Wavelength locker and wavelength variable 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

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

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

[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 of which are tilted to cause a relative phase difference of π / 2 before being 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, making it possible to recognize the direction of wavelength change over a wide band with high resolution.

[0003] Japanese Patent Application Publication No. 2002-202190

[0004] However, the conventional technology has had 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.

[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.

[0007] In the present disclosure, first and second lights having 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 miniaturized.

[0008] 1 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to a first embodiment; FIG. 2 is a diagram showing the frequency dependence of the transmittance of a general etalon; FIG. 3 is a diagram showing the frequency dependence of the transmittance slope of a general etalon; FIG. 4 is a diagram showing the temperature dependence of the transmittance of a general etalon; FIG. 5 is a diagram showing a wavelength locker according to a comparative example; FIG. 6 is a diagram showing the frequency dependence of the transmittance of an etalon of a comparative example; FIG. 7 is a diagram showing the frequency dependence of the transmittance slope of an etalon of a comparative example; FIG. 8 is a diagram showing the relationship between the angle of incidence of light to an etalon and the transmittance of an etalon; FIG. 9 is a diagram showing the frequency dependence of the transmittance of an etalon according to the first embodiment; FIG. 10 is a diagram showing the frequency dependence of the transmittance slope of an etalon according to the first embodiment; FIG. 11 is a diagram showing the relationship between the angle of incidence of light to an etalon according to the first embodiment and the transmittance of an etalon; FIG. 12 is a diagram showing the frequency dependence of the transmittance of an etalon according to the first embodiment; Fig. 1 is a diagram showing the frequency dependence of the transmittance of the etalon of embodiment 1. Fig. 2 is a diagram showing the frequency dependence of the slope of the transmittance of the etalon of embodiment 1. Fig. 3 is a diagram showing the frequency dependence of the transmittance of the etalon of embodiment 1. Fig. 4 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to embodiment 2. Fig. 5 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to embodiment 3.

[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] 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. The output of the laser light source 1 is controlled by adjusting the current or voltage of the laser light source 1.

[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 6 a and 6 b that are parallel to each other. The first light 5 is incident on the etalon 6 from the end face 6 a. The etalon 6 transmits a portion of the first light 5 and reflects the remainder of the first light 5 at the end face 6 b 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 with respect to the etalon 6 by angle θ, and different optical path lengths within the etalon 6. Therefore, the first light 5 and the second light 9 have different transmission wavelengths transmitted 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] Fig. 2 is a diagram showing the frequency dependence of the transmittance of a typical etalon. Fig. 3 is a diagram showing the frequency dependence of the slope of the transmittance of a typical etalon. FWHM is the half-width of the peak of the transmission waveform of the etalon. FSR is the period of the transmission waveform of the etalon. 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 reflectance R of the etalon. When finesse is high, the slope of the maximum transmittance becomes large, so that a slight change in wavelength changes the current in the light receiving element, 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. However, 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] 4 and 5 are diagrams showing the temperature dependence of the transmittance of a typical etalon. FIG. 4 shows the case where the finesse is constant but the frequency of light is different. FIG. 5 shows the case where the frequency of light is constant but the finesse is different. The transmittance of the etalon changes when the temperature of the etalon is changed. When the frequency of light 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 temperature of the etalon 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 the 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] 9 is a diagram showing 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, has an FSR of 100 GHz, and a π / 2 angle of 1.9 degrees. The data on the left and right represent a case where the frequency of the emitted light from the laser light source 1 or the temperature of the etalon 6 is shifted by ½ FSR. 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 half a 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] 14 is a diagram showing the relationship between the angle of incidence of light on the etalon of embodiment 1 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 left and right data represent a case where the frequency of the emitted light from the laser light source 1 or the temperature of the etalon 6 is shifted by ¼ FSR. By setting the difference θ in the angles of incidence between the first light 5 and the second light 9 to π / 4, 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, the characteristics of the etalon 6 with any peak position can be obtained.

[0025] Figures 15 and 17 are diagrams showing the frequency dependence of the transmittance of the etalon of embodiment 1. Figures 16 and 18 are diagrams showing the frequency dependence of the slope of the transmittance of the etalon of embodiment 1. Figures 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. Figures 15 and 16 show the case where the light receiving elements 10 and 11 are separated. Figures 17 and 18 show the case where light is received collectively 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 of 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. That is, 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 double period is obtained, so even if the etalon length is halved, transmission characteristics with the same period as the conventional one can be obtained. Therefore, the length of the etalon can be shortened, which enables the wavelength locker to be miniaturized.

[0027] As described above, in this embodiment, the first light 5 and the second light 9 having different angles of incidence on the etalon 6 are used. This widens the range in which the gradient of the transmittance of the etalon 6 relative to the change in wavelength of the light is large, and this 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 finesse of the etalon 6 is increased. By increasing the finesse, the gradient of the transmittance relative to the wavelength and temperature can be increased, enabling highly accurate wavelength control. In addition, the controllable temperature range of the etalon can be widened. This reduces the amount of temperature adjustment of 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] Second Embodiment Fig. 20 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to a 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 is 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 re-enters the etalon 6. 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 the light-receiving elements 10 and 11.

[0031] Embodiment 3. Fig. 21 is a diagram showing a wavelength tunable light source with a built-in wavelength locker according to embodiment 3. One large light receiving element 16 receives the first light 5 and the second light 9 transmitted through the etalon 6. The other configuration is the same as in embodiment 2. Even in this case, the same effects as in embodiments 1 and 2 can be obtained.

[0032] REFERENCE SIGNS LIST 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 causes the reflected light to be incident on the etalon as a second light; a light receiving element for receiving each of the first light and the second light transmitted through the etalon, A wavelength locker, characterized in that the first light and the second light have different angles of incidence 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; and a temperature adjustment unit that adjusts the temperature of the laser light source based on an output signal from the light receiving element to control the oscillation wavelength of the laser light source to be constant.