JPH0716982Y2 - Optical frequency change measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION << Industrial Application Field >> The present invention relates to improvement of an apparatus for measuring a spectrum change of a semiconductor laser with high accuracy.

<< Prior Art >> If the chirp width of a DFB laser can be narrowed, chromatic dispersion can be reduced, and thus long-distance, large-capacity communication and transmission by wavelength multiplexing become possible. For this reason, chirping is being suppressed by various methods, but its evaluation is also important. Conventionally, a diffraction grating type spectroscope has been used for this purpose, but it has been difficult to measure the spectrum change with high resolution.

FIG. 5 shows a conventional example in which a diffraction grating type spectrometer and a Fabry-Perot resonator are combined. The measured light is a diffraction grating 41
Directly into the light receiving element 43 through the optical switch 42
Alternatively, the light enters the light receiving element 45 via the Fabry-Perot resonator 44. From the light receiving element 43, a frequency spectrum having a coarse resolution is obtained, and from the light receiving element 45, a frequency spectrum having a relatively high resolution is obtained.

<Problems to be solved by the device> However, the incident conditions of the Fabry-Perot resonator are difficult, and mechanical stability also becomes a problem. Further, when the change in frequency becomes large, the relationship between the light intensity and the optical frequency becomes non-linear. Also, changes in the intensity of the light source affect the measurement. Furthermore, the structure is complicated.

The present invention has been made to solve such a problem, and an object thereof is to realize an optical frequency change measuring device capable of measuring the spectrum change of a laser to be measured with high resolution.

<< Means for Solving the Problem >> An optical frequency change measuring apparatus according to the present invention includes a first optical fiber coupler for entering a light to be measured and branching the light in two directions, and a first optical fiber coupler having two components. A second optical fiber coupler for making the lights branched to two output ports incident on different input ports to interfere with each other, and the first optical fiber coupler.
An ultrasonic optical modulator for frequency shifting output light from one output port of the coupler; a delay fiber for delaying output light from one or the other output port of the first optical fiber coupler by a predetermined time; A light receiving element that detects the optical beat signal from the second optical fiber coupler, an oscillator that drives the ultrasonic optical modulator, and a phase difference between the signal corresponding to the output of this oscillator and the output of the light receiving element. It is characterized by comprising a phase comparator for detecting and a signal processing device for calculating the frequency change of the light under measurement from the output of the phase comparator.

<< Operation >> Since the frequency change of the measured light is converted into the phase change of the optical beat signal, the frequency change can be calculated by the signal processing device after the phase difference with the oscillator output is detected by the phase comparator.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical frequency change measuring device according to the present invention. In the light source unit A, 1 is a laser to be measured, such as a semiconductor laser, to which a DC bias voltage V _B is connected via an inductance L, and 2 is a laser for applying a modulation signal fin to the laser 1 to be measured via a capacitor C. An input terminal 3, a collimator lens for collimating the emitted light of the laser to be measured 1 into parallel light, 4 an optical isolator for preventing the return light which makes the output light of the lens 3 incident, and 5 condenses the emitted light of the optical isolator 4. A condenser lens, 6 is an optical input connector for guiding the output light of the lens 5 to an optical fiber, 22 is a constant temperature bath for storing the laser to be measured 1, and 23 is a temperature control circuit for keeping the temperature of the constant temperature bath 22 constant. 7 is an optical input connector 6
The light from the light source unit A output from the first input port 71
Is composed of a part of the polarization-preserving fiber 8 and a polarization-maintaining fiber 8 into which the light emitted from the second port 74 of the optical coupler 7 is incident. Delay fiber, 12 is a polarization-maintaining fiber that inputs the light emitted from the first output port 73 of the first optical coupler, 13 is a collimator lens that makes the light emitted from the polarization-maintaining fiber 12 parallel light, and 14 is a lens An ultrasonic optical modulator that applies a frequency shift to the output light of 13, an optical lens 15 that collects the output light of the ultrasonic light modulator 14, a polarization-maintaining fiber 16 that inputs the output light of lens 15, and a polarization-preserving fiber 11 The outgoing light of the wave preserving fiber 8 is made incident on the first input port 111, and the outgoing light of the polarization preserving fiber 16 is made second.
Second optical coupler incident on the input port 112 of the optical coupler 11, 17 is a light receiving element such as a photodiode for receiving the light output from the second output port 114 of the optical coupler 11, and 18 is an output electric signal of the light receiving element 17. A light receiving circuit for current / voltage conversion / amplification, 19 is a high frequency oscillator for driving the ultrasonic optical modulator 14, 20 is a phase comparator for outputting a phase difference signal between the output of the high frequency oscillator 19 and the output of the light receiving circuit 18, and 21 is a phase It is a signal processing display device for calculating and displaying the frequency change of the measured light from the output of the comparator 20. The second input port 72 of the optical coupler 7 and the optical coupler 11
The first output port 113 of is not used. Optical coupler 7,11
A polarization-maintaining coupler is preferred as the above.

The operation of the optical frequency change measuring device having the above configuration will be described below. The frequency of the light under measurement emitted from the light source unit A changes with time, and its complex amplitude is expressed by the following equation.

E ₁ = E ₁₀ exp [j2π {f ₀ t + ∫F (t) dt}] (1) where E is the amplitude of light, f ₀ is the initial value of the optical frequency, and F is
(T) is a frequency change of light. This light is an optical coupler 7
Is split into two optical paths, and the delay fiber 9
Then, a delay of a predetermined time is caused, and in the other optical path, after being frequency-shifted by f _{1 in the} ultrasonic optical modulator 14, they are mixed in the optical coupler 11 and the optical beat signal is detected by the light receiving element 17. Two
Assuming that the optical path difference between the two optical paths is L and the time delay due to the optical path difference is τ, the complex amplitude of the delayed light is expressed by the following equation. That is, the complex amplitude of the light delayed by the delay fiber 9
When _{E 2, E 2 = E 20} exp [j2π {f 0 (t-τ) + ∫F (t-τ) d
t}] (2) On the other hand, assuming that the complex amplitude of the light frequency-shifted by the ultrasonic optical modulator 14 is E ₂ ′, E ₂ ′ = E ₂₀ ′ exp [j2π {f ₀ t + f ₁ t + ∫F ( t) dt}]
... (2) Light of 'E ₂ and E _2' are superimposed by the optical coupler 11. The intensity I of the combined light is expressed by the following equation.

I∝ ｜ E ₂ ＋ E ₂ ′ ｜ ² ＝ E ₂₀ ² ＋ E ₂₀ ′ ² ＋ 2E ₂₀・ E ₂₀ ′ cos
[2π {∫ (F (t) −F (t−τ)) dt + f ₁ t + f
₀ τ}] (3) When F (t) does not change much during the time τ, it can be approximated as F (t) −F (t−τ) ≈τdF (t) / dt (4), Expression (3) is as follows. I∝
E ₁₀ ² + E ₂₀ ² + 2E ₁₀ E ₂₀ cos [2π {τF (t) + f ₁ t + f ₀
τ}] (5) Therefore, the phase of the optical beat signal of the equation (5) is 2πτ.
It is {F (t) + f ₀ }. Since the initial value f ₀ of the optical frequency is constant, when the delay time τ is constant, the phase change of the optical beat signal depends only on the frequency change F (t) of the light. Under such a condition, if the delay time τ is known, the optical beat signal is photoelectrically converted by the light receiving element 17, and the phase change thereof is electrically compared by the phase comparator 20 with the output signal of the oscillator 19. To detect. At this time, since the relationship between the optical frequency change amount and the phase angle changes linearly within the phase change of 2π as shown in FIG. 2, the signal processing display device 21 calculates the optical frequency change amount F (t). ,indicate. By calibrating the phase change amount against the frequency change amount in advance,
An accurate display value can be obtained.

According to the optical frequency change measuring device having such a configuration, it is possible to measure the change in the optical frequency with high resolution. Also, the deviation of the modulated light from the constant modulation frequency can be measured in the same manner.

Further, since the optical system is configured by using the optical fiber and the optical coupler, a stable measurement system can be realized.

It is also easy to construct a chirp suppression feedback loop using the optical frequency change signal.

The light input unit including the optical isolator 4 of the light source unit A may be a laser module for communication having a similar function in which a laser and an optical fiber are coupled.

FIG. 3 is a partial block diagram showing a second embodiment of the optical frequency change measuring device according to the present invention. Utilizing that the measuring range of frequency change is determined by the delay fiber length,
In place of the delay fiber 9 of the apparatus shown in FIG. 1, a plurality of delay fibers 25 to 27 having different delay fiber lengths are used as an optical switch.
The optical path is switched by 24 and 28 so that a wide measurement range can be selected.

FIG. 4 is a block diagram showing a third embodiment of the optical frequency change measuring device according to the present invention. The same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. An ultrasonic optical modulator 30 is inserted in the middle of the optical fiber 8 instead of the delay fiber 9 to cause a frequency shift f ₂ and a frequency shift of | f ₁ −f ₂ | is generated as a whole. is there. With this configuration, the reference frequency of the phase comparator can be set to a low frequency. In addition, 10 is a delay fiber provided in the middle of the optical fiber 16, 29 is a lens that makes the light of the fiber 8 enter the ultrasonic light modulator 30 as parallel light, and 31 is the ultrasonic light modulator 30.
The optical fiber 8 collects the frequency-shifted light emitted from the
And a mixer 33 for inputting the output of the second oscillator 32 for driving the oscillator 19 and the second ultrasonic optical modulator 30 at the frequency f ₂ and outputting the difference frequency signal to the phase comparator 20. Is.

<< Advantages of the Invention >> As described above, according to the present invention, an optical frequency change measuring device capable of measuring the spectrum change of the laser under measurement with high resolution can be realized with a simple configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the optical frequency change measuring device according to the present invention, FIG. 2 is a diagram for explaining the operation of the device of FIG. 1, and FIG. 3 is an optical frequency according to the present invention. 4 is a partial configuration block diagram showing a second embodiment of the change measuring apparatus;
FIG. 5 is a block diagram showing the configuration of a third embodiment of an optical frequency change measuring device according to the present invention, and FIG. 5 is a configuration block diagram showing an example of a conventional optical frequency change measuring device. 7 …… First optical fiber coupler, 9,10,25,26,27 ……
Delay fiber, 11 ... Second optical fiber coupler, 14 ...
… Ultrasonic light modulator, 17 …… Light receiving element, 19 …… Oscillator, 20
...... Phase comparator, 21 …… Signal processor, 73,74 …… Output port, 111,112 …… Input port.

Claims (1)

[Scope of utility model registration request] 1. A first device which receives light to be measured and splits the light in two directions.
Optical fiber coupler, a second optical fiber coupler that causes lights branched from two output ports of the first optical fiber coupler to enter different input ports and interfere with each other, and the first optical fiber An ultrasonic optical modulator that frequency shifts the output light from one output port of the coupler,
A delay fiber for delaying output light from one or the other output port of the first optical fiber coupler for a predetermined time; a light receiving element for detecting an optical beat signal from the second optical fiber coupler; An oscillator that drives the acoustic wave modulator, a phase comparator that detects the phase difference between the signal corresponding to the output of this oscillator and the output of the light receiving element, and the frequency change of the measured light from the output of this phase comparator. An optical frequency change measuring device, comprising: a signal processing device for calculating.