RU2664692C1 - Measuring phase noise of narrow-band laser based on the mach-zehnder interferometer consisting of the rm-fiber - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: invention relates to devices for measuring phase noise by the frequency discriminator method, which is a Mach-Zehnder interferometer, and can be used for attestation of narrow-band, highly stable lasers used in communication lines, hydrophones, lidar systems, and phase-sensitive reflectometry. Phase noise meter of narrow-band lasers includes: an optical coupler forming two channels: the first, recording the instantaneous values of the power of the source, and the second, recording two interference signals; a polarizer in the second channel, which forms linearly polarized radiation at the entrance to an unbalanced Mach-Zehnder fiber interferometer, made of a fiber optic splitter coupler (1×2) on the basis of a fiber with preservation of the polarization state at the input of the interferometer, additional fiber with preservation of the polarization state introducing the phase difference, and a combining fiber-optic coupler (2×1) on the basis of fiber with preservation of the polarization state at the output of the interferometer; located after the Mach-Zehnder interferometer, a polarizing beam splitter separating two interference signals from orthogonally polarized waves; three receivers of optical radiation, two of which are located after the polarizing beam splitter in the second channel, and one – in the first channel; an analog-to-digital converter that receives signals from all three receivers, and thereafter a digital signal processing unit for calculating the spectral power density of the phase noise by performing the functions in the following order: normalization of the interference signal to instantaneous values of the signal proportional to the laser power, in order to compensate for the relative noise of the laser intensity; high-frequency filtering to compensate for temperature instability; calculation of phase fluctuations and calculation of the spectral power density of phase noise.EFFECT: minimization of the error in measuring the phase noise of a narrow-band laser.1 cl, 2 dwg

Description

Technical field

The invention relates to devices for measuring phase noise by the method of frequency discriminator, which is a Mach-Zehnder interferometer, and can be used to certify narrow-band highly stable lasers used in communication lines, hydrophones, lidar systems, as well as in phase-sensitive reflectometry.

State of the art

Frequency instability caused by spontaneous transitions, optical loss, and technical noise associated with vibrations and temperature fluctuations in the laser cavity has a significant effect on the quality of communication lines, limiting the spectral line width, on systems for monitoring the state of objects using coherent phase-sensitive reflectometry. The most common characteristic of laser frequency instability is the spectral power density of phase fluctuations — phase noise. One of the main methods for measuring phase noise is the coherent frequency discriminator method, the advantages of which are the ability to measure the extremely narrow line width of the laser spectrum, measure the characteristics in a wide spectral range, and also the ability to measure frequency changes over time. In the method of a frequency discriminator, fluctuations in the frequency of laser radiation due to the shoulder difference in the fiber interferometer are converted into fluctuations in the phase difference of the interfering waves. This phase difference determines the intensity of the interference pattern. By adjusting the implementation time, you can vary the frequency resolution; an increase in implementation time allows the spectral density of phase noise to be measured at low frequencies. An additional measurement of the laser power makes it possible to take into account the relative intensity noise and compensate for its influence on the result of the calculation of phase noise.

In US patent US 5995223A (publ. 30.11.1999) describes a device for fast phase interferometry. This device performs phase reconstruction of the interference signal from a test medium with a spatially varying optical length. The device also uses the principle of separation according to the state of polarization and the introduction of an additional path difference in one of the polarization components. The recorded interference signals are phase-shifted by 90 degrees, which allows reconstructing the interferometric phase picture of the test medium from 0 to 2π radians. The proposed scheme can be used to measure phase noise, however, in this patent this is not the main task of the device. The advantage of this device is its resistance to vibrations and instabilities of environmental conditions.

Its main drawback, due to the scope of the device, is its spatial implementation, which, compared to fiber, has large dimensions and large losses of radiation power, as well as a very complex periodic alignment.

US patent application US 20110122906 A1 (publ. May 26, 2011) describes an apparatus using a fiber unbalanced Mach-Zehnder interferometer and a dedicated reference signal from a laser to measure the wavelength of the radiation source.

However, the proposed circuit in this patent is not used to measure the phase noise of the laser, but only to control the radiation emitted by the laser. Also, the patent drawback is the presence of only one Mach-Zehnder interferometer, and this will lead to high measurement instability in the presence of external factors.

The device described in US patent US 5671301 A (publ. 23.09.1997), based on measuring the phase difference between the reference and measuring signals and processing the received data. The device is based on a Mach-Zehnder interferometer, in the supporting arm of which there is a phase shifter controlled by a piezoelectric element through a signal processing unit. Thus, feedback is formed, which negatively affects the cost of this device.

In the patent of the Russian Federation RU 2569052 C1 (publ. 20.11.2015) a method of compensating for the frequency drift of a reference radiation source in a spectrometric device based on a Fourier interferometer is presented. The compensation method is based on the acquisition and accumulation of data representing the reference interferogram, which depends on the radiation frequency of the reference source; as well as data representing a signal interferogram recorded by a Fourier interferometer. The reference and signal interferograms are compared in an arithmetic unit to determine the phase shift, then mathematical transformations are performed to control the action of the spectrometer to obtain an interferogram of an unknown sample.

This scheme allows you to compensate for laser noise, but does not directly measure the spectral density of the phase noise of the laser.

As a prototype, the installation described in US patent US 4918373 (publ. 04.17.1990) was selected. In this installation, the signal from the source is divided by the coupler into two components, one of which enters the fiber delay line, introducing a certain delay into this signal. The other part passes through an adjustable phase shifter, which biases the phase of the supplied signal. Both signals are then fed to the fiber connector, the result of the mixing is amplified by a low-noise amplifier and then fed to an electric spectrum analyzer, which displays the frequency distribution of the phase noise. The result of signal mixing depends on the relative phase difference and represents the amplitude changes. In other words, the system converts the phase noise of the signal under study into fluctuations in the amplitude of the interference signal, which are then studied on a spectrum analyzer.

The main disadvantage of the prototype is the need for a highly stable reference radiation source, as well as a feedback circuit for holding the operating point of the interferometer in the quadrature region.

Disclosure of invention

The objectives of the invention are to minimize the measurement error of the phase noise of a narrow-band laser in the absence of the need to use a highly stable radiation source and feedback circuit providing the operating point of the interferometer.

This problem is solved by the proposed phase-noise meter of narrow-band lasers, which includes: an optical coupler that forms two channels: the first, which records the instantaneous values of the source power, and the second, which records two interference signals. The polarizer in the second channel generates linearly polarized radiation at the input to the unbalanced Mach-Zehnder fiber interferometer made of a 1 × 2 fiber-optic coupler based on a fiber with preservation of the polarization state (PM fiber) at the input of the interferometer, an additional fiber with preservation of the polarization state (PM fiber), introducing a phase difference, and a fiber optic coupler 2 × 1 based on the fiber while maintaining the state of polarization (PM fiber) at the output of the interferometer. After the Mach-Zehnder interferometer, there is a polarizing beam splitter that separates two interference signals from orthogonally polarized waves. Next, there are three optical radiation receivers, two of which are located after the polarization beam splitter in the second channel, and one in the first channel, and an analog-to-digital converter (ADC), which receives signals from all three receivers, the outputs of which are connected to its digital processing unit signals for calculating the spectral power density of phase noise power by performing functions in the following order: normalization of the interference signal to instantaneous values of the signal proportional to the laser power, with to compensate for the relative noise of the laser intensity; high-pass filtering to compensate for temperature instability; calculation of phase fluctuations and calculation of the spectral power density of phase noise.

The possibility of making correct measurements without a reference radiation source is achieved by generating two interference signals for orthogonally polarized waves using an unbalanced Mach-Zenden interferometer consisting of a fiber with the polarization state preserved (PM fiber), further separation of interference signals using a polarizing beam splitter in a fiber execution and their subsequent analysis. A linearly polarized laser radiation, oriented at an angle of 45 ° relative to the fast and slow axes of the PM fiber, formed after the laser beam passes through the polarizer, enters the interferometer. The unbalanced Mach-Zehnder fiber interferometer includes two 1x2 couplers made of PM fiber and sharing radiation in equal proportions, as well as an additional part of the PM fiber included in one of the arms of the interferometer. Part of the radiation from the test source through the 1 × 2 fiber coupler goes directly to the radiation receiver to measure the relative noise of the studied laser intensity and compensate for its influence on the phase noise estimate, the second part passes through a polarizer, an interferometer and a polarization beam splitter, after which interference signals from orthogonally polarized waves the outputs of the polarizing beam splitter also go to radiation receivers. The division ratio of the first coupler is selected so that the signal value at all three receivers is of the same order. Three signals digitized using the ADC form realizations that are processed according to a certain algorithm in order to calculate the spectral power density of the phase noise power.

Reducing the measurement error is achieved by reducing the influence of temperature instabilities and vibrations on the interferometer by linearly filtering interference signals, and also by compensating for the relative noise of the laser intensity, which is measured by the proposed device by recording the instantaneous values of the laser radiation power. The proposed scheme can be considered as two interferometers for independent polarization states. Since the interferometer is actually single, then the same external conditions influence both output interference signals: temperature changes, vibrations, pressure. The listed external influences are rather slow processes, in connection with which the interferograms will correlate with each other, which allows high-frequency filtering to be applied during digital processing and thereby compensate for the effect of interferometer noise on the measurement of phase noise. Also, due to the implementation of two interferometers, it is possible to compensate for the nonlinearity of the working section of the interferometer, which occurs in circuits with one interferometer without feedback and leads to a distortion of the interference signal due to oscillations at additional frequencies.

List of figures

In FIG. 1 shows a diagram of the proposed meter.

In FIG. 2 shows graphs of phase noise obtained experimentally.

The implementation of the invention

In FIG. 1 shows a diagram of the proposed meter. The meter contains an optical connector 1, to which the investigated laser is connected; fiber optic coupler for two channels (1 × 2) 2; polarizer 3 with an output made of PM fiber; fiber optic coupler (1 × 2) with preservation of the state of polarization 4, additional PM fiber 5; fiber optic coupler (2 × 1) with preservation of the state of polarization 6 to combine the two signals; polarizing beam splitter 7; three radiation receivers 8; an analog-to-digital converter 9 and a digital signal processing unit 10. All components from the polarizer 3 to the radiation detectors 8 are made of PM fiber (fibers with preservation of the polarization state), which allows transmitting radiation with orthogonal polarization states independently of each other and thereby form two interference signals, which are subsequently separated by a polarizing beam splitter 7.

, где

- длина волокна, n - показатель преломления), что позволяет измерять фазовые шумы лазера, нормированные на единицу оптической длины. Излучение после прохождения плеч интерферометра объединяется волоконно-оптический ответвителем (2×1) 6, выход которого соединен с поляризационным светоделителем 7. Интерференционные сигналы от ортогонально поляризованных волн, сформированных на выходах поляризационного светоделителя 7, регистрируются приемниками излучения 8, подключенными к АЦП 9. Оцифрованные сигналы с трех каналов АЦП передаются в блок цифровой обработки сигнала 10, в котором рассчитывается спектральная плотность фазового шума за счет выполнения функций в следующем порядке: нормировка интерференционного сигнала на мгновенные значения сигнала, пропорционального мощности лазера, с целью компенсации относительного шума интенсивности лазера; высокочастотная фильтрация с целью компенсации температурной нестабильности; вычисление флуктуаций фазы и расчет спектральной плотности мощности фазового шума.The studied narrow-band laser is connected to an optical connector 1 connected to a fiber optic coupler (1 × 2) 2, dividing the radiation energy in fractions, determined from the condition that the signal-to-noise ratio is sufficient for correct operation, while the signal value at all three receivers should to be of the same order. One of the outputs of the coupler 2 is connected to the radiation polarizer 3, the second is directly connected to one of the radiation receivers 8. The output of the polarizer 3 is connected to the optical (1 × 2) radiation coupler based on the PM fiber 4, which divides the radiation between the two arms of the interferometer in a ratio of 50 % by 50%. In this case, the slow axis of the input fiber of the coupler (1 × 2) 4 should be oriented at an angle of 45 ° to the direction of polarization of the radiation after the polarizer, that is, to the slow or fast axis of the output PM fiber. An RM fiber 5 is included in one of the arms of the interferometer, introducing a certain phase difference between the interfering waves, thereby providing the operating point of the interferometer. The length of the fiber 5 is selected so that changes in the intensity of the interference signal due to fluctuations in the frequency of the laser can be detected. With a fiber length of 70 cm, the optical path difference in the arms of the interferometer is 1 m (

where

is the fiber length, n is the refractive index), which makes it possible to measure the phase noise of the laser, normalized to a unit of optical length. The radiation after passing through the arms of the interferometer is combined with a fiber optic coupler (2 × 1) 6, the output of which is connected to a polarizing beam splitter 7. Interference signals from orthogonally polarized waves generated at the outputs of a polarizing beam splitter 7 are recorded by radiation receivers 8 connected to the ADC 9. Digitized signals from the three ADC channels are transmitted to the digital signal processing unit 10, in which the spectral density of phase noise is calculated by performing the functions in the next step order: normalization of the interference signal to the instantaneous signal proportional to the laser power in order to compensate for the relative noise of the laser intensity; high-pass filtering to compensate for temperature instability; calculation of phase fluctuations and calculation of the spectral power density of phase noise.

Thus, part of the radiation from the studied source through the fiber coupler (1 × 2) with a known fission factor goes directly to the radiation receiver, the second part passes the polarizer, interferometer and polarizing beam splitter, after which interference signals from orthogonally polarized waves from the outputs of the polarizing beam splitter radiation receivers. A linearly polarized laser radiation, oriented at an angle of 45 ° relative to the fast and slow axes of the PM fiber, formed after the laser beam passes through the polarizer, enters the interferometer. The unbalanced Mach-Zehnder fiber interferometer includes (1 × 2) and (2 × 1) couplers made of PM fiber and sharing radiation in equal proportions, as well as an additional portion of the PM fiber included in one of the arms of the interferometer. Three ADC-digitized signals form realizations that are processed according to the above algorithm in order to calculate the spectral power density of phase noise power.

The interferometer converts small changes in frequency into changes in intensity, which are further converted by receivers into changes in photocurrent and, accordingly, voltage. Since at the input of the interferometer the direction of the electric field vector of the polarized radiation is deflected by 45 degrees relative to the fast and slow axes of the PM fiber, two orthogonal components E _x and E _y propagate at different speeds along the fiber, which, when a fiber passes through a fiber of length L _w, leads to the occurrence of phase differences between the two arms of the interferometer, different for orthogonal polarizations. Thus, the proposed meter circuit can be considered in the form of two interferometers for independent polarization states. Received interference signals are described by the following expressions:

where I _{in (t)} is the input signal, ν (t) is the laser frequency, s is the speed of light, Δ ₁ (t) and Δ ₂ (t) are the optical path differences at the arms of the interferometer for the orthogonal components E _x and E _y , respectively.

Δ ₁ (t) = n _x L _w ,

Δ ₂ (t) = n _y L _w = (n _x + Δ _n ) L _w ,

Δn = λ / BL

where L _w is the length of the PM fiber, λ is the laser wavelength, n _x, n _y are the refractive indices of the slow and fast axes, respectively, Δn is the difference in refractive indices for orthogonal components, BL = 3 ... 5 mm is the length of the beats ( the parameter of the RM fiber, which determines the distance at which the phase difference between the orthogonal polarized modes is 2π).

, которая может использоваться для компенсации влияния шумов интенсивности лазера на результат интерференции. Данную компенсацию амплитудной нестабильности предлагается выполнять после вычисления коэффициента пропорциональности, отвечающего за отношение средних потоков от лазера и интерферометра:In addition to interference signals, a signal proportional to the laser power is recorded during the measurement time interval

, which can be used to compensate for the effect of laser intensity noise on the result of interference. It is proposed to perform this compensation of amplitude instability after calculating the proportionality coefficient, which is responsible for the ratio of the average fluxes from the laser and the interferometer:

> - среднее значение мощности лазера.where <I _x > is the average value of the intensity of the interference signal from the component polarized along the fast axis of the PM fiber, <

> is the average laser power.

Next, the constant component is subtracted from the interference signal, taking into account the intensity noise:

,

,

- колебания интерференционного сигнала для х-компоненты относительно среднего значения. Для получения гармонической составляющей необходимо нормировать на мгновенные значения интерференционного сигнала с учетом мощности лазера:Where

- oscillations of the interference signal for the x-component relative to the average value. To obtain the harmonic component, it is necessary to normalize to the instantaneous values of the interference signal, taking into account the laser power:

To compensate for the influence of temperature instabilities of the interferometer, the Fourier spectra of two interference signals are analyzed and filtered in the region of greatest correlation. After the operations performed, the phase fluctuations of the interference signal are calculated as an argument of the harmonic function:

,

,

- фильтрованный интерференционный сигнал, Ф₁ - флуктуации фазы.Where

- filtered interference signal, Ф ₁ - phase fluctuations.

The spectral power density of phase noise is calculated by the formula:

,

,

where T is the time of the recorded implementation; Δt = 1 / ƒ _∂ - ADC sampling period, ƒ _∂ - sampling frequency. By varying the parameters of the ADC reading: the sampling frequency and the recording time, it is possible to calculate the spectral density in the desired spectral range and with the necessary frequency resolution.

The effect of temperature compensation is achieved by filtering interference signals in the region of the highest correlation of their spectra. This is due to the fact that a single interferometer is implemented, that is, the phase difference of both interference patterns changes in the same way under the influence of external noise: temperature changes, vibrations, pressure.

Reducing the effect of noise intensity of the source is achieved by introducing an additional channel that records the instantaneous values of the laser power and the subsequent use of these data in the digital signal processing algorithm.

The proposed meter also allows you to avoid the possible insensitivity of the interferometer due to the fact that two interference signals are recorded, which are phase shifted relative to each other by a fixed amount, independent of changes in external conditions.

The proposed processing algorithm was applied to interference signals from several lasers. In an article by Pnev Alexey B .; Stepanov Konstantin V .; Dvoretskiy Dmitriy A .; Zhimov Andrei A .; Nesterov Evgeny T .; Sazonkin Stanislav G .; Chemutsky Anton O .; Shelestov Dmitriy A .; Fedorov Aleksey K .; Svelto Cesare; Karasik Valeriy E. Minimization of errors in narrowband laser phase noise measurements based on reference measurement channels // International journal of advanced biotechnology and research. (2016), - V. 7, - I. 4, - PP. 1445-1451 presents the results of measuring the spectral density of the phase noise of a Redfem Integrated Optic laser (Fig. 2). The measured spectral densities of the phase noise were compared with the spectral density of the phase noise presented in the documentation for this laser. The characteristic obtained without the use of the algorithm for compensating the relative noise of the intensity (Chart 11) is several orders of magnitude higher than the spectral density of the phase noise presented in the manufacturer's documentation (Chart 13). In this case, the measurement results using the algorithm for compensation of relative intensity noise (Figure 12) coincide with the manufacturer's data. Such a qualitative assessment shows that the processing of interference signals using the presented algorithm leads to a decrease in the error, which allows us to conclude that it is operable. The measurement error is minimized due to the fact that the instantaneous values of the interference signal are reduced in proportion to the instantaneous values of the laser power.

Claims (1)

A phase-noise meter for narrow-band lasers, including: an optical coupler that forms two channels: the first, which records the instantaneous values of the source power, and the second, which records two interference signals; a polarizer in the second channel that generates linearly polarized radiation at the input to the unbalanced Mach-Zehnder fiber interferometer made of a fiber optic dividing coupler (1 × 2) based on fiber with preserving the polarization state at the input of the interferometer, an additional fiber preserving the polarization state, introducing a phase difference, and combining a fiber optic coupler (2 × 1) based on the fiber while maintaining the state of polarization at the output of the interferometer; a polarization beam splitter located after the Mach-Zehnder interferometer separating the two interference signals from orthogonally polarized waves; three optical radiation receivers, two of which are located after the polarization beam splitter in the second channel, and one in the first channel; an analog-to-digital converter, to which signals from all three receivers arrive, and after it a digital signal processing unit for calculating the spectral power density of phase noise by performing functions in the following order: normalization of the interference signal to instantaneous values of the signal proportional to the laser power, with the aim of compensation of relative noise of laser intensity; high-pass filtering to compensate for temperature instability; calculation of phase fluctuations and calculation of the spectral power density of phase noise.

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