Room‐temperature quantum cascade laser packaged module at ∼8 μm designed for high‐frequency response

packaged module at ∼8 μm designed for high-frequency response Ke Yang,1,2 Junqi Liu,1,2,✉ Shenqiang Zhai,1 Jinchuan Zhang,1 Ning Zhuo,1 Lijun Wang,1,2 Shuman Liu,1,2 and Fengqi Liu1,2,3 1Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Beijing, China 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China 3Beijing Academy of Quantum Information Sciences, Beijing, China ✉Email: jqliu@semi.ac.cn

Introduction: High-speed quantum cascade lasers (QCLs) are important for mid-infrared free-space optical communication (FSOC) because of their small size, low power consumption, and high modulation speed. QCLs emitting in long-wave infrared (LWIR) are especially attractive because of lower attenuation in the atmosphere [1]. Due to their intrinsic high-speed response the modulation bandwidth of QCLs can theoretically achieve 100 GHz level [2], but is usually limited by electric parasitic in practice. Several different device structures are applied to improve the modulation speed of LWIR QCLs. QCLs emitting at 8 μm using chalcogenide glass as insulating layer response flat up to roughly 7 GHz at 20 K [3]. An almost flat frequency response up to 14 GHz at 77 K was obtained for QCLs embedded into the microstrip line [4]. However, there are few reports on high-speed LWIR QCLs working at room temperature, especially with the packaged module.
Some previous studies [3,5,6] focused on reducing the parasitic capacitance of QCLs; however, the differential resistance of QCLs also contributes to the RC constant and thus influences the high-frequency performance. In this letter, we presented a packaged high-speed QCL module with improving high-frequency performance without changing the device structure.
Design and fabrication: The QCL wafer used in this experiment was grown on an n-doped (Si, 2 × 10 17 cm −3 ) InP substrate by solid-source molecular beam epitaxy (MBE) based on a bound-to-continuum structure [7]. The laser chip was prepared as a semi-insulating InP (SI-InP, Fe-doped) buried heterostructure device with a ridge width of 8 μm. The ridge waveguide was deep etched (∼9 μm) using wet chemical etching and then the SI-InP was regrown thick enough (> 7 μm) on both sides of the ridge by metal-organic chemical vapor deposition (MOCVD) to reduce the parasitic capacitance. Next, a 450-nm thick SiO 2 layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) for electrical insulation. After an injection window was opened, a Ti/Au layer was deposited by e-beam evaporation for electrical contact and subsequently an additional 5 μm-thick gold layer was electroplated to improve the heat dissipation. After substrate thinning, contact metal deposition, and annealing, the wafer was cleaved into 0.5mm-long laser bars. Such a short cavity length is expected to reduce parasitic capacitance, and thus improve modulation bandwidth. At the same time, short cavity lowers electric power consumption. To reduce mirror losses, the back facet of the QCL was coated with high reflective film of ZrO 2 /Ti/Au/Ti/Al 2 O 3 for reflectivity about 100%, and the front facet coated with Al 2 O 3 /Ge/Al 2 O 3 /Ge for reflectivity about 92%. Finally, the prepared QCL chip was mounted epi-layer side down on a specially designed SiC ceramic heat sink to reduce electric parasites. The module is based on a high-heat-load (HHL) package with a matching printed circuit board (PCB) and a Sub-Miniature Version A (SMA) connector for transmitting radio frequency (RF) signals, as shown in Figure 1. The RF signal is injected from the SMA connector and the DC current from the pins on the opposite side. They are then combined on the matching PCB and sent to the QCL chip. In a sense, the matching PCB behaves like a bias tee, where the DC current and the RF signal are combined. A thermoelectric cooler (TEC) and a temperature sensor are used for temperature control. The electrical circuit schematic diagram of the module is shown in Figure 2. The values of C 1 , C 2, and C 3 are all 11 nF in this experiment but each of them can also be larger. C 2 and C 3 are used to block direct current (DC) and can be seen as short circuits to RF signal. The role of R 1 and C 1 is to increase the -3 dB cut-off frequency, and C 1 can also filter the noise from the DC current source. R 2 should match the impedance of the module to the RF source impedance R s , so R 2 = R s − R1RQCL R1+RQCL . The simplified small-signal model of a QCL bare chip consists of a differential resistor R QCL and a parasitic capacitance C QCL in parallel [3], in which only the current flowing through R QCL contributes to the light emission. After injecting the radio frequency (RF) signal the RF current flowing through R QCL (I RF,QCL ) will be: where P RF is the setting output power of RF source. Let I RF,QCL (ω −3dB ) = 1 √ 2 I RF,QCL (0) and then it can be calculated that the -3 dB cut-off frequency is After adding the PCB, the absolute value of impedance of C 1 (∼0.14 at 100 MHz) is much smaller than R 1 , which can be approximated as a short circuit, so R 1 is equivalently connected in parallel with R QCL , which reduces the RC constant of the whole packaged module to After injecting the RF signal, the RF current flowing through R QCL (I RF,QCL ) will be and the -3 dB cut-off frequency is Though smaller R 1 results in higher -3 dB cut-off frequency and smaller DC voltage drop on R 1 , it also causes smaller RF current going through R QCL at low frequency. As a compromise, R 1 can be selected close to R QCL .
Experiments and discussion: Figure 3 shows the continuous-wave (CW) power-current-voltage (P-I-V) characteristics of the QCL bare chip and the packaged module. The inset shows emission spectra of the packaged module at a temperature of 298 K. According to the I-V characteristic curve of the bare chip, it can be calculated that the differential resistance above the threshold current is about 19 . Accordingly, R 1 = 21.5 ≈ R QCL and R 2 = 39.2 are selected. It can be seen from Figure 3 that the threshold current of the packaged module is equal to that of the bare chip, which are both 110 mA. The corresponding threshold current density is 2.75 kA/cm 2 , which is slightly larger than that in Ref. [7], indicating a small mirror loss. The decrease in output power after packaging may be due to the limitation of collection efficiency of the lens. The bias voltage of the packaged module is greater than that of the bare chip under the same current, which is mainly due to the DC voltage drop on R 1 . At 298 K and when DC current is 120 mA, the emission wavelength of the packaged module is about 8.14 μm.
To characterize the high-frequency performance of the bare chip and the packaged module, we applied a microwave rectification technology [3]. The testing circuit of the bare chip is similar to that described in [5] and that of the packaged module as shown in Figure 2. The parameters used in the test are: the RF source setting output power is 0 dBm, the amplitude of the RF signal is modulated at 1021 Hz, and the DC current is 120 mA. For the testing circuit of the packaged module, the input of the lock-in amplifier is connected in parallel with C 1 whose absolute value of impedance to the rectified signal (∼14.2 k at 1021 Hz) is much greater than R 1 + R QCL , so C 1 will have little impact on the rectified signal.
It should be noted that the rectified voltage V rect is proportional to the square of the RF current (I RF,QCL or I RF,QCL ) flowing through R QCL [8], i.e.  (2) and (4), respectively. The test results show that the rectification response of packaged module is relatively flat up to 2 GHz and the -3 dB cut-off frequency is about 2.2 GHz, while that of the bare chip is about 0.9 GHz. The deviation of the test result from the simulation result may be caused by the nonideality of leads connected to the QCL and of the components on the PCB.

Conclusion:
We have demonstrated a high-speed QCL module with a CW emitting wavelength of 8.14 μm and a CW output optical power about 2 mW at 298 K. The RC constant is reduced by inserting a matching printed circuit, and the -3 dB cut-off frequency is increased from about 0.9 to 2.2 GHz. The module can be used in short-distance FSOC in the future, such as data exchange in the data centre.