulse distortion, and temporal coherence degradation. Even though the previous studies were performed for the case of pulse-mode SC evolution, the findings can also be used for the explanation of the significant output RIN mechanism of our CW SC case due to the fact that the CW SC evolution in our SC laser or most of the demonstrated CW SC generating schemes is based on noisy Raman solitonic structure formation initiated by MI. Finally, we investigated the long-term stability of the output spectrum generated from our SC laser. The output spectrum was recorded every 1 h for 4 h whilst the pump power was set to be a maximum value of 5.5 W. Fig. 4(a) shows the measured output spectra and their close-up views are shown in Fig. 4(b). The resolution bandwidth of the OSA was 0.2 nm. No significant spectral fluctuations were observed although a linear increase of the output spectral power existed with a rate of 0.025 dB/h owing to temperature dependence of our fabricated pump LD driver circuit. It is clearly evident from this measurement that a stable SC laser was readily achieved. 3. Conclusion
(a)
(b) Fig. 4. (a) Measured optical spectra of the supercontinuum laser output recorded every 1 h for 4 h at a maximum pump power of 5.5 W. (b) Their close-up views.
We have experimentally performed a further study on our proposed CW SC ring laser scheme. This time, a clad pumped Er/Yb fiber-based SC laser was constructed for the demonstration of a practical benefit in terms of lowcost, compact, and self-contained device implementation. Using a simple fiber ring laser structure incorporating an Er/Yb fiber and a HNL-DSF, we readily achieved a stable SC laser output with a spectral bandwidth larger than 470 nm. A relatively low output power of 53.4 mW compared to that of conventional single pass CW SC sources was obtained; however, the power level is believed to be high enough for OCT or optical sensing applications. Relative-intensity-noise (RIN) property of the generated SC is then measured and compared with that of a pumped Er/Yb fiber amplified spontaneous emission (ASE) and the SC laser is found to have a much higher RIN than the fiber ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process. Further investigation is required to fully understand the high RIN mechanism and to figure out a proper method for the suppression of the excess intensity noise. Although the excess intensity noise problem still needs to be sorted out, we believe that such a simple, low-cost, and ultra-broad band SC source should find a
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