rus
Issue #4/2020
V. P. Duraev S. V. Medvedev, S. A. Voronchenko
Single-Frequency Ring Semiconductor Lasers With a Fiber Cavity & Their Application
Single-Frequency Ring Semiconductor Lasers With a Fiber Cavity & Their Application
DOI: 10.22184/1993-7296.FRos.2020.14.4.308.318
The results of studies of tunable single-frequency ring semiconductor lasers at a wavelength of 1550 and 1650 nm with a fiber cavity are presented. The principles of designing ring semiconductor lasers with a fiber cavity with conservation of polarization and without conservation of polarization are stated. The single-frequency operation and tuning of the wavelength of a semiconductor ring laser is substantiated. The main characteristics of semiconductor ring lasers and their applications are discussed.
The results of studies of tunable single-frequency ring semiconductor lasers at a wavelength of 1550 and 1650 nm with a fiber cavity are presented. The principles of designing ring semiconductor lasers with a fiber cavity with conservation of polarization and without conservation of polarization are stated. The single-frequency operation and tuning of the wavelength of a semiconductor ring laser is substantiated. The main characteristics of semiconductor ring lasers and their applications are discussed.
Теги: diode laser spectroscopy (dls) matching a laser diode to a fiber optical communication lines semiconductor ring laser диодная лазерная спектроскопия (dls) оптические линии связи полупроводниковый кольцевой лазер согласование лазерного диода со световодом
Single-Frequency Ring Semiconductor Lasers With a Fiber Cavity & Their Application
V. P. Duraev S. V. Medvedev, S. A. Voronchenko
New Laser Technology Joint Stock Company, Moscow, Russia
The results of studies of tunable single-frequency ring semiconductor lasers at a wavelength of 1550 and 1650 nm with a fiber cavity are presented. The principles of designing ring semiconductor lasers with a fiber cavity with conservation of polarization and without conservation of polarization are stated. The single-frequency operation and tuning of the wavelength of a semiconductor ring laser is substantiated. The main characteristics of semiconductor ring lasers and their applications are discussed.
Received on: 14.05.2020
Accepted on: 11.06.2020.
INTRODUCTION
The basis of a semiconductor ring laser (SRL) [1] is a semiconductor optical amplifier (SOA) of a traveling wave. In its design, a ring-locked optical fiber with or without polarization conservation is used as a cavity. The superluminescent emitter (SLE) serves as an active element of the SOA, and, accordingly, in SRL [2].
Semiconductor ring lasers are of particular interest in that they are relatively inexpensive to manufacture and can be used in many fields of science and technology. In particular, in optical communication lines, in optical information processing devices [3], in high-resolution spectroscopy [4], in navigation systems, in gyroscopy, as a master oscillator in radiofrequency and microwave generators and in many other devices [5].
One of the important differences between SRLs with a fiber cavity and other lasers is the ability to incorporate a wide range of optical devices and components into the cavity. The design of the SRL allows the use of a fiber cavity of arbitrary length from a few centimeters to tens or even hundreds of kilometers. It is thanks to these possibilities that SRLs with ultra-long cavitys are created.
In contrast to linear lasers, SRL can be carried out in the SRL cavity, both the standing wave mode and the traveling wave mode. In SRL, two waves can simultaneously exist, propagating towards each other in the cavity. This property allows us to use SRL as the basis for creating a laser gyroscope using the Sagnac effect [6].
In recent years, in the field of fiber technology, significant success has been achieved in the production of single-mode optical fibers, in the development of welding technology, the creation of optical fibers with the preservation of polarization, fiber optic insulators, element base (fiber couplers, polarizers, electro-optical modulators, etc.). This made it possible to seriously consider the creation of many optical devices based on a semiconductor ring laser.
EXPERIMENTAL PART
General view and design elements of the SRL are presented in Fig. 1. To fabricate the active SRL element, we used InGaAsP / lnP based epitaxial structures (radiation wavelength 1550, 1650 nm) with quantum-well layers fabricated by the method of MOS‑hydride epitaxy.
The active element of the SRL had a cavity length of 1200–1600 μm, and the width of the active strip was 3 μm. To reduce back reflections, the front and rear faces were beveled at an angle of 7 ° to the active waveguide, and two-layer antireflection coatings (Al2O3 and ZrO2) were applied to them. Coatings provide a reflection coefficient of less than 0.05% (Fig. 2). The power of superluminescent radiation was 3–7 mW at a pump current of 300 mA and a spectral width of 30 nm or more.
One of the main difficulties in creating a SRL is the assembly and the problem of introducing radiation into a single-mode fiber. There are different ways to match a laser diode to a fiber. In this work, we used a method of matching a radiation source with a fiber using a microlens formed at the end of a fiber [1, 7]. Microlenses can be made by melting and grinding.
The melting method is based on drawing the fiber under the action of an electric arc and then melting the end of the fiber to form the radius of the lens. This method does not allow you to create lenses with a very small radius, so the maximum value of the radiation input coefficient into the fiber is 40%.
The grinding procedure consists of creating guides at the end of the fiber by grinding on coarse powder (6–9 μm) and polishing on fine powder (1–3 μm) on a fleecy fabric to create a fillet at the end of the fiber. Production is controlled by a projected on-screen image of a red laser emitting from a fiber through a lens. This method allows you to get a cylindrical lens with a radius of about 5 microns (with an angle between the guides of 90°). It is known that the geometrically radiating region of the end face of the SRL crystal is a rectangle with dimensions of 3 × 0.1 μm. Therefore, the beam divergence angle in one plane is much larger than in the other and amounts to 40–45°. A polished cylindrical lens provides a radiation input coefficient of up to 80%. A fiber with a microlens was aligned on a three-coordinate table and fixed with solder on a micro-furnace (current-conducting board).
The SRL fiber leads are terminated by FC / APC optical connectors, which allow the module to dock with other elements of the optical system. In this case, the ends of the connector are ground at an angle of 7°, which reduces the amount of reflection from the fiber-air interface, thus suppressing undesirable feedback optical.
The type of fiber used as a cavity is also determined by the scope of the laser. For example, in gyroscopic applications it is necessary to use fiber with conservation of polarization, which ensures the greatest stability of the generation frequency of SRL in a resting state. In the SRL, an optical fiber of the Panda type was used (Fig. 3). This fiber is able to maintain linear polarization of radiation launched along one of its axes (fast or slow) with an extinction of more than 40 dB. The conservation of polarization in such a fiber is due to the presence of mechanical stresses caused by special rods (see Fig. 3, position 2). These stresses result in birefringence along the fiber core.
The main condition for proper alignment and termination is the combination of the direction of linear polarization and the slow axis of the Panda fiber. When docking the active element of the SRL with the fiber, the fiber must be positioned so that the plane p-n transition of the SRL crystal is aligned with the slow axis of the fiber. When terminating with FC / APC connectors, the connector key must be aligned with the slow axis of the fiber. In the work, a passive fiber orientation method was used. The passive method is based on the geometric orientation of the axis of the PM fiber with visual identification of the position of the tensioning elements in the transverse section of the fiber.
The output of radiation for registration at the photodetector is carried out using a fiber directional coupler. The branch coefficient is selected depending on the specific application of the laser. For use in a gyroscope, it is sufficient to derive 1–5% of the radiation. When using a ring laser as a radiation source, large branch coefficients are used. Optical fiber couplers are the waveguide equivalent of conventional optical translucent mirrors. Couplers are most often made either by polishing the sides or by welding. The optical couplers used in this work were made of single-mode fiber with preservation of polarization.
As a photodetector, on which interference and detection of difference frequencies of optical radiation occurs, a photodetector at a wavelength of 1.55 μm was used. This photodetector was a PIN photodiode on InGaAs / lnP quadruples, that is, on the same compounds from which the SRL chip is made. The sensitivity of the photodiode at a wavelength of 1.55 μm was 0.9 A / W.
During operation, the active element of the SRL, through which currents of 100–300 mA are passed, is very hot. This leads to a decrease in quantum efficiency, the destruction of the waveguide and a change in parameters such as the center of the gain line and gain. In this regard, the active element is placed on the Peltier cooling element. Which, together with a thermistor built into the module and an external temperature stabilization circuit, allows you to maintain a constant crystal temperature with an accuracy of 0.1 °C. As a pump current source and stabilization circuit, the DLC‑1300 driver was used.
It was found that SRL, depending on the cavity fiber used, can operate in both multimode and single-mode modes. When using fiber as a cavity, a single-frequency lasing regime can be achieved. There are two mutually perpendicular waves with a PM fiber in the cavity: one traveling along the slow axis, the other along the fast axis. The oscillations in these two waves acquire a certain phase difference, and, consequently, the corresponding difference in stroke, defined by the expression:
,
where nо and ne are the refractive indices of the ordinary and extraordinary rays, respectively, L is the fiber length.
Due to the mutual perpendicularity of the oscillations, the ordinary and extraordinary waves cannot interfere with each other. This results in light polarized elliptically. If, after the exit of these two waves from the crystal plate, they are passed through the polarizer, then the device from each wave passes only those components that are polarized in one plane, i. e. distinguishes oscillations of one direction from both coherent waves. Next, the waves will interfere with each other depending on the difference in stroke obtained by them in the fiber. Therefore, the light intensity will depend on the phase difference acquired in the fiber by both waves. The role of the polarizer is played by the active element of the SOA, which enhances basically only one polarization.
The free spectral interval equal to the distance between adjacent maximums of the filter transmission depends on the fiber length while maintaining the polarization:
.
Figure 4 shows the optical spectrum of SRLs with a fiber length while maintaining a polarization of 1 m.
With such a fiber length, the free spectral range is 4.8 nm, and the gain required for generation is provided for only one maximum transmission filter. The emission line width is 0.013 nm, which is close to the resolution of the spectrum analyzer. The SRL generation frequency can be smoothly tuned by changing the pump current and the temperature of the active element of the laser [8].
Results
One of the most important measuring instruments that determine the level of development of the navigation technology of aircraft for various purposes are rotation sensors. Currently, there are both rotation sensors based on microelectromechanical systems, and sensors that do not have moving parts: fiber-optic gyroscopes and gyroscopes based on gas ring lasers based on the Sagnac effect [1]. The problems of creating a rotation sensor based on a semiconductor ring laser (SRL) are still being solved [9]. The potential of such a rotation sensor promises a decrease in weight and size characteristics and cost, ease of manufacture, and, possibly, a decrease in sensitivity to other influences other than rotation.
In fig. Figure 5 shows the current-voltage characteristics of the SOA (at the output of one of the fiber terminals of the SOA before the ring closes) (Fig. 5a) and SRL (at the output of the coupler) (Fig. 5b) at a wavelength of 1550 nm. The power of the superluminescent radiation of the SOA was 3.3 mW at a pump current of 250 mA. In the ring closure mode, the threshold SRL generation current was 65 mA, and the radiation power at a pump current of 200 mA was 15 mW, which means that the SRL operates in the generation mode.
The optical spectrum analyzer “ANDO” measured the optical emission spectra of SRLs before (Fig. 5c) and after ring closure (Fig. 5d). The superluminescent emission spectrum (before ring closure) had a half-width of the emission line of 35 nm at a wavelength of 1548 nm, while the SRL generation spectrum had a half-width of the line of less than 0.02 nm at a wavelength of 1549.2 nm.
Since the SRL cavity length is usually from 3.5 meters or more, it follows that the frequency interval between adjacent modes is 60 MHz or less. It is clear that there is no such optical spectrum analyzer that could resolve modes with such a small distance between them.
Therefore, the optical spectra that are presented above are essentially envelopes of the real SRL spectrum. The longitudinal modes of a semiconductor ring laser can be observed using the method of spectral analysis of a radio frequency signal from a photodetector.
Fig. 6 shows a setup diagram for measuring the response of a SRL to rotation. The SRL cavity had a radius of 15 cm and a length of 800 m. Directional coupler 1 was used to output part of the radiation of counterpropagating waves from the cavity. Two counterpropagating waves interfered in coupler 2 and entered the photodetector. The signal from the photodetector was fed to the GW‑lnstek 7830 radio frequency spectrum analyzer. The SRL was placed on a rotating table. The table allows rotation with angular velocities in the range of 0–1000 deg / s. In the ongoing studies, the maximum rotation speed reached 180 deg / s. Information is acquired through sliding contacts built into the rotating table. A radio frequency spectrum analyzer was connected to the sliding contacts. As is known, a laser ring cavity is sensitive to nonreciprocity, because, due to the Sagnac effect, the frequencies of counterpropagating waves are split in the absence of capture. Due to the presence of very significant scattering in the active element of the SRL, one should expect a large region of capture of the frequencies of counterpropagating waves, which was found to be 1 deg / s.
Figure 7 shows the optical and radio-frequency spectrum of a signal from a photodetector recording the radiation of a stationary SRL, for which the optical frequencies of the opposing waves coincide. The photodetector has a quadratic nonlinearity and distinguishes the beat spectrum of a large number of optical modes falling into the radio frequency range. The distance between the optical modes with a SRL ring cavity length L = 800 m is 255 kHz, and the line width is 3 kHz.
Single-frequency semiconductor tunable ring lasers are especially in demand in spectroscopy and gas sensing, where a narrow laser generation line scans individual gas absorption lines with a very high resolution (Fig. 8). A narrow emission line width allows one to completely resolve individual gas absorption lines (with a typical line width of several GHz at atmospheric pressure) [4]. The measurement is carried out by comparing the central absorption peak with a zero level on both sides of the line.
Below is the spectral characteristic of SRLs at a wavelength of 1650 nm (Fig. 9). In the ring closure mode, the threshold SRL generation current was 60 mA, and the radiation power at a pump current of 180 mA was 8 mW. The SRL generation spectrum had a line half-width of less than 0.02 nm at a wavelength of 1649.6 nm.
CONCLUSION
Thus, tunable single-frequency semiconductor ring lasers with a fiber cavity with conservation of polarization at the wavelengths of 1550 and 1650 nm are presented in this work. The lasers studied in the work had a radiation output power of the fiber in the range from 8 to 15 mW. The cavity length of the presented ring lasers varied from a few millimeters to hundreds of meters. The radiation line width of a ring laser with a wavelength of 1550 nm and a fiber cavity length of 800 meters was 3 kHz. SRLs have a large set of properties that allow you to find more and more new applications for practical purposes. The possibility of using SRLs at a wavelength of 1650 nm for diode laser spectroscopy (DLS) and at a wavelength of 1550 nm for gyroscopy is shown.
REFERENCES
Patent RU41924. Kol’cevoj lazer / Duraev V. P., Nedelin E. T., Nedobyvajlo T. P., Sumarokov M. A.
Patent RU119938. Superlyuminescentnyj izluchatel’ / Akparov V.V., Duraev V. P., Nedelin E. T., Nedobyvajlo T. P., Sumarokov M. A.
Duraev V. P., Medvedev S. V., Voronchenko S. A. Optoelektronnye komponenty dlya cifrovyh informacionnyh sistem. Foton-ekspress. 2019; 3(155); 2–5.
Lynch S. G. et al. Bragg-grating-stabilized external cavity lasers for gas sensing using tunable diode laser spectroscopy. Novel In-Plane Semiconductor Lasers XIII. – International Society for Optics and Photonics. 2014; 9002: 900209. DOI: 10.1117/12.2039971.
Duraev V. P., Medvedev S. V. Fibre ring cavity semiconductor laser. Quantum Electronics. 2013; 43(10): 914–916.
Akparov V. V., Dmitriev V. G., Duraev V. P., Kazakov A. A. A semiconductor ring laser: study of its characteristics as a rotation sensor. Quantum Electronics. 2010; 40(10):851–854.
Duraev V. P. Medvedev S. V. Fiziko-matematicheskij model’ poluprovodnikovogo opticheskogo usilitelya. Obozrenie prikladnoj i promyshlennoj matematiki. 2019; 26(2): 105–118.
Chen H. Dynamics of widely tunable single-frequency semiconductor fiber ring laser. Physics Letters A. 2004; 320 (5–6): 333–337. DOI: 10.1016/j.physleta.2003.11.038.
Sakharov V. K. Model of a laser gyroscope with frequency dithering. Quantum Electronics. 2016; 46(6): 567–573.
ABOUT AUTHORS
Duraev V. P., Doctor of Technical Sciences, JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID:0000-0002-2701-0335
Medvedev S. V., Cand. of Technical Sciences, JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID:0000-0001-6289-3228
Voronchenko S.A., JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID: 0000-0002-3913-1097
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.
V. P. Duraev S. V. Medvedev, S. A. Voronchenko
New Laser Technology Joint Stock Company, Moscow, Russia
The results of studies of tunable single-frequency ring semiconductor lasers at a wavelength of 1550 and 1650 nm with a fiber cavity are presented. The principles of designing ring semiconductor lasers with a fiber cavity with conservation of polarization and without conservation of polarization are stated. The single-frequency operation and tuning of the wavelength of a semiconductor ring laser is substantiated. The main characteristics of semiconductor ring lasers and their applications are discussed.
Received on: 14.05.2020
Accepted on: 11.06.2020.
INTRODUCTION
The basis of a semiconductor ring laser (SRL) [1] is a semiconductor optical amplifier (SOA) of a traveling wave. In its design, a ring-locked optical fiber with or without polarization conservation is used as a cavity. The superluminescent emitter (SLE) serves as an active element of the SOA, and, accordingly, in SRL [2].
Semiconductor ring lasers are of particular interest in that they are relatively inexpensive to manufacture and can be used in many fields of science and technology. In particular, in optical communication lines, in optical information processing devices [3], in high-resolution spectroscopy [4], in navigation systems, in gyroscopy, as a master oscillator in radiofrequency and microwave generators and in many other devices [5].
One of the important differences between SRLs with a fiber cavity and other lasers is the ability to incorporate a wide range of optical devices and components into the cavity. The design of the SRL allows the use of a fiber cavity of arbitrary length from a few centimeters to tens or even hundreds of kilometers. It is thanks to these possibilities that SRLs with ultra-long cavitys are created.
In contrast to linear lasers, SRL can be carried out in the SRL cavity, both the standing wave mode and the traveling wave mode. In SRL, two waves can simultaneously exist, propagating towards each other in the cavity. This property allows us to use SRL as the basis for creating a laser gyroscope using the Sagnac effect [6].
In recent years, in the field of fiber technology, significant success has been achieved in the production of single-mode optical fibers, in the development of welding technology, the creation of optical fibers with the preservation of polarization, fiber optic insulators, element base (fiber couplers, polarizers, electro-optical modulators, etc.). This made it possible to seriously consider the creation of many optical devices based on a semiconductor ring laser.
EXPERIMENTAL PART
General view and design elements of the SRL are presented in Fig. 1. To fabricate the active SRL element, we used InGaAsP / lnP based epitaxial structures (radiation wavelength 1550, 1650 nm) with quantum-well layers fabricated by the method of MOS‑hydride epitaxy.
The active element of the SRL had a cavity length of 1200–1600 μm, and the width of the active strip was 3 μm. To reduce back reflections, the front and rear faces were beveled at an angle of 7 ° to the active waveguide, and two-layer antireflection coatings (Al2O3 and ZrO2) were applied to them. Coatings provide a reflection coefficient of less than 0.05% (Fig. 2). The power of superluminescent radiation was 3–7 mW at a pump current of 300 mA and a spectral width of 30 nm or more.
One of the main difficulties in creating a SRL is the assembly and the problem of introducing radiation into a single-mode fiber. There are different ways to match a laser diode to a fiber. In this work, we used a method of matching a radiation source with a fiber using a microlens formed at the end of a fiber [1, 7]. Microlenses can be made by melting and grinding.
The melting method is based on drawing the fiber under the action of an electric arc and then melting the end of the fiber to form the radius of the lens. This method does not allow you to create lenses with a very small radius, so the maximum value of the radiation input coefficient into the fiber is 40%.
The grinding procedure consists of creating guides at the end of the fiber by grinding on coarse powder (6–9 μm) and polishing on fine powder (1–3 μm) on a fleecy fabric to create a fillet at the end of the fiber. Production is controlled by a projected on-screen image of a red laser emitting from a fiber through a lens. This method allows you to get a cylindrical lens with a radius of about 5 microns (with an angle between the guides of 90°). It is known that the geometrically radiating region of the end face of the SRL crystal is a rectangle with dimensions of 3 × 0.1 μm. Therefore, the beam divergence angle in one plane is much larger than in the other and amounts to 40–45°. A polished cylindrical lens provides a radiation input coefficient of up to 80%. A fiber with a microlens was aligned on a three-coordinate table and fixed with solder on a micro-furnace (current-conducting board).
The SRL fiber leads are terminated by FC / APC optical connectors, which allow the module to dock with other elements of the optical system. In this case, the ends of the connector are ground at an angle of 7°, which reduces the amount of reflection from the fiber-air interface, thus suppressing undesirable feedback optical.
The type of fiber used as a cavity is also determined by the scope of the laser. For example, in gyroscopic applications it is necessary to use fiber with conservation of polarization, which ensures the greatest stability of the generation frequency of SRL in a resting state. In the SRL, an optical fiber of the Panda type was used (Fig. 3). This fiber is able to maintain linear polarization of radiation launched along one of its axes (fast or slow) with an extinction of more than 40 dB. The conservation of polarization in such a fiber is due to the presence of mechanical stresses caused by special rods (see Fig. 3, position 2). These stresses result in birefringence along the fiber core.
The main condition for proper alignment and termination is the combination of the direction of linear polarization and the slow axis of the Panda fiber. When docking the active element of the SRL with the fiber, the fiber must be positioned so that the plane p-n transition of the SRL crystal is aligned with the slow axis of the fiber. When terminating with FC / APC connectors, the connector key must be aligned with the slow axis of the fiber. In the work, a passive fiber orientation method was used. The passive method is based on the geometric orientation of the axis of the PM fiber with visual identification of the position of the tensioning elements in the transverse section of the fiber.
The output of radiation for registration at the photodetector is carried out using a fiber directional coupler. The branch coefficient is selected depending on the specific application of the laser. For use in a gyroscope, it is sufficient to derive 1–5% of the radiation. When using a ring laser as a radiation source, large branch coefficients are used. Optical fiber couplers are the waveguide equivalent of conventional optical translucent mirrors. Couplers are most often made either by polishing the sides or by welding. The optical couplers used in this work were made of single-mode fiber with preservation of polarization.
As a photodetector, on which interference and detection of difference frequencies of optical radiation occurs, a photodetector at a wavelength of 1.55 μm was used. This photodetector was a PIN photodiode on InGaAs / lnP quadruples, that is, on the same compounds from which the SRL chip is made. The sensitivity of the photodiode at a wavelength of 1.55 μm was 0.9 A / W.
During operation, the active element of the SRL, through which currents of 100–300 mA are passed, is very hot. This leads to a decrease in quantum efficiency, the destruction of the waveguide and a change in parameters such as the center of the gain line and gain. In this regard, the active element is placed on the Peltier cooling element. Which, together with a thermistor built into the module and an external temperature stabilization circuit, allows you to maintain a constant crystal temperature with an accuracy of 0.1 °C. As a pump current source and stabilization circuit, the DLC‑1300 driver was used.
It was found that SRL, depending on the cavity fiber used, can operate in both multimode and single-mode modes. When using fiber as a cavity, a single-frequency lasing regime can be achieved. There are two mutually perpendicular waves with a PM fiber in the cavity: one traveling along the slow axis, the other along the fast axis. The oscillations in these two waves acquire a certain phase difference, and, consequently, the corresponding difference in stroke, defined by the expression:
,
where nо and ne are the refractive indices of the ordinary and extraordinary rays, respectively, L is the fiber length.
Due to the mutual perpendicularity of the oscillations, the ordinary and extraordinary waves cannot interfere with each other. This results in light polarized elliptically. If, after the exit of these two waves from the crystal plate, they are passed through the polarizer, then the device from each wave passes only those components that are polarized in one plane, i. e. distinguishes oscillations of one direction from both coherent waves. Next, the waves will interfere with each other depending on the difference in stroke obtained by them in the fiber. Therefore, the light intensity will depend on the phase difference acquired in the fiber by both waves. The role of the polarizer is played by the active element of the SOA, which enhances basically only one polarization.
The free spectral interval equal to the distance between adjacent maximums of the filter transmission depends on the fiber length while maintaining the polarization:
.
Figure 4 shows the optical spectrum of SRLs with a fiber length while maintaining a polarization of 1 m.
With such a fiber length, the free spectral range is 4.8 nm, and the gain required for generation is provided for only one maximum transmission filter. The emission line width is 0.013 nm, which is close to the resolution of the spectrum analyzer. The SRL generation frequency can be smoothly tuned by changing the pump current and the temperature of the active element of the laser [8].
Results
One of the most important measuring instruments that determine the level of development of the navigation technology of aircraft for various purposes are rotation sensors. Currently, there are both rotation sensors based on microelectromechanical systems, and sensors that do not have moving parts: fiber-optic gyroscopes and gyroscopes based on gas ring lasers based on the Sagnac effect [1]. The problems of creating a rotation sensor based on a semiconductor ring laser (SRL) are still being solved [9]. The potential of such a rotation sensor promises a decrease in weight and size characteristics and cost, ease of manufacture, and, possibly, a decrease in sensitivity to other influences other than rotation.
In fig. Figure 5 shows the current-voltage characteristics of the SOA (at the output of one of the fiber terminals of the SOA before the ring closes) (Fig. 5a) and SRL (at the output of the coupler) (Fig. 5b) at a wavelength of 1550 nm. The power of the superluminescent radiation of the SOA was 3.3 mW at a pump current of 250 mA. In the ring closure mode, the threshold SRL generation current was 65 mA, and the radiation power at a pump current of 200 mA was 15 mW, which means that the SRL operates in the generation mode.
The optical spectrum analyzer “ANDO” measured the optical emission spectra of SRLs before (Fig. 5c) and after ring closure (Fig. 5d). The superluminescent emission spectrum (before ring closure) had a half-width of the emission line of 35 nm at a wavelength of 1548 nm, while the SRL generation spectrum had a half-width of the line of less than 0.02 nm at a wavelength of 1549.2 nm.
Since the SRL cavity length is usually from 3.5 meters or more, it follows that the frequency interval between adjacent modes is 60 MHz or less. It is clear that there is no such optical spectrum analyzer that could resolve modes with such a small distance between them.
Therefore, the optical spectra that are presented above are essentially envelopes of the real SRL spectrum. The longitudinal modes of a semiconductor ring laser can be observed using the method of spectral analysis of a radio frequency signal from a photodetector.
Fig. 6 shows a setup diagram for measuring the response of a SRL to rotation. The SRL cavity had a radius of 15 cm and a length of 800 m. Directional coupler 1 was used to output part of the radiation of counterpropagating waves from the cavity. Two counterpropagating waves interfered in coupler 2 and entered the photodetector. The signal from the photodetector was fed to the GW‑lnstek 7830 radio frequency spectrum analyzer. The SRL was placed on a rotating table. The table allows rotation with angular velocities in the range of 0–1000 deg / s. In the ongoing studies, the maximum rotation speed reached 180 deg / s. Information is acquired through sliding contacts built into the rotating table. A radio frequency spectrum analyzer was connected to the sliding contacts. As is known, a laser ring cavity is sensitive to nonreciprocity, because, due to the Sagnac effect, the frequencies of counterpropagating waves are split in the absence of capture. Due to the presence of very significant scattering in the active element of the SRL, one should expect a large region of capture of the frequencies of counterpropagating waves, which was found to be 1 deg / s.
Figure 7 shows the optical and radio-frequency spectrum of a signal from a photodetector recording the radiation of a stationary SRL, for which the optical frequencies of the opposing waves coincide. The photodetector has a quadratic nonlinearity and distinguishes the beat spectrum of a large number of optical modes falling into the radio frequency range. The distance between the optical modes with a SRL ring cavity length L = 800 m is 255 kHz, and the line width is 3 kHz.
Single-frequency semiconductor tunable ring lasers are especially in demand in spectroscopy and gas sensing, where a narrow laser generation line scans individual gas absorption lines with a very high resolution (Fig. 8). A narrow emission line width allows one to completely resolve individual gas absorption lines (with a typical line width of several GHz at atmospheric pressure) [4]. The measurement is carried out by comparing the central absorption peak with a zero level on both sides of the line.
Below is the spectral characteristic of SRLs at a wavelength of 1650 nm (Fig. 9). In the ring closure mode, the threshold SRL generation current was 60 mA, and the radiation power at a pump current of 180 mA was 8 mW. The SRL generation spectrum had a line half-width of less than 0.02 nm at a wavelength of 1649.6 nm.
CONCLUSION
Thus, tunable single-frequency semiconductor ring lasers with a fiber cavity with conservation of polarization at the wavelengths of 1550 and 1650 nm are presented in this work. The lasers studied in the work had a radiation output power of the fiber in the range from 8 to 15 mW. The cavity length of the presented ring lasers varied from a few millimeters to hundreds of meters. The radiation line width of a ring laser with a wavelength of 1550 nm and a fiber cavity length of 800 meters was 3 kHz. SRLs have a large set of properties that allow you to find more and more new applications for practical purposes. The possibility of using SRLs at a wavelength of 1650 nm for diode laser spectroscopy (DLS) and at a wavelength of 1550 nm for gyroscopy is shown.
REFERENCES
Patent RU41924. Kol’cevoj lazer / Duraev V. P., Nedelin E. T., Nedobyvajlo T. P., Sumarokov M. A.
Patent RU119938. Superlyuminescentnyj izluchatel’ / Akparov V.V., Duraev V. P., Nedelin E. T., Nedobyvajlo T. P., Sumarokov M. A.
Duraev V. P., Medvedev S. V., Voronchenko S. A. Optoelektronnye komponenty dlya cifrovyh informacionnyh sistem. Foton-ekspress. 2019; 3(155); 2–5.
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ABOUT AUTHORS
Duraev V. P., Doctor of Technical Sciences, JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID:0000-0002-2701-0335
Medvedev S. V., Cand. of Technical Sciences, JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID:0000-0001-6289-3228
Voronchenko S.A., JSC “New Laser Technology”, nolatech@mail.ru, http://nolatech.ru, Moscow, Russia.
ORCID: 0000-0002-3913-1097
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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