rus
Issue #5/2021
P. O. Yakushenkov, E. A. Cheshev, I. M. Tupitsin
Investigation of Mode Locking of the Diode-Pumped Laser for the Generator of a Carrier Train in Photonic Circuits
Investigation of Mode Locking of the Diode-Pumped Laser for the Generator of a Carrier Train in Photonic Circuits
DOI: 10.22184/1993-7296.FRos.2021.15.5.420.426
Lasers are used in radio photonics as a generator, like an electronic one in radio electronics. Mode-locked lasers are used to generate a carrier train in photonic circuits. The report is devoted to the study of the diode pumping for the mode-locked laser and its comparison with the results fiber pumping.
Lasers are used in radio photonics as a generator, like an electronic one in radio electronics. Mode-locked lasers are used to generate a carrier train in photonic circuits. The report is devoted to the study of the diode pumping for the mode-locked laser and its comparison with the results fiber pumping.
Теги: diode pumping high frequency pulse train kerr lens mode-locked laser saturable absorber высокочастотная последовательность импульсов диодная накачка керровская линза лазер с синхронизацией мод насыщающийся поглотитель
Investigation of Mode Locking of the Diode-Pumped Laser for the Generator of a Carrier Train in Photonic Circuits
P. O. Yakushenkov, E. A. Cheshev, I. M. Tupitsin
Lebedev Physical Institute of the RAS, Moscow, Russia
Lasers are used in radio photonics as a generator, like an electronic one in radio electronics. Mode-locked lasers are used to generate a carrier train in photonic circuits. The report is devoted to the study of the diode pumping for the mode-locked laser and its comparison with the results fiber pumping.
Key words: diode pumping, mode-locked laser, high frequency pulse train, Kerr lens, saturable absorber.
Received on: 20.08.2021
Accepted on: 02.09.2021
INTRODUCTION
Last 10 years the volume of the transmitted information has increased 50 times. The need of SHF devices of detection, transmission and processing the signal is increasing. The operating speed of the silicon integrated circuits has stopped at 3.5 GHz, and last 10 years increasing by realizing the parallel calculations, in a number of areas electronics has approached the physical limit. In the 21‑st century the radio photonics becomes the priority direction of the development.
Now, the large element base of the integrated optics has been developed, associated with signal processing in fiber-optic communication lines: splitters and combiners, spectral, mode and polarization filters, modulators, switches, deflectors. Laser in photonic integrated circuits is serving as a generator, as the signal-to-noise ratio is very important in photonic information systems, then, due to spontaneous emission and spikes, mode-locked lasers are used, generating ultra short powerful pulses with a high repetition rate.
There are many ways to achieve mode locking: active mode locking, uses an oscillator [1, 2], pulse repetition rate 5 GHz, pulse duration 48 ps; passive mode locking – Kerr Lens Mode-locking (KLM), mode-locking, using the nonlinear-optical Kerr effect, pulse duration 82 fs, pulse repetition rate 4 GHz [3]; saturable absorbers with pulse duration 20 ps, pulse repetition rate 10 GHz [4]; SESAM (Semiconductor saturable-absorber mirror) pulse duration 3 ps, pulse repetition rate 160 GHz [5] the shortest pulse duration 6 fs, pulse repetition rate 86 MHz (Ti: Sapphire) [6]; and MIXEL (Modelocked integrated external-cavity surface emitting laser) pulse duration 500 fs, pulse repetition rate 100 GHz. All of them are to achieve the same phase for different modes.
For integrated photonic circuits, such a laser can be a semiconductor one; it is possible to grow a heterostructure with a semiconductor laser with a pumping diode directly on an integrated circuit, the radiation from which will immediately go out into the waveguides on it; or the radiation can be fed into the circuit from an external source with better characteristics, for example, from a fiber laser, but with high losses. There are very high-quality external fiber sources of laser pulses with a duration of the order of femtoseconds and with repetition rates of the order of hundreds of gigahertz. Also, fiber pumping can form a better Kerr lens due to a more optimal intensity distribution along the spot radius. The modes can be synchronized by the Kerr lens effect, in which a thermal lens is formed due to absorption.
The Kerr lens is making the refractive index so, that the modes are in the same phase on the same part of the optical path, generating a higher intensity, that enhances the effect till the modes are synchronizing. This is a third-order nonlinear effect similar to self-focusing. The effective focal length of the Kerr lens can be roughly estimated as f = r2 / 4n2I, where r is the spot radius, n2 is the nonlinear refractive index, I is the intensity, since this is a thermal lens, then the law is parabolic; it can be more accurately estimated as [7]:
,
where A is the absorption capacity of the medium;
I0 is the intensity of the incident radiation;
a = λ / ρc is the thermal conductivity, the probability integral function , the error function
. In this case, the radius of the mode spot can be calculated as , where L = l / n is the effective length, l is the length of the nonlinear crystal. In one of the works [8], due to such self-focusing, the spot size varied from 0,2 to 0,05 mm.
The optical properties of the solid-state laser in the process of generating radiation are significantly changed due to the appearance of thermopotic inhomogeneities during uneven heating during the pumping process. The appearance of the thermo lens can be simulated by a lens, the optical force of which depends on the pump power.
So, the purpose of this work was to study the generation of a high-frequency (GHz) pulse train on a diode-pumped Nd : YVO4 nonlinear crystal.
DIODE PUMPING
To obtain a high-frequency pulse train on a diode-pumped nonlinear Nd : YVO4 crystal, the scheme on fig. 1 was used.
The laser diode was powered from a control unit, which also maintained a given temperature using a temperature sensor and a Peltier element, a cylindrical lens Л1 was glued to the diode, and lens Л2 focused the pump on the active element. Mirror З1 was flat and located close up to the active element, mirror З2 was spherical with a curvature radius of 300 mm and 95% reflection, the cavity length was 15 cm, and the frequency corresponding to its round trip time was 1 GHz. The Nd : YVO4 active element had a length of 0,5 mm and a doping level of 0,5%, the signal from the receiver was analyzed on an oscilloscope, a laser pointer was used to align the elements along the optical axis, and the spectrum of the output radiation of the diode and laser was analyzed separately using a spectrometer. The threshold current of the diode was 1.7–1.8 A, for pumping the active element, the spectrum of the diode was tuned to the maximum absorption of the element – 808 nm, for this the diode temperature was maintained at 28 °C.
The mode spot radius was calculated using the formulas for a Gaussian beam, with such resonator as in the experiment, it was about 200 μm (Fig. 2), pumping to the active element was focused into a spot of about 150 μm (from experience, it is known that for efficient energy transfer to the radiation mode, it is better to focus the pumping into a spot slightly less than the mode radius, so as not to create unnecessary thermopotic effects). The laser emission spectrum was 1 064 nm. On the oscilloscope (Fig. 3) we observed a high-frequency pulse train with a 1 GHz frequency, corresponding to the cavity round-trip time. Fig. 4 shows intermode beats corresponding to partial mode locking – by making the Fourier transform of such signal, we will notice, that some of the modes are in the same phase, in contrast to a signal without synchronized modes with different random phases, where we would observe uniform white noise. Next, the length of the resonator in the stability region was changed from 15 cm to 30 cm; the program ‘reZonator’ was used to calculate the stability region of the resonator.
At 22 cm cavity length, closer to the calculated boundary of the stability region, we also observed partial mode locking (Fig. 5). The pulse repetition rate corresponded to the cavity round-trip time – about 800 MHz. With such cavity length, it was not possible to achieve complete mode locking at different pump power levels, because unstable modes cannot be synchronized, as in the case of 15 cm cavity.
At 17 cm cavity length, almost complete mode locking was observed at a pump power of 2.7 A, and at a pumping of 3 A, there was complete synchronization; at a pumping of 3.5 A, a partial synchronization was again observed with the appearance of new modes that were not synchronized. We planned to increase the pump power for better synchronization, we thought, that it would enhance self-focusing and the Kerr lens effect. To continue increasing the effect with the appearance of new modes, it was planned to take a crystal with a lower doping (0,2%) and accordingly a higher nonlinear refractive index. The optimal doping level depends on the cavity configuration, the pump spot and more.
Taking a crystal with concentration less, making the nonlinear refractive index higher, we had fewer generated modes, because threshold increased. During experiment all generated modes were completely synchronized, but when all modes are synchronized, the self-focusing process ends, since no new modes appear in this case, in contrast to the case with a more doped crystal. We observed appearing and immediately disappearing fully synchronized modes.
Having such results, we decided to use saturable absorber for better self-focusing (Fig. 9), then we began to observe stable fully synchronized modes – self-focusing was stable. In the article “Compact efficient multi-GHz Kerr-lens modelocked diode-pumped Nd : YVO4 laser” [8], the authors wrote about the experiment in which they observed mode locking using fiber-pumping.
On a photonic integrated circuit, diode pumping can be performed directly in the circuit, however, fiber pumping with higher losses can form a better Kerr lens due to better intensity distribution over the pumping spot radius. However, the authors note, that with fiber pumping, mode locking and generation of stable high-frequency pulses were much worse than with diode pumping and saturable absorber.
CONCLUSION
Comparing diode and fiber pumping, we come to the conclusion, that the main for mode locking and generation of stable high-frequency pulses is a saturable absorber, so that the difference in the Kerr lens under diode and fiber pumping is not visible.
ABOUT AUTHORS
Yakushenkov P. O., junior researcher, Laboratory of Photonics of Molecules, Lebedev Physical Institute P. N. Lebedev, yakushenkovpo@lebedev.ru, Moscow, Russia.
AuthorID: 934131
Cheshev E. A., Cand.of Sc. (Phys.&Math), Leading Researcher, Laboratory of Semiconductor Lasers, Lebedev Physical Institute P. N. Lebedev, cheshevea@lebedev.ru, Moscow, Russia.
Researcher ID: N‑1588-2015
AuthorID: 160546
Tupitsyn I. M., junior researcher, Laboratory of Semiconductor Lasers, Lebedev Physical Institute P. N. Lebedev, tupitsynim@lebedev.ru, Moscow, Russia.ResearcherID: R‑5257-2017
Contribution of authors
The experiments were done within the framework of the dissertation of Yakushenkov P. O. under the leadership of Czech E. A. and assistance in the experimental part of Tupitsyn I. M.
P. O. Yakushenkov, E. A. Cheshev, I. M. Tupitsin
Lebedev Physical Institute of the RAS, Moscow, Russia
Lasers are used in radio photonics as a generator, like an electronic one in radio electronics. Mode-locked lasers are used to generate a carrier train in photonic circuits. The report is devoted to the study of the diode pumping for the mode-locked laser and its comparison with the results fiber pumping.
Key words: diode pumping, mode-locked laser, high frequency pulse train, Kerr lens, saturable absorber.
Received on: 20.08.2021
Accepted on: 02.09.2021
INTRODUCTION
Last 10 years the volume of the transmitted information has increased 50 times. The need of SHF devices of detection, transmission and processing the signal is increasing. The operating speed of the silicon integrated circuits has stopped at 3.5 GHz, and last 10 years increasing by realizing the parallel calculations, in a number of areas electronics has approached the physical limit. In the 21‑st century the radio photonics becomes the priority direction of the development.
Now, the large element base of the integrated optics has been developed, associated with signal processing in fiber-optic communication lines: splitters and combiners, spectral, mode and polarization filters, modulators, switches, deflectors. Laser in photonic integrated circuits is serving as a generator, as the signal-to-noise ratio is very important in photonic information systems, then, due to spontaneous emission and spikes, mode-locked lasers are used, generating ultra short powerful pulses with a high repetition rate.
There are many ways to achieve mode locking: active mode locking, uses an oscillator [1, 2], pulse repetition rate 5 GHz, pulse duration 48 ps; passive mode locking – Kerr Lens Mode-locking (KLM), mode-locking, using the nonlinear-optical Kerr effect, pulse duration 82 fs, pulse repetition rate 4 GHz [3]; saturable absorbers with pulse duration 20 ps, pulse repetition rate 10 GHz [4]; SESAM (Semiconductor saturable-absorber mirror) pulse duration 3 ps, pulse repetition rate 160 GHz [5] the shortest pulse duration 6 fs, pulse repetition rate 86 MHz (Ti: Sapphire) [6]; and MIXEL (Modelocked integrated external-cavity surface emitting laser) pulse duration 500 fs, pulse repetition rate 100 GHz. All of them are to achieve the same phase for different modes.
For integrated photonic circuits, such a laser can be a semiconductor one; it is possible to grow a heterostructure with a semiconductor laser with a pumping diode directly on an integrated circuit, the radiation from which will immediately go out into the waveguides on it; or the radiation can be fed into the circuit from an external source with better characteristics, for example, from a fiber laser, but with high losses. There are very high-quality external fiber sources of laser pulses with a duration of the order of femtoseconds and with repetition rates of the order of hundreds of gigahertz. Also, fiber pumping can form a better Kerr lens due to a more optimal intensity distribution along the spot radius. The modes can be synchronized by the Kerr lens effect, in which a thermal lens is formed due to absorption.
The Kerr lens is making the refractive index so, that the modes are in the same phase on the same part of the optical path, generating a higher intensity, that enhances the effect till the modes are synchronizing. This is a third-order nonlinear effect similar to self-focusing. The effective focal length of the Kerr lens can be roughly estimated as f = r2 / 4n2I, where r is the spot radius, n2 is the nonlinear refractive index, I is the intensity, since this is a thermal lens, then the law is parabolic; it can be more accurately estimated as [7]:
,
where A is the absorption capacity of the medium;
I0 is the intensity of the incident radiation;
a = λ / ρc is the thermal conductivity, the probability integral function , the error function
. In this case, the radius of the mode spot can be calculated as , where L = l / n is the effective length, l is the length of the nonlinear crystal. In one of the works [8], due to such self-focusing, the spot size varied from 0,2 to 0,05 mm.
The optical properties of the solid-state laser in the process of generating radiation are significantly changed due to the appearance of thermopotic inhomogeneities during uneven heating during the pumping process. The appearance of the thermo lens can be simulated by a lens, the optical force of which depends on the pump power.
So, the purpose of this work was to study the generation of a high-frequency (GHz) pulse train on a diode-pumped Nd : YVO4 nonlinear crystal.
DIODE PUMPING
To obtain a high-frequency pulse train on a diode-pumped nonlinear Nd : YVO4 crystal, the scheme on fig. 1 was used.
The laser diode was powered from a control unit, which also maintained a given temperature using a temperature sensor and a Peltier element, a cylindrical lens Л1 was glued to the diode, and lens Л2 focused the pump on the active element. Mirror З1 was flat and located close up to the active element, mirror З2 was spherical with a curvature radius of 300 mm and 95% reflection, the cavity length was 15 cm, and the frequency corresponding to its round trip time was 1 GHz. The Nd : YVO4 active element had a length of 0,5 mm and a doping level of 0,5%, the signal from the receiver was analyzed on an oscilloscope, a laser pointer was used to align the elements along the optical axis, and the spectrum of the output radiation of the diode and laser was analyzed separately using a spectrometer. The threshold current of the diode was 1.7–1.8 A, for pumping the active element, the spectrum of the diode was tuned to the maximum absorption of the element – 808 nm, for this the diode temperature was maintained at 28 °C.
The mode spot radius was calculated using the formulas for a Gaussian beam, with such resonator as in the experiment, it was about 200 μm (Fig. 2), pumping to the active element was focused into a spot of about 150 μm (from experience, it is known that for efficient energy transfer to the radiation mode, it is better to focus the pumping into a spot slightly less than the mode radius, so as not to create unnecessary thermopotic effects). The laser emission spectrum was 1 064 nm. On the oscilloscope (Fig. 3) we observed a high-frequency pulse train with a 1 GHz frequency, corresponding to the cavity round-trip time. Fig. 4 shows intermode beats corresponding to partial mode locking – by making the Fourier transform of such signal, we will notice, that some of the modes are in the same phase, in contrast to a signal without synchronized modes with different random phases, where we would observe uniform white noise. Next, the length of the resonator in the stability region was changed from 15 cm to 30 cm; the program ‘reZonator’ was used to calculate the stability region of the resonator.
At 22 cm cavity length, closer to the calculated boundary of the stability region, we also observed partial mode locking (Fig. 5). The pulse repetition rate corresponded to the cavity round-trip time – about 800 MHz. With such cavity length, it was not possible to achieve complete mode locking at different pump power levels, because unstable modes cannot be synchronized, as in the case of 15 cm cavity.
At 17 cm cavity length, almost complete mode locking was observed at a pump power of 2.7 A, and at a pumping of 3 A, there was complete synchronization; at a pumping of 3.5 A, a partial synchronization was again observed with the appearance of new modes that were not synchronized. We planned to increase the pump power for better synchronization, we thought, that it would enhance self-focusing and the Kerr lens effect. To continue increasing the effect with the appearance of new modes, it was planned to take a crystal with a lower doping (0,2%) and accordingly a higher nonlinear refractive index. The optimal doping level depends on the cavity configuration, the pump spot and more.
Taking a crystal with concentration less, making the nonlinear refractive index higher, we had fewer generated modes, because threshold increased. During experiment all generated modes were completely synchronized, but when all modes are synchronized, the self-focusing process ends, since no new modes appear in this case, in contrast to the case with a more doped crystal. We observed appearing and immediately disappearing fully synchronized modes.
Having such results, we decided to use saturable absorber for better self-focusing (Fig. 9), then we began to observe stable fully synchronized modes – self-focusing was stable. In the article “Compact efficient multi-GHz Kerr-lens modelocked diode-pumped Nd : YVO4 laser” [8], the authors wrote about the experiment in which they observed mode locking using fiber-pumping.
On a photonic integrated circuit, diode pumping can be performed directly in the circuit, however, fiber pumping with higher losses can form a better Kerr lens due to better intensity distribution over the pumping spot radius. However, the authors note, that with fiber pumping, mode locking and generation of stable high-frequency pulses were much worse than with diode pumping and saturable absorber.
CONCLUSION
Comparing diode and fiber pumping, we come to the conclusion, that the main for mode locking and generation of stable high-frequency pulses is a saturable absorber, so that the difference in the Kerr lens under diode and fiber pumping is not visible.
ABOUT AUTHORS
Yakushenkov P. O., junior researcher, Laboratory of Photonics of Molecules, Lebedev Physical Institute P. N. Lebedev, yakushenkovpo@lebedev.ru, Moscow, Russia.
AuthorID: 934131
Cheshev E. A., Cand.of Sc. (Phys.&Math), Leading Researcher, Laboratory of Semiconductor Lasers, Lebedev Physical Institute P. N. Lebedev, cheshevea@lebedev.ru, Moscow, Russia.
Researcher ID: N‑1588-2015
AuthorID: 160546
Tupitsyn I. M., junior researcher, Laboratory of Semiconductor Lasers, Lebedev Physical Institute P. N. Lebedev, tupitsynim@lebedev.ru, Moscow, Russia.ResearcherID: R‑5257-2017
Contribution of authors
The experiments were done within the framework of the dissertation of Yakushenkov P. O. under the leadership of Czech E. A. and assistance in the experimental part of Tupitsyn I. M.
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