Tunable Laser Device with Frequency Stabilization and Low-Frequency Noise

Abstract

A tunable laser device comprising a tunable fiber laser and a laser control module, wherein the tunable fiber laser is configured to generate laser light having a center wavelength controlled by one or more Bragg gratings in a first fiber, and the laser control module is configured to receive at least a part of the laser light generated by the tunable fiber laser, generate a stabilization control signal, and feedback the control signal to the tunable fiber laser to stabilize the center wavelength. The laser control module comprises a wavelength discrimination element having an etalon with one or more etalon resonances, the etalon being formed by one or more Bragg gratings in a second fiber, and the etalon resonance being controlled by the one or more Bragg gratings in the second fiber. A tunable laser device is disclosed.

Description

Technical Field

[0001] The present disclosure generally relates to tunable laser devices. More particularly, the present disclosure relates to providing a frequency-stabilized tunable laser device. More particularly, the present disclosure relates to providing a tunable laser device having low frequency noise.

Background Art

[0002] Tunable laser devices are known in the art. Further, it is generally known that a single-frequency fiber laser generates laser light having very high frequency stability and a correspondingly very low level of frequency noise. An example of a tunable laser device such as a fiber laser with a frequency discrimination element that is frequency-stabilized and exhibits low frequency noise is disclosed in Patent Document 1. Preferably, the frequency discrimination element of Patent Document 1 is made of solid silica such as a solid silica-based Fabry-Perot interferometer having a mirror and is disposed outside the laser cavity. When the frequency discrimination element is a solid silica-based Fabry-Perot interferometer, a combination of high finesse and an appropriate free spectral range can be obtained using a small interferometer. When designing a frequency discrimination element, it is well known that such an element has a high slope steepness obtained by increasing the finesse of the interferometer or decreasing the free spectral range of the interferometer. However, it is difficult to manufacture a small tunable interferometer having a very high slope steepness.

[0003] To solve this problem, high-finesse interferometers have been manufactured in fibers having a Bragg grating inside. In other words, it has been proposed to use a fiber-based Bragg grating configured to operate as a Fabry-Perot interferometer as a frequency discrimination element.

[0004] Non-Patent Document 1 discloses an example of a tunable laser device based on a diode laser combined with a frequency discrimination element in the form of a fiber Bragg grating Fabry-Perot cavity. The setup of this paper depends on a complex self-injection locking system, which reports the control of the laser frequency and a significant reduction in phase / frequency noise.

[0005] Another example of a tunable laser device combined with a frequency discrimination element in the form of a fiber Bragg grating is disclosed in Patent Document 2. This disclosure relates to controlling the laser frequency based on the frequency discrimination element.

[0006] A third example of a tunable laser device combined with a frequency discrimination element in the form of a fiber Bragg grating, although it is a narrowband phase-shifted fiber Bragg grating, is disclosed in Patent Document 3. This disclosure also relates to controlling the laser frequency based on the frequency discrimination element.

[0007] In the prior art as described above, for example, the stabilization of the laser after wavelength tuning requires a long time and / or depends on a complex setup. An ytterbium-doped fiber ring laser comprising a fiber Bragg grating (FBG) and a fiber Fabry-Perot (FFP) is disclosed in Non-Patent Document 2. In this paper, the cavity is locked to the FFP using the well-known Pound-Drever-Hall (PDH) technique. A desired setup for replacing the FFP with a phase-shifted FBG is described, and this setup includes two FBGs, namely, a first FBG in the ring laser cavity and a second FBG in the PDH loop. The PDH loop forms part of the ring laser cavity and is susceptible to both noise and stability effects.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Document

[0009]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] The object of the present disclosure is to provide a tunable laser device that overcomes the problems of the prior art. Furthermore, the object of the present disclosure is to reduce the frequency / phase noise of a tunable laser device, particularly a tunable fiber laser.

[0011] Furthermore, an object of the present disclosure is to provide a tunable laser device that generates laser light and includes a tunable fiber laser and a laser control module that stabilizes the frequency of the laser and provides high frequency stability of the laser light while maintaining the tunability of the device.

[0012] Finally, and in particular, an object of the present disclosure is to provide a tunable laser device that efficiently and easily stabilizes the frequency of a tunable laser device.

Means for Solving the Problems

[0013] These and other objects are defined in the claims and solved by a tunable laser device as described below in the present disclosure. In one aspect of the present disclosure, a tunable laser device, comprising a tunable fiber laser and a laser control module, wherein the tunable fiber laser is configured to generate laser light having a center wavelength controlled by one or more Bragg gratings in a first fiber, the laser control module is configured to receive at least a part of the laser light generated by the tunable fiber laser, generate a stabilization control signal, and feedback the control signal to the tunable fiber laser to stabilize the center wavelength, the laser control module includes a wavelength discrimination element including an etalon having one or more etalon resonances, the etalon is formed by one or more Bragg gratings in a second fiber, and a tunable laser device is disclosed in which the etalon resonance is controlled by the one or more Bragg gratings in the second fiber.

[0014] The first fiber and the second fiber are configured to be controlled by a fiber control module, and the fiber control module is configured to transmit a tuning control signal related to determining a first temperature of the first fiber and a second temperature of the second fiber.

[0015] In this way, the fiber control module shifts the center wavelength by modifying the one or more Bragg gratings in the first fiber, and shifts the etalon resonance by modifying the one or more Bragg gratings in the second fiber, and thus plays both roles.

[0016] Therefore, the shift of the etalon resonance can advantageously be shifted by an efficient and easy method, similar to the center wavelength. It is understood that the first fiber generates an optical signal that is part of the laser light.

[0017] In a preferred embodiment, a part of the laser light received by the laser control module is not sent back to the tunable fiber laser. In other words, the laser control module is configured to only receive the laser light. Therefore, the laser control module is configured to block and / or prevent the laser light from being sent back to the tunable fiber laser. The inventors have found that in this preferred embodiment, the system is less affected by noise compared to a system described in Non-Patent Document 2, for example, where the laser light from the control module is sent back to the laser ring cavity using a 4-port circulator. Therefore, the tunable laser device of the present disclosure is different from that of the paper at least by this feature. Further, a tunable laser device as described in the paper uses a piezoelectric device to stretch the fiber of the ring laser cavity so that the mode matches the mode of the FBG. Thus, the FBG is not used to adjust the wavelength as in the present disclosure. As described in Non-Patent Document 2, the laser is not optimal and needs improvement. The laser device disclosed herein operates completely differently from that of the laser described in Non-Patent Document 2 and provides a much improved performance.

[0018] The fiber control module according to the present disclosure is configured to transmit a tuning control signal related to determining a first temperature of a first fiber and a second temperature of a second fiber. In this context, it is understood that the tuning control signal transmitted to both the first fiber and the second fiber results from a single signal. The single signal provides efficient and easy control of the two fibers. Clearly, a single original signal may be split into two signals, for example, when the two fibers are separated from each other and not in thermal contact with each other. However, most preferably, the single signal is retained as a single signal, for example, in embodiments when the two fibers are in thermal contact with each other. The two fibers may be in thermal contact with each other via one or more substrates and / or via one or more heat carriers, as described below.

[0019] Typically, the fiber laser wavelength is controlled or tuned by only a single signal independent of the signal transmitted to control the etalon. Examples of such are disclosed in Patent Document 1 and Patent Document 2. However, the fiber laser may also be controlled by two or more signals, such as those disclosed in Patent Document 2, and such signals are also independent of the signal transmitted to control the etalon.

[0020] In contrast to such setups, the present disclosure provides a fiber laser in which, in addition to having a laser phase noise reduced by a stabilization control signal, the laser wavelength is controlled by a tuning control signal that is also used to control the etalon resonance. In other words, the present disclosure provides a fiber laser that is controlled or tuned by a signal that also depends on the signal transmitted to control the etalon. This dependence provides efficient and easy control of the two fibers.

[0021] Surprisingly, the inventors of the present disclosure have found that using a control module that serves both to shift the central wavelength of a laser and to shift the etalon resonance of an etalon can be applied not only to fiber lasers but also to various types of lasers.

[0022] Accordingly, in a second aspect of the present disclosure, there is provided a tunable laser device, comprising a tunable laser and a laser control module, wherein the tunable fiber laser is configured to generate laser light having a central wavelength, the laser control module is configured to receive at least a part of the laser light generated by the tunable laser, generate a stabilization control signal, and feedback the control signal to the tunable laser to stabilize the central wavelength, the laser control module comprises a wavelength discrimination element having an etalon with one or more etalon resonances, the tunable laser and the etalon are configured to be controlled by a common control module, and the common control module is configured to transmit a tuning control signal related to determining a first temperature of the tunable laser and a second temperature of the etalon, whereby the common control module serves both to shift the central wavelength of the tunable laser and to shift the etalon resonance of the etalon, and a tunable laser device is provided.

[0023] In a preferred embodiment of the second aspect of the present disclosure, the tunable laser is a planar waveguide laser, and the etalon is formed of a material having the same material properties as the planar waveguide laser, such as the planar waveguide and the etalon having the same thermal expansion. In a first preferred embodiment, the materials of the planar waveguide laser and the etalon are formed of InP or another low-loss passive material.

[0024] In another preferred embodiment of the second aspect of the present disclosure, the tunable laser is a laser comprising one or more Bragg gratings, such as a fiber laser, and the etalon is formed of a material having the same material properties as the tunable laser, such as the same thermal expansion. For example, the tunable laser and the etalon may be formed of the same material, such as a low-loss passive material such as InP.

[0025] In some embodiments of the second aspect of the present disclosure, the tunable laser is a diode laser or a solid-state laser. In these embodiments and the previous preferred embodiments, the common control module may transmit a tuning control signal related to determining the first temperature of the tunable laser and the second temperature of the etalon due to the tunable laser and the etalon being in thermal contact with each other. When the materials of the tunable laser and the etalon have the same thermal expansion, both the thermal tuning of the central wavelength and the thermal tuning of the etalon wavelength resonance are efficiently controlled together and dependently.

[0026] In the most preferred embodiment of the second aspect, a portion of the laser light received by the laser control module is not sent back to the tunable fiber laser. In other words, the laser control module is configured to only receive the laser light. Accordingly, the laser control module is configured to block and / or prevent the laser light from being transmitted back to the tunable fiber laser. The inventors have found that in this preferred embodiment, the system is less susceptible to noise.

[0027] The above and / or additional objects, features, and advantages of the present disclosure will be further described by the following exemplary and non-limiting detailed description of the embodiments of the present disclosure with reference to the accompanying drawings.

Brief Description of the Drawings

[0028]

Figure 1

Embodiments for Carrying Out the Invention

[0029] Tuning control signal In one embodiment of a tunable laser device, the tuning control signal is communicated to a thermal carrier that is in thermal contact with both the first fiber and the second fiber.

[0030] In a preferred embodiment of the tunable laser device, the tuning control signal is an electrical signal that is communicated to one or more thermoelectric elements that are in thermal contact with the first fiber and the second fiber. The thermoelectric element may be, for example, a Peltier element, a thin film heating element, or another resistive heating element.

[0031] Preferably, the tuning control signal may be communicated to a single substrate configured to hold the first fiber and the second fiber. When using a single substrate, the single substrate may be in thermal contact with the thermal carrier.

[0032] More preferably, the tuning control signal may be communicated to a first substrate configured to hold the first fiber, and the tuning control signal may be communicated to a second substrate configured to hold the second fiber. When using two substrates, the first substrate and the second substrate may be in thermal contact with the thermal carrier so as to provide thermal contact between the thermal carrier and both the first fiber and the second fiber. Thus, the tuning control signal may simply be transmitted to the thermal carrier, for example, to a Peltier element disposed inside the thermal carrier, such that the temperature of the Peltier element is distributed to both the thermal carrier and the fiber.

[0033] Using a single substrate or two substrates configured to hold the first and second fibers can ensure that accurate wavelength tuning is achieved. This can be ensured because, for example, the temperature of the first fiber, i.e., the tunable fiber laser, can be determined via the temperature of a single substrate or the temperatures of two substrates.

[0034] The relationship between the first fiber and the substrate holding the first fiber is determined by the combination of the thermo - optical effect and the elasto - optical effect caused by the strain from the thermal expansion of the substrate and is described as follows.

[0035]

Equation

[0036] Here, T is the temperature, λ is the center wavelength of the laser, and α s is the thermal expansion coefficient of the substrate. The first fiber has a thermo - optical coefficient (dn / dT) / n, where n is the refractive index of the first fiber, and p e is the elasto - optical coefficient of the first fiber, and α f is the thermal expansion of the first fiber.

[0037] In other words, the center wavelength of the first fiber may thus be determined and tuned by the temperature of a single or the first substrate, most preferably in combination with thermal carriers. According to the above - described embodiment, a thermal carrier is defined, and the thermal carrier may be made of a material having a high thermal conductivity such as aluminum or copper. The thermal carrier can ensure that heat is efficiently and uniformly distributed along the substrate and / or the fiber.

[0038] In the most preferred embodiment of the tunable laser device, the first and second substrates are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same such that the first and second substrates expand thermally in the same manner. As described immediately above, the heat carriers may serve to distribute heat such that the temperatures of the first and second substrates become the same. In other words, the heat carriers may serve to provide that the first temperature and the second temperature are the same. The first and second substrates may be formed of, for example, ceramic. In this way, the central wavelength of the laser is affected by the temperature of the first fiber according to the relationship given by Equation (1), and the etalon resonance of the second fiber is affected by the temperature of the second fiber according to the same Equation (1). As described, at least α of the first substrate s is the same as the α of the second substrate s . The term "the same" is understood in the present disclosure to be within a maximum difference of 10%, such as a difference of less than 5%, or for example a difference of less than 1%.

[0039] In connection with the above preferred embodiment, it may also be further advantageous to have a first fiber and a second fiber formed of a material having the same thermal expansion. When the temperatures of the two substrates are the same, it means that the first and second fibers expand thermally in the same manner.

[0040] However, in other embodiments not necessarily related to the above preferred embodiment, the first fiber and the second fiber are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same such that the first and second fibers expand thermally in the same manner.

[0041] The same temperature need not be defined by the hot carrier as described above. Instead, the same temperature may in principle be provided directly via a single substrate or directly via the first and second substrates. For example, the substrate may be formed of aluminum and may be controlled by one or more thermoelectric elements disposed within the substrate or in direct contact with the substrate, thereby directly controlling the temperature of the substrate.

[0042] As can be understood from the above, the hot carrier is an example of indirectly providing temperature to the substrate. Thus, both the first temperature and the second temperature according to the present disclosure can be provided directly or indirectly to the fiber and / or the substrate.

[0043] In some embodiments, the first and second substrates are formed of materials having different thermal expansions, and the defined first temperature and the defined second temperature are the same so that the first and second substrates expand similarly. In other words, the thermal expansions of two substrates having different thermal expansions may expand similarly by having different heating, i.e., different signals to the thermoelectric elements of the two substrates. Thus, the signals may be defined to compensate for the different thermal expansions of the two substrates. In such embodiments, the fact that the first defined temperature of the first fiber and the second defined temperature of the second fiber are the same may be provided, for example, by splitting a tuning control signal into two different signals, the two different signals being defined by the thermal expansion of the substrate. However, these embodiments are less preferred than the preferred embodiments where the first and second substrates are formed of materials having the same thermal expansion and the first defined temperature and the second defined temperature are the same so that the first and second substrates expand similarly.

[0044] Similarly, in some embodiments, the first fiber and the second fiber may be formed of materials having different thermal expansions, and the first temperature and the second temperature may be different such that the first and second fibers are similarly thermally expanded. In such embodiments where the first temperature of the first fiber and the second temperature of the second fiber are the same, for example, it may be provided by splitting a tuning control signal into two different signals, the two different signals being defined by the thermal expansion of the substrate and the fiber. Also in this case, these embodiments are less preferred than the preferred embodiments where the first and second fibers are formed of materials having the same thermal expansion and the first temperature and the second temperature are the same such that the first and second substrates are similarly thermally expanded.

[0045] Less preferred embodiments such as those described above may require a good understanding of the thermal expansion of the substrate and the fiber, but when this understanding is provided, the characteristics of the substrate can be utilized to further optimize both the shift of the central wavelength and the shift of the etalon resonance in combination with each other.

[0046] Regardless of how the same temperature of the fiber is obtained, the same thermal expansion of the fiber and / or the substrate serves to synchronously shift the central wavelength and the etalon resonance of the tunable fiber laser in the most preferred embodiments.

[0047] As described above, even if the thermal expansions of the fiber and / or the substrate are different, that is, for example, even if the thermal expansions of two fibers and / or the fibers differ by more than 10%, it is also possible to obtain the same temperature of the fiber. However, when different fibers and / or different substrates are optimized to provide the same temperature of the fiber, the different thermal expansions of the fiber and / or the substrate serve to synchronously shift the central wavelength of the tunable fiber laser and the etalon resonance in the most preferred embodiments.

[0048] A tunable laser device is provided herein that maintains the relative distance between the center wavelength and the resonant wavelength very efficiently by synchronous wavelength shifting. Typically, in the prior art, when tuning a tunable laser, the resonant wavelength is not shifted very efficiently. That is, there is no dependence between the signal for controlling the tuning of the laser and the signal for controlling the etalon. Therefore, the solution presented in this disclosure provides an optimized tunable laser device.

[0049] To determine the center wavelength of the laser, it may be important to know the temperature of at least the first fiber and / or the substrate for the first fiber. A thermoelectric element may be used to indirectly measure the temperature, but it may be advantageous to directly measure the temperature.

[0050] Therefore, in the most preferred embodiment, the substrate comprises one or more temperature sensors configured to measure the temperature of the substrate. The temperature sensor may be one or more of a negative temperature coefficient (NTC) thermistor, a positive temperature coefficient (PTC) thermistor, a resistance temperature detector (RTD), or a thermocouple. The temperature sensor may be disposed, for example, inside a hole in the substrate.

[0051] The First Fiber and the Tunable Fiber Laser In a preferred embodiment of the tunable laser device, the tunable fiber laser is a fiber DFB (distributed feedback) laser or a DBR (distributed Bragg reflector) fiber laser.

[0052] According to the tunable laser device disclosed herein, the tunable laser device comprises a tunable fiber laser. A fiber laser as disclosed herein is known to constitute a class of small lasers that exhibit lower levels of frequency noise compared to many other types of similar small lasers.

[0053] Although the fiber laser itself has low-level frequency noise, the present disclosure provides an optimized tunable laser device with even lower-level frequency noise. This is by means of an etalon as described and is also described below.

[0054] Second Fiber and Etalon Resonance In one embodiment of the tunable laser device, the etalon in the second fiber is configured with a free spectral range of less than 10 GHz, preferably less than 5 GHz, more preferably less than 2.5 GHz, and most preferably about 2.0 GHz. As disclosed herein, using a low free spectral range has very specific advantages. This is because fast tuning of the laser center wavelength by the stabilization control signal is only possible over a limited frequency range, and thus the free spectral range of the etalon is smaller than or the same as the fast tuning range of the laser. For example, a low free spectral range may allow the laser frequency to be stabilized at less than 1000 ms, such as less than 100 ms, such as less than 10000 ms. A free spectral range as disclosed herein typically cannot be practically provided in a small etalon such as a solid silica-based Fabry-Perot interferometer with mirrors. Therefore, in addition to the first fiber, using a second fiber that includes an etalon functions to enable a low free spectral range, which may enable the laser frequency to be stabilized very efficiently and rapidly. Further, as described above, the second fiber also functions to provide an etalon that can be controlled in synchronization with the center wavelength of the first fiber by controlling the fiber temperature. Therefore, the combination of both using the second fiber as an etalon and controlling it using a fiber control module that also controls the tunable fiber laser provides a highly improved tunable laser device.

[0055] In another embodiment of the tunable laser device, the full width at half maximum (FWHM) bandwidth of the etalon resonance is configured to be less than 300 MHz, preferably less than 100 MHz, and most preferably about 50 MHz. As disclosed herein, using a low resonance FWHM bandwidth has very specific advantages. For example, the etalon resonance suppresses frequency noise within the lock bandwidth to less than 10000 Hz 2 / Hz, preferably less than 100 Hz 2 / Hz, more preferably less than 1 Hz 2 / Hz. This provides a highly improved tunable laser device with very low frequency noise. In combination with the low free spectral range of the etalon, the present disclosure provides a tunable laser device that is efficient in stabilizing and also has low frequency noise.

[0056] More generally, according to the present disclosure, when a tunable laser device is used with a fiber laser and a fiber having a fiber-based Fabry-Perot interferometer, the tunable laser device is configured to very efficiently lock the central wavelength of the fiber laser to one of the resonances of the laser control module. Further, the present disclosure provides a tunable laser device in which locking to the etalon resonance is achieved while the tunable laser is temperature-adjusted, i.e., while the wavelength-variable laser is wavelength-shifted via temperature tuning. This produces a tunable laser device having a very low level of frequency noise over a wide range of frequencies.

[0057] Therefore, the tunable laser according to the present disclosure has a low level of frequency noise suitable for applications such as quantum computing, quantum optics, quantum sensing, optical clocks, gravitational wave detection, high-resolution spectroscopy, and ultra-precise time and frequency transfer.

[0058] Stabilization control signal In one embodiment of the tunable laser device, the stabilization control signal fed back to the tunable fiber laser is via one or more actuators configured to change the tension of the Bragg grating in the first fiber. The one or more actuators may be one or more of a piezoelectric actuator, a magnetostrictive actuator, and / or a thermal expansion actuator. Preferably, the one or more actuators may be disposed on a substrate that holds the first fiber. Most preferably, only a single actuator is disposed on the substrate that holds the first fiber. Using only a single actuator can provide easy control of the fiber laser. Further, in some embodiments, it has been found that when using an etalon having a free spectral range as described above, only a single actuator is needed to efficiently stabilize the fiber laser. In other words, the inventors of the present disclosure have identified how it is possible to use only a single actuator to stabilize the frequency of the fiber laser as disclosed herein.

[0059] Alternatively, or in addition, the stabilization control signal is via an acousto-optic modulator configured to change the wavelength of the fiber laser so that it is fed back to lock the tunable fiber laser to the etalon resonance.

[0060] In a preferred embodiment of the tunable laser device, the tunable laser device further comprises a balanced detector for generating the stabilization control signal, and the stabilization control signal is based on the difference in power between a reference signal and the power transmitted through a frequency discriminator element.

Example

[0061] FIG. 1 shows an example of an embodiment of a tunable laser device 1 according to the present disclosure. The tunable laser device 1 includes a tunable fiber laser 2 and a laser control module 3. The tunable fiber laser 2 is configured to generate laser light 4 that is finally transmitted as a laser output. The laser light 4 has a center wavelength controlled by one or more Bragg gratings 5 in a first fiber 6. The laser control module 3 is configured to receive at least a part of the laser light 4 generated by the tunable fiber laser 2, generate a stabilization control signal 7, and feedback the control signal 7 to the tunable fiber laser 2 to stabilize the center wavelength. As can be seen from FIG. 1, at least a part of the laser light generated by the tunable fiber laser 2 first enters a polarization tab / isolator 8 and is received by the laser control module 3. The polarization tab / isolator 8 splits the light into light transmitted to the laser output and light transmitted to a 3 dB splitter 9. From the 3 dB splitter 9, a part of the light is transmitted to the laser control module 3 via a fiber. Another part of the light is transmitted to a wavelength discrimination element 10 via another fiber. Accordingly, the tunable laser device includes the wavelength discrimination element 10. The wavelength discrimination element 10 includes an etalon having one or more etalon resonances. The etalon is formed by one or more Bragg gratings 11 in a second fiber 12 such that the etalon resonance is controlled by the one or more Bragg gratings 11 in the second fiber 12.

[0062] The first fiber 6 and the second fiber 12 are configured to be controlled by a fiber control module 13, and the fiber control module 13 is configured to transmit a tuning control signal 14 related to determining a first temperature of the first fiber 6 and a second temperature of the second fiber 12. In this example, the tuning control signal 14 is a single signal generated from the fiber control module 13. Here, the single signal is transmitted to a heat carrier 20 which is a plate made of aluminum. The plate is shown here with rounded corners and is disposed on the tunable laser device 1. Thus, the plate is shown here as an invisible plate. However, the heat carrier 20 has a defined thickness and is configured to distribute heat. The heat is generated from one or more Peltier elements controlled by the tuning control signal 14. The Peltier element 14 is in thermal contact with the heat carrier 20. In this way, the tuning control signal 14 is communicated to the heat carrier. Further, the heat carrier is in thermal contact with both the first fiber 6 and the second fiber 12. Thus, the fiber control module 13 serves both to shift the center wavelength by modifying one or more Bragg gratings 5 in the first fiber 6 and to shift the etalon wavelength by modifying one or more Bragg gratings 5 in the first fiber 6.

[0063] In FIG. 1, the tuning control signal 14 is communicated to a first sub-state 15 configured to hold the first fiber 6. Further, the tuning control signal 14 is also communicated to a second sub-state 16 configured to hold the second fiber 12.

[0064] Thus, the first substrate 15 and the second substrate 16 are in thermal contact with a heat carrier 20 that is in thermal contact with both the first fiber 6 and the second fiber 12. As can be seen from FIG. 1, the fiber laser 2 is pumped by a pump laser 17 via a wavelength division multiplexing (WDM) element 18.

[0065] As can be further seen from FIG. 1 of the setup of the tunable laser device 1, the present disclosure provides a compact tunable laser device 1. This is achieved in part by using two fibers (6 and 12), but also because the two fibers (6 and 12) according to the present invention can be arranged very close to each other, in particular in thermal contact with each other. Thermal contact between the etalon and the laser is typically not desired and is mostly avoided by thermal insulation. The present disclosure adopts a completely different approach and utilizes arranging two fibers in thermal contact with each other.

[0066] In this example, the Bragg gratings (5 and 11) of the first fiber 6 and the second fiber 12 are fixed under tension by an adhesive to substrates (15 and 16) made of the same material. The substrate 15 including the laser Bragg grating 5 further has a high-speed actuator 19 embedded in the substrate. This actuator 19 enables fast tuning of the laser wavelength and thus enables locking the laser wavelength to the etalon resonance using the stabilization control signal 7. Since the substrates (15 and 16) are in thermal contact with the heat carrier 20, the laser wavelength and the etalon resonance change synchronously as a reaction to the tuning control signal 14. This setup enables stabilizing the laser wavelength while the wavelength is being changed by the tuning control signal because the actuator 6 provides tuning of a wavelength much faster than the tuning from the tuning control signal 14.

[0067] Also, as can be seen from FIG. 1, a part of the laser light received by the laser control module 3 is not sent back to the tunable fiber laser 2. In other words, the laser control module is configured to only receive the laser light. Therefore, the laser control module 3 is configured to block and / or prevent the laser light from being transmitted back to the tunable fiber laser 2. As described above, at least a part of the laser light generated by the tunable fiber laser 2 first enters the polarization tab / isolator 8 and is received by the laser control module 3, and the polarization tab / isolator 8 splits the light into the laser output and the light transmitted to the 3 dB splitter 9. From the 3 dB splitter 9, a part of the light is transmitted to the laser control module 3 via a fiber. Another part of the light is transmitted to the wavelength discrimination element 10 via another fiber. The 3 dB splitter 9 prevents a part of the laser light received by the laser control module 3 from being sent back to the tunable fiber laser 2.

[0068] Item 1. A tunable laser device, comprising: a tunable fiber laser and a laser control module; the tunable fiber laser is configured to generate laser light having a center wavelength controlled by one or more Bragg gratings in a first fiber; the laser control module is configured to receive at least a part of the laser light generated by the tunable fiber laser, generate a stabilization control signal, and feedback the control signal to the tunable fiber laser to stabilize the center wavelength; the laser control module includes a wavelength discrimination element having an etalon with one or more etalon resonances, the etalon is formed by one or more Bragg gratings in a second fiber, and the etalon resonance is controlled by the one or more Bragg gratings in the second fiber; The first fiber and the second fiber are configured to be controlled by a fiber control module, and the fiber control module is configured to transmit a tuning control signal related to determining a first temperature of the first fiber and a second temperature of the second fiber, whereby the fiber control module, shifts the center wavelength by modifying the one or more Bragg gratings in the first fiber, and shifts the etalon resonance by modifying the one or more Bragg gratings in the second fiber, and a tunable laser device that serves both roles.

[0069] 2. The tunable laser device according to item 1, wherein the tuning control signal is communicated to a thermal carrier that is in thermal contact with both the first fiber and the second fiber.

[0070] 3. The tunable laser device according to item 1 or 2, wherein the tuning control signal is an electrical signal communicated to one or more thermoelectric elements that are in thermal contact with the first fiber and the second fiber.

[0071] 4. The tunable laser device according to item 1, wherein the tuning control signal is communicated to a single substrate configured to hold the first fiber and the second fiber.

[0072] 5. The tunable laser device according to item 4, wherein the single substrate is in thermal contact with a thermal carrier that is in thermal contact with both the first fiber and the second fiber. 6. The tunable laser device according to item 1, wherein the tuning control signal is communicated to a first substrate configured to hold the first fiber, and the tuning control signal is communicated to a second substrate configured to hold the second fiber.

[0073] 7. The tunable laser device according to item 6, wherein the first substrate and the second substrate are in thermal contact with a heat carrier that is in thermal contact with both the first fiber and the second fiber.

[0074] 8. The tunable laser device according to item 2, 5 or 7, wherein the heat carrier is made of aluminum or copper. 9. The tunable laser device according to item 6 or 7, wherein the first substrate and the second substrate are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same so that the first substrate and the second substrate expand thermally in the same manner.

[0075] 10. The tunable laser device according to item 6 or 7, wherein the first substrate and the second substrate are formed of materials having different thermal expansions, the first temperature and the second temperature are the same, the tuning control signal compensates for the different thermal expansions, and the first substrate and the second substrate expand thermally in the same manner.

[0076] 11. The tunable laser device according to any one of items 2 to 10, wherein the substrate includes one or more temperature sensors configured to measure the temperature of the substrate.

[0077] 12. The tunable laser device according to any one of items 1 to 11, wherein the first fiber and the second fiber are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same so that the first fiber and the second fiber expand thermally in the same manner.

[0078] 13. The first fiber and the second fiber are formed of materials having different thermal expansions, the first temperature and the second temperature are different, the tuning control signal compensates for the different thermal expansions, and the first fiber and the second fiber expand thermally in the same manner. The tunable laser device according to any one of items 1 to 12.

[0079] 14. The same thermal expansion serves to synchronously shift the center wavelengths of the tunable fiber laser and the etalon resonance. The tunable laser device according to item 9 and / or 12.

[0080] 15. The different thermal expansions serve to synchronously shift the center wavelengths of the tunable fiber laser and the etalon resonance. The tunable laser device according to item 10 and / or 13.

[0081] 16. The second fiber is configured to have a free spectral range of less than 10 GHz, preferably less than 5 GHz, more preferably less than 2.5 GHz, and most preferably about 2.0 GHz. The tunable laser device according to any one of items 1 to 15.

[0082] 17. The free spectral range serves to stabilize the frequency of the laser at less than 1000 ms, such as less than 100 ms, such as less than 10000 ms. The tunable laser device according to item 16.

[0083] 18. The full width at half maximum (FWHM) bandwidth of the etalon resonance is configured to be less than 300 MHz, preferably less than 100 MHz, and most preferably about 50 MHz. The tunable laser device according to any one of items 1 to 17.

[0084] 19. The etalon resonance reduces the frequency noise within the lock bandwidth to less than 10000 Hz 2 / Hz, preferably less than 100 Hz 2 / Hz, more preferably less than 1 Hz2 The tunable laser device according to item 18, which plays a role of suppressing to less than / Hz.

[0085] 20. The tunable laser device according to any one of items 1 to 19, wherein the stabilization control signal returned to the tunable fiber laser passes through one or more actuators configured to change the tension of the Bragg grating in the first fiber.

[0086] 21. The tunable laser device according to any one of items 1 to 20, wherein the stabilization control signal returning to the tunable fiber laser passes through an acousto-optic modulator configured to change the wavelength of the fiber laser.

[0087] 22. The tunable laser device according to any one of items 1 to 21, further comprising a balanced detector for generating the stabilization control signal, wherein the stabilization control signal is based on the power difference between a reference signal and the power transmitted through the frequency discrimination element.

Claims

1. A tunable laser device, comprising a tunable fiber laser and a laser control module, wherein the tunable fiber laser is configured to generate laser light having a central wavelength controlled by one or more Bragg gratings in a first fiber, the laser control module is configured to receive at least a part of the laser light generated by the tunable fiber laser, generate a stabilization control signal, and feedback the control signal to the tunable fiber laser to stabilize the central wavelength, the laser control module includes a wavelength discrimination element having an etalon with one or more etalon resonances, the etalon is formed by one or more Bragg gratings in a second fiber, and the etalon resonance is controlled by the one or more Bragg gratings in the second fiber, the first fiber and the second fiber are configured to be controlled by a fiber control module, and the fiber control module is configured to transmit a tuning control signal related to determining a first temperature of the first fiber and a second temperature of the second fiber, whereby the fiber control module shifts the central wavelength by modifying the one or more Bragg gratings in the first fiber, and plays both roles of shifting the etalon resonance by modifying the one or more Bragg gratings in the second fiber, A tunable laser device, wherein a part of the laser light received by the laser control module is not sent back to the tunable fiber laser.

2. The tunable laser device according to claim 1, wherein the tuning control signal is communicated to a heat carrier in thermal contact with both the first fiber and the second fiber.

3. The tunable laser device according to claim 1 or 2, wherein the tuning control signal is an electrical signal communicated to one or more thermoelectric elements in thermal contact with the first fiber and the second fiber.

4. The tuning control signal is communicated to a first sub-state configured to hold the first fiber, and the tuning control signal is communicated to a second substrate configured to hold the second fiber. The tunable laser device according to claim 1.

5. The first substrate and the second substrate are in thermal contact with a thermal carrier that is in thermal contact with both the first fiber and the second fiber. The tunable laser device according to claim 4.

6. The thermal carrier is made of aluminum or copper. The tunable laser device according to claim 2 or 5.

7. The first substrate and the second substrate are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same so that the first substrate and the second substrate expand thermally in the same manner. The tunable laser device according to claim 4 or 5.

8. The substrate includes one or more temperature sensors configured to measure the temperature of the substrate. The tunable laser device according to any one of claims 2 to 7.

9. The first fiber and the second fiber are formed of a material having the same thermal expansion, and the first temperature and the second temperature are the same so that the first fiber and the second fiber expand thermally in the same manner. The tunable laser device according to any one of claims 1 to 8.

10. The same thermal expansion serves to synchronously shift the center wavelengths of the tunable fiber laser and the etalon resonance. The tunable laser device according to claim 7 or 9.

11. The second fiber is configured to have a free spectral range of less than 2.5 GHz, most preferably about 2.0 GHz. The tunable laser device according to any one of claims 1 to 10.

12. The free spectral range serves to stabilize the frequency of the laser at less than 1000 ms, such as less than 100 ms, such as less than 10000 ms. The tunable laser device according to claim 11.

13. The full width at half maximum (FWHM) bandwidth of the etalon resonance is configured to be less than 300 MHz, preferably less than 100 MHz, and most preferably about 50 MHz, the tunable laser device according to any one of claims 1 to 12.

14. The etalon resonance suppresses frequency noise within the lock bandwidth to less than 10000 Hz ２ / Hz, preferably less than 100 Hz ２ / Hz, more preferably less than 1 Hz ２ / Hz, and plays a role in suppressing the same, and the tunable laser device according to claim 13

15. The stabilization control signal returned to the tunable fiber laser is via one or more actuators configured to change the tension of the Bragg grating in the first fiber, the tunable laser device according to any one of claims 1 to 14.

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