JP3031396B2 - Broadband optical frequency sweeper - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband optical frequency sweeping apparatus for sweeping the frequency of a light wave in a wide band.

[0002]

2. Description of the Related Art Conventional optical frequency control, modulation and sweeping are performed by a direct modulation method by controlling physical parameters of a light source or by an external modulation method by controlling a modulation signal of an external modulator. I was However, in any of the methods, it is difficult to achieve high frequency sweep accuracy with the conventional configuration. Therefore, in optical communication, optical measurement, optical signal processing, and the like, the use of optical frequencies has been limited to a very narrow range.

In order to cope with such a situation, a completely new technique capable of sweeping the frequency of a light wave stepwise with extremely high precision has been developed (Japanese Patent Application No. 4-36181). As shown in FIG. 5, the optical frequency sweeping device includes an optical pulse modulator 31 that converts incident continuous light into a pulse light having a predetermined pulse width, and an optical pulse modulator including an acousto-optic frequency shifter 32 and an optical amplifier 33 therein. A pulse circuit output from the optical pulse modulator 31 is guided to the optical loop circuit via the optical directional coupler 34. The optical directional coupler 34 outputs half of the pulse intensity to the outside,
The other half is sent to the optical amplifier 33 of the optical loop circuit.

The optical amplifier 33 amplifies the pulse light,
The acousto-optic frequency shifter 32 gives a predetermined frequency shift to the amplified pulse light and guides it to the optical directional coupler 34.
The optical directional coupler 34 again divides the pulse light into an external output and a circulating pulse light. Similarly, a predetermined frequency shift is given to the pulse light every time the light travels around the optical loop circuit. Thus, the frequency of the light output from the optical directional coupler 34 to the outside can be changed stepwise on the time axis with the shift frequency as a unit.

The control circuit 35 includes the optical pulse modulator 3
1 and the acousto-optic frequency shifter 32 are synchronously controlled to periodically repeat the frequency sweep. By making the pulse width of the pulse light equal to the loop length (circulation time) of the optical loop circuit, pseudo continuous light whose frequency changes stepwise can be output. If the optical amplifier 33 has sufficient gain to compensate for optical loss in the optical directional coupler and the optical loop circuit,
The optical loop circuit can be circulated a plurality of times, and a relatively large frequency shift can be given to the subsequent pulsed light.

[0006]

In the configuration of the optical frequency sweeping device of the prior application, spontaneous emission noise (ASE) is generated in an optical amplifier disposed in an optical loop circuit. Since the spontaneous emission noise light is accumulated each time the pulsed light goes around the optical loop circuit, it is practically difficult to sweep the frequency over a wide range by making the pulsed light go around a plurality of times.

That is, if the spontaneous emission noise light is at a predetermined level with respect to the signal light, the population inversion formed in the amplification medium of the optical amplifier is also consumed for amplifying the spontaneous emission noise light. Even if the total intensity of the light summed with the spontaneous emission noise light is kept constant for each orbit, the intensity of the signal light decreases each time, and the frequency can be swept over a wide range by increasing the number of orbits. Did not.

An object of the present invention is to provide a wide-band optical frequency sweeping device which can solve such inconveniences and sweep the frequency of a light wave with a wide band and high accuracy.

[0009]

SUMMARY OF THE INVENTION The present invention provides an optical pulse modulator for converting continuous light incident from the outside into pulsed light having a predetermined pulse width and outputting the pulsed light, and a first input port or a second input port. Incident light of the first output port and the second output port.
It is configured to output the demultiplexed light to the output port.
An optical directional coupler that takes in the input port and uses the first output port as an output port to the outside;
An optical amplifier that couples between the output port and the second input port and amplifies the pulse light in the optical path, a frequency shifter that gives a predetermined frequency shift to the frequency of the pulse light, and turns on and off the pulse light In a broadband optical frequency sweeping device including an optical loop circuit having an optical switch and an optical pulse modulator and a control circuit for synchronously controlling the optical switch, the pulse light is transmitted through the optical path of the optical loop circuit. ,
It has a transmission band characteristic of removing spontaneous emission noise light generated in the optical amplifier, and inserts a frequency variable bandpass filter whose center transmission frequency changes according to an external control signal, and the control circuit includes an optical pulse modulator and It is a configuration that synchronously controls a frequency variable bandpass filter together with an optical switch and outputs an external control signal that changes the center transmission frequency of the frequency variable bandpass filter in units of a shift frequency given by a frequency shifter in accordance with the circulation of pulsed light. It is characterized by the following.

[0010]

The broadband optical frequency sweeping device of the present invention provides an optical amplifier in the optical path of the optical loop circuit when a predetermined frequency shift is given by the frequency shifter of the optical loop circuit and the pulse light is circulated while being amplified by the optical amplifier. By inserting a frequency-variable band-pass filter for removing the spontaneous emission noise light generated in the above, the intensity of the spontaneous emission noise light can be significantly reduced.

Further, the center transmission frequency of the frequency variable band-pass filter is changed in units of shift frequency given by the frequency shifter in accordance with the circulation of the pulse light, so that the frequency always coincides with the frequency of the pulse light (signal light) circulating. be able to. Therefore, it is possible to perform high-precision frequency sweep over an extremely wide range over the entire variable band while suppressing spontaneous emission noise light.

[0012]

FIG. 1 is a block diagram showing the configuration of an embodiment of a broadband optical frequency sweeping apparatus according to the present invention.

In FIG. 1, an optical pulse modulator 31 takes in continuous light generated by a laser light source or the like from an external input port 36, converts the continuous light into pulse light having a predetermined pulse width, and converts the continuous light into a pulse light having a predetermined pulse width. Output to one input port. An external output port 37 is connected to the first output port of the optical directional coupler 34, and the second output port and the second input port are connected via a loop-shaped optical waveguide 38. The optical waveguide 38 removes the spontaneous emission noise light generated in the frequency shifter 11, the optical amplifier 33, and the optical amplifier 33 for giving a predetermined frequency shift to the pulse light demultiplexed by the optical directional coupler 34, and is externally controlled. An optical loop circuit is configured by inserting a frequency variable bandpass filter 12 and an optical switch 13 that change the center transmission frequency according to a signal.

The control circuit 35 synchronously controls the on / off state of the frequency variable bandpass filter 12, the optical switch 13, and the optical pulse modulator 31, so that the last round pulse light and the externally input pulse light can be controlled. Light can be prevented from overlapping in time, and the optical frequency sweep can be repeated periodically.

The position of the frequency shifter 11 may be located after the variable frequency band-pass filter 12, and by using the acousto-optic frequency shifter 32 shown in FIG. They can also be used together. Further, as the frequency shifter 11, a high-speed phase modulator or a high-speed intensity modulator can be used.

The frequency variable band-pass filter 12 which can be externally controlled by the control circuit 35 is a liquid crystal variable wavelength filter (Hirabayashi et al., "Having a narrow band width and wide variable range") whose center transmission frequency is changed by externally applied voltage control. Liquid crystal sub-literal etalon type variable wavelength filter ", Proceedings of the 1991 IEICE Spring Conference, C-219), or a mechanical variable wavelength filter whose center transmission frequency is changed by driving a motor with electric power. The liquid crystal variable wavelength filter is particularly excellent in that it has high-speed response. This liquid crystal variable wavelength filter has a structure in which the refractive index of the liquid crystal inserted into the cavity is changed by external voltage control to control the center transmission frequency.
The frequency variable range is as wide as about 130 nm in wavelength, and the transmission bandwidth is sufficiently narrow as 0.1 nm or less. Furthermore, because the response speed of the liquid crystal is high,
It is possible to sweep the transmission frequency over the entire variable bandwidth in a time of about 1 ms.

With such a configuration, the pulse light circulating in the optical loop circuit is given a predetermined frequency shift each time the pulse light circulates, and the frequency of the light output from the optical directional coupler 34 to the external output port 37 is changed. Can be changed stepwise on the time axis with the shift frequency f as a unit. By making the pulse width of the pulse light emitted from the optical pulse modulator 31 equal to the loop length (circulation time) of the optical loop circuit, the pulse light extracted to the external output port 37 is as shown in FIG. A pseudo continuous light whose frequency changes stepwise on the time axis in units of a shift frequency f can be obtained.

Here, the feature of the present invention is that the spontaneous emission noise light generated in the optical amplifier 33 is removed by the frequency variable band-pass filter 12, and the signal light is increased even if the number of rounds of the pulse light increases. Component degradation can be minimized.

Further, the frequency variable bandpass filter 1
As shown in FIG. 3, the center transmission frequency of 2 is changed every shift of the pulse light with the shift frequency f as a unit. At this time, if the frequency of the pulse light is made to match the center transmission frequency of the frequency variable bandpass filter 12 first, the pulse light always passes through the transmission band of the frequency variable bandpass filter 12 for each rotation. In addition, an extremely wide band optical frequency sweep over the entire variable bandwidth can be performed at high speed. In the above example, 130
Optical frequency sweeping down to nm is possible.

Further, since the frequency of the pulse light is strictly shifted to an integer multiple of the shift frequency, and the duration at each frequency is the optical loop circulating time, a high-precision optical frequency sweep with extremely high frequency accuracy with respect to time is performed. Can be realized.

When a high-speed intensity modulator having a modulation frequency of 10 GHz is used as the frequency shifter 11, a +10 GHz modulation component is used as the frequency shift light. At this time, by shifting the center transmission frequency of the frequency variable bandpass filter 12 by 10 GHz, an optical frequency sweep over 1000 GHz (1 THz) can be realized with 100 rounds. Further, as shown in FIG.
If the transmission bandwidth of the frequency variable bandpass filter 12 is, for example, about 10 GHz, other frequency components accompanying the high-speed intensity modulation (the signal light component before modulation and the modulation component of -10 GHz).
And an ideal optical frequency sweep without other frequency components can be realized.

[0022]

As described above, according to the present invention, the spontaneous emission noise light generated in the optical amplifier of the optical loop circuit can be effectively removed by the frequency variable bandpass filter. By changing the center transmission frequency in units of the shift frequency of the pulse light, it is possible to perform high-precision frequency sweep over an extremely wide range over the entire variable band. That is, it is possible to perform high-speed optical frequency sweep over an extremely wide band while maintaining high frequency sweep accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a broadband optical frequency sweeping device of the present invention.

FIG. 2 is a diagram showing the time dependence of the frequency of pulsed light extracted to an external output port 37.

FIG. 3 is a diagram showing the time dependence of the center transmission frequency of the frequency variable bandpass filter 12.

FIG. 4 is a diagram showing an operation example of a frequency shifter 11 and a frequency variable bandpass filter 12.

FIG. 5 is a block diagram showing a configuration example of an optical frequency sweep device in the prior application.

[Explanation of symbols]

 Reference Signs List 11 frequency shifter 12 frequency variable bandpass filter 13 optical switch 31 optical pulse modulator 32 acousto-optic frequency shifter 33 optical amplifier 34 optical directional coupler 35 control circuit 36 external input port 37 external output port 38 optical waveguide

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-232540 (JP, A) 1992 IEICE Autumn Conference Proceedings [Section 4] Communication and Electronics (September 15, 1992 Issued) p. 4-126 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 2/02 H01S 3/13-3/137 H04B 10/00-10/04 JICST file (JOIS)

Claims (1)

(57) [Claims] An optical pulse modulator configured to convert continuous light incident from the outside into a pulse light having a predetermined pulse width and output the pulse light; and to convert incident light from a first input port or a second input port into a first input port.
A branch output to an output port and a second output port, wherein the pulse light is taken into a first input port;
An optical directional coupler having an output port as an external output port, coupling between a second output port and a second input port of the optical directional coupler, and transmitting the pulsed light in an optical path thereof. An optical amplifier for amplifying, a frequency shifter for giving a predetermined frequency shift to the frequency of the pulse light,
An optical loop circuit in which an optical switch for turning off is inserted, and a control circuit for synchronously controlling the optical pulse modulator and the optical switch, a broadband optical frequency sweeping device, wherein in the optical path of the optical loop circuit, Inserting a frequency-variable bandpass filter having a transmission band characteristic of transmitting pulsed light and removing spontaneous emission noise light generated in the optical amplifier, and having a center transmission frequency that changes in accordance with an external control signal; The circuit controls the frequency variable bandpass filter synchronously with the optical pulse modulator and the optical switch, and shifts the center transmission frequency of the frequency variable bandpass filter by the frequency shifter according to the rotation of the pulse light. A wide-band optical frequency sweeping device characterized in that it outputs an external control signal that changes in frequency units.