Light source device and optical pulse tester

Abstract

To provide a light source device capable of easily keeping the center wavelength within a predetermined range, and an optical pulse tester including the light source device.SOLUTION: A light source device includes a semiconductor laser having a first end face and a second end face parallel to each other, which form a first resonator, and emitting laser light from the first end face, and an optical system that is arranged on an optical path of the laser beam emitted from the semiconductor laser, forms a second resonator with the second end face of the semiconductor laser, and whose reflectance with respect to light having a predetermined wavelength width centered on the specified center wavelength of the semiconductor laser is higher than that of the first end face.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a light source device and an optical pulse tester. [Background technology]

[0002] An optical time domain reflectometer (OTDR) is a device that injects optical pulses into an optical fiber under test and tests or measures the characteristics of the optical fiber based on the return light from the optical fiber. Examples of optical time domain reflectometers include OTDR (Optical Time Domain Reflectometer), BOTDR (Brillouin Optical Time Domain Reflectometer), and ROTDR (Raman Optical Time Domain Reflectometer).

[0003] An OTDR is an instrument that measures the transmission loss of an optical fiber and the distance to a fault point based on Rayleigh scattered light and Fresnel reflected light that occur within the optical fiber. A BOTDR is an instrument that measures the strain and temperature distribution of an optical fiber based on the backscattered light of Brillouin scattering that occurs within the optical fiber. A ROTDR is an instrument that measures the temperature distribution of an optical fiber based on the backscattered light of Raman scattering that occurs within the optical fiber.

[0004] The following Patent Document 1 discloses a conventional optical pulse tester that can output optical pulses and highly stable continuous light (or modulated light) from a single semiconductor laser. Also, the following Patent Document 2 discloses a semiconductor laser module in which a wavelength-selective filter is provided in a cavity formed by a reflective film formed on the front end face (or rear end face) of a semiconductor laser element and the exposed surface of the incident end face (or the reflective film formed on the incident end face) of an optical fiber. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-13346 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-141609 Summary of the Invention [Problem to be solved by the invention]

[0006] The optical time domain reflectometers generally use Fabry-Perot semiconductor lasers because they are relatively inexpensive and can produce high output power. However, the center wavelength of individual Fabry-Perot semiconductor lasers varies, and the center wavelength also changes with temperature. For this reason, it is difficult to keep the center wavelength of Fabry-Perot semiconductor lasers within a predetermined range (for example, within 15 nm of the specified value) unless the individual Fabry-Perot semiconductor lasers are carefully selected and temperature controlled.

[0007] The present invention has been made in view of the above circumstances, and has as its object to provide a light source device that can easily keep the center wavelength within a predetermined range, and an optical time domain tester equipped with such a light source device. [Means for solving the problem]

[0008] In order to solve the above problem, a light source device (21, 21B, 21C, 22) according to one embodiment of the present invention comprises a semiconductor laser (LD) having a first end face (E1) and a second end face (E2) parallel to each other and forming a first resonator, and emitting laser light from the first end face, and an optical system (21c, 21e, 21f, 21h) arranged on the optical path of the laser light emitted from the semiconductor laser, forming a second resonator (RS2) with the second end face of the semiconductor laser, and having reflection characteristics in which the reflectance for light of a predetermined wavelength range centered on a specified central wavelength of the semiconductor laser is higher than the reflectance of the first end face.

[0009] Here, in a light source device according to one embodiment of the present invention, the optical system comprises an optical element (21c) having a first surface (PL1) provided with a reflective film having the reflective characteristics, and a second surface (PL2) provided with an anti-reflection film against the laser light emitted from the semiconductor laser.

[0010] Alternatively, in a light source device according to one aspect of the present invention, the optical system comprises a first optical element (21e) having the reflection characteristics and arranged obliquely with respect to the optical path of the laser light, and a second optical element (21f) that reflects and transmits the laser light that has passed through the first optical element at a predetermined ratio.

[0011] Furthermore, the light source device according to one aspect of the present invention includes a collimating optical system (21b) that is provided on the optical path of the laser light between the semiconductor laser and the optical system and that collimates the laser light emitted from the semiconductor laser.

[0012] Alternatively, in a light source device according to one aspect of the present invention, the optical system includes a reflecting member (21h) on which a grating (GR) having the reflection characteristics is formed, and a focusing optical system (21g) is provided on the optical path of the laser light between the semiconductor laser and the reflecting member, and focuses the laser light emitted from the semiconductor laser onto the reflecting member.

[0013] An optical pulse tester (1) according to one aspect of the present invention is an optical pulse tester (1) that tests characteristics of an optical fiber (FUT) based on return light obtained by injecting an optical pulse into the optical fiber, and includes: a bidirectional module (11) having any of the above-described light source devices that emit the optical pulses and a light receiving device (26) that receives the return light; and a signal processing unit (14) that performs processing to determine characteristics of the optical fiber based on the light reception result of the light receiving device. [Effects of the Invention]

[0014] According to the present invention, there is an advantage that the central wavelength can be easily kept within a predetermined range. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a block diagram showing the configuration of a main part of an optical pulse tester according to a first embodiment of the present invention; [Figure 2] 1 is a diagram showing a configuration of a main part of a bidirectional module according to a first embodiment of the present invention; [Figure 3] 1 is a diagram showing a configuration of a main part of a light source device according to a first embodiment of the present invention. [Figure 4] 1 is a diagram showing an example of the reflection and transmission characteristics of a notch filter used in the first embodiment of the present invention. [Figure 5] FIG. 4 is a diagram showing a configuration of a main part of a light source device according to a second embodiment of the present invention. [Figure 6] 10 is a diagram showing an example of the reflection and transmission characteristics of a bandpass filter and a half mirror used in the second embodiment of the present invention. FIG. [Figure 7] FIG. 10 is a diagram showing the configuration of a main part of a light source device according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a light source device and an optical time domain tester according to embodiments of the present invention will be described in detail with reference to the drawings. First, an overview of the embodiments of the present invention will be described, followed by a detailed description of each embodiment of the present invention.

[0017] 〔overview〕 An embodiment of the present invention makes it possible to easily set the center wavelength of a light source device within a predetermined range. The JIS standard (JIS C 6823) specifies that the center wavelength of a light source device used in an optical time domain reflectometer (OTDR) must be within 15 nm of a specified value (e.g., 1310 nm or 1550 nm). The Fabry-Perot semiconductor lasers typically used in optical time domain reflectometers have variations in center wavelength between individual devices. Therefore, in order to set the center wavelength of the light source device within the range specified by the JIS standard, it is necessary to select devices whose center wavelength is close to the specified value, which increases costs.

[0018] Furthermore, the central wavelength of a Fabry-Perot semiconductor laser also changes with temperature. For example, a wavelength shift of approximately 0.4 nm / °C occurs with temperature changes. Therefore, to keep the central wavelength of the light source device within the range specified by the JIS standard, it is necessary to strictly control the temperature of the light source device, for example by installing a Peltier element in the light source device. However, since OTDRs are often used outdoors, such as at construction sites, they are generally battery-powered. In environments with large temperature changes, the power consumption of the Peltier element increases, drastically shortening the battery operating time.

[0019] Distributed Feed-Back (DFB) semiconductor lasers have the advantage of small variations in center wavelength between individual lasers and wavelength shift due to temperature changes of approximately 0.1 nm / °C, which is significantly smaller than that of Fabry-Perot semiconductor lasers. Therefore, if a DFB semiconductor laser is used instead of a Fabry-Perot semiconductor laser, it is conceivable that the center wavelength of the light source device can easily fall within the range specified by the JIS standard. However, DFB semiconductor lasers have low optical output and are therefore unable to achieve a high dynamic range. Furthermore, their small spectral half-width generates phasing noise (phase noise), which degrades the OTDR waveform.

[0020] In an embodiment of the present invention, an optical system is disposed on the optical path of a laser beam emitted from a first end face of a semiconductor laser, the optical system having a reflectance characteristic that is higher than the reflectance of the first end face of the semiconductor laser for light of a predetermined wavelength band centered on a specified center wavelength of the semiconductor laser, and a resonator is formed by the second end face of the semiconductor laser and the optical system, thereby making it possible to easily keep the center wavelength of the light source device within a predetermined range.

[0021] [First embodiment] Optical pulse tester Fig. 1 is a block diagram showing the main components of an optical time domain reflectometer according to a first embodiment of the present invention. As shown in Fig. 1, the optical time domain reflectometer 1 of this embodiment comprises a bidirectional module 11, an LD driver 12, a sampling unit 13, a signal processing unit 14, a display unit 15, and a connector 16. The optical time domain reflectometer 1 tests or measures the characteristics of an optical fiber FUT based on the return light obtained by injecting an optical pulse into the optical fiber FUT. The optical time domain reflectometer 1 is also called an OTDR.

[0022] The bidirectional module 11 outputs an optical pulse (laser light) to be incident on the optical fiber FUT based on a drive signal DS output from the LD driver 12, and also receives return light obtained from the optical fiber FUT and outputs a received light signal RS. The bidirectional module 11 will be described in detail later.

[0023] The LD driver 12 outputs a drive signal DS for driving the bidirectional module 11 under the control of the signal processor 14. That is, the LD driver 12 outputs the drive signal DS for causing the bidirectional module 11 to output an optical pulse to be incident on the optical fiber FUT. The sampling unit 13 samples the received light signal RS output from the bidirectional module 11 under the control of the signal processor 14.

[0024] The signal processing unit 14 controls the LD driving unit 12 and the sampling unit 13, and performs calculations required to determine the characteristics of the optical fiber FUT using the signal sampled by the sampling unit 13. The display unit 15 includes a display device such as a liquid crystal display device, and displays the results of calculations performed by the signal processing unit 14. The connector 16 is used to connect one end of the optical fiber FUT to the optical pulse tester 1.

[0025] <Two-way module> Fig. 2 is a diagram showing the configuration of the main parts of a bidirectional module according to a first embodiment of the present invention. As shown in Fig. 2, the bidirectional module 11 according to this embodiment includes light source devices 21 and 22, a multiplexing / demultiplexing filter 23, a beam splitter 24, a lens 25, and a light receiving device 26. Such a bidirectional module 11 can output optical pulses having center wavelengths defined in the JIS standard (JIS C 6823). For example, the bidirectional module 11 can output an optical pulse (hereinafter sometimes referred to as a "first optical pulse") having a center wavelength of 1550 nm within ±15 nm, and an optical pulse (hereinafter sometimes referred to as a "second optical pulse") having a center wavelength of 1310 nm within ±15 nm.

[0026] The light source device 21 includes a pulse light source 21a, a collimating lens 21b (collimating optical system), and a notch filter 21c (optical system, optical element), and outputs a first light pulse based on a drive signal DS output from the LD driver 12. The light source device 22 includes a pulse light source 22a, a collimating lens 22b (collimating optical system), and a notch filter 22c (optical system, optical element), and outputs a second light pulse based on a drive signal DS output from the LD driver 12. The light source devices 21 and 22 will be described in detail later.

[0027] The multiplexing / demultiplexing filter 23 multiplexes the first optical pulse output from the light source device 21 and the second optical pulse output from the light source device 22. If the light source devices 21 and 22 are driven simultaneously, the first optical pulse and the second pulse are multiplexed by the multiplexing / demultiplexing filter 23. If only one of the light source devices 21 and 22 is driven, either the first optical pulse or the second pulse is guided to the beam splitter 24 via the multiplexing / demultiplexing filter 23.

[0028] The beam splitter 24 splits the incident light at a predetermined splitting ratio (for example, 1:1). For example, the beam splitter 24 transmits 50% of the first optical pulse or the second optical pulse guided from the multiplexing / demultiplexing filter 23 and reflects the remaining 50%. The beam splitter 24 also reflects 50% of the return light obtained from the optical fiber FUT and transmits the remaining 50% indirectly. The lens 25 couples the first optical pulse or the second optical pulse that has passed through the beam splitter 24 to one end of the coupling optical fiber FB. Note that one end of the coupling optical fiber FB is connected to the bidirectional module 11 and optically coupled to the lens 25, and the other end is connected to the connector 16. That is, one end of the optical fiber FUT is connected to the other end of the coupling optical fiber FB.

[0029] The light receiving device 26 includes a lens 26a and a photodetector 26b, receives the return light reflected by the beam splitter 24, and outputs a light receiving signal RS. The lens 26a focuses the return light reflected by the beam splitter 24 onto the photodetector 26b. The photodetector 26b includes a light receiving element such as an avalanche photodiode (APD), performs photoelectric conversion on the return light incident on the light receiving surface of the light receiving element, and outputs a light receiving signal RS corresponding to the return light incident on the light receiving surface.

[0030] <Light source device> FIG. 3 is a diagram showing the main configuration of a light source device according to a first embodiment of the present invention. As shown in FIG. 3, light source device 21 has a configuration in which collimating lens 21b and notch filter 21c are arranged in this order on the optical path of an optical pulse emitted from pulsed light source 21a. Note that while FIG. 3 shows light source device 21, light source device 22 also has a similar configuration. In other words, if light source device 21, pulsed light source 21a, collimating lens 21b, and notch filter 21c shown in FIG. 3 are replaced with light source device 22, pulsed light source 22a, collimating lens 22b, and notch filter 22c, respectively, the configuration of light source device 22 is obtained. However, the center wavelength of light source device 22 is 1310 nm.

[0031] 3, the pulse light source 21a includes a semiconductor laser LD. This semiconductor laser LD is, for example, a Fabry-Perot semiconductor laser having a first end face E1 and a second end face E2 that are parallel to each other. The first end face E1 of the semiconductor laser LD is coated with, for example, an AR coating (Anti-Reflection coating) having a reflectance of about 5% or less for the first optical pulse. The second end face E2 of the semiconductor laser LD is coated with, for example, an HR coating (High Reflection coating) having a reflectance of about 90% or less for the first optical pulse.

[0032] When the semiconductor laser LD receives the drive signal DS output from the LD driver 12 shown in Fig. 1, it emits an optical pulse having a center wavelength of approximately 1550 nm from the first end face E1. Here, the central wavelength of a Fabry-Perot semiconductor laser varies between individual lasers, and the central wavelength also changes with temperature, so it is said to "emit an optical pulse having a center wavelength of approximately 1550 nm."

[0033] Collimating lens 21b is provided on the optical path of the optical pulse between pulse light source 21a and notch filter 21c, and collimates the optical pulse output from pulse light source 21a to form parallel light. Notch filter 21c is an optical element that forms a resonator together with second end face E2 of semiconductor laser LD, and is provided to keep the center wavelength of light source device 21 within a predetermined range (a range of ±15 nm centered at 1550 nm).

[0034] The notch filter 21c is a parallel-plate glass member having a first surface PL1 on which a reflective film having the reflection characteristics shown in Fig. 4 is formed, and a second surface PL2 on which an AR coating is formed. The notch filter 21c is made of a glass material such as BK7 or B270. The reason for using the notch filter 21c made of such a glass material is to reduce the temperature dependency of the reflection wavelength.

[0035] Fig. 4 is a diagram showing an example of the reflection and transmission characteristics of the notch filter used in the first embodiment of the present invention. In the graph shown in Fig. 4, the horizontal axis represents wavelength and the vertical axis represents transmittance. Here, if transmittance is T [%], then reflectance R can be expressed by the relational expression R = 100 - T [%]. Since the vertical axis represents transmittance, the graph shown in Fig. 4 directly shows the transmission characteristics of notch filter 21c, but also indirectly shows the reflection characteristics of notch filter 21c from the above relational expression.

[0036] 4, the transmittance of the notch filter 21c is lowest at the specified center wavelength (1550 nm) of the semiconductor laser LD, for example, at about 70%. In other words, the reflectance of the notch filter 21c is highest at the specified center wavelength (1550 nm) of the semiconductor laser LD, for example, at about 30%. Furthermore, the transmittance of the notch filter 21c gradually increases as the wavelength moves away from the specified center wavelength (1550 nm) of the semiconductor laser LD. In other words, the reflectance of the notch filter 21c gradually decreases as the wavelength moves away from the specified center wavelength (1550 nm) of the semiconductor laser LD.

[0037] In the example shown in FIG. 4, the wavelength width (full width at half maximum: FWHM) (predetermined wavelength width) of the portion where the reflectance is half of the reflectance at the center wavelength is about 10 nm. This wavelength width may be 15 nm, which is specified as the error of the center wavelength in the JIS standard (JIS C 6823). The notch filter 21c has reflection characteristics such that the reflectance for light in at least this wavelength width is higher than the reflectance of the first end facet E1 of the semiconductor laser LD. The reflectance of the reflective film formed on the first surface PL1 of the notch filter 21c needs only to be higher than the reflectance of the first end facet E1 of the semiconductor laser LD, and is preferably, for example, about 20 to 70%.

[0038] 3, in this embodiment, a first resonator RS1 is formed by the first end face E1 and the second end face E2 of the semiconductor laser LD, and a second resonator RS2 is formed by the second end face E2 of the semiconductor laser LD and a reflective film formed on the first surface PL1 of the notch filter 21c. In other words, a composite resonator consisting of the first resonator RS1 and the second resonator RS2 is formed in the light source device 21.

[0039] As described above, the reflectance of the notch filter 21c is higher than the reflectance of the first end face E1 of the semiconductor laser LD. Therefore, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2 formed by the second end face E2 of the semiconductor laser LD and the reflective film formed on the first face PL1 of the notch filter 21c.

[0040] As described above, the notch filter 21c is made of a glass material, and therefore the temperature dependency of the reflected wavelength is smaller than that of the semiconductor laser LD (for example, by about one order of magnitude smaller). Therefore, even if the ambient temperature changes, the change in the central wavelength of the pulsed light output from the light source device 21 can be made extremely small. This allows the central frequency of the optical pulse output from the light source device 21 to be kept within a predetermined range (a range of ±15 nm centered at 1550 nm).

[0041] <Operation of the optical time domain tester> When the operation of the optical pulse tester 1 is started, first, the signal processing unit 14 shown in Fig. 1 controls the LD driver 12, which outputs a drive signal DS. The drive signal DS output from the LD driver 12 is supplied to, for example, a light source device 21 (see Fig. 2) of the bidirectional module 11. When the drive signal DS is supplied to the light source device 21, laser oscillation occurs in the semiconductor laser LD (a semiconductor laser LD having a resonator RS1 formed therein as shown in Fig. 3) provided in the pulse light source 21a, and an optical pulse (laser light) is emitted from a first end face E1 of the semiconductor laser LD.

[0042] The optical pulse emitted from the first end face E1 of the semiconductor laser LD is converted into parallel light by the collimator lens 21b and then enters the notch filter 21c, where a portion of the parallel light is reflected by a reflective film formed on the first face PL1 of the notch filter 21c, and the remainder is transmitted through the notch filter 21c. The parallel light reflected by the notch filter 21c is collected by the collimator lens 21b and enters the semiconductor laser LD from the first end face E1. The optical pulse incident on the semiconductor laser LD is reflected by the second end face E2 of the semiconductor laser LD, and then a portion of the parallel light is again emitted from the first end face E1 of the semiconductor laser LD.

[0043] The optical pulse emitted from the first end face E1 of the semiconductor laser LD is converted into parallel light by the collimator lens 21b and then enters the notch filter 21c, where a portion of the optical pulse is reflected by the reflective film formed on the first face PL1 of the notch filter 21c, and the remainder is transmitted through the notch filter 21c. In this manner, the optical pulse travels back and forth within the second resonator RS2 formed by the second end face E2 of the semiconductor laser LD and the reflective film formed on the first face PL1 of the notch filter 21c. As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, and a first optical pulse (an optical pulse having a central wavelength within ±15 nm of 1550 nm) is output from the light source device 21.

[0044] The first optical pulse output from the light source device 21 passes through the multiplexing / demultiplexing filter 23 and the beam splitter 24 in this order, and then enters the optical fiber FUT connected to the connector 16. As the first optical pulse propagates through the optical fiber FUT, Rayleigh scattered light and Fresnel reflected light are generated within the optical fiber FUT. These propagate as returned light in the reverse direction (the opposite direction to the propagation direction of the first optical pulse) through the optical fiber FUT.

[0045] The return light output from the optical fiber FUT is received by the optical receiver 26 provided in the bidirectional module 11, and the optical receiver 26 outputs a received light signal RS. This received light signal RS is sampled by the sampling unit 13 shown in FIG. 1. The signal sampled by the sampling unit 13 is input to the signal processing unit 14 and used in calculations required to determine the characteristics of the optical fiber FUT. The signal processing unit 14 performs calculations to determine, for example, the distance from the optical pulse tester 1 to a fault point in the optical fiber FUT based on, for example, the time from when a first optical pulse is output from the light source device 21 to when the return light is received by the optical receiver 26. The calculation results obtained by the signal processing unit 14 in this manner (for example, the transmission loss of the optical fiber FUT, the distance to the fault point, etc.) are displayed on the display unit 15.

[0046] As described above, in this embodiment, a notch filter 21c having a reflectance characteristic for light in a predetermined wavelength range (e.g., ±15 nm) centered on a specified center wavelength (e.g., 1550 nm) of the semiconductor laser LD is disposed on the optical path of the optical pulse (laser light) emitted from the first end face E1 of the semiconductor laser LD. The notch filter 21c has higher reflectance than the first end face E1 of the semiconductor laser LD. The second end face E2 of the semiconductor laser LD and the notch filter 21c form a second resonator RS2. As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, so that the center wavelength of the light source device 21 can easily fall within a predetermined range.

[0047] In this embodiment, the semiconductor laser LD, which includes the first resonator RS1, also oscillates, preventing the spectral width from becoming extremely narrow and eliminating the influence of waveform noise due to phasing noise. Furthermore, if the distance between the semiconductor laser LD and the notch filter 21c (external resonator length) becomes extremely long, the gain may be insufficient and laser oscillation may not occur if the optical pulse width is narrow. In contrast, in this embodiment, the notch filter 21c can be positioned immediately after the collimator lens 21b, ensuring sufficient laser oscillation even when the optical pulse width is narrow. Furthermore, in this embodiment, a temperature control device such as a Peltier element is not used to keep the center wavelength of the light source device 21 within a predetermined range. Therefore, even when the optical pulse tester 1 is used outdoors, the battery life is not shortened.

[0048] Second Embodiment Optical Time Domain Reflectometer, Bidirectional Module The essential configuration of the optical time domain tester of this embodiment is similar to that of the optical time domain tester 1 shown in Fig. 1. Furthermore, the essential configuration of the bidirectional module included in the optical time domain tester of this embodiment is such that the light source devices 21 and 22 of the bidirectional module 11 shown in Fig. 2 are replaced with those shown in Fig. 5. Therefore, a description of the configuration of the optical time domain tester and bidirectional module of this embodiment will be omitted.

[0049] <Light source device> Fig. 5 is a diagram showing the main configuration of a light source device according to a second embodiment of the present invention. Fig. 5 shows light source device 21B, which is provided in place of light source device 21 shown in Fig. 3, but light source device 22B (not shown), which is provided in place of light source device 22 shown in Fig. 3, has the same configuration. However, the center wavelength of light source device 22B is 1310 nm.

[0050] As shown in Fig. 5, light source device 21B of this embodiment differs from light source device 21 shown in Fig. 3 in that it includes a band-pass filter 21e (optical system, first optical element) and a half mirror 21f (optical system, second optical element) instead of notch filter 21c in Fig. 3. That is, light source device 21B of this embodiment has a configuration in which collimator lens 21b, band-pass filter 21e, and half mirror 21f are arranged in this order on the optical path of the light pulse emitted from pulse light source 21a.

[0051] Here, as shown in FIG. 5, the bandpass filter 21e is disposed at an angle with respect to the optical path of the optical pulse emitted from the pulse light source 21a. This is to prevent the optical pulse reflected by the bandpass filter 21e from entering the semiconductor laser LD. In other words, this is to prevent a resonator from being formed by the second end face E2 of the semiconductor laser LD and the bandpass filter 21e. The angle of inclination of the bandpass filter 21e with respect to the optical path of the optical pulse emitted from the pulse light source 21a is, for example, 5°. In the light source device 21B of this embodiment, a first resonator RS1 is formed by the first end face E1 and second end face E2 of the semiconductor laser LD, and a second resonator RS2 is formed by the second end face E2 of the semiconductor laser LD and the half mirror 21f.

[0052] Fig. 6 is a diagram showing an example of the reflection and transmission characteristics of a bandpass filter and a half mirror used in the second embodiment of the present invention. Fig. 6(a) is a diagram showing an example of the reflection and transmission characteristics of a bandpass filter, and Fig. 6(b) is a diagram showing an example of the reflection and transmission characteristics of a half mirror. In the graph shown in Fig. 6, similar to the graph shown in Fig. 4, the horizontal axis represents wavelength and the vertical axis represents transmittance.

[0053] 6(a), the bandpass filter 21e has a transmission characteristic of transmitting only light of a predetermined wavelength width centered on the specified central wavelength (1550 nm) of the semiconductor laser LD. For example, the wavelength width (full width at half maximum: FWHM) (predetermined wavelength width) through which the bandpass filter 21e transmits light is approximately 10 to 15 nm. Note that the transmittance of the bandpass filter 21e at the central wavelength (1550 nm) is preferably as high as possible, and is preferably, for example, approximately 100%.

[0054] As shown in Fig. 6(b), the half mirror 21f has a substantially constant reflectance for light in the wavelength range of 1550 to 1600 nm. It is sufficient that the half mirror 21f has a substantially constant reflectance for at least the first optical pulse (an optical pulse having a central wavelength of 1550 nm within ±15 nm). The reflectance of the half mirror 21f is preferably, for example, 30 to 40%.

[0055] In the light source device 21B of this embodiment, the optical pulse emitted from the first end face E1 of the semiconductor laser LD is converted into parallel light by the collimator lens 21b and then enters the bandpass filter 21e. Of the light that enters the bandpass filter 21e, only light having a predetermined wavelength width centered on the specified central wavelength (1550 nm) of the semiconductor laser LD passes through the bandpass filter 21e and enters the half mirror 21f.

[0056] Of the light incident on the half mirror 21f, a portion (e.g., 30% of the light) is reflected by the half mirror 21f, and the remainder (e.g., 70% of the light) is transmitted through the half mirror 21f. The parallel light reflected by the half mirror 21f is transmitted through the bandpass filter 21e, then condensed by the collimator lens 21b, and enters the semiconductor laser LD from the first end face E1. The optical pulse incident on the semiconductor laser LD is reflected by the second end face E2 of the semiconductor laser LD, and then a portion of the light is emitted again from the first end face E1 of the semiconductor laser LD.

[0057] The optical pulse emitted from the first end face E1 of the semiconductor laser LD is converted into parallel light by the collimator lens 21b, then passes through the bandpass filter 21e and enters the half mirror 21f. Of the light that enters the half mirror 21f, a portion (e.g., 30% of the light) is reflected by the half mirror 21f, and the remainder (e.g., 70% of the light) passes through the half mirror 21f. In this way, the optical pulse travels back and forth within the second resonator RS2 formed by the second end face E2 of the semiconductor laser LD and the half mirror 21f. As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, and a first optical pulse (an optical pulse having a central wavelength within ±15 nm of 1550 nm) is output from the light source device 21B.

[0058] As described above, in this embodiment, an optical system consisting of a bandpass filter 21e and a half mirror 21f is disposed on the optical path of the optical pulse (laser light) emitted from the first end face E1 of the semiconductor laser LD. This optical system has a reflectance characteristic in which the reflectance for light in a predetermined wavelength range (e.g., ±15 nm) centered on a specified central wavelength (e.g., 1550 nm) of the semiconductor laser LD is higher than the reflectance of the first end face E1 of the semiconductor laser LD. The second end face E2 of the semiconductor laser LD and the half mirror 21f constituting the optical system form a second resonator RS2.

[0059] As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, so that the central wavelength of the light source device 21B can be easily kept within a predetermined range. Also, in this embodiment, as in the first embodiment, sufficient laser oscillation is possible even if the pulse width of the optical pulse is narrow, and the battery driving time is not shortened.

[0060] Third Embodiment Optical Time Domain Reflectometer, Bidirectional Module The essential configuration of the optical time domain tester of this embodiment is similar to that of the optical time domain tester 1 shown in Fig. 1. Furthermore, the essential configuration of the bidirectional module included in the optical time domain tester of this embodiment is such that the light source devices 21 and 22 of the bidirectional module 11 shown in Fig. 2 are replaced with those shown in Fig. 7. Therefore, a description of the configuration of the optical time domain tester and bidirectional module of this embodiment will be omitted.

[0061] <Light source device> Fig. 7 is a diagram showing the main configuration of a light source device according to a third embodiment of the present invention. Fig. 7 shows light source device 21C, which is provided in place of light source device 21 shown in Fig. 3, but light source device 22C (not shown), which is provided in place of light source device 22 shown in Fig. 3, has the same configuration. However, the center wavelength of light source device 22C is 1310 nm.

[0062] As shown in Fig. 7, light source device 21C of this embodiment differs from light source device 21 shown in Fig. 3 in that it includes condenser lens 21g (condensing optical system), reflecting member 21h (optical system), and collimating lens 21i instead of collimating lens 21b and notch filter 21c shown in Fig. 3. That is, light source device 21C of this embodiment has a configuration in which condenser lens 21g, reflecting member 21h, and collimating lens 21i are arranged in this order on the optical path of the light pulse emitted from pulse light source 21a.

[0063] The condenser lens 21g is provided on the optical path of the optical pulse between the pulse light source 21a and the reflecting member 21h, and condenses the optical pulse output from the pulse light source 21a onto one end of the reflecting member 21h. The reflecting member 21h is a member having the same reflection / transmission characteristics (reflection / transmission characteristics shown in FIG. 4) as the notch filter 21c shown in FIG. 3. The reflecting member 21h is, for example, a member in which a grating GR is formed in the core of an optical fiber built into a fiber stub. The collimating lens 21i collimates the optical pulse output from the other end of the reflecting member 21h to form parallel light. In the light source device 21C of this embodiment, a first resonator RS1 is formed by the first end face E1 and the second end face E2 of the semiconductor laser LD, and a second resonator RS2 is formed by the second end face E2 of the semiconductor laser LD and the grating GR formed on the reflecting member 21h.

[0064] In the light source device 21C of this embodiment, an optical pulse emitted from a first end face E1 of the semiconductor laser LD is focused by a focusing lens 21g onto one end of a reflecting member 21h. The focused optical pulse propagates through the core of an optical fiber built into the reflecting member 21h and enters a grating GR formed in the core of the optical fiber. A portion of the light that enters the grating GR is reflected by the grating GR, and the remainder passes through the grating GR. The light that passes through the grating GR is converted into parallel light by a collimating lens 21i and output.

[0065] The light reflected by the grating GR propagates in the opposite direction through the core of the optical fiber built in the reflecting member 21h, and is then focused by the focusing lens 21g and incident on the semiconductor laser LD from the first end face E1. The optical pulse incident on the semiconductor laser LD is reflected by the second end face E2 of the semiconductor laser LD, and then a part of the light is emitted again from the first end face E1 of the semiconductor laser LD.

[0066] The optical pulse emitted from the first end face E1 of the semiconductor laser LD is focused by the focusing lens 21g, then propagates through the core of the optical fiber built in the reflecting member 21h, and enters the grating GR formed in the core of the optical fiber. A portion of the light that enters the grating GR is reflected by the grating GR, and the remainder passes through the grating GR. The light that passes through the grating GR is converted into parallel light by the collimating lens 21i and output.

[0067] In this way, the optical pulse travels back and forth within the second resonator RS2 formed by the second end face E2 of the semiconductor laser LD and the grating GR formed on the reflecting member 21h. As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, and the first optical pulse (an optical pulse having a central wavelength within ±15 nm of 1550 nm) is output from the light source device 21.

[0068] As described above, in this embodiment, the reflecting member 21h on which the grating GR is formed is disposed on the optical path of the optical pulse (laser light) emitted from the first end face E1 of the semiconductor laser LD. This grating GR has a reflectance characteristic in which the reflectance for light in a predetermined wavelength range (e.g., ±15 nm) centered on a specified center wavelength (e.g., 1550 nm) of the semiconductor laser LD is higher than the reflectance of the first end face E1 of the semiconductor laser LD. The second end face E2 of the semiconductor laser LD and the grating GR of the reflecting member 21h form a second resonator RS2.

[0069] As a result, the main oscillation mode (longitudinal mode) of the semiconductor laser LD is determined by the second resonator RS2, so that the central wavelength of the light source device 21C can be easily kept within a predetermined range. Also, in this embodiment, as in the first embodiment, sufficient laser oscillation is possible even if the pulse width of the optical pulse is narrow, and the battery driving time is not shortened.

[0070] Although the light source device and optical time domain reflectometer according to the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be freely modified within the scope of the present invention. For example, in the above-mentioned first to third embodiments, an optical time domain reflectometer including a light source device that outputs optical pulses with a wavelength of 1550 nm and a light source device that outputs optical pulses with a wavelength of 1310 nm has been described. However, the optical time domain reflectometer may include only one of a light source device that outputs optical pulses with a wavelength of 1550 nm and a light source device that outputs optical pulses with a wavelength of 1310 nm.

[0071] In the first to third embodiments, an optical pulse tester is described that outputs an optical pulse having a wavelength of 1550 nm and an optical pulse having a wavelength of 1310 nm, but the wavelength of the optical pulse output from the optical pulse tester may be a wavelength other than 1550 nm or 1310 nm. Furthermore, the optical pulse tester is not limited to one that outputs optical pulses of two wavelengths, but may also be one that outputs optical pulses of one wavelength, or one that outputs optical pulses of three or more wavelengths.

[0072] Furthermore, the light source device of the first embodiment described above includes the notch filter 21c having the reflection / transmission characteristics shown in Fig. 4. However, instead of this notch filter 21c, a VHG (Volume Holographic Grating) having similar reflection / transmission characteristics may be included. [Explanation of symbols]

[0073] 1 Optical Time Domain Reflectometer 11 Bidirectional Module 14 Signal processing section 21 Light source device 21B Light source device 21C light source device 21a Pulsed light source 21b Collimating lens 21c Notch Filter 21d Notch Filter 21e Bandpass Filter 21st floor half mirror 21g Condenser lens 21h Reflective material 22 Light source device 26 Light receiving device E1 1st end surface E2 2nd end face FUT optical fiber GR Grating LD Semiconductor Laser PL1 1st page PL2 2nd page RS1 1st resonator RS2 2nd resonator

Claims

1. a semiconductor laser having a first end face and a second end face parallel to each other and forming a first resonator, the semiconductor laser emitting laser light from the first end face; an optical system that is disposed on an optical path of laser light emitted from the semiconductor laser, that forms a second resonator together with the second end face of the semiconductor laser, and that has a reflectivity for light in a predetermined wavelength range centered on a predetermined central wavelength of the semiconductor laser that is higher than the reflectivity of the first end face; Equipped with the optical system includes an optical element having a first surface provided with a reflective film having the reflection characteristics, and a second surface provided with an anti-reflection film for preventing reflection of the laser light emitted from the semiconductor laser, the reflectance of the second end face with respect to light in the predetermined wavelength range is approximately 90% or less; Light source device.

2. 2. The light source device according to claim 1, further comprising a collimating optical system provided on an optical path of the laser light between the semiconductor laser and the optical system, for collimating the laser light emitted from the semiconductor laser.

3. 1. An optical time domain tester for testing characteristics of an optical fiber based on return light obtained by injecting an optical pulse into the optical fiber, comprising: a bidirectional module having a light source device according to claim 1 or 2 that emits the optical pulse and a light receiving device that receives the returned light; a signal processing unit that performs processing to determine the characteristics of the optical fiber based on the light receiving result of the light receiving device; An optical time domain tester comprising:

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