Liquid-Crystal Optical Switches and Signal Processors

Transcription

1 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 Liquid-Crystal Optical Switches and Signal Processors ABSTRACT Chongchang Mao, Ming Xu, Wei Feng, Jianyu Liu, and Jung-Chih Chiao Chorum Technologies 133 E. Arapaho Road, Richardson, TX 7581, USA. In this work, we presented several different types of liquid-crystal WDM (wavelength-divisionmultiplexing) signal processors including broadband optical switches, voltage-controlled variable attenuators and optical harmonic equalizers. Keywords: Liquid-crystal, WDM, optical components, optical switches, optical variable attenuators, optical equalizers. INTRODUCTION With increasing interest and required features in dynamically reconfigurable network architectures [1], compact non-mechanical optical signal processors are in great demand to develop reliable, fast-reconfiguring, energy/space-efficient network elements such as dynamic add/drop multiplexers, dynamic optical routers and all-optical cross connects with signal regulation functionality. Liquid crystal technology has been drawing a lot of attention to as an option for all-optical signal processors because of its unique abilities to provide optical performance comparable to optomechanical devices, but with the reliability and efficiency of solid-state devices. In this paper, several optical signal processors, including optical switches, optical attenuators and optical gain equalizers, using liquid-crystal technologies, are discussed. The measurement results of optical performance are presented. OPERATION PRINCIPLES The advantages of liquid-crystal optical devices have been widely recognized [2]. Liquid-crystal devices provide solid-state switching reliability, low control voltages and power consumption. They also have low insertion losses and low polarization-dependent losses since liquid-crystal materials have high transmittance at the near-ir wavelengths. Liquid-crystal materials also have wide operating spectral bandwidth, making them suitable for WDM applications. Output 1 Output Rotate Polarization Polarization Beam Splitter Output 2 Analog Polarization Rotation _ E Input LC _ E Biasing Voltage Figure 1: The concept of a liquid-crystal 1x2 switch. Input LC _ E Variable Voltage Figure 2: The concept of a liquid-crystal attenuator.

2 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 By applying a voltage on a liquid-crystal spatial modulator, liquid crystal molecules realign and rotate the polarizations of lasers passing through them. With a sufficient voltage, the laser polarization rotates to an orthogonal state. We utilize the discrete characteristics for optical switching by routing the electrically-controlled polarizations to the desired ports, as shown in Fig. 1. With a variable voltage, a desired portion of the input power can be adjusted to the output port by rotating the polarization. We utilize the analog characteristics for optical attenuation, as shown in Fig. 2 LIQUID-CRYSTAL SWITCHES In the past, two factors have limited the applications of liquid-crystal optical devices in practical networks: high temperature-dependent losses and relatively low operation speeds. We have demonstrated new optical architectures for temperature-insensitive fast switching. By optimizing liquid-crystal material parameters and improving device architectures, without heaters or temperature controllers, the temperature dependent loss is greatly reduced to less than.2db for operation at the temperature range of -5 o 7 o C over the entire C band. Figure 3 shows the switching crosstalk, insertion loss and polarization-dependent loss for a typical liquid-crystal 1x2 switch from -5 o C to 7 o C at 155nm. The switching isolation is better than 47.5dB, the insertion loss is less than 1.1dB and the PDL is less than.15db in the entire temperature range. The variation of insertion loss is less than.1db over the -5 o -7 o C range in this device. Fig. 4 shows the wavelength-dependent variation of insertion loss. The wavelengthdependent loss is less than.1db over a wavelength range of nm. Conventional liquid-crystal modulators have relatively slow operation times, typically more than 15ms. With our improved optical design and liquid-crystal materials, the switching time is reduced to less than 1ms, which includes the electrical triggering time. Fig. 5 shows a maximum switching time of 1ms in a temperature range of -5 o -7 o C for a typical liquid-crystal 1x2 switch. Several different types of liquid-crystal switches, such as 1x2, 2x2, 1x8, and 2x2 add/drop switches, have been demonstrated. These demonstrations show similar characteristics of lower insertion losses, lower polarization dependent losses, lower temperature dependent losses, low polarization mode dispersion and wider operation spectral ranges Switching Crosstalk, db Crosstalk Insertion loss Loss, db Insertion Loss, db PDL Temperature, o C Figure 3: Switching crosstalk, insertion losses and polarization-dependent losses for a typical liquid-crystal 1x2 switch Figure 4: Insertion loss over a wavelength range of nm for a typical liquidcrystal 1x2 switch.

3 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 Figure 5: The maximum switching time for a typical liquid-crystal 1x2 switch. LIQUID-CRYSTAL ATTENUATORS Fig.6 shows the insertion losses for a typical liquid-crystal attenuator at -5 o C, 25 o C and 7 o C from 152nm to 157nm. The wavelength-dependent loss is less than.1db across the nm range at any specified temperature. Figure 7 shows the flatness over a wavelength range of nm for a liquid-crystal attenuator at different attenuation levels. The flatness is within.1db,.3db,.35db,.3db and.33db for different attenuation levels up to 21-dB attenuation. The flatness data shown here includes power variation of input signals and noise in measurements. The control voltage for a 2-dB attenuation is less than 3V. Since the liquid-crystal attenuators manipulate polarization states to achieve desired power adjustment, the overall polarization-dependent loss and polarization mode dispersion can be minimized in the architecture. Figure 8 shows the polarization-dependent loss (PDL) as a function of attenuation. The PDL is less than.1db for attenuation up to 25dB and.2db up to 3-dB attenuation. Figure 9 shows the polarization mode dispersions (PMD) less than.6ps over a wavelength range of nm.. Loss, db o C 25 o C 7 o C Relative Power, db W avelength, nm Figure 6: Insertion losses for a typical liquid-crystal attenuator at -5 o C, 25 o C and 7 o C Figure 7: Spectral flatness over a wavelength range of nm for a typical liquidcrystal attenuator at different attenuation levels.

4 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November PDL, db.3.2 PMD, ps Attenuation, db Figure 8: Polarization-dependent loss as a function of attenuation for a typical liquidcrystal attenuator Figure 9: Polarization mode dispersion as a function of wavelength for two liquid-crystal attenuators. OPTICAL HARMONIC EQUALIZERS In wavelength-division-multiplexing (WDM) optical links, it is important to keep all lasers in the same fiber at the same power levels in order to avoid signal-to-noise-ratio degradation due to the power-dependent and wavelength-dependent gain characteristics in optical amplifiers [3]. The non-flat gain profiles over the desired spectral ranges in optical amplifiers cause variations in power levels for different lasers. With a cascade of optical amplifiers in a WDM link, lower accumulated gain in some certain wavelengths reduces signal-to-noise ratios. Therefore, it limits the transmission distance. This issue may be resolved by installing fixed-gain compensators or filters with each amplifier to achieve a flatten gain. However, the gain profiles in amplifiers change with the numbers and power levels of input lasers. In a dynamically reconfigurable WDM network, where lasers are dynamically added or dropped from nodes or cross connects, the gain profiles of optical amplifiers will vary with network reconfigurations. Even for simple point-topoint fixed-add/drop WDM systems, there are design considerations concerning future addition of lasers or reduction of WDM wavelength spacings. The gain profiles will also change as channel numbers vary. Therefore, with fast growing interests in dynamic reconfigurable WDM networks and upgradability considerations, dynamically-controlled all-optical gain equalizers become Liquid crystal optical harmonic gain equalizer Input Liquid crystal attenuator 1 st stage 2 nd stage N th stage Liquid crystal wavelength shifter Output Liquid crystal biasing lines Manual control Interface Driver circuit DSP Automatic firmware equalization Optical spectrum analyzer Figure 1: Configuration of a liquid-crystal optical harmonic equalizer with optical amplifiers (s).

5 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 essential elements for the next generation WDM networks. Several works have been reported on optical power equalizers [4-8]. In this work, we demonstrated a harmonic-based approach using liquid-crystal modulators. Figure 1 shows the configuration of a liquid-crystal optical harmonic equalizer (OHE) used with optical amplifiers. In this demonstration, the equalizer has multiple stages of harmonics. Figure 11: An optical harmonic equalizer with electronic board. The optical spectrum analyzer obtains the power profile at the output. The digital signal processor (DSP) compares the data to the user setting, which can be accessed through the user interface, and calculates the required transfer function that needed to flatten the power profile. The DSP then calculates the amplitude and wavelength shift for each harmonic filter by expanding the required transfer function in a Fourier series. Then the circuit applies accurate biasing voltages on the liquid-crystal modulators. Each stage of filter includes a liquid-crystal harmonic filter to adjust the wavelength shift and a liquid-crystal attenuator to adjust the amplitude. The amplitude attenuation range for each harmonic filter is more than 1dB and the tunable wavelength change is more than 1 free spectral range. The response times of liquid-crystal devices are in the millisecond and sub-millisecond ranges. The required biasing voltage magnitudes for liquidcrystal modulators are less than 5V. Figure 11 shows a photo of an integrated optical harmonic -1-1 Relative pow er, db Relative pow er, db (a) (b) Relative pow er, db Relative pow er, db (c) (d) Figure 12: Equalization results and the input power profiles of s.

6 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 (a) (b) Figure 13: (a) The multiplexed lasers with the OHE in the through mode and (b) the automatic equalization result. equalizer, which contains an optical cavity mounted on a DSP circuit board. Figures 12 shows the automatic equalization results for four different gain profiles. In these tests, an amplified spontaneous emission (ASE) source is used as the input signal of erbium-doped fiber amplifiers () in the input of OHE, without a second at the output of OHE. The flattened results were measured at the output of OHE. Fig.12 (a), (b), (c) and (d) show flatness of ±.15dB, ±.3dB, ±.3dB and ±.15dB in the wavelength ranges of nm, nm, nm and nm, respectively. Generally speaking, the flattened output profiles can be reached within less than ±.3dB for the required flatness levels in less than four equalization iterations. The insertion loss in the through status is around 5dB. The dynamic range for setting user-desired gain profiles is more than 1dB. Figure 13 shows the equalization for discrete channels with multiplexed WDM lasers. The multiplexed lasers, with the optical harmonic equalizer in the through mode, have a power variation of 6dB in the wavelength range of nm and 1556nm, as shown in Fig 13(a). The spacing between lasers was tuned to be uneven in order to demonstrate the flexibility in the.3 Insertion Loss (db) Wavelength (nm) Figure 14: Insertion loss and PDL for the through state PDL (db) Polarization M ode dispersion, ps.2.1 Through state state 1-dB attenuation state Figure 15: Polarization mode dispersions in different states.

7 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 harmonic-based equalization approach for the case that some of WDM signals might be added or dropped. The flatness can be reached within ±.15dB in this case, as shown in Fig. 13(b). Figures 14 shows the insertion loss and polarization-dependent loss (PDL) for the through-state. The insertion loss is around 5dB and PDL is around.3db. For up to 1-dB attenuation in the transfer function, PDL is less than.5db. Figure 15 shows the measured polarization mode -4 dispersion (PMD) under three different states í through-state, flattened state, and a transfer function with 1-dB attenuation. Fig. 15 shows Figure 16: and tilting results PMDs less than.14ps. Typically, the polarization with the input power profile of. mode dispersion is less than.3ps under all operating conditions. Chromatic dispersions were also measured for three different cases. The chromatic dispersion is less than ±7ps/nm. Furthermore, the DSP circuit provides a variable power setting, which allows the power levels at both ends of the passband to be dynamically adjusted. The DSP then generates the required transfer functions to achieve the desired output levels with a linear profile. This functionality provides a gain-tilting profile that may add more operation flexibility for different types of optical amplifiers or gain-characteristics requirement. Two possible tilting profiles are shown in Fig. 16 along with an equalized profile. It shows a tilting of ±4dB at the edges of passband. In our design, the tilting slope can be ±1dB across the C band. Since the liquid-crystal modulators are broadband devices and the Fourier harmonic approach is wavelength-independent, the liquid-crystal optical harmonic gain equalizers can also be used in other wavelength ranges such as S-band or L-band, in addition to C-band. Figure 17 (a) shows the multiplexed lasers in L-band with the OHE in the through mode and (b) the automatic equalization result. Relative power, db Tilted #1 Tilted #2 (a) (b) Figure 17: (a) The multiplexed lasers in L-band with the OHE in the through mode and (b) the automatic equalization result.

8 21 Asia-Pacific Optical and Wireless Communications Conference, Beijing, China, November 21 CONCLUSIONS The concept and performance for liquid-crystal optical attenuators, optical switches, and optical harmonic equalizers have been demonstrated. The attenuator and switch demonstrations show lower insertion losses, lower polarization dependent losses, lower temperature dependent losses, low polarization mode dispersion and wider operation spectral ranges compared with other types of solid-state devices; as well as faster switching speeds, smaller sizes and less reliability concern compared to optomechanical devices. Functionality - include-ing gain equalization and tilting, and optical performance, such as spectral flatness, polarization dependent losses, polarization mode dispersion and chromatic dispersion, have been demonstrated for the optical harmonic equalizers. The equalization performance has been verified with continuous-spectrum gain profiles and discrete multiplexed narrow-band lasers. The results show great promises for dynamic, quick manipulation of signal power profiles in WDM systems. ACKNOWLEDGMENTS The author greatly appreciates all the colleagues, including Tizhi Huang, Pin Siu, Jeff Huang, Yueai Liu, Yatao Yang, Minchun Li, Hong Jiang, Mei Zhang, David Coburn, and Kuang-Yi Wu, in Chorum Technologies for their contributions in this paper. REFERENCES [1] Multiwavelength reconfigurable WDM/ATM/SONET network testbed, G.K. Chang, G. Ellinas, J. Gamelin, M. Iqbal and C. Brackett, Journal of Lightwave Technology, Vol.14, No. 6, pp.132, [2] Optical free-space multichannel switches composed of liquid-crystal light-modulator arrays and birefringent crystals, K. Noguchi, Journal of Lightwave Technology, Vol.16, pp.1473, [3] G. Keiser, A review of WDM technology and applications, Optical Fiber Tech., Vol.5, pp.3, [4] Y. Li and C. Henry, Silica-based Optical Integrated Circuits, IEE Proceedings Optoelectronics, Vol.143, No.5, Oct [5] S. Parry, J. King, K. Roberts, N. Jolley, R. Keys and J. Mun, Dynamic gain equalization of s with Fourier filters, The Optical Amp. & Their App., June 9, [6] S. Yun, B. Lee, H. Kim and B. Kim, Dynamic erbium-doped fiber amplifier based on active gain flattening with fiber acoustooptic tunable filters, IEEE Photonics Technology Letters, Vol. 11, No. 1, Oct [7] B. Offrein, F. Horst, G. Bona, R. Germann, H. M. Salemink and R. Beyeler, Adaptive gain equalizer in high-index-contrast SiON technology, IEEE Photonics Technology Letters, Vol.12, No.5, 2. [8] Dynamic spectrum equalizer, WDM Solutions June 2.