Wavelength lock and wavelength-variable light source with built-in wavelength lock

Abstract

A beam splitter (3) splits a laser beam (2) to generate first light (5). The etalon (6) transmits a portion of the first light (5) and reflects the remaining portion of the first light (5) at an end face to generate reflected light (7). The reflection unit (8) reflects the reflected light (7) and enters the etalon (6) as second light (9). The light receiving elements (10, 11) receive the first light (5) and the second light (9) that have passed through the etalon (6), respectively. The first light (5) and the second light (9) have different incident angles with respect to the etalon (6), and have different optical path lengths inside the etalon (6).

Description

Technical Field

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

[0002] In an optical system, a wavelength monitor that splits light into two, tilts the optical axes, and makes them incident on an etalon with a phase difference of π / 2 to be received by a light-receiving element respectively, and a wavelength-variable light source incorporated therein have been proposed (for example, refer to Patent Document 1). Thus, two signals can be obtained with a single etalon, and the wavelength change direction can be identified with high resolution over a wide band.

[0003] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-202190

[0004] However, in the prior art, there is a problem that the optical system for splitting light into two is enlarged, and the etalon is also enlarged. Summary of the Invention

[0005] The present disclosure is made to solve the above-described problems, and an object thereof is to obtain a small wavelength locker and a wavelength-variable light source with a built-in wavelength locker capable of performing high-precision wavelength control over a wide band.

[0006] The wavelength locker of the present disclosure is characterized by including: a beam splitter that splits a laser beam to generate first light; an etalon that transmits a part of the first light and reflects the remaining part of the first light at an end face to generate reflected light; a reflecting portion that reflects the reflected light and makes it incident on the etalon as second light; and a light-receiving element that receives the first light and the second light that have passed through the etalon, wherein the first light and the second light have different incident angles with respect to the etalon, and have different optical path lengths inside the etalon.

[0007] In the present disclosure, the first light and the second light having different incident angles with respect to the etalon are used. As a result, the range in which the slope of the transmittance of the etalon with respect to the change in the wavelength of light is large becomes wider, and thus this can be used to expand the controllable range of wavelength locking of the laser light source. Therefore, even if the fineness of the etalon is increased, the controllable range can be ensured. By increasing the fineness, the slope of the transmittance with respect to the wavelength and temperature can be increased, and thus high-precision wavelength control can be performed. In addition, since the second light is obtained by using the reflected light of the etalon, the optical system and the etalon can be miniaturized. Brief Description of the Drawings

[0008] Figure 1 It is a diagram showing a wavelength-variable light source with a built-in wavelength locker according to Embodiment 1.

[0009] Figure 2 It is a graph showing the frequency dependence of the transmittance of a general etalon.

[0010] Figure 3 It is a graph showing the frequency dependence of the slope of the transmittance of a general etalon.

[0011] Figure 4 It is a graph showing the temperature dependence of the transmittance of a general etalon.

[0012] Figure 5 It is a graph showing the temperature dependence of the transmittance of a general etalon.

[0013] Figure 6 It is a graph showing the wavelength locker of the comparative example.

[0014] Figure 7 It is a graph showing the frequency dependence of the transmittance of the etalon of the comparative example.

[0015] Figure 8 It is a graph showing the frequency dependence of the slope of the transmittance of the etalon of the comparative example.

[0016] Figure 9 It is a graph showing the relationship between the angle of light incident on the etalon and the transmittance of the etalon.

[0017] Figure 10 It is a graph showing the frequency dependence of the transmittance of the etalon of Embodiment 1.

[0018] Figure 11 It is a graph showing the frequency dependence of the slope of the transmittance of the etalon of Embodiment 1.

[0019] Figure 12 It is a graph showing the frequency dependence of the transmittance of the etalon of Embodiment 1.

[0020] Figure 13 It is a graph showing the frequency dependence of the slope of the transmittance of the etalon of Embodiment 1.

[0021] Figure 14 It is a graph showing the relationship between the angle of light incident on the etalon of Embodiment 1 and the transmittance of the etalon.

[0022] Figure 15 It is a graph showing the frequency dependence of the transmittance of the etalon of Embodiment 1.

[0023] Figure 16 It is a graph showing the frequency dependence of the slope of the transmittance of the etalon of Embodiment 1.

[0024] Figure 17It is a diagram showing the frequency dependence of the transmittance of the etalon according to Embodiment 1.

[0025] Figure 18 It is a diagram showing the frequency dependence of the slope of the transmittance of the etalon according to Embodiment 1.

[0026] Figure 19 It is a diagram showing the frequency dependence of the transmittance of the etalon according to Embodiment 1.

[0027] Figure 20 It is a diagram showing the wavelength locker built-in wavelength tunable light source according to Embodiment 2.

[0028] Figure 21 It is a diagram showing the wavelength locker built-in wavelength tunable light source according to Embodiment 3. Detailed Embodiment

[0029] The wavelength locker and the wavelength locker built-in wavelength tunable light source according to the embodiment will be described with reference to the accompanying drawings. The same or corresponding components are denoted by the same reference numerals, and repeated descriptions may be omitted.

[0030] Embodiment 1.

[0031] Figure 1 It is a diagram showing the wavelength locker built-in wavelength tunable light source according to Embodiment 1. A laser beam 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.

[0032] A beam splitter 3 splits the laser beam 2 into an output light 4 and a first light 5. The output light 4 is output to the outside through the beam splitter 3. 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 thereto.

[0033] An etalon 6 has end faces 6a, 6b that are parallel to each other. The first light 5 enters the etalon 6 from the end face 6a. The etalon 6 transmits a part of the first light 5 and reflects the remaining part of the first light 5 at the end face 6b to generate a reflected light 7.

[0034] A high reflector 8 reflects the reflected light 7 and causes the reflected light 7 to enter the etalon 6 again at an angle as a second light 9. The light receiving elements 10, 11 are, for example, photodiodes, and receive the first light 5 and the second light 9 that have passed through the etalon 6, respectively.

[0035] The incident angles of the first light 5 and the second light 9 with respect to the etalon 6 differ by an angle θ, and the optical path lengths inside the etalon 6 are different. Therefore, for the first light 5 and the second light 9, the transmission wavelength through the etalon 6 is different from the transparent temperature of the etalon 6.

[0036] 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, and controls the oscillation wavelength of the laser light source 1 to be constant. The etalon 6 is disposed above 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 (ThermoElectric Cooler) using Peltier elements. In addition, a temperature measurement unit such as a thermistor or a thermocouple that measures the temperature of the etalon 6 is disposed above the etalon 6, or above the temperature adjustment unit 13 and near the etalon 6. The temperature measurement unit is also provided in the laser light source 1.

[0037] Figure 2 It is a graph showing the frequency dependence of the transmittance of a general etalon. Figure 3 It is a graph showing the frequency dependence of the slope of the transmittance of a general etalon. FWHM is the peak half-value width of the transmission waveform of the etalon. FSR is the period of the transmission waveform of the etalon. The finesse F is defined by F = FSR / FWHM.

[0038] When the reflectivity of the etalon is set to R, F = πR 1 / 2 / (1 - R). Therefore, when the reflectivity R of the etalon is increased, the finesse can be increased. When the finesse is high, since the slope of the maximum transmittance becomes large, the current of the light receiving element changes due to a small wavelength change, so wavelength control can be performed with high precision. On the other hand, wavelength locking needs to be controlled in a region where the slope of the transmittance with respect to wavelength change is large, but when the finesse is high, the region where the slope of the transmittance is large becomes narrow, so wavelength locking is likely to deviate.

[0039] Figure 4 and Figure 5 It is a graph showing the temperature dependence of the transmittance of a general etalon. Figure 4 It is the case where the finesse is constant and the frequency of light is different. Figure 5 It is the case where the frequency of light is constant and the finesse is different. By changing the temperature of the etalon, the transmittance of the etalon will change. When the frequency of light is different, the peak position of the transmittance of the etalon with respect to temperature will change. Therefore, in order to perform wavelength locking at a desired wavelength, the temperature of the etalon is changed to a temperature at which the slope of the transmittance is large.

[0040] However, if the temperature of the etalon deviates from the ambient temperature, a large amount of power is required to keep it at a constant temperature. Therefore, expanding the controllable range of wavelength locking and suppressing the power consumption of the temperature control of the etalon become design items that conflict with high-precision wavelength control.

[0041] Figure 6It is a diagram of the wavelength locker showing a comparative example. The light receiving element 11 directly receives the reflected light 7 of the etalon 6. Figure 7 It is a diagram showing the frequency dependence of the transmittance of the etalon in the comparative example.

[0042] Figure 8 It is a diagram showing the frequency dependence of the slope of the transmittance of the etalon in the comparative example. In the first light 5 and the reflected light 7, the sign of the slope of the transmittance of the etalon 6 is reversed. However, the region where the slope of the controllable transmittance for wavelength locking is large does not expand.

[0043] In contrast, in the present embodiment, by using the first light 5 and the second light 9 having different incident angles with respect to the etalon 6, the region where the slope of the controllable transmittance for wavelength locking is large is expanded. This will be described in detail below.

[0044] Figure 9 It is a diagram showing the relationship between the angle at which light is incident on the etalon and the transmittance of the etalon. The material of the etalon is synthetic quartz, the FRS is 100 GHz, and the π / 2 angle is 1.9 deg. The data on the left and right are the cases where the frequency of the emitted light of the laser light source 1 or the temperature of the etalon 6 is offset by 1 / 2 FSR. By setting the difference θ between the incident angles of the first light 5 and the second light 9 to the π / 2 angle, the characteristics of the etalon 6 with respect to the second light 9 become characteristics that deviate by a half cycle compared to the characteristics of the etalon 6 with respect to the first light 5.

[0045] Figure 10 and Figure 12 It is a diagram showing the frequency dependence of the transmittance of the etalon in Embodiment 1. Figure 11 and Figure 13 It is a diagram showing the frequency dependence of the slope of the transmittance of the etalon in Embodiment 1. Figures 10 to 13 It is the case where the difference θ between the incident angles of the first light 5 and the second light 9 is the π / 2 angle. Figure 10 and Figure 11 It is the case where the light receiving elements 10 and 11 are separated. Figure 12 and Figure 13 It is the case where light is received by one light receiving element together. By using not only the first light 5 but also the second light 9, the controllable range for wavelength locking can be expanded.

[0046] Figure 14It is a diagram showing the relationship between the angle at which light is incident on the etalon in Embodiment 1 and the transmittance of the etalon. The material of the etalon is synthetic quartz, the FRS is 100 GHz, and the π / 4 angle is 1.3 deg.. The data on the left and right are the cases where the frequency of the emitted light from the laser light source 1 or the temperature of the etalon 6 is offset by 1 / 4 FSR. By setting the difference θ between the incident angles of the first light 5 and the second light 9 to the π / 4 angle, the characteristics of the etalon 6 with respect to the second light 9 become characteristics that deviate by 1 / 4 cycle compared to the characteristics of the etalon 6 with respect to the first light 5. Thus, by adjusting the difference θ between the incident angles, etalon 6 characteristics with an arbitrary peak position can be obtained.

[0047] Figure 15 and Figure 17 It is a diagram showing the frequency dependence of the transmittance of the etalon in Embodiment 1. Figure 16 and Figure 18 It is a diagram showing the frequency dependence of the slope of the transmittance of the etalon in Embodiment 1. Figures 15 to 18 This is the case where the difference θ between the incident angles of the first light 5 and the second light 9 is the π / 4 angle. Figure 15 and Figure 16 This is the case where the light receiving elements 10 and 11 are separated. Figure 17 and Figure 18 This is the case where light is received by one light receiving element together. Compared with the case where the difference θ between the incident angles is the π / 2 angle, the controllable range of wavelength locking can be concentrated.

[0048] Figure 19 It is a diagram showing the frequency dependence of the transmittance of the etalon in Embodiment 1. 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, FSR is inversely proportional to the length d of the etalon. In this embodiment, by setting θ = π / 2 angle, a transmittance characteristic with a double period can be obtained. Therefore, even if the length of the etalon is set to 1 / 2, a transmittance characteristic with the same period as before can be obtained. Therefore, the length of the etalon can be shortened, and thus the wavelength locker can be miniaturized.

[0049] As described above, in the present embodiment, the first light 5 and the second light 9 having different incident angles with respect to the etalon 6 are used. Thereby, the range where the slope of the transmittance of the etalon 6 with respect to the change in the wavelength of the light is large is expanded, and thus the controllable range of the wavelength locking of the laser light source can be expanded using this point. Therefore, even if the fineness of the etalon 6 is increased, the controllable range can be ensured. By increasing the fineness, the slope of the transmittance with respect to the wavelength and temperature can be increased, and thus high-precision wavelength control can be performed. In addition, the temperature range of the controllable etalon can be expanded. Therefore, since the temperature adjustment amount of the etalon can be reduced, power consumption can be suppressed. In addition, since the reflected light of the etalon 6 is used to obtain the second light 9, the optical system and the etalon can be miniaturized.

[0050] Embodiment 2.

[0051] Figure 20 FIG. is a diagram showing a wavelength locker-integrated wavelength tunable light source according to Embodiment 2. In the present embodiment, instead of the high reflector 8 of Embodiment 1, a high reflection film 14 is provided on the side surface of the beam splitter 3. The high reflection film 14 reflects the reflected light 7, and the reflected light 7 is incident on the etalon 6 again as the second light 9 at an angle. Other configurations are the same as those in Embodiment 1.

[0052] If the optical axis of the laser beam 2 is inclined by θ / 2 and incident on the beam splitter 3, the difference in the incident angles of the first light 5 and the second light 9 with respect to the etalon 6 becomes θ. Therefore, for the first light 5 and the second light 9, the transmission wavelength through the etalon 6 and the transparent temperature of the etalon 6 are different. Therefore, the same effect as in Embodiment 1 can be obtained.

[0053] In addition, consider the case where the second reflected light 15 of the etalon 6 is reflected by the high reflection film 14 and incident on the etalon 6 again. Such light becomes the remaining component of the reflected lights 7 and 15, and thus becomes a complex transmission shape. It is difficult to perform wavelength locking control based on such light. Therefore, the outer dimensions and configurations of the beam splitter 3 and the etalon 6 are set so that the reflected light 15 does not become the third incident light on the etalon 6. Alternatively, the outer dimensions and configurations 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.

[0054] Embodiment 3.

[0055] Figure 21 FIG. is a diagram showing a wavelength locker-integrated wavelength tunable light source according to Embodiment 3. A single large light receiving element 16 receives the first light 5 and the second light 9 that have passed through the etalon 6. Other configurations are the same as those in Embodiment 2. Even in this case, the same effects as in Embodiments 1 and 2 can be obtained.

[0056] Description of Reference Numerals

[0057] 1... Laser light source; 2... Laser beam; 3... Beam splitter; 5... First light; 6... Etalon; 7... Reflected light; 8... High reflector (reflective part); 9... Second light; 10, 11, 16... Light receiving elements; 12... Temperature adjustment unit; 14... High reflection film (reflective part).

Claims

1. A wavelength locker, characterized in that, it comprises: a beam splitter that splits a laser beam to generate a first light; an etalon that transmits a part of the first light and reflects the remaining part of the first light at an end face to generate a reflected light; a reflection part 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 passed through the etalon respectively, wherein the incident angles of the first light and the second light with respect to the etalon are different, and the optical path lengths inside the etalon are different.

2. The wavelength locker according to claim 1, characterized in that, the reflection part is a highly reflective mirror that reflects the reflected light and makes it incident on the etalon at an angle as the second light.

3. The wavelength locker according to claim 1, characterized in that, the reflection part is a highly reflective film provided on a side surface of the beam splitter.

4. The wavelength locker according to claim 3, characterized in that, the outer dimensions and configurations of the beam splitter and the etalon are set in such a way that the light reflected from the end face of the etalon by a part of the second light does not enter the etalon again or does not enter the light receiving element.

5. The wavelength locker according to any one of claims 1 to 4, characterized in that, one of the light receiving elements receives the first light and the second light that have passed through the etalon.

6. A wavelength variable light source with a built-in wavelength locker, characterized in that, it comprises: the wavelength locker according to any one of claims 1 to 5; a laser light source that emits the laser; and a temperature adjustment part that adjusts the temperature of the laser light source based on the output signal of the light receiving element and controls the oscillation wavelength of the laser light source to be constant.

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