CS-LDTC07 WAVELENGTH | Alldatasheet
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demonstrated by researchers at Harvard University are also discussed. higher harmonics of the cavity free spectral range (FSR). used to generate OFCs are not easily executed or possible. have been employed to create OFCs up to the THz range. eliminate external optics while downconverting. equidistant-spaced frequency peaks (Figure 1). Figure 1. Frequency Comb Illustration communication, data transmission and spectroscopy.
Case Study CS-LDTC07 Rev. A Page 4 © 2020 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com chip, the stability of the THz comb is greatly improved from the frequency tuning rates that are shared. 1 The THz comb tuning rates of the emitting frequency and carrier frequency are estimated to be -.56cm -1/A and 6.9 MHz/mA. These values are over ten times smaller than the mid-IR comb values, increasing stability.1 The DFB QCL design has the increased current dynamic range for harmonic comb operation and increased reproducibility while operating at room temperature. It has also decreased complexity in the setup - no cryogenic cooling or external optical elements are needed. Further research and experimentation could enable monolithic control and tuning of emission of harmonic combs as well as realization of fundamental THz frequency comb through the DFB QCL design. ALTERNATIVE APPLICATIONS selF-starting harmonic Frequency comb Researchers at Harvard University, Massachusetts have developed frequency combs using QCLs for other applications based on a self-starting comb generation design.2 Researchers from Harvard University have developed a terahertz harmonic frequency comb using a QCL device. This experiment studies the harmonic comb state of the QCL and its possibilities. Two FP QCLs are used to produce and confirm the presence of a harmonic comb. Injected current is increased from the lasing threshold of single-mode operation to harmonic comb state. The second FP QCL has a higher injection current to operate at the fundamental comb state for reference to verify the equidistant spacing between modes in the harmonic comb QCL. This multiheterodyne beating technique allows the measurement of terahertz-scale beatnote frequency of the harmonic state. This allows for the sample spectrum to be down-converted from the optical domain to the radiofrequency (RF) domain. The RF comb that is created can be measured and verified using electronic frequency counters. The harmonic comb QCL operates with a repetition rate of 400 GHz, and the reference fundamental comb QCL operated with a repetition rate of 7.7GHz. The light emitted from the harmonic comb QCL is passed through the reference QCL. This enables extraction of the multiheterodyne signal for verification. The optical carrier frequency of the laser was found to be fc = 66.7 THz with improvements of the stability of the signal due to the self-detection design using a reference QCL. This concept utilizes intermodal comb spacing of hundreds of gigahertz up to the terahertz range. Applications for this compact comb device range from terahertz wireless communication systems, telecommunications, radioastronomy, quantum optics, to spectroscopy seen in the previous design. Due to the QCL's versatile composition, microwaves can be generated and modulated to wirelessly transmit information.2 radio Frequency transmitter Other research from Harvard University realizes a compact radio frequency transmitter based on a QCL frequency comb.4 With demand for wireless communication and devices increasing, the need for higher frequency operation also increases. Extremely narrow linewidth can be generated at room temperature, and modulation and emission of subterahertz waves are attainable. This can compensate the growing need for high-frequency communication technology with high-speed data transfer. In this experiment, a FP QCL operating in the fundamental frequency comb state is used with a narrow linewidth beat note at fB = 5.5 GHz. An Antenna is attached to the top of the QCL and connected to two top laser contacts, and a gap is created in the top electrode. This allows researchers to use the radio frequency alternating currents from within the QCL to generate into the antenna enabling wireless microwave emission as well as the mid-IR radiation output from the QCL. This creates the Laser Radio Transmitter (LRT). An audio analog signal can modulate the laser current which modulates the laser beat note frequency. This encodes the baseband information onto the 5.5 GHz carrier wave which is received by a horn antenna away from the laser. This signal is filtered and down-converted to fit the bandwidth of a software-defined radio. The audio track can be retrieved after demodulation. This device shows the success of a radio frequency transmitter using a QCL device. Further advancements show wireless frequency sensitivity capabilities with the LRT. This would allow the laser beat note to be wirelessly injection locked to an external microwave reference. This single device can be used in applications similar to the ones previously listed.4
Case Study CS-LDTC07 Rev. A Page 5 © 2020 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com KEYWORDS Quantum Cascade Laser, Frequency Comb, Room Temperature, Terahertz, Harmonic, Distributed-feedback Grating, Mid-IR, Multiheterodyne Spectroscopy PRODUCT USED QCL2000 LAB, PTC10K-CH, TC5 LAB
REVISION HISTORY
Document Number: CS-LDTC07 REVISION DATE NOTES A June 2020 Initial Release REFERENCES terahertz semiconductor frequency comb. Nat Commun 10, 2403 (2019). doi.org/10.1038/s41467-019-10395-7 2. Kazakov, D. et al. Self-starting harmonic frequency comb generation in a quantum cascade laser. Nat. Photonics 11, 789–792 (2017). doi.org/10.1038/s41566-017-0026-y https://www.rp-photonics.com/frequency_combs.html 4. Piccardo, M. et al. Radio frequency transmitter based on a laser frequency comb. Proc. Natl Acad. Sci. USA 116, 9181 (2019). doi.org/10.1073/pnas.1903534116 USEFUL LINK
- QCL2000 LAB Product Page
- PTC10K-CH Product Page
- TC5 LAB Product Page OPEN ACCESS The figures and data used for this case study were obtained from Reference 1. The article (Ref. 1) is distributed under terms of Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided that you give appropriate credit to the original authors and the source, provide a link to the Creative Commons license, and indicate if changes were made. Figures 2 and 4 were cropped or the format was changed. No changes were made to the other images. They are presented here in their original form. The captions for Figures 2 and 4 have been modified from their original form. WAVELENGTH'S ROLE The QCLs operation state is highly dependent on the injection current. Because the harmonic comb state is accomplished with slightly more current than single-mode state, it is crucial that the QCLs are properly and accurately driven and have stable temperature control. Wavelength Electronics' QCL2000 LAB QCL drivers (Figure 6 left) provide up to 2.0 A of current to the QCLs with noise current as low as 0.4 µA (RMS). The average noise density of 4 nA/√Hz is necessary for the design of the room temperature frequency comb. These high-precision and ultra-low noise current sources provide the necessary stability for harmonic frequency combs. Wavelength Electronics' PTC10K-CH temperature controller (Figure 6 right) provides up to ±10 A of current to the TEC to stabilize the temperature of the QCLs. The PTC has temperature stability of less than 0.0012ºC which surpasses the required temperature stability of less than 10 mK in the experiment. The compact and accurate design of the PTC enables total temperature control of the lasers. At Harvard University, both studies use Wavelength Electronics' TC5 LAB (Figure 6 top) to control the temperature of the QCLs. The TC5 LAB has temperature stability as precise as 0.0002ºC which also surpasses the precision of better than 10 mK required. QCL2000 LABs are also used to drive the QCLs at the varying current levels with the same noise levels listed for the Northwestern University study.
Figure 6. Wavelength Electronics' TC5 LAB (top),