Development of functional optical fiber demultiplexers (1994 – 1998)

1. Introduction
Array waveguide grating (AWG) devices with a demultiplexing function are currently in widespread use for wavelength division multiplexing (WDM) systems. Around 1990, however, optical fiber demultiplexers using a single gradient index (GRIN) rod lens had just been developed.
In this work, we fabricated and tested functional optical fiber multiplexers that have a multi-position switching function or a signal distribution function (star coupler function).

2. Optical fiber demultiplexer with a multi-position switching function [1]
Figure 1 shows a schematic illustration of the functional device, which consists of a star-shaped input/output optical fiber array, a gradient index (GRIN) rod lens, and a rotatable grating prism (GRISM).

Fig. 1 Schematic illustration of the demultiplexer with a multi-position switching function.

The wavelength-multiplexed light from λ1 to λ7 enters the input optical fiber located at the center of the optical fiber array. The input light is collimated through the GRIN rod lens, enters the GRISM, and is then diffracted at different angles according to wavelength. The diffracted light propagates in the reverse direction and is focused on the different output optical fibers (CH1-CH7), which are aligned along one radial direction. Therefore, one can obtain the demultiplexed beams from the seven output fibers. One can select an individual set of output fibers (SW1-SW6) by rotating the GRISM.
Figure 2 shows a photograph of the fabricated device and an image of the transmitted light through the input/output optical fiber array. In our experiments, we aligned the optical components using large high-precision translation stages. Downscaling the device can be expected with the continuing development of packaging technology.

Fig. 2 Photograph of the fabricated device and an image of the transmitted light through the input/output fiber array.

The main characteristics of the fabricated device were as follows: 7 demultiplexing channels, 6 switching channels, working wavelength region (0.64 – 0.88 μm), 3 dB bandwidth (18 – 22 nm), demultiplexing channel spacing (35 – 42 nm), minimum transmission loss for each demultiplexing channel (7.0 – 12.6 dB), channel crosstalk ( < -10 dB).

3. Optical fiber demultiplexer with a star coupler function[2]
Figure 3 shows a schematic illustration of the functional device. It consists of a star-shaped input/output optical fiber array, a GRIN rod lens, and a GRISM. Although this device is similar to that described in Section 2, the grating prism consists of a hexagonal pyramid prism and six grating facets.

Fig. 3 Schematic illustration of the demultiplexer with a star-coupler function.

The wavelength multiplexed light from λ1 to λ7 enters the input optical fiber located at the center of the optical fiber array. The input light is then collimated through the GRIN rod lens and enters the GRISM, where it is divided into six beams by the six grating facets. The six beams are then diffracted at different angles according to wavelength. The diffracted beams propagate in the reverse direction and are focused on the different output optical fibers (CH1-CH7), which are aligned along the radial direction. Accordingly, one can simultaneously obtain demultiplexed output beams from the six fan-out channels (F1-F6).
Figure 4 shows a photograph of our experimental setup for evaluating the device characteristics, and an image of the transmitted light through the input/output optical fiber array. Higher fabrication accuracy is necessary for the input/output fiber array used in this device compared to that for the optical fiber demultiplexer with the multi-position switching function described in Sec. 2. The fabrication accuracy was improved in this fabrication.

Fig. 4 Photograph of our experimental setup for evaluating the characteristics of the fabricated device, and an image of the light transmitted through the input/output fiber array.

The main characteristics of the fabricated device were as follows: 7 demultiplexing channels, 6 fan-out channels, working wavelength region (0.62 – 0.88 μm), 3 dB bandwidth (13 – 24 nm), demultiplexing channel spacing (36 – 45 nm), minimum transmission loss for each demultiplexing channel (7.4 – 17.2 dB), channel crosstalk ( < -10 to -15 dB).

Acknowledgements
I would like to thank my collaborator, Prof. T. Sakamoto of Okayama Prefectural University, for his proposal and design of the functional devices. I also want to thank Sumitomo Electric Industries, Ltd. for providing me with the optical fiber cables. This work was partially supported by the Electric Technology Research Foundation of Chugoku.

References
[1] T. Fukushima and T. Sakamoto, A 7×6 optical fiber grating demultiplexer-multiposition switch for 0.64-0.88 μm band, Journal of Lightwave Technology, vol. 14, no. 5, pp. 867-872, 1996.
[2] T. Fukushima, T. Yokota, and T. Sakamoto, Fabrication of 7×6 multimode optical fiber grating demultiplexer-star coupler using a single GRIN-rod lens, Journal of Lightwave Technology, vol. 15, no. 10, pp. 1938-1946, 1997.