Wavelength locker and wavelength tunable light source with built-in wavelength locker

Abstract

A beam splitter (3) divides a laser beam (2) to generate a first beam (5). An etalon (6) transmits therethrough a portion of the first beam (5), and reflects the remaining portion of the first beam (5) on an end surface thereof to generate a reflected beam (7). A reflection unit (8) reflects the reflected beam (7) and causes the same to enter the etalon (6) as a second beam (9). Reception elements (10, 11) respectively receive the first beam (5) and the second beam (9) transmitted through the etalon (6). The first beam (5) and the second beam (9) have different angles of entry with respect to the etalon (6), and are different in optical path length in the etalon (6).

Description

[Technical Field]

[0001] The present disclosure relates to a wavelength locker and a wavelength tunable light source with a built-in wavelength locker. [Background technology]

[0002] A wavelength monitor and a wavelength tunable light source incorporating such a monitor have been proposed (see, for example, Patent Document 1), in which light is split into two by an optical system, the optical axes are tilted to shift the phase difference between the two light beams relative to each other by π / 2, and the two light beams are then incident on an etalon, and each light is received by a light receiving element. This allows two signals to be obtained from the etalon alone, and the direction of wavelength change can be recognized with high resolution over a wide band. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2002-202190 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the conventional technology has the problem that the optical system that splits the light into two becomes large, and the etalon also becomes large.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide a compact wavelength locker and a wavelength tunable light source with a built-in wavelength locker that are capable of wavelength control over a wide band with high precision. [Means for solving the problem]

[0006] The wavelength locker according to the present disclosure includes a beam splitter that splits laser light to generate a first light, an etalon that transmits a portion of the first light and reflects the remainder of the first light at an end surface to generate reflected light, a reflecting section that reflects the reflected light and makes it incident on the etalon as a second light, and a light receiving element that receives the first light and the second light that have transmitted through the etalon, and is characterized in that the first light and the second light have different incident angles with respect to the etalon and different optical path lengths inside the etalon. [Effects of the Invention]

[0007] In this disclosure, first and second lights with different angles of incidence on the etalon are used. This widens the range in which the gradient of the etalon's transmittance with respect to changes in the wavelength of the light is large, which can be used to widen the controllable range of wavelength locking of the laser light source. Therefore, the controllable range can be secured even if the etalon's finesse is increased. Increasing the finesse increases the gradient of the transmittance with respect to wavelength and temperature, enabling highly accurate wavelength control. Furthermore, since the second light is obtained using the reflected light from the etalon, the optical system and etalon can be made smaller. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to a first embodiment. [Figure 2] FIG. 1 is a diagram showing the frequency dependence of the transmittance of a typical etalon. [Figure 3] FIG. 1 is a diagram showing frequency dependence of the slope of the transmittance of a typical etalon. [Figure 4] FIG. 1 is a diagram showing the temperature dependence of the transmittance of a typical etalon. [Figure 5] FIG. 1 is a diagram showing the temperature dependence of the transmittance of a typical etalon. [Figure 6] FIG. 10 is a diagram illustrating a wavelength locker according to a comparative example. [Figure 7] FIG. 10 is a diagram showing the frequency dependence of the transmittance of an etalon of a comparative example. [Figure 8]FIG. 10 is a diagram showing the frequency dependence of the transmittance slope of the etalon of the comparative example. [Figure 9] FIG. 10 is a diagram showing the relationship between the angle of incidence of light to an etalon and the transmittance of the etalon. [Figure 10] FIG. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of the first embodiment. [Figure 11] FIG. 4 is a diagram showing frequency dependence of the slope of the transmittance of the etalon of the first embodiment. [Figure 12] FIG. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of the first embodiment. [Figure 13] FIG. 4 is a diagram showing frequency dependence of the slope of the transmittance of the etalon of the first embodiment. [Figure 14] 4 is a diagram showing the relationship between the angle of incidence of light to the etalon and the transmittance of the etalon according to the first embodiment. FIG. [Figure 15] FIG. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of the first embodiment. [Figure 16] FIG. 4 is a diagram showing frequency dependence of the slope of the transmittance of the etalon of the first embodiment. [Figure 17] FIG. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of the first embodiment. [Figure 18] FIG. 4 is a diagram showing frequency dependence of the slope of the transmittance of the etalon of the first embodiment. [Figure 19] FIG. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of the first embodiment. [Figure 20] FIG. 10 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to a second embodiment. [Figure 21] FIG. 10 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] A wavelength locker and a wavelength tunable light source with a built-in wavelength locker according to an embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0010] Embodiment 1 1 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to embodiment 1. Laser light 2 is emitted from a laser light source 1. When the current or voltage of the laser light source 1 is adjusted, the output of the laser light source 1 is controlled.

[0011] The beam splitter 3 splits the laser light 2 into an output light 4 and a first light 5. The output light 4 passes through the beam splitter 3 and is output to the outside. The first light 5 is reflected by the beam splitter 3 in a direction different from that of the output light 4. The reflection angle is, for example, 90°, but is not limited to this.

[0012] The etalon 6 has end faces 6a and 6b that are parallel to each other. The first light 5 is incident on the etalon 6 from the end face 6a. The etalon 6 transmits a portion of the first light 5 and reflects the remainder of the first light 5 at the end face 6b to generate reflected light 7.

[0013] The high-reflection mirror 8 reflects the reflected light 7 and makes it incident again at an angle as second light 9 into the etalon 6. The light-receiving elements 10 and 11 are, for example, photodiodes, and receive the first light 5 and the second light 9 that have passed through the etalon 6, respectively.

[0014] The first light 5 and the second light 9 have different incident angles to the etalon 6 by angle θ, and therefore have different optical path lengths inside the etalon 6. Therefore, the first light 5 and the second light 9 have different transmission wavelengths that pass through the etalon 6 and different transparent temperatures of the etalon 6.

[0015] The temperature adjustment unit 12 adjusts the temperature of the laser light source 1 based on the output signal of the light receiving element 10 or the output signal of the light receiving element 11, thereby controlling the oscillation wavelength of the laser light source 1 to be constant. The etalon 6 is disposed on the temperature adjustment unit 13. The temperature adjustment unit 13 adjusts the temperature of the etalon 6. The temperature adjustment units 12 and 13 are, for example, thermoelectric coolers using Peltier elements. A temperature measurement unit such as a thermistor or thermocouple that measures the temperature of the etalon 6 is disposed on the etalon 6 or near the etalon 6 on the temperature adjustment unit 13. A temperature measurement unit is also provided in the laser light source 1.

[0016] Figure 2 shows the frequency dependence of the transmittance of a typical etalon. Figure 3 shows the frequency dependence of the slope of the transmittance of a typical etalon. FWHM is the half-width of the peak of the etalon's transmission waveform. FSR is the period of the etalon's transmission waveform. Finesse F is defined as F=FSR / FWHM.

[0017] If the reflectance of the etalon is R, then F=πR 1 / 2 / (1-R). Therefore, finesse can be increased by increasing the etalon reflectance R. When finesse is high, the slope of the maximum transmittance becomes larger, so that even a slight change in wavelength changes the current in the photodetector, allowing for highly accurate wavelength control. On the other hand, wavelength locking must be controlled within a region where the slope of the transmittance relative to wavelength change is large, but when finesse is high, the region where the slope of the transmittance is large becomes narrower, making it easier for wavelength lock to be released.

[0018] Figures 4 and 5 are diagrams showing the temperature dependence of the transmittance of a typical etalon. Figure 4 shows the case where the finesse is constant but the optical frequency is different. Figure 5 shows the case where the optical frequency is constant but the finesse is different. The transmittance of the etalon changes when the etalon temperature is changed. When the optical frequency is different, the peak position of the transmittance with respect to the etalon temperature changes. Therefore, in order to wavelength lock at the desired wavelength, the etalon temperature is changed to set a temperature at which the slope of the transmittance is steep.

[0019] However, if the etalon temperature deviates from the ambient temperature, a large amount of power is required to maintain a constant temperature. Therefore, widening the controllable range of wavelength locking and reducing the power consumption for etalon temperature control are design issues that contradict high-precision wavelength control.

[0020] FIG. 6 is a diagram showing a wavelength locker according to a comparative example. A light receiving element 11 directly receives reflected light 7 from an etalon 6. FIG. 7 is a diagram showing the frequency dependence of the transmittance of the etalon according to the comparative example. FIG. 8 is a diagram showing the frequency dependence of the slope of the transmittance of the etalon according to the comparative example. The positive and negative signs of the slope of the transmittance of the etalon 6 are reversed for the first light 5 and the reflected light 7. However, the region where the slope of the transmittance is large and wavelength locking controllable does not expand.

[0021] In contrast to this, in this embodiment, the region where the gradient of the transmittance that can be controlled by wavelength locking is large is widened by using the first light 5 and the second light 9 that have different angles of incidence on the etalon 6. This will be described in detail below.

[0022] Figure 9 shows the relationship between the angle of incidence of light onto the etalon and the transmittance of the etalon. The etalon is made of synthetic quartz, with an FSR of 100 GHz and a π / 2 angle of 1.9 degrees. The data on the left and right are for cases where there is a ½ FSR shift in the frequency of the light emitted from the laser light source 1 or the temperature of the etalon 6. By setting the difference θ in the angles of incidence between the first light 5 and the second light 9 to a π / 2 angle, the characteristics of the etalon 6 for the second light 9 are shifted by a half period compared to the characteristics of the etalon 6 for the first light 5.

[0023] 10 and 12 are graphs showing the frequency dependence of the transmittance of the etalon of embodiment 1. FIGS. 11 and 13 are graphs showing the frequency dependence of the slope of the transmittance of the etalon of embodiment 1. FIGS. 10 to 13 show the case where the difference θ in the angle of incidence between the first light 5 and the second light 9 is π / 2. FIGS. 10 and 11 show the case where the light receiving elements 10 and 11 are separated. FIGS. 12 and 13 show the case where the light is received collectively by a single light receiving element. By using not only the first light 5 but also the second light 9, the controllable range of wavelength locking can be expanded.

[0024] FIG. 14 is a diagram showing the relationship between the angle of incidence of light on the etalon of the first embodiment and the transmittance of the etalon. The etalon is made of synthetic quartz, has an FSR of 100 GHz, and a π / 4 angle of 1.3 degrees. The data on the left and right are for a case where there is a shift of ¼ FSR due to the frequency of the emitted light from the laser light source 1 or the temperature of the etalon 6. By setting the difference θ in the angles of incidence between the first light 5 and the second light 9 to a π / 4 angle, the characteristics of the etalon 6 for the second light 9 are shifted by ¼ period compared to the characteristics of the etalon 6 for the first light 5. By adjusting the difference θ in the angles of incidence in this way, it is possible to obtain the characteristics of the etalon 6 with any peak position.

[0025] Figs. 15 and 17 are diagrams showing the frequency dependence of the transmittance of the etalon of embodiment 1. Figs. 16 and 18 are diagrams showing the frequency dependence of the slope of the transmittance of the etalon of embodiment 1. Figs. 15 to 18 show the case where the difference θ in the angles of incidence between the first light 5 and the second light 9 is π / 4. Figs. 15 and 16 show the case where the light receiving elements 10 and 11 are separated. Figs. 17 and 18 show the case where all the light is received at once by a single light receiving element. The controllable range of wavelength locking can be concentrated compared to the case where the difference θ in the angles of incidence is π / 2.

[0026] 19 is a diagram showing the frequency dependence of the transmittance of the etalon according to the first embodiment. When the wavelength of light is λ, the refractive index of the etalon is n, and the length of the etalon is d, FSR=λ 2 / 2nd. In other words, the FSR is inversely proportional to the etalon length d. In this embodiment, by setting the angle θ=π / 2, a transmission characteristic with a doubled period is obtained, so even if the etalon length is halved, transmission characteristics with the same period as before can be obtained. Therefore, the etalon length can be shortened, making it possible to miniaturize the wavelength locker.

[0027] As described above, this embodiment uses the first light 5 and the second light 9, which have different angles of incidence on the etalon 6. This widens the range in which the transmittance gradient of the etalon 6 is large relative to changes in the wavelength of the light, thereby expanding the controllable range of wavelength locking of the laser light source. Therefore, the controllable range can be secured even if the finesse of the etalon 6 is increased. Increasing the finesse increases the gradient of the transmittance relative to wavelength and temperature, enabling highly accurate wavelength control. Furthermore, the controllable temperature range of the etalon can be expanded. This reduces the amount of temperature adjustment required for the etalon, thereby reducing power consumption. Furthermore, since the second light 9 is obtained using the reflected light from the etalon 6, the optical system and etalon can be made smaller.

[0028] Embodiment 2 20 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to the second embodiment. In this embodiment, a high-reflection film 14 is provided on the side surface of the beam splitter 3 instead of the high-reflection mirror 8 of the first embodiment. The high-reflection film 14 reflects the reflected light 7 and makes it re-enter the etalon 6 at an angle as second light 9. The other configurations are the same as those of the first embodiment.

[0029] When the optical axis of the laser light 2 is tilted by θ / 2 and the light is incident on the beam splitter 3, the difference in the angle of incidence of the first light 5 and the second light 9 with respect to the etalon 6 becomes θ. Therefore, the first light 5 and the second light 9 have different transmission wavelengths that pass through the etalon 6 and different transparent temperatures of the etalon 6. Therefore, the same effects as those of the first embodiment can be obtained.

[0030] It is possible that the second reflected light 15 from the etalon 6 is reflected by the high-reflection film 14 and enters the etalon 6 again. This light becomes the remaining component of the reflected light 7 and 15, resulting in a complex transmission shape. It is difficult to control wavelength locking based on this light. Therefore, the outer sizes and positions of the beam splitter 3 and etalon 6 are set so that the reflected light 15 does not become the third incident light to the etalon 6. Alternatively, the outer sizes and positions of the light-receiving elements 10 and 11 are adjusted so that the third incident light does not enter them.

[0031] Embodiment 3 21 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to the third embodiment. One large light receiving element 16 receives the first light 5 and the second light 9 transmitted through the etalon 6. The other configurations are the same as those of the second embodiment. Even in this case, the same effects as those of the first and second embodiments can be obtained. [Explanation of symbols]

[0032] 1 laser light source, 2 laser light, 3 beam splitter, 5 first light, 6 etalon, 7 reflected light, 8 high-reflection mirror (reflection section), 9 second light, 10, 11, 16 light receiving element, 12 temperature adjustment section, 14 high-reflection film (reflection section)

Claims

1. a beam splitter that splits the laser light to generate a first light; an etalon that transmits a portion of the first light and reflects the remainder of the first light at an end surface to generate reflected light; a reflecting section that reflects the reflected light and makes it incident on the etalon as a second light; a light-receiving element configured to receive the first light and the second light transmitted through the etalon, The wavelength locker is characterized in that the first light and the second light have different incident angles with respect to the etalon and different optical path lengths within the etalon.

2. 2. The wavelength locker according to claim 1, wherein the reflecting portion is a high-reflection mirror that reflects the reflected light and makes it incident on the etalon at an angle as the second light.

3. 2. The wavelength locker according to claim 1, wherein the reflecting portion is a highly reflective film provided on a side surface of the beam splitter.

4. 4. The wavelength locker according to claim 3, wherein the outer sizes and arrangements of the beam splitter and the etalon are set so that a portion of the second light reflected by the end surface of the etalon does not re-enter the etalon or does not enter the light receiving element.

5. 5. The wavelength locker according to claim 1, wherein one of the light receiving elements receives the first light and the second light transmitted through the etalon.

6. A wavelength locker according to any one of claims 1 to 4; a laser light source that emits the laser light; a temperature adjusting unit that adjusts the temperature of the laser light source based on the output signal of the light receiving element to control the oscillation wavelength of the laser light source to be constant.

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