JP3354630B2 - Optical transmission characteristics 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 an optical transmission characteristic for measuring transmission characteristics of optical components such as an optical wavelength filter used for optical communication and measurement, that is, amplitude and phase frequency characteristics such as transmission, absorption and reflection. The present invention relates to a measuring device, and more particularly to an optical transmission characteristic measuring device with improved accuracy of a measuring frequency.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional optical transmission characteristic measuring device (see FIG.
3 of what is commonly referred to as a light wave network analyzers. FIG. The frequency-variable semiconductor laser 1 generates frequency-swept light. The frequency sweeping is performed by the frequency sweeping means 2 controlling the temperature of the semiconductor laser 1 using a Peltier element or the like to change the oscillation operating point of the semiconductor laser 1. Semiconductor laser 1
The light emitted from the light enters the optical branching circuit 3, where it is branched into two paths, one to the acousto-optic modulator 4 (hereinafter AOM4) and the other to the optical coupler 8 as reference light. Is emitted.

The AOM 4 is driven by a drive signal output from a drive signal source 5 and shifts the light incident from the optical branching circuit 3 by a drive frequency f (referred to as diffracted light) to be used as measurement light. The light is emitted to the measurement object 6. The drive frequency f of the drive signal is set to, for example, 100 MHz so that the AOM 4 can obtain a sufficient diffraction efficiency.

The measuring light transmitted through the device under test 6 enters the optical coupler 8. The optical coupler 8 combines the measurement light with the reference light from the optical branching circuit 3 and emits interference light. The photoelectric converter 9 converts the interference light into an electric beat signal and outputs the signal to the amplitude detector 10 and the phase comparator 11. The frequency of the beat signal is the driving frequency f of the AOM4.
And 100 MHz in the case of the above example. Note that the optical coupler 8 and the photoelectric converter 9 constitute the optical interference unit 7.

[0005] The amplitude detector 10 detects a beat signal and detects an amplitude decay rate given by the DUT 6 to the measurement light. Further, the phase comparator 11 compares the phase of the drive signal of the drive signal source 5 (which is a reference for phase comparison) with the beat signal, and detects the amount of phase transition given to the measurement light by the device under test 6. These detection results are displayed on a display (not shown) or the like in correspondence with the frequency sweep of the semiconductor laser 1, so that the frequency of the amplitude and phase of the device under test (for example, an optical wavelength filter or the like) is displayed. Properties can be measured. Alternatively, the beat signal output from the photoelectric converter 9
Signal, instead of the amplitude detector 10 and the phase comparator 11,
If the frequency analysis device receives and analyzes the frequency,
It is possible to measure the structure and state in the longitudinal direction.

[0006]

However, in the conventional method of changing the oscillation operating point by controlling the temperature of the frequency-variable semiconductor laser 1 as the frequency sweeping method, the frequency accuracy during the frequency sweeping is as low as several hundred MHz. Therefore, a problem arises when an accuracy of several tens of MHz or less is required, such as when measuring an object to be measured in a narrow band such as a Fabry-Perot resonator (FIG. 4A). That is, it is not easy to set the frequency only once. Even if the frequency can be set again and the measurement can be performed several times, the peak points of the two amplitudes of the Fabry-Perot resonator ((
B) There was a problem that the frequency of (1) cannot be accurately grasped.

Further, since the frequency sweep is performed by the method described above, the reproducibility and stability of the frequency are poor, and the thermal time constant of the mount portion of the semiconductor laser 1 and the Peltier device is limited. And the sweep time is 20 seconds / 20
There is a problem as a practical measuring device, such as a low GHz (measuring time is long). Frequency solution received by frequency analyzer
When performing analysis, the frequency accuracy and sweep speed
Affect.

[0008]

In order to solve the above-mentioned problems, according to the present invention, an optical loop path for circulating an optical pulse is formed between an acousto-optic modulator and a return path, and the optical loop path is formed in the optical loop path. A frequency sweep is performed in a stepwise manner by introducing an optical pulse having a fixed frequency and rotating the optical pulse, and shifting the frequency by the driving frequency f of the acousto-optic modulator for each rotation.
Further, the acousto-optic modulator also serves as a frequency shifting means for frequency sweeping and a branching means for branching the light pulse into undiffracted light and diffracted light different from the undiffracted light by a driving frequency f.

[0009]

The frequency during the frequency sweep is an integral multiple of the drive frequency f of the acousto-optic modulator based on the fixed optical frequency included in the light pulse, so that the relative frequency accuracy during the sweep is driven. It can be as high as the accuracy of the frequency f. Since the sweep time is determined mainly by the time required for the optical pulse to circulate in the optical loop, the speed can be increased. Further, by making the acousto-optic modulator serve as both the frequency shifting means and the branching means, not only diffracted light necessary for frequency sweeping but also undiffracted light can be effectively used. As a result, the reference light and the measuring light can be used. Can be simultaneously generated by the acousto-optic modulator.

[0010]

FIG. 1 is a block diagram of an optical transmission characteristic measuring apparatus showing an embodiment of the present invention. The same components as those in the conventional example are denoted by the same reference numerals, and detailed description of those components will be omitted. The optical pulse generating means 12 includes a light source 13 and an optical pulse modulator 14, and generates an optical pulse including light of a fixed frequency (light that has not been frequency-swept). That is, the fixed-frequency light source 13 continuously generates light having a stable frequency and emits the light to the optical pulse modulator 14. The optical pulse modulator 14 generates an optical pulse by applying a pulse modulation to the light incident from the light source 13 with a pulse signal input from the control device 19.

The above-mentioned optical pulse is incident on the AOM 4 via the optical coupler 16 and the waveform shaper 17. AOM4
Is a drive signal (frequency f) output from the drive signal source 5
The optical pulse (1) is frequency-shifted by the driving frequency f by the incident optical pulse.
And a light pulse that does not shift in frequency (referred to as zero-order light or undiffracted light), and these light pulses are split into diffracted light and undiffracted light to be spatially different. Emit in the direction. That is, the light pulse of the diffracted light is emitted to the optical branch circuit 18, and the light pulse of the undiffracted light is emitted to the DUT 6 as measurement light.

In this case, assuming that the frequency of the light pulse incident on the AOM 4 is f ₀ , the frequency of the undiffracted light is the same as that of the incident light pulse, and the frequency of the diffracted light is f ₀ .
₀ + f. Note that f ₀ + f is the case where the + 1st-order light is used as the diffracted light, and becomes f ₀ −f when the −1st-order light is used.
Hereinafter, a case where + 1st-order light is used will be described.

Next, the light pulse of the diffracted light that has entered the optical branching circuit 18 is branched here, one of which is emitted to the optical interference means 7 as reference light and the other is emitted to the optical coupler 16. The optical pulse of the diffracted light incident on the optical coupler 16 is incident again on the AOM 4 via the waveform shaper 17.
Then, the AOM 4 splits the incident light pulse into diffracted light and undiffracted light again and emits them. At this time, the respective frequencies are f ₀ + f for the non-diffracted light and f ₀ + 2f for the diffracted light.

From the frequency change described above, every time the diffracted light emitted from the AOM 4 enters the AOM 4 via the optical branching circuit 18, the optical coupler 16 and the waveform shaper 17, and is output from the AOM 4, a non-diffused light is output. It can be seen that the frequencies of the diffracted light and the diffracted light increase by the frequency f, that is, the frequency is swept.

Therefore, the optical branch circuit 18, the optical coupler 1
6, a feedback path 15 is formed by the waveform shaper 17, and an optical loop path is formed by the return path 15 and the AOM 4, and an optical pulse from the optical pulse generating means 12 is introduced into the optical loop path. The frequency is shifted by the driving frequency f of the AOM 4 for each rotation, so that a stepwise frequency sweep is performed. Although not shown in FIG. 1, the return path 15 may include an optical fiber.

Note that the waveform shaper 17 has a function of A in the optical loop path.
It not only compensates for the loss of the OM 4, the optical branching circuit 18 and the optical coupler 16, but also functions as a limiter due to gain saturation when the light quantity increases too much.
It is composed of a D amplifier and the like.

The operation of the above-described frequency sweep will now be described with reference to FIG. The light source 13 generates continuous light having a frequency f ₀ as shown in FIG. The optical pulse modulator 14 applies pulse modulation to the continuous light with a pulse signal (pulse width Tp, cycle Ts) as shown in FIG. An optical pulse having a frequency f ₀ and a pulse width Tp is generated. This optical pulse is transmitted through the optical coupler 16 and the waveform shaper 17 to the AOM.
4 is incident.

The AOM 4 is driven by a drive signal (frequency f) from the drive signal source 5, and splits an incident light pulse into undiffracted light and diffracted light and emits the same. The frequencies of the undiffracted light and the diffracted light emitted first are shown in FIG.
As shown in (f), they are f ₀ and f ₀ + f, respectively. Then, the optical pulse of the diffracted light emitted from the AOM 4 enters the AOM 4 again via the return path 15 (circulates the optical loop path once), and is emitted again from the AOM 4 (second time). The frequencies of the undiffracted light and the diffracted light emitted this second time are, as shown in (e) and (f),
They are f ₀ + f and f ₀ + 2f, respectively. Further on,
When the optical pulse circulates around the optical loop path as described above, the frequency of the undiffracted light and the frequency of the diffracted light become (e),
As shown in (f) and (f), each of them increases by f.

Then, the light pulse that has entered the AOM 4 while circulating next to the above is emitted only as non-diffracted light (no generation of diffracted light) as shown in FIG. In other words, the first sweep (starting from t ₀ ) ends in the period of 、, and in に お い て, the unnecessary light pulse remaining in the optical loop path and emitted in the next sweep (starting from t ₃ ) is emitted. ing. For this, the controller 19 controls the driving signal source 5 by a control signal as shown in (b), t ₂
Period ~t ₃ is as the drive signal is not supplied to the AOM4. Note that the drive signal is supplied to the AOM 4 during the period from t _{0 to} t ₂ . By repeating the above operation, the second, third,... Frequency sweep is performed.

The pulse width Tp of the light pulse must be less than the light propagation time of the optical loop. FIG.
(E) and (f) show examples in which the pulse width Tp is made equal to the light propagation time to make pseudo continuous light. As a method of making the pulse width Tp equal to the light propagation time, there is a method of adjusting the pulse width Tp or adjusting the light propagation time by adding (or changing the length of) an optical fiber to the return path 15. is there. In this way, the reference light and the measurement light are frequency-swept in a stepwise manner and emitted from the optical branch circuit 18 and the AOM 4, respectively.

The reference light and the measurement light transmitted through the device under test 6 are incident on an optical interference means 7 composed of an optical coupler 8 and a photoelectric converter 9 and are converted into an electric beat signal of a frequency f. Converted and output. The amplitude detector 10 detects the beat signal and detects the amplitude characteristic of the device under test. Further, the phase comparator 11 performs a phase comparison between the beat signal and the drive signal (frequency f) to detect a phase characteristic of the device under test. Therefore, with the optical transmission characteristic measuring apparatus described above, the steep characteristic of the Fabry-Perot resonator shown in FIG. 4B can be easily and accurately measured.

In the embodiment shown in FIG. 1, the light pulse generating means 12 is constituted by the light source 13 and the light pulse modulator 14. However, the present invention is not limited to this. An optical pulse generator that generates an optical pulse may be used. Further, in the above-described embodiment, an example in which the frequency is sequentially increased has been described. However, in the case of sequentially decreasing the frequency, the above-described -1st-order light may be used as the diffracted light.

In the above embodiment, the case where the amplitude / phase characteristics of the DUT 6 are measured has been described. However, when the reflection characteristics are measured, as shown in FIG.
An optical directional coupler 20 may be added between the device and the device under test 6. Further, in the above embodiment, the undiffracted light is used as the measurement light,
Although the diffracted light is used as the reference light, the reverse may be used.

[0024]

The frequency during the frequency sweep is an integer multiple of the drive frequency f of the acousto-optic modulator (AOM) with respect to the fixed optical frequency included in the light pulse. The frequency accuracy was as high as several Hz equal to the accuracy of the drive frequency f. By using a reference light source for the light pulse generation means, the absolute accuracy is 1.
10 MHz or less was achieved in the 5 μm band. As a result, frequency reproducibility and stability were also excellent.

The sweep time is determined mainly by the time required for the optical pulse to circulate in the optical loop path.
The speed could be increased to 20 ms / 20 GHz with respect to 0 GHz. In this sweep time, the driving frequency of the acousto-optic modulator is 10 MHz, and the optical path length of the optical loop is 3 k.
m (the optical fiber length of the return path is about 2 km). In addition, by making the acousto-optic modulator serve both as a frequency shift unit and a branching unit, not only diffracted light required for frequency sweeping but also undiffracted light can be effectively used,
As a result, the reference light and the measurement light could be simultaneously generated by the acousto-optic modulator.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical transmission characteristic measuring apparatus showing one embodiment of the present invention;

FIG. 2 is a timing chart showing an operation state of each unit of the present invention;

FIG. 3 is a block diagram showing a conventional example;

FIG. 4 is a diagram showing a configuration and characteristics of a Fabry-Perot resonator;

FIG. 5 is a view showing another embodiment of the present invention.

[Explanation of symbols]

4... Acousto-optic modulator (AOM), 5... Drive signal source,
6... DUT, 7... Optical interference means, 8, 16... Optical coupler, 9... Photoelectric converter, 10... Amplitude detector, 11.
..Phase comparator, 12 ... light pulse generating means, 13 ... light source, 14 ... light pulse modulator, 15 ... feedback path, 17 ...
..Waveform shaper, 18 optical branch circuit, 19 control device, 20 optical directional coupler.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01J 9/02 G01M 11/00 G01B 9/02 G02F 2/02

Claims (1)

(57) [Claims] 1. An optical transmission characteristic measuring device for measuring a transmission characteristic of a device under test from a reference light and a measuring beam passing through the device under test, comprising: an optical pulse generating means (12) for generating a light pulse; The pulse rotates as an input, and a part of the light amount is
A 1-input 2-output optical loop path for output
Optical coupler for inputting a signal to the optical loop path (16)
Diffracts the light pulse circling the optical loop path.
Frequency shift and undiffracted light
An acousto-optic modulator (4) for outputting from the loop,
A part of the light amount of the light pulse circling the loop
An optical branching means (18) for outputting the light from the optical loop path;
An optical pulse disposed at an arbitrary position and circulating through the optical loop path
And a waveform shaper (17) that compensates for the loss of light amount.
The optical loop path and one of two outputs of the optical loop path pass through the device under test.
<br/> optical transmission characteristic measuring apparatus characterized by comprising an optical interference means (7) and the measuring light to obtain interference with other a is the reference light of the two outputs.