A Mounting Method on Configuring a Wavelength- Voltage Relation of an Optical Tunable Bandpass Filter (TBF) for Actuation & Control A.H.Poh a , F.R. Mahamd Adikan a , M.H. Abu Bakar a a Photonics Research Group, Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Abstract- Although commercial tunable optical filters with integrated electronic controls are available, it costs much more than a manually tunable filter. This paper aims to demonstrate a simple procedure on mounting a manually tunable bandpass filter (TBF), with a mathematical model to represent the relation between the sensor’s output and the central wavelength of the filter. A position sensor is used to match the central wavelength of the filter and the sensor’s output. This results in a simple mount to actuate the TBF and the position of the wavelength is simply translated in terms of the output voltage. Although the accuracy of the system is not commercial grade, a rudimentary setup with promising results can be observed. We present the mount using very simple components, utilizing motorized actuation, with an analog voltage output which can bear any desired resolution for digitization. Finally, a mathematical model is developed to represent the wavelength- voltage relation. This setup paves way for researchers to log the central wavelength from tunable filters without the need for any equipment, at a much lower cost compared to its commercial counterparts. I. INTRODUCTION Optical Tunable Filters are essential for a myriad of optical circuit setups, allowing only a limited bandwidth to be undisturbed. Applications range from tunable lasers [1], Wavelength Division Multiplexing systems [2], monitoring channels [3] , notch filters [4], fiber interrogation [5], and so on. There are various optical filters with different tuning mechanisms. One of the most popular types are acousto- optic tunable filters, with even more recent designs with increased accuracy [6]. There are designs employing elasto- optic effects [7], and thermo-optic tuning [8]. Motorized ones are commercially available, produced by Dicon Fibreoptics Inc. [9]. Although all of the aforementioned components are much more precise, they are also much more costly compared to a manually tunable filter. In our research, where data is logged from 5 minute-intervals to a few days, the tuning rate (especially within the C-band) is not crucial; therefore a slower system is acceptable. This fact is resonated by even recent works where tunable filters can be used to interrogate fiber sensors, especially for structural health monitoring (SHM)[10], temperature sensing [11] and corrosion sensing [12]. All these applications do not require real-time scanning. Also, the only crucial parameter to be investigated is the central wavelength of the filter, especially in tunable ring lasers. An optimized system which conforms to the limitations mentioned above is very desirable, while maintaining minimal cost. The TBF used in this setup costs around RM3000 (Ã$1000 USD) from Alnair Labs. [13] II. THEORY The mounting scheme is shown below: Figure 1: A TBF mount design. From the design above, a 10k potentiometer acts as a position sensor and a voltage divider. When the sensor is turned along with the tuning of the TBF, while assigning 5V to terminal 1 and 0V to terminal 3, the resistance across terminals 1 and 2, or 2 and 3 changes. It obeys the voltage division rule: V out = V 2 = 琢 迭┸鉄 琢 東搭盗 x V ref , for 0.1V ref 判V out 判 0.9V ref (1) While R 1,2 is the resistance between terminals 1 and 2, V ref as the reference voltage, is set at 5V, and R pot as the potentiometer’s full resistance is 10kµ for this case. The change of the output voltage is linear with the tuning degree. V out is limited to a minimum of 10% of the V ref , and maximum at 90% V ref . The setup included the rail voltage of the sensor into consideration, since tuning the positioning can only take place in a limited number of revolutions, and when this limit is transgressed, the position of the bandwidth will shift according to the excess tuning beyond the range. Therefore the starting output voltage was preset at 0.5V and ends at 4.5V, with a reference to 5V-0V rail. As per the datasheet of Alnair Labs’ TBF, the wavelength is tuned via an internal micrometer, where the wavelength changes linearly with the radius of the turn. As both of the components change its corresponding parameters linearly, we expect to see a linear correlation between the output voltage, V out and the TBF’s central wavelength via an OSA. 3rd International Conference on Photonics 2012, Penang, 1-3 October 2012 978-1-4673-1463-3/12/$31.00 ©2012 IEEE 327