Experimental Results for the OSA Module and Components Implementation Part 2

Bidirectional optical signal transmission in full-duplex mode

To confirm the full-duplex bidirectional data transmission performance, eye-diagrams and frequency responses were measured and compared to those that occur during the half-duplex mode of operation.

For the measurement of the half-duplex mode, the idle transmitter and receiver parts in the up- and down-link modules are turned off, whereas for the full-duplex mode the idle parts are turned on. The eye-diagram openings at 10 Gb/s are clear for both the half-duplex and the full-duplex mode with a peak-to-peak output of 240 mV, as shown in Fig. 1

Fig. 1 Measured eye-diagrams of the bidirectional optical link at 10 Gb/s for (a) half-duplex and (b) full-duplex signal transmissions with 24.4 ps/div.

The eye-diagrams shown in Fig. 1 show that the peak-to-peak jitter increases from 33.6 ps in the half-duplex mode to 35.2 ps in the full-duplex mode. In contrast, the rise/fall times increased from 20.2/26.3 ps in the half-duplex mode to 22.8/29.4 ps in the full-duplex mode. These results indicate that the jitter and rise/fall times of the full-duplex bidirectional operation are slightly degraded compared to those of the half-duplex mode. This degradation might be caused by the inter-mode crosstalk between the up- and down-link originated from the idle electrical and optical components.

Fig 2 The measured S-parameters of the bidirectional optical link for full-duplex and half-duplex signal transmission (without and with crosstalk between the up- and down-link, respectively).

Figure 3 The measured frequency responses with the crosstalk of the unidirectional up-link

(a) Crosstalk from Ch1 to other channels

(b) Crosstalk from Ch2 to other channels,

(c) Crosstalk from Ch3 to other channels,

(d) Crosstalk from Ch4 to other channels.

Figure 2 shows the S-parameter measurements of the bidirectional optical link for full-duplex and half-duplex signal transmission. The difference in the 3-dB bandwidth is about 0.1 GHz between these two modes, suggesting that the inter-mode crosstalk is negligible. The frequency response is stable and uniform with a 3-dB bandwidth of 7.5 GHz, which is suitable for 10 Gb/s full-duplex operation. These results indicate that the full-duplex signal transmission undergoes negligible degradation of the bandwidth.

40G 850nm 150m QSFP+ SR4 Fiber Optic Transceiver Module

From the results shown in Figs. 1 and 2, the slightly higher jitter and bandwidth reduction for full-duplex operation may stem from the small amount of interference between the two widely separated wavelengths of 850 nm and 1060 nm. The crosstalk between the up- and down-link due to the full-duplex operation mode is negligible compared to the inter-channel crosstalk discussed in Fig. 3. Hence, full-duplex operation was successfully demonstrated with the proposed bidirectional OSA module design.

Conclusion

A 40 Gb/s optical link was demonstrated using a four-channel bidirectional OSA module. The OSA module was designed with individual SiOBs for VCSEL, PD and M-PD array chips; the deflection of light to and from the VCSEL/PD component was realized using 45°-mirrors formed in the fiber, which were then placed under the SiOBs for the VCSEL and the PD.

In order to split the up- and down-link light signals with low crosstalk, we selected two wavelengths of 850 nm and 1060 nm, which are widely separated. The up- and down-link light is split with wavelength filters inserted with the mirror under the VCSEL. These structures provide low crosstalk of less than −22.6 dB between neighboring channels in the full-duplex operation mode.

The frequency responses for the full-duplex and half-duplex operation modes demonstrated 3-dB bandwidths of 7.55 GHz and 7.65 GHz, respectively, suggesting that the inter-mode crosstalk is negligible. The fabricated bidirectional OSA modules demonstrated successful 40 Gb/s full-duplex optical signal transmission for the up- and down-links, in both cases.

The BER measurement was less than 10−12 up to 10 Gb/s/ch for each channel at the minimum input power of −8 dBm, which is suitable for data center applications. From the measurement results, the proposed 40 Gb/s OSA module is applicable for high-speed bidirectional data transmission through rack-to-rack and board-to-board optical links with negligible inter-channel and inter-mode crosstalk.

Notices: This article is reprinted from: opticsinfobase.org/oe/fulltext.cfm?uri=oe-22-2-1768&id=277085

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