rus
Issue #3/2023
P. A. Itrin, D. I. Sementsov, A. B. Petrov, M. A. Kozlyakov, V. A. Ribenek
High-frequency Ηarmonic Mode Locking in a Frequency-Shifted Fiber Ring Laser With an Acousto-Optic Modulator
High-frequency Ηarmonic Mode Locking in a Frequency-Shifted Fiber Ring Laser With an Acousto-Optic Modulator
DOI: 10.22184/1993-7296.FRos.2023.17.3.184.194
The possible development of a soliton ring laser with the hybrid harmonic mode locking that provides the generation of a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter, is demonstrated. The mode locking is based on the effect of intracavity frequency shift. The development is based on the assumption that, under certain conditions, an acousto-optic frequency shift, supplemented by the spectral filtration, can lead to stabilization and an increase in the quality of a high-frequency pulse sequence.
The possible development of a soliton ring laser with the hybrid harmonic mode locking that provides the generation of a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter, is demonstrated. The mode locking is based on the effect of intracavity frequency shift. The development is based on the assumption that, under certain conditions, an acousto-optic frequency shift, supplemented by the spectral filtration, can lead to stabilization and an increase in the quality of a high-frequency pulse sequence.
Теги: acousto-optic modulator fiber ring laser mode locking time jitter ultrashort pulse generation акустооптический модулятор волоконный кольцевой лазер временной джиттер генерация ультракоротких импульсов синхронизация мод
High-frequency Ηarmonic Mode Locking in a Frequency-Shifted Fiber Ring Laser With an Acousto-Optic Modulator
P. A. Itrin1, D. I. Sementsov1, A. B. Petrov2, M. A. Kozlyakov2, V. A. Ribenek1
Ulyanovsk State University, Ulyanovsk, Russia
Nordlase Ltd., Saint-Petersburg, Russia
The possible development of a soliton ring laser with the hybrid harmonic mode locking that provides the generation of a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter, is demonstrated. The mode locking is based on the effect of intracavity frequency shift. The development is based on the assumption that, under certain conditions, an acousto-optic frequency shift, supplemented by the spectral filtration, can lead to stabilization and an increase in the quality of a high-frequency pulse sequence.
Keywords: fiber ring laser, mode locking, acousto-optic modulator, ultrashort pulse generation, time jitter
Article received on: 20.04.2023
Article accepted on: 05.05.2023
Introduction
It is well-known that the laser sources of high-frequency pulse sequences are required in a wide range of applications of modern photonics, in particular, in the problems of optical communications, spectroscopy, metrology, etc. [1, 2]. One of the most popular designs of such sources in recent decades are the soliton fiber lasers with harmonic mode locking (HML), in the cavity of which many pulses are located periodically [3]. This approach to the implementation of high-frequency pulse generators combines the possible achievement of a multi-GHz pulse repetition rate with the fundamental advantages of fiber lasers, namely the compactness compatible with the high (up to 30 dB and higher) gain value, high quality of the output beam, flexible fiber output, reliability and ease of operation [4].
There are several classification options of the fiber lasers with HML. The most obvious one uses the mode locking achievement method as the main feature, namely the nonlinear polarization rotation (NPR) [5, 6] or special saturable absorbers based on carbon nanotubes, graphene, etc. [7, 8]. According to the HML implementation method, the sources can be divided into two large groups. In the first of them, the HML occurs due to the incorporation of an additional high-Q filter into the cavity with a free spectral range (FSR) that is a multiple of the FSR of the main cavity [9], while this filter selectivity should be sufficient to select the individual cavity modes [10, 11]. The lasers from the second group are distinguished by the fact that the required periodic pulse arrangement is obtained automatically due to the mutual repulsion [12]. The pulse interaction mechanism in each particular case is not always obvious; it can be the interaction through the saturable and relaxing dissipative parameters [13, 14], interaction through the dispersive waves or a continuous component [15], through acoustic waves, through electrostriction [16], etc. A common property of all these interactions is their very low intensity, in many cases only slightly exceeding the level of noise impact (for example, related to the noise in the active medium, thermal effects, vibrations, etc.) on pulses. These noise impacts cause fluctuations in the pulse position, namely the time jitter, and its value is much higher than that of the lasers operating at the fundamental frequency [17]. Reduction in the time jitter and stability improvement of the pulse sequences generated by the HML lasers will significantly increase their attractiveness in the applications, in particular, in the problems of comb spectrum generation that are especially important at the moment.
Recently, the fiber lasers with hybrid harmonic mode locking, where the saturable absorption effect is accompanied by a frequency shift performed by an optical modulator [18–20], generate the considerable interest, while the modulator frequency being much lower than the pulse repetition rate, i. e., there is not active, but passive mode locking with a frequency shift of the soliton pulse. A number of results show that this approach has not only a local effect, leading to a single pulse generation, but also affects the entire pulse sequence, while changing the interpulse interaction intensity [21–25]. These results have motivated the present paper, namely the development of a soliton ring laser with the hybrid harmonic mode locking using the intracavity frequency shift effect. Based on the assumption that, in certain cases, an acousto-optical frequency shift, supplemented by the spectral filtration, can lead to the stabilization and increase in the quality of high-frequency pulse sequence, the task was set to develop and demonstrate a harmonic mode-locked fiber laser capable of generating a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter.
Experimental setup
The considered layout of the fiber laser is shown in Fig. 1. The ring cavity includes an Er-doped fiber (EDF – EY‑305) with a dispersion of D = +9 ps/(nm km) and a length of 5.75 m, pumped by two laser diodes with a maximum power of 550 mW at a wavelength of 1480 nm, through two WDM 1480/1550. The total cavity length is L=64.92 m that corresponds to a fundamental repetition frequency of 3.19 MHz. All fiber elements are made of standard SMF‑28 fiber (D = +17 ps/(nm km) for 1 550 nm). The unidirectional propagation mode is provided by an optical insulator; a 90/10 coupler is used to output the radiation. The cavity includes an acousto-optic modulator (MT80-IIR30-Fio-SM0) operating in the frequency shift mode Δν = 80 MHz. This cavity element is sensitive to the polarization of radiation, and the polarization ectinction coefficient is ~ –20 dB. Moreover, the standard layout of the ring fiber laser resonator includes an additional control element, namely a tunable filter OZ Optics (BTF‑11-11-1525/1570) that allows the signal filtration with tuning of the bandwidth and center frequency in the band of 1 525–1 570 nm. During the experiments, the filter width is set to a value close to the minimum one (slightly more than 1 nm). A feature of the filter is the lack of sensitivity to polarization.
For spectrum measurements, an Ando AQ6317B optical spectrum analyzer with a resolution of 0.02 nm is used. To indicate the RF signal, a Tektronix RSA607A RF spectrum analyzer with a 15 GHz UPD‑15-IR2-FC photodetector (for the pulse repetition rates up to 7.5 GHz) and a 20 GHz Keysight UXR0204A oscilloscope with a Keysight 33 GHz photodetector (for the repetition rates over 7.5 GHz) are used.
Experimental results
The experimental results are recorded in the filter tuning band from 1528 to 1548 nm. In the entire tuning band, the mode locking at the fundamental capacity frequency (3.19 MHz) relevant to the single-pulse generation regime, occurs when the pumping power reaches ~200 mW. As the pump power increases, the laser switches to the multipulse generation mode with a uniform distribution of individual pulses over the cavity, i. e. to the harmonic mode locking regime.
The tuning range under consideration can be divided into two bands: short-wave and long-wave (Fig. 2). In the first one (1528–1535 nm), the mode locking is specified by high stability. The mode locking obtained in this band by adjusting the polarization controller at a low repetition rate is retained as the pumping level is increased. In this case, the pulse repetition rate, as well as the output power, are increased proportionally to the pumping (Fig. 3), while it is important to note that no additional polarization adjustment is performed.
Any changes in the optical spectra with an increase in pumping and a relevant growth of the repetition rate for two different values of the central wavelength of the tunable filter λ0 are shown in Fig. 4.
The stability of the generated pulse sequences can be characterized using the RF spectra. Fig. 5 shows the radio frequency (RF) spectra obtained for the sequence with a repetition rate of 4.5 GHz at the filter’s central transmission wavelength λ0 = 1532.7 nm.
The sequence has a fairly high level of supermode noise suppression (>35 dB). In the high-resolution RF spectrum in the range of tens of kHz, a high level (>60 dB) of the signal-to-noise ratio can also be noted. The change in the supermode noise suppression level and the signal-to-noise ratio with the increased pumping and a relevant growth of the repetition rate is shown in Fig.4(b). As can be seen, over the entire tuning range, the noise specifications of the pulse sequence remain approximately at the same level. We should note once again that only the pumping value is varied during tuning, and no additional polarization adjustment is performed.
The sequences generated in the long-wave (1535–1548 nm) region of the filter tuning are generally less stable. For some values of the central transmission wavelength, it is possible to obtained the pulsed oscillation with a high (> 1 GHz) repetition rate (the right part of Fig. 4), however, these regimes are not stable in relation to changes in the pumping level. In contrast to the previous case, the laser reacts to the changes in pumping not by adjustment of the pulse repetition rate, but by failure of the mode locking. To restore the pulse sequence generation for each pumping level, it is necessary to perform an additional polarization adjustment. However, it is in this tuning range where it is possible to achieve the high repetition rates exceeding 10 GHz (Fig. 7). At the central transmission wavelength of the filter λ0 = 1546.5 nm, a pulse sequence with a repetition rate of 12 GHz is obtained with a high level of supermode noise suppression >40 dB. At the filter transmission wavelength λ0 = 1545.5 nm, a repetition rate of 13 GHz is achieved, however, the RF spectrum type of this pulse sequence and a significantly lower level of supermode noise suppression indicate its weak stability (Fig. 7 – top row). For comparison, Fig. 7 also shows the oscillogram and RF spectrum of a 9 GHz pulse sequence obtained in the short-wave region of the filter (λ0 = 1529 nm) (Fig. 7 – bottom row).
Discussion of results
and conclusions
The most important feature of the laser under consideration is the stable implementation of harmonic mode locking in the multipulse generation mode. This fact means availability of a stable interpulse repulsive force that ensures a uniform pulse distribution over the ring cavity. Based on the fact that the cavity has a significant abnormal dispersion, i. e. it generates the soliton-type pulses without any significant frequency modulation, the spectrum width can be used to estimate the duration of an individual pulse. The duration at half maximum is about τFWHM ≈ 12.5 ps that corresponds to the sech pulse duration of τ ≈ 7 ps. Thus, the average interpulse distance for a sequence with a repetition rate of ~1–10 GHz is many times greater than the pulse duration. It excludes the direct interpulse interaction from the possible repulsion causes. The frequency shift followed by filtration should suppress the possible generation of a narrow-band continuous component of the cavity radiation. The optical spectrum type confirms this assumption. Thus, the interaction through the continuous component and dispersive waves can also be excluded from the causes of pulse repulsion. As a result, the saturable and relaxing amplification remains the most probable candidate for the role of the interpulse interaction carrier [8, 9].
In terms of the mode locking type, the laser can be referred to the hybrid type. The saturable absorption mechanism required for mode locking can be either HML or shifting the radiation frequency followed by the filtration that underlies the mode locking of so-called frequency-shifted feedback lasers [20, 26]. While analyzing any changes in the output power and pulse repetition rate in the short-wave region of the filter tuning (Fig. 3), it is possible to conclude that an increase in pumping leads to the generation of additional pulses, while the energy of each individual pulse Ep = Wout / νrep remains constant. In this case, in a fairly wide repetition frequency range, the pulse spectrum width is changed insignificantly (Fig. 4) indicating that the main pulse specifications (duration, peak power) also remain almost constant with any changes in pumping. Thus, it can be argued that in this case, a standard multisoliton generation process is developed, where the HML is used as a mode locking mechanism [27]. In this case, the saturable and relaxing amplification ensures the mutual pulse repulsion and implementation of harmonic mode locking [28]. Having considered the low noise level and high sequence stability, it can be assumed that in this case the frequency shift and filtration are applied as an additional stabilization mechanism that provides a high level of supermode noise suppression [21]. A necessary condition for such stabilization is the low frequency shift Δν in relation to the carrier frequency ν0.
While analyzing the mode locking when the filter is shifted to the long-wave side (1535–1548 nm – spectra No.5–9 in Fig. 2), in the general case, it is possible to note the lower stability of the pulse sequences. We believe that this may be due to an increase in the ratio Δν/ν0. When it reaches a certain threshold value, the dissipative balance in the system becomes much more complicated. The frequency shift and filtration from a small perturbation in the HML scheme that provides energy equalization and pulse unification [26], becomes an independent mode locking mechanism [19]. For various values of the system parameters (pumping variations, polarization controller orientations, central filter wavelength, etc.), one mode locking mechanism or another is dominated, providing the generation of pulse sequences with various specifications (duration and energy of an individual pulse, repetition rate, etc.). These conditions lead to the possible sudden rearrangements of the generation properties upon transition from the region dominated by one mode-locking mechanism to the region where another mechanism plays the main role, as well as to possible mode locking failures at some intermediate parameter values. However, maintenance of the harmonic mode locking supported by the saturable and relaxing gains and stabilized by a frequency shift, allows in this region at a certain pumping level and certain polarization settings to achieve a repetition rate of ~12 GHz with a supermode noise suppression level of >40 dB.
In conclusion, it should be noted that the acousto-optical frequency shift implemented in the cavity of a ring fiber laser together with the spectral filtration can lead to the stabilization and increase in the quality of a high-frequency pulse sequence, and this assumption has been experimentally confirmed. In particular, the experimental results show that the proposed layout is promising for generating the stable (with a low supermode noise level) high-frequency pulse sequences with the wide capabilities of repetition frequency tuning and local wavelength tuning. The use of a tunable filter with a narrow (~ 1 nm) band width makes it possible to implement the harmonic mode locking with a repetition rate of 10 GHz or more, while generation of the continuous component and dispersive waves is completely suppressed. The disadvantages of this method include a relatively long duration (~ 10 ps) and low energy (~1.5 pJ) of the generated pulses. However, the proposed source can easily be included in the cascade amplification circuits that increase the pulse energy by orders of magnitude [22, 29].
Acknowledgement
The paper is supported by the Russian Science Foundation (project No. 23-79-30017).
AUTHORS
Itrin Pavel A., Junior Researcher, e-mail: itrin@mail.ru, Post-graduate student, S. P. Kapitsa Nonlinear and Microwave Photonics Laboratory, Ulyanovsk State University (UlSU), Ulyanovsk, Russia
ORCHID: 0000-0002-7198-0646
Sementsov Dmitry I., Dr. of Sciences (Phyth.&Math.), Professor, Ulyanovsk State University, Ulyanovsk, Russia.
ORCHID: 0000-0001-6760-0156
Petrov Andrey B., Cand. of Sciences (Engin.), engineer, a.petrov@ nordlase.ru, Nordlase LLC, St. Petersburg, Russia.
ORCHID: 0000-0001-9219-1040
Kozlyakov Mikhail S., m.kozliakov@nordlase.ru, Nordlase LLC, St. Petersburg, Russia.
ORCHID: 0000-0003-2616-4532
Ribenek Valeria A., Junior Researcher, Post-graduate student, S. P. Kapitsa Nonlinear and Microwave Photonics Laboratory, Ulyanovsk State University (UlSU), Ulyanovsk, Russia.
ORCHID: 0000-0002-9233-5339
Conflict of interest
The authors declare no conflicts of interest. All the authors took part in the manuscript preparation, each in his own part, discussing the results, making suggestions.
P. A. Itrin1, D. I. Sementsov1, A. B. Petrov2, M. A. Kozlyakov2, V. A. Ribenek1
Ulyanovsk State University, Ulyanovsk, Russia
Nordlase Ltd., Saint-Petersburg, Russia
The possible development of a soliton ring laser with the hybrid harmonic mode locking that provides the generation of a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter, is demonstrated. The mode locking is based on the effect of intracavity frequency shift. The development is based on the assumption that, under certain conditions, an acousto-optic frequency shift, supplemented by the spectral filtration, can lead to stabilization and an increase in the quality of a high-frequency pulse sequence.
Keywords: fiber ring laser, mode locking, acousto-optic modulator, ultrashort pulse generation, time jitter
Article received on: 20.04.2023
Article accepted on: 05.05.2023
Introduction
It is well-known that the laser sources of high-frequency pulse sequences are required in a wide range of applications of modern photonics, in particular, in the problems of optical communications, spectroscopy, metrology, etc. [1, 2]. One of the most popular designs of such sources in recent decades are the soliton fiber lasers with harmonic mode locking (HML), in the cavity of which many pulses are located periodically [3]. This approach to the implementation of high-frequency pulse generators combines the possible achievement of a multi-GHz pulse repetition rate with the fundamental advantages of fiber lasers, namely the compactness compatible with the high (up to 30 dB and higher) gain value, high quality of the output beam, flexible fiber output, reliability and ease of operation [4].
There are several classification options of the fiber lasers with HML. The most obvious one uses the mode locking achievement method as the main feature, namely the nonlinear polarization rotation (NPR) [5, 6] or special saturable absorbers based on carbon nanotubes, graphene, etc. [7, 8]. According to the HML implementation method, the sources can be divided into two large groups. In the first of them, the HML occurs due to the incorporation of an additional high-Q filter into the cavity with a free spectral range (FSR) that is a multiple of the FSR of the main cavity [9], while this filter selectivity should be sufficient to select the individual cavity modes [10, 11]. The lasers from the second group are distinguished by the fact that the required periodic pulse arrangement is obtained automatically due to the mutual repulsion [12]. The pulse interaction mechanism in each particular case is not always obvious; it can be the interaction through the saturable and relaxing dissipative parameters [13, 14], interaction through the dispersive waves or a continuous component [15], through acoustic waves, through electrostriction [16], etc. A common property of all these interactions is their very low intensity, in many cases only slightly exceeding the level of noise impact (for example, related to the noise in the active medium, thermal effects, vibrations, etc.) on pulses. These noise impacts cause fluctuations in the pulse position, namely the time jitter, and its value is much higher than that of the lasers operating at the fundamental frequency [17]. Reduction in the time jitter and stability improvement of the pulse sequences generated by the HML lasers will significantly increase their attractiveness in the applications, in particular, in the problems of comb spectrum generation that are especially important at the moment.
Recently, the fiber lasers with hybrid harmonic mode locking, where the saturable absorption effect is accompanied by a frequency shift performed by an optical modulator [18–20], generate the considerable interest, while the modulator frequency being much lower than the pulse repetition rate, i. e., there is not active, but passive mode locking with a frequency shift of the soliton pulse. A number of results show that this approach has not only a local effect, leading to a single pulse generation, but also affects the entire pulse sequence, while changing the interpulse interaction intensity [21–25]. These results have motivated the present paper, namely the development of a soliton ring laser with the hybrid harmonic mode locking using the intracavity frequency shift effect. Based on the assumption that, in certain cases, an acousto-optical frequency shift, supplemented by the spectral filtration, can lead to the stabilization and increase in the quality of high-frequency pulse sequence, the task was set to develop and demonstrate a harmonic mode-locked fiber laser capable of generating a multi-GHz pulse sequence in combination with a high level of supermode noise suppression and low time jitter.
Experimental setup
The considered layout of the fiber laser is shown in Fig. 1. The ring cavity includes an Er-doped fiber (EDF – EY‑305) with a dispersion of D = +9 ps/(nm km) and a length of 5.75 m, pumped by two laser diodes with a maximum power of 550 mW at a wavelength of 1480 nm, through two WDM 1480/1550. The total cavity length is L=64.92 m that corresponds to a fundamental repetition frequency of 3.19 MHz. All fiber elements are made of standard SMF‑28 fiber (D = +17 ps/(nm km) for 1 550 nm). The unidirectional propagation mode is provided by an optical insulator; a 90/10 coupler is used to output the radiation. The cavity includes an acousto-optic modulator (MT80-IIR30-Fio-SM0) operating in the frequency shift mode Δν = 80 MHz. This cavity element is sensitive to the polarization of radiation, and the polarization ectinction coefficient is ~ –20 dB. Moreover, the standard layout of the ring fiber laser resonator includes an additional control element, namely a tunable filter OZ Optics (BTF‑11-11-1525/1570) that allows the signal filtration with tuning of the bandwidth and center frequency in the band of 1 525–1 570 nm. During the experiments, the filter width is set to a value close to the minimum one (slightly more than 1 nm). A feature of the filter is the lack of sensitivity to polarization.
For spectrum measurements, an Ando AQ6317B optical spectrum analyzer with a resolution of 0.02 nm is used. To indicate the RF signal, a Tektronix RSA607A RF spectrum analyzer with a 15 GHz UPD‑15-IR2-FC photodetector (for the pulse repetition rates up to 7.5 GHz) and a 20 GHz Keysight UXR0204A oscilloscope with a Keysight 33 GHz photodetector (for the repetition rates over 7.5 GHz) are used.
Experimental results
The experimental results are recorded in the filter tuning band from 1528 to 1548 nm. In the entire tuning band, the mode locking at the fundamental capacity frequency (3.19 MHz) relevant to the single-pulse generation regime, occurs when the pumping power reaches ~200 mW. As the pump power increases, the laser switches to the multipulse generation mode with a uniform distribution of individual pulses over the cavity, i. e. to the harmonic mode locking regime.
The tuning range under consideration can be divided into two bands: short-wave and long-wave (Fig. 2). In the first one (1528–1535 nm), the mode locking is specified by high stability. The mode locking obtained in this band by adjusting the polarization controller at a low repetition rate is retained as the pumping level is increased. In this case, the pulse repetition rate, as well as the output power, are increased proportionally to the pumping (Fig. 3), while it is important to note that no additional polarization adjustment is performed.
Any changes in the optical spectra with an increase in pumping and a relevant growth of the repetition rate for two different values of the central wavelength of the tunable filter λ0 are shown in Fig. 4.
The stability of the generated pulse sequences can be characterized using the RF spectra. Fig. 5 shows the radio frequency (RF) spectra obtained for the sequence with a repetition rate of 4.5 GHz at the filter’s central transmission wavelength λ0 = 1532.7 nm.
The sequence has a fairly high level of supermode noise suppression (>35 dB). In the high-resolution RF spectrum in the range of tens of kHz, a high level (>60 dB) of the signal-to-noise ratio can also be noted. The change in the supermode noise suppression level and the signal-to-noise ratio with the increased pumping and a relevant growth of the repetition rate is shown in Fig.4(b). As can be seen, over the entire tuning range, the noise specifications of the pulse sequence remain approximately at the same level. We should note once again that only the pumping value is varied during tuning, and no additional polarization adjustment is performed.
The sequences generated in the long-wave (1535–1548 nm) region of the filter tuning are generally less stable. For some values of the central transmission wavelength, it is possible to obtained the pulsed oscillation with a high (> 1 GHz) repetition rate (the right part of Fig. 4), however, these regimes are not stable in relation to changes in the pumping level. In contrast to the previous case, the laser reacts to the changes in pumping not by adjustment of the pulse repetition rate, but by failure of the mode locking. To restore the pulse sequence generation for each pumping level, it is necessary to perform an additional polarization adjustment. However, it is in this tuning range where it is possible to achieve the high repetition rates exceeding 10 GHz (Fig. 7). At the central transmission wavelength of the filter λ0 = 1546.5 nm, a pulse sequence with a repetition rate of 12 GHz is obtained with a high level of supermode noise suppression >40 dB. At the filter transmission wavelength λ0 = 1545.5 nm, a repetition rate of 13 GHz is achieved, however, the RF spectrum type of this pulse sequence and a significantly lower level of supermode noise suppression indicate its weak stability (Fig. 7 – top row). For comparison, Fig. 7 also shows the oscillogram and RF spectrum of a 9 GHz pulse sequence obtained in the short-wave region of the filter (λ0 = 1529 nm) (Fig. 7 – bottom row).
Discussion of results
and conclusions
The most important feature of the laser under consideration is the stable implementation of harmonic mode locking in the multipulse generation mode. This fact means availability of a stable interpulse repulsive force that ensures a uniform pulse distribution over the ring cavity. Based on the fact that the cavity has a significant abnormal dispersion, i. e. it generates the soliton-type pulses without any significant frequency modulation, the spectrum width can be used to estimate the duration of an individual pulse. The duration at half maximum is about τFWHM ≈ 12.5 ps that corresponds to the sech pulse duration of τ ≈ 7 ps. Thus, the average interpulse distance for a sequence with a repetition rate of ~1–10 GHz is many times greater than the pulse duration. It excludes the direct interpulse interaction from the possible repulsion causes. The frequency shift followed by filtration should suppress the possible generation of a narrow-band continuous component of the cavity radiation. The optical spectrum type confirms this assumption. Thus, the interaction through the continuous component and dispersive waves can also be excluded from the causes of pulse repulsion. As a result, the saturable and relaxing amplification remains the most probable candidate for the role of the interpulse interaction carrier [8, 9].
In terms of the mode locking type, the laser can be referred to the hybrid type. The saturable absorption mechanism required for mode locking can be either HML or shifting the radiation frequency followed by the filtration that underlies the mode locking of so-called frequency-shifted feedback lasers [20, 26]. While analyzing any changes in the output power and pulse repetition rate in the short-wave region of the filter tuning (Fig. 3), it is possible to conclude that an increase in pumping leads to the generation of additional pulses, while the energy of each individual pulse Ep = Wout / νrep remains constant. In this case, in a fairly wide repetition frequency range, the pulse spectrum width is changed insignificantly (Fig. 4) indicating that the main pulse specifications (duration, peak power) also remain almost constant with any changes in pumping. Thus, it can be argued that in this case, a standard multisoliton generation process is developed, where the HML is used as a mode locking mechanism [27]. In this case, the saturable and relaxing amplification ensures the mutual pulse repulsion and implementation of harmonic mode locking [28]. Having considered the low noise level and high sequence stability, it can be assumed that in this case the frequency shift and filtration are applied as an additional stabilization mechanism that provides a high level of supermode noise suppression [21]. A necessary condition for such stabilization is the low frequency shift Δν in relation to the carrier frequency ν0.
While analyzing the mode locking when the filter is shifted to the long-wave side (1535–1548 nm – spectra No.5–9 in Fig. 2), in the general case, it is possible to note the lower stability of the pulse sequences. We believe that this may be due to an increase in the ratio Δν/ν0. When it reaches a certain threshold value, the dissipative balance in the system becomes much more complicated. The frequency shift and filtration from a small perturbation in the HML scheme that provides energy equalization and pulse unification [26], becomes an independent mode locking mechanism [19]. For various values of the system parameters (pumping variations, polarization controller orientations, central filter wavelength, etc.), one mode locking mechanism or another is dominated, providing the generation of pulse sequences with various specifications (duration and energy of an individual pulse, repetition rate, etc.). These conditions lead to the possible sudden rearrangements of the generation properties upon transition from the region dominated by one mode-locking mechanism to the region where another mechanism plays the main role, as well as to possible mode locking failures at some intermediate parameter values. However, maintenance of the harmonic mode locking supported by the saturable and relaxing gains and stabilized by a frequency shift, allows in this region at a certain pumping level and certain polarization settings to achieve a repetition rate of ~12 GHz with a supermode noise suppression level of >40 dB.
In conclusion, it should be noted that the acousto-optical frequency shift implemented in the cavity of a ring fiber laser together with the spectral filtration can lead to the stabilization and increase in the quality of a high-frequency pulse sequence, and this assumption has been experimentally confirmed. In particular, the experimental results show that the proposed layout is promising for generating the stable (with a low supermode noise level) high-frequency pulse sequences with the wide capabilities of repetition frequency tuning and local wavelength tuning. The use of a tunable filter with a narrow (~ 1 nm) band width makes it possible to implement the harmonic mode locking with a repetition rate of 10 GHz or more, while generation of the continuous component and dispersive waves is completely suppressed. The disadvantages of this method include a relatively long duration (~ 10 ps) and low energy (~1.5 pJ) of the generated pulses. However, the proposed source can easily be included in the cascade amplification circuits that increase the pulse energy by orders of magnitude [22, 29].
Acknowledgement
The paper is supported by the Russian Science Foundation (project No. 23-79-30017).
AUTHORS
Itrin Pavel A., Junior Researcher, e-mail: itrin@mail.ru, Post-graduate student, S. P. Kapitsa Nonlinear and Microwave Photonics Laboratory, Ulyanovsk State University (UlSU), Ulyanovsk, Russia
ORCHID: 0000-0002-7198-0646
Sementsov Dmitry I., Dr. of Sciences (Phyth.&Math.), Professor, Ulyanovsk State University, Ulyanovsk, Russia.
ORCHID: 0000-0001-6760-0156
Petrov Andrey B., Cand. of Sciences (Engin.), engineer, a.petrov@ nordlase.ru, Nordlase LLC, St. Petersburg, Russia.
ORCHID: 0000-0001-9219-1040
Kozlyakov Mikhail S., m.kozliakov@nordlase.ru, Nordlase LLC, St. Petersburg, Russia.
ORCHID: 0000-0003-2616-4532
Ribenek Valeria A., Junior Researcher, Post-graduate student, S. P. Kapitsa Nonlinear and Microwave Photonics Laboratory, Ulyanovsk State University (UlSU), Ulyanovsk, Russia.
ORCHID: 0000-0002-9233-5339
Conflict of interest
The authors declare no conflicts of interest. All the authors took part in the manuscript preparation, each in his own part, discussing the results, making suggestions.
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