JP6039744B1 - Noise figure measurement method, noise figure measurement apparatus, and measurement system - Google Patents

Abstract

The noise figure of an optical amplifier phase sensitive optical amplifier is measured with high accuracy. A noise figure measurement method according to the present invention includes a relative intensity noise (RINin) of input light measured based on electric power obtained by photoelectric conversion of input light inputted to an optical amplifier phase sensitive optical amplifier, and Based on the relative intensity noise (RINtotal) of the amplified light measured based on the electric power obtained by photoelectrically converting the amplified light amplified by the optical amplifier phase sensitive optical amplifier, A noise figure (NF) of an optical amplifier phase sensitive optical amplifier including noise based on emitted light and excess noise based on factors other than spontaneously emitted light is calculated. [Selection] Figure 1

Description

  The present invention relates to a noise figure measurement method, a noise figure measurement apparatus, and a measurement system for measuring a noise figure of an optical amplifier, for example, for measuring a noise figure of a phase sensitive optical amplifier (PSA). The present invention relates to a noise figure measurement method.

  Conventionally, an optical receiver used in an optical communication system needs to detect an optical signal attenuated by propagating through an optical fiber with high light receiving sensitivity. Therefore, the optical receiver generally receives detection light amplified using a laser amplifier by a photodetector (hereinafter also referred to as “PD”) and converts it into an electrical signal, and the converted electrical signal is converted into an electrical signal. A configuration in which the electric signal (digital signal) is identified by a discriminator after being amplified by an electric amplifier as necessary.

  As the laser amplifier, for example, a fiber laser amplifier or a semiconductor laser amplifier that amplifies signal light by making excitation light incident on an optical fiber to which a rare earth element such as erbium or praseodymium is added is used. In general, since fiber laser amplifiers and semiconductor laser amplifiers amplify signal light as it is, there is no limitation on electrical processing speed. Therefore, in a long-distance optical transmission system that is required to detect a very high-speed modulated signal with high sensitivity, an optical receiver having the above-described configuration is often used. The laser amplifier is also widely used as a linear repeater in an optical communication system because of its relatively simple device configuration.

  However, in these laser amplifiers, unavoidably and randomly generated spontaneous emission light is mixed regardless of the signal component, so that the SN ratio of the signal light is reduced by at least 3 dB before and after amplification. This decrease in the S / N ratio of the signal light is a factor that limits the S / N ratio in the optical receiver and limits the reception sensitivity. In addition, when a laser amplifier is used as a linear repeater, it is a factor that degrades transmission quality.

  In recent years, research on a phase sensitive amplifier (hereinafter referred to as “PSA”) has been advanced as a means to overcome the limitations of such a laser amplifier. When the signal light and the excitation light are incident on the phase-sensitive light amplification unit (optical parametric amplification mechanism) and the phases of the two coincide with each other, the PSA amplifies the input signal light, and the two phases are shifted by 90 degrees. The input signal light is attenuated. Using this characteristic, the phase of the excitation light and the signal light are matched so that the amplification gain is maximized, and the light is incident on the phase-sensitive light amplification unit to minimize the spontaneous emission light of the signal light and quadrature phase. It is possible to suppress and amplify the signal light without substantially degrading the SN ratio.

  By the way, as one of the important indexes for evaluating the performance of PSA, there is a noise figure (hereinafter also referred to as “NF”) indicating the degree of deterioration of the gain of the amplifier and the SN ratio of the input / output of the amplifier. In optical communication, noise degrades the SNR of a transmission signal and decreases transmission capacity. Therefore, it is important to measure NF with high accuracy in evaluating the performance of PSA.

  In particular, the PSA is an amplifier that is extremely low noise in principle as compared with the conventional laser amplifier, and since the NF is very small, the measurement error greatly affects the noise figure. Therefore, it is extremely important to accurately measure the noise figure in evaluating the performance of the PSA, and a highly accurate noise figure measurement method is desired.

  Further, when the optical amplifier is actually used, in addition to the spontaneous emission light noise that is generated in principle in the optical amplifier, noise and the like due to the above components are generated. For example, excessive noise such as noise due to multipath interference (MPI), noise due to excitation light, and noise due to return light is added. In particular, in addition to an optical parametric amplification (OPA) mechanism that actually performs amplification, the PSA generates a phase synchronization mechanism for synchronizing the phase of signal light and pumping light and high-power pumping light. It has a feature that there are many components compared to the conventional laser amplifier. For this reason, the influence of excess noise generated by components other than the OPA mechanism is greater than that of a conventional laser optical amplifier. Since these excessive noises become noise in the RF baseband frequency region, that is, the signal frequency band after photoelectric conversion, in order to degrade the signal waveform after reception, all the noise of the optical amplifier including these excessive noises is reduced. It is practically important to measure.

As methods for measuring the noise figure of an optical amplifier, the following methods are conventionally known.
For example, as a noise figure measurement method of a laser optical amplifier, an electrical spectrum in the RF band after photoelectric conversion of input light and amplified light of the laser optical amplifier is measured using an electrical spectrum analyzer (ESA), and the measured value is used. The ESA method for calculating the noise figure, and the optical spectrum of the input light and the amplified light of the laser optical amplifier are measured using an optical spectrum analyzer (OSA), and the amplified spontaneous emission (Amplified Spontaneous Emission: ASEASE) level of the amplified light is measured. The OSA method for calculating the noise figure by measuring the gain is known (for example, see Non-Patent Document 1).

  In the OSA method, which is a measurement method in the optical frequency domain, it is extremely difficult to observe the influence of excess noise in the RF baseband frequency domain as a change in the optical spectrum, and therefore, excess noise cannot be accurately evaluated. There is a problem. On the other hand, the ESA method is an accurate measurement method that can measure the noise figure including the above-described influence of excessive noise that cannot be evaluated by the OSA method.

  However, in the conventional laser amplifier, since the occurrence of excessive noise is negligible, the OSA method that can easily evaluate the noise figure is generally employed. For example, the OSA method is standardized as a noise figure measurement method of an optical fiber amplifier that is a typical laser amplifier for optical communication (IEC-61291).

  For this reason, conventionally, in the PSA as well as the laser amplifier, the noise figure is measured by the OSA method. However, as described above, the PSA has many components such as a phase synchronization mechanism and a mechanism for generating high-output pumping light. Therefore, the PSA is used for the influence of MPI and pumping light generation as compared with the conventional laser amplifier. The influence of noise of an erbium-doped optical fiber amplifier (EDFA) is large. That is, PSA is more affected by excess noise than noise caused by spontaneous emission light, compared to conventional laser amplifiers. Therefore, with the general OSA method as a noise figure evaluation method of a laser amplifier, the above excessive noise in the PSA cannot be accurately evaluated.

  In recent years, in order to avoid the problem related to the noise evaluation of PSA, research for evaluating the total noise of the amplifier including the excess noise other than the noise caused by the spontaneous emission light of the PSA has been advanced. For example, Non-Patent Document 2 uses a reference optical fiber amplifier whose noise level and noise figure on the RF spectrum after photoelectric conversion are known, and the noise level of the reference optical fiber amplifier. A method for relatively evaluating the noise of PSA by comparing the noise level of PSA with PSA is disclosed.

D.M.Baney, P. Gallion, and R.S.Tucker, “Theory and measurement techniques for the noise figure of optical amplifiers,” Optical fiber technology, vol.6, no.2, pp.122-154, 2000. M. Asobe, T. Umeki, and O. Tadanaga, “Phase sensitive amplification with noise figure below the 3 dB quantum limit using CW pumped PPLN waveguide,” Optics Express 20, 13164-13172, 2012.

  According to the method of evaluating the noise figure using the reference optical fiber amplifier disclosed in Non-Patent Document 2 described above, the RF base is obtained by photoelectrically converting the amplified light and measuring the noise level on the electrical spectrum. It is possible to evaluate the noise including the excess noise of the amplifier in the band frequency domain.

  However, in the method disclosed in Non-Patent Document 2, the noise figure of the reference optical fiber amplifier uses the value determined by the OSA method described above, and it cannot be said that the excessive noise is accurately included. . That is, the method disclosed in Non-Patent Document 2 is a mixed measurement method of the ESA method and the OSA method, and it cannot be said to be a complete electrical evaluation, and there is a problem in accuracy as a method of measuring the noise figure of PSA. .

  Further, PSA and conventional laser amplifiers are completely different in the principle of generation of noise generated by the amplifier, and the amount of noise caused by spontaneous emission is also greatly different. Therefore, conventional ESA that has been adopted as a noise figure measurement method for laser amplifiers. The method cannot be adopted as it is as a PSA noise figure measurement method.

  The present invention has been made in view of the above problems, and an object of the present invention is to measure the noise figure of a phase-sensitive optical amplifier with high accuracy.

The noise figure measurement method according to the present invention includes a relative intensity noise (RINin) of input light measured based on power obtained by photoelectric conversion of input light input to the phase sensitive optical amplifier (2), and phase sensitive. Based on the relative intensity noise (RINtotal) of the amplified light measured based on the electric power obtained by photoelectrically converting the amplified light amplified by the optical amplifier, the noise and natural noise based on the spontaneous emission light in the phase sensitive optical amplifier A noise figure measurement method for calculating a noise figure (NF) of a phase sensitive optical amplifier including excess noise based on factors other than emitted light, the first step of obtaining a measured value of relative intensity noise of input light (S1) ), A second step (S2) for obtaining a measured value of the relative intensity noise of the amplified light, a third step (S3) for obtaining a measured value of the average optical power of the input light, and the gain of the phase sensitive optical amplifier Take measurements The obtained fourth step (S4), the measured value of the relative intensity noise of the input light obtained in the first step, the measured value of the relative intensity noise of the amplified light obtained in the second step, and the input obtained in the third step the measured value of the average optical power of the light, based on the measured value of the gain of the phase sensitive optical amplifier obtained in the fourth step, a fifth step (S5) and the free Mukoto to calculate the noise figure (NF) Features.

The noise figure measurement method according to the present invention includes a relative intensity noise (RINin) of input light measured based on power obtained by photoelectric conversion of input light input to the phase sensitive optical amplifier (2), and phase sensitive. Based on the relative intensity noise (RINtotal) of the amplified light measured based on the electric power obtained by photoelectrically converting the amplified light amplified by the optical amplifier, the noise and natural noise based on the spontaneous emission light in the phase sensitive optical amplifier A noise figure measurement method for calculating a noise figure (NF) of a phase sensitive optical amplifier including excess noise based on factors other than emitted light, the first step of obtaining a measured value of relative intensity noise of input light (S1) ), A second step (S2) for obtaining a measured value of the relative intensity noise of the amplified light, a third step (S3) for obtaining a measured value of the average optical power of the input light, and the input obtained in the first step Light relative A noise factor is calculated based on the measurement value of the intensity noise, the measurement value of the relative intensity noise of the amplified light acquired in the second step, and the measurement value of the average optical power of the input light acquired in the third step. 4 and a step (S5), characterized in including Mukoto.

  The noise figure measurement apparatus (10) according to the present invention measures the relative intensity noise of the input light measured based on the electric power obtained by photoelectrically converting the input light input to the phase sensitive optical amplifier (2). A first relative intensity noise acquisition unit (104) for acquiring a value, and a measured value of the relative intensity noise of the amplified light measured based on the electric power obtained by photoelectrically converting the amplified light amplified by the phase sensitive optical amplifier A second relative intensity noise acquisition unit (105) for acquiring a gain, a gain acquisition unit (102, 103, 107) for acquiring a measured value of the gain of the phase sensitive optical amplifier, and a measured value of the average optical power of the input light Input light power acquisition unit (103) that performs measurement of the relative intensity noise of the input light acquired by the first relative intensity noise acquisition unit, and measurement of the relative intensity noise of the amplified light acquired by the second relative intensity noise acquisition unit Value and phase acquired by the gain acquisition unit Based on the measured value of gain of the optical amplifier and the measured value of the average optical power of the input light acquired by the input optical power acquisition unit, noise based on spontaneous emission in the phase sensitive optical amplifier and factors other than the spontaneous emission And a noise figure calculation unit (108) for calculating a noise figure including excess noise based on the above.

  The noise figure measurement apparatus (10) according to the present invention provides a measured value of relative intensity noise of input light measured based on electric power obtained by photoelectric conversion of input light input to the phase sensitive optical amplifier (1). Acquires a measured value of the relative intensity noise of the amplified light measured based on the electric power obtained by photoelectrically converting the amplified light amplified by the first relative intensity noise acquiring unit (104) and the phase sensitive optical amplifier. The second relative intensity noise acquisition unit (105), the input light power acquisition unit (103) that acquires a measurement value of the average optical power of the input light, and the relative intensity of the input light acquired by the first relative intensity noise acquisition unit Based on the measured value of the noise, the measured value of the relative intensity noise of the amplified light acquired by the second relative intensity noise acquisition unit, and the measured value of the average optical power of the input light acquired by the input optical power acquisition unit, the phase Based on spontaneous emission in a sensitive optical amplifier. Characterized in that it has noise factor calculation unit that calculates a noise figure containing excessive noise based on factors other than noise and spontaneous emission light and (108).

  A measurement system (50) according to the present invention includes a light source (1) that emits light, an optical power measurement device (7) that measures the average optical power of light emitted from the light source, and a phase sensitive optical amplifier (2). A photodetector (4) for converting the received light into an electrical signal, a relative intensity noise measuring device (5) for measuring relative intensity noise based on the electrical signal converted by the photodetector, and an output from the light source The light that is input to the phase sensitive optical amplifier and the light that is output from the phase sensitive optical amplifier is input to the photodetector, or the light that is output from the light source is not input to the phase sensitive optical amplifier. The above-mentioned noise figure measurement in which the optical switch (8, 9) that forms a path to be input to the optical fiber, the average optical power measured by the optical power measuring instrument, and the relative intensity noise measured by the relative intensity noise measuring instrument are input. Device (10) and Characterized in that it comprises.

  In the above description, as an example, constituent elements on the drawing corresponding to the constituent elements of the invention are represented by reference numerals with parentheses.

  According to the present invention, the noise figure of a phase sensitive optical amplifier can be measured with high accuracy.

FIG. 1 is a diagram showing a procedure of a noise figure measurement method according to an embodiment of the present invention. FIG. 2 is a diagram showing an outline of a configuration of a measurement system for realizing the noise figure measurement method according to the embodiment of the present invention. FIG. 3 is a diagram showing an actual evaluation result of the noise figure measured by the noise figure measurement method according to the embodiment of the present invention. FIG. 4 is a diagram showing a specific configuration of a measurement system including a noise figure measurement device according to an embodiment of the present invention. FIG. 5 is a diagram showing an internal configuration of the noise figure measurement apparatus according to the embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

≪Principle of noise figure measurement method according to this embodiment≫
The principle of an optical amplifier noise figure measurement method according to an embodiment of the present invention will be described.
In the following description, it is assumed that the optical amplifier is a phase sensitive amplifier (PSA).

First, assuming that the phase of the signal light input to the PSA and the phase of the excitation light for amplification are perfectly synchronized, the average number of photons of the amplified light when coherent light is incident on the PSA as the signal light n _psa and photon number dispersion σ ² _psa are expressed by equations (1) and (2).
Here, g is the gain of the PSA, n _in is the average number of input photons of the signal light input to the PSA, and gn _in is the average number of photons of the amplified light. Further, n _{asa_psa} is the average number of photons of spontaneous emission in the PSA, and can be expressed by the equation (3) when the gain of the PSA is g. Σ ² _{sig_ase} represents the beat noise component between the signal (input light) and the ASE of the PSA, and σ ² _{ase_ase} represents the beat noise component between the ASE of the PSA.

In addition, when the band of the parametric spontaneous emission light is cut by the optical filter and the band of the optical filter is Δνopt, σ ² _{sig_ase} and σ ² _{ase_ase} are respectively expressed by Expression (4) and Expression (5). Can do.

In general, since the signal light output from the signal light source includes intensity noise, the signal light is in an excessive noise state deviated from the coherent state.
Therefore, in the following, it is assumed that intensity noise due to spontaneously emitted light from the laser is mixed in the signal light source. In this case, the photon number dispersion σ ² _rin of the input light input to the PSA is expressed by Expression (6). Here, β is an excess noise figure of the input light and represents a deviation from the coherent state of the signal light source.

  Incidentally, relative intensity noise (RIN) representing temporal fluctuation of the intensity of laser light is a ratio of dispersion of optical power and average optical power, and is represented by Expression (7). Here, the denominator of (Expression 7) represents the average optical power, and the numerator represents the dispersion of the optical power.

  Further, the relative intensity noise RIN has a relationship represented by the equation (8) between the photon number dispersion σ and the average photon number n.

When signal light including noise with an excess noise figure β is amplified by PSA, noise contained in the amplified light, that is, photon number dispersion becomes (σ ² _psa + σ ² _arin ). Here, σ ² _arin is expressed by Expression (9).

Further, as described above, in practice, excessive noise is added as noise of PSA in addition to noise caused by parametric spontaneous emission in these PSA and noise caused by intensity noise of signal light. When the dispersion of the number of output photons due to this excessive noise is σ ² _ex , the total number of photon dispersion after amplification is expressed by Expression (10). Here, σ ² _arin represents a noise component generated as a result of amplification of excess noise of the signal light source. Also, σ ² _rin is the photon number dispersion (= shot noise + excess noise component) of the signal light source, and if the excess noise index β = 0, it becomes coherent light with only shot noise. Also, σ ² _ex represents all excess noise components (for example, noise such as MPI, return light, excitation light, residual excitation light shot noise).

As understood from the equation (10), the noise figure NF determined by the noise component (σ ² _psa + σ ² _ex ) including excessive noise, which is difficult to accurately measure by the conventional OSA method, is quantified. (Σ ² _{total −σ} ² _arin ) may be calculated for _efficient evaluation.
Therefore, in the following, the noise figure NF caused by (σ ² _{total −σ} ² _arin ) is _expressed as a measurable parameter, that is, the relative intensity noise RIN _{in of the} signal light input to the PSA, and the amplified light output from the PSA. It is assumed that the relative intensity noise RIN _total , PSA gain g, average number of input photons n _{in and the} like are used.

  First, the relative intensity noise RIN is defined by the power after being received by the photodetector (PD). That is, when the signal power of the input light detected by the PD is ps and the noise power of the input light detected by the PD is pn, Expression (8) can be rewritten as Expression (11).

Further, pn and ps can be expressed by Expression (12) and Expression (13), respectively. Here, B is the baseband band of the receiving system, RL is the load resistance, and e is the elementary charge.

Next, consider the relative intensity noise RINin of the input light of the PSA.
When the signal power of the input light when the input light of the PSA is received by the PD is ps _in and the noise power of the input light is pn _in , the relative intensity noise RIN _in of the input light of the PSA is obtained from the above equation (11). Can be represented by formula (14). Further, pn _in and ps _in can be expressed by Expression (15) and Expression (16), respectively. Here, n _sse is the average number of photons of spontaneous emission in the signal light source.

Next, the relative intensity noise RINtotal of the amplified light by PSA is considered.
When the amplified light signal power when the amplified light by the PSA is received by the PD is ps _total and the noise power of the input light is pn _total , the relative intensity noise RIN _total of the amplified light of the PSA is obtained from the above equation (11). Can be expressed by Equation (17). Moreover, pn _total and ps _total can be represented by the formula (18) and the formula (19), respectively.

Next, consider the signal-to-noise ratio.
The input optical signal-to-noise ratio of the PSA, i.e., signal-to-noise ratio snr _in the signal light source is represented by the formula (20).

On the other hand, after amplification by PSA, since the total photon number dispersion is expressed by Equation (10), the signal-to-noise ratio snr _{psa +} of the amplified light by PSA when only dispersion due to PSA noise including excess noise is considered. _ex can be expressed by Equation (21) using (σ ² _psa + σ ² _ex = σ _{total −σ} ² _arin ). Here, pn _{psa + ex} is the noise power of the amplified light by the PSA including excess noise, and ps _{psa + ex} is the signal power of the amplified light by the PSA, and are expressed by the equations (22) and (23), respectively. be able to.

Therefore, the noise figure f _{psa + ex} of the PSA including excess noise can be expressed by Expression (24). Here, snr _in is a signal-to-noise ratio when the input light of the PSA is coherent light, and since the photon number dispersion is equal to the average photon number, snr _in = n _in / (2 · B).

From the above relationship, the noise figure f _{psa + ex} of the PSA including excess noise is expressed by Expression (25) using the relative intensity noise RIN _in of the input light of the PSA and the relative intensity noise RIN _{total of the} amplified light. be able to.

Here, when the spontaneous emission light of the signal light source is sufficiently small, or when the spontaneous emission light of the signal light source is sufficiently suppressed by the optical filter and inputted to the PSA, the noise figure f _{psa + ex of the} PSA including excessive noise. Can be expressed by a simplified expression (26).

Further, when the spontaneous emission of the signal light source described above is sufficiently small (or the spontaneous emission is suppressed) and the gain g of the PSA is sufficiently large, the noise figure f _{psa + ex of the} PSA including excessive noise is _obtained. Can be expressed by equation (27), which is a further simplification of equation (26).

As understood from the equations (26) and (27), the noise figure of the PSA including excess noise is the relative intensity noise of the signal light output from the signal light source and the relative intensity noise of the amplified light by the PSA. It can be calculated by measuring the input optical power and the gain of the PSA. That is, Equation (26) and Equation (27) indicate that the noise figure of PSA including excess noise can be evaluated electrically.
Hereinafter, a PSA noise figure measurement method based on the above principle will be described in detail.

<< Specific example of noise figure measurement method according to this embodiment >>
FIG. 1 is a diagram showing a procedure of a noise figure measurement method according to an embodiment of the present invention.
FIG. 2 is a diagram showing an outline of a configuration of a measurement system for realizing the noise figure measurement method according to the embodiment of the present invention.

  As shown in FIG. 1, in the noise figure measurement method according to the embodiment of the present invention, the step S1 of measuring the relative intensity noise RINin of the input light to be input to the PSA and the amplified light output from the PSA are measured. It comprises step S2 for measuring the relative intensity noise RINtotal, step S3 for measuring the average optical power of the input light, step S4 for measuring the gain of the PSA, and step S5 for calculating the noise figure of the PSA.

(1) Step S1
In step S1, the relative intensity noise RIN _in of the input light to be input to the PSA is measured. Specifically, as shown in FIG. 3, an optical filter 3 for removing beat noise between ASEs generated from the PSA 2 without passing through the PSA 2 from the light output from the external light source 1 as the input light of the PSA 2 is provided. The light receiving current of the input light generated by being incident on the light detector (photodetector) 4 and received by the light detector 4 is input to the RIN measuring device 5, whereby the relative intensity noise RIN _{in of the} input light is input. Measure. At this time, the light output from the light source 1 may be incident on the optical filter 3 and the photodetector 4 via the optical filter 0, or the spontaneous emission light included in the light output from the light source 1 is sufficient. If it is small, the light output from the light source 1 may enter the optical filter 3 and the photodetector 4 without passing through the optical filter 0.

  Here, the light source 1 is, for example, a resonator laser light source that continuously outputs laser light, and the input light is, for example, CW (Continuous wave) light. The RIN measuring device 5 is a known measuring device capable of measuring the relative intensity noise of light based on the electric power obtained by photoelectric conversion by the photodetector 4. For example, the RIN measuring device 5 calculates the relative intensity noise by converting the light intensity noise power density into the noise current density and converting the average light power into the average received light current. The RIN measurement device 5 separates thermal noise in the photodetector 4 and noise (in-device noise) generated by an electric amplifier provided in the RIN measurement device 5 and only noise included in the input light. It is desirable to have a correctly calibrated device that can measure accurately.

(2) Step S2
In step S2, the relative intensity noise RIN _total of the amplified light output from the PSA is measured. Specifically, as shown in FIG. 3, the light output from the light source 1 is input to the PSA 2, and the amplified light output from the PSA 2 is incident on the photodetector 4 through the optical filter 3. Then, the light receiving current of the amplified light generated by receiving light by the photodetector 4 is input to the RIN measuring device 5 to measure the relative intensity noise RIN _in of the amplified light.

  Here, PSA2 is a phase sensitive optical amplifier using periodically poled lithium niobate (PPLN). Here, it is assumed that the optical filter 3 is an optical bandpass filter having a bandwidth of 2 nm.

(3) Step S3
In step S3, the average optical power of the input light of PSA2 is measured. Specifically, the average optical power of the input light of the PSA 2 output from the light source 1 is measured by using, for example, a known optical power meter. Thereby, the input average photon number n _in of the input light can be easily calculated using the measured value of the average light power of the input light, the frequency of the input light, and the Planck constant. An optical device with an optical power meter function, such as an optical spectrum analyzer, may also be used.

(4) Step S4
In step S4, the PSA gain is measured. Specifically, for example, the gain of PSA2 is calculated based on the measured value of the optical spectrum of the input light of PSA2 measured by using a known optical spectrum analyzer and the measured value of the optical spectrum of the amplified light by PSA2. .

(5) Step S5
In step S5, the noise figure of PSA is measured using the measured values in steps S1 to S4. Specifically, the measured value of the relative intensity noise RIN _in of the input light measured in step S1, the measured value of the relative intensity noise RIN _total of the amplified light by PSA2 measured in step S2, and the input light measured in step S3. The noise figure of PSA2 is calculated based on the input average photon number n _{in of} the input light calculated from the measured value of the average optical power and the measured value of the gain g of PSA2 measured in step S4.

More specifically, based on the measured value of the relative intensity noise RIN _in , the measured value of the relative intensity noise RIN _total , the calculated average number of input photons n _{in of} the input light, and the measured value of the gain g described above. The noise figure of PSA2 is calculated by solving equation (26).

Alternatively, when the gain g of the PSA 2 is sufficiently large, based on the measured value of the relative intensity noise RIN _in , the measured value of the relative intensity noise RIN _total , and the calculated average number of input photons n _in of the input light, The noise figure of PSA2 is calculated by solving the above equation (27). In this case, step S4 need not be performed.

Next, actual evaluation results obtained by the above-described noise figure measurement method are shown below.
Here, an external resonator laser light source is used as the light source 1, and the noise figure of PSA2 when the CW light output from the light source is amplified by PSA2 using periodically poled lithium niobate (PPLN) is commercially available. Evaluation was performed using the RIN evaluation apparatus 5. As the optical filter 3, an optical bandpass filter having a bandwidth of 2 nm was used.

First, as step S 1, the light output from the light source 1 is irradiated onto the photodetector 4 through the optical filter 3, thereby measuring the relative intensity noise RIN _in of the input light with the RIN measuring device 5. At this time, as step S3, the average optical power of the input light output from the light source 1 was set to 0 dBm, and the input average photon number n _in of the input light was set to “7.8 × 10 ¹⁵ ”. As a result of the above measurement, the relative intensity noise RIN _in of the input light was “−159.8 dB / Hz” at a frequency of 15 GHz.

Next, as step S2, the light output from the light source 1 is input to the PSA 2, and the amplified light output from the PSA 2 is irradiated to the photodetector 4 through the optical filter 3, thereby causing the relative intensity noise of the amplified light. RIN _total was measured with the RIN measuring device 5. At this time, the gain of PSA2 was set to 20 dB as step S5. As a result of the above measurement, the relative intensity noise RIN _total of the amplified light was “−157.5 dB / Hz” at a frequency of 15 GHz.

Next, the noise figure (NF) calculated by substituting the measured value of the relative intensity noise RIN _in of the input light and the intensity noise RIN _total of the amplified light obtained in the above measurement into the above (Equation 27) is , About 1.96 dB.

Further, when the frequencies were 5 GHz and 0.2 GHz, the relative intensity noise RIN _in of the input light and the intensity noise RIN _total of the amplified light were measured in the same manner as described above, and the noise index was calculated. Thus, the relative intensity noise RIN _total of the amplified light at frequencies of 0.2 GHz (200 MHz) and 5 GHz is “−156.8 dB / Hz” and “−157.3 dB / Hz”, respectively, and the noise figure is respectively “2.58 dB” and “2.14 dB”.

FIG. 2 is a diagram showing an actual evaluation result by the above-described noise figure measurement method.
FIG. 2 shows the frequency dependence 201 of the relative intensity noise RIN _total of the amplified light obtained by the above evaluation, and the frequency dependence 202 of the noise figure of PSA2 obtained by the above evaluation.
FIG. 2 shows a frequency dependence 203 of the noise figure when a PSA 2 having the same configuration is measured by a conventional optical evaluation method, as a comparative example. The frequency dependence 203 of the noise figure by this optical evaluation method as a comparative example is the gain of PSA2, the spontaneous emission light level in the optical spectrum after amplification, and the theory of noise figure by the OSA method obtained by the equation (28). The value of the noise figure in the OSA method is calculated using the equation.

  Here, the first term on the right side of Equation (28) is a shot noise term, the second term on the right side of Equation (28) is the beat noise term of the amplified signal-ASE, and Pase is the ASE level after amplification, h is the Planck constant, ν is the optical frequency, and Δν is the resolution of the OSA. As a result of the calculation by the above equation (28), the noise figure (NF) of PSA2 by the OSA method as a comparative example was about 1.9 dB.

  As shown in FIG. 2, the noise figure of PSA2 by the noise figure measurement method according to the present embodiment approaches the noise figure of PSA2 by the OSA method in a high frequency region, and that of PSA2 by the OSA method in a low frequency region. Deviation from noise figure. From the characteristics of the frequency dependence of the noise figure of PSA2, it is understood that the cause of the divergence is due to the influence of multipath interference noise caused by internal reflection in the EDFA used to generate the pump light of PSA2. . That is, the evaluation result of the noise figure of PSA2 by the noise figure measurement method according to the present embodiment is unique to PSA generated from components such as excitation light noise that could not be accurately evaluated by the conventional OSA method. It is understood that the influence of excessive noise is reflected.

<< Effects of noise figure measurement method according to this embodiment >>
As described above, according to the noise figure measurement method according to the present embodiment, the relative intensity noise of the input light and the relative intensity noise of the amplified light are measured, and the PSA is calculated based on the above formula (26) or formula (27). The noise figure (NF) is measured accurately, including the effects of excessive noise such as MPI noise generated from components important for PSA noise evaluation and noise mixed from the laser amplifier used to generate pump light. Noise figure can be measured. That is, according to the noise figure measurement method according to the present embodiment, the noise figure can be evaluated by all-electric measurement, so that the measurement accuracy is higher than that of the conventional measurement method in which the ESA method and the OSA method are mixed. Can be improved.

  In particular, when the PSA gain is sufficiently large, the noise figure can be calculated based on the above equation (27), so that the number of measurement items is further reduced and the measurement is further facilitated.

<< Noise figure measuring device according to this embodiment >>
Next, a noise figure measurement apparatus for realizing the noise figure measurement method according to the present embodiment will be described.
FIG. 4 is a diagram showing a specific configuration of a measurement system including a noise figure measurement device according to an embodiment of the present invention. In the figure, the same components as those in FIG. 3 described above are denoted by the same reference numerals, and detailed description thereof is omitted.

  A measurement system 500A shown in FIG. 4 includes a light source 1, a noise evaluation target PSA2, an optical filter 3, a photodetector 4, a RIN measuring device 5, an optical spectrum analyzer (with optical power measurement function) 7, and optical switches 8 and 9. , A noise figure measuring device 10 and a monitor 11.

  Light output from the light source 1 (input light of the PSA 2) is input to the optical switch 8. The optical spectrum analyzer 7 inputs the input light output from the light source 1 through the optical switches 8 and 9, measures the average optical power of the input light, and measures the average optical power (electrical data) 24. Is supplied to the noise figure measurement apparatus 10. Here, in the measurement of the average optical power of the input light, the optical loss in the optical switches 8 and 9 is measured in advance, and the average optical power of the correct input light is calculated by correcting using the loss information. .

  The optical switches 8 and 9 are components for switching the light propagation direction. As shown in FIG. 4, for example, the optical switch 8 is provided between the light source 1 and the PSA 2, and the optical switch 9 is provided between the PSA 2 and the optical filter 3. Thereby, the light output from the light source 1 (input light of the PSA 2) is input to the PSA 2 and the amplified light output from the PSA 2 is input to the photodetector 4, and the light output from the light source 1 is input to the PSA 2. A path to be input to the photodetector 4 without being input, a path to input the light output from the light source 1 to the PSA 2, a path to input the amplified light output from the PSA 2 to the optical spectrum analyzer 7, and an output from the light source 1 It is possible to form any one of the paths for inputting the processed light to the optical spectrum analyzer 7 without inputting the light to the PSA 2. Switching of the optical switches 8 and 9 is controlled by, for example, a control signal from a noise figure measurement device 10 described later.

  The noise figure measurement device 10 includes the measurement result 24 of the average optical power of the input light measured by the optical spectrum analyzer 7, the optical spectrum 23 of the input light, the optical spectrum 25 of the amplified light, and the input measured by the RIN measurement device 5. The measurement result 21 of the relative intensity noise RINin of the light and the measurement result 22 of the relative intensity noise RINtotal of the amplified light measured by the RIN measuring device 5 are input, and the noise figure of the PSA 2 is calculated based on the input measurement value. It is a device for measuring.

  The noise figure measurement device 10 is realized by a computer that is a hardware resource and a program installed in the computer. For example, the computer includes a program processing device such as a CPU, a storage device such as a RAM (Random Access Memory), ROM, and HDD (Hard Disc Drive), a keyboard, a mouse, a pointing device, operation buttons, a touch panel, and the like. An input device for inputting information from the outside, and a communication device for performing transmission and reception of various information via a communication line such as the Internet, LAN (Local Area Network), WAN (Wide Area Network), etc. PC etc. provided.

  The noise figure measurement device 10 is connected to a display device 11 such as an LCD (Liquid Crystal Display), for example, and can display various measurement results on the display device 11. The noise figure measurement device 10 is connected to a network (not shown) such as a LAN, for example, and can output various measurement results to an external device via the network.

FIG. 5 is a diagram illustrating an internal configuration of the noise figure measurement apparatus 10.
As shown in FIG. 5, the noise figure measurement apparatus 10 includes a control unit 100, an input light spectrum acquisition unit 101, an amplified light spectrum acquisition unit 102, a first relative intensity noise acquisition unit 104, and a second relative intensity noise acquisition unit 105. A storage unit 106, a gain calculation unit 107, and a noise figure calculation unit 108.

  Control unit 100, input light spectrum acquisition unit 101, amplified light spectrum acquisition unit 102, input light power acquisition unit 103, first relative intensity noise acquisition unit 104, second relative intensity noise acquisition unit 105, storage unit 106, gain calculation unit 107 and the noise figure calculation unit 108 are functional units realized, for example, when the hardware resource (CPU or the like) executes processing according to a program.

  The control unit 100 is a functional unit that controls each component constituting the measurement system 500 such as the optical switches 8 and 9 and the light source 1. For example, the control unit 100 controls the light output from the light source 1, the power of the output light, and the like by controlling the light source 1. Further, the control unit 100 switches the propagation path of the light output from the light source 1 by controlling the optical switches 8 and 9.

  The amplified light spectrum acquisition unit 102 is a functional unit for acquiring a measured value of the optical spectrum of the amplified light by the PSA 2. Specifically, the amplified light spectrum acquisition unit 102 acquires the optical spectrum measurement result 23 of the amplified light by the PSA 2 measured by the optical spectrum analyzer 7 having the optical power measurement function.

  The input light spectrum acquisition unit 101 is a functional unit for acquiring a measured value of the optical spectrum of the PS2 input light. Specifically, the input light spectrum acquisition unit 101 acquires the measurement result 25 of the optical spectrum of the input light measured by the optical spectrum analyzer 7 having a function of measuring optical power.

  The input optical power acquisition unit 103 is a functional unit for acquiring a measured value of optical power of PS2 input light. Specifically, the input optical power acquisition unit 103 acquires the measurement result 24 of the average optical power of the input light measured by the optical spectrum analyzer 7 having the optical power measurement function used for the input optical spectrum acquisition unit.

  The first relative noise acquisition unit 104 is a functional unit that acquires the measurement result 21 of the relative intensity noise RINin of the input light measured by the RIN measurement device 5. The second relative noise acquisition unit 105 is a functional unit that acquires the measurement result 22 of the relative intensity noise RINtotal of the amplified light measured by the RIN measurement device 5.

  The gain calculation unit 107 calculates the gain of the PSA 2 based on the measurement result 23 of the amplified light spectrum acquired by the amplified light spectrum acquisition unit 102 and the measurement result 24 of the spectrum of the input light acquired by the input light spectrum acquisition unit 101. Is a functional unit for calculating The gain calculation unit 107 calculates the gain (amplification factor) g of the PSA 2 by dividing the spectrum of the amplified light by the spectrum of the input light, for example.

  The storage unit 106 is a functional unit that stores various data for calculating the noise figure. In the storage unit 106, for example, the first arithmetic expression data 1061 corresponding to the above-described expression (26), the second arithmetic expression data 1062 corresponding to the above-described expression (27), and the noise figure are calculated. Other parameters are stored.

  The noise figure calculation unit 108 measures the measurement result 21 of the relative intensity noise RINin of the input light acquired by the first relative noise acquisition unit 104 and the measurement result of the relative intensity noise RINtotal of the amplified light acquired by the second relative noise acquisition unit 105. 22, the input average photon number nin of the input light calculated based on the measurement result 24 of the average light power of the input light acquired by the input light power acquisition unit 103, and the gain g calculated by the gain calculation unit 107. Thus, the noise figure of PSA2 is calculated by performing calculations according to the first calculation formula data 1061 or the second calculation formula data 1062 stored in the storage unit 106.

The processing procedure of the noise figure measurement by the noise figure measurement apparatus 10 in the measurement system 500 is as follows.
First, the control unit 100 in the noise figure measurement apparatus 10 controls the light source 1 to generate input light having a desired optical power, and the optical power meter 7 measures the measured value of the average optical power of the input light. The noise figure measurement apparatus 10 acquires the measurement result 24 of the average optical power of the input light by the input optical power acquisition unit 103.

  Next, the control unit 100 in the noise figure measurement apparatus 10 controls the optical switches 8 and 9 to irradiate the photodetector 4 with the input light output from the light source 1 without passing through the PSA 2 and the optical filter 3. Forming a light propagation path. Thereafter, the RIN measuring device 5 measures the relative intensity noise RINin of the input light. The noise figure measurement apparatus 10 acquires the measurement result 21 of the relative intensity noise RINin of the input light by the first relative intensity noise acquisition unit 104.

  Next, the control unit 100 in the noise figure measurement apparatus 10 controls the optical switches 8 and 9 so that the input light output from the light source 1 is input to the PSA 2 and the amplified light output from the PSA 2 is input to the optical filter 3. A light propagation path for irradiating the light detector 4 via is formed. Thereafter, the RIN measuring device 5 measures the average optical power of the amplified light and the relative intensity noise RINin of the amplified light. The noise figure measurement apparatus 10 acquires the measurement result 23 of the average optical power of the amplified light by the amplified light power acquisition unit 102 and the measurement result 22 of the relative intensity noise RINtotal of the amplified light by the second relative intensity noise acquisition unit 105. get. At this time, if necessary, the gain calculation unit 107 in the noise figure measurement apparatus 10 calculates the gain g based on the measurement result 23 of the average optical power of the amplified light and the measurement result 24 of the average optical power of the input light. To do.

  Next, the noise figure calculation unit 108 in the noise figure measurement apparatus 10 calculates the noise figure. For example, the noise figure calculation unit 108 calculates the measurement result 21 of the relative intensity noise RINin of the input light acquired by the first relative noise acquisition unit 104 and the relative intensity noise RINtotal of the amplified light acquired by the second relative noise acquisition unit 105. The input average photon number nin of the input light calculated based on the measurement result 22, the measurement result 24 of the average optical power of the input light acquired by the input light power acquisition unit 103, and the gain g calculated by the gain calculation unit 107. The noise figure of PSA2 is calculated by substituting it into the arithmetic expression (expression (26)) related to the first arithmetic expression data 1061.

  When the gain g is large, the calculation formula (formula (27)) according to the second calculation formula data 1062 is used instead of the first calculation formula data 1061 without using the gain g calculated by the gain calculation unit 107. The noise figure of PSA2 may be calculated by substituting the above measurement result into. In this case, the noise figure measurement device 10 does not have to include the gain calculation unit 107 and the amplified optical power acquisition unit 102.

  The value of the noise figure calculated by the noise figure calculation unit 108 is displayed on, for example, the display device 11 and stored in a storage device (not shown) provided inside or outside the noise figure measurement device 10.

  As described above, according to the measurement system using the noise figure measurement apparatus 10, the noise figure measurement apparatus 10 can control the light source 1 and switch the light propagation path. It becomes easy and the efficiency of the measurement work can be improved.

  Although the invention made by the present inventors has been specifically described based on the embodiments, it is needless to say that the present invention is not limited thereto and can be variously modified without departing from the gist thereof. Yes.

  For example, in the above-described embodiment, the case where the optical amplifier to be evaluated for noise figure is PSA has been described as an example. However, the noise figure can be measured for optical amplifiers other than PSA by the same method.

  In the noise figure measurement method according to the above embodiment, steps S1 to S4 for obtaining various parameters for calculating the noise figure are not limited to the order shown in FIG. For example, as long as each parameter can be measured by the step of calculating the noise figure (S5), the order of steps S1 to S4 can be changed.

  In the above embodiment, the case where the RIN measuring device 5 and the photodetector 4 are provided separately is illustrated, but an RIN measuring device in which the photodetector 4 is integrated may be used.

  Further, in the measurement system 500 using the noise figure measurement device 10, the case where the optical switch 9 is provided at the subsequent stage of the optical filter 3 is illustrated, but the optical switch 9 may be provided between the PSA 2 and the optical filter 3. Further, the arrangement of components such as the optical splitter 6, the optical switches 8, 9 and the optical power meter 7 is not limited to that shown in FIG. That is, as long as the above parameters (relative intensity noise RINin, Rtotal, etc.) necessary for the arithmetic processing based on the above equations (26) and (27) can be acquired, the arrangement of each component is different from that shown in FIG. It may be.

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... PSA, 3, 6 ... Optical filter, 4 ... Optical detector, 5 ... RIN measuring device, 7 ... Optical spectrum analyzer, 8, 9 ... Optical switch, 10 ... Noise figure measuring device, 11 ... Display Device: 21 ... Measurement result of relative intensity noise RINin of input light, 22 ... Measurement result of relative intensity noise RINtotal of amplified light, 23 ... Measurement result of average optical power of amplified light, 24 ... Measurement result of optical spectrum of input light 25 ... Measurement result of optical spectrum of amplified light, 100 ... Control unit, 101 ... Input light spectrum acquisition unit, 102 ... Amplified light spectrum acquisition unit, 103 ... Input light power acquisition unit, 104 ... First relative intensity noise acquisition unit 105 ... second relative noise acquisition unit, 106 ... storage unit, 107 ... gain calculation unit, 108 ... noise figure calculation unit, 1061 ... first calculation formula data, 1062 ... second calculation formula data, 00 ... measurement system.

Claims (7)

Obtained by photoelectrically converting the relative intensity noise of the input light measured based on the power obtained by photoelectrically converting the input light input to the phase sensitive optical amplifier and the amplified light amplified by the phase sensitive optical amplifier. The phase including noise based on spontaneous emission in the phase-sensitive optical amplifier and excess noise based on factors other than the spontaneous emission based on the relative intensity noise of the amplified light measured based on the measured power A noise figure measurement method for calculating a noise figure of a sensitive optical amplifier ,
A first step of obtaining a measurement of relative intensity noise of the input light;
A second step of obtaining a measured value of the relative intensity noise of the amplified light;
A third step of obtaining a measured value of the average optical power of the input light;
A fourth step of obtaining a measurement of the gain of the phase sensitive optical amplifier;
The measured value of the relative intensity noise of the input light acquired in the first step, the measured value of the relative intensity noise of the amplified light acquired in the second step, and the average of the input light acquired in the third step A noise figure measurement comprising: a fifth step of calculating the noise figure based on a measured value of optical power and a measured value of the gain of the phase sensitive optical amplifier obtained in the fourth step Method. The noise figure measurement method according to claim 1,
The fifth step includes
The noise figure and f _{psa + ex,} an input average number of photons based on the average optical power of the input light and n _in, the relative intensity noise of the input light and RIN _in, the relative intensity noise of the amplified light RIN _total And the noise figure is calculated based on the equation (A), where g is the gain of the phase sensitive optical amplifier.
The noise figure measurement method characterized by the above-mentioned.

Obtained by photoelectrically converting the relative intensity noise of the input light measured based on the power obtained by photoelectrically converting the input light input to the phase sensitive optical amplifier and the amplified light amplified by the phase sensitive optical amplifier. The phase including noise based on spontaneous emission in the phase-sensitive optical amplifier and excess noise based on factors other than the spontaneous emission based on the relative intensity noise of the amplified light measured based on the measured power A noise figure measurement method for calculating a noise figure of a sensitive optical amplifier,
A first step of obtaining a measurement of relative intensity noise of the input light;
A second step of obtaining a measured value of the relative intensity noise of the amplified light;
A third step of obtaining a measured value of the average optical power of the input light;
The measured value of the relative intensity noise of the input light acquired in the first step, the measured value of the relative intensity noise of the amplified light acquired in the second step, and the average of the input light acquired in the third step And a fourth step of calculating the noise figure based on an optical power measurement value . The noise figure measurement method according to claim 3 ,
The fourth step includes
The noise figure and f _{psa + ex,} an input average number of photons based on the average optical power of the input light and n _in, the relative intensity noise of the input light and RIN _in, the relative intensity noise of the amplified light RIN _total Is a step of calculating the noise figure based on the formula (B).
The noise figure measurement method characterized by the above-mentioned.

A first relative intensity noise acquisition unit for acquiring a measurement value of the relative intensity noise of the input light measured based on the electric power obtained by photoelectrically converting the input light input to the phase sensitive optical amplifier;
  A second relative intensity noise acquisition unit for acquiring a measurement value of the relative intensity noise of the amplified light measured based on electric power obtained by photoelectrically converting the amplified light amplified by the phase sensitive optical amplifier;
  A gain acquisition unit for acquiring a measured value of the gain of the phase sensitive optical amplifier;
  An input optical power acquisition unit that acquires a measurement value of the average optical power of the input light;
  The measurement value of the relative intensity noise of the input light acquired by the first relative intensity noise acquisition unit, the measurement value of the relative intensity noise of the amplified light acquired by the second relative intensity noise acquisition unit, and the gain acquisition unit Based on the measured value of the gain of the phase-sensitive optical amplifier obtained in step 4 and the measured value of the average optical power of the input light obtained by the input optical power acquisition unit, based on the spontaneous emission light in the phase-sensitive optical amplifier A noise figure calculation unit for calculating a noise figure including excess noise based on factors other than noise and the spontaneous emission light,
  A noise figure measurement apparatus characterized by the above. A first relative intensity noise acquisition unit for acquiring a measurement value of the relative intensity noise of the input light measured based on the electric power obtained by photoelectrically converting the input light input to the phase sensitive optical amplifier;
A second relative intensity noise acquisition unit for acquiring a measurement value of the relative intensity noise of the amplified light measured based on electric power obtained by photoelectrically converting the amplified light amplified by the phase sensitive optical amplifier;
An input optical power acquisition unit that acquires a measurement value of the average optical power of the input light;
The measured value of the relative intensity noise of the input light acquired by the first relative intensity noise acquisition unit, the measured value of the relative intensity noise of the amplified light acquired by the second relative intensity noise acquisition unit, and the input light power Based on the measurement value of the average optical power of the input light acquired by the acquisition unit, a noise index including noise based on spontaneous emission light and excess noise based on factors other than the spontaneous emission light in the phase sensitive optical amplifier is calculated. And a noise figure calculation unit . A light source that emits light;
An optical power measuring device for measuring the average optical power of the light emitted from the light source;
The phase sensitive optical amplifier;
A photodetector that converts the received light into an electrical signal;
A relative intensity noise measuring device for measuring relative intensity noise based on the electrical signal converted by the photodetector;
The light output from the light source is input to the phase sensitive optical amplifier and the light output from the phase sensitive optical amplifier is input to the photodetector, or the light output from the light source is the phase sensitive light. An optical switch that forms a path for input to the photodetector without being input to an amplifier;
The noise figure measurement device according to claim 5 or 6 , wherein the average optical power measured by the optical power measuring device and the relative intensity noise measured by the relative intensity noise measuring device are input. A noise figure measurement system.

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