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Research Article

Vol. 1, No. 6 / December 2014 / Optica

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Improved power conversion efficiency in high-performance photodiodes by flip-chip bonding on diamond XIAOJUN XIE,* QIUGUI ZHOU, KEJIA LI, YANG SHEN, QINGLONG LI, ZHANYU YANG, ANDREAS BELING, AND JOE C. CAMPBELL Department of Electrical and Computer Engineering, University of Virginia, 351 McCormick Road, Charlottesville, Virginia 22904, USA *Corresponding author: [email protected] Received 14 July 2014; revised 6 November 2014; accepted 6 November 2014 (Doc. ID 216817); published 17 December 2014

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 100% [30]. In this work, modulation depth is enhanced based on a Mach– Zehnder modulator by adjusting the bias point of the modulator and the power of the input RF signal delivered to the modulator. Figure 6 shows a block diagram of the setup. The modulation depth of the optical signal can be expressed as mω  2 sinβV dc J 1 βV ac ∕1  cosβV dc J 0 βV ac ; (2)

where β  π∕V π , and V π , V dc , and V ac are the half-wave voltage, bias voltage, and RF signal voltage of the modulator, respectively. The DC and AC optical power decrease as the modulator is biased away from quadrature toward its null point (βV dc :π∕2 → π), but the DC optical power decreases faster initially and then slower than the AC optical power [31]. Therefore, the modulation depth will initially increase, peak at a specific bias point, and finally decrease rapidly. This is illustrated in the inset of Fig. 5(b). The maximum mω is 1.414 when βV dc  3.12 (close to null bias point) and βV ac  0.0314 (small input RF signal). The maximum PCE for the modulation-depth-enhancement technique is ∼70%. When the modulator is biased at the quadrature point, the photodiode works at Class A condition, i.e., the input RF signal to the modulator is impressed on the signal to the photodiode. When the modulator is biased toward its null point, a part of the optical envelope (which corresponds with the input RF signal to the modulator) will be cut off, or “clipped,” and the photodiode operates in the Class AB condition. When the operation of a transistor changes from Class A to Class AB, the PCE increases. The amplitude of fundamental signal will increase at first and then drop beyond a specific point, and the DC component will decrease [1]. In other words, the

Research Article

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Fig. 6. Experimental setup for the modulation-depth-enhancement technique.

modulation depth is enhanced when the operation changes from Class A to Class AB. The blue dashed line in Fig. 5(c) illustrates the relationship between Class AB PCE and average photocurrent. By optimizing the bias voltage of the modulator (βV dc ≈ 3.1) and the RF signal voltage (βV ac  0.0314), 60%, 52%, and 51% PCE with 28.3, 27.7, and 27.6 dBm RF output power at 6, 8, and 10 GHz, respectively, have been achieved [Fig. 5(b)], which approaches the class AB PCE limit based on optoelectronic modulator (63.5% at 100 mA), which is plotted as the blue dashed line in Fig. 5(c). 6. CONCLUSION

We have demonstrated 1.8 W output power at 10 GHz and 60% PCE at 6 GHz with a flip chip bonded on diamond photodiode chip without active cooling. These CC-MUTC photodiodes achieved RF output powers of 29.6, 28.2, and 26.2 dBm at 15, 20, and 25 GHz, respectively. The failure power density on the diamond submount was 300% and 50% higher than that of photodiodes without flip chip bonding and bonded to an AlN submount, respectively. Based on the high-power and high-frequency MUTC photodiode on diamond submount and modulation-depth-enhancement technique, power conversion efficiencies of 51%–60% at 6– 10 GHz with ∼27.8 dBm RF output power were achieved. FUNDING INFORMATION

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