JP2903975B2 - Optical receiver characteristic measuring method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring characteristics of an optical receiver, and more particularly, to an optical communication device using an optical fiber, comprising an optical detector and an amplifier for amplifying the detection output of the optical detector. The present invention relates to an optical receiver characteristic measuring method and apparatus capable of easily measuring a frequency characteristic expressed by a receiving frequency versus output power characteristic of an optical receiver including the optical receiver.

[0002]

2. Description of the Related Art In optical communication for transmitting high-speed digital signals using an optical fiber, the characteristics of an optical receiver must be accurately measured over a wide frequency range in order to secure optical communication at a recent transmission speed of about gigabit / second. It is important for the system construction to understand this. The performance of the optical receiver includes a transimpedance frequency characteristic and a frequency characteristic of an equivalent input noise current density (noise frequency characteristic).

As a method of measuring these frequency characteristics,
There is a method of measuring by using output power when light from a light source having a wide line width, for example, a light emitting diode (LED) or a super luminescent diode (SLD) is received by an optical receiver. This method utilizes the fact that the photodetector generates a shot noise current and a beat noise current which are white noises at the time of receiving light by the optical receiver, and the output noise of the receiver generated by the shot noise current and the beat noise current expresses a transimpedance frequency characteristic, and The noise frequency characteristic is derived by utilizing the fact that an output is generated based on the shot noise current and the beat noise current in addition to the receiver noise generated during non-light reception during light reception. As an improved measuring method, the frequency characteristics of beat noise generated when spontaneous emission light emitted from a semiconductor amplifier is received and the frequency of beat noise when spontaneous emission light is received by a standard optical receiver There is a method of deriving from characteristics (for example, JP-A-3-220467).

[0004]

In this conventional method for measuring the characteristics of an optical receiver, it is necessary to use a light source having a wide line width in order to eliminate the influence of intensity noise of the light source at the time of measurement.
However, since high output cannot be obtained with the above-described LED or semiconductor amplifier, it is not possible to secure a sufficiently large receiver output as compared with measuring instrument noise, and there is a problem that accurate measurement is difficult. In addition, in the SLD that can obtain a high output, there is a problem that the device is deteriorated quickly and the life is short. Further, the measuring method as disclosed in Japanese Patent Application Laid-Open No. 3-220467 requires a standard optical receiver for performing calibration at the time of measurement, and a calibration procedure at the time of measurement. In this case, there are problems such as an increase in cost due to having a standard optical receiver for each device, maintenance for maintaining the standard, and the need for a calibration procedure, which complicates the measurement.

An object of the present invention is to solve the above-mentioned problems,
Using a light source that can secure a large light output with a simple configuration,
An object of the present invention is to provide a simple calibration optical receiver characteristic measuring method and apparatus capable of ensuring accurate measurement without requiring a calibration reference receiver and a calibration procedure.

[0006]

According to the method for measuring the characteristics of an optical receiver according to the present invention, spontaneous emission light generated by exciting a rare-earth-doped optical fiber is inputted to a measured optical receiver whose characteristics are to be measured and detected. A first step of obtaining a ratio between a photoelectric flow rate and an output power over a measurement frequency range to derive a transimpedance frequency characteristic of the optical receiver under test, and reducing the amount of spontaneous emission light from the rare earth-doped optical fiber by at least 2 The noise frequency characteristic of the optical receiver under measurement is changed based on the photoelectric flow rate and the output power over the measurement frequency range and the wavelength band of the spontaneous emission light when input to the optical receiver under measurement after switching to a stage. Deriving a second step.

The optical receiver characteristic measuring apparatus of the present invention comprises an erbium-doped optical fiber doped with erbium as a rare earth element and at least one excitation light source for exciting the erbium-doped optical fiber. A light source means for emitting the spontaneous emission light having a known wavelength band as a light source by being incident on an erbium-doped optical fiber; and a light source switching means for switching an output light amount of the spontaneous emission light emitted from the light source means in at least two stages, The spontaneous emission light emitted from the light source means is input to the measured light receiver whose characteristics are to be measured, and the detected photoelectric flow rate and the spontaneous emission light emitted from the light source means are input to the measured light receiver. In this case, the noise frequency characteristic of the optical receiver under test is included without including a calibration procedure based on the output power at each frequency in the measurement frequency range. And a characteristic deriving means for obtaining.

[0008]

FIG. 4 is an explanatory diagram showing the principle of measuring the transimpedance frequency characteristic of the optical receiver according to the present invention. As an optical fiber laser amplifier using an optical fiber doped with a rare earth element, an optical fiber doped with erbium is used as a laser active material, and light from an erbium fiber light source that is pumped with a semiconductor laser as an excitation light source contains intensity noise. Therefore, the noise current generated in the optical receiver under test 13 includes the shot noise current Ishot and the beat noise current Ib.
eat. These noise current sources are white noise currents as shown in FIG. This white noise current is shown in FIG.
Upon receiving the characteristic of the transimpedance Z _T of the measured light receiver 13 shown in (b) as shown in FIG. 4 (c), characteristic of the transimpedance Z _T to the output power characteristic appears directly. Here, the root mean square per unit frequency of the shot noise current and the beat noise current are (1) and
It is expressed by equation (2).

<Ishot ² > = 4 (G−1) · nsp · Δν · e (ηe) (1) <Ibeat ² > = 2 (G−1) ² · nsp ² · Δν · (ηe) ² (2) At this time, the current Ipd flowing through the built-in photodetector when the measured optical receiver 13 receives light is expressed by the following equation (3).

Ipd = 2 (G−1) · nsp · Δν · (ηe) (3) where G is the amplification factor of the erbium-doped fiber amplifier,
nsp is the population inversion coefficient, Δν is the wavelength width of the output light, η is the quantum efficiency of the optical receiver, and e is the charge amount. Using the equation (3), the equations (1) and (2) are transformed as the following equations (4) and (5), respectively.

<Ishot ² > = 2 · e · Ipd [A ² / Hz] (4) <Ibeat ² > = Ipd ² / (2Δν) [A ² / Hz] (5) Assuming that the result of measuring the output power of the measuring optical receiver 13 is P (f) [W / Hz], the transimpedance characteristic Z _T (f) is given by the following equation (6).

Z _T (f) = P (f) ^1/2 * R / (<Ibeat ² > + <Ishot ² > + <Icir ² >) [Ω] (6) where R is a measuring instrument And Icir is the equivalent input noise current density of the optical receiver. If the input light from the light source is increased, Icir is much smaller than Ishot and Ibeat, so that the influence of Icir can be ignored. By using the above procedure, the transimpedance characteristic of the receiver is derived.

Next, the principle of measuring the noise frequency characteristic of the optical receiver will be described with reference to FIG. In the measurement, the amount of light incident from the erbium fiber light source to the optical receiver under measurement is switched to two steps using an optical shutter or the like (usually two steps when light is incident and when light is not incident) are used.
This is done by measuring the output power. When light is being received by the optical receiver under measurement, the sum of noise power due to the shot noise, beat noise, and circuit noise described above is observed in the output of the optical receiver under measurement. When the output noise power at this time is N _1, N ₁ is expressed by the following equation (7).

N ₁ = Z _T (<Ishot ² > + <Ibeat ² > + <Icir ² >) [W] (7) Next, the output power N of the measured optical receiver when no light is received ₂ is expressed by the following equation (8).

N ₂ = Z _T (<Icir ² >) [W] (8) From the ratio of these equations (7) and (8), <Icir ² >
Is represented by the following equation (9).

<Icir ² > = (<Ishot ² > + <Ibeat ² >) / (N ₁ / N ₂ −1) [A ² / Hz] (9) Here, from the photocurrent Ipd at the time of light reception. , (4) and (5) yield <Ishot ² > and <Ibeat ² >.
Further, a noise frequency characteristic is derived from Expression (9).

Incidentally, the present invention is characterized in that an erbium fiber light source is used as a light source used for measurement. This is because in the erbium fiber light source shown in FIG. 3A, a large output can be obtained relatively easily by adjusting the amount of the excitation light source by the excitation light source 3 and the length of the erbium-doped optical fiber 2. is there. FIG.
When a sufficient output cannot be obtained with a single unit configuration as shown in FIG. 3A, the light source unit is replaced with an optical isolator 12 as shown in FIG.
By cascade-connecting via the above, a desired large output can be easily obtained.

[0018]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a first embodiment of the present invention. In the first embodiment, an erbium-doped optical fiber 2 as a medium to be excited, an excitation light source 3 using a semiconductor laser, an erbium fiber light source 1 as a light source means having an incident optical system 10, and an erbium fiber light source 1 The variable optical attenuator 4 as a light source switching means for switching the light amount of the output light 101 in two stages, the measured light receiver 5 having the photodetector 8 and the amplifier 9, and the received light flowing through the measured light receiver 5 An ammeter 6 for measuring the photocurrent at the time, and a power spectrum measuring device 7 which constitutes a characteristic deriving means together with the ammeter 6 and measures the output power of the optical receiver 5 to be measured.

An output 101 having a known wavelength band is transmitted from the erbium fiber light source 1, passes through the variable attenuator 4, and enters the optical receiver under test 5. An ammeter 6 is connected to the photodetector 8 of the optical receiver 5 to be measured, and measures a photocurrent when light is incident. A power spectrum measuring device 7 is connected to the output of the measured optical receiver 5 and measures the output power spectrum.

The transimpedance characteristic is measured by improving the signal-to-noise (N) ratio S / N with the noise generated by the power spectrum measuring device 7 and the noise frequency characteristic of the optical receiver 5 to be measured. In order to eliminate the influence of the above, the attenuation of the variable attenuator 4 is reduced to give a sufficient amount of received light to the optical receiver 5 to be measured. In this state, the erbium fiber light source 1
Using the wavelength band of the output light 101, the photoelectric flow rate measured by the ammeter 6, and the power spectrum of the output 501 of the measured optical receiver 5 measured by the power spectrum measuring device 7.
The transimpedance is derived based on the equations (4), (5) and (6).

The measurement of the noise frequency characteristic of the measured optical receiver 5 is performed by measuring the photoelectric flow rate by the ammeter 6 and the output power spectrum N by the power spectrum measuring instrument 7 in a state where sufficient light reception is given to the measured optical receiver 5. ₁ Measure (output noise power).
Next, the output power spectrum N ₂ when the light reception of the measured optical receiver 4 is completely cut off by the variable optical attenuator 4 is measured. Based on these measured values and the wavelength width of the input light,
The noise frequency characteristic is derived from the equations (4), (5) and (9).

FIG. 2 is a block diagram of a second embodiment of the present invention. In the second embodiment, instead of the variable optical attenuator 4 in the first embodiment shown in FIG. 1, the input light quantity to the optical receiver under test 5 is switched by the injection current of the erbium fiber light source 1a to the excitation light source 3. Of the variable current source 11 in the erbium fiber light source 1a.
To switch the injection current. In this configuration, although the deterioration of the excitation light source 3 is accelerated to switch the injection current to the excitation light source 3, the variable optical attenuator 4 as a mechanical control mechanism for switching the amount of incident light is not required.

[0023]

As described above, according to the present invention, the spontaneous emission light generated by exciting the rare-earth-doped optical fiber is input to the optical receiver under measurement, and the ratio between the detected photoelectric flow rate and the output power is determined. Measure transimpedance frequency characteristics by measuring over the measurement frequency range, and change the noise frequency characteristics based on the photoelectric flow rate, output power, and wavelength band measured by switching the amount of spontaneous emission light from the rare-earth-doped optical fiber in at least two stages. The derivation has an effect of realizing a simple configuration and accurate optical receiver characteristic measuring method and apparatus that does not require the configuration reference receiver and the calibration procedure.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a second embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a configuration example of an erbium fiber light source in the first and second embodiments of the present invention.

FIG. 4 is an explanatory diagram illustrating a measurement principle of a transimpedance frequency characteristic of the optical receiver.

FIG. 5 is an explanatory diagram illustrating a measurement principle of a noise frequency characteristic of the optical receiver.

[Explanation of symbols]

 Reference Signs List 1, 1a Erbium fiber light source 2 Erbium-doped optical fiber 3 Excitation light source 4 Variable optical attenuator 5 Optical receiver under measurement 6 Ammeter 7 Power spectrum amplifier 8 Photodetector 9 Amplifier 10 Incident optical system 11 Variable current source 12 Optical isolator

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Claims (2)

(57) [Claims] 1. A spontaneous emission light generated by exciting a rare-earth-doped optical fiber is input to an optical receiver under measurement whose characteristics are to be measured, and the ratio between the detected photoelectric flow rate and the output power is determined over the measurement frequency range. A first step of deriving a transimpedance frequency characteristic of the measured optical receiver, and a case where the amount of spontaneous emission light from the rare-earth-doped optical fiber is switched to at least two stages and input to the measured optical receiver. A second step of deriving a noise frequency characteristic of the optical receiver under measurement based on the photoelectric flow rate and the output power over the measurement frequency range and a wavelength band of the spontaneous emission light. Receiver characteristic measurement method. 2. An erbium-doped optical fiber doped with erbium as a rare earth element, and at least one pump light source for pumping the erbium-doped optical fiber. Light source means for emitting light using the known spontaneous emission light as a light source, light source switching means for switching an output light amount of the spontaneous emission light emitted from the light source means in at least two stages, and the spontaneous emission light emitted from the light source means When the photoelectric flow and the spontaneous emission light emitted from the light source means are input to the optical receiver under measurement whose characteristics are to be measured and are input to the optical receiver under measurement, at each frequency of the measurement frequency range Characteristic deriving means for obtaining a noise frequency characteristic of the optical receiver under measurement without including a calibration procedure based on the output power. An optical receiver characteristic measuring device, characterized in that: