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Tunable single-longitudinal-mode Ytterbium all fiber laser with saturable-absorber-based auto-tracking filter
Optics Communications 285 (2012) 2702–2706
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable single-longitudinal-mode Ytterbium all fiber laser with saturable-absorber-based auto-tracking filter Feifei Yin, Sigang Yang ⁎, Hongwei Chen, Minghua Chen, Shizhong Xie Tsinghua National Laboratory for Information Science and Technology (TNList) Department of Electronic Engineering, Tsinghua University, 100084, Beijing, China
a r t i c l e
i n f o
Article history: Received 9 October 2011 Received in revised form 28 December 2011 Accepted 4 February 2012 Available online 18 February 2012 Keywords: Fiber ring lasers Single longitudinal mode Tunable lasers Ytterbium lasers
a b s t r a c t A broadband tunable, single-longitudinal-mode (SLM) Ytterbium fiber laser with unpumped Ytterbium-doped Sagnac loop is proposed and demonstrated experimentally. The unpumped Ytterbium-doped Sagnac loop is employed as a saturable absorber based auto-tracking filter to ensure single-longitudinal-mode oscillation. And a tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. With 1m Ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 160-mW pump power. The laser is very stable with output power of about 3 dBm and optical signal to noise ratio of higher than 50 dB in all the 60-nm tuning range. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Wavelength-tunable single-longitudinal-mode (SLM) lasers with broad tuning range are versatile tools for many applications, including sensing, bio-medical technology and spectroscopy [1]. In a variety of different wavelengths, 1-μm optical sources have attracted more and more attention for their applications in ophthalmologist for Optical Coherence Tomography (OCT). In the swept-source OCT ophthalmologist, the light source should be with narrow linewidth and broadband tunability. For broad tuning range, Ytterbium (Yb) doped fibers, which can be directly pumped by laser diodes, with their broad gain bandwidth of 100 nm around 1 μm wavelength offer almost ideal gain medium for the generation and the amplification of wavelength-tunable laser at 1-μm band [2,3]. These days, several techniques have been developed for generating a SLM operation at 1-μm band, such as fiber Bragg grating (FBG) based mode discriminator [4], saturableabsorber-based mode filters [5,6], and distributed feedback mechanism based on phase shifted FBG [7], etc. Moreover, tunable 1060-nm Ybdoped fiber lasers have also been demonstrated with tuning range of 20.4 nm [8] and 26 nm [9]. In these lasers, the mode filters ensure SLM operation functioned simultaneously as the wavelength tuning components, which limited the tuning range of the lasers. In ref. [5], by using semiconductor optical amplifier, 40-nm tuning range was achieved which is still not large enough. In ref. [6], a spatial-hole burning (SHB) stabilized SLM Yb-doped fiber laser is investigated where the SHB is in the same standing-
⁎ Corresponding author. E-mail address: [email protected] (S. Yang). 0030-4018/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2012.02.007
wave laser cavity as the laser pump. In this paper, a unidirectional ring cavity is applied with a separate unpumped Yb-doped Sagnac loop to override the SHB in the gain medium. The Sagnac loop functions as a SHB induced auto-tracking filter to ensure SLM oscillation. By using Yb-doped fiber as the gain medium, an all fiber tunable SLM laser is demonstrated. A tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. By using only 1-m Yb-doped fiber as the gain medium, a SLM fiber laser with 60-nm tuning range at 1060-nm band is realized. The behaviors of the output power, the threshold and slope efficiency and the wavelength stabilities, have also been studied. 2. Experimental setup and principle The experimental setup of our widely tunable SLM Yb-doped fiber laser is shown in Fig. 1. The laser is pumped at 976 nm via a 980/1060 wavelength division multiplexer (WDM) coupler. The cavity gain is provided by a 1-m-long Yb-doped single cladding single-mode fiber (YDF, NUFERN SM-YSF-HI) with 7.5-μm core diameter at 1060 nm and 250-dB/m absorption at 975 nm. And the operating wavelength is tuned by a 1060-nm tunable band pass optical filter (T-BPF, Agiltron, FOTF) based on thin film cavity filter technology. The T-BPF has 3-dB bandwidth of 0.8 nm in the whole tuning range from 1010 nm to 1100 nm promising a large tuning range of our SLM laser. A polarization controller (PC) PC1 is applied to adjust the polarization state of light inside the cavity and stabilize the laser. A 90/10 fiber splitter is applied as an output coupler via the 10% port after the filter in order to achieve a better optical signal to noise ratio (OSNR) with respect to background noise. The unpumped Yb-doped Sagnac loop is composed of a 3-dB coupler, two polarization
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with channel spacing of Δλ = λ 2/(2neffΔL) = 0.08 nm forms the response of the Sagnac filter. As RFBG has a narrow bandwidth less than 28.1 MHz, the FWHM of the Sagnac filter is 28.1 MHz. Although the longitudinal mode space of the fiber ring cavity is about 25 MHz, the SLM operation condition is satisfied considering the mode competition. As the Yb-doped fiber has wide band absorption, when the TBPF was tuned, the fiber loop mirror based self-induced FBG filter could dynamically track the wavelength tuning of the tunable filter. As a result, by using the fiber loop mirror, a large tuning range can be achieved. Fig. 1. Proposed experimental setup of the Yb fiber laser with unpumped ytterbium-doped Sagnac loop based Saturable Absorber. OC: optical, PC: polarization controller, WDM: wavelength division multiplexer, T-BPF: tunable band pass optical filter.
controllers PC2, PC3 and 1-m Yb-doped fiber which is the same as the gain fiber. The circulator has 35-dB isolation from port 3 to port 2 as well as from port 2 to port 1; thus it acts as an isolator to ensure unidirectional propagation and suppress undesired reflections. All the couplers used in our experiment are fused tapered which has a wide band to ensure a large tuning range of our fiber laser. All the components in our experiment are pigtailed with Hi-1060 single-mode fiber (SMF). And in order to minimize the cavity loss and reducing undesired reflections, all the components are fusion spliced together. In the Sagnac fiber loop mirror, two counter-propagating waves form a standing wave and induce spatial-hole burning (SHB) in the unpumped Yb-doped fiber. The refractive index of the unpumped Ybdoped fiber changes spatially due to the SHB. This results in periodical spatial variation of refraction index, given by the Kramers–Kronig relation [10] Δα z; ω′ c ω2 ′ Δnðz; ωÞ ¼ P:V:∫ω1 2 dω π ω′ −ω2
ð1Þ
where P.V. is the principal value of the integral derived over the frequency range ω1 b ω b ω2. Δα is the variation of saturable absorption coefficient. The spatial period is λ/2neff from the standing wave theory, where λ is the central wavelength and neff is the effective refraction index of the Yb-dope fiber. In our experiment, the maximum intracavity power is about 30 mW, and the induced refraction index change is estimated to be Δn b 2 × 10− 7. So, a weakly couple fiber Bragg grating (FBG) is self-induced with a period of Λ = λ/2neff. The reflection of the FBG is:
RFBG
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sinh2 κ 2 −δ2 Lg pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ cosh2 κ 2 −δ2 Lg −δ2 =κ 2
3. Results and discussion 3.1. SLM lasing To verify the sidemode suppression performance of the unpumped Ytterbium-doped Sagnac loop, the self-beating radio frequency (RF) spectrum by applying the laser output to a 10-GHz photo detector (PD) is measured using a 40-GHz spectrum analyzer (Anritsu MS2668C) from dc to 1 GHz span with the resolution bandwidth of 100 kHz. Fig. 2 shows the RF spectra of the proposed laser without and with the unpumped ytterbium-doped Sagnac loop filter respectively. Without the Sagnac loop filter the RF spectrum is very noisy and unstable due to the mode hopping, and the laser is multiple-longitudinal-mode. As Fig. 2 shows, the free spectral range (FSR) of the laser cavity is 25.2 MHz. With the Sagnac loop filter connected, as shown in Fig. 2, the beating signal disappeared and no spike signals were observed and the relative intensity noise (RIN) is less than 100 dBc/Hz RBW kHz, Only the direct current peak can be observed which indicates that a SLM operation with a side-mode suppression of at least up to 1 GHz can be achieved by the proposed configuration. The optical output of the proposed SLM Yb laser is measured by an optical spectrum analyzer (OSA, Ando AQ6317B) with a 0.01-nm resolution. Fig. 3 is the optical spectrum of the SLM fiber laser output at 1050 nm. The OSNR is about 55 dB showing the SLM laser output with good sidemode suppression. 3.2. Flat SLM over 60 nm As the T-BPF was tuned, the optical output of the proposed SLM Yb laser is measured by an OSA with a 0.01-nm resolution. Fig. 4 shows the overlapped SLM laser spectra from 1020 nm to 1090 nm by a
ð2Þ
where δ = β − π/λ is the detuning, κ = 2Δn/(neffλ) is the coupling coefficient of the FBG, Lg is the grating length. The full-width-half maximum (FWHM) bandwidth of the self-induced FBG can be written as: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 u c u Δn 2 Λ þ Δf ¼ κ t λ 2neff Lg
ð3Þ
In our experiment, λ = 1050 nm, Lg = 1 m, neff = 1.45 and Δn b 2 × 10 − 7, from Eq. (3) we can estimate the FWHM of the FBG as Δf b 28.1 MHz. If we neglect the transmission of the FBG, the reflection of the Sagnac loop incorporated with the FBG can be written as: 2
R ¼ RFBG ðλÞ sin ðβΔLÞ
ð4Þ
ΔL is difference between L1 and L2 which is optimized to 4 mm. So the combination of the reflection of the FBG and the sinusoidal response
Fig. 2. Electrical spectra measured by applying the laser output to a PD. RBW = 100 KHz without and with the unpumped ytterbium-doped Sagnac loop filter with RIN spectrum inserted.
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Fig. 3. Optical spectrum of the fiber laser output at 1050 nm with an OSNR of 55 dB (0.01-nm resolution).
step of 2-nm at the pump power of 160 mW. Fig. 5 is the corresponding output power and the OSNR. If the power variation is within 3 dB, the tuning range is greater than 60 nm from 1027 nm to 1087 nm. In the 60-nm-wide tuning range, the output power is about 3 dBm and the OSNR is higher than 50 dB (0.01-nm resolution). To get SLM operation and high OSNR, it is necessary to adjust the PCs at some wavelength as the components in our experiment are not polarization maintaining. And if the cavity is implemented with all polarization maintaining components, the use of the PC would be avoided. Fig. 6 is the amplified spontaneous emission (ASE) spectra of the 1-m Yb-doped fiber at different pump powers. It is clearly that the ASE is not as flat as the output power curve as shown in Fig. 5. The peak of the ASE spectrum is 1030 nm which means the maximum gain wavelength is 1030 nm. As shown in Fig. 5, the output power is grown with the wavelength increasing when the wavelength is below 1030 nm, which agrees well with the ASE spectrum as shown in Fig. 6. However, in the 1030 nm to 1080 nm band, the output power is almost flat but the ASE spectrum is monotonically decreasing. This is mainly because of the reabsorption of the ASE at shorter wavelength [11]. Another reason for the flat output power curve is the homogeneous broadening.
Fig. 5. Output power and OSNR when the wavelength is tuned from 1020 to 1090 nm.
Fig. 6. The ASE spectrum of 1-m Yb-doped fiber at pump powers of 120 mW and 130 mW.
Fig. 7 shows the evolution of the SLM output power vs. the pump power at 1030 nm, 1040 nm, 1050 nm, 1060 nm, 1070 nm and
1080 nm and their linearity fitting. It is clear that, different operation wavelengths have different slope efficiency and different threshold pump power. Among the six wavelengths, the smallest threshold pump power is at 1040 nm. And the threshold increased with the wavelength increasing from 1040 nm to 1080 nm, which agrees well with the ASE spectrum as shown in Fig. 6.
Fig. 4. Output spectra of the proposed laser with the wavelength tuned from 1020 to 1090 nm.
Fig. 7. The SLM output power vs. the pump power at different wavelength and their linearity fitting.
3.3. Threshold and slope efficiency
F. Yin et al. / Optics Communications 285 (2012) 2702–2706
Also it is clear from Fig. 7, the slope efficiency increased with the wavelength increasing from 1030 nm to 1080 nm. The smallest slope efficiency is only 2.4% at 1030 nm, and the largest is 4.9% at 1080 nm. Furthermore from 1030 nm to 1080 nm, the slope efficiency increases linearly while the ASE spectrum is monotonically decreasing. From Fig. 6, when the pump power is larger than 120 mW the pump light cannot be absorbed completely. So when the laser system lasing at wavelength longer than 1030 nm the residual will be absorbed and the ASE at wavelength shorter than the lasing wavelength will be reabsorbed as well. The longer the lasing wavelength is, the more ASE will be reabsorbed. As a result, the slope efficiency increases with the lasing wavelength increases. The slope efficiency is below 5% in the entire tuning range which is because of the large cavity loss. The components in the laser system have inevitable insertion loss especially the large loss introduced by the Yb-doped fiber loop filter. Another reason for the low efficiency is the output coupler ratio. It is believed that an optimized output coupler ratio and ring cavity ratio would lead to a higher output.
3.4. Power and wavelength stability The whole laser system is mounted on an iron panel to reduce the influence of temperature on the stability of the laser system and get better performance. To analyze the power and wavelength stability of our SLM laser, the system is operated in room environment for periods of 60 min at six different wavelengths from 1030 nm to 1080 nm with a 10-nm step. The fluctuations of the output power and wavelengths at the six wavelengths are shown in Fig. 8. The wavelength fluctuation at 1030 nm is 12 pm which is worse than 1040 nm and 1050 nm. The reason is that, at 1030 nm the effect of SHB is reduced because of the large absorption, which has been pointed out in ref. [6]. At 1080 nm, the wavelength fluctuation is 12 pm which is
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worse than 1060 nm and 1070 nm. The reason is the degradation of the absorption at 1080 nm. In all the monitored wavelengths, the largest wavelength shift is 12 pm and the largest power fluctuation is 0.1 dB (2.3%). It should be noted that the power meter (Ando AQ2140 Multimeter with Ando AQ2732 Sensor Unit) has a total power accuracy of ±5% (±0.22 dB). So the wavelength shift is very close to the 0.01-nm resolution and the power fluctuation is within the measurement accuracy of the power meter, confirming that the proposed fiber laser has a high lasing stability. 4. Conclusion We have proposed and demonstrated an all fiber SLM 1060-nm band laser with 60 nm tuning range. The SLM is achieved by using a ytterbium-doped Sagnac loop as an auto-tracking filter. The output power over the 60 nm tuning range is very flat because of the homogeneous broadening of Yb-doped fiber and the reabsorption of the ASE at shorter wavelength. And the SMSR is higher than 50 dB in the entire tuning range. The threshold and slope efficiency have also been investigated at different wavelengths. The slope efficiency increases with the operation wavelength which is also caused by the reabsorption of the ASE at wavelength shorter than the lasing wavelength. The power and wavelength stability have been studied at different emission wavelengths, showing our laser a very stable all fiber SLM laser system. Acknowledgments This work is supported by the National Basic Research Program of China (973 Program) under Contract 2010CB327606, 2012CB315703 and by the National Nature Science Foundation of China under Contract 61108007, 61090391.
Fig. 8. Fluctuations of the output power and wavelength over periods of 60 min at six different wavelengths.
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References [1] G.A. Ball, W.H. Glenn, Journal of Lightwave Technology 10 (10) (1992) 1338. [2] D.C. Hanna, R.M. Percival, I.R. Perry, R.G. Smart, P.J. Suni, A.C. Tropper, Journal of Modern Optics 37 (4) (1987) 517. [3] H.M. Pask, R.J. Carman, D.C. Hanna, A.C. Tropper, C.J. Mackechnie, P.R. Barber, J.M. Dawes, IEEE Journal of Selected Topics in Quantum Electronics 1 (1) (1995) 2. [4] W. Guan, J.R. Marciante, IEEE Photonics Technology Letters 19 (5) (2007) 261. [5] M. Tang, X.L. Tian, X.N. Lu, S.N. Fu, P.P. Shum, Z.R. Zhang, M. Liu, Y. Cheng, J. Liu, Optics Letters 34 (14) (2009) 2204.
[6] R. Paschotta, J. Nilsson, L. Reekie, A.C. Trooper, D.C. Hanna, Optics Letters 22 (1) (1997) 40. [7] A. Asseh, H. Storoy, J.T. Kringlebotn, W. Margulis, B. Sahlgren, S. Sandgren, R. Stubbe, G. Edwall, Electronics Letters 31 (12) (1995) 969. [8] A. Wang, L. Feng, J. Huang, C. Gu, X. Lu, H. Ming, J. Xie, Chinese Journal of Quantum Electronics 22 (4) (2005) 607. [9] M. Engelbrecht, A. Ruehl, D. Wandt, D. Kracht, Optics Express 15 (8) (2007) 4617. [10] S. Fleming, T. Whitley, IEEE Journal of Quantum Electronics 32 (1996) 1113. [11] Luís A. Gomes, Lasse Orsila, Tomi Jouhti, Oleg G. Okhotnikov, IEEE Journal of Selected Topics in Quantum Electronics 10 (1) (2004) 129.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable single-longitudinal-mode Ytterbium all fiber laser with saturable-absorber-based auto-tracking filter Feifei Yin, Sigang Yang ⁎, Hongwei Chen, Minghua Chen, Shizhong Xie Tsinghua National Laboratory for Information Science and Technology (TNList) Department of Electronic Engineering, Tsinghua University, 100084, Beijing, China
a r t i c l e
i n f o
Article history: Received 9 October 2011 Received in revised form 28 December 2011 Accepted 4 February 2012 Available online 18 February 2012 Keywords: Fiber ring lasers Single longitudinal mode Tunable lasers Ytterbium lasers
a b s t r a c t A broadband tunable, single-longitudinal-mode (SLM) Ytterbium fiber laser with unpumped Ytterbium-doped Sagnac loop is proposed and demonstrated experimentally. The unpumped Ytterbium-doped Sagnac loop is employed as a saturable absorber based auto-tracking filter to ensure single-longitudinal-mode oscillation. And a tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. With 1m Ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 160-mW pump power. The laser is very stable with output power of about 3 dBm and optical signal to noise ratio of higher than 50 dB in all the 60-nm tuning range. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Wavelength-tunable single-longitudinal-mode (SLM) lasers with broad tuning range are versatile tools for many applications, including sensing, bio-medical technology and spectroscopy [1]. In a variety of different wavelengths, 1-μm optical sources have attracted more and more attention for their applications in ophthalmologist for Optical Coherence Tomography (OCT). In the swept-source OCT ophthalmologist, the light source should be with narrow linewidth and broadband tunability. For broad tuning range, Ytterbium (Yb) doped fibers, which can be directly pumped by laser diodes, with their broad gain bandwidth of 100 nm around 1 μm wavelength offer almost ideal gain medium for the generation and the amplification of wavelength-tunable laser at 1-μm band [2,3]. These days, several techniques have been developed for generating a SLM operation at 1-μm band, such as fiber Bragg grating (FBG) based mode discriminator [4], saturableabsorber-based mode filters [5,6], and distributed feedback mechanism based on phase shifted FBG [7], etc. Moreover, tunable 1060-nm Ybdoped fiber lasers have also been demonstrated with tuning range of 20.4 nm [8] and 26 nm [9]. In these lasers, the mode filters ensure SLM operation functioned simultaneously as the wavelength tuning components, which limited the tuning range of the lasers. In ref. [5], by using semiconductor optical amplifier, 40-nm tuning range was achieved which is still not large enough. In ref. [6], a spatial-hole burning (SHB) stabilized SLM Yb-doped fiber laser is investigated where the SHB is in the same standing-
⁎ Corresponding author. E-mail address: [email protected] (S. Yang). 0030-4018/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2012.02.007
wave laser cavity as the laser pump. In this paper, a unidirectional ring cavity is applied with a separate unpumped Yb-doped Sagnac loop to override the SHB in the gain medium. The Sagnac loop functions as a SHB induced auto-tracking filter to ensure SLM oscillation. By using Yb-doped fiber as the gain medium, an all fiber tunable SLM laser is demonstrated. A tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. By using only 1-m Yb-doped fiber as the gain medium, a SLM fiber laser with 60-nm tuning range at 1060-nm band is realized. The behaviors of the output power, the threshold and slope efficiency and the wavelength stabilities, have also been studied. 2. Experimental setup and principle The experimental setup of our widely tunable SLM Yb-doped fiber laser is shown in Fig. 1. The laser is pumped at 976 nm via a 980/1060 wavelength division multiplexer (WDM) coupler. The cavity gain is provided by a 1-m-long Yb-doped single cladding single-mode fiber (YDF, NUFERN SM-YSF-HI) with 7.5-μm core diameter at 1060 nm and 250-dB/m absorption at 975 nm. And the operating wavelength is tuned by a 1060-nm tunable band pass optical filter (T-BPF, Agiltron, FOTF) based on thin film cavity filter technology. The T-BPF has 3-dB bandwidth of 0.8 nm in the whole tuning range from 1010 nm to 1100 nm promising a large tuning range of our SLM laser. A polarization controller (PC) PC1 is applied to adjust the polarization state of light inside the cavity and stabilize the laser. A 90/10 fiber splitter is applied as an output coupler via the 10% port after the filter in order to achieve a better optical signal to noise ratio (OSNR) with respect to background noise. The unpumped Yb-doped Sagnac loop is composed of a 3-dB coupler, two polarization
F. Yin et al. / Optics Communications 285 (2012) 2702–2706
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with channel spacing of Δλ = λ 2/(2neffΔL) = 0.08 nm forms the response of the Sagnac filter. As RFBG has a narrow bandwidth less than 28.1 MHz, the FWHM of the Sagnac filter is 28.1 MHz. Although the longitudinal mode space of the fiber ring cavity is about 25 MHz, the SLM operation condition is satisfied considering the mode competition. As the Yb-doped fiber has wide band absorption, when the TBPF was tuned, the fiber loop mirror based self-induced FBG filter could dynamically track the wavelength tuning of the tunable filter. As a result, by using the fiber loop mirror, a large tuning range can be achieved. Fig. 1. Proposed experimental setup of the Yb fiber laser with unpumped ytterbium-doped Sagnac loop based Saturable Absorber. OC: optical, PC: polarization controller, WDM: wavelength division multiplexer, T-BPF: tunable band pass optical filter.
controllers PC2, PC3 and 1-m Yb-doped fiber which is the same as the gain fiber. The circulator has 35-dB isolation from port 3 to port 2 as well as from port 2 to port 1; thus it acts as an isolator to ensure unidirectional propagation and suppress undesired reflections. All the couplers used in our experiment are fused tapered which has a wide band to ensure a large tuning range of our fiber laser. All the components in our experiment are pigtailed with Hi-1060 single-mode fiber (SMF). And in order to minimize the cavity loss and reducing undesired reflections, all the components are fusion spliced together. In the Sagnac fiber loop mirror, two counter-propagating waves form a standing wave and induce spatial-hole burning (SHB) in the unpumped Yb-doped fiber. The refractive index of the unpumped Ybdoped fiber changes spatially due to the SHB. This results in periodical spatial variation of refraction index, given by the Kramers–Kronig relation [10] Δα z; ω′ c ω2 ′ Δnðz; ωÞ ¼ P:V:∫ω1 2 dω π ω′ −ω2
ð1Þ
where P.V. is the principal value of the integral derived over the frequency range ω1 b ω b ω2. Δα is the variation of saturable absorption coefficient. The spatial period is λ/2neff from the standing wave theory, where λ is the central wavelength and neff is the effective refraction index of the Yb-dope fiber. In our experiment, the maximum intracavity power is about 30 mW, and the induced refraction index change is estimated to be Δn b 2 × 10− 7. So, a weakly couple fiber Bragg grating (FBG) is self-induced with a period of Λ = λ/2neff. The reflection of the FBG is:
RFBG
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sinh2 κ 2 −δ2 Lg pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ cosh2 κ 2 −δ2 Lg −δ2 =κ 2
3. Results and discussion 3.1. SLM lasing To verify the sidemode suppression performance of the unpumped Ytterbium-doped Sagnac loop, the self-beating radio frequency (RF) spectrum by applying the laser output to a 10-GHz photo detector (PD) is measured using a 40-GHz spectrum analyzer (Anritsu MS2668C) from dc to 1 GHz span with the resolution bandwidth of 100 kHz. Fig. 2 shows the RF spectra of the proposed laser without and with the unpumped ytterbium-doped Sagnac loop filter respectively. Without the Sagnac loop filter the RF spectrum is very noisy and unstable due to the mode hopping, and the laser is multiple-longitudinal-mode. As Fig. 2 shows, the free spectral range (FSR) of the laser cavity is 25.2 MHz. With the Sagnac loop filter connected, as shown in Fig. 2, the beating signal disappeared and no spike signals were observed and the relative intensity noise (RIN) is less than 100 dBc/Hz RBW kHz, Only the direct current peak can be observed which indicates that a SLM operation with a side-mode suppression of at least up to 1 GHz can be achieved by the proposed configuration. The optical output of the proposed SLM Yb laser is measured by an optical spectrum analyzer (OSA, Ando AQ6317B) with a 0.01-nm resolution. Fig. 3 is the optical spectrum of the SLM fiber laser output at 1050 nm. The OSNR is about 55 dB showing the SLM laser output with good sidemode suppression. 3.2. Flat SLM over 60 nm As the T-BPF was tuned, the optical output of the proposed SLM Yb laser is measured by an OSA with a 0.01-nm resolution. Fig. 4 shows the overlapped SLM laser spectra from 1020 nm to 1090 nm by a
ð2Þ
where δ = β − π/λ is the detuning, κ = 2Δn/(neffλ) is the coupling coefficient of the FBG, Lg is the grating length. The full-width-half maximum (FWHM) bandwidth of the self-induced FBG can be written as: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 u c u Δn 2 Λ þ Δf ¼ κ t λ 2neff Lg
ð3Þ
In our experiment, λ = 1050 nm, Lg = 1 m, neff = 1.45 and Δn b 2 × 10 − 7, from Eq. (3) we can estimate the FWHM of the FBG as Δf b 28.1 MHz. If we neglect the transmission of the FBG, the reflection of the Sagnac loop incorporated with the FBG can be written as: 2
R ¼ RFBG ðλÞ sin ðβΔLÞ
ð4Þ
ΔL is difference between L1 and L2 which is optimized to 4 mm. So the combination of the reflection of the FBG and the sinusoidal response
Fig. 2. Electrical spectra measured by applying the laser output to a PD. RBW = 100 KHz without and with the unpumped ytterbium-doped Sagnac loop filter with RIN spectrum inserted.
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Fig. 3. Optical spectrum of the fiber laser output at 1050 nm with an OSNR of 55 dB (0.01-nm resolution).
step of 2-nm at the pump power of 160 mW. Fig. 5 is the corresponding output power and the OSNR. If the power variation is within 3 dB, the tuning range is greater than 60 nm from 1027 nm to 1087 nm. In the 60-nm-wide tuning range, the output power is about 3 dBm and the OSNR is higher than 50 dB (0.01-nm resolution). To get SLM operation and high OSNR, it is necessary to adjust the PCs at some wavelength as the components in our experiment are not polarization maintaining. And if the cavity is implemented with all polarization maintaining components, the use of the PC would be avoided. Fig. 6 is the amplified spontaneous emission (ASE) spectra of the 1-m Yb-doped fiber at different pump powers. It is clearly that the ASE is not as flat as the output power curve as shown in Fig. 5. The peak of the ASE spectrum is 1030 nm which means the maximum gain wavelength is 1030 nm. As shown in Fig. 5, the output power is grown with the wavelength increasing when the wavelength is below 1030 nm, which agrees well with the ASE spectrum as shown in Fig. 6. However, in the 1030 nm to 1080 nm band, the output power is almost flat but the ASE spectrum is monotonically decreasing. This is mainly because of the reabsorption of the ASE at shorter wavelength [11]. Another reason for the flat output power curve is the homogeneous broadening.
Fig. 5. Output power and OSNR when the wavelength is tuned from 1020 to 1090 nm.
Fig. 6. The ASE spectrum of 1-m Yb-doped fiber at pump powers of 120 mW and 130 mW.
Fig. 7 shows the evolution of the SLM output power vs. the pump power at 1030 nm, 1040 nm, 1050 nm, 1060 nm, 1070 nm and
1080 nm and their linearity fitting. It is clear that, different operation wavelengths have different slope efficiency and different threshold pump power. Among the six wavelengths, the smallest threshold pump power is at 1040 nm. And the threshold increased with the wavelength increasing from 1040 nm to 1080 nm, which agrees well with the ASE spectrum as shown in Fig. 6.
Fig. 4. Output spectra of the proposed laser with the wavelength tuned from 1020 to 1090 nm.
Fig. 7. The SLM output power vs. the pump power at different wavelength and their linearity fitting.
3.3. Threshold and slope efficiency
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Also it is clear from Fig. 7, the slope efficiency increased with the wavelength increasing from 1030 nm to 1080 nm. The smallest slope efficiency is only 2.4% at 1030 nm, and the largest is 4.9% at 1080 nm. Furthermore from 1030 nm to 1080 nm, the slope efficiency increases linearly while the ASE spectrum is monotonically decreasing. From Fig. 6, when the pump power is larger than 120 mW the pump light cannot be absorbed completely. So when the laser system lasing at wavelength longer than 1030 nm the residual will be absorbed and the ASE at wavelength shorter than the lasing wavelength will be reabsorbed as well. The longer the lasing wavelength is, the more ASE will be reabsorbed. As a result, the slope efficiency increases with the lasing wavelength increases. The slope efficiency is below 5% in the entire tuning range which is because of the large cavity loss. The components in the laser system have inevitable insertion loss especially the large loss introduced by the Yb-doped fiber loop filter. Another reason for the low efficiency is the output coupler ratio. It is believed that an optimized output coupler ratio and ring cavity ratio would lead to a higher output.
3.4. Power and wavelength stability The whole laser system is mounted on an iron panel to reduce the influence of temperature on the stability of the laser system and get better performance. To analyze the power and wavelength stability of our SLM laser, the system is operated in room environment for periods of 60 min at six different wavelengths from 1030 nm to 1080 nm with a 10-nm step. The fluctuations of the output power and wavelengths at the six wavelengths are shown in Fig. 8. The wavelength fluctuation at 1030 nm is 12 pm which is worse than 1040 nm and 1050 nm. The reason is that, at 1030 nm the effect of SHB is reduced because of the large absorption, which has been pointed out in ref. [6]. At 1080 nm, the wavelength fluctuation is 12 pm which is
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worse than 1060 nm and 1070 nm. The reason is the degradation of the absorption at 1080 nm. In all the monitored wavelengths, the largest wavelength shift is 12 pm and the largest power fluctuation is 0.1 dB (2.3%). It should be noted that the power meter (Ando AQ2140 Multimeter with Ando AQ2732 Sensor Unit) has a total power accuracy of ±5% (±0.22 dB). So the wavelength shift is very close to the 0.01-nm resolution and the power fluctuation is within the measurement accuracy of the power meter, confirming that the proposed fiber laser has a high lasing stability. 4. Conclusion We have proposed and demonstrated an all fiber SLM 1060-nm band laser with 60 nm tuning range. The SLM is achieved by using a ytterbium-doped Sagnac loop as an auto-tracking filter. The output power over the 60 nm tuning range is very flat because of the homogeneous broadening of Yb-doped fiber and the reabsorption of the ASE at shorter wavelength. And the SMSR is higher than 50 dB in the entire tuning range. The threshold and slope efficiency have also been investigated at different wavelengths. The slope efficiency increases with the operation wavelength which is also caused by the reabsorption of the ASE at wavelength shorter than the lasing wavelength. The power and wavelength stability have been studied at different emission wavelengths, showing our laser a very stable all fiber SLM laser system. Acknowledgments This work is supported by the National Basic Research Program of China (973 Program) under Contract 2010CB327606, 2012CB315703 and by the National Nature Science Foundation of China under Contract 61108007, 61090391.
Fig. 8. Fluctuations of the output power and wavelength over periods of 60 min at six different wavelengths.
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