A 0.77-V drive voltage (V π ) electro-optic modulator with bandwidth exceeding 67 GHz is described. Modulator is a compound semiconductor device fabricated using substrate removal technology. This allows placement of metal electrodes on both sides of an optical waveguide containing a p-i-n diode. Hence ohmic losses are reduced significantly. Electrode gap is essentially the same as i layer thickness, which can be kept very uniform and small. Waveguide core also contains a MQW, which improves electro-optic efficiency. Lack of p doping in the waveguide and large detuning between MQW absorption peak and operating wavelength keep the propagation loss low. Large size of the waveguide also helps to keep coupling loss low. Modulator is designed as a traveling wave device using the loaded line approach, which is used for velocity matching. Combination of these approaches yields a device with the lowest V π and widest bandwidth.

Novel electro-optic modulators in compound semiconductor epilayers using substrate removal techniques are reported. Epilayer consists of a p-i-n junction in which i layer is composed of an InGaAlAs/InAlAs MQW. This creates an optical mode with very strong vertical confinement and overlapping very well with the large electric field of the reverse biased p-i-n junction. This approach combined with the large quadratic electro-optic coefficient due to MQW improves efficiency of modulation significantly. Mach-Zehnder electro-optic modulators fabricated using this approach has 0.2 V (0.6 V) V π for 3 (1) mm long electrodes at 1.55 μm under push pull drive corresponding to record modulation efficiency of 0.06 V•cm. Index Terms-Compound semiconductor modulators, integrated optics, optical modulators. I. INTRODUCTIONO PTICAL modulator is a key component for a wide range of applications requiring electrical to optical conversion. These include fiber optic communications, RF photonics, optical signal processing and instrumentation. Electro-optic modulators are among the most commonly used modulator types. One of the key metrics for an electro-optic modulator is the drive voltage needed to switch the modulator from on to off state or vice versa. This parameter is also known as V π . Electrical power required to turn the modulator on and off is proportional to the square of this parameter. Clearly a low V π modulator is needed to reduce electrical power consumption. Electro-optic modulators are fabricated in different material platforms such as LiNbO 3 [1], [2], polymer [3] and compound semiconductors [4]. Such modulators typically have V π values of about a few volts and higher. Presently most commonly used electro-optic modulators are made in LiNbO 3 . They have low frequency V π of around 3 V with bandwidths around 20 GHz. This value increases with frequency and bandwidth. In an electro-optic modulator, there is a tradeoff between V π and bandwidth. There are reports of very wide bandwidth LiNbO 3 modulators approaching 100 GHz [1]. But these devices have V π of about 10 V. High V π values Manuscript

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This Letter is on wide-bandwidth electrodes suitable for ultralow voltage substrate-removed compound semiconductor electro-optic modulators. Using a loaded line approach, traveling wave electrodes suitable for wide-bandwidth and low-voltage operation were studied theoretically and experimentally up to 35 GHz. Using staircase waveguides and n-i-p-i-n epilayer designs, electrode capacitance and resistance were reduced significantly. Experimental and modeling results are found to agree very well. Theoretical and experimental results indicate that subvolt modulators with electrical to optical bandwidths in excess of 30 GHz are possible.

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