Thin-film filters (TFs) are widely used in fiber-optic devices. They are included in most of the 1 × 2 add-drop filters currently available. Larger N-channel add-drop filters can also be constructed, whether by combining several 1 × 2 filters or by arranging N TFs in a zigzag configuration similar to TF-based wavelength division multiplexers/demultiplexers. Dropping N channels thus seems to require a large number of TFs, boosting cost and packaging size. Highly desirable would be a single TF that can separate several desired wavelength optical beams from the main transmission medium.
Taking advantage of the fact that a TF is sensitive to angular orientation, we have developed a scheme by which a multi-wavelength optical beam falls on the TF several times, each instance at a different angle. The beams are thus effectively separated in space.1
Figure 1 shows our proposed TF-based 1 × N add-drop filter arranged in a reflective architecture for a number of drop ports (N ≥ 3). Key components include the TF, a lens with an array of 2N-3 single mode optical fibers (SMFs), N-1 single fiber-optic collimators (FOCs), and one optical circulator. As indicated, SMF 1 receives a multi-wavelength optical beam as obliquely incident to the TF at the widest possible angle. In this state, only the λ1 optical beam, the furthest downshifted from the center wavelength (λ0), passes through the TF and enters a FOC. Meanwhile, the TF reflects the remaining beams to SMF 2. Advantages inherent to optical fiber flexibility enable SMF 2 to be easily connected to SMF 3, which in turn allows the wavelength optical beams to again fall on the TF, but at a smaller incident angle. As a result, only the λ2 (i.e., Δλ = λ2 - λ1) wavelength optical beam passes through the TF.
Figure 1. Components of our proposed thin film (TF) filter-based 1 × N add-drop filter include one TF lens with an array of 2N-3 single mode optical fibers (SMFs), N-1 single fiber-optic collimators (FOCs), and one optical circulator.
Meanwhile, all remaining wavelength optical beams are still directed from the TF to SMF 4. The operation is repeated several times until the remaining beams reach the (N-1)th SMF. At this point they pass through the optical circulator, enter the the Nth SMF, and are normally incident on the TF. The λ0 wavelength optical beam then passes through the TF and is coupled to the last single FOC while the λN wavelength optical beam is again retro-reflected from the TF to the Nth SMF and emerges at the optical circulator.
Based on the above operations, only one TF is required to isolate N wavelength optical beams. This should entail lower cost, smaller packaging size, and fewer component devices. With a typical 0.23-pitch graded-index lens, commonly used in most fiber-optic collimators, we calculate that our proposed structure is theoretically capable of separating 51 wavelength channels with channel spacing of 0.25 nm.
To prove our concept, we set up an experiment for our reflective TF-based 1 × 3 add-drop filter via use of a triple FOC and a commercial four-cavity wavelength filter specified for 0.8 nm channel spacing systems. As wavelength optical beams reached the TF at various angles, a two-dimensional image sensor enabled us to observe the desired beams separating in space (see Figure 2). The optical loss values of 0.67 dB and 1.66 dB were also determined at Drop1 and Drop2, respectively. The average loss for the remaining beams at the Drop3 measured 2.59 dB. In addition, optical crosstalk was < -18 dB. A polarization dependent loss of < 0.08 dB was also investigated.
Figure 2. Snapshots show three optical beams in space: (a) 1548.37 nm at Drop1, (b) 1549.17 nm at Drop2, and (c) 1552 nm at Drop3.
Various factors that could have affected our results include TF performance, the number of times the optical beam passed through the fiber adapters, free-space coupling efficiency, and the optical circulator.
At this stage, it is clear that angle multiplexing enhances TF performance by enabling isolation of all wavelength channels as needed. Our experimental proof of concept, carried out with a 1 × 3 add-drop filter module using a triple fiber-optic collimator, holds promise for future implementation of the 1 × N add-drop filter.