rus
Issue #6/2025
A.I.Vergules, D.M.Zhigunov, D.S.Zemtsov, A.S.Smirnov, A.K.Zemtsova, K.N.Garbuzov, S.S.Kosolobov V.P.Drachev, D.A.Korobko, A.A.Fotiadi, A.V.Ivanov, M.A.Ladugin, A.S.Dudin, P.I.Lazarenko, V.V.Kovalyuk, E.P.Kitsyuk, V.V.Svetukhin
NARROW LINEWIDTH LASER SOURCE BASED ON DFB LASER AND INTEGRATED SILICON RING RESONATOR
NARROW LINEWIDTH LASER SOURCE BASED ON DFB LASER AND INTEGRATED SILICON RING RESONATOR
DOI: https://doi.org/10.22184/1993-8578.2025.18.6.356.365
We demonstrated the narrowing of the distributed feedback laser linewidth from 210 kHz to 150 Hz via self-injection locking with a photonic integrated circuit-based external Si resonator. This opens the prospects for compact narrow lasers for pure microwave signal generation.
We demonstrated the narrowing of the distributed feedback laser linewidth from 210 kHz to 150 Hz via self-injection locking with a photonic integrated circuit-based external Si resonator. This opens the prospects for compact narrow lasers for pure microwave signal generation.
Теги: df laser narrow linewidth laser radiation poc-лазер pure microwave signal ring resonator кольцевой резонатор малошумный микроволновой сигнал узкополосное лазерное излучение
Received: 3.09.2025 | Accepted: 10.09.2025 | DOI: https://doi.org/10.22184/1993-8578.2025.18.6.356.365
Original paper
NARROW LINEWIDTH LASER SOURCE BASED ON DFB LASER AND INTEGRATED SILICON RING RESONATOR
A.I.Vergules¹, Intern Researcher, ORCID: 0000-0002-5673-4148 / aleksandra.vergules@skoltech.ru
D.M.Zhigunov¹, Cand. of Sci. (Physics and Mathematics), Leading Researcher, ORCID: 0000-0003-1714-1208
D.S.Zemtsov¹, Cand. of Sci. (Physics and Mathematics), Senior Researcher, ORCID: 0000-0002-8774-3338
A.S.Smirnov¹, Engineer, ORCID: 0000-0002-1573-1100
A.K.Zemtsova¹, Junior Researcher, ORCID: 0000-0001-6072-9520
K.N.Garbuzov¹, Research Engineer, ORCID: 0000-0003-3851-6339
S.S.Kosolobov¹, Doct. of Sci. (Physics and Mathematics), Docent, ORCID: 0000-0002-1814-8877
V.P.Drachev¹, Doct. of Sci. (Physics and Mathematics), Prof., ORCID: 0000-0002-6205-7497
D.A.Korobko², Cand. of Sci. (Physics and Mathematics), Leading Scientist, ORCID: 0000-0002-1582-7567
A.A.Fotiadi², Cand. of Sci. (Physics and Mathematics), Leading Scientist, ORCID: 0000-0002-8086-8218
A.V.Ivanov³, Head of department, ORCID: 0009-0007-6420-9777
M.A.Ladugin³, Doct. of Sci. (Physics and Mathematics), Head of the scientific and production complex, ORCID: 0000-0002-4086-7229
A.S.Dudin³, 2nd class Design Engineer, ORCID: 0009-0006-8170-3083
P.I.Lazarenko⁴, Cand. of Sci. (Tech), Head of laboratory, ORCID: 0000-0003-4309-3481
V.V.Kovalyuk5, Cand. of Sci. (Physics and Mathematics), Head of laboratory, ORCID: 0000-0002-1529-4481
E.P.Kitsyuk6, Cand. of Sci. (Tech), Head of laboratory, ORCID: 0000-0002-4166-8408
V.V.Svetukhin6, Doct. of Sci. (Physics and Mathematics), Director, Corresponding Member of RAS, ORCID: 0000-0003-0831-9254
Abstract. We demonstrated the narrowing of the distributed feedback laser linewidth from 210 kHz to 150 Hz via self-injection locking with a photonic integrated circuit-based external Si resonator. This opens the prospects for compact narrow lasers for pure microwave signal generation.
Keywords: narrow linewidth laser radiation, DF laser, ring resonator, pure microwave signal
For citation: A.I. Vergules, D.M. Zhigunov, D.S. Zemtsov, A.S. Smirnov, A.K. Zemtsova, K.N.Garbuzov, S.S.Kosolobov, V.P. Drachev, D.A.Korobko, A.A. Fotiadi, A.V. Ivanov, M.A. Ladugin, A.S. Dudin, P.I. Lazarenko, V.V. Kovalyuk, E.P. Kitsyuk, V.V. Svetukhin. Narrow linewidth laser source based on dfb laser and integrated silicon ring resonator. NANOINDUSTRY. 2025. Vol. 18. No. 6. PP. 356–365. https://doi.org/
10.22184/1993-8578.2025.18.6.356.365.
INTRODUCTION
Highly coherent laser sources of radiation of telecommunication C-band wavelengths (1530–1565 nm) are widely used in modern optical systems, where laser frequency stability, spectral width of its generation line and laser noise level determine the consumer characteristics of the whole system. For example, in coherent optical communications, the spectral linewidth of the laser directly affects the phase stability of the signal, its demodulation and achievable data rate, while in generation of spectrally pure microwave signals based on photonic techniques, phase noise of the laser limits the minimum achievable spectral width of the generated RF signals.
Semiconductor distributed feedback lasers (DFLs) are compact, energy-efficient and low-cost sources of single-frequency optical radiation with relatively high output power and ability to be integrated into photonic integrated circuits and fibre devices. However, the typical spectral linewidth of DFL lasers at the level of a few MHz limits their wide application in microwave photonics, where sources with a higher degree of coherence are required. Further increase of coherence of the output radiation of DFL lasers is hindered by the relatively small length and low goodness of the laser resonator, which is unable to filter out noise caused by current and temperature fluctuations, directly affecting stability of the laser generation frequency.
One of the ways to overcome these limitations is to use the effect of self-capture by the laser of natural frequency of an external high-frequency resonator. The essence of the effect is that part of the radiation generated by the laser is passed through an external high-frequency resonator acting as a frequency-selective element, and then directed back into the laser, reducing the threshold of laser generation at the frequency of the captured mode of the resonator. Once the frequency is captured, any slow change in the mode of a resonator (e.g., due to temperature variations) results in a synchronous change in laser generation frequency. Laser operation stability in the self-capture mode is determined by the power and stability of the phase of the return wave from the external resonator entering the laser.
The external resonator can be fibre ring resonators [1], linear fibre resonators based on fibre Bragg gratings (FBGs) [2], monolithic microresonators with whispering gallery modes [3], and resonators based on photonic integrated circuits (PICs) [4]. Microresonators with whispering gallery modes traditionally exhibit the highest goodness of order 108–1011, but in real applications they require regular alignment of the optical components providing light input-output. Fibre ring resonators can have quite high goodness of fit, but due to their relatively long length they are extremely sensitive to external influences leading to frequent laser generation failures due to jumps in the trapped frequency between neighbouring modes of the external resonator. Linear resonators based on FBG are compact and have similar goodness of fit as ring resonators, but they require labour-intensive and time-consuming fabrication and therefore are not suitable for mass production. Micro-ring resonators based on PICs can be a promising solution. They have a goodness-of-fit of up to 106, are compact, stable, and can be mass-produced due to the compatibility of the process with the widespread complementary metal-oxide-semiconductor (CMOS) technology. In a number of works it was shown that such resonators make it possible to achieve the narrowing of the laser line to hertz units [4–6]. This opens up new possibilities for manufacturing the compact narrowband lasers suitable for coherent multiplexed data transmission, generation of low-noise microwave signals, high-resolution spectroscopy, and precision optical sensing.
The purpose of this work is to develop a narrowband laser source in the telecommunication C-band, realised on the basis of a DFL laser and an integrated silicon micro-resonator made on a "silicon-on-insulator" (SoI) platform.
RESEARCH METHODS
Fabrication of an integral ring resonator on the SoI platform
The external resonator for the DFL laser was fabricated on a standard silicon-on-insulator substrate with a silicon layer thickness of 220 nm. The image of the ring resonator obtained using a scanning electron microscope (SEM) is presented in Fig.1a. A stadium-type ring resonator in a double feedback ("drop-add") configuration with a radius of 500 μm, a coupling region length of 1200 μm, and a waveguide-to-ring gap width of 140 nm (Fig.1b) was fabricated by electron beam lithography and inductively coupled plasma etching techniques. In fabricated configuration, all waveguides are equipped with tapers smoothly extending to a width of 15 µm. At the ends of the tapers, radiation input-output gratings with incomplete etching are formed (Fig.1c). The chip surface is additionally coated with a 1 µm thick SiO2 layer by electron-beam deposition.
To the input and output ports of the ring resonator are docked optical fibres with polarisation preservation, the ends of which are polished and form an angle of 40⁰, which provides redirection of the signal propagating in the fibre to the area of I/O gratings on the chip due to the complete internal reflection from the bevelled end.
To position the optical fibres relative to the chip, 6-axis manipulators with holders were used to rotate the fibre around its axis. Alignment was controlled visually using microscopes providing side and top views. To control the transmission level of the fabricated resonator, the optical fibres were connected to a BOSA optical spectrum analyser (Aragon Photonics) with a maximum spectral resolution of 10 MHz. After adjusting the optical fibre position to the maximum transmittance, it was fixed on the chip using a UV curable adhesive with refractive index close to that of SiO2. The image of the ring resonator with glued fibres obtained using an optical microscope is presented in Fig.2.
In Fig.3a the scheme of the external resonator and its connection to the laser is highlighted by a dotted frame. Using a fibre splitter, laser output radiation is divided into two parts: 75 per cent is directed to the optical feedback system and 25 per cent to the laser spectrum measurement path. The polarisation of radiation is rotated by 90⁰ using a polarisation rotator (PR) to match the TE polarisation for which the gratings and passive components of the chip are made. The radiation is then separated by a 50/50 power ratio optical directional tap whose channels are connected to optical fibres docked to the chip. The open terminal is terminated with an absorber. A silicon ring on the chip provides spectrally selective feedback to the laser resonator. In this configuration, the 50/50 splitter allows the external resonator response to be enhanced by constructive interference.
Self-heterodyne delay interferometry for laser spectral linewidth measurement
The spectral width of the laser line was measured using a delayed self-heterodyne interferometer [7], the schematic of which is also shown in Fig.3a. Studied laser radiation passing through an optical isolator (OI) to eliminate parasitic back reflections, is divided by a directional splitter into two signals – a reference and a delayed one. The reference signal passes through an acousto-optic modulator (AOM), which shifts the emission frequency by the modulation frequency – 120 MHz. The delayed signal passes through a polarisation controller (PC) and a 50 km long fibre line, at the output of which phase fluctuations due to laser noise cease to correlate with fluctuations in the reference arm. For a reliable measurement, the delay time in the fibre should significantly exceed the expected coherence time of the studied laser. A second splitter combines the reference and delayed signals, and the interference beats are detected by a photodetector (PD) and displayed on an Anritsu electrical spectrum analyser (ESA).
The radiation spectral line shape obtained at the output of the circuit is usually a Voigt profile, which is the result of convolution of the Lorentzian and Gaussian circuits, as shown in Fig.3b. The Gaussian component of the spectrum is due to flicker noise, which is inversely proportional to the detuning from the centre frequency and arises from current drift and temperature fluctuations in the laser controller. The Lorentzian component associated with the quantum nature of laser radiation characterises the laser line itself, including the process of spontaneous emission in the active region of the laser, and is a randomly distributed quantity. By extracting the Lorentzian and Gaussian components from the Voigt profile, it is possible to estimate the laser line width. Thus, the fundamental width of the laser line is equal to half the width of the Lorentzian component at the 3 dB level, and the Gaussian width is determined by multiplying the width of the spectrum of the Gaussian component at the 3 dB level by the multiplier √2/2 [8].
RESULTS
Optical spectrum of an external integrated ring resonator on the SoI platform
The optical spectrum of the fabricated silicon ring resonator measured at the probe station is shown in Fig.4. According to the spectrum obtained, the spectral width of the ring resonance was 9.6 pm (1.2 GHz at a wavelength of 1550 nm), the free dispersion region was 115 pm (14.37 GHz), the goodness of fit was 161,000, and the sharpness was 12. Based on the results of the ring transmission spectrum measurements before and after the optical fibres were glued, the total docking loss of both ports at 1530 nm wavelength was about 2.3 dB.
Characterisation of the spectral width of the DFL laser in the free mode and in the frequency self-capture mode
As a source, a DFL laser with a fibre output with polarisation preservation at a wavelength of 1530 nm was used, having a maximum power of 102 mW at a current of 554.6 mA in continuous mode. Figure 5a shows the dependence of the output optical power of the laser on the pump current. The threshold of laser generation is observed at a current of about 70–72 mA, after which the output power smoothly increases with increasing current.
The laser spectrum measured by self-heterodyne interferometry is shown in Fig.5b. At a current close to the threshold value, a single spectral line exceeding the noise level by more than 35 dB is observed in the spectrum. With increasing current, a noise line appears on the sides of the main line due to the excitation of additional laser modes exceeding the threshold level. Thus, at a current of 400 mA the ratio of the laser peak amplitude to the noise illumination is reduced to 15 dB. The spectrum of the laser operating in the free mode at a control current of 300 mA and temperature (TEK) of 26 ⁰C is shown in Fig.6a. From the approximation of the spectrum by the Voigt function presented in Fig.6b, it follows that the Lorentzian width of the laser line is 210 kHz and the Gaussian width is 2.7 MHz.
The spectrum of the laser in the frequency self-capture mode at a current of 300 mA is also shown in Fig.6a. Comparing it with the spectrum of a freely operating laser, we can conclude that the use of an external resonator on the chip allows us to effectively narrow the laser generation line and suppress additional modes appearing at high pump currents. The approximation of this spectrum by the Voigt function is shown in Fig.7c. According to the approximation, the Lorentzian width of the laser generation line was 150 Hz, while the Gaussian width has a value of 36.6 kHz. Thus, the spectral line of the laser in the assembly with the PIC resonator is narrowed by a factor of 1400. The power of the laser output radiation in the frequency self-capture mode is about 1 mW. It is also important to note that duration of continuous operation of the laser in the frequency self-capture mode before jumping to another mode was about 1.5 minutes, and this despite absence of temperature stabilisation of the external resonator and active feedback phase adjustment.
CONCLUSIONS
Thus, in this work, a narrowband laser source based on a DFL laser and an integrated silicon ring resonator with a goodness-of-fit of 1.61 · 105, fabricated on a SoI platform, is realised. The use of the resonator as an external filter made it possible to achieve a narrowing of the laser spectral line from 210 kHz to 150 Hz in the frequency self-capture mode, which corresponds to a 1400-fold improvement in coherence. The obtained results confirm the promising use of integrated resonators on the SoI platform for manufacturing the compact highly coherent lasers of telecommunication C-band wavelengths.
ACKNOWLEDGEMENTS
The research was performed under RSF grant No. 23-91-06301, "Compact high coherence laser generators", and 23-79-30017 "Development of the element base of microwave photonics", and with the financial support of the Ministry of Science and Higher Education of the Russian Federation (FNRM-2025-0014).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Original paper
NARROW LINEWIDTH LASER SOURCE BASED ON DFB LASER AND INTEGRATED SILICON RING RESONATOR
A.I.Vergules¹, Intern Researcher, ORCID: 0000-0002-5673-4148 / aleksandra.vergules@skoltech.ru
D.M.Zhigunov¹, Cand. of Sci. (Physics and Mathematics), Leading Researcher, ORCID: 0000-0003-1714-1208
D.S.Zemtsov¹, Cand. of Sci. (Physics and Mathematics), Senior Researcher, ORCID: 0000-0002-8774-3338
A.S.Smirnov¹, Engineer, ORCID: 0000-0002-1573-1100
A.K.Zemtsova¹, Junior Researcher, ORCID: 0000-0001-6072-9520
K.N.Garbuzov¹, Research Engineer, ORCID: 0000-0003-3851-6339
S.S.Kosolobov¹, Doct. of Sci. (Physics and Mathematics), Docent, ORCID: 0000-0002-1814-8877
V.P.Drachev¹, Doct. of Sci. (Physics and Mathematics), Prof., ORCID: 0000-0002-6205-7497
D.A.Korobko², Cand. of Sci. (Physics and Mathematics), Leading Scientist, ORCID: 0000-0002-1582-7567
A.A.Fotiadi², Cand. of Sci. (Physics and Mathematics), Leading Scientist, ORCID: 0000-0002-8086-8218
A.V.Ivanov³, Head of department, ORCID: 0009-0007-6420-9777
M.A.Ladugin³, Doct. of Sci. (Physics and Mathematics), Head of the scientific and production complex, ORCID: 0000-0002-4086-7229
A.S.Dudin³, 2nd class Design Engineer, ORCID: 0009-0006-8170-3083
P.I.Lazarenko⁴, Cand. of Sci. (Tech), Head of laboratory, ORCID: 0000-0003-4309-3481
V.V.Kovalyuk5, Cand. of Sci. (Physics and Mathematics), Head of laboratory, ORCID: 0000-0002-1529-4481
E.P.Kitsyuk6, Cand. of Sci. (Tech), Head of laboratory, ORCID: 0000-0002-4166-8408
V.V.Svetukhin6, Doct. of Sci. (Physics and Mathematics), Director, Corresponding Member of RAS, ORCID: 0000-0003-0831-9254
Abstract. We demonstrated the narrowing of the distributed feedback laser linewidth from 210 kHz to 150 Hz via self-injection locking with a photonic integrated circuit-based external Si resonator. This opens the prospects for compact narrow lasers for pure microwave signal generation.
Keywords: narrow linewidth laser radiation, DF laser, ring resonator, pure microwave signal
For citation: A.I. Vergules, D.M. Zhigunov, D.S. Zemtsov, A.S. Smirnov, A.K. Zemtsova, K.N.Garbuzov, S.S.Kosolobov, V.P. Drachev, D.A.Korobko, A.A. Fotiadi, A.V. Ivanov, M.A. Ladugin, A.S. Dudin, P.I. Lazarenko, V.V. Kovalyuk, E.P. Kitsyuk, V.V. Svetukhin. Narrow linewidth laser source based on dfb laser and integrated silicon ring resonator. NANOINDUSTRY. 2025. Vol. 18. No. 6. PP. 356–365. https://doi.org/
10.22184/1993-8578.2025.18.6.356.365.
INTRODUCTION
Highly coherent laser sources of radiation of telecommunication C-band wavelengths (1530–1565 nm) are widely used in modern optical systems, where laser frequency stability, spectral width of its generation line and laser noise level determine the consumer characteristics of the whole system. For example, in coherent optical communications, the spectral linewidth of the laser directly affects the phase stability of the signal, its demodulation and achievable data rate, while in generation of spectrally pure microwave signals based on photonic techniques, phase noise of the laser limits the minimum achievable spectral width of the generated RF signals.
Semiconductor distributed feedback lasers (DFLs) are compact, energy-efficient and low-cost sources of single-frequency optical radiation with relatively high output power and ability to be integrated into photonic integrated circuits and fibre devices. However, the typical spectral linewidth of DFL lasers at the level of a few MHz limits their wide application in microwave photonics, where sources with a higher degree of coherence are required. Further increase of coherence of the output radiation of DFL lasers is hindered by the relatively small length and low goodness of the laser resonator, which is unable to filter out noise caused by current and temperature fluctuations, directly affecting stability of the laser generation frequency.
One of the ways to overcome these limitations is to use the effect of self-capture by the laser of natural frequency of an external high-frequency resonator. The essence of the effect is that part of the radiation generated by the laser is passed through an external high-frequency resonator acting as a frequency-selective element, and then directed back into the laser, reducing the threshold of laser generation at the frequency of the captured mode of the resonator. Once the frequency is captured, any slow change in the mode of a resonator (e.g., due to temperature variations) results in a synchronous change in laser generation frequency. Laser operation stability in the self-capture mode is determined by the power and stability of the phase of the return wave from the external resonator entering the laser.
The external resonator can be fibre ring resonators [1], linear fibre resonators based on fibre Bragg gratings (FBGs) [2], monolithic microresonators with whispering gallery modes [3], and resonators based on photonic integrated circuits (PICs) [4]. Microresonators with whispering gallery modes traditionally exhibit the highest goodness of order 108–1011, but in real applications they require regular alignment of the optical components providing light input-output. Fibre ring resonators can have quite high goodness of fit, but due to their relatively long length they are extremely sensitive to external influences leading to frequent laser generation failures due to jumps in the trapped frequency between neighbouring modes of the external resonator. Linear resonators based on FBG are compact and have similar goodness of fit as ring resonators, but they require labour-intensive and time-consuming fabrication and therefore are not suitable for mass production. Micro-ring resonators based on PICs can be a promising solution. They have a goodness-of-fit of up to 106, are compact, stable, and can be mass-produced due to the compatibility of the process with the widespread complementary metal-oxide-semiconductor (CMOS) technology. In a number of works it was shown that such resonators make it possible to achieve the narrowing of the laser line to hertz units [4–6]. This opens up new possibilities for manufacturing the compact narrowband lasers suitable for coherent multiplexed data transmission, generation of low-noise microwave signals, high-resolution spectroscopy, and precision optical sensing.
The purpose of this work is to develop a narrowband laser source in the telecommunication C-band, realised on the basis of a DFL laser and an integrated silicon micro-resonator made on a "silicon-on-insulator" (SoI) platform.
RESEARCH METHODS
Fabrication of an integral ring resonator on the SoI platform
The external resonator for the DFL laser was fabricated on a standard silicon-on-insulator substrate with a silicon layer thickness of 220 nm. The image of the ring resonator obtained using a scanning electron microscope (SEM) is presented in Fig.1a. A stadium-type ring resonator in a double feedback ("drop-add") configuration with a radius of 500 μm, a coupling region length of 1200 μm, and a waveguide-to-ring gap width of 140 nm (Fig.1b) was fabricated by electron beam lithography and inductively coupled plasma etching techniques. In fabricated configuration, all waveguides are equipped with tapers smoothly extending to a width of 15 µm. At the ends of the tapers, radiation input-output gratings with incomplete etching are formed (Fig.1c). The chip surface is additionally coated with a 1 µm thick SiO2 layer by electron-beam deposition.
To the input and output ports of the ring resonator are docked optical fibres with polarisation preservation, the ends of which are polished and form an angle of 40⁰, which provides redirection of the signal propagating in the fibre to the area of I/O gratings on the chip due to the complete internal reflection from the bevelled end.
To position the optical fibres relative to the chip, 6-axis manipulators with holders were used to rotate the fibre around its axis. Alignment was controlled visually using microscopes providing side and top views. To control the transmission level of the fabricated resonator, the optical fibres were connected to a BOSA optical spectrum analyser (Aragon Photonics) with a maximum spectral resolution of 10 MHz. After adjusting the optical fibre position to the maximum transmittance, it was fixed on the chip using a UV curable adhesive with refractive index close to that of SiO2. The image of the ring resonator with glued fibres obtained using an optical microscope is presented in Fig.2.
In Fig.3a the scheme of the external resonator and its connection to the laser is highlighted by a dotted frame. Using a fibre splitter, laser output radiation is divided into two parts: 75 per cent is directed to the optical feedback system and 25 per cent to the laser spectrum measurement path. The polarisation of radiation is rotated by 90⁰ using a polarisation rotator (PR) to match the TE polarisation for which the gratings and passive components of the chip are made. The radiation is then separated by a 50/50 power ratio optical directional tap whose channels are connected to optical fibres docked to the chip. The open terminal is terminated with an absorber. A silicon ring on the chip provides spectrally selective feedback to the laser resonator. In this configuration, the 50/50 splitter allows the external resonator response to be enhanced by constructive interference.
Self-heterodyne delay interferometry for laser spectral linewidth measurement
The spectral width of the laser line was measured using a delayed self-heterodyne interferometer [7], the schematic of which is also shown in Fig.3a. Studied laser radiation passing through an optical isolator (OI) to eliminate parasitic back reflections, is divided by a directional splitter into two signals – a reference and a delayed one. The reference signal passes through an acousto-optic modulator (AOM), which shifts the emission frequency by the modulation frequency – 120 MHz. The delayed signal passes through a polarisation controller (PC) and a 50 km long fibre line, at the output of which phase fluctuations due to laser noise cease to correlate with fluctuations in the reference arm. For a reliable measurement, the delay time in the fibre should significantly exceed the expected coherence time of the studied laser. A second splitter combines the reference and delayed signals, and the interference beats are detected by a photodetector (PD) and displayed on an Anritsu electrical spectrum analyser (ESA).
The radiation spectral line shape obtained at the output of the circuit is usually a Voigt profile, which is the result of convolution of the Lorentzian and Gaussian circuits, as shown in Fig.3b. The Gaussian component of the spectrum is due to flicker noise, which is inversely proportional to the detuning from the centre frequency and arises from current drift and temperature fluctuations in the laser controller. The Lorentzian component associated with the quantum nature of laser radiation characterises the laser line itself, including the process of spontaneous emission in the active region of the laser, and is a randomly distributed quantity. By extracting the Lorentzian and Gaussian components from the Voigt profile, it is possible to estimate the laser line width. Thus, the fundamental width of the laser line is equal to half the width of the Lorentzian component at the 3 dB level, and the Gaussian width is determined by multiplying the width of the spectrum of the Gaussian component at the 3 dB level by the multiplier √2/2 [8].
RESULTS
Optical spectrum of an external integrated ring resonator on the SoI platform
The optical spectrum of the fabricated silicon ring resonator measured at the probe station is shown in Fig.4. According to the spectrum obtained, the spectral width of the ring resonance was 9.6 pm (1.2 GHz at a wavelength of 1550 nm), the free dispersion region was 115 pm (14.37 GHz), the goodness of fit was 161,000, and the sharpness was 12. Based on the results of the ring transmission spectrum measurements before and after the optical fibres were glued, the total docking loss of both ports at 1530 nm wavelength was about 2.3 dB.
Characterisation of the spectral width of the DFL laser in the free mode and in the frequency self-capture mode
As a source, a DFL laser with a fibre output with polarisation preservation at a wavelength of 1530 nm was used, having a maximum power of 102 mW at a current of 554.6 mA in continuous mode. Figure 5a shows the dependence of the output optical power of the laser on the pump current. The threshold of laser generation is observed at a current of about 70–72 mA, after which the output power smoothly increases with increasing current.
The laser spectrum measured by self-heterodyne interferometry is shown in Fig.5b. At a current close to the threshold value, a single spectral line exceeding the noise level by more than 35 dB is observed in the spectrum. With increasing current, a noise line appears on the sides of the main line due to the excitation of additional laser modes exceeding the threshold level. Thus, at a current of 400 mA the ratio of the laser peak amplitude to the noise illumination is reduced to 15 dB. The spectrum of the laser operating in the free mode at a control current of 300 mA and temperature (TEK) of 26 ⁰C is shown in Fig.6a. From the approximation of the spectrum by the Voigt function presented in Fig.6b, it follows that the Lorentzian width of the laser line is 210 kHz and the Gaussian width is 2.7 MHz.
The spectrum of the laser in the frequency self-capture mode at a current of 300 mA is also shown in Fig.6a. Comparing it with the spectrum of a freely operating laser, we can conclude that the use of an external resonator on the chip allows us to effectively narrow the laser generation line and suppress additional modes appearing at high pump currents. The approximation of this spectrum by the Voigt function is shown in Fig.7c. According to the approximation, the Lorentzian width of the laser generation line was 150 Hz, while the Gaussian width has a value of 36.6 kHz. Thus, the spectral line of the laser in the assembly with the PIC resonator is narrowed by a factor of 1400. The power of the laser output radiation in the frequency self-capture mode is about 1 mW. It is also important to note that duration of continuous operation of the laser in the frequency self-capture mode before jumping to another mode was about 1.5 minutes, and this despite absence of temperature stabilisation of the external resonator and active feedback phase adjustment.
CONCLUSIONS
Thus, in this work, a narrowband laser source based on a DFL laser and an integrated silicon ring resonator with a goodness-of-fit of 1.61 · 105, fabricated on a SoI platform, is realised. The use of the resonator as an external filter made it possible to achieve a narrowing of the laser spectral line from 210 kHz to 150 Hz in the frequency self-capture mode, which corresponds to a 1400-fold improvement in coherence. The obtained results confirm the promising use of integrated resonators on the SoI platform for manufacturing the compact highly coherent lasers of telecommunication C-band wavelengths.
ACKNOWLEDGEMENTS
The research was performed under RSF grant No. 23-91-06301, "Compact high coherence laser generators", and 23-79-30017 "Development of the element base of microwave photonics", and with the financial support of the Ministry of Science and Higher Education of the Russian Federation (FNRM-2025-0014).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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