RU2498468C1 - Method for differential control of population inversion of laser medium and apparatus for realising said method (versions) - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method for differential control of population inversion of the laser medium involves spectral analysis of the tapped portion of spontaneous radiation power during pumping through relative comparison of optical spectrum densities of spontaneous radiation in two regions: long-wave and short-wave. Spectral regions in the vicinity of maximum spectrum densities are compared.

EFFECT: high stability of operation of the laser and higher power thereof.

6 cl, 7 dwg

Description

The field of technology.

The invention relates to methods for protecting optical amplifiers and pulsed lasers from optical breakdown due to redundancy in the population of inverse levels. Namely, to methods of controlling the acceptable level of inversion of active optical media used in telecommunication, surgery, and metalworking devices.

The prior art.

An important factor in the stable and stable operation of quantum fiber amplifiers and high-power pulsed lasers is the reliable control of the degree of inversion of the laser medium or the gain of the medium.

As a rule, in a number of inventions, the amplification of the laser medium of a working high-power pulsed laser or optical amplifier is judged by the power of amplified spontaneous emission (ASE) measured by a photodetector in the intervals between pulses or, in the so-called quasi-stationary mode, characteristic of optical communication amplifiers. In US 2010/7817921 B2, photodetectors are used to determine ASE, which measure the integrated optical power and compare its value with a tabular value. It is clear that such a method is subject to a number of systematic errors characteristic of proportional integral systems. In particular, errors can be caused by various uncontrolled losses in the coupler from a high-power fiber (for example, during contact deformation due to heating or aging of the welding site at high powers, etc.), which in the best case leads to large-scale distortions in the control and unstable operation of the system, and worst of all, to its failure.

Similar problems arise in the method of protecting optical amplifiers in US 2007/7177174 B2, in which only part of the ASE, but again of a proportional type, is incident on the photodetector.

US 2005/0225846 A1 also proposes to prevent the instability of a high-power pulsed fiber amplifier by using only a proportional power signal at the input of the photodetector to measure ASE. And in this case, it is necessary to calibrate the ASE power branch coefficient every time - this is problematic and each time complicates the operation of the system.

Description of the invention

For stable operation of high-power pulsed laser systems, it is necessary to carry out more reliable control of the population inversion of the laser medium during the pumping process, and, therefore, the associated ASE power value under conditions when the branching part of the ASE power signal from the main channel is reliably unknown, for example, due to strong heating of the place of contact with the main powerful waveguide under the action of the radiation itself.

Therefore, the aim of the present invention is to develop a more reliable method and device for controlling the population inversion of the active medium of a pulsed laser or optical amplifier based on the differential control of a branch arbitrary part of the ASE, without the need to calibrate the branch coefficient. Moreover, the ASE power measurements are carried out at intervals between pulses for the case of a pulsed laser or optical amplifier.

The method is based on measuring the spectral components of ASE (applicable for both a pulsed laser and an optical amplifier), reduces to measuring the differential spectra of the optical power of the ASE at the input of two photodetectors in the branch part of the laser radiation. The spectral method is implemented in two devices - for a pulsed laser and for an optical amplifier.

The essence of the invention is as follows.

To simplify the discussion, we consider a fiber version of a pulsed laser (Figure 1) and an optical amplifier. (Figure 2).

The essence of the differential spectral method is as follows. It is proposed to use differential measurements of the ASE optical power spectra at the input of two photodetectors. With this scheme of switching on photodetectors with different ASE spectrum bandwidths, data processing eliminates common-mode systematic errors in the uncertainty of constant background power levels of ASE, systematic current pickups from powerful electrical signals, as well as errors in bending low-frequency mechanical vibrations at the point of removal of the fiber in contact with the main waveguide radiation.

In Fig.1 and Fig.2 after the active fiber 10, with the mirrors of the resonator 11 and 12 (absent in the optical amplifier - Fig.2), with the pump 20 through the optical couplers 50, 51 two photoelectric converters 40, 41 with narrow-band filters 60 are connected , 61 at the inputs for measuring the spectral density of the spectrum of the ASE. Filters with a passband of the order of 2 nm can be made on the basis of Bragg gratings. Filters are passed in opposite parts of the spectrum: one in the long wavelength and the other in the short wavelength region near the ASE spectrum density maxima. The spontaneous emission ASE filtered in this way is fed to the input of the photoelectric converters 40, 41 and then in the form of electrical signals 70, 71 to the controller 80, which, after matching the signals, generates a command in the form of electrical signals 90 and 100 to control the master oscillator 30 and pump 40 , respectively. The master oscillator 30 (FIG. 2) generates a radiation pulse 110, which reaches the input of the active fiber and amplifies as it passes to the output 120.

Figure 3 shows a typical picture of the change in power P (ASE) in the branch portion 50 in front of the filters 60 and 61 at the moments of the passage of pulses from the master oscillator at different levels of inversion of the active medium (different amplifications of the pumped medium), in the passive mode of power control and pulse repetition rate, i.e. with constant pumping and a constant repetition rate of triggering pulses of the master oscillator.

Figure 3 shows that the level of luminescence before the even and odd pulses is different (sharp emissions in the figure correspond to optical pulses). Odd pulses (if counted from the left) corresponds to a lower value of the gain and inversion. The developing optical pulse has a relatively low output energy, so that the level of inversion changes slightly after it (only a small subsidence of ASE is observed). Until the next (even) pulse, the inversion will increase again, so that the gain before the even pulse is much larger than before the odd one. As a result, a large-energy pulse is formed, which greatly squanders the inversion. This unwanted effect can be eliminated by switching on the active control mode, repetition rate by comparing the ratio of spectral density in the short-wave and long-wave regions of the ASE spectrum. Let us explain it below.

Figure 4 schematically shows the ASE spectrum at 4 different levels of diode pumping: curves 1-4, as the pump current of the Yb amplifier increases. For different active media, the appearance of the spectrum may differ, but two peaks will be one way or another characteristic of the spectral region of wavelengths of 1030 nm (short-wavelength part of ASE) and 1060 nm (long-wavelength part of ASE). Therefore, it seems convenient to select a certain ratio of the amplitudes of the maxima in the ASE spectrum in the short-wave ASE_H and long-wave ASE_L spectral regions as a criterion for the sufficiency of pumping to create the necessary and safe level of inversion (gain). This ratio is selected for a certain type of active fiber, since the energy band structure can differ from fiber to fiber depending on the technology of its production and the type of impurities introduced.

A similar pattern of the ASE spectrum is also observed in the active fiber with other rare-earth metals, for example, in Er-amplifiers, which are widely used in telecommunications. In Fig. 5, at spontaneous emission wavelengths in the short-wavelength spectral region of 1545 nm and long-wavelength - 1562 nm, the maxima ASE_H and ASE_L are observed, respectively. For more or less uniform amplification of WDM communication channels, it is advisable to choose the pump from the condition of equality of these maxima. This may be a criterion for the optimal tuning of the optical amplifier, on the one hand, and, say, an excess of ASE_H over ASE_L by 3 dB can be interpreted by the controller as a condition for disabling pumping and protecting the amplifier from overload (in the absence of a WDM communication session).

Thus, by measuring the density of the spectrum in at least two regions of the ASE spectrum (near the maximums ASE_H and ASE_L), you can control the situation and make the right decision - turn off the pump and prevent optical breakdown or turn on the discharge pulse, or do both at the same time. Technically, this boils down to comparing the spectral density of ASE in two different regions of the spectrum as the population inversion increases (Fig. 6) at different times t1-t4. Here, time acts as a parameter, for each of its values (in our example: t1-t4), ASE_H (t1) and ASE_L (t1) (synchronous measurements) can be compared. Thus, in the spectral method of differential control of the population inversion of the laser medium,

1. Spectral analysis of the branch portion of the ASE power during pumping by relative comparison of the ASE spectrum density in two areas: long-wave and short-wave, by narrow-band filtering at the inputs of two optoelectronic converters connected to the ac a controller that controls pumping and / or the master oscillator.

2. Compare sections of the spectrum from two regions in the vicinity of the maximum spectral densities ASE_H and ASE_L.

3. The relative comparison is synchronous over two regions of the ASE spectrum in the form of the ratio of the amplitudes ASE_H and ASE_L, the corresponding density of the spectrum or in the form of a difference on a decibel scale.

4. Pumping a laser medium in which one of a number of laser materials is used

- glass

- single crystal,

- optical fiber, quartz or sapphire, or polymer.

5. Using laser materials containing rare earth metals, in particular Er, Yb.

6. With control of critical parameters of the optical gain of the laser medium, for example, exceeding the permissible pump level or reaching the optimum level, in the conditions of continuous or pulsed laser generation, or optical amplification.

7. With control by a controller receiving through a.c.p. output signals of optoelectronic converters and comparing the ratios of their amplitudes with a previously set table value.

8. Using the signal from the DAC controller for feedback on the pump level when controlling the population inversion of the active laser medium by changing the pump level under the control of the controller.

9. Using the pulse limitation of the growth of the population inversion at the start of the radiation pulse of the master oscillator, triggered by the control signal of the controller, resetting the population inversion to a low level after laser generation with each pulse.

The following is a description of two devices for implementing the above methods of differential control of the inversion of the laser medium in geometry:

1 - pulsed laser using a spectral method (Figure 1),

2 - optical amplifier using a spectral method (Figure 2),

In the first device (Figure 1) for the differential control of the population inversion of the laser medium 10 of a pulsed laser during the pumping process, the following elements are used:

- two optoelectronic converters 4041, the inputs of which through narrow-band optical filters 60, 61 are connected in parallel to the optical Y-coupler 51, the common input of which is connected to the optical coupler 50 of the ASE power of the laser medium,

- controller 80 with two input a.c.p. 83, 84, for cyclic reception in the intervals between pulses of the output signals of optoelectronic converters, calculating the ratio of their amplitudes, comparing the magnitude of the ratio with the tabular value and controlling the actuators by means of a. 81, 82,

- the first actuator is a pump device 20, which changes the level of pumping, up to shutdown to protect against optical breakdown,

- the second actuator is a master oscillator 30 that generates an input start pulse of radiation, resetting the inversion of the population of the medium to its original state.

In the second device (Figure 2) for the differential control of the population inversion of the laser medium 10 of the optical amplifier during the pumping process, the following elements are used:

- two optoelectronic converters 40, 41, the inputs of which through narrow-band optical filters 60, 61 are connected in parallel to an optical Y-coupler 51, the common input of which is connected to an optical coupler 50 of the ASE power of the laser medium,

- controller with two input a.c.p. 83, 84, for receiving, in the intervals between pulses, and differentiating in time the output signal of the optoelectronic converter, calculating the ratio of two maximum amplitudes, comparing the ratio with the table value and controlling the actuators by means of a.a.p. 81,

- Executive device - a pump device 20, which changes the pump level, up to shutdown to protect against optical breakdown.

A brief description of the drawings.

Figure 1. Block diagram of a pulsed laser with two spectral control channels.

10 - active fiber,

20 - pumping,

30 - master oscillator,

40, 41 - optoelectronic converters,

50 - optical power coupler,

51 - optical Y-coupler,

60, 61 - selective optical filters,

70, 71 - output electrical signals of optoelectronic converters,

80 - controller

81, 82 - c.a.p.,

83, 84 - A.ts.p.,

90 - control signal to the master oscillator,

100 - control signal to the pump,

110 - input radiation of an optical amplifier,

120 - output radiation of an optical amplifier.

Figure 2. Block diagram of an optical amplifier with two spectral control channels.

10 - active fiber,

20 - pumping,

40, 41 - optoelectronic converters,

50 - optical power coupler,

51 - optical Y-coupler,

60, 61 - selective optical filters,

70, 71 - output electrical signals of optoelectronic converters,

80 - controller

81 - c.a.p.,

83, 84 - A.ts.p.,

100 - control signal to pump,

110 - input radiation of an optical amplifier,

120 - output radiation of an optical amplifier.

Figure 3. Oscillogram of the optical radiation of a pulsed laser (output signal of a photoelectronic converter) at a constant pump level and pulse repetition rate of the master oscillator.

Figure 4. Spectrum (schematically) of ASE at 4 different levels of diode pumping: curves 1–4, as the pump current of the Yb amplifier increases.

Figure 5. Spectrum (analyzer - Anritsu) of ASE in Er - optical amplifier.

6. Kinetics of the increase in signal P of power ASE in time in the intervals between pulses.

7. Oscillogram of the output pulses of a fiber laser at a repetition rate of 1 kHz.

The implementation of the invention

The proposed methods and devices have been tested in relation to fiber pulsed lasers and optical amplifiers with active fibers of various types in laser devices manufactured by NTO IRE-Polyus LLC (Russia), IPG Photonics (USA), IPG Laser (Germany).

As the first optical power coupler 50 (Figs. 1, 2 and 7), you can use a 1% coupler (you can use an additional attenuator of 10-100 dB when working with high powers). Note that knowledge of the attenuation coefficient in the present invention is not required, this greatly simplifies the technical side of the issue of radiation abstraction and allows the use of a non-calibrated coupler of less than 1%.

The second optical coupler is best used - a 50% Y coupler. The fibers and the coupler itself must be carefully laid together with filters (for example, based on fiber Bragg gratings) in the tray and can be filled with compound).

Optoelectronic converters are based on photodetectors, for example, the EPM6xx series (JDSU).

Microcontrollers were applied by SILICON LABORATORIES C8051F06x series. The microcontroller receives electrical signals at the inputs of the analogue digital converter, performs digital filtering, differentiation, the necessary calculations and comparisons with the data stored in its memory, using the digital analogue converter. controls diode pumping and master oscillator, changes the pump power and prevents from optical breakdown.

The master oscillator is a semiconductor laser with direct modulation or with an external modulator.

Figure 9 shows the waveform of the output pulses of a fiber laser at a repetition rate of 1 kHz, a wavelength of 1.06 μm at two different pump levels at a peak power of up to 10 kW.

Industrial applicability.

The proposed methods and devices can be widely used in the laser industry, in communication devices with optical amplifiers, medical pulsed lasers, and in new developments.

Claims (6)

1. A method of differential control of the population inversion of a laser medium, consisting of spectral analysis of a branch of the spontaneous emission power during pumping, by relative comparison of the densities of the optical spectrum of spontaneous radiation in two regions: long-wavelength and short-wavelength, whereby sections of the spectrum are compared in the vicinity of the maximum spectrum densities. 2. The method of differential control of the population inversion of the laser medium according to claim 1, in which restrict the growth of population inversion by changing the pump. 3. The method of differential control of the population inversion of the laser medium according to claim 2, in which the growth of the population inversion is limited by an external pulse from the master oscillator, which reduces the level of population inversion. 4. The method of differential control of the population inversion of the laser medium according to claim 3, in which the spectral analysis is carried out using a controller that controls the pump and the master oscillator. 5. A device for differential control of the population inversion of a laser medium of a pulsed laser by spectral analysis of a branch of the spontaneous emission power during pumping, including:
- two optoelectronic converters, the inputs of which through narrow-band optical filters are connected in parallel to the optical Y-coupler, the common input of which is connected to the optical power coupler of the spontaneous emission of the laser medium,
- controller with two input a.c.p. for cyclic reception in the intervals between pulses of the output signals of optoelectronic converters, calculating the ratio of their amplitudes, comparing the ratio with the table value and controlling the actuators by means of a.a.p.,
- the first actuator is a pump device that changes the pump level, up to shutdown to protect against optical breakdown,
- the second actuator is a master oscillator that generates an input start pulse, resetting the inversion of the population of the medium to its original state with a low level of inversion. 6. A device for differential control of the population inversion of the laser medium of an optical amplifier by spectral analysis of a branch of the spontaneous emission power during pumping, including:
two optoelectronic converters, the inputs of which through narrow-band optical filters are connected in parallel to the optical Y-coupler, the common input of which is connected to the optical coupler of the spontaneous radiation power of the laser medium,
- controller with two input a.c.p. for receiving the output signals of the optoelectronic converters, calculating the ratio of their amplitudes, comparing the ratio with the tabular value and controlling the actuator by means of a.a.
- Executive device - a pump device that changes the pump level, up to shutdown to protect against optical breakdown.

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