Wide Spectral Characteristics of Si Photonic Crystal Mach-Zehnder Modulator Fabricated by Complementary Metal-Oxide-Semiconductor Process

Abstract

Optical modulators for optical interconnects require a small size, small voltage, high speed and wide working spectrum. For this purpose, we developed Si slow-light Mach-Zehnder modulators via a 180 nm complementary metal-oxide-semiconductor process. We employed 200 µm lattice-shifted photonic crystal waveguides with interleaved p-n junctions as phase shifters. The group index spectrum of slow light was almost flat at n g « 20 but exhibited˘10% fluctuation over a wavelength bandwidth of 20 nm. The cutoff frequency measured in this bandwidth ranged from 15 to 20 GHz; thus, clear open eyes were observed in the 25 Gbps modulation. However, the fluctuation in n g was reflected in the extinction ratio and bit-error rate. For a stable error-free operation, a 1 dB margin is necessary in the extinction ratio. In addition, we constructed a device with varied values of n g and confirmed that the extinction ratio at this speed was enhanced by larger n g up to 60. However, this larger n g reduced the cutoff frequency because of increased phase mismatch between slow light and radio frequency signals. Therefore, n g available for 25 Gbps modulation is limited to up to 40 for the current device design.

Integrated Optical Modulator Based on Transition between Photonic Bands

“…Other method for using silicon based devices for modulation is to utilize the plasma dispersion effect, which is the change of refractive index with the free carrier (electrons and holes) concentration 46 , 47 . This process could be performed with injection 19 , 33 , depletion 18 , 20 – 25 , 34 , 36 , 37 or accumulation 17 , 26 , 27 , 30 , 32 of the carriers.…”

“…Recently, a large number of studies on MZI based modulators have been performed with phase shifter structures composed of traditional pn, p-i-n diodes or MOS structures and as a consequence, high operation speeds up to 50 Gb/s were obtained 17 – 27 . However, among all these designs, the shortest phase shifter length seems to be 200 µm 17 , 20 , 21 and the lowest voltage swing for a proper modulation is 1.75V pp with −0.9 V DC offset 17 , 20 . Furthermore, in order to increase the relative phase shift (Δ ϕ ) between arms of MZI and reduce the length of phase shifter, in some designs push-pull drive has been employed 17 .…”

“…Slow light, which is associated with lowering the group velocity of the light, is offered by PhCs as well 15 . Therefore, PhCs are quite useful in terms of manipulating refractive index of the materials and there are several PhC based optical modulator designs available in the literature for enhanced performance 17 , 20 , 21 , 28 , 60 – 63 . For instance, length of the phase shifter was reduced to 80 µm for 1 Gb/s operation speed with 2 V on-state voltage 28 .…”

Adiabatic wavelength redshift by dynamic carrier depletion using p−i−n -diode–loaded photonic crystal waveguides

“…Figure 2 is a magnified view of the center of the LSPCW and its corresponding mode profile. To obtain the low-dispersion band at an approximate wavelength of 1550 nm, we set the lattice constant a, hole diameter 2r, and third-rows lattice shift s 3 to 400, 215, and 110 nm, respectively [20]. This device yielded n g ~ 24 in the low-dispersion band.…”

Slow light with interleaved p-n junction to enhance performance of integrated Mach-Zehnder silicon modulators

“…[35]. We should notice that slow-light MZ modulators with interleaved (or, more generally, modulated) p-n junctions have been deeply studied in the context of 2D PhC waveguides for wavelengths in the C-band (λ = 1.55 μm) [36,37]. The grating waveguides considered here have the advantage of being more compatible with standard silicon photonic circuits on widespread industrial platforms, especially because the structure is simpler and less demanding in terms of optical design and fabrication, and low-loss tapers with standard rib waveguides are easily available.…”