JP2001284707A - Semiconductor laser light source and device for measuring reflection of optical frequency region - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser light source having a small width of an oscillation spectrum line and superior linearity of sweep and capable of high-speed sweep at THz/s or above. SOLUTION: A light beam emitted from an LD 11 driven by a driving current source 12 is changed into a parallel light beam by a lens 13a, and passed through an optical phase modulator 32 and fed back to the LD 11 to generate a state of self injection synchronization. The optical phase modulator 32 has a refractive index changing proportionally to an applied voltage from an optical phase modulator driving unit 33, and thereby the feedback optical length changes and the frequency of the emitting light from the LD 11 is swept.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser light source device capable of sweeping an optical frequency and an optical frequency domain reflection measuring device for measuring the optical reflection characteristics of an optical transmission line.

[0002]

2. Description of the Related Art An optical frequency sweep light source capable of optical frequency sweep has been conventionally used as a light source used for optical measurement and the like. As one method for obtaining a high spatial resolution in an optical reflection measuring device for diagnosing an optical transmission line, an optical frequency domain reflection measuring device (OFDR: Optical Frequency).
Domain Reflectometry) is promising.

[0003] The basic principle of OFDR is as follows.
The photodetector causes reflected light or backscattered light from the measured light transmission path disposed on one of the optical paths of the interferometer to interfere with the reference light. The interference signal has a beat frequency that is proportional to the optical path length difference between the reference light and the reflected light when the frequency of the light source is swept linearly in time, and the optical transmission path to be measured is diagnosed by spectral analysis of the beat frequency. Becomes possible. OFDR is
It can be classified into two types: a phase correlation type when the optical path length of the measured optical transmission path is shorter than the coherent distance of the light source, and a phase decorrelation type when the optical path length is long. The spatial resolution [delta] L, when the phase correlation type, δL = c / (2nΔF) next, when the phase non-correlation type, and _{δL = cΔν / (4nf m ΔF} ). Here, the spectral line width of the light source is Δν, the optical frequency sweep width is ΔF, the sweep frequency is f _m , the speed of light is c, and the refractive index of the optical fiber is n. Therefore, for the phase correlation type,
The spatial resolution improves as the optical frequency sweep width increases. However, since the measurement range is limited to a coherent distance that is inversely proportional to the spectral line width of the light source, a light source with a narrow spectral line width is required to expand the measurement range. In the case of the phase decorrelation type, when the spectral line width of the light source is narrow,
When the optical frequency sweep width is large and the sweep frequency is high, that is, when the optical frequency sweep speed is high, the spatial resolution is improved.
Regardless of the phase correlation type or the phase non-correlation type, the resolution deteriorates unless the optical frequency sweep is performed linearly in time. Therefore, in order to expand the measurement range of OFDR and improve the spatial resolution, it is necessary to perform optical frequency sweep linearly at high speed and time using a narrow line width laser light source.

A mirror or the like is arranged outside a semiconductor laser (hereinafter, LD: Laser Diode) that oscillates in a single longitudinal mode, and a part of light from the LD is fed back to the LD to obtain a narrow line width. The injection locking phenomenon is known. A light source utilizing self-injection locking has a narrow line width of about 10 kHz, and is used for special applications such as spectroscopic measurement of atoms and molecules. In order to perform optical frequency sweep, it is necessary to change the length of the resonator, that is, the length of the return optical path.

FIG. 7 shows a conventional semiconductor laser light source device capable of sweeping an optical frequency. The LD light source unit 1 includes an LD 11, a drive current source 12, and a lens 13a. LD11
Is a distributed feedback semiconductor laser (DFB-LD), and the output light of the LD 11 passes through the optical feedback means 2 to which a PZT (piezoelectric actuator) 31 is fixed, and then returns to the LD.
Returned to 11. By slightly displacing the optical feedback means 2 in the optical axis direction by the PZT 31, the oscillation frequency can be swept in a range up to about several GHz.

[0006]

As described above, a conventional LD light source using self-injection locking can sweep an optical frequency of several GHz with a narrow line width. Since frequency sweeping has been performed, high-speed sweeping at THz / s or higher is difficult, and linearity of sweeping cannot be ensured. For this reason, the optical reflection measuring device OF
When the conventional optical frequency sweep light source using PZT is applied to DR, high spatial resolution cannot be obtained.
The problem to be solved by the present invention is that an LD light source utilizing self-injection locking has a narrow oscillation spectrum line width and a T
It is an object of the present invention to enable a high-speed sweep at Hz / s or higher and to improve the linearity of the sweep. Another object of the present invention is to improve the spatial resolution by using a light source capable of sweeping at a high speed of THz / s or higher according to the present invention and having excellent linearity of sweeping.

[0007]

According to the present invention, there is provided an LD light source using self-injection locking, wherein an optical phase modulator is used as a means for changing an external feedback optical path length. And That is, the semiconductor laser light source device of the present invention is provided with a semiconductor laser unit that oscillates laser light in a longitudinal single mode, and is disposed at a predetermined fixed position with respect to the semiconductor laser unit, and at least a part of the laser light Light feedback means for returning to the semiconductor laser means;
A light which is arranged on an optical path between the semiconductor laser means and the optical feedback means and utilizes an electro-optical effect for changing the oscillation frequency of the laser light by modulating the phase of the laser light. A phase modulator.

The optical phase modulator uses a light-transmitting dielectric crystal whose refractive index changes in proportion to an applied electric field. This phenomenon is called a first-order electro-optic effect. Since the optical path length of light passing through the crystal is the product of the refractive index and the transmission distance, changing the refractive index changes the optical path length, that is, the phase of the transmitted light. By using the optical phase modulator, the semiconductor laser light source device of the present invention,
High-speed sweep at THz / s or more can be performed, and the linearity of the sweep can be improved. The electro-optic effect has a response speed of several THz in theory, and is several G as an optical phase modulator.
Hz response speed is realized. Therefore, by using the optical phase modulator utilizing the electro-optic effect, sweeping can be performed at a much higher speed than the conventional configuration using PZT, and since mechanical displacement is not required, it is mechanically robust. A configuration with excellent stability can be realized.

Further, the optical frequency domain reflection measuring device of the present invention uses the semiconductor laser light source device of the present invention as a light source. That is, the optical frequency domain reflection measuring apparatus of the present invention comprises a light source capable of sweeping an optical frequency, receives the output light from the light source, splits the light into two waves, outputs one to the reference optical path, and transmits the other to the optical transmission line to be measured. Demultiplexing means for outputting to the path, multiplexing means for multiplexing the output light from the reference optical path with the reflected light and the backscattered light from the measured light transmission path, and the output light from the multiplexing means An optical frequency domain reflection measuring apparatus comprising: a light detecting unit for receiving and converting the electric signal into an electric signal; and a frequency analyzing unit for performing frequency analysis of the electric signal output from the light detecting unit. Is a semiconductor laser light source device according to claim 1.

[0010]

FIG. 1 shows the basic structure of a semiconductor laser light source device according to the present invention. The semiconductor laser light source device according to the present invention includes an LD light source unit 1 as a semiconductor laser unit capable of oscillating in a single longitudinal mode alone, and a light feedback unit 2 for feeding back at least a part of light emitted from the LD light source unit 1. An optical phase modulator 3 disposed in the feedback optical path and changing the feedback optical path length
It is composed of At least a part of the emitted light from the LD light source unit 1 is fed back by the light feedback unit 2, and self-injection locking occurs. The optical frequency of the emitted light is swept by changing the feedback optical path length by the optical phase modulator 3.

FIG. 2 is a configuration diagram of a first embodiment of the semiconductor laser light source device according to the present invention. LD light source unit 1 is LD
11, a drive current source 12, and a lens 13a. The LD 11 is a distributed feedback semiconductor laser (DFB-L
D), one of the emission end faces 11a is provided with a cleavage plane or HR coating, and the other emission end face 11b is provided with several percent AR coating. The drive current source 12 supplies an injection current to drive the LD 11. The light from the emission end face 11b is
a is a parallel light. This light passes through the optical phase modulator 32, passes through the phase modulator 32 again via the optical feedback unit 2, and is fed back to the LD 11.

The optical phase modulator includes a bulk type optical phase modulator and a waveguide type optical phase modulator. Bulk type optical phase modulator using dielectric crystal as it is, thickness of several mm, width of several mm
A half-wave voltage is required to be several hundred volts in order for light to propagate in a region of the order of magnitude. On the other hand, the waveguide type optical phase modulator can confine and propagate light in a narrow region having a thickness of several μm and a width of several μm. In the present embodiment, a bulk-type optical phase modulator is used as the optical phase modulator 32. An end face 32a of the optical phase modulator 32,
32b is provided with an AR coating. The optical phase modulator driving unit 33 applies a voltage to the optical phase modulator 32.
The refractive index of the light passing through the optical phase modulator 32 changes in proportion to the applied voltage. This is equivalent to changing the return optical path length. In order to maximize the optical frequency sweep width, it is necessary to change the feedback optical path length by a half wavelength. At this time, an applied voltage of several hundred volts is required.

The light feedback means 2 is constituted by a reflection mirror.
The reflection mirror is arranged at a position separated from the LD 11 by about several tens cm. The external resonator length required to obtain light having an oscillation line width of, for example, 10 kHz is 30 cm to 40 cm. The end face 32b of the optical phase modulator 32 has a high reflectance (HR)
A configuration is also possible in which a coating is applied to form a reflection mirror, and this mirror is used as the light feedback means 2. Further, a half mirror can be used as the light feedback means 2 to extract output light.

FIG. 3 is a configuration diagram of a second embodiment of the semiconductor laser light source device according to the present invention. In the present embodiment,
A waveguide type optical phase modulator is used as the optical phase modulator 32. L
The D light source unit 1 has almost the same configuration as that of the first embodiment. However, since a waveguide type optical phase modulator is used, the light from the emission end face 11b of the LD 11 is coupled to the spherical fiber 14. Further, a configuration in which the fiber is coupled via a lens without using the spherical fiber may be used. The output light of the LD 11 is guided to the optical phase modulator 32 through the spherical fiber 14, passes through the optical phase modulator 32 again through the optical feedback means 2, and
It is returned to D11. The waveguide-type optical phase modulator has a significantly smaller electrode spacing than the bulk-type optical phase modulator.
The voltage applied by the optical phase modulator driving unit 33 is several volts.
The degree is good. The configuration of the light feedback means 2 is the same as in the first embodiment.

FIG. 4 is a configuration diagram of a third embodiment of the semiconductor laser light source device according to the present invention. LD light source unit 1
This is the same configuration as the embodiment. A Fabry-Perot interferometer 21 is used for the optical feedback means 2. In order to prevent unnecessary reflected light from returning to the LD 11, the Fabry-Perot interferometer 21 is arranged to be inclined with respect to the optical axis of the light emitted from the LD light source unit 1. As the optical phase modulator 32, a bulk type optical phase modulator is used. The optical phase modulator 32 is placed between the Fabry-Perot interferometers 21 and has end faces 32a, 32b.
Has an AR coating.

FIG. 5 is a block diagram of a fourth embodiment of the semiconductor laser light source device according to the present invention. LD light source unit 1 is L
In this configuration, a diffraction grating 15 as a wavelength selection element is placed outside D11. The LD 11 is a Fabry-Perot LD, and one emission end face 11a is provided with an AR coating. The LD 11 is operated by the drive current source 12.
The output light from the end face 11a is made parallel by the lens 13b,
The light enters the diffraction grating 15. The diffraction grating 15 diffracts light having a wavelength corresponding to the arranged angle in the direction of the LD 11, and causes laser oscillation with the end face 11b. Such an LD light source unit 1 is called an external cavity laser. LD11
The output light from the end face 11b is converted into parallel light by the lens 13a, passes through the optical phase modulation unit 3, and is returned via the optical feedback unit 2. The optical feedback means 2 and the optical phase modulation unit 3
This is the same configuration as the embodiment.

FIG. 6 shows an embodiment of the optical frequency domain reflection measuring apparatus of the present invention. An optical frequency sweeping light source 4 using the semiconductor laser light source device of the present invention, and an output light from the optical frequency sweeping light source 4 are split into two by a fiber coupler 5 as a splitter, and one of the splitters is sent to a reference optical path 6. The other end is connected to a measured optical fiber 7 which is a measured optical transmission line. Reference optical path 6
The reflected light from the optical fiber 7 and the reflected light from the optical fiber 7 to be measured are multiplexed and interfered by the fiber coupler 5 as multiplexing means, and are incident on a pin photodiode (PIN-PD) 8.
The spectrum of the interference signal is analyzed using a spectrum analyzer 9 as a frequency analyzer. As an embodiment of the optical frequency domain reflection measuring apparatus, in addition to the configuration as in the present embodiment, for example, there is an apparatus using a beam splitter as a demultiplexing unit or a multiplexing unit.

[0018]

According to the semiconductor laser light source device of the present invention, in the LD light source using self-injection locking, an optical phase modulator is used as a means for changing an external feedback optical path length. High-speed sweeping is enabled, and the linearity of optical frequency sweeping can be improved. Further, it is possible to realize a light source that is mechanically more robust and smaller than before. Further, the optical frequency domain reflection measurement device of the present invention,
Since the semiconductor laser light source device of the present invention is used as an optical frequency sweep light source, the spatial resolution is improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of a semiconductor laser light source device of the present invention.

FIG. 2 is a configuration diagram showing a first embodiment of the semiconductor laser light source device of the present invention.

FIG. 3 is a configuration diagram showing a second embodiment of the semiconductor laser light source device of the present invention.

FIG. 4 is a configuration diagram showing a third embodiment of the semiconductor laser light source device of the present invention.

FIG. 5 is a configuration diagram showing a fourth embodiment of the semiconductor laser light source device of the present invention.

FIG. 6 is a configuration diagram showing an embodiment of an optical frequency domain reflection measurement device of the present invention.

FIG. 7 is a diagram showing a configuration of a conventional semiconductor laser light source device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 LD light source part (semiconductor laser means) 2 Optical feedback means 3 Optical phase modulation part 4 Optical frequency sweep light source (light source) 5 Fiber coupler (Demultiplexing means, multiplexing means) 6 Reference optical path 7 Optical fiber to be measured (Light to be measured) Transmission line) 8 pin photodiode (light detecting means) 9 spectrum analyzer (frequency analyzing means) 11 LD (semiconductor laser) 11a end face (emission end face) 11b end face (emission end face) 12 drive current source 13a lens 13b lens 14 tip Spherical fiber 15 Diffraction grating 21 Fabry-Perot interferometer 31 PZT 32 Optical phase modulator 32a End face (Optical phase modulator end face) 32b End face (Optical phase modulator end face) 33 Optical phase modulator driving section

Continued on the front page F term (reference) 2H079 AA02 BA03 CA24 DA02 EA03 EA11 HA08 HA11 KA01 KA11 KA14 KA18 5F073 AA64 AA67 AA83 AB21 AB25 AB27 AB28 AB29 BA09 EA29

Claims (2)

[Claims] A semiconductor laser means for oscillating a laser beam in a single longitudinal mode; a semiconductor laser means disposed at a predetermined fixed position with respect to the semiconductor laser means; An optical feedback means (2) for returning the laser beam to the semiconductor laser means and the optical feedback means, and changing an oscillation frequency of the laser light by modulating a phase of the laser light. A semiconductor laser light source device comprising: an optical phase modulator (32) utilizing an electro-optical effect for causing the laser to emit light. 2. A light source (4) capable of sweeping an optical frequency, receiving the output light from the light source, splitting the received light into two, outputting one to a reference optical path, and the other to a measured optical transmission path. Demultiplexing means (5); multiplexing means (5) for multiplexing output light from the reference light path with reflected light and backscattered light from the measured light transmission path; and output from the multiplexing means. An optical frequency domain reflection measuring device comprising: a light detecting means (8) for receiving light and converting it into an electric signal; and a frequency analyzing means (9) for analyzing the frequency of the electric signal output from the light detecting means. An optical frequency domain reflection measuring device, wherein the optical frequency sweepable light source is the semiconductor laser light source device according to claim 1.