Modified Uni-traveling Carrier Research Articles

Thermal dissipation enhancement in flip-chip bonded uni-traveling carrier photodiodes.

The thermal properties of modified uni-traveling carrier (MUTC) photodiode flip-chip bonded to AlN and diamond are simulated. The thermal impedance of InGaAs is the primary source of internal heating. An n-down epitaxial structure is designed to improve thermal dissipation. Compared to the conventional p-down configuration, the n-down MUTCs bonded to diamond, or AlN submounts achieved 145% and 110% improvement in dissipated power density at thermal failure, respectively. The improved thermal characteristics presage higher RF output power before thermal failure.

Impact of nonlinearity including bleaching in MUTC photodetectors on RF-modulated electro-optic frequency combs.

We use the drift-diffusion equations to calculate the responsivity of a modified uni-traveling carrier (MUTC) photodetector (PD) with a frequency comb input that is generated by a series of short optical pulses. We first use experimental results for the responsivity of the MUTC PD to obtain an empirical model of bleaching in pulsed mode. We incorporate our empirical bleaching model into a drift-diffusion model to calculate the impact of nonlinearity in an MUTC PD on RF-modulated electro-optic frequency combs. We quantify the nonlinearity using the second- and third-order intermodulation distortion powers (IMD2 and IMD3), from which we calculate the second- and third-order output intercept points (OIP2 and OIP3). In contrast to a continuous wave (CW) input for which there is a single IMD2 and IMD3 and hence a single OIP2 and OIP3, each comb line n has its own IMD2n, IMD3n, OIP2n, and OIP3n associated with it. We determine the IMD2n, IMD3n, OIP2n, and OIP3n, and we compare the results with and without bleaching. We find that the impact of bleaching is complex and, somewhat surprisingly, not always detrimental. The principal effect of bleaching is to lower the responsivity, which decreases the nonlinearity due to space charge. While bleaching always reduces the OIP2n and OIP3n, we find that bleaching leads to a decreased distortion-to-signal ratio for large n.

Comparison of the impact of nonlinearity in a p-i-n and an MUTC photodetector on electro-optic frequency combs.

We calculate the impact of nonlinearity in both a p-i-n photodetector (PD) and a modified uni-traveling carrier (MUTC) PD on an RF-modulated frequency comb generated using 100-fs optical pulses with a 50-MHz repetition rate. We take into account bleaching (nonlinear saturation) that is due to the high peak-to-average-power ratio and contributes to the device nonlinearity. Nonlinear impairment of an RF-modulated continuous wave is typically characterized by the second- and third-order intermodulation distortion products (IMD2 and IMD3). In contrast, an RF-modulated frequency comb must be characterized by a distinct IMD2n and IMD3n for each comb line n. We calculate IMD2n and IMD3n in both p-i-n and MUTC PDs and compare the results. We also calculate the ratio of the IMD2n power and the IMD3n power to the fundamental power Sin in both p-i-n and MUTC PDs. We find that nonlinear distortion has a greater impact at high frequencies in the MUTC PD than in the p-i-n PD.

High-Gain Ka-Band Analog Photonic Link Using High-Power Photodiode at 1064 nm

We report high-gain, low-noise-figure, and high linearity analog photonic links at 1064 nm using InP/InGaAs modified uni-traveling-carrier (MUTC) photodiodes. Balanced photodiodes reach output powers of 17.7 dBm, 19.8 dBm, 20.7 dBm, and 22 dBm at 38 GHz, 29 GHz, 27 GHz, and 24.5 GHz, respectively. High common-mode rejection ratio (CMRR) of 25 dB and good linearity with a third-order intercept point (OIP3) of up to 34 dBm were measured at 38 GHz for 10-μm-diameter balanced photodiodes. We demonstrate two analog photonic links with different noise reduction techniques without electronic amplification. In the first link, we use balanced detection with a quadrature-biased Mach–Zehnder modulator (MZM) for noise cancellation. Link gain of 6.8 dB, noise figure (NF) of 25.8 dB and spurious free dynamic range (SFDR3) of 114 dB·Hz2/3 at 38 GHz were achieved. The second link works at low bias modulation for improved signal-to-noise ratio with a single photodetector. Record-high gain (19.3 dB at 26 GHz and 17 dB at 38 GHz) and low noise figure (14.5 dB at 26 GHz and 17 dB at 38 GHz) were demonstrated.

High-Speed Evanescently-Coupled Waveguide Type-II MUTC Photodiodes for Zero-Bias Operation

We demonstrate evanescently-coupled GaAs0.5Sb0.5/In0.53AlyGa0.47-yAs modified uni-traveling carrier (MUTC) waveguide photodiodes (PDs) with high bandwidth (BW) of 66 GHz at zero bias and over 100 GHz bandwidth under low bias condition. An InxAlyGa1-x-yAs dual-integrated waveguide-drift layer which can facilitate efficient light coupling from the waveguide into the absorber over a short length is utilized. A 7-μm long waveguide PD has an internal responsivity of 0.48 A/W at 1550 nm and clearly open eye diagrams at 40 Gb/s are demonstrated at zero bias.

Microwave Photonic Link With Improved SFDR Using Two Parallel MZMs and a Polarization Beam Combiner

This article presents a linearized microwave photonic link by means of two parallel Mach-Zehnder modulators (MZMs) and a polarization beam combiner (PBC). The proposed link exhibits significant suppression in third-order nonlinearity, which results in fifth order limited performance. Experiments are conducted and measured results demonstrate 17-dB improvement in spurious-free dynamic range (SFDR), resulting in a fifth order limited SFDR of 124 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4/5</sup> . An Erbium Doped Fiber Amplifier (EDFA) and a high-power Modified Uni-Traveling Carrier (MUTC) photodiode are used to realize a link gain of 6 dB at 60 mA photocurrent for operation at 1 GHz. LTE signals with various instantaneous bandwidths centered at 1 GHz are delivered by the linearized link where spectral regrowth is dramatically suppressed.

Calculation of the impulse response and phase noise of a high-current photodetector using the drift-diffusion equations.

We describe a procedure to calculate the impulse response and phase noise of high-current photodetectors using the drift-diffusion equations while avoiding computationally expensive Monte Carlo simulations. We apply this procedure to a modified uni-traveling-carrier (MUTC) photodetector. In our approach, we first use the full drift-diffusion equations to calculate the steady-state photodetector parameters. We then perturb the generation rate as a function of time to calculate the impulse response. We next calculate the fundamental shot noise limit and cut-off frequency of the device. We find the contributions of the electron, hole, and displacement currents. We calculate the phase noise of an MUTC photodetector. We find good agreement with experimental and Monte Carlo simulation results. We show that phase noise is minimized by having an impulse response with a tail that is as small as possible. Since, our approach is much faster computationally than Monte Carlo simulations, we are able to carry out a broad parameter study to optimize the device performance. We propose a new optimized structure with less phase noise and reduced nonlinearity.

Phase-Modulated Analog Photonic Link With a High-Power High-Linearity Photodiode

We investigate phase-modulated analog photonic link performance using interferometric detection with a Mach–Zehnder interferometer (MZI) under different bias conditions and a high-power high-linearity modified unitraveling carrier (MUTC) photodiode (PD). Expressions for radio frequency (RF) small signal gain and the optimal MZI bias point for RF gain are derived, and noise figure (NF) and third-order spurious free dynamic range (SFDR3 ) are analyzed. Using a 28- μ m-diameter MUTC single PD with 23 dBm RF output power under 155-mA saturation current at 10 GHz, we measured a record-high link gain of 25 dB, 18 dB NF, and ${\text{114}}\;{\text{dB}} \cdot {\rm{H}}{{\rm{z}}^{{\text{2/3}}}}$ SFDR3 at 130 mA photocurrent with an optimally biased MZI. By using a 24- μ m-diameter balanced MUTC PD pair with 21 dBm RF output power at 108 mA at 10 GHz, we obtained a 16 dB link gain, 16 dB NF, and ${\text{118}}\;{\text{dB}} \cdot {\rm{H}}{{\rm{z}}^{{\text{2/3}}}}$ DR3 at 100 mA total dc photocurrent when the MZI was biased at quadrature. The RF gain at both MZI bias points is in good agreement with our calculations.

Low dark current III-V on silicon photodiodes by heteroepitaxy.

Top-illuminated PIN and modified uni-traveling carrier (MUTC) photodiodes based on InGaAs/InAlAs/InP were epitaxially grown on Si templates. Photodiodes with 30-μm diameter have dark currents as low as 10 nA at 3 V corresponding to a dark current density of only 0.8 mA/cm2. The responsivity, 3-dB bandwidth, output power and third-order output intercept point (OIP3) were 0.79 A/W, 9 GHz, 2.6 dBm and 15 dBm, respectively.

Nonlinearities of High-Speed p-i-n Photodiodes and MUTC Photodiodes

The linearity, the RF output power, and the bandwidth for p-i-n photodiodes (PDs) and modified uni-traveling carrier (MUTC) PDs are analyzed for a variation of bias voltage and optical input power. Based on an equivalent circuit, equations for analytic calculation of these PD parameters are derived. Good agreement between the measurements and the calculated results of a p-i-n PD and a MUTC PD could be observed for frequencies up to 135 GHz. As a result, the linearity is improved and its photocurrent dependence can be reduced when the bandwidth of the PD is limited by the RC time constant.

Frequency dependent harmonic powers in a modified uni-traveling carrier photodetector.

We use a drift-diffusion model to study frequency dependent harmonic powers in a modified uni-traveling carrier (MUTC) phododetector. The model includes external loading, incomplete ionization, the Franz-Keldysh effect, and history-dependent impact ionization. In three-tone measurements, the bias voltage at which a null occurs (bias null) in the second-order intermodulation distortion (IMD2) is different for the sum frequency and difference frequency. We obtained agreement with the experimental results. The bias null that appears in the IMD2 is due to the Franz-Keldysh effect. The bias voltage at which the bias null is located depends on the electric field in the intrinsic region, and the difference in the location of the bias null for the sum frequency and difference frequency is due to the displacement current in the intrinsic region. When the frequency is large, the displacement current is large and has a large effect on the harmonic powers. We also found that the bias null depends on the recombination rate in the p-absorption region because the electric field decreases in the intrinsic region when the recombination rate in the p-region decreases.

Characterization of Amplitude Noise to Phase Noise Conversion in Charge-Compensated Modified Unitravelling Carrier Photodiodes

The phase noise, i.e., amplitude modulation to phase modulation (AM-PM) in the photodetector. In general, AM-PM of charge-compensated modified uni-travelling carrier (CC-MUTC) photodiodes originates from the combined nonlinearities of the detector impedance and carrier transit times, which depend on the electric field distribution and the velocity-field characteristics of carrier transport. In this work, we report measurements of AM-PM in transit-time-limited CC-MUTC photodiodes. A Crosslight-based simulation of the photodetection process is used to quantitatively estimate the AM-PM of CC-MUTC photodiodes. Simulated AM-PM of a 15-GHz microwave signal shows good agreement with experimental results. The measured and simulated AM-PM versus photocurrent exhibit a null point that originates from the competition between hole deceleration due to the collapsed electric field in the depleted absorption region and electron acceleration in the self-induced electric field in the undepleted absorption region.

High-Power Photodiodes for Analog Applications

High-power, high-speed photodiodes are being used in an increasing number of applications including fiber optic antenna links, radio frequency (RF) over fiber, and photonic generation of low phase noise microwave signals. The fact that the photodiode (PD) can be operated at high photocurrent levels provides several improvements in these systems including high dynamic range, high link gain, and low noise figure. In photonic wireless systems an antenna integrated photodiode can help to increase radiated RF power without the need for electronic amplification and hence simplify the RF circuitry at the antenna unit. To achieve high RF output power, various photodiode structures have been developed [1]–[4] among which the uni-traveling carrier (UTC) photodiode [3] has demonstrated high saturation current and high bandwidth. We have developed charge-compensated modified uni-traveling carrier (MUTC) photodiodes flip-chip bonded on high-thermal conductivity substrates to address the two primary effects that limit the RF output power of photodiodes, space-charge and thermal.

Improved power conversion efficiency in high-performance photodiodes by flip-chip bonding on diamond

Recently, microwave photonic techniques have emerged to address the challenges that microwave systems face under high-frequency or wideband conditions. To a large extent, the performance of microwave photonic systems depends on the performance of individual optoelectronics devices, such as high-power and high power conversion efficiency photodiodes. Here, we report on a flip chip bonded on a diamond InP/InGaAs modified unitraveling carrier (MUTC) photodiode with record RF output powers of 32.7 dBm (1.86 W), 29.6 dBm, 28.2 dBm, and 26.2 dBm at 10, 15, 20, and 25 GHz, respectively, without active cooling. The corresponding dissipated powers are 34 dBm (2.5 W), 32.3 dBm, 30.4 dBm, and 28.3 dBm, respectively. Compared with previously reported RF power, the device on the diamond submount achieves >80% higher RF output power. Using the high-power and high-frequency MUTC photodiode on diamond submount, a record power conversion efficiency of 50.7%–60% at 6–10 GHz with ∼27.8 dBm RF output power has been achieved as compared to previously reported efficiencies in the <40% range in the corresponding frequency band.

High Power Photodiodes for Microwave Applications

High Power Photodiodes for Microwave Applications

Photonic microwave generation with high-power photodiodes

We utilized and characterized high-power, high-linearity modified unitraveling carrier (MUTC) photodiodes for low-phase-noise photonic microwave generation based on optical frequency division (OFD). When illuminated with picosecond pulses from a repetition-rate-multiplied gigahertz Ti:sapphire modelocked laser, the photodiodes can achieve a 10 GHz signal power of +14 dBm. Using these diodes, we generated a 10 GHz microwave tone with less than 500 attoseconds absolute integrated timing jitter (1 Hz-10 MHz) and a phase noise floor of -177 dBc/Hz.We also characterized the electrical response, amplitude-to-phase conversion, saturation, and residual noise of the MUTC photodiodes.

High-power flip-chip mounted photodiode array

Four-element modified uni-traveling-carrier (MUTC) photodiode arrays (PDA) flip-chip bonded onto transmission lines on AlN substrates are demonstrated. High RF output powers of 26.2 dBm and 21.0 dBm are achieved at 35 GHz and 48 GHz, respectively, using a PDA with 28-μm diameter photodiodes. A systematic comparison between a PDA with four 20 μm-diameter elements and a discrete detector with the same active area (40-μm diameter) is presented. The PDA achieved higher output power and thermal dissipation compared to its discrete counterpart.

High Responsivity and High Power UTC and MUTC GaInAs-InP Photodiodes

We have developed a high-performance uni-traveling-carrier (UTC) and a modified uni-traveling-carrier (MUTC) photodiode (PD). We report a comparison between the two devices comprising both a 1.5- μm-thick absorption layer followed by a 0.5-μm-thick transparent collector layer. Both devices showed simultaneously a high responsivity (larger than 0.92 A/W at 1.55 μm), a high saturation current (larger than 100 mA at 10 GHz), and a high linearity (OIP3 of 35 dBm at 10 GHz). Thanks to a partly depleted absorber, the MUTC-PD is demonstrated to achieve a higher bandwidth (more than 20 GHz at high current), while the UTC-PD is demonstrated to achieve a higher saturation current and a less voltage dependent radio-frequency and linearity characteristics.

High-power high-linearity flip-chip bonded modified uni-traveling carrier photodiode

We demonstrate a flip-chip bonded modified uni-traveling carrier (MUTC) photodiode with an RF output power of 0.75 W (28.8 dBm) at 15 GHz and OIP3 as high as 59 dBm. The photodiode has a responsivity of 0.7 A/W, 3-dB bandwidth > 15 GHz, and saturation photocurrent > 180 mA at 11 V reverse bias.

Characterizing and Modeling Nonlinear Intermodulation Distortions in Modified Uni-Traveling Carrier Photodiodes

We present a physics-based model for nonlinear analysis of InGaAs/InP modified uni-traveling carrier (MUTC) photodiodes. At low frequencies (<;3 GHz), the Franz-Keldysh effect has been identified as the primary nonlinear mechanism in these MUTC photodiodes. The output third-order intercept point (OIP3) is used as a figure of merit for characterizing the nonlinear intermodulation distortion. The OIP3 behavior of the photodiode simulated using this model agrees well with measurements.