JPH03220467A - Method and apparatus for measuring characteristic of photo detector - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] Table of Contents Overview Industrial Application Fields Prior Art Problems to be Solved by the Invention Means for Solving the Problems Implementation Examples Summary of Effects of the Invention Method and Device for Measuring Characteristics of Optical Receivers With the aim of establishing a simple measurement method and providing a low-cost measurement device, the measurement method involves switching the bias current value of a semiconductor optical amplifier to at least two values, and measuring the amount of light from the semiconductor optical amplifier. Measure the output noise power of the optical receiver under measurement under each bias current to find the Y factor,
A first step of deriving the noise characteristics of the optical receiver under test from the Y factor and the excess noise ratio due to spontaneous emission repeat noise under each bias current, and determining the bias current value of the semiconductor optical amplifier at least two times. Measure the output noise power of the optical receiver to be measured, which is manually powered by the light from the semiconductor optical amplifier, and the optical receiver for calibration, whose load resistance is known, under their respective bias currents, and from the measured values. and a second step of deriving the transimpedance of the optical receiver to be measured.

Industrial applications The present invention relates to a method and apparatus for measuring characteristics of an optical receiver.

Optical communications are attracting attention as a means of transmitting large amounts of information in a highly information-oriented society, and demands for higher transmission speeds (bit rates) are increasing year by year. In the intensity noise/direct detection method (IM-DD method), 1.
The transmission speed of 6GBPS or 2°4GBPS has reached the practical stage, and at the research level, 1QG8PS has become a reality. Furthermore, a coherent optical communication system is being researched as a system that theoretically enables long-distance and large-capacity transmission compared to the IM-DD system, and a transmission speed of 4 GBPS has been achieved in this system as well. As the transmission speed in optical communications continues to increase,
Understanding and cleaning the characteristics of an optical receiver over a wide band is important in system design, for example, when determining how much signal level deterioration can be tolerated and setting relay intervals.

Conventional techniques Generally, methods for measuring the characteristics of optical receivers include measuring the error rate of a received signal and determining noise characteristics.

The former is often performed when an optical receiver is incorporated into a system, and when measuring the characteristics of an optical receiver independently,
The latter is common.

The noise characteristics of an optical receiver are usually measured by converting various noises occurring in the signal path within the optical receiver into an input current, etc., and determining its frequency characteristics. That is, a technical issue is how to determine the frequency characteristics of the input equivalent noise of an optical receiver.

FIG. 6 is a block diagram of a general optical receiver.

The optical receiver 61 includes a photoreceiver 62 that generates a photocurrent proportional to the square of the electric field amplitude of human-powered light, a load resistor 63 through which the photocurrent flows, and an electric circuit that amplifies the voltage drop across the load resistor 63. 64. When the photocurrent generated in the photoreceiver 62 is I and the output terminal voltage is E, the transimpedance Z of the photoreceiver 61 is expressed as Z=E/I. Note that this transimpedance is used when converting the measured output noise into input.

A conventional method for measuring frequency characteristics of input-referred noise will be explained with reference to FIG.

First, the frequency characteristics of the output noise as shown in FIG. 2(a) are measured. This characteristic applies to frequency-selective level meters, L
Combination with PF (low pass filter) and power meter,
Alternatively, this can be done relatively easily by measuring the output noise level for each frequency domain using an NF meter.

Next, the frequency characteristics of the transimpedance of the optical receiver as shown in FIG. 2(b) are measured.

This measurement consists of measuring the absolute value of the transimpedance in a relatively low frequency region and measuring the relative frequency characteristics for extrapolating (extracting) the characteristics measured in the low frequency region to the high frequency region. The method for measuring the absolute value of transimpedance in the low frequency region is as follows: (1) The S parameter of the electric circuit section 64 is measured using, for example, a network analyzer, and the transimpedance is derived from the S parameter.

(2) Light from an LD (semiconductor laser) that has been analog intensity modulated by a sine wave signal is input to the photoreceiver 62, and is calculated from the photocurrent and output terminal voltage at that time.

etc. In method (2), when measuring, the light receiver 62
However, in optical receivers designed for high bit rates, it is actually difficult to separate the optical receiver and the electric circuit section, which are connected by a strip line or the like. In method (2), the measurement is complicated because the extinction ratio of the LD needs to be corrected, and there is an upper limit to the measurement frequency due to the frequency characteristics of the LD.

On the other hand, as a method for measuring relative frequency characteristics, ■ Input light from an LD that can be stably modulated over a wide band into the optical receiver 61, and measure using a Sukasi network analyzer as a black box including the LD. do.

■ Light from two LDs with close oscillation frequencies are simultaneously input to the light receiver 62, and a hetero gain detection signal of the frequency difference is generated using the square law detection characteristic of the light receiver 62, and the oscillation frequency of one LD is changed. By scanning the frequency of the heterodyne detection signal, the frequency characteristics of the response level are obtained (heterodyne method).

etc. In method (2), there is an upper limit to the measurement frequency due to the characteristics of the LD, and in method (2), there is a lower limit to the measurement frequency due to the spectral width of the LD, so generally it is necessary to use methods (2) and (3) together, and the measurement is difficult. It's complicated.

Through the above procedure, the frequency characteristics of the output noise (Figure 7 (a)
))'F and the frequency characteristics of transimpedance (Fig. 7(b)) are obtained, and based on these,
It is possible to obtain the frequency characteristic of input conversion noise (input conversion noise current) as shown in FIG. 7(C).

Problems to be Solved by the Invention As described above, the conventional method is complicated, requires various expensive measuring equipment, and is not common. This is considered to be due to the fact that the optical receiver is located at the forefront of the optical receiver, and the signal input must be based on light.

The present invention was created in view of these circumstances, and aims to establish a simple measuring method and provide a low-cost measuring device.

Means to Solve the Problem When measuring the noise characteristics of an amplifier in the microwave band, for example, which does not have a photoreceiver at the front stage, the characteristics can be easily measured using the Y-factor method. In this method, the Y factor is determined by using two noise sources with different noise temperatures (such as avalanche diodes) or by turning on and off a noise source with a predetermined noise temperature, and the noise figure, etc. is derived from this Y factor. This is how I did it. However, the Y-factor method cannot be directly applied to an optical receiver having a light receiver at the front stage. This is because it is meaningless unless the noise characteristics of the optical receiver that performs optical-to-electrical conversion are measured.

Therefore, in the present invention, a semiconductor optical amplifier (
A semiconductor laser type optical amplifier) is used, and the spontaneously emitted beat noise (hereinafter sometimes simply referred to as "beat noise") generated thereby is used.

FIG. 2 is an explanatory diagram of an example of beat noise generated in an optical receiver by using a semiconductor optical amplifier.

The vertical axis is the relative output noise (dB), and the horizontal axis is the input optical intensity (dB).
m). Dashed lines 11 and 12 are the beat noise when the bias current of the semiconductor optical amplifier is 40 mA and 46 mA, respectively, and solid lines 13 and 14 are the total noise when the bias current is the same <40 mA and 46 mA, respectively.

Further, a dashed line 15 indicates thermal noise caused by the load resistance of the optical receiver. In this way, beat noise is equal to or larger than thermal noise, and since beat noise changes depending on the bias current and does not change depending on the input light intensity, it is possible to set the bias current value of a semiconductor optical amplifier to a different value. By setting , it is possible to obtain at least two noise sources necessary for implementing the Y-factor method.

FIG. 1 shows a basic configuration diagram of the device of the present invention.

In the figure, l is a semiconductor optical amplifier, 2 is a bias circuit for the semiconductor optical amplifier 1 that can switch the bias current value between at least two values, and 3 is a circuit that generates beat noise when the light from the semiconductor optical amplifier 1 is photoelectrically converted. 4 is an optical receiver to be measured, and 5 is an optical receiver to be measured which transmits the light from the semiconductor optical amplifier 1 to the optical receiver to be measured 4 and the optical receiver for calibration. 3 is an output noise detection unit that measures these output noise voltages when manually applied.

The method for measuring characteristics of an optical receiver according to the present invention is based on a method for measuring characteristics of a semiconductor optical amplifier 1.
The bias current value of is switched to at least two values, and the output noise power of the optical receiver to be measured 4 to which the light from the semiconductor optical amplifier 1 is input is measured under each bias current to determine the Y factor. The first step is to derive the noise characteristics of the optical receiver under test 4 from the Y factor and the excess noise ratio due to the spontaneous emission beat noise under each bias current.
step, and the bias current value of the semiconductor optical amplifier 1 is changed to a small value (both are changed to two values), and the optical receiver 4 to be measured to which the light from the semiconductor optical amplifier 1 is input, and the calibration light whose load resistance is known. The output noise power of the receiver 3 is measured under each bias current, and based on the measured value, the output noise power of the optical receiver 4 to be measured is determined.
and a second step of deriving the transimpedance of.

For production FIG. 3 is an explanatory diagram of the measurement principle according to the present invention.

FIG. 3(a) shows a measurement system in the first step of the method of the present invention, and FIG. 2(b) shows a measurement system in the second step. It is assumed that the bias current of the semiconductor optical amplifier 1 can be switched between a first bias current and a second bias current.

First, the first step of deriving the noise characteristics of the optical receiver 4 to be measured will be explained with reference to FIG. 3(a). The noise power measured as the output of the optical receiver under test 4 when the semiconductor optical amplifier 1 is driven with the first bias current is given by the following equation.

N, -(2T'/4R) ・(kl3,12)
+ <1AXP2> ) (1) Also, the output noise power of the optical receiver under test 4 at the second bias current is similarly given by the following equation.

~2=(Za'/4R) ・((i5P2'> +
<iA, IP”> ) ... [2] Here, 2 is the transimpedance of the optical receiver under test 4, R is the human power impedance of the output noise power detector 5 (usually 50Ω), <l5PI'> is the input-referred beat noise current at the first bias current, < 1 SP2') is the input-referred beat noise current at the second bias current, (I A)IP'') is the input-referred input of the optical receiver 4 under test. The noise current is .

From equation (1), if the beat noise is considered as input noise and the circuit noise generated in the optical receiver under test 4 is considered as internal noise, then the noise figure F is defined as follows.

Therefore, the relationship between internal noise N6 and noise figure F is N,
=ZT' ・ <l AXP”> / 4 R(F
-1)・ZT2・<! sp1'> / 4
R (4), and the input-referred noise current, which is generally used to express the noise characteristics of an optical receiver, is given by the following equation using the noise figure.

<IAIIP") (F-1) ・ <ISP+'> ... (5) From formulas (2) and (4), NF (dB) = ENR-10LOG (Y-1)
...6 mouths N2 = (Zt'/4R)
・((F-1) ・<!sp+”>”<!
spa''> )...(6) is obtained, so from equations (3) and (6), the Y factor Y is as follows. Therefore, the noise figure F is given by the following equation.

The excess noise ratio of beat noise is Then, the noise figure NF is as follows.

Therefore, if the excess noise ratio ENR of beat noise is given, the noise characteristics of the optical receiver under test 4 can be determined by measuring the Y factor. The excess noise ratio ENR of the beat noise can be obtained, for example, by turning on and off light from a semiconductor optical amplifier input to a light receiver connected to a known load resistance.

Next, measurement of the transimpedance of the optical receiver 4 to be measured will be explained using FIG. 3 et al.

In this case, it is necessary to take into account the gain of the measurement system (output noise detector 5, etc.). Measurement system with 0 gain
It is assumed that the noise power is measured including the output noise, assuming that the amplifier has an internal noise of N, /. 1st
If the noise at the second bias current is N1' and the noise at the second bias current is N, /, then N, , = N, +N, ・G ・
...Ql)2=N.

10N,・G ・・・αri holds true. Therefore, the gain of the measurement system is.

Consider a case where the output of the semiconductor optical amplifier 1 is connected to the calibration optical receiver 3. Semiconductor optical amplifier 1 with first bias current
When driving, the noise power measured for the output of the calibration optical receiver 3 is given by the following equation.

N+ = ((isp+”>・R/4)
・G+kTBG+N, ... α4) Here, R is the load resistance in the calibration optical receiver 3 (for example, 5
0Ω), kTB is thermal noise. Further, the output noise power of the calibration optical receiver 3 at the second bias current can be similarly given by the following equation.

N2 = ((1sp2') ・R/4) ・G
-kTBG・～, ...α9N, -N.

=((<1--22><i,,,">) ・R/4) ・G...αO
becomes. On the other hand, from equations (1), (2), and α, N, −
! i.

= (< i sP2”> −(i-Pl”
) ) ・Zt"/4R= (N, -N,
')/G ・・・α. Therefore, ZT2 is expressed by the following formula.

From the 0e formula, by substituting this into the Shin formula, we get
1] From formula 021, it becomes. Therefore, by measuring the output noise power of the optical receiver under test 4 and the optical receiver for calibration 3 when the semiconductor optical amplifier 1 is driven with two different bias currents, the transimpedance of the optical receiver under test 4 can be determined. can be found.

As described above, according to the present invention, it is possible to establish a simple measurement method, and it is also possible to provide a low-cost measurement device that does not require expensive measurement equipment such as a network analyzer.

Example The present invention will be explained in detail below.

FIG. 4 is a block diagram of an optical receiver characteristic measuring device showing an embodiment of the present invention. In this figure, 21 is a main body of the measuring instrument, and 4 is an optical receiver to be measured which is electrically and optically connected to the main body 21 of the measuring instrument. The optical receiver to be measured 4 is
It consists of a photodetector 22 such as a photodiode, a load resistor 23 (usually high impedance) through which the photocurrent generated in the photodetector 22 flows, and an amplifier 24 that amplifies the voltage across the load resistor 23. 25 is a bias circuit for the photoreceiver 22.

A part of the output light of the semiconductor optical amplifier 1 provided in the measuring instrument main body 21 is inputted to the light receiver 22 of the optical receiver to be measured via the optical fiber 38 for connection.

Further, a part of the output light of the semiconductor optical amplifier 1 is also input to the optical receiver 26 of the optical receiver 3 for calibration. If the light receivers 22 and 26 cannot be supplied with light of sufficient intensity for measurement, the semiconductor optical amplifier 1 is manually supplied with light of an appropriate intensity, and the semiconductor optical amplifier 1 is used in an optically amplified state. It's okay. In the optical receiver 3 for calibration, 27 is the optical receiver 26
This is a load resistance through which the photocurrent generated by the current flows, and in this embodiment, a 50Ω resistor is used. 28 is load resistance 27
29 is an amplifier that amplifies the voltage across both ends of the light receiver 26.
This is a bias circuit.

The output noise detector 5 measures the output noise power of either the calibration optical receiver 3 or the optical receiver under test 4 selected by the switch 30 . In the following description, it is assumed that this output noise detection section 5 is a frequency selective level maker. 3
1 is a display device such as a CRT device for displaying measurement results.

In this embodiment, a control circuit is provided to control each part based on the procedure of the measuring method of the present invention, and its configuration will be explained. 32 is a 170 circuit, and this I10 circuit 32
takes in the output noise signal measured by the frequency selection level meter 5. A CPU 33 performs calculations based on the captured signals according to a predetermined program, and also forms signals for switching the bias current in the bias circuit 2 and the switch 30.

34 is a RAM that temporarily stores calculation results etc. in the CPU 33, and 35 is an R in which the above programs etc. are stored.
It's OM.

36 is an I10 circuit having a demultiplexer function, which sends a control signal to the bias circuit 2 and switch 30, sends a band designation signal to the frequency selection level meter 5, and sends a display signal to the display device 31 according to the calculation result of the CPU 33. Send out.

37 is a data bus, which connects the CPU 33 and the I10 circuit 32.
, 36, RAM 34 and ROM 35 are interconnected.

The operation of this measuring device will be explained.

First, the bias current of the bias circuit 2 and the switch 30
By performing the switching, the output noise of the optical receiver for calibration 3 and the optical receiver to be measured 4 is measured, and the noise figure NF is determined by performing calculations according to the α0 formula or the like.

Next, a similar switching operation is performed, and the transimpedance Z is determined by calculation according to formula (to).

is required.

Then, the noise figure and transimpedance are determined for each band (frequency band selected by the frequency selection level meter 5), and the input conversion noise determined for each band is displayed on the display device 31. It is possible to automatically obtain the same frequency characteristics of input-referred noise in a short time. In addition, the characteristics of the optical receiver 26 in the optical receiver for calibration and the measuring instrument main body 2
Since the optical coupling efficiency, etc. within 1 is stable over a relatively long period of time, the measurement results for the calibration optical receiver 3 are R
It can be stored in the AM 34 or a storage device (not shown). This eliminates the need to measure the calibration optical receiver 3, further simplifying the measurement.

FIG. 5 is a graph showing an example of the characteristics to be measured, in which the vertical axis represents the input equivalent noise power of the optical receiver 4 to be measured, and the horizontal axis represents the frequency. Conventional measurement methods are inaccurate for the part corresponding to 1/f noise, where input-referred noise increases at relatively low frequencies, and the part corresponding to f2 noise, where input-referred noise increases at relatively high frequencies. In contrast to conventional methods, in which measurements are complicated, the method of the present invention makes it possible to optimally set at least two different bias currents to increase the effective number of solutions to each simultaneous equation. In particular, it is possible to easily and accurately measure portions corresponding to 1/f noise and f2 noise.

As described in detail, according to the present invention, it is possible to establish a simple measuring method and provide a low-cost measuring device for measuring the characteristics of an optical receiver.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of the device of the present invention, Fig. 2 is an explanatory diagram of beat noise between spontaneously emitted lights, Fig. 3 is an explanatory diagram of the measurement principle in the present invention, and Fig. 4 is an illustration of an embodiment of the present invention. FIG. 5 is a block diagram of an apparatus for measuring characteristics of an optical receiver. FIG. 5 is a graph showing an example of characteristics to be measured. FIG. 6 is a block diagram of an optical receiver. FIG. 7 is an explanatory diagram of a conventional method. DESCRIPTION OF SYMBOLS 1... Semiconductor optical amplifier, 2... Bias circuit of semiconductor optical amplifier, 3... Optical receiver for calibration, 4... Optical receiver under test, 5... Output noise detection section (frequency selection level meter).

Claims (1)

[Claims] 1. The bias current value of the semiconductor optical amplifier (1) is switched between at least two values, and the output of the optical receiver to be measured (4) into which the light from the semiconductor optical amplifier (1) is input. The noise power is measured under each bias current to determine the Y factor, and the noise characteristics of the optical receiver under test (4) are determined from the Y factor and the excess noise ratio due to beat noise between spontaneously emitted light under each bias current. The first step is to derive the bias current value of the semiconductor optical amplifier (1) by at least 2
The output noise power of the optical receiver under test (4) into which the light from the semiconductor optical amplifier (1) is input and the optical receiver for calibration (3) whose load resistance is known is set to the respective bias values. 1. A method for measuring characteristics of an optical receiver, comprising: measuring under current, and deriving the transimpedance of the optical receiver to be measured (4) from the measured value. 2. A semiconductor optical amplifier (1), a bias circuit (2) for the semiconductor optical amplifier (1) capable of switching a bias current value between at least two values, and a photoelectric converter for the light from the semiconductor optical amplifier (1). a calibration optical receiver (3) with a known load resistance, which is equipped with an optical receiver that generates beat noise between spontaneous emission lights when (
4) and an output noise detection unit (5) that measures the output noise power of the optical receiver to be measured (4) and the optical receiver for calibration (3) when input to the optical receiver for calibration (3); An optical receiver characteristic measuring device comprising: