Theoretical and experimental investigation of low-volgage and low-loss 25-Gbps Si photonic crystal slow light Mach–Zehnder modulators with interleaved p/n junction

Abstract

In this study, we investigated the performance of Si lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators theoretically and experimentally. The LSPCW increases the phase shift in modulator to 2.3-2.5 times higher, which allows for size reduction and high performance. On-chip passive loss was reduced to less than 5 dB by optimizing each component. We obtained 25 Gbps clear open eye and 3 dB extinction ratio at a drive voltage of 1.5-1.75 V for 200 μm phase shifter with linear p/n junction when we added a modulation loss of 7 dB. This modulation loss was reduced to 0.8 dB, maintaining other performance, by employing interleaved p/n junction and optimizing doping concentrations.

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

“…Moreover, an interleaved p-n junction was employed, which increases the overlap of a depletion region with the slow-light mode and thus increases ∆n eq . This succeeded in the 25 Gbps modulation in a device with L = 200 µm with a drive voltage of V pp = 1.75 V (on the panel of pulse pattern generator, PPG) [15]. In this study, we observed an ER of 3 dB, although the excess modulation loss (ML), which usually helps to increase the ER, was moderately suppressed to 0.8 dB.…”

“…In general, f3dB is constrained by the RC time constant. We have reported the calculation of the resistance R and capacitance C at the p-n junction using the commercial simulator Lumerical DEVICE in Reference [15]. Modeling the Si layer sandwiched by the SiO2 BOX layer and top SiO2 cladding (both insulators), they were calculated to be 96 Ω (146 Ω including 50 Ω internal resistance in the PPG connected in series with the R) and ~60 fF at Vb = −2.2 × 2 V, respectively, for the current design.…”

“…To overcome this constraint, we have developed photonic crystal waveguide (PCW) MZ modulators. They are constructed via a CMOS process, with a phase shifter length L significantly reduced by the slow-light effect [10][11][12][13][14][15]. For slow light with a low group velocity υ g , the slowdown factor is usually expressed by the group index n g " c/υ g , where c is the light velocity in a vacuum.…”

“…Values of n g are observed to be between 3 and 4 for Si rib waveguides, while Si PCWs easily produce n g > 20 [10]. In a simplified model, the phase shift ∆φ in a waveguide with an equivalent modal index change induced by carrier plasma dispersion, ∆n eq , is expressed as [15,16] ∆φ " k 0 n g ∆n eq n eq ζ L , ζ " n eq ω b dω b dn eq « 1…”

“…Figure 1a shows the schematic of the Si LSPCW MZ modulator with the interleaved p-n junction. The details of the structure, theoretical modal behaviors, and fundamental modulation characteristics can be seen in Reference [15]. Third-row-shifted LSPCWs with a central p-n junction and SiO 2 cladding as phase shifters were employed; these exhibit typical LD slow light.…”