RU2425338C2 - Fast-operation laser radiation wavelength meter for fiber optic data transfer systems - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: proposed device comprises laser source with its radiation splitted by light splitter into two beams. Each beam is reflected by appropriate reflector moving along optical axis by means of drive to pass said light splitter and get to photo cell input. Photo cell input is connected to ADC input. ADC output is connected to computer input to display measured data on its monitor. Additionally, interferometer comprises calibration channel incorporating reference laser source, three extra different-length arms and fast switch to connect relevant arm to interferometer.

EFFECT: faster measurement, expanded dynamic range.

5 cl, 4 dwg

Description

The invention relates to measuring equipment in the field of spectrometry and is a high-speed measuring instrument for the wavelength of laser radiation propagating through a fiber waveguide, based on a two-channel Michelson interferometer.

There are many devices known for measuring the wavelength of laser radiation, therefore, it is advisable to indicate the closest analogue described in patent US 005422721 A (publ. 06.06.1995). This patent proposes a Fourier spectrometer (see FIG. 1), dividing the incoming radiation into a plurality of optical pairs and at the same time changing the optical path difference (hereinafter, OPX) between the beams. The Fourier spectrometer is designed to study the spectral composition of non-monochromatic radiation without using a reference channel.

The device operates as follows. The radiation from a point source 1 is converted by a collimating lens 2 into a plane wave, which is divided into two beams by a beam splitter 3. Then, one beam, reflected from a beam splitter 3, falls onto a plane mirror 4, whose position is determined by a piezo drive 8. The second beam, passing through a beam splitter 3, is reflected from a set of flat mirrors 5. Each beam reflected from a flat mirror hits the beam splitter 3, after reflection from which it passes through the corresponding focusing lens 6 and enters the corresponding photodetector (hereinafter - Ф PU) 7, which, in turn, is connected to an analog-to-digital converter 9. The outputs of the analog-to-digital converters are connected to a computer that displays information on screen 11. By taking readings for each pair of beams: a beam reflected from a plane mirror 4, and one of the beams reflected from each of the planar mirrors 5, the spectrometer generates many different interferograms and determines the spectral composition of the incoming radiation, performing the Fourier transform of the total interferogram obtained from the areas corresponding to each th of the pairs to provide spectral resolution, much greater than what could be obtained by treating a single interferogram obtained from a single scan range.

The main disadvantage of this analogue is the low level of output signal intensity, associated with the need to separate the initial beam into many channels, while with a small number of photodetectors, the energy loss is most significant. Figure 2 illustrates the view of the beams in the plane of the FPU in the case of constructing a spectrometer using two photodetectors. We assume that the beam reflected from the mirror 4 is of unit radius. Then we calculate the loss as the ratio of the area of the beam reflected from the mirror 4 to the areas of the beams reflected from the mirrors 5.

where I _loss is the intensity component of the loss;

I ₀ - the intensity of the beam passing through the beam splitter:

S ₄ - the area of the beam reflected from the mirror 4, in the plane of the FPU;

S ₅ - the area of the beam reflected from each of the mirrors 5 in the plane of the FPU.

It can be seen that the losses amount to 50% of the intensity of the beam transmitted through the beam splitter.

Such significant energy losses make it difficult to use a device for measuring the wavelength of low-intensity radiation and for working in fiber-optic systems.

Another significant drawback is associated with the low speed of the device due to the need to scan a large number of points of the interference pattern, which will significantly increase the signal processing time by an analog-to-digital converter and a computing device.

The technical result to which the invention is directed is to increase the speed of measuring the wavelength of laser radiation, as well as to increase the dynamic range of the measured signals.

The specified technical result is achieved due to the fact that the device, made according to the scheme of a two-channel Michelson interferometer, contains the studied and measured laser sources, the radiation of which is divided by a beam splitter into two beams, each of which is reflected from the corresponding reflectors, one of which is biased along the optical axis by means of a drive, the beam splitter passes again and enters the input of the photodetector, the output of which is connected to the input of the analog-to-digital converter The indexer, the output of which is connected to the input of the electronic computer, which displays the measured information on the screen.

In contrast to the analogue under consideration, a calibration channel is added to the interferometer circuit, which contains a reference laser source, as well as three additional arms of different lengths and a fast-acting electro-optical switch that allows connecting the corresponding arm to the interferometer with high speed.

As a result of the serial connection of shoulders of different lengths, the measured wavelength is subsequently refined by more accurately determining the phases of interferograms due to the number of periods of the interference pattern known from the previous measurement that fit into the travel difference introduced by the connected shoulder of a different length during the current measurement.

The invention is further described in more detail using the drawings.

Figure 1 shows the structural diagram of the device is the closest analogue.

Figure 2 shows the configuration of the beams in the plane of the FPU in the case of using two flat mirrors 5.

Figure 3 shows the structural diagram of the proposed device.

Figure 4 shows a view of beams of rays in the plane of the FPU when using four FPU.

Figure 2 conditionally shows a structural diagram of the proposed device. To implement the invention, a frequency-stabilized laser must be used as a reference radiation source 12 in the calibration channel of the interferometer. The radiation from the reference and measured laser is divided into two beams by a beam splitter 14, which is a translucent plate. To create the optimal signal-to-noise ratio, it is advisable to perform a beam splitter with a beam splitting coefficient of 50% / 50%. The resulting beams, reflected from the corresponding reflectors, enter the input of the corresponding FPUs 22 and 23, where they form an interference pattern.

To measure the period of the interference pattern, it is necessary to shift it by a predetermined integer number of periods, and then measure the phase of the shift corresponding to the fractional part of the period. To carry out this measurement, a two-channel interferometer with one stationary arm with a reflector 15 and a set of four connected arms with reflectors 18, 19, 20 and 21 is implemented. The interference pattern is shifted using switch 17 by connecting the arms with different lengths in series to the interferometer, and the measurement of the magnitude of the shift of the pattern corresponding to the fractional part of the period occurs by changing the length of the stationary shoulder using the piezo drive of the reflector 15 installed in this player e.

To measure the wavelength of the measured laser 13, the following operations are performed.

1. The reflector 15 is rebuilt using a piezo drive to a distance DS in steps of DS / k, where k is selected, as a rule, in the range of 10–20 and means the number of points to be selected on one interferogram period. The range of movement of the reflector 1 is selected in accordance with the formula:

where OPD 0 _1-2 is the introduced ORX, λ _max is the upper limit of the operating range, q> 0.5 is a coefficient guaranteeing a shift in the interference pattern by more than one period.

2. After each step of the piezo drive, a synchronous intensity measurement is carried out in the channel of the reference laser 12 and the measured laser 13. The digitized ADC 24 data is recorded in two arrays I1 _et and I2 _ism and fed to the input of the computer 25.

3. The phases of the first and kth points in the channel of the reference laser 12 and the measured laser 13 are calculated. The phases can be determined, for example, using a 5-point algorithm [P. Hariharan, B. F. Oreb and T. Eiju Digital phaseshifting interferometry : a simple error-compensating phase calculation algorithm. Applied optics, Vol. 26, No. 13, 1987]. According to this algorithm, the phase of each Nth point can be calculated by the formula:

where ϕ is the phase of the Nth point;

I _N , I _{N + 1} , I _{N + 2} , I _{N + 3} , I _{N + 4} - intensities of 5 successively following interferogram points.

4. An integer number of periods (wavelengths) is determined in the channel of the reference and measured laser.

5. The wavelength of the measured laser is calculated according to the formula based on a comparison of the periods of interferograms [Optical measurements / A.N. Andreev, E.V. Gavrilov, G. G. Ishanin, etc.: Textbook. - M .: University book; Logos, 2008. - 416 p., P. 353-354]:

where N _et is the integer number of periods in the interferogram of the reference laser;

N _ISM is the integer number of periods in the interferogram of the measured laser;

φ _{em_1m} is the phase of the first point in the interferogram of the reference laser;

φ _{em_km} is the phase of the kth point in the interferogram of the reference laser;

φ _{ISM_1m} - phase of the first point in the interferogram of the measured laser;

φ _{ism_km} is the phase of the k-th point in the interferogram of the measured laser.

6. The maximum possible error in measuring the wavelength when scanning an interferogram of length DS can be estimated using the dependence of the wavelength of the measured laser on a number of parameters, see (4):

Using the formula for the total standard uncertainty [Fundamentals of optical radiometry / under. ed. prof. A.F. Kotyuk. - M .: FIZMATLIT, 2003. - 544 p., P. 36], we obtain the following formula for the wavelength error of the measured laser:

where δφ is the error of phase measurement. Thus, for example, if λ _et = 632 nm (wavelength of a frequency-stabilized He-Ne laser), δφ = 3 ° is a typical error of phase measurement using a five-point algorithm [P. Hariharan, B. F. Oreb and T. Eijn Digital phase-shifting interferometry: a simple error-compensating phase calculation algorithm. Applied optics, Vol. 26, No. 13, 1987], N _et = 1, then the error after scanning the reflector in the DS range will be δλ = 5.2 nm.

The possible initial introduction of the ORF between the reflectors 15 and 18 is set in accordance with the following inequality:

which means that the uncertainty of the wavelength can lead to a change in the number of wavelengths that fit into the ORX by no more than one, i.e. wavelength uncertainty can lead to a change in only a fraction of the interferogram period. From (7), it is possible to approximately estimate the value of ORX if, for example, the lower boundary of the working spectral range is λ _{meas_min} = 1200 nm:

If at the same time the initial ORX was measured with an error of not more than 0.2 · λ _et , then it becomes possible to increase the measurement accuracy by more than 30 times by performing calculations using the following formula:

where mod is the operation of determining the integer part of a number. The corresponding error after clarification can be determined from the following expression:

Thus, if, for example, _λfl = 632 nm, then the error after refinement will be δλ _{meas_m1} = 0.012 nm.

7. After the measurement has been refined, the reflector 15 returns to its original position. Moreover, the return is carried out exactly the same step by step with a step DS / k. After that, the error of returning to the initial position is calculated using the following expression:

where φ _{em_1m_tax} is the phase of the 1st point of the interferogram after returning to its original position, calculated using the five-point algorithm.

8. Then, the reflectors are switched using the switch 17. Now the interferometer is formed using the reflectors 15 and 19. The initial ORF between the reflectors 15 and 19 should be set equal to:

and should be measured with an error of not more than 0.2 · λ _et .

9. Now, to reduce the error in measuring the wavelength, the reflector 15 is rebuilt using a piezoelectric drive to a distance DS with a step DS / k.

After each step of the piezo drive, a synchronous intensity measurement in the channels of the reference and measured lasers is carried out. The data digitized by the analog-to-digital converter 24 is recorded in two arrays I2 _et and I2 _meas .

10. An integer number of periods (wavelengths) is determined in the arrays I2 _et and I2 _rev .

11. The specified wavelength is calculated using the following expression:

After that, the error in measuring the wavelength is estimated using the following expression:

Consecutively carrying out procedures (8) - (11) when connecting the arms with reflectors 20 and 21 and varying the corresponding parameters, it is easy to obtain, at the last fourth stage, when connecting the arm with reflector 21, the error value δλ _{meas_уm4} = 1 pm, which corresponds to the best samples of commercially available meters wavelengths.

We calculate the dynamic range gain in comparison with the analogue. Consider the case when the analog uses four photodetectors and, accordingly, four positions of the reflectors (see figure 4). In this case, the gain in energy of the developed device before the analog will be the ratio of the total beam area, which is fully used in the proposed scheme, to the area of four beams of diameter d used in the analog. Numerically, the gain is:

This means that the proposed device is capable of measuring the wavelength of a laser source of about 1.5 times less power.

We calculate the gain in speed obtained through the use of another method of signal processing. Since the analogue uses continuous recording of the entire interferogram with its breakdown into equal sections, the registration of which is carried out simultaneously, the registration time limited by the ADC will be:

where N _ism is the number of whole periods in the interferogram of the measured laser, which is necessary for measuring the wavelength with a given accuracy, K _FPU = 4 is the number of photodetectors, ƒ _{ADC disk} is the ADC sampling frequency, m is the number of points per interferogram period necessary to identify the period (m > 2 based on the Nyquist criterion).

In relation to the proposed device, the minimum registration time is:

where q is the coefficient guaranteeing the shift of the interference pattern by more than one period, i is the number of consecutive refinements of the measured wavelength (the number of connected arms), NS is the selected number of steps, p is the number of switches within one measurement cycle, t _per is the switching time reflectors. In the case under consideration, we take q = 0.75, i = 4, NS = 20, p = 3. It is not possible to directly compare the maximum achievable time, since there is also a switching time in the calculation of the measurement time of the proposed device, however, a comparison can be made based on the technical characteristics of the devices existing today.

Let the required number of periods N _ism = 300000, m = 3, ƒ _{ADC_disk} = 500 MHz, t _per = 200 ns [Boston Applies Technologies Inc. NanonaTM High Speed & Low Loss Optical Switch, http://www.bostonati.com/products/PI-FOS.pdf] , then the measurement time in the considered analog:

The measurement time in the proposed device:

Thus, the proposed high-speed laser wavelength meter for fiber-optic information transmission systems provides an increase in the speed of measurement of the wavelength of laser radiation, as well as an increase in the dynamic range of the measured signals.

Claims (5)

1. A high-speed laser wavelength meter for fiber-optic information transmission systems, made according to the Michelson interferometer scheme, contains a laser source under study, the radiation of which is divided by a beam splitter into two beams, each of which is reflected from the corresponding reflectors, one of which is made with the possibility displacement along the optical axis using the drive, the beam splitter passes again and enters the input of the photodetector, the output of which is connected to the analog-to-digital input a transducer, the output of which is connected to the input of a computing device that provides measured information on the screen, characterized in that the calibration channel containing a reference laser source is additionally introduced into the interferometer circuit, as well as three additional arms of different lengths and a high-speed switch allowing high speed sequentially connect the corresponding arm to the interferometer. 2. The device according to claim 1, characterized in that the beam splitter is configured to separate the power of the incident radiation with a ratio of 50% / 50%. 3. The device according to any one of claims 1 and 2, characterized in that a frequency-stabilized helium-neon laser is used as a reference laser source. 4. The device according to any one of claims 1 and 2, characterized in that a computer is used as a computing device. 5. The device according to any one of claims 1 and 2, characterized in that an electro-optical switch is used as a high-speed switch.

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