Anand Ramaswamy

Integrated Coherent Receivers for High-Linearity Microwave Photonic Links

In this paper, we present a coherent receiver based on an optical phase-locked loop (PLL) for linear phase demodulation. The receiver concept is demonstrated at low frequency. For high-frequency operation, monolithic and hybrid integrated versions of the receiver have been developed and experimentally verified in an analog link. The receiver has a bandwidth of 1.45 GHz. At 300 MHz, a spurious free dynamic range (SFDR) of 125 dB ldr Hz2/3 is measured.

Highly Linear Coherent Receiver With Feedback

We propose and demonstrate a novel coherent receiver with feedback for high-linearity analog photonic links. In the proposed feedback receiver, a local phase modulator tracks the phase change of the signal and reduces the effective swing across the phase demodulator without reducing the transmitted signal. The signal-to-noise-ratio is thus maintained while linearity is improved. Up to 20-dB improvement in spur-free dynamic range (SFDR) is achieved experimentally. At 3.13 mA of average photocurrent per photodiode, the measured SFDR is 124.3 dBmiddotHz2/3, which corresponds to an SFDR of 131.5 dBmiddotHz2/3 when the link is shot-noise-limited

High Output Saturation and High-Linearity Uni-Traveling-Carrier Waveguide Photodiodes

Waveguide uni-traveling-carrier photodiodes (UTC-PDs) have been fabricated and tested. Output saturation currents greater than 40 mA at 1 GHz are demonstrated for a 10 mumtimes150mum photodiode (PD). The third-order intermodulation distortion is also measured and exhibits a third-order output intercept point of 43 dBm at 20 mA and 34 dBm at 40 mA for this same PD. UTC-PDs with geometries of 5 mumtimes100 mum and 10 mumtimes100 mum are also compared and it is shown that a wider waveguide PD has improved saturation characteristics due to the lower optical power density which reduces the saturation at the front end of the device

High Power Silicon-Germanium Photodiodes for Microwave Photonic Applications

We demonstrate high current operation of an evanescently coupled Ge waveguide n-i-p photodetector grown on top of a Si rib waveguide. A 7.4 μm × 500 μm device was found to dissipate 1.003 W of power (125.49 mA at -8 V). 2-D thermal simulations of the device show that the relatively high thermal conductivities of the intrinsic Ge region and the p+ doped Si layer result in efficient heat transfer and hence, lower absorber temperatures when compared to a similar InP based waveguide photodiode. Additionally, to determine the feasibility of these devices for analog photonic applications, we performed large signal and small signal radio frequency (RF) measurements as well as linearity measurements. At 1 GHz and 40 mA of photocurrent, a third order output intercept point (OIP3) of 36.49 dBm is measured. The maximum RF power extracted at 1 GHz is 14.17 dBm at 60 mA of photocurrent and 7 V reverse bias.

SFDR Improvement of a Coherent Receiver Using Feedback

A novel coherent optical receiver is proposed and experimentally demonstrated by using a feedback technique capable of reducing the nonlinear distortion in a traditional receiver while retaining the signal to noise ratio.

Output Saturation and Linearity of Waveguide Unitraveling-Carrier Photodiodes

Waveguide unitraveling-carrier photodiodes (UTC-PDs) with different absorber and collector layer doping levels have been fabricated and characterized. These photodiode (PD) structures are fabricated on a platform that allows for the monolithic integration of multiquantum-well optical phase modulators and couplers for realizing novel coherent receivers. Compared to PD A, PD B has a lower and more graded p-doping profile in the absorber layer and also a higher n-doping level in the collector layer. For PD B a larger field is induced in the absorber layer at high photocurrent levels. Also the higher n-doping in the collector layer is adequate for providing charge compensation. For PD B, there is an enhancement in the RF response as the photocurrent level is increased. At a frequency of 1 GHz, the saturation current for PD A is around 65 mA and that for PD B is around 63 mA. For PD B, the third-order output intercept point at photocurrent levels of 30 and 40 mA is 37.2 and 34.9 dBm, respectively. That for PD A is 35.8 and 30.4 dBm. PD B is, therefore, favorable for linear operation at high current levels.

Nonlinear modeling of waveguide photodetectors

A new method of simulating photodiode nonlinearities is proposed. This model includes the effects of non-uniform absorption in three dimensions, self-heating, and is compatible with circuit components defined in the frequency domain, such as transmission lines. The saturated output power and third order output intercept points of two different waveguide photodiodes are simulated, with excellent agreement between measurement and theory. The technique is then used to provide guidance for the future design of linear waveguide-based photodetectors.

Three-tone characterization of high-linearity waveguide uni-traveling-carrier photodiodes

The IMD3 of waveguide UTC photodiodes are characterized using a three-tone technique that is independent of harmonics from the optical sources. At 1 GHz, an OIP3 of 46.2 dBm at 60 mA of photocurrent is measured.

Novel Optical Phase Demodulator Based on a Sampling Phase-Locked Loop

A novel phase-locked coherent demodulator, based on a sampling phase-locked loop, is presented and investigated theoretically. The demodulator is capable of operating at high frequencies, by using optical sampling to downconvert the high-frequency input radio-frequency signal to the frequency range of the baseband loop. We develop a detailed theoretical model of the (sampling) phase-locked coherent demodulator and perform detailed numerical simulations. The simulation results show that the operation of the sampling demodulator resembles the operation of the baseband demodulator for very short optical pulses (<2 ps). Furthermore, we investigate how the signal-to-noise ratio of the demodulator is affected by timing and amplitude jitter of the pulsed optical source

Monolithically Integrated Balanced Uni-Traveling-Carrier Photodiode with Tunable MMI Coupler for Microwave Photonic Circuits

A monolithically integrated balanced uni-traveling-carrier photodiode (UTC-PD) with a tunable 2times2 multimode interference (MMI) coupler has been fabricated and tested. Two waveguide UTC-PDs are electrically isolated using high-energy Helium implantation, and then connected in series using a monolithic metal interconnect. On chip metal-insulator-semiconductor (MIS) capacitors provide some DC decoupling. The tunable MMI coupler allows for tuning of the power balance in the PDs. Output saturation currents greater than 40 mA at 1 GHz are demonstrated for a single 10 mum times 150 mum UTC-PD. The third order intermodulation distortion (IMD3) is also measured and exhibits an output intercept point (OIP3) of 43 dBm at 20 mA and 34 dBm at 40 mA for this same UTC-PD. In the balanced configuration, the OIP3 values are therefore 49 dBm and 40 dBm. The balanced UTC-PD is also highly symmetric; the common mode rejection ratio (CMRR) was measured to be around 40 dB.