Generation of 10-Gbit/s Optical Millimeter-Wave Signal using a 125-GHz Mode-Locked Laser Diode Integrated with an Electro-Absorption Modulator

We fabricated a compact optical millimeter-wave (MMW) transmitter by monolithically integrating an active mode-locked laser diode (MLLD) and high-mesa electro-absorption (EA) modulators. From the frequency domain measurements of the 125-GHz optical MMW signal, we obtained a linewidth of less than 10 Hz and a timing jitter of 0.1 ps. As for the integrated EA modulator, a high extinction ratio (20 dB) and a large 3-dB-down bandwidth (16 GHz) were achieved by employing high-mesa waveguide structure buried with polyimide. In bit-error-rate measurements of the 10-Gbit/s MMW signal generated using the optical MMW transmitter, error-free operation was achieved.

I. Introduction

Wireless transmission of multi-gigabit-per-second data enables several attractive services, such as wireless transmissions of multi-channel non-compressed high-definition television signals for relaying broadcasts of live events and telemedicine. Transmitting such a high-bit-rate signal requires millimeter waves (MMWs) as carrier signal. Photonic techniques provide several advantages for handling MMW signal, such as the availability of optical amplifiers, low transmission loss, and the wide bandwidth of optical fiber. Especially advantageous is that once the MMW signal is superimposed on the optical carrier, the MMW signal can be easily modulated with high-bit-rate data signal using an optical intensity modulator. Recently, 10-Gbit/s data signal has been transmitted on a 125-GHz carrier using photonic techniques [1]. In this method, an optical MMW source is a key device. Most optical MMW sources employ complicated configurations to achieve high frequency stability and accuracy [2], [3], which are required for wireless communications systems. In contrast, an active mode-locked laser diode (MLLD) is a simple device that can generate stable optical MMW signal synchronized with an electrical RF input [4], [5]. These optical MMW sources require an external optical modulator to modulate their output by multi-gigabit-per-second data signal. In fiber transmission systems, an electro-absorption (EA) modulator monolithically integrated with a DFB laser is widely used as a low cost and compact optical source [6]. Thus, a monolithic integration of the MLLD and EA modulator would be very advantageous in constructing a simple low-cost optical MMW transmitter. In addition, an active MLLD that integrates an EA modulator for mode locking in its cavity [4], [5] is suitable for the integration of another modulator for encoding. In this paper, we report the generation of 125-GHz optical MMW signal modulated by 10-Gbit/s data signal using a compact transmitter comprising an active MLLD monolithically integrated with a high-mesa EA modulator. Fig. 1. (a) Schematic drawing of the MLLD integrated with a high-mesa EA modulator, and (b) a photograph of the optical-MMW-transmitter module.