Optimization of electrically tunable VCSEL with intracavity nematic liquid crystal

We optimize the wavelength tuning range of a Vertical-Cavity Surface-Emitting Laser with an intracavity layer of nematic Liquid Crystal (LC-VCSEL) lasing around 1.3 μm. The tunability is obtained by applying voltage to the liquid crystal layer, which esentially is to vary the refractive index from the extraordinary to the ordinary. We achieve 71.6 nm continuous tuning (without mode hopping) with liquid crystal thickness of about 3.2 μm. We investigate the impact of ambient temperature on the LC-VCSEL tuning range and show that mode-hop tuning can be achieved in the temperature range from −10◦C to 50◦C where the LC is in nematic phase. © 2015 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (140.7260) Vertical cavity surface emitting lasers; (160.3710) Liquid crystals. References and links 1. C. F. R. Mateus, M. C. Y. Huang, C. J. Chang-Hasnain, J. E. Foley, R. Beatty, P. Li, and B. T. Cunningham, “Ultra-sensitive immunoassay using VCSEL detection system,” Electron. Lett. 40, 649–651(2004). 2. M. Lewander, A. Fried, P. Weibring, D. Richter, S. Spuler, L. Rippe, “Fast and sensitive time-multiplexed gas sensing of multiple lines using a miniature telecom diode laser between 1529 nm and 1565 nm,” Appl. Phys. B 104, 715(2011). 3. M. Lackner, M. Schwarzott, F. Winter, B. Kogel, S. Jatta, H. Halbritter, and P. Meissner, “CO and CO2 spectroscopy using a 60 nm broadband tunable MEMS-VCSEL at 1.55 μm,” Opt. Lett. 31, 3170–3172(2006). 4. V. Jayaraman, J. Jiang, B. Potsaid, G. Cole, J. Fujimoto, and A. Cable, “Design and performance of broadly tunable, narrow line-width, high repetition rate 1310nm VCSELs for swept source optical coherence tomography,” Proc. SPIE 8276, 82760D (2012). 5. C. Gierl, K. Zogal, S. Jatta, H. A. Davani, F. Kuppers, P. Meissner, T. Grundl, C. Grasse, M.-C. Amann, A. Daly, B. Corbett, B. Kgel, A. Haglund, J. Gustavsson, P. Westbergh, A. Larsson, P. Debernardi, and M. Ortsiefer, “Tuneable VCSEL aiming for the application in interconnects and short haul systems,” Proc. SPIE 7959, 795908 (2011). 6. C.J. Chang-Hasnain, “Tunable VCSEL,” IEEE J. Sel. Top. Quantum Electron. 6, 978–987 (2000). 7. Y. Zhou, M. Huang, and C.J. Chang-Hasnain, “Tunable VCSEL with ultra-thin high contrast grating for highspeed tuning,” Opt. Express 16, 14221–14226 (2008). 8. R. Yi, Y. Weijian, C. Chase, M.C.Y. Huang, D.D.P. Worland, S. Khaleghi, M.R. Chitgarha, M. Ziyadi, A.E. Willner, and C.J. Chang-Hasnain, “Long-wavelength VCSEL using high-contrast grating,” IEEE J. Sel. Top. Quantum Electron. 19 1701311 (2013). #234966 $15.00 USD Received 19 Feb 2015; revised 17 Apr 2015; accepted 6 May 2015; published 5 Jun 2015 © 2015 OSA 15 Jun 2015 | Vol. 23, No. 12 | DOI:10.1364/OE.23.015706 | OPTICS EXPRESS 15706 9. M.S Wu, E.C. Vail, G.S. Li, W. Yuen, and C.J. Chang-Hasnain, “Tunable micromachined vertical cavity surface emitting laser,” Electron. Lett. 31, 1671–1672 (1995). 10. D. Vakhshoori, P. Tayebati, C.-C. Lu, M. Azimi, P. Wang, J.-H. Zhou, and E. Canoglu, “2m WCW single-mode operation of a tunable 1550 nm vertical cavity surface emitting laser with 50 nm tuning range,” Electron. Lett. 35, 900–901 (1999). 11. A. Syrbu, V. Iakovlev, G. Suruceanu, C.-A. Berseth, A. Rudra, A. Mircea, A. Mereuta, and E. Kapon, “1 mW CW 38 nm tunable 1.5 μm VCSELS with tuning voltage below 4 V,” Opt. Commun. 5, 1–2 (2002). 12. A. Syrbu, V. Iakovlev, G. Suruceanu, A. Caliman, A. Rudra, A. Mircea, A. Mereuta, S. Tadeoni, C.-A. Berseth, M. Achtenhagen, J. Boucart, and E. Kapon, “1.55-μm optically pumped wafer-fused tunable VCSELs with 32nm tuning range,” IEEE Photon. Technol. Lett. 16, 1991–1993 (2004). 13. V. Jayaraman, G. D. Cole, M. Robertson, A. Uddin, and A. Cable, “High-sweep-rate 1310 nm MEMS-VCSEL with 150 nm continuous tuning range,” Electron. Lett. 48, 867–869 (2012). 14. F. Riemenschneider, M. Maute, H. Halbritter, G. Boehm, M.-C. Amann, and P. Meissner, “Continuously tunable long-wavelength MEMS-VCSEL with over 40-nm tuning range,” IEEE Photon. Technol. Lett. 16, 2212–2214 (2004). 15. M. Maute, B. Kgel, G. Bohm, P. Meissner, and M.-C. Amann,“MEMS-tunable 1.55-μm VCSEL with extended tuning range incorporating a buried tunnel junction,” IEEE Photon. Technol. Lett. 18, 688–690 (2006). 16. S. Jatta, B. Kogel, M. Maute, K. Zogal, F. Riemenschneider, G. Bohm, M.-C. Amann, and P. Meisner, “BulkMicromachined VCSEL At 1.55 μm with 76-nm Single-Mode Continuous Tuning Range,” IEEE Photon. Technol. Lett. 21, 1822–1824 (2009). 17. C. Gierl, T. Gruendl, P. Debernardi, K. Zogal, C. Grasse, H. Davani, G. Bhm, S. Jatta, F. Kppers, P. Meiner, and M. Amann, “Surface micromachined tunable 1.55 μm-VCSEL with 102 nm continuous single-mode tuning,” Opt. Express 19, 17336–17343 (2011). 18. C. Gierl, K. Zogal, S. Paul, and F. Kppers, “Tunable MEMS-VCSEL with > 140-nm tuning range using tuning range using SiO2/SiC-based MEMS-DBR,” Proc. SPIE 9001, 900107 (2014). 19. C. Levallois, B. Caillaud, J. de Bougrenet de la Tocnaye, L. Dupont, A. Lecorre, H. Folliot, O. Dehaese, and S. Loualiche, “Nano-polymer-dispersed liquid crystal as phase modulator for a tunable vertical-cavity surfaceemitting laser at 1.55 μm,” Appl. Opt. 45, 8484–8490 (2006). 20. O. Castany, L. Dupont, A. Shuaib, J.P. Gauthier, C. Levallois, and C. Paranthoen, “Tunable semiconductor vertical-cavity surface-emitting laser with an intracavity liquid crystal layer,” Appl. Phys. Lett. 98, 161105 (2011). 21. O. Castany, C. Paranthoen, C. Levallois, A. Shuaib, J.P. Gauthier, N. Chevalier, O. Durand, L. Dupont, and A. Le Corre, “Demonstration of a 34 nm monolithic continuously tunable VCSEL at 1.55 μm combined with liquid crystal,” Compound Semiconductor Week (CSW/IPRM), 23rd International Conference on Indium Phosphide and Related Materials (May 2011) pp. 1–4. 22. Y. Xie, J. Beeckman, W. Woestenborghs, K. Panajotov, and K. Neyts, “VCSEL with photo-aligned liquid crystal overlay,” IEEE Photon. Technol. Lett. 24, 1509–1512 (2012). 23. Y. Xie, J. Beeckman, K. Panajotov, and K. Neyts, “Vertical-cavity surface-emitting laser with a liquid crystal external cavity,” Opt. Lett. 39, 6494–6497 (2014). 24. K. Panajotov and H. Thienpont, “Vertical-cavity surface-emitting laser with liquid crystal overlay,” Opt. Express 19, 16749–16759 (2011). 25. K. Panajotov, Y. Xie, M. Dems, C. Belmonte, H. Thienpont, J. Beeckman, K. Neyts, “Vertical-cavity surfaceemitting laser emitting circularly polarized light,” Laser Phys. Lett. 10, 105003 (2013). 26. Y. Xie, J. Beeckman, K. Panajotov, K. Neyts, “Vertical-cavity surface-emitting laser with a chiral nematic liquid crystal overlay, IEEE Photon. J. 6, 1500010 (2014). 27. N. Volet, T. Czyszanowski, J. Walczak, L. Mutter, B. Dwir, Z. Mickovi, P. Gallo, V. Iakovlev, A. Sirbu, A. Caliman, A. Mereuta, and E. Kapon, “Improved single-mode emission characteristics of long-wavelength waferfused vertical-cavity surface-emitting lasers by intra-cavity patterning,” Proc. SPIE 8639, 86390S (2013). 28. X.H. Li, V.M. Kozenkov, F.S.Y. Yeung, P.Z. Xu, V.G. Chigrinov, and H.S. Kwok, “Liquid-crystal photoalignment by super thin azo dye layer,” Jpn. J. Appl. Phys. 45, 203–205 (2006). 29. M. Born and E. Wolf, Principles of Optics (Wiley, 1970). 30. J. Li, S.-T. Wu, S. Brugioni, R. Meucci, and S. Faetti, “Infrared refractive indices of liquid crystals,” J. Appl. Phys. 97, 073501 (2005). 31. J. Li, C. Wen, S. Gauza, R. Lu, and S. Wu, “Refractive indices of liquid crystals for display applications,” J. Display Technol. 1, 51 (2005). 32. T. Czyszanowski, M. Dems, R. Sarzaa, K. Panajotov, K. D. Choquette, “Photonic crystal VCSELs: detailed comparison of experimental and theoretical spectral characteristics,” IEEE J. Sel. Top. Quantum Electron. 19, 1701908 (2013). 33. R. Sarzala, T. Czyszanowski, M. Wasiak, M. Dems, L. Piskorski, W. Nakwaski, K. Panajotov, “Numerical self-consistent analysis of VCSELs advances in optical technologies,” Advances in Optical Technologies 2012, 689519 (2012). 34. T. Czyszanowski, M. Dems, K. Panajotov, “Single mode condition and modes discrimination in photonic-crystal #234966 $15.00 USD Received 19 Feb 2015; revised 17 Apr 2015; accepted 6 May 2015; published 5 Jun 2015 © 2015 OSA 15 Jun 2015 | Vol. 23, No. 12 | DOI:10.1364/OE.23.015706 | OPTICS EXPRESS 15707 1.3 μm AlInGaAs/InP VCSEL,” Opt. Express 15, 5604 (2007) 35. T. Czyszanowski, N. Volet, J. Walczak, M. Dems, R.P. Sarzala, V. Iakovlev, A. Sirbu, A. Mereuta, A. Caliman, E. Kapon, “Numerical analysis of mode discrimination by intracavity patterning in long-wavelength wafer-fused vertical-cavity surface-emitting lasers,” IEEE J. Quantum Electron. 50, 732–740 (2014).


Introduction
Tunable semiconductor lasers operating in the wavelength region around 1310 nm and 1550 nm are of great interest for various applications, such as biological and gas sensing [1,2], spectroscopy [3], tomography [4] or telecommunications [5][6][7][8]. Many groups have demonstrated wavelength tuning of electrical or optically pumped VCSEL combined with Micro-Electro-Mechanical-System (MEMS) technology, using a moving membrane like a mirror to change the optical cavity length and hence provide wavelength tuning. MEMS movable reflector are typically cantilever or membrane structures using high contrast gratings or DBRs and are actuated by electrostatic, electrothermal or piezo-electric forces.
MEMS-VCSEL, in which the top DBR comprises a micromachined cantilever (c-VCSEL), was first demonstrated in 1995 in [9]. Wavelength tuning range (TR) of 15 nm was achieved for a voltage of 5.7 V. In 1999, a tunable 1.55 μm VCSEL with a 50 nm TR was developed [10]. The device top membrane with a curved mirror is pulled down by electrostatic force, when applying voltage from 0 to 39 V between the top membrane and the bottom mirror. In 2002, a wafer-fused optically pumped tunable VCSEL with output power up to 1 mW at 1.55-μm and 38 nm TR for a voltage change of 4 V was reported [11]. In 2004, the same group achieved an output power of 2 mW reducing the TR to 32 nm for a voltage below 4 V [12]. TR is further increased to 150 nm in optically pumped MEMS-VCSEL around 1.3 μm in 2012 [13].
For the first time, an electrically pumped tunable MEMS-VCSEL with a TR of 40 nm and a maximum output power of 100 μW was developed in 2004 [14]. The VCSEL is based on a two-chip concept, where the movable mirror membrane is fabricated on a separate chip, avoiding compromise between MEMS and VCSEL designs. Using a similar device, the same group improved the output power up to 1.7 mW by reducing the top mirror reflectivity, but reduced the TR till 28 nm [15]. The TR is significantly increased to 60 nm in 2006 [15] and to 76 nm in 2009 [16] using an antireflection coatings. The upper MEMS mirror is a SiO 2 -SiN concave DBR which can be actuated electrothermally. The maximum output power measured is 1.31 mW. A high performance electrically pumped single-mode MEMS tunable VCSEL with a TR of 102 nm with a maximum output power of 3.2 mW was developed in 2011 [17]. The technology used is based on low temperature plasma enhanced chemical vapor deposition without wafer-bonding or wafer-gluing and is not restricted to 1.55 μm only. The SiO x -SiN y based movable top DBR is a membrane, which is suspended on four flexible beams. When the current flows through the metallization of the top mirror membrane, the beams of the membrane are expanded, increasing the air-gap cavity length. The largest TR of 140 nm for electrically pumped MEMS-VCSEL was reported in 2014 [18]. The tunable VCSEL is based on the same structure as in [17] however, with high refractive index contrast SiO 2 /SiC layers movable DBR that broadens the spectral width of the reflectivity and increases the TR.
Despite the excellent and large TR, MEMS tunable VCSELs suffer from a rather complex technological process, and the separate MEMS are sensitive to mechanical vibrations. Another way to obtain wavelength tuning in VCSELs is to change its refractive index, using LC as an electro-optic material located in the cavity, forming a device without any movable part. LC-VCSELs have many advantages, such as simpler technological process, insensitivity to mechanical disturbances, small tuning response time and high reliability. Because of these impressive achievements, question arises if the TR of intracavity LC-VCSEL can be further increased, especially for structures that allow for electrical pumping. We answer this question here, by carrying out an optimization of an LC-VCSEL lasing around 1.31 μm and achieve a TR of 71.6 nm. Furthermore, we investigate the impact of temperature on the TR and show that mode-hop free TR can be maintained from −10 • C to 50 • C for LC thickness of 2.7 μm on the expense of decreasing the TR from 77.2 nm at −10 • C to 38.2 nm at 50 • C.

Device structure
In Fig. 1 a schematic cross section of the tunable LC-VCSEL is shown, with the LC placed in-between a Half VCSEL (bottom DBR and Active Region) and a top DBR.  Fig. 1. Schematic of the cross section of a tunable LC-VCSEL with intracavity nematic LC. Detailed description of the structure with labels denoted is given in Table 1. Our structure is based on [27], where a wafer-fusion VCSEL has been optimized for low threshold current and wide stop-band of more than 100 nm. The bottom DBR consists of 35 pairs of nid GaAs/Al 0.9 Ga 0.1 As. The bottom DBR is wafer-fused to a 5/2λ cavity grown on an InP substrate. The active region consists of six compressively strained AlGaInAs QWs with seven tensile strained barriers of AlGaInAs. The p-doped AlInAs layer acts as a current blocking layer and is followed by a buried highly doped P+ N+ tunnel junction. On top of the N-InP spacer we place the LC layer sandwiched between two alignment layers of azo dye SD1 [28]. The top DBR consists of N top = 25 pairs of nid-GaAs/Al 0.9 Ga 0.1 As and 1 pair of n-doped (N = 5 · 10 18 cm −3 ) GaAs/Al 0.9 Ga 0.1 As. We also study the cases with N top = 20 and N top = 16.
Finally, two intracavity highly doped contact layers achieve a proper electrical injection. The doped layer in the top DBR acts as a third intracavity contact, which is used to apply a voltage to the LC. The detailed description of the LC-VCSEL structure is presented in Table 1. The current is uniformly distributed thanks to N-doped InP layers (1) and (2) before injection into the cavity starting with the nid InP layer (3). The layers (4) and (7) (P-AlInAs and N-InP, respectively) are spacers to adjust the tunnel junction. A uniform current injection is necessary for avoiding any filamentation of the lasing mode, which is why the current injected into the tunnel junction is spread by the N-doped spreading layers (8) and (9). Layers (1) and (9) are much thicker than the rest of the layers and are used for fine tuning the cavity thickness for the emission wavelength. In our device the tunnel junction consists of two AlGaInAs layers of 15 nm; the first one is N-type doped (N = 5 · 10 19 cm −3 ) and the second is P-type doped (P = 5 · 10 19 cm −3 ). We implement a nematic E7 LC placed between the layer (9) and the top DBR. The layer of the top DBR in contact with the LC is highly-doped and it is the cathode contact to apply the voltage to the LC. Alignment layers are required on both sides of the LC to control the orientation of its molecules. When a voltage is applied to the LC, its molecules tilt from a planar to a nearly homeotropic orientation which means that the refractive index goes from the extraordinary to the ordinary, and the laser emission wavelength of the LC-VCSEL decreases.

Theoretical results
Our procedure of optimizing the LC-VCSEL for large TR is based on the transfer matrix method [24, 29] to find the resonant wavelengths and the corresponding threshold gains. Three different variables of our structure are varied: the LC thickness, the number of pairs in the top DBR and the transition from the extraordinary refractive index (n e ) to the ordinary refractive index (n o ) of the LC when voltage is applied. This transition will determine the tuning spectra of our LC-VCSEL. The refractive indices for light traveling along Z axis and linearly polarized along the X and Y axes are determined by [24] n x = n o , where θ is the angle at which the molecules are aligned with respect to the z axis. We carry out the optimization procedure by performing a sweep of the LC thickness, with fixed number of DBR and refractive index of the LC, either the ordinary or the extraordinary (n o or n e ). Figure  2 shows the evolution of the lowest threshold gain mode and the corresponding resonance wavelength, with the thickness of the LC. The number of pairs of the top DBR is fixed to 25. The period of the solid blue curve for the ordinary refractive index (n o =1.5036) is 440 nm and for the dashed red line for extraordinary refractive index (n e =1.6899) is 387 nm, which correspond to half of the optical wavelength of λ 0 = 1.31μm (n o L o ≈ n e L e ≈ λ 0 /2). Due to the fact that the two periods are different, there is a situation when the peaks for the n o and n e refractive indices coincide (e.g. at L ≈ 3.3 μm and L ≈ 6.6 μm). This modulation is better seen in Fig.  3 where the differences between the two curves from Fig. 2 are shown. The periodicity of the envelope of the modulation in Fig. 3 corresponds to L = 3.51 μm, which is given approximately by L ≈ λ /(2( n e -n o )). From Fig. 3 is clearly seen that the maximum mode-hop free TR of about 71.6 nm occurs when the LC thickness is around 3.2 μm. A comparison of the three threshold gains corresponding to the maximum TR as a function of the LC refractive index for different numbers of top DBR (25, 20 and 16) can be seen in Fig.  4(a). The TR as a function of the LC refractive index for the three different cases is shown in Fig. 4(b). As can be seen, the lowest threshold gain with the largest TR is achieved with N top = 25 pairs. Quite importantly, the TR is not strongly impacted by the reflectivity of the top mirror, remaining approximately around 72 nm. where From Fig. 3 it can be seen that the maximum TR occurs when the thickness of the LC is around 3.2 μm. To study the temperature effect on the wavelength tuning, we take this thickness and check the TR from −10 • C to 50 • C. Figure 5 shows the TR for three different LC thicknesses of L LC = 3.2 μm, 2.7 μm and 2.3 μm, where we obtain a mode-hop free TR in temperature ranges of ΔT = 25 • C, 40 • C and 60 • C, respectively. As can be seen, mode-hop free TR can be maintained from −10 • C to 50 • C for LC thickness of 2.7 μm on the expense of decreasing the TR from 77.2 nm at −10 • C to 38.2 nm at 50 • C.   In the calculation method employed so far the internal heating of the VCSEL by the injection current has not been taken into account. Question therefore arises if this heating can prevent the wavelength tuning when electro-optically changing the LC refractive index. In order to answer this question, we have carried out 3D calculation of the LC-VCSEL system that accounts self-consistently for the injection current distribution, generated temperature distribution, its impact on the gain and the resonant optical mode at the lasing threshold [32-34]. The model was already used in simulations of similar structures and revealed very good agreements with experiment [35]. Example of such calculations for LC thickness of 2.315 μm is presented in Fig. 6, where the top panel presents the resonant wavelength, the middle panel the thresholds gain and the bottom panel the temperature rise in the LC above the ambient temperature as a function of LC refractive index. As can be seen from Fig. 6(a), the wavelength tuning properties of the LC-VCSEL are reproduced well by these self-consistent calculations revealing threshold gain change with LC refractive index ( Fig. 6(b)), similar to the one in Fig. 4(a) obtained by the simple 1D optical solver. Quite importantly, the self-consistent LC-VCSEL model reveals that the temperature rise in the LC layer is only about 10 • C for a current of 6.43 mA (Fig. 6(c)) and can not therefore impact severely the wavelength tuning properties of LC-VCSEL system. Finally, Fig. 7 shows an example of temperature distribution in a vertical cross-section of the LC-VCSEL. In the intracavity contacted VCSEL considered here, heat is generated close to the active region and is mostly vertically transferred to the bottom DBR and the substrate and spread laterally in the semiconductor material.

Conclusion
An optimization has been carried out to achieve the maximum wavelength tuning range (TR) of a VCSEL lasing around 1.3 μm with an intracavity nematic LC (E7). A TR of 71.6 nm at room temperature (25 • C) is obtained for a LC thickness of 3.2 μm: when the refractive index of the LC goes from the extraordinary to the ordinary the laser emission wavelength decreases from 1342.7 nm to 1271 nm. The threshold gain remains lower than 2500 cm −1 (3200 cm −1 ) in the whole TR for a top DBR with N top = 25 (20) pairs. Decreasing the top DBR reflectivity increases the threshold gain, however it does not impact the tuning range. We have also checked the impact of the ambient temperature on the tuning range. For the E7 LC studied here, the temperature range of the nematic phase is from −10 • C to 60 • C. We have shown that a modehop free TR can be maintained from −10 • C to 50 • C for LC thickness of 2.7 μm on the expense of decreasing the TR from 77.2 nm at −10 • C to 38.2 nm at 50 • C. Finally, we ascertain our findings by a 3D calculation of the LC-VCSEL system that accounts self-consistently for the injection current distribution, generated temperature distribution, its impact on the gain and the resonant optical mode at the lasing threshold.