JP2907140B2 - Modulation distortion measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation distortion measuring apparatus, and more particularly to a measuring apparatus for measuring the modulation distortion of a semiconductor laser for analog modulation used as a light source for a cable television (CATV) or a mobile communication system.

[0002]

2. Description of the Related Art As a conventional method for measuring the modulation distortion of a semiconductor laser for analog modulation, as shown in FIG. 3, a differential IL in a direct current (DC) driven (non-modulated) state is used.
There is a method of judging from the shape of the (current-light output) curve.
As shown in FIG. 3B, when the analog modulation semiconductor laser is non-defective with respect to modulation distortion, this measuring method is used.
Since the light output of the differentiated IL waveform is constant irrespective of the increase in the current, in the case of other differentiated IL waveforms, for example, a waveform that decreases nonlinearly as shown in FIG. As shown in FIG. 7C, when a waveform that increases nonlinearly is obtained, the analog modulation semiconductor laser is evaluated as defective with respect to modulation distortion.

As another method for measuring the modulation distortion of a semiconductor laser for analog modulation known in the art, a distortion characteristic is obtained by measuring the power of a distortion component in a state where the laser is actually modulated. There is a way. In this measuring method, a modulation distortion measuring device as shown in the configuration diagram of FIG. 4 is used.

In FIG. 1, a matrix generator 1
A multiplexed signal of a plurality of channels of carriers generated by 1 is supplied from a DC power supply 13 to a biased laser diode (LD) 15 via a coil 14 to drive the laser diode (LD) 15. As a result, light modulated by the multiplexed signal generated by the matrix generator 11 is emitted from the LD 15, is condensed by the lens 16, propagates through the optical fiber 17, is received by the light receiving element 18, and is photoelectrically converted. .

[0005] The electric signal obtained by the light receiving element 18 is subjected to low distortion amplification by a wide band low distortion amplifier 19 as a reception signal, and then input to a spectrum analyzer 20.
The power of the distortion component is measured. The distortion characteristics are evaluated based on the power measurement results.

[0006]

However, the conventional measuring method shown in FIG. 3 has a feature that it is very simple and requires only a small-scale measuring device, but it is a rather indirect measurement. There is a problem that valid data having a strong correlation with the value of the modulation distortion cannot be easily obtained.

On the other hand, the conventional measuring apparatus shown in FIG. 4 actually measures distortion in a state very close to the operating state of the LD 15, so that direct and most reliable modulation distortion measurement data is obtained. On the other hand, the matrix generator 11 is configured to generate a multiplexed signal of a carrier for 80 channels for CATV, for example, so that the matrix generator 11 is very expensive. In addition, there is a problem that the measurement takes a long time because obtaining the measurement result when one of the plurality of channels constituting the multiplexed signal is cut is repeated for all the channels.

[0008] The present invention has been made in view of the above points,
It is an object of the present invention to provide a modulation distortion measuring device capable of obtaining effective data with high reliability with respect to modulation distortion with a relatively small and inexpensive configuration.

Another object of the present invention is to provide a modulation distortion measuring apparatus capable of easily obtaining valid data on highly reliable modulation distortion in a short time.

[0010]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a DC power supply for supplying an arbitrary DC bias current to a semiconductor laser to be measured, and a DC power supply having a weaker level than the DC bias current . Actually emitted light of semiconductor laser
Signal supply means for superimposing a high frequency signal of a single frequency in a frequency band for modulating a DC bias current to a semiconductor laser, and receiving an optical output of the semiconductor laser to produce an electric signal
And a monitor for monitoring the power of the high-frequency signal component superimposed on the electric signal using a spectrum analyzer, and changing the high-frequency signal component by the monitor while changing the DC bias current from the DC power supply. The power is monitored.

Here, the monitoring means in the present invention comprises: a light receiving element for receiving the laser light emitted from the semiconductor laser and converting the laser light into an electric signal; and detecting the power of a high frequency signal component contained in the electric signal output from the light receiving element. Receiving the DC bias current from the DC power supply and the output detection value of the spectrum analyzer as input signals,
And a waveform drawing device for plotting the DC bias current on the horizontal axis or the vertical axis and the detected value on the vertical axis or the horizontal axis. Further, the signal supply means changes the frequency of the high-frequency signal to an arbitrary value each time the power of the high-frequency signal component is monitored by the monitoring means.

In the present invention, a high frequency signal of a single frequency at a constant level weaker than the DC bias current is superimposed on the DC bias current, supplied to the semiconductor laser, and driven. Here, as shown by a curve I in FIG. 2A, the semiconductor laser has a driving current-light output characteristic in which the light output increases nonlinearly at a predetermined threshold current or more. Accordingly, when the DC bias current has the value indicated by B1 in FIG. 2A, the optical output of the semiconductor laser when the constant-level high-frequency signal S1 is superimposed and supplied to the semiconductor laser is shown in FIG. As shown by L1, a relatively high level high frequency signal component is included. On the other hand, the DC bias current
The optical output of the semiconductor laser when the high-frequency signal S2 of a certain level is superimposed and supplied to the semiconductor laser at a large value indicated by B2 in (a) is relatively as indicated by L2 in FIG. A low-level high-frequency signal component is included.

The power of the high-frequency signal component included in the optical output is proportional to the slope efficiency of the semiconductor laser if the high-frequency signal level supplied to the semiconductor laser is kept constant. Therefore, while changing the DC bias current while keeping the level of the high-frequency signal supplied to the semiconductor laser constant, the power of the high-frequency signal component in the optical output is measured by the monitoring means, and the high-frequency signal component is compared with the DC bias current. Is plotted, as shown in FIG.
The differential I- at the signal frequency that actually modulates the semiconductor laser
An L waveform is obtained.

[0014]

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

FIG. 1 shows a configuration diagram of an embodiment of the present invention. In the figure, the same components as those in FIG. 4 are denoted by the same reference numerals. As shown in FIG. 1, in this embodiment, a signal generator 21 for generating a weak high-frequency signal (RF signal) and a direct current (DC) for applying a direct current bias voltage to a semiconductor laser (LD) 15 to be measured. (DC) A power supply 23 and a coil 14, a photodiode (PD) 22 for receiving light from the LD 15 and performing photoelectric conversion, a spectrum analyzer 24 for detecting power of an output electric signal of the PD 22, and an example of a waveform drawing apparatus XY recorder 25. In a CATV or mobile communication system, the spectrum analyzer 24 can measure the power of a signal in the 50 MHz to 900 MHz band, which is a frequency at which light emitted from a semiconductor laser is actually modulated.

Next, the operation of this embodiment will be described. R in the 50 MHz to 900 MHz band, which is the frequency that actually modulates the emitted light of the semiconductor laser in the CATV or mobile communication system generated by the signal generator 21.
The F signal is passed through the capacitor 12 to the LD 15 to be measured.
Is supplied as a drive current. A DC bias current is supplied to the LD 15 from the DC power supply 23 via the coil 14 simultaneously with the RF signal. The RF signal is set to a constant level that is weaker than the DC bias current.

The LD 15 emits light when driven by the RF signal superimposed on the DC bias current. The emitted laser light is received by the PD 22 and converted into an electric signal. The output electric signal of the PD 22 is supplied to the spectrum analyzer 24, where the power (spectral intensity) of the RF signal component superimposed on the optical output is detected.

The spectrum analyzer 24 supplies the detected peak value of the spectrum intensity to the Y-axis of the XY recorder 25. The X-axis of the XY recorder 25 has a DC
A voltage proportional to the current value of the power supply 23 is supplied. With such a configuration, while the level of the RF signal applied to the LD 15 is kept constant and the value of the DC bias current supplied from the DC power supply 23 is continuously changed, the power of the RF signal component included in the light emitted from the LD 15 is reduced. Detected and plotted on the XY recorder 25.

As described above, when the drive current vs. optical output characteristic of the LD 15 is shown by the curve I in FIG. 2A, the power of the RF signal component included in the optical output depends on the RF signal applied to the LD 15. If the level is kept constant, it is proportional to the slope efficiency of the LD 15. Therefore, the R applied to the LD 15
By changing the DC bias current while keeping the F signal level constant, measuring the power of the RF signal component in the optical output, and plotting the RF signal component against the DC bias current, FIG. As shown in (1), a differentiated IL waveform at the RF signal frequency is obtained.

Therefore, as described above, the DC power source 2 shown in FIG.
The output of the signal generator 21 is detected by detecting the power of the RF signal component contained in the light emitted from the LD 15 and plotting it on the XY recorder 25 while continuously changing the value of the DC bias current supplied from 3. The differential IL waveform at the RF signal frequency can be recorded by the XY recorder 25.

Further, the RF output from the signal generator 21
The XY recorder 25 can record the differential IL waveform at each RF signal frequency by changing the signal frequency within the frequency range actually used and performing the same measurement as described above. Such a differential IL waveform at an actual modulation frequency has a stronger correlation with distortion characteristics and higher reliability than a differential IL waveform obtained by a measurement method in a conventional DC drive state. A lot of useful information can be obtained. For example, the value of the second-order distortion can be estimated from the slope of the waveform, and the value of the third-order distortion can be estimated from the curvature.

Further, according to this embodiment, since an expensive device such as a matrix generator is not used, and the XY recorder 25 is not so expensive, the conventional device shown in FIG. Compared to the simple configuration, it is inexpensive (for example, about 1/5 times that of the apparatus shown in FIG. 4), and the measurement is completed in a short time because the high-frequency signal has a single frequency.

The present invention is not limited to the above embodiment. For example, the output DC voltage of the DC power supply 23 is set on the Y axis of the XY recorder 25, and the detected value of the spectrum analyzer 24 is set on the X axis. May be supplied. XY
Instead of the recorder 25, another waveform drawing device such as a liquid crystal display device or a display device of a personal computer can be used.

[0024]

As described above, according to the present invention,
The differential I- at the signal frequency that actually modulates the semiconductor laser
Since the L waveform is obtained, compared with the differential IL waveform obtained by the measurement method in the conventional DC driving state, the correlation with the distortion characteristic is strong, and many useful measurement information with high reliability can be obtained. it can.

In addition, compared with a conventional measuring device using a matrix generator or an optical fiber, a simple and inexpensive configuration can provide a measurement result of modulation distortion in a short time.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating the principle of the present invention.

FIG. 3 is a diagram illustrating an example of a conventional modulation distortion determination method.

FIG. 4 is a configuration diagram of an example of a conventional modulation distortion measuring device.

[Explanation of symbols]

 Reference Signs List 15 semiconductor laser (LD) 21 signal generator 22 photodiode (PD) 23 direct current (DC) power supply 24 spectrum analyzer 25 XY recorder

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01M 11/00 H01S 3/18

Claims (3)

(57) [Claims] A DC power supply for supplying an arbitrary DC bias current to a semiconductor laser to be measured ;
A signal supply unit that superimposes a high frequency signal of a single frequency in a frequency band that modulates light emitted from the semiconductor laser on the DC bias current and supplies the same to the semiconductor laser; and receives an optical output of the semiconductor laser. To electrical signals
Monitoring means for monitoring , using a spectrum analyzer, the power of the high-frequency signal component superimposed on the electric signal , while changing the DC bias current from the DC power supply, A modulation distortion measuring device for monitoring the power of the high-frequency signal component. 2. The monitor means for receiving a laser beam emitted from the semiconductor laser and converting the laser beam into an electric signal, and detecting the power of the high-frequency signal component included in the electric signal output from the light-receiving element. Receiving the DC bias current from the DC power supply and the output detected value of the spectrum analyzer as input signals, and receiving the DC bias current on the horizontal axis or vertical axis, and the detected value on the vertical axis or horizontal axis. 2. The modulation distortion measuring device according to claim 1, further comprising a waveform drawing device for plotting on an axis. 3. The signal supply unit according to claim 1, wherein the signal supply unit changes the frequency of the high-frequency signal to an arbitrary value each time the monitoring unit monitors the power of the high-frequency signal component. Modulation distortion measurement device.