rus
Issue #4/2021
A. B. Ustinov, I. Yu. Tatsenko, A. A. Nikitin, A. V. Kondrashov, A. V. Shamray , A. V. Ivanov
Principles of Constructing Optoelectronic Microwave Oscillators. Part II
Principles of Constructing Optoelectronic Microwave Oscillators. Part II
DOI: 10.22184/1993-7296.FRos.2021.15.4.334.346
The article confronts the readers with the principles of creating optoelectronic microwave generators. The physical processes underlying the operation of various types of oscillators are briefly considered in the first part of the review (see PHOTONICS RUSSIA. 2021; 15(3):228–237. DOI: 10.22184/1993-7296.FRos.2021.15.3.228.237). Optoelectronic microwave generators based on active ring resonant systems and a comparison of the phase noise of different types of optoelectronic generators are considered in the second part of the review.
The article confronts the readers with the principles of creating optoelectronic microwave generators. The physical processes underlying the operation of various types of oscillators are briefly considered in the first part of the review (see PHOTONICS RUSSIA. 2021; 15(3):228–237. DOI: 10.22184/1993-7296.FRos.2021.15.3.228.237). Optoelectronic microwave generators based on active ring resonant systems and a comparison of the phase noise of different types of optoelectronic generators are considered in the second part of the review.
Principles of Constructing Optoelectronic Microwave Oscillators. Part II
A. B. Ustinov , I. Yu. Tatsenko , A. A. Nikitin , A. V. Kondrashov , A. V. Shamray , A. V. Ivanov
Saint Petersburg Electrotechnical University (LETI), Saint Petersburg, Russia
A. F. Ioffe Physical and Technical Institute of RAS, Saint Petersburg, Russia
M. F. Stelmakh Research Institute «Polyus» JSC, Moscow, Russia
The article confronts the readers with the principles of creating optoelectronic microwave generators. The physical processes underlying the operation of various types of oscillators are briefly considered in the first part of the review (see PHOTONICS RUSSIA. 2021; 15(3):228–237. DOI: 10.22184/1993-7296.FRos.2021.15.3.228.237). Optoelectronic microwave generators based on active ring resonant systems and a comparison of the phase noise of different types of optoelectronic generators are considered in the second part of the review.
Key words: Optoelectronic Oscillator, Microwave Oscillator, Optical frequency comb, Integrated ring resonators
Received on: 15.04.2021
Accepted on: 11.05.2021
4. OPTOELECTRONIC MICROWAVE OSCILLATORS BASED ON ACTIVE RING RESONANT SYSTEMS
4.1. Single-ring OEO
The configuration of the Optoelectronic Oscillator (OEO) with the Mach-Zehnder modulator is most often studied in the scientific literature [28–42]. Such a generator is a ring circuit with positive feedback, in which the optical radiation is modulated with a microwave signal using an electro-optical modulator, and a band-pass microwave filter is used as a frequency-selective element (Fig. 10). For the first time, such a generator circuit was proposed in [29].
To generate a monochromatic microwave signal, it is necessary to implement the selection of resonant frequencies. Selection can be implemented, for example, with a band-pass microwave filter. To tune the generation frequency, it is advisable to use frequency-tunable filters. Thus, for example, in works [31–35] filters on spheres of yttrium iron garnet (YIG) were used (Fig. 11a).
Also, a tunable microwave filter can be made on the basis of an epitaxial YIG film (Fig. 11b). The work of the latter is based on the excitation, propagation and reception of spin waves in a film [43]. Due to the fact that spin waves have a relatively low group velocity of the order of 104–106 m / s, such a filter, in addition to frequency filtering of the signal, introduces an additional delay time.
The authors of [38] used a microwave filter based on a tangentially magnetized YIG film. Frequency tuning of such a filter is carried out by changing the external magnetic field. Fig. 12 shows the frequency tuning of the filter in the 4–12 GHz range by increasing the strength of the external magnetic field.
Fig. 13a shows typical spectra of phase noise for a spin-wave optoelectronic generator with a fiber optic delay line length of 200 m and 4 km, at a generation frequency of 10 GHz. At a 10 kHz offset frequency, the phase noise was –146.5 dBc / Hz. Fig. 13b shows the dependence of the phase noise on the length of the fiber optic delay line at different offset frequencies.
A distinctive feature of spin-wave optoelectronic generators is their rich nonlinear dynamics. At high power levels in such systems, the generation of a periodic signal and chaos develops [39, 40]. An optoelectronic oscillator with phase-lock loop (PLL) is described in [41].
Synchronization of the optoelectronic oscillator with a highly stable crystal oscillator using a PLL loop allows a highly stable signal to be obtained. The temperature instability of the generated signal frequency is determined by the temperature instability of the reference high-stability crystal oscillator. In the temperature range from + 5 °C to + 40 °C, the relative instability of the optoelectronic heterodyne did not exceed 3 · 10–9. The phase noise of the optoelectronic generator with a PLL loop was about –140 dBc / Hz at 10 kHz offset from the carrier (Fig. 14).
The described single-ring circuits are characterized by the fact that with an increase in the length of the optical fiber delay line, the phase noise decreases, but at the same time the interval between adjacent harmonics decreases. This increases the bandwidth requirements of the filter to effectively suppress adjacent resonant frequencies. One of the ways to solve this problem is to use multi-ring OEO circuits.
4.2. Multi-ring OEO
Monochromatic microwave oscillators based on a two-ring circuit formed by parallel-connected fiber lines of various lengths were first proposed in [42]. Fig. 15 (a) shows a schematic diagram of such a generator. Fiber-optic delay lines (FODLs), connected in parallel, act as an optical interferometer, which makes it possible to suppress lateral harmonics in the spectrum of the ring resonator. The resonant frequencies of a ring resonator containing optical fibers with delay times τ1, τ2 ... τN is determined as follows f = m / τ1 = p / τ2 =... = q / τN [44–46]. The principle of suppression of lateral harmonics is shown in Fig. 15 (b). The resonant harmonics of rings with optical fibers of lengths 10l, 5l and l, are shown in the upper part of Fig. 15 (b). It can be seen that for the cases of a two-ring circuit containing fibers of length 10l and 5l, as well as 10l and l, parallel connection of additional rings provides the appearance of transfer characteristic zeros at frequencies satisfying the condition of antiphase interference. A three-ring circuit formed by lines of length 10l, 5l and l, the transfer characteristic of which is shown in the lower part of Fig. 15 (b), provides additional suppression of lateral resonant harmonics and significantly expands the frequency distance between resonant harmonics [47].
In [34], it was experimentally demonstrated that an optoelectronic oscillator based on fiber lines 4.4 km, 3 km, and 1.2 km in length provides a phase noise level of –128 dBc / Hz at 10 kHz offset from the carrier at a side harmonic level of –93 dB in the carrier frequency tuning range from 6 to 12 GHz. The disadvantage of suppressing lateral frequency harmonics by parallel connection of FODLs is a decrease in the total Q-factor, and, consequently, an increase in the phase noise level, due to the addition of short FODLs necessary to suppress spurious harmonics. So, for example, the level of phase noise in a two-ring OEO on a FODL with a length of 8.4 km and 2.2 km (–140 dBc / Hz with a offset from the carrier at 10 kHz) is equivalent to a single-ring OEO on a FODLs with a length of 4.4 km (–136 dB / Hz at 10 kHz offset from the carrier) [34].
One of the ways to solve this problem was proposed in [48], in which it was shown that control of the gain makes it possible to suppress the lateral harmonics in the generation spectrum of two ring OEO on FODL 10 km and 1 km long. Another way to suppress the lateral harmonics of a high-Q OEO on a long FODLwithout decreasing its Q-factor is to use coupled OEO [49]. In this configuration, the microwave signal generated by the low-noise OEO on the long FODL is branched off into the OEO on the short FODL, providing frequency and phase locking. The passband of the microwave filter in the slave OEO provides suppression of lateral harmonics, the position of which is determined by the length of the short FODL. Thus, the proposed design provides low phase noise, which is typical for OEO on a long FODL, as well as a relatively large frequency distance between harmonics in the generation spectrum, which is typical for OEO on short FODLs [50, 51]. The diagram of such a generator is shown in Fig. 16. The length of the fiber-optic delay line of the master OEO was ~6 km, and the slave OEO was ~50 m. The high-Q master OEO determines the spectral characteristics of the generated signal, and the slave OEO is used to isolate the generated harmonic [49]. The use of such a phase-locked circuit made it possible to achieve a phase noise level of –130 dBc / Hz and –150 dBc / Hz at 1 kHz and 10 kHz offsets, respectively, at a level of suppression of the nearest parasitic harmonic of 140 dB.
Let us consider one more method for suppressing side harmonics due to series connection of ring resonators [52]. Such a configuration is shown in Fig. 17a and is an OEO, in the optical path of which several additional FODLs are connected to the FODL with polarization support, closed in a ring and performing the functions of optical resonators [53]. Fig. 17b shows the transfer characteristics of single-ring, double-ring and three-ring on 10l, 5l and l optical fibers. As can be seen from this figure, the series connection of additional ring resonators provides frequency selectivity of resonant harmonics and, as a consequence, an increase in the distance between resonant harmonics. An increase in the number of rings leads to a weakening of those harmonics that do not satisfy the resonance condition in each of the rings [45, 53]. Thus, additional rings perform a frequency-selective function without decreasing the overall Q-factor of the circuit and, consequently, without increasing the phase noise level [46, 54].
Besides, optical resonators with WGMs can be used as a narrow-band filter in the schemes of such OEOs instead of filters based on ring FODLs [55–57]. In [58], an optoelectronic generator based on a FOLZ was investigated, in which a resonator with a WGM is used as a frequency selective element. A schematic representation of such an OEO is shown in Fig. 18. In [58], experimental studies of three designs were carried out: OEO only on optical fiber 4 km long; OEO only on a resonator with a WGM made of MgF2 with a loaded Q-factor of 1.68 · 108; OEO on FODL and on a resonator with WGM. As a result, it was shown that the introduction of resonators with WGMs into the optical path ensures the suppression of side harmonics to a level of –53 dB while maintaining the phase noise level characteristic of OEO on optical fiber, which was –124 dBc / Hz at a 10 kHz offset of the generation frequency of 6.25 GHz.
4.3. OEO with optical amplification
One of the main sources of noise in an optoelectronic generator is the microwave amplifier, which is used to compensate for losses in the circuit. Therefore, to reduce the phase noise of the generator, an optical amplifier based on erbium-doped fiber can be used. Figure 19 shows a schematic of an oscillator with an optical amplifier [59].
In [59], a comparison of the phase noise spectrum for OEO configurations with optical and microwave amplifiers at a generation frequency of 10 GHz was carried out. It is shown that in the range from 100 Hz to 100 kHz, the phase noise level of a generator with an optical amplifier is 10–15 dBc / Hz lower than that of a generator with a microwave amplifier. Paper [60] presents the results of a study of an optical amplifier based on erbium-doped fiber for use in an optoelectronic microwave generator based on delay lines instead of a microwave amplifier with low phase noise. The generation frequency was 8 GHz, the length of the erbium-doped fiber was 10 m. At 10 and 100 Hz offsets from the carrier, the phase noise was –50 and –85 dBc / Hz, respectively. At 1 and 10 kHz offsets, the phase noise was –110 and –130 dBc / Hz.
4.4. Industrial OEOs from OEwaves
OEwaves is a commercial manufacturer of modular and integrated optoelectronic oscillators. According to information from the company’s website www.oewaves.com for 2020, the OEO in modular design has a phase noise of –138 dBc / Hz, at 10 kHz offset from the carrier, and the available oscillation frequencies are in the range of 8–12 GHz. For an integrated OEO (generation frequency 28–36 GHz), the phase noise is –110 dBc / Hz, at 10 kHz offset from the carrier. Also, OEwaves manufactures laboratory OEOs that can be mounted in a 19" (48.26 cm) wide telecommunication rack. The phase noise of the laboratory OEO is –163 dBc / Hz at 6 kHz offset, and about –155 dBc / Hz at 10 kHz offset. Photos of the proposed generators and their characteristics are shown in Fig. 20.
5. COMPARISON OF PHASE NOISE OF OPTOELECTRONIC OSCILLATORS OF VARIOUS TYPES
This final section summarizes the main results achieved to date in the development of OEO. Oscillators with the lowest phase noise are presented. They are divided according to the frequency range of microwave signal generation.
In the 1–8 GHz range, the lowest phase noise is possessed by an optoelectronic oscillator with frequency tuning, in which a delay line on a YIG film is used as a frequency-selective element (a band-pass filter based on surface spin waves) [38]. At 10 kHz offset, the phase noise of the generator was –146 dBc / Hz. These low phase noise values have been achieved through the use of low noise components and a 4 km fiber optic delay line.
Paper [61] describes an OEO with a generation frequency of 10 GHz and having the lowest phase noise in the 8–12 GHz range. The phase noise values were –163 dBc / Hz at 6 kHz offset and about –155 dBc / Hz at 10 kHz offset from the carrier. As in the previous case, the design of the generator uses low-noise components, as well as a fiber-optic delay line with a length of 16 km.
In papers [62–64], tunable optoelectronic generators with a generation frequency above 10 GHz are presented. [62] showed the generation of a signal with frequencies from 8.28 GHz to 25 GHz, with phase noise below –120 dBc / Hz at 10 kHz offset. For an optoelectronic oscillator tunable in the 8–14 GHz range [63], the phase noise at 10 kHz offset was –121 dBc / Hz. The OEO presented in [64] had the lowest phase noise of –123 dBc / Hz at 10 kHz offset from the generation frequency of 13.2 GHz. The phase noise at the same offset at other lasing frequencies in the range of 8.6–15.2 GHz did not exceed –111 dBc / Hz.
In the frequency range 18–27 GHz, the optoelectronic generator shown in [65] has the lowest phase noise. The main feature of this generator is the use of phase-locked loop frequency. At 10 kHz offset from the carrier, the phase noise was –134 dBc / Hz.
For Ka (27–40 GHz) and W (75–110 GHz) bands, the OEO presented in [66] has the lowest phase noise, which contains two coupled ring circuits (optical and electrical). The presented oscillator allows you to generate signals with frequencies of 30 and 90 GHz The phase noise at 10 kHz offset was –130 dBc / Hz (30 GHz) and –120 GHz (90 GHz).
The generator presented in [67] at a generation frequency of 51 GHz has a phase noise of –105 dBc / Hz at 10 kHz offset from the carrier. At present time, this is the lowest phase noise value for generators operating in the 40–75 GHz range.
This work was partially financed by the Ministry of Science and Higher Education of the Russian Federation (State Assignment project, grant No. FSEE‑2020-0005).
ABOUT AUTHORS
Alexey B. Ustinov, Doctor of Physical and Mathematical Sciences, Leading researcher, Department of Physical Electronics and Technology, Electrotechnical University, ETU «LETI», St. Petersburg Russia.
ORCID: 0000-0002-7382-9210
Nikitin Andrey Aleksandrovich, Candidate of Physical and Mathematical Sciences, Department of Physical Electronics and Technology, ETU «LETI»; e-mail: and.a.nikitin@gmail.com, St. Petersburg, Russia.
ORCID: 0000-0002-4226-4341
Kondrashov Alexander Viktorovich, Candidate of Physical and Mathematical Sciences, Department of Physical Electronics and Technology, ETU «LETI», St. Petersburg, Russia.
ORCID: 0000-0003-4192-4480
Tatsenko Ivan Yurievich, postgraduate student, Department of Physical Electronics and Technology, ETU «LETI», St. Petersburg, Russia.
ORCID: 0000-0001-6320-9352
Shamrаi Alexander Valerievich, Doctor of Physical and Mathematical Sciences, e-mail: Achamrai@mail.ioffe.ru, Head. lab. of Quantum Electronics Physicotechnical Institute named after A. F. Ioffe, St. Petersburg, Russia.
ORCID: 0000-0003-0292-8673
Ivanov Andrey Viktorovich, Head of Department, JSC «Research Institute «Polyus» named after M. F. Stelmakh», Moscow, Russia
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
A. B. Ustinov , I. Yu. Tatsenko , A. A. Nikitin , A. V. Kondrashov , A. V. Shamray , A. V. Ivanov
Saint Petersburg Electrotechnical University (LETI), Saint Petersburg, Russia
A. F. Ioffe Physical and Technical Institute of RAS, Saint Petersburg, Russia
M. F. Stelmakh Research Institute «Polyus» JSC, Moscow, Russia
The article confronts the readers with the principles of creating optoelectronic microwave generators. The physical processes underlying the operation of various types of oscillators are briefly considered in the first part of the review (see PHOTONICS RUSSIA. 2021; 15(3):228–237. DOI: 10.22184/1993-7296.FRos.2021.15.3.228.237). Optoelectronic microwave generators based on active ring resonant systems and a comparison of the phase noise of different types of optoelectronic generators are considered in the second part of the review.
Key words: Optoelectronic Oscillator, Microwave Oscillator, Optical frequency comb, Integrated ring resonators
Received on: 15.04.2021
Accepted on: 11.05.2021
4. OPTOELECTRONIC MICROWAVE OSCILLATORS BASED ON ACTIVE RING RESONANT SYSTEMS
4.1. Single-ring OEO
The configuration of the Optoelectronic Oscillator (OEO) with the Mach-Zehnder modulator is most often studied in the scientific literature [28–42]. Such a generator is a ring circuit with positive feedback, in which the optical radiation is modulated with a microwave signal using an electro-optical modulator, and a band-pass microwave filter is used as a frequency-selective element (Fig. 10). For the first time, such a generator circuit was proposed in [29].
To generate a monochromatic microwave signal, it is necessary to implement the selection of resonant frequencies. Selection can be implemented, for example, with a band-pass microwave filter. To tune the generation frequency, it is advisable to use frequency-tunable filters. Thus, for example, in works [31–35] filters on spheres of yttrium iron garnet (YIG) were used (Fig. 11a).
Also, a tunable microwave filter can be made on the basis of an epitaxial YIG film (Fig. 11b). The work of the latter is based on the excitation, propagation and reception of spin waves in a film [43]. Due to the fact that spin waves have a relatively low group velocity of the order of 104–106 m / s, such a filter, in addition to frequency filtering of the signal, introduces an additional delay time.
The authors of [38] used a microwave filter based on a tangentially magnetized YIG film. Frequency tuning of such a filter is carried out by changing the external magnetic field. Fig. 12 shows the frequency tuning of the filter in the 4–12 GHz range by increasing the strength of the external magnetic field.
Fig. 13a shows typical spectra of phase noise for a spin-wave optoelectronic generator with a fiber optic delay line length of 200 m and 4 km, at a generation frequency of 10 GHz. At a 10 kHz offset frequency, the phase noise was –146.5 dBc / Hz. Fig. 13b shows the dependence of the phase noise on the length of the fiber optic delay line at different offset frequencies.
A distinctive feature of spin-wave optoelectronic generators is their rich nonlinear dynamics. At high power levels in such systems, the generation of a periodic signal and chaos develops [39, 40]. An optoelectronic oscillator with phase-lock loop (PLL) is described in [41].
Synchronization of the optoelectronic oscillator with a highly stable crystal oscillator using a PLL loop allows a highly stable signal to be obtained. The temperature instability of the generated signal frequency is determined by the temperature instability of the reference high-stability crystal oscillator. In the temperature range from + 5 °C to + 40 °C, the relative instability of the optoelectronic heterodyne did not exceed 3 · 10–9. The phase noise of the optoelectronic generator with a PLL loop was about –140 dBc / Hz at 10 kHz offset from the carrier (Fig. 14).
The described single-ring circuits are characterized by the fact that with an increase in the length of the optical fiber delay line, the phase noise decreases, but at the same time the interval between adjacent harmonics decreases. This increases the bandwidth requirements of the filter to effectively suppress adjacent resonant frequencies. One of the ways to solve this problem is to use multi-ring OEO circuits.
4.2. Multi-ring OEO
Monochromatic microwave oscillators based on a two-ring circuit formed by parallel-connected fiber lines of various lengths were first proposed in [42]. Fig. 15 (a) shows a schematic diagram of such a generator. Fiber-optic delay lines (FODLs), connected in parallel, act as an optical interferometer, which makes it possible to suppress lateral harmonics in the spectrum of the ring resonator. The resonant frequencies of a ring resonator containing optical fibers with delay times τ1, τ2 ... τN is determined as follows f = m / τ1 = p / τ2 =... = q / τN [44–46]. The principle of suppression of lateral harmonics is shown in Fig. 15 (b). The resonant harmonics of rings with optical fibers of lengths 10l, 5l and l, are shown in the upper part of Fig. 15 (b). It can be seen that for the cases of a two-ring circuit containing fibers of length 10l and 5l, as well as 10l and l, parallel connection of additional rings provides the appearance of transfer characteristic zeros at frequencies satisfying the condition of antiphase interference. A three-ring circuit formed by lines of length 10l, 5l and l, the transfer characteristic of which is shown in the lower part of Fig. 15 (b), provides additional suppression of lateral resonant harmonics and significantly expands the frequency distance between resonant harmonics [47].
In [34], it was experimentally demonstrated that an optoelectronic oscillator based on fiber lines 4.4 km, 3 km, and 1.2 km in length provides a phase noise level of –128 dBc / Hz at 10 kHz offset from the carrier at a side harmonic level of –93 dB in the carrier frequency tuning range from 6 to 12 GHz. The disadvantage of suppressing lateral frequency harmonics by parallel connection of FODLs is a decrease in the total Q-factor, and, consequently, an increase in the phase noise level, due to the addition of short FODLs necessary to suppress spurious harmonics. So, for example, the level of phase noise in a two-ring OEO on a FODL with a length of 8.4 km and 2.2 km (–140 dBc / Hz with a offset from the carrier at 10 kHz) is equivalent to a single-ring OEO on a FODLs with a length of 4.4 km (–136 dB / Hz at 10 kHz offset from the carrier) [34].
One of the ways to solve this problem was proposed in [48], in which it was shown that control of the gain makes it possible to suppress the lateral harmonics in the generation spectrum of two ring OEO on FODL 10 km and 1 km long. Another way to suppress the lateral harmonics of a high-Q OEO on a long FODLwithout decreasing its Q-factor is to use coupled OEO [49]. In this configuration, the microwave signal generated by the low-noise OEO on the long FODL is branched off into the OEO on the short FODL, providing frequency and phase locking. The passband of the microwave filter in the slave OEO provides suppression of lateral harmonics, the position of which is determined by the length of the short FODL. Thus, the proposed design provides low phase noise, which is typical for OEO on a long FODL, as well as a relatively large frequency distance between harmonics in the generation spectrum, which is typical for OEO on short FODLs [50, 51]. The diagram of such a generator is shown in Fig. 16. The length of the fiber-optic delay line of the master OEO was ~6 km, and the slave OEO was ~50 m. The high-Q master OEO determines the spectral characteristics of the generated signal, and the slave OEO is used to isolate the generated harmonic [49]. The use of such a phase-locked circuit made it possible to achieve a phase noise level of –130 dBc / Hz and –150 dBc / Hz at 1 kHz and 10 kHz offsets, respectively, at a level of suppression of the nearest parasitic harmonic of 140 dB.
Let us consider one more method for suppressing side harmonics due to series connection of ring resonators [52]. Such a configuration is shown in Fig. 17a and is an OEO, in the optical path of which several additional FODLs are connected to the FODL with polarization support, closed in a ring and performing the functions of optical resonators [53]. Fig. 17b shows the transfer characteristics of single-ring, double-ring and three-ring on 10l, 5l and l optical fibers. As can be seen from this figure, the series connection of additional ring resonators provides frequency selectivity of resonant harmonics and, as a consequence, an increase in the distance between resonant harmonics. An increase in the number of rings leads to a weakening of those harmonics that do not satisfy the resonance condition in each of the rings [45, 53]. Thus, additional rings perform a frequency-selective function without decreasing the overall Q-factor of the circuit and, consequently, without increasing the phase noise level [46, 54].
Besides, optical resonators with WGMs can be used as a narrow-band filter in the schemes of such OEOs instead of filters based on ring FODLs [55–57]. In [58], an optoelectronic generator based on a FOLZ was investigated, in which a resonator with a WGM is used as a frequency selective element. A schematic representation of such an OEO is shown in Fig. 18. In [58], experimental studies of three designs were carried out: OEO only on optical fiber 4 km long; OEO only on a resonator with a WGM made of MgF2 with a loaded Q-factor of 1.68 · 108; OEO on FODL and on a resonator with WGM. As a result, it was shown that the introduction of resonators with WGMs into the optical path ensures the suppression of side harmonics to a level of –53 dB while maintaining the phase noise level characteristic of OEO on optical fiber, which was –124 dBc / Hz at a 10 kHz offset of the generation frequency of 6.25 GHz.
4.3. OEO with optical amplification
One of the main sources of noise in an optoelectronic generator is the microwave amplifier, which is used to compensate for losses in the circuit. Therefore, to reduce the phase noise of the generator, an optical amplifier based on erbium-doped fiber can be used. Figure 19 shows a schematic of an oscillator with an optical amplifier [59].
In [59], a comparison of the phase noise spectrum for OEO configurations with optical and microwave amplifiers at a generation frequency of 10 GHz was carried out. It is shown that in the range from 100 Hz to 100 kHz, the phase noise level of a generator with an optical amplifier is 10–15 dBc / Hz lower than that of a generator with a microwave amplifier. Paper [60] presents the results of a study of an optical amplifier based on erbium-doped fiber for use in an optoelectronic microwave generator based on delay lines instead of a microwave amplifier with low phase noise. The generation frequency was 8 GHz, the length of the erbium-doped fiber was 10 m. At 10 and 100 Hz offsets from the carrier, the phase noise was –50 and –85 dBc / Hz, respectively. At 1 and 10 kHz offsets, the phase noise was –110 and –130 dBc / Hz.
4.4. Industrial OEOs from OEwaves
OEwaves is a commercial manufacturer of modular and integrated optoelectronic oscillators. According to information from the company’s website www.oewaves.com for 2020, the OEO in modular design has a phase noise of –138 dBc / Hz, at 10 kHz offset from the carrier, and the available oscillation frequencies are in the range of 8–12 GHz. For an integrated OEO (generation frequency 28–36 GHz), the phase noise is –110 dBc / Hz, at 10 kHz offset from the carrier. Also, OEwaves manufactures laboratory OEOs that can be mounted in a 19" (48.26 cm) wide telecommunication rack. The phase noise of the laboratory OEO is –163 dBc / Hz at 6 kHz offset, and about –155 dBc / Hz at 10 kHz offset. Photos of the proposed generators and their characteristics are shown in Fig. 20.
5. COMPARISON OF PHASE NOISE OF OPTOELECTRONIC OSCILLATORS OF VARIOUS TYPES
This final section summarizes the main results achieved to date in the development of OEO. Oscillators with the lowest phase noise are presented. They are divided according to the frequency range of microwave signal generation.
In the 1–8 GHz range, the lowest phase noise is possessed by an optoelectronic oscillator with frequency tuning, in which a delay line on a YIG film is used as a frequency-selective element (a band-pass filter based on surface spin waves) [38]. At 10 kHz offset, the phase noise of the generator was –146 dBc / Hz. These low phase noise values have been achieved through the use of low noise components and a 4 km fiber optic delay line.
Paper [61] describes an OEO with a generation frequency of 10 GHz and having the lowest phase noise in the 8–12 GHz range. The phase noise values were –163 dBc / Hz at 6 kHz offset and about –155 dBc / Hz at 10 kHz offset from the carrier. As in the previous case, the design of the generator uses low-noise components, as well as a fiber-optic delay line with a length of 16 km.
In papers [62–64], tunable optoelectronic generators with a generation frequency above 10 GHz are presented. [62] showed the generation of a signal with frequencies from 8.28 GHz to 25 GHz, with phase noise below –120 dBc / Hz at 10 kHz offset. For an optoelectronic oscillator tunable in the 8–14 GHz range [63], the phase noise at 10 kHz offset was –121 dBc / Hz. The OEO presented in [64] had the lowest phase noise of –123 dBc / Hz at 10 kHz offset from the generation frequency of 13.2 GHz. The phase noise at the same offset at other lasing frequencies in the range of 8.6–15.2 GHz did not exceed –111 dBc / Hz.
In the frequency range 18–27 GHz, the optoelectronic generator shown in [65] has the lowest phase noise. The main feature of this generator is the use of phase-locked loop frequency. At 10 kHz offset from the carrier, the phase noise was –134 dBc / Hz.
For Ka (27–40 GHz) and W (75–110 GHz) bands, the OEO presented in [66] has the lowest phase noise, which contains two coupled ring circuits (optical and electrical). The presented oscillator allows you to generate signals with frequencies of 30 and 90 GHz The phase noise at 10 kHz offset was –130 dBc / Hz (30 GHz) and –120 GHz (90 GHz).
The generator presented in [67] at a generation frequency of 51 GHz has a phase noise of –105 dBc / Hz at 10 kHz offset from the carrier. At present time, this is the lowest phase noise value for generators operating in the 40–75 GHz range.
This work was partially financed by the Ministry of Science and Higher Education of the Russian Federation (State Assignment project, grant No. FSEE‑2020-0005).
ABOUT AUTHORS
Alexey B. Ustinov, Doctor of Physical and Mathematical Sciences, Leading researcher, Department of Physical Electronics and Technology, Electrotechnical University, ETU «LETI», St. Petersburg Russia.
ORCID: 0000-0002-7382-9210
Nikitin Andrey Aleksandrovich, Candidate of Physical and Mathematical Sciences, Department of Physical Electronics and Technology, ETU «LETI»; e-mail: and.a.nikitin@gmail.com, St. Petersburg, Russia.
ORCID: 0000-0002-4226-4341
Kondrashov Alexander Viktorovich, Candidate of Physical and Mathematical Sciences, Department of Physical Electronics and Technology, ETU «LETI», St. Petersburg, Russia.
ORCID: 0000-0003-4192-4480
Tatsenko Ivan Yurievich, postgraduate student, Department of Physical Electronics and Technology, ETU «LETI», St. Petersburg, Russia.
ORCID: 0000-0001-6320-9352
Shamrаi Alexander Valerievich, Doctor of Physical and Mathematical Sciences, e-mail: Achamrai@mail.ioffe.ru, Head. lab. of Quantum Electronics Physicotechnical Institute named after A. F. Ioffe, St. Petersburg, Russia.
ORCID: 0000-0003-0292-8673
Ivanov Andrey Viktorovich, Head of Department, JSC «Research Institute «Polyus» named after M. F. Stelmakh», Moscow, Russia
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
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