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Single mode compound microsphere laser
Optics Communications 420 (2018) 1–5
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single mode compound microsphere laser Muqiao Li, Jiulin Gan *, Zhishen Zhang, Wei Lin, Qilai Zhao, Shanhui Xu, Zhongmin Yang State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, Guangzhou 510641, China Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510641, China
ARTICLE
INFO
Keywords: Lasers Single mode Vernier effect
ABSTRACT Single mode laser is demonstrated based on compound microspheres. The two coupled microspheres, corresponding to the diameters of 32.7 μm and 49.2 μm, are both made from Er3+ /Yb3+ co-doped phosphate glass. Based on the Vernier effect, single mode laser emission has been realized at 1544.43 nm with the side-mode suppression ratio of 45.1 dB and the threshold of only 163 μW. This compound microspheres scheme can effectively select one laser emission mode among various modes, which can potentially used in micro-size laser source, high precision sensor and other frontier fields.
1. Introduction Single mode laser, which owns the merits of higher monochromaticity and better beam quality, plays a significant role in high precision measurement and manipulation research field, such as optical frequency standards [1], laser cooling [2], and single nanoparticle detection [3]. In recent years, single mode laser based on microresonators, especially whispering gallery mode (WGM) microcavities [4–12], has attracted widely interest in relevant research fields. The rotational symmetrical structure of the WGM microcavity can well confine energy by the total internal reflection law, which can be established in various shapes, such as microring [7,8], microdisk [9,10], microcylinder [11], and microsphere [12]. This principle guarantees high energy density, long photons life in WGM microcavity, which can lead to high cavity quality (𝑄) factor and small mode volume. For example, the microsphere can achieve high 𝑄-factor up to 1010 [13], which is a highly competitive component for sensing, lasing and nonlinear optics. Therefore, WGM microcavity, especially microsphere, has been demonstrated as an excellent candidate for constructing low threshold and narrow linewidth lasers. Microcavity laser based on single active microsphere has been thoroughly and deeply researched [12–21], where the microspheres have been fabricated by using many different gain materials for various wavelength lasing emission, such as Er3+ /Yb3+ co-doped phosphate glass [14,15], Ho3+ /Tm3+ co-doped silica [16], Nd3+ -doped borosilicate glass [17], Bi-doped germanate glass [18] and so on. However, multi-mode emission operation and relative low side mode suppress ratio based on the single microsphere laser has restricted its further application. In order to realize single mode laser operation in microsphere, *
shortening cavity length for larger free spectral range (FSR) would be a single and direct scheme. However, microsphere for lasing is typically fabricated to be larger than 15 μm in diameter with acceptable radiation loss [19], where the corresponding FSR is the same order of magnitude as he gain bandwidth of the Er3+ doped gain materials. Although there are only a few fundamental modes within the whole gain bandwidth, tens of higher order modes will also oscillate in microsphere. As a result, both fundamental modes and higher order modes will be excited simultaneously. The single microsphere laser generally exhibits multilongitudinal and transverse mode operation [20–22]. Therefore, assistive technology is urgently needed for effectively achieving single mode operation in microsphere laser. Conventional technologies, such as optical feedback [23] and injection seeding [24], are too complicate and bulky for the microsphere laser. Microcavity with metallic coating has been demonstrated to reduce cavity dimensions considerably smaller than the wavelength of light and achieved single mode lasing [25]. However, extremely high gain material is desired to enable lasing due to high loss in metal. Microcavities with parity-time (PT) symmetry have also been demonstrated for mode selection, where the breaking of PT-symmetry condition will induce the desired mode to have a higher gain while suppressing other modes [26]. However, elaborate manipulating the interplay between gain and loss in PTsymmetry microcavities is complicated and the use of such a structure would greatly increase the fabrication complexity. Different from the complicated schemes as mentioned above, compound microcavity structure, typically consists of two microcavities with various materials and
Corresponding author. E-mail address: [email protected] (J. Gan).
https://doi.org/10.1016/j.optcom.2018.03.022 Received 19 December 2017; Received in revised form 11 February 2018; Accepted 8 March 2018 0030-4018/© 2018 Published by Elsevier B.V.
M. Li et al.
Optics Communications 420 (2018) 1–5
is vertical hung and heated in the middle using CO2 laser with high power, and it has been drawn to be several microns in diameter under gravity. Secondly, this microrod is cut in the middle using high power laser pulse. Finally, the end of microrod has been melted and curled to be a microsphere naturally through surface tension. The diameters of active microspheres used in experiment are 32.7 μm and 49.2 μm respectively. 3. Results and discussion With respect to the fundamental modes in cavity, the resonant condition can be approximately written as: 𝜋𝑛𝑒𝑓 𝑓 𝐷 = 𝑙𝜆
(1)
where 𝑛𝑒𝑓 𝑓 is the effective refractive index of the resonant fundamental mode; D is the diameter of the microsphere; 𝜆 is the resonant wavelength; l is an integer giving the number of field maxima in a round trip of the resonator. FSR is defined as wavelength spacing 𝛥𝜆𝐹 𝑆𝑅 of two adjacent fundamental modes in a resonator. It can be given by expression:
Fig. 1. Schematic diagram of compound microspheres laser. The fiber taper is used for launching the pump light (yellow arrows) and collecting the signal light (green arrows). The dashed arrows in the microsphere represent the WGM of the pump and signal light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
𝛥𝜆𝐹 𝑆𝑅 ≈
𝜆2 𝜋𝑛𝑒𝑓 𝑓 𝐷
(2)
According to Eq. (2), FSR will increase with R decreasing. The 𝛥𝜆𝐹 𝑆𝑅 is calculated to be 15.2 nm for 32.7 μm diameter microsphere used in the experiment. Theoretically, within about 60 nm gain bandwidth of the Er3+ /Yb3+ co-doped phosphate glass, there are about four fundamental modes satisfying resonant condition. Many higher order modes will also have chance to be excited. Thus, the Vernier effect based on compound microcavity is induced for enhancing FSR and suppressing most of laser modes. The FSR of each microsphere is different, and the common resonant wavelength will be selected based on the Vernier effect. The relationship of the FSR between compound microspheres and related single microsphere could be written as:
shapes, can realize single mode lasing operation based on the Vernier effect and intracavity modulation [27]. Vernier-scale consists of two scales with different periods and the overlap between lines on the two scales, which can be used to perform high accuracy measurement. The Vernier effect concluded from Vernier-scale, which is a compact and suitable approach for mode management, has been applied in photonic devices and various kinds of compound microcavity for single mode lasing operation [28–31]. In this paper, compound microspheres exploiting the Vernier effect are composed of two coupled Er3+ /Yb3+ co-doped glass microspheres with different diameters. The single mode lasing operation is achieved at 1544.43 nm with side-mode suppression ratio 45.1 dB and laser threshold of 163 μW. This single mode compound microspheres laser can be potentially used in bio-sensing, nanoprocessor and single photon light source.
𝐹 𝑆𝑅1,2 = 𝑛1 𝐹 𝑆𝑅1 = 𝑛2 𝐹 𝑆𝑅2
(3)
where 𝐹 𝑆𝑅1,2 is the FSR of compound microspheres; 𝐹 𝑆𝑅1 and 𝐹 𝑆𝑅2 are the FSR of single microsphere separately; 𝑛1 and 𝑛2 are integers. Combining Eq. (1) with Eq. (2), the relationship between the ratio of 𝑛1 and 𝑛2 and the diameters of two microspheres can be written as:
2. Experiment devices
𝐷 𝑛1 = 1 𝑛2 𝐷2
Fig. 1 shows the experimental diagram of compound microspheres laser configuration. The fiber taper is selected for launching the pump light and collecting the signal light [32]. The condition of phasematching can be obtained by adjusting the diameters of microfiber and microsphere in coupling area. Meanwhile, proper distance between them will enhance coupling efficiency. The pump light can be effectively coupled into and out of the microsphere through evanescent field. This coupled method by microfiber possesses higher coupling efficiency than free space, prism and many other coupling media, which would bring the advantages of significant reduction in lasing threshold. The microfiber with 1.1 μm diameter is directly drawn from single mode fiber (HI1060, Corning) by the flame-brushing technique [33]. Through this method, the microfiber used in experiment has less than 0.1 dB/mm loss and 3 dB insertion loss. Active microsphere is fabricated from homemade Er3+ /Yb3+ codoped phosphate glass with concentrations of 3.0 mol% for Er3+ and 5.0 mol% for Yb3+ respectively. Its refractive index is about 1.54 around 1.55 μm wavelength. The effective gain bandwidth of this Er3+ /Yb3+ co-doped phosphate material covers the range from 1520 nm to 1580 nm. More details about this gain material can be found in our previous work [34]. Active microsphere has been fabricated by laser thermoforming technique. Firstly, Er3+ /Yb3+ co-doped glass rod
where 𝐷1 and 𝐷2 are the diameters of two microspheres. Microspheres used in this experiment are 32.7 μm and 49.2 μm. Therefore, the mode spacing in compound microspheres is at least about 45.6 nm. The Vernier effect enhanced compound microspheres are helpful for enlarging the interval of lasing modes and offering much higher possibility for achieving single mode lasing operation. The diameters of microspheres used in this experiment are 32.7 μm and 49.2 μm, which are marked as ‘‘microsphere A’’ and ‘‘microsphere B’’ respectively in paper for simplification. As shown in Fig. 1, the smaller ‘‘microsphere A’’ is directly coupled with microfiber for obtaining pump light and exporting signal light. The larger ‘‘microsphere B’’ is placed close enough to ‘‘microsphere A’’ for evanescent field coupling. These two microspheres are assembled to be compound microspheres (marked as ’’microsphere A+B). Fine adjustment has been executed to ensure that the microfiber and the two equatorial planes of the microspheres are located at the same plane. A 976 nm continuewave laser with 10 MHz linewidth (DBR976S, Thorlabs corporation) is adopted as pump source, which is launched into compound microspheres through evanescent field coupling. When pump power is above the laser threshold, the laser will be oscillating and propagating along clockwise and counter-clockwise directions within microsphere. The laser emission can be coupled out from both ends of the tapered fiber and recorded by optical spectrum analysis (OSA, Anritsu MS9710C) 2
(4)
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Optics Communications 420 (2018) 1–5
Fig. 2. The optical spectrum and power curve of compound microspheres (microsphere A+B). (a) Optical spectrum of compound microspheres laser. Inset figure ( ) is the partially enlarged drawing of (a) within 2 nm spectrum range. (b) Power curve of compound microspheres. Inset image (ii) is the real object photo of microsphere A+B.
Fig. 3. The relationship of pump power and central wavelength of ‘‘compound microspheres A+B’’.
Fig. 4. F–P etalon data of single mode laser.
According to Fig. 5, both ‘‘microsphere A’’ and ‘‘microsphere B’’ exhibit multi-wavelength lasing emission. As indicated by the black arrows in the two spectra, two microsphere lasers have nearly the same resonant wavelength at 1544.4 nm. So based on the Vernier effect, great majority of modes will be suppressed and the common mode which is located at 1544.43 nm would be selected for the single mode lasing in the compound ‘‘microspheres A+B’’. It is interesting to note that the finally resonant mode need not be the fundamental mode. The wavelength shifting between the theory and experiment is due to the thermal effect since the coupling states of each microsphere are much different. In fact, the central lasing wavelength of ‘‘microsphere A’’, ‘‘microsphere B’’ and ‘‘microsphere A+B’’ cavity will shift with the pump power changing. The laser thresholds of ‘‘microsphere A’’, ‘‘microsphere B’’, and ‘‘microsphere A+B’’ laser are compared and analyzed. As shown in Fig. 6, the thresholds of ‘‘microsphere A’’ and ‘‘microsphere B’’ are 137.6 μW and 83.4 μW respectively. Thresholds of compound microspheres laser are a little higher, which is due to compound induced larger pump absorption and inevitable coupling loss between the two microspheres. The compound microspheres structure is an efficient approach for single mode selection. It is worth noticing that the single mode laser can be achieved by assembling two arbitrary different sizes of microspheres. In this scheme, the Vernier effect based compound microcavity can enlarge FSR and suppress a majority of laser modes supported in single microsphere. The common resonant wavelength of two microspheres need not be the fundamental mode because there are multiple higher order modes oscillating within one FSR. Also, the compound microspheres structure can be implemented with various gain material range from visible to infrared wavelength for single mode lasing application, which is a prospective component for bio-sensing, nanoprocessor, and so on.
with a wavelength resolution of 0.02 nm and power meter. The optical spectrum and power curve of the compound microspheres are shown in Fig. 2(a) and Fig. 2(b) respectively. The strong green light (upconverted photoluminescence of Er3+ /Yb3+ co-doped phosphate glass) has been observed in both microspheres, which is shown in the inset (ii) of Fig. 2(b). Based on the intensity of fluorescence, the optimal coupling can be realized easily. A single mode lasing emission has been achieved at 1544.43 nm based on the compound ‘‘microsphere A+B’’. The side-mode suppression ratio of this single mode laser is 45.1 dB. No other modes have been observed at expanded scanning range from 1520 nm to 1620 nm which is presented in Fig. 2(a). The output power versus the pump power is shown in Fig. 2(b), where the pump power is measured without subtracting the large transmission and coupling loss. The lasing threshold of the compound microsphere cavity is about 163 μW. The central lasing wavelength of ‘‘microsphere A+B’’ cavity will shift with the pump power changing. When increasing the pump power, the lasing wavelength produced by compound ‘‘microsphere A+B’’ shows redshift, as shown in Fig. 3, while the single mode lasing feature has been kept. Because of relative low resolution of the optical spectrum analysis (0.02 nm), a scanning Fabry–Perot (F–P) interferometer (SA210-9A) with a FSR of 1.5 GHz and a finesse of 300 is used to confirm the single mode feature of the compound microsphere laser. As shown in Fig. 4, within a scanning cycle, laser operates in single frequency stably without mode hop and mode competition. This lasing emission wavelength is believed to be the common mode of ‘‘microsphere A’’ and ‘‘microsphere B’’ alone separately. In order to prove it, the lasing spectra of single microsphere containing ‘‘microsphere A’’ and ‘‘microsphere B’’ are separately shown in Fig. 5(a) and Fig. 5(b), respectively 3
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Optics Communications 420 (2018) 1–5
Fig. 5. Optical spectra of ‘‘microsphere A’’ (a) and ‘‘microsphere B’’ (b). Inset image ( ) and ( ) are the real object photos of ‘‘microsphere A’’ (a) and ‘‘microsphere B’’ (b) respectively.
Fig. 6. The power curves of microspheres laser. (a) The power curve of ‘‘microsphere A’’ alone; (b) the power curve of ‘‘microsphere B’’ alone.
4. Conclusion
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In conclusion, single mode compound microspheres laser based on compound microcavity has been demonstrated. Because of higher order modes oscillation, single mode laser operation could not easily be realized based on one microsphere cavity. Based on the Vernier effect, single mode lasing emission has been achieved at 1544.43 nm by constructing the compound microspheres microcavity. This single mode compound microsphere laser owns the merits of high side-mode suppression ratio (45.1 dB) and low lasing threshold (163 μW), which indicates its potential application in high performance light source, high-precision sensor and other frontier fields in the future. Acknowledgments This work is supported by National Key Research and Development Program of China (2016YFB0402204), NSFC (61575064, U1609219), the Science and Technology Project of Guangdong (2015B090926010), Tiptop Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2015TQ01X322), the Fundamental Research Funds for Central Universities (2015ZP019), and the High level Personnel Special Support Program of Guangdong Province (2014TX01C087). References [1] T. Wu, X. Peng, W. Gong, Y. Zhan, Z. Lin, B. Luo, H. Guo, Observation and optimization of 4 He atomic polarization spectroscopy, Opt. Lett. 38 (6) (2013) 986– 988. [2] S.S. Sané, S. Bennetts, J.E. Debs, C.C.N. Kuhn, G.D. McDonald, P.A. Altin, J.D. Close, N.P. Robins, 11 W narrow linewidth laser source at 780 nm for laser cooling and manipulation of Rubidium, Opt. Express 20 (8) (2012) 8915–8919. [3] J. Zhu, Ş.K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, L. Yang, On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator, Nat. Photon. 4 (1) (2010) 46–49. [4] L. He, S.K. Ozdemir, J. Zhu, W. Kim, L. Yang, Detecting single viruses and nanoparticles using whispering gallery microlasers, Nat. Nanotechnol. 6 (7) (2011) 428–432.
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[21] Y. Ruan, K. Boyd, H. Ji, A. François, H. Ebendorff-Heidepriem, J. Munch, T.M. Monro, Tellurite microspheres for nanoparticle sensing and novel light sources, Opt. Express 22 (10) (2014) 11995–12006. [22] A. François, N. Riesen, H. Ji, V.S. Afshar, T.M. Monro, Polymer based whispering gallery mode laser for biosensing applications, Appl. Phys. Lett. 106 (3) (2015) 031104. [23] H. Ghafouri-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters, second ed., John Wiley & Sons Ltd., Chichester, 2003, p. 172. [24] F. Wei, B. Lu, J. Wang, D. Xu, Z. Pan, D. Chen, H. Cai, R. Qu, Precision and broadband frequency swept laser source based on high-order modulation-sideband injectionlocking, Opt. Express 23 (4) (2015) 4970–4980. [25] M.T. Hill, Y.-S. Oei, B. Smalbrugge, Y. Zhu, T. de Vries, P.J. van Veldhoven, F.W.M. van Otten, T.J. Eijkemans, J.P. Turkiewicz, H. de Waardt, E.J. Geluk, S.-H. Kwon, Y.-H. Lee, R. Nötzel, M.K. Smit, Lasing in metallic-coated nanocavities, Nat. Photon. 1 (10) (2007) 589–594. [26] H. Hodaei, M.-A. Miri, M. Heinrich, D.N. Christodoulides, M. Khajavikhan, Paritytime–symmetric microring lasers, Science 346 (6212) (2014) 975–978. [27] H.D. Lu, X.J. Sun, M.H. Wang, J. Su, K.C. Peng, Single frequency Ti: sapphire laser with continuous frequency-tuning and low intensity noise by means of the additional intracavity nonlinear loss, Opt. Express 22 (20) (2014) 24551–24558.
[28] S. Lei, L. Liu, L. Xu, Single-frequency coupled asymmetric microcavity laser, Opt Lett. 33 (10) (2008) 1150–1152. [29] V.D. Ta, R. Chen, H. Sun, Coupled polymer microfiber lasers for single mode operation and enhanced refractive index sensing, Adv. Opt. Mater. 2 (3) (2014) 220–225. [30] L. Ge, H.E. Türeci, Inverse Vernier effect in coupled lasers, Phys. Rev. A 92 (1) (2015) 013840. [31] H. Wang, S. Liu, L. Chen, D. Shen, X. Wu, Dual-wavelength single-frequency laser emission in asymmetric coupled microdisks, Sci. Rep. 6 (2016) 38053. [32] L. Mescia, P. Bia, M. De Sario, A. Di Tommaso, F. Prudenzano, Design of mid-infrared amplifiers based on fiber taper coupling to erbium-doped microspherical resonator, Opt. Express 20 (7) (2012) 7616–7629. [33] G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. Sessions, E. Koukharenko, X. Feng, G. Murugan, J. Wilkinson, D. Richardson, Optical fiber nanowires and microwires: fabrication and applications, Adv. Opt. Photon. 1 (1) (2009) 107–161. [34] S.H. Xu, Z.M. Yang, T. Liu, W.N. Zhang, Z.M. Feng, Q.Y. Zhang, Z.H. Jiang, An efficient compact 300 mW narrow-linewidth single frequency fiber laser at 1.5 μm, Opt. Express 18 (2) (2010) 1249–1254.
5
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single mode compound microsphere laser Muqiao Li, Jiulin Gan *, Zhishen Zhang, Wei Lin, Qilai Zhao, Shanhui Xu, Zhongmin Yang State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, Guangzhou 510641, China Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510641, China
ARTICLE
INFO
Keywords: Lasers Single mode Vernier effect
ABSTRACT Single mode laser is demonstrated based on compound microspheres. The two coupled microspheres, corresponding to the diameters of 32.7 μm and 49.2 μm, are both made from Er3+ /Yb3+ co-doped phosphate glass. Based on the Vernier effect, single mode laser emission has been realized at 1544.43 nm with the side-mode suppression ratio of 45.1 dB and the threshold of only 163 μW. This compound microspheres scheme can effectively select one laser emission mode among various modes, which can potentially used in micro-size laser source, high precision sensor and other frontier fields.
1. Introduction Single mode laser, which owns the merits of higher monochromaticity and better beam quality, plays a significant role in high precision measurement and manipulation research field, such as optical frequency standards [1], laser cooling [2], and single nanoparticle detection [3]. In recent years, single mode laser based on microresonators, especially whispering gallery mode (WGM) microcavities [4–12], has attracted widely interest in relevant research fields. The rotational symmetrical structure of the WGM microcavity can well confine energy by the total internal reflection law, which can be established in various shapes, such as microring [7,8], microdisk [9,10], microcylinder [11], and microsphere [12]. This principle guarantees high energy density, long photons life in WGM microcavity, which can lead to high cavity quality (𝑄) factor and small mode volume. For example, the microsphere can achieve high 𝑄-factor up to 1010 [13], which is a highly competitive component for sensing, lasing and nonlinear optics. Therefore, WGM microcavity, especially microsphere, has been demonstrated as an excellent candidate for constructing low threshold and narrow linewidth lasers. Microcavity laser based on single active microsphere has been thoroughly and deeply researched [12–21], where the microspheres have been fabricated by using many different gain materials for various wavelength lasing emission, such as Er3+ /Yb3+ co-doped phosphate glass [14,15], Ho3+ /Tm3+ co-doped silica [16], Nd3+ -doped borosilicate glass [17], Bi-doped germanate glass [18] and so on. However, multi-mode emission operation and relative low side mode suppress ratio based on the single microsphere laser has restricted its further application. In order to realize single mode laser operation in microsphere, *
shortening cavity length for larger free spectral range (FSR) would be a single and direct scheme. However, microsphere for lasing is typically fabricated to be larger than 15 μm in diameter with acceptable radiation loss [19], where the corresponding FSR is the same order of magnitude as he gain bandwidth of the Er3+ doped gain materials. Although there are only a few fundamental modes within the whole gain bandwidth, tens of higher order modes will also oscillate in microsphere. As a result, both fundamental modes and higher order modes will be excited simultaneously. The single microsphere laser generally exhibits multilongitudinal and transverse mode operation [20–22]. Therefore, assistive technology is urgently needed for effectively achieving single mode operation in microsphere laser. Conventional technologies, such as optical feedback [23] and injection seeding [24], are too complicate and bulky for the microsphere laser. Microcavity with metallic coating has been demonstrated to reduce cavity dimensions considerably smaller than the wavelength of light and achieved single mode lasing [25]. However, extremely high gain material is desired to enable lasing due to high loss in metal. Microcavities with parity-time (PT) symmetry have also been demonstrated for mode selection, where the breaking of PT-symmetry condition will induce the desired mode to have a higher gain while suppressing other modes [26]. However, elaborate manipulating the interplay between gain and loss in PTsymmetry microcavities is complicated and the use of such a structure would greatly increase the fabrication complexity. Different from the complicated schemes as mentioned above, compound microcavity structure, typically consists of two microcavities with various materials and
Corresponding author. E-mail address: [email protected] (J. Gan).
https://doi.org/10.1016/j.optcom.2018.03.022 Received 19 December 2017; Received in revised form 11 February 2018; Accepted 8 March 2018 0030-4018/© 2018 Published by Elsevier B.V.
M. Li et al.
Optics Communications 420 (2018) 1–5
is vertical hung and heated in the middle using CO2 laser with high power, and it has been drawn to be several microns in diameter under gravity. Secondly, this microrod is cut in the middle using high power laser pulse. Finally, the end of microrod has been melted and curled to be a microsphere naturally through surface tension. The diameters of active microspheres used in experiment are 32.7 μm and 49.2 μm respectively. 3. Results and discussion With respect to the fundamental modes in cavity, the resonant condition can be approximately written as: 𝜋𝑛𝑒𝑓 𝑓 𝐷 = 𝑙𝜆
(1)
where 𝑛𝑒𝑓 𝑓 is the effective refractive index of the resonant fundamental mode; D is the diameter of the microsphere; 𝜆 is the resonant wavelength; l is an integer giving the number of field maxima in a round trip of the resonator. FSR is defined as wavelength spacing 𝛥𝜆𝐹 𝑆𝑅 of two adjacent fundamental modes in a resonator. It can be given by expression:
Fig. 1. Schematic diagram of compound microspheres laser. The fiber taper is used for launching the pump light (yellow arrows) and collecting the signal light (green arrows). The dashed arrows in the microsphere represent the WGM of the pump and signal light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
𝛥𝜆𝐹 𝑆𝑅 ≈
𝜆2 𝜋𝑛𝑒𝑓 𝑓 𝐷
(2)
According to Eq. (2), FSR will increase with R decreasing. The 𝛥𝜆𝐹 𝑆𝑅 is calculated to be 15.2 nm for 32.7 μm diameter microsphere used in the experiment. Theoretically, within about 60 nm gain bandwidth of the Er3+ /Yb3+ co-doped phosphate glass, there are about four fundamental modes satisfying resonant condition. Many higher order modes will also have chance to be excited. Thus, the Vernier effect based on compound microcavity is induced for enhancing FSR and suppressing most of laser modes. The FSR of each microsphere is different, and the common resonant wavelength will be selected based on the Vernier effect. The relationship of the FSR between compound microspheres and related single microsphere could be written as:
shapes, can realize single mode lasing operation based on the Vernier effect and intracavity modulation [27]. Vernier-scale consists of two scales with different periods and the overlap between lines on the two scales, which can be used to perform high accuracy measurement. The Vernier effect concluded from Vernier-scale, which is a compact and suitable approach for mode management, has been applied in photonic devices and various kinds of compound microcavity for single mode lasing operation [28–31]. In this paper, compound microspheres exploiting the Vernier effect are composed of two coupled Er3+ /Yb3+ co-doped glass microspheres with different diameters. The single mode lasing operation is achieved at 1544.43 nm with side-mode suppression ratio 45.1 dB and laser threshold of 163 μW. This single mode compound microspheres laser can be potentially used in bio-sensing, nanoprocessor and single photon light source.
𝐹 𝑆𝑅1,2 = 𝑛1 𝐹 𝑆𝑅1 = 𝑛2 𝐹 𝑆𝑅2
(3)
where 𝐹 𝑆𝑅1,2 is the FSR of compound microspheres; 𝐹 𝑆𝑅1 and 𝐹 𝑆𝑅2 are the FSR of single microsphere separately; 𝑛1 and 𝑛2 are integers. Combining Eq. (1) with Eq. (2), the relationship between the ratio of 𝑛1 and 𝑛2 and the diameters of two microspheres can be written as:
2. Experiment devices
𝐷 𝑛1 = 1 𝑛2 𝐷2
Fig. 1 shows the experimental diagram of compound microspheres laser configuration. The fiber taper is selected for launching the pump light and collecting the signal light [32]. The condition of phasematching can be obtained by adjusting the diameters of microfiber and microsphere in coupling area. Meanwhile, proper distance between them will enhance coupling efficiency. The pump light can be effectively coupled into and out of the microsphere through evanescent field. This coupled method by microfiber possesses higher coupling efficiency than free space, prism and many other coupling media, which would bring the advantages of significant reduction in lasing threshold. The microfiber with 1.1 μm diameter is directly drawn from single mode fiber (HI1060, Corning) by the flame-brushing technique [33]. Through this method, the microfiber used in experiment has less than 0.1 dB/mm loss and 3 dB insertion loss. Active microsphere is fabricated from homemade Er3+ /Yb3+ codoped phosphate glass with concentrations of 3.0 mol% for Er3+ and 5.0 mol% for Yb3+ respectively. Its refractive index is about 1.54 around 1.55 μm wavelength. The effective gain bandwidth of this Er3+ /Yb3+ co-doped phosphate material covers the range from 1520 nm to 1580 nm. More details about this gain material can be found in our previous work [34]. Active microsphere has been fabricated by laser thermoforming technique. Firstly, Er3+ /Yb3+ co-doped glass rod
where 𝐷1 and 𝐷2 are the diameters of two microspheres. Microspheres used in this experiment are 32.7 μm and 49.2 μm. Therefore, the mode spacing in compound microspheres is at least about 45.6 nm. The Vernier effect enhanced compound microspheres are helpful for enlarging the interval of lasing modes and offering much higher possibility for achieving single mode lasing operation. The diameters of microspheres used in this experiment are 32.7 μm and 49.2 μm, which are marked as ‘‘microsphere A’’ and ‘‘microsphere B’’ respectively in paper for simplification. As shown in Fig. 1, the smaller ‘‘microsphere A’’ is directly coupled with microfiber for obtaining pump light and exporting signal light. The larger ‘‘microsphere B’’ is placed close enough to ‘‘microsphere A’’ for evanescent field coupling. These two microspheres are assembled to be compound microspheres (marked as ’’microsphere A+B). Fine adjustment has been executed to ensure that the microfiber and the two equatorial planes of the microspheres are located at the same plane. A 976 nm continuewave laser with 10 MHz linewidth (DBR976S, Thorlabs corporation) is adopted as pump source, which is launched into compound microspheres through evanescent field coupling. When pump power is above the laser threshold, the laser will be oscillating and propagating along clockwise and counter-clockwise directions within microsphere. The laser emission can be coupled out from both ends of the tapered fiber and recorded by optical spectrum analysis (OSA, Anritsu MS9710C) 2
(4)
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Optics Communications 420 (2018) 1–5
Fig. 2. The optical spectrum and power curve of compound microspheres (microsphere A+B). (a) Optical spectrum of compound microspheres laser. Inset figure ( ) is the partially enlarged drawing of (a) within 2 nm spectrum range. (b) Power curve of compound microspheres. Inset image (ii) is the real object photo of microsphere A+B.
Fig. 3. The relationship of pump power and central wavelength of ‘‘compound microspheres A+B’’.
Fig. 4. F–P etalon data of single mode laser.
According to Fig. 5, both ‘‘microsphere A’’ and ‘‘microsphere B’’ exhibit multi-wavelength lasing emission. As indicated by the black arrows in the two spectra, two microsphere lasers have nearly the same resonant wavelength at 1544.4 nm. So based on the Vernier effect, great majority of modes will be suppressed and the common mode which is located at 1544.43 nm would be selected for the single mode lasing in the compound ‘‘microspheres A+B’’. It is interesting to note that the finally resonant mode need not be the fundamental mode. The wavelength shifting between the theory and experiment is due to the thermal effect since the coupling states of each microsphere are much different. In fact, the central lasing wavelength of ‘‘microsphere A’’, ‘‘microsphere B’’ and ‘‘microsphere A+B’’ cavity will shift with the pump power changing. The laser thresholds of ‘‘microsphere A’’, ‘‘microsphere B’’, and ‘‘microsphere A+B’’ laser are compared and analyzed. As shown in Fig. 6, the thresholds of ‘‘microsphere A’’ and ‘‘microsphere B’’ are 137.6 μW and 83.4 μW respectively. Thresholds of compound microspheres laser are a little higher, which is due to compound induced larger pump absorption and inevitable coupling loss between the two microspheres. The compound microspheres structure is an efficient approach for single mode selection. It is worth noticing that the single mode laser can be achieved by assembling two arbitrary different sizes of microspheres. In this scheme, the Vernier effect based compound microcavity can enlarge FSR and suppress a majority of laser modes supported in single microsphere. The common resonant wavelength of two microspheres need not be the fundamental mode because there are multiple higher order modes oscillating within one FSR. Also, the compound microspheres structure can be implemented with various gain material range from visible to infrared wavelength for single mode lasing application, which is a prospective component for bio-sensing, nanoprocessor, and so on.
with a wavelength resolution of 0.02 nm and power meter. The optical spectrum and power curve of the compound microspheres are shown in Fig. 2(a) and Fig. 2(b) respectively. The strong green light (upconverted photoluminescence of Er3+ /Yb3+ co-doped phosphate glass) has been observed in both microspheres, which is shown in the inset (ii) of Fig. 2(b). Based on the intensity of fluorescence, the optimal coupling can be realized easily. A single mode lasing emission has been achieved at 1544.43 nm based on the compound ‘‘microsphere A+B’’. The side-mode suppression ratio of this single mode laser is 45.1 dB. No other modes have been observed at expanded scanning range from 1520 nm to 1620 nm which is presented in Fig. 2(a). The output power versus the pump power is shown in Fig. 2(b), where the pump power is measured without subtracting the large transmission and coupling loss. The lasing threshold of the compound microsphere cavity is about 163 μW. The central lasing wavelength of ‘‘microsphere A+B’’ cavity will shift with the pump power changing. When increasing the pump power, the lasing wavelength produced by compound ‘‘microsphere A+B’’ shows redshift, as shown in Fig. 3, while the single mode lasing feature has been kept. Because of relative low resolution of the optical spectrum analysis (0.02 nm), a scanning Fabry–Perot (F–P) interferometer (SA210-9A) with a FSR of 1.5 GHz and a finesse of 300 is used to confirm the single mode feature of the compound microsphere laser. As shown in Fig. 4, within a scanning cycle, laser operates in single frequency stably without mode hop and mode competition. This lasing emission wavelength is believed to be the common mode of ‘‘microsphere A’’ and ‘‘microsphere B’’ alone separately. In order to prove it, the lasing spectra of single microsphere containing ‘‘microsphere A’’ and ‘‘microsphere B’’ are separately shown in Fig. 5(a) and Fig. 5(b), respectively 3
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Fig. 5. Optical spectra of ‘‘microsphere A’’ (a) and ‘‘microsphere B’’ (b). Inset image ( ) and ( ) are the real object photos of ‘‘microsphere A’’ (a) and ‘‘microsphere B’’ (b) respectively.
Fig. 6. The power curves of microspheres laser. (a) The power curve of ‘‘microsphere A’’ alone; (b) the power curve of ‘‘microsphere B’’ alone.
4. Conclusion
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