Open Access
Add to BibSonomy
Abstract
This article shows a dual frequency comb in the 2 µm wavelength region using mutually injection locked gain-switched semiconductor lasers. Strained InGaAs multi-quantum-well discrete mode lasers and gain switching were used to generate two optical frequency combs with repetition rates of 2 GHz and 2.0001 GHz respectively, centred at 2.002 µm. Each optical comb spanned approximately 100 GHz. Through mutual injection locking to an edge comb line common in both combs, a phase locked dual frequency comb was demonstrated with 44 beating tones unique to single comb line pair interactions. This scheme allows for the comb information to be compressed into a 5 MHz detection bandwidth and captured with millisecond acquisition times, which could be of benefit to a number of sensing applications.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
Over the last decade, optical combs have become an essential part of research and applications in many scientific fields. Optical frequency combs (OFCs) offer a stable, coherent and periodic signal which has been creatively used in applications including coherent optical communications [1,2], metrology [3], optical clocks [4], distance ranging [5] and optical sensing [6,7]. From the outset, OFCs have been generated using a variety of methods and optical sources [8]. They can also be generated at many wavelengths, with the majority focusing on the 1.55 $\mu$m wavelength region due to the availability of good quality optical sources, borrowed from optical fibre communications. Although optical combs offer higher resolution and broader spectral coverage in sensing applications when compared to their single wavelength counterparts [9], the key factors limiting their wide implementation within community-based sensing or distributed sensing networks are the cost, size and complexity of the optical components required [10]. Hence, research efforts have focused on the miniaturisation of comb sources, aiming towards photonic integrated circuits (PICs) [11–13]. One approach to achieve compact low complexity OFC generation is gain switching of semiconductor diode lasers [14]. Gain switching is a process used to achieve pulsed emission from a single mode laser source through direct modulation of the laser driving current. Due to the nonlinear response of the laser under this modulation regime, the pulses generated are compressed, with pulse widths much less than the period of the applied modulation [15]. The repetition rate of the pulses determines the frequency spacing between the optical comb lines, allowing for robust control of the OFC free spectral range (FSR). Such schemes have shown picosecond pulse generation resulting in combs with bandwidths reaching up to 250 GHz in the 1.55 $\mu$m region [16].
In recent years, the potential of dual optical frequency comb architecture for spectroscopic applications has been demonstrated [17]. Dual frequency combs (DFCs) enable near real-time spectroscopy by compressing the information of the OFCs into a narrow frequency bandwidth detected on a photodetector in the electrical domain. DFCs can be used for spectroscopic sensing without mechanical moving parts, removing one of the key challenges preventing the downsizing of modern spectroscopic equipment. Figure 1 describes the principles of DFC frequency down-conversion to the electrical domain. The key feature is the interaction between the two OFCs which generates a beating tone spectrum in the electrical domain containing the amplitude and frequency information of the original combs. With a dual comb scheme, two optical combs are generated with repetition frequencies $f_1$ and $f_2$, where $f_2$ = $f_1$ + $\Delta f$ and $\Delta f$ is the difference in repetition frequency. When the two optical combs are simultaneously detected by a photodetector, beating tones are generated between neighbouring comb lines. These neighbouring comb lines, shown by lines sharing the same colour in Fig. 1(b), are referred to as comb line pairs. The heterodyne beating tones between comb line pairs are generated at RF frequencies which are multiples of $\Delta f$ and, ideally, individual to each comb line pair. This allows for the conversion of the amplitude and frequency information of the comb lines to the electrical domain, compressed by a factor of $f_1$/$\Delta f$. One of the greatest advantages of this scheme is that it removes the requirement for high speed photodetectors to measure the entire comb bandwidth, and greatly reduces the acquisition time for measurements when compared to direct spectroscopy methods [18]. As with gain-switched OFCs, dual comb spectroscopy has been shown mostly for wavelengths around 1.55 $\mu$m, due to the availability of sources in this waveband [19–21]. The challenge is extending beyond this waveband to enable key sensing applications.
Fig. 1. Description of the dual comb interaction where (a) two OFCs of slightly different repetition rates $f_1$ and $f_2$=$f_1$+$\Delta f$ (b) are combined to form a collection of comb line pairs which are frequency offset by multiples of $\Delta$f, resulting in (c) an RF tone spectrum containing the beating tones generated between comb line pairs separated by $\Delta f$.
The 2 $\mu$m wavelength region is of interest due to its potential in a variety of applications. For example, communications at 2 $\mu$m were demonstrated, when exploring the low loss of hollow core photonic bandgap fibre, enabling dense wavelength division multiplexing [22]. Silicon photonics could also benefit from the advancements in 2 $\mu$m optical technologies, as silicon has favourable properties such as low two-photon absorption and high Kerr coefficient around 2 $\mu$m [23]. Moreover, 2 $\mu$m waveband can be used for optical sensing of key gases, such as carbon dioxide, ammonia and water vapour. An optical comb source operating in the 2 $\mu$m wavelength region would enable the identification of such gases, with the potential for faster acquisition speeds and higher accuracy than currently available single optical carrier sensing techniques [24–26]. To the best of our knowledge, the development of comb sources operating at 2 $\mu$m has been limited [6]. Progress has been made in the development of 2 $\mu$m OFC sources for high power applications, using Thulium doped fibre components [27,20]. However, the focus is now being shifted towards integrable, small scale and low power alternatives. In recent years, improvements in strain management in multiple quantum well (MQW) lasers has led to the development of strained InGaAs MQW discrete mode (single frequency) lasers in the 2 $\mu$m waveband on an InP platform. These lasers have demonstrated a side-mode-suppression ratio of 40 dB, with 20 mA threshold current and optical linewidths $\approx$2 MHz [28]. The availability of such lasers opens up an avenue for frequency comb generation in the 1.55 $\mu$m to 2.1 $\mu$m wavelength range at low cost, small form factor and with the potential for future integration. In this article, we demonstrate OFC generation at 2.002 $\mu$m, spanning 100 GHz, using gain-switched discrete mode semiconductor lasers. Furthermore, we also demonstrated a coherent dual frequency comb through mutual optical injection locking of the OFCs, with millisecond acquisition of high resolution beating tones compressed into a narrow detection bandwidth of 5 MHz with over 30 dB beat tone signal to noise ratio (SNR).
2. Experiment
Figure 2 depicts the experimental setup used to generate and analyse the DFC proposed here. The three lasers used (Primary Laser/ Laser 1/ Laser 2) had a strained InGaAs/AlInGaAs MQW structure, with single frequency emission achieved through etching of slots in the waveguide layer to promote optical feedback of a single laser mode. Figure 2(a) shows the primary laser, which was used to mutually injection lock both comb sources, enabling a common phase relationship between the two OFCs. Injection locking is required as uncorrelated phase noise between the two lasers would cause excessive broadening of the RF beating tones. Injection locking also allows for phase stabilisation of the pulses produced through gain switching, which can be negatively impacted by spontaneous emission when the lasers are modulated below the laser threshold. A circulator, in combination with a 50/50 bi-directional coupler, was used to couple the primary laser to both comb sources (laser 1 and laser 2) via port P2. The primary laser polarisation was controlled to allow for optimal injection locking to laser 1 and laser 2. Figure 2(b) and 2(c) depict the two gain-switched OFC sources. Polarization controllers were used on both OFC sources to create a parallel polarisation between the two optical fields. This was done to maximize the amplitude of the RF beating tones generated between the two combs. The laser sources were biased, using two independent current sources, and thermally tuned to the same centre wavelength to allow for maximum overlap of the optical combs. The lasers were modulated using two synthesizers, which were locked to a common 10 MHz reference. The rationale for the use of a synthesizer was to enable change of frequency and amplitude applied during experiments, but this could easily be implemented with low-cost RF oscillators. The OFCs were generated under the laser driving conditions shown in Table 1. The difference in modulation amplitude is due to slight variations in the threshold current induced by the different operating temperatures of the two lasers. At the receiver, the combined optical signal from the merged combs, port P3 from the circulator, was amplified using a Thulium-doped fibre amplifier (TDFA) and filtered using a tunable optical filter with a 5 nm transmission window around 2 $\mu$m [29]. Filtering wasrequired to remove out-of-band amplified spontaneous emission produced by the TDFA, preventing saturation of the photodetector. The combined optical signal was then monitored in the optical domain using a Yokogawa long-wavelength spectrum analyser (OSA - AQ6375B), with a resolution of 6 GHz. An InGaAs photodetector (EOT ET-5000F) with 12.5 GHz bandwidth was also used to detect the combined optical signal, enabling detection of the beating tones generated between the two combs. An Agilent electrical spectrum analyser (ESA - E4407b) and Keysight sampling oscilloscope (86100D Infiniium DCA-X Wide-Bandwidth) were used to view the signals in the electrical domain. The scope was triggered with one of the synthesizer’s signals in combination with a precision timebase reference module.
Fig. 2. Experimental setup for mutually injection locked dual comb generation with lasers (a) primary laser; (b) comb source 1 and (c) comb source 2.
Table 1. Laser driving parameters.
3. Results
Figure 3(a) shows the LI curves recorded for laser 1 and laser 2. Due to the temperatures difference required to operate both lasers at 2.002 $\mu$m, the threshold current is slightly lower for laser 2. This led to a difference in the switching voltage required for OFC generation in each laser. Figure 3(b) shows the measured frequency response of the two laser sources. Laser 1 demonstrated a 3 dB RF bandwidth of approximately 3 GHz. Laser 2 suffered a larger drop off at low frequencies and frequency response roll-off around 3 GHz. It has been shown that gain switching at frequencies far below the relaxation oscillation (RO) frequency leads to a reduction in achievable OFC bandwidth and an increase in the asymmetry of the OFC spectrum due to laser pulses formation through adiabatic chirp instead of dynamic chirp [30]. With this in mind, the lasers were modulated near the laser RO frequencies at 2 GHz and 2.0001 GHz, allowing for narrow comb line spacing to be achieved while achieving an optimal comb bandwidth. Although 2 GHz was found to be an optimal repetition frequency, the laser sources exhibited a tunable OFC spacing up to 6 GHz, with reduced spectral power and an increase in the required switching voltage. This range could be increased if the laser frequency response was improved. Having a low repetition frequency is of importance for DFC technologies as the comb spacing determines the spectroscopic resolution for molecular sensing applications. The 2 GHz repetition frequency offers a desirable low free spectral range while maintaining optimal spectral bandwidth for the proposed OFC sources. The repetition frequency difference ($\Delta f$) was selected to be 100 kHz as it allowed for the comb to be compressed to a narrow detection bandwidth with fast sweep times still resolving the dual comb beating tones. Reducing the $\Delta f$ value below 100 kHz could be achieved, but did not improve data acquisition time. Mutual optical injection locking of laser 1 and laser 2 is achieved through locking a comb line near the edge of the comb bandwidth, common in both comb spectra, to the primary laser. Injection locking is required as a tight phase relationship between the OFCs leads to the generation of narrow beating tones which can then be compressed into a small detection bandwidth without unwanted overlap. The two OFC laser sources were each injected with -8 dBm of optical power from the primary laser. Figure 4(a) and 4(b) show the optical pulses generated by laser 1, detected in the time domain without and with optical injection. In the free running case, without optical injection, there is a large timing jitter and amplitude noise due to spontaneous emission introduced when the device is switched below the lasing threshold [15,31]. Through optical injection, the pulses are seeded by the primary laser, reducing the effect of spontaneous emission on pulse-to-pulse coherence. This negated much of the jitter resulting in an estimated 3 ps of RMS timing jitter for optical pulses with $\approx$70 ps pulse width. Figure 4(c) and 4(d) show a beating tone generated between a single comb line pair from the OFCs in a regime where the combs are free running (blue) and injection locked (red). For this measurement the frequency difference ($\Delta f$) between the OFCs was increased from 100 kHz to 60 MHz, allowing for easy viewing of a single beating tone. In the free running case, phase noise between the comb sources was uncorrelated, resulting in a broadening of the generated beat tone. The broadened beat tone had a full width half maximum (FWHM) of 0.4 MHz. Through mutual injection and locking of the phase relationship between the two combs, the beating tone generated sees a reduction in FWHM to 11 Hz. The low beating tone FWHM suggests a tight phase relationship between the two comb sources when optically injected.
Fig. 3. (a) LI curves for laser 1 and laser 2 operated at 22$^{\circ }$C and 14$^{\circ }$C respectively and (b) S21 frequency response at 30mA bias current.
Fig. 4. (a) and (b) show pulses generated by laser 1 when switched at 2GHz with (red) and without (blue) optical injection; (c) and (d) show a beating tone generated between a single comb line pair in the dual frequency comb when no optical injection is present (blue) and when the comb sources are mutually injected (red).
Figure 5(a) describes the comb edge injection locking in the optical domain with symmetry of frequencies around the injection wavelength, labelled $\lambda _{Inj}$. This mirroring of beating frequencies around the injection frequency causes issues in the RF detection as not all beating tones are generated by single comb line pairs, see Fig. 5(b). To minimise the repetition of beating frequencies, the primary laser was swept through the OFCs to the shortest wavelength which produces stable injection locking. This was found to be 2001.25 nm, as in Fig. 5(c). Attempting to inject at shorter wavelengths caused the laser side modes to be less suppressed, reducing the side mode suppression ratio (SMSR) by 10 dB, measured with the OSA. In this setup it is important to keep laser SMSR high as the comb produced around the main mode is replicated around the laser side modes, leading to further repetition of beating frequencies. Once the optical power of these repeated beating frequencies is kept sufficiently low, they have minimal effect on the detected RF beating spectrum, as we will show later. Figure 5(c) shows the optical spectrum of the injected dual comb captured with 6 GHz resolution. This is the maximum resolution of the OSA, which is too low to resolve individual comb lines but gives an approximation of the comb profile. With optical injection, an SMSR of 30 dB was achieved with a comb spanning approximately 2 nm centred at 2.002 $\mu$m. The injection wavelength is shown by the dashed line in Fig. 5(c). The shaded region around this value is an estimation of the wavelength range where beating frequencies are mirrored, generating shared beating frequencies. Figure 6(a) shows the electrical domain beating spectrum obtained from the DFC shown in Fig. 5(c). The electrical spectrum was captured with 10 kHz resolution, averaging 10 sweeps each 30 ms long, giving a total integration time of 300 ms. The spectrum shows 49 well defined beating tones separated by 100 kHz with over 30 dB SNR. The beating tone spectrum was detected in a 5 MHz bandwidth. This equated to a compression factor of 20000 relative to the optical domain bandwidth of approximately 100 GHz.
Fig. 5. (a) Dual comb optical spectrum with optical injection to a common frequency tone near the edge of both combs; (b) Electrical domain frequency spectrum showing beating tones generated from the injected dual comb where red tones are the result of multiple beating tones with the same frequency and blue tones are individual to one comb line pair; (c) recorded optical spectrum for injected dual comb.
Fig. 6. (a) RF beating tone spectrum generated between the two OFCs in the mutually coherent dual comb; Spectra of 100 sweeps with 30 ms sweep time and no averaging of (b) first 5 beating tones from dual frequency comb; $\Delta P$ represents the difference between the highest and lowest measured beat tone power for each tone; and (c) frequency comb beating spectrum between 1.65 MHz and 2.25 MHz.
To tackle the issue of mirrored frequencies around the injection wavelength some methods of dual comb generation use acousto-optic modulators (AOMs) to create a small frequency shift between the two combs, removing the symmetry and repetition of beat frequencies [19]. Removing this symmetry allows for the maximum number of individual beating frequencies to be generated, maximising the effective sensing bandwidth of the comb. In this experiment, a frequency shifting AOM was not included, as they add additional complexity and cost while only returning a small improvement in the number of beating tones detected. When describing the portion of the comb which is functional for spectroscopic applications, we looked for interference causing amplitude fluctuations in the RF domain beating tones. It was found that RF beating tones generated by more than one comb line pair showed amplitude fluctuation due to interference at the shared beating frequency. In Fig. 6(a), RF beating tones labelled 1-6 were monitored for fluctuations in beating tone amplitude. This was done through recording 100 sweeps, each taking 30 ms without averaging, to see the maximum fluctuation of the signal power ($\Delta$P), as shown in Fig. 6(b). Table 2 shows the results obtained when measuring values of $\Delta$P for beating tones 1 to 6. It was found that the first 5 RF beating tones displayed a large amplitude fluctuation, showing multiple comb line pairs were contributing to the generation of these beating frequencies. As the injection wavelength is near the edge of the frequency comb spectrum, the amplitude of the frequency comb lines at shorter wavelengths begin to reduce, following the spectral profile of the OFC. This reduction in optical power reduces their contribution to amplitude fluctuations in the RF domain, as seen in the decreasing $\Delta$P values. The amplitude fluctuation for comb lines beyond the first 5 represents, approximately, the beating of two comb line pairs where the amplitude difference in the optical domain is approximately 30 dB, the SMSR of the frequency combs. For all RF beating tones beyond 5 the value of $\Delta$P was $\approx$2 dB, see Fig. 6(c). Through averaging of 10 sweeps, the power fluctuations, $\Delta$P$_{10}$, for beating tones 6 and above could be reduced to $\leq$0.3 dB. The 5 beating tones affected by large amplitude fluctuations would not be useful in sensing applications, leading to a 10$\%$ reduction in functional comb bandwidth. The inclusion of a frequency shifting AOM for one of the frequency combs would recover this 10$\%$ of the comb bandwidth; however, this inclusion would increase the complexity and cost of potential integrated dual comb devices and may not be necessary depending on the detection bandwidth required by the application.
Table 2. Fluctuation in RF beating tone power where $\Delta P$ is power fluctuation over 100 electrical spectrum sweeps; $\Delta P_{10}$ is the power fluctuation for 100 beating spectra averaged every 10 sweeps.
4. Conclusion
We have demonstrated for the first time, the generation of a dual frequency comb using gain-switched semiconductor lasers in the 2 $\mu$m wavelength region. The dual frequency comb spanned 100 GHz with a comb line spacing of 2 GHz. Through optical injection locking, a high phase correlation between the two combs was achieved, resulting in an 11 Hz beating tone linewidth between the OFCs. Through the selection of a repetition frequency offset of 100 kHz between the OFCs, the DFC spectrum was compressed by a factor of 20000 to a 5 MHz detection region in the electrical domain. Taking into account the symmetric beating frequencies around the injection wavelength, 45 comb lines were generated which had frequencies unique to a single comb line pair interactions. Future ambitions for this technology would be to realise an integrated architecture as has been done with similar sources in the 1.55 $\mu$m wavelength region [11]. The primary drawback of gain-switched lasers is the limited OFC bandwidth. By combining the demonstrated comb sources with recently developed micro-resonator technologies, an integrable low power OFC source could potentially address gain-switched OFC bandwidth limitations [32,33]. The demonstrated DFC in the 2 $\mu$m wavelength region using semiconductor laser sources and a low complexity setup, shows promise as a potential solution for high accuracy sensing application in the fields of medicine, air quality monitoring and industrial bioreactor sensing.
Funding
Science Foundation Ireland (IPIC(12/RC/2276-P2)); Irish Research Council (GOPIG/2017/385).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. L. Lundberg, M. Mazur, A. Mirani, B. Foo, J. Schröder, V. Torres-Company, M. Karlsson, and P. A. Andrekson, “Phase-coherent lightwave communications with frequency combs,” Nat. Commun. 11(1), 201 (2020). [CrossRef]
2. P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle, M. H. P. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos, “Microresonator-based solitons for massively parallel coherent optical communications,” Nature 546(7657), 274–279 (2017). [CrossRef]
3. J. Ye, H. Schnatz, and L. W. Hollberg, “Optical frequency combs: from frequency metrology to optical phase control,” IEEE J. Sel. Top. Quantum Electron. 9(4), 1041–1058 (2003). [CrossRef]
4. S. B. Papp, K. Beha, P. Del’Haye, F. Quinlan, H. Lee, J. K. Vahala, and S. A. Diddams, “Microresonator frequency comb optical clock,” Optica 1(1), 10–14 (2014). [CrossRef]
5. P. Trocha, M. Karpov, D. Ganin, M. H. P. Pfeiffer, A. Kordts, S. Wolf, J. Krockenberger, P. Marin-Palomo, P. Weimann, S. Randel, W. Freude, T. J. Kippenberg, and C. Koos, “Ultrafast optical ranging using microresonator soliton frequency combs,” Science 359(6378), 887–891 (2018). [CrossRef]
6. N. Picqué and T. W. Hänsch, “Frequency comb spectroscopy,” Nat. Photonics 13(3), 146–157 (2019). [CrossRef]
7. S. Diddams, L. Hollberg, and V. Mbele, “Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb,” Nature 445(7128), 627–630 (2007). [CrossRef]
8. M. Imran, P. M. Anandarajah, A. Kaszubowska-Anandarajah, N. Sambo, and L. Potí, “A Survey of Optical Carrier Generation Techniques for Terabit Capacity Elastic Optical Networks,” IEEE Commun. Surv. Tutorials 20(1), 211–263 (2018). [CrossRef]
9. A. Foltynowicz, P. Masłowski, T. Ban, F. Adler, K. C. Cossel, T. C. Briles, and J. Ye, “Optical frequency comb spectroscopy,” Faraday Discuss. 150, 23–31 (2011). [CrossRef]
10. T. Fortier and E. Baumann, “20 years of developments in optical frequency comb technology and applications,” Commun. Phys. 2(1), 153 (2019). [CrossRef]
11. J. K. Alexander, L. Caro, M. Dernaika, S. P. Duggan, H. Yang, S. Chandran, S. P. Martin, A. A. Ruth, P. M. Anandarajah, and F. H. Peters, “Integrated dual optical frequency comb source,” Opt. Express 28(11), 16900–16906 (2020). [CrossRef]
12. X. Xu, J. Wu, M. Tan, T. G. Nguyen, S. T. Chu, B. E. Little, R. Morandotti, A. Mitchell, and D. J. Moss, “Broadband Microwave Frequency Conversion Based on an Integrated Optical Micro-Comb Source,” J. Lightwave Technol. 38(2), 332–338 (2020). [CrossRef]
13. X. Xu, J. Wu, T. G. Nguyen, S. T. Chu, B. E. Little, R. Morandotti, A. Mitchell, and D. J. Moss, “Broadband RF Channelizer Based on an Integrated Optical Frequency Kerr Comb Source,” J. Lightwave Technol. 36(19), 4519–4526 (2018). [CrossRef]
14. C. Lin, P. L. Liu, T. C. Damen, D. J. Eilenberger, and R. L. Hartman, “Simple picosecond pulse generation scheme for injection lasers,” Electron. Lett. 16(15), 600–602 (1980). [CrossRef]
15. P. M. Anandarajah, R. Maher, Y. Q. Xu, S. Latkowski, J. O’Carroll, S. G. Murdoch, R. Phelan, J. O’Gorman, and L. P. Barry, “Generation of Coherent Multicarrier Signals by Gain Switching of Discrete Mode Lasers,” IEEE Photonics J. 3(1), 112–122 (2011). [CrossRef]
16. P. D. Lakshmijayasimha, A. K. Anandarajah, E. P. Martin, P. Landais, and P. M. Anandarajah, “Expansion and phase correlation of a wavelength tunable gain-switched optical frequency comb,” Opt. Express 27(12), 16560–16570 (2019). [CrossRef]
17. I. Coddington, N. Newbury, and W. Swann, “Dual-comb spectroscopy,” Optica 3(4), 414–426 (2016). [CrossRef]
18. B. Bernhardt, A. Ozawa, P. Jacquet, M. Jacquey, Y. Kobayashi, T. Udem, R. Holzwarth, G. Guelachvili, T. W. Hänsch, and N. Picqué, “Cavity-enhanced dual-comb spectroscopy,” Nat. Photonics 4(1), 55–57 (2010). [CrossRef]
19. C. Quevedo-Galán, V. Durán, A. Rosado, A. Pérez-Serrano, J. M. G. Tijero, and I. Esquivias, “Gain-switched semiconductor lasers with pulsed excitation and optical injection for dual-comb spectroscopy,” Opt. Express 28(22), 33307–33317 (2020). [CrossRef]
20. C. Quevedo-Galán, A. Pérez-Serrano, I. E. López-Delgado, J. M. G. Tijero, and I. Esquivias, “Dual-Comb Spectrometer Based on Gain-Switched Semiconductor Lasers and a Low-Cost Software-Defined Radio,” IEEE Access 9, 92367–92373 (2021). [CrossRef]
21. E. P. Martin, S. Chandran, A. Rosado, E. P. Soderholm, J. K. Alexander, F. H. Peters, A. A. Ruth, and P. M. Anandarajah, “Mutually Injection Locked Gain Switched Optical Frequency Combs for Dual Comb Spectroscopy of H2S,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, 2020), paper AM4K.2.
22. H. Zhang, N. Kavanagh, Z. Li, J. Zhao, N. Ye, Y. Chen, N. v. Wheeler, J. P. Wooler, J. R. Hayes, S. R. Sandoghchi, F. Poletti, M. N. Petrovich, S. U. Alam, R. Phelan, J. O’Carroll, B. Kelly, L. Grüner-Nielsen, D. J. Richardson, B. Corbett, and F. C. Garcia Gunning, “100 Gbit/s WDM transmission at 2 µm: transmission studies in both low-cost hollow core photonic bandgap fiber and solid core fiber,” Opt. Express 23(4), 4946–4951 (2015). [CrossRef]
23. A. D. Bristow, N. Rotenberg, and H. M. van Driel, “Two-photon absorption and Kerr coefficients of silicon for 850–2200nm,” Appl. Phys. Lett. 90(19), 191104 (2007). [CrossRef]
24. J. A. Nwaboh, O. Werhahn, P. Ortwein, D. Schiel, and V. Ebert, “Laser-spectrometric gas analysis: CO2–TDLAS at 2µm,” Meas. Sci. Technol. 24(1), 015202 (2013). [CrossRef]
25. M. E. Webber, R. Claps, F. V. Englich, F. K. Tittel, J. B Jeffries, and R. K. Hanson, “Measurements of NH3 and CO2 with distributed-feedback diode lasers near 2.0 µm in bioreactor vent gases,” Appl. Opt. 40(24), 4395 (2001). [CrossRef]
26. U. N. Singh, M. Petros, T. F. Refaat, C. W. Antill, R. Remus, and J. Yu, “Airborne lidar for simultaneous measurement of column CO2 and water vapor in the atmosphere,” Proc. SPIE 10006, 1000602 (2016). [CrossRef]
27. A. Parriaux, K. Hammani, and G. Millot, “Two-micron all-fibered dual-comb spectrometer based on electro-optic modulators and wavelength conversion,” Commun. Phys. 1(1), 17 (2018). [CrossRef]
28. R. Phelan, J. O’Carroll, D. Byrne, C. Herbert, J. Somers, and B. Kelly, “In0.75Ga0.25As/InP Multiple Quantum-Well Discrete-Mode Laser Diode Emitting at 2 µm,” IEEE Photonics Technol. Lett. 24(8), 652–654 (2012). [CrossRef]
29. E. Russell, N. Kavanagh, K. Shortiss, and F. C. Garcia Gunning, “Development of thulium-doped fibre amplifiers for the 2µm waveband,” Proc. SPIE 10683, 106832Q (2018). [CrossRef]
30. A. Rosado, A. Pérez-Serrano, J. M. G. Tijero, A. V. Gutierrez, L. Pesquera, and I. Esquivias, “Numerical and Experimental Analysis of Optical Frequency Comb Generation in Gain-Switched Semiconductor Lasers,” IEEE J. Quantum Electron. 55(6), 1–12 (2019). [CrossRef]
31. S. P. Ó. Dúill, R. Zhou, P. M. Anandarajah, and L. P. Barry, “Analytical Approach to Assess the Impact of Pulse-to-Pulse Phase Coherence of Optical Frequency Combs,” IEEE J. Quantum Electron. 51(11), 1–8 (2015). [CrossRef]
32. W. Weng, K. Kaszubowska-Anandarajah, J. Liu, P. M. Anandarajah, and T. J. Kippenberg, “Frequency division using a soliton-injected semiconductor gain-switched frequency comb,” Sci. Adv. 6(39), 39 (2020). [CrossRef]
33. W. Weng, A. Kaszubowska-Anandarajah, J. He, P. D. Lakshmijayasimha, E. Lucas, J. Liu, P. M. Anandarajah, and T. J. Kippenberg, “Gain-switched semiconductor laser driven soliton microcombs,” Nat. Commun. 12(1), 1425 (2021). [CrossRef]
