Optical signal is coupled out using fiber grating

The development of high speed optical telecommunication systems has been driven by a need for bandwidth. Currently, the main focus is on getting fiber into metro and access networks and to individual residences (fiber-to-the-home, FTTH). In particular, for FTTH to succeed, the cost of any optical communication systems to be installed must be reasonable. Small and cheap optical devices, such as receivers and transmitters, are therefore required. Fiber gratings are well-suited to this purpose: they have many attractive features that make them viable alternatives to conventional optical devices. In particular, their small size makes them ideal for integrated optics, and their wavelength-selective nature makes them a good match for wavelength-division-multiplexed communications systems.

Our goal was to design a fiber grating that could be use to replace optics for signal out-coupling. We noted that tilted fiber Bragg gratings (TFBGs) are good for extracting light from the tightly-confined core mode into the cladding (less-confined) and/or radiation (free-space) modes. This property has been used to sense the core-mode power: by directly coupling the core mode to the radiation mode, in-line fiber spectrometers^1–3 and monitors⁴ have been implemented. However, these have mainly had coupling efficiencies in the 0.1∼3% range: unsuitable for high sensitivity optical receivers.

To overcome this problem, we etch the cladding. As a result, we take advantage of two kinds of coupling processes. First, the strong core-to-cladding mode coupling is forced by the TFBGs. Then, a highly efficient out-coupling of the cladding mode through the v-groove structure takes place. Unfortunately, the core-to-cladding mode coupling creates an undesirable crosstalk effect that results in some degradation of to the optical communication,⁵ but so far this cannot be avoided.⁶

Figure 1. (a) Schematic of the device and (b) the overall mode coupling process.

Figure 2. Fabricated v-grooved cladding by the femtosecond laser: (a) top view, (b) side view, and (c) infrared camera view of the light of the out-coupled cladding mode.

The device structure and the mode coupling processes are depicted in Figure 1, where the two types of coupling can be seen clearly. Importantly, through total internal reflection (TIR), the cladding mode is outcoupled through the v-grooved cladding. At the same time, the tilt angle of the TFBG has been adjusted to maximize the coupling efficiency so that most of the core mode power is coupled to the cladding mode. The TFBG is fabricated in a hydrogen-loaded photosensitive fiber using a KrF excimer laser. The tilt angle of the phase mask is set to 2°, which corresponds to 2.97° for the actual grating tilt angle.⁷

Figure 2 shows the fabricated v-groove and how it emits light. It is made by precisely removing a portion of the cladding by appropriately focusing a high-power (femtosecond) laser beam.

Once the core mode is coupled to the cladding, it propagates in the cladding region and is tightly guided by the air-cladding boundary. Since the refractive index of the air is lower than that of the cladding, when the light then meets the v-groove, TIR occurs as long as the incident angle is higher than the critical angle. Therefore, most of the cladding mode can be coupled out to the detector as shown in Figure 2(c).

Figure 3. Measured data: the solid line is the photocurrent of the avalanche photodiode and the dotted line is the transmission spectrum.

In our experiment, we measured the output photocurrent of an avalanche photodiode (APD) and the transmitted power with respect to the wavelength, and then we calculated the maximum coupling efficiency of the device. Figure 3 shows the measured data. The solid and open circles are photocurrents of the APD and transmitted power, respectively. Since there is no core-cladding mode coupling in the longer wavelength region, no photocurrent is evident. The actual power launched into the device is estimated to be about 0.73mW, as obtained from the transmission spectrum in Figure 3. The maximum photocurrent is about 0.4mA and corresponds to 0.4mW, in which the multiplying factor M=9 is applied for the APD. The out-coupling efficiency is therefore about 54.8%. In our experience, the out-coupling efficiency can be improved by optimizing the grating and the v-groove in the cladding.

We have developed a fiber device in which the optical signal can be coupled out directly coupled out without using any lenses, filters, or splitters. Since this device has wavelength-division-multiplexing characteristics inherited from the TFBG, a receiver array for WDM signals can be implemented by having cascading grating structures, each with different grating parameters. This kind of in-line optical system also makes possible a new range of small optical communication devices such as transceivers, receivers, and monitoring devices.