External cavity semiconductor mode-locked laser and its adjustment method

Abstract

In view of the requirements for low-noise, high-repetition frequency, and miniaturized integrated semiconductor mode-locked lasers in the fields of optical communication technology, optical sensing technology, and microwave photonics, the present invention provides an external cavity semiconductor mode-locked laser and its adjustment method, which includes a semiconductor gain chip, a laser external cavity, an optical isolator, a coupling lens, and an output optical fiber. Since the semiconductor gain chip belongs to an inhomogeneous broadening gain medium and supports multiple longitudinal modes to emit light simultaneously, when transmitting in a highly nonlinear medium, the energies of each longitudinal mode are exchanged with each other through the four-wave mixing effect, thereby realizing the phase locking of each longitudinal mode and forming a pulse output. By adopting an external cavity structure, the cavity length of the semiconductor laser is increased, the noise of the semiconductor laser is reduced, and mode-locked pulses with a high signal-to-noise ratio can be achieved. The solution provided by the present invention has the advantages of small volume, low cost, the repetition frequency can be adjusted by changing the external cavity length, and the emission wavelength is easy to control, etc., and has important practical value in the fields of microwave technology, optical communication systems, etc.

Description

Technical Field

[0001] The present invention relates to a semiconductor laser, specifically to a mode-locked semiconductor laser, and more specifically to an external cavity semiconductor mode-locked laser and its adjustment method. Background Art

[0002] Semiconductor mode-locked lasers have become a research hotspot for ultra-short pulse sources due to their series of advantages such as compact and robust structure, low power consumption, low cost, and high repetition frequency, and have important application values in many application fields such as optical communication, optical frequency comb, spectroscopy, metrology, optical sensing, generation of microwave and terahertz waves. A typical semiconductor mode-locked laser realizes the locking of laser modes by using a saturable absorber placed in the laser resonator cavity. Traditional electrically pumped semiconductor mode-locked lasers usually adopt a two-section waveguide structure, where one section adopts a p-i-n structure and is applied with a forward voltage to act as a gain medium; the other section is applied with a reverse bias voltage and serves as a fast saturable absorber for realizing pulse compression. In the past few decades, semiconductor mode-locked lasers based on saturable absorbers have achieved a series of achievements in manufacturing processes and application research. However, the use of saturable absorbers increases the excitation threshold of the laser and reduces the energy conversion efficiency. At the same time, the additional reverse bias voltage also increases the complexity of operation.

[0003] In recent years, semiconductor mode-locked lasers without saturable absorbers based on the nonlinear effects in gain waveguides have been demonstrated. By using the high power density in the gain waveguide to enhance its nonlinear effects, the locking of laser modes is realized based on efficient four-wave mixing, and then ultra-short optical pulses are generated. So far, monolithic integrated single quantum dot semiconductor mode-locked lasers with sub-picosecond pulse widths and dozens of gigahertz repetition frequencies have been reported. However, due to the relatively long interaction length of on-chip integrated semiconductor mode-locked lasers, a relatively large spontaneous emission noise is formed, which limits their applications in microwave and high-precision metrology.

[0004] A short gain medium and an external cavity structure can minimize the noise level and realize the adjustment of the repetition frequency. At present, mode-locked integrated external cavity surface-emitting lasers based on an optical pumping scheme have been demonstrated. However, their system structures are complex and are not conducive to the coupling between the output laser and the optical fiber.

[0005] In summary, the huge application prospects of semiconductor mode-locked lasers have promoted the rapid development of semiconductor mode-locked lasers. To enhance their application values in fields such as optical communication, optical sensing systems, and microwave photonics, it is urgent to develop semiconductor mode-locking technologies with high integration and ultra-low noise. Summary of the Invention

[0006] In view of the requirements for low-noise, high-repetition frequency, and miniaturized integrated semiconductor mode-locked lasers in the fields of optical communication technology, optical sensing technology, and microwave photonics, the present invention provides an integrated external cavity semiconductor mode-locked laser and its adjustment method. This laser has the characteristics of ultra-low noise, narrow mode linewidth, and flexible repetition frequency adjustment, and its structure is simple, stable and reliable, low-cost, and easy for large-scale production.

[0007] The technical solution of the present invention is to provide an external cavity semiconductor mode-locked laser, which is characterized in that it includes a semiconductor gain medium, a laser external cavity, an optical isolator, an optical coupling lens, and an output optical fiber arranged in sequence along the optical path; the semiconductor gain medium is a non-uniform broadening gain medium, and together with the laser external cavity, it forms a mode-locked laser oscillation cavity, and the total dispersion of the mode-locked laser oscillation cavity is negative dispersion.

[0008] The laser external cavity includes a beam collimating lens and a filtering reflection element arranged in sequence along the optical path. The beam collimating lens is used to collimate the output beam of the semiconductor gain medium into parallel light; the filtering reflection element is used to select the output center wavelength and spectral width of the laser and reflect part of the optical signal back to the semiconductor gain medium.

[0009] The optical isolator is used to prevent external reflected light from entering the mode-locked laser oscillation cavity.

[0010] The optical coupling lens is used to shape the output optical field of the mode-locked laser oscillation cavity to match the mode field of the output optical fiber.

[0011] Further, the beam collimating lens is a cylindrical lens; the filtering reflection element includes an optical bandpass filter and an optical partial reflector arranged in sequence along the optical path; the optical bandpass filter is used to select the output center wavelength and spectral width of the laser; the optical partial reflector is used to reflect part of the optical signal back to the semiconductor gain medium.

[0012] Further, the optical bandpass filter is a spatial optical bandpass filter with an optical bandwidth of 0.02 - 30 nanometers; the reflectivity of the optical partial reflector is greater than 30%; the optical isolator is a spatial optical isolator with an isolation degree greater than 15 dB.

[0013] Further, the beam collimating lens can also be a fiber lens; the filtering reflection element is a fiber grating; the output mode field of the fiber lens matches the output mode field of the semiconductor gain medium, and realizes low-loss coupling with the semiconductor gain medium. At this time, a fiber-type optical isolator can be selected for the optical isolator, and its isolation degree is greater than 15 dB.

[0014] Further, the bandwidth of the fiber grating is 0.02 - 30 nanometers, and the reflectivity is greater than 30%.

[0015] Further, the external cavity semiconductor mode-locked laser further includes a temperature control unit composed of a semiconductor refrigerator and a temperature sensor, which is used to monitor and lock the operating temperature of the laser.

[0016] Further, the semiconductor gain medium is a high-gain semiconductor gain chip, the small-signal gain of which is greater than 15 dB; an optical antireflection film is coated on the end face close to the external laser cavity; an optical high-reflection film is coated on the end face far from the external laser cavity.

[0017] Further, the semiconductor gain medium is an InP / InGaAsP chip.

[0018] Further, the external cavity semiconductor mode-locked laser further includes a photodetector located at the end of the semiconductor gain medium coated with a high-reflection optical film. The photodetector is a PIN-type photodetector and is used for real-time monitoring of the emission power.

[0019] Further, the optical coupling lens is a convex lens to achieve low-loss coupling of the output light of the mode-locked laser oscillation cavity to the output optical fiber; the output optical fiber is a single-mode optical fiber or a polarization-maintaining single-mode optical fiber.

[0020] The present invention also provides an adjustment method for an external cavity semiconductor mode-locked laser, which is characterized in that it includes the following steps:

[0021] Step 1: Adjust the temperature of the laser to stabilize its operating temperature at 25 °C;

[0022] Step 2: By adjusting the drive current of the semiconductor gain chip, enhance the optical field in the mode-locked laser oscillation cavity;

[0023] Step 3: Increase the drive current. When the drive current exceeds the threshold, the gain in the mode-locked laser oscillation cavity is greater than the loss, and multiple longitudinal modes oscillate simultaneously to emit laser;

[0024] Step 4: Further increase the drive current, and the laser obtains greater gain in the mode-locked laser oscillation cavity to generate more longitudinal modes; as the drive current increases, the nonlinear effect in the semiconductor gain chip is enhanced, and the phases of each longitudinal mode are mutually locked through the four-wave mixing effect, thereby outputting a regular pulse sequence.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1. The external cavity semiconductor mode-locked laser provided by the present invention utilizes a mature commercial semiconductor gain medium, has high efficiency and high product reliability; at the same time, by adopting an external cavity structure, flexible adjustment of the laser repetition frequency and wavelength and ultra-low noise can be achieved; and the core device has a small volume, enabling ultra-small volume packaging, and has important application values in the fields of optical sensing, communication technology, and microwave photonics.

[0027] 2. All components adopted by the present invention have mature manufacturing processes and can be manufactured through standardized manufacturing processes. Therefore, it has a low manufacturing cost and can be mass-produced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic structural diagram of an external cavity semiconductor mode-locked laser in one embodiment of the present invention;

[0029] Figure 2 It is a working principle diagram of an external cavity semiconductor mode-locked laser in one embodiment of the present invention;

[0030] Figure 3 It is a diagram of the laser evolution process of an external cavity semiconductor mode-locked laser in one embodiment of the present invention; (a) is the spectral diagram of continuous laser; (b) is the waveform diagram of continuous laser; (c) is the spectral diagram of unstable laser pulses; (d) is the waveform diagram of unstable laser pulses; (e) is the spectral diagram of mode-locked laser pulses; (f) is the waveform diagram of mode-locked laser pulses;

[0031] Figure 4 It is an experimental result diagram of an external cavity semiconductor mode-locked laser in one embodiment of the present invention. (a) is the spectral diagram of mode-locked laser pulses with a repetition frequency of 255 MHz; (b) is the spectral diagram of mode-locked laser pulses with a repetition frequency of 10 GHz; (c) is the waveform diagram of mode-locked laser pulses with a repetition frequency of 255 MHz; (d) is the waveform diagram of mode-locked laser pulses with a repetition frequency of 10 GHz; (e) is the radio frequency spectrum of mode-locked laser pulses with a repetition frequency of 255 MHz; (f) is the radio frequency spectrum of mode-locked laser pulses with a repetition frequency of 10 GHz;

[0032] Figure 5 It is the fundamental frequency spectrum diagram of an external cavity semiconductor mode-locked laser in one embodiment of the present invention, and its 3 dB spectral width is 573 Hz.

[0033] Figure 6 It is the mode linewidth test diagram of an external cavity semiconductor mode-locked laser in one embodiment of the present invention, and the measured linewidth is 5.4 kHz.

[0034] The reference numerals in the figure are:

[0035] 1 - Semiconductor gain medium; 1-1 - Semiconductor gain chip; 1-2 - Optical antireflection film; 1-3 - Optical high reflection film; 2 - Beam collimating lens; 3 - Optical bandpass filter; 4 - Optical partial reflector; 5 - Optical isolator; 6 - Optical coupling lens; 7 - Output optical fiber; 8 - Semiconductor refrigerator; 9 - Temperature sensor; 10 - Photoelectric detector. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0037] Example 1

[0038] As Figure 1 shown, the external cavity semiconductor mode-locked laser of this embodiment is composed of a semiconductor gain medium 1, a beam collimating lens 2, an optical band-pass filter 3, an optical partial reflector 4, an optical isolator 5, an optical coupling lens 6, an output optical fiber 7, a semiconductor refrigerator 8, a temperature sensor 9 and a photodetector 10. The semiconductor gain medium 1, the beam collimating lens 2, the optical band-pass filter 3, the optical partial reflector 4, the optical isolator 5, the optical coupling lens 6 and the output optical fiber 7 are arranged in sequence along the optical path. The beam collimating lens 2, the optical band-pass filter 3 and the optical partial reflector 4 form a laser external cavity. The semiconductor gain medium 1 and the laser external cavity form a laser oscillation cavity, and the total dispersion of the oscillation cavity is negative dispersion. All devices are encapsulated in a standard 14-pin butterfly package housing, and all electrical signals are connected to an external control circuit through the pins.

[0039] The semiconductor gain medium 1 is a non-uniform broadening gain medium, and a semiconductor gain chip 1-1 can be used, whose small-signal gain is greater than 15 dB; the end face close to the laser external cavity is coated with an optical antireflection film 1-2, whose transmittance is greater than 99%; the end face far from the laser external cavity is coated with an optical high-reflection film 1-3, whose reflectivity is greater than 70%. Specifically, an InP / InGaAsP chip can be used in this embodiment, adopting a quantum well structure, as the gain medium and mode-locking medium of the entire mode-locked laser; its gain bandwidth is greater than 80 nm, the gain center wavelength is 1550 nm, the saturated output power is greater than 18 dBm, and the small-signal gain is 30 dB; the end face close to the laser external cavity is coated with an optical antireflection film with a reflectivity less than 0.5%; the end face far from the laser external cavity is coated with an optical high-reflection film with a reflectivity of 90%.

[0040] The beam collimating lens 2 is a cylindrical lens, which is used to collimate the output light spot of the semiconductor gain medium 1 into approximately parallel light. The optical band-pass filter 3 is a spatial optical band-pass filter with a passband bandwidth of 0.02 - 30 nm, which is used to select the output center wavelength and spectral width of the laser. The optical partial reflector 4 is a partial light reflector with a reflectivity greater than 30%, which is used to reflect part of the optical signal back to the gain medium. The optical isolator 5 is a spatial optical isolator with an isolation degree greater than 15 dB, which is used to prevent external reflected light from entering the mode-locked laser oscillation cavity and affecting the stable operation of the laser. The optical coupling lens 6 is a convex lens, which is used for shaping the output optical field of the semiconductor mode-locked laser to match the mode field of the laser output fiber 7. The output fiber 7 is a single-mode fiber or a polarization-maintaining single-mode fiber. The semiconductor refrigerator 8 and the temperature sensor 9 together constitute the laser temperature control unit to realize the locking of the laser operating temperature. The semiconductor refrigerator 8 can be a micro semiconductor refrigerator, which is used for controlling the laser operating temperature with a maximum operating current of 2 A; the temperature sensor 9 can be a thermistor chip, which is used for real-time detection of the laser operating temperature and realizes closed-loop control of the laser operating temperature in combination with the semiconductor refrigerator. The photodetector 10 is a PIN-type photodetector, which is used for real-time monitoring of the laser emission power and is located at the end of the semiconductor gain medium 1 coated with a high-reflection optical film.

[0041] Embodiment 2

[0042] The difference between this embodiment and Embodiment 1 is that the external laser cavity in this embodiment is composed of an optical fiber lens and an optical fiber grating. The output mode field of the optical fiber lens matches the output mode field of the semiconductor gain medium 1, realizes low-loss coupling with the semiconductor gain medium 1, its light spot is 2 μm, the divergence angle is 20 - 28°, and the coupling loss with the semiconductor gain chip is less than 2 dB. The optical fiber grating in this embodiment simultaneously has the functions of a band-pass optical filter and a partial reflector, its center wavelength is 1550 nm, the 3 dB bandwidth is 0.11 nm, and the reflectivity is 80%. Correspondingly, the optical isolator 5 in this embodiment is a fiber-type optical isolator with an isolation degree of 35 dB, which is used to prevent external optical signal reflection from entering the laser and affecting the normal operation of the laser. All the optical fibers in this embodiment are single-mode polarization-maintaining optical fibers.

[0043] The following combines Figure 2 and Figure 3 to elaborate in detail on the working principle of the external cavity semiconductor mode-locked laser in Embodiment 2:

[0044] As Figure 2, the semiconductor gain chip used in the external cavity semiconductor mode-locked laser of the above embodiments is unevenly expanded. When the gain in the oscillation cavity is greater than the loss, multiple longitudinal modes will oscillate simultaneously and emit laser. However, the phases of these longitudinal modes are random, and due to mode competition, the intensities of these longitudinal modes also have large fluctuations. At this time, the total emission power of the laser is a continuous light, but with large noise, as shown in Figure 3 as shown in (a) and (b). When the drive current of the semiconductor gain chip increases, the laser obtains greater gain in the oscillation cavity, exciting more laser modes. And as the drive current of the semiconductor gain chip increases, its nonlinear effect is significantly enhanced, and a four-wave mixing effect occurs between the longitudinal modes, making the phases between the longitudinal modes correlated and forming laser pulses in the time domain. When the four-wave mixing effect is not sufficient to lock the phases of all longitudinal modes, the pulses emitted by the laser are randomly distributed, as shown in Figure 3 as shown in (c) and (d); when the four-wave mixing effect is strong enough to lock the phases of all laser modes, the laser then emits laser pulses with a continuous and uniform distribution, as shown in Figure 3 as shown in (e) and (f). The mode-locking mechanism based on the four-wave mixing effect is as shown in Figure 2 .

[0045] To illustrate the feasibility of the repetition rate adjustment of the external cavity semiconductor mode-locked laser of the present invention, two external cavity semiconductor mode-locked lasers with different repetition rates are constructed. The repetition rate of the laser is determined by the length of the laser cavity: f rep =m*c / 2nL, where c is the speed of light, nL is the optical path of the entire laser cavity, and m is a positive integer representing the harmonic mode-locking order of the semiconductor mode-locked laser. For the fundamental mode-locked laser, m = 1. Figure 4 is the experimental result diagram of the constructed external cavity semiconductor mode-locked laser, with repetition rates of 255 MHz and 10 GHz respectively, and corresponding pulse widths of 36 ps and 34 ps respectively. Figure 4 In (a) is the spectral diagram of the mode-locked laser pulse with a repetition rate of 255 MHz; (b) is the spectral diagram of the mode-locked laser pulse with a repetition rate of 10 GHz; (c) is the waveform diagram of the mode-locked laser pulse with a repetition rate of 255 MHz; (d) is the waveform diagram of the mode-locked laser pulse with a repetition rate of 10 GHz; (e) is the radio frequency spectrum of the mode-locked laser pulse with a repetition rate of 255 MHz; (f) is the radio frequency spectrum of the mode-locked laser pulse with a repetition rate of 10 GHz.

[0046] To illustrate the ultra-low noise characteristics of the external cavity semiconductor mode-locked laser of the present invention, the radio frequency spectrum and longitudinal mode linewidth of the external cavity semiconductor mode-locked laser with a repetition rate of 10 GHz are tested, and the test results are distributed as shown in Figure 5 and Figure 6As shown, the signal-to-noise ratio of its fundamental frequency reaches 81.1 dB, the 3 dB bandwidth of the fundamental frequency spectrum is 573 Hz; the linewidth of a single longitudinal mode of the mode-locked laser is 5.4 kHz.

[0047] In summary, the external cavity semiconductor mode-locked laser of the present invention has the advantages of flexible adjustable repetition frequency, low noise, simple structure, small volume, low cost, easy mass production and high reliability. It provides a reliable and low-cost laser light source for many application fields such as optical communication, optical frequency comb, spectroscopy, metrology, optical sensing, and the generation of microwave and terahertz waves.

Claims

1. An external cavity semiconductor mode-locked laser, characterized in that: it includes a semiconductor gain medium (1), a laser external cavity, an optical isolator (5), an optical coupling lens (6) and an output optical fiber (7) arranged in sequence along the optical path; the semiconductor gain medium (1) is a non-uniform broadening gain medium, and forms a mode-locked laser oscillation cavity with the laser external cavity, and the total dispersion of the mode-locked laser oscillation cavity is negative dispersion; the laser external cavity includes a beam collimating lens (2) and a filtering and reflecting element arranged in sequence along the optical path, wherein the beam collimating lens (2) is used to collimate the output beam of the semiconductor gain medium (1) into parallel light; the filtering and reflecting element is used to select the output center wavelength and spectral width of the laser and reflect part of the optical signal back to the semiconductor gain medium (1); the optical isolator (5) is used to prevent external reflected light from entering the mode-locked laser oscillation cavity; the optical coupling lens (6) is used to shape the output optical field of the mode-locked laser oscillation cavity to match its mode field with that of the output optical fiber (7); the beam collimating lens (2) is a cylindrical lens; the filtering and reflecting element includes an optical band-pass filter (3) and an optical partial reflector (4) arranged in sequence along the optical path; the optical band-pass filter (3) is used to select the output center wavelength and spectral width of the laser; the optical partial reflector (4) is used to reflect part of the optical signal back to the semiconductor gain medium (1); it also includes a temperature control unit composed of a semiconductor cooler (8) and a temperature sensor (9), which is used to monitor and lock the working temperature of the laser; the semiconductor gain medium (1) is a high-gain semiconductor gain chip, and its small-signal gain is greater than 15 dB; an optical antireflection film (1-2) is coated on the end face close to the laser external cavity; an optical high-reflection film (1-3) is coated on the end face far from the laser external cavity; it also includes a photodetector (10) located at the end face of the semiconductor gain medium (1) coated with a high-reflection optical film, and the photodetector (10) is a PIN-type photodetector, which is used for real-time monitoring of the emission power.

2. The external cavity semiconductor mode-locked laser according to claim 1, characterized in that: the optical band-pass filter (3) is a spatial optical band-pass filter, and its optical bandwidth is 0.02 - 30 nanometers; the reflectivity of the optical partial reflector (4) is greater than 30%; the optical isolator (5) is a spatial optical isolator, and its isolation degree is greater than 15 dB.

3. The external cavity semiconductor mode-locked laser according to claim 1, characterized in that: the beam collimating lens (2) is an optical fiber lens; the filtering and reflecting element is a fiber grating; wherein the output mode field of the optical fiber lens matches the output mode field of the semiconductor gain medium, and realizes low-loss coupling with the semiconductor gain medium (1); the optical isolator (5) is a fiber-type optical isolator, and its isolation degree is greater than 15 dB.

4. The external cavity semiconductor mode-locked laser according to claim 3, characterized in that: the bandwidth of the fiber grating is 0.02 - 30 nanometers, and the reflectivity is greater than 30%.

5. The external cavity semiconductor mode-locked laser according to claim 4, characterized in that: the semiconductor gain chip is an InP / InGaAsP chip.

6. The external cavity semiconductor mode-locked laser according to claim 5, characterized in that: The optical coupling lens (6) is a convex lens, which realizes the low-loss coupling of the output light of the mode-locked laser oscillator cavity to the output optical fiber (7); the output optical fiber (7) is a single-mode optical fiber or a polarization-maintaining single-mode optical fiber.

7. A method for adjusting an external cavity semiconductor mode-locked laser, using the external cavity semiconductor mode-locked laser according to claim 1, characterized in that it includes the following steps: Step 1, adjust the temperature of the laser and stabilize its operating temperature at 25 °C; Step 2, by adjusting the drive current of the semiconductor gain chip, enhance the optical field in the mode-locked laser oscillator cavity; Step 3, increase the drive current. When the drive current exceeds the threshold, the gain in the mode-locked laser oscillator cavity is greater than the loss, and multiple longitudinal modes oscillate simultaneously to emit laser; Step 4, further increase the drive current, and the laser obtains greater gain in the mode-locked laser oscillator cavity, generating more longitudinal modes; as the drive current increases, the nonlinear effect in the semiconductor gain chip enhances, and the phases of each longitudinal mode are locked to each other through the four-wave mixing effect, thereby outputting a regular pulse sequence.

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