JPH0359428A - Method and device for measuring frequency modulation characteristic of semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain frequency modulation characteristics by performing frequency discrimination by utilizing interference between light beams passed through two propagation paths corresponding to light frequencies. CONSTITUTION:The light of a semiconductor laser 4 to be measured is made incident on one input port of an optical branch circuit 6 through lenses 22 and 24 and an optical isolator 26 and its output port is connected to one input port of an optical coupler 10 through a path 8a. Then the other output port of the circuit 6 is connected to the other input port of the optical coupler 10 through a path 8b and a polarization controller 28, two output ports of the optical coupler 10 are coupled optically with photodetectors 12a and 12b of a double balanced type photodetector, and the difference signals between photocurrents generated by the elements 12a and 12b is amplified 30 and inputted to a network analyzer 20. At this time, amplitude-modulated components of the output currents of the elements 12a and 12b are removed and the frequency-modulated components are double in amplitude as compared with a case wherein the light is photodetected by a single element. Therefore, a frequency shift quantity corresponding to a modulated current supplied from the analyzer 20 to the laser 4 accurately is measured.

Description

[Detailed Description of the Invention] Table of Contents Overview Industrial Application Fields Prior Art (Figs. 7 and 8) Means for Solving the Problems to be Solved by the Invention (Fig. 1, Functions (Fig. 1) Fig. 3〉 Embodiment (Figs. 4 to 6) Effects of the invention Fig. 2) Overview Regarding a method and apparatus for measuring frequency modulation characteristics of a semiconductor laser, it is easy to obtain accurate frequency modulation characteristics of a semiconductor laser. For example, the purpose of the present invention is to provide a measurement method and apparatus capable of splitting light from a semiconductor laser to be measured that is driven by modulation, delaying one of the branched lights relative to the other, and inputting the branched light into an optical coupler. After the two optical outputs of the optical coupler are each subjected to optical-to-electrical conversion, a difference signal is obtained and this is compared with the modulation signal.

INDUSTRIAL APPLICATION FIELD The present invention relates to a method and apparatus for measuring frequency modulation characteristics of a semiconductor laser.

In recent years, in the fields of communication and measurement, the properties of light as waves,
That is, research and development of systems that actively utilize coherence is underway. For example, in a coherent optical communication system using the FSK method, frequency modulation of light is realized by directly modulating the bias current of a semiconductor laser as a transmitting side light source. Therefore, it is necessary to measure the frequency modulation characteristics of the semiconductor laser. Here, the frequency modulation characteristic of a semiconductor laser refers to a parameter on which the oscillation frequency of the semiconductor laser depends, for example, the ratio of frequency change to a unit change in bias current or its frequency characteristics.

BACKGROUND OF THE INVENTION A conventional method for measuring frequency modulation characteristics of a semiconductor laser will be explained with reference to FIG. 7(a). 102 is a semiconductor laser to be measured that is modulated and driven; 104 is a semiconductor laser to be measured 102
106 and 110 are optical fibers through which the branched lights from the optical coupler 104 are propagated with one delayed relative to the other, and 108 is the light from the optical fibers 106 and 110. 112 is a photoreceiver for optical-to-electrical conversion of the light from the optical coupler 108, and 114 is an amplifier for amplifying the received light signal from the photoreceiver 112. The signal from the amplifier 114 is input to the network analyzer 116, and the modulation current of the semiconductor laser 102 to be measured is provided by the network analyzer 116.

Optical coupler 104.108 and optical fiber 106°110
constitutes a so-called Matsuhatsu-Enda type optical interferometer, and its output characteristic with respect to optical frequency is as shown by the curve 124 in FIG. 8(a). When the optical frequency is changed at the portion A where the positive slope is the largest in the characteristic curve 124, the output level also changes accordingly, that is, frequency discrimination is performed, so the frequency modulation characteristics of the semiconductor laser can be grasped. be able to. However, in practice, when the bias current of a semiconductor laser is changed in order to change the optical frequency, the optical output intensity changes accordingly, making it impossible to grasp accurate frequency modulation characteristics.

The following methods can be used to suppress this undesired amplitude modulation component.

(2) In addition to point A on the characteristic curve 124, measurement is also carried out at point B, where the negative slope is maximum.

Then, focusing on the fact that the discrimination waveform 126 at point A and the discrimination waveform 128 at point B are in opposite phase, whereas the amplitude modulation waveform is in phase, the respective measurement results are stored in the network analyzer 116. By subtracting vectorwise, the amplitude modulation component is canceled out and accurate frequency modulation characteristics are obtained.

■ A phase shifter such as a Babinet-Soleil phase plate is connected to the optical fiber 1.
By inserting it into the middle of one of the fibers 06 and 110 and shifting the phase of the light propagating through the fiber by π, a characteristic curve 124' having a symmetrical shape at point A is obtained, as shown in FIG. By utilizing the fact that the discrimination curve 126' is in opposite phase to the discrimination curve 126 with respect to the characteristic curve 124, the amplitude modulation component is canceled out as in the case (2).

Another example of the conventional frequency modulation characteristic measuring method will be explained with reference to FIG. 7b). In this example, a polarization-maintaining fiber (polarization-maintaining optical fiber) 120 that propagates light while preserving the polarization plane is used, and the polarization plane of the light from the semiconductor laser 102 is, for example, relative to the main axis of the polarization-maintaining fiber 120. The plane of polarization is adjusted by the polarization controller 118 so that it is tilted at 45 degrees, and the light emitted from the polarization constant fiber 120 is made to enter the fiber polarizer 122. The fiber type polarizer 122 propagates only polarized light having a plane of polarization inclined at 45 degrees with respect to the main axis of the polarization constant fiber 120, and does not propagate polarized light having other planes of polarization. The light emitted from the fiber polarizer is then received by a light receiver 112.

In this conventional example as well, since an inter-mode delay occurs in the polarization-controlled fiber 120, it is possible to measure the frequency modulation characteristics according to the principle shown in FIG. 8(a) or (9). In addition, when following the principle of FIG.
It may be rotated.

Problems to be Solved by the Invention In conventional measurement methods, in order to eliminate errors caused by amplitude modulation components, double measurements are performed by changing the driving conditions of the semiconductor laser, or the setting of the optical system is changed. It is not easy to measure because it is necessary to redo the measurement and take a double measurement. Furthermore, if the driving conditions of the semiconductor laser are changed by bias current, temperature, etc., it is not possible to measure the characteristics under constant conditions, which becomes a problem when the characteristics measurement results under constant conditions are required. .

The present invention was created in view of such technical problems, and aims to provide a measuring method and apparatus that can easily obtain accurate frequency modulation characteristics of a semiconductor radar.

Means for Solving the Problems FIG. 1 is a principle block diagram showing a first configuration of the invention.

Reference numeral 6 denotes an optical branching circuit that branches the light from the semiconductor laser to be measured f4 which is modulated and driven by the modulation circuit 2, and 8 propagates the branched light from the optical branching circuit 6 by delaying one of them with respect to the other. an optical delay circuit; IO is an optical coupler into which the light from the optical delay circuit 8 is input; 12 is a double-balanced photoreceiver that converts the light from the optical coupler 10 into electricity and outputs a difference signal; 14 is a comparison means for comparing the output of the double-balanced photoreceiver 12 with the modulation signal.

When performing measurements using the measuring device having the first configuration, the light from the semiconductor laser 4 to be measured that is modulated and driven is split, one of the branched lights is delayed relative to the other, and an optical coupler is used. After the two optical outputs of the optical coupler 10 are subjected to optical-to-electrical conversion, a difference signal is obtained, and this is compared with the modulation signal.

FIG. 2 is a principle block diagram showing a second configuration of the invention.

16 is a constant polarization fiber into which light from the semiconductor laser 4 to be measured that is modulated and driven by the modulation circuit 2 is incident with a constant polarization plane; 18 is a polarization separator that separates the polarization of the light emitted from the constant polarization fiber 16; Reference numeral 12 denotes a double-balanced photoreceiver that photoelectrically converts the light from the polarization splitter 18 and outputs a difference signal; 14 is a comparison means that compares the output of the double-balanced photoreceiver 12 with a modulation signal. .

When performing measurements using the measuring device with the second configuration, the light from the semiconductor laser 4 to be measured that is modulated and driven is incident on the constant polarization fiber 16 with a constant polarization plane. After the output light is polarized and subjected to optical-to-electrical conversion, a difference signal is obtained, and this is compared with the modulation signal.

The first and second configurations of the invention measure frequency modulation characteristics by using the fact that light passing through two propagation paths given a delay time difference interferes depending on the optical frequency for frequency discrimination. The common technical idea is to use a double-balanced photoreceiver to eliminate the influence of the amplitude modulation component that occurs at that time. However, in the first configuration, an optical delay circuit consisting of equal optical fibers of different lengths is used to provide a delay time difference, whereas in the second configuration, an optical delay circuit is used to provide a delay time difference. The difference is that a polarization-constant fiber with birefringence is used. Common points between the first and second configurations will be explained below with reference to the model diagram shown in FIG.

FIG. 3 shows a measurement system configured according to the first configuration.

It is assumed that the optical delay circuit 8 is composed of a first path 8a made of an optical fiber or the like, and a second path 8b also made of an optical fiber or the like and having a longer optical path than the first path 8a. The optical coupler 1O consists of a half mirror 10a. Further, the double-balanced photodetector 12 is configured such that a first photodetector 12a and a second photodetector 12b are connected in series, and a difference signal is extracted from the connection point. Furthermore, the network analyzer 20 functions as the comparison means 14 and the modulation circuit 2.

Now, the electric field of the light emitted from the semiconductor laser 4 to be measured is expressed as E
ls The electric field of the light that passes through the first bus 8a, is reflected by the half mirror 10a, and enters the light receiving element 12a is expressed as E2, the second bus 8
E3 is the electric field of the light that passes through b, passes through the half mirror 1Oa, and enters the first light receiving element 12a, and E3 is the electric field of the light that passes through the first path 8a, passes through the half mirror 10a, and enters the second light receiving element 12b. Let E4 be the electric field of the light that passes through the second bus 8b, is reflected by the half mirror 10a, and enters the second light receiving element 12b. Since E, is the electric field of amplitude-modulated and frequency-modulated light, E+ =2 (1+
Msin2πf, to)E.

+(27r(iloto+Δ ν5in(2π f, t
o)), E2 to E are each expressed as follows, ignoring loss.

Here, M is the modulation index of amplitude modulation, fm is the modulation frequency, ω0 is the optical frequency, Δν is the amount of frequency deviation, and τ is the delay time difference between the first bus 8a and the second bus 8b. Also, E.S.
The sign of is reversed because when the light from the second bus 8b is reflected by the half mirror 10a, the phase is shifted by π according to the difference in the refractive index of the constituent materials of the half mirror.

At this time, the photocurrent h2 generated in the first light receiving element 12a and the photocurrent 2 generated in the second light receiving element 12b are expressed as follows.

11 =(E2+E3)(E2"+E3")=K C(
A(t)) '+ (A(t+τ))2+ A(t)
A(t+r) Bs1n(2πf, t+φ)] ・
(6) 1, =K [: (A(t,))'+(A(t,)
+τ))2- A(t) A(t+ r) Bs1n(
2yrf, t+φ)]−(7) Here, K is a photoelectric conversion coefficient, B is a conversion coefficient from frequency modulation to amplitude modulation, and φ is a phase difference. Therefore, the output 1 from the double-balanced receiver
out is expressed as follows.

1out=1 1 12 2 A(t)A(t+ r)Bsin(2πf, t+
φ)-(8) As is clear from the equation (8), most of the amplitude modulation components are canceled out, and the measurement error caused by the amplitude modulation components is significantly reduced. As a result, extremely accurate frequency modulation characteristics can be measured. Further, in this case, complicated work such as reassembling the optical system is not necessary.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 4 is a block diagram of a measuring device showing an embodiment of the first configuration. The light emitted from the semiconductor laser 4 to be measured is
For example, the light is inputted to one input port of the optical branching circuit 6 consisting of a one-to-one optical coupler via lenses 22 and 24 arranged at approximately confocal positions and an optical isolator 26 inserted between them. The other input port of the optical branch circuit 6 is not used. One output port of the optical branch circuit 6 is directly connected to one input port of the optical coupler IO by the first path 8a of the optical delay circuit, and the other output port of the optical branch circuit 6 is connected to the second bus 8b of the optical delay circuit. and the polarization controller 28 to the other input port of the optical coupler IO. Here, the polarization controller 28 is used when the first bus 8a and/or the second path 8b constituting the optical delay circuit are ordinary optical fibers that do not have polarization maintaining properties. This is to prevent interference efficiency from decreasing. Hikari Kabra 10-2
The two output ports (2) and (2) are made of, for example, tapered spherical fibers, and are optically coupled to the light receiving elements 12a, 12b of the double-balanced light receiver with the same optical path length, respectively. and,
Difference signals between photocurrents generated in the light receiving elements 12a and 12b are input to the network analyzer 20 via an amplifier 30. At this time, the light receiving element 12a
, 12b respectively correspond to the modulated waveforms at point A in Fig. 8(b), so the output current of the double-balanced photoreceiver has the amplitude modulation component removed. Furthermore, the amplitude of the frequency modulation component is doubled compared to when the light is received by a single light receiving element. Therefore, it is possible to accurately measure the amount of frequency deviation corresponding to the modulation current given from the network analyzer 20 to the semiconductor laser 4 under test in real time, and by looking at the frequency characteristics, the frequency deviation of the semiconductor laser 4 under test can be measured accurately. Modulation characteristics can be easily and accurately measured.

In the configuration of this embodiment, the branching ratio of the optical coupler and optical coupler 10 used as the optical branching circuit 6 can be set to 1:1, and by doing so, the photocurrent generated by interference can be maximized and the measurement sensitivity can be improved. can be increased.

FIG. 5 is a block diagram of a measuring device showing an embodiment of the second configuration. The polarization plane of the light from the optical isolator 26 is controlled by the polarization controller 32 so that the light from the semiconductor laser 4 to be measured enters the polarization constant fiber 16 with a constant polarization plane, and is output from the polarization constant fiber 16. The polarized light separator 18 configured using a dielectric multilayer filter etc.
The polarization is separated into two polarization components whose polarization planes are orthogonal to each other. The polarized light beams enter the light receiving elements 12a and 12b with the same optical path length, respectively.

The plane of polarization of the light incident on the polarization constant fiber 16 is set at an angle of 45° with respect to the principal axis of the polarization constant fiber 16 so that two independent fundamental modes of the polarization constant fiber 16 are equally generated from this incident light. It is set to do the following. This is for the same reason as the one-to-one optical coupler used in the previous embodiment.

FIG. 6 shows P-polarized light and S-polarized light separated by the polarization separator 18.
FIG. 3 is a diagram for explaining the relationship between the polarization plane of polarized light and the main axis of the polarization constant fiber 16. In the stress birefringence type polarization constant fiber, its main axis PA is on the plane connecting the center of the core 16a of the polarization constant fiber 16 and the center of the stress applying part 16b. or the polarization plane of S-polarized light by 45°. By doing this, the output port (■) of the polarization separator 18.

From ■ is the output port of the optical coupler lO in the previous embodiment■
Since signal light equivalent to the signal light from , (2) can be obtained, the frequency characteristics of the semiconductor laser 4 to be measured can be measured.

As described in detail of the invention, according to the measuring method or apparatus according to the first or second configuration of the invention, it becomes possible to accurately and easily measure the frequency modulation characteristics of a semiconductor laser. This effect is achieved.

[Brief explanation of drawings]

Fig. 1 is a principle block diagram showing the first configuration of the invention, Fig. 2 is a principle block diagram showing the second configuration of the invention, Fig. 3 is a model diagram explaining the principle of the invention, and Fig. 4 is a principle block diagram showing the first configuration of the invention. Figure 5 is an example diagram of the second configuration. Figure 6 is a diagram showing the polarization plane (p) of polarized light separated by the polarization separator.
. 2... Modulation circuit, 4... Semiconductor laser to be measured, 6... Optical branch circuit, 8... Optical delay circuit, 10
...optical coupler, 12...double balanced receiver,
14... Comparison means, 16... Polarization constant fiber, 18... Polarization separator, 20... Network analyzer.

Claims (4)

[Claims] (1) Modulation driven (2) Semiconductor laser under test (
4) is branched (6), one of the branched lights is delayed (8) relative to the other, and input to an optical coupler (10);
A frequency semiconductor laser characterized in that the two optical outputs of the optical coupler (10) are each subjected to optical-to-electrical conversion to obtain a difference signal (12), which is compared (14) with a modulation signal. Modulation characteristic measurement method. (2) Optical branching circuit (6) that branches the light from the semiconductor laser under test (4) that is modulated and driven by the modulation circuit (2).
), an optical delay circuit (8) that propagates the branched lights from the optical branch circuit (6) by delaying one of them with respect to the other, and the light from the optical delay circuit (8) is inputted. Optical coupler (1
0), and a double-balanced photoreceiver (12) that converts the light from the optical coupler (10) into electricity and outputs a difference signal.
A frequency modulation characteristic measuring device for a semiconductor laser, comprising comparing means (14) for comparing the output of the double-balanced photodetector (12) with a modulation signal. (3) Modulation driven (2) Semiconductor laser to be measured (
4) is input into a constant polarization fiber (16) with a constant plane of polarization, and the output light from the constant polarization fiber (16) is polarized and separated (18), and after optical-to-electrical conversion, the difference signal is obtained. (12) and compares this with a modulation signal (14). (4) a constant polarization fiber (16) into which light from the semiconductor laser to be measured (4) modulated and driven by the modulation circuit (2) is incident with a constant polarization plane; a polarization separator (18) that polarizes the emitted light; and a double-balanced photoreceiver (12) that converts the light from the polarization separator (18) into electricity and outputs a difference signal. A frequency modulation characteristic measuring device for a semiconductor laser, comprising comparing means (14) for comparing the output of the double-balanced photodetector (12) with a modulation signal.