Phase compensation type Bragg fiber grating external cavity laser and wavelength tuning method

Abstract

The invention discloses a phase compensation type Bragg fiber grating external cavity laser and a wavelength tuning method, and relates to the technical field of lasers. The laser comprises a gain chip, a phase shifter, a lens coupling optical fiber, a Bragg grating and a stretching module. The gain chip has an inclination angle and emits light obliquely. The phase shifter is vertically aligned with the light emitting direction of the gain chip; the light of which the phase is adjusted by the phase shifter is coupled by the lens coupling optical fiber and enters the Bragg grating; the Bragg grating is positioned at the tail end of the lens coupling optical fiber; the lens coupling optical fiber is attached to the surface of the stretching module which is used for stretching the cavity length of the Bragg grating. The wavelength tuning method comprises the following steps: performing phase compensation by using the phase shifter, and stretching the cavity length of the Bragg grating by using the stretching module to realize continuous mode-hopping-free tuning. The invention has important application value in the field of optical sensing.

Description

Technical Field

[0001] The invention relates to the technical field of lasers, and in particular to a phase-compensated Bragg fiber grating external cavity laser and a wavelength tuning method. Background Art

[0002] Traditional external cavity lasers have the characteristics of narrow linewidth, single longitudinal mode and wide tuning range. In an external cavity diode laser (ECDL), a diode is used as the gain medium, usually a gain chip specially designed for ECDL. The reflection grating in the Littrow or Littman-Metcalf external cavity laser structure acts as a wavelength selection element, and the wavelength can be tuned by rotating the grating. However, such ECDLs are large and require precise design to maintain the alignment of the optical path over a wide tuning range.

[0003] Therefore, many iterations have been made on the basis of the above to find alternatives. Among them, the more popular ones are Bragg grating lasers, in which the wavelength selection element is composed of a fiber Bragg grating (FBG). The manufacture of the wavelength selection element is separated from the manufacture of the gain medium, potentially allowing greater flexibility in wavelength selection and reducing the investment required for short-term operation of customized wavelength devices. However, this method is easily limited by environmental and structural stability factors, and during the tuning process, the external cavity length of the laser and the Bragg grating wavelength have high requirements for continuous matching over a large range, making it difficult to achieve a mode-hop-free tuning range of more than 0.5nm. Summary of the invention

[0004] In view of the above problems, the present invention proposes a phase-compensated fiber Bragg grating external cavity laser and a wavelength tuning method, that is, a narrow-linewidth, single-longitudinal-mode external cavity laser with Bragg grating feedback combined with phase compensation, which can achieve a mode-hop-free tuning range of more than 1 nm.

[0005] According to one aspect of the present invention, a phase-compensated fiber Bragg grating external cavity laser is provided, the laser comprising: a gain chip 1, a phase shifter 2, a lens-coupled fiber 3, a Bragg grating 4, and a stretching module 5; wherein:

[0006] The gain chip 1 has a tilt angle and outputs light at an angle; the phase shifter 2 is used to adjust the phase of the light wave; the lens-coupled optical fiber 3 is an optical fiber with a lens coupled at the front end and a Bragg grating 4 coupled at the rear end; the lens-coupled optical fiber 3 is attached to the surface of the stretching module 5, and the stretching module 5 is used to stretch the cavity length of the Bragg grating 4; the phase shifter 2 is vertically aligned with the light output direction of the gain chip 1, and the light after the phase is adjusted by the phase shifter 2 is coupled into the Bragg grating 4 through the lens-coupled optical fiber 3.

[0007] Furthermore, the left side of the gain chip 1 is made of high reflectivity material with a reflectivity greater than or equal to 95%, and the right side has a certain inclination angle for emitting light, and the angle of the output light deviating from the center is equal to 20°.

[0008] Furthermore, the front end coupling lens of the lens-coupled optical fiber 3 is a conical microlens.

[0009] Furthermore, the Bragg grating 4 is an equidistant grating engraved on the tail end of the lens-coupled optical fiber 3 by photolithography technology.

[0010] Furthermore, the stretching module 5 is a piezoelectric ceramic controller or a semiconductor refrigerator. When stress is used to stretch the cavity length, the stretching module 5 is a piezoelectric ceramic controller; when temperature is used to stretch the cavity length, the stretching module 5 is a semiconductor refrigerator.

[0011] Furthermore, the material of the phase shifter 2 includes silicon material and LiNbO3 material.

[0012] According to another aspect of the present invention, a wavelength tuning method is proposed, the method comprising: simultaneously using the phase shifter 2 in the phase-compensated fiber Bragg grating external cavity laser to perform phase compensation and using the stretching module 5 to stretch the cavity length of the Bragg grating 4 to achieve continuous mode-hop-free tuning; specifically comprising: continuously adjusting the phase compensation by voltage-controlled phase shifter 2, thereby obtaining a wavelength increased by the phase compensation part; continuously stretching the cavity length of the Bragg grating 4 by voltage-controlled stretching module 5, thereby obtaining a Bragg grating diffraction wavelength.

[0013] Further, the increased portion of the Bragg grating diffraction wavelength and the increased wavelength of the phase compensation portion satisfy the following relationship:

[0014]

[0015] In the formula, φ b represents the phase added by Bragg grating 4; φ ps Represents the phase of phase shifter 2; n b represents the effective refractive index of the Bragg grating 4; L b represents the length of the Bragg grating 4; n1 represents the refractive index of the gain chip 1; n2 represents the refractive index of the external cavity; L1 represents the length of the gain chip 1; L2 represents the length of the external cavity.

[0016] The beneficial technical effects of the present invention are:

[0017] The present invention proposes a phase-compensated Bragg fiber grating external cavity laser and a wavelength tuning method, wherein the laser comprises a gain chip, a phase shifter, a lens-coupled optical fiber, a Bragg grating, and a stretching module; wherein the phase shifter is used for phase compensation, and the stretching module is used to stretch the cavity length of the Bragg grating, so that continuous mode-hopping-free tuning can be achieved. The present invention has important application value in the field of optical sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood by reading the detailed description below with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present invention are shown in an exemplary and non-limiting manner, in which:

[0019] Figure 1 is a schematic structural diagram of a fiber Bragg grating external cavity laser according to an embodiment of the present invention;

[0020] Figure 2 Schematic diagram of the relationship between the laser external cavity mode wavelength and the phase shifter in an embodiment of the present invention;

[0021] Figure 3 Schematic diagram of the cavity length structure of a fiber Bragg grating external cavity laser in an embodiment of the present invention;

[0022] Figure 4 It is a flow chart of wavelength tuning of a fiber Bragg grating external cavity laser in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided only to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0024] In order to solve the problems of large line width, limited non-mode hopping tuning range and large size of traditional Littrow or Littman-Metcalf external cavity lasers, the present invention proposes a small-size laser with narrow line width, single longitudinal mode and butterfly package with phase compensation.

[0025] The embodiment of the present invention provides a phase-compensated fiber Bragg grating external cavity laser, such as Figure 1 As shown, the proposed external cavity laser includes a gain chip 1, a phase shifter 2, a lens-coupled optical fiber 3, a Bragg grating 4, and a stretching module 5; wherein,

[0026] The gain chip 1 has a tilt angle and outputs light at an angle; the phase shifter 2 is used to adjust the phase of the light wave; the lens-coupled optical fiber 3 is an optical fiber with a lens coupled at the front end and a Bragg grating 4 coupled at the rear end; the lens-coupled optical fiber 3 is attached to the surface of the stretching module 5, and the stretching module 5 is used to stretch the cavity length of the Bragg grating 4; the phase shifter 2 is vertically aligned with the light output direction of the gain chip 1, and the light after the phase is adjusted by the phase shifter 2 is coupled into the Bragg grating 4 through the lens-coupled optical fiber 3.

[0027] In this embodiment, preferably, the left side of the gain chip 1 is made of high reflectivity material with a reflectivity greater than or equal to 95%, and the right side has a certain inclination angle for emitting light, and the output light deviates from the center by an angle of 20°.

[0028] In this embodiment, preferably, the front end coupling lens of the lens-coupled optical fiber 3 is a conical microlens.

[0029] In this embodiment, preferably, the Bragg grating 4 is an equidistant grating engraved on the tail end of the lens-coupled optical fiber 3 by photolithography technology.

[0030] In this embodiment, preferably, the stretching module 5 is a piezoelectric ceramic controller or a semiconductor refrigerator. When stress is used to stretch the cavity length, the stretching module 5 is a piezoelectric ceramic controller; when temperature is used to stretch the cavity length, the stretching module 5 is a semiconductor refrigerator.

[0031] In this embodiment, preferably, the material of the phase shifter 2 includes silicon material and LiNbO3 material. Silicon material has a high thermo-optical coefficient, which is about 1.8×10 -4 K -1 . Only a limited increase in temperature is required to achieve a significant change in the waveguide refractive index, which in turn affects the change in cavity length. At the same time, the thermal conductivity of silicon material is approximately 149W / mK. This feature ensures that the silicon-based thermo-optical phase shifter can have a faster response speed, and the switching time is generally within tens of microseconds. The heater is mainly realized through high-resistivity metal or doped waveguides, and the waveguide is heated from the top or both sides of the waveguide. For single-waveguide thermo-optical phase shifters, the phase shift efficiency of these two heating methods is close, approximately between 20-30mW / π. The LiNbO3 material applies voltage to the electrodes on both sides of the waveguide, causing the refractive index of the waveguide to change, thereby causing additional phase changes in the transmitted light in the waveguide. If the modulator length and modulation frequency are different, the corresponding half-wave voltage is different.

[0032] Specifically, the left side of the gain chip 1 is made of high reflectivity material with a reflectivity of 95%; the right side has a certain inclination angle to output light, and the output light deviates from the center angle by 20°; the output power is greater than 40mW, and the output wavelength is 1550nm. The phase shifter 2 is vertically aligned with the light output direction of the gain chip 1, and the light after the phase adjustment by the phase shifter 2 is coupled into the Bragg grating 4 through the lens coupling optical fiber 3; the phase shifter 2 can achieve a 2π optical phase change and can be controlled by voltage, for example, a voltage of 150V can change the phase by about 2.5π. The lens coupling optical fiber 3 is a conical microlens, which can improve the coupling efficiency. The Bragg grating 4 is an equidistant grating realized by photolithography technology at the tail end of the lens coupling optical fiber 3. The lens-coupled optical fiber 3 is attached to the surface of the stretching module 5. The stretching module 5 is used to stretch the cavity length of the Bragg grating 4. The length of the stretching module 5 must cover the length of the Bragg grating 4. The stretching module 5 includes a piezoelectric ceramic controller or a TEC (semiconductor refrigerator). When the stress is used to stretch, the stretching module 5 is a piezoelectric ceramic controller. When the temperature is used to stretch, the stretching module 5 is a TEC. That is, the external cavity length of the external cavity laser is controlled by the stretching module 5. The cavity length of the Bragg grating 4 can be stretched by stress or temperature. The stress method uses piezoelectric ceramic stretching. For example, the laser output wavelength is detected by a wavelength meter or a hyperfine spectrometer. For a 10mm fiber grating, when the stretching deformation is 20μm, the laser output wavelength can move 2nm. The temperature method uses TEC to control the temperature change. Usually, a temperature change of 10°C corresponds to a laser output wavelength change of 1nm.

[0033] like Figure 2 As shown, the laser proposed in the present invention will experience wavelength jump, i.e. mode hopping, after experiencing the phase period π of the Bragg grating 4 before compensation by the phase shifter 2. Therefore, a wider range of mode-hop-free tuning can be achieved through phase compensation of the phase shifter 2.

[0034] like Figure 3 As shown, the laser cavity length L3 proposed in the present invention is composed of three parts, namely, the cavity length 6 (length L1) of the gain chip 1, the cavity length 7 corresponding to the phase compensation part of the phase shifter 2, and the length 8 (external cavity length L2) of the Bragg grating 4. Mode-hop-free tuning requires the phase shifter 2 to keep the wavelength of the Bragg grating 4 synchronized with the oscillation wavelength of the laser.

[0035] Another embodiment of the present invention provides a wavelength tuning method, which includes: simultaneously using the phase shifter 2 in the phase-compensated fiber Bragg grating external cavity laser to perform phase compensation and using the stretching module 5 to stretch the cavity length of the Bragg grating 4 to achieve continuous mode-hop-free tuning; specifically including: continuously adjusting the phase compensation by voltage-controlled phase shifter 2 to obtain the wavelength increased by the phase compensation part; continuously stretching the cavity length of the Bragg grating 4 by voltage-controlled stretching module 5 to obtain the Bragg grating diffraction wavelength.

[0036] According to an embodiment of the present invention, Figure 4 As shown, after the gain chip 1 is turned on and emits light, the phase compensation is continuously adjusted by the LiNbO3 phase shifter 2 or the Si phase shifter 2, and then the Bragg grating 4 is stretched by the piezoelectric ceramic controller 5 or the TEC 5, thereby continuously tuning the output wavelength of the laser.

[0037] When the wavelength λ of the Bragg grating 4 b The wavelength of the laser external cavity mode λ c When they overlap, the hybrid laser produces maximum output power. Mode-hop-free tuning requires the same shift for each wavelength, i.e., δλ b =δλ c The tuning of the laser wavelength is achieved by stretching the phase part and the Bragg grating 4. Changing the phase part of the phase shifter 2 will only change the wavelength of the laser external cavity mode, while stretching the cavity length of the Bragg grating 4 will change the diffraction wavelength of the Bragg grating 4 and the wavelength of the laser external cavity mode at the same time.

[0038] Since the round-trip phase increase of the light in the Bragg grating 4 is 2π, it can be intuitively shown that the wavelength change λ of the Bragg grating 4 b , plus the phase φ increased by the Bragg grating 4 b , the change relationship is:

[0039]

[0040] The free spectral range of the Bragg grating 4 is:

[0041]

[0042] Where λ represents the wavelength of Bragg grating 4; n b (λ) represents the effective refractive index of the Bragg grating 4; L b represents the length of the Bragg grating 4 .

[0043] The compensation relationship between phase shifter 2 and the wavelength of the laser external cavity mode is:

[0044]

[0045] In the formula, φ psrepresents the phase of phase shifter 2;

[0046] The free spectral range of the output wavelength of the external cavity laser is:

[0047]

[0048] Wherein, n1(λ) represents the refractive index of the gain chip 1; n2(λ) represents the refractive index of the external cavity; L1 represents the length of the gain chip 1; and L2 represents the length of the external cavity.

[0049] Under this condition, the tuning sensitivity is defined as F λ for:

[0050]

[0051] In order to meet the no-mode-hop tuning condition, the phase added by the Bragg grating 4 must have a fixed ratio to the phase added by the phase shifter 2, expressed as:

[0052]

[0053] Formula (6) gives the phase tuning ratio. To achieve the tuning sensitivity given by formula (5), the phase tuning ratio formula (6) needs to be satisfied.

[0054] The present invention proposes a phase-compensated Bragg fiber grating external cavity laser and a wavelength tuning method, which uses a phase shifter to perform phase compensation and uses a stretching module to stretch the cavity length of the Bragg grating, thereby achieving continuous mode-hopping-free tuning. The present invention has important application value in the field of optical sensing.

[0055] Although the operations of the method of the present invention are described in a specific order in the drawings, this does not require or imply that the operations must be performed in this specific order, or that all the operations shown must be performed to achieve the desired results. Additionally or alternatively, some steps may be omitted, multiple steps may be combined into one step, and / or one step may be decomposed into multiple steps.

[0056] Although the spirit and principle of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the disclosed specific embodiments, and the division of various aspects does not mean that the features in these aspects cannot be combined to benefit, and such division is only for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included in the spirit and scope of the attached claims.

Claims

1. A phase-compensated fiber Bragg grating external cavity laser, characterized in that: include: A gain chip (1), a phase shifter (2), a lens-coupled optical fiber (3), a Bragg grating (4), and a stretching module (5); wherein: The gain chip (1) has an inclination angle and outputs light at an angle; the phase shifter (2) is used to adjust the phase of the light wave; the lens-coupled optical fiber (3) is an optical fiber having a lens coupled at the front end and a Bragg grating (4) coupled at the rear end; the lens-coupled optical fiber (3) is attached to the surface of the stretching module (5), and the stretching module (5) is used to stretch the cavity length of the Bragg grating (4); the phase shifter (2) is vertically aligned with the light output direction of the gain chip (1), and the light after the phase is adjusted by the phase shifter (2) is coupled into the Bragg grating (4) through the lens-coupled optical fiber (3).

2. A phase-compensated fiber Bragg grating external cavity laser according to claim 1, characterized in that: The left side of the gain chip (1) is made of a high reflectivity material with a reflectivity greater than or equal to 95%, and the right side has a certain inclination angle for emitting light, with the output light deviating from the center by an angle of 20°.

3. The phase-compensated fiber Bragg grating external cavity laser according to claim 1, characterized in that: The front end coupling lens of the lens-coupled optical fiber (3) is a conical microlens.

4. The phase-compensated fiber Bragg grating external cavity laser according to claim 1, characterized in that: The Bragg grating (4) is an equidistant grating engraved on the tail end of the lens-coupled optical fiber (3) by photolithography technology.

5. The phase-compensated fiber Bragg grating external cavity laser according to claim 1, characterized in that: The stretching module (5) is a piezoelectric ceramic controller or a semiconductor refrigerator. When stress is used to stretch the cavity length, the stretching module (5) is a piezoelectric ceramic controller; when temperature is used to stretch the cavity length, the stretching module (5) is a semiconductor refrigerator.

6. The phase-compensated fiber Bragg grating external cavity laser according to claim 1, characterized in that: The materials of the phase shifter (2) include silicon material and LiNbO3 material.

7. A wavelength tuning method, characterized in that: At the same time, the phase shifter (2) in the phase-compensated Bragg fiber grating external cavity laser described in any one of claims 1 to 6 is used to perform phase compensation, and the cavity length of the Bragg grating (4) is stretched using a stretching module (5) to achieve continuous mode-hop-free tuning; specifically, the method comprises: continuously adjusting the phase compensation by controlling the phase shifter (2) by voltage, thereby obtaining the wavelength increased by the phase compensation part; and continuously stretching the cavity length of the Bragg grating (4) by controlling the stretching module (5) by voltage, thereby obtaining the Bragg grating diffraction wavelength.

8. A wavelength tuning method according to claim 7, characterized in that: The increased portion of the Bragg grating diffraction wavelength and the increased wavelength of the phase compensation portion satisfy the following relationship: In the formula, φ b represents the phase added by the Bragg grating (4); φ ps represents the phase of the phase shifter (2); n b represents the effective refractive index of the Bragg grating (4); L b represents the length of the Bragg grating (4); n1 represents the refractive index of the gain chip (1); n2 represents the refractive index of the external cavity; L1 represents the length of the gain chip (1); and L2 represents the length of the external cavity.

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