Development of LiNbO 3 for CATV Transmission Systems

Transcription

1 Development of LiNbO 3 Modulators for CATV Transmission Systems Norikazu Miyazaki, Takashi Noguchi, and Toshio Sakane (Optoelectronics Research Div., New Technology Research Laboratory) ABSTRACT We have developed two types of external LiNbO 3 (LN)modulators designed for fiber- optic CATV links. One modulator is an electrically linealized modulator which combines a phase modulator and an intensity modulator integrated on the same LN chip, and the other is an optically linearized modulator consisting of two intensity modulators. Both types of modulators have good characteristics for CATV analog transmission applications. 1. Introduction CATV system networks, conventionally constructed with coaxial lines, are being replaced by fiber-optics because of non-induction, large transmission capacities and other advantages of optical communication. Fiber-optic CATV system networks have become one of the most attractive industrial areas as their large transmission capacity is considered applicable not only to CATV systems but also to telecommunications and multi-media links. The fiber-optic CATV networks originally relied on a 1.3µm band but are now shifting to a 1.5µm band 1) because the fiber-optic loss is less and direct amplification of light is possible in a 1.5µm band system using an erbium doped fiber amplifier (EDFA). This presents a new horizon for building fiber-optic networks stretching over longer distances, an important advantage especially for CATV trunk lines. However, most of the existing fiber-optic lines use 1.3µm single-mode fibers which, when used in a 1.5µm band system, are susceptible to the presence of relatively large dispersion. Since direct modulation of LD contains a large chirp, dispersion presents the problem of waveform distortion, making it impractical for transmission distances over 30km. This situation calls for low-chirp, external modulators, and the authors were inspired to the development of modulators for CATV applications using LiNbO 3 (LN). Since modulators are based on the Mach-Zehnder interferometer, their output characteristics will be like sine-wave functions, and the modulators thus require correction of distortion to get linearity in the output optical signals. This need was met by the development of two types of modulators in which the distortion was corrected electrically and optically. 2. CATV application of LN modulators Our company has long been engaged in the development and commercialization of LN modulators for digital communications. The differences between the modulators for digital communications and the newly-developed modulators for CATV use are described below. 2.1 Compatibility with analog signals 1) Flat characteristics of frequency response The modulators for digital communications have a wide frequency band ranging from DC to high frequencies (several to several tens GHz) which makes fast operation possible. But they do not account for signal distortion that presents a problem for the modulators for CATV use that are designed for analog communications. For this reason, the digital modulators contain small ripples in their frequency response and cannot be used readily for CATV systems. Thus, the main development theme was to implement a flat frequency response by minimizing ripples caused by a phenomenon called an acoustic effect. Since LN is a piezoelectric material, it generates acoustic wave when a voltage is applied and this generates ripples at frequencies where resonance is caused by the sound wave in the modulator. Resonance is affected especially by the thickness orientation of the LN substrate and the parallelism of the structure between the HOT and GND electrodes. 2) 3) For the purpose of the new modulators, the fre- 16 TECHNICAL REPORT 1999

2 quency response characteristics were improved by means of roughing the bottom of the substrate and varying the width of the HOT electrode. 2) Correction of distortion The LN light-intensity modulators follow the Mach- Zehnder composition and the transfer function of the MZ intensity modulators can be expressed as a sine wave-like function of the input voltage. For this reason, signal distortion is always found in a modulator output. In CATV transmission, many channels are allocated within the band and signal distortion in a channel can cause an interference problem with other channels. Signal distortion can be intermodulation resulting from the correlation of frequencies of plural channels, and cross modulation (XM) in which the amplitude of a signal wave is overridden by the signal of another modulation frequency. Intermodulation presents a problem mainly because of the composite second order (CSO) beat which is a composite of second order distortion, and composite triple beat (CTB) which is a composite of triple distortion by the sums and differences of three component frequencies. Second and other high even-order distortions of the MZ modulators can be eliminated by optimization of the driving bias point while triple distortion requires special corrective measures. The latter can take the forms of electrical linearization of the input signal to the intensity modulator, and optical linearization based on multiplexing of modulators. This paper reports the development of two types of modulators based on these two corrective methods. 3) Impact of light reflection on signal distortion Another major problem in a CATV system is the distortion caused by multiple reflection of the optical signal. In the conventional modulators for digital communications, the fibers are cut perpendicular to the end surface for butt connection of the LN chip. In the newly-developed modulators, fibers are ground obliquely so that the intensity of the returned light is reduced to a very low level. 2.2 Use of directional coupler output The LN modulators for digital communications use Y- branching in the output stage for optical switching. The newly-developed s use balanced bridge interferometer (BBI) structure incorporating directional couplers rather than the Y-branching. With the Y-branching interferometer, the portion of optical power that does not contribute to output is radiated within the modulator substrate. With the BBI configuration, the optical power can be taken out from the other output port, thus providing two complementary outputs. The second output can be used simply for power branching or for monitoring signal light, thus realizing efficient use of optical power. In addition, the received optical signal of the complementary output may be differentiated for the purpose of C/N enhancement. 4) Another advantage is that the influence of the DC drift which can present a problem in an LN modulator can be reduced since the bias voltage for minimizing the abovementioned second order distortion can basically be reduced to 0V. It must be noted, however, the characteristics of directional couplers are dependent, more so than those of the Y-branching, on the condition of the waveguide. This leads to a need for some strict control in fabrication, inevitably resulting in a lower yield than the Y-branching type. 3. Electrically linearized CATV modulator 3.1 Main features of the electrically linearized Figure 3.1 shows a schematic diagram of the electrically linearized modulator for CATV applications. The modulator basically consists of the phase modulator (PM) and the intensity modulator. The latter is a BBI consisting of 3dB couplers. This modulator type requires a special circuit for external electrical linearization but is simple in design as a bias point adjusting circuit is the only control circuit required. Fig. 3.1 Schematic diagram of an electrically linearized modulator 3.2 Role of the phase modulator The phase modulator (PM) is used for controlling stimulated Brillouin scattering (SBS) in the fiber. The SBS threshold depends on the optical power and the spectral band width. An SBS causes noise, distortion, etc., which restricts input power to the fiber. The SBS triggering power threshold can be raised by phase modulation at frequencies higher than the CATV signal band, thereby broadening the spectral band 5) 6) width. 3.3 Output characteristics of the modulator The transfer function of the output light intensity of this type of modulator is a simple sine-wave function. It can be determined by multiplying the transfer matrix of elements that constitute the modulator, 7) and may be expressed as following: 17

3 I 1 = 1/2 [1 - sin2 γ sin (φ + φ DC + Ψ)]... (1) where, I is the intensity of output light, γ is the coupling angle of the directional coupler, φ is the phase change due to the signal voltage, φ DC is the phase change due to a bias voltage, and Ψ is the differential phase between the BBI arms. From expression (1), second- or higher even-order harmonic distortion can be eliminated from the output when the bias DC is set equal to φ DC + Ψ, although triple or higher-order distortion still exists. The input electrical signal thus needs to be linearized, which is achieved by the pre-distortion and feed-forward methods.8) As expression (1) indicates, the coupling angle γ of the directional coupler affects the on/off extinction ratio of the optical output, the maximum ratio being found when γ = 45 degrees. Furthermore, since the directional coupler is wavelength dependent, the practical range of wavelength is limited. 3.4 Fabrication of the modulator The modulator was fabricated using an X-cut LN wafer, with the waveguide being formed by Ti thermal diffusion. Figure 3.2 shows an external view of the fabricated electrically linearized modulator. newly-developed modulator, with very flat (1dBe) frequency characteristics over a band up to 1GHz. Figure 3.5 shows a sample of wavelength dependence of the on/off extinction ratio. A ratio of over 20dB is evident over the wavelength range of several ten nanometers. Table 3.1 Targeted specifications and measured values of sample modulators Characteristic Specification Sample value Insertion loss 5dB 3.6dB Differential insertion loss between ports Light reflection attenuation Drive voltage (Vp) Frequency response On/off extinction ratio 1dB 50dB 4Vp-p@100MHz 1GHz 0.1dB Input 60dB Output1 57dB Output2 60dB 3.2V 17.6V 0.9dB 22dB Fig. 3.3 Frequency response of a modulator without anti-acoustic design Fig 3.2 External view of the electrically linearized modulator 3.5 Characteristics of the electrically linearized Table 3.1 shows the targeted specifications and the measured values of sample modulators. The wavelength of 1550nm was used for most of the measurements. Some samples with perpendicularly cut fibers had a light reflection attenuation value of 50dB or less, while attenuation over 50dB was achieved with stability on samples with obliquely ground fibers. Figure 3.3 shows an example of frequency response without the aforementioned analog signal compatibility. It reflects signal deterioration due to many ripples. Figure 3.4 shows frequency response of the Fig. 3.4 Frequency response of a modulator with anti-acoustic design 18 TECHNICAL REPORT 1999

4 I 1 = 1/2 [1 + sin2γ 1 cos2γ 2 sinφ 1 sin2 γ 2 cosφ 2 sinφ 1 + 2sin2γ 2 cos2γ 1 sinφ 1 cosφ 2 +sin2γ 2 sinφ 2 cosφ 1 ]... (2) Fig. 3.5 Wavelength dependence of the on/off extinction ratio (sample) 4. Optically linearized 4.1 Main features of the optically linearized Figure 4.1 shows a schematic diagram of the optically linearized modulator for CATV applications. The modulator consists of two cascaded BBI intensity modulators. This modulator type is unique in that it provides characteristic stability over an extended period without an electrical linearization circuit. Another advantage over the electrically linearized type is the wider dynamic range made available by the deeper depth of modulation. It must be noted, however, that it requires complex control circuitry as two bias points and RF signal power need to be adjusted. It is also subject to more strict fabrication conditions as two directional couplers are contained in the modulator. where, γ 1 and γ 2 are the coupling angles of the respective directional couplers and φ 1 and φ 2 are the differential phases between the BBI stages. The dynamic range of the modulator is determined by the coupling angles, and the optical output is linearized by electrical signals reaching the two BBIs respectively at an appropriate voltage ratio. Figure 4.2 shows an example of output change of expression (2) when γ 1 = γ 2 = 60 and φ 1 = kφ 2 while k = 0 and k = 0.5, respectively. It must be noted, however, it will be crucial that the coupling angle be reproduced accurately during fabrication since the voltage ratio for optimum operation and other characteristics can be affected substantially by the coupling angle. Fig. 4.2 Output transfer function of the optically linearized modulator 4.3 Fabrication of the modulator The modulator was fabricated using an X-cut LN wafer, as was the electrically linearized type, with the waveguide being formed by Ti thermal diffusion. Figure 4.3 shows an external view of the fabricated optically linearized modulator. Fig. 4.1 Schematic diagram of an optically linearized modulator 4.2 Linearization by 2-stage BBI configuration The optical output of a single-stage BBI modulator can be expressed by the product of simple sine-wave functions as in expression (1), but a two-stage BBI configuration requires a complex product of sine and cosine wave 7) 9) functions as shown in expression (2). Fig 4.3 External view of the optically linearized modulator 19

5 Table 4.1 Measured values of a sample modulator 4.4 Characteristics of the optically linearized Table 4.1 shows the characteristics of a fabricated modulator sample. The wavelength of 1550nm was used for most of the measurements. Figures 4.4 and 4.5 show examples of frequency response of the modulator, with 1.4dB deterioration in amplitude response and a 5 phase deterioration at the frequency band from 40M to 860MHz. Distortion is also affected significantly by the difference in characteristics between the two modulator stages. The amplitude fluctuation was 0.15dB and phase fluctuation was 0.5 in the above-mentioned frequency range. 5. Problems Repeatability of the coupling angle of the coupler is the hardest fabrication problem for both modulator types. In addition, the optically linearized modulator requires that the coupling angles of the two directional couplers be as equal as possible. Naturally, the waveguide fabrication process needs to be determined very carefully and the coupling angle characteristics of the modulator elements must be verified by measurement. Since this measurement requires optical and electrical signals to the elements, characteristic evaluation can be performed only after the final wafer fabrication process. These conditions call for the development of a new trimming technique for correcting the coupling angle of the directional couplers at the end of the final wafer fabrication process. At the present, use of the excimer laser is being considered toward this goal, in which the waveguide between the couplers is trimmed by the excimer laser so that the waveguide structure is modified, thereby adjusting the coupling angle. This trimming method has been tested with a modulator module, and its feasibility has been verified for both the correction of the coupling angle and characteristic stability after the correction. Fig. 4.4 Frequency response of a sample modulator 6. Concluding remarks Two types of modulators for CATV applications were developed based on two different methods of linearization. The newly-developed modulators proved to meet the characteristic requirements for analog communications. Digital broadcasting and co-existence of analog and digital data are on the rise today, and the industry is moving toward the use of higher frequency carriers than those used in conventional analog broadcasting. The new CATV modulators reported in this paper were targeted for signal bands up to 1GHz, but a broader band application is expected to be the next agenda for development. Fig. 4.5 Differential frequency response between two BBIs Acknowledgements We wish to express our special appreciation to Mr. Minowa, group leader of Optoelectronics Research Group, for his advice on the development project. We are also thankful to the members of the Product Development 20 TECHNICAL REPORT 1999

6 Room, Optoelectronics Operation Div., for their assistance in the sample fabrication, and to Mr. Sakamoto for his cooperation in the wafer fabrication. References 1) W. Muys, J. C. van der Plaats, F. W. Willems, H. J. van Dijk, J. S. Leong, and A. M. J. Koonen, A 50-channel Externally Modulated AM-VSB Video Distribution System with Three Cascaded EDFAs Providing 50dB Power Budget over 30km of Standard Single-mode Fiber, IEEE Photon. Technol. Lett., Vol. 7, No. 6, pp , ) R. L. Jungerman, and C. A. Flory, Low-frequency Acoustic Anomalies in Lithium Niobate Mach-Zehnder Interferometers, Appl. Phys. Lett., Vol. 53, No. 16, pp , ) J. L. Brooks, G. S. Maurer, and R. A. Becker, Implementation and Evaluation of a Dual Parallel Linearization System for AM- SCM Video Transmission, J. of Lightwave Technol., Vol. 11, No. 1, pp , ) D. Piehler, X. Zou, C. Y. Kuo, A. Nilsson, J. Kleefeld, G. Garcia, J. D. Ralston, and A. Mathur, 55dB CNR over 50km of Fibre in an 80-channel Externally-modulated AM-CATV System without Optical Amplification, Electron. Lett., Vol. 33, No. 3, pp , ) F. W. Willems, J. C. van der Plaats, and W. Muys, Harmonic Distortion Caused by Stimulated Brillouin Scattering Suppression in Externally Modulated Lightwave AM-CATV Systems, Electron. Lett., Bo. 30, No. 4, pp , ) F. W. Willems, and J. S. Leong, Simultaneous Suppression of Stimulated Brillouin Scattering and Interferometric Noise in Externally Modulated Lightwave AM-SCM Systems, IEEE Photon. Technol. Lett., Vol. 6, No. 12, pp , ) H. Skeie, and R. V. Johnson, Linearization of Electro-optic Modulators by a Cascade Coupling of Phase Modulating Electrodes, SPIE, Vol. 1, 1583, pp , ) M. Nazarathy, J. Berger, A. J. Ley, L. M. Levi, and Y. Kagan, Progress in Externally Modulated AM-CATV Transmission Systems, J. of Lightwave Technol., Vol. 11, No. 1, pp , ) H. Skeie, An Optically Linearized Modulator for CATV Applications, SPIE, Vol. 2291, pp ,