JPH0820333B2 - Method for measuring frequency characteristics of optical modulator and measuring apparatus therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention accurately measures the frequency response characteristics of an external optical modulator and an optical switch required in optical communication and optical signal processing over an ultra-wide band. The present invention relates to a measuring method and its device.

[Conventional technology]

FIG. 2 shows a basic configuration of a conventional frequency response measuring method for an optical modulator. In the figure, 21 is an optical input section, 22
Is an optical modulator to be measured, 23 is an oscillator, 24 is a high-speed light receiving circuit, and 25 is a spectrum analyzer. In this method, the ω _s component of the light whose intensity is modulated by the optical modulator 22 at the electrical input frequency ω _s from the oscillator 23 is received by the high-speed light receiving circuit 24, and its output signal intensity is The frequency characteristic of the optical modulator is measured from the signal level on the spectrum analyzer when it is detected by the spectrum analyzer 25 and ω _s is changed.

[Problems to be Solved by the Invention]

However, with such a conventional method, although it is possible to measure the frequency characteristic of the optical modulator relatively easily, the frequency characteristic measured by the spectrum analyzer is not limited to the optical modulator but the frequency of the light receiving circuit 24. The characteristics will also be included. On the other hand, in recent years, is being promoted faster optical modulator, it has been developed modulator having a bandwidth of more than 10GH _z. If you try to measure the properties of such a broadband optical modulator accurately, but the light receiving circuit, it is necessary to use a wider than the bandwidth of the modulator, the band of the light receiving circuit is also 10GH _z before and after, more However, there is a drawback in that the frequency response of the light receiving circuit is overlapped and accurate measurement becomes difficult when trying to measure the frequency response.

The present invention has been made in view of the above points, and an object thereof is to solve the problem that the influence of the frequency characteristic of the light receiving circuit influences the measurement of the characteristic of the optical modulator and the measurement becomes inaccurate. An object of the present invention is to provide a method and apparatus for accurately measuring the frequency characteristic of a modulator.

[Means for solving the problem]

In order to achieve the above-mentioned object, the frequency characteristic measuring method of an optical modulator according to the present invention combines the output lights of two single wavelength oscillation semiconductor lasers and inputs them to the optical modulator to be measured. However, a physical change is applied to at least one of the two semiconductor lasers to control the beat frequency, which is the oscillation frequency difference between the two semiconductor lasers, so as to be close to the electrical input frequency of the optical modulator. The light receiving circuit receives the output light of the optical modulator, and measures the intensity of the frequency difference component between the beat frequency and the electric input frequency contained in the output photocurrent.

A frequency characteristic measuring method according to another invention of the present invention is the above method, in which both frequencies are swept while the frequency difference between the beat frequency and the electrical input frequency of the optical modulator is kept substantially constant, and this sweep is performed. The output light of the optical modulator is received by the light receiving circuit at each frequency, and the intensity of the frequency difference component contained in the output photocurrent is measured.

Furthermore, an optical modulator frequency characteristic measuring device according to another invention of the present invention is an object to be measured by multiplexing two single wavelength oscillation semiconductor lasers and output lights of the two semiconductor lasers. An optical multiplexer input to the optical modulator, an oscillator for generating a signal of a predetermined frequency for driving the optical modulator, and a physical change in at least one of the two semiconductor lasers, and the two The controller for controlling the beat frequency, which is the oscillation frequency difference of the semiconductor laser, so as to be close to the frequency of the electric input of the optical modulator, the light receiving circuit for receiving the output light of the optical modulator, and the light receiving circuit A spectrum analyzer for measuring the intensity of the frequency difference component between the beat frequency contained in the output photocurrent and the electric input frequency is provided.

[Action]

Therefore, in the present invention, even when the electric input frequency to the optical modulator, that is, the modulation frequency ω _s is changed by using the optical beat signal generated by two continuous lights having different oscillation optical frequencies of the semiconductor laser. , The frequency component to be observed is the difference (ω _s −ω _b ) between the modulation frequency and the beat frequency ω _b which is the difference between the oscillation frequencies of the two semiconductor lasers.
In order constant it is, without being affected by the frequency characteristics of the light receiving circuit, performed by One cotton accurate measurements over a wide band 10GH _z.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 is a block diagram showing an embodiment of the frequency characteristic measuring apparatus for an optical modulator according to the present invention. In the figure, 1
1, 12 are oscillating light frequency variable semiconductor lasers, 13 is an oscillating frequency controller for controlling the oscillating light frequency of these semiconductor lasers 11, 12, and 14 is each of the semiconductor lasers 11, 12
An optical multiplexer that combines the output light from the, 15 is an optical modulator to be measured, 16 is an oscillator that drives the optical modulator 15, 17
Is a light receiving circuit for receiving the output light of the optical modulator 15, and 18 is a spectrum analyzer.

Next, the operation of this embodiment will be described. First, the output lights of the two semiconductor lasers 11 and 12 are combined by the optical combiner 14. At this time, the oscillation frequencies of the two semiconductor lasers 11 and 12 are set to ω ₁ and ω ₁ + ω _b , respectively, and the beat frequency ω _b , which is the difference between the oscillation frequencies of the semiconductor lasers 11 and 12, is driven to the optical modulator 15. The oscillating light frequency of the semiconductor laser 11 or 12 is controlled by the controller 13 so that it becomes a value close to the oscillating frequency ω _s of the oscillator 16. Semiconductor laser
A semiconductor DFB laser is suitable as 11,12. This DFB
The laser has a wide optical frequency variable range and is capable of continuous sweep of the oscillation optical frequency, which is convenient for measurement. The optical frequency sweeping method at this time can be realized by changing the temperature or bias current of the semiconductor laser. A detailed method of this frequency sweep is described, for example, in Japanese Patent Application No. 60-274412 of the same applicant.

Then, the output lights of the two semiconductor lasers 11 and 12 multiplexed by the optical multiplexer 14 are incident on the optical modulator 15. On the other hand, the light intensity of the optical modulator 15 is modulated by the output of the oscillator 16 with the electric signal of frequency ω _s . Therefore, the output light intensity P (t) of the optical modulator 15 is expressed by the following equation.

However, a is the modulation depth () of the optical modulator, and Fm (ω) is a parameter to be measured by the frequency characteristic of the optical modulator.
When this output light is received by the light receiving circuit 17, ω ₁ , ω
_Since the components of ₁ + ω _b and 2ω ₁ + ω _b have a very high frequency, the light receiving circuit 17 outputs a DC component in response to the average power. Therefore, the output photocurrent I of the light receiving circuit 17
(T) is expressed as follows.

However, F _d (ω) is the frequency characteristic of the light receiving circuit.

Therefore, the measurement of the present embodiment is performed by measuring the level of the component (ω _s −ω _b ), which is the last term of the equation (2). That is, first, the modulation frequency ω _s of the optical modulator 15 is set to a certain desired frequency. Then 2
By changing the oscillation frequency of one or both of the semiconductor lasers 11 and 12 of the stage, the value of the beat frequency ω _b is changed to (ω _s −
The value of ω _b ) is set to a low frequency such that it falls within the flat response frequency range of the light receiving circuit 17. That is, F _d (ω _s −
ω _b ) is always constant. In this case, the amplitude of the (ω _s −ω _b ) component when the modulation frequency ω _s of the oscillator 16 is successively changed is proportional to Fm (ω _s ), and the optical modulator at the modulation frequency ω _s It represents 15 modulation characteristics. When the level of the (ω _s −ω _b ) component is measured by the spectrum analyzer 18, the squared value of the output photocurrent is measured, so the measured frequency response is F ² _m ( Since it is proportional to ω), the measured value can be converted to obtain the characteristic of F _m (ω).

As described above, according to the present invention, the optical signal having the beat frequency ω _b is input to the optical modulator 15, and the difference between the modulation frequency ω _s of the optical modulator and the beat frequency ω _b is kept constant at a low frequency. By measuring the level of this (ω _s −ω _b ) component, it becomes possible to accurately measure the frequency characteristic of the optical modulator over a wide band.

〔The invention's effect〕

As described above, according to the present invention, by using two semiconductor lasers and using the optical beat signal generated by two continuous lights having different oscillation optical frequencies of these semiconductor lasers, the high frequency of the optical modulator can be achieved. Since the frequency response characteristic can be obtained by always measuring the level of a constant low frequency component, there is no possibility of being affected by the frequency characteristic of the light receiving circuit. Moreover, the frequency difference omega _b of the two semiconductor lasers used for measurement, since it is possible to cotton connexion swept just easily several 10GH _z or more ultra wideband changing a few degrees the temperature of the semiconductor laser, conventional of it is possible to accurately measure the frequency characteristics of Atsuta number 10GH _z or more ultra-broadband optical modulator difficult in the method, there is excellent effect with a high-speed optical transmission, and the like.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a frequency characteristic measuring device for an optical modulator according to the present invention, and FIG. 2 is a block diagram showing a configuration of a conventional frequency characteristic measuring method for an optical modulator. 11, 12 ... Semiconductor laser, 13 ... Oscillation frequency controller, 14 ... Optical multiplexer, 15 ... Optical modulator, 16 ... Oscillator,
17: Light receiving circuit, 18: Spectrum analyzer.

Claims (3)

[Claims] 1. Combined output lights of two single wavelength oscillation semiconductor lasers and inputting them to an optical modulator to be measured,
A physical change is applied to at least one of the two semiconductor lasers to control a beat frequency, which is an oscillation frequency difference between the two semiconductor lasers, so as to be close to a frequency of an electric input of the optical modulator. A frequency characteristic of the optical modulator, characterized in that the output light of the optical modulator is received by a light receiving circuit, and the intensity of the frequency difference component between the beat frequency and the electric input frequency contained in the output photocurrent is measured. Measuring method. 2. The method according to claim 1, wherein both frequencies are swept while the frequency difference between the beat frequency and the electric input frequency is kept substantially constant, and the output light of the optical modulator is received by the light receiving circuit at each of the swept frequencies. Then, the intensity of the frequency difference component contained in the output photocurrent is measured to measure the frequency characteristic of the optical modulator. 3. Two single wavelength oscillation semiconductor lasers, and
An optical multiplexer that multiplexes the output light of the semiconductor lasers to be incident on the optical modulator to be measured, an oscillator that generates a signal of a predetermined frequency for driving the optical modulator, and 2
A controller that applies a physical change to at least one of the two semiconductor lasers to control a beat frequency, which is an oscillation frequency difference between the two semiconductor lasers, so as to be close to a frequency of an electric input of the optical modulator. A light receiving circuit for receiving the output light of the optical modulator; and a spectrum analyzer for measuring the intensity of the frequency difference component between the beat frequency and the electric input frequency contained in the output photocurrent of the light receiving circuit. Characteristic optical modulator frequency characteristic measuring device.